FIELD OF THE INVENTION

[0001] The present invention relates to optical security devices and methods for their manufacture. In particular, the present invention relates to optical security devices which include a reflecting surface relief structure or structures in their construction.

BACKGROUND TO THE INVENTION

[0002] Optical security devices are commonly used in security documents as a means of avoiding unauthorised duplication or forgery of such documents. Typically, such devices produce optical effects which may be difficult for a potential counterfeiter to replicate.

[0003] Counterfeiting of banknotes and other valuable documents has become an increasingly important issue in recent times due to ready availability of colour photocopiers and computer scanning equipment. This technology provides counterfeiters with a much easier route to copying of valuable documents issued using traditional security printing technologies. In response central banks and banknote printers have turned to technologies which produce images that vary with changing angle of view, and which therefore cannot be easily photographed.

[0004] Such devices known collectively as optically variable devices (OVDs) have proven to be very successful in reducing incidence of counterfeiting using computer scanning equipment. However, the counterfeiters have not been idle during this time and some groups have adopted alternative holographic approaches to counterfeiting or simulating diffractive optically variable devices produced by banknote printing groups.

[0005] Therefore, there is a need for alternative OVD technologies which produce images well differentiated in their image characteristics from diffractive optically variable devices. [0006] PCT/AU02/00551 entitled "An Optical Device and Methods of Manufacture" describes a method of producing an optical device based on arrays of micromirrors wherein each micromirror may be formed with a particular angle of inclination and dimensions so as to reflect light of a particular intensity into a particular direction such that the ensemble of micromirrors spanning the device collectively generate an optical image which varies with angle of view. In the case of the present invention, it has been recognised that via the use of binocular vision, it is possible to configure arrays of micromirrors in such a way that a three dimensional image may be generated in the sense that the observed image appears to float above the surface of the device.

[0007] The present invention may provide an alternative OVD technology based on reflective, rather than diffractive optical principles. The present invention may also provide a method for manufacturing optical security devices including such OVD technology.

SUMMARY OF THE INVENTION

[0008] The present invention provides an optical device including a surface relief structure, or structures, which reflects incident light in a controlled manner to generate an optical effect having one or more images which appear to float above the surface of the device. The floating images may be due to a stereoscopic effect or perception of depth and/or 3-dimensional structure based on visual information derived by an observer with normally developed binocular vision.

[0009] Because the eyes of humans, and many animals, are located at different lateral positions on the head, binocular vision results in two slightly different images projected to the retinas of the eyes. The differences are due to different projection of objects as seen by each eye. These positional differences are referred to as horizontal disparities, or more generally, binocular disparities. Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real 3-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".

[0010] Stereoscopic depth or a floating image effect may be generated more strongly for incident sources of light having a relatively narrow beam angle (i.e. not diffused light, but originating from a source having a relatively small angular width - e.g. from 2 to 20 degrees). The surface relief structure or structures of the present invention may also be arranged to produce a non-floating image, as for example described in PCT/AU02/00551 , which is viewable when the device is rotated by 90 degrees about an axis perpendicular to the plane of the device. The non-floating image capability may include an image switching option wherein the non-floating image may switch to a different type of non-floating image as the device is rocked back and forth about an axis within the plane of the device.

[0011] The non-floating images may be observable under a much greater range of light sources as these images do not involve a stereoscopic effect and therefore do not rely on binocular vision ability of the observer. By combining a floating image effect with a non-floating image effect, the optical device of the present invention may be authenticated under a much greater range of light sources as well as providing an increased level of security due to two different types of optical effects being utilized.

[0012] Since the operating mechanism of the optical device is based on reflection rather than diffraction, the optical effects produced by the device are achromatic and are therefore easily distinguishable from rainbow light optical effects produced by the more commonly available diffractive optically variable devices. This high level of differentiation from diffractive devices, some of which may be simulated or counterfeited using dot matrix hologram techniques, therefore promises a higher level of security when the device is used as an anti-counterfeiting feature on valuable documents.

[0013] According to one aspect of the present invention there is provided an optical device comprising an array of reflecting surface relief structures projecting from or recessed into a base plane of the device, wherein the optical device generates an optical effect including a floating image at least when the device is viewed by an observer from a first position, and wherein the floating image changes to another type of floating image or to a non-floating optical image when the device is rotated about an axis perpendicular to the base plane of the device, wherein said array of reflecting surface relief structures includes at least a first group of reflecting surfaces oriented such that they reflect incident light into the left eye of the observer, but substantially do not reflect incident light into the right eye of the observer, when the device is viewed from the first position.

