The invention relates to a process for the operation of an tympanic clinical thermometer, in which a sensor (8) for detecting infrared radiation is arranged in a housing (1) with a radiation inlet (23). Electronic evaluation means (13) is associated with the sensor (8). To measure the temperature the radiation inlet (23) is covered with a measuring cap (5) allowing the passage of infrared radiation. Any operation without a measuring cap (5) is detected by the release of an acoustic warning signal or light signal (14) after the thermometer is switched on.
