GB2401823A - Optically coded security markings - Google Patents

Abstract

In a method of providing security, authentication or copy protection markings, these are provided in the form of refractive index variations or other variations in light managing properties, in a layer or sheet of light-transmitting material. In carrying out the method a light-transmitting photopolymerisable material is exposed to polymerising radiation, e.g. UV, through at least one mask or stencil to provide variations or surface relief variations, in the material due to selective polymerisation, such that said layer or sheet has a plurality of different diffusion tones. A security or authentication marking comprises a layer or sheet of light transmitting material consisting of overt and/or covert images formed, using at least one shadow mask with a plurality of diffusion tones or using at least one stencil with a plurality of perforations of different density. Disclosed also is a method of producing an overt security or authentication marking by forming a speckle pattern on a photographic plate or with a metal stencil, using collimated radiation and a diffuser. Other methods are also disclosed.

Description

240 1 823 "Optically Coded Security Markings" This invention relates to

security or authentication marking of documents, such as currency notes or other valuable documents, or of high value products such as performancecritical parts or luxury products. 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 documents or products.

Counterfeiting is a major global problem that causes immense damage to industrial and financial sectors of the global economy. The counterfeit market can be divided into three main segments: security printed documents (e.g. banknotes, passports, driving licenses), brand protection (e.g. software, pharmaceuticals, music, video, perfumes) and plastic cards (e.g. credit cards, magnetic strips, chip cards).

Approaches currently used to prevent counterfeiting of bank notes include the use of several overt (visible to the eye) and covert (not easily seen by the eye but readable by some other means) anti-counterfeiting measures. Plastic card forgery is thought to equal that of security printed documents in terms of annual losses. By a large margin, the most significant segment is brand protection with annual losses due to counterfeiting in the region of USD 350 billion. Branded goods that are being counterfeited include software CDs.

Overt measures currently used to prevent counterfeiting of brands include laser coding, unique bar codes and holographic seals while covert measures include photocopier sensitive security inks, unique chemical signatures and digital watermarks. Manufacturers are now utilising a combination of covert and overt methods for brand protection and governments are using the technology to protect national and voter identity cards, licenses and tax stamps.

The use of polymer films for the prevention of counterfeiting has been reported and methods include: 1) A transparent window feature of a polymer banknote that allows the incorporation of transmission based optical devices on a banknote so that the user, by folding the note over on itself and looking through an optical device which is a part of the note itself, can visually inspect and verify certain security features on the banknote (Zientek, Proc. SPIE 3314, p272-4, 1998).

2) Reflective film technology provides a method for the manufacture of a sheet element having a reflective anti-counterfeiting device. In particular, it provides a method for the manufacture of a security document. The method comprises the steps of applying a continuous band of a foil material in the form of a repetitive pattern to a portion of the surface of the sheet element and overprinting at least a part of the foil material with a security tint. This technology envisages the use of hot foil stamping or a similar method such as a transfer process to apply a metallic foil or similarly reflective film on to a surface of the material to be used for printing a security document. This technology provides security documents having improved protection against counterfeiting and is particularly applicable to the manufacture of bank notes (EP0093009).

3) The use in printed documents of dyes which change in colour when photocopied! or scanned has potential in anti-counterfeiting of secure documents such that documents cannot be scanned or photocopied.

4) The application of light management films has previously been disclosed for I overt coding for products in all segments of the anticounterfeiting market mentioned above.

PCT/GB02/05868 (WO/03055692) discloses the use of light management films on documents or products such that persons may be assured of their authenticity and conversely such that it is made more difficult for counterfeits of such documents or products to be produced which will pass as authentic products or documents, whereby variations in refractive index or other light managing properties of a sheet of light-transmitting material provide a security or authentication marking.

It is an object of this invention to provide security or authentication markings, and a method of producing such, on documents or products by which persons receiving the same may be assured of their authenticity.

