Optical security device

Details Optical security device Optical security device

2002-07-26

2004-10-19

Le, Thien M. (Department: 2876)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C359S015000, C359S002000, C349S127000, C349S129000

Reexamination Certificate

active

06806930

ABSTRACT:

This invention relates to an optical security device for applying to or incorporating in commercial items. Such items are found principally in the fields of document security (banknote, stamp, card and tickets applications), brand protection (pharmaceuticals, flavours, liquors), secure packaging of articles, software, spare parts for vehicles, etc., or packaging therefor; however the invention is not limited to such fields.
Optical security devices are commonly used for authenticating articles to which they are applied. A first type of such device comprises holograms, kinograms, watermarks, micro-perforations, optical variable inks, etc. Such devices can be inspected with the naked eye (first level inspection) and provide an authentication, having regard to the difficulty a counterfeiter would have in providing a similar device; such security devices will be referred to herein as “first level” security devices.
A second type of such security device provides a greater degree of security or authentication than the first type and is viewable with a cheap and easily available tool, for example, a polariser sheet, a magnifying glass, a black lamp (UV), etc (second level inspection). Examples of such security elements are micro-printing devices, fluorescent inks, and devices based on polarisation effects as disclosed in our earlier application WO 98/52077. Application WO 98/52077 discloses devices based on photo-oriented polymer network (PPN) layer, which is disposed on a substrate and is selectively oriented in different directions in different local regions over its surface. The PPN layer is covered by a layer of cross-linked liquid-crystal monomers; this layer, which is optically anisotropic and exhibits birefringence provides an optical retarder layer. The liquid crystal nature of the retarder layer follows the selective orientation of the PPN layer to enable the manufacturing of phase retarder images which may be seen with the aid of polariser sheets. Such security devices will be referred to herein as “second level” security devices.
A third type of security device comprises elements which can be visualised or detected only with special, expensive tools such as photo-spectrometers, polarising microscopes, etc. (third level inspection). In addition, electronic techniques are known for inserting, and subsequently identifying, watermarks in an image or totally encrypting an image. Further examples for such security devices are elements made with special fluorescent inks, digital scrambled indicia. All these devices have in common that they can only be received with special decrypting tools, and will be referred to herein as “third level” security devices.
The concept of the present invention is to provide a new optical security device which exhibits third level security levels and combines, if desired, in a single device all three first, second and third levels of security.
In a first aspect, the invention provides an optical authentication device in combination with an inspection tool, wherein the authentication device comprises at least a first layer of optically anisotropic material which is selectively oriented in different orientations over its area such as to encrypt an object therein; and wherein the inspection tool includes a further layer of optically anisotropic material which is selectively oriented in different orientations over its area for decrypting the object such that the object can be viewed when the authentication device is viewed through the inspection tool.
In a second aspect, the invention provides an optical authentication device in combination with an inspection tool, wherein the authentication device comprises at least a first layer of optically anisotropic material which is selectively oriented in different orientations over its area such as to encrypt an object therein; and wherein the inspection tool is such as to provide a light path or light beam having an optical parameter non-uniform over its width such as to co-operate with said optical anisotropic material to enable viewing of the encrypted object.
In a third aspect, the invention provides an optical authentication device comprising at least a first layer of optically anisotropic material, which is structured to provide first, second and third optical inspection levels, namely a first inspection level where an optical effect can be discerned with the naked eye, a second level in which a disguised object can be discerned with the aid of an optical inspection tool, and a third level in which an encrypted object can be discerned with a decrypting optical inspection tool.
For the purposes of this specification, “object” is used in the sense of an object which may be viewed optically and forms an image in the eye. Such object will usually be a simple recognisable object such as a pattern or design, or alphanumeric characters, digits etc. representing a code. Thus the object essentially carries information which can be discerned visually. In accordance with the invention, the encrypted object provides a first predetermined item of information, and the disguised object provides a second predetermined item of information, different from the first.
As preferred, said first layer of optically anisotropic material comprises a structured arrangement of phase retarding elements, preferably comprising adjacent elemental areas which are selectively phase retarded relative to one another. The resulting viewable objects created by such a phase retarder depends on the polarisation directions and spectral wavelength distribution of the in and out-coming light. A disguised object can be viewed with sheet polariser(s). When viewed with a decrypting optical tool, an encrypted object can be viewed.
As preferred, the first layer comprises a Liquid Crystal Polymer (LCP) layer, which preferably can be aligned and structured/patterned by a further layer of linear photopolymer (LPP)—synonymous with photooriented polymer network (PPN).
The first layer may be of a structurally-self-supporting material. Alternatively a substrate may be provided mounting the first layer.
The first inspection level which may be optional, is preferably provided by a layer, which gives the reflected light a particular property caused by an optical effect such as diffraction, refraction, wavelength selective reflection, etc. Where the layer comprises cholesteric material (which may be additional to the first layer) the in-coming light is selectively reflected with a wavelength dependent on viewing angle. Within a selective wavelength band the reflected light is circularly polarised, the transmitted light is also circularly polarised but in the opposite helical sense. Outside the selective wavelength band the polarisation state of the light remains unchanged. By tilting the cholesteric layer a colour shift can be perceived with the naked eye e.g. from red to green or from green to blue.
The second and third inspection levels is provided by LCP material forming a first layer. The LCP material form a structured phase retarder layer. Adjacent areas of the structured LCP retarder layer show from one area to the other at least two different orientations of their optical axes into which at least an encrypted and optionally at least a non-encrypted hidden image is stored. The non-encrypted hidden information/images or objects—if present—can be visualised with a normal sheet polariser (second inspection level). The encrypted images can be discerned with a decrypting optical tool as mentioned above (third inspection level).
In accordance with a preferred form of the invention an optical authentication device comprises a substrate, and at least an encrypted, preferably photo-oriented LCP layer which provides third level security. Such a third level device can be discerned with a decrypting optical polarising inspection tool which consists of decrypting LCP layer and therefore can be considered also as a security device. Such an optical authentication device may enable a combined usage for second and third level inspection: visualisation of the second level f

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