A noncontact infrared tympanic thermometer which does not require environmental stabilization or waveguide temperature control because it utilizes an optically stabilized infrared detector for detecting the infrared energy emitted by the tympanic membrane. The optical stabilization renders the instrument insensitive to ambient temperature effects and allows it to read true tympanic membrane temperatures unaffected by the temperature of the side walls of the detector package. This selectivity is accomplished by locating a neutral density filter over half of the optical aperture of the detector package such that the neutral density filter "shadows" only one of two thermopile channels for detecting the infrared energy emitted by the tympanic membrane. The two thermopile channels are connected in series opposition such that any optical signal equally present in both channels will yield a zero net output. Since the infrared energy emitted by the walls of the detector package reach each detector equally, the effects of these emissions on the temperature measurement are eliminated. The neutral density filter prevents the energy from the tympanic membrane from falling equally on the two thermopile detectors and similarly cancelling. Instead, the resulting differential signal may be processed along with the ambient temperature signal to determine the temperature of the tympanic membrane. The infrared detector of the invention may also be incorporated into an otoscope.