Title of Invention	SECURITY ELEMENT HAVING TWO STACKED LAYERS COMPOSED OF CHOLESTERIC LIQUID CRYSTAL MATERIAL AND METHOD FOR MANUFACTURING THE SAME
Abstract	The present invention relates to a security element (12) for security papers, value documents and the like, having two stacked layers (22, 24) composed of cholesteric liquid crystal material that each reflect, in a first viewing direction normal to their surface, selectively light (30, 34) from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light (32, 36) from a second, shorter-wave spectral range. According to the present invention, a first of the two layers (22) is present in the form of a motif to be depicted and reflects light (34) from the green spectral range in the first viewing direction, while the second of the two layers (24) reflects light (32) from the green spectral range in the second viewing direction.

Title of Invention

SECURITY ELEMENT HAVING TWO STACKED LAYERS COMPOSED OF CHOLESTERIC LIQUID CRYSTAL MATERIAL AND METHOD FOR MANUFACTURING THE SAME

Abstract

The present invention relates to a security element (12) for security papers, value documents and the like, having two stacked layers (22, 24) composed of cholesteric liquid crystal material that each reflect, in a first viewing direction normal to their surface, selectively light (30, 34) from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light (32, 36) from a second, shorter-wave spectral range. According to the present invention, a first of the two layers (22) is present in the form of a motif to be depicted and reflects light (34) from the green spectral range in the first viewing direction, while the second of the two layers (24) reflects light (32) from the green spectral range in the second viewing direction.

