Method for monitoring the size variation and the focus shift of a weld pool in laser welding

In a method for monitoring the size variation and the focus shift of a weld pool simultaneously or either the size variation or the focus shift of a weld pool in a laser welding wherein a laser beam is transmitted through an aperture which limits a size of a laser beam, said transmitted laser beam is focused with at least one lens with some chromatic aberration but minimum spherical aberration on a workpiece, a thermal radiation from a weld pool is measured through the at least one lens and through the aperture or any other aperture limiting a size of a weld pool, the thermal radiation and the laser beam reflected from the workpiece is separated with a dichromatic mirror, the thermal radiation is split with dichromatic beam splitters into three spectral bands, each spectral band of the thermal radiation is filtered with a narrow band-pass optical filter each narrow band-pass filtered thermal radiation is detected with a single-element detector as an electrical signal, electrical signals are sampled as detector signals from the single-element detectors digitally, a plurality of digitized detector signals are processed as processed signals so that the processed signals generate a digital signal, a size variation signal, and/or another digital signal, a focus shift signal, and a size error signal representative of the size variation of a weld pool and/or a focus error signal representative of the focus shift of a weld pool is generated from the size variation signal and the focus shift signal, the improvement comprising the steps of:

• selecting optimized wavelengths of said spectral bands experimentally so that said focus shift signal is generated independently from a laser power variation;
• and
• compensating a focus shift dependence in size variation monitoring experimentally so that said size variation signal is generated independently from a focus shift of said weld pool.