1. School of Optometry and Vision Sciences, University of Cardiff, Redwood Building, King Edward VII Avenue, Cathays Park, Cardiff CF10 3NB, Wales, UK, European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, 38043 Grenoble, France, School of Conservation, Esplanaden 34, DK-1263, Copenhagen, Denmark, National Museums Liverpool, Laser Technology, Conservation Centre, Liverpool L1 6HZ, UK, and School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UK