Three-lambda metrology

Abstract

The state-of-the-art technique for measuring discontinuous surface profiles, e.g. diffractive optical elements (DOE) is white-light interferometry. Compared to single wavelength phase-shifting interferometry conventional white-light-interferometry is rather slow, because the number of frames to be evaluated is about ten times greater than in phase-shifting-interferometry. Therefore white-light-interferometry needs more memory capacity and computer time. Single wavelength phase-shifting interferometry cannot be used for the mentioned task since the order of the interference fringes cannot be determined. But if three wavelengths, e.g. a red, a green, and a blue one are used which preferably have no common factor it is possible to determine the interference order of the fringes or the absolute optical path difference (OPD) of the interferometer. The interference patterns are simultaneously recorded by a color CCD-camera having 3 separate chips. The OPD is calculated for each pixel from the three phase values mod 2π . The algorithms used and experimental results will be presented.