The goal of ellipsometry is to determine the optical and structural constants of thin films and flat surfaces from the measurements of the ellipse of polarization in reflected or transmitted light. In the absence of birefringence and optical activity a flat surface, a single-layer film, or a thin-film stack may be characterized by the complex reflection coefficients rp = |rp|exp(iφrp) and rs = |rs|exp(iφrs) for p- and s-polarized incident beams, as well as by the corresponding transmission coefficients tp = |tp|exp(iφtp) and ts = |ts|exp(iφts).

Strictly speaking, an ellipsometer is a device that measures the complex ratios rp /rs and/or tp /ts. The amplitude ratios are usually deduced from the angles ψr and ψt, which are defined by tan ψr = |rp|/|rs| and tan ψt = |tp|/|ts|. In practice, measuring the individual reflectivities Rp = |rp|2, Rs = |rs|2 or transmissivities Tp = |tp|2, Ts = |ts|2 does not require much additional effort. Measuring the individual phases, of course, is difficult, but the relative phase angles φrp − φrs and φtp − φts can be readily obtained by ellipsometric methods. The values of Rp, Rs, φrp − φrs, ψr, Tp, Ts, φtp − φts and ψt may be measured as functions of the angle of incidence, θ, or as functions of the wavelength of the light, λ, or both.

The results of ellipsometric measurements are fed to a computer program that searches the space of unknown parameters to find agreement between the measured data points and theoretical calculations.