WO2019153043A1

WO2019153043A1 - Optically variable device having tonal effect

Info

Publication number: WO2019153043A1
Application number: PCT/AU2019/050091
Authority: WO
Inventors: Karlo Jolic
Original assignee: Ccl Secure Pty Ltd
Priority date: 2018-02-09
Filing date: 2019-02-08
Publication date: 2019-08-15
Also published as: AU2018100185B4; AU2018100185A4

Abstract

The invention is directed to a method of producing a micro-structure for an optically variable device. The micro-structure has a plurality of micro-structure pixels. The method comprises determining the tonal range of an image, the image having a plurality of image pixels; assigning a tonal value for one or more image pixels based on the tonal range; determining a height value of each micro-structure pixel based on the tonal value of a corresponding image pixel or corresponding group of image pixels; and forming each micro-structure pixel at the determined height value to create the micro-structure.

Description

Optically Variable Device Having Tonal Effect

Technical Field

[0001] The invention is directed to a micro-structure for an optically variable device and a method of producing the micro-structure, as well as an optical variable device including the micro-structure. The optical variable device may include various integral image devices and devices in which elements of the micro-structure are determined based on an interlaced/interleaved image processing method (interlaced type integral image device), or a digital image projection method (projection type integral image device).

Background of Invention

[0002] Current methods of providing micro-optical effect images for integral image devices include providing one or more binary images in or on an image layer (binary image layer) and an array of focusing elements (such as micro-lenses) which work together with the image layer under different illumination conditions to project one or more different viewpoint images.

[0003] In order to provide the binary images required for the image layer, coloured or grayscale raw images are converted into binary images using various binary dithering schemes. For example, amplitude modulation halftone screening involves simulating the grey-tones in an image using dots of variable sizes located on a constant pitch grid. Another example dithering scheme is frequency modulated screening (also known as diffusion dithering) where the grey-tones in an image are simulated by dots of a constant size distributed with varying frequencies across the image.

[0004] Light interacting with each point in the binary imagery layer is therefore attenuated into one of two possible brightness levels (which are at least

approximately discrete - either high or low brightness) that are subsequently sampled and magnified by the micro-lenses to project a magnified viewpoint image which appears tonal or grayscale to the user. In reality, the viewpoint image projected to the user is based on one or more binary images in the binary image layer. Since the dots that make up the viewpoint image are generally too small to be resolved by the naked eye, the viewer perceives an illusion of tonality.

[0005] A problem with existing methods of using such a binary image layer is that it can often be difficult to implement the associated integral image devices as thin micro-optical effect security features, such as those required on banknotes and other thin security documents. This is because each micro-lens implementing a micro- optical effect on a banknote must be very small, to ensure it is able to focus through the small thickness of the banknote, on to the imagery on the image layer (typically on the reverse side of the banknote). The thinner the security document, the smaller the size of the micro-lenses that can be used. The small size of each micro-lens therefore limits the amount of binary information that can be placed in the optical footprint of each lens in the image layer. It can therefore be difficult to implement integral image devices having a relatively complex tonal imagery in the binary image layer as security devices on security documents having a limited thickness, such as

banknotes.

[0006] Embodiments of the present invention may provide a micro-structure for an optically variable device and a method of producing the micro-structure which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides the consumer with a useful choice.

Summary of Invention

[0007] According to one aspect of the invention, there is provided a method of producing a micro-structure for an optically variable device, the micro-structure having a plurality of micro-structure pixels, the method comprising determining a tonal range of an image, the image having a plurality of image pixels; assigning a tonal value for one or more image pixels based on the tonal range, determining a height value of each micro-structure pixel based on the tonal value of a corresponding image pixel or corresponding group of image pixels, forming each micro-structure pixel at the determined height value to create the micro-structure.

[0008] Advantageously, by varying height to provide varying tonal effects, the micro-structure is capable of depicting a grayscale image without having to simulate the grayscale image using binary values (i.e. via image dithering), thereby improving fidelity and quality of the projected viewpoint images based on the micro-structure.

[0009] Any suitable method may be used to determine the height of each micro structure pixel. For example, look-up tables, and various linear or non-linear functions may be used.

[0010] In one embodiment, the step of determining a height value may include determining a height value based on a look-up table. The look-up table may provide a corresponding height value for each tonal value, or a corresponding height value for each tonal value falling within a predetermined tonal range.

[001 1] In one embodiment, the step of determining a height value may include determining a height value based on a linear function of the tonal value of one or more corresponding image pixels. For example: hi = ^ x H_max (1 ) wherein, hi is the height value of micro-structure pixel i; ti is the tonal value of a corresponding image pixel;

Tis the tonal range of the image, for a grayscale image the tonal range refers to the total number of tonal levels minus one (e.g. for an 8 bits per pixel grayscale image, the tonal range is 2^s— 1 = 255); and

Hma is the maximum height of the micro-structure (e.g. 10 microns). [0012] In another embodiment, the step of determining a height value may include determining a complementary height value based on a linear function of the tonal value of one or more corresponding image pixels. For example:

wherein, hi is the height value of micro-structure pixel i; ti is the tonal value of a corresponding image pixel;

Vis the tonal range of the image, for a grayscale image the tonal range refers to the total number of tonal levels minus one (e.g. for an 8 bits per pixel grayscale image, the tonal range is 2^s— 1 = 255); and

H_ma is the maximum height of the micro-structure (e.g. 10 microns).

[0013] For a micro-structure having a maximum height of 10 microns depicting a grayscale image having 256 grayscale levels (i.e. 0 to 255), the above linear formula (1 ) will provide a height value for each micro-structure pixel between 0 and 10 microns for creating the micro-structure capable of provide varying tonal effects.

[0014] In another embodiment, the step of determining a height value may include determining a height value based on a non-linear function of the tonal value of one or more corresponding image pixels. For example:

wherein, hi is the height value of micro-structure pixel i; ti is the tonal value of a corresponding image pixel; Vis the tonal range of the image, for a grayscale image the tonal range refers to the total number of tonal levels minus one (e.g. for an 8 bits per pixel grayscale image, the tonal range is 2^s— 1 = 255); and

H_ma is the maximum height of the micro-structure (e.g. 10 microns).

[0015] In another embodiment, the step of determining a height value may include determining a complementary height value based on a non-linear function of the tonal value of one or more corresponding image pixels. For example:

[0016] For a micro-structure having a maximum height of 10 microns depicting a grayscale image having 256 grayscale levels, the above non-linear formula (2) will also provide a height value for each micro-structure pixel between 0 and 10 microns for creating the micro-structure capable of providing varying tonal effects. The advantage of the squared function (2) is that for micro-structures depicting some images, the height differences between micro-structure pixels depicting different tonal values will generally be greater to provide greater contrasting tonal effects.

[0017] In one embodiment, the step of determining a height value includes determining a height value that is directly proportional to a complementary tonal value of one or more corresponding image pixels. In another embodiment, the step of determining a height value includes determining a height value that is inversely proportional to the tonal value of the one or more corresponding image pixels.

[0018] Any suitable number of height values can be used to define each pixel of the micro-structure. In the examples including functions (1 ) and (2) above, up to 256 different height values may be used to create the micro-structure. In some

embodiments, the step of determining a height value includes determining a height value from a range of at least three discrete height values.

[0019] Preferably, the step of determining a height value may include determining a height value between 2 to 15 microns. Preferably, the step of determining a height value may include determining a height value between 2 to 12 microns. In some embodiments, a maximum height value may be between 12 to 15 microns.

[0020] The optically variable device may be an integral image device. In some embodiments, the optically variable device may be an integral image device produced using an interlaced/interleaved image processing method (herein after referred to as an interlaced type integral image device). In another embodiment, the optically variable device may be an integral image device produced using a digital image projection (herein after referred to as a projection type integral image device). In another embodiment, the optically variable device may be an integral image device produced using an interlaced/interleaved image processing method applied to a plurality of digital image projections.

