EP1945465B1 - Transparent security element and method for its production - Google Patents

Abstract

The invention relates to a double-sided transparent security element (20) for protection of valuable objects having motifs in the form of patterns, characters or codings introduced into the security element, comprising a first optically active layer (24) composed of a cholesteric liquid-crystal material, which selectively reflects light with a predetermined circular polarization in a predetermined wavelength range, and a second (22) and third (26) optically active layer, which are arranged on both sides of the first optically active layer (24) and each form a phase-shifting layer for light from the predetermined wavelength range. The second and third optically active layers (22, 26) in this case contain motifs which are independent of one another, in order to verify the security element (20) from opposite sides.

Description

The invention relates to a double-sided see-through security element for the protection of valuables, with introduced into the security element motifs in the form of patterns, characters or codes. The invention further relates to a method for producing such a security element as well as a valuable object equipped with such a security element.

Review windows have long been known for banknotes in the field of polymer notes. Providing a banknote with a see-through window on its own, however, does not yet provide additional counterfeit security for the note. Therefore, various security features have been proposed for see-through windows and partly also converted into banknotes.

So are from the publication AU 488 652 Security documents known in which security features can be checked by means of transmitted light inspection. Between plastic webs, an optically variable security element is arranged, which can be viewed through a transparent see-through window in the overlying cover sheet.

As security elements often optically variable elements are used, which give the viewer a different image impression, for example a different color impression, from different viewing angles. From the publication EP 0 435 029 A2 is such a security element with a plastic-like layer of a liquid crystal polymer is known which shows at room temperature a pronounced color change game. The optically variable effects of the liquid crystal polymers can be combined with a print image of black color, which can produce patterns that appear black in transmitted light. In the reflected light however, only the areas provided with the print image show a color change.

The publication WO 03/006261 describes a document in the form of a banknote containing a rectangular polymer sheet with two opposing surfaces for carrying respective information. In the sheet, a rectangular security window is arranged, which contains an optically variable element. This has a rectangular polarizing element with two opposing surfaces containing a predetermined first and a predetermined second birefringent pattern.

Other valuables, such as branded goods, documents of value, such as certificates, vouchers, checks, or other forgery-prone papers, such as passports and other identity documents, are often provided with security elements for security, which allow a verification of the authenticity of the object of value and at the same time as protection to serve unauthorized reproduction.

Based on this, the present invention seeks to provide a security element of the type mentioned above with high security against counterfeiting, which avoids the disadvantages of the prior art.

This object is achieved by the security element and the manufacturing method having the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

• eine erste optisch aktive Schicht aus einem cholesterischen flüssigkristallinen Material, die in einem vorbestimmten Wellenlängenbereich Licht mit einer vorbestimmten zirkularen Polarisation selektiv reflektiert, und
• eine zweite und dritte optisch aktive Schicht, die beiderseits der ersten optisch aktiven Schicht angeordnet sind, und die für Licht aus dem vorbestimmten Wellenlängenbereich jeweils eine phasenschiebende Schicht bilden.

According to the invention contains a generic see-through security element

• a first optically active layer of a cholesteric liquid crystalline material which selectively reflects light with a predetermined circular polarization in a predetermined wavelength range, and
• a second and third optically active layer disposed on either side of the first optically active layer, and for light from the each predetermined wavelength range form a phase-shifting layer.

The second and third optically active layers contain mutually independent motifs for verification of the security element from opposite sides. Independent from one another means that there are two separate motifs, which can each be checked from opposite sides of the security element, and which need not be related to each other.

Phase shifting layers are optically active layers which act on the phase of a transmitted light wave. The partial beams of an incident polarized light wave receive a path difference and thus a phase difference due to different refractive indices. If the phase difference of the two partial beams is just half or quarter wavelengths, so-called λ / 2 or λ / 4 layers are obtained. However, the phase shifting layers of the present invention are not limited to these values, but can produce any phase difference. In the context of the invention, one or both of the phase-shifting layers is preferably formed from nematic liquid-crystalline material.

As explained in detail below, the security element can be viewed for verification, for example, with a linear polarizer or a circular polarizer, or illuminated with appropriately polarized light. While when viewing the security element in transmitted light, typically none of the motifs of the phase-shifting layers is recognizable, the motifs are made clearly visible by means of a corresponding polarization filter. It is understood that the presence of the motifs can also be checked by machine. The following Effects are particularly evident against a dark background.

In a preferred embodiment of the invention, one or both of the phase-shifting layers forms a λ / 4-layer for light from the predetermined wavelength range at least in partial regions. As explained further below, these embodiments are particularly designed for verification by means of linear polarizers.

In other, likewise preferred embodiments of the invention, one or both of the phase-shifting layers for light from the predetermined wavelength range forms a λ / 2-layer at least in partial regions. This embodiment is particularly aligned to the verification by means of circular polarizers.

In all embodiments, the motifs may be advantageously formed by partially different orientation of one or both of the phase-shifting layers. The motivating effect is based on a different depending on the orientation of the interaction of the incident polarized light with the phase-shifting layers. For example, incident linearly polarized light may be converted to right or left polarized light depending on the layer orientation, as explained in more detail below. It is understood that other interactions between the phase shifting layers and the polarization of the incident light can be exploited.

For aligning the phase-shifting layers one or more photo-alignment layers are expediently provided. Alignment layers (alignment layers) of linear photopolymers exposed by exposure can be patterned with polarized light, are known. In an alignment layer, for example, two alignment directions with photo-resolution can be predetermined by exposing the alignment layer in a first step through a mask with linearly polarized UV radiation. In a second step, the mask is removed and the previously unexposed areas are exposed with 90 ° rotated linearly polarized UV radiation. If nematic liquid crystals are applied to such an alignment layer, then they respectively orient themselves at the local orientation of the alignment layer.

