OPTICALLY VARIABLE SECURITY ELEMENT, DATA CARRIER AND MANUFACTURING METHOD

Description

[0001] The invention relates to an optically variable security element for securing security documents, valuables, and other data carriers, comprising a lenticular image that displays at least two different appearances from different viewing angles. The invention also relates to a method for manufacturing such a security element and a data carrier equipped with such a security element.

[0002] Data carriers, such as valuables or identification documents, but also other valuables like branded goods, are often equipped with security features to ensure authenticity. These features allow verification of the carrier's authenticity and simultaneously serve as protection against unauthorized reproduction. Security features with viewing-angle-dependent effects play a particularly important role in authenticity assurance, as these cannot be reproduced even with the most modern copying equipment. These security features are equipped with optically variable elements that present a different visual impression to the viewer from different angles, displaying, for example, a different color or brightness impression and / or a different graphic motif depending on the viewing angle.

[0003] In this context, it is known to protect data carriers by applying laser-engraved reversible images. Two or more different markings, such as a serial number and an expiration date, are laser-engraved into the data carrier at different angles using an arrangement of cylindrical lenses. The laser radiation creates a local darkening of the data carrier, making the engraved markings visually apparent. When viewed from a particular angle, only the marking engraved from that direction is visible, so that tilting the data carrier perpendicular to the axis of the cylindrical lenses creates an optically variable reversible effect.

[0004] Well-known lens-shift images are often based on lens foils that have a color print and a full-surface metallization on the side opposite the lenses, with the color print and the metallization together forming a single image. When such a lens foil is illuminated with laser radiation from a specific angle, the focusing effect of the lenses concentrates the laser beam into a focal spot in the plane of the printed image, causing a portion of the color and / or the metallization to ablate.

[0005] One difficulty lies in the fact that the laser intensity must be adjusted so that in one area only the metal is ablated, while in another area the metal is ablated along with the paint. Modulating the laser energy to suit the specific conditions is not possible. Furthermore, the paint used must be appropriately selected to absorb the laser radiation as effectively as possible.

[0006] Consequently, this method of producing lens-shift images sometimes results in incomplete ablation of the color or metal, leading to an unsatisfactory visual impression. Furthermore, the metal at the edges of the laser beam is often not ablated, but rather its structure is altered in such a way that it can no longer be removed, resulting in an undesirable string-of-pearls effect.

[0007] An example of a conventional lens tilt image with metallic and printed representation is described in publication WO 2017 / 097430 A1.

[0008] EP 3 967 508 A1 discloses an optically variable security element for securing security papers, valuable documents and other data carriers, with an image that shows at least two different appearances from different viewing directions, wherein the image contains a radiation-sensitive, color-shifting motif layer, the radiation-sensitive, color-shifting motif layer contains a plurality of modification areas generated by radiation exposure, the unmodified original areas and the modification areas of the radiation-sensitive motif layer have a different visual appearance, the modification areas are arranged in the form of a predetermined motif, and wherein the radiation-sensitive, color-shifting motif layer is a three-layer system with a lower metallic reflective layer.comprising a middle dielectric spacer layer and an upper semitransparent layer made of a phase-change material which has a different refractive index in its crystalline and amorphous states, wherein the phase-change material is in an amorphous state in the modified regions and in a crystalline state in the original regions, or vice versa. EP 3 967 508 Al does not disclose a lens array.

[0009] Based on this, the invention aims to provide an optically variable security element of the type mentioned above with an attractive visual appearance. The invention also provides a method for manufacturing such a security element and a data carrier equipped with such a security element.

[0010] This problem is solved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0011] According to the invention, in a generic optically variable safety element, the lens grid image comprises a lens grid consisting of a plurality of microlenses and a radiation-sensitive, color-tilting motif layer spaced apart from the lens grid.

[0012] The radiation-sensitive, color-shifting motif layer in turn contains a multitude of modification areas created by radiation exposure, each precisely aligned with the microlenses of the lens array, as well as unmodified original areas.

[0013] The unmodified original areas, i.e., the areas of the radiation-sensitive, color-shifting motif layer not modified by radiation exposure, and the modified areas of the radiation-sensitive motif layer exhibit different visual appearances. Furthermore, the modified areas are arranged in the form of a predefined motif, which is visible through the lens grid when viewing the security element from a specific viewing angle.

[0014] The radiation-sensitive, color-shifting motif layer contains a three-layer system with a lower metallic reflective layer, a middle dielectric spacer layer, and an upper semi-transparent layer made of a phase-change material that exhibits different refractive indices in its crystalline and amorphous states. The phase-change material exists in an amorphous state in the modified regions and in a crystalline state in the original regions, or vice versa.

