spectral-hemispherical emissivity

[] ε(λ,T ) (of an element of surface of an opaque temperature radiator that has an optical smooth surface)
The ratio of its spectral radiant exitance to that of a blackbody at the same temperature:

epsilon left ( lambda ,T right )=left ( 1/pi right )int _{2pi } epsilon left ( lambda ,theta ,phi ,T right ) cos theta domega
equiv M_{lambda }left ( lambda ,T right )/M_{lambda, blackbody }left ( lambda ,sqrt{T} right )
Note: Hemispherical emissivity is frequently called “total” emissivity. “Total” by itself is ambiguous and should be avoided since it may also refer to “spectral-total” (all wavelengths) as well as “directional-total” (all directions). (See Section, spectral-total hemispherical emissivity.) « Back to Definitions Index