## 3 Results and Discussion
### Image Quality Assessment  
The quality of the images presented in Figure 8 was assessed using the BRISQUE technique. As shown in Table 4, the BRISQUE technique yields low and high scores for high- and low-quality images, respectively, as explained in Section 2.4. The 420, 508, and 660 nm images exhibited low quality owing to strong noise, resulting in high scores. In the 752 nm image, the molten pool was partly observable; however, its width could not be discerned because of the persistent interference of the arc plasma. Conversely, the 808 nm and 830 nm images demonstrated high quality with relatively low scores, with the 830 nm image exhibiting the highest quality. Adjustments to the digital camera settings were necessary for capturing the feature information of the molten pool within the same wavelength band. It is advisable to set the gain and gamma to minimum and maximum values of 0 and 2.2, respectively, to distinctly observe the boundary between the molten pool and base metal sections. As shown in Table 5, when the gamma value was 1.0, the molten pool appeared dark because of low contrast. At a gamma value of 2.2, the molten pool was clear, and the boundary with the base metal is evident owing to its high contrast. When the BRISQUE technique was employed, a gamma value of 2.2 resulted in a score of 56.09, indicating the highest quality. Figure 9: Molten pool image obtained by combining an 830 nm band-pass filter and ND filters during mild-steel GMAW: (**a**) 830 nm band-pass filter + OD0.5 ND filter; (**b**) 830 nm band-pass filter + OD1.0 ND filter; (**c**) 830 nm band-pass filter + OD2.0 ND filter. Table 6 presents the images captured when the 830 nm bandpass filter was combined with the ND filter to minimize the noise generated by the saturation of the arc plasma. When combined with the OD0.5 ND filter, the saturation area of the arc plasma was reduced, and interference with the edge was prevented despite variations in the bead width depending on the welding conditions. The combination that included the OD1.0 ND filter decreased the overall image brightness. However, the saturation area of the arc plasma was further reduced, and the light reflected around the edge of the molten pool was reduced, enabling a clear area division. Therefore, this is the most suitable image for performing semantic segmentation in the welding quality monitoring model, which will be included in a future study. Owing to excessive sensitivity reduction, observations were impossible with the combination that included the OD2.0 ND filter. Therefore, the clearest observations were obtained when images were captured by combining the 830 nm bandpass and OD1.0 ND filters. The image obtained with the 830 nm band-pass alone served as the reference image, and the image quality for each OD value was assessed using BRISQUE. The OD1.0 ND filter combination yielded the highest quality, with a score of 15.30. Figure 10 shows that the interference from the arc plasma was minimized, and the boundary of the molten pool is clearly observed in the image acquired when an 830 nm band-pass and OD1.0 ND filters were combined. Figure 11 shows the image captured for 1 s under the most suitable imaging conditions for monitoring the molten pool during mild-steel gas-metal arc welding. Triggering devices such as field-programmable gate arrays can cause problems including time delays and inaccurate synchronization. The vision system proposed in this study captured high-quality molten pool images during DC pulse welding without dividing the peak current and base current sections. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Optical Density** & 0.5 (transmission: 32\%) & 1.0 (transmission: 10\%) & 2.0 (transmission: 1\%) \\ \hline
**Image** & & & \\ \hline
**Score** & 42.85 & 15.30 & 34.51 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: BRISQUE image quality score of combining the 830 nm band-pass filter with each ND filter.