AU2017100679A4 - Optical security device and method - Google Patents

Optical security device and method Download PDF

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AU2017100679A4
AU2017100679A4 AU2017100679A AU2017100679A AU2017100679A4 AU 2017100679 A4 AU2017100679 A4 AU 2017100679A4 AU 2017100679 A AU2017100679 A AU 2017100679A AU 2017100679 A AU2017100679 A AU 2017100679A AU 2017100679 A4 AU2017100679 A4 AU 2017100679A4
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substrate
focusing
lens
image
security
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AU2017100679A
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Michael Hardwick
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CCL Security Pty Ltd
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CCL Security Pty Ltd
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Abstract

Abstract A security device is disclosed including a substrate such as a banknote foundation and one or more focusing elements or lens structures located on one side of the substrate. The focal length of at least some of the focusing elements or lens structures is arranged so that it is greater than the distance to the opposite side of the substrate by a pre-determined margin to facilitate detecting image elements that are not located on the substrate. Each image element may be located in an object plane associated with a machine reader and may be associated with a respective focusing element or lens structure. L7 \ 12113114 15 \ / I 13a

Description

1 2017100679 08 Jun2017
OPTICAL SECURITY DEVICE AND METHOD CROSS REFERENCE TO RELATED APPLICATION
[0001] The present invention is related to the following international patent application, the disclosure of which is incorporated herein by cross reference. PCT/AU2011/001063 entitled Multichannel Optically Variable Device published under WO2012/024718.
FIELD OF THE INVENTION
[0002] The present invention relates to optical security devices and methods for their manufacture and/or verification. In particular the present invention relates to optical security devices which include focusing elements or lens structures in their construction to generate machine readable optical effects and/or features.
BACKGROUND TO THE INVENTION
[0003] Optical security devices are commonly used in connection with valuable documents as a means of avoiding unauthorised duplication or forgery of such documents. Typically, such security devices may produce optical effects and/or features which may be difficult for a potential counterfeiter to replicate. The optical effects and/or features may also be used as a means to assist verification of the valuable documents.
[0004] Counterfeiting of banknotes and other valuable documents has become an increasingly important issue in recent times due to ready availability of colour photocopiers and computer scanning equipment. This technology provides counterfeiters with an easier route to copying of valuable documents issued using traditional security printing technologies. In response central banks and banknote printers have turned to technologies which produce images that vary with changing angle of view, and which therefore cannot be easily photographed.
[0005] Such devices known collectively as optically variable devices (OVDs) have proven to be very successful in reducing incidence of counterfeiting using computer scanning equipment. However, the counterfeiters have not been idle 2 2017100679 08 Jun2017 during this time and some groups have adopted alternative holographic approaches to counterfeiting or simulating diffractive optically variable devices produced by banknote printing groups.
[0006] There is a need to increase security of valuable documents such as polymer banknotes using an alternative technology incorporating lens based features rather than diffractive optical principles.
[0007] There is also a need to develop a banknote authentication method to address potential edge detection and/or feeding issues associated with polymer banknotes that contain edge to edge windows.
[0008] The present invention may provide a machine readable feature or features suitable for use in a public verification station such as automatic teller machine (ATM) or other banknote accepting machine.
SUMMARY OF THE INVENTION
[0009] The present invention may be based on optically variable features similar to “Horizon” which makes use of lens based features and “interlaced images” to generate flipping or moving images associated with a security device or banknote (refer commemorative Singapore $50 note-http://www.mas.gov.sg/News-and-Publications/Media-Releases/2015/Commemorative-Notes-to-Celebrate-SGSG50.aspx).
[0010] According to one aspect of the present invention there is provided a security device including a substrate such as a banknote foundation and one or more focusing elements or lens structures located on one side of said substrate, wherein the focal length of at least some of said focusing elements or lens structures is greater than the distance to the opposite side of the substrate by a pre-determined margin to facilitate detecting an image or image elements that are not located on the substrate.
[0011] The focusing elements or lens structures may be printed or formed via a soft emboss technology as described herein to produce a machine readable feature 3 2017100679 08 Jun2017 or features. The Machine readable feature or features may be incorporated in automatic teller machines (ATM) or other banknote accepting machines.
[0012] The or each focusing element or lens structure may include a printed/embossed lens in a windowed area (i.e. transparent on the opposite side of the lens) of the substrate. The or each focusing element or lens structure may contain a focal point which does not focus on the opposite side of the substrate as is the case with “Horizon” but may instead focus on an image or image element located a pre-determined distance away from the opposite side of the substrate.
