GB2580126A

GB2580126A - Security devices and methods of manufacture thereof

Info

Publication number: GB2580126A
Application number: GB1821040.1A
Authority: GB
Inventors: Lister Adam
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2018-12-21
Filing date: 2018-12-21
Publication date: 2020-07-15
Also published as: GB201821040D0

Abstract

A security device 10 comprises an at least semi-transparent substrate 12 having opposed first and second sides 12a, 12b. The substrate defining an image display area on or in which are provided a plurality of first image regions 14 and a plurality of second image regions 16 which are relatively less transmissive and relatively more reflective than the first image regions. Each image region of the first image regions and the second image regions is sized such that said image region is not individually perceptible to the human eye. The first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions. The first image and second image are different. The image regions may be less than 100µm. There is also provided a method of manufacturing the security device.

Description

SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

FIELD OF INVENTION

Embodiments of the present invention relate generally to security devices and methods of manufacturing such security devices. Security devices are used to confirm value of security articles and security documents such as banknotes, cheques, passports, identify cards, driver's licences and the like. Improved security documents are provided which offer high levels of security and may be manufactured quickly, efficiently and accurately.

BACKGROUND

Security documents such as banknotes, cheques, passports, identity cards, driver's licences, credit cards, certificates of authenticity, fiscal stamps and the like are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein.

Typically such security documents (also termed "documents of value") are provided with security devices or security articles (which can comprise one or more security devices) for checking the authenticity of the document and to deter counterfeiting or tampering. By "security device" we mean a feature which it is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment. 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 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 or covert effects such as magnetic materials may also be employed.

Since the visual effects exhibited by security devices are extremely difficult to imitate or tamper with a person inspecting a security document can be assured that if a security device on the security document exhibits the correct appearance or effect that the security document is genuine. Therefore, the security of a document is increased.

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

However, as security devices have become increasingly complex, the methods required to produce security devices and security documents have also increased in complexity. Consequently, known methods of producing security devices and security documents are slow, expensive and require large and complicated machinery. Hence, there is also a need for security devices which may be manufactured quickly, easily and accurately.

As a result, there is a pressing need for complex security devices, security articles and security documents which may be quickly and efficiently 20 manufactured.

SUMMARY OF INVENTION

According to an aspect of the invention there is provided a security device comprising: an at least semi-transparent substrate comprising opposed first and second sides, the substrate defining an image display area on or in which are provided: a plurality of first image regions; and a plurality of second image regions which are relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the first image regions and the second image regions is sized such that said image region is not individually perceptible to the human eye; the first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions; and wherein the first image and second image are different.

Therefore, a security device according to this aspect of the invention exhibits different visual appearances (e.g. differences in colour, shape or form) depending on whether the security device is viewed in transmission or reflection. This change in appearance is seen from at least one side (and preferably both sides) of the security device.

Although the visual effect is complex, it may be easily observed by a person inspecting the security device. An authenticator may observe the security device in transmission by (for instance) holding the security device up to a light source, and may observe the security device in reflection by (for instance) holding the security device against an opaque background. A change in the appearance of the security device between reflection and transmission provides the authenticator with a clear indication of whether a document is authentic or not. Therefore, security devices according to the present invention offer increased security.

Furthermore, given that the security device exhibits a complex change in appearance, would-be counterfeiters will struggle to understand the structure of the security device and will be discouraged from attempting to imitate the security device. For instance, the complex visual effect offered by the present invention cannot be reproduced using conventional reproduction techniques (e.g. scanning or photocopying). As a result, security devices according to the present invention are highly secure. Nevertheless, the security device may be efficiently manufactured -e.g. by printing inks with appropriate optical properties onto a substrate to form the first and second image regions.

It will be appreciated that the visual effects discussed above are created by the relative differences in transmittance and reflectance between the first image regions and second image regions. However, it will also be understood that the second image regions may not be opaque and that the first image regions may not transmit all light.

Nevertheless, when the security device is viewed in transmission significantly more light is transmitted by the first image regions than the second image regions. Since an observer will receive substantially more light from the first image regions than the second image regions the appearance of the security device will be dominated by the light transmitted by the first image regions such that the image display region of the security device strongly or predominantly exhibits the first image. Having said this, the first image may be seen most clearly in transmission when the second image regions are substantially opaque and the amount of light transmitted by the second image regions is negligible in comparison to the light transmitted by the first image regions.

Equally, when the security device is viewed in reflection significantly more light is reflected by the relatively reflective second image regions than the first image regions. Consequently, the appearance of the security device to an observer is dominated by the light reflected by the second image regions (rather than the relatively smaller amount of light reflected by the first image regions). Therefore, the image display region strongly or predominantly exhibits the second image to an observer. In preferred examples the second image regions are sufficiently reflective that the amount of light reflected by the relatively transmissive first image regions is negligible in comparison to the amount of light reflected by the second image regions.

The phrase "viewed from the first side of the substrate" used herein will be understood to mean that the security device is viewed by an observer such that the first side (or surface) of the substrate is positioned closer to the observer than the second side (or surface). In other words, the security device is positioned such that the first side of the substrate faces the viewer.

Equally, the phrase "viewed from the second side of the substrate" used herein will be understood to mean that the security device is viewed by an observer such that the second side (or surface) of the substrate is positioned closer to the observer than the first side (or surface). In other words, the security device is positioned such that the second side of the substrate faces the viewer.

The first and second image regions are sized such that they are not individually perceptible to the human eye in regular use and cannot be readily observed on a macro-scale. In other words, the image regions are provided on a micro-scale and each image region may not be distinguished or resolved from the neighbouring image regions without close inspection, or using a microscope or alternative magnification device. In contrast, on the macro-scale (as would be visible to a human observer in normal use without assistance) the first image regions combine to exhibit the first image, whereas the second image regions combine to exhibit the second image. This is analogous is to how individual pixels in conventional display screens cannot be resolved by the naked eye in use and instead combine to exhibit an apparently continuous image to a viewer. For instance, where the security devices of the present invention are to be incorporated into traditional security documents such as banknotes and passports which are typically inspected from distances of between 0.3 and 1m the image regions may have a dimension of less than 100 pm in at least one direction in the plane of the security device.

By "being configured" it will be understood that one or more properties of the first and second image regions are selected so as to achieve the necessary visual effects. These properties can include one or more of the transmittance, reflectance, extents, materials, thicknesses, and positions of the first and second image regions.

For instance, the relative strength of the first image and second image under any given light condition will depend on the proportion of light transmitted and reflected to the viewer, as discussed above. Indeed, the appearance of the security device under transmission and reflection (or in mixed contexts where neither transmissive nor reflective effects dominate) can be controlled by varying the relative lateral extents of the first and second image regions. For instance, where the combined extent of the first image regions is increased relative to the combined extents of the second image regions the first image will be relatively more intense (i.e. brighter and/or more easily visible) in both reflection and transmission.

Equally, the appearance of the first and second images may also be modified by varying the reflectivity and transmissivity of the first and second image regions. In practice this may be achieved by (for instance) changing the composition of first and second image regions or by applying the first and second image regions in different thicknesses. For example, the second image regions may be made less transmissive and more opaque by applying an ink used to make the second image regions in a thicker layer. Following this change, the first image will be more easily observed in transmission since less light will be transmitted through second image regions and proportionally more light will be transmitted through the first image regions.

Changes to the surface finish of the first and second image regions may also affect the reflectivity of the first and second image regions (and hence the appearance of the first and second images).

By at least semi-transparent it is understood that the substrate transmits sufficient light in the visible spectrum that the first image regions may be observed in transmission through the security device. For instance, the substrate may be translucent (i.e. light scattering) or substantially transparent.

Preferably the substrate is a substantially transparent and colourless polymer. In particularly preferred embodiments, the substrate may be polyethylene terephalate (PET), biaxially orientated polypropylene (BOPP), polycarbonate (PC), polyvinylchloride (PVC), polybutlylene terephalate (PBT), nylon or acrylic.

The substrate may be a unitary or multi-layered substrate.

In particularly preferred examples substrate is substantially transparent and colourless. For instance, the semi-transparent substrate may have an optical density of less than or equal to 0.1, preferably less than or equal to 0.05, more preferably less than or equal to 0.02. The optical density of the substrate may be measured using a transmission densitometer, with an aperture area equivalent to that of a circle with a 1mm diameter. A suitable transmission densitometer is the MacBeth 1D932.

Preferably the relative variation in the transmittance and reflectance between the first and second image regions is exhibited in the visible spectrum (i.e. under electromagnetic radiation between approximately 400nm and approximately 700nm). In this case the security device will exhibit different appearances in transmission and reflection when seen in the visible spectrum (e.g. by a human observer). Additionally, or alternatively, the first and second image regions may exhibit different transmittance or reflectance to light outside the visible range such as light in the ultraviolet spectrum (electromagnetic radiation between 10nm and 400nm) or infrared spectrum (electromagnetic radiation between 700nm to 1 mm). The differences in appearance caused by such variations in optical properties outside the visible range will not be visible to the human eye but may be observed with the help of a machine reader.

The image display area is an area of the security device on which the first and second image regions are provided. The image display area (and hence the first and second image regions) may extend across the full area of the security device or the substrate of the security device. Alternatively, the image display area, and the first and second image regions, may only extend across a portion of the security device.

In preferred embodiments the first and second image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; or graphic. In particular, it is preferred that the two images may be clearly distinguished from one another such that the authenticity of the devices may be readily observed.

In particularly preferred embodiments the first image is of a constant first tint and first hue (i.e. a first colour), and the second image is of a constant second tint and second hue (i.e. a second colour). In other words, the first image regions preferably transmit light of a first wavelength or range of wavelengths corresponding to the first colour, whilst the second image regions preferably reflect light of a second wavelength or range of wavelengths corresponding to the second colour. The term "colour" is intended to include all hues detectable by the human eye, including black, grey, white, silver etc., as well as red, green, blue, etc. The colour change between the first and second images is easily observed by a human observer or a machine reader. These security devices may therefore be easily authenticated.

Alternatively, or additionally, the first and second images may comprise differences not being visible to the human eye. For instance, the first and second images may exhibit differences in the ultraviolet and/or infrared spectra. One or more of the first and second image regions may comprise fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

Preferably, the first image regions comprise an at least semi-transparent material provided on the substrate. By "on" it is understood that the first image regions and the semi-transparent material forming the first image regions are provided in contact with or above either side (i.e. either surface) of the substrate.

Equally the second image regions may comprise an at least semi-opaque material provided on the substrate. Again, by "on" it is understood that the second image regions and the semi-transparent material comprised within the second image regions are provided in contact with or above either side (i.e. either surface) of the substrate. In the latter case one or more intermediate layers may be provided between the substrate and the first and/or second image regions. Thus the first and/or second image regions may be quickly and easily applied to the substrate using any method of providing layers of material to a substrate such as printing or vapour deposition.

In preferred embodiments the first image regions and second image regions are provided on the same side of the substrate. In these examples it is easy to register the first and second image regions -i.e. apply the image regions in the correct location relative to one another. This is not essential, and in alternative embodiments the first and second image regions may be provided to opposing sides of the substrate, and/or at least one of the first and second image regions may be provided integrally within the substrate.