Full Text	Security Element Having Liquid Crystal Layers The present invention relates to a security element, for security papers, value documents and the like, having two stacked layers composed of cholesteric liquid crystal material that each reflect, in a first viewing direction normal to their surface, selectively light from a first, longer-wave spectral range, and reflect, in a second viewing direction at an angle to the normal to their surface, selectively light from a second, shorter-wave spectral range. For protection, valuable articles such as branded articles and value documents are often furnished with security elements that permit the authenticity of the valuable article to be verified, and that simultaneously serve as protection against unauthorized reproduction. In many cases, the particular properties of liquid crystal materials are exploited for this purpose, and especially the viewing-angle-dependent color impression of liquid crystal layers, which is referred to in the following as a color-shift effect. Due to the physical properties, the color-shift effect of a single liquid crystal layer always causes a shift in the reflected light wavelength from a longer- wave range to a shorter-wave range when the liquid crystal layer is tilted out of the vertical viewing direction. To increase the spectrum of achievable color effects, it has thus been proposed to combine multiple liquid crystal layers having different color- shift effects. For example, publication EP 0 720 753 Bl describes a colored material having two stacked chiral liquid crystal polymer lamina, one lamina reflecting infrared light when viewed normal to its surface, and red light when viewed at an angle to the normal to its surface. The second lamina reflects blue light when viewed normal to its surface, and ultraviolet light when viewed at an angle to the normal to its surface. If such a layer arrangement is viewed normal to its main surface, it appears blue, since the first lamina reflects infrared light and thus appears transparent. In contrast, when viewed obliquely, the layer arrangement appears red, since then the second lamina reflects ultraviolet light and thus appears transparent. Overall, the layer arrangement thus displays a reverse color-shift effect compared with normal liquid crystal layers, from blue to red. However, what is problematic in many security elements having liquid crystal layers is that contrasting motif portions in which the brightness of different motif portions changes upon tilting, or the contrast ratios reverse, and in practice are sometimes difficult to perceive. Other embodiments in which liquid crystal layers are combined with conventional semitransparent ink layers require great effort for good depiction quality. Based on that, the object of the present invention is to avoid the disadvantages of the background art and especially to specify a security element of the kind cited above having a conspicuous visual tilt effect and high counterfeit security. This object is solved by the security element having the features of the main claim. A corresponding manufacturing method and a data carrier furnished with such a security element are specified in the coordinated claims. Developments of the present invention are the subject of the dependent claims. According to the present invention, in a generic security element, it is provided that a first of the two liquid crystal layers is present in the form of a motif to be depicted and reflects light from the green spectral range in the first viewing direction, and that the second of the two liquid crystal layers reflects light from the green spectral range in the second viewing direction. As explained in greater detail in the following, the inventive coordination of the color-shift effects of the liquid crystal layers with the dependence of the sensitivity of the human eye on the light wavelength facilitates a surprising and visually impressive tilt effect: When the security element is viewed from above, the motif to be depicted appears for a viewer with brightly shining green against a clearly contrasting background. If the viewer tilts the security element out of the vertical in an oblique viewing direction, then the motif disappears and a uniform green surface without perceptible contrast differences appears as the image impression. Here, in the context of the present invention, the green spectral range comprises all wavelengths that evoke in the viewer a substantially green color impression. The first layer is preferably developed such that it reflects in the first viewing direction light from the wavelength range between 490 nm and 590 nm, particularly preferably between 520 nm and 570 ran. Similarly, the second layer is preferably developed such that it reflects in the second viewing direction light from the wavelength range between 490 nm and 590 nm, particularly preferably between 520 nm and 570 nm. In an advantageous embodiment, the first layer is developed such that it reflects in the second viewing direction light from the blue or ultraviolet spectral range. The second layer advantageously reflects in the first viewing direction light from the red or infrared spectral range. Both layers are expediently arranged on a substrate, the first layer being arranged underneath the second layer in a first variant of the present invention. However, the sequence of the two liquid crystal layers plays no role for the effect according to the present invention, such that, alternatively, the first layer can also be arranged over the second layer. To increase the visual conspicuousness of the color-shift effects, the substrate preferably includes a light-absorbing background. The substrate can especially include a dark, for example black, background or, to be able to systematically influence the color impression of the liquid crystal layers, also a light-absorbing colored background. Also, a substrate having a light-diffractive background, especially having a diffractive black background, may be used since, through such a background, the color effect of the liquid crystals is likewise emphasized. Advantageously, at least one of the two liquid crystal layers is applied contiguously as a film coating and, if applicable, patterned in the form of a motif. Alternatively or additionally, at least one of the two liquid crystal layers is formed by imprinting liquid crystal pigments embedded in a binder matrix. According to an advantageous development, the security element includes at least one further security feature, especially a layer having magnetic, electrically conductive, phosphorescent, fluorescent or other luminescent feature substances, or a polarizing or phase-shifting layer. The further layer can be arranged over, under or between the two layers composed of liquid crystal material. To promote the alignment of the liquid crystal material, also an alignment layer can be provided between the two layers composed of liquid crystal material. It is also understood that the security elements according to the present invention are not limited to embodiments having two liquid crystal layers, but rather that also further liquid crystal layers can be provided to achieve additional effects or impacts. The present invention also comprises a method for manufacturing a security element, for security papers, value documents and the like, in which two layers composed of cholesteric liquid crystal material are stacked that each reflect, in a first viewing direction normal to their surface, selectively light from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light from a second, shorter-wave spectral range, wherein, in the method, a first of the two layers is produced in the form of a motif to be depicted and is developed in such a way that it reflects light from the green spectral range in the first viewing direction, and the second of the two layers is developed in such a way that it reflects light from the green spectral range in the second viewing direction. The present invention further comprises a data carrier, especially a value document, such as a banknote, an identification card or the like, that is furnished with a security element of the kind described. At this point, it is to be noted that the concept according to the present invention can, in principle, also be realized in that, in a generic security element, a first of the two liquid crystal layers is present in the form of a motif to be depicted and reflects light from the green spectral range in the second viewing direction, and that the second of the two liquid crystal layers reflects light from the green spectral range in the first viewing direction. In an advantageous embodiment for this, the second layer is developed such that it reflects light from the blue or ultraviolet spectral range in the second viewing direction. The first layer reflects advantageously light from the red or infrared spectral range in the first viewing direction. As a result, in the above-described embodiment of the concept according to the present invention, the motif is not perceptible in top view (first viewing direction), since the second layer reflects light from the green spectral range in the first viewing direction, the first layer reflects light from the red or infrared spectral range in the first viewing direction, and the human eye is significantly more sensitive to green light. For the viewer, there thus appears in the first viewing direction a uniformly green surface without perceptible contrast differences, while when the security element is tilted out of the vertical in an oblique viewing direction, the green motif to be depicted appears against a contrasting blue background. Also in this embodiment, the effect according to the present invention is based on the coordination of the color-shift effects of the two liquid crystal layers with each other and with the spectral sensitivity of the human eye, which is explained in greater detail below. In practice, however, it has been shown that, in this alternative embodiment of the concept according to the present invention, the contrast differences do not show up quite as well for the viewer, which is why this alternative embodiment is presently not claimed. Moreover, it is to be