GB2382546A

GB2382546A - Security device incorporating light collecting dye

Info

Publication number: GB2382546A
Application number: GB0124337A
Authority: GB
Inventors: Robert Tweed Dick
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2001-10-10
Filing date: 2001-10-10
Publication date: 2003-06-04
Anticipated expiration: 2021-10-10
Also published as: GB2382546B; AU2002301508B2; GB0124337D0

Abstract

A security design 2, such as a guilloche or latent image, is formed by printing or embossing on a substrate 1 using an ink or other material which includes a light collecting dye. When light from a source 3 is incident on the design 2 it is absorbed by the light collecting dye which then fluoresces so as to emit light at a (visible) wavelength. This is internally reflected within the material 4 until it emerges from the edges 5,6. Little fluorescent light is emitted through major surface 7. Thus the design 2 is only faintly visible when viewed normally. It becomes more visible when viewed at a shallow angle 8. The effect may be enhanced by creating multiple upstanding edges (25, figure 5) through which light emission is concentrated resulting in a broad image when viewed at the shallow angle.

Description

SECURITY DEVICE The invention relates to a security device, typically for authenticating an article, such as a security document, on which it is provided.

A wide variety of optical security devices have been developed to protect security documents and other articles against unauthorised reproduction, particularly counterfeiting. These often involve the use of holograms or diffraction gratings which make documents very difficult to reproduce by photocopying and the like.

As counterfeiters develop more sophisticated techniques for overcoming these problems, there is a continuing need to develop new security devices.

In accordance with the present invention, a security device comprises a security design formed in a material including a light collecting dye, the material and dye cooperating together, in use, to collect incident light and to cause light to be internally reflected in the material and emitted at edges of the material.

We have realised that a new type of security device can be constructed using a combination of a light collecting dye and a suitable material in which the dye is provided which concentrates light output along edges of the material. This assists in enabling the device easily to be authenticated. In addition, in some examples, a latent image can be enhanced making the device very difficult to copy while the structure of the device itself is difficult to reproduce.

The device can be fabricated in a variety of ways.

For example, the material may comprise an ink printed or otherwise provided on a substrate. This printing may itself define a security design but in the preferred approach, a relief structure is formed in the material, typically by embossing. Thus, in one example, a light collecting dye is added to a resin which is coated onto a substrate and this is then embossed by use of a blind

embossing technique on an intaglio press. Alternatively, the light collecting dye could be added to a screen/flexo/gravure ink system and printed as a discrete patch or print, which is subsequently embossed. Another approach would be to use a coarse screen print design or any print process capable of laying down a thick film of ink and use edges of the print i. e. without any embossing step.

Any embossing carried out does not need to be restricted to the use of an intaglio press but any suitable technique could be used. Embossing can take place before or after application of the ink or coating containing the light collecting dye.

In an alternative approach, the material comprises a substrate. This is particularly advantageous where it is desired to incorporate the security device as an integral feature of the article or document to be protected. The light collecting dye may be diffused into or coated on the substrate and preferably the substrate is transparent or semi-transparent. By semi-transparent we mean with sufficient clarity to still allow internal reflection to occur within the substrate or at least collection of light through the substrate to allow the desired edge effect.

In this case, the substrate will typically be provided with a security design in the form of a relief pattern which again may be embossed into it.

As mentioned above, by suitably constructing the security design (with or without embossing) it is possible to view the bright edges of the design from specific view points which leads to the ability to generate a latent type image. Thus, holding the security device normally, either viewing in reflection or transmission, the colour will be weak or non-apparent while holding the device at a shallow angle will enable a bright intense colour to be observed. The effect is enhanced by holding the device facing a light source viewing the edge effect at a shallow angle with a dark background behind it.

A further benefit is achieved by placing the material next to an unprinted area of a document close to its edge. Any light collected by the material is then transmitted by total internal reflection to the edge of the document.

The cooperation between the material and light collecting dye can be achieved in a variety of ways. In the preferred approach, the light collecting dye comprises a substance which fluoresces in response to incident visible light. By suitably choosing the material, the interface between the material and air together with the fluorescence wavelength can be selected so that internal reflection is caused. However, light collecting dyes which emit light at wavelengths similar to the incident light could also be used.

Any conventional security design can be implemented with this invention, examples including security indicia such as guilloche patterns, microprint, fine line, latent images and the like.

The security device can be provided as a label which is affixed to an article or may be formed as part of the article. Articles such as packaging are envisaged but the invention is particularly suitable for use with security documents including any document of value such as banknotes, cheques (whether bank or travellers), bonds, share certificates, licences, some types of identity cards, passports, visas, tickets, passbooks, vouchers, deeds, tamper revealing seals and labels, brand authenticity marks or labels and the like. The device may also of course be used on conventional plastic card materials such as relatively rigid PVC.

Some examples of security documents according to the invention will now be described with reference to the accompanying drawings, in which :- Figure 1 is a schematic, cross-section through a first example;

Figure 2A and Figure 2B illustrate the appearance of the document shown in Figure 1 under different viewing conditions ; Figure 3 is a view similar to Figure 1 but of a second example ; and, Figures 4 to 8 are views similar to Figure 1 but of third to seventh examples respectively.

The example shown in Figure 1 comprises an opaque, plastic or paper substrate 1 having a thickness in the range 75-120 microns and on which is printed using a conventional printing technique such as screen/flexo/gravure etc. a security design 2. The substrate may require pre-coating to allow a base for the dye containing ink to lay on. This prevents loss of edge height due to absorption into the substrate. The design 2 is printed with an ink comprising a colourless and transparent medium (or vehicle) into which a light collecting dye is incorporated, the ink having a thickness of 5-50 microns, preferably 10-15 microns. Suitable dyes are described in US-A-4492648. When light from a source 3 is incident on the printed ink 2 it is absorbed by the light collecting dye which then fluoresces so as to emit light at a different, visible wavelength which is then internally reflected within the ink thickness as shown schematically at 4 until it exits at edges 5,6.

