1 SECURITY DEVICE INCLUDING A DIFFRACTIVE OPTICAL ELEMENT AND A FILTER FIELD OF THE INVENTION [0001] The invention generally relates to the field of security devices for documents, for example verification devices included 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 produce images that vary with angle of view of the device, angle of illumination by an external light source, and/or nature of the external light source. Because the image on the device varies in this way, it cannot be copied by conventional photographic, computer scanning or other reprographic printing technologies. The incorporation of such optically variable device (OVD) into security documents therefore acts as a deterrent against counterfeiting of the document. [0003] At the present time, most of the OVD features used on banknotes and other security documents for this purpose are of a diffractive type. That is, the devices consist of complex patterns of finely engraved grooves which interact with the incoming light to produce images via the optical mechanism of diffraction. There are many types of OVD technologies operating in this way. These include conventional types of classical holograms, as well as more sophisticated dot matrix holograms. Special proprietary diffractive OVD technologies have also been produced over the years, including Kinegrams, Exelgrams, Alphagrams, etc. The common aspect of these proprietary diffractive technologies is that they have been developed via specialised and secret manufacturing methods to generate some unique, and tightly controlled, optical effects, which are more secure from counterfeiting because these more secure optical effects cannot be copied or 2 simulated via publicly available conventional holographic or dot matrix techniques. [0004] Nevertheless, over recent years as counterfeiting groups have become better organised and more technically competent, and the high returns from counterfeiting, in spite of the risks, have become more readily appreciated by unscrupulous groups, the attempts at simulation of genuine devices 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. SUMMARY OF THE INVENTION [0005] It would be desirable to provide a security device including a hidden diffractive security effect as well as a hidden non-diffractive security effect, which can be viewed independently or simultaneously. [0006] According to a first aspect of the present invention, there is provided a security device including a diffractive optical element (DOE) and a hidden image element, wherein the DOE is provided within a DOE region and is configured to provide a DOE visual effect, and wherein the hidden image element is provided within a hidden image region, wherein the hidden image element includes an arrangement of hidden grating elements and is configured to reveal a hidden image visual effect when viewed in-line with a verification device including an arrangement of verification grating elements, wherein the hidden image visual effect and the DOE visual effect are viewable simultaneously. The DOE visual may be further viewable separately to the hidden image visual effect and the hidden image visual effect may be further viewable separately to the DOE visual effect. [0007] Preferably, the security device includes different types of hidden grating elements including background grating elements and foreground grating 3 elements. Preferably, the background grating elements are arranged in a repeating arrangement within a background region, and wherein the foreground grating elements are arranged in a repeating arrangement within a foreground region, and wherein the foreground region and the background region are non overlapping. The foreground grating elements may be arranged to provide the hidden image when viewed in-line with the verification device. The background grating elements may be longitudinally extending and arranged in a linear repeating arrangement, and the foreground grating elements may also be longitudinally extending and arranged in a linear repeating arrangement. Alternatively, the background grating elements may be arranged in a 2 dimensional repeating arrangement, and the foreground grating elements may be arranged in a 2-dimensional repeating arrangement. [0008] Preferably, each hidden grating element has an identical width in a first direction, and there is an identical spacing between adjacent hidden grating elements of the same type in the first direction, and the position of each foreground grating element is offset in the first direction from the position of a corresponding background grating element. The offset in the first direction may be equal to the width of a hidden grating element. Preferably, the verification grating elements of the verification device each have a width equal to the width of the hidden grating elements, and the spacing between adjacent verification grating elements is equal to the spacing between adjacent hidden grating elements of the same type. [0009] When the background grating elements and the foreground grating elements are arranged in a 2-dimensional repeating arrangement, preferably each hidden grating element has an identical width in a second direction perpendicular to the first direction, and there is an identical spacing between adjacent hidden grating elements of the same type in the second direction, and the verification grating elements of the verification device each have a width in a second direction equal to the width in the second direction of the hidden grating elements, and the spacing between adjacent verification grating elements in the 4 second direction is equal to the spacing between adjacent hidden grating elements of the same type in the second direction. [0010] In an alternative, preferably