Patent Grant
11225102
The present invention relates to security devices suitable for use in security documents such as banknotes, identity documents, passports, certificates and the like, as well as methods for manufacturing such security devices.
To prevent counterfeiting and to enable authenticity to be checked, security documents are typically provided with one or more security devices which are difficult or impossible to replicate accurately with commonly available means such as photocopiers, scanners or commercial printers.
One well known type of security device is one which uses a colour shifting element to produce an optically variable effect that is difficult to counterfeit. Such a colour shifting element generates a coloured appearance which changes dependent on the viewing angle. Examples of known colour shifting structures include photonic crystals, liquid crystals, interference pigments, pearlescent pigments, structured interference materials or thin film interference structures including Bragg stacks.
It is also known in the art that the optical effect produced by a colour shifting element can be modified by introducing a film comprising a surface relief over the colour shifting element, wherein the surface relief comprises a plurality of angled facets that refract the light incident to, and reflected from, the colour shifting element so as to provide a different optical effect to the viewer. For example, such an additional “light control layer” may produce colour shifting effects which are visible closer to a normal angle of viewing with respect to the device, and may enable more colours to be viewed on tilting the device as compared to the colour shifting element in isolation.
In order to increase the difficulty of counterfeiting such a security device, it is beneficial for the security device to exhibit more than one colour shifting effect. The amount of refraction of light by a surface relief positioned above a colour shifting element (and therefore the exhibited colour shifting effect) may be manipulated by using a surface relief having varying facet angles. The use of different facet angles allows for different amounts of refraction and, correspondingly, different colour shifting effects. However, although this is beneficial for security, it is difficult, time consuming and costly to produce a surface relief having a plurality of different facet angles.
In accordance with a first aspect of the invention there is provided a security device comprising: a colour shifting element that exhibits different colours dependent on the angle of incidence of light impinging upon the colour shifting element, and; an at least partially transparent light control layer covering at least a part of the colour shifting element and comprising a surface relief adapted to modify the angle of light incident upon the light control layer, wherein; the light control layer comprises at least first and second functional regions having different refractive indices such that light incident upon the first functional region impinges upon the colour shifting element at a first angle of incidence, and light incident upon the second functional region impinges upon the colour shifting element at a second, different, angle of incidence.
The inventors have realised that they can provide a security device that provides a striking visual effect to a viewer through a combination of the optical effects generated by the colour shifting element and the surface relief of the light control layer. Particularly advantageously, the first and second functional regions having different refractive indices allows control and manipulation of the visual effect exhibited to a viewer of the security device without having to vary the geometry of the surface relief in different areas of the light control layer which is both difficult and time-consuming to do. The light incident upon the first and second functional regions impinges upon the colour shifting element at different angles of incidence due to different amounts of refraction as a result of the different refractive indices.
Typically, the first and second functional regions each comprise a surface relief. The surface relief of the first functional region may be different to the surface relief of the second functional region. In particularly advantageous embodiments, the first and second functional regions comprise substantially the same surface relief. For example, in some embodiments the light control layer may be in the form of a substantially uniform surface relief such that the first and second functional regions comprise substantially the same surface relief.
The expression “surface relief” is used to refer to a structure of elevations and depressions. This may be described as a non-planar part of the outwardly facing surface of light control layer. The surface relief typically has a plurality of facets angled with respect to the colour shifting element so as to define a plurality of elevations and depressions. Light incident upon the light control layer is refracted at the interface between the (typically) air and the angled facets of the surface relief so as to modify the angle of light subsequently incident upon the colour shifting element. The surface relief typically has a pitch (e.g. the distance between adjacent elevations) in the range of 1-100 μm, more preferably 5-70 μm, and structure depth (e.g. the height of an elevation) in the range of 1-100 μm, more preferably 5-40 μm. The light control layer is at least partially transparent, which may also include “translucent”. The light control layer covers at least a part of the colour shifting element. In some examples the light control layer may cover substantially the entirety of the colour shifting element. The light control layer is typically colourless.
The expression “colour shifting element” is used to refer to any material which can selectively reflect or transmit incident light to create an optically variable effect, in particular an angularly dependent coloured reflection or transmission. It is envisaged that at least at one viewing angle, under illumination by visible light, the wavelength (or range of wavelengths) of light exhibited by the colour shifting element will be in the visible light range and therefore seen by the naked human eye as a visible colour. At at least one viewing angle, under illumination by visible light, the wavelength (or range of wavelengths) of light exhibited by the colour shifting element may be in the non-visible light range, for example the infra-red range of the electromagnetic spectrum. In such an instance the colour shifting element will appear black. In the context of the present specification, black is taken to be a colour. Under non-visible light illumination, the wavelength (or range of wavelengths) of light exhibited by the colour shifting element may be in the non-visible light range.
