Patent GB-1160923-A (GB)
Title: Optical Relay for Television Purposes
IPC: G02F1/03
Filed: 1967-01-23
Abstract: 1,160,923. Television display systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 23 Jan., 1967 [26 Jan., 1966 (2)], No. 3283/67. Heading H4F. [Also in Divisions G3 and H1] In a projection television system, Fig. 1, a plate 12, which exhibits the Pockels effect is positioned between crossed polarizers 14, 16 and utilized to modulate a beam of light from a source 6 in accordance with a video signal applied to a control electrode 32 or anode 30 so that a television image is projected on to screen 2, the plate 12 being scanned by an electron beam from gun 20. The plate 12 comprises an acid salt material in which the hydrogen has been enriched with deuterium, e.g. KD 2 PO 4 , and is utilized at a temperature approaching the Curie temperature, i.e. the temperature below which it becomes ferroelectric, and is held at the Curie temperature by means of a thermal control device 18. The K-ion may be replaced by Rb or Cs and the PO 4 -ion by AsO 4 . A direct voltage is applied to anode 30 which although shown as a plate parallel to the light beam it is preferably arranged parallel to the face of plate 12, e.g. in the form of a copper grid. The control electrode to which the video signal is applied in the described embodiment is formed by a thin metal layer (gold, silver, chromium) covered by metal oxide layers for adhesion to plate 12. In operation secondary electrons are released from plate 12 by the scanning electron beam and collected by anode 30 resulting in the potential of each elemental point scanned reaching a limit value with respect to the anode potential. Thus with constant anode potential each passage of the electron beam fixes the potential of any point on the face of the plate at a value V 0  and if V A  is the potential of the control-electrode 32 at the instant of passage of the electron beam the charge and thus the potential difference between the two faces of the plate is V o  - V A . The resulting electric field which is at rightangles to the plate 12 and electrode 32 is constant between two passages of the electron beam and is governed during the passages by the video signals. The constance of the electric field prevents flickering of the image. In a detailed embodiment, Fig. 2, the light beam indicated by arrows 40 striking the plate 12 is reflected on the rear face of the plate by a mirror 42, Fig. 3, comprising seven layers alternately of zinc sulphide and cryolite. To increase the secondary-emission coefficient a cryolite layer 44 of double the thickness is added which is scanned by the electron beam from gun 20, the secondary electrons being collected on grid anode 30 (Fig. 3). The crossed polarizers are arranged in front of the tube or alternatively a "Brace" prism, Fig. 4 (not shown), or just a single polarizer may be used. The control electrode 32, Fig. 3, is formed by a metal layer (gold, silver, chromium) covered with one or more metal oxide layers 321, 322. Grid 30 may be coated with an insulating layer and stuck to the modulating plate 12. The plate 12 is stuck to a fluorine plate 66 arranged in a copper trough 62 which is cooled by a hollow ring 80 in order to maintain the temperature of plate 12 constant. The hollow ring 80 communicates with a Joule- Thompson cryostat 81 to which nitrogen is supplied under pressure from a container 84, Fig. 7. The cryostat comprises a narrow tube 82 which terminates in a small aperture and is wound inside an inner tube 83. The gas expands through the aperture so that it is cooled and then escapes along the inner tube cooling the incoming gas thus gradually reducing the temperature of ring 80. The supply of nitrogen from container 84 is regulated by a device 86 which is controlled by an electromagnetic device 100 in turn controlled by the variations in the capacitance of a plate 88 of the same material as plate 12, this plate 88 being in thermal contact with the anode grid support and thus plate 12. When the light beams are incident at an angle to the plate 12 the phase shift produced by the double refraction of the plate is cancelled by positioning a crystal plate, e.g. quartz, in the optical path parallel to plate 12 having a double refraction of opposite sign. Since quartz exhibits optical activity the plate is divided into two plates, one dextrorotatory and the other levorotatory. The light beam may be directed on to the plate 12 by means of a mirror or total reflection prism, Fig. 8 (not shown), or a "Glazebrook" prism may be used, Fig. 9 (not shown).