When electromagnetic radiation impinges upon a surface of an object, the radiation is partially absorbed, partially reflected, and partially transmitted. If all of the electromagnetic radiation is absorbed by the object, the object is called a blackbody.
Planck's law and the Stefan-Boltzmann law are derived assuming blackbody properties. In reality, most common materials are not 100% black. As a result, the measured temperature Tm (assuming blackbody properties) is slighter smaller than the true temperature T.
The emittance e is introduced to describe the difference in radiation absorption between common objects and blackbodies, according to the equation,
If the emittance is independent of wavelength (i.e. el = e = constant) the object is called a graybody.
Another term called emissivity is similar to emittance in describing the difference in radiation intensity between real materials and blackbodies. However, emissivity is a material property usually defined only for highly polished surfaces or controlled conditions. In other words, emittance is the general concept of the radiation mismatch between an object and a blackbody, whereas the emissivity is the emittance of a particular material under a certain condition.