The Signal Corps Role in the Development of Continuous-Tone Electrophotography

This paper discusses the reasons which lead the Signal Corps to investigate different nonsilver halide photographic processes, and to select electrophotography as the subject of a major research and development effort. The problems inherent in the process are discussed and several methods with which it was attempted to evolve a workable procedure are shown. In particular, the research directed toward increasing the spectral response and over-all sensitivity of selenium layers, as well as of other semiconductors, is outlined. Other problems discussed are spontaneous decay of the charge in the dark, plate fatigue, and after-images. The different approaches to the conversion of the electrostatic latent image into a visible, permanent image are discussed, including liquid spray and immersion methods, dry ``cascade'' and powder cloud development. Emphasis in the discussion is on the powder cloud method and its related problems: selection of the most suitable powder material, uniformity of particle size, polarity and amount of charge of the particles, design of suitable cloud generators, particle agglomeration. It is shown that certain problems inherent in the powder development on selenium plates can be simplified by first transferring the electrostatic latent image from the reusable selenium to the permanent carrier, and developing on this carrier. The paper finally discusses the possibilities for future developments and applications, and calls attention to a comprehensive list of pertinent patents available to readers on request.