[0021] Various different processes may be used to form the micro-structure. The step of forming each micro-structure pixel may include forming the micro-structure pixels using an embossing process or micro-structure printing process. The step of forming each micro-structure pixel may include forming the micro-structure pixels using embossable radiation curable ink. Hot embossing methods can also be used.

[0022] In some embodiments, the micro-structure may be formed via a UV embossing process. The UV embossing process may include creating a micro structure mould on an embossing tool (e.g. embossing roller or plate). The embossing process may further include applying a UV curable ink to a substrate, and then placing the substrate in contact with the embossing tool. Once and/or after contact is made, the UV curable ink is cured by passing UV light through the substrate.

[0023] In some embodiments, the micro-structure may be formed via a micro structure printing process. In particular, a micro-structure mould on a roller or plate. The step of forming each micro-structure pixel may include forming a micro-structure mould on an embossing roller or shim or on a micro-structure printing roller or shim. The step of forming each micro-structure pixel may further include applying a UV curable ink to the micro-structure mould, applying a substrate to the roller or plate and curing the UV curable ink. The step of forming each micro-structure pixel may include forming the micro-structure pixels using radiation curable ink. [0024] The step of forming each micro-structure pixel may include forming the micro-structure pixels using transparent or translucent ink and/or coloured ink.

[0025] In some embodiments, the image is an 8-bit grayscale image having 256 different tonal values. In some embodiments, a portion of the image may include a binary image having two tonal values. For example, a raw grayscale image may be divided into two or more sections, and one or more sections of the raw grayscale image may be converted into a binary image via image dithering. When creating a micro-structure to depict the raw image, portions of the micro-structure corresponding to the grayscale sections of the image may include micro-structure pixels having three or more height values to provide varying tonal effects, and portions of the micro structure corresponding to the binary sections may include micro-structure pixels having two discrete heights to represent the binary information.

[0026] According to a further aspect of the invention, there is provided a micro structure of an optically variable device, the micro-structure being manufactured using a method as described above.

[0027] According to another aspect of the invention, there is provided an optically variable device comprising: an image layer carrying a micro-structure, the micro-structure having a plurality of micro-structure pixels, each micro-structure pixel having a height corresponding to a tonal effect associated with the pixel; a plurality of focusing elements; the plurality of focusing elements and the image layer being configured to project one or more viewpoint images based on the micro-structure, and wherein the micro-structure pixels have three or more discrete height values to create a varying tonal effect in the viewpoint images.

[0028] An optically variable device may include a micro-structure as previously described. For example, the height values may be determined based on a look-up table. The micro-structure may depict one or more raw greyscale images. The height values may be determined based on a linear function of tonal values of corresponding image pixels in the raw images. The micro-structure may depict one or more raw images, and the height values may be determined based on a non-linear function of tonal values of corresponding image pixels in the raw images. The height values may be directly proportional to the tonal values of the corresponding image pixels. The height values may be directly proportional to complementary tonal values of the tonal values of the corresponding image pixels. The height value may be between 2 to 12 microns. The optically variable device may be an interlaced type integral image device. The optically variable device may be a projection type integral image device. The optically variable device may be a moire magnification device.

[0029] The micro-structure may be transparent or translucent. The micro-structure may be opaque or semi-opaque. The micro-structure pixels may have at least three different discrete height values. The micro-structure pixels may have less than or equal to 256 different discrete height values.

[0030] An optically variable device may further include one or more opaque or semi-opaque or transparent or translucent layers for at least partially covering the micro-structure.

[0031] The focusing elements may be a 1 D array of focusing elements, for example a one-dimensional array of cylindrical or elliptical micro-lenses. Alternatively, the focusing elements may be a 2D array of focusing elements, for example a two- dimensional array of round lenses with a spherical or elliptical surface profile.

[0032] According to a further aspect of the invention, there is provided a method of producing a micro-structure for an optically variable device, the micro-structure being defined by a plurality of micro-structure coordinates, the method comprising determining a tonal range of an image, the image having a plurality of image datasets for defining the image; assigning a tonal value for one or more image datasets based on the tonal range, determining a height value associated with each coordinate of the micro structure based on the tonal value of a corresponding image dataset or corresponding group of image datasets, forming a micro-structure based on the height values determined for each coordinate.

[0033] The image datasets may include image pixels for defining the image. In one embodiment, the image datasets may include vector images for defining the image.

[0034] In some embodiments, the raw image may be a vector image or 3D vector model. Image datasets from the vector images or models may be translated into height values at each coordinate in a coordinate system defining the shape of the micro-structure.

[0035] The step of forming a micro-structure may include creating a micro structure mould based on the height values determined for each coordinate.

[0036] Embodiments of the invention therefore enable the production of thin integral image devices having a micro-structure image layer capable of depicting true grayscale images. True grayscale here means the degree of light reflection projected by each micro-lens to each point in the projected/magnified viewpoint image is non binary i.e. the projected brightness is not one of a set of 2 (either high or low) brightness levels or tones, rather it is one of a set of 3 or more brightness levels or tones, or is part of a continuous spectrum of brightness/tones.

[0037] In other words, the tones in the projected viewpoint image are not achieved through binary dithering. Advantageously, the projected viewpoint images produced can be more complex and more realistic, since the tones in the viewpoint image are not simulated through a composition of binary image pixels, rather each micro structure pixel can depict a true grayscale tone. As such, the brightness level at each micro-structure pixel in the viewpoint image is a true grayscale brightness level, and not an impression of tonal variation as simulated by binary information. [0038] Reference throughout this specification to 'one embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristic described herein may be combined in any suitable manner in one or more combinations.

Definitions

Security Document or Token

[0039] 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.

[0040] 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

[0041] 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. [0042] 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".

Region

[0043] A region, as used herein, corresponds to an area of a surface of a security document or a substrate. For example, a first region located on a first side of a substrate is a different region to a second region located on a second side of the same substrate, even when the two regions are opposite one another. Two regions can be: opposite, wherein each region is located in the same area of the security document or substrate but on opposite surfaces; partially opposite, wherein one region includes a portion opposite all or a portion of the other region; and non opposite, wherein the regions are entirely not opposite each other.

Opacifying layers

[0044] 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 < L₀, where U 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 or oxidising cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Security Device or Feature

[0045] 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). 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 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 viewpoint 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).

Embossable Radiation Curable Ink

[0046] 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.

[0047] 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.

[0048] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.

[0049] 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, e.g. nitro-cellulose.

[0050] The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing micro-structures, including diffractive structures such as diffraction gratings and holograms, and micro-lenses and lens arrays.

However, they may also be embossed with larger relief structures, such as non diffractive optically variable devices.

[0051] 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.

[0052] 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. [0053] 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.

Optically variable image or device (OVD)

[0054] An optically variable image or device is a security feature or device that changes in appearance. OVDs provide an optically variable effect, for example, when the device substrate viewed in reflection or transmission, is tilted and/or when the viewing angle of the observer relative to the OVD changes. The image of an OVD may also be changed by aligning a verification device over the security feature or device. An OVD may be provided by a printed area, e.g. an area printed with metallic inks or iridescent inks, by an embossed area, and by a combination of a printed and embossed feature. OVDs may be provided by integral image devices such as moire magnification devices and devices in which elements of the micro-structure are determined based on an interlaced/interleaved image processing method, or a digital image projection method, or an interlaced/interleaved image processing method applied to a plurality of digital image projections. An OVD may also be provided by a diffractive device, such as a diffraction grating or a hologram and may include arrays of micro-lenses and lenticular lenses.

Depictions and Images

[0055] The terms "depiction" and "image" are used herein to describe

arrangements of ink on a surface of the optical device which may be viewed by a viewer (such as a typical human user). The terms are used in a synonymous sense, and should not be construed as signifying differences in concept.

[0056] Depictions and images can include the following: symbols; characters; numbers; portraits; codes; encrypted and/or scrambled visual information; patterns; geometric shapes; microimages; etc.

