HK1068416B - Optical component, element and device for protection against forgery or copying comprising the same - Google Patents

Description

Optical element and anti-counterfeiting or anti-copying component comprising same

The present invention relates to an optical element comprising an optically anisotropic film layer, wherein the film layer has at least two regions of different molecular orientation. For example, the anisotropic layer may be a retarder layer formed of a cross-linked liquid crystal monomer.

The particular use of the element according to the invention is in the field of forgery prevention and copy prevention.

The need for security protection of security articles such as banknotes, credit cards, certificates, identity cards and the like is constantly increasing due to the availability of high quality copying techniques. In addition, counterfeit branded products and copied copyrighted products (e.g., read-only optical disks, computer software, electronic device chips, etc.) have been produced in low-payroll countries and exported around the world. As the number of counterfeits is increasing, the need for new elements for protection against counterfeiting and that can be identified visually and by machine is also considerable.

In the field of copy-protected banknotes, credit cards and the like, there are already a large number of authentication elements. Depending on the value of the document to be protected, very simple or rather complex elements are used. Some countries are satisfied with providing banknotes with metal strips that produce a black color when photocopied. Although this prevents them from being copied, this type of element is very easy to copy. In contrast, there are also more complex authentication elements, such as holograms and images (cinegrams). Authentication elements of this type are based on the diffraction of light through a grid and require observation at different viewing angles in order to verify their authenticity. These diffractive elements produce three-dimensional images, color variations or motion effects that depend on the viewing angle and have to be examined on the basis of predetermined criteria or rules. In practice, it is not possible to read information (such as images or numbers) encoded with this technique using a machine. Furthermore, the information capacity of these elements is very limited and only optical experts can clearly distinguish between authenticity.

Finally, one should not ignore the fact that optical diffraction effects have been used over time outside the security field, in particular for consumer products (e.g. wrapping paper, toys, etc.), and that the production methods of such elements have also become well known over time and are accordingly directly emulated.

In addition to the above described diffractive elements, also other elements are known, which are suitable for optimal copy protection. These elements include optical elements such as those disclosed in EP-a689 '084 or EP-a 689' 065, i.e. optical elements comprising an anisotropic liquid crystal layer, which elements will have a local molecular alignment structure.

These elements are based on a hybrid film structure comprising an alignment layer and a film layer in contact therewith and consisting of liquid crystal monomers which are cross-linked to each other. In this case, the alignment layer is composed of a photo-orientable polymer network (PPN) (synonymous with LPP used in other documents) which in the oriented state defines a number of regions of altered orientation by a predetermined array. In particular, this orientation, characterised by a spatial variation of the direction of the optical axis, is fixed by a subsequent cross-linking step, which after cross-linking forms a cross-linked optically structured liquid crystal with a predetermined orientation (LCP stands for liquid crystal polymer). The orientation pattern itself and the information written into the liquid crystal are initially invisible without the aid of viewing before the liquid crystal monomers are crosslinked. These film layers have a transparent appearance. If the substrate, on which the film layers are positioned, is transmissive to light, then the orientation of the LCP or information that has been written will become visible by placing the optical element between two polarizers. If a birefringent liquid crystal layer is positioned on the reflective layer, the pattern or corresponding information is made visible with only one polarizer held above the element. The LCP authentication element may in fact store information in the form of text, images, photographs and combinations thereof without limitation. In contrast to prior art authentication elements, LCP elements can even be identified by non-professional verification of the authenticity of the security feature, since learning how to identify complex color changes or motion effects is not paramount. Since reading LCP authentication elements is very simple, reliable and quick, machine readable and visible information can be incorporated in the same authentication element.

It is also well known that the complexity of LCP discriminator elements can be further enhanced by tilting the optical axis of the LCP layer uniformly or with local variations relative to the plane of the layer. This can be done in a known manner by creating a layer of PPN with local variations in the tilt angle over the surface. This further provides a tilting effect, that is to say that the information contained in the birefringent layer can be seen, whether the positive or negative contrast will depend on the viewing angle. It is now an object of the present invention to provide further possible film structures of the above-mentioned type for optical elements, for electro-optical devices, in particular for copy protection elements.

This is achieved according to the invention in that the physical parameters and configuration of the crosslinked liquid crystal layer can be varied and/or different film layers with different optical properties can be combined with various substrates. Since the film layers used are generally transparent, they can be applied successfully to known permanently visible authentication elements, such as watermarks, holograms or images. Thus, the retarder pattern of the liquid crystal layer can be seen in addition to the permanently visible authentication element when viewed with a linear polarizer.

In the case of the transmissive birefringent layer described in EP-a 689' 084, it is necessary to arrange a polarizer on each side of the element in order to read out the stored information or to make it visible. In this case, since two polarizers are involved in positioning above and below the authentication element, it is difficult to quickly inspect an identification card or the like. According to the invention, the complementary integration of at least one polarizer in the film layer structure can overcome this disadvantage. For example, if there is a polarizing layer below the birefringent layer, only one external polarizer held above the element is sufficient to make the stored optical information visible.

According to EP-a 689' 084, the polarizing layer integrated in the authentication element can be designed as a dichroic LCP layer. It is also possible to apply PPN and LCP layers on the substrate using a polarizer as the substrate.

Where a reflector is present (which may be omitted in accordance with the invention), the polarisers may be polarisers for incoming rays and analyser for outgoing rays, which may not always be required.

Another disadvantage of the authentication element described in EP-a 689' 084 is that the polarization state of light passing through the substrate may be affected by arranging a polarizer underneath the substrate. For example, if inexpensive polymer substrates are used which, because of their manner of production, lend themselves to birefringent properties, then, since the birefringent properties of these substrates are a random result of manufacture and differ from place to place, the birefringent properties of the LCP layers may be cancelled out in extreme cases, with the result that the information of the authentication element may no longer be readable. Furthermore, strongly scattering materials (such as paper) are rejected from consideration as substrates, and also do not carry any encoded information, since polarized light will be immediately depolarized by these materials, so that the polarization state of light passing through the second polarizer and analyzed with it is not recognizable.

