Rapid shape measurement based on the measurement of the mutual coherence function using a shear interferometer (Gamma-Profilometry)

• Applicant: Professor Dr. Ralf Bernhard Bergmann
• Subject Area: Measurement Systems
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 265388903

Shape measurement is an indispensable tool for industrial quality assurance of micro parts. For an aim of 100% quality assessment in mass production, a precise and robust measurement technique is needed that has to provide measurement rates below one second per piece. Currently, there is no technique available that offers these capabilities for micro parts with rough surfaces and with dimensions of several 100 micrometers in all spatial directions.Tactile and confocal approaches are much too slow. Geometrical-optical approaches such as micro fringe projection only provide, without averaging over many data points, comparably large measurement uncertainties of 5 to 10 µm. Due to the scanning process white light interferometers are too slow as well. Finally, interferometric multi-wavelengths methods are not practicable for micro objects that have sizes of several 100 micrometers, because the corresponding speckle fields decorrelate due to the required large difference between the wavelengths.The aim of the project is to close this gap in the state of the art and to provide such a measurement technique. The basic idea of the proposed concept is to perform shear interferometry under illumination with short coherence length. A shear interferometer superposes light originating from two positions x and x+s on the object plane separated by the shear s. Especially when using light with a short coherence length, the mutual coherence function Gamma(x, x+s), i.e. the contrast and the phase of the interference figure, depends on the finite difference of the surface profile between the positions separated by the shear. Hence, if the mutual coherence function Gamma(x, x+s) can be determined at any position x, it should be possible to calculate the surface profile of the object by means of numerical integration.The proposed technique shares larges similarities with white light interferometry (or coherence radar, respectively) and should yield similar results with respect to the measurement uncertainty. However, in contrast to these methods it is not required to sequentially scan over the entire height of the specimen under investigation. Additionally, due to the common-path-configuration there is no need for a reference wave, thereby eliminating the need for a mechanically stable environment. These benefits enable the simultaneous achievement of the industrially required goals of a substantially higher measurement speed and an increased robustness.

• DFG Programme: Research Grants