Editing Ellipsometry (section)

===Data analysis===
Ellipsometry is an indirect method, i.e. in general the measured <math>\Psi</math> and <math>\Delta</math> cannot be converted directly into the optical constants of the sample. Normally, a model analysis must be performed, for example the [[Forouhi–Bloomer model|Forouhi Bloomer model]]. This is one weakness of ellipsometry. Models can be physically based on energy transitions or simply free parameters used to fit the data. 
 
Direct inversion of <math>\Psi</math> and <math>\Delta</math> is only possible in very simple cases of [[isotropic]], [[wiktionary:Homogeneity|homogeneous]] and infinitely thick films. In all other cases a layer model must be established, which considers the optical constants ([[refractive index]] or [[dielectric function]] tensor) and thickness parameters of all individual layers of the sample including the correct layer sequence. Using an iterative procedure (least-squares minimization) unknown optical constants and/or thickness parameters are varied, and <math>\Psi</math> and <math>\Delta</math> values are calculated using the [[Fresnel equations]]. The calculated <math>\Psi</math> and <math>\Delta</math> values which match the experimental data best provide the optical constants and thickness parameters of the sample.