However, if an integrated polarizer is positioned between the substrate and the LCP layer as proposed by the present invention, the substrate does not have any effect on the polarization state of the light as it passes through the LCP layer. Thus, on the one hand it is possible to use inexpensive polymer substrates which, because of their manner of production, themselves have birefringent properties, while on the other hand the substrates do not need to be transparent. In this case even scattering backing materials (such as paper etc.) may be suitable.

There are a variety of products, such as paint, documents, photographs, compact discs, semiconductor chips, in which the authentication element is not required to be visible, as this would detract from the appearance of the product or would draw the attention of potential product counterfeiters of the authentication element. In these cases, the present invention suggests incorporating orientable fluorescent dyes into the transmissive LCP film layer structure.

Still other optical effects may be used in liquid crystal authentication elements. Examples include those effects produced by cholesteric filters. A known characteristic of these filters is that they refract a portion of the visible spectrum by circular polarization in a band of wavelengths depending on the physical parameters, while the light not reflected is transmitted (see: Schadt M.F)unfschilling j.jpn.j.appl.phys.29(1990) 1974). The effect of this feature is that the transmitted and reflected light rays have different colors. To produce this visual effect, the range of reflected wavelengths must be selected so that it falls within the visible range. Of course, for applications where the information is read by a machine, it is possible to have the refraction band fall outside the visible band.

Different types of optical elements, which can also be used as authentication elements in the field of copy protection, are based on the combination of a linear polarizer and a cholesteric filter sheet. This type of configuration makes it possible to produce different colors (discussed further below), for which purpose a second linear polarizer is used, which is arranged on the opposite side of the cholesteric filter plate to the first polarizer.

Finally, the tilting effect described at the outset can also be produced in a manner different from the known manner. It is thus possible according to the invention to produce an identification element whose tilting effect is more pronounced and whose production method is technically simpler. This is particularly achieved since at least one birefringent film LCP layer in the element is constructed in such a way that its effective birefringence depends on the viewing angle. In this case, the optical axis can lie in the plane of the film layer, i.e. without incurring the additional expense of tilting the optical axis out of the plane in a defined manner.

According to the present invention, there is provided an optical element comprising at least two film layers characterized by a retarder layer and a polarizing layer, the retarder layer having at least two regions with different optical axes. It is preferred that the retarder layer comprises an anisotropic layer of cross-linked liquid crystal monomers. The retarder layer may be disposed on the alignment layer, and the alignment layer may be in contact with the polarizing layer. The alignment layer preferably comprises a photo-aligned polymer network (PPN). A polarizing layer may be placed on the substrate. Optionally, a second polarizing layer may be arranged on the liquid crystal layer, and another alignment layer and another liquid crystal layer may be arranged on the second polarizing layer, thus constituting a second liquid crystal layer. A polarizing layer may be further arranged on the second liquid crystal layer, and a third alignment layer and a third liquid crystal layer may be disposed on this polarizing layer, and then the third liquid crystal layer may be also constructed. The element for protection against forgery and/or copying may have the above-mentioned optical element and an external linear or circular polarizer, the liquid crystal layer encoding information that can be analyzed with the external polarizer. The element is characterized by at least two liquid crystal layers, each liquid crystal layer encoding a part of the information content, and the parts together forming the complete information content. In such an element, the liquid crystal layer may be designed as a retarder layer and is preferably placed on a substrate, characterized in that the substrate encodes a part of the information content. Preferably, the outer polarizer is constructed and both the liquid crystal layer and the outer polarizer each encode a portion of the information content.

The optical element may feature at least one circularly polarizing layer or preferably two overlapping circularly polarizing layers with a left-hand and a right-hand rotation. The element for protection against forgery and/or copying may comprise such an optical element and an external linear or circular polarizer for analyzing the encoded information.

The invention also provides an optical element comprising an optically anisotropic film formed of liquid crystal molecules, characterized in that the optically anisotropic film comprises fluorescent molecules and preferably has at least several regions with different optical axes. The invention also extends to a security and/or copy protection element comprising such an optical element.

The invention also provides an optical element comprising at least two film layers, which element is characterized by a cholesteric film layer and a linear polarizing layer, and preferably by an optically anisotropic film layer, which may have several regions with different optical axes. The optically anisotropic film layer may be composed of cross-linked liquid crystal molecules. The cholesteric film layer and the optically anisotropic film layer are preferably disposed on the same side of the linear polarizing layer that can be in contact with the cholesteric film layer. A linearly polarizing layer may be arranged on the substrate, a cholesteric film layer being in contact with the linearly polarizing layer, an alignment layer may be disposed on the cholesteric film layer, and an optically anisotropic layer formed of cross-linked liquid crystal monomers may be disposed on the alignment layer, the (optically anisotropic) liquid crystal layer forming regions having different molecular orientations. An element for protection against forgery and/or copying has such an optical element and an external linear polarizer for analyzing information encoded with the liquid crystal layer and/or the cholesteric film layer.

The invention also provides an optical element comprising a birefringent liquid crystal layer having at least two regions with different optical axes, characterised in that the dependence of the optical retardation of the liquid crystal layer on the viewing angle is different in the different regions. Such elements may be designed in such a way that the color of the element locally differs when viewed through the polarizer and may be biaxial; preferably the birefringent layer is biaxial. The element for protection against forgery and/or copying can have such an optical element. According to the invention, a further element for protection against forgery and/or copying comprises a polarizing layer having at least two regions with different directions of polarization.

The element is arranged on a substrate and comprises an optically anisotropic film layer having at least two regions with different optical axes, and the substrate is a reflective polarizer.

The invention also provides a device for protection against forgery and/or copying, in which the elements of any of the types described above and the analyzer are arranged on the same substrate (e.g. a license or a banknote).

Some of which may be considered as documents carrying invisible evidence of authenticity, usually in the form of polarized light. Some of these documents lack a reflective layer and can be authenticated by illumination from below (through the document towards the viewer). Some such documents may omit an integrated polarizing layer.

