The study of watermark bar code recognition with light transmission theory
Fan Zhang, Tiegen Liu, Lianxiang Zhang, Xiaojun Zhang · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2004
Watermark bar code is one of the latest anti-counterfeiting technologies, which is applicable to the security of various financial documents, especially banknotes. With the machine-readable watermark bar code, we can not only distinguish the genuine from the fake, but also recognize different face values of banknotes. So it is necessary to develop an effective method to read it. With watermark bar codes embedded in euro banknotes as samples, we design a system for watermark bar code recognition based on light transmission theory. An 808nm LD serves as light source. By receiving the transmitted light from behind the banknote with a high-speed photodetector, light transmission curves in different parts of different face value euro banknotes are obtained and analyzed in order to get the unique characteristic of watermark bar code. The correlation coefficient is then introduced for two reasons. For one thing, the high correlation coefficient derived from the light transmission curves shows the coherence of vertically placed watermark bar code area in different horizontal parts of the same face value banknote, which testifies the consistency and repeatability of the experiment. For another, the low correlation coefficient shows the irrelevance of watermark bar code area and other area of banknotes. What’s more, it also shows the difference of watermark bar code area of different face value banknotes. As a result, the machine-readable watermark bar code is made the unique characteristic of certain face value banknotes, which can distinguish them from both the counterfeit ones and other face value genuine banknotes.