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  • Review Article
  • Open access
  • Published: 25 October 2021

Augmented reality and virtual reality displays: emerging technologies and future perspectives

  • Jianghao Xiong 1 ,
  • En-Lin Hsiang 1 ,
  • Ziqian He 1 ,
  • Tao Zhan   ORCID: 1 &
  • Shin-Tson Wu   ORCID: 1  

Light: Science & Applications volume  10 , Article number:  216 ( 2021 ) Cite this article

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  • Liquid crystals

With rapid advances in high-speed communication and computation, augmented reality (AR) and virtual reality (VR) are emerging as next-generation display platforms for deeper human-digital interactions. Nonetheless, to simultaneously match the exceptional performance of human vision and keep the near-eye display module compact and lightweight imposes unprecedented challenges on optical engineering. Fortunately, recent progress in holographic optical elements (HOEs) and lithography-enabled devices provide innovative ways to tackle these obstacles in AR and VR that are otherwise difficult with traditional optics. In this review, we begin with introducing the basic structures of AR and VR headsets, and then describing the operation principles of various HOEs and lithography-enabled devices. Their properties are analyzed in detail, including strong selectivity on wavelength and incident angle, and multiplexing ability of volume HOEs, polarization dependency and active switching of liquid crystal HOEs, device fabrication, and properties of micro-LEDs (light-emitting diodes), and large design freedoms of metasurfaces. Afterwards, we discuss how these devices help enhance the AR and VR performance, with detailed description and analysis of some state-of-the-art architectures. Finally, we cast a perspective on potential developments and research directions of these photonic devices for future AR and VR displays.

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Recent advances in high-speed communication and miniature mobile computing platforms have escalated a strong demand for deeper human-digital interactions beyond traditional flat panel displays. Augmented reality (AR) and virtual reality (VR) headsets 1 , 2 are emerging as next-generation interactive displays with the ability to provide vivid three-dimensional (3D) visual experiences. Their useful applications include education, healthcare, engineering, and gaming, just to name a few 3 , 4 , 5 . VR embraces a total immersive experience, while AR promotes the interaction between user, digital contents, and real world, therefore displaying virtual images while remaining see-through capability. In terms of display performance, AR and VR face several common challenges to satisfy demanding human vision requirements, including field of view (FoV), eyebox, angular resolution, dynamic range, and correct depth cue, etc. Another pressing demand, although not directly related to optical performance, is ergonomics. To provide a user-friendly wearing experience, AR and VR should be lightweight and ideally have a compact, glasses-like form factor. The above-mentioned requirements, nonetheless, often entail several tradeoff relations with one another, which makes the design of high-performance AR/VR glasses/headsets particularly challenging.

In the 1990s, AR/VR experienced the first boom, which quickly subsided due to the lack of eligible hardware and digital content 6 . Over the past decade, the concept of immersive displays was revisited and received a new round of excitement. Emerging technologies like holography and lithography have greatly reshaped the AR/VR display systems. In this article, we firstly review the basic requirements of AR/VR displays and their associated challenges. Then, we briefly describe the properties of two emerging technologies: holographic optical elements (HOEs) and lithography-based devices (Fig. 1 ). Next, we separately introduce VR and AR systems because of their different device structures and requirements. For the immersive VR system, the major challenges and how these emerging technologies help mitigate the problems will be discussed. For the see-through AR system, we firstly review the present status of light engines and introduce some architectures for the optical combiners. Performance summaries on microdisplay light engines and optical combiners will be provided, that serve as a comprehensive overview of the current AR display systems.

figure 1

The left side illustrates HOEs and lithography-based devices. The right side shows the challenges in VR and architectures in AR, and how the emerging technologies can be applied

Key parameters of AR and VR displays

AR and VR displays face several common challenges to satisfy the demanding human vision requirements, such as FoV, eyebox, angular resolution, dynamic range, and correct depth cue, etc. These requirements often exhibit tradeoffs with one another. Before diving into detailed relations, it is beneficial to review the basic definitions of the above-mentioned display parameters.

Definition of parameters

Taking a VR system (Fig. 2a ) as an example. The light emitting from the display module is projected to a FoV, which can be translated to the size of the image perceived by the viewer. For reference, human vision’s horizontal FoV can be as large as 160° for monocular vision and 120° for overlapped binocular vision 6 . The intersection area of ray bundles forms the exit pupil, which is usually correlated with another parameter called eyebox. The eyebox defines the region within which the whole image FoV can be viewed without vignetting. It therefore generally manifests a 3D geometry 7 , whose volume is strongly dependent on the exit pupil size. A larger eyebox offers more tolerance to accommodate the user’s diversified interpupillary distance (IPD) and wiggling of headset when in use. Angular resolution is defined by dividing the total resolution of the display panel by FoV, which measures the sharpness of a perceived image. For reference, a human visual acuity of 20/20 amounts to 1 arcmin angular resolution, or 60 pixels per degree (PPD), which is considered as a common goal for AR and VR displays. Another important feature of a 3D display is depth cue. Depth cue can be induced by displaying two separate images to the left eye and the right eye, which forms the vergence cue. But the fixed depth of the displayed image often mismatches with the actual depth of the intended 3D image, which leads to incorrect accommodation cues. This mismatch causes the so-called vergence-accommodation conflict (VAC), which will be discussed in detail later. One important observation is that the VAC issue may be more serious in AR than VR, because the image in an AR display is directly superimposed onto the real-world with correct depth cues. The image contrast is dependent on the display panel and stray light. To achieve a high dynamic range, the display panel should exhibit high brightness, low dark level, and more than 10-bits of gray levels. Nowadays, the display brightness of a typical VR headset is about 150–200 cd/m 2 (or nits).

figure 2

a Schematic of a VR display defining FoV, exit pupil, eyebox, angular resolution, and accommodation cue mismatch. b Sketch of an AR display illustrating ACR

Figure 2b depicts a generic structure of an AR display. The definition of above parameters remains the same. One major difference is the influence of ambient light on the image contrast. For a see-through AR display, ambient contrast ratio (ACR) 8 is commonly used to quantify the image contrast:

where L on ( L off ) represents the on (off)-state luminance (unit: nit), L am is the ambient luminance, and T is the see-through transmittance. In general, ambient light is measured in illuminance (lux). For the convenience of comparison, we convert illuminance to luminance by dividing a factor of π, assuming the emission profile is Lambertian. In a normal living room, the illuminance is about 100 lux (i.e., L am  ≈ 30 nits), while in a typical office lighting condition, L am  ≈ 150 nits. For outdoors, on an overcast day, L am  ≈ 300 nits, and L am  ≈ 3000 nits on a sunny day. For AR displays, a minimum ACR should be 3:1 for recognizable images, 5:1 for adequate readability, and ≥10:1 for outstanding readability. To make a simple estimate without considering all the optical losses, to achieve ACR = 10:1 in a sunny day (~3000 nits), the display needs to deliver a brightness of at least 30,000 nits. This imposes big challenges in finding a high brightness microdisplay and designing a low loss optical combiner.

Tradeoffs and potential solutions

Next, let us briefly review the tradeoff relations mentioned earlier. To begin with, a larger FoV leads to a lower angular resolution for a given display resolution. In theory, to overcome this tradeoff only requires a high-resolution-display source, along with high-quality optics to support the corresponding modulation transfer function (MTF). To attain 60 PPD across 100° FoV requires a 6K resolution for each eye. This may be realizable in VR headsets because a large display panel, say 2–3 inches, can still accommodate a high resolution with acceptable manufacture cost. However, for a glasses-like wearable AR display, the conflict between small display size and the high solution becomes obvious as further shrinking the pixel size of a microdisplay is challenging.

To circumvent this issue, the concept of the foveated display is proposed 9 , 10 , 11 , 12 , 13 . The idea is based on that the human eye only has high visual acuity in the central fovea region, which accounts for about 10° FoV. If the high-resolution image is only projected to fovea while the peripheral image remains low resolution, then a microdisplay with 2K resolution can satisfy the need. Regarding the implementation method of foveated display, a straightforward way is to optically combine two display sources 9 , 10 , 11 : one for foveal and one for peripheral FoV. This approach can be regarded as spatial multiplexing of displays. Alternatively, time-multiplexing can also be adopted, by temporally changing the optical path to produce different magnification factors for the corresponding FoV 12 . Finally, another approach without multiplexing is to use a specially designed lens with intended distortion to achieve non-uniform resolution density 13 . Aside from the implementation of foveation, another great challenge is to dynamically steer the foveated region as the viewer’s eye moves. This task is strongly related to pupil steering, which will be discussed in detail later.

A larger eyebox or FoV usually decreases the image brightness, which often lowers the ACR. This is exactly the case for a waveguide AR system with exit pupil expansion (EPE) while operating under a strong ambient light. To improve ACR, one approach is to dynamically adjust the transmittance with a tunable dimmer 14 , 15 . Another solution is to directly boost the image brightness with a high luminance microdisplay and an efficient combiner optics. Details of this topic will be discussed in the light engine section.

Another tradeoff of FoV and eyebox in geometric optical systems results from the conservation of etendue (or optical invariant). To increase the system etendue requires a larger optics, which in turn compromises the form factor. Finally, to address the VAC issue, the display system needs to generate a proper accommodation cue, which often requires the modulation of image depth or wavefront, neither of which can be easily achieved in a traditional geometric optical system. While remarkable progresses have been made to adopt freeform surfaces 16 , 17 , 18 , to further advance AR and VR systems requires additional novel optics with a higher degree of freedom in structure design and light modulation. Moreover, the employed optics should be thin and lightweight. To mitigate the above-mentioned challenges, diffractive optics is a strong contender. Unlike geometric optics relying on curved surfaces to refract or reflect light, diffractive optics only requires a thin layer of several micrometers to establish efficient light diffractions. Two major types of diffractive optics are HOEs based on wavefront recording and manually written devices like surface relief gratings (SRGs) based on lithography. While SRGs have large design freedoms of local grating geometry, a recent publication 19 indicates the combination of HOE and freeform optics can also offer a great potential for arbitrary wavefront generation. Furthermore, the advances in lithography have also enabled optical metasurfaces beyond diffractive and refractive optics, and miniature display panels like micro-LED (light-emitting diode). These devices hold the potential to boost the performance of current AR/VR displays, while keeping a lightweight and compact form factor.

Formation and properties of HOEs

HOE generally refers to a recorded hologram that reproduces the original light wavefront. The concept of holography is proposed by Dennis Gabor 20 , which refers to the process of recording a wavefront in a medium (hologram) and later reconstructing it with a reference beam. Early holography uses intensity-sensitive recording materials like silver halide emulsion, dichromated gelatin, and photopolymer 21 . Among them, photopolymer stands out due to its easy fabrication and ability to capture high-fidelity patterns 22 , 23 . It has therefore found extensive applications like holographic data storage 23 and display 24 , 25 . Photopolymer HOEs (PPHOEs) have a relatively small refractive index modulation and therefore exhibits a strong selectivity on the wavelength and incident angle. Another feature of PPHOE is that several holograms can be recorded into a photopolymer film by consecutive exposures. Later, liquid-crystal holographic optical elements (LCHOEs) based on photoalignment polarization holography have also been developed 25 , 26 . Due to the inherent anisotropic property of liquid crystals, LCHOEs are extremely sensitive to the polarization state of the input light. This feature, combined with the polarization modulation ability of liquid crystal devices, offers a new possibility for dynamic wavefront modulation in display systems.

The formation of PPHOE is illustrated in Fig. 3a . When exposed to an interfering field with high-and-low intensity fringes, monomers tend to move toward bright fringes due to the higher local monomer-consumption rate. As a result, the density and refractive index is slightly larger in bright regions. Note the index modulation δ n here is defined as the difference between the maximum and minimum refractive indices, which may be twice the value in other definitions 27 . The index modulation δ n is typically in the range of 0–0.06. To understand the optical properties of PPHOE, we simulate a transmissive grating and a reflective grating using rigorous coupled-wave analysis (RCWA) 28 , 29 and plot the results in Fig. 3b . Details of grating configuration can be found in Table S1 . Here, the reason for only simulating gratings is that for a general HOE, the local region can be treated as a grating. The observation of gratings can therefore offer a general insight of HOEs. For a transmissive grating, its angular bandwidth (efficiency > 80%) is around 5° ( λ  = 550 nm), while the spectral band is relatively broad, with bandwidth around 175 nm (7° incidence). For a reflective grating, its spectral band is narrow, with bandwidth around 10 nm. The angular bandwidth varies with the wavelength, ranging from 2° to 20°. The strong selectivity of PPHOE on wavelength and incident angle is directly related to its small δ n , which can be adjusted by controlling the exposure dosage.

figure 3

a Schematic of the formation of PPHOE. Simulated efficiency plots for b1 transmissive and b2 reflective PPHOEs. c Working principle of multiplexed PPHOE. d Formation and molecular configurations of LCHOEs. Simulated efficiency plots for e1 transmissive and e2 reflective LCHOEs. f Illustration of polarization dependency of LCHOEs

A distinctive feature of PPHOE is the ability to multiplex several holograms into one film sample. If the exposure dosage of a recording process is controlled so that the monomers are not completely depleted in the first exposure, the remaining monomers can continue to form another hologram in the following recording process. Because the total amount of monomer is fixed, there is usually an efficiency tradeoff between multiplexed holograms. The final film sample would exhibit the wavefront modulation functions of multiple holograms (Fig. 3c ).

Liquid crystals have also been used to form HOEs. LCHOEs can generally be categorized into volume-recording type and surface-alignment type. Volume-recording type LCHOEs are either based on early polarization holography recordings with azo-polymer 30 , 31 , or holographic polymer-dispersed liquid crystals (HPDLCs) 32 , 33 formed by liquid-crystal-doped photopolymer. Surface-alignment type LCHOEs are based on photoalignment polarization holography (PAPH) 34 . The first step is to record the desired polarization pattern in a thin photoalignment layer, and the second step is to use it to align the bulk liquid crystal 25 , 35 . Due to the simple fabrication process, high efficiency, and low scattering from liquid crystal’s self-assembly nature, surface-alignment type LCHOEs based on PAPH have recently attracted increasing interest in applications like near-eye displays. Here, we shall focus on this type of surface-alignment LCHOE and refer to it as LCHOE thereafter for simplicity.

The formation of LCHOEs is illustrated in Fig. 3d . The information of the wavefront and the local diffraction pattern is recorded in a thin photoalignment layer. The volume liquid crystal deposited on the photoalignment layer, depending on whether it is nematic liquid crystal or cholesteric liquid crystal (CLC), forms a transmissive or a reflective LCHOE. In a transmissive LCHOE, the bulk nematic liquid crystal molecules generally follow the pattern of the bottom alignment layer. The smallest allowable pattern period is governed by the liquid crystal distortion-free energy model, which predicts the pattern period should generally be larger than sample thickness 36 , 37 . This results in a maximum diffraction angle under 20°. On the other hand, in a reflective LCHOE 38 , 39 , the bulk CLC molecules form a stable helical structure, which is tilted to match the k -vector of the bottom pattern. The structure exhibits a very low distorted free energy 40 , 41 and can accommodate a pattern period that is small enough to diffract light into the total internal reflection (TIR) of a glass substrate.

The diffraction property of LCHOEs is shown in Fig. 3e . The maximum refractive index modulation of LCHOE is equal to the liquid crystal birefringence (Δ n ), which may vary from 0.04 to 0.5, depending on the molecular conjugation 42 , 43 . The birefringence used in our simulation is Δ n  = 0.15. Compared to PPHOEs, the angular and spectral bandwidths are significantly larger for both transmissive and reflective LCHOEs. For a transmissive LCHOE, its angular bandwidth is around 20° ( λ  = 550 nm), while the spectral bandwidth is around 300 nm (7° incidence). For a reflective LCHOE, its spectral bandwidth is around 80 nm and angular bandwidth could vary from 15° to 50°, depending on the wavelength.

The anisotropic nature of liquid crystal leads to LCHOE’s unique polarization-dependent response to an incident light. As depicted in Fig. 3f , for a transmissive LCHOE the accumulated phase is opposite for the conjugated left-handed circular polarization (LCP) and right-handed circular polarization (RCP) states, leading to reversed diffraction directions. For a reflective LCHOE, the polarization dependency is similar to that of a normal CLC. For the circular polarization with the same handedness as the helical structure of CLC, the diffraction is strong. For the opposite circular polarization, the diffraction is negligible.

Another distinctive property of liquid crystal is its dynamic response to an external voltage. The LC reorientation can be controlled with a relatively low voltage (<10 V rms ) and the response time is on the order of milliseconds, depending mainly on the LC viscosity and layer thickness. Methods to dynamically control LCHOEs can be categorized as active addressing and passive addressing, which can be achieved by either directly switching the LCHOE or modulating the polarization state with an active waveplate. Detailed addressing methods will be described in the VAC section.

Lithography-enabled devices

Lithography technologies are used to create arbitrary patterns on wafers, which lays the foundation of the modern integrated circuit industry 44 . Photolithography is suitable for mass production while electron/ion beam lithography is usually used to create photomask for photolithography or to write structures with nanometer-scale feature size. Recent advances in lithography have enabled engineered structures like optical metasurfaces 45 , SRGs 46 , as well as micro-LED displays 47 . Metasurfaces exhibit a remarkable design freedom by varying the shape of meta-atoms, which can be utilized to achieve novel functions like achromatic focus 48 and beam steering 49 . Similarly, SRGs also offer a large design freedom by manipulating the geometry of local grating regions to realize desired optical properties. On the other hand, micro-LED exhibits several unique features, such as ultrahigh peak brightness, small aperture ratio, excellent stability, and nanosecond response time, etc. As a result, micro-LED is a promising candidate for AR and VR systems for achieving high ACR and high frame rate for suppressing motion image blurs. In the following section, we will briefly review the fabrication and properties of micro-LEDs and optical modulators like metasurfaces and SRGs.

Fabrication and properties of micro-LEDs

LEDs with a chip size larger than 300 μm have been widely used in solid-state lighting and public information displays. Recently, micro-LEDs with chip sizes <5 μm have been demonstrated 50 . The first micro-LED disc with a diameter of about 12 µm was demonstrated in 2000 51 . After that, a single color (blue or green) LED microdisplay was demonstrated in 2012 52 . The high peak brightness, fast response time, true dark state, and long lifetime of micro-LEDs are attractive for display applications. Therefore, many companies have since released their micro-LED prototypes or products, ranging from large-size TVs to small-size microdisplays for AR/VR applications 53 , 54 . Here, we focus on micro-LEDs for near-eye display applications. Regarding the fabrication of micro-LEDs, through the metal-organic chemical vapor deposition (MOCVD) method, the AlGaInP epitaxial layer is grown on GaAs substrate for red LEDs, and GaN epitaxial layers on sapphire substrate for green and blue LEDs. Next, a photolithography process is applied to define the mesa and deposit electrodes. To drive the LED array, the fabricated micro-LEDs are transferred to a CMOS (complementary metal oxide semiconductor) driver board. For a small size (<2 inches) microdisplay used in AR or VR, the precision of the pick-and-place transfer process is hard to meet the high-resolution-density (>1000 pixel per inch) requirement. Thus, the main approach to assemble LED chips with driving circuits is flip-chip bonding 50 , 55 , 56 , 57 , as Fig. 4a depicts. In flip-chip bonding, the mesa and electrode pads should be defined and deposited before the transfer process, while metal bonding balls should be preprocessed on the CMOS substrate. After that, thermal-compression method is used to bond the two wafers together. However, due to the thermal mismatch of LED chip and driving board, as the pixel size decreases, the misalignment between the LED chip and the metal bonding ball on the CMOS substrate becomes serious. In addition, the common n-GaN layer may cause optical crosstalk between pixels, which degrades the image quality. To overcome these issues, the LED epitaxial layer can be firstly metal-bonded with the silicon driver board, followed by the photolithography process to define the LED mesas and electrodes. Without the need for an alignment process, the pixel size can be reduced to <5 µm 50 .

figure 4

a Illustration of flip-chip bonding technology. b Simulated IQE-LED size relations for red and blue LEDs based on ABC model. c Comparison of EQE of different LED sizes with and without KOH and ALD side wall treatment. d Angular emission profiles of LEDs with different sizes. Metasurfaces based on e resonance-tuning, f non-resonance tuning and g combination of both. h Replication master and i replicated SRG based on nanoimprint lithography. Reproduced from a ref. 55 with permission from AIP Publishing, b ref. 61 with permission from PNAS, c ref. 66 with permission from IOP Publishing, d ref. 67 with permission from AIP Publishing, e ref. 69 with permission from OSA Publishing f ref. 48 with permission from AAAS g ref. 70 with permission from AAAS and h , i ref. 85 with permission from OSA Publishing

In addition to manufacturing process, the electrical and optical characteristics of LED also depend on the chip size. Generally, due to Shockley-Read-Hall (SRH) non-radiative recombination on the sidewall of active area, a smaller LED chip size results in a lower internal quantum efficiency (IQE), so that the peak IQE driving point will move toward a higher current density due to increased ratio of sidewall surface to active volume 58 , 59 , 60 . In addition, compared to the GaN-based green and blue LEDs, the AlGaInP-based red LEDs with a larger surface recombination and carrier diffusion length suffer a more severe efficiency drop 61 , 62 . Figure 4b shows the simulated result of IQE drop in relation with the LED chip size of blue and red LEDs based on ABC model 63 . To alleviate the efficiency drop caused by sidewall defects, depositing passivation materials by atomic layer deposition (ALD) or plasma enhanced chemical vapor deposition (PECVD) is proven to be helpful for both GaN and AlGaInP based LEDs 64 , 65 . In addition, applying KOH (Potassium hydroxide) treatment after ALD can further reduce the EQE drop of micro-LEDs 66 (Fig. 4c ). Small-size LEDs also exhibit some advantages, such as higher light extraction efficiency (LEE). Compared to an 100-µm LED, the LEE of a 2-µm LED increases from 12.2 to 25.1% 67 . Moreover, the radiation pattern of micro-LED is more directional than that of a large-size LED (Fig. 4d ). This helps to improve the lens collection efficiency in AR/VR display systems.

Metasurfaces and SGs

Thanks to the advances in lithography technology, low-loss dielectric metasurfaces working in the visible band have recently emerged as a platform for wavefront shaping 45 , 48 , 68 . They consist of an array of subwavelength-spaced structures with individually engineered wavelength-dependent polarization/phase/ amplitude response. In general, the light modulation mechanisms can be classified into resonant tuning 69 (Fig. 4e ), non-resonant tuning 48 (Fig. 4f ), and combination of both 70 (Fig. 4g ). In comparison with non-resonant tuning (based on geometric phase and/or dynamic propagation phase), the resonant tuning (such as Fabry–Pérot resonance, Mie resonance, etc.) is usually associated with a narrower operating bandwidth and a smaller out-of-plane aspect ratio (height/width) of nanostructures. As a result, they are easier to fabricate but more sensitive to fabrication tolerances. For both types, materials with a higher refractive index and lower absorption loss are beneficial to reduce the aspect ratio of nanostructure and improve the device efficiency. To this end, titanium dioxide (TiO 2 ) and gallium nitride (GaN) are the major choices for operating in the entire visible band 68 , 71 . While small-sized metasurfaces (diameter <1 mm) are usually fabricated via electron-beam lithography or focused ion beam milling in the labs, the ability of mass production is the key to their practical adoption. The deep ultraviolet (UV) photolithography has proven its feasibility for reproducing centimeter-size metalenses with decent imaging performance, while it requires multiple steps of etching 72 . Interestingly, the recently developed UV nanoimprint lithography based on a high-index nanocomposite only takes a single step and can obtain an aspect ratio larger than 10, which shows great promise for high-volume production 73 .

The arbitrary wavefront shaping capability and the thinness of the metasurfaces have aroused strong research interests in the development of novel AR/VR prototypes with improved performance. Lee et al. employed nanoimprint lithography to fabricate a centimeter-size, geometric-phase metalens eyepiece for full-color AR displays 74 . Through tailoring its polarization conversion efficiency and stacking with a circular polarizer, the virtual image can be superimposed with the surrounding scene. The large numerical aperture (NA~0.5) of the metalens eyepiece enables a wide FoV (>76°) that conventional optics are difficult to obtain. However, the geometric phase metalens is intrinsically a diffractive lens that also suffers from strong chromatic aberrations. To overcome this issue, an achromatic lens can be designed via simultaneously engineering the group delay and the group delay dispersion 75 , 76 , which will be described in detail later. Other novel and/or improved near-eye display architectures include metasurface-based contact lens-type AR 77 , achromatic metalens array enabled integral-imaging light field displays 78 , wide FoV lightguide AR with polarization-dependent metagratings 79 , and off-axis projection-type AR with an aberration-corrected metasurface combiner 80 , 81 , 82 . Nevertheless, from the existing AR/VR prototypes, metasurfaces still face a strong tradeoff between numerical aperture (for metalenses), chromatic aberration, monochromatic aberration, efficiency, aperture size, and fabrication complexity.

On the other hand, SRGs are diffractive gratings that have been researched for decades as input/output couplers of waveguides 83 , 84 . Their surface is composed of corrugated microstructures, and different shapes including binary, blazed, slanted, and even analogue can be designed. The parameters of the corrugated microstructures are determined by the target diffraction order, operation spectral bandwidth, and angular bandwidth. Compared to metasurfaces, SRGs have a much larger feature size and thus can be fabricated via UV photolithography and subsequent etching. They are usually replicated by nanoimprint lithography with appropriate heating and surface treatment. According to a report published a decade ago, SRGs with a height of 300 nm and a slant angle of up to 50° can be faithfully replicated with high yield and reproducibility 85 (Fig. 4g, h ).

Challenges and solutions of VR displays

The fully immersive nature of VR headset leads to a relatively fixed configuration where the display panel is placed in front of the viewer’s eye and an imaging optics is placed in-between. Regarding the system performance, although inadequate angular resolution still exists in some current VR headsets, the improvement of display panel resolution with advanced fabrication process is expected to solve this issue progressively. Therefore, in the following discussion, we will mainly focus on two major challenges: form factor and 3D cue generation.

Form factor

Compact and lightweight near-eye displays are essential for a comfortable user experience and therefore highly desirable in VR headsets. Current mainstream VR headsets usually have a considerably larger volume than eyeglasses, and most of the volume is just empty. This is because a certain distance is required between the display panel and the viewing optics, which is usually close to the focal length of the lens system as illustrated in Fig. 5a . Conventional VR headsets employ a transmissive lens with ~4 cm focal length to offer a large FoV and eyebox. Fresnel lenses are thinner than conventional ones, but the distance required between the lens and the panel does not change significantly. In addition, the diffraction artifacts and stray light caused by the Fresnel grooves can degrade the image quality, or MTF. Although the resolution density, quantified as pixel per inch (PPI), of current VR headsets is still limited, eventually Fresnel lens will not be an ideal solution when a high PPI display is available. The strong chromatic aberration of Fresnel singlet should also be compensated if a high-quality imaging system is preferred.

figure 5

a Schematic of a basic VR optical configuration. b Achromatic metalens used as VR eyepiece. c VR based on curved display and lenslet array. d Basic working principle of a VR display based on pancake optics. e VR with pancake optics and Fresnel lens array. f VR with pancake optics based on purely HOEs. Reprinted from b ref. 87 under the Creative Commons Attribution 4.0 License. Adapted from c ref. 88 with permission from IEEE, e ref. 91 and f ref. 92 under the Creative Commons Attribution 4.0 License

It is tempting to replace the refractive elements with a single thin diffractive lens like a transmissive LCHOE. However, the diffractive nature of such a lens will result in serious color aberrations. Interestingly, metalenses can fulfil this objective without color issues. To understand how metalenses achieve achromatic focus, let us first take a glance at the general lens phase profile \(\Phi (\omega ,r)\) expanded as a Taylor series 75 :

where \(\varphi _0(\omega )\) is the phase at the lens center, \(F\left( \omega \right)\) is the focal length as a function of frequency ω , r is the radial coordinate, and \(\omega _0\) is the central operation frequency. To realize achromatic focus, \(\partial F{{{\mathrm{/}}}}\partial \omega\) should be zero. With a designed focal length, the group delay \(\partial \Phi (\omega ,r){{{\mathrm{/}}}}\partial \omega\) and the group delay dispersion \(\partial ^2\Phi (\omega ,r){{{\mathrm{/}}}}\partial \omega ^2\) can be determined, and \(\varphi _0(\omega )\) is an auxiliary degree of freedom of the phase profile design. In the design of an achromatic metalens, the group delay is a function of the radial coordinate and monotonically increases with the metalens radius. Many designs have proven that the group delay has a limited variation range 75 , 76 , 78 , 86 . According to Shrestha et al. 86 , there is an inevitable tradeoff between the maximum radius of the metalens, NA, and operation bandwidth. Thus, the reported achromatic metalenses at visible usually have limited lens aperture (e.g., diameter < 250 μm) and NA (e.g., <0.2). Such a tradeoff is undesirable in VR displays, as the eyepiece favors a large clear aperture (inch size) and a reasonably high NA (>0.3) to maintain a wide FoV and a reasonable eye relief 74 .

