Patent Application
20200150586
The invention relates to a holographic security device, for example for use on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents. Methods of manufacturing such a security device are also disclosed.
Articles of value, and particularly documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data therein. Typically such documents are provided with a number of visible security devices for checking the authenticity of the object. Holographic devices are widely used as such security devices and provide an optically variable effect to an observer, meaning that the appearance of the device is different at different angles of view. Holographic security devices are particularly effective since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices.
Conventional holograms comprise a surface relief where the diffraction of incident light from the surface relief generates the holographic effect. Typically the holographic effect is the projection of the image of a three-dimensional object with parallax, which provides a striking optically variable effect for a viewer that is easily authenticatable. However, with the increasing sophistication of counterfeiters, simple three-dimensional holograms are no longer as secure as they once were.
As a response to this, holographic security devices were provided as described in WO 93/24333, where the holographic effect exhibited a moiré pattern produced from a pair of overlapping, regular arrays of lines or dots having very similar form, and with each array having a line of symmetry. Furthermore the lines of symmetry of the two arrays were aligned. A viewer of such a device therefore observed a moiré pattern that remained substantially uniform in form on changing the viewing position (i.e. tilting the device). The precision required to ensure that the axes of symmetry were aligned in order to provide the substantially uniform holographic image was greater than that which could be provided by counterfeiters, and as such these devices had a high security level.
However, counterfeiting technology has inevitably improved, and there is therefore the need to provide holographic security devices having increased security.
In accordance with a first aspect of the invention there is provided a holographic security device comprising a holographic image layer which when illuminated exhibits the optically variable effect of viewing first and second overlapping patterns of elements, wherein; the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and the pitches and relative locations of the first and second patterns of elements are such that, upon illumination of the device; at a first viewing position of the security device the first set of image elements are exhibited by the holographic image layer and at a second, different viewing position of the security device the second set of image elements are exhibited by the holographic image layer.
The security device of the present invention is a holographic security device in that the effect exhibited to a viewer upon viewing the device is generated by the diffraction of light from the holographic image layer. The holographic image layer exhibits the optically variable effect of viewing first and second patterns of elements without the patterns of elements needing to be physically present in the device. This dramatically increases the ease with which the security device may be incorporated into a security document (such as a passport), as the document itself does not need to be modified in order to exhibit the effect. Furthermore, the difficulty of counterfeiting is increased as it is difficult for a would-be counterfeiter to re-create the holographic image layer.
Typically, the holographic image layer comprises a holographic recording of the optically variable effect of viewing the first and second overlapping patterns of elements. Preferably the optically variable effect exhibited by the holographic image layer is a variable image; the image being variable in that it is perceived to change dependent on viewing position of the device. The variable image seen when viewing the security device at varying viewing positions corresponds to the variable image that would be seen when viewing the first and second patterns of elements. In other words, the device replays a holographic reproduction (or “holographic image”) of the effect that would be observed when viewing physical first and second patterns of elements.
The first and second patterns of elements are typically arranged parallel to each other and are spaced apart in a direction perpendicular to the planes of the patterns. This means that, on tilting the security device to observe parallax effects due to the separation of the patterns, a viewer will observe, at a first viewing position, the first set of image elements which typically cooperate together to form a first recognisable image and observe, at a second viewing position, the second set of image elements which similarly typically cooperate together to form a second recognisable image. The first and second viewing positions may be referred to as different viewing angles of the device. Typically, at least one of the first and second patterns of elements, or both in combination, define indicia, preferably a letter, digit, geometric shape, symbol, image, graphic or alphanumerical text. It is envisaged that for the majority of applications, only the first pattern of elements (comprising the first and second sets of image elements) will define indicia. For example, the first set of image elements may define a star shape, and the second set of image elements may also define a star shape but of a smaller size. Tilting the device would then exhibit a dynamic effect of the star growing and shrinking in size. In such a situation, the first and second sets of image elements may be interleaved with each other, such that the star shapes appear in substantially the same spatial location at the first and second viewing positions, with the only dynamic effect being the apparent change in size.
The first set of image elements may define indicia at a first spatial location and the second set of image elements may define indicia at a second spatial location such that upon changing viewing positions (for example by tilting the device relative to the viewer) a viewer perceives animation of said indicia from the first to the second spatial locations.
This so-called “phase interference” effect provided by the first aspect of the invention provides a memorable effect to a viewer, and enhanced security of the device.
Typically, the viewing position of the device is changed by tilting the device, relative to an observer, about tilt axis substantially within the plane of the device. The different viewing positions are typically different viewing angles of the device. It will be appreciated that substantially the same effect may be observed by an observer moving with the device remaining stationary.
For example, the first and second viewing positions may be positioned along a first axis, wherein the first and second patterns of elements are arranged such that the first and second sets of image elements are exhibited to a viewer when the device is tilted along a tilt axis not parallel to the first axis. Typically, the first axis and the tilt axis are substantially perpendicular, with the tilt axis preferably lying substantially in the plane of the security device.
Preferably, the first pattern of elements further comprises a third set of image elements that are exhibited at a third viewing position of the security device.
Similarly to above, the third set of image elements will together preferably define a third recognisable image, enabling more complex dynamic effects (such as animation) to be exhibited to a viewer when tilting the device. Further preferred embodiments may include fourth and further sets of image elements.
The first and second patterns of elements are provided in an overlapping manner, and may be fully overlapping or partially overlapping. Importantly, there must be at least partial overlap of the patterns of elements such that an interference effect generated by the two patterns can be recorded in the holographic image layer. The first pattern of elements can be thought of as an “information” or “artwork” pattern, and the second pattern of elements as a “decoding” or “sampling” pattern. Typically, when recording the holographic image layer, an object beam is directed firstly through the artwork pattern and subsequently the sampling pattern, with the resulting interference effect being recorded in the holographic image layer.
