Apparatus for determining geometrical parameters of a laser beam includes an optical system, a device for output coupling radiation, a beam diagnostic device, and a reflector element. The optical system focuses the laser beam into a processing region. The device for output coupling radiation couples out radiation that runs through the optical system in a direction opposite to a direction of the laser beam. The reflector element has a first surface which is partially reflecting and curved, where the curvature is equal to a mean curvature of a wave front of the laser beam in a positioning region of the reflector element.