WO2011017741A1

WO2011017741A1 - Optically variable devices and method of manufacture

Info

Publication number: WO2011017741A1
Application number: PCT/AU2010/001006
Authority: WO
Inventors: Robert Arthur Lee
Original assignee: Securency International Pty Ltd
Priority date: 2009-08-10
Filing date: 2010-08-09
Publication date: 2011-02-17
Also published as: DE112010003249T5; GB2486994B; GB201202110D0; GB2486994A; CH703994B1

Abstract

An optically variable device, including a plurality of groups of surface relief elements. The groups are interlaced and/or stacked. A first group of relief elements is characterised by a first height or depth and produces a first type of optical and/or electromagnetic effect. A second group of relief elements is characterised by a second height or depth which is different to the first height or depth. The second group produces a second different type of optical and/or electromagnetic effect than that produced by the first group.

Description

OPTICALLY VARIABLE DEVICES AND METHOD OF MANUFACTURE FIELD OF THE INVENTION

The present invention relates to security devices capable of producing multiple optically variable effects, to security documents or tokens incorporating such devices, and to methods of manufacturing such devices.

DEFINITIONS

Security Document or Token

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 licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

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.

Substrate

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.

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

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.

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.

A partly transparent or translucent area, hereinafter referred to as a "half- window", 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.

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

Opacifying layers applied to a transparent substrate may comprise any one of more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed with 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 substantially opaque material to which indicia may be subsequently printed or otherwise applied. Security Element or Feature

As used herein the term security element 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, photochrome, 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).

Diffractive Optical Elements (DOEs)

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).

DOEs are sometimes referred to as computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms, Fresnel holograms and volume reflection holograms. BACKGROUND

It is known to provide diffractive optically variable devices on security documents and other articles of value as security features. These are usually provided in the form of a metallic foil patch or stripe which is affixed to the security document or article by hot stamping. Because the image on the patch or stripe varies with angle of view or angle of illumination, the image cannot be reproduced using computer scanning and printing techniques. Many banknotes around the world now use diffractive optically variable device (OVD) images as protection against counterfeiting.

Examples of OVD technologies include the diffractive optically variable device (DOVD) described in US patents 5,825,547 and 6,088,161 , and the DOVD described in European patents EP 330738 and EP 1059099. These devices are examples of foil based diffractive structures that have proven to be highly effective deterrents to the counterfeiting of official documents.

Although a diffractive security device alone may act to discourage or prevent counterfeiting of a security document, it is desirable to provide multiple distinct optical effects within the same security device to enhance the recognisability and security effectiveness of the device.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia, or elsewhere, before the priority date of each claim appended hereto.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides an optically variable device, including a plurality of groups of surface relief elements, the groups being interlaced and/or stacked, wherein a first said group is characterised by a first height or depth and produces a first type of optical and/or electromagnetic effect, and a second said group is characterised by a second height or depth which is different to the first height or depth, the second group producing a second, different, type of optical and/or electromagnetic effect than that produced by the first group. By applying the relief elements in such a way that they are interlaced and/or stacked, it is possible to produce a composite structure which has one optical effect when viewed under certain conditions, and a second, distinct optical effect visible in substantially the same region of the device when the device is viewed under different conditions. For example, the first optical effect may be observable in transmission, whilst the second is visible in reflection.

In a second aspect of the present invention, there is provided a method of producing an optically variable device including a plurality of groups of surface relief elements, the method including the steps of:

applying a first said group of surface relief elements to a substrate, the first group being characterised by a first height or depth and producing a first type of optical and/or electromagnetic effect, and

applying a second said group of surface relief elements which are interlaced and/or stacked with the first group,

the second group being characterised by a second height or depth which is different to the first height or depth,

whereby the second group produces a second, different, type of optical and/or electromagnetic effect than that produced by the first group.

Preferably, the groups of surface relief elements are formed in a radiation curable material applied to a substrate.

In a particularly preferred embodiment, the first and second groups are formed in a radiation curable material applied to a substrate.

Preferably, the first type of optical and/or electromagnetic effect is a non- diffractive effect and the second type of optical and/or electromagnetic effect is a diffractive optically variable effect.

In one preferred embodiment, the first type of optical and/or electromagnetic effect is one or more of one of the following effects: reflective, refractive, diffusely scattering, electrical. Advantageously, this reduces or substantially eliminates optical cross-talk between the optical effects produced by the device, since the diffractive effect is observable mostly in higher diffraction orders while the reflective, refractive or diffusely scattering effect is observable only in the zero-order. Preferably, a third group of surface relief elements is interlaced and/or stacked with the first and second groups. The relief elements of the third group may be formed in a surface of the relief elements of the first group or the second group. Alternatively, the relief elements of the third group may be interlaced with the relief elements of the first group or the second group.

The first type of optical and/or electromagnetic effect may be a greyscale optically invariable image. In this embodiment, the greyscale image may be visible in reflection from a first side of the device, and a negative version of the greyscale image visible in transmission from the first side of the device. The grey levels of the greyscale image may be determined by the surface density of the relief elements of the first group in the plane of the device. Alternatively, the grey levels may be determined by the depths of the relief elements of the first group.

Preferably, the first height or depth is at least five times greater, and more preferably at least 10 times greater, than the second height or depth.

The minimum height or depth of the relief elements of the first group is preferably greater than about 5 microns, and the maximum height or depth is preferably less than about 120 microns.

The relief elements of the second group preferably have a maximum height or depth which is less than about 4 microns. The minimum height or depth of the relief elements of the second group is preferably greater than about 0.1 microns.

By having two groups of relief elements which differ significantly in scale, it is possible to ensure that the optical effects generated by the two groups are observably distinct.

