## 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
No pressure loss in the building envelope after installation of the measuring system could be detected. Figure 14a shows a recorded image of one camera from a series of measurements. It exactly shows what the camera observes coded in gray values without the fringe pattern. Cube shaped test consolidations were built with a size of 10 mm \(\times\) 10 mm for each part with varying process parameters such as laser power and scanning speed, for details, see Table A1 of Appendix A. The scanning direction for all cubes is parallel to the y-axis of the SLM machine coordinate system (approximately vertical in Figure 14). A track is the result of the laser beam scanning along a straight line on the powder bed with a constant speed. The results presented below were taken from the layer 10 and 20 as stated in the following text and corresponding figures. In the complete printing procedure 40 layers have been formed. Figure 12: Principle of the fringe-projection technique used to analyze the PBF-LB/M process. A structured pattern is projected onto the powder bed by the projection unit. A stereo camera system (cameras 1 and 2) records this pattern. With the mathematical combination of the vision rays at a common point of intersection (sketched in blue as an example), an (x, y, z)-value of the surface can be obtained by triangulation for each pixel pair of the cameras. Reprinted with permission from Reference [53], where more information on the measurement principle can be found. As the melting tracks shown in Figure 14a have a different appearance depending on the camera's perspective, one needs to calculate the point cloud of the final (x, y, z) coordinates of the structures created. These geometrical data are the product of the evaluation of both camera perspectives. The geometric coordinates of the point cloud are shown in Figure 14b in a view similar to that of the camera image in Figure 14a. It can be seen that the melting signatures of the consolidations are similar to the height profile. A major goal of the research presented here is to derive conclusions about the quality of the printed parts from the measured dimensional values. In purely qualitative terms,  
Figure 14: Results of printing depending on process parameters showing melted tracks of layer 10 with a size of 10 \(\times\) 10 mm. (**a**) Perspective view of one measurement camera. (**b**) Visualized 3D data from the calculated point cloud. The measured geometry values show a high degree of similarity to the image of the camera. For height information, see the color scale. Process and sample parameters are given in Table A1 of Appendix A.  
Figure 13: In-process measurement system. (**a**) CAD-Drawing of the dimensions and positions of the measuring system within the installation space. The calculated vision rays of the cameras on the building board are drawn in blue. For best practice, both cameras should capture the same field of view (here for the size of 150 \(\times\) 150 mm). (**b**) Installation and test of the measuring system inside the Realizer 250 system of BIAS. A 3D printed, temperature-stable hood (white) shields the process. The reference pattern is projected from outside through a viewing window. it can be determined from the measurement data shown in Figure 14b that the tracks created by various process parameters have straight melted traces (e.g., upper and lower left), interruptions of the melted traces (e.g., middle row, center and right) or irregular patterns (middle row left or lower row right). In order to determine the pore density as an important quantitative parameter of the process, we use X-ray computed tomography (X-CT). As described in Figure 11, one would expect a dependence of the structure height on the pore density. Figure 15 therefore shows the results of the porosity measured by X-CT as a function of the thickness difference between the structure with the highest porosity and the other structure heights. The height level of the structure with the highest porosity is therefore used as a reference level and set to zero.