Alternatively, structured, in particular embossed, alignment layers, which are subdivided into regions with different alignment directions, can also be provided in another way for aligning the phase-shifting layers. In an expedient embodiment, the embossed alignment layer has a diffractive structure. If nematic liquid crystals are applied to such an alignment layer, they are oriented, as in the case of the above-described photoalignment layer, respectively at the local alignment of the alignment layer.

The motifs can also be formed by a region-wise different thickness of one or both of the phase-shifting layers. For example, the degree of phase rotation may be proportional to the layer thickness, so that the influence of the polarized light on the layer thickness can be adjusted specifically.

One or both of the phase-shifting layers can also be present only in regions in the form of a motif. This design presents itself as an extreme case of the aforementioned embodiment, when the recessed Regions of the phase-shifting layers are considered as layers with a layer thickness of zero.

In particular, if one or both of the phase-shifting layers are only partially present, they are advantageously arranged immediately above or below a full-surface transfer assist layer which serves to produce the transfer of the partial layer to a target substrate. The transfer assist layer expediently has greater adhesion to the partial layer than to a substrate to be detached, so that the latter can be removed after transfer to the target substrate without damaging the phase-shifting layer.

In this way, very complex layer structures can be created by repeatedly transferring individual layers or layer composites to one another, whereby optimum production conditions can be selected for each layer or layer composite by the separate production. Thus, according to the invention, it is also possible to combine layered composites which require mutually exclusive production conditions or interfering substrates, since these can be removed during or after the joining of the partial layer composites.

As a transfer assist layer preferably a UV-curing lacquer layer is applied, in particular printed. The UV-curable lacquer layer expediently contains photoinitiators, it being necessary to choose a balance between sufficiently high adhesion of the transfer assist layer to the layer to be transferred and sufficiently low adhesion to the substrate to be removed in order to select the optimum photoinitiator.

In a further preferred embodiment, a layer of cholesteric liquid-crystalline material is applied, for example printed, as the transfer assist layer. In a particularly preferred embodiment, this function is taken over by the first optically active layer so that it simultaneously forms a transfer assist layer for one of the phase-shifting layers.

In expedient embodiments of the security elements according to the invention, one or more optically substantially isotropic adhesive layers and / or one or more optically substantially isotropic adhesion promoter layers are provided.

To further increase the security against counterfeiting, the security elements can also have negative information in the form of patterns, characters or codes which are formed by recesses in one or more of the optically active layers. Also, non-transparent regions may be provided in the form of patterns, characters, or codes formed by partially applying ink or magnetic ink on one or both sides of the first optically active layer.

In an advantageous variant of the invention, the optically active layers are arranged on both sides of a carrier film. Alternatively it can be provided that the optically active layers are arranged on one side of a carrier film. The optically active layers can also be arranged between two carrier films in order to achieve a particularly high protection of the optically active layer sequence.

The carrier foil or the carrier foils are preferably optically substantially isotropic for light from the predetermined wavelength range. You can For example, consist of cycloolefin copolymers or be formed by a combination of two or more differently oriented plastic films.

Alternatively, the carrier film (s) for light from the predetermined wavelength range have a defined optical anisotropy with a path difference constant over the extent of the security element. In particular, carrier films with a path difference of n * λ, with n from the natural numbers, and especially with a path difference of 1 * λ are preferred because the polarization of light when passing through such a film, as in an optically isotropic film substantially remains unchanged.

At least one of the phase-shifting layers can advantageously be printed on the carrier film in the form of a motif.

The invention further includes a method for producing a double-sided see-through security element having motifs in the form of patterns, characters or codes. In the method of the present invention, there is provided a first optically active layer of a cholesteric liquid crystalline material which selectively reflects light having a predetermined circular polarization in a predetermined wavelength range, and becomes second and third optically active layers respectively for light of the predetermined wavelength range form phase-shifting layer, arranged on both sides of the first optically active layer. The second and third optically active layers are provided with mutually independent motifs for verifying the security element from opposite sides.

One or both of the phase-shifting layers are advantageously formed of nematic liquid-crystalline material. In particular, the liquid-crystalline layers are printed by gravure printing, screen printing, flexographic printing, Knifecoating or curtain coating.

One or more of the optically active layers are preferably produced on a release film (release film), which is removed after combining the optically active layers of the resulting layer composite. Alternatively or additionally, one or more of the optically active layers is produced on an optically substantially isotropic carrier film which remains in the resulting layer composite after combining the optically active layers. As mentioned above, instead of an optically isotropic carrier film, it is also possible to use a carrier film having a defined optical anisotropy and a path difference constant over the extent of the security element.

At least one of the phase-shifting layers is advantageously printed on a release film, in particular partially in the form of a motif.

The invention also includes a valuable article, such as a branded article, a value document or the like, which is equipped with a double-sided see-through security element of the type described. The see-through security element is expediently arranged in or above a window area or a continuous opening of the object of value. The valuable item may be, for example, a security paper, a value document or a product package.

The invention further includes a method for checking the authenticity of a see-through security element or article of value of the type described above, in which the see-through security element is checked for the presence of predetermined motifs by means of a linear polarizer or a circular polarizer from one or both sides, and the authenticity of the security element on the basis of the test result is judged.

