**3. Experimental System**

In-plane displacement measurement experiments of a disc were carried out to verify that the proposed method can improve the accuracy of DIC measurements.

The schematic diagram of the experimental system is shown in Figure 4. The system consists of a camera, a lens, a hot plate, and a metal disc with black spots on the surface.

**Figure 4.** Schematic diagram of the experimental system.

There is one camera to do the 2D DIC measurements. The camera is perpendicular to the surface of the measured object. The camera model is POINT GREY 60S6M (FLIR System, Inc., Portland, OR, USA). The resolution of the camera is 13 fps at 2736 × 2192 pixels. The lens used is a Schneider lens with a focal length of 29.3 mm and a F-number of 2. The measured object is a metal disc, as shown in Figure 5, and the surface is sprayed with the black spots that can be used for DIC measurement. The spots pattern can also be used for the BOS technique. There is a micrometer screw bar on the backside of the disc. The screw bar can rotate the disc to load the in-plane displacement to the disc. The resolution of the rotation is 0.05 degrees. The purpose of the experiment was to measure the

in-plane displacement loaded to the disc accurately. During the experiment, the fluorescent light on the ceiling was the only source of illumination, and the laboratory was closed with no additional airflow interference. A hot plate was placed between the measured object and the camera, closer to the measured object. The top of the hot plate (heated portion) had dimensions of 200 × 100 mm in plane, and the height of the hot plate was 25 mm. The temperature of the hot plate can be accurately controlled by the control box up to 350 ◦C. Heat waves are formed in the air region above the hot plate, and the specimen is imaged through the heat waves. The acquired images were processed using MATLAB (The MathWorks, Inc., Natick, MA, USA). DIC measurement was performed with the open-source 2D DIC software Ncorr [29]. The experimental device diagram is shown in Figure 6.

**Figure 5.** Measured object.

**Figure 6.** Experimental device diagram: (**a**) The camera is facing vertically to the measured object; (**b**) Temperature control box; (**c**) Hot plate in front of the measured object.
