Patent Publication Number: US-8528941-B2

Title: Security element having a laser marking

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a security element for security papers, value documents and the like, having a laser-markable transparent or translucent marking layer into which, through the action of laser radiation, visually perceptible identifiers in the form of patterns, letters, numbers or images are introduced. The present invention also relates to a security paper and a data carrier having such identifiers, and a manufacturing method for a corresponding security element, security paper or a corresponding data carrier. 
     2. Description of the Related Art 
     Identification cards, such as credit cards or personal identity cards, have long been provided with an individual identifier by means of laser engraving. In marking by laser engraving, through suitable guidance of a laser beam, the optical properties of the card material are irreversibly changed in the form of a desired marking. For example, in publication DE 30 48 733 A1 is described an identification card having applied information and exhibiting, on one surface, different colored layer regions that are stacked and that are at least partially interrupted by visually perceptible personalization data. 
     In addition to identification cards, also other value documents that are at risk of counterfeiting, such as banknotes, stocks, bonds, certificates, vouchers, checks, admission tickets, but also security elements for application to such data carriers, are often provided with laser-generated, individualizing marks, such as a serial number. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a security element of the kind mentioned above, exhibiting laser-generated identifiers of high counterfeit security. To further increase the security and perceptibility, the identifiers are intended to exhibit especially a viewing-angle-dependent visual appearance. 
     This object is solved by the security element having the features of the main claim. A security paper, a data carrier and a corresponding manufacturing method are specified in the coordinated claims. Developments of the present invention are the subject of the dependent claims. 
     According to the present invention, the identifiers of a generic security element each comprise a lamellar structure composed of a plurality of substantially parallel lamella that extend into the depth of the marking layer and that are characterized by the parameters color, width, height, lateral orientation, tilt angle and spacing. 
     Here, according to a preferred variant of the present invention, the marking layer is arranged on an opaque base layer whose intrinsic color is at least partially perceptible when viewed parallel to the lamella of a lamellar structure. According to another likewise preferred variant of the present invention, the marking layer is arranged on a transparent or translucent base layer such that the security element is at least partially light-transmitting when viewed parallel to the lamella of a lamellar structure. The security element can then be used, for example, over a transparent region of a data carrier, or it can, at certain viewing directions, reveal the view of a data carrier lying thereunder. 
     The marking layer can also be arranged between the base layer and an effect ink layer, since, as explained in greater detail below, the identifier need not begin on the surface of the security element, but rather, through suitable choice of the laser parameters, can also be introduced in a deeper region of the security element. For this, the wavelength, intensity and focus of the laser radiation, for example, are set such that the threshold for a visually perceptible change in the laser-exposed material is exceeded only in the desired layer depth. 
     In an advantageous embodiment, the lamellar structures of different identifiers differ at least in their lateral orientation in order to achieve a different visual appearance upon a rotation of the security element. 
     Additionally or alternatively, the lamellar structures of different identifiers can differ at least in their tilt angle in order to achieve a different visual appearance upon a tilting of the security element. 
     The lamellar structures of different identifiers can also differ in at least one of the parameters color, width, height and spacing to produce regions having a different visual appearance within the security element. These parameter differences can be combined with different tilt angles or different lateral orientations. The lamellar structures of different identifiers can, for example, also be staggered. 
     According to a preferred embodiment of the present invention, the height of at least a portion of the lamella is less than the layer thickness of the marking layer. Here, the height of the lamella can be set as desired by controlling the laser energy. In particular, the lamella can begin at the base layer and reach up to a maximum height that is less than the layer thickness of the marking layer. 
     It is also possible to have the lamella begin at a certain height above the base layer such that, upon movement of the security element, additionally, a parallax effect occurs. This can be achieved, for example, through different laser sensitivities in different layer regions of the marking layer, or through a pre-sensitization of the material of the marking layer in some regions. The latter approach even makes it possible to easily introduce parallax images into a homogeneous layer. For this purpose, through a first lasering, visually substantially non-perceptible, pre-sensitized regions are produced in the marking layer. Here, the pre-sensitized regions can especially themselves be developed in the form of lamellar structures. Through a second lasering from another irradiation direction, visually perceptible identifiers are then produced in the overlap region within the pre-sensitized regions. 
