Benchmarking Coaxial and Angular Optical Emission Spectroscopy With Recommendations for Reliable Compositional In Situ Monitoring During Laser Powder Bed Fusion

ABSTRACT Real‐time insight into local chemistry is critical for reliable part quality in additive manufacturing, especially laser powder bed fusion (PBF‑LB/M), where rapid thermal cycles and localized evaporation can undermine part performance. Optical emission spectroscopy (OES) offers non‑intrusive, in situ plume monitoring, but detection geometry strongly affects precision. We address this through geometry‐ and physics‐based approaches and spectral measurements with angular (off‐axis) and coaxial (on‐axis) configurations, exploring how detection configurations influence signal stability, spatial consistency, and chemical precision, by varying laser power, integration time, scan position, and pattern on an Nd‐Fe‐based hard magnet alloy. By quantifying wavelength‐dependent efficiency and plume anisotropy, we establish a decision framework to choose the most suitable in situ OES setup for applications ranging from defect detection to reliable high‐precision chemical mapping. Coaxial OES offers superior detection sensitivity for low‐ionization‐energy elements like Nd, due to the forward‐peaked emission profile and high detection efficiency, enabling quantitative detection below ±0.2 at.% under optimized conditions. Angular OES is easier to integrate and reveals lateral plume structure and processing defects, but yields two‐fold lower compositional accuracy under identical conditions. These findings provide a foundation for selecting OES geometries in PBF‐LB/M and represent a clear step toward robust, material‐sensitive closed‐loop monitoring in industrial PBF‐LB/M.