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Title:
SECURITY DEVICE BASED ON CUSTOMISED MICROPRISM DEVICE
Document Type and Number:
WIPO Patent Application WO/2008/029128
Kind Code:
A3
Abstract:
A security device comprises a transparent substrate (1 ) provided with a prismatic surface structure (2) defining an array of substantially planar facets. An adhesive layer (3) extends around the perimeter of one surface of the substrate (1) such that the security device can be adhered to a further substrate, wherein at least some parts of the said surface are exposed to the air whereby those parts will be spaced from the further substrate in use to define a transparent substrate/air interface such that corresponding parts of the prismatic surface structure (2) forms a reflector due to total internal reflection when viewed at at least one first viewing angle and are transparent when viewed at at least one second viewing angle.

Inventors:
COMMANDER LAWRENCE GEORGE (GB)
EASTELL CHRISTOPHER JOHN (GB)
ISHERWOOD ROLAND (GB)
Application Number:
PCT/GB2007/003342
Publication Date:
June 26, 2008
Filing Date:
September 05, 2007
Export Citation:
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Assignee:
RUE DE INT LTD (GB)
COMMANDER LAWRENCE GEORGE (GB)
EASTELL CHRISTOPHER JOHN (GB)
ISHERWOOD ROLAND (GB)
International Classes:
G02B5/124; B29D11/00; B42D15/00; B29C41/28; B29C41/32; B42D101/00; B44F1/04
Domestic Patent References:
WO2006095161A22006-09-14
Foreign References:
US5591527A1997-01-07
JP2004262144A2004-09-24
US5643400A1997-07-01
Attorney, Agent or Firm:
GILL JENNINGS & EVERY LLP (7 Eldon Street, London EC2M 7LH, GB)
Download PDF:
Claims:

CLAIMS

1. A security device comprising a transparent substrate provided with a prismatic surface structure defining an array of substantially planar facets, wherein an adhesive layer extends around the perimeter of one surface of the substrate such that the security device can be adhered to a further substrate, wherein at least some parts of the said surface are exposed to the air whereby those parts will be spaced from the further substrate in use to define a transparent substrate/air interface such that corresponding parts of the prismatic surface structure form a reflector due to total internal reflection when viewed at at least one first viewing angle and are transparent when viewed at at least one second viewing angle.

2. A device according to claim 1 , wherein the first viewing angle is orthogonal to the security device and the second viewing angle is offset from the orthogonal direction.

3. A device according to claim 1 or claim 2, wherein the adhesive layer has a width of at least 1 mm and preferably at least 3mm.

4. A device according to any of the preceding claims, wherein the adhesive is on the surface of the transparent substrate provided with the prismatic surface structure.

5. A device according to any of the preceding claims, wherein the adhesive layer extends over part of the prismatic surface structure.

6. A method according to any of claims 1 to 4, wherein the adhesive layer is laterally spaced from the prismatic surface structure. 7. A method according to any of the preceding claims, wherein the adhesive layer is applied as a screen, such as a dot screen.

8. A device according to claim 7, wherein the dots of the dot screen cannot be resolved by the naked eye.

9. A device according to any of the preceding claims, wherein the adhesive layer is applied in a pattern so that the part(s) of the transparent substrate not provided with adhesive define an image or images.

10. A device according to any of the preceding claims, wherein the adhesive layer itself defines an image or images.

11. A device according to any of the preceding claims, wherein areas within the perimeter layer of adhesive which have been left free of adhesive define an image or images.

12. A device according to claim 11 , wherein said adhesive-free areas define a total area coverage of no more than 50%, preferably 40%, most preferably 30%, of the area of the perimeter layer.

13. A device according to any of claims 9 to 12, wherein the images are selected from patterns, symbols, alphanumeric characters and combinations thereof. 14. A device according to any of the preceding claims, wherein the adhesive includes a functional component that reacts to an external influence. 15. A device according to claim 14, wherein the functional component is one or more of fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic. 16. A device according to any of the preceding claims, wherein the substrate provided with the prismatic surface structure comprises a polymeric film.

17. A device according to any of the preceding claims, wherein the prismatic surface structure comprises one or more arrays of parallel linear prisms, tetrahedra, square pyramids, corner-cube structures, and hexagonal-faced corner-cubes.

18. A device according to claim 17, wherein the prismatic surface structure comprises at least two linear arrays of prisms arranged at an angle to one another, preferably orthogonally.

19. A device according to any of the preceding claims, wherein the pitch of the prisms in the prismatic surface structure is in the range 1-100μm, preferably 5-

40μm.

20. A device according to any of the preceding claims, wherein the facets of the prismatic surface structure extend at substantially 45° to the plane of the substrate. 21. A device according to any of the preceding claims, wherein the facets are arranged at substantially 90° to each other.

22. A device according to any of the preceding claims, wherein the prismatic surface structure is formed in at least two regions with different refractive indices.

23. A device according to claim 22, wherein the regions are defined by different resin materials or similar resin materials with different refractive indices.

24. A device according to claim 22 or claim 23, wherein one of said regions defines an image and the other of said regions defines a background surrounding the one region, the refractive index of the one region being higher than that of the other region such that when the security device is viewed normally both regions reflect light so that the device will appear uniform, when the device is viewed from a first oblique angle the other region will become transparent while the one region remains reflecting, and when the device is viewed from a second oblique angle both regions will appear transparent.

25. A device according to any of claims 22 to 24, wherein the difference in refractive index between the two regions is at least 0.1, preferably at least 0.15 and most preferably at least 0.2.

26. A device according to any of the preceding claims, wherein the refractive index of the material defining the prismatic surface structure is preferably at least 1.4, more preferably at least 1.5, and most preferably at least 1.6. 27. A device according to any of the preceding claims, having a thickness no greater than 50μm, preferably no greater than 40μm.

28. A security document provided with a security device according to any of the preceding claims.

29. A security document according to claim 28, wherein the security device defines one of a patch, stripe or thread, for example a windowed thread.

30. A security document according to any of claims 28 or claim 29, wherein the security device defines or is provided with an image or indicia relating to another image or indicia on the security document.

31. A document according to any of claims 28 to 30, wherein the security device is located over all or part of an underlying image or images on the document which is or are revealed when the security device is viewed at the second viewing angle.

32. A document according to claim 31 , wherein the L* value of the or each underlying image is less than 50, preferably less than 25, most preferably less than 10.

33. A document according to claim 31 or claim 32, wherein the or each underlying image has a softened edge.

34. A security document according to any of claims 31 to 33, when dependent on at least claim 22, wherein the first region of the security device overlies a first image and the second region of the security device overlies a second image.

35. A security document according to any of claims 31 to 34, wherein the images which the security device overlies are selected from patterns, symbols, alphanumeric characters, serial numbers and the like.

36. A security document according to any of claims 28 to 35, wherein the security device is overprinted with indicia.

37. A security document according to claim 36, wherein the overprinted indicia is in register with other indicia on the security document.

38. A security document according to claim 36 or claim 37, wherein the overprinted indicia extends continuously from the surface of the security document over the security device.

39. A security document according to any of claims 36 to 38, when dependent on any of claims 31 to 35, wherein the overprinted indicia are registered with an image or images under the security device.

40. A security document according to claim 39, wherein the indicia printed over the security device define a pattern complementary to a pattern underneath the security device.

41. A security document according to claim 40, wherein elements of one of the patterns fall within gaps between elements of the other of the patterns.

42. A security document according to any of claims 36 to 41 , wherein the overprinted indicia are provided using a functional ink, for example a fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive or piezochromic ink.

43. A security document according to any of claims 28 to 42, wherein the security device is embossed with a non-diffractive line structure. 44. A security document according to claim 43, wherein the embossed line structure is in register with indicia on the security document.

45. A security document according to any of claims 28 to 44, wherein the adhesive layer is applied in a pattern such that one or both of the first and second parts define an, preferably identifying, image or images related to an image on the document.

46. A security document according to claim 45, wherein said related image on the document is located under the security document.

47. A security document according to any of claims 28 to 46, wherein the security device is provided in or on a window of the security document.

48. A security document according to claim 47, wherein the window is transparent so that the security device can be viewed from either side of the document.

49. A security document according to any of claims 28 to 48, wherein the document is made of paper or plastics.

50. A security document according to any of claims 28 to 49, the document comprising a document of value such as a banknote.

51. A method of manufacturing a security device, the method comprising forming a prismatic surface structure on a transparent substrate; and applying an adhesive around the perimeter of one surface of the substrate such that the security device can be adhered to a further substrate, wherein at least some parts of the said surface are exposed to the air whereby those parts will be spaced from the further substrate in use to define a transparent substrate/air interface such that corresponding parts of the prismatic surface structure form a reflector due to total internal reflection when viewed at at least one first viewing angle and are transparent when viewed at at least one second viewing angle.

52. A method according to claim 51 , wherein the adhesive is applied to the surface of the transparent substrate provided with the prismatic surface structure.

53. A method according to claim 51 or claim 52, wherein the transparent substrate is removably attached to a carrier, such as a polymeric film.

54. A method according to claim 53, wherein the carrier has a thickness of at least 12μm.

55. A method according to any of claims 51 to 54, wherein the step of forming a prismatic surface structure comprises providing at least two materials, for example resins, of different refractive indices on the substrate in laterally spaced regions; and impressing the prismatic surface structure into the materials.

56. A method according to claim 55, wherein the step of providing the at least two materials comprises printing, using at least one non-contact printing process, the materials onto the substrate. 57. A method according to claim 55 or claim 56, further comprising curing the at least two materials following the impressing step.

58. A method of manufacturing a security device according to any of claims 51 to 57, the security device being constructed in accordance with any of claims 1 to 27.

Description:

SECURITY DEVICE BASED ON CUSTOMISED MICROPRISM DEVICE

The present invention relates to improvements in security devices that can be used in varying shapes and sizes for various authenticating or security applications, particularly a device comprising a prismatic film customised to display identifying information.

Background

Security documents such as banknotes now frequently carry optically variable devices such as diffraction gratings or holographic optical microstructures as a security feature against copy and counterfeit. This has been motivated by the progress in the fields of computer-based desktop publishing and scanning, which renders conventional security print technologies such as intaglio and offset printing more prone to attempts to replicate or mimic. Examples of such holographic structures and their manufacturing techniques can be found in EP0548142 and EP0632767 filed in the name of De La Rue Holographies Ltd.

The use of diffraction gratings or holographic optical microstructures has become more prevalent in recent years and consequently the underlying component technologies/sciences have become increasingly accessible to would be counterfeiters.

Optically variable devices can also be created using non-holographic micro- optics. One advantage is that mechanical copying of micro-optical components, such as microprisms, typically with a size range of 1-50μm, is very difficult to achieve because any variation in dimension or geometrical distortion leads to a decline or extinction of the required optical properties.

The use of prismatic films to generate optical security devices is known. A grooved surface, a ruled array of tetrahedra, square pyramids or corner cube structures are examples of prismatic structures observed in such films. There is a significant volume of prior art on devices that utilise the retroreflective nature of prismatic structures. One example is EP1047960, which describes a reflective article with a concealed retroreflective pattern in which indicia are substantially hidden under normal viewing conditions but easily detectable under retroreflective lighting conditions. The general use of such devices is limited because in order to

ensure correct verification of the hidden image the use of a directional light beam source is required which is typically in the form of handheld viewer.

An alternative application of prismatic structures in the field of optical security articles has been described in US5591527. In the preferred embodiment a substantially totally internal reflecting film, defined by a series of parallel linear prisms having planar facets, is adhered to a security document. A film comprising a plurality of parallel linear prisms can be used to produce an optically variable device using the phenomena of total internal reflection (TIR). A cross- section of a prismatic film defined by a series of parallel linear prisms is illustrated in Figure 1. First consider the case where the film in Figure 1 is viewed such that the light is incident upon the smooth surface i.e. the prismatic array is in a "prisms-down" configuration relative to the viewer. When the angle between facets is 90°, light incident upon the smooth surface at an angle G 1 to the normal of the smooth surface (ray 1) will be totally internally reflected at each face of the prism and exit back through the smooth surface when the incident light is refracted by the smooth surface and then strikes the facets of the structured surface (points a and b) at angles Ci 1 and α 2 respectively, with respect to the normal of the facet, which are greater than the critical angle. The critical angle for a material, in air, is defined as the arc sine of the reciprocal of the index of refraction of the material. In addition, a significant portion of the incident light striking the smooth surface at an angle Q 2 to the normal of the smooth surface which produces refracted light that strikes the structured surface, for example at point c, at an angle, βi, less than the critical angle will be transmitted through the prismatic film (ray 2) and the remainder of the incident light will be reflected by the smooth surface. The switch angle, θ spd , for the prisms-down configuration is the smallest angle of incidence with respect to the normal of the smooth surface at which the incident light is not totally internally reflected within the prism structure. The prismatic film in Figure 1 , when in the prisms-down configuration, exhibits an optical switch by being alternatively totally reflecting (bright "metallic" appearance) at angles of view less than the switch angle or transparent at angles of greater than the switch angle. In the totally reflecting state the film will exhibit a bright "metallic" appearance (i.e. exhibiting a lustre similar to that of metals), which is solely a result of the high reflectivity of the prismatic film. The film does not require a physical metallic layer, for example a vapour deposited metallised layer or a layer of metallic ink, to generate the bright metallic appearance.

