AU2018100183B4

AU2018100183B4 - Methods for multi-channel image interlacing

Info

Publication number: AU2018100183B4
Application number: AU2018100183A
Authority: AU
Inventors: Karlo Jolic
Original assignee: CCL Security Pty Ltd
Current assignee: CCL Security Pty Ltd
Priority date: 2018-02-09
Filing date: 2018-02-09
Publication date: 2018-08-16
Anticipated expiration: 2026-02-09
Also published as: AU2018100183A4; WO2019153041A1; FR3077767A1

Abstract

Abstract A method is provided for generating one or more optically variable devices, said optically variable device including a lens array provided on one surface of a substrate and an array of image elements provided on an opposing surface of the substrate, the method including the following steps: providing an original interlaced image formed by interlacing a plurality of images, said interlaced image including an array of image pixels and blank pixels, selectively blanking one or more image pixels to create a continuum of blank pixels, thereby forming a modified interlaced image, wherein an extent of the continuum of blank pixels is determined based on a resolution limit of a printing technique used to apply the image elements on the substrate based on the modified interlaced image.

Description

Technical Field [0001] The present invention relates to methods for producing optically variable image elements on a substrate to be viewed through a lens array or other lens microstructure.

Background of Invention [0002] Security devices are applied to security documents or similar articles, such as identity cards, passports, credit cards, bank notes, cheques and the like and may take the form of diffraction gratings and similar optically detectable microstructures. Such security devices are difficult to falsify or modify, and are easily damaged or destroyed by any attempts to tamper with the document. Some of these security devices include focussing elements, such as micro lenses, which act to sample and magnify image elements and project imagery which is observable to a user for authentication purposes.

[0003] Micro-image elements in thin security documents typically have a small number of image channels, particularly if the image is applied to the substrate using standard print methods such as flexo-printing, offset printing or gravure printing. This is because standard printing methods have a limited print resolution, and additionally because the micro lenses used in security documents, such as bank notes, must be very small in order to maintain the desired thinness of the document, and maintain a small focal length, i.e. to enable the micro lenses to focus on micro-imagery deployed within their focal plane.

[0004] Conventional multi-channel optically variable imagery security features may be achieved by applying micro-image elements underneath an array of lenticular lenses, usually in their focal plane, or substantially close to it. The lens width (or the lens pitch - if this is greater than the lens width) is roughly an integer multiple of the minimum width of an image element, which means that the maximum number of image channels possible is equal to the width (or pitch - if this is greater than the lens

2018100183 09 Feb 2018 width) of each micro-lens divided by the minimum width of an image element. For example, if the lenses are each 50 pm wide, and the minimum image element size is 25 pm, only two image channels can be produced using this method.

[0005] A disadvantage of this approach is that the maximum number of image channels is limited by the ratio of the lens width (or the lens pitch - if this is greater than the lens width) to the minimum image element size. However, the fewer the number of image channels, the easier it is for the security feature to be counterfeited.

[0006] The ever increasing sophistication of counterfeiting operations requires continuous improvement in the design of security devices for protecting documents against such forgery. One such improvement would be to print multi-channel images. However, to be able to print multi-channel (i.e. more than two image channels) using conventional printing techniques, i.e. using the same print resolution or dot size, requires variations to the known method for creating the imagery.

[0007] It would be desirable to provide a method for producing a multi-channel micro-optic device, which projects an optically variable image to the user, wherein the multi-channel device can be easily and/or inexpensively manufactured.

[0008] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention [0009] According to an aspect of the present invention, there is provided a method for generating an optically variable device, said optically variable device including a lens array provided on one surface of a substrate and an array of image elements provided on an opposing surface of the substrate, the method including the following steps:

providing an original interlaced image formed by interlacing a plurality of images, said interlaced image including an array of image pixels and blank pixels;

2018100183 09 Feb 2018 selectively blanking one or more image pixels to create a continuum of blank pixels, thereby forming a modified interlaced image, wherein an extent of the continuum of blank pixels is determined based on a resolution limit of a printing technique used to apply the image elements on the substrate based on the modified interlaced image.

[0010] In a particular embodiment, the extent of the continuum of blank pixels is equal to or greater than a size of the resolution limit of the printing technique.

[0011] In another embodiment, the continuum of blank pixels and the resolution limit of the printing technique lie in the same direction.

[0012] The array of pixels, i.e. the image pixels and the blank pixels, may comprise a plurality of image sections, wherein each image section corresponds to one of the plurality of interlaced images. The array of pixels may be arranged in rows and columns and the one or more image pixels are selectively blanked such that the continuum of blank pixels extends at least from one image pixel lying in a row or column to another non adjacent image pixel lying in the same row or column.

[0013] In some embodiments, the image pixels are selectively blanked in image sections which are arranged vertically. In other embodiments, the image pixels are selectively blanked in image sections which are arranged horizontally.

[0014] In some forms of the invention, the step of selectively blanking one or more image pixels from more than one image section to create a continuum of blank pixels to form a modified interlaced image is repeated, such that if in the first instance the image pixels are selectively blanked in image sections which are arranged vertically, the pixels are selectively blanked in image sections which are arranged horizontally when the step is repeated.

[0015] In other forms of the invention, the step of selectively blanking one or more image pixels from more than one image section to create a continuum of blank pixels to form a modified interlaced image is repeated, such that if in the first instance the

2018100183 09 Feb 2018 pixels are processed in image sections which are arranged horizontally, the pixels are processed in image sections which are arranged vertically when the step is repeated.

[0016] The array of pixels may comprise a binary image consisting of a plurality of pixels of a first colour and a plurality of pixels of a second colour. The method may further comprise the step of overprinting one or more pixels of the first or second colour with a contrasting colour.

[0017] In some embodiments, the first colour is black and the second colour is white. In one embodiment, the image pixels are black pixels and the blank pixels are white pixels. In another embodiment, the image pixels are white pixels and the blank pixels are black pixels.

[0018] Each image section may be one pixel wide and each pixel may have a width that substantially equals to half of a width or half of a pitch of a single lens. More generally, each pixel has a width such that an integral multiple of widths are equal to the lens width or the lens pitch.

