Three-dimensional shape measurement of transparent objects by pattern transmission photogrammetry

• Applicant: Dr.-Ing. Marcus Petz
• Subject Area: Measurement Systems
• Term: from 2008 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 106026994

The objective of the first project phase has been the development of a deflectometric measurement system for the geometric measurement of transparent objects in transmission. This type of measurement strategy offers significant advantages compared to established measurement techniques which separately measure the front and rear surface of such a specimen in reflection. The geometric relation of both surfaces and the effect of refraction are intrinsically contained in the measurement data so that it is possible to reconstruct the complete geometry and the index of refraction simultaneously.During the first project phase a deflectometric measurement system, optimized for accuracy and resolution, has been set up. Instead of widely-used consumer-grade computer monitors, high-end radiologic grayscale displays have been used for this setup. For the measurement system different kinds of reconstruction algorithms have been implemented. A pointwise evaluation of correspondences between incident and refracted rays did not yet yield satisfying results because the deviations of the measured surface can become up to 1 mm. However, with a model of the specimen based on Zernike-polynomials it has been possible to implement a global reconstruction algorithm and to simultaneously measure the geometry and the index of refraction for an aspheric ophthalmic lens blank. The deviation of the form-defining parameters was less than 2%, corresponding to a maximum deviation of the measured surface of 30 µm.Based on the previous findings the potential for further development of the technique must be rated as high. In the current state, however, the specifications of the system are not competitive compared to established measuring systems. In particular the systematic deviations and the measurement time are an order of magnitude to high. The objective of the proposed continuation of the project therefore is a significant improvement of the specifications by means of a combination of enhanced algorithms and novel measurement strategies. Besides an analysis and minimization of systematic deviations in the global reconstruction, specifically the pointwise reconstruction has to be developed further, because it features a considerable potential with respect to the flexibility of the measuring technique. It is to be expected, that especially the possibility of a direct measurement of both geometry and index of refraction opens various applications. An automated 100%-testing of complex formed transparent objects would be feasible as soon as the specifications have been significantly improved.

• DFG Programme: Research Grants