For ellipsometry, polarised light is shone on to a sample surface at an oblique angle of incidence. The plane of incidence of the light is the plane that contains both incident and reflected beams. The polarisation of light reflected parallel (p) and perpendicular (s) to the plane of incidence is measured. This allows the relative phase change (delta) and relative amplitude change (psi) from the reflected surface to be determined. Ellipsometry can be used to measure film thicknesses and with variable wavelength, dielectric properties can also be estimated.

Reflectance Difference Spectroscopy (also known as Reflectance Anisotropy Spectroscopy) is similar to ellipsometry except that normal incidence is used. The difference of reflectance between 2 axes of a crystal surface can be determined. This difference can be very small. RDS is used for in-situ monitoring of crystal growth.