Patent GB-413954-A (GB)
Title: Improvements in television
IPC: H01J31/40
Filed: 1933-12-13
Abstract: 413,954. Television. HENROTEAU, F. C. P., 150, Elgin Street, Ottawa, Canada. Dec. 13, 1933, No. 35104. Convention date, Dec. 15, 1932. [Classes 39 (i) and 40 (iii).] An optical image is impressed on a photo-sensitive surface 65 to form an electrostatic reproduction, and a surface 56, other than the sensitive surface, is scanned to emit an electronic stream of constant magnitude, which is picked up at an anode 22 modulated by the electrostatic reproduction. Transmission is carried out in the following four stages. (a) A plate 21 with many small apertures 68 is given a low positive potential by a commutator 24, and a second apertured plate 20 and the anode 22 consisting of a circle of wire are earthed by commutators 25, 26. An image of the view is projected through an aperture 16 in a rotating disc 15, the end 2 of the cell, which is transparent through the inner metallic coating 9 being there a mesh, and the openings 68 in the first plate to the photosensitive surface 65 of caesium on the second plate. Each part emits a number of electrons depending on the light falling on it, and these are picked up by the nearest of insulated conductive elements 61 on the first plate, owing to its positive potential. The elements thus carry, in negative potentials, an electrostatic equivalent of the view. (b) The light is interrupted by the disc 15, the first plate is brought to a very low positive potential, the second plate remains earthed, and the anode is raised to a high positive potential by the commutators 24, 25, 26 respectively. A beam 55 of cathode rays from a cathode 6 is moved by electromagnets 7, 8 to scan the surface 56 of the second plate facing the anode and causes a secondary electronic emission. These electrons are subjected in part to the attraction of the positive potential of the first plate less the negative potential of the nearest conductive element 61. The number reaching the positive anode is thus modulated according to the view. The modulated potential of the anode is impressed on the grid of a valve 41 and transmitted. To prevent the high-speed electrons, caused by the incidence of the cathode-beam, from producing interfering secondary emission, these electrons are picked up by a grid 12 at high positive potential in an earthed screen 11. (c) Scanning is stopped by earthing the cathode 6, the two plates and the anode are earthed, and light is admitted through the opposite transparent end 3 of the cell by an aperture in a rotating disc 19. A filter 18 allows only the passage of light of a wave length which, on striking the thin photosensitive surfaces 62 on the conductive elements 61 of the first plate, causes the emission of slowly moving electrons. As the elements 61 are negative, these electrons are picked up by other members in the cell. The insulating coating 58 between the elements 61 is aluminium oxide, which passes current from positive to negative more readily than from negative to positive. Thus, as soon as an element 61 reaches a positive potential, owing to the emission of electrons, leakage current prevents any increase in positive potential, and they all reach a uniform slightly positive potential. (d) The electrodes are kept earthed, and the light is shut off by the disc 19. All the elements 61 are then reduced by leakage current to a uniform zero potential. According to a modification a continuous transparent insulating plate is inserted between the apertured plate 21 and the conductive elements 61, Fig. 4 (not shown). By increasing the length of exposure of the plate 20 to the view, faint images may be reproduced distinctly, for example those obtained in telescopes or microscopes. At the receiver the plates 20, 21 are replaced by a transparent phosphorescent screen, and the period of phosphorescence is shortened and the brightness increased by submitting the screen to the action of infra-red light through the opening 3 between each scanning. Three similar apparatus may be used in conjunction so that the transmission is continuous. Specifications 335,958 and 335,995, [both in Class 39 (i)], are referred to.