WO2004062938A2

WO2004062938A2 - Improvements in or relating to security or authentication markings and the like

Info

Publication number: WO2004062938A2
Application number: PCT/GB2004/000069
Authority: WO
Inventors: Rifat Iqbal; Dosten Baluch; Jonathan Michael Blackledge
Original assignee: Durand Technology Limited
Priority date: 2003-01-10
Filing date: 2004-01-09
Publication date: 2004-07-29
Also published as: GB0300555D0; WO2004062938A3

Abstract

A security or authentication marking on a document or product takes the form of refractive index variations or other variations in light managing properties, in a layer or sheet of light-transmitting material provided on or in at least part of the document or product. The light-transmitting material may be in the form of a sheet of said light-transmitting material extended over a light-transmitting aperture or window in the document or product, or may be extended in a layer across a light-transmitting part of the product, or over a light-reflecting part of said document or product.

Description

Title: "Improvements in or relating to security or authentication markings and the like"

THIS INVENTION relates to security, authentication or copy protection marking of documents, such as currency notes or cheques or other valuable documents, or of high value products such as performance-critical parts, pharmaceuticals or luxury products such as containers of perfume etc. That is to say, the invention relates to the provision on such documents or products of a means by which persons receiving the same may be assured of their authenticity and, conversely, of a means which will make it more difficult for counterfeits of such documents or products to be produced which will pass as authentic products or documents.

In various embodiments of the present invention, this is achieved through the use of sheets, films or layers of light-transmitting materials which can be applied to or incorporated in such a document or product and which utilise light refracting or light reflecting properties to provide a security or authentication marking or to provide a critical property of such security or authentication marking. Such sheets or layers are also referred to herein as "light management films".

According to one aspect of the present invention there is provided a document or product incorporating a security or authentication marking in the form of refractive index variations or other variations in light managing properties, in a layer or sheet of light-transmitting material provided on or in at least part of said document or product. According to another aspect of the invention there is provided a method of providing a security or authentication marking on a document or product comprising applying to such document or product a light-transmitting sheet or layer incorporating refractive index variations or other variations in light managing properties.

Embodiments of the invention are described below by way of example with reference to the accompanying drawings in which;

Figure 1 illustrates a currency note provided with a security or authentication marking in accordance with the invention;

Figure 2 is a sectional view, to an enlarged scale, illustrating another embodiment ofthe invention;

Figures 3a, 3b and 3c illustrate successive stages in the production of a marking in accordance with another embodiment ofthe invention; and

Figure 4 is a view similar to Figure 3 a illustrating a further embodiment.

In embodiments ofthe invention described below, use is made of silicone-based photopolymerisable systems of the kind disclosed in our co-pending International patent application PCT/GBO 1/04978, the specification and drawings of which are attached hereto marked "Annex".

These photopolymerisable systems enable, inter alia, the production of what are herein referred to, for convenience, as "light management films" which can be applied by a coating or moulding technique to currency notes or documents and can be engineered to provide effects, as described below, useful in relation to security or authentication markings. Such mixtures or systems incorporate organic prepolymers which, when exposed to appropriate forms of electromagnetic radiation, undergo polymerisation in the areas exposed, whereby, after any necessary processing steps, a light-transmitting sheet or layer is obtained, which may be characterised by refractive index variations and/or by variations in layer thickness (i.e. by surface relief features, or by combinations of such features).

The present invention is concerned with the application of such materials in, for example, marking or otherwise distinguishing currency notes or certificates, credit cards, CDs, CD ROMS, DVDs, bank cards, ID cards and the like in such a way as to provide effects which will allow holders or users to assure themselves that the document or product is genuine and not a counterfeit and which, accordingly, will make it much more difficult for counterfeiters to produce convincing counterfeit documents or products.

