As Figure 5 shows, even a slight shift in focus position causes a significant change in power density. This in turn directly influences the quality of the weld seam, which, especially in sensitive welding processes, means that only very narrow tolerances can be permitted for changes in power density. Here too, the crux of the issue are the high powers involved.
For this purpose, Ophir developed a non-contact measurement method based on the Rayleigh scattering, which describes how electromagnetic waves scatter as they are deflected off particles in the air that are smaller than the radiation's wavelength, e.g. oxygen or nitrogen molecules. The electric field of the laser radiation induces an oscillation in the dipole molecule at the laser's frequency, thus leading to elastic scattering at that same frequency. The scattered laser light is imaged from the side using a telecentric lens assembly on a CCD or CMOS camera. Each individual pixel in a single line of the CCD camera detects the scattered light as a measuring point of intensity in the beam profile. From these measurements, and using an integrated software with high accuracy, it is possible to calculate beam and beam-quality parameters according to ISO 13694 and ISO 11146 standards, including focus diameter, focus position, divergence, ellipticity, M² (1/k) and beam parameter product (BBP).
Since the systems of the BeamWatch® product line are based on this measuring principle, they allow for real-time monitoring of the beam profile. This makes changes to the focus visible. A comparison of different measurement methods recently showed that non-contact measurement technology, although not yet documented in the ISO standards, is fully ISO compliant. Users thus obtain ISO-compliant measurement results that are reliable and repeatable, since there is neither influence on the beam caused by the measuring instrument nor deterioration of the measuring instrument caused by the beam. In the manufacturing process, these measurements can also be combined with the aforementioned power measurements: As a rule, the laser power is briefly tested and then, after a specified production period,