AU2012100572A4

AU2012100572A4 - Optical security device producing different images in reflection and transmission

Info

Publication number: AU2012100572A4
Application number: AU2012100572A
Authority: AU
Inventors: Michael Hardwick
Original assignee: Securency International Pty Ltd
Current assignee: Securency International Pty Ltd
Priority date: 2012-05-10
Filing date: 2012-05-10
Publication date: 2012-06-07
Anticipated expiration: 2020-05-10
Also published as: WO2013166561A1; AU2012100572B4; FR2990525A1

Abstract

An optical security device 200, has a relief structure 204 provided on a transparent or partially transparent substrate 202, and a coating 206 provided 5 over the relief structure 204, wherein under incident light the relief structure 204 produces a first image 102 and a second image 104, such that when the device 200 is viewed in transmission the first image 102 and second image 104 are visible, and when the device 200 is viewed in reflection the first image 102 is visible and the second image 104 is either weakly visible or not visible. This may 10 be achieved by providing a coating 206 of such a high refractive index that the depth of the portion 210 of the relief structure 204 that produces the second image 104 is insufficient for the second image 104 to be visible in reflection. zoo -F Sa 30 44l q0 402. a /\ FiytAr Set FirmA~ S

Description

1 OPTICAL SECURITY DEVICE PRODUCING DIFFERENT IMAGES IN REFLECTION AND TRANSMISSION FIELD OF THE INVENTION 5 This invention relates to optical security devices having relief structures such as may be used to provide optical effects for the verification of security documents. BACKGROUND TO THE INVENTION 10 A wide variety of diffractive structures are available to add security to a document. Such diffractive structures may include, for example, holograms, nanogravure, and reflective/refractive structures. A diffractive structure may be provided on a transparent substrate, and be coated with a high refractive index coating, thereby producing a diffractive effect that is viewable in both reflection 15 and transmission. Such diffractive structures provide some degree of protection against counterfeiters. However, it is desirable to develop improved diffractive structures which provide additional security features. 20 SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided an optical security device, including a relief structure provided on a transparent or partially transparent substrate, the relief structure having a first portion which under incident light produces a first image and a second portion which under 25 incident light produces a second image; and a coating provided over the relief structure; wherein when the device is viewed in transmission the first image and second image are visible, and when the device is viewed in reflection the first image is visible and the second image is substantially not visible. Preferably, the first portion of the relief structure has a greater depth than 30 the second portion of the relief structure. Preferably the refractive index of the coating is sufficiently high that the depth of the second portion of the relief structure is insufficient to provide for the second image to be clearly visible in reflection.

2 The relief structure may be a binary diffraction grating. Alternatively the relief structure is a multilevel diffractive structure. Where the relief structure is either a binary diffraction grating or a multilevel diffractive structure, the first portion of the relief structure preferably has a depth of approximately double that 5 of the depth of the second portion of the relief structure. Further preferably, where the relief structure is a binary diffraction grating the first portion of the relief structure has a depth of approximately 1000nm, and the second portion of the relief structure has a depth of approximately 450nm. Alternatively, where the relief structure is a multilevel diffractive structure, it is 10 further preferable that the first portion of the relief structure has a depth of approximately 2400nm, and the second portion of the relief structure has a depth of approximately 1200nm. In an alternative embodiment, the relief structure may be a diffractive structure. Where the relief structure is a diffractive structure, it is preferable that 15 the first portion of the relief structure has a depth of approximately seven times that of the depth of the second portion of the relief structure. It is further preferable that the first portion of the relief structure has a depth of approximately 50pm, and the second portion of the relief structure has a depth of approximately 7pm. 20 The relief structure may be formed by embossing an embossable radiation curable ink. A device according to the invention may be provided in or on a security document, and further may be provided in a window of the security document. The security document may be a banknote. 25 According to another aspect of the invention, there is provided a method of producing an optical security device, including the steps of providing a relief structure on a transparent or partially transparent substrate, the relief structure having a first portion which under incident light produces a first image and a second portion which under incident light produces a second image; and 30 providing a coating over the relief structure; such that when the device is viewed in transmission the first image and second image are visible, and when the device is viewed in reflection the first image is visible and the second image is substantially not visible.

