## 1 Introduction
### Previous Approaches of Defect Detection by Measuring the Powder Bed Surface  
Several scientific approaches address sensor-only concepts of measuring the powder bed surface in PBF-LB/P by generating 3D data as part of laser profilometry and fringe projection, as well as optical coherence tomography (OCT), contact image sensor (CIS) application, and microscopy. In [17], [PERSON] et al. managed to successfully integrate a laser profilometry system by [PERSON] (Osaka, Japan) into a DTM Sinterstation 2000 using Duraform P12 powder material. The profilometer was mounted at the coating system and required N\({}_{2}\) cooling due to temperatures of up to 170 \({}^{\circ}\)C. Within the paper, several issues were investigated, such as measuring the powder layer quality in different conditions, highlighting the impacts of adapting the recoating speed, preheating temperature, and particle shape. This approach enabled the calculation of the effective layer thickness (ELT), the evaluation of layer density, and the measurement of curling from 70 \(\upmu\)m to 350 \(\upmu\)m. Although the work showed an effective way of in situ process monitoring in PBF-LB/P by detecting geometrical variations with sub-micrometer vertical resolution, only sub-areas of the powder bed could be observed due to the scanning range of the profilometer and the mounting position. Further, no coherences were investigated in terms of thermal fields and temperature distributions on the powder bed surface, leading to defects such as curling. Considering the results presented in [30] by [PERSON] et al., a laser profilometry sensor and a thermal camera were installed at a fused deposition modelling (FDM) system. By automated adjustment of material flow and build speed, three types of defects were mitigated: low layer times, underfill, and overfill. The control system handled the build parameters to compensate for height deviations from \(-\)100% underfill to 50% overfill and low layer times of 15 s. No detailed information about the profilometer or thermal camera system was given. [PERSON] et al. [31] introduced a further approach for curling defect analysis in PBF-LB/P by investigating the feasibility of deploying a fringe projection system to measure the powder bed surface of an EOS P100 PBF-LB/P machine. The sensor was mounted outside the machine. This enabled large areas of the powder bed to be covered with an xyz resolution of 75 \(\upmu\)m. In this approach, an effective method was presented of controlling curling defects by tuning process parameters. A lack of information on data acquisition time and surface data analysis raised questions regarding implementation as a real-time process monitoring system. In [32], [PERSON] et al. investigated the influence of varying laser power on the penetration depth in selective laser sintering by applying optical coherence tomography (OCT). OCT enables subsurface imaging and defect detection based on low coherence interferometry by encoding wavelength depth information with an axial resolution of between 1 \(\upmu\)m and 20 \(\upmu\)m. The aim of this paper was not the implementation of a real-time monitoring system, but rather on observing subsurface melt pool changes and defects such as increasing melt pool depth due to excessive laser power. A further approach to surface texture analysis and subsurface defect detection in PBF-LB/P by using OCT was presented by [PERSON] et al. in [33]. A spectral domain phase-sensitive OCT system (PhS-OCT) was used to quantify simulated defective regions with graduated porosity embedded in fully dense regions. Loose powder detection and differentiation between densities of \(\pm\)5% at a sub-surface depth of 350 \(\upmu\)m was achieved. Further optical measurements by focus variation microscopy (FVM) were implemented to compare the ability of OCT to detect surface profile and areal texture with that of FVM. A simple and cost-efficient solution of scanning the powder bed surface in PBF-LB/P was presented by [PERSON] and [PERSON] [34].