Focusing elements

[0057] The plurality of focusing elements may include any devices previously reported to be suitable for viewing micro-image elements on a substrate, particularly a substrate of a security document. In some embodiments, the focusing elements comprise reflective focussing micro-structures such as parabolic micro-mirrors, refractive micro-lens structures, including conventional micro-lenses and Fresnel lenses. In other embodiments, diffractive focusing elements, such as zone plates or photon sieves, may be employed. Fresnel lenses and diffractive focusing elements may be particularly suited for integration into a security feature of a security document, because such focusing elements are thinner than conventional micro-lens structures for a given focal length.

[0058] The plurality of focusing elements are generally in an ordered, repeating array, such as lenticular, rectangular or hexagonal configurations. The focusing elements may be registered in exact or offset alignment with the array of micro-image elements on the substrate, depending on the nature of the optical effect to be produced.

[0059] The plurality of focusing elements may be produced as a separate sheet which is adhered to the substrate. Flowever, in preferred embodiments, the plurality of focusing elements is produced by applying a transparent radiation-curable coating to the substrate, and embossing and curing the coating with radiation to form the focusing elements. The transparent coating into which the focusing elements are embossed may optionally have the same composition as the embossed coating of the relief layer of the micro-image elements. [0060] The focusing elements are generally disposed at a distance from the micro-image elements that is equal to or less than the focal length of the focusing elements. For use in security documents, the focal lengths of the focusing elements are preferably in the range of from 20 to 200 pm, more preferably from 65 to 170 pm, more preferably from 65 to 1 15 pm, corresponding with or exceeding the typical thickness of transparent substrates for security documents.

Integral Image Device

[0061] The term“Integral Image Device” is used herein to refer to an optically variable device that may project a magnified and/or‘floating’ viewpoint image, or a collection of different images when the device substrate viewed in reflection or transmission, is tilted and/or when the viewing angle of the observer relative to the device changes.

[0062] Integral Image devices may include moire magnification devices and devices in which visual elements of the micro-structure are determined based on an interlaced/interleaved image processing method, or a digital image projection method, or an interlaced/interleaved image processing method applied to a plurality of digital image projections.

[0063] A moire magnification device may include a light-transmitting polymeric substrate, an arrangement of micro image elements on or within the substrate or image layer, and an arrangement of focusing elements (e.g. micro-lenses). The micro image elements can be formed in a periodically repeating array. The projected moire magnified image can appear magnified and‘floating’ above or below the substrate. The magnification is determined by the relationship between the distance P₀ between the centre points of two adjacent micro images and the distance Pi between the centre points of adjacent focusing elements, (i.e. the relationship between the array pitches).

[0064] Accordingly, a pitch mismatch between the array of micro images and the array of micro-lenses can be introduced to create the phenomena of moire

magnification. [0065] It is also possible to introduce a small rotational misalignment between the array of micro images and the array of micro-lenses; or use a combination of small pitch mismatch and a small rotational misalignment to create the phenomena of moire magnification and enable the generation of moving images upon changing the viewing angle / tilting the substrate.

[0066] An integral image device produced using the interlaced/interleaved image processing method (referred to herein as an interlaced type integral image device) typically also includes a light-transmitting polymeric substrate, an arrangement of micro image elements on or within the substrate or image layer, and an arrangement of focusing elements (e.g. micro-lenses). The micro image elements may be formed on the substrate based on a collection of inter-related images, for example, by allocating relevant digitised portions of each inter-related image to a corresponding region of the substrate associated with each focusing element. The resulting effects may include, among other things, a moving 3D object, a dynamic (e.g. morphing or transforming) 3D object, a contrast switching 2D or 3D image, a flipping 2D or 3D image, an animation effect with 2D or 3D images, a dynamic design of curves, abstract designs, shapes, photographs, 3D objects and images.

[0067] An integral image device produced using a digital image projection method (referred to herein as a projection type integral image device) also includes a light- transmitting polymeric substrate, an arrangement of micro image elements on or within the substrate or image layer, and an arrangement of focusing elements (e.g. micro-lenses). The positions of the micro image elements on the image layer are determined by mapping portions of a projected image as viewed from a projection origin onto a virtual image plane for each focusing element. The projected images from an integral image device produced using a digital image projection can also produce a floating effect similar to that of a moire magnification device, as well as 2D/3D image effects including contrast switching, flipping, animation and morphing.

Comprise

[0068] The term“comprise” and variants of that term such as“comprises” or “comprising” are used herein to denote the inclusion of a stated integer or integers but not to exclude any other integer or any other integers, unless in the context or usage an exclusive interpretation of the term is required.

Prior Art

[0069] Reference to prior art disclosures in this specification is not an admission that the disclosures constitute common general knowledge.

[0070] In order that the invention may be more readily understood and put into practice, one or more preferred embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of Drawings

[0071] Figure 1 A illustrates an example 8-bit per pixel grayscale raw image having a size of 1 X 1 inch;

[0072] Figure 1 B is an enlarged illustration of Figure 1 A;

[0073] Figure 2A illustrates the raw image of Figure 1 A after it has been

resampled to a 50 pixel by 50 pixel image;

[0074] Figure 2B is an enlarged illustration of Figure 2A;

[0075] Figure 3A is illustrates the raw image of Figure 1 A after it has converted to a binary image via diffusion dithering;

[0076] Figure 3B is an enlarged illustration of Figure 3A;

[0077] Figures 4A to 4D are cross sectional views of a unit of an integral image device according to embodiments of the invention;

[0078] Figure 5 is a plan view of an image layer of a moire magnification device according to one embodiment of the present invention;

[0079] Figure 6 is a plan view of a micro-structure on the image layer as shown in Figure 5; [0080] Figure 7 illustrates a A-A cross sectional profile of the micro-structure of Figure 6;

[0081] Figure 8 is a cross sectional view of a single unit of the moire magnification device including the micro-structure of Figures 6 and 7;

[0082] Figure 9 is a schematic diagram illustrating the concept for capturing different views of a 3D model to create an interlaced type integral image device according to another embodiment of the invention;

[0083] Figure 10 illustrates different images captured from the different views as shown in Figure 9;

[0084] Figure 1 1 illustrates a plan view of a micro-structure according to one embodiment of the invention created by interlacing the different images as shown in Figure 10;

[0085] Figure 12 is a schematic diagram illustrating a plan view of a single unit of an interlaced type integral image device associated with Figures 9 to 1 1 ;

[0086] Figures 13 to 15 illustrate schematic plan views of a subset of an interlaced type integral image device according to different embodiments of the invention;

[0087] Figure 16 illustrates a plan view of a micro-structure for an interlaced type integral image device having a 1 D array of focusing elements according to one embodiment of the invention;

[0088] Figure 17 is a schematic diagram illustrating the concept for capturing different views of a 3D model to create a projection type integral image device according to another embodiment of the invention;

[0089] Figure 18 illustrates different images captured from the different views as shown in Figure 17;

[0090] Figure 19 illustrates a plan view of an example projection type integral image device associated with Figures 17 and 18; [0091] Figure 19A illustrates a plan view of an example projection type integral image device having a 1 D array of micro-lenses;

[0092] Figure 20 is a process flow diagram illustrating a method of producing an optically variable device according to one embodiment of the invention.

Detailed Description

[0093] Figures 1 A and 1 B each illustrate an 8 bits per pixel grayscale raw image 100 suitable for transformation onto an image layer of an integral image device. The tone of the pixels in raw image 100 includes 256 grey levels (i.e. from 0 to 255). The tonal range for raw image 100 is therefore 255 and a tonal value of each pixel in the raw image has an integer value that lies in the range from 0 to 255. The raw image 100 is defined by 315 X 315 pixels.

[0094] Figure 1 A illustrates raw image 100 having a size of roughly 1 inch X 1 inch, which is a typical size for a projected viewpoint image of a moire magnification device. Figure 1 B illustrates raw image 100 at 300% magnification, having a size of roughly 7.5 cm by 7.5 cm.

[0095] Due to size and resolution limitations of forming a micro-structure on the image layer, the raw image 100 needs to be resampled to matching a resolution that is achievable for micro integral image device applications.

