GB481094A - Improvements in and relating to cathode ray devices - Google Patents

Abstract

481,094. Cathode-ray tubes. BRITISH THOMSON-HOUSTON CO., Ltd., and GABOR, D. June 4, 1936, No. 15673. [Class 39 (i)] A cathode-ray tube for producing persistent records on a fluorescent or other recording member, contains a surface capable of emitting secondary electrons which is struck by two beams, a fast recording beam of small crosssection and a diffuse flooding beam of low velocity. The surface normally repels the flooding beam, but when struck by the recording beam assumes a positive equilibrium voltage which allows the flooding beam to strike it and pass to a fluorescent or other recording screen, the positive equilibrium voltage being maintained by the flooding beam after the recording beam has passed. In one form, the recording beam from cathode 18, Fig. 4, is deflected in two directions by a double plate system so that the beam always passes through an aperture 22 and strikes a grid 16 of metal, such as aluminium, zirconium or tantalum, which is oxidized or otherwise coated with insulating material and preferably also with insulated particles of secondaryemitting metal. The aperture 22 is surrounded by the annular main cathode 17 which emits under the influence of focusing electrode 28 a beam of electrons diverging uniformly over the whole of the grid 16. Each element of the grid, when struck by the recording beam emits an excess of secondary electrons and assumes a positive voltage dependent only on the emissive characteristic of the grid and the resistance of the insulation thereon, and not on the intensity of the beam. This voltage permits the passage of the flooding beam which is focused by a coil 19 upon the fluorescent screen 20. As the voltage is maintained by the flooding electrons after the recording beam has passed, the curve traced by the recording beam will persist until the flooding beam is cut off. For television purposes the recording beam is modulated in diameter and covers a number of mosaic elements. The contrast may then be increased by superimposing an alternating voltage on the line scanning voltage so that the scanning beam moves in jerks from one picture point to the next, producing an effect similar to a half. tone screen. If the flooding beam is switched off at the end of each picture, the last points of the picture will be of smaller intensity than the first. To avoid this, the flooding beam may be arranged as a band several lines wide, which follows the scanning beam in the frame scanning direction. Alternatively, the flooding beam may be in the form of two rectangles, each the size of the picture, separated by a thin dark band which immediately precedes the scanning line and wipes out the picture. During fly-back the dark band may be rendered inoperative by defocusing. In one such construction, shown in Fig. 2, the scanning beam from cathode 50 is focused by a system 51 which is supported in an insulating member 52 within the tubular screen 53. A double deflection system, the vertical set of which is shown at 54, 55 and 56, 57, deflects the beam so as to pass always through the aperture 62. The aperture 62 is also illuminated by a diffuse beam from the annular cathode 59 and has a thin cross-bar 63, the image of which in the lens system 64, 65 wipes out the picture when deflected by plates 66, 67. In another modification, the fluorescent screen itself acts as a " gate " having embedded in it a metal mosaic the elements of which repel electrons from the flooding beam until brought to a positive voltage by the secondary emission of the recording beam. The fluorescent material is deposited as separate grains on the mosaic.