Accommodation-convergence Conflict Research Articles

Spatial perception in stereoscopic augmented reality based on multifocus sensing

In many areas ranging from medical imaging to visual entertainment, 3D information acquisition and display is a key task. In this regard, in multifocus computational imaging, stacks of images of a certain 3D scene are acquired under different focus configurations and are later combined by means of post-capture algorithms based on image formation model in order to synthesize images with novel viewpoints of the scene. Stereoscopic augmented reality devices, through which is possible to simultaneously visualize the three dimensional real world along with overlaid digital stereoscopic image pair, could benefit from the binocular content allowed by multifocus computational imaging. Spatial perception of the displayed stereo pairs can be controlled by synthesizing the desired point of view of each image of the stereo-pair along with their parallax setting. The proposed method has the potential to alleviate the accommodation-convergence conflict and make augmented reality stereoscopic devices less vulnerable to visual fatigue.

Metalens Eyepiece for 3D Holographic Near-Eye Display.

Near-eye display (NED) systems for virtual reality (VR) and augmented reality (AR) have been rapidly developing; however, the widespread use of VR/AR devices is hindered by the bulky refractive and diffractive elements in the complicated optical system as well as the visual discomfort caused by excessive binocular parallax and accommodation-convergence conflict. To address these problems, an NED system combining a 5 mm diameter metalens eyepiece and a three-dimensional (3D), computer-generated holography (CGH) based on Fresnel diffraction is proposed in this paper. Metalenses have been extensively studied for their extraordinary capabilities at wavefront shaping at a subwavelength scale, their ultrathin compactness, and their significant advantages over conventional lenses. Thus, the introduction of the metalens eyepiece is likely to reduce the issue of bulkiness in NED systems. Furthermore, CGH has typically been regarded as the optimum solution for 3D displays to overcome limitations of binocular systems, since it can restore the whole light field of the target 3D scene. Experiments are carried out for this design, where a 5 mm diameter metalens eyepiece composed of silicon nitride anisotropic nanofins is fabricated with diffraction efficiency and field of view for a 532 nm incidence of 15.7% and 31°, respectively. Furthermore, a novel partitioned Fresnel diffraction and resample method is applied to simulate the wave propagations needed to produce the hologram, with the metalens capable of transforming the reconstructed 3D image into a virtual image for the NED. Our work combining metalens and CGH may pave the way for portable optical display devices in the future.

Visual acuity response when using the 3D head-up display in the presence of an accommodation-convergence conflict

Visual discomfort and fatigue due to accommodation-convergence (AC) conflict in stereoscopic displays has been widely reported, but little is known about its impact on visual acuity, particularly when automotive three-dimensional (3D) head-up displays (HUDs) are involved. This paper presents a study on the visual acuity response when an indigenously developed 75% transparent retroreflective screen is used as a windshield 3D HUD. The simulated optical collimation technique was used to provide the virtual content at a farther depth (i.e. on the road while driving). Two user test experiments were performed. The first test was performed under the see-through condition, where the real scene (i.e. roadside view) was perceived through the 3D HUD, while the second test was performed under the simulated collimation condition, where a stereo-collimated virtual content was projected on the HUD at a farther depth. The results showed a slightly declining trend (from 20/20 to 20/25) in visual acuity response when the HUD screen was placed between the viewer and the scene. An inverse relation between the amount of AC conflict and visual acuity was observed under the simulated collimation condition. The >100 cm user-to-screen distance was found to be comfortable, providing the highest acuity response.

Multi-plane augmented reality display based on cholesteric liquid crystal reflective films.

To address the accommodation-convergence conflict problem in conventional augmented reality (AR) head-mounted displays, we propose a compact multi-plane display design based on cholesteric liquid crystal (CLC) reflective films and a polarization switch. Because of the polarization selectivity of CLC films, circularly-polarized light with different handedness is reflected by different CLC films, resulting in different optical path lengths and different image depths by the lens. A flicker-free dual-plane prototype with correct focus cues and relatively low operating voltage has been implemented. Moreover, a multi-plane AR display scheme with more than 2 depth planes is proposed by stacking multiple CLC films and polarization switches together.

Hybrid light field head-mounted display using time-multiplexed liquid crystal lens array for resolution enhancement.

In recent years, head-mounted display technologies have greatly advanced. In order to overcome the accommodation-convergence conflict, light field displays reconstruct three-dimensional (3D) images with a focusing cue but sacrifice resolution. In this paper, a hybrid head-mounted display system that is based on a liquid crystal microlens array is proposed. By using a time-multiplexed method, the display signals can be divided into light field and two-dimensional (2D) modes to show comfortable 3D images with high resolution compensated by the 2D image. According to the experimental results, the prototype supports a 12.28 ppd resolution in the diagonal direction, which reaches 82% of the traditional virtual reality (VR) head-mounted display (HMD).

