GB2547236B

GB2547236B - Improvements in security devices

Info

Publication number: GB2547236B
Application number: GB1602479.6A
Authority: GB
Inventors: Lister Adam
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2016-02-11
Filing date: 2016-02-11
Publication date: 2019-09-11
Anticipated expiration: 2036-02-11
Also published as: GB201602479D0; EP3414103A1; WO2017137719A1; GB2547236A

Description

Improvements in Security Devices

The present invention relates to improvements in security devices for various authenticating or security applications and, in particular, to an optically variable security device which has a security feature which utilises a colourshift effect.

The increasing popularity of colour photocopiers and other imaging systems and the improving technical quality of colour photocopies has led to an increase in the counterfeiting of secure documents, such as banknotes, passports, identification cards and the like. There is, therefore, a need to add additional authenticating or security features to existing security features. Steps have already been taken to introduce optically variable features that cannot be reproduced by a photocopier into substrates used to make such documents. There is also a demand for security features which are discernible by the naked eye, but which are "invisible" to, or viewed differently by, a photocopier. Since a photocopying process typically involves scattering high-energy light off an original document containing the image to be copied, one solution is to incorporate one or more features into the document which have a different perception in different viewing conditions, such as in reflected and transmitted light, or at different viewing angles.

It is well known in the field of security documents to use liquid crystal materials, thin film interference structures, multilayer polymeric structures, photonic crystal structures and the like to generate angularly dependent coloured reflection to provide a distinctive optical characteristic. EP-A-0435029, for example, describes a data carrier, such as an identification card, which comprises a liquid crystal polymer layer or film. Examples of security devices utilising thin film interference structures are described in US-B-4186943 and US-A-2005/0029800. Examples of security devices utilising multilayer polymeric structures are described in EP-A-1047549.

It is therefore an object of the present invention is to provide an alternative security feature which is difficult to counterfeit and which has a visually distinctive optically variable appearance.

The invention therefore comprises a security device comprising a colourshifting layer, and at least one luminescent coloured region and at least one non-luminescent coloured region located on one side of the colourshifting layer, wherein:- at a first angle of incidence, there is a first difference between the reflection spectrum of the at least one non-luminescent coloured region and the combined emission and reflection spectra of the at least one luminescent coloured region within the band pass of the colourshifting layer such that the security device has a first appearance; and at a second angle of incidence, there is a second difference between the reflection spectrum of the at least one non-luminescent coloured region and the combined emission and reflection spectra of the at least one luminescent coloured region within the band pass of the colourshifting layer such that the security device has a second appearance, which is different from the first appearance.

In a wavelength range between 450-650nm, the first difference at at least one wavelength is preferably less than or equal to 30%, and more preferably less than or equal to 20%.

In a wavelength range between 450-650nm, the second difference at at least one wavelength is preferably greater than 40%, more preferably greater than 50%, and more preferably still greater than 60%.

Preferably, at the first angle of incidence, the first appearance of the security device is substantially uniform in that the at least one luminescent coloured region and the at least one non-luminescent coloured region are substantially indistinguishable from each other and the first angle of incidence is preferably a normal angle of incidence.

Preferably, at the second angle of incidence, the second appearance of the security device is such that the at least one luminescent coloured region and the at least one non-luminescent coloured region are distinguishable from each other.

In a preferred embodiment the at least one luminescent coloured region and/or the at least one non-luminescent coloured region define indicia which are visible at the second angle of incidence and the indicia may be positive or negative.

The at least one luminescent coloured region and the at least one non-luminescent coloured region may be at least partially contiguous.

There are preferably gaps between the at least one luminescent coloured region and the at least one non-luminescent coloured region.

Alternatively the at least one luminescent coloured region and the at least one non-luminescent coloured region are at least partially superimposed and may be in register with each other.

The security device preferably further comprises a layer separating the at least one luminescent coloured region and the at least one non-luminescent coloured region.

The at least one luminescent coloured region and/or the at least one non-luminescent coloured region may be applied directly to the colourshift layer.

There is preferably a layer provided between the at least one luminescent coloured region and/or the at least one non-luminescent coloured region and the colourshift layer.

