Time evolution of photon production rates in H I (black), He II (blue) and O VI (yellow-green) at five metallicities, from an instantaneous burst of total mass 10⁶ M_⊙ occurring at an age of zero. At each metallicity three single-break power law (α_u = α_m = −1.8, −2.35, −2.70) IMF models are shown at two upper mass limits (M_u = 100, 300 M_⊙), with line style indicating different slopes and line colour different upper mass cut-offs as indicated in the key. The rest of the models lie within the bounds of those shown at fixed age.