GB2538491A

GB2538491A - Security documents and methods of manufacture thereof

Info

Publication number: GB2538491A
Application number: GB1507966.8A
Authority: GB
Inventors: Isherwood Roland
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2015-05-11
Filing date: 2015-05-11
Publication date: 2016-11-23
Anticipated expiration: 2035-05-11
Also published as: GB2538491B; AU2016261440C1; EP3294567A1; GB201507966D0; AU2016261440B2; MX2017014413A; EP3294567B1; AU2016261440A1; WO2016181109A1

Abstract

A security document 1 has an at least semi-transparent or translucent window region 2 of lower optical density than an adjacent non-window region 3 of the security document. The window region is defined by at least one opacifying layer 6a, 6b of the security document which is present in the non-window region and not in the window region of the document. The security document further includes an optically variable security device 10 disposed at least partially in the window region, comprising a first ink layer 11 and a second ink layer 12 which at least partially overlap one another in the window region, wherein the first ink layer comprises a non-iridescent, semi-transparent ink composition with a visible colour which does not vary with viewing angle, and the first ink layer has a different composition from the at least one opacifying layer of the security document. The second ink layer comprises an iridescent, semi-transparent ink composition, the appearance of which varies according to the viewing angle.

Description

Security Documents and Methods of Manufacture thereof This invention relates to security documents having optically variable security devices thereon, and methods for their manufacture. Security devices are used for example on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents, in order to confirm their authenticity.

Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of iridescent (e.g. colour-shifting) or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed.

The use of iridescent inks on security documents is well known. For example, the use of semi-transparent or transparent iridescent pigments in windows on a polymer banknote is described in WO-A-2006/133512, which discloses a colour-shifting ink composition applied in a window region of a polymer banknote. The single ink composition comprises both an iridescent pigment and a non-iridescent transparent organic pigment. The use of an organic pigment in the same formulation allows greater control of the colour palette.

There is a constant need to develop new security devices with more complex visual effects in order to stay ahead of would-be counterfeiters.

In accordance with the present invention, a security document is provided having an at least semi-transparent or translucent window region of lower optical density than an adjacent non-window region of the security document, the window region being defined by at least one opacifying layer of the security document which is present in the non-window region and not in the window region of the document, and an optically variable security device disposed at least partially in the window region, comprising a first ink layer and a second ink layer which at least partially overlap one another in the window region, wherein the first ink layer comprises a non-iridescent, semi-transparent ink composition with a visible colour which does not vary with viewing angle, the first ink layer having a different composition from the at least one opacifying layer of the security document and the second ink layer comprises an iridescent, semi-transparent ink composition, the appearance of which varies according to the viewing angle.

It should be appreciated that the at least one opacifying layer could take various different forms depending on the construction of the security document. In some cases the document may comprise a transparent substrate, e.g. polymer, and the at least one opacifying layer could be disposed on one or both sides of the substrate, comprising for example a polymeric coating (e.g. white ink) or a fibrous layer (e.g. paper). In other cases the substrate of the security document could itself form the opacifying layer, e.g. where the document is based on a paper or other non-transparent substrate. Where the security document as a whole includes more than one opacifying layer (e.g. multiple coatings on one or each side of a transparent substrate), one or more of those layers may extend into the window region such that it takes the form of a half window (rather than full window), at least in places, but at least one of the layers will not extend into the window region in order that the window region is more transparent or translucent than the non-window region. One or more of the opacifying layers could also extend between the first and second ink layers at least in places. More details of preferred document constructions will be given below.

The present inventor has realised that the security of this category of security device can be increased by utilising two (or more) ink layers where at least one of the ink layers is a non-iridescent conventional pigmented ink (the "first ink layer"), and at least one is an iridescent ink (the "second ink layer"). The use of separate ink compositions maintains the advantages disclosed by WO-A-2006/133512 in being able to more widely control the colour palette but in addition it enables a number of advantages over the single ink feature described in the prior art. Firstly, the appearance of the device can be configured to be different depending on which side the device is viewed from, at least when viewed in reflected light. This is because when viewed from the side of the iridescent ink, the iridescent effect will act to conceal the underlying non-iridescent colour at some viewing angles, whereas from the other side the non-iridescent colour will be visible at all angles and will modify the iridescent effect seen therethrough.

Secondly, in preferred implementations, the appearance of the device can be varied in a localised manner by having regions with and without the standard conventional pigment. The use of the two ink layers allows the creation of multiple regions with different optical effects significantly increasing the counterfeit resistance of the document and increases the memorability of the security feature to the general public. It also increases the aesthetic nature of the device by opening up more design possibilities. Hence, preferably the first and second ink layers only partially overlap one another such that in a first part of the optically variable security device both the first and second ink layers are present, one lying over the other, and in a second part of the optically variable security device only the first or the second ink layer is present.

Depending on the desired design, the non-overlapping parts of the device could be located outside the window region in the non-window region of the document.

However, in particularly preferred embodiments, the first part and the second part of the optical variable security device extend across respective portions of the window region. This enables the two parts to be compared with one another in transmitted as well as in reflected light more readily, since the window region is more translucent than the non-window region.

