Why is emissivity different with different cameras?
This is difficult for people to fully understand. To be honest, emissivity in general takes some time to begin to get a good grasp of, for me the process of getting a strong understanding took a few years, a lot of reading and some experimentation.
Emissivity changes with temperature, angle and wavelength. It is important to understand that for a blackbody, emissivity will always be 1. A greybody will have an emissivity of less than 1, but it will be constant regardless of wavelength. Realbodies however, will have an emissivity that varies with wavelength. Both blackbodies and Greybodies are ideal definitions, although you can get close, in reality everything is “real”.
When you do an emissivity measurement with a camera you get a “Grey Emissivity Equivalent” value. This is an average value throughout the spectral response of that particular camera (between λ1 & λ2, (say 8-12μm)). If you use a camera that has a different spectral response (say 8-14μm), you get a different average value.
Will there be much of a difference? Maybe, maybe not. Certainly two cameras of the same type, with the same type of detector should measure the same value on the same material under the same conditions. However it can change drastically between a LWIR camera and a MWIR camera, and it can change between a LWIR camera of one type and a LWIR camera of a different type on some materials.
There really are no shortcuts, emissivity must be measured, with your own camera. Emissivity tables, while they are quite good on some materials (usually non-metals), should not be relied upon.