METHODS OF MANUFACTURE

FIELD OF THE INVENTION

[0001 ] The present invention is directed to security devices including highly reflective areas and methods of manufacture. In particular, the invention relates to a method of manufacturing security devices including highly reflective areas with metallic particle or nanoparticle inks.

BACKGROUND TO THE INVENTION

[0002] Security devices are typically used in security documents as a means of ensuring that documents can be authenticated and avoid forgery or

unauthorised duplications of those documents.

[0003] A variety of reflective security devices are known which rely on the application of reflecting surfaces and relief structures to produce these security devices. It is particular advantageous for the relief structures or surfaces to have a high reflectivity such as those from a metallic surface.

[0004] Conventionally, techniques to produce security devices include embossing a diffraction pattern into a polymer layer to form a surface relief pattern. A thin reflective layer of metal can then be applied over the pattern to form a reflective relief structure. An image is produced that is viewable in reflection from the resultant metallic diffractive pattern.

[0005] One way of producing the thin reflective metal layer is to use a vacuum deposition process. The material to be coated is placed into a vacuum chamber and a metal is vaporised. The vaporised metal then condenses onto the material, forming a metallic layer. While effective, vacuum deposition methods are expensive due to the procurement and maintenance of vacuum equipment. [0006] Another way to produce thin metal layers on a substrate is to use metallic inks. The application of these inks is substantially less costly than the use of vacuum deposition processes while providing thin metal layers.

[0007] However, metallic inks such as silver inks have several disadvantages. First, metallic inks have low adhesive properties which can result in a low adherence to the substrate or material to which they are applied, and thus security devices formed from such metallic inks are less durable. This may require the addition of layers under or over the layer of metallic ink to improve adhesion and hence improve the durability of the security document. An additional coating or layer over the ink or metallic layer can reduce its reflectivity or other optical properties.

[0008] Secondly, once an ink is applied, it is typically air-dried so that the ink changes from a liquid to a solid to form a metallic layer on the material. However, this has been known to reduce the reflectivity of the metallic layer.

[0009] Further, the air-drying process of the ink layer is sensitive to external environmental factors such as humidity and elevated temperatures. This also can affect the reflectivity and other properties of the metallic ink layer.

[0010] Lastly, some previous use of metallic inks has not been viable due the time it takes to cure the ink after application. Typical processing times for metallic inks have been on the order of hours.

[001 1 ] It is thus desirable to provide an improved method of manufacturing a security device with at least one highly reflective layer. It is also desirable to provide a security device including at least one metallic ink layer with enhanced reflectivity. SUMMARY OF THE INVENTION

[0012] According to a first aspect of the present invention, there is provided a method of manufacturing a security device including the steps of:

providing a substrate;

applying a sinterable metallic particle or nanoparticle ink including metal ions to at least one surface of the substrate in at least one area;

activating the ink by a redox reaction to convert the metal ions to metal; and sintering the ink to form at least one highly reflective area

[0013] According to a second aspect of the present invention, there is provided a security device including a substrate and at least one highly reflective area applied to at least one side of the substrate, wherein the highly reflective area comprises a sinterable metallic particle or nanoparticle ink including metal ions activated by a redox reaction to convert the metal ions to metal and sintered to form at least one highly reflective area.

[0014] A process for applying a sinterable metallic particle or nanoparticle ink to form a reflective area or areas is much preferred to a vacuum deposition method due to the lower costs and efficiency. A vacuum deposition process is expensive due to the procurement and maintenance of vacuum equipment.

Further, it is difficult to integrate a vacuum deposition process into a standard printing process, which reduces the efficiency of the production of the security devices. Having a metallic ink with highly reflective properties that can be integrated into standard security device production processes greatly increases efficiency and reduces costs.

[0015] In addition, a sinterable metallic particle ink provides a reflective surface that has a high reflectivity when sintered, as the metallic particles bind together during the sintering process. The sintering of the metallic particle ink provides an additional durability to the reflective area or areas, and can reduce or remove the requirement for an additional overcoat for protection. [0016] The metallic particle or nanoparticle ink, may be sintered by applying radiation, preferably ultraviolet radiation or alternatively microwave, x-ray or infra red radiation. The radiation, in particular UV radiation, allows for faster and improved processing, such as quicker drying for thicker coatings and printing at high speeds.

