FASTWIND model predictions for the line intensities used for luminosity criteria in late O-type stars as a function of surface gravity (log g) for different T_eff−log Q pairs and ɛ_Si = 7.0 dex. Lines used in the primary and secondary criteria (see Table 7) are shown in the upper and lower panels, respectively. Line profiles were broadened using v sin i = 100 km s⁻¹, and a spectral resolving power of R = 4000 (used to classify the VFTS spectra). For all profiles, v_mic was fixed at 10 and 5 km s⁻¹ for the NLTE and formal integral computations, respectively. The log g range for our sample is indicated by the grey rectangle.