[0014] In one form, the array of reflecting structures also includes a second group of reflecting surfaces oriented such that they reflect incident light into the right eye of the observer, but substantially do not reflect incident light into the left eye of the observer, when the device is viewed from the first position.

[0015] The non-floating image generated by the device may change to another non-floating image when the device is rotated, tilted or rocked back and forwards about an axis lying within the base plane of the device.

[0016] In one form each reflecting surface relief structure may include, for example, a square based or rectangle based pyramid, wherein each of four sides associated with each pyramid forms a micro-mirror pixel.

[0017] By "image pixel" here we mean a "picture element", which is the smallest area of an image point as observed by the naked eye of the observer. By "micro- mirror pixel" here we mean the associated surface relief structure of the device whose parameters such as angle of reflection and area of reflection may be varied independently across the device at the origination phase so as to generate the greatest possible range of images.

[0018] Each pyramid may have base dimensions of 500 microns or less and the height of the or each pyramid may be 250 microns or less. [0019] In another form each reflecting surface relief structure may include a triangular prism, wherein each of two inclined sides associated with each prism forms a reflecting surface, stripe or pixel. By "triangular prism" here we mean a prism structure which has a geometry consisting of a triangular cross-section and three rectangular sides, wherein two of the three rectangular sides are used as reflecting surfaces.

[0020] The above arrangement may generate an optical effect in the form of a stereoscopic or floating image at least when the surface relief structures are viewed from the first position relative to an axis perpendicular to the base plane of the device.

[0021] The floating image generated by the device may include a greyscale portrait of a face, scene, logo, alphanumeric character or any other graphic design wherein relative intensity of light reflected from the device is fixed or varies from point to point across the image.

[0022] The non-floating image generated by the device may include a greyscale portrait of a face, scene, logo, alphanumeric character or any other graphic design wherein relative intensity of light reflected from the device is fixed or varies from point to point across the image. The non-floating image may be generated by a diffractive or non-diffractive structure or by an ink that changes colour depending on viewing angle or position of the observer relative to the plane of the device.

[0023] Each projecting or recessed surface relief structure may be produced in any one of a number of ways. In one form the or each structure may be produced by applying a layer of embossable radiation curable ink to a substrate and embossing the layer prior to curing the ink via ultraviolet radiation.

[0024] One way to form the reflecting surface may be to use a vacuum metal deposition process. In this process, the surface to be coated may be placed in a vacuum, and the metal may be vaporised. When the vaporised metal contacts the surface, it may condense to form a metallic layer on the surface. [0025] An alternative to a vacuum metal deposition process may be to utilise a metallic nanoparticle ink to coat the required surface. The application of such an ink may be achieved at substantially reduced cost compared to the vacuum deposition process, while still providing a thin coating that may be highly reflective, or semi- transparent with a high refractive index, depending on the composition of the ink.

[0026] The optical security device may be embossed or attached to a valuable document such as a banknote, passport, credit card, cheque, etc. in order to prevent or inhibit counterfeiting of that document.

[0027] According to a further aspect of the invention, there is provided a method of manufacturing an optical device including forming an array of reflecting surface relief structures projecting from or recessed into a base plane of the device, to generate an optical effect in the form of a floating image at least when the device is viewed by an observer in a first position, and wherein the floating image changes to another type of floating image or to a non-floating image when the device is rotated about an axis perpendicular to the base plane of the device, wherein said array of reflecting surface relief structures includes at least a first group of reflecting surfaces oriented such that they reflect incident light into the left eye of the observer, but substantially do not reflect incident light into the right eye of the observer, when the device is viewed from the first position.

[0028] The method may include a step of applying a layer of embossable radiation curable ink to a substrate prior to being embossed while soft and curing the ink by radiation to form the array of reflecting surface relief structures.

[0029] The method may include a step of applying a coating of metallic nanoparticle ink to the relief structures to produce the reflective (or at least partially reflective) relief structures. The coating may be applied as a curable coating.

[0030] The ink may be applied as a silver nanoparticle ink. Where this is the case, the silver nanoparticle ink preferably has less than 40% silver. Alternatively, the method may include applying an aluminium nanoparticle ink or a gold nanoparticle ink.

