## 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  
Additive manufacturing allowing layer-by-layer shaping of complex structures is of rapidly increasing interest in production technology. Especially for laser-based powder bed fusion processes of metals (PBF-LB/M), the authors of Reference [46] show a growing demand in production technology. As a consequence, there is a lively demand for quality inspection, process control and monitoring. There are more than 50 process parameters, adjustable or fixed concerning additive manufacturing (AM) technologies [47]. Among others, laser and scanning parameters, powder material and powder bed properties and recoating parameters are important for the PBF-LB/M. Many in-process sensing techniques such as cameras or pyrometers [48] for continuous temperature monitoring primarily focus on monitoring melt pool signatures. In Reference [49], a two-channel-pyrometer, coaxially integrated into the machine laser beam guidance, is implemented to identify thermal indicators with a resolution better than 10 \(\upmu\)m. Other non-contact methods, for example, optical coherence tomography in selective laser melting [50] and selective laser sintering [51], are also applied. However, none of the methods mentioned have shown that the measured parameters have a direct relation to the internal quality or target geometry. A review of process defects and in situ monitoring methods in metal powder bed fusion may be found in [52]. As one of the important quality parameters, the presence of pores created during laser beam melting has to be avoided or minimized. However, currently there is no method for obtaining information on the pore density or its change during the process. As the density of the material decreases with an increasing fraction of pores, it should, however, be possible to detect a change of pore density by a precise height measurement of the structures. Figure 11 shows a schematic representation of the corresponding situation:  
Figure 10: Fringe Projection measurements on wings. (**a**) Measurement of the BLADE GKN wing. Aerospace wing (front nose section and lower cover). Due to the size of the measuring field, the strongly curved front nose section, the subsequent geometry of the wing and recordings under daylight, we show here an example of an extreme measuring situation for the FP-technology. (**b**) Measurement of the BLADE Saab wing (front nose section and upper cover). The setup is equipped with retro-reflectors mounted on CFRP sticks for interacting with the laser tracker system. Reprinted with permission from Reference [44]. During the process, the volume of the initial powder shrinks due to the laser beam melting process. Therefore, the height of the recrystallized structure is lower than that of the powder bed surface. Due to the density dependence of the volume of recrystallized material on the volume of embedded pores, there is an additional slight height difference between structures that have a low and a high porosity. A measurement of this height difference should therefore give an indication for a change in the pore density of the current process. In Reference [53], we demonstrated a new approach to evaluate 3D laser printed parts in powder bed fusion-based AM in-line within a closed space. It was shown that high-precision metrology based on structured light is suitable for dimensional in-process sensing. The accuracy of the system presented was qualified to be below 10 \(\upmu\)m in the z-coordinate perpendicular to the surface of the powder bed. This value is sufficient to reliably detect errors in powder coating or consolidation at a layer thickness of 50 \(\upmu\)m or higher. The basic idea of the measurement technology to be used in the PBF-LB/M process is the application of the Fringe Projection method. With this dimensional measurement technique, one can, for example, derive the shape, size and position as well as waviness and roughness. Figure 12 illustrates the method.