It is based on the principle that the intensity of light received by the observer depends upon the distance of the  observer from the source and the temperature of the distant source. A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature T) is related to the thermal radiation or irradiance  of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan–Boltzmann constant and the emissivity ε of the object:
This output is used to infer the object's temperature from a distance, with no need for the pyrometer to be in thermal contact with the object; most other thermometers (e.g. thermocouples and resistance temperature detectors (RTDs)) are placed in thermal contact with the object and allowed to reach thermal equilibrium.
Pyrometry of gases presents difficulties. These are most commonly overcome by using thin-filament pyrometry or soot pyrometry. Both techniques involve small solids in contact with hot gases.