Diffractive device with embedded light source mechanism

Abstract

An optical device comprising an at least substantially transparent substrate with a first side comprising a source layer with an arrangement of source elements, and a second side comprising a layer of an optically variable device (OVD) with a corresponding arrangement of diffractive elements, wherein each source element is configured to provide an integrated light source for an associated diffraction element when the first side is illuminated, and wherein the diffractive elements are configured to produce an optical effect that is observable when the diffractive elements are viewed by an observer, such as the naked eye when illuminated by the source elements.

Description

The invention generally relates to the field of optical Devices, in particular those used to improve the security against forgery of documents. BACKGROUND OF THE INVENTION Optically variable devices, such as holograms, diffractive optical elements, microlens devices, interference pigment devices, etc., are known to improve the security against forgery of documents. Typically, the optically variable device is molded onto or attached to the document and provides protection against conventional copying techniques, such as photocopying, since such techniques are unable to accurately reproduce the variable appearance of the device. In response to improvements made by counterfeiters in reproduction, optically variable devices that are more difficult to counterfeit have been developed using advanced techniques or at least satisfactory imitations of existing optically variable devices. For an optically variable device to be useful in providing protection, users of documents to which the device is attached should be able to easily identify the device and the optical effect it provides. Desirable properties of optically variable devices typically include... Devices brightness, memorability, Ease of use, etc. 2 / 38 • · · · ·· · · · · · ·· ····· ·· · · · • · · · · · · · ······ · · ··· ······ ··· · ·· ·· ·· ·· ·· Improvements to such devices are therefore necessary to continuously increase the deterrent against counterfeiting. BRIEF SUMMARY OF THE INVENTION Existing diffractive devices, such as diffractive optical elements (DOEs), generate optically variable effects through diffractive interaction with incident light. The optically variable effect often depends strongly on the nature of the incident light source, for example, whether it is diffuse light or a point light source, and on the shape of the light source. In view of this, according to one aspect of the present invention, an optical device is provided comprising an at least substantially transparent substrate with a first side comprising a source layer with an arrangement of source elements, and a second side comprising a layer of an optically variable device (OVD) with a corresponding arrangement of diffractive elements, wherein each source element is configured to provide, when the first side is illuminated by an external light source, an integrated light source which is a light source that provides light to an associated diffraction element substantially independently of the external light source, and wherein the diffractive elements are configured to produce an optical effect that is observable when the diffractive elements are viewed by an observer, such as the naked eye when illuminated by the source elements, wherein each 3 / 38 Diffraction element according to the shape of its associated Source element is configured. Preferably one or both of: a) the source elements define images that are varied or fixed between the source elements; and b) the surface relief of the diffractive elements between the diffractive elements is varied so that the observed image or images appear to change magnification; move; change shape; change brightness; change contrast; and / or change hue when the viewing angle is changed. Each source element can define a source image and each diffractive element can define a diffractive focusing element, preferably a diffractive element with a circular or cylindrical zone plate, configured to provide an enlarged and / or shifted projection of the source image of the associated source element. Typically, the substrate comprises a characteristic thickness, and the surface relief of each diffractive element is partially determined by the characteristic thickness. Each diffractive element is preferably uniquely assigned to a source element. Preferably, each source element has at least one linear dimension that is smaller than a space between 4 / 38 the source element and its associated diffractive element, preferably about half the space between them. According to another aspect of the present invention, a document, preferably a security document, is provided which includes the optical device of the previous embodiment. Preferably, the document includes a transparent Document substrate, one region of which is the same The substrate corresponds to the optical device, wherein the document preferably also comprises opacifying layers on each side of the document substrate, each being absent in overlapping regions, thereby defining a window in which the optical device is located. Alternatively, the optical device can be separately molded onto the document and attached to the document in a window region, wherein the window is either a transparent section of the document or corresponds to a distant section of the document. According to a further aspect of the present invention, a method for manufacturing the optical device of the first aspect is