Ellipsometry measures a change in polarization as light reflects or transmits from a material structure. The polarization change is represented as an amplitude ratio, Ψ, and the phase difference, Δ. The measured response depends on optical properties and thickness of individual materials. Thus, ellipsometry is primarily used to determine film thickness and optical constants.

The instrument is a null type ellipsometer. Nulling is achieved by adjusting the analyzer and polarizer so that all reflected light off of the substrate is extinguished.