Patent Publication Number: US-11040565-B2

Title: Method for manufacturing a security element having a lens grid image

Description:
BACKGROUND 
     The present invention relates to a method for manufacturing a security element having a lenticular image for depicting one or more target images that are visible only from predetermined viewing directions and whose motifs are formed by visually perceptible, contrasting metallic and demetalized sub-regions of a motif layer. 
     For protection, data carriers, such as value or identification documents, but also other valuable objects, such as branded articles, are often provided with security elements that permit the authenticity of the data carrier to be verified, and that simultaneously serve as protection against unauthorized reproduction. 
     Security elements having viewing-angle-dependent effects play a special role in safeguarding authenticity, as said elements cannot be reproduced even with the most modern copiers. Here, the security elements are furnished with optically variable elements that, from different viewing angles, convey to the viewer a different image impression and, depending on the viewing angle, display for example another color or brightness impression and/or another graphic motif. 
     It has long been known, for instance, to personalize identification cards, such as credit cards and identity cards, by means of laser engraving. In a personalization by laser engraving, the optical properties of the substrate material of the identification cards are irreversibly altered through suitable guidance of a laser beam in the form of a desired marking. 
     Document EP 0 219 012 A1 describes an identification card having a partial lens grid pattern through which desired pieces of information are inscribed in the card at different angles with a laser. Subsequently, when viewed, said pieces of information can also be perceived only at said angle, such that the different pieces of information appear when the card is tilted. 
     If a lenticular image includes a metallic motif layer, then the depicted motifs can be formed by local demetalizations in the metallic motif layer. Here, various possibilities are known for introducing a design into a metalization with a laser through demetalization. The demetalization can be done, for example, through direct inscription in that a laser beam is guided over the metallic motif layer by means of a suitable scanning unit, or also by a large-area laser impingement using a mask. In both cases, producing demetalized lines of a desired width in the motif layer poses a particular challenge. 
     If, for demetalization, the metallic motif layer is successively impinged on from various angles, and thus at different locations in the focal plane, with a finely focused laser beam until the sub-regions having the desired line width are each demetalized, then the scanning of the entire area of the lenticular image is normally very complex and laborious. Thus, to shorten the process duration, it was recommended to arrange the metallic motif layer outside the focal plane of the (micro-)lenses such that an expanded image of the incident laser radiation results in the plane of the motif layer upon laser demetalization. In this case, the demetalization can be performed significantly faster, but due to the defocusing, blurred tilt images having image changes that are no longer clearly defined are produced. 
     SUMMARY 
     Proceeding from this, it is the object of the present invention to specify a method of the kind mentioned above that avoids the disadvantages of the background art and that facilitates, especially at high production speed, a production of sharply delimited demetalized sub-regions of selectable line width in a lenticular image. 
     Said object is solved by the features of the independent claim. Developments of the present invention are the subject of the dependent claims. 
     According to the present invention, in a method of the kind cited above,
         a lenticular image having a lens grid composed of a plurality of microlenses and a metallic motif layer arranged spaced apart from the lens grid is provided,   the refractive effect of the microlenses defining a focal plane and the metallic motif layer being arranged substantially in said focal plane,   a line width is chosen for the demetalized sub-regions to be produced in the metallic motif layer,   a marking laser source having a laser wavelength λ is selected such that the resolving power D(λ) of the microlenses of the lenticular image at the selected laser wavelength λ substantially corresponds to the line width of the demetalized sub-regions to be produced, and   the metallic motif layer is impinged on through the microlenses with laser radiation of the selected marking laser source to produce demetalized sub-regions in the metallic motif layer.       

