# Full-Field Vibration Measurements by Using High-Speed Two-Dimensional Digital Image Correlation
## 3 Experiment and Result  
### In-Plane and Out-of-Plane Translation Tests  
To validate the effectiveness of characterizing full-field vibrations by using the proposed HS-2D-DIC, in-plane and out-of-plane translation tests were firstly carried out. As shown in Figure 3, a flat plate, marked by a random speckle pattern, was fixed on a two-dimensional linear translation stage as the specimen. In front of the plate, a pair of blue light sources (center wavelength of 465 nm) were used for illumination, and a camera (Revelater M220, Fuhuang AgileDevice Ltd., Hefei, China, maximum frame rate of 2000 _fps_ at a resolution of 1920 \(\times\) 1080 pixels) equipped by an imaging lens (Sigma Art series DG HSM(A014), Sigma Co., Ltd., Kanagawa, Japan, aperture F/1.4 and focal length of 50 mm) and a band-pass filter was used to capture the speckle images. It is noted that the speckle pattern is important for DIC measurements; i.e., a speckle pattern with the appropriate speckle size (3\(\sim\)5 pixels per speckle), high contrast, and random distribution are required for a high-resolution and precise DIC measurements [31]. Therefore, in this experiment, the speckle pattern was numerically generated based on the Gaussian simulation algorithm [32]. It features a speckle size of 3-6 pixels and 10% speckle density. In addition, the combination of the blue monochromatic light source and band-pass filter used by the imaging system is used to eliminate the influence of ambient light [33]. The distance between the camera and the plate was set as 500 mm. Before the measurement, the imaging lens was carefully adjusted to make the camera capable of capturing high-contrast speckle images. In the translation tests, the plate was moved from 0 mm to 5 mm with 0.5 mm intervals, controlled by the two-dimensional translation stage, and a speckle image was captured at each position using the camera for the 2D-DIC calculation. Since the tested sample/region may not have a regular shape, a specific region of interest (ROI) has to be specified before applying a DIC calculation. During the DIC calculation, the subset size and grid step, which will affect the spatial resolution, measurement accuracy, and computation time, have to be selected. In general, a large subset size and small grid step can result in a high resolution and good accuracy, but low computational efficiency. In this experiment, a rectangular region of 600 \(\times\) 600 pixels was selected as the region of interest (ROI). A subset size of 51 \(\times\) 51 pixels and a grid step of 5 were chosen as the DIC calculation parameters. The measured in-plane and out-of-plane displacements are shown in Figure 4. From the results, it is observed that the measured _U_-displacement for the in-plane translation test and _W_-displacement for the out-of-plane translation test are in good agreement with the prescribed ones. The error fluctuations are caused by imperfect translations and the limited measurement precision. The absolute values of the mean errors are less than  
Figure 3: Experimental arrangement of in-plane and out-of-plane translation tests. 0.1 mm, validating the effectiveness of the proposed HS-2D-DIC method for measuring both in-plane and out-of-plane displacements. From Figure 4b, it can be seen that the standard deviation increased with the out-of-plane displacement. This could be due to the relatively large out-of-plane motion of the specimen induced by the out-of-focus camera. Therefore, if the magnitude of the out-of-plane displacement is larger than the DOF, the defocusing results in the appearance of measurement errors. This issue can be addressed in the following two ways are provided: (1) increasing the speckle size--the relatively large out-of-plane displacement can turn to be out of focus, but increasing the speckle size ensures that the speckle can still be tracked when defocusing.