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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.
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