Mean intensity of the adopted radiation field for two sets of parameters {G₀, W}. The FUV and the visible components are in dotted blue and dotted red lines, respectively and the total spectrum is in a black solid line. (a) Mean intensity of the radiation field adopted in Sect. 3 corresponding to {G₀ = 10⁴, W = 5 × 10⁻³}. (b) Mean intensity with the same FUV component but a weaker visible one ({G₀ = 10⁴, W = 5 × 10⁻⁹}) overlaid on the Mathis ISRF scaled by G₀ = 10⁴.