AU2009278275B2 - Method for producing microlenses - Google Patents

Abstract

A method for producing microlenses (4) comprises the steps of providing a carrier substrate (5), prestructuring the carrier substrate using the intaglio printing method, such that on a front of the carrier substrate raised regions are produced and on a back opposite of the front of the carrier substrate, recesses corresponding to the raised regions are produced, and applying translucent plastic (7, 8) on at least one side of the carrier substrate in the region of the raised regions or recesses for forming microlenses.

Description

A method for producing microlenses [0001] This invention relates to a microlens substrate which can be used as a verifi cation means or security feature for a data carrier, and to a production method for mi crolenses and for such a microlens substrate. [00021 Data carriers, such as value documents or identification documents, or also other objects of value, such as branded articles, are often provided for safeguarding purposes with security elements which permit a check of the authenticity of the object and which at the same time serve as protection from unauthorized reproduction. 10003] The term "data carrier" also includes precursors of such data carriers that are unfit for circulation and are present, for example in the case of security paper, in quasi endless form and are processed further at a later time. Data carriers according to the present invention are in particular bank notes, shares, bonds, deeds, vouchers, checks, high-quality admission tickets, but also other papers at risk of forgery, such as pass ports and other identification documents, and also card-shaped data carriers, in particu lar chip cards, and product protection elements, such as labels, seals, packages and the like. 10004] A security element can be embedded in such a data carrier, for example in a bank note or a chip card, or be configured as a self-supporting transfer element, for example as a patch or as a label, which after its production is applied to a data carrier or other object to be secured. [0005] Security elements frequently produce a readily visible optical impression, so that such security elements, besides their function as protection means, are sometimes also employed exclusively as decorative elements. [00061 To prevent a forgery or imitation of security elements for example with high-quality color photocopiers, security elements frequently have optically variable elements which convey a different pictorial impression to the viewer from different viewing angles, showing e.g. a different color impression or also different graphical motifs. In this connection, it is known for example to employ diffractive optical micro- -2 or nanostructures in the form of embossed holograms or other hologram-like diffrac tive structures. 100071 For producing optically variable elements it is further known to employ mi crolens arrangements. For example, EP 0 219 012 A2 discloses a regular arrangement of parallel, abutting cylindrical lenses which shows only a strip-shaped area below each cylindrical lens in dependence on the viewing direction because of the converg ing effect of the cylindrical lenses. Below the cylindrical lenses there are arranged im ages broken down in a strip shape which combine to form a certain overall image for a viewer according to the viewing direction. According to the viewing angle, different images become visible upon horizontal arrangement of the cylindrical lenses, thereby making it possible to produce tilt images and animations. When the cylindrical lenses are in the vertical direction, stereoscopic parallaxes can be incorporated in the image, thereby making it possible to produce a three-dimensional impression for the viewer. [00081 Besides the use of cylindrical lenses, it is also known to employ a regular ar rangement of spherical lenses as is employed for example in a moir6 magnification arrangement. [0009] US 5 712 731 A relates to the use of such a moir6 magnification arrange ment as a security feature. The security apparatus described therein has a regular ar rangement of substantially identical printed microimages, as well as a regular two dimensional arrangement of substantially identical spherical microlenses. The micro lens arrangement has substantially the same pitch as the microimage arrangement. When the microimage arrangement is viewed through the microlens arrangement, one or several magnified versions of the microimages are produced for the viewer in the areas where the two arrangements are substantially in register. 10010] The basic mode of function of such moird magnification arrangements is de scribed in the article "The moird magnifier", M.C. Hutley, R. Hunt, R.F. Stevens and P. Savander, Pure Appl. Opt. 3 (1994), pp. 133-142. Very briefly, moir6 magnification accordingly designates a phenomenon occurring upon the viewing of a grid of identi cal image objects through a lenticular grid with approximately the same grid measure. As with any pair of similar grids, this results in a moird pattern which appears in this -3 case as a magnified and optionally rotated image of the repeated elements of the image grid. Further design variants and effects that are based on this mechanism are de scribed for example in the article "Properties of moird magnifiers", Kamal et al., Opti cal Engineering 37 (11), pp. 3007-3014 (November 1998). 10011] Regular microlens arrangements can also be employed as verification means for security elements, as is described in EP 1 147 912 B 1. Here, certain structures of a security element become visible to the user only upon viewing through such a verifica tion element, so that the function of the security element can be hidden to a casual viewer. [0012] For producing such microlens arrangements there are different technologies known in the prior art. In EP 1 878 584 A2 there is disclosed for this purpose the print ing of an optical lacquer on a carrier substrate by means of a gravure printing plate. The gravure printing plate has depressions made therein which constitute the negative form of the desired lens arrangement. Further, this print also discloses the use of the gravure printing plates as an embossing tool by means of which the desired microlens arrangement is formed for example into a lamination layer. Such methods are also de scribed in EP 0 698 256 B2, there being disclosed therein alternatively also the use of photoresist layers in connection with suitable masks for producing microlenses. [00131 In DE 10 2006 003 798 Al there is stated, as a further alternative, the partial application of a primer layer which produces a change in surface tension in certain areas, which can be employed for producing the microlenses. [00141 From the last-mentioned print, and also from WO 2006/016265 Al, there is known, as a further alternative, an ink jet method wherein a translucent plastic, for ex ample an optical lacquer, is positioned at the desired places in the form of microdrop lets by means of an ink jet printing head on a rough surface. In this method, the em ployed materials must have suitable surface tensions in order to avoid a flow-out of the applied liquid microdroplets. [0015] All known microlens arrangements show a multilayer structure and require accordingly complex and multi-stage production methods. Further, in particular with 4 ink jet methods, the substrate and the plastic applied for producing the micro lenses must show certain properties in interaction, for example with regard to surface tension, in order to avoid an unintentional flow-out on the substrate of the plastic that is liquid upon application. In the further known, above-described gravure printing and embossing methods, the exact image, i.e. the negative form of the micro-lenses to be produced, must be brought into the gravure printing plate, which likewise involves extensive and complex preparation steps for providing such a gravure printing plate prior to the production of a micro-lens arrangement. 10015AI Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application. [0015B] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. [0015C] In a first aspect there is provided a method for producing micro-lenses, comprising the steps of: - supplying a carrier substrate, and - prestructuring the carrier substrate, so that elevations arise on a front side of the carrier substrate, and depressions corresponding substantially to the elevations arise on a back side of the carrier substrate opposing the front side, characterized by the step of: - applying translucent plastic to at least one side of the carrier substrate in the area of the elevations or depressions for forming micro-lenses. [0015D] In a related aspect there is provided a micro-lens substrate, comprising - a carrier substrate with incorporated elevations on a front side of the carrier substrate and depressions corresponding substantially to the elevations on a back side of the carrier substrate opposing the front side, and - a plurality of micro-lenses which are arranged on at least one side of the carrier substrate in the area of the elevations or depressions.

