Illustration of the geometry of 1D PDR models made with the Meudon PDR code. We show the two-component radiation field composed of a beamed part representing the stellar radiation field and an isotropic part representing the ISRF. This configuration yields the typical chemical stratification, with the ionization front denoting the beginning of the PDR, followed by the dissociation front, or H/H₂ transition, and the C⁺/C/CO transition deeper in the cloud. H₂O appears even deeper.