AU2015100393B4 - Modulated Surface Relief Structure - Google Patents

Abstract

A hidden image security method is disclosed including the steps of obtaining first and second parts of an encoded hidden image corresponding to an original image; providing a light transmissive substrate; applying the first part of the encoded hidden image to the light transmissive substrate in the form of a plurality of focussing elements; displaying the second part of the encoded hidden image to a display screen of an electronic device; and superposing the plurality of focussing elements of the security document or device over the second part of the encoded hidden image on the display screen to decode and reveal the encoded hidden image.

Description

1 MODULATED SURFACE RELIEF STRUCTURE FIELD OF THE INVENTION [0001] The invention generally relates to a modulated surface relief structure and optically variable devices, suitable for incorporation in banknotes, passports and other security documents formed from a substrate to which one or more layers of printing are applied. DEFINITIONS Security Document or Token [0002] 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. [0003] 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. Substrate [0004] 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 2 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. [0005] 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 [0006] 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. [0007] A window area may be formed in a polymeric security document which has 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. [0008] 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 3 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. [0009] 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 [0010] Opacifying layers applied to a transparent substrate 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 substantially opaque material to which indicia may be subsequently printed or otherwise applied. Security Device or Feature [0011] 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 4 structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs). Diffractive Optical Elements (DOEs) [0012] 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). BACKGROUND OF THE INVENTION [0013] Security devices used in security documents or the like may be overt security devices, such as holograms or diffraction gratings, which can be viewed readily by the viewer in normal lighting conditions, or covert security devices which require a separate authenticating device to reveal a latent or hidden image. [0014] One known type of covert security device consists of an array of lines printed on the security document which may be authenticated by a separate line screen or a lenticular lens array.

5 [0015] WO 2009/121141 Al and WO 2009/152580 Al of the Commonwealth Scientific and Industrial Research Organization both describe techniques for encoding and decoding hidden images. The contents of both WO 2009/121141 Al and WO 2009/152580 Al are incorporated herein by reference. [0016] WO 2009/121141 Al discloses a hidden image method and apparatus in which encoded latent images are formed as a plurality of modulated lines on carrier devices, the latent images being hidden until revealed by a corresponding line decoder. [0017] WO 2009/152580 Al discloses a method of decoding hidden images in which a first image comprising a plurality of lines is printed onto a light transmissive substrate and a second image of modulated lines is displayed on an electronic display, the first and second images providing between them a hidden image so that when the substrate is superposed on the display the hidden image is decoded and revealed. This technique is referred to as Decode-on-Monitor or 'DOM'. The DOM technique may use a black and white screen to reveal a greyscale hidden image, or a colour display monitor to reveal a coloured hidden image (known as Colour DOM or C-DOM). A wide variety of electronic displays may be used in the DOM and C-DOM techniques, such as monitors and screens of liquid crystal displays (LCDs), mobile telephones, personal digital assistants, fixed and portable entertainment systems, electronic game terminals, instrument readouts, mp3 players, global positioning system (GPS) units, point of display monitors, automatic teller machines (ATMs), electronic checkout systems, etc. [0018] One limitation of DOM and C-DOM printed security features is that they could benefit from additional brightness when authenticated by an illuminated black and white or colour monitor. The colour brightness that can be obtained from a C-DOM printed security feature is dependent upon matching the resolution of the C-DOM feature to the authenticating device. If the resolution does not match, the colour monitor, such as an LCD screen, may not be able to display the coloured image accurately.

6 [0019] Another limitation of DOM or C-DOM printed security features is that the latent image encrypted into the printed C-DOM image may be at least partly visible to the naked eye. SUMMARY OF THE INVENTION [0020] According to one aspect of the present invention, there is provided a hidden image security method including: obtaining first and second parts of an encoded hidden image corresponding to an original image; providing a light transmissive substrate; applying the first part of the encoded hidden image to the light transmissive substrate in the form of a plurality of focussing elements; displaying the second part of the encoded hidden image to a display screen of an electronic device; and superposing the plurality of focussing elements of the security document or device over the second part of the encoded hidden image on the display screen to decode and reveal the encoded hidden image. [0021] The plurality of focussing elements forming the first part of the encoded hidden image preferably comprise lenticular structures. The second part of the encoded hidden image is preferably displayed on the display screen as a plurality of lines. [0022] Preferably, the method includes the step of modulating at least one parameter of the lines of the second part of the encoded hidden image. The at least one modulated parameter may be one or more of: line width; line spacing; line angle; line phase. In one preferred embodiment, at least one corresponding parameter of the focussing elements is also modulated. [0023] Accordingly, the plurality of focussing elements magnify, and, therefore, display the second part of the encoded image. Effectively, the portions which are not magnified are not viewed through the focusing elements and an encoded image is viewable instead.

