1 A POLARISING DEVICE AND METHOD FOR PRODUCING A POLARISING DEVICE FIELD OF THE INVENTION The invention generally relates to polarising elements for security devices, 5 the polarising element including a liquid crystal based polariser. BACKGROUND It is well known that many of the world's banknotes, as well as other security documents, carry optical security elements which produce images that vary depending on whether the optical security elements are viewed through a 10 decoding screen or not. The incorporation of such optical security elements into security documents therefore acts as a deterrent against counterfeiting of the document. Nevertheless, over recent years as counterfeiting groups have become better organised and more technically competent, and the high returns from 15 counterfeiting, in spite of the risks, have become more readily appreciated by unscrupulous groups, the attempts at simulation of genuine elements have become more and more successful. This problem is exacerbated by the fact that the authentication process for the banknote by members of the public has long been recognised as the weakest point in the security system. Not only do 20 members of the public spend very little, if any, time authenticating their banknotes, which makes it easier for simulations to pass through. Furthermore, a common feature of optical security elements is that the images created are viewable by directly viewing the optical security element. Therefore, although it is difficult to reproduce the optical effect of the optical security element, it is easily 25 possible to create a passable forgery that may appear similar to the optical effect. The use of polarises and liquid crystals to produce security documents including a revealable hidden image has been described in WO 2011/017749. In this document, liquid crystals are used to create a polarising security feature that is then revealed by use of a standard polariser. However, such polarisers can be 30 relatively expensive and difficult to implement on a security document.
2 Security Document or Token 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 5 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 10 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 elements described herein may also have application in other products, such as packaging. Substrate 15 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 20 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. 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 25 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 30 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 3 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. 5 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 10 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. 15 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 < 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 20 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 25 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. 30 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 4 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; 5 optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs). SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a method for manufacturing a polarising element for a security document, including the steps 10 of: a) providing a transparent substrate; b) embossing an arrangement of grooves into a first surface of the substrate, the grooves having a groove spacing; c) applying a liquid crystal material to the first surface, such that the liquid 15 crystal material covers at least a portion of the arrangement of grooves; and d) curing the liquid crystal material, wherein the groove spacing of the arrangement of grooves is such that the liquid crystal molecules within the liquid crystal material substantially align such that the liquid crystal layer polarises transmitted light, wherein the polarising element is 20 configured to polarise one or more specified range of wavelengths, and wherein the liquid crystal material includes one or more dichroic dyes, such that each range of wavelengths is associated with one or more of the dichroic dyes. Preferably, the grooves are arranged in a linear repeating pattern. Preferably, the spacing between adjacent grooves is between 100 and 300 25 nm. The liquid crystal material may be cured using UV light. The one or more specified range of wavelengths may be confined to infra-red wavelengths. As an alternative, the one or more specified range of wavelengths may be confined to visible wavelengths. As a further alternative, the one or more specified 30 range of wavelengths is confined to a subset of the visible wavelengths. As a further alternative, the one or more specified range of wavelengths is confined to ultra-violet wavelengths. Preferably, the liquid crystal material includes a first dichroic dye. Each dichroic dye particle of the first dichroic dye may be attached to a liquid crystal 5 molecule. Alternatively, the first dichroic dye may be mixed into the liquid crystal material. Preferably, the liquid crystal material includes a second dichroic dye configured to operate over a different range of wavelengths to the first dichroic dye. 5 The first dichroic dye and the second dichroic dye may be selected such that they do not interfere with each other. Alternatively, the first dichroic dye and the second dichroic dye may be selected such that they do interfere with each other. Preferably, the polarising element is transparent or translucent. Preferably, the liquid crystal material includes a solvent. The solvent may be 10 removed before or during curing of the liquid crystal material. According to a second aspect of the present invention, there is provided a polarising element including a transparent substrate, an arrangement of grooves on a surface of the substrate, at least a portion of the arrangement of grooves covered by a liquid crystal material, and wherein the arrangement of grooves having a groove 15 spacing such that the liquid crystal molecules within the liquid crystal material are substantially aligned such that the liquid crystal layer polarises transmitted light, wherein the polarising element is configured to polarise one or more specified range of wavelengths, and wherein the liquid crystal material includes one or more dichroic dyes, such that each range of wavelengths is associated with one or more of the 20 dichroic dyes. Preferably, the grooves are arranged in a linear repeating pattern. The spacing between adjacent grooves may be between 100 and 300 nm. Preferably, the liquid crystal material is cured using UV light. The one or more specified range of wavelengths may be confined to infra-red 25 wavelengths. As an alternative, the one or more specified range of wavelengths may be confined to visible wavelengths. As a further alternative, the one or more specified range of wavelengths may be confined to a subset of the visible wavelengths. As a further alternative, the one or more specified range of wavelengths may be confined to ultra-violet wavelengths. 30 Preferably the liquid crystal material includes a first dichroic dye. Preferably, each dichroic dye particle of the first dichroic dye is attached to a liquid crystal molecule. Alternatively, the first dichroic dye is mixed into the liquid crystal material.
