GB2448604A

GB2448604A - Copy-Protected Documents

Info

Publication number: GB2448604A
Application number: GB0806995A
Authority: GB
Inventors: Jack Silver; Robert Withnall; George Fern; Emma Barrett; Paul Marsh; Neil Macnab
Original assignee: Governor and Co of Bank of England
Current assignee: Governor and Co of Bank of England
Priority date: 2007-04-18
Filing date: 2008-04-17
Publication date: 2008-10-22
Anticipated expiration: 2028-04-17
Also published as: GB0806994D0; GB0806995D0; GB201200640D0; GB2448605B; GB2484233A; GB2448604B; GB0806998D0; GB2448605A; GB0707420D0; GB2448606A; GB2484233B; GB0806996D0; GB2448603B; GB2448603A

Abstract

The invention relates to documents of value incorporating features that prevent them being accurately copied by image reproduction means such as a scanner or photocopier. A document of value (10) bears an image comprising a material doped with a rare-earth element wherein the image has a first appearance under daylight and has a second appearance under synthetic white light. A copy (14) of the original document (10) is not colour identical to the original document. Typical colour changes are from yellow/green to yellow or orange to pink and purple to pale green when reproduction is attempted. Preferred rare-earth elements are erbium, neodymium and holmium which are doped into an yttium lattice. An appropriate printing ink is also disclosed.

Description

Title: CQpvProtected Documents

Field of the Invention

This invention relates to documents incorporating features that prevent them from being accurately copied by an image reproduction means, such as a scanner or photocopier.

Background to the Invention

Documents of value, such as banknotes and bank drafts, incorporate a variety of security features which are difficult to replicate. Image reproduction means, such as photocopiers or scanners, have the capability to replicate an image to a fair degree of accuracy. Documents of value are protected from copying by incorporating features that do not reproduce when the original document is copied by an image reproduction means. These copy-protected documents rely on a number of different features to prevent copying, some of which are based around the use of variable line spacing and variable colour density. When copied, the resolution of the image production means is not sufficient to copy the features accurately, and in some cases the copy reveals a hidden message warning of the non-authenticity of the document. However, image reproduction means are operating to ever improving resolution making these features less effective.

It is an aim of the present invention to provide copy-protected documents with improved security features.

$wnmary of the Invention In accordance with the present invention, there is provided a document of value bearing an image comprising a material doped with a rare-earth element wherein the image has a first appearance under daylight and has a second appearance under synthetic white light, wherein the first and second appearances are different.

It is known that synthetic white light, as generated by an image reproduction means, typically has a spectra which is quite different to that of daylight. However for most documents this difference in spectra does not produce any noticeable differences in colour between the original and the copy. The present inventors have discovered that certain rare-earth doped materials can appear as a different colour under synthetic white light and that this property can be used to indicate that copying has taken place.

Thus, attempts to copy such a document by use of a device which illuminates the document with synthetic white light (e.g. photocopiers and scanners) will result in the image being reproduced as a different colour to that in the original. Copies can therefore be detected by visible inspection of the document without the need for a detection apparatus.

At least one of the first and second appearances is as a visible colour, one of the appearances possibly being non-visible. However, preferably the first appearance is a first visible colour and the second appearance is a second visible colour. The first and second colours are different colours and enable the image to be visible under daylight and synthetic white light respectively. The first and second colours may each be any visible colour, including white or black, provided only that the image contrasts with the surrounding part of the document so that it is visible. For example a white image on a green document is a visible image, whereas a white image on a white background is not a visible image.

The change in appearance may arise directly from the change in emission of the doped material. Alternatively the doped material may be added to a substance to produce different colour changes than those exhibited by the doped material on its own. In this way a doped material exhibiting one colour change can be used to produce a range of colour changes. For example a doped material changing from yellow to transparent mixed with a substance coloured blue gives a green to blue colour change but when added to a substance coloured red gives an orange to red colour change.

The change in the colour of light emitted by the doped materials is due to narrow absorption bands in the visible region of the doped material. Absorption of light at or around a main excitation radiation wavelength of synthetic white light can give emission of that light at a different wavelength, which has the effect of the doped material, and therefore the image it is present in, having a different appearance, e.g. having a different colour.