In some advantageous implementations, the optically variable security device is located wholly inside the window region. However in other cases it is desirable to be located in parts of the device outside the window region since this enables the appearance of those parts to be compared against the pads inside the window. Due to the transparency of the inks, different effects will be exhibited inside and outside the window region. Preferably, therefore, the optically variable security device is located partially in the window region and partially in the non-window region. The different areas of the device could be spaced apart from one another but more preferably the optically variable security device extends across the boundary between the window and non-window regions. This continuity permits the two areas of the device to be compared directly against one another at the window boundary.

Unlike in prior art devices, it is particularly desirable that any portions of the device located outside the window should still be visible in the finished document, including the iridescent effect. As such, it is preferred that in the non-window region the optically variable security device is located on the outer surface of the security document such that from one side of the security document the first and second ink layers can be viewed unobstructed by the non-window region of the document. However, as mentioned above, in half-window regions an opacifying layer may extend between the first and second ink layers.

In order to enhance the complexity, and hence security level, of the device it is preferred that either the first ink layer or the second ink layer is arranged so as to define indicia, the other of the first ink layer and the second ink layer being arranged to extend across at least part of the indicia and beyond the indicia so as to obscure the indicia from at least one viewing angle. For example, in a particularly preferred implementation, the first ink layer is arranged to define the indicia and the second ink layer is configured to obscure the indicia from at least one viewing angle, preferably when viewed on-axis in reflected light. It has been found particularly effective where the indicia are negative indicia, defined by one or more gaps in the first or second ink layer. Most preferably, the indicia are obscured by the second ink layer in reflected light at at least some viewing angles, and are visible in transmitted light. The indicia could take any form including alphanumerical characters, symbols, logos, graphics or the like.

The first and second ink layers could be disposed on opposite surfaces of a transparent substrate forming (all or part of) the document. However, in more preferred embodiments, the first and second ink layers are disposed on the same surface of a substrate forming the security document, the first ink layer preferably being disposed between the substrate and the second ink layer where the first and second ink layers overlap.

Advantageously, the second ink layer exhibits a colour when viewed in transmitted light which is complementary to the colour exhibited by the second ink layer when viewed in reflected light at the same angle.

The security level of the device can be enhanced further by incorporating one or more additional ink layers. Hence in a preferred embodiment, the optically variable security device further comprises a third ink layer, comprising an iridescent, semi-transparent ink composition having an appearance different from that of the second ink layer at at least some viewing angles, the third ink layer preferably being laterally offset from the second ink layer and more preferably at least partially overlapping the first ink layer. The third ink layer may or may not also partially overlap the second ink layer, but if so should not wholly overlap it.

In a particularly preferred example, the second and third inks are selected such that, when viewed in reflected light the second ink layer exhibits the same colour as the third ink layer exhibits in transmitted light, and vice versa. This enables a "switching" effect to be observed with the two inks appearing to swap colours upon changing from reflected light viewing to transmitted light.

Iridescent pigments suitable for the second ink layer of the current invention include the widely available mica based interference pigments for example mica flakes coated with a thin layer of titanium dioxide or other metal oxide coated pigments. The thickness of the titanium dioxide coating is varied to obtain different colours as is well known in the art. Iridescent pigments suitable for the current invention preferably exhibit a colour in transmission which is complementary of the reflective colour. The iridescent pigment may or may not exhibit a change between two visible colours upon changing the reflective viewing angle, e.g. green to gold. In preferred cases the iridescent pigment may exhibit a change from a visible colour at some viewing angles, preferably when viewed substantially along the normal to the document, to substantially colourless (transparent) at other viewing angles.

For the first ink layer, any conventional coloured pigment can be used, but organic pigments are preferred.

It is desirable that in the non-window region of the security document, the document is light in colour (e.g. white, off-white or light grey), and bright so that when the device is viewed in reflection, light is reflected back through any parts of the device located in the non-window region of the document to render the device more visible. In preferred implementations, the non-window region of the document has a brightness L* in CIE Va*b* colour space of at least 70, preferably at least 80 and more preferably at least 90.

The security document can have various different constructions. For instance, the security device can be employed in a transparent region of either a paper or polymer secure document. In preferred embodiments the security document has a polymer substrate. Hence, preferably the security document comprises a transparent polymeric substrate, the at least one opacifying layer being disposed on one or both of its surfaces in the non-window region, the or each opacifying layer preferably comprising a polymeric coating or a fibrous layer (e.g. paper). Typically the polymer substrate will comprise a base substrate polymer film, e.g. of BOPP, and then additional print receptive opacifying coatings are applied on one or both sides of the polymer substrate to create a print receptive layer. Transparent or translucent window region(s) are defined by a gap in at least one of the opacifying layer(s). The one or more opacifying layers can be provided on either side of the polymer substrate and the various window regions may also be defined on either or both sides. The security feature of the current invention can be applied either solely in a fully transparent region or in a half window region where an opacifying layer is present on one side of the substrate or the other, or regions of the design forming the security feature can be applied over both the fully transparent region and the opacifying coating. Where the opacifying layer(s) comprise inks, these inks will be of a different composition, and preferably a different colour, from the inks forming the first and second ink layers.

