WO2010121293A1 - Optically variable device and security document including same - Google Patents

Optically variable device and security document including same Download PDF

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Publication number
WO2010121293A1
WO2010121293A1 PCT/AU2010/000435 AU2010000435W WO2010121293A1 WO 2010121293 A1 WO2010121293 A1 WO 2010121293A1 AU 2010000435 W AU2010000435 W AU 2010000435W WO 2010121293 A1 WO2010121293 A1 WO 2010121293A1
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WO
WIPO (PCT)
Prior art keywords
elements
dzp
security document
microimage
transparent
Prior art date
Application number
PCT/AU2010/000435
Other languages
French (fr)
Inventor
Robert Arthur Lee
Original Assignee
Securency International Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to AU2009901719A priority Critical patent/AU2009901719A0/en
Priority to AU2009901719 priority
Application filed by Securency International Pty Ltd filed Critical Securency International Pty Ltd
Publication of WO2010121293A1 publication Critical patent/WO2010121293A1/en

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Classifications

    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00Optical elements other than lenses
    • G02B5/18Diffraction gratings
    • G02B5/1876Diffractive Fresnel lenses; Zone plates; Kinoforms
    • G02B5/188Plurality of such optical elements formed in or on a supporting substrate
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B42BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
    • B42DBOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
    • B42D25/00Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
    • B42D25/40Manufacture
    • B42D25/405Marking
    • B42D25/41Marking using electromagnetic radiation
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B42BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
    • B42DBOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
    • B42D25/00Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
    • B42D25/20Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof characterised by a particular use or purpose
    • B42D25/21Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof characterised by a particular use or purpose for multiple purposes
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B42BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
    • B42DBOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
    • B42D25/00Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
    • B42D25/20Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof characterised by a particular use or purpose
    • B42D25/29Securities; Bank notes
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B42BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
    • B42DBOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
    • B42D25/00Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
    • B42D25/30Identification or security features, e.g. for preventing forgery
    • B42D25/328Diffraction gratings; Holograms
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B42BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
    • B42DBOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
    • B42D25/00Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
    • B42D25/30Identification or security features, e.g. for preventing forgery
    • B42D25/342Moiré effects
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B42BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
    • B42DBOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
    • B42D25/00Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
    • B42D25/40Manufacture
    • B42D25/405Marking
    • B42D25/425Marking by deformation, e.g. embossing
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B41PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41MPRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
    • B41M3/00Printing processes to produce particular kinds of printed work, e.g. patterns
    • B41M3/14Security printing
    • B42D2035/20
    • B42D2035/44

Abstract

There is provided a security document or device (95) comprising a substantially transparent or translucent material (60), an array of micro image elements (91a, 91b) provided on one side (62) of the transparent or translucent material (60), and a diffractive lens array (92) including a plurality of linear diffractive zone plate (DZP) elements provided on the opposite side (61) of the substantially transparent or translucent material (60). The linear DZP elements form diffractive lenses each arranged to focus incident radiation (98) onto a micro image element of the array of micro image elements to produce a magnified image. The individual DZP elements have at least one characteristic, such as focal length or off-axis distance, modulated with respect to an adjacent DZP element.

Description

OPTICALLY VARIABLE DEVICE AND SECURITY DOCUMENT INCLUDING
SAME FIELD OF THE INVENTION
The present invention relates to optically variable devices, and to the use of such optically variable devices as security devices in security documents. BACKGROUND
It is known to include security devices having an optically variable effect as an authentication means for a security document. A person viewing the security document can verify that the document is authentic by changing the viewing angle, and observing a change in the image or images produced by the security device. For example, the security device may be a hologram or other diffractive structure which changes with viewing angle due to the different orders in the diffraction pattern.
Another type of optically variable device, as exemplified by WO 2006/125224, uses refractive structures in the form of an array of microlenses, focussed on an array of microimage elements. The microlenses collectively image the array of microimages to produce a magnified image by moire magnification, the magnified image appearing to move as the viewing angle changes. The use of such refractive structures necessarily adds to the thickness of the device, possibly substantially so unless the base diameter of the microlenses is made very small, in which case the fabrication of the microlenses may pose serious challenges.