In an advantageous variant of the invention, the linear polarizer or circular polarizer is provided in a window area or a continuous opening of the object of value. The object of value is expediently flexible, so that the see-through security element and the linear or circular polarizer can be laid one on top of the other by bending or folding the object of value for self-authentication.

Further embodiments and advantages of the invention are explained below with reference to the figures. For better clarity, a scale and proportioned representation is omitted in the figures.

Fig. 1

eine schematische Darstellung einer Banknote mit einem Durchsichtsbereich, über dem ein erfindungsgemäßes Sicher- heitselement angeordnet ist,

Fig. 2

schematisch den prinzipiellen Schichtaufbau eines Sicherheits- elements nach einem Ausführungsbeispiel der Erfindung zur Erläuterung dessen grundsätzlicher Funktionsweise,

Fig. 3

die Herstellung eines doppelseitigen Durchsichtssicherheits- elements nach einem Ausführungsbeispiel der Erfindung, wo- bei (a) einen ersten und zweiten Schichtverbund vor dem Ka- schieren und (b) das fertige Sicherheitselement mit entfernter Releasefolie zeigt,

Fig. 4

ein Durchsichtssicherheitselement nach einem anderen Aus- führungsbeispiel der Erfindung, bei dem die optisch aktiven Schichten auf beiden Seiten durch Folien geschützt sind,

Fig. 5

die Herstellung eines doppelseitigen Durchsichtssicherheits- elements nach einem weiteren Ausführungsbeispiel der Erfin- dung, wobei (a) einen ersten und zweiten Schichtverbund vor dem Kaschieren und (b) das fertige Sicherheitselement zeigt,

Fig. 6

schematisch den grundlegenden Schichtaufbau eines erfin- dungsgemäßen Sicherheitselements, bei dem die Motive druck- technisch erzeugt werden,

Fig. 7

die Herstellung eines doppelseitigen Durchsichtssicherheits- elements nach einem weiteren Ausführungsbeispiel der Erfin- dung, wobei (a) das fertige Sicherheitselement und (b) die sepa- rat hergestellten Schichten bzw. Schichtverbünde vor dem Ka- schieren zeigt,

Fig. 8

ein Durchsichtssicherheitselement nach einem weiteren Aus- führungsbeispiel der Erfindung,

Fig. 9

die Herstellung eines auf einer Seite einer Trägerfolie aufgebau- ten Sicherheitselements nach einem weiteren Ausführungsbei- spiel der Erfindung, wobei (a) die separat hergestellten Schich- ten bzw. Schichtverbünde vor dem Kaschieren, und (b) das fer- tige Sicherheitselement zeigt,

Fig. 10

in (a) die Herstellung und in (b) das fertige Sicherheitselement für eine Abwandlung des Ausführungsbeispiels der Fig. 9,

Fig. 11

ein zwischen zwei isotropen Trägerfolien aufgebautes Durch- sichtssicherheitselement nach einem weiteren Ausführungsbei- spiel der Erfindung,

Fig. 12

einen Aufbau eines erfindungsgemäßen Durchsichtssicher- heitselements, der besonders für leicht anisotrope Trägerfolien geeignet ist, und

Fig. 13

die Herstellung eines Durchsichtssicherheitselements nach noch einem weiteren Ausführungsbeispiel der Erfindung.

Show it:

Fig. 1

1 a schematic representation of a banknote with a see-through region, above which a security element according to the invention is arranged,

Fig. 2

2 shows the basic layer structure of a security element according to an exemplary embodiment of the invention for explaining its basic mode of operation,

Fig. 3

the production of a double-sided see-through security element according to an embodiment of the invention, wherein (a) shows a first and second layer composite before the shingling and (b) the finished security element with the release sheet removed,

Fig. 4

a see-through security element according to another embodiment of the invention, in which the optically active layers are protected on both sides by foils,

Fig. 5

the production of a double-sided see-through security element according to a further exemplary embodiment of the invention, wherein (a) shows a first and second layer composite prior to laminating and (b) the finished security element,

Fig. 6

1 schematically the basic layer structure of a security element according to the invention, in which the motifs are produced by printing technology,

Fig. 7

the production of a double-sided see-through security element according to a further exemplary embodiment of the invention, wherein (a) shows the finished security element and (b) the separately produced layers or layer composites before being laminated,

Fig. 8

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

Fig. 9

the production of a security element constructed on one side of a carrier foil according to a further exemplary embodiment of the invention, wherein (a) the separately produced layers or laminations prior to laminating, and (b) the finished security element,

Fig. 10

in (a) the production and in (b) the finished security element for a modification of the embodiment of Fig. 9 .

Fig. 11

a see-through safety element constructed between two isotropic carrier foils according to a further exemplary embodiment of the invention,

Fig. 12

a structure of a see-through security element according to the invention, which is particularly suitable for slightly anisotropic carrier films, and

Fig. 13

the production of a see-through security element according to yet another embodiment of the invention.

The invention will be explained below using the example of a banknote. Fig. 1 shows a schematic representation of a banknote 10, which contains a see-through area 12 in a partial area of the note. The see-through area 12 can be, for example, a continuous opening or a transparent partial area of the banknote 10. In or above this see-through area 12, a security element 14 according to the invention is arranged, the security features of which can be tested from both sides of the banknote 10.

The basic mode of operation of security elements according to the invention will now be described on the basis of the basic layer structure of FIG Fig. 2 which only shows the layers which are absolutely necessary in the context of the invention.