[0031] The method may also include the step of providing at least one opacifying layer as an opacifying coating, preferably an opacifying ink layer. In addition or alternatively, the method may include the step of providing a pigmented ink layer over the reflecting surface relief structures, to selectively reflect incident light that is within a certain wavelength range, thereby providing a coloured and more evident optical effect.

[0032] Further aspects of the invention are directed to a security document, such as a banknote including the optical security device as described in any of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Specific embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0034] Figure 1 shows an array of reflecting micro-pyramid structures according to one embodiment of the invention.

[0035] Figure 2 shows a cross sectional view of reflecting micro-pyramid structures.

[0036] Figure 3 shows observation geometry relative to a substrate plane.

[0037] Figure 3a shows how the inclination angle of a micro-mirror can be calculated based on observation geometry of Figure 3.

[0038] Figures 4a to 4c show artwork comprising two interleaved tracks of micro- mirror pixels according to an embodiment of the invention.

[0039] Figures 5a and 5b show respective right and left eye micro-mirrors represented in 3D coordinate space. [0040] Figure 6 shows right and left eye micro-mirrors represented in 2D coordinate space with grey scales representing depth into a resist.

[0041] Figure 7 shows a schematic representation of an array of reflecting structures according to another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

DEFINITIONS

Security document

[0042] 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 licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0043] 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 can also have application in other products, such as packaging.

Security Device or Feature

[0044] 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, and alteration or tampering. Security devices or features can 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 can 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) comprising reflective optical structures including reflecting surface relief structures and diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Metallic Nanoparticle Ink

[0045] As used herein, the term metallic nanoparticle ink refers to an ink having metallic particles of an average size of less than one micron.

Diffractive Optical Elements (DOEs)

[0046] As used herein, the term diffractive optical element refers to a numerical- type diffractive optical element (DOE). Numerical-type diffractive optical elements (DOEs) rely on the mapping of complex data that reconstruct in the far field (or reconstruction plane) a two-dimensional intensity pattern. Thus, when substantially collimated light, e.g. from a point light source or a laser, is incident upon the DOE, an interference pattern is generated that produces a projected image in the reconstruction plane that is visible when a suitable viewing surface is located in the reconstruction plane, or when the DOE is viewed in transmission at the reconstruction plane. The transformation between the two planes can be approximated by a fast Fourier transform (FFT). Thus, complex data including amplitude and phase information has to be physically encoded in the micro-structure of the DOE. This DOE data can be calculated by performing an inverse FFT transformation of the desired reconstruction (i.e., the desired intensity pattern in the far field).

[0047] DOEs are sometimes referred to as computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms.

Embossable Radiation Curable Ink

[0048] 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.

[0049] 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 sub- wavelength gratings, transmissive diffractive gratings and lens structures.

[0050] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.

[0051] 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.

[0052] The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be embossed with larger relief structures, such as nondiffractive optically variable devices.

[0053] 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. [0054] Referring to the drawings, Figure 1 shows an optical device according to one embodiment of the present invention comprising an array of reflecting surface relief structures provided in the form of micro-pyramid structures 10. Faces A and D on each micro-pyramid structure 10 correspond to floating image channels, that is, faces A and D are configured to generate a floating image when viewed by an observer from a first viewing position; while Faces C and B on each micro-pyramid structure 10 correspond to non-floating image switching channels, that is, they are configured to generate a non-floating switching image when viewed by the observer from a different viewing position. Switching image generally means an image which switches to a different type of non-floating image as the device is rocked back and forth about an axis within the plane of the device. Alternatively, Faces C and B can be configured to generate a second, different floating image. In one form, the optical device may include a plurality of image channels wherein each image channel is associated with a viewing angle of the observer. As the device is rotated, the observer is expected to see a series of image changes as the viewing angle changes.

[0055] In another embodiment, Faces A and B on each micro-pyramid structure 10 correspond to floating image channels whereas Faces C and D on each micro- pyramid structure 10 correspond to non-floating image switching channels.

[0056] While not shown in the drawings, it should be appreciated that the reflecting surface relief structures can be provided in other forms and not just limited to micro-pyramid structures such as that shown in Figure 1 . For example, the number of side faces on each micro-pyramid structure may be other than 4 but greater than 3. For a floating image to be generated, preferably there is an even number of opposing reflecting facets so that each pair of facets reflects two crossed light paths to the two eyes of the observer.