Illustrative embodiments of the invention are now described with reference to the drawings. In accordance with a simplified schematic representation of the method,

fig. 1 shows a film layer structure of an optical element, comprising a polarizing layer, a PPN layer and an LCP layer, and an associated analyzer;

FIG. 2 shows the LCP structure of the element shown in FIG. 1;

FIG. 3 shows a foldable document having elements of the type shown in FIG. 1;

FIG. 4 shows a film layer structure constructed in a different manner than the structure shown in FIG. 1, with additional PPN and LCP layers, and an analyzer arranged behind the film layer structure in the direction of light travel;

FIG. 5 shows a film layer structure constructed in a different manner than the structure shown in FIG. 1, with additional PPN and LCP layers, and an analyzer arranged in front of the film layer structure in the direction of light travel;

FIG. 6 shows a film layer structure constructed in a different manner than the structure shown in FIG. 1, with two additional PPN and LCP layers, and two external polarizers on either side;

figures 7a and 7b show an LCP element with locally different orientations and cholesteric filters, and a polarizer arranged behind the element in the direction of travel of the light;

FIGS. 8a and 8b show a film layer structure of the type shown in FIG. 7, but with a polarizer arranged in front of the element in the direction of light travel;

FIGS. 9a and 9b show a film structure of the type shown in FIG. 7, but with a complementary cholesteric filter;

FIGS. 10a and 10b show a film layer structure of the type shown in FIG. 7, but in which the cholesteric filter and polarizer are interchanged;

FIG. 11 shows a two-layer authentication element consisting of a cholesteric filter and a first linear polarizer and an associated analyzer;

fig. 12 shows the film layer structure shown in fig. 11, but with an additional retarder layer.

A substrate 1 made of a transparent material (e.g., glass) or a scattering material (e.g., paper) is shown with a schematic sectional view through a film layer structure according to a first illustrative embodiment of the present invention shown in fig. 1. On the substrate there is a linear polarizing layer 2 on which there is a photo-oriented polymer network (PPN) film layer 3 with local variations in network orientation (e.g. image formation) on its surface close to the substrate. Examples of suitable materials include cinnamic acid derivatives as described in EP-A525 ' 478 or US-A5 ' 389 ' 698. They are oriented and simultaneously crosslinked by selective exposure to linearly polarized ultraviolet light.

An anisotropic layer 4 of cross-linked liquid crystal monomers is adjacent to the film layer 3. In this case, the LCP film layer 4 is made up of molecular arrangements whose orientation is predetermined by the orientation of the underlying film layer 2. The LCP layer 4 is photo-cross-linked using light of an appropriate wavelength, thereby fixing the molecular orientation defined by the PPN layer 3. The optical information (i.e. the image) directed to the picture or stored can be made visible by means of the external polarizer 5, for which purpose the light passes through the element denoted by 7 from below upwards in the direction of the arrow 6, while the polarizer 5 (functioning as an analyzer in this case) remains above the element 7.

Fig. 2 shows the preferred mutual orientation of the optical axes of adjacently constructed partial areas of the LCP layers 4. To produce maximum contrast, the optical axes of adjacent regions are at a 45 ° angle.

Fig. 3 shows a variant according to the invention, which is suitable for simplifying the verification of such LCP security items. In this case, the second (external) polariser 5 is mounted on a flexible, light-transmissive substrate 8, such as a document or banknote. The result of this is that the polariser 5 can be positioned over elements 7 located elsewhere on the same banknote by folding or bending the banknote 8 so that the image stored in the LCP is visible when viewed through the polariser 5. In this way, the two polarizers required for the identification of the stored identification elements are present on the same substrate, with the result that no external polarizer is necessary and therefore no auxiliary tools are required for analyzing the information.

Of course, the second polarizer 5 itself may also form part of the film layer structure, which in turn carries the LCP layers. On the one hand, there are two LCP layer structures carrying information content at the same time on one substrate, which information content can be seen separately as separate patterns from each other with the external polarizer. On the other hand, if the substrate is bent, the optical anisotropy phenomena of the two LCP layers can also be combined with each other and can be seen through the two polarizers. In this case, a third pattern different from those two independent patterns will be generated.

In accordance with the present invention, complexity, surprise, optical quality and information content are all enhanced by fabricating a film structure in which two information-carrying LCP layers sandwich a polarizing layer. The information content or information content that can be seen will depend on whether an external second polarizer is arranged above or below the film layer structure. The arrangement of the film elements corresponding to the elements is schematically shown in fig. 4 and 5. In this case, two sets of paired PPN and LCP film layers are denoted by 11a and 11b or 12a and 12b, respectively, and the polarizing layer arranged between the two pairs is denoted by 13. The external polarizer functioning as an analyzer is here denoted 14a or 14b, while the direction of the light rays is denoted 15.

However, if one outer polarizer, in the imagined 14a and 14b (not shown), is arranged above and below, both kinds of information will be seen at the same time. If one or both of the outer polarizers are rotated by 90 deg., the information content will be inverted each independently, i.e. represented as a negative image. For example, the image may be stored in one of the two LCP layers and the corresponding text information may be written in the other LCP layer. Thus, by selecting the arrangement of the polarizers, it is possible that only the image becomes visible or only the text becomes visible, or both become visible at the same time.

Similar to the examples described above with reference to fig. 1 to 5, the number of PPN and LCP layers may be further increased. In the case of the element 29 having a three-layer structure (fig. 6), the film layers 21a/21b, 22a/22b and 23a/23b are separated from each other by means of two orthogonal polarizing layers 24 and 25. In such a film layer structure, the central LCP layer 22b arranged between the two polarizing layers 24 and 25 can be produced according to the method described in EP-A689' 084. When a light ray 26 is incident at an angle normal to the plane of the film layers, the information in the central layer 22b is in this case permanently visible, and arranging the outer polarisers 27 or 28 above or below the elements 29 as described above will cause the information in the upper or lower LCP layer 23b or 21b, respectively, to become visible. If both polarizers 27 and 28 are placed on both sides of element 29, the information in the three LCP layers may become visible at the same time. For example, an image may be broken up in this manner and assigned to the three LCP layers 21b, 22b and 23b, respectively. By arranging only one or two external polarizers, the parts of the image will recombine to form the original image.

However, the information in the central LCP layer may also be encoded by locally varying the tilt angle or by tilt effects of those types described below (e.g. spatially varying the direction of the optical axis relative to the plane of the film layer). For a film layer system consisting of three LCP layers with a polarizing layer disposed therebetween, this has the result that the information in the central image is initially not visible when the film layer is viewed at an orthogonal angle. The information in the central layer becomes visible only when viewed at oblique angles, due to the different birefringence of the regions having different tilt directions of the optical axis. By using one or both outer polarizers the information content of the upper and/or lower layer becomes visible at the same time as the information of the central layer.