To overcome this limitation, Li et al. 87 proposed a novel zone lens method. Unlike the traditional phase Fresnel lens where the zones are determined by the phase reset, the new approach divides the zones by the group delay reset. In this way, the lens aperture and NA can be much enlarged, and the group delay limit is bypassed. A notable side effect of this design is the phase discontinuity at zone boundaries that will contribute to higher-order focusing. Therefore, significant efforts have been conducted to find the optimal zone transition locations and to minimize the phase discontinuities. Using this method, they have demonstrated an impressive 2-mm-diameter metalens with NA = 0.7 and nearly diffraction-limited focusing for the designed wavelengths (488, 532, 658 nm) (Fig. 5b ). Such a metalens consists of 681 zones and works for the visible band ranging from 470 to 670 nm, though the focusing efficiency is in the order of 10%. This is a great starting point for the achromatic metalens to be employed as a compact, chromatic-aberration-free eyepiece in near-eye displays. Future challenges are how to further increase the aperture size, correct the off-axis aberrations, and improve the optical efficiency.

Besides replacing the refractive lens with an achromatic metalens, another way to reduce system focal length without decreasing NA is to use a lenslet array 88 . As depicted in Fig. 5c , both the lenslet array and display panel adopt a curved structure. With the latest flexible OLED panel, the display can be easily curved in one dimension. The system exhibits a large diagonal FoV of 180° with an eyebox of 19 by 12 mm. The geometry of each lenslet is optimized separately to achieve an overall performance with high image quality and reduced distortions.

Aside from trying to shorten the system focal length, another way to reduce total track is to fold optical path. Recently, polarization-based folded lenses, also known as pancake optics, are under active development for VR applications 89 , 90 . Figure 5d depicts the structure of an exemplary singlet pancake VR lens system. The pancake lenses can offer better imaging performance with a compact form factor since there are more degrees of freedom in the design and the actual light path is folded thrice. By using a reflective surface with a positive power, the field curvature of positive refractive lenses can be compensated. Also, the reflective surface has no chromatic aberrations and it contributes considerable optical power to the system. Therefore, the optical power of refractive lenses can be smaller, resulting in an even weaker chromatic aberration. Compared to Fresnel lenses, the pancake lenses have smooth surfaces and much fewer diffraction artifacts and stray light. However, such a pancake lens design is not perfect either, whose major shortcoming is low light efficiency. With two incidences of light on the half mirror, the maximum system efficiency is limited to 25% for a polarized input and 12.5% for an unpolarized input light. Moreover, due to the existence of multiple surfaces in the system, stray light caused by surface reflections and polarization leakage may lead to apparent ghost images. As a result, the catadioptric pancake VR headset usually manifests a darker imagery and lower contrast than the corresponding dioptric VR.

Interestingly, the lenslet and pancake optics can be combined to further reduce the system form. Bang et al. 91 demonstrated a compact VR system with a pancake optics and a Fresnel lenslet array. The pancake optics serves to fold the optical path between the display panel and the lenslet array (Fig. 5e ). Another Fresnel lens is used to collect the light from the lenslet array. The system has a decent horizontal FoV of 102° and an eyebox of 8 mm. However, a certain degree of image discontinuity and crosstalk are still present, which can be improved with further optimizations on the Fresnel lens and the lenslet array.

One step further, replacing all conventional optics in catadioptric VR headset with holographic optics can make the whole system even thinner. Maimone and Wang demonstrated such a lightweight, high-resolution, and ultra-compact VR optical system using purely HOEs 92 . This holographic VR optics was made possible by combining several innovative optical components, including a reflective PPHOE, a reflective LCHOE, and a PPHOE-based directional backlight with laser illumination, as shown in Fig. 5f . Since all the optical power is provided by the HOEs with negligible weight and volume, the total physical thickness can be reduced to <10 mm. Also, unlike conventional bulk optics, the optical power of a HOE is independent of its thickness, only subject to the recording process. Another advantage of using holographic optical devices is that they can be engineered to offer distinct phase profiles for different wavelengths and angles of incidence, adding extra degrees of freedom in optical designs for better imaging performance. Although only a single-color backlight has been demonstrated, such a PPHOE has the potential to achieve full-color laser backlight with multiplexing ability. The PPHOE and LCHOE in the pancake optics can also be optimized at different wavelengths for achieving high-quality full-color images.

Vergence-accommodation conflict

Conventional VR displays suffer from VAC, which is a common issue for stereoscopic 3D displays 93 . In current VR display modules, the distance between the display panel and the viewing optics is fixed, which means the VR imagery is displayed at a single depth. However, the image contents are generated by parallax rendering in three dimensions, offering distinct images for two eyes. This approach offers a proper stimulus to vergence but completely ignores the accommodation cue, which leads to the well-known VAC that can cause an uncomfortable user experience. Since the beginning of this century, numerous methods have been proposed to solve this critical issue. Methods to produce accommodation cue include multifocal/varifocal display 94 , holographic display 95 , and integral imaging display 96 . Alternatively, elimination of accommodation cue using a Maxwellian-view display 93 also helps to mitigate the VAC. However, holographic displays and Maxwellian-view displays generally require a totally different optical architecture than current VR systems. They are therefore more suitable for AR displays, which will be discussed later. Integral imaging, on the other hand, has an inherent tradeoff between view number and resolution. For current VR headsets pursuing high resolution to match human visual acuity, it may not be an appealing solution. Therefore, multifocal/varifocal displays that rely on depth modulation is a relatively practical and effective solution for VR headsets. Regarding the working mechanism, multifocal displays present multiple images with different depths to imitate the original 3D scene. Varifocal displays, in contrast, only show one image at each time frame. The image depth matches the viewer’s vergence depth. Nonetheless, the pre-knowledge of the viewer’s vergence depth requires an additional eye-tracking module. Despite different operation principles, a varifocal display can often be converted to a multifocal display as long as the varifocal module has enough modulation bandwidth to support multiple depths in a time frame.

To achieve depth modulation in a VR system, traditional liquid lens 97 , 98 with tunable focus suffers from the small aperture and large aberrations. Alvarez lens 99 is another tunable-focus solution but it requires mechanical adjustment, which adds to system volume and complexity. In comparison, transmissive LCHOEs with polarization dependency can achieve focus adjustment with electronic driving. Its ultra-thinness also satisfies the requirement of small form factors in VR headsets. The diffractive behavior of transmissive LCHOEs is often interpreted by the mechanism of Pancharatnam-Berry phase (also known as geometric phase) 100 . They are therefore often called Pancharatnam-Berry optical elements (PBOEs). The corresponding lens component is referred as Pancharatnam-Berry lens (PBL).

Two main approaches are used to switch the focus of a PBL, active addressing and passive addressing. In active addressing, the PBL itself (made of LC) can be switched by an applied voltage (Fig. 6a ). The optical power of the liquid crystal PBLs can be turned-on and -off by controlling the voltage. Stacking multiple active PBLs can produce 2 N depths, where N is the number of PBLs. The drawback of using active PBLs, however, is the limited spectral bandwidth since their diffraction efficiency is usually optimized at a single wavelength. In passive addressing, the depth modulation is achieved through changing the polarization state of input light by a switchable half-wave plate (HWP) (Fig. 6b ). The focal length can therefore be switched thanks to the polarization sensitivity of PBLs. Although this approach has a slightly more complicated structure, the overall performance can be better than the active one, because the PBLs made of liquid crystal polymer can be designed to manifest high efficiency within the entire visible spectrum 101 , 102 .

figure 6

Working principles of a depth switching PBL module based on a active addressing and b passive addressing. c A four-depth multifocal display based on time multiplexing. d A two-depth multifocal display based on polarization multiplexing. Reproduced from c ref. 103 with permission from OSA Publishing and d ref. 104 with permission from OSA Publishing

With the PBL module, multifocal displays can be built using time-multiplexing technique. Zhan et al. 103 demonstrated a four-depth multifocal display using two actively switchable liquid crystal PBLs (Fig. 6c ). The display is synchronized with the PBL module, which lowers the frame rate by the number of depths. Alternatively, multifocal displays can also be achieved by polarization-multiplexing, as demonstrated by Tan et al. 104 . The basic principle is to adjust the polarization state of local pixels so the image content on two focal planes of a PBL can be arbitrarily controlled (Fig. 6d ). The advantage of polarization multiplexing is that it does not sacrifice the frame rate, but it can only support two planes because only two orthogonal polarization states are available. Still, it can be combined with time-multiplexing to reduce the frame rate sacrifice by half. Naturally, varifocal displays can also be built with a PBL module. A fast-response 64-depth varifocal module with six PBLs has been demonstrated 105 .

The compact structure of PBL module leads to a natural solution of integrating it with above-mentioned pancake optics. A compact VR headset with dynamic depth modulation to solve VAC is therefore possible in practice. Still, due to the inherent diffractive nature of PBL, the PBL module face the issue of chromatic dispersion of focal length. To compensate for different focal depths for RGB colors may require additional digital corrections in image-rendering.

Architectures of AR displays

Unlike VR displays with a relatively fixed optical configuration, there exist a vast number of architectures in AR displays. Therefore, instead of following the narrative of tackling different challenges, a more appropriate way to review AR displays is to separately introduce each architecture and discuss its associated engineering challenges. An AR display usually consists of a light engine and an optical combiner. The light engine serves as display image source, while the combiner delivers the displayed images to viewer’s eye and in the meantime transmits the environment light. Some performance parameters like frame rate and power consumption are mainly determined by the light engine. Parameters like FoV, eyebox and MTF are primarily dependent on the combiner optics. Moreover, attributes like image brightness, overall efficiency, and form factor are influenced by both light engine and combiner. In this section, we will firstly discuss the light engine, where the latest advances in micro-LED on chip are reviewed and compared with existing microdisplay systems. Then, we will introduce two main types of combiners: free-space combiner and waveguide combiner.

Light engine

The light engine determines several essential properties of the AR system like image brightness, power consumption, frame rate, and basic etendue. Several types of microdisplays have been used in AR, including micro-LED, micro-organic-light-emitting-diodes (micro-OLED), liquid-crystal-on-silicon (LCoS), digital micromirror device (DMD), and laser beam scanning (LBS) based on micro-electromechanical system (MEMS). We will firstly describe the working principles of these devices and then analyze their performance. For those who are more interested in final performance parameters than details, Table 1 provides a comprehensive summary.

Working principles

Micro-LED and micro-OLED are self-emissive display devices. They are usually more compact than LCoS and DMD because no illumination optics is required. The fundamentally different material systems of LED and OLED lead to different approaches to achieve full-color displays. Due to the “green gap” in LEDs, red LEDs are manufactured on a different semiconductor material from green and blue LEDs. Therefore, how to achieve full-color display in high-resolution density microdisplays is quite a challenge for micro-LEDs. Among several solutions under research are two main approaches. The first is to combine three separate red, green and blue (RGB) micro-LED microdisplay panels 106 . Three single-color micro-LED microdisplays are manufactured separately through flip-chip transfer technology. Then, the projected images from three microdisplay panels are integrated by a trichroic prism (Fig. 7a ).

figure 7

a RGB micro-LED microdisplays combined by a trichroic prism. b QD-based micro-LED microdisplay. c Micro-OLED display with 4032 PPI. Working principles of d LCoS, e DMD, and f MEMS-LBS display modules. Reprinted from a ref. 106 with permission from IEEE, b ref. 108 with permission from Chinese Laser Press, c ref. 121 with permission from Jon Wiley and Sons, d ref. 124 with permission from Spring Nature, e ref. 126 with permission from Springer and f ref. 128 under the Creative Commons Attribution 4.0 License

Another solution is to assemble color-conversion materials like quantum dot (QD) on top of blue or ultraviolet (UV) micro-LEDs 107 , 108 , 109 (Fig. 7b ). The quantum dot color filter (QDCF) on top of the micro-LED array is mainly fabricated by inkjet printing or photolithography 110 , 111 . However, the display performance of color-conversion micro-LED displays is restricted by the low color-conversion efficiency, blue light leakage, and color crosstalk. Extensive efforts have been conducted to improve the QD-micro-LED performance. To boost QD conversion efficiency, structure designs like nanoring 112 and nanohole 113 , 114 have been proposed, which utilize the Förster resonance energy transfer mechanism to transfer excessive excitons in the LED active region to QD. To prevent blue light leakage, methods using color filters or reflectors like distributed Bragg reflector (DBR) 115 and CLC film 116 on top of QDCF are proposed. Compared to color filters that absorb blue light, DBR and CLC film help recycle the leaked blue light to further excite QDs. Other methods to achieve full-color micro-LED display like vertically stacked RGB micro-LED array 61 , 117 , 118 and monolithic wavelength tunable nanowire LED 119 are also under investigation.

Micro-OLED displays can be generally categorized into RGB OLED and white OLED (WOLED). RGB OLED displays have separate sub-pixel structures and optical cavities, which resonate at the desirable wavelength in RGB channels, respectively. To deposit organic materials onto the separated RGB sub-pixels, a fine metal mask (FMM) that defines the deposition area is required. However, high-resolution RGB OLED microdisplays still face challenges due to the shadow effect during the deposition process through FMM. In order to break the limitation, a silicon nitride film with small shadow has been proposed as a mask for high-resolution deposition above 2000 PPI (9.3 µm) 120 .

WOLED displays use color filters to generate color images. Without the process of depositing patterned organic materials, a high-resolution density up to 4000 PPI has been achieved 121 (Fig. 7c ). However, compared to RGB OLED, the color filters in WOLED absorb about 70% of the emitted light, which limits the maximum brightness of the microdisplay. To improve the efficiency and peak brightness of WOLED microdisplays, in 2019 Sony proposed to apply newly designed cathodes (InZnO) and microlens arrays on OLED microdisplays, which increased the peak brightness from 1600 nits to 5000 nits 120 . In addition, OLEDWORKs has proposed a multi-stacked OLED 122 with optimized microcavities whose emission spectra match the transmission bands of the color filters. The multi-stacked OLED shows a higher luminous efficiency (cd/A), but also requires a higher driving voltage. Recently, by using meta-mirrors as bottom reflective anodes, patterned microcavities with more than 10,000 PPI have been obtained 123 . The high-resolution meta-mirrors generate different reflection phases in the RGB sub-pixels to achieve desirable resonant wavelengths. The narrow emission spectra from the microcavity help to reduce the loss from color filters or even eliminate the need of color filters.

LCoS and DMD are light-modulating displays that generate images by controlling the reflection of each pixel. For LCoS, the light modulation is achieved by manipulating the polarization state of output light through independently controlling the liquid crystal reorientation in each pixel 124 , 125 (Fig. 7d ). Both phase-only and amplitude modulators have been employed. DMD is an amplitude modulation device. The modulation is achieved through controlling the tilt angle of bi-stable micromirrors 126 (Fig. 7e ). To generate an image, both LCoS and DMD rely on the light illumination systems, with LED or laser as light source. For LCoS, the generation of color image can be realized either by RGB color filters on LCoS (with white LEDs) or color-sequential addressing (with RGB LEDs or lasers). However, LCoS requires a linearly polarized light source. For an unpolarized LED light source, usually, a polarization recycling system 127 is implemented to improve the optical efficiency. For a single-panel DMD, the color image is mainly obtained through color-sequential addressing. In addition, DMD does not require a polarized light so that it generally exhibits a higher efficiency than LCoS if an unpolarized light source is employed.

MEMS-based LBS 128 , 129 utilizes micromirrors to directly scan RGB laser beams to form two-dimensional (2D) images (Fig. 7f ). Different gray levels are achieved by pulse width modulation (PWM) of the employed laser diodes. In practice, 2D scanning can be achieved either through a 2D scanning mirror or two 1D scanning mirrors with an additional focusing lens after the first mirror. The small size of MEMS mirror offers a very attractive form factor. At the same time, the output image has a large depth-of-focus (DoF), which is ideal for projection displays. One shortcoming, though, is that the small system etendue often hinders its applications in some traditional display systems.

Comparison of light engine performance

There are several important parameters for a light engine, including image resolution, brightness, frame rate, contrast ratio, and form factor. The resolution requirement (>2K) is similar for all types of light engines. The improvement of resolution is usually accomplished through the manufacturing process. Thus, here we shall focus on other three parameters.

Image brightness usually refers to the measured luminance of a light-emitting object. This measurement, however, may not be accurate for a light engine as the light from engine only forms an intermediate image, which is not directly viewed by the user. On the other hand, to solely focus on the brightness of a light engine could be misleading for a wearable display system like AR. Nowadays, data projectors with thousands of lumens are available. But the power consumption is too high for a battery-powered wearable AR display. Therefore, a more appropriate way to evaluate a light engine’s brightness is to use luminous efficacy (lm/W) measured by dividing the final output luminous flux (lm) by the input electric power (W). For a self-emissive device like micro-LED or micro-OLED, the luminous efficacy is directly determined by the device itself. However, for LCoS and DMD, the overall luminous efficacy should take into consideration the light source luminous efficacy, the efficiency of illumination optics, and the efficiency of the employed spatial light modulator (SLM). For a MEMS LBS engine, the efficiency of MEMS mirror can be considered as unity so that the luminous efficacy basically equals to that of the employed laser sources.

As mentioned earlier, each light engine has a different scheme for generating color images. Therefore, we separately list luminous efficacy of each scheme for a more inclusive comparison. For micro-LEDs, the situation is more complicated because the EQE depends on the chip size. Based on previous studies 130 , 131 , 132 , 133 , we separately calculate the luminous efficacy for RGB micro-LEDs with chip size ≈ 20 µm. For the scheme of direct combination of RGB micro-LEDs, the luminous efficacy is around 5 lm/W. For QD-conversion with blue micro-LEDs, the luminous efficacy is around 10 lm/W with the assumption of 100% color conversion efficiency, which has been demonstrated using structure engineering 114 . For micro-OLEDs, the calculated luminous efficacy is about 4–8 lm/W 120 , 122 . However, the lifetime and EQE of blue OLED materials depend on the driving current. To continuously display an image with brightness higher than 10,000 nits may dramatically shorten the device lifetime. The reason we compare the light engine at 10,000 nits is that it is highly desirable to obtain 1000 nits for the displayed image in order to keep ACR>3:1 with a typical AR combiner whose optical efficiency is lower than 10%.

For an LCoS engine using a white LED as light source, the typical optical efficiency of the whole engine is around 10% 127 , 134 . Then the engine luminous efficacy is estimated to be 12 lm/W with a 120 lm/W white LED source. For a color sequential LCoS using RGB LEDs, the absorption loss from color filters is eliminated, but the luminous efficacy of RGB LED source is also decreased to about 30 lm/W due to lower efficiency of red and green LEDs and higher driving current 135 . Therefore, the final luminous efficacy of the color sequential LCoS engine is also around 10 lm/W. If RGB linearly polarized lasers are employed instead of LEDs, then the LCoS engine efficiency can be quite high due to the high degree of collimation. The luminous efficacy of RGB laser source is around 40 lm/W 136 . Therefore, the laser-based LCoS engine is estimated to have a luminous efficacy of 32 lm/W, assuming the engine optical efficiency is 80%. For a DMD engine with RGB LEDs as light source, the optical efficiency is around 50% 137 , 138 , which leads to a luminous efficacy of 15 lm/W. By switching to laser light sources, the situation is similar to LCoS, with the luminous efficacy of about 32 lm/W. Finally, for MEMS-based LBS engine, there is basically no loss from the optics so that the final luminous efficacy is 40 lm/W. Detailed calculations of luminous efficacy can be found in Supplementary Information .

Another aspect of a light engine is the frame rate, which determines the volume of information it can deliver in a unit time. A high volume of information is vital for the construction of a 3D light field to solve the VAC issue. For micro-LEDs, the device response time is around several nanoseconds, which allows for visible light communication with bandwidth up to 1.5 Gbit/s 139 . For an OLED microdisplay, a fast OLED with ~200 MHz bandwidth has been demonstrated 140 . Therefore, the limitation of frame rate is on the driving circuits for both micro-LED and OLED. Another fact concerning driving circuit is the tradeoff between resolution and frame rate as a higher resolution panel means more scanning lines in each frame. So far, an OLED display with 480 Hz frame rate has been demonstrated 141 . For an LCoS, the frame rate is mainly limited by the LC response time. Depending on the LC material used, the response time is around 1 ms for nematic LC or 200 µs for ferroelectric LC (FLC) 125 . Nematic LC allows analog driving, which accommodates gray levels, typically with 8-bit depth. FLC is bistable so that PWM is used to generate gray levels. DMD is also a binary device. The frame rate can reach 30 kHz, which is mainly constrained by the response time of micromirrors. For MEMS-based LBS, the frame rate is limited by the scanning frequency of MEMS mirrors. A frame rate of 60 Hz with around 1 K resolution already requires a resonance frequency of around 50 kHz, with a Q-factor up to 145,000 128 . A higher frame rate or resolution requires a higher Q-factor and larger laser modulation bandwidth, which may be challenging.

Form factor is another crucial aspect for the light engines of near-eye displays. For self-emissive displays, both micro-OLEDs and QD-based micro-LEDs can achieve full color with a single panel. Thus, they are quite compact. A micro-LED display with separate RGB panels naturally have a larger form factor. In applications requiring direct-view full-color panel, the extra combining optics may also increase the volume. It needs to be pointed out, however, that the combing optics may not be necessary for some applications like waveguide displays, because the EPE process results in system’s insensitivity to the spatial positions of input RGB images. Therefore, the form factor of using three RGB micro-LED panels is medium. For LCoS and DMD with RGB LEDs as light source, the form factor would be larger due to the illumination optics. Still, if a lower luminous efficacy can be accepted, then a smaller form factor can be achieved by using a simpler optics 142 . If RGB lasers are used, the collimation optics can be eliminated, which greatly reduces the form factor 143 . For MEMS-LBS, the form factor can be extremely compact due to the tiny size of MEMS mirror and laser module.

Finally, contrast ratio (CR) also plays an important role affecting the observed images 8 . Micro-LEDs and micro-OLEDs are self-emissive so that their CR can be >10 6 :1. For a laser beam scanner, its CR can also achieve 10 6 :1 because the laser can be turned off completely at dark state. On the other hand, LCoS and DMD are reflective displays, and their CR is around 2000:1 to 5000:1 144 , 145 . It is worth pointing out that the CR of a display engine plays a significant role only in the dark ambient. As the ambient brightness increases, the ACR is mainly governed by the display’s peak brightness, as previously discussed.

The performance parameters of different light engines are summarized in Table 1 . Micro-LEDs and micro-OLEDs have similar levels of luminous efficacy. But micro-OLEDs still face the burn-in and lifetime issue when driving at a high current, which hinders its use for a high-brightness image source to some extent. Micro-LEDs are still under active development and the improvement on luminous efficacy from maturing fabrication process could be expected. Both devices have nanosecond response time and can potentially achieve a high frame rate with a well-designed integrated circuit. The frame rate of the driving circuit ultimately determines the motion picture response time 146 . Their self-emissive feature also leads to a small form factor and high contrast ratio. LCoS and DMD engines have similar performance of luminous efficacy, form factor, and contrast ratio. In terms of light modulation, DMD can provide a higher 1-bit frame rate, while LCoS can offer both phase and amplitude modulations. MEMS-based LBS exhibits the highest luminous efficacy so far. It also exhibits an excellent form factor and contrast ratio, but the presently demonstrated 60-Hz frame rate (limited by the MEMS mirrors) could cause image flickering.

Free-space combiners

The term ‘free-space’ generally refers to the case when light is freely propagating in space, as opposed to a waveguide that traps light into TIRs. Regarding the combiner, it can be a partial mirror, as commonly used in AR systems based on traditional geometric optics. Alternatively, the combiner can also be a reflective HOE. The strong chromatic dispersion of HOE necessitates the use of a laser source, which usually leads to a Maxwellian-type system.

Traditional geometric designs

Several systems based on geometric optics are illustrated in Fig. 8 . The simplest design uses a single freeform half-mirror 6 , 147 to directly collimate the displayed images to the viewer’s eye (Fig. 8a ). This design can achieve a large FoV (up to 90°) 147 , but the limited design freedom with a single freeform surface leads to image distortions, also called pupil swim 6 . The placement of half-mirror also results in a relatively bulky form factor. Another design using so-called birdbath optics 6 , 148 is shown in Fig. 8b . Compared to the single-combiner design, birdbath design has an extra optics on the display side, which provides space for aberration correction. The integration of beam splitter provides a folded optical path, which reduces the form factor to some extent. Another way to fold optical path is to use a TIR-prism. Cheng et al. 149 designed a freeform TIR-prism combiner (Fig. 8c ) offering a diagonal FoV of 54° and exit pupil diameter of 8 mm. All the surfaces are freeform, which offer an excellent image quality. To cancel the optical power for the transmitted environmental light, a compensator is added to the TIR prism. The whole system has a well-balanced performance between FoV, eyebox, and form factor. To release the space in front of viewer’s eye, relay optics can be used to form an intermediate image near the combiner 150 , 151 , as illustrated in Fig. 8d . Although the design offers more optical surfaces for aberration correction, the extra lenses also add to system weight and form factor.

figure 8

a Single freeform surface as the combiner. b Birdbath optics with a beam splitter and a half mirror. c Freeform TIR prism with a compensator. d Relay optics with a half mirror. Adapted from c ref. 149 with permission from OSA Publishing and d ref. 151 with permission from OSA Publishing

Regarding the approaches to solve the VAC issue, the most straightforward way is to integrate a tunable lens into the optical path, like a liquid lens 152 or Alvarez lens 99 , to form a varifocal system. Alternatively, integral imaging 153 , 154 can also be used, by replacing the original display panel with the central depth plane of an integral imaging module. The integral imaging can also be combined with varifocal approach to overcome the tradeoff between resolution and depth of field (DoF) 155 , 156 , 157 . However, the inherent tradeoff between resolution and view number still exists in this case.

Overall, AR displays based on traditional geometric optics have a relatively simple design with a decent FoV (~60°) and eyebox (8 mm) 158 . They also exhibit a reasonable efficiency. To measure the efficiency of an AR combiner, an appropriate measure is to divide the output luminance (unit: nit) by the input luminous flux (unit: lm), which we note as combiner efficiency. For a fixed input luminous flux, the output luminance, or image brightness, is related to the FoV and exit pupil of the combiner system. If we assume no light waste of the combiner system, then the maximum combiner efficiency for a typical diagonal FoV of 60° and exit pupil (10 mm square) is around 17,000 nit/lm (Eq. S2 ). To estimate the combiner efficiency of geometric combiners, we assume 50% of half-mirror transmittance and the efficiency of other optics to be 50%. Then the final combiner efficiency is about 4200 nit/lm, which is a high value in comparison with waveguide combiners. Nonetheless, to further shrink the system size or improve system performance ultimately encounters the etendue conservation issue. In addition, AR systems with traditional geometric optics is hard to achieve a configuration resembling normal flat glasses because the half-mirror has to be tilted to some extent.

Maxwellian-type systems

The Maxwellian view, proposed by James Clerk Maxwell (1860), refers to imaging a point light source in the eye pupil 159 . If the light beam is modulated in the imaging process, a corresponding image can be formed on the retina (Fig. 9a ). Because the point source is much smaller than the eye pupil, the image is always-in-focus on the retina irrespective of the eye lens’ focus. For applications in AR display, the point source is usually a laser with narrow angular and spectral bandwidths. LED light sources can also build a Maxwellian system, by adding an angular filtering module 160 . Regarding the combiner, although in theory a half-mirror can also be used, HOEs are generally preferred because they offer the off-axis configuration that places combiner in a similar position like eyeglasses. In addition, HOEs have a lower reflection of environment light, which provides a more natural appearance of the user behind the display.

figure 9

a Schematic of the working principle of Maxwellian displays. Maxwellian displays based on b SLM and laser diode light source and c MEMS-LBS with a steering mirror as additional modulation method. Generation of depth cues by d computational digital holography and e scanning of steering mirror to produce multiple views. Adapted from b, d ref. 143 and c, e ref. 167 under the Creative Commons Attribution 4.0 License

To modulate the light, a SLM like LCoS or DMD can be placed in the light path, as shown in Fig. 9b . Alternatively, LBS system can also be used (Fig. 9c ), where the intensity modulation occurs in the laser diode itself. Besides the operation in a normal Maxwellian-view, both implementations offer additional degrees of freedom for light modulation.

