Holographic displays are widely regarded as the pinnacle of three-dimensional (3D) visualization technology. In these displays, real objects must be either photographed or converted into 3D models, which are then processed through neural networks or sophisticated algorithms to generate 3D holograms. To address this challenge, we propose an end-to-end 3D hologram generation strategy that integrates the Transport of Intensity Equation (TIE) phase retrieval technique with the Double Phase-Amplitude Coding (DPAC) method. Under coherent light illumination, phase-only holograms containing depth information can be directly generated by capturing out-of-focus amplitude maps of object light waves propagating to the holographic plane via a camera. The TIE module processes the two out-of-focus amplitude maps to resolve the phase and subsequently generates a phase-only hologram through DPAC. We further conduct simulations to validate the phase retrieval capability of the TIE on complex holograms and demonstrate the feasibility of our proposed strategy.
In holographic near-eye displays, enhancing the user experience by expanding the eyebox without compromising the field of view (FOV) is crucial. Current technologies face limitations due to optical etendue, making it difficult to simultaneously achieve a large eyebox and a wide FOV. This paper presents a novel portable augmented reality holographic near-eye display system that expands the exit pupil without reducing the FOV, using exit pupil scanning technology. The system replaces conventional eyepieces and beam splitters with holographic optical elements, employs point light source illumination instead of collimated illumination, and utilizes an off-axis angular spectrum diffraction propagation model between parallel planes tailored to human visual characteristics. This approach effectively mitigates the trade-off between FOV and eyebox. Compared to traditional systems, the proposed design resolves this trade-off in simulations and reduces the form factor, offering a promising new approach for practical holographic near-eye display applications.
Holography always records the 3D object information via interference and then reconstructs the 3D object via diffraction. A holographic optical element (HOE) can be fabricated by analog holographic exposure or digital holographic printing. Since HOE can be recorded on a thin plate or film of holographic material, it is always utilized as a combiner for augmented reality near-eye displays to achieve a compact form factor. The traditional method of HOE fabrication is to divide a coherent light beam into both a signal beam and a reference beam, and then make both beams meet at the holographic material for interference. The optical path of the traditional fabrication system employs more optical devices with complex optical designs. Therefore, a HOE fabrication system based on common optical path interference is proposed in this paper. This paper presents the eyeglass-style HOE fabrication method based on common optical path interference. The HOE fabrication system with a common optical path is designed with fewer optical devices and less vibration effect. As the reference beam, the incident beam illuminates the hologram film. Part of the incident beam passes through the film and is reflected by the curved mirror to illuminate the hologram film from the other side, which works as the signal beam. The crucial device of the proposed system is the curved mirror. We analyzed the effects on HOE fabrication using an off-axis parabolic mirror and a spherical mirror. The experiment finally fabricated the HOEs and verified them for the AR near-eye display.
In holographic near-eye displays, the generation of computational holograms requires using various free-space propagation numerical methods. Traditional free-space propagation is mainly focused on the propagation calculation between parallel planes, which is widely utilized in on-axis holographic displays. But for practical usage of the wearable holographic near-eye displays, the propagation between non-parallel planes is crucial for the hologram generation of off axis projection to the holographic-optical-element (HOE) based combiner. However, at current few methods are reported for generating holograms with non-parallel-plane propagation. This paper proposes a novel method with Wirtinger derivatives to solve the problem of phase hologram retrieval between non-parallel planes. Using the Wirtinger derivatives can transfer the hologram phase retrieval to a quadratic problem. With different loss functions and stochastic optimization methods, this quadratic problem can be minimized by first-order optimization, resulting in a phase-only hologram of the tilted plane. The proposed method using the Wirtinger derivatives to calculate the tilted plane phase hologram can improve image quality significantly. The proposed method can achieve better PSNR and lower computational cost in the simulation than the traditional GS algorithm. The diffraction calculation of the tilted plane can further extend the propagation from two-dimensional(2D) to three-dimensional(3D), which will provide a new way for the hologram generation of 3D scenes.
Currently most of the holographic waveguide near-eye display are based on flat waveguide structure. It is always required to add curved goggles for head mounted display (HMD) or helmet display. And consequently, the human eyes have to see through both the flat waveguide and the curved goggles when viewing see-through background. It suffers from bulky form-factor, low transmittance and limited application. Therefore, this paper proposes a near-eye display system based on curved holographic waveguide, which integrates both flat waveguide and curved goggles. This design includes cylindrical holographic waveguide and corresponding micro projection module. To ensure the rays transmitted regularly in the waveguide, we designed special holographic optical elements (HOEs) attached on the surface of cylindrical waveguide to achieve large exit pupil and field of view (FOV). Besides, the aspheric lenses in the micro projection module are employed to correct aberration caused by cylindrical waveguide. This novel design of the proposed HMD with cylindrical waveguide can achieve a binocular FOV of about 42° in diagonal, an eye relief of 15mm and exit pupil of 10mm×10mm. For the central FOV, the modulation transfer function is close to the diffraction limit, and the intensity is uniformly distributed at the exit pupil with high light efficiency. For the edge FOV, the image quality is still good and suitable for the human eye. This novel design integrates the waveguide with curved goggles into one curved waveguide based near-eye display, which can provide a new approach for portable near-eye display applied in the helmet or some other related usages.
In this paper, a novel method with high-speed angular steering device is presented to realize high angular resolution for the multi-projection based multi-view and light field 3D displays. The state-of-art multi-projection based multi-view and light field 3D displays always employ projector array with multi-layer structure to achieve high angular resolution, but the projector’s physical size still limits the angular resolution of 3D displays. To enhance the angular resolution of multiview and light field displays, a special designed angular steering optical device is integrated into the screen part of the display system, which is capable to deflect rays in a limited angular range to increase the density of projectors equivalently. A high-speed angular steering device is developed to deflect rays in a small angle based on the birefrigent effect of liquid crystal (LC), which mainly includes cascade structure of the pi cell LC and the birefringent LC microprism array. Furthermore, the steering angle control of this device can be well synchronized with the frame rate of projector array. With time-multiplexing of the steering angle, high angular resolution of multi-view and light field 3D displays with single-layer projector array are implemented. In our experiments, the developed angular steering device with steering angles of 0° and ±1° was successfully integrated into twelve-projector based multi-view and light field display systems to enhance the angular resolution respectively. The experimental results show that the angular resolution of the two different types of 3D displays is significantly enhanced to provide better visual experience for users.
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