Abstract:
A verifiable/copy-protected document features a combination of nearly identical line-screen patterns for embedding latent images within visually integrated settings. The latent images can be detected for purposes of verification with a matching viewer but are indistinguishable from their visually integrated settings under ordinary viewing conditions. The line-screen patterns, which can be incorporated into document artwork, are printed at certain combinations of line frequencies and print densities so that the line-screen patterns digitally reproduce as a largely undifferentiated solid tint.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. application Ser. No. 10/895,822, filed on Jul. 22, 2004, as a Continuation of International Application No. PCT/US2003/32159, filed on Oct. 9, 2003, under the common title “Document Containing Security Images” and both prior applications are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates to the protection of documents, including various forms of printed matter, and to security features incorporated into documents for such purposes as authenticating or validating originals and distinguishing unauthorized copies.  
       BACKGROUND OF THE INVENTION  
       [0003]     Secure documents have been produced in the past by letterpress, offset, and intaglio printing processes, including such processes adapted for embedding hidden information or security images. The hidden information or security images, often referred to as latent images, are printed so as to be effectively invisible to the unaided eye but can be detected using various optical enhancements. Document embedded security features deter counterfeiting of valuable papers, important records, ID cards, and financial instruments such as checks, currency, so that original documents can be verified and unauthorized copies of these documents can be readily distinguished from the originals.  
         [0004]     The latent images, which often contain covert information or other indicia such as security codes, graphics, or information within the original documents, can be hidden within visually integrated settings so as to be practically invisible to the unaided eye under normal viewing conditions. Such settings for hiding the latent images range from simple background tints to the remainders of composite images within which the latent images are hidden. However, the latent images are otherwise distinguishable through the use of a visual aid, such as a magnifier or reader. Upon reproduction, such as by scanning or copying of the originals, the latent images can be arranged either to disappear or to become more pronounced. That is, either the absence of the latent image or the visible presence of the latent image in a copy can be construed as an indication of a non-original document.  
         [0005]     Such latent images can be produced by embedding one line-screen pattern within another line-screen pattern. Each of the line-screen patterns can be formed by an array of print elements, such as dots, dashes, line segments, or other elemental shapes, in an arrangement of systematically spaced lines. The two line-screen patterns can be formed or otherwise camouflaged so as to be indistinguishable from one another to the unaided eye, but can differ in some respect that can be optically exploited for viewing the latent image. For example, the two line-screen patterns can be printed in the same color and at the same print density but can differ in orientation or line frequency.  
         [0006]     A first line-screen pattern can be printed in defined areas having one or more shapes for forming the latent image. A second line-screen pattern can be printed throughout an adjoining or an enlarged area forming a visually integrated setting, such as a complementary background or a remaining portion of a composite image. Each of the two line patterns can be formed by a plurality of line patterns in different colors so long as the multiple line patterns that form the latent image match the color and overall appearance of the one or more line patterns that form the visually integrated setting.  
         [0007]     Although invisible to the unaided eye, the latent images can be detected by various optical enhancements such as through optical overlays that increase contrast between the latent images and their visually integrated settings. For example, viewers containing screen patterns that match one of the underlying screen patterns of the latent images or their visually integrated settings can fill in or obscure one line pattern with respect to another.  
         [0008]     In addition to the authentication and validation of originals, the latent images can also be used to distinguish copies from originals. For example, the latent images themselves can be rendered visible by copying for revealing messages such as “void” or “unauthorized copy” on the reproductions. Alternatively, the latent images can be arranged so as not to reproduce, so the absence of the latent image indicates a copy.  
         [0009]     Other developments for purposes of providing document protection are disclosed in the patent literature, as for example, in U.S. Pat. No. 5,018,767 issued May 28, 1991; U.S. Pat. No. 5,193,853 issued Mar. 16, 1993; and U.S. Pat. No. 3,675,948 issued Jul. 11, 1972; and U.S. Pat. No. 4,143,967 issued Mar. 13, 1979, all to Ralph C. Wicker. All of these patents, which are hereby incorporated by reference, disclose various means for providing methods and products to enable copies of documents to be distinguished from the originals, as for example, by a “large dot-small dot pattern”, a “close line-spaced pattern”, and images or indicia which are screen printed at minutely varied spaces and/or angles on the originals and are intended to produce a highly visible moiré pattern effect on the unauthorized copies. In this specification, the words “print”, “printed” and “printing” are used to refer to the making of an original document by any of a number of known printing means, including transferring images from one source to another, typically a paper medium, using a transfer agent such as ink or toner. The words “copy” and “copying” are used to refer to making copies from an original printed document also by any number of known copying means including by such means as digitally copying or scanning for output to a printer.  
