Interferometric measurement of phase change on reflection.

We show an interferometric method for measuring phase change with known uncertainty. Because this measurement uses the backreflection from a sample, the height is intrinsically removed, and only the phase change is measured. The uncertainty in the phase change measurement is +/-3.8 degrees and is dominated by the background subtraction method. We also investigate the effect of the phase change on the interferometric radius measurement. The theoretical worst-case error in the interferometric radius measurement due to the phase change is 30 nm.