each background grating element has an identical width in a first direction, and there is an identical spacing between adjacent background grating elements in the first direction, and each foreground grating element has an identical width in a first direction different to the width of the background grating elements in the first direction, and there is an identical spacing between adjacent foreground grating elements in the first direction different to the spacing between adjacent background grating elements. [0011] Preferably, the security device includes a first side and a second side, and the DOE is located on the first side and the hidden grating elements are located on the second side. In an alternative, preferably the security device includes a first side and a second side, and both the DOE and the hidden grating elements are located on the first side, and the DOE region and the hidden image region are non-overlapping. [0012] Preferably, the security device includes a substrate. The hidden grating elements may be formed from an embossed radiation curable ink applied to a surface of the substrate. The DOE may be formed from an embossed radiation curable ink applied to a surface of the substrate. [0013] According to a second aspect of the present invention, there is provided a security document including a substrate, a security device according to the first aspect located in a first region of the security document, and further including the verification device for revealing the hidden image in a second region of the security document, wherein the first region and second region of the security document are non-overlapping. [0014] Preferably, the security document includes a first opacifying layer applied to a first surface of the security document and a second opacifying layer 5 applied to a second surface of the security document. Each of the first opacifying layer and the second opacifying layer may include a window region corresponding to the verification device. Each of the first opacifying layer and the second opacifying layer may include a window region corresponding to the security device. Alternatively, the first opacifying layer may include a window region corresponding to the security device and the second opacifying layer may include a reflective region corresponding to the security device. [0015] Preferably, the security document is a banknote. [0016] According to a third aspect of the present invention, there is provided a method for producing a security device according to the first aspect, including the steps of: applying a radiation curable ink to at least one surface of a substrate; and embossing and curing the radiation curable ink to form the hidden image region and the DOE region. [0017] According to a fourth aspect of the present invention, there is provided a method for producing a security document according to the second aspect, including the steps of: applying a radiation curable ink to at least one surface of a substrate in a first region; embossing and curing the radiation curable ink in the first region to form the hidden image region and the DOE region; applying a radiation curable ink to the same or the opposite surface of the substrate in a second region; and embossing and curing the radiation curable ink in the second region to form verification device. Security Document or Token [0018] 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, 6 entrance cards and tickets, birth, death and marriage certificates, and academic transcripts. [0019] 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 [0020] 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 [0021] 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.
7 Transparent Windows and Half Windows [0022] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area. [0023] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area. [0024] A partly transparent or translucent area, hereinafter referred to as a "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. [0025] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area. Opacifying layers [0026] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT < Lo, where Lo is the amount of light incident on the document, and LT 8 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. Embossable Radiation Curable Ink [0027] The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink may be cured by other forms of radiation, such as electron beams or X-rays. [0028] Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings may be based on other compounds, eg nitro-cellulose. [0029] The ink is preferably embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a Gravure printing process. [0030] Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The 9 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. [0031] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a 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. Metallic Nanoparticle Ink [0032] As used herein, the term metallic nanoparticle ink refers to an ink having metallic particles of an average size of less than one micron. Diffractive Optical Elements (DOEs) [0033] As used herein, the term diffractive optical element refers to a numerical-type diffractive optical element (DOE). Numerical-type diffractive optical elements (DOEs) rely on the mapping of complex data that reconstruct in the far field (or reconstruction plane) a two-dimensional intensity pattern. Thus, when substantially collimated light, e.g. from a point light source or a laser, is incident upon the DOE, an interference pattern is generated that produces a projected image in the reconstruction plane that is visible when a suitable viewing surface is located in the reconstruction plane, or when the DOE is viewed in