Top left: relative differences δ_ext between extinction optical depth τ_ext and dichroic optical depth τ_dic for hypothetical example with density and grain alignment increasing linearly with increasing path length. The different curves correspond to different input Stokes vectors. In each case the photon package is initially fully linearly polarised, but the Stokes vector orientation θ increases in steps of 5°. The red lines corresponds to θ = 0° (Q₀/I₀ = 1) and the purple line to θ = 90° (Q₀/I₀ = −1), respectively. Bottom left: similar to top left panel, but now showing relative difference δ_tot between total optical depth τ_tot and dichroic optical depth. Right: same as panels on left, but now for model with uniform grain alignment.