For a SLM-based system, there are several options to arrange the SLM pixels 143 , 161 . Maimone et al. 143 demonstrated a Maxwellian AR display with two modes to offer a large-DoF Maxwellian-view, or a holographic view (Fig. 9d ), which is often referred as computer-generated holography (CGH) 162 . To show an always-in-focus image with a large DoF, the image can be directly displayed on an amplitude SLM, or using amplitude encoding for a phase-only SLM 163 . Alternatively, if a 3D scene with correct depth cues is to be presented, then optimization algorithms for CGH can be used to generate a hologram for the SLM. The generated holographic image exhibits the natural focus-and-blur effect like a real 3D object (Fig. 9d ). To better understand this feature, we need to again exploit the concept of etendue. The laser light source can be considered to have a very small etendue due to its excellent collimation. Therefore, the system etendue is provided by the SLM. The micron-sized pixel-pitch of SLM offers a certain maximum diffraction angle, which, multiplied by the SLM size, equals system etendue. By varying the display content on SLM, the final exit pupil size can be changed accordingly. In the case of a large-DoF Maxwellian view, the exit pupil size is small, accompanied by a large FoV. For the holographic display mode, the reduced DoF requires a larger exit pupil with dimension close to the eye pupil. But the FoV is reduced accordingly due to etendue conservation. Another commonly concerned issue with CGH is the computation time. To achieve a real-time CGH rendering flow with an excellent image quality is quite a challenge. Fortunately, with recent advances in algorithm 164 and the introduction of convolutional neural network (CNN) 165 , 166 , this issue is gradually solved with an encouraging pace. Lately, Liang et al. 166 demonstrated a real-time CGH synthesis pipeline with a high image quality. The pipeline comprises an efficient CNN model to generate a complex hologram from a 3D scene and an improved encoding algorithm to convert the complex hologram to a phase-only one. An impressive frame rate of 60 Hz has been achieved on a desktop computing unit.

For LBS-based system, the additional modulation can be achieved by integrating a steering module, as demonstrated by Jang et al. 167 . The steering mirror can shift the focal point (viewpoint) within the eye pupil, therefore effectively expanding the system etendue. When the steering process is fast and the image content is updated simultaneously, correct 3D cues can be generated, as shown in Fig. 9e . However, there exists a tradeoff between the number of viewpoint and the final image frame rate, because the total frames are equally divided into each viewpoint. To boost the frame rate of MEMS-LBS systems by the number of views (e.g., 3 by 3) may be challenging.

Maxwellian-type systems offer several advantages. The system efficiency is usually very high because nearly all the light is delivered into viewer’s eye. The system FoV is determined by the f /# of combiner and a large FoV (~80° in horizontal) can be achieved 143 . The issue of VAC can be mitigated with an infinite-DoF image that deprives accommodation cue, or completely solved by generating a true-3D scene as discussed above. Despite these advantages, one major weakness of Maxwellian-type system is the tiny exit pupil, or eyebox. A small deviation of eye pupil location from the viewpoint results in the complete disappearance of the image. Therefore, to expand eyebox is considered as one of the most important challenges in Maxwellian-type systems.

Pupil duplication and steering

Methods to expand eyebox can be generally categorized into pupil duplication 168 , 169 , 170 , 171 , 172 and pupil steering 9 , 13 , 167 , 173 . Pupil duplication simply generates multiple viewpoints to cover a large area. In contrast, pupil steering dynamically shifts the viewpoint position, depending on the pupil location. Before reviewing detailed implementations of these two methods, it is worth discussing some of their general features. The multiple viewpoints in pupil duplication usually mean to equally divide the total light intensity. In each time frame, however, it is preferable that only one viewpoint enters the user’s eye pupil to avoid ghost image. This requirement, therefore, results in a reduced total light efficiency, while also conditioning the viewpoint separation to be larger than the pupil diameter. In addition, the separation should not be too large to avoid gap between viewpoints. Considering that human pupil diameter changes in response to environment illuminance, the design of viewpoint separation needs special attention. Pupil steering, on the other hand, only produces one viewpoint at each time frame. It is therefore more light-efficient and free from ghost images. But to determine the viewpoint position requires the information of eye pupil location, which demands a real-time eye-tracking module 9 . Another observation is that pupil steering can accommodate multiple viewpoints by its nature. Therefore, a pupil steering system can often be easily converted to a pupil duplication system by simultaneously generating available viewpoints.

To generate multiple viewpoints, one can focus on modulating the incident light or the combiner. Recall that viewpoint is the image of light source. To duplicate or shift light source can achieve pupil duplication or steering accordingly, as illustrated in Fig. 10a . Several schemes of light modulation are depicted in Fig. 10b–e . An array of light sources can be generated with multiple laser diodes (Fig. 10b ). To turn on all or one of the sources achieves pupil duplication or steering. A light source array can also be produced by projecting light on an array-type PPHOE 168 (Fig. 10c ). Apart from direct adjustment of light sources, modulating light on the path can also effectively steer/duplicate the light sources. Using a mechanical steering mirror, the beam can be deflected 167 (Fig. 10d ), which equals to shifting the light source position. Other devices like a grating or beam splitter can also serve as ray deflector/splitter 170 , 171 (Fig. 10e ).

figure 10

a Schematic of duplicating (or shift) viewpoint by modulation of incident light. Light modulation by b multiple laser diodes, c HOE lens array, d steering mirror and e grating or beam splitters. f Pupil duplication with multiplexed PPHOE. g Pupil steering with LCHOE. Reproduced from c ref. 168 under the Creative Commons Attribution 4.0 License, e ref. 169 with permission from OSA Publishing, f ref. 171 with permission from OSA Publishing and g ref. 173 with permission from OSA Publishing

Nonetheless, one problem of the light source duplication/shifting methods for pupil duplication/steering is that the aberrations in peripheral viewpoints are often serious 168 , 173 . The HOE combiner is usually recorded at one incident angle. For other incident angles with large deviations, considerable aberrations will occur, especially in the scenario of off-axis configuration. To solve this problem, the modulation can be focused on the combiner instead. While the mechanical shifting of combiner 9 can achieve continuous pupil steering, its integration into AR display with a small factor remains a challenge. Alternatively, the versatile functions of HOE offer possible solutions for combiner modulation. Kim and Park 169 demonstrated a pupil duplication system with multiplexed PPHOE (Fig. 10f ). Wavefronts of several viewpoints can be recorded into one PPHOE sample. Three viewpoints with a separation of 3 mm were achieved. However, a slight degree of ghost image and gap can be observed in the viewpoint transition. For a PPHOE to achieve pupil steering, the multiplexed PPHOE needs to record different focal points with different incident angles. If each hologram has no angular crosstalk, then with an additional device to change the light incident angle, the viewpoint can be steered. Alternatively, Xiong et al. 173 demonstrated a pupil steering system with LCHOEs in a simpler configuration (Fig. 10g ). The polarization-sensitive nature of LCHOE enables the controlling of which LCHOE to function with a polarization converter (PC). When the PC is off, the incident RCP light is focused by the right-handed LCHOE. When the PC is turned on, the RCP light is firstly converted to LCP light and passes through the right-handed LCHOE. Then it is focused by the left-handed LCHOE into another viewpoint. To add more viewpoints requires stacking more pairs of PC and LCHOE, which can be achieved in a compact manner with thin glass substrates. In addition, to realize pupil duplication only requires the stacking of multiple low-efficiency LCHOEs. For both PPHOEs and LCHOEs, because the hologram for each viewpoint is recorded independently, the aberrations can be eliminated.

Regarding the system performance, in theory the FoV is not limited and can reach a large value, such as 80° in horizontal direction 143 . The definition of eyebox is different from traditional imaging systems. For a single viewpoint, it has the same size as the eye pupil diameter. But due to the viewpoint steering/duplication capability, the total system eyebox can be expanded accordingly. The combiner efficiency for pupil steering systems can reach 47,000 nit/lm for a FoV of 80° by 80° and pupil diameter of 4 mm (Eq. S2 ). At such a high brightness level, eye safety could be a concern 174 . For a pupil duplication system, the combiner efficiency is decreased by the number of viewpoints. With a 4-by-4 viewpoint array, it can still reach 3000 nit/lm. Despite the potential gain of pupil duplication/steering, when considering the rotation of eyeball, the situation becomes much more complicated 175 . A perfect pupil steering system requires a 5D steering, which proposes a challenge for practical implementation.

Pin-light systems

Recently, another type of display in close relation with Maxwellian view called pin-light display 148 , 176 has been proposed. The general working principle of pin-light display is illustrated in Fig. 11a . Each pin-light source is a Maxwellian view with a large DoF. When the eye pupil is no longer placed near the source point as in Maxwellian view, each image source can only form an elemental view with a small FoV on retina. However, if the image source array is arranged in a proper form, the elemental views can be integrated together to form a large FoV. According to the specific optical architectures, pin-light display can take different forms of implementation. In the initial feasibility demonstration, Maimone et al. 176 used a side-lit waveguide plate as the point light source (Fig. 11b ). The light inside the waveguide plate is extracted by the etched divots, forming a pin-light source array. A transmissive SLM (LCD) is placed behind the waveguide plate to modulate the light intensity and form the image. The display has an impressive FoV of 110° thanks to the large scattering angle range. However, the direct placement of LCD before the eye brings issues of insufficient resolution density and diffraction of background light.

figure 11

a Schematic drawing of the working principle of pin-light display. b Pin-light display utilizing a pin-light source and a transmissive SLM. c An example of pin-mirror display with a birdbath optics. d SWD system with LBS image source and off-axis lens array. Reprinted from b ref. 176 under the Creative Commons Attribution 4.0 License and d ref. 180 with permission from OSA Publishing

To avoid these issues, architectures using pin-mirrors 177 , 178 , 179 are proposed. In these systems, the final combiner is an array of tiny mirrors 178 , 179 or gratings 177 , in contrast to their counterparts using large-area combiners. An exemplary system with birdbath design is depicted in Fig. 11c . In this case, the pin-mirrors replace the original beam-splitter in the birdbath and can thus shrink the system volume, while at the same time providing large DoF pin-light images. Nonetheless, such a system may still face the etendue conservation issue. Meanwhile, the size of pin-mirror cannot be too small in order to prevent degradation of resolution density due to diffraction. Therefore, its influence on the see-through background should also be considered in the system design.

To overcome the etendue conservation and improve see-through quality, Xiong et al. 180 proposed another type of pin-light system exploiting the etendue expansion property of waveguide, which is also referred as scanning waveguide display (SWD). As illustrated in Fig. 11d , the system uses an LBS as the image source. The collimated scanned laser rays are trapped in the waveguide and encounter an array of off-axis lenses. Upon each encounter, the lens out-couples the laser rays and forms a pin-light source. SWD has the merits of good see-through quality and large etendue. A large FoV of 100° was demonstrated with the help of an ultra-low f /# lens array based on LCHOE. However, some issues like insufficient image resolution density and image non-uniformity remain to be overcome. To further improve the system may require optimization of Gaussian beam profile and additional EPE module 180 .

Overall, pin-light systems inherit the large DoF from Maxwellian view. With adequate number of pin-light sources, the FoV and eyebox can be expanded accordingly. Nonetheless, despite different forms of implementation, a common issue of pin-light system is the image uniformity. The overlapped region of elemental views has a higher light intensity than the non-overlapped region, which becomes even more complicated considering the dynamic change of pupil size. In theory, the displayed image can be pre-processed to compensate for the optical non-uniformity. But that would require knowledge of precise pupil location (and possibly size) and therefore an accurate eye-tracking module 176 . Regarding the system performance, pin-mirror systems modified from other free-space systems generally shares similar FoV and eyebox with original systems. The combiner efficiency may be lower due to the small size of pin-mirrors. SWD, on the other hand, shares the large FoV and DoF with Maxwellian view, and large eyebox with waveguide combiners. The combiner efficiency may also be lower due to the EPE process.

Waveguide combiner

Besides free-space combiners, another common architecture in AR displays is waveguide combiner. The term ‘waveguide’ indicates the light is trapped in a substrate by the TIR process. One distinctive feature of a waveguide combiner is the EPE process that effectively enlarges the system etendue. In the EPE process, a portion of the trapped light is repeatedly coupled out of the waveguide in each TIR. The effective eyebox is therefore enlarged. According to the features of couplers, we divide the waveguide combiners into two types: diffractive and achromatic, as described in the followings.

Diffractive waveguides

As the name implies, diffractive-type waveguides use diffractive elements as couplers. The in-coupler is usually a diffractive grating and the out-coupler in most cases is also a grating with the same period as the in-coupler, but it can also be an off-axis lens with a small curvature to generate image with finite depth. Three major diffractive couplers have been developed: SRGs, photopolymer gratings (PPGs), and liquid crystal gratings (grating-type LCHOE; also known as polarization volume gratings (PVGs)). Some general protocols for coupler design are that the in-coupler should have a relatively high efficiency and the out-coupler should have a uniform light output. A uniform light output usually requires a low-efficiency coupler, with extra degrees of freedom for local modulation of coupling efficiency. Both in-coupler and out-coupler should have an adequate angular bandwidth to accommodate a reasonable FoV. In addition, the out-coupler should also be optimized to avoid undesired diffractions, including the outward diffraction of TIR light and diffraction of environment light into user’s eyes, which are referred as light leakage and rainbow. Suppression of these unwanted diffractions should also be considered in the optimization process of waveguide design, along with performance parameters like efficiency and uniformity.

The basic working principles of diffractive waveguide-based AR systems are illustrated in Fig. 12 . For the SRG-based waveguides 6 , 8 (Fig. 12a ), the in-coupler can be a transmissive-type or a reflective-type 181 , 182 . The grating geometry can be optimized for coupling efficiency with a large degree of freedom 183 . For the out-coupler, a reflective SRG with a large slant angle to suppress the transmission orders is preferred 184 . In addition, a uniform light output usually requires a gradient efficiency distribution in order to compensate for the decreased light intensity in the out-coupling process. This can be achieved by varying the local grating configurations like height and duty cycle 6 . For the PPG-based waveguides 185 (Fig. 12b ), the small angular bandwidth of a high-efficiency transmissive PPG prohibits its use as in-coupler. Therefore, both in-coupler and out-coupler are usually reflective types. The gradient efficiency can be achieved by space-variant exposure to control the local index modulation 186 or local Bragg slant angle variation through freeform exposure 19 . Due to the relatively small angular bandwidth of PPG, to achieve a decent FoV usually requires stacking two 187 or three 188 PPGs together for a single color. The PVG-based waveguides 189 (Fig. 12c ) also prefer reflective PVGs as in-couplers because the transmissive PVGs are much more difficult to fabricate due to the LC alignment issue. In addition, the angular bandwidth of transmissive PVGs in Bragg regime is also not large enough to support a decent FoV 29 . For the out-coupler, the angular bandwidth of a single reflective PVG can usually support a reasonable FoV. To obtain a uniform light output, a polarization management layer 190 consisting of a LC layer with spatially variant orientations can be utilized. It offers an additional degree of freedom to control the polarization state of the TIR light. The diffraction efficiency can therefore be locally controlled due to the strong polarization sensitivity of PVG.

figure 12

Schematics of waveguide combiners based on a SRGs, b PPGs and c PVGs. Reprinted from a ref. 85 with permission from OSA Publishing, b ref. 185 with permission from John Wiley and Sons and c ref. 189 with permission from OSA Publishing

The above discussion describes the basic working principle of 1D EPE. Nonetheless, for the 1D EPE to produce a large eyebox, the exit pupil in the unexpanded direction of the original image should be large. This proposes design challenges in light engines. Therefore, a 2D EPE is favored for practical applications. To extend EPE in two dimensions, two consecutive 1D EPEs can be used 191 , as depicted in Fig. 13a . The first 1D EPE occurs in the turning grating, where the light is duplicated in y direction and then turned into x direction. Then the light rays encounter the out-coupler and are expanded in x direction. To better understand the 2D EPE process, the k -vector diagram (Fig. 13b ) can be used. For the light propagating in air with wavenumber k 0 , its possible k -values in x and y directions ( k x and k y ) fall within the circle with radius k 0 . When the light is trapped into TIR, k x and k y are outside the circle with radius k 0 and inside the circle with radius nk 0 , where n is the refractive index of the substrate. k x and k y stay unchanged in the TIR process and are only changed in each diffraction process. The central red box in Fig. 13b indicates the possible k values within the system FoV. After the in-coupler, the k values are added by the grating k -vector, shifting the k values into TIR region. The turning grating then applies another k -vector and shifts the k values to near x -axis. Finally, the k values are shifted by the out-coupler and return to the free propagation region in air. One observation is that the size of red box is mostly limited by the width of TIR band. To accommodate a larger FoV, the outer boundary of TIR band needs to be expanded, which amounts to increasing waveguide refractive index. Another important fact is that when k x and k y are near the outer boundary, the uniformity of output light becomes worse. This is because the light propagation angle is near 90° in the waveguide. The spatial distance between two consecutive TIRs becomes so large that the out-coupled beams are spatially separated to an unacceptable degree. The range of possible k values for practical applications is therefore further shrunk due to this fact.

figure 13

a Schematic of 2D EPE based on two consecutive 1D EPEs. Gray/black arrows indicate light in air/TIR. Black dots denote TIRs. b k-diagram of the two-1D-EPE scheme. c Schematic of 2D EPE with a 2D hexagonal grating d k-diagram of the 2D-grating scheme

Aside from two consecutive 1D EPEs, the 2D EPE can also be directly implemented with a 2D grating 192 . An example using a hexagonal grating is depicted in Fig. 13c . The hexagonal grating can provide k -vectors in six directions. In the k -diagram (Fig. 13d ), after the in-coupling, the k values are distributed into six regions due to multiple diffractions. The out-coupling occurs simultaneously with pupil expansion. Besides a concise out-coupler configuration, the 2D EPE scheme offers more degrees of design freedom than two 1D EPEs because the local grating parameters can be adjusted in a 2D manner. The higher design freedom has the potential to reach a better output light uniformity, but at the cost of a higher computation demand for optimization. Furthermore, the unslanted grating geometry usually leads to a large light leakage and possibly low efficiency. Adding slant to the geometry helps alleviate the issue, but the associated fabrication may be more challenging.

Finally, we discuss the generation of full-color images. One important issue to clarify is that although diffractive gratings are used here, the final image generally has no color dispersion even if we use a broadband light source like LED. This can be easily understood in the 1D EPE scheme. The in-coupler and out-coupler have opposite k -vectors, which cancels the color dispersion for each other. In the 2D EPE schemes, the k -vectors always form a closed loop from in-coupled light to out-coupled light, thus, the color dispersion also vanishes likewise. The issue of using a single waveguide for full-color images actually exists in the consideration of FoV and light uniformity. The breakup of propagation angles for different colors results in varied out-coupling situations for each color. To be more specific, if the red and the blue channels use the same in-coupler, the propagating angle for the red light is larger than that of the blue light. The red light in peripheral FoV is therefore easier to face the mentioned large-angle non-uniformity issue. To acquire a decent FoV and light uniformity, usually two or three layers of waveguides with different grating pitches are adopted.

Regarding the system performance, the eyebox is generally large enough (~10 mm) to accommodate different user’s IPD and alignment shift during operation. A parameter of significant concern for a waveguide combiner is its FoV. From the k -vector analysis, we can conclude the theoretical upper limit is determined by the waveguide refractive index. But the light/color uniformity also influences the effective FoV, over which the degradation of image quality becomes unacceptable. Current diffractive waveguide combiners generally achieve a FoV of about 50°. To further increase FoV, a straightforward method is to use a higher refractive index waveguide. Another is to tile FoV through direct stacking of multiple waveguides or using polarization-sensitive couplers 79 , 193 . As to the optical efficiency, a typical value for the diffractive waveguide combiner is around 50–200 nit/lm 6 , 189 . In addition, waveguide combiners adopting grating out-couplers generate an image with fixed depth at infinity. This leads to the VAC issue. To tackle VAC in waveguide architectures, the most practical way is to generate multiple depths and use the varifocal or multifocal driving scheme, similar to those mentioned in the VR systems. But to add more depths usually means to stack multiple layers of waveguides together 194 . Considering the additional waveguide layers for RGB colors, the final waveguide thickness would undoubtedly increase.

Other parameters special to waveguide includes light leakage, see-through ghost, and rainbow. Light leakage refers to out-coupled light that goes outwards to the environment, as depicted in Fig. 14a . Aside from decreased efficiency, the leakage also brings drawback of unnatural “bright-eye” appearance of the user and privacy issue. Optimization of the grating structure like geometry of SRG may reduce the leakage. See-through ghost is formed by consecutive in-coupling and out-couplings caused by the out-coupler grating, as sketched in Fig. 14b , After the process, a real object with finite depth may produce a ghost image with shift in both FoV and depth. Generally, an out-coupler with higher efficiency suffers more see-through ghost. Rainbow is caused by the diffraction of environment light into user’s eye, as sketched in Fig. 14c . The color dispersion in this case will occur because there is no cancellation of k -vector. Using the k -diagram, we can obtain a deeper insight into the formation of rainbow. Here, we take the EPE structure in Fig. 13a as an example. As depicted in Fig. 14d , after diffractions by the turning grating and the out-coupler grating, the k values are distributed in two circles that shift from the origin by the grating k -vectors. Some diffracted light can enter the see-through FoV and form rainbow. To reduce rainbow, a straightforward way is to use a higher index substrate. With a higher refractive index, the outer boundary of k diagram is expanded, which can accommodate larger grating k -vectors. The enlarged k -vectors would therefore “push” these two circles outwards, leading to a decreased overlapping region with the see-through FoV. Alternatively, an optimized grating structure would also help reduce the rainbow effect by suppressing the unwanted diffraction.

figure 14

Sketches of formations of a light leakage, b see-through ghost and c rainbow. d Analysis of rainbow formation with k-diagram

Achromatic waveguide

Achromatic waveguide combiners use achromatic elements as couplers. It has the advantage of realizing full-color image with a single waveguide. A typical example of achromatic element is a mirror. The waveguide with partial mirrors as out-coupler is often referred as geometric waveguide 6 , 195 , as depicted in Fig. 15a . The in-coupler in this case is usually a prism to avoid unnecessary color dispersion if using diffractive elements otherwise. The mirrors couple out TIR light consecutively to produce a large eyebox, similarly in a diffractive waveguide. Thanks to the excellent optical property of mirrors, the geometric waveguide usually exhibits a superior image regarding MTF and color uniformity to its diffractive counterparts. Still, the spatially discontinuous configuration of mirrors also results in gaps in eyebox, which may be alleviated by using a dual-layer structure 196 . Wang et al. designed a geometric waveguide display with five partial mirrors (Fig. 15b ). It exhibits a remarkable FoV of 50° by 30° (Fig. 15c ) and an exit pupil of 4 mm with a 1D EPE. To achieve 2D EPE, similar architectures in Fig. 13a can be used by integrating a turning mirror array as the first 1D EPE module 197 . Unfortunately, the k -vector diagrams in Fig. 13b, d cannot be used here because the k values in x-y plane no longer conserve in the in-coupling and out-coupling processes. But some general conclusions remain valid, like a higher refractive index leading to a larger FoV and gradient out-coupling efficiency improving light uniformity.

figure 15

a Schematic of the system configuration. b Geometric waveguide with five partial mirrors. c Image photos demonstrating system FoV. Adapted from b , c ref. 195 with permission from OSA Publishing

The fabrication process of geometric waveguide involves coating mirrors on cut-apart pieces and integrating them back together, which may result in a high cost, especially for the 2D EPE architecture. Another way to implement an achromatic coupler is to use multiplexed PPHOE 198 , 199 to mimic the behavior of a tilted mirror (Fig. 16a ). To understand the working principle, we can use the diagram in Fig. 16b . The law of reflection states the angle of reflection equals to the angle of incidence. If we translate this behavior to k -vector language, it means the mirror can apply any length of k -vector along its surface normal direction. The k -vector length of the reflected light is always equal to that of the incident light. This puts a condition that the k -vector triangle is isosceles. With a simple geometric deduction, it can be easily observed this leads to the law of reflection. The behavior of a general grating, however, is very different. For simplicity we only consider the main diffraction order. The grating can only apply a k -vector with fixed k x due to the basic diffraction law. For the light with a different incident angle, it needs to apply different k z to produce a diffracted light with equal k -vector length as the incident light. For a grating with a broad angular bandwidth like SRG, the range of k z is wide, forming a lengthy vertical line in Fig. 16b . For a PPG with a narrow angular bandwidth, the line is short and resembles a dot. If multiple of these tiny dots are distributed along the oblique line corresponding to a mirror, then the final multiplexed PPGs can imitate the behavior of a tilted mirror. Such a PPHOE is sometimes referred as a skew-mirror 198 . In theory, to better imitate the mirror, a lot of multiplexed PPGs is preferred, while each PPG has a small index modulation δn . But this proposes a bigger challenge in device fabrication. Recently, Utsugi et al. demonstrated an impressive skew-mirror waveguide based on 54 multiplexed PPGs (Fig. 16c, d ). The display exhibits an effective FoV of 35° by 36°. In the peripheral FoV, there still exists some non-uniformity (Fig. 16e ) due to the out-coupling gap, which is an inherent feature of the flat-type out-couplers.

figure 16

a System configuration. b Diagram demonstrating how multiplexed PPGs resemble the behavior of a mirror. Photos showing c the system and d image. e Picture demonstrating effective system FoV. Adapted from c – e ref. 199 with permission from ITE

Finally, it is worth mentioning that metasurfaces are also promising to deliver achromatic gratings 200 , 201 for waveguide couplers ascribed to their versatile wavefront shaping capability. The mechanism of the achromatic gratings is similar to that of the achromatic lenses as previously discussed. However, the current development of achromatic metagratings is still in its infancy. Much effort is needed to improve the optical efficiency for in-coupling, control the higher diffraction orders for eliminating ghost images, and enable a large size design for EPE.

Generally, achromatic waveguide combiners exhibit a comparable FoV and eyebox with diffractive combiners, but with a higher efficiency. For a partial-mirror combiner, its combiner efficiency is around 650 nit/lm 197 (2D EPE). For a skew-mirror combiner, although the efficiency of multiplexed PPHOE is relatively low (~1.5%) 199 , the final combiner efficiency of the 1D EPE system is still high (>3000 nit/lm) due to multiple out-couplings.

Table 2 summarizes the performance of different AR combiners. When combing the luminous efficacy in Table 1 and the combiner efficiency in Table 2 , we can have a comprehensive estimate of the total luminance efficiency (nit/W) for different types of systems. Generally, Maxwellian-type combiners with pupil steering have the highest luminance efficiency when partnered with laser-based light engines like laser-backlit LCoS/DMD or MEM-LBS. Geometric optical combiners have well-balanced image performances, but to further shrink the system size remains a challenge. Diffractive waveguides have a relatively low combiner efficiency, which can be remedied by an efficient light engine like MEMS-LBS. Further development of coupler and EPE scheme would also improve the system efficiency and FoV. Achromatic waveguides have a decent combiner efficiency. The single-layer design also enables a smaller form factor. With advances in fabrication process, it may become a strong contender to presently widely used diffractive waveguides.

Conclusions and perspectives

VR and AR are endowed with a high expectation to revolutionize the way we interact with digital world. Accompanied with the expectation are the engineering challenges to squeeze a high-performance display system into a tightly packed module for daily wearing. Although the etendue conservation constitutes a great obstacle on the path, remarkable progresses with innovative optics and photonics continue to take place. Ultra-thin optical elements like PPHOEs and LCHOEs provide alternative solutions to traditional optics. Their unique features of multiplexing capability and polarization dependency further expand the possibility of novel wavefront modulations. At the same time, nanoscale-engineered metasurfaces/SRGs provide large design freedoms to achieve novel functions beyond conventional geometric optical devices. Newly emerged micro-LEDs open an opportunity for compact microdisplays with high peak brightness and good stability. Further advances on device engineering and manufacturing process are expected to boost the performance of metasurfaces/SRGs and micro-LEDs for AR and VR applications.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper. Additional data related to this paper may be requested from the authors.

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J.X. conceived the idea and initiated the project. J.X. mainly wrote the manuscript and produced the figures. E.-L.H., Z.H., and T.Z. contributed to parts of the manuscript. S.W. supervised the project and edited the manuscript.

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Xiong, J., Hsiang, EL., He, Z. et al. Augmented reality and virtual reality displays: emerging technologies and future perspectives. Light Sci Appl 10 , 216 (2021).

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Received : 06 June 2021

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research paper on augmented reality


The past, present, and future of virtual and augmented reality research: a network and cluster analysis of the literature.

\r\nPietro Cipresso,*

  • 1 Applied Technology for Neuro-Psychology Lab, Istituto Auxologico Italiano, Milan, Italy
  • 2 Department of Psychology, Catholic University of the Sacred Heart, Milan, Italy
  • 3 Instituto de Investigación e Innovación en Bioingeniería, Universitat Politècnica de València, Valencia, Spain

The recent appearance of low cost virtual reality (VR) technologies – like the Oculus Rift, the HTC Vive and the Sony PlayStation VR – and Mixed Reality Interfaces (MRITF) – like the Hololens – is attracting the attention of users and researchers suggesting it may be the next largest stepping stone in technological innovation. However, the history of VR technology is longer than it may seem: the concept of VR was formulated in the 1960s and the first commercial VR tools appeared in the late 1980s. For this reason, during the last 20 years, 100s of researchers explored the processes, effects, and applications of this technology producing 1000s of scientific papers. What is the outcome of this significant research work? This paper wants to provide an answer to this question by exploring, using advanced scientometric techniques, the existing research corpus in the field. We collected all the existent articles about VR in the Web of Science Core Collection scientific database, and the resultant dataset contained 21,667 records for VR and 9,944 for augmented reality (AR). The bibliographic record contained various fields, such as author, title, abstract, country, and all the references (needed for the citation analysis). The network and cluster analysis of the literature showed a composite panorama characterized by changes and evolutions over the time. Indeed, whether until 5 years ago, the main publication media on VR concerned both conference proceeding and journals, more recently journals constitute the main medium of communication. Similarly, if at first computer science was the leading research field, nowadays clinical areas have increased, as well as the number of countries involved in VR research. The present work discusses the evolution and changes over the time of the use of VR in the main areas of application with an emphasis on the future expected VR’s capacities, increases and challenges. We conclude considering the disruptive contribution that VR/AR/MRITF will be able to get in scientific fields, as well in human communication and interaction, as already happened with the advent of mobile phones by increasing the use and the development of scientific applications (e.g., in clinical areas) and by modifying the social communication and interaction among people.


