AU2015100642A4 - An optical device and method for producing an optical device - Google Patents

Abstract

A method for producing an optical device, and the optical device produced by said method, having a substrate including: providing at least one area of optical structures on the substrate; covering portions of the at least one area of optical structures with a design layer, wherein the design layer prevents the optical structures that are covered from operating in the same manner as the uncovered optical structures. The optical device produced in this manner allows greater design freedom whilst using existing methods and processes. Figure 1A Figure 1B

Description

1 AN OPTICAL DEVICE AND METHOD FOR PRODUCING AN OPTICAL DEVICE FIELD OF THE INVENTION [0001] The invention relates generally to the field of optical devices, and particularly, but not exclusively, optical security devices such as, for example, those used on banknotes. BACKGROUND TO THE INVENTION [0002] It is well known that many of the world's banknotes, as well as other security documents, carry optical devices which act as security elements. Some optical security elements produce optical effects that vary depending on whether the optical security elements are viewed through a decoding screen or not. The incorporation of such optical variable devices, as security elements, into security documents, therefore, acts as a deterrent against counterfeiting of the document. [0003] It is known to provide optically variable devices in which arrays of microlenses image an object plane containing image elements. In some devices, where repeating images are provided on the object plane in a pre-defined manner, the image elements provide a Moire effect when viewed through the microlenses. In other optically variable devices, image elements are interlaced sections of two or more distinct images, so that as the person viewing the device through the microlenses changes the angle of view, a different image becomes visible. [0004] Microlenses produced in a flexible format are traditionally produced as continual lenses covering an entire area. One method of producing the microlenses involves embossing the lenses into a coating or surface. To enable 2 this, a roller is engraved with a negative version of the microlenses and then used to emboss the shape of the lenses into the coating or surface. [0005] For cylindrical lens arrays, also known as lenticular arrays, this is usually achieved by the use of a diamond stylus which is pressed into the surface of an electroplated steel cylinder to engrave the desired shaped. [0006] Typically, for a cylindrical microlens, the diamond stylus tool is programmed to make a single cut around the circumference of the cylinder to produce a negative relief of the lens surface. Once one lens had been completed, the tool then indexes across to a new position along the axis of the cylinder, a predefined distance (giving the lens pitch) from the first lens, and repeats the process. This process is repeated until the entire cylinder surface is engraved. [0007] For circular or elliptical lens arrays, the tool engraves a negative relief of the desired lens surface, and then indexes to the next datum, typically a predefined distance either along the axis or around the circumference to create a two dimensional lens array. Both the tooling and the process are relatively expensive making the production of custom design lens layouts inordinately expensive. [0008] In the application of lenses as security devices in banknotes today, all instances of are in a simple cylindrical lens stripe format. A stripe implementation can be seen as sub-optimal, as the region allocated to the lenses may be difficult to integrate aesthetically into the banknote or other security document. Lack of aesthetic integration can be seen to reduce the security benefit of the note, as a simple implementation increases the ease of simulation. SUMMARY OF THE INVENTION 3 [0009] According to a first aspect of the present invention, there is provided a method for producing an optical device having a substrate including: providing at least one area of optical structures on the substrate; covering portions of the at least one area of optical structures with a design layer, wherein the design layer prevents the optical structures that are covered from operating in the same manner as the uncovered optical structures. [0010] According to a second aspect of the present invention, there is provided an optical device, including: a substrate; at least one area of optical structures on the substrate; and a design layer covering portions of the at least one area of optical structures, wherein the design layer prevents the optical structures that are covered from operating in the same manner as the uncovered optical structures [0011] Preferably, the substrate is transparent. Also, the optical device may be a security device. [0012] In an embodiment, the design layer is transparent or translucent. Where the design layer is transparent or translucent, the design layer, preferably, has similar reflective and/or refractive properties to the optical structures. Preferably, the design layer has the same or similar refractive index, such that the difference is not noticeable for the particular application, as the optical structures. [0013] In an embodiment, the design layer is substantially opaque, at least, in reflection. Optionally, the design layer can be arranged to allow aspects of the optical structures covered by the design layer to be viewable in transmission. [0014] In an embodiment, the optical structures are selected from one or more of the following: microlenses, Fresnel lens(es), diffraction gratings, diffractive optical elements or diffractive zone plates.