In some embodiments of the invention, there is applied to a document or product a UV-curable or thermally curable photopolymer blend containing a prismatic structure or a lens structure adapted to modify the passage of light therethrough in such a way that, for example, a legend, picture, device or watermark is visible only at a certain angle of view. This is particularly useful for the formation of security-encoding films. The marking itself, (highly visible) may also be encrypted or encoded, for example comprising a serial number or other marking which can provide an assurance of authenticity to an individual (or apparatus) provided with an appropriate "key" but such that counterfeiters are unable to devise corresponding markings which will pass the "authenticity test".

In accordance with one embodiment ofthe invention, a light management film, formed from one or other of the photopolymer systems disclosed in the annex, is moulded onto a document, such as a currency note, which is provided with a hole or window, the arrangement being such that the light management film extends over said hole or window. In this case the light management film may incorporate a security code or "watermark" only readable at a certain angle of view.

In general, there is a first class of embodiments of the present invention in which the security marking is actually incorporated in the light management film and a second class of embodiments in which the light management film obscures a security marking provided on the document or product underlying the film, (for example, printed on the surface of the document or product), except from a predetermined angle of view. It will be understood, of course, that a particular security marking may have the characteristics of both of these classes, possibly even in the same area ofthe document or at least interspersed within a particular area. Whilst some embodiments of the invention rely upon the production of predetermined refractive index variations within a light management film, others may rely upon surface relief configuration to produce refraction at such surface or, more generally, upon interface relief, where a material of a second refractive index is applied as a coating over a material of a first refractive index and the interface between the two materials is contoured in a predetermined fashion so that refraction, and possibly even reflection, of light occurs at the interface between the two materials.

As indicated in the annex to this specification and in the specification of PCT/GBO 1/04978, it has been found that where a silicone-based photopolymerisable system of the kind disclosed therein is exposed to polymerising radiation which is substantially collimated, even when such exposure is effected directly, rather than through an optical mask, the exposed material polymerises to form a diffuser which has the curious property that, when viewed in a direction corresponding to the direction of the original polymerising radiation, it acts as a normal diffuser, (and thus has a translucent but misty appearance), whilst, when viewed from certain other directions it appears as a clear transparent sheet. Thus, for example, where the photopolymerisable material is exposed by collimated UV light directed along the normal to the plane of the photopolymerisable film, the product appears as an ordinary diffuser when viewed along such normal but, when viewed obliquely, appears as a transparent film. The same photopolymerisable material, when exposed to non-collimated light, cures to form a film which acts as a simple light diffuser regardless of the direction in which it is viewed. It will be understood that these characteristics can be used in the production of a security marking in accordance with the invention.

Thus, for example, a film of such photopolymerisable material may be exposed to collimated radiation through an optical mask which is, for example, opaque apart from transparent lettering or numbering, the film, after such exposure to collimated light, in each case, being subjected to blanketing UV radiation to polymerise the previously unexposed regions accordingly. With such an arrangement, the lettering or numbering will be substantially invisible when viewed in the direction of collimation ofthe collimated radiation since it will effectively be "misty" lettering or numbering against the "misty" background, but when viewed at the appropriate oblique angle the lettering or numbering will be discernible (assuming that the film is applied over a suitably contrasting part ofthe surface ofthe document or product concerned), as being grey lettering or numbering against a clear background or clear lettering or numbering against an opaque background.

It will be appreciated, of course, that other modes of exposure of the photopolymerisable material are possible. For example, some portions of the material may be exposed in such a way as to form a hologram and other portions exposed in some other way and so on. The method of applying a security marking, (or applying a light management film to an existing security marking) in accordance with the invention may involve either applying the photopolymerisable system (e.g. as a viscous liquid) to the document or product concerned and exposing it to polarising radiation, (e.g. ultra violet light), ofthe appropriate nature, (e.g. collimated with or without a mask; coherent hologram- forming radiation, etc.) or alternatively the light management film may be processed separately, for example as described in PCT/GBO 1/04978 or the annex hereto with reference to Figures 8 to 10 of that specification or annex, and subsequently applied, e.g. by means of a transparent adhesive, to the document or product concerned. The latter technique is employed in the embodiments ofthe invention described below.

