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Cross Polarization for Improved Digital Image Correlation

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Abstract

Digital image correlation (DIC) is a surface deformation measurement technique for which accuracy and precision are sensitive to image quality. This work presents cross polarization, the use of orthogonal linear polarizers on light source(s) and camera(s), as an effective method for improving optical DIC measurements. The benefits of cross polarization are characterized through quantitative and statistical comparisons from two experiments: rigid body translation of a flat sample and uniaxial tension of a superelastic shape-memory alloy (SMA). In both experiments, cross polarization eliminated saturated pixels that degrade DIC measurements, and increased image contrast, which enabled higher spatial precision by using smaller subsets. Subset sizes are usually optimized for correlation confidence interval (typically with subsets of 21×21 px or larger), but can be decreased to achieve the highest possible spatial precision at the expense of increased correlation confidence intervals. Smaller subset sizes (such as 9×9 px) require better images to maintain correlation within error thresholds. By comparing DIC results from a uniaxial SMA tension test with unpolarized and cross-polarized images, we show that for 9×9 px subsets, the loss of valid DIC data points was reduced almost ten-fold with cross polarization. The only disadvantage we see to cross polarization is the decrease in specimen illumination due to transmission losses through the polarizers, which can easily be accommodated with sufficiently intense light sources. With the installation of relatively inexpensive linear polarizing filters, an optimum optical DIC setup can provide even better DIC measurements by delivering images without saturated pixels and with higher contrast for increased DIC spatial precision.

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Notes

  1. We have chosen “3-D DIC” to describe our DIC measurements with three dimensions of displacement. While 3-D DIC does provide all three displacements, it is still a surface measurement that only provides in-plane gradients of displacement (thus, four components of the deformation gradients, or equivalently three strain components and one rotation). It should not be confused with volumetric DIC (DVC), which can provide the full 3-D deformation gradient (nine components, or six strain components and three rotations).

  2. The terminology is unfortunate in this context. In the optics community, “specular” reflections are defined as those reflections that maintain their polarity. This should not be confused with the “speckle” DIC pattern, which is defined as a random (non-periodic), black-white pattern of dots.

  3. Of course, cross polarization could adversely affect an under-illuminated setup, but that is easily avoided.

  4. Blooming occurs in imaging when fringes extend beyond a bright feature into a dark feature.

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Acknowledgments

We gratefully acknowledge financial support from the National Science Foundation (CAREER Award, CMMI-1251891) and the Department of Defense (NDSEG Fellowship).

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Authors and Affiliations

  1. Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA

    William Scott LePage

  2. Departments of Mechanical Engineering/Materials Science, Engineering, University of Michigan, Ann Arbor, MI, 48109, USA

    Samantha Hayes Daly

  3. Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI, 48109, USA

    John Andrew Shaw

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  1. William Scott LePage
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  2. Samantha Hayes Daly
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  3. John Andrew Shaw
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Correspondence to John Andrew Shaw.

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LePage, W.S., Daly, S.H. & Shaw, J.A. Cross Polarization for Improved Digital Image Correlation. Exp Mech 56, 969–985 (2016). https://doi.org/10.1007/s11340-016-0129-2

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