Electroholography enables the projection of three-dimensional (3-D) images using a spatial-light modulator. The extreme computational complexity and load involved in generating a hologram make real-time production of holograms difficult. Many methods have been proposed to overcome this challenge and realize real-time reconstruction of 3-D motion pictures. We review two real-time reconstruction techniques for aerial-projection holographic displays. The first reduces the computational load required for a hologram by using an image-type computer-generated hologram (CGH) because an image-type CGH is generated from a 3-D object that is located on or close to the hologram plane. The other technique parallelizes CGH calculation via a graphics processing unit by exploiting the independence of each pixel in the holographic plane.
KEYWORDS: Field programmable gate arrays, Microscopy, Image processing, Real time image processing, Optical imaging, Imaging systems, Cameras, Particles, Image resolution, Digital signal processing, Prototyping
High-speed imaging is an indispensable technique, particularly for identifying or analyzing fast-moving objects. The serial time-encoded amplified microscopy (STEAM) technique was proposed to enable us to capture images with a frame rate 1,000 times faster than using conventional methods such as CCD (charge-coupled device) cameras. The application of this high-speed STEAM imaging technique to a real-time system, such as flow cytometry for a cell-sorting system, requires successively processing a large number of captured images with high throughput in real time. We are now developing a high-speed flow cytometer system including a STEAM camera. In this paper, we describe our approach to processing these large amounts of image data in real time. We use an analog-to-digital converter that has up to 7.0G samples/s and 8-bit resolution for capturing the output voltage signal that involves grayscale images from the STEAM camera. Therefore the direct data output from the STEAM camera generates 7.0G byte/s continuously. We provided a field-programmable gate array (FPGA) device as a digital signal pre-processor for image reconstruction and finding objects in a microfluidic channel with high data rates in real time. We also utilized graphics processing unit (GPU) devices for accelerating the calculation speed of identification of the reconstructed images. We built our prototype system, which including a STEAM camera, a FPGA device and a GPU device, and evaluated its performance in real-time identification of small particles (beads), as virtual biological cells, owing through a microfluidic channel.
KEYWORDS: Computer generated holography, Cameras, Remote sensing, 3D image reconstruction, Image sensors, 3D image processing, 3D displays, Integral imaging, Fourier transforms, Imaging systems
This paper shows the method to calculate a computer-generated hologram (CGH) for real scenes under natural light using a commercial light field camera, and shows the results of color reconstruction of the synthesized CGHs. The CGH calculation using light field camera is performed by converting four-dimensional light field captured with a light field camera into a complex amplitude distribution, and the converted complex amplitude distribution is propagated so as to generate an interference pattern. In color reconstruction, we calculated three CGHs with red, green and blue wavelengths and superposed reconstructed red, blue and green images to obtain reconstructed color images. We verified that color three-dimensional images were reconstructed by numerical and optical reconstructions of the synthesized CGHs.
We report frequency estimation of loudspeaker diaphragm vibrating at high speed by parallel phase-shifting digital holography which is a technique of single-shot phase-shifting interferometry. This technique records multiple phaseshifted holograms required for phase-shifting interferometry by using space-division multiplexing. We constructed a parallel phase-shifting digital holography system consisting of a high-speed polarization-imaging camera. This camera has a micro-polarizer array which selects four linear polarization axes for 2 × 2 pixels. We set a loudspeaker as an object, and recorded vibration of diaphragm of the loudspeaker by the constructed system. By the constructed system, we demonstrated observation of vibration displacement of loudspeaker diaphragm. In this paper, we aim to estimate vibration frequency of the loudspeaker diaphragm by applying the experimental results to frequency analysis. Holograms consisting of 128 × 128 pixels were recorded at a frame rate of 262,500 frames per second by the camera. A sinusoidal wave was input to the loudspeaker via a phone connector. We observed displacement of the loudspeaker diaphragm vibrating by the system. We also succeeded in estimating vibration frequency of the loudspeaker diaphragm by applying frequency analysis to the experimental results.
KEYWORDS: Computer generated holography, RGB color model, 3D image reconstruction, Holograms, Diffraction, Chromium, Digital holography, 3D image processing, Spatial light modulators, 3D displays
A calculation reduction method for color digital holography (DH) and computer-generated holograms (CGHs) using color space conversion is reported. Color DH and color CGHs are generally calculated on RGB space. We calculate color DH and CGHs in other color spaces for accelerating the calculation (e.g., YCbCr color space). In YCbCr color space, a RGB image or RGB hologram is converted to the luminance component (Y), blue-difference chroma (Cb), and red-difference chroma (Cr) components. In terms of the human eye, although the negligible difference of the luminance component is well recognized, the difference of the other components is not. In this method, the luminance component is normal sampled and the chroma components are down-sampled. The down-sampling allows us to accelerate the calculation of the color DH and CGHs. We compute diffraction calculations from the components, and then we convert the diffracted results in YCbCr color space to RGB color space. The proposed method, which is possible to accelerate the calculations up to a factor of 3 in theory, accelerates the calculation over two times faster than the ones in RGB color space.
We present a special-purpose computer named HORN (HOlographic ReconstructioN) for fast calculation of computer-
generated holograms (CGHs). The HORN can realize parallel processing of the CGH calculation by using field-
programmable gate arrays. The latest version of HORNs, HORN-7, can reconstruct holographic images more clearly than previous HORNs because HORN-7 can make CGHs as a phase-only hologram (kinoform). In addition, the HORN-
7 can directly output calculated CGHs on a spatial-light modulator via Digital Visual Interface. In this paper, we demonstrate real-time reconstruction of holographic motion pictures by the HORN-7. We calculated CGHs, which
consist of 1,920 × 1,080 pixels, from the object data of ~6,000 points, and succeeded in reconstructing holographic motion pictures from the calculated CGHs at the rate of ~7 frames per second.
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