In some embodiments, the first image regions and/or second image regions are provided in the substrate. For instance, the first image regions and/or second image regions may comprise one or more of an integral marking, ink, pigment, dye or a colorant in the substrate. An integral marking may be achieved by (for instance) applying a laser to the substrate such that the properties of the material forming the substrate are modified. VVhere both the first and second image regions are provided in the substrate the substrate will exhibit differences in material properties or composition in the different image regions within the image display area. In these embodiments the security device offers increased security as it is more difficult for a would-be counterfeiter to imitate or tamper with regions or markings provided internally within a continuous or unitary layer.

Additionally, or alternatively, the first image regions and second image regions may be regions of a single layer. For instance, the single layer may be formed of two materials with different transmissivities, reflectivities or other optical properties which are interlaced or otherwise alternated. Alternatively, different surface treatments may be applied to the single layer in the different regions to achieve the necessary differences in opacity, transmittance and/or reflectance. The single layer may be the substrate in the case that both the first and second image regions are provided in the substrate, or a separate layer (e.g. a printed layer) applied to either side of the substrate. Providing the first image regions and second image regions in the same layer can simplify manufacture and make it easier to register the first and second image regions relative to each other. For instance, this single layer may be a printed layer and the first and second image regions may be applied in a single print working.

In further examples, the security device may comprise a continuous layer which is a least semi-transparent and extends across at least 75% of the image display area, wherein the second image regions comprise portions of an at least semi-opaque material which overlaps the continuous layer, and wherein the first image regions are defined by the spacing between the portions of at least semi-opaque material. In these examples an exact register is achieved between the first and second image regions since the first image regions and second image regions are defined by the boundaries of the at least semi-opaque material. By "overlap" it is understood that the portions of the at least semi-opaque material at extend at least partially over the region of the security device on which the continuous layer is provided. However, the semi-opaque material and the continuous layer may be separated by one or more intermediate layers or components -e.g. the semi-opaque material and the continuous layer may be provided on opposing sides of the substrate.

In the examples discussed above, the image exhibited by the image display area changes when the security device is viewed in transmission and reflection from at least one side of the substrate of the security device (i.e. from at least one side of the security device). However, further improvements to the security offered by a security device can be achieved where the image display area exhibits a change in appearance under transmission and reflection from each side of the security device.

In particularly preferred embodiments, the first and second image regions are configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions. In other words, one or more properties of the first and second image regions (e.g. transmissivity, reflectivity, size, position, thickness, extents and/or surface finish) may be chosen such that, when viewed in transmission from the second side of the substrate, the first image will be observed. This increase in the complexity of the appearance of the security device achieves an increase in the security of the device. Alternatively, where the substrate is only semi-transparent or translucent or otherwise semi-transparent, the relatively transmissive first image regions may not be visible through the substrate (i.e. from the second side of the substrate in which the substrate is positioned between the first image regions and an observer).

More preferably still, the first and second image regions are configured such that when the security device is viewed from the second side of the substrate in reflection the image display area exhibits the second image formed by the second image regions. This is achieved with the appropriate selection or choice, one or more properties of the first and second image regions (e.g. size, position, transmissivity and reflectivity). In such cases the change in the appearance of the security device between reflection and transmission may be observed from either side of the security device (i.e. regardless of the orientation of the security device or a security document to which the device is applied). This double-sided effect, in which a security device exhibits the same change in appearance from both sides, results in a security device which may be more easily authenticated but is more difficult to imitate. Therefore, the security of the security device is increased.

In preferred embodiments of security devices one or more of the second image regions may comprise a second image sub-layer and a reflection enhancing sub-layer; wherein the second image sub-layers are configured to form the second image; and wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective reflection enhancing sub-layers and a viewer.

In other words, the second image regions comprise two sub-layers: a second image sub-layer which is configured to exhibit the second image and a reflection enhancing sub-layer which is provided behind the second image sub-layer when viewed from the first side of the substrate. As a whole the second image regions will be highly reflective and relatively opaque, even if the second image sub-layer is itself semi-transparent. Therefore, the differences between the security device in transmission and reflection are more readily apparent to a viewer, meaning that the security device may be more easily authenticated. Therefore the security of the security device is increased.

In further preferred embodiments one or more of the second image regions comprise a second image sub-layer, and a third image sub-layer; wherein the second image sub-layers are configured to form the second image; wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective third image sub-layers and the viewer; the sub-layers being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device viewed from the second side of the substrate in reflection the image display area exhibits the third image formed by the third image sub-layers; and wherein the first image and third image are different.

When viewed in transmission from either side of the substrate, these security device exhibit the first image (i.e. substantially more light is transmitted by the first image regions than the second image regions such that the appearance of the device is dominated by the first image). VVhereas, when viewed in reflection from the first side of the substrate the security device exhibits the second image. Finally, when the security device is viewed in reflection from the second side of the substrate the security device exhibits the third image. This complex, double-sided effect can be easily seen by a human observer but is difficult to imitate or tamper with. Therefore, the security of the security device is increased.

Additionally, one or more of the second image regions may further comprise a reflection enhancing sub-layer provided between the second image sub-layer and the third image sub-layer. As a result the opacity and reflectivity of the second image regions is increased and the appearance of the first image in transmission and the second or third images in reflection (depending on which side of the security device is seen) become stronger. Therefore, the security device is easier to authenticate. Furthermore, it is difficult for would-be counterfeiters to accurately register (i.e. print in alignment, in corresponding lateral locations) multiple sub-layers within the second image regions. Together these effects mean that these security devices offer increased security.

The reflection enhancing sub-layers discussed above may comprise: a metal (e.g. aluminium, copper, or any other suitable metal); a metal alloy; a metallic or reflective ink; a binder comprising reflective particles; an optical interference thin film structure; a photonic crystal layer; or a liquid crystal layer; or any other suitable reflective substance or assembly. A metal or alloy layer may be applied as a foil or through vapour deposition Suitable metallic inks include Meta!star° inks sold by Eckart In some examples the second image sub-layers and/or the third image sub-layers are at least semi-transparent. This is particularly suitable where the security device is provided with a reflection enhancing layer provided between the second and third image layers.

An alternative example of a security device which exhibits a similar double-sided effect comprises: a plurality of third image regions provided on or in the image display area, the third image regions being relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the third image regions is sized such that said third image region is not individually perceptible to the human eye; wherein the image regions are arranged such that, when viewed from the second side of the substrate, the third image regions are between the second image regions and the viewer; the first, second and third image regions being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device is viewed from the second side of the substrate in reflection the image display area exhibits a third image formed by the third image regions; and wherein the first image and third image are different.

Therefore, when viewed from either side of the substrate in transmission the image display area exhibits the first image. In contrast, when viewed from the first side of the substrate in reflection the image display area exhibits the second image. Finally, when viewed from the second side of the substrate in reflection the image display area exhibits the third image. This complex effect can be easily seen by a human observer and provides a clear indication that the device is genuine, but is difficult to imitate or tamper with.

To achieve this effect, it should be noted that the image regions must be configured such that the third image regions do not wholly overlap the first image regions, so that light may be transmitted through the first image regions. Indeed, in preferred embodiments the third image regions have the same lateral extents as the second image regions, but are provided on the opposing side of the security device. This will maximise the light transmitted through the first image regions and provide a similar effect to the security devices discussed above which comprise multilayer second image regions which comprise second and third image sub-layers.

Alternatively, each third image region may be arranged to partially overlap (i.e. extend laterally over) one or more of the first and second image regions. For instance, where the first, second and third image regions are line regions (i.e. elongate regions which extend in the plane of the security device) and the first and second image regions are interlaced, the third image regions may extend in a direction which is skewed relative to the direction in which the first and second image regions extend, and preferably extend in a direction which is substantially perpendicular to the direction in which the first and second image regions extend (so as to increase the amount of light which may be transmitted through the first image regions). These examples in which the orientation of the third regions is skewed or rotated relative to the first and second image regions are preferred in practice since there is no need to register the third regions to the interlaced first and second image regions and manufacture is simplified.

The third image regions may comprise any of the features of the second image regions discussed above. For instance, in preferred embodiments the third image regions comprise a reflection enhancing sub-layer. In some embodiments the third image regions may be provided on the opposing side of the security device from the first and second image regions. However, this is not essential.

In the embodiments above which exhibit a third image when viewed in reflection from the second side of the substrate, it is preferred that the first image and third image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph. In preferred embodiments the first image is a flat tint of a first colour, and the third tint is a flat tint of a third colour. Alternatively the first and third images may differ by any other visual or structural property. However, this is not essential.

Additionally, or alternatively, the second image and third image are different, and preferably provide different visual appearances, wherein more preferably the second image and third image differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph. In preferred embodiments the second image is a flat tint of a second colour, and the third tint is a flat tint of a third colour. However, this is not essential and the second and third images may be identical -e.g. this may be preferred where the second image region comprises a second image sub-layer and a third image sub-layer separated by a reflection enhancing layer so as to provide a strong visual effect from both sides of the security device.

In preferred embodiments one or more of the first image regions, second image regions and third image regions are printed regions. For instance, image regions may be applied by printing, coating or laminating, optionally in more than one working, preferably by any of: laser printing, inkjet printing, lithographic printing, gravure printing, intaglio printing, flexographic printing, letterpress, dye diffusion, or thermal transfer printing. Alternatively any other printing method may be used. Alternatively, one or more regions could initially be formed on a separate layer and then laminated to the substrate.

In preferred examples each image region has a dimension of less than 100 pm in at least one direction in the plane of the security device, preferably less than 80 pm, more preferably less than 50 pm, more preferably still less than 25 pm. Adjacent regions with these dimensions cannot be easily distinguished from one another in conventional use (i.e. at a distance of approximately 0.3m to 0.5m) by the human eye. Therefore, on a macro-scale the regions will be seen in combination. Alternatively the image regions may be larger than 100 pm, especially where the security device is not intended to be viewed in close proximity. Smaller image regions are more difficult for would-be counterfeiters to manufacture and offer increased security.

In all of the embodiments discussed above, the differences between the reflectance and transmittance of the first image regions and the second and/or third image regions (where present) are preferably sufficiently large so that the first image, and the second and/or third image are distinctly exhibited and readily observed under transmission and reflection, respectively. In particularly preferred embodiments the reflectance of the second and/or third image regions is on average at least 20% greater than the reflectance of the first image regions for light with wavelengths between 400nm and 700nm; more preferably at least 30% greater; more preferably still at least 50% greater. In particularly preferred embodiments the transmittance of the first image regions is on average at least 50% greater than the transmittance of the second and/or third image regions for light with wavelengths between 400nm and 700nm; more preferably at least 100% greater; more preferably still at least 120% greater.

In some examples one or more of the first image regions, second image regions and third image regions are line regions (i.e. elongate regions), square regions or circular regions. Security devices with geometric image regions are easy to manufacture and can be used to create strong visual effects. Alternatively, image regions may be arrays of square, rectangular or circular regions arranged in an alternating chequerboard or grid pattern. Furthermore, the image regions may be arranged in a "polka dot pattern" such that one plurality of image regions (e.g. the first image region) will be observed as an array of circles against a continuous background formed by another plurality of image regions (e.g. the second and/or third image regions). Alternatively any other geometric, random or pseudo-random distribution of image regions may be used.

In particularly preferred embodiments the first image regions are interlaced with the second image regions and/or the third image regions. In these examples the first image regions will alternate with the second and/or third image regions in at least one direction parallel to the plane in which the security device extends.

This is particularly preferred when the image regions in question are line regions.

As mentioned above, where the first image regions, second image regions and third image regions are line regions and the first and second image regions are be interlaced, the third image regions may be orientated substantially perpendicular to the second image regions. This offers a strong change in visual appearance from either side of the security device.

