## 4 Experimental Results and Discussion
### Evaluation of the Objective Color Correction Network
#### 4.2.1 Robustness on Different Lighting Conditions  
Our capture devices provide different lighting conditions by adjusting the brightness. There are five conditions, including 1328 lux, 1415 lux, 1968 lux, 2226 lux, and 2424 lux. They are denoted as ILL1 to ILL5 (ILluminance Level). The top row of the Figure 6 shows examples of test samples under different lighting conditions. After color correction, the visual perception of color patches will be closer, as shown in the bottom row of Figure 6. Figure 7 shows the comparison results of tongue images captured under five different illuminations before and after correction. To enhance the color difference of the tongue, segmentation results are shown. Subjectively, our method allows the tongue image to show a consistent rendering after correction. We can conclude that our OCC network can eliminate inconsistencies in five different lighting conditions.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**\#of Hidden Layers** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** \\ \hline \(\Delta E_{mean}\) & 2.30 & 2.15 & 1.93 & 1.99 & 1.92 & 1.94 & 1.95 & 1.97 \\ Time(s) & 3.3 & 4.2 & 7.0 & 8.3 & 10.7 & 12.1 & 14.7 & 15.6 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison results of different hidden layers.  
To obtain objective results, we re-selected \(10\times 10\) pixel color patches in each color block to calculate the color difference. The results are shown in Table 3, where \(\Delta E_{mean}\) and \(\Delta E_{max}\) are the average color difference and the maximum color difference of the color patches, respectively. As shown in Table 3, under five lighting conditions, \(\Delta E_{mean}\) and \(\Delta E_{max}\) are greatly reduced. Furthermore, all \(\Delta E_{mean}\) of the corrected images are less than 3, which means there is no visual difference. Therefore, our OCC network achieves high accuracy correction results for objective evaluation.  
We have calculated the average difference of the same color patches in the region of interest before and after correction in the CIE Lab color space, as shown in Table 4. Before correction, the average color difference of the same color patch in the color checker image ranged from 2.71 to 18.17. After color correction, the range is reduced to [0.67, 1.21].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Illumination** & & **ILL1** & **ILL2** & **ILL3** & **ILL4** & **ILL5** \\ \hline \multirow{3}{*}{ILL1} & Before & – & 4.95 & **18.17** & 18.02 & 17.17 \\  & After & – & 0.78 & 0.85 & 0.86 & 0.69 \\ \cline{1-1}  & Before & 4.95 & – & 15.05 & 14.43 & 13.97 \\ \cline{1-1}  & After & 0.78 & – & 1.04 & 1.21 & 0.93 \\ \cline{1-1}  & Before & 18.17 & 15.05 & – & 4.26 & 3.55 \\ \cline{1-1}  & After & 0.85 & 1.04 & – & 1.19 & 0.82 \\ \cline{1-1}  & Before & 18.02 & 14.43 & 4.26 & – & **2.71** \\ \cline{1-1}  & After & 0.86 & **1.21** & 1.19 & – & 0.83 \\ \cline{1-1}  & Before & 17.17 & 13.97 & 3.55 & 2.71 & – \\ \cline{1-1}  & After & **0.67** & 0.93 & 0.82 & 0.83 & – \\ \hline \hline \end{tabular}
\end{table}
Table 4: Average color difference between region of interest under different illuminances before and after correction.  
Figure 7: The captured tongue images (upper row) and their corrected results (bottom row) under five different illuminations. From left to right, they are captured under ILL1 to ILL5.