EP4648974A1

EP4648974A1 - Security document and method for its manufacture

Info

Publication number: EP4648974A1
Application number: EP24701036.6A
Authority: EP
Inventors: Rebecca LOCKE; Alice Mesnage
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2023-01-13
Filing date: 2024-01-09
Publication date: 2025-11-19
Also published as: CN120513169A; AU2024208532A1; WO2024149987A1; GB202400314D0; MX2025007405A; CO2025010647A2; GB2629233A; GB2629233B; GB202300529D0

Abstract

A security document is disclosed, comprising: a transparent or translucent polymeric substrate; one or more opacifying layers disposed on the first and/or second sides of the transparent or translucent polymeric substrate, the one or more opacifying layers defining a substrate window region across which at least one of the one or more opacifying layers is absent; and, across at least part of the substrate window region, an array of image elements and an overlapping array of focussing elements are provided. The array of image elements is located substantially in the focal plane of the focussing elements. The array of image elements comprises one or more sets of image elements predominantly or wholly having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at a selected location on the security document. The or each set of image elements is arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defines a respective window cover image. The array of focusing elements are configured to direct light from selected ones of the array of image elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of image elements in combination across the at least part of the substrate window region within a respective range of viewing angles. The security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

Description

SECURITY DOCUMENT AND METHOD FOR ITS MANUFACTURE

FIELD OF THE INVENTION

This invention relates to security documents such as banknotes, identity documents, passports, certificates and the like, as well as methods for manufacturing such security documents.

BACKGROUND

To prevent counterfeiting and enable authenticity to be checked, security documents are typically provided with one or more security features which are difficult or impossible to replicate accurately with commonly available means, particularly photocopiers, scanners or commercial printers. Some types of security element are formed on the surface of a document substrate, for example by printing onto and/or embossing into a substrate such as to create fine-line patterns or latent images revealed upon tilting, whilst others including diffractive optical elements and the like are typically formed on an article such as a security thread or a transfer foil, which is then applied to or incorporated into the document substrate. A still further category of security element is that in which the security feature is integrally formed in the document substrate itself. Awell-known example of such a feature is the conventional watermark, formed in paper document substrates by controlling the papermaking process to as to vary the density of the paper fibres as they are laid down in accordance with a desired image.

Polymer document substrates, comprising typically a transparent or translucent polymer substrate with at least one opacifying layer coated on each side to receive print, have a number of benefits over conventional paper document substrates including increased lifetime due to their more robust nature and resistance to soiling. Polymer document substrates also lend themselves well to certain types of security features such as transparent or translucent windows regions which are more difficult to incorporate in paper-based documents. Window regions provide a strong security level due to their ease of recognition (often presenting a significant contrast with the surrounding material) and their inability to be replicated (or even closely imitated) by photocopying or other scanning/printing methods. Some examples of polymer security document constructions which include opacifying layers and window regions are disclosed in WO-A- 2017/055823, WO-A-2019/197798 and WO-A-2017/009616.

Window regions are typically formed during the bulk manufacture of the document substrate, by the substrate manufacturer. This is conventionally achieved by applying the opacifying layer(s) in such a way as to leave one or more gaps in their coverage of the polymer substrate, in accordance with a predetermined design. Depending on the type of window region to be formed, all of the opacifying layers may be formed with a gap at the same location with the result that the transparent or translucent polymer substrate is exposed on both sides across a region. This is termed a “full” window region, which will be transparent if the polymer substrate is transparent. Alternatively, one or more of the opacifying layers may continue across the region where there is a gap in one or more of the other opacifying layers. In a preferred arrangement, the opacifying layer(s) on one side of the polymer substrate may be absent across the region while at least one (typically all) of those on the other side of the polymer substrate may continue across the region. This is termed a “half’ window region, which will not be transparent but will still be more translucent than the document substrate is elsewhere (in the non-window regions, where all opacifying layers are present). Both full window and half window regions provide valuable security effects since neither can be imitated well by photocopying.

New security features are constantly sought, in order to stay ahead of would-be counterfeiters.

SUMMARY OF INVENTION

In accordance with the present invention, a security document is provided, comprising: a transparent or translucent polymeric substrate; one or more opacifying layers disposed on the first and/or second sides of the transparent or translucent polymeric substrate, the one or more opacifying layers defining a substrate window region across which at least one of the one or more opacifying layers is absent; across at least part of the substrate window region, an array of image elements and an overlapping array of focussing elements are provided, the array of image elements being located substantially in the focal plane of the focussing elements, the array of image elements comprising one or more sets of image elements predominantly or wholly having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at a selected location on the security document, the or each set of image elements being arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defining a respective window cover image; wherein the array of focusing elements are configured to direct light from selected ones of the array of image elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of image elements in combination across the at least part of the substrate window region within a respective range of viewing angles; whereby the security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

By providing the security document with one or more window cover images in this way, formed of image elements configured to look like an area of the note carrying opacifying layer(s), and which are selectively displayed by an array of focussing elements (e.g. lenses or mirrors), the apparent window region exhibited by the security document varies depending on the viewing angle. That is, the lateral extent of the apparent window region changes or moves as the user changes their position relative to the security document (e.g. by tilting the document), and/or the apparent window region disappears/reappears, giving the impression that parts of the security document are becoming more translucent (or vice versa). This is a new and surprising visual effect especially when compared with conventional security documents in which window regions are known to be static and unchanging in terms of location, size and extent (even if security elements located laterally inside those window regions might be optically active). The significant visual impact achieved, together with the inherent complexity of the required structure, provides the presently-disclosed security document with an increased security level.

It will be appreciated that the apparent window region, like any other window region, is a region which appears relatively transparent or translucent compared to its surroundings (at the viewing angle at which the apparent window region in question is displayed).

A further benefit of the disclosed security document lies in the manner in which the apparent window region (i.e. the window region which the viewer perceives to be provided on the security document) is the result of contributions from two different components: the opacifying layer(s), which define the bounds of the substrate window region (which are fixed), and the window cover image(s), when visible (which are optically variable). This provides added manufacturing flexibility. For example, a large volume of document substrate can be manufactured in which all of the substrate window regions produced (i.e. regions where at least one opacifying layer has a gap) are the same. Then, different batches of the resulting document substrate can be provided with different window cover images in the above-described arrangement, so that the apparent window regions displayed by security documents produced from each batch appear different. For instance, in one batch the window cover image(s) applied could result in an apparent window region which appears square (at least at some viewing angles) and in another batch different window cover image(s) may be applied so as to result in an apparent window region which appears circular (at least at some viewing angles), whilst the underlying substrate window regions in both batches are the same (e.g. rectangular). This is useful since it enables the same base document substrate, produced in large volumes, subsequently to be tailored differently for different applications, giving the appearance of different base document substrates having been used. In practice, the base document substrate may be manufactured (including providing the opacifying layer(s) and forming the substrate window region) by a first entity or at a first site, and then the image elements and focussing elements may be applied in one or more separate processes, e.g. by a second entity or at a second site.

The variable window effect is achieved, as set out above, by providing at least one window cover image which predominantly appears (at those viewing angles where it is visible) substantially visually indistinguishable from the opacifying layers provided elsewhere on the security document. The at least one window cover image effectively modifies the window region that is visible in practice, by extending the area where the opacifying layer(s) appear to be present, thereby concealing at least part of the substrate window region, at least when the substrate is viewed in reflected light. To achieve this, the image elements from which the at least one window cover image is formed (predominantly or wholly) are of substantially the same visual appearance (e.g. in terms of colour and/or optical density) as that of the opacifying layers in combination, at a selected location on the security document (where at least one, preferably more, of the opacifying layers are present). It is the combined visual appearance of the at least one opacifying layer present at that selected location that is of relevance, since this is what the viewer will perceive in practice (rather than the appearance of any one individual opacifying layer by itself - it is the accumulative appearance that is relevant). If the combined visual appearance of the opacifying layers is different from different sides of the security document, it will be appreciated that it is the combined visual appearance seen from the same side as that on which the at least one window cover image will be exhibited that is relevant. Whether the variable window effect is visible from just one or both sides of the security document will depend on factors such as the lighting conditions and the optical density of the image elements. As a baseline, the variable window effect will be visible in reflected light from at least one side of the security document. However, in many cases the variable nature of the window will also be apparent in transmitted light, and hence from both sides of the security document when viewed under illumination conditions which include at least some transmitted light.

As indicated already, the image elements of each set will be predominantly or wholly configured to have substantially the same visual appearance as that of the (combined) opacifying layer(s) at the selected location. This means that this appearance (e.g. colour) at least dominates the solid area(s) of the or each window cover image and in preferred cases the or each window cover image may be wholly of that appearance (e.g. colour). However, in some cases it may be desirable to include a small proportion of other image elements (with different appearances) in the window cover image, for instance to provide detailing in other colours (such as a border line around the apparent window region). Nonetheless it is important that the window cover image appears to be a continuation of the opacifying layer(s) and so if there are additional materials provided by image elements of different appearance, these must form only a minor part of the window cover image.

In the present context, “substantially the same visual appearance” means that the relevant visual appearances are substantially indistinguishable to the naked eye, when viewed in reflected light under standard illumination conditions (e.g. white light illumination). In some cases it may be desirable for the image elements to have substantially the same optical density as the collective optical density of the opacifying layer(s) at the selected location, in which case the apparent window region would appear to change and/or disappear in a similar manner under both reflective and transmissive viewing conditions (upon varying the viewing angle). However, in practice this is not essential and the image elements may have a substantially lower optical density compared to that of opacifying layers collectively, in which case the window variation may be less visible (or not visible) in transmissive viewing conditions. Nonetheless, the reflective visual appearance of the image elements will still be substantially the same as that of the opacifying layers collectively at the selected location (e.g. colour). The selected location which the appearance of the image elements is matched to can be any point on the security document where at least one of the opacifying layer(s) is present (provided it is sufficiently distinct in appearance from the substrate window region so that the window cover images appear different from the window region itself). Preferably, however, the selected location is a location at the boundary of the substrate window region, or just outside the substrate window region. This is desirable so that the appearance of the window cover image closely matches of the security document substrate closest to it, and appears as a continuation of the opacifying layer(s) there.

In some preferred embodiments, the selected location is outside the substrate window region, all of the one or more opacifying layers preferably overlapping at the selected location. That is, the selected location is in a non-window region where the maximum number of opacifying layers are present. If the image elements are matched to the collective appearance of the full set of opacifying layers, this will enable the window cover image (or parts thereof) to also appear as a non-window region when visible. This provides a particularly strong visual effect. Alternatively the selected location may be at a position where less than all of the one or more opacifying layers are present. For instance, in another particularly preferred embodiment, the selected location is in a boundary area of the substrate window region, less than all of the one or more opacifying layers preferably overlapping at the selected location. For registration regions it is common to “step back” the outermost opacifying layer around the edges of a window region, such that there is a narrow boundary area where an underlying opacifying layer is revealed. It can be desirable to match the image elements to the appearance of this boundary area since this may abut the window cover image when displayed. This approach may be particularly desirable in cases where the opacifying layers are not all of the same colour as one another.

In this disclosure, it should be noted that where a component is described as being “on” (or applied “onto”) another component, there may or may not be direct contact between those components. For instance, the opacifying layers are disposed on the polymer substrate but this may be via an intermediate layer such as a primer. Similarly, the image elements and the focussing elements may be disposed directly on the surface of the polymer substrate, or there may be an intervening layer such as a secondary substrate, a primer and/or a transparent coloured layer (e.g. a gravure print) between them and the polymer substrate.

The term “transparent” is used herein to mean that the material in question is substantially visually clear, such that an item on one side of the material can be seen sharply through the material from the other side. Therefore transparent materials should have low optical scatter. However, transparent materials may nonetheless be optically detectable (defined below), e.g. carrying a coloured tint and/or a taggant. “Translucent”, on the other hand, means that the material transmits a significant degree of light (i.e. is not opaque), but there is a significant degree of scattering such that the material is not visually clear.

It should be noted that the substrate window region and/or the apparent window region could be surrounded by non-window regions on all sides, which would appear as a fully enclosed window. However this is not essential and in some cases the substrate window region and/or the apparent window region could be partially bordered by an edge of the security document - i.e. part of the window boundary is defined by the edge of the security document. This would appear as an edge window or cut-out region.

The manner in which the apparent window region appears to vary can be controlled through design of the one or more window cover images. In preferred embodiments, the or each window cover image is configured such that, upon changing the viewing angle, the apparent window region appears to change in terms of its shape, size, position and/or the number of apparent window regions. The change could be a discrete “switch” effect between two or more different states, or could appear more gradual. The latter can be assisted by increasing the number of different window cover images built into the security document, as described below. The array of image elements and the array of focussing elements will overlap one another in at least part of the substrate window region, as indicated above. However it is not essential for the two arrays to wholly overlap one another. For example, the lateral extent of array of image elements may be different from the lateral extent the array of focussing elements.

Preferably, the array of image elements is configured such that when the security document is viewed from the respective range of viewing angles, the corresponding window cover image appears as a continuation of the one or more opacifying layers, being contiguous therewith. This can be achieved in some embodiments by (i) arranging that the at least part of the substrate window region across which the array of image elements and the (at least partially overlapping) array of focussing elements are provided includes a portion of the periphery of the substrate window region, and the or each window cover image is configured such that when the security document is viewed from the respective range of viewing angles, the corresponding window cover image appears as a continuation of the one or more opacifying layers, being contiguous therewith. Alternatively, in other embodiments, this can be achieved by (ii) configuring the array of image elements to include an area which even in the absence of focussing elements appears substantially continuous to the naked eye at all viewing angles (e.g. there being no discernible spaces between image elements in this area), the area being located between an optically variable part of the window cover image and the periphery of the substrate window region (e.g. abutting or overlapping at least a portion of that periphery). Since such an area will not co-operate with focussing elements to generate an optically variable effect, it is not necessary for the array of focussing elements to overlap this area, which acts to visually connect the optically variable part within the substrate window region to the non-window region surrounding it. Such an area may arise as a result of two or more window cover images being interlaced which each have solid portions in the same part of the substrate window region, or by inserting a static area into the array of image elements in the form of a continuous or pseudo-continuous structure such as a flood print or a half-tone pattern. In both cases, the area of the image element array will be formed of the same material (or other colour-generating structure) as the rest of the image elements and will thus have the same visual appearance as discussed above.

Both approaches (i) and (ii) have the result that at least some of the periphery of the substrate window region (the “real” window) is concealed (either by the respective window cover image when it is displayed in option (i), or at all viewing angles in option (ii)), thereby better conveying the impression of change and/or better hiding the underlying substrate window region at some or all viewing angles. However in some cases it may be possible to achieve this impression whilst not concealing the periphery, for instance by designing the window cover image(s) to include a pattern which appears to incorporate the shape of the periphery.

There may also be other good reasons for configuring the image element array to include a continuous or pseudo-continuous area, whether or not it is overlapped by focussing elements and irrespective of its position within the substrate window region. For example, this could be used to create a static non-transparent area of the substrate window region which persists at all viewing angles, thereby enhancing the contrast with the optically variable region(s). Further, since continuous or pseudo-continuous areas of the image element array are not required to be overlapped by the focussing element array, their provision reduces the lateral extent of focussing element array needed, and hence also the amount of material used to make it. It will be understood that such a continuous or pseudo- continuous area will typically have a lateral extent equal to or greater than an area covered by a plurality of the focusing elements, and consequentially it will be bigger than any individual image element, in order to appear continuous to the naked eye.

Preferably, the or each window cover image includes at least one solid zone across which the image elements are present, the predominant appearance of the solid zone substantially matching the collective visual appearance of the one or more opacifying layers at the selected location, the solid zone preferably having a generally uniform appearance substantially matching the collective visual appearance of the one or more opacifying layers at the selected location. It will be understood that the or each solid zone appears solid (i.e. contiguous) due to the action of the focusing elements when the window cover image is displayed, but is in fact formed of discontinuous image elements, arranged periodically as described above. While in especially preferred cases, the solid zone will be wholly defined by the image elements having an appearance which substantially matches that of the opacifying layers, this is not essential. As noted above, the image element sets could also include a small proportion of other image elements, e.g. in another colour, in which case the solid zone may exhibit a pattern or other detailing. However, if any other type of image elements are provided (e.g. in a different colour), these should preferably cover no more than 20% of the area of the solid zone, more preferably no more than 10% of the area of the solid zone, still preferably no more than 5% of the area of the solid zone and most preferably no more than 1 % of the area of the solid zone. This helps to ensure that the appearance of the window cover image still predominately matches that of the opacifying layers at the selected location. Where the array of image elements is configured such that when the security document is viewed from the respective range of viewing angles, it is preferably a solid zone of the corresponding window cover image that appears as a continuation of the one or more opacifying layers, being contiguous therewith. For example, in preferred embodiments the at least one solid zone may be located so as to appear to abut or partially overlap part of the periphery of the substrate window region. It is possible for one or more of the window cover image(s) to have a solid zone configured to cover the whole of the substrate window region, in which case when that window cover image is visible it may cause the apparent window region to disappear.

