MX2014004574A - Security device. - Google Patents

Abstract

An optical security device (10; 700) is provided which includes a transparent or translucent substrate (15; 705), at least one first array of repeating elements (20; 720) in or on a first side (16; 706) of the substrate (15; 705), at least one second array of repeating elements (30; 730) on a second side of the substrate. The second array of repeating elements (20; 720) is substantially in register with the first array of elements (30; 730), whereby a first image (130; 725) is visible when viewing the device from the first side, and a second image (200; 735) is visible when viewing the device from the second side. The brightness or colour levels of repeating image elements (30; 720, 730) on at least one side of the substrate may be modulated region-wise to produce a greyscale or coloured image. In one embodiment, the first array of repeating elements in or on a first side of the substrate is an array of focussing elements (20), and the second array of repeating elements in or on the second side of the substrate is an array of image elements (30) having substantially identical shape to each other, such that, when viewing the device from the first side, a magnified image including at least one magnified version of the image element shape is visible, and when viewing the device from the second side, the greyscale or coloured image is visible. In another embodiment, the repeating elements of both the first and second arrays are partially or fully opaque image elements (720, 730) which respectively form the first and second images (725;735).

Description

SECURITY DEVICE Field of the Invention The present invention relates to optical security devices, their use in security documents, and methods of their manufacture.

Definitions Security document As used herein, the term "security document" includes all types of documents and vouchers and identification documents, which include, but are not limited to, the following: currency items, such as banknotes and coins, cards credit, checks, passports, identification cards, security certificates and actions, driver's licenses, property titles, travel documents, such as airline and train tickets, cards and entrance tickets, marriage certificates, death certificates and birth certificates , and academic records.

Transparent windows and half windows As used herein the term "window" refers to a transparent or translucent area in the security document as compared to the substantially opaque region to which the printing is applied. The window can be completely transparent, in such a way as to allow the transmission of substantially unaffected light, or it can be partially transparent or partially translucent allowing the transmission of light, but without allowing the objects to be seen clearly through the window area .

An area of the window may be formed of a polymer security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, omitting the at least one opacifying layer in the region that forms the area of the window. If the opacifying layers are applied on both sides of a transparent substrate and completely in the transparent window it can be formed by omitting the opacifying layers on both sides of the transparent substrate in the area of the window.

A translucent or transparent area, hereinafter referred to as a "half window", can be formed in a polymer security document which has opacifying layers on both sides omitting the opacifying layers on only one side of the security document in the area of window so that the "half window" is not completely transparent, but allows some light to pass through without allowing the objects to be seen clearly through the half-window.

Alternatively, it is possible for the substrates to be formed of a substantially opaque material, such as paper or fibrous material, with an insert of clear plastic material inserted into a cutout, or recessed into the paper or fibrous substrate to form a transparent window or a translucent half-window area. Opacifying layers One or more opacifying layers can be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT < Lo, where Lo is the amount of incident light in the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated crosslinkable polymeric material. Alternatively, a substrate of clear plastic material may be sandwiched between the opacifying layers of paper or other substantially or partially opaque material whose mark may be subsequently printed or otherwise applied.

Focal point size or focal point width H As used herein, the term "focal point size" refers to the dimensions, usually an effective diameter or effective width, of the geometric distribution of the points at which rays refracted through a lens intercepted with a plane of the object at a particular angle of view. The focal point size can be inferred from theoretical calculations, simulations of ray traces, or real measurements.

Focal length s In the present specification, focal length, when used in reference to microlenses in a lens array, means the distance from the vertex of the microlenses to the focus position given by the location of the maximum power density distribution when the radiation collimated from the lens side of the matrix (see T. Miyashita, "Standardization for microlenses and microlens arrays" (2007) Japanese Journal of Applied Physics 46, page 5391).

Thickness of caliber t The gauge thickness is the apex distance of a lens on one side of the transparent or translucent material to the surface on the opposite side of the translucent material in which the image elements are provided that substantially coincide with the plane of the object.

Frequency and lens separation The lens frequency of a lens array is a number of lenses at a given distance along the surface of the lens array. The separation is the distance from the apex of a lens to the apex of the adjacent lens. In a uniform lens array, the separation has an inverse relationship to the lens frequency.

Width of lens W The width of a lens in a microlens matrix is the distance from one edge of the lens to the opposite edge of the lens. In a lens array with semicylindrical or hemispherical lenses, the width will be equal to the diameter of the lenses.

Radius of curvature R The radius of curvature of a lens is the distance from a point on the surface of the lens to a point at which the normal surface of the lens intersects a line that extends perpendicularly through the apex of the lens (the axis of the lens).

Bending height s The height of flexure or flexure of surface s of a lens is the distance from the apex to a point on the axis intercepted by the shortest line of the edge of a lens that extends perpendicularly through the axis. refractive index n The refractive index of a medium n is the ratio of the speed of light in vacuum to the speed of light in the medium. The refractive index n of a lens determines the amount by which the light rays reaching the surface of the lens will be refracted, according to Snell's law: ?? * Sin (a) = n * Sin (?), where a is the angle between the incident ray and the normal one at the point of incidence on the surface of the lens, T is the angle between the refracted ray and the normal one at the point of incidence, and neither is the refractive index of the air (as an approximation nor can it take the value of 1).

Conical constant P The conical constant P is an amount that describes conical flexions, and is used in geometric optics to specify spherical (P = 1), elliptical (0 <p <1, or P> 1), parabolic (P = 0) lenses ), and hyperbolic (P <0). In some references the letter K is used to represent the conic constant. K is related to P by K = P - 1.

Lobe angle The angle of lobe of a lens is the total angle of vision formed by the lenses.

Abbe number The Abbe number of a transparent or translucent material is a measurement of the dispersion (variation of the refractive index with wavelength) of the material. A proper choice of Abbe number for a lens can help to minimize chromatic aberration.

Background of the Invention The present invention seeks to provide a security device which has an attractive appearance that can be inexpensively manufactured while also providing improved resistance to counterfeiting.

It is known to employ ordered microlens matrices for focusing on the corresponding arrays of identical microimages in order to produce optically variable effects. A particular striking effect can be obtained by a slight defective registration of the microlenses and microimages, such that a series of Moire stripes is produced. Moire stripes take the form of elongated versions of the microimages. This effect, known as "Moire enlargement," has been previously described by Hutley et al (Pure and Applied Optics 3, pages 133-142, 1994) and by Amidror ("The Theory of the Moire Phenomenon", Kluwer, Dordrecht, 2000).

