AT518727A2 - Optical device with zero order image - Google Patents

Abstract

An optical device including: a first surface; and an array of pixels on the first surface, wherein a plurality of the pixels include a zero order diffraction element such that each zero order diffraction element is configured to provide a zero order diffractive effect.

Description

Field of the invention

The invention relates generally to optical devices, in particular security devices, for documents such as banknotes.

Background of the invention

It is well known that security features, such as banknotes, are integrated into documents which require a degree of security. Such security features may take many forms, with particularly useful features being those that are visually apparent and therefore relatively easy to inspect.

However, unscrupulous counterfeiting is now better organized and technically more competent, and the high revenues from counterfeiting are now more easily appreciated regardless of the risks. Over the last few years, attempts to mimic real devices have become increasingly successful. This problem is exacerbated by the fact that the banknote authentication process by members of the public has long been recognized as the weakest point in the security system. Often, such security features require that inspection by members of the public be appropriate, but it may be excessively complicated to see them, or they may not provide a strong effect that is easily recognizable. This diminishes the suitability of such features to allow the public to play an active role in reducing counterfeiting costs.

Therefore, it is desirable to provide security features that are difficult to reproduce and falsify while at the same time involving the public so that true authentication of banknotes can occur. Security features that provide a surprising visual effect that, for example, reveal a hidden image that is not normally visible, and that do not require special equipment, are particularly desirable.

Brief description of the invention

According to one aspect of the present invention, there is provided an optical device, preferably a security document security device, comprising: a first surface; and an array of pixels on the first surface, each pixel

The zero order diffraction element includes, so that each zero order diffraction element is designed to provide a zero order diffractive effect.

Preferably, the size of each pixel is the same. Each pixel may have a dimension ranging from 5 to 100 microns.

In embodiments, each pixel has an associated brightness. The associated brightness of each pixel may be selected from a finite number of brightness levels, such as 16 brightness levels. Alternatively, the associated brightness of each pixel may be selected from a continuous range of brightness levels. The zeroth-order diffraction element of each pixel may be located within an active region of the pixel arranged such that the brightness of each pixel is determined by the size of the active region of the pixel. The optical device may further include one or more non-diffractive pixels, each non-diffractive pixel corresponding to a minimum brightness level.

Optionally, each zero-order diffraction element includes a periodic array of grating elements. The periodicity of the arrangement of the grating elements for each zero-order diffraction element may be the same. Preferably, the grating periodicity is not more than 500 nm, more preferably not more than 300 nm, and most preferably not more than 250 nm. In embodiments, a color is associated with each zero order diffraction element and the periodicity of the array of grating elements is zero order for each diffraction element is at least partially determined based on the color associated therewith. The color associated with each zero-order diffraction element may correspond to the appearance of the zero-order diffraction element when the optical device is viewed from a common position.

The grating elements of the optical device may have grating heights or depths of 500 nm or less, preferably between 60 and 250 nm. In one embodiment, the grid elements may have grid heights or depths of between 60 and 150 nm. Such a range of grid heights or depths may be used to produce zero order color special effects depending on other factors such as grating periodicity.

In another embodiment, the grid elements may have grid heights or depths of between 120 and 250 nm. The range of grid heights or depths can provide very bright diffraction efficiencies for high spatial frequency gratings, for example with a grating periodicity of 250 nm or less.

Optionally, the optical device further includes a first opaque layer, optionally black or white, preferably white, which is applied to a second surface of the substrate opposite the first surface.

In an alternative option, the optical device further includes a microlens array formed on a second surface of the substrate, the microlenses of the microlens array being configured to view the pixel array. Optionally, the optical device may further include a second opaque layer, optionally black or white, preferably white, which is applied to the pixel array and thereby covers the pixel array.

According to a second aspect of the present invention, there is provided an optical system including an optical device according to the first aspect and an inspection device, wherein the inspection device includes a microlens array including an array of microlenses, the microlens array being configured to provide an optical effect, preferably a moiré effect or an image change effect, provided that it is positioned to receive the optical

Device overlaps so that the microlenses make the image recognizable.

According to a third aspect of the present invention, there is provided a document, preferably a security document such as a bill, which includes the optical device or the optical system of the preceding aspects.

