GB665737A - Improvements in television systems - Google Patents

Abstract

665,737. Cathode-ray tubes. BRITISH THOMSON-HOUSTON CO., Ltd. July 25, 1947 [Jully 25, 1946], No. 20112/47. Class 39 (i). [Also in Group XL (b)] The photo-sensitive or electrosensitive mosaic of a pick-up tube has its charge reduced to datum level by means of an auxiliary electron stream comprising fast electrons of high voltage, e.g. a few thousand volts, such that the coefficient of secondary emission of the sensitive plate is lower than unity, i.e. at a working point which is the second intersection of the secondary emission characteristic. The auxiliary cathode is either thermionic with a control grid or Wehnelt cylinder, photo-sensitive or secondary emitting. The tube 1contains a mosaic 2 comprising an insulating sheet with a signal plate backing upon which is projected an optical image of an object S during intervals permitted by the shutter 9. The charge image formed on the mosaic by photo-emission is scanned by the gun 3. the wall coating anode 4 during the open shutter period receiving a positive pulse from terminal 23 biased by diode 25 and creating an accelerating field for photo-electrons from 2. In the period just, prior to the opening of the shutter 9, the grid 11 is positively pulsed from terminal 17 to permit an electron stream from thermionic cathode 10 to flow to the mosaic and neutralize the charge image left from the previous scan. The potentiometer 12-14 and biasing diode 16 also control the electron stream. The signal plate of the mosaic is connected to an amplifier 20 to which is applied a blocking potential outside the period of scanning by the electron gun 3, the latter being similarly blocked during the illuminating and flooding periods. The subject, which may be a film or a scene, is illuminated bv intense flashes of light which are so rapidly periodic as to allow some flashes to correspond with the opening of the shutter 9 and yet avoid perceptible flicker. A modification, Fig. 3 (not shown), comprises an image-type tube wherein the optical image falls on a photo-cathode and the electron image is accelerated on to the mosaic. In this form the thermionic auxiliary cathode may be omitted and the flooding electronic stream produced by the photocathode using the same or a different shutter in flooding with light and changing and controlling the voltage applied to the photo-cathode to give the fast electrons. In other modifications, both the shutter 9 and the grid 11 may be omitted, their control being effected by suitable potentials applied synchronously to the photocathode and the thermionic cathode, respectively.