Original domain (solid thick lines) and calculation domain (dotted thick lines) at a given z-plane, for a medium that is periodic in the horizontal (x, y) plane, and for a radiation propagating in the first octant, i.e., along increasing x, y, and z. Both domains span one period in x-direction and one period in y-direction. The nodes of the original domain are (x₀,x₁,...,x_Nx) and (y₀,y₁,...,y_Ny). The nodes of the calculation domain, where the specific intensity is computed, are (x_i0,x_i0 + 1,...,x_i0 + Nx) and (y_j0,y_j0 + 1,...,y_j0 + Ny). The indices i₀ and j₀ are chosen such that Δx_i0 = Δx_max and Δy_j0 = Δj_max. See text in Sect. 7 for a discussion.