(1-0)S(1) and (1-0)S(0) emission brightness (top) and their ratio (bottom-left) for pure FUV photoexcitation as computed by the isobaric MEUDON PDR models (Le Petit et al. 2006). The line brightness are presented in the parameter space of the cloud’s total visual extinction, A_V, max, vs. the cloud’s thermal pressure, P_th. The points are the locations where the PDR models were computed. The red circle is the fiducial model which represents a typical cloud for our observations, for which we also show the thermal structure of the cloud (bottom-right).