1 DIFFRACTION GRATING Field of the Invention The invention generally relates to diffraction gratings and optically variable devices, and to security documents and tokens, and other products which may 5 incorporate such diffraction gratings and optically variable devices. 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 10 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 15 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. Substrate 20 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 25 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. Background to the Invention Optically variable devices (OVD) are used to provide authenticity to security documents, such as banknotes and the like. OVDs typically involve an 30 image or pattern formed onto a reflective surface, such that the image changes when looked at from different positions. Often, these images change in colour when viewed from different directions, and there may be different images viewable from different positions.
2 Various methods have previously been proposed for generating white or achromatic images from surface relief microstructures. A blazed low line density grating with many overlapping orders may be used to produce an image which looks whitish or achromatic. Alternatively, an achromatic image can be created 5 using a multiplicity of diffuse scattering non grating elements to generate diffusely scattered light over a controlled region in the plane of the device. Diffuse scattering features generating an achromatic or whitish image can also be produced using arrays of micrographic picture or text elements. Achromatic images generated by some of these methods are only optically variable in the 10 sense that they can produce an image which is three dimensional in appearance. It is desirable to provide new kinds of optically variable devices that have an achromatic or whitish appearance for use in security documents and other products. Summary of the Invention 15 In one aspect of the present invention, there is provided a diffraction grating, having a diffractive structure including a plurality of grating elements each having a blazed profile positioned on a surface, wherein the spacing between each grating element is varied within a predefined range, such that, at a predefined viewing angle, a maximum diffracted intensity composed of a range of 20 wavelengths is present. The diffraction grating may exhibit a substantially achromatic or whitish effect at the predefined viewing angle. The effect is preferably two-dimensional in appearance. Preferably, the spacing between adjacent grating elements is the 25 difference in the position of each element, wherein the position of each grating element is determined from an offset and a nominal position, wherein the nominal position is determined from: a) a predefined nominal wavelength; and b) the predefined viewing angle. 30 Preferably the offset for each grating element is varied. Preferably the predefined nominal wavelength is selected from within the range of wavelengths, and the nominal position of each grating element is defined as the position for the grating element required to produce a nominal diffraction 3 angle composed of the predetermined nominal wavelength at the predefined viewing angle. The nominal diffraction angle may be a positive first order diffraction angle. The nominal positions of each grating element may define a pattern with 5 regular spacing. Preferably the offset for each grating element is random. The offset for each grating element may be determined from a random function or pseudo random function including predefined maximum and minimum values, such that the combination of the nominal spacing and the maximum and minimum offset 10 values corresponds to the range of possible spacing values between adjacent grating elements. The random function may be a Gaussian random number distribution function. Alternatively the offset for each grating element may be defined by a set of known or predetermined rules. 15 Preferably the offset is substantially less than the distance between adjacent nominal positions. Preferably the grating elements are curved. Preferably the dimensions of each grating element are substantially the same. 20 Preferably each grating element includes an angled top surface, wherein the slope of the surface is equivalent to the blaze angle. Preferably the range of wavelengths corresponds to the wavelengths of the visible spectrum. Specifically, the range of wavelengths may correspond to the range of 400 to 650 nm. Alternatively, the range of wavelengths may correspond 25 to a subset of the visible wavelengths. Alternatively, the range of wavelengths may include non visible wavelengths. Alternatively, the range of wavelengths may include two or more sub-ranges of wavelengths. The achromatic or whitish effect produced by the diffraction grating is preferably visible only when viewing the diffraction grating at angles 30 corresponding to or close to the predefined viewing angle. The predefined viewing angle may be defined for light incident the diffraction grating at substantially 90 degrees to the surface of the diffraction grating. In this case, the effect would be viewable at a substantially perpendicular 4 viewing angle but would disappear as the viewing angle changes to a more inclined viewing angle, such as when the diffraction grating is tilted. Alternatively, the predefined viewing angle could be an acute viewing angle so that the effect produced by the diffraction grating is visible within a 5 narrow range of angles around the predefined acute viewing angle, but disappears when viewed at a substantially perpendicular viewing angle. In a second aspect of the present invention, there is provided a diffraction pixel including a diffraction grating according to the first aspect of the present invention. 