Mean ionizing photon energy in units of 13.6 eV as a function of metallicity, shown for SFR = const. (thick lines) and for the ZAMS (thin lines). Results for different IMFs are shown using the same colour codes as in Fig. 1 (cf. Table 1). Lined and labels on the left indicate the corresponding blackbody temperatures. Z_coll shows the approximate metallicity limit below with collisional effects lead to significant departures from case B.