Body temperature is routinely monitored in clinical settings with infrared ear thermometers which measure the infrared energy emitted from the patient,s eardrum in a calibrated length of time. A short tube with a protective sleeve is inserted into the ear, and a shutter is opened to allow radiation from the tympanic membrane to fall on an infrared detector for a period which is typically from 0.1 to 0.3 seconds in the varieties surveyed. The device beeps when data collection is completed and a readout of temperature is produced on a liquid crystal display.
This kind of temperature from the eardrum has been found to be a clinically reliable indicator of body core temperature. The eardrum is located close to the hypothalmus, which is the body temperature regulator. The membrane itself is thin and almost transparent in the visible, so you would presume that it reliably tracks the temperature inside the membrane so that the infrared energy it emits gives a good indication of the inside temperature.
The infrared energy falls on a thin pyroelectric crystal which develops a charge proportional to that collected energy. Discharging the crystal sends a current pulse through filters and conversion circuits which compare the signal to tabulated data on temperature and calculate a body temperature for the display.