In accordance with a further aspect of the invention there is provided a security article comprising a security device in accordance with the aspects of the invention discussed above. Preferably the security article is a thread, strip, thread, stripe, patch, foil, transfer foil or insert.

In accordance with a further aspect of the present invention there is provided a security document comprising a security device or a security article in accordance with the aspects of the invention discussed above. Preferably the security document is a banknote, cheque, identification document, certificate, share, stamp, visa, passport, licence, bank card, or ID card. In a particularly preferred In a particularly preferred embodiment, the substrate of the security devices discussed above also forms the substrate of a security document or security article. In these cases the first, second and third regions (where present) are provided on or in the substrate of the security document or article as previously described.

In further preferred embodiments the security document may comprise an at least semi-transparent window through which the security device or security article may be observed. Alternatively, the security device may be provided in an at least semi-transparent half-window within the security document. These security documents are increasingly difficult for would-be counterfeiters to imitate and offer increased security. For instance, the at least semi-transparent window may be provided in an otherwise substantially opaque security document. The window may be formed by a gap in or the absence of an opaque layer -i.e. the window may be defined by a region of a document in which an opacifying layer or a ply of paper has been omitted.

According to a further aspect of the invention there is provided a method of manufacturing a security device comprising the steps of: providing an at least semi-transparent substrate comprising opposed first and second sides, the substrate defining an image display area; providing in or on the image display area: a plurality of first image regions; and a plurality of second image regions which are relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the first image regions and the second image regions is sized such said image region is not individually perceptible to the human eye; and the first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions; and wherein the first image and second image are different.

The advantages of the resulting security device are discussed above with reference to the earlier aspects of the invention. The method may further include providing any of the preferred or additional features discussed above.

In the embodiments of security device discussed above comprising multi-layer image regions, one or more of the sub-layers may be configured to act as a resist for the other sub-layers. In these cases a highly accurate registration may be achieved between the sub-layers.

For instance, the step of providing one or more second image regions may cornprise: providing a reflection enhancing sub-layer continuously across the image display area on the substrate; applying the second image sub-layer on the reflection enhancing sub-layer in a pattern corresponding to the desired extents of the second image regions; and removing portions of the reflection enhancing layer outside the desired extents of the second image region, wherein the second image sub-layer is used as a resist.

More broadly, in further embodiments the method may comprise the step of removing a portion of an image sub-layer and/or the reflection enhancing sublayer, wherein either the second image sub-layer or the third image sub-layer is used as a resist.

Preferably, providing at least one of the pluralities of image regions is performed using one or more of: printing; inkjet printing; dye sublimation; laser printing; lithographic printing; flexographic printing; intaglio printing; gravure printing; screen printing; letterpress printing; dye diffusion thermal transfer; or vapour deposition.

According to a further aspect of the invention there is provided a method of manufacturing a security article as discussed above.

According to a further aspect of the invention there is provided a method of manufacturing a security document as discussed above.

The security devices, security articles and security documents manufactured to these methods will provide corresponding benefits and advantages as discussed above with reference to the earlier aspects of the invention.

BRIEF DESCRIPTION OF FIGURES

Figure 1 shows a security device in accordance with an embodiment of the invention in cross-section.

Figures 2a, 2b, 2c show plan views of alternative security devices consistent with the cross section shown in Figure 1.

Figures 3a and 3b show further cross-sections of a security device according to Figure 1. Figure 3a shows the security device of Figure 1 as illuminated by light sources (not shown) facing a second side of the security device; Figure 3b shows the security device of Figure 1 as illuminated by light sources (not shown) facing a first side of the security device.

Figure 4 shows a security device in accordance with an embodiment of the invention in cross-section.

Figure 5 shows a security device in accordance with an embodiment of the invention in cross-section.

Figure 6 shows a security device in accordance with an embodiment of the invention in cross-section.

Figure 7 shows a security device in accordance with an embodiment of the invention in cross-section.

Figures 8a, 8b, Sc and 8d illustrate steps of an exemplary method of producing the exemplary security device of Figure 6 in cross-section.

Figures 9a and 9b show a security device in accordance with an embodiment of the invention in perspective and cross-section respectively Figures 10a and 10b show a security device in accordance with an embodiment of the invention in perspective and cross-section respectively.

Figure 11a to 11e show details concerning the formation of image regions according to an embodiment of the invention.

Figures 12a to 12c show details concerning the formation of image regions according to an embodiment of the invention.

Figures 13a and 13b show a security document according to the present invention in plan view and cross section, respectively.

Figures 14a and 14b show a security document according to the present invention in plan view and cross section, respectively.

Figures 15a and 15b show a security document according to the present invention in plan view and cross section, respectively.

Figures 16a, 16b and 16c show a security document according to the present invention in front plan view, rear plan view and cross section, respectively.

Figure 17a and 17b show embodiments of systems for manufacturing a security device, security article and/or security document according to the present invention.

DETAILED DESCRIPTION

Figure 1a shows in cross-section a security device 10 in accordance with the present invention which exhibits a first image when viewed in transmission from either side of the security device 10, and exhibits a second image when viewed in reflection from either side of the security device 10.

The security device 10 comprises an at least semi-transparent substrate 12 with opposed first and second sides (i.e. surfaces) 12a, 12b. A plurality of first image regions 14 comprising an at least semi-transparent material 14a and a plurality of second image regions 16 comprising an at least semi-opaque material 16a are provided on the first side 12a of the substrate 12. More specifically, as shown, the material of the first image regions 12 and second image regions 14 is provided in contact with the first side 12a of the substrate 14.

The at least semi-transparent material 14a defining the first image regions 14 are more transmissive and less reflective than the at least semi-opaque material 16a defining the second image regions 16. Consequently, the first image regions 14 are relatively more transmissive and relatively less reflective than the second image regions 16.

The first and second image regions 14, 16 are alternated along the x-direction (a direction parallel to the plane of the section shown in Figure 1). However, the first and second image regions 14, 16 are sized so that they may not be individually distinguished on the macro-scale. In other words, a human observer will not be able to distinguish a given first image region 14 from the adjacent second image regions 16 in use without inspecting the security device 10 from a very short distance or by using magnification. Instead, the appearance of the adjacent image regions 14, 16 will be merged or perceived together by a human observer. In other words, a person inspecting the security device 10 will see the light from the security device 10 (whether reflected or transmitted) as a whole.

On the macro-scale the first image regions 14 combine to exhibit a first image, whereas the second image regions 16 combine to exhibit a second image. As discussed above, the first and second image regions 14, 16 may have widths in the x-direction of less than 100 pm.

As shown, the image regions 14, 16 (and therefore an image display area which corresponds to the extent of these regions 14, 16) extend across the entire width of the security device 10 in the x-direction. However, this is not essential and in alternative structures the image display area and the image regions 14, 16 defined therein may only extend across a portion of the security device 10.

It will be appreciated that the cross-section shown in Figure 1 may be formed by security devices 10 with a variety in the layout of the first and second image regions 14, 16. Three security devices 10 with different configurations of the first and second image regions 14, 16 which produce the cross section shown in Figure 1 are shown in Figures 2a, 2b and 2c.

Figure 2a shows a security device 10a in which the first image regions 14 and the second image regions 16 are interlaced line regions (i.e. elongate regions) which extend in the plane of the substrate 12 of the security device 10. Consequently, the elongate first and second image regions 14, 16 cannot be individually distinguished by a human observer along the x-direction (i.e. in the direction in which the image regions 14, 16 alternate).

Figure 2b shows a security device 10 where the first and second image regions 14, 16 are square regions arranged in a chequered pattern wherein the squares alternate between first and second image regions 14, 16 along both the x-direction and z-direction (i.e. in two perpendicular directions in the plane of the substrate 12 of the security device 10b).

Figure 2c shows a security device 10 where the first and second image regions are formed in a 'polka dot' pattern in which the second image regions 16 are formed as a regularly distributed array of circular regions of the same size across the face of the security device 10c. The first image regions 14 extend continuously between the circular second image regions 16.

In each of the examples above, the first and second image regions 14, 16 respectively cover approximately 50% of the security devices 10a, 10b, 10c.

However, this is not essential and may not be preferred where (for instance) the second image regions 16 are particularly reflective or particularly opaque. Furthermore, in Figures 3a, 3b and 3c the first and second image regions 14, 16 are arranged in geometric, repeating patterns. However, in further variants the first and second image regions 14, 16 are arranged in a random or pseudo-random distribution of dots or other regions across the security device.

As mentioned above, the security devices 10 shown in Figures 1 and 2 exhibit different appearances in reflection and transmission when viewed from both sides 12a, 12b of the substrate. This is achieved due to the differences in the reflective and transmissive properties of the materials 14a, 16a forming the first and second image regions 14, 16. These differences in properties are preferably observed under visible light, but this is not essential.

The changes in the appearance of the security device 10 under transmission and reflection will now be discussed with reference to Figures 3a and 3b.

Figure 3a shows schematically the security device 10 as illuminated by light from one or more light sources (not shown) positioned facing the second side 12b of the substrate 12. Figure 3b shows schematically the security device 10 as illuminated by light from one or more light sources (not shown) positioned facing the first side 12a of the substrate 12.

For simplicity, in each picture the light incident on the security device has been separated into light L1 which is initially incident on the first image regions 14, and light L2 which is initially incident on the second image regions 16. In addition, to provide increased clarity, the light reflected by the first image regions 14 and light transmitted by the second image regions 16 are not shown in Figures 3a and 3b. In practice relatively small amounts (and in preferred embodiments negligible amounts) of the light L1 incident on the relatively transmissive first image regions 14 is reflected, and relatively small (preferably negligible) amounts of light L2 incident on the relatively reflective first image regions 14 is transmitted.

In each figure a first observer 01 is positioned on a first side of the security document such that the first observer 01 faces the first side 12a of the substrate 12, and a second observer 02 is positioned on a second side of the security device such that the observer faces the second side 12b of the substrate 12.

The first observer 01 in Figure 3a sees the security device 10 in transmission from the first side 12a of the substrate 12. This is because the light reaching 01 is predominantly transmitted through the security device 10.

In this case, since the first image regions 14 are relatively more transmissive than the second image regions 16, relatively more light L1 which is incident on the first image regions 14 reaches the first observer 01 than the light L2 which is incident on the second image regions 16. Since more light reaches the first observer 01 from the first image regions 14 than the second image regions 16, this light (and therefore the appearance of the first image regions 14) will dominate the appearance of the security device 10. Since the first observer 01 is unable to distinguish the individual image regions 14, 16 which are provided on a micro-scale, the first observer 01 will observe an image formed on the macro-scale by the first image regions 14 in combination. Thus, when viewed in transmission from a first side 12a of the substrate 12, the security device 10 will exhibit the first image.

In contrast, when the security device is primarily illuminated with light from the first side 12a of the substrate 12 -as shown in Figure 3b -the same observer 01 will see the security device in reflection. This is because in Figure 3b the light reaching the first observer 01, is predominantly light emitted by light sources (not shown) on the first side 12a of the substrate 12 that is reflected back to the first observer 01 positioned on the same side 12a of the substrate 12.

As the second image regions 16 are relatively more reflective than the first image regions 14, relatively more of the light L2 which reaches the second image regions 16 is reflected than the light L1 that is incident on the first image regions 14. Therefore, more light is reflected to the first observer 01 by the second image regions 16 than the first image regions 14. As the image regions are not individually perceptible or resolvable to the unaided first observer 01, the first observer 01 will see the light reaching them in combination. Thus, since substantially more light is reflected by the second image regions 16 than the first image regions 14, the light L2 reflected by the second image regions 16 will dominate the appearance of the security device 10. Therefore, the security device 10 exhibits the second image formed by the second image regions 16 in combination.

