AU2018102067A4 - A security device and method - Google Patents

Abstract

A security device, comprising: a substrate including a first and a second surface, a plurality of focusing elements provided on the first surface of the substrate, an image layer provided on the second surface of the substrate, arranged to be sampled by the plurality of focusing elements to produce a multicolour image when the security device is viewed with a suitable illumination source, wherein the image layer comprises a liquid crystal layer having a thickness, the thickness of the liquid crystal layer being modulated such that the thickness at any given location is determined by a colour that is to be observed in the multicolour image, and birefringent properties of a liquid crystal material that is used to form the liquid crystal layer. C> K. C> K ~>

Description

A SECURITY DEVICE AND METHOD

FIELD OF THE INVENTION [0001] The invention relates generally to the field of optical security devices, for example as used on banknotes.

BACKGROUND TO THE INVENTION [0002] It is well known that many of the world’s banknotes, as well as other security documents, carry optical devices, which act as security elements for authentication purposes. Some optical security elements produce optical effects that vary depending on the viewing angle or require a predetermined optical illumination source in order to reveal the optical effects. The incorporation of such optical security elements into security documents therefore acts as a deterrent against counterfeiting of the document.

[0003] Some optical security devices, for example lens based images, interlaced images, stereograms, integral images, magnifying moires and the like suffer from a number of similar problems. For example, the limited resolution of pixels, the addressability of the pixels and the registration of different colours relative to one another. The physical size of lenses used in security device application is usually determined by a number of factors including sag height of the lens and focal length of the lens (intimately related to the thickness of the material upon which the lens will be formed and the distance to the focusing surface, usually the obverse side to the lens).

[0004] The issues of high-resolution images and colour registration (especially multicolour images) have been approached in the past by various different methods.

2018102067 14 Dec 2018 [0005] One method involves using diffractive imagery elements, where colours are created by diffraction elements located within a single surface. With this method, different colours are produced by changing the spacing between parallel diffractive grating elements to preferentially diffract one wavelength of light at a given angle of viewing.

[0006] Another method involves plasmonic structures whereupon conductive surfaces with sub wavelength periodic structures are created so that standing (resonant) waves of a particular frequency are created between the structures.

[0007] Another method involves using laser to alter structures of interference layers in a vacuum-deposited multilayer structure.

[0008] Another method involves creating structural colour by blending chiral and nematic liquid crystals. Whist the use of these two materials has long been known to create colour pairs at a given angle, the colour created is achieved by controlling the ratio of the pair of liquid crystals. The helical pitch of the materials is controlled by the ratio of the two materials and this in turn created the perceived colour pair. OPSEC Security (www.opsecurity.com) created a process which controls the pitch by controlling the amount of exposure to a given frequency of light. As the quantum of light increases, the colour shifts from one end of the spectrum to the other. It is envisaged that this effect is achieved through a greyscale mask being used to control the degree of light exposure.

[0009] All of these methods have certain drawbacks.

[0010] With the diffractive imagery method, the colour of the image varies as a function of viewing angle. The diffraction efficiency varies as a function of pixel size and importantly when used in conjunction with a lens, the device only works when the diffractive gratings are at 90 degrees to the lens direction, i.e. it only works in conjunction with cylindrical lenses and not round lenses, limiting this effect to only one plane.

2018102067 14 Dec 2018 [0011] Plasmonic devices require highly conductive, metallic surfaces to work effectively. They are relatively low in colour strength and tend to produce subdue hues rather than vibrant colours. Their ability to be integrated into high-speed manufacturing process is limited due to the high aspect ratio of the structures as well as the need to vacuum metallise the structure to achieve the required surface conductivity.

[0012] The interference stack process requires the multi-layer refractive stack be produced using a magnetron deposition process. Then each individual pixel must be separately written using laser. This limits the technology to a batch process with a very slow throughput for writing (even though the laser can have relatively high speed write rates, a large number of pixels would require tens of seconds of writing for each image, if not minutes).

[0013] The UV cholesteric nematic pair via the light exposure route includes the added complexity of controlling the degree of light exposure not only through a mask but also the aging of the light source as a function of time. Any variance will result in the variance of the colour of the images. It requires the device to be exposed to be in registration with the surface of a material upon which it is deposited, which further complicates the manufacturing process.

[0014] It is therefore an object of the present invention to provide an optical device and method for the formation thereof which addresses one or more limitations of the prior art, or at least provide an alternative choice for the general public.

SUMMARY OF THE INVENTION [0015] According to a first aspect of the invention, there is provided a security device comprising:

a substrate including a first and a second surface,

2018102067 14 Dec 2018 a plurality of focusing elements provided on the first surface of the substrate, an image layer provided on the second surface of the substrate, arranged to be sampled by the plurality of focusing elements to produce a multicolour image when the security device is viewed with a suitable illumination source, wherein the image layer comprises a liquid crystal layer having a thickness, the thickness of the liquid crystal layer being modulated such that the thickness at any given location is determined by a colour that is to be observed in the multicolour image, and birefringent properties of a liquid crystal material that is used to form the liquid crystal layer.