W094/29768 describes a method of exposing a photographic plate with a micro- scale random speckle pattern to produce an optical mask with a similar speckle field modulation of the photographic optical density. It is already known that a polymeric diffuser can be replicated from such a speckle mask by contact copying into a UV-curable polymer film. This produces micro-scale features within the polymer that have graded refractive index (GRIN) and thus behave like lenses with very short focal length. Each GRIN lens feature focuses and expands light incident upon it. On a macro-scale this causes the film to appear diffuse. As this mechanism of diffusion does not involve back scattering or significant absorption, the film is highly efficient at managing light. Changing production process variables, in particular the mask, leads to diffusers with different characteristics. This can be achieved by changing the speckle size in the mask. It is possible to create a mask with areas of different speckle size, and thus form one diffuser with areas of different diffusion properties that can be seen clearly by the eye. By structuring the areas using shadow masks, it is possible to create images with a multitude of diffusion tones'.

Thus, in accordance with one aspect of the invention, there is provided a method of providing 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, the method including exposure of a layer or sheet of light-transmitting photopolymerisable material to polymerising radiation through at least one mask to cause selective polymerization of said material, and thereby induce refractive index variations within said material and/or surface relief variations, such that said layer or sheet has a plurality of different diffusion tones.

In accordance with another aspect of the invention, there is provided a security, authentication or copy protection marking provided by the lastnoted method.

Embodiments of the invention are described below with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration showing apparatus for forming a speckle mask, Figures 2A and 2B show respective shadow masks which may be used in the apparatus of Figure 1 in carrying out the invention, Figure 3 is a diagrammatic illustration showing another apparatus which may be used in carrying out the invention, Figure 4 illustrates an elongate dot or short slot or strip formed on, for example, a photographic plate in operation of the apparatus of Figure 3, Figure 5 is a diagrammatic illustration showing apparatus in use exposing a photographic plate through a shadow mask, Figure 6 is a diagrammatic illustration showing yet another apparatus, similar in some respects to that of Figure 3, which may be used in carrying out the invention, Figure 7 is a plan view of a stencil or mask, Figure 8 is a view of a photopolymer film produced using the stencil or mask of Figure 7, Figure 9 is a schematic sectional view illustrating production of a diffuser film or light management film using a mask or stencil such as illustrated in Figure Figures lOA to lOD illustrate successive stages in forming a diffusion film or light management film using a mask or stencil such as illustrated in Figure 7, by a variant method, Figure lOE is a schematic plan view of a light management film or diffuser produced by the method of Figure 9 or of Figures 1 OA to 1 OD, Figure lOF is a schematic plan view, to an enlarged scale as compared with Figure 1 OK, of a detail, Figure 11 shows, to an enlarged scale, part of a film produced by the method of Figure 9 or of Figures lOA to lOD, Figure 12 shows, to an enlarged scale, part of a metal stencil with a hole or aperture therethrough, having a feature or pattern on a side or wall of such hole or aperture, and Figure 13 is a fragmentary perspective view, partly in section which shows, again to an enlarged scale, part of another metal stencil with holes or apertures therethrough, having a feature or pattern on a side or wall of one such hole or aperture.

In some methods embodying the invention, a photographic plate is first produced which can be used as, or as the basis for, an optical mask through which a layer of s light transmitting photopolymerisable material can be exposed to polymerising radiation, such as UV light, to produce refractive index variations andlor surface relief variations as a result of polymerization of said material. Thus, referring to Figure 1, a speckle mask may be produced, in a manner similar to that described in W094/29768, by directing coherent light through a ground glass diffuser plate onto a photographic plate. A suitable apparatus is shown in Figure 1 in which a laser beam 10 passes from a beam expanding telescope 12 to a folding mirror 14 and thence down to a ground glass diffuser plate 16 (mounted diffuse side downwards). The laser beam is weakly scattered by the diffuser 16 and forms a fine speckle pattern on a photographic plate 18 below. In one embodiment of this invention, overt images are produced by the following technique. A first shadow mask, such as that shown at Figure 2a, preferably but not necessarily made from thin card, is placed on top of the photographic plate 18 so that different areas of the plate can be exposed differently. The laser beam 10 is scanned across the diffuser plate 16 in a raster scan so that the whole photographic plate, or rather that part of the photographic plate not masked by the shadow mask, is exposed with the speckle pattern. The shadow mask of Figure 2a is then removed and replaced by another, preferably but not necessarily with its conjugate mask such as shown in Fig 2b. The glass diffuser 16 is placed at a higher elevation than for the first exposure before the plate 18 is exposed again, in the same manner as before.

