GB2480227A

GB2480227A - Optical security device with spaced microlines

Info

Publication number: GB2480227A
Application number: GB1003399A
Authority: GB
Inventors: Brian William Holmes; Lawrence George Commander
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2010-03-01
Filing date: 2010-03-01
Publication date: 2011-11-16
Also published as: GB201003399D0

Abstract

An optical security device is for securing documents of value, such as banknotes, against counterfeiting. It has a transparent substrate 2 defining opposed, parallel surfaces 4,6 and having a first array 8 of opaque, spaced microlines on or in one surface, and a second array 10 of spaced microlines on or in the opposite surface. The two arrays 8,10 are located laterally relative to one another such that when the device is viewed at a first viewing angle (Figure 1b) with the first array nearer the viewer, the microlines of the second array fill the spaces between the microlines of the first array, while when the device is viewed at a second viewing angle (Figure 1a), different from the first viewing angle, the microlines of the second array do not fill the spaces between the microlines of the first array.

Description

OPTICAL SECURITY DEVICE

The invention relates to an optical security device, for example for securing articles such as documents of value, including banknotes, against counterfeiting and the like.

In order to provide a commercially valuable security device, it is important to devise such a device which is difficult to manufacture but yet relatively easy to verify. Many optical security devices exist such as holograms, lenticular devices and the like. These work well but are relatively expensive and there is a need for a cheaper device.

In accordance with a first aspect of the present invention, an optical security device comprises a transparent substrate defining opposed, parallel surfaces and having a first array of opaque, spaced microlines on or in one surface, and a second array of spaced microlines on or in the opposite surface, the two arrays being located laterally relative to one another such that when the device is viewed at a first viewing angle with the first array nearer the viewer, the microlines of the second array fill the spaces between the microlines of the first array, while when the device is viewed at a second viewing angle, different from the first viewing angle, the microlines of the second array do not fill the spaces between the microlines of the first array.

In accordance with a second aspect of the present invention, we provide a method of manufacturing an optical security device comprising providing a transparent substrate defining opposed, parallel surfaces; and providing a first array of opaque, spaced microlines on or in one surface, and a second array of spaced microlines on or in the opposite surface, the two arrays being located laterally relative to one another such that when the device is viewed at a first viewing angle with the first array nearer the viewer, the microlines of the second array fill the spaces between the microlines of the first array, while when the device is viewed at a second viewing angle, different from the first viewing angle, the microlines of the second array do not fill the spaces between the microlines of the first array.

In accordance with a third aspect of the invention an optical security device comprises a transparent substrate defining opposed, parallel surfaces and having a first array of opaque, spaced microlines on or in one surface, and a second array of spaced microlines on or in the opposite surface, the two arrays being located laterally relative to one another such that when the device is viewed a moire magnified image is observed.

With this new device, we avoid the need for complex holographic structures or structures utilizing lenses and instead provide a device which achieves this optical effect by virtue of alignment and misalignment between arrays of microlines. The advantage of the device as a security device is that it is difficult to reproduce because of the difficulty of providing the microlines.

Conventional printing techniques cannot be used to generate such microlines in most cases.

In some cases, the microlines of the second array will be opaque but in others, they could be translucent to allow underlying information or colours to be visible therethrough.

The microlines can be provided in a variety of ways. In some cases, microprinting using for example a litho process could be used. Widths down to approximately 10 microns can be achieved.

However, in order to achieve smaller microlines, alternative techniques involving the formation of a relief pattern are needed. This also enables much thinner devices to be constructed which is particularly beneficial when used with security documents.

The relief patterns can be formed by embossing or cast-curing. Of the two processes mentioned, cast-curing provides higher fidelity of replication.

A variety of different relief structures can be used as will described in more detail below. However, the microlines could simply be created by embossing/cast-curing the images as diffraction grating areas. Differing parts of the microlines could be differentiated by the use of differing pitches or different orientations of grating providing regions with a different diffractive colour.

Alternatively the microlines could be formed from anti-reflection structures such as moth-eye (see for example WO-A-2005/106601), zero-order diffraction structures, stepped surface relief optical structures known as Aztec structures (see for example WO-A-2005/115119) or simple scattering structures. For most applications, these structures could be partially or fully metallised to enhance brightness and contrast.

