β colour maps from dust opacity models. Each figure represents a grid of dust size distributions varying minimum and maximum grain sizes (n(a) ∝ a^−q with q=3.0). Coloured by the values of β derived from the opacities for (a) pure absorption (β_abs), (b)scattering (β_sca), (c) extinction (β_ext), and (d) effective extinction (β_eff; see text). Contours are marked by black lines at β = 0.0 and β_ism = 1.38). Pattern effects are artefacts of the interpolation used.