### equation of radiative transfer

**(RTE, transfer equation, radiative transfer equation)**
(equation describing energy transfer by EMR)

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 electromagnetic radiation (EMR) through a gas or plasma, taking into account
the absorption and emission of photons by the
gas particles. The equations generally represent the EMR
along a **beam**, in this case, *beam* meaning the EMR
directed along some straight line, including whatever EMR
is added and/or removed along the way, as long as it is
EMR following that line.
Such an equation can be put in many forms, one general form
being:

rate of change in intensity
= rate of emission due to excitation
- rate of absorption
- rate of scattering of photons out of the beam
+ rate of scattering of photons into the beam
+ rate of generated EMR (within stars, that due to fusion)

In this form, each term's "rate" is a rate over distance along the
beam. Some of the terms are functions of intensity and/or of density, and
some incorporate opacity.

If each term can be calculated (or its value looked
up in a pre-calculated table), the equation can be used to create
tractable stellar-structure models.
Some forms of the equation may combine some of these terms,
e.g., considering the effects of
scattering as part of the emission and absorption terms.
Some common approximating assumptions:
equilibrium (no change over time, thermodynamic equilibrium
or local thermodynamic equilibrium),
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 along a single dimension
(plane-parallel atmosphere).

(*equation,physics,EMR*)
**Further reading:**

http://en.wikipedia.org/wiki/Radiative_transfer

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

**Referenced by pages:**

absorption coefficient

radiative transfer (RT)

source function (S)

Index