Efficient, simultaneous vision ray calibration and system orientation for high precision geometric-optical 3D-measurement systems

• Applicant: Professor Dr. Ralf Bernhard Bergmann
• Subject Area: Measurement Systems
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 289307220

Imaging optical metrology allows high-density, highly accurate, fast and non-contact acquisition of 3D data for quality control. The majority of the methods used are characterised by light rays propagating in straight lines and can be summarised under the term geometric-optical measurement technique. Since the image captured by the camera in the measuring system image is two-dimensional, no direct point-to-point mapping to 3D coordinates is possible; instead, a half-line, the so-called vision or sight ray, emanates from every pixel. Usually these assignments are determined by a camera calibration based on the so-called conventional pinhole camera model. It is assumed that the sight rays from all pixels run through a common point, the so-called pinhole aperture. Real lens systems, however, cannot be adequately characterised by this model. A more recent approach describes the sight rays for each pixel individually. Thus the calibration provides an individual 3D-half-line for each pixel, which offers the highest possible accuracy but is computationally very complex. Also, the smallest possible number of reference recordings is not known, and the possibility for theoretical predictions is curtailed by the quasi-model-free approach of the vision-ray calibration technique. Still, the most common method of camera calibration for metrological purposes is based on an extended pinhole camera model. For low-uncertainty geometric-optical measuring instruments, this approach is unsatisfactory because certain distortion and lens errors cannot be captured and corrected in this way.The aim of the proposed project is the realisation of a new method for the simultaneous implementation of vision-ray camera calibration and multi-camera measurement system orientation by developing new, numerically efficient and geometrically optimal calibration algorithms and strategies that will greatly simplify the calibration of multi-camera 3D coordinate measurement systems based on fringe projection. The project is divided into three sub-objectives: 1. Creation of a complete multi-camera simulation model of sight-ray calibration to investigate various model components, to test numerical optimization methods used and to explore multi-camera calibration methods. 2. Derivation of a specification for the camera line of sight calibration describing the minimal set of reference display positions that are necessary to maintain the uncertainty achieved with currently used methods. The measurement time shal be reduced from several hours to a maximum of 30 min. 3. Development of a calibration method for multi-camera set-ups that significantly reduces the number of recording steps by simultaneous calibration and orientation. Here the measurement effort shall be reduced to 45 min for a two-camera system and to 60 min in a system with four cameras.

• DFG Programme: Research Grants