Preferably a plurality of luminescent coloured regions and/or non-luminescent coloured regions are located on the one side of the colourshifting layer. A layer of adhesive may be applied to one or both sides of the security device, which may be clear and transparent.

Preferably the colourshifting layer comprises a multilayer polymeric film or a liquid crystal film.

The colourshifting layer may comprise a colourshifting material applied to carrier layer.

The at least one luminescent coloured region and/or the at least one non-luminescent coloured region may be formed by a printing process and/or a coating.

Preferably the at least one luminescent coloured region and/or the at least one non-luminescent coloured region comprises a machine readable material.

The security device preferably further comprises a magnetic layer and/or a reflection enhancing layer.

The reflection enhancing layer may be a partially metallised or a partially demetallised layer having at least one metal free area and the at least one metal free area may be in at least partial register with gaps in the at least one luminescent coloured region and/or the at least one non-luminescent coloured region.

Alternatively the reflection enhancing layer may be a diffractive or a reflective relief structure.

The invention further provides a security substrate comprising a base substrate and the security device as described above.

The security device is preferably applied to a surface of the base substrate or is at least partially embedded in the base substrate.

The security device may be exposed in windows in one or both surfaces of the base substrate or may be wholly exposed at one surface of the base substrate.

The invention further provides a security document made from the security substrate described above and the indicia visible in the second appearance may be linked by content and registration to a design or identifying information on the secure document.

The security document preferably is a document of value or means of identification, such as a banknote, a cheque, a certificate, a passport, a passport page, an identification card or a driver’s licence.

Preferred embodiments of the security device of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a plan view of a security substrate incorporating a partially embedded security device of the present invention;

Figure 2 is a cross-sectional side elevation of a security device of the present invention;

Figures 3a and 3b are plan views of the security device of Figure 2 viewed at difference angles of incidence;

Figures 4a and 4b illustrate example reflectance spectra for the security device of Figure 2 viewed at the same angles of incidence of Figures 3a and 3b respectively; and

Figures 5 to 8 are cross-sectional side elevations of alternative security devices of the present invention. A first embodiment of the security device 10 is shown in Figure 2. The security device 10 comprises a colourshifting layer 14, on one side of which are located at least one luminescent coloured region 15 and at least one non-luminescent coloured region 16. Any further reference herein to region in the singular is to also be interpreted as covering multiple regions. The structure comprising the colourshifting layer 14 and coloured regions 15,16 will be referred to herein as the basic structure 20, which has a first side 18, which is the side on which just the colourshifting layer 14 is present, and a second side 19, which is the side on which the coloured regions 15,16 are present. A layer of adhesive 17 is preferably applied to one or both sides 18, 19 of the basic structure. The layer(s) of adhesive 17 are preferably clear and colourless, and help to secure the security device 10 to, or within, the secure substrate 11.

The colourshifting layer 14 is made from a material or a structure which exhibits an angularly dependant coloured reflection. Such materials or structures are so-called iridescent amplitude interference materials or structures, which display different colours at different viewing angles. Examples include thin-film interference structures, interference pigments, pearlescent pigments, liquid crystal film and pigments, photonic crystals and the like. Thin film interference structures comprise repeating layers of different refractive indices; examples can include purely dielectric stacks (metal oxide or polymer) or those composed of alternate dielectric and metallic layers. Thin film interference structures are also known as Bragg stacks or 1D photonic crystals. What all of the above examples have in common is the provision of two or more closely spaced interfaces, at least one of which partially reflects and partially transmits incident light, i.e. the amplitude of the incident light is split. The transmitted portion is reflected at the second or subsequent interfaces and interferes with the portion reflected from the first or earlier interfaces, leading to constructive interference of some wavelengths and destructive interference of others, and hence a characteristic colour which varies with viewing angle.

The colourshifting layer 14 may be formed from a material which is self supporting, and therefore acts as a carrier layer for the other elements of the security device 10.