As a result, due to the difference in refractive index between the first and second functional regions of the light control layer, light incident upon the first and second functional regions at the same angle will impinge upon the colour shifting element at different angles, thereby generating light reflected from the colour shifting element having different wavelengths. Therefore, light corresponding to the first and second functional regions will exhibit different colours. By controlling the refractive index of the first and second functional regions, the exhibited colours may be controlled to provide a desired visual effect at at least one viewing angle.
Furthermore, upon changing viewing angle between first and second viewing positions (typically by “tilting” the device relative to a viewer), different colours may be exhibited by the same functional region, for example a red to green colour shift. Typically, the device is tilted relative to a viewer along a first tilt axis. The first tilt axis typically lies substantially in the plane of the security device. Alternatively or in addition, a viewer may change their position to change the viewing angle.
Examples of such a colour shifting element include photonic crystals, liquid crystals, interference pigments, pearlescent pigments, structured interference materials or thin film interference structures including Bragg stacks. A particularly suitable material for the colour shifting element is a liquid crystal film.
In general the colour shifting element may be substantially opaque or partially transparent (with various examples having been described above). A partially transparent colour shifting element (for example a liquid crystal film) transmits at least some of the light that is incident upon it as well as providing an optical effect in reflection. An example of a substantially opaque colour shifting element is an optically variable pigment. Optically variable pigments having a colour shift between two distinct colours, with the colour shift being dependent on the viewing angle, are well known. The production of these pigments, their use and their characteristic features are described in, inter-alia, U.S. Pat. Nos. 4,434,010, 5,059,245, 5,084,351, 5,135,812, 5,171,363, 5,571,624, EP-A-0341002, EP-A-0736073, EP-A-668329, EP-A-0741170 and EP-A-1114102.
Optically variable pigments having a viewing angle-dependent shift of colour are based on a stack of superposed thin-film layers with different optical characteristics. The hue, the amount of colour-shifting and the chromaticity of such thin-film structures depend inter alia on the material constituting the layers, the sequence and the number of layers, the layer thickness, as well as on the production process. Generally, optically variable pigments comprise an opaque totally reflecting layer, a dielectric layer with an index of refraction of 1.65 or less deposited on top of the opaque layer, and a semi-transparent partially reflecting layer applied on the dielectric layer.
The security device may be viewed in reflection or transmission. If the device is intended to be viewed in reflection and comprises a partially transparent colour shifting element such as a liquid crystal film, it is preferable that the security device further comprises an absorbing element comprising a light-absorbing material positioned on a distal side of the colour shifting element with respect to the light control layer (i.e. such that the colour shifting element is positioned between the light-absorbing material and the viewer) and operable to at least partially absorb light transmitted through the colour shifting element. Such a light-absorbing element positioned under the colour shifting element substantially absorbs light that is transmitted through the colour shifting element (and absorbs light originating from behind the colour shifting element with respect to the viewer), and therefore light reflected from the colour shifting element dominates. In the case where a substantially opaque colour shifting element is used, such an absorbing element is not required. In some embodiments, such an absorbing element may be provided in the form of indicia, such that, when viewed in reflected light, the colour shifting element is visible in the form of the indicia.
Throughout this specification, the term “light” refers to both visible light (see below) and non-visible light outside the visible spectrum, such as infra-red and ultraviolet radiation. “Visible light” refers to light having a wavelength within the visible spectrum, which is approximately 400 to 750 nm. It is most preferable that the visible light is white light, i.e. contains substantially all the visible wavelengths in more or less even proportion. The ultra-violet spectrum typically comprises wavelengths from about 200 nm to about 400 nm, and the infra-red spectrum typically comprises wavelengths from about 750 nm to 1 mm.
Throughout this specification, the term “colour” means a colour which can be seen by the naked human eye under the stated illumination conditions. This includes achromatic hues such as black, grey, white, silver etc., as well as chromatics such as red, blue, yellow, green, brown etc. “Substantially the same” colours are those which appear the same as one another in a cursory inspection (by the naked human eye) although they may not be an exact match under close examination. By the same logic, “different” colours are those which clearly present a contrast to one another that is visible to the naked human eye even without a close inspection. The difference might be in terms of the colour's hue or tone or both.