The relief elements of the second group may be formed at least partly in an upper or lower surface of the relief elements of the first group. In a particularly preferred embodiment, the relief element of the second group are formed in upper surfaces of the relief elements of the first group. This makes it possible, if desired, to preferentially coat the second, shallower, group with a patterned layer of a material, such as a metallic ink, so that the pattern is in perfect register with the second group.

The first and/or the second groups may be metallised. For example, at least some of the relief elements of the second group may be metallised, so that the second group produces a diffractive optically variable effect in reflection, while the first group produces a diffractive or non-diffractive effect in transmission. Alternatively, at least some of the relief elements of the first group and the second group may be metallised so that both effects are observable in reflection.

If a third group of relief elements is present, the third group may also, al alternatively, be metallised.

The second group may be metallised in a pattern which forms at least one passive electrical component. Preferably, the second group is metallised in a pattern which forms a passive radio-frequency antenna, for example an RFID tag. A further, non-optical, authentication means may thus be incorporated into the security device.

In another embodiment of the invention, the relief elements in the second group are preferably spaced from each other at a distance which is sufficiently small that light transmitted or reflected by the second group is polarised. The separation of the relief elements of the second group is preferably in the range 100 nm to 300 nm. In this embodiment, the second height or depth is preferably in the range 5 microns to 10 microns.

The device may further include a layer or coating applied to the first group and/or the second group and, if applicable, the third group. The layer or coating may be a protective layer or coating. Preferably, the refractive index of the layer or coating is different from the refractive index of the material in which the relief elements are formed. If the layer or coating is applied to both groups of relief elements, each of the optical effects may be visible in transmitted light.

Preferably, the first group includes microprisms having a lower surface, an upper surface and at least one inclined sidewalk In one embodiment, the first group includes two or more sets of microprisms, and each set of microprisms is characterised by a different angle of inclination of the inclined sidewall, whereby different images or different regions of an image can be viewed through each set of microprisms of the optically variable device.

The device may include an image layer bearing an image composed of two or more interleaved sub-images, with each set of microprisms viewing one of the sub-images. The interleaved sub-images may be arranged in bands or stripes. The image layer is preferably a printed layer. Each of the microprisms may correspond to a pixel of a grayscale bitmap image, the area of each microprism being related to the gray level of the corresponding pixel.

The device may further include a spacing layer between the first and second group of relief elements. Preferably, the spacing layer has a refractive index which is substantially different to the refractive index of the material in which the relief elements are formed.

In one preferred embodiment, the first group includes microlenses. Preferably, the device further includes a layer of microprinting, the microprinting being viewable through the microlenses to produce the first optical effect.

The second group of relief elements may at least partly take the form of micrographic elements, wherein the microprinting is viewable through the microlenses from an on-axis position and micrographic elements are viewable through the microlenses from an off-axis position.

The device may further include a transparent or translucent substrate. If the relief elements are formed in a radiation curable material applied to the substrate, the radiation curable material is preferably applied by printing.

In a particularly preferred embodiment of the method of the present invention, the first and second groups of relief elements are applied simultaneously.

The method may further include the step of applying a second layer of radiation curable material overlying the first group of relief elements, whereby the second group of relief elements is formed in the second layer of radiation curable material. The second layer of radiation curable material is preferably applied by printing.

In a particularly preferred embodiment, the layer or layers of radiation curable material are embossed to form the first and second groups of relief elements and the radiation curable material is cured substantially at the same time as the embossing step. Such an embossing method is described in WO 2008/031 170, the contents of which are incorporated herein by reference.

The curing step is preferably performed by means of actinic radiation, such as that selected from the group comprising X-ray, electron beam and UV radiation. The method preferably further includes the step of applying the radiation curable material, for example by a printing method, to a transparent or translucent substrate.

According to a third aspect of the invention, there is provided a security element including the optically variable device of the first aspect, or manufactured according to the second aspect of the invention.

According to a fourth aspect of the invention, there is provided a security document including the security element of the third aspect of the invention.

In a particularly preferred embodiment, the optically variable device is provided in a window or half-window area of the security document.

In a fifth aspect of the invention, there is provided an embossing tool including a plurality of groups of surface relief elements or recesses, the groups being interlaced and/or stacked, wherein a first said group is characterised by a first height or depth and a second said group is characterised by a second height or depth which is different to the first height or depth, whereby when a transparent, translucent or reflective material is embossed with the embossing tool, areas embossed with the first group of relief elements produce a first type of optical effect, and areas embossed with the second group of relief elements produce a second, different, type of optical effect.

The embossing tool may further include a third group of surface relief elements or recesses interlaced with the first group or the second group, whereby areas embossed with the third group produce a third type of optical effect.

In a sixth aspect, there is provided a method of creating an embossing tool for applying a plurality of groups of surface relief elements to a transparent, translucent or reflective material, the method including the steps of:

embossing a first group of surface relief elements on a substrate, the first group being characterised by a first height or depth to produce a first type of optical effect;

embossing a second group of surface relief elements on the substrate so that the first and second groups are interlaced and/or stacked, the second group being characterised by a second height or depth which is different to the first height or depth to produce a second, different, type of optical effect; and electroplating or electroforming a replica of the embossed substrate to form the embossing tool.

If the material is transparent or translucent it may be a polymeric substrate or a transparent radiation curable material applied to a substrate. Reflective material, may include a metallized layer applied to a substrate.

In a seventh aspect of the present invention, there is provided an embossing tool manufactured according to the sixth aspect of the invention.

