Abstract:
A watermark can be embedded in an image that has the property of being relatively indecipherable under normal light by including a distraction pattern, and yet remains decipherable under infrared illumination when viewed by a suitable infrared sensitive instrument. This infrared mark comprises, a substrate reflective to infrared radiation, a foreground colorant mixture printed as an image upon the substrate, a background colorant mixture and a distraction colorant mixture. A resultant collocated image rendered substrate suitably exposed to an infrared illumination, will yield a discernable image evident as a infrared mark to a suitable infrared sensitive device, but remain undecipherable under normal ambient light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Cross-reference is made to the following applications which are incorporated by reference for their teachings in their entirety herein: Eschbach et al., U.S. patent application Ser. No. 11/758,344, filed simultaneously herewith, entitled “INFRARED ENCODING OF SECURITY ELEMENTS USING STANDARD XEROGRAPHIC MATERIALS”; Bala et al., U.S. patent application Ser. No. 11/708,313, filed Feb. 20, 2007, entitled “SUBSTRATE FLUORESCENCE MASK UTILIZING A MULTIPLE COLOR OVERLAY FOR EMBEDDING INFORMATION IN PRINTED DOCUMENTS”; Bala et al., U.S. patent application Ser. No. 11/382,897, filed May 11, 2006, entitled “SUBSTRATE FLUORESCENCE MASK FOR EMBEDDING INFORMATION IN PRINTED DOCUMENTS”; Bala et al., U.S. patent application Ser. No. 11/382,869, filed May 11, 2006, entitled “SUBSTRATE FLUORESCENCE PATTERN MASK FOR EMBEDDING INFORMATION IN PRINTED DOCUMENTS”; and Bala et al., U.S. patent application Ser. No. 11/754,702, filed May 29, 2007, entitled “SUBSTRATE FLUORESCENT NON-OVERLAPPING DOT PATTERNS FOR EMBEDDING INFORMATION IN PRINTED DOCUMENTS”. 
     BACKGROUND AND SUMMARY 
     This disclosure relates generally to methods and systems for steganographically embedding information, and more particularly to a system and method for utilizing a multiple color overlay to embed infrared information in documents and/or images collocated with human visible information. 
     Current digital document counterfeit prevention systems are mainly based on the use of digital watermarks, a technique which permits the insertion of information (e.g., copyright notices, security codes, identification data, etc.) to digital image signals and documents. Such data can be in a group of bits describing information pertaining to the signal or to the author of the signal (e.g., name, place, etc.). Most common watermarking methods for images work in spatial or frequency domains, with various spatial and frequency domain techniques used for adding watermarks to and removing them from signals. 
     For spatial digital watermarking the simplest method involves flipping the lowest-order bit of chosen pixels in a gray scale or color image. This works well only if the image will not be subject to any human or noisy modification. A more robust watermark can be embedded in an image in the same way that a watermark is added to paper. Such techniques may superimpose a watermark symbol over an area of the picture and then add some fixed intensity value for the watermark to the varied pixel values of the image. The resulting watermark may be visible or invisible depending upon the value (large or small, respectively) of the watermark intensity. 
     Spatial watermarking can also be applied using color separation. In this approach, the watermark appears in only one of the color bands. This type of watermark is visibly subtle and difficult to detect under normal viewing conditions. However, when the colors of the image are separated for printing or xerography, the watermark appears immediately. This renders the document useless to the printer unless the watermark can be removed from the color band. This approach is used commercially for journalists to inspect digital pictures from a stock photo agency before buying un-watermarked versions. 
     Alternatively, another approach uses infrared (IR) ink rendering to encode a watermark that is not visible under normal illumination, but revealed under IR illumination to a suitable infrared sensitive device such as a infrared sensitive camera. The traditional approach, often used, is to render a watermark with special infrared (IR) inks and to subsequently identify the presence or absence of the watermark in a proffered document using simple infrared illumination and sensing. However, these inks are costly to employ, generally requiring additional workflow steps, and as a result are typically economically viable only in offset printing scenarios, and therefore only truly avail themselves to long print runs. Additionally, these materials are often difficult to incorporate into standard electrophotographic or other non-impact printing systems like solid ink printers, either due to cost, availability or physical/chemical properties. This in turn particularly discourages their use in variable data printing arrangements, such as for redeemable coupons, for example. 
     There is well established understanding in the printing industry regarding the utilization of infrared material inks in combination with infrared light sources and sensors as employed for security marks, particularly as a technique to deter counterfeiting. However, there remains a long standing need for an approach to such a technique which will provide the same benefit but with lower complexity and cost, particularly in a digital printing environment, using only common consumables. 
     All U.S. patents and published U.S. patent applications cited herein are fully incorporated by reference. The following patents or publications are noted: 
     U.S. Patent Application Publication No. 2005/0078851 to Jones et al. (“Multi-channel Digital Watermarking”) describes a system for providing digital watermarks through multiple channels. The channels can include visible, ultraviolet and infrared channels. The non-visible channels can be selected to respond either in the visible or IR/UV spectrums upon the appropriate illumination in the infrared or ultraviolet spectrums. The watermarks in the various multiple channels can cooperate to facilitate watermark detection or to authenticate an object in which the watermarks are embedded. 
