Low intensity light is incident upon a photodiode whose output is coupled to an integrator. The output of the integrator is coupled to an input of an A/D converter, whose output is coupled to a microprocessor, whose output is coupled to a filter. A second output of the microprocessor is coupled to a gate electrode of a FET to provide a reset signal to the FET to reset the integrator. The microprocessor compares the digital samples of the integrated signal from the A/D converter and, firstly, generates the reset signal if a sample is beyond a set limit, and, secondly, calculates delta values between adjacent samples and interpolates the delta values for the reset periods so as to provide a continuous data stream which can be filtered by a filter matched to the form of the original signal.