4 [0015] In an embodiment, the optical structures are embossed. Preferably, the optical structures are embossed into a radiation curable ink. Preferably, the radiation curable ink is printed onto the substrate. Preferably, the radiation curable ink is UV curable ink. [0016] In an embodiment, the optical structures are formed in a stripe on the substrate. [0017] In an embodiment, the design layer is printed. Preferably, the design layer is printed by gravure printing. [0018] According to a third aspect of the invention, there is provided a method for producing a security document, including the step of providing a document substrate including, in a region of the document substrate, an optical device produced according to the method of the first aspect. [0019] In an embodiment, the substrate of the optical device is different to the document substrate, and the optical device is formed separately and subsequently attached to the document substrate. In an alternative embodiment, the substrate of the optical device is the same as the document substrate. [0020] The method preferably includes the step of applying a first opacifying layer to a side of the document substrate, the first opacifying layer including a window region such that the optical device is located in the window region. Also, preferably, the method includes the step of applying a second opacifying layer to a different side of the document substrate to the first opacifying layer, the second opacifying layer including a window region such that the optical device is located in the window region, such that the optical device is located in a window of the security document. Alternatively, the method includes the step of applying a second opacifying layer to a different side of the document substrate to the first opacifying layer, the second opacifying layer partially or entirely covering the 5 optical device, such that the optical device is located in a half-window of the security document. [0021] According to a fourth aspect of the present invention, there is provided a security document, including a document substrate including, in a region of the document substrate, an optical device according to the third aspect. [0022] In an embodiment, the substrate of the optical device is different to the document substrate, and wherein the optical device is formed separately and subsequently attached to the document substrate. In an alternative embodiment, the substrate of the optical device is the same as the document substrate. [0023] The security document preferably includes a first opacifying layer applied to a side of the document substrate, the first opacifying layer including a window region such that the optical device is located in the window region. [0024] Also preferably, the security document includes a second opacifying layer applied to a different side of the document substrate to the first opacifying layer, the second opacifying layer including a window region such that the optical device is located in the window region, such that the optical device is located in a window of the security document. Alternatively, the security document includes a second opacifying layer applied to a different side of the document substrate to the first opacifying layer, the second opacifying layer partially or entirely covering the optical device, such that the optical device is located in a half-window of the security document. [0025] The security document may also include a polariser formed in a region of the document substrate different to the location of the optical device, such that the optical device can be viewed through the polariser by twisting, folding, or other manipulation of the document substrate.

6 [0026] The security document produced by the method of the third aspect, or according to the fourth aspect, may be a banknote. Security Document or Token [0027] 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. [0028] 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 [0029] 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).

7 Substrate [0030] 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. Transparent Windows and Half Windows [0031] 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. [0032] 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. [0033] 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.

8 [0034] 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 [0035] 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 < 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. Refractive index n [0036] The refractive index of a medium n is the ratio of the speed of light in vacuum to the speed of light in the medium. The refractive index n of a lens determines the amount by which light rays reaching the lens surface will be refracted, according to Snell's law: [0037] n * Sin (a) = n * sin (9), [0038] where c is the angle between an incident ray and the normal at the point of incidence at the lens surface , 0 is the angle between the refracted ray and the normal at the point of incidence, and n 1 is the refractive index of air (as an approximation n 1 may be taken to be 1).

9 Embossable Radiation Curable Ink [0039] 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. [0040] 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. [0041] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating. [0042] 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. [0043] 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 non-diffractive optically variable devices.