[0013] Each image or image element may be located in an object plane associated with a machine reader rather than the security device or banknote and may be associated with a respective focusing element or lens structure. In one form each image or image element may be located a pre-determined distance away from a banknote containing a security device. If the machine is an ATM, each image or image element may be located a pre-determined distance from a transport roller and opposite a line camera to coincide with a focal length of a focusing elements or lens structure to facilitate detecting the or each image or image element not located on the substrate. Each image or image element may facilitate optical effects and/or features and/or may include one or more subregions and/or may be paired with complementary image elements.
[0014] The above arrangement may generate optical effects in the form of high contrasting switches and/or contrast switching movements at least when elements of the image or images are viewed from a first position relative to an axis perpendicular to the base plane of the device. The image or image elements may include a greyscale portrait of a face, scene, logo, alphanumeric character or any other graphic design.
[0015] In another form the one or more focusing elements or lens structures may include at least a first and a second array of focusing elements or lens structures, wherein the focal length of at least some focusing elements or lens structures in the first array is greater than the distance to the opposite side of the of the substrate by a first pre-determined margin, and wherein the focal length of at least some focusing 2017100679 08 Jun2017 4 elements or lens structures in the second array is greater than the distance to the opposite side of the of the substrate by a second pre-determined margin.
[0016] Each image or image element may be located in respective first and second object planes associated with a machine reader. Each image or image element in the first object plane may be associated with a respective focusing element or lens structure in the first array. Each image or image element in the second object plane may be associated with a respective focusing element or lens structure in the second array. The first and second pre-determined margins may be utilized to detect a feature of a valuable document that incorporates the security device, such as a denomination of a banknote.
[0017] Each focusing element or lens structure may be produced in any one of a number of ways. In one form the or each focusing element or lens structure may be produced by applying a layer of embossable radiation curable ink to a substrate and soft embossing the layer prior to curing the ink via ultraviolet radiation.
[0018] The optical security device may be embossed or attached to a valuable document such as a banknote, passport, credit card, cheque, etc. in order to prevent or inhibit counterfeiting of that document.
[0019] According to a further aspect of the invention, there is provided a method of forming a security device, including the steps of: providing a substrate having at least one transparent portion and applying one or more focusing elements or lens structures to a first surface of the transparent portion, wherein the focal length of at least some of said focusing elements or lens structures is greater than the distance to the opposite side of the substrate by a pre-determined margin to facilitate detecting image elements that are not located on the substrate.
[0020] The method may include a step of applying a layer of embossable radiation curable ink to the substrate prior to being embossed while soft and curing the ink by radiation to form the one or more focusing elements or lens structures on one side of the substrate. 2017100679 08 Jun2017 5 [0021] The method may also include the step of providing at least one opacifying layer as an opacifying coating, preferably an opacifying ink layer.
[0022] Further aspects of the invention are directed to a security document, such as a banknote including the optical security device as described in any of the embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Specific embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: [0024] Figure 1 shows a cross sectional view of an optical security device according to one embodiment of the present invention.
[0025] Figure 2 shows a cross sectional view of an optical security device being detected by a machine reader.
[0026] Figure 3 shows a cross sectional view of an optical security device according to another embodiment of the present invention.
[0027] Figure 4 shows a cross sectional view of an optical security device according to another embodiment of the present invention. 2017100679 08 Jun2017 6
DESCRIPTION OF PREFERRED EMBODIMENTS DEFINITIONS
Security document [0028] 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 licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.
[0029] The invention is particularly, but not exclusively, applicable to security documents or tokens 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 can also have application in other products, such as packaging.
Security Device or Feature [0030] 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, and alteration or tampering. Security devices or features can 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 can 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) comprising reflective optical structures including reflecting surface relief structures and diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs). 7 2017100679 08 Jun2017
Transparent Windows and Half Windows [0031] 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.
[0032] 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.
[0033] A partly transparent or translucent area, hereinafter referred to as a "halfwindow", 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.
[0034] 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 [0035] 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 l_T < L-o, 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 8 2017100679 08 Jun2017 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.