transmission at the reconstruction plane. The transformation between the two 10 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). [0034] 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. Binary DOEs and Multi-level DOEs [0035] A multi-level DOE is a diffractive optical element with a discrete number of phase levels wherein the number is an integer greater than one. A multi-level DOE with two discrete phase levels may be referred to as a binary DOE. Binary level DOEs are normally symmetrical, but with multi-level DOEs having more than two phase levels, it is possible to generate asymmetrical DOEs in addition to symmetrical DOEs. Further, it is possible to store more data, including encypted data, in a multi-level DOE, and a multi-level DOE has improved performance with regard to the projected image. Larger, brighter, higher contrasting and animated DOEs can be generated with multi-level DOEs produced by printing and embossing a radiation curable ink in accordance with the invention. BRIEF DESCRIPTION OF THE DRAWINGS [0036] 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: [0037] Figure 1 a shows a security document including a security device and a verification device; 11 [0038] Figure 1 b shows a side-on view of the security document, showing a substrate and two opacifying layers; [0039] Figure 2a shows a first arrangement of regions of the security device; [0040] Figure 2b shows a second arrangement of regions of the security device; [0041] Figure 3 shows an arrangement of grating elements of the security device; [0042] Figure 4 shows an appearance of the security device due to a diffractive optical element; [0043] Figure 5 shows an arrangement of grating elements of the verification device; [0044] Figure 6a shows a security document including an optional grating alignment direction; [0045] Figure 6b shows a security document including another optional grating alignment direction; [0046] Figure 6c shows a security document including yet another optional grating alignment direction; [0047] Figure 7a shows an appearance of the security device when viewed with the verification device in-line; [0048] Figure 7b shows another appearance of the security device when viewed with the verification device in-line; 12 [0049] Figure 8a shows a variation of the hidden grating arrangement of a security device; [0050] Figure 8b shows a variation of the verification grating element arrangement of a verification device; [0051] Figure 9a shows a variation of the hidden grating arrangement of a security device; [0052] Figure 9b shows a variation of the verification grating element arrangement of a verification device; [0053] Figure 1 0a shows an appearance of the security device in a first configuration; [0054] Figure 1 Ob shows an appearance of the security device in a second configuration; [0055] Figure 1 Oc shows an appearance of the security device in a third configuration; [0056] Figure 1 Od shows an appearance of the security device in a fourth configuration; [0057] Figure 11 a shows a variation of the appearance of the security device in a third configuration; [0058] Figure 11 b shows a variation of the appearance of the security device in a fourth configuration; and 13 [0059] Figure 12 shows a variation of the hidden grating arrangement of a security device. DESCRIPTION OF PREFERRED EMBODIMENT [0060] Referring to Figure 1 a, there is shown a top-down view of a security document 2 including a security device 4 and a verification device 6. The security document 2 may also include further security features (not shown in Figure 1), for example holograms, multi-level DOEs, Moir6 effects, etc. Figure 1 b shows a side on view of the security document 2, including a substrate 8 and opacifying layers 10, 12 applied to each side of the security document 2. The opacifying layers 10, 12 each include window regions 11 corresponding to both the security device 4 and the verification device 6. In alternative arrangements (not shown), one of the two window regions 11 corresponding to the security device 4 and/or one of the two window regions corresponding to the verification device 6 can be omitted. For example, where the security device 4 is reflective, then an opacifying layer 10 or 12 (optionally a silvered or otherwise reflective material) can be located opposite the viewing surface of the security device 4. In another example, the verification device 6 is reflective. The substrate 8 is preferably a polymer substrate (such as biaxial polypropylene). Alternatively, the substrate 8 can be a paper or paper composite substrate. [0061] Referring to Figure 2a, the security device 4 includes a DOE region 13, including a DOE 14 for providing a DOE visual effect, and a hidden image region 16 including a hidden image element 17 for providing a hidden image visual effect. For the purposes of this disclosure, a hidden image visual effect is an image that is hidden when the security device 4 is viewed directly (unaided), and visible when the security device 4 is viewed through the verification device 6 and/or when the verification device 6 is viewed through the security device 4. This arrangement of security device 4 and verification device 6 is herein referred to as viewing the security device 4 "in-line" with the verification device 6. Preferably, the 14 hidden image visual effect is revealed when the verification device 6 is in contact, or is close proximity to, the security device 4. [0062] It is appreciated that the DOE visual effect is also "hidden" under normal viewing conditions, as the DOE visual effect is revealed when the DOE 14 is illuminated with a point light source. In the example shown in Figure 2a, the DOE 14 is located within and surrounded by the hidden image region 16, and