Improvement of grayscale representation of the horizontally scanning holographic display

Enlargements of Viewing Zone and Screen Size of Holographic Displays Using MEMS SLM Combined with Scanning Systems

The problems of conventional holographic display techniques, which are the requirements of a sub-micron pixel pitch and ultra-high resolution for spatial light modulators (SLMs) to enlarge the viewing zone and screen size, can be addressed using microelectromechanical systems (MEMS) SLMs combined with spatial scanning systems. Various scanning systems have been efficiently combined with high-speed image generation of MEMS SLMs based on the time-multiplexing technique. The horizontal scanning system enlarged the viewing zone and screen size, the circular scanning system provided 360° three-dimensional (3D) images, and the RGB scanning system generated color 3D images. The screen size can be increased scalably using a multichannel system based on the space-multiplexing technique. The use of a short laser pulse illumination system eliminates the mechanical scanning system and greatly simplifies the display system. The measurements of the accommodation responses of human eyes showed that 3D images generated by the screen scanning holographic display have a possibility to solve the visual fatigue issue caused by the vergence–accommodation conflict, which prevents the long-time usage of conventional 3D displays.

Holobricks: modular coarse integral holographic displays

Here, we propose and demonstrate a modular holographic display system that allows seamless spatial tiling of multiple coarse integral holographic (CIH) displays called "holobricks". A holobrick is a self-contained CIH module enclosing a spatial light modulator (SLM), a scanner, and periscopic coarse integral optics. Modular CIH uses a coarse pitch and small area but high-bandwidth SLM in conjunction with periscopic coarse integral optics to form the angularly tiled 3D holograms with large viewing areas and fields of view. The creation of periscopic coarse integral optics prevents the optical system from being larger than the holographic image and allows the holographic fringe pattern to fill the entire face of the holobrick. Thus, multiple holobricks can be seamlessly abutted to form a scalable spatially tiled holographic image display capable of both wide field-of-view angle and arbitrary large-size area. We demonstrate an initial prototype that seamlessly tiles two holobricks each with 1024 × 768 pixels, 40° FOV, full color, 24 fps, displaying 2D, 3D holographic stereograms, and full parallax 3D CGI Fresnel holograms.

Comparison of wavefront recording plane-based hologram calculations: ray-tracing method versus look-up table method

In this study, we compare the ray-tracing method with the look-up table (LUT) method in order to optimize computer-generated hologram (CGH) calculation based on the wavefront recording plane (WRP) method. The speed of the WRP-based CGH calculation largely depends on implementation factors, such as calculation methods, hardware, and parallelization method. Therefore, we evaluated the calculation time and image quality of the reconstructed three-dimensional (3D) image by using the ray-tracing and LUT methods in the central processing unit (CPU) and graphics processing unit (GPU) implementations. Thereafter, we performed several implementations by changing the number of object points and the distance from 3D objects to the WRP. Furthermore, we confirmed different characteristics between CPU and GPU implementations.

Addressable spatial light modulators for eye-tracking autostereoscopic three-dimensional display using a scanning laser

A scanning laser-based back light three-dimensional (3D) display capable of rendering full-resolution, low crosstalk, and vivid 3D depth perception has been developed by incorporating time-sequential multiplexing and eye-tracking technologies. This system includes three main subsystems: a scanning laser module, a relay transfer unit created by combining multiple transmissive-type electrically addressed ferroelectric liquid crystal spatial light modulators (FLC-SLMs), and a dual-directional transmission screen (DDTS) unit that can produce different angular magnification factors in both the tangential and sagittal planes. The light beam is directed by the DDTS after transmission through FLC-SLMs, and left and right eye viewing zones are produced sequentially in accordance with the locations of clear apertures in the FLC-SLM that are controlled based on data from the eye-tracking system. Owing to the persistence of human vision, 3D images are formed as a result of the high-speed scanning backlight and fast response characteristics of the FLC-SLM. A prototype of the proposed 3D display was designed and built, and experiments were carried out. The experimental results verify the feasibility of the proposed scheme, and full-resolution images with natural 3D perception are demonstrated by the prototype.

Computer-generated hologram generation method to increase the field of view of the reconstructed image

In this paper, a computer-generated hologram (CGH) generation method is proposed to increase the field of view (FOV) in the holographic display. The CGH is generated through accumulating interference patterns of all object points. The size of each interference pattern is equal to the sum of the size of the recorded object and the spatial light modulator (SLM), so the size of the interference pattern is increased. The position of the interference pattern is related to that of the corresponding recorded object point. In the reconstruction process, three SLMs in a planar configuration are used to load the CGH. The seams between the SLMs are eliminated by beam splitters. Meanwhile, the boundaries of the diffraction light of all interference patterns are parallel with each other. Compared with the holographic display method using one SLM, the experiments demonstrate that the proposed method can reduce the demand for viewing distance and expand the FOV obviously in the horizontal direction, especially for the large-sized recorded object.

