Measures the refractive index and the thickness of semi-transparent thin films. The instrument relies on the fact that the reflection at a dielectric interface depends on the polarization of the light while the transmission of light through a transparent layer changes the phase of the incoming wave depending on the refractive index of the material. An ellipsometer can be used to measure layers as thin as one nanometer up to layers which are several microns thick. Applications include the accurate thickness measurement of thin films, the identification of materials and thin layers, and the characterization of surfaces.