provided, comprising the following steps: manufacturing a substrate component in a reverse profile on a required OVD layer profile; determining a print pattern corresponding to a required source layer; applying a radiation-curable ink to a surface of a transparent substrate; Embossing the radiation-curable ink with the Underlay component, and curing of the radiation-curable ink, thereby forming the diffraction layer; and 5 / 38 Printing the print pattern onto an opposite surface of the substrate, preferably congruent with the surface profile of the diffraction layer. The embossing and printing steps are preferably carried out essentially simultaneously. Optionally, the transparent substrate includes opacifying layers located on each surface, with the opacifying layers not being present in the region of the radiation-curable ink, thus defining a window encompassing the optical device. Security document or token In the sense used here, the term includes Security documents and tokens include all types of documents and tokens of value and identification documents, including but not limited to the following: currency elements such as banknotes and coins, credit cards, passports, identity cards, securities and share certificates, driver's licenses, deeds of ownership, travel documents such as plane and train tickets, admission tickets and passes, birth, death and marriage certificates and academic transcripts. The invention is applicable in particular, but not exclusively, to security documents or tokens such as banknotes or identification documents such as identity cards or passports, which are formed from a substrate onto which one or more printing layers are applied. The diffraction gratings and optically variable devices described herein can also be used in other products such as packaging. 6 / 38 Safety device or feature As used herein, the term encompasses Security device or feature: any one of a large number of security devices, elements, or features designed to protect the security document or token from forgery, copying, alteration, or falsification. Security devices or features can 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 they can 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). substrate In the sense used here, the term substrate refers to the starting material from which the security document or token is formed. The starting material can be paper or another fibrous material such as cellulose, a plastic or polymer material, including, but not limited to, polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), biaxially oriented polypropylene (BOPP); or a composite material made of two or more materials. 7 / 38 more materials, such as a laminate of paper and at least one plastic material or of two or more polymer materials. Transparent windows and half-windows In the sense used here, the term "window" refers to a transparent or translucent area in the security document as opposed to the essentially opaque region onto which printing is applied. The window may be completely transparent, allowing the transmission of light essentially unaffected, or it may be partially transparent or translucent, allowing some light transmission without allowing objects to be clearly seen through the window area. A window area can be formed in a polymeric security document that has at least one layer of transparent polymeric material and one or more opaque layers applied to at least one side of a transparent polymeric substrate, or by omitting at least one opaque layer in the region forming the window area. If opaque layers are applied to both sides of a transparent substrate, then a completely transparent window can be formed by omitting the opaque layers on both sides of the transparent substrate in the window area. A partially transparent or translucent area, hereinafter referred to as a "half-window", can be formed in a polymeric security document that provides opaque coverage. 8 / 38 • · · · · · · ···· · · ····· ·· ··· • ·· · · ·· · ·· · • ·· ·· · · ···· ······ ··· · ·· · · · · · · · ·· ·· The document has layers on both sides, with the opaque layers omitted only on one side of the security document in the window area, so that the "half-window" is not completely transparent, but allows some light to pass through without allowing objects to be clearly seen through the half-window. Alternatively, the substrates can be made from an essentially opaque material such as paper or fiber material, with an insert of transparent plastic material placed into a cutout or recess in the paper or fiber substrate to form a transparent window or a translucent half-window area. Covering layers One or more opaque layers can be applied to a transparent substrate to increase the opacity of the security document. An opaque layer is such that LT < Lo, where Lo is the amount of light incident on the document and LT is the amount of light transmitted through the document. An opaque layer can comprise one or more of any variety of opaque coatings. For example, the opaque coatings can include a pigment such as titanium dioxide, which is in a The substrate is dispersed in a binder or carrier substance of a heat-activated crosslinkable polymer material. Alternatively, a substrate of transparent plastic material could be arranged between opaque layers of paper or another partially or substantially opaque material. 