Single SLM full-color holographic 3-D display based on sampling and selective frequency-filtering methods

Analysis of reconstruction quality for computer-generated holograms using a model free of circular-convolution error

Continuous complex-amplitude computer-generated holograms (CGHs) are converted to discrete amplitude-only or phase-only ones in practical applications to cater for the characteristics of spatial light modulators (SLMs). To describe the influence of the discretization correctly, a refined model that eliminates the circular-convolution error is proposed to emulate the propagation of the wavefront during the formation and reconstruction of a CGH. The effects of several significant factors, including quantized amplitude and phase, zero-padding rate, random phase, resolution, reconstruction distance, wavelength, pixel pitch, phase modulation deviation and pixel-to-pixel interaction, are discussed. Based on evaluations, the optimal quantization for both available and future SLM devices is suggested.

Color dynamic holographic display based on complex amplitude modulation with bandwidth constraint strategy

In this Letter, we introduce a multiplexing encoding method with a bandwidth constraint strategy to realize a color dynamic holographic display based on complex amplitude modulation (CAM). The method first uses the angular spectrum method (ASM) with a bandwidth constraint strategy to calculate the diffracted wavefronts of red, green, and blue channels. Then the diffracted wavefronts of three channels are synthesized into a color-multiplexed hologram (CMH) based on the double-phase method after they interfere with off-axis reference lights. The color 3D objects can be reconstructed when the combination of red, green, and blue lights is used to illuminate the double-phase CMH, and a 4f system with a slit filter is introduced to extract the desired spectrums. Numerical simulations and optical experiments are performed to verify the effectiveness of the proposed method and the results show that it can achieve a color holographic display with high quality. Our proposal is simple and fast, and the display system is compact. It is expected that our proposed method could in future be widely used in the holographic field.

Review of computer-generated hologram algorithms for color dynamic holographic three-dimensional display

Holographic three-dimensional display is an important display technique because it can provide all depth information of a real or virtual scene without any special eyewear. In recent years, with the development of computer and optoelectronic technology, computer-generated holograms have attracted extensive attention and developed as the most promising method to realize holographic display. However, some bottlenecks still restrict the development of computer-generated holograms, such as heavy computation burden, low image quality, and the complicated system of color holographic display. To overcome these problems, numerous algorithms have been investigated with the aim of color dynamic holographic three-dimensional display. In this review, we will explain the essence of various computer-generated hologram algorithms and provide some insights for future research.

Direct binary search method for high-resolution holographic image projection

Complex-amplitude modulation of light fields with a digital micromirror device (DMD) has been widely used in holographic image projection. DMD is a binary-amplitude modulator, and its use for complex field modulation in a 4f configuration requires low-pass filtering. However, the reconstructed fields suffer from low resolution due to the limited bandwidth for the existing methods such as the Lee and superpixel methods. Here, we report a direct binary search (DBS) method to design high-resolution complex-amplitude holograms. The method is able to increase the spatial bandwidth up to twice that of the superpixel method. Numerical simulations and experiments are presented to demonstrate the method, which show that the errors are reduced by about 60% and 40% respectively for the test fields compared to the superpixel method. Furthermore, the measured efficiency of laser light can be improved by a maximum of 60%.

See-through holographic display with randomly distributed partial computer generated holograms

Holographic displays have the feature to show images out of the plane of the device itself, which is especially favored for augmented reality (AR) applications where the images need to be merged with the real world. In existing cases of AR holographic display, a combiner is used to converge the light path of the display image and surrounding scene toward the viewer's eye. In this paper, the idea of combining the holographic device and the combiner has been proposed, resulting in a see-through holographic display. In order to maintain the see-through quality of the device, the concept of partial hologram has been introduced, which means only a part of the area on the device has the holographic fringe pattern while leaving the rest fully transparent. Experiment and theoretical investigation shows that an evenly yet randomly distributed partial hologram provides the best holographic image quality assuming a fixed percentage of the holographic area on the device. A passive computer generated hologram (CGH) with two phase levels has been designed and fabricated for the verification. With partial hologram sharing 25% of the whole area, the CGH exhibits 90.9% of total transmission and 72.2% of parallel transmission. The demonstration shows a high see-through quality while providing a clear holographic image.

Compact full-color holographic 3-D display based on undersampled computer-generated holograms and oblique projection imaging

A compact full-color electro-holographic three-dimensional (3-D) display with undersampled computer-generated holograms (US-CGHs) and oblique projection imaging (OPI) is proposed. For its realization, undersampling conditions of the CGH enabling the complete recovery of image information are derived, and the OPI-based longitudinal-to-lateral depth conversion (LTL-DC) scheme allowing the simple reconstruction of full-color images is also proposed. Three-color off-axis US-CGHs are generated with their center-shifted principle fringe patterns (CS-PFPs) of the novel look-up table (NLUT) method, where center-shifts are calculated with the derived undersampling conditions of the CGH based on the generalized sampling theorem, and then multiplexed into the color-multiplexed hologram (CMH). The CMH is loaded on a SLM (spatial light modulator) and reconstructed by being illuminated with a multi-wavelength light source, where an original full-color image is reconstructed being spatially separated from the other color-dispersed images on the projected image plane with the OPI-based LTL-DC process, which enables us to view the original full-color image just with a simple filter mask. Performance analysis and successful experiments with the test 3-D objects in motion confirm the feasibility of the proposed system.