In one embodiment, the sampling pattern of elements comprises a periodic or quasi-periodic array of substantially opaque and substantially transparent regions, typically arranged as a one dimensional line screen pattern or dot screen pattern. The term “transparent” means that light is transmitted through the transparent regions of the sampling pattern with low optical scattering such that the image elements of the artwork pattern can be viewed through the sampling pattern with minimal obscuration. Conversely, the term “opaque” means that light does not pass through the opaque regions such that the image elements of the artwork pattern cannot be viewed through the opaque regions. Therefore, the second (“sampling”) pattern of elements controls which parts of the first (“artwork”) pattern are visible at certain viewing angles when tilting the device. Furthermore, the size ratio between the opaque and transparent regions of the sampling pattern controls the number of frames at which the artwork pattern replays (and therefore the number of discrete frames exhibited when viewing the final device). For example, the sampling pattern may comprise a one-dimensional line screen comprising 600 μm wide substantially opaque rectangular regions separated by 100 μm wide substantially transparent regions, which would provide seven frames exhibited at seven corresponding viewing angles. The artwork pattern of elements would then preferably comprise seven sets of interleaved (or “interlaced”) image elements, with each image set displaced by 100 μm along the viewing direction from adjacent interlaced image sets and intended to be viewable through the transparent regions of the sampling plate at different viewing angles.
In general, for N image channels, the first pattern of elements will be comprised of N interlaced strip patterns of width RD/N (where RD is the repeat distance (or pitch) of the second pattern of elements). In the above example, this is 100 μm. Thus each interlaced strip of the first pattern of elements matches a transparent region of the second pattern of elements and the holographic security device behaves like a low brightness N channel lenticular device.
The width of the substantially transparent regions of the second pattern of elements may be increased to advantageously increase transmissive brightness of the device and/or reduce the visibility of the second pattern of elements in the final replayed image when viewing the device, but this will be at the cost of greater image overlap.
In some preferred embodiments, the second pattern of elements comprises a one dimensional or two dimensional array of focussing elements, which advantageously increases the brightness of the final replayed image. This will be explained in more detail below.
In accordance with a second aspect of the present invention there is provided a holographic security device comprising a holographic image layer which when illuminated exhibits the optically variable effect of viewing first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that, upon illumination of the device; the holographic image layer exhibits a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; at least one of the first and second patterns of elements comprises a first area having a first pitch along at least one axis and a second area having a second, different pitch along said axis, whereby the moiré effect causes different degrees of magnification of the first pattern of elements to occur, such that the holographic image layer exhibits areas of different depth corresponding to the first and second areas.
This aspect of the invention advantageously uses the effect of moiré magnification to provide a holographic image layer that exhibits different perceived depths in order to provide a memorable, easily authenticatable image to a viewer of the security device. This is particularly beneficial as the different depth effects can be exhibited using first and second patterns of elements that are positioned substantially parallel to each other (for example the patterns of elements may typically be provided on transparencies or substantially transparent plates, e.g. a flat emulsion coated glass) rather than generating different depth effects by non-parallel positioning of the patterns of elements.
The moiré magnification factor depends upon the difference between the periodicities or pitches of the first and second patterns of elements. (As with the first aspect of the invention the first pattern of elements can be thought of as the “artwork” pattern and the second pattern of elements thought of as the “sampling” pattern.) A pitch mismatch between the two patterns of elements along an axis can also conveniently be generated by rotating one pattern relative to the other, such that the two patterns have a rotational misalignment.
Preferably, as in the first aspect, the first or second patterns of elements, or both in combination, define indicia, preferably a letter, digit, geometric shape, symbol, image, graphic or alphanumerical text.
Typically, the first pattern of elements comprises an array of image elements that are compressed along at least the axis along which magnification occurs due to the moiré effect. The compression factor of the image elements is determined such that the magnified image elements exhibited upon viewing the holographic image layer of the device have the desired aspect ratio. As it is the relative pitch mismatch (or rotational misalignment) between the two patterns of elements that gives rise to the moiré magnification, the array of image elements may have a constant pitch along an axis with the second pattern of elements comprising regions of different pitch along the same axis in order to provide apparent magnification. Alternatively or in addition, the array of image elements may have varying pitch along an axis, with the second pattern of elements having a constant pitch. This second scenario is generally preferred as it allows the same second (“sampling”) pattern to be used for a variety of different artwork patterns, which beneficially allows for efficient personalisation of the security devices. The apparent depth difference between objects within the final hologram image is a particularly striking effect.
At least one image element may comprise at least two sub-elements configured to have different degrees of magnification such that a viewer such that a viewer of the device perceives the image element to have a three dimensional appearance.
In the case where the first pattern of elements comprises an array of image elements, tilting of the device exhibits apparent fast movement of the image elements along an axis not parallel with a tilting axis, due to progressive sampling across individual image elements. Typically the axis along which the image elements appear to move is substantially perpendicular to the tilt axis.
This fast movement of image elements in the exhibited holographic image upon tilting of the device provides a distinctive effect to a viewer, especially in combination with the perceived depth of the image elements.
The pitch of the array of image elements may vary continuously along at least one axis of at least one region, whereby the moiré effect causes different degrees of magnification of the image elements to occur, such that the viewer perceives that the magnified image elements are located on a first image surface that is tilted or curved with respect to the surface of the security device. This provides a device where the holographic image viewed by an observer has an image plane or surface that is appears noticeably tilted or curved relative to the plane of the device. This visual effect significantly enhances the visual appearance of the security device and, moreover, enhances the security level associated with the device since the necessary pitch requirements of the first and second patterns of elements increases the complexity of manufacture and deters would-be counterfeiters.
The pitch of the array of image elements may vary in a linear (constant gradient) or non-linear (variable gradient) manner.
It should be noted that, due to the potential for the holographic images exhibited by the device to appear curved, the term “image surface” will generally be used in place of “image plane”. However, in places where the latter term is used, it will be appreciated that the term “plane” is not limited to being flat unless otherwise specified.
The term “continuously varies” in this context means that the pitch variation across the array of image elements is such that the resulting image surface on which the magnified image elements are perceived in the variable holographic image appears smooth to the human eye.
The image elements in the array can all be identical in size, in which case the varying magnification levels across the device will cause size distortion. This can be used as a visual effect in itself. However, in preferred embodiments, the size of the image elements varies in a corresponding manner such that the viewer perceives that the magnified image elements have substantially the same size as each other on the first image surface.