     In the plane of the marking layer, the lamella can be formed in the shape of straight lines, curved lines, broken lines and/or in the shape of lines having a varying width. 
     According to a further advantageous embodiment of the present invention, color areas, especially gray areas, are arranged between the lamella of at least one identifier. These color areas do not change their color or gray value upon rotation and/or tilting of the security element. 
     It can further be provided that the identifiers of the marking layer yield, together with other identifiers of the security element, especially with identifiers imprinted on the security element, an aggregate piece of information. The aggregate piece of information is then perceptible only from certain viewing angles. 
     Instead of a single marking layer, also multiple marking layers can be provided, each having a layer thickness between about 50 μm and about 300 μm. The marking layers can also be spaced apart such that identifiers can be produced at different depths of the security element. For at least partially identical identifiers at different depths, a parallax effect is likewise created, since the identical identifiers are precisely stacked only from a certain viewing direction, while the identifiers appear broadened or duplicated from other viewing directions. 
     The lamella advantageously exhibit a height between about 50 μm and about 150 μm. Their width is limited downwards by the focus diameter of the laser beam and is preferably between about 20 μm and about 150 μm, particularly preferably between about 70 μm and about 120 μm. 
     The present invention also comprises a security paper for manufacturing security or value documents, such as banknotes, checks, identification cards, certificates or the like, and a data carrier, especially a branded article, a value document or the like. The security paper or the data carrier exhibits a laser-markable transparent or translucent marking layer into which, through the action of laser radiation, visually perceptible identifiers in the form of patterns, letters, numbers or images are introduced. The identifiers each comprise a lamellar structure composed of a plurality of substantially parallel lamella that extend into the depth of the marking layer and that are characterized by the parameters color, width, height, lateral orientation, tilt angle and spacing. 
     For this, the security paper or the data carrier can either be furnished with a security element of the kind described above, or itself provided with such identifiers. Also in the latter case, the identifiers are advantageously developed in the manner already described in greater detail above. 
     The present invention further comprises a method for manufacturing a security element, security paper or data carrier having a laser-markable transparent or translucent marking layer in which, through the action of laser radiation, visually perceptible identifiers in the form of patterns, letters, numbers or images are introduced into the marking layer. The identifiers are each formed having a lamellar structure composed of a plurality of substantially parallel lamella that extend into the depth of the marking layer and that are characterized by the parameters color, width, height, lateral orientation, tilt angle and spacing. 
     For marking, an infrared laser in the wavelength range between 0.8 μm and 3 μm, especially a Nd:YAG laser, or a related laser, such as a Nd:glass laser, a Nd:YVO 4  laser or the like, is preferably used. The identifiers are expediently introduced with pulsed laser radiation, for example with an output between 3 W and 150 W, preferably between 3 W and 50 W. 
     The lamellar structures according to the present invention can be developed to be very fine and be produced very precisely by the high precision of the beam control. The freedom of the beam control facilitates high variability of the producible identifiers, which give the designer great freedom of design. As explained, the identifiers can also be introduced into a security element, security paper or a data carrier subsequently and depth-selectively through already existing layers. 
     The material for the laser-markable marking layer and the laser radiation used for marking are optimally coordinated. For example, suitable laser-markable plastics, such as polyethylene (PE), polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyproyplene (PP) and polyamide (PA), are known to the person of skill in the art. Further, the plastic can be stretched monoaxially or biaxially. The stretching of the plastic causes it, among other things, to gain polarizing properties that can be used as a further security feature. The aids required to take advantage of these properties, such as polarization filters, are known to the person of skill in the art. 
     Furthermore, the marking layer can also include additives that absorb laser radiation very well, such as TiO 2  or infrared absorbers, to be able to introduce the markings at low beam intensity. 
