A body temperature thermometer has a probe head assembly (8) which includes a first sensor (10), such as a thermopile, for generating a first output signal representative of an amount of infrared radiation impinging thereon The probe head assembly further includes a waveguide (16) and a surrounding hollow probe (14) for receiving infrared radiation emitted by a biological surface tissue, such as the external ear canal and tympanic membrane of a patient, and directing the radiation to the first sensor. A second sensor (12) is mounted in the probe head assembly for generating a second output signal representative of a temperature of the first sensor. A third sensor (26) is thermally coupled to the waveguide for providing a third output signal representative to the temperature thereof. A processor (30) is connected to the sensors for processing the output signals to determine a body temperature of the patient utilizing a calibration mapping of a plurality of target temperatures to corresponding output signals taken over a predetermined range of ambient temperatures. A display (34) is driven by the processor for providing a user with a visual indication of the determined body temperature.