Projection-type see-through near-to-eye display with a passively enlarged eye-box by combining a holographic lens and diffuser

Eyebox enlargement in holographic AR glasses

Despite the high output characteristics of holographic optical elements (HOEs), there are still no affordable holographic AR glasses that combine qualities such as a wide field of view (FOV) and a large eyebox (EB). In this study, we propose an architecture for holographic augmented reality glasses that covers both needs. Our solution is based on the combination of an axial HOE with a directional holographic diffuser (DHD) illuminated by a projector. A transparent-type DHD redirects the light from the projector, increasing the angular aperture of the image beams and providing a large EB. A reflection-type axial HOE redirects the light, transforming the spherical beams into parallel ones and providing a wide FOV for the system. The main feature of our system is the coincidence of the DHD position with the planar intermediate image of the axial HOE. This unique condition makes the system free of off-axial aberrations and ensures high output characteristics. The proposed system has a horizontal FOV of 60 deg and an EB width of 10 mm. We used modeling and a prototype to prove our investigations.

Multiplane holographic augmented reality head-up display with a real-virtual dual mode and large eyebox

We propose a multiplane augmented reality (AR) head-up display (HUD) with a real-virtual dual mode based on holographic optical elements (HOEs). The picture generation unit (PGU) is only a single free-focus projector, and the optical combiner includes a HOE lens (HOEL) for long-distance virtual image display and a HOE diffuser (HOED) for in-plane real image display. A HOED with directional scattering characteristics in the real image mode can significantly increase the size of the eyebox (EB) without increasing the size of the HOE, and a HOEL with a flexible design for the optical focal length in the virtual image mode can be used to achieve a different depth of the AR display. The proposed AR HUD system, which has a compact structure and offers high light transmittance, high energy usage, a multiplane display, and a large EB, is expected to be widely used in the future.

Compact near-eye display with a wide FOV and a wide eye box using 24 multiple-exposure holograms

A compact near-eye display with a 60° horizontal field of view, wide eye box of 5 mm, and high resolution of 720 p is proposed and developed by combining a transmission hologram that duplicates the beam of a scanning display and a reflection hologram that reflects duplicated beams toward the user's eye. The feasibility of the proposed near-eye display is demonstrated by examining the specifications and exposure of 24 multiple holograms. A compact NED that can display images with a horizontal FOV of 60° and that has a 6.2 mm × 4.8 mm eye box and 720 pixels vertical resolution is achieved.

Analysis on image quality of a holographic lens with a non-converging signal wave for compact near-eye displays

We analyze an image quality of a holographic lens (HL) in order to implement compact near-eye displays using a flat-panel-type micro-display panel. The proposed method utilizes a non-converging signal wave in a fabrication process of the HL, so that it provides affordable eye-box size with minimizing the aberration due to rays in the off-Bragg condition. For analyzing and optimizing the HL based on the non-converging signal wave, we introduce a comprehensive analysis model for an assessment of the image quality in the HL. The analysis model, inspired from the conventional lens design strategy for near-eye displays, evaluates the focal spot quality for incident rays forming each pixel with considering the on- and off-Bragg diffraction. The theoretical analysis is validated by simulation results using a volume hologram model in Zemax OpticStudio. As experimental verifications, we realize a prototype system using photopolymer-based HLs in a green color with the high transmittance of 89.3%. The image quality of the HLs is analyzed, which coincides well with the proposed analysis and assessment metric. By building a compact experimental setup employing the HL and a micro-organic light emitting diode display, we present see-through images with 8.0 mm of eye-box with reduced aberrations.

Design of off-axis reflective imaging systems based on freeform holographic elements

Holographic optical element (HOE) can be used in many areas in optics due to its characteristics of thin structure, flexible wavefront reconstruction/control ability and angular/wavelength selectivity. In this paper, we propose a design method of off-axis reflective imaging systems based on freeform HOEs, which are fabricated by freeform wavefronts. The freeform HOEs offer many degrees of design freedom and can correct the aberrations in nonsymmetric imaging systems. The initial imaging system with freeform HOEs is generated using a point-by-point design approach, and is used for the preliminary design of the imaging system and the freeform recording system of each HOE. Then a joint optimization is conducted for all the systems, simultaneously considering the imaging performance, the diffraction efficiency, the system constraints and fabrication to get the final design results. To validate the feasibility and effectiveness of the proposed method, an off-axis reflective head-up display system with good performance based on freeform HOEs is designed and fabricated. Detailed procedures of the design and development process of the prototype are demonstrated.

Resolution-preserving passive 2D/3D convertible display based on holographic optical elements

We propose and demonstrate a resolution-preserving passive 2D/3D convertible display by two individual wavelengths. It uses a holographic optical element to generate two images and passively separate the exit pupils for these two wavelengths, which forms two viewpoints for each of the observer's eyes. Due to Bragg-mismatched reconstruction of two similar but distinct wavelengths, the images are separated in space. They can be fused into one through the convergence function of human eyes. By switching the input image source, the conversion between 2D and 3D mode can be realized. This method is resolution-preserving and 2D/3D convertible with no extra active components. For experimental verification, a proof-of-concept projection-type prototype is assessed.