Comparison of the ionizing spectra of different models for age 1 Myr. Shown is the cumulative number of ionizing photons emitted above energy E, Q(> E), as a function of the energy (in eV). All ionizing spectra are normalized to unity at the Lyman limit (13.6 eV), to allow for easy comparison of their relative hardness. Vertical lines mark the ionization potentials of several important ions.