Spectral energy distributon of the radiation field: at the entrance of the HII region (dotted line), at the ionization front (dashed line), compared to the same initial BC03 radiation field simply truncated at the Lyman break (solid line), (a) for log U = −2, n_H = 100 cm⁻³ and Z_stars = 0.02, (b) as a function of Z_stars, (c) as a function of U for n_H = 100 cm⁻³) and Z_stars = 0.02, (d) and as a function of n_H for log U = −2 and Z_stars = 0.02.