EP2219168A2 - Transmission security element - Google Patents

Abstract

The security element (40) has a thin film element (42) i.e. two-layered thin film element, with color shift effect and comprising a metallic layer (44) and a dielectric layer (46), where the metallic layer is semitransparent and the security element exhibits a color in transmitted light at an angle between 0 and 40 degrees. Thickness of the metallic layer lies between 5 and 70 nm. The metallic layer forms a selective absorbent optical element in a visible spectral range. An independent claim is also included for a method for manufacturing a transmission security element.

Description

The invention relates to a see-through security element for security papers, documents of value and the like, a method for producing such a see-through security element and equipped with such a see-through security element data carriers.

Data carriers, such as valuables or identity documents, or other valuables, such as branded articles, are often provided with security elements for the purpose of security, which permit verification of the authenticity of the data carriers and at the same time serve as protection against unauthorized reproduction. The security elements may be in the form of, for example, a security thread embedded in a banknote, a tearing thread for product packaging, an applied security strip, a cover sheet for a banknote having a through opening or a self-supporting transfer element, such as a patch or label after its manufacture is applied to a document of value.

For some years, see-through windows have proven to be attractive security features in polymer and, more recently, in paper banknotes as they allow the use of a variety of security features. Security elements with viewing-angle-dependent effects play a special role in the authentication of authenticity since they can not be reproduced even with the most modern copiers. The security elements are thereby equipped with optically variable elements that give the viewer a different image impression under different viewing angles and, for example, show a different color or brightness impression and / or another graphic motif depending on the viewing angle.

The object of the present invention is therefore to provide a generic see-through safety element which avoids the disadvantages of the prior art. In particular, the see-through security element should combine an appealing visual appearance with high security against counterfeiting.

This object is achieved by the see-through security element, the manufacturing method and the data carrier having the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

According to the invention, a see-through security element of the type mentioned in the beginning has a thin-film element with a color-shift effect which contains precisely one metal layer and at least one dielectric layer, the metal layer being semitransparent. By "semitransparency" is meant translucency, in particular a light transmission between 3% and 80%.

As explained in more detail below, the see-through security element in reflected light on a metallic reflective and in transmitted light a colored visual impression. In particular, the see-through security element appears metallically specular and color-neutral in reflected light and appears colored in the transmitted light. Particularly preferably, the colored transmission is combined with a color shift effect, so that the see-through security element appears in transmitted light depending on the viewing angle with a different color and thus has a semi-transparent color shift effect.

The color of transmitted or reflected light can be quantified by specifying a color locus in a color system. In the context of this The colouration F is indicated by the distance of the standard chromaticity coordinates x, y of the color to the white point (x = y = 1/3) in the CIE standard valence system.

In the incident light, the see-through security element according to the invention preferably has a colouration F <0.10, particularly preferably F <0.05, when viewed perpendicularly. The see-through security element then shows in reflected light only weak, almost completely desaturated colors, which hardly visually stand against the background of the metallic reflection of the semitransparent metal layer.

Advantageously, the color impression always remains weak when tilting the see-through security element, the see-through security element preferably always having a color F <0.10 in reflected light in the angular range between 0 ° and 60 °.

Unlike the reflection, the transmission of the see-through security element is colored, wherein the see-through security element in transmitted light when viewed perpendicularly preferably has a color F> 0.05, particularly preferably F> 0.10. Advantageously, the strong color impression is retained for a wide range of the visible color in transmitted light or the visible in the transmitted light color shift effect. In particular, the see-through security element in transmitted light in the angular range between 0 ° and 40 ° always a colouration F> 0.05, preferably F> 0.10.

The layer thickness of the semitransparent metal layer is preferably between 5 nm and 70 nm, wherein the thickness range between 15 nm and 50 nm for silver layers, the thickness range between 5 nm and 15 nm for aluminum layers and the thickness range between 30 nm and 70 nm is particularly preferred. In addition to the metals silver, aluminum and gold, the semitransparent metal layer may in particular also be formed from chromium, nickel, copper, potassium, tungsten or an alloy of these metals. A particularly color-intensive transmission can be achieved with semitransparent metal layers of silver, aluminum, gold, potassium or copper.

In general, for the semitransparent metal layer, in particular those metals are considered whose plasma frequency lies in the ultraviolet, visible or near-infrared spectral range. The plasma frequency ω _p is the frequency of the longitudinal oscillations of the electron gas relative to the ion bodies. For example, the plasma frequency of silver corresponds to a wavelength of 311 nm (ultraviolet), the plasma frequency of gold to a wavelength of 560 nm (visible), and the plasma frequency of aluminum to a wavelength of 830 nm (near infrared).

