If the check-box indicating that the radiation mesh is 
not forming a closed domain has been checked,
 the temperature (in degre C) 
of the exterior has to be specified in that field.
In that situation, one notices that
the external domain behaves as 
a black body (emissivity of 1 can be specified)
If possible authors suggest to use preferably the imposed temperature 
functionality. It has the drawback to oblige users to mesh explicitely 
the opening but then the general algorithms
 are used ensuring conservativity at the radiation level 
and an emissivity different from 1 can be specified and can vary from 
one region of the opening to another.
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 SYRTHES version   : VERSION_SYRTHES
 Name help file  : Bcr_Rpa_le.html
 I. Rupp, C. Peniguel  -  may 2014
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