The use of a transparent or translucent material as the substrate for the security device or security document makes these substrates suitable as a vehicle for devices of the type described above. For example, the microimages can be applied to one side of the substrate, and the microlenses applied to the opposite side of the substrate, which thus acts as an optical separator, as described for example in US 5,712,731.

An alternative way of producing a security device or security document that exhibits a Moire magnification effect is to provide a separate screen in the form of a microlens array. The screen can be an independent element, or it can be incorporated as part of the security device or document and put into register with the microimages, which are placed elsewhere in the document, folding the document.

Brief Description of the Invention According to a first aspect of the present invention, an optical security device is provided, which includes: a transparent or translucent substrate, at least a first matrix of repetitive elements on or on a first side of the substrate, at least a second matrix of repetitive elements on a second side of the substrate, wherein the at least one second array of repeating elements is substantially in register with the at least one first array of elements, and where a first image is visible when the device is viewed from the first side, and a second image is visible when the device is viewed from the second side.

Preferably, the repetitive elements on at least one side of the substrate are modulated in the direction of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image.

In one embodiment, the repetitive elements on at least one side of the substrate can be modulated in amplitude. In another embodiment, the thickness or surface area of the repeating elements on at least one side of the substrate can be modulated.

At least one of the first and second images may be optically variable. In one embodiment, the first image may be an optically variable image and the second image may be an optically invariable image.

In another embodiment, both of the first and second images are optically invariant images.

The security optical device may include two or more arrays of repeating elements on or on at least one side of the substrate.

The repetitive elements on at least one side of the substrate may be patterned elements. In one embodiment, the elements embossed on at least one side of the substrate form repeating image elements. In this case, each stamped image element has a depth, and the depth is preferably modulated in the direction of the region.

The repetitive elements on at least one side of the substrate can be printed image elements. In another embodiment, the repetitive elements on both sides of the substrate are printed image elements.

The repetitive elements on at least one side of the substrate can be diffraction or sub-wavelength structures forming image elements. The diffraction or sub-wavelength structures may be surrounded by a non-diffractive background.

In a further embodiment, the repetitive elements on at least one side of the substrate are non-diffractive image elements surrounded by a background area comprising a diffraction or sub-wavelength meshed structure, and a background region of the image on that side of the substrate exhibiting an optically variable color effect.

In a preferred embodiment, at least a first array of repeating elements on or on a first side of the substrate is a matrix of focusing elements, and at least one second array of repeating elements on or on the second side of the substrate is a matrix. of image elements that have a substantially identical shape to each other, wherein the image elements of the at least one second matrix is modulated in the direction of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image, so that, when viewing the device from the first side, an enlarged image including at least one enlarged version of the image element form is visible, and when the device is viewed from the second side, the image is grayscale or of color is visible.

According to the second aspect, the present invention provides an optical security device, which includes: a transparent or translucent substrate, a matrix of elements that focus on or on a first side of the substrate, and at least one matrix of repeating image elements that are substantially identical to each other and that are arranged on or on a second side of the substrate, wherein the matrix of repeating image elements is substantially in register with the array of elements that are focused, and wherein the picture elements are modulated in the sense of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image, such that, when viewing the device from the first side, an enlarged image including at least one enlarged version of the image element form is visible, and when the device is viewed from the second side, the color image or scale of gray is visible.

Preferably, the image elements are modulated in amplitude. In a preferred form of the invention, the amplitude modulation is performed by varying the thickness of the line or the surface area of the picture elements.

Alternatively, the image elements can be frequency modulated. For example, the repeating image elements can substantially have the same total period as the matrix of the focusing elements, but can be omitted in some areas so that variations in brightness occur when the second is seen. side of the device. This results in a slightly degraded enhanced image quality, but an improved contrast for displaying the image in grayscale or color.

The effect produced by the security device consequently includes a magnification effect when the device is viewed from the first side, while the device (unexpectedly for a transparent area) produces a completely different optical effect, such as so that an invariably optical image (for example, a portrait), when the device is viewed from the opposite side. This combination of two different types of optical effects within the same area of the device provides a more recognizable security feature with greater security over known security devices.

The device also has an increased manufacturing facility, since a matrix of image elements on a single surface of the device, and applied in a single manufacturing step (eg, by stamping), when used to produce the two different effects .

In a particular preferred embodiment, the enlarged image is an optically variable image and the grayscale or color image is an optically invariable image.

A monochromatic macroscale image can be produced through the interaction of diffractive and non-diffractive elements. In lighting conditions when multiple or diffuse light sources, the image appears as a negative representation (inverted contrast) of the input image in reflection, and as a positive representation of the input image in transmission. In general, a purely diffraction device under these lighting conditions can produce a very weak image of very low diffraction efficiency, in some circumstances, which is so weak as not to be recognizable at all.

The optical security device must include two or more arrays of repeating image elements. Accordingly, for example, an enlarged image may be visible when a first region of the device is viewed from the first side, while a second, different, enlarged image is visible when a second region of the device is viewed from the first side. When the device is viewed from the second side, a grayscale or color tonal image is visible. The addition of additional arrays of image elements that are repeated consequently adds to the complexity of the visual effect produced by the device, further increasing the difficulty for the counterfeiter.

In a preferred embodiment, the thickness of the focal point of the elements that focus on a plane of the object located on the second side of the device is approximately the same as, or within 20% of, the thickness of the pixels. This allows larger image elements that can be used with a given substrate thickness, while still providing the desired enlarged image effect.

Preferably, the enlargement of the image elements in the enlarged image is controlled by a separation difference and / or a rotational misalignment between the matrix of the focusing elements and the array of the pixels.

In one embodiment, the image elements are stamped image elements, but the image elements can also be printed image elements. Embossed image elements are particularly preferred because of the higher resolution that can be achieved with the stamping processes, resulting in a sharper image when the device is viewed from the first side. In a method for applying the amplitude modulation of the image elements, each stamped image element has a depth, and the depth is modulated in the direction of the region.

Printing techniques can also be used provided that the resolution of the printing is sufficiently high for the image elements that are to be accommodated under the focusing elements.

In a particular preferred embodiment, the image elements comprise diffraction or sub-wavelength meshed elements surrounded by a non-diffractive background area, wherein the enlarged image exhibits optically variable effects of color. Alternatively, the elements of the image may be non-diffractive image elements, surrounded by a background area comprising meshed diffraction or sub-wavelength elements, wherein a background region of the enlarged image exhibits an effect optically variable color.