[0015] As explained in more detail below, the layer made of the phase change material takes on the role of the partially transparent absorber layer in a conventional three-layer system and, due to its special properties, allows the creation of image motifs with particularly high edge sharpness.

[0016] A particular advantage is that the phase-change material layer is located outside the focal plane of the microlenses. Specifically, this means that the distance of the phase-change material layer from the focal plane is between 5% and 30% of the microlens' focal length. Since the phase-change material is not ablated during the laser treatment used to create the modification zones, but only heated to a melt and thus brought to a phase transition, a position within the focal plane is unnecessary. Furthermore, the significantly lower energy input into the phase-change material layer resulting from the defocusing enables the creation of sharply defined modification zones and, consequently, the generation of edge-to-edge images.

[0017] The refractive index difference of the phase-change material in its crystalline and amorphous states is advantageously greater than 0.2, particularly greater than 0.4 or even greater than 0.6. Such a refractive index difference allows for good contrast or color differentiation between the original regions and the modified regions.

[0018] The phase change material preferably contains GeSbTe or AgInSbTe, in particular Ge₂Sb₂Te₅ or Ag₃In₄Sb₇6Te₁₇. However, other phase change materials, such as VO₃₆, NbO₃₆, GeTe, GeSb, GaSb, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, or AlSb, can also be used.

[0019] The phase-change material is preferably present in a layer thickness between 3 nm and 100 nm, particularly between 3 nm and 20 nm. With such a layer thickness, the phase-change material layer is well suited to act as an absorber layer in the color-shifting three-layer structure, and also exhibits a sufficiently large difference in appearance between its two material states.

[0020] In an advantageous embodiment, the lens array comprises or represents a one-dimensional arrangement of microlenses, in particular cylindrical lenses. It is also advantageous for the lens array to comprise or represent a two-dimensional arrangement of microlenses, in particular spherical or aspherical lenses.

[0021] For the purposes of this description, microlenses are defined as lenses whose size in at least one lateral direction is below the resolution limit of the naked eye. Microlenses can be cylindrical, but spherical or aspherical lenses are also suitable. The latter preferably have a diameter between 5 µm and 100 µm, particularly between 10 µm and 50 µm, and most preferably between 15 µm and 20 µm. Microcylindrical lenses preferably have a width between 5 µm and 100 µm, particularly between 10 µm and 50 µm, and most preferably between 15 µm and 20 µm. The length of the microcylindrical lenses is arbitrary; for example, when used in safety threads or transfer elements, it can correspond to the total width of the thread or transfer element and be several millimeters or several centimeters.

[0022] Several different motifs for different viewing angles can be specified for the optically variable security element. Accordingly, the radiation-sensitive, color-shifting motif layer can then contain several groups of modification areas, each visible from the corresponding specified viewing direction and generating the corresponding specified motif.

[0023] The invention also includes a method for manufacturing an optically variable safety element of the type described, in which a lenticular image with a lenticular grid and a spaced-apart radiation-sensitive, color-tilting motif layer is provided, wherein the radiation-sensitive, color-tilting motif layer comprises a three-layer system with a lower metallic reflective layer, a middle dielectric spacer layer, and an upper semi-transparent layer of a phase-change material, and wherein the phase-change material in the semi-transparent layer is either (i) entirely in an amorphous material state or (ii) entirely in a crystalline material state, and the radiation-sensitive, color-tilting motif layer is irradiated through the lenticular grid to generate modification regions in the phase-change material.in which the material state of the phase-change material is transformed from the amorphous to the crystalline material state in case (i) and from the crystalline to the amorphous material state in case (ii).

[0024] The radiation-sensitive, color-shifting motif layer, in the form of a pre-selected motif, is exposing the laser radiation to a predetermined viewing direction.

[0025] A particular advantage is achieved when the laser is defocused relative to the plane of the phase-change material during the generation of the modification zones, with a laser defocus of 0.25 mm to 3 mm proving especially effective. Specifically, a defocus of 0.5 mm to 1 mm, or even a significant defocus of 1.5 mm to 3 mm, can be advantageously selected for generating the modification zones.

[0026] For laser application to generate the modification areas, laser parameters in the following areas have proven particularly effective: a laser power of 0.38 to 1.15 W, a pulse duration of 85 ns to 250 ns, a Rayleigh length of 0.35 mm to 1.05 mm, a focus diameter of 0.01 mm to 0.03 mm, and a defocus of 0.25 mm to 3 mm, in particular of 0.5 mm to 1 mm.