[0019] The original interlaced image may comprise a first image and a second image interlaced together, the first image being divided into a first set of image sections and the second image being divided into a second set of image sections.

[0020] In some embodiments, the modified interlaced image is formed by positioning image sections from the first set of image sections beneath every other lens in the lens array and positioning image sections from the second set of image sections beneath every alternate other lens, and wherein a first displacement of each image section from the first set of image sections to a first optical axis of the respective every other lens less a second displacement of each image section from the second set of image sections to a second optical axis of the respective alternate every other lens equals plus or minus half the width or half of the pitch of a single lens. This embodiment may be referred to as a two-channel example.

[0021] In another embodiment, the modified interlaced image is formed by positioning the first set of image sections beneath every other lens associated with every alternate row of pixels, and positioning the second set of images sections

2018100183 09 Feb 2018 beneath every alternate other lens in every alternate other row of pixels, and wherein a first displacement of each image section from the first set of image sections to a first optical axis of the respective lens less a second displacement of each image section from the second set of image sections to a second optical axis of the respective alternate every other lens equals plus or minus half of the width or half of the pitch of a single lens. This embodiment may be referred to as a two-channel checkerboard example.

[0022] In yet another embodiment, the modified interlaced image is formed by positioning the first and second sets of image sections beneath every alternate lens, wherein the alternate lens selected for each set of image sections in a row comprises a random binary selection, and wherein a first displacement of each image section from the first set of image sections to a first optical axis of the respective lens less a second displacement of each image section from the second set of image sections to a second optical axis of the respective alternate lens equals plus or minus half of the width or half of the pitch of a single lens. This embodiment may be referred to as a two-channel with random interleaving example.

[0023] In some forms of the invention, N binary images are interlaced to form N image sections per lens, wherein N is an integer equal to or greater than 3, each image section being one pixel wide and each pixel has a width that is substantially equal to 1/N of a width or a pitch of a single lens.

[0024] The modified interlaced image maybe formed by positioning the image sections beneath the lenses such that a displacement of each image section of a set of image sections S to a respective lens optical axis of an associated lens, less a second displacement of each image section of an adjacent set of image sections (S+1) to a respective lens optical axis of the associated lens, wherein S is a positive integer not exceeding (N-1), equals plus or minus 1/N of a width or a pitch of one lens, and wherein the image sections are arranged in a repeating sequence of two adjacent sections followed by (N - 1) blank adjacent sections. This embodiment may be described as a generic N-channel per lens example having two adjacent sections followed by (N-1) blank sections.

2018100183 09 Feb 2018 [0025] The displacement of a first image section in each row of pixels relative to a reference axis is equal to R/N of the width or the pitch of one lens, wherein R is a random integer from 0 to N. This embodiment may be described as a generic Nchannel per lens example having two adjacent sections followed by (N-1) blank sections including randomisation.

[0026] In some of the foregoing embodiments, N is equal to 4.

[0027] In some embodiments, a height of each row of pixels is at least equal to a minimum size of the image element. Row height is set to a minimum size to ensure the pixels can be printed, however the size selected is preferably sufficiently small that it cannot be resolved with the naked eye. This ensures the image fidelity is not compromised, for example, so that the checkerboard effect cannot be seen with the naked eye. If the minimum row height is too large, then the checkerboard may become visible (undesirable). Vertical interlacing artefacts are an undesirable consequence of blanking the image sections. Vertical interlacing artefacts can be eliminated using a checker board interlacing approach as described, but if the minimum size height of each row of pixels is too large then checkerboard artefacts may replace the vertical interlacing artefacts. Setting the pixel row height to be at least equal to a minimum image element size, that is not easily resolved with the naked eye, ameliorates this problem.

[0028] The image sections from a first set of image sections and a second set of image sections may be derived from a first binary image and the sections from a third set of image sections and a fourth set of image sections may be derived from a second binary image that is different from the first binary image, and the first set of image sections are arranged to be adjacent to the second set of image sections, and the second set of image sections are arranged to be adjacent to the third set of image sections, and the third set of image sections are arranged to be adjacent to the fourth set of image sections. This embodiment may be described as a two channel image example using four channels.

[0029] In some embodiments, the image sections from alternating sets of image sections are blank image sections. This particular embodiment may be described as a

2018100183 09 Feb 2018 two channel image example using four channels, with alternating channels that are blank.

[0030] The optically variable image elements may be formed on the substrate by one or more of the following methods of printing: gravure; flexographic; or offset. Preferably, the minimum size of the image element is equal to or greater than a minimum size that can be printed using the printing techniques.

[0031] According to another aspect of the present invention, there is provided an optically variable device including a lens array provided on one surface of a substrate and an array of image elements provided on an opposing surface of the substrate, the array of image elements are printed on the substrate by a selected printing technique based on a modified interlaced image, said modified interlaced image being formed by:

interlacing a plurality of images into an array of image pixels and blank pixels, thereby forming an original interlaced image;

selectively blanking one or more image pixels to form a continuum of blank pixels, thereby forming the modified interlaced image, wherein an extent of the continuum of blank pixels is determined based on a resolution limit of the printing technique.

[0032] In a particular embodiment, the extent of the continuum of blank pixels is equal to or greater than a size of the resolution limit of the selected printing technique.

[0033] In another embodiment, the continuum of blank pixels and the resolution limit of the printing technique lie in the same direction.

[0034] The array of pixels, i.e. the image pixels and the blank pixels, may be arranged in rows and columns and the continuum of blank pixels extends at least from one image pixel lying in a row or column to another non-adjacent image pixel lying in the same row or column.

2018100183 09 Feb 2018 [0035] The array of image pixels and blank pixels may comprise a plurality of image sections, wherein each image section corresponds to one of the plurality of interlaced images. In some embodiments, the image pixels are selectively blanked in image sections which are arranged vertically. In other embodiments, the image pixels are selectively blanked in image sections which are arranged horizontally.

[0036] The array of pixels may comprise a binary image consisting of a plurality of pixels of a first colour and a plurality of pixels of a second colour. The one or more pixels of the first or second colour may be overprinted with a contrasting colour.

[0037] In some embodiments, the first colour is black and the second colour is white.

