AU2016100413B4 - Ink free gaps in optical security device - Google Patents

Abstract

An optical security device, including a substrate (12) having a first surface and a second surface; and a layer of ink (14) provided in at least one zone on the first surface to produce a patch or patches of ink; wherein the patch or patches of ink are subject to defects and wherein an ink free gap or gaps (13) is/are deliberately introduced in the layer of ink to resemble and/or mask the defect or defects. The layer of ink may include a layer or reflective metallic nanoparticle ink.

Description

INK FREE GAPS IN OPTICAL SECURITY DEVICE FIELD OF THE INVENTION

[0001] The present invention relates to optical security devices and methods for their manufacture. In particular the present invention relates to optical security devices which include a nanoparticle ink in their construction.

BACKGROUND TO THE INVENTION

[0002] Optical security devices are commonly used in security documents as a means of avoiding unauthorised duplication or forgery of such documents. Typically, such devices produce optical effects which may be difficult for a potential counterfeiter to replicate.

[0003] A wide range of optical security devices are known in the art. Frequently, such devices rely upon application of a reflective coating or a semi-transparent coating with a high refractive index in order to display an optical effect. For example, it is not uncommon for an optical security device to be constructed by embossing a diffraction pattern into a UV cured polymer layer to form a surface relief pattern. A thin reflective metal layer may be provided over the pattern. In this manner, an optical effect created by the diffraction pattern may be viewable in reflection. Alternatively, the metal layer may be substituted for a transparent layer with a high refractive index, allowing a diffractive effect to be viewed but also allowing any information behind the device to be visible.

[0004] A thin reflective metal layer may be provided in a number of ways. One way may be to use a vacuum deposition process. In this process, the surface to be coated may be placed in a vacuum, and the metal may be vaporised. When the vaporised metal contacts the surface, it may condense and form a metallic layer on the surface. This procedure may be effective in providing a reflective layer but it is relatively costly.

[0005] An alternative to a vacuum deposition process may be to utilise a metallic nanoparticle ink to coat the required surface. The application of such an ink may be achieved at substantially reduced cost compared to the vacuum deposition process, while still providing a thin coating that may be highly reflective, or semi-transparent with a high refractive index, depending on the composition of the ink.

[0006] Use of reflective metallic nanoparticle inks has previously been problematic; as such inks may display weak adhesion to polymer layer surfaces including embossed polymer layer surfaces, to which they are applied. Moreover defects and/or artefacts may be present in the material and/or surface of the polymer layer and/or substrate itself. The defects may be especially prevalent at an edge or edges of the polymer layer, such as a radiation curable ink, that is used to construct or emboss a diffraction pattern.

[0007] The defects may include pinhole sized defects (approximately 0.1 to 1.0mm) in the layer of metallic ink. The defects may include relatively small areas wherein the ink has failed to adhere to the polymer layer or substrate and/or where the structure of the polymer layer has prevented an even coating. The defects may arise during application or printing of inks over such polymer layers, such as when a metallic nanoparticle ink is used. Moreover the defects may become visible to the naked eye and/or may detract from security features that may be provided within the optical security device. Consequently, despite their attractive optical properties, it has proved difficult to effectively use these types of inks in producing optical security devices.

[0008] It is therefore desirable to provide an optical security device that addresses the difficulties presented by the defects and/or artefacts that may be present in the material and/or surface of the substrate. It is also desirable to provide a method for manufacturing such optical security devices.

SUMMARY OF THE INVENTION

[0009] According to one aspect of the present invention there is provided an optical security device, including a substrate having a first surface and a second surface; and a layer of ink provided in at least one zone on the first surface to produce a patch or patches of ink; wherein the patch or patches of ink are subject to defects and wherein an ink free gap or gaps is/are deliberately introduced in the layer of ink to resemble and/or to mask the defect or defects.

[0010] The layer of ink may include reflective metallic nanoparticle ink. The patch or patches of ink may at least partly overlie a diffractive relief structure. The diffractive relief structure may include embossable radiation curable ink. The diffractive relief structure may be provided on the first surface of the substrate. Alternatively, the diffractive relief structure may be provided on the second surface of the substrate. The relief structure may include a diffractive optical element. The or each ink free gap may be introduced at or near an edge of the diffractive relief structure.

[0011] A translucent or transparent coating may be applied directly to at least part of the or each diffractive relief structure where the patch or patches of ink is/are absent. The refractive index of the transparent or translucent coating may be substantially the same as the refractive index of the or each diffractive relief structure.

