Photoplastic Recording

a method of recording and reproducing optical information. The data carriers used in photo-plastic recording are, with rare exceptions, thin transparent oil-base, thermoplastic (seeTHERMOPLASTIC RECORDING), or gelatinous (seeGELS) layers. Such a storage layer is part of a laminated structure that usually consists of two or three layers.

In a two-layer structure, the storage layer is a disperse system consisting of a photosensitive semiconductor material (see) in a polymer binder and is applied to a thin layer of conducting material. In a three-layer structure, a dielectric storage layer is applied to a photosensitive semiconductor layer that lies between the storage layer and a conducting layer. In most cases, all the layers are transparent, so that the recording and reproduction of information is carried out by transillumination. In some structures, light is reflected either from a specular conducting substrate or from the opaque surface of a photosensitive semiconductor storage layer.

Before recording, the structure is sensitized by uniformly charging the storage layer and grounding the conducting substrate, thereby forming a sort of capacitor in which the charged storage surface serves as a plate. In a two-layer structure, a light signal causes part of the surface charge to discharge onto the substrate (the stronger the illumination of a given microsection of the surface, the more complete the discharge). However, in a three-layer structure, a positive charge passes from the substrate to the photosensitive semiconductor surface adjacent to the storage layer. In both types of structures, the electrostatic attractive forces of opposite charges deform the surface of the soft storage layer—often after heating the layer, a process called thermal development—to form a relief. The depth distribution of the relief corresponds to the distribution of the radiation flux over the surface of the storage layer; that is, the optical information is encoded in the resulting relief. When the recorded information is read, differences in the thickness of the relief cause variations in the phase of the reading light wave. Such differences are not perceived by the eye or by photodetectors. For this reason, the phase variations are converted into variations in the amplitude of the light wave, that is, in the intensity of the reading beam; the amplitude variations are detected by both photodetectors and the human eye. Such conversion is carried out at the present time (the 1970’s) by the schlieren method but, in theory, may also be performed by a method similar to phase contrast in microscopy (seeMICROSCOPE: Methods of illumination and observation [microscopy]). The structures employed in photoplastic recording may be used more than once, since a recording that is no longer needed may be erased by heat treatment.

The main advantage of photoplastic recording is the possibility of reading information very soon after recording. Hence, photoplastic recording may be used for the virtually instantaneous transmission and conversion of images. For example, in television, the images may be projected onto screens having an area of several square meters and intended for either personal or public use. The high resolving power and high speed characteristic of photoplastic recording make the method a promising technique for holography, for various types of optical image processing, and for use in electronic computers—for example, in computer memories or in data input and output.