4A 10015E] In a third aspect there is provided the use of a carrier substrate which has been prestructured, preferably embossed, particularly preferably blind embossed by gravure printing, in particular by intaglio printing, so that elevations are located on a front side and depressions corresponding substantially to the elevations on a back side opposing the front side, as a lens mandrel for producing micro-lenses. 100161 In an advantageous aspect, the invention may avoid the disadvantages of the prior art and provide in particular a micro-lens substrate which can also be used as a verification means or security feature for a data carrier and which is simply constructed, as well as a production method for micro-lenses and for such a micro lens substrate which permits a simplified production. 100171 . The dependent claims relate to preferred embodiments and developments of the invention. [00181 The invention is based on the finding that a prestructured carrier substrate with elevations and depressions can be employed as a mandrel for producing micro-lenses. Such a prestructured carrier substrate can be provided by embossing a suitable carrier substrate. For this purpose there can be employed embossing tools known per se. Preferably, the carrier substrate is blind embossed by gravure printing. In so doing, there are provided in a gravure printing plate employed for blind embossing, depressions, in particular engravings, which produce suitable embossed structures on the carrier substrate. It is possible here to influence in targeted fashion the form and dimension of the elevations and depressions, and thus of the micro-lenses to be produced, for example by means of the engraving depth and engraving width.

-5 100191 In blind embossing, embossing is done by means of a gravure printing plate without ink filling, that is, no printing ink is provided in the depressions of the gravure printing plate. Embossing is done here under very high contact pressure, that is, be tween the gravure printing plate and a counterpressure plate, which can be covered with a special thin rubber cloth, there act very high forces, which guarantee a perma nent embossing of the carrier substrate. [00201 The gravure printing process is preferably a line gravure or intaglio printing process, wherein there are engraved in the gravure printing plate or intaglio printing plate depressions, for example in the form of lines, in the printing plate [sic] by means of a rotating graver guided manually or by machine. However, is it also possible to use a halftone gravure method wherein the depressions are etched into the gravure printing plate. Furthermore, a gravure printing plate can of course also be provided with the intended engravings by means of a suitable laser. 100211 In the gravure printing process, the carrier substrate is embossed at high pressure by means of the gravure printing plate and a counterpressure plate. On ac count of the depressions in the gravure printing plate there arise elevations on the front side of the carrier substrate which faces the gravure printing plate, and corresponding depressions on the opposing back side of the substrate. The depressions in the gravure printing plate are configured such that the elevations and/or the depressions of the em bossed substrate are suitable for molding microlenses. There is thus provided a simple possibility for producing elevations and/or depressions suitable for molding micro lenses. [0022] Microlenses are then produced by applying a translucent plastic suitable for forming the microlenses to at least one of the two sides of the blind embossed carrier substrate in the area of the elevations or depressions. Preferably, the depressions on the back side of the carrier substrate are filled with such a translucent plastic and/or the elevations on the front side of the carrier substrate are covered over the entire surface with such a translucent plastic. Within the framework of the present description, "translucent" means that light can shine through to a certain extent or completely, thus also including transparency. A translucent layer permits objects therebehind or there- -6 under to be perceived, even if the brightness of the objects can be reduced and/or the color of the objects changed through the translucent layer. Translucent plastic is ac cordingly understood to be a transparent plastic or a semitransparent plastic, in particu lar a glazing plastic. [0023] Upon the production of microlenses by filling the depressions on the back side of the carrier substrate with the translucent plastic, the spatial extension of the lens on the carrier substrate is determined not only by material parameters, such as the surface tension of the translucent plastic on the carrier substrate, but advantageously also by the spatial geometry of the depression produced in the substrate. This results in more degrees of freedom in the selection of the materials. Additionally, the spatial po sition of the applied plastic is also determined by the depression, and in this manner the microlens to be formed is centered at the intended position. Thus, a lower location accuracy is necessary upon application of the translucent plastic. [00241 The surface form of the microlens is determined on its side facing the carrier substrate by the form of the depression. In this connection, the invention is based on the finding that the back side of a carrier substrate embossed by intaglio printing or halftone gravure with an accordingly configured gravure printing plate has an almost spherical or cylindrical embossed structure suitable for forming microlenses. Such an embossed structure results on the back side of the carrier substrate both in the case of spherical, pyramidal or line-shaped depressions in the gravure printing plate and in the case of differently designed engravings or etchings. By depressions in the gravure printing plate in the form of concentric rings there can also be provided on the back side of the carrier substrate an embossed structure which is suitable for forming a Fresnel lens. [00251 Upon filling of the preferably spherical or cylindrical embossed structure on the back side of the carrier substrate there thus results a spherically or cylindrically symmetric lens surface on the side of the carrier substrate. Additionally, for example the width and the height of the lens can be determined by the form of the depression in the gravure printing plate. Suitable technologies for incorporating almost any desired engraving depths and engraving widths are known from the prior art.

