AU2017101673A4 - Authentication means using deliberate misregistration of print elements - Google Patents

Abstract

A security feature for an article of value 10 is disclosed. The article 10 is one of a set of similar articles of value, and the security feature has a substrate 24 and, an image element (e.g. 14, 12, 20, 30 or 32) formed on the substrate. The substrate has a reference feature (e.g. 15, 28) defining a registered position for the image element, and the image element is formed at a misregistered position (e.g. X1, X2 , X3 or X4 ). The misregistered position is deliberately offset from the registered position such that the deliberate offset differs from deliberate offsets respectively defining the misregistered positions of corresponding image elements on other valuable articles in the set. The offset(s) may be small enough to go unnoticed to the casual observer but machine readable to provide a means to authenticate the article as genuine. (Fig. 1 to Accompany Abstract) 2017 8675 309 Figure 4 26 12 18 16 Figure 5 - - 26 A I. - Figure 6

Description

Authentication Means using Deliberate Misregistration of Print

Elements

Field of the Invention [0001] The invention relates generally to the field of security features and anticounterfeit measures used to authenticate articles of value. In particular, the invention relates to security features that use the image elements shown on security features to authenticate a valuable article.

Background of the Invention [0002] To guard against counterfeits, security features are applied to articles of value such as security documents or tokens. Security features are often difficult to replicate and may provide evidence of tampering. Security features applied to documents such as identity cards, passports, credit cards, banknotes, cheques, vouchers, tickets and certificates will incorporate features that are overt (readily apparent to the observer) or covert (not visible or apparent to the observer). Overt security features are complex to falsify and/or easily damaged by removal from the document. This type of first level security feature allows members of the public or untrained staff to perform a basic authentication check of the document or article without needing any additional specialised equipment (e.g. UV light etc.).

[0003] One commonly used overt security feature is the application of a reflective foil to the article. This provides a basic but effective measure to prevent forgeries using colour photocopiers. The specular reflection provided by a metallic foil is not reproduced by the coloured inks or toners used by a photocopier.

[0004] More sophisticated counterfeiters will sometimes attempt to remove the foil from one security document, such as a banknote, and apply it to another counterfeit document (sometimes referred to as ‘upgrading’ banknotes). To counter this, the document and foil are often overprinted with a pattern. To remove the foil from a genuine document and apply it to a counterfeit will require precise registration between the patterns printed on the underlying document and the pattern overprinted on the foil (see for example US 6,508,489). This is a complex and time consuming process that provides a substantial disincentive for many counterfeiters.

[0005] With the development of polymer banknotes, the use of transparent windows as a security feature has become common. The transparent window is an effective and readily apparent (overt) security device that cannot be reproduced with a colour photocopier. However, as counterfeiters become more sophisticated, there is an ongoing imperative to improve security features on documents and articles of value. In light of this, additional security features may be incorporated into the windows of the polymer documents. A foil patch may be applied to the window of a polymer document to improve its counterfeit resistance and this foil may itself support additional security features, such as holograms or other diffractive microstructures. Unfortunately, applying an overprint to a foil within the window of a polymer document is problematic.

[0006] Polymer security documents with window areas, such as polymer banknotes, are constructed using a transparent polymer substrate (typically biaxially orientated polypropylene (BOPP)) and applying opacifying ink layers on the front and back. The opacifying ink is omitted from the window area on one or both sides of the substrate and is typically applied in the required pattern by a gravure printing process.

[0007] These opaque inks are specifically formulated to strongly adhere to the polymer surface. Polymers such as biaxially oriented polypropylene (BOPP) have a low surface energy which do not ‘wet out’ properly for the majority of inks leading to problems with unintentional ink transfer and drying times. These specially formulated opaque inks overcome these issues but are not used for any subsequent printing of images and indicia on the banknote. The subsequently applied inks are printed on the opaque sections via lithographic, offset and/or intaglio printing processes.

[0008] Printing these inks on the BOPP window, or a foil applied within the BOPP window, results in ‘ink set-off where the printed ink unintentionally transfers from one document to another. In light of this, the foils applied within windows are not overprinted and therefore susceptible to removable and re-application to counterfeit documents. As discussed above, overprinting the foil and surrounding surface with an intricate pattern is a substantial disincentive for would be counterfeiters.

