GB2328180A

GB2328180A - Security device comprising a birefringent film

Info

Publication number: GB2328180A
Application number: GB9717284A
Authority: GB
Inventors: David William Thomas; Anthony David Harman; Timothy Andrew Large
Original assignee: De la Rue International Ltd; Thomas De la Rue and Co Ltd
Current assignee: De la Rue International Ltd
Priority date: 1997-08-14
Filing date: 1997-08-14
Publication date: 1999-02-17
Anticipated expiration: 2017-08-14
Also published as: GB2328180B; GB9717284D0

Abstract

The film 3 has birefringent regions 10 of different thickness such that when a material is viewed through a polarising viewer, a pattern is exhibited. The film may be patterned with security indicia using a CO2 laser which melts or removes the regions 10 so as to destroy the birefringent property of the film. The device may be coated with a lacquer layer 12 and a metal layer 11. The device may be applied to a banknote or incorporated into a label or hot stamp film (Fig 6).

Description

SECURITY DEVICE The invention relates to a security device and a method of manufacturing such a security device.

Security devices are used to secure a variety of articles against tampering and counterfeiting. Examples include articles of value such as banknotes, cheques, share certificates and the like but such devices can also be used with other types of article including bottled spirits and other items.

Ideally, a security device is difficult to counterfeit or tamper with but easy to identify. Examples of known security devices include holograms which can be provided in a hot stamping foil for ease of use, watermarks, biometric data, security printing and the like.

Recently, attempts have been made to utilize a birefringent material as a security device. An example is described in US-A-5284364 in which use is made of the property of a quarter waveplate to rotate plane polarised light so that when the material is illuminated with plane polarised light and viewed under reflection through a plane polarising plate, the underlying substrate will appear dark or light depending upon the presence or absence of the quarter waveplate. Where the quarter waveplate exists, the underlying substrate will appear dark since the plane of polarisation of the light will be rotated by the quarter waveplate so that it can no longer pass back through the plane polarising plate whereas those areas of the substrate where the quarter wave plate is absent will not rotate the light and thus appear light. US-A-5284364 discloses a number of different methods for generating security devices using a quarter waveplate. These primarily relate to mechanical methods in which a quarter waveplate film is cut to completely remove parts of the film and chemical/ radiation techniques in which dyes are introduced into the film which can be bleached selectively and thus cause the bleached regions to lose their polarising effect.

The problem with the mechanical approach is that the pattern of the removed areas of film can be seen relatively easily with the naked eye and thus could be detected and reproduced fraudulently while the radiation/chemical techniques require the introduction of dyes and are thus complex to manufacture. Furthermore, this known device yields only a monochromatic image.

US-A-4659112 discloses an alternative approach to modifying the structure of a quarter waveplate film. In this case, the film is hot stamped selectively to define an invisible marking which can be viewed through a suitable polarising analyser. We have found that the use of hot stamping techniques is not ideal since the melted material can transfer to the hot stamping dye making this approach unsuitable for mass production techniques.

GB-A-2283455 discloses the use of a laminate structure, each layer in the structure affecting the polarisation of light so as to result in a particular, predetermined pattern which can be authenticated. The drawback of this approach is the need for a laminate which leads to a relatively thick construction which is not desirable for security purposes.

US-A-4659112 discloses an ID card comprising an information-bearing substrate made of a flexible but substantially rigid material, a partial light reflector superposed over said substrate which produces a substantially nondepolarizing light reflection, and a substantially transparent 900 retarder superposed over said reflector. This disclosure describes the use of polyvinyl alcohol which has no strength and must therefore be supported. The polyvinyl alcohol is chemically modified.

In accordance with one aspect of the present invention, a security device comprises a birefringent film having birefringent regions of different thickness such that when the material is viewed through a polarising viewer, a pattern is exhibited.

In contrast to the known approaches, we have managed to produce a device with much higher security, in which certain regions of a single film have different thicknesses from other regions but all retain their birefringent property. As will be explained in more detail below, varying the thickness of the birefringent material while maintaining its birefringence causes the material to produce a colour and/or intensity variation. In particular the film may act as a colour filter, the intensity and colour varying as the retardation of the filter varies.

It will be appreciated that by suitably controlling the thicknesses, a predetermined colour pattern can be generated which can then be authenticated.

In contrast to US-A-4659112, the birefringent film will typically be physically modified which increases precision.