According to a fourth aspect of the present invention, there is provided a method of manufacturing an optical device according to the first aspect, the method comprising the steps of: applying radiation-curable ink (RCI) to a first surface of a substrate; Embossing the RCI using a high resolution embossing device; and hardening the RCI.

The high resolution embossing apparatus can be manufactured using a method involving electron beam lithography. Electron beam lithography can be used to create a master master, which in turn is needed to make the high resolution embossing device.

The method optionally includes a step of forming a microlens array, preferably an embossed microlens array, a second surface of the substrate such that the microlenses of the microlens array are configured to view an image associated with the RCI.

According to a fifth aspect of the present invention, there is provided a method of producing a document according to the third aspect, comprising the steps of: applying, in a region of a substrate, radiation-curable ink (RCI) to a first surface of a substrate, embossing the RCI with a high-resolution embossing device; and hardening the RCI; and applying an opaque layer to one or both of a first surface and a second surface of the substrate, wherein the one or both opaque layers are applied such that the RCI is visible from at least one side of the substrate.

Optionally, the method further includes the step of forming a microlens array, preferably an embossed microlens array on another portion of the substrate, onto the RCI such that when the bill is folded or otherwise deformed so that the microlens array is overlying the RCI, microlenses of the microlens array for viewing an image associated with the RCI. Alternatively, the method may further include a step of forming a microlens array, preferably an embossed microlens array, a second surface of the substrate that overlaps the RCI so that the microlenses of the microlens array are configured to view an image associated with the RCI.

Security document or token

As used herein, the term security documents and tokens includes all types of value-added documents and tokens including, but not limited to, currency items such as banknotes and coins, credit cards, passports, passports, securities and stock certificates, driver's licenses, title deeds, travel documents such as airline and train tickets, tickets and tickets, birth, death and marriage certificates and academic transcripts.

The invention is particularly, but not exclusively, applicable to security documents or tokens, such as banknotes or identification documents, such as badges or passports, formed from a substrate to which one or more print layers are applied.

substratum

As used herein, the term substrate refers to the starting material from which the security document or token is formed. The starting material may be paper or another fibrous material such as cellulose, a plastic or a polymer material including, but not limited to, polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinylchloride (PVC),

Polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

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 a print is applied. The

Window may be completely transparent so as to allow the transmission of light substantially unimpaired, or it may be partially transparent or translucent, partially enabling the transmission of light without allowing objects to be clearly seen through the window area ,

A window area may be in a polymeric

A security document comprising at least one layer of transparent polymeric material and one or more covering layers applied to at least one side of a transparent polymeric substrate by omitting at least one opaque layer in the region forming the window region , 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 region.

A partially transparent or translucent area, hereinafter referred to as a "half-window", may be formed in a polymeric security document having covering layers on both sides by omitting the covering layers only on one side of the security document in the window area, so that the " Half Window "is not completely transparent, but allows some of the light to pass through without allowing objects to be clearly seen through the half window.

Alternatively, it is possible for the substrates to be made of a substantially opaque material, such as paper or paper

Fiber material are formed, wherein an insert of transparent plastic material is inserted into a cutout or a 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 may be applied to a transparent substrate to increase the opacity of the security document. An opaque layer is such that LT <L0, where L0 is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opaque layer may comprise one or more of a variety of opaque coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed in a binder or carrier of a heat activated crosslinkable polymer material. Alternatively, a substrate of transparent plastic material could be placed between opaque layers of paper or other partially or substantially opaque material onto which indicia may subsequently be printed or otherwise applied.

Safety device or feature

As used herein, the term security device or feature includes any of a wide variety of security devices, elements, or features intended to prevent the security document or token from being counterfeited.

Protect copy, modification or manipulation. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document are; Safety inks, such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochrome inks; printed and embossed features, including relief structures; Interference layers; Liquid crystal devices; Lenses and

Lentikularstrukturen; optically variable devices (OVD), such as diffractive devices, including diffraction gratings, holograms, and diffractive optical elements (DOE).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings. It should be understood that the embodiments are presented for purposes of illustration only, and the invention is not limited by this illustration. For the drawings:

Figures la to lc each show a document containing an optical device;

Figure 2 shows an optical device according to an embodiment;

FIGS. 3a and 3b show pixels according to various embodiments;

Figure 4 shows an arrangement of grid elements of a zero order pixel;

Figure 5 shows pixels arranged in groups comprising pixels of different colors; and

Figures 6a to 6b show embodiments involving arrangements of microlenses.

