WO2018166653A1 - Security element with reflective colour filter properties - Google Patents

Abstract

The invention relates to a security element (2) with reflective colour filter properties for producing value documents, such as bank notes (1), cheques or similar, having a profile structure (6) with elevations (7) over a base surface (10) and having a metallic upper side. The elevations (7) have a cover surface (8) which is parallel to the base surface (10) and, when seen from the cross-section, each one has at least one step and are distributed in a quasi-static manner with respect to the position and/or the extension on the base surface (10). The step height (t) is between 100 nm and 350 nm.

Description

 S i c h e r i a n d i n d i o n t i o n s e rv e n t i o n s S e c tio n s

The invention relates to a security element with reflective color filter properties for the production of value documents, such as banknotes, checks or the like, which has a profile structure which has elevations over a base area and has a metallic upper side. The invention further relates to a production method for a security element for the production of value documents, such as banknotes, checks or the like, wherein a profile structure is produced which has elevations over a base surface and has a metallic upper side. Subwavelength gratings can be used to produce color effects at the finest spatial resolution. From the generic

DE 102011101635 A1 discloses a 2-dimensional periodic nanostructure which delivers colors in reflection and in transmission. This is an embossed structure which is vapor-deposited only with an aluminum layer. The disadvantage is that such nanostructures for series production with high structural integrity at low defect rate must be able to be molded. Known nanostructures for color effects often have an aspect ratio greater than 1 and periods of less than 300 nm. For these structures, the embossing process for defect-free duplication is a challenge.

Other so-called Aztec structures are known in the literature (see US 20080309996 A1, JJ Cowan, "Aztec surface-relief volume diffractive structure.", JOSA A, 7.8 (1990): 1529-1544). These staircase-shaped relief structures are arranged such that the light paths from adjacent ones Stairs form a path difference and interfere. This retardation leads to constructive for certain wavelengths to destructive for other wavelengths. The light interference is used in these writings to form volume holograms. Furthermore, it is known that phase holograms can also be formed therewith (compare US 4,888,260 Bl, E. Hasman et al., "Efficient multilevel phase holograms for C0 ₂ lasers", Optics letters, 16.6 (1991): 423-425). Furthermore, there are attempts to replicate by such structures the blue color of the Morpho butterfly (see JJ Cowan, "Advances in holographic replication with the Aztec structure", Proc. Internat. Conf Holography, (2006), S. Kinoshita et al., "Physics of structural colors.", Reports on Progress in Physics, 71.7 (2008): 076401). WO 2015188908 A1 describes a method for producing security elements with angle-independent structure colors. The colors are formed by randomly arranged microspheres which are coated. A. Yetisen et al., Color-Selective 2.5 D Holograms on Large-Area Flexible

Substrates for Sensing and Multilevel Security. ", Advanced Optical Materials, 4.10 (2016): 1589-1600, show the application of color-selective holograms as security features that are based on stair-shaped structures and can be duplicated in an embossing process on flexible substrates.

The invention has for its object to develop an embossed structure, which is easier to manufacture and yet shows a good color effect.

The invention is defined in claims 1, 6 and 7.

The security element provides reflective color filter properties that can be used to produce value documents, such as banknotes, checks or the like. can be used to protect against counterfeiting. It has a profile structure that has elevations over a base area. An upper side of the profile structure is metallic, whereby the entire profile structure can also be metallic. The elevations are each seen in cross-section at least one stage. They have a step height that is uniform at least in areas and is between 100 nm and 350 nm. The surveys are distributed in location and / or extent on the floor area quasi-statistically. In a preferred embodiment, the surveys are multi-level and have two to twenty levels. The one- or multi-stage properties of the elevations are also shown by the fact that the top surfaces of the elevations are parallel to the base area. In the case of a multi-level structure, the top surfaces in the steps correspond to the treads of a conventional staircase. Even the highest level of the survey is parallel to the base area.

The term "one-step" describes a survey that has only one step from the base to the single top surface, the number of steps being equal to the number of levels for top surfaces, for a single-level survey there is only one top surface at one level In a multi-level survey, there are several top surfaces, where the distance between successive top surfaces in a survey is always the same, in other words, the step height is constant within a survey.