[0016] According to a second aspect of the present invention, there is provided a security device comprising:

a substrate including a first and a second surface, a plurality of focusing elements provided on the first surface of the substrate, a plurality of image elements provided on the second surface of the substrate, the focusing elements and the image elements being arranged to produce a polarisation image when the security device is viewed with a suitable illumination source, wherein at least some of the image elements comprise a liquid crystal element having a thickness determined by a colour that is to be produced by its associated image element for the polarisation image.

2018102067 14 Dec 2018 [0017] Preferably, the suitable illumination source required to reveal the image is a linearly polarised light source.

[0018] Preferably, the polarisation image is a multicolour polarisation image.

[0019] Preferably, the thicknesses of the liquid crystal elements are modulated such that different image elements include liquid crystal elements of varying thicknesses, thereby producing multiple colours which collectively form a multicolour image.

[0020] In one form, the thicknesses of the liquid crystal elements are modulated to vary discretely, or continuously. Alternatively, the liquid crystal element may be formed at a substantially similar thickness, so that the image elements all produce a similar colour but at differing brightness levels and the image generated by the security device is a binary, or a greyscale image.

[0021] Preferably, the thicknesses of the liquid crystal elements are determined in accordance with birefringent properties of the liquid crystal material used to form the liquid crystal elements.

[0022] In one form, the liquid crystal elements are formed from aligned nematic liquid crystals.

[0023] In one form, the security device includes an alignment layer for aligning the liquid crystal elements in a desirable orientation. The alignment layer includes one or more surface relief structures, such as grooves, recesses, or voids with predetermined spacing, depth, and orientation to receive the liquid crystal material while it is soft and in a liquid form, and aligns the liquid crystal elements in the same direction as the orientation of the surface relief structures. After the liquid crystal elements are aligned, they can then produce a predetermined polarisation pattern when the suitable illumination source is used.

2018102067 14 Dec 2018 [0024] In another embodiment, the liquid crystal material may be printed over the alignment layer in a liquid form, and then embossed while it is soft so that a second relief structure is formed in an upper surface which is not in direct contact with the alignment layer. In this way, it is possible to configure the security device as a two-sided security device which reveals two polarisation images. The first polarisation image is revealed when the image layer is viewed through the focusing elements from the first side of the substrate, and a second polarisation image can be revealed when the image layer is directly viewed from the second side of the substrate.

[0025] The alignment layer may be formed from a radiation curable ink or the like. For example, the relief structure may be formed by embossing a transparent radiation curable ink with an embossing tool, and subsequently or simultaneously curing the radiation curable ink to retain the relief structures in the alignment layer.

[0026] In some embodiments, the alignment layer may be arranged to produce a non-polarised imagery effect viewable under normal lighting conditions.

[0027] In some embodiments, the alignment layer, when it is arranged to produce a non-polarised imagery effect, includes a diffractive grating, preferably at least with respect to visible wavelengths of light.

[0028] In some embodiments, the alignment layer includes relief structures that extend in different directions, such that different polarisation images, or different portions of an image become visible as the security device is rotated relative to an axis which is substantially orthogonal to the substrate plane. Such different polarisation images may be configured to produce an animated image sequence.

[0029] In a preferred embodiment, the alignment layer includes alignment structures in which the orientation of the structures continuously changes. More

2018102067 14 Dec 2018 preferably, the alignment structures correspond to diffraction gratings that continuously change their orientation. In this embodiment, as the security device is viewed under normal lighting condition and as it is rotated relative to an axis perpendicular to the substrate, a continuously changing diffractive imagery effect is perceivable. As it is viewed under a polarised light source and as it is rotated relative to the same axis, a continuously changing polarisation image or polarisation image sequence is viewable. This provides a particularly strong forgery resistant security feature.

[0030] Preferably, the liquid crystal material is applied to the alignment layer after forming the relief structures, by a printing process, such as gravure printing, intaglio printing, offset printing, silk screen printing, or inkjet printing.

[0031] Alternatively, the liquid crystal elements may be formed as microstructures and directly deposited on the second surface of the substrate.

[0032] The focusing elements may be lenticular lenses, round lenses, zone plates, Fresnel lenses, refractive elements, or the like.

[0033] In some embodiments, the focusing elements are formed by embossing and curing a substantially transparent curable resin, for example a conventional free radical or cationic acrylate system or a radiation curable resin, preferably, a UV curable resin.

[0034] Alternatively, the focusing elements could also be formed by embossing and curing a radiation curable nematic liquid crystal, preferably, a UV curable nematic liquid crystal.

