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11 November 2010 Determination of the optimal marker positions for optical extensometer considering lens distortion
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Besides the requirement on high contrast and low noise, the more important aspect to affect the accuracy of the optical extensometer is the pixel resolution of the digital image. The resolution of the marker positioning algorithms is often evaluated as pixel, and the higher pixel resolution will induce a higher strain measurement resolution in optical extensometer. Since the algorithm error is always seen as a constant, the strain error is decreased with the distance of the measured region increases when using optical extensometer. On the other hand, the lens distortion will affect the accuracy of the marker positioning algorithms. If the distance of the measured region increases, the lens distortion error will also increase for that the lens distortion is much evident at the image margin. Therefore, when applying optical extensometer, a balance should be found between the algorithm error and the high distortion of the marker image. In order to obtain the best measurement accuracy, the marker positions for optical extensometer should be optimized. An optimization model for choosing best marker position for optical extensometer is constructed on the considering of the pixel resolution and second order lens distortion. It can be concluded that the extensometer using markers located at the position optimized from the optimization model would give the best measurement results, and the law that the best distance between the two markers will change with the size of the strain if the distortion parameters of the camera is given has been found.
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Zilong Zhao, Xian Wang, Jiazhi Pang, and Shaopeng Ma "Determination of the optimal marker positions for optical extensometer considering lens distortion", Proc. SPIE 7855, Optical Metrology and Inspection for Industrial Applications, 785508 (11 November 2010);


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