Therefore, preferably the doped material has an absorption band maximum at a first wavelength in the visible spectrum and an associated emission band maximum at a second wavelength in the visible spectrum, different to the first wavelength.

Daylight includes radiation at significant levels at all frequencies in the visible spectrum and beyond. Typical light emitting diode (LED) scanners produce synthetic white light having main excitation wavelengths around 465, 525 and 635nm, which when combined produce a synthetic white light. Photocopiers and scanners operating with fluorescent light have main excitation radiation wavelengths at around 436nm, 490nm, 546nm, 58mm and 613nm, again resulting in a synthetic white light. The synthetic white light may be TL83 or TL84.

The doped material according to the invention is preferably responsive to LED scanners and reproduction means based on cold cathode fluorescent light or ultraviolet light to produce the colour change, i.e. by absorbing light at one of the main excitation wavelengths and re-emitting it at a different wavelength.

In order to exploit these main excitation radiation wavelengths in this way, preferably the doped material has an absorption band maximum to within lOnin, preferably within Snm, of one of the wavelengths 465nm, 525nm and 635nm. Alternatively, the doped material may have an absorption band maximum to within lOnm, preferably within Sum, of one of the wavelengths 436nm, 490nm, 546nm, 587nm and 613nm.

Preferably the active substance comprises an Yttrium-based lattice e.g. YNbO4 doped with at least one rare-earth element, or Y2W06 doped with at least one rare-earth element. By doping such a host lattice with rare-earth elements, for suitable rare-earths, colour changes can be seen at rare-earth concentrations as low as 2% or even 1%. Particularly preferred rare-earths for doping within YNbO4 or Y2W06 are Holmium, or Neodynium or Erbium. YNbO4 or Y2W06 doped with rare-earth elements are particularly suitable for exhibiting a colour change in response to an LED scanner.

Particularly preferred materials are YNbO4: Ho, YNbO4: Nd, Y2W06: Ho and Y2W06: Nd.

Preferably the doped material is incorporated in a printing ink, for ease of placement on the document of value.

Alternatively the doped material may be on or incorporated into a substrate of the document of value by any means, such as inclusion of fibres, planchettes, particles, threads, foils or pieces of polymer incorporating the doped material and carried on or within the document of value.

The invention also lies in a printing ink incorporating a doped material wherein the ink has a first appearance under daylight and has a second appearance under synthetic white light.

Where the doped material is used in printing ink, preferably the doped material has a particle size of 5 microns or less, more preferably 0.9-0.001 microns.

Such an ink may be printed as a series of dots within a region of colour matched to that of the ink but without the doped material, so producing a very visible speckled effect when replication of the document is attempted by an image reproduction means. The visual effect of the colour change on scanning is then apparent over a larger area without needing to use large amounts of the doped material.

Alternatively, the image may comprise the doped material colour matched to a colour pigment when irradiated. Thus before radiation, the document may show a speckled region resulting from printing separately with colour pigment and ink including the doped material. On irradiation, the doped material appears as the same colour as the pigment, such that the copy then shows a solid block of colour over the relevant area.

The invention will now be described, by way of example, and with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of a first embodiment of a copy-protected document in accordance with the present invention and its image as replicated by an image reproduction means; and Figure 2 shows a second embodiment.

Description

Figure 1 shows a simplified document of value 10, incorporating a plurality of coloured rectangles 12, wherein an ink pigment used to print the coloured rectangles incorporates an doped material which is visible in daylight as a first colour, shown as grey scale in Figure 1 daylight. When reproduction of the genuine document 10 is attempted in a photocopier, scanner or other reproduction means, the doped material appears as a different colour during scanning by radiation emitted by the scanner or photocopier, such that a copy 14 originating from the scanner is visually very different to the authentic document 10. The rectangular areas on the copy 14 show as a different colour, which for the sake of easy illustration is shown as black in this example. The genuine document 10 only exhibits a colour change during the scanning process: once it is no longer irradiated by the scanner, the doped material displays its usual daylight colour.

