GB807992A - Improvements in or relating to cathode ray image pick-up devices - Google Patents

Abstract

807,992. Television pick-up tubes. ELECTRIC & MUSICAL INDUSTRIES Ltd. June 16, 1955 [July 3, 1954], No. 19544/54. Class 39(1). A cathode ray pick-up tube circuit arrangement comprises a target 2 of slightly conductive glass having a separate photocathode 10 for projecting a composite stream of photoelectron on to one side of the target 2 with such a high velocity that the secondary emission ratio exceeds unity to form a positive charge image which is read off by a low velocity scanning beam from gun 3, 4, 5, "sticking, " which occurs after some hours of operation is removed and the tube "rejuventated" by operation for a short period by changing the electrode potentials so that the scanning beam has a high velocity and the photoelectrons from the photocathode have such a low velocity that the secondary emission ratio is less than unity lay down a negative charge image on the target. 10 hours of normal operation in the cps regime may be followed by one hour's operation in the high velocity regime to give "rejuvenation." This short period of rejuvenation can be achieved by increasing the scanning beam current (by increasing potential of 4) or by increasing the amount of light falling on the photo-electric cathode 10 (the photocathode being uniformly illuminated during rejuvenation). Sticking is defined as lag or persistence of signal output after cut off of light, the undesired output being often of reversed polarity, and is stated to be due to electrolysis in the glass target, which is usually of soda-lime glass, the sodium ions migrating from one side of the target to the other, proportionally to the photocathode illumination ; rejuvenation causes current flow in the opposite direction. Suitable electrode potentials for cps and high velocity electron beam scanning obtained from potentiometers 7 and 8 are given, switch over from one regime to the other being effected manually or by relays. The scanning gun 3, 4, 5 is surrounded by multiplier 9 from which the output signal is taken during normal low velocity operation, and is followed by persuader 6, shield 7, decelerating electrode 8, while the photocathode 10 is followed by accelerator 11 and electrode 13 connected to mesh 12. The focusing solenoid 16 ensures normal approach of the beam to the target 2, and 14, 15 are horizontal and vertical scanning coils.