     In one preferred method variant, the lenticular image is adapted for depicting n≥2 target images, and for the demetalized sub-regions to be produced, a line width is chosen that is between 0.6*d ML /n and 1.4*d ML /n, preferably between 0.8*d ML /n and 1.2*d ML /n, particularly preferably between 0.9*d ML /n and 1.1*d ML /n, where d ML  is the diameter of the microlenses. Here, the number n of target images to be depicted is especially 2, 3, 4 or 5. 
     Here, within the scope of this description, lenses whose size in at least one lateral direction lies below the resolution limit of the naked eye are referred to as microlenses. In principle, the microlenses can be developed to be spherical or aspherical, but currently the use of plano-convex cylindrical lenses is preferred such that, in the said method, a lenticular image having a lens grid composed of a plurality of plano-convex micro-cylindrical lenses is advantageously provided. With micro-cylindrical lenses, the term “diameter” always refers to the dimension perpendicular to the cylinder axis. The length of the micro-cylindrical lenses is arbitrary; for instance, when used in security threads, it can equal the total width of the thread and be several millimeters. 
     According to the present invention, the metallic motif layer of the lenticular image is arranged substantially in the focal plane of the microlenses, which especially means that the distance of the metallic motif layer from the focal plane is less than 25%, preferably less than 10% and particularly preferably less than 5% of the focal length of the microlenses. 
     The resolving power D of the microlenses of the lenticular image is advantageously determined by the Airy formula D(λ)=2.44*λ*f/d ML , where f is the focal length of the microlenses, λ the light wavelength and d ML  the diameter of the microlenses. The marking laser source is then advantageously selected such that the resolving power D(λ) differs from the line width of the demetalized sub-regions to be produced by less than 15%, preferably by less than 10%. 
     Here, advantageously, an easily available laser source is used as the marking laser source, such as a Nd:YAG laser, a frequency-doubled Nd:YAG laser, a frequency-tripled Nd:YAG laser or an Er:glass laser. In principle, also other laser sources having other wavelengths can, of course be used, such as the diode laser, which is available for numerous wavelengths, as long as they are suitable only for demetalizing the metallic motif layer. If two or more different laser sources of differing wavelengths are used, then line widths of differing sizes can easily be realized in one security element. 
     In one advantageous development of the present invention, it is provided that, for fine control, the laser power of the marking laser source is adjusted to adapt the line width of the produced demetalized sub-regions to the chosen line width. 
     A lenticular image is advantageously provided whose lens grid comprises microlenses having a lens diameter between 5 μm and 20 μm and whose lens period is between 100% and 125% of the lens diameter. 
     The lens grid can adjoin air, but it can especially also be embedded in an embedding layer whose refractive index preferably differs from the refractive index of the microlenses by 0.2 or more. 
     Further exemplary embodiments and advantages of the present invention are explained below by reference to the drawings, in which a depiction to scale and proportion was dispensed with in order to improve their clarity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Shown are: 
         FIG. 1  a schematic diagram of a banknote having an inventive security element in the form of a window security thread that includes a tilt image having three different target images, 
         FIG. 2  schematically, the structure of the window security thread in  FIG. 1 , in cross section, 
         FIG. 3  a schematic drawing of a lenticular image to explain the principle used according to the present invention, and 
         FIG. 4  schematically, the structure of a window security thread according to another exemplary embodiment of the present invention, in cross section. 
     