[0038] In one embodiment, the image pixels are black pixels and the blank pixels are white pixels. In another embodiment, the image pixels are white pixels and the blank pixels are black pixels.

[0039] Each image section may be one pixel wide and each pixel may have a width that is substantially equal to half of a width or a pitch of a single lens. More generally, each pixel has a width such that an integral number of widths are equal to the lens width or the lens pitch.

[0040] The plurality of images interlaced together may comprise a first image and a second image interlaced together, the first image being divided into a first set of image sections and the second image being divided into a second set of image sections.

[0041] In some embodiments, image sections from the first set of image sections are positioned beneath every other lens in the lens array and image sections from the second set of images sections are positioned beneath every alternate other lens, and a first displacement of each image section from the first set of image sections to a first optical axis of the respective every other lens less a second displacement of each image section from the second set of image sections to a second optical axis of the respective alternate every other lens equals plus or minus half the width or the pitch of a single lens. This embodiment may be referred to as a two-channel example.

2018100183 09 Feb 2018 [0042] In another embodiment, the first set of image sections are positioned beneath every other lens associated with every alternate row of pixels, and the second set of images sections are positioned beneath every alternate other lens in every alternate other row of pixels, and wherein a first displacement of each image section from the first set of image sections to a first optical axis of the respective lens less a second displacement of each image section from the second set of image sections to a second optical axis of the respective alternate every other lens equals plus or minus half of the width or the pitch of a single lens. This embodiment may be referred to as a two-channel checkerboard example.

[0043] In yet another embodiment, the first and second sets of image sections are positioned beneath every alternate lens, wherein the alternate lens selected for each set of image sections in a row comprises a random binary selection, and wherein a first displacement of each image section from the first set of image sections to a first optical axis of the respective lens less a second displacement of each image section from the second set of image sections to a second optical axis of the respective alternate lens equals plus or minus half of the width or the pitch of a single lens. This embodiment may be referred to as a two-channel with random interleaving example.

[0044] In some forms of the invention, N binary images are interlaced to form N image sections per lens, wherein N is an integer equal to or greater than 3, each image section being one pixel wide and each pixel has a width that is substantially equal to 1/N of a width or a pitch of a single lens.

[0045] The image sections may be positioned beneath the lenses such that a displacement of each image section of set of image sections S to a respective lens optical axis of an associated lens, less a second displacement of each image section of an adjacent set of image sections (S+1) to a respective lens optical axis of the associated lens, wherein S is a positive integer not exceeding (N-1), equals plus or minus 1/N of a width or a pitch of one lens, and wherein the image sections are arranged in a repeating sequence of two adjacent sections followed by (N - 1) blank adjacent sections. This embodiment may be described as a generic N-channel per lens example having two adjacent sections followed by (N-1) blank sections.

2018100183 09 Feb 2018 [0046] The displacement of a first image section in each row of pixels relative to a reference axis is equal to R/N of the width or the pitch of one lens, wherein R is a random integer from 0 to N. This embodiment may be described as a generic Nchannel per lens example having two adjacent sections followed by (N-1) blank sections including randomisation.

[0047] In some of the foregoing embodiments, N is equal to 4.

[0048] The image sections from a first set of image sections and a second set of image sections may be derived from a first binary image and the sections from a third set of image sections and a fourth set of image sections are derived from a second binary image that is different from the first binary image, and the first set of image sections are arranged to be adjacent to the second set of image sections, and the second set of image sections are arranged to be adjacent to the third set of image sections, and the third set of image sections are arranged to be adjacent to the fourth set of image sections. This embodiment may be described as a two channel image example using four channels.

[0049] In some embodiments, the image sections from alternating sets of image sections are blank image sections. This particular embodiment may be described as a two channel image example using four channels, with alternating channels that are blank.

[0050] The height of each row of pixels is preferably at least equal to a minimum size of the image element. In one embodiment, the height of each row is 100 microns or less. In another embodiment, the height of each row is 70 microns or less.

[0051 ] In some embodiments, the optically variable image element includes one or more of the following means of implementing an optically variable image: flip; contrast switch; animation; morphing; three dimensional; magnifying moire images; and integral images.

[0052] Optionally, the image element and the lens array may be integrated into a unitary structure.

2018100183 09 Feb 2018 [0053] The lens array may include one or more of the following: lenticular lenses; round lenses; and parallax barrier screens.

[0054] In some embodiments, the image element is implemented in one of the following forms: as a printed image; as an embossed image; or as a micro-structure.

Definitions

Security Document or Token [0055] As used herein the term security document includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0056] The invention is particularly, but not exclusively, applicable to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Substrate [0057] 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.

[0058] 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

2018100183 09 Feb 2018 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”.

Security Device or Feature [0059] 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).

Brief Description of Drawings [0060] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be understood that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings:

[0061] Figure 1A shows an exemplary original interlaced image.

[0062] Figure 1B shows the original interlaced image of Figure 1A with a plurality of black pixels selected to be “blanked” or removed from the image.

[0063] Figure 1C shows the modified interlaced image which has been processed according to an embodiment of the present invention.

[0064] Figures 2A and 2B show two separate images to be interlaced to form a two channel flipping image.

2018100183 09 Feb 2018 [0065] Figure 2C shows the two separate images interlaced to form an original interlaced image.

[0066] Figure 2D shows an enlarged version of a portion of the original interlaced image, shown in the box in Figure 2C.

[0067] Figure 2E shows a modified interlaced image generated in accordance with an embodiment of the present invention.

[0068] Figure 2F shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 2E.

[0069] Figure 2G shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 2F.

[0070] Figure 3A shows a modified interlaced image generated using the “checkerboard” approach in accordance with the method of the present invention.

[0071] Figure 3B shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 3A.

[0072] Figure 3C shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 3B.

[0073] Figure 4A shows a modified interlaced two channel flipping image generated using the “randomisation” approach in accordance with the method of the present invention.

[0074] Figure 4B shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 4A.

[0075] Figure 4C shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 4B.

[0076] Figures 5A to 5D show four images to be interlaced, to generate a four channel flipping image.

2018100183 09 Feb 2018 [0077] Figure 5E shows the four images shown in Figures 5A to 5D interlaced to form an original interlaced image.

