Just read an article that explained emissivity and I thought it really got the point accross. For all you that use temperature as a way to guage severity of a fault you may be surprise on how accurate it really is.
Emissivity is, in fact, a material property that defines the efficiency with which a surface emits energy. Metals tend to be inefficient while non-metals are much more so. Inefficient emitters are also termed "low" emissivity surfaces. Further, low emissivity surfaces tend to also reflect their thermal surroundings.
Emissivity is typically given as a number between zero and one with zero having no emission and one have 100% emission (compared to a perfect emitter at the same temperature). Human skin, interestingly, regardless of skin tone, has an emissivity of approximately 0.98. Most shiny metals have emissivity values below 0.20.
Here's a simple experiment that clearly shows this: obtain a fry pan with a shiny metal side and a coated, non-stick side. Heat it until it is smoking hot. Put your hand NEAR the shiny side; it will feel slightly warm. Now put your hand near the coated side; it will feel VERY warm. The difference? The efficiency with which the surface emits radiation.
Infrared cameras (and infrared spot thermometers) "see" the same radiation your hand is feeling. So when I look at the shiny side of the pan, it does not look very hot compared to the coated side. Of course, both sides are the same temperature.
When thermographers are inspecting equipment with low emissivities, i.e. metals, they encounter two problems. First, the surface does not look hot when it is. Second, it is virtually impossible to accurately measure the surface temperature radiometrically. It is possible to use an "emissivity correction factor," BUT for measurements on metals, even those that are oxidized, the results will be inconsistent and inaccurate.
Despite what many people think, it is NOT simply a matter of dialing in a correction value and getting the correct temperature! If you don't believe me, try it on the frying pan ten times at varying temperatures and see what your results are. You can trust the temperature measurements on the coated side, but the shiny side will just lie to you!
One last thing: the accuracy of temperature DIFFERENCE measurements is also impacted by emissivity. It is NOT possible to make these types of measurements on low emissivity surfaces.
If you are using thermography or interfacing with thermographic data in any way, it is essential that you understand emissivity. Get out that frying pan and see for yourself!