Alternatively, the opacifying layer of the security document may be a fibrous substrate (e.g. paper) having an aperture defining the window region, and an insert having a transparent polymeric substrate is incorporated onto or into the fibrous substrate across the window region. The insert could be a patch, thread or strip formed of a polymer film for example. The insert could be incorporated into the fibrous substrate during paper making or could be affixed to a surface of the substrate at a later stage.

In particularly preferred examples, the window region includes a full window portion (extending across all or part of the window region) comprising a region of transparent substrate without an opacifying layer on either surface. Alternatively or in addition, the window region may include a half window portion (extending across all or part of the window region) comprising a region of transparent substrate with an opacifying layer on only one of its surfaces. In this case it is preferred that the second ink layer be visible through the half window as the top layer of the security device, i.e. the second ink layer should preferably be located between the first ink layer and the viewer, so that the iridescent effect can be viewed in reflection without obscuration by the opacifying layer or modification by the first ink layer. Alternatively, in the half window portion, the opacifying layer (forming the half window) may be between the first ink layer and the second ink layer on the same surface of the substrate. Here, the first ink layer is preferably between the opacifying layer and the substrate with the second ink layer outermost so that its iridescent effect can be viewed without obstruction.

Preferably, the visible colour of the first ink layer is different from that of the second ink layer when viewed in reflection from any angle and when viewed in transmission.

In order to obtain a variety of visual effects it is important that each of the ink layers is of sufficient transparency. Preferably, the first and second ink layers each individually have an optical density of less than 0.2, preferably less than 0.1. Advantageously, where the first and second ink layers overlap, in combination the first and second ink layers have an optical density of less than 0.3, preferably less than 0.2. For the purpose of the current invention the optical density for both the iridescent and non-iridescent ink is measured on a transmission densitometer, with an aperture area equivalent to that of a circle with a 1mm diameter. A suitable transmission densitometer is the MacBeth TD932.

In addition the security feature may also comprise any of: a luminescent substance, a fluorescent substance, a phosphorescent substance, a magnetic substance, a piezochromic substance or a thermochromic substance. The substance(s) could be included in one or more of the ink layers already described, or could be provided in one or more additional layers.

The present invention also provides a method of manufacturing a security document according to any of the preceding claims, comprising, in any order: forming an at least semi-transparent or translucent window region on a security document substrate, the window region having a lower optical density than an adjacent non-window region of the security document, the window region being defined by at least one opacifying layer of the security document substrate which is present in the non-window region and not in the window region of the document; applying a first ink layer to the security document substrate at least partially in the window region; applying a second ink layer to the security document substrate at least partially in the window region; the first and second ink layers forming an optically variable security device disposed at least partially in the window region; wherein the first ink layer and the second ink layer at least partially overlap one another, the first ink layer comprising a non-iridescent, semi-transparent ink composition with a visible colour which does not vary with viewing angle, the first ink layer having a different composition from the at least one opacifying layer of the security document, and the second ink layer comprising an iridescent, semitransparent ink composition, the appearance of which varies according to the viewing angle.

The resulting security device provides the advantages already discussed above.

In particularly preferred implementations wherein the first and second ink layers are applied to the same surface of the substrate, the second ink layer is applied after the first ink layer. In this way the iridescent ink layer forms the outermost layer of the security device, thereby enabling unobstructed viewing of the iridescent effect.

Advantageously, the security document substrate comprises a transparent polymeric substrate, the window region being formed by the application of one or more opacifying layers to the transparent polymeric substrate in the non-window region, the first and second layers of ink being applied after the application of the one or more opacifying layers. However, the security device can also be applied to paper-based documents as previously mentioned.

In a particularly preferred embodiment, the first layer of ink is applied before the application of the one or more opacifying layers to the same surface of the substrate. In this and other preferred embodiments, the second layer of ink is applied after the application of at least one of the opacifying layers to the same surface of the substrate. Thus, at least one of the opacifying layers may be between the first and second ink layers, at least in a half-window region of the device.

The window region may include a full window portion and/or a half window portion.

The first and second ink layers can be applied by any suitable process but advantageously are each applied by printing, most preferably by gravure printing, although any conventional printing process can be used. Advantageously, the first and second ink layers are applied in register with one another. For instance the first and second ink layers may be applied in the same in-line process. The opacifying layer(s) defining the window region may or may not be applied in the same in-line process. For instance if the opacifying layer(s) comprise a polymeric coating, e.g. ink, these may also be printed, e.g. by gravure.

The method can be adapted to manufacture a security document having any of the preferred features described above.

Examples of security documents in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 depicts a portion of a first embodiment of a security document in accordance with the present invention, Figure 1(a) showing a cross-section of the security document and Figures 1(b), (c) and (d) showing a plan view of a security device on the security document under three different viewing conditions; Figure 2 depicts a portion of a second embodiment of a security document in accordance with the present invention, Figure 2(a) showing a cross-section of the security document and Figures 2(b), (c) and (d) showing a plan view of a security device on the security document under three different viewing conditions; Figure 3 depicts a portion of a third embodiment of a security document in accordance with the present invention, Figure 3(a) showing a cross-section of the security document and Figures 3(b), (c) and (d) showing a plan view of a security device on the security document under three different viewing conditions; Figure 4 depicts a portion of a fourth embodiment of a security document in accordance with the present invention, Figure 4(a) showing a cross-section of the security document and Figures 4(b), (c) and (d) showing a plan view of a security device on the security document under three different viewing conditions; and Figures 5 and 6 show two further embodiments of security documents in accordance with the present invention, in cross-section.