The minimum security element 20 of the Fig. 2 contains as a first optically active layer a layer 24 of cholesteric liquid crystalline material. The cholesteric layer 24 selectively reflects light of a predetermined circular polarization in a predetermined wavelength range, depending on the employed twisting device. For further explanation, it is assumed that the cholesteric layer 24 reflects right circularly polarized light. Light opposite polarization direction, in the embodiment, therefore, left-circularly polarized light, is passed by the cholesteric layer 24, however, without substantial absorption.

Furthermore, a second optically active layer 22 and a third optically active layer 26, which in the exemplary embodiment each consist of nematic liquid-crystalline material, are arranged on opposite sides of the cholesteric layer 24. They each form a phase-shifting layer for light from the predetermined wavelength range, wherein for explanation of the Fig. 2 It is assumed that the two phase-shifting layers 22 and 26 each form a λ / 4-layer due to their layer thickness. The side of the security element on which the first phase-shifting layer 22 is applied is referred to below as the front side, the side of the second phase-shifting layer 26 as the rear side.

The two phase-shifting layers 22 and 26 contain mutually independent motifs in the form of patterns, characters or codes, the serve the verification of the security element from opposite sides.

The motifs can be introduced into the security element 20 in different ways. For the general explanation of the Fig. 2 It is assumed that the phase-shifting layers 22, 26 have first and second regions 22-1, 22-2, and 26-1, 26-2, respectively, in which the nematic liquid crystal material is rotated 90 degrees from each other. In other embodiments, the first or second regions may also be formed, for example, by other orientations, by thickness variations of the phase-shifting layers, or by-recesses in the phase-shifting layers, as described below.

Coming back to the presentation of the Fig. 2 For example, the security element 20 (that is, the bill or value document containing the security element 20) is considered for verification with a linear polarizing filter placed on the front or back. By rotation of the polarizing filter, the polarization direction of the incident light can be set arbitrarily. The effects described below appear particularly clear against a dark background.

When viewing the security element 20 in transmitted light, none of the motifs of the layers 22, 26 can be seen, the security element appears only slightly tinted.

On the other hand, if the security element 20 is viewed in plan view with a linear polarization filter suitably placed on its front side, then the motif of the first phase-shifting layer 22 emerges with a clear contrast.

The isotropic light incident on the security element is linearly polarized by the overlying polarization filter. In the first phase shift layer 22, the linearly polarized light is then converted into right circular or left circular polarized light depending on the local orientation of the nematic λ / 4 layer. For example, the polarization vector of the light may be such that the light in the regions 22-1 is converted to right circularly polarized light, in the regions 22-2 to left circularly polarized light.

Only one of the two types of light, in the embodiment, the right circularly polarized light of the areas 22-1, is reflected by the cholesteric layer 24, while the left circularly polarized radiation portion of the areas 22-2 is transmitted. The reflected right circularly polarized light is converted back into linearly polarized light by the nematic λ / 4 layer 22 when it is read again, the linear polarization obtained just corresponding to the original polarization of the light, so that it is transmitted by the overlying polarization filter without significant absorption.

In the areas 22-1, the security element thus largely radiates the incident radiation back, they appear to the viewer in supervision bright. The areas 22-2, however, appear dark, since the incident there light passes through the security element without reflection. A possible interaction of the left circularly polarized light transmitted by the cholesteric layer 24 with the second phase-shifting layer 26 does not occur because the transmitted light leaves the security element without further reflection and is absorbed by a dark background.

Overall, the observer or a machine recording system can thus perceive the motif of the first phase-shifting layer 22 formed by the regions 22-1 and 22-2 with high contrast, while the motif of the opposite phase-shifting layer 26 does not appear when viewing the front side. Starting from the described position of the polarization filter, the motif appears with a rotation of the linear polarization filter by 90 ° as a negative image. It is understood that the correct position of the polarizing filter does not have to be known beforehand (and is often unknown), because the viewer easily finds a position with correct scene reproduction by turning the filter.

If the security element 20 (or the banknote containing the security element 20) is turned over and viewed in plan view with a polarization filter suitably placed on its rear side, the motif of the second phase-shifting layer 26 can be seen. In this case, analogously to the beam path described when viewed from the front side, the linearly polarized light enters the second phase-shifting layer 26 and, depending on the local orientation of the nematic λ / 4-layer in the regions 26-1 and 26- 2 converted into right circular or left circular polarized light. Only the right circularly polarized light, for example, the areas 26-1, is reflected by the cholesteric layer 24, the left circularly polarized radiation component is transmitted. The reflected right circularly polarized light is converted by the nematic λ / 4 layer 26 in the retransmission in linearly polarized light, and transmitted by the overlying polarization filter without appreciable absorption, since the resulting linear polarization just corresponds to the original polarization of the light.

The motif of the second phase-shifting layer 26 formed by the regions 26-1 and 26-2 thus emerges with high contrast, since the security element 20 reflects back the incident radiation only in the regions 26-1, so that these regions are bright, the transmitting regions 26-2, on the other hand, appear dark. Again, by rotation of the linear polarizing filter by 90 °, a negative image of the subject can be obtained. When viewed from the rear side, the motif of the opposite first phase-shifting layer 22 does not appear. The cholesteric layer 24 thus functions as a motif-dependent mirror which, together with the nematic λ / 4 layers 22 and 26, reflects a different motif image, depending on the viewing direction.

Fig. 3 schematically illustrates the production of a double-sided see-through security element 30 according to a specific embodiment of the invention. It will, as in Fig. 3 (a) a first layer composite 32 is produced from a release film 34, for example an untreated PET film, a first alignment layer 36, for example a photoalignment layer, a first phase-shifting layer 38 of a nematic liquid crystal material and an optically active layer 40 of cholesteric liquid crystal material.