A "sub-wavelength" or a zero-order mesh element is a relief surface or a masked microstructure that produces light only in zero-order diffraction under illumination by light of a given wavelength. In general, these zero-order structures have a periodicity which is less than the desired wavelength of the incident light. For this reason, zero-order diffraction gratings are sometimes known as sub-wavelength meshes.

The use of diffraction or sub-wavelength image elements (or of non-diffractive image elements in a diffraction background) advantageously provides a device which produces a striking visual effect under both both the specular reflection and the conditions in low light or diffuse.

The elements that focus can be diffractive microlenses. Alternatively, these may be Fresnel lenses, diffractive zone plates, or photon screens. An example photon screen is one in which a series of openings is pseudo-randomly distributed along the Fresnel zones or a Fresnel zone plate, as described for example in US 7,368,744.

In another preferred embodiment of the first aspect of the invention, the at least one first array of repeating elements on or on the first side are image elements that form a first image, the at least one second array on or on the second side are elements of image forming a second image, the image elements of at least one of the first and second arrays of repeating elements are at least partially opaque, whereby the first image is visible when the device is seen in reflection from the first side, and the second image is visible when the device is seen in reflection on the second side.

According to a third aspect of the invention, an optical security device is provided, which includes: a transparent or translucent substrate, at least a first matrix of repeating elements on or on a first side of the substrate forming a first image, at least a second array of repetitive elements on a second side of the substrate forming a second image, wherein the at least one second array of repeating elements is substantially in register with the at least one first array of repeating elements, and the at least one first array and / or the at least one second array of repeating elements are at least partially opaque, whereby the first image is visible when the device is seen in reflection of the first side, and the second image is visible when the device is seen in reflection from the second side.

Preferably, the image elements of both the first and second matrices are at least partially opaque. The image elements of at least one of the matrices can be completely opaque. In one embodiment, the image elements of at least one of the arrays are partially opaque and partially transparent, such that the first and second images are combined to form a third image which is visible when the security device is viewed in transmission.

The image elements of at least one of the first and second matrices can be color image elements which are modulated in the direction of the region according to the color or brightness levels of the corresponding regions of an input image of the region. color.

Alternatively, or additionally, the image elements of at least one of the first and second arrays can be grayscale image elements which modulate in the direction of the region according to the brightness levels of the corresponding regions of a grayscale input image.

The elements of the image on at least one side of the substrate can be printed image elements. The elements on both sides of the substrate can be printed image elements. Alternatively, the image elements on at least one side of the substrate can be stamped picture elements. In other embodiments, the image elements on at least one side of the substrate can be diffraction structures or sub-wavelength structures.

In a further embodiment, the image elements that are repeated on at least one side of the substrate are non-diffractive image elements surrounded by a background area comprising a meshed diffraction or sub-wavelength structure, wherein a region of Background of the image is visible when viewing the device from that side that exhibits an optically variable color effect.

In a fourth aspect of the invention, there is provided a method of manufacturing a safety device, which includes the steps of: forming at least a first matrix of repetitive elements on or on a first side of the transparent or translucent substrate; Y forming at least one second array of elements on or on a second side of the substrate, wherein the at least one second array of repeating elements is substantially in register with the at least one first array of repetitive elements, so that when the device is viewed from the first side, a first image is visible, and when the device is viewed from the second side, a second image is visible.

Preferably, the method includes the step of modulating the repetitive elements on at least one side of the substrate in the direction of the region according to the brightness or color levels of a color or grayscale input image.

The method preferably includes the step of printing the repetitive elements on at least one side of the substrate to form repeating image elements. In one embodiment, the method includes the step of printing the repeating elements on both sides of the substrate to form repeating image elements. Various printing methods can be used to print repeating image elements, including offset, flexographic printing, intaglio printing and gravure printing. Simultaneous printing is a particularly preferred printing method which can be used to form repeating image elements in the register on opposite sides of a substrate simultaneously.

The method may include the step of forming the repetitive elements on at least one side of the substrate as diffraction image elements.

The method may include the step of stamping the repetitive elements on at least one side of the substrate.

In a particularly preferred method, the steps of forming the at least one first array of repeating elements and forming the at least one second array of pixels is performed substantially simultaneously.

In a fifth aspect, the present invention provides a method of manufacturing a security device, which includes the steps of: forming a matrix of elements that focus on or on a first side of a transparent or translucent substrate; Y forming at least one array of repetitive elements that have a substantially identical shape to one another and that accommodate on or on a second side of the substrate, wherein the arrangement of repeating image elements are substantially in register with the array of elements that are focused, and wherein the image elements are modulated in the sense of the region according to the brightness levels of the corresponding regions of a color or grayscale image, so that, when viewing the device from the first side, at least one enlarged version of the form of image elements is visible, and when the device is viewed from the second side, the color or grayscale image is visible .

The method further includes the step of applying a curable radiation curable ink to the first side and / or the second side.

Preferably, the method further includes the step of forming the focusing elements in the curable radiation curable ink on the first side by stamping. The method may also include the step of forming the imaging elements in the curable radiation curable ink on the second side by stamping.

Stable radiation curable ink The term stamped radiation curable ink used herein refers to any ink, varnish or other coating which can be applied to the substrate in a printing process, and which can be stamped, as smooth to form a structure in relief and cure by radiation to fix the structure or stamped relief. The curing process does not take place before the ink is cured by radiation, but it is possible for the curing process to take place either after stamping or substantially at the same time as the stamping process. The radiation curable ink is preferably ultraviolet (UV) curable. 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 formed from a transparent resin material. This transparent or translucent ink is particularly suitable for the printing of light transmitting security elements, such as wavelength sub-wave meshes, transmitting diffraction meshes and lens structures.

In a particular preferred embodiment, the transparent or translucent ink preferably comprises a UV curable acrylic lacquer or curable coating.

These UV curable varnishes can be obtained from different manufacturers, including Kingfisher Ink Limited, ultraviolet product of type UVF-203 or similar. Alternatively, the radiation curable stamped coatings may be based on other compounds, for example, nitro cellulose.

The radiation curable inks and varnishes used herein have been found to be particularly suitable for stamping microstructures, including diffraction structures, such as diffraction gratings or holograms, and microlenses and lens arrays. However, these can also be embossed with larger relief structures, such as non-diffractive optically variable devices.

The ink is preferably stamped and cured by ultraviolet (UV) radiation at substantially the same time. In a particular preferred embodiment, the radiation curable ink is applied and stamped at substantially the same time in a gravure printing process.