noted that, for this presently not claimed embodiment of the security element, of course all features of the dependent claims can be combined analagously, as long as a feature combination is not logically precluded. The presently not claimed security element according to the described alternative embodiment can be manufactured through a generic method, provided that a first of the two layers is produced in the form of a motif to be depicted and is developed in such a way that it reflects light from the green spectral range in the second viewing direction, and the second of the two layers is developed in such a way that it reflects light from the green spectral range in the first viewing direction. Here, too, all features of the dependent method claims can be realized analogously, as long as the feature combination is not logically precluded. Further exemplary embodiments and advantages of the present invention are explained below by reference to the drawings, in which a depiction to scale and proportion was omitted in order to improve their clarity. Shown are: Fig. 1 a schematic diagram of a banknote having a security element according to the present invention, Fig. 2 a cross section through the security element in fig. 1, together with the specification of the color (Red, Green or Blue) of the light reflected in different viewing directions by the liquid crystal sublayers, Fig. 3 the realization of the image impression when the security element in fig. 2 is viewed from above, wherein (a) shows the cross section in fig. 2 with the controlling reflection colors for the view from above, and (b) shows the image impression when viewed from above, Fig. 4 the realization of the image impression when the security element in fig. 2 is viewed at an oblique angle, wherein (a) shows the cross section in fig. 2 with the controlling reflection colors for the view at an oblique angle and (b) shows the image impression when viewed obliquely, Fig. 5 a cross section like fig. 2 through an inventive security element having the reverse sequence of the liquid crystal layers, and Fig. 6 a cross section like fig. 2 through a security element according to a further exemplary embodiment of the present invention. The invention will now be explained using a banknote as an example. For this, fig. 1 shows a schematic diagram of a banknote 10 that is provided with an inventive security element 12 that exhibits a conspicuous visual tilt effect. When viewed from above, the security element 12 displays a high-contrast green motif against a red background that, when the security element is tilted out of the vertical, changes into a monochrome green surface without perceptible contrast differences. With reference to the cross section in fig. 2, the security element 12 exhibits an absorbing, especially black background 20 and, arranged on the black background 20, two layers 22, 24 composed of cholesteric liquid crystal material. Here, the first liquid crystal layer 22 is applied in the form of the desired motif, for example the crest motif shown in fig. 3(b). In the exemplary embodiment, the second liquid crystal layer 24 is arranged contiguously over the first liquid crystal layer 22. In other embodiments, however, also the second liquid crystal layer itself can be developed in the form of a motif, as long as it provides only a background for the motif of the first liquid crystal layer. In manufacturing the security element, the liquid crystal layers 22, 24 can first be applied contiguously as a film coating, at least the first liquid crystal layer 22 being patterned in the form of the motif to be depicted after application by known methods. Alternatively, the liquid crystal layers can also be produced by imprinting liquid crystal pigments embedded in a binder matrix. To promote the alignment of the cholesteric liquid crystal material, alignment layers can be provided between the background 20 and the first liquid crystal layer 22 and/or between the two liquid crystal layers 22 and 24. At this point, let it be noted that the security element is not necessarily fabricated by applying the layers directly to a suitable data carrier. Rather, it is also possible to use a so-called transfer method, in which, for example, at least the two liquid crystal layers are applied to a suitable substrate, for example a foil, the layer then located on the outside is provided with an adhesive layer and, with the aid of the adhesive layer, the two layers are applied to an absorbing background to a data carrier. Thereafter, the foil is removed and the security element is arranged on a data carrier with the layer sequence shown in the figures. A number of suitable transfer methods with which a data carrier according to the present invention can be obtained are known to the person of skill in the art. Due to its crystal structure, the cholesteric liquid crystal layers 22, 24 each exhibit a defined color-shift effect, so each reflect, in a first viewing direction normal to their surface, selectively light from a first, longer-wave spectral range, and reflect in a second viewing direction, at an angle to the normal to their surface, selectively light from a second, shorter-wave spectral range. The effect according to the present invention is now based on the coordination of the color-shift effects of the two liquid crystal layers with each other and with the spectral sensitivity of the human eye. Here, advantage is taken of the fact that cholesteric liquid crystal layers reflect circularly polarized light in the corresponding relevant spectral range with a reflection factor of up to 50%. In the longer-wave range, the bands become wider and the extent of the reflection maximum decreases somewhat. Overall, however, the corresponding reflected light quantity in the optical range remains approximately the same. In contrast to this spectral uniformity, the human eye exhibits a pronounced maximum sensitivity in the green spectral range. According to the relative spectral sensitivity luminosity function V(\) of the eye of a standard observer, which was defined by the International Commission on Illumination (CIE), the light-adjusted eye exhibits a maximum sensitivity at a wavelength of 555 nm. At a wavelength of about 475 nm in the blue spectral range, or at a wavelength of about 650 nm in the red spectral range, the sensitivity of the eye in each case is already just 10% of the maximum sensitivity, and decreases further toward the edges of the visible spectral range. For the dark-adapted eye, the luminosity function V'(X) is shifted somewhat into the blue, and the maximum sensitivity there is 507 nm. To exploit this high green sensitivity of the eye to design a surprising visual tilt effect, according to the present invention, the second liquid crystal layer 24 is developed in such a way that it exhibits a color-shift effect from red to green, so reflects, in the first viewing direction normal to its surface, selectively light 30 from the red spectral range (R in fig. 2), and in a second viewing direction at an angle to the normal to its surface, selectively light 32 from the green spectral range (G). Here, according to the preceding embodiments, the green spectral range comprises all wavelengths that evoke in an average viewer a substantially green color impression, especially the wavelength range between 490 nm and 590 nm, and particularly preferably between 520 nm and 570 nm. Further, the first liquid crystal layer 22 that is applied as a motif is developed such that it exhibits a color-shift effect from green to blue, so reflects, in the first viewing direction normal to its surface, selectively light 34 from the green spectral range (G), and in a second viewing direction at an angle to the normal to its surface, selectively light 36 from the blue spectral range (B). When the security element 12 is viewed vertically from above, for the viewer, overall a high-contrast image impression results, as illustrated with reference to figures 2 and 3(a). In the background region 26, in which only the second liquid crystal layer 24 is present, the aggregate color impression 40 is determined exclusively by the red light 30 reflected by the second liquid crystal layer 24 in the first viewing direction, such that the background 50 of the security element 12 appears red. In the motif region 28 in which the two liquid crystal layers 22 and 24 overlap, the aggregate color impression 42 results from the superimposition of the red light 30 reflected by the second liquid crystal layer 24 and the green light 34 reflected by the first liquid crystal layer 22. Due to the significantly higher spectral sensitivity of the human eye for green light compared with red light, the green color impression of the first liquid crystal layer 22 dominates for the viewer, such that the interior of the crest motif 52 appears in brightly shining green. When viewing the security element 12 vertically from above, the viewer thus perceives a clearly contrasting green crest motif 52 against a red background 50, as depicted in fig. 3(b). If the viewer tilts the security element 12 in an oblique viewing direction, then the crest motif disappears and gives way to a substantially uniformly green surface without perceptible contrast differences, as illustrated with reference to figures 2 and 4(a). In the background region 26, in which only the second liquid crystal layer 24 is present, the aggregate color impression 44 is again determined only by the green light 32 reflected by the second liquid crystal layer 24 in the viewing direction, so the background 54 of the security element 12 appears green when viewed obliquely. In the motif region 28 in which the two liquid crystal layers 22 and 24 overlap, the aggregate color impression 46 results from the superimposition of the green light 32 reflected by the second liquid crystal layer 24 and the blue light 36 reflected by the first liquid crystal layer 22. Due to the significantly higher spectral sensitivity of the human eye for green light compared with blue light, the green color impression of the second liquid crystal layer 24 dominates for the viewer, such that the interior of the crest motif 56 likewise appears green. Thus, when the security element 12 is viewed