GB529410A - Improvements in or relating to television transmitting cathode ray tubes - Google Patents

Abstract

529,410. Photo-electric cells. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 30, 1939, No. 15964. Convention date, May 31, 1938. [Class 39 (i)] A television transmitting tube has a mosaic electrode and a photocathode adjacent to and substantially coplanar with each other and means for scanning the mosaic with a beam of electrons from the photocathode. In the form shown in Fig. 1, an optical image of the object 5 is focused on to a single-sided mosaic screen 4, the back coating 15 of which is earthed at 16 and connected to a triode amplifier 17. The scanning pattern on the fluorescent screen of a cathode-ray tube 8 is focused at 9 on to the earthed photocathode 7 and the two electrodes are separated by a coarse mesh accelerating electrode 10 (or in its place a ring electrode) which is maintained slightly positive with respect to earth. A focusing coil 12 of diameter equal to or greater than the maximum distance between the remote edges of the electrodes 4, 7 produces a substantially semicircular field, as shown in dotted lines, which intersects the electrodes 4, 7 normally to their surfaces. A recess 11 in the tube receives the coil so as to give this normal intersection. The slow velocity electrons emerging from the photo-electric layer 19 under the action of the scanning beam of the tube 8 form in effect a record scanning beam and are directed by the electrode 10 on to the mosaic 4, giving signals in proportion to the intensity of the image of the object 5. Since any low velocity electrons from the photocathode reach the mosaic, "dark spot" signals are not produced, and the coplanar disposition of the two electrodes 4, 7 precludes light from the flying light spot from reaching the mosaic. Fig. 2 shows a form in which the mosaic and photocathode 4, 7 are placed over a common flat part of the tube above the poles of a horse-shoe electromagnet 20.