For example, in preferred embodiments, two colours will be considered substantially the same as one another if the Euclidean distance ΔE*ab between them in CIELAB colour space (i.e. the CIE 1976 L*a*b* colour space) is less than 3, more preferably less than 2.3. The value of ΔE*ab is measured using the formula
ΔE*ab=√{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}
Where ΔL*, Δa* and Δb* are the distance between the two colours along the L*, a* and b* axes respectively (see “Digital Color Imaging Handbook” (1.7.2 ed.) by G. Sharma (2003), CRC Press, ISBN 0-8493-0900-X, pages 30 to 32). Conversely, if ΔE*ab is greater than or equal to 3 (or, in more preferred embodiments, greater than or equal to 2.3), the two colours will be considered different. The colour difference ΔE*ab can be measured using any commercial spectrophotometer, such as those available from Hunterlab of Reston, Va., USA.
Typically, at least at one viewing angle, the first and second functional regions exhibit different colours.
Typically, the surface relief of the light control layer is further adapted to modify the angle of light from the colour shifting element. In the same manner that light incident upon the security device is refracted at the interface between the air and the light control layer, fight from the colour shifting element is also refracted at the interface between the light control layer and the air. As a result, the presence of the light control layer having the surface relief means that a different colour is exhibited to an observer viewing the security device at a first viewing angle than would be exhibited to an observer viewing the colour shifting element in isolation at that viewing angle.
Typically, the first functional region has a refractive index in the range of 1.2 to 1.8, preferably in the range of 1.35-1.7. Similarly, the second functional region has a refractive index in the range of 1.2 to 1.8, preferably in the range of 1.35-1.7. As discussed above, the first functional region has a different refractive index to the second functional region. The difference in refractive index between the first and second functional regions is typically at least 0.1, preferably at least 0.2 and even more preferably at least 0.3.
In preferred embodiments, at least one of the first and second functional regions defines indicia. This provides a particularly striking effect due to the fact that at least at one viewing angle, the first and second functional regions exhibit different colours, such that the indicia will appear coloured against a different colour background. Furthermore, upon tilting the device relative to the viewer, these colours may change due to the characteristics of the colour shifting element. Typically such indicia comprises at least a digit, letter, geometric shape, symbol, image, graphic or alphanumerical text. A particularly striking effect is exhibited if, at one viewing angle (typically a normal viewing angle with respect to the security device), both the first and second functional regions exhibit substantially the same colour such that they are not distinguishable. On tilting the device, the first and second regions exhibit different colours, thus providing a “hidden image” effect. Both the first and second functional regions may exhibit substantially the same colour at a viewing angle even though light incident upon the light control layer subsequently impinges upon the colour shifting element at different angles, due to the fact that a range of wavelengths of light reflected by the colour shifting element may be perceived by the naked eye as the same colour. On tilting the device, the difference in angles of incidence (and therefore the wavelengths of reflected light) on the colour shifting element may increase such that the different colours are exhibited.
In some embodiments the first and second functional regions substantially abut each other. In other words there is no gap between the first and second functional regions. In alternative embodiments, the first and second functional regions may be spaced apart. In such embodiments the first and second functional regions are typically laterally spaced apart. In yet further embodiments the light control layer may comprise three or more functional regions, with some functional regions substantially abutting one another and some being spaced apart. In the cases where the functional regions are spaced apart, the region between the functional regions may be described as a “non-functional” region of the light control layer in that is does not substantially modify the angle of light to or from the colour shifting element. Such a non-functional region does not comprise a surface relief. The non-functional region may therefore comprise a substantially planar portion substantially parallel with the colour shifting element (i.e. does not comprise a surface relief), or may comprise no light control layer material, such that the colour shifting element is exposed between the first and second regions. In this second case the spaced apart first and second functional regions are still part of the same light control layer. The use of first and second functional regions spaced apart by a non-functional region provides the ability to exhibit further coloured effects.
