  In this paper we study the application of a simplified method to solve the
dynamic radiative transfer problem in expanding envelopes. The method, which
requires a computational effort similar to that of the diffusion approximation,
is based on the use of a generalization of the Eddington closure relationship
allowing the inclusion of scattering and relativistic corrections to order
(v/c). We apply this method to the calculation of light curves of type Ia
supernovae, showing that it gives much more accurate results than the diffusion
approximation, and that the latter is seriously in error when applied to
determine emergent flux and its spectral distribution.
