Emissivity compares the thermal emittance of an object to that of which of the following?

Emissivity is a measure of how effectively a surface emits thermal radiation compared to a perfect black body at the same temperature. A black body is an idealized physical object that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence, and it also emits radiation at the maximum possible intensity for a given temperature.

When comparing the thermal emittance of an object, using a black body as the reference provides a clear standard because a black body is characterized by having an emissivity of 1. This means that any real object will have an emissivity value ranging between 0 and 1, indicating how closely it approaches the radiation emission capabilities of a black body. Therefore, by comparing an object’s emissivity to that of a black body, one can determine how much thermal energy it can emit relative to the ideal maximum.

In contrast, the other options do not serve as standard references for thermal emittance. A white body might reflect more heat than it emits, a reflective body is not defined by its thermal emittance, and a heated body may not necessarily represent a fixed emissivity value, as its thermal emission can vary widely. Thus, the relationship of emissivity is fundamentally linked to the characteristics of a black body.