In recent years, laser welding has found increasing use in various industries. One reason is the enhancement of economic efficiency compared to competing techniques. The realization of this aspect necessitates automated process control strategies. One method of on-line process monitoring during laser beam welding is the use of spatially resolved detectors. Normally CMOS cameras are preferred to CCD cameras because, compared to CCD cameras, they have a higher dynamic and direct pixel access, which results in high frame rates when working with subframes.
The main difficulty of camera based process monitoring during welding of steel and aluminum is the optical resolution of the large temperature range from the melting point to the evaporation point. The temperature range that can be observed with a camera, on the one hand, depends on the dynamic and the spectral response of the camera and, on the other hand, on the detection wavelength. However because of disturbances by emissions from the excited metal vapor above the work piece not each wavelength range is useful for the process monitoring.
Experiments for process monitoring during laser beam welding with a CMOS camera in different spectral ranges show the influence of the selected wavelength on the detection result and, therefore, also on the significance of the different indicators for the process control. Extensive spectroscopic investigations of the emissions of the welding process give information about the feasible spectral ranges. The result is a kind of measurement instruction for the camera based process monitoring.