Accordingly, it is desirable to provide an alternative security device which is capable of providing a great variety of optically variable effects, whilst having a lesser thickness than known refractive devices. DEFINITIONS
Security Document
As used herein the term security document 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. The invention is particularly, but not exclusively, applicable to security documents 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.
Substrate
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); 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.
The use of plastic or polymeric materials in the manufacture of security documents pioneered in Australia has been very successful because polymeric banknotes are more durable than their paper counterparts and can also incorporate new security devices and features. One particularly successful security feature in polymeric banknotes produced for Australia and other countries has been a transparent area or "window".
Transparent Windows and Half Windows
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.
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. 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.
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
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
Lγ < Lo , where Lo 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.
Security Device or Feature
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).
Embossable Radiation Curable Ink
The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink may be cured by other forms of radiation, such as electron beams or X-rays.
The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as numerical-type DOEs and lens structures.
In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.
Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings may be based on other compounds, eg nitro-cellulose.
The radiation curable inks and lacquers used in the invention have been found to be particularly suitable for embossing microstructures, including diffractive structures such as DOEs, diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be embossed with larger relief structures, such as non-diffractive optically variable devices.
. The ink is preferably embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a Gravure printing process. Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.
With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a cross-linker, for example a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-lined or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; ziconium complexes; aluminium acetylacetone; melamines; and carbodi-imides.
The type of primer may vary for different substrates and embossed ink structures. Preferably, a primer is selected which does not substantially affect the optical properties of the embossed ink structure.
In another possible embodiment the radiation curable ink may include metallic particles to form a metallic ink composition which is both printable and embossable. Such a metallic ink composition may be used to print a reflective security element, such as a diffraction grating or hologram. Alternatively, a transparent ink, e.g. formed from a clear resin, may be applied on one side of the substrate, with or without an intermediate primer layer, the transparent ink then being embossed and cured with radiation and a metallic ink composition subsequently applied to the embossed transparent ink in a printing process, if it is desired to form a reflective security element as part of the security device.
It is also possible for the metallic ink composition to be applied in a layer which is sufficiently thin to allow the transmission of light. When a metallic ink is used, it preferably comprises a composition including metal pigment particles and a binder. The metal pigment particles are preferably selected from the group comprising: aluminium, gold, silver, platinum, copper, metal alloy, stainless steel, nichrome and brass. The metallic ink preferably has a low binder content and a high pigment to binder ratio. Examples of metallic ink compositions suitable for use in the present invention are described in WO2005/049745 of Wolstenholme International Limited, which describes coating compositions suitable for use in coating a diffraction grating comprising metal pigment particles and a binder, wherein the ratio of pigment to binder is sufficiently high as to permit the alignment of the pigment particles to the contours of the diffraction grating. Suitable binders may comprise any one or more selected from the group comprising nitrocellulose, ethyl cellulose, cellulose acetate, cellulose acetate propionate (CAP), cellulose acetate butyrate (CAB), alcohol soluble propionate (ASP), vinyl chloride, vinyl acetate co-polymers, vinyl acetate, vinyl, acrylic, polyurethane, polyamide, rosin ester, hydrocarbon, aldehyde, ketone, urethane, polyethyleneterephthalate, terpene phenol, polyolefin, silicone, cellulose, polyamide and rosin ester resins. In one particularly preferred metallic ink composition, the binder comprises nitro cellulose and polyurethane.
The pigment to binder ratio preferably falls substantially within the range from about 5:1 to about 0.5:1 by weight, and more preferably falls substantially within the range from about 4:1 to about 1 :1 by weight.
The metal pigment content by weight of the composition is preferably less than about 10%, and more preferably less than about 6%. In particularly preferred embodiments, the pigment content by weight of the composition falls substantially in the range from about 0.2% to about 6%, and more preferably from about 0.2% to about 2%.
The average particle diameter may be in the range from about 2μm to about 20μm, preferably in the range from about 5μm to about 20μm, and more preferably in the range from about 8μm to about 15μm.
