Schematic view of the subdivision of a plane-parallel shock into zones. The red curve displays a typical temperature profile induced by an interstellar shock wave in arbitrary units as a function of the distance in logscale. A shock profile is divided into N zones. Each zone is labelled with an index i and is supposed to extend between the boundaries z_i−1 and z_i which are not homogeneously separated (as the scheme would lead to believe). The classical definitions of the angle θ and its cosine μ are used to account for the angular dependance of the radiation field in a plane-parallel geometry.