Panels a) to e): ZAHB locus for Y = 0.246 and Z = 0.001 (black lines) in the theoretical plane (M_bol vs. T_eff) and in four UV-bands (Johnson-Cousin U vs. U − V; Strömgrem u vs. (u − y); HST/WFPC2 F336W vs. (F336W − F555W); Sloan u vs. u − g). Dashed and dotted lines indicate the location at the ZAHB locus of the following temperatures: 6125 K (red line), 6600 K (green line), and 7800 K (blue line). Panel f): spectra of stars at the ZAHB level with the properties (T_eff, log g) of dashed/dotted lines computed with ATLAS9 and SYNTHE packages. Transmission curves of the filters used in left panels are also shown: Johnson U − V (gray lines), Strömgren u − y (black lines), HST/WFPC2 F336W−F555W (dark green lines), and Sloan u − g (cyan lines). Transmission curves are multiplied arbitrarily by 4. To compare these spectra, fluxes are multiplied by the surface area of the respective star. Panels g) to j): differential convolved spectra depending on the transmission curves compared to the 6600 K spectra.