Alternatively or in addition to the array of image elements continuously varying in pitch, the pitch of the second pattern of elements may vary continuously along at least one axis of at least one region. In the same manner as above, this causes different degrees of magnification of the image elements to occur, such that the viewer perceives that the magnified image elements are located on a first image surface that is tilted or curved with respect to the surface of the security device. It is envisaged that typically the magnification effects will be generated by varying the pitch of the first pattern of elements (the “artwork” array) while using a second pattern of elements having a constant pitch, as this allows for ease of personalisation of the security devices simply by changing the first pattern of elements as appropriate.
In some embodiments the pitches of the first and second patterns of elements and their relative locations are such that a first image surface is positioned in front of or behind the surface of the security device. In other advantageous implementations, the pitches of the first and second patterns of elements and their relative locations are such that a first image surface intersects the surface of the security device.
In particularly advantageous embodiments of the present invention, the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and; the pitches and relative locations of the first and second patterns of elements are such that at a first viewing position of the device the first set of image elements are exhibited by the holographic image layer and at a second, different viewing position of the device the holographic image layer exhibits the second set of image elements. A combination of the phase interference effects described above in the first aspect of the invention together with the moiré magnifier effects described in the second aspect provides complex holographic images that are exhibited to an observer of the device. This complexity of the holographic security device significantly increases its security level as would-be counterfeiters are deterred by the difficulty in reproducing such a holographic image layer.
The invention is primarily intended for use with white light viewable holographic image layers (“holograms”). Here white light comprises the visible part of the electromagnetic spectrum, i.e. between approximately 390 nm and 700 nm. Embossed holograms are one class of white light viewable holograms.
Embossed holograms are formed by surface relief patterns which diffract light in order to create the holographic effect. Such surface relief patterns may be replicated into polymeric layers in order to mass produce the article, using a master plate which exhibits the (inverse) surface relief pattern. Replication can be done by plastic deformation of a plastic film under heating, or by curing a polymeric composition under the influence of UV light or an electron beam while the composition is in intimate contact with a master plate. The holographic image layer of the security device may be principally reflective in its viewing such as being formed as a surface relief pattern (embossed hologram) in a transparent plastic film, which patterned surface is selectively metallised for example with a substantially opaque layer of aluminium. Alternatively, the holographic image layer may be partly reflective and partly transparent such as when the above transparent film would be coated with a thin layer of a material having a higher refractive index than the plastic such as zinc sulphide or titanium dioxide: such transparent holographic layers can be used as overlays for passport photographs and the like.
The inventors have discovered that the security device of the present invention advantageously provides additional security characteristics dependent upon the nature of the light used to illuminate the device (and therefore the holographic image layer). When illuminated with diffuse white light, there is a tendency towards a reduction in the chromatic saturation (i.e. loss of colour) of the final replayed image and/or a mixing or overlap of frames and parallax views. However, under spotlight or point source (for example a torch), only certain frames replay at a particular viewing angle, meaning that the holographic effect (such as apparent animation or different depths) appears clearly the viewer.
This phenomenon advantageously provides a “two-level” security characteristic of the security device, as the device will appear different under different lighting conditions (i.e. diffuse or spotlight). Indeed, the security device may be manufactured to deliberately replay an unrecognisable image when illuminated by anything other than a point source of light, and only replay the desired holographic effect under spotlight or a point light source.
In both the first and second aspects, the second pattern of elements may take a variety of forms. For example, it may comprise a one dimensional line screen pattern or dot screen pattern, and this is particularly suitable for the case where the holographic image layer comprises an embossed rainbow hologram created using a Benton slit and a H1/H2 recording process as is known in the art. Such a hologram exhibits parallax in a direction parallel with the length of the Benton slit and a colour rainbow variation in a direction orthogonal to the slit direction. In the present invention, the one dimensional sampling pattern will preferably be aligned along a direction orthogonal to the slit direction so that the different image elements are visible at different viewing positions when tilting the device, with the tilt axis typically being aligned perpendicular to the length of the Benton slit.
Alternatively, the sampling pattern may comprise a two dimensional line screen pattern or dot screen pattern. In such a scenario, the artwork pattern may also comprise a two dimensional pattern, and the security device will exhibit parallax effects to a viewer when tilted about two different tilt axes (typically perpendicular to each other). In addition to embossed holograms described above, volume (or “Lippmann”) holograms are another class of white light viewable holograms that may be used in the present invention. With a volume hologram, the hologram image is generated by Bragg reflection off a series of refractive index modulated planes within the volume of the material. Volume holograms are both wavelength and angularly selective with regard to the incident light and so do not show mixing of parallax views in the holographic image to the same degree as with embossed holograms. Although volume holograms may be used for one-dimensional embodiments, they are particularly suitable for use where the sampling pattern comprises a two dimensional pattern due to their wavelength and angle selectivity.
In the examples above, we have discussed the case where the second pattern of elements comprises a line screen (for example an array of horizontal and/or vertical lines) and/or dot screen pattern. More generally however, more complex effects can be generated using curved lines—for example the second pattern of elements may comprise a series of substantially opaque concentric circles separated by substantially transparent regions. In some embodiments, at least one of the first and second patterns of elements comprises a one dimensional line screen pattern or a one dimensional pattern of indicia. In other embodiments, at least one of the first and second patterns of elements comprises a two dimensional line screen pattern or dot screen pattern, or a two dimensional pattern of indicia.
Here, “one dimensional” is used to describe device that exhibit an optically variable effect on tilting about one axis, whereas “two dimensional” means that the device exhibits an optically variable effect on tilting about more than one axis.
In alternative embodiments of both the first and second aspects of the invention, the second pattern of elements may comprise a one dimensional or two dimensional array of focussing elements, typically microlenses. Where the second (“sampling”) pattern of elements comprises a line array or a dot array as described above, these lines or dots may be visible in the holographic image replayed by the security device, which may detract from the overall visual impression exhibited to a viewer (although it could be used as a visual effect in its own right). The use of an array of focussing elements advantageously means that the final variable holographic image does not contain any apparent lines originating from the second pattern of elements, whilst still maintaining striking visual effects on tilting. For example, the use of an array of (typically cylindrical) microlenses allows for replay of the different image arrays of the first pattern of elements at different viewing angles. Furthermore, moiré magnification can be exhibited by making use of an array of focussing elements (such as lenses or micromirrors) as the second pattern or elements and a corresponding array of microimage elements as the first pattern of elements. Each microimage element is a complete, miniature version of the image which is ultimately observed on viewing the device, and the array of focussing elements acts to select and magnify a small portion of each underlying microimage element, which portions are combined by the human eye such that the whole, magnified image is visualised. The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device.
