Evaluation of Scanner-Based Focus Finding Methods on Rough Surfaces

摘要

Automated laser beam focus positioning for precise work piece alignment is highly important in laser processing. Ideally, the laser used for processing of the work piece should also be used for fo-cus finding. For this a confocal camera can be used in front of a galvanometric scanner and objec-tive. However, in typical laser processing applications the roughness of the sample in combination with the laser spot size and the coherence of the laser provide a confocal camera image with a dis-tinct speckle pattern. This speckle pattern can impede the determination of the focal position with respect to the work piece, especially if simple evaluation algorithms are applied. In this work we in-vestigate the performance of several more advanced image evaluation methods on confocal camera images for work pieces of different materials, different roughness under normal and tilted incidence of the laser beam. We show that two investigated algorithms provide a positioning uncertainty that is well below the Rayleigh length of the laser beam.