[0009] Covert security features operate on the premise that counterfeiters cannot replicate a feature they are unaware of. Common covert features involve printing areas with ink that reflects (or absorbs) light outside the visible spectrum such as infrared (IR) or ultra violet (UV). These features are easily detected with the appropriate light source and viewing filter, and hence well suited to automatic detection in document handling equipment such as automatic teller machines. The use of these inks is known and even unsophisticated counterfeiters will check for IR and UV images.

[0010] Any reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of the Invention [0011] With the above issues in mind, a first aspect of the present invention provides a security feature for an article of value, the article being one of a set of articles of value, the security feature comprising: a substrate; and, an image element formed on the substrate; wherein, the substrate has a reference feature defining a registered position for the image element, and the image element is formed at a misregistered position, the misregistered position being deliberately offset from the registered position such that the deliberate offset differs from deliberate offsets respectively defining the misregistered positions of corresponding image elements on other valuable articles in the set.

[0012] As discussed above, the inadvertent misregistration of overprints on foils has fortuitously provided a tamper evident feature in the past. The present invention uses a deliberate or contrived misregistration of an image element to provide an authentication feature that is effectively covert to the casual observer but machine readable without (necessarily) using expensive IR or UV inks.

[0013] Preferably, the reference feature is one or more peripheral edges of the substrate. Optionally, the reference feature is one or more features selected from: • a second image element formed on the substrate; • a window or half window formed on the substrate; • a lens array; and, • a foil attached to the substrate.

[0014] Preferably, the deliberate offset does not exceed a predetermined maximum offset. Preferably, the predetermined maximum offset of the image element is small enough that the misregistered position is not immediately apparent to a casual observer. In a further preferred form, the deliberate offset from the registered position is randomly determined in terms of direction and length. Optionally, the deliberate offset is calculated. In some forms, the misregistered position is a unique identifier for the article of value. Preferably, the set of articles each have an individual serial number and the deliberate offset is calculated using an algorithm applied to the serial number such that the misregistration uniquely identifies the article of value.

[0015] According to a second aspect, the present invention provides a security feature for an article of value, the security feature comprising: a substrate with a surface for receiving print elements; a foil applied to an area of the surface; a print element applied to the surface, the print element having a first portion applied to the foil and a second portion that borders on the area with the foil; wherein the first portion is printed with a printing technique that differs from that used to print the second portion, such that there is a misregistration between the first and second portions.

[0016] Overprinting the foil with a different, more suitable print technique effectively addresses the problem of ink set off discussed above. However, the Applicants have also recognised this provides a mechanism to introduce a deliberate misregistration between the overprint on the foil and print on the surrounding note. This misregistration is machine readable but can still be small enough that it is not immediately apparent during normal use. This provides an effectively covert authentication feature.

[0017] Preferably, the subsrate is a polymer substrate, and in some forms, the polymer is biaxially oriented polypropylene (BOPP).

[0018] The process of printing sheets or continuous web with a polymer substrate has been found to be more stable than printing paper sheets or web. That is, the position and movement of the polymer sheets and webs through the print press is more precise and finely toleranced. This allows greater control of the deliberate misregistration between the first and second portions of the overprint. Better control of the misregistration allows small variations that are indicative of the banknote, such as the denomination or a unique identity.

[0019] Optionally, the misregistration between the first and second print portions is indicative of a type of the article.

[0020] Preferably, the misregistration between the first and second print portions is a unique identifier for the article.

[0021] Preferably, the offset is algorithmically generated to uniquely identify the article.

[0022] In some embodiments, the article is associated with a unique serial number which is used by the algorithm to generate the degree of misregistration.

[0023] In some embodiments of the first and second aspects of the invention, the article of value is a security document, the substrate of the security feature is a polymer substrate that provides a polymer substrate for the security document.

[0024] Preferably, the print element is formed by printing or embossing.

[0025] Optionally, the print element is printed using one or more of: laser printing; inkjet printing; gravure; intaglio printing; offset printing; toner transfer; or lithographic printing.

[0026] In relation to the second aspect of the invention, the first portion of the print element on the foil is formed by laser printing, embossing, gravure, ink-jet or toner transfer.

[0027] Preferably, the second portion of the print element is printed by offset printing, lithographic printing, or intaglio printing.

