Patent GB-841453-A (GB)
Title: Stabilized image scanner
IPC: H04N5/257
Filed: 1958-08-05
Abstract: 841,453. Colour television. HAZELTINE CORPORATION. Aug. 5, 1958 [Aug. 14, 1957], No. 25045/58. Class 40(3). [Also in Groups XXXV and XL(c)] In an image scanning system including a light source and a photo-electric pick-up device, means are provided for periodically developing electrical signals representative of the intensity of the light source and for utilizing the signals for stabilizing the product of the light source intensity and the signal gain of the scanning system. The invention is described as applied to an electronic film previewer, Fig. 1, of the type described in Specification 841,125 in which a negative film 11  is scanned by a cathode-ray tube flying spot scanner 10 to develop red, green and blue component signals in photo-multiplier pick-up devices 16R, 16G, 16B, the signals in the respective channels being passed through amplifiers 37R, 37G, 37B to electronic processing apparatus 81 the output from which is employed to reproduce a positive image on cathode-ray tube 82. In accordance with the invention, scanner 10 is caused to overscan the area of film 10 so that periodic samples of the light are by-passed to the pick-up device to introduce reference pulses, Fig. 3, into the resulting signal trains. The reference pulse in each channel is then applied to a differential feed-back circuit 38R, 38G, 38B in which it is gated out by a control pulse applied at a terminal X and compared in amplitude with a reference voltage to derive a control signal. As illustrated, each control signal controls the gain of the associated photo-multiplier 16. As an alternative, or in addition, each control voltage may control the gain of the associated channel amplifier 37. As a further alternative, or in addition, the control signals may control the intensity of the scanning spot in cathode-ray tube 10. If a three-gun, threecolour, scanner tube is employed, each control signal may control the intensity of the corresponding colour component of the scanning light. If a single-gun scanner tube is employed, the three-control signals are combined to form a composite signal for application to the single control electrode. Fig. 2 illustrates a circuit in which the reference pulses are employed to control the gain of a photo-multiplier pick-up device. The composite signal train from the output of the channel amplifier 37 is restored to a reference level by diode 41, which responds to pulses produced by blanking during the retrace intervals, and the reference pulses are then separated out and applied to grid 46 of pentode 45 by way of a four-diode gate 53 which is opened under the control of pulses applied at terminal X. The cathode 47 of the pentode is maintained at a reference potential determined by voltage reference tubes 40, 73. An integrating capacitor 75 maintains the grid potential at the peak level of the reference pulses applied through gate 53 with the result that the pentode passes a steady current determined by the pulse amplitude. The pentode current controls the potential drop across a network 30 which supplies the operating voltages to the secondary-emission electrodes 18-27 of the photo-multiplier device 16.