Experimental verification of accommodation-convergence conflict in viewing integral photography

We experimentally verified the depth perception and accommodation-convergence conflict in viewing integral photography. For comparison, the same measurements were performed with binocular stereoscopic images and real objects. First, the depth perception in viewing an integral three-dimensional (3D) target was measured at three display resolutions: 153, 229, and 458 ppi. The results showed that the depth perception was dependent on the display resolution. The results were also evaluated in a statistical test at a significance level of 5%. The results showed that the recognized depth perception ranges were 180, 240, and 330 mm when the display resolutions were 153, 229, and 458 ppi, respectively. The results were also analyzed in terms of image resolution. This suggested that depth perception occurred at over 1.0 cpd. The accommodation and convergence responses in viewing an integral 3D target displayed on a 3D display with 458 ppi were measured using PowerRef 3. The experimental results were evaluated with a multiple comparison test. It was found that 6 of the 10 observers did not have an accommodation-convergence conflict when viewing the integral 3D target inside and outside the depth of field. In conclusion, integral photography can provide a natural 3D image that looks like a real object.

6.1: Invited Paper: Dynamic Optical Devices for AR/VR Applications

Light field near eye displays (LFNED) can produce lightweight devices and address the accommodation-convergence conflict. However, low spatial resolution and narrow FoV creates a poor immersive experience in LFNED. In this paper, the dynamic optical components are proposed to enhance image resolution and widen the FoV of LFNED. The proposed devices are the flat and thin component with only few mm of thickness, which could be placed in the gap of LFNED without increasing the thickness. The results show that the proposed components could be easily integrated in LFNED with the effect of resolution and FoV enhancement..

P‐95: Full Resolution 3D Display on Computer Screen Free from Accommodation‐convergence Conflict

Existing commercial 3D display technologies for computer screens are based on binocular parallax theory. The inherent accommodation‐convergence (AC) conflict makes them suffering the visual fatigue problem. The present work developed a full resolution 3D display prototype based on Super Multi‐view technology for the computer screen through sequentially gating two liquid crystal light valves before each eye of the viewer, being free from the AC conflict.

Measurement of static convergence and accommodation responses to images of integral photography and binocular stereoscopy.

Static convergence and accommodation responses were measured by comparing integral photography images, binocular stereoscopic images, and real objects in a measurement range from 450 to 900 mm. The experimental results were evaluated with a multiple comparison test. It was found that six of the ten observers did not have an accommodation-convergence conflict in viewing integral photography in the range. Moreover, the required resolution was found to be 0.7 or more and less than 1.4 cycles per degree for inducing accommodation. In conclusion, integral photography can provide a natural 3D image that looks like a real object.

Three-dimensional display on computer screen free from accommodation-convergence conflict

Existing commercial three dimensional (3D) display technologies for computer screens, such as stereoscopic and auto-stereoscopic methods, are based on binocular parallax theory. Due to the inherent accommodation-convergence conflict, (auto-) stereoscopic displays are always suffering the visual fatigue problem. Through sequentially gating two different segments of each eye's aperture of the viewer and refreshing the display contents simultaneously, a 3D display based on Super Multi-view technology for the computer screen is proposed in this paper, which is free from the accommodation-convergence conflict. Employing custom liquid-crystal-valve array for each eye, a proof-of-concept prototype is set up and a 3D display free from accommodation-convergence conflict gets demonstrated.

Review of Holographic Printers for Computer-Generated Holograms

Two different types of holographic printers have been developed that can output holograms on photosensitive materials from digital data calculated as computer-generated holograms to display three-dimensional (3-D) images. The first one is the holographic fringe printer that can write a thin transmission hologram. The other one is the volume hologram printer to write a thick reflection hologram. These printers can output holograms that can provide all physiological depth cues the same as real objects. Therefore, reconstructed images are very natural and do not provide the accommodation-convergence conflict like other 3-D display systems, such as stereoscopic displays. This paper introduces basics of printers, improvements, and reconstructed images.

Monocular 3D see-through head-mounted display via complex amplitude modulation.

The complex amplitude modulation (CAM) technique is applied to the design of the monocular three-dimensional see-through head-mounted display (3D-STHMD) for the first time. Two amplitude holograms are obtained by analytically dividing the wavefront of the 3D object to the real and the imaginary distributions, and then double amplitude-only spatial light modulators (A-SLMs) are employed to reconstruct the 3D images in real-time. Since the CAM technique can inherently present true 3D images to the human eye, the designed CAM-STHMD system avoids the accommodation-convergence conflict of the conventional stereoscopic see-through displays. The optical experiments further demonstrated that the proposed system has continuous and wide depth cues, which enables the observer free of eye fatigue problem. The dynamic display ability is also tested in the experiments and the results showed the possibility of true 3D interactive display.