[0028] Preferably, the print element is an overprint pattern.

[0029] Preferably, the polymer substrate is a biaxially oriented polypropylene and the foil has a reflective layer such as a thin metallic layer.

[0030] Preferably, a primer is applied to the first surface of the polymer substrate prior to applying the foil.

[0031] Preferably, an opacifying ink layer is printed on the first surface of the polymer substrate in a pattern defining a window in which the opacifying layer is not present, such that the area in which the foil is applied is defined by the window.

[0032] Preferably, the window further comprises an optically variable device selected from: a diffraction grating; a hologram; a volume hologram; a diffractive optical element (DOE); a liquid crystal polymer layer; and/or a moire magnification device.

[0033] Preferably, the optically variable device is within the foil.

[0034] Preferably, the security document is a banknote.

[0035] Preferably, the misregistration is associated with the denomination of the banknote.

[0036] Preferably, the misregistration is associated with the serial number of the banknote.

[0037] In a third aspect, the present invention provides a method of producing a set of articles of value, each article in the set having a security feature, the method comprising: providing a substrate for each of the security features such that the substrate has a reference feature used to define a registered position for an image element; and, determining a misregistered position for the image element that is deliberately offset from the registered position such that the deliberate offset differs from deliberate offsets respectively defining the misregistered positions of corresponding image elements on other valuable articles in the set.

[0038] According to a fourth aspect, the present invention provides a method of producing a security feature for an article of value, the method comprising the steps of: providing a substrate with a surface for receiving a visible print; applying a foil to an area of the surface; applying a print element on the first surface by printing a first portion of the visible print on the foil using a first printing technique and printing a second portion of the visible print using a second printing technique that differs from the first printing technique, wherein the second portion borders on the area in which the foil is applied, and a degree of misregistration exists between the first and second portions.

[0039] According to a fifth aspect, the present invention provides a set of security documents, each security document comprising: a substrate; and, an image element formed on the substrate; wherein, the substrate has a reference feature defining a registered position for the image element, and the image element is formed at a misregistered position, the misregistered position being deliberately offset from the registered position such that the deliberate offset differs from deliberate offsets respectively defining the misregistered positions of corresponding image elements on other security documents in the set.

[0040] According to a sixth aspect, the present invention provides a security document comprising: a polymer substrate with a surface for receiving print elements; a foil applied to an area of the surface; a print element applied to the first surface, the print element having a first portion applied to the foil and a second portion that borders on the area with the foil; wherein the first portion is printed with a printing technique that differs from that used to print the second portion, such that there is a misregistration between the first and second portions.

Definitions

Security Document or Token [0041] As used herein the term security document includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0042] The invention is particularly, but not exclusively, applicable to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Substrate [0043] As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

[0044] The use of plastic or polymeric materials in the manufacture of security documents pioneered in Australia has been very successful because polymeric banknotes are more durable than their paper counterparts and can also incorporate new security devices and features. One particularly successful security feature in polymeric banknotes produced for Australia and other countries has been a transparent area or “window”.

Transparent Windows and Half Windows [0045] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[0046] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

[0047] A partly transparent or translucent area, hereinafter referred to as a “half-window,” may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the “half-window” is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.

[0048] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying Layers [0049] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that Lj<L0 where Lo is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Security Device or Feature [0050] As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Embossable Radiation Curable Ink [0051] The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink maybe cured by other forms of radiation, such as electron beams or X-rays.

[0052] The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as sub-wavelength gratings, transmissive diffractive gratings and lens structures.

[0053] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating, [0054] Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings maybe based on other compounds, e.g. nitro-cellulose.

[0055] The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be embossed with larger relief structures, such as non-diffractive optically variable devices.

[0056] The ink is preferably embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a Gravure printing process.

[0057] Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.

[0058] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a crosslinker, for example a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetyl acetone; melamines; and carbodi-imides.

Comprise, Comprises, Comprised or Comprising [0059] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

Brief Description of the Drawings [0060] Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a banknote with a security feature according to one embodiment the present invention;

Figure 2 is an enlarged schematic sectional view along section 2-2 shown in Figure 1;

Figure 3 is an enlarged view of Inset A shown in Figure 1;

Figure 4 is a schematic representation of a banknote with a security feature according to another embodiment of the invention;

Figure 5 is an enlarged schematic sectional view along section 5-5 shown in Figure 4;

Figure 6 is an enlarged view of Inset A shown in Figure 4; and

Figure 7 is a flowchart of the main steps in the production of a security device according to the invention.