Typically, the width of each image strip or microline formed by a relief or by printing is less than 100 microns, preferably less than 50 microns, most preferably in the range 5-25 microns Ink can be incorporated in the relief pattern either by filling recesses of the relief pattern or onto raised features of the relief pattern. Relief patterns could, for example, be created by cast-curing or embossing and then the recesses or pits filled by a liquid ink, the excess being removed by a doctor blade or the like. The ink could be a gravure type or ink jet type ink.

In the case of raised areas, these could be inked by methods analogous to offset litho printing or flexographic printing. The inking of raised areas has the advantage that it is better suited to multiple colours since the doctoring process would inevitably mix different inked areas. Multiple colours allow different coloured elements to pass by each other in a movement type design.

Particularly attractive is to use a wet litho process to ink the raised areas since this would allow some simple colour based effects (e.g. image flip or simple moire) with the higher resolution raised image effects.

The microlines will comprise one or more inks or metals to achieve the required opacity and they could be of the same or different colours.

Typically, the spaces between the microlines of the second array will be fully transparent by simply not providing any ink or metallisation. However, these spaces could be translucent or even provided with patterns such as symbols, characters, security patterns and the like.

The security device can be used in a wide variety of forms, for example as an integral part of a document of value such as a banknote. In that case, the substrate of the device would be formed by a transparent portion of the substrate of the article such as a transparent window. In addition, it could be provided in the form of a label or security thread for inclusion into a document of value.

An example of an optical security device according to the invention will now be described with reference to the accompanying drawings, in which:-Figures IA and lB illustrate schematically a first example of a security device according to the invention when viewed at different viewing angles; and, Figures 2A-2G illustrate different examples of relief structures defining m icrol ines.

Figures 1A and 1 B illustrate an example of a security device according to the invention comprising a transparent substrate 2 which is typically a transparent polymeric material, for example bi-axial PET or polypropylene. On opposite surfaces 4,6 of the substrate 2 are provided respective first and second arrays of microlines 8,10. The arrays are slightly misaligned. The microlines 8 are separated by spaces 12 having a width similar to the width of the microlines 8. The microlines 10 are separated by spaces 14 having a width similar to that of the microlines 10.

As explained above, the microlines 8,10 could be printed on the surfaces 4,6, for example by litho printing, gravure printing or the like. However, it is preferred that the microlines are provided by relief patterns which are then selectively inked or metallised. Relief patterns can be obtained by micro-embossing, cast-curing or photo-ablation. These will then be selectively inked or metallised as mentioned earlier.

In any event, the microlines 8 are opaque so as to define a mask while the microlines 10 could be opaque or transparent/translucent.

The lateral positions of the arrays are such that when the security device is viewed perpendicularly to the surfaces 4,6 (Figure 1A), the viewer will see through the aligned spaces 12,14 so that the device appears partially transparent.

On the other hand, when the device is viewed at an acute angle, typically 45° as shown in Figure IB, the line of sight through the spaces 12 will be obscured by the microlines 10 so that the device appears opaque.

The thickness of the substrate 2 will determine in angular terms how rapidly the microlines move in and out of alignment as the viewing angle (or angle of tilt changes). It is generally preferable that the substrate 2 has a thickness that ensures that the device presents only one alignment view and one misalignment view over the typical viewing angle.

A variety of different security devices can be fabricated depending upon the nature of the microlines 8,10.

In one example, the microlines 10 are metallic so that at the viewing angle shown in Figure 1 B, the device appears metallic.

The microlines 10 could be formed of black or coloured opaque ink so that again at the angle shown in Figure 16, the device would appear opaque but either black or coloured.

If the two microline arrays are of the same colour and the device is applied over a back coated with a complementary colour (say red), then the device can provide a binary colour shift.

The microlines 8,10 could have the same widths (as shown), or different widths.

In a further embodiment of the current invention the microlines can be arranged such that the device creates an image switch on tilting. An example structure is shown in Figure 3A and Figure 3B. The device comprises a transparent substrate 32 which is typically a transparent polymeric material, for example bi-axial PET or polypropylene. On opposite surfaces of the substrate 32 are provided respective first and second arrays of microlines 30 and 31. The arrays are registered such that the spaces in one array are substantially filled by the lines in the second array. In this example the array 30 is a uniform grid as shown in Figure3B and the array 31 is in the form of the text "DLR".

