AU2011101251A4 - Optically variable device - Google Patents

Abstract

5 An optically variable device, including a plurality of surface relief image elements, each including a surface profile, located on or in a substrate and configured to reflect and/or refract incident light as to form part or all of an image, such that one or more of the surface relief image elements produces a reflected or refracted wavefront which is structurally stable with respect to small perturbations or 10 distortions of that part of the surface of the device incorporating the image element. 100 102 115 108 Figure Ia

Description

1 OPTICALLY VARIABLE DEVICE FIELD OF THE INVENTION The invention relates generally to optically variable devices operating in reflection or refraction mode, and security documents incorporating such optically 5 variable devices. BACKGROUND 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 or angle of illumination by an external light source. 10 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. 15 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 20 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 25 some unique, and tightly controlled, optical effects, which are more secure from counterfeiting because these more secure optical effects cannot be copied or simulated via publicly available conventional holographic or dot matrix techniques. Nevertheless, over recent years as counterfeiting groups have become 30 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 2 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 members of the public spend very little, if any, time authenticating their banknotes, which makes it easier for simulations to pass through, but the banknote circulation 5 process itself causes difficulties through crinkling degradation of the banknote surface and its corresponding OVD images, which again makes image recognition more difficult, and therefore enhancing the prospects for counterfeit or simulated OVD images to be accepted. Various types of diffractive optically variable devices have been 10 constructed around diffraction catastrophe and structural stability principles. Diffraction catastrophes are the interfering wave patterns associated with structurally stable caustics. A "caustic" in the optics sense is a general term used to describe the focussing properties of families of light rays. Familiar examples of caustics are 15 the bright point focus formed when a parallel beam of light passes through a lens, the cusp shaped line pattern formed by a point source of light on the surface of a cup of coffee and the bright complex wavy line pattern formed on the bottom of a swimming pool in direct sunlight. Certain types of caustics which possess a property known as structural stability can be classified into a hierarchy of well 20 defined topological shapes. "Structural stability" is a term used to describe the behaviour of caustics undergoing smooth perturbations. A caustic is said to be structurally stable if it reacts to perturbations in such a way as to retain its basic structural form. For example, the point focus of a lens is structurally unstable because the slightest 25 perturbation in the positioning or shape of the lens will destroy the point focus that is, change the focus from being that of a point to being some smeared out two dimensional figure. The inherent structurally unstable property of a point or line focus is the reason that the optical engineering of lens making is such a difficult art. Other examples of structurally unstable caustics include the reflection 30 patterns generated by initially flat reflecting surfaces which have become slightly crumpled or distorted. In general, for a structurally unstable caustic, the slightest distortion of the surface can change the topological form of the caustics markedly.