In the examples described below, samples of photopolymer for application as light engineering films to documents or products were prepared using silicone - based ( eg. silicone acrylate based) photopolymerisable systems selected from the systems indicated in PCT/GBO 1/04978 as being effective. Such a UV- curable photopolymerisable system or blend of appropriate rheology as characterised by rheological studies was coated onto a substrate in the form of a glass tile or plate, for example. As the polymerisation of such photopolymerisable systems is in general inhibited by contact with oxygen, it is necessary to exclude oxygen, either by providing a covering layer of impermeable material such as mylar or by a blanket of inert gas (e.g. nitrogen) during exposure. The coated substrate is passed under a nitrogen blanket whilst being exposed to collimated UV light for a period. Alternatively a cover sheet of mylar could be placed on top ofthe coating followed by UV-curing. Where an optical mask of the kind referred to was to be used, the photopolymerisable material was first exposed through the mask (a "contact printing" technique). The tile or plate was then given a "blanket" exposure to UV from the side opposite the mask (and thus not through the mask), turned over and similarly exposed for a further period. With or without a mask the exposure to collimated radiation produces a film which appears as a normal light-diffuser, (i.e., appears misty) when viewed on-axis, i.e. along the direction of the collimated radiation, but which is transparent when viewed at a predetermined angle off-axis. The film is provided, (eg., as described above), with a certain pattern during the curing, for example so that the marking becomes visible when the film and the product or document is viewed off-axis. This unique marking makes the film ideal for anti-counterfeit applications such as currency, CDs, bank cards, etc. Ideally the marking itself may incorporate some encryption, as an additional layer of security.

In one example, (Figure 1), a patch of Sumitomo No. 7 adhesive was applied to the bottom right hand corner of a Bank of England ten pound note. A hole 15 was punched through the note and the adhesive patch. A light management film was prepared as described above using formulation MCL 77 described in Table 1 in the Annexe (opaque on-axis, transparent off-axis), the collimated exposure being effected through a mask configured to produce a predetermined security marking or code. This film was fixed over the adhesive so as to extend over the hole. The code in the light management film, in the region within the hole, could only be read at a certain angle.

In another example, there was applied to the central clear part of a CD a circular disk of Sumitomo no. 7 adhesive, with a circular hole in the middle in register with the hole in the middle of the CD. A circular disk of light management film, of the same character as described in relation to the preceding example, (opaque on-axis, transparent off-axis) and again prepared using MCL 77, was applied over the adhesive disk, the film having a central aperture registering with the hole in the middle ofthe CD. The code in the film again could be read only at a certain angle.

In a third example a hole was punched in the top right hand corner of a bank card, to which a patch of Sumitomo No. 7 adhesive had also been fixed, the hole likewise being punched through the adhesive patch. Again a patch of light management film, of the same character as described in relation to the preceding examples, (opaque on-axis, transparent off-axis) and again prepared using MCL 77, was applied over the adhesive. Again, the code within the region ofthe light management film extending over the hole could only be read at a certain angle.

In other embodiments, light management films cut to appropriate size and treated with suitable adhesion promoters were molded onto currency notes, CDs and bank cards by applying a pressure in a molding press in the range 3 to 20 tons per square inch. Results similar to those noted above were obtained. In other embodiments ofthe invention a transparency bearing a legend, picture, or "watermark" or the like was placed on the underside of the mylar or glass substrate coated with the UV-curable photopolymer blend. The coated substrate was passed under a nitrogen blanket, (or was covered by a cover sheet of mylar, placed on top of the coating), whilst being exposed, from below, to collimated UV light for a period. Where an optical mask ofthe kind referred to was used, this formed the substrate and the transparency bearing the legend, picture, or watermark was placed on the non-gelatine side of the mask and the photopolymerisable material was first exposed through the mask and transparency. The glass tile or plate was then turned over and exposed for a further period to provide a "blanket" exposure. With or without a mask, taking the collimation direction as on-axis, a diffuser was formed that was transparent off-axis. It was found that an image was, in effect, imprinted into the film during the curing according to the legend, picture, or watermark used. This unique marking makes the film ideal for anti-counterfeiting marking for currency, CDs, bank cards, and the like.