[0078] Figure 5F shows an enlarged version of a portion of the original interlaced image, shown in the box in Figure 5E.

[0079] Figure 5G shows the modified interlaced image generated in accordance with an embodiment of the present invention.

[0080] Figure 5H shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 5G.

[0081] Figure 5J shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 5H.

[0082] Figure 6A shows a modified interlaced four channel flipping image generated using the “randomisation” approach in accordance with the method of the present invention.

[0083] Figure 6B shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 6A.

[0084] Figure 6C shows an enlarged version of a portion of the modified interlaced image, shown in the box in Figure 6B.

[0085] Figures 7A to 7D shows a series of four binary images of a tennis player superimposed on an array of the numeral “20” wherein each of the binary images shown in Figures 7A to 7D represents one of four image channels.

[0086] Figure 7E shows an interlaced image generated by interlacing the binary images according to an embodiment of the present invention utilising four image channels and a randomisation approach.

[0087] Figure 7F shows an enlarged version of a portion of the interlaced image, shown in the box in Figure 7E.

[0088] Figure 8A shows a standard interlaced image having 16 image channels.

2018100183 09 Feb 2018 [0089] Figure 8B shows the principles of the present invention applied to an interlaced image having 16 image channels.

[0090] Figure 8C shows a pixel array representing interlaced images with no randomisation applied.

[0091] Figure 8D shows a pixel array representing interlaced images with randomisation applied to the vertical interlacing phase according to an embodiment of the present invention.

[0092] Figure 8E shows a pixel array representing interlaced images with randomisation applied to the vertical interlacing phase followed by randomisation applied to the horizontal interlacing phase according to another embodiment of the present invention.

[0093] Figure 8F shows the modified interlaced image generated in accordance with the method described by reference to Figure 8E.

[0094] Figure 8G shows an annotated version of Figure 8A.

Detailed Description [0095] The present invention is directed to a method for generating an optically variable device, and an optically variable device formed on a substrate by such a method, which ameliorates at least some deficiencies of the prior art by increasing the number of images that may be interlaced together, i.e. image channels, to produce an optically variable image for observation through a lens array, using conventional print methods such as gravure, flexographic and offset printing, which offer standard print resolutions.

[0096] The innovative method of generating such optically variable devices to be viewed using an array of lenses to produce the desired optically variable effect, such as a three dimensional or animation effect, involves generating a modified interlaced image by selectively blanking or removing image pixels from the interlaced image sections to form a continuum of blank pixels having an extent that is at least equal to or greater than a resolution of a printing technique used to generate the image

2018100183 09 Feb 2018 element. The resolution limit of the printing technique used to apply the image elements on the substrate determines a minimum size of the image element.

[0097] Referring now to Figure 1 A, there is shown an exemplary 10x10 pixel array representing a portion of a binary image 100 consisting of a plurality of black pixels 105 and a plurality of white pixels 110, i.e. blank pixels and image pixels. The image is generated by interlacing four images, that is, the pixel array is divided into four image sections or slices 115, 120, 125 and 130, wherein each image section corresponds to one of the interlaced images. The portion of the binary image 100 spans 2.5 lenses of the lens array 135 through which the binary image 100 is to be observed. That is, each lens 140, 145 and 150 spans one set of four image sections 115, 120, 125 and 130. In the illustrated example, it is to be understood that the print resolution provides a minimum feature size of three pixels wide, i.e. equating to 75% of the width of one lens 140 and 145. In this particular example, the white pixels 110 correspond to the image elements that need to be printed on the substrate by the selected printing technique, which has a resolution limit of 3 pixels in the horizontal direction.

[0098] Referring now to Figure 1B, a selection of the plurality of black pixels 105 of Figure 1A is depicted as grey pixels, i.e. 155. In accordance with the inventive method, these selected pixels 155, are “blanked” or removed from the binary image 100. Selection of the pixels 155 to be blanked, is based on fulfilling the requirement that pixels are blanked from the interlaced image sections 115, 120, 125 and 130 so as to form a continuum of blank pixels (see Figure 1C). The continuum of blank pixels and the resolution limit of the printing technique lie in the same direction. More specifically, in the illustrated case, the continuum extends perpendicularly to the direction of the image sections (which are positioned vertically in the illustrated example), to an extent that is equal to or greater than a resolution of a printing technique used to generate the image element. As was stated in relation to Figure 1 A, in this case, the minimum image element size enabled by the print resolution is three pixels wide.

[0099] Referring now to Figure 1C, there is shown the resulting modified interlaced image 160, which has been processed according to the method of the

2018100183 09 Feb 2018 present invention. That is, a continuum of blank pixels which equals or exceeds the minimum image element size of three pixels and is provided perpendicularly to the direction of the image sections 115, 120, 125 and 130. Accordingly, assuming that there is no limitation of the vertical print resolution for the purposes of this example, all features (corresponding to the continuum of blank pixels) can be printed satisfactorily using the resolution provided by conventional printing techniques.

[0100] In practice, the vertical print resolution will be limited for some image element designs. In such a case, the steps of the method for generating the image element may be repeated, i.e. on the resulting modified interlaced image 160, but rather than processing the pixels to selectively blank pixels in vertical image sections, they are processed in horizontal sections. The same could be achieved by turning the modified interlaced image 160 90 degrees and then processing the pixels to selectively blank pixels in vertical image sections in accordance with the initial process.

[0101] It will be appreciated that the example illustrated in Figures 1A to 1C and the subsequent examples are depicted such that black pixels are selectively blanked from the pixel array to form a continuum of white image pixels. However, it will be understood that the image element of the present invention could equally be achieved by selectively blanking white pixels from the pixel array to form a continuum of black “blank” pixels. That is, the method achieves a continuum of blank pixels that is equal to or exceeds the minimum printable feature size in that direction.

[0102] In one embodiment, the continuum of blank pixels is printed in a first colour and optionally overprinted in a second contrasting colour. For example, if the continuum of pixels is printed on a reverse side of substrate to that upon which the lenses are disposed, in white ink, and the white ink is subsequently overprinted with a second contrasting colour, e.g. red ink, micro-imagery elements are formed in the colour red on a white background, and magnified by the micro-lenses to create the desired optical effect.

