## 3 LPBF Sensing Techniques
### Optical Techniques
#### 3.1.3 Techniques Using a Visible Light Source  
**Optical microscopy (OM, OT)** utilizes visible light in combination with a system of lenses to produce magnified images of the sample. OM is called a transmission light microscope (TLM) or light microscope. The advantages of OM are that it is typically easy to use, small and lightweight, unaffected by electromagnetic fields, requires no radiation, and requires little training and maintenance. The disadvantages of OM are that it cannot  
Figure 21: Example of XRD pattern results provided by [89], showing results for (**a**) the powder and (**b**) Z-scan direction, (**c**) Y-scan direction, and (**d**) 67\({}^{\circ}\) rotation scanning direction. Figure 20: Example of XCT cross-sections of coupon samples with various porosities provided by [88]. The increase in brittle fracture is attributed to the porosity regardless of defect type. magnify to their level and produces lower-resolution pictures. Figure 22 displays an example of optical microscopy images of various defects in an LPBF SS316L [90]. **Confocal laser scanning microscopy (CLSM)** uses laser light to scan the sample using different patterns and generate imagery. The laser penetrates the sample deeper than light from a standard lightbulb, allowing for 3D imagery of controlled depth. The advantages of CLSM are that it can provide high spatial-resolution imagery with reduced background noise and good accuracy. The disadvantages of CLSM are that it is more expensive than light microscopes, has limited penetration depth capability, and has a time-consuming sample preparation procedure. An example of CLSM showing high surface roughness in a Ti6 Al4V LPBF-printed part is provided in Figure 23 [91]. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Material** & **XCT** & **XRD** \\ \hline \multicolumn{4}{c}{Stainless-Steel-Based Alloys} \\ \hline
1.2767L Tool Steel [52], SS316L [47; 92], RAFM steel [93], 18 Ni & ✓ & ✓ \\ \hline Custom austentic SS. [36], SS316L [90; 95], AISI 420 Steel [59; 96] & & ✓ \\ \hline A800H [97], SS316L [98; 99; 100] & ✓ & \\ \hline Titanium-Based Alloys & & \\ \hline Ti-TiB [101; 102], Ti6 Al4V [88; 103; 104], Ti64 [105] & ✓ & \\ \hline TiNi [61], Ti6 Al4V [91; 103], Ti-43 Al-9V-0.5Y [64], Ti6 Al7 Nb [106] & & ✓ \\ \hline NiTi [62], Ti6 Al4V [107; 108] & ✓ & ✓ \\ \hline Nickel-Based Alloys & & \\ \hline IN625 [109], IN718 [69; 110; 111], Ni10 Cr6W1 Fe9 Ti11 [112] & & ✓ \\ \hline Hastelloy X [66], nickel-based superalloy [113] & ✓ & \\ \hline IN718 [114] & ✓ & ✓ \\ \hline Aluminum-Based Alloys & & \\ \hline AlMgScZr [115], AISi10 Mg [116; 117] & ✓ & ✓ \\ \hline Al-33 Cu [75], AISi10 Mg [77] & & ✓ \\ \hline AlSi10 Mg [118; 119; 120] & ✓ & \\ \hline \multicolumn{4}{c}{Other alloys/elements} \\ \hline Ta [82], CoCrMo [121], Nb [84], Cu10 Zn [122] & & ✓ \\ \hline Zn-alloy [83], Ag [85], CoCr [123] & ✓ & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Combinations of optical techniques using an X-ray source for various customized or less investigated LPBF-printing powder materials. Figure 22: Example of OM imagery provided by [90] showing several types of microstructural defects at the (**a**) front, (**b**) argon outlet, (**c**) back, and (**d**) argon inlet locations of an SS316L part. **Digital single-lens reflect (DSLR)** cameras combine optics and the single-lens reflex camera mechanism with an image sensor to record the images. A DSLR uses the reflex scheme, where light travels through several lenses and is then reflected by a mirror that sends the image through a camera prism. The reflex design scheme is the main difference between a DSLR and other digital cameras. The advantages of DSLR cameras are that they are widely available and provide better image quality than other cameras. The disadvantage of DSLR cameras is that they are relatively expensive. A comparison of the CAD model and DSLR imagery of an SS316L LPBF-printed part is provided in Figure 24[125].