[1] Kings Coll London, Randall Div, London WC2R 2LS, England
[2] Univ Cambridge, Dept Engn, Cambridge CB2 1PZ, England
[3] Comp Vis Consulting Ltd, Woking, Surrey, England
[4] NIH, Cell Biol & Metab Branch, Bethesda, MD 20892 USA
[5] Univ Jena, Inst Phys Chem, D-6900 Jena, Germany
[6] Inst Photon Technol, Jena, Germany