Astigmatism correction and quality optimization of computer-generated holograms for holographic waveguide displays

In this manuscript, the astigmatism of the waveguide combiner with a pair of symmetry HOEs was analyzed. The light field can be predicted by the modified convolution formulation of Fresnel diffraction when the information of light passes through the astigmatism causing element. Then the astigmatism can be corrected. The theory was experimentally proved by the system with a phase-only SLM and a diffraction planar waveguide. Furthermore, the image quality of astigmatism corrected phase-type CGHs can be improved via the iteration process. Since the coherence of light source was employed, the temporal averaging method was utilized to avoid speckle noise.

Full-color computer-generated holographic near-eye display based on white light illumination

We propose a full color computer generated holographic near-eye display (NED) based on white light illumination. The method inspired from color rainbow holography is used for calculation of 2D and 3D color holograms. The parameters of the color hologram calculation are designed based on the parameters of the spatial light modulator (SLM) with 4K resolution. A slit type spatial filter is designed in frequency domain to extract red, green and blue frequency components for full color display. A NED system including a white light source, an achromatic collimating lens, a 4K SLM, a 4f optical filtering system, and an achromatic lens as eyepiece is designed and developed. The main contribution of this paper is the first time to apply the rainbow holography concept to the dynamic full color NED with a compact display system. The optical experiments prove the feasibility of the proposed method.

Demonstration of a low-crosstalk super multi-view light field display with natural depth cues and smooth motion parallax

Due to lack of the accommodation stimulus, an inherent drawback for the conventional glasses-free stereoscopic display is that precise depth cues for the human monocular vision is rent, which results in the well-known convergence-accommodation conflict for the human visual system. Here, a super multi-view light field display with the vertically-collimated programmable directional backlight (VC-PDB) and the light control module (LCM) is demonstrated. The VC-PDB and the LCM are used to form the super multi-view light field display with low crosstalk, which can provide precisely detectable accommodation depth for human monocular vision. Meanwhile, the VC-PDB cooperates with the refreshable liquid-crystal display panel to provide the convergence depth matching the accommodation depth. In addition, the proposed method of light field pick-up and reconstruction is implemented to ensure the perceived three dimensional (3D) images with accurate depth cues and correct geometric occlusion, and the eye tracker is used to enlarge the viewing angle of 3D images with smooth motion parallax. In the experiments, the reconstructed high quality fatigue-free 3D images can be perceived with the clear focus depth of 13 cm in the viewing angle of ± 20°, where 352 viewpoints with the viewpoint density of 1 mm^-1 and the crosstalk of less than 6% are presented.

Performance enhancement of integral imaging based Fresnel hologram capturing by the intermediate view reconstruction

A method aiming at improving the performance of integral imaging (II) based Fresnel hologram is proposed, which is generated by using the intermediate view reconstruction (IVR). The conventional integral holograms are generally generated through Fourier transforming the elemental images (EI) of II into hogels. However, a trade-off between the angular resolution and the spatial resolution of II is inevitable within the generation of integral hologram. The IVR is introduced to enhance the angular spectrum of II-based Fresnel hologram while keeping a compact image size and being free from moving the lenslet array. Multiple elemental image array (EIA) sequences are generated with the IVR and transformed to the corresponding holograms. All the generated hologram sequences shift depending on the relative position of the virtual lens array and are added together to synthesize the Fresnel hologram with a high angular spectrum. The synthesized hologram can reconstruct the 3D image with the combined light fields of all the integral hologram sequences. Finally, both the simulation with multiple objects and experiments of real 3D object are numerically and optically conducted. The high matching results among them confirm this work a better performance over the conventional methods.

Implementation of full-color holographic system using non-uniformly sampled 2D images and compressed point cloud gridding

A multiple-camera holographic system using non-uniformly sampled 2D images and compressed point cloud gridding (C-PCG) is suggested. High-quality, digital single-lens reflex cameras are used to acquire the depth and color information from real scenes; these are then virtually reconstructed by the uniform point cloud using a non-uniform sampling method. The C-PCG method is proposed to generate efficient depth grids by classifying groups of object points with the same depth values in the red, green, and blue channels. Holograms are obtained by applying fast Fourier transform diffraction calculations to the grids. Compared to wave-front recording plane methods, the quality of the reconstructed images is substantially better, and the computational complexity is dramatically reduced. The feasibility of our method is confirmed both numerically and optically.