[0096] As shown in Figure 2A, the raw image 100 has been resampled to 50 X 50 pixels to form resampled raw image 102 having a size of roughly 1 inch X 1 inch. Resampled raw image 102 can be matched to the resolution of a pixel raster grid associated with each micro-lens. In this particular example, the size of resampled raw image 102 is determined based on being compatible for use with a 2D micro-lens array, each lens having a 50 micron pitch. In practice, the micro-structure tooling can be originated at 25400 dpi resolution such that each pixel in an associated tooling raster grid is 1 micron in size. Accordingly, 50 X 50 pixels are available per lens in which to place a moire image. The resampled raw image 102 having 50 X 50 pixels is therefore compatible with the micro-structure tooling adopted. Figure 2B illustrates resampled raw image 102 at 300% magnification, having a size of roughly 7.5 cm by 7.5 cm, to more clearly illustrate the resampling effects on image quality.

[0097] The resampled raw image 102 remains an 8 bits per pixel grayscale image having 256 grey levels. The resampled raw image 102 as shown in Figure 2A would still be acceptable for a moire magnification device for thin film security documents such as banknotes, since the tonal effect of the grayscale image allows reasonable image quality and the identity of the person in the image 102 is still recognisable.

[0098] Figures 3A and 3B illustrate the resultant binary image 104 after the resampled raw image 102 is converted via diffusion dithering. The binary image 104 remains 50 X 50 pixels, however only 2 grey levels are used to define the binary image 104. In Figure 3A, the binary image 104 has a size of roughly 1 inch X 1 inch, and Figure 3B illustrates binary image 104 at 300% magnification, having a size of roughly 7.5 cm by 7.5 cm, to more clearly illustrate the diffusion dithering effects on image quality.

[0099] Even though the resolution of binary image 104 remains 50 X 50 pixels, the quality of the binary image 104 illustrates an obvious reduction when compared with grayscale resampled raw image 102, to the extent that the identity of the person in the binary image 104 is no longer recognisable. Figures 3A and 3B therefore further illustrate difficulties in implementing micro integral image devices for thin film applications using known methods involving binary images 104. For relatively complex images, it can often be difficult to achieve a desirable image quality once a raw image 100 is resampled and converted to a binary image 104 suitable for application to a micro image layer.

[0100] Embodiments of the present invention provide a micro-structure 202a depicting the resampled grayscale image 102 for an optically variable device such as an integral image device to provide tonal effects in the projected/magnified viewpoint image(s) of the integral image device. The ability to provide tonal effects in the micro structure 202a allows higher quality complex raw images to be used to implement the optical variable devices. [0101] A cross sectional view of a unit 200 of such an integral image device is shown in Figure 4A. An integral image device may include any suitable number of focusing elements, for example, in a 2D or 1 D array as discussed in further detail below. In practice, hundreds or thousands of micro-lenses may be used to implement an integral image device.

[0102] Unit 200 illustrates a substrate 205 having an image layer 204 on which the micro-structure 202a is formed. A focusing element in the form of a micro-lens 206 spaced from the image layer 204 is formed on an opposite side of the substrate 205.

A plane of the micro-lens 206 is generally parallel to that of the image layer 204.

Distance between the micro-lens 206 and the image layer is defined by the thickness of the substrate 205. Typically, the image layer 204 generally coincides, or is slightly within the focal plane of the micro-lens 206.

[0103] In suitable illumination conditions, the micro-lens 206 focuses on different areas of the micro-structure 202a on the image layer 204 to create various optical effects. The properties of the micro-lenses, the spatial relationship between the micro lenses and the micro-structures 202a on the image layer 204, and the configuration of the micro-structure 202a can be designed to project different types of viewpoint images in different integral image devices, as explained in further detail below with reference to Figures 5 to 19.

[0104] To provide the tonal effects of each image pixel in resampled raw image 102, the micro-structure 202a includes a plurality of micro-structure pixels 208 having varying height values to represent varying tonal values of each corresponding image pixel 103 of resampled raw image 102. For example, each micro-structure pixel 208 may correspond to one image pixel 103 or a group of image pixels. The height value of each micro-structure pixel 208 can be proportional or inversely proportional to the tonal value (i.e. 0 to 255) of the corresponding image pixel(s) 103. As explained in further detail below, the height value can also be a function of the tonal value or be determined based on a look-up table.

[0105] In one example, the height value of each micro-structure pixel 208 is a linear function of the tonal value (i.e. 0 to 255) of a corresponding image pixel 103. In particular, a height value for each micro-structure pixel 208 can be determined based on formula (1 ) below:

wherein, hi is the height value for micro-structure pixel i;

ti is the tonal value for a corresponding image pixel;

Tis the tonal range, for grayscale image 102 the tonal range is 255; and

H_ma is the maximum height of the micro-structure (e.g. 10 microns).

[0106] In this example, the micro-structure 208 has a maximum height of 10 microns. Using formula (1 ), a height value from 0 to 10 microns for each micro structure pixel 208 defines the micro-structure representation of raw image 102.

[0107] The different height values of the micro-structure pixels 208 allows different amounts of light to be reflected or transmitted from the pixels 208, thereby achieving a ‘true’ grayscale tonal effect. The depth versus grey-level correspondence can also be optimised using various mathematical models or empirically, to achieve an optimised visual perception of grey-level distribution in the projected viewpoint image.

[0108] In another example, the height value of each micro-structure pixel 208 is a non-linear function of the tonal value (i.e. 0 to 255) of a corresponding image pixel 103. In particular, a height value for each micro-structure pixel 208 can be determined based on formula (2) below. For example:

wherein, hi is the height value for micro-structure pixel i;

ti is the tonal value for a corresponding image pixel; Tis the tonal range, for grayscale image 102 the tonal range is 255; and

H_ma is the maximum height of the micro-structure (e.g. 10 microns).

[0109] In some embodiments, the tonal range of a raw image 102 may be scaled up or down to a larger (e.g. more than 256 grey levels) or smaller tonal range (e.g. less than 256 grey levels), and the tonal value of the image pixels 103 can be determined as a function of the old and new tonal range and the original grayscale values (e.g. from 0 to 255). The function may be a linear function or a non-linear function.

[0110] In some embodiments, a lookup table can be used to define the

relationship between the height value of a micro-structure pixel 208 and the tonal value of corresponding image pixel 103. An example lookup table is provided below.

[0111] The tonal value range in the lookup table above is between 0 and 7. In practice, any suitable number of tonal values having any suitable tonal range can be used to provide a micro-structure of sufficient quality.

[0112] As illustrated in Figures 4A to 4D, the micro-structures 202a to 202e can be formed using any suitable material in any suitable manner according to different embodiments of the invention. As mentioned, the micro-structures 202a, 202e can be implemented in the focal plane (not shown) or slightly inside the focal plane of the array of lenses 206.

[0113] Figure 4A illustrates that, according to one embodiment, the micro structure 202a is implemented as a transparent or translucent micro-structure, for example using embossable radiation curable ink, on a light transmitting polymer substrate 205. As light can travel through the micro-structure 202 and substrate 205, the projected viewpoint image(s) can be viewable in transmission as well as reflection.

[0114] According to another embodiment as shown in Figure 4B, the micro structure 202b includes a transparent or translucent micro-structure base 210 having micro-structure pixels 212 each having a height value corresponding to a desired tonal effect (e.g. corresponding to a desired tonal value). The micro-structure base 210 is overprinted with one or more opaque or semi-opaque layers 214 (e.g. with a layer of semi-opaque coloured ink such as red ink) so as to fill the space between the micro-structure pixels 212. The base 210 and the opaque layer 214 creates a micro structure 202b having coloured tonal effects. The projected viewpoint image(s) can be viewable in transmission as well as reflection, more preferably in transmission. The projected viewpoint image(s) consist of different tones of the colour of layer 214.

[0115] Optionally, the micro-structure 202b can include an additional outer opaque layer 216 printed using coloured ink of a contrasting colour (e.g. white) to that of intermediate layer 214. The projected viewpoint image(s) can be viewed in reflection. Layer 216 has the effect of increasing the contrast in the projected image. The projected viewpoint image(s) consist of different tones of the colour of layer 214. The background in the projected viewpoint image(s) has the colour of layer 216.