Further complexity can be added by adding an LCP layer separated from the remaining film layers by a polarizing layer. Thus, information can be stored in each LCP layer in different ways (e.g. by locally changing the direction of the optical axis in-plane and out-of-plane). Thus, the information content in the various film layers can be seen independently of each other in terms of viewing angle and the layout of the outer polarizer.

Linear polarizing layers can also be produced using LCP layers containing dichroic dye molecules. In such a film the dichroic molecules are oriented according to the local orientation of the LCP molecules, so that in the film the light is locally linearly polarized, that is to say according to the orientation of the dichroic dye molecules. By constructing the doped LCP layer it is possible to create thereby a polarizing layer with local differences in polarization direction. The brightness and/or color of the birefringent layer between the two polarizers depends on the direction of the optical axis of the retarder layer and the transmission direction of the two polarizers, one (or both) of which can be constructed as the polarizer needed to visualize the retarder pattern and thereby carry information. The patterns in the retarder layer and the polarizer can then be matched to each other. Thus, it is possible to place a part of the information in the LCP layer and another part in the polarizer. Thus, only individuals who can provide a structured polarizer that matches the retarder layer can read the entire information content. If there is a reflector under the structured retarder layer, then a second (optional unstructured) polarizer under the retarder layer is no longer needed for reading the information. However, just as a part of the information may be placed in the analyzer, a part of the information may already be permanently present on the substrate. For example, a photograph can be divided in this way into a portion that is permanently visible on the substrate and a portion that is initially invisible, the latter being placed in the retarder layer and only visible using the polarizer. In case of LCP patterns on the reflector, a further variation might be to structure the reflector itself. The supplemental information stored in the structural retarder layer can be seen in the reflective areas when viewed through the polarizer.

As already mentioned above, there are various products, such as paint coatings, documents, photographs, read-only optical discs and semiconductor chips, in which the identification elements are not designed to be visible. Transmissive structural retarder layers will satisfy this condition, but in order to visualize the information they contain, polarizers are placed in front of and behind the retarder layer, which is only possible if the substrate does not change the polarization state of the light. In contrast, in the case of reflective elements based on a structural retarder layer, it is necessary to have a reflector under the retarder layer that is generally always visible.

In the case of various situations of this type, a further object is to provide an authentication element which, although carrying recoverable information, cannot be seen under normal conditions. This is achieved according to the invention by incorporating into the structural LCP layer a orientable fluorescent dye which either fluoresces anisotropically or (and) absorbs light anisotropically and has an absorption band in the ultraviolet band. If the fluorescent molecules are chosen appropriately, rather than being exposed to polarized uv light, those molecules with transition moments parallel to the polarization direction of the exciting uv light are preferentially excited. In an LCP layer, where the fluorescent molecular partitions are perpendicular to each other according to the LCP orientation, only those areas oriented parallel to the polarization direction of the uv light will necessarily fluoresce, which will make it possible for one to see information stored in the film layer that is not visible in the absence of uv excitation.

Alternatively, the doped LCP layer can also be excited with isotropic light. If the fluorescent molecules are chosen appropriately, they will emit fluorescence with a polarization, the polarization direction of which is determined by the molecular orientation. With a polarizer it is possible to distinguish between areas with different fluorescence polarization, which will make it possible for a person to see the information in the film layer.

Fig. 7 to 10 show optical elements having at least one cholesteric filter, which, as already described above, can also be used for authentication elements having crosslinked liquid crystal molecules.

In a first illustrative embodiment of this type of element (fig. 7a and 7b), a structured LCP retarder layer 31 with an optical retardation or optical path difference of λ/4 is used and information is encoded by means of zones with mutually perpendicular optical axes. If the reflection wavelength λ is to be selected_RA cholesteric filter 33 in the visible range is placed under this structure blocker layer 31 or under the PPN alignment layer 32, and light passing through the cholesteric filter from bottom to top in the direction of arrow 34 will first produce circular polarization in the selected wavelength range. The circularly polarized light passes through the structure retarder layer 31 due to lambda_RThe optical retardation of/4 will be converted to linearly polarized light. Since the optical axes of the differently oriented regions are perpendicular to each other (as shown in fig. 7b), the polarization directions of the linearly polarized light after passing through the respective regions are also rotated by 90 ° with respect to each other. If a linear polarizer 35 (having a transmission angle β of 45 ° measured with respect to the direction of the mutually perpendicular optical axes of the retarder layer 31) is maintained above this alignment, colored and colorless regions will be seen. When polarizer 35 is rotated 90, the optical properties of the zones will be interchanged.

On the other hand, if light enters the element not through the filter 33, the PPN layer 32 and the retarder layer 31 in this order, but is incident from above through the linear polarizer (as shown in fig. 8), the pattern that has been written will appear complementary in color by reflected light. In this way it is possible to produce a high information content authentication element in which the information appears as a complementary colour and depends on whether transmitted or reflected light is observed.

Both the circular polarizer and the linear polarizer may form part of the film layer structure, in which case they are permanently present. However, it is also possible to arrange them above or below the film layer only when reading information. For example, the circular polarizing layer may be made of chiral LCP material that is only a few microns thick.

The design of the cell shown in fig. 9a is similar to the cell shown in fig. 7, with a structural retarder layer 41 having an optical retardation of about λ/4. In this case, too, the information is encoded by means of zones whose optical axes are perpendicular to one another, as shown in fig. 9 b. However, in this illustrative embodiment, a left-handed and a right-handed cholesteric filter 42 and 43, respectively, are arranged in order below the PPN layer 44 belonging to the retarder layer 41. The maxima of the selective reflection bands of the two filters 42 and 43 fall in different wavelength bands. If the linear polarizer 45 is again held above the structural retarder layer, the areas with mutually perpendicular optical axes will appear in different colors. When the polarizer or retarder layer is rotated 90, the colors of the zones are interchanged.

