Dependence of the ratio R_f = F_15.5/F_11.4 on the grain size distribution. The ratio R_f is deduced from the model after integration of the flux in the considered size range. R_f is plotted for a selection of different size ranges, all following a distribution in a^−3.5. Each segment depicts a range that is plotted in blue for silicate grains and in orange for graphite grains. The dots correspond to single sizes of silicate grains. The two retained ranges are indicated by thicker dash-dot segments. The range of ratios deduced from observations is indicated by shaded cyan rectangle.