METHODS OF MANUFACTURING AN IMAGE PATTERN FOR A SECURITY DEVICE

Abstract

A method of manufacturing an image pattern for a security device is disclosed. The method comprises: (a) providing a metallised substrate comprising a substrate material having a first metal layer thereon on a first surface of the substrate material, the first metal layer being soluble in a first etchant substance; (b) applying a first resist layer to the first metal layer, the first resist layer comprising a resist material; (c) bringing the first resist layer into contact with a relief structure comprising a support carrying one or more protrusions thereon, the one or more protrusions each extending away from the support to a distal tip, whereupon at least one of the protrusion(s) extends into the first resist layer; (d) while the first resist layer and the relief structure are in contact, controlling the metallised substrate and/or the relief structure to achieve relative movement between the metallised substrate and at least the tip of the at least one of the protrusion(s) along a movement direction, such that the at least one of the protrusion(s) extending into the first resist layer expels a corresponding at least one portion of the resist material from a corresponding at least one region of the metallised substrate; (e) separating the first resist layer from the relief structure such that the at least one of the protrusion(s) is removed from the first resist layer, leaving the resist material remaining on the metallised substrate outside the at least one region, thereby forming a pattern of one or more first pattern elements in which the resist material is present and one or more second pattern elements, corresponding to the at least one region, in which the resist material is substantially absent; and (f) applying the first etchant substance to the metallised substrate whereupon the second pattern elements of the first metal layer are dissolved, the remaining first pattern elements of the first metal layer forming an image pattern.

Claims

1. A method of manufacturing an image pattern for a security device, the method comprising: (a) providing a metallised substrate comprising a substrate material having a first metal layer thereon on a first surface of the substrate material, the first metal layer being soluble in a first etchant substance; (b) applying a first resist layer to the first metal layer, the first resist layer comprising a resist material; (c) bringing the first resist layer into contact with a relief structure comprising a support carrying one or more protrusions thereon, the one or more protrusions each extending away from the support to a distal tip, whereupon at least one of the protrusion(s) extends into the first resist layer; (d) while the first resist layer and the relief structure are in contact, controlling the metallised substrate and/or the relief structure to achieve relative movement between the metallised substrate and at least the tip of the at least one of the protrusion(s) along a movement direction, such that the at least one of the protrusion(s) extending into the first resist layer expels a corresponding at least one portion of the resist material from a corresponding at least one region of the metallised substrate; (e) separating the first resist layer from the relief structure such that the at least one of the protrusion(s) is removed from the first resist layer, leaving the resist material remaining on the metallised substrate outside the at least one region, thereby forming a pattern of one or more first pattern elements in which the resist material is present and one or more second pattern elements, corresponding to the at least one region, in which the resist material is substantially absent; and (f) applying the first etchant substance to the metallised substrate whereupon the second pattern elements of the first metal layer are dissolved, the remaining first pattern elements of the first metal layer forming an image pattern.

2. A method according to claim 1, wherein the resist material is a curable resist material.

3. (canceled)

4. A method according to claim 2, further comprising, during or after steps (d) and/or (e) and before step (f), curing the resist material remaining on the metallised substrate outside the first region(s).

5. A method according to claim 1, wherein in step (b) the first resist layer is applied to a thickness which is less than or equal to the height of the at least one protrusion of the relief structure.

6. (canceled)

7. (canceled)

8. A method according to claim 1, wherein in step (b), the resist material has a first viscosity level when applied to the metallised substrate to form the resist layer, and step (b) further includes subsequently increasing the viscosity of the resist material to a second viscosity level.

9. (canceled)

10. (canceled)

11. A method according to claim 1, wherein the resist material is a curable material, further comprising, after step (b) and before step (c): (b1) partially pre-curing the first resist layer.

12. A method according to claim 1, wherein the one or more protrusions each have a base on the support and a distal tip, the sides of the protrusion being angled on each side of the tip such that the tip is pointed.

13-15. (canceled)

16. A method according to claim 1, wherein the one or more protrusions each have a lateral shape of which at least part is arranged along a direction which is not parallel with the movement direction.

17. A method according to claim 1, wherein the one or more protrusions each have a lateral shape in the form of a rectilinear line, a curved line, a dot or an indicia such as alphanumeric characters, symbols, geometric shapes or graphics.

18. A method according to claim 1, wherein the one or more protrusions comprises a plurality of protrusions.

19-21. (canceled)

22. A method according to claim 1, wherein the one or more protrusions each have a base on the support and a distal tip, at least the tip of the protrusion being formed of a flexible material such that the tip deforms during step (d).

23. (canceled)

24. A method according to claim 1, wherein in step (d) the metallised substrate and the relief structure are both conveyed in the same sense along the movement direction, at the same or different speeds from one another.

25. A method according to claim 1, wherein during step (d) a pressure applied between the relief structure and the metallised substrate is sufficient that the tip(s) of the at least one protrusion extends through the first resist layer and contacts the first metal layer.

26. A method according to claim 22, wherein during step (d) a pressure applied between the relief structure and the metallised substrate is sufficient that the tip(s) of the at least one protrusion extends through the first resist layer and contacts the first metal layer; and the pressure applied between the relief structure and the metallised substrate is such that the tip(s) of the at least one protrusion are deformed against the first metal layer, thereby causing relative movement between the tip(s) of the at least one protrusion and the metallised substrate.

27. (canceled)

28. A method according to claim 1, wherein the one or more second pattern elements have a dimension in the movement direction of 50 microns or less.

29. (canceled)

30. (canceled)

31. A method according to claim 1, further comprising providing a colour layer on the first or second surface of the substrate material, the colour layer comprising at least one optically detectable substance provided across the first and second pattern elements in at least one zone of the pattern, such that when viewed from one side of the substrate web, the colour layer is exposed in the second pattern elements between the first pattern elements of the first metal layer.

32-40. (canceled)

41. A method according to claim 1, wherein the pattern of first and second pattern elements is periodic in at least a first dimension and either the first pattern elements are substantially identical to one another and/or the second pattern elements are substantially identical to one another.

42-48. (canceled)

49. A method according to claim 1, wherein the pattern of first and second pattern elements defines sections of at least two images interleaved with one another periodically in at least a first dimension.

50. A method according to claim 1, wherein in step (a) the metallised substrate further comprises a second metal layer on the second surface of the substrate material, and the method further comprises manufacturing a second image pattern by applying a second resist layer to the second metal layer and performing steps (b) to (f) on the second resist layer.

51-53. (canceled)

54. A method of manufacturing a security device, comprising: (i) manufacturing a first image pattern using the method of claim 1; and (ii) providing a viewing component overlapping the first image pattern; wherein the first image pattern and the viewing component are configured to co-operate to generate an optically variable effect.

55. A method according to claim 54, wherein the viewing component comprises a focusing element array, a masking grid or a second image pattern.

56-68. (canceled)

69. An image pattern for a security device manufactured in accordance with claim 1.

70. A security device manufactured in accordance with claim 54.

71. A security article comprising an image pattern according to claim 69, wherein the security article is security thread, strip, foil, insert, transfer element, label or patch.

72. A security document comprising an image pattern according to claim 69, wherein the security document is a banknote, cheque, passport, identity card, driver's licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity.

Description

[0098] Examples of security devices, image patterns therefor and their methods of manufacture in accordance with the present invention will now be described and contrasted with conventional examples, with reference to the accompanying drawings, in which:

[0099] FIG. 1 schematically illustrates apparatus for performing a method of manufacturing an image pattern in accordance with a first embodiment of the invention;

[0100] FIG. 2 is a flow diagram showing steps of the method according to the first embodiment;

[0101] FIGS. 3(a) to (e) are schematic cross-sections of an exemplary substrate at various steps in the method according to the first embodiment;

[0102] FIG. 4 schematically illustrates apparatus for performing a method of manufacturing an image pattern in accordance with a second embodiment of the invention;

[0103] FIG. 5 is a flow diagram showing steps of the method according to the second embodiment;

[0104] FIG. 6 schematically illustrates apparatus for performing a method of manufacturing an image pattern in accordance with a third embodiment of the invention;

[0105] FIG. 7 is a flow diagram showing steps of the method according to the third embodiment;

[0106] FIG. 8(a) schematically depicts an exemplary relief structure as may be utilised in methods according to any embodiment of the invention, in perspective view; FIG. 8(b) showing an enlarged portion of the exemplary relief structure, in cross-section; and FIG. 8(c) illustrating the exemplary relief structure in combination with an exemplary substrate, in use during a method according to an embodiment of the invention;

[0107] FIG. 9(a) shows in plan view an exemplary substrate prior to the removal of any resist material, illustrating the positions at which protrusions of the relief structure make contact with the substrate in an embodiment of the invention; FIG. 9(b) showing the same substrate after relative movement between the substrate and the relief structure, to illustrate the regions from which the resist material has been removed; and FIG. 9(c) showing the substrate of FIG. 9(b) in perspective view;

[0108] FIGS. 10, 11 and 12 show three further exemplary substrates in plan view, (a) prior to the removal of any resist material and illustrating the positions at which protrusions of the relief structure make contact with the substrate, and (b) after relative movement between the substrate and the relief structure, illustrating the regions from which the resist material has been removed, in three different embodiments of the invention;