Alternatively, the photographic plate may, in this stage, be exposed in its entirety to the speckle pattern produced by the laser light, without any second mask being used.

In a variant of the method described, overt images may be produced by changing the exposure intensity. Thus, rather than changing the separation of diffuser 16 from the photographic plate 18, the laser intensity may be changed between the two exposures.

In a further variant, the photographic plate is first exposed with one shadow mask and then exposed again with the conjugate mask using a different laser intensity.

In a yet further variant, the photographic plate is first exposed to the laser light speckle pattern without a shadow mask and is then exposed again with a shadow mask using the same or a different laser intensity, thus twice exposing the area defined by the mask. In this instance it is essential that the glass diffuser is not moved between the two exposures as the aim is to reinforce the speckle features already written in mask. Moving the diffuser would create a different speckle field.

The shadow mask may consist of block images, for example simple shapes or letters, or more complicated images, e.g. photographs. A more complicated image could be derived from the varying optical density in a photographic negative, or from a laser-printed grey scale on a transparency.

Covert images, i.e. features that cannot be seen easily by the eye due to their micron-scale size, but can be detected by some other means, can be used for a second line of security. By writing a regular array of features on the micron scale, a covert image is produced. The features cannot be seen by the eye, besides a slight dispersion effect of white light but can be detected by illuminating with collimated monochromatic coherent light (e.g. laser) to produce a diffraction pattern.

In a further embodiment of this invention, using apparatus illustrated in Figure 3, microdot arrays necessary for developing covert images are generated using, for example, a two-axis CNC positioning system equipped with an optical system including a laser 30, preferably a Nd:YAG laser, and a fibre optic delivery system 32 (Figure 3). The CNC positioning system is arranged to produce a scan of the laser beam relative to the photographic plate. The optical system includes a chopper 34 for stroking the laser, the strobe effect being intended to expose dots on the photographic plate as the scanning moves continuously over the surface of the photographic plate 18. As the dots are formed during a continuous scan, the dots are effectively short slots. By reducing the exposure time, the length of slot can be small relative to the radius of the dot (see Figure 4). The length of the exposed region (dot length) is a function of beam spot diameter, optical chopper mark- space ratio and scan velocity. Similarly, the spacing between the exposed regions on the plate 18 is also related to these same parameters but primarily to mark-space ratio and scan speed where the spot diameter is much smaller than the dot spacing. The laser spot size defines the minimum size of dot and is controlled by the objective focus.

In this embodiment of the invention, the covert array is written onto the photographic plate by first focusing the laser optics so that the collimated laser beam forms a spot of the desired diameter on the surface of the photographic plate and centralising it on the laser beam. The laser beam intensity is set to give the required dot size and parameters such as scan speed and step size are entered once the control code is loaded into the CNC positioning machine. The optical chopper is set running, the photographic plate is positioned emulsion side up, and the exposure cycle is initiated. After exposure the photographic plate is then developed.

To reverse the image and obtain the dark dots as transparent regions through which the replicating photopolymer can be exposed, the covert image is contact copied from the master. The covert image master mask is placed on top of a photographic plate with the image surface of the master mask downwards, in contact with the emulsion of the photographic plate below, and pressed down gently to expel the air film between the plates. The scanning optics arrangement is set up as shown in Figure 5, in a manner similar to that shown in Figure 1 for overt arrays except that the ground glass diffuser plate is not needed. The laser scanning cycle is initiated and the photographic plate subsequently developed.

