All objects with a temperature above -459.67 °F (0 °K) radiate some level of infrared energy. The object’s ability to emit this energy is known as emissivity (ε). Many factors influence the emissivity of the object including material and surface finish. A polished metal has a much lower emissivity than the same metal with a rough surface, for example. Emissivity information is available from internet searches, textbooks, etc., but the values in practice can vary due to target surroundings, shape and other factors. This table shows some examples.
Emissivity is the ratio of emitted radiation to a theoretical perfect emitter known as a “black body”. A true black body has an emissivity factor of ε = 100, while all real objects have a factor of ε < 100. The duller and darker a material is, the closer its emissivity is to 100. The more reflective a material is, the lower its emissivity.
