Left panel: example BPASS v2.1 model spectra of a single-burst stellar population after 5 Myr from the star burst. The single (dashed) and binary (continuous) model predictions are shown for different IMFs (Γ = −1.0, −1.35, −1.70) with a high-mass IMF cutoff at 300 M_⊙. The dashed black vertical lines mark λ = 228 Å, 353 Å, 508 Å, and 912 Å, below which He⁺, O⁺⁺, C⁺⁺, and H ionising photons are produced. Right panel: zoomed-in region λ <  275Å clearly showing the difference in flux around He⁺ ionising limits.