In the last 5 years, virtual reality (VR) and augmented reality (AR) have attracted the interest of investors and the general public, especially after Mark Zuckerberg bought Oculus for two billion dollars ( Luckerson, 2014 ; Castelvecchi, 2016 ). Currently, many other companies, such as Sony, Samsung, HTC, and Google are making huge investments in VR and AR ( Korolov, 2014 ; Ebert, 2015 ; Castelvecchi, 2016 ). However, if VR has been used in research for more than 25 years, and now there are 1000s of papers and many researchers in the field, comprising a strong, interdisciplinary community, AR has a more recent application history ( Burdea and Coiffet, 2003 ; Kim, 2005 ; Bohil et al., 2011 ; Cipresso and Serino, 2014 ; Wexelblat, 2014 ). The study of VR was initiated in the computer graphics field and has been extended to several disciplines ( Sutherland, 1965 , 1968 ; Mazuryk and Gervautz, 1996 ; Choi et al., 2015 ). Currently, videogames supported by VR tools are more popular than the past, and they represent valuables, work-related tools for neuroscientists, psychologists, biologists, and other researchers as well. Indeed, for example, one of the main research purposes lies from navigation studies that include complex experiments that could be done in a laboratory by using VR, whereas, without VR, the researchers would have to go directly into the field, possibly with limited use of intervention. The importance of navigation studies for the functional understanding of human memory in dementia has been a topic of significant interest for a long time, and, in 2014, the Nobel Prize in “Physiology or Medicine” was awarded to John M. O’Keefe, May-Britt Moser, and Edvard I. Moser for their discoveries of nerve cells in the brain that enable a sense of place and navigation. Journals and magazines have extended this knowledge by writing about “the brain GPS,” which gives a clear idea of the mechanism. A huge number of studies have been conducted in clinical settings by using VR ( Bohil et al., 2011 ; Serino et al., 2014 ), and Nobel Prize winner, Edvard I. Moser commented about the use of VR ( Minderer et al., 2016 ), highlighting its importance for research and clinical practice. Moreover, the availability of free tools for VR experimental and computational use has made it easy to access any field ( Riva et al., 2011 ; Cipresso, 2015 ; Brown and Green, 2016 ; Cipresso et al., 2016 ).

Augmented reality is a more recent technology than VR and shows an interdisciplinary application framework, in which, nowadays, education and learning seem to be the most field of research. Indeed, AR allows supporting learning, for example increasing-on content understanding and memory preservation, as well as on learning motivation. However, if VR benefits from clear and more definite fields of application and research areas, AR is still emerging in the scientific scenarios.

In this article, we present a systematic and computational analysis of the emerging interdisciplinary VR and AR fields in terms of various co-citation networks in order to explore the evolution of the intellectual structure of this knowledge domain over time.

Virtual Reality Concepts and Features

The concept of VR could be traced at the mid of 1960 when Ivan Sutherland in a pivotal manuscript attempted to describe VR as a window through which a user perceives the virtual world as if looked, felt, sounded real and in which the user could act realistically ( Sutherland, 1965 ).

Since that time and in accordance with the application area, several definitions have been formulated: for example, Fuchs and Bishop (1992) defined VR as “real-time interactive graphics with 3D models, combined with a display technology that gives the user the immersion in the model world and direct manipulation” ( Fuchs and Bishop, 1992 ); Gigante (1993) described VR as “The illusion of participation in a synthetic environment rather than external observation of such an environment. VR relies on a 3D, stereoscopic head-tracker displays, hand/body tracking and binaural sound. VR is an immersive, multi-sensory experience” ( Gigante, 1993 ); and “Virtual reality refers to immersive, interactive, multi-sensory, viewer-centered, 3D computer generated environments and the combination of technologies required building environments” ( Cruz-Neira, 1993 ).

As we can notice, these definitions, although different, highlight three common features of VR systems: immersion, perception to be present in an environment, and interaction with that environment ( Biocca, 1997 ; Lombard and Ditton, 1997 ; Loomis et al., 1999 ; Heeter, 2000 ; Biocca et al., 2001 ; Bailenson et al., 2006 ; Skalski and Tamborini, 2007 ; Andersen and Thorpe, 2009 ; Slater, 2009 ; Sundar et al., 2010 ). Specifically, immersion concerns the amount of senses stimulated, interactions, and the reality’s similarity of the stimuli used to simulate environments. This feature can depend on the properties of the technological system used to isolate user from reality ( Slater, 2009 ).

Higher or lower degrees of immersion can depend by three types of VR systems provided to the user:

• Non-immersive systems are the simplest and cheapest type of VR applications that use desktops to reproduce images of the world.

• Immersive systems provide a complete simulated experience due to the support of several sensory outputs devices such as head mounted displays (HMDs) for enhancing the stereoscopic view of the environment through the movement of the user’s head, as well as audio and haptic devices.

• Semi-immersive systems such as Fish Tank VR are between the two above. They provide a stereo image of a three dimensional (3D) scene viewed on a monitor using a perspective projection coupled to the head position of the observer ( Ware et al., 1993 ). Higher technological immersive systems have showed a closest experience to reality, giving to the user the illusion of technological non-mediation and feeling him or her of “being in” or present in the virtual environment ( Lombard and Ditton, 1997 ). Furthermore, higher immersive systems, than the other two systems, can give the possibility to add several sensory outputs allowing that the interaction and actions were perceived as real ( Loomis et al., 1999 ; Heeter, 2000 ; Biocca et al., 2001 ).

Finally, the user’s VR experience could be disclosed by measuring presence, realism, and reality’s levels. Presence is a complex psychological feeling of “being there” in VR that involves the sensation and perception of physical presence, as well as the possibility to interact and react as if the user was in the real world ( Heeter, 1992 ). Similarly, the realism’s level corresponds to the degree of expectation that the user has about of the stimuli and experience ( Baños et al., 2000 , 2009 ). If the presented stimuli are similar to reality, VR user’s expectation will be congruent with reality expectation, enhancing VR experience. In the same way, higher is the degree of reality in interaction with the virtual stimuli, higher would be the level of realism of the user’s behaviors ( Baños et al., 2000 , 2009 ).

From Virtual to Augmented Reality

Looking chronologically on VR and AR developments, we can trace the first 3D immersive simulator in 1962, when Morton Heilig created Sensorama, a simulated experience of a motorcycle running through Brooklyn characterized by several sensory impressions, such as audio, olfactory, and haptic stimuli, including also wind to provide a realist experience ( Heilig, 1962 ). In the same years, Ivan Sutherland developed The Ultimate Display that, more than sound, smell, and haptic feedback, included interactive graphics that Sensorama didn’t provide. Furthermore, Philco developed the first HMD that together with The Sword of Damocles of Sutherland was able to update the virtual images by tracking user’s head position and orientation ( Sutherland, 1965 ). In the 70s, the University of North Carolina realized GROPE, the first system of force-feedback and Myron Krueger created VIDEOPLACE an Artificial Reality in which the users’ body figures were captured by cameras and projected on a screen ( Krueger et al., 1985 ). In this way two or more users could interact in the 2D-virtual space. In 1982, the US’ Air Force created the first flight simulator [Visually Coupled Airbone System Simulator (VCASS)] in which the pilot through an HMD could control the pathway and the targets. Generally, the 80’s were the years in which the first commercial devices began to emerge: for example, in 1985 the VPL company commercialized the DataGlove, glove sensors’ equipped able to measure the flexion of fingers, orientation and position, and identify hand gestures. Another example is the Eyephone, created in 1988 by the VPL Company, an HMD system for completely immerging the user in a virtual world. At the end of 80’s, Fake Space Labs created a Binocular-Omni-Orientational Monitor (BOOM), a complex system composed by a stereoscopic-displaying device, providing a moving and broad virtual environment, and a mechanical arm tracking. Furthermore, BOOM offered a more stable image and giving more quickly responses to movements than the HMD devices. Thanks to BOOM and DataGlove, the NASA Ames Research Center developed the Virtual Wind Tunnel in order to research and manipulate airflow in a virtual airplane or space ship. In 1992, the Electronic Visualization Laboratory of the University of Illinois created the CAVE Automatic Virtual Environment, an immersive VR system composed by projectors directed on three or more walls of a room.

More recently, many videogames companies have improved the development and quality of VR devices, like Oculus Rift, or HTC Vive that provide a wider field of view and lower latency. In addition, the actual HMD’s devices can be now combined with other tracker system as eye-tracking systems (FOVE), and motion and orientation sensors (e.g., Razer Hydra, Oculus Touch, or HTC Vive).

Simultaneously, at the beginning of 90’, the Boing Corporation created the first prototype of AR system for showing to employees how set up a wiring tool ( Carmigniani et al., 2011 ). At the same time, Rosenberg and Feiner developed an AR fixture for maintenance assistance, showing that the operator performance enhanced by added virtual information on the fixture to repair ( Rosenberg, 1993 ). In 1993 Loomis and colleagues produced an AR GPS-based system for helping the blind in the assisted navigation through adding spatial audio information ( Loomis et al., 1998 ). Always in the 1993 Julie Martin developed “Dancing in Cyberspace,” an AR theater in which actors interacted with virtual object in real time ( Cathy, 2011 ). Few years later, Feiner et al. (1997) developed the first Mobile AR System (MARS) able to add virtual information about touristic buildings ( Feiner et al., 1997 ). Since then, several applications have been developed: in Thomas et al. (2000) , created ARQuake, a mobile AR video game; in 2008 was created Wikitude that through the mobile camera, internet, and GPS could add information about the user’s environments ( Perry, 2008 ). In 2009 others AR applications, like AR Toolkit and SiteLens have been developed in order to add virtual information to the physical user’s surroundings. In 2011, Total Immersion developed D’Fusion, and AR system for designing projects ( Maurugeon, 2011 ). Finally, in 2013 and 2015, Google developed Google Glass and Google HoloLens, and their usability have begun to test in several field of application.

Virtual Reality Technologies

Technologically, the devices used in the virtual environments play an important role in the creation of successful virtual experiences. According to the literature, can be distinguished input and output devices ( Burdea et al., 1996 ; Burdea and Coiffet, 2003 ). Input devices are the ones that allow the user to communicate with the virtual environment, which can range from a simple joystick or keyboard to a glove allowing capturing finger movements or a tracker able to capture postures. More in detail, keyboard, mouse, trackball, and joystick represent the desktop input devices easy to use, which allow the user to launch continuous and discrete commands or movements to the environment. Other input devices can be represented by tracking devices as bend-sensing gloves that capture hand movements, postures and gestures, or pinch gloves that detect the fingers movements, and trackers able to follow the user’s movements in the physical world and translate them in the virtual environment.

On the contrary, the output devices allow the user to see, hear, smell, or touch everything that happens in the virtual environment. As mentioned above, among the visual devices can be found a wide range of possibilities, from the simplest or least immersive (monitor of a computer) to the most immersive one such as VR glasses or helmets or HMD or CAVE systems.

Furthermore, auditory, speakers, as well as haptic output devices are able to stimulate body senses providing a more real virtual experience. For example, haptic devices can stimulate the touch feeling and force models in the user.

Virtual Reality Applications

Since its appearance, VR has been used in different fields, as for gaming ( Zyda, 2005 ; Meldrum et al., 2012 ), military training ( Alexander et al., 2017 ), architectural design ( Song et al., 2017 ), education ( Englund et al., 2017 ), learning and social skills training ( Schmidt et al., 2017 ), simulations of surgical procedures ( Gallagher et al., 2005 ), assistance to the elderly or psychological treatments are other fields in which VR is bursting strongly ( Freeman et al., 2017 ; Neri et al., 2017 ). A recent and extensive review of Slater and Sanchez-Vives (2016) reported the main VR application evidences, including weakness and advantages, in several research areas, such as science, education, training, physical training, as well as social phenomena, moral behaviors, and could be used in other fields, like travel, meetings, collaboration, industry, news, and entertainment. Furthermore, another review published this year by Freeman et al. (2017) focused on VR in mental health, showing the efficacy of VR in assessing and treating different psychological disorders as anxiety, schizophrenia, depression, and eating disorders.

There are many possibilities that allow the use of VR as a stimulus, replacing real stimuli, recreating experiences, which in the real world would be impossible, with a high realism. This is why VR is widely used in research on new ways of applying psychological treatment or training, for example, to problems arising from phobias (agoraphobia, phobia to fly, etc.) ( Botella et al., 2017 ). Or, simply, it is used like improvement of the traditional systems of motor rehabilitation ( Llorens et al., 2014 ; Borrego et al., 2016 ), developing games that ameliorate the tasks. More in detail, in psychological treatment, Virtual Reality Exposure Therapy (VRET) has showed its efficacy, allowing to patients to gradually face fear stimuli or stressed situations in a safe environment where the psychological and physiological reactions can be controlled by the therapist ( Botella et al., 2017 ).

Augmented Reality Concept

Milgram and Kishino (1994) , conceptualized the Virtual-Reality Continuum that takes into consideration four systems: real environment, augmented reality (AR), augmented virtuality, and virtual environment. AR can be defined a newer technological system in which virtual objects are added to the real world in real-time during the user’s experience. Per Azuma et al. (2001) an AR system should: (1) combine real and virtual objects in a real environment; (2) run interactively and in real-time; (3) register real and virtual objects with each other. Furthermore, even if the AR experiences could seem different from VRs, the quality of AR experience could be considered similarly. Indeed, like in VR, feeling of presence, level of realism, and the degree of reality represent the main features that can be considered the indicators of the quality of AR experiences. Higher the experience is perceived as realistic, and there is congruence between the user’s expectation and the interaction inside the AR environments, higher would be the perception of “being there” physically, and at cognitive and emotional level. The feeling of presence, both in AR and VR environments, is important in acting behaviors like the real ones ( Botella et al., 2005 ; Juan et al., 2005 ; Bretón-López et al., 2010 ; Wrzesien et al., 2013 ).

Augmented Reality Technologies

Technologically, the AR systems, however various, present three common components, such as a geospatial datum for the virtual object, like a visual marker, a surface to project virtual elements to the user, and an adequate processing power for graphics, animation, and merging of images, like a pc and a monitor ( Carmigniani et al., 2011 ). To run, an AR system must also include a camera able to track the user movement for merging the virtual objects, and a visual display, like glasses through that the user can see the virtual objects overlaying to the physical world. To date, two-display systems exist, a video see-through (VST) and an optical see-though (OST) AR systems ( Botella et al., 2005 ; Juan et al., 2005 , 2007 ). The first one, disclosures virtual objects to the user by capturing the real objects/scenes with a camera and overlaying virtual objects, projecting them on a video or a monitor, while the second one, merges the virtual object on a transparent surface, like glasses, through the user see the added elements. The main difference between the two systems is the latency: an OST system could require more time to display the virtual objects than a VST system, generating a time lag between user’s action and performance and the detection of them by the system.

Augmented Reality Applications

Although AR is a more recent technology than VR, it has been investigated and used in several research areas such as architecture ( Lin and Hsu, 2017 ), maintenance ( Schwald and De Laval, 2003 ), entertainment ( Ozbek et al., 2004 ), education ( Nincarean et al., 2013 ; Bacca et al., 2014 ; Akçayır and Akçayır, 2017 ), medicine ( De Buck et al., 2005 ), and psychological treatments ( Juan et al., 2005 ; Botella et al., 2005 , 2010 ; Bretón-López et al., 2010 ; Wrzesien et al., 2011a , b , 2013 ; see the review Chicchi Giglioli et al., 2015 ). More in detail, in education several AR applications have been developed in the last few years showing the positive effects of this technology in supporting learning, such as an increased-on content understanding and memory preservation, as well as on learning motivation ( Radu, 2012 , 2014 ). For example, Ibáñez et al. (2014) developed a AR application on electromagnetism concepts’ learning, in which students could use AR batteries, magnets, cables on real superficies, and the system gave a real-time feedback to students about the correctness of the performance, improving in this way the academic success and motivation ( Di Serio et al., 2013 ). Deeply, AR system allows the possibility to learn visualizing and acting on composite phenomena that traditionally students study theoretically, without the possibility to see and test in real world ( Chien et al., 2010 ; Chen et al., 2011 ).

As well in psychological health, the number of research about AR is increasing, showing its efficacy above all in the treatment of psychological disorder (see the reviews Baus and Bouchard, 2014 ; Chicchi Giglioli et al., 2015 ). For example, in the treatment of anxiety disorders, like phobias, AR exposure therapy (ARET) showed its efficacy in one-session treatment, maintaining the positive impact in a follow-up at 1 or 3 month after. As VRET, ARET provides a safety and an ecological environment where any kind of stimulus is possible, allowing to keep control over the situation experienced by the patients, gradually generating situations of fear or stress. Indeed, in situations of fear, like the phobias for small animals, AR applications allow, in accordance with the patient’s anxiety, to gradually expose patient to fear animals, adding new animals during the session or enlarging their or increasing the speed. The various studies showed that AR is able, at the beginning of the session, to activate patient’s anxiety, for reducing after 1 h of exposition. After the session, patients even more than to better manage animal’s fear and anxiety, ware able to approach, interact, and kill real feared animals.

Materials and Methods

Data collection.

The input data for the analyses were retrieved from the scientific database Web of Science Core Collection ( Falagas et al., 2008 ) and the search terms used were “Virtual Reality” and “Augmented Reality” regarding papers published during the whole timespan covered.

Web of science core collection is composed of: Citation Indexes, Science Citation Index Expanded (SCI-EXPANDED) –1970-present, Social Sciences Citation Index (SSCI) –1970-present, Arts and Humanities Citation Index (A&HCI) –1975-present, Conference Proceedings Citation Index- Science (CPCI-S) –1990-present, Conference Proceedings Citation Index- Social Science & Humanities (CPCI-SSH) –1990-present, Book Citation Index– Science (BKCI-S) –2009-present, Book Citation Index– Social Sciences & Humanities (BKCI-SSH) –2009-present, Emerging Sources Citation Index (ESCI) –2015-present, Chemical Indexes, Current Chemical Reactions (CCR-EXPANDED) –2009-present (Includes Institut National de la Propriete Industrielle structure data back to 1840), Index Chemicus (IC) –2009-present.

The resultant dataset contained a total of 21,667 records for VR and 9,944 records for AR. The bibliographic record contained various fields, such as author, title, abstract, and all of the references (needed for the citation analysis). The research tool to visualize the networks was Cite space v.4.0.R5 SE (32 bit) ( Chen, 2006 ) under Java Runtime v.8 update 91 (build 1.8.0_91-b15). Statistical analyses were conducted using Stata MP-Parallel Edition, Release 14.0, StataCorp LP. Additional information can be found in Supplementary Data Sheet 1 .

The betweenness centrality of a node in a network measures the extent to which the node is part of paths that connect an arbitrary pair of nodes in the network ( Freeman, 1977 ; Brandes, 2001 ; Chen, 2006 ).

Structural metrics include betweenness centrality, modularity, and silhouette. Temporal and hybrid metrics include citation burstness and novelty. All the algorithms are detailed ( Chen et al., 2010 ).

The analysis of the literature on VR shows a complex panorama. At first sight, according to the document-type statistics from the Web of Science (WoS), proceedings papers were used extensively as outcomes of research, comprising almost 48% of the total (10,392 proceedings), with a similar number of articles on the subject amounting to about 47% of the total of 10, 199 articles. However, if we consider only the last 5 years (7,755 articles representing about 36% of the total), the situation changes with about 57% for articles (4,445) and about 33% for proceedings (2,578). Thus, it is clear that VR field has changed in areas other than at the technological level.

About the subject category, nodes and edges are computed as co-occurring subject categories from the Web of Science “Category” field in all the articles.

According to the subject category statistics from the WoS, computer science is the leading category, followed by engineering, and, together, they account for 15,341 articles, which make up about 71% of the total production. However, if we consider just the last 5 years, these categories reach only about 55%, with a total of 4,284 articles (Table 1 and Figure 1 ).

TABLE 1. Category statistics from the WoS for the entire period and the last 5 years.

FIGURE 1. Category from the WoS: network for the last 5 years.

The evidence is very interesting since it highlights that VR is doing very well as new technology with huge interest in hardware and software components. However, with respect to the past, we are witnessing increasing numbers of applications, especially in the medical area. In particular, note its inclusion in the top 10 list of rehabilitation and clinical neurology categories (about 10% of the total production in the last 5 years). It also is interesting that neuroscience and neurology, considered together, have shown an increase from about 12% to about 18.6% over the last 5 years. However, historic areas, such as automation and control systems, imaging science and photographic technology, and robotics, which had accounted for about 14.5% of the total articles ever produced were not even in the top 10 for the last 5 years, with each one accounting for less than 4%.

About the countries, nodes and edges are computed as networks of co-authors countries. Multiple occurrency of a country in the same paper are counted once.

The countries that were very involved in VR research have published for about 47% of the total (10,200 articles altogether). Of the 10,200 articles, the United States, China, England, and Germany published 4921, 2384, 1497, and 1398, respectively. The situation remains the same if we look at the articles published over the last 5 years. However, VR contributions also came from all over the globe, with Japan, Canada, Italy, France, Spain, South Korea, and Netherlands taking positions of prominence, as shown in Figure 2 .

FIGURE 2. Country network (node dimension represents centrality).

Network analysis was conducted to calculate and to represent the centrality index ( Freeman, 1977 ; Brandes, 2001 ), i.e., the dimension of the node in Figure 2 . The top-ranked country, with a centrality index of 0.26, was the United States (2011), and England was second, with a centrality index of 0.25. The third, fourth, and fifth countries were Germany, Italy, and Australia, with centrality indices of 0.15, 0.15, and 0.14, respectively.

About the Institutions, nodes and edges are computed as networks of co-authors Institutions (Figure 3 ).

FIGURE 3. Network of institutions: the dimensions of the nodes represent centrality.

The top-level institutions in VR were in the United States, where three universities were ranked as the top three in the world for published articles; these universities were the University of Illinois (159), the University of South California (147), and the University of Washington (146). The United States also had the eighth-ranked university, which was Iowa State University (116). The second country in the ranking was Canada, with the University of Toronto, which was ranked fifth with 125 articles and McGill University, ranked 10 th with 103 articles.

Other countries in the top-ten list were Netherlands, with the Delft University of Technology ranked fourth with 129 articles; Italy, with IRCCS Istituto Auxologico Italiano, ranked sixth (with the same number of publication of the institution ranked fifth) with 125 published articles; England, which was ranked seventh with 125 articles from the University of London’s Imperial College of Science, Technology, and Medicine; and China with 104 publications, with the Chinese Academy of Science, ranked ninth. Italy’s Istituto Auxologico Italiano, which was ranked fifth, was the only non-university institution ranked in the top-10 list for VR research (Figure 3 ).

About the Journals, nodes, and edges are computed as journal co-citation networks among each journals in the corresponding field.

The top-ranked Journals for citations in VR are Presence: Teleoperators & Virtual Environments with 2689 citations and CyberPsychology & Behavior (Cyberpsychol BEHAV) with 1884 citations; however, looking at the last 5 years, the former had increased the citations, but the latter had a far more significant increase, from about 70% to about 90%, i.e., an increase from 1029 to 1147.

Following the top two journals, IEEE Computer Graphics and Applications ( IEEE Comput Graph) and Advanced Health Telematics and Telemedicine ( St HEAL T) were both left out of the top-10 list based on the last 5 years. The data for the last 5 years also resulted in the inclusion of Experimental Brain Research ( Exp BRAIN RES) (625 citations), Archives of Physical Medicine and Rehabilitation ( Arch PHYS MED REHAB) (622 citations), and Plos ONE (619 citations) in the top-10 list of three journals, which highlighted the categories of rehabilitation and clinical neurology and neuroscience and neurology. Journal co-citation analysis is reported in Figure 4 , which clearly shows four distinct clusters.

FIGURE 4. Co-citation network of journals: the dimensions of the nodes represent centrality. Full list of official abbreviations of WoS journals can be found here: .

Network analysis was conducted to calculate and to represent the centrality index, i.e., the dimensions of the nodes in Figure 4 . The top-ranked item by centrality was Cyberpsychol BEHAV, with a centrality index of 0.29. The second-ranked item was Arch PHYS MED REHAB, with a centrality index of 0.23. The third was Behaviour Research and Therapy (Behav RES THER), with a centrality index of 0.15. The fourth was BRAIN, with a centrality index of 0.14. The fifth was Exp BRAIN RES, with a centrality index of 0.11.

Who’s Who in VR Research

Authors are the heart and brain of research, and their roles in a field are to define the past, present, and future of disciplines and to make significant breakthroughs to make new ideas arise (Figure 5 ).

FIGURE 5. Network of authors’ numbers of publications: the dimensions of the nodes represent the centrality index, and the dimensions of the characters represent the author’s rank.

Virtual reality research is very young and changing with time, but the top-10 authors in this field have made fundamentally significant contributions as pioneers in VR and taking it beyond a mere technological development. The purpose of the following highlights is not to rank researchers; rather, the purpose is to identify the most active researchers in order to understand where the field is going and how they plan for it to get there.

The top-ranked author is Riva G, with 180 publications. The second-ranked author is Rizzo A, with 101 publications. The third is Darzi A, with 97 publications. The forth is Aggarwal R, with 94 publications. The six authors following these three are Slater M, Alcaniz M, Botella C, Wiederhold BK, Kim SI, and Gutierrez-Maldonado J with 90, 90, 85, 75, 59, and 54 publications, respectively (Figure 6 ).

FIGURE 6. Authors’ co-citation network: the dimensions of the nodes represent centrality index, and the dimensions of the characters represent the author’s rank. The 10 authors that appear on the top-10 list are considered to be the pioneers of VR research.

Considering the last 5 years, the situation remains similar, with three new entries in the top-10 list, i.e., Muhlberger A, Cipresso P, and Ahmed K ranked 7th, 8th, and 10th, respectively.

The authors’ publications number network shows the most active authors in VR research. Another relevant analysis for our focus on VR research is to identify the most cited authors in the field.

For this purpose, the authors’ co-citation analysis highlights the authors in term of their impact on the literature considering the entire time span of the field ( White and Griffith, 1981 ; González-Teruel et al., 2015 ; Bu et al., 2016 ). The idea is to focus on the dynamic nature of the community of authors who contribute to the research.

Normally, authors with higher numbers of citations tend to be the scholars who drive the fundamental research and who make the most meaningful impacts on the evolution and development of the field. In the following, we identified the most-cited pioneers in the field of VR Research.

The top-ranked author by citation count is Gallagher (2001), with 694 citations. Second is Seymour (2004), with 668 citations. Third is Slater (1999), with 649 citations. Fourth is Grantcharov (2003), with 563 citations. Fifth is Riva (1999), with 546 citations. Sixth is Aggarwal (2006), with 505 citations. Seventh is Satava (1994), with 477 citations. Eighth is Witmer (2002), with 454 citations. Ninth is Rothbaum (1996), with 448 citations. Tenth is Cruz-neira (1995), with 416 citations.

Citation Network and Cluster Analyses for VR

Another analysis that can be used is the analysis of document co-citation, which allows us to focus on the highly-cited documents that generally are also the most influential in the domain ( Small, 1973 ; González-Teruel et al., 2015 ; Orosz et al., 2016 ).

The top-ranked article by citation counts is Seymour (2002) in Cluster #0, with 317 citations. The second article is Grantcharov (2004) in Cluster #0, with 286 citations. The third is Holden (2005) in Cluster #2, with 179 citations. The 4th is Gallagher et al. (2005) in Cluster #0, with 171 citations. The 5th is Ahlberg (2007) in Cluster #0, with 142 citations. The 6th is Parsons (2008) in Cluster #4, with 136 citations. The 7th is Powers (2008) in Cluster #4, with 134 citations. The 8th is Aggarwal (2007) in Cluster #0, with 121 citations. The 9th is Reznick (2006) in Cluster #0, with 121 citations. The 10th is Munz (2004) in Cluster #0, with 117 citations.

The network of document co-citations is visually complex (Figure 7 ) because it includes 1000s of articles and the links among them. However, this analysis is very important because can be used to identify the possible conglomerate of knowledge in the area, and this is essential for a deep understanding of the area. Thus, for this purpose, a cluster analysis was conducted ( Chen et al., 2010 ; González-Teruel et al., 2015 ; Klavans and Boyack, 2015 ). Figure 8 shows the clusters, which are identified with the two algorithms in Table 2 .

FIGURE 7. Network of document co-citations: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank, and the numbers represent the strengths of the links. It is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past VR research to the current research.

FIGURE 8. Document co-citation network by cluster: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing reports the name of the cluster with a short description that was produced with the mutual information algorithm; the clusters are identified with colored polygons.

