OPTICALLY VARIABLE SECURITY DEVICES

Abstract

An optical device is formed by hot stamping a demetallized hologram to an optically variable foil or to a coating of optically variable ink. In another embodiment a hologram is hot stamped to a banknote or document printed with a color-shifting ink.

Claims

1-24. (canceled)

25. A method of manufacturing a device for providing an image having an optically variable feature, comprising: a) providing a diffractive structure for forming at least a portion of the image; b) providing an optically variable structure distinct from the diffractive structure, for providing the optically variable feature to the image, wherein the optically variable structure changes color with a change in angle of incident light, and wherein the optically variable structure comprises a first reflector layer, an absorber layer, and a dielectric layer between the reflector and absorber layers; c) covering the diffractive structure or the optically variable structure with an adhesive; d) after steps (a)-(c), activating, with energy, the adhesive and coupling the diffractive structure and the optically variable structure together in a predetermined mutual arrangement, wherein the energy activated adhesive forms a layer; wherein the diffractive structure comprises a relief pattern and a high refraction index layer made of a material having an index of refraction no less than 1.65; and wherein the diffractive structure comprises a patterned opaque coating so that the diffractive structure has opaque regions and light transmissive regions.

26. The method as defined in claim 25, wherein the opaque regions for preventing light incident on the diffractive structure from propagating through to the optically variable structure via the layer of the energy activated adhesive, and light transmissive regions for allowing light incident on the diffractive structure to propagate through to the optically variable structure via the layer of the energy activated adhesive.

27. The method as defined in claim 25, wherein in step (a) comprises providing a second reflector layer to the diffractive structure.

28. The method as defined in claim 27, wherein the second reflector layer is partially demetallized.

29. The method as defined in claim 28, wherein the second reflector layer is segmented so that the reflector layer has one or more light transmissive windows, so that the optically variable structure is visible through said windows.

30. The method as defined in claim 27 wherein the optically variable structure comprises a coating with a plurality of multilayer optical interference flakes therein or thereon.

31. The method as defined in claim 25, wherein step (a) comprises providing a substrate having a first side supporting the diffractive structure, wherein the first side of the substrate has a first region wherein the diffractive structure is therein or thereon, and the first side of the substrate has a second region wherein the diffractive structure is absent, and in step (c) the layer of the energy activated adhesive is adjoined to the first side of the substrate so that the optically variable structure is adhesively bound solely to the substrate and coupled to the diffractive structure by the surrounding adhesive.

32. A device manufactured by the method defined in claim 25.

33. The method as defined in claim 25, wherein the diffractive structure includes a grating to form a light transmissive region for allowing light incident on the diffractive structure to propagate through to the optically variable structure via the layer of the energy activated adhesive.

34. The method as defined in claim 25, wherein the diffractive structure includes indicia.

35. The method as defined in claim 25, wherein the diffractive structure includes a substrate having the grating impressed on a surface of the substrate.

36. The method as defined in claim 25, wherein the diffractive structure includes a protective coating.

37. The method as defined in claim 25, wherein the adhesive includes covert flakes.

38. The method as defined in claim 37, wherein the covert flakes include charms, taggants, shaped pigments, magnetic flakes, or fluorescent pigments.

39. The method as defined in claim 25, wherein in step c) the adhesive covers the diffractive structure or the optically variable structure in a pattern.

40. The method as defined in claim 39, wherein the diffractive structure has regions not bonded to the optically variable structure based upon the pattern.

41. The method as defined in claim 39, wherein the pattern of adhesive forms a frame around the diffractive structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. It is to be appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0030] FIG. 1a is a cross sectional drawing illustrating the manufacture of a portion of chromagram in accordance with the teachings of this invention.

[0031] FIG. 1b is a cross sectional drawing illustrating the manufacture of a portion of chromagram in accordance with the teachings of this invention.

[0032] FIG. 2a is a cross-sectional view of a Chromagram with an adhesive between an optically variable (OV) foil and demetallized hologram.

[0033] FIG. 2b is a cross-sectional view of a Chromagram with an adhesive.

[0034] FIG. 2c is a cross-sectional view of a Chromagram with a clear adhesive between the foil and demetallized hologram, wherein the adhesive contains covert flakes or a low concentration of optically variable flakes or optically variable magnetic flakes.

[0035] FIG. 2d is a cross-sectional view of a Chromagram wherein an OV structure includes a laser ablated insignia.

[0036] FIG. 2e is a cross-sectional view of a Chromagram according to another embodiment of this invention.