Advantageously, as the focussing elements are substantially transparent to light, the use of an array of focussing elements as the second pattern of elements increases the overall transmission efficiency of light through the patterns of elements as compared to an array of substantially opaque elements. Indeed, the use of an array of focussing elements may provide an increase in optical brightness of in excess of 50% over an array of substantially opaque elements.
In the case where an array of lenses is used as the second (“sampling”) pattern of elements, the first and second patterns of elements are typically separated by a distance substantially equal to the focal length of the lenses.
Volume holograms are particularly suited to embodiments where an array of focussing elements is used as the second pattern of elements.
In accordance with a third aspect of the invention, there is provided a holographic security device comprising a holographic image layer which when illuminated exhibits the optically variable effect of viewing first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that, upon illumination of the device; the holographic image layer exhibits a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; the holographic image layer comprises a volume hologram.
In accordance with a fourth aspect of the present invention, there is provided a security article comprising a security device according to any of the preceding aspects, wherein the security article is preferably a security thread, strip, patch, label or transfer foil.
In accordance with a fifth aspect of the present invention, there is provided a security document comprising an article according to the fourth aspect, wherein the 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. The security document may be a passport, banknote, security label, identification card, driving licence or other document of value.
In accordance with a sixth aspect of the invention, there is provided a method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the optically variable effect generated by illuminating the first and second overlapping patterns of elements, wherein; the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and; the pitches and relative locations of the first and second patterns of elements are such that, upon illumination of the holographic image layer, at a first viewing position of the holographic image layer the first set of image elements are exhibited and at a second, different viewing position of the holographic image layer the second set of image elements are exhibited.
It is envisaged that the recording of the optically variable effect (typically a variable image) in the recording medium may be done as is conventionally known in the art. For example, the holographic image layer may be formed through a conventional H1/H2 recording process, wherein the object image is recorded into an intermediate transmission hologram known as the H1 and then the image from the H1 is holographically projected onto or near the surface of second hologram known as the H2, here forming the holographic image layer. For the case of an embossed hologram the H2 would typically be comprised of a substrate coated in photo-resist. Following chemical processing of the H2 resist it would coated with a thin sub-100 nm layer of conductive metal such as Silver and then Nickel replicas from using electroplating. The holographic image layer is primarily intended to be viewed in white light, in which case the holographic image within the H1 may be confined to a Benton slit which in projection onto the H2 hologram sacrifices vertical parallax to form a rainbow hologram.
Alternatively, the hologram may comprise a volume hologram recorded as is known in the art, either via projection from an intermediate H1, wherein the reference beam impinges on the opposite side of the H2 to the H1 object beam, or by directly recording the object image into the volume master hologram. The recording of the holographic image layer may be performed using on-axis or off-axis geometry, and the holographic image layer may be intended to be viewed in transmission or reflection.
By the recording of the optically variable effect here, we mean that the interference effect generated by directing light through the overlapping patterns of elements is recorded in the holographic image layer such that, when the holographic image layer is illuminated, it replays the same variation with viewing angle that would be experienced by viewing the patterns of elements directly.
In the case of an embossed hologram, the holographic image layer is formed by surface relief patterns which diffract light in order to create the holographic effect and generate the resultant variable optical effect. Such surface relief patterns may be replicated into polymeric layers in order to mass produce the article, using a master plate which exhibits the (inverse) surface relief pattern. Replication can be done by plastic deformation of a plastic film under heating, or by curing a polymeric composition under the influence of UV light or an electron beam while the composition is in intimate contact with a master plate.
The first (“artwork”) pattern of elements comprises first and second sets of image elements. By using a corresponding second (“sampling”) pattern of elements, a distinctive “switching” effect can be exhibited by the holographic image layer, wherein at the first viewing position the first set of image elements combine to form a recognisable shape or image and at the second viewing position the second set of image elements combine to form a different recognisable shape or image. The contrast between the two sets of image elements provides a distinctive effect to a user. In some preferred embodiments, the first pattern of elements comprises a third set of image elements that that are exhibited at a third viewing position of the holographic image layer. This advantageously allows for more complex variable images to be exhibited, such as perceived animation if the sets of image elements are at different spatial locations.
Preferably, at least one of the first and second patterns of elements, or both in combination, define indicia, preferably a letter, digit, geometric shape, symbol, image, graphic or alphanumerical text.
This so-called “phase interference” effect provided by the sixth aspect of the invention provides a memorable effect to a user, and enhanced security.
In accordance with a seventh aspect of the invention, there is provided a method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the optically variable effect generated by illuminating the first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that, upon illumination of the holographic image layer, the holographic image layer exhibits a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; at least one of the first and second patterns of elements comprises a first area having a first pitch along at least one axis and a second area having a second, different pitch along said axis, whereby the moiré effect causes different degrees of magnification of the first pattern of elements to occur, such that the holographic image layer exhibits areas of different depth corresponding to the first and second areas.
As with the sixth aspect, it is envisaged that the recording of the resultant variable image in the recording medium may be performed as is conventionally known in the art. For example, the holographic image layer may be formed through a conventional H1/H2 recording process as is known in the art, wherein the object image is recorded into an intermediate transmission hologram known as the H1 and then the image from the H1 is holographically projected onto or near the surface of second hologram known as the H2. For the case of an embossed hologram the H2 would typically be comprised of a substrate coated in photo-resist. Following chemical processing of the H2 resist it would be coated with a thin sub-100 nm layer of conductive metal such as Silver and then Nickel replicas from using electroplating. The holographic image layer is primarily intended to be viewed in white light, in which case the holographic image within the H1 may be confined to a Benton slit which in projection onto the H2 hologram sacrifices vertical parallax to form a rainbow hologram.