Alternatively, the colourshifting layer 14 may comprise a carrier layer to which is applied a colourshifting material. Although all types of colourshifting materials and structures, as listed above, may be used to provide the colourshifting layer 14 one particularly suitable class of materials for the colourshifting layer 14 are multilayer polymeric films, such as are described in EP-A-1047549, US-B-5089318 and WO-A-9619377, an example of which is a giant birefringent optics (GBO) film, such as CM592 or CM550 produced by 3M. Alternatively pigmented coatings, for example a pigmented liquid crystal coating, may be applied to a carrier layer of a suitable polymeric substrate such as polyethylene terephthalate (PET) or bi-axially oriented polypropylene (BOPP).

Liquid crystal films which would be suitable for use in the present invention would generally have a 20-25% light reflectance, although this is polarisation selective so a maximum 50% is achievable. Multilayer polymeric films generally have higher light reflectances and can be greater than 50%, and even approaching 100%. This means that the latent image and colourshift effects will be brighter for such films over liquid crystal films. The chemical mechanical durability of multilayer polymeric films is also better than that of liquid crystal films. In particular the liquid crystal films can be frangible, unless protected by additional protective layers, making them vulnerable to physical hazards when security documents incorporating them are in circulation. However the additional of extra layers adds to the cost and time of production and adds additional bulk to the security device 10.

The coloured regions 15,16 may be applied directly to a surface of the colourshifting layer 14, as shown in Figure 2, or in an alternative configuration as described below. The coloured regions 15, 16 are preferably applied by a printing or coating process, or another suitable process for applying an ink or coating. More preferably the coloured regions 15,16 are printed using inks. Any reference herein to inks should be interpreted to include all forms of pigmented/coloured coatings.

When the security device 10 is viewed in incident light at different angles it provides a distinctive colourshifting security effect, which can be used to expose a latent image, as shown in Figures 3a and 3b. The colourshifting layer 14 and the coloured regions 15,16 are selected so that, at a first angle of incidence, there is a first difference between the reflection spectrum of the non-luminescent coloured region 16 and the combined emission and reflection spectra of the luminescent coloured region 15 within the band pass of the colourshifting layer 14 such that the security device 10 has a first appearance at this angle of incidence. At a second angle of incidence, there is a second difference between the reflection spectrum of the non-luminescent coloured region 16 and the combined emission and reflection spectra of the luminescent coloured region 15 within the band pass of the colourshifting layer 14 such that the security device 10 has a second appearance at this angle of incidence, which is different from the first appearance.

In a preferred embodiment (illustrated in Figures 3a and 3b), when the security device 10 is viewed at a first angle of incidence from the first side 18 (in this example a normal angle of incidence), the coloured regions 15,16 are perceived by the viewer as substantially the same colour (Figure 3a) and therefore not distinguishable from each other. However when the security device 10 is viewed from the first side 18 at a second angle of incidence, which is different from the first angle of incidence (Figure 3b), the coloured regions 15,16 are perceived by the viewer to be of different colours and are therefore easily distinguishable from each other.

When incident light strikes the colourshifting layer 14 on the first side 18 (i.e. an orientation in which the colourshifting layer 14 overlies the coloured regions 15,16), some of the incident light is reflected, and some of the light is transmitted. The colourshifting layer 14 acts as an optical band pass filter which selectively transmits certain wavelengths of light and reflects the rest. The wavelengths, and therefore colour, of the reflected and transmitted light depend on the structure and composition of the colourshift material/structure. The wavelengths of the reflected and transmitted light are also dependent on the angle of incidence, which results in a colour change perceived by the viewer as the security device 10, and therefore the colourshifting layer 14, is tilted.

The light which is transmitted through the colourshifting layer 14 strikes the coloured regions 15, 16, which in turn absorb some wavelengths of the transmitted light and reflect other wavelengths of light back through the colourshifting layer 14. A non-luminescent ink (dye or pigment), such as may be used for the coloured region 16, will generally diffusely reflect light which is the colour of the ink and absorb all the other colours of the spectrum. The colour of the light reflected is generally independent of the angle of view. A luminescent ink such as may be used for the coloured region 15, absorbs light in a special region of its spectrum and, in addition to reflecting light of certain wavelengths, will also emit light of a different wavelength (or range of wavelengths), and therefore colour, to that of the reflected light. The colour of the light reflected is also generally independent of the angle of view. The emission characteristics of the luminescent ink are such that it emits its light in all directions. This is also true in the case of a normal absorbing ink, but there will be a slight specular attribute to a normal ink whereby there may be a slight dependency where there is a light source but this is quite small. Luminescence is caused by chemical, biochemical, or crystallographic changes, the motions of subatomic particles, or radiation-induced excitation of an atomic system. Typically, for the present invention, it will be a result of electronic processes within the atomic structures of the dye or pigment of the ink, which results in the emitted light having a longer wavelength or lower energy than the absorbed light. The colourshifting layer 10 also acts as an optical band pass filter for the light reflected and emitted by coloured regions 15,16.

