Abstract: The position of the focus of a laser beam arrangement relative to a reference surface is determined by (A) irradiating a laser beam on the reference surface, (B) measuring the intensity of reflection light generated by the surface due to the laser beam, wherein in (C), (A) and (B) are repeated for a plurality of different, effective distances between the surface and the arrangement, and (D) that effective distance is determined as the focal distance representative for the position of the focus for which the measured or an interpolated intensity of reflection light is extremal, if, (E) every time (A) and (B) are performed, the arrangement and surface are moved relative to each other so the laser beam sweeps across a surface area of the reference surface that has a stronger direct reflection and weaker diffuse reflection, and sweeps across a structure formed in its interior that has a stronger diffuse reflection and weaker direct reflection.