The first layer composite 32 contains a first motif which is produced in the exemplary embodiment as follows: On the release film 34, a photostructurable layer 36 of polyvinyl cinnamate or polyimide is applied, which in the manner described above by exposure to polarized light corresponding to the first desired motif can be structured. On the structured layer 36, a nematic liquid crystal layer 38 is applied, which is in the areas 38-1 and 38-2, respectively, in accordance with the respectively predetermined by the layer 36 orientation oriented. The layer 36 acts as an alignment layer for the nematic liquid crystal layer 38, so that the imprinted motif of the photoalignment layer 36 continues into the liquid crystal layer 38.

In addition, a second layer composite 42 having an optically substantially isotropic film 44, a second alignment layer 46, for example a photoalignment layer, and a second phase-shifting layer 48 made from a nematic liquid crystal material is produced. The second layer composite 42 contains a second motif, which can be generated by means of the photoalignment layer 46 as described above.

The first layer composite 32 is then applied via an adhesive layer 50 (FIG. Fig. 3 (b) ) is laminated on the free rear side of the optically isotropic film 44 of the second layer composite 42, as indicated by the arrow 52. Subsequently, the release film 34 is removed by separation winding, so that a security element with the in Fig. 3 (b) created layer sequence arises.

For some applications, it may be advantageous if the stable film does not, as in the embodiment of the Fig. 3 , lies in the middle of the layer structure, but when the optically active layers are protected on one or both sides by films. Fig. 4 schematically shows such a double-sided see-through security element 60 according to another embodiment of the invention.

To produce the see-through security element 60, on a suitably pretreated, optically substantially isotropic film 62 by successively coating a first photoalignment layer 64, a first phase-shifting layer 66 of a nematic liquid crystal material, an optically active layer 68 of cholesteric liquid crystal material, a second photo-alignment layer 70 and a second phase-shifting layer 72 of nematic liquid crystal material are deposited.

Analogous to the embodiment of Fig. 3 In this case, by patterning the photoalignment layers 64 and 70, motifs are generated in the phase-shifting layers 66 and 72. If necessary, primers or adhesion promoter layers 74 can be applied between the film 62 and the first photoalignment layer 64, as well as between the cholesteric layer 68 and the second photoalignment layer 70.

A second, optically substantially isotropic film 78 is then applied to this layer composite by means of a laminating adhesive 76, so that the see-through security element 60 is protected from both sides. The operation of the security element is not affected by the optically isotropic films 62, 78.

The double-sided see-through security element of Fig. 5 corresponds with its layer structure largely the in Fig. 4 However, shown embodiment, differs from this in the sequence of applied to the optically isotropic films layers. For producing the security element of Fig. 5 For example, a first layer composite 65 is formed by applying to an optically substantially isotropic film 62 an optional adhesion promoter layer 74, a first photoalignment layer 64, a first phase shift layer 66 of nematic liquid crystal material, and an optically active layer 68 of cholesteric liquid crystal material, as shown in FIG Fig. 5 (a) shown.

In addition, a further layer composite 75 is produced by applying an optional adhesion promoter layer to a second optically substantially isotropic film 78 74, a second photo-alignment layer 70 and a second phase-shifting layer 72 is applied. The second layer composite 75 is then applied via an adhesive layer 76 (FIG. Fig. 5 (b) ) is laminated to the cholesteric layer 68 of the first layer composite 65 as indicated by reference numeral 79. The two layer composites 65, 75 may additionally be provided with adhesion promoters in order to improve the laminating resistance with the laminating adhesive. The optical mode of operation is due to the modified manufacturing process of Fig. 5 not changed.

In the exemplary embodiments described so far, the motifs are introduced into the phase-shifting layers by aligning the nematic liquid crystals with the aid of suitably pretreated alignment layers. The following will be with reference to the FIGS. 6 to 13 described further embodiments of the invention, in which the motives are generated by printing without such alignment-promoting measures. The underlying principle is first based on the schematic representation of Fig. 6 explained.

The security element 80 of Fig. 6 contains as the first optically active layer, a layer 84 of cholesteric liquid-crystalline material which selectively reflects in a predetermined wavelength range depending on the used Verdriller light of a predetermined circular polarization. By way of illustration, it is again assumed that the cholesteric layer 84 reflects right circularly polarized light and transmits light of opposite polarization direction without substantial absorption.

A second 82 and third 86 optically active layer of nematic liquid crystal material are disposed on opposite sides of the cholesteric layer 84. They form for light from the predetermined wavelength range in each case a phase-shifting layer, wherein for the explanation of Fig. 6 It is assumed that the two phase-shifting layers 82, 86 each represent a λ / 2 layer due to their layer thickness in the relevant wavelength range. The side of the first phase-shifting layer 82 will hereinafter be referred to as the front side, the side of the second phase-shifting layer 86 as the rear side.

The phase-shifting layers 82, 86 are now not printed over the entire surface, but only partially, wherein the shape and arrangement of the printed areas 82-1, 86-1 and the recessed areas 82-2, 86-2 two independent motives in the form of patterns , Characters or encodings that serve to verify the security element 80 from opposite sides.

Unlike the embodiments described above, the verification of the security element 80 is not performed with linear polarizing filters, but with the aid of circular polarizers which transmit only light of a specific circular polarization. Such circular polarizers can be formed for example by a linear polarizer and a downstream λ / 4 plate.

Without aids, the motifs of the phase-shifting layers 82, 86 are not recognizable to the viewer. On the other hand, if the security element 80 is illuminated with isotropic light and viewed in plan view by a circular polarizer, depending on the position of the security element 80, the motif of the first or second phase-shifting layer clearly emerges.