-7 [0026] The form of the microlens on the side facing away from the carrier substrate is determined in general by different properties of the carrier substrate and of the trans lucent plastic, such as by the form and the volume of the depression, the amount of translucent plastic filled into such a depression, and the material properties of carrier substrate and translucent plastic. Advantageously, the surface tension of the translu cent plastic on the carrier substrate is so adjusted that there results for the lens surface on the side facing away from the carrier substrate a surface suitable for optical pur poses, for example a lens surface with a spherical arcuate or parabolic slice plane. Thus, said lens surface is not determined by an embossing tool, as is known from the prior art. This makes it possible to dispense with the hitherto necessary provision of such a high-precision tool, on the one hand, and with a corresponding embossing step, on the other hand, so that the production method as a whole is shortened and simpli fied. 10027] Alternatively or additionally, the front side of the carrier substrate can also be covered with such a translucent plastic at least in certain areas. Thus, concave bulges form in the applied translucent plastic in the area of the elevations, i.e. in the area of the positive structure resulting from the gravure printing process, whereby mi crolenses likewise arise. Such a covering of the front side of the carrier substrate serves at the same time to produce a plane surface on the front side of the carrier sub strate, to mechanically strengthen the carrier substrate, and/or to influence in targeted fashion the optical refractive power of the overall arrangement. 10028] The carrier substrate preferably comprises paper and/or a carrier foil, in par ticular a translucent carrier foil. In the simplest case, the carrier substrate consists completely either of paper or of plastic. The carrier substrate can, however, also con sist of different materials in certain areas, and consist in particular of paper in one area and at the same time of plastic, preferably of a translucent carrier foil, in another area. This makes it possible to emboss different materials as a carrier substrate in one opera tion. A translucent carrier foil is understood here to be either a transparent or a semi transparent carrier foil, for example a glazing carrier foil, which comprises for exam ple polyamide, polyester, polyethylene or biaxially oriented polypropylene (BOPP).

100291 Preferably, the filling of the depressions on the back side of the carrier sub strate is carried out such that piano- or biconvex lenses form on the back side of the carrier substrate. The desired form of the lenses can be adjusted here by means of the employed translucent plastic and its drying behavior, in particular by a volume change accompanying drying, for example a volume reduction (volume shrinkage). The vol ume shrinkage occurring upon polymerization or upon curing of UV-curable lacquers favors the formation of biconvex lenses. [00301 Biconvex lenses show a greatest possible positive refractive power at a given amount of applied translucent plastic or at a predefined build-up height of the micro lenses produced on the back side, and thus possess improved optical properties com pared with piano-convex lenses due to the double-sided curvature. When further trans lucent plastic is now additionally applied to the front side of a translucent carrier sub strate, there result in the area of the elevations concave, in particular plano-concave, microlenses which have a negative refractive power. Thus, the positive refractive power of the biconvex lenses produced on the back side can be weakened and in this manner the focal length of the overall arrangement be adjusted in targeted fashion. This can be done by the choice of the refractive index of the translucent plastic em ployed for producing the front-side microlenses, whereby in general the refractive power of a lens increases with an increasing refractive index of the lens material. [00311 The produced microlenses can be formed on the front side and/or back side of the carrier substrate in principle both as spherical lenses and as cylindrical lenses, in particular as curved cylindrical lenses. [00321 Preferably, the microlenses arranged in the depressions on the back side of the carrier substrate are arranged spaced apart and spatially separated from each other. Thus, a microlens forms in each depression independently, without there being any mutual influencing of neighboring microlenses. The spacing of neighboring micro lenses is expediently chosen here to be as small as possible in order to guarantee as high an area coverage as possible and thus a high-contrast representation in interaction for example with a security element explained more closely hereinafter. However, it is -9 also possible to arrange the microlenses contiguously, resulting in a continuous, all over layer of translucent plastic which comprises the microlenses. 100331 The filling of the depressions on the back side of the carrier substrate and/or the covering of the front side of the carrier substrate with the translucent plastic is ad vantageously done by printing technology, for example by flexographic printing or screen printing. This permits both the amount and the site of the emitted transparent plastic to be controlled well. In particular, this makes it possible to produce the indi vidual microlenses on the back side of the carrier substrate with different colors. In general, parameters of the microlenses that are variable by the printing can be chosen individually for each of said microlenses. Moreover, flexographic printing and also screen printing are common printing methods, which permit printing to be done in a simple way and thus cost-efficiently, as well as at high speed. [00341 Preferably, the applied plastic has a high refractive index and contains for this purpose for example nanoscalic, transparent particles with a high inorganic pig ment content, e.g. titanium dioxide or zinc oxide. The refractive index of the applied plastic can also be increased by adding zinc ions and/or calcium ions. A further possi bility is to incorporate transparent, strongly optically refractive spheres or hollow spheres which are preferably formed from PMMA (polymethylmethacrylate). Further, the translucent plastic can also be dyed and/or contain optically variable effect pig ments. Moreover, there can also be employed monomers with a high refractive index. [0035] Materials with a high refractive index preferably possess a refractive index of more than 1.5, preferably more than 1.6 and particularly preferably more than 1.7. As such materials there come into consideration for example organic compounds and combinations of organic compounds which can be processed to a lacquer system and undergo a polyreaction upon irradiation, in particular upon UV or electron irradiation, and crosslink or cure to a polymeric material, that is, to a polymer, a copolymer or a mixture of polymers and/or copolymers with a high refractive index. Possible organic compounds in this connection are halogenated acrylates, methacrylates or aromatic acrylates.