3 Security document As used herein, the term security document includes all types of documents and tokens of value and identification documents including, but not 5 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. 10 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 15 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 20 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 25 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. 30 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.

4 Diffractive Optical Elements (DOEs) 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 5 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 10 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 15 in the far field). DOEs are sometimes referred to as computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms, Fresnel holograms and volume reflection holograms. Refractive index n 20 The refractive index of a medium n is the ratio of the speed of light in vacuo 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: nI* Sin (a)= n * Sin (0), 25 where cc 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, is the refractive index of air (as an approximation ni may be taken to be 1). Embossable Radiation Curable Ink 30 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 5 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 5 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 sub wavelength gratings, transmissive diffractive gratings and lens structures. 10 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. 15 Alternatively, the radiation curable embossable coatings may be based on other compounds, eg nitro-cellulose. 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. 20 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 25 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 30 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.

6 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 5 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 10 cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium 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 15 properties of the embossed ink structure. BRIEF DESCRIPTION OF THE DRAWINGS Figure la is a representation of a combined image produced by a relief structure. 20 Figure lb shows the separate image components of the image of Figure 1 a. Figure 2 is a schematic cross section of an optical security device having a relief structure in the form of a hologram with varying depths. Figure 3 is a schematic cross section of an optical security device having a 25 relief structure in the form of a nanogravure or multilevel structure with varying depths. Figure 4 is a schematic cross section of an optical security device having a relief structure in the form of a reflective/refractive structure with varying depths. Figures 5a and 5b are representations of the image visible when the 30 optical security devices of Figures 2 to 4 are viewed in reflection and transmission respectively.