[0027] Alternatively or in addition to defocusing the laser, the layer of phase change material can be arranged outside the focal plane of the microlenses, wherein the distance of the layer of phase change material from the focal plane is in particular between 5% and 30% of the focal length of the microlenses.

[0028] The invention also includes a data carrier, in particular a security document, a security paper, an identity card, a branded item or the like, with a security element of the type described.

[0029] Further embodiments and advantages of the invention are explained below with reference to the figures, in the representation of which a scale and proportion-accurate reproduction has been omitted in order to increase clarity.

[0030] They show: Fig. 1 a schematic representation of a banknote with an optically variable security element according to the invention, Fig. 2 schematically the layer structure of the security element of the Fig. 1 in cross-section, and Fig. 3 strongly schematically illustrates the laser application to the safety element of the Fig. 1 for generating the modification areas in the phase change material layer.

[0031] The invention will now be explained using the example of security features for banknotes. Figure 1 Figure 1 shows a schematic representation of a banknote 10 which is provided with a security element 12 according to the invention in the form of an affixed transfer element. Figure 2schematically shows the layer structure of the safety element 12 according to the invention in cross-section.

[0032] In the exemplary embodiment, the safety element 12 represents a lenticular image with a reversible image, which shows one of two different appearances 14A, 14B depending on the viewing direction. When viewed perpendicularly (viewing direction P in Figures 1 and 2 The security element 12 exhibits a uniform, metallic-glossy appearance 14A. When the security element is tilted 16, a motif-like appearance 14B becomes visible within a certain angular range around an oblique viewing direction (viewing direction S). In the exemplary embodiment, this motif displays the value "10" against a uniform metallic-glossy background. Furthermore, when the security element 12 is tilted back and forth 16, a slight color shift effect occurs in the reflected metallic-glossy color.

[0033] It is understood that the invention is not limited to the transfer elements shown for illustration purposes in banknotes, but can also be used, for example, in security threads, wide security strips, or cover films. Furthermore, in practice, security elements according to the invention can display not only alphanumeric character sequences, but also more complex motifs such as geometric patterns, portraits, codes, numbering, architectural, technical, or natural motifs.

[0034] Returning to the presentation of the Fig. 2 The security element 12 contains a transparent carrier film 20, for example a polyethylene terephthalate (PET) film approximately 20 µm thick. The carrier film 20 has opposing first and second main surfaces, the first main surface being provided with a lens grid 22 consisting of a plurality of microlenses 24 embossed in a varnish layer.

[0035] The embossed microlenses 24 can, for example, be designed as rod, cylinder, or spherical lenses arranged in a hexagonal or rectangular lens grid. A hot or UV embossing process can be used, in particular, to manufacture the microlenses 24.

[0036] On the second main surface of the carrier film 20, a laser-sensitive, color-shifting motif layer 30 is arranged, which comprises a three-layer system with a lower metallic reflective layer 32, a middle dielectric spacer layer 34, and an upper semi-transparent layer 36 made of a phase-change material. The terms "bottom" and "top" refer to the intended viewing direction of the motif layer on the top side of the security element 12, i.e., through the lens array 22.

[0037] In the exemplary embodiment, the metallic reflective layer 32 is opaque and forms a solid mirror with a metallic luster. The reflective layer 32 can, for example, be made of aluminum or silver and have a thickness of 10 nm or more, advantageously 20 nm or more.

[0038] The dielectric spacer layer 34 is advantageously made of SiO₂, but can also consist of ZnS, MgF₂, TiO₂, ZnO, or Al₂O₃. The thickness of the dielectric spacer layer 34 essentially determines the color impression of the color-shifting motif layer 30 and is typically between 100 nm and 500 nm.

[0039] The uppermost layer 36 of the three-layer system is unique in that it is formed by a thin layer of a phase-change material which has a different refractive index in its crystalline and amorphous material states.

[0040] For example, GeSbTe or AgInSbTe, in particular Ge₂Sb₂Te₅ or Ag₃In₄Sb₆Te₁₇, can be used as the phase change material. The thickness of the phase change material layer is typically between 3 nm and 100 nm, preferably between 3 nm and 20 nm. The phase change material layer acts as a partially transparent absorber layer, a role usually fulfilled by a thin chromium layer in conventional color-shifting thin-film systems.