[0017] Activating the ink by a redox reaction to convert the metal ions to solid metal has the advantage that the metallic particle or nanoparticle ink includes metal ions in solution and the redox reaction causes the ink to solidify quickly. This allows for fast processing and printing of the security devices.

[0018] Preferably, the metallic ink includes metallic nanoparticles. Metallic nanoparticles are easier and more effective to sinter than larger metallic particles.

[0019] The metallic particles or nanoparticles may include one or more of a selection of silver, gold, platinum, copper, metal alloy, stainless steel, nichrome, brass, aluminium or titanium.

[0020] The metallic particle or nanoparticle ink is preferably applied in a printing process, for example by gravure or intaglio printing or by offset or screen printing.

[0021 ] In one preferred embodiment, the metallic particle or nanoparticle ink includes an optically variable pigment. The optically variable pigment may provide a colour shift between two distinct colours with the colour shift being dependent upon the viewing angle.

[0022] At least one relief structure may be formed in the at least one highly reflective area on a first and/or second surface of the substrate.

[0023] Preferably, the method includes curing the metallic particle or nanoparticle ink. The method can also include forming at least one relief structure and curing in single or separate steps. Preferably, the method includes sintering the metallic particle or nanoparticle ink before, during or after curing. The method can also include activating the metallic particle or nanoparticle ink by redox reaction before, during or after curing the metallic particle or nanoparticle ink.

[0024] The method can include forming the at least one relief structure and curing the metallic particle ink in a single step. The activation of the metallic particle ink by redox reaction and curing may be a single step. Further, the method can include activating the metallic particle ink by redox reaction, sintering and curing the metallic particle ink in a single step.

[0025] The metallic particle or nanoparticle ink is preferably a radiation curable ink, which may be cured with ultra-violet or infra-red radiation.

[0026] The action of sintering, activating the metallic particle ink by redox reaction and curing in one or simultaneous steps, greatly increases the printing speeds such as to be almost instantaneous. The processing speeds can be at least 50 m per second, with preferred speeds of 90 to 120 m per second. A protective coating may not be necessary, as the process of sintering, activating the metallic particle ink by redox reaction and curing provides for a particularly durable and highly reflective layer.

[0027] The at least one relief structure may be formed by embossing, preferably in an intaglio or gravure printing process. Alternatively, the at least one relief structure may be formed by engraving, laser ablation or hot stamping.

[0028] A transparent or translucent layer may be applied to a first and/or second surface of the substrate. The at least one relief structure may be formed in the transparent or translucent layer. The metallic particle or nanoparticle ink may be applied to at least part of the at least one relief structure formed in the transparent or translucent layer. Alternatively, the at least one relief structure may be formed in the transparent or translucent layer on the first surface of the substrate, with the metallic particle or nanoparticle ink applied to the second surface of the substrate.

[0029] In an alternative embodiment, the at least one relief structure may be formed in the metallic particle or nanoparticle ink, for example by embossing.

[0030] Preferably, applying the metallic particle or nanoparticle ink and forming the at least one relief structure occurs in a single step. This allows for a more efficient printing process and for higher speeds in processing.

[0031 ] In a preferred embodiment, the at least one relief structure includes at least one diffractive relief structure. Preferably, the at least one diffractive relief structure may include at least one of: a diffractive optical element (DOE), a diffractive beam splitter, a diffractive diffuser, a diffractive corrector, a diffraction grating or a hologram. The at least one diffractive relief structure may be reflective or transmissive.

[0032] In another embodiment, the at least one relief structure includes at least one non-diffractive relief structure. The at least one non-diffractive relief structure may be reflective or transmissive.

[0033] At least one reflective relief structure may include one or more mirror or micro-mirror elements, such as a retro-reflector. Transmissive relief structures can include a lens or microlens array, such as a Fresnel lens or a spherical, aspherical, or cylindrical microlens array.

[0034] A transparent or translucent coating may be applied directly to at least part of the at least one relief structure. Preferably, the refractive index of the transparent or translucent coating is substantially the same as the refractive index of the at least one relief structure. By applying a translucent or transclucent coating with substantially the same refractive index, it is possible to render invisible part of the coated relief structure as required. [0035] The substrate is preferably a transparent or translucent substrate. At least one opacifying layer may be applied to at least part of the substrate. The at least one opacifying layer may be applied intermittently to the at least one highly reflective area to form at least part of indicia or an image. The at least one opacifying layer may be an opacifying coating, preferably an opacifying ink layer.

