FIELD OF THE INVENTION

The present invention relates to improvements in liquid crystal devices and methods of manufacture of liquid crystal devices. In one form, the invention may be used to create security devices and security documents including such security devices, but it will be understood that the invention may have wider applicability.

DEFINITIONS

Security Document

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 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

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 < L ₀ , where L ₀ is the amount of light incident on the document, and L _τ 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

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).

Diffractive Optical Elements (DOEs)

As used herein, the term diffractive optical element or diffractive optical projection 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.

Overt Security Devices

Overt security devices are those which are apparent to a person handling the banknote and include devices such as security threads embedded in layers of the security document and visible at least in transmission when a person holds the security document up to the light; printed features which are visible in reflection and/or transmission; embossed features, including relief structures, which may be tactile so that they can be detected by a person feeling the tactile area of the note; and optically variable devices (OVDs). OVDs provide an optically variable effect when the banknote is tilted and/or when the viewing angle of the observer relative to the OVD changes. An OVD may be provided by a printed area, eg an area printed with metallic inks or iridescent inks, by an embossed area, and by a combination of a printed and embossed feature. An OVD may also be provided by a diffractive device, such as a diffraction grating or a hologram.

Covert Security Devices

A covert security device is one which is not apparent to a person handling the banknote without the use of external verification or authentication means. Covert security devices include features such as microprinting, which requires a magnifying lens to authenticate the microprinting; and features formed by photoluminescent inks and phosphorescent inks which require illumination by electromagnetic radiation of a particular wavelength, eg infra-red (IR) or ultra- violet (UV) radiation, for the ink to luminesce or phosphoresce; and photochromic, thermochromic, hydrochromic or piezochromic inks.

BACKGROUND OF THE INVENTION

Manufacturers of security documents are constantly seeking to provide security devices with increased resistance to counterfeiting. Technological advances, particularly in relation to printing technology, have made it easier for casual counterfeiters to mimic many types of optical effect.

The use of different forms of liquid crystals, both nematic and cholesteric, as security devices has previously been proposed. For example, US 5 602 661 discloses an optical component which has an orientation layer comprising a photoorientable polymer network (PPN) in contact with a film of cross-linked nematic liquid crystal monomers with varying local orientation of the liquid crystal molecules. The liquid crystal monomers are oriented by interaction with the PPN layer and the orientation is fixed in a subsequent cross-linking step.

The security of a document can be increased by providing more than one security element on the same document, particularly through the combination of an overt and a covert security feature. For example, EP1613988 describes a multilayer film in which a replication layer is embossed with a diffractive structure under heat and pressure. A layer of liquid crystal material is then applied to the replication layer so that the liquid crystal molecules are oriented in accordance with the diffractive structure. There is thus produced a film having an overt security feature (diffractive effect) as well as a covert security feature (polarisation effect). However, application of the liquid crystal material to the diffractive structure may greatly reduce or even remove the diffractive effect, since the liquid crystal material has a similar refractive index to the replication layer, and the security of the film thus resides mostly in the covert security feature.

Another type of liquid crystal film is described in US 6 627 270. A liquid crystal polymer film having a cholesteric or chiral smectic orientation is manufactured by preparing an alignment film, such as a rubbed polyimide film, on a substrate, and then applying a liquid crystal to the alignment film, whereby the liquid crystal is aligned according to the orientation of the alignment film. A diffraction pattern is then formed in the aligned liquid crystal film under heat and pressure. Thus multiple layers and multiple processing steps are required to form the liquid crystal device, increasing the complexity and expense of manufacture.

In view of the aforementioned deficiencies of known methods of forming liquid crystal devices, it is desirable to provide a method which is simpler and less expensive to implement, and which is more efficient.

It is also desirable to provide devices with multiple security features, in which the effectiveness of one security feature is not compromised by the presence of the others.

It is further desirable to provide a security document or device of a more complex nature, which can produce different visual effects and/or be used for different purposes, such as for verifying a security feature at another part of the security document.

Further, many security documents, such as banknotes, have several different security features in different areas of the note. This can be confusing for the public, but is a necessity imposed by applying security features by different methods which requires relatively large tolerances, typically of at least 1 .5-2.0 mm so that the features stand by themselves. It is therefore desirable to provide a process for forming multiple security features in a single area that does not require large tolerances and which is difficult to replicate.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a polarising liquid crystal device, including at least one relief structure in an embossed area or areas of a radiation curable liquid crystal material, wherein the liquid crystal material is aligned with a predetermined polarisation pattern in the embossed area or areas, the predetermined polarisation pattern including two or more areas having different polarisations.