     U.S. Pat. No. 7,127,112 to Sharma et al. (“Systems for Spectral Multiplexing of Source Images to Provide a Composite Image, for Rendering the Composite Image, and for Spectral Demultiplexing of the Composite Image by Use of an Image Capture Device”) provides methods and systems for spectrally-encoding plural source images and for providing the spectrally-encoded plural source images in a composite image, for rendering the composite image on a substrate, and for recovering at least one of the encoded source images from the rendered composite image. A desired source image is recovered when the rendered composite image is subjected to illumination by one or more illuminants and the desired source image is detected by one or more sensors in an image capture device. The spectral characteristics of the colorants, illuminants, and sensors are employed to spectrally encode the source image in the composite image. 
     The disclosed embodiments which follow below provide examples of improved solutions to the problems noted in the above background discussion and the art cited therein. There is shown in these examples a method for creation of an infrared mark with distraction pattern to be printed by a printing device as an image on a substrate for embedding information in printed documents. The method comprises providing a substrate reflective of infrared, and selecting at least one background color mixture for the infrared mark. The method also teaches selecting at least one foreground color mixture for the infrared mark, the at least one foreground color mixture exhibiting low contrast against the at least one background color mixture under normal illumination, and high contrast against the at least one background color mixture under infrared illumination. The method further comprises selecting at least one distraction color mixture for the infrared mark. This at least one distraction color mixture is comprised of at least two colors. The at least one distraction color mixture as selected has a substantially negligible effect on the infrared response of the foreground and background color mixtures, as well as having a substantially noticeable effect of the visual response of the least one foreground color mixture and the least one background color mixture. The method also comprises creating an infrared mark using the foreground color mixture against the background color mixture, and creating a distraction pattern using the distraction color mixture as collocated with the infrared mark. 
     In yet another embodiment there is disclosed a method for creation of an infrared mark to be printed by a printing device as an image on a substrate for embedding information in printed documents. The method comprises providing a substrate reflective of infrared and selecting at least one background color mixture for the infrared mark. The method further comprises selecting at least one foreground color mixture for the infrared mark, the at least one foreground color mixture exhibiting low contrast against the at least one background color mixture under normal illumination, and high contrast against the at least one background color mixture under infrared illumination. The method further comprises selecting at least one distraction color mixture for the infrared mark, the at least one distraction color mixture comprised of at least two colors, the at least one distraction color mixture having a substantially negligent-negligible effect on the infrared response of the foreground and background color mixtures, as well as having a substantially noticeable effect of the visual response of the least one foreground color mixture and the least one background color mixture. The method thus creating an infrared mark using the foreground color mixture against the background color mixture in close spatial proximity, with a distraction pattern using the distraction color mixture as collocated with the foreground color mixture against the background color mixture. 
     In an alternate embodiment there is disclosed a system for an infrared mark as rendered by a conventional printing device as an image on a substrate comprising a substrate reflective of infrared with at least one background color mixture for the infrared mark and at least one foreground color mixture for the infrared mark, the at least one foreground color mixture exhibiting low contrast against the at least one background color mixture under normal illumination, and high contrast against the at least one background color mixture under infrared illumination. The embodiment further includes at least one distraction color mixture for the infrared mark, the at least one distraction color mixture comprised of at least two colors, the at least one distraction color mixture having a substantially negligible effect on the infrared response of the at least one foreground and background color mixtures, as well as having a substantially noticeable effect of the visual response of the at least one foreground color mixture and the at least one background color mixture. Thus an infrared mark is created by using the at least one foreground color mixture imaged as arranged in close spatial proximity against the at least one background color mixture, with a distraction pattern imaged using the at least one distraction color mixture as collocated with the at least one foreground color mixture in close spatial proximity against the at least one background color mixture. 
     