[0036] According to another aspect of the invention, there is provided a security device as manufactured by any of the methods as described above.

[0037] According to yet another aspect of the invention there is provided a security document, such as a banknote including at least one security device according to any one of embodiments described above. Preferably, the at least one security device is incorporated into a window or half-window of the security document.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Specific embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

[0039] Figure 1 is a representative cross section of a security device according to a first embodiment of the invention.

[0040] Figure 2 is a representative cross section of the first security device of Figure 1 after the step of sintering.

[0041 ] Figure 3 is a representative cross section of a security device according to a second embodiment of the invention.

[0042] Figure 4 is a representative cross section of the security device of Figure 3 in a half-window configuration. [0043] Figure 5 is a representative cross section of a security device according to a third embodiment of the invention.

[0044] Figure 6 is a representative cross section of a security device of Figure 5 after the step/s of embossing and sintering.

[0045] Figure 7 is a representative cross section of a security device according to another embodiment of the invention.

[0046] Figure 8 is a representative cross section of the security device of Figure 7 in a half-window configuration.

[0047] Figure 9 is a plan view of a security document, such as a bank note with the security devices of any of the embodiments of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

DEFINITIONS

[0048] Security Document or Token

[0049] As used herein the term security documents and tokens 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.

[0050] 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.

[0051 ] Security Device or Feature

[0052] 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) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

[0053] Embossable Radiation Curable Ink

[0054] The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which can 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 can be cured by other forms of radiation, such as electron beams or X-rays. [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 can also be embossed with larger relief structures, such as non-diffractive optically variable devices.

[0056] Metallic Nanoparticle Ink

[0057] As used herein, the term metallic nanoparticle ink refers to an ink having metallic particles of an average size of less than one micron.

DETAILED DESCRIPTION OF THE DRAWINGS

[0058] Referring to Figures 1 and 2, there is shown a cross section of a security document 10, where there is provided a substrate 12 of transparent polymeric material. The transparent substrate 12 may be formed from a polymeric material, such as one or more layers of bi-axially oriented

polypropylene. However other materials can be used to form the substrate 12 such as polyethylene or polyethyleneterephthalate (PET).

[0059] Opacifying layers 14, 16 are applied to a first and second side of the substrate 12 and but are omitted in a region of the substrate 12 to form a window. The security document is provided with a security device 22 in the window which is formed as described below. The opacifying layers 14, 16 may include one or more coatings of opacifying ink applied to opposite sides of the substrate 12. Alternatively, a hybrid substrate may be formed by applying the opacifying layers 14, 16 as layers of paper or other opaque materials.

[0060] As shown in Figure 1 , a layer of ink containing metallic particles or nanoparticles 18 is then applied to one side of the substrate. The metallic particle or nanoparticle ink 18 is preferably applied during a printing process such as intaglio printing, gravure printing, offset printing or screen printing. Preferred printing methods such as gravure or intaglio are able to achieve a two to three micron layer of metallic ink.

[0061 ] The metallic particle or nanoparticle ink 18 then undergoes a step of sintering. The act of sintering the ink causes the particles 18 to fuse together and create one or more highly reflective areas on the substrate 12, such as the highly reflective layer 20 of Figure 2. In this manner, a security device 22 is applied to the window region of security document 10 which is defined by the opacifying layers 14, 16.

[0062] Due to the sintering process, a highly reflective layer 20 can be produced with at least a gloss value of 250 units with preferred values of 600-700 units, as measured with a Sheen Gloss master gloss meter at an angle of 20°. Furthermore, the highly reflective layer 20 is durable and may not require an overcoat to maintain any desired security document durability properties.

[0063] With a very thin layer of metallic particle or nanoparticle ink, the security device 22 formed by the highly reflective area or areas may appear substantially transparent when viewed in transmission and reflective when viewed in reflection. With a thicker layer of metallic particle or nanoparticle ink, the security device formed by the highly reflective area or areas will not transmit light and will be viewable only in reflection.

