Phase-shifting interferometry (PSI) techniques can be used to measure surface profiles with nanometer level resolutions. A test surface of arbitrary shape is imaged with a Fizeau interferometer capable of producing a controlled phase shift along a Z axis for a PSI acquisition with the chosen algorithm. The optical system is aligned along the Z axis and the surface is imaged onto a camera so each pixel corresponds to a unique position in the XY plane. A laser beam is directed towards a reference surface and the test surface, and the interference between the light beams reflected from the test and reference surfaces are sampled as a function of phase shift and subsequently analyzed with a PSI algorithm to extract the test surface phase map, which is converted into physical units using the known wavelength of the laser beam. A conventional PSI algorithm assumes a constant scanning motion (i.e., constant velocity).
If the scanning motion is not uniform, errors in the measured surface profile occur. Unfortunately, it is often the case that the scanning motion in PSI is not uniform. This can occur due to nonlinear motions of the scanning mechanism, or through vibrations that act on each component of the interferometer differently.