A sensor and method for determining the optical properties of a sample material is disclosed. The sensor comprises a light source that generates a linearly polarized light beam having a predetermined polarization orientation with respect to the plane of incidence. The linearly polarized light beam is reflected off the sample and is split into second and third light beams where the second and third light beam consist of the combined projections of mutually orthogonal components of the first light beam. A signal processor measures the intensity difference between the second and third light beams to calculate the phase difference induced by the sample material.