Diagrams showing the hardness of the ionizing part of the spectrum of stellar populations, constructed by the production efficiencies of H I- and He I- (panel a) or He II-ionizing photons (panel b) (ξ_ion, 0, ξ_ion, 1, ξ_ion, 2). The colored regions represent the time spans indicated by the color bars for co-eval stellar populations and cover the predictions from all metallicities (from solar metallicity down to Z = 0.0002 for stripped stars, see Appendix A and Table A.1). Green shades show predictions for young, single star populations, while the purple and red shades represent the hardness of stellar populations in which stripped stars are included. The case of constant star formation is shown with solid lines and markers (pluses in black for single stars and circles in red for when stripped stars are included), taken after 500 Myr. Above the diagrams, we show the distribution of measured ξ_ion, 0 for three samples of observed unresolved stellar populations from intermediate to high-redshift by Bouwens et al. (2016), Matthee et al. (2017), and Shivaei et al. (2018).