GB2219248A

GB2219248A - Optical security device

Info

Publication number: GB2219248A
Application number: GB8907355A
Authority: GB
Inventors: David Julian Pizzanelli
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-03-31
Filing date: 1989-03-31
Publication date: 1989-12-06
Anticipated expiration: 2009-03-31
Also published as: GB8807820D0; GB8907355D0; GB2219248B

Abstract

An optical security device for preventing unlawful copying or counterfeiting has a readable light diffracting record formed by embossing a relief hologram of diffraction grating into an embossable material in which the embossable material is primed with an at least one tinction of a predetermined chromacity such that coherent monochromatic laser light emissions are blocked or substantially inhibited, or are absorbed and re-emitted as coherent semi-monochromatic light of a different, longer, wavelength.

Description

OPTICAL SECURITY DEVICE This invention relates to an optical security device.

It is known to produce light diffracting patterns, such as holograms or diffraction gratings, and in particular optical patterns formed as surface relief structures which may be embossed into plastic and other embossable materials, and which are subsequently adhered or bonded to the surface of commercial goods or documents to form a means by which the commercial goods or documents may be identified as genuine.

The fabrication of fine line surface relief structures which diffract incident white light, such as diffraction gratings is well known in the prior art; and the method of forming surface relief holograms and diffraction gratings in photoresist material, and the art of growing nickel stamping or embossing dies from the relief surfaces of such holograms and diffraction gratings formed in photoresist materials, and the use of nickel stamping or embossing dies to emboss the holograms and diffraction gratings into embossable materials such as thermoplastic sheets, has been described by a number of authors, such as Bartolini et al in Applied Optics Vol. 9, No. 10, October 1970 pp 2283-2290.

The composition of fine line surface relief structures such as diffraction gratings and holograms into graphic designs for decorative and security usages has been described, for example by Dr. M. C. Hutley in the Newsletter of the National Physical Laboratory 1979 pp 6-7, and by Steven Paul McGrew in P.C.T. Application US/81/01439.

In their simplest form, the security devices described in the prior art are composed of a transparent sheet or film of thermoplastic material which is embossed with a distinctive light diffracting pattern comprising a hologram or diffraction grating, and coated with a metal layer to render the light diffracting pattern reflective, and then coated with a strong adhesive layer so that the embossed material may be firmly bonded to a suitable surface such as paper, card or plastic, thus forming a device which may be used to authenticate commercial goods or documents of value.

One embodiment of such a security device is disclosed in G.B.

Patent 2,129,739B, wherein the light diffracting pattern is embossed into the lacquer of a hot-stamping foil. The lacquer is metallised and coated with heat-activated adhesive and subsequently bonded to the surface of products and documents of value, in the same way as conventional hotstamping foil.

Holograms manufactured by the way disclosed in G.B. Patent 2,129,739B have proved to be a very effective means of authenticating products and articles of value.

It has been reported, however, that the optical security devices manufactured by the methods described in the prior art may be counterfeited by relatively simple methods by holographers skilled in the art (see for example 'Bank and Management' June 1985 pp 60-63 and 'New Scientist' 25 Oct 1984 p 24 'Fake hologram, that will do nicely').

The method which is of primary concern is the 'contact copy' method, first described by Brumm in Applied Optics Vol. 5, No. 12, Dec 1966 pp 1946-1947 and US-A-3,758,186. By this method, a counterfeit copy of an optical security device or security hologram may be made simply by placing a suitable photosensitive material, such as a fine grain silver halide emulsion or high resolution photoresist, in close contact with the hologram and exposing the photosensitive material to coherent monochromatic light such as laser radiation, of a suitable wavelength.

Of particular concern is the method of producing a counterfeit hologram in photoresist or similar materials which may be used to generate a nickel stamping or embossing die. Such a counterfeit stamping or embossing die might be used to manufacture large quantities of counterfeit holograms or optical security devices.

The present invention seeks to provide an optical security device comprising a surface relief hologram or diffraction grating which is protected from optical copying methods such as those described above.

According to one aspect of this invention there is provided an optical security device comprising a readable light diffracting record having an embossable material which is embossed with a hologram or diffraction pattern, the embossable material being primed with at least one tinction of a predetermined chromacity so as to block or substantially inhibit the transmission of coherent monochromatic laser emissions through the embossable material, such that optical reconstruction of the light diffracting record within the hologram or diffraction grating is prevented or substantially inhibited.

According to a further aspect of this invention there is provided an optical security device comprising a readable light diffracting record having an embossable material embossed with a relief hologram or diffraction grating, said embossable material being primed with a tinction of a predetermined chromacity such that coherent monochromatic laser light emissions are absorbed by the tinction in the embossable material and re-emitted as partially coherent semi-monochromatic light of a different wavelength.

