In the last years, parallel to the introduction of laser systems with high focusability the demand for quantitative monitoring systems for laser material processing has increased. Indeed, current laser systems with strong focusability and wavelengths of about 1 µm exhibit a high innovation potential in many application ranges, for example the possibility of adjusting the welding depth to even small material thicknesses. However, the usability of these advantages is limited because the suitable process windows are considerably constricted at increased welding speed. In some applications even more than for lasers emitting radiation of 10 µm. Therefore, a reliable real-time monitoring of thermal material processing is of vital importance. A new promising approach is the exploitation of the polarization-dependent emission characteristics of hot radiating surfaces to get detailed information about geometrical surface structures.
In addition to the dimensions of the melt pool, the raised welding bead or its underfill are important quality characteristics for industrial applications. Generally melt pool structures or seam imperfections result from the geometry and the dynamics of the capillary. This paper introduces a novel monitoring system to determine the three-dimensional keyhole or cutting front geometry based on the polarized thermal emission of the hot surface. Besides the possibility to ascertain the spatial inclination of capillaries, this sensor can be used to monitor melt pool structures or upcoming seam imperfections during laser material processing or to prevent the latter by an in-process control as well.