[0037] FIG. 3a is a cross-sectional view of a Chromagram wherein a grating is supported by, or formed therein, a layer of high refraction index material.

[0038] FIG. 3b is a cross-sectional view of a Chromagram wherein a demet hologram is supported by, a layer of high refraction index material.

[0039] FIG. 4 is a cross-sectional view of a Chromagram wherein an OV structure is adhesively bonded to the substrate supporting a diffractive structure, whereas the OV and diffractive structures are coupled by pressure.

[0040] FIG. 5 is a view of a poker chip made in accordance with the invention wherein covert flakes having symbols thereon are provided within the coating in a similar manner to FIG. 2c.

DETAILED DESCRIPTION

[0041] The present invention is related to optical devices wherein a relief structure providing an optical effect such as a hologram or diffraction grating is coupled to an optically variable structure by an adhesive, which may be an energy activated adhesive. The resulting optical structure exhibits unique optical effects.

[0042] For the purpose of this application, the term “energy activated adhesive” or “energy activated binder”, means a bonding substance that requires an energy source for curing. The energy activated adhesives include, but are not limited to, hot stamp adhesives, UV or e-beam activated adhesives, thermoplastic and thermoset adhesives, paint-based polymeric compositions, varnishes, and staining compositions. By way of example, an adhesive is selected from the group of: polymethacrylate, polyacrylate, polyamide, nitrocellulose, alkyd resin, polyvinyl alcohol, polyvinyl acetate, and polyurethane.

[0043] The methods of activating the adhesives include hot stamping, UV curing, applying heat, pressure, or a beam of electrons. For brevity, an energy activated adhesive, possibly with special flakes therein, is referred to as “an adhesive” hereinbelow where it does not lead to confusion.

[0044] As was described heretofore, in the background of the invention, the field of hot stamping and more particularly, hot stamping of one optical coating or substrate with another is well known. For example, coated substrates bearing images, logos or other indicia are hot stamped onto lottery cards, passports, banknotes, driver’s licenses, poker chips, and a variety of other articles and substrates are well known.

[0045] The adhesive may be printed into patterns or flood coated over the entire surface. If patterned, the product becomes more tamper proof since the product cannot be physically removed in one piece. Attempts to remove the device by dissolving the adhesive using solvents would also be detrimental since the solvent would also attack the hardcoat/release which in turn would destroy the device, making tampering obvious.

[0046] The device disclosed in the present application comprises a diffractive structure, which can take various conventional forms including diffraction patterns such as diffraction gratings, refraction patterns, holographic patterns such as two-dimensional and three-dimensional holographic images, demetallized holograms, coatings with varied index of refraction, light transmissive dielectric coatings with refractive flakes therein or thereon, Kinegram.sup.R devices, Pixelgram.sup.R devices, corner cube reflectors, zero order diffraction structures, moiré patterns, and light interference patterns based on microstructures having dimensions of from about 0.1 .Math.m to about 10 .Math.m.

[0047] In accordance with one embodiment of the present invention, the diffractive structure comprises a reflector layer, having at least a part demetallized. A demet layer can be made of Al, Cu, Ni, and other metals and metal alloys that have been patterned by demetallization. Various techniques may be used to pattern the metal layer, such as chemical etching or oil ablation in vacuum, both done in registration with the relief image.

[0048] In one embodiment of the present invention, the diffractive structure comprises a windowed or segmented opaque layer having one or more light transmissive windows to allow combining of optical effects provided by the diffractive and optically variable structures, so that the optically variable structure is visible through said windows when the device is viewed from the side of the diffractive structure. Preferably, the windowed coating is reflective to provide an additional security feature.

[0049] The diffractive structure may be embossed on an embossable resin layer made of such materials as type G PET, Polycarbonate, polyvinyl chloride or polymethacrylate. An embossable layer may be combined with hardcoat/release layer. An embossing may be either patterned or continuous.

[0050] The diffractive structure may comprise a grating formed in a substrate, preferably a light transmissive or essentially transparent substrate, which may be made of Polyethylene Terephtalate (PET), Oriented Polypropylene (OPP) or other suitable plastic material. By way of example, a PET layer has a thickness of 6-25 microns.

[0051] The diffractive structure may comprise a high refraction index layer coated on a relief pattern, such as an embossed resin layer. The high refraction index layer may be made of a material having the index of refraction no less than 1.65. A high refractive index layer can be made of ZnS, TiO.sub.2, ZrO.sub.2, etc.