In an advantageous embodiment of the invention, the semitransparent metal layer is homogeneous, ie in particular not formed in the form of metallic islands or clusters, and thus allows easy application, for example by vapor deposition in a vacuum coating process.

The semitransparent metal layer forms in particular a selectively absorbing optical element in the visible spectral range. As recognized in the present invention, by combining one or more dielectric layers with a selectively absorbing optical element, a semitransparent see-through security element can be provided whose transmission and reflection colors need not be complementary to one another. The dielectric layers and the selective On the contrary, absorbing optical elements can advantageously be matched to one another in such a way that the see-through security element appears substantially neutral in color in reflection and produces a strong color impression in transmission. In this case, the hue of the transmitted light is not sensitive to the layer thickness of the semitransparent metal layer, so that this parameter is not critical in the production.

In an advantageous variant of the invention, the thin-film element of the see-through security element is a two-layered thin-film element which contains precisely one dielectric layer in addition to the semitransparent metal layer.

In another likewise advantageous variant of the invention, the thin-film element is a three-layered thin-film element which, in addition to the semitransparent metal layer, contains two dielectric layers arranged on opposite sides of the metal layer. The two dielectric layers are advantageously high-refractive with a refractive index n ≥ 1.8.

According to a further likewise advantageous variant of the invention, the thin-film element is a three-layered thin-film element which, in addition to the semitransparent metal layer, contains two dielectric layers arranged one above the other on the same side of the metal layer. In this case, advantageously, the first dielectric layer arranged directly on the semitransparent metal layer is low-refractive with a refractive index n <1.8 and the second dielectric layer arranged on the first dielectric layer is high-refractive with a refractive index n ≥ 1.8.

In general, in the case of the dielectric layer (s) arranged directly on the semitransparent metal layer, the layer thickness d and the refractive index n are preferably matched to one another such that the product d * n is between 300 nm and 800 nm. The second dielectric layer arranged on the first dielectric layer advantageously has a layer thickness between 30 nm and 100 nm.

Examples of high-index dielectric layers are ZnS layers or TiO ₂ layers. Low-refractive dielectric layers can be formed, for example, from SiO ₂ or MgF ₂ .

In all embodiments, the semitransparent metal layer may have recesses in the form of patterns, characters or codes, for example to represent additional information in the see-through security element. Furthermore, in all embodiments, the thin-film element may be combined with a diffractive diffraction structure, wherein the semitransparent metal layer forms the metallization layer of the diffraction structure. The diffractive diffraction structure may in particular represent a hologram, a holographic grating image, a blazed diffraction structure, a computer-generated hologram (CGH) or another hologram-like diffraction structure. In addition, other combinations, such as with blind structures or with achromatic structures, such as a matte structure, a micromirror arrangement, a Blazegitter with a sawtooth-like furrow profile or Fresnellinsen arrangement and / or a micro-optical relief structure in question.

The invention also includes a method for producing a see-through security element of the type described, in which a thin-film element with a color-shift effect is produced by exactly one semitransparent design Metal layer is combined with at least one dielectric layer. The semitransparent metal layer and the at least one dielectric layer are expediently applied to a carrier, for example a film substrate, in particular vapor-deposited by a vacuum coating method. The film substrate can be provided with a desired relief or embossed structure and coated, for example, with a primer / adhesion promoter. After transferring the see-through security element to a target substrate, the carrier can be detached from the security element.

The invention further includes a data carrier with a see-through security element of the type described above, wherein the see-through security element is arranged in particular in or over a transparent window area or a through opening of the data carrier. The data carrier may in particular be a value document, such as a banknote, in particular a paper banknote, a polymer banknote or a film composite banknote, or an identification card, such as a credit card, bank card, cash card, authorization card, identity card or passport personalization page.

Further embodiments and advantages of the invention will be explained below with reference to the figures, the representation has been dispensed to scale and proportionate reproduction in order to increase the clarity. The various embodiments are not limited to use in the specific form described, but can also be combined with each other.

