AU2012100265B4 - An optical security element and method for production thereof - Google Patents

Abstract

An optical security element is disclosed including a diffractive region and a selection device, configured such that the diffraction region is viewable through 5 the selection device, wherein the diffractive region includes a plurality of subregions, each associated with a particular viewing angle, and configured to diffract a particular wavelength or range of wavelengths at a diffraction angle corresponding to the associated viewing angle, and wherein the selection device is configured to display, at each viewing angle, the subregion associated with the 10 viewing angle. Preferably, the diffractive region is a blazed grating and the selection device is a microlens array. In this manner, a diffractive image can be produced which does not necessarily change colour when the viewing angle is changed. SECURENCY INTERNATIONAL PTY LTD WATERMARK PATENT & TRADE MARK ATTORNEYS

Description

1 AN OPTICAL SECURITY ELEMENT AND METHOD FOR PRODUCTION THEREOF FIELD OF THE INVENTION The present invention relates to an optical security element and method of 5 production of an optical security element and particularly, but not exclusively, an optical security element for use in securing banknotes and the like against counterfeiting. BACKGROUND TO THE INVENTION Security features have traditionally been applied to banknotes and other 10 security documents by a number of methods. Printed features are commonplace and are relatively easy for counterfeiters to reproduce. More sophisticated security features such as diffractive optically variable devices (DOVDs) have also been employed in banknotes and the like. A DOVD is harder to counterfeit because its physical structure must be reproduced, for example by contact 15 copying. However, in some circumstances its optical effect can be mimicked, for example by the use of metallic pigmented inks. Some alternative security devices make use of microlenses or lenticular lenses to view graphic elements in order to produce optically variable magnified images. For example, it is known to provide a lenticular lens comprising an array 20 of semi-cylindrical lenses for viewing underlying graphic elements in the form of interlaced strips of a series of images. As the angle of view is changed, strips of different respective images come into view, so that the viewer sees an animation effect in which successive frames correspond to successive images in the series. Another security device employing lenses is disclosed in US 5712731 and 25 includes graphic elements in the form of a two-dimensional array of identical microimages, formed by printing on a surface of a substrate. The array of microimages is viewable through a corresponding two-dimensional array of spherical microlenses. A slight mismatch between the pitch or rotational alignment of the microimage array and microlens array can produce moire fringes 30 in the form of one or more magnified (and possibly rotated) versions of the identical microimages. These known security devices, sometimes referred to in the art as a "moire magnifier", may produce images which appear to move and/or float below or above the plane of the device as the observing angle changes.