The material of the light control layer may be provided by at least one of intaglio printing, gravure, flexo printing, inkjet printing, knife coating, curtain or blade techniques. Typically, the light control layer comprises a polymer. The surface relief of the light control layer may be formed in a single step, for example by an embossing, extrusion or cast curing process. This process typically occurs after the provision of the light control material. For example, it is envisaged that a layer of polymer comprising regions of different refractive index will be applied to a colour shifting element, and an embossing die will subsequently be provided having a surface structure corresponding to the desired light control layer. As discussed, a particular advantage of the present invention is being able to control the optical effect exhibited to a viewer through the use of varying refractive index of the light control layer rather than changing the geometry of the light control layer in particular regions. Therefore, the same embossing die may be used to manufacture a plurality of different security devices that exhibit different optical effects.
The light control layer typically comprises a UV curable material. Suitable UV curable materials may comprise a polymeric material which may typically be of one of two types of polymeric resin, namely:
a) Free radical cure resins, which are typically unsaturated resins or monomers, pre-polymers, oligomers etc. containing vinyl or acrylate unsaturation for example and which cross-link through use of a photo initiator activated by the radiation source employed e.g. UV.
b) Cationic cure resins, in which ring opening (e.g. epoxy types) is effected using photo initiators or catalysts which generate ionic entities under the radiation source employed e.g. UV. The ring opening is followed by intermolecular cross-linking.
The radiation used to effect curing is typically UV radiation but could comprise electron beam, visible, or even infra-red or higher wavelength radiation, depending upon the material, its absorbance and the process used. Examples of suitable curable materials include UV curable acrylic based clear embossing lacquers or those based on other compounds such as nitro-cellulose.
The curable material could be elastomeric and therefore of increased flexibility. An example of a suitable elastomeric curable material is aliphatic urethane acrylate (with suitable cross-linking additive such as polyaziridine).
Examples of UV-curable monomers that may be used to form a polymeric light control layer include 1-Ethoxylated phenol acrylate, 2-Ethoxylated phenol acrylate, Bisphenol A ethoxylated acrylate, Phenoxy benzyl acrylate, 1-Ethoxylated-o-phenylphenol acrylate and Benzyl acrylate, Bisphenol fluorine diacrylate and Modified bisphenol fluorine diacrylate.
Examples of UV-curable oligomers that may be used to form a polymeric light control layer include halogen oligomers such as Bromo epoxy acrylate, and non-halogen oligomers such as Epoxy acrylate and Urethane acrylate.
In some embodiments, a polymeric light control layer may comprise at least one monomer and at least one oligomer.
Examples of UV-curable polymers that may be used to form the light control layer include Poly(pentabromophenyl methacrylate), Poly(pentabromophenyl acrylate), Poly(pentabromobenzyl methacrylate), Poly(pentabromobenzyl acrylate), Poly(2,4,6-tribromophenyl methacrylate), Poly(vinylphenylsulfide), Poly(1-napthyl methacrylate), Poly(2-vinylthiophene), Poly(2,6-dichlorostyrene), Poly(N-vinylphthalimide), Poly(2-chlorostyrene), Poly(pentachlorophenyl methacrylate), Poly(1,1,1,3,3,3-hexafluoroisopropyl acrylate), Poly(2,2,3,3,4,4,4-heptafluorobutyl acrylate), Poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate), Poly(2,2,3,3,3-pentafluoropropyl acrylate), Poly(1,1,1,3,3,3-hexafluoroisopropyl methacrylate), Poly(2,2,3,4,4,4-hexafluorobutyl acrylate), Poly(2,2,3,4,4,4-hexafluorobutyl methacrylate), Poly(2,2,3,3,3-pentafluoropropyl methacrylate), Poly(2,2,2-trifluoroethyl acrylate), Poly(2,2,3,3-tetrafluoropropyl acrylate), Poly(2,2,3,3-tetrafluoropropyl methacrylate) and Poly(2,2,2-trifluoroethyl methacrylate).
Typically, the surface relief of the light control layer comprises at least one microstructure. In preferred embodiments, the microstructure is a linear microprism and the surface relief comprises an array of linear microprisms. However, a number of different surface reliefs of the light control layer are envisaged. For example, the surface relief may comprise two or more arrays of linear microprisms, wherein the long axes of one array are angularly offset from the axes of the other array. Such a surface relief would provide a rotational optical effect as well as the colour shifting effect dependent on a tilt angle of the security device, wherein the rotational effect is dependent on the azimuthal angle of viewing with respect to the arrays of linear micro prisms. The optical effect due to the presence of a microprism array will be more readily observed when the device is viewed in an azimuthal direction perpendicular to the long axes of the array rather than in an azimuthal direction parallel to the long axes of the array.