obliquely from above, both the region of the crest motif 56 and the background region 54 appear green, such that the image motif disappears for the viewer for lack of contrast. The tilted security element 12 appears to the viewer as a uniformly green surface, as illustrated in fig. 4(b). The sequence of the two liquid crystal layers can also be reversed, as shown in fig. 5. In the security element 60, on an absorbing colored background 66 is first imprinted contiguously a layer 64, composed of cholesteric liquid crystal material that, like the second liquid crystal layer of the exemplary embodiment in figures 2 to 4, exhibits a color-shift effect from red to green. On this contiguous layer 64 is then imprinted a further layer 62 composed of cholesteric liquid crystal material in the form of the motif to be depicted. The further layer 62 exhibits, like the first liquid crystal layer of the exemplary embodiment in figures 2 to 4, a color-shift effect from green to blue. In the same way, as described above, also in this security element, the depicted motif stands out in high contrast in top view and disappears when the security element is tilted. In addition, the color impression in top view and when viewed obliquely can be influenced by the inherent color of the absorbing colored background 66. It is understood that the described liquid crystal layers can be combined with further layers that, for example, serve to protect the security element or one of its sublayers, to improve alignment or adhesion of the layers, or to further increase the counterfeit security. In fig. 6 is shown, by way of example, a security element 70 according to a further exemplary embodiment of the present invention, in which is applied, on a diffractive black background 72, a magnetic feature substance 74 in the form of a further motif, for example a machine-readable barcode. On this magnetic layer is then provided a layer sequence composed of two stacked liquid crystal layers 22, 24, as described for the exemplary embodiment in fig. 2. If appropriate, also alignment layers can be provided under the first and/or the second liquid crystal layer to facilitate or to improve the alignment of the liquid crystal material. As already mentioned, according to an alternative, presently not claimed variant of the present invention, the first liquid crystal layer (motif layer) can exhibit a color-shift effect from, for example, red (first viewing direction, top view) to green (second viewing direction) and the second liquid crystal layer a color-shift effect from green (first viewing direction) to, for example, blue (second viewing direction). In this variant, when the security element is viewed vertically from above, a substantially uniformly green area without perceptible contrast differences then results, while the viewer perceives, from an oblique viewing direction, the green motif against a blue background. In other words, the motif is perceptible for the viewer only when the security element is viewed at an oblique angle. With reference to this alternative, not presently claimed variant of the present invention, all details given in the description for the claimed variants apply analogously. This also applies to the illustration of the claimed variants in figures 1 to 6. Claims 1. A security element, for security papers, value documents and the like, having two stacked layers composed of cholesteric liquid crystal material that each reflect, in a first viewing direction normal to their surface, selectively light from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light from a second, shorter-wave spectral range, characterized in that a first of the two layers is present in the form of a motif to be depicted and reflects light from the green spectral range in the first viewing direction, and in that the second of the two layers reflects light from the green spectral range in the second viewing direction. 2. The security element according to claim 1, characterized in that the first layer reflects in the first viewing direction light from the wavelength range between 490 nm and 590 nm, preferably between 520 run and 570 nm. 3. The security element according to claim 1 or 2, characterized in that the second layer reflects in the second viewing direction light from the wavelength range between 490 run and 590 nm, preferably between 520 nm and 570 nm. 4. The security element according to at least one of claims 1 to 3, characterized in that the first layer reflects light from the blue or ultraviolet spectral range in the second viewing direction. 5. The security element according to at least one of claims 1 to 4, characterized in that the second layer reflects light from the red or infrared spectral range in the first viewing direction. 6. The security element according to at least one of claims 1 to 5, characterized in that the two layers are arranged on a substrate and the first layer is arranged underneath the second layer. 7. The security element according to at least one of claims 1 to 5, characterized in that the two layers are arranged on a substrate and the first layer is arranged over the second layer. 8. The security element according to claim 6 or 7, characterized in that the substrate includes a light-absorbing background. 9. The security element according to claim 8, characterized in that the substrate includes a dark, especially black background. 10. The security element according to claim 8, characterized in that the substrate includes a light-absorbing colored background. 11. The security element according to claim 6 or 7, characterized in that the substrate includes a light-diffractive background, especially a diffractive black background. 12. The security element according to at least one of claims 1 to 11, characterized in that at least one of the two liquid crystal layers is applied contiguously as a film coating and, if applicable, is patterned in the form of a motif. 13. The security element according to at least one of claims 1 to 11, characterized in that at least one of the two liquid crystal layers is formed by imprinting liquid crystal pigments embedded in a binder matrix. 14. The security element according to at least one of claims 1 to 13, characterized in that the security element includes at least one further security feature, especially includes a layer having magnetic, electrically conductive, phosphorescent, fluorescent or other luminescent feature substances or a polarizing or phase-shifting layer. 15. The security element according to claim 14, characterized in that the further layer is arranged over, under or between the two layers composed of liquid crystal material. 16. The security element according to at least one of claims 1 to 15, characterized in that an alignment layer is arranged between the two layers composed of liquid crystal material. 17. A method for manufacturing a security element, for security papers, value documents and the like, in which two layers composed of cholesteric liquid crystal material are stacked that each reflect, in a first viewing direction normal to their surface, selectively light from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light from a second, shorter-wave spectral range, characterized in that a first of the two layers is produced in the form of a motif to be depicted and is developed in such a way that it reflects light from the green spectral range in the first viewing direction, and in that the second of the two layers is developed in such a way that it reflects light from the green spectral range in the second viewing direction. 18. The method according to claim 17, characterized in that the two layers are arranged on a substrate and the first layer is arranged underneath the second layer. 19. The method according to claim 17, characterized in that the two layers are arranged on a substrate and the first layer is arranged above the second layer. 20. The method according to at least one of claims 17 to 19, characterized in that at least one of the two liquid crystal layers is applied contiguously as a film coating and, if applicable, is patterned in the form of a motif. 21. The method according to at least one of claims 17 to 19, characterized in that at least one of the two liquid crystal layers is formed by imprinting liquid crystal pigments embedded in a binder matrix. 22. The method according to at least one of claims 17 to 21, characterized in that the security element is combined with at least one further security feature, especially with a layer having magnetic, electrically conductive, phosphorescent, fluorescent or other luminescent feature substances or with a polarizing or phase-shifting layer. 23. The method according to claim 22, characterized in that the further layer is arranged over, under or between the two layers composed of liquid crystal material. 24. The method according to at least one of claims 17 to 23, characterized in that an alignment layer is arranged between the two layers composed of liquid crystal material. 25. A data carrier, especially a value document, such as a banknote, identification card or the like, that exhibits a security element according to at least one of claims 1 to 16 or a security element manufactured according to at least one of claims 17 to 24. 26. The use of a security element according to at least one of claims 1 to 16 or of a security element manufactured according to at least one of claims 17 to 24 or of a data carrier according to claim 25 for securing goods of any kind against counterfeiting. The present invention relates to a security element (12) for security papers, value documents and the like, having two stacked layers (22, 24) composed of cholesteric liquid crystal material that each reflect, in a first viewing direction normal to their surface, selectively light (30, 34) from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light (32, 36) from a second, shorter-wave spectral range. According to the present invention, a first of the two layers (22) is present in the form of a motif to be depicted and reflects light (34) from the green spectral range in the first viewing direction, while the second of the two layers (24) reflects light (32) from the green spectral range in the second viewing direction.

