Effect on the model SED (top) derived from the RT simulations for M 51 assuming a two-phase dust component with a clump fraction f_cl of 0, 0.2 and 0.5, representative for the relative fraction of dust mass distributed in clumps, indicated as a black solid, green dotted and red dashed line, respectively. The bottom panel shows the relative SEDs for RT models with clump fractions f_cl = 0.2 (green dotted line) and f_cl = 0.5 (red dashed line) compared to the RT model with a smooth dust component, which was used for the analysis in this paper.