OPTICAL SECURITY DEVICE

Abstract

The invention relates to an optical device comprising two patterned retarders and a reflective polarizer between the retarders. Information which is stored in the pattern of the retarders can be observed both in transmission and reflection. The device according to the invention can be used as high security element, for example for banknotes.

Claims

1. An optical security device including a stack comprising a first and a second optical retarder layer each with zonewise different orientation of the optical axis, characterized in that a reflective polarizer is between the two retarders, the polarizing area overlapping at least with a part of each retarder.

2. The optical security device according to claim 1, wherein the reflective polarizer (13) is a linear polarizer.

3. The optical security device according to claim 2, wherein the reflective polarizer (13) is a multi-layer birefringent polymeric film.

4. The optical security device according to claim 1, wherein at least one of the optical retarders (11, 12) includes a layer comprising polymerized or cross-linked liquid crystals with locally different optical axes directions.

5. The optical security device according to claim 4, wherein the layer comprising polymerized or cross-linked liquid crystals is in contact with an aligning surface.

6. The optical security device according to claim 5, wherein the aligning surface is the surface of a photo-alignment layer.

7. The optical security device according to claim 5, wherein the aligning surface is an embossed structure in the surface of the substrate or in a layer on the substrate.

8. The optical security device according to claim 1, wherein the zonewise different orientation of the optical axis encodes for an image.

9. The optical security device according to claim 8, wherein the image comprises at least one of a photograph, a text, a microtext, a number, a picture, a bar code, a symbol, a character, an illustration, a graphic.

10. The optical security device according to claim 1, further comprising at least one of a protection layer, a color filter layer, a thin metallic layer, a dielectric layer.

11. A method of manufacturing an optical security device according to claim 1, comprising the steps producing at least one of the first and second retarders in the form of a foil and laminating the retarder or retarders to the reflective polarizer.

12. A method of manufacturing an optical security device according to claim 1, comprising producing the patterned retarder as a thin layer on a substrate, transferring the thin retarder layer to the reflective polarizer by laminating it to the reflective polarizer, removing the substrate from the retarder.

13. A method of manufacturing an optical security device according to claim 1, wherein a reflective polarizer film is used as a substrate and at least one of the patterned retarders is prepared by coating or printing a liquid crystal composition onto the reflective polarizer film.

14. A method of manufacturing an optical security device according to claim 1, comprising arranging an external polarizer between the device and an observer.

15. A method of manufacturing an optical security device according to claim 1, comprising arranging an external polarizer behind the device, as seen by an observer.

16. A method of manufacturing an optical security device according to claim 1, comprising illuminating the device with polarized light and observing the image without a polarizer in the light path between the device and an observer.

17. A method of manufacturing an optical security device according to claim 1, comprising illuminating the device with non-polarized light and observing the device with a polarizer in the light path between the device and an observer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention is further illustrated by the accompanying drawing figures. The drawings are examples only and shall help to understand the invention but shall in no way limit the scope of the invention.

[0020] FIG. 1 shows the layer structure of a device according to the invention.

[0021] FIG. 2 shows the observation of the information encoded in the device in transmission. In FIG. 2.1 the image stored in the pattern of the first retarder layer is visualized by arranging an external polarizer behind the device as seen by an observer. In FIG. 2.2 the image stored in the pattern of the second retarder layer is visualized by arranging an external polarizer in front of the device as seen by an observer.

[0022] FIG. 3 shows the observation of the information encoded in the device in reflection. In FIG. 3.1 the observer is adjacent to the first retarder layer and by illuminating the device with polarized light from the side of the observer, the image stored in the first patterned retarder layer is visualized. In FIG. 3.2 the observer is adjacent to the second retarder layer and by illuminating the device with polarized light from the side of the observer, the image stored in the second patterned retarder layer is visualized.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The layer structure of a device 10 according to the invention is shown in FIG. 1. The device comprises a first optical retarder layer 11 with an orientation pattern encoding a first image, a second optical retarder layer 12 with an orientation pattern encoding a second image and a reflective polarizer 13 between the two retarders. Preferably, the reflective polarizer is a linear polarizer. For the following explanation it is assumed that both the reflective polarizer 13 and the external polarizer 14 are linear polarizers.

[0024] The first image can be visualized by properly arranging an external polarizer 14 adjacent to the first retarder 11, as shown in FIG. 2.1. The observer 30 is positioned on the opposite side of the device. An observer may be a human or a machine, for example a camera, a scanner or any other optical and/or electro-optical system that is able to detect an image. Incident light 21 from behind the device, as seen by the observer, is polarized by the external polarizer. The polarization state of the light is locally changed on transmission through the retarder 11, depending on the local optical axis direction of the patterned retarder. For the observer, the reflective polarizer 13 acts as a transmissive polarizer and therefore can block the light fully or partially or transmits the light, depending on the local polarization state of the light. Hence the image stored in the retarder 11 becomes visible.