Alternatively, the hologram may comprise a volume hologram recorded as is known in the art, either via projection from an intermediate H1, wherein the reference beam impinges on the opposite side of the H2 to the H1 object beam, or by directly recording the object image into the volume master hologram The recording of the holographic image layer may be performed using on-axis or off-axis geometry, and the holographic image layer may be intended to be viewed in transmission or reflection.
As has been described above, by the recording of the optically variable effect, we mean that the interference effect generated by directing light through the overlapping patterns of elements is recorded in the holographic image layer such that, when the holographic image layer is illuminated, it replays the same variation with viewing angle that would be experienced by viewing the patterns of elements directly.
In the case of an embossed hologram, the holographic image layer is formed by surface relief patterns which diffract light in order to create the holographic effect and generate the resultant variable optical effect. Such surface relief patterns may be replicated into polymeric layers in order to mass produce the article, using a master plate which exhibits the (inverse) surface relief pattern. Replication can be done by plastic deformation of a plastic film under heating, or by curing a polymeric composition under the influence of UV light or an electron beam while the composition is in intimate contact with a master plate.
This aspect of the invention advantageously uses the effect of moiré magnification to manufacture a holographic image layer that, when illuminated, exhibits different perceived depths in order to provide a memorable, easily authenticatable image to a viewer of the holographic image layer. This is particularly beneficial as the different depth effects can be exhibited using first and second patterns of elements that are positioned substantially parallel to each other (for example the patterns of elements may typically be provided on transparencies or substantially transparent plates that are positioned in a parallel manner) rather than generating different depth effects by the non-parallel positioning of the patterns of elements.
Similarly to the aspects above, the first or second patterns of elements, or both in combination, preferably define indicia, preferably a letter, digit, geometric shape, symbol, image, graphic or alphanumerical text.
Typically, the first pattern of elements comprises an array of image elements that are compressed along at least the axis along which magnification occurs due to the moiré effect. The compression factor of the image elements is determined such that the magnified image elements exhibited by the holographic image layer have the desired aspect ratio. As it is the relative pitch mismatch (or rotational misalignment) between the two patterns of elements that gives rise to the moiré magnification, the array of image elements may have a constant pitch along an axis with the second pattern of elements comprising regions of different pitch along the same axis in order to provide apparent magnification. Alternatively or in addition, the array of image elements may have varying pitch along an axis, with the second pattern of elements having a constant pitch. This second scenario is generally preferred as it allows the same second (“sampling”) pattern to be used for a variety of different artwork patterns, which beneficially allows for efficient personalisation of the holographic image layer.
At least one image element may comprise at least two sub-elements configured to have different degrees of magnification such that a viewer of the holographic image layer perceives the image element in the resulting variable image to have a three dimensional appearance.
In the case where the first pattern of elements comprises an array of image elements, tilting of the holographic image layer relative to a viewer (i.e. changing viewing position) exhibits apparent fast movement of the image elements along an axis not parallel with a tilting axis, due to different image elements being visible through the second pattern of elements at different viewing angles, with this effect having been recording in the holographic image layer. Typically the axis along which the image elements appear to move is substantially perpendicular to the tilt axis. This fast movement of image elements upon tilting of the device provides a distinctive effect to a viewer, especially in combination with the perceived depth of the image elements.
The pitch of the array of image elements may vary continuously along at least one axis of at least one region, whereby the moiré effect causes different degrees of magnification of the image elements to occur, such that the viewer perceives that the magnified image elements are located on a first image surface that is tilted or curved with respect to the surface of the holographic image layer. This provides a holographic image layer where the variable holographic image viewed by an observer has an image plane or surface that is appears noticeably tilted or curved relative to the plane of the holographic image layer. This visual effect significantly enhances the visual appearance of the holographic image layer and, moreover, enhances the security level associated with the holographic image layer since the necessary pitch requirements of the first and second patterns of elements increases the complexity of manufacture and deters would-be counterfeiters.
It should be noted that, due to the potential for the variable images generated by the holographic image layer to appear curved, the term “image surface” will generally be used in place of “image plane”. However, in places where the latter term is used, it will be appreciated that the term “plane” is not limited to being flat unless otherwise specified.
The term “continuously varies” in this context means that the pitch variation across the array of image elements is such that the resulting image surface on which the magnified image elements are perceived in the variable holographic image appears smooth to the human eye.
The image elements in the array can all be identical in size, in which case the varying magnification levels across the device will cause size distortion. This can be used as a visual effect in itself. However, in preferred embodiments, the size of the image elements varies in a corresponding manner such that the viewer perceives that the magnified image elements have substantially the same size as each other on the first image surface.
Alternatively or in addition to the array of image elements continuously varying in pitch, the pitch of the second pattern of elements may continuously along at least one axis of at least one region. In the same manner as above, this causes different degrees of magnification of the image elements to occur, such that the viewer perceives that the magnified image elements exhibited when viewing the holographic image layer are located on a first image surface that is tilted or curved with respect to the surface of the holographic image layer. It is envisaged that typically the magnification effects will be generated by varying the pitch of the first pattern of elements (the “artwork” array) while using a second pattern of elements having a constant pitch, as this allows for ease of personalisation of the holographic image layer simply by changing the first pattern of elements as appropriate.
In some embodiments the pitches of the first and second patterns of elements and their relative locations are such that a first image surface is positioned in front of or behind the surface of the holographic image layer. In other advantageous implementations, the pitches of the first and second patterns of elements and their relative locations are such that a first image surface intersects the surface of the holographic image layer.
In particularly advantageous embodiments of the method, the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and; the pitches and relative locations of the first and second patterns of elements are such that at a first viewing position of the holographic image layer, the first set of image elements are exhibited by the holographic image layer and at a second, different viewing position of the holographic image layer, the second set of image elements are exhibited by the holographic image layer. A combination of the phase interference effects together with the moiré magnifier effects provides complex holographic images that are exhibited to an observer of the holographic image layer. This complexity of the holographic image layer significantly increases its security level as would-be counterfeiters are deterred by the difficulty in reproducing such a holographic image layer.
