 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 formula_1 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.