An identification card(ID card)is one of the most important devices for physical security systems. Recently, spread of color printers and scanners has increased the risk of abusing ID card copies. Therefore, we have introduced an information embedding method on digital watermark using color difference for printing images. When the printing resolution is larger than 300(dpi), it is hard to sense the color difference of two adjacent pixels for human eyesight. Using this characteristic, digital

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... ermark can be embedded in the printing image as invisible fine color difference modulation. Additionally, we have developed a technique to apply this digital watermark to thermal transfer printing. Evaluations of invisibility and restoring show the good results. Further simulation experiment indicates that constructing sub image information by machine readable code is promising.