Population diagrams for molecules with non-blended emission lines. The diamonds represent the values of N_u/g_u for each transition, derived applying the opacity correction. The solid line shows the fit to the opacity-corrected data, while the dashed line shows the fit assuming optically thin emission. The fitted values for column density (N) and excitation temperature (T_ex) are reported, together with the maximum opacity of the observed transitions. The error bars show 3σ uncertainties.