1. In combination: a circuit node; a radiation sensor having a control terminal to which a control signal may be applied for adjusting the radiation sensitivity of said sensor and having also an output current path connected to said node for supplying an output current thereto, said output current being proportional to the intensity of radiation applied to said sensor at a given control signal level applied to said control terminal; a second current path connected to said circuit node for withdrawing from said node a fixed current level, said second current path accepting the current supplied to said node by said output current path; circuit means for supplying said control signal to said sensor and for maintaining said circuit node at a given virtual reference voltage level, said circuit means including means responsive to any tendency for the current level in said output current path to change, for changing the value of the control signal in a sense to return the flow of current in said output current path to its original value; and low pass filter means for initially receiving said control signal produced by said circuit means, attenuating said control signal and applying the attenuated control signal to said sensor, said low pass filter means having a first and a second, higher, break frequency and providing minimum attenuation of said control signal at frequencies below said first break frequency and a fixed maximum value of attenuation of said control signal at frequencies above said second break frequency.

7. In combination: a differential amplifier having inverting and non-inverting input terminals and an output terminal, said non-inverting terminal being maintained at a reference potential; first means for applying a bias signal of one sense to said inverting input terminal; radiation responsive means for applying a further bias signal of opposite sense to said inverting terminal, the magnitude of said further bias signal being representative of a radiation input signal and a control signal supplied to said radiation responsive means; and filter means for supplying said control signal to said radiation responsive means in response to an output signal produced at said output terminal of said differential amplifier said filter means having a first and a second, higher, break frequency and providing minimum attenuation of said control signal at frequencies below said first break frequency and a fixed maximum value of attenuation of said control signal at frequencies above said second break frequency.