The following description will focus on examples of security documents having polymeric document substrates, such as polymer banknotes. However, in other embodiments the described security devices can instead be formed on security documents based on paper or other fibrous material, with a window region comprising an insert with a polymeric substrate, such as a patch or stripe incorporated into the paper or affixed to the paper.

Figure 1 shows a polymer banknote 1 with a transparent substrate 5, formed for example of BOPP. The substrate 5 may be monolithic or multi-layered and may include other transparent layers such as a primer layer. A window region 2 is created by gaps in opacifying layer(s) 6a, 6b which in this example are provided on both sides of the substrate 5 although this is not essential. The window region 2 in this example is a full window, the opacifying layers 6a, 6b on both sides of the substrate 5 being absent, but in other cases the window region 2 may be a half window, the opacifying layers on one side of the substrate (only) continuing across the window region. Generally, the window region will be more translucent (or transparent) compared with the adjacent non-window region 3 of the document. The window region 2 may be surrounded by the non-window region, or could be located at an edge or corner of the document such that it is not surrounded on all sides.

The opacifying layer(s) 6a, 6b are preferably white or off-white in appearance, for example they may be white with a grey tint due to the presence of an anti-static conductive particle. Using the CIE L*a*b* (CIELAB) colour space the L* brightness value for the opacifying coating is preferably greater than 70 and even more preferably greater than 80 and even more preferably greater than 90. Typically the opacifying layers will comprise polymeric materials which are printed, coated or otherwise applied to the surface(s) of the substrate 5, preferably in a selective manner. In other cases the opacifying layer(s) could comprise fibrous layer(s), e.g. paper, which may be laminated to the substrate 5, resulting in a "hybrid" paper/polymer document.

To form an optically variable security device 10, a conventional (visibly) coloured, semi-transparent non-iridescent ink 11 is then printed onto the substrate 5 (which as noted above may optionally include an additional transparent layer, such as a primer). The composition of the non-iridescent ink 11 will be different from the composition of the opacifying layers 6a, 6b and preferably a different colour (e.g. not white). The non-iridescent ink 11 is applied to regions B of the substrate 5 such that a negative (non-printed) image of indicia, in this case the digit "10", is present in the centre of the security device as indicated by region C in Figure 1(a). A semi-transparent, iridescent ink 12 is then applied over the non-iridescent ink 11 in regions B and C such that it completely covers the non-iridescent ink 11 and the negative image of the digit "10". In this example the iridescent ink 12 has the same outer boundary/profile as the non-iridescent ink 11 however this is not essential and the iridescent ink 12 could extend beyond the non-iridescent ink 11 or vice versa. It will be seen that in some parts of the device (regions B), both inks 11, 12 are present whereas in other parts of the device (region C), only one of the inks, in this case iridescent ink 12, is present.

In the example in Figure 1 the iridescent ink 12 appears blue at normal incidence when viewed in reflection against a dark background and then appears substantially colourless when viewed at an angle away from normal incidence in reflection against a dark background. The iridescent ink 12 appears yellow in transmitted light which is the complementary colour to the blue light observed in reflection. The non-iridescent ink 11 is a red ink. Figure 1(a) shows that by using two separate ink layers 11, 12 three different regions (A, B and C) are present each, exhibiting different optical effects which will be illustrated with reference to Figures 1(b), 1(c) and 1(d).

Figure 1(b) shows the appearance of the window region 2 when the security document is viewed under reflected light at normal incidence against a black background (not illustrated in Figure 1(a)). In region A, across which neither ink layer extends, the underneath black background is unmodified and so the region appears black. In region B, where the first and second ink layers 11, 12 overlap, the iridescence ink 12 dominates the appearance and so this region appears blue. Likewise, region C where only the iridescent ink 12 is present appears blue. The result is that regions B and C have substantially the same appearance and so the indicia "10" cannot be distinguished.

Figure 1(c) shows the appearance of the window region 2 when the security document is viewed under reflected light off-axis (i.e. away from the normal) against a black background. Region A again appears black. In region B, since the iridescent ink 12 now appears colourless, the colour of the non-iridescent ink dominates and the region appears red. In region C, defining the digit "10", only the iridescent ink layer 12 is present and whilst this appears colourless it may cause a slight diffusing / absorbing effect with the result that the black of the background is slightly altered such that the region appears dark grey. Nonetheless this provides a visual contrast between the regions B and C such that the negative indicia "10" is now visible. Hence the presence of this indicia can be revealed and hidden by tilting the security document in reflected light viewing conditions.