therefore the DOE region 13 and the hidden image region 16 are non overlapping. In another example as shown in Figure 2b, the DOE region 13 is not continuous, and therefore the DOE 14 is split into two parts 14a, 14b. Each part 14a, 14b is located entirely within separate areas of the hidden image region 16. For the remainder of the disclosure, reference will be made to arrangements corresponding to Figure 2a. In another configuration, the DOE region 13 is located on the opposite side of the substrate 8 to the hidden image region 16, and therefore the DOE region 13 (and correspondingly the DOE 14) can overlap the hidden image region 16. [0063] The DOE 14 includes a DOE grating including a plurality of DOE grating elements (not shown). The DOE 14 can be a multi-level DOE, for example a binary DOE. The DOE grating elements are arranged to provide a DOE visual effect when illuminated by a point light source, herein referred to as viewing the DOE 14 under correct conditions. In general, the spacing between adjacent DOE grating elements is of the order such that diffractive effects will occur. [0064] Figure 3 shows a portion of the hidden image region 16 according to an embodiment (the DOE region 13 is not shown in this figure). The hidden image region 16 includes a background region 16a and a foreground region 16b. The hidden image region 16 further includes a plurality of hidden image grating elements 18, with each hidden grating element 18 associated with and located within the background region 16a (labelled background grating element 18a) or the foreground region 16b (labelled foreground grating element 18b). Each hidden image grating element 16 is longitudinally extending and arranged parallel 15 each other hidden image grating element 16, as shown in Figure 3. The layout of the foreground region 16b defines the hidden image visual effect. It is noted that the different shading of the foreground grating elements 18b and the background grating elements 18a is simply for clarity. In general, the foreground grating elements 18b and the background grating elements 18a can be the same colour or shade, or a range of colours and/or shades. Preferably, the hidden grating elements 18 are opaque. The space between adjacent hidden grating elements 18 can be transparent, translucent, or opaque, with the requirement that the spacing is different in appearance to the hidden grating elements 18. [0065] Each hidden grating element 18 extends from one edge of the security device 4 to an opposite edge, unless interrupted by a border between the background region 16a and the foreground region 1 6b, or a border between either region 16a, 16b and the DOE region 13 (except in configurations where the DOE region 13 and the hidden image region 16 can overlap). As can be seen in Figure 3, within the background region 16a there are non-interrupted background grating elements 19a and interrupted background grating elements 19b, 19c. Where two hidden grating elements 18 would be the same hidden grating element 18 but for an interruption due to one or more borders, they can be considered the same hidden grating element 18 and are can therefore be referred to as interrupted hidden grating elements 18. For example in Figure 3, the background grating elements 18a labelled 19b and 19c can be considered the same background grating element 18a. Though Figure 3 shows each foreground grating element 18b as being interrupted by a border between the foreground region 16b and the background region 16a, this is not a requirement. [0066] The spacing 20 between adjacent hidden grating elements 18 within the same hidden image region 16 (i.e. between adjacent background grating elements 18a or between adjacent foreground grating elements 18b) is constant. Further, the width 22 of each hidden grating element 18 is preferably equal to the spacing 20 between adjacent hidden grating elements 18. Where a hidden 16 grating element 18 is adjacent a border, the hidden grating element 18 may have a different width (this situation is not shown in Figure 3). [0067] The position of the foreground grating elements 18b is offset with respect to background grating elements 18a. As shown in Figure 3, the foreground grating elements 1 8b are offset to the right an equal amount to the spacing 20 between adjacent hidden grating elements 18 (whether the foreground grating elements 1 8b are considered offset to the right or left is unimportant). It is preferable to have a shift equal to the spacing 20, however this is not an absolute requirement. The width 22 of each hidden grating element 18 can be greater than 10 microns, preferably approximately 30 microns. The spacing 20 between adjacent hidden grating elements 18 is also preferably greater than 10 microns, more preferably 30 microns. The width 22 of the hidden grating elements 18 and the spacing 20 between hidden grating elements 18 are preferably sufficient to avoid visible diffractive effects. [0068] When the security device 4 is inspected without the verification device 6 being in-line, the small spacing 20 of the hidden grating elements 18 results in a blurring effect when viewed with the unaided eye (e.g. as the spacing is below the resolving ability of the unaided eye), such that the difference in positioning of the background grating elements 18a and the foreground grating elements 18b is undetectable, or at least difficult to observe, and therefore the image defined by the foreground grating elements 1 8b is undetectable, or at least difficult to detect. [0069] Furthermore, when the DOE 14 is inspected using a correct viewing arrangement but without the verification device 6 positioned in-line with the security device 4, the DOE 14 effect is seen but the image defined by the