Desktop software and modern printers/scanners no longer use this, they use something based on watermarking. I suspect it may have something to do with the fine

Desktop software and modern printers/scanners no longer use this, they use something based on watermarking. I suspect it may have something to do with the fine pattern of parallel lines, which you will find on almost every currency. I suspect this patent might have something to do with it, but the math is beyond my comprehension: http://www.google.com/patents/US7720249 http://www.google.com/patents/US7720249 This patent was referenced by the above patent and seems even more applicable: http://www.google.com/patents/US6449377 http://www.google.com/patents/US6449377 (references patents 5583614, 4723149, 5633952, 5640467, and 5424807 which may be good further reading). I also remember someone mentioning that they reverse engineered the currency detector code in Photoshop(?) and got something like "fourier mellin transform, followed by checking specific values in an array".