## 3 LPBF Sensing Techniques
### Thermal Techniques
The thermal features for each voxel for spatial and temporal domains were extracted using SWIR imaging during the LPBF printing process. The thermal features extracted from the SWIR data for each voxel could indicate whether the voxel contains pore defects. SWIR tracked the thermal history of different voxels during the LPBF process to determine the location and characteristics of defects in another study [157]. **Single-camera two-wavelength imaging pyrometry (STWIP)** systems utilize a high-speed camera to monitor melt pool dynamics and estimate pool morphology and temperature. In STWIP systems, the melt pool is imaged at two different wavelengths using two photodiodes, and the temperature is estimated using the Imaging Pyrometry method. The estimated temperature is created by extracting the intensity ratio of the melt pool from the STWIP system. STWIP circumvents potential issues caused by multiple camera sensors, such as errors induced by different imaging sensors and the relative location of the melt pool depending on the location of the sensor. [PERSON] and [PERSON] [158] utilized an STWIP system to measure melt pool temperature profiles in NiCr alloy LPBF-printed fatigue bar samples. Disparate temperature profiles in the fatigue bars could indicate potential process anomalies or part defects. An investigation to estimate the melt pool temperature, morphology, and intensity profiles of IN718 LPBF-printed specimens using STWIP was reported [159]. The melt pool temperature signatures collected from the STWIP system were correlated with ex situ microscopy-characterized melt pool depths and average grain lengths. The fusion defects were not observed in the tracks comparing the optical microscopy imaging and STWIP data (see Figure 28). [PERSON] et al. [160] mapped the STWIP-measured melt pool to detect process anomalies and part defects in IN718 LPBF-printed specimens. Figure 27: IR camera imaging highlights the location and shape of various defects [155]. **Infrared pyrometer sensors (IPS)** monitor light emitted from the LPBF melt pool and associate specific signatures with certain printing defects. Infrared pyrometer sensors have a fast response time and are dependably repeatable. It is a non-destructive and physically small sensor. However, they are affected by smoke and dust particles. IR pyrometer and photodiode sensors captured the SS316L melt pool thermal emission wavelengths [161]. The thermal signatures obtained through the IR pyrometer correspond to various melt pool modes and can hint towards pore formations and predict defect creation. [PERSON] et al. [162] collected pyrometry measurements during the LPBF printing process of Inconel. A correlation between the pyrometer signal and pore distribution was established, and a probability curve for a given pyrometer signal was derived. [PERSON] et al. [163] estimated the melt pool temperature of the AlSi10 Mg LPBF-printed part by capturing thermal emission Planck (TEP) process signatures. The individual TEP signatures were obtained using two photodiodes to collect two light intensity measurements. A system monitored the LPBF printing process of SS316L single tracks, increasing laser power and velocity using high-speed infrared diode-based pyrometry optical imaging signals [98]. The pyrometry signal showed distinct signatures of conduction-to-keynhole mode transitions. A probability correlation predicting pore formation was derived. **Differential scanning calorimeters (DSC)** are thermoanalytical devices that measure a material's specific heat capacity change. DSC measures the changes in heat flow (exothermic or endothermic) during metal alloys' heating or cooling processes. These measurements determine the thermodynamic temperatures associated with phase transitions such as solidification and melting [164]. DSC is considered relatively inexpensive but reproducibly difficult. Regarding LPBF printing, DSC has been used to quantify phase transformation temperatures, behaviors, and solidification temperature ranges.