Security is an important concern in the realm of documents and digital production and/or reproduction of same. Known digital image printing/copying systems produce documents of such high quality that a need has been identified to prevent effective printing/copying of certain documents such as high-value printed items including tickets, financial instruments, security passes, and the like. Known techniques include printing the original document in a manner such that it includes a digital “watermark” using only conventional paper and toner or ink. A digital watermark in a broad sense can be defined as information, for example one or more letters, words, symbols or patterns, that is at least partially (preferably fully or at least substantially) hidden in a printed image under normal viewing conditions but that is more clearly discernable under certain specialized viewing conditions. Unauthorized reproduction of documents including such digital watermarks typically degrades or obscures the digital watermark, which can aid in detection of counterfeit documents.
A fluorescence mark is one example of a known digital watermark. Methods and systems are known for including fluorescence marks in printed documents using conventional papers (e.g., ordinary “copy paper” or “printer paper”) and ordinary inks/toners (e.g., CMYK ink/toner), specifically by using metameric colorant mixtures. Under visible lighting conditions [e.g., electromagnetic radiation wavelengths of about 400-700 nanometers (nm)], the different colorant mixtures that are printed on respective adjacent portions of the paper together define an overall printed document region that appears substantially uniform in color. Under ultraviolet (UV) lighting (e.g., electromagnetic radiation wavelengths shorter than about 400 nm), these different colorant mixtures exhibit different UV absorption and, thus, different suppression of UV fluorescence of the optical brightening agents used in conventional printing/copying papers such that the region printed with the colorant mixture that suppresses less of the substrate fluorescence appears as a lighter/brighter region while the adjacent area printed with the colorant mixture that strongly suppresses substrate fluorescence appears as a darker region. These contrast variations under UV lighting are used to create watermark-like patterns, e.g., numbers, letters, symbols, shapes.
An example of this is shown in FIG. 1, wherein a colorant mixture “B” is selected and applied to patch area BP which, in this example, is shaped as the alphanumeric symbol “0”. Further, a colorant mixture “A” is selected and applied to patch area AP arranged here in substantially close spatial proximity to patch area BP, and thereby providing a background around patch area BP. The patch areas AP and BP together define a fluorescence mark region FMR. Both colorant mixture A and colorant mixture B are comprised of one or more suitably selected colorants, but colorant mixtures A and B are different mixtures. Each colorant mixture A or B may be, for example, either a single CMYK colorant or any mixture of CMYK colorants. In the illustrated example, colorant mixture A will be selected so as to provide greater substrate coverage and greater substrate fluorescence suppression as compared to colorant mixture B. The colorant mixtures A and B will also be selected to match each other closely in their average color and luminance when viewed under visible light conditions. As shown at UV in FIG. 1, under UV lighting conditions, patch BP will appear brighter as compared to patch AP, due to the relatively limited suppression of the fluorescence of the optical brightening agents in the paper substrate as compared to the patch AP, thus forming a fluorescence mark FM. In contrast, under visible light conditions as shown at VIS, patches AP,BP are at least substantially indistinguishable. This property of matching color under a first lighting condition (e.g., visible light) but unmatched color under a second lighting condition (e.g., UV light) is referred to as metamerism and the colorant mixtures A and B can be said to define a metameric or approximately metameric pair.
By way of a simplified example, an approximate 50% gray color may be realized with a halftone of black (K) colorant only and used for colorant mixture B to print patch BP. This may then be color-matched against a colorant mixture A comprising cyan (C), magenta (M), and yellow (Y) that yields a similar approximate 50% gray color, which is used to print the patch AP. In general, colorant mixture A will cover more of the paper, thus providing much higher suppression of native substrate fluorescence as compared to the patch BP, so that under UV lighting conditions, the patch BP will be readily apparent as a fluorescence mark FM. The two colorant mixtures will appear quite nearly identical “gray” under normal visible light viewing as shown at VIS in FIG. 1. Thus, when a document including such a fluorescence mark is subjected to UV illumination, the fluorescence mark FM is revealed. A printed “look-alike” document or mere photocopy will not properly reproduce the watermark.
Additional details and variations relating to fluorescence marks are disclosed in U.S. patent application Ser. No. 11/382,897 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Mask for Embedding Information in Printed Documents” and U.S. patent application Ser. No. 11/382,869 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Pattern Mask for Embedding Information in Printed Documents” and the disclosures of both these applications are hereby expressly incorporated by reference into the present specification.
The above and other known systems and methods allow the fluorescence mark FM to represent variable data, e.g., a user-selected string, symbol or pattern. Known systems and methods do not allow for “double layer variable data” wherein additional user-selected variable data is printed in the fluorescence mark region FMR for viewing in visible light, without obscuring or altering the fluorescence mark FM as viewed in UV light. It has been deemed desirable to provide a system and method according to the present development for implementing double layer variable data in a fluorescence mark region FMR, wherein the fluorescence mark region FMR includes both UV light variable data as a fluorescence mark FM and visible light variable data, without the visible light variable data obscuring or altering the fluorescence mark.
One previously considered possibility for creating the first (UV light) and second (visible light) variable data region would be to use colors C1 and C2 as a metameric pair for the fluorescence mark variable data and use color C3 for the visible light variable data if the following relationships could be preserved:                C1(visible light)≈C2(visible light)≠C3 (visible light)        C1(UV light)≠C2(UV light)≈C3 (UV light)Such colorant combinations are not easily generated and identified. As such, an alternative method for implementing double layer variable data in a fluorescence mark region FMR is needed.        