Full Text

Security Element Having Liquid Crystal Layers
The present invention relates to a security element, for security papers, value
documents and the like, having two stacked layers composed of cholesteric
liquid crystal material that each reflect, in a first viewing direction normal to
their surface, selectively light from a first, longer-wave spectral range, and
reflect, in a second viewing direction at an angle to the normal to their
surface, selectively light from a second, shorter-wave spectral range.
For protection, valuable articles such as branded articles and value
documents are often furnished with security elements that permit the
authenticity of the valuable article to be verified, and that simultaneously
serve as protection against unauthorized reproduction. In many cases, the
particular properties of liquid crystal materials are exploited for this
purpose, and especially the viewing-angle-dependent color impression of
liquid crystal layers, which is referred to in the following as a color-shift
effect.
Due to the physical properties, the color-shift effect of a single liquid crystal
layer always causes a shift in the reflected light wavelength from a longer-
wave range to a shorter-wave range when the liquid crystal layer is tilted out
of the vertical viewing direction.
To increase the spectrum of achievable color effects, it has thus been
proposed to combine multiple liquid crystal layers having different color-
shift effects. For example, publication EP 0 720 753 Bl describes a colored
material having two stacked chiral liquid crystal polymer lamina, one lamina
reflecting infrared light when viewed normal to its surface, and red light
when viewed at an angle to the normal to its surface. The second lamina
reflects blue light when viewed normal to its surface, and ultraviolet light
when viewed at an angle to the normal to its surface.

If such a layer arrangement is viewed normal to its main surface, it appears
blue, since the first lamina reflects infrared light and thus appears
transparent. In contrast, when viewed obliquely, the layer arrangement
appears red, since then the second lamina reflects ultraviolet light and thus
appears transparent. Overall, the layer arrangement thus displays a reverse
color-shift effect compared with normal liquid crystal layers, from blue to
red.
However, what is problematic in many security elements having liquid
crystal layers is that contrasting motif portions in which the brightness of
different motif portions changes upon tilting, or the contrast ratios reverse,
and in practice are sometimes difficult to perceive. Other embodiments in
which liquid crystal layers are combined with conventional semitransparent
ink layers require great effort for good depiction quality.
Based on that, the object of the present invention is to avoid the
disadvantages of the background art and especially to specify a security
element of the kind cited above having a conspicuous visual tilt effect and
high counterfeit security.
This object is solved by the security element having the features of the main
claim. A corresponding manufacturing method and a data carrier furnished
with such a security element are specified in the coordinated claims.
Developments of the present invention are the subject of the dependent
claims.
According to the present invention, in a generic security element, it is
provided that

a first of the two liquid crystal layers is present in the form of a motif
to be depicted and reflects light from the green spectral range in the
first viewing direction, and that
the second of the two liquid crystal layers reflects light from the green
spectral range in the second viewing direction.
As explained in greater detail in the following, the inventive coordination of
the color-shift effects of the liquid crystal layers with the dependence of the
sensitivity of the human eye on the light wavelength facilitates a surprising
and visually impressive tilt effect: When the security element is viewed from
above, the motif to be depicted appears for a viewer with brightly shining
green against a clearly contrasting background. If the viewer tilts the security
element out of the vertical in an oblique viewing direction, then the motif
disappears and a uniform green surface without perceptible contrast
differences appears as the image impression.
Here, in the context of the present invention, the green spectral range
comprises all wavelengths that evoke in the viewer a substantially green
color impression. The first layer is preferably developed such that it reflects
in the first viewing direction light from the wavelength range between
490 nm and 590 nm, particularly preferably between 520 nm and 570 ran.
Similarly, the second layer is preferably developed such that it reflects in the
second viewing direction light from the wavelength range between 490 nm
and 590 nm, particularly preferably between 520 nm and 570 nm.
In an advantageous embodiment, the first layer is developed such that it
reflects in the second viewing direction light from the blue or ultraviolet
spectral range. The second layer advantageously reflects in the first viewing
direction light from the red or infrared spectral range.

Both layers are expediently arranged on a substrate, the first layer being
arranged underneath the second layer in a first variant of the present
invention. However, the sequence of the two liquid crystal layers plays no
role for the effect according to the present invention, such that, alternatively,
the first layer can also be arranged over the second layer.
To increase the visual conspicuousness of the color-shift effects, the substrate
preferably includes a light-absorbing background. The substrate can
especially include a dark, for example black, background or, to be able to
systematically influence the color impression of the liquid crystal layers, also
a light-absorbing colored background.
Also, a substrate having a light-diffractive background, especially having a
diffractive black background, may be used since, through such a
background, the color effect of the liquid crystals is likewise emphasized.
Advantageously, at least one of the two liquid crystal layers is applied
contiguously as a film coating and, if applicable, patterned in the form of a
motif. Alternatively or additionally, at least one of the two liquid crystal
layers is formed by imprinting liquid crystal pigments embedded in a binder
matrix.
According to an advantageous development, the security element includes at
least one further security feature, especially a layer having magnetic,
electrically conductive, phosphorescent, fluorescent or other luminescent
feature substances, or a polarizing or phase-shifting layer. The further layer
can be arranged over, under or between the two layers composed of liquid
crystal material.

To promote the alignment of the liquid crystal material, also an alignment
layer can be provided between the two layers composed of liquid crystal
material.
It is also understood that the security elements according to the present
invention are not limited to embodiments having two liquid crystal layers,
but rather that also further liquid crystal layers can be provided to achieve
additional effects or impacts.
The present invention also comprises a method for manufacturing a security
element, for security papers, value documents and the like, in which two
layers composed of cholesteric liquid crystal material are stacked that each
reflect, in a first viewing direction normal to their surface, selectively light
from a first, longer-wave spectral range, and in a second viewing direction at
an angle to the normal to their surface, selectively light from a second,
shorter-wave spectral range, wherein, in the method,
a first of the two layers is produced in the form of a motif to be
depicted and is developed in such a way that it reflects light from the
green spectral range in the first viewing direction, and
the second of the two layers is developed in such a way that it reflects
light from the green spectral range in the second viewing direction.
The present invention further comprises a data carrier, especially a value
document, such as a banknote, an identification card or the like, that is
furnished with a security element of the kind described.