Description of preferred embodiment

With reference to Figures la and lb, a document 2 is provided which includes an optical device 4, such as a security device, and an optional verification feature 6. The document 2 may be a security document 2 such as a banknote. The security document 2 can also be any other document that requires a security measure, for example a credit card or a passport. The document 2 includes a substrate 8 that may include a first opaque layer 10 applied to a first side 11 and a second opaque layer 12 applied to a second side 13. Both the first opaque layer 10 and the second opaque layer 12 are shown to include window regions corresponding to the optical device 4 and the verification device 6, it being understood that one of the first and second opaque layers 10, 12 is in some configurations may be configured to cover one of the optical device 4 and the inspection device 6 shown in FIG. 1c, the second covering layer 12 in the illustration covering the optical device 4. In the case of FIG. 1c, the opaque layer may correspond to an opaque backing layer for the optical element 4, such as a white or black backing layer. In this way, the optical device 4 or the verification device 6 can be formed in a half-window region.

The optical device 4, with reference to FIG. 2, includes a substrate 8 having a first surface 16a and a second surface 16b corresponding to the first and second sides 11, 13 of the document 2, respectively. The first surface 16a includes an array of pixels 14. As shown in the figure and as assumed herein, the array of pixels 14 corresponds to a regular 2D array of pixels 14, however, the array of pixels 14 may generally be any suitable arrangement, including one be irregular arrangement. The pixels 14 are arranged to form an image that is viewable by a user or a hidden image that must be made recognizable by the use of one or more verification devices 6.

It is understood that the image or hidden image may correspond to an array of microimages, such as a recurring ID or 2D pattern of microimages. The pixels 14 may all be the same size, with the "size" of a pixel 14 as used herein corresponding to the area occupied by the pixel 14 on the first surface 16a.

Each pixel 14, with reference to Figures 3a and 3b, includes a zero-order diffraction element 18 (in the figures, the zero-order diffraction element 18 represents the shaded portion of the pixel 14) provided to provide a visual effect of zero-order diffraction , In the embodiment shown in FIG

In the embodiment shown in Figure 3b, each pixel 14 has an active region 20 with the diffractive element 18 disposed within the active region 20. The portion of each pixel 14 that does not include an active region 20 is referred to herein as an inactive region 22 of the pixel. As shown in Figure 3b, various pixels 14a, 14b, 14c may have different sized active regions 20a, 20b, 20c. The different sizes of the active regions 20 result in different brightnesses of the corresponding pixels 14, wherein a large active region 20 is associated with a lighter pixel 14. "Brightness" as used herein corresponds to the relative brightness between pixels 14. Preferably, maximum brightness corresponds to the largest active region 20 associated with a pixel 14. Also shown in Figure 3b is a non-diffractive pixel 15. The non-diffractive pixel 15 corresponds to a pixel 14 having only one inactive region 22. The non-diffractive pixel 14 thus corresponds to a minimum brightness pixel 14. Each pixel 14 may have a brightness selected from a finite range of brightness levels (eg, 16 levels) Alternatively, the brightness of each pixel 14 is selected from a continuous range of brightness levels.

Each zeroth-order diffraction element 18, with reference to FIG. 4, includes an array of grating elements 24. In the illustrated embodiment, the grating elements 24 correspond to protrusions from the first surface 16a of the optical device 4. Other designs include grating elements 24 that have notches or recesses in the grating elements first surface 16a or regions having a different refractive index compared to the substrate 8 in which the grid elements 24 are embedded, or a layer applied to the substrate 8 in which the grid elements 24 are embedded. As shown, the grid elements 24 are in a linear periodic arrangement with a constant grid element periodicity 26 and a constant grid element height or depth. For example, the grating element periodicity is less than 500 nm, preferably less than 300 nm, better still less than 250 nm. The grating heights or depths are preferably 500 nm or less, more preferably between 60 and 250 nm. In some embodiments, grating heights or depths may vary after the required zeroth-order effects between 60 and 150 nm or between 120 nm and 250 nm. Center spacing and widths of the grating elements are preferably 500 nm or less, more preferably between 60 and 250 nm.

