Printed matter – Having revealable concealed information – fraud preventer or... – Utilizing electromagnetic radiation
Patent
1992-04-08
1994-05-10
Bell, Paul A.
Printed matter
Having revealable concealed information, fraud preventer or...
Utilizing electromagnetic radiation
283109, 283111, 359 2, B42D 1500
Patent
active
053102223
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an optical device of the kind comprising a number of layers including an optically diffracting layer and an at least partially reflective layer which together generate a first image. Such devices are hereinafter referred to as of the kind described.
Optically diffracting devices cause incoming visible, infra red or ultra violet light to be diffracted. The devices most commonly encountered comprise an optically diffracting surface.
Such surfaces generally have a very fine microstructure which is subsequently metallised to add reflectivity so that the devices may be viewed by reflection. In addition to such surface relief devices it is also possible to obtain optical diffraction from volume hologram devices and the like.
Optically diffracting devices of the surface relief type may originate from the recording of laser bean interference patterns, or they may be created by other means known in the art.
Optically diffracting devices which are suitable for use in white light such as white light viewable holograms have predominated in the security marking industry. Such optically diffracting devices may more loosely be described as optically interfering devices in the sense that illuminating light is diffracted as distinct from merely absorbed or reflected.
Optical devices of the kind described find frequent use for decorative and brand identification purposes and as security devices on substrates such as transaction cards, credit cards and the like. Examples of typical optical devices include holograms and graphical diffractive patterns.
Typically, devices of the kind described possess a surface relief replica of a master profile. To make these, the fine line recording of an optically interfering pattern, usually stored on a nickel plate, is replicated on a plastic surface. The replicating processes generally used are moulding, embossing, casting or curing. Embossing of a thermoplastic polyer under conditions of elevated temperature and pressure is commonly employed as described for example in W089/03760. Further examples are described in EP-A-0328086 and in "Countermeasures Against Hologram Counterfeiting" by S. McGrew, International Symposium and Product Presentation for Optical Information Storage and Display, Zurich, Switzerland, Oct. 14-16 1987.
WO 88/09252 discloses the making of such optical replicas by ultraviolet curing of a polymerisable monomer while in contact with an optically interfering surface. U.S. Pat. No. 4,758,296 describes a similar system.
Electron beam recording or curing means may alternatively be employed.
Once the optical surface is created it is then reflectively metallised, such as by the vapour deposition of a thin layer of aluminum or other metal. Alternatively a metallised thermoplastic layer may be embossed.
The result is that the embossed surface can be viewed through the thermoplastic layer and against the reflective metallised backing.
The metallised film may at this stage be used for decorative purposes such as wrapping film.
Commonly a further layer is coated onto the metal. This may be a protective lacquer which if the resultant device is to be used for security purposes may be of the same chemical type as the embossed layer.
For certain applications e.g. holographic label stock, adhesive is then applied. This may be a pressure sensitive adhesive, or heat activatable, hot melt or laminating adhesive or a laminating film.
For security purposes the strength of the adhesive will be chosen to be at least that of the device so that the device cannot be peeled from the substrate to which it is affixed without damage.
Commonly the layer into which the optical surface is cast is supported on a carrier. The carrier may be a temporary carrier, such as when the thermoplastic layer is applied to a wax release coating on a polyester film. The use of temporary carriers is common if the optical device is to be stamped onto, say, a credit card.
Alternatively the presence of a discrete release layer, say, of wax, may be avoided by using a polymeric layer which exhibi
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S. McGrew, "Countermeasures Against Hologram Counterfeiting", Optical Security Systems, International Symposium and Product Presentation for Optical Information Storage and Display, Zurich, Switzerland, Oct. 14-16, 1987.
Chatwin Charles E.
Ezra David
Gomme Martin C.
Kay Ralph
Bell Paul A.
De La Rue Holographics Limited
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