2 A known problem with the above devices is achieving the graphic element resolution required to produce high quality optically variable imagery, given the relatively small diameters (of the order of 100 microns or less) which must be used for the microlenses in order to meet thickness specifications for flexible 5 security documents such as banknotes. Traditional printing methods such as gravure, flexographic and intaglio printing cannot, in general, achieve the required resolution. Some methods for applying the graphic elements, such as etching or laser ablation, may be able to provide the required resolution. However, these methods 10 can, in general, only produce monochromatic imagery. Alternative methods for applying graphic elements seek to control the amount of light reflected from the surface bearing the graphic elements. For example, US 20030179364 constructs monochromatic microimages by using light traps to create dark areas of an otherwise reflective surface. Each light trap 15 constitutes a pixel of one of the microimages. Similarly, AU 2005238699 makes use of anti-reflective "moth-eye" structures in order to selectively reduce the reflectivity of a reflective surface. Both of these methods can produce high resolution graphic elements, but are also limited to monochromatic imagery. If printing methods are used to apply the graphic elements, and 20 multicoloured printing is desired, the printer is faced with the problem of precisely registering the colour components of the image to achieve the desired effect. If embossing methods are used, then it is possible to achieve a multicoloured effect by applying diffractive structures as is known in the art. Under illumination by polychromatic light, these provide a changing colour impression 25 with changing viewing angle due to the different diffraction orders of the wavelengths of incident light. However, diffractive structures are generally not suitable for viewing through lenses as part of a security device, because the colour of a diffractive structure under polychromatic (e.g. white light) illumination varies due to the appearance of the first diffraction order for the wavelength 30 components of the incident light according to the viewing angle. The optically variable effect due to the lenses will thus be conflated with the effect due to the diffractive structure, making it extremely difficult for the observer to distinguish an authentic image or series of images from the myriad of coloured images actually 3 observed. In addition, in the case of a lenticular lens device, the degree of cross talk between successive frames may result in unacceptable blurring or noise, reducing the effectiveness of the device. It is therefore desirable to provide an improved security device which can 5 produce multicoloured and high resolution optically variable effects. It is further desirable to provide an improved method for manufacturing such a security device and for incorporating the security device into a security document. Security document As used herein, the term security document includes all types of 10 documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic 15 transcripts. Transparent Windows and Half Windows As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the 20 transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area. A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying 25 layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area. 30 A partly transparent or translucent area, hereinafter referred to as a "half window", may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the "half-window" is not fully 4 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 5 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 10 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 opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. 15 Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied. Focal point size H As used herein, the term focal point size refers to the dimensions, usually 20 an effective diameter or width, of the geometrical distribution of points at which rays refracted through a lens or other focussing element intersect with an object plane at a particular viewing angle. The focal point size may be inferred from theoretical calculations, ray tracing simulations, or from actual measurements. Focal length f 25 In the present specification, focal length, when used in reference to a microlens (or similar focussing element) provided as part of a microlens array, means the distance from the vertex of the microlens to the position of the focus given by locating the maximum of the power density distribution when collimated radiation is incident from the lens side of the array (see T. Miyashita, 30 "Standardization for microlenses and microlens arrays" (2007) Japanese Journal of Applied Physics 46, p 5391). Gauge thickness t 5 The gauge thickness is the distance from the apex of a lenslet on one side of the transparent or translucent material to the surface on the opposite side of the translucent material on which the image elements are provided which substantially coincides with the object plane. 5 SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided an optical security element including: a) a diffraction region; and b) a selection device configured such that the diffraction region is viewable 10 through the selection device, wherein the diffraction region includes a plurality of subregions, wherein each subregion is associated with a particular viewing angle, and wherein each subregion is configured to diffract a particular wavelength or range of wavelengths at a diffraction angle corresponding to the associated viewing angle, 15 and wherein the selection device is configured to display, at each viewing angle, the subregion associated with the viewing angle. The diffraction region may include a blazed diffraction grating, such that the blaze angle of the diffraction grating for each subregion is equal to the diffraction angle associated with the subregion. 20 Preferably, the selection device includes a lens structure, and wherein, for each viewing angle, the lens structure is configured to focus on the associated subregion of the diffraction region. According to another aspect of the present invention, there is provided a method for producing an optical security device including an arrangement of 25 optical security elements according to the first aspect, the method including the steps of: a) providing a transparent or translucent substrate including first and second embossable surfaces; and b) simultaneously embossing the first surface with a diffraction pattern 30 and the second surface with a microlens array such that the lens array and the diffraction pattern are in registration and the diffraction pattern is viewable through the microlens array.