The lateral positions of the arrays are such that when the security device is viewed perpendicularly to the surfaces comprising the array 30 and from the side of the array 30, the viewer will see a fully opaque uniform area.

On the other hand, when the device is tilted away from the perpendicular the image "DLR" will be revealed.

In a further embodiment of the current invention the use of two fine line periodic repeating patterns in an arrangement shown in Figure 1 produces a magnified moire interference pattern.

The moire phenomenon can be used to create interesting optical effects including image movement. The most interesting effect is moire magnification -the magnification of a fine line periodic repeating pattern (the base layer) by a grid or line pattern of similar period (the revealing layer) according to (1). Where m is the magnified size, f, is the separation (pitch) of the images in the base layer, and F is the pitch of the revealing layer m= r 1 A proposed movement design involves using a fine line patterned base layer, and similar resolution line revealing layer as shown in Figure 4 in plan view, In this case the device comprises a revealing layer in the form of an array of lines positioned on one side of a transparent substrate and a base layer comprises an array of lines with variable spacing arranged to form a magnified "5" in the result moire pattern positioned on the opposite side of the substrate.

Due to the nature of a moire magnifier device comprising arrays of microlines a certain proportion of light must be blocked from the structure by the revealing layer making it appear dark. This means in order to properly view the effect the device must be viewed in transmission. In order to maximise light transmission a one-dimensional magnification is preferred. This has the disadvantage that the movement of the magnified image on titling the device is only observed in one direction. However by manipulating the design work this limitation can be overcome.

If the arrays are arranged at 450 to the horizontal axis of the device then a horizontal movement can be induced by vertically tilting the device around the horizontal axis. Changing the angle of the arrays relative to the horizontal axis of the device will result in movement in different directions. Combination of different orientation provides movement in two directions as illustrated in Figure 4 when the device is tilited around the horizontal axis. Rotation of the arrays away from the horizontal will result in the magnified image being skewed and this can be corrected by slightly misaligning the rotating layer relative to the base layer.

The use of opaque line structures, particularly in relation to the moire embodiment results in a security device which operates primarily in transmission and appears very dark and not aesthetically pleasing in reflection. In a further embodiment of the current invention an additional reflective based security feature is added to the security device. For example the microlines could exhibit optically variable properties for example in the form of a diffraction grating or a hologram or in the form of a angular dependent colourshifting material such as a cholesteric liquid crystal material or a thin film interference structure formed either by vapour deposition or a pigmented coating. In this manner the device.

will exhibit the additional optically variable effect in reflection while in transmission it will exhibit primarily the movement of the moire magnifier but the magnified images themselves may well exhibit the optically variable effect as the device is tilted.

In the case of metallic microlines, these can be provided directly onto the transparent substrate 2 or onto a layer or coating applied to the substrate layer.

A first method for generating the metallic line pattern is to first apply a uniform metal coating by vacuum deposition. The regions in between the metal lines can be removed by an etchant -either directly by printing an etchant directly onto the metal or indirectly by first printing a resist mask onto the metal and then immersing in an etchant bath.

A second method for generating the metallic line pattern is to first apply a uniform metal coating by vacuum deposition. The metal is then removed by photo-ablation.

A third method is to use photolithography wherein a high intensity light is first arranged to impinge on a transmission mask containing a transmissive grid pattern -the resulting transmitted field then impinges on a projection lens system which the focuses of de-magnified image of the grid pattern directly onto the back of metal film. The irradiance maxima within this light field ablating the desired metal pattern.

A fourth method is to print a grid of metallic ink lines using a technique such as offset litho.

In a fifth method for generating the patterned microline arrays a uv curable polymeric resin is applied to a metallised polymeric substrate. The device is passed over a transparent cylinder, preferably a quartz cylinder, which comprises a mask in the form of the required microline array. A UV lamp is placed inside the quartz cylinder and the polymeric resin is cured through the mask as it passes over the cylinder. The uncured regions of the polymeric resin are then removed and then the cured regions in the form of microlines are used as a resist mask to prevent metal removal when the film is subsequently immersed in a etchant bath.

In the example shown in Figure 1, the microlines 8,10 will be rectilinear (although this cannot be seen in the drawing). However, other line shapes could be used instead such as curved lines.