Nonetheless, in many embodiments it is advantageous if at least one (preferably all) of the window cover image(s) further includes at least one transparent zone into which the corresponding set of image elements do not extend. In this way, when that window cover image is exhibited, the apparent window region will still be visible but with a different extent (e.g. different outermost periphery) relative to the substrate window region. In some preferred embodiments at least one of the window cover image(s) further includes at least two transparent zones into which the corresponding set of image elements do not extend, the at least two transparent zones being separated from one another by the at least one solid zone. By providing multiple separate transparent zones, this can be used to change the number of apparent window regions. For instance, if the (unmodified) substrate window region is visible at a first viewing angle and then a window cover image having two transparent zones is combined with it at another viewing angle, the number of window regions will able to switch between one and two as the security document is tilted back and forth.

As discussed above, the apparent window region is the result of the substrate window region combined with whichever window cover image is visible at the corresponding viewing angle (if any). In some cases, only part of the periphery of the apparent window region may deviate from that of the substrate window region - for instance one end of the substrate window region may be covered up while the rest remains visible. In such preferred embodiments, at least one of the window cover image(s) is configured such that the apparent window region visible from the corresponding range of viewing angles has a boundary of which a first portion is defined by the periphery of the substrate window region, and a second portion which is defined by the respective window cover image.

It is also possible for the whole of the periphery of the apparent window region to be different from that of the substrate window region. For instance, the bounds of the apparent window region may lie entirely within the bounds of the substrate window region. This may be desirable for instance where the apparent window region is to have a substantially different shape and/or size from the substrate window region. In such preferred embodiments, at least one of the window cover image(s) is configured such that the apparent window region visible from the corresponding range of viewing angles has a boundary which is wholly defined by the respective window cover image.

Whilst in some cases it may be desirable to have all of the window cover images in a certain security document to fall into the same one of the two categories mentioned just above, in other cases a mix may be employed. In this way, at some viewing angles the boundary of the apparent window region will be partially defined by the periphery of the substrate window region whereas at other viewing angles it will be wholly defined by the relevant window cover image.

The array of image elements and/or the overlapping array of focussing elements may each be provided across the whole of the substrate window region or across only a sub-part of the substrate window region. The overlapping of the two arrays could be complete or partial. It should be noted that the area over which the array of image elements extends will usually not be continuously covered by image elements - rather, in places corresponding to solid zones of the corresponding image, the image elements will be present but periodically spaced from one another, whereas in places corresponding to transparent zones of the corresponding image, the image elements will be locally absent. In other words, the cover image will be considered to extend across an area corresponding to the outer bounds of the overlapping arrays (e.g. the whole substrate window region) even though zones within that image may be transparent and thus carry no image elements. It should also be noted that there is no requirement for focussing elements to be provided overlapping areas of the image element array which do not in practice provide an optically variable effect (e.g. where adjacent image elements from different window cover images together appear substantially continuous). As such, the array of focussing elements may preferably have a lateral extent which is smaller than that of the array of image elements. This makes the construction more tolerant of mis-register since it is not essential for the array of focussing elements to fill the substrate window region fully.

The (at least partially overlapping) arrays of image elements and focussing elements may be located wholly within the bounds of the substrate window region (preferably at least the array of image elements abutting at least part of the perimeter of the substrate window region). However in other preferred implementations, the array of image elements and/or the array of focussing elements may extend outside the substrate window region, overlapping at least part of the opacifying layer(s). This arrangement helps to conceal any misalignment of the image elements relative to the opacifying layer(s) and so makes the security document more tolerant of mis-register. While, in preferred cases, the image elements and/or focussing elements will be applied after the opacifying layers, with the result that the opacifying layer(s) are located between the image elements and the polymer substrate (and/or between the focussing elements and the polymer substrate) in areas of overlap, the opposite is also possible.

In some embodiments, it may be desirable to arrange that the substrate window region is not visible (in its unmodified form) at any viewing angle. However in other cases, it is advantageous to display the full substrate window region at at least one angle of view. For instance, this can help to emphasise the variable nature of the apparent window region since the substrate window region defines the largest window region available across all viewing angles. Therefore, to exhibit the full extent of the substrate window region at one viewing angle and then to conceal a significant portion of it at another (using a window cover image) results in an impactful visual effect. To achieve this, preferably the array of image elements includes spaces interspersed with the set(s) of image elements and positioned with the same periodicity as the or each set of image elements, such that, when the security document is viewed from at least one angle, the focussing elements direct light from the spaces to the viewer such that no window cover image is displayed and the apparent window region is the same as the substrate window region.

The security document could be provided with a single window cover image, visible only at a corresponding subset of viewing angles, so that the apparent window region appears to switch between the full extent of the substrate window region and a smaller modified window when the window cover image is visible. However, it is also possible to further increase the complexity (and hence the security level) of the device by providing multiple window cover images, each of which is visible at a different viewing angle. That is, each window cover image occupies its own “channel”. Hence, preferably, the array of image elements includes a plurality of sets of image elements, the respective sets being interspersed with one another and each set being arranged at the same periodicity, each set of image elements defining a respective window cover image at least some of which are preferably different from one another. In this case, it is not essential that the (unmodified) substrate window region be visible at any viewing angle. Each set of image elements will be formed of a material (preferably the same material) having the visual characteristics described above so that each window cover image is indistinguishable from the opacifying layer(s). Each window cover image may have one or more solid zones and/or one or more transparent zones as described above. It will be appreciated that, in areas of the array where all of the interlaced window cover images have solid zones, the image elements of the adjacent sets will together form a continuous area which is not optically variable (and as such there is no need for this area to be overlapped by focussing elements). In some cases, every one of the window cover images provided in a certain security document may be different from one another but this is not essential.

Of course, the greater the number of channels provided, the lower the proportion of the substrate dedicated to displaying each one, and therefore the lower the apparent optical density of each window cover image. As such, a balance between increased optical variability (achieved by more window cover images) and sufficient apparent optical density (achieved by fewer window cover images) is desirable. In preferred embodiments, the image element array could be configured such that it includes a peripheral zone adjacent the periphery of the substrate window region in which there are a first number of interlaced channels (which may be a single channel meaning that the zone appears static, in which case there is no need for the array of focussing elements to overlap this zone), and an inner zone in which there are a second number of interlaced channels, the second number being greater than the first number. In this way, the apparent optical density of the window cover images will be higher adjacent the periphery of the substrate window region than in the interior of the substrate window region. This helps to avoid (or reduce) an obvious drop in optical density between the non-window region and the window cover image, whilst at the same time enabling a higher degree of optical variability in parts of the substrate window region away from the periphery. By providing a plurality of window cover images, more complex visual effects become possible. In particularly preferred embodiments, the respective window cover images define frames of an animation effect, the apparent window region appearing to change in a step-wise or substantially continuous manner as the viewing angle is successively changed. For instance, the animation effect may comprise any of: expansion or contraction of the apparent window region; and/or movement of the apparent window region; and/or opening or closing of the apparent window region; and/or morphing from one shape to another of the apparent window region; and/or increasing or decreasing in number of apparent window regions.

It is particularly advantageous if the animation effect is configured to be cyclic, exhibiting a change in the apparent window region which varies successively from a first state to a second state and then successively from the second state back to the first state, before repeating as the viewing angle is changed in a continuous direction. For instance, the apparent window region could appear to increase and then decrease in size as the security document is tilted to the right, and then to repeat this as tilting continues to the right. Alternative the change could be a morphing from one shape to another and back again. Cyclic animations have the benefit that they are highly tolerant of translational mis-register between the focussing elements and the image elements, since it does not matter at precisely what viewing angle each “frame” of the animation is displayed at. The same cycle will still be displayed upon tilting, with no “jump” between the end of one cycle and the next (because they start and end with the apparent window in the same “state”).

In some implementations it may be desirable to “hide and reveal” the substrate window region (or apparent window region) so that the window appears to substantially disappear at some viewing angles, since this is a distinctive visual effect. However, in other cases it will be preferred to maintain the presence of a window region at all viewing angles given the inherently high security level this confers on the security document. As such, preferably, the or each window cover image is configured such that, at no viewing angle is the substrate window region fully concealed, whereby the security document exhibits the apparent window region at all viewing angles. In particularly preferred embodiments, the or each window cover image is configured such that a sub-region of the substrate window region maintains a constant level of transparency or translucency at all viewing angles, as a result of there being no focusing elements in the sub-region and/or no image elements in the sub-region. The constant level of transparency or translucency will be determined by that of the polymer substrate and any opacifying layer(s) present across the sub-region. For instance, if the polymer substrate is transparent and there are no opacifying layers across the sub-region (i.e. it is a “full window” region), the sub-region will remain transparent at all angles of view.

If desired, the complexity (and hence security level) can be further enhanced by using the arrangement of focussing elements and/or the image element array to also provide one or more other optically variable effects. For instance, the array of image elements may preferably further include a set of image elements having a different visual appearance (e.g. colour) from the collective visual appearance of the one or more opacifying layers at the selected location and defining a feature image which is displayed when the security device is viewed from a corresponding viewing angle, the feature image contrasting with the collective visual appearance of the one or more opacifying layer(s) at the selected location. In this way, the apparent window region may itself appear to change only at certain ranges of viewing angles while at others the feature image may be displayed within or over the apparent window region. The feature image could itself define one or more window regions within it and/or could exhibit information content, such as alphanumeric text, a portrait, a logo, a pattern or another graphic. The feature image could be monochromatic or multi-coloured.

The security of the document could be further enhanced by providing additional features in or alongside the focussing element array and/or the image element array. For example, the focussing element array could be formed from a transparent material comprising a coloured tint and/or a taggant. It is also possible to form the focussing element array from different transparent materials in different areas, e.g. so as to form a pattern of different materials some of which may be tinted or carry a taggant. Additionally or alternatively other relief structures could be formed in the same material, alongside (or between areas of) the focussing elements, such as tactile features. For instance, if the focussing elements are formed by cast-curing one or more curable materials (e.g. resins), these other relief structure(s) may be formed simultaneously with the focussing elements in the same cast-cure process. All of the above options could be located inside or outside the substrate window region. The image element array could also be extended, e.g. to provide a static image alongside the image elements such as a macroimage or microtext. For instance, material forming the image element array could be deposited according to an image such as a portrait or other multi-tonal image (e.g. a half-tone image). Again, this could be within the substrate window region or outside. If inside the substrate window region, such an image may preferably be positioned within the apparent window region at at least some viewing angles, preferably all.

Various different constructions of the security document are possible. For instance, the image elements and the focussing elements may both be arranged on the same side of the polymer substrate, as long as a suitable optical spacing is provided between them (e.g. built into the design of the focussing elements). However, it is preferred that the array of image elements and the overlapping array of focussing elements are disposed on opposite sides of the transparent substrate with preferably no opacifying layer located therebetween. This has manufacturing benefits as will be explained below.

The substrate window region can also have various constructions but in all cases will be more translucent (i.e. have lower optical density) than the adjacent nonwindow region of the security document. The substrate window region is defined by virtue of one or more gaps provided in one or more of the opacifying layer(s). In preferred embodiments, at least part of the substrate window region has a full window construction, across which all of the opacifying layers are absent (i.e. there is a gap in all of the opacifying layers across this part of the region). Most preferably the whole substrate window region has a full window construction. If the polymer substrate is transparent, this at least part of the substrate window region will be transparent with the result that the apparent window region will also be transparent. Alternatively, at least part of the substrate window region may have a half window construction, across which the opacifying layers are absent on one side of the substrate and present on the other side (i.e. the opacifying layers on one side have a gap at the relevant position and those on the other side do not). In some cases, the whole substrate window region may have a half window construction. It is also possible to form the substrate window region of a mixture of full and half window regions alongside one another. This can be beneficial since it adds to the complexity of the security document and optionally the window cover image(s) may be designed to complement or otherwise reflect the pattern formed by the mixture of window types.

It is also possible to provide an optional second focussing element array, which also at least partially overlaps the array of image elements, but on the opposite side. That is, the array of image elements would be located between the two focussing element arrays. The focal length of the second focussing element array would also be selected such that the array of image elements is substantially in its focal plane (as well as remaining in the focal plane of the first focussing element array). Typically, the two focussing element arrays will therefore have different focal lengths although this is not essential. In this way, the variable window effect can be observed in reflected light from both sides of the security document.

Advantageously, each of the opacifying layers comprises a non-fibrous material, preferably comprising a binder and an opacifying pigment. In preferred implementations each of the opacifying layers comprises an ink, preferably a gravure ink. For instance, the or each opacifying layer may comprise a titanium dioxide pigment in a suitable binder. Whilst it is not essential, preferably each of the opacifying layers is of substantially the same colour as one another, preferably white, off-white or light grey. In other cases, the opacifying layers may not all be of the same colour. For example, it may be desirable for internal opacifying layer(s) to be of substantially the same colour as one another (e.g. white, off-white and grey) while the outermost opacifying layer(s) on one or both sides of the security document are of a different colour (e.g. light blue, light green, light pink, light orange, light yellow or light purple). In all cases, one or more of the opacifying layers may contain an anti-static, electrically conductive pigment. Each opacifying layer preferably covers at least 50% of the surface area of one side of the security document, more preferably at least 80%. However, in some cases one or more of the opacifying layers may be localised, e.g. covering less than 50% of the surface area of one side of the security document.

The array of image elements can be formed in various different ways (e.g. via deposition of material, removal of material and/or the formation of a relief structure) as mentioned in more detail below. Preferably, the image elements comprise a material (such as an ink) having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at the selected location on the security document, the image elements preferably being print elements. If the image elements are formed by a material in this way, that material preferably possesses and provides the appearance characteristics which have been ascribed above to the image elements. Preferably, the one or more sets of image elements are each predominantly or wholly formed of this material having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at the selected location on the security document. However, as mentioned it is also possible for the image elements to further include small portions of different colours, e.g. formed of different materials. If the image element array includes any static areas, these will be formed in the same way as the image elements themselves and have the same visual appearance (e.g. colour).

The array of image elements preferably comprises one or more inks, preferably lithographic or flexographic inks. Most preferably the array of image elements is formed by a single ink. However while the visual appearance of this material must be substantially the same as the combined visual appearance of the opacifying layers at the selected location (as discussed above), the material itself is preferably of a different formulation from that of the or each opacifying layer.

Providing substantially the same visual appearance may preferably involve the image elements defining the or each window cover image being of substantially the same visible colour as the collective visible colour of the one or more opacifying layers at the selected location. Desirably, the Euclidean distance AE*_ab between the respective colours in Cl ELAB colour space is less than 5, more preferably less than 3, most preferably less than 2.3, where AE*_ab is measured using the formula where AL*, Aa* and Ab* 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). The colour difference AE*_ab can be measured using any commercial spectrophotometer, such as those available from Hunterlab of Reston, Virginia, USA.

If the security document includes multiple sets of image elements and multiple corresponding window cover images, each set could in theory be provided in a different material and/or via a different colour-generating mechanism (all achieving the required visible appearance). However, more preferably all sets of image elements will be made from the same material and/or colour-generating mechanism. Still preferably the array of image elements is defined by a single print working. This ensures complete registration between the image elements of each set.

It will be appreciated that the security document could, optionally, include more than one substrate window region each equipped with an optically variable arrangement and corresponding window cover image(s) if desired. Typically, the two or more substrate window regions would be spaced from one another by nonwindow region(s) of the security document. The window cover images provided in each substrate window regions may be configured to operate independently of one another or in some co-ordinated manner. For instance, a first substrate window region could be fully visible (i.e. no window cover image displayed) simultaneously (i.e. at the same viewing angle) as a second substrate window region is hidden (i.e. a window cover image is displayed, fully concealing the substrate window), and vice versa.

The so-described construction may be used as a security document by itself. However it may be desirable to provide the security document with further features. In preferred cases the security document further comprises a graphics layer extending over at least part of the opacifying layer(s) and optionally over at least part of the substrate window region on one or both sides of the document. If the graphics layer overlaps the image elements carrying the window cover images, the graphics layer will be on the outer side (i.e. the image elements will be between the graphics layer and the polymer substrate). In the case of a banknote for instance, the graphics layer may include fine-line patterns, guilloches, portraits, currency and/or denomination information. The graphics layer may typically comprise one or more print workings in one or more respective colours, and may be applied using security print techniques such as lithographic, flexographic, intaglio, gravure, screen or letterpress printing. The security document may also be provided with additional security features, e.g. carried by a security thread, foil, stripe or patch which is then applied to the security document, or formed directly thereon. In preferred examples, the security document may be a banknote, a cheque, a bank card, a stamp, a passport, a visa, a certificate, a driving licence, an identification document or the like.