The aperiodic and in particular quasi-statistical arrangement of the surveys with respect to position and / or extent on the base surface leads to surprisingly bright colors in reflection.

The surveys are at least partially uniform in terms of step height; optionally also with regard to the number of steps. From- stretching the surveys need not be uniform. It turned out that the step height influences the reflected color. The number of levels has an influence on the spectral broadband of the color reflection. A partial variation of the step height (and optionally the number of steps) makes it possible to create areas of different color reflections in the security elements and thus to code a motif.

The security element can be produced by simple stamping processes. It is particularly favorable here to form the profile structure in a transparent material and then, in order to obtain the metallic surface, to coat it with a metal layer. Alternatively, the profile structure can already be formed directly in a metallic material, so that it is not only metallic on the surface. With regard to preferred color filter properties as well as low contamination, it is preferable to embed the profile structure in a transparent dielectric and thus to provide the dialectic also on the metallic top. The term "top" is thus not necessarily related to an exposed surface, but on the top of the tread structure, if it is considered that the elevations rise above the base.

The security element can in particular be part of a not yet executable precursor to a value document, which additionally may have further authenticity features. Documents of value are understood here to mean documents that are to be protected against counterfeiting or forgery. Banknotes, banknotes, chip or security cards, such as bank or credit cards or identity cards, are examples of such value documents. The security element can in particular be used in a security tearing, security tape, security tape, patch or label.

It is understood that the features mentioned above and those yet to be explained can be used not only in the specified combinations but also in other combinations or in isolation for embodiments, without departing from the scope of the present invention. The invention will be explained in more detail by way of example with reference to the accompanying drawings, which disclose features essential to the invention. Show it:

1 is a schematic representation of a banknote with a security feature, a sectional view through the security feature in a first embodiment,

Fig. 3 is a sectional view through the security feature

 second embodiment,

Fig. 4 is a sectional view through the security feature

 a third embodiment,

5 is a sectional view through the security feature in a fourth embodiment, 6 is a schematic diagram for illustrating the color-filtering effect of the security element,

Fig. 7 is a schematic representation for illustrating the

 Setting the filtered color with the security element,

8-10 spectral plots of the reflectance for different embodiments of the security element, FIGS. 11 and 12 CIE color diagrams for different embodiments of the security element,

13 shows a diagram to illustrate the incident angle dependence of the color effect,

14 is an associated CIE diagram,

15 and 16 further embodiments of the security element and Fig. 17 is an enlarged plan view of the security element of

 Fig. L

Fig. 1 shows a banknote 1 with a security element 2, which was applied in the present case as a patch on a banknote paper. The security element 2 provides a motif consisting of a foreground element, in this case a star 3, and a background 4 delimited therefrom. Star 3 and background 4 are formed by the fact that the structure of the security element described below differs in geometric parameters, which are also explained below. FIG. 2 shows a sectional view through the structure of the security element 2. A profile structure 6 is arranged on a substrate 5. Fig. 2 shows the cross-sectional view. The profile structure 6 comprises a multiplicity of elevations 7, which are arranged at an irregular and quasi-statistically distributed distance from one another. Fig. 2 shows two different distances di between the middle and the left elevation and d2 between the middle and the right elevation. The elevations 7 of the profile structure 6 are formed stepped. They have a top surface 8 and, since they are in two stages in the embodiment shown in FIG. 2, also a further parallel step surface 9. The elevations 7 are located above a base surface 10. Both top surface 8 and parallel surface 9 are parallel to Base area 10. The step widths are labeled bi and bi. They are the same in the illustration of FIG. 2 and constant over all elevations 7. As will be explained below, this constancy is not absolutely necessary, but the step widths may be varied in embodiments. Within a survey 7, the step heights 1 1 and t _{2 are} always identical. Between different elevations 7, they can vary. This will also be explained below. The elevations 7 and the base 10 are metallic at the top. This can be done by, as shown in Fig. 2, both the base 10 and the elevations 7 are formed from a corresponding metallization. The uniform step height within an elevation 7 is between 100 nm and 350 nm. In this area, metallization layers can be produced without problems and structured, for example, by lithography.

