GB462550A - Improvements in or relating to television transmission systems comprising cathode ray tubes - Google Patents

Abstract

462,550. Cathode-ray tubes; lightsensitive cells. MILLER, H. Sept. 10, 1935, No. 25202. [Glasses 39 (i) and 40 (iii)] Mosaic screens for television transmission utilize the photo-voltaic effect. The screen consists of a semi-conducting layer capacitatively associated with a conducting electrode and separated from it by a " blocking layer " of high resistance. When the junction between the semi-conductor and the blocking layer is illuminated by the image to be transmitted, electrons flow from the semi-conductor to the conducting electrode. Either electrode may be made mosaic and the scanning may be by electron or light beam. In one example, a copper signal plate is coated with a layer of cuprous oxide 0À01 mm. thick, by heating in air and washing with sodium cyanide. The blocking layer is formed by electron or ion bombardment of the cuprous oxide or by coating with resin or by vacuum sublimation of calcium fluoride or quartz. A mosaic of silver elements is evaporated on to the blocking layer through a grid. The mosaic is scanned by a cathode-ray beam producing secondary electrons which restore the mosaic elements to datum potential and induce picture signals in the copper plate. The mosaic potentials and signal impulses are reversed in comparison with a photo-electric mosaic, and " tilt " distortion due to collection of stray electrons by positively charged elements is eliminated. Further, a sudden change of average illumination does not produce a signal impulse. A screen in which the semi-conducting electrode is mosaic may be prepared by oxidizing an aluminium signal plate and spraying this oxide blocking layer with discrete particles of cuprous oxide, sufficiently transparent to allow access of light to the junction with the blocking layer. Signals in this case are in the same sense as the normal photoelectric mosaic. Instead of projecting the image on the scanned side of the screen, the signal plate may be sufficiently transparent to allow the image to be thrown on the opposite side, as described in Specification 455,123. The signal plate may be a semi-transparent layer, upon a mica support sheet, upon which is deposited a layer of cuprous oxide crystals transparent to red light, followed by a blocking layer and a silver mosaic. Or a thin copper plate may be oxidized right through and etched until transparent. The signal layer is then evaporated onto one side and a blocking layer and silver mosaic formed on the other, the whole being supported by a mica sheet adjacent the signal electrode. Alternatively, the mosaic elements may be semi-conducting cuprous oxide crystals as described above. A screen adapted to be scanned by a light beam is shown in Fig. 4, and comprises a signal plate 6 and a semi-conducting layer 7, both transparent, an opaque blocking layer 8, and a mosaic 9 of photo-sensitized elements. The screen is mounted in an evacuated vessel with a ring-shaped collecting anode, the picture is thrown onto the righthand side and the mosaic elements are scanned from the opposite side by a light spot, preferably from an unmodulated cathode-ray tube. Photoelectrons are emitted which discharge the mosaic and are collected bv the anode.