[0109] FIGS. 13 and 14 are photographs showing exemplary image patterns produced using methods in accordance with embodiments of the invention;

[0110] FIGS. 15(a) to (e) are schematic cross-sections of different exemplary substrates carrying image patterns thereon, produced in accordance with various embodiments of the invention;

[0111] FIG. 16 shows a schematic cross-section of another exemplary substrate carrying an image pattern thereon, produced in accordance with another embodiment of the invention;

[0112] FIGS. 17(a) to (c) are schematic cross-sections of three exemplary security devices each comprising an image pattern produced in accordance with embodiments of the invention;

[0113] FIG. 18(a) illustrates in plan view an exemplary image pattern in accordance with an embodiment of the present invention, FIG. 18(b) showing in plan view the appearance of a security device in accordance with an embodiment of the present invention incorporating the image element array of FIG. 18(a), at one viewing angle;

[0114] FIG. 19(a) illustrates an exemplary image pattern in accordance with an embodiment of the invention, and FIG. 19(b) shows the appearance of a security device incorporating the image pattern of FIG. 19(a);

[0115] FIG. 20(a) schematically depicts a security device in accordance with a further embodiment of the present invention, FIG. 20(b) showing a cross-section through the security device, and FIGS. 20(c) and (d) showing two exemplary images which may be displayed by the device at different viewing angles;

[0116] FIGS. 21, 22 and 23 show three further embodiments of security devices in accordance with embodiments of the invention;

[0117] FIG. 24 schematically illustrates apparatus for performing a method of manufacturing an image pattern in accordance with another embodiment of the invention;

[0118] FIG. 25 shows a cross section through a security device in accordance with another embodiment of the invention;

[0119] FIGS. 26, 27 and 28 show three exemplary articles carrying security devices in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and

[0120] FIG. 29 illustrates a further embodiment of an article carrying a security device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.

[0121] The ensuing description will focus on examples of methods of manufacturing image patterns with high resolution, fine detail in the form of image element arrays as required for use in security devices such as moir magnifiers, integral imaging devices and lenticular devices (amongst others). Preferred embodiments of such security devices making use of image element arrays made in accordance with the described method will then be described below. However it should be appreciated that the disclosed methods of manufacturing image patterns can be used to form any high resolution image pattern, as may be suitable for use in other security devices such as microtext or other micro-graphics.

[0122] A first embodiment of a method of manufacturing an image pattern will be described with reference to FIGS. 1, 2 and 3. FIG. 1 illustrates exemplary apparatus 1 for performing the method, FIG. 2 is a flow diagram setting out steps of the method and FIG. 3 shows an exemplary substrate at various stages during the processing thereof in accordance with the method. First, a metallised substrate is provided (step S100), which comprises a (preferably transparent) substrate material 10 carrying a metal layer 11 on one of its surfaces, as shown in FIGS. 1 and 3(a). The substrate material 10 typically comprises at least one transparent polymeric material, such as BOPP, and may be monolithic or multi-layered. The substrate may be of a type suitable for forming the basis of a security article such as a security thread, strip, patch or transfer foil, or of a type suitable for forming the basis of a security document itself, such as a polymer banknote. The substrate may include additional layers, such as a filter layer (described below) and/or a primer layer underlying the metal layer 11. Alternatively the substrate could be designed as a transfer film in which case a release layer such as wax may be provided between the metal layer 11 and the substrate material 10 so that the image pattern produced using the method can later be transferred to another surface. The substrate could also carry additional security features such as an optically variable relief structure, e.g. a diffraction structure such as a hologram, kinegram or diffraction grating, which the metal layer 11 follows, over all or part of the substrate surface. The substrate may be supplied pre-metallised, or the metal layer 11 (and any optional underlying layer(s)) could be applied as part of the presently disclosed method, e.g. by vapour deposition, sputtering or the like. The metal layer 11 may cover the whole area of the substrate material 10 (as shown) or could be provided only across selected portion(s) of the substrate within which the demetallised pattern is to be formed. Suitable metals include aluminium, copper, chromium and alloys of each (including in particular aluminium-copper alloys). The metal layer 11 is preferably substantially opaque to visible light and is desirably as thin as possible whilst achieving this opacity. The thinner the layer, the more accurately it can be etched since it will be less susceptible to lateral spread of the etched region. In preferred examples, the metal layer 11 may have a thickness of between 10 and 100 nm, more preferably between 10 and 50 nm, most preferably between 10 and 25 nm.

[0123] In step S102, a resist material 12 is then applied onto the metal layer 11, by an application station 2, with the result being shown in FIG. 3(b). The resist material 12 may be applied all over the substrate area or could be applied selectively, e.g. to define a large-scale pattern or image (sufficiently large to be visible to the naked eye). Suitable application techniques include printing or coating the resist material onto the metal layer. For example, in the exemplary apparatus of FIG. 1 the application station 2 comprises a gravure roller 2a which receives resist material from a reservoir 2c (or other inking components such as a suitable roller chain) and transfers it onto the metallised substrate as it is conveyed past. The gravure roller 2a is opposed in this example by an impression roller 2b. Examples of suitable resist materials will be given below but typically may comprise a polymeric material such as a resin, gel or an ink binder (with or without pigment). The resist material remains as a liquid, gel or plastically deformable solid after application to the metal layer for patterning of the resist layer 12, which takes place in step sequence S104, S106 and S108 at patterning station 5.

[0124] The resist-coated substrate is conveyed from the application station 2 via a conventional web transport system (not shown, but represented schematically at 3) along a machine direction MD to the patterning station 5 at which the resist layer 12 is brought into contact with a relief structure 6. In this example, the relief structure 6 is carried on the circumferential surface of a cylinder 7 although in other cases the relief structure 6 could be carried on a belt supported between rollers, or in sheet-fed (i.e. non-continuous) implementations of the method the relief structure 6 could be arranged on a plate which is moved towards and away from the substrate. While in contact with one another, the relief structure 6 and the substrate are controlled so as to achieve relative motion between at least the tips of the protrusions and the substrate. In the FIG. 1 example, here the cylinder 7 is driven in the same direction as the substrate (MD) but at a different speed meaning that where the relief structure 6 contacts the resist layer 12, there is relative motion between them in the machine direction MD. In this example, the cylinder 7 is driven more slowly than the substrate. In other examples where the relief structure is formed at least in part of a flexible material (described below) such differential conveyancing speeds are not required and the necessary relative movement can be achieved by flexing of the protrusions against the substrate.

[0125] The relief structure 6 comprises at least one but more preferably a plurality of protrusions 6a, spaced by troughs 6b, arranged on a support 6c which here corresponds to the surface of cylinder 7. The shape and arrangement of the protrusions can be configured as desired in order to obtain the desired image pattern as will be explained further below. In this example, the relief structure comprises a series of protrusions 6a periodically spaced around the surface of the cylinder 7 and hence in the machine direction MD.

[0126] As shown in FIGS. 1 and 3(c), as the relief structure 6 comes into contact with the resist layer 12, at least one of its protrusions 6a extends into the resist layer 12, preferably coming into contact with the underlying metal layer 11. The relative movement between the relief structure 6 and the substrate has the result that the inserted protrusion(s) 6a pushes resist material out of one or more corresponding regions of the substrate, resulting in gaps in the resist layer 12. As shown in FIG. 3(c), the areas where the resist material remains become first pattern elements P.sub.1 and the regions from which the resist material has been removed become second pattern elements P.sub.2. The substrate continues along the transport path such that it is separated from the relief structure, resulting in the substrate carrying a patterned resist layer 12 formed of first elements P.sub.1 where the resist is still present, and intervening second elements P.sub.2 where the resist material is now substantially absent.

[0127] The substrate is then conveyed to an etching station 9 at which one or more etchant substances are applied to the substrate. The etchant substance(s) dissolve the exposed portions of the metal layer 11 in the second pattern elements P.sub.2 whilst the resist material 12 protects the metal layer 11 in the first pattern elements P.sub.1. The particular etchant selected will depend on the metal to be etched. For instance where the metal is aluminium or an alloy thereof, the etchant is preferably alkaline, such as sodium hydroxide. Where the metal is copper or an alloy thereof, the etchant is preferably acidic, such as hydrochloric acid. The etchant can be applied to the substrate by passing the substrate through an etchant bath, or spraying the etchant onto the substrate, for example. The result, shown in FIG. 3(e), is an image pattern P formed in the metal layer 11, of first elements P.sub.1 where the metal remains present, and intervening second elements P.sub.2 where the metal is substantially absent.

[0128] It should be noted that, in the above method, the aim is not to replicate the contours of the relief structure 6 in the resist layer 12. Rather, the protrusions 6a expel the resist material out of regions of the substrate area, pushing the material into neighbouring areas (corresponding to troughs 6b) and/or off the substrate. Preferably, the protrusions act as blades, wiping the metal layer substantially clean of resist material in those regions. As described further below, the expelled resist material could, in some cases, fill the troughs 6b in which case the patterned resist layer 12 may take on the contours of the relief structure 6 to some extent. However generally the relative movement between the two components will mean that any such replication is inexact and typically the resulting profile of the resist layer 12 will be different from (the mirror image of) the relief structure 6.

