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A phase retrieval inspired approach for the determination of optical aberrations in microscopy

23.10.2020

Accurate bidirectional measurements are performed at PTB using UV-microscopy. For this purpose, edge detection algorithms based on the rigorous calculation of the optical imaging problem are employed. Hence, all the parameters of the imaging system and the object must be considered due to their specific impact on theimage formation. In order to further improve the model-based edge detection the small but inevitable optical aberrations of the imaging optics must be included into the simulations.For the determination of the aberrations, microscopic images of state-of-the-art line gratings (shapes verified by AFM) are measured in the focus and several afocal planes of the microscope. This procedure is similar to a phase retrieval method with the only difference that  usually  idealized  pinholes  are  measured  there.  A  particle  swarm  optimisation  (PSO)  method  is  developed  to  determine  the aberration of the used microscope in terms of Zernike polynomials. Therefore, the particles of the PSO are evenly spread over the parameter space that has been expanded by the Zernike polynomials. The optimisation problem consists of the minimization of the difference between the microscopic images in different focal planes and their respective simulated microscopic images. Because the illumination, imaging, and object parameters of the measured and the simulated microscopic images are identical, only a change of the Zernike polynomialsis responsible for a minimization during an iteration of the PSO. First results show, that when convergence is  achieved,  the  difference  between  the  images  is  very  small.  The  particle  responsible  for  the  minimum  then  holds  the  Zernikepolynomials, which characterize the aberrations of the used microscope.

(J. Krüger, FB 4.2, jan.krueger@ptb.de)

https://www.euspen.eu/knowledge-base/ICE20184.pdf