In both the sixth and seventh aspects, the second pattern of elements may take a variety of forms. For example, it may comprise a one dimensional line screen pattern or dot screen pattern, and this is particularly suitable for the case where the holographic image layer comprises an embossed rainbow hologram created using a Benton slit and a H1/H2 recording process as is known in the art. Such a hologram exhibits parallax in a direction parallel with the length of the Benton slit and a colour rainbow variation in a direction orthogonal to the slit direction. In the present invention, the one dimensional sampling pattern will preferably be aligned along a direction orthogonal to the slit direction so that the different image elements are visible at different viewing positions when tilting the device, with the tilt axis typically being aligned perpendicular to the length of the Benton slit (i.e. parallel with the direction of alignment of the sampling pattern).
Alternatively, the sampling pattern may comprise a two dimensional line screen pattern or dot screen pattern. In such a scenario, the artwork pattern may also comprise a two dimensional pattern, and the holographic image layer will exhibit parallax effects to a viewer when tilted about two different tilt axes (typically perpendicular to each other). Such an embodiment is particularly suitable for use where the holographic image layer comprises a volume (or “Lippmann”) hologram.
In the examples above, we have discussed the case where the second pattern of elements comprises a line screen (for example an array of horizontal and/or vertical lines) and/or dot screen pattern. More generally however, more complex effects can be generated using curved lines—for example the second pattern of elements may comprise a series of substantially opaque concentric circles separated by substantially transparent regions. In some embodiments, at least one of the first and second patterns of elements comprises a one dimensional line screen pattern or a one dimensional pattern of indicia. In other embodiments, at least one of the first and second patterns of elements comprises a two dimensional line screen pattern or dot screen pattern, or a two dimensional pattern of indicia.
In alternative embodiments of both the sixth and seventh aspects of the invention, the second pattern of elements may comprise a one dimensional or two dimensional array of focussing elements, typically microlenses. Where the second (“sampling”) pattern of elements comprises a line array or a dot array as described above, these lines or dots may be visible in the final holographic image, which may detract from the overall visual impression exhibited to a viewer (although it could be used as a visual effect in its own right). The use of an array of focussing elements advantageously means that the holographic image replayed by the holographic image layer does not contain any apparent lines from the second pattern of elements, whilst still maintaining striking visual effects on tilting of the holographic image layer. For example, the use of an array of (typically cylindrical) microlenses allows for replay of the different image arrays of the first pattern of elements at different viewing positions. Furthermore, moiré magnification can be exhibited by making use of an array of focussing elements (such as lenses or mirrors) as the second pattern or elements and a corresponding array of microimage elements as the first pattern of elements.
Each microimage element is a complete, miniature version of the image which is ultimately observed on viewing the device, and the array of focussing elements acts to select and magnify a small portion of each underlying microimage element, which portions are combined by the human eye such that the whole, magnified image is visualised. The magnified array appears to move relative to the holographic image layer upon tilting and can be configured to appear above or below the surface of the holographic image layer.
Advantageously, as the focussing elements are substantially transparent to light, the use of an array of focussing elements as the second pattern of elements increases the overall transmission efficiency of light through the patterns of elements as compared to an array of substantially opaque elements. Indeed, the use of an array of focussing elements may provide an increase in optical brightness of in excess of 50% over an array of substantially opaque elements.
In the case where an array of focussing elements is used as the second (“sampling”) pattern of elements, the first and second patterns of elements are typically separated by a distance substantially equal to the focal length of the lenses.
Volume holograms are particularly suited to embodiments where an array of focussing elements is used as the second pattern of elements.
In accordance with an eighth aspect of the invention there is provided a method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the optically variable effect generated by illuminating the first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that, upon illumination of the holographic image layer, the holographic image layer exhibits a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; the holographic image layer comprises a volume hologram.
The holographic image layer produced by the sixth to eighth aspect of the invention, is typically used to form a security device, wherein such a security device comprises the holographic image layer.
In accordance with a further aspect of the invention there is provided a holographic security device comprising a holographic image layer which, when illuminated, generates a variable image produced by first and second overlapping patterns of elements, wherein; the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and; the pitches and relative locations of the first and second patterns of elements are such that the first and second patterns of elements cooperate to exhibit the first set of image elements at a first viewing position and to exhibit the second set of image elements at a second, different viewing position.
In accordance with a yet further aspect of the invention there is provided a holographic security device comprising a holographic image layer which, when illuminated, generates a variable image produced by first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that the first pattern of elements cooperates with the second pattern of elements to generate a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; at least one of the first and second patterns of elements comprises a first area having a first pitch along at least one axis and a second area having a second, different pitch along said axis, whereby the moiré effect causes different degrees of magnification of the first pattern of elements to occur, such that a viewer of the variable image perceives areas of different depth corresponding to the first and second areas.
In accordance with a yet further aspect of the invention there is provided a holographic security device comprising a holographic image layer which, when illuminated, generates a variable image produced by first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that the first pattern of elements cooperates with the second pattern of elements to generate a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; the holographic image layer comprises a volume hologram.
In accordance with a yet further aspect of the invention there is provided a method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the resultant variable image generated by illuminating the first and second overlapping patterns of elements, wherein; the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and; the pitches and relative locations of the first and second patterns of elements are such that the first and second patterns of elements cooperate to exhibit the first set of image elements at a first viewing position of the resultant image and to exhibit the second set of image elements at a second, different viewing position of the resultant variable image.
In accordance with a yet further aspect of the invention there is provided a method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the resultant variable image generated by illuminating the first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that the first pattern of elements cooperates with the second pattern of elements to generate a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; at least one of the first and second patterns of elements comprises a first area having a first pitch along at least one axis and a second area having a second, different pitch along said axis, whereby the moiré effect causes different degrees of magnification of the first pattern of elements to occur, such that a viewer of the resultant variable image perceives areas of different depth corresponding to the first and second areas.
In accordance with a yet further aspect of the invention there is provided a method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the resultant variable image generated by illuminating the first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that the first pattern of elements cooperates with the second pattern of elements to generate a magnified version of at least a part of the first pattern of elements due to the moiré effect, and further wherein; the holographic image layer comprises a volume hologram.
