Detection and deterrence of counterfeiting of documents having a characteristic color

Abstract

A method and apparatus is described for detection and deterrence of counterfeiting that permits one to make legitimate color copies without introducing visual artifacts or experiencing substantial processing delays. The scheme detects a characteristic color of frequently counterfeited documents and alters the characteristic color in the copies. The scheme exploits the fact that conversion between color spaces is frequently accomplished using a color Look-Up Table (LUT). Because it represents no computational burden to alter the values in the LUT, we can efficiently modulate slightly the value of the characteristic color in a way that depends on the value of the counter. For example, at first even banknote green is rendered correctly. However, as the page uses more and more of the ink, a modulation function causes conversion to a darker value than the actual green requested. After using a significant portion of the ink required to render a banknote, the modulation function can switch again, and cause a lighter than requested green to be returned. The net effect of using this modulation function is that distinct bands will appear when an image of a banknote is printed. These visible marks appear on the page independently of the orientation of the note. If we consider a general image with a lot of green, we find that there is no visible difference between versions of the image rendered with and without the modulation scheme. The reason for this is that although many pixels of the general image may meet the criterion of being classified as “banknote green” in the LUT, they are dispersed and a minor increase or decrease in the value is not as noticeable as when a large amount of that ink is used in close proximity, as is the case on the back of a banknote.