Resolution enhancement of holographic printer using a hogel overlapping method

Analog-to-digital conversion of information archived in display holograms: I. discussion

This discussion paper highlights the potential of display holograms in the storage of information about objects' shape. The images recorded and reconstructed from holograms have high visual appeal, and the holographic carrier has far higher information capacity than other storage media. One hindrance to the application of display holograms is the inadequate development of techniques for digitizing information from them, which is compounded by insufficient analysis and discussion of existing approaches. In this review, we provide a historical retrospective of the use of display holography to save comprehensive information on object morphology. We also discuss existing and emerging technologies for converting information into a digital format, addressing one of the most serious challenges to the widespread use of display holography. Potential applications of these technologies are also analyzed.

Holographic printing for generating large-angle freeform holographic optical elements

We propose a holographic printing technique for generating highly efficient large-deflection-angle freeform holographic optical elements (HOEs). For industrial device applications, the optical efficiency and deflection angle of HOEs are critical. To fabricate a high-frequency volume grating in a hogel, we design an optomechanical hogel recording system with a high angle deflection capability, which contrasts with the conventional printing scheme, the wavefront holographic printing technique featuring a paraxial deflection angle. With the proposed system, a large-deflection-angle HOE is experimentally demonstrated, and short-throw holographic caustic projection patterns are realized.

Simplified digital content generation based on an inverse-directed propagation algorithm for holographic stereogram printing

Holographic stereogram (HS) printing requires extensive memory capacity and long computation time during perspective acquisition and implementation of the pixel re-arrangement algorithm. Hogels contain very weak depth information of the object. We propose a HS printing system that uses simplified digital content generation based on the inverse-directed propagation (IDP) algorithm for hogel generation. Specifically, the IDP algorithm generates an array of hogels using a simple process that acquires the full three-dimensional (3D) information of the object, including parallax, depth, color, and shading, via a computer-generated integral imaging technique. This technique requires a short computation time and is capable of accounting for occlusion and accommodation effects of the object points via the IDP algorithm. Parallel computing is utilized to produce a high-resolution hologram based on the properties of independent hogels. To demonstrate the proposed approach, optical experiments are conducted in which the natural 3D visualizations of real and virtual objects are printed on holographic material. Experimental results demonstrate the simplified computation involved in content generation using the proposed IDP-based HS printing system and the improved image quality of the holograms.

Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography

We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik’s coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 μm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.

Efficient Hogel-Based Hologram Synthesis Method for Holographic Stereogram Printing

With the development of the holographic printer, printing synthetic hologram requires smaller holographic element (hogel) size to improve spatial resolution of the reconstruction. On the contrary, a larger hogel size affords higher angular resolution, but it leads to a lower lateral resolution and there exists a trade-off problem. In this paper, a hologram synthesis method based on three-dimensional (3D) rendering of computer-generated holographic stereogram (HS) is proposed to limit the spatial-angular trade-off problem. The perspectives of the 3D scene are captured by re-centering the camera method and transformed into parallax-related images by a proposed pixel re-arrangement algorithm for holographic printing. Unlike the conventional approaches, the proposed algorithm not only improves the angular resolution of the reconstruction while maintaining the hogel size fixed, but also keeps the spatial resolution without degradation. The effectiveness of the proposed method is verified by numerical simulation and an optical experiment.

Holographically customized optical combiner for eye-box extended near-eye display

We propose a customizing method of a holographic optical element (HOE) by using a holographic printer, which extends the eye-box with high field of view (FOV) for a holographic augmented reality near-eye display (AR NED). The holographic printer setup to manufacture HOE is presented and a prototype of the AR NED is implemented. To make a simple AR NED system, we propose a total internal reflection holographic printing method using an index-matching optical frame. As a result, the eye-box of the AR NED is extended in both vertical and horizontal directions and FOV of 50° is achieved at the center of the eye-box. Through the simulations and the experimental results, the feasibility of the proposed method is verified.