[0052] In one embodiment of the present invention, the diffractive structure is visible through the OV structure, so the substrate supporting the diffractive structure may be opaque.

[0053] In one embodiment of the present invention, the diffractive structure is a windowed substrate having a coated pattern thereon, wherein regions that are uncoated form windows therethrough the color shifting background is visible.

[0054] The device disclosed in the present application comprises an optically variable structure which, in one embodiment, is a multilayer optical interference film comprising a reflector layer, an absorber layer, and a dielectric layer between the reflector and absorber layers, as it is known in the art. A reflective layer can be made of any metal that has a reflectance over 20%, preferably aluminum. By way of example, a dielectric layer is made of MgF.sub.2 or other transparent material as known in the art.

[0055] An absorber can be a grey metal with a ratio of n/k about 1, where n is the real part of the refractive index and k is the imaginary part of the reflective index, for example Cr or Ni or other transition metal, or can be a non-selective absorber across the visible spectrum, or can be a cermet, as described in the article entitled “Influence of Nanosized Metal Clusters on the Generation of Strong Colors and Controlling of their Properties through Physical Vapor Deposition (PVD)” by R. Domnick et al., 49th Annual Technical Conference Proceedings (2006), Society of Vacuum Coaters, incorporated herein by reference. By way of example, a cermet material comprises silver islands in a dielectric matrix.

[0056] In another embodiment of the present invention, the optically variable structure is a multilayer optical interference film comprising a first and second absorber layers, and a dielectric layer therebetween. This multilayer film configuration is disclosed in U.S. Pat. No. 5,278,590 to Phillips et al. Such a film structure allows optically variable structure 10b to be transparent to light incident upon the surface thereof.

[0057] In yet another embodiment, the optically variable structure is a multilayer optical interference film comprising alternating low and high refraction index layers, where the individual layers have an index of refraction between 1.38 and 2.3

[0058] In one embodiment of the present invention, the optically variable structure comprises a light transmissive dielectric coating with a plurality of multilayer optical interference flakes therein or thereon. Such flakes are described, for example, in U.S. Pat. No. 6,749,777 granted to Argoitia et al.

[0059] Alternatively, the optically variable structure comprises optically variable ink, comprising optical effect flakes in a carrier, wherein the flakes may have one or more predetermined optical characteristics; for example, flakes may be optically variable changing color with a change in angle of incident light, or flakes may be diffractive, or may have covert symbols therein or thereon, or the flakes may simply be reflective or absorptive. In some instances, optical effect flakes have a combination of optical effects, for example, they may be diffractive and color shifting, or they may be diffractive and reflective, or diffractive and highly absorptive depending upon the desired effect. Furthermore, flakes having different optical effects may be mixed together in desired ratios. Pigments that may be added include those based on interference, for example mica based pigments, Fabry Perot type pigments, liquid crystal type pigments, including those that color shift with viewing angle, non- shifting pigments like gold and nickel, and other metallic flakes.

[0060] In one embodiment of the present invention, the optically variable ink is printed onto a substrate such as a banknote or any other security document.

[0061] The substrate supporting the optically variable structure is either opaque or light-transmissive in various embodiments of the present invention.

[0062] FIG. 1a illustrates a process of manufacturing a first portion of a chromagram in accordance with the teachings of this invention. In diffractive structure 10a, substrate 12 having region 14 impressed with a hologram, is partially coated with a pattern of highly reflective aluminum 16 preventing light from passing therethrough. Therefore, when the structure is viewed from the top side, a highly reflective surface is seen where the Al coating 16 is present. Substrate 12, a resin/hardcoat layer in which the embossing 14 is impressed, may have an optional protective coating 28 on its surface, whereon one or more letters, a logo or other indicia 19 is printed. The uncoated portions 17 of the hologram are substantially light transmissive.

[0063] Generally, in the prior art manufacture of chromagrams, an optical effect coating would be applied directly over the Al, as well as over the uncoated portions of the light transmissive substrate. However, in accordance with this invention an entirely separate structure 10b shown in FIG. 1b is prepared consisting of a substrate and an optically variable coating, such as a color shifting coating.

[0064] FIG. 1b illustrates a process of manufacturing a second portion of a chromagram, optically variable structure 10b, in accordance with the teachings of this invention. A substrate made preferably of PET is shown coated with a reflector layer 22, a dielectric layer 20 and an absorber layer 18. A hot stamp adhesive layer 62 which may be 3-10 .Math.m thick is then applied over the optical stack formed by layers 22, 20, and 18 and dried.

