Imaging apparatus (100, 200, 1200) includes a semiconductor substrate (312) and an array (202) of pixel circuits (1202, 1204), which are arranged in a matrix on the semiconductor substrate and define respective pixels (212) of the apparatus. Pixel electrodes (1208) are respectively coupled to the pixel circuits, and a photosensitive (1206) is formed over the pixel electrodes. A common electrode (1207), which is at least partially transparent, is formed over the photosensitive film. An opaque metallization layer (1214) is formed over the photosensitive film on one or more of the pixels and coupled in ohmic contact to the common electrode. Control circuitry (208, 1212) is coupled to apply a bias to the common electrode via the opaque metallization layer while correcting a black level of the output values from the pixels using the signals received from the one or more of the pixels over which the opaque metallization layer is formed.