Detailed Description of the Preferred Embodiments [0061] Figurel shows an article of value in the form of a banknote 10 with an image element 14 used as a security feature 1. The partial cross-section of the banknote 10 shown in Figure 2, best illustrates the layered structure through the window region. The banknote 10 has a polymer substrate 24 in the form of a biaxially oriented polypropylene (BOPP) transparent layer. On the other surface of the BOPP substrate 24 in the orientation (shown in Figure 2), is a thin layer of primer 34 to improve the adhesion of a foil patch 12. The foil patch 12 may include a reflective layer, such as a metallic reflective layer or simply provide a polymer film in which other image elements or security features are formed. The foil 12 is positioned in a window 20 provided by openings in the opacifying ink layer (or layers) 26 on the upper surface of the substrate 24. As discussed above, the window 20 may be a transparent window without opacifying layers on either side of the substrate, or a ‘half window’ with opacifying ink 22 on one side only.

[0062] Opaque ink layers 22 and 26 are applied by a gravure printing process using inks specifically formulated to adhere to the BOPP substrate 24. These (typically white) ink layers have been developed to bond with substrates with low surface energy, such as BOPP. Normal inks do not ‘wet-out’ properly on the surfaces which causes problems with ink transfer to other substrates and long drying times. As discussed above, a thin primer layer 34 is applied before the foil patch 12 is stamped into the window 20. Stamping, or even hot stamping, a foil into unprimed areas of BOPP does not provide sufficient adhesion.

[0063] The opacifying ink layers 22 and 26 are overprinted with visible images and indicia. These image elements normally include the serial number 30, the denomination 32, any filigree patterning 14 and embossed diffractive features 28. Image elements printed onto the opacifying ink layer 26 are usually applied using lithographic and/or intaglio printing processes. However, printing onto a metallic foil 12 using these techniques is problematic and the ink is prone to unintentional transfer onto other documents (known as ‘ink set off). This is caused by inadequate drying times for intaglio and lithographic inks when printed onto a foil patch. These inks are predominantly formulated for particular colour saturation levels, durability and, importantly, drying times specified for an underlying opaque ink layer. In light of this, the portion of overprint 18 on the foil 12 is applied using a different print technique to that of the overprint portion 16 on the remainder of the banknote 10. However, this is discussed in more detail below in relation to the second embodiment described with reference to Figures 4 to 6.

[0064] As discussed in the background section, the printing of security documents, including banknotes, has traditionally involved a degree of inherent misregistration between print elements that would be considered significant by today’s standards. In today’s continuous web or sheet fed printing processes, the polymer substrate (and even the paper substrate) are more stable with respect to their position on each successive print station. Furthermore, advances in printing technology have allowed techniques such as inkjet printing, laser printing and so on to be incorporated into larger gravure, intaglio, lithographic or offset printing processes. With the improved control of print registration, the Applicants have realised that a controlled reintroduction of some misregistration can be used to provide a security feature for the purposes of authentication. Of course, the misregistration of print elements should be restricted to a level that generally goes unnoticed unless closely scrutinised.

However, the optical sensors used in document handling machines, such as ATMs, are more sensitive and can identify and quantify relatively small misregistrations.

[0065] In Figure 1, the filigree pattern 14 is an image element on the banknote 10 that can be subjected to position misregistration without being immediately apparent to the casual observer. For example, minor shifts in the position of the filigree pattern 14 relative to a reference feature, such as star burst motif 15 or the periphery of the substrate 24, can be machine readable by monitoring the offset Xi (in the case of the star burst motif 15) or X2 (in the case of the substrate edge).