In one embodiment, each zero-order diffraction element 18 of the optical device 4 has a common constant grating element periodicity 26 and a common grating orientation. An image is provided because of the brightness variation of each pixel 14 due to the size of an active region 20, as described with reference to Figure 3b. For example, for a monochromatic 2-color image, each pixel 14 may be selected to have one of two brightness levels. In a particular implementation of this example, one brightness level corresponds to a pixel 14 without inactive region 22, and the other brightness level corresponds to a pixel 14 without active region 20 (i.e., a non-diffractive pixel 15). In another example, a 16-color image may be created, with each pixel 14 having a brightness level selected from one of 16 levels (where the minimum brightness level may correspond to a non-diffractive pixel 15). In this embodiment, the optical device 4 may appear as a monochromatic color image when the optical device 4 is viewed from a predetermined position. The color of the image is determined at least on the basis of the common grating periodicity 26 and can further be determined by a selection of the following: the material of the substrate 8, the material of the grating element 24, the coating between the substrate 8 and the grating elements 24, the coating In general, the color for a given optical device 4 can be determined by routine experiment-based variation of the grating periodicity 26.

Another embodiment corresponds to a variation of the previously described embodiment. In this embodiment, each pixel 14 may have a color selected from two or more colors.

The color of each pixel 14 corresponds to the color of the pixel 14 when viewed from a given common viewing position. In one implementation of this embodiment, each pixel 14 has a color selected from one of four colors, specifically red, green, and blue. Each pixel 14 also has an associated brightness, as previously described. In this way, an RGB image can be generated. As shown in Figure 5, the pixels 14 are arranged in groups 28 which contain pixels 14 associated with each possible color ± u (red, green, blue). To preserve a regular 2D array of pixels 14, two pixels of a color may exist in a group 28 (such as the two green pixels shown in FIG. 5).

The microlens array 30, with reference to FIGS. 6 a to 6 c, is provided for viewing the pixels 14 of a pixel layer 30. In FIG. 6 a, the microlens array 30 is provided on the opposite surface (the second surface 16 b) of the substrate 8 to the pixel layer 30 and is designed to focus on the pixels 14 of the pixel layer 30. In FIG. 6b, the microlens array 30 is provided in a separate section of the substrate 8 to the microlens array 30, whereby a checking element of a checking device 32 is formed.

In FIG. 6 c, the microlens array 30 is provided as a checking device that corresponds to the checking feature 6 of the document 2. In this case, the microlenses of the microlens array 30 are designed to focus on the pixels 14 of the optical device 4 when the document 2 is folded or otherwise deformed such that the microlens array 30 overlaps the optical element 4, preferably in contact with either the first side 11 or the second page 13.

The microlens array 30 is suitable for viewing an arrangement of an image corresponding to an array of microimages. An advantage of pixels 14 with zero-order diffraction element 18 is that high-resolution imaging is possible.

0th order diffraction elements 18 are advantageous over first and higher order diffraction elements, since it has been found that microlenses recombine in operation after first and higher order diffraction effects, thereby reducing the effectiveness of such gratings for use in optical devices based on microlenses and microimages , is reduced. Therefore, zeroth-order diffraction gratings 18 can provide high-contrast, high-resolution microimage. The high-resolution microimage can correspond to pixels with a dimension ranging from 5 to 100 microns. For example, a square pixel may have a length and width of 5 to 100 microns each. A round pixel may have a diameter of 5 to 100 microns. Reducing the pixel size affects the amount of light that reflects each pixel, so the particular application will dictate the ideal size of the pixel.

Since the lattice spacings of the zeroth-order lattice elements 18 of the pixels 14 are relatively small, high-resolution techniques are required to form the pixels 14. In such a method of forming the pixels 14, embossing with a high-resolution embossing apparatus is used. The high-resolution embossing device can be created by a method involving electron beam lithography, which allows the formation of more detailed (and therefore high-resolution) images.

Features on a surface allows. A master master can be created using electron beam lithography, which can then be used to create the high resolution embosser. The array of pixels 14 may be formed by first applying a radiation-curable ink (RCI) to a first surface of the substrate 8 and embossing the radiation-curable ink using the embossing tool. Due to surface tension effects, it may be desirable to cure the RCI before the embossing tool is removed, thus preserving the structure of the zeroth-order grid elements 18. The RCI is preferably cured using a suitable radiation; Thus, a UV-curable ink can be cured by exposure to UV radiation. It is understood that other inks and curing methods may be used, for example, thermosetting inks.

