Patent GB-672767-A (GB)
Title: Electrophotography
IPC: G03G9/08
Filed: 1947-07-30
Abstract: 672,767. Electro-photographic processes. BATTELLE DEVELOPMENT CORPORATION. July 30, 1947 [April 4, 1939], No. 20671/47. Class 98 (ii). [Also in Group XVI] A method of recording a light image comprises producing and storing an electrostatic latent image by first charging the surface of a layer of a " photoconductive insulating material " in the dark or under subdued illumination. and exposing the layer to the light image to produce an electrostatic latent image which is developed by a finely-divided material which adheres to the charged areas. A " photo-conductive insulating material " is normally an insulator but is rendered very slightly conducting by light. Examples of such materials are sulphur, anthracene, anthraquinone, the mixture formed by melting an excess of sulphur with selenium or anthracene, the compound formed by melting sulphur (1 part) with anthracene (3 parts), linseed oil boiled with sulphur and dried in a thin layer. The electrically conducting backing layer is normally metal which may be etched to receive the insulating coating. The latter may be melted or sublimed on. The insulating material may be impregnated into paper which is temporarily supported on a metal sheet for exposure. The impregnation may be by melting or deposition from a solution. Alternatively a paper sheet may first be impregnated, say, with metallic silver, prepared by reducing silver chloride, to render it conducting, and then be coated or impregnated with the insulating layer. Again, a paper sheet may be coated first with a conducting layer of bronze or carbon in a binder and then with the insulating layer. The charging process described is frictional. After charging the photo-conductive layer is exposed to the light image obtained, for example, by contact or projection from an illuminated transparency. The developing powder is applied and the excess removed by air blast. Both of these steps may be effected simultaneously with an atomizer. The powder image may be transferred by contact to a second surface which may be wetted by a liquid which is a solvent for the powder to assist transfer. Alternatively the second surface may be rendered slightly adhesive by coating or impregnation with a wax or resin and the powder fixed in place by heating. Alternatively the powder may be fixed by a lacquer. The process is suitable for reproducing pictorial subjects since the developing powder then adheres to the photo-conducting layer in a density, in any area, proportional to the picture density and charge density. For colour reproduction successive printings may be made with coloured powders from colour separation records. The Specification as open to public inspection describes methods of charging using the apparatus shown. The plate 22 carrying the photoconductive insulator 21 is mounted behind a transparent plate 41 carrying a partially transparent sputtered metal layer 42 between which, and the plate 22, a potential difference is applied from a source 45 while the insulating layer is uniformly illuminated by the lamp 46. The lamp and potential difference are switched off and the charged insulating layer is exposed through the lens 47. Alternatively the potential difference may be reversed during exposure so that the polarity in the illuminated portions is reversed. Electrically charged dust is then applied. Again, the insulating layer may be initially uncharged and may be charged in the illuminated areas by application of the potential difference during exposure. Also described are methods by which, by suitable choice of the powder and the surface to which it is applied, the invention may be used in a number of printing processes. For lithographic reproduction the powder image is transferred to a lithographic metal or paper sheet, the powder being of a resin which is wetted by lithographic ink. Alternatively the powder image may be formed directly on a lithographic metal plate covered with photo-conductive insulator. Heating melts the dust to secure it to the plate and sublimes off the insulator. For typography a metal plate carrying a resin image formed in this way may be etched in the areas not protected by the image. For hectographic repro. duction an image of powdered dye is formed and transferred to a gelatin layer, either flat or-on the cylinder of a rotary machine, on to which sheets of paper are pressed. Alternatively an alcohol soluble dye may be used and transferred to a paper sheet, to be carried on the cylinder of a rotary machine, into contact with which are brought sheets moistened with alcohol.