AN APERIODIC MOIRE SECURITY ELEMENT AND METHOD FOR PRODUCTION THEREOF

Abstract

A moir magnification device for authenticating security articles, the moir magnification device comprising: an array of micro focusing elements; and an array of micro images; wherein the array of micro focusing elements and the array of micro images are correspondingly aperiodic such that the micro focusing elements generate moir magnifications of the micro image when viewing the device at predetermined viewing angles. A device such as this raises the complexity and difficulty of the task facing would-be counterfeiters. The use of an aperiodic micro focusing element array and micro image array is not immediately apparent, as the moir magnifications observed by the viewer appear the same as that of a regular device with periodic arrays. In this way, the aperiodic aspect of the design remains covert until the moir device is more closely and deliberately analysed.

Claims

1. A moir magnification device for authenticating security articles, the moir magnification device comprising: an array of micro focusing elements; and an array of micro images; wherein the array of micro focusing elements and the array of micro images are correspondingly aperiodic such that the micro focusing elements generate moir magnifications of the micro image when viewing the device at predetermined viewing angles.

2. A moir magnification device according to claim 1, wherein the moir magnifications generated by the array of micro focusing elements and the micro image array are equivalent to moir magnifications generated by a periodic micro focusing element array and corresponding periodic micro image array.

3. A moir magnification device according to claim 1, wherein the device further comprises a substrate, such as a transparent polymer, and the aperiodic array of micro focusing elements is embossed on one side of the substrate while the micro images is embossed on the opposing side of the substrate.

4. A moir magnification device according to claim 1, wherein the device comprises a substrate, such as a transparent polymer, and the micro focusing element array and the micro image array are formed on the same side of the substrate, the micro image array comprising individual image elements in registration with one of the micro focusing elements respectively.

5. A moir magnification device according to claim 4, wherein the micro focusing elements are concave micro mirrors and the individual image elements are formed at the surface of the corresponding micro mirrors.

6. A moir magnification device according to claim 5, wherein the individual image elements are embossed into the surface of the corresponding micro mirror.

7. A moir magnification device according to claim 5, wherein the concave surface of the micro mirrors is coated, preferably with a metallic coating.

8. A moir magnification device according to claim 5, wherein the image elements are coated for optical contrast with the concave surface of the micro mirrors.

9. A moir magnification device according to claim 6, wherein the image elements are embossed as diffractive gratings, holographic structures or moth-eye structures in the surface of the concave micro mirrors.

10. A moir magnification device according to claim 1, wherein the micro image array comprises a first set of micro images and a second set of micro images, the first set of micro images and the second set of micro images being aperiodic in accordance with the aperiodicity of the micro focusing element array such that the moir magnifications of the first set of micro images appear to be at a different plane to the moir magnifications of the second set of micro images.

11. A method of producing a moir device for authenticating security articles, the method comprising the steps of: forming an aperiodic array of micro focusing elements; forming an aperiodic array of micro images positioned relative to corresponding micro focusing elements within the aperiodic array of micro focusing elements; such that, the array of micro focusing elements generate moir magnifications when viewing the device is viewed from predetermined angles.

12. A method according to claim 11, further comprising the steps of: providing a substrate, and embossing the micro focusing elements on one side of the substrate while simultaneously embossing the micro images on the other side of the substrate.

13. A method according to claim 11, wherein the micro focusing elements and the micro images are embossed into a radiation curable material applied to the substrate and subsequently or simultaneously cured.

14. A method according to claim 13, wherein the array of micro images comprises individual image elements embossed into respective micro focusing elements within the array of micro focusing elements.

15. A method according to claim 13, wherein the individual image elements and the micro focusing elements are simultaneously embossed on the one side of the substrate.

16. A security article incorporating a moir magnification device according to claim 1.

17. A security article according to claim 16, wherein the micro focusing elements have different sizes and focal lengths.