In an eighth aspect, the present invention provides a method of creating an embossing tool for applying a plurality of groups of surface relief elements to a transparent, translucent or reflective material, the method including:

providing a substitute including a first group of surface relief elements, the first group being characterised by a first height or depth to produce a first type of optical effect;

applying a second group of surface relief elements of sacrificial material to the first group, the second group having a second height or depth which is different to the first height or depth;

applying a third group of surface relief elements in an upper surface of the second group of surface relief elements;

electroplating or electroforming a metallic layer on the first, second and third groups; and

removing the sacrificial material.

The sacrificial material may be photoresist. If so, the second group of surface relief elements may be formed by irradiating the photoresist through a mask. The mask may include apertures with a surface density corresponding to the grey levels of an input greyscale image.

BRIEF DESCRIPTION OF THE FIGURES

Certain preferred embodiments of the invention will now be described, with reference to the accompanying drawings, in which:

Figures 1 (A) to 1 (B) and 2(A) to 2(C) show cross sectional views through different embodiments of the optically variable device;

Figure 3 schematically illustrates generation of two optically variable effects by the device of Figure 2(C); Figures 4(A) and 4(B) demonstrate selective metallisation of a group of relief elements in an exemplary optically variable device;

Figures 5(A) to 5(C) show the formation of an optical variable device in which both optical effects are visible in reflection;

Figure 6 is a cutaway view of an optically variable device including a printed layer;

Figure 7 illustrates an embodiment of the invention in which microlenses are used to produce one of the optical effects;

Figure 8 shows a modification of the embodiment of Figure 7;

Figure 9 illustrates the generation of two different optically variable effects by the device of Figure 8;

Figures 10(A) and 10(B) illustrate embodiments of the invention including a wire grid polariser;

Figures 1 1 (A) and 1 1 (B) show an alternative embodiment in which groups of relief elements are stacked to produce an optically variable device;

Figure 12 shows a yet further embodiment with an interlaced and stacked arrangement of groups of relief elements;

Figures 13(A) to 13(C) and 14 schematically depict two preferred embodiments of a method of creating an embossing tool;

Figure 15 is a schematic sectional view through a security document incorporating an optically variable device in accordance with the invention;

Figure 16 shows another embodiment of a method for creating an embossing tool;

Figure 17 shows a method of producing an optically variable device using the embossing tool of Figure 16; and

Figure 18 shows a person viewing the device of Figure 17.

DETAILED DESCRIPTION OF THE FIGURES

Referring now to Figure 1 (A), there is shown an optically variable device

100 including a first group of relief elements in the form of microprisms 1 10, shown in sectional view, each of which have an inclined sidewall 1 12. A second group of relief elements is formed by a set of microstructures 120 which are formed in upper surface of the microprisms 1 10, thereby forming a composite structure 130 in which two different groups of relief elements having different depths d and D interlaced to produce two different optically variable effects.

The depth of the first group is indicated in Figure 1 (A) by D, and that of the second group by d. D will generally be an order of magnitude or so greater than d, and is preferably in the range from about 5 microns to about 120 microns and more preferably from about 10 microns to about 40 microns. The depth d is preferably in the range from about 0.1 microns to about 4 microns, and more preferably from about 0.2 microns to about 2 microns. The two group of relief elements 1 10, 120 can be formed such that the first group 1 10 produces a primarily refractive effect, while the second group 120 produces a primarily diffractive effect. The second group may, for example, take the form of a diffraction grating, a hologram or a diffractive optical element (DOE).

The composite structure 130 is formed on a transparent substrate 200, and is such that both the diffractive and refractive optically variable effects may be viewed in transmission from either side of the device.

A refractive optically variable effect may be produced by structuring the microprisms such that the slopes of the microprism sidewalls vary. The first group of relief elements 1 10 thus constitutes two or more interlaced subgroups of relief elements in the form of microprism arrays, each array characterised by a particular slope. Each array of microprisms will refract light at a particular range of angles, so that different regions of an input image would be observed in transmission at different angles of view of the light source or different angles of view with respect to the normal to the plane of the transparent substrate 200.

Alternatively, an image switch could be created in transmission if two groups of interlaced prism regions were created having opposite slope, with the set of prisms of one group corresponding to a first input image, and the set of prisms of opposite slope corresponding to a second input image. As the transmitted angle of view or light source angle was changed from one side of the normal to the surface to the other, an image switching effect would be observed.

A third type of transmission mode refractive imaging effect could be observed if the microprisms all had the same slope, but varied in area from region to region according the greyness value of the corresponding region of an input greyscale image, such as a portrait image of a human face. For each of the above three cases of refractive optically variable image effects the overall optical behaviour of the device would be a hybrid of an optically variable diffraction effect observed from the collective behaviour of the second group of relief elements 120 which is formed in the upper surfaces of the microprisms 1 10, together with an optically variable refractive effect, both effects being observed in transmission.

The optically variable device in the form of bi-level microstructure 130 is formed in a radiation-curable material by an embossing process. For example, a transparent UV-curable lacquer is first applied to the polymer substrate 200 by any suitable process, such as a printing process. The lacquer may then be embossed under heat and pressure (hot emboss) with a shim carrying the relief structures 1 10, 120, and cured by UV radiation to simultaneously form the relief elements in the lacquer. Alternatively, the relief elements 1 10, 120 may be embossed into the lacquer while it is still soft, and then cured by UV radiation while the embossing shim is still in contact with the lacquer (soft emboss). The embossing and curing steps may be performed substantially simultaneously.

It is also possible to form the structure shown in Figure 1 (A) in two separate stages. In a first stage, the relief elements 1 10 are formed in the UV- curable lacquer, by either the hot embossing or soft embossing methods described above, using an embossing shim carrying only the first group of relief elements 1 10. A further layer of UV-curable lacquer is then formed on the upper surfaces of the first relief elements 1 10, and the further layer then embossed, in register with the first group of relief elements, with a separate shim carrying only the second relief elements 120.

