A pyrometer is non contacting device intercepting and measuring thermal radiation emitted from an object to determine surface temperature. This is also known as radiation thermometer.
A pyrometer / radiation thermometer consists of an optical system and detector. The optical system focuses the energy emitted by an object into the detector. The output of the detector is proportional to the amount of energy radiated by the target object and response of the detector to the specific radiation wave length. The output is used to inter the object temperature.