A system is configured to measure two separately polarized beams upon diffraction from a substrate in order to determine properties of a grating on a substrate. Linearly polarized light sources are passed via a fixed phase retarder in order to change the phase of one of two orthogonally polarized radiation beams with respect to the other of the two beams. The relative phases of the two radiation beams and other features of the beams as measured in a detector gives rise to properties of the substrate surface. The grating and the initial linear polarization of the radiation beam are angled non-orthogonally relative to each other.