The incident isotropic light is still isotropic even after passing through the first phase-shifting layer 82, since the additional path difference of λ / 2 affects all polarization directions in the same way. The cholesteric layer 84 is the only reflective layer in the layer structure of FIG Fig. 6 It reflects according to the above definition just the right circularly polarized portion of the incident isotropic light, while the left circularly polarized radiation component is transmitted.

The reflected right circularly polarized light is now converted into left circularly polarized light in the printed areas 82-1 or 86-1 of the nematic λ / 2 layer upon re-passage, while it is circularly polarized in the recessed areas 82-2 or 86-2 remains. Therefore, when the security element is viewed through a circular polarizer transmitting only right circularly polarized light, the recess 82-2 or 86-2 appears bright, while the areas 82-1 or 86-1 covered by the nematic λ / 2 layer appear dark. The inverse contrast is when viewed through a circular polarizer which transmits only left circularly polarized light. If, as in the embodiment, no further reflective layers are provided, the visual impression when viewed from the front side (or rear side) is not influenced by a polarization change of the transmitted light by the backside (or front side) λ / 2 layer.

Another possibility of verification is to illuminate the security element 80 with circularly polarized light, for example, by directing isotropic illumination radiation through a circular polarizer, which transmits only right circularly polarized light, to the security element.

In the recessed areas 82-2 or 86-2, the incident right circularly polarized radiation is reflected from the cholesteric layer 84, so that these areas appear bright to a viewer. On the other hand, in the occupied areas 82-1 or 86-1, the incident light is converted from the nematic λ / 2 layer to left circularly polarized light transmitted through the cholesteric layer 84 and absorbed by a preferably dark background. These areas therefore appear obscure to the viewer. Again, the inverted contrast can be obtained by viewing with light of opposite polarization direction.

After the above explanation, it is understood that when using nematic λ / 2 layers, the maximum brightness contrast between occupied and recessed areas is achieved. By using phase-shifting layers of different thickness and thus different path difference, multiple brightness levels can be provided. For example, a subject with 4 levels of brightness can be achieved by using phase shifting layers with a retardation of 0 (recessed areas), λ / 6, λ / 3, and λ / 2 (maximum thickness). Similarly, subjects with a greater number of brightness levels can be realized.

The production of a double-sided see-through security element 90 according to a further exemplary embodiment of this variant of the invention will now be described with reference to FIG Fig. 7 explains Fig. 7 (a) the finished security element 90 and Fig. 7 (b) the separately produced layers or layer composites before laminating schematically shows.

To produce a first layer composite 92, a layer 96 of nematic liquid-crystalline material is partially printed on a smooth plastic film 94 of good surface quality in the form of a first desired motif. On this Nematenschicht 96 and the plastic film 94 is printed over the entire surface a transfer assist layer 98, the adhesion to the Plastic film 94 is lower than the Nematenschicht 96 and the subsequent transfer of the only partially present Nematenschicht 96 is used. As described in greater detail above, this transfer assist layer may be, for example, a UV-crosslinkable lacquer layer.

Similarly, a second layer composite 100 is produced by printing on a smooth plastic film 102 of good surface quality a layer 104 of nematic liquid-crystalline material partially in the form of a second desired motif. Also on the Nematenschicht 104 and the plastic film 102 is printed over the entire surface, a transfer auxiliary layer 106, the adhesion to the plastic film 102 is less than the Nematenschicht 104th

Furthermore, a cholesteric layer 110 is laminated onto an optically largely isotropic carrier foil 108 (reference numeral 112) which, for example, reflects right-circularly polarized light of the predetermined wavelength range. The optically isotropic carrier foil 108 can consist, for example, of cycloolefin copolymers or of a combination of differently stretched plastic films. Then, the first and second layer composite 92 or 100 are laminated on the top or bottom of the layer composite of carrier film 108 and cholesteric layer 110, as indicated by the arrows 114 and 116. Subsequently, the carrier foils 94 and 102 are removed by separating coils, so that the in Fig. 7 (a) shown layer structure, wherein the reference numeral 118 denotes the laminating adhesive layers.

It goes without saying that further layers can be applied to the security element 90 for further processing, depending on the planned field of application. For example, the security element 90 can be heat-sealed on both sides equipped and run like a security thread for a banknote with an opening.

In the preparation described, a transfer step can be saved if the cholesteric layer 110 is also used as a transfer assist layer for the nemate layer 104 of the second layer composite. It is exploited that the cholesteric layer 110, as well as the above-exemplified UV-crosslinkable lacquer layer, a lower adhesion to the plastic film 102 than the Nematenschicht 104. Due to the modified production results in in Fig. 8 shown embodiment in which between the nematic motif areas 104 is cholesteric liquid crystal material, so that the cholesteric layer 110, unlike in the only schematic representation of Fig. 8 can be correspondingly uneven.

For reasons of resistance, it may also be advantageous to construct the entire optically active layer composite on one side of a carrier film, as in US Pat Fig. 9 shown schematically. There, a first layer composite 122 of a smooth plastic film 124, a partially printed in the form of a first desired motif layer 126 of nematic liquid-crystalline material and a UV-crosslinkable lacquer layer 128 is laminated on a substantially largely isotropic carrier film 120 as a transfer auxiliary layer and the plastic film 124 then removed , please refer Fig. 9 (b) ,

In a second layer composite 130, a second motif layer 134 of nematic liquid-crystalline material is printed on a smooth plastic film 132 and a cholesteric layer 136 of suitable thickness is applied as a transfer assist layer. The second layer composite 130 is laminated onto the carrier film 120 with the already applied first layer composite and then removed the second plastic film 132 by separation winding. Overall, the in. Arises Fig. 9 (b) illustrated double-sided see-through security element 140, wherein the reference numeral 138 denotes the laminating adhesive layers.