Other modifications and improvements may be made without departing from the scope of the present invention.

Claims (25)

claims: An optical device including: a first surface; and an array of pixels on the first surface, wherein a plurality of the pixels include a zero order diffraction element such that each zero order diffraction element is configured to provide a zero order diffractive effect, and wherein the array of pixels is configured to provide an image; wherein the image includes an array of microimages. The optical device according to claim 1, wherein the size of each pixel is the same. The optical device of any one of the preceding claims, wherein each pixel has a dimension of 5 to 500 microns. The optical device of any one of the preceding claims, wherein each pixel has an associated brightness. The optical device of claim 4, wherein the associated brightness of each pixel is selected from a finite number of brightness levels, such as 16 brightness levels. The optical device of claim 4, wherein the associated brightness of each pixel is selected from a continuous range of brightness levels. The optical device according to any one of claims 4 to 6, wherein the zero-order diffraction element of each pixel is located within an active region of the pixel arranged to determine the brightness of each pixel by the size of the active region of the pixel. The optical device of any one of claims 4 to 7, further comprising one or more non-diffractive pixels, each non-diffractive pixel corresponding to a minimum brightness level. The optical device of any one of the preceding claims, wherein each zero-order diffraction element includes a periodic array of grating elements. The optical device of claim 9, wherein the periodicity of the array of grating elements is the same for each zero-order diffraction element. The optical device of claim 9, wherein a color is associated with each zero order diffraction element, and wherein the periodicity of the array of the grating elements for each zero order diffraction element is determined based at least in part on the color associated therewith. The optical device according to claim 11, wherein the color associated with each zero-order diffraction element corresponds to the appearance of the zero-order diffraction element when the optical device is viewed from a common position. 13. Optical device according to one of claims 9 to 12, wherein the grid elements have grid depths or heights of 500 nm or less, preferably between 60 and 250 nm. 14. An optical device according to any one of claims 1 to 13, further comprising a first opaque layer, optionally black or white, preferably white, which is applied to a second surface of the substrate opposite the first surface. 15. The optical device of claim 1, further comprising a microlens array formed on a second surface of the substrate, the first and second surfaces corresponding to opposite sides of a transparent or translucent substrate, the microlens array for viewing the array of pixels is designed. 16. An optical device according to any one of claims 1 to 13 or 15, further comprising a first opaque layer, optionally black or white, preferably white, which is applied to the array of pixels and thereby covers the array of pixels. 17. An optical system including an optical device according to any one of claims 1 to 16 and a verification device, wherein the verification device includes a microlens array including an array of microlenses, the microlens array being configured to generate an optical effect, preferably a moire. Effect or image change effect when positioned so as to overlap the optical device. A document, preferably a security document such as a bill, which includes the optical device of any one of claims 1 to 16 or the optical system of claim 17. A method of manufacturing an optical device according to any one of claims 1 to 16, the method comprising the steps of: applying radiation-curable ink (RCI) to a first surface of a substrate; Embossing the RCI using a high resolution embossing device; and hardening the RCI. 20. The method of claim 19, wherein the high resolution embossing device is fabricated using a method involving electron beam lithography. 21. The method of claim 20, wherein the electron beam lithography is used to create a master master, which in turn is used to make the high resolution embosser. The method of any of claims 20 to 22, including a step of forming a microlens array, preferably an imprinted microlens array, a second surface of the substrate so that the microlenses of the microlens array are adapted for viewing an image associated with the RCI. A method of producing a document according to claim 18, including the steps of: - applying, in a region of a substrate, radiation-curable ink (RCI) to a first surface of a substrate, embossing the RCI using a high resolution embossing device; and hardening the RCI; and applying an opaque layer to one or both of a first surface and a second surface of the substrate, wherein the one or both opaque layers are applied such that the RCI is visible from at least one side of the substrate. 24. The method of claim 23, further comprising the step of forming a microlens array, preferably an embossed microlens array on another portion of the substrate on the RCI such that when the bill is folded or otherwise deformed such that the microlens array is disposed overlying the RCI , Microlenses of the microlens array are designed to view an image associated with the RCI. 25. The method of claim 23, including a step of forming a microlens array, preferably an imprinted microlens array, a second surface of the substrate that overlaps the RCI so that the microlenses of the microlens array are configured to view an image associated with the RCI.