WO2019046891A1

WO2019046891A1 - Laser marking and/or ablation to create micro-imagery for micro-lens security features

Info

Publication number: WO2019046891A1
Application number: PCT/AU2018/050956
Authority: WO
Inventors: Karlo Ivan Jolic
Original assignee: Ccl Secure Pty Ltd
Priority date: 2017-09-05
Filing date: 2018-09-05
Publication date: 2019-03-14
Also published as: AU2017101215B4; AU2017101215A4; FR3070627A1

Abstract

A method of manufacturing a security device, including: providing a substrate having a first and a second side; applying an ablative coating on the first side of the substrate; directing laser light through a mask including laser light transmissive portions corresponding to a desired imagery pattern to be ablated from the ablative coating; focusing laser light transmitted though the mask via projection optics to form a focused mask image on the ablative coating, without passing through a first array of micro-lenses forming part of the security device, resulting in the removal of the ablative coating in a plurality of areas to create multiple patterns, each pattern being viewable at a particular viewing angle or range of angles through the first array of micro-lenses.

Description

Laser marking and/or ablation to create micro-imagery for micro-lens security features

Technical Field

[0001] The invention relates generally to security documents in which security elements are used as an anti-counterfeiting measure, and in particular to the manufacture of such security documents.

Background of Invention

[0002] Prior art micro-lens based security devices, in which an imagery

component is created by laser ablation and/or marking, generally have a limited number of image channels. This is because each image channel is created by directing a homogenised laser beam (i.e. laser beam with a "top hat" energy distribution i.e. flat energy profile) through a 2D image mask. The beam is then passed through an array of micro-lenses forming part of the security device. The micro-lenses then focus the beam on to the image plane surface, where laser ablation or marking takes place.

[0003] The laser beam is incident to the plane of micro-lenses at a particular relative angle. The image of the mask can then be observed by the user (as a projected optical image) by viewing the lenses at the same relative angle, thus creating a security image uniquely viewable only at angles in the near vicinity of the angle used to perform the ablation and/or marking.

[0004] An advantage of this approach is that the imagery pattern is perfectly aligned to the micro-lenses since the imagery pattern is created by the micro-lenses focusing the incident laser light. However a disadvantage of this approach is that, if a larger number of image channels are required (for example, in order to increase the counterfeit resistance of the feature, or to create more complex optical effects utilising multiple image channels including animations, morphing, flipping images, interlaced 3D images, integral 3D images and magnifying moire images) then implementation of each additional image channel requires implementation of additional laser beam systems and additional optical beam paths thus making the system very complex and expensive from both capital and operational perspective and ultimately unviable as a manufacturing process.

[0005] In the past, multiple image channels in micro-lens security devices have been achieved by printing the imagery on the reverse side of the micro-lenses, for example using gravure printing or flexo printing or offset printing. These printing techniques are generally lower in resolution compared to laser ablation or laser marking, therefore fewer image channels can be created with these techniques i.e. compared to laser ablation / laser marking methods, images that are created using printing methods produce lower complexity optical effects and therefore have lower counterfeit resistance. For example, creating a magnifying moire image with high fidelity is generally not possible with such conventional print methods however it is possible using laser marking/ablation, due to the higher image resolution that is possible with laser. Printing methods also suffer from print defects such as feathering, whereas laser methods do not.

[0006] Multiple image channels in micro-lens security devices have also been achieved by using shims in which the high resolution image pattern (which

implements multiple image channels in micro-lens device) is deployed as a recessed structure in the shim. The manufacturing process involves (i) coating the shim with a thin layer of the UV-curable pigmented ink; (ii) wiping the excess ink away so that only the recessed image structures are filled with ink; (iii) partially curing the pigmented ink left in the recessed image structures; (iv) placing a clear UV-curable "lift-off" layer in contact with the shim; (v) fully curing both the lift-off layer and the pigmented ink; (vi) then lifting the pigmented fully cured ink out of the shim structures. Whilst this method can achieve very high resolution images and a large number of image channels, it is very complex and capital expensive, and its reliability at high production speeds over a wide web is hitherto unproven.