In order to achieve TIR at the planar facet boundary in Figure 1 the prism material must have a higher refractive index than the neighbouring material contacting the facets. US5591527 indicates that the change in refractive index at the planar facet boundary in Figure 1 should be at least 0.1 Rl units and more preferably at least 0.7Rl units. In the security article in US5591527 a significant refractive index difference is obtained by using a separation layer between the adhesive and the prismatic film to provide air pockets. In one embodiment the separation layer is provided in the form of an image in order to create a "flip-flop" image that is only viewable when the angle of view is greater than the critical angle.

Now consider the case where the film in Figure 1 is viewed such that the light is incident upon the faceted surface i.e. the prismatic array is in a "prisms-up" configuration relative to the viewer. Light incident at an angle θ 3 to the normal of the smooth surface (ray 3) is refracted by the faceted surface and then strikes the smooth boundary (point d) at an angle β 2 , with respect to the normal of the smooth boundary, which is less than the critical angle and therefore a significant portion of the incident light is transmitted through the prismatic film. In contrast light incident in a direction substantially parallel to the normal of the faceted surface (ray 4) at an angle θ 4 to the smooth surface is refracted by the faceted surface and then strikes the smooth boundary (point e) at an angle α 3 , with respect to the normal of the smooth boundary, which is greater than the critical angle and therefore undergoes TIR and exits the prismatic film through the faceted surface at point f. The switch angle, θ spu , for the prisms-up configuration is the smallest angle of incidence with respect to the normal of the smooth surface at which incident light is totally reflected by the prismatic structure. It should be noted that for the prisms-up configuration TIR only occurs for a limited angular range above θ spu , and for angles of incidence exceeding this range the film switches back to being substantially transparent. The prismatic film in Figure 1 , when in the prisms-up configuration, exhibits an optical switch by being substantially transparent at angles of view less than the switch angle and becoming totally reflecting (bright "metallic" appearance) at the switch angle and for a limited range above the switch angle and returning to a transparent appearance for angles of view exceeding this range.

A similar type of device to the one described in US5591527 is disclosed in patent applications WO03055692 and WO04062938. In this example a light-transmitting

film with a high refractive index is adhered to a product or document where one surface of the high refractive film has a prismatic structure. The film is placed over an image in the form of a legend, picture or pattern such that when viewed along the normal to the document the prismatic film is opaque and conceals the image but when viewed at an oblique angle the prismatic film is light transmitting allowing the image to be observed. In order for the device in WO03055692 and WO04062938 to function the prismatic material must have a significantly higher refractive index than the neighbouring material typically the adhesive used to adhere it to the document. A refractive index difference of at least 0.6 between the prismatic material and the adhesive is required for a device comprising an array of linear prisms as shown in the prior art. Polymeric adhesives/coatings typically have a refractive index of no less than 1.4 and therefore in order to achieve a difference of at least 0.6 the refractive index of the prismatic material must be at least 2. Prismatic films are typically made from UV curable polymers or thermoplastic polymeric films which have refractive indices in the range 1.4- 1.6. It is very difficult and expensive to increase the refractive index of a polymeric material above 1.7, and therefore the cited prior art does not provide a practical solution.

The security devices described in US5591527, WO03055692, and WO04062938 exhibit a distinct optical switch that is viewable in ambient light and therefore provides an advantage over the retroreflective devices that typically requires handheld viewers. However the examples in the prior art either require complex production processes, as with the addition of a separation layer in US5591527 or the requirement of an expensive and non-practical high refractive index material as in WO03055692 and WO04062938. The first aspect of the current invention solves the problems of the prior art by providing an optically variable device based on a prismatic film with a simple and easy to manufacture construction where the device is only adhered to the document at its perimeter leaving the central region free of adhesive such that a prism/air interface is maintained to generate total internal reflection.

Furthermore the devices described in the cited prior art contain only a simple on- off switch, i.e. the regions containing the prismatic structures switch from totally reflecting to transparent at the same specified angle, which limits the extent to which they can be customised. This limitation provides an advantage to the

counterfeiter who only requires to produce one generic prismatic film that can be used to counterfeit a whole range of security devices. The second aspect of the current invention solves the problems of the prior art by providing an optically variable device based on a prismatic film with a simple and easy to manufacture construction where the device is only adhered to the document at its perimeter leaving the central region free of adhesive such that a prism/air interface is maintained to generate total internal reflection and where the prismatic film comprises regions with different refractive indices creating different optically variable effects across the device enabling the creation of a unique customised prismatic film for each security application.

JP-A-2004-262144 describes another example of a prismatic film being used as a security device. In this case, the trough portions of the prisms are partially filled with a colouring resin to form patterns. In one example, the adhesive is provided on the film in a pattern instead of the colouring resin. The problem with this structure is that it does not provide a particularly durable secure device since it can be detached from the document on which it has been provided and suffers from the risk of ingress of dirt and solvent.

Summary of Invention

In accordance with a first aspect of the present invention, a security device comprises a transparent substrate provided with a prismatic surface structure defining an array of substantially planar facets, wherein an adhesive layer extends around the perimeter of one surface of the substrate such that the security device can be adhered to a further substrate, wherein at least some parts of the said surface are exposed to the air whereby those parts will be spaced from the further substrate in use to define a transparent substrate/air interface such that corresponding parts of the prismatic surface structure form a reflector due to total internal reflection when viewed at at least one first viewing angle and are transparent when viewed at at least one second viewing angle. The inventors have realized that it is possible to utilize an adhesive layer around the perimeter of the device without placing significant constraints on the appearance of the device, particularly if the adhesive is transparent, so as to overcome the problems mentioned above. At the same time, the performance of the device is not significantly harmed, by ensuring the provision of the said air gap.

In this specification, by "transparent" we mean that light can pass through the substrate although in some cases it may be attenuated.

In most cases, in use, the security device will be provided on a security document in the prisms down orientation but it is also possible to provide the security device in the prisms up orientation.

The prismatic surface structure may be provided directly in a surface of the transparent substrate or in a further layer, such as a film, attached to the substrate.

The viewing angle can be varied by tilting and/or rotating the device.

In one example, the security device comprises a substantially transparent layer having a prismatic surface structure on one side with a constant refractive index consisting of an array of substantially planar facets. An adhesive is applied to the perimeter of the security device. The device is applied to a security document such that on viewing the device the prismatic structured regions are in the prisms- down configuration and those not in contact with the adhesive will switch from totally reflecting (brightly "metallic") to transparent as the sample is tilted away from normal incidence. The adhesive can be applied such that the adhesive-free areas of the prismatic structure, which exhibit an optical switch, form an identifying image.

In some cases the prismatic surface structure is formed directly in a surface of the substrate while in other cases it is formed in a layer on the substrate.

In further examples of a second aspect, the security device comprises a substantially transparent layer having a prismatic surface structure on one side, preferably comprising of two or more regions with different refractive indices, consisting of an array of substantially planar facets. An adhesive is applied to the perimeter of the security device. The device is applied to a security document such that on viewing the device the prismatic structured regions are in the prisms- down configuration and those not in contact with the adhesive will switch from totally reflecting (brightly "metallic") to transparent as the sample is tilted away from the normal. The difference in refractive index between the two regions of the

prismatic film is such that they switch from totally reflecting to transparent at different viewing angles.

The optical variability with viewing direction of the two regions can be used to customise the security device. One of the regions could define an identifying image and the second region will form the background. The region defining the identifying image has a higher refractive index than the region defining the background such that the switching angle (θ spd ) for the identifying image is greater than the switching angle (θ spd ) for the background. When viewed at normal incidence the device will appear uniform as both the background and the image will be brightly reflecting with a "metallic" appearance. If the device is now tilted, with the viewing direction perpendicular to the long axes of the linear prisms, the background will switch from brightly reflecting to transparent when the angle of view is greater than the switching angle (θ SPd ) defining TIR for Region 1 , but the image will remain "metallic" until the device is tilted further such that the angle of view is now greater than the switching angle (θ spd ) defining TIR for Region 2 at which point the device will appear uniformly transparent. In this manner the security device can be made where a positive "metallic" latent image is seen to appear and disappear on tilting and where the image is hidden within a metallic background at one viewing angle and a substantially transparent background at a second viewing angle.

Examples of prismatic structures suitable for the current invention include but are not limited to a series of parallel linear prisms with planar facets arranged to form a grooved surface, a ruled array of tetrahedra, an array of square pyramids, an array of corner-cube structures, and an array of hexagonal-faced corner-cubes.

An array of parallel linear prisms is one of the preferred prismatic structures for the current invention because it has very high reflection efficiency and a mirror- like finish and therefore will appear strongly "metallic" within the angular range where the conditions for TIR are satisfied. For a device containing a one- dimensional linear prism structure the viewing angle at which TIR occurs will depend on the angle of rotation of the device in its plane. Two-dimensional prismatic structures such as square pyramids and corner-cubes are less sensitive to the rotation of the substrate, but such structures are not as efficient reflectors as an array of parallel linear prisms with TIR failing at some locations on the

facets. However the switch from the reflective to the transparent state as the angle of view is changed is still distinct enough to enable two-dimensional prismatic structures to be used in the optically variable device of the first aspect of the current invention.

The security device of the current invention can be used to authenticate a variety of substrates but is particularly suitable for application to flexible substrates such as paper and polymeric films and in particular banknotes. The security device can be manufactured into patches, foils, stripes, strips or threads for incorporation into plastic or paper substrates in accordance with known methods. Such a device could be arranged either wholly on the surface of the document, as in the case of a stripe or patch, or may be visible only partly on the surface of the document in the form of a windowed security thread. In a further embodiment the device could be incorporated into the document such that regions of the device are viewable from the both sides of the document. Methods for incorporating a security device such that it is viewable from both sides of the document are described in EP1141480 and WO03054297. Alternatively, the security device of the current invention could be incorporated into a transparent window of a polymer banknote.