[0035] In the embodiments where the focusing elements are formed from a UV curing nematic liquid crystal material, it may optionally contain a dichroic dye that acts as a polarising filter. In this embodiment, the surface of the focusing elements will optionally comprise micron to submicron alignment channels to

2018102067 14 Dec 2018 receive the dichroic dye. The alignment channels are configured such that they do not adversely influence the effectiveness of the focusing elements in sampling the image elements provided on the second surface of the substrate. By creating these alignment channels and providing the dichroic dye to the focusing elements, the focusing elements can filter the optical illumination that passes through and create a polarised illumination. In this embodiment, the image may be viewable without a polarised illumination source, as the focusing elements are capable of creating a polarising filter with the assistance of the dichroic dye. As such, a suitable illumination source in these embodiments includes unpolarised light.

[0036] In some embodiments, the image elements also comprise a dichroic dye. By preselecting a dichroic dye and adding this to the image layer, it is possible to further enhance the effect by making a non-coloured image change to the same image but multicoloured or to a different image and coloured under polarised light.

[0037] In some embodiments, each focusing element has one associated image element. In other embodiments, each focusing element may have a plurality of associated image elements and the plurality of associated image elements are arranged to be observed by a viewer at different viewing angles.

[0038] Preferably, the security device also includes a high refractive index (HRI) layer applied to the plurality of image elements. The HRI layer optionally includes a substantially flat outward facing surface. The HRI layer may be selected to have a refractive index the same as, or substantially the same as, the ordinary refractive index of the liquid crystal elements. Alternatively, the HRI layer may be selected to have a refractive index the same as, or substantially the same as, the extraordinary refractive index of the liquid crystal elements. In a further alternative, the HRI layer may be selected to have a refractive index between the ordinary and extraordinary refractive indices of the liquid crystal elements. In another configuration, however, the HRI layer is selected to have a refractive

2018102067 14 Dec 2018 index substantially greater than the largest refractive index of the liquid crystal elements.

[0039] Preferably, the substrate is transparent.

[0040] According to a third aspect of the present invention, there is provided a security document, preferably a banknote, including a document substrate including, in a region of the document substrate, a security device according to the first or the second aspect.

[0041] In an embodiment, the substrate of the security device is different to the document substrate, and wherein the security device is formed separately and subsequently attached to the document substrate. In an alternative embodiment, the substrate of the optical device is the same as the document substrate.

[0042] The security document preferably includes a first opacifying layer applied to a side of the document substrate, the first opacifying layer including a window region such that the security device is located in the window region.

[0043] Also preferably, the security document includes a second opacifying layer applied to a different side of the document substrate to the first opacifying layer, the second opacifying layer including a window region such that the security device is located in the window region of the security document. Alternatively, the security document includes a second opacifying layer applied to a different side of the document substrate to the first opacifying layer, the second opacifying layer partially or entirely covering the optical device, such that the optical device is located in a half-window of the security document.

[0044] According to a fourth aspect of the present invention, there is provided a method of manufacturing a security device in an in-line process, the method comprising:

2018102067 14 Dec 2018 providing a substrate including a first and a second surface, forming a plurality of focusing elements on the first surface of the substrate, forming an image layer on the second surface of the substrate, said image layer being arranged to be sampled by the plurality of focusing elements to produce a multicolour image when the security device is viewed with a suitable illumination source, wherein the image layer comprises a liquid crystal layer having a thickness, the thickness of the liquid crystal layer being modulated such that the thickness of the liquid crystal layer at any given location is determined by the colour that is to be observed in the multicolour image and birefringent properties of a liquid crystal material that is used to form the liquid crystal layer.

[0045] In a preferred form, the focusing elements and the image layer are both formed by radiation curable microstructure forming processes. Radiation curable microstructure forming processes include: soft embossing, where an embossing tool, having relevant microstructures thereon, is used to emboss into uncured or partially cured radiation curable ink present on a substrate: and microprinting, where radiation curable ink is provided to a tool, having relevant microstructures thereon, which is subsequently brought into contact with a substrate to which the microstructure are to be formed on. In both cases, suitable radiation is provided when the tool and substrate are brought together, or shortly thereafter, fixing the microstructures in the radiation curable ink on the substrate. The preferred radiation curable system is UV, such that the radiation curable ink is UV curable ink and the provided radiation is UV.

[0046] Preferably, the step of forming the plurality of focusing elements includes:

2018102067 14 Dec 2018 depositing a radiation curable ink over a selected region of the substrate, embossing the radiation curable ink with a first embossing tool to form the focusing elements, and simultaneously or subsequently after the embossing step, curing the radiation curable ink to retain the focusing elements on the substrate.

[0047] Alternatively, the step of forming the plurality of focusing elements includes: depositing a radiation curable ink over a selected region of a first microprinting tool containing suitable focusing element structures, bringing the first microprinting tool into contact with the substrate, and simultaneously or subsequently after bringing the first microprinting tool into contact with the substrate, curing the radiation curable ink to retain the focusing elements [0048] Preferably, the step of forming the image layer include:

depositing a radiation curable ink over a selected region of the substrate, embossing the radiation curable ink with a second embossing tool to form an alignment layer, said alignment layer including alignment structures configured and shaped to align a liquid crystal material, applying a liquid crystal material to the alignment layer, allowing the liquid crystal material to be aligned in the same direction as the alignment structures.

