Through an analysis of the state of polarization of the light that is reflected from the sample the technique allows the accurate characterization of a range of properties including the layer thickness, optical constants, chemical composition, crystallinity, anisotropy and uniformity. Thickness determinations ranging from a few angstroms to tens of microns are possible for single layers or complex multilayer stacks.

Ellipsometry consists of the measurement of the change in polarization state of a beam of light upon reflection from the sample of interest. The exact nature of the polarization change is determined by the sample's properties (thickness and refractive index). The experimental data are usually expressed as two parameters Ψ and Δ. 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 parallel to the sample surface, and the p-component is oscillating parallel to the plane of incidence). The intensity of the s and p component, after reflection, are denoted by R_s and R_p. The fundamental equation of ellipsometry is then written: