GB1241404A - Improvements in or relating to cathode ray tube optical data scanners - Google Patents

Abstract

1,241,404. Character recognition. SIEMENS A.G. 1 April, 1970 [2 April, 1969], No. 15356/70. Heading G4R. [Also in Divisions G2-G3] In a cathode-ray tube optical data scanner of the type wherein a lens 6 forms an image of the tube light spot in the plane of a data carrier 3, which reflects light from a read-out field 4 to photo-electric transducers 7, 8 whose summed video signals are passed to an A/D converter 12, light-receiving elements 14 are placed in a viewing position relative to the tube screen corresponding to the position of the transducers 7, 8 relative to the read-out field 4, the combined output of elements 14 are passed to a photo-electric transducer 15 to form a control signal which is compared in a control circuit 16 with a reference signal, and the error signal so produced is used to control the radiation intensity of the tube 5 so that the control signal is maintained as closely as possible to the reference signal. The control system prevents the occurrence in the signals from transducers 7, 8 of undesired modulations caused by the differing sensitivity of the transducers for different angles of incidence of the light. These modulations are compensated by the control system so that the signal supplied to the circuit 12 only contains disturbances produced by the differing reflective properties of the data carrier 3. The geometry of the system containing members 3 ... 6, 7, 8, 14 is analysed to explain how the signals from members 7, 8, 14 are modulated in accordance with the same law relating the various angles and distances of the system, Fig. 2 (not shown). Members 14 may be optical conductors or photo-electric transducers such as electron-multipliers. Members 7, 8 may be replaced by a system similar to members 14, 15. The control circuit 16, Fig. 3 (not shown), comprises a differential transistor amplifier supplied with the control and reference signals and controlling the grid of the tube 5 via the supply 20. The circuit 16 also includes an auxiliary circuit to override the signal from the amplifier so that the tube may be dark modulated at predetermined instants. The scanner scans two data-carrier transport paths alternately.