Diffractive Optical Elements (DOEs) [0036] As used herein, the term diffractive optical element refers to a numerical type diffractive optical element (DOE). Numerical-type diffractive optical elements (DOEs) rely on the mapping of complex data that reconstruct in the far field (or reconstruction plane) a two-dimensional intensity pattern. Thus, when substantially collimated light, e.g. from a point light source or a laser, is incident upon the DOE, an interference pattern is generated that produces a projected image in the reconstruction plane that is visible when a suitable viewing surface is located in the reconstruction plane, or when the DOE is viewed in transmission at the reconstruction plane. The transformation between the two planes can be approximated by a fast Fourier transform (FFT). Thus, complex data including amplitude and phase information has to be physically encoded in the micro-structure of the DOE. This DOE data can be calculated by performing an inverse FFT transformation of the desired reconstruction (i.e., the desired intensity pattern in the far field).
[0037] DOEs are sometimes referred to as computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms.
Embossable Radiation Curable Ink [0038] 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) 9 2017100679 08 Jun2017 radiation. Alternatively, the radiation curable ink may be cured by other forms of radiation, such as electron beams or X-rays.
[0039] 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 subwavelength gratings, transmissive diffractive gratings and lens structures.
[0040] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.
[0041] 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 may be based on other compounds, e.g. nitro-cellulose.
[0042] 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 micro lenses and lens arrays. However, they may also be embossed with larger relief structures, such as non-diffractive optically variable devices.
[0043] 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.
Focal point size H
[0044] As used herein, the term focal point size refers to the dimensions, usually an effective diameter or width, of the geometrical distribution of points at which rays refracted through a lens intersect with an object plane at a particular viewing angle. The focal point size may be inferred from theoretical calculations, ray tracing simulations, or from actual measurements. 10 2017100679 08 Jun2017
Focal length f [0045] In the present specification, focal length, when used in reference to a micro lens in a lens array, means the distance from the vertex of the micro lens to the position of the focus given by locating the maximum of the power density distribution when collimated radiation is incident from the lens side of the array (see T. Miyashita, "Standardization for micro lenses and micro lens arrays" (2007) Japanese Journal of Applied Physics 46, p 5391).
Gauge thickness t [0046] The gauge thickness is the distance from the apex of a lenslet on one side of the transparent or translucent material to the surface on the opposite side of the translucent material on which the image elements are provided which substantially coincides with the object plane.
Lens frequency and pitch [0047] The lens frequency of a lens array is the number of lenslets in a given distance across the surface of the lens array. The pitch is the distance from the apex of one lenslet to the apex of the adjacent lenslet. In a uniform lens array, the pitch has an inverse relationship to the lens frequency.
Lens width W
[0048] The width of a lenslet in a micro lens array is the distance from one edge of the lenslet to the opposite edge of the lenslet. In a lens array with hemispherical or semi-cylindrical lenslets, the width will be equal to the diameter of the lenslets.
Radius of curvature R
[0049] The radius of curvature of a lenslet is the distance from a point on the surface of the lens to a point at which the normal to the lens surface intersects a line extending perpendicularly through the apex of the lenslet (the lens axis).
Sag height s [0050] The sag height or surface sag s of a lenslet is the distance from the apex to a point on the axis intersected by the shortest line from the edge of a lenslet extending perpendicularly through the axis. 11 2017100679 08 Jun2017
Refractive index n [0051] The refractive index of a medium n is the ratio of the speed of light in vacuo to the speed of light in the medium. The refractive index n of a lens determines the amount by which light rays reaching the lens surface will be refracted, according to Snell's law: η * Sin (a ) = n * Sin (Θ ), where a is the angle between an incident ray and the normal at the point of incidence at the lens surface , Θ is the angle between the refracted ray and the normal at the point of incidence, and η is the refractive index of air (as an approximation η may be taken to be 1 ).
Conic constant P
[0052] The conic constant P is a quantity describing conic sections, and is used in geometric optics to specify spherical (P = 1 ), elliptical (0 < P < 1 , or P > 1 ), parabolic (P = 0), and hyperbolic (P < 0) lens. Some references use the letter K\ to represent the conic constant. K\s related to P via K= P -1 .
Lobe Angle [0053] The lobe angle of a lens is the entire viewing angle formed by the lens. Abbe number [0054] The Abbe number of a transparent or translucent material is a measure of the dispersion (variation of refractive index with wavelength) of the material. An appropriate choice of Abbe number for a lens can help to minimize chromatic aberration.
