GB480996A - Improvements in or relating to electro-optical image transmission systems - Google Patents

Abstract

480,996. Photo-electric cells; cathode-ray tubes. ELECTRICAL RESEARCH PRODUCTS, Inc. Feb. 19, 1937, No. 5012. Convention date, March 4, 1936. [Class 39 (i)] A transmitting device for television and like purposes comprises a film of photoconductive material united to a film of photo-emissive material; the image to be transmitted is projected on to the photoconductive fihn through a transparent metal film in contact with it and the photo-emissive film is scanned by a beam of radiation. Preferably the photo-conductive film is of some high resistance material such as mercuric iodide or native sodium chloride ; in this case the photo-emissive film, which must be discontinuous, and the metal film form with the photo-conductive film a number of elementary condensers, the charges of which leak away at different rates corresponding to the conductivity produced by the illumination and the signal currents are produced when the condensers are recharged at the next transit of the scanning-beam. To prevent lateral discharge between these condensers the thickness of the photo-conductive layer must be less than the diameter of the scanning spot, preferably about one-tenth as much. If a photo-conductive material of low resistance, such as selenium, is employed the working currents are produced by the discharge through the selenium; in this case the film should be divided into small areas. The Figure shows a television system comprising such a transmitter. The screen C and a fine mesh electrode 15 are enclosed in an exhausted container 14 and the object focused on the screen by a lens 19. The scanning- beam is produced by a cathode-ray tube with a fluorescent screen 41 the luminous spot on which is projected on to the photo-electric surface of the screen by a lens 26. Scanning is effected by deflecting plates of 42, 45 of the ordinary type connected in the ordinary way to deflecting plates 53, 54 of a receiving tube 50. The electrode 15 and screen Care connected to the grid and cathode of a valve 18 and the resulting oscillations, amplified if necessary transmitted to a transformer 60, one terminal of which is connected to the anode 52 and a grid 56 in the tube 50, and the other to a second group 57 in the tube. By this means the discharge in the tube 50 is modulated in accordance with the signal current. A modification is described in which the scanning is effected by rotating discs instead of cathode-ray tubes.