1 ENCRYPTED OPTICALLY VARIABLE IMAGE FIELD OF THE INVENTION The invention generally relates to security devices for documents, for example banknotes. 5 BACKGROUND TO THE INVENTION It is well known to include security features within documents requiring a level of security, for example banknotes. Such security features can take on a number of forms, however particularly useful features are ones that are visually apparent and therefore inspectable with relative ease. 10 However, 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 15 authentication process for the banknote by members of the public has long been recognised as the weakest point in the security system. Often, such security features require inspection by members of the public to be useful, but may be overly complicated to correctly view or may not provide a strong effect that is easily recognised. This diminishes the usefulness of such features in allowing the 20 public to take an active role in reducing the cost of counterfeiting. Therefore, it is desirable to provide security features which are difficult to reproduce and, therefore, counterfeit, while engaging the public such that regular authentication of banknotes can take place. Security features which provide a surprising visual effect, for example revealing a hidden image that is not normally 25 visible, while not requiring specialist equipment, are particularly desirable. SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a method for creating an encrypted image, having encrypted image pixels, from a halftone image having halftone pixels and a two-tone image having two-tone 30 pixels, wherein each encrypted image pixel has a first region and a second region, wherein for each encrypted image pixel: a) both the first region and the second region have a first colour when a corresponding halftone pixel has a first halftone value; and b) depending on a value of a corresponding two-tone pixel, 2 the first region has one of the first colour and a second colour and the second region has the other of the first colour and the second colour when a corresponding halftone pixel has a second halftone value. The halftone image preferably corresponds to an overt image. The halftone 5 image may be generated from a greyscale image. The two-tone image preferably corresponds to a hidden image. Preferably, for each encrypted image pixel, when a corresponding two tone pixel has a first two-tone value, the first region is the first colour and the second region is the second colour. Also preferably, for each encrypted image 10 pixel, when a corresponding two-tone pixel has a second two-tone value, the first region is the second colour and the second region is the first colour. Preferably, the first and second two-tone values each correspond to a colour. The first two-tone value may correspond to the first colour and the second two-tone value may correspond to the second colour. 15 Preferably, the first and second halftone values each correspond to a colour. The first halftone value may correspond to the first colour and the second halftone value may correspond to the second colour. Preferably, the first colour is darker than the second colour. In this case, the first colour may be black and the second colour may be white. Alternatively, 20 the first colour may be lighter than the second colour. In this case, the first colour may be white and the second colour may be black. In another alternative, one of the first and second colours may be transparent. According to a second aspect of the present invention, there is provided a method for producing a security device, including the steps of: a) creating an 25 encrypted image using the method of the first aspect; and b) applying the encrypted image to a substrate. According to a third aspect of the present invention, there is provided a method for producing a security document including the steps of: a) providing a substrate; and b) applying to a side of the substrate in a region an encrypted 30 image created according to the method of the first aspect. Preferably, the method includes the further step of providing within a different region of the substrate a decoding screen. The decoding screen is preferably a microlens array. In this case, each microlens may have a width equal 3 to the width of a pixel. Furthermore, the microlenses of the microlens array may be lenticular lenses, and the length of each microlens may be at least equal to a dimension of the encrypted image. Alternatively, the decoding screen is a line screen. The substrate is preferably configured for folding such that the encrypted 5 image can be viewed through the decoding screen. According to a fourth aspect of the present invention, there is provided a method for viewing an encrypted image located within a region of a security document, wherein the security image is created according to the first aspect, wherein the encrypted image is viewed through a microlens array configured to 10 reveal a hidden image. According to a fifth aspect of the present invention, there is provided a security document including an encrypted image created according to the method of the first aspect, or a security document created according to the third aspect. The security document is preferably a banknote. 15 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, 20 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 25 identity cards or passports formed from a substrate to which one or more layers of printing are applied. Substrate 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 30 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 4 material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials. Transparent Windows and Half Windows As used herein the term window refers to a transparent or translucent area 5 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. 10 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 15 transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area. 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 20 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. 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 25 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 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 30 LT < Lo , where Lo is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the 5 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 5 material to which indicia may be subsequently printed or otherwise applied. Security Device or Feature As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. 10 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, 15 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). BRIEF 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 a security document including a security device and a 25 verification device; Figure lb shows a security document including a security device, and a separate verification device; Figure 1 c shows another view of the security document of Figure 1, further showing two opacifying layers; 30 Figure 2 shows a greyscale image; Figure 3 shows a two-tone image; 6 Figure 4 shows a halftone image based on the greyscale image of Figure 2, and further shows a comparison between a halftone subregion and the corresponding greyscale subregion; Figure 5 shows example layouts of a subregion of the halftone image and 5 the corresponding two-tone image, and further shows the resulting encrypted image; Figure 6 shows the appearance of a subregion of the encrypted image when viewed through the verification device from different directions; Figure 7 shows the appearance of the encrypted image when viewed 10 directly, when viewed through the verification device from a first direction, and when viewed through the verification device from a second direction; Figure 8 shows a first example configuration of the self-verifying security document; Figure 9 shows a second example configuration of the self-verifying 15 security document; and Figure 10 shows a third example configuration of the self-verifying security document. DESCRIPTION OF PREFERRED EMBODIMENTS There is provided a security document 2 including a security device, as 20 shown in Figures la and 1b. The security device 4 corresponds to a preferably printed or optionally otherwise applied encrypted image. The encrypted image has a first appearance when viewed directly. When viewed through an associated verification device 6, the encrypted image has at least a second appearance, preferably a second and third appearance depending on the angle at which the 25 encrypted image is viewed. The verification device 6 corresponds to a decoding screen including a periodic feature, for example preferably a microlens array or a line-screen. The verification device 6 can be incorporated into the security document 2 as shown in Figure la, and thereby the security document 2 is self-verifying, as 30 the encrypted image can be viewed by folding the security document 2 such that the verification device 6 overlaps the security device 4. Advantageously, the self verifying security document 2 enables a user of the security document 2 to check the authenticity of the security document 2.
