Sunghee Hong

Seamless full color holographic printing method based on spatial partitioning of SLM

The holographic wavefront printer decodes the wavefront coming from a three-dimensional object from a computer generated hologram displayed on a spatial light modulator. By recording this wavefront as an analog volume hologram this printing method is highly suitable for realistic color 3D imaging. We propose in the paper spatial partitioning of the spatial light modulator to perform mosaic delivery of exposures at primary colors for seamless reconstruction of a white light viewable color hologram. The method is verified for a 3 × 3 color partitioning scheme by a wavefront printer with demagnification of the light beam diffracted from the modulator.

Holographic Printing of White-Light Viewable Holograms and Stereograms

Denis Gabor [1] invented the holographic method to improve the resolution of an electron microscope in 1948. Following the invention of the coherent light sources a decade later, the holographic techniques proved their unique potential in many fields as three-dimensional (3D) display technology [2], optical metrology [3], medicine [4], commerce etc. Among them, fullcolor and full parallax high resolution holographic printing as a technique for recording of 3D objects and scenes which are reconstructed under white light illumination is experiencing extensive development. The holographic printing, being a part of the research on 3D imaging of 3D objects from sampled data sets by holographic means, also followed the two main approaches in this area: i) computation of the holographic fringes by numerical simulation of the interference and encoding the resulting pattern onto a suitable medium for further optical display; ii) digital acquisition or computation of a set of discrete perspectives of a scene and their optical multiplexing in a holographic medium for building a stereoscopic pseudo 3D image.

Numerical reconstruction of a full parallax holographic stereogram with radial distortion

Successful commercialization of holographic printers based on holographic stereograms requires a tool for their numerical replaying and quality assessment before the time-consuming and expensive process of holographic recording. A holographic stereogram encodes 2D images of a 3D scene that are incoherently captured from multiple perspectives and rearranged before recording. This study presents a simulator which builds a full parallax and full color white light viewable holographic stereogram from the perspective images captured by a virtual recentering camera with its further numerical reconstruction for any viewer location. By tracking all steps from acquisition to recording, the simulator allows for analysis of radial distortions caused by the optical elements used at the recording stage. Numerical experiments conducted at increasing degree of pincushion distortion proved its insignificant influence on the reconstructed images in all practical cases by using a peak signal-to-noise ratio and the structural similarity as an image quality metrics.

Image-Quality Enhancement for a Holographic Wavefront Color Printer by Adaptive SLM Partitioning

The wavefront printer records a volume-reflection hologram as a two-dimensional array of elemental holograms from computer-generated holograms (CGHs) displayed on a spatial light modulator (SLM). The wavefront coming from the object is extracted by filtering in the spatial-frequency domain. This paper presents a method to improve color reproduction in a wavefront printer with spatial division of exposures at primary colors, by adaptive partitioning of the SLM in accordance with the color content encoded in the input CGHs, and by the controllable change of exposure times for the recording of primary colors. The method is verified with a color wavefront printer with demagnification of the object beam. The quality of reconstruction achieved by the proposed method proves its efficiency in eliminating the stripe artifacts that are superimposed on reconstructed images in conventional mosaic recording.

Comparison of System Properties for Wave-Front Holographic Printers

A conventional analog hologram displays highly realistic 3D images of objects. A holographic printer can record such a hologram onto a holographic emulsion from digital contents. Various holographic printers for recording a holographic stereogram, a fringe pattern or a wave-front have been developed for the recent two decades [1-4]. Multiple perspectives of the scene are used as contents for holographic stereogram recording, and an observer perceives a 3D object by binocular vision. However, the acquired perspectives have longitudinal magnification distortion caused by camera geometry that leads to the distorted 3D space representation over the displayed hologram. Thus the holographic stereogram may not be suitable for specific applications which require actual size display. The fringe printer records computer generated fringe patterns onto a holographic emulsion without any reference wave, and the printed hologram is a thin diffractive optical element which provides non-distorted 3D reconstruction but without color selectivity. The wave-front printing technique overcomes these drawbacks - longitudinal magnification distortion and lack of color selectivity – and makes a wave-front printer a desirable choice of a holographic printer. Two wave-front printers have been recently independently designed [1,2] and the aim of this paper is to analyze and compare their properties.

