Distortion correction of a microscopy lens system for deformation measurements based on speckle pattern and grating

Abstract

Lens distortion will cause errors in the measurement of deformation by digital image correlation when using a microscope at high magnification for image recording. To improve the measurement accuracy of 2D digital image correlation, lens distortion was detected by rigid-body translation tests for a micro speckle pattern using DIC calculation and by phase analysis of a cross-grating using a Fourier transform. Based on the first-order radial distortion model, the coefficients for the lens distortion were determined using the least-squares method. Then, the displacement fields without lens distortion were obtained, and accurate measurements were achieved even for images that were deformed by lens distortion. The results demonstrate that the average standard deviation of the displacement fields decreases by 60% for lens distortion correction by rigid-body translation tests of a micro speckle pattern using DIC calculation, and they decreased by 97% for phase analysis of a cross-grating using a Fourier transform. The experimental results verify that digital image correlation is a powerful tool for micro-scale deformation measurement using a microscope at high magnification as long as one correct for lens distortion.