AU2017101252A4 - Hybrid optically variable moire device - Google Patents

Hybrid optically variable moire device Download PDF

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AU2017101252A4
AU2017101252A4 AU2017101252A AU2017101252A AU2017101252A4 AU 2017101252 A4 AU2017101252 A4 AU 2017101252A4 AU 2017101252 A AU2017101252 A AU 2017101252A AU 2017101252 A AU2017101252 A AU 2017101252A AU 2017101252 A4 AU2017101252 A4 AU 2017101252A4
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layer
optically active
optically variable
pattern
optically
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AU2017101252A
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Robert Lee
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CCL Security Pty Ltd
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CCL Security Pty Ltd
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Abstract

Abstract An optically variable device is provided for producing a hybrid visual effect. The optically variable device includes at least two opposing layers, a bottom layer and a top layer separated from the bottom layer by a distance, wherein the bottom layer and the top layer each include at least one optically active element. Each optically active element is capable of producing a visual effect and superimposing one or more optically active elements on the top layer with one or more optically active elements on the bottom layer produces the hybrid visual effect being a moir6 effect. 115 t L--F 110 p p G2(--- 220 Figure 2

Description

HYBRID OPTICALLY VARIABLE MOIRE DEVICE
Technical Field [0001 ] The invention relates generally to optically variable devices and more particularly to the configuration of such optically variable devices. Such optically variable devices may have application in a number of fields including as an anticounterfeiting measure on security documents such as banknotes, government documents, tickets or security labels.
Background of Invention [0002] Security devices are applied to security documents or similar articles, such as identity cards, passports, credit cards, bank notes, cheques and the like and may take the form of diffraction gratings and similar optically detectable microstructures. Such security devices are difficult to falsify or modify, and are easily damaged or destroyed by any attempts to tamper with the document. Often security devices are designed to be overt features of the document, such that they are observable with the naked eye. This type of public or primary security device enables members of the public to perform some degree of authentication of the document, without the use of any additional viewing apparatus.
[0003] The ever increasing sophistication of counterfeiting operations requires continuous improvement in the design of security devices for protecting documents against forgery. For example, the ready availability of high resolution digital scanners and coloured photocopiers makes it increasingly plausible for counterfeiters to copy security documents issued using conventional security printing technologies. Accordingly, there is a constant need for alternative and improved security devices.
[0004] One such group of improved optical devices, known as optically variable devices, produces images which vary with the angle of view such that they cannot be readily copied or imaged. For this reason, optically variable devices have been very successful at thwarting would be counterfeiters. However, continuous improvement by counterfeiters has resulted in adoption of holographic approaches to enable simulation of optically variable effects produced by genuine security document printing technologies.
[0005] Accordingly, there is now proposed an alternative optically variable device. The optically variable device incudes a moire effect which is known to produce a visual perception occurring when viewing a pattern comprising a series of lines or dots superimposed on another pattern, where the series of lines or dots differ in relative size, angle, or spacing between the superimposed patterns. Superimposing two similar patterns gives rise to a third pattern referred to as the “moire pattern” which when observed from different angles will vary thereby generating an optical effect that is perceived as an animation effect. Whilst optically variable devices including moire effects offer the usual advantages of optically variable devices in that they are not readily copied or imaged, it would be desirable to produce a more complex optically variable device that could deliver an additional level of security.
[0006] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.
Summary of Invention [0007] According to an aspect of the present invention, there is provided an optically variable device for producing a hybrid visual effect, the optically variable device including at least two opposing layers, a bottom layer and a top layer separated from the bottom layer by a distance, wherein the bottom layer and the top layer each include at least one optically active element, each optically active element capable of producing a visual effect and wherein superimposing one or more optically active elements on the top layer with one or more optically active elements on the bottom layer produces the hybrid visual effect being a moire effect.
[0008] The distance separating the bottom and top layers may comprise a region transparent to light.
[0009] The at least one optically active elements may be selected from one or more of a diffuse scattering region, a diffraction grating; a micro prism; a micro mirror and/or a micro lens.
[0010] At least one of the bottom layer and the top layer includes a pattern formed by a plurality of optically active elements. The pattern may for example include a number of optically active elements arranged in a row or grid-like pattern or array.
[0011] In some embodiments, the optically active elements on the bottom layer include diffuse reflecting regions and the optically active elements on the top layer include one or more of a diffraction grating; a micro prism; a micro mirror and/or a micro lens.
