Vertical surface brightness profiles of the simple pillar model described in Sect. 4.5. The three panels correspond to three different values of the total optical depth through the pillar. The horizontal axis indicates the depth through the pillar, measured from the top where the radiation enters. The different lines in each panel correspond to models with a different number of photon packages (ranging from 10⁶ to 10¹⁰) and with and without the application of composite path length stretching.