[0041] In the illustrated embodiment, the phase-change material initially exists in an amorphous state, which can be transformed into a crystalline state by the application of heat, specifically by exposure to a laser beam. This change in the material state is accompanied by a change in the refractive index of the phase-change material and, consequently, a change in the interference color produced by the three-layer system.

[0042] In order to display the desired image motif 14B from the desired viewing direction, the safety element 12 was illuminated with laser radiation in the form of the desired image motif, in this case the value number "10", from the viewing direction S, from which the image motif 14B should later be visible. The shape of the image motif can be defined, for example, by a mask or by scanning the desired shape.

[0043] Due to the focusing effect of the microlenses 24, the phase-change material is locally heated above its melting point by the laser radiation and thereby transformed into its crystalline form in small modification regions 40. Since the laser beam passes through the microlenses 24, the resulting modification regions 40 are precisely aligned with the microlenses 24 of the lens array.

[0044] Due to the altered refractive index, the three-layer system consisting of reflective layer 32, dielectric spacer layer 34, and phase-change material layer 36 exhibits a visual appearance in the modified areas 40 after laser irradiation that differs from the visual appearance of the unmodified original areas 42. From the viewing direction S, the value "10" formed by the modified areas 40 is therefore clearly visible against the background formed by the unmodified original areas 42 (illustration 14B).

[0045] When viewed vertically (viewing direction P) and at other oblique viewing angles, the viewer sees exclusively unmodified original areas through the microlenses 24, so that the security element 12 appears homogeneous from these viewing directions (motif image 14A), although it exhibits the color shift effect of the three-layer system 32, 34, 36. It is understood that several motif images can be generated in the motif layer 30 in the same way, which become visible from different viewing directions.

[0046] A particular advantage of the design according to the invention is that the energy input required for the crystallization of the phase-change material by laser irradiation is significantly lower than the energy input required for the ablation of a metal layer. Compared to conventional designs whose motif generation is based on the ablation of a metal layer, this allows for the creation of motifs with significantly higher edge sharpness. When the safety element 12 is tilted 16, the viewer therefore sees a motif flip that is perfectly matched with a metallic-looking color flip.

[0047] Still with reference to Fig. 2The security element 12 can additionally contain a barrier layer 38 against oxygen or water between the carrier film 20 and the phase-change material layer 36. The barrier layer 38 can, for example, be formed by a thin dielectric layer of SiO₂ with a thickness between 3 nm and 50 nm. On its underside, the security element 12 can contain further layers, for example, a primer layer or an adhesive layer 26 for bonding the security element to a target substrate, such as the banknote 10.

[0048] It has proven particularly advantageous to arrange the phase change material layer 36 outside the focal plane of the microlenses 22. Figure 3Figure 1 schematically illustrates the laser application to the safety element 12 to generate the modification areas 40 in the phase-change material layer 36. The focal plane 50 of the microlenses 24 lies outside the phase-change material layer 36, so that the laser radiation 52 is slightly defocused in the plane of the phase-change material. This defocusing can be achieved, for example, by selecting the focal length of the microlenses 24 or the distance between the microlenses 24 and the phase-change material layer 36. The distance of the phase-change material layer 36 from the focal plane 50 is, in particular, between 5% and 30% of the focal length of the microlenses 24.

[0049] Unlike conventional designs based on the area-wise ablation of a metal layer, focusing the laser beam in the plane of the phase change material is not necessary in the designs according to the invention, since the phase change material is not ablated, but only heated into the melt and thereby brought to a phase transition.

[0050] Alternatively or additionally, the laser itself can be defocused. The resulting lower energy input into the phase-change material layer 36 (and thus heating of the phase-change material only to the melt) also allows for the creation of sharply defined modification regions. In one embodiment, the following laser parameters were used to generate the modification regions: a laser power of 0.76 W, a pulse duration of 170 ns, a Rayleigh length of 0.7 mm, a focus diameter of 0.02 mm, and a defocus of in the range of +0.5 to 1.0 mm. Reference symbol list

[0051] 10 Banknote 12 Security element 14A, 14B Appearances 16 Tilt direction 20 Carrier foil 22 Lens grid 24 Microlenses 30 Radiation-sensitive, color-tilting motif layer 32 Metallic reflective layer 34 Dielectric spacer layer 36 Phase-change material layer 38 Barrier layer 40 Modification areas 42 Unmodified original areas 50 Focal plane 52 Laser radiation