If the optically substantially isotropic carrier film can be printed directly with liquid crystal material, it is possible to dispense with a laminating adhesive layer and a transfer auxiliary layer, as is apparent from the Fig. 10 (a) schematically illustrated. In this case, a Nematen layer 154 of the desired thickness and with the desired first motif is printed directly onto the carrier film 152. The second layer composite 130 is as in the embodiment of Fig. 9 produced, laminated on an adhesive layer 156 on the printed carrier film 152, 154 and then removes the plastic film 132, so that the double-sided see-through security element 150 of Fig. 10 (b) arises.

The entire layer structure can also be arranged between two optically largely isotropic carrier films, as is the case with the exemplary embodiment of FIG Fig. 11 shown schematically. The security element 160 shown there is a variant of the with reference to Fig. 9 described security element 140, so that the matching layers are denoted by the same reference numerals. In addition to the embodiment of the Fig. 9 a second optically largely isotropic carrier foil 162 is provided which, together with the first carrier foil 120, encloses and protects the optically active layer composite. The sequence of laminations is not strictly predetermined: for example, the carrier film 162 via an adhesive layer, not shown on the security element 140 of Fig. 9 (b) or the sequence of the nemate layer 126 and the transfer assist layer 128 can be interchanged without such changes Impact on the optical functioning of the security element.

If there are no ideal carrier films (ie optically largely isotropic films or films with a well-defined anisotropy of the type explained further below), it is possible to use the method described in US Pat Fig. 12 schematically shown possible interference can be reduced by a carrier film with slightly anisotropic optical properties.

The double-sided see-through security element 170 of Fig. 12 contains a carrier film 172 with slight optical anisotropy. On opposite sides of the carrier film 172 are two adhesive layers 174 two layer composites 130 of in Fig. 9 (a) shown with motif-bearing Nematenschichten 134-A and 134-B and similar cholesteric layers 136-A and 136-B laminated. Since each nemate layer 134-A, 134-B cooperates with its own cholesteric layer 136-A, 136-B to achieve the mirror effect, a slight optical anisotropy of the carrier film 172 does not disturb the optical operation of the security element 170. However, the retardation of the carrier film 172 may not correspond to λ / 2 for similar cholesteric layers 136-A or 136-B, since then just a complete reflector would emerge, which no longer reflects only circularly polarized light.

However, if the two cholesteric layers are designed for reflection of opposite circular polarization directions, the carrier film advantageously has a path difference of λ / 2, because then the circularly polarized light transmitted through the first cholesteric layer is just reversed in its polarization direction by the carrier film and so on is also transmitted from the second, designed in the opposite polarization direction cholesteric layer.

A further variant for producing a see-through security element 180 according to the invention is shown in FIG Fig. 13 shown schematically. In this manufacturing process, the nematic layers 184, 188 are printed in the desired thickness and with the desired motifs on a PET film 182, 186 in each case. In addition, the cholesteric liquid crystal layer 190 is printed on one of the nematic layers 188 as a transfer assist layer. The laminate having the nemate layer 188 and the cholesteric liquid crystal layer 190 is then laminated to the film 182 carrying the first nematic layers 184. The backing sheet 186 of the laminated composite can then be peeled off, as in FIG Fig. 13 shown.

If a completely releasable composite is desired, an alignment layer, for example a photoalignment layer of the type described above, can additionally be applied to a layer composite comprising nemates and cholesteric liquid crystals, and the second nematen layer can be printed as a motif on the alignment layer.

In all cases described, in which optically essentially isotropic carrier films were used, according to the invention it is also possible to use films as carrier films or laminating films whose path difference corresponds to an integer multiple of λ. In particular, carrier films with a path difference of 1 * λ are preferred, since then the layer thickness tolerance is maximal. In addition, it is also advantageous for the optical effect if the path difference is approximately constant in the largest possible part of the visible spectrum.

All described embodiments can also be provided with further information by recesses in individual layers or by recesses in applied metallization layers. By partial printing of magnetic ink or simple black ink, non-transparent portions may be formed on one or both sides of the cholesteric liquid crystal layer. The color shift effect of the liquid crystal layers appears more brilliant in these areas because of the absorbing background.

Claims (30)

1. A double-sided see-through security element (20) for securing valuable articles, having, introduced into the security element, motifs in the form of patterns, characters or codes, characterized by

   - a first optically active layer (24) composed of a cholesteric liquid crystal material that, in a predetermined wavelength range, selectively reflects light having a predetermined circular polarization,