[0049] Alternatively, the step of forming the image layer includes: depositing a radiation curable ink over a selected region of a second microprinting tool containing suitable alignment structures, bringing the second microprinting tool into contact with the substrate, and simultaneously or subsequently bringing the second microprinting tool into contact with the substrate, curing the radiation curable ink to retain the focusing elements, to form an alignment layer, said

2018102067 14 Dec 2018 alignment layer including alignment structures configured and shaped to align a liquid crystal material, applying a liquid crystal material to the alignment layer, allowing the liquid crystal material to be aligned in the same direction as the alignment structures.

[0050] Preferably the plurality of focusing elements are formed at a single printing or embossing station. Similarly, the alignment layer is formed at a single printing or embossing station.

[0051] The first and second embossing tools and first and second microprinting tools may be formed by any one of the following methods: extreme UV lithography, EB and ion beam etching, Direct write laser lithography, Nano 3D writing.

[0052] In one form, the step of forming the plurality of focusing elements and the alignment layer may be completed using other manufacturing methods such directly depositing a micro-structured layer over the substrate, wherein the microstructured layer includes elements that correspond to the focusing elements and/or the relief layer.

[0053] The multicolour image that is observable may be configured to provide one or more of the following imagery effects: full colour integral imaging, full colour pseudoscopic magnification, ultra high resolution full colour interlacing, ultra high resolution image animation, and so on.

[0054] In some embodiments, the security device is arranged to provide two imagery effects: a polarisation image that is only viewable with the suitable illumination source, preferably a linearly polarised illumination source, and a nonpolarisation image that is viewable under normal lighting conditions (i.e. nonpolarised light source), for example a diffractive imagery effect enabled by the relief structures in the alignment layer.

2018102067 14 Dec 2018 [0055] In one form, the security device is arranged to provide two-sided polarisation imagery effect: a first polarisation image is revealed when the image layer is viewed via the focusing elements, and a second polarisation image is revealed from an opposite side of the security device without viewing through the focusing elements. Preferably the second polarisation image is produced by directly embossing a polarisation pattern in an upper surface of the liquid crystal material.

[0056] In some embodiments, the multicolour image produced by the security device is achieved by interlacing red, green, blue image elements.

[0057] The suitable illumination source to view the polarisation image can be an existing LCD screen of a mobile phone, a computer, ATM machine, cash register, or the like.

[0058] In some embodiments, rotation of the illumination source by 90 degree relative to an axis substantially perpendicular to the substrate causes a contrast switch (i.e. positive to negative transition) of the image generated by the device.

[0059] In some embodiments, the alignment layer includes alignment structures that extend in different directions, so that the liquid crystal layer is also aligned in different directions. As the security device is rotated around an axis substantially orthogonal to the plane in which the security device resides, the polarisation image may change its appearance, i.e. colour, shape, brightness; or a series of polarisation images may be revealed as the device is rotated, to produce an animation image sequence.

[0060] In a preferred embodiment, the security device is formed in a window area of a document substrate and appears substantially transparent before the suitable illuminations source is applied.

2018102067 14 Dec 2018

Security Document or Token [0061] As used herein the term security documents and tokens 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.

[0062] The invention is particularly, but not exclusively, applicable to security documents or tokens such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Security Device or Feature [0063] 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).

Substrate [0064] 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

2018102067 14 Dec 2018 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.

Windows and Half Windows [0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

2018102067 14 Dec 2018

Opacifying layers [0069] 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 < Lo, where Lo is the amount of light incident on the document, and Lt is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Refractive index n [0070] The refractive index of a medium n is the ratio of the speed of light in vacuum to the speed of light in the medium. The refractive index n of a lens determines the amount by which light rays reaching the lens surface will be refracted, according to Snell’s law:

[0071] Πγ * Sin(a) = n* Sir^0^) , [0072] where a is the angle between an incident ray and the normal at the point of incidence at the lens surface, θ is the angle between the refracted ray and the normal at the point of incidence, and m is the refractive index of air (as an approximation m may be taken to be 1).

Radiation Curable Ink [0073] The term radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be printed or embossed while soft, or semi-soft, to form a relief structure and cured by radiation to fix the relief structure. The curing process, typically, does not take place before the radiation curable ink is printed or embossed, but it is possible for the ink to be partially cured (semi-soft), in some

2018102067 14 Dec 2018 processes, before printing or embossing and also for the curing process to take place either after printing or embossing or at substantially the same time as the printing or embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink may be cured by other forms of radiation, such as electron beams or X-rays. References to UV curable ink(s) in the remainder of the description are by way of example. All embodiments may be replaceable with other radiation curable inks, as long as they can meet the criteria required by the embodiment (such as viscosity prior to curing). Similarly, reference to UV lamps reflect that the description refers to UV curable inks. If an ink curable by electron beam is used, the, clearly, an electron beam device would be used instead of the UV lamps.