The thickness of the pigment particles is preferably less than about 100nm and more preferably less than about 50nm. In one embodiment, the thickness of the pigment particles falls substantially within the range from 10-50nm. In another embodiment, the thickness of the pigment particles falls substantially within the range from 5-35nm, and in another embodiment the average thickness of the pigment particles falls substantially within the range from 5-18nm.
Embossable UV curable ink compositions such as described above have been found to be particularly suitable for embossing to form optically diffractive security devices, such as diffraction gratings, holograms and diffractive optical elements.
In the case of a half-window in which the transparent region is covered on one side by at least one opacifying layer, a security device formed from an embossed metallic ink may be a reflective device which is only visible in the half- window from the opposite side of the substrate, which is not covered by an opacifying layer in the half-window area.
It is also possible for the opacifying layer, which covers the half-window area on one side of the substrates, to allow the partial transmission of light so that the security device formed by the embossed ink is partially visible in transmission from the side, which is covered by the opacifying layer in the half-window area. SUMMARY OF THE INVENTION
In one aspect of the present invention, there is provided a security document or device comprising a substantially transparent or translucent material, an array of modulated microimage elements provided on one side of the transparent or translucent material, and a diffractive lens array including a plurality of diffractive zone plate (DZP) elements provided on the opposite side of the substantially transparent or translucent material, wherein the DZP elements form diffractive lenses arranged to focus incident radiation transmitted through or from the array of microimage elements to produce a magnified image.
The individual DZP elements of the diffractive lens array preferably have at least one characteristic, eg focal length and/or off-axis distance (the offset between the central zone of the array and the geometric centre of the array), modulated with respect to an adjacent DZP element.
By modulating a characteristic such as the focal length and/or off-axis distance of individual DZP elements or pixels across the array, it is possible to produce a wide variety of optical effects which are visible in transmission or reflection. For example, the width or radius of the central zone (and consequently of the zones to either side of the central zone) may be altered in order to change the focal length of a DZP element. A source being imaged by the DZP array (diffractive lens array) will thus appear to "float" above or behind the device as different parts of the device are viewed since the DZP elements in a particular region of the device will produce images at distances according to their focal length.
Advantageously, in addition to the moving and/or floating image effect produced by the DZP array, the DZP elements may also produce a coloured diffractive optically variable effect to provide an additional security feature. The use of diffractive structures as focussing elements also greatly reduces the thickness of the device.
The DZP elements are preferably linear diffractive zone plate elements, though it is possible for circular or elliptical DZP elements or other forms of DZP elements to comprise, or be included, in the diffractive lens array.
Preferably, the diffractive lens array includes a plurality of linear DZP elements, at least one of which is an off-axis linear DZP element. The diffractive lens array may include a first set of off-axis linear DZP elements arranged to focus radiation onto a first set of microimage elements, and a second set of linear DZP elements arranged to focus light onto a second set of microimage elements, whereby a first magnified image can be seen when the security document or device is viewed from a first angle, and a second magnified image can be seen when the security document or device is viewed from a second angle.
In another embodiment, the diffractive lens array may include a plurality of DZP elements with individual elements being characterised by different focal lengths and/or off-axis distances, whereby a moving and/or floating image is observable in a plane which is different from the plane of the microimages. Preferably, the focal lengths and/or off-axis distances of adjacent DZP elements are different from each other.
Alternatively, the off-axis distance of the DZP elements may be modulated so that different parts of the input image are observable at different angles, thus producing a moving image effect. In a further alternative, both the focal length and the off-axis distance may be modulated in order to produce a composite effect whereby the image appears to both float and move as the device is viewed from different positions and/or angles.
In another aspect of the present invention, there is provided a method of manufacturing a security document or device comprising a substantially transparent or translucent material, the method including the steps of: applying an array of microimage elements to one side of the material, and applying a plurality of diffractive zone plate (DZP) elements to the opposite side of the material, and forming the DZP elements such that individual DZP elements have at least one characteristic, such as focal length or off-axis distance, modulated with respect to an adjacent DZP element, whereby the DZP elements form diffractive lenses each arranged to focus incident radiation from a microimage element of the array of microimage elements to produce a magnified image.