[0103] Referring now to Figures 2A to 2G there is shown an exemplary implementation of a two-channel design providing a flipping image effect when

2018100183 09 Feb 2018 observed through a lens array from two different viewing angles. In this case, the imagery has been interlaced using standard prior art methods, and the minimum image element size required to realise the flipping image effect is equal to half of the width (or pitch) of one lens. On thin substrates such as banknotes, which require lenses with very small width or pitch, such minimum image element sizes cannot be achieved using conventional gravure printing techniques. The present invention overcomes this problem, enabling flipping images to be printed on banknotes using standard gravure resolutions. A further advantage is that the resulting image element is substantially free of anomalies such as ghosting and channel cross-talk when magnified and observed through a lens array.

[0104] Using gravure printing as an example of a conventional printing technique, the minimum size of image elements that can be reliably printed is 35 to 45pm. Accordingly, to create a two-channel flipping image, free of anomalies such as ghosting and channel cross-talk, the lens array used to observe the image requires lenses having a width (or pitch, if this is greater than the lens width) of around 90pm. Use of such large lenses for a thin security document is not viable, since they would cause the thickness of the substrate to which they are applied to become too thick.

For example, if the substrate is a security document such as a bank note, then use of such large lenses would adversely impact tactility of the document.

[0105] Using the method of the present invention, smaller image elements, also referred to as micro-imagery can be designed to be positioned underneath smaller lenses. For example, lenses having a width (or pitch) of 63.5pm can be used together with image elements printed to a size of 63.5pm using gravure printing techniques.

[0106] Referring now to Figures 2A and 2B, there are shown two separate images to be interlaced to form a two-channel, flipping image. Figure 2A shows an image 210 of the numeral “111 and Figure 2B shows an image 220 of the numeral “222”. That is, in a two-channel, flipping image, the numeral “111” shown in Figure 2A is observed when the interlaced image (see Figures 2C and 2D) is viewed from one angle and the numeral “222” shown in Figure 2B is observed when the interlaced image is viewed from another angle.

2018100183 09 Feb 2018 [0107] Referring now to Figure 2C, there is shown the interlaced image sections as they would appear in the “original” interlaced image 230, i.e. as produced using interlacing or interleaving techniques known in the art. Figure 2D shows an enlarged version of box 240 in Figure 2C. In this case, the smallest image element 250, i.e. a line having a width of approximately 31.75pm, is generally regarded as being too small to print reliably using conventional gravure printing techniques. That is, to be printed reliably via gravure, the minimum feature size must be at least 35 to 45pm. Accordingly, printing the interlaced image 230 shown in Figure 2C using conventional gravure printing techniques is likely to give rise to anomalies such as ghosting and cross-talk, that is, both image channels 210 and 220 will be simultaneously observable at some angles of observation, i.e. generally corresponding to where the image sections for the two channels overlap.

[0108] Referring now to Figures 2E and 2G, there is shown a modified interlaced image 260 generated in accordance with the method of the present invention. In this case, image pixels have been selectively blanked, such that channel one pixels, i.e. in the first image section, are present for every odd lens 280, and channel two pixels, i.e. in the second image section, are present for every even lens 290. The selective blanking of pixels can result in entire image sections or slices being removed to form the modified interlaced image. This ensures that the continuum of blank pixels that lies perpendicular to the image sections has an extent that is equal to or greater than one lens width or pitch, i.e. 63.5pm. Significantly, this size lies well within the printing capabilities of conventional printing techniques such as gravure, flexographic or offset printing, enabling two channel flipping images to be printed reliably using conventional techniques without adversely impacting image quality by ghosting or other anomalies.

[0109] The example illustrated in Figures 2E to 2G, may exhibit parallel vertical lines that are void of colour, i.e. corresponding to the columns of pixels that have been blanked from the modified interlaced image. This may be addressed by dividing the original interlaced image into rows, and for each row, place the channel one pixels, i.e. those in the first image section, under every odd lens and the channel two pixels, i.e. those in the second image section, under every even lens. For every alternate row, place the channel one pixels, under every even lens and the channel

2018100183 09 Feb 2018 two pixels, under every odd lens. Using this approach, the contrast of the image is maintained, at 50%, but rather than observing parallel vertical lines that are void of colour in the magnified image, a checkerboard of void colour is created, maintaining image quality. This approach can also be shown as :

standard interleaved slices:	12 12 12 12 12 12 12 12	12 12 12 12 12 12	12 12 12 12
12	12
new interleaved slices:	1-	-2	1-	-2	1-
	-2	1-	-2	1-	-2
	1-	-2	1-	-2	1-
	-2	1-	-2	1-	-2

[0110] Assuming the height of each row is relatively small, e.g. 35 pm, preferably, at least equal to the minimum printable feature size in the vertical direction, the checkerboard effect will not be visually discernible when the modified interlaced image is observed, since the size of each rectangle within the checkerboard is very small, i.e. 63.5 pm X 35 pm. Accordingly, the resulting modified interlaced image is a high fidelity image having no undesirable vertical interlacing artefacts.

[0111] Referring now to Figures 3A to 3C, there is shown an exemplary implementation of the “checkerboard approach to generating a modified interlaced image. The image 300 has been divided into rows comprising odd rows and even rows. Pixels have been selectively blanked in odd rows, such that channel one pixels, i.e. in the first image section, are present under every even lens 320 and channel two pixels, i.e. in the second image section, are present under every odd lens 330, and selectively blanked in even rows, such that channel one pixels, are present under every odd lens and channel two pixels, are present under every even lens. This approach avoids the selective blanking of image pixels resulting in entire image sections or slices being removed to form the modified interlaced image, but instead generates a checkerboard effect. It can be seen in Figure 3C that the extent of the continuum of blank pixels 340, positioned perpendicular to the direction of the image sections, is at least one lens 320, 330 width or pitch, i.e. 63.5pm, wherein the width or pitch of one lens lies well within the resolution of conventional printing techniques.