3 An example of a structurally stable caustic can be observed on the surface of a cup of coffee. Perturbations in the positioning of the light source, the volume of coffee in the cup, or the angle at which the cup is held, do not change the topology of the caustic. The cusp shape may become larger or smaller, or one 5 side may become longer than the other, but it still remains a cusp. The classification of structurally stable caustics into a hierarchy of topologically different forms was made possible by a theorem published in 1972 by the French mathematician Rene Thom and later applied and expanded in his book "Structural stability and morphogenesis" (R. Thom, 1975, Benjamin). 10 Thom's theorem is a piece of pure mathematics which deals with the classification of structurally stable singularities generated by gradient mappings. For surface relief structures of slightly rolling hills and valleys, a gradient map is best visualised by drawing perpendicular lines at every point on this undulating surface, each line being perpendicular to the local gradient of the surface at a 15 particular point, and then allowing this network of lines to intersect a plane placed some distance away from the undulating surface and roughly parallel to it. The gradient map of the surface then corresponds to the set of intersections of the endpoint of the lines with the observation plane. Because the initial surface is gently undulating, it can be seen that the network of local perpendiculars will 20 contain regions where two or more perpendicular lines intersect each other. If the initial surface is smoothly undulating then the points of intersection of the local perpendiculars will in general trace out lines of intersection in the space above the initial surface. These lines of intersection of the local perpendiculars are singular lines of the gradient map. Thom's theorem showed that gradient maps of 25 this type can generate only a finite number of structurally stable singularities, or caustics in the case of optics. The actual number of structurally stable types depends on the dimensionality of the control space above the surface which generates the gradient map. Thom showed that in a control space of four dimensions there can only be seven different types of structurally stable 30 singularities. These are known as fold, cusp, swallowtail, butterfly, elliptic umbilic, parabolic umbilic, and hyperbolic umbilic caustics. These singularities were also called catastrophes by Thom because they represent sudden changes in the observed behaviour of the system as a 4 singularity is crossed. For example, in the case of the diver in a swimming pool, if he or she were to look at the sun from the bottom of the pool, then as his or her eyes cross a caustic the image of the sun would change from a single image to a double image or vice versa. This sudden change in the number of observed 5 images when the eye crosses a caustic shows that the word "catastrophe" is a particularly appropriate description of phenomena involving caustics. A similar mechanism is responsible for the sparkling of sunlight on the surface of the sea. In this case also the sparkling light patterns correspond to multiple images of the sun. 10 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 cards, cheques, passports, identity cards, securities and share certificates, 15 driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts. The invention is particularly, but not exclusively, applicable to security documents or tokens such as banknotes or identification documents such as 20 identity cards or passports formed from a substrate to which one or more layers of printing are applied. The optically variable devices described herein may also have application in other products, such as packaging. SUBSTRATE As used herein, the term substrate refers to the base material from which 25 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 30 plastic material, or of two or more polymeric materials. SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided an optically variable device, including a plurality of surface relief image elements, 5 each including a surface profile, located on or in a substrate and configured to reflect and/or refract incident light as to form part or all of an image, such that one or more of the surface relief image elements produces a reflected or refracted wavefront which is structurally stable with respect to small perturbations or 5 distortions of that part of the surface of the device incorporating the image element. Preferably, the height of each surface relief image element at any point (x,y), thereby determining the surface relief elements surface profile, is described by a function f(xy) which has the property that its Hessian of second derivatives 10 H(xy) does not vanish identically across the surface of the element and wherein this condition is expressed mathematically by the expression; H(x,y) = 2 2 2 2 #0, except at particular points (x,y), or line regions (xj,yj) - (x 2 ,y 2 ) of non-zero length, and where the gradient mapping of such Hessian vanishing regions produces corresponding singular caustic regions 15 in the gradient mapping space (u,v) of the function f(x,y), wherein the coordinates (u,v) of the mapping space are related to the coordinates (xy) of function space according to the gradient mapping equations; u(x,y) = R,(xy)+ c ax v(x,Y) = R2(x,Y)+ c2, where Rj(xy) and R2(x,y) are predetermined functions of (xy) and cj and c2 are 20 predetermined constants independent of (xiy), and wherein the functions u(xy) and v(xy) describe the direction of optical beams reflected from or refracted through the surfacefx,y) to the eye of an observer. Preferably, the surface profile of each surface relief element is substantially the same. The surface profile of each surface relief element may be 25 defined by a height function f(xy), and wherein the height function is: f(x,y)= A cos( )cos( ), 4 4 where A is a coefficient which determines the size of the caustic regions. Alternatively, the surface profile of each surface relief element may be defined by a height function f(xy), and wherein the height function is 6 f(x,y)= A cos 2cos 2 + 0.667 cos2 + cos where A is a coefficient which determines the size of the caustic regions. Preferably, the function f(x,y), describing each of the surface relief elements, also includes one or more independent parameters which may be used 5 to encode image information representing the image according to a mapping algorithm whereby one or more of the independent parameters may be mapped in one to one correspondence with the parameters used to describe the image information. The image information may consist of an array or multiplicity of regions of particular light intensity described by individual light intensity or 10 greyscale parameters which are mapped in one to one correspondence with independent parameters in the individual surface relief element functions f,(x,y). Preferably, the functions fg(xy) belong to a special class of functions which upon being gradient mapped into the (u,v) space produce strong focussing or light caustic regions in the form of "folds", "cusps", "swallowtail", "butterfly", "elliptic 15 umbilic", parabolic umbilic" or "hyperbolic umbilic" forms or combinations thereof, the terms "folds", "cusps", "swallowtail", "butterfly", "elliptic umbilic", parabolic umbilic" or "hyperbolic umbilic". Preferably, the independent parameter is a scaling factor, such that the maximum slope of a surface relief element is proportional to the scaling factor of 20 the surface relief element. The scaling factor may be inversely proportional to the perceived maximum brightness of the associated surface relief element. Preferably, the surface profile of each surface relief element is defined by a height function, and wherein the height function is of the form; fij (x, y) = aijfo (x, y) , 25 wherein a is the scaling factor for the surface profile, and wherein fo(xy) is the height function of the common base profile, i and j being the coordinates of the particular surface relief element. Preferably, the height function for the surface profile of each surface relief element is; 30 f; (x, y)= a, cos c .