Some examples of security devices and methods according to the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a cross-section through a prismatic film;

Figure 2 illustrates a film comprising a linear prism array;

Figures 3a and 3b illustrate cross-sections through first and second examples for use in the prisms down and prisms up orientation respectively;

Figure 4 is a polar plot showing the reflectivity of a typical linear prism film;

Figure 5 is a view similar to Figure 4 but for an alternative orientation of prisms;

Figure 6 illustrates an asymmetrical linear prismatic structure;

Figure 7 illustrates two polar plots relating to symmetrical and asymmetrical structures respectively;

Figure 8 illustrates a truncated asymmetrical structure;

Figure 9 is a polar plot relating to the structure shown in Figure 8;

Figure 10 illustrates a security document carrying a security patch and a security stripe in accordance with the examples of the invention;

Figures 11a and 11b illustrate the appearance of a cross-section through either the patch or stripe of Figure 10 when provided with a device according to Figure 3a or Figure 3b respectively;

Figures 12a and 12b show, in plan view, two examples of a security device according to the invention when viewed normally (Figure 12a) and off-axis (Figure 12b) respectively;

Figure 13 is a cross-section through a further example of a security device according to the invention;

Figures 14a and 14b illustrates two examples of security devices based on the Figure 13 construction when viewed normally (Figure 14a) and off-axis (Figure 14b) respectively;

Figure 15 shows an example of a security device applied to a document using an adhesive in the form of a high resolution line pattern;

Figures 16a and 16b illustrate, in plan form, an example of a security device with images in the adhesive layer when viewed normally (Figure 16a) and off-axis (Figure 16b);

Figures 17a and 17b illustrate, in plan, two further examples of security devices according to the invention;

Figures 18a and 18b illustrate, in plan view, another example of a security device when viewed normally (Figure 18a) and off-axis (Figure 18b) respectively;

Figures 19 and 20 are similar to Figures 13 and 14 but of a third example;

Figures 21a and 21b are polar plots illustrating the conditions for TIR for different refractive index materials;

Figure 22 is a cross-section through a fourth example of a security device according to the invention utilising different refractive index materials;

Figures 23a-23c illustrate the appearance of a security device according to the Figure 22 example when viewed normally, 10° tilt off-axis, and 20° tilt off-axis respectively;

Figure 24 illustrates successive stages in the production of an example of a security device according to the invention;

Figures 25a and 25b illustrate stages in transferring a security device onto a document;

Figure 26 illustrates an embodiment including an additional buffer layer;

Figures 27a and 27b illustrate the appearance of security device based on the Figure 3 construction, applied over a printed image, when viewed normally and off-axis respectively;

Figures 28a and 28b illustrate, in plan, a further example of a security device having the construction shown in Figure 3a applied over a printed image when viewed normally (Figure 28a) and off-axis (Figure 28b) respectively;

Figures 29a-29c illustrate a device based on the Figure 18 construction when viewed normally, 10° tilt off-axis, and 20° tilt off-axis respectively;

Figures 30a and 30b illustrate, in plan, an example of a security device according to the invention overlying a printed image on a document when viewed in plan (Figure 30a) and off-axis (Figure 30b) respectively;

Figures 31a-31c illustrate an example of a security device, the device when provided on a security document and viewed normally, and the device on the security document when viewed off-axis respectively;

Figure 32 illustrates an image suitable for providing underneath the security device on a document;

Figures 33a-33c are similar to Figure 29a-29c but illustrating further examples of security devices based on the Figure 22 construction;

Figures 34a and 34b are similar to Figures 27a and 27b but of a further example;

Figures 35a-35c are similar to Figures 29a-29c but of yet another example;

Figures 36a and 36b illustrate, in plan, another method of integrating a security device according to the invention into a security document when viewed in plan (Figure 36a) and off-axis (Figure 36b) respectively; Figures 37a-37b are similar to Figures 27a-27b but of a further example;

Figures 38a and 38b illustrate, in plan, another method of integrating a security device according to the invention into a security document when viewed in plan (Figure 38a) and off-axis (Figure 38b) respectively;

Figures 39a and 39b illustrate the incorporation of a security device into a transparent area of a security document;

Figure 40 illustrates the appearance of the security device of Figure 39 under different viewing conditions;

Figure 41 illustrates an example of a security device according to the invention provided as a patch over a hole or aperture in a security document;

Figure 42 illustrates the incorporation of a security device as a windowed thread;

Figure 43 is a cross-section through an example of a windowed thread suitable for the Figure 42 example;

Figures 44 and 45 are cross-sections through further examples of security devices suitable for use as windowed threads;

Figures 46a-46c illustrate the appearance of the windowed thread of Figure 32 when viewed normally, 10° tilt off-axis and 20° tilt off-axis respectively;

Figure 47 is a cross-section through an example of a security device according to the invention with orthogonally arranged linear prismatic arrays; and,

Figures 48a-48d illustrate the appearance of a security device based on the Figure 47 example when viewed normally, 10° tilt off-axis, 45° to the long axis, and 90° parallel with the long axis respectively.

Examples of prismatic structures for the current invention include both one- dimensional and two-dimensional prismatic structures. A one-dimensional structure is defined as a structure with a constant cross-section and where the surface height of the structure only varies in one direction. An example of a one- dimensional prismatic structure is a series of parallel linear prisms with planar facets arranged to form a grooved surface. A two-dimensional structure is defined as one where the surface height varies in two directions and the cross-section is not constant. Examples of two-dimensional prismatic structures include but are not limited to a ruled array of tetrahedra, an array of square-based pyramids, an array of corner-cube structures and an array of hexagonal-faced corner-cube structures. As indicated previously the above structures will be substantially reflective via TIR if the prism material has a higher refractive index than the neighbouring material contacting either the facets (prisms-down) or the smooth surface (prisms-up) and the angle of incidence upon the facets or the smooth surface exceeds the critical angle. The structure of the security device of the current invention is such that the neighbouring material will be air with a refractive index of 1.0. The refractive index of the prismatic materials is preferably greater than 1.4 and more preferably greater than 1.5, and even more preferably greater than 1.6 The higher the refractive index of the prismatic material the more efficient is the reflection efficiency and the greater is the angular range over which total internal reflection occurs.

The security device of the first or second aspect of the current invention can be further customised by having a localised prismatic surface structure preferably comprising of two or more arrays of a prismatic structure, where the reflective properties of the arrays are dependent on the angle of rotation of the layer and where the arrays are rotated relative to each other within the plane of the layer. A preferred prismatic structure for this aspect of the invention is a series of parallel linear prisms. The brightly reflecting to transparent switch of a prismatic film comprising of an array of parallel linear prisms is sensitive to the rotation of the film and is dependent on the angle between the viewing direction and the long axis of the linear prisms. Referring to the cross-section in Figure 1 , when viewed normally in the prisms-down configuration the film will be brightly reflecting with a "metallic" appearance. Figure 2 illustrates a film comprising a linear prism array based on the cross-section in Figure 1 in the prisms-down configuration. If the film is now tilted with the viewing direction perpendicular to the long axes of the

linear prisms (direction A) the film will switch from brightly reflecting to transparent when the angle of view is greater than the switching angle (θ spc j) defining TIR. However if the film is rotated such that the viewing direction is parallel to the long axes of the linear prisms (direction B) the film remains brightly reflecting with a "metallic" appearance at all viewing angles.

This variability with viewing direction can be used to customise the security device by having two arrays of a series of parallel linear prisms where the arrays are rotated relative to each other by substantially 90° within the plane of the substrate. One of the linear prism arrays could be applied in the form of an identifying image and the second array will form the background. When viewed at normal incidence the device will appear uniform as both the background and the image will be brightly reflecting with a "metallic" appearance. If the device is now tilted, with the viewing direction perpendicular to the long axes of the linear prisms forming the image, the image will switch from brightly reflecting to transparent when the angle of view is greater than the switching angle (θ spd ) defining TIR, but the background will remain "metallic" at all viewing angles. However if the device is rotated and tilted, such that the viewing direction is parallel to the long axes of the linear prisms forming the image, the image remains brightly reflecting with a "metallic" appearance at all viewing angles and the background will switch from brightly reflecting to transparent when the angle of view is greater than the switching angle (θ spc j) defining TIR. In this manner the security device can be made to reveal a negative "metallic" latent image on tilting at one rotational orientation and a positive "metallic" latent image when tilting at a second substantially perpendicular rotational orientation.

Referring now to Figure 3a there is illustrated a cross-section of a substrate typical of the construction of the current invention for use in security or authenticating devices. The construction comprises a substantially clear polymeric film 1 of polyethylene terephthalate (PET) or the like. A prismatic surface structure 2, comprising an array of substantially planar facets, is formed on one surface of the clear polymeric film. An adhesive 3 is applied around the perimeter of the prismatic array to enable the device to be adhered to a secure document (not shown) while still maintaining a prism/air interface (airgap) in the central regions of the device. When viewed from the top of the device the prismatic array is in the prisms-down configuration.

Figure 3b shows an alternative cross-section of a substrate typical of the construction of the current invention for use in security or authenticating devices. The construction again comprises a substantially clear polymeric film 1 of polyethylene terephthalate (PET) or the like. A prismatic surface structure 2, comprising an array of substantially planar facets, is formed on one surface of the clear polymeric film. An adhesive 3 is applied around the perimeter of the opposite surface of the polymeric film 1 to enable the device to be adhered to a secure document (not shown) while still maintaining a prism/air interface (airgap) in the central regions of the device. When viewed from the top of the device the prismatic array is in the prisms-up configuration. An array of parallel linear prisms is the preferred prismatic structure for the current invention because it has very high reflection efficiency and therefore will appear strongly "metallic" within the angular range where the conditions for TIR are satisfied. The prism pitch is preferably in the range 1-100μm and more preferably in the range 5-40μm and where the facets makes an angle of approximately 45° with the base substrate and the angle between the facets is approximately 90°. For a device containing an array of parallel linear prisms the viewing angle at which TIR occurs will depend on the angle of rotation of the substrate in its plane. Figure 4 is a polar plot showing the reflectivity of a typical linear prism film where the angle of rotation of the substrate in its plane is represented circumferentially and the angle of incidence light is represented radially (90° to -90°). The centre of the plot corresponds to light entering the film at normal incidence. For the example shown, the refractive index of the prism film is 1.5 and the prisms are in contact with air, which has a refractive index of ~1. In this example the prism pitch is 20μm and the prism height is 10μm. The prismatic film is oriented such that the apexes of the prism are pointing away from the viewer (i.e. prisms-down configuration). If the radius is defined as the distance of a point from the centre of the plot, then each radius corresponds to the degree of tilt away from normal incidence. The rotation angle is the angle between the direction of tilt and the long axes of the linear prisms. For example in Figure 4, arc 1 illustrates the condition where the direction of tilt is parallel to the long axes of the linear prisms and arc 2 illustrates the condition where the direction of tilt is perpendicular to the long axes of the linear prisms. The horizontal scale on the plot represents the angles of incidence along arc 2 and the vertical scale represents the angles of incidence along arc 1. For simplicity the scales representing the angles of

incidence for the other rotational orientations are not shown. In the polar plot the values at each point correspond to reflectivity where reflectivity has a value between 0 and 1 where 0 is equivalent to 0% reflectivity and 1 is equivalent to 100% "metallic" reflectivity. For the current invention, the film will be totally reflecting and exhibit a "metallic" appearance if the reflectivity is greater than 0.7 and preferably greater than 0.8 and more preferably greater than 0.9. In order to simplify the plot, the light shaded area on the diagram indicates the angular conditions at which the reflectivity is greater then 0.8 and therefore illustrates the approximate angular range exhibiting TIR. The dark shaded area in Figure 4 indicates the angular range in which the film is substantially transparent i.e. areas with a reflectivity of less than 0.4, however it should be noted that there is a small transitional area between the totally reflecting and substantially transparent states not shown in Figure 4 or any of the subsequent polar plots. The size of this transitional area is normally such that in practice the viewer will observe a sharp switch from the totally reflecting to the substantially transparent state. Figure 4 shows that when the direction of tilt is parallel to the long axes of the linear prisms (i.e. arc 1) TIR occurs at all angles of incidence, however when the direction of tilt is perpendicular to the long axes of the linear prisms TIR occurs at normal incidence and angles of incidence up to approximately 5° away from the normal. As the angle between the direction of tilt and the long axes of the linear prism changes from perpendicular to parallel the angular range at which TIR occurs increases i.e. the film remains totally reflecting at increasingly oblique angles.

Figure 5 shows an equivalent polar plot to Figure 4, using the same prismatic structure and refractive indices, for the prisms-up orientation. Figure 5 shows that when the direction of tilt is perpendicular to the long axes of the linear prisms (arc 2) TIR reflection occurs for angles of incidence between approximately 40-55° and outside this range the film is substantially transparent. However when the direction of tilt is parallel to the long axes of the linear prisms TIR occurs at a significantly more oblique angle of incidence approximately in the range 60-65°.

The device is preferably orientated such that the optical switch occurs at the preferred viewing position of the authenticator. For example on a secure document such as a banknote the device could be oriented such that the long axes of the prisms are parallel to the long axes of the banknote such that the

optical switch from totally reflecting to transparent is easily observed by tilting around the long axis of the banknote.

Two-dimensional prismatic structures such as square pyramids, cornercubes and hexagonal-faced corner cubes are less sensitive to the rotation of the substrate, but such structures are not as efficient reflectors as an array of parallel linear prisms with TIR failing at some locations on the facets. However the switch from the reflective to the transparent state as the angle of view is changed is still distinct enough to enable two-dimensional prismatic structures to be used in the optically variable device of the current invention. The facets of the two- dimensional prismatic structures are typically in the region of 1-100μm across and more preferably in the region of 5-40μm. For the square pyramids the facets are typically disposed at an angle of ~45° to the base substrate and the angle between the facets is approximately 90°. For the corner-cubes and the hexagonal-faced corner-cubes the facets are typically disposed at an angle of -55° to the base substrate and the angle between the facets is approximately 90°. One advantage of the corner-cube and hexagonal-faced corner-cube structures over an array of parallel linear prisms is that a lower refractive index difference between the prismatic material and the neighbouring material is required to exhibit TIR. For example a device comprising an array of corner-cube structures with a refractive index difference of 0.4 would exhibit total internal reflection over a greater range of viewing angles than a device comprising an array of parallel linear prisms with a refractive index difference of 0.4.