Alternatively, a direct positive image can be formed by over exposure of the original latent image. This is achieved by first forming the image in a negative plate followed by flood exposure of the whole plate in a controlled manner. The originally exposed areas of the image become overexposed, causing the emulsion to reverse, whilst the originally exposed levels will be exposed to the desired level controlled by the flood exposure process.

Alternatively, reversal processing of a negative latent image can be used to produce a positive image. By developing the original latent image, followed by a bleaching process to remove the negative image, the unexposed emulsion is left, to form the positive image. The bleached image is flood exposed and developed to produce a positive developed image.

In the above methods, after exposure and development of the photographic plate to form the optical mask, a photopolymer sheet or film is formed by exposure of a photopolymerisable material, through the optical mask, to collimated polymerising radiation such as W light. In other embodiments of the invention, a photopolymer material is exposed directly, i.e. by substituting a layer of photopolymerisable material for the photographic plate in the arrangements described above.

In a further embodiment of this invention, the chopper wheel 34 of Figure 3 is replaced with an optical modulator or like device by which the collimated light beam 10 is pulsed. Such devices allow irregularly spaced rows of dots to be written on the photographic plate, as well as regularly spaced rows of dots. The relative alignment of adjacent rows of dots can also be controlled.

In another embodiment of this invention, parallel lines, similar to a bar code, are written onto the photographic plate. Referring to Figure 6, by comparison with Figure 3, an optical modulator 38 is used in place of the chopper wheel and is triggered by the CNC controller, such that the lines are placed at the required intervals on the photographic plate. The lines may be regularly or irregularly spaced, with the distribution of spacings providing a means of authentication. The width of the lines may also be used for authentication purposes.

A further embodiment of this invention combines the use of both overt and covert images. The superposition of an overt image on a covert image can distract from the covert image. This may be useful for aesthetic reasons or the overt image functionality may be used, for example a 2D barcode in the overt image.

In this embodiment, a covert array is exposed onto a photographic plate as described previously and an overt image is written over the top. In an alternative embodiment a covert array is written on top of an overt image. In all of the arrangements described above, the laser intensity for each exposure is chosen so as to avoid over or under exposure which causes the speckle feature contrast to be too high or too low thus affecting the diffuser-making capability of the optical mask.

In a further embodiment of this invention, a combined overt and covert image is produced which removes the requirement for over-writing the covert image with an overt speckle image. A regular array of modulated features produces a diffraction pattern without overtly revealing the nature of the modulation. The modulation of the feature acts as a second line of security that would be read by other means. For example, the feature could be a barcode, with some aspect of the barcode being modulated between occurrences of the barcode in the array. Such aspects of the barcode could be the spacing, length or thickness of the lines, or the length or orientation of the barcode. Alternatively, the feature could be a dot or group of dots, with the size or shape modulated or, in the case of a group of dots, the spacing of the dots within the feature. It will be understood by a person skilled in the art that the features are not limited to those mentioned here, and similarly the modulations.

In another embodiment, a suitable reflective layer above the photographic plate is deformed at either the covert or overt writing stage to create a custom interference pattern. Alternatively, the plate, which may be made of any transparent or semi- transparent medium of higher refractive index than air, for example glass, quartz or plastic, may be deformed or moulded, or a moulded surface cast onto a plate of suitably transmissive medium. The effect is caused by coherent light being reflected by two plates separated by a thin film of air. When the separation is nonuniform, for example when a meniscus or lens shape is defined by the surfaces, a wedge effect gives rise to multiple refraction leading to an interference pattern of lines of equal thickness across the emulsion. This modulates the intensity of the speckle mask.

For a watermark ten fringes deep, a deflection of 2.5 micrometres is needed when using a laser wavelength of 522nm.

