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 and gray component replacement (GCR) theory, this paper introduces a black component generation method for black ink hiding infrared security image. Firstly, we establish a color lookup table for GCR and find the substitution relation between K and CMY. Secondly, the source image is color-separated with the mask which is the security image. The source image is firstly converted to CMY color format data words. The black value of each pixel for security image is detected and treated as GCR value of the corresponding pixel for source image. Then CMY values are found in GCR LUT and subtracted from three primary CMY values. And an adjustment coefficient is employed for color correction. Thirdly, the color separation image 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 black ink hiding IR security image.
Determination of the effect of the degree of gray component replacement on color differences
