Photographic camera with exposure control utilizing a phase controllable crystal

Claim 1

An exposure-terminating arrangement for photographic cameras capable of making flashbulb exposures, the arrangement comprising, in combination, a pair of parallel planar polarization filters, a planar phase element located intermediate the filters and parallel thereto, the filters and the phase element being arranged in the path of light entering the camera normal thereto, the phase element having two control electrodes and being operative for rotating the polarization direction of polarized light passing therethrough in dependence upon the potential difference across its two control electrodes, a light-integrating device operative for receiving scene light and generating an integral signal corresponding to the time integral of the received light, and a threshold detector circuit having an input connected to the light-integrating circuit for receipt of the integral signal and having a pair of output terminals, each control electrode of the phase element being connected only to a respective one of the threshold detector circuit output terminals and the operation of the phase element accordingly being determined exclusively by the potential difference across the threshold detector circuit output terminals, the threshold detector circuit being capable of assuming only two states, the first being a state in which it applies across the control electrodes of the phase element a potential difference causing the phase element to permit polarized light emerging through the first filter to pass also through the second filter, the second being a state in which the threshold detector circuit applies across the control electrodes of the phase element a potential difference causing the phase element to prevent polarized light emerging through the first filter from passing through the second filter, the threshold detector being in the first state when the integral signal is below a predetermined value, the threshold detector assuming its second state and accordingly causing the phase element to terminate the exposure when the integral signal reaches the predetermined value, whereby when the integral signal quickly reaches the predetermined value due to high flashbulb scene brightness levels the exposure can be immediately terminated without dependence upon the response times of moving parts.