6 Preferably, the liquid crystal material includes a second dichroic dye configured to operate over a different range of wavelengths to the first dichroic dye. The first dichroic dye and the second dichroic dye may be selected such that they do not interfere with each other. 5 Alternatively, the first dichroic dye and the second dichroic dye may be selected such that they do interfere with each other. Preferably, the polarising element is transparent or translucent. According to a third aspect of the present invention, there is provided a security document including: a polarising element manufactured according to the 10 first aspect of the present invention; or a polarising element according to the second aspect of the present invention. Preferably, the security document further includes a security element. DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described with reference to the 15 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: Figure 1 shows a security document including a polarising element according to an embodiment; 20 Figure 2 shows a polarising element according to an embodiment; Figure 3 shows a polarising element including a liquid crystal material containing liquid crystal molecules; Figure 4a shows an arrangement for embossing a substrate; Figure 4b shows an embossing step for embossing a substrate; 25 Figure 4c shows an embossed substrate including an arrangement of grooves; Figure 5 shows an embossed substrate including an arrangement of grooves filled with liquid crystal material; Figure 6 shows curing liquid crystal material using UV light; 30 Figure 7a shows a liquid crystal material including dichroic dye particles attached to liquid crystal molecules; Figure 7b shows dichroic dye articles dispersed within a liquid crystal material; and 7 Figure 8 shows a groove containing liquid crystal material including two different dichroic dyes. The following description describes a polarising element which is relatively inexpensive and easy to implement onto a security document and, especially, a 5 polymer substrate based security document. A polarising element of this type has a number of advantages over prior art polarisers. In particular, polarisers are challenging to manufacture on paper substrates due to high cost and profile, or thickness constraints. Prior art polarisers are not pliable and relatively thick, which can impact implementation into a security document, such as a banknote. For 10 example, thick areas will, potentially cause problems in the feeding of substrates through processing and printing machines, and durability is an issue with non pliable substrates. The disadvantages in prior art polariser manufacturing technology involve the use of a stretched polymer and lamination steps for metallized coatings. 15 Metalized coatings are not robust physically or chemically to enable use in banknote applications. Other polarizers made of nanowires and newer technology which may or may not involve metallization, are expensive and complex to re produce. Robustness is not guaranteed. Ultimately, the main impediment is integration into the substrate, the extra thickness of the conventional polarizer 20 (thickness required due to the nature of the technology, and in order to impart robustness), makes it unsuitable for banknote applications. In reference to figure 1, a security document 100 is provided which includes a polarising element 102 and an optional secondary security element 104. The polarising element 102 is preferably transparent or at least translucent, 25 and can be located with a window region of the security document 100. If present, the secondary security element 104 may be transparent, translucent or opaque and may be located within a window or half-window region of the security document. Referring to figure 2, the polarising element 102 includes a substrate 204, 30 an arrangement of grooves 200 located within the substrate 204, and a layer of liquid crystal material 202 applied to the substrate. According to the specific embodiment illustrated in figure 2, the grooves 200 are arranged in a linear repeating pattern, known as a grating, such that each groove has an identical or 8 substantially identical groove width 206. The grooves 200 shown include a square-like cross-section; however any appropriate cross-section suitable for aligning liquid crystals may be used. The groove spacing 208 is measured from the same point on adjacent grooves 200 and is a parameter of the pattern. In an 5 embodiment, the groove spacing 208, or frequency, is preferably between 250nm and 1pm, and more preferably between 100nm and 300nm. Preferably the groove spacing 208 is selected such that the grooves do not produce diffraction effects at the observed frequencies. Referring to figure 3, the liquid crystal material 202 is shown in a cured 10 state, such that the liquid crystal molecules 300 within the liquid crystal material 202 are fixed in place, with the liquid crystal molecules 300 aligning substantially with the length of each groove 200. The liquid crystal material may include a binder or other supplement to provide for fixing the liquid crystal molecules 300 in place. 