Fig.1

eine schematische Darstellung einer Banknote mit einem erfindungsgemäßen Durchsichtssicherheitselement,

Fig. 2

schematisch einen Schnitt entlang der Linie II-II von Fig.1 bei Betrachtung im Auflicht,

Fig. 3

schematisch einen Schnitt entlang der Linie II-II von Fig.1 bei Betrachtung im Durchlicht,

Fig. 4

ein Durchsichtssicherheitselement nach einem Ausführungsbeispiel der Erfindung,

Fig. 5

ein Ausführungsbeispiel der Erfindung mit einem zweischichtigen Dünnschichtelement,

Fig. 6

ein erfindungsgemäßes Durchsichtssicherheitselement mit einem Dünnschichtelement mit Dreischichtaufbau,

Fig. 7

die Verwendung einer Variante des Durchsichtssicherheitselements der Fig. 6 zur Abdeckung einer durchgehenden Öffnung einer Banknote,

Fig. 8

die Ergebnisse einer numerischen Simulation für dünne Silberschichten (d = 5 nm bis 40 nm) in einer dielektrischen Umgebung mit Brechungsindex n = 1,5 bei senkrechter Betrachtung in Reflexion ((a), (b), (c)) bzw. in Transmission ((d), (e), (f)), wobei
(a) und (d) jeweils die CIE-xy-Farbtafel,
(b) und (e) den Anteil des reflektierten Lichts R bzw. transmittierten Lichts T in Abhängigkeit von der Schichtdicke d der Silberschicht, und
(c) und (f) die Farbigkeit F des reflektierten bzw. transmittierten Lichts in Abhängigkeit von der Schichtdicke d der Silberschicht zeigen,

Fig. 9

die Ergebnisse einer numerischen Simulation für eine dünne Silberschicht (d = 25 nm), die über einer niedrigbrechenden Dielektrikumsschicht in Form einer 500 nm dicken MgF₂-Schicht, einer hochbrechenden Dielektrikumsschicht in Form einer 50 nm dicken ZnS-Schicht und einem Dielektrikum mit n = 1,5 angeordnet ist, bei Betrachtung in Reflexion ((a), (b), (c)) bzw. in Transmission ((d), (e), (f)), wobei
(a) und (d) jeweils die CIE-xy-Farbtafel,
(b) und (e) den Anteil des reflektierten Lichts R bzw. transmittierten Lichts T in Abhängigkeit von der Lichteinfallsrichtung 0, und
(c) und (f) die Farbigkeit F des reflektierten bzw. transmittierten Lichts in Abhängigkeit von der Lichteinfallsrichtung θ zeigen,

Fig. 10

eine Polymerbanknote mit einem erfindungsgemäßen Durchsichtssicherheitselement,

Fig. 11

schematisch einen Schnitt entlang der Linie XI-XI von Fig. 10,

Fig. 12,13

zwei Varianten, bei denen erfindungsgemäße Durchsichtssicherheitselemente mit Hybridsubstraten kombiniert sind, und

Fig. 14,15

zwei Varianten, bei denen erfindungsgemäße Durchsichtssicherheitselemente bei Karten eingesetzt sind.

Show it:

Fig.1

a schematic representation of a banknote with a see-through security element according to the invention,

Fig. 2

schematically a section along the line II-II of Fig.1 when viewed in reflected light,

Fig. 3

schematically a section along the line II-II of Fig.1 when viewed in transmitted light,

Fig. 4

a see-through security element according to an embodiment of the invention,

Fig. 5

An embodiment of the invention with a two-layer thin-film element,

Fig. 6

an inventive see-through safety element with a thin-film element with three-layer structure,

Fig. 7

the use of a variant of the see-through security element of Fig. 6 for covering a through opening of a banknote,

Fig. 8

the results of a numerical simulation for thin silver layers (d = 5 nm to 40 nm) in a dielectric environment with refractive index n = 1.5 when viewed vertically in reflection ((a), (b), (c)) or in transmission ((d), (e), (f)), where
(a) and (d) each the CIE xy color chart,
(b) and (e) the proportion of the reflected light R or transmitted light T as a function of the layer thickness d of the silver layer, and
(c) and (f) show the color F of the reflected or transmitted light as a function of the layer thickness d of the silver layer,

Fig. 9

the results of a numerical simulation for a thin silver layer (d = 25 nm), over a low-refractive dielectric layer in the form of a 500 nm thick MgF ₂ layer, a high refractive index dielectric layer in the form of a 50 nm thick ZnS layer and a dielectric with n = 1.5, when viewed in reflection ((a), (b), (c)) and in transmission ((d), (e), (f)), respectively
(a) and (d) each the CIE xy color chart,
(b) and (e) the proportion of the reflected light R and transmitted light T as a function of the light incident direction 0, and
(c) and (f) show the chromaticity F of the reflected or transmitted light as a function of the light incident direction θ,

Fig. 10

a polymer banknote with a see-through security element according to the invention,

Fig. 11

schematically a section along the line XI-XI of Fig. 10 .

Fig. 12,13

two variants in which see-through security elements according to the invention are combined with hybrid substrates, and

Fig. 14, 15

two variants in which inventive see-through security elements are used in maps.