[0055] Referring to the drawings, Figure 1 shows a cross sectional view of an optical security device 10, wherein a layer of embossable radiation curable ink is applied to a transparent or windowed area of substrate 11 prior to being embossed while soft to form lenticular focusing structures 12-15. The ink may be cured by radiation to fix the embossed lenticular focusing structures 12-15 located on one side 16 of substrate 11. Each focusing structure 12-15 is formed such that its focal length is greater than the distance D1 to the opposite side 17 of substrate 11 by a predetermined margin M1. The focal point of each focusing structure 12-15 12 2017100679 08 Jun2017 (represented by focal point 13a of focusing structure 13) facilitates detecting image elements 20-23 as shown in Figure 2 by means of line camera 18.
[0056] Referring to Figure 2, image elements 20-23 are not located on substrate 11 but are instead located on surface 24 associated with a machine such as an ATM, banknote sorting or other purpose built machine. Image elements 20-23 may be placed a pre-determined margin M1 away from substrate 11. If the associated machine is an ATM, this may be a pre-determined distance from transport rollers (not shown) and opposite line camera 18 to coincide with the focal point 13a of lenticular focusing structure 13. When image elements 20-23 are moved into the focal point 13a of lenticular focusing structure 13, an optical effect may be detected, validated and/or analysed. The optical effect may include a moving, 3D or other lens effect and may be validated in any suitable manner and by any suitable means via dedicated software.
[0057] Figure 3 shows a cross sectional view of an optical security device 30 wherein a layer of embossable radiation curable ink is applied to a transparent or windowed area of substrate 31 prior to being embossed while soft to form lenticular focusing structures 32-35. The ink may be cured by radiation to fix the embossed lenticular focusing structures 32-35 located on one side 36 of substrate 31.
[0058] Each focusing structure 32, 34 is formed such that its focal length is substantially equal to the distance D2 to the opposite side 37 of substrate 31. This may produce an optical effect similar to Florizon which may be visible to the eye 38 through lenticular structures 32, 34 focused on the back 37 of substrate 31.
[0059] Each focusing structure 33, 35 is formed such that its focal length is greater than the distance D2 to the opposite side 37 of substrate 31 by a predetermined margin M1. The focal point of each focusing structure 33, 35 (represented by focal point 33a of focusing structure 33) facilitates detecting image elements 20-23 as shown in Figure 2 by means line camera 39.
[0060] The embodiment of figure 3 provides an optical security device which may produce a primary optical effect visible to members of the public through focusing 13 2017100679 08 Jun2017 structures 32, 34 which focus on the back 37 of substrate 31, but may also produce a secondary optical effect as described above with reference to figure 2.
[0061] Lenticular focusing structures 32-35 may be interlaced as shown in figure 3, or may be located in two discrete areas on a lens patch with substantially no gap between each area on the lens patch.
[0062] Figure 4 shows a cross sectional view of an optical security device 40 wherein a layer of embossable radiation curable ink is applied to a transparent or windowed area of substrate 41 prior to being embossed while soft to form lenticular focusing structures 42-45. The ink may be cured by radiation to fix the embossed lenticular focusing structures 42-45 located on one side 46 of substrate 41.
[0063] Each focusing structure 42, 44 is formed such that its focal length is greater than the distance D3 to the opposite side 47 of substrate 41 by a first predetermined margin M2. The focal point of each focusing structure 42, 44 (represented by focal point 42a of focusing structure 42) facilitates detecting image elements placed a predetermined margin M2 away from substrate 41 (such as elements 20-23 shown in Figure 2) by means line camera 48.
[0064] Each focusing structure 43, 45 is formed such that its focal length is greater than the distance D3 to the opposite side 47 of substrate 41 by a second predetermined margin M3. The focal point of each focusing structure 43, 45 (represented by focal point 43a of focusing structure 43) facilitates detecting image elements placed a predetermined margin M3 away from substrate 41 (such as elements 20-23 shown in Figure 2) by means line camera 49.
[0065] The embodiment of figure 4 provides an optical security device which may provide an option to authenticate specific banknotes including specific denominations of banknotes. In this embodiment different image structures 20-23 may be placed at separate focal points 42a, 43a on separate surfaces associated with a machine such as an ATM. When a bank note passes these focal points, one of two different image elements 20-23 may come into focus depending on which focusing structure 42, 43 passes an associated camera 48, 49. 14 2017100679 08 Jun2017 [0066] Lenticular focusing structures 42-45 may be interlaced as shown in figure 4, or may be located in two discrete areas on a lens patch with substantially no gap between each area on the lens patch.
