![]() The display module was implemented using a digital micromirror device (DMD) and a high-dynamic range illumination source using a light-emitting diode (HDR LED). 25 reported volumetric displays that have a large number of focal depths, i.e., 40 and 280, respectively. However, it is difficult to cover a wide depth of field while providing continuous focus cues. A state-of-the-art display, which is refreshed at 240 Hz, may optimally reconstruct four focal planes by sacrificing the frame rate. The most feasible approach for increasing the focal plane number is to employ temporal multiplexing 9, 11, 12, 15, 21 with synchronization of a display module and focus-tunable optics. However, it is challenging to increase the number of focal planes without declining the frame rate or form factor. In multi-plane displays, it is important to achieve a large number of focal planes for high resolution, wide depth of field, and continuous accommodation cues. Multi-plane displays are also able to provide users with depth information by floating multiple discrete focal planes. In addition, holographic displays possess challenging issues, such as speckle noise and large computational demand. Nevertheless, holographic displays suffer from a trade-off between the field of view and exit pupil, which is related to the limited bandwidth of spatial light modulators. Several holographic near-eye display prototypes were analyzed thoroughly regarding enhancements in form factor 17, 20 and tolerance 23. Recently, holographic displays have been spotlighted, especially in near-eye display applications, as an alternative approach to light field displays. Although each approach has distinct advantages, they share a common limitation (i.e., trade-offs) that comes from the large amount of information required for the reconstruction of light fields. Several optical systems have been introduced to reconstruct four-dimensional light fields using lens arrays 1, multi-projections 2, 6, or layered structures 3, 5, 6, 7. For instance, light field displays with focus cues suffer from a trade-off among the spatial resolution, angular resolution, depth of field 22, and frame rate. Mostly, the ability of the display system to reproduce physiological cues involves sacrificing the resolution 1, 5, 6, 8, 10, 14, 15, 21, frame rate 2, 9, 11, 12, viewing window 16, 20, or eye box 13, 17, 19. However, it has been challenging to provide all of the convergence, accommodation, and motion parallax without sacrificing the display performance. Several display technologies have been introduced and studied to reconstruct physiological cues. To induce physiological cues, a specific display system is required, such as light field displays 1, 2, 3, 4, 5, 6, 7, stereoscopes with focus cues 8, 9, 10, 11, 12, 13, 14, 15, and holographic displays 16, 17, 18, 19, 20. On the other hand, physiological cues refer to the physical states of the two eyes and objects in terms of convergence, accommodation, and motion parallax. With advancements in 3D rendering and computer graphics, these psychological cues can be reproduced via ordinary two-dimensional (2D) display panels. Psychological cues are related to the visual effects that are usually observed in daily life, including shading, perspective, illumination, and occlusion. The human visual system understands the real world and perceives depth information of 3D objects via psychological and physiological cues. To approach the ideal 3D display system, the following points should be considered: first, the visual characteristics of a real object that make it look real second, alleviation of the artificiality that comes from state-of-the-art displays finally, those essential features must be realized without sacrificing figures of merit, such as the resolution, frame rate, and eye box. An ideal three-dimensional (3D) display system provides an immersive and realistic experience.
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