7 It is noted that a verification device 6 present on a second security document 2 can be used to verify the security device 4 of a first security document 2. Alternatively, the security document 2 does not include a verification 5 device, as shown in Figure 1b, and is therefore not self-verifying. For example, a separate verification device 8 including a periodic feature, for example a microlens array or a line-screen can be positioned overlapping the security device 4, allowing for decryption of the encrypted image. Advantageously, this alternative allows for authorised individuals or organisations in possession of an appropriate 10 verification device 8 to verify the security document 2. Referring to Figure 1c, the security document 2 (in this case, a self verifying security document 2 including a verification device 6) includes a substrate 7, which can be transparent, and the security device 4 can correspond to a printed area of the substrate 7. The security document 2 can include one or 15 more opacifying layers 9a, 9b. Referring to Figure 2, there is provided a greyscale image 10 including a plurality of greyscale image pixels with a specified resolution, for example 400 dots-per-inch (dpi). Each greyscale image pixel corresponds to a 'dot', or an element of the greyscale image 10. For example, a greyscale image 10 that has a 20 width of one inch and a height of three inches with the aforementioned resolution will have an arrangement of greyscale image pixels such that there are 400 greyscale image pixels from the left side to the right side along any one line, and 1,200 greyscale image pixels from the top to the bottom along any one line. The greyscale image 10 can be provided on a computer running appropriate software, 25 for example Adobe® Photoshop®. There is further provided, as shown in Figure 3, a two-tone image 12 including a plurality of two-tone image pixels, wherein each two-tone image pixel is associated with one of two values (e.g. each value is a distinct colour). Preferably, the arrangement of two-tone image pixels is the same as the 30 arrangement of greyscale image pixels of the greyscale image 10. Therefore, the resolution of the two-tone image 12 is the same as the greyscale image 10 and, preferably, there is a one-to-one correspondence between the greyscale image pixels of the greyscale image 10 and two-tone image pixels of the two-tone image 8 12. In one example, as shown in the figure, one colour is black (grey level 0) and one colour is mid-grey (grey level 127). As for the greyscale image 10, the two tone image 12 can be provided on a computer running appropriate software. It is understood that the choice of colours for the two-tone image 12 is arbitrary; 5 however for convenience it is preferable to choose two colours with sufficient contrast such that the two-tone image can be readily identifiable when depicted. It is understood that the colours of the two-tone image 12 can be selected from colours and/or shades. It is envisaged that the two-tone image 12 can correspond to a halftone image (being a different half-tone image to that described below), 10 where the halftone image includes the appearance of more than two tones due to the arrangement of pixels of two different colours (e.g. black and white pixels). The same principles that are described below with reference to the two-tone image 12 of Figure 3 will apply to such a halftone version of the two-tone image 12. 15 The greyscale image 10 is converted into a halftone image 14 as shown in Figure 4, for example by using a computer algorithm such as the halftone filter provided with Adobe Photoshop. The halftone image 14 can be created using amplitude modulation, frequency modulation, or any other suitable modulation. The halftone image 14 corresponds to an arrangement of halftone image pixels 20 wherein each halftone image pixel is selected from one of two values, for example each value is a distinct colour, preferably white (grey level 255) and black (grey level 0). Halftone images are known in the art, and provide an appearance of different shades depending on the concentration of the two colours. Figure 4 also shows a comparison between corresponding subregions of 25 the halftone image 16 and the greyscale image 18. As can be seen, darker regions of the greyscale image subregion 18 are represented by higher black pixel densities in the corresponding halftone image subregion 16. Colour halftone images 14 can be used, for example where instead of black a particular colour is used (e.g. red). 30 Figure 5 shows the creation of an encrypted image 20 based on (subregions of) the halftone image 14 and the two-tone image 12. The encrypted image 20 corresponds to a modification to the halftone image 14. It is noted that the encrypted image 20 may replace the original halftone image 14 or be created 9 in addition to the original halftone image 14. As can be seen, there are 64 pixels for each subregion, arranged in eight rows of eight pixels. The colour of each encrypted image pixel is selected by modifying the corresponding halftone image pixel based on the value (e.g. colour) of the 5 corresponding two-tone image pixel. Each encrypted image pixel includes a first region and a second region. Preferably the arrangement of the first region and the second region is the same for each encrypted image pixel. For each black halftone image pixel, both the first region and the second region of the corresponding encrypted image pixel is assigned black. 