The resulting view to an observer will be the result of additive colour mixing of the light reflected by the colourshifting layer 14 and that which is reflected/emitted by the coloured regions 15,16 and transmitted through the colourshifting layer 14. In order to achieve the aforementioned effect, the inks for the regions 15,16 must be carefully selected such that, at the first angle of incidence, the reflection spectrum of the non-luminescent coloured region 16 is substantially the same as the combined emission and reflection spectra of the luminescent coloured region 15 within the band pass of the colourshifting layer 14 (i.e. the range of wavelengths of light which the colourshifting layer will transmit) at that angle of incidence. In addition, at a second angle of incidence, the reflection spectrum of the non-luminescent region 16 is different from the combined emission and reflection spectra of the luminescent coloured region 15 within the band pass of the colourshifting layer 14 at that angle of incidence.

The following is a specific non-limiting example, for the purposes of illustrating the principle of the invention. In this example the colourshifting layer 14 is a multilayer polymer film, which will typically comprise alternate layers of two polymeric materials of different refractive indices. The selection of the materials will depend on the required reflection and transmission optical characteristics but typical examples of materials can be found in US-B-6531230. In this example the multilayer polymer film has reflection characteristic such that, when the film is viewed at normal incidence to the plane of the substrate, it reflects light predominantly in the wavelength range 550-750nm and transmits all other wavelength of visible light outside of this range. The wavelength ranges in which light is transmitted is called the band pass. In this example the multilayer polymeric film comprises alternating layers of PET (polyethylene terephthalate) and PMMA (polymethyl methacrylate).

The non-luminescent coloured region 16 is a conventional liquid based gravure ink with a pantone colour reference P124U and the luminescent coloured region 15 is a conventional fluorescent daylight ink with a pantone colour reference P804C. Daylight fluorescence occurs when light close to or within the ultra-violet range of the electromagnetic spectrum is absorbed and then emitted at wavelengths longer than those of the absorption. For example, blue light is reflected as yellow or red light and yellow light is reflected as red light. Furthermore the addition of the emitted light component and the conventional reflected components increases the intensity and brightness of these inks. In this example the applied coat weight of the conventional ink, which forms the non-luminescent coloured region 16, is 3 gsm and the applied coat weight for the daylight fluorescent ink, which forms the luminescent coloured region 15, is 3.5 gsm. Typically the coat weight for either ink will be in the range 2-5gsm.

Figure 4a shows the reflectance spectra for both the luminescent coloured region 16 and the non-luminescent coloured region 15 when the security device 10 is viewed at normal incidence. Note that, for the luminescent coloured region 16, the reflectance spectra is a result of both emitted light component and the conventional reflected light component. In addition the wavelength range, inside which light is reflected back from the colourshifting layer 14 when the layer is viewed at normal incidence, is shown by the region between the dotted lines. This is the reflectance band 21 and, outside this range, visible light will be transmitted though the colourshifting layer 14, i.e. the band pass. When viewing at normal incidence in ambient daylight, the colourshifting layer 14 will reflect back most of the peak reflectance of the two coloured regions 15,16. It can be seen (and illustrated by the hatched region 22) that outside of the reflectance band 21 and within the wavelength range 450-650nm, which is the most sensitive range for the naked eye, there is little difference in the overall reflectance for the coloured regions 15 and 16. The low difference in reflectance for coloured regions 15,16 results in these coloured regions 15,16 being indistinguishable when the security device 10 is viewed at normal incidence.

