## 3 Results  
### Investigation of the Influence of Calibration Parameters on the Results of Strinin/Stress Analysis  
The calibration process of the camera(s) of the Dantec Dynamics correlation systems has to be carried out for each measurement separately (either before or after the measurement), while the Istra4D V.4.3.0 program includes a tool providing information about the current accuracy of the obtained calibration parameters. This tool is the so-called calibration residuum, indicating the average inaccuracy of the search for the so-called nodal points of the calibration target. The value of the correlation residuum, together with the cameras' intrinsic and extrinsic parameters, are displayed in the calibration module of Istra4D from the target's fourth (and every higher) registered position. These parameters are gradually updated depending on the number of different calibration target positions captured during the calibration of the cameras. The manufacturer claims that eight frames are sufficient for accurate calibration and that the residuum value should not exceed 0.2-0.5 px. Even a small residuum value does not guarantee that the calibration was carried out correctly. However, too large a value (>1.0 px) indicates that some error has occurred in the camera calibration process [46]. Since the standard calibration process runs relatively quickly (takes about 5 min), the user who changes the positions of the calibration target in the field of view of the cameras cannot continuously check the obtained calibration parameters and can only follow the recommendation of the manufacturer of the correlation devices Dantec Dynamics to monitor the value of the calibration residuum. For that reason, this parameter was also registered with each saved calibration file. Figure 7 shows the changes in the calibration residua determined by Istra4D during the three calibration processes. The residuum is the lowest (ca. 0.35-0.45 px) and approximately the same for both calibration targets supplied by Dantec Dynamics (1.5 mm and 3.0 mm) for the entire calibration process. In the case of the 3.0 mm target, a constant trend can be observed from the 10 th recorded target position. The calibration residuum is significantly higher in the case of the target manufactured with less precision (2.0 mm). Its value (ca. 0.7 px) exceeds the level of 0.2-0.5 px, which Dantec Dynamics set as the level characterising a correctly performed calibration. Based on the information presented, the group of calibration targets used is interesting mainly in the following aspects. We have two high-precision calibration targets (1.5 mm and 3.0 mm), where the 3.0 mm target size corresponds approximately to the size of the evaluated part of the specimen (recommended calibration target size), and 1.5 mm is relatively small with respect to the cameras' field of view in both, calibration and correlation mode. The size of the 2.0 mm target is more suitable than the 1.5 mm target. On the other hand, the accuracy of its manufacture is lower (as confirmed by the obtained data on calibration residua). The tensile loading measurements described above were performed in correlation mode at an image resolution of 1800 \(\times\) 1800 px (pixel density approx. 56 px/mm). The  
Figure 7: Plot of calibration residua obtained when calibrating the correlation system using three different calibration targets. reference images with the evaluation area marked are shown in Figure 8. For the evaluation, the facet size was set to 20 \(\times\) 20 px with an overlap of 5 px. As the correlation of the images is carried out across facets, it is necessary to smooth the acquired fields for each measurement using the Dantec Dynamics correlation systems. Otherwise, the obtained result is affected by single noise values. Istra4D V4.3.0 offers two smoothing options. The first one is the ACSP filter for local regression, and the second option is the global Smoothing Spline filter [47].