- 10 100361 The translucent plastic applied for forming the microlenses is thus preferably an optically active lacquer which, in a further production step, is dried or, in the case of UV curing, cures directly after application and is thereby fixed. Drying is effected in the case of solvent-based lacquers by evaporation of the solvent, whereby such sol vent-based lacquers are not unconditionally suitable for making microlenses on ac count of their drying mechanism. It is hence preferable to use curable lacquers, where drying is effected by curing, for example at an elevated temperature or by UV irradia tion, because the drying process in this case involves no, or only a slight, volume change of the applied optical lacquer and thus supports the formation of symmetric microlenses, such as biconvex lenses. This effect is based on surface and volume shrinkage effects during the drying process. [00371 As an optical lacquer it is suitable here to use an unmatted lacquer curing by UV irradiation, i.e. a lacquer containing no matting agents. For example, the optical lacquer is an acrylate system with.about 5 to 10% of monomers, about 3 to 7% of photoinitiators and about 0.5 to 1% of silicone-based or mineral oil-based defoamer. For use in a flexographic printing process, the viscosity of the optical lacquer upon printing is preferably between about 0.1 and 1 Pas. For use in a screen printing proc ess, the viscosity preferably amounts to about 0.5 to 5 Pas. [0038] For example, the optical lacquer is an acrylate system with about 5 to 25% of oligomers (preferably polyurethane acrylates or polyester acrylates), about 5 to 25% of bi- or multifunctional monomers based on acrylate or methacrylates, about 5 to 7% of photoinitiators, about 5 to 25% of reactive diluent (based on acrylates or methacry lates such as 1,6-hexadiol diacrylate), about 5 to 15% of transparent fillers and up to about 5% of additives. The viscosity of such an optically active UV screen printing lacquer is preferably in the range of 0.5 to 5.0 Pas at a shear rate of d= 250 s- and a temperature of 20*C. [00391 It is in principle possible to choose the materials of the carrier substrate and of the microlenses in such a way that they can be easily separated from each other to produce individual microlenses. For producing a microlens substrate, however, the microlenses are firmly connected to the carrier substrate. For this purpose it is expedi- - 11 ent when the two materials enter into a physical or optionally chemical connection, either directly or for example by providing an additional primer layer. [00401 Advantageously, for prestructuring by blind embossing, there are provided in a gravure printing plate identical depressions that are regular, for example arranged in a grid, thereby making it possible to obtain a regular arrangement of identical mi crolenses and thus obtaining a microlens array. The blind embossed carrier substrate is a part of the microlens substrate here. Optionally, the microlens substrate is cut to the desired size. [00411 Alternatively, there can also be provided depressions with different widths and/or depths in the gravure printing plate. Likewise, there can be provided a grid-like or lattice-like arrangement of the depressions with different angles in certain areas, or a partial arrangement of such depressions in geometrical forms or motifs. This makes it possible to provide accordingly different depressions and elevations in the carrier substrate. [00421 When the prestructured carrier substrate consists of a translucent carrier foil in certain areas or completely, this provides a microlens substrate or a microlens foil. It can be employed as a verification means for a data carrier. For use as a verification means, the microlens foil is preferably provided with one or many spacers or an addi tional spacer layer in order to ensure a suitable distance between the microlens foil and a security element to be verified. Preferably, the spacer layer additionally comprises strongly optically refractive spheres or hollow spheres in order to increase the refrac tive power of the microlens foil of the verification means. [00431 The microlens foil is preferably of mirror-symmetric construction, whereby the symmetry plane extends through the center plane of the biconvex microlenses, par allel to the carrier substrate of the microlens foil. It is in particular of optically sym metric construction, so that the microlens foil can serve as a verification means, inde pendently of the direction from which one looks through the microlens foil during verification. In other words, one can view through the microlens foil a subjacent secu rity element from both sides. This optically symmetric or mirror-symmetric structure can be provided by the carrier substrate playing no, or only a small, part for the beam - 12 path within the microlens foil, which can be ensured for example by a sufficiently thin layer thickness of the carrier substrate. Such a symmetric verification means permits, on the one hand, a simplified handling of the microlens foil as a verification means, because verification can take place independently of the viewing direction through the microlens foil. On the other hand, such a symmetric verification means can preferably be used for self-verification on data carriers, in particular bank notes, or also for verifi cation of other security elements applied to data carriers. [00441 The above remarks on symmetric or optically symmetric microlens foils hold fundamentally also for asymmetric microlens foils. However, e.g. plano-convex lenses have a greater focal length compared with biconvex lenses. Therefore, micro lens foils with symmetric lenses are particularly preferred within the framework of the present invention, in particular for verification independent of the viewing direction through the microlens foil. [00451 For self-verification, the data carrier, preferably a bank note, is folded and the verification means brought into congruence with a security element. In particular in the case of a symmetric structure of the verification means, the verification means can advantageously be folded in different directions and thereby interact with different security elements for verification. The handling of the verification means is especially easy for the viewer in the case of a symmetric structure of the microlens foil, because the microlens foil has the same focal length for both viewing directions. For example, the verification means can be folded around a folding axis selectively onto a front side or a back side of the data carrier. Further, several folding axes can also be provided in the data carrier, whereby the verification means can interact with two security ele ments per folding axis. [0046] The microlens substrate according to the invention, in particular in the form of a microlens foil, can also be employed for producing a complete security feature for a data carrier. For this purpose, the translucent plastic of the microlenses and/or the carrier substrate is preferably provided, in particular printed, in the area of the back side depressions with a suitable grid-like microimage structure. This makes it possible - 13 to provide a microrefraction image, preferably employing the moird magnification ef fect. 100471 The invention also relates to the use of a carrier substrate which has been prestructured, preferably embossed, particularly preferably blind embossed by gravure printing, in particular by intaglio printing, so that elevations are located on a front side and depressions substantially corresponding to the elevations on a back side opposing the front side, as a lens mandrel for producing microlenses. 100481 Finally, the invention also includes a data carrier, in particular a value document, a branded article or the like, having a microlens substrate of the above described type. The microlens substrate can be arranged over the entire surface, on partial surfaces or in a window area of the data carrier. 100491 The present invention is characterized by a very much simpler structure and an accordingly simple production method compared with security features known from the prior art. It is in particular possible to provide multicolor motifs and tilt motifs in simple fashion. [00501 Further embodiment examples and advantages of the invention will hereinaf ter be explained with reference to the figures. For more clarity, the figures do without a representation that is true to scale and to proportion. [00511 There are shown: Fig. I a schematic representation of a bank note with security features; Figs. 2a and 2b a plan view and a cross section of a lens structure according to the invention; Figs. 3a to 3f in each case different embodiment examples of a microlens foil ac cording to the invention; and Figs. 4a to 4e in each case different embodiment examples of a security element ac cording to the invention.

