Patent ID: 8072610

Claim:
In a Mirau interferometer including a source of linearly polarized illumination of a predetermined bandwidth and a predetermined plane of polarization, wherein said illumination is directed toward a test surface and a totally reflective reference mirror that obscures a portion of test and reference beams directed toward a detector, the improvement comprising: a beamsplitting element splitting said linearly polarized illumination into linearly polarized test and reference beams; an achromatic quarter-wave retarder placed in an optical path of the test beam; and an optical element in an optical path of said test and reference beams directed toward said totally reflective reference mirror, said optical element having dispersive characteristics matching said quarter-wave retarder at said predetermined bandwidth of the linearly polarized illumination; wherein the retarder is set with a principal axis at a 45-degree angle with respect to the plane of polarization of the linearly polarized illumination, so as to produce orthogonally polarized test and reference output beams.