In a second aspect, there is provided a polarising liquid crystal device, including at least one relief structure in an embossed area or areas of a radiation curable liquid crystal material, wherein the liquid crystal material is aligned with a predetermined polarisation pattern in the embossed area or areas, wherein the at least one relief structure includes a diffractive relief structure.

In a further aspect, the invention provides a method of forming a polarising liquid crystal device, including the steps of:

applying a radiation curable liquid crystal material to a substrate;

embossing at least one relief structure in a radiation curable liquid crystal material,

the embossing step aligning the liquid crystal material with a predetermined polarisation pattern, and

curing the embossed liquid crystal material, wherein the embossing and curing steps are performed substantially simultaneously.

By aligning the liquid crystal material via the embossing step, the manufacturing process is greatly simplified, since the relief structure, which provides a first security feature, is formed simultaneously with the covert security feature provided by the polarisation effect of the predetermined polarisation pattern. The two security features are formed in the same area of the device, and are automatically in register with each other. Furthermore, the optical effect produced by the relief structure is not compromised by the application of a further layer of material, since the relief structure is formed in the liquid crystal material itself. The process thus forms a two-in-one security device in a single layer in a single step.

Simultaneously embossing and curing the liquid crystal material may further improve the simplicity and efficiency of manufacture since the same tool may be used to both emboss and cure the liquid crystal material. Thus effectively only two steps are required to produce the liquid crystal device: applying the liquid crystal material to a substrate (e.g. by printing), and embossing-curing the desired structure in the liquid crystal material. The polarisation pattern may include a first area or areas having a first polarisation, and a second area or areas having a second polarisation, wherein the first polarisation is orthogonal to the second polarisation. When viewed under a filter at one angle, only the first area or areas may be visible, and when the filter is rotated and the device viewed at a different angle, for example at 90 degrees to the first angle, only the second area or areas may be visible. This embodiment thus provides for a polarisation effect which is variable when viewed under different conditions.

The polarisation pattern may be black or coloured. Coloured effects may be achieved with the use of suitable UV curable liquid crystal materials, such as the polymerisable and/or mesogenic compound-containing formulations described in US 2003/0104144, EP 1669431 and WO 2006/027076, which are incorporated herein by reference in their entirety. Other suitable liquid crystal materials include the cross-linkable diacrylate compositions described in US 6,160,597, the contents of which are also incorporated herein by reference.

The liquid crystal material may be a cross-linkable monomer or oligomer, or a polymer. If it is a monomer or oligomer, it may be cross-linked to form a polymer by curing the aligned liquid crystal material with radiation.

Preferably, the method further includes the step of curing the liquid crystal material with actinic radiation, preferably selected from the group including X- rays, UV radiation and electron beam radiation. In some embodiments, the liquid crystal material may also include a dichroic dye, such as a dye having the structure described in US 6,160,597. Polarising liquid crystal devices formed from liquid crystal materials containing such dyes may have a variety of applications, including as liquid crystal displays for electronic devices such as televisions, cell phones or head-up displays, photographic filters, and so on.

In one embodiment, a light-absorbing layer may at least partly overlie the relief structure. This serves to enhance the contrast between the polarisation effect and the effect due to the relief structure itself, whether this be diffractive, refractive or otherwise. In some embodiments, the effect due to the relief structure may be enhanced, particularly if the relief structure is a diffractive structure.

The light-absorbing layer is preferably a high-gloss black layer. The reflectivity of the layer is preferably greater than 60% when measured at 20 degrees incidence using ASTM test method D523, more preferably greater than 90%.

Another way to increase the contrast between the polarisation effect and the effect due to the relief structure is to apply a reflective coating, preferably a reflective metallic coating, which at least partly overlies the relief structure. This may be done in addition to, or instead of, the application of a light-absorbing layer. It is thought that the presence of a reflective coating tends to mute the colour-change effect of the polarisation pattern, whilst preserving the effect due to the relief structure.

At least one of the relief structures may be diffractive. The relief structure may be a lens structure, such as a Fresnel lens or an array of part-cylindrical or part-spherical lenses.