TABLE 2. Cluster ID and silhouettes as identified with two algorithms ( Chen et al., 2010 ).

The identified clusters highlight clear parts of the literature of VR research, making clear and visible the interdisciplinary nature of this field. However, the dynamics to identify the past, present, and future of VR research cannot be clear yet. We analysed the relationships between these clusters and the temporal dimensions of each article. The results are synthesized in Figure 9 . It is clear that cluster #0 (laparoscopic skill), cluster #2 (gaming and rehabilitation), cluster #4 (therapy), and cluster #14 (surgery) are the most popular areas of VR research. (See Figure 9 and Table 2 to identify the clusters.) From Figure 9 , it also is possible to identify the first phase of laparoscopic skill (cluster #6) and therapy (cluster #7). More generally, it is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past VR research to the current research.

FIGURE 9. Network of document co-citation: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing on the right hand side reports the number of the cluster, such as in Table 2 , with a short description that was extracted accordingly.

We were able to identify the top 486 references that had the most citations by using burst citations algorithm. Citation burst is an indicator of a most active area of research. Citation burst is a detection of a burst event, which can last for multiple years as well as a single year. A citation burst provides evidence that a particular publication is associated with a surge of citations. The burst detection was based on Kleinberg’s algorithm ( Kleinberg, 2002 , 2003 ). The top-ranked document by bursts is Seymour (2002) in Cluster #0, with bursts of 88.93. The second is Grantcharov (2004) in Cluster #0, with bursts of 51.40. The third is Saposnik (2010) in Cluster #2, with bursts of 40.84. The fourth is Rothbaum (1995) in Cluster #7, with bursts of 38.94. The fifth is Holden (2005) in Cluster #2, with bursts of 37.52. The sixth is Scott (2000) in Cluster #0, with bursts of 33.39. The seventh is Saposnik (2011) in Cluster #2, with bursts of 33.33. The eighth is Burdea et al. (1996) in Cluster #3, with bursts of 32.42. The ninth is Burdea and Coiffet (2003) in Cluster #22, with bursts of 31.30. The 10th is Taffinder (1998) in Cluster #6, with bursts of 30.96 (Table 3 ).

TABLE 3. Cluster ID and references of burst article.

Citation Network and Cluster Analyses for AR

Looking at Augmented Reality scenario, the top ranked item by citation counts is Azuma (1997) in Cluster #0, with citation counts of 231. The second one is Azuma et al. (2001) in Cluster #0, with citation counts of 220. The third is Van Krevelen (2010) in Cluster #5, with citation counts of 207. The 4th is Lowe (2004) in Cluster #1, with citation counts of 157. The 5th is Wu (2013) in Cluster #4, with citation counts of 144. The 6th is Dunleavy (2009) in Cluster #4, with citation counts of 122. The 7th is Zhou (2008) in Cluster #5, with citation counts of 118. The 8th is Bay (2008) in Cluster #1, with citation counts of 117. The 9th is Newcombe (2011) in Cluster #1, with citation counts of 109. The 10th is Carmigniani et al. (2011) in Cluster #5, with citation counts of 104.

The network of document co-citations is visually complex (Figure 10 ) because it includes 1000s of articles and the links among them. However, this analysis is very important because can be used to identify the possible conglomerate of knowledge in the area, and this is essential for a deep understanding of the area. Thus, for this purpose, a cluster analysis was conducted ( Chen et al., 2010 ; González-Teruel et al., 2015 ; Klavans and Boyack, 2015 ). Figure 11 shows the clusters, which are identified with the two algorithms in Table 3 .

FIGURE 10. Network of document co-citations: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank, and the numbers represent the strengths of the links. It is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past AR research to the current research.

FIGURE 11. Document co-citation network by cluster: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing reports the name of the cluster with a short description that was produced with the mutual information algorithm; the clusters are identified with colored polygons.

The identified clusters highlight clear parts of the literature of AR research, making clear and visible the interdisciplinary nature of this field. However, the dynamics to identify the past, present, and future of AR research cannot be clear yet. We analysed the relationships between these clusters and the temporal dimensions of each article. The results are synthesized in Figure 12 . It is clear that cluster #1 (tracking), cluster #4 (education), and cluster #5 (virtual city environment) are the current areas of AR research. (See Figure 12 and Table 3 to identify the clusters.) It is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past AR research to the current research.

FIGURE 12. Network of document co-citation: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing on the right hand side reports the number of the cluster, such as in Table 2 , with a short description that was extracted accordingly.

We were able to identify the top 394 references that had the most citations by using burst citations algorithm. Citation burst is an indicator of a most active area of research. Citation burst is a detection of a burst event, which can last for multiple years as well as a single year. A citation burst provides evidence that a particular publication is associated with a surge of citations. The burst detection was based on Kleinberg’s algorithm ( Kleinberg, 2002 , 2003 ). The top ranked document by bursts is Azuma (1997) in Cluster #0, with bursts of 101.64. The second one is Azuma et al. (2001) in Cluster #0, with bursts of 84.23. The third is Lowe (2004) in Cluster #1, with bursts of 64.07. The 4th is Van Krevelen (2010) in Cluster #5, with bursts of 50.99. The 5th is Wu (2013) in Cluster #4, with bursts of 47.23. The 6th is Hartley (2000) in Cluster #0, with bursts of 37.71. The 7th is Dunleavy (2009) in Cluster #4, with bursts of 33.22. The 8th is Kato (1999) in Cluster #0, with bursts of 32.16. The 9th is Newcombe (2011) in Cluster #1, with bursts of 29.72. The 10th is Feiner (1993) in Cluster #8, with bursts of 29.46 (Table 4 ).

TABLE 4. Cluster ID and silhouettes as identified with two algorithms ( Chen et al., 2010 ).

Our findings have profound implications for two reasons. At first the present work highlighted the evolution and development of VR and AR research and provided a clear perspective based on solid data and computational analyses. Secondly our findings on VR made it profoundly clear that the clinical dimension is one of the most investigated ever and seems to increase in quantitative and qualitative aspects, but also include technological development and article in computer science, engineer, and allied sciences.

Figure 9 clarifies the past, present, and future of VR research. The outset of VR research brought a clearly-identifiable development in interfaces for children and medicine, routine use and behavioral-assessment, special effects, systems perspectives, and tutorials. This pioneering era evolved in the period that we can identify as the development era, because it was the period in which VR was used in experiments associated with new technological impulses. Not surprisingly, this was exactly concomitant with the new economy era in which significant investments were made in information technology, and it also was the era of the so-called ‘dot-com bubble’ in the late 1990s. The confluence of pioneering techniques into ergonomic studies within this development era was used to develop the first effective clinical systems for surgery, telemedicine, human spatial navigation, and the first phase of the development of therapy and laparoscopic skills. With the new millennium, VR research switched strongly toward what we can call the clinical-VR era, with its strong emphasis on rehabilitation, neurosurgery, and a new phase of therapy and laparoscopic skills. The number of applications and articles that have been published in the last 5 years are in line with the new technological development that we are experiencing at the hardware level, for example, with so many new, HMDs, and at the software level with an increasing number of independent programmers and VR communities.

Finally, Figure 12 identifies clusters of the literature of AR research, making clear and visible the interdisciplinary nature of this field. The dynamics to identify the past, present, and future of AR research cannot be clear yet, but analyzing the relationships between these clusters and the temporal dimensions of each article tracking, education, and virtual city environment are the current areas of AR research. AR is a new technology that is showing its efficacy in different research fields, and providing a novel way to gather behavioral data and support learning, training, and clinical treatments.

Looking at scientific literature conducted in the last few years, it might appear that most developments in VR and AR studies have focused on clinical aspects. However, the reality is more complex; thus, this perception should be clarified. Although researchers publish studies on the use of VR in clinical settings, each study depends on the technologies available. Industrial development in VR and AR changed a lot in the last 10 years. In the past, the development involved mainly hardware solutions while nowadays, the main efforts pertain to the software when developing virtual solutions. Hardware became a commodity that is often available at low cost. On the other hand, software needs to be customized each time, per each experiment, and this requires huge efforts in term of development. Researchers in AR and VR today need to be able to adapt software in their labs.

Virtual reality and AR developments in this new clinical era rely on computer science and vice versa. The future of VR and AR is becoming more technological than before, and each day, new solutions and products are coming to the market. Both from software and hardware perspectives, the future of AR and VR depends on huge innovations in all fields. The gap between the past and the future of AR and VR research is about the “realism” that was the key aspect in the past versus the “interaction” that is the key aspect now. First 30 years of VR and AR consisted of a continuous research on better resolution and improved perception. Now, researchers already achieved a great resolution and need to focus on making the VR as realistic as possible, which is not simple. In fact, a real experience implies a realistic interaction and not just great resolution. Interactions can be improved in infinite ways through new developments at hardware and software levels.

Interaction in AR and VR is going to be “embodied,” with implication for neuroscientists that are thinking about new solutions to be implemented into the current systems ( Blanke et al., 2015 ; Riva, 2018 ; Riva et al., 2018 ). For example, the use of hands with contactless device (i.e., without gloves) makes the interaction in virtual environments more natural. The Leap Motion device 1 allows one to use of hands in VR without the use of gloves or markers. This simple and low-cost device allows the VR users to interact with virtual objects and related environments in a naturalistic way. When technology is able to be transparent, users can experience increased sense of being in the virtual environments (the so-called sense of presence).

Other forms of interactions are possible and have been developing continuously. For example, tactile and haptic device able to provide a continuous feedback to the users, intensifying their experience also by adding components, such as the feeling of touch and the physical weight of virtual objects, by using force feedback. Another technology available at low cost that facilitates interaction is the motion tracking system, such as Microsoft Kinect, for example. Such technology allows one to track the users’ bodies, allowing them to interact with the virtual environments using body movements, gestures, and interactions. Most HMDs use an embedded system to track HMD position and rotation as well as controllers that are generally placed into the user’s hands. This tracking allows a great degree of interaction and improves the overall virtual experience.

A final emerging approach is the use of digital technologies to simulate not only the external world but also the internal bodily signals ( Azevedo et al., 2017 ; Riva et al., 2017 ): interoception, proprioception and vestibular input. For example, Riva et al. (2017) recently introduced the concept of “sonoception” ( ), a novel non-invasive technological paradigm based on wearable acoustic and vibrotactile transducers able to alter internal bodily signals. This approach allowed the development of an interoceptive stimulator that is both able to assess interoceptive time perception in clinical patients ( Di Lernia et al., 2018b ) and to enhance heart rate variability (the short-term vagally mediated component—rMSSD) through the modulation of the subjects’ parasympathetic system ( Di Lernia et al., 2018a ).

In this scenario, it is clear that the future of VR and AR research is not just in clinical applications, although the implications for the patients are huge. The continuous development of VR and AR technologies is the result of research in computer science, engineering, and allied sciences. The reasons for which from our analyses emerged a “clinical era” are threefold. First, all clinical research on VR and AR includes also technological developments, and new technological discoveries are being published in clinical or technological journals but with clinical samples as main subject. As noted in our research, main journals that publish numerous articles on technological developments tested with both healthy and patients include Presence: Teleoperators & Virtual Environments, Cyberpsychology & Behavior (Cyberpsychol BEHAV), and IEEE Computer Graphics and Applications (IEEE Comput Graph). It is clear that researchers in psychology, neuroscience, medicine, and behavioral sciences in general have been investigating whether the technological developments of VR and AR are effective for users, indicating that clinical behavioral research has been incorporating large parts of computer science and engineering. A second aspect to consider is the industrial development. In fact, once a new technology is envisioned and created it goes for a patent application. Once the patent is sent for registration the new technology may be made available for the market, and eventually for journal submission and publication. Moreover, most VR and AR research that that proposes the development of a technology moves directly from the presenting prototype to receiving the patent and introducing it to the market without publishing the findings in scientific paper. Hence, it is clear that if a new technology has been developed for industrial market or consumer, but not for clinical purpose, the research conducted to develop such technology may never be published in a scientific paper. Although our manuscript considered published researches, we have to acknowledge the existence of several researches that have not been published at all. The third reason for which our analyses highlighted a “clinical era” is that several articles on VR and AR have been considered within the Web of Knowledge database, that is our source of references. In this article, we referred to “research” as the one in the database considered. Of course, this is a limitation of our study, since there are several other databases that are of big value in the scientific community, such as IEEE Xplore Digital Library, ACM Digital Library, and many others. Generally, the most important articles in journals published in these databases are also included in the Web of Knowledge database; hence, we are convinced that our study considered the top-level publications in computer science or engineering. Accordingly, we believe that this limitation can be overcome by considering the large number of articles referenced in our research.

Considering all these aspects, it is clear that clinical applications, behavioral aspects, and technological developments in VR and AR research are parts of a more complex situation compared to the old platforms used before the huge diffusion of HMD and solutions. We think that this work might provide a clearer vision for stakeholders, providing evidence of the current research frontiers and the challenges that are expected in the future, highlighting all the connections and implications of the research in several fields, such as clinical, behavioral, industrial, entertainment, educational, and many others.

Author Contributions

PC and GR conceived the idea. PC made data extraction and the computational analyses and wrote the first draft of the article. IG revised the introduction adding important information for the article. PC, IG, MR, and GR revised the article and approved the last version of the article after important input to the article rationale.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer GC declared a shared affiliation, with no collaboration, with the authors PC and GR to the handling Editor at the time of the review.

Supplementary Material

The Supplementary Material for this article can be found online at:

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Keywords : virtual reality, augmented reality, quantitative psychology, measurement, psychometrics, scientometrics, computational psychometrics, mathematical psychology

Citation: Cipresso P, Giglioli IAC, Raya MA and Riva G (2018) The Past, Present, and Future of Virtual and Augmented Reality Research: A Network and Cluster Analysis of the Literature. Front. Psychol. 9:2086. doi: 10.3389/fpsyg.2018.02086

Received: 14 December 2017; Accepted: 10 October 2018; Published: 06 November 2018.

Reviewed by:

Copyright © 2018 Cipresso, Giglioli, Raya and Riva. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Pietro Cipresso, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

Augmented Reality: A Comprehensive Review

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  • Published: 20 October 2022
  • Volume 30 , pages 1057–1080, ( 2023 )

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research paper on augmented reality

  • Shaveta Dargan 1 ,
  • Shally Bansal 2 ,
  • Munish Kumar   ORCID: 1 ,
  • Ajay Mittal 3 &
  • Krishan Kumar 4  

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Augmented Reality (AR) aims to modify the perception of real-world images by overlaying digital data on them. A novel mechanic, it is an enlightening and engaging mechanic that constantly strives for new techniques in every sphere. The real world can be augmented with information in real-time. AR aims to accept the outdoors and come up with a novel and efficient model in all application areas. A wide array of fields are displaying real-time computer-generated content, such as education, medicine, robotics, manufacturing, and entertainment. Augmented reality is considered a subtype of mixed reality, and it is treated as a distortion of virtual reality. The article emphasizes the novel digital technology that has emerged after the success of Virtual Reality, which has a wide range of applications in the digital age. There are fundamental requirements to understand AR, such as the nature of technology, architecture, the devices required, types of AR, benefits, limitations, and differences with VR, which are discussed in a very simplified way in this article. As well as a year-by-year tabular overview of the research papers that have been published in the journal on augmented reality-based applications, this article aims to provide a comprehensive overview of augmented reality-based applications. It is hard to find a field that does not make use of the amazing features of AR. This article concludes with a discussion, conclusion, and future directions for AR.

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Dargan, S., Bansal, S., Kumar, M. et al. Augmented Reality: A Comprehensive Review. Arch Computat Methods Eng 30 , 1057–1080 (2023).

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Received : 11 September 2022

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In-Depth Review of Augmented Reality: Tracking Technologies, Development Tools, AR Displays, Collaborative AR, and Security Concerns

Toqeer ali syed.

1 Faculty of Computer and Information Systems, Islamic University of Madinah, Medina 42351, Saudi Arabia

Muhammad Shoaib Siddiqui

Hurria binte abdullah.

2 School of Social Sciences and Humanities, National University of Science and Technology (NUST), Islamabad 44000, Pakistan

3 Malaysian Institute of Information Technology, Universiti Kuala Lumpur, Kuala Lumpur 50250, Malaysia

4 Department of Computer Science, Bacha Khan University Charsadda, Charsadda 24420, Pakistan

Abdallah Namoun

Ali alzahrani, adnan nadeem, ahmad b. alkhodre.

Augmented reality (AR) has gained enormous popularity and acceptance in the past few years. AR is indeed a combination of different immersive experiences and solutions that serve as integrated components to assemble and accelerate the augmented reality phenomena as a workable and marvelous adaptive solution for many realms. These solutions of AR include tracking as a means for keeping track of the point of reference to make virtual objects visible in a real scene. Similarly, display technologies combine the virtual and real world with the user’s eye. Authoring tools provide platforms to develop AR applications by providing access to low-level libraries. The libraries can thereafter interact with the hardware of tracking sensors, cameras, and other technologies. In addition to this, advances in distributed computing and collaborative augmented reality also need stable solutions. The various participants can collaborate in an AR setting. The authors of this research have explored many solutions in this regard and present a comprehensive review to aid in doing research and improving different business transformations. However, during the course of this study, we identified that there is a lack of security solutions in various areas of collaborative AR (CAR), specifically in the area of distributed trust management in CAR. This research study also proposed a trusted CAR architecture with a use-case of tourism that can be used as a model for researchers with an interest in making secure AR-based remote communication sessions.

1. Introduction

Augmented reality (AR) is one of the leading expanding immersive experiences of the 21st century. AR has brought a revolution in different realms including health and medicine, teaching and learning, tourism, designing, manufacturing, and other similar industries whose acceptance accelerated the growth of AR in an unprecedented manner [ 1 , 2 , 3 ]. According to a recent report in September 2022, the market size of AR and VR reached USD 27.6 billion in 2021, which is indeed estimated to reach USD 856.2 billion by the end of the year 2031 [ 4 ]. Big companies largely use AR-based technologies. For instance, Amazon, one of the leading online shopping websites, uses this technology to make it easier for customers to decide the type of furniture they want to buy. The rise in mobile phone technology also acted as an accelerator in popularizing AR. Earlier, mobile phones were not advanced and capable enough to run these applications due to their low graphics. Nowadays, however, smart devices are capable enough to easily run AR-based applications. A lot of research has been done on mobile-based AR. Lee et al. [ 5 ] developed a user-based design interface for educational purpose in mobile AR. To evaluate its conduct, fourth-grade elementary students were selected.

The adoption of AR in its various perspectives is backed up by a prolonged history. This paper presents an overview of the different integrated essential components that contribute to the working framework of AR, and the latest developments on these components are collected, analyzed, and presented, while the developments in the smart devices and the overall experience of the users have changed drastically [ 6 ]. The tracking technologies [ 7 ] are the building blocks of AR and establish a point of reference for movement and for creating an environment where the virtual and real objects are presented together. To achieve a real experience with augmented objects, several tracking technologies are presented which include techniques such as sensor-based [ 8 ], markerless, marker-based [ 9 , 10 ], and hybrid tracking technologies. Among these different technologies, hybrid tracking technologies are the most adaptive. As part of the framework constructed in this study, the simultaneous localization and mapping (SLAM) and inertial tracking technologies are combined. The SLAM technology collects points through cameras in real scenes while the point of reference is created using inertial tracking. The virtual objects are inserted on the relevant points of reference to create an augmented reality. Moreover, this paper analyzes and presents a detailed discussion on different tracking technologies according to their use in different realms i.e., in education, industries, and medical fields. Magnetic tracking is widely used in AR systems in medical, maintenance, and manufacturing. Moreover, vision-based tracking is mostly used in mobile phones and tablets because they have screen and camera, which makes them the best platform for AR. In addition, GPS tracking is useful in the fields of military, gaming, and tourism. These tracking technologies along with others are explained in detail in Section 3 .

Once the points of reference are collected after tracking, then another important factor that requires significant accuracy is to determine at which particular point the virtual objects have to be mixed with the real environment. Here comes the role of display technologies that gives the users of augmented reality an environment where the real and virtual objects are displayed visually. Therefore, display technologies are one of the key components of AR. This research identifies state-of-the-art display technologies that help to provide a quality view of real and virtual objects. Augmented reality displays can be divided into various categories. All have the same task to show the merged image of real and virtual content to the user’s eye. The authors have categorized the latest technologies of optical display after the advancements in holographic optical elements (HOEs). There are other categories of AR displays, such as video-based, eye multiplexed, and projected onto a physical surface. Optical see-through has two sub-categories, one is a free-space combiner and the other is a wave-guide combiner [ 11 , 12 ]. The thorough details of display technologies are presented in Section 4 .

To develop these AR applications, different tools are used depending on the type of application used. For example, to develop a mobile-based AR application for Android or iOS, ARToolKit [ 13 ] is used. However, FLARToolKit [ 14 ] is used to create a web-based application using Flash. Moreover, there are various plug-ins available that can be integrated with Unity [ 15 ] to create AR applications. These development tools are reviewed in Section 6 of this paper. Figure 1 provides an overview of reviewed topics of augmented reality in this paper.

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Overview of AR, VR, and collaborative AR applications, tools, and technologies.

After going through a critical review process of collaborative augmented reality, the research has identified that some security flaws and missing trust parameters need to be addressed to ensure a pristine environment is provided to the users. Hackers and intruders are always active to exploit different vulnerabilities in the systems and software, but the previous research conducted on collaborative augmented reality did not depict reasonable efforts made in this direction to make secure collaboration. To address the security flaws and to provide secure communication in collaborative augmented reality, this research considered it appropriate to come up with a security solution and framework that can limit danger and risks that may be posed in the form of internal and external attacks. To actualize the secure platform, this study came up with an architecture for presenting a secure collaborative AR in the tourism sector in Saudi Arabia as a case study. The focus of the case study is to provide an application that can guide tourists during their visit to any of the famous landmarks in the country. This study proposed a secure and trustful mobile application based on collaborative AR for tourists. In this application, the necessary information is rendered on screen and the user can hire a guide to provide more information in detail. A single guide can provide the services to a group of tourists visiting the same landmark. A blockchain network was used to secure the applications and protect the private data of the users [ 16 , 17 ]. For this purpose, we performed a thorough literature review for an optimized solution regarding security and tracking for which we studies the existing tracking technologies and listed them in this paper along with their limitations. In our use case, we used a GPS tracking system to track the user’s movement and provide the necessary information about the visited landmark through the mobile application.

Observing the fact that AR operates in an integrated fashion that combines different technologies including tracking technologies, display technologies, AR tools, collaborative AR, and applications of AR has encouraged us to explore and present these conceptions and technologies in detail. To facilitate researchers on these different techniques, the authors have explored the research previously conducted and presented it in a Venn diagram, as shown in Figure 2 . Interested investigators can choose their required area of research in AR. As can be seen in the diagram, most research has been done in the area of tracking technologies. This is further divided into different types of tracking solutions including fiducial tracking, video-based tracking, and inertial tracking. Some papers lie in several categories for, example some papers such as [ 18 , 19 , 20 ] fall in both the fiducial tracking and sensor categories. Similarly, computer vision and display devices have some common papers, and inertial tracking and video-based tracking also have some papers in common. In addition, display devices share common papers with computer vision, mobile AR, design guidelines, tool-kits, evaluation, AR tags, and security and privacy of AR. Furthermore, visualization has different papers in common with business, interior design, and human–robot communication. While education shares some paper with gaming, simulation, medicine, heritage, and manufacturing. In short, we have tried to summarize all papers and further elaborate in their sections for the convenience of the reader.

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Classification of reviewed papers with respect to tracking, display, authoring tools, application, Collaborative and security.

Contribution: This research presents a comprehensive review of AR and its associated technologies. A review of state-of-the-art tracking and display technologies is presented followed by different essential components and tools that can be used to effectively create AR experiences. The study also presents the newly emerging technologies such as collaborative augmented reality and how different application interactions are carried out. During the review phase, the research identified that the AR-based solutions and particularly collaborative augmented reality solutions are vulnerable to external intrusion. It is identified that these solutions lack security and the interaction could be hijacked, manipulated, and sometimes exposed to potential threats. To address these concerns, this research felt the need to ensure that the communication has integrity; henceforth, the research utilizes the state-of-the-art blockchain infrastructure for the collaborating applications in AR. The paper further proposes complete secure framework wherein different applications working remotely have a real feeling of trust with each other [ 21 ].

Outline : This paper presents the overview of augmented reality and its applications in various realms in Section 2 . In Section 3 , tracking technologies are presented, while a detailed overview of the display technologies is provided in Section 4 . Section 6 apprises readers on AR development tools. Section 7 highlights the collaborative research on augmented reality, while Section 8 interprets the AR interaction and input technologies. The paper presents the details of design guidelines and interface patterns in Section 9 , while Section 10 discusses the security and trust issues in collaborative AR. Section 12 highlights future directions for research, while Section 13 concludes this research.

2. Augmented Reality Overview

People, for many years, have been using lenses, light sources, and mirrors to create illusions and virtual images in the real world [ 22 , 23 , 24 ]. However, Ivan Sutherland was the first person to truly generate the AR experience. Sketchpad, developed at MIT in 1963 by Ivan Sutherland, is the world’s first interactive graphic application [ 25 ]. In Figure 3 , we have given an overview of the development of AR technology from the beginning to 2022. Bottani et al. [ 26 ] reviews the AR literature published during the time period of 2006–2017. Moreover, Sereno et al. [ 27 ] use a systematic survey approach to detail the existing literature available on the intersection of computer-supported collaborative work and AR.

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Augmented reality advancement over time for the last 60 years.

2.1. Head-Mounted Display

Ens et al. [ 28 ] review the existing work on design exploration for mixed-scale gestures where the Hololens AR display is used to interweave larger gestures with micro-gestures.

2.2. AR Towards Applications

ARToolKit tracking library [ 13 ] aimed to provide the computer vision tracking of a square marker in real-time which fixed two major problems, i.e., enabling interaction with real-world objects and secondly, the user’s viewpoint tracking system. Researchers conducted studies to develop handheld AR systems. Hettig et al. [ 29 ] present a system called “Augmented Visualization Box” to asses surgical augmented reality visualizations in a virtual environment. Goh et al. [ 30 ] present details of the critical analysis of 3D interaction techniques in mobile AR. Kollatsch et al. [ 31 ] introduce a system that creates and introduces the production data and maintenance documentation into the AR maintenance apps for machine tools which aims to reduce the overall cost of necessary expertise and the planning process of AR technology. Bhattacharyya et al. [ 32 ] introduce a two-player mobile AR game known as Brick, where users can engage in synchronous collaboration while inhabiting the real-time and shared augmented environment. Kim et al. [ 33 ] suggest that this decade is marked by a tremendous technological boom particularly in rendering and evaluation research while display and calibration research has declined. Liu et al. [ 34 ] expand the information feedback channel from industrial robots to a human workforce for human–robot collaboration development.

2.3. Augmented Reality for the Web

Cortes et al. [ 35 ] introduce the new techniques of collaboratively authoring surfaces on the web using mobile AR. Qiao et al. [ 36 ] review the current implementations of mobile AR, enabling technologies of AR, state-of-art technology, approaches for potential web AR provisioning, and challenges that AR faces in a web-based system.

2.4. AR Application Development

The AR industry was tremendously increasing in 2015, extending from smartphones to websites with head-worn display systems such as Google Glass. In this regard, Agati et al. [ 18 ] propose design guidelines for the development of an AR manual assembly system which includes ergonomics, usability, corporate-related, and cognition.

AR for Tourism and Education: Shukri et al. [ 37 ] aim to introduce the design guidelines of mobile AR for tourism by proposing 11 principles for developing efficient AR design for tourism which reduces cognitive overload, provides learning ability, and helps explore the content while traveling in Malaysia. In addition to it, Fallahkhair et al. [ 38 ] introduce new guidelines to make AR technologies with enhanced user satisfaction, efficiency, and effectiveness in cultural and contextual learning using mobiles, thereby enhancing the tourism experience. Akccayir et al. [ 39 ] show that AR has the advantage of placing the virtual image on a real object in real time while pedagogical and technical issues should be addressed to make the technology more reliable. Salvia et al. [ 40 ] suggest that AR has a positive impact on learning but requires some advancements.

Sarkar et al. [ 41 ] present an AR app known as ScholAR. It introduces enhancing the learning skills of the students to inculcate conceptualizing and logical thinking among sevemth-grade students. Soleiman et al. [ 42 ] suggest that the use of AR improves abstract writing as compared to VR.

2.5. AR Security and Privacy

Hadar et al. [ 43 ] scrutinize security at all steps of AR application development and identify the need for new strategies for information security, privacy, and security, with a main goal to design and introduce capturing and mapping concerns. Moreover, in the industrial arena, Mukhametshin et al. [ 44 ] focus on developing sensor tag detection, tracking, and recognition for designing an AR client-side app for Siemen Company to monitor the equipment for remote facilities.

