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In this paper we present an optical derotator for scanning vibrometer measurements on rotating objects. The main part of an optical derotator is a rotating prism. Several concepts are known from literature. We have chosen a Dove prism because it can derotate the rotation of the specimen by simply watching through the prism, which rotates with half the speed. The design of our derotator is presented in this paper as well as a discussion of the system performance. In addition we show experimental measurement results on a fan rotating with 3000 rpm.

We discuss how the results obtained from a white-light interferometer can be compared to tactile measurements. The core idea to achieve comparability is to determine a short cut-off wavelength up to which the spatial frequency components of the surface topography are measured with less than 3 dB attenuation. We demonstrate for different interferometers that the data has to be filtered to achieve a linear transfer characteristic which allows to define the short cut-off wavelength. In addition, we demonstrate investigations of the error sources in shape measurements that we have identified. Results of our work are influencing a VDI/VDE calibration guideline for shape measurements which is currently under development. We show in this paper how the procedure developed for the guideline can be employed to real measurement devices. Uncertainty contributions to the error budget are also discussed and measurements on shape standards are presented.

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