Since little fluorescent light is emitted through the major surface 7, when the document is viewed normally (Figure 2A), the security design is only faintly visible but when the document is viewed at a shallow angle as shown at 8 the design becomes visible (Figure 2B). In practice when the ink is coated over a transparent surface the coating is"substantially invisible"but when coated onto an opaque background the coating is lightly coloured. If the embossed design contains areas of solids, these can be made to emit colour from the faces of the solids by introducing a fine embossing over the surface of the face

e. g. using an ink retaining screen as found in intaglio plates.

Internal reflection is caused by selecting a material for the ink 2 which is optically denser than air, the refractive index of the ink being selected so that the majority of the fluorescent light is internally reflected.

In the Figure 3 example, the opaque substrate 1 is replaced by a transparent, plastics substrate 10 having a similar thickness. A similar security design 2 is printed on the substrate 10. In this example, light from the source 3 is collected not only by the ink 2 but also from other parts of the transparent substrate and is channelled to the ink 2 where it is absorbed by the light collecting dye. Some internal reflection of the resultant, fluorescent light will thus occur within the substrate 10 as shown at 11 as well as within the ink layer 2. The result is a much stronger and dramatic effect when the document is viewed at the shallow angle 8.

In the examples described so far, the ink layer 2 has been printed in the form of the security design on the substrate. It would also be possible to print a substantially continuous ink layer 2 on the substrate and then provide it with a relief by, for example, embossing.

In that case, when viewed at the shallow angle 8 a bright image will be viewed as a result of light being emitted through the embossed edges.

In the Figure 4 example, the light collecting dye 20 is absorbed into a transparent substrate 21. When illuminated by the source 3, the light collecting dye will fluoresce as before and internal reflection as shown at 22 will convey light to the edges 23 of the dye and also to the edges of the substrate. This will achieve a limited effect when viewed at the shallow angle. A much more enhanced effect, as shown in Figure 5, can be achieved by embossing the diffused dye 20 as shown at 24. This creates upstanding edges 25 through which light emission is

concentrated thus leading to a broad image when viewed at the shallow angle.

A variety of security designs can be printed and/or embossed including conventional security features as guilloche patterns but also other known latent images and simple features such as indicia or logos. The presence of the design could be hidden by incorporating it in conjunction with a secondary relief or print working to

disguise its presence.

Example A 0. 1% Lumogen F Red 300 (available from BASF) was < added to a Sericol Metallic Ink Medium C0382 and coated

onto a 80/im polypropylene substrate using a kbar no. 6. The resultant coating thickness is approximately 15 m. The uncoated side was then embossed by passing through an intaglio proofing press.

A further example is illustrated in Figure 6. In this case, the light collecting dye 20 is printed onto one surface 30 of a transparent substrate 21. The opposite surface 31 is embossed at 32. Under normal illumination from light source 3, the light collecting dye will fluoresce as before and internal reflection will occur within the dye 20. Some light will leak across the transparent substrate 21 to the embossing 32 where it will become visible when the device is viewed at a shallow angle as seen at 33.

Another example of the invention which may constitute a polymer banknote is shown in Figure 7. In this example, a transparent substrate of orientated polypropylene 40 is provided having a thickness of about 75 microns and, on its rear surface, being provided with a pair of opacifying coatings 41,42. The front surface of the substrate 40 carries a first opacifying coating 43 having a thickness of about 1 micron and a second opacifying coating 44 also having a thickness of about 1 microns but defining a window 45 into which a light collecting dye layer 46 is provided.

A further example is shown in Figure 8 in which a partially metallised layer 50 is provided on a PET or oriented polypropylene layer 51, a light collecting dye layer 52 being provided on the partial metallisation. In one case, this would correspond to the provision of the light collecting dye on a security thread.

Claims

CLAIMS 1. A security device comprising a security design formed in a material including a light collecting dye, the material and dye cooperating together, in use, to collect incident light and to cause light to be internally reflected in the material and emitted at edges of the material.
2. A device according to claim 1, wherein the material comprises an ink provided on a substrate.
3. A device according to claim 1, wherein the material comprises a substrate, the dye being coated on the substrate.
4. A device according to claim 1, wherein the material comprises a substrate, the light collecting dye being diffused into the substrate.
5. A device according to any of claims 2 to 4, wherein the substrate is transparent or semi-transparent.
6. A device according to claim 5, wherein the substrate is a plastics material.
7. A device according to any of claim 2 to 6, wherein the substrate is coated.
8. A device according to any of the preceding claims, wherein the security design is in the form of a relief pattern.
9. A device according to claim 8, wherein the security design is embossed into the material.
10. A device according to any of claims 1 to 7, wherein the security design is a printed pattern.
11. A device according to any of the preceding claims, wherein the light collecting dye comprises a substance which fluoresces in response to incident visible light.
12. A security device according to any of the preceding claims, wherein the security design comprises one or more of security indicia guilloche, fine line, microprint, latent, embossed, or printed structures.

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13. A security device substantially as hereinbefore described with reference to any of the examples shown in the accompanying drawings.
14. An article provided with a security device according to any of the preceding claims.
15. A document provided with a security device according to any of claims 2 to 7, wherein the substrate of the security device is provided by a substrate of the document.
16. A document according to claim 14, wherein the document comprises one of a banknote, cheque (whether bank or travellers), bond, share certificate, licence, identity card, passport, visa, ticket, passbook, voucher, deed, tamper revealing seal or label, and brand authenticity mark or label.