[0057] Referring to Figure 2, there is shown a cross section of an optical security device 20 including an array of reflecting micro-pyramid structures such as that shown in Figure 1 . To form this layered structure, a layer of embossable radiation curable ink 22 is applied in an area of a surface of substrate 21 prior to being embossed while soft to form the micro-pyramid relief structures 23. The radiation curable ink 22 may be cured by UV radiation to fix the embossed micro-pyramid relief structure 23. A metallic nanoparticle ink may be applied to form a thin reflective coating 24 over the relief structure 23 to produce a reflecting or at least partially reflecting relief structure.

[0058] Nanoparticle ink may provide reflectivity which is equivalent to that achieved by vacuum metallisation, but may be provided more cheaply and efficiently as the ink may be applied by a printing method. In one embodiment of the present invention, the metallic nanoparticle ink includes a silver nanoparticle, having less than 40% silver. However, a range of other metallic nanoparticle inks may also be suitable for use in accordance with the invention, for example, silver nanoparticle inks with greater than 40% silver, aluminium nanoparticle inks and gold nanoparticle inks.

[0059] A protective coating 25 such as a transparent varnish may be applied over the reflective or partially reflective relief structure 23 and 24. Protective coating 25 may be applied over the reflective coating 24 and the relief structures 23 as well as over other areas of substrate 21 in which the relief structures 23 and the reflective coating 24 are not present. The latter areas (not shown) may contain a non-floating image that may be visible when the device is rotated about an axis perpendicular to the plane of the device or is rocked back and forth about an axis within the plane of the device. The non-floating image(s) may comprise a diffractive or non-diffractive structure such as a colour changing ink.

[0060] Preferably, the protective coating 25 is a high refractive index (HRI) coating, as this may assist in ensuring that the optical effect produced by the reflecting relief structures 23 and 24 remains visible even if metallic nanoparticle ink is applied in a very thin layer. However, in other embodiments possible protective coatings may include a transparent, non-high refractive varnish.

[0061] It will be appreciated that a suitable coating should demonstrate one or all of the following attributes: good adhesion to the substrate, highly transparent, generally colourless, and robust. As mentioned above, possible coatings may include a transparent, non-high refractive varnish. Varnish may denote a material that results in a relatively durable and protective finish. Exemplary transparent varnishes may include, but are not limited to, nitrocellulose and cellulose acetyl butyrate. Alternatively, the coating may be a high refractive index coating, being a coating having a metal oxide component of small particle size and high refractive index dispersed in a carrier, binder or resin. Such a high refractive index coating may contain solvent as it is a dispersion. Where a high refractive index coating of this type is used, it may be air cured or UV cured.

[0062] Alternatively, a high refractive index coating utilising a non-metallic polymer, such as sulphur-containing or brominated organic polymers may also be used.

[0063] The metallic nanoparticle ink or the reflective coating 24 is preferably applied to the surface of the relief structure 23 in a repeating pattern, such as a plurality of substantially parallel lines, or a plurality of substantially circular spots. By "repeating pattern" here we mean that the image is comprised of pairs of opposing reflecting facets repeated across the area of the device. An example of a particular surface relief structure here would be an array of "tracks" or "gutters" as shown in Figure 7 where the sides of the gutters vary in angle along the length of each gutter according to the picture information required to be generated. These angles may even be 0 or 90 degrees so that the gutters appear to have gaps in them at particular points along their length.

[0064] The reflective coating 24 including the metallic nanoparticle ink may be applied by any one of several techniques that will be apparent to the person skilled in the art. Preferably, the metallic nanoparticle ink is applied by a gravure printing process, however it may also be applied by other suitable techniques such as flexography or offset printing.

[0065] Referring to Figures 3 and 3a, the theory behind the stereoscopic or floating 3D image effect using a micro-mirror array may be illustrated with reference to a test image involving a simple piece of artwork in the form of a $ sign. The 3D effect required is for the image plane 30 which contains the $ sign to float above substrate plane 31 of the device at a height D of approximately 1 cm when viewed by an observer's right and left eyes 32, 33 at a viewing distance H of 21 cm. The separation E between the observer's eyes 32, 33 is assumed to be 8 cm and the area of the optical device is of dimensions less than 2cm x 2 cm.

