Patent Publication Number: US-6991260-B2

Title: Anti-counterfeiting see-through security feature using line patterns

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to anti-counterfeiting patterns on a document and, more particularly, to line patterns on the front and back surfaces of a document which allow a document holder to verify the authenticity of the document and which have enhanced security protection against copying of the document. 
     A great number of printed documents require highly reliable means of ensuring their authenticity. These documents include currency, negotiable instruments, stock certificates, checks, tickets and the like. The means employed to indicate authenticity for the document should be permanent, durable, and difficult to replicate to allow the public at large to rely on the authenticity of the documents. This latter quality is particularly important to preclude, or at least to dissuade attempts at counterfeiting the documents in order to ensure a maximum degree of confidence in the original document. In the case of banknotes, passports, checks, and other intrinsically valuable documents, confidence in the authenticity of the document is especially important, as any member of the public might become a holder or user of the document at any time. 
     The criteria for an effective document security feature are relatively easy to formulate. Such features should be difficult to replicate to deter potential counterfeiters. The features should permit ready detection by means available to ordinary holders or users of the final document. For banknotes and other documents on whose authenticity the public at large relies, the features should be discernible and verifiable under ordinary light conditions. 
     The increasing popularity of color photocopiers and other imaging systems, and the improving technical quality of color photocopiers, has led to an increase in the counterfeiting of such documentation. 
     A wide variety of security features for documents have been proposed previously. Examples of such security features include: optically variable devices, such as holograms and diffraction gratings; security threads or strips; microprint; watermarks; fine line or ‘filigree’ patterns; or color-shifting inks, fluorescent inks, and phosphorescent inks. 
     These measures naturally add to the complexity and production cost of the documents. 
     A disadvantage is that several of these document security features may require an optical filter or other external equipment, to provide the required lighting condition for verification of the security device. For example, fluorescent inks may require a source of ultraviolet light for their verification, and microprint, fine line and filigree patterns may require a magnifying lens for verification or may only be machine readable. 
     To prevent unauthorized duplication or alteration of documents, frequently special indicia or a background pattern are provided for document sheet materials. The indicia or background pattern is imposed upon the sheet material usually by some type of printing process such as offset printing, lithography, letterpress or other like mechanical systems, by a variety of photographic methods, by xerographic printing, and a host of other methods. Most of these patterns placed on sheet materials depend upon complexity and resolution to avoid ready duplication. Consequently, they add an increment of cost to the sheet material without being fully effective in many instances in providing the desired protection from unauthorized duplication or alteration. 
     It is an object of the present invention to provide a low cost, anti-counterfeiting pattern on a document which is easy to manufacture and yet difficult to counterfeit. 
     It is another object of the present invention to provide an anti-counterfeiting pattern on a document which a document user or holder with no additional external equipment can verify the authenticity of the document. 
     SUMMARY OF THE INVENTION 
     According to the present invention, line patterns are aligned on the front and back surfaces of a document to provide an anti-counterfeiting security device. The document is sufficiently transparent to allow see-through of the partial image pattern on the back of the document to be superimposed on the partial image pattern on the front of the document to form a complete image if the patterns are properly aligned. The patterns will not form a complete pattern if misaligned. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale. 
         FIG. 1  is a front view of the anti-counterfeiting feature on a document of the present invention. 
         FIG. 2  is a top view of the anti-counterfeiting feature on a document of  FIG. 1 . 
         FIG. 3  is an illustration of the first pattern of the anti-counterfeiting feature on a document of  FIG. 1 . 
         FIG. 4  is an illustration of the second pattern of the anti-counterfeiting feature on a document of  FIG. 1 . 
         FIG. 5  is a schematic view of light transmission through the see-through anti-counterfeiting feature on a document of  FIG. 1 . 
         FIG. 6  is an illustration of the first partial image and the line pattern background of the first pattern of the anti-counterfeiting feature on a document of  FIG. 1 . 
         FIG. 7  is an illustration of the second partial image and the line pattern background of the second pattern of the anti-counterfeiting feature on a document of  FIG. 1 . 
         FIG. 8  is an illustration of the superimposition of the first pattern and the second stochastic pattern when aligned to form a complete authentication image. 
         FIG. 9  is an illustration of the superimposition of the first pattern and the second pattern when misaligned to cause the disappearance of the authentication image. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numeric ranges are provided for various aspects of the embodiments described. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the embodiments. These recited materials are to be treated as exemplary, and are not intended to limit the scope of the claims hereof. In addition, the figures are not drawn to scale for ease of understanding the present invention. 
     In the present invention, gray image data may be characterized as image signals, each pixel of which is defined at a single level or optical density in a set of ‘c’ optical density levels, the number of members in the set of levels being larger than desired. Each pixel will be processed in the manner described herein below, to redefine each pixel in terms of a new, smaller set of ‘d’ levels In this process, ‘c’ and ‘d’ are integer values representing pixel depth, or a number of signal levels at which the pixel may appear. One common case of this method includes the conversion of data from a relatively large set of gray levels to one of two legal or allowed binary levels for printing in a binary printer. 
