Temperature is measured remotely by supplying a measuring pulse of a first wavelength via optical fibres to a sensor element formed of a material exhibiting temperature dependent absorption at this first wavelength. A reference pulse at a second wavelength is also directed through the sensor element, the second wavelength being selected to be transmitted through the sensor element substantially independently of temperature variation. A receiver determines the intensity of the measuring and reference pulses after transmission through the sensor element to generate signals V1 and V2 respectively. The receiver output V3 during a dwell period in the absence of either measuring or reference pulses is also measured. A value Z representing the sensor temperature is determined from Z = (V1 - V3)/(V2 - V3). Wavelength dependent errors are avoided by keeping the light source at a constant temperature. Optical feedback ensures the light source is driven to produce pulses of constant intensity.
