## 3 Results and Discussion
### Analysis of Fire Severity Classification Accuracy for Different Indices  
As shown in Figure 11, the classification accuracy of the three VIs derived from the post-fire image is generally highest in areas where spectral separability was most pronounced, as analyzed earlier. WF1 for DI category, using the combination of the near-infrared band at 757 nm and the visible band at 611 nm, reached its peak value of 79.26% (Table 1). The regions with higher classification accuracy for dVIs from thepre- and post-fire image combination were largely consistent with those from the post-fire image, both focusing on areas with higher spectral separability. Compared to original VIs, the classification accuracy of dDI showed a significant decline, while dRI accuracy slightly decreased. However, the classification accuracy of dNDI improved, with the highest value of WF1 (83.39%) achieved for the combination of shortwave infrared band 2048 nm and near-infrared band 1106 nm (Table 1). The classification accuracy for distinguishing between unburned and burned areas was markedly higher, while accuracy varied for different fire severities. High severities showed the highest classification accuracy, followed by moderate severities, with light severities showing the lowest accuracy. This variation in classification accuracy was a key factor contributing to the differences in overall index performance. In conclusion, the dNDI category, derived from the pre- and post-fire image combination, offered the highest classification accuracy, making it the most effective method for fire severity classification. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Ranking**} & \multicolumn{2}{c}{**DI Category**} & \multicolumn{2}{c}{**RI Category**} & \multicolumn{2}{c}{**NDI Category**} \\ \cline{2-7}  & **Combination/nm** & **WF1/\%** & **Combination/nm** & **WF1/\%** & **Combination/nm** & **WF1/\%** \\ \hline
1 & 757–611 & 79.26 & 2216–1728 & 75.08 & 2216–1728 & 74.90 \\
2 & 705–645 & 79.11 & 2216–1577 & 74.97 & 2216–1712 & 74.71 \\
3 & 705–654 & 79.04 & 2216–1543 & 74.76 & 2216–1577 & 74.70 \\
4 & 714–637 & 78.94 & 2216–1560 & 74.68 & 2216–1594 & 74.69 \\
5 & 731–619 & 78.92 & 2216–1712 & 74.65 & 2216–1745 & 74.60 \\
6 & 731–594 & 78.78 & 2216–1627 & 74.55 & 1998–1577 & 74.59 \\
7 & 757–559 & 78.74 & 2031–1510 & 74.45 & 2216–1678 & 74.54 \\
8 & 775–611 & 78.66 & 2031–1678 & 74.39 & 2216–1560 & 74.47 \\
9 & 722–645 & 78.65 & 2199–1543 & 74.39 & 2031–1577 & 74.44 \\
10 & 783–619 & 78.65 & 2199–1661 & 74.35 & 2199–1543 & 74.44 \\ \hline \end{tabular}
\end{table} 
Table 1: The TOP 10 classification accuracies of different VIs for post-fire image. Figure 10: The accuracy of spectral and spectral difference classification for different fire severities. (**a**) Post-fire image; (**b**) Difference of the Pre- and post-fire image. Figure 11: The classification accuracy of different VIs in the post-fire image and different dVis in the pre- and post-fire image combination. (**a**) DI category; (**b**) RI category; (**c**) NDI category; (**d**) dDI category; (**e**) dRI category; (**f**) dNDI category. The TOP 10 WF1 for different VIs based on the post-fire image are shown in Table 1. Among the DI category, the highest accuracy was achieved with the combination of the near-infrared band at 757 nm and the red band at 611 nm, with an accuracy of 79.26%. Among the RI category, the highest accuracy was achieved with the combination of the shortwave infrared bands at 2216 nm and 1728 nm, with an accuracy of 75.08%. Among the NDI category, the highest accuracy was achieved with the combination of the shortwave infrared bands at 2216 nm and 1728 nm, with an accuracy of 74.90%. A comparison of the post-fire image results revealed that the DI category provided the best classification accuracy, while the RI and NDI category showed similar but relatively lower accuracy. The optimal classification accuracy for VIs was 79.26%.