
396 OPTICS FOR ENGINEERS
cavity at some temperature is
¯
E(T)
incident
, and the amount absorbed is
a
¯
E(T)
incident
(the rest
being reected), the radiant exitance must be the same:
¯
M(T)
emitted
=
a
¯
E(T)
incident
. (12.73)
Otherwise, the wall would change its temperature. In this equation,
a
is the absorption coef-
cient. The rest of the light is reected either specularly or diffusely, and the total reectance
is 1 −
a
. We state that “the absorption is equal to the emission,” and refer to this statement as
the principle of detailed balance.
Third, detailed balance must be true wavelength-by-wavelength. If it were not, we could
use a clever arrangement of ...