The microimage elements may be applied by a printing process, or by any other suitable method, such as laser marking or embossing. In one preferred embodiment, the microimage elements are applied by directing a laser through the DZP elements from the opposite side of the material so that the microimage elements are formed on the first side of the material.
The security document or device may include a highly reflective layer, such as a metallic layer. For example, the metallic layer may be applied on the same side of the material as the DZP array, so that light diffracted from the DZP elements is specularly reflected and focussed whereby the optically variable effect is visible in reflection.
The diffractive lens array may be applied to the material by any of a variety of different methods, including an embossing process, an engraving process, or by electron beam lithography. In one embodiment, the DZP elements may be embossed directly into the material. Alternatively, the DZP elements may be embossed into an additional layer applied to the material. The additional layer is preferably applied by a printing process such as a rotogravure process.
The additional layer may be a radiation curable material, preferably a transparent or translucent radiation curable material. The method may further include the step of curing the radiation curable material. Preferably, this is done at substantially the same time as embossing the DZP elements into the radiation curable material.
Applying the DZP elements by embossing enables the diffractive lens array to be formed on the security document or device in a relatively cheap and fast process which can be incorporated into in-line processes which are used in banknote manufacture, for example.
The embossing tool or shim may be formed by any convenient method, such as engraving, but is preferably formed by electron beam lithography.
One or both sides of the substantially transparent or translucent material of the security document or device may include an opacifying layer applied thereto. If the opacifying layer is applied to both sides, it does not cover the entire surface of the document or device, but is omitted at least in the region containing the DZP elements. Preferably, the opacifying layer is also omitted in the region containing the microimage elements.
One or both sides of the security document or device may have additional security elements or indicia applied thereto, such as text, printed artwork and so on. The additional indicia may be applied at least partly in the same region as the DZP elements. BRIEF DESCRIPTION OF THE FIGURES
Various embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is an enlarged plan view of a circular diffractive zone plate;
Figure 2 is an enlarged plan view of a linear diffractive zone plate;
Figure 3 shows examples of eight modulated linear diffractive zone plate elements;
Figure 4 is an enlarged plan view of a diffractive lens array comprising a plurality of modulated linear diffractive zone pixel elements;
Figure 5 is a schematic view showing formation of a magnified image when viewing a microimage element through a linear diffractive zone plate;
Figure 6 is a schematic sectional view through a transparent substrate provided with a microimage element and a linear diffractive zone array showing formation of a magnified virtual image; Figure 7 is an example of two off-axis linear diffractive zone plate elements of opposite focussing bias;
Figure 8 is an enlarged plan view of a two channel diffractive lens array including two sets of off-axis linear DZP elements such as shown in Figure 7;
Figures 9 (a), (b) and (c) are schematic diagrams showing three alternative ways of configuring a two-channel linear DZP array;
Figure 10 is an example of a microimage array including interlaced microimage elements for a two channel linear DZP array;
Figure 11 is an enlarged view of an area of an alternative microimage array including interlaced microimage elements;
Figure 12 is a schematic section through a security document incorporating an optically variable device in accordance with the invention; and
Figure 13 is a schematic view showing a method of forming microimages for a multichannel diffractive zone plate array. DETAILED DESCRIPTION OF THE FIGURES
Referring to Figures 1 (a) and 1 (b), there is shown enlarged plan and cross- sectional views of a circular diffractive zone plate (DZP) 10. The circular DZP 10 may be formed from a periodical grating structure of alternating opaque circular zones 12a, 12b, 12c, etc and transparent circular zones 14 a, 14b, 14c, etc. as shown in Figure 1 b. Alternatively, a circular DZP may be formed as a three- dimensional grating structure having grooves or raised areas corresponding to the transparent circular zones 14a, 14b, 14c, etc and raised areas or grooves corresponding to the opaque circular zones 12a, 12b, 12c etc. As shown by Figures 1 , the period or width of the zones 12a, 14a, 12b, 14b, 12c, 14c, etc decreases successively with increasing radius.