Preferably, in order to be suitable for the process for gravure printing, the radiation curable ink has a viscosity that falls substantially in the range of about 20 to about 175 centipoise, and more preferably about 30 to 50 centipoise. Approximately 150 centipoises. The viscosity must be determined by measuring the time to drain the varnish from a Zahn # 2 cup. A sample that is drained in 20 seconds has a viscosity of 30 centipoise, and a sample that drains in 63 seconds has a viscosity of 150 centipoise.

With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the addition of the stamping structure formed by the ink to the substrate. The intermediate layer preferably comprises a first layer, and more preferably, the first layer includes a polyethylene imine. The first layer may also include a crosslinking agent, for example, a multifunctional isocyanate. Examples of other primer layers suitable for use in the invention include: hydroxyl-terminated polymers; copolymers based on hydroxyl-terminated polyester; crosslinking or non-crosslinking hydroxylated acrylates; polyurethanes; and cationic or anionic acrylates curable by UV. Examples of suitable crosslinking agents include: isocyanates; polyaziridines; zirconium complexes; aluminum acetylacetone; melamines; and carbodi-imides.

The type of primer layer can be varied by different substrates and stamping ink structures. Preferably, a first primer layer is selected so as not to substantially affect the optical properties of the stamped ink structure.

The steps for forming the array of elements that focus and form the matrix of image elements are preferably performed sequentially. However, in some embodiments, for example, when the focusing elements and image elements are applied by stamping, the steps can be performed simultaneously.

The method must also include the step of curing the curable ink by stamped radiation. This is preferably done substantially simultaneously with the stamping step.

In a preferred embodiment, the image elements comprise mesh elements of diffraction or sub-wavelength, around a non-diffractive background area. Alternatively, the image elements may be non-diffractive image elements, around a background area comprising mesh diffraction or sub-wavelength elements.

The image elements can be formed by printing or stamping the image element form as a non-diffractive structure on a background of mesh diffraction or sub-wavelength elements. Alternatively, the image elements can be formed by stamping diffraction or sub-wavelength meshed elements on a non-diffractive background.

In a further aspect, a security document is provided that includes a security device according to the first or second or third aspects of the invention, or a security device manufactured according to the fourth or fifth aspects of the invention. The security device can be formed in, or applied to, a security document window.

Brief Description of the Figures Preferred embodiments of the invention will be described with reference to the accompanying Figures, in which: Figure 1 is a cross-sectional view through one embodiment of a security device according to the invention; Figure 2 shows the security device of Figure 1 as part of a security document; Figure 3 shows a series of picture elements for use with a security device according to an embodiment of the invention; Figure 4 shows the image elements of Figure 3 as viewed through a matrix of focusing elements; Figures 5 and 6 show a variation of the embodiment of Figures 3 and 4; Figure 7 shows a further embodiment of the security device in which the picture elements have been printed; Figure 8 shows an optically invariable image which is visible to a person viewing the security device of Figure 3 from the opposite side of the matrix of the focusing elements; Y Figure 9 shows an embodiment of an apparatus suitable for manufacturing security devices or security documents according to the above modalities; Figure 10 shows a schematic sectional view of a security device according to another embodiment of the invention; Figure 11 is a schematic sectional view of a security document with the security device of Figure 10 in a window of the document; Figure 12 shows the security document of Figure 11 with a first visible image when viewed in reflection from a first side; Figure 13 shows the security document of Figure 11 with a second image visible when seen in reflection from the second side; Figure 14 shows an enlarged view of the window area and security device of the security document of Figure 12; Figure 15 shows an enlarged view of the window area and security device of the security document of Figure 13; Y Figure 16 shows a modified embodiment of the security document of Figures 12 and 15 with a third image visible when viewed in transmission.

Detailed description of the invention In Figure 1, a partial cross section is shown through a security device 10 having a transparent or translucent substrate 15 having a first side 16 and a second side 17. A matrix of elements that focus on the shape of microlenses of spherical part 20 and formed on the first side 16, and a corresponding array of repeating image elements 30 is formed on the second side 17.

The focusing elements 20 can be formed directly on the surface of the first side 16 of the substrate 15, but preferably they are formed in a curable radiation curable ink which is applied to the first side 16, for example by gravure printing.

The image elements 30 may be printed image elements, which are applied, for example, by flexographic printing to the surface of the second side 17. Preferably, through them, these are patterned image elements formed by the application of a curable ink to the surface. stamped radiation to the second side 17, stamping a matrix of relief structures in the curable radiation curable ink, and curing the ink.

Figure 2 is a partial cross-section through a security document 100 which includes the security device 10. The security document includes a transparent or translucent substrate 105 having a first side 106 and a second side 107. ink opacifiers 108, 109 that are applied to the first side 106 and the second side 107, respectively, apart in a window region in which the security device 10 is placed. The opacifying ink 108, 109 is preferably applied before the security device 10 is formed in the area of the window, for easier registration of the security device 10 with the region of the window.

Referring now to Figures 3, 4 and 8, additional details of the security device 10 are shown. The image element array 30 which is applied to the second side 17 of the device 10 is generated by creating a distorted or halftone version of a input image 200. In the example shown in Figure 8, each pixel of the monochromatic bitmap 200 is assigned to one of three brightness levels, each brightness level corresponding to a particular line thickness for the picture elements (amplitude modulation) as shown to the right in the views enlarged regions 211, 212, 213 of the image element array 210. In an alternative embodiment, the spatial distribution of the image elements 30 on the second side 17 of the substrate can be modulated according to the brightness levels of an input image 200 (frequency modulation), but amplitude modulation is preferred.

Each image region 211, 212, 213 comprises pixels 25 which are square areas having dimensions corresponding to the dimensions of the superimposed partially spherical lenses 20. The pixels 25 will generally be in the order of 45 microns square to 65 microns square, although it will be appreciated that larger or smaller dimensions can be chosen to suit the particular application. It will also be appreciated that pixels do not need to be square, but in many applications it is convenient to choose square pixels.

The region of the image 211 corresponds to a part of the bright region of an input image 200 and as such, includes stamped image elements 30 which produce the greatest amount of reflected or transmitted light, i.e. they have the greatest thickness of line. The image region 212 is within the next brighter part of the image 200, and then the image elements 31 have a slightly reduced line thickness compared to the picture elements 30. Similarly, the image region 213 which is within the darkest areas of an input image 200 has pixels 32 having the minimum line thickness.

