A process for producing an interface unit and also a group of such interface units are specified. The interface unit exhibits a first reference surface for beaming in radiation, a second reference surface for emitting the radiation, and an axis extending in the direction from the first to the second reference surface. The production process comprises the steps of setting an optical path length of the interface unit between the first and second reference surfaces along the axis and the fixing of the set optical path length of the interface unit. The optical path length of the interface unit is set in such a way that radiation of a defined numerical aperture beamed in at the first reference surface exhibits a focus location that is predetermined with respect to the second reference surface in the direction of the axis. A precise and uniform focus location with respect to the second reference surface is obtained.
