How do ear thermometers work?

The eardrum is a very accurate point to measure body temperature because it is recessed inside the head (not unlike the tongue). The problem with the eardrum is that it's especially fragile. You don't want to touch the eardrum with a thermometer.

This makes detection of the eardrum's temperature a remote sensing problem. Technically, the eardrum is not that remote -- just a centimeter or so down the ear canal. Still, it's remote nonetheless.

The remote sensing of an object's temperature is done with that object's infrared radiation. This technique provides a good way to detect the temperature of someone's eardrum.

All the objects around you radiate infrared energy. Human beings don't have sensors to detect subtle differences in infrared. Skin, however, can detect objects radiating lots of infrared energy. When you stand next to a fire to warm yourself, the "warmth" is infrared energy you are absorbing. The temperature sensor in the ear thermometer is a device that is very sensitive to subtle changes in infrared emission. A common sensor is the thermopile, which is accurate to 1/10th of a degree. The thermopile sees the eardrum and can measure its infrared emissions. The emission is then converted into a temperature and displayed on a small LCD screen.

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