[0116] The layers 214, 216 can also serve to protect the pixel base 210 from wear and tear to thereby preserve the integrity of viewpoint image(s) projected therefrom over time. The layers 214, 216 can also serve to protect the pixel base 210 from mechanical copying.

[0117] According to another embodiment as shown in Figure 4C, the micro structure 202c includes a single layer printed using opaque or semi-opaque ink. The micro-structure pixels 218 being formed in opaque or semi-opaque ink and each having a height value corresponding to a desired tonal effect. The projected viewpoint image(s) are viewable in reflection or transmission, more preferably in transmission.

[0118] According to a further embodiment as shown in Figure 4D, the micro structure 202d includes an opaque or semi-opaque micro-structure base 220 having micro-structure pixels 212 each having a height value corresponding to a desired tonal effect. The micro-structure base 220 is overprinted with one or more opaque layers 224 (e.g. using coloured ink in a contrasting colour to the base 220) so as to fill the space between the micro-structure pixels 212 and cover the micro-structure base 220. The projected viewpoint image(s) are viewable in reflection and consist of different tones of the colour of layer 220. The background in the projected viewpoint image(s) has the colour of layer 224. The layer 224 can also serve to protect the pixel base 220 from wear and tear to thereby preserve the integrity of viewpoint image(s) projected therefrom over time. The layer 224 can also serve to protect the base 220 from mechanical copying.

[0119] In another embodiment, rather than layer 224 being an opaque layer, the layer 224 may be a substantially transparent/translucent layer, allowing the viewpoint image(s) to be viewed in either reflection or transmission, more preferably in transmission.

[0120] Whilst Figures 4A to 4D illustrate micro-structures according to different embodiments of the invention, other different combinations of transparent/translucent and opaque and/or semi-opaque layers can also be used to create different tonal effects in accordance with other embodiments of the invention.

[0121] In practice, for integral image devices to be viewed in transmission lighting (from the lens side), the micro-structure pixels will typically be formed using

transparent/translucent ink, or a semi-opaque coloured ink without any opaque overprinted layers. In these embodiments, the brightness of each micro-structure pixel will depend on the height of the micro-structure pixel. For example, if the micro structure pixels are formed in red ink, then a low corresponding height value will project a light red tone to the viewer; and a high corresponding height value will project a dark red tone to the viewer.

[0122] For integral image devices to be viewed in reflection lighting (from the lens side), the micro-structure pixels will typically be formed using opaque or semi-opaque coloured ink. One or more layers of opaque overprinted layers can be used to cover the micro-structure pixels in a contrasting colour. In these embodiments, the brightness of each pixel will again depend on the height value of each micro-structure pixel. For example, if the colour used for the micro-structure pixels is red, then a low height value sampled by the lens will project a light red tone to the viewer, and a high height value will project a dark red tone to the viewer.

[0123] The micro-structure according to embodiments of the invention can be applied to different integral image devices to create different visual effects.

[0124] Each of the different types of integral image devices may include a light- transmitting polymeric substrate, an arrangement of micro image elements (e.g. formed by micro-structure pixels) as described above on or within the substrate or image layer, and an arrangement of focusing elements (e.g. micro-lenses).

[0125] The spatial relationship between adjacent focusing elements and adjacent micro image elements, and/or the arrangement of the micro-structure pixels on an image layer can be altered and configured to create different optical effects according to the different types of integral image devices as described in further detail below. Whilst the examples below are provided in relation to micro-lenses, other types focusing elements can be used. Moreover, the spherical micro-lenses or cylindrical (lenticular) micro-lenses may be used.

Moire magnification Device

[0126] In a moire magnification device, the micro-structure pixels form a periodically repeating array of micro-images. By configuring the image layer and the micro-lenses according to moire magnification principles previously described, the moire magnification device can create a projected viewpoint image which appears to be‘floating’ above or below the substrate. [0127] Currently available techniques, such as direct laser writing methods (mask less laser lithography, grey tone lithography), are capable of achieving 256 different depths in a micro-structure 202. It is therefore possible to encode an 8 bits per pixel grayscale digital image into a micro-structure 202 in accordance with the present invention.

[0128] Figure 5 depicts a magnified plan view of an image layer 204 including a periodically repeating 5X5 array of micro-structures 300. The micro-structures 300 are in the form of repeating portraits 300 depicting the raw image 102 as shown in Figure 2A.

[0129] In one embodiment, the pitch P₀ of the portraits 300 is 50.1 microns, and the pitch Pi for a corresponding micro-lens array is 50 microns, to achieve a

magnification factor of 500 and therefore a magnified viewpoint image having a size of 25 mm X 25 mm.

[0130] Each portrait 300 can be provided using a micro-structure 300 having a plurality of micro-structure pixels 208, 212, 218, 222 as shown in Figures 4A to 4E. In this example, the maximum height of the micro-structure is 12 microns. Each pixel will therefore have an associated height value which is one of up to 256 different values between 0 and 12 microns.

[0131] As each portrait 300 depicts a grayscale image in which the white areas are defined by a grey level of 255 and the black areas are defined by a grey level of 0, the height value of the corresponding micro-structure pixels will be directly

proportional to the difference between the maximum grey level value (i.e. 255) and the grey levels of the raw image 102.

[0132] Accordingly, the white areas of the raw image 102 will correspond to micro structure pixels having a height of 0, and the black areas of the raw image 102 will correspond to micro-structure pixels having a height of 12 microns. Using formula (1 ) as defined above, a raw image pixel having a grey level of 64 will have a

corresponding micro-structure pixel height given by (255-64) / 255 X 12 = 9 microns. The same formula can be used to calculate the corresponding micro-structure pixel height values for all micro-structure pixels required to depict raw image 102. [0133] Figure 6 illustrates a magnification of a micro-structure portrait 300 having section A-A, and Figure 7 illustrates the micro-structure height profile of the cross- section A-A. As illustrated in Figures 6 and 7, a white region along A-A is depicted by corresponding height values of 0. The height values shown in the height profile graph of Figure 7 range from 0 to 12 microns.

[0134] Figure 8 illustrates a cross-sectional unit of the moire magnification device 302 implementing the micro-structure 300 for the micro-structure portrait 300. The cross-section of the device 302 shown in Figure 8 also corresponds cross-section A-A of Figure 6.

[0135] The micro-structure 300 is formed on an image layer on one side of a substrate 205, and a micro-lens is formed on an opposite side of the substrate 205. The moire magnification device 302 in the present example includes 5 X 5 of the units illustrated in Figure 8. Flowever, in practice, any suitable number of units can be implemented to provide a moire magnification device.

[0136] In some embodiments, the micro-structures 300 on the image layer 204 can be formed using UV embossing or micro-printing techniques as further described in relation to Figure 20. Essentially, once the desired height values for each micro structure pixel are determined, the height values can be converted to depth values for engraving on a roller or plate to create a micro-structure mould.

[0137] In one embodiment, the roller or plate is an embossing roller or plate and a UV embossing technique is used. In this embodiment, UV curable ink is applied to a substrate which is then brought into contact with the embossing roller or plate and UV light is passed through the substrate to cure the UV curable ink.

[0138] In another embodiment, UV curable ink can be directly applied to the roller or plate so that the ink fills the recesses in the engraving with minimal excess ink. The ink on the roller or plate is then partially cured (and optionally any excess can be removed from the roller or plate). The substrate is then applied to the roller or plate and the ink is fully cured through the substrate while it is in contact with the roller or plate. Once curing is complete, the substrate is removed from the roller or plate with the cured ink forming micro-structures thereon. The cured ink takes the shape of the engraving and thereby forms the micro-structures 300. As explained in reference to Figures 4A to 4D, any coloured or transparent/translucent ink can be used with or without additional overprint layers.

