Pyrometer

Pyrometers are non-contact thermometers that determine the temperature of the measured body from thermal radiation. They mainly measure temperatures in the range of −50 °C to +3500 °C.

Principle
The pyrometer captures and evaluates the electromagnetic radiation emitted by the measured object. The cause of this radiation is the internal mechanical movement of molecules, the intensity of which depends precisely on the temperature of the object. The theoretical basis on which pyrometers work is the Stefan–Boltzmann law.
 * The black body radiation intensity Me is directly proportional to the fourth power of the thermodynamic temperature T of the black body, i.e. Me = σT4, where the Stefan-Boltzmann constant σ takes on the value σ=5.67 × 10−8 Wm−2 K−4.

For non-contact temperature measurement, it is necessary to know the emissivity of the object. The emissivity ε is equal to the ratio of the radiation intensities of a real surface and an absolute black body. It can take values ​​from 0 to 1.0. A shiny mirror would have a value of 0, for example, and a completely black body would have a value of 1.0.

Advantages and disadvantages of pyrometers
Benefits include:
 * measurement speed
 * the possibility of measuring even moving objects
 * easy temperature measurement in hard-to-reach places
 * trouble-free measurement of very high temperatures
 * the measured object is not affected
 * the measurement takes place without the risk of contamination and mechanical effects on the surface of the object being measured

Disadvantages include:
 * the measured object must be optically visible to the thermometer
 * solid obstacles only allow surface measurement (not internal temperature)
 * the need to protect the sensor from dust and condensing liquids
 * only the surface temperature of the materials is measured and their emissivity must be taken into account

Related articles

 * Temperature measurement
 * Temperature measurement/Catalog of methods in biophysics
 * Measurement and assessment of body temperature

Source
https://cs.wikipedia.org/wiki/Pyrometr

http://www.allforpower.cz/UserFiles/file/termokamery_1.pdf