Top panels: BPASS stellar population predictions for the evolution of the number of ionisation photons produced by a single instantaneous star burst (10⁶ M_⊙) stellar population as a function of time. From left to right: H and He⁺ ionisation photons computed by integrating the spectra at λ = 228 Å and 912 Å, respectively. The models are computed at Z = 0.0001, 0.001, 0.002, 0.01,  and 0.02 for binary models with different IMFs (Γ = −1.0, −1.35, and  − 1.70) and IMF upper mass cutoffs (100 M_⊙ and 300 M_⊙). Bottom panels: similar to the top panels, but showing the evolution of ξ_ion of H and He⁺ as a function of time.