Other forms of microprismatic structures are envisaged, for example structures comprising microprisms having an asymmetrical structure or a repeating faceted structure.
The microstructure may be a one dimensional microstructure. By “one dimensional” it is meant that the optical effect provided by the microstructure is primarily observed in one rotational viewing direction with respect to an individual microstructure, typically perpendicular to a long axis of the microstructure. However, a surface relief comprising a two dimensional microstructure is also envisaged wherein the optical effect due to the presence of the microstructure is readily observed at two or more rotational viewing directions. Examples of such a two-dimensional microstructure include corner cubes and pyramidal structures. The surface relief may alternatively comprise a lenticular array having a curved surface structure.
In accordance with a second aspect of the present invention there is provided a security article comprising a security device according to the first aspect, wherein the security article is preferably a security thread, strip, patch, label, transfer foil or a polymer substrate. The polymer substrate may be a data page for a passport, for example.
In accordance with a third aspect of the present invention there is provided a security document comprising a security device according to the first aspect, or a security article according to the second aspect. The security device or security article may be located in a transparent window region of the document, or inserted as a window thread, or affixed to a surface of the document. Such a security document preferably comprises a banknote, identity document, passport, cheque, visa, licence, certificate or stamp. Where the security article is a polymer substrate, the polymer substrate is typically a laminate for a data page of security document such as a passport or identification card. Another scenario is that the polymer substrate could be the substrate of a polymer banknote i.e. the security device is formed directly on the polymer banknote substrate.
In accordance with a fourth aspect of the present invention there is provided a method of manufacturing a security device, the method comprising: providing an at least partially transparent light control layer so as to cover at least a part of a colour shifting element that exhibits different colours dependent on the angle of incidence of light impinging upon it, wherein; the light control layer comprises a surface relief adapted to modify the angle of light incident upon the light control layer, and further wherein; the light control layer comprises at least first and second functional regions having different refractive indices such that light incident upon the first functional region impinges upon the colour shifting element at a first angle of incidence, and light incident upon the second functional region impinges upon the colour shifting element at a second, different, angle of incidence.
As in the first aspect, the expression “colour shifting element” is used to refer to any material which can selectively reflect or transmit incident light to create an optically variable effect, in particular an angularly dependent coloured reflection or transmission. It is envisaged that at least at one viewing angle, under illumination by visible light, the wavelength (or range of wavelengths) of light exhibited by the colour shifting element will be in the visible light range and therefore seen by the naked human eye as a visible colour. At at least one viewing angle, under illumination by visible light, the wavelength (or range of wavelengths) of light exhibited by the colour shifting element may be in the non-visible light range, for example the infra-red range of the electromagnetic spectrum. In such an instance the colour shifting element will appear black. Under non-visible light illumination, the wavelength (or range of wavelengths) of light exhibited by the colour shifting element may be in the non-visible light range.
Typically, the material of the light control layer is provided by at least one of intaglio printing, gravure, flexo printing, inkjet printing, knife coating, curtain or blade techniques. The material may be any of the materials set out above in the first aspect of the invention for example.
The surface relief of the light control layer may be formed by one of embossing, extrusion or cast curing, typically subsequently to the provision of the light control layer material to a colour shifting element.
In some embodiments the surface relief is provided by cast curing, and the first and second functional regions are cured substantially simultaneously. In other embodiments the surface relief is provided by cast curing, and the first functional region and second functional region are cured at different times.
In accordance with a fifth aspect of the invention there is provided a method of forming a security article comprising the method of forming a security device according to the fourth aspect, wherein the security article is preferably a security thread, strip, patch, label, transfer foil or a polymer substrate.
In accordance with a sixth aspect of the invention there is provided a method of forming a security document comprising the method of forming a security device according to the fourth aspect, or the method of forming a security article according to the fifth aspect, wherein the security device or security article is preferably located in a transparent window region of the document, or is inserted as a window thread, or is affixed to a surface of the document. Such a security document preferably comprises a banknote, identity document, passport, cheque, visa, licence, certificate or stamp.
Examples of the invention will now be described with reference to the attached drawings, in which:
As is understood in the art, when light is incident upon the colour shifting element 10, some of the light is reflected and undergoes Bragg reflection. The wavelength (and hence the colour exhibited to the viewer 50) of the reflected light is dependent on the structure and composition of the colour shifting element. In this particular example, as the colour shifting element is partially transparent, the absorbing element is used to substantially absorb the light transmitted through the colour shifting element in order to enhance the optical effect exhibited to the viewer when viewing the colour shifting element in reflection.