[0129] Examples of suitable resist materials 12 which can be used in the above process include inks based on polymeric binders, and resins such as novolak resin. One exemplary resin that may be used is VMCA from UCAR which is a vinyl acetate/vinyl chloride copolymer. What is important is that the resist material is sufficiently deformable so as to allow patterning by the relief structure whilst exhibiting low inherent spreading so that the desired pattern will be retained in the resist layer during etching. This could be achieved for example by applying the resist material 12 as a solution which gradually dries throughout the process, such that only solids remain after patterning, thereby arresting any spreading of the resist. In another example, the resist material could be heated (or reheated) just before or as it comes into contact with the relief structure in order to increase its deformability, and then cooled while in contact with the relief structure and/or after separation in order to re-harden the material and fix its shape. For instance, where the resist material is a thermoplastic resin (for example VMCA from UCAR which as mentioned above is a vinyl acetate/vinyl chloride copolymer), this could be applied to the metal layer as a solution (i.e. dissolved in a solvent) or alternatively in a melted state. After the resist layer has been formed, the resin can then be heated (e.g. to approximately 120 degrees) prior to coming into contact with the relief cylinder, so that the material is deformable as the resin comes into contact with the relief structure. During patterning a pressure of around 5 bar may be applied between the relief cylinder and the substrate. The resin is then cooled quickly to fix the pattern and prevent spreading, which can be achieved by arranging the relief structure on a chiller roller/cylinder. It can then subsequently go on to be etched as described above.

[0130] However, it is more preferable to use a resist material 12 which is curablethat is, upon appropriate treatment the resist material undergoes a chemical reaction which causes it to harden and become more solid. Typically this involves the formation of cross-links between polymer chains. The curable resist material 12 could be a radiation-curable resist material (e.g. UV-curable), or could be heat-curable (or a combination of both). Radiation-curable materials are particularly preferred due to the fast speed with which curing can be achieved. An example of a heat-curable resist material is a polyester polyol binder such as Stepanol PC-020-01 from Alfa Chemicals alongside N75 isocyanate cross linker. An example of a suitable UV-curable resist material is Lumogen OVD Primer from BASF. More generally, two examples of acrylate compounds commonly used in photocurable polymerisation are tripropylene glycol diacrylate (TPGDA) and dipentaerythritol hexaacrylate (DPHA). Upon reaction with an excited species each of the terminal double bonds in these acrylate compounds is able to be broken and can thus form a bond with other similar compounds to create a cross-linked network. TPGDA is classed as a difunctional monomer, exhibits low viscosity and offers a high reactivity. DPHA can be seen to be a hexafunctional monomer. Multifunctional monomers generally exhibit excellent reactivity and high cross-link density.

[0131] A second embodiment of the invention in which such a curable resist material is used will now be described with reference to FIGS. 4 and 5. The appearance of the substrate at the various stages of processing is much the same as in the first embodiment and so FIGS. 4 and 5 can also be read in conjunction with FIG. 3. FIGS. 4 and 5 use like reference numerals for items already described in the preceding embodiment. Components shown in dashed lines in FIG. 4 are optional, as are steps shown in dashed lines in FIG. 5.

[0132] Thus, the exemplary apparatus 1 used to perform the method of the second embodiment, shown in FIG. 4, is the same as that of FIG. 1 save for the features which will now be described. The resist material applied at application 2 (step S102) is a curable resist material and is applied to the metal layer 11 in its uncured state, such that the so-formed resist layer 12 is a fluid or deformable solid. The resist layer 12 is then brought into contact with the relief structure 6 in the same manner as described previously (step S104). During patterning (i.e. whilst the resist layer 12 and the relief structure are in contact) in step S106a, the resist layer 12 is exposed to energy from a curing source 8a, such as radiation or heatmost preferably UV radiation. For example, the cylinder 7 and at least part of the relief structure may be transparent to the curing radiation so that the curing source 8a may be located inside the cylinder 7. Hence, the resist layer is caused to harden such that, once patterned, any lateral spreading of the material is reduced or, preferably, eliminated.

[0133] As an alternative or in addition to curing the resist material during patterning by curing source 8a, the resist material could be cured after patterning, i.e. after separation of the resist material from the relief structure (step S109, post-curing). FIG. 4 illustrates a curing source 8b which may be provided for this purpose. For instance, it may be desirable to use curing source 8a to partially cure the resist material 12 such that it is near-solid at the point of separation but the cure is incomplete. This assists in the removal of the protrusions 6a from the resist material since adhesion of one to the other can be reduced or avoided. The resist material can then be fully cured by post-curing source 8b.

[0134] It may also (or alternatively) be desirable to perform a partial pre-cure of the resist material 12 after it has been applied to the metal layer 11 and before patterning takes place. Whatever type of resist material is used, the present inventors have found that the results of patterning are improved by the use of a thin resist layer 12, and in particular one with a thickness less than the height of the protrusions 6a of the relief structure 6. For instance, particularly good results have been achieved where the thickness of the applied resist layer is of the order of 1 to 2 microns, or 2 gsm. This will be described in further detail below but for the time being it is sufficient to note that the use of a curable resist material can assist in enabling the provision of a thin resist layer 12 since the material can be applied in a highly fluid, low viscosity form at application station 2c. This promotes spreading of the material over the area of the substrate 10 and hence helps to achieve a thinner resist layer 12. However, patterning of such a low viscosity resist material will be difficult due to the spreading and so preferably the viscosity of the resist material may be increased prior to patterning, by partially pre-curing the resist material (step S103). FIG. 4 shows a pre-curing source 8c provided for this purpose. Of course, the pre-cure must not be complete since the resist material should still be sufficiently deformable for patterning in the manner described above.

[0135] It should be noted that whilst the various curing sources 8a, 8b and 8c may preferably be of the same type (e.g. all UV radiation sources) this is not essential. In other cases the curable resist material could contain more than one different types of initiator, such as a heat-activated initiator in addition to a radiation-sensitive initiator. Hence the pre-curing source 8c could for instance be a heat source, activating the thermal initiator to achieve a first part of the cure, and then curing sources 8a and/or 8b (whichever is/are provided) could comprise radiation sources to initiate the photosensitive initiator and complete the cure.

[0136] As just mentioned, it is desirable to apply the resist material to the substrate at a relatively low viscosity and then increase its viscosity prior to patterning. One way to achieve this is by partial pre-curing as illustrated above. Alternative techniques will now be illustrated with reference to FIGS. 6 and 7, which relate to a third embodiment of the invention. The appearance of the substrate at the various stages of processing is much the same as in the first and second embodiments and so FIGS. 6 and 7 can also be read in conjunction with FIG. 3. FIGS. 6 and 7 use like reference numerals for items already described in the preceding embodiments. Components shown in dashed lines in FIG. 6 are optional, as are steps shown in dashed lines in FIG. 7. The exemplary apparatus 1 used to perform the method of the third embodiment, shown in FIG. 6, is the same as that of FIG. 1 save for the features which will now be described.

[0137] Whether or not the resist material is a curable resist material, it may preferably be applied in the form of a solution, the solid components of the resist material being dispersed in a suitable solvent. In this way the viscosity of the fluid at the point of application will depend (at least in part) on the proportion of solid components to solvent and this can be adjusted as desired to achieve the required resist layer thickness. Once applied to the metal layer 11, the viscosity of the resist layer 12 is increased (step S102a) by either active or passive drying of the resist material to evaporate some or all of the solvent. This can be promoted by the provision of a heater 2e, such as an oven through which the substrate is conveyed. This approach can be used in conjunction with a curable resist material, such as a UV curable material, the drying of step S102a being used either in addition to or instead of the pre-curing step S103 described above. For instance, the present inventors have found that particularly good results can be achieved using this approach with a resist material composition comprising a mixture of the following components: [0138] 1 gramLumogen OVD Primer 301 from BASF; [0139] 0.01 gramSurfactant (such as BYK-020 or BYK-055); and [0140] 0.2 gramvinyl acetate/vinyl chloride-based resist (to promote substrate adhesion).

[0141] The above composition was diluted with MEK (methyl ethyl ketone) to yield a 10% solids solution for application to metal layer 11. The solution had a viscosity of around 43 mPa.Math.s (measured using zahn cup or cone and plate rheometer (22 degrees C.)) and was found to spread easily and quickly so as to form a thin resist layer 12. The MEK solvent was then dried off (e.g. using an oven at about 80 degrees C. for 1 to 2 seconds), leaving a very thin layer of pure, curable resist material.

[0142] In another alternative, the resist material may be heated to reduce its viscosity prior to application to the substrate 10 (evidently this option is not suitable for a thermally-curable resist material). In this case, a heater 2d may be provided to heat the resist material in reservoir 2c (or on roller 2a) as it is applied to the substrate. Once the resist layer 12 has been formed, the viscosity of the material can be increased by cooling the substrate. Again this may be passive or active, and in the latter case a cooling module 2f may be provided, such as a refrigerator or a coolant spray.

[0143] The manner in which the resist material 12 is patterned by the relief structure 6 will now be described in more detail with reference to FIGS. 8 to 12. FIG. 8(a) shows an exemplary relief structure 6 having much the same form as that referred to above in connection with the first, second and third embodiments, arranged on the circumferential surface of a cylinder 7. The relief structure 6 comprises a plurality of protrusions 6a which extend away from a support 6c, which here corresponds to the cylinder surface. In this example, the protrusions 6a are spaced from one another at their bases, but this is not essential. Either way, troughs 6b will be defined between adjacent protrusions 6a.