         [0010]     Copier and computer scanner-printer technologies continue to evolve in ways that can thwart intentions for rendering the latent images irreproducible in their original form. A goal of these technologies is to obtain copies as good as an original. Copiers, scanners, and other duplicating equipment, including desktop publishing systems, have become increasingly sophisticated in color reproduction, including the color matching of copies to color standards such as the Pantone Color Matching System.  
         [0011]     Many of these document protection methods and products were developed before such significant improvements in copier and computer reproduction technologies and are no longer as effective for distinguishing copies from originals. Thus, further improvements in the area of document protection are needed, especially for distinguishing copies from originals to prevent unauthorized copying of valuable documents.  
       SUMMARY OF THE INVENTION  
       [0012]     We have discovered unique formulations for both authenticating and preventing unauthorized reproduction of latent images containing hidden information or other indicia within original documents. Such latent images together with their visually integrated settings are reproduced as generally solid tones formed by altered or otherwise transformed screen patterns no longer encoding the latent images in their original form. Copies of protected documents are rendered obvious by the alteration or transformation of the screen patterns from which the latent images and their visually integrated settings were originally formed.  
         [0013]     One version of the invention as a verifiable document includes a latent image embedded within a visually integrated setting of the document. The latent image and the visually integrated setting, which can be incorporated into the document in various forms such as bars, banners, borders, meaningful shapes, or as portions of art works or other graphic images, are formed by patterns of relatively oriented print elements. The print element patterns of the latent image and the visually integrated setting have common colors, common print densities, and common line frequencies within at least one of two combinations of print densities and line frequencies. The first is a combination of print densities of at least 20 percent and line frequencies of at least 175 lines per inch, and a second is a combination of print densities of at least 50 percent and line frequencies of less than 175 lines per inch. The commonalities among the colors and print densities of the print element patterns do not need to be exact but should be close enough so that the latent image remains at least partially indistinguishable from its visually integrated setting to the unaided eye.  
         [0014]     For example, the common print densities are preferably within approximately 5 percent of each other, and the common line frequencies are preferably within approximately 20 percent of each other and more preferably within 10 percent of each other. However, the print element patterns of the latent image and the visually integrated setting preferably differ in angular orientation by at least 5 degrees and more preferably by at least 30 degrees so that the latent image is distinguishable from the visually integrated setting with a matching visual aid.  
         [0015]     The matching visual aid can be a viewer oriented to exploit the different angular orientations of the print element patterns of the latent image and the visually integrated setting in the verifiable document to produce contrast between the two patterns. The viewer can contain a line-screen pattern with a line frequency having an integer multiple relationship (i.e., an even multiple or even quotient) with the line frequency of the latent image or visually integrated setting. The angular difference between the print element patterns of the latent image and the visually integrated setting produces contrasting moiré effects (e.g., more or less fringes) when the screen pattern of the viewer is aligned with the print element pattern of one or the other of the latent image and visually integrated setting.  
         [0016]     The above-described combinations of print densities and line frequencies of the latent image and visually integrated setting interfere with the digital reproduction of the verifiable document. As reproduced by a majority of commercial digital copiers, the latent image and the visually integrated setting are not similarly contrastable by the matching viewer. For example, the latent image and the visually integrated setting can be reproduced as a solid tone on the digitally reproduced copies. The solid tone often has a print density noticeably above or below the common print density of the print element patterns of the latent image and the visually integrated setting. The original print element patterns of the latent image and visually integrated setting are at least partially replaced by print elements in other (including random) orientations or frequencies, which tend to blur differences between the latent image and visually integrated setting and partially or even completely fill in spaces not previously occupied by the original print elements. As a result, the contrast between the latent image and the visually integrated setting through the matching viewer is significantly reduced in the copies.  
