## 2 Materials and Methods
### Analysis of Spectroscopic Characteristics and Wavelength Band Selection  
The strong radiation emitted during gas-metal arc welding makes it impossible to observe the weld using conventional vision systems. This necessitates an appropriate wavelength band for image acquisition using a vision camera to monitor the molten pool and metal during gas-metal arc welding. As illustrated in Figure 1, the radiation emitted from the arc plasma causes image saturation and hinders clear observation. Therefore, the strong radiation must be removed and filtered, and its characteristics must be analyzed to determine the appropriate conditions. Furthermore, the spectral radiance of the source that emits radiation and the quantum efficiency of the image sensor used for image acquisition must be considered. Radiation refers to the movement of heat when a certain material emits electromagnetic energy, even when the media are not in direct contact. It represents the conversion of the internal energy of a material into electromagnetic energy, which is also referred to as thermal radiation. During GMAW, thermal radiation enables observation of the molten pool using a passive vision sensor without an auxiliary light source. [PERSON]'s law for black-body radiation is expressed by Equation (1):  
\[B_{v}(T)=\frac{2 hv^{3}}{c^{2}}\frac{1}{e^{h\
u/kT}-1} \tag{1}\]  
where \(B_{v}(T)\) is the spectral radiance; \(h\) is the [PERSON] constant; \(v\) is the frequency of the electromagnetic radiation; \(c\) is the speed of light in vacuum; \(k\) is the Boltzmann constant; and \(T\) is the absolute temperature of the material. As the temperature of the material increases, the peak of the emitted spectral radiance shifts to shorter wavelengths. According to the theory of black-body radiation in thermodynamics, all materials emit electromagnetic radiation at absolute zero or higher temperatures. A black body is an ideal object that absorbs all incident electromagnetic radiation regardless of the frequency and incident angle. A black body emits electromagnetic radiation at a constant temperature, referred to as black-body radiation, and its spectrum is determined solely by the temperature rather than the shape or material. Figure 1 depicts [PERSON]'s law for black-body radiation. Depending on temperature \(T\), the molten pool in gas-metal arc welding emits more radiation than that emitted at the melting temperature of steel. According to [PERSON] [24], the peak temperature \begin{table}
\begin{tabular}{c c c c c} \hline
**C** & **Si** & **Mn** & **P** & **S** \\ \hline
0.1852 & 0.343 & 1.355 & 0.011 & 0.0044 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the base metal ISO 630 E355 (wt.%). \begin{table}
\begin{tabular}{c c c c c} \hline
**C** & **Si** & **Mn** & **P** & **S** \\ \hline
0.07 & 0.65 & 1.14 & 0.015 & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of the filler wire ISO 636-A W 42 5 2 Si (wt.%). of a molten pool of mild steel (Q235) is 3169 K, and its spectral radiance reaches a peak near the near-infrared (NIR) wavelength of 800 nm. According to [PERSON] [25], the temperature of the arc plasma ranged from 7000 to 20,900 K when an Ar 90%-CO\({}_{2}\) 10% mixed gas was used as a protective gas. Therefore, the spectral radiance peak shifted from 600 nm to shorter wavelengths with increasing temperatures. To acquire feature information from the molten pool during GMAW, the area where the arc plasma is formed and the metal melts must be captured. Therefore, the spectroscopic characteristics must be analyzed by measuring the radiation from both the arc plasma and molten pool, and the sum of all radiation emitted during GMAW must be considered. Therefore, contrary to a previous study [19], the irradiation from the arc plasma and the molten pool were simultaneously measured without dividing the arc plasma and the molten pool sections. The radiation emitted from mild-steel GMAW was measured using an Ocean Optics HR4000 CG-UV-NIR spectrometer (Ocean Optics, Orlando, FL, USA).