Centered-camera-based effective perspective images' segmentation and mosaicking method for full-parallax holographic stereogram printing

The centered-camera-based effective perspective images' segmentation and mosaicking (CCEPISM) method is proposed to improve the previous EPISM-based printing of full-parallax holographic stereograms. The idea of pixel mapping from target camera images to centered camera images is analyzed, and backward pixel mapping as well as forward pixel mapping are modeled and formulated to transform the perspective images captured by different camera strategies. The principle of the proposed CCEPISM is introduced in detail along with its specific implementation. The experimental results validate the proposed method and demonstrate that CCEPISM is an effective method for printing full-parallax holographic stereograms, and the quality of optical reconstruction is the same as that of EPISM-based holographic stereograms. The whole 3D scene is captured by a centered camera image, and the pixel waste caused by simple camera capture and the number of sampled perspective images is reduced significantly.

Analysis on the reconstruction error of EPISM based full-parallax holographic stereogram and its improvement with multiple reference plane

To reduce the reconstruction error of holographic stereogram fabricated by effective perspective images' segmentation and mosaicking method (EPISM), a multiple-reference-plane (MRP) approach is proposed and validated. The reconstruction error for traditional EPISM is analyzed, and the results indicate that the distortion as well as the blur will be involved for object points located far away from the reference plane. A new method by introducing multiple reference planes is proposed, which divides the 3D scene into several parts along its depth direction, and sets a reference plane for each of the object part. By resynthesizing all the effectively synthetic perspective images referred to their own reference planes of the object parts, the finally effectively synthetic perspective image exposed to one holographic elemental by only once exposure is generated. The optically experimental results demonstrate the validity of the proposed method, and the reconstruction error of full-parallax holographic stereogram printed by MRP based EPISM can be reduced evidently while the displayed depth range of 3D scene can be extended, compared to the traditional EPISM approach.

Holographic Element-Based Effective Perspective Image Segmentation and Mosaicking Holographic Stereogram Printing

Effective perspective image segmentation and mosaicking (EPISM) method is an effective holographic stereogram printing method, but a mosaic misplacement of reconstruction image occurred when focusing away from the reconstruction image plane. In this paper, a method known as holographic element-based effective perspective image segmentation and mosaicking is proposed. Holographic element (hogel) correspondence is used in EPISM method as pixel correspondence is used in direct-writing digital holography (DWDH) method to generate effective perspective images segments. The synthetic perspective image for holographic stereogram printing is obtained by mosaicking all the effective perspective images segments. Optical experiments verified that the holographic stereogram printed by the proposed method can provide high-quality reconstruction imagery and solve the mosaic misplacement inherent in the EPISM method.

Characteristic and improvement on the reconstructed quality of effective perspective images' segmentation and mosaicking-based holographic stereogram

Based on the effective perspective images' segmentation and mosaicking (EPISM) printing method, the influence of the holographic element (hogel) size on the reconstructed quality is analyzed to improve the reconstructed quality of holographic stereogram. The flipping effect and diffraction effect in the spatial domain are also discussed, and the optical transfer function of the holographic stereogram is used in the spectrum domain to evaluate the reconstructed quality. Theoretical analysis and optical experimental results show that the hogel size plays an important role on the flipping effect as well as the clarity of the reconstrued 3D perspective images of the EPISM-based holographic stereogram, and the flipping effect can be improved significantly with optimized hogel size.

Off-axis virtual-image display and camera by holographic mirror and blur compensation

We propose off-axis virtual-image display and camera systems, which integrate a vertically-standing holographic off-axis mirror, blur-compensation optical systems, and digital imaging devices. In the system, the holographic mirror is used for an off-axis reflector, which realizes an upright and thin screen for virtual-image formation. By combining it with a display unit, an off-axis virtual-image display is realized, where the virtual image can be seen behind the upright holographic mirror. Simultaneously, by combining it with a camera unit, an off-axis camera is implemented, which realizes frontal shooting of objects by a camera placed at an off-axis position. Since both the off-axis display and the camera can be implemented by a single holographic mirror, it can be applied to a two-way visual-telecommunication system with a thin screen, which implements eye contact and the observer--image distance. A problem with the proposed system is image blur, which is caused by the chromatic dispersion of the holographic mirror. To solve this, we designed optical blur-compensation systems using a diffractive optical element and a diffuser or a lens. Experimental results verify the concept of the proposed systems with clarifying the effect of designed blur-compensation methods.