A Fast Computer-Generated Holographic Method for VR and AR Near-Eye 3D Display

A fast computer-generated holographic method with multiple projection images for a near-eye VR (Virtual Reality) and AR (Augmented Reality) 3D display is proposed. A 3D object located near the holographic plane is projected onto a projection plane to obtain a plurality of projected images with different angles. The hologram is calculated by superposition of projected images convolution with corresponding point spread functions (PSF). Holographic 3D display systems with LED as illumination, 4f optical filtering system and lens as eyepiece for near-eye VR display and holographic optical element (HOE) as combiner for near-eye AR display are designed and developed. The results show that the proposed calculation method is about 38 times faster than the conventional point cloud method and the display system is compact and flexible enough to produce speckle noise-free high-quality VR and AR 3D images with efficient focus and defocus capabilities.

Single SLM full-color holographic three-dimensional video display based on image and frequency-shift multiplexing

A single spatial-light-modulator (SLM) full-color holographic 3-D video display based on image and frequency-shift multiplexing (IFSM) is proposed. In the frequency-shift multiplexing (FSM), three-color holograms are multiplied with their respective phase factors for shifted-separations of their corresponding frequency-spectrums on the Fourier plane. This FSM process, however, causes three-color images to be reconstructed at the center-shifted locations depending on their multiplied phase factors. Center-shifts of those color images due to the FSM can be balanced out just by generation of three-color holograms whose centers are pre-shifted to the opposite directions to those of the image shifts with the novel-look-up-table (NLUT) based on its shift-invariance property, which is called image-shift multiplexing (ISM). These image and frequency-shifted holograms are then multiplexed into a single color-multiplexed hologram and loaded on the SLM, and from which a full-color 3-D image can be reconstructed on the optical 4-f lens system without any color dispersion just by employing a simple pinhole filter mask. Fourier-optical analysis and experiments with 3-D objects in motion confirm the feasibility of the proposed system.

Accelerated generation of holographic videos of 3-D objects in rotational motion using a curved hologram-based rotational-motion compensation method

A new curved hologram-based rotational-motion compensation (CH-RMC) method is proposed for accelerated generation of holographic videos of 3-D objects moving on the random path with many locally different arcs. All of those rotational motions of the object made on each arc can be compensated, just by rotating their local curved holograms along the curving surfaces matched with the object's moving trajectory without any additional calculation process, which results in great enhancements of the computational speed of the conventional hologram-generation algorithms. Experiments with a test video scenario reveal that average numbers of calculated object points (ANCOPs) and average calculation times for one frame (ACTs) of the CH-RMC-based ray-tracing, wavefront-recording-plane and novel- look-up-table methods have been found to be reduced by 73.10%, 73.84%, 73.34%, and 68.75%, 50.82%, 66.59%, respectively, in comparison with those of their original methods. In addition, successful reconstructions of 3-D scenes from those holographic videos confirm the feasibility of the proposed system.

Fast generation of full analytical polygon-based computer-generated holograms

A fast calculation method to obtain the full-analytical frequency spectrum of a spatial triangle based on the three-dimensional (3D) affine transformation is presented. Computer-generated holograms (CGHs) of an object can then be generated rapidly using the angular spectrum for propagation. The derivation process in the theory, which has more preciseness, indicates a difference from previous methods based on affine transformations ([Appl. Opt.47, 1567 (2008)Appl. Opt.52, A290 (2013)]). The proposed method to achieve 3D transformation from an arbitrary triangle to a primitive triangle includes two steps: 3D rotation and 2D affine transformation. The overall transform matrix is given by the product of a rotation matrix and a 2D affine matrix. A modified back-face culling is also introduced based on exterior normal for correct occlusion relation. Several complex 3D objects are implemented successfully using the proposed method in numerical simulations and optical experiments. The resulting computation time demonstrates that the efficiency of the proposed method is enhanced as compared to that of previous works.

Quality enhancement and GPU acceleration for a full-color holographic system using a relocated point cloud gridding method

The calculation of realistic full-color holographic displays is hindered by the high computational cost. Previously, we suggested a point cloud gridding (PCG) method to calculate monochrome holograms of real objects. In this research, a relocated point cloud gridding (R-PCG) method is proposed to enhance the reconstruction quality and accelerate the calculation speed in GPU for a full-color holographic system. We use a depth camera to acquire depth and color information from the real scene then reconstruct the point cloud model virtually. The R-PCG method allows us to classify groups of object points with the same depth values into grids in the red, green, and blue (RGB) channels. Computer-generated holograms (CGHs) are obtained by applying a fast Fourier transform (FFT) diffraction calculation to the grids. The feasibility of the R-PCG method is confirmed by numerical and optical reconstruction.