([O I] 63 µm + [C II] 158 µm) / I_FIR Intensity Ratio emitted from the surface of a PDR as a function of the cloud density n and the FUV flux incident on the cloud G₀. The models in the wk2006 ModelSet are based Kaufman et al. 1999 and Kaufman et al. 2006 . They use these parameters. More details are in the FITS headers.