## 2 Materials and Methods
### System Configuration  
#### 2.3.1 Vision Camera  
The configuration of the proposed vision system is shown in Figure 3. The system includes a vision camera, lens, ND filter, bandpass filter, and ultraviolet (UV) filter and is located at the rear of the welding direction. Because the molten pool head was covered by the gas nozzle when the welding torch and camera were parallel, the angle between the welding torch and camera was set to 45\({}^{\circ}\) to acquire images of the entire molten pools. As outlined in Section 2.2, a wavelength band was selected based on the spectroscopic analysis. Images suitable for weld observation were captured using a filter that only allowed the passage of the selected wavelength band. When images are captured during arc welding, the red-green-blue sensor captures images with higher sharpness and signal-to-noise ratio than a monochrome sensor [26]. The vision camera used in this experiment was the IDS UI-3271 LE (IDS Imaging, Obersulm, Germany), which uses a Sony IMX265 global-shutter CMOS color sensor (Sony, Tokyo, Japan). Its quantum efficiency is illustrated in Figure 4.  
Table 3 shows the specifications of the vision camera and the parameter values set in the experiment. The vision camera exhibited the highest quantum efficiency near the blue, green, and red wavelengths of 450 nm, 530 nm, and 610 nm, respectively. Because relatively good color quality is exhibited in each peak wavelength band, quantum efficiency is important for selecting imaging bands. In addition, when images are captured, parameters such as gain and gamma must be properly adjusted. The gain, a parameter that adjusts the brightness of an image, represents the overall brightness level. The image becomes brighter and darker when this value increases and decreases, respectively. The gamma  
Figure 3: Vision system configuration during GMAW: (**a**) schematic; (**b**) experimental system.  
ratio indicates the contrast between dark and bright regions of an image. The contrast increases with the gamma ratio, making the dark part darker and bright parts brighter. If it decreases, the level of color representation decreases owing to a reduction in contrast. The exposure time indicates the time at the image sensor is exposed to light. A long exposure time yields a bright image because a large amount of light is input to the sensor. Likewise, a short exposure time yields a dark image because less light is supplied. Additionally, if the exposure time is long, many movements are captured, whereas only fast-moving subjects are captured if the exposure time is short. The boundary between the molten pool and base metal must be clear to acquire feature information via image capture during GMAW. An increase in the gain increases the brightness of the overall image. If the value is extremely high, noise is generated in the image. Therefore, its value must be reduced. The gamma value must be set as high as possible to increase contrast and clearly demarcate the boundary between the bright and dark parts. If the exposure time is long, a large amount of light enters the sensor and saturates the image; therefore, the pixel clock value decreases depending on the camera specifications, limiting image capture at high frame rates. Therefore, an appropriate level was selected. The parameters were adjusted to improve the sharpness of the acquired images. Additionally, the system was connected to an industrial computer environment set up with Intel(r) Core(tm) 15-10500, 16 GB DDR4 RAM (Intel, Santa Clara, CA, USA), and NVIDIA GeForce RTX 3060 (NVIDIA, Santa Clara, CA, USA).