The 'hidden image' effect in security holograms and its personalization by laser demetallization

摘要

The given work investigates principles of recording, calculation, and security aspects of 'hidden image' effect in digital holograms that are intended for security applications. Dot-matrix and image-matrix technologies of optical recording can be widely used for recording protective holograms with such type of security features. When a collimated laser beam falls on and then is reflected from the section of holograms, containing a protective 'hidden image' element, a graphic image can be seen in the projection of diffracted light on the frosted screen. The present work also discusses a method of personalizing the 'hidden image' effect with the help of laser demetallization. In this way the hidden image can be individualized for each hologram sticker and contain additional information such as a number, text or logotype. The attractiveness of this method is in the possibility of achieving a considerable increase of the protective characteristic of holograms and incorporating additional variable information in them, as well as in providing both visual and automatic ways of checking authenticity of a hologram.