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
     The invention will now be explained using the example of security elements for banknotes and other value documents. For this,  FIG. 1  shows a schematic diagram of a banknote  10  that is furnished with an inventive security element in the form of a window security thread  12 . The window security thread  12  emerges at the surface of the banknote  10  in window regions  14 , while it is embedded in the interior of the banknote  10  in the ridge regions  16  lying therebetween. 
     In the window regions  14 , the security thread  12  displays a tilt image that, from three different viewing directions  30 A,  30 B,  30 C, presents to the viewer in each case a different target image  18 A,  18 B or  18 C. Here, the target images  18 A- 18 C each display a motif that is formed from visually perceptible and contrasting metallic motif portions  20  and demetalized motif portions  22 A,  22 B,  22 C. 
     Specifically, the window security thread  12  of the exemplary embodiment displays, when viewed obliquely  30 A from above, a sequence of euro symbols  22 A against a shiny metallic background  20 , while when viewed perpendicularly  30 B, a sequence of crest motifs  22 B is visible against a shiny metallic background  20 , and when viewed obliquely  30 C from below, a sequence of numeral motifs  22 C in the form of the denomination “10” is visible against a shiny metallic background  20 . Upon tilting the banknote, the appearance of the window security thread  12  in the window regions  14  changes back and forth between the three target images  18 A,  18 B and  18 C depending on the viewing direction. 
       FIG. 2  shows, schematically, the structure of the window security thread  12  in  FIG. 1  in cross section. The window security thread  12  comprises a carrier  32  in the form of a transparent plastic foil, for example a PET foil. The top of the carrier  32  is furnished with a lens grid in the form of a plurality of parallel plano-convex cylindrical lenses  34  that have a radius of curvature R=4 μm and a lens diameter d ML =7 μm and are arranged in a lens grid having a lens period of L=8 μm. In the exemplary embodiment in  FIG. 2 , the lens grid adjoins air such that the cylindrical lenses having n lens =1.5 and n air =1 have a focal length of f=3R=12 μm. 
     On the bottom of the carrier  32  is formed, composed of aluminum, a motif layer  40  that comprises demetalized sub-regions  42  spaced apart in the grid of the cylindrical lenses  34 . The carrier  32 , the cylindrical lenses  34  and the motif layer  40  are coordinated with each other in such a way that the motif layer  40  is located in the focal plane of the cylindrical lenses  34 . 
     For illustration,  FIG. 2  shows a section of the lenticular image in which the motif layer  40  includes demetalized sub-regions  42  only in the regions  44 B that are visible when viewed perpendicularly  30 B. The regions  44 A and  44 C that are visible when viewed obliquely from above (viewing direction  30 A) or obliquely from below (viewing direction  30 C) have no demetalizations in the displayed section such that from these directions, in each case, the viewer views metal regions of the motif layer  40 . Although the individual demetalized sub-regions  42  constitute narrow strips arranged in the grid of the cylindrical lenses, due to the focusing effect of the cylindrical lenses  34 , they assemble to compose the desired sequence of motifs  18 A- 18 C when viewed from the different viewing directions. 
     Due to the small dimensions of the cylindrical lenses  34 , a large number of metallic or demetalized sub-regions interact in each case in reconstructing the motifs  18 A- 18 C. For example, at a height of the demetalized motif portions  22 A- 22 C of 2 mm and a lens period of the cylindrical lenses of L=8 μm, the demetalized sub-regions  42  that participate in the reconstruction of the “euro symbol,” “crest” and “number string 10” motifs are distributed over an area of the motif layer  40  that is covered by 2 mm/8 μm=250 cylindrical lenses. 
     As likewise depicted in  FIG. 2 , the window security thread  12  typically includes further layers, such as a contiguous ink layer  45 , which permits a coloring of the demetalized motif portions  22 A- 22 C, an opaque white layer  46  and a heat seal coating layer  48 . However, said layers or other functional layers are not significant for the present invention and are thus not described in greater detail. 
     In designing the motif image of a lenticular image for depicting three target images, it has proven to be particularly advantageous when the line width D real  of the demetalized sub-regions  42  is substantially one-third of the diameter d ML  of the microlenses  34 . Analogously, the advantageous line width of the demetalized sub-regions in a lenticular image for depicting two target images is substantially half of the microlens diameter, and generally for a number n of target images to be depicted, substantially an n-th of the diameter d ML  of the microlenses. In this way, on one hand, the available area of the motif layer is used to optimum advantage, and on the other hand, a clearly defined jumping around between the different target images is achieved when the lenticular image is tilted. 
     Conventionally, to achieve said advantageous line width, the motif layer  40  is, for example, scanned from different angles with a finely focused laser beam until sub-regions  42  of the desired width are demetalized, or, to increase the process speed, the motif layer is arranged outside the focal plane of the microlenses  34  such that, upon laser demetalization, an expanded and thus wider image of the incident laser radiation results in the plane of the motif layer. However, both variants have disadvantages as regards the process duration or the quality of the target images produced, as already explained above. 
     To remedy this, the solution according to the present invention uses the wavelength-dependent resolving power of the optical system formed by the microlenses to obtain, without defocusing, through a targeted selection of the wavelength of the laser radiation used for the demetalization, a desired line width. 
     To explain the principle used in greater detail, with reference to  FIG. 3 , due to diffraction effects, even a parallel light beam  50  is not imaged to a point or, in the case of cylindrical lenses, to an infinitely narrow line by the microlenses  34 , but produces an Airy disk or an elongated diffraction line  52  having a diameter
 