The form of the polarising viewer will depend upon the nature of the security device and in particular depending upon whether the security device is viewed in transmission or reflection. In the case of transmission, it is necessary to polarise light impinging on the film and then to view the transmitted light through an analyser on the other side of the film, the polarisation effect of the polariser and analyser being suitably chosen to enable the pattern to be viewed. For example, each could comprise a plane polariser; or in some cases, the polariser could comprise a circular polariser or a higher order waveplate laminated to a plane polariser. The circular polariser avoids the pattern disappearing at certain angles of view and the higher order waveplate enhances colour effects.

One of the advantages of incorporating some birefringents into the viewer is that the film can then be made very thin.

In reflection mode, a single viewing device can be used, typically a plane polariser through which light passes before impinging on the security device and then being reflected back through the same plane polariser to enable the security device to be viewed.

All references to a "polarising viewer" in the specification should be interpreted therefore to include both reflection and transmission type viewers, as appropriate.

The regions of different thickness could be connected to form a continuous pattern but will typically comprise a number of discrete recesses. Typically, the depths of the regions vary substantially continuously throughout a predetermined range.

A particular advantage of the device according to the first aspect of the invention is the continuous nature of the birefringent material which should be contrasted with the material described in the prior art when sections are mechanically removed.

An alternative approach which maintains the continuity aspect involves a security device according to a second aspect of the present invention, the device comprising a birefringent film bonded to a higher melting point layer, selected regions of the birefringent film having been melted and/or removed to destroy the birefringent property of the film, whereby when the material is viewed through a polarising viewer, a pattern is exhibited.

This aspect of the invention avoids the need for additional dyes to destroy the birefringent property, by causing the birefringent film itself to melt and/or be removed, but maintains the integrity of the film by providing the film on a higher melting point layer.

Melting the birefringent film relieves stresses in the polymer and thus destroys the birefringent property but the device as a whole maintains its integrity since the higher melting point layer will not melt and so holds the birefringent film substantially in place. Maintenance of a continuous film also assists the subsequent provision of an adhesive or metal coating.

In general, a device according to the second aspect of the invention will also exhibit a colour variation although it will usually be of a binary nature as compared with the multiple colour variation obtainable with the first aspect.

An intensity variation as well as or instead of a colour variation is also envisaged.

The higher melting point layer typically comprises a radiation, e.g. UV, curable lacquer or a thermoset material such as an epoxy resin, phenol formaldehyde resin, or phenolic epoxy.. These materials are particularly suitable for use with birefringent polymer films such as PET, for example Mylar C manufactured by Du Pont and Hoechst Diafoil.

The pattern exhibited by security devices according to either aspect of the invention can define a variety of appearances but it is particularly convenient if it defines indicia, for example security indicia. Examples of security indicia are chosen from the group comprising lines, line segments, dots, letters, numbers, characters, logos, guilloches and bar codes.

When the device is to be viewed in reflection, a variety of different methods can be used to provide a reflective layer. These include the use of materials with a different refractive index from the birefringent film, such as zinc sulphide and the like, but conveniently the reflective layer comprises metallisation, for example an aluminium or stainless steel layer typically sputter coated or vacuum deposited, or a printed reflective material.

In the case of the first aspect of the invention, the reflective layer may be provided on the birefringent film before its thickness is varied but in the case of the second aspect of the invention, preferably the reflective layer is provided after the birefringent property of the selected regions has been destroyed. The reason in this latter case is that the melting action could also affect the reflective layer.

It should also be noted that in some cases, a transparent security device could be used in conjunction with an article presenting a reflective surface so that the device can be viewed in reflection.

In some cases, the device further comprises a protective cover layer, for example a polyurethane based lacquer, to reduce soiling and to infill depressions in the film surface.

In a particularly preferred aspect, the device further comprises an adhesive layer to enable the device to be adhered to a substrate. This allows the device to be fabricated in the form of a label or a hot stamp film.

Alternatively, however, the adhesive could be provided on the substrate.

In other applications, the birefringent film may itself define the article to be protected, with the material carrying further indicia separate from the security device defining the nature of the article. A typical example might be a security document such as a banknote with the birefringent film forming the or one of the primary layers of the banknote. Part of the birefringent film is incorporated into a security device while the remainder carries conventional printed indicia.

Security devices according to the invention can be manufactured in a variety of ways but we have devised certain particularly preferred methods which allow rapid manufacture and accurate generation of patterns for mass production. The method allows these patterns to be easily varied between devices so that they can be customised to the document to be protected.