2018100183 09 Feb 2018 [0112] Another option involves dividing the original interlaced image into rows. Within each row, channel one pixels are placed under alternate lenses to channel two pixels. However, the alternate lens selected for each channel is a random binary selection of either every odd lens or every even lens. The effect of randomisation is that the vertical lines in the modified interlaced image are removed without comprising contrast or image brightness (which is maintained at around 50% as in the previously described examples). Accordingly, the resulting modified interlaced image is a high fidelity image having no undesirable vertical interlacing artefacts.

[0113] Referring now to Figures 4A to 4C, there is shown an exemplary implementation of the “randomised” approach to generating a modified interlaced image in accordance with an embodiment of the present invention. Referring now to Figure 4A, there is shown a modified interlaced image 400 generated in accordance with the method of the present invention. In this case, the image 400 has been divided into rows, comprising odd rows and even rows. Pixels have been selectively blanked, such that a completely random selection of pixels from respective image channels are positioned under each odd lens or each even lens, provided that the extent of the continuum of blank pixels, positioned perpendicular to the direction of the image sections, has an extent at least equal to the width or pitch of one lens 420. Figure 4B shows box 410 of the image 400 enlarged, and Figure 4C shows box 410 enlarged again, more clearly showing the resulting effect.

[0114] The embodiments shown in Figures 3A to 3C and Figures 4A to 4C are particularly useful for implementing two-channel flipping images using standard print resolutions. The resulting modified interlaced images cannot be replicated using commercially available interlacing software, ultimately making it difficult for the image designs to be copied by a counterfeiter.

[0115] It should be understood by those skilled in the relevant art, that the method of the invention, and particularly the step of selectively blanking image pixels from image sections, can be understood to constitute either selective blanking or removal of image pixels, from a binary image consisting of interlaced image sections, or alternatively can be considered in terms of selective placement of image pixels, in a binary image consisting of interlaced image sections. In the first case, the pixels could

2018100183 09 Feb 2018 be selectively removed in accordance with one or more masks or templates, that define which pixel, section or channel is to be removed from beneath each lens in the lens array. For example, using the “randomised” approach the template or templates are offset in the horizontal direction by a random integral number of image sections or slices. The end result of this approach is the same as the “randomised” approach described with reference to Figures 4A to 4C.

[0116] In other embodiments, it may be desirable to interlace more than two images to generate an interlaced image comprising three or more image channels.

For example, referring now to Figures 5A to 5D there are shown four images 502,

504, 506 and 508 to be interlaced, to generate a four-channel flipping image. Channel 1 is shown in Figure 5A, channel 2 is shown in Figure 5B, channel 3 is shown in Figure 5C and channel 4 is shown in Figure 5D.

[0117] Referring now to Figure 5E, the four images 502, 504, 506 and 508 are interlaced to form original interlaced image 510. In this example the lens width or pitch is 63.5 microns and the smallest image feature, i.e. a black or white line is 63.5/4 = 15.875pm wide. This feature size is too small to be printed reliably using conventional printing methods such as gravure. However, by generating a modified interlaced image using the method of the present invention, an image element is produced that can be printed using conventional printing methods. Figure 5F is an enlargement of box 512 in Figure 5E and shows a portion of the original interlaced image.

[0118] Referring now to Figure 5G, there is shown the result of interlacing the four images in accordance with an embodiment of the present invention. That is, using N image sections per lens, wherein N is equal to or greater than 3, retain two adjacent pixels, blank or omit the next (N minus 1) pixels, retain the next two pixels, then blank or omit the next (N minus 1) pixels, and so on. In the example where N = 4, (i.e. four image sections are allocated per lens), this involves retaining two adjacent pixels, blanking or omitting the next three pixels, retaining the next two pixels, then blanking or omitting the next three pixels, and so on. This process ensures that the horizontal extent of any continuum of blank pixels is at least equal to the width (N minus 1) = 3 pixels, and are at least % X 63.5 = 47.625pm wide. This ensures that all gaps between image features are of sufficient size to be printed using conventional printing

2018100183 09 Feb 2018 techniques. Moreover, this approach ensures that each of the four image channels is represented equally within the modified interlaced image, i.e. that the number of image sections in the modified interlaced image is approximately equal in respect of each of the four interlaced images.

[0119] It can be seen from Figure 5G when compared with Figure 5E, that implementation of the inventive method may result in a reduction in contrast. In this example, the brightness of the modified interlaced image shown in Figure 5G is 40% of the brightness of the original interlaced image shown in Figure 5E which has no selective blanking of pixels. Generally speaking, where N image sections are allocated per lens, wherein N is equal to or greater than 3, then the reduction in brightness provided by the method of generating an optically variable image element in accordance with the present invention is given by the equation:

Brightness reduction (%) = 100 - 100X2/(N+1) (Equation 1)

Equation 1 is derived since the interlacing method of the present invention repeats after (N+1) image sections have been traversed, and only two sections per (N+1) image sections are maintained (or not blanked). Therefore, the brightness of the modified interlaced image, relative to an original interlaced image where all pixels have been retained, is 100 X 2/(N+1). This is illustrated in the representation below, wherein a “ - “ indicates a blanked pixel:

standard interleaved slices: 1234 1234 1234 1234 1234 new interleaved slices: 12— -23- --34 4 1

In this example, the minimum width of the continuum of blank pixels is (N-1 )/N X the lens width (or the lens pitch, whichever is greater). N is selected such that the width of (N-1) pixels equals or exceeds the minimum printed feature size. The minimum printed feature size will be print method dependent. As N increases, the print resolution required decreases. Whilst this is advantageous for production, the brightness of the image will decrease.

[0120] It will be understood by those skilled in the art that without application of the inventive method, it is not possible to print a four channel flipping image for 63.5pm lenses using conventional gravure print methods. Moreover, the image

2018100183 09 Feb 2018 element design shown in Figures 5G to 5J, i.e. the modified interlaced image, cannot be readily replicated using standard commercially available interlacing software.

[0121] As with the exemplary embodiments described with reference to Figures 2E to 2G, the modified interlaced image in Figure 5J may be perceived as exhibiting parallel vertical lines void of colour. As described with regard to the example described with reference to Figures 2E to 2G, the occurrence of such lines void of colour can be eliminated from the four channel flipping image, by applying a process of randomisation to the selection of pixels to be blanked.

