Editing Ellipsometry (section)

===Data acquisition===
Ellipsometry measures the complex reflectance ratio <math>\rho</math> of a system, which may be parametrized by the amplitude component <math>\Psi</math> and the phase difference <math>\Delta</math>. 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 perpendicular to the plane of incidence and parallel to the sample surface, and the ''p'' component is oscillating parallel to the plane of incidence). The amplitudes of the ''s'' and ''p'' components, after [[Reflection (physics)|reflection]] and normalized to their initial value, are denoted by <math>r_s</math> and <math>r_p</math> respectively. The angle of incidence is chosen close to the [[Brewster angle]] of the sample to ensure a maximal difference in <math>r_p</math> and <math>r_s</math>.<ref>Butt, Hans-Jürgen, Kh Graf, and Michael Kappl. "Measurement of Adsorption Isotherms". Physics and Chemistry of Interfaces. Weinheim: Wiley-VCH, 2006. 206-09.</ref> Ellipsometry measures the complex reflectance ratio <math>\rho</math> (a complex quantity), which is the ratio of  <math>r_p</math> over <math>r_s</math>:
: <math>\rho = \frac{r_p}{r_s} = \tan \Psi \cdot e^{i\Delta}.</math>
Thus, <math>\tan\Psi</math> is the amplitude ratio upon [[Reflection (physics)|reflection]], and <math>\Delta</math> is the phase shift (difference). (Note that the right side of the equation is simply another way to represent a [[complex number]].) Since ellipsometry is measuring the ratio (or difference) of two values (rather than the absolute value of either), it is very robust, accurate, and reproducible. For instance, it is relatively insensitive to scatter and fluctuations and requires no standard sample or reference beam.