15 Referring now to figures 4a and 4b, a method for manufacturing a polarising element 400 includes providing a substrate 402 including a polarising element region 404. An embossed arrangement of grooves is created in the polarising element region 404 using an embossing device 406. The embossing device 406 includes an arrangement of embossing elements 408 that is the 20 inverse of the arrangement of grooves 200, located on an embossing surface 410. The embossing step includes pressing the embossing surface 410 into the polarising element region 404, followed by removal of the embossing surface 410. The substrate 402 can be heated before embossing, and further can be cooled after embossing. The substrate 402 can include an embossing layer including an 25 embossable material that is different to the substrate 402. The result is a substrate 402 including an arrangement of grooves 412 located in a polarising region 404 as shown in figure 4c. In reference to figure 5, liquid crystal material 500 is applied to the polarising region 404 such that the liquid crystal material 500 covers the grooves 30 (not shown). The liquid crystal material 500 is preferably in a liquid crystalline state, such that the liquid crystal material 500 as able to flow and cover each of the grooves such that the liquid crystal molecules 300 within the liquid crystal material 202 align substantially with the length of the grooves 412. The liquid 9 crystal material 500 may include a solvent for assisting in applying the liquid crystal molecules 300 to the grooves 412. The liquid crystal material 500 can be applied to the grooves using a printing technique, for example flexographic printing or gravure printing. Preferably, the technique used to apply the liquid 5 crystal material 500 has minimal impact on the alignment of the liquid crystal molecules 300, leaving the liquid crystal molecules 300 free to align with the grooves 412. In reference to figure 6, after application of the liquid crystal material 500 to the each of the grooves 412, the liquid crystal material 500 is cured. In one 10 embodiment, curing is achieved using UV light from a UV light source 600, which is directed onto the surface 404 of the polarising element 400. After an appropriate length of time, the liquid crystal material 500 is cured such that the liquid crystal molecules 300 are restrained from changing orientation. The result is that light transmitted through the liquid crystal material 500 is polarised by the 15 liquid crystal molecule alignment. When a solvent is used to assist with application of the liquid crystal molecules 300 to the grooves 412, preferably the curing step includes removal of the solvent either before or during the curing of the liquid crystal material 500. In an embodiment as shown in figures 7a and 7b, a dichroic dye is present 20 within each groove 412. The dichroic dye can correspond to dichroic dye particles 800 attached to the liquid crystals molecules 300 (see figure 7a), or alternatively the dichroic dye particles 800 can be dispersed within the liquid crystal material 500 (see figure 7b). The effect of the dichroic dye is to limit the polarising effect of the aligned liquid crystal molecules 300 to specified wavelengths by removing 25 other wavelengths from the incident light. Example wavelength ranges include infra-red light, visible light, and ultra-violet light. In an embodiment as shown in figure 8, the liquid crystal material 500 includes two or more different dichroic dyes 900, 902 (shown dispersed within the liquid crystal material). The different dichroic dyes 900, 902 are selected to 30 operate over different ranges of wavelengths, such that the polarising element 102 is able to selectively polarise wavelengths located within the two or more different ranges of wavelengths. In this embodiment, the secondary security element 104 can include a separate security feature corresponding to each range 10 of wavelengths. For example, the polarising element 102 includes a first dichroic dye 900 operating over a range of infra-red wavelengths and a second dichroic dye 902 operating over a range of visible wavelengths. In this example, the secondary security device 104 can include a machine readable security feature 5 operating over the infra-red wavelengths, and a human readable security feature operating over the visible wavelengths, where each security feature corresponds to an unrelated image. Further modifications and improvements may be made without departing from the scope of the present invention. For example, the arrangement of 10 grooves may include grooves following a curved path. As a further example, the cross-sectional shape of the grooves may take on a variety of forms, as long as the grooves are able to function to assist alignment of the liquid crystal molecules.