[0144] As explained above, the protrusions act to expel resist material from certain regions of the substrate during relative motion in the machine direction (MD) between the substrate and the relief structure. Each protrusion preferably acts as a blade, wiping the surface of metal layer 11 clean of resist material in these regions. To improve the precision with which these regions are formed, it is desirable that when the relief structure comes into contact with the resist layer, the protrusions cut through the resist material cleanly upon insertion (to avoid compression of the material between the protrusion and the substrate which could lead to spreading). As such it is preferable for each protrusion to have a sharp tip, and most preferably a tip which is narrower than the base of the protrusion. This is shown more clearly in FIG. 8(b) which is a cross-section through three of the protrusions 6a of the relief structure 6 shown in FIG. 8(a), all of which are identical in this example. Thus, each protrusion 6a has a substantially triangular cross-section with a tip 6a which has a width W.sub.t that is narrower than the width W.sub.b at the base 6a of the protrusion. The height h of the protrusion 6a is measured between its tip 6a and base 6a in the direction normal to the support surface 6c (which here is a radius of the cylinder 7). In this example each protrusion extends laterally in a straight line along the length of the cylinder so as to form a prism. The side faces of each prism are straight, intersecting at the tips 6a to provide a sharp edge for cleanly cutting through the resist material as already described. In alternative embodiments the cross-sections of the protrusions need not be triangular. For instance the main part of each protrusion could be parallel-sided, but preferably a sharp tip with angled sides is still provide at the distal end. Nonetheless, a wider base as provided by a triangular cross section is beneficial for improved durability and hence longevity of the relief structure.

[0145] The relief structure could be made of any material such as polymer, ceramic, glass or metal. However, it is particularly preferred that at least the tips of the protrusions 6a, and preferably the whole of each protrusion, is made of a flexible material such as an elastomeric polymer, such that when the protrusions contact the metal layer 11 on substrate 10, there is deformation of the protrusion tip 6a. For instance, materials with a typical shore hardness of 40 to 70 on the A scale are preferred. Thus, the whole relief structure could for example be moulded in a flexible material or each protrusion could be formed of a first body portion of a relatively inflexible material and a second tip portion of a relatively flexible material. Such flexing of the protrusion tips against the metallised substrate can be used by itself to give rise to the necessary relative movement between the tips of the relief structure and the substrate, or in combination with conveying the relief structure and substrate at different relative speeds.

[0146] In addition, especially if a radiation-curable resist material is to be used, it is preferable that at least part of the relief structure 6 is formed of a material which is transparent at least to the radiation wavelength(s) in question, e.g. UV. At least the support 6c should be transparent since as shown below it is the troughs 6b where the resist material needs to be cured and hence if the protrusions are spaced it may be adequate if the radiation reaches the resist material through the support 6c only. However, more preferably the protrusions 6a are also formed of such a transparent material. In particularly preferred examples the relief structure 6 is formed of an optically transparent elastomer such as PDMS (polydimethylsiloxane), or a silicone material. These are polymer materials with a low Young's modulus. A master mold can be made using conventional patterning techniques and the soft relief structure material is able to replicate the negative of the master. Some additional advantages of using a soft, i.e. flexible, material for the relief structure 6 are listed below: [0147] The flexibility of the relief structure allows it conform to non-flat substrates (for example curved, warped or bowed surfaces). [0148] Cost reduction, as soft relief structures are less expensive to make, and one can produce many inexpensive soft molds from one expensive master. [0149] Since the relief structure can deform around any contaminant particles, the structure becomes much less sensitive to these, thus prolonging the structure's life. [0150] Flexible relief structure materials generally have a low surface energy.

[0151] This stops the resist adhering to the relief structures instead of adhering to the substrate.

[0152] FIG. 8(c) shows the relief structure 6 in contact with an exemplary substrate carrying resist layer 12 during patterning. In this example, the cylinder 7 and the substrate 10 are both conveyed in the same machine direction MD with the same speed as one another. As the substrate advances, a protrusion 6a makes contact and cuts into the resist layer 12 (illustrated by the protrusion labelled (i)). As the movement continues, the protrusion extends through the full thickness of the resist layer 12 and contacts the metal layer 11. Preferably the pressure applied between the relief structure 6 and the substrate 10 is such that the tip of the protrusion 6a becomes pressed against the metal layer 11 and, still preferably, is deformed against the surface as shown by protrusion (ii). For instance the tip 6a may become flexed against the metal surface, as shown, or may become spread out against the surface. The flexing of the protrusion amounts to relative movement between the protrusion tip and the metal surface as the tip effectively slides across the metal surface under the transverse pressure applied between the cylinder 7 and the substrate 10 in order to adopt its flexed profile. The deformation of the protrusion 6a against the metal surface (and hence close contact between the two components) also helps to ensure that the resist material is more completely removed from the metal. As it flexes, the protrusion pushes the resist material out of a region of the substrate and into a neighbouring area.

[0153] As the substrate continues, the protrusion 6a is released from compression against the metal layer 11 (see protrusion (iii)), having expelled the resist material from a region P.sub.2 of the substrate. The length of region P.sub.2 in the machine direction will depend on a number of factors including the speed of relative movement and the applied pressure. Eventually, the protrusion lifts out of the resist layer (see protrusion (iv)), leaving a pattern of elements P.sub.1 where the resist material remains present, and intervening elements P.sub.2 where the resist material is absent, corresponding to the regions from which it has been expelled by the relief structure 6.

[0154] It should be noted that in a variant of this embodiment the degree of relative movement between the relief structure tips 6a and the substrate could be adjusted by additionally conveying the relief cylinder 7 and the substrate 10 at different relative speeds from one another (as in the first embodiment).

[0155] It will be appreciated that to achieve full removal of resist material from the regions, the tips of the protrusions preferably make good contact with the metal layer. It is therefore desirable that the resist layer 12 be applied to the metal layer 11 at a thickness t.sub.1 less than the height h of the protrusions, as mentioned above. For instance, particularly good results have been achieved where the thickness of the applied resist layer is of the order of 1 to 2 microns, or 2 gsm, with a protrusion height of around 6 microns. This not only avoids the resist layer spacing the relief structure too far from the substrate, but also reduces the volume of expelled resist material that will be pushed off the substrate area during patterning. As shown in FIG. 8(c), as each region is cleared of resist material, it is pushed into the neighbouring area corresponding to a trough of the relief structure and if this has sufficient empty volume, the thickness of the resist layer can be increased to t.sub.2 in these areas. This improves the effectiveness of the resist material remaining in elements P.sub.1 during later etching.

[0156] FIG. 9 illustrates an exemplary substrate subjected to the same patterning procedure as shown in FIG. 8. FIGS. 9(a) and (b) show the same substrate in plan view before and after patterning, respectively, while FIG. 9(c) shows the patterned substrate in perspective view. In FIG. 9(a), the solid lines marked 6a represent the positions where the tips of the protrusions 6a make contact with the metal layer 11 although it will be appreciated that in practice this will not usually happen simultaneously but rather one at a time. The dashed lines on either side represent the base 6a of each protrusion 6a. Thus, the protrusions represented in FIG. 9(a) are straight, parallel triangular prims as shown previously in perspective view in FIG. 8(a). FIG. 9(b) shows the same substrate after the relative motion (along machine direction MD) has been completed and the relief structure separated from the resist 12. It will be seen that each protrusion 6a has cleared the resist material from a corresponding region P.sub.2 leaving the resist material confined to intervening areas P.sub.1. The same result is shown in perspective view in FIG. 9(c). The width of each first pattern element, d.sub.1, and that of each second pattern element, d.sub.2, will depend on factors such as the relative speed of motion during patterning and the pressure applied, as well as the relief structure itself but preferably these dimensions are of the order of 100 microns or less, more preferably 50 microns or less, most preferably 20 microns or less. In tests, the inventors have achieved line widths of 5 to 6 microns.

[0157] The formation of line patterns such as that illustrated above finds many applications such as in the construction of lenticular devices, venetian blind devices and moir interference devices as will be exemplified below. However, the presently disclosed methods can be used to form any desired image pattern through design of the relief structure and control of the processing parameters, and some further examples are shown in FIGS. 10, 11 and 12. In each case, Figure (a) shows the substrate prior to patterning, the relief structure being illustrated by solid and dashed lines in the manner already explained with reference to FIG. 9(a), and Figure (b) shows the patterned substrate.

[0158] In FIG. 10, the relief structure 6 comprises an array of straight prism segments arranged in an alternating manner. The result of relative movement in the machine direction MD is a checkerboard arrangement of first and second pattern elements P.sub.1, P.sub.2. Such arrangements find particular utility in two-dimensional lenticular devices, for example.

[0159] In FIG. 11, the relief structure 6 defines a microimage in the form of an alphanumerical character, here the letter A. In practice the relief structure may comprise a plurality of identical microimages arranged to form a regular orthogonal or hexagonal array, but only one is depicted here for clarity. After relative movement between the relief structure 6 and the resist material 12, a resist-free region P.sub.2 having the shape of a letter A is formed on the substrate, however, this is elongated in the machine direction relative to the original form of the letter on the relief structure. This is due to the movement of the relief structure through the resist material. It will be noted that all parts of the letter A have a component of their direction in the direction orthogonal to the machine direction MD, with the result that all parts of the letter are transferred to the substrate pattern.