- 14 [00521 In Fig. 1 there is depicted as a data carrier a bank note 1. It comprises a secu rity feature in the form of a security element 2 and of a verification means 3. The veri fication means 3 consists of a microlens foil, while the security element 2 is con structed as a grid-like microimage structure in the embodiment example. The verifica tion means 3 can be brought into congruence with the security element 2 by folding along the folding axis of the bank note 1, depicted in Fig. I by a dash line, thereby re sulting in a security feature as depicted in the lower part of Fig. 1. The microimage structure of the security element 2 is recognizable here for example magnified due to the moird magnification effect. Such an authenticity check can be designed in many different variants. For example, one can look through the verification means 3 from different sides depending on the structure. 100531 For this purpose, the microlens foil of the verification means 3, due to a bi convex shaping of the microlenses, has a mirror symmetry relative to a substrate plane extending through the center plane of the biconvex microlenses 4. If such a mirror symmetric structure of the verification means 3 is present, an observation of a subja cent security element 2 can be effected through the verification means 3 in both direc tions, this being especially easily possible for the viewer due to the mirror-symmetric structure of the verification means 3, as already mentioned above, i.e. such a verifica tion means has especially good handling. Accordingly, it is provided that the verifica tion means 3, for self-verification by folding the bank note, can be folded both for wards and backwards to then be brought into congruence selectively with a security element 2 applied to the front side of the bank note I or one applied to the back side thereof. Upon the interaction of verification means 3 and the respective security ele ment 2 there is formed the respectively desired security feature. 100541 As the substrate material for the data carrier there comes into consideration in the case of a bank note 1 any type of paper, in particular cotton vellum paper. It is of course also possible to use paper containing a content x of polymeric material, where x can be between 0 and 100 wt%. [00551 The substrate material of the bank note 1, or in general of a data carrier, can also be a plastic foil, such as a polyester foil. The foil can be stretched uniaxially or - 15 biaxially. A stretching of the foil results, inter alia, in it acquiring light-polarizing properties, which can be utilized as a further security feature. The substrate material can also be a multilayer composite which contains at least one layer of paper or of a paper-like material. Such a composite, which can also be used as a substrate material for bank notes, is characterized by an exceptionally great stability, which is of great advantage for the durability of the note or data carrier. 10056] As a substrate material there further comes into consideration a multilayer, paper-free composite material, which can be advantageously used in particular in some climatic regions of the earth. [0057] Generally, the verification means according to the invention can advanta geously be arranged in a transparent/translucent area of the substrate. The transpar ent/translucent area can be realized by providing in an opaque substrate material, e.g. paper, a window-like aperture (through opening), which is closed substantially over the entire surface by a verification means according to the invention, e.g. a microlens foil. The through opening can be produced during the production of the substrate (so called "vat hole") or subsequently by cutting or punching, in particular laser beam cut ting. 100581 All substrate materials can contain accessory agents which can serve as au thenticity features. There come into consideration here in particular luminescent sub stances which are preferably transparent in the visible wavelength range and can be excited in an invisible wavelength range by suitable aids, such as a source emitting UV radiation or IR radiation, to produce a luminescence radiation that is directly visible or detectable with aids. 10059] The security element 2 to be verified can generally be constructed in differ ent ways, for example as a micro-printed area with a fine dot or line grid structure, as a structure grid with hidden information, as multicolored, grid structures placed one over the other and/or as an embossed grid structure with and without an ink-carrying ele ment, for example with a reflective, metallic layer.

- 16 [00601 A strong interference or magnification effect is achieved with a line grid whose order of magnitude of periodicity corresponds to the order of magnitude of pe riodicity of the verification means 3, amounting for example to 300 pm. The width of the lines amounts to in each case 150 ptm here, and the width of the respectively inter jacent, for example unprinted, white space likewise amounts to 150 Rm. [0061] In the case of a picture motif integrated in the security element 2, the line modulation from the light to the dark picture areas preferably amounts to 100 to 150 pm. Besides a simple line grid, there can also be employed an offset grid. [00621 Instead of a line grid, the security element 2 can also be present as a periodi cally recurring motif or symbol. Its repeat (periodicity) is created in dependence on the embossed structures. [0063] A further effect that is strongly perceptible visually upon verification is achieved with a line grid in multicolor printing. For this purpose, colored lines (for example, CMY, cyan, magenta, yellow) are printed edge to edge, that is, directly con tiguously. 100641 Upon viewing of such structures through a verification means 3 with a grid like microlens arrangement with spherical lenses there results at right angles to the ground lines a strong magnification with a three-dimensional appearance. Tilting pro duces the impression of a flowing motion, the motion being effected in each case at right angles to the tilting direction. The underlying mechanism is based on the mecha nism described in the above-mentioned articles "The moird magnifier" and "Properties of moird magnifiers". [00651 The microlens foil 6 shown in the following embodiment examples accord ing to Figures 2a to 3f can be used as such a verification means 3 for data carriers 1 according to Fig. 1. [0066] Fig. 2a shows a schematic plan view of two neighboring spherical micro lenses 4 on a carrier substrate not depicted. The microlenses 4 possess, in the embodi ment example, a diameter d of about 500 gm and a spacing a of about 2 [tm. The spa- - 17 tial periodicity of such a microlens arrangement thus amounts to approximately 500 gm. In Fig. 2b there is depicted a schematic cross section of the microlens arrange ment. Besides the microlenses 4 there is also depicted schematically the carrier sub strate 5. In the depicted embodiment examples the carrier substrate 5 is blind em bossed by intaglio printing. The microlenses 4 possess a height h of about 60 Im. The engraving depth of the depressions in the gravure printing plate employed for emboss ing the carrier substrate 5 amounts to about 100 pm. The thickness of the carrier sub strate 5 is in the range of about 15 to about 100 ptm and is negligible in the ideal case. 10067] Generally, in the embodiment examples the diameters or, in the case of cy lindrical lenses, the width d of the microlenses is in the range between about 50 and about 500 ptm. The engraving depths of the depressions in the gravure printing plate are in the range between about 20 and about 200 pm, and the engraving widths in the range between about 50 and about 500 pm. Because the back-side embossed structures of the carrier substrate 5 are filled substantially completely with optical lacquer, the width d of the microlenses corresponds substantially to the engraving width. This holds in particular when the thickness of the carrier substrate 5 is negligible in com parison. 100681 Fig. 3a shows schematically a first embodiment example for a microlens ar rangement 6 which can be employed as a verification means. The carrier substrate 5 here is a transparent carrier foil which has been embossed by gravure printing. In the depressions thereby resulting on the back side of the carrier foil 5, i.e. the side previ ously facing away from the gravure printing plate or intaglio printing plate, there are arranged microlenses 4 which consist of an optically active, translucent lacquer 7. The lacquer 7 can be transparent or, as long as it is at least translucent, also colored. The microlenses 4 here are arranged spaced apart. [0069] Alternatively, however, the microlenses 4 can also be arranged within a con tinuous, all-over layer of optical lacquer, as depicted in Fig. 3b. The production of mi crolenses in a continuous layer of optical lacquer is generally easier to realize than the production of a multiplicity of individual microlenses.