Claims

1. Optically variable security element (12) for safeguarding security papers, valuable documents and other data carriers (10), comprising a lens grid image showing at least two different appearances (14A, 14B) from different viewing directions, wherein - the lens grid image contains a lens grid (22) composed of a plurality of microlenses (24) and a radiation-sensitive, colour-shifting motif layer (30) arranged at a distance from the lens grid (22), - the radiation-sensitive, colour-shifting motif layer (30) contains a multiplicity of modification regions (40) generated by action of radiation, which are arranged in each case with register accuracy with respect to the microlenses of the lens grid, - the unmodified original regions (42) and the modification regions (40) of the radiation-sensitive motif layer (30) have a different visual appearance, - the modification regions (40) are arranged in the form of a predefined motif visible when the security element is viewed from a predefined viewing direction through the lens grid (22), and wherein - the radiation-sensitive, colour-shifting motif layer (30) comprises a three-layered layer system having a lower metallic reflection layer (32), a middle dielectric spacer layer (34) and an upper semitransparent layer (36) composed of a phase change material having a different refractive index in crystalline and amorphous material states, and - wherein the phase change material is present in the modification regions (40) in the amorphous material state and is present in the original regions (42) in the crystalline material state, or vice versa.

2. Security element according to Claim 1, characterized in that the layer composed of the phase change material lies outside the focal plane of the microlenses, wherein the distance between the layer composed of the phase change material and the focal plane is in particular between 5% and 30% of the focal length of the microlenses.

3. Security element according to Claim 1 or 2, characterized in that the difference in refractive index of the phase change material in crystalline and amorphous states is greater than 0.2, in particular greater than 0.4 or even greater than 0.6.

4. Security element according to at least one of Claims 1 to 3, characterized in that the phase change material contains GeSbTe or AgInSbTe, in particular Ge2Sb2Te5 or Ag3In4Sb76Te17.

5. Security element according to at least one of Claims 1 to 4, characterized in that the phase change material is present in a layer thickness of between 3 nm and 100 nm, in particular between 3 nm and 20 nm.

6. Security element according to at least one of Claims 1 to 5, characterized in that the lower metallic reflection layer is opaque.

7. Security element according to at least one of Claims 1 to 6, characterized in that the lower metallic reflection layer consists of aluminium or silver and / or has a layer thickness of 10 nm or more, in particular of 20 nm or more.

8. Security element according to at least one of Claims 1 to 7, characterized in that the middle dielectric spacer layer is formed from SiO2 and / or has a layer thickness of between 100 nm and 500 nm.

9. Security element according to at least one of Claims 1 to 8, characterized in that the lens grid and the radiation-sensitive, colour-shifting motif layer are arranged on opposite sides of a carrier film, in particular a transparent plastic film.

10. Security element according to at least one of Claims 1 to 9, characterized in that a barrier layer, preferably a thin dielectric layer having a layer thickness of between 3 nm and 50 nm, is arranged between the carrier film and the layer composed of the phase change material.

11. Security element according to at least one of Claims 1 to 10, characterized in that the security element contains a plurality of different motifs for different viewing angles, wherein the radiation-sensitive, colour-shifting motif layer contains a plurality of groups of modification regions, which are each visible from the associated predefined viewing direction and generate the associated motif.

12. Method for producing an optically variable security element (12) comprising a lens grid image showing at least two different appearances (14A, 14B) from different viewing directions, wherein in the method - a lens grid image comprising a lens grid (22) and a radiation-sensitive, colour-shifting motif layer (30) arranged at a distance is provided, wherein the radiation-sensitive, colour-shifting motif layer contains a three-layered layer system having a lower metallic reflection layer (32), a middle dielectric spacer layer (34) and an upper semitransparent layer (36) composed of a phase change material, wherein the phase change material in the semitransparent layer is present completely either (i) in the amorphous material state or (ii) in the crystalline material state, and - the radiation-sensitive, colour-shifting motif layer (30) is impinged on by laser radiation (52) through the lens grid (22) in order to generate modification regions (4) in the phase change material, in which regions the material state of the phase change material is converted from the amorphous to the crystalline material state in case (i) and from the crystalline to the amorphous material state in case (ii).

13. Method according to Claim 12, characterized in that the radiation-sensitive, colour-shifting motif layer in the form of a preselected motif is impinged on by the laser radiation from a predefined viewing direction.

14. Method according to Claim 12 or 13, characterized in that the laser is defocused relative to the plane of the phase change material.

15. Method according to at least one of Claims 12 to 14, characterized in that the layer composed of the phase change material is arranged outside the focal plane of the microlenses, wherein the distance between the layer composed of the phase change material and the focal plane is in particular between 5% and 30% of the focal length of the microlenses.

16. Data carrier comprising an optically variable security element according to any of Claims 1 to 11.