[0007] It would be desirable to provide a simpler and/or more economical method for achieving multiple image channels in a micro-lens security device. [0008] It would be desirable to provide a method of manufacturing a security device that ameliorates and/or overcomes one or more disadvantages of known manufacturing methods.

Summary of Invention

[0009] One aspect of the invention provides a method of manufacturing a security device, including: providing a substrate having a first and a second side; applying an ablative coating on the first side of the substrate; directing laser light through a mask including laser light transmissive portions corresponding to a desired imagery pattern to be ablated from the ablative coating; focusing laser light transmitted though the mask via projection optics to form a focused mask image on the ablative coating, without passing through a first array of micro-lenses forming part of the security device, resulting in the removal of the ablative coating in a plurality of areas to create multiple patterns, each pattern being viewable at a particular viewing angle or range of angles through the first array of micro-lenses.

[0010] In one or more embodiments, the ablative coating includes a first layer of pigmented ink which substantially absorbs laser light.

[001 1] In one or more embodiments, the first layer of pigmented ink substantially absorbs UV laser light including any one or more of 157nm or 193 nm or 248 nm or 266 nm or 308 nm or 355 nm UV laser light.

[0012] In one or more embodiments, the first layer forms part of an opacifying layer or a design layer of a security document bearing the security device.

[0013] In one or more embodiments, the ablative coating further includes at least a second layer of pigmented ink which substantially absorbs laser light, each of the first and at least second layers of pigmented ink having different colours. [0014] In one or more embodiments, the second layer of pigmented ink

substantially absorbs UV laser light including any one or more of 157nm or 193 nm or 248 nm or 266 nm or 308 nm or 355 nm UV laser light.

[0015] In one or more embodiments, the at least second layer forms part of the opacifying layer or design layer.

[0016] In one or more embodiments, prior to passing through the mask, the laser light is homogenised via a homogeniser so it has a substantially uniform spatial energy density in a plane perpendicular to incident laser light direction.

[0017] In one or more embodiments, the focused mask image is directed onto the ablative coating at a normal angle of incidence.

[0018] In one or more embodiments, the thickness of the ablative coating is selected so that a single pulse of the laser beam has sufficient energy density to effect ablation.

[0019] In one or more embodiments, the projection optics de-magnifies and focuses the mask image on the ablative coating so that the energy density in the focused mask image is sufficient to effect ablation with a single laser beam pulse.

[0020] In one or more embodiments, the substrate is optically clear.

[0021] In one or more embodiments, the first array of micro-lenses are formed on the second side of the substrate before removal of the ablative coating in the plurality of areas.

[0022] In one or more embodiments, the first array of micro-lenses are formed on the first side of the substrate before removal of the ablative coating in the plurality of areas, the micro-lenses not overlying areas to be removed by ablation.

[0023] In one or more embodiments, the first array of micro-lenses are formed on the first or second side of the substrate after removal of the ablative coating in the plurality of areas. [0024] In one or more embodiments, the substrate is substantially transmissive to laser light, and wherein the laser light passes through the substrate to focus on the ablative coating.

[0025] In one or more embodiments, after laser ablation has been performed, a second array of micro-lenses is applied to an opposite side of the substrate to the first array of micro-lenses, such that the second array of micro-lenses focus light on the laser ablated areas.

[0026] Another aspect of the invention provides a method of manufacturing a security device, including: providing a substrate having a first and a second side, the substrate including laser- light absorbing additives at a surface on the first side of the substrate, or in a plane below the surface; directing laser light through a mask including laser light transmissive portions corresponding to a desired imagery pattern to be marked; focusing laser light transmitted though the mask via projection optics to form a focused mask image on the surface on the first side of the substrate, or on a plane beneath the surface, without passing through a first array of micro-lenses forming part of the security device, resulting in marking of the surface on the first side of the substrate or the plane beneath the surface in a plurality of areas to create multiple patterns, each pattern being viewable at a particular viewing angle or range of angles through the array of micro-lenses.

[0027] In one or more embodiments, the laser-light absorbing additives are within a skin layer at the first side of the material or within a layer below the surface of the substrate.

