WO1990006539A1

WO1990006539A1 - Security marking

Info

Publication number: WO1990006539A1
Application number: PCT/GB1989/001443
Authority: WO
Inventors: Stephen Charles Wallace
Original assignee: TRAQSON Ltd
Current assignee: TRAQSON Ltd
Priority date: 1988-12-01
Filing date: 1989-12-01
Publication date: 1990-06-14
Anticipated expiration: 1991-06-01
Also published as: AU4744790A; CA2004423A1; IL92524A0; ZA899183B; GB8828091D0

Abstract

A security mark has a photochromic compound covered by a controlled absorption layer comprising a saturable absorber. Light of a wavelength serving to convert the photochromic compound is at low intensity absorbed by the saturable absorber. A photochromic compound converted by UV can thus be sunlight protected. At high intensities, the absorption of the saturable absorber drops to permit conversion of the photochromic compound.

Description

SECURITY MARKING

This invention relates to security marking and in particular to the use of marking substances which include a photochromic compound. These substances can be arranged to take a colourless form when applied to an article requiring authentication, for example a personal identification document. The photochromic compound can be converted to a coloured form by irradiation with UV, the optically detectable colour change in the marking substance then serving to authenticate the document.

Security marking substances as outlined above are known and reference is directed by way of example to WO 88/06306; JP 62227794 and WO 88/01288.

It is known that exposure to UV over a period of time can destroy the photochromic effect in certain compounds. The life of a particular marking substance will depend upon the flux of UV to which it is subjected whether during the authentication procedure or extraneously. The UV applied during the authentication step is liable to be a critical factor in determining the life of a marking substance only in the case of items requiring very frequent authentication. Extraneous UV from sunlight will be an important factor in many cases where no control can be exercised over the manner in which the marked item is stored.

It is one object of this invention to provide a method of marking offering increased lifetimes.

In applications where a relatively high degree of security is required, it would be advantageous to provide a more demanding authentication step, preferably without significantly increasing the complexity of the authentication apparatus. It is thus a further object of the present invention to provide an improved method of marking offering closer control over the essential requirements of the authentication step. Accordingly, the present invention consists in one aspect in a method of marking an item with a security mark which exhibits a reversible and optically detectable change on exposure to light of a specific wavelength, comprising the steps of applying to the article a marking substance comprising a photochromic compound convertible from one photochrome to another on exposure to light of said wavelength, said compound being covered by a controlled absorption layer containing a saturable absorber at said wavelength, whereby there is defined a threshold intensity of light at said wavelength beneath which threshold there is by reason of absorption in said controlled absorption layer no detectable change in the mark within a given time period, a detectable change occuring substantially immediately at intensities above said threshold.

Preferably, the saturable absorber comprises a laser dye having at said wavelength, an absorbtion cross section in the excited state which small compared with the absorbtion cross section in the ground state.

In another respect, the present invention consists in a security mark comprising a body of marking substance comprising a photochromic compound and a sunlight protection layer covering said marking substance, the sunlight protection layer comprising a saturable absorber.

It should be recognised that the term "light" as used herein is not to be regarded as restricted to visible light.

A variety of saturable absorber compounds-are known. They have the property that below a threshold light intensity, an equilibrium is established between molecular processes of photon absorption and relaxation, such that the transmissivity at the specific wavelength of operation is very low. Above the threshold intensity, the photon absorption process is saturated, with all absorber molecules being held in the higher energy state. Above this threshold intensity, therefore, the substance has significant transmissivity at the specific wavelength. Saturable absorbers are known for use as high speed switches in lasers and reference is directed in this regard to "Lasers - Light amplifiers and oscillators" - Dieter Ross - 199 Academic Press.

In the practice of the present invention, it can be arranged, by appropriate selection of a saturable absorber and the concentration and thickness of the controlled absorption layer, that the transmissivity of the layer to UV at normal sunlight intensities is very low but that at a UV intensity achievable - with careful design - in authentication apparatus, the transmissivity is high.

The invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a diagramatic section through a security mark in accordance with this invention, and

Figure 2 is a diagramatic section through an alternative security mark according to this invention.

Referring to Figure 1, a marking substance 10 is applied to a substrate 12 by printing or any other suitable application technique. The marking substance includes a photochromic compound which is preferably a fulgide but which could take other forms such as a spiropyran. The detailed structure of the photochromic compound forms no part of the present invention and it will suffice to make reference to the known art, examples of which are GB-A-1464 603, GB-A-2002752 and GB-A-2051813.

The photochromic compound is incorporated in the marking substance by any of a variety of appropriate techniques. For example, the photochromic compound may be dissolved or dispersed in a solution of polymeric material which is cast, coated or otherwise applied to the substrate. For example, a photochromic fulgide can be dissolved in a solution of cellulose acetate in acetone with a film of the marking _ n _

substance being formed upon evaporation of acetone. Alternative polymers are polyesters e.g. polyethylene terephthalate; acrylates, e.g. PMMA; olefin polymers or co-polymers; vinyl polymers, e.g. PVA or PVC; polycarbonates and polyamides. Alternative organic solvents are toluene and chlorinated hydrocarbons.

