Coefficient of Emission or Emissivity

The coefficient of emission, or emissivity, $e$, of a given surface is defined as the ratio of that surface's emissive power $R$ to the emissive power $R_B$ of a perfect black surface, both measured at the same temperature:

Since a perfect blackbody has the maximum possible emissive power at any given temperature (Section 3.12), it follows that:

• for a perfect blackbody, $e = 1$;
• for a perfect reflector, $e = 0$ (it emits essentially nothing, consistent with the emitter-absorber symmetry of Section 3.11). …