     As used herein, the term “dot pattern” refers to a product or an image resulting from a screening process. A “screen cell”, as used herein, refers to the set of pixels which together will form the dot pattern, while the term “screen matrix” will be used to describe the set of values which together make up the set of threshold to be applied. A “pixel” refers to an image signal associated with a particular position in an image, having a density between white and black. Accordingly, pixels are defined by intensity and position. A dot pattern is made up of a plurality of pixels. These terms are used for simplification and it should be understood that the appropriate sizing operations have to be performed for images where the input resolution in terms of scan pixels is different from the output resolution in terms of print pixels. 
     The present invention allows for a gray pattern to be used on a document, where the gray pattern can be generated using a halftoning process to produce a desirable gray. 
     Each location in an image may be called a “pixel.” In an array defining an image in which each item of data or image signal provides a value, each value indicating the color of a location may be called a “pixel value”. Each pixel value is a bit in a “binary form” of an image, a gray scale value in a “gray scale form” of an image, or a set of color space coordinates in a “color coordinate form” of an image, the binary form, gray scale form, and color coordinate form each being a two-dimensional array defining the image. 
     Reference is now made to  FIGS. 1 and 2 , wherein there is illustrated see-through line patterns  10 ,  12  on a document  14  for an anti-counterfeiting security feature  16  in accordance with this invention. 
     A first pattern  10  is on the front surface  18  of document  14 . As shown in  FIG. 3 , the first pattern  10  has a plurality of pixels  20  characterizing gray image data. 
     A second pattern  12  is on the back surface  22  of document  14 . As shown in  FIG. 4 , the second pattern  10  has a plurality of pixels  24  characterizing gray image data. The first pattern  10  and its image data is different from the second pattern  12  and its image data. The first pattern  10  and the second pattern  12  are aligned on opposite surfaces of the document. The first and second patterns can be formed by halftoning. 
     The first and second patterns  10 ,  12  only cover a portion  16  of the front and back surface  18 ,  20  of the document  14 . The document  14  will carry conventional printing (not shown) adjacent to the security feature portion  16 . 
     The patterns  10 ,  12  can be provided in any conventional manner using conventional inks such as black inks, colored inks, white inks, metallic inks, or optically variable inks. 
     An important aspect of the see-through patterns  10 ,  12  on the document  14  is its ability to permit verification of authenticity by any holder and under normal light conditions. 
     The document  14  will be transparent enough, or alternately the security feature portion  16  with the patterns  10 ,  12  will be transparent enough, to permit see-through under normal light  26  by a document holder. The document  14  will typically be a paper such as rag paper and the like but could also comprise a plastics material such as a plastics film or other material such as credit card material, non-wovens and the like. Alternately, the security feature portion  16  will be defined by a plastic insert within a surrounding paper document  14 . 
     A light beam  26 , such as visible light in the range of wavelengths between about 380 and 720 nanometers, from a light source  28 , either natural or artificial, is incident on the document  14 . The light beam  26  is either transmitted through the document, absorbed by the document, or reflected from the document. As represented by the line  30  in  FIG. 5 , transmitted light  26  enters the document through back surface  22 , passes through the document  14 , and emerges from the front surface  18  to be seen by observer  32 . 
     When overlapping the second pattern  12  during see-through, the first pattern  10 , as seen in  FIG. 6 , has a first partial image  34  and a background pattern of a plurality of vertical parallel lines  36  formed from the gray image data. When overlapping the first pattern  10  during see-through, the second pattern  12 , as seen in  FIG. 7 , has a second partial image  38  and a background pattern of a plurality of vertical parallel lines  40  formed from the gray image data. 
     Returning to  FIG. 5 , an observer  32  viewing the document  14  from the front side  18  with the light  26  behind the back side  22  of the document will “see through” the document  14  and view the second pattern  12  on the back surface  22  aligned with and superimposed on the first pattern  10  on the front surface  18 . 
     As shown in  FIG. 8 , the vertical parallel lines  36  of the pattern  10  on the front surface  18  are superimposed on the vertical parallel lines  40  of the pattern  12  on the back surface  22 . The second partial image  38  on the back surface  22  is aligned with the complementary first partial image  34  on the front surface  18  to form a complete image  42 . The resulting complete image  42  serves as an authentication mark for the document. 
     Preferably, the partial images  34 ,  38  of the first and second patterns  10 ,  12  each define a characteristic image. The first and second pattern define recognizable patterns (such as security patterns) or images such as geometric shapes, graphic illustrations, alphanumeric characters and other curvilinear patterns. This enables the document easily to be authenticated either by the eye of the holder or by a machine in the case of a machine readable image. 
