## 3 Results and Analysis of Radiometric Thermal Data
### Basic Calibration When Using Several Different Cameras
The instruments were divided into two groups depending on the angle of the optics, paying attention to possible sources of reflected radiation and the influence of the operator. Figure 21 shows a blackbody Voltcnraft IRS-350 with a set temperature value of 50 \({}^{\circ}\)C (green diodes) and a current value of 44.9 \({}^{\circ}\)C (red diodes). The active area where the camera calibration is performed is shown in red in the thermogram. The calibration process is performed when the blackbody enters a steady state. For this reason, we decided to perform the calibration in seven values, from 27 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, with a step of 3 \({}^{\circ}\)C, which is the first larger amount compared to the specified measurement uncertainty of the camera. The specified interval covers the values for early detection of hotspots, where accuracy and precision are important. Higher temperature values quickly become noticeable, and the detection algorithm had no problems with them. Using the raw data, Figure 22 shows the detailed temperature distribution on the active surface of another blackbody whose surface is not flat, unlike the Voltcnraft. The image shows a maximum value of 50.774 \({}^{\circ}\)C, a minimum value of 50.006 \({}^{\circ}\)C, and an average value of 50.41068 \({}^{\circ}\)C, which was indicated on the display with a value of 50.4 \({}^{\circ}\)C.  
In Figure 22, a circular area can be seen with slight deviations in temperature, which were not real or physically possible. This is a result of the surface geometry, which, due to the different emissivity of individual circle areas, resulted in different emissivities and different amounts of radiation while the blackbody had a steady temperature-value camera presenting slight differences in temperature values. We can conclude that the camera does not measure temperature but registers infrared radiation, based on which the temperature values are assigned in accordance with the calibration data. In addition to the changes in geometry, a change in a surface emissivity is also possible and occurs in practice (degradation in paint due to the sunlight UV exposure or partial surface oxidation). In order to take into account all influences on the measurement uncertainty of the results, the camera displays temperature values with one decimal place. During the measurement, it  
Figure 21: Voltcnraft IRS-350 in operation. Figure 22: Temperature distribution on the active blackbody surface. was necessary for the spatial resolution IFOV (Instantaneous Field of View), i.e., the viewing angle of a single pixel of the sensor, be completely within the range of the reference surface. Figure 23 shows the maximum distance the camera could be from the blackbody for the measurement to be accurate. The maximum distance of the camera was compared with the field of view (FOV) that the camera would have at the specified distance, depending on its resolution. In the measurement, we tried to fill the frame as much as possible with the active element of the blackbody to make it easier to determine the mean, as shown in Figure 23. To determine the mean value, it was not enough to read 10 different readings from the camera, and it was necessary to change the camera for each measurement so that the fluctuations of the temperature of the blackbodies were reduced to a minimum when determining the mean value. Table 5 shows the mean values of the measurements made according to the indicated procedure.