[0074] The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as subwavelength gratings, transmissive diffractive gratings and lens structures.

[0075] The transparent or translucent ink preferably comprises an acrylic based UV curable clear lacquer or coating. Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable ink may be based on other compounds, eg nitro-cellulose.

[0076] The radiation curable inks and lacquers used herein have been found to be particularly suitable for printing or embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be printed or embossed with larger relief structures, such as non-diffractive optically variable devices.

[0077] The ink is preferably printed or embossed and cured by ultraviolet (UV) radiation at substantially the same time.

2018102067 14 Dec 2018 [0078] Preferably, in order to be suitable for Gravure printing, which is the preferred method of applying the radiation curable ink when it is subsequently embossed, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.

[0079] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a cross-linker, for example a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetylacetone; melamines; and carbodi-imides.

BRIEF DESCRIPTION OF THE DRAWINGS [0080] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be appreciated that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings:

[0081] Figure 1 is a schematic diagram of a security document including a security device of the invention;

[0082] Figure 2a shows a first embodiment of the invention from a side view;

2018102067 14 Dec 2018 [0083] Figure 2b shows a second embodiment of the invention from a side view;

[0084] Figure 2c shows a third embodiment of the invention from a side view;

[0085] Figure 3a shows a fourth embodiment of the invention;

[0086] Figure 3b shows a fifth embodiment of the invention;

[0087] Figure 4 shows a sixth embodiment of the invention;

DESCRIPTION OF PREFERRED EMBODIMENT [0088] For the purposes of the following discussion, the figures are to be considered illustrative and not to scale, unless otherwise indicated. The figures illustrate simplified depictions of the embodiments described.

[0089] “Incident light” or “Incident illumination” is light from a light source incident onto a side of the substrate, and is in general considered to be nonpolarised white light (for example, as sourced from an incandescent or fluorescent light source), unless otherwise stated.

[0090] A “visual effect” is an image, pattern, or other visually identifiable effect. A visual effect can be a hidden visual effect, which is only visible under certain conditions, or an overt visual effect, which is visible under normal viewing conditions. A visual effect can also be a diffractive visual effect or a non-diffractive visual effect.

[0091] “Colour” as used herein refers to a colour as perceived, and may correspond to a single range of wavelengths or a mixing of different ranges of wavelengths.

2018102067 14 Dec 2018 [0092] It should be noted that throughout the present disclosure, ‘multicolour’ is used to mean at least two different colours, and preferably, a broad range of different colours. In addition, if a polarisation image shows, for example a motif, a number, or an icon, then the motif, number or icon itself must include a plurality of different colours in order to be considered as a multicolour image.

Principles of construction [0093] It is known that materials incorporating liquid crystals are birefringent. Incident light onto a surface including a liquid crystal material will experience different refractive indices depending on the polarisation of the incident light.

[0094] In general, the colour change of an incident illumination source after passing through a liquid crystal material is a function of the refractive indices difference over the optical path. This is known as the differential interferometry. When the sample observed is placed between two crossed linear polarisers, the optical retardation is given by r = 2d(n_e-n_o) for reflective arrangement and r = d(n_e-n_o) for transmissive arrangement, where d is the thickness of the birefringent material. Typical value of the extraordinary (n_e) and ordinary indices (n₀) can reach up to Δη = 0.2. This allows substantially thinner films to be used to achieve desired effect. Namely, about 0.8pm of film can act as λ/2 plate, thus 0.4pm thick film for λ/4 plate. As seen from the table below, it can be approximately stated, that in the first order, the desired liquid crystal material thickness obeys d = r/Δη.

[0095] For example, blue colour would occur for a liquid crystal element having a thickness of dbiue = 0.23/0.2 = 1.15pm. Similarly, red colour would occur for a liquid crystal element having a thickness of dred= 0.33/0.2= 1.65pm. Using this technique, it is possible to determine a range of different thicknesses required for liquid crystal elements to produce a range of colours for a polarisation image, thereby producing a multicolour polarisation image.

2018102067 14 Dec 2018

Colour	Wavelength [nm]	r(n=0)	r(n=1)
Red	620 - 750	310-380	930- 1120
Orange	580 - 620	295-310	890 - 930
Green	500 - 570	250 - 285	745 - 855
Blue	450 - 500	225 - 248	675 - 740

Exemplary embodiments of the invention [0096] Referring to Figure 1, a first embodiment of a security device 110 of the present invention is provided on a security document 100.

