Abstract: A machining head of a laser machining apparatus comprises focusing optics which focus laser radiation. A measuring device measures changes of the focal length of the optics. The measuring device has a light source for generating measuring light different from the laser radiation. A light-exit window for the measuring light is imaged, with assistance of the focusing optics or a part thereof, onto a reflecting reference surface arranged to be stationary relative to the exit window and the focusing optics. A light sensor detects measuring light that has emerged from the exit window, has passed through the focusing optics or the part thereof, was reflected on the reflecting surface, and has again passed through the focusing optics or the part thereof. An evaluating device ascertains the change in the focal length of the focusing optics from measuring signals made provided by the light sensor.

Abstract: A machining head of a laser machining apparatus comprises focusing optics which focus laser radiation in a focal spot. A measuring device measures changes of the focal length of the focusing optics. The measuring device has a light source for generating measuring light that is different from the laser radiation. A light-exit window for the measuring light is imaged, with the assistance of the focusing optics or a part thereof, onto a reflecting reference surface which is arranged to be stationary relative to the light-exit window and the focusing optics. A light sensor detects measuring light that has emerged from the light-exit window, has passed through the focusing optics or the part thereof, was reflected on the reflecting surface, and has again passed through the focusing optics or the part thereof. An evaluating device ascertains the change in the focal length of the focusing optics from measuring signals made provided by the light sensor.