Automatic brightness control for image intensifier tube

Claim 1

An automatic image brightness control system for a light imaging device comprising an image intensifier tube, a power supply circuit and means for coupling said power supply to said tube, said tube being adapted for operation over the range of input light illumination levels from 10 5 to 10 2 foot candles; wherein said tube includes a plurality of cascaded image intensifier stages each stage including a photocathode electrode and an anode electrode wherein each anode electrode is coupled to the succeeding photocathode electrode, the photocathode electrode of the first of said stages being a photocathode input electrode for emitting electrons in response to the intensity of light applied thereto, and the anode electrode of the last of said stages including a phosphor display screen output electrode providing an image display in accordance with said electrons impinging thereon; wherein said power supply includes a DC voltage source, an oscillator circuit having an input and output, a first resistive impedance serially coupling said DC voltage source to said input of said oscillator circuit wherein the alternating voltage at said output of said oscillator circuit decreases in proportion to the direct current being supplied to said oscillator circuit, and a diode-capacitor voltage multiplier having an input terminal and a plurality of discrete DC voltage output terminals, said input terminal of said voltage multiplier being coupled to said output of said oscillator circuit; and wherein said means for coupling said power supply to saId tube includes stepped connections respectively coupling output terminals of said voltage multiplier to said stages of said tube, the one of said connections coupling a given one of said output terminals of said voltage multiplier to said photocathode input electrode of said tube including a serially connected second resistive impedance to individually control the voltage applied to the first of said stages in response to the intensity of the light being applied to said photocathode input electrode, said second resistive impedance having a given fixed value which at input light illumination levels substantially below 10 2 foot candles results in only a negligible voltage drop existing across said second resistive impedance and at input light illumination levels substantially above 10 2 foot candles results in a voltage drop across said second resistive impedance sufficient to substantially reduce the voltage applied to the first of said stages.