[0012] In other embodiments, the optically active elements on the bottom layer are selected from one or more of a diffraction grating; a micro prism; a micro mirror and/or a micro lens and the optically active elements on the top layer include one or more of a diffraction grating; a micro prism; a micro mirror and/or a micro lens.
[0013] In alternative embodiments, an optically active element on the bottom layer is a reflective surface and the visual effect produced by the bottom layer is a reflection of the one or more optically active elements provided on the top layer of the device. The top layer includes at least one optically active element selected from one or more of a diffraction grating; a micro prism; a micro mirror and/or a micro lens.
[0014] The device may be formed on a substrate, the substrate having two opposing surfaces, forming the bottom layer and the top layer respectively wherein the distance separating the bottom layer and top layer is determined at least in part by a thickness of the substrate.
[0015] In some embodiments, the device is configured to be applied to or incorporated into a security document.
[0016] The distance separating the top layer and the bottom layer may be between 5pm and 500pm. In one specific embodiment wherein the optically variable device is used as a security device on a security document such as a bank note, e.g. a polymer bank note, the thickness of the substrate is around 70 microns.
[0017] The optically active elements on each of the bottom layer or the top layer may substantially form grid lines having a width of between 10pm and 200pm.
Definitions
Security Document or Token [0018] 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.
[0019] The invention is particularly, but not exclusively, applicable to security documents 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 may also have application in other products, such as packaging.
Substrate [0020] As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including, but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.
[0021] The use of plastic or polymeric materials in the manufacture of security documents pioneered in Australia has been very successful because polymeric banknotes are more durable than their paper counterparts and can also incorporate new security devices and features. One particularly successful security feature in polymeric banknotes produced for Australia and other countries has been a transparent area or “window”.
Transparent Windows and Half Windows [0022] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.
[0023] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.
[0024] A partly transparent or translucent area, hereinafter referred to as a “halfwindow,” may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the “half-window” is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.
[0025] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.
Opacifying Layers [0026] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT<L0 where L0 is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.
Security Device or Feature [0027] 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, alteration or tampering. Security devices or features may 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 may 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).
Embossable Radiation Curable Ink [0028] 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 maybe cured by other forms of radiation, such as electron beams or X-rays.
[0029] 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.
[0030] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.
[0031] Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings may be based on other compounds, e.g. nitro-cellulose.
[0032] 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 nondiffractive optically variable devices.
[0033] 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.
[0034] Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.
[0035] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a crosslinker, for example, a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetyl acetone; melamines; and carbodi-imides.
Optically Active Element [0036] The term optically active element used herein refers to a feature or structure having the capacity to produce an optical or visual effect when a beam of light is transmitted through or reflected by the feature as perceived by a human eye. The optical or visual effect might be a diffractive, reflective or refractive effect. Examples of optically active elements that can produce such optical or visual effects in the context of the present invention include diffuse scattering regions, diffraction gratings, micro prisms, micro mirrors and micro lenses.
Comprise, Comprises, Comprised or Comprising [0037] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.
Brief Description of Drawings [0038] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be understood that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings: [0039] Figure 1 is a schematic side view of an optically variable device according to an embodiment of the present invention.
[0040] Figure 2 shows a more detailed side view of the optically variable device of Figure 1 illustrating how a moire effect is perceived by an observer when observing the optically variable device from an angle.
[0041 ] Figures 3A to 3E shows a schematic side view of an optically variable device showing various options for providing the patterns on the bottom layer and the top layer of the device that produce the visual effects perceived by an observer according to various embodiments of the present invention.
[0042] Figures 4A to 4E show another schematic side on view of a an optically variable device showing various options for providing the patterns on the bottom layer and the top layer of the device that produce the visual effects perceived by an observer according to various other embodiments of the present invention.
Detailed Description [0043] Referring firstly to Figure 1, there is shown a cross section of an optically variable device for producing a hybrid visual effect 100 in accordance with an embodiment. The optically variable device 100 includes two layers. A bottom layer 110 and a top layer 120. The bottom layer 110 and the top layer 120 are to be substantially opposing as shown.
[0044] The bottom layer 110 includes at least one optically active element 130 each of which is independently capable of producing a visual effect such as a diffractive, reflective or refractive effect. For example, such optically active elements may comprise diffraction gratings, micro mirrors, micro prisms, lenses, diffuse scattering regions or the like. The top layer 120 similarly includes at least one optically active element 130. One of or both of the top layer 120 and the bottom layer 110 may include a pattern 125 and 115 respectively, formed by arranging a plurality of optically active elements 130.The visual effect generated by the optically active elements 130 will depend on the nature of the optically active elements selected to make up the pattern 115,125.
