Ellipsometry measures the complex reflectance ratio  of a system, which may be parametrized by the amplitude component  and the phase difference . The polarization state of the light incident upon the sample may be decomposed into an s and a p component (the s component is oscillating perpendicular to the plane of incidence and parallel to the sample surface, and the p component is oscillating parallel to the plane of incidence). The amplitudes of the s and p components, after reflection and normalized to their initial value, are denoted by  and  respectively. The angle of incidence is chosen close to the Brewster angle of the sample to ensure a maximal difference in  and . Ellipsometry measures the complex reflectance ratio  (a complex quantity), which is the ratio of   over :
Thus,  is the amplitude ratio upon reflection, and  is the phase shift (difference). (Note that the right side of the equation is simply another way to represent a complex number.) Since ellipsometry is measuring the ratio (or difference) of two values (rather than the absolute value of either), it is very robust, accurate, and reproducible. For instance, it is relatively insensitive to scatter and fluctuations and requires no standard sample or reference beam.