At this point, it is to be noted that the concept according to the present
invention can, in principle, also be realized in that, in a generic security
element,
a first of the two liquid crystal layers is present in the form of a motif
to be depicted and reflects light from the green spectral range in the
second viewing direction, and that
the second of the two liquid crystal layers reflects light from the green
spectral range in the first viewing direction.
In an advantageous embodiment for this, the second layer is developed such
that it reflects light from the blue or ultraviolet spectral range in the second
viewing direction. The first layer reflects advantageously light from the red
or infrared spectral range in the first viewing direction.
As a result, in the above-described embodiment of the concept according to
the present invention, the motif is not perceptible in top view (first viewing
direction), since the second layer reflects light from the green spectral range
in the first viewing direction, the first layer reflects light from the red or
infrared spectral range in the first viewing direction, and the human eye is
significantly more sensitive to green light. For the viewer, there thus appears
in the first viewing direction a uniformly green surface without perceptible
contrast differences, while when the security element is tilted out of the
vertical in an oblique viewing direction, the green motif to be depicted
appears against a contrasting blue background. Also in this embodiment, the
effect according to the present invention is based on the coordination of the
color-shift effects of the two liquid crystal layers with each other and with
the spectral sensitivity of the human eye, which is explained in greater detail
below.

In practice, however, it has been shown that, in this alternative embodiment
of the concept according to the present invention, the contrast differences do
not show up quite as well for the viewer, which is why this alternative
embodiment is presently not claimed.
Moreover, it is to be noted that, for this presently not claimed embodiment of
the security element, of course all features of the dependent claims can be
combined analagously, as long as a feature combination is not logically
precluded.
The presently not claimed security element according to the described
alternative embodiment can be manufactured through a generic method,
provided that
a first of the two layers is produced in the form of a motif to be
depicted and is developed in such a way that it reflects light from the
green spectral range in the second viewing direction, and
the second of the two layers is developed in such a way that it reflects
light from the green spectral range in the first viewing direction.
Here, too, all features of the dependent method claims can be realized
analogously, as long as the feature combination is not logically precluded.
Further exemplary embodiments and advantages of the present invention
are explained below by reference to the drawings, in which a depiction to
scale and proportion was omitted in order to improve their clarity.
Shown are:

Fig. 1 a schematic diagram of a banknote having a security element
according to the present invention,
Fig. 2 a cross section through the security element in fig. 1, together
with the specification of the color (Red, Green or Blue) of the
light reflected in different viewing directions by the liquid
crystal sublayers,
Fig. 3 the realization of the image impression when the security
element in fig. 2 is viewed from above, wherein (a) shows the
cross section in fig. 2 with the controlling reflection colors for
the view from above, and (b) shows the image impression
when viewed from above,
Fig. 4 the realization of the image impression when the security
element in fig. 2 is viewed at an oblique angle, wherein (a)
shows the cross section in fig. 2 with the controlling reflection
colors for the view at an oblique angle and (b) shows the
image impression when viewed obliquely,
Fig. 5 a cross section like fig. 2 through an inventive security
element having the reverse sequence of the liquid crystal
layers, and
Fig. 6 a cross section like fig. 2 through a security element according
to a further exemplary embodiment of the present invention.
The invention will now be explained using a banknote as an example. For
this, fig. 1 shows a schematic diagram of a banknote 10 that is provided with

an inventive security element 12 that exhibits a conspicuous visual tilt effect.
When viewed from above, the security element 12 displays a high-contrast
green motif against a red background that, when the security element is
tilted out of the vertical, changes into a monochrome green surface without
perceptible contrast differences.
With reference to the cross section in fig. 2, the security element 12 exhibits
an absorbing, especially black background 20 and, arranged on the black
background 20, two layers 22, 24 composed of cholesteric liquid crystal
material. Here, the first liquid crystal layer 22 is applied in the form of the
desired motif, for example the crest motif shown in fig. 3(b). In the
exemplary embodiment, the second liquid crystal layer 24 is arranged
contiguously over the first liquid crystal layer 22. In other embodiments,
however, also the second liquid crystal layer itself can be developed in the
form of a motif, as long as it provides only a background for the motif of the
first liquid crystal layer.
In manufacturing the security element, the liquid crystal layers 22, 24 can
first be applied contiguously as a film coating, at least the first liquid crystal
layer 22 being patterned in the form of the motif to be depicted after
application by known methods. Alternatively, the liquid crystal layers can
also be produced by imprinting liquid crystal pigments embedded in a
binder matrix. To promote the alignment of the cholesteric liquid crystal
material, alignment layers can be provided between the background 20 and
the first liquid crystal layer 22 and/or between the two liquid crystal layers
22 and 24.
At this point, let it be noted that the security element is not necessarily
fabricated by applying the layers directly to a suitable data carrier. Rather, it
is also possible to use a so-called transfer method, in which, for example, at

least the two liquid crystal layers are applied to a suitable substrate, for
example a foil, the layer then located on the outside is provided with an
adhesive layer and, with the aid of the adhesive layer, the two layers are
applied to an absorbing background to a data carrier. Thereafter, the foil is
removed and the security element is arranged on a data carrier with the
layer sequence shown in the figures. A number of suitable transfer methods
with which a data carrier according to the present invention can be obtained
are known to the person of skill in the art.
Due to its crystal structure, the cholesteric liquid crystal layers 22, 24 each
exhibit a defined color-shift effect, so each reflect, in a first viewing direction
normal to their surface, selectively light from a first, longer-wave spectral
range, and reflect in a second viewing direction, at an angle to the normal to
their surface, selectively light from a second, shorter-wave spectral range.
The effect according to the present invention is now based on the
coordination of the color-shift effects of the two liquid crystal layers with
each other and with the spectral sensitivity of the human eye. Here,
advantage is taken of the fact that cholesteric liquid crystal layers reflect
circularly polarized light in the corresponding relevant spectral range with a
reflection factor of up to 50%. In the longer-wave range, the bands become
wider and the extent of the reflection maximum decreases somewhat.
Overall, however, the corresponding reflected light quantity in the optical
range remains approximately the same.
In contrast to this spectral uniformity, the human eye exhibits a pronounced
maximum sensitivity in the green spectral range. According to the relative
spectral sensitivity luminosity function V(\) of the eye of a standard
observer, which was defined by the International Commission on
Illumination (CIE), the light-adjusted eye exhibits a maximum sensitivity at a

