Holographic Retinal Projection Display with Enhanced Depth Cues

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Introduction

In recent years, with the launch of wearable near-eye display devices such as Apple’s Vision Pro and Microsoft’s Hololens, virtual reality (VR) and augmented reality (AR) display technologies have gradually had a profound impact in fields such as industry, education, and healthcare. Near-eye display technology, as an important technical support for VR/AR devices, has received widespread attention and rapid development in recent years. Currently, binocular parallax 3D display is still a commonly used 3D display technology in near-eye displays, which suffers from the vergence-accommodation conflict (VAC) problem, causing eye fatigue and discomfort during prolonged viewing. Retinal projection display (RPD) alleviates this issue by limiting the exit pupil diameter of the imaging system, using converging fine light beams to directly project images onto the human retina, thus expanding the depth of field perceived by the human eye. However, RPD near-eye displays struggle to provide defocus blur of multi-focal images, resulting in a lack of monocular depth cues and difficulty in achieving natural near-eye 3D display.

To address this issue, Professor Lü Guoqiang and Associate Researcher Wang Zi from Hefei University of Technology proposed a method to precisely control the beam width and depth of field of holographic RPD, enabling the direct projection of multi-focal images onto the retina, achieving lens-free holographic RPD near-eye display with significant depth cues. Recently, this research was published in the journal Liquid Crystal and Display (indexed in ESCI and Scopus, a core Chinese journal) in the 2024, Issue 7, and was selected as the cover article for that issue.

Holographic Retinal Projection Display with Enhanced Depth Cues

Fig. 1: Cover image of Liquid Crystal and Display, Issue 7, 2024

Fig. 2(a) illustrates the reconstruction process of traditional holographic PRD. The converging spherical wavefront generated by the hologram converges at the pupil, which then directly projects the multi-focal image information onto the retina. Fig. 2(b) shows the simulation and experimental results of the reconstructed image of traditional holographic RPD. It can be seen that due to the large depth of field characteristic of RPD, images at different preset depths can be clearly imaged, failing to provide defocus blur and focus cues. Therefore, it is necessary to regulate the depth of field of holographic RPD.

Fig. 2: (a) Reconstruction process of traditional holographic RPD (b) Simulation and experimental reconstruction images of holographic RPD at different focusing depths

Image source: Liquid Crystal and Display, 2024, 39(7): 901-908. Fig.1-3

The depth of the reproduced image is determined by the distance of the imageL and the beam width entering the pupile [Daniel G. Green, 1980], while the beam width entering the pupile is determined by the divergence angle of the object lightθ and the distance of the imageL, as shown in Fig. 3(a). Therefore, by changing the divergence angle of the object lightθ, the depth of the reproduced image can be controlled. The divergence angle of the object light is proportional to sin^-1(λv), whereλ andν are the wavelength and spatial frequency [Tomoyoshi Shimobaba, 2015]. The high-frequency part has a larger divergence angle, while the low-frequency part has a smaller divergence angle. The research team achieved precise control of the divergence angle of the object lightθ by introducing band-limited random phase. As shown in Fig. 3(b), the band-limited random phase is obtained by aperture limiting the spectrum of completely random phase, where the spectral aperturep is calculated based on the depthL of the target image and the required beam width at the pupil planee. In the hologram calculation process, the amplitude of the target image is multiplied by the spherical wave phase and the band-limited random phase, and then diffracted to the holographic surface to encode it as a hologram. Fig. 3(c) shows the simulation results of the beam width of the object light at the pupil under different random phases, verifying that the band-limited random phase can flexibly control the beam width entering the human eye. Fig. 4 illustrates the simulation and experimental results of holographic RPD based on band-limited random phase reconstructed at different depths, showing significant defocus blur, providing focus cues for the observer while maintaining high image quality with low speckle.

Fig. 3: (a) Geometric relationship between the divergence angle of the object light and the beam width at the pupil plane (b) Relationship between band-limited random phase and spectrum

Image source: Liquid Crystal and Display, 2024, 39(7): 901-908. Fig.4,6,9

Fig. 4: Simulation and experimental reconstruction images of holographic RPD based on band-limited random phase at different focusing depths

Image source: Liquid Crystal and Display, 2024, 39(7): 901-908. Fig.7

Conclusion and Outlook

To address the issue of RPD’s inability to provide defocus blur and depth cues, this study proposes a holographic RPD scheme based on band-limited random phase. This method allows for free adjustment of the beam width entering the pupil, which can adapt to the pupil diameter, providing observers with significant defocus blur depth information while maintaining high imaging quality with low speckle. The results of this research can directly project multi-focal images onto the retina, effectively solving the monocular depth perception problem associated with RPD, and are expected to be applied in the field of AR/VR near-eye displays.

Paper Information

Tu Kefeng, Sun Fei, Wang Zi, et al. Holographic retinal projection display with enhanced depth cues[J]. Liquid Crystal and Display, 2024, 39(7): 901-908.

https://cjlcd.lightpublishing.cn/thesisDetails#10.37188/CJLCD.2023-0242

Corresponding Author Information

Holographic Retinal Projection Display with Enhanced Depth Cues

Wang Zi, Associate Researcher at Hefei University of Technology, master’s supervisor. He obtained his bachelor’s and doctoral degrees in physics from the University of Science and Technology of China in 2012 and 2017, respectively, and mainly conducts research in computational holography, 3D display, and near-eye display. He has undertaken multiple research projects including the National Natural Science Foundation, National Engineering Technology Research Center projects, and major projects in Anhui Province. He has published over 30 SCI academic papers in prestigious journals such as Optics Letters, Optics Express, and Applied Physics Letters.

E-mail: [email protected]

Supervised by: Zhang Ying, Zhao Yang

Editor: Zhao Wei

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