Figure 1(d) shows the appearance of the window region 2 in transmitted light (i.e. when viewed on-axis against a backlight). Region A now appears transparent and colourless. In region B, where the two ink layers 11, 12 overlap, an orange tint results from the yellow colour of the iridescent layer 12 when viewed in transmission (complementary to its blue reflective colour), combined with the red colour of the non-iridescent layer 11. In region C, defining the digit "10", only the iridescent layer 12 is present and so this appears transparent with a yellow tint. Again, the presence of the negative indicia "10" can be discerned.

Instead of defining the indicia "10" in the non-iridescent ink layer 11, the indicia could be defined by appropriate gaps in the iridescent layer 12. In this case the indicia would be apparent in on-axis reflected light and hidden in off-axis reflected light, and remain visible in transmitted light.

In another modification, an additional dark background layer (e.g. black ink) may be provided in parts of the device either under the two ink layers 11, 12 or on the opposite side of the substrate. This will act in the same way as the dark background referenced above in relation to the viewing conditions of Figures 1(b) and (c), causing the iridescent ink 12 to dominate the appearance of those regions in reflected light, even if the document as a whole is not placed against a dark background. In such regions the transmissive colour described above (e.g. orange) would not be visible in transmitted light due to the opacity of the dark background layer. The inclusion of such regions can be used to create more complex designs and so further increase the security level of the document. This applies to all embodiments.

Iridescent pigments suitable for the second ink layer 12 of the current invention include the widely available mica based interference pigments for example mica flakes coated with a thin layer of titanium dioxide or other metal oxide coated pigments. The thickness of the titanium dioxide coating is varied to obtain different colours as is well known in the art. Iridescent pigments suitable for the current invention exhibit a colour in transmission which is complementary of the reflective colour as is illustrated in Figure 1.

The iridescent and non-iridescent inks can be applied by any conventional printing process, but typically this is a gravure printing process.

Typical ink formulations suitable for the current invention are as follows: Non-iridescent ink (layer 11): 12% pigment(s) -conventional pigment (preferably an organic pigment) varied as necessary to achieve the desired colour 5% Binder (for example Vinyl acetate / vinyl chloride copolymer) 83% Ethyl Acetate For example, to form a red non-iridescent ink, any of the following red pigments could be used (the concentration of each in the binder to be varied as necessary to achieve the desired optical density): Para Reds, Toluidine Red ('CI Pigment Red 3 No. 12120'), ["Permanent Red "Rim], Carmine F.B., Naphthol Reds and Rubines, Permanent Red FRC, Bordeaux FRR, Rubine Reds, Lithol Reds, BON Red, Lithol Rubine 4B, BON Maroon, Rhodamine 6G, Lake Red C, BON Arylamide Red, Quinacrinone Magentas, Copper Ferrocyanide Pink, Benzimidazolone Carmines and Reds (e.g. 'CI Pigment Red 176 No. 12515', 'CI Pigment Red 185 No. 12516' or 'CI Pigment Red 208 No. 12514'), Azo Magenta G, Anthraquinone Scarlet, and Madder Lakes.

It will be appreciated that the abbreviation 'Cl' used makes reference to the Society of Dyers and Colorists' Color Index.

Iridescent ink (laver 12): 24% -iridecscent (e.g. mica) particles -conventional particles selected to create the desired interference generated colour.

11% -binder -(e.g. Vinyl acetate / vinyl chloride copolymer) 65% -Ethyl Acetate In order to obtain the effects described with reference to Figure 1 (and all embodiments described below) the optical density of the printed inks is important. Typically the optical density for each of the iridescent ink 12 and non-iridescent ink 11 when measured on a transmission densitometer, with an aperture area equivalent to that of a circle with a 1mm diameter, is preferably less than 0.2, more preferably less than 0.1. A suitable transmission densitometer is the MacBeth TD932. The optical density in regions where both layers overlap when measured on a transmission densitometer, with an aperture area equivalent to that of a circle with a 1mm diameter, is preferably less than 0.3, more preferably less than 0.2.

Figure 2 shows a second embodiment of a security document 1 in accordance with the present invention. Features already described with reference to Figure 1 are denoted using like reference numerals in Figure 2. In this case both the iridescent and non-iridescent inks 11, 12 extend into the non-window region 3 of the security document and part of the security device 10 is therefore printed onto the opacifying layers 6a, 6b. The iridescent ink 12 extends beyond the non-iridescent ink 11. Two further regions of the security device D and E are now created which exhibit different optical effects to the regions A, B and C described with reference to figure 1. In region E the high reflectivity of the opacifying layers 6a, 6b (defined above) is important as it allows the light transmitted through the iridescent ink 12 to be reflected back to the observer such that they observe the complementary colour to that observed in region D in reflected light. This is achieved by using an opacifying layer 6a, 6b (preferably an ink) with a high L* value as described above.

Figures 2(b), (c) and (d) show the security device 10 in plan view in the same viewing conditions as in Figures 1(b), (c) and (d) above. However it should be noted that the views in Figure 2 include a portion of the non-window region 3 of the document as well as the window region 2, the periphery of which correspond to that of region B. Hence in reflected light at normal incidence against a black background (Figure 2(b)), regions B and D both appear blue since the iridescent ink 12 dominates.