Interlaced type integral image device

[0139] In interlaced type integral image devices, the image layer is made up of an array of domains, each domain generally corresponding to each focusing element’s optical footprint. Each domain is also divided up into a number of discrete cells. The total number of discrete cells is equal to the number of interlaced images. As explained in further detail below, the images are interlaced such that for a particular viewing angle, each focusing element focuses on one of the discrete cells of its corresponding domain. When the viewing angle changes, each focusing element focuses on a different one of the discrete cells of its corresponding domain. The cumulative effect across the entire interlaced type integral image device is that a complete viewpoint image is projected for each viewing angle, and a different viewpoint image can be projected when the viewing angle changes. If the interlaced images are different, different viewpoint images can be projected for different viewing angles.

[0140] Figures 9 to 12 illustrate the concept behind an interlaced type integral image device using 9 interlaced images to simulating a 3D view of a grayscale cube 400.

[0141] In the example illustrated in Figure 9, the 9 interlaced images are created by capturing different images (frames) of the cube 400 from 9 different points of view (viewing angles). Each point of view is denoted by a camera 402a to 402i in Figure 9. All views intersect at a point that lies in the plane of the micro-lenses, this plane being parallel to the X-Y plane of the coordinate system in Figure 9. Portions of the cube 400 that lie in front of this intersection point will appear to float in front of the micro lenses, and portions that lie behind this intersection point will appear to float behind the micro-lenses.

[0142] As shown in Figure 10, each of the 9 image frames is a 2D raw image 404a to 404i in grayscale. In practice, to achieve the desired 3D effect, at least 4 X 4 = 16 frames would be required. However, the present example only uses 9 frames for simplicity of illustration.

[0143] For thin film security document applications, such as banknote

applications, the pitch of the micro-lenses used for integral image devices are limited to a certain range to enable the micro-lenses to focus across the thickness of the substrate on an image layer provided on an opposite side of the substrate. In the present example, micro-lenses each having a pitch of 60 microns are used. A 2D array of several hundreds or thousands of micro-lenses can be used.

[0144] A plan view of an image layer 406 on which a micro-structure 408 is formed is illustrated in Figure 1 1 . The view shown is looking directly at the image layer from the lens side of the substrate. The micro-structure 408 includes a plurality of micro-structure pixels having different height values to depict the grayscale image frames 404a - 404i after the frames are interlaced.

[0145] To interlace the image frames 404a - 404i to create the 3D effect, each domain 410 on the image layer 406, for example generally occupying the optical footprint of each micro-lens 412 as shown in Figure 12, is divided up into 9 discrete cells 414a to 414i. A grayscale image pixel at location (xi, y\) of each of the 9 image frames 404a to 404i is mapped to corresponding cell 414i to 414a of each domain 410. Each domain 410 is therefore capable of representing one image pixel in each one of the 9 grayscale frames 404a to 404i. For simplicity, in this example we assume a square lens array and each domain is the same size as one image frame pixel.

[0146] For example, a micro-structure 408 subset formed in domain 410 of the image layer 406 representing the image pixel at location (1 , 1 ) of each of the frames 404a to 404i would be constructed as follows:

• Discrete cell 414a of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404i, a micro-structure pixel having a height value which

corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404i is allocated to cell 414a; • Discrete cell 414b of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404h, a micro-structure pixel having a height value which corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404h is allocated to cell 414b;

• Discrete cell 414c of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404g, a micro-structure pixel having a height value which corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404g is allocated to cell 414c;

• Discrete cell 414d of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404f, a micro-structure pixel having a height value which

corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404f is allocated to cell 414d;

• Discrete cell 414e of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404e, a micro-structure pixel having a height value which corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404e is allocated to cell 414e;

• Discrete cell 414f of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404d, a micro-structure pixel having a height value which corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404d is allocated to cell 414f;

• Discrete cell 414g of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404c, a micro-structure pixel having a height value which

corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404c is allocated to cell 414g;

• Discrete cell 414h of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404b, a micro-structure pixel having a height value which corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404b is allocated to cell 414h; • Discrete cell 414i of domain 410 corresponds to the image pixel at location (1 , 1 ) of frame 404a, a micro-structure pixel having a height value which

corresponds to the grayscale value of the image pixel at location (1 , 1 ) of frame 404a is allocated to cell 414i.

[0147] The discrete cells 414a to 414i correspond with image pixels in frames 404a to 404i in an‘upside down and reverse’ correlation. This ensures that the viewer sees the intended frame 404 when observing the device from a specific viewpoint. For example, a viewer looking straight on to the interlaced type integral image device should view a projection of frame 404e. If the viewer moves to the right, the viewer should view a projection of frame 404f. Therefore, as the viewer moves to the right, the viewpoint image that is seen should change from 404e to 404f. However, the focal point of the micro-lenses would move to the left when the viewer moves to the right. For this reason, the correlation between the cells 414 and frames 404 are in reverse order for this particular 3D example.

[0148] Micro-structure pixel locations and height values can be determined in a similar manner for an adjacent domain (not shown) corresponding to the image pixels at location (1 , 2) of each of the frames 404, and so forth. Once the micro-structure pixels to all available domains of the image layer 406 are mapped and the respective height values determined, the micro-structure can be formed using a number of different processes as explained further below, for example using UV embossing, micro-structure printing and the like.

[0149] As illustrated in Figure 13, the width of each micro-lens 412 may generally correspond with the width of each corresponding rectangular domain 410. However, in other embodiments, the micro-lenses and the domains may be arranged differently to achieve different optical effects.

[0150] For example, as illustrated in Figure 14, the width of each micro-lens 412’ is larger than the width of each corresponding rectangular domain 410’. This configuration provides a smaller field of view in which the viewpoint images are projected. The viewpoint image is thus visible over a smaller range of viewing angles. [0151] As illustrated in Figure 15, the width of each micro-lens 412” is smaller than the width of each corresponding rectangular domain 410”, and the spacing between adjacent micro-lenses 412” is greater. This configuration provides a larger field of view in which the viewpoint images are projected. The viewpoint image is thus visible over a larger range of viewing angles.

[0152] In other embodiments, the shape of each domain 410 may be rectangular, hexagonal, or of any other suitable shape.

[0153] In some embodiments, a 1 D array of lenses may be used for the interlaced type integral image device, for example using lenticular lenses.

[0154] Figure 16 provides an example plan view of a micro-structure 420 depicting 4 interlaced frames of raw image 102 using the same principles as discussed above with reference to Figures 9 to 12. In each frame the position of raw image 102 is horizontally displaced relative to its position in the previous adjacent frame. The frames in the example of Figure 16 employ the same raw image 102 for illustrative purposes only. Frames of different images can be used to create any suitable optical effect, such as 3D, flipping, morphing and moving images.

[0155] Raw image 102 of Figure 2 is a grey scale image of 8 bits per pixel and 50dpi. In the current example, if 400 LPI lenticular lenses are to be used, the 50 dpi raw images 102 can be resampled to 400dpi. Four frames can then be interlaced to create the interlaced design illustrated in Figure 16.

Projection type integral image device

[0156] In projection type integral image devices, each device typically includes an image layer carrying a micro-structure having micro-structure pixels, and an array of focusing elements occupying a plane that is generally parallel to the image layer. The focusing elements and the image layer work together to project one or more viewpoint images based on the micro-structure. Each focusing element (e.g. micro-lens) has a respective domain in the image layer, and each domain may occupy a region substantially equal to the optical footprint of the respective focusing element. [0157] The configuration of the micro-structure pixels is determined based on a digital projection representation of an object. To simulate the digital projection representation, subsets of a projected image of the object captured from a plurality of virtual projection origins are allocated to each domain under each focusing element (e.g. a spherical micro-lens). The combination of the projected image subsets of each domain of the image layer results in the digital projection representation. The perceived“float” or“depth” of each object point approximately equals the

perpendicular distance from the object point to the plane in which the virtual projection origins are located.

[0158] An example will now be described with reference to Figure 17 to 19 to explain the projection principles underlying the projection type integral image device.

[0159] Figure 17 illustrates a virtual object 500 in the form of a grayscale 3D cube. In the present example, it is desirable to create a projection type integral image device capable of projecting a 3D viewpoint image of the cube 500 that appears to “float” above the micro-lens array.

