Temperature sensor, the most commonly used non-contact thermometer is based on the basic law of black body radiation, called radiation thermometer. Radiation thermometry includes luminance method (see optical pyrometer), radiometric method (see radiation pyrometer) and colorimetric method (see colorimetric thermometer). Various radiation temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured for a black body (object that absorbs all radiation and does not reflect light) is the true temperature.

Advantages of non-contact temperature measurement: The upper limit of measurement is not limited by the temperature resistance of the temperature sensing element, so there is no limit to the maximum measurable temperature in principle. For high temperatures above 1800 °C, non-contact temperature measurement methods are mainly used.
With the development of infrared technology, the radiation temperature sensor has gradually expanded from visible light to infrared.