7 [0024] The display screen of the electronic device is preferably an illuminated liquid crystal display (LCD), although electronic display screens using other technologies are also applicable. There are many different types of electronic devices that have illuminated electronic display screens suitable for use in the invention, including laptop computers, mobile (cellular) telephones, notebook and tablet computers, digital cameras, etc. Preferably, the focussing elements perform "pixel" sampling of the electronic display. That is, the focussing elements display, or sample, pixels of the second part of the encoded image. It is possible to match the encoded image to the resolution of the display, such that the focussing elements are directly sampling the physical electronic pixels, or sub-pixels, of the electronic display. However, it is also possible to match the focussing elements to the resolution of the encoded image, where an image "pixel" relates to the resolution of the image and the image pixel may encompass many physical electronic pixels. [0025] In a particularly preferred embodiment, the light transmissive substrate forms part of a security document, and the method is used to verify the authenticity of the security document. [0026] According to a second aspect of the invention, there is provided a security document or device including: a light transmissive substrate, and a first part of an encoded hidden image in the form of a plurality of focussing elements applied to the light transmissive substrate, wherein when the plurality of focussing elements is superposed over an electronic display screen provided with a second part of the encoded hidden image, the encoded hidden image is decoded and revealed. [0027] Various limitations of existing DOM and C-DOM images can be addressed by using focussing elements instead of line barriers (that is, opaque printed lines) that block the display of individual pixels of the authenticating device.

8 [0028] There are multiple benefits that are contingent upon use of suitable focussing elements. [0029] First, by using focussing elements instead of opaque printed lines to perform LCD pixel, or sub-pixel, sampling, an individual LCD pixel can be selected and magnified to fill the width of the focussing element, resulting in increased brightness of projected LCD pixel. The limited brightness of existing DOM and C-DOM images is due to the use of opaque printed lines in the DOM or C-DOM image. The lines act as a barrier, which screens the selection and display of individual LCD pixels thereby reducing overall brightness. [0030] Sub-pixels are the individual colour elements that together make up a colour pixel. Traditionally, on a colour screen there are one green sub-pixel, one red sub-pixel and one blue sub-pixel, although modern screens use different combinations of the numbers of each colours and can include other colours. [0031] Second, by using focussing elements instead of opaque printed lines to perform LCD pixel sampling, the first part of the encoded hidden image will appear transparent, thereby rendering the encrypted image less obvious to the naked eye. [0032] Consequently, preferred embodiments of the invention provide enhanced colour brightness of the DOM / C-DOM feature, transparency of the DOM / C-DOM feature leading to enhanced security. Preferred embodiments also offer relative ease of manufacture, and sufficient flexibility to integrate the DOM / C-DOM feature with complementary security features, thus offering additional protection. [0033] The suitable focussing elements of the invention are preferably provided in the form of a lenticular structure which replaces the lines and gaps in a conventional DOM / C-DOM. This enables viewing an enhanced colour image 9 on an illuminated LCD screen. The encrypted latent image is also less apparent to the naked eye. [0034] The plurality of focussing elements are preferably applied to the light transmissive substrate in an embossing process. In a particularly preferred embodiment, the plurality of focussing elements are embossed in a radiation curable layer applied to the light transmissive substrate. [0035] Our International Patent Application W02008/031170A1, the contents of which are incorporated herein by reference, describes a "soft embossing" process which is particularly suitable for forming the focussing elements in the method of the present invention. With such a soft embossing process, the focussing elements may be embossed into an embossable radiation curable ink layer and substantially simultaneously cured with radiation such as UV radiation, X-rays or electron beams. Embossable Radiation Curable Ink [0036] 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. [0037] 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.

10 [0038] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating. [0039] 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. [0040] 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. [0041] 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. [0042] 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. [0043] The lenticular structures used in the invention may be fabricated in different resolutions (lenses per inch) using a specially made lens roller, though any suitable fabrication technique can be used. While a variety of lenticular structures have previously been proposed for security documents, the application 11 of lenticular structures with a C-DOM feature has not previously been contemplated. Brief Description of the drawings [0044] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [0045] Figure 1 is a flow chart of a preferred method in accordance with the invention; [0046] Figure 2 is a schematic diagram of a security document having first part of an encoded hidden image in a window of the document; [0047] Figure 3 is a section on the line A-A of Figure 2; [0048] Figure 4 is a schematic diagram of an electronic device displaying a second part of the encoded hidden image; [0049] Figure 5 shows the security document laid over the display screen of the electronic device of Figure 4 to authenticate the security document. DESCRIPTION OF PREFERRED EMBODIMENT [0050] A preferred method in accordance with the invention is shown in Figure 1 and involves obtaining a source image 10 that will correspond to the image that will ultimately be revealed to authenticate an article. The source image is used to generate first and second parts of an encoded hidden image 11 by using one of a number of suitable transformative techniques described in further detail below.

