A Security Printing Method by Black Ink Hiding Infrared Image

Abstract

In the multi-color printing, the four inks used are cyan, magenta, yellow and black, which have different response in the near infrared spectral area, in which the black with CMK color inks have no response while the black with K color ink has a good response. But two kinds of black are both visible and equivalent under daylight. Based on this principle, this paper introduces a new method of security printing by K ink hiding infrared image. To hide infrared image in source image, the Yule-Nielsen modified spectral Neugebauer equations is employed, the Yule-Nielsen parameter is found based on a least squares regression over a training set of spectral measurements, then the source image is color-separated with the mask which is the security image. The security image must be a grey-scale map. The security information is stored in the K channel of the color separation image which is printed. In daylight the visual effect of printed image is the same as that of source image, but the security image is detected by IR camera. Finally, a series of experiments are performed on HP Color LaserJet CP2035 printer. Experimental results show that the proposed method is promising and feasible for preventing from falsifying presswork.