The doped material thus prevents a visually identical copy being reproduced and the copy has such major visible features of difference that it would be readily apparent to any party that the document of value was not authentic. Whilst most commercial establishments receiving money have detectors to check for more complex security features that are invisible under normal daylight, the general public does not have such facilities available to them. However, where documents are protected against simple colour graphic reproduction in accordance with the present invention, the public is readily alerted to the lack of validity of the copy document.

Figure 2 shows an alternative embodiment where the doped material is printed onto the document of value 10 with an ink pigment which has been colour matched with the body colour of the doped material such that when the document of value is viewed under daylight, there is an area 20 of one colour. When replication of the document of value 10 is attempted in a colour scanner or photocopier, the pattern of the active colour changing substance within the colour-matched pigment becomes apparent. In this case, on the copy 14 black stripes become apparent next to grey stripes, the non-active pigment being reproduced as its original on the copy 14.

The person skilled in the art will understand that the schematic representations given are purely by way of example and that generally more complex graphical images would be used on the document of value. The examples given here are for illustration of the general concept of the technique. The colours of grey and black are for the purposes of illustration. Typical colour changes, by way of example, include yellow/green to yellow, orange to pink and purple to pale green.

The colour variation exhibited between the authentic document and the copied document will vary depending on the doped material used and also the radiation used by the scanner or photocopier.

Certain rare-earth elements when doped into a YNbO4 or Y2W06 lattice produced visible colour changes in response to scanners and photocopiers, at rare-earth concentrations as low as 2%. Other lattices for doping with rare-earth elements were tried, e.g. Y203, but none were found to give a colour change.

Twelve rare-earths were investigated, Ce34, Pr34, Nd34, Sm3, Eu34, Gd34, Th3, Dy3, Ho34, Er34, Tm3 and Yb3 However, only E?4, Nd34 and Ho3 when doped gave any useful colour change. Nd34 and Ho34 are particularly preferred.

YNbO4:Ho, YNbO4:Nd, YNbO4:Pr all exhibited colour change properties when scanned. The YNbO4:Ho exhibited a change from pale orange when visible under normal light, to green when duplicated by an LED scanner. For YNbO4:Nd, a change from purple under ordinary daylight to pink when scanned is exhibited.

For incorporation into ink pigments, ideally the doped materials have a particle size of around microns as this range of particle sizes remains suspended within the ink and prints more effectively than larger particle sizes. Sub-micron particle sizes are also appropriate, although generally less desirable.

Incorporation of the materials referred to above in documents of value can assist in automatic verification of documents of value Machines that automatically detect the presence of genuine documents of value are widely used. These machines may interrogate documents at any rate of speed by manual or automatic insertion of single or bunches of documents. The materials described in this specification offer an additional means for validating the authenticity of such documents of value when used with these machines.

There are many types of machines that automatically validate documents of value. Machines that use reflected and or transmitted light with a silicon detector or a camera as a means of interrogation are particularly relevant. These machines normally consist of a synthetic visible light source and a means of detecting either reflected and or transmitted visible light which is first incident on the surface of a document. If the light source chosen to interrogate the document is one that a doped material within or on the document of value responds to and the detector used is one that is responsive to the reflected and or transmitted light from the doped material, the machine will be able to detect a unique response from the doped material on the document as it passes the sensor. Alternatively, by switching the light source on and off at a selected scanning rate, the machine would be able to detect the occurrence of the colour change itself.

Typically the synthetic light source used in the validation machine will be that as used in image reproduction means, such as scaimers and photocopiers, for example cold cathode fluorescent light or LEDs. Where LEDs are used, there may be a combination of different colour LEDs to ensure the colour change occurs. Using LEDs with different emission characteristics ensures that the colour change can be stimulated, and so validity of the document assessed, even where there are a variety of documents of value incorporating different doped materials and responsive to different wavelengths.

