What is emittance?
Among the three basic forms of heat transfer – conduction, convection, and radiation – the term emittance refers to a material’s ability to release heat through radiation.
The term necessarily refers to the heat exchange at surface of a material because heat exchange through a solid would be conductive and heat exchange through a fluid would be convective.
Emittance is the ratio of the radiation emitted by a given material to that emitted by a “black body” at the same temperature. This unitless value would be 0.0 if no emittance was possible and 1.0 at the ideal maximum – the theoretical “black body” (i.e., a material with an emittance of unity).
The lower the emittance, the lower the radiative heat exchange. Typically, emittance is related to absorptance (i.e., highly absorptive materials will usually have high emittance) – but they are not the same characteristic. With regard to opaque materials, absorptance directly relates to reflectance.
What is reflectance? How is it different than albedo?
Recall that solar radiation striking a surface will either be reflected, absorbed, or transmitted. The combination of those three phenomena will equal 100% (irregardless of emittance). When referring to opaque materials, solar radiation will not transmit. It will only be absorbed or reflected.
Reflectance is the ratio of reflected light to incident light.
Note that reflectance refers specifically to light – the visible portion of the solar electromagnetic radiation spectrum. Light reflectance value (LRV) is a more accurate term for this ratio. The term albedo refers to overall solar radiation reflectance – not just the visible portion of the spectrum.