The present invention further provides a method of manufacturing a security document, comprising:

(a) providing a transparent or translucent polymeric substrate having one or more opacifying layers disposed on the first and/or second sides of the transparent or translucent polymeric substrate, the one or more opacifying layers defining a substrate window region across which at least one of the one or more opacifying layers is absent; and (b) applying an array of image elements and an overlapping array of focussing elements across at least part of the substrate window region, the array of image elements being located substantially in the focal plane of the focussing elements, the array of image elements comprising one or more sets of image elements formed predominantly or wholly of a material having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at a selected location on the security document, the or each set of image elements being arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defining a respective window cover image; wherein the array of focusing elements are configured to direct light from selected ones of the array of image elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of image elements in combination across the at least part of the substrate window region within a respective range of viewing angles; whereby the security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

The method results in a security document having all the benefits already discussed above. Any of the preferred features of the disclosed security document mentioned above can be provided by appropriate implementation of the method.

It should be noted that in the context of this manufacturing method, the terms “on” and “onto” are used in the same way as mentioned above. That is, the terms do not require direct contact between the components in question.

As discussed above, the security document may be made in stages, with a first entity making the base document substrate by applying the opacifying layer(s) to the polymer substrate (leaving gaps to form the substrate window regions) and then a second entity applying the focussing elements and image elements. Therefore step (a) may simply involve obtaining the so-produced document substrate from a suitable supplier. Alternatively, the whole manufacturing process could be carried out by one entity, in which case step (a) preferably comprises applying the one or more opacifying layers onto the first and/or second sides of the transparent or translucent polymeric substrate. Either way, the opacifying layer(s) are preferably applied to the polymer substrate by a first printing technique, the first printing technique most preferably being gravure printing.

The array of image elements could be applied onto the polymer substrate using various techniques, including the possibility of forming the image elements in or on a secondary substrate (typically transparent) which can then be affixed to the transparent or translucent polymer substrate, e.g. using an adhesive or by lamination. However, in preferred implementations, in step (b) the array of image elements is applied by forming the image elements on the transparent or translucent polymer substrate in situ.

The array of image elements can be formed via various different processes, such as any of: printing (optionally plus curing), partial removal of a material, filling of a recess with material, diffractive effects or structural colour such as plasmonic mechanisms), micro-intaglio and so on. Most preferably, the image elements are print elements and are formed by printing. Advantageously, this is achieved by a second printing technique different from the first printing technique. Preferably the second printing technique is a lithographic printing technique, most preferably offset lithographic printing. The lithographic printing technique could be wet or dry lithography (offset or otherwise). Another preferred example of suitable second printing techniques is flexographic printing.

The array of focussing elements could also be formed on another secondary substrate which is then affixed to the transparent or translucent polymer substrate. However, preferably in step (b) the array of focussing elements is formed on the transparent or translucent substrate, most preferably by any of: a cast-cure technique; by embossing of the transparent or translucent substrate or a layer applied thereto; or by printing (e.g. as described in WO-A-2013/167887). “Cast- curing” involves forming a surface relief profile in a curable material, the surface relief defining the focussing elements, and curing the material so as to retain the profile. Specifically, in a cast-curing process, the focussing elements may be formed by:

(i) applying at least one transparent curable material either onto the transparent or translucent polymer substrate or onto a casting tool carrying a surface relief corresponding to the focussing elements,

(ii) bringing the substrate and the casting tool together so as to form the transparent curable material(s) with the casting tool; and

(iii) curing the transparent curable material(s) in the first region and in the second region, so as to retain the surface relief in the first region.

The transparent curable material (e.g. resin) can have a coloured tint and/or comprise a taggant as mentioned earlier. It is also possible to use more than one such material, e.g. arranged so as to form a pattern. The same cast-cure process can be used to form additional features such as tactile elements or other relief structures if desired.

As discussed above, the image elements and the focussing elements could be located on the same side of the polymer substrate. However, preferably, in step (b) the array of image elements is printed onto the first side of the transparent or translucent substrate and the array of focussing elements is formed on the second side of the transparent or translucent substrate. This enables the thickness of the substrate to be used as an optical separator between the image elements and the focussing elements.

Arranging the image elements and focussing elements on opposite sides of the substrate also has the particular advantage that the two sides of the substrate can be accessed at the same time during the manufacturing process. Hence in an especially preferred embodiment, the printing of the array of image elements and the forming of the array of focussing elements are performed simultaneously on opposite sides of the transparent or translucent substrate at the same position along the transparent or translucent substrate along the machine direction. This has the benefit of being able to achieve extremely high registration between the focussing elements and the image elements. Some techniques for implementing this are disclosed in more detail in WO-A-2017/009616.

Whether simultaneous application is used or not, it is still preferable that in step (b) the array of image elements and the overlapping array of focussing elements are applied in register with one another. In particular, it is advantageous if the image elements and focussing elements are registered to one another to a greater degree than either is registered to the opacifying layers.

BRIEF DESCRIPTION OF DRAWINGS

Examples of security documents will now be described with reference to the accompanying drawings, in which:

Figures 1 (a) and 1 (b) show the appearance of a first embodiment of a security document in plan view under reflected light, from first and second viewing angles respectively;

Figure 2 shows a cross-section through the security document of Figures 1 (a) and 1 (b), along the line X-X’;

Figures 3(a) and 3(b) each schematically depict a portion of the security document of Figures 1 (a) and 1 (b), at an enlarged scale, showing illustrate light rays directed to a viewer at the first and second viewing angles, respectively;

Figure 3(c) shows an exemplary window cover image as used in the security document of Figures 1 (a) and 1 (b), while Figure 3(d) shows an exemplary substrate window region;

Figures 4(a) to 4(c) illustrate a security document according to a second embodiment of the invention, Figures 4(a) and 4(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 4(c) showing a cross-section through the security document along the line Y-Y’;

Figures 5(a) to 5(d) illustrate a security document according to a variant of the second embodiment of the invention, Figures 5(a) and 5(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figures 5(c) and 5(d) showing two alternative cross-sections through the security document along the line Y-Y’ according to different implementations;

Figures 6(a) to 6(c) illustrate a security document according to a third embodiment of the invention, Figures 6(a) and 6(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 6(c) showing a cross-section through the security document along the line Y-Y’;

Figures 7(a) to 7(c) illustrate a security document according to a fourth embodiment of the invention, Figures 7(a) and 7(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 7(c) showing a cross-section through the security document along the line Y-Y’;

Figures 8(a) to 8(c) illustrate a security document according to a fifth embodiment of the invention, Figures 8(a) and 8(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 8(c) showing a cross-section through the security document along the line Y-Y’;

Figures 9(a) to 9(c) illustrate a security document according to a sixth embodiment of the invention, Figures 9(a) and 9(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 9(c) showing a cross-section through the security document along the line Y-Y’;

Figures 10(a) to 10(c) illustrate a security document according to a seventh embodiment of the invention, Figures 10(a) and 10(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 10(c) showing a cross-section through the security document along the line Y-Y’;

Figure 10(d) shows a cross-section through a security document according to a variant of the seventh embodiment;

Figures 11 (a) to 11 (d) illustrate a security document according to a eighth embodiment of the invention, Figures 11 (a), 11 (b) and 11 (c) showing a portion of the security document in plan view at first, second and third viewing angles respectively, and Figure 11 (d) showing a cross-section through the security document along the line Y-Y’;

Figures 12(a) to 12(c) illustrate a security document according to a ninth embodiment of the invention, Figures 12(a) and 12(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 12(c) showing a cross-section through the security document along the line Y-Y’;

Figures 13(a) to 13(c) illustrate a security document according to a tenth embodiment of the invention, Figures 13(a) and 13(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 13(c) showing a cross-section through the security document along the line Y-Y’;

Figures 14(a) to 14(d) illustrate a security document according to a eleventh embodiment of the invention, Figures 14(a), 14(b) and 14(c) showing a portion of the security document in plan view at first, second and third viewing angles respectively, and Figure 14(d) showing a cross-section through the security document along the line Y-Y’;

Figures 15(a) to 15(e) illustrate a security document according to a twelfth embodiment of the invention, Figures 15(a), 15(b), 15(c) and 15(d) showing a portion of the security document in plan view at first, second and third viewing angles respectively, and Figure 15(e) showing a cross-section through the security document along the line Y-Y’;

Figures 16(a) to 16(d) illustrate a security document according to a thirteenth embodiment of the invention, Figures 16(a), 16(b) and 16(c) showing a portion of the security document in plan view at first, second and third viewing angles respectively, and Figure 16(d) showing a cross-section through the security document along the line Y-Y’;

Figures 17(a) to 17(d) illustrate a security document according to a fourteenth embodiment of the invention, Figures 17(a), 17(b) and 17(c) showing a portion of the security document in plan view at first, second and third viewing angles respectively, and Figure 17(d) showing a cross-section through the security document along the line Y-Y’;

Figures 18(a) to 18(d) illustrate a security document according to a fifteenth embodiment of the invention, Figures 18(a), 18(b) and 18(c) showing a portion of the security document in plan view at first, second and third viewing angles respectively, and Figure 18(d) showing a cross-section through the security document along the line Y-Y’; Figures 19(a) to 19(f) show further embodiments of security documents, in crosssection;

Figures 20(a) to 20(e) illustrate a security document according to another embodiment of the invention, Figure 20(a) showing a cross-section through the security document, Figures 20(b) and 20(c) showing the security document in plan view as seen by a first observer on a first side of the security document at two different viewing angles, and Figures 20(d) and 20(e) showing the security document in plan view as seen by a second observer on a second side of the security document at two different viewing angles;

Figures 21 (a) to 21 (c) illustrate a security document according to a further embodiment of the invention, Figure 21 (a) showing a cross-section through the security document, and Figures 21 (b) and 21 (c) showing the security document in plan view as seen by a first observer on a first side of the security document at two different viewing angles;

Figure 22 is a flowchart showing steps in an exemplary method of manufacturing a security document in accordance with an embodiment of the invention;

Figures 23(a) and 23(b) schematically illustrate an exemplary process for forming a focussing element array for use in embodiments of the invention;

Figures 24 and 25 schematically show two embodiments of apparatus for simultaneously applying a focussing element array and an array of image elements to a substrate, suitable for use in methods according to the invention;

Figure 26 schematically shows an embodiment of apparatus for sequentially applying a focussing element array and an array of image elements to a substrate, suitable for use in methods according to the invention;

Figures 27(a) to 27(j) show examples of image elements;

Figure 28 shows a further embodiment in plan view;

Figure 29 shows another embodiment of a security document in cross-section;

Figures 30(a) and 30(b) show the security document of Figure 29 in one variant in plan view, from two different viewing angles respectively;

Figures 31(a) and 31(b) show the security document of Figure 29 in another variant in plan view, from two different viewing angles respectively;

Figures 32(a) to 32(c) illustrate a security document according to a first comparative example, Figures 32(a) and 32(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 32(c) showing a cross-section through the security document along the line Y-Y’; and

Figure 33(a) to 33(c) illustrate a security document according to a second comparative example, Figures 33(a) and 33(b) showing a portion of the security document in plan view at first and second viewing angles respectively, and Figure 33(c) showing a cross-section through the security document along the line Y-Y’.

DETAILED DESCRIPTION

Figures 1(a) and 1 (b) show an exemplary security document 100 in accordance with a first embodiment of the invention. Here, the security document 100 is depicted as a banknote but the same structure can be applied to any other form of security document, such as a cheque, a certificate, a passport, an ID card, a bank card or any other document which requires authentication. The security document 100 exhibits an apparent window region 10, which appears more translucent than the remainder of the security document 100 and preferably is transparent. In this example the security document 100 is also provided with several optional features, including a graphics layer 90 and security devices 93 and 95. The graphics layer 90 may for instance exhibit fine-line patterns or other secure markings, images such as a portrait and/or information such as a denomination indicator. The security features 93 and 95 could be of any type and may be formed directly on the security document 100 or could be supplied in the form of a security article (e.g. a security strip, patch or foil) and affixed to the security document 100. Examples include diffractive devices such as holograms, micro-optical devices based on lenses or prisms, latent images, UV or other fluorescent features, magnetic devices and so on. In the present embodiment, security device 93 is inside the apparent window region 10 while security device 95 is outside the apparent window region 10. It should be appreciated that any of the optional features described here could equally be applied to any of the embodiments set out below. As can be appreciated from a comparison of Figures 1 (a) and 1 (b), which show the security document 100 from different viewing angles, the apparent window region 10 varies depending on the angle of view. When the security document is observed from a first viewing angle (Figure 1(a)), the apparent window region 10 has an oval-shaped periphery 11. When the security document 100 is observed from a second viewing angle (Figure 1 (b)), for instance having been tilted about the y-axis, the apparent window region 10 now covers a larger area and has a rectangular periphery 11. If the viewing position is alternated between the first and first and second viewing angles, the apparent window region 10 appears to switch repeatedly between oval and rectangular shapes.

The mechanism behind this variable window effect is best explained with reference to Figures 2, 3 and 4. Figure 2 shows a cross-section through security document 100 along the line X-X’. It will be seen that the security document 100 comprises a polymer substrate 1 (which is preferably transparent but at least translucent to visible light). At least one opacifying layer 2a, 2b is provided on one or both surfaces of the polymer substrate 1 , across the majority of the security document 100. In this case opacifying layers 2a, 2b are provided on both surfaces of the polymer substrate 100 which is typical but not essential. In practice, several opacifying layers may be coated on each surface of the polymer substrate, e.g. up to 5 or 7 opacifying layers, which may or may not have the same lateral extent as one another. Each opacifying layer preferably covers at least 50% of the surface area of one side of the security document, more preferably at least 80%. However, in some cases one or more of the opacifying layers may be localised, e.g. covering less than 50% of the surface area of one side of the security document. Typically the opacifying layers 2a, 2b will be a light colour such as white, off-white or grey, so as to provide a suitable background for printing. While in many cases all of the opacifying layers will be of substantially the same colour as one another, this is not essential and an example where this is not the case will be provided below. At least one of the opacifying layers 2a, 2b (here, all of them) is omitted across a region 5 of the substrate, which therefore constitutes a substrate window region. In this example the substrate window region 5 is rectangular, having a periphery 6 defined by the border between the area where the opacifying layers are present and the area in which they are absent (i.e. the gap in the opacifying layers). There may be a boundary region (not shown) immediately outside the periphery 6 in which one or more of the opacifying layers is “stepped back” by a small amount away from the window region (e.g. by 0.5 mm or less) for registration regions and so less than all of the opacifying layers are present here. The region outside the substrate window region 5 (and any boundary region provided) where all the opacifying layers are present may be referred to as the non-window region 7.

In at least part of the substrate window region 5, the security document 100 is provided with an optically variable arrangement comprising an array of focussing elements 12 and an array of image elements 14. The two arrays 12, 14 at least partially overlap one another and the array of image elements 14 is arranged approximately in the focal plane of the focussing elements. In the present example, the focussing elements 12 are located on a first surface of the polymer substrate 1 and the image elements 14 are located on the opposite (second) surface of the polymer substrate 1 , which is preferred but not essential. In combination, the array of focussing elements 12 and the array of image elements 14 co-operate to display a window cover image 20a when the security document 100 is viewed from the first viewing angle (Figure 1 (a)) but not when the security document is viewed from the second viewing angle (Figure 1 (b)). The image elements 14 have substantially the same visual appearance as that of the opacifying layers 2a, 2b (in combination) at a predetermined location on the security document 1 where at least one of the opacifying layer(s) is present, e.g. the predetermined location may be in the non-window region 7 (or in a boundary region, if provided). For example, if the stack of opacifying layers 2a, 2b appears white in the non-window region 7, the material forming image elements 14 preferably also appears substantially white. As such, the window cover image 20a appears (when visible) as an extension of the non-window region 7 (or of the boundary region). Thus the apparent window region 10 actually exhibited by the security document 100 at a particular angle of view is defined by the combination of the substrate window region 5 and any window cover image 20a visible at that viewing angle. For instance, in the Figure 1 embodiment, the window cover image 20a comprises an oval-shaped transparent zone 22a surrounded by a solid area 21a which will have substantially the same appearance as that of the opacifying layers 2a, 2b (at least predominantly). This is shown in Figure 3(c). The window cover image 20a here is sized to fill the substrate window region 5.