The image elements 30, 31, 32 are all substantially the same shape, and differ only in their line thicknesses. The line thickness can be modulated to the extent allowed by the resolution of the process used to apply the image elements. For flexographic printing, the smallest resolution is approximately 7 microns. For a stamping process, the smallest resolution is limited by the resolution of the electron beam or the process used to create the stamping original, and may be of the order of nanometers. The stamping processes are consequently preferred because they allow a finer gradation between the thickness of the line, and therefore the brightness levels, consequently producing the impression of a smooth transition between areas of different colors or brightnesses.

The lens array 20 and the array of image elements 30, 31, 32 can be made to produce an enlarged Moire image when the two arrays have slightly different spacings or are rotationally misaligned. The degree of magnification for a lens array of period a and an image element matrix of b is given by a2 / A, where? = a-b is the separation difference. If the lens array and the image matrix have periods identical to, but have axes which are at an angle T to each other, the magnification is approximately l / (l-cos T).

The pixel grid of 3x3 25 in Figure 3 includes three of each of image elements 30, 31 and 32. Each image element 30, 31 or 32 is a non-diffractive element printed or stamped around an area 35, 36 or 37, respectively, which comprises an embossed structure of embossed surface. The raised relief structures in bottom areas 35, 36 and 37 must have the same parameters (stamping depth, spatial frequency, azimuth deviation) with each other, or the relief parameters of the surface may vary between regions, if desired .

It will also be appreciated that the image elements 30-32 can be formed by unstamped regions of pixels 25, that is, the entire pixel 25 is stamped, apart from a region that has a boundary with the shape of the pixels. , 31 or 32.

The relief structure of the surface can be diffraction to brilliantly produce a colored background area which changes with the angle of observation. Alternatively, the relief structure of the surface may be a mesh (zero order), of sub-wavelength having a particular color, at all viewing angles. Advantageously, the sub-wavelength structures in general also produce strong polarization effects, and therefore vary (for example) the azimuthal deviation of the relief structure between the pixels which may result in a characteristic of additional authentication which can be seen under polarizing filters.

The optical effects produced by device 10 of Figures 1, 3, 4 and 8 are as follows.

When the device 10 is seen from the first side 16 of the substrate 15, at least one enlarged rotated version 130 of the individual picture elements 30, 31, 32 are seen due to the Moire magnification effect produced by the lenses 20. Although the elements 30, 31, 32 are not exactly identical, they can be made to differ in the thickness of the lines by a sufficiently small degree in such a way that the collective produced by sampling of the individual image elements by the lenses 20 is a " average "of the individual sampled images. The image may also appear to float above or below the plane of the device 10, and / or exhibit orthopalactic movement as the device 10 is tilted back and forth or from one side to the other by the viewer. If the diffraction background areas 35, 36 and 37 are used, the enlarged images 130 may also have an optically variable background and bright colors.

When the device 10 is seen from the second side 17 of the substrate 15, the individual image elements 30, 31, 32 do not expand, and are of a dimension which is too small to be perceived by the naked eye (preferably of the order of 150 microns or less, more preferably less than 70 microns, so that it is imperceptible at a viewing distance of 20 cm), collectively produce the printing of a color or monochromatic "" image 200.

The device 10 consequently counter-intuitively produces a floating image or movement employed optically variable, when viewed from the first side 16, and a monochromatic or optically invariable full color image, when viewed from the second side 17, the transparent substrate or translucent 15.

Figures 5 and 6 show an alternative embodiment in which the picture elements are patterned 40, 41 and 42 within the pixels 25 are surrounded by non-diffractive areas 45. The picture elements 40, 41 and 42 have different thickness, and in this embodiment, they also comprise relief structures of diffraction or sub-wavelength surface. The relief structures of surface may have identical parameters in each image element (embossing depth, spatial frequency, curvature, azimuthal deviation), or the parameters may vary from each pixel to pixel or between different thicknesses of the image element. The embodiment of Figures 5 and 6 produces, when viewed through the lenses 20, at least one enlarged and rotated version 140 of the individual picture elements 40, 41, 42 which are well color according to the color of the individual image elements that are sampled. As for Figures 3 and 4, when viewed from the second side 17 of the transparent or translucent substrate 15, the device of Figures 5 and 6 exhibits an optically invariable monochromatic or full-color tonal image as shown in Figure 8.

Figure 7 shows part of a further embodiment of a device produced by flexographic printing, in which a grid of 3x3 pixels includes image elements 50, 51, 52 composed of flexographic points 60 and which are surrounded by diffraction background regions. 55, 56, 57, respectively. The image elements 50, 51, 52 have substantially the same shape (of a letter "A"), but differ in the number of flexographic points of which they are composed.

As for the modalities of Figures 3 and 5, when viewed from the second side 17 of the transparent or translucent substrate 15, the device exhibits an optically invariable monochromatic or full-color tonal image as shown in Figure 8.

Referring now to Figure 9, there is shown an embodiment of an apparatus for manufacturing security documents, including devices of the above type.

The apparatus for printing and printing 500 is schematically shown in Figure 9 which includes a supply unit 502 for supplying a sheet-like substrate 501 to various printing and printing stations, including a 504 opacifying station, a first printing station 506, a stamping station 510, a second printing station 606, and a second stamping station 610, and a third printing station 514.

The substrate 501 is preferably made of a substantially transparent or translucent polymeric material, such as biaxially oriented polypropylene (BOPP) and can be supplied continuously to the opacifying station 504 from a roll 503 of the material in the supply unit 502. The opacifying station 504 includes opacifying means for applying at least one opacifying layer to at least one side of the substrate 501. The opacifying means is preferably in the form of a printing unit, eg, one or more rotogravure printing cylinders. 505 to apply one or more ink opacifying coatings to one or both sides of the substrate. However, it is possible for the opacifying station 504 to include opacifying means in the form of a lamination unit for applying one or more sheet-like layers to at least one partially opaque material, such as paper or other fibrous materials at least one side of the transparent substrate.

Preferably, the opacifying means 505 in the opacifying station 504 is arranged to omit at least one opacifying layer on one or both sides of the substrate in at least one region to form a window or half-window area.

The first printing station 506 includes a printing medium 507, 508 for applying a curable radiation curable ink to the substrate 501. The printing means may comprise at least one printing cylinder 507, for example a printing cylinder for rotogravure, with the opacified transparent substrate fed between the printing cylinder 507 and the corresponding roll or roll 508 on the opposite side of the substrate.

