The spectral radiative heat flux, 
,
passing through a surface at some location r with
a unit vector normal n is:
 | (8–13) |
Integrating the equation of transfer over solid angles, the divergence of the spectral radiative heat flux is given by:
 | (8–14) |
where G v is the spectral incident radiation, given by:
 | (8–15) |
The total radiative flux is obtained by integrating Equation 8–14 over the spectrum:
 | (8–16) |
In the case of pure scattering, 
. Therefore 
, as it should because in this case no energy is
lost from the radiation field; clearly this is also true in thermodynamic
equilibrium.
Optical thickness is a dimensionless quantity that represents the ability of a given path length of gas to attenuate radiation of a given wavelength. Optical thickness is given by:
 | (8–17) |
where 
is the optical thickness (or opacity)
of the layer of thickness 
and is a function of all the values
of 
between 0
and 
. A large value of 
means
large absorption of radiation. Note that this definition of optical
thickness is different from that traditionally found in the optics
literature where the optical thickness is a property of the material.