Ellipsometry is an optical technique widely used to make accurate measurements of the thickness and optical properties of thin films. It measures the change in polarization of an electromagnetic wave as it is reflected from the boundary between two (or more) media of differing refractive index.
In reflection ellipsometry, a sample is illuminated at an angle Φ with light of a known polarization state. The resulting reflected light is analyzed to measure the change in the polarization state caused by reflection off the sample. The two main ellipsometric parameters are Ψ and Δ, defined by the relation
where rs and rp are the amplitude reflection coefficients of the components of incident light with their electric vectors aligned perpendicular (s) and parallel (p) to the plane of incidence respectively. Typically, elements of a mathematical model of the sample (e.g. thickness, refractive index) are adjusted to produce values of Ψ and Δ to match with those measured experimentally.
A good tutorial on the subject can be found at the following link:Ellipsometry tutorial
We are fortunate enough to have a spectroscopic ellipsometer in our labs, made by Woollam.