Optically and machine-detectable copying security elements

摘要

Abstract: Since their introduction at the SPIE-conference 'Holographic Optical Security Systems' in The Hague in 1991, the printed security structures like SAM (screen angle modulation) and FREM (frequency modulation) have proven their value in the protection against digital color-copiers. Over the past years the SAM security structures have been used extensively in security documents, not only as a anti-copying structure but increasingly as an optically or machine detectable security feature, using respectively a SAM-screener and Laplace-filtering. This last feature of optical- or machine-detectability is becoming of more importance due to the fact that publicly available image processing software is getting more and more advanced. Once a document has been scanned into a computer, the use of digital filtering techniques may well overrule, imitate or simulate many security features. By increasing the frequency of the SAM- structures to a higher level, leading to so-called $mu@SAM (micro SAM) or the use of sample band image coding (SABIC), structures are obtained that cannot be reproduced with standard equipment. Moreover, these structures have been designed such that their presence in a security document is easily checked, either by simple optical means or during scanning by a simple image processor. For different application areas different techniques have been developed such as symmetry-breaking ($mu@SAM screener), (one-sided) Laplace-filtering, envelope-detection and optical Fourier domain-filtering. !14