Grayscale performance enhancement for time-multiplexing light field rendering

One of the common approaches to compensate for the grayscale performance limitation in time-multiplexing light field displays is to employ a halftone technique. We propose an ordered-dithering halftone algorithm based on a 3-dimension super-mask to increase the gray levels of the time-multiplexing light field display. Our method makes full use of the overlapping perceived pixels which are caused by the time-multiplexing design, such that effectively trading-off the spatial resolution and color performance. A real-time rendering time-multiplexing display prototype is built to validate the proposed halftone algorithm. We conducted a user study to evaluate the quality of display scenes dithered by different super-mask configuration, which showed the consistency with the parameters we pre-calculated. The 3D ordered-dithering algorithm is able to present better visual perception than the conventional halftone algorithms with respect to grayscale representation, and flexible to be applied in different time-multiplexing light field display systems.

Color image generation for screen-scanning holographic display

Horizontally scanning holography using a microelectromechanical system spatial light modulator (MEMS-SLM) can provide reconstructed images with an enlarged screen size and an increased viewing zone angle. Herein, we propose techniques to enable color image generation for a screen-scanning display system employing a single MEMS-SLM. Higher-order diffraction components generated by the MEMS-SLM for R, G, and B laser lights were coupled by providing proper illumination angles on the MEMS-SLM for each color. An error diffusion technique to binarize the hologram patterns was developed, in which the error diffusion directions were determined for each color. Color reconstructed images with a screen size of 6.2 in. and a viewing zone angle of 10.2° were generated at a frame rate of 30 Hz.

Table screen 360-degree holographic display using circular viewing-zone scanning

A table screen 360-degree holographic display is proposed, with an increased screen size, having an expanded viewing zone over all horizontal directions around the table screen. It consists of a microelectromechanical systems spatial light modulator (MEMS SLM), a magnifying imaging system, and a rotating screen. The MEMS SLM generates hologram patterns at a high frame rate, the magnifying imaging system increases the screen of the MEMS SLM, and the reduced viewing zones are scanned circularly by the rotating screen. The viewing zones are localized to practically realize wavefront reconstruction. An experimental system has been constructed. The generation of 360-degree three-dimensional (3D) images was achieved by scanning 800 reduced and localized viewing zones circularly. The table screen had a diameter of 100 mm, and the frame rate of 3D image generation was 28.4 Hz.

Viewing-zone scanning holographic display using a MEMS spatial light modulator

Horizontally scanning holography using a spatial light modulator based on microelectromechanical system, which we previously proposed for enlarging both the screen size and the viewing zone, utilized a screen scanning system with elementary holograms being scanned horizontally on the screen. In this study, to enlarge the screen size and the viewing zone, we propose a viewing-zone scanning system with enlarged hologram screen and horizontally scanned reduced viewing zone. The reduced viewing zone is localized using converging light emitted from the screen, and the entire screen can be viewed from the localized viewing zone. An experimental system was constructed, and we demonstrated the generation of reconstructed images with a screen size of 2.0 in, a viewing zone width of 437 mm at a distance of 600 mm from the screen, and a frame rate of 60 Hz.

Holographic 3D display observable for multiple simultaneous viewers from all horizontal directions by using a time division method

A holographic three-dimensional display system with a viewing angle of 360°, by using a high-speed digital micromirror device (DMD), has been proposed. The wavefront modulated by the DMD enters a rotating mirror tilted vertically downward. The synchronization of the rotating mirror and holograms displayed on the DMD allows for the reconstruction of a wavefront propagating in all horizontal directions. An optical experiment has been demonstrated in order to verify our proposed system. Binocular vision is realized from anywhere within the horizontal plane. Our display system enables simultaneous observation by multiple viewers at an extremely close range.

Improvement of gray-scale representation of horizontally scanning holographic display using error diffusion

Horizontally scanning holography can enlarge both screen size and viewing zone angle. A microelectromechanical-system spatial light modulator, which can generate only binary images, is used to generate hologram patterns. Thus, techniques to improve gray-scale representation in reconstructed images should be developed. In this study, the error diffusion technique was used for the binarization of holograms. When the Floyd-Steinberg error diffusion coefficients were used, gray-scale representation was improved. However, the linearity in the gray-scale representation was not satisfactory. We proposed the use of a correction table and showed that the linearity was greatly improved.

