## 3 DIC Method for Deformation Measurement
By changing the technical parameters of the camera or the size of the studied area, these values are adjusted, and the parameters of the experiment are optimized by maximizing the number of efficiently used pixels. Figure 3: General pattern requirements for the DIC method (according to recommendations in [62]). Then, accepting a certain size of dots in the numerical expression \(a\) (pixels), the researcher determines the optimal size of the dots that make up the pattern on the studied physical surface:  
\[d=a\cdot\xi. \tag{3}\]  
The optimal value of \(a\) depends on the specific features of a particular engineering problem and belongs to a wide range of values. Although this parameter is not strictly limited, application of the most optical speckle size will give the best flexibility. In general, when the pattern is formed from speckles that are too large, or if it is too sparse, it is possible that certain subsets may be entirely on a region of black or a region of white. In such case, good correlation will be hard to obtain, as in any part of the region there will be an exact match. It is possible to eliminate this problem by increased size of subsets, which will probably reduce the spatial resolution [62]. However, for the case of a specifically small size of pattern, the resolution of the camera may not be enough for accurate specimen representation. Thus, the pattern indicates the jitter as it interacts with the sensor pixels, rather than smooth moving. Official software guides recommend using dots of at least 5 pixels in size. [62]. The authors in [63] recommend the use of a randomly formed pattern of the same number of dots (10, 20 and 30 pixels). The method of application to the investigated surface significantly influences obtaining the desired pattern. A variety of methods can be  
Figure 4: Schematic diagram of experimental setup with application of the DIC technique. used to achieve a good speckle pattern. Among the most common are stamps, aerosols, printing, lithography, UV photolithography and laser marking [64]. The official software recommendations [62] highlight the following: Correlated Solutions Speckle Kit, spray paint, sharpie, printed patterns, etc. (see Figure 5). Correlated Solutions recommend using a specifically developed Speckle Pattern Application Kit, in which an arrow of stamp rollers enables producing optimal speckle patterns Ink application includes rolling the dots to the studied surface, and the stamp rockers are pressed onto the specimen. It is recommended to first apply white paint on the surface to create the base coat. Dot sizes and their amounts may vary, depending on camera resolution. [62]  
Among the simplest and most common techniques, which can be used for metal, ceramic or composite specimens of intermediate size (within 25 mm and 1.25 m) is spray paint. The surface is prepared by coating in white, after which the spray is streamed. It is possible to create speckles of different scales, either by moving the spray stream across the surface or by holding the specimen above the spray stream. Although this method is rather simple, it has some sufficient limitations. In many cases, it is almost impossible to reach the desired contrast of the pattern, which is critical for strains near the DIC noise floor. In addition, spray paint is not the optimal choice for applications where the specimen would be stiffened or chemically altered [62]. While applying the paint or ink on the surface, the researcher has to thoroughly control the amount of paint used. Thus, a sufficient coat includes thin, light layers. Heavy coats are the possible reason of surface distortion and shape changes. In many cases for membrane and thin structures, it is appropriate to use the ink without an additional base coat. Otherwise, the material will be stiffened, and extra mass will be added, which will cause negative influence on the results. For samples with complex geometry and textures, a pattern can be created with a sharpie marker.