The Woollam tool uses ellipsometry to characterize thin films. Ellipsometry detects the phase change in polarized light as it reflects from or transmits through samples. The Woollam does not directly measure thickness and optical constants, but rather uses collected data to solve models which estimate the thickness and optical parameters of interest. The general sequence of operation is: 1) Measure, 2) Create Model, 3) Fit, 4) Results. The Woollam measures Ψ and Δ which characterize the change in polarization as the light is reflected from the surface. Ψ characterizes the amplitude and Δ characterizes the phase difference.