Therefore, when the security document is viewed at the first viewing angle, the window cover image 20a is displayed and appears to modify the (rectangular) lateral extent of the substrate window region 5, resulting in an oval-shaped apparent window region 10. Of course, the substrate window region 5 is not actually changed (being fixed as a result of the placement of the opacifying layers), but it appears to be changed by the super-position of the window cover image 20. It should be noted that in Figure 1 (a) the periphery 6 of the substrate window region 5 is depicted in dashed lines so that it can be compared with the periphery 11 of the apparent window region. However, in practice the original periphery 6 will preferably not be apparent to the user at the first viewing angle (at least not all of it), since it will appear substantially hidden in the non-window region 7. When the security document 100 is viewed from the second viewing angle (Figure 1(b)), the window cover image 20a is no longer displayed and so the whole of the substrate window region 5 is visible. In other words, at this viewing angle, the apparent window region 10 is the same as the substrate window region 5 (and so are their respective peripheries 11 and 6), since there is no contribution from any window cover image and so no modification made.

As noted above, the image elements 14 have substantially the same visual appearance as the collective appearance of the opacifying layers 2a, 2b (at a predetermined location), meaning that that the relevant visual appearances are substantially indistinguishable to the naked eye, when viewed in reflected light under standard illumination conditions (e.g. white light illumination). In some cases it may be desirable for the material forming the image elements to have substantially the same optical density as the collective optical density of the opacifying layer(s) at the relevant location, in which case the apparent window region would appear to change and/or disappear under both reflective and transmissive viewing conditions (upon varying the viewing angle). However, in practice this is not essential and the material forming the image elements may have a substantially lower optical density compared to that of the opacifying layers collectively, in which case the window variation may be less visible (or not visible) in transmissive viewing conditions. Nonetheless, the reflective visual appearance of the image elements will still be substantially the same as that of the opacifying layers collectively (e.g. colour) such that the variable window effect is visible at least under reflective viewing conditions. It should be noted that despite the similarity in appearance between the image elements 14 and the opacifying layers 2a, 2b, the materials and/or mechanisms forming those appearances are preferably different, e.g. in terms of their chemical formulation if two different materials are used. For instance, in preferred examples the opacifying layers 2a, 2b will each be formed of a gravure ink while the image elements 14a (if printed) may each be formed of an offset (e.g. litho) ink.

Providing substantially the same visual appearance may preferably involve image elements 14a, being of substantially the same visible colour as the collective visible colour of the one or more opacifying layers 2a, at the selected location. Desirably, the Euclidean distance AE*_ab between the respective colours in Cl ELAB colour space is less than 5, more preferably less than 3, most preferably less than 2.3, where AE*_ab is measured using the formula where AL*, Aa* and Ab* 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). The colour difference AE*_ab can be measured using any commercial spectrophotometer, such as those available from Hunterlab of Reston, Virginia, USA.

Figures 3(a) and 3(b) schematically show an enlarged portion of the security document 100 within the substrate window region 5, in order to explain the mechanism by which the above-described optically variable effect is generated. In this example the focussing elements 12 are depicted as elongate lenses (e.g. cylindrical lenses) with their long axes aligned along the y-axis of the document 100. The focussing elements 12 are periodic in the orthogonal (x-axis) direction. The array of image elements 14 here comprises one set of image elements 14a, which are image slices taken from the window cover image 20a (shown in Figure 3(c)). The image elements 14a are arranged with substantially the same periodicity as the focussing elements 12 in the x-axis direction. In this example the image elements 14a are spaced from one another by gaps 15a which are empty, corresponding to a blank image 15 (shown in Figure 3(d)).

The configuration is such that, when the device is viewed by a first observer Oi at a first viewing angle, as shown in Figure 3(a), each focussing element in the array 12 directs light from one of the image elements 14a in the set corresponding to window cover image 20a to the viewer. Thus the first window cover image 20a is displayed to observer Oi by the set of image elements 14a in combination with one another. When the device is viewed by a second observer O2 at a second viewing angle (Figure 3(b)), the focussing elements now direct light from the spaces 15a to the viewer, with the result that no image is displayed (or, equivalently, a blank image 15 is displayed). Thus the arrangement is a form of two-channel lenticular device.

The window cover image 20a can take many different forms, more examples of which will be given below. In general, it is preferred that the window cover image 20a includes at least one solid zone 21a, which is a non-transparent contiguous area the predominant appearance of which is substantially the same as that of the opacifying layers in the non-window region 7, and most preferably also at least one transparent zone 22a. However, the latter is not essential.

In order that the window cover image 20a appears as an extension of the nonwindow region in the manner described, the image elements 14a will be at least predominantly of the said appearance substantially matching that of the opacifying layers in the selected location. Most preferably, the image elements 14a will be wholly of this appearance (as in the first embodiment described herein), in which case the solid zone 21a will have a substantially uniform, block colour appearance (when viewed via the focussing elements - in practice the actual image layer may comprise a line or dot screen but it will appear solid to the naked eye), substantially matching the appearance of the opacifying layers in combination at the selected location. However, this is not essential and in some embodiments it may be desirable that the sets of image elements 14a also include a small proportion of some other image elements (e.g. a different colour), in order for the window cover image 20a to exhibit a pattern or other detailing, which may for instance appear to overlie the matched colour in the solid zone of the window cover image. An example of this will be provided in relation to Figure 16 below but it should be appreciated that this variation can be applied to all embodiments.

Further embodiments will now be described with reference to Figures 4 to 18. In each case the construction of the security document and the operation principles are the same as described above with respect to the first embodiment. The image elements are configured to have substantially the same appearance as that of the opacifying layers (collectively), at a point just outside the substrate window region. It will be appreciated that the Figures show only a portion of the security document in each case (that in the vicinity of the apparent window region 10) and in practice the rest of the security document could be as shown in Figures 1 (a) and 1 (b). It should also be noted that the cross-sections shown in the Figures are highly schematic and not to scale. In particular, the periodic relationship between the image elements and the focussing elements is not depicted accurately. In some cross-sectional Figures, the image elements are shaded and a key is provided to correlate the shading pattern to a window cover image (e.g. “A” corresponds to 20a, “B” to 20b and so on) which is defined by the image elements having that shading pattern. Thus the shading is provided solely to identify the relationships between the image elements and the window cover images, and should not be construed as indicative of any other characteristics of the image elements. This applies for example to Figures 4(c), 6(c), 7(c), 8(c), 9(c), 10(c), (d), 11 (d), 12(c), 13(c), 14(d), 15(e), 16(d), 17(d) and 18(d).

Figure 4 depicts a second embodiment of the invention. Here, as in the first embodiment, the array of image elements 14 comprises a single set 14a of image elements corresponding to a window cover image 20a, spaced by gaps 15a. The arrangement is such that, at the first viewing angle, as shown in Figure 4(a), the gaps 15a are direct to the viewer with the result that the window cover image 20a is not visible and the apparent window region 10 is the same as the substrate window region 5, here an elongate “lozenge” shape. The window cover image 20a consists wholly of a solid zone configured to fill the substrate window region 5 entirely. As such, at the second viewing angle, there is no apparent window region 10 (i.e. it has disappeared), because the now-visible window cover image 20a conceals the whole of the substrate window region 5. In other words, the original substrate window region 5 has been fully turned “off’.

A variant of the second embodiment is shown in Figure 5. Again, a single set 14a of image elements is provided, corresponding to a window cover image 20a, spaced by gaps 15a. However, in this case the image element array 14 further includes a static area 14(i) extending between its optically variable area 14(ii) and the periphery 6 of the substrate window region 5. The optically variable area 14(ii) corresponds to the area of the apparent window region 10, which in this example is rectangular. Here, the image elements 14a and gaps 15a are interleaved in the aforementioned manner such that they co-operate with the focussing elements to generate an optically variable effect. Outside the periphery 11 of the apparent window region, in static area 14(i), the image element array 14 is configured to have a substantially continuous structure 25 which will not co-operate with the focussing element array to generate an optically variable effect, and appears substantially solid to the naked eye with or without the presence of focussing elements 12. For instance, structure 25 could be formed as a half-tone pattern or as a flood print. The continuous (or pseudo-continuous) structure 25 is formed of the same material (or colour-generating structure) as image elements 14a so that it has the same visual appearance as image elements 14a, and is preferably manufactured in the same process, e.g. a printing process, most preferably lithographic printing.

When the security document is viewed from a first viewing angle, shown in Figure 5(a), the rectangular apparent window region 10 is displayed because, in this area, the focussing elements direct the gaps 15a to the viewer and the window cover image 20a is not visible. At the same time, the continuous area 14(ii) of the image element array, which extends to the periphery of the substrate window region 5, appears contiguous with that periphery and therefore conceals the size and shape of the underlying substrate window region 5. When the security document is viewed from a second viewing angle, shown in Figure 5(b), the window cover image 20a is now displayed to the viewer. Here, the window cover image 20a consists wholly of a solid zone configured to fill the optically variable part 14(ii) of substrate window region 5 entirely. As such, at the second viewing angle, there is no apparent window region 10 (i.e. it has disappeared), because the now-visible window cover image 20a together with the static part 14(i) of the image element array conceals the whole of the substrate window region 5. In other words, the original substrate window region 5 has again been fully turned “off’.

Two alternative constructions of the security document are shown in cross-section in Figure 5(c) and 5(d). In both cases, the focussing element array 12 is arranged to overlap only part of the image element array 14 and does not fill the whole of substrate window region 5. This is beneficial since the arrangement is more tolerant of potential misregistration between the focussing element array 12 and the opacifying layers 2a, 2b defining the substrate window region. The focussing element array 12 need only overlay the part 14(ii) of the image element array 14 which is configured to give rise to an optically variable effect, which in this case corresponds to the extent of rectangular apparent window region 10. The focusing element array 12 may also extend over part (but preferably not all) of the continuous structure 25 of the image array 14 in its static area 14(i) in order to reduce translational registration requirements between the focussing element array 12 and the image element array 14. Of course, it is also possible for the focussing element array 12 to fill the whole of the substrate window region 5 if preferred. In the Figure 5(c) construction, the continuous or pseudo-continuous structure 25 of the image element array 14 is configured to abut the periphery 6 of the substrate window region 5 (or may be spaced from it by a uniform keyline). In the Figure 5(d) implementation, the continuous or pseudo-continuous structure 25 of the image element array 14 extends outside the substrate window region 5, overlapping its periphery 6, having been laid down after the opacifying layer 2b. Both constructions achieve the same desired aim of making the continuous or pseudo-continuous structure 25 appear as a contiguous extension of the opacifying layers 2a, 2b, thus helping to conceal the shape and size of the substrate window region 5.

In another variant of the second embodiment, shown in Figure 6 (constituting a third embodiment), the focussing element array 12 performs a dual purpose. An optically variable device is provided by placing additional image elements 91 in the focal plane of the array 12 which are configured to co-operate with the focussing elements to generate a contrasting optically active visual effect. This could take the form of any known type of device, such as a lenticular device or a moire magnifier. In the example shown, the device 91 is a two-channel lenticular device displaying an image of a star which appears to rotate between a first orientation 91a visible at the first viewing angle (Figure 6(a)) and a second orientation 91b visible at the second viewing angle (Figure 6(b)). This can be achieved by interlacing image slices from corresponding first and second images showing the star shape at the two orientations, in known manner. Preferably the image elements 91 will be provided in one or more colours which strongly contrast with the image elements 14a. When the security document 100 is viewed from the first viewing angle, the optically variable device 91 will appear to be located in apparent window region 10, whereas when viewed from the second viewing angle, it will now appear to be located in a non-window region (since the apparent window region 10 has been caused to disappear as explained above). It should be noted that an optically variable device 91 can be added to any of the embodiments described below in the same manner as shown here.

Figure 7 shows a fourth embodiment. As in the previous two embodiments, here the array of image elements 14 comprises a single set 14a of image elements corresponding to a window cover image 20a, spaced by gaps 15a. The arrangement is such that, at the first viewing angle, as shown in Figure 7(a), the gaps 15a are direct to the viewer with the result that the window cover image 20a is not visible and the apparent window region 10 is the same as the substrate window region 5, here an elongate “lozenge” shape. The window cover image 20a, visible in Figure 7(b), is configured to fill only a portion of the substrate window region 5, here the top third. This is achieved by providing the window cover image 20a with a solid zone 21a having shape and size corresponding to the top third of the substrate window region 5, and an adjacent transparent zone 22a. It should be noted that since only the part of image element array 14 corresponding to the area of solid zone 21a is optically variable (the remainder of the substrate window region 5 being visible at all viewing angles), the focussing element array 12 need not extend over the whole of the substrate window region 5 (as shown) but could be provided only over a lesser area provided it includes the optically variable part (similar to the arrangement shown in Figure 2). Thus, the apparent window region 10 appears to change in terms of both size and shape (becoming smaller, and less elongate) when the viewing angle is changed from the first to the second. In other words, the original substrate window region 5 has been partially turned “off’ (in a spatial sense). It will also be noted that here, the periphery 11 of the apparent window region viewed from the second viewing angle (Figure 7(b)) is defined in part by the periphery 6 of the substrate window region 5 and in part by the border 23a between the solid and transparent zones in the window cover image 20a.

The fifth embodiment, shown in Figure 8, is much the same as the fourth embodiment. However, here, the window cover image 20a, visible at the second viewing angle, is configured to fill both the top third and the bottom third of the “lozenge” shape of the substrate window region 5. As such, the apparent window region 10 now appears to switch between the full lozenge-shaped area at the first viewing angle (Figure 8(a)) and a smaller, rectangular area at the second viewing angle (Figure 8(b)). This is achieved by configuring the window cover image 20a to have two solid zones 21a’, 21a” and an intervening transparent zone 23a. It should be noted that since only the parts of image element array 14 corresponding to the areas of solid zones 21a’, 21a” are optically variable (the remainder of the substrate window region 5 being visible at all viewing angles), the focussing element array 12 need not extend over the whole of the substrate window region 5 (as shown) but could be provided only over a lesser area provided it includes the optically variable parts (similar to the arrangement shown in Figure 2). Again, the periphery 11 of the apparent window region is defined in part by the periphery 6 of the substrate window region (the “verticals” of the rectangular window 10), and in part by the borders 23a in the window cover region (the “horizontals” of the rectangular window 10).

It is also possible for the window cover image 10 to wholly define the periphery 11 of the apparent window region 10 (at the corresponding angle of view). This was the case in the first embodiment above and is also provided in the sixth embodiment, shown in Figure 9. Here, the window cover image 20a, visible at the second viewing angle (Figure 9(b)), comprises an approximately square transparent zone 22a surrounded on all four sides by a solid zone 21a. Thus when the security document is viewed from the second viewing angle, all of the periphery 6 of the substrate window region is concealed. The entire periphery 11 of the apparent window region 10 is defined by the border 23a between the zones 21a, 22a in the window cover image 20a. Upon tilting, the apparent window 10 appears to switch between the full lozenge shaped area of the substrate window region at the first viewing angle, and a significantly smaller square region at the second viewing angle.

In the embodiments so far, a single window cover image has been provided. However, it is also possible to configure the security document with two or more window cover images, each visible at a different viewing angle. This is demonstrated in the seventh embodiment, shown in Figures 10(a) to (c). The image element array 14 now comprises two sets of image elements 14a, 14b, each corresponding to a different respective window cover image 20a, 20b, and each having substantially the same colour as the opacifying layers in the selected location (i.e. just outside the substrate window region, in this case). The image elements from each set are interleaved with one another and have the same periodicity. When the security document is viewed from the first viewing angle (Figure 10(a)), the first window cover image 20a is displayed, whereas when the security document is viewed from the second viewing angle (Figure 10(b)), the second window cover image 20b is displayed. In this example, the first window cover image 20a has a square transparent zone 22a surrounded by a solid zone 21a on all four sides (as in the sixth embodiment), whereas the second window cover image 20b has a circular transparent zone 22b surrounded by a solid zone 21 b in all directions. Each window cover image is sized to fill the substrate window region 5 such that its original periphery 6 is hidden at all viewing angles. Thus, upon tilting, the apparent window region 10 appears to switch shape between square and circular. In this case, the periphery 11 of the apparent window region 10 is wholly defined by the window cover images 20a, 20b at all viewing angles.

It will be noted that, in the seventh embodiment, where the solid zones 21a and 21b of the first and second window cover images overlap one another, the image elements 14a, 14b forming the cover images together form a substantially continuous structure of the image element array which is not itself optically variable - that is, this area of the substrate window region 6 will appear the same at all viewing angles despite the presence of focussing elements 12. In practical terms, this area amounts to having a continuous or pseudo-continuous structure similar to that discussed with reference to Figure 5 above.

