The invention is related to a method for producing a collimator comprising an X-ray transparent substrate. The innovative method comprises the steps of: forming a slit in the substrate, wherein the slit has first and second side walls; filling the slit with an X-ray absorbing material so that the absorbing material extends from the first side wall to the second side wall; removing part of the X-ray absorbing material thereby forming a second slit that extends from the remaining absorbing material to the second side wall; filling the second slit with X-ray transparent material; removing part of the X-ray transparent material, thereby forming a third slit extending from the remaining transparent material to the second side wall; and finally filling the third slit with X-ray absorbing material. In accordance with the present invention a collimator can be produced having any desired aspect ratio.