The last element within this category is shown in fig. 10a and 10 b. In this case too, a lambda/4 structured retarder layer 51 is used, in which the information is encoded by means of zones whose optical axes are perpendicular to one another. Unlike the example shown in fig. 7, linear polarizer 52 and cholesteric filter 53 are interchanged in this case. Light incident from below in the direction of arrow 54 will first undergo uniform linear polarization by linear polarizer 52 and will become left-handed or right-handed circularly polarized light depending on the optical axis direction upon passing through structure retarder layer 51. If the cholesteric filter 53, which functions as a circular polarizer, is held on top, either left-handed or right-handed circularly polarized light will be transmitted depending on the handedness of the circular polarizer, while light of the opposite handedness will be reflected. Thus, the pattern of the writing retarder layer 51 encoded by the different optical axis directions will appear as a bright colored region pattern.

In this particular case, if the circular polarizer 53 is replaced by a second linear polarizer, the pattern cannot be seen because the polarization state of light after passing through the regions of the retarder layer 51 is either a right-handed or left-handed circular polarization state.

The fact that the individual retarder zones, whose optical axes are perpendicular to each other, cannot be distinguished by means of a linear polarizer offers the possibility of writing different information contents into the LCP layer, and it is also possible to read them out without interfering with each other by means of different auxiliary tools. To do this, the first information may be encoded using regions whose optical axes are perpendicular to each other, as described for the illustrative embodiment shown in fig. 10, for example. Then, the second information is encoded using regions whose optical axes are at 45 ° to the mutually perpendicular axes in the first regions. If a linear polarizer is placed under the retarder layer and illumination is provided through it to the film layer, as described in the illustrative embodiment of figure 10, only the second information is visible when viewed with the second linear polarizer remaining above the element formed by the linear polarizing layer, PPN layer and LCP layer. Instead, the first information is only visible at normal viewing angles (as explained before) when a circular polarizer is placed on top of the retarder layer instead of a linear polarizer, in which case the second information is also visible with reduced intensity. Thus, for example, in the authentication element, it is possible to have a linear polarizer layer permanently integrated under the structural retarder layer, so that it is sufficient to see different information contents if only a linear polarizer and a circular polarizer are placed one after the other on the element in order to verify the authenticity of the element.

Finally, in this connection it will be pointed out that there is another possibility of using at least one cholesteric filter, i.e. to visualize the retarder structure without using any linear polarizer but only a circular polarizer. For this purpose, information is recorded by establishing an optical retardation in the retarder layer, it then being possible to have the optical axis in the same direction throughout the plane of the film layer. If this type of retarder layer is placed between cholesteric filters with overlapping selective reflection bands, the written information will be visible or readable.

As already described above, there is the possibility of further developing an optical authentication element which essentially consists of a cholesteric filter and two linear polarizers.

This is because if a second linear polarizer, acting as an analyzer, is arranged on the other side of the cholesteric filter of the first polarizer, it will be possible for the linear polarizer in combination with the cholesteric filter to produce a different color.

In the simplest case, the authentication element using this effect would consist of only one cholesteric filter. In order to produce an optical effect that can be used in the authentication element, two linear polarizers are necessary, which will be maintained above or below the cholesteric film layer as desired. In this simple case, the cholesteric film layer may be applied only to a transparent substrate (e.g., glass). However, if the authentication element is to be applied to a depolarized diffusive substrate, the first polarizer may be permanently integrated in the authentication element. This type of authentication element is shown in fig. 11. This element consists of a cholesteric film layer 61, a substrate 62 and a first polarizing layer 63 arranged between the substrate 62 and the film layer 61. The second polarizer required to view the stored information is indicated at 64 and should be held above the element when required.

The color of light passing through the linear polarizing layer 63 in the direction of arrow 65 in the cholesteric filter 61 is determined first by the selective reflection wavelength of the cholesteric filter 61. If the outer linear polarizer 64 remains above the cholesteric film layer, the color changes when the polarizer 64 is rotated. For example, if a cholesteric filter 61 that reflects green light is used, the transmitted light first appears red violet. Conversely, if the film layer is viewed through the second polarizer 64, a yellow, green, red or blue color can be seen when this polarizer is rotated.

If a uniaxial optical retardation layer (e.g., with an optical path difference of lambda/2) is placed between the cholesteric filter 61 and the second polarizer 64, the color will also change by rotating the retardation layer when the position of the polarizer 64 is unchanged. It is possible in this way to produce a wide range of color modulation by appropriate selection of the reflection wavelength and bandwidth of the cholesteric filter and by appropriate selection of the optical path difference and the optical axis direction of the retardation layer. The retardation layer may also be positioned between the input polarizer 63 and the cholesteric filter 61, rather than between the cholesteric filter 61 and the polarizer 64.

As long as a non-structured retardation layer is used, the color effect does not differ very significantly from what is achieved by using a single cholesteric film layer between the two polarizers. However, when using a different arrangement of the structured retarder layers for each of the zoned optical axes, there is a possibility of local color differences. It consists of a first polarizing layer 72 disposed on a substrate 71, a cholesteric film layer 73, and a structural LCP retarder layer 74 associated with a PPN alignment layer 75. If this element is placed under an external polarizer 76 (e.g., with the polarizer orientation perpendicular to the polarization orientation of the polarizing layer 72), then different colors can be seen, the number of different colors depending on the structure of the retarder layer 74 and determined by the number of optical axes that are oriented differently. In this way, information can be represented in color. This impressive optical effect is further enhanced by the separate changes in the colors as the polarizer 76 is rotated. Furthermore, the colour of this type of authentication element is significantly dependent on the viewing angle in view of the dependence of these selective reflection wavelengths on the viewing angle and in view of the optical path differences in the retarder layer.

In addition to establishing the optical axis direction, it is also possible to establish an optical retardation in the retarder layer. Whereby it is possible to optimize the color with complementary parameters.

Although it is possible for a cholesteric filter in combination with a light retardation layer to exhibit a large number of colors, it is not possible to adjust the brightness of the color in this arrangement over the full range from dark to light. However, this can be achieved by constructing the cholesteric filters in different ways, for example by locally removing the cholesteric film layer by means of photolithography, or by locally changing the optical path length in the visible wavelength band during production to change the reflection wavelength of the cholesteric filters. Since the cholesterol-type filter is not present or is optically isotropic at the spots treated in this way, only the retarder layer determines the optical behavior of the spots. For example, in the case of crossed polarizers, it is possible to have the optical axis parallel to one of the polarizers, just as this would block the light at this point, and thus darken. By changing the ratio of the dark area and the colored area, it is possible to control the brightness of each color patch (mosaic pattern).