[0066] A controlled misregistration of the pattern 14 from a nominal position, with respect to the reference feature, provides an effectively covert security feature that goes unnoticed to the untrained eye. Furthermore, the security feature can be enhanced by using the misregistration of more than one image element relative to one or more reference features. For example, Figure 1 shows the image element in the form of pattern 14 with a controlled misregistration Xi relative to the star burst motif 15 and a second misregistration X2, relative to the left edge of the substrate 24. Then in Figure 3, the foil overprint portion 18 of the pattern 14 is the print element with a controlled horizontal misregistration X3 and a controlled vertical misregistration X4 relative to the star shaped diffractive embossing 28. With four separate machine readable misregistrations, each possible combination of Xi, X2, X3 and X4’ may be used to indicate an attribute of the banknote 10, such as the denomination 32 or even its unique identity which can be confirmed using the serial number 30. Indeed the serial number 30 can be used to determine one or more of the misregistrations Xi to X4. For example, by applying an appropriate algorithm to the serial number 30, or portions of the serial number 30, one or more of the misregistrations Xi to X4, are calculated and therefore provide a unique identifier for the banknote 10. Distributing a number of misregistrations across the banknote 10 provides additional variables for more precise authentication, and/or allows each individual misregistration to be smaller in magnitude, and therefore less noticeable.

[0067] Figure 4 shows an article of value in the form of a banknote 10 with a security feature 1 according to another embodiment of the invention. A partial cross section of the banknote 10 through line 5-5 is shown in Figure 5. The banknote 10 has a transparent BOPP substrate 24. On a first side of the polymer substrate 24, is a thin layer of primer 34 to improve the adhesion of a metallic foil patch 12. The foil 12 is positioned in a window 20 formed in one or more opacifying ink layers 26. The second side of the polymer substrate 24 may also have one or more opacifying ink layers 22. The window 20 may be a transparent window defined by areas without opacifying layers on either side, or a ‘half window’ with opacifying material on one side only.

[0068] The opaque ink layers 22 and 26 are applied by a gravure printing process and these inks are specifically formulated to adhere to the BOPP substrate 24. Opacifying ink layers 22 and 26 are typically white and formulated for use with substrates with low surface energy such as BOPP. Most inks do not ‘wet out’ properly on these surfaces giving rise to issues with ink transfer and ink drying. The foil patch 12 requires a thin primer layer 34 before being hot stamped into the window 20. Hot stamping a foil onto unprimed areas of BOPP or the white opacifying ink 26 will not properly adhere.

[0069] The opacifying white ink layers 22 and 26 are printed with visible images and indicia, including elements such as the serial number 30, the denomination 32 and any patterning 14, such as fine filigree. These images and indicia are usually printed on the white opacifying layer using lithographic and intaglio printing techniques. As discussed above, printing the outer surface of a metallic foil 12 using these techniques often results in ink set off and spoilage from unintentional transfer onto other documents. This is because intaglio inks and lithographic inks are formulated for particular colour saturation levels, durability and so on. These inks will usually have a particular drying mechanism which makes them unsuitable for application to a foil patch in light of the ink set off issue. In view of these problems, polymer banknotes have not previously had overprint patterns extending across the foil. Some paper notes, such as that shown in US6508489, have a foil patch with an overprint pattern. However, in paper banknotes, the foil is inserted into a window cut through the paper substrate. Prior to insertion into the window, the foil is coated with a primer layer to be more receptive to the ink.

[0070] Polymer banknotes create windows by omitting the opacifying layers on one or both sides of the transparent polymer substrate. The application of a primer layer to the outside of a foil patch is not well suited to the process of hot stamping the foil onto the BOPP substrate 24.

[0071] It has been found that polymer banknotes are more stable than paper banknotes during the printing process. The movement of the polymer sheet along the media feed path is more precise and allows the printing tolerances to be smaller. It light of this, the portion 18 of the overprint 14 extending over the foil 12 can be formed using a different and more suitable printing technique. Similarly, the overprint 18 may be embossed onto the foil 12. In this way, the overprint pattern 14 is formed using two different techniques, the first used to print the portion 18 across the foil 12 and the second technique used to print the portion 16 across the opacifying layers 26. The print technique used on the portion 18 across the foil 12 may be inkjet, toner, laser or embossing. These print techniques are relatively easy to into a continuous web printing process. As discussed above, the portion 16 of the pattern 14 printed across the opacifying layers 26 is typically lithographic or intaglio printing.