[0064] The metallic particle or nanoparticle ink 18 may be applied in one or more regions as shown in Figure 1 . It may be embossed to produce a relief structure which can include diffractive or non-diffractive, transmissive or reflective structures. These diffractive structures can include diffractive optical element (DOE), diffractive lens, diffractive beam splitter, diffractive diffuser, diffractive corrector, diffraction grating or hologram. Reflective structures could include mirrors or micro mirror elements while transmissive structures can include a lens or microlens array, such as Fresnel lens or a spherical, aspherical or cylindrical microlens array. [0065] The metallic particle or nanoparticle ink 18 preferably includes metallic nanoparticles. It has been shown that the sintering step of the ink containing metallic nanoparticles can be shorter and require less energy.

[0066] The process of sintering can be activated in many ways, preferably by radiation, such as ultraviolet (UV) radiation, although other kinds of radiation can be used such as microwave, x-ray or infra-red. Alternatively, the metallic ink can be self-sintering.

[0067] The ink 18 may contain metallic particles or nanoparticles of gold or silver, although other metallic particles of aluminium, titanium, brass, platinum, copper, metal alloy, stainless steel, nichrome can be used.

[0068] The ink 18 can also include optically variable pigments such as a nacreous or pearlescent pigment, such as iriodin. Other optically variable pigments can be included that provide a colour shift between two distinct colours with the colour shift being dependent upon viewing angle.

[0069] In a preferred method of manufacture, the ink 18 also undergoes a step of activation, such as an oxidation-reduction reaction, known commonly as a redox reaction. In the example of Figures 1 to 3, it is envisaged that an ink 18 including metallic particles of silver will be used. The silver ions of the ink 18 are converted by the addition of electrons to silver metal during the activation step. Other metals can also undergo redox reactions in the same way, such as copper or gold, in this embodiment.

[0070] The steps of processing the metallic particle ink 18 including one or more of sintering, activating by redox reaction and curing preferably occur instantaneously or substantially instantaneously. This allows the printing of metallic inks 18 with these methods onto security documents at typical printing speeds. For example, for banknote printing, it is preferred that the printing speeds be on the order of hundreds of metres per second. However, the steps of processing can occur separately, in any order, as can be designed by a person skilled in the art.

[0071 ] The security device 22 now has the effect of showing a highly reflective surface in the window region formed by the opacifying layers 14, 16. The ink 18 may be applied in one or more regions of the window to form a pattern or indicia, and/or may contain a relief structure such as a diffractive or non-diffractive pattern. It can be understood that the security device 22 can be viewed upon reflection from either side of the substrate 12 or in transmission by holding the security device 22 up to a source of light. Optionally, an opacifying layer (not shown) can be applied over the highly reflective layer 20 to form a half-window which prevents the security device 22 from being viewed in transmission and is thus only viewable from one side of the substrate 12.

[0072] In an alternate embodiment of the invention, as shown in Figures 3 and 4, a cross sectional view of a security document 23 is shown having a transparent substrate 24 with opacifying layers 26, 28, and a window region defined by an area where the opacifying layers 26, 28 are omitted. In similar manner as described previously, a sinterable metallic particles or nanoparticle ink containing particles or nanoparticles of silver ions is applied to one side of the substrate. A highly reflective layer 30 is formed by activating the silver ions by a redox reaction and sintering the silver particles or nanoparticle ink to form a solid silver layer.

[0073] In addition to the highly reflective layer 30, a UV embossable transparent or translucent ink 32 can be applied to the opposite side of the substrate 24. During the step or before the step of curing the ink, the ink 32 can be embossed with a relief structure 34, as seen in Figures 3 and 4. The relief structure 34 can include diffractive structures and/or non-diffractive structures to form a transitory embossed image 36.

[0074] When the transitory image 36 is viewed within a range of angles above the transitory image, the highly reflective layer 30 formed by the activated and sintered ink produces relatively coherent reflections, except where the surface is interrupted by the transitory embossed image 36. The transitory embossed image 36 is caused by interruption of the reflected light from the highly reflected layer 30 (see dotted lines of Figure 3) which causes scattering of the reflected light and some transmittance. The contrast between the coherent reflections and scattered light allows the embossed image 36 to be viewed.

[0075] However, when the image 36 of Figures 3 and 4 is viewed at oblique angles, which do not receive any of the reflected light from the highly reflective layer 30, there is no contrast between the reflected and scattered light off the image 36 and thus the transitory embossed image 36 is nearly invisible. This means that the transitory embossed image 36 is only visible at range of angles and thus can be seen to switch on and off depending on the angle of view. This provides for a security device 38 that exhibits effects that are difficult to reproduce or photocopy.