[0045] The top layer 120 of the optically variable device 100 is separated from the from the bottom layer by a distance t which is transparent to light. This transparent gap t, together with transparent regions existing between optically active elements 130 on the top layer 120 and the bottom layer 110 produces the hybrid visual effect described below. The distance separating the top and bottom layers may be anywhere between 5pm and 500pm. The thickness twill generally be determined by the thickness of the substrate. For example, in an embodiment wherein the optically variable device is used as a security device on a security document such as a bank note, e.g. a polymer bank note, the thickness of the substrate will typically be around 70 microns. This thickness twill dictate the dimensions of any optically active elements forming the pattern and the periodicity of the pattern that is required to produce the hybrid moire effect.
[0046] Hybrid visual effects are achieved when the optically active elements 130 on the bottom layer 110 and the top layer 120 are superimposed. For example, in the case where the optically active elements 130 are arranged on a layer so as to form a pattern, and the patterns are superimposed, where the two patterns differ in relative size, angle, or spacing, they will be generally understood by those skilled in the relevant art to produce a moire effect due to interference occurring between the superimposed patterns. That is, superimposing the first and second patterns formed of an arrangement of optically active elements gives rise to a moire pattern, which will vary when observed from different angles. In this case, it will be understood that the optically active elements themselves that form the patterns independently produce their own optical effects, thereby giving rise to a hybrid optically active moire device.
[0047] Various versions of such an optically variable device are envisaged, with various types and/or combination of types of optically active elements 130 arranged on the top layer 120 and the bottom layer 110 to achieve various hybrid visual effects.
[0048] The reference to “bottom” and “top” is to be understood in the context of the orientation of the optically variable device when viewed by an observer. That is the bottom layer 110 bearing optically active elements 130 arranged in a first pattern 115 is on the underside so that light is transmitted through the top layer 120 and the optically active elements 130 arranged in a second pattern 125 to give rise to the desired moire effect.
[0049] Referring now to Figure 2, there is shown how a moire effect is perceived when an observer 210 observes the optically variable device 200 from an angle. That is, whether the light reflected by the bottom layer 220 and through the top layer 230 will interfere constructively will depend on the thickness / between the bottom layer and the top layer, the dimensions of any optically active elements 240 forming the respective first pattern 250 and the second pattern 260, i.e. the width of the grid lines formed by consecutively arranged optically active elements. The grid lines may have a width of between 10pm and 200pm. In the case illustrated in Figure 2, the grid lines of the first pattern 250 and second pattern 260 are formed by diffraction gratings. Accordingly, the hybrid optically active device 200 will give rise to both diffractive and moire visual effects.
[0050] More specifically, in the case where the optically variable device is used as a security device on a bank note formed on a transparent polymer substrate of approximately 70 microns, the grid lines forming the patterns 250, 260 on the bottom layer 220 and the top layer 230 are about 30 microns wide with spacings between grid lines having a similar magnitude. This configuration gives rise to strong moire effects in accordance with the principles detailed below.
[0051] In reference to Figure 2, it will be understood by those skilled in the relevant art that if x and y are the coordinates of a point in the top layer 230 of the optically variable device 200 containing a second pattern 260 consisting of a pattern of optically active elements arranged in grids or lines, then the second pattern may be defined by the equation Gl(x, y) - n, where n is an integer 0,1,2,3,... etc or -1,-2, -3,... etc. In this instance the y direction is parallel to the bottom layer 220 and the top layer 230. If p is the angle of observation, with respect to the normal of the optically variable device as seen by the eye of the observer 210, then after transmission through the bottom layer 220 with first pattern 250 G2(x,y) consisting also of optically active elements arranged in grids or lines, the first pattern G2(x,y) will appear to be displaced by a distance d, where d - tTan(p'), when the visual effect or moire pattern is observed from above the top layer 230 bearing the second pattern grid pattern Gl(x,y), where t is the thickness of the substrate and p' is the angle after refraction of the light into the film.
[0052] The general equation for a moire fringe pattern resulting from two superimposed patterns, Gl(x,y) - n and G2(x,y) - m is given by n-m-k, where k is the fringe index of the moire fringes, k - 0, ±1, ±2,..., etc. In the present case G2(x,y) will appear to be displaced a distance d ~ tTan(p'), in the y direction when observed from the side Gl(x,y). Therefore the general expression for the resultant moire pattern is given by:
where k - 0,+1,±2, ±3,... and Sin(p)/Sin(p) -n, where n is the refractive index of the substrate, and the refractive index outside of the substrate is assumed to be unity.
