Three-dimensional digital image correlation (DIC) has found widespread popularity for strain measurements due to its excellent accuracy, robustness and ease of use. However, 3D measurements have been difficult to obtain on specimens where high magnification is required. This is mainly due to the lack of optics with sufficient depth-of-field to acquire two high magnification images from different viewing angles.
Stereo microscopes overcome these depth-of-field limitations. However, the internal construction of stereo microscopes prevents proper correction of image distortions using traditional models, such as Seidel lens distortions. These uncorrected images will result in severely biased shape and strain measurements. In fact, it is not uncommon to observe bias levels of several thousand microstrain.
To overcome this problem, Correlated Solutions, Inc., has developed an easy-to-use calibration method that does not suffer from the problems associated with traditional parametric distortion models. The calibration method computes the non-parametric distortion fields of the stereo microscope and has been shown to completely eliminate shape and strain bias from the measurements.