Ellipsometry measures the change of polarization upon reflection or transmission. Typically, ellipsometry is done only in reflection setup. The exact nature of the polarization change is determined by the analysis of the sample's properties like thickness, complex refractive index, etc. Although optical techniques are inherently diffraction limited, ellipsometry exploits phase information and polarization state of light and hence can achieve angstrom level resolution. In its simplest form, the technique is applicable to thin films with thickness less than a micrometer to several nanometers.