What is Emissivity?
Does emissivity change with temperature? In order to answer this question, first we must understand emissivity. Emissivity is the ratio of the energy radiated from a materials surface to that radiated from a perfect emitter. A blackbody is an example of a perfect emitter, it has an emissivity of 1, while a perfect reflector or white body has an emissivity of 0. Most natural objects are considered “graybodies” as they emit a fraction of their maximum possible blackbody radiation at a given temperature.
Does Emissivity Change With Temperature?
Yes, Emissivity changes with temperature because of energy that is tied up in the behavior of the molecules that form the surface. Following Plancks law, the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths. The energy emitted at shorter wavelengths increases more rapidly with temperature.
As the material gets to a higher temperature, the molecules move more and more, this means they will usually emit more energy. But that’s just blackbody radiation. The differences from black body radiation have to do with the energy levels of the atoms making up the molecules and their modes of vibration. The surface does not behave exactly as a blackbody, and the difference is due to those subtle effects of what’s really going on inside the molecule, which changes with temperature.
Single-wavelength technology relies on a constant emissivity setting in order to produce an accurate temperature reading, if the emissivity is variable across temperatures than the pyrometer will read in error unless the operator adjusts the emissivity setting in real-time. Williamson pyrometers equipped with ratio technology especially dual-wavelength sensors are specifically designed to automatically compensate for these changes in emissivity because they not only measure temperature but also emissivity. Together both of these real-time readings provide confidence that the sensor is reporting the correct temperature value.