The surface to surface radiation is implemented in
*CrysMAS* by so called view factors radiation. The quality of the
view factors is important for the quality of the computation result: If there are
artificial heat sources or sinks due to inaccurate view factors, increase the view
factor precision (i.e. decrease the value specified). View factors are always
calculated before temperature, either in an independent run or automatically in
the same computation run as temperature.

*CrysMAS* can handle heat transfer by radiation in perfectly
transparent media and for media which show certain transparency windows in a given
wavelength domain (Visibility Bands). The physical model used by
*CrysMAS* is that of the gray body radiation: Surfaces of
a cavity are emitting a hemispherical flux proportional to
σ
T ^{4} and proportionality coefficient
ε
is called emissivity.

The view factor is defined
between two elements of the cavities surface. If i and j are two such
elements, the view factor from i to j , F
_{ji} represents the fraction of the energy that leaves
element i and arrives to j if the assumption of black body radiation.
For the gray body, the
Γ
matrix is of interest. It is defined in the following way:

q _{i} ^{net} =
Γ
^{.} q
^{em} is thereby the difference between absorbed and emitted
energy at surface element i .
Γ
can be directly computed from the view factors
and the emissivities. q^{net} is taken as a heating source density.

The energy leaving element i must arrive at the other elements without loss, therefore the view factors have the following property:

In the numerical treatment this sum does not exactly equal 1 and the energy inside
the furnace is not conserved completely.
*CrysMAS* provides a renormation function for the
Γ
matrix on the
**View factors** tab of the
**Numerical parameters** dialog.

View factors have to be recomputed, if geometry or emissivity were changed.
In either case *CrysMAS* computes them automatically.