9 / 38, onto which information can subsequently be printed or otherwise applied. Refractive index n The refractive index of a medium, n, is the ratio of the speed of light in a vacuum to the speed of light in the medium. The refractive index n² of a lens determines the degree to which light rays reaching the lens surface are refracted, according to Snell's law of refraction: ni.sinCöi) = n2.sin(02) where is the angle between an incident ray and the normal ray at the point of incidence on the lens surface, , 02 is the angle between the refracted ray and the normal ray at the point of incidence, and Πγ the The refractive index of air is (as an approximation, a value of 1 can be taken). Embossable, radiation-curable ink The term "embossable radiation-curable ink" used here refers to any ink, varnish, or other coating that can be applied to the substrate using a printing process and that can be embossed while still soft to create a print. The relief structure is formed and can be hardened to fix the embossed relief structure. The hardening process does not take place before the radiation-curable ink is embossed. 10 / 38 • · • · ·· • ·«·· ·· • · • · • ·· • • « • · • · • · · • • • · • · • · • * • • • · · • However, it is possible for the curing process to take place either after embossing or essentially at the same time as the embossing step. The radiation-curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation-curable ink can be cured by other forms of radiation, such as electron beams or X-rays. The radiation-curable ink is preferably a transparent or translucent ink made from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing on light-transmitting security elements such as... Suitable for subwavelength gratings, permeable diffractive gratings and lens structures. In a particularly preferred embodiment, the transparent or translucent ink preferably comprises a UV-curable clear embossable acrylic-based varnish or a coating of this type. Such UV-curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product Ultraviolet Type UVF-203, or similar. Alternatively, radiation-curable embossable coatings can be based on other compounds, e.g., nitrocellulose. It was found that the radiation-curable inks and varnishes used here are particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, as well as microlenses and lens arrays. However, they can also be used with larger 11 / 38 *· Φ· «· w ··»· ·· *···· ​​·· er« • « · · ··· · · · · • · · · « · · · ··· *··♦«· · « · · »* ·· ·· ··· ·· « w Relief structures, such as non-diffractive optically variable devices, can be embossed. The ink is preferably embossed and cured by ultraviolet (UV) radiation essentially simultaneously. In a particularly preferred embodiment, the radiation-curable ink is applied and embossed essentially simultaneously using an intaglio printing process. Preferably, to be suitable for gravure printing, the radiation-curable ink has a viscosity that is essentially in the range of approximately 20 to approximately 175 centipoise, or better, from approximately 30 to approximately 150 centipoise. The viscosity can be determined by measuring the time it takes to drain the varnish from a No. 2 toothed cup. A sample drained in 20 seconds has a viscosity of 30 centipoise, and a sample drained in 63 seconds has a viscosity of 150 centipoise. For some polymer substrates, it may be necessary to apply an intermediate layer to the substrate before applying the radiation-curable ink to improve the adhesion of the embossed structure formed on the substrate with the aid of the ink. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethyleneimine. The primer layer may also include a crosslinking agent, for example, a multifunctional isocyanate. Examples of other primers suitable for use in accordance with the invention include: hydroxyl-terminated polymers; hydroxyl-terminated copolymers 12 / 38 polyester-based; crosslinked or uncrosslinked hydroxylated acrylates; polyurethanes; and UV-curable anionic or cationic acrylates. Examples of suitable Crosslinking agents include: isocyanates; polyaziridines; zirconium complexes; aluminium acetylacetone; melamines; and carbodiimides. Ink made from metallic nanoparticles In the sense used here, the term metallic nanoparticle ink refers to an ink containing metallic particles with an average size of less than one micrometer. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described with reference to the accompanying drawings. It is understood that these embodiments are presented for illustrative purposes only and that the invention is not defined by them. Illustration is not restricted. For the 'Drawings' applies: Figures aa and bb show documents with optical Devices according to various embodiments of the invention; Figure 2 shows a simplified representation of an optical device according to the present invention; Figure 3 shows a source layer and an OVD layer according to one embodiment; 13 / 38 Figure 4 shows the interaction between a light source, a grid structure and an eye; Figure 5 shows a source element configured as a slit and a corresponding diffraction element grid; Figure 6 shows an external light source illuminating a source element and a diffraction element configured to provide a magnified equivalent of the source element's image; and Figure 7 shows a method for manufacturing an optical device. DESCRIPTION OF PREFERRED EXECUTION FORM Figures 1a and 1b each show a document 2 with an optical device 4 according to embodiments of the invention. The optical device 4 comprises a transparent (or at least substantially transparent) substrate 8. The document 2 also comprises a substrate (herein Document substrate 9). In the embodiment shown in Figure 1a, the two substrates 8 and 9 are the same, meaning that the optical device 4 and the document 2 share the same substrate 8 and 9. In the embodiment shown