Holographic optical security systems [electronic resource] : proceedings, ECO4, 14-15 March 1991, The Hague, The Netherlands / William F. Fagan, chair/editor ; sponsored by the congress of EPS--European Physical Society, Europtica--the European Federation for Applied Optics, SPIE--the International Society for Optical Engineering ; cooperating organizations, ANRT--Association nationale de la recherche technique [and others].