The reflective properties of an array of prismatic structures of the type described in the current invention can be modified by varying the prismatic structure such that it no longer has a symmetrical cross-section. For example consider an array of parallel linear prisms where the facets make an angle of approximately 45° with the base substrate and the angle between the facets is approximately 90°. If the structure is altered such that one of the facets makes an angle of 35° to the base substrate and the other facet makes an angle of 55° to the base substrate, as illustrated in Figure 6, the apex is shifted to create an asymmetrical structure but the angle between the facets remains at 90°. The polar plots in Figure 7 show how the angular range in which TIR occurs is altered by the creation of this asymmetrical structure when the structures are viewed in the prisms-down configuration. For this example the refractive index of the prismatic material is 1.5

which is in contact with air with a refractive index of 1. For the symmetrical structure when the direction of tilt is perpendicular to the long axes of the linear prisms (along arc 2) TIR occurs at normal incidence and angles of incidence up to approximately 2-3° away from the normal. In contrast for the asymmetrical structure, when the direction of tilt is perpendicular to the long axes of the linear prisms (along arc 2), the angular range in which TIR occurs is shifted such that it occurs for angles of incidence in the range 15-20° away from the normal. However the angular range exhibiting TIR is very small and does not offer a practical solution.

The asymmetrical linear prismatic structure in Figure 6 is limited by the fact that light incident on the longer facet close to the base substrate does not reflect back out of the prismatic film even though it undergoes TIR when incident on the longer facet. This is illustrated in Figure 6. Light ray 1 is refracted on entering the film at point a and is incident on the longer facet at an angle α to the normal such that it undergoes TIR at both the long and short facet and exits back through the smooth surface. However light ray 2 is refracted on entering the film at point b and is incident on the longer facet at the same angle α as ray 1 but at a point close enough to the base substrate that the reflected ray is now incident on the smooth surface rather than the shorter facet. Light ray 2 undergoes TIR at the smooth surface and does not exit the film and therefore is not reflected. Ray 3 is the limiting case in that it shows the location on the longer facet below which the incident light ray is no longer reflected onto the shorter facet and therefore a non- reflecting region is created. A solution to this problem is to create a truncated version of the asymmetrical structure as shown in Figure 8, in which the structure is truncated at the limiting point defined by ray 3 in Figure 6. The truncated angle φ is equal to 90-χ where χ is the angle between the normal to the smooth surface and the bisector of the apex angle as indicated on Figure 8. The polar plot in Figure 9 shows that the angular range for the truncated structure in which TIR occurs is significantly greater than the angular range for the non-truncated structure (Figure 7). For the truncated structure TIR occurs for angles of incidence between 10-20° away from the normal when viewed perpendicularly to the long axes of the linear prisms (along arc 2)

The use of a truncated asymmetrical structure enables the tilt angle at which the "metallic" to transparent switch occurs to be controlled making the device more

difficult to counterfeit and allows embodiments where different areas of the film could have different switch angles resulting in different parts of the device switching on and off as the device is tilted.

The use of asymmetrical prismatic structures is equally applicable to corner- cubes and hexagonal-faced corner-cubes. Corner-cube based structures are retroreflective and therefore the "metallic" state is best viewed when there is a light source directly behind the viewer. In most practical situations the person viewing the device will be positioned off-axis from the light source and will not easily observe the highly reflective "metallic" state. The use of asymmetric corner-cube based structures enables the divergence of the retroreflected light such that the "metallic" state can be viewed off-axis from the light source. This divergence can be achieved by having at least one facet of the corner-cube structure tilted at an angle that differs from the angle which would be required for all dihedral angles within the corner-cube structure to be orthogonal. For example one of the facets of a hexagonal corner-cube structure could be disposed at an angle of 50° to the base substrate and the other two facets disposed at an angle of 55° to the base substrate.

Films comprising a surface prismatic structure can be produced by a number of industry standard methods including UV casting, micro-embossing and extrusion. The preferred methods for the prismatic films used in the current invention are UV casting and micro-embossing.

The first stage of the UV casting process is the formation of a master structure in the form of a production tool. A negative version of the final prismatic structure is created in the production tool using well known techniques such as diamond turning, engraving, greyscale photolithography and electroforming. The production tool can typically be in the form of a sheet, a cylinder or a sleeve mounted on a cylinder. A preferred method for the production tool is diamond turning. In this process a very sharp diamond tool is used to machine a negative version of the required prismatic structure in a metallic material such as copper, aluminium and nickel.

In a typical UV casting process a flexible polymeric film is unwound from a reel, where a UV curable polymer is then coated onto the substrate film. If required, a drying stage then takes place to remove solvent from the resin. The film is then

held in intimate contact with the production tool in the form of an embossing cylinder, whereby the prismatic structure defined on the production tool is replicated in the resin held on the substrate film. UV light is used at the point of contact to cure and harden the resin, and as a final stage, the reel of flexible prismatic film is rewound onto a reel. UV casting of prismatic structures is, for example, described in US3689346.

Flexible polymeric films suitable for the UV casting process include polyethylene teraphthalate (PET), polyethylene, polyamide, polycarbonate, poly(vinylchloride) (PVC), poly(vinylidenechloride) (PVdC), polymethylmethacrylate (PMMA), polyethylene naphthalate (PEN), and polypropylene.

UV curable polymers employing free radical or cationic UV polymerisation are suitable for the UV casting process. Examples of free radical systems include photo-crosslinkable acrylate-methacrylate or aromatic vinyl oligomeric resins. Examples of cationic systems include cycloaliphatic epoxides. Hybrid polymer systems can also be employed combining both free radical and cationic UV polymerization. Further examples of polymer systems suitable for the formation of prismatic films by UV casting are given in US4576850 and US5591527.

An alternative process for the production of films comprising a surface prismatic structure is micro-embossing. Suitable micro-embossing processes are described in US4601861 and US6200399. In US4601861 a method is described for continuously embossing a corner-cube structure in a sheeting of thermoplastic material, where the actual embossing process takes place at a temperature above the glass transition temperature of the sheeting material. Suitable thermoplastic materials include polyethylene teraphthalate (PET), polyethylene, polyamide, polycarbonate, poly(vinylchloride) (PVC), poly(vinylidenechloride) (PVdC), polymethylmethacrylate (PMMA), polyethylene naphthalate (PEN), polystyrene, polysulphone and polypropylene.

In one embodiment, the current invention can be incorporated into a security document 10 as a security patch 12 or stripe 11 , as illustrated in Figure 10. Figure 11a illustrates an example cross-section of a security document 10 in the region where the security patch 12 or stripe 11 has been applied. In this example a prismatic array 13 is replicated over the whole surface of a polymeric film 14, but the adhesive 3 used to adhere the security device to the document is only

applied around the perimeter of the security device such that an air/prism interface is maintained by virtue of an air gap 15 in the central regions of the device. This construction allows the employment of conventional polymeric materials for both the adhesive 3 and the prismatic structure 13, i.e. polymeric materials having a refractive index between 1.4 and 1.7. The device will not exhibit TIR in the perimeter regions where the adhesive contacts the prism film because of the insufficient difference in the refractive index between the adhesive and the prismatic structure, but TIR will be exhibited in the central region because of the higher refractive index difference observed at the air/prism interface. In this example the prismatic array 13 consists of an array of parallel linear prisms with a prism pitch of 20μm and a prism height of 10μm. In order to ensure TIR does not occur in the adhesive regions the difference in refractive index between the adhesive and the materials forming the prismatic structure should be no greater than 0.2, and preferably no greater than 0.05 and even more preferably no greater than 0.02.

The adhesive 3 is provided adjacent to the boundary of the security device such that it extends to all parts of the boundary. The adhesive may be applied as continuous solid layer or as a screen. A particularly convenient screen is defined by an array of dots, although other screens such as linear, circular or sinusoidal could be used. Indeed, the term "screen" should be construed broadly to encompass many different shapes of screen elements.

The adhesive 3 forms a complete perimeter layer. The presence of a complete perimeter layer of adhesive such that the device is uniformly sealed to the substrate around its perimeter improves the durability of the device when the secure document 10 is being handled by the general public. The sealed perimeter area provides a barrier to chemicals penetrating under the prismatic film and degrading the observed "metallic" effect generated by TIR. In addition a complete perimeter adhesive layer results in no part of the edge of the device being accessible to act as an initiation point to tear the device off the document. Preferably the width of the perimeter adhesive layer is at least 1mm and preferably at least 3mm.

The device construction in Figure 11a comprises a prismatic array 13 formed on one surface of the clear polymeric film 14 where the prismatic array is an array of linear parallel prisms and the refractive index of the material forming the prismatic

array has a sufficiently high refractive index that total internal reflection occurs at the prism/air interface over certain angles of view. When viewed from the top of the security device the prismatic array 13 is in the prisms-down configuration relative to the viewer and the passage of light though the structure is as defined for rays 1 and 2 in Figure 1 when viewed perpendicularly to the long axes of the linear prisms. A light ray travelling along direction C is incident on prismatic array 13 at an angle of incidence that is less than the switching angle θ spd and all the light is reflected. If the device is now tilted such that the light is travelling along direction D, the angle of incidence on the prismatic array is now greater than the switching angle θ spd and therefore the majority of the light is transmitted via refraction.

The optical properties of the prismatic array enables an optically variable effect to be generated such that on viewing the device in Figure 11a from above the secure document 10 and normal to the plane of the clear polymeric film 14 (direction C) the prismatic array 13 is totally reflecting and appears "metallic". If the device is now tilted such that the light is travelling along direction D, the angle of incidence on the prismatic array 13 is now greater than the switching angle θ spc ι and therefore the majority of the light is transmitted via refraction and the prismatic array appears transparent.

In an alternative embodiment, illustrated in Figure 11b, the device could be applied to the document such that when viewed from the top of the security device the prismatic array 13 is in the prisms-up configuration relative to the viewer and the passage of light though the structure is as defined for rays 3 and 4 in Figure 1 when viewed perpendicularly to the long axes of the linear prisms. A light ray travelling along direction C is incident on the prismatic array 13 at an angle less than the switching angle θ spu and therefore the majority of the light is transmitted via refraction. If the device is now tilted such that the light is travelling along direction D such that the angle of incidence is now greater than the switching angle θ spu and within the angular range for TIR all of the light is reflected by prismatic array 13 which appears "metallic". The following illustrative examples in the remaining Figures are for when the device is adhered to the document such that the prisms are viewed in the prisms-down configuration (Figure 11a), but equivalent examples are possible when the prisms are viewed in the prisms-

up configuration with the only difference being that the prismatic structure is in its totally reflecting or transparent state at different angles of view.

Figure 12 shows, in plan view, two examples of the device illustrating possible formats in which they can be applied to the secure document. In one example the security device is in the form of a rectangle 20 and is adhered to the document using an adhesive 3 along all four edges of the device such that the adhesive forms a complete perimeter around the device. The refractive index difference between the prismatic material and the adhesive is such that TIR does not occur at the prism/adhesive interface, and therefore the optical effect is not generated within the perimeter region with the device remaining transparent at all angles of view. In a second and preferred embodiment a prismatic array 21 is replicated onto a clear polymeric film and the adhesive 3 is applied such that the regions where the prism/air interface is maintained is in the form of an identifying image or images. In the example shown in Figure 12 the image is a star. Preferably, as shown in the example in Figure 12, the adhesive layer still covers a complete perimeter around the edge of the device.

When the secure document is viewed normally along direction C the prismatic array, in the form of the rectangle or the star, is totally reflecting and appears "metallic" On tilting the document, such that the tilt angle is greater than θ spd , and viewing along direction D perpendicular to the long axes of the prisms, the prismatic array, in the form of the rectangle or the star, appears substantially transparent. The appearance of the perimeter adhesive layer remains constant irrespective of viewing angle, and preferably the adhesive is a substantially transparent layer

The following examples illustrated in Figures 13-20 are based on a linear prismatic array where the linear prisms have a pitch of 20μm and a prism height of 10μm. The described features are achieved when tilting the device and viewing the linear prisms such that their long axes are perpendicular to the viewing direction.

