## 3 LPBF Sensing Techniques
### Optical Techniques
#### 3.1.1 Techniques with Electron Source  
**Scanning electron microscopy (SEM)** uses a focused beam of electrons to analyze the specimen's surface topography and material composition. The focused electron beam passes through scanning coils in the electron column that deflect the beam so the object can be scanned. Once the electron beams interact, random scattering and absorption cause the electrons to lose energy. High-energy electrons are reflected by elastic scattering, secondary emission by inelastic scattering, and electromagnetic radiation emission. The scattered electrons are captured using detectors, and the electron signals are converted to voltage signals using photomultipliers. Collecting the measured intensity of the detected signal and the electron beam's recorded position enables the construction of a 2D image of the part. SEM technologies are frequently equipped with multiple detectors to detect specific emissions. Electron backscatter diffraction (EBSD) and energy-dispersive X-ray spectroscopy (EDS) are often paired with SEM technology to provide further information on the specimen. The advantages of using SEM include its high versatility, high image resolution, easy operation, fast scanning rate, and non-destructive nature. The disadvantages of SEM include its large size, high price, vacuum environment requirements, minor radiation exposure risk, long sample prep time, and sensitivity to interferences (electrical, magnetic, and vibrational). An example of SEM imagery of an LPBF-printed part highlighting un-melted spherical powder is provided in Figure 14[36]. **Transmission electron microscopy (TEM)** is comparable to the SEM in that a focused beam of electrons is directed toward the sample. However, in TEMs, the sample is located in the middle of the column. The transmitted electrons passing through the sample before they are collected provide the sample's internal structure, including crystalline structure, morphology, and stress state information. Once the transmitted electrons pass through the sample, they travel through a series of intermediate and projector lenses below the sample and directly produce an image on a fluorescent screen or charge-coupled device (CCD) camera. Unlike SEMs, which only produce 2D surface topographical imagery, TEMs can also produce 2D images of internal structures. SEMs have a much larger maximum field of view (FOV) than TEMs, though the magnification capabilities of TEMs are much higher than those of SEMs. EDS has often been coupled with TEM to produce elemental and atomic-bonding information. TEMs have been widely used to analyze interfacial grain boundaries, dislocation networks and densities, and particle precipitation. The advantages of using TEM are practically the same as those of SEM. The disadvantages of using TEM  
Figure 14: Example of SEM imagery provided by [36], highlighting un-melted spherical Ti6 Al4V powders (**a**) inside of the LPBF specimen post-build and (**b**) on the free surface of the specimen. are also the same as those of SEM, with the added negative of TEM being destructive to the sample. An example of TEM imagery is provided in Figure 15, showing the location of stacking faults in an LPBF-printed austenitic stainless-steel sample [37]. **Electron backscatter diffraction (EBSD)** detector is an SEM-compatible technique where the electron beam is scanned across the surface of a tilted crystalline sample. The electronic source is provided by a field emission gun rather than a thermionic emission mechanism. A similar technology often associated with EBSD is electron channeling contrast imaging (ECCI). ECCI is geared explicitly towards imaging and characterizing lattice defects, while EBSD is geared towards more detailed crystallographic analysis. ECCI has been used in LPBF printing analysis to visualize precipitates in the printing process due to the non-equilibrium between the melting and solidification processes [38; 39].