Thus, the security device 10 exhibits a first image when viewed in transmission and a security image when viewed in reflection by the first observer 01 on the first side 12a of the substrate 12.

The second observer 02 located on the second side 12b of the substrate will see a similar change in the appearance of the security device 10.

In Figure 3b, in which the security device is illuminated by light sources (not shown) facing the first side 12a of the security device 10, the second observer 02 sees the security device 10 in transmission. As discussed with reference to the first observer 01 of Figure 3a, relatively more of the light L1 incident on the relatively transmissive first image regions 14 is transmitted through the security device than the light L2 incident on the relatively reflective second image regions 16. Therefore, the second observer 02 receives more light through the first image regions 14 than from the second image regions 16. Hence, the security device 10 exhibits the first image formed by the combination of the first image regions 14 because the light Li transmitted through the first image regions 14 dominates the appearance of the security device 10.

In contrast, the second observer 02 shown in Figure 3a views the security device 10 in reflection. In this case a relatively larger proportion of the light L2 incident on the second image regions 16 is reflected than the light L1 incident on the first image regions 14. Therefore, the second observer 02 (who is unable to distinguish individual image regions 14, 16 from one another in normal use) will see the second image formed by a combination of the second image regions 16 because the second image regions 16 dominate the appearance of the security device 10.

A wide variety of materials and techniques may be used to manufacture the security devices 10 shown in Figures 1,2 and 3.

As an example, the at least semi-transparent material 14a of the first image regions 14 may be formed solely of Sicura Flex 39-8 Blue 072 C E01, a relatively transmissive blue ink from Siegwerk (RTM). The relatively reflective and at least semi-opaque material 16a within the second image regions 16 may be formed wholly of USFR8489 Magenta, a magenta ink produced by Tameside Ink (RIM). In such an example the substrate is formed of substantially transparent polyethylene terephalate (PET) with a thickness of 23 pm. The inks may be printed onto the substrate 10 at approximately 2 grams per square meter (gsm) so as to form first and second image regions 14, 16 which are approximately 1.5 to 2 pm thick (in the z-direction) Therefore, the security device 10 manufactured using these two exemplary inks will exhibit a blue tint when viewed in transmission from either side 12a, 12b of the substrate 12 and a magenta tint when viewed in reflection from either side 12a, 12b of the substrate 12.

The relative reflectance -i.e. the proportion of light reflected from a surface -of these two inks within the visible range was tested using a Shimadzu UV-3600 Plus spectrophotometer (which measures reflectivity at a 8% angle between the detector and illuminator) and a HunterLab LabScan XE colour measurement system (which measures at a 45% detector to illumination angle). For this testing, two samples of PET of a thickness of 23 pm were used, each coated with one of the two inks at 2 gsm, such that the ink was approximately 1.5 to 2 pm thick, as is present in the embodiment of the security device described above.

Using the Shimadzu, the blue ink (Sicura Flex 39-8 Blue 072 C E01) was measured to have an average reflectance of 2.01% for light between 400 and 700 nm (i.e. light across the visible spectrum), whilst the magenta ink (USFR8489 Magenta) was measured to have an average reflectance of 2.61% between 400 and 700nm (i.e. across the visible spectrum).

Using the HunterLab, Sicura Flex 39-8 Blue 072 C E01 was measured to have an average reflectance of 1.10% across the visible spectrum between 400 and 700nm (i.e. across the visible spectrum), whereas USFR8489 Magenta was measured to have an average reflectance of 1.57% between 400 and 700nm (i.e. across the visible spectrum).

Therefore, as measured with the Shimadzu UV-3600 Plus the relatively reflective magenta ink (USFR8489 Magenta) has a reflectance which is approximately 30% greater than the blue ink (Sicura Flex 39-8 Blue 072 C E01). Whereas, when measured using the HunterLab LabScan XE system the magenta ink has a reflectance which is approximately 40% greater than the blue ink.

The relative transmittance (i.e. the proportion of light transmitted by a sample) was also measured using the Shimadzu spectrophotometer and the same samples as used in the reflectance testing discussed above. The average transmittance of the blue ink across the visible spectrum between 400nm and 700nm was measured at 37.0%. The average transmittance of the magenta ink across the visible spectrum between 400nm and 700nm was measured at 16.2%. Therefore, the blue ink has a transmittance which is approximately 130% greater than the magenta ink, as measured by the Shimadzu UV-3600 Plus spectrophotometer.

The PET substrate 12 was measured to have an optical density of 0.02 using a MacBeth TD932 optical densitometer. However, substantially any semi-transparent substrate could be used. In particular, many polymer materials are suitable for this purpose (e.g. biaxially orientated polypropylene (BOPP), polycarbonate (PC), polyvinylchloride (PVC), polybutlylene terephalate (PBT), nylon or acrylic).

In this example the security device exhibits a single colour from each side in both reflection and transmission. However, this is not essential and the first and second image regions 14, 16 may be printed with multiple inks in order to create graphics, logos, indicia, photos, portraits or other images, so long as the relative differences in the transmissivity and reflectivity of the first and second image regions 14, 16 are maintained.

Indeed, any other inks or materials which exhibit a detectable difference in reflectivity and transmissivity may be applied to the substrate 10 to form the first and second image regions 14, 16. For instance, the inventor has achieved similarly successful results when the magenta ink (USFR8489 Magenta) described above is replaced by Fast Red 032, a red ink produced by Siegwerk (RTM).

Inks are preferred for the security devices 10 since they may be easily and accurately applied using a variety of printing methods including gravure, flexographic, lithographic, intaglio, ink jet, laser jet or dye diffusion thermal transfer printing. Where the first and second image regions 14, 16 are printed in this manner they may be applied in a single print working or in two separate print workings.

Figure 4 shows a further security device 20 in accordance with the present invention in cross section. The security device 20 exhibits different appearances when viewed in transmission and reflection from both sides of the security device. However, unlike the security devices 10 shown in Figures 1 to 3, the appearance of the security device 20 when viewed in reflection is different from either side of the security device 20.

The security device 20 comprises an at least semi-transparent (and preferably substantially transparent) substrate 22 with opposed first and second sides 22a, 22b. An at least semi-transparent continuous layer 28 is provided in contact with the first side 22a of the substrate 22. The continuous layer 28 has the same lateral extents as the substrate 22 -i.e. the continuous layer extends across the whole of the first side 22a of the substrate -however, this is not essential.

The security device 20 further comprises an array of portions (i.e. regions) of an at least semi-opaque material 26a which is relatively more reflective and relatively less transmissive than the continuous layer 28. As shown, the portions of at least semi-opaque material 26a are provided in contact with the side of the continuous layer 28 which does not contact the substrate 22 (i.e. the portions of the reflective and at least semi-opaque material 26a are provided over the continuous layer 28 relative to the substrate 22). The portions of at least semi-opaque material 26a are laterally spaced from each other -i.e. the portions of at least semi-opaque material 26a are provided in an array with gaps or spacing between the portions of at least semi-opaque material 26a.

Therefore, it will be appreciated from Figure 4 that the security device 20 is divided into two sets of alternating image regions 24, 26 along the x-direction (a direction extending in the plane of the security device and parallel to the plane of the cross-section shown in Figure 4). A plurality of relatively reflective and relatively less transmissive second image regions 26 are defined by the extent of the at least semi-opaque material 26a provided or applied onto the security device 20 (as discussed above). Whereas, a plurality of relatively transmissive and relatively less reflective first image regions 24 are defined by the lateral spacing or gaps between the at least semi-opaque material 26a (and by definition the spacing between the second image regions 26). In other words, the first image regions 24 comprise the relatively transmissive portions of the security device 20 which are not provided with at least semi-opaque material 26a.

In summary, the extents of the relatively reflective second image regions 26 are defined by the extents of the at least semi-opaque material 26a, whereas the relatively transmissive first image regions 24 are defined by the absence of this at least semi-opaque material 26a. Therefore, the first and second image regions 24, 26 of Figure 4 are by definition exactly registered together. This can be contrasted with the security devices 10 of Figures 1 to 3 where the first and second image regions 14, 16 are defined by the boundary of inks, materials or layers applied to the security device and may be mis-registered as a result of errors in printing.

Thus it is straightforward to achieve accurate registration between the first and second image regions 24, 26 for the security device shown in Figure 4 in comparison to the security device 10 shown in Figure 1.

As with Figure 1, the first and second image regions 24, 26 are sized such that they cannot be distinguished from one another by a human authenticator without magnification or close inspection (i.e. they are not individually perceptible to the human eye in typical use or on a macro-scale).

Together the first image regions 24 combine to exhibit a first image. For instance, the continuous layer 28 may comprise one or more pigments, dyes, markings or colorants to form the first image within the first image regions. Whereas a second image in the second image regions 26 is formed by the appearance of the portions of at least semi-opaque material 26a in combination on a macro-scale. The at least semi-opaque material 26a may comprise one or more one or more pigments, dyes, markings or colorants to form the second image.

As discussed in reference to the security device 10 shown in Figure 3, when the security device 20 shown in Figure 4 is viewed in transmission from either side of the security device 20, the security device 20 will exhibit the first image. This is because relatively more light is transmitted through the relatively transmissive first image regions 24 than the reflective second image regions 26.

When viewed in reflection relatively more light is reflected by the second image regions 26 than by the first image regions 24. However, the security device 20 may exhibit different appearances from each side of the security device 20 in reflection.

From the first side 22a of the substrate 22 (i.e. when the security device 20 is arranged such that the portions of at least semi-opaque material 26a are placed between the observer and the continuous layer 28) an observer will see a second image formed by light reflected to the observer from the reflectively reflective material 26a within the second image regions 26. This second image is formed by the light reflected by the second image regions 26 (which are not individually perceptible) when combined on the macro-scale.

When viewed in reflection from the second side 22b of the substrate (i.e. when the security device 20 is arranged such that the substrate 22 and continuous layer 28 is placed between the portions of at least semi-opaque material 26a) light is again predominantly reflected by the at least semi-opaque material 26a. However, the light incident on the second image regions 26 from the second side 22b of the security device 20 must be transmitted through the substrate 22 and continuous layer 28 before it reaches the at least semi-opaque material 26a.

Equally, after the light is reflected by the at least semi-opaque material 26a it must be transmitted through the substrate and the continuous layer 28 before it may exit the security device 20. As a result the light which reaches an observer on the second side 22b of the substrate under reflection (and hence the appearance of the security device 20) is modified by the appearance of the substrate and the continuous layer 28. Where the continuous layer 28 and substrate 22 are substantially visually transparent and colourless in the second regions 26, the appearance of the second image will not be modified and the security device 20 will exhibit the second image from both sides of in reflection. However, in other variants in which the continuous layer 28 and/or substrate 22 are optically effective (e.g. coloured or translucent) within the second regions 26, the security device 20 will instead exhibit in reflection from the second side 22b of the substrate 22 the second image formed by the at least semi-opaque material 26a, but where it has been modified or altered by the combined effects of the substrate 20 and the continuous layer 28.

