A lead sulphide photoconductive cell is used as the balance detector in a low temperature pyrometer employing a null method. The cell is alternately exposed to radiation from the unknown hot body and from a lamp contained in the instrument. The output from the cell which is amplified by a tuned amplifier is adjusted to zero by changing the current through the lamp. The lamp current can be calibrated in terms of black body temperature. The instrument described has three temperature ranges - 150-220° C, 210-330° C and 300-500° C - obtained by inserting different absorption screens in the path of the radiation from the lamp. Corresponding effective wavelengths are 2.45 μ, 2.35 μ and 2.20 μ.A modification of the instrument to produce a temperature range of 100-175° C is also described. The measurement accuracy throughout these ranges is ± 5° C for a single reading.