GB896123A - Improvements in or relating to cathode ray tubes - Google Patents

Abstract

896,123. Cathode-ray tubes. GENERAL ELECTRIC CO. Ltd. Oct. 14. 1960 [Nov. 5, 1959], No. 37624/59. Class 39(1). A cathode-ray tube designed to operate with post-deflection acceleration of the electron beam comprises an electrically-conducting mesh electrode 14 in the vicinity of, and parallel to, an electrically-conducting target screen 8, the surface of the mesh 14 remote from the screen being coated with a layer of dielectric material 16 whose secondary electron emission properties are such that the layer is charged positively with respect to the mesh in normal use of the tube, whereby electric fields are formed within the meshes of electrode 14 which prevent the low velocity secondary electrons from passing through to screen 8. A hollow, open-ended, conducting shield 18 of appreciable axial length is disposed on the side of the mesh 14 remote from the screen, with one end adjacent to but insulated from the mesh, which extends radially outwards beyond the adjacent end of the shield around the whole of its periphery. As shown, the metallic wire mesh 14 is carried by a frame 15 which is spaced from shield 18 by insulating supports 20, and which is provided with an external contact 17 whereby the mesh is maintained at a potential slightly, e.g. 50 V, negative with respect to shield 18. The shield 18 may be frusto-conical instead of cylindrical and may contact the tube wall by means of a radial flange instead of being secured thereto by members 19 as shown, but is in each case maintained at the same potential, e.g. 1 kV positive with respect to the cathode, as the final anode 11 of gun 5 by means of a graphite or silver wall coating 10. The conducting layer 8 on luminescent layer 6 is 10 kV positive with respect to the cathode and is connected to coating 10 by a resistive coating 13 of chromic oxide. The dielectric layer 16 may be of silica, alumina or magnesia and during operation is maintained approximately at the potential of shield 18. The phosphor layer 6 may be replaced by three superposed transparent phosphor layers which emit in different colours with or without intermediate transparent layers of electron energy absorbing material. Voltage colour signals from a colour synchronizer circuit are applied to the aluminium film 8 to cause the electron beam to penetrate to each of the phosphor layers in an appropriate sequence. Specification 896,122 is referred to.