- 18 [00701 In Fig. 3c there is again schematically depicted an embossed carrier foil, wherein the front side is covered with a layer 9 of optical lacquer 8. The exterior sur face of the optical lacquer layer 9 is flat, and the optical lacquer layer 9 reproduces the positive structure resulting from the embossing of the carrier foil 5, so that plano concave microlenses arise in the area of the elevations of the carrier substrate 5. [00711 In Fig. 3d there is depicted a combination of the embodiment examples from Figures 3a and 3c. Here, at the same time, the depressions on the back side of the transparent carrier foil 5 are filled with optical lacquer 7 for forming the microlenses 4, and the front side of the carrier foil 5 is covered over the entire surface with a layer 9 of optical lacquer 8. Thus, the optical refractive power of the microlenses 4 is weak ened, whereby a positive refractive power of the overall arrangement nevertheless re mains, because the biconvex microlenses 4 on the back side of the carrier foil 5 have a stronger positive refractive power than the plano-concave microlenses in the front-side lacquer layer 9. In this manner there can be obtained, inter alia, a targeted influencing of the optical refractive power of the microlens arrangement. [0072] The embodiment example depicted in Fig. 3e corresponds in its spatial struc ture to the embodiment example depicted in Fig. 3a. In the optical lacquer 7 of the mi crolenses 4 produced on the back side, however, there are incorporated transparent, strongly optically refractive spheres or hollow spheres which have a high refractive index and are made for example of PMMA (polymethylmethacrylate), polystyrene or polycarbonate. The size of the spheres is in the range between 1 and 50 pm and amounts to for example 2, 3, 5, 10, 20 or 30 Jim, depending on the existing dimensions of the microlenses containing them. 10073] The embodiment example depicted in Fig. 3f builds again on the embodi ment example depicted in Fig. 3a. Additionally there is provided here a spacer layer 10 as a spacer. Said layer can, as depicted, consist of strongly optically refractive spheres or hollow spheres with a diameter of for example about 50 pm. However, it can also consist of a suitable plastic foil, for example in the form of a label. The spacer layer 10 can be applied to the front side and/or to the back side of the carrier foil 5.

- 19 [00741 Alternatively to the embodiment example shown in Fig. 3f, a spacer can be realized on the front side of the substrate by an elevation with great height. Such eleva tions, which are higher than the elevations formed by the carrier-substrate front-side prestructuring according to the invention, are produced e.g. by accordingly deeply en graved gravure printing plates. [0075] In Fig. 4a there is depicted a first embodiment example of a security feature 11, wherein a security element 2 and a verification element 3, for example a microlens foil 6, are combined to a complete security feature 11. With the depicted security fea ture 1 1 there can be obtained for example a moire magnification effect. For producing a security feature I 1 according to the invention, a transparent carrier foil 5 is blind embossed with an intaglio printing plate having hemispherical depressions, and the back-side depressions filled with translucent lacquer 7 and dried to produce spherical microlenses 4. Additionally, the microlenses 4 are printed with a single- or multicolor grid structure 12. For printing the grid structure 12 it is suitable to use for example off set, gravure, flexographic or screen printing processes. [0076] The microlenses 4 are arranged here in a grid and form a two-dimensional Bravais lattice with a preselected symmetry. The Bravais lattice can have for example a hexagonal lattice symmetry, but lattices with a lower symmetry, in particular the symmetry of a parallelogram lattice, are also possible. [00771 The spacing of neighboring microlenses 4 is preferably chosen as small as possible to guarantee as high an area coverage as possible and thus a high-contrast rep resentation. The spherically configured microlenses 4 preferably have a diameter be tween about 50 pm and about 500 pim, preferably greater than 200 pm. [0078] The grid structure 12 printed on the side of the microlenses 4 facing away from the carrier foil 5 contains a grid-like arrangement of identical microimage ele ments. The grid structure 12 also forms a two-dimensional Bravais lattice with a prese lected symmetry, for example with hexagonal lattice symmetry or the symmetry of a parallelogram lattice.