As already stated above, it is also possible to generate the tilting effect in the birefringent layer of the kind described at the outset in a manner different from the known manner, whereby it is possible to produce authentication elements in which the tilting effect is more pronounced and which can be produced more easily.

This is achieved according to the invention in that at least one birefringent layer in the layer structure is constructed such that its effective retardation depends on the viewing angle. In this case, the optical axis may be in the plane of the film layer, i.e. without paying the additional costs required to tilt the optical axis out of the plane in a prescribed manner. The optical retardation is equal to the product of the thickness of the film layer and the optical anisotropy of the material, so that for a given material the optical retardation can be adjusted by the thickness of the film layer. If the effect of the light retardation depends on the viewing angle, the grey value or color changes accordingly with the viewing angle. For example, where the material has a well-defined uniaxial optical anisotropy, the retardation can be adjusted in such a way that the film layer shows a violet color when viewed perpendicularly. However, if the film is viewed obliquely in such a way that the viewing angle and the optical axis form a plane, the color changes from violet to yellow. However, if viewed obliquely from a direction perpendicular to the optical axis, the color changes from violet to blue. Thus, for the respective optical axis positions, it is possible to achieve an effect that the color changes from violet to yellow when the film layer is tilted upward or downward, and that it changes to blue when the film layer is tilted rightward or leftward.

This angular dependence of the optical retardation can be used to produce structured LCP authentication elements for writing information that has an angular dependent characterization. If, for example, the LCP layer is constructed as described in EP-a689084 in accordance with the optical axis of the different regions of the information to be expressed or parallel or perpendicular to a reference axis in the plane of the film layer, the information is initially not visible when viewed perpendicularly with crossed polarisers. It is only possible to see the written pattern when the film is viewed obliquely, since the viewing angles are different in respect of the areas where the optical axes are perpendicular to each other. If the film thickness is also adjusted in such a manner that the light retardation appears violet when viewed perpendicularly, the color in the region where the optical axis is parallel to the reference axis changes from violet to blue while the color in the other regions changes from violet to yellow when tilted about the reference axis. If the film is tilted up and down, yellow information will be presented on a blue background, and if the film is tilted left and right, blue information will be presented on a yellow background. Of course, it is equally possible to set other colors, grey values or combinations of colors and grey values by means of the film layer thickness. When using grey values, a black and white effect is obtained instead of a color effect.

Both uniaxial birefringent materials and biaxial birefringent materials are suitable for producing birefringent layers with a visible image that varies with viewing angle. However, viewing angle dependence can be further enhanced with optically biaxial materials. For example, if the index of refraction normal to the plane of the film layer is less than the index of refraction in the plane of the film layer, the change in optical retardation and corresponding tilt effect during tilted viewing is much greater than using uniaxial materials.

Instead of biaxial materials, a strong dependence on the viewing angle can also be achieved by a film structure consisting of two or more uniaxial film layers, for example in one film layer the optical axis is parallel or oblique to the film plane, while in the second film layer it is perpendicular to the film plane. By appropriately selecting the film thickness ratio, the tilt effect can be enhanced or reduced. Furthermore, if the film layer is also structured such that the optical axis is parallel or tilted with respect to the film layer plane, i.e. the projected sections of the optical axis in the film layer plane are directed in different orientations, the color or gray value of the pattern seen when viewed obliquely under crossed polarizers will vary considerably with only a slight variation in the viewing angle.

In another illustrative embodiment, a strong dependence on the viewing angle can also be achieved by a film layer structure comprising a non-structured optically biaxial layer and a structured birefringent layer of optically uniaxial material, which can be produced very simply by, for example, applying the structured birefringent layer directly to an optically biaxial sheet.

The use of a substrate that enables the polarization of incident light to be varied with angle also allows the fabrication of viewing angle dependent authentication elements. This would be the case, for example, with a non-metallic smooth surface such as glass or plastic. At least a portion of obliquely incident light rays are polarized after being reflected by the surface of the material. At a specific angle of incidence (brewster angle) depending on the respective material, the reflected light is in fact completely linearly polarized. If such a material with a polarization effect that varies with angle is used as the substrate for the structural retarder layer, then obliquely incident light reflected by the substrate surface will be polarized before passing through the retarder layer again. The polarization state of which varies with the local optic axis direction so that the pattern in the corresponding structural retarder layer can be seen when this type of film layer is viewed obliquely through the polarizer. The best contrast is obtained if the film is viewed at the brewster angle. This pattern does not appear at all when the viewing angle is a right angle.

It is also possible to use several layers that absorb light anisotropically instead of birefringent layers to produce a tilting effect. For example, this type of film layer may be fabricated with an LCP layer incorporating dichroic dyes. Since the dichroic dye is oriented with the LCP molecules, different orientations can be imparted to the dichroic dye partitions by the structural orientation of the LCP molecules as well. Depending on the dye used, it is possible to polarize light in the visible range or only in a single wavelength band, so that the film layers appear grey or colored. If the film layer is viewed through a linear polarizer, the written pattern can be seen.

The LCP layer containing the dichroic dye exhibits absorption that is dependent on the viewing angle. If the dichroic dye doped uniaxially oriented LCP layer is tilted around the orientation direction of the LCP or dye molecules, the film layer appears darker than when viewed normal, as the optical path length increases with increasing tilt angle. However, if the film layer is tilted about an axis lying in the plane of the film layer perpendicular to the orientation of the LCP, the film layer appears brighter, since in this case the absorption axis of the dye molecules is tilted with respect to the incident direction of the light, resulting in a smaller proportion of the light being absorbed. To see these changes in brightness due to tilt, viewing through a polarizer is not absolutely necessary. For example, if the LCP molecules in different regions form an LCP layer parallel or perpendicular to each other, those regions of the LCP which are oriented parallel to the tilt axis appear darker when the film layer is tilted about one of the two preferred directions, while the remaining regions appear lighter. Conversely, if the film is tilted about another preferred axis, the brightness of the regions is reversed. This effect is also seen without the use of an additional polarizer, so it is particularly suitable for applications where it is desired to inspect the authentication element without the use of an auxiliary tool.