[0053] The present invention builds on these known principles by replacing the grid lines Gl(x,y) and G2(x,y) by optically active elements. In the example illustrated in Figure 2, the optically active elements are diffractive regions manufactured by soft embossing diffractive grating structures into the relevant regions. Since diffraction effects subtract light from the zero order (and put this light energy into the various diffraction orders) the patterned regions will appear dark with respect to the adjacent non-patterned or non-embossed regions when the optically variable device is observed in zero order or straight through illumination. Accordingly, moire interference effects will be observed in a similar manner as if the grid lines had been printed on the substrate with dark ink rather than being embossed with diffraction grating structures. This means that in addition to the moire effects produced by the moire interference, diffractive effects will also be observed off-axis in various diffraction orders.
[0054] At particular angles of view, it should be possible to observe a combination of both moire and diffractive effects, i.e. a hybrid visual effect. In one exemplary embodiment, the moire fringe pattern is arranged to depict a birdcage, and the diffraction effects are arranged to generate an image of a bird in first order. In the zero order the birdcage alone would be perceived by an observer, whilst in first order the bird in the birdcage would be perceived by the observer due to a hybrid visual effect generated by the combined moire and diffractive effects.
[0055] In further embodiments of the invention, the optically active elements may comprise optically active structures other than diffractive gratings. One alternative arrangement is to replace the diffractive grating structures with micro mirror structures. Again the patterned regions will appear dark when observed at normal incidence in transmission, i.e. since the grid lines comprising the patterns will reflect rather than diffract the incident light away, causing them to appear dark to an observer.
[0056] In another embodiment, the diffractive grating structures are replaced by micro prism structures. Again the patterned regions will appear dark when observed at normal incidence in transmission, i.e. since the grid lines comprising the patterns will refract rather than diffract or reflect the incident light away, causing them to be perceived as dark regions by an observer.
[0057] In yet another embodiment, the diffractive grating structures are replaced by lenslet array structures. Again the patterned regions will appear dark when observed at normal incidence in transmission, i.e. since the grid lines comprising the patterns will refract rather than diffract or reflect the incident light away, causing them to be perceived as dark regions by an observer.
[0058] In still another embodiment, use of various combinations of the different types of optically active elements are envisaged.
[0059] With the exception of the micro mirror and metallised diffraction grating grid lines, all of the other moire effect options can be achieved by soft embossing particular types of surface relief microstructures into the substrate. In the case of pure shadow moire effects, (i.e. no diffraction grating, micro prism or micro lens effects) the grid lines forming the first pattern and/or the second pattern may be produced by soft embossing the patterns such that the non-transparent regions are comprised of diffuse scattering surface relief structures. In the case of transparent micro diffraction gratings, micro prisms or micro lenses, the corresponding patterned regions take the form of surface relief gratings, micro prisms or micro lenses. In the case of the metallised micro diffraction gratings and micro mirrors, the corresponding patterned regions could be uniformly metallised by printing over the entire surface and then selectively demetallised in the non-surface relief regions by selective demetallisation. However, in this example, registration issues could arise, i.e. in matching the demetallisation to the flat regions in between the surface relief regions. Examples of some of the hybrid moire options are shown schematically in Figures 3A to 3E.
[0060] Referring now to Figures 3A to 3E, various examples of combinations of optically active elements forming the patterned regions to produce hybrid moire visual effects are schematically shown. Turning first to Figure 3A, there is shown an optically variable device 300 having a micro prism 330 grid pattern on the top layer 310 superimposed over a diffuse embossed 340 grid pattern on the bottom layer 320 of the device. Figure 3B shows an example of a micro lens 350 grid pattern on the top layer 310 superimposed over a diffuse embossed 340 grid pattern on the bottom layer 320. Figure 3C shows an example of a micro diffraction grating 360 pattern on the top layer 310 superimposed over a diffuse embossed 340 grid pattern on the bottom layer 320. Figure 3D shows an example of a mixed micro prism 330 and micro diffraction grating 360 pattern on the top layer 310 superimposed over a micro diffraction 360 grating pattern on the bottom layer 320. Finally, Figure 3E shows an example of a mixed micro prism 330 and micro diffraction grating 360 pattern on the top layer 310 superimposed over a diffuse embossed 340 grid pattern on the bottom layer 320.
