Extended depth from focus using white light interferometer

Abstract

A new approach to 3-D profilometry for the white light interferometer (WLI) is presented. We detail a simple way to construct a profiler that uses the fringe contrast degradation of the white light Mirau interferometer. The amount of fringe contrast degradation in the Mirau interferometer can be founded by the depth from focus (DFF) method. In the method, the variance of the mismatch function and the modified local variance function are used as the focus measures. The proposed method is named as the extended depth from focus (EDFF) that determines the zero optical path difference (OPD) from the amount of fringe contrast degradation of the white light interferometer. The method has a theoretically unlimited range and can profile with subpixel accuracy both optically rough and smooth surfaces without changing the algorithm.