The efficiency ${\eta }_{LC}$ of linear-to-circular polarization conversion when light is reflected at a dielectric–conductor interface is determined as a function of the principal angle $\overline{\varphi }$ and principal azimuth $\overline{\psi }$ . Constant- ${\eta }_{LC}$ contours are presented in the $\overline{\varphi }$ , $\overline{\psi }$ plane for values of ${\eta }_{LC}$ from 0.5 to 1.0 in steps of 0.05, and the corresponding contours in the complex plane of the relative dielectric function ϵ are also determined. As specific examples, efficiencies $⩾88\%$ are obtained for light reflection by a Ag mirror in the visible and near-IR $(400–1200\mathrm{nm})$ spectral range, and $\ge 40\%$ for the reflection of extreme ultraviolet (EUV) and soft x-ray radiation by a $\mathrm{SiC}$ mirror in the $60–120\mathrm{nm}$ wavelength range.