[0160] FIG. 12 shows another example where this is not the case. Here the relief structure 6 defines the letter O (again, it may in fact carry an array of these microimages). Now, after relative movement, the resist has a resist-free region P.sub.2 again in the shape of an elongated O but its two sets of opposing sides are different in line width. This is because those portions of the relief structure running approximately parallel to the machine direction MD do not present a surface which can push resist material out of the region during the relative movement. In contrast, those portions running in the orthogonal direction are able to engage with the resist material in the manner described previously and clear the respective regions of the substrate.

[0161] For the above reason, in general terms it is preferred that the relief structure includes at least some portions that have a component of their direction in the direction orthogonal to the machine direction. In other words, the entire protrusion should not lie parallel to the machine direction (although some parts of it may).

[0162] FIGS. 13 and 14 are photographs showing two exemplary image patterns formed using methods in line with those described above. Both photographs are transmission views with the metallised areas (elements P.sub.1) appearing dark and the demetallised areas (elements P.sub.2) appearing light. In FIG. 13, an array of straight lines has been formed using a relief structure corresponding to that of FIGS. 8(a) and 9(a). The pitch between the protrusion tips was 20 microns. Metallised lines of between 5 and 6 microns have been achieved. In FIG. 14, an array of curved lines has been formed using a different relief structure in which the protrusions follow corresponding curved lines extending generally in the direction orthogonal to the machine direction MD. Here, the pitch between the protrusion tips was about 120 microns. Metallised lines of about 54 microns have been achieved.

[0163] Various further optional features of the method will now be described. FIGS. 15(a) to (e) show cross-sections through different substrates carrying image patterns formed according to different embodiments of the invention. Figure 15(a) is identical to FIG. 3(e) and shows the substrate after etching (step S110), as previously discussed. Here the resist layer 12 is shown to remain in place in the first pattern elements 12 and this can be particularly desirable if the resist material is optically detectable, e.g. carrying a coloured tint or a luminescent substance. In still further cases the remaining resist 12 could have different optical characteristics in different areas of the substrate, e.g. different colours, so as to define a large-scale macro pattern or image. This could be achieved by patterning gravure roller 2a and providing multiple application stations 2, one for each resist material.

[0164] However in other embodiments it is preferred to remove the remaining resist material 12 once etching step S110 has been completed and this can be achieved by carrying out a further etching step using a different etchant which dissolves the resist but not the metal layer 11. In this case (and if the resist 12 remains but is transparent or single-coloured) the first pattern elements P.sub.1 will all have the same appearance (corresponding to that of metal layer 11), and the second pattern elements P.sub.2 will be transparent. This may be desirable in some implementations of security devices. However in many cases it is preferable to modify the optical characteristics of the second pattern elements P.sub.2 and this can be achieved, in one example, by applying a colour layer 13 over the patterned metal layer 11, as shown in FIG. 15(b). The colour layer 13 comprises at least one optically detectable substance and is applied over at least a zone of the array. Whilst in preferred cases the colour layer will have a visible colour (i.e. visible to the naked eye), this is not essential since the at least one optically detectable substance could be, for example, a luminescent substance which emits outside the visible spectrum and is only detectable by machine. In general, the colour layer may comprise any of: one or more visible dyes or pigments; luminescent, phosphorescent or fluorescent substances; metallic pigments; interference layer structures or interference layer pigments (e.g. mica, pearlescent pigments, colour-shifting pigments etc.), for example. Substances such as these may be dispersed in a binder to form an ink, for example, suitable for application by printing or coating, or could be applied by other means such as vapour deposition. Most preferably the colour layer is applied by a printing technique such as: laser printing, inkjet printing, lithographic printing, gravure printing, flexographic printing, letterpress or dye diffusion thermal transfer printing. Since the high resolution detail of the image element array is provided by the metal layer 11, the colour layer 13 does not need to be applied using a high resolution process and can if desired be applied in more than one working. It should be noted that a colour layer 13 can also be applied over remaining elements of the resist layer 12 if these remain in place.

[0165] Since the colour layer 13 does not need to be applied at high resolution, it can be made relatively thick and therefore may possess sufficiently high optical density to produce a good quality image by itself. However, in some cases it is desirable to increase the optical density by applying a substantially opaque backing layer 14 over the colour layer 13 as depicted in FIG. 15(b). The backing layer 14 most preferably comprises a further metal layer, e.g. of aluminium. The provision of a backing layer 14 reduces the amount of light transmitted through the device which could otherwise confuse the final image, thereby improving the visual appeal and (in the case of a metal backing layer) making the colour of the first pattern elements, provided by colour layer 13, more reflective and therefore more intense.

[0166] In still further embodiments the colour layer could be located differently within the device structure, provided that from one side of the structure both the metal pattern elements P.sub.1 and the portions of the colour layer 13 in the second pattern elements P.sub.2 can be seen alongside one another. FIGS. 15(c) and (d) show two further exemplary substrates with different structures to illustrate this.

[0167] In FIG. 15(c) the patterned metal layer 11 has been formed on the first surface of a transparent substrate 10 using the same method as previously described. The colour layer 13 is provided on the second surface of the transparent substrate so that when the structure is viewed from the side of the metal layer 11, the colour layer is visible through the gaps in the first pattern elements. Optionally, a substantially opaque backing layer 14 may be provided over the colour layer 13 on the second surface of the substrate as previously described.

[0168] In FIG. 15(d) the colour layer 13 is provided on the first surface of substrate 10 before the patterned metal layer 11 is formed on the same surface. That is, the colour layer 13 is disposed on the metallised substrate web between the substrate and the metal layer 11 provided in step S100 of the above-described method. A substantially opaque backing layer 14 may optionally also be provided under the colour layer 13 on the first surface, or on the second surface of the substrate 10 (not shown). In these embodiments, the substrate 10 need not be transparent since the image element array will not be viewed through it in the finished device.

[0169] Embodiments in which the demetallised pattern is formed on a substrate with a pre-existing colour layer 13 (whether located on the first or second surface of the substrate) are better adapted for use in circumstances where no registration is desired between the colour layer 13 and the demetallised pattern, since it is technically more straightforward to register the application of the colour layer 13 to an existing demetallised pattern than vice-versa.

[0170] In a still further embodiment, shown in FIG. 15(e), in place of printing or coating the colour layer 13 onto substrate 10, the colour layer 13 (and optional backing layer 14) could also be formed on another substrate 19 and then laminated to or transferred onto the metal layer 11.

[0171] In many implementations, the uniformly metallic appearance of the first pattern elements P.sub.1 will be desirable. However, the specularly reflective nature of the metal layer 11 can have the result that the appearance of the elements will depend significantly on the nature of illumination. As such in some embodiments it is preferred to reduce the degree of specular reflection by providing a filter layer 15 (FIG. 16) in the form of a light diffusing layer which will ultimately sit between the metal layer and the viewer, acting to diffuse the light reflected by the metal pattern elements P.sub.1 and hence improve the light source invariance of the finished device. The light diffusing layer 15 is located between the transparent substrate 10 and the metal layer 11 and may therefore be incorporated already in the metallised substrate web provided at the start of the method. Alternatively, if the metallisation is carried out as part of the method, the light diffusing layer 15 may be applied to the substrate in an earlier step. The light diffusing layer can comprise a scattering pigment dispersed in a binder and may be coloured or colourless. The layer can be applied by coating or printing, preferably flexographic, gravure, lithographic or digital printing, and may optionally be a radiation-curable material, e.g. requiring UV-curing. In some embodiments, the appearance of the light diffusing layer 15 may be uniform across the image element array. However in other cases the light diffusing layer could comprise multiple different materials arranged as a multi-coloured pattern or image. The light-diffusing layer need not be applied with high resolution and so can be formed of multiple workings if desired.

[0172] In still further embodiments, the filter layer 15 may not be light-diffusing (i.e. optically scattering), but may comprise a clear, coloured material which can be used to modify the appearance of the metal pattern elements. For example, by providing a filter layer 15 having an orange/brown tint in combination with a metal layer 11 of aluminium, the metal takes on the appearance of copper. The tinted filter layer 15 could be applied to selected regions only (optionally with a clear colourless layer in other areas) to give a bimetallic effect.

[0173] The filter layer 15 will typically not be soluble in the etchant used in step S110 and so will typically remain across the whole image array once the metal layer 11 has been patterned, as shown in FIG. 16. If the filter layer is sufficiently translucent such that a contrast can still be observed between the first and second pattern elements P.sub.1, P.sub.2, this may be acceptable and the light diffusing layer may remain across both sets of elements in the final array. However, generally it is preferred to remove the filter layer 15 from the first pattern elements and this can be achieved by applying a suitable further etchant in which the filter is soluble. A colour layer 13 can then be applied if desired, followed by an optional backing layer 14 (both as described above).

