Ultraviolet photon flux (integrated from 911 to 2400 Å) as a function of the total hydrogen column density (bottom panel) and the visual extinction (top panel) calculated with the Meudon PDR code (a), by Tielens & Hollenbach (1985; b), and with the Paris-Durham shock model set in advection mode (c–f) for an isochoric gas of density n_H = 10⁴ cm⁻³ with an incident radiation field G₀ = 10². The predictions of the Paris-Durham model are shown for pure graphite grains with a size distribution defined by Table A.1 (standard model, labelled c), for silicate grains with the same size distribution (d), graphite grains with a minimal radius set to 3 × 10⁻⁶ cm (e), and graphite grains with a minimal radius set to 3 × 10⁻⁶ cm where the grain absorption coefficients are scaled to impose a N_H∕A_V ratio of 1.87 × 10²¹ cm⁻² (f).