A Tangible Floating Display System for Interaction

A tangible floating display that can provide different perspective views without special glasses being introduced. The proposed system can display perspective floating images in the space in front of the system with the help of concave mirrors. In order to avoid wearing special equipment to interact and deliver the sense of touch, the proposed system adopted an ultrasound focusing technology. For providing an immersive experience to the viewers, the proposed system consists of a tangible floating display system and a multiple-view imaging system for generating three lenticular displays in front of the users.

Color Holographic Wavefront Printing Technique for Realistic Representation

This paper presents design and implementation of a color wavefront holographic printer. The printer output is a white light viewable volume hologram composed as a two-dimensional (2-D) array of elemental holograms. The three-dimensional (3-D) input data for the printer are encoded as computer generated holograms (CGHs) displayed in succession on an amplitude type spatial light modulator. The printer optical head extracts and demagnifies the beam with the 3-D object information from the beam diffracted by the modulator to produce a small size elemental hologram. This allows for mosaic delivery of exposures at primary colors with each elemental hologram acting as a single color channel. Computer generation of digital contents is accelerated by design of a phase-added stereogram approach based on partitioning of each CGH. Bright 3-D reconstruction with a motion parallax at saturated colors from holograms of test objects is presented. The holograms are printed on a silver-halide emulsion. The achieved high quality proves feasibility of recording analog color volume holograms from digital contents by mosaic delivery of the primary colors. Two quality enhancement approaches are proposed and tested.

Color holographic wave-front printing technique

The paper presents design and implementation of a color wavefront holographic printer for white light viewable holograms of three-dimensional (3D) objects from digital contents. The printed hologram was composed as a two-dimensional (2D) array of elemental holograms. The 3D information was encoded in computer generated holograms displayed in succession on an amplitude spatial light modulator. Filtering of the light beam diffracted from the modulator extracted the beam coming from the object. Its demagnified version was recorded onto the holographic emulsion. The small size elemental hologram made possible application of mosaic delivery of exposures at primary colors. A modified phase-added stereogram approach was proposed to accelerate computer generation of digital contents. We achieved bright 3D reconstruction with a motion parallax at saturated colors from holograms of test objects that were printed on a silver-halide emulsion. Two quality enhancement approaches were developed.

Three-dimensional imaging of cultural heritage artifacts with holographic printers

Holography is defined as a two-steps process of capture and reconstruction of the light wavefront scattered from three-dimensional (3D) objects. Capture of the wavefront is possible due to encoding of both amplitude and phase in the hologram as a result of interference of the light beam coming from the object and mutually coherent reference beam. Three-dimensional imaging provided by holography motivates development of digital holographic imaging methods based on computer generation of holograms as a holographic display or a holographic printer. The holographic printing technique relies on combining digital 3D object representation and encoding of the holographic data with recording of analog white light viewable reflection holograms. The paper considers 3D contents generation for a holographic stereogram printer and a wavefront printer as a means of analogue recording of specific artifacts which are complicated objects with regards to conventional analog holography restrictions.

Gaze contingent hologram synthesis for holographic head-mounted display

Development of display and its related technologies provides immersive visual experience with head-mounted-display (HMD). However, most available HMDs provide 3D perception only by stereopsis, lack of accommodation depth cues. Recently, holographic HMD (HHMD) arises as one viable option to resolve this problem because hologram is known to provide full set of depth cues including accommodation. Moreover, by virtue of increasing computational power, hologram synthesis from 3D object represented by point cloud can be calculated in real time even with rigorous Rayleigh-Sommerfeld diffraction formula. However, in HMD, rapid gaze change of the user requires much faster refresh rate, which means that much faster hologram synthesis is indispensable in HHMD. Because the visual acuity falls off in the visual periphery, we propose here to accelerate synthesizing hologram by differentiating density of point cloud projected on the screen. We classify the screen into multiple layers which are concentric circles with different radii, where the center is aligned with gaze of user. Layer with smaller radius is closer to the region of interest, hence, assigned with higher density of point cloud. Because the computation time is directly related to the number of points in point cloud, we can accelerate synthesizing hologram by lowering density of point cloud in the visual periphery. Cognitive study reveals that user cannot discriminate those degradation in the visual periphery if the parameters are properly designed. Prototype HHMD system will be provided for verifying the feasibility of our method, and detailed design scheme will be discussed.