[0097] The security document 100 includes a document substrate 120, which is the main carrier of various security and design features of the security document 100. For ease of illustration, only one security feature 110 is shown in all the embodiments of the present disclosure, but it is well recognised that security documents, typically, have multiple security features. The document substrate 110, which is typically made from a transparent polymeric material, includes a first surface 130 and an opposing second surface 140, and are both substantially planar. One or more opacifying layers 150 may be provided in selected regions of the security document 100, particularly when the substrate 110 is substantially transparent, so that designs patterns, solid colours, text, or similar thereof can be directly formed on the opacifying layers. A window region 160 is created by omitting the opacifying layers in a selected region of the document substrate 120. The security device 110 of the invention is located within the window region 160 of the security document 100 in Figure 1, but this is not essential. As the security device 110 is integrally formed as part of the security document 100, the document substrate 110 also acts as the substrate of the security device 110.

2018102067 14 Dec 2018 [0098] In other embodiments, the security device 110 is formed, for example, as a transfer film for applying to the substrate 120 of the security document 100.

[0099] Figure 2a shows a detailed side-view of the first embodiment of the invention. A plurality of focusing elements 111 are provided on the first surface 130 of the substrate 120, and an imagery layer 114 is provided on the second surface 140 of the substrate 120, comprising a plurality of image elements 113a, 113b, 113c, 113d, 113e (herein 113a-e) arranged to be sampled by their associated focusing elements 111. The image elements 113a-e are located within a focal range of the focusing elements 111, or defers from the focal range of the focusing elements 111 by a reasonable amount, for example such that each focussing element is capable of sampling the respective image element in a distinguishable manner. When a suitable illumination source 170 is used, a polarisation image is revealed, preferably a multicolour polarisation image (not shown). Preferably the size of the focusing elements directly relate to the focal length of the elements and is within ±20% of the thickness of the optical spacer between the focusing element and the image layer, i.e. the thickness of the substrate.

[00100] In this embodiment, the imagery layer 114 is a multi-layered structure including at least an alignment layer 112, and a liquid crystal layer 113 including a plurality of liquid crystal elements 113a-e each associated with one of the focusing elements 111. As mentioned, the thickness of the liquid crystal layer 113 is modulated based on birefringence properties of the liquid crystal material used to form the liquid crystal elements 113a-e, in order to produce different colours for the multicolour image. It should be appreciated that the thickness variation of the liquid crystal layer 113 can be at discrete levels, for example it may only contain 3 distinct thicknesses, corresponding to image elements which generate red, green and blue colours respectively. By interlacing the red, green and blue image elements in a suitable pattern, it is possible to generate a coloured image from the three primary colours.

2018102067 14 Dec 2018 [00101] It should be appreciated that these image elements are micro-image elements, which means they have to be viewed through the focusing elements 111 and are not individually discernible by naked eye. As such, their dimension (e.g. width) should be kept sufficiently small, and is preferably smaller than 250pm, or 100pm, and ideally in the range of 12-80pm. This is so that the focusing elements required to sample these image elements are also sufficiently small, to keep the overall thickness of the security device 110 thin so it can be easily incorporated into the security document 100.

[00102] Alternatively, the thickness variation of the liquid crystal layer 113 are gradual and continuous, which means colours other than red, green, or blue can be directly produced based on birefringent properties of the liquid crystal material. Producing a coloured polarisation image in this way significantly increases the difficulty to counterfeit the security device, as colours are produced by micron or nanometer range liquid crystal structures, rather than by printed colour inks. The invention enables accurate registration of different colours in an image, which has been extremely difficult to achieve in the past with printed colours. This is because a single printing unit could only print one colour, producing an image that includes multiple colours means a number of printing units have to be used to apply the colours sequentially, which not only increases the production cost, but also increases the possibility of registration errors between different printing units.

[00103] In a preferred form, the liquid crystal elements 113a-e are formed from nematic liquid crystals, which is dissolved in a solvent and mixed in a polymer resin. The liquid crystal resin is printed over an alignment structure, and then subsequently cured to retain a polarisation pattern therein. In this embodiment, the alignment structure is provided in the alignment layer 112 in the form of a relief structure 113. In addition to aligning the liquid crystal elements 113a-e, the relief structure 113 could provide a second layer of security by incorporating, for example, a diffractive grating, which can provide a diffractive effect viewable under normal lighting conditions.

2018102067 14 Dec 2018 [00104] Figures 2b and 2c show two alternative embodiments of the alignment layer 112 and liquid crystal elements 113a-e.

[00105] Figure 2b shows an embodiment where the thickness dt of the liquid crystal layer 113 is modulated at discrete levels, wherein each thickness dt level corresponds to a colour that is to be produced by the corresponding image element.

[00106] Figure 2c shows an alternative embodiment of the invention. The alignment layer 112 is divided into two regions A and B, where region A comprises a relief structure 112b in the form of a diffraction grating, and arranged to provide a diffractive imagery effect when it is sampled by the focusing elements 111. In region B, the alignment layer 112 is configured to having a different surface relief 112a which is adapted to primarily act as an alignment structure so as to align liquid crystal elements 113 to its predetermined polarisation. In addition to its alignment function, the surface relief 112a may also be adapted to provide a diffractive imagery effect similar to that of 112b.

