Wearable near-eye displays for virtual and augmented reality (VR/AR)
have seen enormous growth in recent years. While researchers are
exploiting a plethora of techniques to create life-like
three-dimensional (3D) objects, there is a lack of awareness of the
role of human perception in guiding the hardware development. An
ultimate VR/AR headset must integrate the display, sensors, and
processors in a compact enclosure that people can comfortably wear for
a long time while allowing a superior immersion experience and
user-friendly human–computer interaction. Compared with other 3D
displays, the holographic display has unique advantages in providing
natural depth cues and correcting eye aberrations. Therefore, it holds
great promise to be the enabling technology for next-generation VR/AR
devices. In this review, we survey the recent progress in holographic
near-eye displays from the human-centric perspective.
Holographic displays have great potential to realize mixed reality by modulating the wavefront of light in a fundamental manner. As a computational display, holographic displays offer a large degree of freedom, such as focus cue generation and vision correction. However, the limited bandwidth of spatial light modulator imposes an inherent trade-off relationship between the field of view and eye-box size. Thus, we demonstrate the first practical eye-box expansion method for a holographic near-eye display. Instead of providing an intrinsic large exit-pupil, we shift the optical system's exit-pupil to cover the expanded eye-box area with pupil-tracking. For compact implementation, a pupil-shifting holographic optical element (PSHOE) is proposed that can reduce the form factor for exit-pupil shifting. A thorough analysis of the design parameters and display performance are provided. In particular, we provide a comprehensive analysis of the incorporation of the holographic optical element into a holographic display system. The influence of holographic optical elements on the intrinsic exit-pupil and pupil switching is revealed by numerical simulation and Wigner distribution function analysis.
We introduce an augmented reality near-eye display dubbed "Retinal 3D." Key features of the proposed display system are as follows: Focus cues are provided by generating the pupil-tracked light field that can be directly projected onto the retina. Generated focus cues are valid over a large depth range since laser beams are shaped for a large depth of field (DOF). Pupil-tracked light field generation significantly reduces the needed information/computation load. Also, it provides "dynamic eye-box" which can be a break-through that overcome the drawbacks of retinal projection-type displays. For implementation, we utilized a holographic optical element (HOE) as an image combiner, which allowed high transparency with a thin structure. Compared with current augmented reality displays, the proposed system shows competitive performances of a large field of view (FOV), high transparency, high contrast, high resolution, as well as focus cues in a large depth range. Two prototypes are presented along with experimental results and assessments. Analysis on the DOF of light rays and validity of focus cue generation are presented as well. Combination of pupil tracking and advanced near-eye display technique opens new possibilities of the future augmented reality.
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