in Figure 1b, the document substrate 9 differs from the substrate 8 of the optical device 4. In any case, the document includes two first and second opacifying layers, 7a and 7b. The opacifying layers 7a and 7b 14 / 38 act to reduce or eliminate the transparency of document 2 in the regions where layers 7a, 7b are present. In the embodiments shown, both opacifying layers 7a, 7b are not present in the area of ​​the optical device 4, causing the optical device 4 to be located within a The window region of document 2 is located there. It is also possible that the document 2 is inherently opaque (or essentially opaque), for example, where the document substrate 9 is paper or a paper composite material. In this case, the opacifying layers 7a, 7b are not necessarily required. The optical device 4 is still located in a window region of the document 2, which can be achieved using known methods, such as forming the optical device 4 as a film and applying the film to a cut-out area of ​​the opaque document substrate 9. The optical device 4 typically represents a The security function is ready, meaning that the optical device 4 is working to reduce the susceptibility of document 2 to forgery. Optical device 4 can be referred to as a 'security device' or 'security token' when used for this purpose. A document 2 that requires protection against forgery is often referred to as a 'security document'. Figures la and lb also show further Security features 6 (6a in Figure 1a, 6b in Figure 1b) which reduce the susceptibility of the document 2 to forgery in combination with the optical device 4 15 / 38 can support. In figure la, the further Security feature 6a is implemented in a window region of document 2, with the further security feature 6b being implemented in an opaque (i.e., non-window) region of document 2 in Figure 1b. The illustrated arrangements are merely examples, and in general, document 2 can include one or more security features 6, each implemented in a window, half-window, or opaque region of document 2. Further examples are shown. Safety features 6 include: optically variable Devices, such as diffractive optical elements, Kinograms®, microlens-based features, holograms, etc.; watermark images, small print, etc. As shown in Figures 1a and 1b and in more detail in Figure 2, the optical device 4 generally includes a substrate 8 with a source layer 10 on a first side 16a and an OVD layer 12 opposite the source layer 10 on a second side. Figure 3 shows the source layer 10 and the OVD layer 12 in more detail. The source layer 10 comprises an array of source elements 18. The source elements 18 typically correspond to a pixelated printed source pattern, meaning that they are generated by selective printing on areas of the source layer 10, with each source element 18 representing a "pixel" of the source pattern. The array can be as shown; that is, a rectangular square matrix. According to one implementation, the array is selected such that the source elements 18 are arranged in any repeating manner, for example, by arranging them according to one of the five two-dimensional Bravais grids. In a 16 / 38 In alternative implementations, the arrangement of the source elements 10 need not be repetitive. Each source element 18 of the source layer 10 defines an image consisting of a transparent and an opaque section. Typically, the opaque section defines at least one boundary of the image, such that the entire transparent section of the source element 10 lies within the boundary. The source elements 10 are typically created using a printing process, such as gravure, screen printing, intaglio printing, etc., where ink is applied only to the opaque sections. In this way, the source elements 18 define transparent images. Figure 3 also shows a specific example of a Source element 18, which is source element 18a, which has an image in the form of a transparent line or transparent slit surrounded by an opaque printed border. In one embodiment, each source element 18 is identical. For this reason, the arrangement of the source elements 18 represents an arrangement of identically printed source pixels. In another embodiment, not shown, the source layer 10 comprises different source elements 18, that is, the source layer 10 contains at least two different images. Due to different Source elements are enabled to change their appearance when the viewing position is changed. It is understood that the images defined by the source elements 18 are based on very simple concepts, for example: 17 / 38 • · ·· · · · ···· · · ····· ·· ··· • · · ···· · ·· · ······ · · ··· ······ ··· t ·· · · ·· · · · · ·· ·· a line or dot pattern, or complicated concepts such as characters, symbols or representations can be selected. An external light source 30 is positioned to illuminate the source layer 10. The external light source 30 has an arbitrary shape, for example, a point source, a fluorescent tube, a uniformly cloudy sky, etc. Furthermore, the external light source can illuminate the source layer 10 from an arbitrary angle or direction. Each source element 18 transmits the light incident from the light source only through the non-opaque regions of the source element 18. The overall effect is that each source element 18 acts as an embedded light source with a predefined shape corresponding to the image of the source element, for example the slit shown in Figure 3. The substrate 8 is transparent, allowing light incident on each source element 18 to propagate from the first side 14a of the substrate 8 to the second side 14b. The substrate 8 acts as an interlayer between the source layer 10 and the OVD layer 12. Typically, the substrate 8 is made of a solid material and has a characteristic thickness. For example, a biaxially oriented polypropylene material used in polymer banknotes typically has a thickness between 70 and 100 pm. With further reference to Figure 3, the OVD layer 12 includes an arrangement of the diffractive elements 26. 