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DFG project: Optical surface metrology with spatially and temporally partially coherent light wave fields (OPAL)

Within the framework of a joint project between the Bremer Institut für angewandte Strahltechnik (BIAS) and the Physikalisch-Technische Bundesanstalt (PTB), new procedures to determine the form of optical surfaces were investigated and developed. The activities were funded by the Deutsche Forschungsgemeinschaft (DFG).

The project focused on developing a shearing interferometer with multiple light sources to measure aspherical and freeform surfaces. The measurement principle for transmission is shown in Fig. 1. Using a spatial light modulator (SLM) as a shearing element provides great advantages: Nearly equal paths of propagation of the interference fields place very low requirements on the coherence of the light, so that it is possible to use LEDs. This enables, in particular, the simultaneous illumination of the measuring area with several LEDs without disturbing interferences occurring in the overlapping areas. Due to the individual positioning of the light-emitting diodes, it is possible to acquire data for areas which are difficult to measure (such as surfaces which exhibit asymmetries or large differences in the edge steepness) without having to use special lens systems or computer-generated holograms (CGH). The known advantages of shearing interferometry, which comprise its compact design and high robustness against vibrations, remain unchanged.

Figure 1: Measurement principle of partially coherent shearing interferometry with multiple light sources

A model of the measurement system based on ray tracing can be used not only to estimate the measurement accuracy, but also represents an elementary part of the validation procedure.

Within the scope of the project, it was possible to show that multiple light sources (Fig. 2a), consisting of several partially coherent light sources, yield a coherent interferogram (see Fig. 2b) when the entire measuring area is illuminated at the same time. In addition, form measurements in transmission were performed [Hag20].

Figure 2: a) Realization of 7-fold multiple light sources (with the specimen in the foreground), b) recorded shearing interferograms. Due to the partial coherence of the individual light sources, the different light sources cannot interfere with each other: The individual interferograms are perfectly complementary and can be merged into a complete global interferogram.

Since industry increasingly demands measurements in reflection, the measurement procedure was extended to cover measurements in reflection during the second phase of the project [Muel20]. Transferring this procedure to industry is now planned.


DFG project website: https://gepris.dfg.de/gepris/projekt/258565427?language=en

Project partner: www.bias.de

Selected publications

[Muel20] A. Müller, C. Falldorf, M. Lotzgeselle, G. Ehret, R. B. Bergmann: “Multiple Aperture Shear-Interferometry (MArS):a solution to the aperture problem for the formmeasurement of aspheric surfaces,” Optics Express 28 (23) (2020) pp. 34677-34690,  doi.org/10.1364/OE.408979

[Hag20] J. Hagemann: “Die wechselseitige Kohärenzfunktion teilkohärenter optischer Lichtwellenfelder und ihre Anwendung zur Formmessung”, Dissertation an der Universität Bremen (2020), doi.org/10.26092/elib/283

[Hag17] J.-H. Hagemann, C. Falldorf, G. Ehret, R. B. Bergmann: „Error influences of the shear element in interferometry for the form characterization of optics,“ Proc. SPIE 10329, Optical Measurement Systems for Industrial Inspection X, 103291T (2017); doi.org/10.1117/12.2269932

[Fal17] C. Falldorf, J.-H. Hagemann, G. Ehret, R. B. Bergmann: Sparse light fields in coherent optical metrology, Applied Optics 56 (13), pp. F14-F19, 2017, doi.org/10.1364/AO.56.000F14

[Hag16] J.-H. Hagemann, G. Ehret, R. B. Bergmann, C. Falldorf: “Realization of a shearing interferometer with LED multipoint illumination for form characterization of optics,” DGaO Proceedings (2016). www.dgao-proceedings.de

[Fal14] C. Falldorf, A. Simic, G. Ehret, M. Schulz, C. v. Kopylow, R. B. Bergmann: “Precise optical metrology using computational shear interferometry and an LCD monitor as light sourc.,” Fringe 2013: 7th International Workshop on Advanced Optical Imaging and Metrology: 729 -734, (2014). doi.org/10.1007/978-3-642-36359-7_134


Dr.-Ing. Gerd Ehret

Phone: +49 531 592-4220
E-Mail: gerd.ehret(at)ptb.de