## 3 LPBF Sensing Techniques
### Thermal Techniques  
Thermal techniques utilize differences in temperature and light to assess part quality and detect defects. Thermal techniques can be implemented in situ to monitor the melt pool behavior in LPBF printing. Infrared and thermal imaging cameras (1400-3000 nm and 3000-15,000 nm wavelength range) and pyrometers are used. After solidifying the melt pool, they provide indicative information regarding the specific defects' location, type, and size. Other observed techniques, such as differential scanning calorimeters (DSC), can monitor the changes in heat flow during metal alloys' heating or cooling processes. The heat flow enables tracking temperatures associated with phase transitions to point out potential defect formations. **Thermal imaging cameras (TIC)** provide data on temperature variations concerning the subject's absolute temperature and surroundings. Typically, filters are applied to two high-speed cameras that capture two wavelength intensities in thermal imaging. The intensity ratio between the two images can be correlated to temperature values. TIC enabled the analysis of the melt pool dynamics, tracking ejected particles from the LPBF printing process, as shown in Figure 26[149]. **Infrared thermography cameras (IRTC)** capture images by detecting short IR energy wavelengths emitted by an object and converting them to temperature values. Defects are detected by locating disturbances in the temperature profile. [PERSON] et al. [150] utilized an IR camera to detect defects in LPBF components. A similar study monitored the melt pool characteristics of Ti6 Al4V, AlSi10 Mg, and NiTinol LPBF-printed parts using a high-speed NIR thermal CMOS camera [151]. Observations made using the NIR camera were compared with XCT measurements of the cut samples, indicating excellent compatibility between the actual defects and what was observed from in situ quality control. A forward-looking infrared (FLIR) camera was used to capture temperature measurements of the molten pool surface of SS316L [152]. Unstable melt track characteristics were observed using the IR camera, which showed spherical formations akin to balling. The evolution of the generated thermal footprint during the LPBF printing process using an IR camera to detect artificially created laser defects was evaluated [153]. The IRTC detected the typical micrometric defects inside the AM part. Artificially created micro-drilled defects in titanium alloy LPBF-printed acetabular cup prostheses using an FLIR camera were captured in a  
Figure 26: The surface temperature development during the LPBF scanning of an overhanging feature using TIC. Thermal imaging shows ejection particle ejection that can help indicate the location of potential defects [149]. study by [PERSON] et al. [154]. Subsurface defects were detected up to a depth of 1.3 mm in the part. The IRTC captured plume and spatter signatures using near-infrared (NIR) imagery. The intensity and dynamic nature of the plume and spatter during the LPBF printing process were caused by the melting instabilities, including defects such as lack of fusion, cracks, warps, or delamination, which were successfully captured [154]. A method for reliable characterization of the calorimetric signature of fatigue damage was developed using IRTC [25]. Figure 27 shows IR camera images capturing the location of defects and damage to the LPBF-printed part [155]. **Short-wave infrared imaging (SWIR)** cameras capture wavelengths between 900 and 2500 nm. The SWIR cameras are superior to alternative thermal cameras for capturing fast dynamic events at high temperatures, such as melt pool dynamics [156]. The radiance sensitivity of the SWIR wavelength band regarding the blackbody temperature is much higher than that of NIR, as well as medium-wave and long-wave infrared imagery cameras. [PERSON] et al. [156] captured the fast melt pool dynamics critical to the microstructure formation of SS304 LPBF-printed parts.