in place of the electronic shutter system of the preferred embodiment the capacitor 10 now discharges through the image intensifier tube 3 with the tube 3 acting as a variable resistance in the discharge circuit the variable resistance is a function of the light intensity received by the photocathode 4 the current through the image intensifier tube 3 will exponentially decay as shown in the waveform 32 of fig2 with the rate of decay dependent on the light intensity incident to the photocathode the capacitance and operating voltage of the variable capacitor are chosen such that they correspond to the energy required to provide proper exposure through the image intensifier tube 3 and onto the photographic emulsion 7 the photographic emulsion requires s ergs cm 2 of exposure to an activated image intensifier phosphor screen 6 to record an image the total exposure received over the exposed emulsion will then be u03c0d 2 s 4 s 4 ergs where d is the diameter of the phosphor screen in cm if the radiant efficiency of the phosphor screen is e ergs out ergs in then u03c0d 2 s 4e ergs of energy must be supplied to the device for proper exposure the amount of energy can be supplied by the variable capacitor when 1 2cv 2 u03c0 d 2 s 4e where c is the capacitance and v is the voltage further it is obvious to one skilled in the art that in applying the formulas to determine the capacitance allowance must be made for reactance losses phosphor screen dead voltage losses and the like from this it can be seen that in practical application the voltage from the.