[0012] Preferably the first coating and the transparent or translucent coating may have substantially the same refractive index. Even more preferably, the coatings may be the same, preferably applied at the same time. The coating may include a curable coating.

[0013] The ink free gaps may be introduced in a predetermined pattern or patterns. The predetermined pattern or patterns may be adapted to be machine readable. The predetermined pattern or patterns may include an embedded code.

[0014] In one embodiment each ink free gap may be substantially circular in shape. In another embodiment each ink free gap (or at least some of them) may be substantially square in shape. In another embodiment each ink free gap (or at least some of them) may be substantially triangular in shape.

[0015] In yet another embodiment each ink free gap (or at least some of them) may include lines of ink separated by ink free strips. Each ink free gap may be substantially 0.1 to 1 mm in size and preferably 0.5 to 1 mm in size.

[0016] A first coating may be applied over the zone or zones in which the ink is provided. The first coating may adhere to the first surface where the ink is absent including the ink free gaps, thereby retaining the ink between the first surface and the first coating.

[0017] The ink may be provided in a plurality of substantially parallel lines. Where the ink is provided in this manner, preferably each line has a width of 1 nm to 200 pm, and further preferably, the lines are spaced apart by 1 nm to 200 pm.

[0018] Alternatively, the ink may be provided in a plurality of substantially circular spots. Where the ink is provided in this manner, preferably each substantially circular spot may have a diameter of 1 nm to 200 pm, and further preferably the spots may be spaced apart by 1 nm to 200 pm.

[0019] Preferably the size and spacing of the substantially parallel lines or substantially circular spots produces an optical density of greater than 0.1.

[0020] The ink may form a substantially opaque, reflective layer. Alternatively the ink may form a semitransparent layer with a refractive index greater than that of the relief structure.

[0021] The ink may include a silver nanoparticle ink. Where this is the case, the silver nanoparticle ink preferably has less than 40% silver.

[0022] Alternatively, the ink may include an aluminium nanoparticle ink. Further alternatively the ink may include a gold nanoparticle ink.

[0023] The substrate of the optical security device may be transparent or translucent. The optical security device may include at least one opacifying layer applied to at least part of the first surface of the transparent or translucent substrate. Further, the optical security device may include at least one opacifying layer applied to at least part of the second surface of the transparent or translucent substrate. The at least one opacifying layer may include an opacifying coating, preferably an opacifying ink layer.

[0024] Preferably, the at least one opacifying layer is at least partly omitted to form a window or half window on at least one of the first and second surface of the substrate in the area where the ink and high refractive index coating are provided.

[0025] Even more preferably, at least one of the opacifying layers may be provided intermittently to the second surface of the substrate in the region of the ink to form indicia or an image.

[0026] The first coating may include a high refractive index coating. Alternatively, the first coating may include a transparent, non-high refractive varnish.

[0027] According to a further aspect of the invention, there is provided a method of manufacturing an optical security device including a substrate having a first surface and a second surface; said method including applying a layer of ink in at least one zone on the first surface of said substrate to produce a patch or patches of ink; wherein the patch or patches of ink are subject to defects, and including the step of deliberately introducing in the layer of ink an ink free gap or gaps to resemble and/or to mask the defect or defects.

[0028] The layer of ink may include reflective metallic nanoparticle ink. The method may also further include the step of applying the patch or patches of ink to at least partly overlie a diffractive relief structure, the diffractive relief structure being provided on the first or second surface of the substrate. The diffractive relief structure may include embossable radiation curable ink. The ink free gaps may be introduced at or near an edge of the diffractive relief structure.

[0029] The method preferably includes the step of providing the diffractive relief structure on the first or second surface of the substrate prior to applying the ink. The relief structure may be provided as a diffractive optical element.

[0030] The method may also include the step of applying a transparent or translucent coating directly to at least part of the or each relief structure where the patch or patches of ink is/are absent, and wherein the refractive index of the transparent or translucent coating is substantially the same as the refractive index of the or each relief structure.

[0031] Preferably, the first coating and the transparent or translucent coating may have the same refractive index. Even more preferably, the coatings may be applied at the same time.

[0032] The method may include the step of introducing the ink free gaps in a predetermined pattern or patterns. The predetermined pattern or patterns may be adapted to be machine readable. The predetermined pattern or patterns may include an embedded code.

[0033] The method may include the step of introducing each ink free gap in a substantially circular in shape. Alternatively the method may include the step of introducing each ink free gap in substantially square in shape. Alternatively the method may include the step of introducing each ink free gap in substantially triangular shape. Each ink free gap may include lines of ink separated by ink free strips. Each ink free gap may be substantially 0.1 to 1mm in size and preferably 0.5 to 1mm in size.