[0065] A novel and inventive aspect of this invention is the manufacture of a windowed structure, such as diffractive structure 10a, and a separate optically variable structure, such as structure 10b, wherein the two structures can be married or bonded together forming a chromagram by the application of heat and pressure via hot stamping. Each of the first and second structures can vary; several non-limiting examples are given throughout this application.

[0066] In embodiments of the present application the hot stamp adhesive can be applied and dried upon either a diffractive structure, structure 10a in the aforedescribed example, or an optically variable structure, such as 10b, or both structures, prior to bonding the two structures together. The thickness for hot stamp adhesive may be between 3 .Math.m and 10 .Math.m, with preferable range 3-7 .Math.m.

[0067] Further described embodiments of the present invention shown in FIGS. 2a and 2b have different optically variable coatings on light transmissive substrate 64. Substrate 64 can be either opaque or light transmissive. Light transmissive substrate 64 provides for viewing the device from both sides, which is advantageous if the optically variable structure is light transmissive, for example made with a light transmissive OV ink or an all dielectric Ab/D/Ab optical stack.

[0068] In embodiment shown in FIG. 2a, substrate 64 is coated with reflective layer 22, dielectric layer 20 and absorber layer 18 forming optically variable color shifting foil 23. Substrate 12, which can be a resin/hardcoat layer, has grating 14 thereon partially coated with a pattern of highly reflective coating 16 in contact with portions of the grating 14, for preventing light from passing therethrough. Resin layer 12 is optionally covered with protective light transmissive layer 28 with opaque indicia 19 printed thereon.

[0069] FIG. 2a is the result of adhesively joining structures 10a and 10b together. Preferably, grating 14 is embossed onto substrate 12 and covered with patterned demetallized aluminum 16, then the demet hologram is hot stamped or hot roll nipped to the optical stack using clear hot stamp adhesive 62. All that is required to form a chromagram once the structures 10a and 10b are brought together is aligning substrates 12 and 64 so that they are in registry and then the application of heat and pressure in a hot stamp machine.

[0070] Preferably, reflective layer 16 is windowed, so that substrate 12 has one or more regions 100 thereon embossed and covered with reflective material, said regions separated by regions 17 shown in FIG. 1a, not covered with aluminum and can be either embossed or not-embossed. It is easier to emboss the whole region and then pattern the Al and then put down the optically matching adhesive. Alternatively, the continuous reflective coating is shifted to one side, the resulting structure has planar reflective coating over non-embossed regions providing highly reflective mirror-like portions of the image as an additional optical effect.

[0071] The structure shown in FIG. 2a, when viewed from the top, provides a combined optical effect including a holographic image having windows wherein a color shifting background shows. The combined optical effect is added by printed indicia. When the structure shown in FIG. 2a is viewed through substrate 64, which should be light transmissive for this purpose, just a metallic look arising from the opaque reflector is visible. If the reflector is semi-transmitting, i.e. the thickness of the reflector layer is below the opaque point, then one would see a color shift with a faint reflection hologram showing through at certain locations where aluminum 16 is present. In between the faint reflection hologram replicated by aluminum 16 down one would see images from indicia 19.

[0072] Another embodiment of the present invention shown in FIG. 2b is similar to the embodiment shown in FIG. 2a in many respects, however has first substrate 64, preferably made of PET, coated with color shifting flakes 35 in carrier 34, by way of example an adhesive or acrylic- or urethane-based ink, hot stamped to the same upper structure as in FIG. 2a using a hot stamp adhesive 62. After the ink has dried and cured, thus forming a color shifting coating, a hot stamp adhesive 62 is applied and cured. To form a Chromagram, the coated first substrate having the hot stamp adhesive 62 is bonded with a second substrate covered with the same layers as in the embodiment shown in FIG. 2a.