Further information about Zone Plate Optics of circular DZPs can be found, for example, in D. Attwood, "Soft X-rays and extreme ultraviolet radiation", University Press, Cambridge (1999).
Figure 2 shows an enlarged view of a linear diffractive zone plate (LDZP) 20. The LDZP 20 may be formed from a periodical linear grating structure of alternating transparent linear zones 22a, 22b, 22c etc and opaque linear zones 24a, 24b, 24c etc. Alternatively a linear DZP may be formed as a three- dimensional grating structure having raised areas or grooves corresponding to the transparent linear zones 22a, 22b, 22c etc and grooves or raised areas corresponding to the opaque linear zones 24a, 24b, 24c etc. In similar manner to the circular DZP of Figure 1 , the period or width of the zones 22a, 24a, 22b, 24b, 22c, 24c, etc of the LDZP 20 decreases successively moving outwardly from the central zone 22a.
The LDZP 20 forms a diffractive lens structure which functions in a similar manner to a conventional cylindrical refractive lens, except for the addition of multiple focussing lines or spots which are much weaker than the main focal spots.
Figure 3 shows a linear diffractive zone plate (LDZP) palette of 8 elements or pixels which may be used to form security devices in accordance with the invention. The LDZP element 31 or pixel LDZP001 is a symmetric zone plate 31 having a central transparent linear zone 31a and the same number of transparent linear zones 31b, 31c etc and opaque linear zones 32a, 32b, 32c etc on each side of the central linear zone 31a. In the schematic example shown in Figure 3, the LDZP element 31 has 11 opaque linear zones 32a, 32b, 32c, etc and 11 transparent linear zones 31 b, 31c on each side of the central linear zone 31a, but it will be appreciated that the number of transparent and opaque linear zones 31a, 31 b, 31c, 32a, 32b, 32c may vary for different applications or requirements. For instance, the LDZP element of Figure 2 has about 40 transparent and opaque linear zones on each side of the central zone 22a.
The linear diffractive zone plate elements 33, 35, 37 or pixels LDZP002, LDZP003 and LDZP004 are off-axis asymmetric variations or modulations of the LDZP element 31 or pixel LDZP001 , each having its larges transparent first linear zone 33a, 35a, 37a offset from the central axis of the LDZP element 33, 35, 37 and a different number of opaque and transparent linear zones 34a, 33b, 34b, 33c, 34c etc on opposite sides of the first linear zone 33a, 35a, 37a. In the LDZP elements 33, 35, 37, the first linear zones 33a, 35a, 37a are displaced from the central axis by a progressively larger distance.
The linear diffractive zone plate (LDZP) elements 131 , 133, 135 or pixels LDZP005, LDZP006 and LDZP007 represent focal variations or modulations of the LDZP element 31 or pixel LDZP001. Each LDZP element 131 ,133,135 is a symmetric zone plate having the same number of transparent linear zones 131b, 131c etc and opaque linear zones 132a, 132b, 132c, etc on each side of the central transparent first linear zone 131a, 133a, 135a. The LDZP elements 131 , 133, 135 differ from the LDZP element 31 in that their central first linear 131a, 133a, 135a are wider than the central first linear zone 31a, so that the focal lengths of the LDZP elements 131 , 133, 135 are modulated relative to the focal length of the LDZP elements 31. Also, the number of transparent and opaque linear zones of each of the LDZP elements 131 , 133, 135 is different from those of LDZP element 31.
The linear diffractive zone plate (LDZP) element 137 or pixel LDZP 008 is another asymmetric zone plate being an off axis modulation of LDZP element 133 (pixel LDZP006). LDZP element 137 is similar to LDZP elements 33, 35, 37 in that it has its largest first transparent linear zone 138a offset from the central axis of the element 137 and a different number of opaque and transparent linear zones 139a, 138b, 139b, 138c, 139c on opposite sides of the first linear zone 138a.
