Effect on the model SED (top) derived from the RT simulations for M 51 assuming a dust scale height h_z,d = 150, 225 and 450 pc or, thus, a dust-to-stellar scale height of , and 1, indicated as a black solid, green dotted and red dashed line, respectively. The bottom panel shows the relative SEDs for RT models with dust scale heights of h_z,d = 150 pc (green dotted line) and h_z,d = 450 pc (red dashed line) compared to the RT model with h_z,d = 225 pc, which was used for the analysis in this paper.