5a Preferably, the method includes the step of applying a first opacifying layer to the first surface that includes a window region corresponding to the microlens array. BRIEF DESCRIPTION OF THE DRAWINGS 5 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 1 shows an optical security device according to an aspect of the 10 present invention; Figure 2A shows the appearance of a diffractive region at a first viewing angle according to an embodiment; Figure 2B shows the appearance of a diffractive region at a second viewing angle according to an embodiment; 15 Figure 2C shows the appearance of a diffractive region at a third viewing angle according to an embodiment; Figure 2D shows the appearance of a diffractive region at a fourth viewing angle according to an embodiment; Figure 3A shows the appearance of a diffractive region at a first viewing 20 angle according to an embodiment; Figure 3B shows the appearance of a diffractive region at a second viewing angle according to an embodiment; Figure 3C shows the appearance of a diffractive region at a third viewing angle according to an embodiment; 25 Figure 3D shows the appearance of a diffractive region at a fourth viewing angle according to an embodiment; Figure 4A shows the visible subregion of a plurality of subregions at a first viewing angle of a diffractive region according to an embodiment; Figure 4B shows the visible subregion of a plurality of subregions at a 30 second viewing angle of a diffractive region according to an embodiment; 6 Figure 4C shows the visible subregion of a plurality of subregions at a third viewing angle of a diffractive region according to an embodiment; Figure 4D shows the visible subregion of a plurality of subregions at a fourth viewing angle of a diffractive region according to an embodiment; 5 Figure 5A shows a blazed diffraction grating according to an embodiment; Figure 5B shows a blaze angle of a blazed diffraction grating; Figure 6 shows an arrangement of a plurality of diffractive regions according to an embodiment; Figure 7A shows the appearance of an arrangement of a plurality of 10 diffractive regions when viewed at a first viewing angle according to an embodiment; Figure 7B shows the appearance of an arrangement of a plurality of diffractive regions when viewed at a second viewing angle according to an embodiment; 15 Figure 7C shows the appearance of an arrangement of a plurality of diffractive regions when viewed at a third viewing angle according to an embodiment; Figure 7D shows the appearance of an arrangement of a plurality of diffractive regions when viewed at a fourth viewing angle according to an 20 embodiment; Figure 8A shows a schematic drawing of the operation of a security device at a first viewing angle according to an embodiment; Figure 8B shows a schematic drawing of the operation of a security device at a second viewing angle according to an embodiment; 25 Figure 8C shows a schematic drawing of the operation of a security device at a third viewing angle according to an embodiment; Figure 8D shows a schematic drawing of the operation of a security device at a fourth viewing angle according to an embodiment; Figure 9 shows a variable intensity arrangement according to an 30 embodiment; Figure 10 shows an arrangement of variable intensity colour pixels according to an embodiment; 7 Figure 11 shows a security document according to an aspect of the present invention; Figure 12 shows a method of manufacturing a security device according to an aspect of the present invention; and 5 Figure 13 shows an alternative embodiment including a mask layer. DESCRIPTION OF PREFERRED EMBODIMENT Referring to Figure 1, a schematic representation of an image layer 10 of an optical security device 60 is shown having a normal 12 and viewing angles 14, 16, 18 and 20 representing different angular sectors from the normal 12. Each 10 viewing angle 14, 16, 18, 20 is associated with a particular colour or range of colours, or more generally, a particular wavelength or range of wavelengths, such that when the optical security device 60 is viewed at each of said viewing angles 14, 16, 18, 20, the associated colour, wavelength, or range thereof of each viewing angle 14, 16, 18, 20 is visible. It should be appreciated that a viewing 15 angle 14, 16, 18, 20 can represent substantially the centre of a range of angles at which viewing is possible. In this example, each range of viewing angles is approximately equal to the angular separation between adjacent viewing angles. According to an embodiment as shown in Figures 2A, 2B, 2C and 2D, the image layer 10 includes a diffractive region 21 including a diffractive structure 20 which is configured such that, when the image layer 10 is viewed at each of the viewing angles 14, 16, 18 and 20, the diffractive region 21 appears to be substantially the same colour. According to another embodiment as shown in Figures 3A, 3B, 3C and 3D, the image layer 10 includes a diffractive region 21 including a diffractive structure 25 configured such that the apparent colour of the diffraction region 21 at each viewing angle 14, 16, 18, 20 can be different. For example, the apparent colour of the diffraction region 21 at the viewing angles 14, 16 and 20 represented by Figures 3A, 3B and 3D is a first colour (for example, green), and the apparent colour of the diffraction region 21 at the viewing angle 18 represented by Figure 30 3C is a second colour (for example, red). Each colour may be chosen from a selection of colours (e.g. two colours) or an infinite number of colours (e.g. the visible spectrum), such that each colour is sufficiently distinct to be distinguished either visually or by the use of a suitable device.

