Abstract: The radiometric measurement of the temperature of remote targets is usually done with a radiometer having a detector whose field of view is alternately switched between the target and a reference temperature source. The detector output is then used to change the reference source temperature to equal the target temperature. According to the invention the reference source temperature is not changed by the measuring apparatus. The difference in detector signals obtained from the target and reference source is used to calculate the target temperature from the known laws of thermal radiation and the known detectivity and spectral response of the detector. The invention can be incorporated in thermal imaging apparatus using the radiation detector scanned to form the thermal image as the radiometer detector. In such radiometric thermal imaging apparatus an array of detectors is scanned across the scene in a raster and a corresponding visual raster is produced.