The printing means 507, 508 is arranged to apply the radiation curable ink to the first side 16 of the substrate 501 in which the lenses 20 are to be stamped in the stamping station 510. The lenses can be applied in a window area formed by areas that are omitted from the opacifying ink applied in the opacifying station 504.

The stamping station 510 includes stamping means preferably in the form of a plate cylinder 511 and the printing cylinder 512. The stamping means 511, 512 includes stamping portions arranged to stamp different areas of the substrate as it passes through. of the pass between the clisé and printing cylinders 511, 512.

The stamping station 510 can also include radiation curable means 513 to cure the stamping, the radiation curable ink substantially simultaneously or almost immediately after the ink has been stamped, to form the lenses 20. Alternatively, It can provide a separate healing station. The radiation curable means preferably comprises an ultraviolet (UV) curable unit for curing a UV curable ink, but other types of units for curing, for example, units curable by X-ray or electron beam (EB) can be Use for radiation curable inks for X-ray or EB.

The second printing station 606 includes printing means 607, 608 for applying a curable radiation curable ink to the second side of the substrate 501. The printing means may comprise at least one printing cylinder 607, for example a printing cylinder for rotogravure. transparent opacified fed between the printing cylinder 607 and a corresponding roller or roller 608 on the opposite side 16 of the substrate 501. The printing means 607, 608 are arranged to apply the radiation curable ink to the second side of the substrate 501 in a region directly opposite and in register with the lenses 20. If the lenses 20 are applied in a window region, the radiation curable ink is applied in the window region on the opposite side of the substrate 501.

The stamping station 610 includes stamping means preferably in the form of a carrier cylinder 611 and a printing cylinder 612 the holding cylinder 611 supports the structures of the picture elements 30-32 and / or 40-42, and also , if the patterned background areas 35-37, 45 or 55-57 are used, the structures of those background areas. After the substrate 501 passes through the stamping station 610, which holds a matrix of lens structures 20 on its first surface 16, and a corresponding matrix of image elements 30-32 or 40-42, which is substantially in register with the lens array 20.

The stamping station 610 may also include radiation curable means 613 to cure the stamping, the radiation curable ink substantially simultaneously or almost immediately after the ink has been stamped, to form the cured stamped picture elements 30-32 or 40-42.

It will also be appreciated that the second printing station 606 and the second printing station 610 can be replaced by a printing station, for example, a flexographic printing station, if the printed image elements 50-52 are to be used.

The third printing station 514 includes printing means for applying printed features to the substrate. The printed media preferably includes a printing cylinder 516 such as a printing cylinder for rotogravure, offset and gravure printing and can be used to apply a wide variety of printed features to the substrate. For example, the printing cylinder 516 in the second printing station 514 can be used to apply security features printed on the register with, adjacent to or surrounding the stamped security element.

In the operation of the apparatus, the transparent substrate 501 is supplied to the supply unit 502 through the opacifying station 504 where at least one opacifying layer is applied to at least one side of the substrate 501. The at least partially opacified substrate 501 is it then feeds through the first printing station 506, where the curable radiation curable ink is applied to an area (eg, a window area) to which it is to be stamped to form the lenses 20.

The substrate 501 is then fed through the stamping station 510, where the previously applied area of ink is stamped to form the microlenses 20 on the first side of the substrate 501. The radiation curable ink is then cured by radiation, so Preferred in the stamping station 510 for fixing the stamped lenses 20.

The substrate 501 is fed through the second printing station 606 and the second printing station 610 in order to form embossed image elements 30-32 or 40-42 on the second side 17 of the substrate 501, substantially in registration with stamped lenses 20.

The apparatus 500 may also further include printing stations or stamping stations (not shown) for applying additional features printed or stamped to the substrate 501.

It is also possible for an opacifying station to be placed after the stamping station 510, with this opacifying station which applies at least one opacifying layer to at least one side of the substrate 501 except in the area of the stamped lenses 20 and the pixels. 30-32 or 40-42 to form a window.

In some embodiments, it is also possible to simultaneously print both sides of the substrate, so that the lenses 20 and the image elements 30-32 or 40-42 are formed substantially in register on opposite sides of the substrate at the same time. weather.

Figures 10 to 16 illustrate additional embodiments of security devices and security documents according to the present invention which produce different images when viewed in reflection from opposite sides of the device, and in transmission.

In Figure 10 a partial cross section is shown through a security device 700 having a transparent or translucent substrate 705 having a first side 706 and a second side 707. A first matrix 712 of repeating elements in the form of elements 720 are formed on the first side 706, and a second array 713 of repeating elements in the form of pixels 730 are formed on the second side 707 substantially in register with the pixels 730 of the first array 713.

The image elements 720, 730 of the first and second matrices 712, 713 are preferably formed as lines or fine points, with spaces 721, 722 between the points which allow the transmission of light through the transparent or translucent substrate. Where the lines are used, they can be straight, curved, wavy or take other forms. When the dots are used, the dots are preferably round, but they can take regular or irregular shapes. In each case the image elements 730 of the second matrix 713 are preferably substantially the same shape as the image elements 720 of the first matrix 712, as well as being substantially in register.

The image elements 720, 730 of the first and second matrices 712, 713 are color or grayscale image elements which are modulated in the direction of the region according to the color or brightness levels of the corresponding regions of a color or grayscale input image.

Figure 11 is a partial cross section through a security document 800 which includes a security device 700. The security document includes a transparent or translucent substrate 805 having a first side 806 and a second side 807. They are applied layers of opacifying ink 808, 809 to the first side 806 and the second side 807, respectively, apart from a window region 810 in which the security device 700 is placed. The opaque ink 808, 809 is preferably applied before the security device 700 is formed in the window 710, for a greater ease of registering the security device 700 with the window.

Figures 12 and 13 show the security document 800 when viewed in reflection from the first and second side 806, 807 respectively. Figures 14 and 15 show, respectively, enlarged views of the window region 810 and the security device 700 of the security document shown in Figures 12 and 13.

The image elements 720, 730 are at least partially opaque. Therefore, when the security device 700 is viewed in reflection in a direction substantially perpendicular to the plane of the substrate from the first side 806, only the image elements 720 of the first matrix 712 are visible so that a first color image or gray scale 725 formed of the image elements 720 is visible as shown in Figures 12 and 14. Likewise, when the security device 700 is viewed in reflection in a direction substantially perpendicular to the plane of the substrate from the second side 807 , only the image elements 730 and the first matrix 713 are visible so that a first color or grayscale image 735 formed of the image elements 730 is visible as shown in Figures 13 and 15.

