## 3 In Situ Sensors Used in the L-PBF Processes
### Temperature Sensors  
The fast cooling rate and temperature fields in the meltpool are critical to the quality of parts manufactured using the L-PBF process [88]. Temperature changes might cause metal phase variations that can influence the mechanical properties and induce defects  
Figure 6: Schematic illustration of acoustic emission sensor location: (**a**) structure-borne acoustic emissions [79], (**b**) airborne acoustic emissions [81].  
Figure 7: Schematic illustration of different vision sensor locations for in situ monitoring of the L-PBF process. (**a**) Represents a coaxial setup where the camera can follow meltpool locations [86]. (**b**) Off-axis location of the camera, where the camera is positioned on the side window of the chamber [87].  
in the fabricated parts. Monitoring the L-PBF process temperature while manufacturing the part can help understand the parts' thermal history [67]. The meltpool temperature profile influences the microstructure of the L-PBF parts and affects the defect formation in the parts [54]. Temperature sensors can be categorized into contact and noncontact devices. Contact devices such as thermocouples can be used to provide an estimate of the overall build platform temperature. However, special drilling holes must be machined in the build plate to install the thermocouple sensors, as shown in Figure 8[89]. thermocouples have a slow response time and might not be suitable for monitoring the small scale of meltpool. Noncontact thermal sensors are more common to use during L-PBF processes monitoring, especially if the main goal is to control the performance.  
Noncontact temperature measurement depends on electromagnetic radiation, as they are directly related to the fundamental nature of the process [53]. The key issue is to find the appropriate sensor that can detect the radiation from the processing zone without detecting back radiation from the laser beam itself. The L-PBF process irradiates a broad frequency of spectrum (UV to IR), and photodiodes can be employed to capture the radiation emitted during the melting process [91]. However, photodiodes are single-point sensors and can only provide little information about the spatial distribution of temperature [92]. Optical emission spectroscopy has been implemented in laser welding and directed energy deposition (DED) processes to monitor the process by correlating the collected with the defect formation [93]. If the spectrometer is installed coaxial to the laser beam, it can collect information from any point on the build plate [94]. Spectrometers were used to evaluate the temperature during the L-PBF process; however, compared to pyrometers, it was found to be slower in response and reproduced a large amount of data that might be hard to interpret [95].  
A pyrometer measures the temperature of an object or surface from the thermal radiation emitted. It is similar to photodetectors because they absorb energy and measure the wave intensity at any wavelength. Although pyrometers are limited in their spatial resolution, installing them coaxially would also resolve the issue of monitoring the meltpool temperature over the total distance of the powder bed. Thermal cameras are different from visible light cameras as they observe infrared radiation over visible light. The thermal or infrared cameras capture the infrared energy from the meltpool and create an image  
Figure 8: Different combinations of temperature sensors: a thermocouple drilled in the baseplate, an off-axis thermal camera to detect overall powder bed temperature and heat distribution, and a coaxial thermal camera to detect meltpool temperature [90].  
related to temperature. The resolution of thermal cameras is usually lower than that of visible light cameras as the thermal detector sensors are larger in size. However, the setup and position of an infrared camera would be similar to that of the visible light cameras.