The utilization of the photovoltaic properties of CdSe-Ge n-n heterojunction as a wide range pyrometer is described; the method is a variation of the two colour technique. The optical response of the heterodiode depends on the bias voltage in such a way that the photovoltage produced by white radiation can be biased to zero, thus providing a null detector. For a specific spectral distribution of energy produced by a black body radiator within the bandwidth of the device there exists a unique bias voltage for which the photovoltage is zero, and the bias voltage can be calibrated directly in terms of temperature. The sensitivity of the detection is 0·6 mV K⁻¹ at 1000 K, 0·1 mV K⁻¹ at 2000 K and 0·05 mV K⁻¹ at 3000 K.