3. Tracking Technology of AR

Tracking technologies introduce the sensation of motion in the virtual and augmented reality world and perform a variety of tasks. Once a tracking system is rightly chosen and correctly installed, it allows a person to move within a virtual and augmented environment. It further allows us to interact with people and objects within augmented environments. The selection of tracking technology depends on the sort of environment, the sort of data, and the availability of required budgets. For AR technology to meet Azuma’s definition of an augmented reality system, it must adhere to three main components:

  • it combines virtual and the real content;
  • it is interactive in real time;
  • is is registered in three dimensions.

The third condition of being “registered in three dimensions” alludes to the capability of an AR system to project the virtual content on physical surroundings in such a way that it seems to be part of the real world. The position and orientation (pose) of the viewer concerning some anchor in the real world must be identified and determined for registering the virtual content in the real environment. This anchor of the real world may be the dead-reckoning from inertial tracking, a defined location in space determined using GPS, or a physical object such as a paper image marker or magnetic tracker source. In short, the real-world anchor depends upon the applications and the technologies used. With respect to the type of technology used, there are two ways of registering the AR system in 3D:

  • Determination of the position and orientation of the viewer relative to the real-world anchor: registration phase;
  • Upgrading of viewer’s pose with respect to previously known pose: tracking phase.

In this document, the word “tracking” would define both phases as common terminology. There are two main types of tracking techniques which are explained as follows (depicted in Figure 4 ).

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Categorization of augmented reality tracking techniques.

3.1. Markerless Tracking Techniques

Markerless tracking techniques further have two types, one is sensor based and another is vision based.

3.1.1. Sensor-Based Tracking

Magnetic Tracking Technology: This technology includes a tracking source and two sensors, one sensor for the head and another one for the hand. The tracking source creates an electromagnetic field in which the sensors are placed. The computer then calculates the orientation and position of the sensors based on the signal attenuation of the field. This gives the effect of allowing a full 360 range of motion. i.e., allowing us to look all the way around the 3D environment. It also allows us to move around all three degrees of freedom. The hand tracker has some control buttons that allow the user to navigate along the environment. It allows us to pick things up and understand the size and shape of the objects [ 45 ]. In Figure 5 we have tried to draw the tracking techniques to give a better understanding to the reader.

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Augmented reality tracking techniques presentation.

Frikha et al. [ 46 ] introduce a new mutual occlusion problem handler. The problem of occlusion occurs when the real objects are in front of the virtual objects in the scene. The authors use a 3D positioning approach and surgical instrument tracking in an AR environment. The paradigm is introduced that is based on monocular image-based processing. The result of the experiment suggested that this approach is capable of handling mutual occlusion automatically in real-time.

One of the main issues with magnetic tracking is the limited positioning range [ 47 ]. Orientation and position can be determined by setting up the receiver to the viewer [ 48 ]. Receivers are small and light in weight and the magnetic trackers are indifferent to optical disturbances and occlusion; therefore, these have high update rates. However, the resolution magnetic field declines with the fourth power of the distance, and the strength of magnetic fields decline with the cube of the distance [ 49 ]. Therefore, the magnetic trackers have constrained working volume. Moreover, magnetic trackers are sensitive to environments around magnetic fields and the type of magnetic material used and are also susceptible to measurement jitter [ 50 ].

Magnetic tracking technology is widely used in the range of AR systems, with applications ranging from maintenance [ 51 ] to medicine [ 52 ] and manufacturing [ 53 ].

Inertial Tracking: Magnetometers, accelerometers, and gyroscopes are examples of inertial measurement units (IMU) used in inertial tracking to evaluate the velocity and orientation of the tracked object. An inertial tracking system is used to find the three rotational degrees of freedom relative to gravity. Moreover, the time period of the trackers’ update and the inertial velocity can be determined by the change in the position of the tracker.

Advantages of Inertial Tracking: It does not require a line of sight and has no range limitations. It is not prone to optical, acoustic, magnetic, and RE interference sources. Furthermore, it provides motion measurement with high bandwidth. Moreover, it has negligible latency and can be processed as fast as one desires.

Disadvantages of Inertial Tracking: They are prone to drift of orientation and position over time, but their major impact is on the position measurement. The rationale behind this is that the position must be derived from the velocity measurements. The usage of a filter could help in resolving this issue. However, the issue could while focusing on this, the filter can decrease the responsiveness and the update rate of the tracker [ 54 ]. For the ultimate correction of this issue of the drift, the inertial sensor should be combined with any other kind of sensor. For instance, it could be combined with ultrasonic range measurement devices and optical trackers.

3.1.2. Vision-Based Tracking

Vision-based tracking is defined as tracking approaches that ascertain the camera pose by the use of data captured from optical sensors and as registration. The optical sensors can be divided into the following three categories:

  • visible light tracking;
  • 3D structure tracking;
  • infrared tracking.

In recent times, vision-based tracking AR is becoming highly popular due to the improved computational power of consumer devices and the ubiquity of mobile devices, such as tablets and smartphones, thereby making them the best platform for AR technologies. Chakrabarty et al. [ 55 ] contribute to the development of autonomous tracking by integrating the CMT into IBVS, their impact on the rigid deformable targets in indoor settings, and finally the integration of the system into the Gazebo simulator. Vision-based tracking is demonstrated by the use of an effective object tracking algorithm [ 56 ] known as the clustering of static-adaptive correspondences for deformable object tracking (CMT). Gupta et al. [ 57 ] detail the comparative analysis between the different types of vision-based tracking systems.

Moreover, Krishna et al. [ 58 ] explore the use of electroencephalogram (EEG) signals in user authentication. User authentication is similar to facial recognition in mobile phones. Moreover, this is also evaluated by combining it with eye-tracking data. This research contributes to the development of a novel evaluation paradigm and a biometric authentication system for the integration of these systems. Furthermore, Dzsotjan et al. [ 59 ] delineate the usefulness of the eye-tracking data evaluated during the lectures in order to determine the learning gain of the user. Microsoft HoloLens2’s designed Walk the Graph app was used to generate the data. Binary classification was performed on the basis of the kinematic graphs which users reported of their own movement.

Ranging from smartphones to laptops and even to wearable devices with suitable cameras located in them, visible light tracking is the most commonly used optical sensor. These cameras are particularly important because they can both make a video of the real environment and can also register the virtual content to it, and thereby can be used in video see-through AR systems.

Chen et al. [ 60 ] resolve the shortcomings of the deep learning lightning model (DAM) by combining the method of transferring a regular video to a 3D photo-realistic avatar and a high-quality 3D face tracking algorithm. The evaluation of the proposed system suggests its effectiveness in real-world scenarios when we have variability in expression, pose, and illumination. Furthermore, Rambach et al. [ 61 ] explore the details pipeline of 6DoF object tracking using scanned 3D images of the objects. The scope of research covers the initialization of frame-to-frame tracking, object registration, and implementation of these aspects to make the experience more efficient. Moreover, it resolves the challenges that we faced with occlusion, illumination changes, and fast motion.

3.1.3. Three-Dimensional Structure Tracking

Three-dimensional structure information has become very affordable because of the development of commercial sensors capable of accomplishing this task. It was begun after the development of Microsoft Kinect [ 62 ]. Syahidi et al. [ 63 ] introduce a 3D AR-based learning system for pre-school children. For determining the three-dimensional points in the scene, different types of sensors could be used. The most commonly used are the structured lights [ 64 ] or the time of flight [ 65 ]. These technologies work on the principle of depth analysis. In this, the real environment depth information is extracted by the mapping and the tracking [ 66 ]. The Kinect system [ 67 ], developed by Microsoft, is one of the widely used and well-developed approaches in Augmented Reality.

Rambach et al. [ 68 ] present the idea of augmented things: utilizing off-screen rendering of 3D objects, the realization of application architecture, universal 3D object tracking based on the high-quality scans of the objects, and a high degree of parallelization. Viyanon et al. [ 69 ] focus on the development of an AR app known as “AR Furniture" for providing the experience of visualizing the design and decoration to the customers. The customers fit the pieces of furniture in their rooms and were able to make a decision regarding their experience. Turkan et al. [ 70 ] introduce the new models for teaching structural analysis which has considerably improved the learning experience. The model integrates 3D visualization technology with mobile AR. Students can enjoy the different loading conditions by having the choice of switching loads, and feedback can be provided in the real-time by AR interface.

3.1.4. Infrared Tracking

The objects that emitted or reflected the light are some of the earliest vision-based tracking techniques used in AR technologies. Their high brightness compared to their surrounding environment made this tracking very easy [ 71 , 72 ]. The self-light emitting targets were also indifferent to the drastic illumination effects i.e., harsh shadows or poor ambient lighting. In addition, these targets could either be transfixed to the object being tracked and camera at the exterior of the object and was known as “outside-looking-in” [ 73 ]. Or it could be “inside-looking-out”, external in the environment with camera attached to the target [ 74 ]. The inside-looking-out configuration, compared to the sensor of the inside-looking-out system, has greater resolution and higher accuracy of angular orientation. The inside-looking-out configuration is used in the development of several systems [ 20 , 75 , 76 , 77 ], typically with infrared LEDs mounted on the ceiling and a head-mounted display with a camera facing externally.

3.1.5. Model-Based Tracking

The three-dimensional tracking of real-world objects has been the subject of researchers’ interest. It is not as popular as natural feature tracking or planner fiducials, however, a large amount of research has been done on it. In the past, tracking the three-dimensional model of the object was usually created by the hand. In this system, the lines, cylinders, spheres, circles, and other primitives were combined to identify the structure of objects [ 78 ]. Wuest et al. [ 79 ] focus on the development of the scalable and performance pipeline for creating a tracking solution. The structural information of the scene was extracted by using the edge filters. Additionally, for the determination of the pose, edge information and the primitives were matched [ 80 ].

In addition, Gao et al. [ 81 ] explore the tracking method to identify the different vertices of a convex polygon. This is done successfully as most of the markers are square. The coordinates of four vertices are used to determine the transformation matrix of the camera. Results of the experiment suggested that the algorithm was so robust to withstand fast motion and large ranges that make the tracking more accurate, stable, and real time.

The combination of edge-based tracking and natural feature tracking has the following advantages:

  • It provides additional robustness [ 82 ].
  • Enables spatial tracking and thereby is able to be operated in open environments [ 83 ].
  • For variable and complex environments, greater robustness was required. Therefore, they introduced the concept of keyframes [ 84 ] in addition to the primitive model [ 85 ].

Figen et al. [ 86 ] demonstrate of a series of studies that were done at the university level in which participants were asked to make the mass volume of buildings. The first study demanded the solo work of a designer in which they had to work using two tools: MTUIs of the AR apps and analog tools. The second study developed the collaboration of the designers while using analog tools. The study has two goals: change in the behavior of the designer while using AR apps and affordances of different interfaces.

Developing and updating the real environment’s map simultaneously had been the subject of interest in model-based tracking. This has a number of developments. First, simultaneous localization and map building (SLAM) was primarily done for robot navigation in unknown environments [ 87 ]. In augmented reality, [ 88 , 89 ], this technique was used for tracking the unknown environment in a drift-free manner. Second, parallel mapping and tracking [ 88 ] was developed especially for AR technology. In this, the mapping of environmental components and the camera tracks were identified as a separate function. It improved tracking accuracy and also overall performance. However, like SLAM, it did not have the capability to close large loops in the constrained environment and area ( Figure 6 ).

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Hybrid tracking: inertial and SLAM combined and used in the latest mobile-based AR tracking.

Oskiper et al. [ 90 ] propose a simultaneous localization and mapping (SLAM) framework for sensor fusion, indexing, and feature matching in AR apps. It has a parallel mapping engine and error-state extended Kalman filter (EKF) for these purposes. Zhang et al.’s [ 91 ] Jaguar is a mobile tracking AR application with low latency and flexible object tracking. This paper discusses the design, execution, and evaluation of Jaguar. Jaguar enables a markerless tracking feature which is enabled through its client development on top of ARCoreest from Google. ARCore is also helpful for context awareness while estimating and recognizing the physical size and object capabilities, respectively.

3.1.6. Global Positioning System—GPS Tracking

This technology refers to the positioning of outdoor tracking with reference to the earth. The present accuracy of the GPS system is up to 3 m. However, improvements are available with the advancements in satellite technology and a few other developments. Real-time kinematic (RTS) is one example of them. It works by using the carrier of a GPS signal. The major benefit of it is that it has the ability to improve the accuracy level up to the centimeter level. Feiner’s touring machine [ 92 ] was the first AR system that utilized GPS in its tracking system. It used the inclinometer/magnetometer and differential GPS positional tracking. The military, gaming [ 93 , 94 ], and the viewership of historical data [ 95 ] have applied GPS tracking for the AR experiences. As it only has the supporting positional tracking low accuracy, it could only be beneficial in the hybrid tracking systems or in the applications where the pose registration is not important. AR et al. [ 96 ] use the GPS-INS receiver to develop models for object motion having more precision. Ashutosh et al. [ 97 ] explore the hardware challenges of AR technology and also explore the two main components of hardware technology: battery performance and global positioning system (GPS). Table 1 provides a succinct categorization of the prominent tracking technologies in augmented reality. Example studies are referred to while highlighting the advantages and challenges of each type of tracking technology. Moreover, possible areas of application are suggested.

3.1.7. Miscellaneous Tracking

Yang et al. [ 98 ], in order to recognize the different forms of hatch covers having similar shapes, propose tracking and cover recognition methods. The results of the experiment suggest its real-time property and practicability, and tracking accuracy was enough to be implemented in the AR inspection environment. Kang et al. [ 99 ] propose a pupil tracker which consists of several features that make AR more robust: key point alignment, eye-nose detection, and infrared (NIR) led. NIR led turns on and off based on the illumination light. The limitation of this detector is that it cannot be applied in low-light conditions.

Summary of tracking techniques and their related attributes.

Moreover, Bach et al. [ 118 ] introduce an AR canvas for information visualization which is quite different from the traditional AR canvas. Therefore, dimensions and essential aspects for developing the visualization design for AR-canvas while enlisting the several limitations within the process. Zeng et al. [ 119 ] discuss the design and the implementation of FunPianoAR for creating a better AR piano learning experience. However, a number of discrepancies occurred with this system, and the initiation of a hybrid system is a more viable option. Rewkowski et al. [ 120 ] introduce a prototype system of AR to visualize the laparoscopic training task. This system is capable of tracking small objects and requires surgery training by using widely compatible and inexpensive borescopes.

3.1.8. Hybrid Tracking

Hybrid tracking systems were used to improve the following aspects of the tracking systems:

  • Improving the accuracy of the tracking system.
  • Coping with the weaknesses of the respective tracking methods.
  • Adding more degrees of freedom.

Gorovyi et al. [ 108 ] detail the basic principles that make up an AR by proposing a hybrid visual tracking algorithm. The direct tracking techniques are incorporated with the optical flow technique to achieve precise and stable results. The results suggested that they both can be incorporated to make a hybrid system, and ensured its success in devices having limited hardware capabilities. Previously, magnetic tracking [ 109 ] or inertial trackers [ 110 ] were used in the tracking applications while using the vision-based tracking system. Isham et al. [ 111 ] use a game controller and hybrid tracking to identify and resolve the ultrasound image position in a 3D AR environment. This hybrid system was beneficial because of the following reasons:

  • Low drift of vision-based tracking.
  • Low jitter of vision-based tracking.
  • They had a robust sensor with high update rates. These characteristics decreased the invalid pose computation and ensured the responsiveness of the graphical updates [ 121 ].
  • They had more developed inertial and magnetic trackers which were capable of extending the range of tracking and did not require the line of sight. The above-mentioned benefits suggest that the utilization of the hybrid system is more beneficial than just using the inertial trackers.

In addition, Mao et al. [ 122 ] propose a new tracking system with a number of unique features. First, it accurately translates the relative distance into the absolute distance by locating the reference points at the new positions. Secondly, it embraces the separate receiver and sender. Thirdly, resolves the discrepancy in the sampling frequency between the sender and receiver. Finally, the frequency shift due to movement is highly considered in this system. Moreover, the combination of the IMU sensor and Doppler shift with the distributed frequency modulated continuous waveform (FMCW) helps in the continuous tracking of mobile due to multiple time interval developments. The evaluation of the system suggested that it can be applied to the existing hardware and has an accuracy to the millimeter level.

The GPS tracking system alone only provides the positional information and has low accuracy. So, GPS tracking systems are usually combined with vision-based tracking or inertial sensors. The intervention would help gain the full pose estimation of 6DoF [ 123 ]. Moreover, backup tracking systems have been developed as an alternative when the GPS fails [ 98 , 124 ]. The optical tracking systems [ 100 ] or the ultrasonic rangefinders [ 101 ] can be coupled with the inertial trackers for enhancing efficiency. As the differential measurement approach causes the problem of drift, these hybrid systems help resolve them. Furthermore, the use of gravity as a reference to the inertial sensor made them static and bound. The introduction of the hybrid system would make them operate in a simulator, vehicle, or in any other moving platform [ 125 ]. The introduction of accelerators, cameras, gyroscopes [ 126 ], global positioning systems [ 127 ], and wireless networking [ 128 ] in mobile phones such as tablets and smartphones also gives an opportunity for hybrid tracking. Furthermore, these devices have the capability of determining outdoor as well as indoor accurate poses [ 129 ].

3.2. Marker-Based Tracking

Fiducial Tracking: Artificial landmarks for aiding the tracking and registration that are added to the environment are known as fiducial. The complexity of fiducial tracking varies significantly depending upon the technology and the application used. Pieces of paper or small colored LEDs were used typically in the early systems, which had the ability to be detected using color matching and could be added to the environment [ 130 ]. If the position of fiducials is well-known and they are detected enough in the scene then the pose of the camera can be determined. The positioning of one fiducial on the basis of a well-known previous position and the introduction of additional fiducials gives an additional benefit that workplaces could dynamically extend [ 131 ]. A QR code-based fudicial/marker is also proposed by some researchers for marker-/tag-based tracking [ 115 ]. With the progression of work on the concept and complexity of the fiducials, additional features such as multi-rings were introduced for the detection of fiducials at much larger distances [ 116 ]. A minimum of four points of a known position is needed for determining for calculating the pose of the viewer [ 117 ]. In order to make sure that the four points are visible, the use of these simpler fiducials demanded more care and effort for placing them in the environment. Examples of such fiducials are ARToolkit and its successors, whose registration techniques are mostly planar fiducial. In the upcoming section, AR display technologies are discussed to fulfill all the conditions of Azuma’s definition.

3.3. Summary

This section provides comprehensive details on tracking technologies that are broadly classified into markerless and marker-based approaches. Both types have many subtypes whose details, applications, pros, and cons are provided in a detailed fashion. The different categories of tracking technologies are presented in Figure 4 , while the summary of tracking technologies is provided in Figure 7 . Among the different tracking technologies, hybrid tracking technologies are the most adaptive. This study combined SLAM and inertial tracking technologies as part of the framework presented in the paper.

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Steps for combining real and virtual content.

4. Augmented Reality Display Technology

For the combination of a real and the virtual world in such a way that they both look superimposed on each other, as in Azuma’s definition, some technology is necessarily required to display them.

4.1. Combination of Real and the Virtual Images

Methods or procedures required for the merging of the virtual content in the physical world include camera calibration, tracking, registration, and composition as depicted in Figure 7 .

4.2. Camera vs. Optical See Through Calibration

It is a procedure or an optical model in which the eye display geometry or parameters define the user’s view. Or, in other words, it is a technique of complementing the dimensions and parameters of the physical and the virtual camera.

In AR, calibration can be used in two ways, one is camera calibration, and another is optical calibration. The camera calibration technique is used in video see-through (VST) displays. However, optical calibration is used in optical see-through (OST) displays. OST calibration can be further divided into three umbrellas of techniques. Initially, manual calibration techniques were used in OST. Secondly, semi-automatic calibration techniques were used, and thirdly, we have now automatic calibration techniques. Manual calibration requires a human operator to perform the calibration tasks. Semi-automatic calibration, such as simple SPAAM and display relative calibration (DRC), partially collect some parameters automatically, which usually needed to be done manually in earlier times by the user. Thirdly, the automatic OST calibration was proposed by Itoh et al. in 2014 with the model of interaction-free display calibration technique (INDICA) [ 132 ]. In video see through (VST), computer vision techniques such as cameras are used for the registration of real environments. However, in optical see through (OST), VST calibration techniques cannot be used as it is more complex because cameras are replaced by human eyes. Various calibration techniques were developed for OST. The author evaluates the registration accuracy of the automatic OST head-mounted display (HMD) calibration technique called recycled INDICA presented by Itoh and Klinker. In addition, two more calibration techniques called the single-point active alignment method (SPAAM) and degraded SPAAM were also evaluated. Multiple users were asked to perform two separate tasks to check the registration and the calibration accuracy of all three techniques can be thoroughly studied. Results show that the registration method of the recycled INDICA technique is more accurate in the vertical direction and showed the distance of virtual objects accurately. However, in the horizontal direction, the distance of virtual objects seemed closer than intended [ 133 ]. Furthermore, the results show that recycled INDICA is more accurate than any other common technique. In addition, this technique is also more accurate than the SPAAM technique. Although, different calibration techniques are used for OST and VST displays, as discussed in [ 133 ], they do not provide all the depth cues, which leads to interaction problems. Moreover, different HMDs have different tracking systems. Due to this, they are all calibrated with an external independent measuring system. In this regard, Ballestin et al. propose a registration framework for developing AR environments where all the real objects, including users, and virtual objects are registered in a common frame. The author also discusses the performance of both displays during interaction tasks. Different simple and complex tasks such as 3D blind reaching are performed using OST and VST HMDs to test their registration process and interaction of the users with both virtual and real environments. It helps to compare the two technologies. The results show that these technologies have issues, however, they can be used to perform different tasks [ 134 ].

Non-Geometric Calibration Method

Furthermore, these geometric calibrations lead to perceptual errors while converting from 3D to 2D [ 135 ]. To counter this problem, parallax-free video see-through HMDs were proposed; however, they were very difficult to create. In this regard, Cattari et al. in 2019 proposes a non-stereoscopic video see-through HMD for a close-up view. It mitigates perceptual errors by mitigating geometric calibration. Moreover, the authors also identify the problems of non-stereoscopic VST HMD. The aim is to propose a system that provides a view consistent with the real world [ 136 , 137 ]. Moreover, State et al. [ 138 ] focus on a VST HMD system that generates zero eye camera offset. While Bottechia et al. [ 139 ] present an orthoscope monocular VST HMD prototype.

4.3. Tracking Technologies

Some sort of technology is required to track the position and orientation of the object of interest which could either be a physical object or captured by a camera with reference to the coordinate plan (3D or 2D) of a tracking system. Several technologies ranging from computer vision techniques to 6DoF sensors are used for tracking the physical scenes.

4.4. Registration

Registration is defined as a process in which the coordinate frame used for manifesting the virtual content is complemented by the coordinate frame of the real-world scene. This would help in the accurate alignment of the virtual content and the physical scene.

4.5. Composition

Now, the accuracy of two important steps, i.e., the accurate calibration of the virtual camera and the correct registration of the virtual content relative to the physical world, signifies the right correspondence between the physical environment and the virtual scene which is generated on the basis of tracking updates. This process then leads to the composition of the virtual scene’s image and can be done in two ways: Optically (or physically) or digitally. The physical or digital composition depends upon the configuration and dimensions of the system used in the augmented reality system.

4.6. Types of Augmented Reality Displays

The combination of virtual content in the real environment divides the AR displays into four major types, as depicted in Figure 8 . All have the same job to show the merged image of real and virtual content to the user’s eye. The authors have categorized the latest technologies of optical display after the advancements in holographic optical elements HOEs. There are other categories of AR display that arealso used, such as video-based, eye multiplexed, and projection onto a physical surface.

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Types of augmented reality display technologies.

4.7. Optical See-Through AR Display

These kinds of displays use the optical system to merge the real scenes and virtual scene images. Examples of AR displays are head-up display HUD systems of advanced cars and cockpits of airplanes. These systems consist of the following components: beam splitters, which can be of two forms, combined prisms or half mirrors. Most beam splitters reflect the image from the video display. This reflected image is then integrated with a real-world view that can be visualized from the splitter. For half mirrors as a beam splitter, the working way is somewhat different: the real-world view is reflected on the mirror rather than the image of the video display. At the same time, the video display can also be viewed from the mirror. The transport projection system is semi-transparent optical technology used in optical display systems. Their semi-transparent property allows the viewer to witness the view at the back of the screen. Additionally, this system uses diffused light to manifest the exhibited image. Examples of semi-display optical systems are transparent projection film, transparent LCDs, etc. Optical combiners are used for the combination of virtual and real scene images. Optical see-through basically has two sub-categories, one is a free-space combiner and the other is a wave-guide combiner [ 140 ]. Additionally, now the advancement of technology has enabled technicians to make self-transparent displays. This self-transparent feature help in the miniaturization and simplification of the size and structure of the optical see-through displays.

4.7.1. Free-Space Combiners

Papers related to free space combiners are discussed here. Pulli et al. [ 11 ] introduce a second-generation immersive optical see-through AR system known as meta 2. It is based on an optical engine that uses the free-form visor to make a more immersive experience. Another traditional geometric display is ultra-fast high-resolution piezo linear actuators combined with Alvarez’s lens to make a new varifocal optical see-through HMD. It uses a beamsplitter which acts as an optical combiner to merge the light paths of the real and virtual worlds [ 12 ]. Another type of free-space combiner is Maxwellian-type [ 112 , 113 , 114 , 141 ]. In [ 142 ], the author employs the random structure as a spatial light modulator for developing a light-field near-eye display based on random pinholes. The latest work in [ 143 , 144 ] introduces an Ini-based light field display using the multi-focal micro-lens to propose the extended depth of the field. To enhance the eyebox view there is another technique called puppil duplication steering [ 145 , 146 , 147 , 148 , 149 , 150 ]. In this regard, refs. [ 102 , 151 ] present the eyebox-expansion method for the holographic near-eye display and pupil-shifting holographic optical element (PSHOE) for the implementation. Additionally, the design architecture is discussed and the incorporation of the holographic optical element within the holographic display system is discussed. There is another recent technique similar to the Maxwellian view called pin-light systems. It increases the Maxwellian view with larger DoFs [ 103 , 104 ].

4.7.2. Wave-Guide Combiner

The waveguide combiner basically traps light into TIR as opposed to free-space, which lets the light propagate without restriction [ 104 , 105 , 106 ]. The waveguide combiner has two types, one is diffractive waveguides and another is achromatic waveguides [ 107 , 152 , 153 , 154 , 155 ].

4.8. Video-Based AR Displays

These displays execute the digital processes as their working principle [ 156 ]. To rephrase, the merging of the physical world video and the virtual images, in video display systems, is carried out by digital processing. The working of the video-based system depends upon the video camera system by which it fabricates the real-world video into digital. The rationale behind this system is that the composition of the physical world’s video or scenario with the virtual content could be manifested digitally through the operation of a digital image processing technique [ 157 ]. Mostly, whenever the user has to watch the display, they have to look in the direction of the video display, and the camera is usually attached at the back of this display. So, the camera faces the physical world scene. These are known as “video see-through displays" because in them the real world is fabricated through the digitization (i.e., designing the digital illusion) of these video displays. Sometimes the design of the camera is done in such a way that it may show an upside-down image of an object, create the illusion of a virtual mirror, or site the image at a distant place.

4.9. Projection-Based AR Display

Real models [ 158 ] and walls [ 159 ] could be example of projection-based AR displays. All the other kinds of displays use the display image plan for the combination of the real and the virtual image. However, this display directly overlays the virtual scene image over the physical object. They work in the following manner:

  • First, they track the user’s viewpoint.
  • Secondly, they track the physical object.
  • Then, they impart the interactive augmentation [ 160 ].

Mostly, these displays have a projector attached to the wall or a ceiling. This intervention has an advantage as well as a disadvantage. The advantage is that this does not demand the user to wear something. The disadvantage is that it is static and restricts the display to only one location of projection. For resolving this problem and making the projectors mobile, a small-sized projector has been made that could be easily carried from one place to another [ 161 ]. More recently, with the advancement of technology, miniaturized projectors have also been developed. These could be held in the hand [ 162 ] or worn on the chest [ 163 ] or head [ 164 ].

4.10. Eye-Multiplexed Augmented Reality Display

In eye-multiplexed AR displays, the users are allowed to combine the views of the virtual and real scenes mentally in their minds [ 72 ]. Rephrased, these displays do not combine the image digitally; therefore, it requires less computational power [ 72 ]. The process is as follows. First, the virtual image gets registered to the physical environment. Second, the user will get to see the same rendered image as the physical scene because the virtual image is registered to the physical environment. The user has to mentally configure the images in their mind to combine the virtual and real scene images because the display does not composite the rendered and the physical image. For two reasons, the display should be kept near the viewer’s eye: first, the display could appear as an inset into the real world, and second, the user would have to put less effort into mentally compositing the image.

The division of the displays on the basis of the position of the display between the real and virtual scenes is referred to as the “eye to world spectrum”.