As the viewing angle ⊖va increases (and therefore the angle of incidence ⊖I of the incident light increases), the wavelength of the reflected light generally decreases due to a change in path length such that a colour change is exhibited to the viewer on a change of viewing angle (e.g. “tilting” the colour shifting element). Typically this may be a red to green colour shift or an infra-red (i.e. the colour shifting element exhibits a black colour) to red colour shift.
For the following discussion, we will consider the combination of the colour shifting element and surface relief as a security device 100, with a viewing angle ⊖va1 of the device being defined with respect to the normal of the colour shifting element, as in the example of
At a viewing angle ⊖va1, light is incident upon the device at an angle of incidence ⊖I, as shown by light ray 1. When light ray 1 is incident upon facet 22 of the surface relief, it is refracted due to the difference in refractive index between the medium in which incident ray 1 travels (typically air with a refractive index nair˜1) and the material of the surface relief. In the present example, and using the notation seen in
nair sin ⊖i=n1 sin ⊖r, (1)
where ⊖i and ⊖r are the angles of incidence and refraction, respectively, of the incident light ray 1 with respect to the normal Nsr.
We can now see that the refracted light ray is incident on the colour shifting element 10 with larger angle of incidence ⊖va′ (with respect to the normal Ncs) than the angle of incidence ⊖I if the surface relief were not present. As a result, the wavelength of light reflected from the colour shifting element 10 (shown at light ray 3) is different (i.e. exhibits a different colour) as compared to if the surface relief were not present. The reflected light ray 3 is refracted at facet 24 and observed by viewer at viewing angle ⊖va1.
In the example shown in
As it is the angle of incidence of a light ray on the colour shifting element that determines the wavelength, and therefore the perceived colour, of light exhibited to a viewer of the device 100, we can see that the presence of the surface relief positioned between the colour shifting element and the viewer enables the control of the colour exhibited by the device.
Moreover, we can see that ⊖r (and therefore ⊖va′) is dependent upon the refractive index of the surface relief material. The inventors have advantageously realised that they can control the optical effect exhibited to a viewer of such a device by varying the refractive index of the surface relief material. This is particularly advantageous as the surface relief itself may be uniform across the device, with only the refractive index of the surface relief material varying.
Table 1 below shows the effect that different refractive indices of the surface relief material have on the angle of incidence ⊖va′ upon the colour shifting element in comparison with the angle of incidence of light upon the device, ⊖I. The numerical figures in Table 1 are for a symmetrical triangular linear prism with a top angle αtop of 90°, as described above in
Similarly, Table 2 shows the effect that different refractive indices of the surface relief material have on the angle of incidence ⊖va′ upon the colour shifting element for a symmetrical triangular prism with a top angle atop of 60°.
As can be seen from Tables 1 and 2, for a given angle of incidence upon the device ⊖I, as the refractive index of the surface relief material increases, so does the angle of incidence of the light incident upon the colour shifting element. In general, as the angle of incidence upon the colour shifting element increases, the wavelength of light reflected from the colour shifting element decreases (i.e. red to blue). Therefore, the colour exhibited to a viewer of a device changes with refractive index. This can be used to allow a device to exhibit regions of different colour at a given viewing angle, while the device still maintains its colour shifting properties upon tilting. Examples of this will now be explained below.
The light control layer 20 comprises a microprismatic structure comprising an array of linear microprisms that define a surface relief, as described above with reference to
As can be seen in
The visual effect of device 200 will be explained with reference to two viewing angles ⊖va1 and ⊖va2, shown in
The visual effect exhibited to a viewer by the device 200 at viewing angle ⊖va1 is schematically illustrated in
Furthermore, although the surface relief of the light control layer will refract the incident light for normal viewing such that the light is incident on the colour shifting element 10 at an angle of incidence ⊖va′ that is not 0°, typically this incident angle ⊖va′ will be small (of the order of <5°) such that the reflected light from the colour shifting element will still be in the infra-red range of the electromagnetic spectrum. Therefore, at a normal angle of viewing ⊖va1, both functional regions A and B, and region C, will exhibit a black colour to a viewer. The device 200 will therefore appear a substantially uniform black colour with substantially imperceptible colour differences between functional regions, as schematically illustrated by the uniform shading across regions A, B and C in
Upon tilting the device 200 and viewing the device at viewing angle ⊖va2, region C will exhibit a red colour due to the colour shifting properties of the colour shifting element. Furthermore, due to the refraction of light incident upon the light control layer as described above in
This change in appearance of the uniform black colour to the three regions of different colour upon changing the viewing angle provides a striking visual effect to the viewer that is easy to authenticate, and yet difficult to replicate. It is particularly advantageous that the different optical effects can be provided by the different functional regions of the light control layer without having to vary the facet angles. A particularly interesting effect would be seen if one of the regions defined indicia (such as a digit, letter, geometric shape, symbol, image, graphic or alphanumerical text). Such indicia would not be distinguishable at a normal angle of viewing, and would only be revealed upon tilting the device.
