DIFFRACTIVE DEVICE WITH ENHANCED ANTI-COPYING PROTECTION

Abstract: When a diffractive device (2) is illuminated by a light source, it generates one or more diffraction images which are observable from particular ranges of viewing angles around the device. The device comprises a substrate (6), a diffractive structure (3) embossed into or adhered onto the substrate, a first substantially transparent layer (7) over the diffractive structure, light splitting means (8, 10, 11) over the first substantially transparent layer, and a second substantially transparent layer (9) over the light splitting means. The light splitting means protects the diffractive device against replication using near field or far field imaging procedures which require coherent light illumination.

Description: DIFFRACTIVE DEVICE WITH ENHANCED ANTI-COPYING PROTECTION This invention relates to a diffractive device with enhanced anti-copying protection. It relates particularly but not exclusively to a diffractive device whose diffractive structure is protected against replication using near-field or far-field imaging procedures which require coherent light illumination. It has recently become common for diffractive devices to be used as an anti-forgery security measure on bank notes, credit cards, cheques, share certificates and other similar documents. It is relatively easy for a putative forger using a colour photocopier and other commonly available technology to create a passable imitation of a bank note or other valuable document, but it is more difficult to reproduce a diffractive device attached to the surface of the bank note or other document. A diffractive device typically generates an optically variable diffractive image, such that, when the orientation of the diffractive device is moved relative to the observer and/or source of illumination, the image changes. Well known examples of this are the holographic devices embodied in Visa credit cards and Mastercard credit cards. It is of course much more difficult to reproduce a diffractive device which embodies particular optically variable images than it is to produce a colour photocopy of a bank note. However, it is possible for a determined forger to make a reproduction of a diffractive device which is of acceptable quality. Such a reproduction requires the making of a fairly accurate image of the diffractive structure of the diffractive device, and requires the use of a coherent illumination source because of the finely detailed structure of a diffractive surface. Incoherent illumination techniques do not result in acceptable quality reproduction. It is an object of the present invention to provide a diffractive device which is protected against imaging techniques which use coherent illumination. According to the present invention, there is provided a diffractive device which, when illuminated by a light source, generates one or more diffraction images which are observable from particular ranges of viewing angles around the device, comprising: (a) a substrate; (b) a diffractive structure embossed into or adhered onto the substrate; (c) a first substantially transparent layer over the diffractive structure; (d) light splitting means over the first substantially transparent layer; and (e) a second substantially transparent layer over the light splitting means. The diffractive structure on the substrate may be any suitable type of diffractive structure. In some embodiments it may comprise a patterned surface relief structure. In other embodiments such as reflective volume holograms, the diffractive structure may have no surface relief structure. The light splitting means causes partial reflection of light reflected from the diffractive structure, resulting in a multiplicity of the original image (at least two images and possibly more, depending upon the characteristics of the light splitting means). The light splitting means can be designed such that the multiple image interference effect is almost unperceivable when the diffractive device is illuminated by ordinary light, but the interference effect becomes quite pronounced when illuminated by coherent radiation, thereby preventing imaging of acceptable quality using coherent radiation techniques. The light splitting means may be any suitable light splitting means. One preferred type of light splitting means is a partially reflecting plane. A partially reflecting plane may be substantially uniform, or it may vary in degree of reflectivity over the plane in such a way as to produce a visually observable result under coherent illumination, such as a simple message or line indicating non- originality. Another preferred type of light splitting means is a second diffractive structure. A second diffractive structure may comprise wholly or partially opaque lines forming a grating or shapes arranged in any pattern which achieves a suitable diffractive result. Alternatively, a second diffractive structure may comprise a three-dimensional patterned relief structure, which may optionally itself generate one or more diffraction images. The invention will hereinafter be described in greater detail by reference