In a variant of the seventh embodiment, shown in Figure 10(d), this is made use of by arranging the focusing element array 12 to overlap only part of the image element array 14. Here, the focussing element array 12 does not extend to the periphery of the substrate window region 5 but rather is spaced from it, which makes the construction more tolerant of misregister between the focussing element array 12 and the opacifying layers 2a, 2b. The focussing element array 12 is positioned so as to overlap at least the part 14(ii) of the array of image elements 14 which gives rise to an optically variable effect, which in this case corresponds approximately to the area containing the square transparent zone 22a of the first window cover image (the circular transparent zone 22b falling inside zone 22a). The focussing elements are absent over at least some of the area 14(i) of the array 14 where the interleaved image elements 14a, 14b are continuous with each other. Since this area 14(i) of the image element array 14 appears substantially visually continuous to the naked eye, it provides visual continuity between the optically variable apparent window region 10 and the surrounding opacifying layers 2a, 2b, helping to conceal the periphery 6 of the substrate window region 5 at all viewing angles. Preferably, the focussing element array 12 is larger than the optically variable part 14(ii) (and thus overlaps some of the continuous area 14(i)) of the image element array 14 to also improve tolerance of mis-register between the focussing element array 12 and the image element array 14. It should be noted that similar considerations can be applied to all embodiments disclosed herein - that is, where parts of the array of image elements 14 are substantially continuous (and thus would not give rise to an optically variable effect), there is no need for the focussing element array to extend over them. Moreover it may be advantageous for the focussing element array 12 not to extend over the whole substrate window region 5 for the reasons just explained.

It is also possible to provide the security document with more than two window different appearances of the apparent window region, by increasing the number of “channels” that can be displayed across the whole range of viewing angles. The eighth embodiment, shown in Figure 11 , is an example of a three-channel device suitable for this. Here, three window cover images 20a, 20b, 20c are provided. The image element array 14 comprises three corresponding sets of image elements, 14a, 14b, 14c, all of the same colour, each having the same periodicity and interlaced with one another in the direction of periodicity. The first and second window cover images 20a, 20b are the same as in the previous embodiment. The third window cover image 20c defines a triangular transparent zone 22c, surrounded by a solid zone 21c. Thus, from a first viewing angle (Figure 11 (a)), the apparent window region 10 appears square, from a second viewing angle (Figure 11 (b)), the apparent window region appears circular, and from a third viewing angle the apparent window region appears triangular. At all viewing angles the periphery 11 of the apparent window region 10 is defined wholly by the window cover images 20 while the periphery 6 of the substrate window region 5 remains hidden. It will be appreciated that, as in the seventh embodiment, it is not essential for the focussing element array 12 to extend across the whole of the substrate window region 5. Rather, the focussing element array 12 need only overlap the part of the image element array 14 which co-operates with the focussing elements to generate an optically variable effect, which here corresponds approximately to the extent of square transparent zone 22a of the first window cover image. For instance, in a variant of the eighth embodiment, the cross section of the structure could be the same as shown in Figure 10(d).

In the above examples, the window cover image(s) have been configured such that, upon changing the viewing angle the apparent window region 10 changes in shape and/or size, or disappear/reappears. Alternatively or in addition the window cover image(s) can be configured such that the apparent window region 10 changes in term of number. That is, the number of apparent window regions 10 visible simultaneously (i.e. at the same viewing angle) may vary with viewing angle. An example of this is shown in the ninth embodiment, depicted in Figure 12. Here, the security device is provided with two window cover images , the construction being the same as in the seventh embodiment. In this case the first window cover image 20a and the second window cover image 20b each define a square transparent zone 22a, 22b having the same size and location such that this part of the apparent window region 10 does not appear to change upon tilting. However, while the first window cover image 20a (Figure 12(a)) comprises a solid zone 21a which fills the whole of the substrate window region 5 outside the square transparent zone 22a, the second window cover image 20b (Figure 12(b)) defines two further transparent zones 22b’ and 22b”. Each of these additional transparent zones 22b’ and 22b” is circular and is separated from the square transparent zone 22b by the solid zone 21 b. Thus, on changing between the first viewing angle and the second viewing angle the number of apparent window regions change from one to three and back, as the two circular window regions appear and disappear. As in the preceding examples, it is not necessary for the focussing element array 12 to extend over the whole of the substrate window region 5, but only those parts of the image element array 14 which give rise to an optically variable effect. In the present case, this includes the locations corresponding to the transparent zones 22b’ and 22b” of the second cover image (all other parts of the image element array 14 being static).

It is also possible to display multiple apparent window regions at more than one viewing angle. For instance, Figure 13 shows a tenth embodiment which is of the same construction as the ninth embodiment, except that here both the first and the second window cover images define three discrete transparent zones, and hence three apparent window regions are displayed at both the first and second viewing angles. However, the shape of the two small windows is configured to vary. At the first viewing angle (Figure 13(a)), the first window cover image 20a is displayed, in which the two smaller transparent zones are circular, meaning that the security document has one relatively large square apparent window region 10, and two relatively small circular apparent window regions 10. On changing to the second viewing angle (Figure 13(b)), the larger square apparent window region 10 appears unchanged, but the two smaller apparent window regions have changed from circular to square. As in the preceding examples, it is not necessary for the focussing element array 12 to extend over the whole of the substrate window region 5, but only those parts of the image element array 14 which give rise to an optically variable effect. In the present case, this includes the locations corresponding to the square transparent zones 22b’ and 22b” of the second window cover image (the circular transparent zones 22a’ and 22a” of the first window cover image falling inside them).

As mentioned above, more complex visual effects can be supported via the incorporation of three or more window cover images, each visible at a different viewing angle. Aside from switching effects, such three (or more) channel devices can advantageously be configured to provide animation effects, each frame of the animation being displayed by one of the window cover images. In this way, the apparent window region 10 can be configured to change in a gradual or step-wise manner (depending on the number of images). For instance, the window cover images could be configured to provide animation effects such as: an expansion or contraction of the apparent window region; and/or movement of the apparent window region; and/or opening or closing of the apparent window region; and/or morphing from one shape to another of the apparent window region; and/or increasing or decreasing the number of apparent window regions. The eleventh embodiment, shown in Figure 14, provides an example of this. Here, three channels are provided, each one displaying a respective window cover image 20a, 20b, 20c. Each of the window cover images defines one circular transparent zone 22a, 22b, 22c respectively, which are each centred on the same position but are of decreasing diameter. Hence when the security document is viewed from the first viewing angle (Figure 14(a)), one relatively large circular apparent window region 10 is visible. Upon tilting in one direction, the apparent window region 10 appears to shrink in size in two steps (Figures 14(b) then (c)). On tilting in the reverse direction, the apparent window region appears to expand once more. It would be possible to make the change more gradual and less step-wise by inserting more window cover images defining circular windows with diameters intermediate those shown. As in the preceding examples, it is not necessary for the focussing element array 12 to extend over the whole of the substrate window region 5, but only those parts of the image element array 14 which give rise to an optically variable effect. In the present case, this corresponds to the circular transparent zone 22a of the first window cover image (the smaller transparent zones 22b, 22c falling inside it).

In a particular preferred variant of this embodiment (not shown), the animation effect is configured to be cyclic. That is, the window cover images are configured to display a change in the apparent window region from a first state to a second state and back again, as the security document is tilted in a continuous direction. For instance, this could be achieved by adding a fourth window cover image to the set shown in Figure 14, to be displayed fourth in the sequence shown. Thus, upon tiling in one direction, starting from the first viewing angle (Figure 14(a)) the circular apparent window region 10 would appear to decrease in size in two steps, reaching a minimum size as shown in Figure 14(c), before increasing again to the intermediate size displayed by the fourth window cover image (matching that in Figure 14(b)) as tilting continues in the same direction. This is beneficial because the cyclical nature avoids any visible “jump” in the state of the apparent window region 10 when the viewing angle reaches the end of the sequence. The resulting device is also highly tolerant of lateral misregister. These effects, and more examples of suitable cyclic sequences, are described in the context of lenticular security devices in WO-A-2012/153106, the principles of which can be applied equally to the window cover images in the present disclosure. Figures 15 to 18 show further examples of animation effects which can be incorporated into security documents in accordance with embodiments of the present invention. In each case, the animation effect can be converted into a cyclic animation effect by insertion of one or more additional window cover images, repeating one or more of the frames shown so as to return the apparent window region 10 to its “starting” state.

In the twelfth embodiment, shown in Figure 15, four channels are provided but only three window cover images 20a, 20b, 20c. The first channel, visible from the first viewing angle (Figure 15(a)) corresponds to a blank image 15 achieved by inserting spaces 15a into the image element array 14 at the same periodicity as each of the three sets of image elements 14a, 14b, 14c. Therefore, at the first viewing angle, no window cover image is displayed and the apparent window region 10 is the same as the substrate window region 5, which here has an elongate lozenge shape. At the second viewing angle (Figure 15(b)), the first window cover image 20a is displayed, which fills the top third of the substrate window region 5, resulting in a shorter apparent window region 10. As tilting continues, at the third viewing angle (Figure 15(c)), the second window cover image 20b is displayed which fills the top two-thirds of the substrate window region, leaving only a small, almost square apparent window region 10. On still further tiling, at a fourth viewing angle (Figure 15(d)), the third window cover image 20c is displayed which fills the whole substrate window region 5 such that the apparent window region 10 disappears. Hence the animation appears as a sliding or shuttering effect with the window gradually being turned “off” when the viewing angle is changed in one direction, and “on” in the opposite direction.

The thirteenth embodiment, shown in Figure 16, is a three-channel device similar to that shown in Figure 14. Here the three window cover images 20a, 20b, 20c are configured to generate the appearance of a camera shutter closing as the viewing angle is changed. Thus the approximately-square apparent window region 10 decreases in size from the first viewing angle (Figure 16(a)) to the second (Figure 16(b)), before disappearing (Figure 16(c)). Unlike the first to twelfth embodiments, however, in the present embodiment, the image elements 14a, 14b, 14c are not wholly of an appearance which matches the opacifying layers (e.g. white), and thus the solid zones 21a, 21 b, 21c of the respective window cover images 20a, 20b, 20c are not entirely uniform in appearance. Rather, each cover image 20a, 20b, 20c includes one or more features 24a, 24b, 24c in another colour (e.g. black) which may be provided by forming the image elements with a small proportion of another material (e.g. black ink) in addition to the predominant material which matches the opacifying layers (or any equivalent manner of achieving different colour effects, as appropriate for the manner in which the image elements 14 are implemented). In this example, the features 24a, 24b, 24c are fine line details used to create the appearance of a camera shutter surround the apparent window region 10. It will be noted that the features 24a, 24b, 24c occupy only a small proportion of the area of the respective solid zones. This is in order to ensure that the predominant appearance of the window cover image still matches the opacifying layers and thus that it appears as an extension of the non-window region. In preferred examples any additional visually contrasting parts of the image elements of any one window cover image should occupy no more than 20% of the area of the solid zone(s) of that image, more preferably no more than 10%, still preferably no more than 5%, most preferably no more than 1 % (as in the present example). In an alternative implementation, features such as the details 24a, 24b and 24c could instead be provided in window cover images by leaving gaps in the coverage of the image elements 14. As in some of the preceding examples, it is not necessary for the focussing element array 12 to extend over the whole of the substrate window region 5, but only those parts of the image element array 14 which give rise to an optically variable effect.

The window cover images can also be configured to give the impression of movement to the apparent window region 10. In a fourteenth embodiment, shown in Figure 17, three window cover images 20a, 20b, 20c are provided. In each case the window cover image comprises a solid zone 21a, 21b, 21c having the shape of a chevron, and two transparent zones 22a’, 22a”, 22b’, 22b”, 22c’, 22c” above and below the chevron. The position of the chevron different in one image to the next, such that as the security document is tilted, the chevron appears to move down the security document. The chevron appears as a portion of the non- window region of the note dividing two apparent window regions (above and below the chevron) from one another. Thus the size and shape of the two apparent window regions changes according to the angle of view.

Figure 18 shows a fifteenth embodiment which is the same as the previous embodiment, but in negative. Thus, the moving chevron now corresponds to the apparent window region 10, being defined by a transparent zone 22a, 22b, 22c in each of the three window cover images with the rest of each image carrying a solid zone. In this case, the end result is the appearance of a chevron-shaped apparent window region 10 which does not change in shape or size upon changing the viewing angle, but rather changes in position.

While in each of the embodiments shown above, the substrate window region 5 and the apparent window region 10 are configured such that they are surrounded by non-window regions on all sides, appearing as a fully enclosed window, this is not essential. In other cases the substrate window region 5 and/or the apparent window region 10 could be partially bordered by an edge of the security document - i.e. part of the window boundary is defined by the edge of the security document. This would appear as an edge window or cut-out region.

In all of the above embodiments in which two or more window cover images are provided, each comprises image elements 14a, 14b... predominantly of an appearance substantially matching that of the opacifying layer(s) in combination at the selected location (e.g. in the non-window region 7). However, it is also possible to provide a contrasting feature image in one of the channels, in one or more colours different from the opacifying layers. For example, in the Figure 10 or 11 embodiments, one of the window cover images could be replaced by a feature image extending over all or part of the substrate window region 5. Such a feature image could have information content such as alphanumeric text, a symbol, a portrait, a pattern or another graphic. For instance, in the Figure 10 embodiment, the second window cover image 20b could be replaced by a feature image with the letters “DLR” and a logo aligned down the line Y-Y’. Thus upon tilting, at the first viewing angle the apparent window region 10 would be a small square region as shown in Figure 10(a), whereas at the second viewing angle the full substrate window region 5 would be revealed, with the feature image (“DLR” + logo) exhibited inside it. A feature image such as this could be added to any of the embodiments described above.

For explanation purposes, the same exemplary document structure has been shown in all the embodiments described above. However, in practice, the same results can be achieved via a wide variety of different arrangements, some of which will now be described with reference to Figures 19(a) to (f). It should be appreciated that any of the above embodiments can be implemented using any one of the constructions depicted in Figure 19(a) to (f). Further, while Figures 19(a) to (f) depict the focussing element array 12 and image element array 14 as extending across the whole of the substrate window region 5, as discussed above this is not essential. In particular, in any of the example constructions shown it may be desirable for the focussing element array 12 to extend over only part (and not all) of the substrate window region 5, preferably being provided wholly within the substrate window region.

Figure 19(a) shows an exemplary construction which is the same as that described in the previous embodiments, with the focussing elements 12 and image elements 14 applied to opposite sides of the transparent or translucent polymer substrate 1. However, in this case the array of focussing elements 12 and the array of image elements 14 are not provided only in the substrate window region 5, but also extend into the non-window region 7, where they overlap the opacifying layers 2a, 2b. This may be desirable since it increases tolerance to mis-register between the gap in the opacifying layers which defines the substrate window region 5, and the focusing element I image element combination. Since the opacifying layers 2a, 2b may be applied in a separate process from the application of the focusing elements and image elements (i.e. not in-line) there is potential for significant mis-register. By arranging for the focusing elements and image elements to continue beyond the periphery 6 of the substrate window region 5, the likelihood of any portion of the substrate window region being inadvertently omitted is reduced. In variants, only one or the other of the focussing element array 12 and the image element array 14 might be arranged to extend into the non-window region.

It will be noted that in the Figure 19(a) embodiment, where the focussing elements 12 and image elements 14 overlap the opacifying layers 2a, 2b, the opacifying layers are located between the arrays 12, 14 and the polymer substrate 1. This reflects the preference for applying the opacifying layers 2a, 2b before the focussing elements 12 and image elements 14 are applied. However, it is also possible to perform manufacture in the reverse order, in which case the arrays 12, 14 would be covered by the opacifying layers 2a, 2b in the non-window region.

The material from which the focussing elements 12 are formed could optionally also be used to form other relief structures alongside the focussing elements 12, e.g. non-focussing structures such as pedestals or tactile elements. Such structures could be located inside the substrate window region 5 (e.g. between areas of focussing elements) and/or in the non-window region 7.

