Editing Pyrometer (section)

== Principle ==

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]] <math>j^\star</math> 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:
:<math>
 j^\star = \varepsilon \sigma T^4.
</math>
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. [[thermocouple]]s and [[resistance temperature detector]]s (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.{{citation needed|date=December 2016}}