Particles, which contaminate the substrate during thin film deposition, are prone to cause irremovable defects and demand special attention in the field of high precision laser optics as they can lead to localized absorption and laser damage. In this contribution, we present a camera based fully in-vacuum device which continuously monitors the coating surface of a transparent test substrate under dark field illumination. We show the possibilities of this setup regarding the sensitivity to small particles (diameter 1 μm). As the first operational test of the particle monitor, singe layers of HfO 2 are grown on fused silica. By analyzing the evolution of the scattering intensity of the particles, we derive their position in the substrate-coating-system. Therefore, this in situ particle detection concept can deliver data on which process step is responsible for particle generation in multilayer films and aims to be a tool to minimize coating defects. © COPYRIGHT SPIE.