Fig. 3 shows a modification of the embodiment of Fig. 2. The second embodiment shown there differs on the one hand with respect to the structure of the profile structure 6, on the other hand in that the profile structure is completely embedded in a dielectric. In the second embodiment Form is the profile structure 6 formed by the fact that on the substrate 5, not a metallization was applied, but an embossing lacquer 10. This is provided by an embossing process with the geometry of the profile structure 6 and then coated with a metallization 12 to the metalized upper side to produce the profile structure 6. Subsequently, the profile structure 6 is provided with a lamination 14 and covered with a cover sheet 15. The profile structure 6 thus has a dielectric VM on its upper side due to the metal layer 12 and is embedded in a dielectric material with the refractive indices m, n ₂ and n ₃ .

First and second embodiments of FIGS. 2 and 3, respectively, show an optional feature that is suitable for all constructions of the security element 2, namely that the step widths b are chosen such that the adjacent area proportions of the individual steps are approximately equal. It has been found that this achieves the best possible color effect.

4 and 5 show illustrations of a third and fourth embodiment corresponding to FIGS. 2 and 3. However, here the elevations 7 are formed with five stages. The step heights h to ts of these five stages are again uniform.

Fig. 6 illustrates the effect of the stepped elevation 7. It shows a single-stage elevation 7 on the left, a two-stage elevation 7 in the middle and a five-level elevation on the right. It can be seen that the stepped elevations 7 above the base surface 10 behave in a similar manner to an alternating refractive index multilayer structure. As the proportion of levels increases, color saturation for reflected radiation R incident at an angle Θ is favored. Fig. 6 shows this on the basis of the light paths for the individual stages. Constructive interference forms in a gangway difference 2δ = ηλ, where n is an integer. The wavelengths for constructive interference at one stage then result according to the equation λ = 2 t / n sin Θ. The step height can be used to adjust the color effect. This is illustrated by FIG. 7, which exemplarily shows three two-stage elevations 7 over the base area 10 with different step heights t. They are chosen so that the colors blue B (left), green G (middle) and red Ro (right) are formed in reflection.

Subsequently, the spectral reflection in the visible wavelength range at normal incidence for unpolarized light on a security element 2 with a two-stage profile structure 6 according to FIG. 3 is investigated. The metallization 12 consists of aluminum and the refractive indices of the surrounding dielectrics are the same and amount to 1.52. FIG. 9 shows spectrally the degree of reflection R, wherein, unlike the third embodiment, the elevations 7 in the case of that examined in FIG

Profile structure 6 are arranged periodically with a fixed distance d. The distance d is varied in Fig. 8; the step height is uniformly 150 nm. The curves show a pronounced maximum in blue light, namely a wavelength of 450 nm. The position of this maximum is independent of the period d. The longer-wavelength light component of the zeroth diffraction order is suppressed for increasing periods and intensifies the color saturation in the blue. This shows that a periodic arrangement of the elevations 7 is disadvantageous. It is therefore provided in the embodiments that the bumps are non-periodically, e.g. B. quasi-statistically arranged.

FIG. 9 shows the zeroth-order spectral reflectance, again for an aluminum metallization of a two-stage tread structure with a uniform step height, wherein the step height in FIG. 9 is varied. It can be clearly seen that, as already explained with reference to FIG. 7, the step height clearly the color of the reflected radiation R sets. The mentioned step heights form the colors blue, green and red.

FIG. 10 shows the same conditions for a profile structure 6 with five-step elevations 7 and otherwise identical geometry parameters. It turns out that the higher number of steps produces sharper reflection maxima.

FIGS. 11 and 10 show the color characteristics of the profile structures measured in FIGS. 9 and 10 in the CIE-1931 color space. The color properties were obtained by folding the calculated reflection spectra with the emission curve of a D65 standard lamp and the sensitivity of the human eye and converting them into color coordinates x, y, z. The diagrams show that in both cases the profile structure 6 with stepped elevations 7 represents the RGB colors. It can also be seen that the five-step survey profile analyzed in FIG. 12 provides a particularly saturated green hue.

