Interferometric measurement apparatus and method having increased measuring range

Claim 1

Interferometric measurement apparatus having an increased measurement range, comprising: means for splitting a measuring beam into two linearly polarized beams laterally spaced from each other, wherein said beams are linearly polarized perpendicularly to each other; means for directing said beams onto an object surface at different angles of incidence, said beams thus experiencing a shift in phase with respect to each other which is a function of the topographic height variations of the object surface which is irradiated; means for combining said beams after interaction with the object surface to produce a combined beam; means (12) for periodically varying the phase relationship between said two beams; and means (13,14,15) for sensing the amplitude of the component portion of the combined beam which is linearly polarized in a predetermined direction, wherein the amount of change in the phase relationship between said two beams caused by said phase varying means when said amplitude sensing means senses a maximum or minimum amplitude is a measure of the topographic height variations of the object surface which is irradiated.