Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-08
2004-05-11
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S096000, C349S098000, C349S124000
Reexamination Certificate
active
06734936
ABSTRACT:
The invention relates to an optical component containing an optically anisotropic layer, which latter has at least two regions with different molecular orientations. The anisotropic layer may, for example, be a retarder layer formed by cross-linked liquid-crystal monomers.
A particular use of the components according to the invention is in the field of protection against forgery and copying.
The demand for safeguarding banknotes, credit cards, securities, identity cards and the like against forgery is increasing constantly on account of the high-quality copying techniques which are available. Furthermore, in low-wage countries, imitations of branded products and copies of copyright-protected products, for example compact discs, computer software, electronics chips, etc. have been produced and exported worldwide. Because of the increasing number of forgeries, there is therefore a great need for new elements which are safeguarded against forgery and can be identified both visually and by machine.
In the field of copy-protecting banknotes, credit cards etc, there are already a considerable number of authentication elements. Depending on the value of the document to be protected, very simple or relatively highly complex elements arc employed. Some countries are content to provide banknotes with metal strips which come out black on a photocopy. Although this prevents them from being copied, elements of this type are very easy to imitate. In contrast to this, there are also more complex authentication elements, for example holograms and cinegrams. Authentication elements of this type are based on the diffraction of light by gratings and need to be observed under different viewing angles in order to verify their authenticity. These diffracted elements produce three-dimensional images, colour variations or kinematic effects which depend on the angle of observation and have to be checked on the basis of predetermined criteria or rules. It is not practically possible to use machines for reading information, for example images or numbers, encoded using this technique. Furthermore, the information content of these elements is very limited, and only an optical specialist will be capable of discriminating definitively between forgeries and an original.
Lastly, one should not ignore the fact that diffractive optical effects have in the course of time also been used outside the field of security, in particular for consumer articles such as wrapping paper, toys and the like, and the production methods for such elements have in the course of time become known to a large group of people and are correspondingly straightforward to imitate.
Further to the diffractive elements mentioned above, other components are also known which are suitable for optimum copy protection. These include optical components, as disclosed for example by EP-A 689,084 or EP-A 689,065, that is to say components with an anisotropic liquid-crystal layer, which latter has local structuring of the molecular orientation.
These components are based on a hybrid layer structure which consists of an orientation layer and a layer which is in contact with it and consists of liquid-crystal monomers cross-linked with one another. In this case, the orientation layer consists of a photo-orientable polymer network (PPN)—synonymous with LPP used in other literature which, in the oriented state, through a predetermined array, defines regions of alternating orientations. During the production of the liquid-crystal layer structure, the liquid-crystal monomers are zonally oriented through interaction with the PPN layer. This orientation which, in particular, is characterized by a spatially dependent variation of the direction of the optical axis, is fixed by a subsequent cross-linking step, after which a cross-linked, optically structured liquid crystal (LCP for liquid crystal polymer) with a preestablished orientation pattern is formed. Under observation without additional aids, both the orientation pattern itself and the information written into the liquid crystal before the liquid-crystal monomers are cross-linked, are at first invisible. The layers have a transparent appearance. If the substrate on which the layers are located transmits light, then the LCP orientation pattern or the information which has been written become visible if the optical element is placed between two polarizers. If the birefringent liquid-crystal layer is located on a reflecting layer, then the pattern, or the corresponding information, can be made visible using only a single polarizer which is held over the element. LCP authentication elements make it possible to store information, virtually without restriction, in the form of text, images, photographs and combinations thereof. In comparison with prior art authentication elements, the LCP elements are distinguished in that the authenticity of the security feature can be verified even by a layman since it is not first necessary to learn how to recognise complicated colour changes or kinematic effects. Since LCP authentication elements are very simple, reliable and quick to read, machine-readable as well as visual information can be combined in the same authentication element.
As is likewise already known, the complexity of LCP authentication elements can be increased further by inclining the optical axis of the LCP layer relative to the plane of the layer, uniformly or with local variation. This can be done in known fashion by producing a PPN layer with a locally varying tilt angle on the surface. This further provides a tilt effect, that is to say the information contained in the birefringent layer is seen with positive or negative contrast depending on the angle of observation. The object of the invention is now to provide further possible layer structures of the above-mentioned type for optical components, electro-optical devices and, in particular, for copy protection elements.
According to the invention, this is achieved in that the physical parameters and the configuration of the cross-linked liquid-crystal layer are varied and/or different layers, as well as a variety of substrates, with differing respective optical properties are combined. Since the layers which are used are generally transparent, they can also be applied successfully to already known, permanently visible authentication elements, for example watermarks, holograms or cinegrams. The retarder pattern of the liquid-crystal layer can then be seen in addition to the permanently visible authentication element on observation using a linear polarizer.
When using the transmissive birefringent layers described in EP-A 689,084, it is necessary to arrange one polarizer on each side of the element in order to read or make visible the information which is stored. A quick check of identity cards and the like is in this case made difficult by the involved positioning of the two polarizers above and below the authentication element. This disadvantage can be removed according to the invention by additionally integrating at least one polarization layer in the layer structure. If there is, for example, a polarization layer below the birefringent layer, then one external polarization sheet, held over the element, is sufficient for making the stored optical information visible.
A polarization layer integrated in the authentication element can, according to EP-A 689,084, be designed as a dichroic LCP layer. It is also possible to use a polarization sheet as a substrate for the PPN and LCP layers applied to it.
Where a reflector is present, which can be omitted according to this invention, the polariser sheet may possibly be the polariser for ingoing light and the analyser for outgoing light, which may not always be desirable.
A further disadvantage of the authentication elements described in EP-A 689,084 is that, when arranging a polarizer below the substrate, the polarization state of the light on passing through the substrate can be affected. If, for example, use is made of inexpensive polymer substrates which, by virtue of the way in which they are produced,
Schadt Martin
Seiberle Hubert
Chowdhury Tarifur R.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Rolic AG
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