8 Now referring to Figures 4A, 4B, 4C and 4D, the diffractive region 21 is shown including a plurality of subregions 24, 26, 28 and 30. The diffractive region 21 has a corresponding lens structure (not shown in Figures 4A, 4B, 4C and 4D) which alters the portion of the diffractive region 21 which is viewable at different 5 viewing angles. In this way, only one subregion 24, 26, 28, 30 is viewable for any single viewing angle, except for a possible transition region in which a viewing angle includes a portion of two adjacent subregions 24, 26, 28, 30. Therefore, the lens structure is a selection device which allows each subregion to be viewed at certain viewing angles. Each subregion 24, 26, 28, 30 includes a distinct 10 diffractive structure constituting a portion of the diffractive structure of the diffractive region 21. In an embodiment, the diffractive structure of the diffractive region 21 is entirely composed of the diffractive structures of the subregions 24, 26, 28, 30. In an alternative embodiment, the diffractive structure of the diffractive region 21 includes diffractive structure not associated with a subregion 24, 26, 28, 15 30, for example structure constituting transition zones between adjacent subregions 24, 26, 28, 30. The transition zones can be configured to provide a smooth transition in grating spacing between adjacent subregions 24, 26, 28, 30, or alternatively the transition zones can correspond to non-diffractive regions. Figure 4A shows the subregion 24 of the diffractive region 21 being 20 viewable which corresponds to the viewing angle 14 of Figure 1. Similarly, Figures 4B, 4C and 4D show subregions 26, 28 and 30 of the diffractive region 21 viewable at viewing angles 16, 18 and 20 of Figure 1, respectively. As discussed previously, each subregion 24, 26, 28, 30 is associated with a colour. The diffractive angle of the colour (in general, though not exclusively, 25 the first order diffraction angle) is configured to correspond to the focal angle of the lens when the lens is viewed at the viewing angle 14, 16, 18, 20. The focal angle can be the same as the viewing angle 14, 16, 18, 20 but this is not a requirement. For the purposes of the present discussion, each viewing angle 14, 16, 18, 20 has an associated focal angle and therefore diffraction angle to 30 achieve the correct colour. For convenience, the diffractive angle will be referred to as a viewing angle 14, 16, 18, 20, which is to be taken to be the diffractive angle associated with the viewing angle 14, 16, 18, 20.

9 Referring to Figure 5A, the diffractive region 21 can include a blazed diffraction grating 100. The blazed diffraction grating 100 includes regions of different grating spacing and blaze angle, corresponding to the subregions 24, 26, 28, 30. Referring to Figure 5B, the blaze angle 96 is defined as the angle of the 5 top surface of each grating element 98 as measured from the plane of the diffraction grating 100. The blaze angle 96 of each subregion 24, 26, 28, 30 can correspond to the viewing angle of the subregion 24, 26, 28, 30. A blazed diffraction grating 100 has the advantage of selecting a preferred diffraction angle, equal to the blaze angle, which corresponds to a diffraction 10 intensity maximum. Away from the blaze angle, the diffraction intensity is subdued when compared to a non-blazed diffraction grating with equivalent grating spacing. Therefore, each subregion 24, 26, 28, 30 of the blazed diffraction grating 100, as shown in Figure 5A, has a blaze angle 96 equal to the viewing angle 14, 16, 18, 20 of the subregion 24, 26, 28, 30. The result is an increased 15 visual effect (i.e higher intensity) when compared to a non-blazed grating. As a further benefit, cross-talk from adjacent subregions 24, 26, 28, 30 to the subregion associated with the particular viewing angle 14, 16, 18, 20, is much reduced. For example, when the optical security device 60 is viewed at viewing angle 16, cross-talk from subregions 24, 28 and 30 is reduced when a blazed 20 diffraction grating 100 is used when compared to a non-blazed diffraction grating. According to an embodiment, substantially the same colour is viewable at each viewing angle. To achieve this effect, a grating spacing of the diffractive structure corresponding to each subregion 24, 26, 28, 30 of the diffractive region 21 must be calculated. The grating spacing can be calculated from the following 25 equation: d= nA sin(-) where d is the groove spacing, c- is the required diffraction angle, n is the order number and A is the wavelength. For the purposes of the present discussion, the order number will be 1 for all calculations (i.e. each subregion 24, 30 26, 28, 30 will be configured to provide a first order diffraction effect at the corresponding viewing angle 14, 16, 18, 20).

