A temperature measuring device comprises an optical temperature probe (20) and a control module (21). Thermal emission from a recess (28a) in the tip end portion (23) of an optical fiber (22) is transmitted to the remote end of the fiber for detection by the control module. The recess provides an improved sensor capability with regard to maintaining constant emissivity and rapid response time. The recess is suitable for both single crystal optical fiber, and optical fiber with a core-and-cladding construction.