Patent GB-598802-A (GB)
Title: Improvements in or relating to cathode ray light control devices
IPC: H04N5/74
Filed: 1945-09-11
Abstract: 598,802. Light valves ; cathode-ray tubes. EHRENBERG, W. Sept. 11, 1945, No. 23353. [Classes 39 (i) and 40 (iii)] In a cathode-ray light control device in which the deformation of a screen by electrostatic forces arising from scanning by the cathode-ray beam is used to modulate a beam of light means are provided for charging an additional area of the screen near part of the boundary of the scanned area so that undesired deformations are moved away from the boundary. As applied to apparatus for reproducing television signals a deformable screen 1 consisting of a thin semi-conducting film of oil spread on a glass disc 2 is scanned by two electron beams 4, 29 from cathode-ray guns 3, 25 associated with deflecting means 5, 6 and 26, 27, controlled by oscillators .31, 32 respectively. The beam 4, which is modulated by the television signals, scans the patch 7 and the resulting electrostatic force between this patch and a thin transparent conducting layer acting as a counter-electrode to film 1 and placed between it and the glass disc deforms the film in accordance with the modulation of the vision signals. This deformation controls a beam of light from a source 8 directed on to patch 7 by reflectors 9, 10 through a grating 12 and lens system 14 which is adjusted to focus the light passing between the bars of the grating 12 exactly on to the bars of grating 13 when the patch 7 is undeformed. Deformation of 7 then effectively varies the focus of lens system 14 so that each elemental area of the deformed patch allows a quantity of light, proportional to the deformation and therefore to the vision signals, to pass through grating 13 and thence by means of lens system 19 and reflector 20 to be projected on to a screen 21. The electron beam 29 from gun 25, which is unmodulated, is deflected over a patch 28 overlapping patch 7, means, not shown, being provided for blanking the beam when it is passing over patch 7 so that only the margin 30 is scanned and thus receives a charge density substantially equal to the mean charge density applied to patch 7 by beam 4. The boundary discontinuity of patch 7 is consequently removed from the region scanned by beam 4, since the margin is deformed uniformly to an extent equal approximately to the average deformation of 7. To prevent permanent deformation of the scanned patch, the glass disc 2 is slowly rotated in the direction of the arrow 23, the patch being first cooled beneath cover 24 and then rendered plane by means of a knife 22. Satisfactory results are obtained when the area charged by beam 29 lies only along the edges 33 of patch 7 which extend in the effective direction of motion of the disc across the centre of the scanned area, and instead of extinguishing the beam 29 while passing over patch 7 it may be reduced to low intensity or displaced rapidly from one edge of the patch to the other. More than one gun such as 25 may be employed to charge the additional area or the gun 25 may be replaced by a suitable source which continuously sprays this area with electrons. Specifications 543,485 and 548,643 are referred to.