Voxel characteristic estimation of integral imaging display system using self-interference incoherent digital holography.

Three-dimensional (3D) images reconstructed by integral imaging display are captured as a complex hologram using self-interference incoherent digital holography (SIDH) and analyzed for the volumetric image characteristics. The integrated images can present 3D perception through not only binocular disparity but also volumetric property, which is represented in forming a volume picture element, called 'voxel', and an important criterion to distinguish the integral imaging from the multiview 3D display. Since SIDH can record the complex hologram under incoherent lighting conditions, the SIDH camera system has the advantage to measure the voxel formed with the incoherent light fields. In this paper, we propose a technique to estimate and analyze the voxel characteristics of the integral imaging system such as the depth location and resolution. The captured holograms of the integrated images are numerically reconstructed by depth for the voxel analysis. The depth location of the integrated image can be calculated and obtained using the autofocus algorithms and the focus metrics values, which also show the modalities of depth resolution. The estimation method of this paper can be applied to the accurate and quantitative analysis of the volumetric characteristics of light field 3D displays.

Wide-viewing holographic stereogram based on self-interference incoherent digital holography.

We propose a holographic stereogram synthesis method which uses holograms that are optically captured by self-interference incoherent digital holography (SIDH) based on a geometric phase lens. SIDH is a promising solution for hologram acquisition under low-coherence lighting condition. A mechanical scanning system is constructed to acquire multiple perspective holograms. Numerical simulations and experimental analyses conducted using high-resolution diffractive optical element demonstrate that the proposed method can produce a wide-viewing hologram which can realize realistic 3D scenarios with depth cues such as accommodation and motion parallax. The future objectives include the implementation of a multiple-camera system for holographic videos.

Digital Incoherent Compressive Holography Using a Geometric Phase Metalens.

We propose a compressive self-interference incoherent digital holography (SIDH) with a geometric phase metalens for section-wise holographic object reconstruction. We specify the details of the SIDH with a geometric phase metalens design that covers the visible wavelength band, analyze a spatial distortion problem in the SIDH and address a process of a compressive holographic section-wise reconstruction with analytic spatial calibration. The metalens allows us to realize a compressive SIDH system in the visible wavelength band using an image sensor with relatively low bandwidth. The operation of the proposed compressive SIDH is verified through numerical simulations.

Design of ghost-free floating 3D display with narrow thickness using offset lens and dihedral corner reflector arrays.

A floating 3D display with a dihedral corner reflector array (DCRA) is presented to improve space efficiency and eliminate ghost images. Floating displays using a DCRA have the space efficiency problem of having a system thickness equal to the height of the floating image and the problem of a ghost image interrupting the visibility of the floating display. The DCRA is analyzed to find the ghost image region. Based on the analysis, an off-axis integral floating display is placed outside the ghost image region to avoid the ghost image. To increase space efficiency, the optical path is folded using a mirror. In addition, the off-axis integral floating display is used to create a tilt angle for projecting the input image onto the DCRA in a proposed confined and narrow system to observe the complete 3D image. The effectiveness of the system was verified through simulations and experiments.

Michelson-interferometric-configuration-based incoherent digital holography with a geometric phase shifter.

The phase-shifting method is a simple and efficient approach to extract complex hologram information free of bias and twin-image noise. In this study, the geometric phase-shifting method is utilized for a self-interference incoherent digital holographic recording system based on the Michelson-type interferometer. The phase-shifting module consists of a horizontal polarizer, and two achromatic quarter-wave plates are employed inside the interferometer, replacing conventional phase-shifting devices, such as the piezo-actuated mirror. Since the phase-shifting amount of the introduced method herein is theoretical, regardless of the input wavelength, the simultaneous recording of step-wise phase-shifted interferograms for different color channels is available. Therefore, the multi-color hologram recording is achieved with fewer numbers of exposures. The demonstration of multi-color hologram recording and reconstruction are presented to validate the proposed idea.

Compact self-interference incoherent digital holographic camera system with real-time operation.

The video recording-capable compact incoherent digital holographic camera system is proposed. The system consists of the linear polarizer, convex lens, geometric phase lens, and the polarized image sensor. The Fresnel hologram is recorded by this simple configuration in real time. The system parameters are analyzed and evaluated to record a better-quality hologram in a compact form-factor. The real-time holographic recording and its digitally reconstructed video playback are demonstrated with the proposed system.

Monocular depth estimation method using a focus tunable lens.

We propose a method for the passive-type estimation of depth using a focus tunable lens. The proposed method utilizes a lens group and a focus tunable lens and charge coupled device. The target object is imaged by the group of lenses and measured by changing the focal length of the focus tunable lens. The method aims to measure depth information in the image domain and not in the object domain. An autofocusing algorithm finds the best focus position of the target object through the focus value calculated by the Sobel operator. We believe that the proposed method is applicable to depth measurement systems because it offers a simple configuration without any active light source and can operate in real time. The experiment, performed for comparison with theoretical calculations, confirms the feasibility of the proposed method.