Figure 19(b) shows an embodiment in which at least the image elements 14 have been applied before at least the opacifying layer 2b on the same side of the polymer substrate 1 , which extends over the array of image elements 14 across the substrate window region 5. Unlike the previous examples, here the substrate window region 5 is of a half-window construction, having opacifying layer(s) present on one side of the polymer substrate 1 but not the other. (All previous embodiments above have utilised full-window constructions of the substrate window region 5). In this case the variable window effect will be more subtle (at least under transmitted light illumination conditions) since the apparent window region 10 will be a half-window and so the contrast between it and the window cover image(s) contributing to its definition will be less strong. Under reflective viewing conditions the variable window effect may be enhanced by such an arrangement since the opacifying layer present in the half window reduces the amount of light transmitted and so increases the perceived reflected light. Arrangements in which the focussing elements 12 and image elements 14 are applied to opposite sides of the transparent or translucent polymer substrate 1 (including all the embodiments described so far) are preferred since this facilitates the attainment of extremely high register between the focussing elements and the image elements 14, as will be described below. Also, the full thickness of the polymer substrate 1 can be used to provide the necessary optical spacing between the focussing elements and the image elements. However, this is not essential and in other implementations, both arrays could be disposed on the same side of the polymer substrate 1 . An embodiment of this sort is shown in Figure 19(c). Here, again, the substrate window region 5 is a half window although it could alternatively be a full window if a gap is inserted into opacifying layer 2b. The image elements 14 are applied to a first surface of the polymer substrate, followed by the focussing element array 12 on the same surface, on top of the image elements. In this case, the focussing element array 12 may be designed with an integral optical spacer 12a to maintain the correct distance between the focusing elements 12 and the image elements 14.

Figure 19(d) shows an arrangement in which the image elements 14 and focussing element array 12 are both applied to the first surface of the polymer substrate in the same way as just described. However, in this case the substrate window region 5 includes a part 5’ which is a full-window and parts 5” which are half-windows. This is possible in all embodiments and could be used to increase the complexity and security level of the device, especially if the lateral arrangement of the parts 5’, 5” is reflected in the design of the window cover images. In this way any mis-register between the opacifying layers and the image elements would be highlighted thus presenting a significant challenge to a would- be counterfeiter.

In most embodiments, it is preferred that the focussing elements and/or image elements are formed “in situ”, i.e. on the polymer substrate 1 (optionally via a primer layer or a transparent coloured print layer, such as a gravure tint, or the like). For instance, the focussing elements may be formed by cast-curing onto the polymer substrate 1 , and/or the image elements 14 may be formed by printing onto the polymer substrate 1. These options will be described in more detail below. However, it is also possible to form one or both of the components on a respective secondary substrate, e.g. in the form of a strip, patch or transfer foil, and then affix them to the polymer substrate 1. Figure 19(e) shows an example in which the focussing elements have been formed on a secondary substrate 13, and the image elements have been formed on a secondary substrate 16, which have then been affixed to opposite sides of the polymer substrate 1 . It is also possible for one of the arrays to be formed on a secondary substrate (e.g. the focussing elements) and the other to be formed in-situ (e.g. the image elements). Alternatively, as shown in Figure 19(f), both arrays could be formed on opposite sides of the same secondary substrate 13, which is then affixed over all or part of the substrate window region 5.

In all of the above embodiments, the optically variable window effect has only been viewable (under purely reflective illumination conditions) from one side of the security document (that from which the image elements 14 can be viewed via the focussing elements 12). (Although when viewed with some transmitted light the above embodiments will exhibit a variable effect from both sides). However it is also possible to arrange for the apparent window region 10 to exhibit the described changes under reflective illumination conditions on both sides of the security document, by providing a second focussing element array. Figure 20 shows a further embodiment of a security document 100 in which this is the case. It will be appreciated that, as in the previous embodiments, here the drawings are highly schematic and not to scale. The embodiment employs all the same principles as described in the previous embodiments and so features with like reference numerals will not be described again in detail here.

As shown in the cross-section of Figure 20(a), in addition to the focussing element array 12 and image element array 14 already described, the security document 100 is provided with a second focussing element array 18 on the opposite side. The second focussing element array 18 also overlaps the print element array 14, such that the image element array is located between the first focussing element array 12 and the second focussing element array 18. The focal length of the second focussing array 18 is selected so that the image element array 14 approximately coincides with its focal plane, whilst also remaining in the focal plane of the first focussing element array 12. As such, in this example the focal length of the second focussing element array 18 is smaller than that of the first focussing element array 12. The necessary optical spacing may either be built into the design of the second focussing element array 18 or may be provided by a secondary substrate 13 via which the second focussing element array 18 could be affixed to the document 100.

The variable window effect will now be visible from both sides of the document. Figures 20(b) and 20(c) show the security document from a first side, that of observer Oi, at two different viewing angles. In this case the security document is provided with a single window cover image 20a in the same manner as the first embodiment above and so upon tiling the document the apparent window region 10 appears to switch between rectangular, corresponding to the full extent of the substrate window region 5 (Figure 20(b)), and oval, corresponding to window cover image 20a (Figure 20(c)). When viewed from the other side of the document, that of observer O2, again the same switch is exhibited as the document is tilted, as shown in Figures 21 (d) and 21 (e).

While this dual-sided approach has been illustrated as applied to a security document of the short disclosed in the first embodiment above, it should be appreciated that the same can be applied to any of the embodiments disclosed herein, by the provision of a second focussing element array.

In the above embodiments, the security document 100 has been provided with a single substrate window region 5. However, additional substrate window regions 5 can be provided if desired and these may also be equipped with respective focussing element arrays and image element arrays in order to exhibit the variable window effects described above. Figure 21 shows a further embodiment of a security document 100 in which this is the case. As shown in the cross-section of Figure 21 (a), here the security document is provided with two substrate window regions 5a, 5b spaced from one another by a non-window region. Each substrate window region 5a, 5b is provided with a respective focussing element array 12 and image element array 14 in the manner described in any of the preceding embodiments. Each region 5a, 5b will therefore exhibit a variable window effect when viewed from one side of the document 100, upon tilting. Any of the various effects described in the preceding embodiments could be employed and those in the respective windows 5a, 5b could be configured independently of one another or to work in co-ordination with one another.

In the particular preferred example illustrated, the two window regions are configured to display a co-ordinated switching effect. Each window 5a, 5b is equipped with a window cover image 20a which fills the whole substrate window region 5a, 5b when visible, as already described in relation to Figure 4 above. The respective devices are configured so that, on at least some viewing angles, one of the windows 5a, 5b is visible (or “on”), while the other is concealed (or “off’) by the window cover image 20a. This is shown in Figures 21 (b) and 21 (c): Figure 21 (b) shows the security document at a first viewing angle from which the first window region 5a is wholly visible since its window cover image 20a is not being directed to the viewer, while simultaneously the second window region 5b is hidden since its window cover image is being directed to the viewer. At a second viewing angle shown in Figure 21 (c), now the first window region 5a is hidden since its window cover image 20a is being directed to the viewer, while simultaneously the second window region 5b is visible, since its window cover region 20a is not being directed to the viewer. Thus upon tiling the window 5a, 5b appear to switch on and off in an alternating manner, which can give rise to the impression that the window region has moved from one part of the document to another.

Figure 22 is a flowchart showing steps in an exemplary method of manufacturing a security document in accordance with the present invention, such as any of the security documents described above. In a first step S101 , a transparent or translucent polymer substrate 1 is provided. Suitable examples include polymeric materials such as polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof. The polymer substrate 1 may be monolithic, e.g. formed from a single one of the above materials, or multi-layered, e.g. having multiple layers of the same type of polymer (optionally with different orientations) or layers of different polymer types. As mentioned previously, by “transparent” it is meant that the polymer substrate 1 may be substantially visually clear, although it may carry a coloured tint and/or another optically detectable substance such as a fluorescent material. Alternatively, if the polymer substrate 1 is “translucent” it will cause some light scattering but is not opaque.

One or both surfaces of the polymer substrate 1 may be treated to improve adhesion I retention of subsequently applied materials. For example, a primer layer may be applied to all or part of either surface of the polymer substrate 1 , e.g. by printing or coating. The primer layer is preferably transparent and again could be tinted or carry another optically detectable material. Suitable primer layers include compositions comprising polyethylene imine, hydroxyl terminated polymers, hydroxyl terminated polyester-based co-polymers, cross-linked or uncross-lined hydroxylated acrylates, polyurethanes and UV curing anionic or cationic acrylates. Alternatively or in addition to the application of a primer layer, the surface of the polymer substrate 2a may be prepared for onward processing by controlling its surface energy. Suitable techniques for this purpose include plasma or corona treatment.

Next, in step S102, one or more opacifying layers 2a, 2b are applied to the first and/or second surfaces of the polymer substrate 1 , leaving a gap in at least one of the layers in order to define the substrate window region 5. The opacifying layer(s) 2a, 2b each comprise a non-transparent material, the primary purpose of which is usually to provide a suitable background for later printing of graphics thereon. Thus, preferably, the opacifying layers comprise polymeric, non-fibrous material containing at least a light scattering substance such as a pigment. The opacifying layers 2a, 2b are preferably light in colour, most preferably white or another light colour such as off-white or grey so that a later-applied graphics layer will contrast well against it. In preferred examples, the opacifying layers each have a brightness L* in CIE L*a*b* colour space of at least 70, preferably at least 80 and more preferably at least 90. For example, each opacifying layer may comprise a resin such as a polyurethane based resin, polyester based resin or an epoxy based resin and an opacifying pigment such as titanium dioxide (TiO2), silica, zinc oxide, tin oxide, clays or calcium carbonate. One or more opacifying layers 2a, 2b may be applied to each surface of the polymer substrate 1 , in order to achieve the necessary opacity. The optical density of each layer by itself may typically be around 0.2 to 0.5. Preferably, two or three or more layers are applied to each surface, overlapping one another. In a preferred embodiment, at least one of the opacifying layers (preferably one on each surface of the polymer substrate 1) is made electrically conductive, e.g. by the addition of a conductive pigment thereto. This reduces the effect of static charges which may otherwise build up on the security document during handling.

The opacifying layers 2a, 2b are preferably applied to the polymer substrate 1 using a first printing process such as gravure printing, screen printing, flexographic printing, or lithographic (e.g. offset lithographic) printing, although in other cases the opacifying layers could be coated onto the substrate, or applied by any other convenient method. One or more of the opacifying layers will be omitted across gap(s) on one or both surfaces of the polymer substrate to form the substrate window region 5 (which may be a full window part or a half window, or a mixture of both) as described above. This can be achieved through appropriate patterning of the opacifying layers during the application process. Suitable apparatus/methods for applying opacifying layers to a substrate are disclosed in WO-A-2017/009616 (section 4), including the formation of window regions.

Next, in step S103, the array of focussing elements 12 and the array of image elements 14 are applied across at least part of the substrate window region 5 in an overlapping manner. This can be done in either order, or simultaneously if the two arrays 12, 14 are to be located on opposite sides of the polymer substrate 1 . An arrangement having the image element array 14 on one side of the substrate 1 and the focussing element array 12 on the other means that it is possible to apply both simultaneously. This achieves extremely high registration between the image elements and the focussing elements since there is no movement of the substrate between the application of the two components: they are both applied to opposite surfaces of the substrate at the same position along the substrate (in the direction along which the substrate moves through the manufacturing apparatus - the machine direction), at the same time. The substrate 1 could be in the form of a sheet or a web. Examples of suitable manufacturing processes and apparatus for applying the focussing elements and image elements will be given below. However, typically the image elements 14 are applied via a second print process that is different from the print process via which the opacifying layers 2a, 2b were applied. For instance, the image elements 14 may be formed by a lithographic printing technique, most preferably offset lithographic printing. The lithographic printing technique could be wet or dry lithography (offset or otherwise). Other preferred examples of suitable printing techniques include flexographic printing. The ink(s) used could be curable ink(s), in which case a curing step may be added, e.g. an LED UV cure.

It should be noted that the order of steps S102 and S103 could be swapped.

Next, the security document may optionally be provided with further features in optional steps S104 and S105. In step S104, a graphics layer 90 may be provided on one or both sides of the security document, over the opacifying layers 2a, 2b. Typically this is achieved via one or more security print processes, such as lithographic printing, flexographic printing or intaglio printing. It should be noted that if the graphics layer 90 is to be formed by the same printing technique as that by which the image elements 14 are formed, the graphics layer 90 and image elements 14 could optionally be formed in the same processing step as one another, e.g. simultaneously or at least by the same printing unit. The graphics layer 90 may include fine-line patterns, such as guilloches, portraits or other images, and information which may be static (e.g. denomination information) or personalised (e.g. passport holder bibliographic data). In step S105, additional security features such as 93, 95 described with respect to Figure 1 may be affixed to the security document 100. It will be appreciated that the order of steps S104 and S105 may change depending on the nature of the additional security features to be applied.

The exemplary manufacturing method shown in Figure 22 could all be performed by the same entity. However, in preferred implementations, steps S101 and S102 may be performed by a first entity (the substrate manufacture), and steps S103 onward by a second entity (often referred to as the security document printer). Thus, the second entity may source a suitable polymer document substrate, already equipped with opacifying layer(s) and having the substrate window region pre-defined, from a supplier and then proceed with steps S103 onwards.

Whilst in the embodiments described above, the focusing elements 12 have been implemented as elongate lenses (e.g. cylindrical lenses), this is not essential. In all of the above embodiments, the array of elongate lenses could be replaced by a two-dimensional array of focussing elements which need not be elongate but could for example be spherical, aspherical, hexagonal or square. There would be no requirement to change the layout of the image element array 14. Effectively, a plurality of the focusing elements in the array arranged along a straight line performs the same function as each of the elongate lenses described above. It is also possible to reconfigure the array of image elements 14 to have two- dimensional periodicity, whereby the or each set of image elements 14a will be interlaced with spaces 15a and/or other sets of image elements 14b, 14c etc in two directions. Two-dimensional lenticular constructions such as this have the advantage that they will be optically active whichever axis the security document is tilted about. It should also be appreciated that while in many cases the focussing elements will comprise lenses, it is also possible to implement the focussing elements as mirrors.

In order for the desired optically variable effects to be exhibited, the image elements 14 must be correctly located within the optical footprints of the focussing elements. Hence, each image element 14a must be narrower than the pitch of the focussing element array 12, which as discussed above is typically no more than 100 microns, usually less. For example, if the diameter of the focusing elements is 30pm then each image element 14a may be around 15pm wide or less. Alternatively for a smooth animation effect it is preferable to have as many different interleaved images as possible, typically at least five but ideally as many as thirty. In this case the size of the image elements 14a should be in the range 0.1 to 6pm.

The focussing elements 12 can be formed via any convenient process including embossing or printing. However, cast-curing is most preferred. Suitable apparatus, materials and methods for forming relief structures defining the focussing element array 12 disclosed herein are described in WO-A-2018/153840 and WO-A-2017/009616. In particular, the focussing elements can be formed by the in-line casting devices detailed in WO-A-2018/153840 (e.g. that designated 80 in Figure 4 thereof), using an embossing tool 85 carrying an appropriately designed micro-optical structure from which can be cast the desired focussing element array shape. Similarly, the cast-curing apparatuses and methods disclosed in section 2.1 of WO-A-2017/009616 (e.g. in Figures 4 to 8 thereof) can also be used to form the presently disclosed relief structures.

Whichever casting apparatus is used, the curable material(s) from which the relief structure is cast may be applied either directly to the tool carrying the desired relief shape (e.g. to the embossing tool 85 of WO-A-2018/153840 or to the casting tool 220 of WO-A-2017/009616), or the curable material(s) may be applied directly to the substrate on which the relief structure is to be formed, and then brought into contact with the tool (e.g. by impressing the tool onto the deposited curable material). Both options are described in the aforementioned documents. Preferably, the latter option is employed and the curable material(s) are applied to the substrate by screen printing as detailed in WO-A-2018/153840, before being formed into the desired relief structure. If the former option is employed, it should be noted that there is preferably no wiping of the casting tool surface relief between applying the curable material to it, and bringing it into contact with the substrate, so that a base layer of curable material remains connecting the focussing elements together on the substrate. Suitable curable materials are disclosed in WO-A-2017/009616, section 2.1. UV- curable materials are most preferred. Curing of the material(s) preferably takes place while the casting tool is in contact with the curable material, against the substrate. The radiation used to effect curing will typically be 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. A suitable UV curable lacquer is the product UVF-203 from Kingfisher Ink Limited or photopolymer NOA61 available from Norland Products. Inc, New Jersey.

An example of a suitable cast-cure process for forming focussing element array 12 suitable for use in the security document disclosed herein will be described with reference to Figures 23(a) and (b) hereto, which show the focussing element array 12 only schematically. The process is shown as applied to a document substrate 201 , comprising a transparent or translucent polymer substrate 1 carrying opacifying layers 2a, 2b (not shown). Figure 23(a) depicts the apparatus from a side view, and Figure 23(b) shows the document substrate 201 in a perspective view, the manufacturing apparatus itself being removed for clarity.

