Emissivity is a surface property that determines how much radiation an object emits at a given temperature compared to a blackbody at the same temperature. Emissivity (along with background thermal radiation) is a primary source of errors in infrared temperature measurement. Emissivity can be more easily understood if it is realized that infrared has similar properties to visible light.
?
Mirrors figure prominently in the discussion of heat radiation and emissivity*. Since heat and light radiation behave similarly, what we see with our eyes is similar to what the IRt/c sees.
?
When you look in a mirror with your eyes, you see only reflections, nothing of the mirror itself. If the mirror is perfect, it has 100% reflectivity. Therefore, it emits nothing because it reflects everything. 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 example, we saw 90% of the mirror as a perfect reflector and 10% as imperfections, 90% of the mirror would reflect; the remaining 10% would emit. Therefore, the emissivity equals 0.1.
?
Consider for a moment the exact opposite of a perfect mirror, which is a perfect emitter. The eye looks at a perfect emitter and sees no reflection at all, only the emitting surface. Since 100% of the surface emits, and 0% reflects, the emissivity equals 1.0. This type of object is called a blackbody.
Finally, consider a good emitter. The eye sees a small amount of reflection interspersed with the large amount emitting. If 10% of the surface did not emit and instead reflected, we would have 10% reflecting and the remaining 90% emitting. Therefore, the emissivity equals 0.9. Accordingly, we can state the following rule of emissivity:
?
The emissivity of a surface is simply the percentage of the surface that emits. The remaining percentage of the surface reflects.
?
Shiny metal surfaces act like mirrors, with emissivity in the range of 0.05 to 0.2. Accordingly, they have only a 4% to 25% emitting area compared to the reflecting area, and for that reason are difficult to measure with infrared methods. Non-metals, organic materials, and coated metals have emissivity in the range of 0.8 to 0.95 and thus have 400% to 1900% emitting area compared to reflecting area, and thus are much more easily measured successfully.
?
