Temperature measurement

A method of measuring temperature comprises the steps of consecutively launching (1, 2) input pulses of light at two different wavelengths into a temperature sensing element in the form of an elongate optical fibre (3); passing Raman scattered light from said element and deriving from said input pulses to intensity detectors (7, 8); obtaining from the detectors (7, 8) output signals indicative of the intensity of the Raman scattered light; and processing said output signals to provide a temperature measurement. A known position (10) along the optical fibre (3) is maintained at a known temperature to provide a reference for deriving temperature measurements at other positions along the optical fibre.