10 [0044] 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. [0045] 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. [0046] 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-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetylacetone; melamines; and carbodi-imides. BRIEF DESCRIPTION OF THE DRAWINGS [0047] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be appreciated that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings: 11 [0048] Figure 1 shows a security document including a security device; [0049] Figure 2a and 2b shows a prior art microlens based optical device; [0050] Figure 3a and 3b shows a prior art method of producing a microlens based optical device; [0051] Figure 4a and 4b shows an optical device according to the present invention; [0052] Figure 5a and 5b shows a security document including an optical device according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENT [0053] For the purposes of the following discussion, the figures are to be considered illustrative and not to scale, unless otherwise indicated. The figures illustrate simplified depictions of the embodiments described. [0054] A "visual effect" or "optical effect" is an image, pattern, or other visually identifiable effect. A visual effect can be a hidden visual effect, which is only visible under certain conditions, or an overt visual effect, which is visible under normal viewing conditions. A visual effect can also be a diffractive visual effect or a non-diffractive visual effect. [0055] Referring to Figure 1A and 1 B, a security document 2 includes a first optical device 4 and second optical device 6. The security document 2 includes a substrate 8 including a first side 10 and a second side 12. The first side 10 and/or the second side 12 can include an opacifying layer 14,16. The opacifying layer can have provided (e.g. printed) onto it designs and/or patterns and/or solid colours and/or text, amongst other elements. In this example, the first optical 12 device 4 is in a window region of opacifying layer 14. The opacifying layer 14 does not have a corresponding window region and, therefore, the first optical device 4 is located in a "half-window". In the case of second optical device 6, both opacifying layers 14, 16 include a window region corresponding to the second optical device 6 and, therefore the second optical device 6 is in a full window region. [0056] The first and second optical devices 4, 6 also include a substrate, which can be the same substrate 8 as the whole of the security document 2 (as is assumed herein). In other embodiments, the first and second optical devices 4, 6 are formed, for example, as a transfer film for applying to the substrate 8 of the security document 2. [0057] As such, the security document 2 can include one or more optical devices 4, 6 in windows or half-windows, as well as other security features. [0058] In the embodiments described herein, optical devices provide a security function in respect of a security document, and, therefore, can be interchangeably referred to as security devices. [0059] Referring now to figure 2A and 2B, a prior art document 20 is shown, which could be a security document, having a substrate 22, a cylindrical microlens array 24 and an image layer 26. The document 20 has regions 28 in which lenses have not been formed. The substrate 22 is transparent such that an optical effect is generated through the interaction of the microlens array 24 and the image layer 26. [0060] Referring now to figure 3A, during manufacture of the document 20, a UV curable ink layer 30 is printed in an area in which the microlens array 24 is desired. The UV curable ink layer 30 is then embossed with an embossing tool 32, as shown in figure 3B. The embossing tool 32 has a negative relief of the 13 desired lenses, such that a positive relief of the lenses is generated on embossing, but the pattern of the negative relief is larger than the printed UV curable ink layer 30 and may, indeed, be larger than the substrate 22. [0061] One example of an embossing tool is a cylinder with engraved microlenses covering the maxim width of a surface to be embossed. For example, the present maximum size sheet which can be processed through a sheet-fed process for security printing (nominally 980mm x 880mm). [0062] In a subsequent step, or whilst the embossing tool is in the process of embossing, the UV curable ink layer 30 is exposed to UV, which hardens the ink layer 30 and ensures that the microlens array 24 retains its shape. Figure 3C shows the microlens array 24 after the embossing tool 32 has been removed. [0063] As demonstrated above, a microlens array can be generated at a specific location on a document through providing an embossing layer, such as the UV curable ink layer 30, and applying a general embossing tool, which simply needs to be larger than the desired area that requires to be embossed. As described above, the expense involved in created an embossing tool, such as a cylinder for a printing press, means embossing tools have typically been created with lens arrays that expand across the majority of the embossing tool. This has meant that complicated lens designs have not been used on security documents, or elsewhere, due to the expense in making them, despite the desire to create complicated designs for aesthetic and security reasons. [0064] To obviate or mitigate this problem, a design layer can be applied on top of the microlenses such that areas of the microlenses are covered and other areas uncovered in a desired design arrangement. [0065] Other embossing tools which create optical structures other than microlenses can also take advantage of this concept. Where it is prohibitively 14 expensive to create a custom design of an optical structure or prohibitively expensive to create more than one of a particular embossing tool (thereby creating multiple designs on the same embossing tool) and it is possible to "select" part of the embossed optical structure by covering the remaining part of the optical structure, the disclosed concept is appropriate. [0066] Whilst some embodiments, and particularly the embodiment described below, require a transparent substrate, alternative embodiments do not require the substrate to be transparent / translucent. For example, if the optical structures are formed over a reflective layer or a reflective layer is formed over the optical structures, optical effects can be viewable on substrates which are opaque. [0067] In addition, for most optical structures, the concept of covering the undesired portions of optical structures provides a surprising further security feature. The uncovered portion of optical structure, if designed to do so, will provide an overt security feature for most users. However, the covered portion can act as a security feature in itself. In some cases, due to the difference in thicknesses of the design layer where it covers the optical structures, a pattern can be seen in transmission. The pattern may only be viewable under particular lighting conditions, increasing the security of the document by providing a covert security feature. [0068] Referring now to Figure 4A and Figure 4B, an optical device 40 is shown having a substrate 42 and an array of microlenses 44. In addition, a design layer 46 is provided over portions of the microlenses. Figure 4A is a cross section of Figure 4B along the line A-A. As such, it can be seen that layer 46 is provided over the microlenses 44. By selectively providing the layer 46 in the region of the array of microlenses 44, portions of the array of microlenses 44 can be left uncovered. In one embodiment, layer 46 is an opacifying layer, such that only the uncovered portions of the array of microlenses 44 can be effectively used. In an alternative embodiment, layer 46 is transparent or translucent and has a reflective and refractive properties the same, or similar to, that of the 15 microlenses 44. In this case, the microlenses 44 which are covered by the layer 46 do not operate as microlenses due to layer 46 being of similar refractive index. [0069] In a preferred embodiment, layer 46 is printed on to the microlenses 44. Whether layer 46 is printed or provided by another process, it is preferred, especially where layer 46 is transparent or translucent, that layer 46 fills in the areas between the lenses to a height almost equivalent to the height of the lenses themselves. [0070] In this manner, a custom lens design can be made without having to create custom lens tools. Instead, a lens array is manufactured in a known manner and then overprinted with a custom design layer. Design layer 46 may be applied such that it creates an aesthetic effect sympathetic to the banknote design. The layer 46 may also be applied in such a way that the lenses no longer appear as a contiguous region but as separate islands of lenses, again which may reflect a particular design aspect. [0071] Furthermore, as foreshadowed above, when an opacifying coating or ink is used as the design layer 46, and the transmission opacity of the design layer 46 is designed appropriately (as is known in the art of creating shadow images, see, for example, Australian banknotes issued since 1992, where, when the banknote is held up to the light, an image of the Australian Coat of Arms can be seen faintly, under other printing), the variations in thicknesses of the design layer 46, can be seen as a pattern when viewed in transmission. [0072] Typically, where the design layer is printed over the microlenses and requires to be thicker than 5 microns, gravure or silkscreen printing would be the preferred application process, whereas for thicknesses of less than 5 microns flexography, offset or gravure printing are most applicable .