Characteristic and optimization of the effective perspective images' segmentation and mosaicking (EPISM) based holographic stereogram: an optical transfer function approach

Based on our proposed method for holographic stereogram printing using effective perspective images’ segmentation and mosaicking (EPISM), we analyze the reconstructed wavefront errors, and establish the exit pupil function model of proposed printing system. To evaluate the imaging quality, the optical transfer function (OTF) of the holographic stereogram is modelled from the aspect of frequency response. The characteristic of the OTF with respect to the exit pupil size and the aberration are investigated in detail. We also consider the flipping effect in spatial domain. The optimization of hogel sizes, i.e., the sampling interval of original perspective images and the printing interval of synthetic effective perspective images, are given for the optimized reconstruction. Numerical simulations and optical experiments are implemented, and the results demonstrate the validity of our analysis, and the optimized parameters of hogel sizes can improve the imaging quality of full parallax holographic stereogram effectively.

Method of single-step full parallax synthetic holographic stereogram printing based on effective perspective images' segmentation and mosaicking

With the principle of ray-tracing and the reversibility of light propagation, a new method of single-step full parallax synthetic holographic stereogram printing based on effective perspective images' segmentation and mosaicking (EPISM) is proposed. The perspective images of the scene are first sampled by a virtual camera and the exposing images, which are called synthetic effective perspective images, are achieved using the algorithm of effective perspective images' segmentation and mosaicking according to the propagation law of light and the viewing frustum effect of human eyes. The hogels are exposed using the synthetic effective perspective images in sequence to form the whole holographic stereogram. The influence of modeling parameters on the reconstructed images are also analyzed, and experimental results have demonstrated that the full parallax holographic stereogram printing with the proposed method could provide good reconstructed images by single-step printing. Moreover, detailed experiments with different holographic element sizes, different scene reconstructed distances, and different imaging planes are also analyzed and implemented.

One-step real-virtual combined reflection hologram: a 4f relay approach

The production of conventional optical reflection holograms can be classified into the one-step method or the two-step method. In the one-step method, only the diverging light of an object can be recorded, and the reconstructed scene is a virtual one behind the recording medium. In the two-step method, the diverging light or the converging light can be recorded alternatively. However, the process is complicated considering double exposures. The object is first imaged by a 4f system, and then the interference patterns are recorded by single exposure. The reconstructed image can be either a virtual image behind the recording medium or a real image in front of the recording medium. The ideal imaging property of a 4f system has been demonstrated theoretically, and the proposed method has been verified experimentally.

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.

3D holographic printer: fast printing approach

This article describes the general operation principles of devices for synthesized holographic images such as holographic printers. Special emphasis is placed on the printing speed. In addition, various methods to increase the printing process are described and compared.

Vignetting-free computer-generated Fresnel holograms by mask shifting

In computer-generated Fresnel holography, direct sampling (DS) and simple shading (SS) are two common ways to generate sampled Fresnel zone plates (FZPs) on the hologram plane. Nevertheless, either aliasing or vignetting, or both, will occur in the reconstructed image when the DS method or the SS method is applied. To avoid vignetting together with aliasing in the two sampling methods, either the object size or the object distance must be restricted in generating the holograms. In this paper we propose a mask-shifting (MS) method to generate the sampled FZPs. The main concept of the MS method is that the center of the FZP can be shifted relative to the center of the mask against the FZP when the FZP is at the margin of the hologram. The shifting of the mask will result in only a phase shift and will not change the intensity distribution of the reconstructed point. Thus, by using the MS method, aliasing and vignetting are simultaneously alleviated in any combination of object size and object distance.