In a further embodiment there is disclosed a method for creation of an infrared mark to be printed by a printing device as an image on a substrate for embedding information in printed documents. The method comprises providing a substrate reflective of infrared, selecting at least one background color mixture for the infrared mark and selecting at least one foreground color mixture for the infrared mark, the at least one foreground color mixture exhibiting low contrast against the at least one background color mixture under normal illumination, and high contrast against the at least one background color mixture under infrared illumination. The method further includes selecting at least one distraction color mixture for the infrared mark, the at least one distraction color mixture comprised of at least two colors, the at least one distraction color mixture having a substantially negligible effect on the infrared response of the foreground and background color mixtures, as well as having a substantially noticeable effect of the visual response of the least one foreground color mixture and the least one background color mixture. The method also comprises printing the image background with the at least one background color mixture, along with printing the image foreground with the at least one foreground color mixture, and overprinting a distraction pattern with the at least one distraction color mixture to create an infrared mark using the at least one foreground color mixture against the at least one background color mixture, with a distraction pattern using the at least one distraction color mixture as collocated with the at least one foreground color mixture against the at least one background color mixture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the embodiments described herein will be apparent and easily understood from a further reading of the specification, claims and by reference to the accompanying drawings in which: 
         FIG. 1  is a conceptual illustration depicting two color which match under normal illumination but differ under infrared illumination to a suitable device. 
         FIG. 2  is a further conceptual illustration similar to  FIG. 1  but introducing a distraction color. 
         FIG. 3A  is an illustration of the teachings herein for a multicolor substrate Infrared mark under normal illumination. 
         FIG. 3B  is an illustration of the teachings herein for a multicolor substrate Infrared mark under IR illumination viewed by a suitable device. 
         FIG. 4A  is an illustration for a multicolor substrate Infrared mark under normal illumination. 
         FIG. 4B  is an illustration for a multicolor substrate Infrared mark and introduces a fourth colorant mixture to the mark of  FIG. 4A . 
         FIG. 5  depicts one exemplary embodiment example of colorant mixtures suitable in for an Infrared mark. 
         FIG. 6  depicts a flowchart that illustrates one process for creating a Infrared mark in accordance with the teachings herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense. 
     For the purposes of clarity, the following term definitions are provided:
         Color: A color can be uniquely described by three main perceptual attributes: hue, denoting whether the color appears to have an attribute according to one of the common color names, such as red, orange, yellow, green, blue, or purple (or some point on a continuum); colorfulness, which denotes the extent to which hue is apparent; and brightness, which denotes the extent to which an area appears to exhibit light. Light sources used to illuminate objects for viewing are typically characterized by their emission spectrum and to a reduced degree by their color temperature, which is primarily relevant for characterization of sources with a spectrum similar to a black body radiator. See, for instance, Hunt, R. W. G.,  Measuring colour , Ellis Horwood, 1991, and Billmeyer and Saltzman,  Principles of Color Technology,  3 rd  Ed. (Roy S. Berns), John Wiley &amp; Sons, 2000.   Colorant: A dye, pigment, ink, or other agent used to impart a color to a material. Colorants, such as most colored toners, impart color by altering the spectral power distribution of the light they receive from the incident illumination through two primary physical phenomenon: absorption and scattering. Color is produced by spectrally selective absorption and scattering of the incident light, while allowing for transmission of the remaining light. For example, cyan, magenta and yellow colorants selectively absorb long, medium, and short wavelengths respectively in the spectral regions. Some colorants, such as most colored toners, impart color via a dye operable in transmissive mode. Other suitable colorants may operate in a reflective mode.   Metameric rendering/printing: the ability to use multiple colorant combinations to render a single visual color, as can be achieved when printing with more than three colorants.   Infrared Mark: A watermark embedded in the image that has the property of being relatively indecipherable under normal light, and decipherable under infrared light using suitable infrared sensitive devices.   Image: An image may be described as an array or pattern of pixels that are mapped in a two-dimensional format. The intensity of the image at each pixel is translated into a numerical value which may be stored as an array that represents the image. An array of numerical values representing an image is referred to as an image plane. Monochromatic or black and white (gray scale) images are represented as a two-dimensional array where the location of a pixel value in the array corresponds to the location of the pixel in the image. Multicolor images are represented by multiple two-dimensional arrays.   Illuminant: A source of incident luminous energy specified by its relative spectral power distribution.   Image plane: A two-dimensional representation of image data. For example the uppercase letters C, Y, M, K are used to indicate two-dimensional arrays of values representing cyan, magenta, yellow and key (black) components of a polychromatic (multicolor) image. Two-dimensional arrays of values may also be referred to as “planes”. For example, the Y plane refers to a two-dimensional array of values that represent the yellow component at every location (pixel) of an image.   Composite Image: An array of values representing an image formed as a composite of plural overlaid (or combined) colorant image planes. Source images may be encoded as described herein and the resulting image planes are combined to form a composite image.   Imaging Device: A device capable of generating, capturing, rendering, or displaying an image; including devices that store, transmit, and process image data. A color imaging device has the capability to utilize color attribute information.   Luminance: A photometric measure describing the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. Luminance indicates how much luminous power will be perceived by the human eye looking at the surface from a particular angle of view. It is therefore an indicator of how bright a surface will appear.   Security document: A paper or document having a value so as to render it vulnerable to attempts at counterfeiting or unauthorized copying, be this value real or perceived.       