Preferably the tinction is provided in a predetermined pattern and the readable light diffracting record is embossed in registration with the predetermined pattern of the tinction.

Advantageously the light diffracting record is an achromatic hologram.

Preferably the light diffracting record is a laser transmission hologram.

Thus, in this invention, by priming the otherwise transparent embossable material into which the light diffracting surface relief structure is to be embossed with certain optical filters or modifiers, efficient reconstuction of the hologram or diffraction grating using laser light of given wavelengths is prevented.

This invention takes advantage of the fact that certain dyes have the property of absorbing coherent monochromatic light emissions of a particular wavelength and, having reached a threshold of absorption, reemit the energy as partially coherent, semi-monoc.omatic light of different, longer wavelengths.

It is this property of these rare dyes which forms the basis of dye laser systems, where continuous wave emissions from, for example an EXCIMER or Ion laser, are focussed onto a stream of dye causing an extremely high level of flourescence. Such emission is broadband, typically exhibiting a 40GHz linewidth.

The invention also takes advantage of the fact that certain dyes and pigments are particularly effective at absorbing monochromatic radiation of certain specific waelengths, and that there are a limited number of wavelengths commonly available to the potential counterfeiter and that certain wavelengths in the visible spectrum are difficult to obtain, whilst dyes operating in the same regions are very effective at blocking or absorbing those wavelengths which are commonly available.

The invention will now be described by way of example with reference to the accompanying drawing which shows a schematic crosssection of an optical security device in accordance with this invention.

In the drawing the optical security device is formed by a hot transfer foil composed of the following layers: a carrier layer of polyester 1 such as P.V.C. typically about 12-25pm thick, a separation layer 2 such as a wax layer of about 0.5 to 1pm thick a scratchresistant transparent lacquer layer 3 of about lpm thickness, an embossable transparent thermoplastic lacquer layer 4 of about 0.5pom thick which incorporates selectively printed further layers of transparent embossable lacquer of about 0.5 to lwn tinctured with dyes 5 of a predetermined chromacity such that they absorb coherent monochromatic laser light emissions at certain wavelengths and re-emit them as partially coherent semi-monochromatic light of different, longer wavelengths, or such that they substantially inhibit the transmission of coherent monochromatic laser light emissions through the embossable lacquer layer; an embossed surface relief 6 forming a distinctive light diffracting pattern such as a hologram or diffraction grating, a metallic layer 7 of not more than 1000A to make the diffracting pattern retroreflective; adhesive layer 8; and substrate 9 of, for example, plastics, paper or card.

If now a ray of coherent monochromatic laser light 14 is applied to the optical security device, the rays are absorbed by the dye tinction 5 in the embossable transparent lacquer layer 4 and upon reaching a threshold of absorption the tinction re-emits rays 15 of partially coherent and semi-monochromatic light of a longer wavelength.

By way of example the following dyes are suitable for use in this invention.

To protect against the 441.6 nanometer line of a He-Cd laser being used to make a counterfeit hologram in photoresist or similar bluesensitive materials, a suitable dye would be Coumarin 343 which has peak absorption at 446 nanometers and re-emits semi-coherent semimonochromatic light at approximately 519 nanometers, a wavelength to which fine resolution photoresist materials are not sensitive.

To protect against the 457.9 nanometer line of an Argon-Ion laser being used to make a counterfeit hologram in photoresist or similar materials, a suitable dye would be Coumarin 334 which has a peak absorption at 452 nanometers and re-emits light at approximately 521 nanometers, or Coumarin 6 which peaks at 458 and emits at 540 nanometers, wavelengths to which fine resolution photoresist maerials are not sensitive.

To protect against the 514.5 nanometer line of an Argon-Ion laser being used to make a counterfeit hologram in Green-sensitive fine grain silver halide emulsions or similar materials a suitable dye would be 2',7'Dichlorofluoroscein which has a peak absorption at 512.8 nanometers and re-emits light at approximately 571 nanometers, or Rhodamine 110 which has a peak absorption at 510.6 and re-emits light at approximately 570 nanometers, to which wavelengths fine grain silver halide emulsions are largely insensitive.

To protect against the 632.8 nanometer line of Helium-Neon laser being used to make a counterfeit hologram in Red-sensitive fine grain silver halide emulsions or similar materials, a suitable dye would be Nile Blue A Perchlorate, which has a peak absorption at 628 nanometers and re-emits light at approximately 690 nanometers, a wavelength to which Red-sensitive fine grain silver halide emulsions are largely insensitive.

As an alternative to protect against copying by such a laser, Naphthol Green B could be used.