[0122] In the same manner as with the previous example of a “randomised” approach to generating a modified interlaced image, the original interlaced image is divided into rows, and for each row, selection of the pixels to be blanked from the image sections is randomised. More specifically, the offset of the first retained (i.e. non blanked) pixel, relative to the beginning of each row, equals R times the width of one image section, wherein R is a random integer in the range from 0 to N. In the example of a four channel flipping image, integer R is randomly equal to one of 0, 1,

2, 3 or 4. This effectively “locks in” the position of each of the other pixels retained (i.e. not blanked) in this particular row. In other words, randomising the “phase” of the interlacing process for each row of the image. The previously described approach of retaining two pixels, then blanking the next three, etc., is preserved throughout the entire row and in each row, however the starting position of the interlacing process is random for each row. This “randomised” approach has the effect of avoiding the occurrence of vertical lines void of colour in the modified interlaced image without further compromising the image brightness, which is maintained at 40%. The resulting four channel flipping image, is of high visual fidelity provided that the row height chosen is sufficiently small.

[0123] Referring now to Figures 6A to 6C, there is shown the application of the “randomised” approach for generating a modified four channel flipping image. Figure 6A shows the modified four channel interlaced image, whilst Figure 6B shows box 610, also shown in Figure 6A enlarged. Figure 6C shows box 610 enlarged more still, showing that the interlacing is repeated every five lenses, but has a different, or random, starting position in each row of the pixel array.

2018100183 09 Feb 2018 [0124] The randomised approach may also be applied to simulate images with a smaller number of image channels, by interlacing a larger number of image channels For example, a two channel flipping image can be generated, using four image channels, by assigning image A to channels 1 and 2 and image B to channels 3 and 4, and optionally using the randomisation approach to generate the modified interlaced image. This approach provides an additional layer of security, since if a counterfeiter was to attempt to replicate a two channel flipping image, they would be unlikely to implement the replication using four image channels together with a randomised approach. Accordingly, it is likely that a counterfeit could be detected by microscopic examination of the image element.

[0125] The use of four channels to implement a two channel flipping image confers additional advantage, in that it is possible to more clearly distinguish two image channels on observation, by introducing blank image channels into the image channel viewing sequence. For example, a two channel flipping image could be implemented using four image channels by assigning image A to image channel 1, a blank image to image channel 2, image B to image channel 3 and assigning another blank image to image channel 4. When the resulting interlaced image is observed through a lens array while tilting the image element, at an initial viewing angle the observer will see image A, followed by image A disappearing to a blank image, followed by image B appearing, followed by image B disappearing to another blank image and so on. This implementation is advantageous because, not only is there no cross-talk between images A and B, but the image flipping effect appears slower, making the two image channels easier to distinguish and authenticate by the observer.

[0126] This method of interlacing images to generate optically variable image effects, may be applied to generate other optically variable effects such as contrast switching, morphing, animation and 3D effects. Referring now to Figure 7A there is shown a first binary image 710 consisting of a tennis player 715 and an array of the numeral “20” 720. Figure 7B shows a second binary image 725 which is the same as the first 710 except the tennis player 715 has been displaced to the right by a first displacement and the numeral array has been displaced to the left by a second

2018100183 09 Feb 2018 displacement. Figure 7C shows a third binary image 730 which is the same as the second 725 except the tennis player 715 has been displaced to the right by a first displacement and the numeral array has been displaced to the left by a second displacement. Figure 7D shows a fourth binary image 735 which is the same as the third 730 except the tennis player 715 has been displaced to the right by a first displacement and the numeral array 720 has been displaced to the left by a second displacement.

[0127] Referring now to Figure 7E, there is shown an interlaced image 740 resulting from interlacing the four image channels shown in Figures 7A to 7D respectively, using the method of the invention according to an embodiment using four image channels per lens and a randomisation of the interlacing phase in each row of the image. Figure 7F shows an enlarged portion of Figure 7E, designated as box 750.

[0128] When imagery generated in this manner is positioned in the focal plane of a lens array, the magnified image comprises an image of the tennis player that appears to float above the lenses, and an image of the array of the numeral “20” that appears to float beneath the lenses. In both cases, the distance at which the images appear to “float” above or beneath the lenses appears to be constant. This type of 3D effect is easily recognisable by an observer and enables easy authentication. Even if such a 3D image were attempted to be replicated by the counterfeiter, a counterfeit version could be readily detected by microscopic examination of the image element.

[0129] The optically variable image element of the present invention may be employed to generate integrated lens and imagery structures, and in both singlesided and double-sided lens based optical security devices. Moreover, the principles described may be applied to two dimensional lens and imagery arrays. For example, referring now to Figure 8A, there is shown for comparison a standard interlacing scheme for 16 image channels, that can be implemented for a rectangular array of round lenses. In this case, rather than dividing each image channel into image sections, wherein the width of the image section is determined such that an integral number of the image sections will fit beneath each lens in the array, instead each image channel, that is to be interlaced, is divided into image sections or zones 810,

2018100183 09 Feb 2018 so that an integral number of image sections fit beneath each lens, in this example a round lens, of the 2D rectangular lens array. In Figure 8A, 16 discrete image sections are shown, each corresponding to the footprint of each of the 5 X 5 = 25 lenses in the lens array. Each image zone comprises a square pixel in a 4 X 4 pixel array. Such an image, if intended for a thin security document such as a banknote, cannot be printed using conventional printing techniques due to inadequate print resolution.

[0130] Referring now to Figure 8B, there is shown the principles of the present invention applied to image elements designed for a rectangular array of round lenses. In this case, again, the 16 image channels are arranged so that data from each image channel is be positioned beneath each lens in the rectangular array in a location that is associated with the particular image channel.

[0131] However, in the case of Figure 8B, the approach described with reference to Figures 5G to 5J has been applied in a second dimension. That is, the image sections 820 that lie on two adjacent rows 830 and two adjacent columns 840 have been selected not to be blanked, or to be retained, and the pixels in the three rows 850 and columns 860 adjacent to the retained pixels are selected to be blanked. As previously described, this approach provides a continuum of blank pixels (indicated as “XX”) that enable the image features to be printed using the print resolution available using conventional printing techniques.