     It is known to utilize infrared material inks to ensure document security. See, for example, U.S. Patent Application No. 2007/0017990 A1 to Katsurabayashi et al., which is herein incorporated by reference in its entirety for its teachings. However, these inks are costly and are often difficult to incorporate into standard electrophotographic or other non-impact printing systems, such as solid ink printers, due to cost, availability or physical/chemical properties. 
     An alternate approach is to incorporated the different characteristics of standard colorants in the infrared domain through a selective and guided use of common metameric printing. In this approach, use is made of the high reflectance of common paper substrates under infrared illumination. Coupled with the high transmissivity of standard chromatic colorants (e.g.: standard cyan, magenta and yellow) to infrared wavelength and the high absorption of standard black colorant—commonly carbon black—to infrared wavelength, sets of colorant mixtures can be found with an reasonable identical visual response, but a distinctively different infrared response, as described in co-pending application Ser. No. 11/758,344 “Infrared Encoding Of Security Elements Using Standard Xerographic Materials” incorporated by reference above. 
     An infrared watermark (termed herein as an “Infrared Mark”) is embedded in a printed document by selectively masking the substrate reflectance to infrared with standard C, M, Y, K colorants as used in digital color printing. A challenge in this approach is to design two colors that match under normal illumination, and yet exhibit significant contrast under infrared light. This is conceptually illustrated in  FIG. 1 , where it is assumed, for the simplicity of illustration, that some form of luminance component (labeled Y) is used to describe the color of the patterns. In the standard case, the two colors  110  and  120  should match under a normal illuminant (for example, illuminant A), but one color  110  would be considerably lighter than the other color  120  under infrared illumination to a suitable device. 
     Suitable devices in the context of infrared marks are sensors that can distinguish the amount of infrared radiation being reflected from a surface like a printing substrate, or being transmitted from a substrate. Note that for the purpose of simplicity of description herein but not to be limited to such, we assume for example an infrared reflective substrate such as common printing paper or the like. 
     As can be seen from  FIG. 1 , colors  110  and  120  might be functionally identical when typically perceived by the human eye in passing, but closer scrutinizing inspection might nevertheless reveal a difference in the two colors as patterned. Particularly if the observer knows to look for it. The reasons for this can for example be caused by an incorrect color match due to printer imprecision/drift, and/or an incorrect match due to inherent calibration limitations, or alternatively as based on other observational differences involving other colorant rendering attributes, such as gloss. What is described herein below overcomes these issues by teaching a further technique which makes an infrared mark that is increasingly difficult and even impossible for an unaided eye to discern absent the necessary infrared set-up, by virtue of the incorporation of a second variable information print substantially collocated with the infrared mark. 
     This collocated variable information will be printed in the same physical area as the infrared mark and in such a manner that the variable information is clearly visible and stronger than any anticipated contrast of the metameric infrared mark caused by any of the above mentioned error sources. 
     The encryption approach described herein employs a minimum set of three colors in the infrared mark: background color B, Infrared Mark color IR v , and distraction color D designed with the following properties. Under normal light, the infrared mark color blends into the background, while the distraction text exhibits high contrast against the background and is thus strongly visible. Under infrared light, the situation is reversed—the distraction color blends into the background and the infrared mark/text exhibits high contrast, becoming highly visible. This is illustrated in  FIG. 2 . As shown in  FIG. 2 , the contrast of the distraction color  225  against the background color  120  and the infrared mark color  110  under illuminant A, normal light, is sufficiently significant that any imprecision in the match between the infrared mark and background under illuminant A can be substantially masked by the high contrast “noise” distraction text/pattern of distraction color  225 . Under infrared illumination, the situation reverses and the grouping changes, effectively turning the “noise” color into signal. Here the contrast between the distraction color  225  and the background color  120  is not significant and the infrared color  110  becomes readily visible. 
