Conference proceedings
Dispersion optimized white-light interferometer based on a Schwarzschild objective

Publication Details
Kühnhold, P.; Lehmann, P.; Niehues, J.
Peter H. Lehmann, Wolfgang Osten, Kay Gastinger
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Optical Measurement Systems for Industrial Inspection
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White light interferometry provides a fast and highly precision possibility for three-dimensional measurement of topographies. Two kinds of interferometer are mainly used because of their compact design and high stability: the Michelson-Interferometer especially for low magnifications, and the Mirau-Interferometer. White light interferometers are very sensitive to the influence of dispersion [1, 2, and 3]; these kinds of effects may cause significant errors. Lehmann [3, 4] has shown that plane glass plates used in a Mirau-interferometer are a significant source for dispersion effects. Especially by the measurement of curved structures errors occur, which are caused by dispersion effects. Measuring a sinusoidal structure with an amplitude of 0.1 µm and a wavelength of 8 µm with a common scanning white light interferometers (SWLI) results in the correct value for the wavelength but as twice as high for the amplitude. Taking this effect as basis we have build a dispersion optimized white light interferometer. The design corresponds to a Mirau-interferometer, but in order to reduce the disturbing effects for measurement, the sources of dispersion should be avoided. For this purpose no lenses and no plane glass plates were used in this construction. The magnification and image were obtained by a reflective microscope objective. So called pellicles, which are 3 µm thick are polymermembranes with optical layers, were used to adjust the light intensity within the measurement and the reference beam path. Results have shown that in spite of those measures the dispersion of the pellicle has still negative effects on the measurement results. The amplitude of the sinusoidal normal could only be measured correctly by the evaluation of the correct phasing with an unwrapping algorithm. Systematic discrepancies were demonstrated between the results of the envelope evaluation by which the altitude of an object is depending of the coherence length of the used light and the phase evaluation by which the height information is calculated from the phasing of the interference signal. This leads to the conclusion, that even the slight differences of less than 1 µm of the optical path in the material of the measure and reference beam path of a white light interferometer causes errors of the measured altitude.
[1] R. Berger, et al, „Measurement errors of mirrorlike, tilted objects in white light interferometry", SPIE 2007
[2] F. Gao, et al, „Surface measurement errors using commercial scanning white light interferometers", Meas. Sci. Technol. 19 (2008)
[3] P. Lehmann, „Vertical scanning white-light interference microscopy on curved microstructures", Optics Letters /Vol No.35, No. 11 (2010)
[4] P. Lehmann, J. Niehues, „Limitations and Optimization of Low-coherence Interferometry for High Precision Microscopic Form Measurement ", Fringe 2009


Last updated on 2019-25-07 at 17:09