Electrostatic Recording

a process in which various kinds of information represented by electric signals are placed and preserved on a dielectric medium. The process entails creating on the medium a particular distribution of electric charges, known as a charge pattern, which constitutes a “latent image” of the recorded information.

Electrostatic recording systems may be divided into two basic groups, depending on the methods used for recording and reproducing information. In systems of the first group, the recording component is either an electrode head or a cathode-ray tube with a metal-filament screen. A latent image element area is formed by the transfer of charges from the electrodes (or filaments) of the recording component to the dielectric medium across an air gap 5–20 micrometers wide. The transfer takes place as a result of an electric discharge that occurs when a voltage of 700–900 volts is applied to the electrodes. The latent image produced on the dielectric medium as a consequence of the relative displacement of the recording component and the medium is then converted to a visible image by means of electrophotography.

A record of the image may be obtained on electrostatically charged paper, which consists of an electrically conductive base upon which a dielectric layer has been applied; either dry or liquid electrographic developers may be used to render the latent image visible. A record may also be obtained on a dielectric-coated drum, from which the image, developed with powder, is subsequently transferred to plain paper.

The advantages of the systems in the first group include a high information rate (the rate is approximately 10,000–20,000 symbols per second for digital information and several tens of kilo-hertz for analog information) and the ability to record different kinds of information, including halftone images, and to convert the information to visible form almost immediately. Also among the advantages are the absence of chemical and impact effects on the dielectric medium during recording and reproduction, the systems’ insensitivity to light, and the relatively low cost of the materials used for recording. The systems that constitute the first group may be used to record computer output and to record processes in experimental physics and measurement technology.

The second group includes systems that record electric signals by focusing a scanning electron beam on a dielectric medium in an evacuated chamber and that reproduce information also in the form of electric signals, which are then converted into a television image or a document. In such systems, the dielectric medium—a tape 35 mm or 70 mm wide—consists of three layers: a base of polyethylene terephthalate (Lavsan) 50–80 micrometers thick, a thin metallic layer that may be as much as 1 micrometer thick, and a dielectric layer that may be up to 10 micrometers thick. The electron beam is formed with an electron gun that produces a low-intensity beam. To reproduce information, an electron beam from the same gun or from an additional low-intensity gun scans the surface of the dielectric medium. Secondary electrons (seeSECONDARY ELECTRON EMISSION) that are dislodged from the dielectric medium by the beam are channeled into an electron multiplier, and the density-modulated flux of secondary electrons is converted into a video signal.

By comparison with magnetic recording systems, the second group of electrostatic recording systems offers the advantages of having a frequency band that is as much as 20 megahertz wider, a higher recording density, and a higher quality of reproduction. The disadvantages of the systems of the second group are complexity of design and the need to use materials that have been produced in vacuo and to evacuate the chamber after each tape change. Systems of the second group are used to transmit images from space. Thermoplastic recording is a form of electrostatic recording.