     Thus a minimum of only three colors need to be defined, with effectively less stringent requirements on color matching. Simultaneously, the distraction amplitude under nominally room ambient illuminant A is effectively eliminated under infrared light, leading to a higher signal-to-noise ratio. An additional advantage to the three-color overlay approach as disclosed herein is that distraction patterns may be utilized, since they effectively disappear under infrared illumination. A distraction pattern may be chosen to itself convey semantic content. Examples of semantic distraction patterns include text strings or icons. The advantage is that the user is more likely to be drawn towards a semantic distraction pattern than low-level image variations, and is thus less likely to notice and decipher a infrared mark under normal light. This enables greater tolerance and robustness in the design of the infrared mark. 
       FIGS. 3A and 3B  provide depiction of one example of this semantic distraction pattern provided here as text semantic content. Here the background is rendered using one metameric colorant mixture  310  provided as having a high infrared reflectance caused by high infrared transmittance of the chromatic colorants and high substrate reflection from the paper. For both cases depicted in  FIG. 3A  and  FIG. 3B  respectively, the background colorant mixture serves as reference under illuminant A in  FIG. 3A  and infrared illumination in  FIG. 3B . The second metameric colorant mixture  320  of low infrared reflectance e.g.: caused by high infrared absorption of common carbon black toner, is printed in substantially the same location as the background. The two metameric colorant mixtures will result in the same color under standard illumination, here for example illuminant A. In substantially the same location, a third color is used to print the semantic overlay  330 . Under normal room ambient illumination, colorant mixtures  310  and  320  are metameric with colorant mixture  330  being visually distinct and thus the text message “Overlay” is visible. Under infrared illumination, the situation changes and colorant mixtures  310  and  330  are metameric for the infrared sensing device, while colorant  320  is distinctive to said device thus displaying the text string message “INFRARED MARK” as is depicted here in  FIG. 3B . 
     The problem with the system described thus far is that for a collocation of infrared mark and distraction patterns, a single location on the substrate can only effectively exhibit one colorant mixture, and the simple schematical arrangement of  FIG. 3  would require two different colors at a single location in areas where the infrared mark and distraction color are physically in the same location. It will be understood to one skilled in the art that an approximation of printing multiple colors to the same location can be achieved by spatial multiplexing following the methods disclosed in Ser. No. 11/758,344 in combination with the method outlined in U.S. patent application Ser. No. 11/708,313 incorporated by reference above. What is described herein below is a method to derive a set of four colors that to a very good approximation provide the above described relationship, while at the same time solving the printing issue well understood by those skilled in the art, that it is cumbersome if not impossible in standard printing systems to alter the printing color in any complex form as a function of previously printed colors. 
     This problem is further described by  FIG. 4A  and B, where  FIG. 4A  provides a depiction scenario simplified from that of  FIG. 3 , with  410  here indicating the background printed in a metameric match to the infrared mark colorant mixture  420  and a overlay colorant mixture  430  printed which exhibits a strong contrast to colorant mixture  410  and thus colorant mixture  420  under normal illumination, but is metameric to colorant mixture  410  under infrared illumination to a suitable sensing device. As can be seen in  4 A, the intersection of colorant mixture  420  and colorant mixture  430  has to be printed in one colorant mixture and for the standard imaging model typical of digital printing, this will be the last requested colorant mixture, in this case colorant mixture  430 . Under infrared illumination, the rectangle rendered with colorant mixture  420  will have a break where colorant mixture  430  is overlaid, since colorant mixture  430  is an infrared metameric to colorant mixture  410 . In  FIG. 4B , a fourth colorant mixture  440  is introduced at the overlap of colorant mixtures  420  and  430 . The requirement for this colorant mixture is to be an infrared metameric to colorant mixture  420  and normal illuminant metameric to colorant mixture  430 . Mathematically this can be described as:
 
