Abstract: The invention relates to detectors of penetrating radiation, in particular x-radiation. The detector comprises a photoconductive material to which two electrical signals are applied. An output signal is detected and in the presence of radiation incident on the detector this includes a component formed by mixing of the two input signals, an harmonic if they are of the same frequency. The amplitude of that component is proportional to the intensity of the incident radiation. There is also a background component which should either be reduced to insignificant proportions or removed. Advantageous electrode arrangements for such a device are also disclosed.