[0076] Notably, as the highly reflective layer 30 has been activated, sintered, and cured it is particularly durable, and so it may not be necessary to apply an additional protective coating, such as a transparent gloss varnish.

[0077] In Figure 4, the highly reflective layer 30 is only viewable from one side of the substrate, as the opacifying layer 28 substantially covers the entire region of that side of the substrate 24. This forms the security device 22 in a half window configuration, where the security device 22 is only viewable from one side of the security document 23.

[0078] Further in Figure 4, an opacifying layer 37 can be applied to the relief structure 34. The opacifying layer 37 at least partly covers the relief structure 34 and can form indicia or at least part of an image. One or more opacifying layers 37 can be applied on the relief structure 34 to form the security device 38. The opacifying layer or layers 37 can be applied to relief structures 34 on either side of substrate 24. [0079] One or more transparent or translucent layers 39 can also be applied directly to at least part of the relief structures 34 on one or both sides of the substrate 24. The transparent or translucent layers 39 can be a polymer or an optical lacquer. If the transparent or translucent layer or layers 39 has an index or refraction that substantially the same as the index of refraction of the relief structure 34, then translucent or transparent layer 39 has the effect of rendering at least part of the relief structure 34 invisible as required. The translucent or transparent layer or layers 39 can be a high refractive index (HRI) coating.

[0080] Referring to Figures 5 and 6, an alternate security device 40 can be formed in a security document 42 by first printing the ink including the metallic particles or nanoparticles 46 onto one side of the transparent or translucent substrate 44 in one or more preferred regions. In this example, the ink 46 includes silver ion particles or nanoparticles.

[0081 ] The ink 46 can then be embossed and sintered either simultaneously or in separate steps to form the embossed relief structure 54 in a highly reflective layer 52 as shown in Figure 6. The ink can be also activated before or simultaneously during the embossing and sintering step. The activation step consists of a redox reaction, where the silver ions are converted to silver. In a preferred embodiment, the sintering of the silver ink 46 is performed by UV radiation, although other types of radiation can be used, such as infra red, x-ray or microwave.

[0082] The embossed relief structure 54 can consist of one or more non- diffractive or diffractive structures such as DOEs, holograms or diffraction gratings. This provides a complex security device 40 which can be formed as a composite of several diffractive and n on -diffractive and highly reflective structures. The relief structure 54 can also include mirror or micro-mirror elements, such as a retro-reflector. [0083] An opacifying layer 53 can be applied to the relief structure 54, seen in Figure 6. The opacifying layer 53 at least partly covers the relief structure 54 and can form indicia or at least part of an image. One or more opacifying layers 53 can be applied on the relief structure 54 to form the security device 40. The opacifying layer or layers 53, which can be an opacifying ink can be applied to relief structures 54 on either side of substrate 44, not shown in Figure 6.

[0084] At least one transparent or translucent layer 55 may be applied directly to at least part of the relief structures 54 on one or both sides of the substrate 54. The transparent or translucent layer 55 may be a polymer or an optical lacquer. If the transparent or translucent layer has an index or refraction that is substantially the same as the index of refraction of the relief structure 54, then the translucent or transparent layer 55 has the effect of rendering at least part of the relief structure 54 invisible as required. The translucent or transparent layer 55 may be a high refractive index coating (HRI).

[0085] Referring to Figure 7, there is shown another security device 56 in a security document 58. First, layers of embossable UV curable inks 66 and 70 are applied to opposite sides of a transparent or translucent substrate 60 within a window region where no opacifying layers 62 and 64 are applied. In Figure 7, the embossable UV curable ink layer 66 is a transparent or translucent ink, and the UV curable ink layer 70 is a metallic particle or nanoparticle ink. The inks 66 and 70 can be applied by a gravure or intaglio process. Then in either simultaneous or separate steps, the inks 66 and 70 can be embossed, and cured. The embossing can be applied by an inkless intaglio or gravure printing method.

[0086] The metallic particle or nanoparticle ink 70 of the security device 56 can be activated by redox reaction, sintered, embossed and cured substantially instantaneously. This allows the printing of metallic inks 66, 70 on security documents with these methods at typical printing speeds. For example, for banknote printing, it is preferred that the printing speeds be on the order of hundreds of metres per second. However, it is expected that the activating by redox reaction, sintering, and curing can occur in any order as can be necessary.

