Results from radiative transfer calculations. Apparent OPR as a function of LOS mean temperature and column density (A, B), and as a function of the attenuation factor (C). Plots A and B refer to calculations with an effective OPR of 2.5 and 3, respectively, with a colour-coding according to the derived apparent OPR. For C plot, the colour-coding is according to the column density (in base-10 log scale).