What is the theoretical emissivity value of a perfect emitter?

The theoretical emissivity value of a perfect emitter is 1. Emissivity is a measure of an object's ability to emit infrared energy compared to that of a perfect black body, which is defined as an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

A perfect black body would emit radiation with maximum efficiency, and thus it is assigned an emissivity of 1. This means that for any temperature, it emits the highest possible amount of thermal radiation. Emissivity values range from 0 to 1, where a value of 0 would signify a perfect reflector that emits no thermal radiation, and values closer to 1 indicate materials that are more efficient at radiating heat. Therefore, the choice of 1 signifies that the emitter being described is ideal, emitting the maximum possible thermal radiation.

In practical terms, most real materials will have emissivity values less than 1, as they do not perfectly emit or absorb thermal radiation, but a perfect emitter, by definition, would always have an emissivity of 1.