Abstract: An optical temperature sensor has an outer probe with a sapphire element at its forward end within a stagnation chamber through which hot gas flows and heats a thermally-emissive coating on the element. A lens focusses radiation emitted by the coating onto one end of a fibre-optic cable that extends within the rear of the probe. A gas passage along the probe enables cooling gas to flow from an inlet at the rear end, around the fibre optic cable, lens and through an outlet rearwardly of a transparent thermal barrier which protects the sapphire element from the cooling gas.