Ellipsometry is a non-destructive metrology method, allowing measurement of material and geometrical properties of micro and nano systems. A target is illuminated by light with well-defined polarization states and a number of field components of the reflected and/or transmitted light are measured. Basically an ellipsometer measures the change of polarization of an incident beam caused by a sample.
Since the measured light carries no direct information about the measured system, inverse techniques have to be used to reconstruct parameters of the system. Therefore, simulations for different configurations of the systems are performed and the best matching results to the experimental measurement are determined. Fitted quantities are thereby certain components of the Mueller or Jones Matrix.
An example can be found here.