An integral imaging-based 2D/3D convertible display system is proposed by using a lens-array holographic optical element (LAHOE), a polymer dispersed liquid crystal (PDLC) film, and a projector. The LAHOE is closely attached to the PDLC film to constitute a projection screen. The LAHOE is used to realize integral imaging 3D display. When the PDLC film with an applied voltage is in the transparent state, the projector projects a Bragg matched 3D image, and the display system works in 3D mode. When the PDLC film without an applied voltage is in the scattering state, the projector projects a 2D image, and the display system works in 2D mode. A prototype of the integral imaging-based 2D/3D convertible display is developed, and it provides 2D/3D convertible images properly.

Integral Imaging based 2D/3D Convertible Display System by Using Holographic Optical Element and Polymer Dispersed Liquid Crystal

Holography refers to the process of recording a complete wave field of interfered coherent beams into a medium (hologram), which can be used to reproduce the original wave field. Since its invention by Dennis Gabor in 1948, the quality of recorded holograms has been dramatically improved due to advancements in materials and recording methodology. As a result of the unique property of wavefront manipulation, holographic optical elements (HOEs) have since found pervasive applications in the fields of data storage, solar concentration, imaging, and display. Lately, augmented reality (AR) has gained unprecedented research interest in both academia and industry because of its potential to become the next-generation display, which could fundamentally transform our daily lives. The basic concept of AR is to seamlessly blend virtual digital contents with real surrounding environments. However, the see-though capability of AR, along with the requirement of delivering high-fidelity images to the viewer’s eyes, poses great challenges to optical designs in terms of field of view (FOV), eye box size, image contrast ratio, and generation of correct focus cues, just to name a few. Generally, AR systems with traditional geometric optics are based on a partialmirror combination. Freeform surfaces can be adopted for aberration correction and achieving higher image quality, but the tradeoff between system form factor and the product of FOV and eye box remains a huge obstacle toward commercialization. Recently, HOE-based AR systems have gained increasing momentum because of the diverse functions of HOEs and large degrees of freedom in design and choice of materials. Various systems have been proposed to resolve the issues related to focus cue generation, system form factor, FOV, and eye box size. In this review, we will first introduce holography methods and the underlying physics of HOE formation. Next, we will describe some unique optical properties of HOEs and their functionalities. After that, we will briefly review the applications of HOEs in several AR display systems and discuss their pros and cons. Finally, we will cast some perspectives on future developments of HOEs for AR displays.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adpr.202000049

Holographic Optical Elements for Augmented Reality: Principles, Present Status, and Future Perspectives

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.

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

Computer-generated holography can obtain the wavefront required for constructing arbitrary intensity distributions in space. Currently, speckle noises in holography remain an issue for most computational methods. In addition, there lacks a multiplexing technology by which images from a single hologram and light source can be switched by a lens. In this work, we first come up with a new algorithm to generate holograms to project smoother images by wavevector filtering. Thereupon, we propose a unique multiplexing scheme enabled by a Fourier lens, as the incident light can be decomposed either by a superposition of spherical waves or plane waves. Different images are obtained experimentally in the spatial and wavevector domains, switchable by a lens. The embedded wavevector filtering algorithm provides a new prospective for speckle suppression without the need for postprocessing. The multiplexing technology can double the capacity of current holographic systems and exhibits potential for various interesting display applications.

/pdf/domain-multiplexed-computer-generated-holography-by-embedded-4i39nmj4wa.pdf

Domain multiplexed computer-generated holography by embedded wavevector filtering algorithm

Review on tabletop true 3D display

In this paper, we propose a dual-view-zone tabletop 3D display system based on integral imaging by using a multiplexed holographic optical element (MHOE) that has the optical properties of two sets of microlens arrays. The MHOE is recorded by a reference beam using the single-exposure method. The reference beam records the wavefronts of a microlens array from two different directions. Thus, when the display beam is projected on the MHOE, two wavefronts with the different directions will be rebuilt and the 3D virtual images can be reconstructed in two viewing zones. The MHOE has angle and wavelength selectivity. Under the conditions of the matched wavelength and the angle of the display beam, the diffraction efficiency of the MHOE is greatest. Because the unmatched light just passes through the MHOE, the MHOE has the advantage of a see-through display. The experimental results confirm the feasibility of the dual-view-zone tabletop 3D display system.

Dual-view-zone tabletop 3D display system based on integral imaging.

In this paper, we propose a tabletop augmented reality (AR) three-dimensional (3D) display system based on integral imaging by using a holographic optical element (HOE) that performs the optical function of a microlens array. The microlens array HOE records the wavefronts of a set of tilted spherical waves based on the theory of reflective volume holograms. In the reconstruction, an elemental image array projected by a projector and collimated by relay optics satisfies Bragg matching conditions and reconstructs the tilted spherical waves. Based on the theory of integral imaging, a 3D image is generated by the tilting spherical waves in an oblique viewing angle, which presents the tabletop 3D feature. Meanwhile, because of the wavelength and angular selectivity of the reflective volume hologram, the Bragg mismatched lights emitted from the real-world scene directly pass through the microlens array HOE. The 3D image and the real-world scene are seamlessly combined, which presents the AR feature. The experimental results confirm that the proposed system can provide a 3D virtual image in an oblique viewing angle and the real-world scene behind the system can also be clearly presented.

Tabletop augmented reality 3D display system based on integral imaging

An augmented reality (AR) three-dimensional (3D) display based on one-dimensional integral imaging (1DII), by using a lenticular lens array holographic optical element (LLA-HOE), is proposed. The 3D image of the 1DII has higher vertical resolution compared with the image of conventional integral imaging whose resolution is sharply reduced for providing quasi-continuous viewpoints in both the horizontal and vertical directions. The proposed 3D display consists of a projector and an LLA-HOE and is compact. As an image combiner, the LLA-HOE can diffract Bragg-matched light rays that have the same wavelength and incident angle as the original reference wave; it can also function as a lenticular lens array to reconstruct a 3D image but transmit other light rays emitted from the surroundings. In the experiment, an LLA-HOE of 80  mm×80  mm size is recorded, and a combination of a high-resolution 3D virtual image and a real 3D object is presented by the proposed AR 3D display.

High-resolution augmented reality 3D display with use of a lenticular lens array holographic optical element

We propose a dual-view integral imaging 3D display based on a multiplexed lens-array holographic optical element (MHOE). A MHOE is a volume holographic optical element obtained by multiplexing technology, which can be used for dual-view integral imaging 3D display due to the angle selectivity of the volume HOE. In the fabrication of the MHOE, two spherical wavefront arrays with different incident angles are recorded using photopolymer material. In the reconstruction, two projectors are used to project the elemental image arrays (EIA) with corresponding angles for two viewing zones. We have developed a prototype of the dual-view integral imaging display. The experimental results demonstrate the correctness of the theory.

Min-Yang He

Papers

Integral Imaging based 2D/3D Convertible Display System by Using Holographic Optical Element and Polymer Dispersed Liquid Crystal

Dual-view-zone tabletop 3D display system based on integral imaging.

Tabletop augmented reality 3D display system based on integral imaging

High-resolution augmented reality 3D display with use of a lenticular lens array holographic optical element

Dual-View Integral Imaging 3D Display Based on Multiplexed Lens-Array Holographic Optical Element