The production of PPN and LCP layers which can be used according to the invention and the production of authentication elements with a tilting effect will be explained in more detail below.

Production of PPN layers:

suitable PPN materials include cinnamic acid derivatives. For the fundamental study of the present invention, PPN materials with a high glass transition temperature (Tg 133 ℃), i.e. polymers, were selected:

spin-coat with a 5% solution of PPN material in NMP at 2000rpm on a glass plate for 1 minute. The coating was then allowed to dry on a heated platen for 1 hour at 130 ℃ and then in vacuo for 1 hour. The film was then exposed to linearly polarized light (200W high pressure mercury lamp) for 5 minutes at room temperature. Then, the film layer is used as an alignment layer of liquid crystal.

2. Mixture of crosslinkable LC monomers for LCP layer:

in these examples, the following diacrylate compounds were used as the crosslinkable LC monomer:

monomer 1

Monomer 2

Monomer 3

Using this compound, a supercooled nematic mixture M was developed with a particularly low melting point (TM. approx.35 ℃ C.)_LCPThereby making it possible to prepare the LCP layer at room temperature.

The diacrylate monomers were present in the mixture with the following composition:

180 percent of monomer

215% of monomer

Monomer 35%

In addition, 2% of the Ciba-Geigy photoinitiator IRGACURE 369 was added to the mixture.

Then, the mixture MLCP was dissolved in anisole. By means of the concentration of MLCP in anisole, it is possible to adjust the thickness of the LCP layer in a wide range.

To photoinitiate the LC monomer for crosslinking, the film was exposed to isotropic light from a 150W xenon lamp in an inert atmosphere for approximately 30 minutes.

3. Discriminating element with tilting effect

The PPN-coated glass plate was divided in half and exposed to polarized uv light, respectively, with the polarization direction of the light being rotated by 90 ° relative to the first exposure when the second half was exposed. In each case, the other half is covered during exposure. This results in two regions in which the planar orientation directions are perpendicular to each other.

Production of M at a concentration of 5% in anisole_LCPAnd (3) solution. The solution was spin coated onto a PPN layer that had been exposed to light in different ways. Spin coating parameters: spin coating was carried out for 2 minutes at 1000 rpm. To optimize the orientation of the LC monomer, the coated substrate was heated to just above the clearing point (Tc 67 ℃). The film was then cooled to 3 ℃ below the clearing point at a rate of 0.1 ℃/min.

After crosslinking of the LC monomer, a thickness of the LCP layer of approximately 80nm was obtained.

If this film layer is arranged between crossed polarizers such that the orientation direction of the LCP layer forms a 45 ° angle with the transmission direction of the polarizers, the LCP layer appears uniformly gray. However, if the film is viewed obliquely so that the viewing direction forms a plane with the orientation of the left half of the panel, then the left half of the panel appears darker and the right half of the panel appears brighter.

To conclude, it should be noted that the optical effects described above and the corresponding film layer structure and material composition represent only one option according to the invention in a large number of embodiments and that they can be combined in various ways in order to develop the identification element.

It is of course possible, therefore, to place into the optical element instead of the LCP layer any other type of birefringent layer that is likely to produce an optical effect that can be used by products such as authentication elements.

Furthermore, it is also possible to use different alignment layers for the above examples (instead of PPN alignment layers), where the alignment layers will have the same or similar properties as the PPN layer in terms of the desired optical properties and resolution. It is also possible to produce the required orientation of the retarder layer with a correspondingly structured substrate. Structured substrates of this type can be produced, for example, by embossing, etching and scoring.

Finally, it should be noted that the multilayer structure according to the invention can be used not only for security and copy protection elements, but also for the production of products such as electro-optical liquid crystal cells, in which the LCP layers will perform a wide variety of optical and orientational functions.

In summary, with reference to the drawings, the present invention discloses at least the following subject matter:

1. an optical element comprising at least two film layers, characterized by a retarder layer (4, 11b, 12b, 21b, 22b, 23b, 31, 41, 51, 74) and a polarizing layer (2, 13, 24, 25, 33, 42, 43, 52), wherein the retarder layer (4, 11b, 12b, 21b, 22b, 23b, 31, 41, 51, 74) has at least two regions with different optical axes.

2. The optical element according to item 1 above, characterized in that the retarder layer (4, 11b, 12b, 21b, 22b, 23b, 31, 41, 51, 74) comprises an anisotropic layer (4) comprising a cross-linked liquid crystal monomer.

3. An optical element according to item 2 above, characterized in that the retarder layer (4, 11b, 12b, 21b, 22b, 23b, 31, 41, 51, 74) is placed on the orientation layer (3, 11a, 12a, 21a, 22a, 23a, 32, 44, 75) and the orientation layer (3, 11a, 12a, 21a, 22a, 23a) is in contact with the polarizing layer (2, 14, 11b, 12b, 21b, 22b, 23 b).

4. Optical component according to the above item 2 or 3, characterized in that the orientation layer (3, 11a, 12a, 21a, 22a, 23a) comprises a photo-oriented polymer network (PPN).

5. An optical element according to any of the preceding claims 2 to 4, characterized in that the polarizing layer is structured.

6. The optical element according to any of the above items 2 to 5, characterized in that the linearly polarizing layer is placed on the substrate (1, 8).

7. An optical element according to item 6 above, characterized in that a second polarizing layer (13, 24) is arranged above the liquid crystal layer (11b, 21b), and a further orientation layer and a further liquid crystal layer (12a/12 b; 22a/22b) are arranged above this second polarizing layer and constitute a second liquid crystal layer.

8. An optical element according to item 7 above, characterized in that a further polarizing layer (25) is arranged above the second liquid crystal layer (22b), and a third alignment layer and a third liquid crystal layer (23a/23b) are arranged above this further polarizing layer and constitute the third liquid crystal layer.

9. An element for protection against forgery and/or copying, characterized in that the information encoded with the liquid crystal layer can be analyzed by means of an external polarizer (5, 14, 28, 27) according to the optical element of any one of the above-mentioned items 2 to 8 and an external linear or circular polarizer (5, 14, 28, 27).

