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Experimental Mechanics

, Volume 56, Issue 6, pp 969–985 | Cite as

Cross Polarization for Improved Digital Image Correlation

  • William Scott LePage
  • Samantha Hayes Daly
  • John Andrew Shaw
Article

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.

Keywords

Digital image correlation DIC Cross polarization Optical imaging 

Notes

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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Copyright information

© Society for Experimental Mechanics 2016

Authors and Affiliations

  • William Scott LePage
    • 1
  • Samantha Hayes Daly
    • 2
  • John Andrew Shaw
    • 3
  1. 1.Department of Mechanical EngineeringUniversity of MichiganAnn ArborUSA
  2. 2.Departments of Mechanical Engineering/Materials Science, EngineeringUniversity of MichiganAnn ArborUSA
  3. 3.Department of Aerospace EngineeringUniversity of MichiganAnn ArborUSA

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