[0025] If the external polarizer is arranged between the device and the observer, such that it is adjacent to the patterned retarder 12, as shown in FIG. 2.2, then the incident light 21 is polarized by the reflective polarizer 13, which in this case again acts as a linear polarizer. Depending on the local optical axis directions in retarder layer 12, the polarization state of the light is changed. The polarizer 14 blocks the light fully or partially or transmits the light, depending on the local polarization state of the light. Hence the image stored in the retarder 12 becomes visible.

[0026] In both of the above transmissive cases, the external polarizer 14 is preferably arranged with the polarization direction parallel or perpendicular to the polarization direction of the reflective polarizer 13. However, the image becomes also visible for almost every other angle, but the optical contrast of the visualized image may be lower.

[0027] It is also possible to use two external polarizers at the same time in order to observe the images stored in the first and the second retarder simultaneously.

[0028] Instead of changing the position of the polarizer from front to behind and vice versa it is also possible to rotate the device around an axis through the plane of the device.

[0029] In FIG. 3.1 the image stored in the pattern of retarder layer 11 is observed in reflection. The state of polarization of incident polarized light 22, which, for example, has a polarization direction 25 within the drawing plane, is locally changed in retarder layer 11, depending on the local optical axis direction. The reflective polarizer in this case works in reflection and hence acts as an analyzer. Therefore, the intensity of the locally reflected light depends on the change of the local polarization state of the light. After reflection, the polarization state of the still polarized light may further be changed but as there is no additional analyzer, the intensity of the light is not further changed. Hence, the observer 23 can see the visualized image encoded in the pattern of the retarder layer 11.

[0030] When the observer is adjacent to the retarder layer 12, as in FIG. 3.1, the image stored in the pattern of retarder layer 12 can be observed in reflection in the same way as described above.

[0031] Instead of illuminating the device with polarized light 22, it could also be observed in reflection with incident non-polarized light. In this case the light reflected from the device can be analyzed with an external polarizer, which also makes the stored image visible.

[0032] If the angle between the optical axis of the retarder and the polarization direction of the reflective polarizer is 45 and the optical retardation of the retarders is that of a half wave plate, which for green light corresponds to about 280 nm, the brightness is at a maximum. For other values of the optical retardation the brightness is lower, but as the dark state is independent from the optical retardation, an image can be observed for any retardation. Preferably, however, the optical retardation for retarders according to the invention is larger than 100 nm, more preferred larger than 140 nm and most preferred larger than 180 nm. The optical retardation is preferably in the range between 180 and 320 nm, more preferred between 200 to 300 nm and most preferred between 220 and 280 nm. By choosing even larger values of retardation, an optical retarder between two polarizers appears colored. In order to achieve a colored appearance the retardation is preferably larger than 250 nm, more preferred larger than 350 nm and most preferred larger than 450 nm.

[0033] A device according to the invention may include additional layers such as alignment layers, protection layers, color filter layers, thin metallic layers, or dielectric layers.

[0034] There are technologies, methods and materials known for the production of retarders with patterned orientation. For example, the retarders may include a layer comprising polymerized or cross-linked liquid crystals with locally different optical axes directions. Such layers are, for example, prepared by applying cross-linkable liquid crystal materials on top of an alignment layer exhibiting locally different alignment directions. The liquid crystal material adopts the local alignment directions of the underlying alignment layer and is then cross-linked to fix the orientation. With regard to the preparation of such optical elements reference is made to WO09112206, which is hereby incorporated by reference.

[0035] There are different methods that can be used to produce a device according to the invention. Preferably the patterned retarders are made by applying a cross-linkable liquid crystal material, for example by coating, onto a substrate with an aligning surface having the desired orientation pattern. The liquid crystal material adopts the local alignment direction of the underlying aligning surface and is then cross-linked to fix the orientation. The aligning surface may be the surface of an aligning layer. An alignment layer with locally different alignment directions can easily be prepared by the photo-alignment technique, where a layer of a material, which is sensitive to the polarization of light, is exposed to linearly polarized light. Patterned alignment is achieved by changing the polarization direction of the light for the exposure of different regions of the photo-alignment layer. Besides using a photo-alignment layer to provide the orientation pattern for the liquid crystal material, other techniques may be used, such as embossing a structure capable of aligning liquid crystals in the surface of a substrate or a layer on the substrate.

[0036] There are different ways to combine the retarders and the reflective polarizer. The patterned retarders may be produced separate from the reflective polarizer. For example, one or both patterned retarders may be produced in the form of a foil which is/are then laminated to the reflective polarizer. If the patterned retarder is produced as a thin layer on a substrate the thin retarder layer may be transferred to the reflective polarizer by, for example, laminating it to the reflective polarizer and then removing the substrate from the retarder. The same process can be used for the combination of the first retarder with the reflective polarizer and for the combination of the second retarder with the reflective polarizer. Preferably, a reflective polarizer film is used as a substrate and at least one of the patterned retarders is prepared by coating or printing a liquid crystal composition onto the reflective polarizer. There may be a separate layer on the polarizer which provides the alignment information for the liquid crystal material. The liquid crystal material adopts the local alignment direction of the underlying alignment layer and is then cross-linked to fix the orientation.