GB1430614A - Radiation sensitive scanning system - Google Patents

Abstract

1430614 Scanning system INTERNATIONAL BUSINESS MACHINES CORP 21 Nov 1973 [18 Dec 1972] 53978/73 Heading H4F [Also in Division G1] To compensate for variations in the sensitivities of the individual photo-diodes in a photodiode array, e.g. a self-scanned linear array 25 of photo-diodes analysing light reflected from a moving document 14, the array is initially arranged to scan a standard background or white level which may be in the form of a document or a bedplate, and the resulting sensitivity video signal which represents non-uniformity in sensitivity and in illumination is stored and used to correct subsequent video information signals. The integrated 30 sensitivity video signal has a fixed grey level signal from source 32 subtracted, 35, therefrom, is digitized in A-D converter 40 and stored in memory 50, e.g. a shift register. During scanning of an information document these stored signals are read-out from the memory 50, D-A converted, 55, and added, 60, to the fixed grey level signal from source 32, the difference between resulting sensitivity video signal and the serial video information signal being derived in differential amplifier 65 and then divided by the sensitivity video signal in circuit 70 to produce at output 75 a compensated serial video information signal which may be fed via a threshold circuit to a utilization system such as a recognition, reproduction or processing system. To compensate for photo-diode leakage current non-uniformity, correction signals derived by initially scanning a black level may also be stored and subsequently read-out. Memory 50 and converters 40 and 55 may be replaced by an analogue shift register.