In preferred embodiments of the invention such as described above and below, a light managing, (e.g. light diffusing) security marking or layer is produced by exposing, to W light, through the optical mask produced as described, a photopolymerisable medium such as a photopolymerisable monomer. The photopolymerisable material used may be a silicone-based photopolymerisable system of the kind disclosed in WO02/39184. These photopolymerisable systems enable, inter alla, 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 polymerization in the areas exposed, whereby, after any necessary processing steps, a lighttransmitting sheet or layer can be obtained, which may be characterized by refractive index variations and/or by variations in layer thickness (i. e. by surface relief features, or by combinations of such features).

In accordance with one embodiment, a non-destructive energy source such as x- rays, ultrasonics, eddy current or magnetic means may be used to map the microstructure of the GRIN diffuser thereby allowing detection of for example, a covert barcode. Alternatively, in another embodiment, a high resolution camera combined with a suitable light source may also be used for mapping the correct pattern.

In an alternative embodiment an overt image is incorporated into a film with covert features. The covert features can be read via the mathematical code generated from the diffraction pattern as disclosed in WO/03055692. The overt image is produced using a metal stencil containing perforations. Such a stencil is shown in plan in Figure 7. The diffuser film produced has the overt image of the metal stencil as shown in Figure 8.

The film depicted in Figure 8 has GRIN ( graded refractive index) structure, which is strongly defined in the areas outside of the image produced by the stencil, and weaker GRIN structure from the image itself. Referring to Figure 8, it will be understood that each of the dots in the array of rows and columns of dots visible is much larger in size than the GRIN microlenses referred to and that each such dot is formed by a respective clump of much smaller GRIN microlenses. Each dot visible in Figure 8, of course, corresponds to a respective aperture or hole in the stencil shown in Figure 7.

The manner in which the film shown in Figure 8 is produced is described below with reference to Figure 9. In Figure 9, layer 1 represents Mylar (polyester) film of 50 micrometres thickness, which is placed on a glass plate, (not shown), onto which is placed a Mylar film template (layer 2, lOO micrometres thick). The silicone based photopolymerisable formulation referred to above (which is liquid before polymerization and is referenced 40 in Figure 9), is placed within a recess defined by a central aperture in the template and a cover sheet of Mylar placed on top (layer 3, 50 micrometres thick). A roller is then applied to spread the photopolymerisable material evenly. The metal template or stencil is placed underneath the glass and the film cured from above by collimated W light directed onto the Mylar/photopolymer package. Alternatively, the photopolymerisable material may be exposed to collimated UV light through the metal stencil.

As disclosed in WO02/39184, 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.

Referring to Figures lOA to lOE, in a variant of the method described with reference to Figure 9, a layer 40 of photopolymerisable material, which may be contained between Mylar sheets, (not shown in Figure lOA to lOE) in the same way as described with reference to Figure 9, is, in a first stage, illustrated in Figure lOA, exposed to collimated W light, (represented by the vertical arrows in Figure lOA), through an apertured shadow mask, e.g. in the form of an apertured metal plate 2c forming a metal stencil, placed on top of the layer, (or rather on top of the upper Mylar sheet (not shown) bounding the layer 40, assuming a Mylar/photopolymer package as described with reference to Figure 9). For the duration (1 - 2 minutes) of the first stage (stage 1) illustrated in Figure lOA, the package comprising the layer 40, its bounding Mylar layers and the metal stencil 2c is arranged with the planes of the major surfaces of its components and thus of the metal stencil, Mylar sheets and layer 40, inclined at an angle to the perpendicular to the direction of incidence of the collimated ultra-violet light. This angle may, as shown, be 15 , for example. The W exposure, carried out for 1 - 2 14, minutes, cures those areas of the film where the light enters the perforations or apertures in the metal stencil.