foreground region 16b (the hidden image visual effect) is not seen. This is shown in Figure 4, where the DOE visual effect is the appearance of "50". [0070] The verification device 6 is preferably the same or approximately the same size (by area) as the security device 4. Referring to Figure 5, the 17 verification device 6 includes an arrangement of verification grating elements 24, wherein each verification grating element 24 is preferably identical to each other verification grating element 24. Each verification grating element 24 extends longitudinally from one edge to an opposite edge of the verification device 6, and is equally spaced. The spacing of the verification grating elements 24 is preferably the same as the spacing 20 of the hidden grating elements 18, and the widths of the verification grating elements 24 is preferably the same as the width 22 of the hidden grating elements 18. [0071] The verification elements 24 and the hidden grating elements 18 can extend perpendicular or parallel to the length of the security document 2, as shown in Figures 6a and 6b respectively (the alignment of the verification elements 24 and hidden grating elements 18 is indicated by the direction of the arrows 30 and 32, respectively). Alternatively, the verification elements 24 and hidden grating elements 18 can extend in other directions. The requirement is that the security device 4 and the verification device 6 can be positioned in-line such that the verification elements 24 and the hidden grating elements 18 extend in substantially parallel directions. In the example of Figure 6c, the verification grating elements 24 extend in a direction that is rotated 45 degrees with respect to the length of the security document 2, and the hidden grating elements 18 extend in a direction that is perpendicular to the direction of the verification grating elements 24. This means that when the security document 2 is folded such that the security device 4 and the verification device 6 are in-line, the verification grating elements 24 and the hidden grating elements 18 are aligned parallel. An advantage of the configuration of Figure 8c is that stacks of two or more security documents 2 are unlikely to reveal the hidden image. [0072] When the security device 4 is viewed through the verification device 6, the verification grating elements 24 can align with the background grating elements 18a, with the resulting image shown in Figure 7a, or instead with the foreground grating elements 18b, with the resulting image shown in Figure 7b. For clarity, the DOE 14 is not shown. A user will be able to shift the verification 18 device 6 with respect to the security device 4, to shift the perceived image between that shown in Figure 7a and that shown in Figure 7b. Where the verification device 6 is not aligned as per Figure 7a or 7b, an intermediate effect is viewable (not shown). [0073] Referring to Figures 8a and 8b, another embodiment is shown wherein the hidden grating elements 18 are arranged in a two-dimensional arrangement. In the example shown, each of the foreground grating elements 18b and background grating elements 18a are square in shape (see Figure 8a). Furthermore, referring to Figure 8b, each of the verification grating elements 24 of the associated verification device 6 is square in shape and arranged in an identical two-dimensional arrangement to the foreground grating elements 18a (as if the arrangement were extended over the entire surface of the verification device 6). Figure 8c shows the effect, similar to that previously described, when the verification device 6 is in-line with the security device 4 such that the verification elements 24 are aligned with the background grating elements 18a. [0074] Another embodiment is shown with reference to Figures 9a and 9b. Similar to the first described embodiment, the security device 4 and the verification device 6 includes a linear arrangement of longitudinally extending background grating elements 18a and foreground grating elements 18b. However, the spacing between adjacent hidden grating elements 18a, 18b is not constant over the extent of the security device 4 and verification device 6, and the width of each hidden grating element 18a, 18b and verification grating element 24 is not constant. Preferably, the variation in grating width and spacing repeats, as shown in the example of Figures 9a and 9b. Preferably, the spacing between two elements is equal to the width of one of the elements (for example, in Figure 9a the spacing between adjacent elements is equal to the width of the left element). Figure 9b shows the verification device 6, where the arrangement of verification grating elements 24 corresponds to the reverse of the arrangement of background elements 18b. Alternatively, when the verification device 6 is flipped in order to be positioned in-line with the security device 4, the arrangement of 19 verification grating elements 24 may be the same as the background grating elements 18a. [0075] Figures 1 0a to 1 Od are illustrative of the appearance of the security device 4 under different viewing configurations. For clarity, relative shading of the different configurations is not shown, and the visual effect of the DOE 14 is simplified. The example shown in Figures 1 0a to 1 Od includes non-overlapping DOE region 13 and hidden image region 16. The hidden image visual effect "$ AU", and the DOE visual effect is "50". [0076] Figure 1 0a corresponds to the appearance of the security device 4 when the verification device 6 is not positioned in-line, and the DOE 14 is not being correctly inspected. In this instance, there is no apparent visual effect. [0077] Figure 1 Ob corresponds to the appearance of the security device 4 again without the