Figure 13 shows a further embodiment of a security device, shown in cross- section, of the current invention applied to the surface of a security document 29. In this example a prismatic array 30 is replicated over the whole surface of a polymeric film 31 , but the adhesive used to adhere the security device to the

document is applied around the perimeter 32 of the security device and in localised areas 33 within the central areas of the device such that an air/prism interface is maintained by virtue of air gaps 34, and therefore TIR is exhibited, in the central regions of the device without the adhesive. The use of a patterned adhesive in the central region of the device enables the creation of more complex patterns which switch from a totally reflecting "metallic" appearance to substantially transparent on tilting.

Figure 14 shows two example designs created by the use of localised areas of adhesive in the central region of the device. In both examples the adhesive is applied to form a complete layer 3 around the perimeter. In addition in the first example the adhesive is applied in the form of stars 33, such that when viewed at normal incidence (Figure 14a) the stars will not exhibit TIR and will remain transparent appearing as a negative image defined by a totally reflecting "metallic" background resulting from the TIR exhibited by the prismatic structures in the adhesive-free regions. On tilting the device (Figure 14b) the background switches from totally reflecting to transparent with the stars remaining transparent such that they disappear into the background. In the second example the adhesive has also been applied in the central region with the adhesive-free regions defining a portrait 34, such that the portrait switches from "metallic" to transparent on tilting the device while the background remains transparent at all angles of view (Figures 14a and 14b).

In a further embodiment the adhesive is printed in a design comprising a fine line spacing. The switching effect from totally reflecting to transparent will be visible between the spacings of the printed adhesive lines and therefore provides the appearance of a fine-line structure. This enables the creation of prismatic totally reflecting high-resolution line patterns. High resolution line patterns are commonly observed on banknotes and referred to as filigree line structures. The line spacings are preferably in the range 50-2000μm and even more preferably in the range 100-1000μm. Figure 15 shows an example of a security device applied to the document using an adhesive which is applied to form a high resolution line pattern. In Figure 15 the device is illustrated in its totally reflecting state such that a metallic pattern is observed against a transparent background.

The adhesive layer can be applied to the film using any of the standard security printing processes including one or all of the following; wet or dry lithographic

printing, intaglio printing, letterpress printing, flexographic printing, screen printing, and/or gravure printing. Preferably the adhesive is applied using screen- printing. The total thickness of the adhesive layer is dependent on the height of the prism structure, but typically the thickness of the adhesive layer measured from the top of the prismatic structure will be in the range 1-1 Oμm.

In the examples illustrated in Figures 13,14 and 15 it is also envisaged that the adhesive in the central regions of the device can be replaced with a standard printing ink or resin. In this type of structure the adhesion is provided by the perimeter adhesive layer and the function of the printing ink or resin is simply to prevent TIR from occurring by substantially matching the refractive index of the material used to form the prismatic array. As with the adhesive the difference in the refractive index of the printing ink and the prismatic structure should be no greater than 0.1 , and preferably no greater than 0.05 and even more preferably no greater than 0.02. The advantage here is that the printing ink only has to cover the prismatic structure and does not have to be so thick that it fills the grooves of the prismatic structure and extends beyond them to provide an adhesive bond. The use of a thinner layer enables the creation of finer structures and more detailed designs. Taking Figure 15 as an example the perimeter layer could be screen-printed while the high-resolution line pattern can be applied using lithographic printing.

In a further embodiment negative designs are created within the perimeter adhesive layer, i.e. the adhesive is omitted in certain regions. For viewing conditions where the prismatic film is totally reflecting, for example when viewing from normal incidence, the negative images in the adhesive layer will be totally reflecting and appear "metallic" against the transparent background of the adhesive layer. On tilting the device to a viewing condition where the prismatic film is not totally reflecting the negative images in the perimeter adhesive layer switch from totally reflecting to transparent and are indistinguishable from the rest of the perimeter layer. Figure 16 illustrates an example where images are created within the perimeter adhesive layer. The device is constructed as shown in Figure 3a, except that in the corner regions of the device adhesive is omitted in localised regions 200 within the perimeter adhesive layer 202 to define the numeral "50". When viewed at normal incidence (Figure 16a) the central region 204 of the device, which is adhesive-free, appears "metallic" due to TIR. When

viewed at normal incidence the perimeter region 202 appears substantially transparent apart from the adhesive-free numerals "50" 200 which appear metallic due to TIR. On tilting the device away from normal incidence (Figure 16b) the central region 204 and the numerals "50" 200 switch from totally reflecting to transparent providing the device with a uniform transparent appearance.

The area coverage of the adhesive-free regions should be sufficiently small such that it has negligible impact on the adhesion of the device to the secure document. Preferably the area coverage of the adhesive-free regions within the perimeter layer should be less than 50% and more preferably less than 40% and even more preferably less than 30%. In calculating this area the width of the perimeter layer (x) is defined as the smallest distance between the edge of the device and the edge of the main image(s) in the centre of the device. The distance (x) is illustrated in Figures 17a and 17b for two different examples of images 206,208 defined negatively by the adhesive. Preferably the designs are in the form of images such as patterns, symbols and alphanumeric characters and combinations thereof. Possible characters include those from non-Roman scripts of which examples include but are not limited to, Chinese, Japanese, Sanskrit and Arabic. The stem width of the characters are preferably in the range 50-1500μm but more preferably in the range 100-1000μm. The adhesive may also comprise discontinuous regions which may include for example line patterns, fine filigree line patterns, dot structures and geometric patterns. Preferably the adhesive-free regions within the perimeter layer should not break the continuous adhesive layer around the device.

The adhesive layer or the materials used to generate the prismatic structures in any of the embodiments can contain functional components that react to an external stimulus. Components of this type include, but are not limited to, fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic. Figure 18 illustrates an example where the adhesive contains a fluorescent material such that a visible colour is observed when viewed under UV illumination. The example in Figure 18 is the same as the first example illustrated in Figure 14 except that the adhesive contains a fluorescent material that appears red when viewed under UV illumination. The adhesive is applied as a perimeter layer 3' and in the form of stars 33' in the central region, such that when viewed at normal incidence (Figure

18a) and under UV illumination the perimeter and the stars will appear red while the rest of the device is totally reflecting and appears metallic. On tilting the device, and viewing under ambient conditions, the adhesive-free areas switch from totally reflecting to transparent with the stars and the perimeter remaining transparent such that they disappear into the background. However when tilted and viewed under UV illumination (Figure 18b) the stars and the perimeter layer appear red and are readily apparent. The example in Figure 18 provides a further advantage to the one shown in Figure 14 in that the negative identifying image that is observed to disappear on tilting, when viewed under ambient conditions, reappears as a positive image when the tilted device is viewed under UV illumination In a further embodiment the adhesive could be applied as a screen rather than a continuous coating. A particularly convenient screen is defined by an array of dots, although other screens such as linear, circular or sinusoidal could be used. Indeed, the term "screen" should be construed broadly to encompass many different shapes of screen elements. Figures 19 and 20 illustrate an example where a screened adhesive 40 is used to adhere central regions of a security device 41 to a secure document substrate 42. As with the previous embodiments a complete layer of adhesive 43 is applied around the perimeter of the device. In the central region of the device adhesive 40 is applied to a prismatic film 44 in the form of a screen such that it defines the image of a star. In this example the screen comprises an array of dots with a size and period that cannot be resolved with the naked eye. When viewed at normal incidence (Figure 20a) the star will be totally reflecting and appear "metallic" in the spaces between the adhesive dots where an air gap 45 defines an air/prism interface and will appear transparent where the adhesive dots are present. The small non- resolvable size of the adhesive dots and the totally reflecting "metallic" spaces between the dots results in the device appearing uniformly "metallic". The reflectivity of the star will not be as great as the adhesive-free background region due to the presence of the small non-reflecting regions, and therefore an additional optical variability is introduced into the sample leading to an increase in the counterfeit resistance. On tilting the device (Figure 20b) the background and the adhesive-free regions of the stars switch from totally reflecting to transparent such that the device now appears uniformly transparent. The advantage of using a screened adhesive in at least a part of the central region of the device is that the adhesion and durability is improved compared to a sample with just an

adhesive layer around the perimeter. In a further embodiment a screened adhesive can be applied all over the device although this would compromise the reflectivity of the device.

The designs generated by the application of the adhesive are preferably in the form of images such as patterns, symbols and alphanumeric characters and combinations thereof. Possible characters include those from non-Roman scripts of which examples include but are not limited to, Chinese, Japanese, Sanskrit and Arabic.

The angle at which the security device of the current invention switches from a totally reflecting "metallic" state to a substantially transparent state can be varied by changing the refractive index difference between the prismatic film and the neighbouring material. In the current invention the neighbouring material is air and therefore the change in the switching angle is achieved by varying the refractive index of the material used to form the prismatic film. Figure 21 highlights how the switching angle θ spd for an array of linear prisms at a certain rotational orientation can be modified by varying the refractive index. In both examples the prism pitch is 20μm and the prism height is 10μm. The prismatic film used to generate the polar plot in Figure 21a has a refractive index of 1.5 and the prismatic film used to generate the polar plot in Figure 21b has a refractive index of 1.7. In both cases the prismatic film is oriented such that the apexes of the prism are pointing away from the viewer (i.e. prisms-down configuration). Figure 21a shows that the switch angle θ spd , when tilted perpendicularly to the long axes of the linear prisms, is approximately 3° when the prismatic film has a refractive index of 1.5 and there is a prism/air interface. Figure 21b shows that by increasing the refractive index of the prismatic film to 1.5 and maintaining the prism/air interface the switch angle θ spd , when tilted perpendicularly to the long axes of the linear prisms, has been increased to 15°.

Moving the switch angle further away from normal incidence is beneficial as it provides a greater range of angles over which the material is totally reflecting and appears "metallic". In a second aspect of the current invention the prismatic film comprises at least two regions which form a prism/air interface where the refractive index of the at least two regions are different. The difference in refractive index of the two prismatic regions enables the generation of an optically

variable security device based on a prismatic film where different regions of the film exhibit a different optically variable effect.

Figure 22 shows another example device structure according to the current invention. The construction comprises a substantially clear polymeric film 50 of polyethylene terephthalate (PET) or the like. Regions 1 , each comprising a prismatic surface structure 51 comprising an array of substantially planar facets, are formed from resin 1 on one surface of the clear polymeric film 50 (shown shaded in Figure 22). Regions 2, each comprising a prismatic surface structure 52 comprising an array of substantially planar facets, are formed from resin 2 on the same surface of the clear polymeric film 50 as regions 1 in the areas not occupied by regions 1. An adhesive 53 is applied to the edge regions of the prismatic array to enable the device to be adhered to a secure document 54 while still maintaining a prism/air interface 55 (airgap) in the central regions of the device. When viewed from the top of the device the prismatic array is in the prisms-down configuration. Resin 1 has a refractive index of 1.5 and therefore the variation of reflectivity with viewing angle is as shown in Figure 21a. Resin 2 has a refractive index of 1.7 and therefore the variation of reflectivity with viewing angle is a shown in Figure 21b.

The difference in the switching angle (θ spd ) of the prismatic structure in region 1 compared to the prismatic structure in region 2 can be used to generate a customised security device. Figure 23 shows an example switching sequence in which resin 2 has been applied in the form of a portrait 60 defining region 2 and resin 1 has been applied to form the background defining region 1. At normal incidence (Figure 23A) both the portrait and the background are totally reflecting and the device appears "metallic" concealing the portrait. On tilting the device a few degrees off-axis (~10°) (Figure 23B) and viewing perpendicularly to the long axes of the linear prisms the background switches to substantially transparent but the portrait remains "metallic" and is therefore revealed. On tilting further off-axis,

(-20°) (Figure 23C) the portrait also switches to substantially transparent and is hidden within a uniform transparent film.

The complexity of the optically variable nature of the Figure 22 device can be increased by having multiple regions which switch from "metallic" to transparent

at different angles of view by forming each region from a resin with a different refractive index.

For the case with two prismatic regions exhibiting different switching angles (θ spd ) at a prism/air interface, then the refractive index difference between resin 1 and resin 2 should preferably be at least 0.1 and more preferably at least 0.15 and even more preferably at least 0.2. Increasing the refractive index difference between resin 1 and resin 2 increases the angular range over which the image defined by region 2 (formed by resin 2) is viewable and therefore aids authentication.

