## 4 Analysis and Discussion
### Polycrystalline Silicon Solar Panel  
#### 4.2.1 Bias Voltage  
At the emission angle of 40\({}^{\circ}\), the forward bias voltage increases from 9.5 V to 15 V with the step of 0.5 V. Image acquisition is carried out at the same time of each voltage change. The images I and DOP of the polycrystalline photovoltaic panel were obtained as shown in Figures 8 and 9, respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Image** & **IE** & **AG** & **SD** & **SF** & **EI** \\ \hline I (0\({}^{\circ}\)) & 5.2738 & 2.5139 & 22.3112 & 5.1122 & 17.2192 \\ I (60\({}^{\circ}\)) & 5.2769 & 2.5012 & 22.3881 & 5.0838 & 17.1187 \\ I (120\({}^{\circ}\)) & 5.2752 & 2.5131 & 22.3823 & 5.1059 & 17.1947 \\ I & 5.7001 & 3.4125 & 31.1978 & 6.8641 & 23.1996 \\ DOP & 4.2414 & 14.3442 & 35.2736 & 31.2222 & 80.5407 \\ AOP & 3.299 & 71.2911 & 72.914 & 134.6397 & 292.2719 \\ \hline \end{tabular}
\end{table} 
Table 1: Objective evaluation index of polycrystalline silicon solar panel. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Image** & **IE** & **AG** & **SD** & **SF** & **EI** \\ \hline I (0\({}^{\circ}\)) & 5.7211 & 2.8818 & 17.4242 & 5.8161 & 22.1181 \\ I (60\({}^{\circ}\)) & 5.7366 & 2.8868 & 17.3715 & 5.8409 & 22.1573 \\ I (120\({}^{\circ}\)) & 5.7335 & 2.8862 & 17.4171 & 5.8285 & 22.1839 \\ I & 6.34 & 4.4121 & 27.0704 & 8.8893 & 33.7119 \\ DOP & 5.1754 & 12.2685 & 30.811 & 27.7195 & 64.3701 \\ AOP & 3.7124 & 65.1589 & 68.8897 & 118.9183 & 259.8149 \\ \hline \end{tabular}
\end{table} 
Table 2: Objective evaluation index of monocrystalline silicon solar panel. Figure 6: SWIR Polarization Image of polycrystalline silicon solar panel. Figure 7: SWIR polarization image of monocrystalline silicon solar panel. It can be seen from Figure 8 that when the bias voltage is below 10 V, the panel is not conductive and the electroluminescence intensity is very small and can almost be ignored; therefore, there is no luminous area shown in the image. With the increase in bias voltage, the intensity of electroluminescence increases gradually. When the bias voltage reaches 11.5 V, the contrast of the image is significantly enhanced, and the difference between the defect and normal area becomes obvious. With a further increase in bias voltage, an overexposure area appears in the image when the bias voltage exceeds 12.5 V. At this moment, it is difficult to distinguish the defect area from other areas. If the bias voltage continued to increase, the solar panel may be damaged heavily. In general, with increases  
Figure 8: Images I of polycrystalline silicon photovoltaic panel with different bias voltages. Figure 9: Images DOP of polycrystalline silicon photovoltaic panel with different bias voltages. in bias voltage, the electroluminescence intensity is continuously enhanced; this is basically consistent with the mathematical model given in Formula (3). From DOP image in Figure 9, when the bias voltage is less than 10 V, the degree of polarization is almost equal to 0, as is the intensity. With the increase in bias voltage, DOP gradually increases and reaches a peak value at about 12 V. This is mainly due to the increase in minority carriers involved in radiation recombination; the difference between the parallel and vertical components of the outgoing light increases until the degree of polarization reaches a maximum value. When the bias voltage exceeds 12 V, the DOP value decreases rapidly with the bias voltage. This is mainly because the number of injected carriers increases, and the numbers of photons participating in radiation recombination in the vertical and horizontal directions approaches equal, which makes the parallel and vertical components of outgoing light almost equal. Additionally, the DOP value decreased, meaning a depolarization effect begins to appear. In short, there exist many overexposed areas in the electroluminescent intensity image if the bias voltage is too large, and these areas cannot appear in DOP images.