## 3 Results
### Investigation of the Influence of Image Brightness on the Results of Strain/Stress Analysis  
All of the reference images acquired during the 12 measurements mentioned in Section 2.2 can be seen in Figure 12. All these images were exported to MATLAB R2020a (MathWorks, Natick, MA, USA), where they were processed to evaluate their mean grey values (MGV). The obtained information about MGV characterising the image brightness is written in the lower right part of the captured images shown in Figure 12. A similar procedure to the calibration influence analysis was established, i.e., the mean relative differences in the evaluated equivalent von Mises stresses were determined to assess the influence of the changing image brightness. As a reference for comparing the results, a field of equivalent von Mises stress was chosen, obtained from the images of the seventh measurement, with a determined MGV = 122, which approximately corresponds to the middle intensity of the evaluated images. The analysis of the differences in the fields mentioned above due to the change of the image brightness was carried out using all the calibration files obtained during the calibration process of the correlation system cameras with 1.5 mm, 2.0 mm, and 3.0 mm targets, resulting in the 3D plots shown in Figure 13.  
Figure 11: Influence of the estimated calibration parameters on the evaluated equivalent von Mises stresses: (**a**) \(F\) = 400 N, (**b**) \(F\) = 800 N, (**c**) \(F\) = 1200 N, (**d**) \(F\) = 1600 N.  
Figure 12: Images of the analysed specimen acquired under changing exposure time of the lenses subjected to analysis of the effect of image brightness on the strain/stress analysis results. Note: MGV is indicated in the lower right corner of each image. Figure 13: _Cont._  
Based on the analysis of the 3D plots shown in Figure 13, it can be concluded that there is a relatively wide range of mean gray values (approximately from 56 to 171) without significant differences in the evaluated equivalent von Mises stresses. The values of the relative differences in the compared fields of the equivalent von Mises stress for images acquired within the range of mean gray values from 56 to 171 are given in Table 3. The change occurs when the image is too bright (overexposed) with a mean gray value greater than 171. There is also an increase in the deviation for images with a low mean gray value (less than 56). Moreover, the evaluation becomes problematic when the images are too dark (less than 22) or too bright (more than 194) because there are problems with finding the so-called starting points, i.e., the places where the algorithm starts to compare the images. The deviations due to the use of different calibration sets (varying CTP) reach approximately the same levels as for the measurement described above (up to a maximum of 3% within all measurements). However, the levels of \(\Delta\sigma_{\mathrm{mean}}^{\mathrm{Mises}}\) observed for the 12 measurements performed at varying image brightness were significantly larger than those obtained when comparing results from a single measurement. The following analysis was performed to reveal the reason causing the higher differences in equivalent von Mises stresses. For each of the four loading levels, i.e., for \(F=400\) N up to \(F=1600\) N, two measurements were performed, during which 30 images of the loaded specimen were captured under the same measurement conditions (MGV = 122). From the evaluated data, the mean relative measurement error was then determined according to the relation:  
\[ME_{\sigma}^{\mathrm{[PERSON]}}=\mathrm{mean}\left(\left|\frac{\sigma_{\mathrm{i }}^{\mathrm{[PERSON]}}(a,b)-\sigma_{\mathrm{mean}}^{\mathrm{[PERSON]}}(a,b)}{ \sigma_{\mathrm{mean}}^{\mathrm{[PERSON]}}(a,b)}\right|\right)\cdot 100\% \tag{7}\]  
where \(i\) = 1, 2, 3,..., 30 represents the image number, \(\sigma_{\mathrm{mean}}^{\mathrm{Mises}}(a,b)\) is an average value of the equivalent von Mises stress evaluated in the facet centre with coordinates \((a,b)\) calculated from all the 30 images.