Claims

Claims 1. A document of value bearing an image comprising a material

doped with a rare-earth element wherein the image has a first appearance under daylight and has a second appearance under synthetic white light wherein the first and second appearances are different.
2. A document according to claim 1, wherein the first appearance is a first visible colour and the second appearance is a second visible colour.
3. A document according to claim 1 or claim 2, wherein the doped material has an absorption band maximum at a first wavelength in the visible spectrum and an associated emission band maximum at a second wavelength in the visible spectrum, different to the first wavelength.
4. A document according to any one of the preceding claims, wherein the synthetic white light is emitted by a plurality of LEDs.
5. A document according to any one of the preceding claims, wherein the synthetic white light is generated by an image reproduction means.
6. A document according to claim 5, wherein the image reproduction means is a photocopier or a scanner.
7. A document according to any one of the preceding claims, wherein the doped material is incorporated in a printing ink.
8. A document according to any one of claims 3 to 7, wherein the phosphor has an absorption band maximum to within 1 Onm, preferably within Snm, of one of the wavelengths 436nm, 490nm, 546nin, 587nm and 61 3nm.
9. A document according to any one of claims 3 to 7, wherein the doped material has an absorption band maximum to within lOnm, preferably within Snm, of one of the wavelengths, 465nm, 525nm and 635nm.
10. A document according to any one of the preceding claims, wherein the rare-earth element is selected from praseodynium, neodynium and holmium.
11. A document according to claim 10, wherein the material which is doped is selected from YNbO4 and Y2W06.
12. A printing ink incorporating a material doped with a rare-earth element which has a first appearance under daylight and a second appearance under synthetic white light wherein the first and second appearances are different..
13. A document of value according to claim 12, wherein the phosphor has a particle size less than 5 microns.
14. A printing ink in accordance with claim 13 wherein the particle size is in the range 0.9-0.001 microns.
15. A printing ink according to any of claims 12 to 14, wherein the phosphor is combined with a coloured substance to produce different colour changes than those exhibited by the phosphor alone.
16. A document of value and a printing ink substantially as herein described with reference to the accompanying drawings. *S.. *.S. * ** ** $

S *S.

S * **

S SS.$

S * S

16. A substrate for a document of value incorporating a phosphor, which is visible as a first colour in daylight and is visible as a second colour in synthetic white light.

17. A document of value and a printing ink substantially as herein described with reference to the accompanying drawings. AM'1b

Claims 1. A document of value bearing an image comprising a material selected from YNbO4 and Y2W06 doped with a rare-earth element wherein the image has a first appearance under daylight and has a second appearance under synthetic white light wherein the first and second appearances are different.

2. A document according to claim 1, wherein the first appearance is a first visible colour and the second appearance is a second visible colour.

3. A document according to claim 1 or claim 2, wherein the doped material has an absorption band maximum at a first wavelength in the visible spectrum and an associated emission band maximum at a second wavelength in the visible spectrum, different to the first wavelength.

4. A document according to any one of the preceding claims, wherein the synthetic white light is emitted by a plurality of LEDs.

5. A document according to any one of the preceding claims, wherein the synthetic white light is generated by an image reproduction means.

6. A document according to claim 5, wherein the image reproduction means is a photocopier or S...

* . a scanner. *5a S..'

: A document according to any one of the preceding claims, wherein the doped material is *S* * incorporated in a printing ink. * .* I. *

s....: 8. A document according to any one of claims 3 to 7, wherein the phosphor has an absorption * S band maximum to within lOnm, preferably within 5nm, of one of the wavelengths 436nm, 490nm, 546nm, 587nm and 613nm.

9. A document according to any one of claims 3 to 7, wherein the doped material has an absorption band maximum to within lOnm, preferably within 5nm, of one of the wavelengths, 465nm, 525nm and 635nm. I'

10. A document according to any one of the preceding claims, wherein the rare-earth element is selected from praseodynium, neodynium and holmium.

11. A printing ink incorporating a material doped with a rare-earth element which has a first appearance under daylight and a second appearance under synthetic white light wherein the first and second appearances are different..

12. A document of value according to claim 11, wherein the phosphor has a particle size less than 5 microns.

13. A printing ink in accordance with claim 12 wherein the particle size is in the range 0.9-0.001 microns.

14. A printing ink according to any of claims 11 to 13, wherein the phosphor is combined with a coloured substance to produce different colour changes than those exhibited by the phosphor alone.

15. A substrate for a document of value incorporating a phosphor, which is visible as a first colour in daylight and is visible as a second colour in synthetic white light.