FIG. 3 shows the influence of the angular dependence on reflected radiation R. Two angles of incidence, 0 ° and 30 °, are plotted. FIG. 13 is based on a profile structure with four-step elevations. The step height is again 150 nm. The maximum in the blue is hardly displaced by tilting by 30 °. Figure 14 shows the ratios in the CIE-1931 color chart for step heights of 150 nm, 180nm and 210nm. The three colors blue, green and red are preserved when tilted in hue. It shows a high angular tolerance of the reflected colors. This is particularly advantageous for generating a motif, in particular when viewing in ambient conditions, which usually include diffused light. A high angle dependence of the reflected colors would become a unwanted color mixing and thus lead to a significant weakening of a color effect.

In essence, the colors obtained in reflection are characterized by the step height t and the refractive index of the surrounding dielectric. The step width b and the individual distances have a minor influence, as long as a periodic arrangement is avoided.

FIG. 15 shows an embodiment of the security element 2 which has single-level elevations 7 in the profile structure 6, which are arranged aperiodically and are distributed in a quasistatic manner. Fig. 16 shows the same conditions for two-stage surveys, wherein at the same time the step width is varied within the surveys. FIG. 17 shows a plan view of the security element 2 of FIG. 1, here with a profile structure according to FIG. 15. The elevations 7 in both spatial directions are arranged aperiodically, with different step heights being selected for the star 3 and the background 4. Of course, this does not show the top view of FIG. 17. The dividing line, which is entered in the figure between the region of the star 3 and the background 4, serves only to clarify the range limits, since the step height is not recognizable in the plan view.

For the color effect it is sufficient if an aperiodic arrangement of the elevations in one spatial direction is provided, the aperiodicity in two spatial directions according to FIG. 17 improves the results.

The production of the profile structure can take place, as is known for relief structures in the prior art, for example from the above-mentioned patent publications. Multistage elevations are typically obtained by a multi-stage etching process or a multi-stage embossing die in the event that the tread structure is produced by embossing in a material to be embossed, such as a UV-curable embossing lacquer.

The advantage of the security feature is that the color can already be achieved with a simple metal coating. Elaborate multi-layered structures with different refractive indices are no longer required, as would be required with the multi-layer layer system. In addition, the profile structures can realize high-resolution motifs with very good resolution, since the spatial resolution can be set very precisely by the embossing process. In particular, optical effects of the individual regions can thus be realized in perfect register with one another. In this way, spatial effects, motion effects and colored motifs can be combined in a single security feature.

LIST OF REFERENCE NUMBERS

1 banknote

 2 security element

 3 star

 4 background

 5 substrate

 6 profile structure

 7 survey

 8 deck area

 9 tread

 10 base area

 11 embossing lacquer

 12 metallization

 14 lamination

 15 cover sheet

Claims

Patent claims 1. Security element with reflective color filter properties for the production of value documents, such as banknotes (1), checks or the like, which has a profile structure (6) which has elevations (7) over a base area (10) and has a metallic upper side, characterized in that the elevations (7) have a top surface (10) parallel to the top surface (8) and seen in cross section in each case at least one stage with a step height (t) are formed and aperiodically distributed in terms of location and / or extent on the base (10), wherein the step height (t) is uniform at least in regions and the step height (t) is between 100 nm and 350 nm. 2. Security element according to claim 1, characterized in that each elevations (7) has two to twenty stages. 3. Security element according to claim 1 or 2, characterized in that different areas (3, 4) are formed, in which the elevations (7) in each case in number of steps and step height (t) are uniform, wherein the areas (3, 4 ) differ in number of steps and / or step height (t) and / or step width (b). 4. Security element according to one of the above claims, characterized in that the profile structure (6) in a transparent material (11) is formed and coated with a metal layer (12). - 2 - 5. Security element according to one of the above claims, characterized in that the profile structure (6) is embedded in a transparent dielectric (14). 6. document of value with a security element (2) according to one of claims 1 to 5. 7. A manufacturing method for a security element (S) for the production of value documents, such as banknotes (1), checks or the like, wherein a profile structure (6) is generated, the elevations (7) over a Base (10) has and has a metallic top, characterized in that the elevations (7) have a top surface (10) parallel to the top surface (8) and seen in cross section in each case at least one stage with a step height (t) are formed and aperiodisch distributed in terms of location and / or expansion on the base (10) , wherein the step height (t) is at least partially uniform and the step height (t) is between 100 nm and 350 nm.