As in Figure 1, the substrate 12 may be formed of substantially transparent polyethylene terephalate (PET) with a thickness of 23 pm. The continuous layer 28 may be formed wholly of the semi-transparent blue ink discussed above with reference to Figures 1 to 3 (Sicura Flex 39-8 Blue 072 C E01, produced by Siegwerk (RIM)). In addition, the portions of at least semi-opaque material 26a may be formed wholly of the relatively reflective magenta or red inks discussed above with reference to Figures 1 to 3. Particularly good results are achieved when these inks are applied at approximately 1 to 3 gsm, more preferably 1.5 to 2 gsm.

Figure 5 shows a further security device 30 which exhibits similar visual effects as the security device 20 shown in Figure 4 -i.e. the security device 30 may different change in visual appearance under reflection and transmission from each side of the security device 30. In this embodiment the features of the substrate 22 and continuous layer 28 of the security device 20 shown in Figure 4 are combined.

The security device 30 shown in Figure 5 comprises an at least semi-transparent substrate 32 with opposed first and second sides 32a, 32b. As in Figure 4, the security device 30 further comprises a plurality of regions or portions at least semi-opaque material 36a which is relatively reflective and relatively less transmissive than the substrate 22. The portions of at least semi-opaque material 36a are provided to the first side 32a of the substrate 32. The portions of at least semi-opaque material 36a are formed as an array (e.g. as an array of line elements) and are laterally spaced from each other in the plane of the security device 30.

The portions of at least semi-opaque material 36a define a corresponding plurality of relatively reflective and relatively opaque second image regions 36.

A plurality of relatively transmissive, but less reflective, first image regions 34 are defined by the gaps or spacing between the at least semi-opaque material 36a such that the first image regions 34 and second image regions 36 alternate.

When viewed in transmission from either side 32a, 32b of the substrate 32, the security device 30 of Figure 5 will predominately exhibit a first image created the substrate 32 within the first image regions 34 because relatively more light will be transmitted through the first image regions 34 than the second image regions 36.

However, the security device 30 will again exhibit different appearances under reflection from either side of the security device 30. This is analogous to the discussed above with reference to Figure 4. When viewed in reflection from the first side 32a of the substrate 32, light will be strongly reflected from the reflective portions of at least semi-opaque material 36a, such that the security device 30 will exhibit a second image formed by the second image regions 36 in combination. Whereas, when the security device 30 viewed in reflection from the second side 32b of the substrate, an observer will see the second image formed by the portions of at least semi-opaque material 36a, as modified by the intervening portions of the substrate 32. Therefore, when the substrate 32 is substantially colourless and transparent in at least the second regions 36 an observer will simply observe the second image (as from the first side 32a of the substrate 32), whereas if the substrate 32 comprises any markings, colourants or other optically effective material in the second regions 36 the appearance second image will be modified by this optically effective portions of the substrate 32.

Security devices which incorporate reflection enhancing layers such as metals or metallic inks into their second image regions offer more distinct (i.e. more readily apparent) changes in appearance under transmission and reflection. Figures 6 and 7 show two further embodiments of security devices according to the present invention.

The security device 40 shown in Figure 6 is a modification of the security device shown in Figure 1. As with the embodiment of Figure 1, this security device 40 includes a substrate 42 with opposed first and second sides 42a, 42b. A plurality of first image regions 44 and second image regions 46 are provided in contact with the first side 42a of the substrate 42.

The first and second image regions 44, 46 are alternated along the x-direction (a direction parallel to the plane of the section shown in Figure 1). However, the first and second image regions 44, 46 are sized so that they may not be individually distinguished on the macro-scale.

As in Figure 1, the first image regions 44 comprise an at least semi-transparent material 44a. However, unlike the security device 10 shown in Figure 1, the second image regions 46 of the security device 40 shown in Figure 6 are multi-layered. Each second image region 46 comprises a second image sub-layer 46a, and a reflection enhancing sub-layer 46b which have the same lateral extents and are provided in register. The reflection enhancing sub-layer 46b is provided in contact with the first side 42a of the substrate 42, and the second image sub-layer 46a is provided over the reflection enhancing sub-layer 46b, such that when the security device 40 is viewed from the first side 42a of the substrate 42 the reflection enhancing sub-layer 46b is positioned between the second image sub-layer 46a and the substrate. On a macro-scale the second image sub-layers 46a combine to exhibit a second image.

Therefore, the reflection enhancing sub-layers 46b form a reflective backing to the second image sub-layers 46a when viewed from the first side 42a of the substrate 42, such that the second image formed by the second image sub-layers 46a is easily seen. Highly reflective and substantially opaque reflection enhancing sub-layers 46b can be achieved using a metal (e.g. aluminium or copper) or a metallic ink as the reflection enhancing layer 46b. As a consequence multi-layered second image regions 46 may be manufactured which are highly reflective and of low transmissivity, even when the second image sub-layer 46a comprises ink(s) which are substantially transparent.

Since the combined second image regions 46 are highly reflective and substantially opaque when compared to the first image regions 44, the second image regions 46 transmit negligible amounts of light in comparison to the first image regions 44. Thus the first image exhibited by the first image regions 44 is readily observed in transmission from either side 42a, 42b of the substrate.

Equally, a strong second image may be easily observed when the security device 40 is viewed in reflection from the first side 42a of the substrate 42 because relatively large amounts of light are reflected by the second image regions 46 in comparison to the first image regions 44. Thus the difference between the first image (as seen in transmission) and the second image (as seen in reflection) is particularly striking or readily observed when viewing the security device 40 of Figure 6 from the first side 42a of the substrate 42.

When viewed from the second side 42b of the substrate 42 in reflection, an observer will not see the second image created by the second image sub-layer 46a within the second regions 46 because relatively little light will be transmitted through the substantially opaque reflection enhancing sub-layer 46b. Instead, assuming the substrate is substantially transparent and colourless the observer will observe the natural appearance of the reflection enhancing layer 46b.

Where the reflection enhancing layer 46b is a metal or metallic layer the security device 40 will exhibit a silver, shiny appearance when viewed in reflection from the second side 42b of the security device 40.

The at least semi-transparent material 44a in the first regions may be formed of Sicura Flex 39-8 Blue 072 C E01 from Siegwerk (RTM), whilst the second image sub-layer 46a may be formed of USFR8489 Magenta by Tameside Ink (RTM) or Fast Red 032 by Siegwerk (RTM).

Alternatively, since the reflection enhancing sub-layer 46b lends the second image regions 46 greater reflectivity and reduced transmittance in comparison to the first image regions 44, the at least semi-transparent material 44a and the second image sub-layer 46a comprise materials of similar transmissivities and reflecfivities. For instance, the at least semi-transparent material 44a and the second image sub-layer 46a may each be formed of a 5 gsm coating of VMCH (a polymer comprising vinyl chloride and vinyl acetate manufactured by Wacker (RTM)) containing 10% Orasol (RTM) yellow pigment. In this case the first and second image may be created by (for instance) variations in the lateral extents of the first image regions 44 containing the at least semi-transparent material 44a and the second image regions 46 which include the second image sub-layer 46a.

Figure 7 shows a further security device 50 which comprises a reflection enhancing sub-layer 56b. However, in this embodiment the multi-layered second image region 56 further comprises a third image sub-layer 56c such that the exhibits a third image when viewed from the second side 52b of its substrate 52 (instead of the silvery, shiny appearance seen with the security device 40 of Figure 6).

The security device comprises a substrate 52, first image regions 54 and second image regions 56. The first image regions 54 comprise a relatively transparent and relatively unreflective at least semi-transparent material 54a. The second image regions 56 are multi-layered and comprise a second image sub-layer 56a, a reflection enhancing sub-layer 56b, and a third image sub-layer 56c. The sub-layers 56a, 56b, 56c have corresponding lateral extents and are provided in register such that the reflection enhancing sub-layer 56b is positioned between the second image sub-layer 56a and the third image sub-layer 56c. The third image sub-layer 56c is provided in contact with the first side 52a of the substrate. On a macro-scale the second image sub-layers 46a combine to exhibit a second image and the third image sub-layers 46a combine to exhibit a third image.

The reflection enhancing sub-layers 56b are preferably substantially opaque and highly reflective. Thus, the multi-layered second image regions 56 as a whole are again substantially opaque and highly reflective. In more detail, a large proportion of the light which is incident on the second image sub-layer 56a from the first side 52a of the substrate 52 will be reflected by either the second image sub-layer 56a itself or the underlying reflection enhancing sub-layer 56b.

Equally, a large proportion of the light incident on the third image sub-layer 56c from the second side 52a of the substrate 52 will be reflected by either the third image sub-layer 56c or the underlying reflection enhancing sub-layer 56b instead of being transmitted through the second image region 56.

The appearance of the security device 50 from the first side 52a of the substrate 52 is identical to that of the security device 40 of Figure 6. The security device exhibits the first image in transmission and the second image in reflection. From the reverse -i.e. from the second side 52b of the substrate 52 -the security device 50 will exhibit the first image in transmission, but exhibits the third image formed by the third image sub-layer 56c in reflection (assuming the substrate 52 is substantially colourless and transparent). Therefore, the security device 50 exhibits a different change in appearance between reflection and transmission when viewed from each side 52a, 52b of the substrate 52. Where the substrate 52 comprises an optically effective marking or colourant in the second regions 56 the third image exhibited in reflection from the second side 52b of the substrate 52 will be modified by the appearance of the substrate.

In a particular embodiment the at least semi-transparent material 54a of the first region 54 may be Sicura Flex 39-8 Blue 072 C E01 from Siegwerk (RTM), the second image sub-layers 56a may be USFR8489 Magenta by Tameside Ink (RIM) and the third image sub-layers 56c may be Fast Red 032 by Siegwerk (RIM). In this case the security device 50 will exhibit a blue tint in transmission from either side of the security device 50, a magenta tint in reflection from a first side of the security device 50 (corresponding to the first side 52a of the substrate 52) and a red tint in reflection from the second side of the security device 50 (corresponding to the second side 52ba of the substrate 52). In this example the second and third images are different. However, this is not essential and alternatively the second and third image may be identical -i.e. the second and third image sub-layers may both be formed solely of USFR8489 Magenta by Tameside Ink (RTM). Alternatively, one or more of the at least semi-transparent material 54a, second image sub-layer 56a and third image sub-layer 56b may comprise VMCH (a polymer comprising vinyl chloride and vinyl acetate manufactured by Wacker (RIM)) containing 10% Orasol (RIM) yellow pigment.

It will be appreciated that the specific structure shown in Figure 7 is not essential to achieve this double-sided visual effect. For instance, the reflection enhancing sub-layer 56b may be omitted where the second and third image sub-layers 56a, 56b are themselves substantially opaque and reflective. Additionally, or alternatively, on or more of the sub-layers 56a, 56b, 56c may be provided on the second side 52b of the substrate 52 -e.g. the third image sub-layer 56c may be provided in contact with the second side 52b of the substrate 52c, whilst the reflection enhancing sub-layer 56b and the second image sub-layer 56a may be provided on the first side 52a of the substrate.

Where security devices comprise multi-layered second image regions (as in the security devices 40, 50 of Figures 6 and 7) it is possible to achieve highly accurate registration between the first and second image regions where one or more of the sub-layers of the second image regions are used as a resist or mask. Such a manufacturing process will now be described with reference to Figures 8a, 8b, 8c, 8d. Figure 8d shows the final security device 40 which is identical to the security device 40 shown in Figure 6 and discussed above.