16 [0073] Referring now to figures 5A and 5B, a security document 50 is shown in a sectioned view (5A) and a top view (5B). Security document 50 includes two optical devices 40A, 40B, which are as described in relation to figures 4A and 4B, although, in this case, substrate 42 is a common transparent substrate 52 across both optical devices 40A, 40B. The security document 50 includes opacifying layers 54 which cover the substrate 54. The arrangement of the opacifying layers 52 define a window region 56 and half-window region 58, which optical devices 40A, 40B are arranged within. [0074] Window region 56 is larger than optical device 40A and, therefore, regions of the window are not covered by the optical device 40A. The design layer 46, however, may extend across the window region 56, even in areas not covered by optical device 40A and, in fact, may extend over (or under) the opacifying layers, if required. In the window region 56, on the opposite side of from the optical structures of the optical device 40A, is an image layer 60. If design layer 46 is transparent, the window region 56 will have areas in which the optical device 40A provides optical effects (interacting with image layer 60) and regions, where the design layer 46 is arranged, which are simply transparent, with no discernible optical effects, especially as the image layer 60 does not extend to where the design layer 46 is arranged. [0075] The half-window region 58, in this example, includes an image layer 62 which, in combination with optical structures of the optical device 40B, provide an optical effect. The design layer 46 over the optical structures, in this example, is transparent and therefore image layer 64 is viewable directly through the combination of the design layer 46 and the optical structures, without, for example, the magnification that would be provided by microlenses. In this manner, the image layers 62 and 64 can be aesthetically designed to complement each other with areas of optical effects combined with areas of no effects.

17 [0076] It should be noted that the examples of figures 5A and 5B are not limiting and the arrangement described in relation to the window region 56 could also be used in the half-window region 58 and vice-versa. [0077] Further modifications and improvements may be incorporated without departing from the scope of the invention. For example, although microlenses are discussed in the specific embodiment described in relation to Figure 4A and 4B, the embodiment is applicable to other optical structures.

Claims (5)

1. A method for producing an optical device having a substrate including: providing at least one area of optical structures on the substrate; covering portions of the at least one area of optical structures with a design layer, wherein the design layer prevents the optical structures that are covered from operating in the same manner as the uncovered optical structures.

2. An optical device, including: a substrate; at least one area of optical structures on the substrate; and a design layer covering portions of the at least one area of optical structures, wherein the design layer prevents the optical structures that are covered from operating in the same manner as the uncovered optical structures.

3. A method or an optical device as claimed in claim 1 or claim 2, wherein the design layer is transparent or translucent.

4. A method or an optical device as claimed in claim 3, wherein the design layer has the same or similar refractive index as the optical structures.

5. A method or an optical device as claimed in any of claims 1 to 4, wherein the optical structures are selected from one or more of the following: microlenses, Fresnel lens(es), diffraction gratings, diffractive optical elements or diffractive zone plates.