The device may optionally include a protective layer 210, as shown in

Figure 1 (B). The protective layer should have a refractive index which is sufficiently different to the radiation-curable material, the protective layer for example comprising a high refractive index transparent ink, so as not to nullify the optically variable effects produced by the two groups of relief elements.

The main visual difference between the diffractive and refractive optically variable effects generated by this type of device is the appearance of multiple orders and diffraction colour effects in the case of the diffractive optically variable images. The refractive optically variable images generated by the interlaced arrays of microprisms will be achromatic in colour and only the zero order effect will be active. This means that the images generated by the refractive elements will not interfere or "cross talk" with the optically variable images generated by the diffractive elements. A particular benefit offered by this dual transmission effect is the possibility for designing optically variable images where the diffractive and refractive elements produce different components of the same optically variable image. For example, since the refractive effect is achromatic and only occurs in the zero order, and diffractive effects are polychromatic and only occur in the non- zero orders, it is possible to design optically variable images where a central achromatic image element is surrounded by polychromatic image elements with the different elements of the composite image switching on and off as the angle of view or angle of transmission illumination is changed.

Referring now to Figures 2(A) to 2(C), there is shown a modification of the optically variable device of Figure 1. A bi-level interlaced microstructure 130 is formed as before (Figure 2(A)). The microstructure 130 is then selectively metallised in diffractive regions 120 with a metallic ink 125 using, for example, a rotogravure printing process, resulting in the selectively reflective diffraction regions 120' shown in Figure 2(B). The resulting microstructure is then overprinted with a transparent ink or lacquer 135 of different refractive index to the UV cured lacquer (Figure 2(C)). The device is then capable of producing a diffractive optically variable effect observable in reflection, and a refractive or diffractive optically variable effect observable in transmission.

Note that in Figure 2 the various regions are not to scale and there could be a large number of grooves 122 within each metallic region 120', depending on the resolution required for component optically variable image.

For example for diffractive and refractive images each having a smallest pixel (or picture element) size of 30 microns, the diffractive elements would each contain 10 to 50 grooves, depending on the angle of diffraction required for the diffractive effect. A ten groove element would diffract at a much smaller angle than a 50 groove element. Note that because the primary master structure (e.g. used for embossing 2(A)) is generated by precision micromanufacturing processes, such as electron beam lithography, the size and positioning of the individual diffractive and refractive elements can be produced with almost limitless precision - e.g. down to fractions of a micron if necessary.

Referring now to Figure 3, the two observation modes of the device of Figure 2 are shown. In the first mode, an optically variable diffractive image 310 with higher-order diffraction maxima corresponding to angles θi , θ₂ is observed in reflection from the metallised diffractive regions 120'. The diffractive optically variable effect may include image switching, movement effects, portraiture, and so on. The form, location and angle of view of the images generated by diffraction from one of the regions 120' depend on the spacings (spatial frequency) and orientation of the grooves in that region.

In the second mode of operation of the device, a refractive optically variable effect can be observed in transmission by looking at a light source 320 placed behind the substrate 200. The second optically variable effect is produced by refraction from the arrays of microprisms 1 10 to generate the optically variable image. The form, location and angle of view of the refractive images from the regions 1 10 depend on the local microprism angles and orientation in each region.

Because each diffractive region 120' and refractive region 1 10 is small (generally having maximum dimension between about 30 and 60 microns), the individual optical regions 1 10, 120' are not discernable by the naked eye, and it is the collective behaviour of a large number of these elemental regions 1 10, 120' which contribute to the observed macroscopic images, which appear to an observer to occupy substantially the same spatial region of the substrate 200.

In Figure 4, there is shown a schematic outline of the selective metallisation process. In Figure 4(A) a bi-level microstructure 130 is printed with a metallic ink 125 using a printing plate 400 uniformly covered with the metallic ink to a very shallow depth d', which is approximately equal to or greater than the depth d of relief structure 120 but much less than the depth D of relief structure 1 10. Because the deep regions 1 10 of the microstructure are very much deeper than the thickness d' of the ink on the printing plate, these regions will not take up any ink. Only the relatively shallow diffractive regions 120 of the microstructure will accept ink and become reflective (Figure 3(B)). The metallised regions 120' are thus automatically in register with the second group of relief elements, avoiding the problem with known selective metallisation processes in which there is a lack of accurate registration between the printing process and the preferred regions of the microstructure that are to be metallised. The method described here is thus much more accurate and has a much higher intrinsic resolution because its limitations are determined by the accuracy and resolution of the microstructure itself and not that of the printing process.

The metallic layer may be patterned by use of an appropriately patterned printing plate. In one embodiment, it is possible to create optically variable devices with an inbuilt electronic or electromagnetic capability. Examples of such effects include passive RFID antenna responses, where the metallised diffractive regions of the OVD are patterned as antenna-like images which act as conducting paths, producing a unique back response electromagnetic signature to an interrogating electromagnetic wave. Since the metallised regions are also diffractive, and can therefore carry optically variable picture information, the RFID tag and diffractive optically variable effect provide a composite security device in which an optical and non-optical security feature are combined in a single device. The surrounding regions are non-metallised refractive or diffuse scattering regions which can carry a further optically variable effect viewable in transmission.

Other printed substrate electronic components can also be realized by embedding a differentially metallised bi-level microstructure within a polymer film. For example, the microstructure could be embossed directly into the film under heat and pressure, or formed by "soft embossing" a polymer precursor and curing the embossed precursor, and then selectively metallising the microstructure as described above. The back of the film may also have printed metallized regions so that the resulting sandwich structure produces capacitive effects between the top and bottom surface conducting electrodes. Similarly, resistive components and patterns can be created using metallic inks with variable resistance properties determined by the nature of the ink's constituent particles.