In a first step of the manufacturing process there is provided an at least semitransparent substrate 42 with opposed first and second sides 42a, 42b. To the first side 42a of the substrate 40 is provided a continuous reflection enhancing layer 46b' (e.g. a metal or alloy foil, or a metallic ink). As shown in Figure 8a the reflection enhancing layer 46b' extends across the whole of the first side 42a of the substrate. As an aside, it will be noted that in further variants where the image display area occupies only a portion of the lateral extent of the final security device, the reflection enhancing layer may be provided over at least the whole of the image display area, but not over the entire first side of a security device.

As shown in Figure 8b, a second image sub-layer 46a is applied over the reflection enhancing layer 46b'. The extents of second image sub-layer 46a correspond (i.e. are substantially the same as) the desired final extents the of second image regions 46.

Subsequently, the portions of the reflection enhancing layer 46b' outside the desired final extent of the second image regions 46 (as defined by the application of the second image sub-layer 46a) are removed. For instance an etchant suitable for removing the reflection enhancing layer 46b' but not the second image sub-layer 46a may to applied to the assembly shown in Figure 8b. In this process, the second image sub-layer 46a acts as a resist, protecting the underlying portions of the reflection enhancing layer 46b' and preventing these portions from being removed.

Thus, a plurality of multi-layered second image regions 46 are formed on the first side 42a of the substrate 42, as shown in Figure 8c. The second image regions 46 comprise the portions of the reflection enhancing layer 46b' which are not removed (e.g. removed by an etchant) which form the reflection enhancing sub-layer 46b over which is provided a second image sub-layer 46a. An etching process is particularly preferred as the resulting reflection enhancing sub-layer 46b and second image sub-layer 46a are accurately registered together.

Finally, a plurality of first image regions 44 are provided (e.g. printed) between the second image regions 46. This final security device 40 is shown in Figure 8d. As will be seen, the portions of the reflection enhancing layer 46b' removed (e.g. during the etching process) are replaced by the first image regions 44 in the final security device 40.

The final step of providing the first image regions 44 between the multi-layer second image regions may not be necessary where a first image is exhibited by portions of the substrate 42 itself (analogously to the security device 30 shown in Figure 5 in which the substrate 32 may comprise internal markings, colorants or other optically effective substances to generate a first image).

Alternative methods of manufacturing the final security device 40 shown in Figure 6 and Figure 8d includes printing a metallised ink as the reflection enhancing layer 46b, printing the at least semi-transparent material 44a and printing the second image sub-layer 46a such that these printed components are registered to one another. These printing processes may be performed simultaneously or separately (and in substantially any order).

The security devices 60, 70 shown in Figures 9 and 10 exhibit a similar double-sided visual effect to the security device 50 shown in Figure 7. Specifically, these security devices 60, 70 exhibit a change from a first image to a second image when viewing from a first side of a security device, and a switch from a first image to a third image when viewing from a second side of a security device.

Figure 9a and 9b show in perspective view and cross section a security device 60 which comprises similar features of the security device 10 shown in Figure 1. The security device 60 comprises an at least semi-transparent substrate 62 with opposed first and second sides 62a, 62b. As in Figure 1, first and second image regions 64, 66 are provided on the first side 62a of the substrate 62. The first and second image regions 64, 66 are interlaced arrays of elongate line regions which extend in the y-direction, as shown in Figure 8a. As in the examples disclosed above, the second image regions 66 are relatively more reflective and relatively less transmissive when compared to the first image regions 64.

In addition, the security device 60 comprises an array (i.e. a plurality) of elongate third image regions 69 are provided on the second side 62b of the substrate 62. The elongate third image regions 69 extend in the y-direction -i.e. in a direction which is substantially perpendicular to the direction in which the first and second image regions 64, 66 extend and substantially parallel to the direction along which the first and second image regions 64, 66 are interlaced. Each third image region 69 is spaced from the adjacent third image regions 69 (i.e. the third image regions 69 are interleaved with regions in which no third region 69 is provided along the x-direction). As with the second image regions 66, the third image regions 69 are relatively more reflective and relatively less transmissive than the first image regions 64.

The first, second and third image regions 64, 66, 69 are sized so that they cannot be individually perceived by a human observer in normal use (i.e. without close inspection or magnification). Instead, to a human observer, the first image regions 64 combine to exhibit a first image (i.e. the first image regions cooperate to form a first image on the macro-scale), whereas the second image regions 66 combine to exhibit a second image and the third image regions 69 combine to exhibit a third image.

As discussed above, when viewed from the first side 62a of the substrate 60 in transmission the security device exhibits a first image because substantially more light is transmitted through the first image regions 64 than either the second image regions 66 or the third image regions 69. Note that the light which is transmitted through the first image regions 64 will predominantly be light from the second side 62b of the substrate 62 which is incident on the gaps or spaces between the third image regions 69 rather than light which is incident on the relatively reflective and relatively opaque third image regions 69. Similarly, when viewed from the second side 62b of the substrate 60 in transmission, the security device 60 will also exhibit the first image for similar reasons.

When viewed from the first side 62a of the substrate 62 in reflection the security device 60 will exhibit the second image because light reaching an observer will predominately be light reflected from the second image regions 66. Although some light may be reflected by the first image regions 64 or the third image regions 69 positioned on the reverse side 62b of the substrate 62 this amount light is relatively small in comparison to the light reflected by the second image regions 66. Therefore, the light reflected by the second image regions 66 will dominate the appearance of the security device 60 in this situation.

Furthermore, when viewed from the second side 62b of the substrate 62 in reflection, the security device 60 will exhibit the third image because light reaching an observer will predominantly be light reflected from the third image regions 69.

Thus the security device 60 shown in Figure 9a and 9b exhibits a different change in visual appearance under reflection and transmission from either side of the security device.

Figures 10a and 10b show a security device 70 manufactured by assembling (e.g. adhering or laminating) two subsidiary security devices 10, 10' manufactured according to in Figure 1 together back-to-back.

The subsidiary security devices 10, 10' both comprise interlaced elongate first image regions 14, 14' and second image regions 16, 16' provided on a first side 12a, 12a' of a substrate 12, 12'. The subsidiary security devices 10, 10' are attached such that the second sides 12b, 12b' of the subsidiary security devices 10, 10' are in contact, and wherein the direction in which the first and second image regions 14, 16 of the first subsidiary security device 10 extend (the x-direction, as shown) is substantially perpendicular to the direction in which the first and second image regions 14', 16' of the second subsidiary security device 10' extend (the y direction).

The first image regions 14, 14' of both subsidiary security devices 10, 10' are relatively less reflective and relatively more transmissive than the second image regions 16, 16'. The first image regions 14, 14' and second image regions 16, 16' are sized such that they cannot be individually distinguished by the human eye in typical use.

In transmission from either side of the security device 70 shown in Figure 10, the security device 70 will exhibit a first image formed by light transmitted through both of the first image regions 14, 14' of the subsidiary security devices 10, 10'. Whereas, assuming the second image regions 16 of the first subsidiary security device 10 combine to exhibit a different image than the second image regions 16' of the second subsidiary security device 10', a different image will be observed from either side of the overall security device 70 in reflection.

An exemplary method of constructing or forming of the pluralities of image regions used in the security devices discussed above is illustrated in Figure 11a to 11 e. Specifically, these figures show how two interlaced arrays of elongate image regions may be constructed from the desired final images which are to be displayed by a security device.

Figure 11a shows a first image 100 (a star) to be exhibited by the security device in transmission. Figure 11 b shows a second image 150 (the numeral "5") to be exhibited by the security device in reflection.

Figure 11c and 11 d respectively show (i.e. show separately) the arrays of first image regions 100a, and second image regions 150a to be provided on the security device. These are arrays of elongate, line elements taken from the respective final images shown in Figures ha and 11 b. The first and second image regions 100a, 150a extend in a first direction (the x-direction as shown). The array of first image regions 100a, shown in Figure 11c, are interleaved along a perpendicular, second direction (the y-direction) with a set of second image region positions, i.e. with a set of empty image regions or gaps that contain no material. The elongate first image regions 100a positively define the star indicium of Figure 1 la with each image region being a different strip of the indicium. The array of second image regions 100b, shown in Figure 11d, is formed in substantially the same way, being formed of a plurality of elongate image strips taken from the numeral "5" shown in Figure 1 lb which are interleaved or separated by a set of empty first image region positions along the second direction (the y-direction).

Please note that for clarity the dimensions of the first and second image regions 100a, 150a have been increased in Figures 11c and 11d (and Figure 11e, discussed below). In practice the first and second image regions 100a, 150a will be sized such that an individual image region may not be distinguished from the adjacent image regions. As such, the width of the elongate first and second image regions 100a, 150a (i.e. the dimension of the first and second image regions 100a, 150a in the second direction in which they interleaved, the y-direction) will be sufficiently small that a human observer will not be able to individually perceive a given image region in normal use. For instance, when used in security documents such as banknotes, licences, cards and passports which are typically inspected from a distance of between 0.3 m and 1 m the first and second image regions 100a, 150a may have a width of less than 100 pm.

Figure 11e shows the final, interleaved pattern of image regions 100a, 150a which is provided (e.g. printed) on a security device. The array of first image regions 100a and second image regions 150a are interleaved along the second direction (the y-direction). In other words the second image regions 150a are provided in the empty image region positions between the first image regions 100a shown in Figure 11c, whereas the first image regions 100a are provided in the empty image region positions between the second image regions 150a shown in Figure 11d.

In order that the first image 100 is seen in transmission and the second image 150 is seen in reflection, the first image regions 100a may be provided in one of the security devices discussed above using a material, colorant or other marking which is relatively less reflective and relatively more transmissive than the material, colorant or marking by which the second image regions 100b are provided.

A further example of images which may be displayed by security devices according to the present invention, and the image regions applied to such security devices to create these images will now be discussed with reference to Figures 12a, 12b and 12c.

Figure 12a shows a first image 200 exhibited by a security device in transmission, the first image 200 comprising a positive image of the numeral "5".

As shown in Figure 12a, the upper half 210a of the number "5" of the first image 200 is provided in a first colour (i.e. a first hue or tint), and the lower half 210b of the "5" is provided in a second colour (i.e. a second hue or tint). Figure 12b shows a second image 250 exhibited by the security device in reflection, the second image comprises a negatively defined "5" of the same size as the "5" shown in the first image 200. Therefore, as the security device is viewed in transmission and then reflection the appearance of the security device changes from a positively defined "5" in two colours to a negatively defined "5" in a single colour.

Figure 12c shows an interleaved pattern 290 of elongate first and second image regions 200a, 250a which are applied to the image display region of a security device so as to generate the first and second images 200, 250 shown in Figures 12a and 12b respectively. These arrays of first and second image regions 200a, 250a may be constructed using the steps discussed above with reference to Figures 11a to 11e. The first image regions 200a may be printed using two or more materials, colourants or other markings so as to provide the difference in appearance between the upper and lower halves 210a, 210b of the "5" in the first image 200.

Again, in practice the dimensions of the first and second image regions 200a, 250a will be substantially smaller such that the first and second image regions shown in Figure 12c may not be individually distinguished to the naked eye in normal use. The image regions 200a, 250a shown in Figure 12c have been illustrated with relatively larger widths to provide further clarity.

It will be appreciated from the above examples that different aspects of the manufacturing process which results in a complete security device or security document can be performed separately from one another, potentially on different manufacturing lines and possibly by different entities. For instance, the manufacture of the security devices may be carried out by a first entity and the resulting product supplied as a security article such as a thread, strip, foil or patch, to another entity which has produced a security document (including the graphics thereon), which then applies or otherwise incorporates the security article into or onto the document.