     As shown in  FIG. 9 , if the first pattern  10  on the front surface  18  of the document  14  is misaligned or not in perfect registration with the second pattern  12  on the back surface  22 , then an observer will not be able to view the authentication image  42 . The disappearance of the authentication image  42  is caused by the phase shift between the background line patterns and/or the angular rotation of the background line patterns relative to each other. The disappearance of the authentication image  42  serves as a sign of counterfeiting for the document. The second partial image  38  on the back surface  22  is misaligned with the first partial image  34  on the front surface  18  upon see-through of the document  14  with a light behind the document. 
     Printing of the halftoning patterns  10 ,  12  on the document  14  is normally carried out with specialized lithographic presses which allow simultaneous front and back surface  18 ,  22  printing during one printing run. In this way, the tolerances applied to the patterns  10 ,  12  are typically a fraction of a millimeter and any variation caused by counterfeiting by printing both sides  18 ,  22  during different printing runs can be quickly noticed. By printing on both sides  18 ,  22  in a single impression, misregister due to variations in the dimensions and thickness of the document  14  caused by change of moisture content or heating and the like are avoided. In all cases, the first and second patterns  10 ,  12  can be provided by printing such as offset, gravure or screen printing or by any other suitable technique such as a transfer process. 
     The primary advantage of a see-through security feature is the difficulty in counterfeiting such features. Partly, this is due to the need to achieve exact registration between the patterns on each side of the document and partly due to the fact that the counterfeiter may not even realize that the feature exists. 
     A high level of transparency for the document  14  is advantageous since it allows the use of the patterns  10 ,  12  which cannot normally be distinguished due to problems of light diffusion as light passes through the substrate. Specialty colors for the patterns  10 ,  12  are permitted because they are more difficult for a counterfeiter to faithfully reproduce with a color copier, printer or scanner. 
     The front and back partial images of the first and second patterns are printed in perfect registration and alignment. 
     The design of the partial images and the patterns is done so that any slight misalignment would be obvious through the disappearance of the authentication image when viewed in transmission and hence would be an indication that the document was counterfeit. 
     If an almost perfect registration can be achieved in the original printing, the present invention can be applied to detect counterfeit copies that are produced by equipment with less registration accuracy by the disappearance of the authentication image. The present invention provides a better detection resolution. The patterns are highly sensitive to mis-registration and misalignment. 
     Halftoning as used in the present invention refers to techniques that create the visual illusion of gray scale using a dot pattern that has only two levels of gray. A normal printing process is binary in nature in that it cannot adjust the density of ink for each spot on the paper. Rather, it can only either print an ink on a spot or leave it blank. For black ink on white paper, the process makes the spot either black or white. To print pictures with gray tones like the patterns on the document, halftoning must be used. 
     Halftoning is a binary encoding method. The basic idea is to print black points or groups of black points in such a way that the local point density is roughly equal to the average gray value in the corresponding regions of the source picture. The printing is controlled in such a fine fashion that the human eye cannot completely resolve the individual printed points or individual groups of points. The printed picture then appears to have continuous gray tones because of the spatial integration performed by the eye. The high resolution of a printer that cannot be fully perceived by the human eye is used to create an illusion of gray scale. 
     The halftone screen is used to create the halftone patterns  10 ,  12  printed on the document  14 . The frequency of the screen and the printed pattern is high (usually 300 dpi or higher) relative to the resolving capability of the eye. A halftone image can be obtained by thresholding, pixel by pixel, a gray level source image against a uniformly distributed random noise or dither. 
     A stochastic screen can be used to produce the invisible partial images and line patterns of the present invention. A stochastic halftone cell is a large threshold array that produces random appearing patterns in the halftone image. To produce an invisible image pattern, at least one additional stochastic cell is produced and used to incorporate image pattern information into the document. In the following embodiment only one additional stochastic screen cell will be described. This is not intended to limit the number of cells that can be used, since the extension to more stochastic screen cells is straightforward. 
     To produce an invisible image pattern, a first stochastic screen is produced to reproduce a gray image with acceptable image quality. A second stochastic screen is produced that is related to the first. Over most of the two halftone cells, the thresholds are identical, and therefore the patterns they produce are correlated. Over a part of the second halftone cell, the thresholds are randomized so that in this region the two cells are uncorrelated. The locations of the thresholds within this area are optimized a second time to produce pleasing patterns. In this way, the second stochastic cell produces patterns of the same image quality as the first cell. When an image is halftoned with these two cells and the images overlaid, the regions that are uncorrelated will appear darker. By alternating the two halftone cells, image pattern information can be incorporated into the halftoned image. 
     The present invention allows for a gray pattern to be used on a document, where the gray pattern can be generated using a halftoning process to produce a desirable gray. 
     Since the patterns are both based on the same random screen optimization or similar random screen optimizations, the patterns look approximately identical. However, when one screen is superimposed on another screen, as for example, by see-through on a document in alignment and superposition, the correlation and non-correlation between the images becomes apparent. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.