In one form of the invention, the liquid crystal device may include two or more different diffractive relief structures, e.g. selected from a diffraction grating, a hologram and a numerical-type diffractive optical element.

In another form of the invention, the liquid crystal device may include a diffractive relief structure forming one security element, and a non-diffractive relief structure forming another security element. The other security element may be an optically variable non-diffractive relief structure or a relief structure forming a lens, a microlens array or a lenticular array.

In a preferred embodiment, the security device formed from the embossed liquid crystal material may include a verification means for verifying another security feature provided on the document or device.

The liquid crystal material may be provided on a substrate. In a particularly preferred embodiment, the liquid crystal material is applied to the substrate in solution, preferably by a printing process.

By applying the liquid crystal material in solution, it is possible to perform the entire process of forming the liquid crystal device as an in-line process. This greatly simplifies the manufacture of the device. Furthermore, this type of "soft- embossing" process is advantageous because it results in less build-up of material on the embossing tool than occurs with known "hot embossing" processes.

Simultaneously embossing and curing the liquid crystal material may further improve the simplicity and efficiency of manufacture since the same tool may be used to both emboss and cure the liquid crystal material. Thus effectively only two steps are required to produce the liquid crystal device: applying the liquid crystal material to a substrate (e.g. by printing), and embossing-curing the desired structure in the liquid crystal material.

With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable liquid crystal material is applied, to improve the adhesion of the embossed structure formed by the liquid crystal material to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a hydroxyl terminated polymer. The primer layer may also include a cross-linker, for example a multi-functional isocyanate. Examples of primers suitable for use in the invention include: hydroxyl terminated polyester based co-polymers; polyethylene imine; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyazihdines; ziconium complexes; aluminium acetylacetone; melamines; and carbodi-imides. Preferably, the cross-linker added to the primer layer is the same as the cross-linker in the liquid crystal material. The type of primer may vary for different substrates and embossed relief structures. Preferably, a primer is selected which does not substantially affect the optical properties of the embossed relief structure.

The substrate is preferably formed from at least one layer of a transparent material and may have at least one layer of an opacifying coating applied to at least one of its sides. The opacifying coating may be omitted in one or more regions to form one or more window or half-window areas.

In one preferred embodiment, opacifying layers are applied to opposite sides of the substrate with the opacifying layers on both sides of the substrate omitted in one region to form a transparent window in the liquid crystal device.

In another preferred embodiment, at least one opacifying layer is applied to one side of the transparent substrate to completely cover said one side, and at least one opacifying layer is applied to the opposite side of the substrate, except in a region which forms the half-window area.

Preferably, the embossed relief structure is formed at least partly in one of the window or half-window areas. If it is not completely within the window or half- window, and thus overlaps with the boundary of the window or half-window, this may provide a further effective security feature since only part of the relief structure is then available for a counterfeiter to reproduce by contact copying.

In a further aspect of the present invention, there is provided a polarising liquid crystal device manufactured according to the method of the third aspect of the invention.

In a yet further aspect of the present invention, there is provided a security document, including a polarising liquid crystal diffractive device according to any of the above-described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figures 1 , 2 and 3 show cross sections through security documents produced in accordance with three different embodiments of the invention; Figure 4 shows an embodiment of a printing apparatus suitable for producing a polarising liquid crystal device, or a security document including such a device;

Figures 5 and 6(a) to 6(e) show a security document printed using the apparatus of Figure 4;

Figures 7(a) to 7(c) show a method of verifying the authenticity of the security document of Figures 5 and 6;

Figures 8(a) to 8(c) show a self-verifying security document including a polarising liquid crystal device; and

Figures 9(a) to 9(c) show a polarising liquid crystal device in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Figure 1 there is shown a security document 10 comprising a substrate 1 1 of transparent plastic materials having a first or upper side 12 and a second or lower side 13. The transparent substrate is preferably formed from a transparent polymeric material such as a laminated structure of two or more layers of bi-axially oriented polypropylene. It will, however, be appreciated that other transparent or translucent polymeric substrates may be used in the present invention such as polyethylene and polyethyleneterephthalate (PET).

Opacifying layers 14 and 15 are applied respectively to the first and second sides 12 and 13 of the transparent substrate 1 1 with the opacifying layers 14 and 15 omitted in one region of the substrates to form a window area 16 in which a security device 17 is provided.