[0073] With reference to FIG. 2e, in another embodiment of the present invention, substrate 68 is a banknote or any other type security document. A security structure is formed by printing a color shift ink onto banknote 68 as a background color, and then hot-stamping a demet hologram, preferably embossed onto resin/hardcoat layer 12 and supported by carrier sheet 66, over the ink using transparent hot stamp adhesive 62. Optionally, adhesive 62 contains other particles or flakes to modify the color of the color shift ink or as a covert security feature. By way of example, adhesive 62 may contain fluorescent materials, covert charms, such as disclosed in U.S. Pat. Application 20050037192 by Argoitia et al, upshift phosphors, interference pigments like mica-based interference flakes, and non-shifting pigments or dyes. The covert flakes are detectable under a microscope, whereas for the fluorescent and phosphor materials, irradiation of the device by UV or IR light is required to activate those features. Since resin/hardcoat layer 12 is quite thin, preferably less than 3 microns, the UV or IR light is able to penetrate to the particles in adhesive 62. In FIG. 2e, a release layer, resin layer and hard coat are shown as one layer 12 providing functionality of each of the three layers. In other embodiments, layer 12 is replaced by any combination of separate layers: a resin layer for embossing, a release layer for releasing the demet hologram from carrier sheet 66, and a hard coat layer for durability of the transferred device. The adhesive layer 62 which couples the optically variable and diffractive structures is an internal adhesive layer of the Chromagram device; an external layer of adhesive may be applied e.g. to the substrate 68 (its lower surface with reference to FIG. 2e) for adhering the Chromagram device to an object.

[0074] In another embodiment of the present invention, similar to the structures shown in FIGS. 2a and 2b, an OV structure used in place of structure 10b is transparent. By way of example, the transparent OV structure is one having a low density of optically variable flakes incorporating opaque Fabry Perot filters, or one based on all dielectric transparent optically variable flakes, or like one described in U.S. Pat. No. 5,278,590. When this construction is viewed through the OV film, a superposition of the holographic and optically variable effects is visible. In essence, the rainbow of colors that were in the initial hologram has been modified by the OV structure, such as an optical stack, whereby some colors are accentuated and some are suppressed. Actually, the hologram can be viewed from both sides; on one side the original hologram can be seen combined with OV background in windows of the reflective layer covering the hologram, and on the other side, the superposition of the hologram and the optical stack can be seen through the OV film.

[0075] In FIG. 2c, covert flakes 45 that cannot be seen with the unaided eye are mixed into the hot stamp adhesive 40 used to bond the two structures together as in the previous embodiments. The examples of covert flakes include, but not limited to, charms or taggants as taught in United States patent application publication number 2006/0035080 incorporated herein by reference, shaped pigments as disclosed in United States patent application publication number 20060035080, magnetic flakes, fluorescent pigments, standard UV activated to form visible light, or specialized anti-Stokes materials UV activated to form visible light.

[0076] These covert flakes serve as a means of authentication. If the covert flakes provide an optical effect, for example under a microscope or being IR activated, additionally to holographic effects exhibited by this structure, though the windows where the Al coating is missing, covert flakes 45 can be seen on color shifting background.

[0077] In another embodiment, optically variable flakes are added to adhesive 40 at low concentrations so that the OV foil colors are modified when viewing from the top.

[0078] According to another embodiment of the present invention, FIG. 2d, depicts a security article 150 including a light transmissive substrate 12 with an optical interference pattern 14 and a color shifting optical coating 156 that is laminated to substrate 12 by way of an adhesive layer 62. The optical coating 156 includes and absorber layer 18, dielectric layer 20, and a reflector layer 22. The optical coating 156 is deposited on carrier sheet 64 to form a prelaminate structure prior to being laminated to substrate 12. The prelaminate structure is subjected to a laser imaging process to form both laser ablated image 118 as well as laser scribed number 122. As shown in FIG. 2d, the optical coating 156 is laminated to substrate 12 so as to be adjacent to optical interference pattern 14 such as a holographic or diffractive pattern.

[0079] The structure shown in FIG. 2d, when viewed from the top, provides a combination of an opaque image, that may be reflective, formed by pattern 14, a color-shifting background visible in the windows of pattern 14, and covert flakes which may be added to adhesive 62. When the structure is viewed from the opposite side, laser ablated image 118 and laser scribed number 122 are visible on the substrate 64 of the reflective surface of the OV stack if substrate 64 is transparent. If hologram 14 is of the transmissive type, for example relief is coated with a high index layer of ZnS (not shown), then the laser ablated image and the number will also be visible from the top.

[0080] Conveniently, the aforedescribed manufacturing process allows the first and second substrates to be manufactured in two different facilities and stored in rolls to be united later.

[0081] Conveniently, the second substrate may have any type of optical effect coating thereon that can be seen through windows in the first substrate. Conveniently any of these first windowed substrates can be married to these second coated substrates at a later time or immediately, by way of hot stamping the two together or by hot roll laminating.

