Illustration of the effect of beaming on variation of the optical depth of the first five doublet states. The parameters for the calculation are shown in the graph. From left to right, the dashed vertical lines indicate the specific column densities where, respectively, the |$4_{13}{\!-\!}4_{14}$|⁠, |$3_{12}{\!-\!}3_{13}$|⁠, and |$2_{11}{\!-\!}2_{12}$| transitions switch from being inverted to not inverted.