Security devices of the sorts described above can be incorporated into or applied to any product for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc. The image regions and/or the complete security device can either be formed directly on the security document or may be supplied as part of a security article, such as a security thread or patch, which can then be applied to or incorporated into such a document.

Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6mm, are particularly useful as the additional exposed thread surface area enables the presently disclosed visual effects to be more readily observed.

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

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

The security device may also be applied to one side of a paper substrate, optionally so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

Examples of such documents of value and techniques for incorporating a security device therein will now be described with reference to Figures 13 to 16.

Figure 13 depicts an exemplary document of value 500, here in the form of a banknote. Figure 13a shows the banknote in plan view whilst Figure 13b shows a cross-section of the same banknote along the line X-X'. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 510. Two opacifying layers 520a and 520b are applied to either side of the transparent substrate 510, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 510.

The opacifying layers 520a and 520b are omitted across a selected region, which forms a window 530 within which a security device 540 is located. In Figure 13(b), a security device 540 is disposed within window 530, with the first and second image regions 540a, 540b arranged on a first surface 510a of the transparent substrate 510. It will be appreciated that the first and second image regions 540a, 540b could equally be provided on the second surface 510b of the transparent substrate 510, or the first and second image regions 540a, 540b could be provided on opposing surfaces 510a, 510b of the transparent substrate 510 (such that the first image regions 540a are provided on the first side 510a of the substrate, and the second image regions 540b on the second side 510b of the substrate, or vice versa).

It will be appreciate that, if desired, any or all of the window 530 could instead be a "half-window", in which an opacifying layer (e.g. 520a or 520b) is continued over all or part of the first and second image regions 540a, 540b. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers 520a and 520b are provided on both sides.

In Figure 14 the banknote 600 is a conventional paper-based banknote provided with a security article 601 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 620a and 620b lie on either side of the thread. This can be done using the techniques described in EP-A-0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 601 in window regions 630a, 630b, 630c of the banknote.

Alternatively the window regions 630a, 630b, 630c may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. It should be noted that it is not necessary for the window regions to be "full thickness" windows: the thread 601 need only be exposed on one surface if preferred. The security device 640 is formed on the thread 601, which comprises a transparent substrate 601a to which first and second image regions 640a, 640b have been provided on one side. Windows 630a, 630b, 630c reveal parts of the device 640, which may be formed continuously along the thread. In the illustration, the first and second image regions 640a, 640b are depicted as being continuous across the entire length of the thread 601, although in practice this may not be case and the first and second image regions 640a, 640b may only be formed discontinuously within or around the exposed regions of the thread 601. Alternatively several security devices could be spaced from each other along the thread, as in the embodiment depicted, with different or identical images displayed by each.

In Figure 15, the banknote 700 is again a conventional paper-based banknote, provided with a strip element or insert 701. The strip 701 is based on a transparent substrate and is inserted between two plies of paper 720a, 720b.

The security device 740 is formed by first and second image regions 740a, 740b applied to a first side of the strip substrate 701a. The paper plies 720a and 720b are apertured across region 730 to reveal the security device 740, which in this case may be present across the whole of the strip 701 or could be localised within the aperture region 730. It should be noted that one or more of the plies 720a, 720b need not be apertured and could be continuous across the security device.

As shown the strip 701 extends across the entire height of the banknote 700 between the top and bottom edges of the banknote as drawn, but this is not essential. Instead an insert with a similar structure may be provided which does not extend continuously to one or more of the boundaries of the banknote 700 A further embodiment is shown in Figure 16 where Figures 16(a) and (b) show the front and rear sides of a document 800 respectively, and Figure 16(c) is a cross section along line 77'. Security article 801 is a strip or band comprising a security device 840 according to any of the embodiments described above. The security article 801 is formed into a security document 800 comprising a fibrous substrate 820, using a method described in EP-A-1141480. The strip 801 is incorporated into the security document 800 such that it is fully exposed on one side of the document (Figure 16(a)) and exposed in one or more windows 830 on the opposite side of the document (Figure 16(b)). Again, the security device 840 is formed on the strip 801, which comprises a transparent substrate 801a with the first and second image regions 840a, 840b formed thereon or therein.

Alternatively a similar construction can be achieved by providing paper 800 with an aperture 830 and adhering the strip element 801 onto one side of the paper 800 across the aperture 800. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting.

In still further embodiments, a complete security device could be formed entirely on one surface of a security document which could be transparent, translucent or opaque, e.g. a paper banknote irrespective of any window region. The first and second image regions may be provided to a surface of the substrate (e.g. applying it directly thereto, or by forming it on another film which is then adhered to the substrate by adhesive or hot or cold stamping) or provided within the substrate (e.g. through the provision of dyes, pigments or other colorants or through a marking process such as laser marking).

The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

Additional optically variable devices or materials can be included in the security device such as thin film interference elements, liquid crystal material and photonic crystal materials. Such materials may be in the form of filmic layers or as pigmented materials suitable for application by printing. If these materials are transparent they may be included in the same region of the device as the security feature of the current invention or alternatively and if they are opaque may be positioned in a separate laterally spaced region of the device.

Methods of producing the security devices, articles and documents discussed above will now be discussed with reference to Figures 17a and 17b. Figures 17a and 17b show web-fed printing systems 900, 950 in which a web of substrate 900 travels in direction D. The system 900 shown in Figure 17a is capable of applying first and second image regions to a first side 910a of a document substrate 910 (e.g. a polymer or paper substrate).

Two print stations 920, 930 are located in contact with or adjacent to the first side 910a of the substrate web. At each print station 920, 930 one or more print workings or layers are applied to the substrate 910. Therefore, it will be understood that the print stations 920, 930 apply print workings to the first side 910a of the substrate 910 in sequence. The substrate may be the substrate of a security device, article or document.

For instance, when manufacturing a security device 10 as shown in Figures 1 to 3, the first print station 920 in direction D1 may apply a plurality of regions of at least semi-transparent material to the substrate 910 to form relatively transmissive and relatively unreflective first image regions. VVhereas the second print station 930 may apply a plurality of regions of at least semi-opaque material to the substrate 910 to form relatively reflective and relatively non-transmissive second image regions.

Alternatively, both the first and second image regions (and the at least semi-transparent material and at least semi-opaque material forming these regions) may be applied simultaneously to the substrate 900 by the single print station 960 shown in Figure 17b. The system shown 950 in Figure 17b may be more accurate than the distributed system 900 as it is difficult to accurately locate or align the different print station 920, 930 relative to each other.

Additionally, or alternatively, a printing system may comprise further print stations configured to apply print workings to the second side 910b of the substrate 900, or to apply one or more sub-layers of a multi-layered image region. These print workings may be applied simultaneously or sequentially as the remaining features of the security device.

In the examples discussed above, all of the steps described are web-based processes, i.e. layers and print workings are applied to a web of the substrate 910, e.g. in one in-line web-fed process. Typically a web with a large width (e.g. between 0.75 and 1.5 m) is supplied for this purpose.

However, in alternative embodiments of the present invention, security devices and security documents are produced using a combination of web fed and sheet fed process steps. For instance, for some process steps it is desirable to reduce the width of the web, e.g. so that shorter (and hence less costly) processing tools can be used. It is also desirable to carry out certain process steps on individual sheets of the material, rather than on a continuous web.

In some embodiments, at least a first print working is applied using web based processes. These print workings and opacifying layers are applied on one side of a polymer substrate web with a large width (e.g. between 0.75 and 1.5 m). The web is then divided into sheets for sheet fed printing whereby at least a second print working is applied in contact with or above a side of the substrate.

Converting a web substrate to a sheet substrate (i.e. dividing the web into sheets) may involve slitting the initial web along its longitudinal direction so as to reduce its width, subsequent processing steps utilising processing tools of correspondingly shorter width compared with those of the preceding steps. The slit webs are then converted in sheets by cutting it along its cross direction at intervals spaced in the longitudinal direction. This process is sometimes referred to as "sheeting". Each sheet will preferably be sized so as carry a plurality of the final documents. Subsequent printing processes and the application of further print workings may then be performed using sheet-fed machinery. The resulting sheets may be divided further before or after printing in order to produce the final documents as required. In other words, documents according to the present invention may be printed using a combination of web-feed and sheet-fed printing steps.

Alternatively, security devices, articles and documents according to the present invention may be solely constructed using sheet-fed processes. In such processes, all print workings and/or layers are applied to a stationary sheet of substrate. This sheet of substrate may be rotated or flipped to apply print The examples of security devices, articles and documents discussed above may comprise (i.e. be provided with) a variety of other layers including one or more: conductive layers to prevent static build up, protective layers intended to prevent or mitigate damage to or tampering with the first and second regions; adhesive layers; a transparent varnish potentially comprising a matting additive to reduce the gloss of a security document and/or to prevent the security documents sticking together when they are stacked; and spacing layers separating the layers and/or workings discussed above. These layers may be provided as part of a multi-layer substrate, between a substrate and the first and second image regions, separating the first and second region in a direction through the thickness of the security device, or over the first and second image regions relative to the substrate (i.e. such that the first and second image regions are between the substrate and the further layer).

Further aspects of the invention are defined in accordance with the following numbered clauses: Numbered Clause 1. A security device comprising: an at least semi-transparent substrate comprising opposed first and second sides, the substrate defining an image display area on or in which are provided: a plurality of first image regions; and a plurality of second image regions which are relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the first image regions and the second image regions is sized such that said image region is not individually perceptible to the human eye; the first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions; and wherein the first image and second image are different.

Numbered Clause 2. A security device according to any preceding numbered clause wherein the first image and the second image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.

Numbered Clause 3. A security device according to any preceding numbered clause wherein the first image is of a constant first tint and first hue, and the second image is of a constant second tint and second hue.

Numbered Clause 4. A security device according to any preceding numbered clause, wherein the first image regions comprise an at least semi-transparent material provided on the substrate.

Numbered Clause 5. A security device according to any preceding numbered clause wherein the second image regions comprise an at least semi-opaque material provided on the substrate.

Numbered Clause 6. A security device according to any preceding numbered clause, wherein the first image regions and second image regions are provided on the same side of the substrate.

Numbered Clause 7. A security device according to any of numbered clauses 1 to 5, wherein the first image regions and/or second image regions are provided in the substrate.

Numbered Clause 8. A security device according to numbered clause 7, wherein the first image regions and/or second image regions comprise one or more of an integral marking, ink, pigment, dye or colorant in the substrate.

Numbered Clause 9. A security device according to any preceding numbered clause, wherein the first image regions and second image regions are regions of a single layer.

Numbered Clause 10. A security device according to any of numbered clauses 1 to 8, comprising a continuous layer which is a least semi-transparent and extends across at least 75% of the image display area, wherein the second image regions comprise portions of an at least semi-opaque material which overlap the continuous layer, and wherein the first image regions are defined by the spacing between the portions of at least semi-opaque material.

Numbered Clause 11. A security device according to any preceding numbered clauses, wherein the first and second image regions are configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions.

Numbered Clause 12. A security device according to any preceding numbered clauses, wherein the first and second image regions are configured such that when the security device is viewed from the second side of the substrate in reflection the image display area exhibits the second image formed by the second image regions.

Numbered Clause 13. A security device according to any preceding numbered clause, wherein one or more of the second image regions comprises a second image sub-layer and a reflection enhancing sub-layer; wherein the second image sub-layers are configured to form the second image; and wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective reflection enhancing sub-layers and a viewer.