In accordance with a third aspect of the present invention, a method of manufacturing a security device comprises modifying the thickness of selected regions of a birefringent film, the remaining material in each region retaining its birefringence whereby when the film is viewed through a polarising viewer, a pattern is exhibited.

In some cases the film may have a retardation that is too small to produce a readily discernible colour when viewed through a plane polariser, but may produce bright colours when viewed through a circular polariser or polariser/waveplate.

Conveniently, the thickness is modified on exposure to a laser beam. This exposure could be in a continuous form but is preferably a pulsed laser beam (as obtained for example from an excimer laser), each pulse causing a certain thickness of the material to be removed. This allows very accurate control of the thickness to be achieved. Typically, the thickness removed at each step will change the retardation effect by 0.1 wave. Thus, for example in the case of Mylar C whose birefringence is typically 0.042, the 0.1 wave retardation change is equivalent to a thickness change of 0.65 microns when the film is viewed in reflection.

In accordance with a fourth aspect of the present invention, a method of manufacturing a security device comprises providing a birefringent film to which is bonded a higher melting point layer; and melting and/or removing selected regions of the birefringent film so as to destroy the birefringent property of the film, whereby when the material is viewed through a polarising viewer, a pattern is exhibited.

Once again, the melting and/or removal step can be carried out using a laser beam, in this case preferably generated by a carbon dioxide laser.

The higher melting point layer will have a relatively high absorbence at the wavelength of the laser so as to heat up and thus melt the adjacent birefringent film which, compared with the wavelength concerned, typically 10.6 microns, will have a very low thickness (typically 4 microns). The higher melting point backing material avoids mechanical disintegration of the birefringent material.

Typically, the birefringent film will be selfsupporting and will usually be stretched during manufacture.

Some examples of security devices and methods for their manufacture according to the invention will now be described with reference to the accompanying drawings, in which: - Figure 1 is a schematic diagram of a single stage birefringent filter; Figure 2 illustrates the transmission spectrum of a simple transmission filter; Figures 3A and 3B illustrate schematically alternative apparatus for creating different thickness regions in a birefringent film; Figure 4 is a cross-section (not to scale) through a film imaged using the apparatus shown in Figure 3; Figure 5 illustrates part of a pattern obtained using the apparatus shown in Figure 3; Figure 6 is a cross-section (not to scale) through a hot stamp film incorporating the film shown in Figure 4; Figure 7 illustrates the variation of birefringence with temperature of Mylar C; Figure 8 illustrates schematically apparatus for destroying the birefringence in certain regions of a film; Figure 9 illustrates the absorbence spectrum of PET; Figure 10 is a cross-section (not to scale) through the film shown in Figure 8; and, Figure 11 illustrates part of a pattern produced by the apparatus in Figure 8.

As will be appreciated, the invention provides two alternative methods of utilizing a birefringent film to form a security device. Typically, the film will have a thickness in the range 4-10 m. In the first type, the thickness of the birefringent film is varied in different regions which result in the retardation of birefringence varying across the surface of the film, although the birefringence will always be linear. In the second type, the film is selectively melted while being provided on a higher melting point layer.

Selective Thickness Variation In the first type, the thickness of the birefringent material is varied in different regions. The effects of thickness on the birefringent properties of such a material have been analysed in connection with a single film or waveplate of transparent birefringent material 1 (Figure 1) positioned between a pair of aligned plane polarising plates 2. This arrangement or filter produces a sinusoidal filter characteristic, i.e.: I = Io.cos2(En.t.2ff/A) (1) where I is the transmitted intensity, Io is the light transmitted by the first polariser 2 alone (normally this is about 4/10 of unpolarised incident light), Sn is the birefringence (the difference in refractive index for the two polarisation states), t the thickness of the waveplate, and A the wavelength.

Waveplates are often specified as a fraction of a wave (e.g. quarter wave plate), where the fraction, which is known as retardation (here denoted A) is given by: A = En.t/Ao (2) So the filter characteristic of equation (1) is expressed: I = Io.cos(2it.A.o/) (3) Most waveplate applications are in the visible, and waveplates are specified at the peak visible response, i.e.

550nm (green).

Polarisation filters are normally analysed using Jones calculus. Light is specified as a two element vector, the two elements representing the two orthogonal polarisation states, and waveplates and polarisers are specified as two by two matrices.

Appendix 1 shows the Jones calculus applied to a single waveplate transmission filter. The graph in Figure 2 shows the transmission spectrum that can be expected from a two wave "simple" filter.