According to another embodiment ofthe invention a legend, picture, watermark is placed on the underside ofthe substrate being mylar or glass which is coated with the UV-curable photopolymer blend. The coated substrate is passed under a nitrogen blanket whilst being exposed to UV light for a standard period (e.g. two minutes), alternatively a cover sheet of mylar is placed on top of the coating followed by UV-curing. Thus, where an optical mask of the kind referred to was used, a legend, picture, watermark is placed on the non-gelatin side ofthe mask and the photopolymerisable material was first exposed through the mask. The tile or plate was then turned over and similarly exposed for a further period to provide a "blanket" exposure. With or without a mask on-axis a diffuser is formed that is transparent off-axis. An image is imprinted into the film depending upon the legend, picture, watermark used during the curing. This unique encryption makes the film ideal for anti-counterfeit applications such as currency, CDs, Bank cards.

According to another embodiment ofthe invention the substrate being glass or mylar is coated with the UV-curable photopolymer blend and a cover sheet of mylar placed on top. A legend, shape or pattern was placed directly in the path ofthe UV light on top ofthe cover sheet of mylar and exposed to UV light for a standard period (e.g. two minutes). Thus, where an optical mask of the kind referred to was used, the photopolymerisable material was first exposed through the mask. The tile or plate was then turned over, a legend, shape or pattern was placed directly on top of the mylar and similarly exposed for a further period to provide a "blanket" exposure. With or without a mask on-axis a diffuser is formed that is transparent off-axis. An image is imprinted into the film depending upon the legend, picture, watermark used during the curing. This unique encryption makes the film ideal for anti-counterfeit applications such as currency, CDs, Bank cards.

In another embodiment ofthe invention a first light-transmitting material, with a high refractive index, (which material may be, but need not be, a photopolymer produced by UV- curing any of the systems disclosed in the annex hereto), is applied to a product or document and the surface of the first material remote from the surface ofthe product or document is provided with a prismatic structure, (for example, as illustrated in Figure 2, comprising a series of similar parallel isosceles-triangular or pent-shaped ridges with intervening v-section grooves). This prismatic structure may, for example, be formed by embossing the material with a Ni or Cu drum which has a master pattern thereon, or by coating the photoplymerisable blend, in a liquid state, onto such a Ni or Cu drum with a complementary pattern and curing the material by heat or by exposure to UV light whilst still on the drum before peeling the material off the drum. There is applied to the ribbed top surface of the first material a top coat of a light transmitting material of a substantially lower refractive index, which top coat has good adhesion to the polymerised film. The top coat may be applied whilst the first material is still on said drum and subsequently the first material and superimposed top coat may be peeled off, as one, from the drum, or the top coat may be applied after the prismatic first material is in situ on the document or product to be marked. The prism structure is illustrated in Figure 2, which shows the combined prismatic material and top coat in position on, for example, the surface of a document bearing a security marking, such as a code number. In Figure 2, the angle of inclination of the flanks of the prism structure with respect to the lower surface of the first material or the upper surface of the top coat, is indicated at θ. It was found that film in which θ = 41°, when placed on a surface bearing a legend, picture, or pattern was hazy, and significantly obscured the legend or pattern, when viewed along the normal to the planes ofthe upper and lower surfaces ofthe combination ofthe film and top coating. In this case the 41° angle corresponded substantially to the critical angle for total internal reflection, between the two materials. Increasing this angle θ to 80° led to a film which, when placed on a surface bearing such legend, picture or pattern was opaque when viewed along the normal to the planes of the upper and lower surfaces of the combination of the film and top coating, but which allowed the legend, picture or pattern to be viewed at a certain viewing angle. In another embodiment of the invention a legend, picture , watermark, or the like was provided on the underside of an optically transparent carrier (e.g. polyester or polycarbonate). This carrier was coated with a high refractive index UV-curable photopolymer blend and embossed with a prism pattern, achieved by a Ni or Cu drum which has a master pattern. A topcoat which has good adhesion to the polymerised film was applied and cured by a suitable method (e.g. thermal- or UV-curing). The legend, picture, watermark was readable at a certain viewing angle.