         [0017]     The common line frequencies are preferably at least 175 lines per inch (69 lines per centimeter), more preferably between 175 lines per inch and 200 lines per inch (79 lines per centimeter), and even more preferably approximately 180 lines per inch (71 lines per centimeter). The common print densities are preferably at least 20 percent and more preferably between 20 percent and 50 percent. However, at common line frequencies below 175 lines per inch, the common print densities are preferably at least 50 percent. Common line frequencies of at least 65 lines per inch are preferred at the print densities of at least 50 percent. The print element patterns of the latent image and visually integrated setting have angular orientations that preferably differ by at least 5 degrees, more preferably by at least 30 degrees, and most preferably by 45 degrees.  
         [0018]     Another version of the invention as a copy-protection system for verifiable documents includes a document containing a verifiable security feature as a latent image within a visually integrated setting. The latent image and the visually integrated setting are substantially indistinguishable to the unaided eye but are visually contrastable for verification through a visual aid. Print elements of the latent image and the visually integrated setting are arranged in patterns so that the latent image and the visually integrated setting digitally reproduce in altered graphic forms that are not similarly contrastable (e.g., contrast significantly reduced) through the visual aid as originally oriented with respect to the latent image and visually integrated setting of the document.  
         [0019]     The print elements of the latent image and the visually integrated setting preferably have common colors, common print densities, and common line frequencies so as to render the latent image and the visually integrated setting substantially indistinguishable to the unaided eye. However, the print elements of the latent image and the visually integrated setting differ from one another in angular orientation so that the latent image can be distinguished from the visually integrated setting through the visual aid. The differing angular orientations produce contrasting moiré effects (e.g., more or less fringes) that allow the latent image to be viewed for authenticating the document. However, upon digital reproduction, the latent image and the visually integrated setting are not similarly contrastable by the matching viewer as originally oriented with respect to the latent image and visually integrated setting of the document.  
         [0020]     Another version of the invention involves a method of distinguishing a copy of an original copy-protected document from the original copy-protected document. A latent image is embedded in a visually integrated setting of the original copy-protected document using patterns of print elements. The patterns of print elements for the latent image and the visually integrated setting are arranged with common colors, common print densities, and common line frequencies so that the latent image is at least partially indistinguishable from the visually integrated setting to the unaided eye. However, the patterns of print elements for the latent image and the visually integrated setting sufficiently differ in at least one respect, such as being oriented at different angles (e.g., 5 degrees or greater) so that the latent image and the background can be contrasted by a visual aid. The copy-protected document is digitally reproduced, reproducing the latent image and the visually integrated setting in one or more patterns of print elements that differ from the original patterns of the print elements of the latent image and the visually integrated setting so as not to be similarly contrastable by the visual aid as originally oriented with respect to the latent image and visually integrated setting of the copy-protected document.  
         [0021]     Yet another version of the invention involves a method of making a copy-protected document. First and second patterns of print elements are printed on a document in common colors, common print densities within 10 percent of each other, and common line frequencies within 20 percent of each other for hiding a latent image within a visually integrated setting. Nonetheless, the first and second patterns are differentiated so that the latent image of the first pattern is distinguishable from the visually integrated setting of the second pattern with a visual aid. The line frequencies and print densities are arranged so that the latent image and the visually integrated setting reproduce on a majority of commercial digital copiers in an altered form that is not similarly contrastable by the same visual aid as originally oriented with respect to the latent image and visually integrated setting of the copy-protected document.  
         [0022]     For verifying the authenticity of the copy-protected document, the visual aid can be arranged as a viewer can be arranged for distinguishing the latent image from the visually integrated setting, such as on the basis of differing line frequencies or differing orientation angles sufficient to produce differing moiré effects (e.g., more or less fringes) between the latent image that the visually integrated setting. However, upon digital reproduction of the copy-protected document, this basis for distinguishing the latent image from the visually integrated setting is lost, and the copy is not verifiable as an original.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0023]      FIG. 1  is a front view of a truncated copy-protected document in accordance with this invention having a banner containing latent images (i.e., the word “VALID”) formed by line-screen patterns of exaggerated pitch and outlined for rendering the otherwise hidden latent image visible within the banner.  
