Phase measuring deflectometry with active display registration

• Applicant: Professor Dr. Ralf Bernhard Bergmann
• Subject Area: Measurement Systems
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444018140

Highly accurate three-dimensional shape assessment is a fundamental requirement for precise manufacturing and quality control. Phase measuring deflectometry (PMD) provides fast, full-field and contact-free shape assessment of specular surfaces by tracing the path of light from a display via the reflection at the surface under test to a camera. Typically, computer displays are applied for this task, enabling the determination of reference points of reflected camera vision rays by spatial coding using the phase shifting technique.Phase shifting provides two-dimensional localization on the display surface. Therefore, the assessment of three-dimensional spatial coordinates requires the shape of the display to be known. However, the shape is not constant but affected by physical stress such as gravity depending on its orientation and particularly by thermal expansion. This behavior requires a procedure for simultaneous shape determination of the display with each PMD measurement. Such an approach is provided by the stereoscopic fringe projection technique. Here, images captured by two cameras from a surface specially coded by structured illumination are used for identification of transient points, providing the correlated coordinates by determination of intersections of corresponding vision rays.The proposed project aims at the improvement of accuracy and flexibility of PMD by introducing an additional stereoscopic camera system into the setup directly observing the display. Since the display in PMD already provides structured illumination, the fringe projection principle is applicable without a projector. The stereo system provides independent information of the display for improved shape determination of the surface under test with PMD. It additionally enables precise determination of the pose of the display. Therefore, storing pose and shape of the display as system calibration data is no longer required. Instead, the stereo camera system provides absolute coordinates related to the phase data of the PMD measurement. The display can be positioned freely, only confined by the condition that fields of view of all cameras sufficiently overlap on the display surface. Most PMD systems lack the ability to access the full surface of curved samples, since system geometries with all camera vision rays intersecting the display cannot be realized. The proposed PMD setup provides variable positioning of the display and improves access to the surface under test by enabling combined evaluation of multiple phase measurements for varying display poses. Using multiple display poses furthermore allows to solve the ambiguity of deflectometric measurements by providing multiple reference points for reflected vision rays. Finally, the proposed approach avoids systematic errors by providing independent display shape information. Assuming a constant display shape for a calibrated system is not required any more.

• DFG Programme: Research Grants