4.4. Radiative Transfer The photoionization calculations
described above are relatively straight forward, provided the
internally produced radiation can freely escape the cloud. This
is the case in most galactic nebulae, and possibly also in AGN
NLR clouds, but not in the BLR, where the optical depth to line
and continuum radiation is significant. In this case the
calculations must be modified, and the following pages describe
some commonly used ways to do so. 4.4.1 Continuum transfer. The
internally produced recombination (bound-free) radiation can
have a large effect on the degree of ionization by interacting
with the gas far from its point of creation. Sophisticated,
iterative methods have been developed to account for the
propagation of this radiation in the cloud. They depend on the
gas distribution and geometry and cannot be applied, in a simple
way, to all configurations. Approximate methods have been
developed too, to shorten the calculations and reduce the number
of iterations. The "on-the-spot" appr