## 3 Results & Discussion
### Comparison between Complete and Incomplete Cuts
Aluminum, with a melting point of about 933 K, emits almost no radiation in the range of the detectable camera between 400 and 700 nm, especially for the green and blue channels of the camera. The maximum wavelength of the specific spectral radiation at the melting point of aluminum is 3105.9 nm, i.e., in the infrared range. According to [PERSON]'s displacement law, the intensity maximum wavelength shifts to a shorter wavelength at higher temperatures. At an evaporation temperature of about 2743 K for aluminum, the maximum wavelength thus shifts to 1056.4 nm. However, the melt in the kerf emits thermal radiation over a wide spectral region. The spectral distribution depends on the temperature according to [PERSON]'s law, and therefore emissions from the process zone for aluminum can still be recorded, even if the maximum wavelength of the emission for aluminum melt is not directly in the detectable range of the high-speed camera. Figure 6 shows exemplifying images of the melt pool area in aluminum for 1, 3, and 5 mm. A smoother melt flow can be related to the lower viscosity and surface tension of aluminum at higher temperatures compared to steel [37; 38; 39]. Higher feed rates or sheet thicknesses result in an increased cut kerf angle. Due to the increased primary laser  
Figure 5: Signals for the green and blue channels of complete cuts (**a**–**c**) and incomplete cuts (**d**–**f**) and an example image of the corresponding signal highlighted by black arrows for aluminum for sheet thicknesses of 1.0, 3.0, and 5.0 mm. irradiation area, the melt film increases for aluminum, and also, the local temperature in the kerf rises, which leads to vaporization. A higher emission of the vaporization compared to the melt explains the high blue component and the overexposed white areas in the signal curve for complete cuts and especially for incomplete cuts in aluminum. The actual cooler cut front for aluminum is not assumed to be at the front of the detectable melt pool area, as for steel, but further in the direction of the feed rate, i.e., further to the right from the top view. This occurs because, presumably due to the lower viscosity and surface tension, the melt is removed more quickly from the cut kerf front. Nevertheless, geometric and spectral information can be calculated from the recorded images, as the resulting vapor plume also reveals similar signal characteristics. In aluminum, it is also observed that for incomplete cuts, the length of the melt pool area increases compared to complete cuts, and the melt pool area appears more elliptical. The melt pool area of the horizontal axis even extends beyond the field of view of the nozzle. Especially good visible for aluminum is the resulting plasma plume for incomplete cuts, which emerges upwards from the kerf. This plasma plume emits in the UV spectral range, as shown in Figure 5d-f. During an incomplete cut, a white area is visible in the region of the kerf, presumably due to overexposure caused by the longer exposure time due to the shorter sampling rate for aluminum and the addition of the color channels. However, the blue spectral region of the emitting plasma plume is, in addition, clearly visible outside of the kerf in Figure 5d-f. Please note, in cuts for aluminum, a sharp peak can often be detected at the start of each cut, here always at 1.5 s, which is not considered for the measured signal for later observations.