## 1 The Technique and Origin of Digital Image Correlation (DIC)
The material the authors used for the X-ray speckle pattern was a thermally sprayed Tantalum powder with an aluminium mask to generate the speckle pattern. The authors used a thin adhesive layer to put the same speckle pattern for the optical cameras over the X-ray speckle pattern. While the two X-ray cameras (Varex 2520 DX, Varex Imaging, Salt Lake City, Utah, USA) and the two optical cameras (PointGrey Grasshopper, FLIR Systems, Inc., Wilsonville, Oregon, USA) only ran at about five frames per second, the principles of X-ray speckle generation and the advantages shown are valid for higher frame rate X-ray photography. Both the optical and X-ray stereo systems were calibrated independently using the VIC-3D software (Correlated Solutions, Inc., Irmo, SC, USA). The authors conducted two experiments to compare the two systems, (i) pure translation of the speckle plate, and (ii) a stationary target, but with a hot plate in front of the target to cause optical distortions. For the first test, their results show that both the optical and X-ray systems were in good agreement, for both the in-plane and out-of-plane translations, and had similar noise floors. The results for the second experiment that the authors conducted showed significant  
Figure 2: Time sequence, the arrows with * show which images are cross-correlated to find the 3D displacement field. Reproduced from [PERSON] and [PERSON] [6], with permission from Elsevier. displacements with the optical system, even though the sample was not moving. In contrast, the X-ray system recorded only the random noise floor. The errors in the optical system recording displacement through heat distortions were 5-18 higher than the error of the X-ray system. The optical system also measured errors of 2-3 times higher in the strain measurements. This shows that X-ray 3D DIC could be a valuable tool in future to measure displacement and strain in regions, that are either not accessible via optical means, or are optically obscured [9]. One issue that can become pertinent in the use of 3D DIC is the resolving of different scales of phenomenon. Given the finite resolution of high and ultra-high-speed cameras, resolving the strain field for small scale localized deformation events can be at odds with determining the overall global strain field of the object being examined. An example of this is [PERSON] and [PERSON] [10], where the authors examine the evolution of microbuckling in an UHMWPE laminate beam, using 2D DIC and the ARAMIS DIC software (GOM GmbH, Braunschweig, Germany), with a supported length of 152 mm. In comparison, the scale of the microbuckling is in the order of 600 \(\upmu\)m [10]. [PERSON] and [PERSON] [10] used a 100 \(\upmu\)m thickness adhesive transfer tape and copper microparticles to generate a speckle pattern with a mean dot size of 15-20 \(\upmu\)m and an inter-dot spacing of less than 10 \(\upmu\)m. Applying a speckle pattern on a scale that can measure this microbuckling, but also being able to determine global strains can be challenging, especially when examined in three dimensions. Resolving these different scales of phenomenon requires the careful choice of field of view, an increased depth of field and a reliable speckle pattern that will still represent the underlying material deformation during large shear.