Lab(410)≈Lab(420)
 
Lab(430)≈Lab(440)
 
IR(410)≈IR(430)
 
IR(420)≈IR(440)
 
where the shorthand notation “Lab(N)” was used to denote the visual perception for a human observer under normal illumination for colorant mixture indicated as “N” in  FIG. 4 , and “IR(M)” indicates the sensor response to the infrared set-up for colorant mixture indicated as “M” in  FIG. 4 . Also it will be understood by those skilled in the art, that while in an ideal situation a true equality of match between the colorant mixtures would be preferred, but that in real-world applications the match only needs to be performed to a quality level sufficient for the identified application.
 
     An additional problem in implementing the system described above in  FIGS. 3 and 4  is that standard print descriptions in digital printing do not allow for a simple inquiry as to the underlying colors already rendered onto the substrate. This means that a simple system laying down colorant mixture  430  on top of colorant mixture  410 , does not have a readily available process for identifying the transition from an underlying colorant mixture  410  to the metameric colorant mixture  420 . Thus the variable data overlay has no indication of when to switch from colorant mixture  430  to colorant mixture  440 . It will be understood that this problem is compounded by the fact that variable data, such as character string data, is commonly rendered using a single color setting command in its page description. 
     Understanding that the application requirement is an approximate match and not a mathematically perfect match, one can address the above mentioned color requirement by introducing the concept of differential colors, where colorant mixture  430  is a differential to colorant mixture  410  and colorant mixture  440  is a differential to colorant mixture  420 . One exemplary embodiment would have a differential color that is essentially neutral for the infrared set-up, in which case the infrared metameric character would be predominantly maintained. The differential colors can be defined as:
 
 CMYK (430)= f{CMYK (410)}
 
 CMYK (440)= f{CMYK (420)},
 
where CMYK refers to the respective colorant mixtures for the colorant mixtures indicated in  FIG. 4 . It is understood that a higher number of colorant mixtures can be used, and as such is well within the contemplation of this disclosure.
 