The invention will now be explained using the example of security elements for banknotes. Fig.1 shows a schematic representation of a banknote 10 with a through opening 14, which is covered with a see-through security element 12 in the form of a strip. The Figures 2 and 3 show the bill 10 along the line II-II of Fig.1 in cross section when viewed in reflected light ( Fig. 2 ) or transmitted light ( Fig. 3 ).

The see-through security element 12 shows when viewed a visually attractive reflected light / transmitted light effect, which visually enhances the security element compared to conventional metallized embossed holograms and also makes much more difficult to adjust. If the see-through security element 12 is viewed in incident light, for example in front of a dark background 20 (FIG. Fig. 2 ), the see-through security element 12 shows a specular, color-neutral metallic hologram 16 both in vertical viewing 22 and in oblique viewing 24. The semitransparency of the see-through security element 12 does not appear in the top view.

On the other hand, if the bill is viewed in transmitted light 30, as in FIG Fig. 3 shown schematically, the dominating in incident light hologram 16 visually back and there appears a pronounced semi-transparent color shift effect, in which a strong color impression changes with the viewing angle, for example, from magenta oblique view 34 to green when viewed vertically 32.

Such a reflected light / transmitted light effect is in contrast to the visual appearance of known optical interference coatings with a color shift effect, like the one in the publication WO 03/070482 A1 described semitransparent interference elements. The interference elements of WO 03/070482 A1 always show a color shift effect both on the front and on the back. As explained on page 8 of said document, the diffraction effects when viewed in incident light, so when the documents rests for example on a substrate, visually particularly stand out. When viewed in transmitted light, the diffraction effects occur significantly, but the color shift effect is pronounced in reflected light and in transmitted light.

The see-through security element according to the invention may, but does not necessarily have, to have a hologram. It is important in the context of the present invention, only that the see-through security element in the reflected light substantially neutral in color and without Farbkippeffekt metallic reflective and appears in transmitted light with a semi-transparent color shift effect.

As an embodiment without hologram shows Fig. 4 shows the layer structure of a see-through security element 40 according to the invention in cross section. The see-through security element 40 includes a thin-film element 42 with a color-shift effect, which contains exactly one metal layer 44 and exactly one dielectric layer 46.

The metal layer 44 is formed semitransparent with a light transmission between 3% and 70% and may be formed in particular from silver, aluminum, gold, chromium, nickel, copper, potassium, tungsten or an alloy of these metals. In the embodiment of Fig. 4 For example, the metal layer 44 is a 30 nm thick silver layer. The dielectric layer 46 may be a low refractive index dielectric layer having a refractive index n <1.8, such as SiO ₂ or MgF ₂ , or a high refractive index dielectric layer with a refractive index n ≥ 1.8, such as TiO ₂ or ZnS. In the embodiment of Fig. 4 For example, the dielectric layer 46 is a 525 nm-thick SiO ₂ layer.

Will the see-through security element 40 of Fig. 4 viewed in incident light, so only the metallic reflective appearance of the metal layer 44, but no color shift effect can be seen. When viewed in transmitted light, the see-through security element exhibits a semi-transparent color shift effect, for example from magenta to green. The see-through security element 40 has, on the one hand, a simple structure with the two-layer thin-film element 42, and, on the other hand, shows a supervision / see-through effect which contrasts with conventional thin-film elements with a color-shift effect and increases the recognition value and the forgery-proof security of the see-through security element.

For producing a see-through security element of in Fig. 4 As shown, a film substrate 48, for example a PET film, is provided with a primer / adhesion promoter 45. Then, the semitransparent metal layer 44 is evaporated in the form of a generally 25 nm to 35 nm thick silver layer and then the dielectric layer 46 in the form of a generally 500 nm to 550 nm thick SiO ₂ layer. The order of the coating can also be reversed, so that initially a 500 nm to 550 nm thick SiO ₂ layer and then a 25 nm to 30 nm thick silver layer is vapor-deposited on the primer layer 45. The visual impression does not change due to the reversal of the layer order.

Fig. 5 shows an embodiment of the invention in which a two-layer thin-film element 52 next to a semi-transparent metal layer 54 includes a high-refractive dielectric layer 56.

The film substrate 58 of the see-through security element 50 in this exemplary embodiment contains an embossing lacquer layer 55 with a hologram embossing structure. After the application of a primer layer, first the dielectric layer 56 in the form of a 325 nm to 350 nm thick ZnS layer was vapor-deposited onto the film substrate, and then the semitransparent metal layer 54 was vapor-deposited in the form of a silver layer of about 30 nm thickness.

When viewed in incident light, the see-through security element 50 shows an opaque metal hologram without color-shift effect. In transmitted light, a semi-transparent color shift effect is visible, which is visible only in the hologram embossing. The see-through security element 50 thus combines a simple two-layer structure of the thin-film element 52 with a novel supervision / see-through effect.