 D (λ)=2.44*λ* f/d   ML   (1)
 
where λ represents the light wavelength, d ML  the diameter of the microlenses and f the focal length of the microlenses. The variable D is also referred to as resolving power, since two points are just barely resolvable by an optical system when their Airy disks (or diffraction lines in the case of cylindrical lenses) overlap each other halfway. Thus, the diffraction-limited resolving power of the optical system of the microlenses  34  itself results, even in the case of optimum focusing of the incident laser radiation, in a certain laser-wavelength-dependent expansion of the focus region.
 
     While the limited resolving power is traditionally viewed mostly as a limitation and as disadvantageous, the present invention deliberately uses the wavelength-dependent size of the diffraction spot to easily produce demetalizations of a desired line width in the focal plane and thus at maximum image sharpness. 
     Specifically, for example in the exemplary embodiment in  FIG. 2 , the initially still contiguous metallic motif layer  40  of the lenticular image is to be furnished with demetalized sub-regions to produce the target images  18 A- 18 C. Since there are to be three image regions  44 A- 44 C under each microlens  34 , a target line width of
 
 D   target   =d   ML /3=2.3 μm
 
is chosen for the demetalized sub-regions  42 . The equation (1) given above for the diameter D of the diffraction spot  52  can be solved for wavelength using the desired value of the line width D target  for the diameter of the diffraction spot  52  in order to obtain an ideal target laser wavelength:
 
λ target =0.41 *D   target   *d   ML   /f   (2)
 