7 Alternatively, the height function for the surface profile of each surface relief element may be; f ,(x,y)=a Cos -Cos -+ 0.667 Cos -+ Cos . Preferably, the variations in surface relief of the device are greater than 5 0.25 microns. Preferably, there is included one or more secondary security features. The secondary security features may be selected from: micromirror reflective or refractive elements; Fourier plane elements; flat elements arranged to provide a mirror reflection of a user to the user; lenslet elements of a lenslet array; and 10 diffractive elements. According to a second aspect of the present invention, there is provided a security document including a paper or polymer substrate incorporating an optical device according the first aspect, wherein the device is incorporated into or onto the surface of a paper or polymer document as a means for authenticating the 15 genuineness of the document. The security document may be a bank note. According to a third aspect of the present invention, there is provided a method for producing an optically variable device including: generating on or in a substrate a plurality of surface relief image elements, each including a surface profile, located and configured to reflect and/or refract incident light as to form 20 part or all of an image, such that one or more of the surface relief image elements produces a reflected or refracted wavefront which is structurally stable with respect to small perturbations or distortions of that part of the surface of the device incorporating the image element. Preferably, there is included the steps of: constructing a representation of 25 a surface relief height function of the plurality of surface relief image elements; driving an electron beam lithography system, using the representation, to create an embossing tool for hard or soft embossing; and embossing the optically variable device using the embossing tool. Preferably, driving the electron beam lithography system includes the 30 steps of: using the representation to electron radiate an electron beam resist coated plate including a metallisation layer to create a pattern of exposed and unexposed areas corresponding to the representation; developing the electron 8 beam resist coated plate; stripping off the metallisation layer either the exposed or the unexposed areas using a chemical or reactive ion beam etching process so as to produce an optical transmission mask corresponding to the representation; inserting the transmission mask into either a contact print or a projection type 5 photolithography system; exposing a photoresist coated glass plate to UV radiation through the transmission mask; electroplating the resulting photoresist surface relief to obtain a nickel copy of the surface; and using the nickel copy of the surface as the embossing tool. Preferably, the representation is a contour map. Alternatively, the 10 representation may be a dithered bitmap. Preferably the counter map or dithered bitmap includes counter lines or bitmap element spacings with a minimum spacing of 10 nanometers. The representation may be a high resolution representation. Preferably, the surface relief height function is a function of position 15 coordinates. Preferably, the electron beam resist coated plate is a glass or quartz plate. Preferably, step (f) is repeated a plurality of times using the nickel copy produced in step (e). DESCRIPTION OF THE DRAWINGS 20 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: Figure la shows an optically variable device including a single image 25 according to an embodiment; Figure lb shows an optically variable device including two interlaced images according to an embodiment; Figure 1c shows different image types according to different embodiments; Figure 2 shows an optically stable caustic; 30 Figure 3 shows a surface profile of an optically variable device; Figure 4 shows the surface plane and the caustic plane according to an embodiment; Figure 5a shows an example surface profile; 9 Figure 5b shows the corresponding caustic formed by the surface of figure 4a; Figure 6a shows an example surface profile; Figure 6b shows the corresponding caustic formed by the surface of figure 5 5a; Figure 7 shows a number of different sized caustics according to an embodiment; Figure 8 shows an image including different levels of brightness, according to an embodiment; 10 Figure 9 shows two surface relief elements with different underlying slopes, according to an embodiment; Figure 10 shows two images on a single surface according to an embodiment; and Figure 11 shows a method for producing an optically variable device 15 according to an embodiment. The following disclosure introduces structurally stable wavefronts which have been reflected or refracted from surface relief image elements to form part or all of an image. The refractive/reflective versions of the structurally stable wavefronts have been found to have significant advantages over their 20 corresponding diffractive versions. These advantages include; greater angular separation between image channels resulting in less sensitivity to crumpling effects and easier image recognisability, higher image brightness due to single order propagation and increased security due to much higher image differentiation from attempted simulation and greater complexity of the origination 25 process. US7281810, titled "Optical Device and Methods of Manufacture" describes an optical device which generates an optically variable image, the image being optically variable in that it varies according to the position of observation, is manufactured by dividing an optically invariable image into multiple pixels. Colour 30 component values are then determined for each pixel. For each of the pixels of the optically invariable image, there is determined an associated pixel surface structure which has a three dimensional surface shape and curvature which is related via a mathematical or computer algorithm to the colour component values 10 of the associated pixel in the optically invariable image, each pixel surface structure being an individual reflective or diffractive surface structure which produces an observable optical effect. An assembly of the reflective or diffractive pixel surface structures is 5 produced which when illuminated generates a plurality of observable optical effects which combine to form an optically variable reproduction of the optically invariable image. While US7281810 does not disallow reflective pixel surface structures which have the special property of generating structurally stable optical wavefronts, there is no disclosure in US7281810 of the specific shapes and their 10 corresponding mathematical forms of such structurally stable wavefront generating surface structures. Furthermore, there is no disclosure in US 7281810 of how such structures may be formed from a contour map or bitmap representation of such structures - in contrast to the purely individual pixel surface structures described in US7281810. 15 In the case of disclosures relating to microlens array optically variable image devices, such as the type described in US patent application 2009/0008923 Al, entitled "Security element", the common feature relating to all of these microlens array devices is the fact the generic surface element structure is in the form of a lens - either of a cylindrical type or of a spherical type. Being 20 lens-like these structures produce focussing effects which are inherently unstable with respect small changes in the shape of the lens elements or position of the focal plane. Therefore these lens array devices do not disclose inherent structural stability property with regard to the resulting optical caustics and the understanding of such features in terms of diffraction catastrophes. 25 Various types of diffractive optically variable devices have been constructed around diffraction catastrophe and structural stability principles. For example European patent EP 0449 893 B1 describes a diffractive device comprised of continuously connected grating groove patterns which diffract an incoming light beam into a multiplicity of diffraction catastrophes. When a 30 generalised diffraction grating of this type was embossed onto a hot stamping foil and incorporated into a banknote it was demonstrated that the pixellated images observed by the naked eye focussed on the grating were relatively insensitive to crumpling of the banknote surface.