This in Fig. 6 The see-through security element 60 shown has a thin-film element 62 with a three-layer structure which generates a color-shift effect with very strong colors in transmitted light.

The see-through security element 60 comprises a film substrate 68 with or without embossed structure, a primer layer 65 and a thin-film element 62 having a three-layer structure with the layer sequence: semitransparent metal layer 74, low-refractive dielectric layer 76, high-index dielectric layer 78. The semitransparent metal layer 74 was in the form of an approximately 30 nm thick layer of silver evaporated, the low-refractive dielectric layer 76 in the form of a 500 nm to 550 nm thick SiO ₂ layer and the high-index dielectric layer 78 in the form of an approximately 50 nm thick ZnS layer. The visual impression of the see-through security element 60 largely corresponds to that of the see-through security element 40 of FIG Fig. 4 However, the additionally provided high refractive dielectric layer 78 leads to a color shift effect with particularly strong colors.

This is currently believed to be due to the fact that the interface of the dielectric layers 76, 78 each reflects a portion of light back to the metal layer 74, thus causing multiple absorption within a light path. For strong colors, a large refractive index difference, such as for example between ZnS as a high-index dielectric layer and MgF ₂ as a low-index dielectric layer, is advantageous.

Fig. 7 shows the use of a variant of the see-through security element of Fig. 6 to cover a continuous opening of a banknote 10. The illustrated see-through security element 80 has a foil substrate 88 with an embossing lacquer layer 85 with a hologram embossing structure. On the film substrate 88 is a thin film element 82 having a three-layer structure as in FIG Fig. 6 , ie with the layer sequence: semitransparent metal layer 94, low-refractive-index dielectric layer 96 and high-refractive-index dielectric layer 98. As in Fig. 7 shown, the semitransparent metal layer 94 may be present only in regions. The thin-film element 62 is further provided with a protective lacquer layer 84 and applied to the banknote 10 via an adhesive layer 86 in the region of the opening 14.

When viewed in incident light, the see-through security element 80 shows an opaque metal hologram with no color shift effect, as in Fig. 2 described.
In transmitted light, a semitransparent color shift effect with very strong colors, as always with Fig. 3 described.

Without wishing to be bound by any particular explanation, the color transmission, with essentially color-neutral reflection, as currently understood, is as follows: Dielectric layers can block regions of the visible spectrum in the transmission, so that the reflection appears colored. Since no light is absorbed, the transmission includes the complementary color of the reflection. If a dielectric layer is combined with a selectively absorbing optical element which absorbs part of the visible spectral range, then the reflection no longer has to be color-complementary to the transmission. On the contrary, such a combination may appear colorless in reflection, but may have a color in transmission.

According to the invention, the selectively absorbing optical element is formed by a very thin metal layer having a thickness of less than 100 nm. The selective optical absorption in the visible spectral range, according to current understanding, unlike conventional color-shifting structures, relies not on the interference of multiply reflected light beams, but on an intrinsic property of metals, namely the fact that metals are for light with a higher frequency than the plasma frequency of the metal become transparent.

The influence on the transmission in the visible spectral range is greater, the closer the plasma frequency is to the visible range. Light whose frequency corresponds to the plasma resonance is absorbed more intensely and increase the transmission for the same wavelength range. The color effect of the metal layer alone is approximately independent of the angle of incidence of the light or of the viewing angle of the observer. Also, the transmitted color hardly changes when the layer thickness of the metal layer is changed. A change in the layer thickness has primarily only on the color intensity.

The following table shows the results of numerical simulations of the transmission of thin metal layers (d = 5 nm to 60 nm). Indicated here is the respective color impression of the metal layer, the distance of the color from the white point W in the CIE xy chromaticity diagram at 10% intensity (see Fig. 8 ), as well as the wavelengths of maximum transmission Tmax and the absorption peak Pabs. Table 1: metal colour color difference Tmax pabs Ag blue 0.11 330nm 240-280 nm al blue 0.07 950 nm 830 nm Au yellow 0.09 540-600 nm - Cr slightly blue 0.03 - 600 nm Ni slightly blue 0.04 370 nm - Cu reddish 0.05 590 nm 250 nm K blue 12:07 in the blue in the blue W reddish 0.01 red blue

As can be seen from Table 1, the metals silver, gold, aluminum, potassium and copper are particularly color intensive (large color difference to the white point).

