GB1319688A - Multichannel light compensation apparatus - Google Patents

Abstract

1319688 Television shading corrections HAZELTINE CORP 8 Oct 1970 [6 Nov 1969] 47885/70 Headings H3T and H4F In a multi channel picture scanning system having a light source which projects a television type raster on to each of a plurality of pictures (e.g. transparencies), the modulated light being picked up by a separate light detector for each picture, nonuniformities in the light translation characteristic of each channel are corrected by varying the gain of the detector as a function of the position of the light spot in the raster. As shown, Fig. 1, horizontal and vertical sync. pulses from the scanning source are applied to function generators 6, 7 the outputs of which are applied to cross coupling terminal board 12 by means of which suitable horizontal and vertical waveforms are selected and applied to a plurality of dynode drivers 13 to 17 each of which varies the gain of an associated photomultiplier 18 to 22 by varying the voltage applied to one of the dynodes. The generators provided comprise an exponential function generator 8, a parabolic function generator 9 and an additional function generator 10 which can provide other (unspecified) functions. The system is initially set up by scanning a blank picture and selecting appropriate horizontal and vertical functions to obtain a uniform output from the photomultiplier. The terminal board may then be replaced by hard wiring. The exponential function generator comprises a capacitor (25) charged through a resistor (26) a transistor (24) being provided which is switched on by the sync. pulses and discharges the capacitor, Fig. 2 (not shown). Complementary outputs are provided via buffer and inverting amplifiers. The parabolic function generator comprises a ramp generator (30) followed by an integrator (31). The ramp generator comprises a capacitor (33) charged by constant current generator (34) and the integrator comprises an operational amplifier with a capacitor connected in the feedback loop. The ramp generator and integrator capacitors are periodically discharged by transistors (32 and 29) which are switched on by the sync. pulses.