11 The Australian polymer banknote issued in January 1988 carried a diffracting grating of this type to protect the banknote against counterfeiting. US patent 5,428,479 describes another type of diffractive structure known under the trademark term Pixelgram T M . Pixelgram structures are comprised of a multiplicity 5 of diffraction grating regions which each have the property of generating optical fields exhibiting caustics and their associated diffraction catastrophes. In the Pixelgram case the grating regions were also disconnected from each other, and this allowed for the further advantage of forming optically variable portrait type images which were also structurally stable with respect to small crumpling effects. 10 The two key advantages of structurally stable caustic optical fields were therefore encompassed by Pixelgram diffractive structures - relative image stability with respect to surface crumpling effects and the ability to form optically variable local image intensity effects via modulation of the strength of the caustic effects. 15 In reference to figure la, there is provided an optically variable device including a surface 100, further including an array 108 of surface relief elements 102 of a particular type (indicated by the typed letter A). The surface relief elements 102 each correspond to an element 112 of an image 110 (for example, the image of an 'A' shown in figure 1a). 20 In reference to figure 1b, two or more images 114 and 116 may be present on the same surface. This may be achieved by interlacing surface relief elements (e.g. A and B pixel elements) 102 corresponding to the two or more different images 114, 116 on the one surface 100. For the images 114, 116 to be distinguishable, the surface relief elements 102 must be configured so those 25 belonging to one image (e.g. A) are only viewable through one range of viewing angles and the relief elements (e.g. B) are only viewable through a different range of viewing angles such that no two surface relief elements 102 belonging to different images 114, 116 are viewable at the same angle. Referring to figure 1c, different image types may be used, including: 30 Outline 120: The surface relief elements may be positioned within the array in a sparse manner, such that the presence of a surface relief element on the array corresponds to the outline or border of the image; Solid 122: The pixels are arranged to define a solid image; 12 Variable Density 126: A grey scale effect can be created by varying the numbers of surface relief elements over the surface; and Variable Element Intensity 126: Each surface relief element may include a relative brightness, corresponding to a greyscale value for the element. 5 The surface relief elements 102 can be of any shape, for example square, circular, or triangular. Further, the surface relief elements 102 can be directly adjacent (as shown in figure 1a) or alternatively there may be space included between each surface relief element 102. The surface relief elements 102 may be positioned in a regular array, or may be dispersed on the surface as required. 10 Each surface relief element 102 is selected to have the property of producing, upon reflection or refraction of incident light, a reflected or refracted wavefront that is structurally stable and therefore relatively expansive in nature so that the relief element is viewable over a small range of viewing angles and is therefore more resistant to the image distortions resulting from small 15 perturbations in the flatness of the substrate surface. Therefore, the resulting image 110 is less significantly altered due to localised undulations to the surface 100. Such stability also allows for the further option of arranging for different surface relief elements 102 to have different reflected or refracted intensities. For ease of description of the following embodiments, it will be assumed 20 that the surface relief elements 102 are square. The array 108 is a two dimensional array including a depth 106 and a width 104, and each surface relief element 102 therefore occupies a unique position on the array 108. The position of each surface relief element 102 may be described by a unique combination of an X coordinate and a Y coordinate. However, these assumptions are not a 25 requirement and it will be appreciated that the arrangement and shape of the surface relief elements 102 in the following embodiments can be altered without departing from the scope of the embodiments. In reference to figure 2, to achieve the required structural stability of the wavefront generated by a particular surface element, the refracted or reflected 30 wavefront generated by that surface element must be of an expansive type with strongly focussed boundary regions. The refracted or reflected beam from a surface relief element 102 is directed in a direction 206 with an expansion angle 208 and a caustic envelope 204 and observed by the eye 202. The image 13 element 102 is therefore structurally stable with respect to surface perturbations or undulations within the range shown by the expansion angle 208. In reference to figure 3, to achieve the required stability of the surface relief elements 102, each surface relief element 102 includes a surface profile 300. The 5 surface profile 300 of each surface relief element 102 is selected to produce a structurally stable reflection or refraction, due to the production of a structurally stable caustic. The surface profile 300 of each surface relief element 102 is defined by a height functionf(x,y), wherein the height function f(x,y) contains the special 10 property that its Hessian, H(x,y), is not equal to zero except at particular points (x,y) or lines segments (x,vy)-(x2,y2) of non-zero length (these positions are known as 'Hessian vanishing regions'). The Hessian is shown mathematically in Eq 1. H(x,y) = D 2 ay 2 -17(Eq. 1) The gradient mapping of the Hessian vanishing regions produces 15 corresponding singular caustic regions in the gradient mapping space (u,v) on the functionf(x,y). The coordinates (u,v) of the mapping space are related to the coordinates (x,y) of the function space according to the gradient mapping equations Eq. 2 and Eq. 3: Df(x, y') u(x, y) R, (x, y) + ci X, (Eq. 2) Df(x, y') v(x, y) R2 (x, Y) + C 2 , (Eq. 3) Where R,(x,y) and R2(x,y) are predetermined functions of (x,y) and c 20 and c2 are predetermined constants independent of(x,y). The functions u(x,y) and v(x,y) describe the direction of optical beams reflected from the surfacef(x,y). Figure 4 shows the relationship between the reflection of the surface profile as defined by the height function f(x,y) 400 and its corresponding caustic on the (u,v) plane 402.