to the attached drawings which show an example form of the invention. It is to be understood that the particularity of those drawings does not supersede the generality of the preceding description of the invention. SlJBSTITϋTE SHEET Rule 26 Figure 1 is a sketch of a credit card incorporating a diffractive device. Figure 2 is a cross-sectional view of a diffractive device according to the prior art. Figure 3 is a cross-sectional view of a diffractive device according to an embodiment of the present invention. Figure 4 is a cross-sectional view of a diffractive device according to an alternative embodiment of the present invention. Figure 5 is a cross-sectional view of a diffracting device according to a further alternative embodiment of the present invention. Figure 1 shows a typical valuable document, in this case a credit card 1, with diffractive device 2 attached. Although it is a relatively easy operation to make a passable reproduction of document 1 , it is a more difficult operation to make a passable reproduction of diffractive device 2, especially if diffractive device 2 incorporates optical variability, so that the images generated are observed to change when the orientation of diffractive device 2 is changed relative to a source of elimination and/or position of the observer. Figure 2 shows a schematic cross-section of a typical prior art diffractive device 2. Typically, the diffractive surface 3 of a diffractive device 2 is produced from a master by a hot rolling or hot stamping process. Diffractive surface 3 typically consists of a large number of grooves arranged in predetermined patterns in order to generate predetermined diffractive effects. The grooves or gratings typically have spatial frequencies between 500 and 2,000 groove per millimetre. The surface pattern is typically embossed into a soft lacquer 4 which is about Iμm thick, and then coated with aluminium to form diffractive surface 3. Through a hard lacquer layer the diffractive surface is then fused onto a transparent polyester carrier (generally 10-20μm thick) from one side, while from the other side it is glued with an adhesive onto a substrate 6. Although transparent polyester layer 5 effectively prevents mechanical reproduction of diffractive surface 3, coherent illumination near-field and far-field imaging techniques are capable of producing images of diffracting surface 3 which can then be used to make a low quality but still passable reproduction of diffracting surface 3. Figures 3, 4 and 5 show cross-sectional views of different embodiments of a diffractive device according to the present invention. Diffractive device 2 comprises a substrate 6, a diffractive structure 3 embossed into or adhered onto substrate 6, first substantially transparent layer 7 over patterned diffractive surface relief structure 3, light splitting means 8,10,11 over first substantially transparent layer 7, and second substantially transparent layer 9 over light splitting means 8,10,11. Diffractive structure 3 may be any suitable structure. It may be a diffractive structure of the surface relief type as illustrated, or it may have no surface relief structure at all. Most commercially available diffractive surface relief structures are comprised of line gratings, but diffractive effects are not confined to line gratings and other structures such as concentric circular grooves and polygonal indentations or projections can also be used to produce diffraction images. The diffraction images may be holograms or other optically variable images. Diffractive structure 3 may be adhered onto substrate 6 as shown in each of the illustrated embodiments, or alternatively it may be embossed directly into substrate 6. Diffractive structure 3 may be made from any suitable materials. It is preferred that diffractive structure 3 be substantially reflective, and aluminium is one suitable choice as a material for the surface relief structure. First substantially transparent layer 7 may be made of any suitable material. Polyester is one suitable type of material. The thickness of first substantially transparent layer 7 is preferably between Iμm and 5μm, and more preferably about 3μm. Light splitting means 8,10,11 may be any means suitable for dividing the wave front emerging from the diffractive device. In the embodiment illustrated in Figure 5, light splitting means 11 comprises a partially reflective surface. It is preferred that the degree of reflectivity be fairly low, and of the order of 5-10%. Partially reflecting plane 11 may be achieved by ensuring that first substantially transparent layer 7 and second substantially transparent layer g have differing refractive indices. Alternatively, partially reflective plane 11 may comprise reflective material such as a semi-transparent metallic coating. In the embodiment illustrated in Figure 3, light splitting means 8 comprises a diffractive structure having opaque or partially opaque lines and/or shapes. In the embodiment illustrated in Figure 4, light splitting means 10 comprises a second patterned diffractive relief structure. In either case, whether the light splitting means is a