It is worth noting that the use of a light control layer may also allow for a wider range of colours to be exhibited upon tiling the device as compared to a colour shifting element in isolation. This is because light that may have been totally internally reflected at the boundary between the colour shifting element and air may now travel through the light control layer due to the smaller change in refractive index at the colour shifting element boundary.
In
At a normal viewing angle ⊖va1 (seen in
Although the light control layer 20 of device 400 is described as having three functional regions of differing refractive index, it will be appreciated that two of the regions may have the same refractive index (e.g. regions A and C). Furthermore, light control layers having four or more functional regions are envisaged.
In this case the colour shifting element 502 comprises a partially transparent liquid crystal layer exhibiting a black (i.e. the reflected light is in the infra-red part of the EM spectrum) to red colour shift on tilting away from normal viewing. As the device is intended to be viewed in reflection an absorbing element (not shown) is used such that the reflected light dominates the visual impression given to a user.
The light control layer 510 is divided into a plurality of regions 504, 506, 508 which generally define the flag indicia, as shown in
At a normal angle of viewing, the security device 500 appears a uniform black colour, in the same manner as has been explained above with reference to
Although
In order to manufacture a security device according to the invention, the absorbing element (if required) and colour shifting element are first laid down on a suitable polymeric carrier substrate, such as a PET or BOPP foil. Here, all printing methods that are suitable for application of the various layers may be used, such as intaglio printing, gravure, flexo printing, inkjet printing, knife coating, curtain or blade techniques. Subsequently the light control layer is formed, as will be described below with reference to
For ease of description, we will first consider the manufacture of device 200 (illustrated in
The device substrate 200a is then introduced to a nip 3150 defined between the casting cylinder 3100 and first impression roller 3200, such that the material on the casting cylinder is transferred to the device substrate 200a. Having been formed into the correct surface relief structure, the curable material is cured by exposing it to appropriate curing energy such as radiation R from a source 3500. This preferably takes place while the curable material is in contact with the surface relief of the casting cylinder, although if the material is already sufficiently viscous this could be performed after separation. In the example shown, the material is irradiated through the device substrate 200a, although the source 3500 could alternatively be positioned above the device substrate 200a, e.g. inside cylinder 3100 if the cylinder is formed from a suitable transparent material such as quartz.
The device substrate, now comprising the cured light control layer material, passes through second nip 3160 defined by second impression roller 3300, and the light control layer, now affixed to the colour shifting element of the device, separates from the casting cylinder such that device 200 is formed. It will be appreciated that an appropriate registering of the applicators 3310, 3320, and the provision of the device substrate 200a is required in order to provide the desired functional regions A and B of the light control layer.
The device substrate 200a passes through first nip 4150 defined by impression roller 4410 and casting cylinder to form the surface relief of the light control layer in the curable material, wherein subsequently the curable material is cured by radiation R in the same manner as described above in relation to
The device substrate, now comprising the cured light control layer material, passes through second nip 4460 defined by second impression roller 4420, and the light control layer, now affixed to the colour shifting element of the device, separates from the casting cylinder such that device 200 is formed.
Different “tints” in
The use of inkjet printing advantageously allows the arrangement of the curable materials to be changed quickly and easily. For example, one security device may be printed so as the functional regions exhibit a first indicia, and a different security device may be printed to as to exhibit a second, different indicia. This has particular advantages in personalising security devices.
In each of the examples described above, the different curable materials of the light control layer are cured substantially simultaneously. However, it is envisaged that in some embodiments, a first curable material is applied and cured, and then subsequently a second curable material is applied and cured.
The radiation used to effect curing is typically UV radiation but could comprise electron beam, visible, or even infra-red or higher wavelength radiation, depending upon the material, its absorbance and the process used. Examples of suitable curable materials include UV curable acrylic based clear embossing lacquers or those based on other compounds such as nitro-cellulose.
