CO(J=8-7) / CO(J=6-5) 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.