10 For each white halftone image pixel, if the corresponding two-tone image pixel is black, the first region of the encrypted image pixel is assigned black and the second region is assigned white. Otherwise, the first region of the encrypted image pixel is assigned white and the second region is assigned black. A decoding screen is also provided. The decoding screen can be a 15 microlens array 22, a subsection of which is shown in Figure 6. The microlens array 22 can be a lenticular lens array with a lens width equal to the width of an encrypted image pixel. For example, when the resolution of the encrypted image is 400 dpi, the width of a lens is 0.0025 inches (63.5 microns). In a preferred decoding configuration, the microlens array 22 is positioned over the encrypted 20 image 20 such that each microlens 24 covers a single pixel and no other pixels of the encrypted image in a direction perpendicular to the longitudinal axis of the microlens 24. The figure shows the change in appearance of the encrypted image 20 when the viewing direction is moved from a first side 26 of the normal to a second side 28. 25 Figure 7 shows the appearance of the encrypted image 20 (and therefore of the security device 4) when viewed without a corresponding microlens array (30), the appearance of the encrypted image 20 when viewed from a first direction when viewed through a microlens array (32), and the appearance of the encrypted image 20 when viewed from a second direction when viewed through a 30 microlens array (34). As can be seen, the hidden image appears to invert as the viewing direction is changed from one side to another. Alternatively, the decoding screen can be a line-screen. The line-screen includes an arrangement of identical parallel lines. Each line preferably has a 10 length equal, or approximately equal, to a dimension of the encrypted image 20. The lines are arranged in a periodically repeating pattern, preferably with spacing between adjacent lines equal to the width of a line. Referring to the arrangement of Figure 1a, where the security document 2 5 includes the security device 4 and the verification device 6, the encrypted image 20 can be revealed by creating a decoding arrangement corresponding to folding of the security document 2 to cause the verification device 6 to overlay the security device 4. An example decoding arrangement is shown in Figure 8. The microlens 10 array 22 is located within a window region of the security document 2, and the encrypted image 20 is located in an opaque region of the security document (for example, located as part of, or on top of, an opacifying layer). The microlenses 24 are configured for viewing the encrypted image 20 through the substrate 7 as shown, and are therefore located on the opposite side of the security document 2 15 to the encrypted image 20. In another example decoding arrangement as shown in Figure 9, the microlens array 22 is located within a window region of the security document 2, and the encrypted image 20 is located within a half-window region of the security document 2. The encrypted image 20 can be located on either side of the 20 substrate 7, and the microlenses 24 are configured for focussing on whichever side as appropriate. The figure illustrates the difference in configuration when the encrypted image 20 is located on each surface of the substrate 7. Figure 10 shows a configuration that can allow for two decoding arrangements associated with the same security document 2. The microlens 25 array 22 is located within a window region of the security document 2, and the encrypted image 20 is located within a window region of the security document 2. The first decoding arrangement 36 is such that the microlenses 24 operate as convex lenses, and the encrypted image 20 appears decrypted to a user viewing the encrypted image 20 from either side of the security document 2. The second 30 decoding arrangement 38 is such that the microlenses 24 operate as concave lenses, and the encrypted image 20 appears decrypted to a user viewing the encrypted image 20 from either side of the security document 2.
11 In order to ensure a focussing distance between the microlenses 24 and the encrypted image 20, it can be preferable to ensure that for each applicable decoding arrangement 36, 38, there is the same (or approximately the same) distance between the microlens array 22 and the encrypted image 20. 5 The encrypted image 20 can be printed in the same process as opacifying layer 9a or 9b when the opacifying layer 9a or 9b is located on the same side of the substrate 7 as the encrypted image 20. For example, both the opacifying layer 9a and the encrypted image 20 can be printed using a drum-based printing technique using the same drum. 10 In the embodiments described, the first colour is black and the second colour is white. In general, the first colour and the second colour can be selected as corresponding to any two colours, with preference given to colours which contrast well with one another. It is noted that the first colour can be lighter than the second colour, as well as darker as described in the embodiments. 15 One of the colours can optionally be selected to be equal to no colour, or transparent. In this case, the colour corresponds to the colour of the underlying substrate and/or opacifying layer, or the colour of light transmitted through the substrate. For example, instead of the first colour being black as described, the colour is transparent. In another example, a white opacifying layer (or at least, 20 white in the region of the security device 4) is applied to the substrate, and the second colour is selected as transparent but will appear white due to the opacifying layer. Further modifications and improvements may be made without departing from the scope of the present invention. For example, the security document can 25 include a second security device configured for viewing through the same microlens array as the described security element. Such a second security device could be, for example, a Moire magnification based security feature. Further, the colours of the encrypted image and the halftone image are described herein as being the same (i.e. black and white), however in general this not a requirement. 30 In another modification, the two dimensions of the encrypted image have different resolutions (for example, in dpi), and the periodic feature (e.g. microlens array or line-screen) has a resolution equal to one of the two dimensions.