Viewing-angle enlargement in holographic augmented reality using time division and spatial tiling

Viewing angle enlargement is essential for SLM-based 3D holographic display. An idea of constructing equivalent-curved-SLM-array (ECSA) is proposed by linear phase factor superimposition. Employing the time division and spatial tiling (TDST) techniques, an ECSA-based horizontal 4f optical system is designed and built. The horizontal viewing angle of a single SLM is increased to 3.6 times when retaining the same hologram area. An interlaced holographic display technique is developed to remove the flicker effect. Holographic augmented reality is performed using the TDST system. Floating holographic 3D image with parallax and accommodation effects is achieved. Both TDST and interlaced technique may extend to multiple SLMs system to achieve larger viewing angle.

Speckle-free, shaded 3D images produced by computer-generated holography

A hologram display technique that provides speckle-free, shaded reconstructed images is proposed. A three-dimensional object consists of object points; these object points are divided into plural object point groups that are generated in a time-sequential manner. Each object point group consists of a two-dimensional (2D) array of object points that are separated so as to prevent interference among them. Each object point group is generated by displaying a 2D array of zone plates on a high-speed spatial light modulator (SLM). The amplitude distribution of the zone plates is modulated two-dimensionally based on Phong shading to shade the reconstructed images. The 2D amplitude distribution of the zone plates is decomposed into multiple binary patterns that are displayed by the SLM in a time-sequential manner. The proposed method is experimentally verified.

Vertical-bandwidth-limited digital holography

Optical scanning holography (OSH) is a promising technique to acquire a big-size digital hologram. However, the acquisition speed is limited by the mechanical scanner. In this Letter we apply the OSH in conjunction with an anisotropic low-pass filtering pupil to acquire vertical-bandwidth-limited (VBL) holograms. The size and the acquisition time of the VBL hologram can be reduced by one order of magnitude while the horizontal resolution remains the same as the conventional scanning hologram. The VBL hologram can be coded as an off-axis hologram without any postfiltering. Meanwhile, the full horizontal bandwidth of the displaying device can be capitalized.

Accommodation measurements of horizontally scanning holographic display

Eye accommodation is considered to function properly for three-dimensional (3D) images generated by holography. We developed a horizontally scanning holographic display technique that enlarges both the screen size and viewing zone angle. A 3D image generated by this technique can be easily seen by both eyes. In this study, we measured the accommodation responses to a 3D image generated by the horizontally scanning holographic display technique that has a horizontal viewing zone angle of 14.6° and screen size of 4.3 in. We found that the accommodation responses to a 3D image displayed within 400 mm from the display screen were similar to those of a real object.

Complex Fresnel hologram display using a single SLM

We propose a novel optical method to display a complex Fresnel hologram using a single spatial light modulator (SLM). The method consists of a standard coherent image processing system with a sinusoidal grating at the Fourier plane. Two or three position-shifted amplitude holograms displayed at the input plane of the processing system can be coupled via the grating and will be precisely overlapped at the system's output plane. As a result, we can synthesize a complex hologram that is free of the twin image and the zero-order light using a single SLM. Because the twin image is not removed via filtering, the full bandwidth of the SLM can be utilized for displaying on-axis holograms. In addition, the degree of freedom of the synthesized complex hologram display can be extended by involving more than three amplitude holograms.

Zone plate method for electronic holographic display using resolution redistribution technique

The resolution redistribution (RR) technique can increase the horizontal viewing-zone angle and screen size of electronic holographic display. The present study developed a zone plate method that would reduce hologram calculation time for the RR technique. This method enables calculation of an image displayed on a spatial light modulator by performing additions of the zone plates, while the previous calculation method required performing the Fourier transform twice. The derivation and modeling of the zone plate are shown. In addition, the look-up table approach was introduced for further reduction in computation time. Experimental verification using a holographic display module based on the RR technique is presented.

Speckle-free and grayscale hologram reconstruction using time-multiplexing technique

Speckle generation is an inherent problem of holography. A speckle-reduction technique employing a time-multiplexing method is proposed. Object points constituting a reconstructed image are divided into multiple object point groups consisting of sparse object points, and the object point groups are displayed time sequentially. The sparseness and temporal summation enable the suppression of speckle generation. The object point group is decomposed into multiple bit planes to represent the grayscale of object points, and binary holograms are generated from the bit plane patterns by using a half-zone plate technique. The binary holograms are displayed by a high-speed spatial light modulator.