The microlines can also be formed as a relief structure and a variety of different relief structures suitable for this are shown in Figure 2. Thus, Figure 2A illustrates microlines in the form of embossed or recessed lines while the non-embossed lines correspond to the spaces in between the lines). Figure 2B illustrates image regions of the strips (lM) in the form of debossed lines or bumps.

In another approach, the relief structures of the microlines can be in the form of diffraction gratings (Figure 20) or moth-eye/fine pitch gratings (Figure 2D).

The microlines of Figures 2A and 2B can be further provided with gratings as shown in Figures 2E and 2F respectively.

Figure 2G illustrates the use of a simple scattering structure in the microlines providing an achromatic effect.

Further, as explained above, in some cases the recesses of the microlines of Figure 2A could be provided with an ink or the debossed regions or bumps of Figure 2B could be provided with an ink. The latter is shown in Figure 2H where ink layers 100 are provided on the bumps.

Figure 21 illustrates the use of an Aztec structure on the microline regions The height or depth of the bumps/recesses is preferably in the range 0.5- 10pm and more preferably in the range 1-5pm. Typical widths of the bumps/ recesses will be defined by the nature of the artwork but would typically be less than 100pm, more preferably less than 5Opm and even more preferably less than 25 microns.

Claims

CLAIMS1. An optical security device comprising a transparent substrate defining opposed, parallel surfaces and having a first array of opaque, spaced microlines on or in one surface, and a second array of spaced microlines on or in the opposite surface, the two arrays being located laterally relative to one another such that when the device is viewed at a first viewing angle with the first array nearer the viewer, the microlines of the second array fill the spaces between the microlines of the first array, while when the device is viewed at a second viewing angle, different from the first viewing angle, the microlines of the second array do not fill the spaces between the microlines of the first array.
2. A security device according to claim 1, wherein the second viewing angle is perpendicular to the surfaces of the substrate.
3. A security device according to claim 2, wherein the first viewing angle is substantially at 45 to the surfaces of the substrate.
4. A security device according to any of the preceding claims, wherein the shape and spacing of the microlines of the two arrays are substantially identical.
5. A security device according to any of the preceding claims, wherein the spacing between the microlines of each microline array is the same as the width of the microlines in that array.
6. A security device according to any of the preceding claims, wherein the microlines of the two arrays are rectilinear.
7. A security device according to any of the preceding claims, wherein the microlines of the second array are also opaque.
6. A security device according to any of the preceding claims, wherein the microlines comprise ink or metal.
9. A security device according to claim 8, wherein the microlines of the two arrays have the same colour.
10. A security device according to any of the preceding claims, wherein the microlines comprise an inked or metallized relief pattern.
11. A security device according to claim 10, wherein the relief pattern is embossed or cast-cured into the substrate.
12, A security device according to claim 10 or claim 11, wherein the relief pattern comprises diffractive grating structures.
13. A security device according to any of the preceding claims, wherein the width of each microline is less than 100 microns, preferably less than.50 microns, most preferably in the range 5-25 microns.
14. A security device according to any of the preceding claims, wherein the spaces between the microlines of the second array are fully transparent.
15. A security device according to any of claims ito 13, wherein the spaces between the microlines of the second array are translucent.
16. A security device according to any of claims I to 13, wherein the spaces between the microlines of the second array are provided with optically visual patterns such as symbols, characters, security patterns and the like.
17. An article provided with a security device according to any of the preceding claims.
18. An article according to claim 17, wherein the article comprises one of banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents for securing value or personal identity.
19. An article according to claim 17, wherein the article comprises a substrate with a transparent portion, on opposite sides of which the lenticular focusing elements and image strips respectively are provided.
20. A method of manufacturing an optical security device comprising providing a transparent substrate defining opposed, parallel surfaces; and providing a first array of opaque, spaced microlines on or in one surface, and a second array of spaced microlines on or in the opposite surface, the two arrays being located laterally relative to one another such that when the device is viewed at a first viewing angle with the first array nearer the viewer, the microlines of the second array fill the spaces between the microlines of the first array, while when the device is viewed at a second viewing angle, different from the first viewing angle, the microlines of the second array do not fill the spaces between the microlines of the first array.
21. A method according to claims 20, wherein the microlines are printed onto the substrate.
22. A method according to claim 20, wherein the microlines are defined by a relief structure embossed or cast-cured into the substrate.
23. A method according to any of claims 20 to 22 for manufacturing a security device according to any of claims 1 to 16.