= Radiative transfer source function
{title2=$S_\nu$}
= Source function
{synonym}
The <radiative transfer> <source function> is emission per unit extinction. If $j_\nu$ is emission per unit path length and $\rho\kappa_\nu$ is extinction per unit path length, $S_\nu=j_\nu/(\rho\kappa_\nu)$ and the transfer equation is $\mu\partial_{\tau_\nu}I_\nu=I_\nu-S_\nu$. Under thermal absorption in <local thermodynamic equilibrium> it is the <Planck function>; for conservative <coherent isotropic scattering> it is the <mean intensity>.
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