- 20 [00791 To produce the desired moire magnification effect, the Bravais lattice of the microimage elements of the grid structure 12 differs in its symmetry and/or size of its lattice parameters slightly according to the invention from the Bravais lattice of the microlenses 4. The lattice period of the microimage elements is in the same order of magnitude as that of the microlenses 4, i.e. in the range of about 50 gm and about 500 jim, preferably greater than 200 pm. [0080] The optical thickness of the carrier foil 5 and the focal length of the micro lenses 4 are mutually coordinated such that the grid structure 12 and thus the microi mage elements are located approximately at the distance of the lens focal length, or can be brought to the required distance, for example for self-verification. Due to the slightly different lattice parameters, the viewer will see a somewhat different partial area of the microimage elements in each case upon viewing from above through the microlenses 4, so that the multiplicity of the microlenses 4 as a whole produces a mag nified image of the microimage elements. The resulting moire magnification depends here on the relative difference of the lattice parameters of the employed Bravais lat tices. If, for example, the lattice periods of two hexagonal lattices differ by 1%, there results a 100-fold moird magnification. For a more detailed presentation of the mode of function, and for advantageous arrangements of the microimage element grids and the microlens grids, reference is made to the prints DE 10 2005 062 132 Al and WO 2007/076952 A2, whose disclosure is included in the present application to this extent. 100811 The embodiment example depicted in Fig. 4b corresponds in structure to the embodiment example depicted in Fig. 4a, whereby here, however, both the back-side depressions are filled with translucent lacquer 7 and a translucent lacquer layer 9 is applied to the front side of the transparent carrier foil 5. [0082] In the embodiment example depicted in Fig. 4c, in a first method step the single- or multicolor grid structure 12 is printed not on the, for example cylindrical, microlenses 4, but on the transparent carrier foil 5. Subsequently, the carrier foil 5 is embossed by gravure printing and filled for example with transparent lacquer 7 in the back-side depressions for forming microlenses 4. Additionally or alternatively, a layer -21 9 of optical lacquer 8 can also be provided on the front side of the transparent carrier foil 5, as in the embodiment example according to Fig. 4b. 100831 The security feature I1 depicted in Fig. 4d is produced by the following method steps: - blind embossing a cylindrical structure into a translucent carrier foil 5 by intaglio printing, - filling the back-side depressions with translucent lacquer 7 for forming cylindrical lenses 4, and - printing the back side by offset, gravure, flexographic or screen printing with single or multicolor information which is broken down into stripes in accordance with the cylindrical lenses 4. [0084] The periodically recurring letters A, B and C in Fig. 4d indicate schemati cally the information broken down into stripes. It is thus possible to produce different effects, such as flip, zoom, morph or three-dimensional representations. 10085] The embodiment example of the security feature 11 depicted in Fig. 4e is ob tained by the following method steps: - blind embossing a, for example spherical, structure into a translucent carrier foil 5 by intaglio printing, - filling depressions on the back side of the translucent carrier foil 5 with a transparent or colored lacquer 7 to form spherical microlenses, and - applying a printed foil 13, for example in the form of a label. [00861 Because the foil 13 is already printed before application, it can advanta geously be printed in a flat, level state, preferably by an offset printing process or also by a flexographic, gravure or screen printing process. 100871 A further embodiment example of the security feature 11 not shown here is obtained by the following method steps: - blind embossing a, for example spherical, structure into a translucent carrier foil 5 by intaglio printing, - filling depressions in the back side of the translucent carrier foil 5 with transparent or colored lacquer 7 to form spherical microlenses, - 22 - printing a spacer layer 10 on the transparent or colored lacquer 7, for example in the form of solvent-based lacquer or in the form of strongly optically refractive spheres incorporated in a lacquer, and - printing the spacer layer 10 with a grid structure 12. [0088] A further embodiment example of the security feature 1 is obtained by the following method steps: - printing a translucent carrier foil 5 with translucent, UV-curable intaglio printing ink by intaglio printing, - filling the thus produced back-side, for example line-shaped, structure with transpar ent or colored lacquer 7 to form cylindrical lenses, and - printing the transparent or colored lacquer 7 with a grid structure 12 by offset, gra vure, flexographic or screen printing. [0089] By the print of a translucent intaglio printing ink, the thickness of the carrier foil is increased at this place, and thus also the thickness of the lens. In this manner the wall thickness of the lenses can be influenced in targeted fashion. 10090] A further embodiment example of the security feature 1I is obtained by the following method steps: - background printing, for example by offset printing, on a paper substrate, - optionally applying a spacer layer 10, - applying a translucent foil 5 to the background print or the spacer layer 10, - embossing the overall layer structure by intaglio printing or halftone gravure, and - applying a transparent or colored, UV-curable lacquer 7 to form microlenses 4. [0091] The spacer layer 10 strengthens the optical effect here. [0092] A further embodiment example of the security feature 1 is obtained by the following method steps: - blind embossing a, for example spherical, structure into a translucent carrier foil 5 by intaglio printing, - applying to the back side a transparent or colored layer in which laser-ablatable dyes (for example carbon black) are incorporated, - incorporating image information in said layer by means of laser, and - 23 - applying lacquer 7 or 8 to one or both sides to form spherical microlenses. [00931 A further embodiment example of the security feature 11 is obtained by the following method steps: - blind embossing a, for example line-shaped, structure into a translucent carrier foil 5 which contains image information, for example through partial metallization, by inta glio printing, and - applying a transparent or colored lacquer 7 or 8 to one or both sides to form cylindri cal lenses. [00941 A further embodiment example of the security feature I1 is obtained by the following method steps: - blind embossing a, for example spherical, structure in a fully metallized translucent foil by intaglio printing, - incorporating image information in the foil by laser ablation, and - applying a transparent or colored lacquer 7 or 8 to one or both sides to form spherical microlenses. [00951 In a further embodiment example of the security feature 11, pigments with specific microinformation are incorporated in the transparent or colored lacquer 7, 8 for forming the microlenses 4. Said microinformation consists for example of a special pigment form or of a microembossing in the form of a logo. Due to the lens effect of the microlenses 4 a viewer will see the microinformation accordingly magnified. [00961 In all embodiment examples there can be employed as a lacquer 7, 8 for forming the microlenses 4 transparent or colored lacquer 7, 8. In particular, there can be present in a microlens arrangement transparent, colored and also mutually different colored microlenses at the same time. Further, a nanoscalic fluorescence can be incor porated in the lacquer. In this manner it is possible to incorporate additional macro scopic information for a viewer.