wavelength of 555 nm. At a wavelength of about 475 nm in the blue spectral
range, or at a wavelength of about 650 nm in the red spectral range, the
sensitivity of the eye in each case is already just 10% of the maximum
sensitivity, and decreases further toward the edges of the visible spectral
range. For the dark-adapted eye, the luminosity function V'(X) is shifted
somewhat into the blue, and the maximum sensitivity there is 507 nm.
To exploit this high green sensitivity of the eye to design a surprising visual
tilt effect, according to the present invention, the second liquid crystal layer
24 is developed in such a way that it exhibits a color-shift effect from red to
green, so reflects, in the first viewing direction normal to its surface,
selectively light 30 from the red spectral range (R in fig. 2), and in a second
viewing direction at an angle to the normal to its surface, selectively light 32
from the green spectral range (G).
Here, according to the preceding embodiments, the green spectral range
comprises all wavelengths that evoke in an average viewer a substantially
green color impression, especially the wavelength range between 490 nm
and 590 nm, and particularly preferably between 520 nm and 570 nm.
Further, the first liquid crystal layer 22 that is applied as a motif is developed
such that it exhibits a color-shift effect from green to blue, so reflects, in the
first viewing direction normal to its surface, selectively light 34 from the
green spectral range (G), and in a second viewing direction at an angle to the
normal to its surface, selectively light 36 from the blue spectral range (B).
When the security element 12 is viewed vertically from above, for the
viewer, overall a high-contrast image impression results, as illustrated with
reference to figures 2 and 3(a). In the background region 26, in which only
the second liquid crystal layer 24 is present, the aggregate color impression

40 is determined exclusively by the red light 30 reflected by the second liquid
crystal layer 24 in the first viewing direction, such that the background 50 of
the security element 12 appears red.
In the motif region 28 in which the two liquid crystal layers 22 and 24
overlap, the aggregate color impression 42 results from the superimposition
of the red light 30 reflected by the second liquid crystal layer 24 and the
green light 34 reflected by the first liquid crystal layer 22. Due to the
significantly higher spectral sensitivity of the human eye for green light
compared with red light, the green color impression of the first liquid crystal
layer 22 dominates for the viewer, such that the interior of the crest motif 52
appears in brightly shining green.
When viewing the security element 12 vertically from above, the viewer thus
perceives a clearly contrasting green crest motif 52 against a red background
50, as depicted in fig. 3(b).
If the viewer tilts the security element 12 in an oblique viewing direction,
then the crest motif disappears and gives way to a substantially uniformly
green surface without perceptible contrast differences, as illustrated with
reference to figures 2 and 4(a).
In the background region 26, in which only the second liquid crystal layer 24
is present, the aggregate color impression 44 is again determined only by the
green light 32 reflected by the second liquid crystal layer 24 in the viewing
direction, so the background 54 of the security element 12 appears green
when viewed obliquely.
In the motif region 28 in which the two liquid crystal layers 22 and 24
overlap, the aggregate color impression 46 results from the superimposition

of the green light 32 reflected by the second liquid crystal layer 24 and the
blue light 36 reflected by the first liquid crystal layer 22. Due to the
significantly higher spectral sensitivity of the human eye for green light
compared with blue light, the green color impression of the second liquid
crystal layer 24 dominates for the viewer, such that the interior of the crest
motif 56 likewise appears green.
Thus, when the security element 12 is viewed obliquely from above, both the
region of the crest motif 56 and the background region 54 appear green, such
that the image motif disappears for the viewer for lack of contrast. The tilted
security element 12 appears to the viewer as a uniformly green surface, as
illustrated in fig. 4(b).
The sequence of the two liquid crystal layers can also be reversed, as shown
in fig. 5. In the security element 60, on an absorbing colored background 66 is
first imprinted contiguously a layer 64, composed of cholesteric liquid
crystal material that, like the second liquid crystal layer of the exemplary
embodiment in figures 2 to 4, exhibits a color-shift effect from red to green.
On this contiguous layer 64 is then imprinted a further layer 62 composed of
cholesteric liquid crystal material in the form of the motif to be depicted. The
further layer 62 exhibits, like the first liquid crystal layer of the exemplary
embodiment in figures 2 to 4, a color-shift effect from green to blue. In the
same way, as described above, also in this security element, the depicted
motif stands out in high contrast in top view and disappears when the
security element is tilted. In addition, the color impression in top view and
when viewed obliquely can be influenced by the inherent color of the
absorbing colored background 66.

It is understood that the described liquid crystal layers can be combined with
further layers that, for example, serve to protect the security element or one
of its sublayers, to improve alignment or adhesion of the layers, or to further
increase the counterfeit security.
In fig. 6 is shown, by way of example, a security element 70 according to a
further exemplary embodiment of the present invention, in which is applied,
on a diffractive black background 72, a magnetic feature substance 74 in the
form of a further motif, for example a machine-readable barcode. On this
magnetic layer is then provided a layer sequence composed of two stacked
liquid crystal layers 22, 24, as described for the exemplary embodiment in
fig. 2. If appropriate, also alignment layers can be provided under the first
and/or the second liquid crystal layer to facilitate or to improve the
alignment of the liquid crystal material.
As already mentioned, according to an alternative, presently not claimed
variant of the present invention, the first liquid crystal layer (motif layer) can
exhibit a color-shift effect from, for example, red (first viewing direction, top
view) to green (second viewing direction) and the second liquid crystal layer
a color-shift effect from green (first viewing direction) to, for example, blue
(second viewing direction). In this variant, when the security element is
viewed vertically from above, a substantially uniformly green area without
perceptible contrast differences then results, while the viewer perceives,
from an oblique viewing direction, the green motif against a blue
background. In other words, the motif is perceptible for the viewer only
when the security element is viewed at an oblique angle. With reference to
this alternative, not presently claimed variant of the present invention, all
details given in the description for the claimed variants apply analogously.
This also applies to the illustration of the claimed variants in figures 1 to 6.