10. An element according to item 9 above, having an optical element according to item 7 or 8 above, characterized by at least two liquid crystal layers, each liquid crystal layer encoding a part of the information content, which together form the total information content.

11. A component according to item 9 or 10 above, wherein the liquid crystal layer is designed as a retarder layer and is placed on a substrate (1, 8), characterized in that the substrate encodes a part of the total information content.

12. A component according to item 9, 10 or 11 above, characterized in that the outer linear polarizer is structured and both the liquid crystal layer and the outer polarizer each encode a part of the total information content.

13. An optical element according to any one of the above items 2 to 5, characterized by at least one circularly polarizer (33, 43, 53).

14. An optical element according to item 13 above, characterized in that the two circular polarizers (40, 43) are arranged one above the other, one of which is left-handed and the other is right-handed.

15. An element for protection against forgery and/or copying, characterized in that it comprises an optical element according to item 13 or 14 above and an external linear or circular polarizer (53, 64, 76) suitable for analyzing the coded information.

16. An optical element comprising an anisotropic layer of liquid crystal molecules, characterized in that the optically anisotropic film layer comprises fluorescent molecules.

17. The optical element according to item 16 above, wherein the optically anisotropic film layer has at least two regions having different optical axes.

18. An element for protection against forgery and/or copying, characterized by comprising an optical element according to the above item 16 or 17.

19. An optical element comprising at least two layers, characterized by a cholesteric layer (61, 73) and a linear polarizing layer (63, 72).

20. The optical element according to item 19 above, further characterized by comprising an optically anisotropic film layer (74).

21. The optical element according to item 20 above, wherein the optically anisotropic film layer (74) has a plurality of regions having different optical axes.

22. The optical element according to the above 20 or 21, wherein the optically anisotropic film layer (74) is formed of crosslinked liquid crystal molecules.

23. The optical element according to any of the above items 20 to 22, characterized in that the cholesteric film layer (61, 73) and the optically anisotropic film layer (74) are on the same side of the linear polarizing layer (63, 72).

24. The optical element according to any one of the above 19 to 23, wherein the linear polarizing layer (63, 72) is in contact with the cholesteric film layer (61, 73).

25. The optical element according to any one of the above items 20 to 23, characterized in that the linearly polarizing layer (63, 72) is in contact with the optically anisotropic film layer (74).

26. The optical element according to the above 19, wherein the linear polarizing layer (63, 72) is arranged on the substrate (62, 71), characterized in that the cholesteric film layer (61, 73) is in contact with the linear polarizing layer (63, 72), the alignment layer (75) is disposed on the cholesteric film layer (61, 73), the optically anisotropic film layer (74) formed of a crosslinked liquid crystal monomer is disposed on the alignment layer, and the liquid crystal layer (74) is formed into a plurality of regions having different optical axes.

27. An element for protection against forgery and/or copying, characterized in that it comprises an optical element according to any of the above-mentioned items 19 to 26 and an external linear polarizer (64, 76) suitable for analyzing information encoded with the liquid crystal layer (74) and/or the cholesteric film layer (61, 73).

28. An optical element comprising a birefringent liquid crystal layer, wherein the liquid crystal layer has at least two regions with different optical axes, characterised in that the optical retardation of the liquid crystal layer in each region is each differently dependent on the viewing angle.

29. An optical element according to item 28 above, characterized in that it is designed in such a way that the colour of the element locally differs when viewed through the polarizer.

30. The optical element according to item 28 or 29 above, characterized in that it is biaxial.

31. The optical element according to item 30 above, wherein the birefringent liquid crystal layer is biaxial.

32. Element for protection against forgery and/or copying, characterized in that it comprises an optical element according to any one of the above-mentioned items 28 to 31.

33. Element for protection against forgery and/or copying, comprising a polarizing layer having at least two regions with different directions of polarization.

34. Element for protection against forgery and/or copying, which element is arranged on a substrate and comprises an optically anisotropic film layer having at least two regions with different optical axes, characterized in that the substrate is a reflective polarizing layer.

35. Device for protection against forgery and/or copying, characterized in that the element according to item 9, 10, 11, 12, 15, 18, 27, 32, 33 or 34 above and the analyzer are arranged on the same substrate.

Claims (11)

1. Element for protection against forgery or copying, comprising:

-an optical element comprising at least two layers, one being a structured retarder layer (31, 41, 74) and the other being a polarizing layer (33, 42, 43, 72), wherein said polarizing layer is a circular polarizer;

-external linear or circular polarizers (35, 76) for analyzing the encoded information,

wherein the retarder layer (31, 41, 74) is structured by having at least two regions with different optical axes and/or different optical retardations.

2. The element of claim 1, characterized in that said retarder layer (31, 41, 74) comprises an anisotropic layer (4) comprising a cross-linked liquid crystal monomer.

3. An element as claimed in claim 1 or 2, characterized in that the circular polarizers of the polarizing layers comprise two circular polarizers (42, 43) arranged one above the other, one left-handed and the other right-handed.

4. The element according to claim 1 or 2, characterized in that the circular polarizer of the polarizing layer comprises one or more cholesteric film layers (33, 42, 43, 73).

5. The component according to claim 3, characterized in that the two circular polarizers are cholesteric film layers (42, 43), one of which is left-handed and the other of which is right-handed, and have reflection bands with maxima in different wavelength ranges.

6. The element of claim 4, characterized in that it comprises a second linear polarizer (72).

7. Element according to claim 6, characterized in that the cholesteric film layer (73) and the structured retarder layer (74) are on the same side of the second linear polarizer (72).

8. The element according to claim 6, characterized in that said second linear polarizer (72) is in contact with said cholesteric film layer (73).

9. The element of claim 6 characterized in that said second linear polarizer (72) is in contact with said structured retarder layer (74).

10. The element according to claim 6, said second linear polarizer (72) being arranged on a substrate (71), characterized in that said cholesteric film layer (73) is in contact with said second linear polarizing layer (72) and an alignment layer (75) is placed on top of said cholesteric film layer (73) and an optically anisotropic layer (74) of cross-linked liquid crystal monomers forms areas with different molecular orientations and is placed on said alignment layer.

11. The element of claim 1, characterized in that said optical element and said external linear or circular polarizer are arranged on the same substrate.