![]() Similar approaches have been demonstrated in the past for static microscopy applications, but not for cross-correlation based flow measurements. 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. The less sharp a particle image is, the broader is the cross-correlation peak and the larger the uncertainty in determining the spatial shift between two frames (see also Fig. The quality of the particle images is essential for low systematic uncertainties. By evaluating the position shift of the cross-correlation (CC) peak between two consecutive frames, the particle movement and therefore the flow velocity can be determined. Usually a light-sheet illumination is generated, producing a homogenous light plane which is the measurement area from where the seeding particles scatter light to the detector (CCD camera). Seeding particles that follow the flow are added to the fluid. Particle imaging velocimetry is an image-based measurement technique that is used for fluid flow measurements. The latter one leads to a model based sensor-less correction which often uses Zernike modes. Either single elements of the actuator can be addressed or models for the expected optical distortion can be assumed. Iterative methods are expected to be time-consuming due to the large parameter space. Usually an iterative approach is chosen, which does not require an additional sensor beside the imaging camera. However, image based wide-field correction requires a CCD-camera and a more complex optimization strategy. There are also sensor-less concepts in terms of FFT-based aberration correction of low spatial frequencies and by using the second moment of the image Fourier transform. the intensity is maximized and the spot shape is improved (e.g. Wavefront sensor-less adaptive optics is commonly used with a pinhole and a single photodetector allowing single light spot correction i.e. a transmission measurement is not performable, then a sensor based closed-loop setup cannot be applied, except a Fresnel guide star approach is applicable. If the optical access for a wavefront sensor is not available, i.e. for removing turbulence effects or distortion correction based on affine transformation algorithms. Beside active in-situ correction of disturbed wavefronts, also post-processing approaches can be applied to improve the image quality, e.g. Beside interferometric methods and holography-based modal wavefront sensing, the Hartmann-Shack wavefront sensor is a widely spread tool. Several methods exist to determine the wavefront of a light wave. The drawback of intrinsic aberration effects in a measurement system or from a measurement object can be compensated by applying spatial light modulators. Aberrations can also influence the point spread function (PSF) for high-resolution confocal microscopy. In microscopy for biomedical applications, aberrations mainly result from different cell layers. Another field of application is ophthalmology in order to get sharp retina-images within the human eye or effectively perform vision correction. ![]() Adaptive optics is used in astronomy for compensating atmospheric turbulence. Our 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.Īdaptive optics has developed to an established method in many research fields of applied optics. The proposed method allows for the reduction of systematic measurement uncertainties in particle image velocimetry. In this work we outline a sharpness metric based aberration correction with a deformable mirror, applied for the first time to particle image velocimetry. 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. MethodsĪ combination of sharpness metric image evaluation and iterative optimization is demonstrated. The usage of a wavefront sensor can be hindered by disturbing light reflexes or scattering. Such distortions can occur at fluctuating phase boundaries during flow measurement and result from the accompanied refractive index changes. Optical distortions can significantly deteriorate the measurement accuracy in particle image velocimetry systems. ![]()
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