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Proceedings Paper

Affine structured light sensor for measurements through inspection windows: basic concept and direct calibration approach
Author(s): Rüdiger Beermann; Lorenz Quentin; Markus Kästner; Eduard Reithmeier
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Paper Abstract

The realization of 3D triangulation measurements in inhomogeneous media is challenging, as the sensor light path is not necessarily rectilinear anymore and the triangulation principle is violated. An exemplary measurement scenario under inhomogeneous optical conditions is the geometry characterization through an inspection window. The discrete refractive index variation from air to inspection window and back to air (or even water) can require complicated light path modeling approaches in order to triangulate 3D surface data correctly. As commonly used entocentric lenses “fan out” the projected light rays, the rays’ incidence angles onto the refractive index interface are not constant, and the rays are individually deflected. In consequence, the typically used camera pinhole model does not apply anymore, or can only approximate the actual light path under refraction. In this paper, we present a structured light sensor concept for measurements through inspection windows, which does not require a special adaption of the light path model. The key is the application of a telecentric stereo camera pair and high-quality optical inspection windows. It can be shown theoretically, that no additional parameters are necessary to model light refraction induced by a plane-parallel plate – such as a window –, when used in combination with a telecentric lens. Next to the affine stereo camera pair, the sensor comprises an entocentric projector unit as feature generator. In previous work, the projector was calibrated with refraction model in order to provide a 3D point cloud basis for the affine camera calibration.1 In the new approach, the projector is merely used as feature generator to solve the correspondence problem between the affine stereo camera pair. Besides the developed sensor hardware concept, we present an overview on the calibration strategy based on an affine self-calibration approach, the solving of the correspondence problem between the cameras, and first calibration and measurement results. In forthcoming work, the sensor is also meant to be used in three-media refraction scenarios.

Paper Details

Date Published: 1 April 2020
PDF: 11 pages
Proc. SPIE 11352, Optics and Photonics for Advanced Dimensional Metrology, 113520E (1 April 2020); doi: 10.1117/12.2556081
Show Author Affiliations
Rüdiger Beermann, Institute of Measurement and Automatic Control (Germany)
Lorenz Quentin, Institute of Measurement and Automatic Control (Germany)
Markus Kästner, Institute of Measurement and Automatic Control (Germany)
Eduard Reithmeier, Institute of Measurement and Automatic Control (Germany)

Published in SPIE Proceedings Vol. 11352:
Optics and Photonics for Advanced Dimensional Metrology
Peter J. de Groot; Richard K. Leach; Pascal Picart, Editor(s)

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