In order to achieve the refractive index differences between resin 1 and resin 2 and produce a functioning device careful material selection is required. Resin 1 is preferably a standard UV curable polymer employing free radical or cationic UV polymerisation suitable for the UV casting process typically have refractive indices in the range 1.4-1.6. One particular base monomer, with a refractive index of 1.5, suitable for the UV casting process via free radical polymerisation is an ethoxylated (3) trimethylolpropane triacrylate supplied by Sartomer under the name SR454-HP.

If resin 1 has a refractive index of 1.5 then resin 2 should have a refractive index of at least 1.6 but more preferably 1.7. A refractive index of 1.7 for UV curable polymers can be obtained by using UV curable monomers/oligomers with highly conjugated (ring-) structure, heavy element substitution (Br, I), high functionality and high molecular weight. In addition suitable high refractive index materials for the current invention include inorganic-organic hybrids where high refractive index inorganic nanoparticles, for example TiO 2 , are dispersed in a polymer resin suitable for UV casting to produce a transparent high refractive index coating. The polymer resin would be chosen such that it is suitable for UV casting and examples include photo-crosslinkable acrylate or methacrylate oligomeric resins. Examples of cationic systems include cycloaliphatic epoxides. Hybrid polymer systems can also be employed combining both free radical and cationic UV polymerization. Further examples of polymer systems suitable for the formation of prismatic films by UV casting are given in US4576850 and US5591527. Methods for dispersing inorganic nanoparticles into polymer systems suitable for UV casting are described in US20020119304, US6720072 and WO02058928.

The prismatic structures of the current invention may also be formed from inorganic materials. Inorganic materials have the advantage of exhibiting a greater range of refractive indices. Suitable techniques for generating a prismatic structure from an inorganic material include oblique vapour deposition or controlled etching along specific crystallographic planes.

Figure 24 illustrates a preferred production process for a device corresponding to that of Figure 22. A master prismatic structure is created in a production tool using well known techniques such as diamond turning, engraving, greyscale photolithography and electroforming. The production tool can typically be in the form of a sheet, a cylinder or a sleeve mounted on a cylinder. A preferred method for the production tool is diamond turning. In this process a very sharp diamond tool is used to machine a negative version of the required prismatic structure in a metallic material such as copper, aluminium and nickel. The prismatic structure is preferably a uniform array with the same prismatic structure being used for region 1 and region 2. One advantage of this structure is that the customisation arises from the refractive index difference of the two resins forming the prismatic structures and not structural differences in the prismatic structures which would increase the complexity and cost of the production tool.

Referring to Figure 24, step 1 in the production process is the application of resin 1 in a localised pattern to a clear polymeric film 50. Suitable polymeric films include polyethylene teraphthalate (PET), polyethylene, polyamide, polycarbonate, poly(vinylchloride) (PVC), poly(vinylidenechloride) (PVdC), polymethylmethacrylate (PMMA), polyethylene naphthalate (PEN), and polypropylene. Resin 2 is then applied in register with resin 1 to the same surface of the clear polymeric film 50 (Step 2). The thickness of the applied layer comprising resin 1 and resin 2 must be of a sufficient thickness to form a prismatic array of the required height. The preferred height of the prismatic structure is in the range 0.5-50μm and more preferably in the range 2.5-20μm. Any printing or coating technique can be used to apply resin 1 and 2 but preferably, for resin 2, the process is a non-contact printing process, i.e. a process which doesn't involve a cylinder or roller being pressed onto the substrate, such as ink-jet. A non-contact process is preferred because when resin 2 is applied resin 1 has not been cured and therefore could be moved or damaged by a contact with a printing roller or cylinder. Alternatively the process

could involve contact in the areas to be printed but not in the unprinted areas, for example flexographic printing.

The coated film is then held in intimate contact with the production tool 65 in the form of an embossing cylinder, whereby the prismatic structure 66 defined on the production tool is replicated in both resins 1 and 2 held on the substrate film (Step 3). UV light is used at the point of contact to cure and harden the resin, and as a final stage, the flexible prismatic film is released from the embossing cylinder (Step 4). Preferably the process takes place as a reel-to-reel process.

An adhesive layer is then applied to the perimeter regions of the security device. The specific configuration of the adhesive layer will depend on the method used to apply the security device to the document. In an alternative method the adhesive can be applied to the document such that when the device is brought into contact with the document the adhesive layer is activated and bonds the device around its perimeter.

The preferred method is to apply the security device to the document as a surface patch, as shown in Figure 10. Figure 25 illustrates a preferred construction and process for applying a security device of the current invention to a secure document as a patch. This method is applicable to security devices in both prisms up and prisms down configurations, and where one (Figures 3 and 11) or multiple resins (Figure 22) are used to produce the prismatic structure.

Figure 25a illustrates a cross-section of a security device of the current invention suitable for application as a surface patch. In the preferred method a carrier substrate 70, for example a polymeric film, coated with a release layer 71 is laminated to a thin clear polymeric film 72, for example PET or BOPP, using a laminating adhesive 73. Alternatively a thermal release layer could be used instead of the laminating adhesive. This laminate structure is used as the substrate for the prism replication process described with reference to Figure 24. An adhesive 74 is then applied to the perimeter of the security device. In the example shown the adhesive is applied onto the prism structure which continues up to the edge of the device. The total thickness of the adhesive layer 74 is dependent on the height of the prism structure, but typically the thickness of the adhesive layer measured from the top of the prismatic structure will be in the range 1-1 Oμm. Alternatively the replication of the prism structure can be

registered such that the perimeter of the device is free of prisms and the adhesive is applied directly to the thin polymeric film. The sample is then die-cut through the centre of the exposed adhesive layer such that the cut penetrates through to the release layer but not into the carrier substrate, as shown by the dashed line in Figure 25a. The device is then transferred onto the document 76, adhesive-side down, using a conventional hot stamping process. The device is released during the hot stamping process and the thin polymeric film with the prism structure is adhered to the document with an air gap 77 therebetween, the carrier film and waste material outside the die-cut regions are then discarded.

For the device structure illustrated in Figure 25a the die-cutting stage of the present invention has to be tightly controlled such that the cut penetrates through to the release layer but not into the carrier film. If the cut penetrates into the carrier film 70 then it will be difficult to remove the waste material without tearing and damaging the carrier film. Efficient removal of the waste material is particularly important in web based continuous processes. Figure 26 illustrates an alternative embodiment of the present invention where a thin buffer layer 78 is provided between the release layer 71 and the carrier film 70. The buffer layer 78 provides a layer into which the cut can penetrate such that the tolerance of the die-cutting process can be increased without the risk of cutting into the carrier film. On removal of the waste the buffer layer 78 will separate either within the buffer layer or at the boundary with the release layer 71. The buffer layer 78 may take the form of a tie-coat polymeric coating or a thin polymeric film. The thickness of the buffer layer is preferably in the range 1-20μm and even more preferably in the range 2-12μm.

The advantage of the transfer process illustrated in Figure 25 is that the sacrificial carrier substrate 70 supports the thin polymeric film 72 during the replication of the prism structures. Typically a UV cast cure replication process requires a minimum film thickness of 25μm for the base substrate to enable efficient handling. A thickness of 25μm for the base substrate results in a final device thickness for the current invention of approximately 40μm once the prism height and the adhesive thickness is added on. A 40μm thick surface patch is suitable for certain secure documents such as passports and ID cards, but is not ideally suitable for documents which have to be sorted and distributed using automated cash handling equipment. The use of a thin base substrate for the prismatic film,

preferably less than 10μm and even more preferably less than 5μm, reduces the thickness of the final security device to 20-25μm and improves the efficiency of the final document through the automated cash handling machines.

In a further step to improve the handling efficiency of the document the thickness of the device around its perimeter can be graduated such that it is lower at the edge of the security device thus introducing a more gradual change in thickness between the base document and the security device. This gradual change in thickness at the perimeter of the device can be achieved by reducing the prism height of the prismatic structure or the thickness of the adhesive layer.

In a further embodiment the security device of the current invention can also be applied as a surface stripe, as illustrated in Figure 10. The pre-application device structure is as for the patch but the die-cutting step is not required as the stripe is slit to the correct width and then preferably applied using a conventional roll-on transfer process.

The following examples illustrated in Figures 27-46 are based on a linear prismatic array where the linear prisms have a pitch of 20μm and a prism height of 10μm. The described features are achieved when tilting the device and viewing the linear prisms such that their long axes are perpendicular to the viewing direction.

In additional embodiments an enhanced optically variable effect is created by combining the transparent to "metallic" switch effect generated by the security devices of the previous embodiments with a printed image on a security document. The "metallic" to transparent switch can be used to hide and reveal the printed information and to more clearly associate the device with the document. This is particularly applicable to the application of the security device of the current invention as a patch or stripe as it is straightforward to apply a surface security device after a printed image has been applied to a security document. Figure 27 illustrates one example where a security device having the construction shown in Figure 3a is applied over a printed serial number 80. At normal incidence (Figure 27a) the prismatic film 2 is totally reflecting and the serial number is concealed by the "metallic" appearance of the prismatic film. On tilting (Figure 27b) the security document away from normal incidence the prismatic film 2 becomes substantially transparent revealing the serial number 80.

Figure 28 illustrates a further example where a security device having the construction shown in Figure 3a is applied over a printed image. In this example the image created by the prismatic film switching from totally reflecting to transparent, and the image underneath the prismatic film are related by content thereby increasing the familiarity of the device with the general public. In this example the adhesive forms a complete layer around the perimeter of the device and also is used to create a negative pictorial image in the form of the sun, i.e. the sun is defined by the adhesive-free areas. The printed image 220 underneath the device is a pictorial image in the form of a star and is applied using lithographic printing before the security device 222 is applied to the document 224. At normal incidence (Figure 28a) the prismatic film in the adhesive-free areas is totally reflecting and the star is concealed by the "metallic" appearance of the sun. On tilting (Figure 28b) the security document 224 away from normal incidence the prismatic film becomes substantially transparent revealing the star 220. Figure 29 illustrates one example where a security device having the construction shown in Figure 22 is applied over a printed image. This security device comprises two regions of prismatic structures, Region 1 and Region 2, which switch from "metallic" to transparent at different tilt angles as illustrated in Figure 21. Referring to Figure 29 Region 1 (triangles) and Region 2 (circle) combine to form a pictorial image in this case an image of the sun. At normal incidence (Figure 29A) both Regions 1 and 2 are totally reflecting and the Sun appears as a "metallic" image. On tilting the device a few degrees off-axis (-10°) (Figure 296) the triangles defined by Region 1 switch to substantially transparent but the circle defined by Region 2 remains "metallic" and is therefore still visible. On tilting further off-axis, (-20°) (Figure 29C) Region 2 also switches to substantially transparent and a printed image 81 is revealed of a star which is related by content to the original identifying image of the security device.

In a further embodiment the switching image is located within a conventional printed image. Figure 30 illustrates one example where a security device 230 having the construction shown in Figure 3a is applied over a printed image 232 such that when it is totally reflecting the prismatic film substantially conceals part of the printed image. In this example an image 232 of a bird is printed onto the document using lithographic printing. The security device 230 comprising a prismatic film 2 is then applied over the printed image using a perimeter adhesive layer 3, such that in its totally reflecting state, when viewed at normal incidence,

the central region of the device substantially conceals the bird's eye (Figure 30A). On tilting the device (Figure 30B) and viewing away from normal incidence the central region switches from totally reflecting to transparent and the bird's eye is revealed and the full printed image is completed.

Figure 31 shows a further example where the switching image is located within a conventional printed image. In this example the printed image 240 comprises an arrangement of geometrical shapes. The security device comprising the prismatic film has the construction shown in Figure 3a. Figure 31a illustrates the device in plan view showing the position of the perimeter adhesive layer 3 and the triangular adhesive-free region 242. At normal incidence the prismatic film in the adhesive-free area is totally reflecting and a "metallic" triangular shape is observed. The device is the applied over the printed image such that when viewed at normal incidence the "metallic" triangle locates within the printed structure to form a first image (Figure 31b). On tilting the security document away from normal incidence the prismatic film becomes substantially transparent and the printed artwork under the "metallic" triangle is revealed forming a second image (Figure 31c) which is complementary in design and content to the first image.

The examples in Figures 30 and 31 have three secure aspects; firstly the totally reflecting to transparent switch, secondly the revealing of an underlying image and thirdly the registrational requirement to ensure the security device is located within the correct part of the printed image.

