GB674678A - Improvements in or relating to electronic scanning apparatus for television and likesystems - Google Patents

Abstract

674,678. Cathode-ray tubes. COMPAGNIE POUR LA FABRICATION DES COMPTEURS ET MATERIEL D'USINES A GAZ. April 20, 1950 [April 25, 1949], No. 9708/50. Class 39 (i). Electronic scanning apparatus comprises a target electrode consisting of a thin layer of an insulating material which becomes conductive when struck by high velocity electrons placed between a transparent conducting layer and a layer of material of high secondary emission ratio, an electron image of a scene to be transmitted being incident on the target and rendering it locally conductive and a cathode-ray beam scanning the target which has such a positive potential that the coefficient of secondary emission of the beam is less than unity. In Fig. 1, the target comprises three layers, a conductive layer M, a layer I of insulating material, e.g. alumina, silica or mica so thin, e.g. less than 1 micron, that it becomes conductive when impacted by high velocity electrons and a thin layer S of a highly secondary emissive material such as magnesium oxide. High velocity electrons produced at the photocathode P by the image strike layer I and make it conductive. The potential of a source U connected through the load impedance Z to the metallic layer M renders the latter a few volts positive relative to the cathode K and the photo electrons from P have a double effect charging S positively by conduction from M and by the secondary emission of the photo electrons. The scanning beam discharges the charged area owing to the coefficient of secondary emission being less than unity and produces a signal in the plate M both by direct conduction and by capacitative connection between layers M and S, the latter predominating. The scanning beam may be modulated with the carrier frequency, and the load impedance connected to the metallic layer may comprise a tuned circuit resonating at the carrier frequency.