In a typical application for security documents, such as banknotes, the dimension of the LDZP elements or pixels LDZP001-008 may fall substantially in the range from about 10μm to about 80μm, and more preferably from about 30μm to about 60μm. The LDZP elements shown in Figure 3 are square-shaped, but it will be appreciated that they may take other shapes, such as oblong or circular.
A security device in accordance with one embodiment of the invention may be formed from a diffractive lens array comprising a plurality of diffractive zone plate DZP elements such as illustrated in Figures 1 to 3 provided on one side of a transparent material, and an array of microimage elements provided on the opposite side of the transparent plastics material. Figures 5 and 6 illustrate how a magnified virtual image 58 is produced by a linear DZP element or pixel 52 provided on one side 61 of a transparent material comprising a polymeric film 60 with a microscopic image element 54 on the opposite side 62 of the polymeric film 60. As shown in Figure 6, the focal length F of the DZP element 52 is slightly greater than the thickness D of the transparent polymeric film 60. A viewer 56 observing the security device from the side 62 of the polymeric film 60 on which the image bar 54 is provided sees an observed magnified virtual image 58 of the image bar 54. If the distance of the virtual image 58 from the DZP element 52 is V, then
_L 1-_L D + V ~ F and the magnification is
D (D-F)
Figure 5 illustrates image formation optics for a linear diffractive zone plate (LDZP) element 52 and a microscopic image element 54 in the form of an image bar or line in transmission mode. The width of the input graphic image bar 54 determines the image intensity at that point in the array.
In the case of a circular DZP element, the image element would be a circular spot, the diameter of which would determine the intensity of the observed image at that point in the array.
Referring to Figure 4, there is shown a diffractive lens array 40 formed from a plurality of modulated linear diffractive zone plate (LDZP) elements 42 arranged in a rectangular grid 44 within an oval border 46. Each of the LDZP elements 42 comprises one of the LDZP pixels LDZP001-008 of Figure 3 which are off-axis modulations and/or focal modulations of symmetric LDZP elements. The arrangement and modulation of the LDZP elements 42 in the array 40 may be chosen such that the image 58 observed by a viewer is a magnified moving and/or floating image similar to that which is produced by an array of modulated refractive microlenses superimposed over an array of microimages in the effect known as Moire magnification.
Figures 7a and 7b are enlarged views showing an example of two off-axis linear diffractive zone plate (LZDP) elements 70, 75 or pixels of opposite focussing bias which may be used in a security device that produces a switching image when viewed at different angles as will be explained with reference to Figures 8 to 12.
The LDZP elements 70, 75 are off-axis asymmetric LDZP elements similar to the pixel element 35 LDZP003 of Figure 3 in that their largest transparent first linear zone 71a is offset from the central axis 73 of the element, and they have a different number of opaque linear zones 72a, 72b, 72c etc and transparent linear zones 71 b, 71 c, etc on opposite sides of the first transparent linear zone 71 a. As shown in Figure 7a, the centre line 74 of the first transparent linear zone 71a of LDZP element 70 is at an intermediate position approximately mid way between the central axis 73 and the upper edge 76 of the LDZP element 70. The LDZP element 75 of Figure 7b is the mirror image of LDZP element 70 of Figure 7a, with the centre line 77 of the first transparent linear zone 71a at an intermediate position between the central axis 73 and the lower edge 79 of the LDZP element 75. In both of the LDZP elements 70 and 75, there are five opaque and five transparent linear zones 72a, 71b, 72b, 73a , 73c, etc above or below the first transparent linear zone 71a, and about 17 opaque and transparent linear zones 72a, 71b, 72b, 73a, 73b etc below or above the first transparent linear zone 71a. It will, however be appreciated that the size and positions of the first transparent linear zones 71a, and the number and periodicity or width of the opaque linear zones 72a, 72b, 72c etc and the transparent linear zones 71 b, 71c etc may be varied for different applications.