A transparent curable material 205 is first applied to the document substrate 201 using an application module 210 which here comprises a patterned print cylinder 211 which is supplied with the curable material from a doctor chamber 213 via an intermediate roller 212. For example, the components shown could form part of a flexographic printing system. Other printing techniques such as lithographic, screen, or gravure printing could also be used. Print processes such as these are preferred since the curable material 205 can then be laid down on the document substrate 201 only in selected regions 202 thereof, the size, shape and location of which can be selected by control of the print process, e.g. through appropriate configuration of the pattern on cylinder 211 . However, in other cases, an all over coating method could be used, e.g. if the focussing element array 12 is to be formed all over the document substrate 201 . The curable material 205 is applied to the document substrate 201 in an uncured (or at least not fully cured) state and therefore may be fluid or a formable solid.

The document substrate 201 is then conveyed to a casting module 220 which here comprises a casting tool 221 in the form of a cylinder carrying a surface relief 225 defining the shape of the focussing element array 12 which is to be cast into the curable material 205. As each region 202 of curable material 205 comes into contact with the cylinder 221 , the curable material 205 fills a corresponding region of the relief structure, forming the surface of the curable material into the shape defined by the relief. The cylinder 221 may be configured such that the relief structure 225 is only provided at regions corresponding to shape and position of the first regions 202 of curable material 205.

Having been formed into the desired focussing element array shape, the curable material 205 is cured by exposing it to appropriate curing energy such as radiation R from a source 222. This preferably takes place while the curable material is in contact with the surface relief 225 although if the material is already sufficiently viscous this could be performed after separation. In the example shown, the material is irradiated through the document substrate 201 although the source 222 could alternatively be positioned above the document substrate 201 , e.g. inside cylinder 221 if the cylinder is formed from a suitable transparent material such as quartz. In an alternative embodiment, the curable material 205 could be applied directly onto casting tool 221 rather than on to the document substrate 201 . This could be done in an all-over or patternwise manner.

In all embodiments of the invention, the image elements 14 can be applied to the substrate using any convenient printing technique. Gravure, lithographic (e.g. wet or dry offset), flexographic, inkjet or micro-intaglio printing are particularly preferred techniques. However, typically the print process will be different from that used to apply the opacifying layers 2a, 2b.

WO-A-2018/153840 and WO-A-2017/009616 also disclose print stations, which may be disposed downstream of the above-described casting apparatus (but alternatively could be located upstream, or at the same point along the machine direction as explained below). Print stations such as these are suitable for applying the image element array 14 to the opposite side of the substrate from that carrying the focussing element array 12. The apparatus disclosed in WO-A- 2018/153840 can achieve particularly high registration between such cast relief structures and the printed elements.

For example, in preferred methods, the focussing element array 12 and image element array 14 are preferably registered to one another sufficiently accurately that any mis-register is too small to be perceived by the naked eye. Preferably, the translational register (i.e. in the machine direction x or cross direction y) is to within 150 pm (+/- 75pm). Desirably, the skew register (i.e. rotational alignment) is to within 1 degrees (preferably less than 0.1 , more preferably less than 0.05, most preferably less than 0.02 degrees). Advantageously, the pitch register (i.e. the degree to which one component is stretched relative to the other) is to within 0.01 %. The precise registration achieved will depend on the consumables that are used in the machine (substrate, inks, resin, print plates) as well as the actual machine configuration.

It is highly desirable for the focussing element array 12 and image element array 14 to be applied to the opposite surfaces of the substrate 1 simultaneously. That is, at the same position along the transport path in the machine direction. This makes it possible to achieve the highest registration between the two components. Figure 7 of WO-A-2018/153840 shows suitable apparatus for achieving this.

Figure 24 hereto shows a schematic example of this in the case where the focussing element array 12 and image element array 14 are applied to the first and second surfaces, respectively, of a document substrate 201 (which may be a web or a sheet), comprising a transparent or translucent polymer substrate 1 and opacifying layers 2a, 2b applied thereto (not shown). The focussing element array 12 and image element array 14 can be formed using any of the processes described above. For clarity, Figure 24 depicts only selected components of the apparatus used to form focussing element array 12 and image element array 14, namely a casting tool 221 (e.g. as shown in Figure 24) and print roller 302, which is supplied with ink 30a via an inking roller 303a. Other components of the process line are not shown. The curable material(s) may be applied on to the substrate 201 upstream of the casting tool 221 or directly onto the casting tool 221. The casting tool 221 and print roller 302 are arranged on opposite sides of the transport path along which the substrate 201 is conveyed, so as to form a (low pressure) nip through which the substrate 201 passes. At each location along the document substrate 201 , its first surface therefore comes into contact with the casting tool 221 at the same time as its second surface comes into contact with the print roller 302. As a result, the focussing element array 12 and image element array 14 are formed on each point of the substrate simultaneously.

This has the significant advantage that any deformation experienced by the substrate 201 , as a result of changes in processing temperature or the like, will be exactly the same when the focussing element array 12 is applied to the document substrate 2 as it is when the image element array 14 is applied. The substrate 201 has no time to expand or contract between the instant at which the focussing element array 12 is applied and when the image element array 14 is applied, since they occur at the same time. As such, a very high degree of register between the two components is automatically achieved.

The arrangement shown in Figure 24 has the disadvantage that since the nip between the casting tool 221 and the print roller 302 constitutes the first point of contact between the substrate and the casting tool 221 , the transparent curable material 205 from which the focussing element array 12 is formed will be substantially uncured when it enters the nip. As such, the pressure applied between the casting tool 221 and the print roller 302 should be low so as to avoid damage to the cast focussing elements 12.

Figure 25 shows an improved arrangement in which formation of the focussing element array 12 and application of the image elements 14 can still be considered simultaneous because the curable material 205 is still in contact with the surface relief on casting tool 221 at the nip location between the casting tool 221 and the print roller 302. The curable material(s) may be applied on to the substrate 201 upstream of the casting tool 221 or directly onto the casting tool 221. The substrate is wrapped around a portion of the casting tool 221 from a first point on roller 61 , at which casting of the focussing element array 12 begins, until the nip with print roller 302 at which point the focussing element array 12 will be relatively well cured, preferably fully cured. As such, the pressure between the two components 221 , 302 can be increased relative to that in the Figure 24 embodiment since the material 205 is relatively hard and less prone to damage. This improves the quality achieved in the image element array 14 formation process. A further benefit of the arrangement shown is the increased wrap length of the substrate 201 around print roller 302, allowing for prolonged curing here also. The substrate 201 stays in contact with print roller 302 from the nip location until take-off roller 62.

Simultaneous application of the focussing element array 12 and image element array 14 is preferred but not essential. Figure 26 illustrates an exemplary arrangement for sequentially (rather than simultaneously) applying the two components 12, 14 on opposing sides of a substrate 201 (which here is in the form of a sheet). This may be described as forming the two components in-line in the same pass. The arrangement generally comprises a print and cast module 410 for forming the focussing elements 12 and a print station 420. The substrate 201 enters the apparatus at arrow A and exits at arrow B. A curable material 205 is first applied to a first side of the sheet substrate 201 as it passes through a nip formed by screen print cylinder 411a and intermediate roller 412a. However, as previously discussed, other printing techniques such as lithographic, flexographic, offset or inkjet printing could also be used. The sheet 201 is then conveyed to casting tool 421a in the form of a cylinder defining the shape of a surface relief structure which is to be cast into the curable material 205. Having been formed (shaped) into the desired surface relief structure defining the focussing element array 12, the curable material 205 is cured by exposing it to appropriate curing energy such as UV radiation from source 222. This preferably takes place while the curable material is in contact with the surface relief 225 although if the material is already sufficiently viscous this could be performed after separation. The sheet substrate 201 , now carrying the cured focussing element array 12, is then conveyed to the print station 420. In this example, the print station 420 is a lithographic print apparatus, comprising a patterned print cylinder 302 which is selectively supplied with an ink 30a via an inking roller 303a. The set(s) of image elements are transferred from print cylinder 302 to a blanket roller 306 and then onto the substrate 201 at a nip between blanket roller 306 and an impression roller 305. The substrate 201 , now carrying both the focussing element array 12 and the image element array 14 on opposite sides is then conveyed away from the print module 420 via at arrow B.

It should be appreciated that, while each of the exemplary manufacturing methods described with respect to Figures 24, 25 and 26 shows the image elements 14 being formed of only a single material 30a, in other embodiments one or more additional materials could be applied onto the print roller 302 to provide further colours. For example to form the embodiment of Figure 16, an additional supply of (e.g. black) ink may be provided in this way to form the features 24a, 24b, 24c.

Similarly, it is also possible to use two or more different transparent curable materials 205 to form the focussing element array (and/or other relief structures such as tactile features). For instance, multiple transparent curable materials 205 with different visual (or otherwise detectable) characteristics may be used, such as materials with different coloured tints and/or taggants. This could be achieved, for instance, by providing the apparatus shown in Figure 23(a) with one or more additional supplies of resin which are also applied to roller 211 in a patterned manner.

Whilst in the above preferred examples, the image elements 14 are print elements comprising a material having the required appearance characteristics, the image elements can be implemented in any way that achieves the desired appearance. One method which has been put forward as an alternative to the printing techniques mentioned above, and can be employed in embodiments of the invention, is used in the so-called Unison Motion™ product by Nanoventions Holdings LLC, as mentioned for example in WO-A-2005052650. This involves creating image elements (“icon elements”) as recesses in a substrate surface before spreading ink over the surface and then scraping off excess ink with a doctor blade. The resulting inked recesses can be produced with line widths of the order of 2 pm to 3 pm.

A different method of producing high-resolution image elements is disclosed in WO-A-2015/044671 and is based on flexographic printing techniques. A curable material is placed on raised portions of a die form only, and brought into contact with a support layer preferably over an extended distance. The material is cured either whilst the die form and support layer remain in contact and/or after separation. This process has been found to be capable of achieving high resolution and is therefore advantageous for use in forming the image element array 14 in the present application.

Some more particularly preferred methods for generating patterns or micropatterns (i.e. an image array 14) on a substrate are known from US 2009/0297805 A1 and WO 2011/102800 A1 . These disclose methods of forming micropatterns in which a die form or matrix is provided whose surface comprises a plurality of recesses. The recesses are filled with a curable material, a treated substrate layer is made to cover the recesses of the matrix, the material is cured to fix it to the treated surface of the substrate layer, and the material is removed from the recesses by separating the substrate layer from the matrix.

Another strongly preferred method of forming a micropattern is disclosed in WO 2014/070079 A1. Here it is taught that a matrix is provided whose surface comprises a plurality of recesses, the recesses are filled with a curable material, and a curable pickup layer is made to cover the recesses of the matrix. The curable pickup layer and the curable material are cured, fixing them together, and the pickup later is separated from the matrix, removing the material from the recesses. The pickup layer is, at some point during or after this process, transferred onto a substrate layer so that the pattern is provided on the substrate layer. The above-mentioned methods described in US 2009/0297805 A1 , WO 2011/102800 and WO 2014/070079 A1 have been found to produce particularly good results and are therefore particularly preferred for use in forming the image array 14 in embodiments of the invention.

In other examples the image elements 14 can be formed by a relief structure and a variety of different relief structure suitable for this are shown in Figure 27. Thus, Figure 27a illustrates image regions of the image elements (IM), in the form of embossed or recessed regions while the non-embossed portions correspond to the non-imaged regions of the elements (Nl). Figure 27b illustrates image regions of the elements in the form of debossed lines or bumps.

In another approach, the relief structures can be in the form of diffraction gratings (Figure 27c) or moth eye I fine pitch gratings (Figure 27d). Where the image elements are formed by diffraction gratings, the apparent colour can be controlled through factors such as the pitch, depth and/or orientation of the grating structure. Achromatic diffractive white (or off-white) can be achieved for instance via a classical holography approach as disclosed in WO-A-2011/138616, or by combining diffraction gratings of different parameters (e.g. each exhibiting red, green and blue light respectively) as disclosed in WO-A-2022/096892. A preferred method for writing such a grating would be to use electron beam writing techniques or dot matrix techniques. Using a diffractive structure to provide the image elements provides a major resolution advantage: although ink-based printing is generally preferred for reflective contrast and light source invariance, techniques such as modem e-beam lithography can be used generate to originate diffractive image strips down to widths of 1 pm or less and such ultra-high resolution structures can be efficiently replicated using UV cast cure techniques.

Such diffraction gratings for moth eye I fine pitch gratings can also be located on recesses or bumps such as those of Figures 27a and b, as shown in Figures 27e and f respectively. Figure 27g illustrates the use of a simple scattering structure providing an achromatic effect (e.g. white).

Further, in some cases the recesses of Figure 27a could be provided with an ink or the debossed regions or bumps in Figure 27b could be provided with an ink. The latter is shown in Figure 27h where ink layers 200 are provided on bumps 210. Thus the image areas of each image element could be created by forming appropriate raised regions or bumps in a resin layer provided on a transparent substrate. This could be achieved for example by cast curing or embossing. A coloured ink is then transferred onto the raised regions typically using a lithographic, flexographic or gravure process.

Finally, Figure 27i illustrates the use of an Aztec structure, and Figure 27j show the formation of image elements by producing recesses which are then (at least partly) filled by ink or another material to provide the colour..

Additionally, image and non-image areas could be defined by combination of different element types, e.g. the image areas could be formed from moth eye structures whilst the non-image areas could be formed from gratings. Alternatively, the image and non-image areas could even be formed by gratings of different pitch or orientation.

Where the image elements are formed solely of grating or moth-eye type structures, the relief depth will typically be in the range 0.05 microns to 0.5 microns. For structures such as those shown in Figures 27 a, b, e, f, h and i, the height or depth of the bumps/recesses is preferably in the range 0.5 to 10pm and more preferably in the range of 1 to 2pm. The typical width of the bumps or recesses will be defined by the nature of the artwork but will typically be less than 100pm, more preferably less than 50pm and even more preferably less than 25pm. The size of the image elements and therefore the size of the bumps or recesses will be dependent on factors including the type of optical effect required, the size of the focusing elements and the desired device thickness. All of the above alternative methods for forming the image elements 14 could be performed in-situ on the document substrate 1 or on a secondary substrate 13 which is affixed to the document substrate 1 .

Figures 28 to 32 show some further preferred variants which can be applied to any of the embodiments of the invention described above.

While providing multiple “channels” (e.g. multiple interlaced window cover images and/or blank elements) is beneficial in that it permits more complex animation effects to be achieved, the window cover images displayed will decrease in apparent optical density as the number of channels increases. This is because there is less space available for display of each of the images. Too low an apparent optical density may lead to an obvious contrast at the periphery 6 of the substrate window region 5, thereby decreasing the effectiveness of the window cover image in terms of concealing all or part of the substrate window region. Therefore in a preferred embodiment the image element array 14 may be configured to have a different arrangement adjacent the periphery 6 as compared with the interior of the substrate window region. Figure 28 shows an example of this, illustrating the image array 14 itself (rather than its appearance through the focussing elements, as in the previous Figures). It will be seen that in a peripheral zone 8a adjacent the periphery 6 of the substrate window region 5 there is substantially uniform all-over coverage of the image elements - that is, a continuous or pseudo-continuous structure 25 of the sort discussed with reference to Figures 5 and 10 above (e.g. laid down as a halftone dot or line pattern at a resolution which will not cooperate with the focussing elements to generate an optically variable effect). As such this zone 8a appears static and of relatively high optical density helping to hide the transition between this and the non-window region 7. Therefore, there is no need for the focussing element array 12 to extend over peripheral zone 8a in this example (although it may do). In an interior zone 8b of the substrate window region 5, at least one periodic set of image elements is provided, spaced either by another set (from a second image) or by spaces. As such there are at least two channels in this zone, preferably more. Thus in the interior zone 8b the apparent optical density of each window cover image displayed is less than that of the peripheral zone but since this area is spaced from the periphery 6 the result is not immediately apparent. The interior zone 8b is therefore able to display the more complex animation effects mentioned above while peripheral zone 8a smooths the visual transition to the non-window region 7. It should be noted that the peripheral zone 8a need not be static (although in this case the focussing element array 12 will need to extend across it). It could also contain two or more channels, but the number of channels should be less than in the interior zone 8b. There could also be more than two zones providing a more gradual staging in number of channels between the periphery and the interior of the substrate window region 5.

