## 4 Conclusions  
This study introduces a vision system designed to capture clear molten pool images online during GMAW, and a method to construct the system. In addition, digital camera settings, imaging wavelength bands, and optical designs have been proposed to precisely measure the molten pools during GMAW. The proposed vision system allows cost reduction with a straightforward configuration and satisfies the image quality requirements for vision-based online quality monitoring during GMAW. The results are summarized as follows.  
1. Accurate measurement of radiation is crucial for clearly observing the molten pool during welding. In this study, the radiation was measured by aligning the collimating lens of the spectrometer, similar to the distance and angle between the weld and camera.
2. The selection of an appropriate lens in combination with a camera is essential for effective monitoring during welding. Factors such as the \(f\)-number, light throughput, and diffraction limit must be considered. The focal length was selected based on the ROI size and working distance between the image sensor and weld. In this study, a lens with an \(f\)-number of \(f/4\) and focal length of 16 mm was proposed for molten pool monitoring during mild-steel GMAW.
3. During mild-steel GMAW, the imaging wavelength band with the least interference from the arc plasma and the most suitable for molten pool observation was 830 nm, with a BRISQUE score of 52.03.  
Figure 12: Weld quality status observed in the molten pool image in mild-steel GMAW: (**a**) humping; (**b**) magnetic arc blow; (**c**) silicate island; (**d**) burn-through; (**e**) spatter; (**f**) undercut.  
4. To capture the information of the molten pool with less noise in the image during mild-steel GMAW, the combination of an 830 nm band-pass filter and an OD1.0 ND filter is the most suitable. It possessed a BRISQUE score of 15.3, which was the best score.
5. Image-quality assessment using BRISQUE confirmed that the image acquired with the proposed optical design method was suitable for monitoring with the highest quality, consistent with the assessment scores.  
We propose a vision system for real-time weld-quality monitoring and a low-cost system for training unskilled welders. The radiation emitted from a weld varies depending on the welding process, base metal, filler metal, and shielding gas used. Therefore, further research will be conducted to design a vision system suitable for each welding process and welding condition, based on the proposed method. Furthermore, control systems will be developed to enhance welding quality by utilizing feature information, such as bead width, height, and weld failures, acquired from the images of this system.  
Conceptualization, G.-G.K. and J.Y.; methodology, G.-G.K. and D.-Y.K.; validation, G.-G.K. and J.-K.P.; formal analysis, G.-G.K. and D.-Y.K.; investigation, G.-G.K. and D.-Y.K.; resources, J.-K.P. and Y.-M.K.; writing--original draft preparation, G.-G.K. and J.Y.; writing--review and editing, G.-G.K. and Y.-M.K.; supervision, J.P. and J.Y. funding acquisition, J.-K.P. and J.Y. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Material Parts Technology Development Project (20022489, Development and Substantiation of Joining Equipment & Smart Joining Lines for Manufacturing Electric Vehicle Chassis and Battery Cases with High-Strength Materials) funded by the \"Ministry of Trade, Industry and Energy (MOTIE, Korea)\" and \"Korea Planning & Evaluation Institute of Industrial Technology (KEIT)\".  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.  
Author [PERSON] was employed by the company Hwashin. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.