10 Applying the equation to the subregion 24 described in relation to Figure 4A, for the subregion 24 to appear green (wavelength of -0.5 microns) at a diffraction angle of 15 degrees, the required groove spacing, Dgreen, is: D 0.5 _ 0.5 1.95pm sin(15 0 ) 0.257 5 Applying the equation to the subregion 26 described in relation to Figure 4B, for the subregion 26 to appear green at a diffraction angle of 30 degrees requires a groove spacing of: D 0.5 _ 0.5 D,, ____-_ - 1.00pmn sin(30 0 ) 0.5 Applying the equation to the subregion 28 described in relation to Figure 10 4C, for the subregion 28 to appear green at a diffraction angle of 45 degrees requires a groove spacing of: D 0.5 _ 0.5_= 0.707pm sin(45 0 ) 0.707 Applying the equation to the subregion 30 described in relation to Figure 4D, for the subregion 30 to appear green at a diffraction angle of 60 degrees 15 requires a groove spacing of: D 0.5 _ 0.5 = 0.578,um sin(60 0 ) 0.865 Referring to Figure 6, the diffractive region 21 can be chosen to represent individual pixels 32 of a larger security device image 34. For example, all pixels 32 can be the same size, such as, for example, 60 microns x 60 microns. In this 20 case, each subregion 24, 26, 28, 30 (see Figures 4A, 4B, 4C, and 4D) would be approximately 15 by 60 microns, if evenly spaced, which they do not necessarily require to be. Each pixel 32 may be arranged in an array such that each pixel 32 is adjacent another pixel 32 (as shown in Figure 6), or the pixels may be dispersed over the image layer 10. 25 According to an embodiment, an optically variable image may be created by selecting the colour associated with each subregion 24, 26, 28, 30 of each diffractive region 21 (pixel 32) from a selection of colours. In this way, each pixel 32 appears as a particular colour for each viewing angle 14, 16, 18, 20.

