## 2 Materials and Methods
### Experiment  
For the in situ monitoring of the LPBF process, an OT system is installed on EOS M 290, which uses a sCMOS (scientific complementary metal-oxide semiconductor) camera PCO.edge 5.5 and works in a near-infrared (NIR) spectral range. The field of the camera view covers the whole building platform with a size of 250 \(\times\) 250 mm. Additional technical information of the camera is provided in Table 1.  
Radiation during the LPBF process contains the process quality information since it is a representative of the emission energy of the melt pool where the metallic powder melts and consolidates. The monitoring system should enable the capturing of the radiation layer-by-layer during the LPBF process. Thus, a bandpass filter with a 100 nm width and a center wavelength (CWL) of 900 nm is used to block the laser reflection and avoid the saturation of the camera sensor due to high-intensity inputs in the broad spectrum. Monitoring data are acquired continually at each layer during the LPBF process. Since exposure time of the camera is not long enough to capture the complete laser exposure process of a layer, multiple images are taken for each layer with a chosen exposure time of 2 s. All images of a certain layer are then stacked to a single image by summing up all grey values pixel-wise. The acquired OT images are calibrated according to the lookup tables (LUTs) for flat-field, defective pixel, and perspective correction where the LUTs are obtained from camera calibration experiments. These calibrated OT images cover the whole 250 \(\times\) 250 mm building platform of the EOS M290 with a resolution of 2125 \(\times\) 2125 pixels. The corresponding spatial resolution of the OT monitoring system is 250 mm/2125 pixel = 0.118 mm/pixel. \begin{table}
\begin{tabular}{c c} \hline \hline
**Camera Feature** & **Specification** \\ \hline Type of sensor & sCMOS monochrome \\ Bit depth & 16 bits \\ Resolution (h \(\times\) v) & 2560 \(\times\) 2160 active pixel \\ Pixel size (h \(\times\) v) & 6.5 \(\upmu\)m \(\times\) 6.5 \(\upmu\)m \\ Exposure time & 2 s rolling shutter \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical specifications of the sCMOS camera (h: horizontal, v: vertical). The process parameter configuration used is shown in Table 2. The corresponding volume energy density is \(\mathrm{E_{V}=\frac{P_{L}}{d_{s}v_{s}\Delta y}=62.5\,\mathrm{J/mm^{3}}\)[24]. As a rule of thumb, overhang angles less than 45\({}^{\circ}\) can be printed using additive manufacturing without support structure. To increase the probability of encountering anomalies in radiation, 15 parts with different overhang angles between 45\({}^{\circ}\) to 90\({}^{\circ}\) with an interval of 5\({}^{\circ}\) are printed for model training (in Figure 4a). The parts are located on the building platform randomly (Figure 4b). Each designed part consists of a strut section and overhang angle section (in Figure 4c,d). The height of strut sections and overhang sections of each part are different to reduce the coupling in training data. To avoid critical geometric distortion (e.g., warping), which may stop the process, parts with overhang angles 75\({}^{\circ}\), 80\({}^{\circ}\), 85\({}^{\circ}\) and 90\({}^{\circ}\) are cancelled manually after the manufacturing of several overhang layers (in Table 3). Sandvik 17-4 PH powder with a mean particle size 6.7 \(\upmu\)m is used for the print job. To create training data with different behaviors of the printing process, struts and overhangs have varying heights. The print job contains 662 layers, while 652 pairs of data are generated for the training due to layer packaging. All captured monitoring data with the process are presented in Section 2.1. The test part is printed with the identical LPBF parameter configuration as the training set. The designed CAD file in STL is sliced by the Trimesh library in python and packaged according to the application procedure in Section 2.1.