What factors affect the emissivity of an object?
Non-metals, usually have a high emissivity, and are usually fairly constant over time. Metals on the other hand can have a very low emissivity, and as they usually oxidise, their emissivity tends to vary drastically over time.
Polished and shiny surfaces tend to have lower emissivity. Sanded or rough surfaces tend to have a higher emissivity.
Ideally we should avoid viewing materials at the exact perpendicular, but very sharp angles would become more reflective, and therefore emissivity would change, and for some materials the emissivity can be very directional. .
Emissivity is not really a constant, and it can vary from camera to camera. The biggest variation can be when moving fro LWIR to MWIR and vice versa, but in some cases it can vary even between two LWIR cameras.
Cavities tend to have a higher emissivity due to the multiple reflections between the surfaces.
Large variations in temperature can cause a variation in emissivity. The emissivity of metals tends to increase as the temperature increases. The emissivity of non-metals tends to decrease as the temperature increases.