     Another observation, as described above, is that the high infrared transmission of standard chromatic colorants is different from the low infrared transmission of the standard black colorant, often carbon black. Thus it may be found that for the precision required in most infrared marking applications the chromatic colorants can be treated as sufficiently transparent so as to be considered as not influencing the infrared component, and the black colorant can be treated as a sufficiently perfect infrared absorber. In this way the infrared metameric character can be obtained simply by maintaining the K component of the colorant mixture constant while variably modifying the CMY components of the colorant mixture in a predetermined way. 
     This scenario is further described in  FIG. 5 . Colorant mixtures  510  and  520  are visually metameric under normal operation, but clearly distinct for an infrared illumination set-up. For simplicity we may consider colorant mixture  510  to contain only the single colorant black, whereas the colorant mixture  520  contains only chromatic colorants cyan, magenta and yellow. Colorant mixtures  510  and  520  fulfill—for the purpose of the description—the relationships:
 
Lab(510)=Lab(520)
 
and
 
IR(510)&lt;&lt;IR(520),
 
where we use the case that the chromatic colorants transmit the infrared radiation coupled by the infrared reflectivity of the substrate to indicate that the infrared signal derived from mixture  520  is sufficiently larger than the response from mixture  510  as defined by the application requirements.
 
     Colorant mixtures  530  and  540  are derived from colorant mixtures  510  and  520  respectively by altering only the CMY components of the colorant mixtures, maintaining the K component, giving:
 
 CMYK (530)= K (510)+ C (530)+ M (530)+ Y (530),
 
 CMYK (540)= K (520)+ C (530)+ M (530)+ Y (530),
 
giving:
 
Lab(530)≈Lab(540),
 
and
 
IR(530)&lt;&lt;IR(540),
 
while having:
 
Lab(530/540)≠Lab(510/520),
 
where the difference between the colorant mixture pair  530  and  540  on one side and the colorant mixture pair  510  and  520  on the other side is of sufficient contrast to satisfy the application requirements. This is effected by using chromatic colorant amounts CMY( 530 ), that are substantially different from the chromatic colorant amounts CMY( 510 ).
 
     In  FIG. 5  identical chromatic colorant amounts and modifications have been used, but it will be understood by those skilled in the art that any chromatic colorant modification can be used. For example, in a Postscript® page description language environment, the chromatic colorant modification can be obtained using the overprint construct (see PostScript® Language Reference Manual by Adobe Systems Incorporated, ISBN 0-201-18127-4). In that implementation the “Overlay” colorant mixture  330  of  FIG. 3  is created using a chromatic colorant combination in the “overprint” state of the page description. 
     In a further example, using the overprint operator one can replace the cyan and/or magenta and/or yellow separation without influencing the other separations. 
     A simplified example for the colors—assuming linearity—would then be: 
                                             C1 = 20% K + 15% C + 20% M + 30% Y           C2 = 35% C + 40% M + 50% Y           C3 = 20% K + 90% C + 90% M + 90% Y           C4 = 90% C + 90% M + 90% Y                        
where it is clear that C 1 ≈C 2 ≠C 3 ≈C 4  in the visible spectrum, but C 1 ≈C 3 ≠C 2 ≈C 4  in the IR spectrum.
 