In another embodiment of the invention, the processing of which is illustrated in Figured 3 a to 3 c, the underside of a transparent substrate 20 is imprinted (22) with a legend, pattern, picture or the like which is intended to be readable only at a certain viewing angle. As shown in Figure 3 a, a layer 24 of a photopolymerisable material, such as one of those disclosed in the annex to the specification, (e.g. MCL77), is applied on top ofthe substrate 20 and an optical mask 26 applied over the layer 24, after which the layer 24 is exposed to collimated UV light through the mask 26. The mask pattern may, for example, comprise an array of circular holes or windows in an otherwise opaque field or may comprise an array of clear slots in an otherwise opaque field. The UV exposure causes the regions not masked by the opaque parts of the mask to polymerise and solidify. The remainder of the (uncured) material 24 may then be washed away leaving islands (or strips) of polymer upstanding from the substrate as shown in Figure 3b. Finally, the combination is heated, so that as illustrated in Figure 3 c, the islands or strips melt to form rounded, lenticular globules, which act as lenses conditioning the viewing ofthe imprinted pattern of the underside of the substrate so that, as intended, it is readable only at the intended angle. In this case, the unit comprising the substrate, printed legend etc., and the lens array together forms a security marking which may be applied, e.g. by adhesive, to a product or document. A master may be formed which is then replicated and a replicate coated to form the film.

In a variant, the lens array could be formed as described with reference to Figures 3 a to 3 c without the imprinted pattern shown in these figures and turned upside down and placed against a surface bearing a legend, picture or pattern. It would then only be readable at a certain viewing angle. This is illustrated in Figure 4.

Films formed with the photopolymerisable system or mixture via UV-curing or thermal curing of a photopolymer blend can be used in combination with the prismatic structure or lens arrangement of Figures 2 to 4 or can be used alone.

Light management films consisting of graded refractive index materials with both regular and irregular structures, i.e. diffusers, produce a characteristic diffraction or 'speckle' pattern in the transmitted field when a coherent light source (typically a He-Ne laser) is directed at them. By placing an efficient and smooth reflector behind the diffuser, the reflected field can be recorded, as opposed to the transmitted field.

Coherent image formation is based on the following model [1]

where p is the point spread function, / is the object function and <8> ® denotes 2D convolution. Compared with the equivalent model for an incoherent image, i.e.

we see that the 'phase-mixing' associated with the convolution of p with / only occurs in a coherent image. In incoherent imaging, the phase properties of p and / do not affect the output. This 'phase-mixing' property associated with the formation of coherent images leads to such images being characterized by 'speckle'. Given that p is fixed, the characteristics of a speckle pattern depend oil those ofthe object function/ .

Using a statistical optics approach, we can consider the intensity of a coherent field φ say to be generated by a random amplitude/phase walk, i.e.

where A_j , φ₇ and N are taken to have pre-defined distributions. Using this approach, under the condition that these distributions are Gaussian, it is possible to show [2] that the statistics of the intensity = |φ|² of this field is characterized by a r -distribution ofthe form

P(z) = Pr[/(x, j/)] = jβ ^α J^^*-¹ exp(- βz), z > 0; {θ, z ≤ 0

where α > 0 and β > 0 with mean and variance given by μ =αβ and σ ² =αβ ² respectively.

