Emissivity
An important concept needed to understand how temperature is mea- sured using infrared radiation is the one of emissivity. Emissivity is a surface property which determines just how well an object’s tempera- ture can be measured by an infrared device. Emissivity (along with background thermal radiation) is the primary source of errors in infra- red temperature measurement. Emissivity can be more easily under- stood if it is realized that infrared has similar properties to visible light.
Simply stated, emissivity is the opposite of reflectivity. A perfect mirror has a reflectivity of unity and an emissivity of zero. A perfect black body has an emis- sivity of unity and a reflectivity of zero. In actuality, all real bodies (including human ones) have an emissivity between these two limits.
It is not possible to accurately measure the surface temperature of any body with an emissivity of less than 1.0 without making a correction for this source of error. Hu- man skin is near but not equal to 1.0 and, if
1.0not accounted for, can introduce errors in the order of one to two degrees. The cup-
like mirror used in the nosepiece of the DermaTemp scanner removes this source of error by trapping all of the radiation from the skin surface and in effect causing the skin surface to act like a black body with an
emissivity of 1.0.
Blackbody
Emissivity =1.0 Reflectivity = 0.0
1.0
Mirrors figure prominently in the discus- sion of heat radiation and emissivity. Since heat and light radiation behave the same way, we can use what we see with our eyes as examples of what the DermaTemp sees. When you look in the mirror, you see only reflections, nothing of the mirror itself. If the mirror is per- fect, it has 100% reflectivity. Because it reflects everything, it emits nothing. For this condition, the emissivity is zero.
If we consider an imperfect mirror, the eye then sees mostly reflection, but also some of the imperfections on the mirror surface. If, for ex
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