[0067] A protective coating such as a transparent varnish? may be applied over the lenticular focusing structures 12-15, 32-35, 42-45. Protective coating? may be applied over the lenticular focusing structures 12-15, 32-35, 42-45 as well as over other areas of substrate 11,31,41 in which focusing structures 12-15, 32-35, 42-45 are not present. The latter areas (not shown) may contain a non-floating image that may be visible at least when the device is rotated about an axis perpendicular to the plane of the device or is rocked back and forth about an axis within the plane of the device. The non-floating image(s) may comprise a diffractive or non-diffractive structure such as a colour changing ink.
[0068] Preferably, the protective coating ? includes a high refractive index (HRI) coating, as this may assist in ensuring that the optical effect produced by lenticular focusing structures 12-15, 32-35, 42-45 remains visible even if the coating is applied in a very thin layer. However, in other embodiments possible coatings may include a transparent, non-high refractive varnish.
[0069] It will be appreciated that a suitable coating should demonstrate one or all of the following attributes: good adhesion to the substrate, highly transparent, generally colourless, and robust. Possible coatings may include a transparent, non-high refractive varnish. Varnish may denote a material that results in a relatively durable and protective finish. Exemplary transparent varnishes may include, but are not limited to, nitrocellulose and cellulose acetyl butyrate. Alternatively, the coating may include a high refractive index coating, being a coating having a metal oxide component of small particle size and high refractive index dispersed in a carrier, binder or resin. Such a high refractive index coating may contain solvent as it is a dispersion. Where a high refractive index coating of this type is used, it may be air cured or UV cured.
[0070] Alternatively, a high refractive index coating utilising a non-metallic polymer, such as sulphur-containing or brominated organic polymers may also be used. 2017100679 08 Jun2017 15 [0071] The metallic nanoparticle ink 24 may be applied by any one of several techniques that will be apparent to the person skilled in the art. Preferably, the ink is applied by gravure, however may also be applied by other suitable techniques such as flexography or offset printing.
[0072] Advantages of the optical security device of the present invention may include: 1. It may provide a machine readable feature or features. 2. It may provide a banknote validation method for ATMs and other banknote sorting equipment. 3. It may improve security of valuable documents such as banknotes. 4. It may be combined with "Horizon" feature to produce an overt and covert security feature. 5. It may provide an edge guide for edge to edge polymer banknote windows in ATMs. 6. It may provide a denomination detecting feature by changing the focal length of the lens. 7. Lenses having multiple focal lengths may be combined, either interlaced or in close proximity to each other, to provide an extra layer of security to valuable documents such as banknotes.
[0073] Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims (5)

  1. Claims
    1. A security device including a substrate such as a banknote foundation and one or more focusing elements or lens structures located on one side of said substrate, wherein the focal length of at least some of said focusing elements or lens structures is greater than the distance to the opposite side of the substrate by a pre-determined margin to facilitate detecting an image or image elements that are not located on the substrate.
  2. 2. A security element according to claim 1 wherein each image or image element is located in an object plane associated with a machine reader and wherein each image or image element is associated with a respective focusing element or lens structure.
  3. 3. A security element according to claim 1, wherein said one or more focusing elements or lens structure includes at least a first and a second array of focussing elements or lens structure, wherein the focal length of at least some focusing elements or lens structures in said first array is greater than the distance to the opposite side of the of the substrate by a first pre-determined margin, and wherein the focal length of at least some focusing elements or lens structures in said second array is greater than the distance to the opposite side of the of the substrate by a second pre-determined margin.
  4. 4. A method of forming a security device, including the steps of: providing a substrate having at least one transparent portion and applying one or more focusing elements or lens structures to a first surface of the transparent portion, wherein the focal length of at least some of said focusing elements or lens structures is greater than the distance to the opposite side of the substrate by a pre-determined margin to facilitate detecting image elements that are not located on the substrate.
  5. 5. A method of authenticating a valuable document, including the steps of: providing a substrate having at least one transparent portion; applying one or more focusing elements or lens structures to a first surface of the transparent portion, wherein the focal length of at least some of said focusing elements or lens structures is greater than the distance to the opposite side of the substrate by a pre-determined margin; and using said one or more focusing elements or lens structures to detect image elements that are not located on the substrate.
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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3835851A1 (en) * 2019-12-10 2021-06-16 Thales Dis France Sa Laser engravable floating image for security laminates

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3835851A1 (en) * 2019-12-10 2021-06-16 Thales Dis France Sa Laser engravable floating image for security laminates
WO2021115954A1 (en) * 2019-12-10 2021-06-17 Thales Dis France Sa Laser engravable floating image for security laminates

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