To protect against the 694.3 nanometer line of a Ruby pulse laser being used to make a counterfeit hologram in Deep Red-sensitive fine grain silver halide emulsions a suitable dye would be 3,3' Diethyloxatricarbocyanine Iodide (DOTC) which has a peak absorption at 687 nanometers and emits light at 780 nanometers, to which emulsions sensitised for the 694.3 nanometer line of a Ruby Pulse laser are largely insensitive.

As an alternative, the embossable material may be primed with a tinction of a pigment or dye which is highly absorbent of a specific laser wavelength or wavelengths which might be used to counterfeit the security hologram or optical security device. Such pigments or dyes would be chosen to compliment the light modifying pigments or dyes in order to provide maximum protection from the threat of counterfeiting.

Unlike the light modifying pigments or dyes described above, the absorbent dyes or pigments should not re-emit light in the visible spectrum, but rather they should dissipate the light energy absorbed as heat.

Whilst in many instances the optical security device may be protected by a single pigment or dye, either of the light absorbing or light modifying variety, a preferred embodiment of the invention consists in priming or coating the embossable material with a plurality of tinctions of pigments or dyes in selected areas. This method has the advantage of allowing the light diffracting pattern to be more readily viewable in white light, because portions of the light diffracting pattern which have been primed to prevent image reconstruction in one region of the visible spectrum will reconstruct a bright image over the rest of the visible spectrum.

A preferred method of priming the transparent embossable thermoplastic material in selected areas is by roller printing with a fine layer of transparent material of the same chemical and physical characteristics as the embossable material which has undergone tinction with selected dyes by means of dilution with an appropriate solvent.

Dyes such as Rhodamine 110 may be dissolved in Methanol. For example the transparent thermoplastic material may be primed by printing with a thermoplastic ink formed by mixing a transparent thermoplastic ink with the tlnctions or dyes. Printed coatings need not be thicker than lum as the depth of profile of the embossed hologram or diffraction grating which forms the light diffracting pattern in the optical security device is typically less than lum in depth.

If a hologram is employed as the light diffracting pattern then conveniently the hologram is of the Benton or Rainbow type, both as known per se. Alternatively, if the light diffracting pattern is a diffraction grating then preferably the grating is composed of Rainbow diffraction gratings known per se.

Claims

CLAIMS:

1. An optical security device comprising a readable light diffracting record having an embossable material which is embossed with a hologram or diffraction pattern, the embossable material being primed with at least one tinction of a predetermined chromacity so as to block or substantially inhibit the transmission of coherent monochromatic laser emissions through the embossable material, such that optical reconstruction of the light diffracting record within the hologram or diffraction grating is prevented or substantially inhibited.

2. An optical security device comprising a readable light diffracting record having an embossable material embossed with a relief hologram or diffraction grating, said embossable material being primed with a tinction of a predetermined chromacity such that coherent monochromatic laser light emissions are absorbed by the tinction in the embossable material and re-emitted as partially coherent semi-monochromatic light of a different wavelength.

3. An optical security device as claimed in claim 2 wherein the embossable material is primed with a plurality of tinctions each of a predetermined chromacity.

4. An optical security device as claimed in claim 2 or 3 wherein the re-emitted light has a longer wavelength than the light emissions that are absorbed by the tinction primed embossable material.

5. An optical security device as claimed in any preceding claim wherein the embossable material is a transparent thermoplastic material which is primed by printing with a thermoplastic ink formed by mixing a transparent thermoplastic ink with the tinctions.

6. An optical security device as claimed in any preceding claim wherein the tinction is provided in a predetermined pattern and the readable light diffracting record is embossed in registration with the predetermined pattern of the tinction.

7. An optical security device as claimed in any preceding claim wherein the embossed relief hologram or diffraction grating is covered a with a metallic layer less than 1000A thick.

8. An optical security device as claimed in claim 7 wherein a heat sensitive adhesive layer is applied to the side of the metallic layer remote from the embossed diffraction grating or hologram.

9. An optical security device as claimed in any preceding claim wherein the readable light diffraction record is mounted on a substrate of plastics, or paper, or card.

10. An optical security device as claimed in any preceding claim wherein the embossable material is a transparent lacquer layer of hot transfer foil which is primed by printing with a further transparent lacquer mixed with the tinction.

11. An optical security device as claimed in any preceding claim wherein the light diffracting record is an achromatic hologram.

12. An optical security device as claimed in any preceding claim wherein the light diffracting record is a laser transmission hologram.

13. An optical security device as claimed in any preceding claim wherein the light diffracting record is a Benton or Rainbow hologram each as known per se.

14. An optical security device as claimed in any of claims 1 to 12 wherein the light diffracting record is composed of Rainbow diffraction gratings known per se.

15. An optical security device substantially as herein described with reference to and as illustrated in the accompanying drawing.

16. A method of forming an optical security device substantially as herein described with reference to each of the examples described and as shown in the accompanying drawing.