By way of example only, Figures 12 and 14 show a black and white image 725 in the form of a four-pointed star. This image can be formed by having opaque black image elements that form the star shape and opaque white image elements that form the background for the star. Figures 13 and 15 show a simple gray image 735 in the shape of an eight-pointed star formed by having opaque gray lines or dots that form the shape of the star and opaque white lines or dots that form the background for the star. However, more complex gray-scale images can be formed by modulation in the direction of the more complex region of the brightness levels of the gray image elements according to the brightness levels of the corresponding regions of the input image. in gray scale.

It is also possible to form color or multi-color images using color image elements with modulation in the region sense of the color or brightness levels of the color lines or points according to the color or brightness levels of the corresponding regions of a color input image.

When substantially completely opaque image elements are used on both sides, only the first image is visible from the first side in the reflection and transmission, and only the second image is visible from the second side in reflection and transmission. In other embodiments, as illustrated by Figure 16, it is possible for one or more additional images to be formed which are visible in transmission using imaging elements on at least the first and second sides which are partially opaque and partially transparent or translucent to allow some light transmission. For example, if the image elements 720 of the first matrix forming the four-pointed star image 725 are completely opaque and the image elements 730 of the second matrix forming the eight-pointed star 735 are partially opaque and partially transparent , when the device is seen in transmission from the second side, a third combined image 740 consisting of the four-pointed star 725 and the eight-pointed star 735 will be visible, as shown in Figure 16.

The image elements 720, 730 of one or both of the first and second dies can be printed image elements, for example, applied by flexographic or simultaneous printing to the surface or surfaces of the substrate. Alternatively, the image elements 720, 730 of one or both of the first and second arrays can be stamped picture elements. The stamped picture elements can be formed by applying a curable radiation curable ink to the substrate, embossing a matrix of embossed structures in the curable radiation curable ink, and curing the ink. For example, a translucent radiation curable ink can be used to form embossed image elements which are partially transparent, so that a combined image is visible in transmission from at least one side.

In a further modification, the image elements on at least one side of the substrate are diffraction image elements. Optically variable images can be formed by diffraction image elements, while generally printed image elements form optically invariant images. In one embodiment, the image elements of the first matrix can be printed images to form an optically invariable image, and the image elements of the second matrix can be diffraction image elements to form an optically variable image.

In a further embodiment, the image elements on at least one side of the substrate are non-diffractive image elements surrounded by a diffraction or sub-wavelength mesh structure, so that a background region of the images visible from that The substrate side exhibits an optically variable color effect.

The diffraction image elements, or a diffraction or sub-wavelength structure, can be formed by applying a curable radiation curable ink to a surface of the substrate, stamping the diffraction or wavelength structure required in the ink curable by radiation, and curing the ink.

An apparatus for manufacturing security documents similar to that described with reference to Figure 9 can be used to manufacture security documents that include imaging elements or stamped or diffraction structures. When the image elements in the first and second dies are printed elements, a simpler apparatus can be used with a simultaneous printing station that replaces the first and second printing and printing stations 506, 510, 606 and 610 of Figure 9 .

Claims (45)

1. An optical security device, characterized in that it includes: a transparent or translucent substrate, at least a first matrix of repetitive elements on or on a first side of the substrate, at least a second matrix of repetitive elements on a second side of the substrate, wherein the at least one second array of repeating elements is substantially in register with the at least one first array of elements, wherein a first image is visible when the device is viewed from the first side, and a second image is visible when the device is viewed from the second side, and wherein the repetitive elements on at least one side of the substrate are modulated in the sense of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image.

2. An optical security device according to claim 1, characterized in that the repetitive elements on at least one side of the substrate are modulated in amplitude.

3. An optical security device according to claim 2, characterized in that the thickness or surface area of the repetitive elements is modulated.

4. An optical security device according to claim 1, characterized in that the first image is an optically variable image and the second image is an optically invariable image.

5. An optical security device according to claim 1, characterized in that the first and second images are optically invariant images.

6. An optical security device according to claim 1, characterized in that it includes two or more arrays of repeating elements on or on at least one side of the substrate.

7. An optical security device according to claim 1, characterized in that the repetitive elements on at least one side of the substrate are stamped picture elements.

8. An optical security device according to claim 8, characterized in that each stamped image element has a depth, and the depth is modulated in the direction of the region.

9. An optical security device according to claim 1, characterized in that the repetitive elements on at least one side of the substrate are printed image elements.

10. An optical security device according to claim 1, characterized in that the repetitive elements on at least one side of the substrate are diffraction image elements.

11. An optical security device according to claim 1, characterized in that the repetitive elements on at least one side of the substrate are non-diffractive image elements surrounded by a background area comprising a meshed diffraction or sub-wavelength structure , and a background region of the image on that side of the substrate that exhibits an optically variable color effect.

12. An optical security device according to claim 1, characterized in that the at least one first array of repeating elements on or on a first side of the substrate is a matrix of focusing elements, and the at least one second array of elements that repeating on or on the second side of the substrate is a matrix of image elements having substantially identical shapes to each other, so that when the device is viewed from the first side, an enlarged image including at least one enlarged version of the Shape of picture elements is visible, and when the device is viewed from the second side, a grayscale or color image is visible.

13. An optical security device, characterized in that it includes; a transparent or translucent substrate, an array of elements that focus on or on a first side of the substrate, and at least one array of repetitive elements of which substantially have the same shape of one another and which are arranged on or on a second side of the substrate, wherein the array of repeating image elements is substantially in register with the array of focusing elements, so that when the device is viewed from the first side, an enlarged image including at least one enlarged version of the Image element form is visible, and when the device is viewed from the second side, the grayscale or color image is visible.

14. An optical security device according to claim 13, characterized in that the image elements that are repeated on at least one side of the substrate are modulated in the direction of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image.

15. An optical security device according to claim 13 or claim 14, characterized in that a focal point width of the repetitive elements in a plane of the object placed on the second side of the device is approximately the same as, or within 20% of, the width of the image elements.

16. An optical security device according to claim 13, characterized in that the image elements are non-diffractive image elements surrounded by a background area comprising a diffraction or sub-wavelength mesh structure, and a background region of the enlarged image that exhibits an optically variable effect of color.