4.11. Head-Attached Display

Head-attached displays are in the form of glasses, helmets, or goggles. They vary in size from smaller to bigger. However, with the advancement of technology, they are becoming lighter to wear. They work by displaying the virtual image right in front of the user’s eye. As a result, no other physical object can come between the virtual scene and the viewer’s eye. Therefore, the third physical object cannot occlude them. In this regard, Koulieris et al. [ 165 ] summarized the work on immersive near-eye tracking technologies and displays. Results suggest various loopholes within the work on display technologies: user and environmental tracking and emergence–accommodation conflict. Moreover, it suggests that advancement in the optics technology and focus adjustable lens will improve future headset innovations and creation of a much more comfortable HMD experience. In addition to it, Minoufekr et al. [ 166 ] illustrate and examine the verification of CNC machining using Microsoft HoloLens. In addition, they also explore the performance of AR with machine simulation. Remote computers can easily pick up the machine models and load them onto the HoloLens as holograms. A simulation framework is employed that makes the machining process observed prior to the original process. Further, Franz et al. [ 88 ] introduce two sharing techniques i.e., over-the-shoulder AR and semantic linking for investigating the scenarios in which not every user is wearing HWD. Semantic linking portrays the virtual content’s contextual information on some large display. The result of the experiment suggested that semantic linking and over-the-shoulder suggested communication between participants as compared to the baseline condition. Condino et al. [ 167 ] aim to explore two main aspects. First, to explore complex craniotomies to gauge the reliability of the AR-headsets [ 168 ]. Secondly, for non-invasive, fast, and completely automatic planning-to-patient registration, this paper determines the efficacy of patient-specific template-based methodology for this purpose.

4.12. Head-Mounted Displays

The most commonly used displays in AR research are head-mounted displays (HMDs). They are also known as face-mounted displays or near-eye displays. The user puts them on, and the display is represented right in front of their eyes. They are most commonly in the form of goggles. While using HMDs, optical and video see-through configurations are most commonly used. However, recently, head-mounted projectors are also explored to make them small enough to wear. Examples of smart glasses, Recon Jet, Google glass, etc., are still under investigation for their usage in head-mounted displays. Barz et al. [ 169 ] introduce a real-time AR system that augments the information obtained from the recently attended objects. This system is implemented by using head-mounted displays from the state-of-the-art Microsoft HoloLens [ 170 ]. This technology can be very helpful in the fields of education, medicine, and healthcare. Fedosov et al. [ 171 ] introduce a skill system, and an outdoor field study was conducted on the 12 snowboards and skiers. First, it develops a system that has a new technique to review and share personal content. Reuter et al. [ 172 ] introduce the coordinative concept, namely RescueGlass, for German Red Cross rescue dog units. This is made up of a corresponding smartphone app and a hands-free HMD (head-mounted display) [ 173 ]. This is evaluated to determine the field of emergency response and management. The initial design is presented for collaborative professional mobile tasks and is provided using smart glasses. However, the evaluation suggested a number of technical limitations in the research that could be covered in future investigations. Tobias et al. [ 174 ] explore the aspects such as ambiguity, depth cues, performed tasks, user interface, and perception for 2D and 3D visualization with the help of examples. Secondly, they categorize the head-mounted displays, introduce new concepts for collaboration tasks, and explain the concepts of big data visualization. The results of the study suggested that the use of collaboration and workspace decisions could be improved with the introduction of the AR workspace prototype. In addition, these displays have lenses that come between the virtual view and the user’s eye just like microscopes and telescopes. So, the experiments are under investigation to develop a more direct way of viewing images such as the virtual retinal display developed in 1995 [ 175 ]. Andersson et al. [ 176 ] show that training, maintenance, process monitoring, and programming can be improved by integrating AR with human—robot interaction scenarios.

4.13. Body-Attached and Handheld Displays

Previously, the experimentation with handheld display devices was done by tethering the small LSDs to the computers [ 177 , 178 ]. However, advancements in technology have improved handheld devices in many ways. Most importantly, they have become so powerful to operate AR visuals. Many of them are now used in AR displays such as personal digital assistants [ 179 ], cell phones [ 180 ], tablet computers [ 181 ], and ultra-mobile PCs [ 182 ].

4.13.1. Smartphones and Computer tablets

In today’s world, computer tablets and smartphones are powerful enough to run AR applications, because of the following properties: various sensors, cameras, and powerful graphic processors. For instance, Google Project Tango and ARCore have the most depth imaging sensors to carry out the AR experiences. Chan et al. [ 183 ] discuss the challenges faced while applying and investigating methodologies to enhance direct touch interaction on intangible displays. Jang et al. [ 184 ] aim to explore e-leisure due to enhancement in the use of mobile AR in outdoor environments. This paper uses three methods, namely markerless, marker-based, and sensorless to investigate the tracking of the human body. Results suggested that markerless tracking cannot be used to support the e-leisure on mobile AR. With the advancement of electronic computers, OLED panels and transparent LCDs have been developed. It is also said that in the future, building handheld optical see-through devices would be available. Moreover, Fang et al. [ 185 ] focus on two main aspects of mobile AR. First, a combination of the inertial sensor, 6DoF motion tracking based on sensor-fusion, and monocular camera for the realization of mobile AR in real-time. Secondly, to balance the latency and jitter phenomenon, an adaptive filter design is introduced. Furthermore, Irshad et al. [ 186 ] introduce an evaluation method to assess mobile AR apps. Additionally, Loizeau et al. [ 187 ] explore a way of implementing AR for maintenance workers in industrial settings.

4.13.2. Micro Projectors

Micro projectors are an example of a mobile phone-based AR display. Researchers are trying to investigate these devices that could be worn on the chest [ 188 ], shoulder [ 189 ], or wrist [ 190 ]. However, mostly they are handheld and look almost like handheld flashlights [ 191 ].

4.13.3. Spatial Displays

Spatial displays are used to exhibit a larger display. Henceforth, these are used in the location where more users could get benefit from them i.e., public displays. Moreover, these displays are static, i.e., they are fixed at certain positions and can not be mobilized.

The common examples of spatial displays include those that create optical see-through displays through the use of optical beamers: half mirror workbench [ 192 , 193 , 194 , 195 ] and virtual showcases. Half mirrors are commonly used for the merging of haptic interfaces. They also enable closer virtual interaction. Virtual showcases may exhibit the virtual images on some solid or physical objects mentioned in [ 196 , 197 , 198 , 199 , 200 ]. Moreover, these could be combined with the other type of technologies to excavate further experiences. The use of volumetric 3D displays [ 201 ], autostereoscopic displays [ 202 ], and other three-dimensional displays could be researched to investigate further interesting findings.

4.13.4. Sensory Displays

In addition to visual displays, there are some sensors developed that work with other types of sensory information such as haptic or audio sensors. Audio augmentation is easier than video augmentation because the real world and the virtual sounds get naturally mixed up with each other. However, the most challenging part is to make the user think that the virtual sound is spatial. Multi-channel speaker systems and the use of stereo headphones with the head-related transfer function (HRTF) are being researched to cope with this challenge [ 203 ]. Digital sound projectors use the reverberation and the interference of sound by using a series of speakers [ 204 ]. Mic-throughand hear-through systems, developed by Lindeman [ 205 , 206 , 206 ], work effectively and are analogous to video and optical see-through displays. The feasibility test for this system was done by using a bone conduction headset. Other sensory experiences are also being researched. For example, the augmentation of the gustatory and olfactory senses. Olfactory and visual augmentation of a cookie-eating scene was developed by Narumi [ 207 ]. Table 2 gives the primary types of augmented reality display technologies and discusses their advantages and disadvantages.

A Summary of Augmented Reality Display Technologies.

4.14. Summary

This section presented a comprehensive survey of AR display technologies. These displays not only focused on combing the virtual and real-world scenes of visual experience but also other ways of combining the sensory, olfactory, and gustatory senses are also under examination by researchers. Previously, head-mounted displays were most commonly in practice; however, now handheld devices and tablets or mobile-based experiences are widely used. These things may also change in the future depending on future research and low cost. The role of display technologies was elaborated first, thereafter, the process of combining the real and augmented contents and visualizing these to users was elaborated. The section elaborated thoroughly on where the optical see-through and video-based see-through are utilized along with details of devices. Video see-through (VST) is used in head-mounted displays and computer vision techniques such as cameras are used for registration of real environment, while in optical see-through (OST), VST calibration techniques cannot be used due to complexity, and cameras are replaced by human eyes. The optical see-through is a trendy approach as of now. The different calibration approaches are presented and analyzed and it is identified after analysis, the results show that recycled INDICA is more accurate than other common techniques presented in the paper. This section also presents video-based AR displays. Figure 8 present a classified representation of different display technologies pertaining to video-based, head-mounted, and sensory-based approaches. The functions and applications of various display technologies are provided in Table 2 Each of the display technologies presented has its applicability in various realms whose details are summarized in the same Table 2 .

5. Walking and Distance Estimation in AR

The effectiveness of AR technologies depends on the perception of distance of users from both real and virtual objects [ 214 , 215 ]. Mikko et al. performed some experiments to judge depth using stereoscopic depth perception [ 216 ]. The perception can be changed if the objects are on the ground or off the ground. In this regard, Carlos et al. also proposed a comparison between the perception of distance of these objects on the ground and off the ground. The experiment was done where the participant perceived the distance from cubes on the ground and off the ground as well. The results showed that there is a difference between both perceptions. However, it was also shown that this perception depends on whether the vision is monocular or binocular [ 217 ]. Plenty of research has been done in outdoor navigation and indoor navigation areas with AR [ 214 ]. In this regard, Umair et al. present an indoor navigation system in which Google glass is used as a wearable head-mounted display. A pre-scanned 3D map is used to track an indoor environment. This navigation system is tested on both HMD and handheld devices such as smartphones. The results show that the HMD was more accurate than the handheld devices. Moreover, it is stated that the system needs more improvement [ 218 ].

6. AR Development Tool

In addition to the tracking and display devices, there are some other software tools required for creating an AR experience. As these are hardware devices, they require some software to create an AR experience. This section explores the tools and the software libraries. It will cover both the aspects of the commercially available tools and some that are research related. Different software applications require a separate AR development tool. A complete set of low-level software libraries, plug-ins, platforms, and standalones are presented in Figure 9 so they can be summarized for the reader.

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Stack of development libraries, plug-ins, platforms, and standalone authoring tools for augmented reality development.

In some tools, computer vision-based tracking (see Section 3.1.2 ) is preferred for creating an indoor experience, while others utilized sensors for creating an outdoor experience. The use of each tool would depend upon the type of platform (web or mobile) for which it is designed. Further in the document, the available AR tools are discussed, which consist of both novel tools and those that are widely known. Broadly, the following tools will be discussed:

  • Low-level software development tools: needs high technological and programming skills.
  • Rapid prototyping: provides a quick experience.
  • Plug-ins that run on the existing applications.
  • Standalone tools that are specifically designed for non-programmers.
  • Next generation of AR developing tools.

6.1. Low-Level Software Libraries and Frameworks

Low-level software and frameworks make the functions of display and core tracking accessible for creating an AR experience. One of the most commonly used AR software libraries, as discussed in the previous section, is ARToolKit. ARToolKit is developed by Billing Hurst and Kato that has two versions [ 219 ]. It works on the principle of a fiducial marker-based registration system [ 220 ]. There are certain advances in the ARToolKit discussed related to the tracking in [ 213 , 221 , 222 , 223 , 224 ]. The first one is an open-source version that provides the marker-based tracking experience, while the second one provides natural tracking features and is a commercial version. It can be operated on Linux, Windows, and Mac OS desktops as it is written in the C language. It does not require complex graphics or built-in support for accomplishing its major function of providing a tracking experience, and it can operate simply by using low-level OpenGL-based rendering. ARToolKit requires some additional libraries such as osgART and OpenScene graph library so it can provide a complete AR experience to AR applications. OpenScene graph library is written in C language and operates as an open-source graph library. For graphic rendering, the OpenScene graph uses OpenGL. Similarly, the osgART library links the OpenScene graph and ARToolKit. It has advanced rendering techniques that help in developing the interacting AR application. OsgART library has a modular structure and can work with any other tracking library such as PTAM and BazAR, if ARtoolkit is not appropriate. BazAR is a workable tracking and geometric calibration library. Similarly, PTAM is a SLAM-based tracking library. It has a research-based and commercial license. All these libraries are available and workable to create a workable AR application. Goblin XNA [ 208 ] is another platform that has the components of interactions based on physics, video capture, a head-mounted AR display on which output is displayed, and a three-dimensional user interface. With Goblin XNA, existing XNA games could be easily modified [ 209 ]. Goblin XNA is available as a research and educational platform. Studierstube [ 210 ] is another AR system through which a complete AR application can be easily developed. It has tracking hardware, input devices, different types of displays, AR HMD, and desktops. Studierstube was specially developed to subsidize the collaborative applications [ 211 , 212 ]. Studierstube is a research-oriented library and is not available as commercial and workable easy-to-use software. Another commercially available SDK is Metaio SDK [ 225 ]. It consists of a variety of AR tracking technologies including image tracking, marker tracking, face tracking, external infrared tracking, and a three-dimensional object tracking. However, in May 2015, it was acquired by Apple and Metaio products and subscriptions are no longer available for purchase. Some of these libraries such as Studierstube and ARToolKit were initially not developed for PDAs. However, they have been re-developed for PDAs [ 226 ]. It added a few libraries in assistance such as open tracker, pocketknife for hardware abstraction, KLIMT as mobile rendering, and the formal libraries of communication (ACE) and screen graphs. All these libraries helped to develop a complete mobile-based AR collaborative experience [ 227 , 228 ]. Similarly, ARToolKit also incorporated the OpenScene graph library to provide a mobile-based AR experience. It worked with Android and iOS with a native development kit including some Java wrapping classes. Vuforia’s Qualcomm low-level library also provided an AR experience for mobile devices. ARToolKit and Vuforia both can be installed as a plug-in in Unity which provides an easy-to-use application development for various platforms. There are a number of sensors and low-level vision and location-based libraries such as Metaio SDK and Droid which were developed for outdoor AR experience. In addition to these low-level libraries, the Hit Lab NZ Outdoor AR library provided high-level abstraction for outdoor AR experience [ 229 ]. Furthermore, there is a famous mobile-based location AR tool that is called Hoppala-Augmentation. The geotags given by this tool can be browsed by any of the AR browsers including Layar, Junaio, and Wikitude [ 230 ].

ARTag is designed to resolve the limitations of ARToolkit. This system was developed to resolve a number of issues:

  • Resolving inaccurate pattern matching by preventing the false positive matches.
  • Enhancing the functioning in the presence of the imbalanced lightening conditions.
  • Making the occlusion more invariant.

However, ARTag is no longer actively under development and supported by the NRC Lab. A commercial license is not available.

6.3. Wikitude Studio

This is also a web-based authoring tool for creating mobile-based AR applications. It allows the utilization of computer vision-based technology for the registration of the real world. Several types of media such as animation and 3D models can be used for creating an AR scene. One of the important features of Wikitude is that the developed mobile AR content can be uploaded not only on the Wikitude AR browser app but also on a custom mobile app [ 231 ]. Wikitude’s commercial plug-in is also available in Unity to enhance the AR experience for developers.

6.4. Standalone AR Tools

Standalone AR tools are mainly designed to enable non-programmer users to create an AR experience. A person the basic computer knowledge can build and use them. The reason lies in the fact that most AR authoring tools are developed on a graphical user interface. It is known as a standalone because it does not require any additional software for its operation. The most common and major functions of standalone are animation, adding interactive behaviors, and construction. The earliest examples of the standalone tools are AMIRE [ 232 ] and CATOMIR [ 233 ]. However, AMIRE and CATOMIR have no support available and are not maintained by the development team.

This standalone AR authoring tool has the advantage of quickly adding to the development of the AR experience. BuildAR has important characteristics. This allows the user to add video, 3D models, sound, text, and images. It has both arbitrary images and the square marker for which it provides computer vision-based tracking. They use the format of proprietary file format for saving the content developed by the user. BuildAR viewer software can be downloaded for free and it helps in viewing the file. However, BuildAR has no support available and the exe file is not available on their website.

Limitation: It does not support adding new interactive features. However, Choi et al. [ 234 ] have provided a solution to this constraint. They have added the desktop authoring tool that helps in adding new interactive experiences.

6.5. Rapid Prototyping/Development Tools

In order to cope with the limitation of low-level libraries, another more fast and more rapid AR application development tool is required. The major idea behind the development of rapid prototyping was that it rapidly shows the user the prototype before executing the hard exercise of developing the application. In the following paragraphs, a number of different tools are explained for developing rapid prototyping. For the creation of multimedia content, Adobe Flashis one of the most famous tools. It was developed on desktop and web platforms. Moreover, the web desktop and mobile experiences can be prototyped by it. Flash developers can use the FLARManager, FLARToolKit, or any other plug-ins for the development of AR experience. Porting the version of ARToolKit over the flash on the web creates the AR experience. Its process is so fast that just by writing a few lines, the developer can:

  • Activate their camera.
  • The AR markers could be viewed in a camera.
  • The virtual content could be overlaid and loaded on the tracked image.

FLARToolkit is the best platform for creating AR prototyping because it has made it very easy for being operated by anyone. Anyone who has a camera and flash-enabled web browser can easily develop the AR experience. Alternatives to Flash: According to the website of Adobe, it no longer supports Flash Player after 31 December 2020 and blocked Flash content from running in Flash Player beginning 12 January 2021. Adobe strongly recommends all users immediately uninstall Flash Player to help protect their systems. However, some AR plug-ins could be used as an alternative to Flash-based AR applications. For instance, Microsoft Silverlight has the SLARToolKit. HTML5 is also recently used by researchers for creating web-based AR experiences. The major benefit of using HTML5 is that the interference of the third-party plug-in is not required. For instance, the AR natural feature tracking is implemented on WebGL, HTML5, and JavaScript. This was developed by Oberhofer and was viewable on mobile web browsers and desktops. Additionally, the normal HTML, with few web component technologies, has been used by Ahn [ 235 ] to develop a complete mobile AR framework.

6.6. Plug-ins to Existing Developer Tools

For the creation of AR experiences, the software libraries require tremendous programming techniques. So, plug-ins could be used as an alternative. Plug-ins are devices that could be plugged into the existing software packages. The AR functionality is added to the software packages that to the existing two-dimensional or three-dimensional content authoring tools. If the user already knows the procedure of using authoring tools that are supported by plug-ins, then AR plug-ins for the non-AR authoring tools are useful. These tools are aimed at:

  • AR tracking and visualization functions for the existing authoring tools.
  • It depends on the content authoring function supplied by the main authoring tool.

There are certain tools available as plug-ins and standalone through which AR applications can be built comparatively simply. These are commercial and some of them are freely available. As discussed earlier, Vuforia can be installed as a plug-in in Unity [ 236 ] and also has a free version. However, with complete support of tools certain amount needs to be paid. Similarly, ARtoolkit is available standalone and a plug-in for Unity is available. It is freely available for various platforms such as Android, iOS, Linux, and Windows. Moreover, ARCore and ARKit are also available for Android and iOS, respectively, and can work with Unity and Unreal authoring tools as a plug-in. ARCore is available and free for developers. MAXST and Wikitude also can work in integration with Unity, though they have a licensing price for the commercial version of the software. MAXST had a free version as well. All these tools, the abovementioned libraries, and standalone tools are depicted in Figure 9 . Cinema 4D, Maya, Trimble SketchUp 3D modeling software, 3Ds Max, and many others were created by a number of plug-ins that acted as authoring tools for three-dimensional content. While 3D animation and modeling tools are not capable of providing interactive features, it is very productive in creating three-dimensional scenes. SketchUp can utilize the AR plug-in by creating a model for the content creators. This model is then viewable in the AR scene provided by a free AR media player. The interactive three-dimensional graphic authoring tools are also available for the creation of highly interactive AR experiences, for instance, Wizard [ 237 ], Quest3D [ 238 ], and Unity [ 236 ]. All of these authoring tools have their own specific field of operation; however, Unity can be utilized to create a variety of experiences. The following are examples that justify the use of Unity over different solutions available:

  • The AR plug-in of the Vuforia tracking library can be used with Unity 3D. This integration will help Vuforia in the creation of AR applications for the android or iOS platform.
  • Similarly, the ARToolkit for Unity also provides marker-based experiences. It provides both image and marker-based AR visualization and tracking.

In such integrations, the highly interactive experiences are created by the normal Unity3D scripting interface and visual programming. Limitations of AR plug-ins: The following are the limitations accrued with the AR plug-in:

  • The need for proprietary software could arise for the content produced by the authoring tool. The design provided by the authoring tools could restrict the user’s interactive and interface designs.
  • Moreover, the authoring tools can also restrict the configurations of hardware or software within a certain limit.

Moreover, Nebeling et al. [ 239 ] reviewed the issues with the authoring tools of AR/VR. The survey of the tools has identified three key issues. To make up for those limitations, new tools are introduced for supporting the gesture-based interaction and rapid prototyping of the AR/VR content. Moreover, this is done without having technical knowledge of programming, gesture recognition, and 3D modeling. Mladenov et al. [ 240 ] review the existing SDKs and aim to find the most efficient SDK for the AR applications used in industrial environments. The paper reveals that currently available SDKs are very helpful for users to create AR applications with the parameters of their choice in industrial settings.

6.7. Summary

This section presents a detailed survey of different software and tools required for creating an AR experience. The section outlines hardware devices used in AR technology and various software to create an AR experience. It further elaborates on the software libraries required and covers bother the aspects of the commercially available tools. Table 3 provides a stack of software libraries, plug-ins, supported platforms, and standalone authoring tools. The figure also presents details of whether the mentioned tools are active or inactive. As an example, BazAR is used in tracking and geometric calibration. It is an open-source library for Linux or windows available under research-based GPL and can be used for research to detect an object via camera, calibrate it, and initiate tracking to put a basic virtual image on it; however, this library is not active at the present. Commercially used AR tools such as plug-ins have the limitations of only working efficiently in the 2D GUI and become problematic when used for 3D content. The advancement of technology may bring about a change in the authoring tools by making them capable of being operated for 3D and developing more active AR interfaces.

A summary of development and authoring tools for augmented reality application development.

7. Collaborative Research on Augmented Reality

In general, collaboration in augmented reality is the interaction of multiple users with virtual objects in the real environment. This interaction is regardless of the users’ location, i.e., they can participate remotely or have the same location. In this regard, we have two types of collaborative AR: co-located collaborative AR and remote collaborative AR. We mention it further in Figure 10 .

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Collaborative augmented reality research domains.

7.1. Co-Located Collaborative AR

In this type of collaborative AR, the users interact with the virtual content rendered in the real environment while sharing the same place. The participant are not remote in such case. In this regard, Wells et al. [ 241 ] aim to determine the impact on the co-located group activities by varying the complexity of AR models using mobile AR. The paper also discusses different styles of collaborative AR such as:

  • hlActive Discussion: A face-to-face discussion including all participants.
  • Single Shared view: The participants focus on a single device.
  • Disjoint and Shared View: Two to three participants focus on a single device.
  • Disjoint and Distributed View: One to two people focus on their devices while the others are discussing.
  • Distributed View: Participants focus on their devices with no discussion.
  • Distributive View with Discussion: Participants focus on their devices while discussing in the group.

In this paper, the author did not contribute to the technology of co-located collaborative AR, but rather performed analysis on the effectiveness of different collaborative AR.

Grandi et al. [ 242 ] target the development of design approaches for synchronous collaboration to resolve complex manipulation tasks. For this, purpose fundamental concepts of design interface, human collaboration, and manipulation are discussed. This research the spiral model of research methodology which involves the development, planning, analysis, and evaluation. In addition, Dong et al. [ 243 ] introduce “ARVita”, a system where multiple users can interact with virtual simulations of engineering processes by wearing a head-mounted display. This system uses a co-located AR technique where the users are sitting around a table.

7.1.1. Applications of Co-located Collaborative AR

Kim et al. [ 244 ] propose a PDIE model to make a STEAM educational class while incorporating AR technology into the system. Furthermore, the “Aurasma” application is used to promote AR in education. In addition, Kanzanidis et al. [ 245 ] focus on teaching mobile programming using synchronous co-located collaborative AR mobile applications in which students are distributed in groups. The result showed that the students were satisfied with this learning methodology. Moreover, Chang et al. [ 246 ] explore the use of a mobile AR (MAR) application to teach interior design activities to students. The results identified that the students who were exposed to MAR showed more effectiveness in learning than those who were taught traditionally. Lastly, Sarkar et al. [ 247 ] discuss three aspects of synchronous co-located collaboration-based problem-solving: first, students’ perspectives on AR learning activities, either in dyads or individually are determined; second, the approach adopted by students while problem-solving is determined; third, the students’ motivation for using ScholAR is determined. Statistical results suggested that 90.4% students preferred the collaborative AR experience, i.e., in dyads. Meanwhile, motivation level and usability scores were higher for individual experiences. Grandi et al. [ 248 ] introduce the design for the collaborative manipulation of AR objects using mobile AR. This approach has two main features. It provides a shared medium for collaboration and manipulation of 3D objects as well as provides precise control of DoF transformations. Moreover, strategies are presented to make this system more efficient for users in pairs. Akccayir et al. [ 249 ] explore the impact of AR on the laboratory work of university students and their attitudes toward laboratories. This study used the quasi-experimental design with 76 participants—first year students aged 18–20 years. Both qualitative and quantitative methods were used for the analyses of data. A five-week implementation of the experiment proved that the use of AR in the laboratory significantly improved the laboratory skills of the students. However, some teachers and students also discussed some of the negative impacts of other aspects of AR. Rekimoto et al. [ 250 ] propose a collaborative AR system called TransVision. In this system, two or more users use a see-through display to look at the virtual objects rendered in a real environment using synchronous co-located collaborative AR. Oda et al. [ 251 ] propose a technique for avoiding interference for hand-held synchronous co-located collaborative AR. This study is based on first-person two-player shooting AR games. Benko et al. [ 87 ] present a collaborative augmented reality and mixed reality system called “VITA” or “Visual Interaction Tool For Archaeology”. They have an off-site visualization system that allows multiple users to interact with a virtual archaeological object. Franz et al. [ 88 ] present a system of collaborative AR for museums in which multiple users can interact in a shared environment. Huynh et al. [ 252 ] introduce art of defense (AoD), a co-located augmented reality board game that combines handheld devices with physical game pieces to create a unique experience of a merged physical and virtual game. Nilsson et al. [ 253 ] focus on a multi-user collaborative AR application as a tool for supporting collaboration between different organizations such as rescue services, police, and military organizations in a critical situation.

7.1.2. Asynchronous Co-Located Collaborative AR

Tseng et al. [ 254 ] present an asynchronous annotation system for collaborative augmented reality. This system can attribute virtual annotations with the real world due to a number of distinguishing capabilities, i.e., playing back, placing, and organizing. Extra context information is preserved by the recording of the perspective of the annotator. Furthermore, Kashara et al. [ 255 ] introduce “Second Surface”, an asynchronous co-located collaborative AR system. It allows the users to render images, text, or drawings in a real environment. These objects are stored in the data server and can be accessed later on.

7.2. Remote Collaborative AR

In this type of collaborative AR, all the users have different environments. They can interact with virtual objects remotely from any location. A number of studies have been done in this regard. Billinghurst et al. [ 256 ] introduce a wearable collaborative augmented reality system called “WearCom” to communicate with multiple remote people. Stafford et al. [ 257 ] present God-like interaction techniques for collaboration between outdoor AR and indoor tabletop users. This paper also describes a series of applications for collaboration. Gauglitz et al. [ 258 ] focus on a touchscreen interface for creating annotations in a collaborative AR environment. Moreover, this interface is also capable of virtually navigating a scene reconstructed live in 3D. Boonbrahm et al. [ 259 ] aim to develop a design model for remote collaboration. The research introduces the multiple marker technique to develop a very stable system that allows users from different locations to collaborate which also improves the accuracy. Li et al. [ 260 ] suggest the viewing of a collaborative exhibit has been considerably improved by introducing the distance-driven user interface (DUI). Poretski et al. [ 261 ] describe the behavioral challenges faced in interaction with virtual objects during remote collaborative AR. An experiment was performed to study users’ interaction with shared virtual objects in AR. Clergeaud et al. [ 262 ] tackle the limitations of collaboration in aerospace industrial designs. In addition, the authors propose prototype designs to address these limitations. Oda et al. [ 263 ] present the GARDEN (gesturing in an augmented reality depth-mapped environment) technique for 3D referencing in a collaborative augmented reality environment. The result shows that this technique is more accurate than the other comparisons. Muller et al. [ 85 ] investigate the influence of shared virtual landmarks (SVLs) on communication behavior and user experience. The results show that enhancement in user experience when SVLs were provided. Mahmood et al. [ 264 ] present a remote collaborative system for co-presence and sharing information using mixed reality. The results show improvements in user collaborative analysis experience.

