In addition to the Stokes vector, there are two other important radiant quantities: irradiance and mean radiance.
Irradiance, or vector irradiance, is defined as the integral over the
entire spherical solid angle of the normal component of the radiance,
relative to some surface.
Irradiance follows the cosine law in that a collimated beam of photons
intercepting a plane surface produces an irradiance that is proportional
to the cosine of the angle between the photon direction and the
surface normal. It has units of Wcm
and may be written as,
In a plane-parallel atmosphere (see section 2.6),
only the irradiance along the -direction
is relevant so the vector representation is not necessary.
The irradiance in polar coordinates is,
The mean radiance, F, also referred to as actinic flux, mean intensity,
flux density, or flux (Madronich, 1987), describes the number
of photons converging upon a volume element per unit time, area and
The mathematical expression for mean radiance is equivalent to
the zeroth moment of the radiation field,