[0160] Each of the cameras 502a to 502i represents a projection origin for capturing a subsection of the object 500. Each camera 502a to 502i essentially operates like a pin-hole camera and only captures the portion of the object 500 visible through the pin-hole. The current example shows 9 projection origins for simplicity of illustration only. In practice, many thousands of projection origins (corresponding to many thousands of micro-lenses) can be used to construct a projection type integral image device.

[0161] Each one of the plurality of projection origins typically corresponds to the centre of curvature of each one of the plurality of focusing elements (i.e. for spherical micro-lenses) in the projection type integral image device.

[0162] Each image subset 504a to 504i as respectively captured by each of the cameras 502a to 502i is shown in Figure 18. In this example, since the object 500 is intended to“float” above the micro-lenses, each image subset 504a to 504i is rotated 180 degrees then mapped to an image layer domain of a corresponding micro-lens to create the digital projection representation as shown in Figure 19. Figure 19 depicts the view seen when looking directly at the imagery plane i.e. viewing from the non lens side of the substrate.

[0163] Elaborating further,

image subset 504a is rotated 180 degrees then mapped to the domain under micro-lens 506a;

image subset 504b is rotated 180 degrees then mapped to the domain under micro-lens 506b;

image subset 504c is rotated 180 degrees then mapped to the domain under micro-lens 506c;

image subset 504d is rotated 180 degrees then mapped to the domain under micro-lens 506d;

image subset 504e is rotated 180 degrees then mapped to the domain under micro-lens 506e;

image subset 504f is rotated 180 degrees then mapped to the domain under micro-lens 506f;

image subset 504g is rotated 180 degrees then mapped to the domain under micro-lens 506g;

image subset 504h is rotated 180 degrees then mapped to the domain under micro-lens 506h; and

image subset 504i is rotated 180 degrees then mapped to the domain under micro-lens 506i.

[0164] The image subsets 504a to 504i are rotated 180 degrees then mapped to the domains, because the virtual projection origins are located in between the image plane and the object. However, if the viewpoint image is intended to“float” behind the micro-lens array, the 180 degrees rotation is not applied, because the image plane is located in between the object and the virtual projection origins. In the latter case, the image produced is shown in Figure 18 (viewing from the lens side of the substrate). If portions of the object are intended to“float” in front of the micro-lenses, and other portions are intended to“float” behind, the final image can be constructed by summing the non-rotated image subsets with the rotated image subsets of each domain, and then mapping the summed result to each domain. The collection of mapped image subsets 504a to 504i in the domains forms the digital projection representation 508 for an image layer.

[0165] Each domain on the image layer may include a plurality of discrete cells. Each discrete cell may correspond to one or more image pixels in the image subset 504 mapped to the cell as explained above, and a grayscale value can be determined based on the grayscale level of the one or more image pixels. To form a micro structure on the image layer depicting the digital projection representation 508, each micro-structure pixel corresponding to a discrete cell will be allocated height value determined based on the grayscale value for the discrete cell. The collection of height values for each micro-structure pixel corresponding to each cell can be used to form the micro-structure by micro-structure printing or UV embossing techniques as described in further detail below.

[0166] In some embodiments, a 1 D array of lenticular lenses may be used to implement the projection type integral image device in accordance with the same projection principles as described above. In these embodiments, each image subset would represent portions of the object 500 captured by cameras 502 located along one straight line oriented perpendicular to the lenticular lenses.

[0167] In a simple example including 3 lenticular lenses, the mapped image in a domain under a first lens would include image subset 504d wherein the vertical dimensions of 504d are equal to the vertical dimensions of the object; the mapped image in a domain under a second lens would include image subsets 504e wherein the vertical dimensions of 504e are equal to the vertical dimensions of the object; and the mapped image in a domain under a third lens would include image subset 504f wherein the vertical dimensions of 504f are equal to the vertical dimensions of the object. Figure 19A shows the final lenticular integral image as it would appear when viewed directly from the lens side of the substrate.

[0168] The micro-structure for the projection type integral image device having a 1 D array of lenticular lenses can be formed in a similar manner to that described for the devices including a 2D array of micro-lenses. [0169] The above operating principles can also be applied to integral image devices for projecting viewpoint images having animation, morphing, flipping and contrast switching effects.

[0170] It is also possible to create a plurality of digital projection representations 508, corresponding to a plurality of projected scenes, and to then interleave these (using the previously discussed principles) to implement animation, morphing, flipping and contrast switching effects with projection type integral images.

[0171] Whilst the formation of the micro-structures in the above examples have been described with respect to micro-structure pixels, for simplicity of illustration as raw image(s) are typically provided in digital pixel format, it is to be understood that the micro-structures can also be formed in other ways using the same inventive concept, such as using vector images for example.

[0172] The height value of each (x, y) coordinate location on an image layer can be determined using a mathematical function such as formula (1 ) or (2) or a suitable lookup table based on the grayscale level of a corresponding image pixel. Once the height values at each (x, y) location across the entire image layer are determined, an embossing roller or plate (shim) can be prepared (e.g. engraved) based on the height values to form the respective micro-structure via UV embossing.

[0173] A method of producing a micro-structure for an optically variable device such as an integral image device as discussed above will now be described below with reference to Figure 20.

[0174] At step 602, one or more raw images are created. The raw images may be created in any suitable manner. For example, the raw image may be created from one or more graphical designs, artistic works, photographs, they can also be created from rendered views of CAD models and/or any other type of 3D computer models.

[0175] For moire magnification devices, one or more raw images 102 may be used for creating a periodically repeated pattern on the image layer 204.

[0176] For interlaced type integral image devices, a plurality of raw image frames 404a to 404i may be created to provide a 3D effect. In some embodiments, the frames can be selected to create animation, morphing, flipping or contrast switching effects.

[0177] For projection type integral image devices, a plurality of raw image frames 504a to 504i may be created to create the digital projection representation 508 for application to an image layer. In some embodiments, the frames can be selected to create animation, morphing, flipping or contrast switching effects.

[0178] At step 604, the one or more raw images and image frames are resampled or otherwise processed into one or more digital images, the image pixels of the digital images having grayscale levels to define tone. The resolution of the raw images and image frames are resampled to match an achievable resolution of the micro-structure 202. For example, resampled raw image 102 matches the resolution of a pixel raster grid associated with each focusing element, being 50 X 50 pixels. The width of the focusing element 206 may be limited based on a thickness of a substrate 205 carrying the focusing elements 206 and micro-structures 202. This is because the focusing elements 206 must have a focal length generally equal to or slightly greater than the distance from the top of the lens (i.e. lens vertex) to the opposite side of the substrate 205. The number of pixel raster grid points associated with each focusing element 206 therefore also depends on the size of the focusing elements 206, placing limits on the complexity of the projected images.

[0179] At step 606, the location of the micro-structure pixels are determined based on mapping of the resampled raw images 102 and image frames 404, 504 according to the type of integral image device desired.

[0180] For moire magnification devices, the one or more re-sampled raw images 102 are generally mapped to a region in the optical footprint of each focusing element (e.g. micro-lens 206) as shown in Figure 5. The period of the raw images in the image layer and the angular orientation of the raw image array relative to the micro-lens array are configured to achieve a desired magnification and depth in the moire magnified image.

[0181] For interlaced type integral image devices, the re-sampled image frames 404a to 404i are interlaced and the image pixels 414a to 414i are mapped to a corresponding domain 410 on the image layer 406 as described with reference to Figures 9 to 12.

[0182] For projection type integral image devices, the resampled image frames 504a to 504i are mapped to respective domains associated with each focusing element 506a to 506i to form the digital projection representation 508 for application to an image layer as described above with reference to Figures 17 to 19b.

[0183] At step 608, the method 600 determines the tonal range of the raw images 102 and image frames 404, 504 created in step 604. For example, if the raw images 102 are 8 bits per pixel grayscale images, the tonal range would be from 0 to 255 as 256 different grey levels are used to define the tone of the image pixels. The tonal value of each image pixel will be a tonal value from 0 to 255.