[0028] In one or more embodiments, the laser-light absorbing additives

substantially absorb UV laser light including any one or more of 157nm or 193 nm or 248 nm or 266 nm or 308 nm or 355 nm UV laser light, or IR laser light including any one or more of 1060 nm or 1064 nm or 10.6 microns, or visible laser light including 532 nm visible laser light.

[0029] In one or more embodiments, prior to passing through the 2D mask, the laser light is homogenised via a homogeniser so it has a substantially uniform spatial energy density in a plane perpendicular to incident laser light direction.

[0030] In one or more embodiments, the focused mask image is directed onto the surface on the first side of the material or the plane below the surface at a normal angle of incidence.

[0031] In one or more embodiments, the loading of laser-light absorbing additives is selected so that a single pulse of the laser beam has sufficient energy density to effect a local change in contrast or colour in the areas exposed to laser radiation.

[0032] In one or more embodiments, the projection optics de-magnifies and focuses the mask image on the laser-light absorbing additives so that the energy density in the focused mask image is sufficient to effect a local change in contrast or colour in the areas exposed to laser radiation with a single laser beam pulse.

[0033] In one or more embodiments, the substrate is optically clear.

[0034] In one or more embodiments, the first array of micro-lenses is formed on the first or second side of the substrate before laser marking in the plurality of areas.

[0035] In one or more embodiments, the first array of micro-lenses is formed on the first or second side of the substrate after laser marking in the plurality of areas.

[0036] In one or more embodiments, the first array of micro-lenses is formed on the second side of the substrate.

[0037] In one or more embodiments, after laser marking has been performed, a second array of micro-lenses is applied to an opposite side of the substrate to the first array of micro-lenses, such that the second array of micro-lenses focus light on the laser marked areas.

[0038] In one or more embodiments, the mask is a 2D mask. [0039] Another aspect of the invention provides a security device manufactured according to one or more of the methods discussed above.

Brief Description of Drawings

[0040] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be understood that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings:

Figure 1 is a first embodiment of a security device manufactured according to the present invention in which multiple image channels are viewable by a user from different viewing angles;

Figures 2 and 3 depict the manner in which a user is able to view the different image channels in the security device shown in figure 1 ;

Figure 4 is a laser ablation/marking apparatus used for manufacturing the security device shown in figures 1 to 3;

Figures 5 and 6 are schematic diagrams of two different security devices that may be formed by laser ablation;

Figures 7 and 8 are schematic diagrams of two embodiments of a security device that may be formed by laser marking; and

Figure 9 is a magnified image of a pattern that may be ablated or marked in an imagery layer of any of the security devices shown in figures 1 to 3 and 5 to 8.

Detailed Description

Definitions

Security Document or Token [0041] As used herein the term security documents and tokens includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0042] The invention is particularly, but not exclusively, applicable to security documents or tokens such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Security Device or Feature

[0043] As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures;

interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Substrate

[0044] As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), biaxially-oriented polypropylene (BOPP); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

Transparent Windows and Half Windows

[0045] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[0046] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

[0047] A partly transparent or translucent area, hereinafter referred to as a "half- window", may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the "half-window" is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.

[0048] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying Layers [0049] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT<L0, where L0 is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be

subsequently printed or otherwise applied.

Ablation

[0050] In the context of this application, ablation or ablating a material is defined as the removal of that material at the point of ablation. That is to say, the removal of the material in its entirety from the relevant area to form apertures in the ablative layer, or at least partial removal of material from the surface of a layer.

Laser Marking

[0051] In the context of this application, laser marking of a material is defined as

exposure of the material to laser radiation, resulting in a colour change or other similar visible changes to the laser-irradiated area.

Drawings

[0052] Referring to FIG. 1 , a portion of a security device 2 is shown having a substantially transparent substrate 4, an opaque or reflective ablative coating 6 on a first side 8 of the substrate 4 and an array of micro-lenses 10 on a second side 12 of the substrate 4. The array of micro-lenses 10, which are also known as lenticular arrays, can include aspherical or asymmetrical micro-lenses or a suitable mixture of both. Aspherical micro-lenses can be used to better match refractive indices of the micro-lens material and substrate, if they are different, and help reduce spherical aberration. [0053] Apertures 14 and 16 are ablated from the ablative coating 8. A person viewing the security device 2 will be able to view a change in the optical

characteristics of the area over which the array of micro-lenses 10 is arranged. In particular, if a person changes the viewing angle of the security device 2 they will view areas 14 at the first angle Θ and areas 16 at second angle φ respectively. By choosing different patterns for each of the apertures 14 and 16, a "flipping" image can be created. That is, as the security document is tilted or rotated, the visible image changes between pattern or images created at each viewing range.