After the first exposure and the resulting polymerization illustrated in Figure lOB and lOC, the metal stencil is removed, the film turned through 180 and then angled at 15 perpendicular to the direction of incidence of the W light so that this direction of incidence, relative to the layer 40, and the direction of incidence, relative to layer 40 during the first exposure, are oppositely inclined with respect to the major planes of layer 40. With the layer 40 thus rearranged, the layer 40 is exposed again for 1 to 2 minutes to the collimated UV light, (stage 2). In Figure lOB, the arrows represent diffusion of the monomer (in the photopolymerisable material) in the course of polymerization as a result of the exposure to W. within the regions so exposed, whilst in Figure lOC, the hatched regions represent the exposed regions after such polymerization. During stage 1 a strong diffuser is l formed in those areas exposed to the UV light, whilst in stage 2 there is a diffusion of monomer previously in the unexposed regions, forming a weak diffuser in between the strong diffuser area (dots). A schematic representation of the W curing process is shown in Figures lOB and lOC. The film produced from the process is shown in Figure 11.

Figure l OF illustrates schematically a plan view of the exposed photopolymer layer or a portion thereof. The features marked 46 represent the regions exposed in the first stage illustrated in Figure lOA, whilst the features marked 48 represent the regions intermediate the features 46 formed by polymerization of the previously unexposed material in stage 2, (Figure NOD). Figure lOF illustrates, by arrows, diffusion of monomer within the layer 40 towards the middle of the exposed region (bounded by the broken lines).

The angle of the film or layer 40 incident to the light does not have to be specifically at 15 . It can be lower (down to 5 degrees) or higher (up to 45 degrees) than this value. A decrease in the density of the perforations in the metal stencil would lead to the diffuser dots in stage 2 being more defined. There would be more monomer/oligomer available for polymerization.

It will be appreciated that, in the method illustrated in Figures lOA to lOF, no shadow mask (e.g. speckle mask) is utilised because in thisparticular embodiment the silicone-based photopolymerisable material referred to above is used, taking advantage of the property of that material, referred to above, of forming a micro- scale GRIN structure when exposed to collimated UV light, without the need for a speckle mask or the like.

In another embodiment a metal stencil has perforations in which at least one of the perforations has a feature or features within the slope of the perforation at a certain depth as shown in Figure 12. Depending upon the direction of W exposure, e.g. depending upon whether it is straight down, (as indicated by arrow 60 in Figure 12), or at an angle, (as indicated by arrow 62 in Figure 12), the GRIN structure formed will be different. There is the possibility of forming an image visible at different angles within the film. Figure 12 represents a magnified view of a hole or perforation in the metal stencil. There are slopes on either side of the hole, achieved during the cutting process. There will be a depth to the metal of the stencil which will feature a pattern or other feature on one side or both sides of the hole or perforation. For example, a logo or other image 50 may be cut into one side or wall 52 of the hole or perforation. The logo or other feature 50 might be formed, for example, by etching away the surface of the wall of the perforation using a corrosive substance, such as acid.

Figure 13 illustrates the application of this idea to a metal stencil 2e, for the same purpose as the stencils discussed above, this stencil being shown to an enlarged scale in fragmentary perspective view in Figure 13, in vertical section through one of a plurality of holes or perforations 64, 64a through the stencil. As shown, the sectioned hole 64a has side walls 52 which slope inwardly from the upper edge of the hole 64a to the lower edge and a logo or other feature 50 is formed on one of these sloping walls intermediate the upper and lower edges of the hole 64a. Again, the GRIN structure produced in the photopolymer material exposed through the stencil will be different if the collimated W light by which the material is exposed is directed substantially perpendicular to the major planes of the stencil, for example, parallel with arrow 60 in Figure 13, or at an angle to these major planes, for example along arrow 62 in Figure 13.

Claims (34)

1) A security or authentication marking in the form of a layer or sheet of light- kansmitting material consisting of overt or covert images formed using one or more shadow masks with a plurality of diffusion tones.

2) A security or authentication marking in the form of a layer or sheet of light- transmitting material consisting of overt and covert images formed using one or more metal stencils with a plurality of perforations of different density.