     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 omitted in order to improve their clarity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an identification card having a security element having a blind image according to an exemplary embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the security element in  FIG. 1 ; 
         FIG. 3  is a top view of the security element in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of a security element according to another exemplary embodiment of the present invention; 
         FIGS. 5   a  and  5   b  are top views of two examples of security elements having two different lamellar structures; 
         FIG. 6  is a cross-sectional view of a security element according to a further exemplary embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of a security element according to a further exemplary embodiment of the present invention; 
         FIG. 8  is a cross-sectional view of a security element according to a further exemplary embodiment of the present invention; 
         FIG. 9  is a cross-sectional view of a security element according to the present invention in which, as the identifier, a parallax image is introduced into the marking layer; and 
         FIGS. 10   a  through  10   d  are top views of different lamellar structures according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be explained using an identification card as an example. For this,  FIG. 1  shows, schematically, an identification card  10  that typically includes a portrait of the cardholder and further data that is not depicted in the figure. Furthermore, for safeguarding authenticity, the identification card  10  is provided with an inventive security element  12  having a blind image that displays a different visual appearance depending on the viewing direction of the viewer. 
     For this, as becomes clear when looking at the cross-sectional diagram in  FIG. 2  and the top view in  FIG. 3  together, the security element  12  exhibits a transparent marking layer  14  into which, through the action of laser radiation, at least one visually perceptible identifier  16  in the form of patterns, letters, numbers or images is introduced. 
     The identifier  16  exhibits a lamellar structure  18  composed of a plurality of substantially parallel lamella  20  that extend into the depth of the marking layer  14  and that is especially characterized by the parameters color, width, height, lateral orientation, tilt angle and spacing of the lamella  20 . For example, in the exemplary embodiment in  FIG. 2 , the lamella  20  exhibit a width of about 100 μm, a spacing of about 120 μm, a tilt angle of about 50° and a height of about 150 μm. In the simplest case, only a lamellar structure is provided in the security element, and the lamella  20  of the lamellar structure  18  exhibit a uniform lateral orientation, as perceptible, for example, in the top view in  FIG. 3 . 
     The transparent marking layer  14  is arranged on an opaque base layer  22  whose intrinsic color differs considerably from the color of the lamella. For example, the base layer  22  can be formed by a white opaque card foil from which the lamella  20  stand out in contrast as laser-induced blackenings of the marking layer  14 . 
     If the security element  12  is now viewed from one viewing direction  24  parallel to the lamella  20 , then the white intrinsic color of the base layer  22  is easily perceptible between the black lamella  20 . From this viewing direction, the white and black regions alternate in rapid succession such that, for the viewer, the impression is created of a uniformly gray area whose brightness depends on the chosen ratio of lamella width to lamella spacing. 
     From other viewing directions, such as the viewing direction  26 , the tilted lamella  20  block the view of the base layer  22 , as with a blind, such that the viewer perceives only a uniformly black area. 
     The surroundings  28  of the identifier  16  can be developed in a gray tone that corresponds to the gray tone of the identifier  16  at a certain viewing angle such that the identifier  16  is not perceptible from this viewing angle. By tilting the security element  12 , the image information of the identifier  16  can be made to appear or to disappear. 
     After this explanation of the basic principle of the present invention, in the following figures, more complex exemplary embodiments having multiple lamellar structures and/or having additional elements will now be described: 
     As shown in cross section in  FIG. 4 , the security element  30  according to another exemplary embodiment of the present invention includes a marking layer  32  having a first lamellar structure  34  that is characterized by a first set of parameters, and a second lamellar structure  36  that is characterized by a second set of parameters. Here, the first and second lamellar structure  34  or  36  differ in at least one of their characteristic parameters in order to produce a different visual appearance from different viewing directions. 
     As in the exemplary embodiment in  FIG. 2 , the base layer  38  of the security element can be opaque or also transparent or translucent. In the latter case, the security element  30  is partially transparent in viewing directions parallel to one of the lamellar structures  34 ,  36 . This can be used to advantage, for example, for a transmitted light effect, or also only to make visible through the security element  30  a data carrier lying thereunder. 