The curable material could be elastomeric and therefore of increased flexibility. An example of a suitable elastomeric curable material is aliphatic urethane acrylate (with suitable cross-linking additive such as polyaziridine). Further examples of suitable materials for the light control layer were set out above in the summary of the invention section.
Subsequent to the manufacturing of the device, the polymer carrier substrate may be removed.
Security devices of the sort described above can be incorporated into or applied to any article for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc.
The security device or article can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. 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 in one or both surfaces of the base substrate. One method for producing paper with so-called 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 substrate. Wide threads, typically having a width of 2 to 6 mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.
The security device or article may be subsequently incorporated into a paper or polymer based substrate so that it is viewable from both sides of the finished security substrate. Methods of incorporating security elements 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 element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.
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.
The security device may also be applied to one side of a paper substrate so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.
Examples of such documents of value and techniques for incorporating a security device will now be described with reference to
The opacifying layers 2103a and 2103b are omitted across an area 2101 which forms a window within which the security device 100 is located. As shown best in the cross-section of
The device 100 may be viewed in transmission or reflection. In the case where it is to be viewed in reflection it is desirable to use a substantially opaque colour shifting element such as a printed ink comprising an optically variable pigment, although a partially transparent colour shifting element may be used in conjunction with an absorbing element as described above. It should be noted that in modifications of this embodiment the window 2101 could be a half-window with the opacifying layer 2103b continuing across all or part of the window over the colour shifting element 10. The banknote may also comprise a series of windows or half-windows. In this case different areas displayed by the security device could appear in different ones of the windows, at least at some viewing angles, and could move from one window to another upon tilting.
If desired, several different security devices 100 could be arranged along the thread, with different optical effects displayed by each. In one example, a first window could contain a first security device, and a second window could contain a second security device, both devices having light control layer surface reliefs comprising linear microprisms, with the linear microprisms of each device arranged along different (preferably orthogonal) directions, so that the two windows display different effects upon tilting in any one direction. For instance, the central window may be configured to exhibit a colour change effect when the document 100 is tilted about the x axis whilst the devices in the top and bottom windows remain uniform in colour, and vice versa when the document is tilted about the y axis. The light control layers of the security devices may have different arrangements (e.g. refractive indices) such that different windows appear different colours upon tilting.
In
A further embodiment is shown in
In
In the examples of
Therefore, at a normal angle of viewing ⊖va1, functional regions A and B, and region C all exhibit a black colour such that the different regions are not discernible. This is schematically illustrated by the uniform shading in
In some embodiments, a security device according to any of the embodiments described above may be incorporated into a security document in the form of a polycarbonate data page, for example for a passport or identity card. Such a security device may be affixed to the surface of the data page, for example through the use of a pressure-sensitive adhesive. Alternatively, it is envisaged that the surface relief of the light control layer of the device may be formed as part of the polycarbonate page itself.
The above embodiments have been described with respect to the light control layer comprising a microprismatic structure comprising a plurality of linear microprisms.
Opposing end faces of an individual microprism are substantially parallel, and such a microprism is known as a “one-dimensional” microprism. The microprismatic structure 820 shown in
The optical effect exhibited by the light control layer is therefore anisotropic. If the security device comprising the light control layer is rotated within its plane, the exhibited optical effect due to the combination of colour shifting element and light control layer is seen most readily when the device is tilted with the viewing direction perpendicular to the long axes of the microprisms (i.e. along Y-Y′). If the device is rotated such that the viewing direction is parallel with the long axes of the microprisms (i.e. along X-X′), the effect is seen to a lesser extent.
A variety of different functional region surface relief structures can be used for a security device light control layer according to the present invention, as will be highlighted with reference to the following
It is envisaged that a light control layer may comprise a plurality of functional regions offset from each other can be used, as shown in
The light control layer may comprise a series of multi-faceted microprisms (i.e. having more than two facets), as shown in the surface relief 1120 of
To obtain more isotropy in the optical properties of the light control layer, a “two-dimensional” microprismatic structure may be used comprising microprisms that are not as rotationally dependent as the linear microprisms of
| Number | Date | Country | Kind |
|---|---|---|---|
| 1713799 | Aug 2017 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2018/052420 | 8/28/2018 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2019/043366 | 3/7/2019 | WO | A |
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| Number | Date | Country | |
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| 20210070090 A1 | Mar 2021 | US |