[0184] In another example, if the raw images are 4 bits per pixel grayscale images, the tonal range would be from 0 to 15 as 16 different grey levels are used to define the tone of the image pixels. The tonal value of each image pixel will be a tonal integer value from 0 to 15. The maximum tonal range of the raw images or image frames may be limited by the currently available technology. For example, a currently available direct laser writer is capable of achieving 256 different engraving depths. If this was the maximum number of engraving depths achievable for the micro-structure, raw images and image frames higher than 8 bits per pixel (e.g. 32 bits per pixel) may need to be processed to 8 bits per pixel or less.

[0185] At step 610, the height value of each micro-structure pixel required to depict the mapped raw images and image frames of step 606 are determined using a suitable mathematical function (e.g. formula (1 ) or (2)) or look up table previously discussed, based on the tonal value of a corresponding image pixel determined in step 608.

[0186] At step 612, the height values of the micro-structure pixels define the structure of the micro-structure and can be used to engrave an embossing roller or plate (shim) or micro-structure printing roller or plate (shim) to form the micro structure mould(s) required to form the micro-structure(s) on a substrate, such as a light transmitting polymer substrate. Any suitable tooling may be employed to create the micro-structure moulds. For example, direct laser engraving (e.g. for embossing rollers), UV photolithography (e.g. for embossing or micro-structure printing shims), laser micro-machining (e.g. for embossing or micro-structure printing shims), and direct laser writing methods such as mask-less laser lithography, grey tone

lithography (e.g. for embossing or micro-structure printing shims).

[0187] The maximum height of a micro-structure is preferably between 2 to 15 microns, and more preferably between 2 to 12 microns.

[0188] At step 614, a radiation-curable lacquer coating is applied to the micro structure printing roller or plate and a substrate is applied to the roller or plate.

Optionally, any excess lacquer can be wiped off before application to the substrate. The lacquer can be a transparent or translucent UV curable ink, an opaque coloured UV curable ink, or semi-opaque coloured UV curable ink. Alternatively, a radiation- curable lacquer coating is applied to the substrate and the substrate is brought into contact with an embossing roller or plate.

[0189] The lacquer is simultaneously cured through the substrate so that the micro-structures adhere to one side of the substrate and are removed together with the substrate from the embossing roller/plate. The focusing elements can be formed on an opposite side of the substrate using similar processes. Moreover, the focusing elements can be formed onto the substrate using similar processes simultaneously with the micro-structures.

[0190] In some embodiments, the micro-structures and focusing elements may be formed according to the methods described in Australian provisional patent application no. 2017902535.

[0191] At optional step 616, one or more overprint layers may be applied to the micro-structures 212 as described above with reference to Figures 4B to 4D. The one or more overprint layers may be applied using transparent or coloured ink which may or may not be UV curable. A layer of overprint coloured ink may be applied to a transparent/translucent micro-structure to provide coloured structural depth and tonal effects in the projected viewpoint image(s). [0192] In step 616, the amount of excess overprint ink (i.e. excess of the volume required to fill the micro-structures) is sometimes minimised, in order to maximise the contrast of the tonal effects created by the varying heights in the micro-structures.

The one or more layers of overprint ink can be applied using gravure, flexographic, offset, or screen printing techniques.

[01 3] Embodiments of the invention can therefore advantageously provide optical variable devices capable of projecting higher quality, more realistic and higher fidelity images, for example compared to prior art devices using solely binary micro structures. This is achieved by encoding grayscale information into the depth of the micro-structures in the image layer. The ability to encoding grayscale information also enables the implementation of more complex viewpoint images than previously possible with binary dithering image processing techniques.

[0194] The foregoing embodiments are intended to be illustrative of the invention, without limiting the scope thereof. The invention is capable of being practised with various modifications and additions as will readily occur to those skilled in the art.

[0195] Accordingly, it is to be understood that the scope of the invention is not to be limited to the exact construction and operation described and illustrated, but only by the following claims which are intended to include all suitable modifications and equivalents permitted by the applicable law.

Claims

The claims defining the invention are as follows:

1. A method of producing a micro-structure for an optically variable device, the micro-structure having a plurality of micro-structure pixels, the method comprising determining a tonal range of an image, the image having a plurality of image pixels; assigning a tonal value for one or more image pixels based on the tonal range; determining a height value of each micro-structure pixel based on the tonal value of a corresponding image pixel or corresponding group of image pixels; and forming each micro-structure pixel at the determined height value to create the micro-structure.

2. A method of claim 1 , wherein the step of determining a height value includes determining a height value based on a look-up table.

3. A method of claim 1 , wherein the step of determining a height value includes determining a height value based on a linear or a non-linear function of the tonal value of one or more corresponding image pixels.

4. A method according to claim 1 , wherein the step of determining a height value includes determining a height value that is directly proportional to the tonal value of one or more corresponding image pixels, or directly proportional to a complementary tonal value of the tonal value of the one or more corresponding image pixels.

5. A method according to any one of the preceding claims, wherein the step of determining a height value includes determining a height value from a range of at least three discrete height values.

6. A method according to any one of the preceding claims, wherein the step of determining a height value includes determining a height value between 2 to 15 microns, or 2 to 12 microns.

7. A method according to any one of the preceding claims, wherein the optically variable device is one of

an interlaced type integral image device,

a projection type integral image device.

8. A method according to any one of the preceding claims, wherein the step of forming each micro-structure pixel includes forming the micro-structure pixels using an embossing process or micro-structure printing process.

9. A method according to any one of the preceding claims, wherein the step of forming each micro-structure pixel includes forming a micro-structure mould on an embossing roller or shim or on a micro-structure printing roller or shim.

10. A method according to any one of the preceding claims, wherein the step of forming each micro-structure pixel includes forming the micro-structure pixels using radiation curable ink.

1 1. A method according to any one of the preceding claims, wherein the image is an 8-bit grayscale image having 256 different tonal values.

12. A method according to any one claims 1 to 11 , wherein a portion of the image includes a binary image having two tonal values.

13. A micro-structure of an optically variable device, the micro-structure being manufactured using a method according to any one of the claims 1 to 12.

14. An optically variable device comprising: an image layer carrying a micro-structure, the micro-structure having a plurality of micro-structure pixels, each micro-structure pixel having a height corresponding to a tonal effect associated with the pixel; a plurality of focusing elements; the plurality of focusing elements and the image layer being configured to project one or more viewpoint images based on the micro-structure, and wherein the pixels of the micro-structure have three or more discrete height values to create a varying tonal effect in the viewpoint images.

15. An optical variable device of claim 14, wherein the height values are

determined based on a look-up table.

16. An optically variable device of claim 14, wherein the micro-structure depicts one or more raw images, and the height values are determined based on a linear or a non-linear function of tonal values of corresponding image pixels in the raw images.

17. The optically variable device of claim 14, wherein the height values are directly proportional to the tonal values of the corresponding image pixels, or directly proportional to complementary tonal values of the tonal values of the corresponding image pixels.

18. An optically variable device according to any one of claims 14 to 17, wherein the height value is between 2 to 15, or 2 to 12 microns.

19. An optically variable device according to any one of claims 14 to 18, wherein the micro-structure pixels have at least three different discrete height values.

20. An optically variable device according to any one of claims 14 to 18, wherein the micro-structure pixels have less than or equal to 256 different discrete height values.

21. An optically variable device according to any one of claims 14 to 20, further including one or more opaque or semi-opaque or transparent or translucent layers for at least partially covering the micro-structure.

22. An optically variable device according to any one of claims 14 to 21 , wherein the focusing elements are a 1 D and/or a 2D array of focusing elements.

23. A method of producing a micro-structure for an optically variable device, the micro-structure being defined by a plurality of micro-structure coordinates, the method comprising determining a tonal range of an image, the image having a plurality of image datasets for defining the image; assigning a tonal value for one or more image datasets based on the tonal range, determining a height value associated with each coordinate of the micro structure based on the tonal value of a corresponding image dataset or corresponding group of image datasets, forming a micro-structure based on the height values determined for each coordinate.

24. A method of claim 22, wherein the image datasets include image pixels for defining the image.

25. A method of claim 23, wherein the image datasets include image vectors for defining the image.