Examples of the present invention will now be described with reference to the attached drawings, in which:
For ease of reference, the description below will refer to certain directions using the notation depicted in
For simplicity the following description will refer to tilting along either the X or Y axis in the geometry of
For a general literature discussion of holographic H1/H2 transfer techniques a suitable reference text is “Practical Holography”, Graham Saxby, published by Prentice Hall Int. (UK) Ltd. 1988.
For ease of reference in the following, the first patterned plate 100 will be referred to as the “artwork plate”, and the second patterned plate 200 will be referred to as the “sampling plate” as discussed above in the summary of the invention section.
In each of these cases, the resulting hologram 10 is used in the security device 1000 such as that seen in
The invention will be described with reference to a number of example effects exhibited by holographic image layers. However, these are not limiting, and the skilled person will understand that features of different examples may be combined.
The artwork plate 100 illustrated in
Take for example the viewing angle Θ1, where the left star 101a is exhibited at its maximum size, and the right star 101b is exhibited in its minimum size. Referring back to
Right star 101b is composed of sections and segments in a corresponding manner such that the final security device, when tilted about the Y axis, displays the seven frames illustrated in
In contrast to the first and second embodiments, only two frames are visible here, as the sampling plate 210 comprises an array of substantially opaque rectangular elements 211 (see
The artwork plate 120 comprises two arrays 121, 122 of substantially rectangular elements as illustrated in the magnified view of artwork plate in
Again, under diffuse light, the “contrast switch” effect will be minimised, with the general shape of the artwork plate being visible (
The above examples have been directed to examples of phase interference effects that may be utilised in the present invention. Alternatively or in addition, the overlapping artwork and sampling plates can also be used to create moiré magnification effects when viewing the final security device, as will be explained in the following.
The degree of magnification achieved is defined by the expressions derived in “The Moire Magnifier”, M. Hutley, R Hunt, R Stevens & P Savander, Pure Appl. Opt. 3 (1994) pp. 133-142. To summarise the pertinent parts of this expression, suppose the pitch of the elements of the artwork plate is A and the pitch of the elements of the sampling plate is B, then the magnification of the artwork plate elements, M is given by:
M=A/SQRT[(B cos(Theta)−A)2−(B sin(Theta))2], (Eq. 1)
where Theta equals the angle of rotation between the elements of the artwork and sampling plates.
For small Theta such that cos(Theta)˜1 and sin(Theta)˜0 and for the case where B≠A, we have,
M=A/(B−A). (Eq. 2)
As we can see from Eq. 2 therefore, if the artwork plate comprises an array of indicia that are compressed along an axis that is perpendicular to the long axis of the sampling plate elements, then the indicia will appear magnified along that axis when viewed through the sampling plate.
This effect is illustrated in
In this specific example, each “£” symbol has a width (i.e. a dimension along the X axis) of 547 μm, and the spacing of the symbols is a constant 70 μm. The sampling plate is the plate 200 illustrated in
Therefore, when viewing the final hologram image exhibited by the device, the viewer perceives the animation effect of the circles as in the second embodiment, together with 4.1 mm wide “£” symbols appearing to have fast movement along the X axis upon tilting the hologram about the Y axis. The apparent movement of the “£” indicia is due to the fact that changing the viewing angle causes the sampling plate to sample different parts of the artwork plate. The magnification of the “£” symbols also provides perceived depth of the final image, providing a striking effect to the viewer.
However, as also visible in
When viewing the hologram under very diffuse light, there is an inherent mixing of all of the replayed holographic frames, and all of the frames are visible, with the general shape of the array of circles and the “5” being visible as a darker part of the colour background. Under spot light however, only certain frames (and ultimately single frames) replay at any given viewing angle, making the moiré effects appear far more clearly. This difference in exhibited optical effect under different lighting conditions advantageously provides a secondary security feature (“level two security feature”) in addition to the difficulty in reoriginating the hologram.
In the embodiments described above, the sampling plate 200 comprised a plurality of equally-spaced rectangular elements. By varying the spacing of the elements of the sampling plate, we can achieve further optical effects in the final hologram image, such as varying the magnification power, the rate of movement of indicia defined in the artwork plate upon tilting the hologram, and also the apparent depth of the indicia of the artwork plate. The sampling plate can be non-constant and exhibit a variation of the width of the opaque or transparent areas (or both). Taking the previously discussed sampling plate 200 as an example, the opaque rectangular elements 201 and/or the gaps 202 may vary in width (dimension along the X axis). Such variation may be linear, sinusoidal or any other mathematical function, and when combined with an artwork plate having indicia of constant width and spacing, will exhibit variable magnification in the final holographic image.
This depth effect can be explained by using Eq. 2 above, where the absolute magnification of the left-most “£” symbol 143 in a given frame of the hologram image is given by M=270/(268−270)=−135×. The absolute magnification of the right-most “£” symbol 147 is given by M=270/(240−270)=−9×. Note that both of these absolute magnification values are negative, hence the inversion of the “£” indicia in the artwork plate 140 such that they appear correctly orientated in the final hologram image.
The apparent “depth” of the indicia elements in the final image relative to the surface plane (i.e. the plane of the hologram) derives from the familiar lens equation relating magnification of an image located a distance V from the plane of a lens of focal length f, this being,
M=V/f−1. (Eq. 3)
In this instance, the distance between the artwork plate and the sampling plate (which is a constant) substitutes for the focal length in Eq. 3. Therefore, from Eq. 3, we can see that the apparent depth (V) of the left-most indicia symbol 143 is more forward (i.e. more negative) than that of the right-most indicia symbol 145.
An image frame exhibited by the hologram generated by the overlapping artwork plate 150 with the sampling plate 230 is illustrated in
When the security device is tilted about the Y axis, the “£” indicia appear to move along the X axis due to the sampling effect of the sampling plate 230. This provides a particularly striking effect to a viewer. In general, the rate of motion is proportional to the perceived image depth. Therefore, generally, the greater the absolute magnification of the indicia, the faster the apparent movement of the indicia upon tilting of the device.
