Ordering the order: a survey of optical document security features
Rudolf L. van Renesse · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1995
Until the appearance of the color copier, security features were optically invariable devices (OIDs). Nowadays, deceptive color copies of OIDs can be made. Optical variability however, is unattainable by the color copier. Optically variable devices (OVDs) are either based on specular reflection (metallic foils), diffraction (holograms, etc.) or interference (thin films, Bragg structures, etc.). A classification of OIDs and OVDs is presented. It is proposed that the degree of microstructural order of an optical feature is an approximate measure of its practical value for document security.