The security device 17 is formed from an embossable liquid crystal material 18 applied on one side 13 of the substrate within the window area, and the liquid crystal material 18 is embossed with a relief structure 19 to form the security device 17.

The liquid crystal material 18 may be any embossable liquid crystal material in a nematic, smectic or chiral phase. It may also be a combination of two or more different liquid crystal materials, for example two or more layers in which each layer is a different liquid crystal material. Alternatively, the liquid crystal material may be a polymerisable and/or mesogenic compound-containing formulation as described in US 2003/0104144, EP 1669431 or WO 2006/027076, the contents of which are incorporated herein by reference in their entirety.

In a preferred method of manufacturing the security document of Figure 1 , the liquid crystal material is printed onto the side 13 of the transparent plastics substrate 1 1 and is embossed while soft and cured with UV radiation simultaneously before the opacifying layers 13 and 14 are applied to the substrate 1 1.

In some applications, an intermediate primer layer (not shown) may be applied to the surface 13 of the transparent substrate 1 1 before the embossable liquid crystal material 18 is applied to improve the adhesion of the resulting embossed security device to the substrate.

In an alternative method, the opacifying layers 14 and 15 could first be applied to the opposite sides 12 and 13 of the substrate 1 1 with the liquid crystal material 18 being printed on the window area 16 of the substrate 1 1 and then embossed and cured with UV radiation.

The opacifying layers 14 and 15 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, the substrate 1 1 of transparent plastic material could be sandwiched between opacifying layers of paper to which indicia may be subsequently printed or otherwise applied. It is also possible for the security documents to be formed from a paper or fibrous substrate which has an area cut-out with a transparent plastics insert inserted into the cut-out area to form a transparent window to which the liquid crystal material is applied and embossed to form the security device 17.

The security device 17 formed from the embossed liquid crystal material 18 may include one or more of a variety of relief structures to form different security elements, including diffractive structures such as diffraction gratings, holograms and numerical-type diffractive optical elements (DOEs). Alternatively or additionally, the security device 17 may include other relief structures such as lens structures and optically variable non-diffractive relief structures.

In a particularly preferred embodiment, the security device 17 includes a multilayer diffractive optical element (DOE). The method of embossing a radiation curable liquid crystal material when soft and curing the material enables multilayer DOEs and non-centro-symmetric DOEs to be formed by embossing. A multilevel DOE is a diffractive optical element with a discrete number of phase levels wherein the number is an integer greater than one. The discrete phase levels of the binary DOE may be formed by embossing each level to a different surface relief depth.

Multiple phase levels each having a discrete surface relief depth may provide multi-bit storage of data. At least some of the phase levels may generate a projected visual image which is visible in a reconstruction plane when the multi- level DOE is illuminated with collimated light, such as from a laser, an LED or other point light source. It is also possible for at least some phase levels to store encrypted data. A multi-layer DOE enables the DOE to store more information and to be provided over a larger area, thereby providing stronger, brighter and more effective DOEs which may also generate animated or moving images. Further, multi-level DOEs which have more than two phase levels may be asymmetrical or symmetrical, whereas single phase and binary DOEs are limited to symmetrical DOEs.

Figure 2 shows a modified security document 20 which is similar to the security document of Figure 1 , and corresponding reference numerals have been applied to corresponding parts. Figure 2 differs from Figure 1 in that the opacifying layer 15 is applied to cover the side 13 of the transparent substrate completely in the area of an embossed liquid crystal material 18, but the opacifying layer 14 on the opposite side 12 of the substrate 1 1 is omitted in the area of the embossed liquid crystal material 18 to form a half-window area 26.

The security device 27 formed from the embossed liquid crystal material 18 in Figure 2 preferably includes at least one reflective security element which is visible from one side of the security document, i.e. the side corresponding to the side 12 of the substrate 11 to which the opacifying layer 14 is only partially applied. The security device 27 may be either completely invisible or partially visible from the opposite side of the substrate, depending upon the thickness and opacity of the opacifying layer 15 which is applied to the opposite side 13 of the substrate 1 1. The security document 20 may be manufactured by first printing the radiation curable liquid crystal material 18 on the transparent substrate 1 1 in the region which is to become the half-window 26, then embossing and curing the liquid crystal material 18 simultaneously to form the embossed relief structure 19, and then applying the opacifying layers 14 and 15 to the substrate 1 1. Alternatively, a curable liquid crystal material may be printed on one side of the transparent substrate in the half-window region, embossed and simultaneously cured and then a metallic ink composition may be applied to the embossed liquid crystal material before the opacifying layer 15 is applied. As shown in Figure 2, these methods have the advantage that the embossed relief structure 19 is protected by the opacifying layer 15 which completely covers the liquid crystal material 18. However, in an alternative method a metallic ink composition could be applied to the opposite surface 12 of the transparent substrate 1 1 in the half- window area either before, during or after the application of the opacifying layer 14. In this case, it may be necessary to apply a protective coating, such as a transparent gloss varnish over the security device formed from the embossed liquid crystal material. If a protective coating is applied, it should have a different refractive index to the liquid crystal material.

