## 3 Selected Results of Optical Metrology Techniques
### Techniques Based on Geometrical Optics Suitable for In-Process and In Situ Applications
#### 3.2.2 In-Process Measurement and Evaluation of Laser Added Manufacturing Processes for Metals
The surface area, spread with metallic alloy powder and/or laser consolidated parts, is illuminated by structured light. Generally, a one-dimensional sinusoidal fringe pattern distribution is projected onto a surface and the modulated phase is calculated with one of the numerous techniques for analyzing fringes. The measuring principle is based on the phase shift method combined with triangulation [54]. The information of the surface profile is obtained from the deformed fringe pattern following the object shape. A system consisting of two identical CCD-cameras captures the fringe images projected onto the building plate inside the process envelope. The two (or more) cameras do not necessarily have to be housed in the printing chamber. With appropriate viewing windows, the system can also be installed outside. By marking the pattern, individual points on the surface can be recorded and calculated with precision in (x, y, z) coordinates using the camera's vision rays (sketched in blue in the picture). Our vision ray calibration technique for the cameras ensures the necessary high accuracy [41]. One advantage of the two-camera system implemented in this way is that the projector (outside the envelope) does not need to be calibrated. Figure 11: Schematic representation of the height change of structures due to laser beam melting. From left to right: An already existing structure is covered with metal powder ready for the next process step. The volume of the initial powder shrinks due to the processing. Therefore, the height of the recrystallized structure is lower than that of the surrounding powder bed surface. The shrinkage of the volume due to the melting process depends on the fraction of pores included in the treated material. Therefore, during growth of the structure, a small height difference evolves due to the density dependence of the volume of recrystallized material. The following results give insights to the performance of our latest measurement setup and research methods (Obtained within the IGF-Project no. 21.178 N, for further details see section Funding). It demonstrates the state-of-the-art performance of dimensional metrology based on structured light applied to PBF-LB/M processes in-line. Figure 13a shows a CAD-design as a basis for the precise positioning of the cameras in the PBF-LB/M building chamber shown in Figure 13b. The arrangement in the CAD-design relative to the building plate corresponds to the exact scale as it has to be integrated into the PBF-LB/M System (SLM Realizer 250) at BIAS. In order to be able to achieve a very high lateral measurement resolution (in the in-plane \(\times\) and y directions) of 35 \(\upmu\)m, the measurement field should only depict the object to be evaluated. In addition, the fields of view of both cameras on the substrate must match as much as possible for the measurement principle to be implemented. The inner square within the gray ground plate in Figure 13a represents the size of a substrate of 150 \(\times\) 150 mm. The measuring fields for the same system can, after a recalculation for the lenses, be expanded up to 280 \(\times\) 280 mm on demand. The time required for the evaluation of a surface layer for this in-process measurement is about 5 s.  
A special feature in the construction of the measuring system is the integration of the existing viewing windows of the PBF-LB/M system. The cameras are mounted and fixed in their final positions on a so-called \"bridge\" (compare Figure 13a). This camera bridge is installed under one of the two windows within the building envelope. The great advantage here is that the necessary cabling can be routed directly through the cover plate of the SLM chamber. The system closes off with the SLM chamber via a printed, heat-resistant hood (white part in Figure 13b with a height of only 90 mm). The fringe images are projected from outside the installation space through another viewing window of the PBF-LB/M system. That brings the further advantage that the projector does not have to be shielded from the process.