[0072] In light of the precise tolerances associated with printing polymer banknotes, the process can incorporate some degree of random or deliberate misregistration between the portion 16 printed on the opacifying layer and the portion 18 printed on the foil 12. As best seen in Figures 4 and 6, the misregistration, δ, can be small enough that it is not apparent to the casual observer but is discerned using specialised optical equipment (see enlarged inset A, shown in Figure 6).

[0073] Incorporating a deliberate degree of misregistration (δ), not apparent to the casual observer, provides a tamper indication should the foil be replicated or reapplied onto a counterfeit document. Furthermore, the degree of misregistration δ may also be indicative of some aspect of the banknote 10, such as the denomination 32.

[0074] The degree of misregistration may also be a unique identifier for each banknote 10. The misregistration may be derived using an algorithm and optionally, the algorithm may use the serial number 30, which is unique to each bank note. If the foil 12 is removed from a genuine bank note 10 and added to a counterfeit note, the covert association between the degree of misregistration and the serial number 30 will be almost impossible to replicate.

[0075] It will be appreciated that the banknote 10 may include other security devices 28 provided on the foil 12 or elsewhere. These security devices may be diffraction gratings, holograms, volume holograms, diffractive optical elements, liquid crystal polymers or moire magnification devices. In the case of a moire magnification device, it will be necessary to incorporate a micro image array into the foil 12 and a corresponding array of focussing elements such as micro-lenses (not shown) on the opposing side of the polymer substrate 24.

[0076] Figure 7 is a flow chart showing the basic steps for producing the tamper evident feature according to the present invention. Sheets or continuous web of a polymer substrate such as BOPP is first printed with a primer on areas for the foil strip or patch (step 38). As discussed above, metallic foils will not bond properly to bare BOPP, but the specially formulated opacifying inks will adhere to the unprimed surface. The BOPP is gravure printed on one or both sides with at least one layer of opacifying ink. The opacifying ink is patterned to provide a window exposing the surface of the polymer substrate coated with the primer.

[0077] A foil strip or patch is stamped (usually hot stamped) into the window directly onto the primer (step 42). Images and indicia are printed on the opacifying layers using a first printing technique such as lithographic offset or intaglio printing (step 44). Skilled workers will appreciate that applying the foil and the intaglio/lithographic inks can happen in any order.

[0078] The exposed surface of the foil patch or strip is printed using a different printing technique such as ink jet, laser, embossing, gravure or toner transfer. A deliberate misregistration with the print from the first printing technique is incorporated to provide a tamper indication if the document were a counterfeit or the foil were reapplied to a counterfeit document.

[0079] The degree of misregistration is indicative of a characteristic of the bank note, such as its denomination. The degree of misregistration may also be indicative of a unique identifier for the bank note 10 (step 48).

[0080] Similarly, an alternative or additional print element can be printed or embossed with a controlled misregistration from one or more reference features such as the edge of the banknote, a window, a further print element and so on.

[0081] If desired, further security devices such as holograms, diffraction grating or lens based optically variable devices can be added (step 46) to the window, or elsewhere on the document.

[0082] The invention has been described herein by way of example only. Skilled workers in this field will readily recognise many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.

Claims (5)

1. Claims

   1. A security feature for an article of value, the article being one of a set of articles of value, the security feature comprising: a substrate; and, an image element formed on the substrate; wherein, the substrate has a reference feature defining a registered position for the image element, and the image element is formed at a misregistered position, the misregistered position being deliberately offset from the registered position such that the deliberate offset differs from deliberate offsets respectively defining the misregistered positions of corresponding image elements on other valuable articles in the set.
2. 2. A security device according to claim 1 wherein the reference feature is one or more features selected from: • a second image element formed on the substrate; • one or more peripheral edges of the substrate; • a window or half window formed on the substrate; • a lens array; and, • a foil attached to the substrate.
3. 3. A security device according to claim 2 wherein the deliberate offset of the misregistered position is less than a predetermined maximum offset.
4. 4. A security device according to claim 1 wherein the misregistered position is a unique identifier for the article of value.
5. 5. A method of producing a set of articles of value, each article in the set having a security feature, the method comprising: providing a substrate for each of the security features such that the substrate has a reference feature used to define a registered position for an image element; and, determining a misregistered position for the image element that is deliberately offset from the registered position such that the deliberate offset differs from deliberate offsets respectively defining the misregistered positions of corresponding image elements on other valuable articles in the set.