Bin-averaged extinction coefficients (Eq. (6)) as a function of the bin-wise Rosseland optical depth of bins 3 and 5 (as defined in Fig. 1) based on the (3D) sunA09 model. The bin-wise Rosseland optical depth τ_Ross,i is computed from wavelengths belonging to bin i. Extinction coefficients calculated from the binning code implemented in Trampedach et al. (2013) with the Trampedach et al. (2014) opacity dataset are shown in black dotted lines, whereas red solid lines represent α_bin,i used in this work.