[0082] In another embodiment of the present invention, alternatively to hot-stamping, a UV activated adhesive is used to bond the two structures together. By way of example, an optically variable foil printed with an adhesive is brought together with a laminating sheet containing the demet hologram; the adhesive is then cured by irradiating the laminating sheet with UV light, wherein the laminating sheet has a transparent or at least UV light - transmissive substrate. One way to overcome the obstacle to UV light posed by the reflective metal covering the hologram, is to use a patterned or windowed reflective layer wherein areas covered with metal are very narrow, estimated to be less than 2 microns, so that UV light can cure the adhesive by coming in at an angle. Another way is to use a semitransparent reflective layer in the OV structure or to use e-beam curing.

[0083] In one embodiment of the present invention, shown in FIG. 3a, optically variable structure 23 is laminated onto grating 14 using adhesive 62, wherein grating 14 is supported by, or formed therein, hardcoat/resin layer 70 and covered with layer of high refraction index material 50. The structure of this embodiment provides a color shifting holographic image by combining the effects of the optically variable and diffractive structures. A transparent OV foil can be used in place of structure 23.

[0084] In another embodiment of the present invention shown in FIG. 3b, optically variable foil 23 is laminated using adhesive 62 onto grating 14 covered by patterned reflective layer 16, wherein grating 14 is supported by, or formed therein, hardcoat/resin layer 70 and covered with layer of high refraction index material 50. By as of example, OV foil 23 is a transparent all dielectric color shift with angle coating or one based on a semi-transparent/dielectric/semi-transparent color shift filter. When viewed from the top, as shown in FIG. 3b, this structure provides a combined optical effect, wherein a specular holographic image formed by light reflected from opaque metallic layer 16, is complemented by a lower reflecting holographic image with a color shifting background from the OV foil 23 formed by light reflecting from the regions where reflective layer 16 has windows.

[0085] In a structure similar to the structure shown in FIG. 3b but without a high refraction index coating, regions wherein reflective layer 16 has windows may show very weak holographic effects. This is very much dependent upon the of index matching between the coating underlying the non-aluminized portion of the hologram and the adhesive. If there is a refractive index differential of at least 0.2 for the real part of the refractive index, then some holographic effects will be seen. The greater the refractive index differential, the more clearly the hologram will be seen. By way of example, a refractive index differential of 0.8, between a substrate with n=1.5 and high index coating with n=2.3, provides brightness to a transparent hologram. In contrast, if the refractive index of the coating is matched, then no holographic effects will be seen in the windows.

[0086] In reference to FIGS. 3a and 3b, high refraction index coating 50 has an index of refraction no less than 1.65. Suitable examples of such a high index transparent layer include TiO.sub.2 or ZnS.

[0087] In one embodiment of the present invention, adhesive layer is patterned so that the diffractive structure has regions not bonded to the optically variable structure.

[0088] In another embodiment of the present invention, the adhesive layer is patterned so that one of the structures, by way of example a OV foil, is bonded to the substrate supporting the second structure, a hologram in our example, forming a frame, or a part of it, around the hologram.

[0089] In one embodiment, shown in FIG. 4, diffractive structure 60 is adjacent to substrate 12, but smaller, so that the top surface of substrate 12 has regions which are not covered with diffractive structure 60. Instead, at least some of these regions are covered with adhesive material thus forming patterned adhesive layer 63 for bonding optically variable foil 23 to substrate 12, whereas diffractive structure 60 is not adhesively bonded to adjacent layers, but is coupled to them by the surrounding adhesive. The pattern of adhesive layer 63 can vary, in particular, adhesive 63 can form a frame around diffractive structure 60 or, in case of elongated hologram 60, adhesive 63 is applied along two longer sides of hologram 60.

[0090] In one embodiment of the present invention, comprising a demetallized hologram hot stamped onto an optically variable foil, a patterned layer of color shifting ink is deposited underneath the demet hologram. This allows the color shift from the foil to show though to the observer. The flakes of the optically variable ink are generally opaque so that a continuous coating of ink would block out the underlying foil. Alternatively, the color shifting ink has a low concentration of flakes so that the color shift of the foil could still be seen though the optically variable ink; the foil colors are modified by the partially transparent optically variable ink. Alternatively, a patterned layer of the optically variable ink is deposited on the top side of the demet hologram, so that both the color shift from the ink and from the OV foil are visible.

[0091] Referring now to FIG. 5 a poker chip is shown having covert flakes bearing indicia in the form of a Euro symbol and a $ symbol that is only visible to the human eye with about 100 times magnification. Preferably, these covert flakes are provided within the hot stamp adhesive on one of the two optical structures.