Hyon-Gon Choo

360-degree tabletop electronic holographic display

We demonstrate a tabletop holographic display system for simultaneously serving continuous parallax 3.2-inch 360-degree three-dimensional holographic image content to multiple observers at a 45-degree oblique viewing circumference. To achieve this, localized viewing windows are to be seamlessly generated on the 360-degree viewing circumference. In the proposed system, four synchronized high-speed digital micro-mirror displays are optically configured to comprise a single 2 by 2 multi-vision panel that enables size enlargement and time-division-multiplexing of holographic image content. Also, a specially designed optical image delivery sub-system that is composed of parabolic mirrors and an aspheric lens is designed as an essential part for achieving an enlarged 3.2-inch holographic image and a large 45-degree oblique viewing angle without visual distortion.

Hybrid approach for accurate depth acquisition with structured light and stereo camera

In this paper, we propose a hybrid approach for accurate depth acquisition by using a structured light-based method with a stereo matching. By projecting additional light patterns onto a scene, a structured light-based method works well on a textureless region where a stereo matching shows poor performance. In contrast, the patterns projected onto a rich textured region obstruct in estimating reliable depth information in the structured light-based method, while a stereo matching excels. We exploit these complementary characteristics by combining the results from both methods that outperform the one from either alone. In our fusion framework, a hybrid stereo matching is introduced, in which the disparity search range for each pixel is limited based on the initial depth map obtained by the structured light-based method. In addition, we introduce a confidence-based fusion method which combines the depth maps, while incorporating the advantages of each method. The experimental results show that the proposed method achieves to estimate accurate depth information, while other methods fail.

Effects of Depth Map Quantization for Computer-Generated Multiview Images using Depth Image-Based Rendering

This paper presents the effects of depth map quantization for multiview intermediate image generation using depth image-based rendering (DIBR). DIBR synthesizes multiple virtual views of a 3D scene from a 2D image and its associated depth map. However, it needs precise depth information in order to generate reliable and accurate intermediate view images for use in multiview 3D display systems. Previous work has extensively studied the pre-processing of the depth map, but little is known about depth map quantization. In this paper, we conduct an experiment to estimate the depth map quantization that affords acceptable image quality to generate DIBR-based multiview intermediate images. The experiment uses computer-generated 3D scenes, in which the multiview images captured directly from the scene are compared to the multiview intermediate images constructed by DIBR with a number of quantized depth maps. The results showed that there was no significant effect on depth map quantization from 16-bit to 7-bit (and more specifically 96-scale) on DIBR. Hence, a depth map above 7-bit is needed to maintain sufficient image quality for a DIBR-based multiview 3D system.

Depth and texture imaging using time-varying color structured lights

In this paper, a depth and texture imaging method using time-varying color structured lights is introduced. Three structured patterns are periodically projected on a scene and captured with our system to produce depth and texture images simultaneously. The color difference between three patterns is used to identify the projected patterns color, and the texture image can be simultaneously obtained by adding the colors of the patterned images. The experimental results show that the proposed method is suitable for depth and texture imaging for static and slow moving objects.

Histogram based hologram binarization for DMD application

In the paper we investigate hologram binarization method through time multiplexing based on histogram. In the proposed approach input object is divided into N components with equal total intensity distributions. Next, propagation and binarization procedure is employed for each component separately. Presented method is matched to DMD application since it modulates the input light simply by reflection. In the computer simulations the quality of the reconstructed holograms with the proposed method is compared with that of the threshold method for different reconstruction distances. Additionally, efficiency of the technique is verified experimentally during optical reconstructions performed in the holographic display with DMD and LED illumination.

Viewing window position control on holographic projection system by electrically focused tunable lens

Space bandwidth product (SBP) is one of the most significant limitation for displaying the digital holographic display. Due to the SBP problem, the size and viewing angle of displayed holograms cannot be enlarged simultaneously. To overcome the SBP problem, holographic projection system has been researched. It uses a field lens to converge diffracted light from a spatial light modulator (SLM) into a viewing window, where the observer can see whole hologram image displayed on the SLM. However, it has a problem that the viewing distance between the display and observer cannot be controlled and fixed on the viewing window plane. We propose a method to control the position of viewing window formation in the holographic projection system by using an electrically focus tunable lens. We added the focus tunable lens in the holographic projection system, and the position of the viewing window can be controlled by its lens power variation. The principle of controlling viewing window in the proposed system is described, and the relationship among the optical power of focus tunable lens, location and size of the viewing window is analyzed. A computer generated hologram encoding based on Fresnel diffraction theory is developed to generate hologram contents for the proposed system with consideration of varying optical power of the focus tunable lens. Test-bed is built to verify the feasibility of the proposed method, and the experimental results confirm that the proposed method can effectively control the viewing window position of the holographic projection system.

Image blur and visual perception for rainbow holographic display

The rainbow hologram provides observation of the reconstructed object with different spectra over different viewing position. Recently, we’ve proposed a concept of digital rainbow holographic display using diffraction grating and white LED lighting source. In the technique, the slit is implemented numerically by reducing the frequency of the hologram, while the rainbow effect is realized by dispersion of white light source on the diffraction grating. Phase only SLM with 4F imaging system is used for implementation of complex wave fields. For classical rainbow hologram, image blur is known to be very important key point regarding holographic image quality. In this paper, we analyze image blur and visual perception for digital rainbow holographic display. The quality of reconstructed rainbow holograms is investigated under varying viewing conditions regarding visual perception and depth resolution. In experiments, the visual properties of the digital rainbow hologram are analyzed using optical reconstructions for the hologram of 3D and 2D objects of different depth.

Fourier Rainbow Holography

We present Fourier rainbow holographic imaging approach. It involves standard laser holographic recording and novel horizontal parallax only holographic display. In the display, the rainbow effect is introduced in an illumination module by high-frequency diffraction grating and white light LED source. The display is addressed by Fourier rainbow digital hologram (FRDH) encoding defocused object field with removed spatial frequency components in one direction by hologram slitting and without spherical phase factor. Theoretically and experimentally it is shown that the method extends the viewing zone of the classical viewing window display in vertical and longitudinal directions, thus the comfort of observation is improved. It is also numerically and experimentally validated that the numerical slitting applied within FRDH generation improves reconstruction depth of the display, here up to 400 mm.

An optical method for compensating phase discontinuity in a 360-degree viewable tabletop digital holographic display system

In this paper, we use an optical method for the implementation of spatially-tiled digital micro-mirror devices (DMDs) to expand space bandwidth product in general digital holographic display systems. In concatenating more than two spatial light modulators (SLMs) optically, there may exist both phase discontinuity and amplitude mismatching of hologram images emanating from two adjacent SLMs. To observe and estimate those properties in digital holographic display systems, we adopt quantitative phase imaging technique based on transport of intensity equation.

Analysis on digital holographic data representation and compression

Holography is a three-dimensional (3D) imaging technique to reconstruct wave field of the light scattered by real objects. In this paper, we present two representative digital holographic data representations, 3D data-based representation and hologram-based representation, and compare compression efficiency of them.