These devices are also called tympanic thermometers. This is because they take measurements from the tympanic membrane – the ‘drum skin’ of your eardrum. Unlike traditional mercury thermometers, which use the expansion of mercury at higher temperatures to measure those in the body, tympanic thermometers measure infrared radiation. Infrared radiation is a form of electromagnetic radiation, like light or radio waves, that is given off by anything that has a temperature.