Photon fluxes, Φ_UV, as a function of the H₂ column density, N, below and above 180 nm (upper and lower panel, respectively) for χ = 1. Both panels show the integrated flux of CR-generated UV photons as a function of the three models of CR proton flux (ℒ, ℋ and 𝒰) and the ISRF UV flux. The cyan dotted lines show the integrated photon flux from H₂ and Η lines generated by collisional radiative excitation (labelled 'rad. exc.'). The three curves for each model correspond to R_V = 3.1 (lower curve), 4.0 (intermediate curve), and (5.5 upper curve). Arrows on the righthand side of the upper panel show previous estimates of Φ_UV, independent of column density: Prasad & Tarafdar (1983, PT83), Cecchi-Pestellini & Aiello (1992, CPA92), and Shen et al. (2004, S+04).