Figure 4b shows a similar spectra to Figure 4a, but in this case shows the reflectance band 21 for the colourshifting layer 14 when the security device 10 is viewed at an angle of 40° to normal incidence. At this viewing condition the colourshifting layer 14 transmits the reflected light from the coloured regions 15, 16 and there is now a significant difference in the reflectance characteristics of the coloured regions 15,16 within the wavelengths of visible light between 450-650nm which pass through the colourshifting layer 14, i.e. within the band pass, which again is illustrated by the hatched region 22. Therefore at this angle of viewing the viewer observes a clear difference between the coloured regions 15, 16.

In the wavelength range between 450-650nm, when the security device 10 is viewed at the first angle of incidence (for example a normal angle of incidence), the reflectance spectra will be considered to be substantially the same within the bandpass of the colourshifting layer 14 if the difference in reflectance at at least one wavelength is no greater than 30 %, and preferably no greater than 20% and even more preferably no greater than 10%. When viewed at the second angle of incidence (for example 40 degrees from normal incidence) the difference in reflectance at at least one wavelength within the range 450-650nm is greater than 40% %, and more preferably greater than 50% and even more preferably greater than 60%.

Any known method of, and apparatus for, the measurement of reflectance can be used. One particularly suitable apparatus for measuring reflectance is a Shimadzu UV3101 PC IR/Vis/UV spectrophotometer between the wavelength range of 450 and 650 nm. All the measurements are carried out in reflection mode. To obtain a difference in reflection (intensity), one measurement is subtracted from the other.

Referring to Figures 3a and 3b, at normal incidence (Figure 3a) the security device 10 exhibits a uniform appearance, which in this example is a yellow colour primarily due to the reflected light of the colourshifting layer 14. On changing the viewing direction to 40° away from normal incidence (Figure 3b) the contrast between coloured regions 15,16 can now be observed with the colour visible from coloured region 15 being a combination of the reflected light from the colourshifting layer 14 and the luminescent coloured ink and the colour visible from coloured region16 being a combination of the reflected light from the colourshifting layer 14 and the non-luminescent coloured ink. In this example the coloured region 16 defines the background 23 and the coloured region 15 defines indicia 24, such as the alphanumeric characters DLR 500. Thus at normal incidence the indicia 24 are not observed but they are revealed on changing the viewing direction to 40° away from normal incidence.

Different luminescent inks emit coloured light which is visible at different angles.

The coloured regions 15,16 may be fully or partly contiguous. However there may be gaps or partial gaps between the coloured regions 15,16. Most preferably the coloured regions 15,16 are applied in the form of indicia 24, such as words, numerals, patterns and the like, either in combination with each other or independently from each other. The indicia 24 may be positive or negative. Thus, when the security device 10 is viewed at an angle, a different image, indicia, pattern (or combination thereof) will become visible. The coloured regions 15,16 may alternatively be applied in fully or partially superimposed layers, or in non-adjacent layers, with another layer therebetween. As the coloured layers 15,16 are preferably opaque, where they are superimposed with another layer between, there needs to be at least one gap in one or both of the layers providing the coloured regions 15,16 to enable the underlying layer to be visible. Either or both of the coloured layers 15, 16 may be applied to the colourshift layer 14 and there may be other layers between the colourshift layer 14 and the coloured layers 15,16 as long as any such other layers are substantially transparent.

Figure 5 illustrates another embodiment of the present invention, which is a pseudo type of demetallised construction formed by one or more gaps 28 in the coloured layers 15,16. Colourshift layers 14 of the type used in this invention exhibit a colour in transmitted light which is complementary to the reflected colour. Therefore in transmitted light the viewer will observe in the gap 28 the complementary colour to the reflected colour for example, a green reflected colour produces a magenta transmitted colour.

In the example shown in Figure 2, the coloured region 16 slightly overlaps the coloured region 15. In the example shown in Figure 6, the coloured regions 15, 16 are in exact register. This may be preferable if the ink of coloured region 15 is not sufficiently opaque to block out the colour coming from the ink of coloured region 16.

