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Mirau Interferometry

A Linnik interferometer works only when the two path lengths are matched to within the coherence length of the light source. For a low-coherence source such as an LED, the coherence length is on the order of micrometers, so matching the path lengths requires precise adjustment of both the target and reference arms. In a Mirau interferometer, the interferometric system is customized for a single microscope lens, so the path lengths are matched during the manufacturing process. Consequently, taking three-dimensional interferometric images is as simple as changing focus on the microscope stage (in the example shown here, focus is changed by moving the microscope objective). As the path length is changed, the interferometric pattern shifts. Measuring this change provides a calibration between brightness of the image and height of the target.