Once the spectrum of the filter is determined, the CIE eye response functions can be used to predict the visible colour and brightness. Figure 2 shows how the perceived intensity and colour of a single waveplate filter varies as the retardation of the filter increases.

When the colour is plotted on the conventional 1976 CIE colour chart, it can be seen that this filter is good at producing greens, blues, cyans, yellows and magentas.

This analysis can be extended to multiple stage filters such as the Lyot and Solc filters. However, it has been found that single stage filters as shown in Figure 1 provide the best combination of colour contrast and brightness.

One technique for achieving different thickness regions in a birefringent film is illustrated in Figures 3 and 4. In this case, the film comprises Mylar C having a thickness of about 4.5 microns. The film 3 is passed from a storage reel 4 to a take-up reel 5 (Figure 3A) in a continuous fashion. An excimer laser 6 generates very high power pulses of W light 7 which pass through a mask 8 and are imaged by a lens system 9 ont facingzsurface of the film 3. The beam may have a wavelength of about 248nm (produced by the use of KrF gas in the laser cavity) and this short wavelength allows very fine features to be reproduced. Each pulse has an energy of 2J/cm2 and removes about 0.5 micron thickness of material. In this way, recesses 10 can be produced in the film 3 located as defined by the mask 8 and having depths dependent on the number of pulses to which they are exposed, different depths yielding different colours in the resulting image due to the different amounts of retardation.

In Figure 3B, a cylinder lens 9' is used, the laser beam being focused through a contact mask 8' onto the film 3.

Figure 5 illustrates part of a birefringent film which has been imaged with apparatus similar to that of Figure 3B. In this case, the film was written while moving at a speed of lOmm/second using a 35mJ/pulse laser beam delivered at 100Hz. A strip 10mum wide can be continuously patterned using the mask. The resultant patterning which can be achieved using this method is both attractive and difficult to counterfeit and enables both monochrome and multi-coloured patterns to be achieved.

The surface of the film 3 which has not been imaged would typically be provided prior to imaging with a reflective layer such as a metallisation 11 which could be vacuum or sputter deposited. A particular advantage of this method is that the film can be metallised before imaging. This means that commercially available aluminised films can be used as a base. Such films are used in capacitor manufacture and food foil wrap, and are therefore manufactured in large volume at low cost.

Furthermore, an over lacquer layer 12 can optionally be provided following imaging to infill the recesses 10.

The refractive index of the over lacquer layer 12 should be close to that of the film in order to make the machined areas invisible without the use of a reader. For example, the refractive index of Mylar is typically in the range 1.58-1.64 depending on the method of film manufacture.

Polyurethane based resins typically have a refractive index of 1.5-1.6 and are therefore particularly suitable as overlacquers.

The assembly shown in Figure 4 is viewed in reflection. That is, a plane polariser is positioned above the laminate, light passing through the plane polariser then passing through the over lacquer 12 into the birefringent film 3 where the plane of polarisation of the light will be rotated. The amount of rotation will vary depending upon the thickness of birefringent material through which the light passes and hence will vary between the positions 10 of different depths. The light is then reflected by the metallisation 11 and further rotation of the plane of polarisation occurs before the light passes back up through the over lacquered layer 12 and to the plane polariser. Only light whose plane of polarisation coincides with that of the plane polariser will pass through the polariser. The result is a coloured image which can be authenticated.

In some cases, the film 3 can define part of an article which is to be protected, the remainder of the film 3 (not shown in Figure 4) carrying conventional printing and other information relating to the article, such as a banknote.

In other applications, the device shown in Figure 4 can be incorporated into a label and/or a hot stamp film.

Figure 6 illustrates part of a hot stamp film in crosssection. As shown, the hot stamp film includes a carrier 15 (having a thickness of about 12sum) to which the device shown in Figure 4 is adhered via a release layer 16.

Furthermore, an adhesive layer 17 is provided on the metallisation layer 11, the adhesive being heat activatable. The laminate shown in Figure 6 is kiss cut as shown at 18 to define the boundaries of the security device. In use, the laminate shown in Figure 6 is brought into contact with an article to be provided with the device and the hot stamping die contacts the exposed surface of the carrier 15. Heat from the dye activates the adhesive 17 which adheres to the article following which the carrier 15 can be pulled away from the security device from which it is separated via the release layer 16.

