## 3 Results and Discussion  
The study focused on four initial cases, selected to allow the direct comparison between the different scenarios and the process response gathered by the photodiode system. Tests with the ideal process parameters (Table 1) were carried out, independently varying the VIS and IR gain until having enough signal to observe the cutting process in each spectrum. VIS gain 3 worked to correctly monitor all the cases, except 5 mm-thick aluminum, where gain 4 also displayed values that were too low. For the IR spectrum, gain 2 was used during the monitoring, except for stainless steel, where gain 1 was enough. Table 3 displays the monitoring signal mean levels obtained for each case at different gains. Figure 2 allows, as an example, the visual contrasting of the four scenarios for the VIS spectrum with gain 3 and IR spectrum with gain 2. During the gain adjustment tests, the experiment with stainless \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Cutting Parameters** & & **AI** & & **SS** \\ \hline
**Thickness [mm]** & **5** & 10 & 10 & 10 \\ Gas type & N\({}_{2}\) & N\({}_{2}\) & O\({}_{2}\) & N\({}_{2}\) \\ Power [kW] & 5 & 5 & 5 & 5 \\ Velocity [mm/min] & 7000 & 2400 & 680 & 1200 \\ Gas pressure [bar] & 15 & 15 & 10.5 & 12 \\ Focal [mm] & \(-\)1.1 & \(-\)2.4 & \(-\)3 & \(-\)7.2 \\ Standoff [mm] & 0.6 & 0.6 & 0.6 & 0.6 \\ Frequency [Hz] & 5000 & 5000 & 5000 & 5000 \\ Duty minimum [\%] & 100 & 100 & 100 & 100 \\ Duty maximum [\%] & 100 & 100 & 100 & 100 \\ Nozzle diameter [mm] & 2.5 & 3 & 2.5 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cutting parameters for 10 mm-thick stainless steel (SS) plate, and 5 and 10 mm-thick aluminum (AI) plates. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Cutting Parameters** & & **AI** & & **SS** \\ \hline
**Thickness [mm]** & **5** & 10 & 10 & 10 \\ Gas type & N\({}_{2}\) & N\({}_{2}\) & O\({}_{2}\) & N\({}_{2}\) \\ Power [kW] & 4–5 & 4.3–5 & 4.2–5 & 4–5 \\ Velocity [mm/min] & 4500–7500 & 2000–2600 & 500–900 & 600–1400 \\ Gas pressure [bar] & 9–17 & 13–19 & 6–12 & 8–16 \\ Focal [mm] & \(-\)0.6—1.6 & \(-\)2—3.2 & \(-\)2—4 & \(-\)0.6—7.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Ranges of laser cutting parameters tested during the experiments. steel and an IR gain of 2 reached a value of 10 V, the maximum of the system maintaining a good cut quality. In previous studies, this behavior was attributed to a loss of cut. However, in this research it was demonstrated that it was possible to obtain saturated monitoring signals with good quality if the used optical gain was not properly adjusted, provoking false positives in a control system. To demonstrate the influence of the material in the photodiode-based monitoring signal, aluminum (Al) and stainless steel (SS) were chosen, maintaining the other case characteristics constant, specifically, plates of 10 mm-thick cut with N\({}_{2}\) as assist gas and 5 kW of laser power. The photodiode detected the emissions produced in the cutting process, where the registered signal was correlated to the emissivity coefficient (\(\varepsilon\)) of each material. For the same gain value, stainless steel displayed a higher monitoring signal value than aluminum in both the VIS and IR spectrum, 0.6 V vs. 1.5 V and 0.4 V vs. 10 V, respectively. The results obtained in the experiments are in accordance with the literature [26], which indicated that the \(\varepsilon\) is higher for stainless steel, and thus, the corresponding monitoring signal. To analyze the influence of the thickness in the registered signal, two aluminum plates of 5 and 10 mm thickness were cut with N\({}_{2}\) at 5 kW, with the velocity, gas pressure, and focal position specified in Table 1. Comparing VIS monitoring signals, the 10 mm thick plate displayed a higher value than the thinner one (0.6 V vs. 0.15 V), explained as a consequence of more vapor plume formation [25] during the cutting process of the thicker plate, and thus, an increase in VIS signal.