## 3 Results of Field Tests
### The Second Stage Experiments
The outdoor temperature is 28 \({}^{\circ}\)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 50-53 \({}^{\circ}\)C with irradiance of 776 W/m\({}^{2}\). Two panels were inspected with IR images in this case. Figure 13a shows the original IR image of the first panel with a clear and bright area in the top middle that corresponds to a defected cell while the upper middle is the junction box as mentioned before. The dark purple lines around the cells are frames of the panel. Figure 13b shows an image converted to grayscale after Gaussian filtering but the bright region still appears clearly. Figure 13c shows a 3-D color image after Gaussian filtering to tell the temperature difference that the hot spot in the top middle is at about 53 \({}^{\circ}\)C. Panel frames are at the temperature of 43 \({}^{\circ}\)C while the rest of the panel is at around 45-49 \({}^{\circ}\)C. Figure 13d shows a histogram of the temperature distribution. The PDF curve still looks like a beta distribution function. The mean temperature is 42 \({}^{\circ}\)C with the deviation of \(-\)11 \({}^{\circ}\)C. The maximum temperature is 53 \({}^{\circ}\)C and the minimum temperature is 30 \({}^{\circ}\)C. Figure 13e displays the cumulative temperature distribution that the percentage of hot spot area over 50 \({}^{\circ}\)C is 1.6%, indicating there is only one cell exceeding the temperature threshold. Finally, Figures 5e and 13e are superimposed in Figure 13f for comparing normal and abnormal conditions. It is found that the area over 50 \({}^{\circ}\)C is about 1.6%, which is close to the 1.5% threshold presumed for a normal solar module, indicating that there is no abnormality or failure. The above description shows that 98.5% of the panel is within the  
normal area, as shown in Figure 13d,e. This is consistent with Figure 13a-c, which shows that bright area of hot spot is clearly visible. The second panel at the same location has a wide bright area in the upper part as shown in Figure 14a,b and it is a visible light image, not an IR image. This bright area is caused by the direct reflection of sunlight. Figure 14d shows the IR image of the same panel taken at the same time as the visual light image. The wide bright area of reflection disappears but a clear and bright zone with a smaller area still exists in the right middle that corresponds to defected cells. Figure 14e shows an image converted to grayscale after Gaussian filtering and the bright zone still appears clearly. Figure 14f shows a histogram of the temperature distribution. The PDF curve still looks like a beta distribution function and the huge bump with 41 \({}^{\circ}\)C is caused by panel frames. The mean temperature is 43 \({}^{\circ}\)C with the deviation of \(-\)10 \({}^{\circ}\)C. The maximum temperature is 53 \({}^{\circ}\)C and the minimum temperature is 30 \({}^{\circ}\)C. Figure 14g displays the cumulative temperature distribution that the percentage of hot spot area over 50 \({}^{\circ}\)C is 3%, indicating there is one cell with high temperature. Finally, Figures 5e and 14g are superimposed in Figure 14h for comparing normal and abnormal conditions. Figure 14g shows that the area over 50 \({}^{\circ}\)C is about 3%, which is 1.5% higher than threshold presumed for a normal solar module, indicating that there is one cell abnormality or failure. The above description shows that 97% of the panel is within the normal area, as shown in Figure 14f,g. Figure 12: Image of second stage experiment 2 that the solar module has cell defective. (**a**) IR origin image. (**b**) Gaussian filtered grayscale image. (**c**) Gaussian filtered color image. (**d**) Histogram of thermal temperature image. (**e**) Cumulative chart of thermal temperature image. (**f**) Cumulative chart-comparison of normal operation and abnormal conditions of solar modules. However, this is a normal operating panel and there are no defected cells occurred.