As such the two regions are substantially indistinguishable and the boundary of the window region 2 itself is hidden. In region E, where only the iridescent ink 12 is present, light is reflected by the opacifying layer 6a back through the ink 12 such that its transmissive colour, yellow, is displayed.

In reflected off-axis light against a black background (Figure 2(c)), regions B and D are again indistinguishable but now appear red (the colour of non-iridescent ink 11) since the iridescent ink 12 appears colourless. Region E takes on the colour of the opacifying layer 6a, typically white.

When viewed in transmitted light (Figure 2(d)), the boundary of the window region 2 now becomes visible. Region B appears transparent with an orange tint resulting from the yellow colour of the iridescent ink 12 in transmission (complementary to its blue reflective colour), combined with the red colour of the non-iridescent ink 11. Region D also takes on an orange tint by virtue of the same mechanism, but is substantially opaque due to the opacifying layers 6a, 6b. Region E also appears opaque white but with a yellow tint due to the iridescent ink 12.

It will be appreciated that the Figure 2 embodiment could be provided with negative indicia by providing appropriate gaps in one or other of the ink layers 11, 12 in the window region 2 as described with respect to Figure 1.

Figure 3 shows a third embodiment of a security document with a security device having ink layers 11, 12 as described above. In this case, an opacifying portion 6c is provided inside the window region 2 (as may be provided in any of the other embodiments disclosed herein), effectively creating a half window portion 2a inside a full window. The non-iridescent ink 11 is disposed across one half of the window 2 (including half of the opacifying portion 6c) and the iridescent ink 12 is disposed across the other half. Additional portions of each ink 11, 12 are located on top of the other ink forming two star-shaped overlapping regions labelled B and B'.

Figures 3(b), (c) and (d) show the appearance of the window region 2 in the same three viewing conditions as in previous embodiments. When viewed in reflection at normal incidence on a black background (Figure 3(b)), regions F and G, where only the non-iridescent ink 11 is present, both appear red and the boundary with the opacifying portion 6c is substantially indiscernible. However, the star-shaped region B inside region F appears blue since here the iridescent ink 12 dominates. The other half of the opacifying portion 6c (region E) appears yellow resulting from the transmissive colour of the iridescent ink 12 due to reflection from the opacifying layer 6c. In the remainder of the window (region C) the iridescent colour dominates and the region appears blue except for the star-shaped region B' where the blue colour is modified by the overlying red ink 11 and so appears purple. Hence both stars B, B' are visible, but differ in colour.

When viewed in reflection off-axis against a black background (Figure 3(c)), regions F and G remain red. Since the iridescent ink 12 now appears colourless, star-shaped region B also now appears red and cannot be readily distinguished. Region E takes on the colour of opacifying layer 6c (typically white) and the remainder of the window (region C) appears dark grey due to the scattering nature of the colourless iridescent pigment modifying the appearance of the black background, except in star-shaped region B' which appears red due to layer 12. Hence only one of the stars is visible under this viewing condition.

When viewed in transmitted light, regions F and G both appear with a red tint although region F is transparent whilst region G is opaque. Similarly, regions E and C both appear with a yellow tint although region C is transparent whilst region E is opaque. Both star-shaped regions B and B' appear orange due to the combination of the transmissive colours (red and yellow) of the two inks 11, 12. Hence both star-shaped regions B, B' are visible and match.

Figure 4 shows a fourth embodiment of the present invention in which two iridescent inks of different colours are provided. Here, ink layer 13 is a second semi-transparent iridescent ink provided across an area laterally offset from first iridescent ink 12. The two iridescent inks may optionally overlap one another but in any case one or both will overlap the non-iridescent ink 11. In this case, non-iridescent ink 11 is provided at a central portion of the window 2, first iridescent ink 12 overlapping one half of non-iridescent ink 11 and half of the window 2, and second iridescent ink 13 covering the other half of non-iridescent ink 11 and window 2.

In this example the first iridescent ink 12 appears blue in reflected light against a dark background and switches to yellow in transmitted light while the second iridescent ink 13 behaves in the opposite manner and appears yellow in reflected light against a dark background and switches to blue in transmitted light. Other combinations of iridescent inks which do not exhibit this opposite behaviour are possible but the security is further enhanced by this switching nature of the two iridescent inks.

Figures 4(b), (c) and (d) show the appearance of the window region 2 in the same three viewing conditions as in previous embodiments. When viewed in reflection at normal incidence on a black background (Figure 4(b)), the iridescent colours of the two iridescent inks 12, 13 dominate such that regions H and J appear yellow whilst regions B and C appear blue. Hence the presence of non-iridescent ink 11 is hidden (including any indicia defined by that ink).

When viewed in reflection off-axis against a black background (Figure 4(c)), both iridescent inks 12, 13 become colourless. Hence in the regions H and C (outside non-iridescent ink 11) the window 2 appears dark grey due to the scattering effect of the pigments on the appearance of the black background. In the regions J and B, the red colour of the non-iridescent ink 11 dominates and hence any indicia defined by the ink 11 are revealed.