Claims (29)

1. A method for producing microlenses, comprising the steps of: - supplying a carrier substrate, and - prestructuring the carrier substrate, so that elevations arise on a front side of the carrier substrate, and depressions corresponding substantially to the elevations arise on a back side of the carrier substrate opposing the front side, characterized by the step of: - applying translucent plastic to at least one side of the carrier substrate in the area of the elevations or depressions for forming microlenses.

2. The method according to claim 1, characterized in that the prestructuring of the carrier substrate is effected by embossing, in particular by blind embossing by gravure printing, preferably by intaglio printing.

3. The method according to claim I or 2, characterized in that the carrier substrate is supplied as a substrate comprising paper and/or a carrier foil, in particular translucent carrier foil.

4. The method according to at least one of the preceding claims, characterized in that the microlenses in the depressions are configured as plano- or biconvex lenses, in particular as cylindrical or spherical lenses.

5. The method according to at least one of the preceding claims, characterized in that the translucent plastic is applied to the back side of the carrier substrate so as to be spatially interrupted between the microlenses or to form a continuous layer.

6. The method according to at least one of the preceding claims, characterized in that the filling of the depressions is effected by printing technology, preferably by flexographic printing or screen printing. -25

7. The method according to at least one of the preceding claims, characterized in that with the translucent plastic there are applied translucent, strongly optically refractive spheres or hollow spheres.

8. The method according to at least one of claims 2 to 7, characterized in that the blind embossing of the carrier substrate is effected by gravure printing with a gravure printing plate which has depressions with an engraving depth of about 20 ptm to about 200 jm and an engraving width of about 50 pm to about 500 pm, preferably of about 100 pm to about 500 pim, particularly preferably of about 200 ptm to about 500 gm.

9. The method according to at least one of the preceding claims, characterized in that the translucent plastic is an optical lacquer, and the method comprises the further step of: - drying the lacquer, the drying being preferably effected by curing, in particular at elevated temperature or by means of UV irradiation.

10. A method for producing a microlens substrate comprising a method for produc ing microlenses according to at least one of the preceding claims, characterized in that the microlenses are firmly connected to the carrier substrate, and the mi crolens substrate preferably possesses a structure that is mirror-symmetric with respect to a plane of the carrier substrate.

11. The method according to claim 10, characterized in that the elevations or depres sions are produced in a regular, in particular grid-like, arrangement.

12. The method according to claim 10 or 11, characterized in that the carrier sub strate is present as a translucent carrier foil with a thickness of about 15 to about 100 jm, preferably with a thickness of 20, 30, 50 or 80 jim.

13. The method according to at least one of claims 10 to 12, characterized by the fur ther step of: - 26 - applying a spacer layer, preferably comprising strongly optically refractive spheres or hollow spheres.

14. The method according to at least one of claims 10 to 13, characterized by the step of: - printing the translucent plastic and/or the carrier substrate, preferably in the area of the depressions, with a grid structure.

15. A microlens substrate, comprising - a carrier substrate with incorporated elevations on a front side of the carrier sub strate and depressions corresponding substantially to the elevations on a back side of the carrier substrate opposing the front side, and - a plurality of microlenses which are arranged on at least one side of the carrier substrate in the area of the elevations or depressions.

16. The microlens substrate according to claim 15, characterized in that the carrier substrate comprises paper and/or a carrier foil, in particular a translucent carrier foil.

17. The microlens substrate according to claim 15 or 16, characterized in that the microlenses arranged in the depressions are configured as plano- or biconvex lenses, in particular as cylindrical or spherical lenses.

18. The microlens substrate according to at least one of claims 15 to 17, character ized in that the microlenses on the back side of the carrier substrate are spaced apart or are connected to each other over the entire surface.

19. The microlens substrate according to at least one of claims 15 to 18, character ized in that the microlenses contain translucent, strongly optically refractive spheres or hollow spheres. -27

20. The microlens substrate according to at least one of claims 15 to 19, character ized in that the microlenses are formed from a translucent plastic, preferably from a cured optical lacquer.

21. The microlens substrate according to at least one of claims 15 to 20, character ized in that the microlenses are firmly connected to the carrier substrate and have a regular, in particular grid-like, arrangement.

22. The microlens substrate according to claim 21, characterized by a structure that is mirror-symmetric with respect to a plane of the carrier substrate.

23. The microlens substrate according to claim 21 or 22, characterized in that the carrier substrate is present as a translucent carrier foil with a thickness of about 15 to about 100 pm, preferably with a thickness of 20, 30, 50 or 80 gm.

24. The microlens substrate according to at least one of claims 21 to 23, character ized by at least one spacer which is formed by an elevation of the carrier sub strate.

25. The microlens substrate according to at least one of claims 21 to 24, character ized by a spacer layer which preferably comprises strongly optically refractive spheres or hollow spheres.

26. The microlens substrate according to at least one of claims 21 to 25, character ized in that the microlenses and/or the carrier substrate are printed with a grid structure.

27. Use of a carrier substrate which has been prestructured, preferably embossed, particularly preferably blind embossed by gravure printing, in particular by inta glio printing, so that elevations are located on a front side and depressions corre sponding substantially to the elevations on a back side opposing the front side, as a lens mandrel for producing microlenses.

- 28 28. A data carrier, in particular value document, branded article or the like, compris ing a microlens substrate according to any of claims 10 to 26 which constitutes a verification means.

29. The data carrier according to claim 27, characterized by at least a first and a sec ond security element and at least one folding axis along which the data carrier can be folded selectively on both sides, so that the microlens substrate interacts, for verification, selectively with the first or the second security element.