7.2.1. Applications of Remote Collaborative AR

Munoz et al. [ 265 ] present a system called GLUEPS-AR to help teachers in learning situations by integrating AR and web technologies i.e., Web 2.0 tools and virtual learning environments (VLEs) [ 266 ]. Bin et al. [ 267 ] propose a system to enhance the learning experience of the students using collaborative mobile augmented reality learning application (CoMARLA). The application was used to teach ICT to students. The results showed improvement in the learning of the students using CoMARLA. Dunleavy et al. [ 268 ] explore the benefits and drawbacks of collaborative augmented reality simulations in learning. Moreover, a collaborative AR system was proposed for computers independent of location, i.e., indoor or outdoor. Maimone et al. [ 269 ] introduce a telepresence system with real-time 3D capture for remote users to improve communication using depth cameras. Moreover, it also discusses the limitations of previous telepresence systems. Gauglitz et al. [ 270 ] present an annotation-based remote collaboration AR system for mobiles. In this system, the remote user can explore the scene regardless of the local user’s camera position. Moreover, they can also communicate through annotations visible on the screen. Guo et al. [ 271 ] introduce an app, known as Block, that enables the users to collaborate irrespective of their geographic position, i.e., they can be either co-located or remote. Moreover, they can collaborate either asynchronously or synchronously. This app allows users to create structures that persist in the real environment. The result of the study suggested that people preferred synchronous and collocated collaboration, particularly one that was not restricted by time and space. Zhang et al. [ 272 ] propose a collaborative augmented reality for socialization app (CARS). This app improves the user’s perception of the quality of the experience. CARS benefits the user, application, and system on various levels. It reduces the use of computer resources, end-to-end latency, and networking. Results of the experiment suggest that CARS acts more efficiently for users of cloud-based AR applications. Moreover, on mobile phones, it reduces the latency level by up to 40%. Grandi et al. [ 242 ] propose an edge-assisted system, known as CollabAR, which combines both collaboration image recognition and distortion tolerance. Collaboration image recognition enhances recognition accuracy by exploiting the “spatial-temporal" correlation. The result of the experiment suggested that this system has significantly decreased the end-to-end system latency up to 17.8 ms for a smartphone. Additionally, recognition accuracy for images with stronger distortions was found to be 96%.

7.2.2. Synchronous Remote Collaborative AR

Lien et al. [ 273 ] present a system called “Pixel-Point Volume Segmentation” in collaborative AR. This system is used for object references. Moreover, one user can locate the objects with the help of circles drawn on the screen by other users in a collaborative environment. Huang et al. [ 274 ] focus on sharing hand gestures and sketches between a local user and a remote user by using collaborative AR. The system is named “HandsinTouch”. Ou et al. [ 275 ] present the DOVE (drawing over video environment) system, which integrates live-video and gestures in collaborative AR. This system is designed to perform remote physical tasks in a collaborative environment. Datcu et al. [ 276 ] present the creation and evaluation of the handheld AR system. This is done particularly to investigate the remote forensic and co-located and to support team-situational awareness. Three experienced investigators evaluated this system in two steps. First, it was investigated with one remote and one local investigator. Secondly, with one remote and two local investigators. Results of the study suggest the use of this technology resolves the limitation of HMDs. Tait et al. [ 277 ] propose the AR-based remote collaboration that supports view independence. The main aim of the system was to enable the remote user to help the local user with object placement. The remote user uses a 3D reconstruction of the environment to independently find the local user’s scene. Moreover, a remote user can also place the virtual cues in the scene visible to the local user. The major advantage of this system is that it allows the remote user to have an independent scene in the shared task space. Fang et al. [ 278 ] focus on enhancing the 3D feel of immersive interaction by reducing communication barriers. WebRTC, a real-time video communication framework, is developed to enable the operator site’s first-hand view of the remote user. Node.js and WebSocket, virtual canvas-based whiteboards, are developed which are usable in different aspects of life. Mora et al. [ 279 ] explain the CroMAR system. The authors aim to help the users in crowd management who are deployed in a planned outdoor event. CroMAR allows the users to share viewpoints via email, and geo-localized tags allow the users to visualize the outdoor environment and rate these tags. Adcock et al. [ 280 ] present three remote spacial augmented reality systems “Composite Wedge”, “Vector Box”, and “Eyelight” for off-surface 3D viewpoints visualization. In this system, the physical world environment of a remote user can be seen by the local user. Lincoln et al. [ 281 ] focus on a system of robotic avatars of humans in a synchronous remote collaborative environment. It uses cameras and projectors to render a humanoid animatronic model which can be seen by multiple users. This system is called “Animatronic Shader Lamps Avatars”. Komiyama et al. [ 282 ] present a synchronous remote collaborative AR system. It can transition between first person and third person view during collaboration. Moreover, the local user can observe the environment of the remote user. Lehment et al. [ 283 ] present an automatically aligned videoconferencing AR system. In this system, the remote user is rendered and aligned on the display of the local user. This alignment is done automatically regarding the local user’s real environment without modifying it. Oda et al. [ 284 ] present a remote collaborative system for guidance in a collaborative environment. In this system, the remote expert can guide a local user with the help of both AR and VR. The remote expert can create virtual replicas of real objects to guide a local user. Piumsomboon et al. [ 285 ] introduce an adaptive avatar system in mixed reality (MR) called “Mini Me” between a remote user using VR and a local user using AR technology. The results show that it improves the overall experience of MR and social presence. Piumsomboon et al. [ 286 ] present “CoVAR”, a collaboration consisting of both AR and VR technologies. A local user can share their environment with a remote VR user. It supports gestures, head, and eye gaze to improve the collaboration experience. Teo et al. [ 287 ] present a system that captures a 360 panorama video of one user and shares it with the other remote user in a mixed reality collaboration. In this system, the users communicate through hand gestures and visual annotation. Thanyadit et al. [ 288 ] introduce a system where the instructor can observe students in a virtual environment. The system is called “ObserVAR” and uses augmented reality to observe students’ gazes in a virtual environment. Results show that this system is more improved and flexible in several scenarios. Sodhi et al. [ 289 ] present a synchronous remote collaborative system called “BeThere” to explore 3D gestures and spatial input. This system enables a remote user to perform virtual interaction in the local user’s real environment. Ong et al. [ 290 ] propose a collaborative system in which 3D objects can be seen by all the users in a collaborative environment. Moreover, the changes made to these objects are also observed by the users. Butz et al. [ 84 ] present EMMIE (environment management for multi-user information environments) in a collaborative augmented reality environment in which virtual objects can be manipulated by the users. In addition, this manipulation is visible to each user of this system.

7.2.3. Asynchronous Remote Collaborative AR

Irlitti et al. [ 291 ] explore the challenges faced during the use of asynchronous collaborative AR. Moreover, the author further discusses how to enhance communication while using asynchronous collaborative AR. Quasi-systems do not fulfill Azuma’s [ 292 ] definition of AR technology. However, they are very good at executing certain aspects of AR as other full AR devices are doing. For instance, mixed-space collaborative work in a virtual theater [ 268 ]. This system explained that if someone wants two groups to pay attention to each other, a common spatial frame of reference should be created to have a better experience of social presence. In the spatially aware educational system, students were using location-aware smartphones to resolve riddles. This was very useful in the educational system because it supported both engagement and social presence [ 245 , 265 , 269 ]. However, this system did not align the 3D virtual content in the virtual space. Therefore, it was not a true AR system. In order to capture a remote 3D scene, Fuchs and Maimone [ 293 ] developed an algorithm. They also developed a proof of concept for teleconferencing. For capturing images, RGB-D cameras were used. The remote scene was displayed on the 3D stereoscopic screen. These systems were not fully AR, but they still exhibited a very good immersion. Akussah et al. [ 294 ] focus on developing a marker-based collaborative augmented reality app for learning mathematics. First, the system focuses on individual experience and later on expands it to collaborative AR.

7.3. Summary

This section provides comprehensive details on collaborative augmented reality which is broadly classified into co-located collaborative AR, where participants collaborate with each other in geographically the same location, and remote collaboration. The applications of both approaches are presented as well. Co-located collaborative AR is mostly adopted in learning realms for sharing information, for example, in museums. On the other hand, in remote collaborative AR the remote user can explore the scene regardless of the local user’s camera position. The applications of this technology are mostly found in education.

8. AR Interaction and Input Technologies

The interaction and input technologies are detailed in this section. There are a number of input methods that are utilized in AR technologies. First, multimode and 3D interfaces such as speech, gesture and handheld wands. Second, the mouse, and keyboard traditional two-dimensional user interfaces (UI). The type of interaction task needed for the interface defines which input method would be utilized in the application. A variety of interfaces have been developed: three-dimensional user interfaces, tangible user interfaces, multimedia interfaces, natural user interfaces, and information browsers.

8.1. AR Information Browsers

Wikitude and Navicam are one of the most popular examples of AR information browsers. The only problem with AR browsers is that they cannot provide direct interaction with the virtual objects.

8.2. Three-Dimensional User Interfaces

A three-dimensional user interface uses the controllers for providing the interaction with virtual content. By using the traditional 3D user interface techniques, we can directly interact with the three-dimensional object in the virtual space. There are a number of 3D user interface interaction techniques as follows: 3D motion tracking sensors are one of the most commonly used devices for AR interaction. The motion tracking sensors allow the following functions: tracking the parts of the user’s body and allow pointing as well as the manipulation of the virtual objects [ 295 ]. Haptic devices are also used for interacting with AR environments [ 296 , 297 , 298 ]. They mainly used as 3D pointing devices. In addition, they provide tactile and forces feedback. This will create the illusion of a physical object existing in the real world. Thereby, it helps in complementing the virtual experience. They are used in training, entertainment, and design-related AR applications.

8.3. Tangible User Interface

The tangible user interface is one of the main concepts of human–computer interface technology research. In this, the physical object is used for interaction [ 299 ]. It bridges the gap between the physical and the virtual object [ 300 ]. Chessa et al. incorporated grasping behavior in a virtual reality systems [ 301 ], while Han et al. presented and evaluated hand interaction techniques using tactile feedback (haptics) and physical grasping by mapping a real object with virtual objects [ 302 ].

8.4. Natural User Interfaces in AR

Recently, more accurate gesture and motion-based interactions for AR and VR applications have become extensively available due to the commercialization of depth cameras such as Microsoft Kinect and technical advances. Bare-hand interaction with a virtual object was made possible by the introduction of a depth camera. It provided physical interaction by tracking the dexterous hand motion. For instance, the physical objects and the user’s hands were recognized by the use of Kinect Camera, designed by the Microsoft HoloDesk [ 299 ]. The virtual objects were shown on the optical see-through AR workbench. It also allowed the users to interact with the virtual objects presented on the AR workbench. The user-defined gestures have been categorized into sets by the Piumsomboon [ 300 ]. This set can be utilized in AR applications for accomplishing different tasks. In addition, some of the mobile-based depth-sensing cameras are also under investigation. For instance, the SoftKinetic and Myo gesture armband controller. SodtKinetic is aimed at developing hand gesture interaction in mobile phones and wearable devices more accurately, while the Myo gesture armband controller is a biometric sensor that provides interaction in wearable and mobile environments.

8.5. Multimodal Interaction in AR

In addition to speech and gesture recognition, there are other types of voice recognition are being investigated. For example, the whistle-recognition system was developed by Lindeman [ 303 ] in mobile AR games. In this, the user had to whistle the right length and pitch to intimidate the virtual creatures in the game. Summary: The common input techniques and input methods have been examined in this section. These included simple information browsers and complex AR interfaces. The simple ones have very little support for the interaction and virtual content, while the complex interfaces were able to recognize even the speech and gesture inputs. A wide range of input methods are available for the AR interface; however, they are needed to be designed carefully. The following section delineates the research into the interface pattern, design, and guideline for AR experiences.

9. Design Guidelines and Interface Pattern

The previous section detailed the wide range of different AR input and interaction technologies; however, more rigorous research is required to design the AR experience. This section explores the interface patterns and design guidelines to develop an AR experience. The development of new interfaces goes through four main steps. First, the prototype is demonstrated. Second, interaction techniques are adopted from the other interface metaphors. Third, new interface metaphors are developed that are appropriate to the medium. Finally, the formal theoretical models are developed for modeling the interaction of users. In this regard, Wang et al. [ 304 ] employ user-centered AR instruction (UcAI) in procedural tasks. Thirty participants were selected for the experiment while having both the control and experiment groups. The result of the experiment suggested that introduction of UcAI increased the user’s spatial cognitive ability, particularly in the high-precision operational task. This research has the potential of guiding advanced AR instruction designs to perform tasks of high cognitive complexity. For instance, WIMP (windows, icons, menus, and pointers) is a very well-known desktop metaphor. In development, it has gone through all of these stages. There are methods developed that are used to predict the time taken by the mouse will select an icon of a given size. These are known as formal theoretical models. Fitts law [ 305 ] is among those models that help in determining the pointing times in the user interfaces. There are also a number of virtual reality interfaces available that are at the third stage with reference to the techniques available. For example, the manipulation and selection in immersive virtual worlds can be done by using the go-go interaction method [ 306 ]. On the other hand, as evident in the previous section, AR interfaces have barely surpassed the first two stages. Similarly, a number of AR interaction methods and technologies are available; however, by and large, they are only the extensions or versions of the existing 3D and 2D techniques present in mobiles, laptops, or AR interfaces. For instance, mobile phone experiences such as the gesture application and the touch screen input are added to AR. Therefore, there is a dire need to develop AR-specific interaction techniques and interface metaphors [ 307 ]. A deeper analysis and study of AR interfaces will help in the development of the appropriate metaphor interfaces. AR interfaces are unique in the sense that they need to develop close interaction between the real and the virtual worlds. A researcher, MacIntyre, has argued that the definition and the fusion of the virtual and real worlds are required for creating an AR design [ 308 ]. The primary goal of this is to depict the physical objects and user input onto the computer-generated graphics. This is done by using a suitable interaction interface. As a result, an AR design should have three components:

  • The physical object.
  • The virtual image to be developed.
  • An interface to create an interaction between the physical world and the virtual objects.

Use of design patterns could be an alternative technique to develop the AR interface design. These design patterns are most commonly used in the fields of computer science and design interface. Alexander has defined the use of design patterns in the following words: “Each pattern describes a problem that occurs over and over again in our environment, and then describes the core of the solution to that problem in such a way that you can use this solution a million times over, without ever doing it the same way twice” [ 309 , 310 ]. The pattern language approach could be used to enhance AR development, as suggested by Reicher [ 311 ]. This idea has evolved from the earlier research works of MacWilliam [ 312 ]. This approach has two main functionalities. First, it is more focused on the software engineering aspect. Secondly, it suggests ways to develop complex AR systems by combining different modules of design patterns. So, they describe each pattern by the number of its aspects such as name, motivation, goal, description, consequences, known project usage, and general usability. One of the most notable examples of it is the DWARF framework [ 313 ]. DWARF is a component-based AR framework that is developed through the design pattern approach. In contrast to the pattern language approach, the user experience of design in the AR handheld device could be used for developing designs. This was described by Xu and the main concern was pre-patterns. Pre-patterns are the components that bridge the gap between the game design and the interaction design. For determining the method of using of design patterns, seamful design could be used. This suggests that the designer should integrate the AR handheld game design and the technology in such a way that they should blend into each other. Some users need more attention for designing effective AR experiences; therefore, the designing of special needs is another intervention to resolve this discrepancy. For instance, as pointed out by Rand and Maclntyre [ 314 ], in designing an AR system for the age group of 6–9, the developmental stages of the children should be accounted for in it. The research has also suggested that a powerful educational experience could be created through the use of AR. In addition to this development, it was also stated that the developmental stages of the students should be considered [ 315 , 316 ]. However, there is no extensive research that suggests the development of AR experiences for children [ 317 ]. Radu, in his paper, has determined the key four areas that should be considered while designing AR for children: attention, motor, special, logic, and memory abilities [ 318 ].

10. Security, Trust, and Collaborative AR

Security is very important in augmented reality, especially in collaborative augmented reality. While using collaborative AR applications, the data are exposed to external attacks, which increases concerns about security relating to AR technologies. Moreover, if the users who share the same virtual collaborative environments are unknown to each other, it also elevates these issues. In [ 319 ], the basic premise of the research is that the developed abstraction device not only improves the privacy but also the performance of the AR apps, which lays the groundwork for the development of future OS support for AR apps. The results suggested that the prototype enables secure offloading of heavyweight, incurs negligible overhead, and improves the overall performance of the app. In [ 320 ], the authors aim to resolve security and privacy challenges in multi-user AR applications. They have introduced an AR-sharing module along with systematized designs and representative case studies for functionality and security. This module is implemented as a prototype known as ArShare for the HoloLens. Finally, it also lays the foundation for the development of fully fledged and secure multi-user AR interaction. In [ 321 ], the authors used AR smart glasses to detail the “security and safety” aspect of AR applications as a case study. In the experiment, cloud-based architecture is linked to the oil extractor in combination with Vuzix Blade smart glasses. For security purposes, this app sends real-time signals if a dangerous situation arrives. In [ 322 ], deep learning is used to make the adaptive policies for generating the visual output in AR devices. Simulations are used that automatically detect the situation and generate policies and protect the system against disastrous malicious content. In [ 323 ], the authors discussed the case study of challenges faced by VR and AR in the field of security and privacy. The results showed that the attack reached the target of distance 1.5 m with 90 percent accuracy when using a four-digit password. In [ 324 ], the authors provide details and goals for developing security. They discuss the challenges faced in the development of edge computing architecture which also includes the discussion regarding reducing security risks. The main idea of the paper is to detail the design of security measures for both AR and non-AR devices. In [ 325 ], the authors presented that the handling of multi-user outputs and handling of data are demonstrated are the two main obstacles in achieving security and privacy of AR devices. It further provides new opportunities that can significantly improve the security and privacy realm of AR. In [ 326 ], the authors introduce the authentication tool for ensuring security and privacy in AR environments. For these purposes, the graphical user password is fused with the AR environments. A doodle password is created by the touch-gesture-recognition on a mobile phone, and then doodles are matched in real-time size. Additionally, doodles are matched with the AR environment. In [ 327 ], the authors discussed the immersive nature of augmented reality engenders significant threats in the realm of security and privacy. They further explore the aspects of securing buggy AR output. In [ 328 ], the authors employ the case study of an Android app, “Google Translator”, to detect and avoid variant privacy leaks. In addition, this research proposes the foundational framework to detect unnecessary privacy leaks. In [ 329 ], the authors discuss the AR security-related issues on the web. The security related vulnerabilities are identified and then engineering guidelines are proposed to make AR implementation secure. In [ 330 ], the past ten years of research work of the author, starting from 2011, in the field of augmented reality is presented. The main idea of the paper is to figure out the potential problems and to predict the future for the next ten years. It also explains the systematization for future work and focuses on evaluating AR security research. In [ 331 ], the authors presented various AR-related security issues and identified managing the virtual content in the real space as a challenge in making AR spaces secure for single and multi-users. The authors in [ 332 ] believe that there is a dire need of cybersecurity risks in the AR world. The introduction of systemized and universal policy modules for the AR architecture is a viable solution for mitigating security risks in AR. In [ 333 ], the authors discuss the challenge of enabling the different AR apps to augment the user’s world experience simultaneously, pointing out the conflicts between the AR applications.

11. Summary

In this paper, the authors have reviewed the literature extensively in terms of tracking and displays technology, AR, and collaborative AR, as can be seen in Figure 10 . It has been observed that collaborative AR has further two classifications i.e., co-located AR and remote collaboration [ 334 ]. Each of these collocated and remote collaborations has two further types i.e., synchronous and asynchronous. In remote collaborative AR, there are a number of use cases wherein it has been observed that trust management is too important a factor to consider because there are unknown parties that participate in remote activities to interact with each other and as such, they are unknown to each other as well [ 21 , 335 , 336 , 337 , 338 ]. There has been a lack of trust and security concerns during this remote collaboration. There are more chances of intrusion and vulnerabilities that can be possibly exploited [ 331 , 339 , 340 ]. One such collaboration is from the tourism sector, which has boosted the economy, especially in the pandemic era when physical interactors were not allowed [ 341 ]. To address these concerns, this research felt the need to ensure that the communication has integrity and for this purpose, the research utilized state-of-the-art blockchain infrastructure for collaborative applications in AR. The paper has proposed a complete secure framework wherein different applications working remotely are having a real feeling of trust in each other [ 17 , 342 , 343 ]. The participants within the collaborative AR subscribed to a trusted environment to further make interaction with each other in a secure fashion while their communication was protected through state-of-the-art blockchain infrastructure [ 338 , 344 ]. A model of such an application is shown in Figure 11 .

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A model of blockchain-based trusted and secured collaborative AR system.

Figure 12 demonstrates the initiation of the AR App in step 1, while in step 2 of Figure 12 , the blockchain is initiated to record transactions related to sign-up, record audio calls, proceed with payment/subscription, etc. In step 3, when the transaction is established, AR is initiated, which enables the visitor to receive guidance from the travel guide. The app creates a map of the real environment. The created map and the vision provide a SLAM, i.e., SLAM provides an overall vision and details of different objects in the real world. Inertial tracking controls the movement and direction in the augmented reality application. The virtual objects are then placed after identifying vision and tracking. In a collaborative environment, the guides are provided with an option of annotation so they can circle a particular object or spot different locations and landmarks or point to different incidents [ 16 ].

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Object name is sensors-23-00146-g012.jpg

Sharing of the real-time environment of CAR tourist app for multiple users [ 16 ].

12. Directions for Research

The commercialization efforts of companies have made AR a mainstream field. However, for the technology to reach its full potential, the number of research areas should be expanded. Azuma has explained the three major obstacles in the way of AR: interface limitation, technological limitations, and the issue of social acceptance. In order to overcome these barriers, the two major models are developed: first, Roger’s innovation diffusion theory [ 345 ] and the technology acceptance model (developed by Martinez) [ 346 ]. Roger has explained the following major restriction towards the adoption of this technology: limited computational power of AR technology, social acceptance, no AR standards, tracking inaccuracy, and overloading of information. The main research trends in display technology, user interface, and tracking were identified by Zho by evaluating ten years of ISMAR papers. The research has been conducted in a wide number of areas except for social acceptance. This section aims at exploring future opportunities and ongoing research in the field of AR, particularly in the four key areas: display, tracking, interaction, and social acceptance. Moreover, there are a number of other topics including evaluation techniques, visualization methods, applications, authoring and content-creating tools, rendering methods, and some other areas.

13. Conclusions

This document has detailed a number of research papers that address certain problems of AR. For instance, AR tracking techniques are detailed in Section 3 . Display technologies, such as VST and OST, and its related calibration techniques in Section 4 , authoring tools in Section 6 , collaborative AR in Section 7 , AR interaction in Section 8 , and design guidelines in Section 9 . Finally, promising security and trust-related papers are discussed in the final section. We presented the problem statement and a short solution to the problem is provided. These aspects should be covered in future research and the most pertinent among these are the hybrid AR interfaces, social acceptance, etc. The speed of research is significantly increasing, and AR technology is going to dramatically impact our lives in the next 20 years.


Thanks to the Deanship of Research, Islamic University of Madinah. We would like to extend special thanks to our other team members (Anas and his development team at 360Folio, Ali Ullah and Sajjad Hussain Khan) who participated in the development, writeup, and finding of historical data. Ali Ullah has a great ability to understand difficult topics in AR, such as calibration and tracking.

Funding Statement

This project is funded by the Deputyship For Research and Innovation, Ministry of Education, Kingdom of Saudi Arabia, under project No (20/17), titled Digital Transformation of Madinah Landmarks using Augmented Reality.

Author Contributions

Conceptualization of the paper is done by T.A.S. Sections Organization, is mostly written by T.A.S. and S.J.; The protype implementation is done by the development team, however, the administration and coordination is performed by A.A., A.N. (Abdullah Namoun) and A.B.A.; Validation is done by A.A. and A.N. (Adnan Nadeem); Formal Analysis is done by T.A.S. and S.J.; Investigation, T.A.S.; Resources and Data Curation, is done by A.N. (Adnan Nadeem); Writing—Original Draft Preparation, is done by T.A.S. and S.J., Writing—Review & Editing is carried out by H.B.A.; Visualization is mostly done by T.A.S. and M.S.S.; Supervision, is done by T.A.S.; Project Administration, is done by A.A.; Funding Acquisition, T.A.S. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

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AI and holography bring 3D augmented reality to regular glasses

Researchers adjusting holographic augmented reality display.

Researchers in the emerging field of spatial computing have developed a prototype augmented reality headset that uses holographic imaging to overlay full-color, 3D moving images on the lenses of what would appear to be an ordinary pair of glasses. Unlike the bulky headsets of present-day augmented reality systems, the new approach delivers a visually satisfying 3D viewing experience in a compact, comfortable, and attractive form factor suitable for all-day wear.

“Our headset appears to the outside world just like an everyday pair of glasses, but what the wearer sees through the lenses is an enriched world overlaid with vibrant, full-color 3D computed imagery,” said  Gordon Wetzstein , an associate professor of electrical engineering and an expert in the fast-emerging field of spatial computing. Wetzstein and a team of engineers introduce their device in a new paper in the journal  Nature . Additional information about this advance is available at  this website , created by the research team.

Though only a prototype now, such a technology, they say, could transform fields stretching from gaming and entertainment to training and education – anywhere computed imagery might enhance or inform the wearer’s understanding of the world around them.

“One could imagine a surgeon wearing such glasses to plan a delicate or complex surgery or airplane mechanic using them to learn to work on the latest jet engine,”  Manu Gopakumar , a doctoral student in the Wetzstein-led  Stanford Computational Imaging lab  and co-first author of the paper said.

Team of five researchers poses with a prototype of their compact augmented reality glasses.

Barriers overcome

The new approach is the first to thread a complex maze of engineering requirements that have so far produced either ungainly headsets or less-than-satisfying 3D visual experiences that can leave the wearer visually fatigued, or even a bit nauseous at times.

“There is no other augmented reality system out there now with comparable compact form factor or that matches our 3D image quality,” said  Gun-Yeal Lee , a postdoctoral researcher in the Stanford Computational Imaging lab and co-first author of the paper.

To succeed, the researchers have overcome technical barriers through a combination of AI-enhanced holographic imaging and new nanophotonic device approaches. The first hurdle was that the techniques for displaying augmented reality imagery often require the use of complex optical systems. In these systems, the user does not actually see the real world through the lenses of the headset. Instead, cameras mounted on the exterior of the headset capture the world in real time and combine that imagery with computed imagery. The resulting blended image is then projected to the user’s eye stereoscopically.

“The user sees a digitized approximation of the real world with computed imagery overlaid. It’s sort of augmented virtual reality, not true augmented reality,” explained Lee.

These systems, Wetzstein explains, are necessarily bulky because they use magnifying lenses between the wearer’s eye and the projection screens that require a minimum distance between the eye, the lenses, and the screens, leading to additional size.

“Beyond bulkiness, these limitations can also lead to unsatisfactory perceptual realism and, often, visual discomfort,” said  Suyeon Choi , a doctoral student in the Stanford Computational Imaging lab and co-author of the paper.

To produce more visually satisfying 3D images, Wetzstein leapfrogged traditional stereoscopic approaches in favor of holography, a Nobel-winning visual technique developed in the late-1940s. Despite great promise in 3D imaging, more widespread adoption of holography has been limited by an inability to portray accurate 3D depth cues, leading to an underwhelming, sometimes nausea-inducing, visual experience.

The Wetzstein team used AI to improve the depth cues in the holographic images. Then, using advances in nanophotonics and waveguide display technologies, the researchers were able to project computed holograms onto the lenses of the glasses without relying on bulky additional optics.

A waveguide is constructed by etching nanometer-scale patterns onto the lens surface. Small holographic displays mounted at each temple project the computed imagery through the etched patterns which bounce the light within the lens before it is delivered directly to the viewer’s eye. Looking through the glasses’ lenses, the user sees both the real world and the full-color, 3D computed images displayed on top.

Holographic device emitting blue light in dark lab.

Life-like quality

The 3D effect is enhanced because it is created both stereoscopically, in the sense that each eye gets to see a slightly different image as they would in traditional 3D imaging, and holographically.

“With holography, you also get the full 3D volume in front of each eye increasing the life-like 3D image quality,” said  Brian Chao , a doctoral student in the Stanford Computational Imaging lab and also co-author of the paper.

The ultimate outcome of the new waveguide display techniques and the improvement in holographic imaging is a true-to-life 3D visual experience that is both visually satisfying to the user without the fatigue that has challenged earlier approaches.

“Holographic displays have long been considered the ultimate 3D technique, but it’s never quite achieved that big commercial breakthrough,” Wetzstein said. “Maybe now they have the killer app they’ve been waiting for all these years.”

Additional authors are from The University of Hong Kong and NVIDIA. Wetzstein is also member of  Stanford Bio-X , the  Wu Tsai Human Performance Alliance , and the  Wu Tsai Neurosciences Institute .

This research was funded by a Stanford Graduate Fellowship in Science and Engineering, the National Research Foundation of Korea (NRF) funded by the Ministry of Education, a Kwanjeong Scholarship, a Meta Research PhD Fellowship, the ARO PECASE Award, Samsung, and the Sony Research Award Program. Part of this work was performed at the  Stanford Nano Shared Facilities (SNSF)  and  Stanford Nanofabrication Facility (SNF) , supported by the National Science Foundation and the National Nanotechnology Coordinated Infrastructure.

Related : Gordon Wetzstein , associate professor of electrical engineering 

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