12 [0051] The method then involves applying the first part of the hidden image as a plurality of focussing elements to a light transmissive substrate 12. The light transmissive substrate may form part of, or may be attached to, an article that is to be authenticated by the method. [0052] When an article including the light transmissive substrate is required to be authenticated, the second part of the encoded hidden image is displayed on the display screen 13 of an electronic device, and the light transmissive substrate is placed over the display screen and positioned so that the focussing elements are superposed over the second part of the encoded hidden image 14. It is then determined whether this decodes the encoded image 15. If the hidden image is revealed 16, the article is authenticated. If, however, the hidden image is not revealed 17, this indicates that the article including the light transmissive substrate is not authentic. [0053] Figures 2 and 3 schematically show an article in the form of a security document 20 for use in the method described above. The security document 20 is shown in Figure 2 is represented by a banknote, though the invention is equally applicable to other types of security and identification documents, and other articles of value that may require some form of authentication for security purposes. [0054] The security document 20 includes a security device 22 provided in an area of light transmissive substrate that forms a substantially transparent window 24 in the document. The security device 22 includes a first part of an encoded hidden image formed by a plurality of focussing elements 26 applied to the light transmissive substrate. [0055] Referring to Figure 3, the security document 20 preferably includes a substrate 30 formed from a substantially transparent polymeric material. One suitable polymeric material is biaxially-oriented polypropylene (BOPP) which has successfully been used in polymer banknotes produced by Australia and other 13 countries since at least the early 1990s, though other substrates may be used. The banknote 20 includes one or more opacifying layers 31, 32 applied to each side of the substrate 30, except in the area of the substantially transparent window 24. The opacifying layers 31,32 may include one or more coatings of opacifying ink applied to opposite sides of the substrate 30. Alternatively, the opacifying layers 31,32 may be formed from layers of paper or other opaque material laminated to opposite sides of the substrate 30 to form a hybrid security document. The opacifying layers 31,32 may bear printed indicia 28. [0056] The plurality of focussing elements 26 in the window 24 are preferably lenticular structures, such as part-cylindrical lenses or microlenses. The focussing elements 26 are preferably embossed structures and may be formed by embossing a radiation curable layer 34 applied to one side of the substrate 30 in the area of the window 24. [0057] Figure 4 shows an electronic device 40 which includes a display screen 42 for use in the method of the invention. The display screen 42 is preferably an illuminated liquid crystal display (LCD) screen. The LCD screen is arranged to display the second part 44 of the encoded hidden image, preferably in the form of a series of lines. [0058] Figure 5 shows the security document 20 of Figures 2 and 3 placed over the display screen 42 so that the focussing elements 26 in the window 24 of the document are superposed over the second part 44 of the encoded hidden image to reveal the encoded hidden image 50. [0059] Those skilled in the art will appreciate that various modifications of the embodiments described herein are possible without departing from the scope of the invention, as defined by the claims.

Claims (5)

1. A hidden image security method comprising: a) obtaining first and second parts of an encoded hidden image corresponding to an original image; b) providing a light transmissive substrate; c) applying the first part of the encoded hidden image to the light transmissive substrate in the form of a plurality of focussing elements; d) displaying the second part of the encoded hidden image to a display screen of an electronic device; and e) superposing the plurality of focussing elements of the security document or device over the second part of the encoded hidden image on the display screen to decode and reveal the encoded hidden image.

2. A method according to claim 1, wherein the plurality of focussing elements are applied to the light transmissive substrate in an embossing process, preferably by embossing a radiation curable layer applied to the light transmissive substrate.

3. A security document or device including a light transmissive substrate, and a first part of an encoded hidden image in the form of a plurality of focussing elements applied to the light transmissive substrate, wherein when the plurality of focussing elements is superposed over an electronic display screen provided with a second part of the encoded hidden image, the encoded hidden image is decoded and revealed.

4. A method or a security document or device according to any one of the preceding claims, wherein the plurality of focussing elements forming the first part of the encoded hidden image comprise lenticular structures. 15

5. A method according to claim 4, wherein the second part of the encoded hidden image is displayed on the display screen as a plurality of lines, and including the step of modulating at least one parameter of the lines of the second part of the encoded hidden image. INNOVIA SECURITY PTY LTD WATERMARK PATENT AND TRADE MARKS ATTORNEYS UIP1337AU00