It is possible to determine actual emissivity values for a material but it is not practical in the real world. Even if you determine an accurate E value, the next time you scan the item that value will have changed rendering your old value useless. In most infrared applications exact temperature measurement is not necessary. To use our previous example of the circuit we measured, there would most likely be more than one circuit next to each other. If you use the same E value for both circuits they will both be skewed the same amount.
E values become even less of a problem when trending an area over time.
If the same circuit with the reading of 100 F has a reading of 110 F the next time you scan it and a reading of 115 F the next time, with the same emissivity setting, we know a problem is developing regardless of the error introduced by emissivity. Thermal image of fuse clips shows large Delta T Dealing with emissivity is not as hard as it would seem. In the real world you pick an emissivity value that approximates the scene you are imaging and then you record it and maintain that same setting every time you scan that object. We hope this article answers your questions regarding emissivity. A full explanation of emissivity is beyond the scope of this web site.