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 phi and principal azimuth psi . Constant-eta(LC) contours are presented in the phi , 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 are also determined. As specific examples, efficiencies > or = 88% are obtained for light reflection by a Ag mirror in the visible and near-IR (400-1200 nm) spectral range, and > or =40% for the reflection of extreme ultraviolet (EUV) and soft x-ray radiation by a SiC mirror in the 60-120 nm wavelength range.