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An **equation of radiative transfer** (or
**radiative transfer equation** or **RTE** or **transfer equation**)
is an equation that describes radiative transfer, the transfer
of energy by EMR through a gas or plasma, taking into account factors
that can remove photons (absorption) and can
insert them (emission). One form:

1 ∂ ^ — ——I+ Ω˙∇_{ν}I+ (_{ν}k+_{ν,s}k)_{ν,a}I=_{ν}j+ 1/(4π)_{ν}k∫_{ν,s}Ic ∂_{ν}dΩt

- c - speed of light in a vacuum.
- t - time
*I*- intensity_{ν}*ν*- frequency*k*- absorption coefficient*k*- scattering opacity_{ν,s}*k*- absorption opacity_{ν,a}*j*- emission coefficient_{ν}- Ω - solid angle

Variants of the equation assume equilibrium (no change over time), an approximation of the composite of all wavelengths (gray atmosphere, using the Rosseland mean opacity), a directional component (Eddington approximation) and/or that variation in the medium is in a single direction (plane-parallel atmosphere). These are used in models of stellar structure.

http://www.met.reading.ac.uk/~ross/Science/RadTrans.pdf

radiative transfer (RT)

source function (S)