     Two examples of security elements each having two different lamellar structures are depicted in the top views in  FIGS. 5(   a ) and  5 ( b ). 
     In the security element  40  in  FIG. 5(   a ), at least the lateral orientation of the tilted lamella  42  and  44  differs such that the visual appearance of the inscribed identifier changes upon rotation of the security element  40 . If the viewer looks at the security element, for example, from the viewing direction  46 , then he looks parallel to the tilted lamella  42  and thus, in sub-regions, at the base layer arranged beneath the marking layer. The interior of the identifier “ 10 ” thus appears having a first brightness in a first color. This first image impression can especially be chosen as desired through the color of the base layer and the color, width and spacing of the lamella  42 . From the viewing direction  46 , the tilted lamella  44  shade the base layer for the viewer such that the surroundings of the identifier “ 10 ” appear having a second brightness in a second color, this second image impression being given substantially only by the color of the lamella  44 . 
     Seen from the viewing direction  48 , the situation reverses. The viewer now looks parallel to the lamella  44  and thus partially at the base layer, while the lamella  42  block the view of the base layer. In this way, the appearance of the security element  40  changes upon rotation in a predefined manner. 
     The security element  50  in  FIG. 5(   b ) includes two lamellar structures whose lamella  52 ,  54  exhibit the same lateral orientation, but include different tilt angles with the surface normal. In this way, the security element  50  constitutes a tilt image whose visual appearance changes upon tilting about a tilt axis parallel to the lamella. For example, the lamella  52  can exhibit a tilt angle of +30°, the lamella  54  a tilt angle of −40° to the surface normal. 
     If the viewer looks at the security element from the viewing direction  56 , then he looks parallel to the lamella  52  tilted toward him and thus, in sub-regions, at the base layer lying beneath the marking layer. The interior of the identifier “ 10 ” thus appears having a first brightness in a first color. This first image impression can, again, especially be chosen as desired through the color of the base layer and the color, width and spacing of the lamella  52 . 
     From this viewing direction, however, the lamella  54  tilted away from the viewer shade the base layer for the viewer such that the surroundings of the identifier “ 10 ” appear having a second brightness in a second color, the second image impression being given substantially only by the color of the lamella  54 . 
     Seen from the viewing direction  58 , the situation reverses, since the viewer now looks parallel to the lamella  54  and thus partially at the base layer, while the lamella  52  block the view of the base layer. In this way, the appearance of the security element  50  changes upon tilting in a predefined manner. 
     In the security element  60  in  FIG. 6 , between the lamella  62  of an identifier are arranged gray areas  64  that retain their gray value independent of the rotation or tilt of the security element  60 . Such gray areas, or more generally also any color areas, can be combined with all described lamellar structures. 
     The exemplary embodiment in  FIG. 7  shows a security element  70  having lamella of different heights. Here, the higher lamella  72  require smaller tilt angles than the lower lamella  74  in order to shade the base layer  76 . The different height of the lamella can be set at will through corresponding control of the laser energy. 
       FIG. 7  also illustrates a further advantage of the blind images according to the present invention. The wavelength and intensity of the laser radiation can, namely, be so chosen and coordinated with the properties of existing layers, such as an applied printing layer  78 , that the lamella, such as the lamella  72  and  74 , can be introduced through these layers into deeper plies of the security element without ablating the existing layers. The identifiers according to the present invention can thus also be used for the subsequent personalization or individualization of security elements or data carriers. For the laser impingement, for example infrared radiation of a pulse-operated Nd:YAG laser with an output between 3 W and 50 W can be used. 
     The blackening of the marking layer can also occur through an effect layer, as illustrated in  FIG. 8 . In the exemplary embodiment in  FIG. 8 , the marking layer  82  of the security element  80  is arranged between a base layer  86  and an effect ink layer  88 . The effect ink layer  88  can include, for example, optically variable interference pigments, thermal inks or the like. 
