Optical surface metrology with spatially and temporally partially coherent light wave fields (OPaL 2)

• Applicants: Professor Dr. Ralf Bernhard Bergmann; Dr.-Ing. Gerd Ehret
• Subject Area: Measurement Systems
• Term: from 2014 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 258565427

Optical metrology of free form surfaces is a mandatory prerequisite for the process of quality assurance in the optical industry. Cost efficient and precise form measurement of aspheric surfaces is still a challenging and unsolved task. Therefore, within the first funding period, the limits and opportunities offered by shear interferometry towards aspheric and free form testing have been investigated. The great benefit of shear interferometry is that it allows for using multiple independent light sources as illumination at the same time. This enables the investigation of large specimen with steep slopes despite the limited acceptance angle of the interferometers imaging system. Hence, the shearographic approach combines a large illumination aperture, which is typical to geometric optical methods, such as deflectometry, with the high precision offered by interferometry. An important result of the first funding period is an entirely novel approach to measure free form surfaces based on the acquisition of the mutual coherence, which was coined Multi Aperture Shear Interferometry (MArS). All experiments necessary to validate the scientific and technical foundation of MArS were conducted with transparent objects being transmitted by the illuminating light. However, for industrial applications, measurements in reflection direction are mandatory. During the research on the approach of MArS it became clear that the measurement in reflection configuration is far more challenging than initially anticipated. A part of the illumination has to come from the direction of the imaging aperture of the interferometer, which means that light sources have to be positioned within the imaging path. Additionally, the positions of the light sources, which are essential to the evaluation process, cannot be determined by a simple calibration measurement without specimen. Finally, if the specimen is made from glass, two reflections arise, one from the front side and one from the reverse side. As a consequence, the scientific and technical problems let the development of a reflection mode setup within the frame of the first funding period appear unrealistic. Therefore, the aim of the second funding period is to further develop the MArS approach towards a configuration in reflection mode, by tackling the above challenges. The current results at hand indicate that MArS has the potential to initiate a paradigm shift in optical metrology of aspheric lenses. The objective now must be to raise this potential.

• DFG Programme: Research Grants