[0054] In this example, the security device 2 includes two image channels, that is, two images are able to be viewed from different viewing angles. It will be appreciated that additional patterns of apertures can be ablated from the ablative coating 6 to create additional image channels (images that can be viewed from additional viewing angles).

[0055] Referring to FIG. 2, a security document 18 is shown, which in this case is a bank note and includes the security device 2 described in relation to FIG. 1 . The security document 18 is being viewed at an angle to its surface of around the first angle Θ and, therefore, ablated areas 14 are visible through the array of micro- lenses 10, due to the fact that they do not reflect light in the same manner as the ablative layer 8, showing a pattern of the numbers "123".

[0056] FIG. 3 then shows the same security document 18 being viewed at the second angle φ. In this case, it is ablated areas 16 which are visible through the array of micro-lenses 10, showing a pattern of the numbers "456".

[0057] In a preferred embodiment, the ablative layer 8 reflects light when viewed through the array of micro-lenses 10. In this manner, the areas which have been ablated do not reflect light and this provides a distinct contrast when a person changes the viewing angles such that the micro-lenses focus on reflective and non- reflective areas.

[0058] Figure 4 depicts a laser ablation/marking apparatus 30 designed for manufacturing the security document 18 depicted in figures 2 and 3. The apparatus

30 includes a laser 32, a beam delivery system 34 including mirrors 36, 38, 40 and 42, , a beam expander 44 and a homogeniser 46. The apparatus 30 further includes a dielectric mask 48 with micron features, as well as projection optics 50 including, in this example, a 9-element CaF2 transfer lens for achieving image reduction. A computer 52 controls operation of the laser 32, and by using feedback of the surface speed and position of a roller 54 over which a substrate web 56 passes, one or more security devices are positioned in the path of laser light 58. Alternative configurations are also possible, in which the web or sheet of substrate is not in contact with a roller during marking &/or ablation.

[0059] The laser ablation/marking apparatus 30 may be integrated with a web or sheet fed process, such as a printing process for making security documents. This enables laser marking and/or laser ablation to be performed in-line with a security document manufacturing process. In one example of this configuration, the laser may be activated by the computer when the laser marking and/or laser ablation areas are in register to a registration mark on the web or sheet that was applied earlier in the process.

[0060] Figure 5 depicts a security device 70 which may be carried or form part of a security document borne by the web substrate 56.

[0061] The substrate 72 of the security device 70 is coated with a layer 74 of pigmented ink which substantially absorbs laser light, preferably UV laser light, more preferably 1 57nm or 193 nm or 248 nm or 308 nm UV laser light.

[0062] Optionally, the substrate 72 is also coated with one or more layers 76 and 78 of pigmented ink. Each layer has a different colour, overlapping or non-overlapping with one another, which substantially absorb laser light, preferably UV laser light, more preferably 157nm or 193 nm or 248 nm or 308 nm UV laser light.

[0063] Optionally, one or more of the pigmented ink layers 74, 76 and 78 may form part of an opacifying layer or a design layer of the security document.

[0064] During manufacture, the beam 58 of laser light, preferably homogenised via a homogeniser 46 so it has a substantially uniform spatial energy density in a plane perpendicular to the beam direction, is directed through the 2D mask 48 which includes laser-light-transmissive portions corresponding to a desired imagery pattern that is to be ablated.

[0065] After passing through said mask 48, the laser beam 58 is directed via the projection optics 50 to form a focused mask image on the surface of the pigmented ink layer or layers that is to be ablated, preferably at a normal angle of incidence. Because the laser ablation resolution is very high (for example, in the case of a UV laser mask projection system, features as small as a couple of microns can be laser ablated), this means a large number of image channels may be implemented in the imagery pattern to be ablated.