     For the impingement of the sub-region  90 , the wavelength, intensity and focus of the laser radiation are chosen such that the marking layer  82  is provided with lamella  84  without destroying the effect ink layer  88 . In the sub-region  90  marked in this way, then both the viewing-angle-dependent identifier and the optically variable effect of the effect ink layer are present. Of course the laser marking  92  can also be so executed in other sub-regions  94  that the effect ink layer  88  is destroyed locally such that no optically variable effect is perceptible there any longer. 
     In the exemplary embodiment  100  in  FIG. 9  is introduced into the marking layer  102 , as the identifier, a parallax image in which the blackenings  104  are located at a certain height h above the base layer  106 . Thus, upon movement of the security element, in addition to the described tilt or rotation effect, a parallax effect occurs due to the movement of the blackenings  104  against the background of the distanced base layer  106 . 
     According to the present invention, this particular identifier is produced in that the material of the marking layer  102  is pre-sensitized by a first lasering from a certain irradiation direction. The pre-sensitized regions, which are marked in  FIG. 9  with the reference number  108 , are not visually perceptible themselves, but the threshold for producing a visible laser marking is reduced in them. The pre-sensitized regions advantageously include a tilt angle of about 30° to about 50° with the surface normal. 
     Through a second lasering  110  from a different, second irradiation direction, the material of the marking layer  102  is now blackened in the overlap areas with the pre-sensitized regions  108 . Here, through a suitably set laser intensity, it can be ensured that, in the non-pre-sensitized regions, the material is not visually changed by the second lasering. Overall is created in this way a lamellar structure  104  whose vertical position within the marking layer can be chosen largely freely through the relative position of the two laserings. 
     The first and second lasering can occur simultaneously such that a higher laser intensity prevails in the overlap region of the laser beams than outside. Here, the laser intensities are chosen such that, individually, they are not sufficient to blacken the material, but the higher laser intensity in the overlap region is above the blackening threshold. 
     However, the second lasering can also occur temporally after the first lasering. In this case, without the precise processes in the material being important for the present invention, through the first lasering, the blackening threshold of the material is reduced in the pre-sensitized regions, wherein this change itself is not visually perceptible. For the second lasering, the laser intensity is now chosen such that it is above the blackening threshold of the pre-sensitized material, but below the blackening threshold of the non-modified material. In this way, too, the desired effect is achieved. 
     In all described exemplary embodiments, the lamella of the identifiers can be developed to be straight and having a constant width.  FIG. 10(   a ) shows a top view of such a lamellar structure having straight lamella  110  and a constant width b. The achievable width of the lamella is given downwards by the focus diameter of the laser used for marking. The focus diameter is typically between 20 μm and 150 μm, preferably between 70 μm and 120 μm, such that corresponding lamella widths b result. In the same way, lamellar structures can be used that form curved lines in the plane of the marking layer. 
       FIG. 10(   b ) and  FIG. 10(   c ) schematically show exemplary embodiments of lamellar structures having lamella  112  and  114  having a changing width. In this way, upon viewing parallel to the lamella, the visible portion of the base layer changes such that the brightness impression varies along the lamella. It is understood that, unlike in the schematic diagram in  FIG. 10(   b ) and  FIG. 10(   c ), in real exemplary embodiments, this change typically takes place on a considerably larger length scale compared with the spacing of adjacent lamella. 
     Through suitable line shapes or suitably chosen spacings of the lamella, the blind images according to the present invention can, at certain viewing angles, also depict a halftone image. For example, the different gray levels of a halftone image can be produced by lamellar structures  116 ,  118 ,  120  having parallel lamella having different spacings between the lamella, as illustrated in the left half of the image in  FIG. 10(   d ). Alternatively or additionally, different gray levels can be produced by lamella of different widths in the lamellar structures  122 ,  124 , as shown on the right in  FIG. 10(   d ). 
     Any predefined halftone image can easily be depicted with such lamellar structures in that, for example, a small areal region of the security element is associated with each halftone image point, and this areal region is provided with a lamellar structure that corresponds to the brightness of the halftone image point. Upon viewing from a viewing direction parallel to the lamella, the halftone image is then perceptible, and from other viewing directions from which the lamella block the view of the base layer, merely a uniformly colored area is shown.