ratio of the radiant intensity of the specimen to that emitted by a black body radiator at the same temperature and under the same geometric and wavelength conditions
NOTE Differentiation is made between:
- Hemispherical emittance (eh) - conditions for incidence or viewing of flux over a hemispherical region.
- Normal emittance (en) - conditions for incidence or viewing through a solid angle normal to the specimen. Ratio refers to the emissivity normal to the surface of the emitting body.