[0174] Since the filter layer 15 is backed up by metal layer 11, it is not required to be of high optical density, although it should act to diffuse and/or to tint or selectively absorb and reflect different colours. Consequently the filter layer 15 can be made thin and this is preferred in order to minimise undercutting of the filter layer during etching. Preferably, the thickness of the filter layer 15 should be equal to or less than the minimum dimension (e.g. line width) of the pattern elements P.sub.1, P.sub.2, more preferably half that dimension or less. For example, if the pattern elements P.sub.1 or P.sub.2 have a dimension of 1 micron, the filter layer should preferably be no thicker than 1 micron, more preferably no thicker than 0.5 microns.

[0175] Like the (optional) filter layer 15, the colour layer 13 may have a uniform appearance across the array, or at least a zone of the array in which it is provided, in which case the finished image element array will be duotone (unless a multi-coloured light diffusing layer is provided). This will be desirable in certain types of security device. However, to increase the complexity and security level of the device, it is preferred that the colour layer 13 comprises multiple zones each comprising different optically detectable substances, e.g. being of different visible colours. The arrangement of different zones may be highly complex, e.g. representing a photograph, or may comprise a simpler arrangement of larger distinct zones. Preferably the colour layer 13 displays an image or indicia (e.g.

[0176] letters, numbers or symbols) either through the relative arrangement of the zones and/or by the periphery of the whole colour layer (i.e. the combined periphery of the zones). In the ensuing examples, different zones of the colour layer 13 will be described for simplicity as having different colours but as noted above whilst in preferred cases these will be different visible colours, this is not essential as the optically detectable substances could be machine readable only. The term colour is also intended to include achromatic appearances such as black, grey, white, silver etc., as well as red, green, blue, cyan, magenta, yellow etc.

[0177] FIGS. 17(a), (b) and (c) illustrate three exemplary security devices which may be formed in accordance with embodiments of the invention. In each case an image pattern P is combined with a viewing component, here in the form of an array 20 of focussing elements 21 (such as lenses or mirrors), in such a way to give rise to an optically variable effect. For instance, the resulting devices may be moir magnification devices, integral imaging devices or lenticular devices of which examples will be given below. In the FIG. 17(a) device, the image pattern P is formed on a first surface of transparent substrate 10 and the focussing element array 20 is provided on the second surface of the same substrate, e.g. by cast-curing, embossing or printing the focussing elements on with a doming resin. Alternatively, as shown in FIG. 17(b) the focussing element array could be provided on a second transparent substrate 19 which is laminated to substrate 10 on which the image pattern P has been formed. If substrate 10 has been configured as a transfer film with a release layer between the substrate material 10 and the metal layer 11 (not shown), the substrate 10 could ultimately be discarded with the resulting device having much the same structure as that shown in FIG. 17(a), with a single substrate 19 remaining. FIG. 17(c) shows another embodiment of a security device in which the image pattern P has been formed on the first surface of substrate 10 and the focussing elements 20 are provided on the other, as in FIG. 17(a) and then the complete assembly has been laminated to a second substrate 19 carrying a colour layer 13 as described above. It is possible to incorporate focussing element arrays with the image patterns in many different ways. For example a focussing element array could be applied to any of the substrate surfaces shown in the embodiments of FIGS. 15(a) to (e), to thereby form a security device.

[0178] An embodiment of a security device will now be described with reference to FIGS. 18(a) and (b). In this case the security device is a moir magnifier, comprising an image element array P formed using the methods described above defining an array of microimages and an overlapping focussing element array 20 with a pitch or rotational mismatch as necessary to achieve the moir effect. FIG. 8(a) depicts part of the image element array P as it would appear without the overlapping focusing element array, i.e. the non-magnified microimage array (but shown at a greatly increased scale for clarity). In contrast, FIG. 18(b) depicts the appearance of the same portion of the completed security device, i.e. the magnified microimages, seen when viewed with the overlapping focussing element array, at one viewing angle.

[0179] In this example, the microimage array is formed using the methods described above and has a cross section corresponding substantially to that shown in

[0180] FIG. 17(c). FIG. 18(a) shows the patterned metal layer 11 and underlying colour layer 13 in plan view and it will be seen that the second pattern elements P.sub.2 form a regular array of microimages which here each convey the digit 5. In this case all of the microimages are of identical shape and size. The metallic first pattern elements P.sub.1 form a contiguous, uniform background surrounding the microimages. Since the colour layer 13 here has two zones of different colour, the microimages in zone 13a appear in a first colour (here represented as black), whilst those in zone 13b appear in a second colour (here represented as white).

[0181] FIG. 18(b) shows the completed security device 30, i.e. the image element array P shown in FIG. 18(a) plus an overlapping focusing element array 20, from a first viewing angle which here is approximately normal to the plane of the device 30. It should be noted that the security device is depicted at the same scale as used in FIG. 18(a): the apparent enlargement is the effect of the focusing element array 20 now included. The moir effect acts to magnify the microimage array such that magnified versions of the microimages are displayed. In this example just two of the magnified microimages are shown. In practice, the size of the enlarged images and their orientation relative to the device will depend on the degree of mismatch between the focussing element array. This will be fixed once the focusing element array is joined to the image element array. In this example, the first magnified microimage is formed from microimages all within zone 13a and hence appears black whilst the second magnified microimage is from microimages all within zone 13b and hence appears white. Upon tilting the magnified microimages may appear to change colour since their position relative to the device will change and they may cross into the other zone of colour layer 13.

[0182] In the above example security device, the microimages are all identical to one another, such that the devices can be considered pure moir magnifiers. However, the same principles can be applied to hybrid moir magnifier/integral imaging devices, in which the microimages depict an object or scene from different viewpoints. Such microimages are considered substantially identical to one another for the purposes of the present invention. An example of such a device is shown schematically in FIG. 19, where FIG. 19(a) shows the unmagnified microimage array, without the effect of focusing elements 21, and FIG. 19(b) shows the appearance of the finished device, i.e. the magnified image. As shown in FIG. 19(a), the microimages 31 show an object, here a cube, from different angles. It should be noted that the microimages are formed as demetallised lines corresponding to the black lines of the cubes in the Figure, the remainder of the metal layer being opaque although this is shown in reverse in the Figure for clarity. A colour layer 13 is provided, here in the form of a single hexagonal zone, which provides colour to the demetallised lines and is concealed by the metal layer elsewhere. Outside the colour layer 13, the microimages may in practice not be visible due to a lack of contrast between the metal layer 11 and a backing layer 14 as previously mentioned. In the magnified image (FIG. 19(b)), the moir effect generates magnified, 3D versions of the cube labelled 34. In reality, only those lines of the magnified cubes 34 which coincide with the colour layer 13 will be visible whilst those portions outside the coloured zone will be invisible or only weakly visible. As the device is tilted the magnified cubes 34 will appear to move across the device and so enter or leave the coloured zone 13 depending on their location and the degree of tilt. This gives the visual impression of the magnified images appearing and disappearing as they move across the central portion of the device. This, combined with the 3D appearance of the images, amounts to an effect with significant visual impact.

[0183] FIG. 20 depicts a further embodiment of a security device 40, which here is a lenticular device. A transparent substrate 10 is provided on one surface with an array of focussing elements 20, here in the form of cylindrical lenses, and on the other surface with an image element array preferably formed of a patterned metal layer 11 and colour layer 13 as described above. The image array comprises first pattern elements P.sub.1, and second pattern elements P.sub.2. The size and shape of each first pattern element P.sub.1 is substantially identical. The pattern elements in this example are elongate image strips and so the overall pattern of elements is a line pattern, the elongate direction of the lines lying substantially parallel to the axial direction of the focussing elements 20, which here is along the x-axis. The lateral extent of the pattern (including its elements P.sub.1 and P.sub.2) is referred to as the array area.

[0184] As shown best in the cross-section of FIG. 20(b), the pattern formed in metal layer 11 and the focussing element array have substantially the same periodicity as one another in the y-axis direction, such that one first pattern element P.sub.1 and one second pattern element P.sub.2 lies under each lens 21. In this case, as is preferred, the width of each element P.sub.1, P.sub.2 is approximately half that of the lens pitch. Thus approximately 50% of the array area carries first pattern elements P.sub.1 and the other 50% corresponds to second pattern elements P.sub.2. In this example, the image array is registered to the lens array 20 in the y-axis direction (i.e. in the arrays' direction of periodicity) such that a first pattern element P.sub.1 lies under the left half of each lens and a second pattern element P.sub.2 lies under the right half. However, registration between the lens array 43 and the image array in the periodic dimension is not essential.

[0185] The colour layer 13 can take any form, including that of a complex, multi-coloured image such as a photograph.

[0186] When the device is viewed by a first observer O.sub.1 from a first viewing angle, each lens 21 will direct light from its underlying first pattern element P.sub.1 to the observer, with the result that the device as a whole appears uniformly metallic as shown in FIG. 20(d). This is referred to more generally as (first) image I.sub.1 since in other examples if a patterned light-diffusing layer were provided over the metal layer (as described in previous embodiments), the first pattern elements P.sub.1 may collectively display any image according to that provided by the light-diffusing layer. When the device is tilted so that it is viewed by second observer O.sub.2 from a second viewing angle, now each lens 21 directs light from the second pattern elements P.sub.2 to the observer. As such the whole device will now appear to display the appearance of the colour layer 13, which in this case carries a star shaped image as shown in FIG. 20(c) which constitutes a (second) image I.sub.2. Hence, as the security device is tilted back and forth between the positions of observer O.sub.1 and observer O.sub.2, the appearance of the device switches between image I.sub.1 and image I.sub.2.