Figure 8 shows a two channel diffractive lens array 80 formed by interlacing two sets of 81 , 82 of off-axis linear diffractive zone plate (LDZP) elements 70, 75 or pixels, such as those of Figure 7. The LDZP elements 70, 75 are arranged in a rectangular grid 84 of pixels within an oval border 86. The first sets 81 of LDZP elements or pixels 70 extend in horizontal rows 85 across the array 80 with their first transparent linear zones 71a offset above the central axis 73 of each LDZP element or pixel 70. The second sets 82 of LDZP elements or pixels 75 extend in horizontal rows 85 across the array 80 with their first transparent linear zones 71a offset below the central axis 73 of each LDZP element or pixel 75. The first and second rows 83,85 of off-axis LDZP elements or pixels 70, 75 alternate in the vertical direction of the array 80 such that rows 83 of pixels 70 have opposite off-axis bias parameters to the pixels 75 of adjacent rows 85.
In Figure 9, there is shown three different constructions of a security device 95 including a DZP array. Figure 9(a) shows an optically variable effect which is visible in transmission. A transparent substrate 60 has applied on one side 61 a DZP array 92, and a plurality of microimage elements 91a, 91b applied to the other side 62, whereby the DZP array is spacially separated from the microimages. A light source 98 is positioned on the same side of the substrate as the micromage elements 91a,91b. Light transmitted through the image elements 91a is diffracted by the DZP array 92 to form a first image 96a, visible to the observer at a first viewing angle at position 94a. Similarly, light transmitted through image elements 91 b is diffractively focussed by DZP array 92 to form a second image 96b visible to an observer at a second viewing angle at position 94b.
Figure 9(b) shows a modified security device 95' which operates in diffusely reflecting mode. Light from source 98' on the same side as the LDZP array 92 reflects diffusely from microimage elements 91a, 91b and is then diffracted by DZP array 92 and focussed to form images 96a, 96b at first and second viewing angles 94a, 94b respectively.
The device may operate in specularly reflecting mode if the DZP array is metallised, as in Figure 9(c), where light from source 98' is absorbed and reemitted by image elements 91a, 91b, the reemitted light then being diffractively focussed to form images 96a, 96b at first and second viewing angles 94a and 94b respectively.
Referring now to Figure 10, there is shown a microimage array 110 in which the microimage elements 91a, 91b are interleaved. The section 100 in dotted outline is shown in enlarged view at top right, where microimage elements 91a corresponding to the image of the letter 'A' are interleaved with microimage elements 91b corresponding to the image of the letter 'B'.
Figure 11 shows an enlarged view 100' of part of an alternative microimage array, wherein the elements 91a, 91 b are interspersed with each other and also with blank areas 91c which lie between each adjacent pair of image elements 91a, 91b.
Referring now to Figure 12, there is shown a security document 120 including a transparent polymeric substrate 60 and including a DZP array 92 on its upper surface in a window or half-window area 125. Microimage elements 91a, 91 b as in Figures 9 and 10 are provided on the lower surface of the security document in the window or half-window area 125. Preferably, the window or half- window area 125 is formed by applying one or more opacifying layers 123 on one or both sides of the substrate to the transparent substrate 60 except in the window or half-window area 125. The opacifying areas may include layers of opacifying ink. Alternatively, they may be comprise paper or other opaque layers, eg in a laminated construction. Additional printed features 121 , 122 may be applied by various printing methods including intaglio, rotogravure and offset printing, on one of both sides of the document.
Finally, in Figure 13, there is shown a method of forming a security device 140. A DZP array 92 is first formed by embossing into a radiation curable lacquer applied to one surface 61 of a polymer film 60. Laser radiation incident from three different directions 130a, 130b, 130c is then directed towards the DZP array, which focuses rays 131a, 131b, 131c to different areas 134a, 134b, 134c on the opposite side 62 of the film 60. The opposite side 62 has a laser-sensitive material applied to it, which is darkened or ablated as the focussed diffracted rays 132a, 132b, 132c strike it in areas 134a, 134b, 134c respectively. The darkened or ablated areas form three different image areas which are visible when the device is viewed from angles 130a, 130b, 130c respectively.
It will be appreciated that many modifications, variations and combinations of the above described embodiments are possible without departing from the spirit and scope of the invention, as defined in the claims appended hereto.

