Photolithography equipment manufacturers are continuously developing machines with ever increasing resolution and wafer throughput. The development has shown that previously less dominant performance limiting factors are increasingly impacting the conformity between the projected image and the object plane. It is evisaged that active optical elements are required for the correction of these non-conformities. This paper introduces the concept of active transmissive optical element shape control to improve focus whilst minimizing the effect of stress-biregringence and overlay.@en