Patent ID: 7863543

Claim:
A method of preparing an apparatus for material processing by producing optical breakthroughs in or on an object, said apparatus comprising a variable three-dimensionally acting focus adjustment device for focusing pulsed processing laser radiation on different locations in or on the object, wherein a contact element, which is transparent for the processing laser radiation and is to be placed on the object, is mounted to the apparatus, said contact element having, on its side to be placed on the object, a contact surface and, located opposite, an entry surface for the processing laser radiation, each of the contact surface and the entry surface having a known shape; prior to processing the object, the position of the entry surface with respect to the focus adjustment device is determined by irradiation of laser radiation on the entry surface, by focusing measurement laser radiation near or on the entry surface by the variable focus adjustment device, an energy density of the focused measurement laser radiation being too low to produce an optical breakthrough, and the focus of the measurement laser radiation being adjusted in a measurement surface which intersects the expected position of the entry surface, the method comprising: a) confocally detecting radiation scattered back or reflected back from the focus of the measurement laser radiation, b) detecting a position of points of intersection between the measurement surface and the entry surface from the confocally detected radiation and an assigned setting of the variable focus adjustment device, c) repeating the confocally detecting step, if necessary, with a modified measurement surface until a determined number of points of intersection have been detected, d) determining the position of the entry surface from the position of the points of intersection and the previously known shape of the entry surface.