In Figure 5, there is shown an alternative embodiment in which both groups of relief elements are metallised. Beginning again with a bi-level interlaced microstructure 130 (Figure 5(A)), a much thicker layer of metallic ink 125, having depth D' equal to or greater than the depth D of the deep relief elements 1 10, is applied to the printing plate. Both types of relief elements will then be reflective (Figure 5(B)) and the hybrid optically variable effect will be produced by a combination of reflective diffractive elements 120' and reflective microprism elements (micromirror elements) 1 10'. A protective layer 136 of an opaque or transparent material may optionally be applied (Figure 5(C)).

Referring now to Figure 6, there is shown an alternative optically variable device 600 in which an optically invariable image 610 is printed on one side of the substrate 200 adjacent to a high refractive index film layer 210. By an appropriate arrangement or micro-structuring of the interlaced deep relief elements 1 10 it may then be recognised that printed information 610 on the back plane of the substrate 200 may be made to appear only at certain angles of observation determined by the shape of the deep phase refracting elements 1 10. The differentially metallised diffraction grating regions 120' produce an optically variable diffractive image effect, and refraction and diffuse scattering from the combined refractive and printed features 1 10, 610 produce a second optically variable effect the colour dependence of which is a function of the colours used in the printed image 610. If the refractive elements 1 10 are in the form of longitudinal prisms then the printed information 610 is in the form of thin stripes with the stripe axis parallel to the long axis of the microprisms 1 10.

When observed and illuminated through the transparent polymer film 200 and UV lacquer layer 1 15, a hybrid optically switching effect is observed through the interaction of a diffractive optical switch from the metallised diffractive regions 120' and a refractive lenticular image switch from the transparent high refractive index regions 1 10 and the printed background 610. In its simplest form, the printed background 610 may consist of a uniformly metallised background achieved, for example, through printing with metallic inks. Alternatively the printed layer 610 may consist of printed variable information, in which case the device represents a ultra high resolution differentially metallised diffractive OVD. In this case it is not necessary to have a difference in refractive index between the protective film and lacquer layers, because different parts of the printed variable information 610 may be observable in unmetallised regions 1 14 as the observing angle is changed. In another embodiment of the optically variable device of the present invention, the first group of relief elements may include one or more microlens arrays. Referring to Figure 7, the deep level microstructures incorporate refractive part-cylindrical or part-spherical lenses 712 adjacent to shallow diffractive elements 722 in grating sections 720. Each grating section 720 is preferably between 30 and 60 microns wide, with the micro-lenses 712 being similarly dimensioned to the grating sections 720. The diffraction grating regions 720 may be overcoated with a metallic layer 725 using the differential printed metallisation process described earlier, or may remain unmetallised.

The lenslets 712 may be used as magnifying elements for microtext or micrographics 735 printed on the opposite side 730 of the polymer film 200. The hybrid OVD effect therefore consists of an optically variable or image switching diffractive effect due to diffractive regions 720 superimposed on magnified printed microtext or micrographics 735 which will vary with angle due to the change in distance between the microlenses 712 and the microtext or micrographics 735 as the viewing angle is changed.

In yet another embodiment, shown in Figure 8, at least some of the diffractive regions 721 may be formed as micrographic elements 723. In this embodiment, optical magnification of the micrographic elements 723 is observed when viewing through lenses 712 from an off-axis position 801 , in addition to magnification of microtext or different micrographic elements 735 viewed through the lenses 712 from an on-axis position 802 (Figure 9). It is noted that the fidelity of the magnification off-axis is much greater than that observed from the on-axis position 802 due to the fact that the optical accuracy is determined purely by the origination process for the microstructure 720, 721 , and not by any subsequent micro-printing process on the top layer 730 of the substrate, ie on top of the high refractive index (hri) protective film layer 210.

From Figure 9 it can be seen that magnification of the micro-printing 735 occurs on-axis, so that little or no interference should occur between magnification of the micrographic elements 723 off-axis and magnification of the micro-printing elements 735 on-axis.

Further embodiments of the invention are shown schematically in Figure 10(A) and 10(B), in which regions of relatively deep relief elements 860 having high spatial frequency are interlaced with regions of relatively shallow relief elements 850 having a lower spatial frequency. The relief elements are embossed into, for example, a UV lacquer layer 880 applied to substrate 900, the lacquer then being cured. Both of the regions 850 and 860 can be coated with metallic ink 855, as in Figure 10(A). If the spacing between adjacent relief elements in regions 860 is less than the wavelength at which the security device is being observed, and in particular of the order of half of the wavelength, light which is incident on regions 860 will be selectively absorbed in one direction so that regions 860 form a polarising structure, sometimes known as a wire grid polariser. Regions 850 diffract incident light and so produce a diffractive optically variable effect which is visible mostly in the first and second diffraction orders.

The device of Figure 10(B) is similar to that shown in Figure 10(A), except that the diffractive regions 850 remain unmetallised when the thin layer of metallic ink 855 is applied.

The device of either Figure 10(A) or Figure 10(B) is a two-channel optically variable device, in which the first group of relief elements 860 produces a polarisation-dependent image which is observed in the zero order, while the second group 850 produces a diffractive optically variable image which is observed in the first and second propagating diffractive orders. Advantageously, an optically variable effect may thus be observed both under uniformly diffuse lighting conditions, in which normal optically variable devices show little or no normal variation, and also under illumination by normal light sources of finite extent, under which conditions ordinary diffractive optical devices may be observed.

