= Ellipsometry
{wiki}
<Ellipsometry> measures the change in <polarization of an electromagnetic wave> upon <specular reflection> to infer material properties. For a simple planar sample, the complex reflection-coefficient ratio is written $\rho=r_p/r_s=\tan\Psi\,e^{i\Delta}$: $\Psi$ describes a relative amplitude and $\Delta$ a relative <wave phase> between the two incident <polarizations>.
A source, polarization-control optics, sample at a known incidence angle and analyzer with a <photodetector> form a basic instrument. Repeating measurements across <wavelength> and angle constrains <refractive index> and film thickness through a model of the layered sample. The fitted thickness is not read directly from $\Psi$: roughness, layer structure and correlated model parameters can produce ambiguous fits.
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