TY - GEN

T1 - Affine structured light sensor for measurements through inspection windows

T2 - Optics and Photonics for Advanced Dimensional Metrology

AU - Beermann, Rüdiger

AU - Quentin, Lorenz

AU - Kästner, Markus

AU - Reithmeier, Eduard

N1 - Funding Information: The authors would like to thank the German Research Foundation (DFG) for the financial and organisational support of this project, as the presented results were obtained within the Collaborative Research Centre 1153 Process Chain for Manufacturing Hybrid High Performance Components by Tailored Forming.

PY - 2020/4/1

Y1 - 2020/4/1

N2 - 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.

AB - 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.

KW - 3D measurement

KW - Affine camera

KW - Fringe projection

KW - Inhomogeneous optical conditions

KW - Refractive index

KW - Snell’s law

KW - Stereo camera pair

KW - Telecentric camera

UR - http://www.scopus.com/inward/record.url?scp=85120997403&partnerID=8YFLogxK

U2 - 10.1117/12.2556081

DO - 10.1117/12.2556081

M3 - Conference contribution

T3 - Proceedings of SPIE - The International Society for Optical Engineering

BT - Optics and Photonics for Advanced Dimensional Metrology

A2 - de Groot, Peter J.

A2 - Leach, Richard K.

A2 - Picart, Pascal

Y2 - 6 April 2020 through 10 April 2020

ER -