In Figure 1 1 (A), there is shown a further embodiment of the present invention, in which a microprism array structure 920 of relatively high surface relief has been stacked on top of, a relatively low surface relief diffractive structure 910 to form a bi-level microstructure combination. In this case the diffractive microstructure 910 is first soft embossed and UV cured onto a polymer film 900 and then covered with a layer of transparent high refractive index film 915 using, for example, a rotogravure printing process. A layer of UV curable lacquer 917 is then applied, preferably by printing, on top of the high refractive index film 915 and soft embossed with a microprism array microstructure 920 and again UV cured. Finally, a second high refractive index layer 925 is applied on top of the microprism array structure 920 to protect the microprism array. The double soft embossed diffractive and refractive structure shown in Figure 1 1 (A) will produce optically variable refractive effects in the zero order from the microprism arrays 920, in combination with optically variable diffractive effects in the non-zero diffractive orders from the diffraction grating array 910. The combination of the optically variable diffractive effect and optically variable refractive effect is generated from the same area of the multilayer film, increasing the difficulty in a counterfeiter trying to reverse engineer and reconstruct the device.

In Figure 1 1 (B) there is shown a variation of the bi-level microstructure of

Figure 1 1 (A). In Figure 1 1 (B) the refractive component comprises pairs of microprism 920, 920' having respective inclined sidewalls 922, 922' of opposite slope. An image switching effect in the zero order of the transmitted beam is produced due to the image components of one image being refracted from the microprisms 920' of left leaning slope and the second image components being generated by the microprisms 920 of right leaning slope.

Optionally, the diffractive layer 910 could also be designed as a matrix or interlaced track structure of two different spatial frequencies so that image switching with different artwork could be generated from this layer also. The overall effect from the combination of refractive and diffractive layers in this instance would be of a four channel image switching device with the inner diffractive image switches being coloured (as a result of the wavelength dependence of the diffractive orders), and the outer image switches being uncoloured (or achromatic) as a result of the refractive mechanism.

In a yet further embodiment, shown in Fig 12, microprisms 1 120 are again stacked on top of diffractive regions 1 1 10, but in this instance are also in interlaced relationship with the diffractive regions since the prism sidewalls 1 124 overlie non-diffractive regions 1 1 1 1 which separate diffractive regions 11 10. The skilled person will also appreciate that the whole of each microprism 1 120 could overlie each corresponding non-diffractive region 1 1 1 1.

Referring now to Figures 13 and 14, there is shown a schematic of a process for producing an embossing tool which may be used to form an optically variable device according to the present invention. First two separate and complementary embossing tools 1210, 1220 are produced (steps 1310, 1320). The respective relief structures are written into a resist material using electron beam lithography, for example, or by a combination of electron beam lithography and photolithography, depending on the scale of the relief structures and hence the resolution required to produce said structures. Each embossing tool or shim is then produced by electroforming or electroplating onto the respective embossed relief structures.

A polymer substrate, for example comprising Perspex, is prepared and is then embossed (under conditions of elevated temperature and pressure) with the first embossing tool 1210 to form the relief structure 110 in the polymer substrate (Figure 13(A) and step 1322). A thin layer (e.g. 2 microns thick) 1215 of UV curable lacquer is then applied (step 1330) on top of the embossed structure 1 10 using, for example, a rotogravure printing plate uniformly covered with the lacquer to produce the structure 1217 shown in Figure 13(B). The second relief structure 120 is then soft embossed (step 1340) in register on top of the first microstructure, for example by a nano-imprinting process, and cured (step 1350) by UV radiation 1250 as shown in Figure 13(C). The resultant bi-level microstructure 130 is then formed into the final bi-level embossing tool by an electroforming or electroplating technique (step 1360).

Alternatively, the bi-level microstructure 130 could be created in a single step using direct write electron beam lithography techniques, for example by using two level electron beam resist processes. In a further alternative, the bi- level structure 130 could be created using embossing of thick electron beam resists followed by a direct write process for the diffractive relief structure.

In a yet further alternative, the bi-level microstructure could be created by using the first embossing tool 1210 to form the relief structure 1 10 in a layer of a UV-curable lacquer (step 1325), and curing the lacquer (step 1327). This is followed by soft embossing, in register, of a thin layer of UV curable lacquer applied to the relief structure 1 10 by the second embossing tool 1220 as before.

Referring now to Figure 15, there is shown a cross-section through part of a security document, indicated generally by 1400, including a security device in the form of a bi-level interlaced microstructure 100 substantially as shown in Figure 2(C). The security document includes a transparent substrate 200 on which the security device 100 having refractive elements 1 10 and metallised diffractive relief structures 120' is formed. A high refractive index material 210 is applied over the microstructure 100. The security document includes at least one opacifying coating 1410 applied on one or both sides, apart from in window areas 1420a, 1420b in which the opacifying coating(s) 1410 is omitted. An observer viewing the window area 1420b of security document 1400 will see a combination of a refractive effect due to the relief elements 1 10 and a diffractive effect due to the metallised diffractive relief structures 120'.

Turning now to Figure 16, there is shown a substrate 1500 in which a first group of surface relief elements 1510 is formed. The surface relief elements 1510 may be embossed directly into the substrate 1500, or the substrate 1500 may be a preformed structure such as the embossing shim 1220 shown in Figure 13.

The substrate 1500 is then coated with a relatively thick (for example, 30 micron thick) layer of photoresist 1520. The photoresist 1520 is then exposed through a mask 1530 having apertures 1531 formed therein. The radiation source 1525 may be any source suitable for exposing the photoresist 1520, for example, a source of UV radiation.

The exposure step leaves a second group of relief elements in the form of pillars of photoresist 1542 which overlie the first group of surface relief elements 1510. The tops of pillars 1542 are then embossed with a second embossing shim 1550 which may carry a surface relief structure which is the same as or different than the surface relief structure 1510 carried by substrate or embossing shim 1500. This step leaves embossed photoresist pillars 1552 having a third group of surface relief elements 1543 formed in their upper surface.