[0066] Preferably, the thickness of the layer or layers of pigmented ink is chosen so that 1 pulse or 1 shot of the laser beam has sufficient energy density to effect ablation.

[0067] Preferably, the beam delivery optics from the mask 48 to the pigmented ink de-magnifies and focuses the mask image on the pigmented ink so the energy density in the focused mask image is sufficient to effect ablation with 1 pulse or 1 shot of the laser beam.

[0068] Preferably, the substrate 72 is optically clear which allows micro-lenses 80 to be applied on the opposite side of the substrate to the ablative coating, either before or after laser ablation step.

[0069] Optionally, after laser ablation has been performed, a second set of micro- lenses can be applied on the opposite side of the substrate to the first set of micro- lenses, such that the second set of micro-lenses are focused on the laser ablated areas. This configuration produces magnified optical effect images that can be viewed from either side of the security device.

[0070] If the substrate 72 is substantially transmissive to laser light, the beam 58 may optionally pass through the substrate 72, so that it focuses on the interface between the substrate and the pigmented ink i.e. in this configuration ablation takes place by directing the beam to focus through the substrate on to the reverse side of the pigmented ink. [0071] In a variant shown in figure 6, a substrate 82 is made from non-transparent materials, for example opaque materials on which a layer 84 of ink, and optionally one or more further layers, can be printed and then directly laser ablated. In this case, micro lenses 86 can be applied by lamination or another suitable technique to the ablated imagery layer to realise the security device 88.

[0072] In addition to the use of ablation to manufacture a security device, the apparatus 30 can be used to manufacture security devices by laser marking laser- light absorbing additives, such as TiO2, located at the surface or in a layer within a substrate. The layer thickness is preferably less than the depth of field of the focused laser light, for example, up to 5-10 microns max. Polymers with skin layers containing laser-light absorbing additives, such as TiO2, may also be used. Transparent or opaque polymers may be used.

[0073] To ensure sufficient resolution for marking micro-lens imagery for banknote/security document applications, laser light that has a wavelength of 1 micron or less is preferred.

[0074] Preferably UV laser wavelengths such as 157nm or 193 nm or 248 nm or 308 nm and substrates with TiO2 and/or ZnO additives are used.

[0075] During manufacture, the beam 58 of laser light, preferably UV laser light, is preferably homogenised via the homogeniser 46 so it has a substantially uniform spatial energy density in a plane perpendicular to the beam direction, is directed through the 2D mask 48 in which the laser-light-transmissive portions correspond to the desired imagery pattern that is to be marked on the substrate.

[0076] After passing through said mask 48, the laser beam is directed via the projection optics 50 to form a focused mask image on the surface 94 of the substrate 92 that is to be marked, preferably at a normal angle of incidence. Once again, because the laser marking resolution is very high (for example, in the case of a UV laser mask projection marking system, features as small as a few microns can be laser marked on polymer skin layers loaded with TiO2 additives) this means a large number of image channels may be implemented in the imagery pattern to be marked. [0077] Preferably, the percentage loading of the laser-light absorbing additives is chosen so that 1 pulse or 1 shot of the laser beam has sufficient energy density to effect a local change in contrast or colour in the areas exposed to the laser radiation.

[0078] Preferably, the beam delivery optics from the mask 48 to the surface 94 of the substrate with laser-light absorbing additives de-magnifies and focuses the mask image on the surface so the energy density in the focused mask image is sufficient to create a contrasting mark with 1 pulse or 1 shot of the laser beam.

[0079] Preferably, the substrate 92 with laser-light absorbing additives is substantially optically clear, which allows micro-lenses 96 to be applied on the opposite side, either before or after laser marking step.

[0080] In a first variant shown in figure 7, a security device 100 includes a substrate 102 in which the laser light absorbing additives are within a skin layer 104 at a first side 106 of the substrate 102.

[0081] Alternatively, and as shown in figure 8, a security device 1 10 may include a substrate 1 12 in which the laser-light absorbing additives are within a layer 1 14 below the surface 1 16 of the substrate 1 12. In this case, the polymer layer between the laser source and the layer 1 16 should be transmissive to the laser source.

