Calculated hydrogen equilibrium ionization structure as a function of the spectral energy distribution of the ionizing source, exemplified by black bodies with different temperatures but the same number of ionizing photons. Due to lower values of the absorption cross section for photons with higher energies the shell of the “ionization front” becomes thicker for harder ionization spectra. This well-known effect can be reproduced only by models with a sufficient number of frequency points in the ionization continuum. The first panel corresponds to the result one usually obtains if the gas is illuminated by a monochromatic source.