Focus-free head-mounted display based on Maxwellian view using retroreflector film.

In this study, a focus-free retinal projection display based on a Maxwellian view was implemented. The display was constructed using simple and cost-effective passive components, such as a retroreflector film, beam splitter, and a pinhole. As a part of the system, the pinhole and display serve as a pinhole projector, which projects images through a small exit pupil. The retroreflector film and the beam splitter form the conjugate image of the pinhole near an observer. By locating the center of the crystalline lens of an eye at the conjugate pinhole, the projected images are focused on the retina and are seldomly changed by the accommodative response of the lens. For the practical implementation of the system, a collimating lens was used as an imaging lens combined with a pinhole. The point spread function of the system was derived to theoretically demonstrate that the system exhibits focus-free features at accommodation distances of 0.25, 0.5, and 1 m. The experiments conducted to demonstrate the feasibility of the system are presented under the same accommodation condition as the analysis.

Depth estimation method using depth-of-field imaging with a retroreflector.

We propose a method for depth estimation using a tilted retroreflective structure, which consists of a beam splitter and a micro-corner cube array. Existing depth estimation methods commonly use an active light source to detect the depth of an object. However, sunlight interferes with their depth measurements and decreases their accuracy outdoors. The proposed method does not need any active light source because depth information is obtained in the image domain, not in the object domain. The depth of field imaging by a retroreflector indicates the depth position of the object image, immediately. We believe that the proposed method can be applied to depth measurement systems such as LIDAR and time of flight cameras. An experiment is performed, whose results are compared with theoretical calculations, which confirms the feasibility of the proposed method.

Achromatic phase shifting self-interference incoherent digital holography using linear polarizer and geometric phase lens.

A simple Fresnel-type self-interference incoherent digital holographic recording system is proposed. The main part of the system consists of the two linear polarizers and geometric phase lens. The geometric phase lens is employed as a polarization selective common-path interferometer. One of the polarizers is rotated by the motor and serves as a phase-shifter with the geometric phase lens, to eliminate the bias and twin image noise. A topological phase is obtained by the relative angle between the polarizer and geometric phase lens. Since this phase shifting method does not depend on the change of the optical path length, the phase shifting performance is almost constant in the broad spectral range. Using the proposed achromatic phase shifting method, a simultaneous three-color phase shifting digital hologram recording under the incoherent light source is demonstrated.

Angle Dependent 3-D Directional Patterning of Flexible Films by Infrared Laser.

Laser direct patterning (LDP) technology has attracted great attention due to its process simplification and environmental friendliness. It is replacing the existing photo-lithography technology. Infrared (IR) laser equipment also has advantages such as low cost and long duration, although it is difficult to implement fine line width compared to ultraviolet (UV) or excimer laser. Therefore, it is very important to realize 3-D patterning system based on cheap infrared laser system. In this paper, 3-D infrared laser direct patterning system was designed by introducing an etching process on Ag paste/ITO/PET flexible film. Such 3-D patterned Ag paste/ITO/PET flexible films showed feasibility and effectiveness of 3-D laser directional patterning technologies. Etch ratio of the fabricated Ag paste/ITO/PET flexible film after 3-D infrared laser direct patterning was then systematically investigated.

Charge Transport in 2D DNA Tunnel Junction Diodes.

Recently, deoxyribonucleic acid (DNA) is studied for electronics due to its intrinsic benefits such as its natural plenitude, biodegradability, biofunctionality, and low-cost. However, its applications are limited to passive components because of inherent insulating properties. In this report, a metal-insulator-metal tunnel diode with Au/DNA/NiOx junctions is presented. Through the self-aligning process of DNA molecules, a 2D DNA nanosheet is synthesized and used as a tunneling barrier, and semitransparent conducting oxide (NiOx ) is applied as a top electrode for resolving metal penetration issues. This molecular device successfully operates as a nonresonant tunneling diode, and temperature-variable current-voltage analysis proves that Fowler-Nordheim tunneling is a dominant conduction mechanism at the junctions. DNA-based tunneling devices appear to be promising prototypes for nanoelectronics using biomolecules.

Self-interference digital holography with a geometric-phase hologram lens.

Self-interference digital holography (SIDH) is actively studied because the hologram acquisition under the incoherent illumination condition is available. The key component in this system is wavefront modulating optics, which modulates an incoming object wave into two different wavefront curvatures. In this Letter, the geometric-phase hologram lens is introduced in the SIDH system to perform as a polarization-sensitive wavefront modulator and a single-path beam splitter. This special optics has several features, such as high transparency, a modulation efficiency up to 99%, a thinness of a few millimeters, and a flat structure. The demonstration system is devised, and the numerical reconstruction results from an acquired complex hologram are presented.

