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Optical Design & Engineering
Amplified viewer provides three-dimensional images of dark objects
A new viewer can form enhanced pictures of objects in the dark, without the use of a camera or display.
18 January 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0553
A conventional amplified viewer using a single lens on the incident plane of an image intensifier will produce a two-dimensional image of an object located in a dark space. The image undergoes photoelectric conversion and electronic amplification before being formed on the phosphor screen of the exit plane. As the optical image on the incident plane is degraded and becomes two-dimensional, so too the output image. An optical system able to produce 3D images on the exit plane would be highly useful. No such device for observing dark objects is available.
The basic integral method, proposed as a photographic technique,1,2 uses a lens array during pickup and display stages, and has recently been adapted to video systems.3,4 To produce an integral image, a lens array comprising a large number of convex elemental lenses is positioned immediately in front of the pickup device. The integral image, composed of as many small elemental images as equals the number of lenses, is then conveyed to the display device, in front of which is positioned a convex lens. As they pass through the lens array, light rays from the display device retrace the original routes and converge, forming an autostereoscopic 3D image.
Note that the reproduced image is pseudoscopic with reversed depth. To avoid this, the elemental images must be capturedas erect images of an object positioned at a distance. We have proposed the use of radial gradient index (GRIN) lenses for the pickup-lens array.5 Their refractive index decreases after it peaks atthe optical axis. When light rays from a distant object enter the GRIN lens, they curve in the direction of a higher refractive index, so that the ray path meanders cyclically.
A GRIN lens length with length of 3/4 · Lp (where Lp represents one cycle of the ray path) readily forms an erect image on the exit plane for a distant object.6 Another GRIN lens, length 1/4 · Lp, is equivalent to a convex lens. The amplified viewer in which the pickup and display stages are unified thus comprises two GRIN lens arrays with an image intensifier between them.
Figure 1 outlines the optical viewer with the image intensifier as used in our experiment. GRIN lens array specifications are listed in Table 1. Through it we observed two objects, the letters I and E, ina dark space.
Figure 1. Experiment using amplified viewer (side view). The objects are 32-point letters I and E. I is positioned at 30mm, and E at 50mm from array 1. The center of both letters is offset 6mm horizontally. Both images are separated from two object positions by TW (45.5mm), equivalent to the thickness of the amplified viewer.
Table 1. Specifications of GRIN lens arrays
Figure 2(a–d) presents photographs of the images from four viewpoints. The objects have an illuminance of approximately 0.1lx, and image intensifier amplification was adjusted so the output phosphorscreen had a peak luminous emittance of about 200lx (lm/m2). As a result, we could observe 3D images with visible brightness. The resolution was low because the viewer was not equipped with a sufficient number of elemental units. Even so, these photos show the distinctive positions of the letters, indicating their 3D presentation.
Figure 2. Photographs of images formed by the amplified viewer. Four viewpoints are all displaced from the axis of the exit plane by approximately 9°.
In summary, an optical viewer based on the integral method can form observable 3D images of objects placed in a dark space, with resolution affected by diffraction. To observe distant objects, the diameter of the elemental lens is 1.0mm, although it could be smaller for near objects. Such a device would be useful for observing 3D objects in dark spaces. It could, for example, be employed to monitor the habits of nocturnal animals or to view disaster sites at night.
Fumio Okano, Jun Arai
NHK (Japan Broadcasting Corporation) Science & Technical Research Laboratories
Fumio Okano is an executive research engineer at the NHK Science & Technical Research Laboratories. His research interests are optoelectronics systems, including cameras and displays and 3D television systems. He chaired the Three-Dimensional TV, Video, and Display Conference sponsored by SPIE in 2006.
Jun Arai is a research engineer at the NHK Science & Technical Research Laboratories. His research interests include optoelectronics systems and 3D television systems.