Thus, unlike incoherent images which can have an unlimited variety of distributions, coherent images are approximately characterized by a Rayleigh type distribution ofthe general form

P(z) = Z^a QXO(- z)

Note however, that the detailed nature ofthe distribution will change from one coherent imaging system to another and in some cases the distribution may have a significant 'tail' (compounded in a K-distribution [2]). Methods of Characterizing a Speckle Pattern

There are in principle, many approaches to the characterization and quantification of a statistical field. Equation 1 suggests the use of a least squares approach to the determination of and β for example along with conventional statistical parameters such as the mean, standard deviation, mode etc. As an example, we have considered two types of Microsharp 'Riffix™' material namely Riffix™ MCL 189 or '189' and Riffix™ MCL 284 or '284' using a transmission mode. Typical speckle patterns and their histograms (with 256 bins) are shown in Figures 1-3 for Riffix™ MCL 189 (top) and Riffix™ MCL 284 (bottom) for three 'shots'. From these results it is clear that the difference of the 'decay curve' for example (relating to the negative exponential of the Rayleigh distribution) is significantly different for the two diffiisers. This is compounded in the differences of the standard deviations of the two speckle field, namely:

Shot no 189, 284 Shot 1: 7.09, 4.22 Shot 2: 6.26, 5.11 Shot 3: 6.21, 5.53

On this basis alone, a threshold of 6.00 would provide an appropriate limit for identifying the two materials, i.e. by application ofthe simple algorithm

start: if std>6, then output=='Riffix™ MCL 189' else output='Riffix™ MCL 284' end In addition to the standard deviation, the maximum value of the Riffix ™ MCL 189 speckle pattern is consistently (and relatively significantly) lower than the maximum value ofthe Riffix ™MCL 284 material although there is no apparent consistency in the scale ofthe results. However, it is possible for statistical tests other than that using the standard deviation to be used.

Thus, according to another embodiment of this invention, a light-transmitting material is applied either to an efficient and smooth reflector applied to a product or document or on a light-reflecting part of said product or document, to provide a means of authenticating said product or document. On directing a coherent light source at said light-transmitting material, a speckle pattern is obtained in the reflected field which may be recorded with a standard CCD camera or other suitable device. Typically a neutral density filter may be used to reduce the light intensity and/or the camera may be placed at an angle to the diffuser (as required) for the same reason.

According to a further embodiment of this invention, a light-transmitting material is applied to a product or document either on a clear part, such as the central part of a CD, or over a hole, for example a hole punched into a Bank of England ten pound note. On directing a coherent light source at said light- transmitting material, a speckle pattern is obtained in the transmitted field which may be recorded as described above.

The speckle pattern recorded in both the above embodiments is a fingerprint of that diffuser and is critically dependent and highly sensitive to the internal physiochemical structure of the diffuser material which, in turn, is critically dependent on the diffuser type and the batch process used. Thus, a batch of fingerprinted diffuser can be used to authenticate items such as credit cards and, also, to provide copy protection. An authentication system with fixed optics is required and the authentication templates determined for each system prior to their use. Any change in the optical configuration will lead to changes in the characteristics of the speckle pattern that are not material dependent. It is therefore envisaged that an authentication detector system for reading the speckle pattern and calculating a mathematical code will consist of i) a coherent light source ii) CCD camera or other imaging system iii) a microprocessor and iv) a test position in a layout that is fixed for the application.

References

[1] Blackledge J M, Quantitative Coherent Imaging, Academic Press 1990. [2] Blackledge J M, Application of Some Image Processing Techniques to the B-Scan Analysis of Two-Phase Flows, IMA Proc. on Imaging and Image Processing: Mathematical Methods, Algorithms and Applications (Ed. J M Blackledge), Oxford University Press, 1998, 349-395

Claims

1. A document or product incorporating a security, authentication or copy protection marking in the form of refractive index variations or other variations in light managing properties, in a layer or sheet of light-transmitting material provided on or in at least part of said document or product.

