GB672205A - Improvements in or relating to television pick-up tubes or the like - Google Patents

Abstract

672,205. Cathode-ray tubes. COMPAGNIE POUR LA FABRICATION DES COMPTEURS ET MATERIEL D'USINES A GAZ. Nov. 14, 1950 [Nov. 14, 1949], No. 27816/50. Class 39 (i). In a television pick-up tube or the like of the kind in which an electron image corresponding to an optical image to be transmitted is incident on a storage target electrode, the target electrode comprises a conducting mesh electrically connected to the output circuit of the tube and a continuous insulating layer disposed in close proximity to the mesh on one side thereof so as to cover both the mesh and the interstices therein, and adapted to be rendered locally conducting under the control of an electron beam which is arranged to scan over the mesh side of the target electrode, the electron image being arranged to be incident on the side of the said electrode opposite to that scanned by the electron beam. As shown, the storage target comprises a metallic mesh 1 supporting a thin film 2 of aluminium or beryllium which is easily permeable to the fast electrons in a cathode-ray beam 6 emitted by the electron gun. A thin layer 3 of a good insulator, for example quartz or magnesium fluoride, is deposited on the film 2, and the film 2 and mesh 1 are.connected to the output impedance 7. An optical image is focused on a transparent photo-cathode 9 and the electrons produced by photo-electric emission are accelerated by means of the cylindrical anode 10, the electron image being focused on the layer 3 by means of the coil 11. Charges are stored on the layer 3 which are a function of the illumination of each image element, these charges being positive if the secondary emission is greater than unity. When an element of the layer 3 is traversed by the beam 6 it becomes instantaneously conductive, the neighbouring elements retaining their insulating property, but as soon as the beam has left an element the insulation is re-established with a time-constant which is a function of the substance employed and its thickness. If the film 2 is made highly negative with respect to the anode 10, the production of a conductive path in an element of the layer 3 provides an intense video signal. In a modification the mesh 1 is filled with a substance which fluoresces with a short afterglow under the impact of the beam 6, and instead of the layer of quartz a thin layer of cadmium sulphide or other material exhibiting photo-conductivity, covers the surface of the mesh 1 on the electron image side. There will be a storage of positive charges at the surface of the cadmium sulphide layer as in the previous case, but at the instant of scanning an element of the layer is strongly illuminated by the light produced in the fluorescent layer and the resistance of the cadmium sulphide layer at that point falls to provide a conductive path. The photo-conductive target can be improved by depositing at the surface a metallic mosaic, for example comprising silver in the form of microscopic islets insulated one from another.