The ratio of the radiated energy in two wavelength bands received from a hot body is a measure of the body's surface temperature. As only a ratio is involved the measurement is independent of the emissivity of the body if this does not vary with wavelength over the used range, the distance of the body from the source, time of exposure, etc.The pyrometer described employs the above principle, a PbS photoconductive cell being used for radiation detection. Temperatures down to 200° C may be measured with adequate accuracy.