United States Patent m
Chatwin et al.
US005310222A
[ii] Patent Number: [45] Date of Patent:
[54] OPTICAL DEVICE
[75] Inventors: Charles E. Chatwin, Horsham; Ralph Kay, Maidenhead; Martin C. Gomme, Hemel Hempstead; David Ezra, Wallingford, all of Great Britain
[73] Assignee: De La Rue Holographies Limited, London, England
[21] Appl. No.: 856,141
[22] PCT Filed: Oct. 19,1990
[86] PCT No.: PCT/GB90/01613
§ 371 Date: Apr. 8,1992
§ 102(e) Date: Apr. 8,1992 [87] PCT Pub. No.: WO91/06925
PCT Pub. Date: May 16, 1991
[30] Foreign Application Priority Data
Oct. 26, 1989 [GB] United Kingdom 8924111
[51] Int. CI.' B42D 15/00
[52] U.S. CI 283/86; 283/109;
283/111; 359/2
[58] Field of Search 283/86, 117, 109, 111;
359/2
[56] References Cited
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OTHER PUBLICATIONS
S. McGrew, "Countermeasures Against Hologram
Counterfeiting", Optical Security Systems, Interna-
tional Symposium and Product Presentation for Optical
Information Storage and Display, Zurich, Switzerland,
Oct. 14-16, 1987.
Primary Examiner—Paul A. Bell
Attorney, Agent, or Firm—Oliff & Berridge
OPTICAL DEVICE
The invention relates to an optical device of the kind comprising a number of layers including an optically 5 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 de- 10 vices 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 15 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 interfer- 20 ence 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 in- 25 dustry. 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 30 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. 35
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 40 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 "Counter- 45 measures 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 50 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. 55
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 60 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. 65 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.
2
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 exhibits low adhesion to the carrier and is peelable therefrom. Such constructions are found in U.S. 3235395 and U.S. 3589962.
Although the use of these devices leads to a relatively secure product which is difficult to counterfeit, there is a need for even more secure devices to be made.
In accordance with one aspect of the present invention, in an optical device of the kind described, a nonoptically diffracting, second image is provided within the device in association with the first image.
In this new arrangement, we have devised a device in which a non-optically diffracting image is contained within the structure of the device rather than simply being placed on the surface of the device where it may readily be removed.
The term "diffracting" is used to indicate that the path of incident light is changed by the active layer. This includes for example simple diffraction as well as more complex interference which may result in a viewable reconstructed image.
In accordance with a second aspect of the present invention, a method of constructing an optical device of the kind described comprises bonding the layers together, and is characterized by providing a non-optically diffracting, second image within the device in association with the first image.
Typically the association between the images will involve providing the second image over the first or in register with the first so that there is at least partial overlap.
Usually the second image will be provided prior to bonding the layers together although an image could be developed or placed after bonding, eg by laser imaging. Commonly the second image will be applied to the surface of a surface relief optical structure prior to the metallisation.
The second images which may be provided include security information such as security printed indicia which take the form of lines, line segments, dots, letters, numbers, characters, logos, guilloches and other design elements. Bar codes and registration marks may also be included. These indicia may be printed by lithography, intaglio, gravure, flexography, screen or letterpress or a combination, with a single colour or colour combinations including blended ink "rainbow" printing— printed design or decorative patterns.
A visual image is typically provided, viewable or photometrically detectable from the same side as the optical effect, although the use of invisible materials which are detectable by use of ultraviolet and infrared incident radiation is also envisaged.
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