Chapter 14 Using Anti-aliasing Camera Filters for DIC: Does It Make a Difference? PL. Reu Abstract Aliased speckle patterns are a known problem for digital image correlation (DIC). By definition, aliased speckles are smaller than the resolution limit of the camera and add “noise” to images via the spatially-aliased frequency content. Aliased speckles occur quite frequently in practical DIC applications, especially when using spray paint to speckle a surface, where control of the speckle size is difficult at best. This paper compares DIC results from aliased speckle patterns imaged with typical machine cameras with and without physical anti-aliasing filters applied to the camera detectors. Additionally, physical anti-aliasing filters are compared with post-processing, digital low-pass filters of aliased images to quantify the influence of the two types of filters on the quality of DIC results. A key result from this work is that the loss of contrast associated with the addition of physical anti-aliasing filters is generally more detrimental to DIC results than the noise resulting from aliased speckles. Keywords Digital Image Correlation (DIC) • Full-field • Optical methods • Uncertainty quantification (UQ) • Aliasing 14.1 Introduction Digital image correlation (DIC) is a full-field displacement and strain measurement technique that uses a digital image of a “speckle” pattern for tracking the underlying motion [1]. The quality of the results relies on the quality of the acquired images, namely the contrast provided by the speckle pattern. This speckle pattern is often applied using a spray paint can and lightly misting the surface. This can create a nice contrasting surface; however, controlling the speckle size is often difficult, and speckles that are too small often result. Additionally, in attempts to increase the spatial resolution of the measurement, speckles are often created that are at the digital sampling limit of two pixels. In both situations, speckles that are smaller than the minimum of two-pixels occur causing the image to be aliased. There is theoretical work on the influence of aliased and undersized speckles on the interpolant and the corresponding bias errors [2]; however, there has been little or no experimental work on this topic. This brief paper looks at an experimental setup that includes three machine vision cameras with two levels of antialiasing filters and an unfiltered camera to investigate the influence of aliased speckles on the DIC results. 14.2 Experimental Setup The experiment consisted of three PointGrey 5-Megapixel cameras with identical Schneider 35-mm lenses. Camera 1 had no antialiasing filters, Camera 2 had a 2-pixel (2Lambda) birefringent blur filter mounted in front of the detector, and Camera 3 had a 4-pixel (4Lambda) blur filter. These filters were designed for the 3.45- m pixel size of the machine vision camera. To confirm the relative filtering of the antialiasing plates, an Airforce target was used to measure the modulation transfer function (MTF) of the camera and lens system. Figure 14.1 is the MTF of the three systems showing that the resolution of the unfiltered camera is approximately 7.2 line pairs/mm (lp/mm), and the other two are 5.6 and 3.6 lp/mm respectively. The resolution represents the resolving power of the optical system with speckles that are smaller than the resolving limit being filtered out of the image. The cameras were setup to observe a 100-mm field-of-view (FOV) with three different speckle patterns that are observed simultaneously by all three cameras. One pattern was printed with a speckle size of 354 mor approximately 6-pixels across the speckle to be fully resolved by all three cameras. The aliased pattern was printed with PL. Reu ( ) Sandia National Laboratories, 1515 Eubank, Albuquerque, NM, 87123, USA e-mail: plreu@sandia.gov © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_14 89
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