18 / 38 The diffractive elements 26 typically correspond to a pixelated OVD microstructure. Essentially, each diffractive element 18 can represent a pixel with a larger diffractive OVD structure. The diffractive elements 26 are configured to be viewed by a viewer 20, typically the naked eye. Each diffractive element 26 is assigned to a source element 18. Typically, each diffractive element 26 is uniquely assigned to a source element 18 and vice versa (as shown in Figure 3), in which each diffractive element 26 is illuminated by its associated source element 18. However, alternatives are provided; for example, each source element 18 can be uniquely assigned to a fixed number (greater than one) of diffractive elements 26, or each diffractive element 26 can be uniquely assigned to a fixed number (greater than one) of source elements 18. For example, one source element 18 can be arranged to provide an artificial light source for four diffractive elements 26, or one diffractive element 26 can be configured to interact with four separate artificial light sources, each corresponding to a different source element 18. Since the source elements 18 provide an embedded light source with a continuous shape that is independent or at least relatively independent of the external light source 30, it is possible to design each diffractive element 26 according to the specific image of the associated source element 18. Each diffractive element 26 has a surface relief configured to produce an optically variable image when the device is viewed by the naked eye; wherein the image that is optically 19 / 38 • · ···· · ·· · • · ·· · · · · · · **'***1*9 ···*··' ··* is variable, varying with changing viewing angle of the device with respect to shape and / or brightness. Optionally, the surface relief of each diffractive element 26 is configured specifically for that diffractive element 26, so that ultimately diffractive elements 26 can have the same surface relief. In general, it may be preferred that the linear dimensions of the source elements 18 are smaller than the space between the source elements 18 and the diffractive elements 26. Typically, the source elements 18 have linear dimensions that roughly correspond to half the space between the source elements 18 and the diffractive elements 26. For example, when used as a security feature on a banknote, the space between the source elements 18 and the diffractive elements 26 is approximately 70 micrometers. In this example, each source element 18 has two linear dimensions of 30 micrometers. Reference is made to R.A. Lee, “Generalized curvilinear diffraction gratings I. Image diffraction patterns,” OPTICA ACTA, 1983, Vol. 30, No. 3, 267–289 (referred to herein as “GCDG1”), where a general theory for curved diffraction gratings illuminated by an arbitrarily spread diffuse light source is described. Each source element 18 is effectively an arbitrarily spread diffuse light source in the context of GCDG1. Referring to Figure 4, the lattice function for a specific diffractive element 26 is given by W(x,y) and 20 / 38 • · · · ·· · · ·· · ·· ····· ·· ··· • · · ···· · ·· · ······ · · ··· ····· ··· · ·· ·· ·· ·· ·· ·· ·· The lattice grooves of the diffractive element 26 are defined by the characteristic equation of the form W(x,y)=n, where “n” is the groove index number (i.e., n = 1, 2, 3, ...). The The figure shows a generalized relationship between the source element 18 (that is, the light source), the diffraction element 26, and the observer 20. As in RA Lee, “Generalised Curvilinear Diffraction Gratings II, OPTICA ACTA 1983, Vol. 30, No. 3, 291-303 (hereafter referred to as “GCDG2”), describes W(x,y) as the Contour map of an abstract phase surface, which is transferred to a planar light source as it passes through or is diffracted by the grating groove pattern W(x,y). The radiation equations for the diffraction grating of geometric optics for the above situation are given by: X ",01V p>+«Ί+«»> =-G~ih-r. iny ai.dw Pi + Wi + Qoi — (1) (2) where (Qoi,Q02) are the coordinates of the center of the The light source coordinate system consists of (i.e., the center point of the associated source element 18) located at a distance Rs from the center point of the grid, as shown in Figure 4. The coordinates (w^wq) are the coordinates of a specific point on the light source, while (pi,P2) are the 21 / 38 The coordinates of the observation point of an eye (or other observer) are those that change at a distance Ro from the The center of the grating is located, as also shown in Figure 4. The parameter G is defined by G'1 = βθ1 + Rg1, where h is the diffraction order number and λ is the wavelength of the incident light. In GCDG1 and GCDG2, it was shown that the observed stripe pattern (that is, the set of (x,y) points on the The grating plane (which bends light towards the eye at a certain viewing angle) can be described by an equation of the following form: S{Wl,w2}(wi + Qoi,w2 + Q02) = 0 (3) which is the angular shape of the embedded light source with respect to individual points within the light source represented by (wl,w2), which in turn is defined with respect to the center of the light source, defined by (Qoi, Q02) r. The observed or perceived illuminated points on the grating are calculated by substituting the grating ray equations of equations (1) and (2) into equation (3). Consider the example of a generalized A diffraction grating observed at a normal angle to the plane of the grating and propagated by an incoherent polychromatic source in the form of a very thin slit, illuminated by a polychromatic external light source, as shown in Figure 3, aligned in a direction parallel to the x-axis (as in Figure 4). 