   - a second (22) and third (26) optically active layer that are arranged on both sides of the first optically active layer (24) and that each form a phase-shifting layer for light from the predetermined wavelength range,

   the second and third optically active layer (22, 26) including motifs that are independent of one another for verifying the security element from opposing sides.
2. The see-through security element (20) according to claim 1, characterized in that one or both of the phase-shifting layers (22, 26) are formed from nematic liquid crystal material.
3. The see-through security element (20) according to claim 1 or 2, characterized in that one or both of the phase-shifting layers (22, 26) form, at least in sub-regions, a λ/4 layer for light from the predetermined wavelength range.
4. The see-through security element (20) according to at least one of claims 1 to 3, characterized in that one or both of the phase-shifting layers (22, 26) form, at least in sub-regions, a λ/2 layer for light from the predetermined wavelength range.
5. The see-through security element (20) according to at least one of claims 1 to 4, characterized in that the motifs are formed by areally different alignment of one or both of the phase-shifting layers (22, 26).
6. The see-through security element (20) according to claim 5, characterized in that one or more photoalignment layers or one or more embossed alignment layers are provided for aligning one or both of the phase-shifting layers (22, 26).
7. The see-through security element (20) according to at least one of claims 1 to 6, characterized in that the motifs are formed by areally different thickness of one or both of the phase-shifting layers (22, 26).
8. The see-through security element (20) according to at least one of claims 1 to 7, characterized in that one or both of the phase-shifting layers (22, 26) are present only areally in the form of a motif.
9. The see-through security element (20) according to at least one of claims 1 to 8, characterized in that one or both of the phase-shifting layers (22, 26) is arranged immediately over or under a contiguous transfer assist layer.
10. The see-through security element (20) according to claim 9, characterized in that at least one transfer assist layer is formed from a UV-curing lacquer layer.
11. The see-through security element (20) according to claim 9 or 10, characterized in that at least one transfer assist layer is formed from a cholesteric liquid crystal material.
12. The see-through security element (20) according to claim 11, characterized in that the first optically active layer (24) forms a transfer assist layer for one of the phase-shifting layers.
13. The see-through security element (20) according to at least one of claims 1 to 12, characterized in that the security element exhibits negative information in the form of patterns, characters or codes that are formed by gaps in one or more of the optically active layers (22, 24, 26).
14. The see-through security element (20) according to at least one of claims 1 to 13, characterized in that the security element exhibits non-transparent regions in the form of patterns, characters or codes that are formed by discontiguous application of printing ink or magnetic ink on one or both sides of the first optically active layer (24).
15. The see-through security element (20) according to at least one of claims 1 to 14, characterized in that the optically active layers (22, 24, 26) are arranged on both sides of a substrate foil or on one side of a substrate foil or between two substrate foils.
16. The see-through security element (20) according to claim 15, characterized in that at least one substrate foil is optically substantially isotropic for light from the predetermined wavelength range.
17. The see-through security element (20) according to at least one of claims 15 to 16, characterized in that at least one substrate foil exhibits, for light from the predetermined wavelength range, a defined optical anisotropy with a constant phase difference across the expanse of the security element, especially an optical anisotropy with a phase difference of n*λ, with n from the natural numbers, preferably with a phase difference of 1*λ.
18. A method for producing a double-sided see-through security element (20) that exhibits motifs in the form of patterns, characters or codes, characterized by the method steps:

    - providing a first optically active layer (24) composed of a cholesteric liquid crystal material that, in a predetermined wavelength range, selectively reflects light having a predetermined circular polarization, and

    - arranging a second (22) and third (26) optically active layer that each form, on both sides of the first optically active layer (24), a phase-shifting layer for light from the predetermined wavelength range,

    the second and third optically active layer (22, 26) being provided with motifs that are independent of one another for verifying the security element from opposing sides.
19. The method according to claim 18, characterized in that one or both of the phase-shifting layers (22, 26) are formed from nematic liquid crystal material.
20. The method according to claim 19, characterized in that the liquid crystal layers are imprinted by means of intaglio printing, screen printing, flexo printing, knife coating or curtain coating.
21. The method according to at least one of claims 18 to 20, characterized in that one or more of the optically active layers (22, 24, 26) are produced on a release foil that is removed from the resultant layer composite after combining the optically active layers (22, 24, 26).
22. The method according to at least one of claims 18 to 21, characterized in that one or more of the optically active layers (22, 24, 26) are produced on an optically substantially isotropic substrate foil that remains in the resultant layer composite after combining the optically active layers (22, 24, 26).
23. The method according to at least one of claims 18 to 21, characterized in that one or more of the optically active layers (22, 24, 26) are produced on a substrate foil that has a defined optical anisotropy and a phase difference that is constant across the expanse of the security element, and that remains in the resultant layer composite after combining the optically active layers (22, 24, 26).
24. The method according to at least one of claims 19 to 23, characterized in that, for aligning the nematic liquid crystal material, at least one of the phase-shifting layers (22, 26) is applied on an alignment layer, especially a photoalignment layer or an embossed alignment layer.
25. The method according to claim 24, characterized in that a photoalignment layer is applied to a release foil and the photoalignment layer is inscribed by exposing a motif in the form of differently aligned regions.
26. The method according to at least one of claims 21 to 25, characterized in that at least one of the phase-shifting layers (22, 26) is imprinted, especially partially imprinted, on a release foil.
27. The method according to claim 26, characterized in that to the at least one phase-shifting layer (22, 26) is contiguously applied a transfer assist layer whose adhesion to the release foil is lower than to the phase-shifting layer (22, 26).
28. The method according to at least one of claims 22 to 27, characterized in that at least one of the phase-shifting layers (22, 26) is imprinted, especially partially imprinted, on an optically substantially isotropic substrate foil or on a substrate foil having a defined optical anisotropy.
29. A valuable article, such as a branded article, value document or the like, having a double-sided see-through security element (20) according to at least one of claims 1 to 28.
30. A method for checking the authenticity of a see-through security element (20) according to one of claims 1 to 28 or of a valuable article according to claim 29, characterized in that the see-through security element (20) is checked with the aid of a linear polarizer or a circular polarizer from one or from both sides for the presence of predetermined motifs, and the authenticity of the security element is assessed on the basis of the check results.

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