[00107] It can be envisaged that the security device 110 as shown in Figure 2c can provide a very robust and surprising visual effect for the security document it is incorporated in. When viewed under normal illumination conditions, i.e. visible light, regions A and B may only produce a simple diffractive effect. However, when a polarised illumination source 170 is used, the device 110 may reveal a multicolour polarisation image different from the diffractive image, thus providing authentication.

[00108] In some embodiments, the alignment layer 112 is formed by applying a curable resin and preferably a UV curable resin over the substrate 120, and then embossing a surface relief into the resin to form the alignment structures. Alternatively, the relief structures can be directly formed in or on a surface of the substrate opposite to the focusing elements 111. Figure 3a shows a side view of this embodiment. The alignment layer 112 is formed in the lower surface 140 of

2018102067 14 Dec 2018 the substrate by directly embossing, hot stamping, blind intaglio printing, laser ablating, or using any other suitable techniques. A layer of liquid crystal material 113 is then applied (e.g. printed) over the relief structure 112a to become aligned with the relief structure and forming a polarisation pattern.

[00109] Figure 3b illustrates yet another embodiment. The liquid crystal elements 113a-e, are formed as discrete microstructures and then deposited over the lower surface 140 of the substrate 120 at suitable locations.

[00110] Instead of being deposited as discrete microstructures as that shown in Figure 3b, the liquid crystal material can also be dissolved in a solvent and mixed in a curable resin, and directly applied to the substrate surface using a suitable printing technique. After that, the liquid crystal resin is embossed and cured as described previously in relation to a conventional UV curable resin. In this way, the thickness variation required in the liquid crystal layer 113 can be directly created in the embossing step, rather than relying on the underlying alignment structures.

[00111] Figure 4 shows another embodiment of the invention which exhibits two polarisation effects from two opposite surfaces of the device 110, wherein one polarisation effect is a macro polarisation pattern the other polarisation effect is a micro polarisation pattern which must be observed through the focusing elements 111. As mentioned above, it is possible to directly create a surface relief in the liquid crystal layer itself, if the liquid crystal materials are mixed in an embossable and curable resin. Using this method, the surface of the liquid crystal layer 113 that is not in direct contact with the underlying alignment layer 112 can be embossed with a surface relief and thereby forming a secondary polarisation pattern, which forms a macro polarisation pattern and does not need to be viewed through the focusing elements 111. The focusing elements 111 are arranged to focus on the liquid crystal elements aligned by the alignment layer 112, and At of Figure 4, i.e. the separation between the two polarisation patterns are sufficiently large so that the focusing elements 111 can only focus on one but not both of the polarisation patterns.

2018102067 14 Dec 2018 [00112] In one configuration, the security device and the polarisation image are designed to have brightness variations throughout. The brightness of the image can be controlled by modifying the alignment structure so it includes an alignment grating (grating region) and a region excluding grating elements (non-grating region). For each image element, the associated brightness corresponds to the ratio of the area of the grating region to the area of non-grating region. Maximum intensity for an image element corresponds to the entire alignment structure being associated with grating region, and minimum intensity for a pixel corresponds to the entire pixel being associated with non-grating region.

Manufacturing [00113] By way of example, the invention can be manufactured as follows.

[00114] First, a radiation curable resin, preferably UV curable resin, is applied to the second surface 112 of the substrate 120, by a suitable printing process. While the UV curable resin is still soft, a relief structure is embossed into the radiation curable resin by an embossing tool. The embossing tool could be an embossing roller or a shim attached to an embossing roller. The alignment layer including the relief structure can be cured as it is embossed, or shortly thereafter, to thereby retain the surface relief.

[00115] Alternatively, the surface relief may be directly formed in the surface 140 of the substrate by a suitable process. Exemplary processes include laser ablation, embossing, stamping, and similar thereof.

[00116] Secondly, a liquid crystal material of a suitable form (e.g. liquid polymer) is deposited over the alignment layer 112. The liquid crystal material may be formed by mixing a liquid crystal material in a polymer resin, allowing it to be deposited over the alignment layer 112 using a conventional printing process and printing equipment. Example printing techniques include inkjet printing, gravure printing and intaglio printing. This step should deposit a thin layer pf liquid

2018102067 14 Dec 2018 crystal material, for example, approximately 1-10 pm of wet liquid crystal polymer over the relief structure. The wet liquid crystal material conforms to the relief surface of the alignment layer 112 and becomes aligned with the orientation of the alignment structure.

[00117] In a simplest form, the liquid crystal molecules are nematic and are aligned substantially parallel to the underlying alignment structure direction.