22 / 38 defined) of the grid. Applying equation (2) in this situation leads to the expression: p2 + Q02 = y~-Ähd-^ (4) where w2 = 0 as the slit can be approximated by an infinitely thin line. The coordinate Wi is not used for the The calculation was used because the slit can also be approximated by a line of infinite length, so that equation (1) can be applied uniformly to all points in the direction X. Q02 defines the angle of the source in With respect to the direction y and h, the diffraction order number and includes values ​​of h = +1, +2, +3, etc., although usually only the first, and possibly second, order needs to be included in the calculation for the gratings, whose brightness or diffraction efficiency drops rapidly with increasing order number. For the specific case of an OVD with zone plate, where W = j4(x2 + y2), while “A is a constant, equation (4) yields p2 + Q02 = y(p- 2A / h4) or y - (p2 + ¢02) / (7 “ 2AM), which G describes a series of straight lines (one for each value of "h") parallel to the source line. Referring to Figure 5, a detailed view of the interaction between a single source element 18 and the diffractive element 26 is shown. Here, the source element 18 has the shape of the printed slit from Figure 3. 23 / 38 The spaced and opposite source element 18 is the diffractive element 26, which is a series of straight lines parallel to the printed gap (source line) of the source element 18. A particular embodiment is shown in Figure 6. Here, the diffractive elements 26 are configured as diffractive lenses, meaning that they act in a similar way to a concave or convex lens. When coupled to a source element 18 that defines an arbitrary shape (in this case, a star), the observer 20 perceives the same shape (i.e., a star) when the Diffraction element 26 is considered. For a diffractive element 26 configured as a diffractive lens, the grating function can take the form W(x,y) — A(x2 + y2) + Bx + Cy, where “A , “B and “C The constants are defined as follows: “A” defines the focusing property and “B” and “C” define off-axis focal points of the diffractive element 26. For example, if “B” and “C” are both zero, the diffractive element 26 would be a circular diffractive lens, as shown in Figure 6. Equations (5) and (6) are obtained by substituting this expression for the grating function into equations (1) and (2): Pi + Wi+Q01 + ß (5) p2 + w2 + Q02 = y — 2AÄhJ + C (6) and substituting this into equation (3) provides the following: 24 / 38 - 2AlK) + B - Pr.ytG'1 ~ 2AÄh) + C - ρ2) = O where the calculation is applied at each point (νν1(ιν2) within the embedded light source. Note that the source equation, which was originally a function of (i+T.wü) (see equation (3)), is now a function of (x,y) with a linear relationship between the points (w1,w2') and (x,y). The result shows that a diffractive lens matrix, where each diffractive element 26 is described by a lattice function of the form W(x,y) = A(x² + y²) + Bx + Cy, produces an observed diffraction fringe pattern with the same shape as the image defined by its associated source element 18. The only difference is that the diffractive fringe pattern is a magnified and / or shifted version of the image (magnified according to parameter "A" and shifted according to parameters "B" and "C"). The degree of magnification can be calculated by considering the two points (Wx,w2) and (w'i,w'2) on a source element 18 and observed by the corresponding diffractive element 26 at point (Pi,p2). Substituting into equations (5) and (6) yields the observed image points that occur at (x,y) and (x',y'). The degree of magnification can then be determined as follows: (w / w,) (1-2AA / IG) 25 / 38 »·· · < • · ·· - Ϊ5 · ··· • · Since (7_1 = Ro1 + R71) and the observed distance Ro is much larger than the thickness of the document substrate Rs, it is possible to determine the To simplify the enlargement as follows: (Rs-2AÄh) (9) This relationship enables a suitable selection of the Lens focus parameter “A depending on the desired magnification for a specific substrate thickness, wavelength and required image properties. The optical device 4 disclosed herein can be manufactured according to the method shown in Figure 7. A transparent substrate, such as a biaxially oriented polypropylene substrate, is provided, and a radiation-curable ink is applied to one side of the substrate by a printing process in RCI step 100. The radiation-curable ink is then embossed with a backing component and cured in embossing step 101. The backing component has a surface profile that differs from the intended surface profile of the OVD layer 12. The printing step 102 is performed simultaneously with, before, or after the embossing step 101. Printing step 102 corresponds to the creation of the source layer 10 by printing an opaque (or substantially opaque) ink onto the opposite side of the substrate, with the opaque ink being absent in areas defining the source images. Typically, it is necessary to establish a registration between the diffraction elements 26 and the source elements 24. 26 / 38 ·· • · ·· · • ·· · • · • • · • • · • • · • • · • · • · • • · • • · • · • · • · to ensure what can be achieved using known methods.

