Asphere and freeform metrology based on shear-interferometrical non-null-test (AspharSchero)

• Applicant: Professor Dr. Ralf Bernhard Bergmann
• Subject Area: Measurement Systems
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 562022481

For aspherical lens testing, interferometric zero tests have long been established as a reliable technique for inspection and quality control. These tests play a critical role in ensuring the precision and performance of aspherical lenses, which are widely used in various industries such as optics, aerospace and medicine. However, the current state of the art in testing aspherical lenses faces significant obstacles in terms of flexible measurement of different test specimens, mainly related to the adaptability and cost of implementing the required computer-generated holograms (CGH). CGH are essential for testing aspheric lenses because they enable measurement of aspheric surfaces by ensuring that incident light is reflected perpendicularly, resulting in resolvable fringe patterns in the interferogram. One of the biggest challenges is the complexity and cost of manufacturing CGH, which makes them costly to integrate into test systems. In addition, alignment of the CGH and specimen requires extreme precision, which requires significant time and effort. These conditions result in an urgent need for a flexible, and accurate sensor that can be effortlessly and fast adapted to lenses with different degrees of asphericity. The goal of this project proposal is to introduce an approach for aspherical non-zero testing using computational shear interferometry (CoSI). CoSI is a method for reconstructing the amplitude and phase of arbitrarily shaped wavefields by superimposing laterally displaced wavefields on each other. The method thus allows, for example, deformation or phase contrast measurements as well as three-dimensional shape detection with a measurement uncertainty in the nm-range using shear interferometry. With CoSI, the fringe density can be adjusted by the amount of te lateral shear and is not only determined by the difference between object and reference wave, making it a promising alternative for testing aspherical lenses. By using CoSI in a non-zero test setup, a large angular aperture and thus high flexibility is combined with a highly accurate, fast and unambiguous measurement. In classic non-zero tests, retrace errors caused by different optical paths of the reference and object waves are a significant source of error. In the shear interferometric non-zero test, although the reference and object waves are not significantly separated from each other, different optical paths through the measurement system for varying test specimen shapes are a similar source of error. The investigation of these shear interferometric retrace errors and their calibration is a major part of the proposed project.

• DFG Programme: Research Grants