In the embodiments where the totally reflecting prismatic film substantially conceals an underlying image it is preferable that the underlying image is of relatively low contrast. Measurements of examples have shown that the difference in L * between any two non-overlapping regions, 4.5mm in diameter, of the underlying image including areas of exposed substrate is preferably less than 50 and more preferably less than 25 and even more preferably less than 10. L* is the dimension for luminance on the CIELAB colour measurement scale. It is also preferable that the image to be concealed does not comprise any sharp edges but instead the edges should be softened using a variety of well known techniques in the field of graphic arts. One example softening technique is to replace the continuous edge line with a discontinuous edge line, where the

discontinuities could take the form of dots, dashes or geometric shapes. In a further example the edge line remains continuous but its profile is modulated in an irregular manner on a scale not resolvable at normal viewing distance with the naked eye. In a further softening technique the hue, tone or saturation of the image is graduated within the edge region. In a yet further example the edge is softened by reducing the coverage of the ink forming the main image in the edge region, for example by reducing the dot density or the dot size in a conventional halftone image, an example of this is shown in Figure 32 where the dot size has been reduced at the edge of the image.

Figure 33 illustrates one example where a security device having the construction shown in Figure 22 is applied over a printed image. This security device comprises two regions of prismatic structures, Region 1 and Region 2, which switch from "metallic" to transparent at different tilt angles as illustrated in Figure 21. Region 1 defines the background and Region 2 defines the number "70". The security device is applied to the document such that the numeral "70" is registered to a printed image 82 on the security document in this case a dollar symbol. At normal incidence (Figure 33A) both the background and the number "70" are totally reflecting and the device appears uniformly "metallic" concealing the number "70". On tilting the device a few degrees off-axis (~10°) (Figure 33B) the background switches to substantially transparent but the number "70" remains "metallic" and is therefore revealed and in addition combines with the printed image to reveal the value and denomination of the document. On tilting further off-axis (-20°) (Figure 33C) the number "70" also switches to substantially transparent and a printed image 83 is revealed, previously concealed by the number "70", of microprint characters also of the number "70".

The printed image which is substantially concealed by the security device in its totally reflecting state can be printed using an ink containing functional components that react to an external stimulus. Components of this type include, but are not limited to, fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic. In one example the ink contains a fluorescent material such that a visible colour is observed when viewed under UV illumination. With reference to the example in Figure 27 the printed serial number is applied using an ink comprising a fluorescent material. At normal incidence (Figure 27A) the prismatic

film 2 is totally reflecting and the serial number is concealed by the "metallic" appearance of the prismatic film. On tilting (Figure 27B) the security document away from normal incidence the prismatic film 2 becomes substantially transparent revealing the serial number 80. On tilting and viewing under UV illumination the serial number will fluoresce and may appear a different colour to that when viewed under visible illumination. This embodiment will be particularly applicable to authenticating banknotes in darkly light environments but where ambient UV illumination is used such as nightclubs. In this scenario at normal incidence the UV illumination will be totally reflected by the prismatic film and will not stimulate the printed image but on titling and viewing off-axis the UV illumination will pass through the prismatic film and the printed image will fluoresce and be clearly visible. This effect will be particularly striking in a darkly lit environment with ambient UV illumination, as the presence of the prismatic film will not be apparent and the authenticator will simply observe the appearance and disappearance of the fluorescent image on tilting the device.

Following the application of the security device, the security substrate undergoes further standard security printing processes to create a secure document, including one or all of the following; wet or dry lithographic printing, intaglio printing, letterpress printing, flexographic printing, screen-printing, and/or gravure printing.

In order to be effective against counterfeiting, the security device is preferably linked to the document it is protecting by content and registration to the designs and identifying information provided on the document. In a further aspect of the current invention the security device is customised after it has been applied to the substrate. In a preferred embodiment customisation of the security device of the current invention takes place at the same time, and preferably using the same equipment, as the standard security printing processes.

In the following examples the security device is applied to the secure substrate as a surface patch, but in each case, unless stated, the method of customisation is equally applicable to stripes, windowed security threads and partially elongate elements that are viewable from either side of the document.

In one example the customisation of the security device occurs by applying printed information after the security device has been applied to the substrate. The security device may be overprinted with images using any of the conventional printing processes such as intaglio, gravure, ink jet, offset lithography, screen, dye diffusion and flexography. The print may be applied as a single print working in a single colour or as multiple print workings in multiple colours. In a preferred embodiment the images are printed partly on the substrate and partly on the security device such that the design continues uninterrupted between the two surfaces.

In one embodiment, particularly suitable when the security device is applied as a patch, a printed image on top of the security device can be registered to a printed image under the security device. At normal incidence the security device is totally reflecting and only the image on top of the security device is visible. On tilting the security device switches to substantially transparent such that the image underneath the security device is now revealed and can be observed in addition to the image on top of the security device.

Figure 34 illustrates a secure document comprising a security device of the present invention where an image is printed onto the security device such that it registers with an image underneath the prismatic film. A lithographic image 250, in this case an array of stars, is applied to a secure document. A security device 252 having the construction shown in Figure 3a is applied to the document such that it is placed over the central part of the lithographic image. This security device 252 comprises a prismatic film 2 with a perimeter adhesive layer 3 and a central area where the prismatic structures are free of adhesive. An intaglio image 254, in the form of a wavy line, is than printed over both the secure document and the security device 252 in register with the lithographic image 250. At normal incidence (Figure 34A) the prismatic film 2 in the adhesive-free central area is totally reflecting and the stars in the central part of the lithographic image 250 are substantially concealed. The authenticator observes the registered lithographic and intaglio images either side of the security device but only the intaglio printed wavy line is seen to continue uninterrupted across the "metallic" security device. On tilting the document away from normal incidence the central area of the prismatic film 2 switches to substantially transparent revealing the

underlying lithographically printed stars which now form a complete image with the rest of the lithographic image and the intaglio printed image (Figure 34B).

The example in Figure 34 with a printed image on top of the security device registered to a second image underneath the security device provides the counterfeiter with an additional challenge in that firstly he must register the print on top with the print underneath to reveal a complete image when the prismatic film is substantially transparent and secondly he must register the print on the device with the related print adjacent to the security device. Preferably the images that are printed on top of the security device are complementary to and registered with respect to the images underneath the security device. The images may define complementary patterns and conveniently gaps between elements of the image on top of the security device may be filled by elements of the image underneath the security device when the device is tilted. This makes registration between the two indicia easy to verify. In further examples the individual images may not form a recognizable image but the combination of the images forms a recognizable image which can be a piece of identifiable information for example the national flag of a country. The formation of a recognizable image on tilting the security device facilitates the authenticator in identifying counterfeits. In a further embodiment the images are defined in more than one colour.

Figure 35 illustrates a further example where an image is printed onto the security device such that it registers with an identifying image within the prismatic film. In this example a security device having the construction shown in Figure 22 is applied to the document. This security device comprises two regions of prismatic structures, Region 1 and Region 2, which switch from "metallic" to transparent at different tilt angles as illustrated to Figure 21. Region 1 defines the background and Region 2 defines four stars. The security device is applied to the document and then a printed image 85 is applied using one of the standard printing techniques, e.g. lithographic printing. One part 85A of the printed image comprises a wavy line which continues uninterrupted between the document and the security device. A second part 85B of the image is a pattern of stars which is positioned solely on the document but in register with the pattern of stars provided by Region 2 of the prismatic structure. At normal incidence (Figure 35A) both the background and the stars are totally reflecting and the device appears uniformly "metallic". On tilting the device a few degrees off-axis (-10°) (Figure

35B) the background switches to substantially transparent revealing the stars which remain metallic and combine with the printed image to form a complete pattern. On tilting further off-axis, (~20°) (Figure 35C) the stars on the security device also switches to substantially transparent.

Figure 36 illustrates a further method for integrating the device into the secure document once it has been applied to the document. In this example the security device 260 is applied to a secure document as a surface patch and comprises a prismatic film 2 with a perimeter adhesive layer 3 and a central area 262 where the prismatic structures are free of adhesive. The adhesive-free region in the central area is in the form of a pictorial image. The document then undergoes a banknote printing operation, for example intaglio printing, and the printed design 264 continues from the substrate into the device 260 such that it is positioned over the perimeter layer of adhesive 3 (Figure 36A). The perimeter layer of adhesive 3 remains substantially transparent at all angles of view and therefore its presence will not be easily apparent to the viewer, however if the device is tampered with and an attempt is made to remove it from the document the printed image over the perimeter adhesive layer will be removed resulting in the presence of an incomplete image which indicates that the security feature has been removed (Figure 36B).

In a further complementary combination the images comprise line patterns. For example the image on top of the security device comprises a first array of fine lines and the image underneath the security device comprises a second array of fine lines corresponding to the first array such that when the arrays are superimposed the lines combine to produce a visible moire pattern, preferably in the form of an identifying image. The lines are superimposed by simply tilting the device such that the prismatic film separating the two line arrays becomes transparent.

For the embodiments where the prismatic structure is a series of parallel linear prisms with planar facets arranged to form a grooved surface then visible moire patterns can be generated by a combination of the repeating pattern of the linear grooves with an array of fine lines underneath the security device. The moire pattern will be observed when the prism film in not totally reflecting. This provides

an additional security feature over the metallic to transparent switch, in that when the device switches to its transparent state a dynamic moire image is observed. The use of the air gap in the current invention enhances the observed moire effect by increasing the spacing between the linear grooves and the printed line array. The enhancement is observed as an increase in the apparent movement of the observed moire image.

As an alternative to the printing of ordinary coloured inks, it is also possible to print functional inks. By functional inks we mean inks that react to an external stimulus. Inks of this type include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic.

As well as functional inks, it is also possible to print onto the security device of the current invention with other optical effect inks. Optical effect inks include OVI® and Oasis® marketed by Sicpa. Other optical inks include inks containing iridescent, iriodine, pearlescent, liquid crystal and metal-based pigments.

In a further embodiment, the customisation of the security device occurs by embossing the security device with raised line structures. The embossed raised line structures can take any convenient form including straight (rectilinear) or curved such as full or partial arcs of a circle or sections of a sinusoidal wave. The lines may be continuous or discontinuous and, for example, formed of dashes, dots or other shapes. By other shapes we mean the dots or dashes could have a graphical form. The line widths are typically in the range 10-2000 microns, preferably 50-1500 microns. The lines can define any shape or form, for example square, triangle, hexagon, star, flower or indicia such as a letter or number.

The embossed line structures are preferably formed by applying an embossing plate to the security device under heat and pressure. Preferably the embossing process takes place during the intaglio printing process and is carried out using an intaglio plate having recesses defining the line structures. Preferably the security device is blind embossed, i.e. the recesses are not filled with ink. However it is also possible that some of the recesses defining the embossed structure may be filled with ink and others left unfilled. Further intaglio printing or blind embossing may be carried out on regions of the substrate adjacent to the security device using the same intaglio plate so as to achieve precise registration

between the different regions. For example the intaglio plate can be used to apply a blind emboss on the security device that continues in register with a raised printed on the security device. In an alternative method the security device can be embossed during the hot stamping process by varying the relief profile of the stamping die.

Figure 37 shows an example of a security device with a construction as shown in Figure 3a comprising a prismatic film 2 with a constant refractive index which has been customised by embossing the film after it has been applied to the secure document. The embossed line structures 90, formed by a respective set of substantially parallel coarse raised lines, define the Euro symbol "€". When viewed at normal incidence (Figure 37A), both the embossed and non-embossed regions appear metallic. The embossed image is readily apparent because of the high mirror-like reflectivity of the prismatic film and also because at the edge of the embossed line the prismatic structure has been modified by the pressure of the embossing process such that in these regions the film is no longer totally reflecting but is now substantially transparent. The transparent regions forms a thin outline around the embossed lines which draws the attention of the authenticator to the embossed pattern. On tilting the device away from normal incidence (Figure 37B) the prismatic film switches from "metallic" to transparent and the visibility of the embossed structure is significantly reduced because of the change in the reflectivity of the base prismatic film.

The embossed image can be related by content or registered to a further image on the security document or to the image underneath the security device in the same manner as described above for printed images on the security device and illustrated in Figures 33-35.

