Traditional security systems are becoming more and more useless as the digital technology moves on. Digital Graphical Systems and their increasing technology are now seriously forcing the security printing industry to start thinking 'digital'. Terms like digital signal and image processing, trail tracing, spread spectrum and digital watermarking, etc. are steadily moving into the security printers vocabulary. Digital technology should not only be seen as a threat but merely as an opportunity for security printers. This same technology can be used for a better implementation and integration of not just the tradition non-variable security feature but especially variable security features into security documents. Depending on the field of application these digital security features can be integrated into a design using localized spectrum, spread spectrum or full spectrum technology. In this presentation some applications will be explained and it will be shown how these technologies can be used to provide Security Solutions for the next Millennium.
Digital graphical systems, color copiers and their increasing technology have forced the security printing industry to drastically change the ideas about security structures. Complex guilloches, screen traps, etc. do no longer provide sufficient protection and techniques based on limitations of these digital systems, like limited color reproduction or loss of certain gradation levels, are becoming more and more useless as the technology in both hard- and software moves on. A closer study of the problem immediately reveals that for both human perception and digital scanning we are dealing with signal-processing in two dimensions. The different frequency bands that are important in security design are explained and security structures are classified according to these bands. It appears that in security design we are dealing with four frequency bands: visual band, sample band, alias band and upper band. An important observation is that printed security structures, carried out in the frequency band of human perception (i.e. visible structures) do not protect against digital scanning, so protective structures must be of a higher frequency level. Some machine detectable copying security structures are treated according to their different frequency band classification. The basic idea behind the treated structures is to add to a security design some kind of modulation in the upper part of, or above the visual frequency band. With both screen angle modulation (SAM) and micro screen angle modulation ((mu) SAM) the orientation of screen lines is used as modulation, whereas with sample band image coding (SABIC) essentially we have some kind of 'screen-in-screen' modulation. Both techniques satisfy the two major conditions that have to be fulfilled for effective protection of security documents, i.e. they have to be easy to print and easy to detect.
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.
A method for generating half-tone images on binary (i.e., paper and ink) systems using different (frequency-) modulation techniques is described. Security gratings and patterns are designed to generate 'alias' images or detectable codes, due to undersampling by the digital scanner.
Conference Committee Involvement (2)
Optical Security and Counterfeit Deterrence Techniques VI
18 January 2006 | San Jose, California, United States
Optical Security and Counterfeit Deterrence Techniques V
20 January 2004 | San Jose, California, United States
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