[0132] In Figure 8B, the perceived image brightness, when compared with the brightness of a standard interlaced image in which all of the image zones are retained, e.g. see Figure 8A, is equal to 100 X 2/(N+1) X 2/(N+1), i.e. in this example equals 16%. If the resulting contrast is unsatisfactory, an alternative selection of pixels to be blanked to form a continuum of blank pixels can be developed and applied to increase the number of image sections that are not selectively blanked or retained, thereby increasing the brightness.

[0133] As described by reference to one dimensional lenses, randomisation may be applied to eliminate parallel lines void of colour within the image, i.e. running both horizontally and vertically. For example the phase of the interlacing can be

2018100183 09 Feb 2018 randomised with respect to each row and each column of the pixel array. This is shown in Figures 8C and 8D.

[0134] Referring firstly to Figure 8C, there are shown interlaced images with no randomisation applied. In this representation, each black square 870 represents a cluster of 2 X 2 = 4 adjacent interlaced image channels, that is, as shown on Figure 8B. Referring secondly to Figure 8D, there are shown interlaced images with randomisation applied to the vertical interlacing phase. That is, in each column 880 of black squares, when viewed from left to right, the random phase offset used in the interlacing is 0,3,1, and 2, wherein the top left corner of the image is datum point 0, O.This randomisation will ameliorate parallel horizontal lines void of colour in the observed image. Referring now to Figure 8E, there are shown interlaced images with randomisation applied to the vertical interlacing phase followed by randomisation applied to the horizontal interlacing phase. That is, in each column 880 of black squares, when viewed from left to right, the random vertical phase offset used in the interlacing is 0,3,1, and 2, wherein the top left corner of the image is datum point 0, O.The random horizontal phase offset used in the interlacing is 0, 2, 1 and 3. This randomisation will ameliorate parallel horizontal lines void of colour and parallel vertical lines void of colour in the observed image. Referring finally to Figure 8F, there is shown a representation of the resulting interlaced image according to Figure 8E.

[0135] Referring now to Figure 8G, there is shown an annotated version of Figure 8A. The 16 image channels are denoted by square (i, j) wherein i = 0,1,2,3 and j =

0,1,2,3. For example, square (0, 0) is the top left square having the position x = 0 and y = 0. Similarly, square (3, 3) is the bottom right square having the position x = 15 and y = 15. x and y are pixel coordinates for the image sections and i and j are references for the two dimensional array of squares (each square corresponding to 2 x 2 = 4 image sections that are not blanked), i.e.:

X, = x position of square (i, j) = (N_x + 1 )i + R, y, = y position of square (i, j) = (N_y + 1 )j + Rj

Rj = random integer for each row j (ranges from 0 to N_x-1)

2018100183 09 Feb 2018

Ri = random integer for each column i (ranges from 0 to N_y-1)

N_x = number of integral sections per lens in X

N_y = number of integral sections per lens in Y

N_x>3

N_y > 3

The above equations describe the random arrangement shown in Figures 8E and 8F. In the equations, if we set R, and Rj to zero, then we get equations describing the non-random arrangement shown in Figures 8B and 8C. In the equations, if we set R, to zero but retain Rj, then the equations describe arrangement shown in Figure 8D,

i.e. randomisation in one direction only.

[0136] The method of the present invention confers a number of distinct advantages. Specifically, it provides means by which optically variable image effects can be generated using the print resolution available using conventional print techniques, such as gravure, flexographic and offset. Moreover, implementing such optically variable effects using the method of the present invention minimises undesirable artefacts which result in anomalies such as ghosting and image channel cross-talk.

[0137] Where the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0138] While the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternative, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed.

2018100183 09 Feb 2018 [0139] The present application may be used as a basis or priority in respect of one or more future applications and the claims of any such future application may be directed to any one feature or combination of features that are described in the present application. Any such future application may include one or more of the following claims, which are given by way of example and are non-limiting in regard to what may be claimed in any future application.

2018100183 09 Feb 2018

Claims

The claims defining the invention are as follows:

1. A method for generating an optically variable device, said optically variable device including a lens array provided on one surface of a substrate and an array of image elements provided on an opposing surface of the substrate, the method including the following steps:

providing an original interlaced image formed by interlacing a plurality of images, said interlaced image including an array of image pixels and blank pixels;

selectively blanking one or more image pixels to create a continuum of blank pixels, thereby forming a modified interlaced image, wherein an extent of the continuum of blank pixels is determined based on a resolution limit of a printing technique used to apply the image elements on the substrate based on the modified interlaced image.
2. The method for generating an optically variable device according to claim 1, wherein the extent of the continuum of blank pixels is equal to or greater than a size of the resolution limit of the printing technique.
3. The method for generating an optically variable device according to claim 1 or 2, wherein the continuum of blank pixels and the resolution limit of the printing technique lie in the same direction.
4. The method for generating an optically variable device according to any one of claims 1 to 3, wherein the image elements are formed on the substrate by one or more of the following methods of printing:

(a) gravure;

(b) flexographic; and (c) offset.
5. An optically variable device including a lens array provided on one surface of a substrate and an array of image elements provided on an opposing surface of the

2018100183 09 Feb 2018 substrate, the array of image elements being printed on the substrate by a selected printing technique based on a modified interlaced image, wherein the modified interlaced image is formed by:

interlacing a plurality of images into an array of image pixels and blank pixels, thereby forming an original interlaced image;

selectively blanking one or more image pixels to form a continuum of blank pixels, thereby forming the modified interlaced image, wherein an extent of the continuum of blank pixels is determined based on a resolution limit of the printing technique.

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Figure 4C
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Figure 5E

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740

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Figure 7F

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810 standard interleaving:

01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 01 02 03 04 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 05 06 07 08 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 09 10 11 12 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16 13 14 15 16

Figure 8A

840 860 naw\interleawinaschene:

820

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XX XX XX 04 XX XX XX 08 XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX 08 XX XX XX 12 XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX 12 XX XX XX 16 XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX 16 XX XX XX 04 XX XX XX XX XX XX XX XX XX XX XX XX

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Figure 8B

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870 880 880

Figure 8C Figure 8D Figure 8E new interleaving scheme with randomisation of interleaving phase in X and Y:

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Figure 8F

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2018100183 09 Feb 2018

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