Security devices 10 comprising liquid crystal materials, which may be used for the colourshifting layer 14, are inherently machine-readable due to the polarisation properties and wavelength selectivity of the liquid crystal materials. The machine readable-aspect of the security device 10 of the present invention can be extended further by the introduction of detectable materials in the existing liquid crystal, or alternate colourshifting materials, or an absorbing layer or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include, but are not limited to, fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

In one preferred embodiment, a pigment may be added to the ink of one or both of the coloured regions 15,16 which is machine readable, to produce one or more machine- readable or conducting areas. Alternatively a magnetic material or magnetic pigment, such as magnetite, may be used to produce one or more machine-readable magnetic areas.

In a further embodiment, only part of one or both of the coloured regions 15,16 may be provided with a magnetic pigment and the remainder provided with a non-magnetic pigment.

Where the colourshifting layer 14 comprises a carrier layer, a magnetic material may be applied, for example along both longitudinal edges of the carrier substrate. The magnetic material may be applied in the form of tramlines or another design. A suitable magnetic material is FX 1021 supplied by Ferron and this may be applied with a coat weight of, for example, 2-6 gsm.

In an alternative embodiment, a transparent magnetic layer can be incorporated at various positions within the structure of the security device 10. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in WO-A-03091953 and WO-A-03091952.

In a further embodiment of the present invention, the security device 10 may be provided with a reflection enhancing layer. In the embodiment illustrated in Figure 6, the reflection enhancing layer is a partially metallised or demetallised layer 25, formed from a metallised film of PET or a similar light transmitting layer, which has metallised or demetallised indicia. A demetallisation process provides at least one metallised area 26 having one or more gaps 27 therein which form the indicia. A metallisation process provides at least one metallised area 26 in the form of indicia surrounded by areas 27 which are not metallised (in the following these will be covered by the reference to demetallised gaps 27). In addition, the non-luminescent coloured region 16 also has one or more gaps 28. The partially metallised or demetallised layer 25 is applied to the second side 19 of the basic structure 20 (comprising the colourshifting layer 14 and the coloured regions 15,16), so as to cover the coloured regions 15,16, with the demetallised gaps 27 in register with the gaps 28 in the non-luminescent coloured region 16. The gaps 27,28 do not need to have the same dimensions, but there must be at least some partial overlap of the gaps 27,28 as the overlap of the gaps 27,28 will define indicia observed in transmitted light. A suitable adhesive, which is preferably clear and colourless, may be used to adhere the partially metallised or demetallised layer 25 to the basic structure 20. In the gaps 27,28 an observer would see the effect of the complimentary colours (as mentioned above in relation to Figure 5). The gaps 28 could be in either the non-luminescent coloured region 16 (as illustrated) or the luminescent coloured region 15 or in both. As a further alternative a gap could define the boundary between the non-luminescent coloured region 16 (as illustrated) or the luminescent coloured region 15. In the demetallised version of the security device, from the reverse side the security device 10 would appear metallised. The presence of metallised or other reflective areas, which could also be formed from a metallic ink, allows a further optical effect to be generated which is viewable from the reverse side of the device in the form of a diffractive or reflective relief structure.

Figure 6 could be modified by replacing the metallised PET film with a PET substrate coated with a lacquer in which a diffractive or reflective relief structure is formed. A reflection enhancing layer, such as a metal or alloy, may then be deposited across the relief structure, e.g. by vacuum deposition. The remaining layers are then present as shown in Figure 6 with coloured layers 15,16 being applied over the reflection enhancing layer. In this embodiment, an optical effect arising from the diffractive or refractive relief structure is observed from the opposite side of the security device 10 to the colourshifting effect. The diffractive or reflective relief structure can be of any type and can exhibit any desired visual effect. For example, the structure may be a hologram or kinegram with any desired replay image, or could be a diffraction grating or series of reflective facets.

In yet a further embodiment, as shown in Figure 7, the security device 10 is again provided with demetallised indicia (or these could be metallised indicia). However, only the non-luminescent coloured region 16 is applied to the colourshifting layer 14 and the partially demetallised light transmitting layer 25 is applied to cover the non-luminescent coloured region 16. The luminescent coloured region 16 has one or more gaps 28. This embodiment of the security device 10 will appear the same as shown in Figure 2. The metallised layer helps to increase the opacity of the security device 10 and to potentially allow the security device 10 to have machine readable conductive properties.

