This paper gives a thorough treatment of a new effect of non‐steady‐state photo‐electromotive‐force (emf). The effect consists of an alternating electric current arising in a short‐circuited bulk sample of a photoconductor illuminated by a vibrating sinusoidal pattern. As a detailed theoretical analysis shows, the frequency transfer function of the effect is identical with that of a differentiating RC circuit having a time constant equal to the characteristic time of a space‐charge grating formation within the sample volume &tgr;sc. For high excitation frequencies (&ohgr;≳&tgr;−1sc), the photo‐emf signal peaks at spatial frequencyK=L−1D(LDis the mean diffusion length of photoinduced carriers) and the photovoltage amplitude in an open‐circuit sample can be as high askBT/etimes the number of fringes of the interference pattern in the interelectrode spacing. The basic conclusions of the theoretical analysis of the effect are supported by the experimental evidence obtained for cubic photoconducting Bi12SiO20crystals.