[0082] In the variants depicted in figures 7 and 8, micro lenses 108 and 1 18 are respectively applied to the security devices 100 and 1 10 either before or after the laser marking step.

[0083] Optionally, after laser marking has been performed, a second set of micro- lenses can be applied on the opposite side of the substrate to the first set of micro- lenses, such that the second set of micro-lenses are focused on the laser marked areas. This configuration produces magnified optical effect images that can be viewed from either side of the security device.

[0084] Figure 9 depicts a magnified image 120 of laser ablated icons of the letter "A" with a 50 micron pitch (suitable for moire magnification), suitable for producing a moire magnification optical effect in the above described security devices. It will be appreciated this is but one example of a pattern that may be ablated or marked as described hereabove.

[0085] Embodiments of the invention provide a simpler and more economical method for achieving multiple image channels via laser ablation and/or marking in a micro-lens security device than currently exists.

[0086] It is currently counterintuitive to ablate or mark imagery on a lens based device without focusing laser light through an array of micro-lenses forming part of the security device because of industry assumptions that this is required in order to obtain registration.

[0087] However, if a roll to roll process is used to create the lenses and the imagery, it is straightforward to achieve good angular registration i.e. the skew between the lenses and the imagery can be tightly controlled and skew can be minimised to an acceptable level. The registration tolerance of the position of the laser created imagery, relative to the lens array, will depend mainly on the registration tolerance of the lenses (the laser optics positioning should be much more consistent and very accurate).

[0088] In the "soft emboss" process used by the Applicant to produce various security documents, the lens position will typically vary by +/- 0.25 mm i.e. this is much larger than the typical size of a micro lens on a banknote, so in effect it will not be possible to control the relative X-Y register to ensure the same image channel is projected to a viewer at a given viewing angle (whereas the method of passing the laser through the lenses to create the imagery does produce very accurate

registration i.e. the image channel projected to a viewer at a given viewing angle will always be the same, from one banknote to the next). Even so, even if the image channel projected at a given angle changes from one banknote to the next, this is not necessarily a problem, since overall the user will still perceive that the image design projected is the same image from one banknote to the next.

Claims

1 . A method of manufacturing a security device, including:

providing a substrate having a first and a second side;

applying an ablative coating on the first side of the substrate;

directing laser light through a mask including laser light transmissive portions corresponding to a desired imagery pattern to be ablated from the ablative coating; focusing laser light transmitted though the mask via projection optics to form a focused mask image on the ablative coating, without passing through a first array of micro-lenses forming part of the security device, resulting in the removal of the ablative coating in a plurality of areas to create multiple patterns, each pattern being viewable at a particular viewing angle or range of angles through the first array of micro-lenses.

2. A method according to claim 1 , wherein the ablative coating includes a first layer of pigmented ink which substantially absorbs laser light.

3. A method according to claim 2, wherein the first layer of pigmented ink substantially absorbs UV laser light including any one or more of 157nm or 193 nm or 248 nm or 266 nm or 308 nm or 355 nm UV laser light.

4. A method according to either one of claims 2 or 3, wherein the first layer forms part of an opacifying layer or a design layer of a security document bearing the security device.

5. A method according to any one of claims 2 to 4, wherein the ablative coating further includes at least a second layer of pigmented ink which substantially absorbs laser light, each of the first and at least second layers of pigmented ink having different colours.

6. A method according to claim 5, wherein the second layer of pigmented ink substantially absorbs UV laser light including any one or more of 157nm or 193 nm or 248 nm or 266 nm or 308 nm or 355 nm UV laser light.

7. A method according to either one of claims 5 or 6 when dependant on claim 4, wherein the at least second layer forms part of the opacifying layer or design layer.

8. A method according to any one of the preceding claims, wherein, prior to passing through the mask, the laser light is homogenised via a homogeniser so it has a substantially uniform spatial energy density in a plane perpendicular to incident laser light direction.

9. A method according to any one of the preceding claims, wherein the focused mask image is directed onto the ablative coating at a normal angle of incidence.

