Areas enclosed in coloured lines show the spread of the results for model clouds filled with CMM+AMM grains a), b), e), and f) and with CMM+AMMI grains c), d), g), and h). The spread is representative of both variations along the sphere radius and variations in f_back from 0 to 1 (from left to right in all figures, see Sect. 4.1 and footnote 3). a), b), c), and d) 4.5 to 3.6 μm coreshine ratio as a function of the 3.6 μm intensity. The black circles are the coreshine ratios measured by Lefèvre et al. (2014) towards 21 starless cores and the black lines are the scatter in these measurements. e), f), g), and h) Near- to mid-IR ratio as a function of the 4.5 to 3.6 μm coreshine ratio. The solid and dashed boxes show the range of values measured towards the Taurus-Perseus regions and L183/L134 cloud, respectively (Lefèvre et al. 2014, see their Figs. 17 and 18). In Figs. a) and e) (c) and g)), the magenta (blue) lines show the results for the control cloud parameters defined in Sect. 3 (R_out = 0.3 pc, ρ_C = 10⁵ H/cm³) and the orange and red lines for the same set of parameters except that R_out = 0.1 and 0.2 pc, respectively. In Figs. b) and f), d) and h), the red lines show the same parameters as in the previous figures (R_out = 0.2 pc, ρ_C = 10⁵ H/cm³), the cyan lines show the results for ρ_C = 10⁴ H/cm³, the orange lines for ρ_C = 5 × 10⁴ H/cm³, and the yellow lines for ρ_C = 5 × 10⁵ H/cm³.