18. A security article according to claim 16, wherein the security article is a banknote.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0045] FIG. 1 is a schematic plain view of a prior art moir magnification device.

[0046] FIG. 2 is a schematic section view through a prior art moir magnification device.

[0047] FIG. 3 is a schematic plain view of a moir magnification device according to the invention.

[0048] FIG. 4 is a schematic cross-section of a moir magnification device according to the invention.

[0049] FIG. 5 is a schematic cross-section of a moir magnification device according to the invention, in which concave micro mirrors are used as focusing elements.

[0050] FIG. 6 is a schematic representation of the moir magnification device applied to a banknote.

[0051] FIG. 7 is a schematic cross-section of a moir magnification device according to the invention for magnification of two different micro image arrays.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0052] FIGS. 1 and 2 illustrate a basic form of known moir magnification devices 2. A lens array 4 is positioned on graphic elements in the form of a micro image array 6. The micro lens array 4 and the micro image array 6 are both periodic in that the individual lenses 8 and the individual micro images 10 are at regular spacings within their respective arrays.

[0053] As shown in FIG. 1, there is a slight mismatch between the spacing or pitch of the micro lenses 8 and that of the micro images 10. This causes each micro lens 8 to focus on a slightly different part of the underlying micro image compared to its neighbouring micro lenses.

[0054] As illustrated in FIG. 2, the individual magnifications from each micro lens 8 are combined in the eye of the viewer 12 to produce one or more moir magnifications 14 of the micro images 10.

[0055] The micro lens layer 16 needs to be precisely positioned relative to the micro image layer 18 to generate the required degree of magnification. Skilled workers in this field will readily understand that very small changes in the pitch mismatch between the micro lenses 8 and the micro images 10 result in very large changes in the degree of magnification. This level of precision is difficult for counterfeiters to achieve and thus moir magnification devices offer a reasonably effective counterfeit resistance measure. Unfortunately, sophisticated counterfeiters are able to replicate the moir magnification device such that it generates reasonably similar magnifications of the micro images.

[0056] To address this, the moir magnification device 22 shown in FIG. 3 uses an aperiodic micro focusing element array 24 and a correspondingly aperiodic micro image array 26. The micro focusing elements 8 are randomly positioned but still relatively close packed. Knowing the random positions of each micro focusing element 8, a corresponding position for an underlying micro image can be determined which still generates a moir magnification when viewed from a particular angle or range of angles. As shown in FIG. 4, the micro focusing elements are micro lenses 8 that each individually magnify an underlying portion or part of different micro images 10 formed in a micro image layer 32. Using the known moir magnified image 28 to be seen by the eye 12 from viewing angle , it is possible to determine the precise position and shape of the underlying micro images 10 required to generate the magnified image 28.

[0057] Similarly, the micro images 10 can be individually configured and distorted (relative to the micro image that would be used in a periodic moir magnification device) such that the desired moir magnification 28 is visible over a range of viewing angles . To achieve this requires significant computer processing capabilities to accurately determine the configuration of the micro images 10. Similarly, it is necessary to precisely fabricate and position the micro focusing elements 8 relative to the micro images 10. The equipment plant processing capacity needed to accurately generate the desired magnified image 28 is well beyond the typical counterfeiter. Furthermore, the aperiodic nature of the moir magnification device is not immediately apparent. Its operation will closely mimic that of a normal moir magnification device and therefore its aperiodic design becomes a covert security feature.

[0058] The precise registration between the micro focusing elements and the micro images 10 may be provided by a so-called double soft embossing process. This process involves embossing a radiation curable epoxy layer deposited on both sides of a substrate. In FIG. 4, the substrate 30 supports a UV curable epoxy layer 16 on one side and a similar UV curable layer 32 on the opposing side. The moir magnification device us compressed between opposing metal shims with surface relief in the form of a negative of the micro focusing element array 24 and a negative of the micro image array 26. This ensures that each micro focusing element 8 is in precise alignment with its corresponding micro image 10.