Claims

CLAIMS:
1. A security document or device comprising a substantially transparent or translucent material, an array of microimage elements provided on one side of the transparent or translucent material, and a diffractive lens array including a plurality of modulated diffractive zone plate (DZP) elements provided on the opposite side of the substantially transparent or translucent material, wherein the DZP elements form diffractive lenses each arranged to focus radiation transmitted through or from the array of microimage elements to produce a magnified image, and the individual DZP elements have at least one characteristic, such as focal length or off-axis distance, modulated with respect to an adjacent DZP element.
2. A security document or device according to claim 1 , wherein the diffractive lens array includes a plurality of linear DZP elements, at least one of which is an off-axis linear DZP element.
3. A security document or device according to claim 1 or claim 2, wherein the diffractive lens array includes a first set of off-axis linear DZP elements arranged to focus radiation onto a first set of microimage elements, and a second set of linear DZP elements arranged to focus light onto a second set of microimage elements, whereby a first magnified image can be seen when the security document or device is viewed from a first angle, and a second magnified image can be seen when the security document or device is viewed from a second angle.
4. A security document or device according to any one of claims 1 to 3, wherein the DZP elements of the diffractive lens array are characterised by a different focal length and/or off-axis distance, whereby a moving and/or floating image is observable in a plane which is different from the plane of the microimages.
5. A security document or device according to claim 4, wherein adjacent DZP elements have different focal length and/or off-axis distance.
6. A security document or device according to claim 1 , wherein each microimage element is part of an input image, and the off-axis distance of the DZP elements is modulated so that different parts of the input image are observable at different angles, whereby the magnified image appears to move as an angle of observation of the security document or device is changed.
7. A security document or device according to claim 6, wherein the focal length of the DZP elements is modulated so that the magnified image appears to both move and float as the angle of observation is changed.
8. A security document or device according to any one of the preceding claims, further including a highly reflective layer applied to one side of the transparent or translucent material.
9. A security document or device according to claim 8, wherein the highly reflective layer is applied on the same side of the material as the DZP array.
10. A security document or device according to any one of the preceding claims, further including an opacifying layer applied to one or both sides of the transparent or translucent material, except in a region containing the DZP elements.
11. A security document or device according to claim 10, wherein the opacifying layer is also omitted in a region containing the microimage elements.
12. A method of manufacturing a security document or device comprising a substantially transparent or translucent material, the method including the steps of: applying an array of microimage elements to one side of the material, and applying a plurality of diffractive zone plate (DZP) elements to the opposite side of the material, and forming the DZP elements such that individual DZP elements have at least one characteristic, such as focal length or off-axis distance, modulated with respect to an adjacent DZP element, whereby the DZP elements form diffractive lenses each arranged to focus incident radiation from a microimage element of the array of microimage elements to produce a magnified image.
13. A method according to claim 12, wherein the microimage elements are applied by printing.
14. A method according to claim 12, wherein the microimage elements are applied by laser marking.
15. A method according to claim 14, wherein the microimage elements are applied by directing a laser through the DZP elements from the opposite side of the material so that the microimage elements are formed on the first side of the material.
16. A method according to claim 12, wherein the microimage elements are applied by an embossing process.
17. A method according to any one of claims 12 to 16, wherein the DZP elements are applied by embossing, engraving, or electron beam lithography.
18. A method according to claim 16 or claim 17, wherein the microimage elements and/or DZP elements are embossed directly into the transparent or translucent material.
19. A method according to claim 16 or 17, wherein the microimage elements and/or DZP elements are embossed into an additional layer which is applied to the material.
20. A method according to claim 19, wherein the additional layer is applied by a printing process, such as rotogravure printing.
21. A method according to claim 19 or claim 20, wherein the additional layer is a radiation curable material.
22. A method according to any one of claims 19 to 21 , further including the step of curing the radiation curable material.
23. A method according to claim 22, wherein the step of curing the radiation curable material is performed at substantially the same time as embossing the microimage elements and/or DZP elements into the radiation curable material.
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