## 3 Results & Discussion  
### Comparison between Complete and Incomplete Cuts  
Figure 4 shows the signal of the melt pool area, calculated using the above-described image processing step, over time during the cutting of a sample. To visualize the difference between a complete (upper row of diagrams) and an incomplete cut (lower row of diagrams), exemplifying results for such cuts in stainless steel and zinc-coated steel are presented. A cut is classified as incomplete if the bottom edge of the kerf resolidifies. This is the case when the laser power is insufficient to penetrate the material or when the melt cannot be driven out of the kerf and adhering burr seals the bottom of the kerf. In addition to the diagrams, for each case, exemplarily chosen frames recorded by the high-speed camera are shown. The timeline of each recording starts after the piercing of the laser. During the time frame, the drives start to accelerate, which leads to the laser cut and is indicated by an increase in the green and blue melt pool areas. Towards the end of the cut, the feed rate decelerates, and a decrease in the melt pool area is visible. A comparison for incomplete cuts (lower row of Figure 4) reveals remarkable differences in the recorded frames and both the spectral and geometrical information calculated from the melt pool area. During a complete cut, the material is kept molten by the laser and driven out of the kerf by the assist gas pressure. The hottest area of the melt pool, located directly at the cut front, emits more intensely in the blue spectral region and can therefore be seen brighter or white in the recorded image. The cooler regions of the melt, such as the ejected melt further from the cut front, emit less in the blue spectral region and appear darker and more pronounced in the green color channel. In contrast, during an incomplete cut, the melt from the cut front is not completely expelled from the kerf and is partially resolidified in the kerf. In general, an increased melt pool area is observed whenever an incomplete cut is forced by a reduction in laser power or an increase in feed rate. Moreover, for incomplete and complete cuts, different behaviors in the calculated geometries from the melt pool are observed. As described above, the width of the kerf represents the vertical axis and the length of the horizontal axis of the melt pool. Considering complete cuts, the melt pool appears more circular, with the vertical axis (width) and the  
Figure 4: 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 stainless steel with a sheet thickness of 1.5 mm and zinc-coated steel with a sheet thickness of 1.0 and 3.0 mm. horizontal axis (length) of the calculated geometries being almost equal. Incomplete cuts result in a significant change in the melt pool geometry. The not expelled portion of the melt resolidified in the cutting kerf, contributing to the thermal radiation and resulting in a pronounced increase in the horizontal axis of the melt pool length in the route of the feed rate. As, in contrast, the kerf width increases less, the melt pool area appears in an elliptical shape. In addition, a tendency can be observed that for complete cuts, the signal of the green melt pool area slightly exceeds the signal of the blue melt pool area. For incomplete cuts, the signal of the blue melt pool area converges with the signal of the green melt pool area or exceeds it. This can be assigned to an enhanced temperature when closing the kerf due to the increased irradiated area of the laser, and more thermal radiation is directed toward the cutting head, which can lead to overexposure. The recorded frames and calculated melt pool areas for aluminum show significant deviations from stainless steel and zinc-coated steel, as depicted in Figure 5. First of all, for complete cuts, significantly lower process radiation is observed, in particular for cutting 1 mm aluminum, despite the longer exposure time.