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Illumination & Displays
Holographic screen for image projection
Research by the late Yuri Denisyuk and colleagues details a new type of on-axis holographic screen for color image projection systems using a reference-free volume hologram.
24 May 2006, SPIE Newsroom. DOI: 10.1117/2.1200604.0200
One of the emerging applications of holography is in the fabrication of screens on which images can be projected. These can be used, for instance, as displays for driver training. The advantage of this kind of screen is that, when it reconstructs an image, it produces a viewing zone in a definite, limited region of space.
Most projection systems are on-axis symmetrical. One of few ways of creating an on-axis holographic screen was suggested by D. Gabor in 1948 and involves a thin hologram.1 We initially considered using this type of screen, which required that a reference source (a diffuser) be placed in the center of the object being recorded.2 In the resulting centered hologram, the conjugate image and halo did not distort the reconstructed real image. However, the bright point of the reference source caused certain distortions to the projected image. It seemed important, therefore, to develop an on-axis screen capable of suppressing the zeroth order bright point. (Note that the Leith off-axis hologram3 has considerable distortion due to the images being spatially separated and super-imposed on the main images.)
While working on the centered hologram screen we arrived at the idea of using a reference-free volume hologram. During recording, we wondered if phase modulation might suppress the zeroth order artifact in the reconstructed image.4 In order to record such a screen, the diffuser had to be illuminated using coherent radiation. The rays scattered from different points on the diffuser to produce a complicated interference pattern which was then recorded on light-sensitive film. Each point on the diffuser—in this type of hologram—may be considered as a reference beam for all the other points, thus creating speckle. At reconstruction, the image of the diffuser acts as a viewing zone for a two-dimensional image projected onto the screen: the image reconstructs the hologram of the diffuser, which is projected out into space.
The volume recording medium was made of thick (0.10 to 0.15mm) layers of self-developing dichromated gelatin, with glycerin added, sensitive in the blue range.5 The hologram recording process was controlled during exposure. As the exposure increased, the bright point in the zeroth order of the viewing zone diffused. Due to this, the zone was uniformly filled with diffuse light.
Having been recorded in blue, the hologram allows the reconstruction of other wavelengths, including white light. Thus, the hologram can to be recorded in the sensitivity range of photographic film, making it useful in color image projection systems. Figure 1 shows photos of projected images when viewed through (a) a holographic (speckle) screen and (b) a glass diffuser. The projected image is brighter in the former.
Figure 1. Shown are projected images from (a) a holographic (speckle) screen and (b) a glass diffuser. The image on the right is clearly brighter than that on the left.
Thus, inspired by the thin Gabor hologram, we suggest a new hologram type. The reference-free volume hologram can compensate for the distortions of the main reconstructed image typical of a Gabor hologram. This allowed us to produce an on-axis holographic screen that does not transmit the zeroth order bright spot. In addition, this screen does not produce a halo or a conjugate image. The reference-free volume hologram allows recording in one spectral range and reconstruction in another. This feature simplifies the choice of recording medium and makes it possible to project a color image through the screen.
This work was supported by the Russian Foundation for Basic Research (grants no. 04-02-17593 and NSh-98.2003.2).
Yuri Denisyuk, Nina Ganzherli, Irina Maurer, and Dmitry Chernykh
Laboratory of Optoelectronics and Holography, Ioffe Physico-Technical Inst. of Academy of Sciences of Russia
Prof. Yuri N. Denisyuk, who died while this article was in preparation, was the head of the laboratory of Optoelectronics and Holography at the A. F. Ioffe Physical-Technical Institute. His main research was in holography, including studying the properties of holograms recorded in quadratically nonlinear media and optical holographic displays. In addition, he was a Fellow of SPIE. He chaired many sessions at, and wrote many papers for, SPIE conferences on holography.
Dr. Nina M. Ganzherli joined the staff of the Ioffe Institute in 1972 after graduating from the faculty of the Physics department of St. Petersburg University. Her recent research includes methods of image projection and optical holographic displays. In addition, she is a co-author of many papers published in SPIE proceedings on holography and optical information processing, as well as articles in Optical Engineering.
Dr. Irina A. Maurer joined the staff of the Ioffe Institute in 1970. She graduated from the faculty of the Physics department of St. Petersburg University in 1968. Her recent research includes the investigation of properties of thick-layered dichromated gelatin and its applications. In addition, she is a co-author of many papers published in SPIE proceedings on holography.
Dr. Dmitry F. Chernykh joined the staff of the Ioffe Institute in 1966. His recent research includes the investigation of optical projection systems and optical holographic displays.
1. D. Gabor,
Proc. R. Soc. London Ser. A,
Vol: 197, pp. 454, 1949.