11 For example, with reference to Figures 7A, 7B, 7C, and 7D, an optically variable image 35 may be devised by selecting a first colour 36 (say, for example, green) and a second colour 38 (say, for example, red). For each viewing angle (150 in Figure 7A, 300 in Figure 7B, 450 in Figure 7C, and 600 in Figure 7D), each 5 subregion 24, 26, 28, 30 of each diffraction region 21 (pixel 32) is associated with either the first colour 36 or the second colour 38, and the grating spacing of each subregion 24, 26, 28, 30 is selected based on whether the particular diffraction region 21 (pixel 32) is to show the first colour 36 or the second colour 38 at the associated viewing angle 14, 16, 18, 20. So, for example, a particular pixel 39, 10 which is at the centre of the image 35, is green at viewing angles 14, 16 and 18, but red at viewing angle 20. The required grating spacing for a green subregion 24 is 1.95 pm, and 2.32 pm for a red subregion 24 (corresponding to a viewing angle 14 of 150). The required grating spacings for each of the subregions 24, 26, 28, 30 to display the 15 colour red can be calculated from: D - 15 0.6 - 0.6 rD,5 0 s 2.32pm~ sin(15') 0.257 0.6 0.6 sin(30') 0.5 D,,45- 0.6 _ 0.6 = 0.849,um sin(45 0 ) 0.707 D,, 600 0.6 0.6 = 0.693pm sin(60') -0.866 20 In summary, for each viewing angle 14, 16, 18, 20, the corresponding subregion 24, 26, 28, 30 of each diffraction region 21 (pixel 32) has a grating spacing according to the desired colour of the diffraction region 21 (pixel 32) according to the table below. Viewing Angle Subregion Grating Spacing for a Pixel to Appear as Colour Green Red 150 1.95 pm 2.32 pm 300 1.00 pm 1.20 pm 450 0.707 pm 0.849 pm 600 0.578 pm 0.693 pm 12 In general, the number of colours available for each subregion can be limited, for example limited to a choice between red, green, and blue, or unlimited by selecting the groove spacing to correspond to any desired colour. Referring now to Figures 8A, 8B, 8C, and 8D, an optical security device 60 5 is shown including a plurality of diffraction regions 21 (pixels 32), each including four subregions 24, 26, 28, 30 and a plurality of lenses 40, which act as a selection device. At a first viewing angle 14, as shown in Figure 8A, the lenses 40 are configured to focus on a first subregion 24 of each diffraction region 21 (pixel 32). At a second viewing angle 16, as shown in Figure 8B, the lenses 40 are 10 configured to focus on a second subregion 26 of each diffraction region 21 (pixel 32). At a third viewing angle 18, as shown in Figure 8C, the lenses 40 are configured to focus on a third subregion 28 of each diffraction region 21 (pixel 32). Finally, at a fourth viewing angle 20, as shown in Figure 8D, the lenses 40 are configured to focus on a fourth subregion 30 of each diffraction region 21 15 (pixel 32). The overall effect is that the lenses 40 provide a different composite image in discrete steps as the viewing angle 14, 16, 18, 20 is changed. There can be one lens associated with each diffraction region 21. Each lens may be associated with a diffraction region such that each lens is in a fixed relative position with respect to the associated diffraction region. 20 Referring to Figure 9, each lens 40 can optionally be associated with two (or more) diffraction regions 21. For example, when a lens 40 is associated with two diffraction regions 21, the lens 40 is configured to view a first diffraction region 71 when viewed from a set of viewing angles 14, 16, 18, 20 on one side 82 of the normal 12 to the image layer 10, and to view a second diffraction region 72 25 when viewed from the complementary set of viewing angles 74, 76, 78, 80 located on the other side 84 of the normal 12 to the image layer 10. In this manner, the diffraction gratings 100 of each diffraction region 21 can be blazed, while the optical security device 60 can provide an optical effect over a larger range of viewing angles (e.g. viewing angles 14, 16, 18, 20, 74, 76, 78, 80). The 30 image shown on either side of the normal can be the same or different for each viewing angle. For each viewing angle 14, 16, 18, 20, each subregion 24, 26, 28, 30 can be configured to display a desired brightness, such that the brightness of each 13 diffraction region 21 (pixel 32) can vary, both with viewing angle 14, 16, 18, 20 and across the image. This can be achieved, as shown in Figure 10, by reducing the area covered by the diffraction structure of a subregion 24, 26, 28, 30, such that each subregion 24, 26, 28, 30 includes diffractive 42 and non-diffractive 44 5 components. The brightness of each subregion 24, 26, 28, 30 is proportional to the ratio of diffractive 42 to non-diffractive 44 components within the subregion 24, 26, 28, 30. The non-diffractive components 44 can be light absorbing, such that incident light is not reflected back to the viewer by the non-diffractive components 44. 10 Referring to Figure 11, in an embodiment, the diffraction regions 21 (pixels 32) are configured to diffract visible wavelengths, and the diffraction regions 21 (pixels 32) are smaller than the resolving ability of the eye or at least are small enough such that diffracted light from adjacent diffraction regions 21 (pixels 32) appear to mix together. In this embodiment, the diffraction regions 21 (pixels 32) 15 can be arranged into groups (shown spaced apart in Figure 11, though may instead be adjacent one another), wherein each diffraction region 21 (pixel 32) is associated with one of a selection of colours, for example each group includes four diffraction regions 21 (pixels 32) - red 85, two green 86, 87, and blue 88. The brightness of each subregion 24, 26, 28, 30 is controlled, allowing for an image 20 consisting of the combination of the component colours for each viewing angle 14, 16, 18, 20. The result is the appearance of a true colour image with variations in both perceived colour and intensity. Referring now to Figure 12, a security document 58 is provided according to an embodiment including a security element 60 and optionally one or more 25 secondary elements 62. The security document 58 includes a substrate 64 of transparent plastic material and one or more opacifying layers on each side of the substrate 64. The transparent substrate 64 is preferably formed from a transparent polymeric material such as a laminated structure of two or more layers of bi-axially oriented polypropylene. It will, however, be appreciated that 30 other transparent or translucent polymeric substrates 64 may be used in the present invention such as polyethylene and polyethyleneterephthalate (PET). The opacifying layers may include one or more coatings of opacifying ink applied to opposite sides of the substrate 64. Alternatively, the opacifying layers may be 14 formed from layers of paper or other opaque material laminated to opposite sides of the substrate 64 to form a hybrid substrate 64. The security element can be located in a window or half-window region defined by the absence of the opacifying layer in a region of the substrate 64 on both or one side of the 5 substrate 64, respectively. The one or more optional secondary elements 62 can occupy window or half-window regions of the substrate 64, and can be chosen from a suitable security or non-security features, including: micromirror reflective or refractive elements; Fourier plane elements; flat elements arranged to provide a mirror 10 reflection of a user to the user; lenslet elements of a lenslet array; and diffractive elements. According to an embodiment as shown in Figure 13, the substrate 64 is embossable on both surfaces 66, 68, either directly or due to the application of an embossing layer. Both sides 66, 68 are embossed at the same time, one with the 15 lenses and the other with the diffractive grating, therefore ensuring correct relative alignment between the diffractive grating and the lenses. The embossing can be achieved by pressing an embossing surface of an embossing device 69, 70 into the surfaces 66, 68. Where an embossing layer is applied to the surface of the substrate, the embossing layer can correspond to a UV curable layer, and the 20 embossing step includes the step of curing the UV curable layer, either while the embossing device 69, 70 is pressed into the embossing layer, or directly after removal of the embossing device 69, 70. The UV curable layer can correspond to an embossable radiation curable ink, which is an ink, lacquer or other coating which may be applied to the 25 substrate 64 in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is 30 curable by ultraviolet (UV) radiation. The radiation curable ink is can be a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as lens structures, 15 or may be further coated by a reflective coating to creating a reflective diffractive device. In another possible embodiment the radiation curable ink may include metallic particles to form a metallic ink composition which is both printable and 5 embossable. Such a metallic ink composition may be used to print a reflective security element, such as a diffraction grating or hologram. Alternatively, a transparent ink, e.g. formed from a clear resin, may be applied on one side of the substrate, with or without an intermediate primer layer, the transparent ink then being embossed and cured with radiation and a metallic ink composition 10 subsequently applied to the embossed transparent ink in a printing process, if it is desired to form a reflective security element as part of the security device. Referring to Figure 14, an embodiment is shown wherein the lens structure is replaced by a mask 94. The mask 94 includes blocking regions 92 and non 15 blocking regions 90. The blocking regions 92 are configured to block diffracted light leaving the diffraction structure; however, the blocking regions 92 can be configured to allow incident external light through. The non-blocking regions 90 are aligned in an offset manner from the diffraction regions 21 (pixels 32), such that each non-blocking region 90 is associated with a particular diffraction region 20 21 (pixel 32). The arrangement of blocking regions 92 and non-blocking regions 90 is such to allow only a portion of each diffraction region 21 (pixel 32) to be viewable at each viewing angle 14, 16, 18, 20, each portion corresponding to a subregion 24, 26, 28, 30 of the diffraction region 21 (pixel 32). In this manner, the mask 94 25 acts in a similar manner to the lens structure, by selecting a subregion 24, 26, 28, 30 to be visible to a viewer. Further modifications and improvements may be made without departing from the scope of the present invention. For example, the number of subregions may be expanded such that the diffraction surface consists of a continuous or 30 substantially continuous change in groove spacing from one side of the diffraction surface to the other. In another example, the lens array is a lenticular lens array. In a further example, the selection device, described above as a lens structure or 16 lens array, is any suitable structure and/or device for presenting to a viewer only a subregion of each diffractive region (pixel) for each viewing angle.

