Why do thermometers that measure infrared emission from the eardrum work so well?

The eardrum, like all objects above absolute zero, emits thermal radiation. The intensity and spectrum of this radiation carry information about the temperature. Because the eardrum is close to the hypothalamus (the part of the brain that regulates body temperature), the temperature of the eardrum is often assumed to be the same as the body's core temperature. So, in order to obtain an accurate measurement of the internal body temperature, the infrared energy emitted from the eardrum must be accurately measured and converted to a temperature value.

For an accurate measurement, the infrared thermometer's sensor must be able to detect the radiation and filter out other radiation sources, such as ambient radiation and radiation emitted by the ear canal or other parts of the ear. To eliminate the noise caused by the radiation emitted by the ear canal, the sensor is usually positioned deep inside the ear canal, facing the eardrum.

For the conversion from infrared radiation to a temperature value, different methods can be employed. One common approach is to use the Stefan–Boltzmann law, which states that the total power radiated by a blackbody is proportional to the fourth power of its absolute temperature. In other words, the hotter an object, the more infrared energy it emits. By measuring the intensity of infrared radiation emitted by the eardrum and using the Stefan–Boltzmann law, the thermometer converts it to a temperature value.

The accuracy and reliability of infrared ear thermometers are influenced by several factors, such as proper placement of the sensor in the ear canal, ear infections, earwax blockages, and variations in body temperature due to factors like exercise, sleep, or a recent meal. To ensure accurate measurements, it's important to follow the manufacturer's instructions for using the thermometer and to consider any factors that may affect the measurement.