Patent GB-668049-A (GB)
Title: Improvements in or relating to electron discharge tubes
IPC: H01J29/45
Filed: 1950-07-06
Abstract: 668,049. Cathode-ray tubes. PHILIPS ELECTRICAL INDUSTRIES, Ltd. July 6, 1950 [July 9, 1949], No. 16903/50. Class 39 (i). In a television transmitter tube comprising a screen for collecting image producing rays, in which for converting of the image to be transmitted into a varying electric current use is made of the permittivity variations of the light-sensitive layer on to which the image is projected, the light-sensitive layer is applied to the electrically conductive surface of a support and coated with a thin layer of insulating material on the side which is scanned by a directive electron beam. As shown the electrode system 1 is of known type, the electron beam 2 being deflected by coils 14, and the positive terminal of the voltage source 9 is connected to a collecting electrode 10 which may be a conductive coating of the tube wall 4. The electrode 10 is connected through a resistor 11 to the screen 3, the voltage across resistor 11 being supplied to the grid of an amplifying valve and the image of an object 13 to be transmitted is projected on to the screen 3 by means of a lens 12. The projection screen 3 consists of a transparent support 15 of'insulating material, for example, mica, to which is applied a thin transparent layer of electrically conductive material 16 forming the signal plate. The light-sensitive di-electric 17, which may consist of, for example, zinc sulphide, cadmium sulphide, zinc selenide, cadmium selenide, or a mixture of two or more of these substances, is coated with an insulating layer 18, which may consist of silicon dioxide, or manganese dioxide, and should be as thin as possible to allow a maximum portion of image producing rays to pass but prevent the electrons of the beam from passing. If this layer is made transparent the image may be projected on to either side of the screen 3. The coating co-operating with the signal plate 16 to form a capacitor in the multiple capacitor system forming the image electrode is constituted by the point of impact of the electron beam 2, and the electron velocity is adjusted so that the number of secondary electrons released at the point of impact exceeds the number of electrons striking the screen. Initially substantially the whole secondary emission current is carried off to the collecting electrode 10 so that the potential of the surface area struck by the beam increases, and gradually fewer electrons reach the collecting electrode. Secondary emission electrons spread over the surface of the layer 18 causing the potential of the surface to decrease to a value slightly lower than that of the collecting electrode, and in this way a potential jump, which at a constant electron velocity is uniform for any point of the surface, occurs at the surface struck by the beam relative to the remaining part thereof, and the variation of the electric charge of each capacitor element thus produced only depends upon the di-electric constant of the light-sensitive substance at this area. Sensitivity may be increased by including a source of constant voltage in the connection between the electrode 10 and the signal layer 16.