  We consider the radiation transfer problem in the discrete-ordinate,
plane-parallel approach. We introduce two benchmark problems with exact known
solutions and show that for strongly non-homogeneous media the homogeneous
layers approximation can lead to errors of 10% in the estimation of the
intensity. We propose and validate a general purpose numerical method that
transforming the two-boundary problem into an initial boundary problem, using
an adaptative step integration and an interpolation of the local optical
properties, can improve the accuracy of the solution up to two orders of
magnitude. This is furthermore of interest for practical applications, such as
atmospheric radiation transfer, where the scattering and absorbing properties
of the media vary strongly with height and are only known, based on
measurements or models, at certain discrete points.