14 Figures 5a and 5b, and figures 6a and 6b, show two example embodiments. Figure 5a shows the surface profile 300 according to the height profile defined by the equation Eq. 4. f(x, y) = cos( ) cos( ) (Eq. 4) 4 4 5 Figure 5b shows the calculated singular caustic pattern 502 in the (u,v) plane 500 of the surface profile shown in figure 5a. Figure 6a shows the surface profile 200 according to the height profile defined by the equation Eq. 5. f(x,y)=cos( 2 )cos( 2 )+0.667 cos( )+cos( ) (Eq. 5) Figure 6b shows the calculated singular caustic pattern 602 in the 10 (u,v) plane 600 of the surface profile shown in figure 6a. In this embodiment, by requiring the surface profile 300 to have the Hessian property stated previously, structurally stable surface relief elements 102 can be created. These can be used to produce an optically variable device including an optically variable image, wherein the observed image elements are 15 structurally stable with respect to small changes in the angle or slope of the substrate incorporating the surface relief image elements 102. In reference to figure 7, a further embodiment is shown wherein the relative brightness of each surface relief element 102 is varied to produce a greyscale image. To achieve this while maintaining structural stability, the surface profile 300 of each surface relief 20 element 102 is selected from a family of structurally stable surface profiles, wherein each member of the family includes a similar base profile multiplied by a scattering parameter. A larger scattering parameter corresponds to a relatively dimmer surface relief element, as less light is directed towards the viewer (the same amount of light is spread over a wider range). 25 For example, by selecting a family of surface profiles of the form shown in Eq. 6, brightness values can be applied to each individual surface relief element 102 based on selection of the scattering parameteraj. The smaller the value chosen for the scattering parameter, the brighter the surface relief element 102 will appear due to the greater concentration of light within the boundary of the 15 caustic. In the example shown in figure 7, the caustic 700 corresponds to the caustic of highest relative brightness, while the caustic 702 corresponds to the caustic of lowest relative brightness. f 1 (x,y) a fo(x, y) (Eq. 6) Here fo(x,y)is simply the base surface profile function (for example, 5 Eq. 4), and i and j are parameters relating to the X and Y coordinates of the particular surface relief element, respectively. Preferably, the functionsf(x,y) when gradient mapped into the (u,v) space produce strong focussing or light caustic regions in the form of "folds", "cusps", "swallowtail", "butterfly", "elliptic umbilic", parabolic umbilic" or "hyperbolic umbilic" 10 forms or combinations thereof, the terms "folds", "cusps", "swallowtail", "butterfly", "elliptic umbilic", "parabolic umbilic" or "hyperbolic umbilic". These named caustic regions having the meaning described in the "NIST Digital Library of Mathematical Functions", which is also consistent with the definition of these terms described in "Thom's Catastrophe Theory". 15 In one particular example as shown in figure 7, the base surface profile function is Eq. 4. The relative brightness of each surface relief element is inversely proportional to the value of the scaling parameteraj, due to the scaling parameter being proportional to the amount of scatter off the associated surface relief element. By having a higher degree of scatter, the amount of light detected 20 by a point-like viewer, for example a user, decreases. In reference to figure 8, by changing the relative brightness of the surface relief elements 102, a grey scale image 800 can be created. In a further embodiment, as shown in figures 9 and 10, two or more images 1000 (e.g. the image of the letter A), 1002 (e.g. the image of the letter B) 25 may be encoded onto a single surface by use of underlying surface slopes 900, 902. Each surface relief element 102 is attributed to one of a plurality of images 1000, 1002, and is sloped 900, 902 with respect to the plane of the surface 100 in a direction common to other surface relief elements belonging to the same image 1000 or 1002. In this embodiment, the viewing angle of a surface relief element 30 102 can be substantially normal to the underlying surface slope 900, 902 of the surface relief element 102.