3) A security or authentication marking in the form of a layer or sheet of light- transmitting material consisting of overt or covert images formed using one or more metal stencils with perforations in which at least one of the perforations has a feature or features within the slope of the perforation at a certain depth.

4) A method of producing an overt security or authentication marking involving forming a speckle pattern on a photographic plate, using collimated radiation and a diffuser.

5) A method of producing an overt security or authentication marking involving forming a speckle pattern with a metal stencil, using collimated electromagnetic radiation and a diffuser.

6) A method as in Claim 4 where the separation of the diffuser from the photographic plate is changed between two or more exposures of the photographic plate.

7) A method as in Claim 4 or 6 where the photographic plate is exposed through at least one shadow mask.

8) A method as in Claim 7 where the laser intensity is changed for the second exposure.

9) A method as in Claim 7 or 8 where the second mask is the conjugate of the first mask.

10) A method as in Claim 4 where the photographic plate is exposed without a shadow mask and then exposed again with a mask.

11) A method as in any of Claims 1 or 6 to 10 where the shadow mask consists of one or more block images.

12) A method as in any of Claims 1 or 6 to 10 where the shadow mask consists of one or more photographic images.

13) A method of producing a covert security or authentication marking comprising exposing a photographic plate and contact copying the covert image from the master to form transparent regions on a photographic plate through which the replicating photopolymer is then exposed.

14) A method as in Claim 13 where the photographic plate is exposed with a chopped scanning collimated beam of radiation.

15) A method as in Claim 13 where the photographic plate is exposed with an optically modulated scanning collimated beam of radiation.

16) A method of producing a covert security or authentication marking comprising exposing through a metal stencil at a given angle through which the photopolymer is exposed, removing stencil and subjecting sample to a second exposure at a given angle.

17) The angle of exposure for the light or the sample in Claim 16 being in the range of 1 to 180 degrees, preferably 5 to 45 degrees, more preferably 15 degrees.

18) A method as in any of Claims 13 to 16 where the covert security or authentication marking consists of regularly spaced features.

19) A method as in Claim 18 where the regularly spaced features are lines of dots or blocks of regularly spaced lines of dots or blocks of irregularly spaced lines of dots.

20) A method as in Claim 18 where the regularly spaced features are parallel lines or blocks of regularly spaced parallel lines or blocks of irregularly spaced parallel lines.

21) A method as in any of Claims 13 to 16 where the covert security or authentication marking consists of irregularly spaced features.

22) A method as in Claim 21 where the irregularly spaced features are lines of dots.

23) A method as in Claim 21 where the irregularly spaced features are parallel lines.

24) A method as in any of Claims 13 to 16 where the spacing of the lines provides a means of authentication.

25) A method as in any of Claims 13 to 16 where a dimension of the lines provides a means of authentication.

26) A method of producing a covert security or authentication marking comprising exposing a photopolymer with a chopped or optically modulated scanning beam of collimated radiation.

27) A method of producing a combined overt and covert security or authentication marking comprising writing an overt array on top of covert.

28) A method of producing a combined overt and covert security or authentication marking comprising writing a covert array on top of an overt.

29) A method of producing a combined overt and covert security or authentication marking comprising modulating the features of a covert array.

30) A security of authentication marking in the form of a layer or sheet of light- transmitting material comprising the creation of an interference pattern on a photographic plate through which the photopolymer is exposed.

31) A method according to any of the previous claims wherein an energy source is used to read the covert barcode.

32) A method of providing 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, the method including exposure of a layer or sheet of lighttransmitting photopolymerisable material to polymerising radiation through at least one mask to cause selective polymerization of said material, and thereby induce refractive index variations within said material and/or surface relief variations, such that said layer or sheet has a plurality of different diffusion tones.

33) A security, authentication or copy protection marking produced by the method of any of Claims 4 to 32.

34) A method of providing a security, authentication or copy protection marking substantially as herein described with reference to any of the accompanying drawings.