GB1264277A - - Google Patents

Abstract

1,264,277. Image detector. STANDARD TELEPHONES & CABLES Ltd. 30 June, 1970, No. 31499/70. Heading H4F. [Also in Division G1] An image reproducer and/or converter, insensitive to changes in the level of overall or ambient illumination, comprises a parallel connected array of photo-conductive detectors 10, coupled to respective light emitting means 19, e.g. diodes, and fed by a constant current source, such that even though such an overall change causes the conductance of the detectors to vary by equal amounts, there is no change in the current flowing in the detectors and so no change in the brightness of the light emitting means. A localized change in illumination, e.g. that of the object of interest, varies the distribution of current flowing within the array and a corresponding change in the brightness of the respective light emitting means. The constant current source comprises a constant voltage source having its voltages set by potentiometer 17 and comparing its voltage with that set up by the detector currents across current monitoring resistor 15. Departure of the voltages from equality causes an adjustment of the base voltage of transistor 14 to regain said equality. Each detector has a trimming resistor 12 and a load resistor 11, the latter supplying an output voltage for application, via an operational amplifier 18 to the photodiode. The output voltage is compared at the amplifier with a reference voltage derived from the constant current source. The detectors may be infra-red sensitive Mercury/Cadmium/Telluride detectors to give a thermal to visible image conversion.