In alternative approaches, the adhesive 17 could be a self-adhesive which would avoid the need for hot stamping; the adhesive 17 could be provided on the article rather than as part of the security device; and, the release layer 16 could be omitted and, for example, replaced by a corona treatment of the carrier 15.

In a further example, a PET carrier can be used which has been imaged using a laser beam in the manner described above. This carrier would be coated with metallisation such as aluminium either before or after imaging. A layer of heat sensitive adhesive would then be applied to the metal layer. The birefringent film would then be adhered to a substrate by using a combined die cutting/stamping technique, the birefringent material being die cut and stamped in one mechanism using a tool whose area covered the image within the birefringent material.

The images defined by the film 3 can have a variety of forms as described previously and could include logos or other indicia relating to the value of the article such as a banknote. In a further option, if the metallised layer 11 is dis-continuous the device could be laid over underlying information which could then partially be seen through the device.

Selective Destruction of Birefringence In the second type, the birefringence of the material is selectively destroyed. This can be achieved by direct thermal printing, or mechanical hot stamping, but preferably laser writing. Figure 7 shows a curve illustrating the residual birefringence after heat treatment of 4.5 micron thick Mylar C films. This shows that the film changes its birefringence very little up to 200C, although it starts to soften and "crinkle" at rather lower temperatures than this. At 230-240C the film is destroyed. Near this temperature, the birefringence increases, depending upon the direction in which the film is held. Above the melt temperature, all birefringence (and the mechanical integrity of the film) is destroyed.

In order to maintain the integrity of the film, therefore, the film must be backed with a higher melting point material such as a heat resistant thermoset material, for example an epoxy resin or phenol formaldehyde coating.

Figure 8 illustrates a typical arrangement for laser annealing a film 20 which moves continuously from a storage reel 21 to a take-up reel 22. The CO2 laser 23 generates a beam 24 which is fed through a scanner device 25 and is then focused by a lens 26 onto the film 20.

The absorption spectrum of PET is shown in Figure 9.

This shows that the material is largely transparent at 10.6 microns, the wavelength of carbon dioxide lasers. It is also substantially transparent over the other major laser wavelengths, 1.06 microns (Nd:YAG) and 5 microns (carbon monoxide), and visible green (copper vapour).

It is difficult to achieve the required realignment in conventional Mylar using a carbon dioxide laser but we have overcome this problem by forming a film composite incorporating high melting point backing material. This is illustrated in Figure 10. The film 20 is backed by an epoxy resin 27 which has a higher melting point. This avoids mechanical disintegration of the film 20 because the back surface of the laminate remains solid at temperatures required to locally melt the film 20, and furthermore the backing 27 can be chosen to give absorption at the chosen laser wavelength.

Figure 11 shows a series of spots 30 which exhibit a colour different from that of the surrounding material 31 and which have been written with a carbon dioxide laser on a composite film made from 4.5 micron thick Mylar C coated with an epoxy-phenolic resin. The resin was a food grade coating resin which is ideal because of the combination of relatively high absorption at 10.6 microns and excellent thermal stability. Similar results have been obtained with a phenol formaldehyde resin.

The spots in Figure 11 were produced by using a stainless steel mask in contact with the film 20 (instead of the scanner 25) and passing it beneath the carbon dioxide laser 23, the laser in this case simply producing a directed source of heat.

In an alternative approach, the beam from the laser 23 may be scanned using, for example, an acousto-optic deflector as the scanner 25. Acousto-optics allows scanning of the beam over distances of 20mm or so with very rapid random access times which has the advantage of being capable of software control enabling images such as in correcting serial numbers, changes in logos, date of issue information and the like to be obtained.

As with the previous examples, a metallisation or other reflective layer may be included if the device is to be viewed in reflection and a further over laminate, lacquer layer may be included. The device shown in Figure 10 could also be provided with an adhesive layer and mounted on a carrier as in the previous examples.

APPENDIX 1 We can use Jones calculus to predict the transmission of a simple filter, and then calculate from the filter function the perceived colour variation as the waveplate thickness varies.

cos(6) sin(8) A = 400.110..700 Roiaiion(O) = sin(#) cos(#)

AO lexpljnA- 0 kpul 1A2 Waveplale(A,A)= r 0 exp|-3nA-! The overall transmission of the cell is given by the product of the matrices representing the components and the matrices representing their relative rotations:

Transmission(#,#) = Polariser#Rotation#-###Waveplate(#,#)#Rotation####Polariser.Inpuli 4 4 If we use a two wave plate the transmission spectrum will be:

Claims

CLAIMS 1. A security device comprising a birefringent film having birefringent regions of different thickness such that when the material is viewed through a polarising viewer, a pattern is exhibited.
2. A device according to claim 1, wherein the regions are defined by a number of discrete recesses.
3. A device according to claim 1 or claim 2, wherein the depths of the regions are integer multiples of 0.1 wave retardation.
4. A security device comprising a birefringent film bonded to a higher melting point layer, selected regions of the birefringent film having been melted and/or removed to destroy the birefringent property of the film, whereby when the material is viewed through a polarising viewer, a pattern is exhibited.
5. A device according to any of the preceding claims, wherein the pattern exhibits a colour and/or intensity variation.
6. A device according to claim 4 or claim 5, wherein the higher melting point layer comprises a radiation, e.g. W, curable lacquer or a thermoset material.
7. A device according to claim 6, wherein the thermoset material comprises an epoxy resin, phenol formaldehyde resin, or an epoxy - phenolic material.
8. A device according to any of the preceding claims, wherein the pattern defines indicia.
9. A device according to claim 8, wherein the indicia comprise security indicia.
10. A device according to claim 9, wherein the security indicia are chosen from the group comprising lines, line segments, dots, letters, numbers, characters, logos, guilloches and bar codes.
11. A device according to any of the preceding claims, further comprising a reflective layer on the opposite side of the birefringent material to the side from which it is viewed.
12. A device according to claim 11, wherein the reflective layer comprises metallisation, or other reflective material.
13. .A device according to any of the preceding claims, further comprising a protective cover layer, for example a polyurethane based lacquer.
14. A device according to claim 13, wherein the refractive index of the cover layer is substantially the same as that of the birefringent film.
15. A device according to any of the preceding claims, further comprising an adhesive layer to enable the device to be adhered to a substrate.
16. A security device substantially as hereinbefore described with reference to any of the examples shown in the accompanying drawings.
17. An article to which a security device according to any of the preceding claims is attached.
18. An article which is formed in part by a security device according to any of claims 1 to 14 or 16, the birefringent film carrying further indicia separate from the security device defining the nature of the article.
19. An article according to claim 18, wherein the indicia defining the nature of the article are printed on the birefringent film.
20. An article of value according to any of claims 17 to 19.
21. A banknote according to claim 20.
22. A method of manufacturing a security device, the method comprising modifying the thickness of selected regions of a birefringent film, the remaining material in each region retaining its birefringence whereby when the film is viewed through a polarising viewer, a pattern is exhibited.
23. A method according to claim 22, wherein the thickness is modified by ablation.
24. A method according to claim 23, wherein the thickness is modified upon exposure to a laser beam.
25. A method according to claim 24, wherein the laser beam is generated by an excimer laser.
26. A method according to claim 24 or claim 25, wherein the laser beam is generated in pulses having a power of 2J/cm/pulse.
27. A method according to any of claims 24 to 26, wherein the laser beam is generated in pulses and removes substantially 0.5 microns of material per pulse.
28. A method according to any of claims 22 to 27, further comprising a reflective layer on the birefringent film.
29. A method according to claim 28, wherein the reflective layer is provided prior to modifying the thickness of selected regions of the birefringent film.
30. A method of manufacturing a security device, the method comprising providing a birefringent film to which is bonded a higher melting point layer; and melting and/or removing selected regions of the birefringent film so as to destroy the birefringent property of the material whereby when the material is viewed through a polarising viewer, a pattern is exhibited.
31. A method according to claim 29, wherein the melting and/or removal step is carried out using a laser beam.
32. A method according to claim 31, wherein the laser beam is generated by a carbon dioxide laser.
33. A method according to any of claims 30 to 32, further comprising providing a reflective layer.
34. A method according to claim 33, wherein the reflective layer is provided after the selective melting and/or removal step.
35. A method according to any of claims 22 to 34, wherein the pattern exhibits a colour and/or intensity variation.
36. A method according to any of claims 22 to 35, further comprising providing a protective layer on the birefringent material, for example a lacquer layer.
37. A method according to any of claims 22 to 36, wherein the pattern defines indicia.
38. A method according to claim 37, wherein the indicia comprise security indicia chosen from the group of lines, line segments, dots, letters, numbers, characters, logos, guilloches and bar codes.
39. A method according to any of claims 22 to 38, further comprising providing a layer of adhesive.
40. A method of manufacturing a security device substantially as hereinbefore described with reference to any of the examples shown in the accompanying drawings.