[00118] Finally, the liquid crystal material 113 and the alignment layer 112 are fully cured, for example through heat and/or radiation, and the image layer 114 including the alignment layer 112 and the liquid crystal layer 113 is fully formed.

[00119] In some embodiments, the image elements also comprise a dichroic dye. The dichroic dye can be mixed into one of the curable resins during the ink or resin preparation phase. By preselecting a dichroic dye and adding this to the image layer, it is possible to further enhance the effect by making a non-coloured image change to the same image but multi-coloured, or to a different image and multi-coloured under polarised light.

[00120] In some embodiments, achromatic dichroic dyes may be added to the curable resins.

[00121] It should be appreciated that the security device may appear wholly transparent, if the curable resins selected to produce the device are all colourless and substantially transparent. As such, it is possible to make the polarisation image appear in what would be a seemingly wholly transparent area of the security document, i.e. window. A finishing layer, such as varnish, or HRI, could be applied to the image layer to maintain the visual appearance of the window region, i.e. to make it appear smooth and without any surface perturbations.

[00122] The focusing elements 111 are formed using similar methods as the liquid crystal elements as described above. The manufacturing process of the

2018102067 14 Dec 2018 security device can be greatly simplified if soft UV emboss is used in the creation of all the major components of the security device.

[00123] In some embodiments, a high refractive index (HRI) layer is applied onto the liquid crystal layer 113. The HRI layer can be applied to a uniform, or substantially uniform (for example, uniform apart from small variations due to the printing process), height above the liquid crystal layer surface. The HRI layer can extend past the sides of the liquid crystal layer 113, providing a protective coating for the layer. The HRI layer 30 can be either reflective or transmissive. When reflective, the liquid crystal layer 113 is only visible through the substrate 120, and therefore the security device 110 should be located within a transparent region 160 of the substrate 120 (i.e. within a window region). When transmissive, the security device 110 and the liquid crystal layer 113 can be viewable through the substrate 112 and/or directly through the HRI layer, depending on whether the security device 110 is located within a full window or half-window region. In another configuration, the HRI layer is transparent and a reflective surface is applied to the outward facing surface of the HRI layer. The HRI layer 30 can be applied using a known printing process, for example gravure printing.

[00124] In an embodiment, the HRI layer is selected to have a refractive index the same as, or close to, a refractive index of the liquid crystal layer 113. As the liquid crystal layer 113 is birefringent, the refractive index can be selected to be close to the ordinary refractive index, the extraordinary refractive index, or a refractive index between these refractive indices. For example, the refractive index of the HRI layer is the mean of the two refractive indices. The HRI layer acts to both physically and optically “smooth out” the height differences of the liquid crystal layer 113, and therefore presenting a surface with no apparent overt visual effect when viewed without polarisers.

2018102067 14 Dec 2018

Illumination source [00125] As mentioned previously, a suitable illumination source 170 is required to view the polarisation image provided by the liquid crystal layer 113. The illumination source could be a linearly polarised light source, or alternatively, an unpolarised light source could be filtered and polarised by a polariser in order to create a suitable illumination source for authenticating the security device 110.

[00126] Further modifications and improvements may be incorporated without departing from the scope of the invention.

Claims (4)

1. A security device, comprising:

a substrate including a first and a second surface, a plurality of focusing elements provided on the first surface of the substrate, an image layer provided on the second surface of the substrate, arranged to be sampled by the plurality of focusing elements to produce a multicolour image when the security device is viewed with a suitable illumination source, wherein the image layer comprises a liquid crystal layer having a thickness, the thickness of the liquid crystal layer being modulated such that the thickness at any given location is determined by a colour that is to be observed in the multicolour image, and birefringent properties of a liquid crystal material that is used to form the liquid crystal layer.

2. A security device, comprising:

a substrate including a first and a second surface, a plurality of focusing elements provided on the first surface of the substrate, a plurality of image elements provided on the second surface of the substrate, the focusing elements and the image elements being arranged

2018102067 14 Dec 2018 to produce a polarisation image when the security device is viewed with a suitable illumination source, wherein at least some of the image elements comprise a liquid crystal element having a thickness determined by a colour that is to be produced by its associated image element for the polarisation image.

3. A security document, preferably a banknote, including a document substrate including, in a region of the document substrate, a security device according to claim 1 or claim 2.

4. A method of manufacturing a security device in an in-line process, the method comprising:

providing a substrate including a first and a second surface, forming a plurality of focusing elements on the first surface of the substrate, forming an image layer on the second surface of the substrate, said image layer being arranged to be sampled by the plurality of focusing elements to produce a multicolour image when the security device is viewed with a suitable illumination source, wherein the image layer comprises a liquid crystal layer having a thickness, the thickness of the liquid crystal layer being modulated such that the thickness of the liquid crystal layer at any given location is determined by the colour that is to be observed in the multicolour image and birefringent properties of a liquid crystal material that is used to form the liquid crystal layer.