[0187] In order to achieve an acceptably low thickness of the security device (e.g. around 70 microns or less where the device is to be formed on a transparent document substrate, such as a polymer banknote, or around 40 microns or less where the device is to be formed on a thread, foil or patch), the pitch of the lenses must also be around the same order of magnitude (e.g. 70 microns or 40 microns). Therefore the width of the pattern elements is preferably no more than half such dimensions, e.g. 35 microns or less.

[0188] Two-dimensional lenticular devices can also be formed, in which the optically variable effect is displayed as the device is tilted in either of two directions, preferably orthogonal directions. Examples of patterns suitable for forming image arrays for such devices include forming the first pattern elements P.sub.1 as grid patterns of dots, with periodicity in more than one dimension, e.g. arranged on a hexagonal or orthogonal grid. For instance, the first pattern elements P.sub.1 may be square and arranged on an orthogonal grid to form a checkerboard pattern with resulting square second pattern elements P.sub.2 in which the colour layer 13 is visible (as shown in FIG. 10 for example). The focusing elements in this case will be spherical or aspherical, and arranged on a corresponding orthogonal grid, registered to the image array in terms of orientation but not necessarily in terms of translational position along the x or y- axes. If the pitch of the focussing elements is the same as that of the image array in both the x and y directions, the footprint of one focussing element will contain a 2 by 2 array of pattern elements. From an off-axis starting position, as the device is tilted left-right, the displayed image will switch as the different pattern elements are directed to the viewer, and likewise the same switch will be exhibited as the device is tilted up-down. If the pitch of the focusing elements is twice that of the image array, the image will switch multiple times as the device is tilted in any one direction.

[0189] Similar effects can be achieved with other two dimensional arrays of pattern elements, e.g. using second pattern elements P.sub.2 which are circular rather than square. Any other dot shape could alternatively be used, e.g. polygonal.

[0190] Lenticular devices can also be formed in which the two or more images (or channels) displayed by the device at different angles do not correspond exclusively to the first pattern elements on one hand and the second pattern elements on the other. Rather, both pattern elements are used in combination to define sections of two or more images, interleaved with one another in a periodic manner. Thus, in an example the first pattern elements may correspond to the black portions of a first image and those of a second image, whilst the second pattern elements may provide the white portions of the same images, or vice versa. Of course the images need not be black and white but could be defined by any other pair of colours with sufficient contrast. Sections of the first and second images are interleaved with one another in a manner akin to the pattern of lines shown in FIG. 20. When the device is tilted the two images will be displayed over different ranges of angles giving rise to a switching effect. More than two images could be interleaved in this way in order to achieve a wide range of animation, morphing, zooming effects etc. In embodiments such as these the colour layer 13 preferably has a uniform appearance (e.g. single colour) across the array as does any light-diffusion layer provided resulting in a duo-tone appearance.

[0191] In all of the above examples of security devices, a focusing element array is employed to co-operate with the image element array to generate an optically variable effect. However, this is not essential and FIGS. 21, 22 and 23 show some examples of security devices with image element arrays made using the above described methods which do not require focussing element arrays. In these examples, two image element arrays are manufactured using the above-described methods, one on each surface of the substrate material 10, as will be described further below with reference to FIG. 24. However in each case it will be appreciated that just one or other of the described image arrays 11a, 11b need be formed using this technique and the other could be formed using any other available method, e.g. printing.

[0192] FIG. 21 shows a security device 50 which operates on similar principles to those of the lenticular device described above with respect to FIG. 20, but utilising two demetallised image element arrays 11a, 11b rather than a single image element array combined with a focusing element array. In this case, one image element array 11a formed on a first surface of transparent substrate 10 forms a masking grid of metal lines 51 spaced by gaps 52, whilst the other image element array 11b formed on the second surface of transparent substrate 10 exhibits a pattern comprising a sequence of image components, labelled A, B, C, etc. Each of the complete images A, B, C, etc from which the image elements are taken is shown under the cross-section of the device and it will be seen that these comprise a sequence of animation steps depicting a star symbol changing in size. To create the pattern formed in metal layer 11b, the five images A to E are split into elements or slices and interleaved with one another so that a slice of image A is positioned next to a slice of image B, which in turn is positioned next to a slice of image C, and so forth. The resulting pattern is formed as a relief structure 6 and transferred to a resist layer 12 on metal layer 11b in the manner described above, before etching as appropriate. On the opposite side of transparent substrate 10, a masking grid is formed by patterning metal layer 11a using the same method via a different relief structure 6 resulting in a spaced array of visually opaque lines 51 with intervening transparent portions 52 through which the pattern in metal layer 11b may be viewed.

[0193] The device could be designed to be viewed in reflected or transmitted light. Transmitted light is preferred since the contrast in the image can generally be perceived more clearly and in addition the same visual effect can be viewed from both sides of the device. When the device is viewed from above the masking grid 11a, at any one instant, the image slices from only one of the images A to E are visible. For example, in the configuration shown in FIG. 21, when the device is viewed straight-on, only the image slices forming image E will be visible, and thus the device as a whole will appear to exhibit a complete reproduction of image E. Provided the dimensions of the device are correctly selected, when the device is observed from different angles, different images will become visible. For example, when the device is viewed from position A, only the image slices forming image A will be visible through the masking grid 11a, the device as a whole whereby exhibiting the complete image A. Similarly, when the device is viewed from position C only the image slices forming image C will be visible. As such, as the device is tilted and the viewer observes it at different angles, different stages of the animation will be seen and, provided the images are printed in the correct sequence, an animation will be perceived. In the present example this will appear as a star symbol increasing or decreasing as the device is tilted. Thus, in this case the animation is perceived as a zooming in and out but in other cases the images could be arranged to depict, for example, perceived motion (e.g. a horse galloping), morphing (e.g. a sun changing into a moon) or perceived 3D depth (by providing multiple images of the same object, but from slightly different angles). Of course, in other examples, fewer images (e.g. 2) could be interleaved resulting in a switch from one image to another at certain tilt angles, rather than an animation effect.

[0194] In order to achieve this effect, the width of each image slice, X, must be smaller than the thickness, t, of the transparent support layer 10, preferably several times smaller, such that there is a high aspect ratio of the thickness t to image slice width X. This is necessary in order that a sufficient portion of the pattern on metal layer 11b can be revealed through tilting of the device. If the aspect ratio were too low, it would be necessary to tilt the device to very high angles before any change in image will be perceived. In a preferred example, each image slice has a width X of the order of 5 to 10 m, and the thickness t of the support layer 10 is approximately 25 to 35 m. The use of the above-described demetallisation process to form the pattern 11b is therefore particularly advantageous since the high resolution nature of the process allows the formation of image elements at these small dimensions.

[0195] The dimensions of the masking grid 11a are generally larger than those of the pattern elements 11b, requiring opaque stripes of width ((n-1)X) where n is the number of images to be revealed (here, five), spaced by transparent regions of approximately the same width as that of the image slices (X). Thus, in this example the opaque regions 51 of the masking grid 11a have a width of around 20 to 40 m and hence could alternatively be produced using conventional techniques such as printing.

[0196] FIG. 22 shows a further embodiment of a venetian blind-type security device in cross-section, comprising first and second patterned metal layers 11a and 11b positioned on either surface of a transparent substrate 10. Metal layer 11a has been demetallised according to a first pattern P.sub.a whereas metal layer 11b has been exposed to a second pattern P.sub.b. In this example, the device has two laterally offset regions I and II. In region I, the exposed pattern elements of pattern P.sub.a and pattern P.sub.b are identical and aligned with one another. In area II the patterns P.sub.a and P.sub.b are identical in pitch but 180 out of phase with one another such that the remaining regions of the first metal layer 11a forming pattern P.sub.a align with the removed regions of the second metal layer 11b forming second pattern P.sub.b, and vice versa.

[0197] When viewed in transmission from directly above, observer (i) will perceive region A as having a lower optical density than region B where light transmission is blocked by the interplay between the two patterns. In contrast, when viewed from an angle at the position of observer (ii), area A will appear relatively dark compared with area B, since light will now be able to pass through aligned transparent regions of patterns P.sub.a and P.sub.b in area B, whereas it will be blocked by the alignment between pattern elements in area A. This contrast flip between areas A and B provides an easily testable, distinctive effect. In order for the switch to be observable at relatively low tilt angles, the aspect ratio of the support layer thickness relative to the spacing of the pattern elements should again be at least one-to-one. It should be noted that it is not essential to ensure an entirely accurate registration between the two patterns P.sub.a and P.sub.b since provided the sizing of the pattern elements is correct, a switch in contrast between the two regions will still be visible as the device is tilted.

[0198] FIG. 23 shows a further embodiment of a security device in cross-section which here takes the form of a moir interference device. In this embodiment, two patterned metal layers 11a, 11b are provided as on either side of transparent substrate 10 but as in the previous embodiments, one or other of the patterns provided by the metal layers could be provided by other means such as printing.