The metallic ink composition can comprise any suitable ink which produces a reflective effect. Examples of suitable inks include the metallic ink compositions described in WO 2005/051675 and WO 2005/049745, the contents of each of which are incorporated herein by reference. The metallic ink compositions may contain pigment particles of metals, such as aluminium, gold, silver, platinum or copper, or alloys such as stainless steel, nichrome or brass.

Figure 3 shows another embodiment of a security document 50 in accordance with the invention which is similar to the document 20 of Figure 2 and corresponding reference numerals have been applied to corresponding parts. The security document 50 differs from that of Figure 2 in that a security device 57 formed by the embossed liquid crystal material 18 is a composite security device containing two different security elements 51 and 52.

Figure 3 also differs from Figure 2 in that additional layers 34 and 35 are applied to the opacifying layers 14 and 15. The layers 34 and 35 may be additional opacifying layers, e.g. pigmented coatings containing titanium dioxide when it is desired to increase the opacity of the security document except in the half-window area 56. Alternatively, the additional layers 34 and 35 may be layers of printed indicia.

The security document 50 also differs from that of Figure 2 in that the composite security device 57 is provided on the same side of the transparent substrate 1 1 as the half-window 56, with a further security element 58 provided on the opposite side 13 of the transparent substrate 1 1 which is completely covered by the opacifying layers 15 and 35 in the half-window area.

The further security element 58 is preferably in the form of an element which interacts with at least one of the first and second security devices 51 and 52 of the composite security device 57. In one embodiment the first security element 51 may comprise a reflective diffractive structure, such as a DOE or hologram, and the second security element 52 may comprise a lens structure with the further security element 58 comprising a security feature which can be verified, inspected or enhanced by the lens structure 52. For example, the further security element 58 may comprise an area of micro printing, with the second security element comprising a Fresnel lens or a magnifying lens for magnifying a viewing the micro printing. Alternatively, the second security element 52 may comprise a lenticular array, such as an array of microlenses 53 with the second security element comprising an array of micro-images 59 in register with the micro lenses such that the micro-images 59 can be viewed through the lenticular array 52. The micro-images 59 may be formed by a variety of different methods.

The micro images 59 could be printed onto the surface 13 of the transparent substrate; or they could be markings formed with a laser, eg by laser blackening, laser colouration or ablation.

The micro-images 59 may be clear, coloured or black, or a combination of the above. The combination of microlenses 53 and micro-images 59 may produce a magnified image of the individual micro-images by a process known as moire magnification. It is also possible for the combination of microlenses 53 and micro- images 59 to produce moving or floating images.

In another possible embodiment, the micro-images 59 may be replaced by a hologram structure 58, such as an embossed reflective rainbow hologram, which in combination with the array of microlenses 53, can produce some interesting optical effects.

Referring now to Figure 4, there is shown schematically a printing and embossing apparatus 100 including a supply unit 102 for supplying a sheet-like substrate to various printing and embossing stations, including an opacifying station 104, a first printing station 106, an embossing station 1 10, and a second printing station 1 14.

The substrate 101 is preferably made of a substantially transparent polymeric material and may be continuously supplied to the opacifying station 104 from a roll 103 of the material at the supply unit 102. The opacifying station 104 includes opacifying means for applying at least one opacifying layer to at least one side of the substrate 101 . The opacifying means is preferably in the form of a printing unit, eg one or more rotogravure printing cylinders 105 for applying one or more opacifying coatings of ink to one or both sides of the substrate. However, it is possible the opacifying station 104 could include opacifying means in the form of a laminating unit for applying one or more sheet-like layers of at least partly opaque material, such as paper or other fibrous material to at least one side of the transparent substrate.

