Polarization interferometers are widely used in microscopy. In these interferometers, the two beams traverse almost identical paths, so that conventional phase-shifting techniques cannot be used. However, because the two beams leaving a polarization interferometer are orthogonally polarized, it is possible to use a phase shifter operating on the geometric phase to introduce a variable phase shift between the two beams without any change in the optical path difference. Since this phase shift is very nearly independent of the wavelength, small variations of the optical path difference over the field can be mapped accurately, even with white light. Achromatic phase-shifting can also be used with two-wavelength illumination to provide a quick and simple method for profiling surfaces exhibiting steps with heights of a few micrometres.
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