10. A method according to any one of the preceding claims, wherein the thickness of the ablative coating is selected so that a single pulse of the laser beam has sufficient energy density to effect ablation.

1 1 . A method according to any one of the preceding claims, wherein the projection optics de-magnifies and focuses the mask image on the ablative coating so that the energy density in the focused mask image is sufficient to effect ablation with a single laser beam pulse.

12. A method according to any one of the preceding claims, wherein the substrate is optically clear.

13. A method according to claim 12, wherein the first array of micro-lenses are formed on the second side of the substrate before removal of the ablative coating in the plurality of areas.

14. A method according to claim 12, wherein the first array of micro-lenses are formed on the first side of the substrate before removal of the ablative coating in the plurality of areas, the micro-lenses not overlying areas to be removed by ablation.

15. A method according to claim 12, wherein the first array of micro-lenses are formed on the first or second side of the substrate after removal of the ablative coating in the plurality of areas.

16. A method according to any one of the preceding claims, wherein the substrate is substantially transmissive to laser light, and wherein the laser light passes through the substrate to focus on the ablative coating.

17. A method according to any one of the preceding claims, wherein, after laser ablation has been performed, a second array of micro-lenses is applied to an opposite side of the substrate to the first array of micro-lenses, such that the second array of micro-lenses focus light on the laser ablated areas.

18. A method of manufacturing a security device, including:

providing a substrate having a first and a second side, the substrate including laser-light absorbing additives at a surface on the first side of the substrate, or in a plane below the surface;

directing laser light through a mask including laser light transmissive portions corresponding to a desired imagery pattern to be marked;

focusing laser light transmitted though the mask via projection optics to form a focused mask image on the surface on the first side of the substrate, or on a plane beneath the surface, without passing through a first array of micro-lenses forming part of the security device, resulting in marking of the surface on the first side of the substrate or the plane beneath the surface in a plurality of areas to create multiple patterns, each pattern being viewable at a particular viewing angle or range of angles through the first array of micro-lenses.

19. A method according to claim 18, wherein the laser-light absorbing additives are within a skin layer at the first side of the substrate or within a layer below the surface of the substrate.

20. A method according to either one of claims 18 or 19, wherein the laser-light absorbing additives substantially absorb UV laser light including any one or more of 157 nm or 193 nm or 248 nm or 266 nm or 308 nm or 355 nm UV laser light, or IR laser light including any one or more of 1060 nm or 1064 nm or 10.6 microns, or visible laser light including 532 nm visible laser light.

21 . A method according to any one of claims 18 to 20, wherein, prior to passing through the 2D mask, the laser light is homogenised via a homogeniser so it has a substantially uniform spatial energy density in a plane perpendicular to the incident laser light direction.

22. A method according to any one of claims 18 to 21 , wherein the focused mask image is directed onto the surface on the first side of the material or the plane below the surface at a normal angle of incidence.

23. A method according to any one of claims 18 to 22, wherein the loading of laser- light absorbing additives is selected so that a single pulse of the laser beam has sufficient energy density to effect a local change in contrast or colour in the areas exposed to laser radiation.

24. A method according to any one of claims 18 to 23, wherein the projection optics de-magnifies and focuses the mask image on the laser-light absorbing additives so that the energy density in the focused mask image is sufficient to effect a local change in contrast or colour in the areas exposed to laser radiation with a single laser beam pulse.

25. A method according to any one of claims 18 to 24, wherein the substrate is optically clear.

26. A method according to claim 25, wherein the first array of micro-lenses is formed on the first or second side of the substrate before laser marking in the plurality of areas.

27. A method according to claim 25, wherein the first array of micro-lenses is formed on the first or second side of the substrate after laser marking in the plurality of areas.

28. A method according to any one of claims 18 to 25, wherein the first array of micro- lenses is formed on the second side of the substrate.

29. A method according to any one of claims 18 to 28, wherein, after laser marking has been performed, a second array of micro-lenses is applied to an opposite side of the substrate to the first array of micro-lenses, such that the second array of micro- lenses focus light on the laser marked areas.

30. A method according to any one of the preceding claims, wherein the mask is a 2D mask.

31 . A security device manufactured according to claims 1 to 17 and / or claims 18 to 30.