10 Preferably the length of each side of the pixel falls substantially within the range from 30 pm to 60 pm. Preferably the orientation of the diffraction grating with respect to the diffraction pixel and the viewing angle of the diffraction grating define a viewing position of the diffraction pixel. 15 In a third aspect of the present invention, there is provided an array of pixels including a plurality of diffraction pixels according to the second aspect of the present invention. Preferably each diffraction pixel belongs to a set, such that the diffraction pixels within a set include a common viewing position. There are preferably at 20 least two sets. There may be more than two sets. Alternatively, only two sets are provided. Preferably the diffraction pixels within a set are arranged within the array of pixels to define an image. The image is preferably achromatic or whitish in appearance. The image is preferably two-dimensional in appearance. 25 Preferably the common viewing position for each set is substantially different to the common viewing position of other sets. In this way, it is possible for the array to provide at least two different images with a different image viewable at each viewing position. The array of pixels may include one or more background pixels. Each 30 position within the array of pixels that is absent a diffraction pixel may contain a background pixel.
5 Preferably the background pixels are diffusely scattering pixels with a scattering intensity substantially similar to the diffraction pixel intensities when the diffraction pixels are viewed away from their viewing positions. In a fourth aspect of the present invention there is provided an optically 5 variable device including an array of pixels according to the third aspect of the present invention. Preferably the array of pixels is provided on or in the surface of a substrate. The substrate may be one of the following: paper or other fibrous material such as cellulose; a plastic or polymeric material such as polypropylene 10 (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. Each pixel may be formed in the surface of the substrate, or in a layer on 15 the surface of the substrate. Alternatively each pixel may be individual created and then attached to the substrate. The array of pixels may formed by an embossing process. In a preferred method the array of pixels is formed by embossing into a layer of UV curable ink or lacquer applied to the surface of the substrate using an embossing shim 20 bearing the pattern of the array of diffraction pixels and the embossed ink or lacquer layer is cured by UV radiation at substantially the same time or shortly after the embossing step. In a fifth aspect of the present invention there is provided a security document including an optically variable device according to the fourth aspect of 25 the present invention. Preferably the security document is one of the following: an item of currency such as a banknote or coin, a credit card, a cheque, a passport, an identity card, a security or share certificate, a driver's licence, a deed of title, a travel document, such as an airline or train ticket, an entrance card or ticket, a 30 birth, death or marriage certificate, and an academic transcript. Description of the Drawings Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be appreciated that the embodiments are given 6 by way of illustration only and the invention is not limited by this illustration. In the drawings: Figure 1 shows diffraction from a diffraction grating according to the prior art; 5 Figure 2 shows diffraction from a blazed diffraction grating according to the prior art; Figure 3 shows a diffraction grating according to one aspect of the present invention; Figure 4 shows a blazed diffraction grating according to one embodiment; 10 Figure 5 shows diffraction from a blazed diffraction grating according to one embodiment; Figure 6 shows a diffraction pixel according to one aspect of the present invention; Figure 7 shows an array of similar diffraction pixels according to one 15 aspect of the present invention; Figure 8 shows arrays of two types of diffraction pixels interspersed, according to one aspect of the present invention; Figure 9 shows arrays of two types of diffraction pixels and interspersed with background pixels, according to one aspect of the present invention; 20 Figure 10 shows an array of pixels formed from individual pixel components, according to one aspect of the present invention; Figure 11 shows an array of pixels formed on a single base, according to one aspect of the present invention; Figure 12 shows a security document including an optically variable device 25 formed from the array of diffraction pixels of Figure 7 viewed away form the viewing position; Figure 13 shows the security document of Figure 12 when viewed at a viewing angle