- thickness of the slab, cm
- Minimum optical depth in the continuum, and maximum optical depth in the continuum, .
- Constant temperatures (in all axes), K
- Outer boundary condition, and diffusion inner boundary condition for all wavelengths.
- Parameterized coherent & isotropic continuum scattering by
defining

with . and are the continuum absorption and scattering coefficients.

The line of the simple 2-level model atom is parameterized by the ratio of the profile averaged line opacity to the continuum opacity and the line thermalization parameter . For the test cases presented below, we have used and a constant temperature and thus a constant thermal part of the source function for simplicity (and to save computing time) and set to simulate a strong line, with varying (see below). With this setup, the optical depths as seen in the line range from to . We use 32 wavelength points to model the full line profile, including wavelengths outside the line for the continuum. We did not require the line to thermalize at the center of the test configurations, this is a typical situation one encounters in a full 3D configurations as the location (or even existence) of the thermalization depths becomes more ambiguous than in the 1D case.

The slab is mapped onto a Cartesian grid. For the test calculations we use voxel grids with the same number of spatial points in each direction (see below). The solid angle space was discretized in with if not stated otherwise. In the following we discuss the results of various tests. In all tests we use the full characteristic method for the 3D RT solution as described above. Unless otherwise stated, the tests were run on parallel computers using 128 CPUs. For the 3D solver we use points along each axis. The solid angle space discretization uses points.