Mr. Xavier Schwab Profile

Xavier Schwab

at SICPA SA

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Proceedings Article | 26 April 2008 Paper

Improved micro topography measurement by LCoS-based fringe projection and z-stitching

X. Schwab, C. Kohler, K. Körner, N. Eichhorn, W. Osten

Proceedings Volume 6995, 69950Q (2008) https://doi.org/10.1117/12.781822

KEYWORDS: Microscopes, Liquid crystal on silicon, CCD cameras, Zoom lenses, Reflectivity, LCDs, Cameras, Light emitting diodes, Calibration, Sensors

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Fringe projection is a commonly used method for 3D surface metrology. Numerous applications have demonstrated a measurement field from a few millimeters to several meters. To enable the measurement of micro systems with this method, a zoom stereo microscope from Leica was used as the basis for the implementation of a fringe projection microscope. A state of the art twisted nematic WUXGA LCD was used for flexible fringe generation. The high fill factor of this reflective LCoS in combination with a 500 Lumen LED and a 12 bit CCD camera delivers fringe patterns with high contrast. This allows us to measure objects with both a strong reflectivity variation and a low reflectivity. The second main objective was to increase the measurement field and the depth of field. Using the zoom system and exchangeable microscope objectives, the measurement fields could be changed quickly from 4 cm² to less than 1 mm². Depending on the measurement field, the depth of field was between 5.22 mm and 0.018 mm. However, this was often not sufficient to measure the complete depth of a 3D-object. The microscope system also features an integrated high precision motor stage, which is already used for system calibration. Based on this, we implemented a new z-stitching method where n measurements at different well determined z-positions of the motor stage were performed. The n resulting topography maps can be stitched together to get the complete depth map of the entire object. Thus the depth measurement range is only limited by the mechanics of the z-stage.

Proceedings Article | 15 September 2006 Paper

Alignment of master and sample in comparative digital holography

Xavier Schwab, Eugen Meister, Giancarlo Pedrini, Wolfgang Osten

Proceedings Volume 6341, 63410Y (2006) https://doi.org/10.1117/12.695339

KEYWORDS: Holograms, Digital holography, Wavefronts, Fringe analysis, Spatial light modulators, LCDs, Holography, Databases, 3D image reconstruction, Error analysis

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A comparative digital holography system suitable for shape and deformation comparisons between master and sample objects with rough surfaces is described. The innovative aspect of comparative digital holography is the illumination of the sample by the conjugated wavefront of the master, as a type of coherent mask, using a liquid crystal display (LCD). The resulting interferogram indicates directly the shape or the deformation differences between the master and sample. As it is not necessary that both objects to be compared are located at the same place for this technique, remote shape or deformation comparison between a master and a sample is possible. A current research topic is the precise alignment of the sample and the reconstructed master wavefront so that the resulting phase map only contains information of the differences in shape or deformation. The reconstructed master wavefront can be adjusted digitally to correctly illuminate the sample object, by introducing an artificial phase-shift. This phase-shift is induced by the LCD, and offers also the possibility of calibrating precisely the set-up. The value for the phase-shift is obtained by a comparison of the resulting interferogram with a database containing fringes from simulations of misalignments between master and sample objects. Using the iterative algorithm described here, the correction of the sample position can be controlled by an automatic adaptation of the coherent mask.

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