## 3 Results of Field Tests
### The First Stage Experiments
Lastly, Figures 5e and 7e are superimposed to form Figure 7f for comparing normal and abnormal conditions. Figure 7e shows that the area over 50 \({}^{\circ}\)C is about 3.4%, 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 96.6% of the panel is within the normal area, as displayed in Figure 7d-e. This is consistent with Figure 7a-c, showing that the bright area of the hot spot is clearly visible. The third panel has no defected cells but several drops of bird guano can be observed on the central middle of panel. This experiment tested irregular shaped hot spots that are often found in remote area. Figure 8a shows the image of the panel obtained through a visual light camera. It is found that bird droppings could not be easily differentiated from its background by visual inspection because both the bird droppings and cells are of black color. Figure 8b shows the original IR image with a bright color region at the central middle part of the panel which is the location of bird droppings. However, it is not brighter than its surrounding and it could be easily overlooked only by visual inspection of the IR image. Figure 8c shows an image converted to grayscale after Gaussian filtering and it shows a clearer image of bird droppings than that in the original IR image because the grayscale magnified the color difference of image. Figure 8d displays an artificial color image based on the grayscale of each pixel after Gaussian filtering. In Figure 8c,d, the hot spot area is more obvious than that in Figure 8b (as shown in the red circle). Unlike the rectangular shape of hot spot shown in Figure 6c, which is caused by failed cells, the hot spot in Figure 8e is of irregular shape. Figure 8f displays the histogram of the PDF of the temperature values in each pixel and the bump with 45 \({}^{\circ}\)C is panel frames. The mean temperature is 48 \({}^{\circ}\)C with the deviation of \(-\)7 \({}^{\circ}\)C. The maximum temperature is 55 \({}^{\circ}\)C and the minimum temperature is 31 \({}^{\circ}\)C. The cumulative chart of CDF is shown in Figure 8f. The percentage of the hot spot area over 50 \({}^{\circ}\)C can be obtained from the cumulative curve, which is 5.5% in this panel. The above description shows that 94.5% of the panel is within the normal area, as shown in Figure 8e,f. Lastly, Figures 5e and 8f are superimposed to form Figure 8g for comparing normal operation and abnormal conditions. It seems that there are about 3-4 abnormal cells and the hot spot area in Figure 8d is irregular and thus it is concluded that those 3-4 abnormal cells are caused by stains. The innovative finding of this research depicts the following observations. When one cell (about 1.67%) on the solar module exceeds 50 \({}^{\circ}\)C, it means that a hot spot or abnormality occurs in one cell of the module, which may reduce the power generation of the module. The innovative method of this research can find not only hot spots, but also estimate the proportion of hot spots on a solar module. This may avoid the misinterpretation of hot spots caused by some small stains (such as bird droppings, branches, leaves, or dust) as cell failure. If the hot spot proportion is not multiples of 1.67% and the shape of hot spot is irregular, it can be determined that the solar module is contaminated and would only require simple cleaning. In fact, during the operation of the solar module, as long as the hot spots caused by cell failure or stains are distinguished, the correct judgment can be made for cleaning or maintenance. As a result, the service life of solar modules is increased and costs are reduced. Some of the results are presented in the following cases. Figure 7: Image of first stage experiment 2.2 that the solar module has two cells defective. (**a**) IR origin image. (**b**) Gaussian filtered grayscale image. (**c**) Gaussian filtered color image. (**d**) Histogram of thermal temperature Image.