[0034] The method may further include the step of applying a first coating over the or each area in which the ink has been applied. The first coating may adhere to the first surface where the ink is absent including the ink free gaps to thereby retain the ink between the first surface and the first coating.

[0035] The ink may be applied in a plurality of substantially parallel lines on the first surface. Where the ink is applied in this manner, preferably each line has a width of 1 nm to 200 pm, and further preferably, the lines are spaced apart by 1 nm to 200 pm.

[0036] Alternatively, the method includes applying the ink in a plurality of substantially circular spots. Where the ink is applied in this manner, preferably each substantially circular spot has a diameter of 1 nm to 200 pm, and further preferably the spots are spaced apart by 1 nm to 200 pm.

[0037] Preferably the size and spacing of the substantially parallel lines or substantially circular spots produces an optical density of greater than 0.1.

[0038] The method may include the step of applying the ink as a substantially opaque, reflective layer. Alternatively the ink may be applied a semitransparent layer with a refractive index greater than that of the relief structure. The coating may be applied as a curable coating.

[0039] The ink may be applied as a silver nanoparticle ink. Where this is the case, the silver nanoparticle ink preferably has less than 40% silver.

[0040] Alternatively, the method may include applying an aluminium nanoparticle ink or a gold nanoparticle ink.

[0041] The method may further include applying at least one opacifying layer applied to at least part of the first surface of the transparent or translucent substrate. Further, the method may include at least one opacifying layer applied to at least part of the second surface of the transparent or translucent substrate.

[0042] An additional step of the method may include at least partly omitting at least one opacifying layer to form a window or half window on at least one of the first and second surface of the substrate in the area where the ink and high refractive index coating are provided. The method may also include intermittently providing the at least one of the opacifying layers to the second surface of the substrate in the region of the ink to form indicia or an image.

[0043] The method also includes the step of providing at least one opacifying layer as an opacifying coating, preferably an opacifying ink layer.

[0044] 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

[0045] Specific embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: [0046] Figure 1 is a representative cross section of an optical security device according to a first embodiment of the invention.

[0047] Figure 2 is a representative cross section of an optical security device according to an alternative embodiment of the invention.

[0048] Figure 3 is a representative cross section of an optical security device according to a further embodiment of the invention.

[0049] Figure 4 shows an enlarged view of an optical security device including an area absent of ink according to an embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

DEFINITIONS

Security document [0050] 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.

[0051] 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 [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).

Metallic Nanoparticle Ink [0053] As used herein, the term metallic nanoparticle ink refers to an ink having metallic particles of an average size of less than one micron.

Diffractive Optical Elements (DOEs) [0054] 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).

[0055] 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 [0056] 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 may be cured by other forms of radiation, such as electron beams or X-rays.

[0057] 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.

[0058] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.

[0059] 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, eg. nitro-cellulose.

[0060] 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.

[0061] 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.

[0062] As discussed above it may be difficult to eliminate or substantially reduce defects including pinhole defects in security devices incorporating a radiation curable layer over-printed with inks as the source of the defects may be related to the supplied film, artefacts within the film and/or the radiation curable layer. One option may be to improve and/or filter artefacts from the film but this may not always be economical.

[0063] One source of defects can occur in the process of forming the radiation curable layer. As indicated above, a radiation curable ink can be embossed to create a structure. The process of repeatedly embossing, typically using a metal embossing tool or “shim”, can create defects in the generated structure. For example, partially cured UV can build up on the shim or dust or other particles can interfere with the structure.

[0064] Whilst improving processes to minimize occurrence of defects or artefacts is desirable, another option may be to design a layer of ink which contains a pattern to disguise and/or mask the defects. For example by deliberately not printing ink in a particular design area or areas of the substrate, ink free gaps or islands may be created in the layer of ink that may resemble a defect or defects. Thus by deliberately providing gaps or islands that are absent of ink, it may be possible to mask defects that occur randomly from the manufacturing process. The ink free gaps or islands may be typically circular in shape and approximately 0.1 to 1 mm in size. A varnish may be applied to the unprinted areas or gaps. The refractive index of the varnish may be similar to that of the UV cured polymer layer.