         [0024]      FIG. 2  is an enlarged view of a portion of the banner better showing the relatively oriented line-screen patterns used for defining but otherwise hiding the latent image “VALID”.  
         [0025]      FIG. 3  is a similar front view of the copy-protected document with a decoding viewer overlay for revealing the latent image.  
         [0026]      FIG. 4  is a front view of a truncated copy of the copy-protected document as reproduced on a conventional digital copier in which the line-screen patterns of the latent image and the banner reproduce as an uninterrupted solid tint.  
         [0027]      FIG. 5  is a front view of an alternative copy-protected document in truncated form with a decoding viewer overlay for revealing a latent image.  
         [0028]      FIG. 6  is a front view of a copy of the copy-protected document of  FIG. 5  as reproduced on a conventional digital copier in which line-screen patterns of the latent image and the banner reproduce as an uninterrupted solid tint. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     A copy-protected document  10  as shown in  FIGS. 1-3  has a top banner  12  formed by relatively oriented line-screen patterns  14  and  16 . Centered within the banner  12 , as an example of ancillary additional printing structures, is a text or graphics indicium  18 , depicted by the word “LOGO”. The line-screen patterns  14  form latent images  20  and  22  as the word “VALID”, which is at least partially hidden to the unaided eye within a visually integrated setting  24  ( a background) formed by the line-screen pattern  16 . A substrate  28  on which the banner  12  is printed can be any of a variety of print mediums, including paper, plastic, film, in single or multi-ply constructions.  
         [0030]     The two line-screen patterns  14  and  16  are formed by arrays of print elements, which in this case are continuous lines but could also be dots, diamonds, dashes, line segments, and or other elemental shapes, in an arrangement of systematically spaced lines (curved or straight). Except for orientation angle, the two line-screen patterns  14  and  16  are largely indistinguishable as being printed in matching colors, line frequencies, and print densities. For example, the line-screen patterns  14  of the latent images  20  and  22  are printed at 0 degrees (i.e., along the width of the document  10 ), and the line-screen pattern  16  of the visually integrated setting  24  is printed at 45 degrees. In general, the line-screen patterns  14  and  16  with matching line frequencies are preferably separated through an angle of at least 5 degrees, more preferably through an angle of at least 30 degrees, and even more preferably (as shown in this example) at approximately 45 degrees.  
         [0031]     The colors and print densities of the two line-screen patterns  14  and  16  match so that the latent images  20  and  22  are at least partially indistinguishable from the visually integrated setting  24  to the unaided eye. The print densities of the line-screen patterns  14  and  16  preferably match within 10 percent and more preferably within 5 percent of each other. Similarly, the line frequencies of the line-screen patterns  14  and  16  preferably match within 20 percent and more preferably within 10 percent of each other. However, as shown in  FIG. 3 , a viewing device  30 , having a line-screen pattern  32  with a line frequency matching the line frequency of the screen patterns  14  and  16 , is angularly oriented to one or the other of the screen patterns  14  or  16  to provide contrast between the two. The line-screen pattern  32  can be phase shifted to align with the lines of the underlying line-screen patterns  14  or  16  or with the spaces between these lines for making the latent image  22  appear lighter (as shown), darker, or some different frequency combination of the two relative to the visually integrated setting  24 . The overlapping line-screen pattern  32  of the viewing device  30  produces different moiré effects (e.g., more or less fringes) in combination with the screen patterns  14  and  16  of the latent images  20  and  22  and the visually integrated setting  24 . The differing moiré effects produce a clearly distinguishable contrast between one or more of the latent images  20  and  22  and the visually integrated setting  24 .  