[0087] In particular, the layer of UV curable ink 66 can be embossed with at least one or an array of focusing elements, for example, cylindrical or spherical lenses such as lenticules, micro-lenses or Fresnel lenses 68. The UV curable metallic particle or nanoparticle ink layer 70 can be embossed with a relief structure such as one or more diffractive or non-diffractive structures to form an image layer 70. Alternatively, the relief structure of layer 70 may not be integral to the substrate 60 but can be applied as a separate item.

[0088] In Figure 8, the layer of UV curable ink 70 is a transparent or translucent ink layer which is embossed with a relief layer and cured. A metallic particle or nanoparticle ink layer 72 is applied to the relief structure of layer 70. The ink 72 can then be activated by a redox reaction then sintered by UV radiation to form a highly reflective relief structure following the relief structures already embossed into the layer of UV cured ink 70. Thus in this embodiment, the highly reflective layer 72 forms a background or reflective layer.

[0089] The opacifying layer 64 can completely cover one side of the security device 56 as shown in Figure 8 to form a half-window configuration. Further, a protective layer or high refractive coating (HRI) layer (not shown) can be applied to the layer of focusing elements 66.

[0090] The impression to the viewer of the security device 56 is of a magnified view of the image layer which can consist of one or more reflective, diffractive or non-diffractive structures. Further, if multiple diffractive or non-diffractive structures are utilised, these can be repeating across the length and width of the layer 72, to produce a range of three-dimensional, floating or moving effects, such as the Moire magnified effect, as known to a skilled person in the art. [0091 ] In Figure 9, there is shown a plan view of a security document 74, such a bank note including a security device 76 according to any of the above embodiments. A denominator 78 shows the value of the security document 74.

[0092] It will be appreciated that various modifications and alterations can be made to the embodiments of the present invention described above without departing from the scope and spirit of the present invention. For example, the different focusing, image and highly reflective layers of the different embodiment can be interchangeable. Further, while a substrate is present in each

embodiment, it is equally possible that an optical spacer be substituted as necessary.

[0093] Other modifications and alterations can be made to the sintering and activation processes. For example, the sintering of the ink can occur by the application of ultraviolet radiation, but other radiation types can also be used in any of the above embodiments, such as infra-red, x-ray or microwave radiation. It is also possible that the ink can be self-sintering.

[0094] Additionally, while specific embodiments of the security device refer to the use of ink including silver particles or nanoparticles, the ink can include one or more of the metals aluminium, titanium, brass, platinum, copper, metal alloy, stainless steel, nichrome. The metallic ink can also include one or more optically variable inks that include particles of metal oxides or sulphides.

[0095] As a further example, the embossments of the relief structures of any of the above embodiments can include one or more repeating or non-repeating, transmissive, reflective, diffractive or non-diffractive structures. The diffractive structures can include diffraction gratings, diffusers, beam splitters, correctors, DOEs or holograms, as known by a person skilled in the art.

[0096] Other modifications that can be made to the manufacturing of the security device can include the order of the application of the metallic particle ink or inks, production of the relief structures, sintering, activating by redox reaction and curing. While it is preferable that the security device can be manufactured by first applying the ink, sintering and then curing the ink, it is possible that any of the above steps can be performed simultaneously or in separate steps or in any order as known to a person skilled in the art. For example, the ink can be applied and relief structures produced simultaneously or in separate steps. Similarly, the metallic particle ink can be activated by redox reaction, sintered and cured in any order.

[0097] Other examples include opacifying inks or transparent or translucent layers can be applied to at least part of any of the relief structures in the embodiments above. The transparent or translucent layers can have any index of refraction, and in particular, can have an index of refraction that substantially the same as the index or refraction of the relief structures. The opacifying layers can be applied to form at least part of an indicia or image.

[0098] Further, while an embodiment of the present invention can refer to the application of the inks by gravure or intaglio printing, other processes that can be viable include screen printing or offset printing. Additionally, while the relief structures of the UV curable transparent inks can be referred above as being formed by an inkless intaglio method, the relief structures can be made by a variety of known methods including engraving, laser ablation or hot stamping.