In a further embodiment the security device is embossed with a non-diffractive line structure. Non-diffractive line structures are structures which produce an optically variable effect when the angle of incidence light varies, but in which this effect is not caused by interference or diffraction. Security devices based on non- diffractive line structures are known in the prior art for example WO9002658 describes a security device in which one or more transitory images are embossed into a reflective surface. WO9820382 discloses a further security device in which a group of elemental areas in which lines extend at different angles from each other form respective image pixels. US1996539 discloses a decorative device in

which a relief structure is formed in a surface and has an optically variable effect. WO2005080089 discloses a security device which has segments defined by line structures in a reflective portion of a substrate, which cause incident light to be reflected non-diffractively as the angle of incidence changes.

The use of a non-diffractive line structure with a totally reflecting prismatic film has two secure aspects; firstly the optically variable feature generated by the line structure and secondly the "metallic" to transparent switch of the prismatic film.

Figure 38 illustrates a further embodiment of the present invention where the security device 270 is combined with both a printed image 276 and a blind embossed image 274such that different identifying information is presented at different angles of view. Lithographic printing is used to print the word "Fifty" 276 on to the security document. A security device 270 having the construction shown in Figure 3a is applied to the document such that it is placed over the word "Fifty". This security device comprises a prismatic film 2 with a perimeter adhesive layer 3 and a central area where the prismatic structures are free of adhesive. Intaglio printing is then used to print the "$" symbol 272 partly on the document and partly over the edge of the security device 270. During the same intaglio process the intaglio plate is also used to apply a blind emboss on the security device defining the numeral "50" that continues in register with the raised printed image of the "$" symbol. When viewing the document at normal incidence (Figure 38A) the prismatic structure in the adhesive-free central area is totally reflecting and the word "Fifty" 276 is concealed. The blind embossed image of the numeral "50" 274 is readily apparent for reasons explained with reference to Figure 37 and therefore the authenticator sees the complete image "$50" which is a combination of the intaglio printed image and the blind embossed image. On tilting the document away from normal incidence the central area of the prismatic film switches to substantially transparent revealing the underlying lithographically printed word "Fifty" but the blind embossed image is not readily apparent, again for reasons explained with reference to Figure 37, and the authenticator now observes the complete image "$Fifty" which is a combination of the intaglio printed image and the lithographically printed image (Figure 386). In this manner the device exhibits one image when viewing at normal incidence and a second image when viewed at an oblique angle of incidence, thereby providing an interactive highly memorable security feature.

In a further embodiment the security device of the current invention could be incorporated into a secure document such that regions of the device are viewable from both sides of the document. Techniques are known in the art for forming transparent regions in both paper and polymer substrates. For example, WO8300659 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region.

WO0039391 describes a method of making a transparent region in a paper substrate.

Other methods for forming transparent regions in paper substrates are described in EP723501 , EP724519 and WO03054297.

One method for incorporating a security device such that it is viewable from both sides of a fibrous document is described in EP1141480. Here a security thread 100 is selectively exposed on one side of the security document 101 and fully exposed on the second side to produce a transparent area 102, as illustrated in Figure 39a. This method allows for the insertion of considerably wider security threads into documents.

Figure 39b shows a cross-sectional view of an example of a security thread according to the current invention that could be incorporated in the manner described in EP1141480. A prismatic array 105 is replicated on side 2 of a clear polymeric film 106 and an adhesive 107 is coated along the two edges of the security device, in this example in contact with the prismatic array, to promote bonding of the thread to the secure document. The adhesive-free regions 108 in the central region of the device enable TIR to occur at the prism/air interface. The security thread is incorporated into the document 101 such that side 2 is substantially exposed on the front of the document and side 1 is exposed in a transparent area on the back of the document, as shown in Figure 40. When the security device is viewed from the back of the document (side 1) the prismatic array is viewed in the prisms-down configuration and therefore at normal incidence the film is totally reflecting and appears "metallic" and the presence of the transparent area is concealed. If the sample is tilted off-axis, while still

viewing from the back of the document, the film becomes transparent revealing a transparent area. When the security device is viewed from the front of the document (side 2), in a region where it is exposed from both sides, the prismatic array is viewed in the prisms-up configuration and therefore at normal incidence the film is transparent and a transparent area within the document is observed, but on tilting off-axis the film is totally reflecting and appears "metallic" and the presence of the transparent area is concealed. The device is only totally reflecting in the region where it is exposed on both sides, the rest of the device is in intimate contact with the fibres forming the document and therefore in this region a prism/air interface is no longer maintained. The fact that the transparent to "metallic" switch is inverted by viewing from the opposite side of the document enables the document to be easily authenticated by placing the transparent area on a printed image/document. When viewed normally from one side of the document the image will be visible through the transparent aperture, but when the banknote is turned over the image will be concealed by an apparently reflective "metallic" film. As with previous embodiments an identifying image can be incorporated into the prismatic film either by using localised regions of adhesive or by locally varying the refractive index of the prismatic film.

In a further embodiment the security device of the present invention can be applied as a patch over a hole or aperture 284 in a paper substrate. The device 280 comprises a prismatic structure 2 and perimeter adhesive 3 as illustrated in Figure 3a. The device 280 is applied to the document as illustrated in Figure 41 with the perimeter adhesive 3 being used to adhere the device to the paper substrate 282. The prismatic film 2 over the aperture 284 is free of adhesive and TIR occurs at the prism/air interface. Once the security device in Figure 41 is in- situ over the aperture 284 it behaves in the same manner as described for the device over the aperture in Figure 40. Alternatively the device can be applied over a transparent region of a polymeric substrate to achieve the same visual effects.

In one embodiment the current invention could be incorporated into a security paper as a windowed thread. Figure 42 shows a security thread 120, formed by a device according to the invention, with windows 122 of exposed thread and areas 124 of embedded thread in a document 126. EP860298 and WO03095188 describe different approaches for the embedding of wider threads into a paper

substrate. Wide threads are particularly useful as the additional exposed area allows for better use of optically variable devices such as the current invention.An example cross-section of a device suitable for incorporation as windowed thread is shown in Figure 43. The construction comprises a substantially clear polymeric film 130 of polyethylene terephthalate (PET) or the like. A prismatic surface structure 132, comprising an array of substantially planar facets, is formed on one surface of the clear polymeric film 130. An adhesive 134 is applied to the edge regions of the prismatic array. The device is then adhered to the substrate for the security thread, which is typically a polymeric film 136, such that a prism/air interface is maintained (airgap) 138 in the central regions of the device. When viewed from the top of the device the prismatic array is in the prisms-down configuration. An adhesive layer 140 is applied to the outer surfaces of the final thread construction to facilitate the adherence of the thread to the document. The use of the laminate thread structure, in which the prism/air interface is sandwiched between two polymeric films, provides a method for protecting the prism/air interface from contamination with moisture or chemicals when inserted into the document during the papermaking process. This is important because TIR is unlikely to occur in any regions of the prism/air interface contaminated with moisture or chemicals and therefore the "metallic" appearance of the device would be degraded.

As with previous embodiments an identifying image can be incorporated into the prismatic film either by using localised regions of adhesive or by locally varying the refractive index of the prismatic film. In a preferred embodiment the identifying image is repeated along the security thread such that one set of identifying images is always visible in the windowed region of the banknote. The incorporation of the security thread into the paper being controlled such that identifying image is visible from the top surface of the windowed region of the banknote.

In a further embodiment a printed layer of identifying information 142 can be incorporated into the security thread as illustrated in Figure 44 which otherwise has a similar construction to Figure 43. On viewing the device in Figure 44 from above the substrate and normal to the plane of the clear polymeric film the prismatic array is totally reflecting and appears "metallic" and the identifying information 142 is concealed. If the device is now tilted away from the normal and

viewed off-axis the prismatic array is now transparent and the identifying information is revealed.

Figure 45 shows a further example of a security device suitable for application to a secure document as a windowed thread and based on the Figure 22 structure but constructed as a thread similar to Figure 44 in which the polymeric film 50 overlies a thread carrier 136 and has a thread adhesive 140. The structure is second to a document 156. This security device comprises two regions of prismatic structures, Region 1 and Region 2, which switch from "metallic" to transparent at different tilt angles as illustrated in Figure 17. Region 1 and Region 2 are in register with the identifying information 150 on the security thread such that different parts of the information are revealed at different tilt angles. Figure 46 illustrates an example switching sequence for a windowed thread with the construction in Figure 45 where identifying information 1 , under Region 1 of the prismatic film, is in the form of the letters "DLR", and identifying information 2, under region 2 of the prismatic film, is in the form of the number "100". At normal incidence (Figure 46A) both Regions 1 and 2 are totally reflecting and the device appears uniformly "metallic". On tilting the device a few degrees off-axis (~10°) (Figure 46B) Region 1 switches to substantially transparent revealing the letters "DLR". On tilting further off-axis, (-20°) (Figure 46C) Region 2 also switches to substantially transparent revealing the number "100".

Preferably the identifying information 142, 150 on the security thread can be created using known metallisation or demetallisation processes. It is known that metallised films can be produced such that no metal is present in controlled and clearly defined areas. Such partly metallised film can be made in a number of ways. One way is to selectively demetallise regions using a resist and etch technique such as is described in US4652015. Other techniques are known for achieving similar effects; for example it is possible to vacuum deposit aluminium through a mask or aluminium can be selectively removed from a composite strip of a plastic support and aluminium using an excimer laser. Alternatively the identifying information could be printed onto the security thread using lithography, UV cured lithography, intaglio, letterpress, flexographic printing, gravure printing or screen printing. The identifying information can be provided using conventional inks such as coloured inks, white inks, black inks, metallic inks, optically variable inks (such as those incorporating thin film optical interference filters or liquid

crystal pigment) and the like. Thermochromic inks, photochromic inks, magnetic inks, infrared absorbing inks and fluorescing and phosphorescing inks may also be employed. The inks may be employed in rainbow printing fashion.

Figure 47 shows an embodiment of a security device further customised by having a localised prismatic surface structure comprising of two arrays of a prismatic structure where the arrays are rotated relative to each other within the plane of the layer. The construction comprises a substantially clear polymeric film 160 of polyethylene terephthalate (PET) or the like. A localised prismatic surface structure, comprising two arrays 162, 164 of a series of parallel linear prisms (prismatic array 3 and prismatic array 4) where the arrays are rotated relative to each other by ~90° within the plane of the substrate, is formed on the lower surface of the clear polymeric film 160. The linear prisms have a pitch of 20μm and a height of 10μm. An adhesive 166 is applied to the edge regions of the prismatic array to enable the device to be adhered to the secure document while still maintaining a prism/air interface (airgap) in the central regions of the device. When viewed from the top of the device the prismatic arrays are in the prisms- down configuration.

An array of parallel linear prisms is particularly suitable for the example in Figure 47 as the angular viewing conditions at which TIR occurs is dependent on the degree of rotation between the tilt direction and the long axes of the linear prisms. This variation in reflectivity is illustrated using the polar plots in Figure 4. Figure 4 shows that TIR occurs at all angles of incidence when the direction of tilt is parallel to the long axes. In contrast when the direction of tilt is perpendicular to the long axes of the linear prisms (i.e. tilting along arc 2) TIR occurs at normal incidence and for a limited tilt range away form normal incidence.

Figure 48 illustrates a secure document, for example a banknote, containing one example of the optically variable effect that could be generated from the security device in Figure 47. Prismatic array 3 is replicated onto the clear polymeric film in the form of a star and prismatic array 4 is replicated over the active area not covered by prismatic array 3 such that it forms the background area. Prismatic arrays 3 and 4 comprise a series of parallel linear prisms and are replicated such that the long axes of the linear prisms forming the star (prismatic array 3) are substantially perpendicular to the long axes of the prisms forming the background

area (prismatic array 4). The lines in Figure 48 schematically represent the long axes of the linear prisms. The long axes of the prisms forming the background area are parallel to long axis of the secure document and the long axes of the prisms forming the star are parallel to short axis of the secure document. When viewed normally (Figure 48A) both prismatic array 3 and prismatic array 4 are totally reflecting and the film has a uniform "metallic" appearance and the star is not visible. On tilting the device a few degrees off-axis, ~10° (Figure 48B), and viewing parallel to the short axis of the secure document (direction A), the background area becomes transparent but the star remains "metallic" and is therefore revealed. If the device remains off-axis and is rotated such that it is viewed at an angle of 45° to the long axis of the secure document (direction C) (Figure 48C) the star becomes substantially transparent and the background area remains transparent resulting in the image of the star being concealed. If the device remains off-axis and is rotated by a further 45° and viewed along the long axis of the secure document (direction B) (Figure 48D) the image is inverted from that observed along direction A with the star switching from "metallic" to transparent and the background area switching from transparent to "metallic".