The security device 10 of the present invention can have a variety of uses. It is particularly suitable for incorporation into a base substrate, preferably made of paper or polymer, to form a secure substrate 11 from which secure documents can be produced. The security device 10 can be in any of the conventional formats known in the prior art, for example patches, foils, stripes, strips or threads. The security device 10 can be arranged either wholly on the surface of the substrate 11, as in the case of a stripe or patch, or so that it is only partly visible at the surface of the substrate 11. The latter arrangement is typically known as a windowed security thread. Security threads are now present in many secure documents, in particular many of the world’s currencies as well as vouchers, passports, travellers’ cheques and the like. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows 12 at one or both surfaces of the document. One method for producing paper with windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider, partially exposed threads into a paper or other substrate. Wide threads, typically having a width of 2-6mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that used in the present invention. Figure 1 shows the security device 10 incorporated into a secure substrate 11 as a windowed thread. The security device 10 is exposed in windows 12 in the surface of the secure substrate 11 and is covered by the areas between the windows 12, known as bridges 13.

The security device 10 may be subsequently incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished secure substrate 11. Methods of incorporating security devices 10 in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security device 10 is wholly exposed at one surface of the secure substrate 11 in which it is partially embedded, and partially exposed in windows at the other surface of the secure substrate 11.

Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer. Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300659 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. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-0039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501, EP-A-724519, WO-A-03054297 and EP-A-1398174.

Any self-supporting film in the security device 10 of the present invention may form the substrate for a polymer based security document. Opacifying layers may be applied to regions where the optical effect of the security element is not required which are then subsequently overprinted with conventional banknote printing.

In the case of a stripe or patch, the security device 10 may be prefabricated on a transfer strip and subsequently transferred to the substrate. The security device 10 can be applied to the document using an adhesive layer 17, which is applied either to the security device 10 or the surface of the secure substrate 11 to which the security device 10 is to be applied. After transfer, the transfer strip may be removed leaving the security device 10 exposed. Alternatively the transfer strip can be left in place to provide an outer protective layer.

Following the application/incorporation of the security device 10, the secure substrate 11 generally undergoes further standard security printing processes including one or more of the following; wet or dry lithographic printing, intaglio printing, letterpress printing, flexographic printing, screen-printing, and/or gravure printing. The secure substrate 11 is then typically cut into a number of secure documents. In a preferred embodiment, and to increase the effectiveness of the security device 10 against counterfeiting, the design of the security device 10 can be linked to the secure document by content and registration to the designs and identifying information provided on the secure document.

The security substrate 11 can be used to make a wide variety of security documents, such as documents of value and means of identification, including bank notes, passports (or pages thereof), identification cards, certificates, drivers licences and the like.

Claims

CLAIMS:

1. A security device comprising a colourshifting layer, and at least one luminescent coloured region and at least one non-luminescent coloured region located on one side of the colourshifting layer, wherein:- at a first angle of incidence, there is a first difference between the reflection spectrum of the at least one non-luminescent coloured region and the combined emission and reflection spectra of the at least one luminescent coloured region within the band pass of the colourshifting layer such that the security device has a first appearance; and at a second angle of incidence, there is a second difference between the reflection spectrum of the at least one non-luminescent coloured region and the combined emission and reflection spectra of the at least one luminescent coloured region within the band pass of the colourshifting layer such that the security device has a second appearance, which is different from the first appearance.

2. A security device as claimed in claim 1 in which, in a wavelength range between 450-650nm, the first difference at at least one wavelength is less than or equal to 30%.

3. A security device as claimed in claim 2 in which, in a wavelength range between 450-650nm, the first difference at at least one wavelength is less than or equal to 20%.

4. A security device as claimed in any one of the preceding claims in which, in a wavelength range between 450-650nm, the second difference at at least one wavelength is greater than 40%.

5. A security device as claimed in claim 4 in which, in a wavelength range between 450-650nm, the second difference at at least one wavelength is greater than 50%, more preferably greater than 60%.

6. A security device as claimed in any one of the preceding claims in which, at the first angle of incidence, the first appearance of the security device is substantially uniform in that the at least one luminescent coloured region and the at least one non-luminescent coloured region are substantially indistinguishable from each other.