Claims (5)

1. An optical security element including: a) a diffractive region; and b) a selection device configured such that the diffraction region is viewable through the selection device, wherein the diffractive region includes a plurality of subregions, wherein each subregion is associated with a particular viewing angle, and wherein each subregion is configured to diffract a particular wavelength or range of wavelengths at a diffraction angle corresponding to the associated viewing angle, and wherein the selection device is configured to display, at each viewing angle, the subregion associated with the viewing angle.

2. An optical security element as claimed in any one of the previous claims, wherein the diffraction region includes a blazed diffraction grating, and wherein the blaze angle of the diffraction grating for each subregion is equal to the diffraction angle associated with the subregion.

3. An optical security element as claimed in any one of the previous claims, wherein the selection device includes a lens structure, and wherein, for each viewing angle, the lens structure is configured to focus on the associated subregion of the diffractive region.

4. A method for producing an optical security device including an arrangement of optical security elements according to any one of the previous claims including the steps of: a) providing a transparent or translucent substrate including first and second embossable surfaces; and b) simultaneously embossing the first surface with a diffraction pattern and the second surface with a microlens array such that the lens array and the diffraction pattern are in registration and the diffraction pattern is viewable through the microlens array. 18

5. A method as claimed in claim 4 including the step of applying a first opacifying layer to the first surface that includes a window region corresponding to the microlens array.