From Glossary of Meteorology


The ratio of the power emitted by a body at a temperature T to the power emitted if the body obeyed Planck's radiation law.

Strictly, emissivity should be qualified by the frequency, direction, and even polarization state of the emitted radiation. This is recognized in qualifiers such as monochromatic (at a given frequency) as opposed to total (over a broad range of frequencies), and directional (the ratio for a particular direction) as opposed to hemispherical (the ratio for a hemisphere of directions). Contrary to a widespread misconception, the upper limit of emissivity is not 1. This upper limit is valid (approximately) only for bodies large compared with all relevant wavelengths.
See also blackbody, emissive power, emittance.