Computation and Display of 3D Movie From a Single Integral Photography

Integral photography is an auto-stereoscopic technique that allows, among other interesting applications, the display of 3D images with full parallax and avoids the painful effects of the accommodation-convergence conflict. Currently, one of the main drawbacks of this technology is the need of a huge amount of data, which have to be stored and transmitted. This is due to the fact that behind every visual resolution unit, i.e. behind any microlens of an integral-photography monitor, between 100 and 300 pixels should appear. In this paper, we make use of an updated version of our algorithm, SPOC 2.0, to alleviate this situation. We propose the application of SPOC 2.0 for the calculation of complete 3D traveling sequences from a single integral photograph. Specifically, our method permits to generate a sequence of 3D images that simulate the travelling 3D frames captured by a non-static cameraman. In the traveling sequence, we can fix at will, for every frame, the size and position of the field of view, and the parts of the scene that are displayed in front or behind the monitor. Our research is illustrated with experiments in which we generate and display a full traveling sequence.

3-1: A Multi-plane Volumetric Optical See-through Head Mounted 3D Display

We design an optical see-through head mounted display (HMD) employing multi-plane volumetric technique for augmented reality applications. With a stack of fast-switching polymerstabilized liquid crystal scattering shutters, the HMD provides correct depth information and solves the accommodationconvergence conflict.

Super stereoscopy technique for comfortable and realistic 3D displays.

Two well-known problems of stereoscopic displays are the accommodation-convergence conflict and the lack of natural blur for defocused objects. We present a new technique that we name Super Stereoscopy (SS3D) to provide a convenient solution to these problems. Regular stereoscopic glasses are replaced by SS3D glasses which deliver at least two parallax images per eye through pinholes equipped with light selective filters. The pinholes generate blur-free retinal images so as to enable correct accommodation, while the delivery of multiple parallax images per eye creates an approximate blur effect for defocused objects. Experiments performed with cameras and human viewers indicate that the technique works as desired. In case two, pinholes equipped with color filters per eye are used; the technique can be used on a regular stereoscopic display by only uploading a new content, without requiring any change in display hardware, driver, or frame rate. Apart from some tolerable loss in display brightness and decrease in natural spatial resolution limit of the eye because of pinholes, the technique is quite promising for comfortable and realistic 3D vision, especially enabling the display of close objects that are not possible to display and comfortably view on regular 3DTV and cinema.

Near-eye light field displays

We propose near-eye light field displays that enable thin, lightweight head-mounted displays (HMDs) capable of presenting nearly correct convergence, accommodation, binocular disparity, and retinal defocus depth cues. Sharp images are depicted by out-of-focus elements by synthesizing light fields corresponding to virtual objects within a viewer's natural accommodation range. We formally assess the capabilities of microlens arrays to achieve practical near-eye light field displays. Building on concepts shared with existing integral imaging displays and light field cameras, we optimize performance in the context of near-eye viewing. We establish fundamental trade-offs between the quantitative parameters of resolution, field of view, and depth of field, as well as the ergonomic parameters of form factor and ranges of allowed eye movement. As with light field cameras, our design supports continuous accommodation of the eye throughout a finite depth of field; as a result, binocular configurations provide a means to address the accommodation-convergence conflict occurring with existing stereoscopic displays. We construct a complete prototype display system, comprising: a custom-fabricated HMD using modified off-the-shelf parts and real-time, GPU-accelerated light field renderers (including a general ray tracing method and a "backward compatible" rasterization method supporting existing stereoscopic content). Through simulations and experiments, we motivate near-eye light field displays as thin, lightweight alternatives to conventional near-eye displays.

Distance perception for points at equiconvergence and equidistance loci.

Subjects viewed luminescent targets placed at an equidistance (ED) locus or on a comparable equiconvergence locus, the Vieth-Müller circle (VMC). In darkness, subjects moved the index finger to the apparent distance of the visible target. This was repeated at leftward eccentricities of 0 degrees (straight ahead), 15 degrees, 22.5 degrees, 30 degrees, 37.5 degrees, and 45 degrees. Contrary to theoretical expectations, apparent distance to points on the VMC did not describe an equidistance locus, but diminished significantly with increasing eccentricity, whereas ED settings increased slightly but significantly. Binocular and monocular groups performed equivalently. Veridicality of results appears to be the outcome of an accommodation-convergence conflict with a dominant role for accommodation.