The use of a combination of dielectric layers with a selectively absorbing optical element offers, in particular, the advantage that, unlike interference-fringe structures, the hue is not sensitively dependent on the layer thickness, so that this parameter is not critical in the production. Also, the production of homogeneous metal layers is less expensive than, for example, the production of cluster layers, which in principle also come as selective absorbers in question.

Fig. 8 shows the results of a numerical simulation for thin silver layers (d = 5 nm to 40 nm) in a dielectric environment with refractive index n = 1.5 when viewed perpendicularly in reflection (left half of the figure, 8 (a), (b), (c) ) or in transmission (right half of the figure, Fig. 8 (d), (e), (f) ). For the numerical calculation, the emission spectrum of a D65 lamp was used, which roughly corresponds to the color spectrum of daylight.

The show FIGS. 8 (a) and (d) in each case the CIE xy color chart, in which the spectral color band 100, on which spectral colors lie, includes all color gradients perceptible to the human eye, given in each case by hue and saturation, for a specific brightness. For orientation, the colors red at 630 nm, yellow at 570 nm, green at 520 nm and blue at 480 nm are marked on the spectral color train 100 by specifying the wavelength. The spectral color train 100 is closed by the purple line 102. The colors on the spectral color train 100 each have maximum saturation. When approaching the achromatic point or white point W, the saturation decreases until it is completely desaturated at the white point (x = 1/3; y = 1/3).

The FIGS. 8 (b) and (e) show as a function of the layer thickness d of the silver layer, the proportion of the reflected light R and transmitted light T in percent. The reflection or the transmission has already been weighted with the sensitivity of the human eye and therefore corresponds to the L value of the Lab color space. Finally, the show FIGS. 8 (c) and (f) depending on the layer thickness d of the silver layer, the color F of the reflected or transmitted light, expressed by the distance of the respective color locus from the white point W in the CIE-xy color chart.

Referring first to the FIGS. 8 (b) and (e) Curves 104 and 106 show the proportion of the reflected light and the transmitted light, respectively. The proportion of the reflected light increases from about 8% at a layer thickness of d = 5 nm to over 80% at a layer thickness of d = 40 nm (curve 104). Accordingly, the proportion of the transmitted light of about 90% at a layer thickness of d = 5 nm drops to about 10% at a layer thickness of d = 40 nm (curve 106).

At the same time, the color intensity of the transmitted light with the layer thickness of about 0.025 at a layer thickness of d = 5 nm increases to about 0.10 at a layer thickness of d = 40 nm, as indicated by curve 110 in FIG Fig. 8 (f) shown. The color location of the transmitted light is indicated by the curve 114 in FIG Fig. 8 (d) shown. With increasing layer thickness, the color location of the transmission moves from the white point W into the blue, approximately in the direction of the 480 nm mark on the spectral color band 100, so that the transmitted light appears clearly blue at a layer thickness above 25 nm.

In contrast, the color of the reflected light decreases with increasing layer thickness of about 0.07 at a layer thickness of d = 5 nm to about 0.02 at a layer thickness of d = 40 nm, as shown by the curve 108 in FIG Fig. 8 (c) shown. The color location of the reflected light is through the curve 112 in FIG Fig. 8 (a) specified. Starting from a slightly orange color locus at d = 5 nm, the reflection color approaches the white point W, so that the reflected light appears substantially color-neutral at a layer thickness above 25 nm.

The effect of color generation through a very thin metal layer as a selectively absorbing optical element is inventively combined with dielectric layers, on the one hand to enhance the color and on the other hand additionally to realize a color shift effect. Particularly strong effects are achieved when two different dielectric layers with a dielectric interface are provided, which redirects some of the light back to the metal layer as efficiently as possible to effect multiple absorption in a light path, such as those discussed above the FIGS. 6 and 7 described designs.

Fig. 9 shows the results of a numerical simulation for a thin silver layer (d = 25 nm) over a low-refractive dielectric layer in the form of a 500 nm thick MgF ₂ layer, a high refractive index dielectric layer in the form of a 50 nm thick ZnS layer and a dielectric n = 1.5 is arranged. The simulation underlying structure largely corresponds to the see-through security element 60 of Fig. 6 , but with the reverse layer sequence of the thin-film element 62, so that the top view is made on the thin silver layer.

The FIGS. 9 (a) to (f) represent the brightness and color of this layer sequence as a function of the light incidence direction, expressed by the Polar angle θ for reflection in reflection (left half of the figure, Fig. 9 (a), (b), (c) ) or in transmission (right half of the figure, Fig. 9 (d), (e), (f) ).

As in Fig. 8 showed the FIGS. 9 (a) and (d) each color locus in the CIE xy color chart, the FIGS. 9 (b) and (e) the proportion of the reflected light R and transmitted light T and the FIGS. 9 (c) and (f) the color F of the reflected or transmitted light, expressed by the distance of the respective color locus from the white point W in the CIE-xy color chart.