17. An optical security device according to claim 13, characterized in that the image elements comprise meshed diffraction or sub-wavelength structures surrounded by a non-diffractive background area, and the enlarged image exhibits an optically variable color effect .

18. An optical security device according to claim 13, characterized in that the focusing elements are refractive microlenses.

19. An optical security device according to claim 13, characterized in that the focusing elements are Fresnel lenses, diffraction zone plates, or photon screens.

20. An optical security device according to claim 13, characterized in that the at least one first array of repeating elements on or on a first side are image elements forming a first image, the at least one second matrix on or on the second side are image elements forming a second image, the image elements of at least one of the first and second arrays of repeating elements are at least partially opaque, whereby the first image is visible when the device is viewed in reflection from the first side, and the second image is visible when the device is seen in the reflection from the second side.

21. An optical security device, characterized in that it includes: a transparent or translucent substrate, at least a first matrix of image elements repeating on or on a first side of the substrate forming a first image, at least a second matrix of image elements repeating on a second side of the substrate forming a second image, wherein the at least one second array of repeating image elements are substantially in register with the at least one first array of repeating image elements, wherein the image elements that are repeated on at least one side of the substrate are modulated in the direction of the region according to the brightness or color levels of the corresponding regions of a grayscale input image and the at least one first matrix and / or the at least one second matrix of repeating image elements is at least partially opaque, whereby the first image is visible when the device is viewed in reflection from the first side, and the Second image is visible when the device is seen in reflection from the second side.

22. An optical security device according to claim 21, characterized in that the image elements of at least one of the first and second matrices are totally opaque.

23. An optical security device according to claim 21, characterized in that the image elements at least one of the first and second repeating element matrix are partially opaque and partially transparent, so that the first and second images are combined to form a third image which is visible when the security device is seen in transmission.

24. An optical security device according to claim 21, characterized in that the image elements of at least one of the first and second matrix are color image elements which are modulated sense of the region according to the color levels or brightness of the corresponding regions of a color input image.

25. An optical security device according to claim 21, characterized in that the image elements of at least one of the first and second matrices are grayscale image elements which are modulated in the direction of the region according to the brightness levels of the corresponding regions of a grayscale input image.

26. An optical security device according to claim 21, characterized in that the image elements on at least one side of the substrate are printed image elements.

27. An optical security device according to claim 21, characterized in that the picture elements on at least one side of the substrate are stamped picture elements.

28. An optical security device according to claim 21, characterized in that the pixels on at least one side of the substrate are diffraction structures or sub-wavelength structures.

29. An optical security device according to claim 21, characterized in that the image elements that are repeated on at least one side of the substrate are non-diffractive image elements surrounded by a background area comprising a diffraction or subframe mesh structure. -wavelength, wherein a background region of the image is visible when the device is viewed from that side that exhibits an optically variable color effect.

30. A method for manufacturing a safety device, which includes the steps of: forming at least a first array of repeating elements on or on a first side of a transparent or translucent substrate; Y forming at least one second array of repetitive elements on or on a second side of the substrate, wherein the at least one second array of repeating elements is substantially in register with the at least one first array of repetitive elements, so that, when the device is viewed from the first side, a first image is visible, and when the device of the second side is seen, a second image is visible, and wherein the repetitive elements on at least one side of the substrate are modulated in the direction of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image.

31. A method according to claim 30, characterized in that it includes the step of modulating the repetitive elements on at least one side of the substrate in the direction of the region according to the brightness or color levels of a grayscale input image. or of color.

32. A method according to claim 30 or claim 31, characterized in that it includes the step of printing the repetitive elements on at least one side of the substrate to form repeating image elements.

33. A method in accordance with the claim 32, characterized in that it includes the step of printing the repetitive elements on both sides of the substrate to form repeating image elements.

34. A method in accordance with the claim 33, characterized in that simultaneous printing is used to form the repeating image elements.

35. A method according to claim 30, characterized in that it includes the step of forming the repetitive elements on at least one side of the substrate as diffraction image elements.

36. A method according to claim 30, characterized in that it includes the step of stamping the repetitive elements on at least one side of the substrate.

37. A method according to claim 30, characterized by the steps of forming the at least one first array of repeating elements and forming the at least one second array of repeating image elements are substantially simultaneously realized.

38. A method for manufacturing the safety device, which includes the steps of: forming a matrix of elements that focus on or on a first side of a transparent or translucent substrate; Y forming at least one matrix of repeating image elements that substantially have the same shape of one another on or on a second side of the substrate, wherein the matrix of repetitive elements are substantially in register with the matrix of elements that are focused, and wherein the image elements are modulated in the sense of the region according to the brightness or color levels of the corresponding regions of a grayscale or color input image, so that, when viewing the device from the first side, an enlarged image including at least one enlarged version of the shape of the image element is visible, and when the device is viewed from the second side, the image at scale of Gray or colored is visible.

39. A method in accordance with the claim 38, characterized in that it also includes the step of applying a curable radiation-curable ink to the first side and / or the second side.

40. A method in accordance with the claim 39, characterized in that it also includes the step of stamping the focusing elements on the curable radiation curable ink on a first side or on the second side.

41. A method in accordance with the claim 40, characterized in that it also includes the step of curing the curable ink by stamped radiation.

42. A method in accordance with the claim 41, characterized in that the step of curing is substantially carried out simultaneously with the stamping step.

43. A security document that includes a security device according to any of claims 1, 13 or 21.

44. A security device manufactured in accordance with claim 30 or claim 38.

45. A security document, characterized in that the security device of claim 30 or claim 38 is formed in, or applied to, a window of the security document. SUMMARY OF THE INVENTION An optical security device is provided that includes a transparent or translucent substrate, at least a first array of repeating elements on or on a first side of the substrate, at least a second array of repeating elements on a second side of the substrate. The second array of repeating elements is substantially in register with the first array of elements, whereby a first image is visible when the device is viewed from the first side, and a second image is visible when the device is viewed from the second side . The brightness or color levels of the repeating image elements on at least one side of the substrate can be modulated in the direction of the region to produce a grayscale or color image. In one embodiment, the first array of repeating elements on or on a first side of the substrate is a matrix of focusing elements, and the second array of repeating elements on or on the second side of the substrate is an image matrix that has way substantially the same way the one to the other, so that, when the device is viewed from the first side, an enlarged image including at least one enlarged version of the image element form is visible, and when the device is viewed from the second side, the image to Grayscale or color is visible. In another embodiment, the repetitive elements of both of the first and second arrays are partially or totally opaque image elements which respectively form the first and second images.