A system for measuring a periodic array of structures on a sample is provided. The system includes an optical source configured to produce an optical beam; an optical system configured to control the polarization of the optical beam and to focus the optical beam with a first NA1 on a sample surface and to sweep the angle of incidence across a range of angles with an approximately fixed focal position on a sample surface with a second NA2 wherein NA2>NA1; additional optical components configured to receive the optical beam reflected from the sample surface and to focus the reflected beam onto a detector; and a recording system to record the reflectivity of the sample surface as a function of the angle of incidence.