         [0032]     The line frequency of the line-screen pattern  32  can match the common line frequency of the line-screen patterns  14  and  16  on a 1-to-1 basis or as an integer multiple or integer quotient of the common line frequencies. The match does not need to be exact but close enough so that a visible contrast is formed between the latent image  22  and the visually integrated setting  24  as a result of the differing orientations of the line-screen patterns  14  and  16  to the line-screen pattern  32 . Similarly, the common line frequencies among the line-screen patterns  14  and  16  do not need to be exact but should be close enough so that the line-screen pattern  32  can produce effective visual contrast with either of the line-screen patterns  14  or  16  when oriented into alignment with either pattern. In fact, subtle differences in line frequency between the two line-screen patterns  14  and  16 , such as line frequencies of 178 and 182, can be used as a further authenticating feature that would otherwise not affect the ability of the latent image  22  to remain hidden within its visually integrated setting  24  to the unaided eye or the ability of the viewing device  30  to produce the required contrast for distinguishing the latent image  22  from its visually integrated setting  24  based on the different angular orientations or their respective line-screen patterns  14  and  16 .  
         [0033]     The viewing device  30 , which is shown in the form of a transmissive plate, can be held against the copy-protected document  10  and rotated for aligning its line-screen pattern  32  with the line-screen pattern  14  or  16  of the latent image  22  of the visually integrated setting  24 . Approaching an orientation of alignment, interfering fringes (not shown) appear that progressively reduce in number and can disappear entirely if the 1-to-1 matching frequencies are perfectly aligned.  
         [0034]     The substrate  34  of the viewing device  30  can be made of plastic, glass, film or other optically transmissive material. The substrate  34  can also be formed of as a lens, preferably with magnifying power. The line-screen pattern  32  can be printed or embossed in the substrate  34 , such as by silkscreen printing or laser burning. The substrate  34  can be clear, tinted, or lined to assist the viewing of the latent images  20  and  22  and can contain one or more than one distinct areas for viewing of multiple screened areas. Each of the screened areas can be formed by screen patterns of different color, line frequency, print density, and relative angular orientation, and each of the distinct areas of the substrate  34  can have a line frequency corresponding to one or more of the screen patterns.  
         [0035]     Although shown at an exaggerated pitch, i.e., a low line frequency, for purposes of illustration, both of the line-screen patterns  14  and  16  preferably share substantially the same line frequency of at least 175 lines per inch (69 lines per centimeter). The matching line frequencies are more preferably between 175 lines per inch and 200 lines per inch (79 lines per centimeter) and even more preferably approximately 180 lines per inch (71 lines per centimeter). The print densities of the line-screen patterns  14  and  16  are also matched at print densities of at least 20 percent and more preferably between 20 percent and 50 percent. Alternatively, at matching line frequencies below 175 lines per inch, the matching print densities are preferably at least 50 percent. Within the lower range, the matching line frequencies are preferably at least 65 lines per inch, and the required print densities tend to be higher approaching line frequencies of 65 lines per inch than the print densities required for line frequencies approaching 175 lines per inch. For example, print densities of at least 60 percent are preferred at the lower end of the range. As used herein, print density is generally defined as the percent of a printed area covered by the print elements. The remaining space of the printed area is located between the lines or between print elements along the lines. The remaining space is assumed to be white, and the effective print density is adjusted depending upon any color contribution of the remaining space.  
         [0036]     The matching line frequencies and print densities are set in relation to the resolving capabilities of a majority of digital copiers (i.e., at least among the most widely used models) to reproduce the line-screen patterns. The combinations of line frequencies and print densities are set so that the original screen patterns are largely replaced by screen patterns in a different form not matched with the intended viewer. Preferably, the angular difference between the line-screen patterns  14  and  16  is also not accurately reproduced in the copies. The preferred intention of the invention is not for the line-screen patterns  14  or  16  of the latent images  20  and  22  and the visually integrated setting  24  to be entirely undetectable in the copies but to be not reproducible as line-screen patterns with the same line frequencies and orientations. Often, however, the line-screen patterns  14  and  16  reproduce in a common graphic form, such as a solid tint as seen in  FIG. 4 . Print elements within the reproduced line-screen patterns preferably fill in spaces not previously occupied by the print elements of the original line-screen patterns  14  and  16 .  