In the illustrated example, the marking substance 10 comprises a 2.3% concentration of the photochromic fulgide Aberchrome 5^0 (available from Aberchromics Limited, Cardiff) in PMMA cast from a solution in ethyl acetate. The formed security mark has an area of 1 sq cm and the thickness of the marking substance is 5 microns.

There is provided over the marking substance 10 a controlled absorption layer 14. This comprises a 10 concentration of a saturable absorber in an appropriate matrix such as polyvinyl alcohol. In this example the saturable absorber molecule is triazinyl stilbene. For detailed information concerning the proproperties of this known saturable absorber, reference is directed to textbooks such as "Laser Dyes - Properties of organic compounds for dye lasers" - Mitsuo Maeda 1987 Academic Press, especially Table 20. The thickness of the controlled absorption layer is 5 microns and it is calculated that at normal sunlight intensities of UV, the controlled absorption layer will decrease the flux of UV incident upon the marking substance by

2 factor of 10 .

In the authentication step, the security mark is irradiated with high intensity UV so as to convert the photochromic compound to a coloured form. This changing colour may be observed visually. The intensity of UV employed in the activation step is in excess of the threshold at which the saturable absorber is saturated. Thus at an intensity of 15 kw suitably focussed, the controlled absorption layer has a transmissivity of approximately 80%. The high intensity UV may be provided, for example, by a flashlamp with appropriate focusing, or other high intensity pulsed light sources. The use of a saturable absorber in the manner according to this invention has the advantage that the lifetime of the security mark on exposure to sunlight is substantially increased. However, the controlled absorption layer has little effect upon visible - for example, green or red - light so that the photochromic compound can be returned rapidly to its colourless form by irradiation at the appropriate wavelength. This can be done by exposure to sunlight or another white light source. Since the transmissivity of the controlled absorption layer is relatively high above the threshold intensity, the time taken to colour the security mark in the activation step is not substantially increased.

In a modification, illustrated in Figure 2, a base coat 16 is interposed between the marking substance 10 and the substrate 12. This base coat may typically comprise a one to two micron layer of polyvinyl chloride which serves to enhance optical contrast and to protect the photochromic compound from possibly damaging effects of the substrate. The use of such a base coat would, for example, be important in the case of paper or other fibre based substrates having relatively high moisture levels and containing fillers and other reactants having a possibly deleterious effect upon the photochromic compound.

It should be understood that this invention has been described by way of example only and a variety of modifications are possible without departing from the scope of the invention.

Other saturable absorbers can be employed and these can be provided in a controlled absorption layer in a variety of techniques. One example of an alternative saturable absorber molecule useful with fulgide photochromies is 1,4-Bis (o-methylstyryl) benzene. This can be formed as a screen ink using a hydrocarbon solvent. The wavelength band of 3 O to 390"™ is appropriate for a wide range of photochromic fulgides. Beyond the specific saturable absorbers mentioned, it will be possible to employ a range of laser dyes • absorbing in the wavelength range of interest and having an absorption cross section in the excited state which is small compared to that in the ground state^*and thus being capable of saturation. Reference is directed in this regard to Maeda ibid and to "Handbook of fluorescent spectra of aromatic molecules" Isador Berl an - 171 Academic Press.

It is not necessary for the entire security mark to be covered by a single, contiguous controlled absorption layer; separate blocks of the marking substance may be covered by respective controlled absorption layers. The marking substance need not be a polymeric film; it may take the form of, for example, a paint or ink containing a photochromic compound or may in appropriate cases consist of a photochromic compound alone.

Claims

1. A method of marking an item with a security mark which exhibits a reversible and optically detectable change on exposure to light of a specific wavelength, comprising the steps of applying to the article a marking substance comprising a photochromic compound convertible from one photochrome to another on exposure to light of said wavelength, said compound being covered by a controlled absorption layer containing a saturable absorber at said wavelength, whereby there is defined a threshold intensity of light at said wavelength beneath which threshold there is by reason of absorption in said controlled absorption layer no detectable change in the mark within a given time period, a detectable change occuring substantially immediately at intensities above said threshold.

2. A method according to Claim 1, wherein the saturable absorber comprises a laser dye having at said wavelength an absorbtion cross section in the excited state which is small compared with the absorbtion cross section in the ground state.

3. A method according to Claim 1, wherein the saturable absorber comprises triazinyl stilbene.

4. A method according to Claim 1, wherein the saturable absorber comprises 1,4-Bis(o-methylstyryl)benzene.

5. A method according to any one of the preceding claims, wherein the photochromic compound comprises a fulgide.

6. A security mark formed by the method of any one of the preceding claims.

7. A security mark comprising a body of marking substance comprising a photochromic compound and a sunlight protection layer covering said marking substance, the sunlight protection layer comprising a UV saturable absorber.

8. A security mark according to Claim 7_. wherein the saturable absorber comprises a laser dye having a UV absorbtion cross section in the excited state which is small compared with the absorbtion cross section in the ground state.

9. A security mark according to Claim 7, wherein the saturable absorber comprises triazinyl stilbene.

10. A security mark according to Claim 7, wherein the saturable absorber comprises 1, -Bis(o-methylstyryl)benzene.