Referring first to the FIGS. 9 (b) and (e) Curves 124 and 126 indicate the proportion of the reflected light and the transmitted light, respectively. The fraction of the reflected light is always between 70% and 80% for angles of incidence between 0 ° (vertical incidence) and 60 ° (curve 124), the proportion of transmitted light is between 15% and slightly more than 25% (curve 126).

The color of the transmitted light and the color shift effect is from the FIGS. 9 (d) and (f) to recognize. The in Fig. 9 (d) The color locus indicated by the curve 134 changes from the starting point 136 in the blue, which corresponds to the perpendicular incidence direction (θ = 0 °), with increasing tilt angle over the purple and red tones to the end point 138 in the yellow spectral range, which corresponds to an oblique viewing direction (θ = 60 °). As from the color space curve 130 of the Fig. 9 (f) can be seen, the color difference of the transmitted light from the white point W is always greater than 0.10 up to a tilt angle of about 45 °, in some areas even greater than 0.15. The specified layer sequence therefore shows in transmission strong colors and a pronounced color shift effect.

In contrast, the color and the color change of the reflected light are much lower, as in the FIGS. 9 (a) and (c) shown. The in Fig. 9 (a) the color locus indicated by the curve 132 changes from a desaturated orange color locus in a perpendicular direction of incidence with an increasing tilt angle over weak green tones to a pale blue near the white point W in an oblique viewing direction. As from the color space curve 128 of the Fig. 9 (c) to recognize, the color difference of the reflected light from the white point W is always less than 0.08, for the most part even less than 0.05. The specified layer sequence therefore shows in reflection only very weak, almost completely desaturated colors, which visually hardly stand out against the background of the metallic reflection. Due to the small deviation of the color locations from the white point, the color change of the desaturated colors during tilting practically does not occur.

Further possible uses of see-through security elements according to the invention will now be described with reference to FIGS FIGS. 10 to 15 briefly described.

Fig. 10 shows a polymer banknote 140 with a printing area 142 and a transparent area 144 in which a see-through security element 150 according to the invention with the incident light / transmitted-light effect described above is applied.

With particular reference to the Figure 11 which is a section through the polymer banknote 140 of Figure 10 along the line XI-XI, the see-through security element 150 has a releasable film substrate 158, which is provided with an embossing lacquer layer 155 with a hologram embossing structure and a thin-film element 152. The thin-film element 152 has a three-layer structure with the layer sequence: semitransparent Metal layer 164, such as aluminum or silver, low refractive dielectric layer 166, such as MgF ₂ or SiO ₂ , and high refractive dielectric layer 168, such as ZnS.

The semitransparent metal layer 164 may also include recesses 162 in the form of patterns, characters, or a coding, such as those shown in FIG Fig. 10 shown letter sequence "PL", have. The thin-film element 152 is provided with a protective lacquer layer 154 and applied to the polymer banknote 140 via an adhesive layer 156 in the transparent region 144. After the transfer, the film substrate 158 is peeled off, as indicated by the reference numeral 160.

The FIGS. 12 and 13 show two variants in which inventive see-through security elements are combined with hybrid substrates, such as composite film banknotes. Figure 12 1 shows a composite film banknote 170 having a first PET film 172, a paper layer 174 and a second PET film 176. A see-through security element 180 of the type described above is embedded in the paper layer 174 in a window region 178 of the film composite banknote 170.

At the in Fig. 13 In the variant shown, the see-through security element 180 is applied in the region of the window 178 to the second PET film 176 of the film composite banknote 170. The see-through security element 180 includes a releasable film substrate 182 which is peeled off after transfer, as indicated by reference numeral 184.

Transparent safety elements according to the invention can also be used with cards, provided that they are at least partially transparent. Fig. 14 shows a card 190, in which a see-through security element 180 of described above in a transparent area on the surface of the card 190 is applied. The releasable film substrate 182 of the see-through security element 180 is peeled off after the transfer (reference numeral 184). In other configurations, a see-through security element 192 according to the invention may also be embedded in an inner layer 194 of the card 190, as in FIG Fig. 15 shown.

In the exemplary embodiments described, the see-through security element according to the invention has mostly been combined, for example, with a hologram structure. It will be understood, however, that this variant is illustrative only and that combinations with other diffractive microstructures such as holographic grating images or hologram-like diffraction structures may be used, or with micro-optic relief structures such as blazed grids, Fresnel structures, lens or micromirror structures , In addition to microrelief structures on the order of the wavelength of light, microstructures with smaller structural elements are also suitable for such a combination, such as sub-wavelength gratings or moth-eye structures, whose structural elements may also be smaller than 100 nm.

