  In this work, we provide a fully-implicit implementation of the
time-dependent, filtered spherical harmonics (FPN) equations for non-linear,
thermal radiative transfer. We investigate local filtering strategies and
analyze the effect of the filter on the conditioning of the system, showing in
particular that the filter improves the convergence properties of the iterative
solver. We also investigate numerically the rigorous error estimates derived in
the linear setting, to determine whether they hold also for the non-linear
case. Finally, we simulate a standard test problem on an unstructured mesh and
make comparisons with implicit Monte-Carlo (IMC) calculations.