When viewed in transmitted light (Figure 4(d)), all four regions of the device take on different colours. Region H appears blue due to the transmissive colour of the second iridescent ink 13. Region J appears purple resulting from the blue transmissive colour of ink 13 combined with the red colour of non-iridescent ink 11. Region B appears orange due to the yellow transmissive colour of ink 12 combined with the red colour of non-iridescent ink 11. Region C appears yellow due to the transmissive colour of ink 12. It will be noted that the colours of regions H and C appear to swap between the viewing conditions of Figures 4(b) and (d) which is a particularly strong visual effect.

Figure 5 shows a fifth embodiment of a security document in cross section. In this case, the opacifying layers 6a on one surface of the substrate 5 comprise two layers 6a' and 6a", the underneath layer 6a' defining the outer periphery of window region 2 (along with optional layer 6b on the other side of the substrate), and upper layer 6a" extends into the window region 2, forming half-window region 2a. The first ink layer 11 of the security device is applied before the upper opacifying layer 6a" so that in the half window region 2a, the first ink layer is located between the substrate 5 and the upper opacifying layer 6a". The second ink layer 12 is located inside upper opacifying layer 6a", i.e. in the full window region only.

This configuration results in two distinct regions B and K, as shown in the Figure.

Region B displays the same appearances at the different viewing conditions as discussed in previous embodiments, i.e. iridescent blue when viewed in on-axis reflected light on a black background; red when viewed in off-axis reflected light on a black background, and orange when viewed in transmitted light. In the half-window regions K, there is no iridescent ink 12 and the device is concealed by the opacifying layer 6a" in reflected light. However, when viewed from the other side of the substrate in reflected light, region K appears red. Also due to the lower optical density in this region (as compared with non-window region 3), when viewed in transmitted light from either side region K appears with a red tint.

Figure 6 shows a sixth embodiment of a security document in cross-section. The construction is the same as that of Figure 5, except that here the iridescent ink 12 extends across the half-window region 2a and (optionally) into the non-window region 3. Hence, in the half-window region 2a, the opacifying layer 6a" is located between the non-iridescent ink 11 and the iridescent ink 12.

Region B (the full window) has the same appearance as described above, i.e. iridescent blue when viewed in on-axis reflected light on a black background; red when viewed in off-axis reflected light on a black background, and orange when viewed in transmitted light. In the half-window region M, in on-axis reflected light the light passes through the iridescent ink 12 and is reflected by opacifying layer 6a" such that the region appears yellow, and under this viewing condition adjacent region L appears the same. The two regions L, M are also indistinguishable in off-axis reflected light since the iridescent ink 12 is colourless and the regions take on the colour of the opacifying layer 6a". However, when viewed in transmission, region M will appear relatively translucent compared to region L. Region M will also appear with an orange tint (the combination of the transmission colours of layers 11 and 12) whereas if any colour is visible in region L this will be that of layer 12 only, i.e. yellow.

In all of the examples when positioned in the transparent window 2 the iridescent and non-iridescent inks may be on the same or different sides of the transparent polymer substrate 5. When positioned in the non-window regions 3, the iridescent and non-iridescent inks should be located on the same side of the substrate as one another, and at least the iridescent ink 12 should preferably be located on the outer surface of the opacifying layer(s). As described above, different effects can be achieved by arranging the non-iridescent ink 12 to be located inside or outside the opacifying layers.

In addition the security device 10 may also comprise any of: a luminescent substance, a fluorescent substance, a phosphorescent substance, a magnetic substance, a piezochromic substance or a thermochromic substance. The substance(s) could be included in one or more of the layers already described (e.g. layers 11, 12 and/or 13), or could be provided in one or more additional layers.

