is moved away from the region of intersection of the two beams 20 and 21 and an unexposed region of the layer 8 is moved in as mentioned earlier for multi level devices this movement must be carried out with precision in order that accurate overlay can be achieved during lithography following this it may be necessary to readjust the piezo electric transducers 16 supporting the mask 13 in order that the mask 13 remains parallel to the recording layer 8 and at the same distance from the layer 8 in the case where the direction of translation of the recording layer 8 lies in the plane of incidence of the reference beam 21 at the layer 8 the aperture 23 should ideally be shifted longitudinally that is along the optical axis of the relay lens system 24 in order that the image of the aperture remains accurately focussed onto the recording layer 8 the translation of the recording layer 8 with respect to the mask 13 may alternatively be achieved by a displacement of the mask 13 however in this case the object and reference beams 20 and 21 must preferably be displaced as well in order to ensure good reproducibility of exposure energy from exposure to exposure the mechanical arrangement required for implementing this which is not shown in fig2 is more elaborate making this approach less desirable the mechanical shutter 25 is activated again and the fresh part of.