Preferably, the opacifying means 105 at the opacifying station 104 is arranged to omit at least one opacifying layer on one or both sides of the substrate in at least one region to form a window or half-window area.

The first printing station 106 includes printing means 107, 108 for applying an embossable radiation-curable liquid crystal material to the substrate 101 . The printing means may comprise at least one printing cylinder 107, eg a rotogravure printing cylinder, with the opacified transparent substrate fed between the printing cylinder 107 and a corresponding cylinder or roller 108 on the opposite side of the substrate.

The printing means 107, 108 is arranged to apply the radiation curable liquid crystal material to a first area 120 of the substrate of security document 200 (Figure 5) on which a relief structure is to be embossed at the embossing station.

The embossing station 1 10 includes embossing means preferably in the form of a plate cylinder 1 1 1 and impression cylinder 1 12. The embossing means 1 1 1 , 1 12 includes embossing portions arranged to emboss different areas of the substrate as it passes through the nip between the plate and impression cylinders 1 1 1 , 1 12. A first embossing portion is arranged to emboss the first area 120 of the substrate 101 to which the embossable radiation curable liquid crystal material is applied to form the relief structure 123.

The embossing station 1 10 may also include radiation curing means 1 13 for curing the embossable, radiation curable liquid crystal material substantially simultaneously or almost immediately after the liquid crystal material has been embossed to form the relief structure. Alternatively, a separate curing station may be provided. The radiation curing means preferably comprises an ultraviolet (UV) curing unit for curing a UV curable liquid crystal material, but other types of curing units, eg X-ray or electron beam (EB) curing units may be used for X-ray or EB radiation curable liquid crystal materials.

The second printing station 1 14 includes printing means for applying printed features to the substrate. The printing means preferably includes a printing cylinder 1 16 such as a rotogravure, offset or intaglio cylinder, and may be used to apply a wide variety of printed features to the substrate. For instance, the printing cylinder 1 16 at the second printing station 1 14 may be used to apply printed security features in register with, adjacent to or surrounding the relief structure. One example of a printed security feature could include a printed metallic ink applied over the relief structure 123. Another example of a printed security feature applied at the second printing station 1 14 is an area of microprinting which can be viewed or inspected when overlaid by a microlens array.

The apparatus also includes first and second registration stations 131 , 132. Each registration station 131 , 132 includes a respective detector for detecting the positions of registration marks relative to a registration key, and a comparator for comparing the relative positions of the registration marks relative to the registration key. The apparatus also includes a control means 140 in the form of a central processor unit (CPU) for adjusting the relative positions of the printing means 107 and 1 16 of the first and second printing stations 106, 1 14 relative to the embossing means at the embossing station 1 10.

In operation of the apparatus, the transparent substrate 101 is supplied from the supply unit 102 through the opacifying station 104 where at least one opacifying layer is applied to at least one side of the substrate 101 . The at least partly opacified substrate 101 is then fed through the first printing station 106 where the embossable radiation curable liquid crystal material is applied to the first area 120 which is to be embossed to form the relief structure.

The substrate 101 is then fed through the embossing station 1 10 where the first area 120 of the substrate is embossed to form the relief structure 123. The radiation curable liquid crystal material is then cured by radiation, preferably at the embossing station 1 10 to fix the embossed relief structure.

The substrate bearing the relief structure is then fed through the first registration station 131 where the relative positions of the registration key and registration marks are detected and compared. If the registration key and registration marks are not in register, the registration station 131 sends a misregistration signal to the CPU 140 which then sends a control signal to the first printing station 106 to adjust the position of the printing means 107, 108 relative to the embossing means 11 1 at the embossing station 1 10.

After the substrate passes through the first registration station 131 it is fed through the second printing station 1 14 where further printed features are applied to the substrate. It is also desirable for the position of such printed features to be accurately located relative to the relief structure. For this purpose, the position of the printed registration mark applied by the second printing station 1 14 relative to the registration key is detected at the second registration station 132. If the printed registration mark is not in register with the registration key, the registration station 132 sends a signal to the CPU 140 which, in turn sends a control signal to the second printing station 1 14 to adjust the position of the printing means 1 16 at the second printing station 1 14 relative to the embossing means 1 1 1 at the embossing station 1 10.

