Patent GB-687270-A (GB)
Title: Apparatus incorporating a video storage tube and method of operating same
Inventor: THEILE RICHARD
IPC: H04N3/40
Filed: 1949-02-18
Abstract: 687,270. Cathode-ray tubes. CATHODEON, Ltd. Feb. 17, 1950 [Feb. 18, 1949; April 20, 1949; July 2, 1949], Nos. 4447/49, 10537/49 and 17548/49. Class 39(i) In a video storage tube in which either a light image or an electron image is projected on to the storage target, the storage plate is subjected during the flyback periods of the scanning beam and preferably during the frame fiyback periods, to the simultaneous actions of an irradiation adapted to cause the,storage surface to emit electrons and of a field which causes said emitted electrons to return to the storage surface and charge it negatively. In Fig. 1 a storage tube 11 has a photo cathode 12, a storage plate 14 e.g. of mica backed by a signal plate 15, a collector electrode 24 which may be a conductive coating and electromagnetic focusing unit. The plate is scanned by high velocity electrons from a gun 20. Pulses e1 are applied to the control electrode 27 of a miniature cathoderay tube having a fluorescent screen 29 of short afterglow giving pulses of diffuse illuminations at the photo cathode during the frame flyback periods. Simultaneously pulses e2 are applied across resistance 30 to render the collector electrode 24 negative during these periods. Blanking pulses may be applied at the same time to the amplifier to avoid transient effects. All pulses may be derived from a pulse generator deriving its master pulses from the existing frame flyback pulses. Pulses el should have a sloping trailing end to compensate for the afterglow of the fluorescent screen 29. The signal taken from the plate 15 contain D.C. components giving the mean value of light level of the transmitted picture. In a modification, the area of the storage plate subjected to pulsed irradiation lies adjacent to but outside the picture area giving an indirect negative charging of the picture area which does not completely remove the residual charge pattern of the previous scan. To this end the photo cathode is masked and a number of fluorescent tubes are mounted round an opaque funnel surrounding the lens system 18. In another method the frame flyback period is divided into two intervals. In the first, the pulsed negative charging process is applied and in the second larger interval the storage surface is allowed to build up a video charge pattern. A positive pulse may be applied to collector 24 to accelerate the charge storage. In a modification (Fig. 3) employing electron multiplication for output, a fine mesh screen 50 is placed close to the target 14b and a secondary emitting ring 51 with a mesh electrode 52 functions as the collector. The screen 50 is connected to earth and the signal plate. Simultaneously with the application of pulses el to the light source 25b, pulses e3 raise the signal plate to the potential of collector 51 and the potential on the mesh 50 drives the emitted electrons back to the storage plate. During the scanning period, the emitted secondary electrons are drawn through meshes 50, 52, and release secondaries from surface 51 which are collected at 52 giving amplified signal. The intermittent light source may be a gas discharge tube or other light source obturated by rotating or vibrating shutters to give light pulses. Irradiating pulses may also be obtained from an additional electron gun.