[0199] To form a moir interference device, each of the metal layers 11a, 11b carries a pattern of elements, mismatches between the two patterns combining to form moir interference fringes. In the example shown, each of the patterns P.sub.a and P.sub.b consists of an array of line elements, with those of one pattern rotated relative to those of the other. In other cases, the mismatch could be provided by a pitch variation rather than a rotation, and/or isolated distortions within one or other of the patterns. When viewed from above such that the two patterns are viewed in combination with one another, moir interference bands are visible and these will appear to move relative to the device depending on the viewing angle. This is due to the precise portions of the two patterns which appear to overlap changing as the viewing angle changes. For instance, in the example of FIG. 23, when viewed directly from above, portion a of pattern P.sub.a will appear to overlap and therefore interfere with portion b of pattern P.sub.b, whereas at a second viewing angle illustrated by observer (ii), the same portion a of pattern P.sub.a will appear to overlap and therefore interfere with a different portion c of the second pattern P.sub.b. In order to achieve significant perceived motion at relatively low viewing angles, a high aspect ratio of the spacing between the two patterns (represented by the thickness t of support layer 10) relative to the spacing s of the line elements in each of the patterns is required. For example, where the line elements have a width and spacing of around 5 m, a thickness t of around 25 m is suitable. No registration between the two patterns P.sub.a and P.sub.b is required.

[0200] The security device structures shown in FIGS. 21, 22 and 23 are preferably formed by carrying out the above-described demetallisation method on both sides of a transparent substrate. FIG. 24 shows an example of apparatus which may be used to produce both patterned metal layers. As shown, the substrate provided in step S100 may include a second metal layer 11b located on the second surface of the substrate 10, which may be of the same composition as the first metal layer 11a, or may be different. Preferably, however, the second metal layer 11b is soluble in the same etchant substance as the first metal layer 11a. A second resist layer 12b is applied over the second metal layer 11b and again this may be of the same composition as the first resist layer 12a or may be different. Both resist layers 12a, 12b are then brought into contact with respective patterning cylinders 7a, 7b each carrying a relief structure 6 in the manner previously described, preferably simultaneously. The two relief structures may be the same as one another or different, and/or may be laterally offset from one another (in the transport path direction and/or in the orthogonal direction), depending on the desired optical effect. Further the nature of the relative motion between the substrate 10 and each relief structure could be the same or could be different. For example, cylinder 7a could be driven at a speed different from that of cylinder 7b. In this example the two relief structures 6 are shown supported on respective opposing cylinders 7a, 7b in a manner correspond to that described above in relation to FIG. 1 but alternatively one or both of the relief structures could be provided in the form of a belt.

[0201] In still further examples, security devices including those discussed above in relation to FIGS. 21 to 23 could be formed by producing two demetallised image patterns P on separate transparent substrates 10 using the above described method, and then laminating them together such that the two metal layers are spaced apart by the two transparent substrates.

[0202] Security devices of the sorts described above are suitable for forming on security articles such as threads, stripes, patches, foils and the like which can then be incorporated into or applied onto security documents such as banknotes and examples of this will be provided further below. However the security devices can also be constructed directly on security documents which are formed of a transparent document substrate, such as polymer banknotes. In such cases, the image pattern may be manufactured on a first substrate, using the method discussed above, and then transferred onto or affixed to one surface of the document substrate, optionally using a transparent adhesive. This may be achieved by foil stamping, for example. An exemplary structure is shown in FIG. 25 where substrate 46 is the transparent document substrate, e.g. BOPP, and layer 47 is an adhesive used to join the image array comprising metal layer 11, colour layer 13 and backing layer 14 (all formed previously) to the substrate. Alternatively, the demetallised pattern array could be formed directly on the document substrate 46 by providing a metal layer on the surface of the substrate 46 (optionally across selected portions only), and performing the above-described method on substrate 46 to form an image element array thereon. Focusing element array 48 can be applied to the opposite side of document substrate 46, e.g. by embossing or cast-curing, before or after the image element array is applied.

[0203] Security devices of the sorts described above can be incorporated into or applied to any product for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc. The image array and/or the complete security device can either be formed directly on the security document (e.g. on a polymer substrate forming the basis of the security document) or may be supplied as part of a security article, such as a security thread or patch, which can then be applied to or incorporated into such a document.

[0204] Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6 mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

[0205] The security article may be incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate at at least one window of the document. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

[0206] Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer. Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300659 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-0039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501, EP-A-724519, WO-A-03054297 and EP-A-1398174.

[0207] The security device may also be applied to one side of a paper substrate, optionally so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

[0208] Examples of such documents of value and techniques for incorporating a security device will now be described with reference to FIGS. 26 to 29.

[0209] FIG. 26 depicts an exemplary document of value 50, here in the form of a banknote. FIG. 26a shows the banknote in plan view whilst FIG. 26b shows a cross-section of the same banknote along the lines X-X. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 51. Two opacifying layers 53 and 54 are applied to either side of the transparent substrate 51, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 51.

[0210] The opacifying layers 53 and 54 are omitted across a selected region 52 forming a window within which a security device is located. In FIG. 26(b), the security device is disposed within window 52, with a focusing element array 48 arranged on one surface of the transparent substrate 51, and image element array 11 on the other (e.g. as in FIG. 18 above). As described in relation to FIG. 18, the image element array 11 could be manufactured on a separate substrate which is then laminated to the document substrate 51 in the window region, or could be manufactured directly on the document substrate 51 by metallising the substrate 51 (at least in the window region 52, optionally all over the substrate) and then forming a demetallised pattern in the metal layer using the above-described method.

[0211] It will be appreciated that, if desired, the window 52 could instead be a half-window, in which one of the opacifying layers (e.g. 53 or 54) is continued over all or part of the image array 11. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers 53 and 54 are provided on both sides.

[0212] In FIG. 27 the banknote 50 is a conventional paper-based banknote provided with a security article 55 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 56 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 55 in window regions 57 of the banknote. Alternatively the window regions 57 may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. It should be noted that it is not necessary for the window regions 57 to be full thickness windows: the thread 55 need only be exposed on one surface if preferred. The security device is formed on the thread 55, which comprises a transparent substrate a focusing array 21 provided on one side and an image array 11 provided on the other. Windows 57 reveal parts of the device, which may be formed continuously along the thread. Alternatively several security devices could be spaced from each other along the thread, with different or identical images displayed by each.

[0213] In FIG. 28, the banknote 50 is again a conventional paper-based banknote, provided with a strip element or insert 58. The strip 58 is based on a transparent substrate and is inserted between two plies of paper 56a and 56b. The security device is formed by a lens array 21 on one side of the strip substrate, and an image array 11 on the other. The paper plies 56a and 56b are apertured across region 59 to reveal the security device, which in this case may be present across the whole of the strip 58 or could be localised within the aperture region 59. It should be noted that the ply 56a need not be apertured and could be continuous across the security device.

[0214] A further embodiment is shown in FIG. 29 where FIGS. 29(a) and (b) show the front and rear sides of the document 50 respectively, and FIG. 29(c) is a cross section along line Z-Z. Security article 58 is a strip or band comprising a security device according to any of the embodiments described above. The security article 58 is formed into a security document 50 comprising a fibrous substrate 56, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (FIG. 29(a)) and exposed in one or more windows 59 on the opposite side of the document (FIG. 29(b)). Again, the security device is formed on the strip 58, which comprises a transparent substrate with a lens array 21 formed on one surface and a co-operating image array 11 as previously described on the other

[0215] Alternatively a similar construction can be achieved by providing paper 56 with an aperture 59 and adhering the strip element 58 onto one side of the paper 56 across the aperture 59. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting.

[0216] In still further embodiments, a complete security device could be formed entirely on one surface of a security document which could be transparent, translucent or opaque, e.g. a paper banknote irrespective of any window region. The image array 11 can be affixed to the surface of the substrate, e.g. by adhesive or hot or cold stamping, either together with a corresponding focusing element array 20 or in a separate procedure with the focusing array 20 being applied subsequently.

[0217] In general when applying a security article such as a strip or patch carrying the security device to a document, it is preferable to bond the article to the document substrate in such a manner which avoids contact between those focusing elements, e.g. lenses, which are utilised in generating the desired optical effects and the adhesive, since such contact can render the lenses inoperative. For example, the adhesive could be applied to the lens array(s) as a pattern that leaves an intended windowed zone of the lens array(s) uncoated, with the strip or patch then being applied in register (in the machine direction of the substrate) so the uncoated lens region registers with the substrate hole or window.

[0218] The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

[0219] Additional optically variable devices or materials can be included in the security device such as thin film interference elements, liquid crystal material and photonic crystal materials. Such materials may be in the form of filmic layers or as pigmented materials suitable for application by printing. If these materials are transparent they may be included in the same region of the device as the security feature of the current invention or alternatively and if they are opaque may be positioned in a separate laterally spaced region of the device.

[0220] The presence of a metallic layer in the security device can be used to conceal the presence of a machine readable dark magnetic layer, or the metal layer itself could be magnetic. When a magnetic material is incorporated into the device the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term alloy includes materials such as Nickel:Cobalt, Iron:Aluminium:Nickel:Cobalt and the like. Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5-50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10-30 microns and a thickness less than 2 microns.

[0221] In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the device structure. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in WO03091953 and WO03091952.

[0222] Negative or positive indicia visible to the naked eye may additionally be created in the metal layer 11 or in any suitable opaque layer, e.g. backing layer 14, either inside or outside the image pattern area.