two-dimensional or three-dimensional diffractive structure, the effect is that multiplicity of the wave front emerging from the diffractive device occurs, with resulting interference. Where light splitting means 10,11 comprises a diffractive structure, the diffractive structure may be a regular structure designed to give a predetermined interference effect, or alternatively it may be designed so as itself to generate one or more diffractive images observable from particular ranges of viewing angles around the diffractive device. Similarly, where the light splitting means is a partially reflecting plane 11, the plane may be substantially uniform in its reflectivity characteristics or it may be varying so as to achieve a predetermined effect which may be observable on viewing the diffractive device. Second substantially transparent layer 9 may be made from any suitable material or materials. Polyester is a suitable material. It is preferred that the constituents of second substantially transparent layer 9 differ slightly from the constituents of first substantially transparent layer 7 so that the refractive indices of layers 7 and 9 differ in a predetermined manner. Second substantially transparent layer 9 is preferably between 3μm and 30μm in thickness, and more preferably about 10μm. The effect of the reflections produced by the light splitting means is that the image produced by the diffractive surface is doubled with some phase shift which is determined by the thicknesses involved. The coherent addition of the two images results in broad interference bands in which the information from the two images arrives in anti-phase. These bands will be clearly visible on any reproduction made by the use of coherent illumination. On the other hand, when the diffractive devices of the present invention are observed in incoherent light, the doubled images add incoherently and the displacement (a few μm) between the two images is insufficient to be resolved by the eye of the observer. It is to be understood that various alterations, additions and/or modifications may be made to the parts previously described without departing from the ambit of the invention.

Claims: CLAIMS: 1. A diffractive device which, when illuminated by a light source, generates one or more diffraction images which are observable from particular ranges of viewing angles around the device, comprising: (a) a substrate; (b) a diffractive structure embossed into or adhered onto the substrate; (c) a first substantially transparent layer over the diffractive structure; (d) light splitting means over the first substantially transparent layer; and (e) a second substantially transparent layer over the light splitting means. 2. A diffractive device according to claim 1 wherein a pronounced interference effect between light reflected from the light splitting means and light reflected by the diffractive structure is observable when the device is illuminated by coherent radiation, but little or no interference effect is observable when the device is illuminated by non-coherent radiation. 3. A diffractive device according to claim 2 wherein copies of the diffractive device made by illuminating the diffractive device incorporate the interference effect in their copied diffractive structures, indicating to an observer that they are copies. 4. A diffractive device according to claim 2 or claim 3 wherein the interference effect is a legible message. 5. A diffractive device according to any one of claims 1 to 4 wherein the light splitting means is a partially reflective interface. 6. A diffractive device according to claim 5 wherein the degree of reflectivity varies over the interface to produce a visually observable effect under coherent illumination conditions. 7. A diffractive device according to any one of claims 1 to 4 wherein the light splitting means is a second diffractive structure. 8. A diffractive device according to claim 7 wherein the second diffractive structure comprises a three-dimensional patterned relief structure. 9. A diffractive device according to claim 7 wherein the second diffractive structure comprises substantially two-dimensional curved or straight substantially parallel lines. 10. A diffractive device according to claim 7 wherein the second diffractive structure comprises an array of substantially two-dimensional shapes .arranged in a diffracting pattern. 11. A diffractive device according to any one of claims 7 to 10 wherein the second diffractive structure itself generates one or more diffraction images. 12. A diffractive device according to any one of claims 1 to 4 wherein the light splitting means is the interface between two substantially transparent layers which have different refractive indices. 13. A diffractive device according to any one of claims 1 to 12 wherein the light splitting means has a reflectivity of the order of 5 % to 10 %.

Inventors: HEGEDUS ZOLTAN, HEGEDUS, ZOLTAN
Applicants: COMMW SCIENT IND RES ORG, HEGEDUS ZOLTAN, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR, HEGEDUS, ZOLTAN
Application year: 1995
Main classification: B42D15
IPC codes: B42D15, G02B5, G03H1, G06K19