[0066] Therefore, according to the desired floating image effect as described above:

E = 8cm,

H = 21 cm,

D = 1 cm,

Tan (a) = (E/2)/(H-D) = 0.2,

S/D = tan(a)=0.2,

Therefore, S = 0.2cm, wherein S is the parallax displacement.

[0067] With reference to Figures 3 and 3a, for micro-mirror pixels of dimensions 30 micron X 30 micron and a substrate depth of 3.5 microns the micromirrors will be inclined in the y direction at an angle of arctan[3.5/(30/2)] ~ 13 degrees to the plane of the device. The y direction deflection equation is z = [3.5/(30/2)]y = 0.23y. The right eye micromirror deflection equation according to the geometry in Figure 3a is z = tan(c) x = tan (a/2)x = tan[arctan(0.2)/2] ~ 0.1 x. Similarly, the left eye micromirror deflection equation is given by z = -tan[arctan(0.2)/2] x ~ -0.1 x. The combined micromirror deflection equation for the right eye is therefore given by z = 0.23y + 0.1 x, and the corresponding left eye equation is z = 0.23y - 0.1 x.

[0068] It should be noted that the inclination angle of 13 degrees is just used an example for ease of explanation. In other embodiments the inclination angle could range from 1 to 89 degrees depending on the desired floating image, the size of the micromirror pixels and the microstructure depth. The micro-mirrors could also have different base dimensions for example they could have base dimensions ranging from 1 micron to 500 microns. [0069] In accordance with the calculations shown above, for a surface depth of 3.5 microns, micro-mirror pixel size of 30x30 microns, observation distance of 21 cm and 8 cm eye to eye separation, there are two equations describing the two sets of micro-mirrors (note: the y axis here is directed out of the page).

[0070] Left eye micro-mirror set: z = 0.23 y - 0.1 x (1 )

[0071] Right eye micro-mirror set: z = 0.23 y + 0.1 x (2)

[0072] Note that for the 3D effect to be observed, the incident light rays reflected by the arrays of reflecting elements associated with eyes 32, 33 of the observer must cross over each other in plane 30 between substrate plane 31 of the optical device and plane 34 containing eyes 32, 33 of the observer.

[0073] The artwork for the simple test image is shown in Figure 4a and the perceived floating image is generated by two interleaved tracks of micro-mirror pixels shown in figures 4b and 4c respectively. For an optical device of size 2cm x 2cm of 30 micron micro-mirror pixels, the overall size of the artwork is 667 x 667 image pixels. The pixel arrangement of figure 4b correspond to micro-mirror pixels reflecting light into the left eye 33 of the observer and the pixel arrangement of figure 4c correspond to micro-mirror pixels reflecting light into the right eye 32 of the observer.

[0074] The background pixels shown in black in figure 4a correspond to a flat background of the optical device. Note that in the artwork of Figures 4a-4c the x-axis is directed across the page and the y-axis is directed up the page. This is important and means that micro-mirror arrangements associated with right and left eyes 32, 33 must be oriented in the y direction in order for the optical effect to be generated in accordance with the geometry shown in figure 3.

[0075] An illustration showing relative orientation of micro-mirrors based on equation (2) and associated with the right eye 32 is shown in figure 5a. An illustration showing relative orientation of micro-mirrors based on equation (1 ) and associated with the left eye 33 is shown in figure 5b. [0076] An enlarged view of micro mirror orientations in the x-z plane is shown in figure 6. Micro-mirrors 61 associated with right eye 32 are defined by equation (2) and micro-mirrors 62 associated with left eye 33 are defined by equation (1 ).

[0077] In Figure 6 grey scale 63 represents micro mirrors associated with the right eye 32 and grey scale 64 represents micro mirrors associated with the left eye 33. The darker greyscales represent greater surface depth.

[0078] Figure 7 shows an optical device according to another embodiment of the invention. The array of reflecting structures includes several columns of gutters or channels containing micro-mirror structures arranged along each gutter or channel. The cross sections or profiles may vary along each gutter (as shown by way of some examples in the diagram) depending on the desired floating image to be observed by an observer. The gutters may be fully metallised (if full high intensity reflection is required) or partly metallised in thickness and/or position of metal (if a lower intensity reflection and partial transmittance of the light is required.

[0079] Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.