## 4 Results and Discussion
### CDF Comparison  
Figure 13 shows the histogram equalization of both sample images. It can be seen that the grayscale was normalized. Linear normalization by getting the mean was used in this study. As a result, the thermal image provides a more linear CDF, and its range was extended to occupy the entire spectrum of levels. Figure 14 shows a portion of three different solar cell images under three different conditions. All three images were simulated to determine the difference in their histogram domain. Figure 12: Histogram representation of the processed sample images. Figure 13: The CDF representation of the processed sample solar cell images was acquired both from the ordinary and thermal cameras. As shown in Figure 15, the three histogram results of the image from the ordinary camera show a non-bimodal scenario compared with the thermal camera on the panel. The histogram of the non-defective image (thermal image--panel 1) indicates both the histogram equalization and AHE exhibit a better-distributed pixel intensity value than the defective thermal image (panel 2). ### CDF Comparison  
Lastly, as shown in Figure 16, the CDF comparison indicates that the thermal imaging technique provided a more linear CDF compared to the sample image taken from the ordinary camera. Moreover, the CDF under the grayscale of both defective solar panels is not similar to the non-defective sample. Figure 14: (**a**) Solar cell in ordinary image, from panel 1; (**b**) non-defective thermal solar cell image, from panel 1 (Sample 1); (**c**) thermal solar cell with series crack, from panel 2 (Sample 2). Figure 15: Histogram comparison on the three different images in three different conditions. Moreover, just to validate the proposed work, electroluminescence (EL) imaging technique was applied on the solar panel testbed to identify the current health status of the solar panels used in the experimental set-up. Electroluminescence is based on the same concept as light-emitting diode (LED). Normally, a current is fed using a large diode into a solar cell, and the radiative recombination of carriers causes the light emission. As an indirect bandgap semiconductor, most recombination in silicon happens through defects or Auger recombination [51]. While electroluminescence (EL) imaging is a measurement technique used to analyze solar cells and solar modules [52] that requires electrical connections, this means it is only applicable to solar cells and modules in operation [22]. The main advantage of EL imaging in PV applications is detecting micro-cracks and cracks on crystalline silicon solar cells that are invisible to the naked eye. The EL image of the testbed is shown in Figure 17. Moreover, the EL image profile of the solar panels used in the experiment is shown in Figure 18. Figure 19 shows the location of the cell defects on panel 2 for both the thermal and electroluminescence images  
Figure 16: The CDF comparison on the three images under three conditions. Figure 20 shows the different simulated images with the non-defective solar cells with the defective solar cell images. It demonstrates that the irregularities in the results on all defective images, both grayscale, histogram equalization, and adaptive histogram equalization modes are not relatively linear compared to the non-defective solar cell image. Figure 17: The electroluminescence (EL) of the testbed. Figure 19: The equivalent location of cell defects (panel 2) both on the thermal and electroluminescence images. Figure 18: (**a**) The EL image profile of panel 1 in a good condition; (**b**) EL image profile of panel 2 which has a series crack condition. Figure 13:0 Simulated CDF comparison on different thermal solar cell condition with non-defective (panel 1) and defective solar cells (panel 2). It shows that the CDF of the sample in the simulated experiments can be validated using the EL images in determining the health status of the solar panel/cell. Although EL is good at detecting defects, it is very costly to set up, which is mostly used in the laboratory.