Each thermometer will take temperature above absolute zero emit a certain amount of infrared radiation energy based on the object’s temperature. The amount of radiation energy is closely related to the wavelength distribution. When a person’s forehead temperature is 96.8-98.6°F(36-37°C), it emits a wavelength of 9-13 um of infrared radiation. Based on this principle, we are able to measure a person’s actual body temperature just by measuring the forehead’s surface temperature.