Abstract
BackgroundOptical distortions can significantly deteriorate the measurement accuracy in particle image velocimetry systems. Such distortions can occur at fluctuating phase boundaries during flow measurement and result from the accompanied refractive index changes. The usage of a wavefront sensor can be hindered by disturbing light reflexes or scattering.MethodsA combination of sharpness metric image evaluation and iterative optimization is demonstrated. The sharpness metric is used as an indicator for wavefront aberrations in order to correct low-order Zernike modes that influence the image quality of particle image velocimetry.ResultsIn this work we outline a sharpness metric based aberration correction with a deformable mirror, applied for the first time to particle image velocimetry. The proposed method allows for the reduction of systematic measurement uncertainties in particle image velocimetry. ConclusionOur approach offers a new way to reduce static or slowly changing wavefront distortions in a fluid flow measurement setup in which a wavefront sensor is not applicable.
Highlights
Optical distortions can significantly deteriorate the measurement accuracy in particle image velocimetry systems
In the present paper we show that it is possible to effectively reduce measurement uncertainties in particle image velocimetry (PIV) by using a sharpness metric and a systematic linear search algorithm which finds the optimal solution within several seconds by cycling through the deformable mirror modes
Note that the metrics possess different widths of decline, which mainly determines the number of necessary amplitude values that have to be set in order to find the extremum
Summary
Optical distortions can significantly deteriorate the measurement accuracy in particle image velocimetry systems. Such distortions can occur at fluctuating phase boundaries during flow measurement and result from the accompanied refractive index changes. The usage of a wavefront sensor can be hindered by disturbing light reflexes or scattering. Adaptive optics is used in astronomy [1] for compensating atmospheric turbulence. Another field of application is ophthalmology [2] in order to get sharp retina-images within the human eye or effectively perform vision correction. The drawback of intrinsic aberration effects in a measurement system or from a measurement object can be compensated by applying spatial light modulators. Beside active in-situ correction of disturbed wavefronts, post-
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