16 In the example shown in figure 10, each image is assigned surface relief elements 102 labelled either 'A' or 'B' corresponding to viewing directions in one direction ('A') or the opposite direction ('B'). For example all surface relief image elements 902 contributing to the macroscopic image of the letter "A" in figure 10 5 could be assigned a height function described by Eq. 7. f1=by+a cos Cos - + 0.667 os2 + os 2 (Eq.7) while all surface relief image elements 900 contributing to the macroscopic image of the letter "B" in figure 10 could be assigned a height function described by Eq. 8. fg=-by+a CsrCo s c + 0.667 cos 2 + cos 2 (Eq.8) where b is the gradient of the underlying surface slopes. 10 The images of the letters A or B will then be observed at a relatively large difference in viewing or illumination angles due to the slopes of the respective surface relief image elements being opposite to each other. In the particular embodiment referred to in Eq 7 and Eq 8, the viewing angles are symmetrical about the normal to the surface 100, however, symmetry in this regard is not 15 required. Referring back to figures la and 1b, the locations on the substrate absent a surface relief element may correspond to one or more secondary security features 115. For example, the secondary elements can be selected from one or more of the following: 20 e micromirror reflective or refractive elements; e Fourier plane elements; e flat elements arranged to provide a mirror reflection of a user to the user; e lenslet elements of a lenslet array; and 25 e diffractive elements. The surface 100 of the optically variable device can be a surface of a substrate. In one embodiment, the variation of height of each surface relief 17 element 102 between the lowest point in the surface profile 300 and the highest point in the surface profile 300 is at least 0.25 microns. In another embodiment, the size of each surface relief element 102, defined as the surface area of the surface 100 covered by the surface relief 5 element 102, is varied to produce varied reflected brightness. This may be used in conjunction with, or alternatively to, variation of scaling factor. Alternatively, in another embodiment, the size of each surface relief element 102 is fixed. In this embodiment, the length of each side of each surface relief element is preferably between 30 and 60 microns. 10 The optically variable device may be manufactured by any appropriate process. In one embodiment as shown in figure 11, the optically variable device is manufactured according to a process involving a combination of electron beam lithography and photolithography. A contour map or alternatively a bitmap density representation, of each of 15 the surface relief image elements in the image array is constructed, via an ultra high resolution data file representation, is constructed 1100 based on the surface relief height function as a function of its coordinates (xy), preferably the minimum distance between adjacent contour lines or bitmap elements being greater than 10 nanometres 20 The contour map or bitmap representation of the function is used to drive the operation of an electron beam lithography system 1102 to electron radiate an electron beam resist coated metallised glass or quartz plate to a pattern of exposed and unexposed areas of similar form to the contour map or bitmap with alternate regions of the contour map or bitmap receiving doses of electron beam 25 radiation and the regions in between these regions receiving no exposure to the electron beam. The electron beam resist coated plate is developed 1104 after exposure to the contour map or bitmap representation of the array and then the metallisation layer on the glass plate is stripped 1006 from the exposed, or alternatively the 30 unexposed, areas using a chemical or reactive ion beam etching process so as to produce an optical transmission mask of the contour map or bitmap array.