Claims
1. A security element, for security papers, value documents and the like,
having two stacked layers composed of cholesteric liquid crystal material
that each reflect, in a first viewing direction normal to their surface,
selectively light from a first, longer-wave spectral range, and in a second
viewing direction at an angle to the normal to their surface, selectively light
from a second, shorter-wave spectral range,
characterized in that
a first of the two layers is present in the form of a motif to be depicted
and reflects light from the green spectral range in the first viewing
direction, and in that
the second of the two layers reflects light from the green spectral
range in the second viewing direction.
2. The security element according to claim 1, characterized in that the
first layer reflects in the first viewing direction light from the wavelength
range between 490 nm and 590 nm, preferably between 520 run and 570 nm.
3. The security element according to claim 1 or 2, characterized in that
the second layer reflects in the second viewing direction light from the
wavelength range between 490 run and 590 nm, preferably between 520 nm
and 570 nm.
4. The security element according to at least one of claims 1 to 3,
characterized in that the first layer reflects light from the blue or ultraviolet
spectral range in the second viewing direction.

5. The security element according to at least one of claims 1 to 4,
characterized in that the second layer reflects light from the red or infrared
spectral range in the first viewing direction.
6. The security element according to at least one of claims 1 to 5,
characterized in that the two layers are arranged on a substrate and the first
layer is arranged underneath the second layer.
7. The security element according to at least one of claims 1 to 5,
characterized in that the two layers are arranged on a substrate and the first
layer is arranged over the second layer.
8. The security element according to claim 6 or 7, characterized in that
the substrate includes a light-absorbing background.
9. The security element according to claim 8, characterized in that the
substrate includes a dark, especially black background.
10. The security element according to claim 8, characterized in that the
substrate includes a light-absorbing colored background.
11. The security element according to claim 6 or 7, characterized in that
the substrate includes a light-diffractive background, especially a diffractive
black background.
12. The security element according to at least one of claims 1 to 11,
characterized in that at least one of the two liquid crystal layers is applied
contiguously as a film coating and, if applicable, is patterned in the form of a
motif.

13. The security element according to at least one of claims 1 to 11,
characterized in that at least one of the two liquid crystal layers is formed by
imprinting liquid crystal pigments embedded in a binder matrix.
14. The security element according to at least one of claims 1 to 13,
characterized in that the security element includes at least one further
security feature, especially includes a layer having magnetic, electrically
conductive, phosphorescent, fluorescent or other luminescent feature
substances or a polarizing or phase-shifting layer.
15. The security element according to claim 14, characterized in that the
further layer is arranged over, under or between the two layers composed of
liquid crystal material.
16. The security element according to at least one of claims 1 to 15,
characterized in that an alignment layer is arranged between the two layers
composed of liquid crystal material.
17. A method for manufacturing a security element, for security papers,
value documents and the like, in which two layers composed of cholesteric
liquid crystal material are stacked that each reflect, in a first viewing
direction normal to their surface, selectively light from a first, longer-wave
spectral range, and in a second viewing direction at an angle to the normal to
their surface, selectively light from a second, shorter-wave spectral range,
characterized in that

a first of the two layers is produced in the form of a motif to be
depicted and is developed in such a way that it reflects light from the
green spectral range in the first viewing direction, and in that
the second of the two layers is developed in such a way that it reflects
light from the green spectral range in the second viewing direction.
18. The method according to claim 17, characterized in that the two
layers are arranged on a substrate and the first layer is arranged underneath
the second layer.
19. The method according to claim 17, characterized in that the two
layers are arranged on a substrate and the first layer is arranged above the
second layer.
20. The method according to at least one of claims 17 to 19, characterized
in that at least one of the two liquid crystal layers is applied contiguously as
a film coating and, if applicable, is patterned in the form of a motif.
21. The method according to at least one of claims 17 to 19, characterized
in that at least one of the two liquid crystal layers is formed by imprinting
liquid crystal pigments embedded in a binder matrix.
22. The method according to at least one of claims 17 to 21, characterized
in that the security element is combined with at least one further security
feature, especially with a layer having magnetic, electrically conductive,
phosphorescent, fluorescent or other luminescent feature substances or with
a polarizing or phase-shifting layer.

23. The method according to claim 22, characterized in that the further
layer is arranged over, under or between the two layers composed of liquid
crystal material.
24. The method according to at least one of claims 17 to 23, characterized
in that an alignment layer is arranged between the two layers composed of
liquid crystal material.
25. A data carrier, especially a value document, such as a banknote,
identification card or the like, that exhibits a security element according to at
least one of claims 1 to 16 or a security element manufactured according to at
least one of claims 17 to 24.
26. The use of a security element according to at least one of claims 1 to 16
or of a security element manufactured according to at least one of claims 17
to 24 or of a data carrier according to claim 25 for securing goods of any kind
against counterfeiting.

The present invention relates to a security element (12) for security papers, value documents and the like, having two stacked layers (22, 24) composed of cholesteric liquid crystal material that each reflect, in a first
viewing direction normal to their surface, selectively light (30, 34) from a first, longer-wave spectral range, and in a second viewing direction at an angle to the normal to their surface, selectively light (32, 36) from a second, shorter-wave spectral range. According to the present invention, a first of the
two layers (22) is present in the form of a motif to be depicted and reflects light (34) from the green spectral range in the first viewing direction, while the second of the two layers (24) reflects light (32) from the green spectral range in the second viewing direction.

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Patent Number

270431

Indian Patent Application Number

1217/KOLNP/2009

PG Journal Number

52/2015

Publication Date

25-Dec-2015

Grant Date

21-Dec-2015

Date of Filing

31-Mar-2009

Name of Patentee

GIESECKE & DEVRIENT GMBH

Applicant Address

PRINZREGENTENSTRASSE 159, 81677 MÜNCHEN

Inventors:

#	Inventor's Name	Inventor's Address
1	HOFFMÜLLER, WINFRIED	MARKSTRASSE 75, 83646 BAD TÖLZ
2	BURCHARD, THEODOR	BUCHLEITEN 1, 83703 GMUND

PCT International Classification Number

B42D 15/00

PCT International Application Number

PCT/EP2007/008584

PCT International Filing date

2007-10-04

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10 2006 049 951.4	2006-10-19	Germany