An alternating bias is applied to a modified photoconductive detector and photosignal extracted from each detector element. This extraction may be performed using an integrator to produce a dc signal output, or alternatively by using a high-pass filter or phase-sensitive detector to extract a photosignal at an harmonic of the bias frequency. The detector used is provided with elements that have a responsivity that is a different function of bias amplitude for each polarity. The detector element may be shaped with variation in width or depth, to produce this differential responsivity. Alternatively element bias contacts may be of different width to produce field gradient and differential responsivity. It is advantageous to obscure a part of each element area by including an opaque mask.