Claims

1. Optical device comprising an at least substantially transparent substrate with a first side comprising a source layer with an arrangement of source elements, and a second side comprising a layer of an optically variable device (OVD) with a corresponding arrangement of diffractive elements, wherein each source element is configured to provide, when the first side is illuminated by an external light source, an integrated light source which is a light source which provides light to an associated diffraction element substantially independently of the external light source, and wherein the diffractive elements are configured to produce an optical effect which is observable when the diffractive elements are viewed by an observer, such as the naked eye when illuminated by the source elements, wherein each diffraction element is configured according to the shape of its associated source element.

2. Optical device according to claim 1, wherein one or both of: a) the source elements define images that are varied between the source elements; and b) the surface relief of the diffractive elements is varied between the diffractive elements, such that the observed image appears to change magnification and / or shape when the viewing angle is changed. 28 / 38 • · • · • ···· • · • · • · • • · • · • · • · ··· • • • · • • · • • · • *·· • 3. Optical device according to claim 2, wherein only the diffractive elements are varied.

4. Optical device according to claim 2, wherein only the source elements are varied.

5. Optical device according to claim 1, wherein each source element defines a source image and wherein each diffractive element defines a diffractive focusing element, preferably a diffractive element with a circular or cylindrical zone plate, configured to provide an enlarged and / or shifted projection of the source image of the associated source element.

6. Optical device according to claim 1, wherein the substrate comprises a characteristic thickness and wherein the surface relief of each diffractive element is partially determined by the characteristic thickness.

7. Optical device according to claim 1, wherein each diffractive element is uniquely assigned to a source element.

8. Optical device according to claim 1, wherein each source element has at least one linear dimension which is less than a gap between the source element and its associated diffractive element, preferably approximately half the gap.

9. Document, preferably a security document, comprising the optical device according to claim 1. 29 / 38 ·· ·· ·· · «·«· ·· • · ♦ · · ·· · · · • · · · ··· · · · ······ · · *·« ······ · · · · · • · · · ·· ··· · * · · 10. Document according to claim 9, wherein the document comprises a transparent document substrate, a region of which corresponds to the same substrate as the optical device, wherein the document preferably also comprises opacifying layers on each side of the document substrate, each of which is not present in overlapping regions, thereby defining a window in which the optical device is located.

11. Document according to claim 9, wherein the optical device is separately formed onto the document and attached to the document in a window region, the window being either a transparent section of the document or corresponding to a remote section of the document.

12. A method for manufacturing the optical device according to claim 1, comprising the following steps: manufacturing a substrate component in a reversed profile on a required OVD layer profile; determining a print pattern corresponding to a required source layer; applying a radiation-curable ink to a surface of a transparent substrate; embossing the radiation-curable ink with the substrate component, and curing the radiation-curable ink, thereby forming the diffraction layer; and printing the print pattern onto an opposite surface of the substrate, preferably congruent with the surface profile of the diffraction layer. 30 / 38 13. The method of claim 12, wherein the embossing step and the pressing step are carried out substantially simultaneously.

14. The method of claim 12, wherein the transparent substrate includes opacifying layers located on each surface, the opacifying layers being absent in the region of the radiation-curable ink, thereby defining a window encompassing the optical device. Vienna, November 16, 2017 Applicant