Claims (27)

See-through security element for security papers, documents of value and the like with a thin-film element with color shift effect, which contains exactly one metal layer and at least one dielectric layer, wherein the metal layer is formed semitransparent. See-through security element according to claim 1, characterized in that the see-through security element in reflected light appears metallic mirror-like and color-neutral, and colored in transmitted light. See-through security element according to claim 1 or 2, characterized in that the see-through security element in transmitted light appears colored with a semi-transparent color shift effect. See-through security element according to at least one of claims 1 to 3, characterized in that the see-through security element has a colouration F <0.10, preferably F <0.05 in reflected light when viewed vertically, wherein the color F by the distance of the standard color value shares x, y Color is given to the white point in the CIE standard valency system. See-through security element according to claim 4, characterized in that the see-through security element in incident light in the angular range between 0 ° and 60 ° always has a color F <0.10. See-through security element according to at least one of claims 1 to 5, characterized in that the see-through security element in transmitted light when viewed vertically has a color F> 0.05, preferably F> 0.10, wherein the color F by the distance of the standard color value shares x, y of the color is given to the white point in the CIE standard valence system. See-through security element according to claim 6, characterized in that the see-through security element in transmitted light in the angular range between 0 ° and 40 ° always has a color F> 0.05, preferably F> 0.10. See-through security element according to at least one of claims 1 to 7, characterized in that the layer thickness of the semitransparent metal layer is between 5 and 70 nm. See-through security element according to at least one of Claims 1 to 8, characterized in that the semitransparent metal layer is formed from a metal whose plasma frequency lies in the ultraviolet, visible or near-infrared spectral region. See-through security element according to at least one of claims 1 to 9, characterized in that the semitransparent metal layer is homogeneous. See-through security element according to at least one of claims 1 to 10, characterized in that the semitransparent metal layer of silver, aluminum, gold, chromium, nickel, copper, potassium, tungsten or an alloy of these metals is formed. See-through security element according to at least one of claims 1 to 11, characterized in that the semitransparent metal layer forms a selectively absorbing in the visible spectral optical element. See-through security element according to at least one of Claims 1 to 12, characterized in that the thin-film element is a two-layered thin-film element which contains precisely one dielectric layer in addition to the metal layer. See-through security element according to at least one of Claims 1 to 12, characterized in that the thin-film element is a three-layered thin-film element which, in addition to the metal layer, contains two dielectric layers arranged on opposite sides of the metal layer. See-through security element according to Claim 14, characterized in that the two dielectric layers arranged on opposite sides of the metal layer are high-refractive with a refractive index n ≥ 1.8. See-through security element according to at least one of claims 1 to 12, characterized in that the thin-film element is a three-layered thin-film element which contains, in addition to the metal layer, two dielectric layers arranged one above the other on the same side of the metal layer. See-through security element according to claim 16, characterized in that the first dielectric layer arranged directly on the metal layer is low-refractive with a refractive index n <1.8 and the on the first dielectric layer disposed second dielectric layer high refractive index n ≥ 1.8. See-through security element according to at least one of Claims 13 to 17, characterized in that the layer thickness d and the refractive index n are matched to one another in the case of the dielectric layer (s) arranged directly on the metal layer, such that the product d * n is between 300 nm and 800 nm. See-through security element according to claim 16 or 17, characterized in that arranged on the first dielectric layer second dielectric layer has a layer thickness between 30 nm and 100 nm. See-through security element according to at least one of claims 1 to 19, characterized in that the semitransparent metal layer has recesses in the form of patterns, characters or codes. See-through security element according to at least one of claims 1 to 20, characterized in that the thin-film element is combined with a diffractive diffraction structure, wherein the semitransparent metal layer forms the metallization layer of the diffraction structure. See-through security element according to at least one of claims 1 to 20, characterized in that the thin-film element is combined with a micro-optical relief structure. A method for producing a see-through security element according to at least one of claims 1 to 22, wherein a thin-film element is produced with color shift effect by exactly one semitransparent metal layer is combined with at least one dielectric layer. A method according to claim 23, characterized in that the semitransparent metal layer and the at least one dielectric layer are vapor-deposited on a carrier. Data carrier with a see-through security element according to one of claims 1 to 22. A data carrier according to claim 25, characterized in that the see-through security element is arranged in or above a transparent window area or a through opening of the data carrier. A data carrier according to claim 25 or 26, characterized in that the data carrier is a value document, such as a banknote, in particular a paper banknote, a polymer banknote, a composite film banknote or identity card.

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