         [0037]     The line frequencies of at least 175 lines per inch combined with the print densities of at least 20 percent or the line frequencies of less than 175 lines per inch combined with print densities of at least 50 percent overcome the apparent resolving capabilities of digital copiers, including scanners and other digital reproducing technologies, resulting in reproductions that do not similarly discriminate between the original latent images  20 ,  22  and the visually integrated setting  24 . That is, the original print element patterns of the latent image and visually integrated setting are at least partially replaced by print elements in other (including random) orientations or frequencies, which tend to blur differences between the latent images and visually integrated settings and partially or even completely fill in spaces not previously occupied by the original print elements.  
         [0038]      FIG. 4  depicts a digital copy  40  of the copy-protected document  10  in which the banner  12  formed by the line-screen patterns  14  and  16  is reproduced in the copy  40  as a modified banner  42  having a common graphic form throughout the regions originally represented by the different line-screen patterns  14  and  16 . The logo  18  ( FIGS. 1-3 ), which is not formed by one of the combinations of common line frequencies and common print densities of the line-screen patterns  14  and  16 , is accurately reproduced as the logo  44 . The banner  42  can have a solid tint at a shade darker or lighter than the original banner  12  and can be formed by a common graphic pattern throughout, e.g., a common halftone or line-screen pattern. A color, textural, or other graphic overlay (not shown) could be incorporated into the original banner  12  for camouflaging the original orientation difference between the line-screen patterns  14  and  16 . Such overlays can be printed separately or incorporated into the line-screen patterns as higher order variations in amplitude or frequency. Any reproduction of such overlays would be independent of differences between the orientations of the line-screen patterns  14  and  16 .  
         [0039]     An alternative copy-protected document  50  is shown in  FIGS. 5 and 6  with latent images  60  and  62  hidden within a visually integrated setting  64  (as a background)—together forming a banner  52  on a substrate  68 , which can be overprinted by a text or graphics indicium  58 , depicted by the word “LOGO”. The latent images  60  and  62  depicting the word “VALID” are formed by a line-screen pattern  54  oriented at an approximately 45 degree angle to the orthogonal axes of the document  50 . The visually integrated setting  64  is formed by the line-screen pattern  56  also oriented at an approximately 45 degree angle to the orthogonal axes of the document  50 .  
         [0040]     In addition to being oriented at the same angle (e.g., 45 degrees), the two line-screen patterns  54  and  56  are printed in a common color and at a common print density to hide the latent images  60  and  62  within the visually integrated setting  64 . However, for purposes of illustration, the latent image  60  is outlined against the visually integrated setting  64  with white space. The line frequencies of the line-screen patterns  54  and  56  preferably differ by only approximately 5 percent to 20 percent of each other. The difference in line frequency is small enough to help hide the latent images  60  and  62  but is large enough to be exploited by a viewer  70 , which can be similar to the viewer  30  ( FIG. 3 ), for visually contrasting the latent images  60  or  62  from the visually integrated setting  64  as also shown in  FIG. 5 .  
         [0041]     Similar to the copy protected document  10  of  FIGS. 1 -3 , the substrate  68  on which the banner  52  is printed can be any of a variety of print mediums, including paper, plastic, film, in single or multi-ply constructions. The two line-screen patterns  54  and  56  are also formed by arrays of print elements, which are depicted as continuous lines but could also be dots, diamonds, dashes, line segments, and or other elemental shapes, in an arrangement of systematically spaced lines (curved or straight). Although differing slightly in line frequency, the line-screen patterns  54  and  56  substantially match in color, print density, and orientation angle. As a result, the latent images  60  and  62  are at least partially indistinguishable from the visually integrated setting  64  to the unaided eye.  
         [0042]     However, as also shown in  FIG. 5 , the viewing device  70  has a line-screen pattern  72  with a line frequency matching the line frequency of one or the other of the line-screen patterns  54  and  56  to provide contrast between the two patterns  54  and  56 . When matched to the line-screen pattern  54 , the line-screen pattern  72  can be phase shifted to align with the lines of the underlying line-screen pattern  54  or with the spaces between these lines for making the latent image  62  appear lighter (as shown), darker, or some different frequency combination of the two in comparison to the visually integrated setting  64 . The overlapping line-screen pattern  72  of the viewing device  70  produces different moiré effects between the line-screen patterns  54  and  56 . When oriented into alignment with the two line-screen patterns  54  and  56 , the line-screen pattern (e.g.,  56 ) that differs most from the line-screen pattern  72  of the viewing device  70  produces more fringes than the line-screen pattern (e.g.,  54 ) that matches the line-screen pattern  72 . The differing moiré effects produce a clearly distinguishable contrast between one or more of the latent images  60  or  62  and the visually integrated setting  64 .  