18 A transmission mask is used to drive a contact print or projection type photolithography system 1108, and then a photoresist coated glass plate is exposed to UV radiation through the transmission mask 1110. After completing of the optical exposure process and developing the 5 photoresist coated glass plate, the resulting photoresist surface relief is electroplated 1112 to obtain a copy of the surface, typically in nickel, which can be replicated a multiplicity of times via a hard or soft embossing process 1114. Further modifications and improvements may be made without departing from the scope of the present invention. For example, on a device containing two 10 images, one image may define a greyscale image, and another image may define an outline image.

Claims (5)

1. An optically variable device, including a plurality of surface relief image elements, each including a surface profile, located on or in a substrate and configured to reflect and/or refract incident light as to form part or all of an image, 5 such that one or more of the surface relief image elements produces a reflected or refracted wavefront which is structurally stable with respect to small perturbations or distortions of that part of the surface of the device incorporating the image element.

2. An optically variable device as claimed in claim 1 wherein the height of each 10 surface relief image element at any point (xy), thereby determining the surface relief elements surface profile, is described by a function f(x,y) which has the property that its Hessian of second derivatives H(xy) does not vanish identically across the surface of the element and wherein this condition is expressed mathematically by the expression H(x,y) = 2,Y) 2 2 2 # 0, except at 15 particular points (xy), or line regions (x 1 ,yv) - (x2,y2) of non-zero length, and where the gradient mapping of such Hessian vanishing regions produces corresponding singular caustic regions in the gradient mapping space (u,v) of the function fx,y), wherein the coordinates (u,v) of the mapping space are related to the coordinates (xy) of function space according to the gradient mapping equations 20 u(x,y) =R(x,y)+c ' and v(x,y)=R2(x,y)+c2, where R(xy) and R2(x,y) are predetermined functions of (xy) and c 1 and c2 are predetermined constants independent of (xiy), and wherein the functions u(xy) and v(xy) describe the direction of optical beams reflected from or refracted through the surfacefx,y) to the eye of an observer. 25

3. An optical device according to claim 2, wherein the function f(x,y), describing each of the surface relief elements, also includes one or more independent parameters which may be used to encode image information representing the image according to a mapping algorithm whereby one or more of the independent 20 parameters may be mapped in one to one correspondence with the parameters used to describe the image information.

4. A security document including a paper or polymer substrate incorporating an optical device according to any one of the previous claims, wherein the device is 5 incorporated into or onto the surface of a paper or polymer document as a means for authenticating the genuineness of the document.

5. A method for producing an optically variable device including: generating on or in a substrate a plurality of surface relief image elements, each including a surface profile, located and configured to reflect and/or refract incident light as to 10 form part or all of an image, such that one or more of the surface relief image elements produces a reflected or refracted wavefront which is structurally stable with respect to small perturbations or distortions of that part of the surface of the device incorporating the image element. 15 SECURENCY INTERNATIONAL LIMITED WATERMARK PATENT & TRADE MARK ATTORNEYS 20 UIP1287AUOO