The photoresist is then passivated by coating with a metallic layer 1560, for example a nickel coating which is sputtered onto the photoresist. This is followed by electroplating of a thick layer of nickel, after which the photoresist is dissolved away to leave the composite embossing shim 1600. Embossing shim 1600 includes a first group 1601 , a second group 1602, and a third group 1603 of surface relief structures.

Figure 17 shows fabrication of an optically variable device using the shim 1600 of Figure 16. Shim 1600 is brought into contact with embossing medium 1710 (for example, an embossable radiation curable ink) to create first, second and third groups of surface relief structures 1701 , 1702 and 1703. A printing roller carrying a thin layer of metallic ink 1710 is then brought into contact with the tops of pillars 1702 in order to create metalized surface relief structures 1713 which can be viewed in reflection. The surface relief structures are then overcoated with a transparent lacquer 1720 having a different refractive index to embossing medium 1710.

In use, as in Figure 18, a person 1800 viewing the device 1700 from a first side 1810 will see an optically variable image from metalized regions 1713 of the device 1700. If the light source 1805 is placed on the opposite side 1820 to the first side 1810, the viewer 1800 sees a second optically variable image in transmission from regions 1701 of the device 1700.

In the embodiments shown in Figures 16 to 18, the apertures 1531 in photomask 1530 may advantageously be arranged to have a surface density corresponding to the grey levels of an input greyscale image. The viewer 1800 will then see a greyscale image (for example, a dithered portrait) in reflection, corresponding to the pattern of the second group of surface relief elements 1702, whilst in transmission, a negative version of the greyscale image will be apparent to the viewer 1800.

It will be understood by the skilled person that many combinations, variations and modifications of the above methods and devices are possible using the information disclosed herein, without departing from the spirit and scope of the invention as defined in the claims appended hereto. For example, the security document shown in Figure 15 may have printing or other matter, including further security elements, applied to either or both of the opacifying coatings. The coating on the top surface could be applied over the whole surface of the security document, so that the omitted region 1420b forms a half-window area. Further, the security document of Figure 15 may include any of the various types of security device described herein.

Claims

CLAIMS:

1. An optically variable device, including a plurality of groups of surface relief elements, the groups being interlaced and/or stacked, wherein a first said group is characterised by a first height or depth and produces a first type of optical and/or electromagnetic effect, and a second said group is characterised by a second height or depth which is different to the first height or depth, the second group producing a second, different, type of optical and/or electromagnetic effect than that produced by the first group.

2. A method of producing an optically variable device including a plurality of groups of surface relief elements, the method including the steps of:

3. An optically variable device or method according to claim 1 or claim 2, wherein the groups of surface relief elements are formed in a radiation curable material applied to a substrate.

4. An optically variable device or a method according to any one of claims 1 to 3, wherein the first type of optical and/or electromagnetic effect is a non- diffractive effect and the second type of optical and/or electromagnetic effect is a diffractive optically variable effect.

5. An optically variable device or method according to claim 4 wherein the first type of optical and/or electromagnetic effect is one or more of one of the following effects:

reflective, refractive, diffusely scattering, electrical.

6. An optically variable device or a method according to any one of claims 1 to 5, wherein a third group of surface relief elements is interlaced and/or stacked with the first and second groups.

7. An optically variable device or a method according to claim 6, wherein the relief elements of the third group are formed in a surface of the relief elements of the first group or the second group.

8. An optically variable device or a method according to claim 6, wherein the relief elements of the third group are interlaced with the relief elements of the first group or the second group.

9. An optically variable device or a method according to any one of claims 4 to 8, wherein the first type of optical and/or electromagnetic effect is a greyscale optically variable image.

10. An optically variable device or a method according to claim 9, wherein the greyscale image is visible in reflection from a first side of the device, and a negative version of the greyscale image is visible in transmission from the first side of the device.

1 1 . An optically variable device or a method according to claim 9 or claim 10, wherein the grey levels of the greyscale image are determined by the surface density of the relief elements of the first group in the plane of the device.

12. An optically variable device or a method according to any one of claims 9 to 1 1 , wherein the grey levels of the greyscale image are determined by the depths of the relief elements of the first group.

13. An optically variable device or a method according to any one of claims 1 to 12, wherein first height or depth is at least five times greater than the second height or depth.

14. An optically variable device or a method according to claim 13, wherein the first height or depth is at least 10 times greater than the second height or depth.

15. An optically variable device or a method according to any one of the preceding claims, wherein the minimum height or depth of the relief elements of the first group is greater than about 5 microns.

16. An optically variable device or a method according to any one of the preceding claims, wherein the maximum height or depth of the relief elements of the first group is less than about 120 microns.

17. An optically variable device or a method according to any one of the preceding claims, wherein the maximum height or depth of the relief elements of the second group is less than about 4 microns.

18. An optically variable device or a method according to any one of the preceding claims, wherein the minimum height or depth of the relief elements of the second group is greater than about 0.1 microns.

19. An optically variable device or a method according to any one of the preceding claims, wherein the relief elements of the second group are at least partly formed in an upper or lower surface of the relief elements of the first group.

20. An optically variable device or method according to any one of the preceding claims, wherein a reflective metallic layer is applied to the first group and/or the second group of relief elements, and/or, if applicable, the third group of relief elements.

21 . An optically variable device or method according to claim 20, wherein the reflective metallic layer is applied at a thickness which is equal to or greater than the maximum height or depth of the relief elements of the second group but less than the minimum height or depth of the relief elements of the first group whereby only the second group is made reflective.

22. An optically variable device or method according to claim 20 or claim 21 , wherein the reflective metallic layer is applied in a pattern.

23. An optically variable device or method according to claim 22, wherein the pattern forms at least one passive electrical component.