[0065] Referring to Figure 1, there is shown a cross section of an optical security device 10, wherein metallic nanoparticle ink 11 is provided in an area of the first surface of substrate 12 to produce a reflective or partially reflective patch or patches. The reflective patch or patches of metallic nanoparticle ink 11 is/are subject to defects (not shown) which are typically circular in size of the order of 0.1 to 1mm in diameter and are generally referred to herein as "pinhole" defects regardless of their actual size and/or shape. To ameliorate or mask the pinhole defect(s) an ink free gap 13 is deliberately created or introduced in the patch of metallic nanoparticle ink 11 to resemble and/or mask the pinhole defect or defects.

[0066] Coating 14 is applied over substrate 12 as well as over areas in which metallic nanoparticle ink 11 is provided. Coating 14 adheres to the surface of substrate 12 where metallic nanoparticle ink 11 is not present including the ink free gap 13 and areas 15 between the regions of metallic nanoparticle ink 11. In this manner individual regions of metallic nanoparticle ink 11 may be retained in position between the surface of the substrate 12 and coating 14 despite weak adhesion of metallic nanoparticle ink 11 to the surface of substrate 12.

[0067] The regions of metallic nanoparticle ink 11 together produce the reflective or partially reflective patch on substrate 12. Multiple areas of a substrate may be provided with metallic nanoparticle ink in this manner if multiple reflective patches or partially reflective patches are desired.

[0068] In an alternative embodiment of the present invention, metallic nanoparticle ink may be used to apply a thin reflective coating to a relief structure, such as a diffractive structure. Such an arrangement is shown in Figure 2, wherein a diffractive structure 21 is provided on the first surface of a substrate 22. Diffractive structure 21 may be integral with substrate 22, for example by being embossed into a polymer substrate, or alternatively may be applied as a separate element, for example by being embossed into a UV cured polymer layer or coating applied to substrate 22.

[0069] A metallic nanoparticle ink 23 is provided in an area of the diffractive structure 21 to produce a reflective or partially reflective patch or patches. The reflective patch or patches including metallic nanoparticle ink 23 are subject to pinhole defects (not shown). The pinhole defects may be more prevalent at an edge or edges of the polymer layer that is used to construct the or each diffractive structure 21. To ameliorate the pinhole defect(s) an ink free gap 24 is deliberately created or introduced in the patch of metallic nanoparticle ink 23 to resemble and/or mask the pinhole defect or defects.

[0070] Coating 25 may be applied over diffractive structure 21, substrate 22 as well as areas in which the metallic nanoparticle ink 23 is provided. Preferably, coating 25 includes a high refractive index (HRI) coating, as this may assist in ensuring that the optical effect produced by diffractive structure 21 remains visible even if metallic nanoparticle ink 23 is applied in a very thin layer. However, in other embodiments possible coatings may include a transparent, non-high refractive varnish. Coating 23 adheres to diffractive structure 21 in ink free gap 24 and areas 26 between regions of metallic nanoparticle ink 23 where metallic nanoparticle ink 23 is not present. In this manner, a reflective patch or patches may be provided over diffractive structure 21. Where this patch forms a substantially opaque reflective layer, the diffractive effect produced by diffractive structure 21 may be viewed in reflection in the area where the patch or patches are provided.

[0071] Alternatively, as shown in Figure 3, a diffractive structure 31 may be provided on the opposite side of substrate 33 to metallic nanoparticle ink 32. Here metallic nanoparticle ink 32 and coating 35 are provided on the first side of substrate 33, with diffractive structure 31 being provided on the second side of substrate 33. As before the reflective patch or patches metallic nanoparticle ink 32 may be subject to pinhole defects (not shown). To ameliorate the pinhole defect(s) an ink free gap 34 is deliberately created or introduced in the patch of metallic nanoparticle ink 33 to resemble and/or mask the pinhole defect or defects.

[0072] Protective varnish 36 may be applied over the diffractive structure 31. The protective varnish 36 in this case may be a high refractive index coating (having a refractive index different from the substrate 33 by at least 0.2). In this arrangement, it is preferable that at least part of substrate 33 and diffractive structure 31 are transparent, and the patch formed by metallic nanoparticle ink 32 is a semi-transparent layer with a refractive index greater than that of substrate 33 and diffractive structure 31. In this manner, the diffractive effect produced by diffractive structure 31 may be viewed in transmission by a viewer positioned at 37 whilst being visible in reflection by a viewer positioned at 38. This result is possible as use of nanoparticle ink 32 may provide a highly reflective surface, but may also permit enough light through to allow the diffractive effect to be visible in transmission. Furthermore, nanoparticle inks may provide reflectivity which is equivalent to that achieved by vacuum metallisation, but may be provided more cheaply and efficiently as the ink may be applied by a printing method.