[0059] Layers 16 and 32 are uncured and soft prior to embossing. The curing process takes place shortly after or substantially at the same time as the embossing step. The radiation curable material is typically curable by ultra-violet (UV) radiation, however other radiation curable materials may be used which are sensitive to X-rays or electron beams.

[0060] The radiation curable materials used to form layers 16 and 32 are transparent or translucent and preferably include an acrylic based UV curable clear embossable lacquer or coating. Such UV curable lacquers can be obtained from various manufacturers including Kingfisher Ink Ltd, product Ultra-Violet Type UVF-203 or similar. Alternatively, the radiation curable embossable coatings may be based on other compounds, such as nitrocellulose.

[0061] The substrate 30 is typically the base material from which the security document to which the moir magnification device is applied. The base material is a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinylchloride (PVC), polyethylene terephthalate (PET), biaxially-oriented polypropylene (BOPP), or a composite material of two or more materials.

[0062] FIG. 5 shows an embodiment of the moir magnification device 22 in which the micro focusing elements are in the form of concave micro mirrors 34. Skilled workers in this field will readily understand that a micro image (or part of a micro image) 10 applied to the concave surface of a concave micro mirror 34 can provide a magnification of the micro image 10 equivalent to that of a micro lens focusing on an underlying micro image.

[0063] For precise registration between the concave micro mirrors 34 and the micro images 10, the micro mirrors and micro images can be simultaneously embossed into the UV curable material 16. Simultaneously embossing the micro images with micro mirrors requires the metal embossing shim to have a negative surface relief of the micro mirrors overlaid with a negative of the corresponding aperiodic micro images. However this ensures perfect registration (as in relative positioning to achieve the desired magnification) between the aperiodic array of focusing elements 24 and the micro image array 26. The lithographic fabrication of these metal shims (typically from nickel) is an advanced manufacturing technique beyond the reach of most counterfeiters.

[0064] The surface of the concave micro mirrors 34 may include a coating such as vapour deposited metal in order to improve reflection of incident light. Furthermore, it will be appreciated that this embodiment does not rely on transmission of light through the substrate 30. Therefore this form of the moir magnification device can be applied to value documents with opaque substrates 30.

[0065] FIGS. 6 and 7 show another form of the moir magnification device 22 applied to a banknote 36. The micro focusing elements are provided in the form of concave micro mirrors 34 as discussed above in relation to FIG. 5. However the magnification device 22 has two different micro image arrays, the first including micro images 10 and the second including micro images 46. Skilled workers in this field will understand that moir magnification devices can magnify two different images or different views of the same image which can be used to generate a 3D image or a moving image (see, for example, FIG. 2 of U.S. Pat. No. 5,712,731 to Drinkwater et al, and FIG. 29 of AU 2010226869 to Visual Physics LLC). Furthermore, if the aperiodic positioning of micro elements 10 corresponds to a regular moir magnification device in which the period mismatch between the micro lenses and micro images is set at a first value, which determines the orthoparallactic movement (OPM) 42 of the first moir magnification 38 in response to a change in viewing angles. Similarly, the aperiodic positioning of the second micro elements 46 can be such that it is equivalent to a period mismatch of a second value which generates orthoparallactic movement (OPM) 44 of the second moir magnification 40 which noticeably differs from the OPM 42 of the first magnification 38 in response to the same change in viewing angles.

[0066] This added level of complexity not only makes the moir magnification device 22 more visually distinctive when used, but also compounds the difficulty for counterfeiters. Accordingly, the aperiodic array of micro focusing elements 24 may be embossed with three or more separate arrays of micro images to further heighten the complexity and security of the device. Similarly, the size and focal length of the concave micro mirrors can be varied within the array as yet another level of complexity.

[0067] The invention has been described herein by way of example only. Skilled workers in this field will readily recognise any variations or modifications which do not depart from the spirit and scope of the broad inventive concept.