7. A security device as claimed in claim 6 in which the first angle of incidence is a normal angle of incidence.

8. A security device as claimed in any one of the preceding claims in which, at the second angle of incidence, the second appearance of the security device is such that the at least one luminescent coloured region and the at least one non-luminescent coloured region are distinguishable from each other.

9. A security device as claimed in claim 8 in which the at least one luminescent coloured region and/or the at least one non-luminescent coloured region define indicia which are visible at the second angle of incidence.

10. A security device as claimed in claim 9 in which the indicia are positive or negative.

11. A security device as claimed in any one of the preceding claims in which the at least one luminescent coloured region and the at least one non-luminescent coloured region are at least partially contiguous.

12. A security device as claimed in any one of the preceding claims in which there are gaps between the at least one luminescent coloured region and the at least one non-luminescent coloured region.

13. A security device as claimed in any one of claims 1 to 10 in which the at least one luminescent coloured region and the at least one non-luminescent coloured region are at least partially superimposed.

14. A security device as claimed in claim 13 in which the at least one luminescent coloured region and the at least one non-luminescent coloured region are in register with each other.

15. A security device as claimed in any one of the preceding claims further comprising a layer separating the at least one luminescent coloured region and the at least one non-luminescent coloured region.

16. A security device as claimed in any one of the preceding claims in which the at least one luminescent coloured region and/or the at least one non-luminescent coloured region is applied directly to the colourshift layer.

17. A security device as claimed in any one of preceding claims further comprising a layer between the at least one luminescent coloured region and/or the at least one non-luminescent coloured region and the colourshift layer.

18. A security device as claimed in any one of the preceding claims in which a plurality of luminescent coloured regions and/or non-luminescent coloured regions are located on the one side of the colourshifting layer.

19. A security device as claimed in any one of the preceding claims further comprising a layer of adhesive applied to one or both sides of the security device.

20. A security device as claim 19 in which the adhesive is clear and transparent.

21. A security device as claimed in any one of the preceding claims in which the colourshifting layer comprises a multilayer polymeric film.

22. A security device as claimed in of claims 1 to 20 in which the colourshifting layer comprises a liquid crystal film.

23. A security device as claimed in any one of claims 1 to 20 in which the colourshifting layer comprises a colourshifting material applied to carrier layer.

24. A security device as claimed in any one of the preceding claims in which the at least one luminescent coloured region and/or the at least one non-luminescent coloured region are formed by a printing process and/or a coating.

25. A security device as claimed in any one of the preceding claims in which the at least one luminescent coloured region and/or the at least one non-luminescent coloured region comprises a machine readable material.

26. A security device as claimed in any one of the preceding claims further comprising a magnetic layer.

27. A security device as claimed in any one of the preceding claims further comprising a reflection enhancing layer.

28. A security device as claimed in claim 27 in which the reflection enhancing layer is a partially metallised or a partially demetallised layer having at least one metal free area.

29. A security device as claimed in claim 28 in which the at least one metal free area is in at least partial register with gaps in the at least one luminescent coloured region and/or the at least one non-luminescent coloured region.

30. A security device as claimed in claim 27 in which the reflection enhancing layer is a diffractive or a reflective relief structure.

31. A security substrate comprising a base substrate and the security device of any one of the preceding claims.

32. A security substrate as claimed in claim 31 in which the security device is applied to a surface of the base substrate.

33. A security substrate as claimed in claim 31 in which the security device is at least partially embedded in the base substrate.

34. A security substrate as claimed in claim 33 in which the security device is exposed in windows in one or both surfaces of the base substrate.

35. A security substrate as claimed in claim 33 or claim 34 in which the security device is wholly exposed at one surface of the base substrate.

36. A security document made from the security substrate as claimed in any one of claims 31 to 35.

37. A security document as claimed in claim 36 in which indicia visible in the second appearance are linked by content and registration to a design or identifying information on the secure document.

38. A security document as claimed in claim 36 or claim 37 wherein the security document is a document of value or means of identification, such as a banknote, a cheque, a certificate, a passport, a passport page, an identification card or a drivers licence.