The above examples use an artwork plate having array(s) of indicia of constant spacing together with a sampling plate comprising regions of different spacing in order to provide the differing depth effects in the final hologram image. However, it will be appreciated that the equivalent effects may be provided using indicia in the artwork plate having varying spacing and a sampling plate having constant spacing (see for example the sixth embodiment below). Furthermore, in some embodiments both the sampling and artwork plates may comprise elements having varying spacing.
Different apparent depths of indicia exhibited in the hologram image can be utilised in order to display objects which appear three dimensional. Consider an indicia element 161 in the shape of a star (see
The absolute magnification of the outer star elements 164, intermediate star elements 163 and inner star elements 162 is therefore −50×, −25.5× and −13.25× respectively (using Eq.2). Using Eq.3 we can therefore also see that the outer star elements 164 have the strongest absolute magnification and appear very forward with respect to the plane of the hologram, with the inner star elements 162 appearing forward of the plane of the hologram, but less so than the outer star elements. This therefore creates a striking three dimensional effect to a viewer of the hologram image, with parallax upon tilting the security device. It will be appreciated that with suitable gap dimensions between the individual elements of the artwork plate arrays, the hologram image may replay the star indicia appearing in the depth behind the plane of the hologram.
A particularly striking effect is provided to a viewer of the hologram when the holographic image replays a combination of the phase interference and moiré magnification effects that have been described above.
For ease of discussion we will now focus on the top left and bottom left quadrants and how they will replay in the final hologram image. The top left quadrant of the final hologram replays as switching between a “£” symbol and a “5” symbol upon tilting about the Y axis (for example see the top left quadrant in the frames of
For example, at viewing angle Θd, the frame shown at
The bottom left quadrant replays a rotating ball and stick 172 upon tilting the hologram. As described above with respect to the “£” and “5” symbols in the top left quadrant, each frame of the rotating ball and stick is due to an array of ball and stick elements of the artwork plate designed to be revealed through the sampling plate at that particular viewing angle. However, in the case of the ball and stick, the arrays at different viewing angles have different spacings such that the elements appear at different depths within the image at different viewing angles. In this specific example, the ball and stick appears at different forward planes throughout the animation, providing a further striking visual effect on top of the already memorable animation effects.
All of the above examples have used a sampling plate comprising a one dimensional line array, which provide parallax effects about one axis of tilt (e.g. tilting about the Y axis in the view of
The use of complex designs for the artwork and sampling plates further increases the level of security associated with the device, as not only will would-be counterfeiters have to calculate the patterning through which the hologram was made, but also have access to tooling capable of generating the patterning.
In all of the examples described above, the sampling plate comprises a line pattern or a dot pattern. The sampling plate patterning is typically visible in the variable hologram image exhibited by the security device which may be used to create a striking visual impact (such as the “tiger” example of the eighth example above), but may in fact create an unwanted artefact that detracts from the primary effect.
The inventors have found that the effects set out above can also be generated using a sampling plate that comprises an array of focussing elements such as microlenses or micromirrors.
For example, the phase interference effects described in the first, second and third examples above can be generated using the array of microlenses 251. The same artwork plates as described above may be used, and only selected image segments will be directed, by the microlenses, towards the viewer at a given viewing angle, thereby generating the dynamic effects upon tilting the device.
An array of focussing elements may also be used to generate moiré magnification effects. Here, the artwork plate will comprise an array of microimages that is mismatched with the array of focussing elements. Each microimage element is a complete, miniature version of the image which is ultimately observed on viewing the device, and the array of focussing elements acts to select and magnify a small portion of each underlying microimage element, which portions are combined by the human eye such that the whole, magnified image is visualised when viewing the device. The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device.
At the left-hand side of the plate 190, i.e x=0, the pitch A(x=0) between adjacent microimage elements 191 (in the x-direction) is selected to replay at a first image depth. At the right-most side of the plate, i.e. x=X, the pitch A(x=X) between adjacent microimage elements is selected to return a greater image depth. Between x=0 and x=X, the pitch A continuously varies. Preferably, the pitch changes between each adjacent pair of elements 191—for instance, the spacing between elements 191a and 191b is slightly less than that between elements 191b and 191c. In this way, the gradual change in image plane depth when viewing the device will be perceived as a smooth surface to the human eye. However, in some cases the same result can be achieved if two or more adjacent pairs of elements share the same spacing. Equations 1, 2 and 3 described above can be used to determine the pitch of the microimage elements and the pitch of the lens array required to obtain the desired depth effects (here the pitch variation is seen in the artwork plate but it will be appreciated that the pitch of the lens array may vary instead or in addition).
In this example, the pitch variation is only applied along the X axis but in other embodiments the pitch of the microimage element array could instead vary along the Y axis, which would result in a plane appearing to tilt towards the “top” or “bottom” edge of the device rather than the left/right edges. In still further embodiments, the pitch could vary along both the X and Y axes, in which case the image plane would appear to tilt in both directions.
It will be noted that, in
The use of an array of focussing elements as the sampling plate also allows integral imaging effects to be recorded in the holographic image layer of the device. Here, the artwork plate 190 comprises an array of microimages, with each microimage being a miniature version of the final image to be exhibited. However, unlike with moiré magnification, there is no mismatch between the focussing elements and the microimages, and instead the visual effect is created by arranging for each microimage to be a view of the same object but from a different viewpoint. When the device is tilted, different ones of the images are magnified by the lenses such that the impression of a three dimensional image is given to a viewer.
The security device of the present invention may be designed to be viewed in reflection or transmission.
It should be noted that a similar construction could be achieved using a paper/plastic composite banknote in which the opacifying layers 1030a and 1030b are replaced by paper layers laminated (with or without adhesive) to an internal transparent polymer layer 1020. The paper layers may be omitted from the window region from the outset, or the paper could be removed locally after lamination. In other constructions, the order of the layers may be reversed with a (windowed) paper layer on the inside and transparent polymer layers on the outside.
In
Many alternative techniques for incorporating security devices of the sorts discussed above are known and could be used. For example, the above described device structures could be formed on other types of security document including identification cards, driving licenses, bankcards and other laminate structures, in which case the security device may be incorporated directly within the multilayer structure of the document.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1708387.4 | May 2017 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2018/051435 | 5/25/2018 | WO | 00 |
