## 4 Where Next? One of the areas where significant research is being conducted is the enhancement of X-ray DIC techniques to allow for high and ultra-high-speed 3D DIC. [PERSON] et al. [63] was able to achieve up to 1.9 million FPS X-ray phast-contrast imaging (XPCI) for the visualization of crack propagation in glass and shock wave propagation in water. From their techniques, a high spatial resolution (effective pixel size of 8 um) and large field of view (12.8 mm \(\times\) 8 mm) was achieved at MFPS frame rates. While this study was not DIC, the techniques the authors describe could be extended to perform 3D-DIC in dynamic deformation events [63]. [PERSON] et al. [64] extended their previous research by examining three methods of obtaining high and ultra-high-speed synchrotron X-ray imaging, shown in a simplified manner in Figure 4. The method the authors examine is using a frame transfer CMOS camera with in-pixel storage, to achieve frame rates above 1 million FPS without reducing the number of pixels in the image frame. The authors demonstrate this method by showing the wave propagation and attenuation in a laser-shocked epoxy foam. The second method is using a high efficiency X-ray image intensifier combined with a CMOS camera. The experimental the authors used to demonstrate this method was to examine the stress (via XPCI and diffraction imaging) in silicon wafers with heat-induced cracks. Due to the limited acceptance of X-rays by crystalline materials, the diffracted signals used in this method are orders of magnitude weaker than the incident polychromatic X-ray beam, leading to the use of an image intensifier. The authors were able to use a conventional CMOS camera to give thousands of frames needed to follow fracture and failure, as well as corresponding strain dynamics. The third method the authors describe is the use of high resolution sCMOS camera with a gate-able visible light image intensifier. This method is a more sensitive form of XPCI and is achieved by using X-ray optics that directly measure beam refraction in grating interferometry and tracking near-field speckles introduced by randomly structured material. This allowed the authors to obtain an effective pixel size of 1.625 um with a time resolution of 70.4 us [64]. This third method the authors describe is similar to DIC and hence could be adapted to provide high and ultra-high-speed 3D DIC, with very small spatial resolution. Further work has been done with X-ray imagining at the European Synchrotron Radiation Facility by [PERSON] et al. [65] The authors were able to achieve an effective frame rate of 5.68 million FPS, by using three Shimadzu Hyper Vision HPV-X2 cameras (Shimadzu, Kyoto, Japan). The authors converted the X-rays to photons, detectable by the cameras with in-line scintillators. The high frame rate was achieved by using the cameras to capture sequential sets of X-ray photons, allowing for all of the pulses created by the synchrotron to be captured. The images from these sets were then stitched together to give a continuous set of images with an interframe time of 176 ns. Their system was able to capture a pixel size of 32 \(\upmu\)m and given the main features of the object were significantly larger than the characteristic length, the edge contrast in the recorded images was enhanced, while preserving the attenuation projection of the target. The experiment the authors used to test the method was the impact of a gas gun projectile into an additively manufactured aluminium lattice [65]. Other recent work by [PERSON] and [PERSON] [66] has looked into using a UV 3D DIC system to measure the surface of an ablating body in an arc-jet facility. The authors used two UV cameras to record images of the self-luminescent surface of an ablating body, shown in Figure 5. There are two key challenges to performing 3D DIC in high temperature environments, imaging and speckling.