Dust model extinction, τ_i, in the V band (blue) and at 250 μm (green) and 1 mm (red) wavelengths for an assumed column density N_H = 10²¹ H cm^-2. The upper plot shows the variations as a function of the a-C(:H) material band gap, E_g, and the lower plot as a function of the power-law, α, component of the small a-C dust population. The filled data points indicate the values for the best-fit DHGL dust model. The crosses with error bars show τ₂₅₀ = 5.8 ± 1.7 × 10^-4 E(B − V), from Eq. (2), multiplied by 10⁴.