Projection-type integral imaging system using a three-dimensional screen composed of a lens array and a retroreflector film.

We propose an improved projection-type integral imaging system using a three-dimensional (3D) screen consisting of a lens array and a retroreflector film in this paper. The projection-type integral imaging system suffers from the disadvantage of low-visibility images because of the inherently small exit pupil size of the projector. In order to resolve this problem, we adopt a 3D screen to avoid the demerits of a diffuser screen, such as off-screen image blur and loss of parallax. To determine the appropriate configuration of the 3D screen in the system, a simulation based on a ray transfer matrix analysis method was performed. The results show that the 3D screen should be located near the central depth plane of the integral imaging system, which leads to the conclusion that only the real mode is available for the proposed system. Experiments to verify this configuration and the feasibility of the proposed system were conducted using a system constructed with a real mode integral imaging system including a convex mirror array, which can fundamentally eliminate the pseudoscopic problem.

Optical defocus noise suppressing by using a pinhole-polarizer in Fresnel incoherent correlation holography.

We propose the method that suppresses the defocus noise optically in the Fresnel incoherent correlation holographic system using a pinhole-polarizer (PP), which is made by punching a pinhole on the linear polarizer. The system configuration of this suggestion is based on the original system with optical-sectioning capability, which is realized by presenting the phase-pinhole on the phase-only spatial light modulator. In our system, the phase-pinhole is replaced with a PP. The replaced component is no longer programmable yet provides an affordable, simple, and light-efficient system configuration. The feasibility of the system with the PP is analyzed and demonstrated.

Real object pickup method of integral imaging using offset lens array.

We propose the pickup system of integral imaging using the offset lens array (OLA), which is a useful optical component for both the pickup and display processes. The main purpose of our system is resolving the pseudoscopic image problem of integral imaging. Also, the flipped image of integral imaging that has the wrong perspective information can be removed by adding an external barrier in the display process. In this paper, the above properties are explained in detail, and the experimental results to verify the feasibility of the proposed system are presented. We are certain that our system can also be applied to other various pickup systems based on integral imaging.

Analysis on expressible depth range of integral imaging based on degree of voxel overlap.

This paper proposes a practical method to analyze the expressible depth range of an integral imaging system based on image blur at defocused depths, which is one of the most noticeable image degradations, caused by overlaps among voxels in both the real and focused mode. In order to obtain the preferably precise area of overlaps among voxels at each depth, display pixels are regarded as surface light sources in the process of voxel size calculation. As a criterion for determining the range, we determine the tolerable limit of the overlaps among voxels to be at least resolved from each other. Based on this principle, several mathematical expressions about the expressible depth range can be derived in both the real mode and focused mode, and their feasibilities are demonstrated by several experiments. The analyses are processed based on both wave optics and ray optics.

Analysis on image expressible region of integral floating.

We define the image expressible region where three-dimensional (3D) images can be reconstructed in an integral floating display using ray distribution information. Here, the image expressible and viewing regions of the integral floating system, defined using ray distribution based on ray optics, are analyzed to resolve an image-cropping problem known as the viewing window that is due to the virtual aperture. Two experiments for verifying the image expressible and viewing regions are performed, and the results are presented. The obtained analysis is useful to maximize the viewing properties of integral floating.

Implementation of active-type Lamina 3D display system.

Lamina 3D display is a new type of multi-layer 3D display, which utilizes the polarization state as a new dimension of depth information. Lamina 3D display system has advanced properties - to reduce the data amount representing 3D image, to be easily made using the conventional projectors, and to have a potential being applied to the many applications. However, the system might have some limitations in depth range and viewing angle due to the properties of the expressive volume components. In this paper, we propose the volume using the layers of switchable diffusers to implement the active-type Lamina 3D display system. Because the diffusing rate of the layers has no relation with the polarization state, the polarizer wheel is applied to the proposed system in purpose of making the sectioned image synchronized with the diffusing layer at the designated location. The imaging volume of the proposed system consists of five layers of polymer dispersed liquid crystal and the total size of the implemented volume is 24x18x12 mm3(3). The proposed system can achieve the improvements of viewing qualities such as enhanced depth expression and widened viewing angle.

Measurement of accommodation response of human eye to integral floating display.

The dioptric responses of human eyes to floating images at different depths reconstructed by an integral floating display are measured with an open-field autorefractor and analyzed according to observer recognition of the accommodation depth cue. Two targets are randomly presented approximately 1.75 D from the observers and have a separation of 0.15 D, corresponding to approximately 5 cm. Two subjects participate, and over 500 responses are measured during each experimental trial. The frequency distribution profiles of the responses to targets at different locations are obtained and analyzed. The results confirm that the subjects focused on the floating targets and not on the surface of the optical component. In this paper, an analysis of the separation between the peaks of the frequency distribution profiles is presented that confirms that the integral floating display could induce the accommodation response to provide the corresponding depth stimulus.