2. A document or product according to Claim 1 wherein said light- transmitting material is in the form of a sheet of said light-transmitting material extended over a light-transmitting aperture or window in said document or product.

3. A document or product according to Claim 1 wherein said light- transmitting material is extended in a layer across a light-transmitting part of said product.

4. A document or product according to Claim 1 wherein said layer or sheet of light-transmitting material is applied over a light-reflecting part of said document or product.

5. A document or product according to Claim 1 wherein said light- transmitting sheet comprises a first layer of a first refractive index and a second layer of a second, significantly different, refractive index, applied over said first layer, and wherein the interface between the first and second layers is of a contoured or prismatic character.

6. A document or product according to Claim 5 wherein the interface between the first and second layers defines a prism structure with prism faces so inclined with respect to the normal to the surface ofthe document or product to which said light-transmitting sheet is applied that, for light passing from the surface of the product or document to which said light-transmitting sheet is applied, in a direction normal to said surface, total internal reflection occurs at said interface, whereby the underlying surface of the product, bearing an authentication or security marking, is substantially invisible when viewed normally, i.e. perpendicular to said surface, but is visible when viewed obliquely from a certain angle.

7. A document or product according to Claim 1 wherein said light- transmitting sheet or layer is of a graded refractive index material having refractive index variations so configured that said sheet or layer, or a selected portion thereof, is light-diffusing when viewed along the normal to the layer but is transparent when viewed obliquely from a predetermined angle.

8. A document or product according to Claim 1 wherein said light- transmitting sheet or layer is of a graded refractive index material having refractive index variations so configured that said sheet or layer, or a selected portion thereof, is transparent when viewed along the normal to the layer but is light-diffusing when viewed obliquely from a predetermined angle.

9. A document or product according to Claim 7 or Claim 8 wherein said selected portion defines text, graphics, or the like.

10. A document or product according to any preceding claim wherein said layer or sheet of light-transmitting material is a silicone photopolymer disclosed in co-pending PCT Patent Application PCT/GBO 1/04978 and as disclosed in the annex to this specification.

11. A method of providing a security or authentication marking on a document or product comprising applying to such document or product a light- transmitting sheet or layer incorporating refractive index variations or other variations in light refracting properties.

12. A method of providing security or authentication marking on a document or product comprising applying to at least a part of such document or product a layer or coating of a light transmitting material and subsequently modifying the layer or coating to produce predetermined refractive index variations or other variations in light managing properties.

13. A method according to Claim 12 wherein said layer or coating is applied as a photopolymerisable material and said step of subsequently modifying the layer or coating includes exposure ofthe latter to polymerising radiation.

14. A method according to Claim 11 wherein the speckle pattern produced by directing a coherent light source at the light-transmitting sheet or layer is used to authenticate said document or product.

15. A method according to any of Claims 11 to 14 wherein said light transmitting sheet or layer is provided over an aperture or light-transmitting window in the document or product.

16. A method according to any of Claims 11 to 14 wherein said light- transmitting sheet or layer is provided over a light-reflecting part of said document or product.

17. A method according to any of Claims 11 to 13 wherein said light transmitting sheet or layer is provided over a part of the surface of the document or product which bears an authentication or security marking.

18. A method according to any of Claims 11 to 17 wherein said light transmitting sheet or layer is a silicone photopolymer formed from any of the compositions disclosed in PCT/GBO 1/04978 and in the annex to this specification, by exposure of such compositions to polymerising radiation.

19. A detection system for reading the speckle pattern produced by directing a coherent light source at the light-transmitting sheet or layer.

20. A detection system as in Claim 19 comprising a coherent light source, an imaging system and a microprocessor.

21. A document or product incorporating a security or authentication marking substantially as hereinbefore described.

22. A method of providing a security or authentication marking on a document or product substantially as hereinbefore described.

23. Any novel feature or combination of features described herein.