[0073] Figure 4 shows an enlarged view of an ink free gap 43 in layer 42 of nanoparticle silver ink which comprises a reversed out or circular gap approximately 0.642 mm in diameter. The layer of nanoparticle silver ink 42 is subject to pinhole defects as described above. Typically the pinhole defects may be approximately 0.5 to 1mm in size. To address the defects which may become visible to the naked eye and/or may detract from security measures that may be provided within optical security device 40, ink free gaps 43 are deliberately introduced in the layer of silver nanoparticle ink to resemble and/or to mask the pinhole defect or defects.

[0074] The ink free gaps 43 comprise small unprinted areas or islands in which nanoparticle silver ink (typically applied by gravure and/or other techniques such as flexography or offset printing) is deliberately omitted and is not present. The ink free gaps 43 may be placed anywhere in the design of an associated security device or feature. However as the pinhole defects are more prevalent at an edge or edges of the polymer layer that is used to construct the or each diffractive relief structure, the ink free gap 43 may be introduced at or near an edge of the or each diffractive relief structure. In particular, each ink free gap 43 may comprise a circular dot, lines, square, triangle, and/or any shape which may make it more difficult to counterfeit and/or may further enhance the process of manufacturing the optical security device or feature.

[0075] Through generating a deliberate pattern of ink free gaps 43, the optical security device may be improved due to additional complexity associated with the deliberate pattern of small gaps 43 in the layer of nanoparticle silver ink that makes up layer 42.

[0076] It is envisaged that the pattern of ink free gaps 43 may be machine readable. That is, their size may be large enough so that a mobile phone app or other automatic machine reader may identify the pattern of deliberate gaps in the ink and may verify authenticity of the optical security device or feature and associated document.

[0077] Referring to both Figure 2 and 3, the diffractive structure 21 or 31 may be readily be replaced by any desired relief structure such as for example a diffractive optical element. Alternatively, high-resolution or high aspect ratio gratings such as polarisation gratings may be used, in which case nanoparticles less than 100 nm may be utilised.

[0078] In one embodiment of the present invention, the metallic nanoparticle ink includes a silver nanoparticle, having less than 40% silver. However, a range of other metallic nanoparticle inks may also be suitable for use in accordance with the invention, for example, silver nanoparticle inks with greater than 40% silver, aluminium nanoparticle inks and gold nanoparticle inks.

[0079] 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. Alternatively, a high refractive index coating utilising a non-metallic polymer, such as sulphur-containing or brominated organic polymers may also be used.

[0080] The metallic nanoparticle ink is preferably applied to the surface of the substrate in either a plurality of substantially parallel lines, or a plurality of substantially circular spots. If the metallic nanoparticle ink is provided in a plurality of substantially parallel lines, the lines preferably have a width of 1 nm to 200 pm, and preferably spaced apart by 1 nm to 200 pm. If the metallic nanoparticle ink is provided in a plurality of substantially circular spots, the spots preferably have a diameter of 1 nm to 200 pm and are preferably spaced apart by 1 nm to 200 pm. Further preferably, the ink stripes or spots may have a width or diameter of around 100 pm, and may be spaced apart by around 100 to 200 pm. These spacings have been found to provide an appropriate optical density to deliver the desired reflectivity. Preferably, the optical density is greater than 0.1.

[0081] The metallic nanoparticle ink may be applied by 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.

[0082] 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. An optical security device, including a substrate having a first surface and a second surface; and a layer of ink provided in at least one zone on the first surface to produce a patch or patches of ink; wherein the patch or patches of ink are subject to defects and wherein an ink free gap or gaps is/are deliberately introduced in the layer of ink to resemble and/or to mask the defect or defects.
2. 2. The optical security device of claim 1 wherein said layer of ink includes reflective metallic nanoparticle ink.
3. 3. The optical security device according to claim 1 or 2, wherein the or each patch of ink at least partly overlies a diffractive relief structure, the diffractive relief structure being provided on the first or second surface of the substrate.
4. 4. The optical security device according to any one of the preceding claims wherein the ink free gaps are introduced in a predetermined pattern or patterns and the predetermined pattern or patterns is/are adapted to be machine readable.
5. 5. A method of manufacturing an optical security device including a substrate having a first surface and a second surface; said method including applying a layer of ink in at least one zone on the first surface of said substrate to produce a patch or patches of ink; wherein the patch or patches of ink are subject to defects, and including the step of deliberately introducing in the layer of ink an ink free gap or gaps to resemble and/or to mask the defect or defects.