The apparatus 100 may also include further printing stations (not shown) for applying further printed features 125, 127, 129 and registration marks to the substrate 101 , and further registration stations with detectors (not shown) may be provided for detecting the positions of those registration marks relative to the position of the registration key to enable adjustment of the printing means at the further printing stations relative to the embossing means 1 1 1 . In the case where the embossed security element is provided in a window or half-window area formed by the opacifying station 104 omitting the opacifying layer or layers in a region on at least one side of the substrate 101 , the opacifying station 104 may also include means for applying a registration mark to the substrate 101 , with the detector at the first registration station 131 or another detector arranged to detect the position of said registration mark relative to the position of the registration key. The position of the opacifying means 105 at the opacifying station 104 may then be adjusted by the control means 140 in response to signals from the first registration station 131 or other detector to ensure the position of the relief structure 123 is accurately in register within the window or half-window area.

It is also possible for an opacifying station to be located after the embossing station, with this opacifying station applying at least one opacifying layer to at least one side of the substrate except in the area of the relief structure 123 to form a window or half-window. This opacifying station may also apply a printed registration mark which is used to adjust the position of the opacifying means to ensure accurate registration with the security element in the aforesaid manner.

Referring now to Figures 5 and 6, there is shown a security document 200 which may be produced by the apparatus of Figure 4. The security document includes a window area 120 in which an embossed liquid crystal relief structure 123 with a first directional embossing 123a and a second directional embossing 123b has been applied. The different directional embossings align the liquid crystal material so that the areas 123a, 123b produce different polarisations. The security document includes further elements which are preferably applied by printing or another in-line process, including denominator 125, image 127 and serial number 129.

In Figure 6(a) and 6(b), a substrate 101 has applied to it a layer 160 of a liquid crystal material by the printing station 104 of Figure 4. Embossing station 1 10 then embosses and cures a relief structure 123 in the liquid crystal material 160 (Figure 6(c)). An opacifying coating 150 is then applied by printing station 1 14, except in the window area 120 containing the embossed relief structure 123 (Figure 6(d)). Finally, further indicia 125, 127, 129 may be applied by one or more additional printing stations to produce the finished security document 200 (Figure 6(e)).

Alternatively, the opacifying coating 150 could be applied prior to application of the liquid crystal material 160 to the substrate 101.

Figures 7(a) to 7(c) demonstrate the two-in-one nature of the security device 123. In Figure 7(a) a person inspecting the windowed area 120 of security document 200 with the naked eye will observe an overt security feature in the form of an optically variable effect due to diffraction through the embossed areas 123a and 123b. However, when viewed through a linear polariser 210 having axis of polarisation represented schematically by arrows 220, a covert security feature becomes apparent. In Figure 7(b), light emitted from region 223a (corresponding to embossing 123a) is blocked and thus region 223a appears dark, while light from region 223b (corresponding to embossing 123b) is allowed to pass. When the polariser 210 is rotated by 90 degrees, so that the axis of polarisation 220' also rotates, region 223a' appears bright and/or coloured while region 223b' appears dark.

In an alternative embodiment, depicted in Figure 8(a), a security document 300 has polarisers 310a, 310b integrated within a second window on the document. Polarisers 310a, 310b have different axes of polarisation. When the document is folded about the line 301 as in Figure 8(b), polariser 310a can be brought into alignment with the relief structure 123, which appears dark in the central region 223a and bright and/or coloured in the outer region 223b. When the document is folded about line 302 as in Figure 8(c), polariser 310b can be brought into alignment with relief structure 123, whereby central region 223a' appears bright and/or coloured, and outer region 223b' appears dark.

The pattern defined by the relief structure 123 is not restricted to simple shapes. More complicated designs, such as those shown in Figure 9, may be embossed into the liquid crystal material. Here an embossed diffractive structure 400 in the shape of a numeral "9" within concentric circles is formed in a liquid crystal material and a layer of a metallic ink composition is applied over the embossed relief structure. For example, the metallic ink may give the relief structure a golden or silver appearance to an observer viewing it with the naked eye (Figure 9(a)). When viewed under a polariser at one angle, the relief structure 400 takes on the appearance shown in Figure 9(b), with a dark background 410. Upon rotating the polariser to a second angle, for example under rotation by 90 degrees, the relief structure takes on the appearance shown in Figure 9(c), having a light background 420.

It will be appreciated that various modifications and alterations may be made to the embodiments of the present invention described above without departing from the scope and spirit of the present invention as defined in the claims appended hereto.