7 DESCRIPTION OF PREFERRED EMBODIMENT Referring to Figures la and 1b, according to a preferred embodiment of the invention, an optical security device has a relief structure which, under incident light, produces the combined image 100. The combined image 100 is 5 made up of a first image 102, being an image of the letter "A", and a second image 104, being an image of a sun. The relief structure may be any one of a variety of relief structures known in the art which produce an image under incident light, which include, for example, holograms, diffraction gratings, nanogravure, numerical-type diffractive optical elements (DOEs), multi-level DOEs, and 10 reflective/refractive structures, or a combination of such structures. In one embodiment of the invention, as shown in Figure 2, the optical security device 200 includes a substrate 202, a relief structure in the form of a holographic structure 204, and a coating 206. A portion of the holographic structure 208 has a greater depth than another portion 210 of the holographic 15 structure. The portion of greater depth 208 of the holographic structure is responsible for producing the first image 102, while the portion of lesser depth 210 is responsible for producing the second image 104. The substrate 202 is transparent or substantially transparent, such that under incident light, both the first image 102 and the second image 104 are 20 visible in transmission. The relief structure 204 may be produced by any suitable method, such as embossing the relief structure204 into the substrate 202 using an embossable radiation curable ink. The coating 206 is selected with a refractive index that is substantially higher than the refractive index of the substrate 202. It has been found that as 25 the refractive index of the coating over a relief structure increases, the depth of the relief structure must also increase to allow the effect of the relief structure to be viewed in reflection. If the relief structure is not sufficiently deep for the refractive index of the coating, the optical effect produced by the relief structure is diminished, and in some cases, not visible at all. Thus the refractive index of the 30 coating 206 and the depth of the portion 210 of the holographic structure that is responsible for the second image 104 are selected such that the second image 104 is not visible or is only weakly visible when the optical security device 200 is 8 viewed in reflection, due to the insufficient depth of the portion 210 of the structure relative to the refractive index of the coating 206. For a binary diffraction grating, where the refractive index of coating 206 is approximately 1.7, the above effect may be achieved by providing portion 208 5 with a depth of approximately 1000nm, while providing portion 210 with a depth of approximately 450nm. Consequently, when device 200 is viewed in reflection, the first image 102 is visible, however the second image 104 is not visible or only weakly visible, and an observer will only see the image 102 as shown in Figure 5a. However, when 10 the same device is viewed in transmission, both the first image 102 and the second image 104 are visible, and an observer will see the combined image shown in Figure 5b. In an alternative embodiment of the invention, as shown in Figure 3, the relief structure is a nanogravure or multilevel structure 304. The optical security 15 device 300, substrate 302, and coating 306 are analogous to 200, 202 and 206 respectively. The structure 304 has a portion 308 that has a greater depth than another portion 310 of the structure. The portion of greater depth 308 of the structure is responsible for producing the first image 102, while the portion of lesser depth 310 is responsible for producing the second image 104. The 20 refractive index of the coating 306 and the depth of the portion 310 of the structure that is responsible for the second image 104 are selected such that the second image 104 is not visible or is only weakly visible when the optical security device 300 is viewed in reflection, due to the insufficient depth of the portion 310 of the structure relative to the refractive index of the coating 306. 25 For a multilevel diffractive structure, where the refractive index of coating 306 is approximately 1.7, the above effect may be achieved by providing portion 308 with a depth of approximately 2400nm, while providing portion 310 with a depth of approximately 1200nm. Consequently, when device 300 is viewed in reflection, the first image 102 30 is visible, however the second image 104 is not visible or only weakly visible, and an observer will only see the image 102 as shown in Figure 5a. However, when the same device is viewed in transmission, both the first image 102 and the 9 second image 104 are visible, and an observer will see the combined image shown in Figure 5b. In a further alternative embodiment of the invention, as shown in Figure 4, the relief structure is a reflective/refractive structure 404. The optical security 5 device 400, substrate 402, and coating 406 are analogous to 400, 402 and 406 respectively. The structure 404 has a portion 408 that has a greater depth than another portion 410 of the structure. The portion of greater depth 408 of the structure is responsible for producing the first image 102, while the portion of lesser depth 410 is responsible for producing the second image 104. The 10 refractive index of the coating 406 and the depth of the portion 410 of the structure that is responsible for the second image 104 are selected such that the second image 104 is substantially not visible, that is, the second image is not visible at all or is only weakly visible, when the optical security device 400 is viewed in reflection, due to the insufficient depth of the portion 410 of the 15 structure relative to the refractive index of the coating 406. For a refractive structure, where the refractive index of coating 406 is approximately 1.7, the above effect may be achieved by providing portion 408 with a depth of approximately 50pm, while providing portion 410 with a depth of approximately 7pm. 20 Consequently, when device 400 is viewed in reflection, the first image 102 is visible, however the second image 104 is not visible or only weakly visible, and an observer will only see the image 102 as shown in Figure 5a. However, when the same device is viewed in transmission, both the first image 102 and the second image 104 are visible, and an observer will see the combined image 25 shown in Figure 5b. The optical security device may be provided as a security feature on a security document, for example a banknote. The substrate may be formed of any material, and may, for example be a polymer substrate. 30

Claims

1. An optical security device, including: a) a relief structure provided on a transparent or partially transparent substrate, the relief structure having a first portion which under incident light 5 produces a first image and a second portion which under incident light produces a second image; and b) a coating provided over the relief structure; wherein when the device is viewed in transmission the first image and second image are visible, and when the device is viewed in reflection the first 10 image is visible and the second image is substantially not visible.

2. A device according to claim 1, wherein the first portion of the relief structure has a greater depth than the second portion of the relief structure.

3. A device according to claim 2, wherein the refractive index of the coating is sufficiently high that the depth of the second portion of the relief structure is 15 insufficient to provide for the second image to be clearly visible in reflection.

4. A method of producing an optical security device, including the following steps: a) providing a relief structure on a transparent or partially transparent substrate, the relief structure having a first portion which under incident light 20 produces a first image and a second portion which under incident light produces a second image; and b) providing a coating over the relief structure; such that when the device is viewed in transmission the first image and second image are visible, and when the device is viewed in reflection the first 25 image is visible and the second image is substantially not visible.

5. A method according to claim 16, wherein the first portion of the relief structure has a greater depth than the second portion of the relief structure.