[0061 ] Referring now to Figures 4A to 4E, yet another alternative implementation of the invention may be constructed by replacing the soft embossed pattern on the bottom layer 420 by a metallised mirror layer 430. This implementation has the effect of avoiding registration problems between patterns formed by the optically active elements in the top layer 410 and bottom layer 420, i.e. with respect to aligning the top layer 410 pattern with the bottom layer 420 pattern since the top pattern grid lines are reflected directly at fixed distances from the bottom layer irrespective of their position on the top layer. Various optional configurations falling within the scope of this arrangement are shown in Figures 4A to 4E.
[0062] Referring now to Figures 4A to 4E, various examples of combinations of optically active elements forming the patterned region on the top layer of the optically variable device together with a metallised mirror layer on the bottom layer to generate a reflection of the pattern on the top layer, are schematically shown. Turning first to Figure 4A, there is shown an optically variable device 400 having a micro prism 440 grid pattern on the top layer 410 superimposed over metallised mirror layer 430 on the bottom layer 420 of the device. Figure 4B shows an example of a micro lens 450 grid pattern on the top layer 410 superimposed over a metallised mirror layer 430 on the bottom layer 420. Figure 4C shows an example of a micro diffraction grating 460 pattern on the top layer 410 superimposed over metallised mirror layer 430 on the bottom layer 420. Figure 4D shows an example of a mixed micro prism 440 and micro diffraction grating 460 pattern on the top layer 410 superimposed over metallised mirror layer 430 on the bottom layer 420. Finally, Figure 4E shows an example of a mixed diffuse embossed 470, micro diffraction grating 460, micro prism 440 and micro lens 450 pattern on the top layer 410 superimposed over metallised mirror layer 430 on the bottom layer 420.
[0063] The alternative optically variable device configurations in accordance with various embodiments shown in Figures 4A to 4E are described by different equations to those describing the transmissive moire effects since in Figure 4A to 4E the incident light will travel twice the distance through the optically variable device when compared with the transmissive version of the device shown for example in Figures 2 and 3A to 3E. If p is the angle of observation, with respect to the normal of the device as observed by the eye of an observer, after reflection from the bottom layer 420 of the device 4000 the line or grid pattern G(x,y) will appear to be displaced by a distance d, where d ~ 2tTan(p’), where p’ is the angle after refraction of the incident light as shown in Figure 2. Therefore, in this case the general expression for the resultant moire pattern is given by:
Gl(x,y)~ Gl(x,y + 2tTan(p’) ) = k where k — 0,+1,+2,+3,...
[0064] The optically variable device of the present invention provides a higher complexity optically variable device that delivers an innovative additional level of security. The optically variable device delivers different optical effects dictated by the nature of the optically active elements used to generate the pattern on the top and in some embodiments the bottom layer of the device. Moreover, the dual layer nature of the device lends itself to production of moire effects. Accordingly, different optical effects will be observed in different diffraction order and also in combination creating a device capable of generating hybrid optical effects.
[0065] While the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternative, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed.
[0066] The present application may be used as a basis or priority in respect of one or more future applications and the claims of any such future application may be directed to any one feature or combination of features that are described in the present application. Any such future application may include one or more of the following claims, which are given by way of example and are non-limiting in regard to what may be claimed in any future application.

Claims (5)

  1. The claims defining the invention are as follows:
    1. An optically variable device for producing a hybrid visual effect, the optically variable device including at least two opposing layers, a bottom layer and a top layer separated from the bottom layer by a distance, wherein the bottom layer and the top layer each include at least one optically active element, each optically active element capable of producing a visual effect and wherein superimposing one or more optically active elements on the top layer with one or more optically active elements on the bottom layer produces the hybrid visual effect being a moire effect.
  2. 2. The optically variable device according to claim 1, wherein the optically active elements are selected from one or more of a diffuse scattering region, a diffraction grating; a micro prism; a micro mirror and/or a micro lens.
  3. 3. The optically variable device according to claim 1 or 2, wherein at least one of the bottom layer and the top layer includes a pattern formed by a plurality of optically active elements.
  4. 4. The optically variable device according to claim 1 or 2, wherein the optically active element on the bottom layer is a reflective surface and the visual effect produced by the bottom layer is a reflection of the optically active elements on the top layer of the device.
  5. 5. The optically variable device according to any one of the preceding claims, wherein the optically variable device is formed on a substrate, the substrate having two opposing surfaces, forming the bottom layer and the top layer respectively, wherein the distance separating the bottom layer and the top layer is determined at least in part by a thickness of the substrate.
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