## 3 Results of Field Tests
### The Second Stage Experiments  
In the second stage of experiments, modules with unknown conditions were used to test the findings of the first stage. A total of six modules were tested in this stage. Experiments were conducted at different locations in the verification field. In each case, one to three images were selected for verification. Verification locations and results are illustrated in the following cases:  
In the first case, the power generation efficiency is low but no obvious damage was found by visual inspection. The location of these panels is on the GPS coordinates of 24\({}^{\circ}\)12'31\" N and 120\({}^{\circ}\)29'39\" E with an altitude of 5 m. The nominal output of this solar farm is 6486 kWp. The experiment was carried out on 1 April 2020, at 12 a.m. The outdoor temperature is 29 \({}^{\circ}\)C. The surface temperature of the normal operating solar module is 45-49 \({}^{\circ}\)C, and the surface temperature of the abnormal solar module is 50-56 \({}^{\circ}\)C with irradiance of 827 W/m\({}^{2}\). Three panels were inspected in this paper following the procedure mentioned above. The first panel has three hot spots with the brightest spot at the bottom left, followed by the bottom right and the top middle. Figure 9a shows the original IR image with a clear and bright area in the bottom left that corresponds to the defected cell while a less bright area located in the upper middle is the junction box as mentioned before. In addition, a defected cell in the lower right of the panel can also be observed in the IR image with purple lines on the left and right margins as frames of the panel. Figure 9b shows an image converted to grayscale after Gaussian filtering. Grid lines of the panel are blurred due to the filtering effect but the bright region still appears clearly. Figure 9c shows a color image after Gaussian filtering to differentiate the temperature variations that the hot spot is at  
Figure 8: Image of first stage experiment 2.3 that there are defects caused by bird guano on the solar cell module. (**a**) Visible light image. (**b**) IR origin image. (**c**) Gaussian filtered grayscale image. (**d**) Gaussian filtered color image. (**e**) Histogram of thermal temperature image. (**f**) Cumulative chart of thermal temperature image. (**g**) Cumulative chart-comparison of normal operation and abnormal conditions of solar modules. about 56 \({}^{\circ}\)C. Panel frames are at the temperature of 40 \({}^{\circ}\)C and the rest of the panel is at around 40\(\sim\)49 \({}^{\circ}\)C. Figure 9d displays a histogram of the temperature distribution. The PDF curve still looks like a beta distribution function. However, a bump can be found at 38 \({}^{\circ}\)C which was not shown in Figure 5d of a normal operating panel because panel frames were not included in Figure 5. The mean temperature is 45 \({}^{\circ}\)C with the standard deviation of \(-\)11 \({}^{\circ}\)C. The maximum temperature is 56 \({}^{\circ}\)C and the minimum temperature is 30 \({}^{\circ}\)C. Figure 9e displays the cumulative temperature distribution that the percentage of hot spot area over 50 \({}^{\circ}\)C is 3.3%, indicating there are more than two cells with high temperature. Finally, Figures 5e and 9e are superimposed in Figure 9f for comparing normal and abnormal conditions. Figure 9e shows that the area over 50 \({}^{\circ}\)C is about 3.3%, which is higher than the 1.5% threshold presumed for a normal solar module, indicating that more than one cell has abnormality or failure. The above description shows that 96.7% of the panel is within the normal area, as shown in Figure 9d,e. This is consistent with Figure 9a-c, showing that bright area of hot spot is clearly visible. The original IR image of the second panel at the same location is shown in Figure 10a with a clear and bright area in the bottom right that corresponds to defected cells while a less bright area located in the upper middle is the junction box as mentioned before.