Abstract: An optical pyrometer for a gas-trubine engine has a radiation receiving head located to view the blades of the engine and supply radiation from the blades to one end of a fibre-optic cable. At the other end of the cable there is mounted a detector comprising a cylindrical metal casing containing a photodiode and a signal conditioning circuit that amplifies the photodiode output. The output circuit is supplied to an electrical connector mounted on the casing. The detector can be mounted directly on the casing of a processing unit by mating its connector with an electrical connector fixed on the processing unit casing thereby avoiding the need for cables between the two casings. The connector on the detector has a flange that provides good thermal contact with the connector on the processing unit so as to help equalize the temperatures of the two casings. The processingunit casing is cooled by means of a thermal transfer unit supplied with liquid fuel.