Chapter 9 Perspective Compensation of 2D-DIC Measurements by Combination with Speckle Imaging Juuso Heikkinen and Gary S. Schajer Abstract Two-dimensional Digital Image Correlation (2D-DIC) is a popular method for determining object in-plane motion and deformation by tracing surface texture. However, the applicability of the method is limited by perspective errors that may be generated by object surface curvature, out-of-plane motions and non-perpendicular camera alignment. The resulting variations in camera magnification lead to errors in the measured surface motion signals. Speckle imaging is a closely related DIC method where the motions of a laser-illuminated surface are tracked by analyzing the scattered interference speckle pattern. When the camera is in focus, the speckles move at the same rate as the surface. However, if the camera is defocused, the speckle motion rate deviates from the actual surface motion. Speckle imaging measurement sensitivity can thus be controlled by varying the amount of defocus. Interestingly, speckle imaging sensitivity increases with near-focus, in contrast to sensitivity reduction in 2D-DIC caused by perspective change. Therefore, if the surface motions are analyzed using both DIC and speckle imaging, the resulting sensitivity difference can be utilized to compensate any perspective errors present. The amount of defocus must be moderated in order to achieve a balance between sensitivity and sufficient spatial resolution for full-field measurements. Therefore, it is important to study how speckle imaging behavior changes as a function of defocus. Here, the speckle imaging characteristics are experimentally investigated by recording speckle patterns at varying defocus distances and with different lens aperture sizes (f-numbers). The resulting speckle patterns are characterized by computing the average speckle diameters. The results reveal how at small defocus distances, speckle size is linearly dependent on lens f-number and inversely proportional to defocus distance. At large defocus distances, on the other hand, speckle size is independent on f-number and linearly proportional to defocus distance. Keywords DIC · Perspective · Speckle imaging · Defocus · Autocorrelation 9.1 Introduction Two-dimensional Digital Image Correlation (2D-DIC) is a simple and effective method for measuring object in-plane movements and deformations by tracking surface texture [1]. However, because 2D measurements contain no depth information, significant limitations are imposed on the measurable objects and motions. First, the specimen surface needs to be flat and aligned perpendicular to the camera. In addition, the object must remain at a fixed, uniform distance from the camera. These conditions enable the in-plane measurement sensitivity (magnification) to remain constant throughout the imaged area. However, in practice, measurement specimens often have non-flat surfaces and also move out-of-plane. The resulting variations in imaging distance cause corresponding magnification variations in the measured images, thus creating artifacts in the in-plane motion signals. Such perspective-induced effects could be minimized by using telecentric lenses, but such lenses are typically bulky and costly, especially for fields-of-view (FOV) larger than a few centimeters. Speckle imaging is a variant DIC method where the laser speckles on the illuminated surface are used to track surface motions in place of painted speckles [2]. When the camera is focused at the surface, the imaged speckles move as if they were fixed to the surface. However, if the camera focal plane is shifted away from the specimen surface so that the camera becomes defocused, the movement of the observed speckles differs from the actual surface motion. At distances far away from the object, the observed speckle motions become larger with increasing defocus distance, contrary to the reduced magnification J. Heikkinen · G. S. Schajer ( ) Department of Mechanical Engineering, University of British Columbia, Vancouver, BC, Canada e-mail: schajer@mech.ubc.ca © The Society for Experimental Mechanics, Inc. 2021 M.-T. Lin et al. (eds.), Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59773-3_9 73
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