     With this restriction it becomes clear that in a more exemplary approach additional restrictions have to be enforced for the two base colors. Note that since the new colors are created by modifying the CMY components, enough variability has to be available. This is achieved by selecting very light colors where the overprint operator can be used to noticeably increase the CMY components individually, or together. For very dark colors this can be done by setting CMY to small amounts, however, for those skilled in the art it is apparent that this often does not lead to the desired effect, since the dark colors generally have a high K component which makes the removal of CMY more difficult, since the K deposited in the first instance can not be modified. For simplicity, an exemplary approach will therefore restrict itself to light colors, which also means that the CMY component will be small. 
     The particular methods performed for designing a infrared mask comprise steps described below with reference to a flow chart in  FIG. 6 . The flow chart illustrates an embodiment in which the methods constitute computer programs made up of computer-executable instructions. Describing the methods by reference to a flowchart enables one skilled in the art to develop software programs including such instructions to carry out the methods on computing systems. The language used to write such programs can be procedural, such as Fortran, or object based, such as C++. One skilled in the art will realize that variations or combinations of these steps can be made without departing from the scope of the disclosure herein. 
     Turning now to  FIG. 6 , a flowchart illustrates the process for creating a infrared mark in accordance with the disclosure herein. At  610  a suitable printing substrate is provided. The substrate may be any white or colored digital printing substrate of high infrared reflectance. Note that in a transmission scenario this requirement would be replaced by a transmissive substrate, everything else remaining the same. 
     The background color B is selected at  620 . In one example embodiment, the method for color selection is structured such that a user may select B, and the remaining two colors are automatically derived from the B choice. In a more general embodiment, the user can be given a choice of foreground and distraction color in  630  and  640  noting that the foreground color choice is severely limited by the requirement of visual metameric behavior and is thus limited to visually identical colors having different colorant mixtures. In an exemplary embodiment, the foreground color would be directly selected through selection of the background color, while a small number of options will be given for the distraction color for aesthetic and design purposes. It is appreciated that steps  620 ,  630  and  640  can be performed in arbitrary order and that the automatic, semi-automatic or manual selection of the colors or colorant mixtures follows from the functional requirements after the first of the three colors/colorant mixtures are selected. This explicitly includes the case where the user selects a color and not a colorant mixture and foreground and background colorant mixtures are automatically selected from the selected visual color. 
     At step  650  the background pattern is first printed onto the substrate. Note that for practical purposes the background pattern is preferably the largest object spatially enclosing foreground and distraction pattern. At step  660  the foreground pattern is printed, replacing the background pattern on the substrate in all locations where foreground color is deposited. At step  670  the distraction color is deposited in a manner different from the standard printing operation as used in step  660 , such that the infrared active colorant component—usually black—is not modified, while the other colorant components are replaced by new chromatic colorant components. In a Postscript® language environment this is achieved using the “overprint” construct, but this disclosure is intended to be equally applicable to other page description languages having functionally equivalent constructs. 
     In an exemplary embodiment at least one of the foreground or distraction components are created from a variable data stream, perhaps most commonly being a character string derived from relevant desired information. It is understood that both foreground and distraction components can be created from two separate variable data streams that are unrelated or functionally related. For example, the unrelated case might incorporate the company name as the foreground stream and the current date as the distraction stream of text. For a further example, the functionally related case might incorporate the recipient&#39;s first name as background data and the recipient&#39;s last name as a distraction data stream. It will be understood by those skilled in the art that the term distraction is used solely to describe the effect on the human color perception system, being neutral with respect to the actual relevance in terms of human recognizable information content. 
     While the present discussion has been illustrated and described with reference to specific embodiments, further modification and improvements will occur to those skilled in the art. For example, the colors could be generated with other special colorants in addition to the standard C, M, Y, K. Examples of such colorants could include low-load colorants (commonly cyan and magenta), orange, green, violet, etc. 
     It is also understood that in an exemplary embodiment the user is actually selecting visual colors without the need to define, understand or modify colorant mixtures and that the corresponding colorant mixtures are automatically derived or as another alternative that a set of colors is offered to the user for selection where the system has predetermined the corresponding colorant mixtures to fulfill the described criteria. 
     Additionally, “code” as used herein, or “program” as used herein, is any plurality of binary values or any executable, interpreted or compiled code which can be used by a computer or execution device to perform a task. This code or program can be written in any one of several known computer languages. A “computer”, as used herein, can mean any device which stores, processes, routes, manipulates, or performs like operation on data. It is to be understood, therefore, that this disclosure is not limited to the particular forms illustrated and that it is intended in the appended claims to embrace all alternatives, modifications, and variations which do not depart from the spirit and scope of the embodiments described herein. 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, color, or material.