Raster Optical System

an optical system that contains a raster, that is, an array of a large number of small optical elements—such as tiny apertures, lenses, gratings, prisms, or mirrors—that are located on a common surface and act as a single optical device. Each small element of the raster optical system participates in the creation of only a single element of the image formed by the system. Raster optical systems differ in the parameters of their elements, the methods of arrangement of the elements on the common surface, and the shape of this surface. The shape may be flat, conical, cylindrical, or spherical. Multisurface raster optical systems are also used, as are three-dimensional systems, where the elements are arranged in space in a complicated fashion.

Figure 1. Analyzing property of a raster optical system: the raster breaks up the image of the object into a discrete row of elementary images. In the present case, the system consists of a raster and a screen placed in the raster’s focal plane.

Irregular and regular raster optical systems are distinguished according to the type of raster. Examples of regular systems are linear systems, where the raster elements have the form of parallel lines; radial systems, whose elements diverge radially from a common center like rays; ring systems, in which the elements are arranged in the form of concentric zones; cellular systems; and row systems, where the elements are in a checkerboard array.

In the raster optical systems used most frequently in practice, the sequence of elements on the common plane has a constant period. In this case, we speak of rasters with constant spacing.

The basic properties of raster optical systems are focusing, multiplication, analysis, and integration. In focusing, the light from a point source is collected by the raster in a point, line, or three-dimensional region. The multiplying property permits multiple repetition of the same images to be obtained. The analyzing property involves the decomposition of the image into separate elementary images (Figure 1). The integrating property makes possible the reconstruction of the complete image of the object from the elementary images (Figure 2).

The normal reproduction of an optical image can be achieved by a raster optical system through the double conversion of the image, wherein analysis is followed by synthesis, or integration, of the resulting elements. For example, in the simple raster optical system consisting of a combination of a raster with a diffusely reflecting screen, the rays travel first in the forward direction and then in the reverse direction so that analysis is followed by synthesis of the three-dimensional image. The characteristics of raster optical systems depend to a considerable degree on the properties of the screen placed in the focal plane of the raster. It is possible to produce a great variety of raster optical systems by combining various types of rasters and screens.

Figure 2. When the light rays from each elementary image obtained as a result of analysis pass through many adjacent raster elements, a great number of similar three-dimensional images of the object are produced. This process illustrates the integrating, or synthesizing, property of a raster optical system.

Raster optical systems are used for many purposes. For some of these purposes ordinary optical systems can be used, but the use of raster systems may be simpler and easier. At the same time, raster systems permit the solution of problems that cannot be handled by traditional optical methods. Such systems are used in printing, where the analyzing property of raster optical systems is the basis for autotype, intaglio printing, and collotype printing. Raster systems are also employed in the textile industry and in measurement technology, which makes use of what are called raster measuring instruments. Raster optical systems are, however, most widely used in applied optics. They find application in motion-picture filming, including highspeed motion-picture photography, and are also used in color photography and color television. Raster optical systems are used to reproduce stereoscopic images that can be observed without the use of special individual viewing devices, or stereoscopes. Lenticular raster films and raster screens for stereoscopic projection and stereoscopic television are used for this purpose. The multiplying property of raster optical systems permits the obtaining of a parallax-free multiplication of optical images. The integrating property of such systems makes possible the reconstitution of the three-dimensional image of an object by the method of integral photography (seeLIPPMANN PROCESS). Many other special-purpose raster optical systems exist.