Lasers used for welding applications radiate in the infrared spectra, which are not visible to the human eye.  The intensive fiber laser light radiate in the visible spectrum but invisible for humans, is especially dangerous to the eye. Fibre laser radiation penetrate through to the retina which can be destroyed irrevocably by relatively little radiation.
Misdirected laser radiation can come directly from the laser and threaten the eyes as a result of a faulty parameter setting, an opened cover, a displaced mirror etc. Other hazards include skin burn or inflammation from combustible materials as a result of misdirected laserradiation. The greatest hazard, however, usually stems from reflected laser radiation: the major share of the laser radiation is reflected by cold material first. To this we can add reflections of work piece edges, as a result of turbulence in the weld pool etc. 
Misdirected radiation and reflections must be blocked off. That is why the law stipulates that the laser beam and the work zone must be in an enclosure. Beyond that, all those present, and the machine operators in particular, should wear protective goggles that are appropriate for the laser radiation being used.  Fibre laser radiation are very dangerous to the eye and require special protective measures and approved safety goggles.