In the above embodiments, the appearance of the image elements 14 has been configured to be substantially the same as the collective appearance of the opacifying layers at a location in the non-window region 7 of the security document (i.e. at a point where all the opacifying layers overlap). However, this is not essential and Figures 29 to 31 show some alternative embodiments. As shown in the cross-section of Figure 29, in this embodiment the opacifying layers do not all have the same lateral extent as one another. On the first surface 1a of the transparent or translucent substrate 1 , two opacifying layers 2a’ and 2a” are provided and similarly on the second surface 1b, two opacifying layers 2b’ and 2b” are provided. The outermost opacifying layer 2a” and 2b” on each side is stepped back from the substrate window region 5, relative to the underlaying opacifying layers 2a’ and 2b’ for registration reasons. This creates a boundary region 7a around the periphery 6 of the substrate window region 5 in which less than all of the opacifying layers are present. The boundary region may be for instance 1 mm wide or less, e.g. 0.5 mm. In this example, the outermost opacifying layers 2a” and 2b” are each of a different colour (e.g. light blue) from the underlying opacifying layers 2a’ and 2b’ (which may for instance be white), although this is not essential and the same arrangement could be employed where the various opacifying layers are all of substantially the same colour. Depending on the desired visual effect, the appearance of the image elements 14 could be configured to match the collective appearance of the opacifying layers either in the non-window region 7 or in the boundary region 7a. Figure 30 shows an embodiment in which the former option is employed. Thus the image elements 14 are configured to have substantially the same appearance as the collective appearance of the opacifying layers overlapping in the nonwindow region 7. If, as indicated in the above example, the outermost opacifying layer 2a” is light blue, the combined appearance of the layers here in reflected light will typically be a similar colour, i.e. light blue (albeit the exact shade may be influenced by the other opacifying layers underneath). As such the image elements 14 are configured to substantially match this appearance and will also be light blue. Figure 30(a) shows the security document from a first viewing angle at which no window cover image is displayed and here the full substrate window region 5 is visible, surrounded by the boundary region 7a which appears white in this example and then the non-window region 7 which appears light blue. Figure 30(b) shows the same security document from another viewing angle at which a window cover image 20a is visible which here has a solid zone surrounding a triangular transparent zone, resulting in an apparent window region 10 which is triangular. The solid zone surrounding the apparent window region will appear light blue and thus appears to substantially match the non-window region.

Figure 31 shows an embodiment in which the latter option is employed. Thus the image elements 14 are configured to have substantially the same appearance as the collective appearance of the opacifying layers overlapping in the boundary region 7a. If, as indicated in the above example, the underlying opacifying layer 2a’ is white, the combined appearance of the layers here in reflected light will typically be a similar colour, i.e. white (albeit the exact shade may be influenced by the other opacifying layers underneath). As such the image elements 14 are configured to substantially match this appearance and will also be white. Figure 31 (a) shows the security document from a first viewing angle at which no window cover image is displayed and here the full substrate window region 5 is visible, surrounded by the boundary region 7a which appears white in this example and then the non-window region 7 which appears light blue. Figure 31 (b) shows the same security document from another viewing angle at which a window cover image 20a is visible which here has a solid zone surrounding a triangular transparent zone, resulting in an apparent window region 10 which is triangular. The solid zone surrounding the apparent window region will appearwhite and thus appears to substantially match the boundary region 7a.

As noted above, one benefit provided by embodiments of the invention is to enable one type of base document substrate (having a pre-defined substrate window region 5) to be modified such that the apparent window region can have any desired appearance. For instance, different batches of the same base substrate can have different window cover images subsequently applied thereto, resulting in a wide variety of security documents with different apparent window regions. In the above embodiments, this modified appearance is also optically variable with the apparent window region appearing to change depending on the angle of view. However in comparative examples, it is also possible to apply similar principles to provide static apparent window regions 10, of different appearance from the underlying substrate window region 5.

Figure 32 shows a first comparative example in which this is the case. Here, the substrate window region 5, defined by the placement of the opacifying layers 2a, 2b (as before) has an elongate “lozenge” shape shown by the dashed-line periphery 6 in Figures 32(a) and 32(b). Within the substrate window region 5 a static window cover image 25 is provided, which defines a circular transparent zone surrounded on all sides by a solid zone. This effectively corresponds to the continuous or pseudo-continuous structures 25 described in certain embodiments above. The image 25 may be formed by printing a material having substantially the same visual appearance as the opacifying layers in combination at a selected location (e.g. the same material as that may be used to predominantly form image elements 14a in the previous embodiments) onto the transparent or translucent polymer 1 . In this case there is no need to form the image 25 of spaced or periodic elements since it is not required to co-operate with any focussing element array to generate an optically variable effect - indeed, in this comparative example, no focussing element array is provided. Instead, in the solid zone the ink may be applied as an all-over “block” print (e.g. flood print), as fine line print or as a dot screen, for instance. If there is any structure to the print this is preferably too small to be visible to the naked eye. As in the preceding embodiments, a small proportion of other colours could optionally be provided, e.g. to exhibit detailing or a pattern, but the dominant appearance should be substantially the same as that of the opacifying layers in the selected location (e.g. the non-window region). The result is that the periphery 6 of the substrate window region is substantially hidden and the security document appears to have a small, circular window 10. As shown in Figures 32(a) and 32(b) there is no change to the apparent window region 10 upon changing the viewing angle - it appears static. It will be appreciated that other batches of the document substrate having the same lozenge-shaped substrate window region could be provided with apparent window regions 10 of different number, size and/or shape (e.g. two or more windows), via appropriate configuration of the image 25.

In the above comparative example, the focussing element array 12 has been omitted entirely but this is not essential. A focussing element array 12 could be added to the Figure 32 example covering all or part of the substrate window region 5 and, provided the print of image 25 does not have a matching periodicity (e.g. in its dot screen), no optically variable effect will be generated so the static appearance described above will be maintained.

Figure 33 shows a further comparative example where this is the case and the image 25 defining the apparent window region 10 appears unchanged upon tilting. However, in this embodiment an additional optically variable device 91 is provided within the apparent window region. In this example, the optically variable device 21 is of the same sort described above with respect to Figure 6. However it is also possible to insert other forms of optically variable device such as moire magnifier.

Some preferred implementations of the invention are set out below:

A security document, comprising: a transparent or translucent polymeric substrate; one or more opacifying layers disposed on the first and/or second sides of the transparent or translucent polymeric substrate, the one or more opacifying layers defining a substrate window region across which at least one of the one or more opacifying layers is absent; across at least part of the substrate window region, an array of print elements and an overlapping array of focussing elements are provided, the array of print elements being located substantially in the focal plane of the focussing elements, the array of print elements comprising one or more sets of print elements formed predominantly or wholly of a material having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers, the or each set of print elements being arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defining a respective window cover image; wherein the array of focusing elements are configured to direct light from selected ones of the array of print elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of print elements in combination across the at least part of the substrate window region within a respective range of viewing angles; whereby the security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

A method of manufacturing a security document, comprising:

(a) providing a transparent or translucent polymeric substrate having one or more opacifying layers disposed on the first and/or second sides of the transparent or translucent polymeric substrate, the one or more opacifying layers defining a substrate window region across which at least one of the one or more opacifying layers is absent; and

(b) applying an array of print elements and an overlapping array of focussing elements across at least part of the substrate window region, the array of print elements being located substantially in the focal plane of the focussing elements, the array of print elements comprising one or more sets of print elements formed predominantly or wholly of a material having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers, the or each set of print elements being arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defining a respective window cover image; wherein the array of focusing elements are configured to direct light from selected ones of the array of print elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of print elements in combination across the at least part of the substrate window region within a respective range of viewing angles; whereby the security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

Claims

1 . A security document, comprising: a transparent or translucent polymeric substrate; one or more opacifying layers disposed on the first and/or second sides of the transparent or translucent polymeric substrate, the one or more opacifying layers defining a substrate window region across which at least one of the one or more opacifying layers is absent; across at least part of the substrate window region, an array of image elements and an overlapping array of focussing elements are provided, the array of image elements being located substantially in the focal plane of the focussing elements, the array of image elements comprising one or more sets of image elements predominantly or wholly having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at a selected location on the security document, the or each set of image elements being arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defining a respective window cover image; wherein the array of focusing elements are configured to direct light from selected ones of the array of image elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of image elements in combination across the at least part of the substrate window region within a respective range of viewing angles; whereby the security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

2. A security document according to claim 1 , wherein the or each window cover image is configured such that, upon changing the viewing angle, the apparent window region appears to change in terms of its shape, size, position and/or the number of apparent window regions.

3. A security document according to claim 1 or claim 2, wherein the selected location is either:

(i) outside the substrate window region, all of the one or more opacifying layers preferably overlapping at the selected location; or

(ii) in a boundary area of the substrate window region, less than all of the one or more opacifying layers preferably overlapping at the selected location.

4. A security document according to any of the preceding claims, wherein the array of image elements is configured such that when the security document is viewed from the respective range of viewing angles, the corresponding window cover image appears as a continuation of the one or more opacifying layers, being contiguous therewith.

5. A security document according to any of the preceding claims, wherein the or each window cover image includes at least one solid zone across which the image elements are present, the predominant appearance of the solid zone substantially matching the collective visual appearance of the one or more opacifying layers at the selected location, the solid zone preferably having a substantially uniform appearance substantially matching the collective visual appearance of the one or more opacifying layers.

6. A security document according to claim 5, wherein at least one of the window cover image(s) further includes at least one transparent zone into which the corresponding set of image elements do not extend, preferably at least two transparent zones into which the corresponding set of image elements do not extend, the at least two transparent zones being separated from one another by the at least one solid zone.

7. A security document according to any of the preceding claims, wherein at least one of the window cover image(s) is configured such that the apparent window region visible from the corresponding range of viewing angles has a boundary of which a first portion is defined by the periphery of the substrate window region, and a second portion which is defined by the respective window cover image.

8. A security document according to any of claims 1 to 6, wherein at least one of the window cover image(s) is configured such that the apparent window region visible from the corresponding range of viewing angles has a boundary which is wholly defined by the respective window cover image.

9. A security document according to any of the preceding claims, wherein the array of image elements and/or the overlapping array of focussing elements are provided across the whole of the substrate window region or across only a sub-part of the substrate window region.

10. A security document according to any of the preceding claims, wherein the array of image elements and/or the overlapping array of focussing elements extend outside the substrate window region, overlapping at least part of the opacifying layer(s).

11. A security document according to any of the preceding claims, wherein the array of image elements includes spaces interspersed with the set(s) of image elements and positioned with the same periodicity as the or each set of image elements, such that, when the security document is viewed from at least one angle, the focussing elements direct light from the spaces to the viewer such that no window cover image is displayed and the apparent window region is the same as the substrate window region.

12. A security document according to any of the preceding claims, wherein the array of image elements includes a plurality of sets of image elements, the respective sets being interspersed with one another and each set being arranged at the same periodicity, each set of image elements defining a respective window cover image at least some of which are preferably different from one another.

13. A security document according to claim 12, wherein the respective window cover images define frames of an animation effect, the apparent window region appearing to change in a step-wise or substantially continuous manner as the viewing angle is successively changed.

14. A security document according to claim 13, wherein the animation effect comprises any of: expansion or contraction of the apparent window region; and/or movement of the apparent window region; and/or opening or closing of the apparent window region; and/or morphing from one shape to another of the apparent window region; and/or increasing or decreasing in number of apparent window regions.

15. A security document according to claim 13 or 14, wherein the animation effect is cyclic, exhibiting a change in the apparent window region which varies successively from a first state to a second state and then successively from the second state back to the first state, before repeating as the viewing angle is changed in a continuous direction.

16. A security document according to any of the preceding claims, wherein the or each window cover image is configured such that, at no viewing angle is the substrate window region fully concealed, whereby the security document exhibits the apparent window region at all viewing angles, wherein preferably the or each window cover image is configured such that a sub-region of the substrate window region maintains a constant level of transparency or translucency at all viewing angles, as a result of there being no focusing elements in the sub-region and/or no image elements in the sub-region.

17. A security document according to any of the preceding claims, wherein the array of image elements includes a continuous or pseudo-continuous area.

18. A security document according to any of the preceding claims, wherein the array of image elements further includes a set of image elements having a different visual appearance from the collective visual appearance of the one or more opacifying layers and defining a feature image which is displayed when the security device is viewed from a corresponding viewing angle, the feature image contrasting with the collective visual appearance of the one or more opacifying layer(s).

19. A security document according to any of the preceding claims, wherein the array of image elements and the overlapping array of focussing elements are disposed on opposite sides of the transparent substrate with preferably no opacifying layer located therebetween.

20. A security document according to any of the preceding claims, wherein at least part of the substrate window region has a full window construction, across which all of the opacifying layers are absent, wherein preferably the whole substrate window region has a full window construction.

21 . A security document according to any of the preceding claims, wherein at least part of the substrate window region has a half window construction, across which the opacifying layers are absent on one side of the substrate and present on the other side, wherein preferably the whole substrate window region has a half window construction.

22. A security document according to any of the preceding claims, wherein each of the opacifying layers comprises a non-fibrous material, preferably comprising a binder and an opacifying pigment.

23. A security document according to any of the preceding claims, wherein each of the opacifying layers comprises an ink, preferably a gravure ink.

24. A security document according to any of the preceding claims, wherein each of the opacifying layers is of substantially the same colour as one another, preferably white, off-white or light grey.

25. A security document according to any of the preceding claims, wherein the one or more sets of image elements each comprise a material having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at the selected location on the security document, the image elements preferably being print elements.

26. A security document according to claim 25, wherein the one or more sets of image elements are each predominantly or wholly formed of the material having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at the selected location on the security document.

27. A security document according to any of the preceding claims, wherein image elements defining all or a predominant part of the or each window cover image, appear substantially the same visible colour as the collective visible colour of the one or more opacifying layers at the selected location, wherein preferably the Euclidean distance AE*_ab between the respective colours in Cl ELAB colour space is less than 5, more preferably less than 3, most preferably less than 2.3, where AE*_ab is measured using the formula where AL*, Aa* and Ab* are the distance between the two colours along the L*, a* and b* axes respectively.

28. A security document according to any of the preceding claims, wherein the array of image elements is an array of print elements, and preferably is defined by a single print working.

29. A security document according to any of the preceding claims, wherein the array of image elements comprises one or more inks, preferably lithographic or flexographic inks, most preferably a single ink.

30. A security document according to any of the preceding claims, further comprising a graphics layer extending over at least part of the opacifying layer(s) and optionally over at least part of the substrate window region.

31. A security document according to any of the preceding claims, wherein the security document is a banknote, a cheque, a bank card, a stamp, a passport, a visa, a certificate, a driving licence, an identification document or the like.

32. A method of manufacturing a security document, comprising:

(b) applying an array of image elements and an overlapping array of focussing elements across at least part of the substrate window region, the array of image elements being located substantially in the focal plane of the focussing elements, the array of image elements comprising one or more sets of image elements predominantly or wholly having substantially the same visual appearance as the collective visual appearance of the one or more opacifying layers at a selected location on the security document, the or each set of image elements being arranged at a periodicity substantially matched to that of the array of focussing elements in at least one dimension and defining a respective window cover image; wherein the array of focusing elements are configured to direct light from selected ones of the array of image elements in dependence on the viewing angle, the or each window cover image being displayed by the corresponding set of image elements in combination across the at least part of the substrate window region within a respective range of viewing angles; whereby the security document exhibits an apparent window region formed by the substrate window region in combination with any one of the one or more window cover images that is displayed depending on the viewing angle, such that upon changing the viewing angle, the apparent window region appears to change and/or disappear.

33. A method according to claim 32, wherein the selected location is either:

34. A method of manufacturing a security document according to claim 32 or 33, wherein step (a) comprises applying the one or more opacifying layers onto the first and/or second sides of the transparent or translucent polymeric substrate, preferably by a first printing technique, wherein most preferably the first printing technique is gravure printing.

35. A method of manufacturing a security document according to any of claims 32 to 34, wherein in step (b) the array of image elements is applied by printing the image elements onto the transparent or translucent substrate, preferably by a second printing technique different from the first printing technique, wherein most preferably the second printing technique is lithographic printing, such as offset lithographic printing, or flexographic printing.

36. A method of manufacturing a security document according to any of claims 32 to 35, wherein in step (b) the array of focussing elements is formed on the transparent or translucent substrate, preferably by a cast-cure technique, by embossing of the transparent or translucent substrate or a layer applied thereto, or by printing.

37. A method of manufacturing a security document according to claims 35 and 36, wherein in step (b) the array of image elements is printed onto the first side of the transparent or translucent substrate and the array of focussing elements is formed on the second side of the transparent or translucent substrate.

38. A method of manufacturing a security document according to claim 37, wherein the printing of the array of image elements and the forming of the array of focussing elements are performed simultaneously on opposite sides of the transparent or translucent substrate at the same position along the transparent or translucent substrate along the machine direction.

39. A method of manufacturing a security document according to any of claims 32 to 38, wherein in step (b) the array of image elements and the overlapping array of focussing elements are applied in register with one another.

40. A method of manufacturing a security document according to any of claims 32 to 39, wherein the method is configured to provide the security document with the features of any of claims 2 to 31 .