verification device 6 in-line, but with the DOE 14 being correctly inspected. Therefore, the appearance corresponds to the DOE visual effect, i.e. "50". [0078] Figure 1 Oc corresponds to the appearance of the security device 4 when the verification device 6 is in-line, but when the DOE 14 is not being correctly inspected. Therefore, the appearance corresponds to the hidden image visual effect, i.e. "0 K". [0079] Finally, Figure 1 Od corresponds to both the verification device 6 being in-line and the DOE 14 being correctly inspected. Therefore, the appearance corresponds to both the DOE visual effect and the hidden image visual effect, i.e. "0 50 K". The DOE visual effect may appear to be in a different plane to the security device 4 and the hidden image visual effect. [0080] Figures 11 a and 11 b illustrate the appearance of the security device 4 when the hidden image visual effect is inverted (i.e. the verification device 6 is 20 positioned such that the verification grating elements 24 overlap the foreground grating elements 18b). As can be seen, the appearance of the DOE 14 is unaffected. [0081] The security device 4 thereby provides two different visual effects, which can provide for easier and/or more effective inspection by a user for determining the validity of the security document 2. [0082] In another embodiment, one, but not both, of the foreground region 16b and the background region 16a can include hidden grating elements 18 with a width and/or spacing different to that of the verification grating elements 24. For example, as shown in Figure 12, the background region 16a includes background grating elements 18a 50% wider than the foreground grating elements 18b and with spacings 50% greater than the spacings between adjacent foreground grating elements 18b. When viewed through the verification device 6 (which has a verification grating element 24 width equal to the foreground grating element 18b width, and a verification grating element 24 spacing equal to the foreground grating element 18b spacing), the background region 16a will appear to have substantially the same brightness despite the position of the verification device 6, whereas the foreground region 16b will either appear brighter than the background region 16a, darker than the background region 16a, or an interim relative brightness, depending on the position of the verification device 6. In this variation, the foreground grating elements 18b are not necessarily offset from corresponding background grating elements 18a. [0083] The security device 4 and/or the verification device 6 can be formed integrally with the substrate 8 of the security document 2. In one embodiment, the gratings of the security device 4 and the verification device 6 are embossed gratings. In this case, the security device 4 and the verification device 6 can each include an embossable material, for example radiation curable ink. Each grating element corresponds to an embossed structure extending from the surface of the security device 4 or verification device 6.
21 [0084] Advantageously, both the DOE region 13 and the hidden image region 16 of the security device 4 are formed through embossing, and further are formed via the same embossing step. An example method for forming the security device 4 is described below. [0085] A security device master is created using known techniques, wherein the master includes a master surface profile complementary to the surface profile (arrangement of grating elements) of the security device 4. The master surface profile includes both the diffractive arrangement of grating elements corresponding to the DOE 14 and the non-diffractive arrangement of hidden grating elements 18 corresponding to the hidden image element. A verification master is also created, using the same known techniques, and includes a complementary surface profile to the verification device 6. [0086] The radiation curable ink is applied to the area of the security document 2 corresponding to the security device 4, and separately to the area of the security document 2 corresponding to the verification device 6. The radiation curable ink in each area is then either sequentially or simultaneously embossed using the respective masters, and simultaneously or immediately after embossing the radiation curable ink is cured through exposure to UV light. The opacifying layers 10, 12 can be applied before, simultaneously with, or after the embossing steps. [0087] Alternatively, the DOE region 13 and the hidden image region 16 of the security device 4 are each formed through embossing of opposite sides of the substrate 8. Embossing of each side can occur simultaneously or sequentially. In this alternative, two security device masters are created using known techniques, corresponding to the hidden image region 16 for one side of the security device 4 and to the DOE region 13 for the opposite side of the security device 4. [0088] In an embodiment, the embossed grating elements (of both the security element 4 and the verification element 6) are coloured, either in the parts 22 of the grating elements corresponding to the peaks of the grating elements, or in the regions between the grating elements, however it is preferable to only colour one of the peaks or regions between the elements. [0089] Alternatively to embossing, other techniques for forming the structures can be employed. Examples of such techniques including laser ablation or etching. The grating elements of the hidden image region and/or the verification device 6 can be printed grating elements (and therefore substantially two dimensional rather than three dimensional). [0090] Further modifications and improvements may be made without departing from the scope of the present invention.