         [0043]     The line frequency of the line-screen pattern  72  can match the line frequency of either of the line-screen patterns  54  or  56  on a 1-to-1 basis or as an integer multiple or integer quotient of the line frequency. The match does not need to be exact but close enough so that a visible contrast is formed between the latent image  60  or  62  and the visually integrated setting  64  as a result of the differing line frequencies of the line-screen patterns  54  and  56 .  
         [0044]     Although the line-screen patterns  54  and  56  differ from each other in line frequency by as much as 20 percent, the line frequencies of both line-screen patterns  54  and  56  are preferably at least 175 lines per inch (69 lines per centimeter) and even more preferably between 175 lines per inch and 200 lines per inch (79 lines per centimeter). The print densities of the line-screen patterns  54  and  56  are matched at print densities of at least 20 percent, more preferably between 20 percent and 50 percent. Alternatively, the line frequencies of both line-screen patterns  54  and  56  are preferably at least 65 lines per inch and less than 175 lines per inch (69 lines per centimeter) and the print densities of the line-screen patterns  54  and  56  are matched at print densities of at least 50 percent. Print densities of 60 percent or more are preferred for the lower end of the range beginning at 65 lines per inch.  
         [0045]     The combinations of line frequencies and print densities are set beyond the resolving capabilities of a majority of digital copiers (i.e., at least among the most widely used models) to reproduce the line-screen patterns. That is, the combinations of line frequencies and print densities are set so that the original screen patterns are largely replaced by screen patterns in a different form not matched with the intended viewer  70 . Preferably, the line frequency difference between the line-screen patterns  54  and  56  is not accurately reproduced in the copies. That is, the line-screen patterns  54  or  56  of the latent images  60  and  62  and the visually integrated setting  24  need not be entirely indistinguishable in the copies but should be reproduced in different forms no longer distinguishable on the basis of their original line frequencies and orientations. More preferably, the line-screen patterns  54  and  56  are reproduced in a common graphic form, such as a solid tint as seen in  FIG. 6 . Print elements within the reproduced line-screen patterns preferably fill in spaces not previously occupied by the print elements of the original line-screen patterns  54  and  56 .  
         [0046]      FIG. 6  depicts a digital copy  80  of the copy-protected document  50  in which the banner  52  ( FIG. 5 ) formed by the line-screen patterns  54  and  56  is reproduced in the copy  80  as a modified banner  82  having a common graphic form throughout the regions originally represented by the different line-screen patterns  54  and  56 . The logo  58  ( FIG. 5 ), which is not formed by one of the combined line-frequency, print density line-screen patterns  54  and  56 , is accurately reproduced as the logo  84 . The banner  82  has a solid tint, which can be a shade darker (as shown) or lighter than the original banner  52  and is formed by a common graphic pattern throughout, e.g., a common halftone or line-screen pattern. A color, textural, or other graphic overlay (not shown) could be incorporated into the original banner  52  for camouflaging the original orientation difference between the line-screen patterns  54  and  56 . Such overlays can be printed separately or incorporated into the line-screen patterns as higher order variations in amplitude or frequency. Any reproduction of such overlays would be independent of differences between the orientations of the line-screens  54  and  56 .  
         [0047]     Although shown as a banner  12  or  52  in the illustrated embodiments, the common-color, common line frequency and print density combinations within the line-screen patterns can be incorporated into documents in a variety of ways, including as bars, borders, meaningful shapes, or as portions of art works or other graphic images. Although the overall print density in the latent images and their immediate visually integrated settings is at least 20 percent for line frequencies above 175 lines per inch and at least 50 percent for line frequencies below 175 lines per inch, additional variations in print density can be accommodated for purposes of blending into graphic designs, superimposing other graphic designs, or for other appearance reasons.  
         [0048]     The invention has been described in conjunction with specific embodiments thereof, but it is evident that many alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.