substantially corresponding to the viewing position; Figure 14 shows a security document including an optically variable device 30 formed from the array of diffraction pixels of Figure 8 at a first viewing angle; Figure 15 shows a security document including an optically variable device formed from the array of diffraction pixels of Figure 8 at a second viewing angle; and 7 Figure 16 shows a security document including an optically variable device formed from the array of diffraction pixels of Figure 8 at a third viewing angle. Referring to figure 1, it is known in the art that incident light 1, incident to the surface of a diffraction grating 2 having a square or sinusoidal groove profile, 5 will be diffracted at various angles (where each diffraction angle is defined as the angle from the normal 4 to the surface 5), defined as first order 3 and higher order (not shown) angles. For incident light 1 incident at an angle perpendicular to the surface of the diffraction grating 2, the diffraction occurs in equal amounts in the positive and negative directions (i.e. ON = OP). In general, a diffraction grating is 10 defined by a collection of grating elements that are arranged in a spatially repeating grating. Furthermore, referring to figure 2, blazing the diffraction grating 2 so that individual grating elements 6 have a surface which extends at an inclined angle to the support surface for the diffraction grating, the blaze angle, maximises the 15 diffraction intensity at the chosen blaze angle, and reduces the diffraction intensity at all other angles. Typically, the blaze angle is chosen to be equal to a first order diffraction angle 7 of a chosen wavelength. For example, it may be desired to maximise the intensity at the positive first order diffraction peak of green light. The blaze angle will therefore be selected in combination with other 20 parameters, such as grating spacing, so that the diffraction angle 7 corresponding to the green light (or a wavelength equivalent to green light) is equal to the blaze angle. It is important to note that the effect of blazing is to maximise the first order diffraction peak only in one direction. The equivalent first order peak at the negative angle 9 is reduced or removed when compared to a normal grating. 25 Referring to figure 3, in one embodiment, a diffraction grating 10 is shown and is made up of a number of parallel grating elements 11 forming a diffractive structure, where the spacing 12 between each grating element 11 is varied within a predefined range when compared to a nominal spacing 13 of the diffraction grating. Each grating element 11 is substantially identical, and may be a groove, 30 a projection or any other suitable structure. The nominal spacing 13 corresponds to the spacing between grating elements associated with a grating including regular spacing between the grating elements. By providing a varied spacing 12, light is diffracted at slightly different angles by each grating element 11, causing 8 diffracted light from different grooves to mix. As such, achromatic light is viewable, achromatic being defined as light which is a product of more than one wavelength. Other possible diffraction gratings include circular grooves, gratings consisting of vertical and horizontal grooves etc. Further, the diffraction grating 5 profile may be curved, or any other suitable grating profile, such that there includes an array of grating elements 11. The grating elements 11 may correspond to peaks, which may be smooth or pointed. Referring to figure 4, the diffraction grating (10 of figure 3) may be a blazed diffraction grating 10, wherein the grating elements 11 define an approximately 10 saw-tooth profile. Each grating element 11 corresponds to a raised peak 14, which includes a downwards slope 15. The length of the downwards slope 15 for each grating element 11 is the same. Referring to figure 5, the nominal spacing is selected based on a desired viewing angle 16 of the grating 10. The viewing angle 16 is defined as the angle 15 16 from the normal 19 of the plane defined by the diffraction grating 10, at which an intensity maxima 18 is desired, when the incident light is incident normal to the surface of the grating 10. To determine the nominal spacing, a nominal wavelength 20 must be determined. In general, the nominal wavelength 20 may be selected from any wavelength within the range of wavelengths present. In one 20 embodiment, the nominal wavelength 20 is selected as the middle wavelength within the range of wavelengths. Referring back to figures 3 and 4, the position of each grating element 11 of the diffraction grating 10 is offset from the nominal position of the grating element. The nominal position of each grating element may be defined by a 25 regular spacing of the grating elements with a nominal spacing distance 13. The position of each peak is then defined as the nominal position with an offset. The offset for each grating element may be random or defined by a set of known or predetermined rules giving desired wavelength mixing properties. The spacing 12 between adjacent grating elements is the difference between the positions of 30 each element, and is therefore varied from the nominal spacing 13. For a random offset, referring back to figure 3 the position of each peak 12 is calculated using the following equation. x, = md+ a# 9 xm is defined as the location of the peak 13 numbered m, where the location is defined as the distance from the left side 31 of the blazed diffraction grating 10. d is the nominal spacing distance 13. a is a parameter equal to the preselected largest value that the offset is allowed to take. P is a random number, 5 which may be within the range -1 P 1 or any other appropriate range. It is noted that the range may be discontinuous. In one embodiment, the random number generator follows a Gaussian distribution within the specified range. d may be calculated using the well known diffraction equation (for a positive first order peak): 10 d = sin(O) e is the nominal viewing angle. Referring back to figure 5, the viewing angle 16 may be selected to be equal to the nominal wavelength 20 first order diffraction angle 21. In one embodiment, the nominal diffraction angle is also equal to a blaze angle 17. This ensures that the viewing angle 16 corresponds to 15 the angle of maximum intensity. However, unlike normal blazed diffraction gratings, there are a range of wavelengths 14 present when the diffraction grating 10 is viewed at the blaze angle 17. a is selected to be less than d, and is preferably selected to provide an appropriate compromise between providing a range of wavelengths and providing a sufficient diffraction effect. 20 Referring to figure 6, a diffraction pixel 22 is provided with a surface 23 incorporating a diffraction grating 10. In general, the orientation of the grating on the pixel and the nominal spacing of the grating defines the viewing position of the diffraction pixel 22.. The viewing position may be defined by an azimuthal angle and an inclination angle, where the inclination angle is the complement to 25 the viewing angle. In one embodiment the azimuthal angle for each pixel is the same, and therefore the viewing position is defined by the viewing angle of the diffraction pixels. The diffraction grating 10 may encompass the entire surface 23 or a portion of the surface 23. In one embodiment, the length of each side 24, 25 of 30 the diffraction pixel is within the range of 30 to 60 pm. In a further embodiment, the diffraction pixel is configured to operate in the range of visible wavelengths, such that the full spectrum of visible wavelengths are present at the viewing 10 position. The visible range of wavelengths corresponds to approximately 400 650 nm. In another embodiment, a subset of visible wavelengths is selected. In another embodiment, the diffraction pixel is configured to operate within a range of wavelengths including non visible wavelengths, for example within the infra-red 5 or ultra-violet parts of the electromagnetic spectrum. The range of wavelengths may be discontinuous, such that the range may be composed of two or more sub ranges. Figures 7, 8 and 9 each show an array 26 of pixels is provided, wherein the array 26 includes at least one set of diffraction pixels 27. In figures 8 and 9 10 there are two sets of diffraction pixels 27, 28 and each array 26 may also include background pixels 29 as shown in figure 9. Referring to figure 7, a set of diffraction pixels 30 is defined by each diffraction pixel 27 within the set having a common orientation, nominal spacing and viewing angle. In the present figure, the diffraction pixels 27 within the same 15 set of diffraction pixels 30 are labelled 'A'. The arrangement of these diffraction pixels 27 defines an image. In the present example, the image is of a large 'A'. The image will be at maximum visibility when viewed from a viewing position corresponding to the viewing angle of the pixel and the orientation of the diffraction grating on the pixel. At other viewing positions, the image will be less 20 visible comparatively. Figures 12 and 13 show a security document 100 including an optically variable device OVD 102 formed from an array of diffraction pixels 104 as described with reference to Figure 7. Figure 12 shows the security document when viewed at a position away from the viewing position defined by the viewing 25 angle and the orientation of the set of diffraction pixels, showing that the image generated by the OVD 102 is not visible or barely discernible. Figure 13 shows the security document when viewed from a position corresponding to the viewing angle and the orientation, or within a range of angles a few degrees on each side of the viewing angle. In this case, a two-dimensional achromatic or whitish image 30 in the shape of a letter "A" generated by the diffraction pixels 104 is highly visible (in figure 13, the 'A' image is shown as being darker than the background, though usually the image will appear as lighter, or brighter, than the background).