Claims

CLAIMS1. A security document having an at least semi-transparent or translucent window region of lower optical density than an adjacent non-window region of the security document, the window region being defined by at least one opacifying layer of the security document which is present in the non-window region and not in the window region of the document, and an optically variable security device disposed at least partially in the window region, comprising a first ink layer and a second ink layer which at least partially overlap one another in the window region, wherein the first ink layer comprises a non-iridescent, semitransparent ink composition with a visible colour which does not vary with viewing angle, the first ink layer having a different composition from the at least one opacifying layer of the security document, and the second ink layer comprises an iridescent, semi-transparent ink composition, the appearance of which varies according to the viewing angle.
2. A security document according to claim 1, wherein the first and second ink layers only partially overlap one another such that in a first part of the optically variable security device both the first and second ink layers are present, one lying over the other, and in a second part of the optically variable security device only the first or the second ink layer is present.
3. A security document according to claim 2, wherein the first part and the second part of the optical variable security device extend across respective portions of the window region.
4. A security document according to any of the preceding claims, wherein the optically variable security device is located wholly inside the window region.
5. A security document according to any of claims 1 to 3, wherein the optically variable security device is located partially in the window region and partially in the non-window region, the optically variable security device preferably extending across the boundary between the window and non-window regions.
6. A security document according to claim 5, wherein in the non-window region the optically variable security device is located on the outer surface of the security document such that from one side of the security document the first and second ink layers can be viewed unobstructed by the non-window region of the document.
7. A security document according to any of the preceding claims, wherein either the first ink layer or the second ink layer is arranged so as to define indicia, the other of the first ink layer and the second ink layer being arranged to extend across at least part of the indicia and beyond the indicia so as to obscure the indicia from at least one viewing angle.
8. A security document according to claim 7, wherein the first ink layer is arranged to define the indicia and the second ink layer is configured to obscure the indicia from at least one viewing angle, preferably when viewed on-axis in reflected light.
9. A security document according to claim 7 or claim 8, wherein the indicia are negative indicia, defined by one or more gaps in the first or second ink layer.
10. A security document according to any of claims 7 to 9, wherein the indicia are obscured by the second ink layer in reflected light at at least some viewing angles, and are visible in transmitted light.
11. A security document according to any of the preceding claims, wherein the first and second ink layers are disposed on the same surface of a substrate forming the security document, the first ink layer preferably being disposed between the substrate and the second ink layer where the first and second ink layers overlap.
12. A security document according to any of the preceding claims, wherein the second ink layer exhibits a colour when viewed in transmitted light which is complementary to the colour exhibited by the second ink layer when viewed in reflected light at the same angle.
13. A security document according to any of the preceding claims, wherein the optically variable security device further comprises a third ink layer, comprising an iridescent, semi-transparent ink composition having an appearance different from that of the second ink layer at at least some viewing angles, the third ink layer preferably being laterally offset from the second ink layer and more preferably at least partially overlapping the first ink layer.
14. A security document according to claims 12 and 13, wherein when viewed in reflected light the second ink layer exhibits the same colour as the third ink layer exhibits in transmitted light, and vice versa.
15. A security document according to any of the preceding claims, wherein the non-window region of the document has a brightness LW in CIE L*a*b* colour space of at least 70, preferably at least 80 and more preferably at least 90.
16. A security document according to any of the preceding claims, wherein the security document comprises a transparent polymeric substrate, the at least one opacifying layer being disposed on one or both of its surfaces in the non-window region, the or each opacifying layer preferably comprising a polymeric coating or a fibrous layer.
17. A security document according to any of claims 1 to 15, wherein the opacifying layer of the security document comprises a fibrous substrate having an aperture defining the window region, and an insert having a transparent polymeric substrate is incorporated onto or into the fibrous substrate across the window region.
18. A security document according to any of the preceding claims, wherein the window region includes a full window portion comprising a region of transparent substrate without an opacifying layer on either surface.
19. A security document according to any of claims 1 to 18, wherein the window region includes a half window portion comprising a region of transparent substrate with an opacifying layer on only one of its surfaces.
20. A security document according to claim 19, wherein in the half window portion, the opacifying layer is between the first ink layer and the second ink layer on the one of the surfaces of the substrate.
21. A security document according to any of the preceding claims, wherein the visible colour of the first ink layer is different from that of the second ink layer when viewed in reflection from any angle and when viewed in transmission.
22. A security document according to any of the preceding claims, wherein the first and second ink layers each individually have an optical density of less than 0.2, preferably less than 0.1.
23. A security document according to any of the preceding claims, wherein where the first and second ink layers overlap, in combination the first and second ink layers have an optical density of less than 0.3, preferably less than 0.2.
24. A method of manufacturing a security document according to any of the preceding claims, comprising, in any order: forming an at least semi-transparent or translucent window region on a security document substrate, the window region having a lower optical density than an adjacent non-window region of the security document, the window region being defined by at least one opacifying layer of the security document substrate which is present in the non-window region and not in the window region of the document; applying a first ink layer to the security document substrate at least partially in the window region; applying a second ink layer to the security document substrate at least partially in the window region; the first and second ink layers forming an optically variable security device disposed at least partially in the window region; wherein the first ink layer and the second ink layer at least partially overlap one another, the first ink layer comprising a non-iridescent, semi-transparent ink composition with a visible colour which does not vary with viewing angle, the first ink layer having a different composition from the at least one opacifying layer of the security document, and the second ink layer comprising an iridescent, semitransparent ink composition, the appearance of which varies according to the viewing angle.
25. A method of manufacturing a security document according to claim 24, wherein the first and second ink layers are applied to the same surface of the substrate, the second ink layer being applied after the first ink layer.
26. A method of manufacturing a security document according to claim 24 or 25, wherein the security document substrate comprises a transparent polymeric substrate, the window region being formed by the application of one or more opacifying layers to the transparent polymeric substrate in the non-window region.
27. A method of manufacturing a security document according to claim 26, wherein the first layer of ink is applied before the application of the one or more opacifying layers to the same surface of the substrate.
28. A method of manufacturing a security document according to claim 26 or 27, wherein at least the second layer of ink is applied after the application of at least one of the opacifying layers to the same surface of the substrate.
29. A method of manufacturing a security document according to any of claims 24 to 28 wherein the window region comprises a full window portion and/or a half window portion.
30. A method of manufacturing a security document according to any of claims 24 to 29 wherein the first and second ink layers are each applied by printing, preferably by gravure printing.
31. A method of manufacturing a security document according to any of claims 24 to 30 wherein the first and second ink layers are applied in register with one another.
32. A method of manufacturing a security document according to any of claims 24 to 31, adapted to manufacture the security document of any of claims 1 to 23.