24. An optically variable device or method according to claim 22, wherein the pattern forms a passive radio-frequency antenna.

25. An optically variable device or method according to claim 24, wherein the passive radio-frequency antenna is an RFID tag.

26. An optically variable device or method according to any one of claims 20 to 22, wherein relief elements in the second group are spaced from each other at a distance which is sufficiently small that light transmitted or reflected by the second group is polarised.

27. An optically variable device or method according to any one of the preceding claims, wherein a layer or coating is applied to the first group and/or the second group, and, if applicable, the third group.

28. An optically variable device or method according to claim 27 wherein the layer or coating is a protective coating.

29. An optically variable device or method according to claim 27 or claim 28, wherein the refractive index of the layer or coating is different from the refractive index of the material in which the groups of relief elements are formed.

30. An optically variable device or method according to any one of the preceding claims, wherein the first group of relief elements includes microprisms having a lower surface, an upper surface and at least one inclined sidewalk

31 . An optically variable device or method according to claim 30, wherein the first group includes two or more sets of microprisms, and each set of microprisms is characterised by a different angle of inclination of the inclined sidewall, whereby different images or different regions of an image can be viewed through each set of microprisms of the optically variable device.

32. An optically variable device or method according to claim 31 , wherein the device includes an image layer bearing an image composed of two or more interleaved sub-images, and each set of microprisms views one of the sub- images.

33. An optically variable device or method according to claim 32, wherein the interleaved sub-images are arranged in bands or stripes.

34. An optically variable device or method according to claim 32 or claim 33, wherein the image layer is a printed layer.

35. An optically variable device or method according to any one of claims 30 to 32, wherein each microprism corresponds to a pixel of a grayscale bitmap image, and the area of each microprism is related to the gray level of the corresponding pixel.

36. An optically variable device or method according to any one of the preceding claims, wherein a spacing layer is applied between the first and second groups of relief elements.

37. An optically variable device or method according to claim 36, wherein the spacing layer has a refractive index which is substantially different to the refractive index of the material in which the relief elements are formed.

38. An optically variable device or method according to any one of claims 1 to 29, 36 or 37, wherein the first group of relief elements includes microlenses.

39. An optically variable device or method according to claim 38, wherein a layer of microprinting is applied to the device, and the microprinting is viewable through the microlenses to produce the first type of optical effect.

40. An optically variable device or method according to claim 39, wherein a region or regions of micrographic elements is or are applied as part of the second group, whereby the microprinting is viewable through the microlenses from an on- axis position and the region or regions of micrographic elements is or are viewable through the microlenses from an off-axis position.

41 . An optically variable device or method according to claim 3 or any claim dependent therefrom, wherein the radiation curable material is applied to a transparent or translucent substrate.

42. An optically variable device or method according to claim 3 or claim 41 , wherein the radiation curable material is applied to the substrate by printing.

43. A security element, including the optically variable device of any one of the preceding claims.

44. A security document, including a security element according to claim 43.

45. A security document according to claim 44 wherein the security element is provided in a window or half-window area of the document.

46. A method according to any one of claims 2 to 42, wherein the first and second groups of relief elements and, if applicable, the third group, are applied simultaneously.

47. A method according to any one of claims 3 to 42 or claim 46, further including the step of applying a second layer of radiation curable material overlying the first group of relief elements, whereby the second group of relief elements is formed in the second layer of radiation curable material.

48. A method according to claim 47, wherein the second layer of radiation curable material is applied by printing.

49. A method according to any one of claims 3 to 42 or claims 46 to 48, wherein the layer or layers of radiation curable material are embossed to form the first and second groups of relief elements and the radiation curable material is cured substantially at the same time as the embossing step.

50. A method according to any one of claims 3 to 42 or claims 46 to 49, wherein the radiation curable material is cured by actinic radiation.

51 . A method according to claim 50, wherein the actinic radiation is selected from the group comprising X-ray, electron beam and UV radiation.

52. An embossing tool including a plurality of groups of surface relief elements or recesses, the groups being interlaced and/or stacked, wherein a first said group is characterised by a first height or depth and a second said group is characterised by a second height or depth which is different to the first height or depth, whereby when a transparent, translucent or reflective material is embossed with the embossing tool, areas embossed with the first group of relief elements produce a first type of optical effect and areas embossed with the second group of relief elements produce a second, different, type of optical effect.

53. An embossing tool according to claim 52, further including a third group of surface relief elements or recesses interlaced with the first group or the second group, whereby areas embossed with the third group produce a third type of optical effect.

54. A method of creating an embossing tool for applying a plurality of groups of surface relief elements to a transparent, translucent or reflective material, the method including the steps of: embossing a first group of surface relief elements on a substrate, the first group being characterised by a first height or depth to produce a first type of optical effect;

embossing a second group of surface relief elements on the substrate so that the first and second groups are interlaced and/or stacked, the second group being characterised by a second height or depth which is different to the first height or depth to produce a second, different, type of optical effect; and

electroplating or electroforming a replica of the embossed substrate to form the embossing tool.

55. A method according to claim 54, wherein the first relief structure is embossed directly into the substrate.

56. A method of creating an embossing tool for applying a plurality of groups of surface relief elements to a transparent, translucent or reflective material, the method including:

providing a substrate including a first group of surface relief elements, the first group being characterised by a first height or depth to produce a first type of optical effect;

applying a third group of surface relief elements to an upper surface of the second group of surface relief elements;

removing the sacrificial material.

57. A method according to claim 56, wherein the sacrificial material is a photoresist.

58. A method according to claim 57, wherein the second group of surface relief elements is formed by irradiating the photoresist through a mask.

59. A method according to claim 58, wherein the mask includes apertures with a surface density corresponding to the grey levels of an input greyscale image.