11 Thus the OVD 102 of figures 12 and 13 provides a type of "switching" image which is achromatic or whitish in appearance that is highly visible at some angles, and barely discernible at other angles as the security document is tilted to change the viewing angle and viewing position. Owing to the diffractive 5 construction of the OVD and the achromatic or whitish image produced, the OVD is highly recognisable as an overt security element for authentication of the security document and is difficult to counterfeit. It is possible to have more than one type of diffraction pixel present in the array of pixels, each type having a substantially different viewing position such 10 that when the viewing position corresponds to pixels of one type, the other pixels are not visible or are visible with a reduced visibility comparatively. Referring to figure 8, an example using two types of diffraction pixel 27, 28 within the array of pixels 26 is shown. Each type of pixel 27, 28 is defined by the set of diffraction pixels 30, 31 to which it belongs, and by the corresponding 15 nominal spacing and viewing position. In the example shown in figure 8, the viewing angle of the 'A' pixels 27 is 30 degrees and the viewing angle of the 'B' pixels 28 is 50 degrees, and the orientation for each type of pixel is the same. When viewing the surface at an angle of 30 degrees the 'A' pixels 27 will be significantly brighter than the 'B' pixels 28, and the array of pixels will display an 20 image 'A'. When viewing the surface at an angle of 50 degrees, the 'B' pixels 28 will be significantly brighter than the 'A' pixels 27, and the array of pixels will display an image 'B'. Figures 14 to 16 show a security document 120 including an optically variable device OVD 122 formed from an array of diffraction pixels 27, 28 as 25 described with reference to Figure 8. Figure 14 shows the security document when viewed at a position away from the viewing position defined by the viewing angle and the orientation of either the 'A' set or the 'B' set of diffraction pixels, showing that neither image generated by the OVD 102 is visible or easily discernible. Figure 15 shows the 30 security document when viewed from a position corresponding to the viewing angle and the orientation, or within a range of angles a few degrees on each side of the viewing angle, of the 'A' set of pixels. In this case, a two-dimensional achromatic or whitish image in the shape of a letter "A" generated by the 12 diffraction pixels is highly visible. Figure 16 shows the security document when viewed from a position corresponding to the viewing angle and the orientation, or within a range of angles a few degrees on each side of the viewing angle, of the 'B' set of pixels. In this case, a two-dimensional achromatic or whitish image in 5 the shape of a letter 'B' generated by the diffraction pixels is highly visible. The OVD in the security document of figures 14 to 16 therefore provides a another type of switching image in which a first image 'A' is highly visible at a first viewing angle, a second image 'B' is highly visible at a second viewing angle, and at angles which are not within a few degrees of the first and second viewing 10 angles, no image is clearly discernible. Referring to figure 9, background pixels 29 may be incorporated into the array of pixels 26. Background pixels 29 are non-diffraction pixels located on parts of the surface that are absent a diffraction pixel 27, 28. The background pixels 29 are configured to be diffusely reflective, such that the reflectance of the 15 pixels when viewed at different angles is approximately the same. The background pixels 29 may be configured to be approximately as bright as the diffraction pixels 27, 28, when the diffraction pixels 27, 28 are viewed away from their associated viewing position. The background pixels 29 therefore assist in hiding the images defined by the diffraction pixels 27, 28 when the array is viewed 20 from viewing positions different to the viewing positions of the diffraction pixels 27, 28, so that the image switching effects described with reference to Figures 12 to 16 are more pronounced. An optically variable device (OVD) including an array of diffraction pixels in accordance with the invention may be provided on a substrate which is attached 25 to the surface of a security document of other article, such as an article of packaging requiring security protection. pixels defined on the surface of the substrate. Referring to figure 10, the OVD may be made up of individual pixel components, where each pixel 37 is made of a separate substrate 34 (see figure 100. Alternatively, the OVD may be made up of multiple pixels 35 on a common 30 substrate 36 (see figure 11). In either case, the substrate 34 or 36 may be any suitable material for producing a diffraction grating where the properties for each diffraction element may be individually controlled. Example of suitable base materials include polymer substrates, such as polyethylene (PE), polycarbonate 13 (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polypropylene (PP), and biaxially oriented polypropylene (BOPP which has been successfully used in the manufacture of polymer banknotes Referring to both figures 10 and 11, diffraction gratings on adjacent diffraction pixels may be directly adjacent/in 5 contact, or there may be a gap 38 between individual gratings. There are many techniques for creating suitable diffraction gratings on the base substrate. These include various origination techniques using laser beams or electron beams for manufacturing a master grating, and then using the master grating to stamp or emboss metallic foils or other substrates with the required 10 diffractive structure. The master diffraction grating may be designed using computer software. Persons skilled in the art will appreciate that various modifications of the above embodiments are possible without departing from the scope of the invention as defined by the claims. 15