     With a target line width of D target =2.3 μm, the lens diameter d ML =7 μm and the focal length of the microlenses f=12 μm, equation (2) results in a target laser wavelength of λ target =550 nm. 
     Thus, as an easily available marking laser source, a frequency-doubled Nd:YAG laser having a wavelength of λ=532 nm is chosen for the demetalization. At this wavelength, according to equation (1), the diameter of the Airy disk is D=2.2 μm and thus, with a difference of only about 4%, corresponds substantially to the desired target line width D target =2.3 μm. 
     When demetalizing, it can further be taken into account that, in practice, the exact value of D calculated according to equation (1) does not always result for the demetalized line width D real , but rather that the actually achieved line width additionally depends slightly on the laser power used. Specifically, especially that region of the focused laser beam in which the laser intensity exceeds the threshold required to demetalize the metallic motif layer is decisive for the demetalization. Since the laser intensity at the edge of the diffraction spot drops very sharply, only a small variation of the actual line width D real , which, however, in practice is suitable for fine control, can be achieved by increasing or decreasing the laser intensity. 
     In addition to the line width adjustment achieved through the wavelength-dependent resolving power, also the wavelength dependence of the refractive index n of the lens material can be used to achieve a further variation and especially an enlargement of the line width. In this way, with the refractive index n of the lens material, which generally varies depending on the wavelength, also the focal length f of the microlenses used varies depending on the wavelength of the incident radiation. 
     In the present invention, the demetalization occurs in such a way that, in a desired view of the security element in the visible spectral range, the metallic motif layer lies substantially in the focal plane of the microlenses. If the microlenses are impinged on, for example, with an IR laser (so for example a Nd:YAG laser having λ=1064 nm), then, depending on the material used for the microlenses, an additional widening of the lines can result in that the focal length at 1064 nm already differs significantly from the focal length in the visible spectral range. Thus, when the metallic motif layer is impinged on with laser radiation, similar conditions are present as in the known method described above, in which the motif layer is deliberately arranged outside the focal plane of the microlenses. Unlike in this known method, however, in the present invention, an arrangement lies “outside the focal plane” only at the wavelength used for demetalization. 
     After selecting the marking laser source and defining the laser intensity to be used for the demetalization (and, if appropriate, the refractive index of the lens material), the metallic motif layer  40  is impinged on through the microlenses  34  with laser radiation from three irradiation directions  30 A,  30 B,  30 C in the form of the motifs  18 A- 18 C to produce the desired demetalized sub-regions  42  in the metallic motif layer  40 . 
     If, in the lenticular image in  FIG. 2 , demetalizations having different line widths are to be produced in the metallic motif layer  40 , then, as easily available laser sources, for example also a Nd:YAG laser having λ=1064 nm and a focal width of D=4.4 μm, a frequency-tripled Nd:YAG laser having λ=355 nm and a focal width of D=1.5 μm, or also an Er:glass laser having λ=1540 nm and a focal width of D=6.4 μm can be used. By using two or more different laser sources of different wavelengths, different sized line widths can also easily be used in one security element. 
     In a second concrete exemplary embodiment, the lenticular image  60  shown in  FIG. 4  is to be furnished with two target images that become visible when viewed obliquely from above (viewing direction  30 A) or obliquely from below (viewing direction  30 C). 
     The top of the carrier  62  is furnished with a lens grid in the form of a plurality of parallel plano-convex cylindrical lenses  64  that have a radius of curvature R=4 μm and a lens diameter d ML =7 μm and are arranged having a lens period of L=8 μm. In the exemplary embodiment, the lens material of the cylindrical lenses  64  has a refractive index n lens =1.6, and the refractive index of the carrier foil  62  is n foil =1.64. In addition, the cylindrical lenses  64  are embedded in an embedding layer  66  having a refractive index n embedding =1.33. 
     On the bottom of the carrier are arranged, as in the exemplary embodiment in  FIG. 2 , a metallic motif layer  40 , a contiguous ink layer  45 , an opaque white layer  46  and a heat seal coating layer  48 . 
     Since there is to be space for two image regions under each microlens, in the present exemplary embodiment,
 
 D   target   =d   ML /2=3.5 μm
 
is chosen as the target line width for the demetalized sub-regions  42  to be produced. To calculate the target laser wavelength with the aid of the equation (2) specified above, also the focal length of the microlenses  64  is needed, which in the present, embedded case results in
 
 f=n   foil /( n   lens   −n   embedding )* R= 24.3 μm.
 
     With the aid of equation (2), from this data, a target laser wavelength of λ target =410 nm results. 
     For the demetalization, in this case, as an easily available marking laser source, a frequency-tripled Nd:YAG laser having a wavelength of λ=355 nm is chosen. Since the diameter of the Airy disk at said wavelength has, according to equation (1), a somewhat smaller diameter (D=3.1 μm) than the target line width (11% difference), when demetalizing, the marking laser source is operated with high laser intensity to make the demetalized line width D real  somewhat larger and to approach the target line width. 
     If, in the lenticular image in  FIG. 4 , demetalizations having other line widths are to be produced in the metallic motif layer, then also for example a Nd:YAG laser having λ=1064 nm and a focal width of D=9.0 μm, a frequency-doubled Nd:YAG laser having λ=532 nm and a focal width of D=4.7 μm, or an Er:glass laser having λ=1540 nm and a focal width of D=13.0 μm can be used.