Numbered Clause 14. A security device according to any of numbered clauses 1 to 11, wherein one or more of the second image regions comprise a second image sub-layer, and a third image sub-layer; wherein the second image sub-layers are configured to form the second image; wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective third image sub-layers and the viewer; the sub-layers being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device viewed from the second side of the substrate in reflection the image display area exhibits the third image formed by the third image sub-layers; and wherein the first image and third image are different.

Numbered Clause 15. A security device according to numbered clause 14, wherein one or more of the second image regions further comprise a reflection enhancing sub-layer provided between the second image sub-layer and the third image sub-layer.

Numbered Clause 16. A security device according to either numbered clause 13 or 15, wherein the reflection enhancing sub-layer comprises: a metal; a metal alloy; a metallic ink; a binder comprising reflective particles; an optical interference thin film structure; a photonic crystal layer; or a liquid crystal layer.

Numbered Clause 17. A security device according to any of numbered clauses 14 to 16, wherein the second image sub-layers and/or the third image sub-layers are at least semi-transparent.

Numbered Clause 18. A security device according to any of numbered clauses 1 to 13, further comprising: a plurality of third image regions provided on or in the image display area, the third image regions being relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the third image regions is sized such that said third image region is not individually perceptible to the human eye; wherein the image regions are arranged such that, when viewed from the second side of the substrate, the third image regions are between the second image regions and the viewer; the first, second and third image regions being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device is viewed from the second side of the substrate in reflection the image display area exhibits a third image formed by the third image regions; and wherein the first image and third image are different.

Numbered Clause 19. A security device according to any of numbered clauses 14 to 18, wherein the first image and third image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; 20 number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.

Numbered Clause 20. A security device according to any of numbered clauses 14 to 19, wherein the second image and third image are different, and preferably provide different visual appearances, wherein more preferably the second image and third image differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.

Numbered Clause 21. A security device according to any preceding numbered clause wherein one or more of the first image regions, second image regions and third image regions are printed regions.

Numbered Clause 22. A security device according to any preceding numbered clause wherein each image region has a dimension of less than 100 pm in at least one direction in the plane of the security device, preferably less than 80 pm, more preferably less than 50 pm, more preferably still less than 25 5 pm.

Numbered Clause 23. A security device according to any preceding numbered clause, wherein the reflectance of the second and/or third image regions is on average at least 20% greater than the reflectance of the first image regions for light with wavelengths between 400nm and 700nm; more preferably at least 30% greater; more preferably still at least 50% greater.

Numbered Clause 24. A security device according to any preceding numbered clause, wherein the transmittance of the first image regions is on average at least 50% greater than the transmittance of the second and/or third image regions for light with wavelengths between 400nm and 700nm; more preferably at least 100%greater; more preferably still at least 120% greater.

Numbered Clause 25. A security device according to any preceding numbered clause, wherein one or more of the first image regions, second image regions and third image regions are line regions, square regions or circular regions.

Numbered Clause 26. A security device according to any preceding numbered clause, wherein the first image regions are interlaced with the second image regions and/or the third image regions Numbered Clause 27. A security device according to numbered clause 18 or any of numbered clauses 19 to 24 when dependent on at least numbered clause 18, wherein the first image regions, second image regions and third image regions are line regions and wherein the first and second image regions are interlaced, wherein the third image regions extend in a direction which is substantially perpendicular to the direction which the first and second line regions extend.

Numbered Clause 28. A security article comprising a security device according to any preceding numbered clause.

Numbered Clause 29. A security article according to numbered clause 28, wherein the security article is a thread, stripe, patch, foil, transfer foil or insert.

Numbered Clause 30. A security document comprising a security device according to any of numbered clauses 1 to 27 or a security article according to either of numbered clauses 28 or 29.

Numbered Clause 31. A security document according to numbered clause 30, wherein the security document is a banknote, cheque, identification document, certificate, share, stamp, visa, passport, licence, bank card, or ID 15 card.

Numbered Clause 32. A security document according to either of numbered clauses 30 or 31, comprising an at least semi-transparent window through which the security device or security article may be observed, the at least semi-transparent window being formed within an otherwise substantially opaque layer.

Numbered Clause 33. A method of manufacturing a security device comprising: providing an at least semi-transparent substrate comprising opposed first and second sides, the substrate defining an image display area; providing in or on the image display area: a plurality of first image regions; and a plurality of second image regions which are relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the first image regions and the second image regions is sized such said image region is not individually perceptible to the human eye; and the first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions; and wherein the first image and second image are different.

Numbered Clause 34. A method according to numbered clause 33, wherein the first image and the second image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.

Numbered Clause 35. A method according to numbered clause 33 or numbered clause 34, wherein the first image is of a constant first tint and first hue, and the second image is of a constant second tint and second hue.

Numbered Clause 36. A method according to any of numbered clauses 33 to 35, wherein providing the first image regions comprises applying an at least semi-transparent material on the substrate.

Numbered Clause 37. A method according to any of numbered clauses 33 to 36, wherein providing the second image regions comprises applying an at least semi-opaque material on the substrate.

Numbered Clause 38. A method according to any of numbered clauses 33 to 37, wherein the first image regions and second image regions are provided on the same side of the substrate.

Numbered Clause 39. A method according to any of numbered clauses 33 to 38, wherein the first image regions and/or second image regions are provided in the substrate.

Numbered Clause 40. A method according to any of numbered clauses 33 to 39, wherein providing the first image regions and/or second image regions comprises providing an integral marking, ink, pigment, dye or colorant in the substrate.

Numbered Clause 41. A method according to any of numbered clauses 33 to 40, wherein the first image regions and second image regions are interlaced regions of a single layer.

Numbered Clause 42. A method according to any of numbered clauses 33 to 41, further comprising providing a continuous layer which is a least semitransparent and extends across at least 75% of the image display area, wherein the second image regions comprise portions of an at least semi-opaque material which overlap the continuous layer, and wherein the first image regions are defined by the spacing between the portions of at least semi-opaque material.

Numbered Clause 43. A method according to any of numbered clauses 33 to 42, wherein the first and second image regions are configured such that when viewed from the second side of the substrate in transmission the security device exhibits the first image formed by the first image regions.

Numbered Clause 44. A method according to any of numbered clauses 33 to 43, wherein the first and second image regions are configured such that when viewed from the second side of the substrate in reflection the security device exhibits the second image formed by the second image regions.

Numbered Clause 45. A method according to any of numbered clauses 33 to 44, wherein one or more of the second image regions comprises a second image sub-layer and a reflection enhancing sub-layer; wherein the second image sub-layers and are configured to form the second image; and wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective reflection enhancing sub-layers and a viewer.

Numbered Clause 46. A method according to numbered clause 45 wherein providing a plurality of second image regions comprises: providing a reflection enhancing sub-layer continuously across the image display area on the substrate; applying the second image sub-layer on the reflection enhancing sub-layer in a pattern corresponding to the desired extents of the second image regions; and removing portions of the reflection enhancing layer outside the desired extents of the second image region, wherein the second image sub-layer is used as a resist.

Numbered Clause 47. A method according to any of numbered clauses 33 to 46, wherein one or more of the second image regions comprises a second image sub-layer, and a third image sub-layer; wherein the second image sub-layers and are configured to form the second image; wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective third image sub-layers and the viewer; the sub-layers being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device viewed from the second side of the substrate in reflection the image display area exhibits a third image formed by the third image sub-layers; and wherein the first image and third image are different.

Numbered Clause 48. A method according to numbered clause 47, wherein one or more of the second image regions further comprise a reflection enhancing sub-layer provided between the second image sub-layer and the third image sub-layer.

Numbered Clause 49. A method according to any of numbered clauses 45, 46 and 48, wherein the reflection enhancing sub-layer comprises: a metal; a metal alloy; a metallic ink; a binder comprising reflective particles; an optical interference thin film structure; a photonic crystal layer; or a liquid crystal layer.

Numbered Clause 50. A method according to any of numbered clauses 43 to 49, wherein the second image sub-layers and/or the third image sub-layers are at least semi-transparent.

Numbered Clause 51. A method according to any of numbered clauses 45 to 50, wherein providing the second image regions comprises the step of removing a portion of an image sub-layer and/or the reflection enhancing sublayer, wherein either the second image sub-layer or the third image sub-layer is used as a resist.

Numbered Clause 52. A method according to any of numbered clauses 33 to 46, further comprising: providing a plurality of third image regions on or in the image display area, the third image regions being relatively less transmissive and relative more reflective than the first image regions; wherein each image region of the third image regions is sized such that said image region is not individually perceptible to the human eye; wherein the image regions are arranged such that, when viewed from the second side of the substrate, the third image regions are between the second image regions and the viewer; the first, second and third image regions being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device is viewed from the second side of the substrate in reflection the image display area exhibits a third image formed by the third image regions.

wherein the first image and third image are different.

Numbered Clause 53 A method according to any of numbered clauses 47 to 52, wherein the first image and third image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or 5 photograph.

Numbered Clause 54. A method according to any of numbered clauses 47 to 53, second image and third image are different, and preferably provide different visual appearances, wherein more preferably the second image and third image differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.

Numbered Clause 55. A method according to any of numbered clauses 33 to 54, wherein providing at least one of the pluralities of image regions is performed using one or more of: printing; inkjet printing; dye sublimation; laser printing; lithographic printing; flexographic printing; intaglio printing; gravure printing; screen printing; letterpress printing; dye diffusion thermal transfer; or vapour deposition.

Numbered Clause 56. A method according to any of numbered clauses 33 to 55, wherein each image region has a dimension of less than 100 pm in at least one direction in the plane of the security device, preferably less than 50 pm, more preferably less than 25 pm.

Numbered Clause 57. A method according to any of numbered clauses 33 to 56, wherein the reflectance of the second and/or third image regions is on average at least 20% greater than the reflectance of the first image regions for light with wavelengths between 400nm and 700nm; more preferably at least 30% greater; more preferably still at least 50% greater.

Numbered Clause 58. A method according to any of numbered clauses 33 to 57, wherein the transmittance of the first image regions is on average at least 50% greater than the transmittance of the second and/or third image regions for light with wavelengths between 400nm and 700nm; more preferably at least 100% greater; more preferably still at least 120% greater.

Numbered Clause 59. A method according to any of numbered clauses 33 to 58, wherein one or more of the first image regions, second image regions and third image regions are line regions, square regions or circular regions.

Numbered Clause 60. A method according to any of numbered clauses 33 to 59, wherein the first image regions are interlaced with the second image regions and/or the third image regions.

Numbered Clause 61. A method according to numbered clause 52 or any of numbered clauses 53 to 60 when dependent on at least numbered clause 52, wherein the first image regions, second image regions and third image regions are line regions and the first and second image regions are interlaced, and wherein the third image regions extend in a direction which is substantially perpendicular to the direction which the first and second line regions extend.

Numbered Clause 62. A method of manufacturing a security article comprising a security device manufactured according to any of numbered clauses 33 to 61.

Numbered Clause 63. A method according to numbered clause 62, wherein the security article is a thread, stripe, patch, foil, transfer foil or insert.

Numbered Clause 64. A method of manufacturing a security document comprising a security device manufactured according to any of numbered clauses 33 to 61 or a security article manufactured according to any of numbered clauses 62 to 63.

Numbered Clause 65. A method according to numbered clause 61, wherein the security document is a banknote, cheque, identification document, certificate, share, stamp, visa, passport, licence, bank card, or ID card.

Numbered Clause 66. A method according to either of numbered clauses 64 or 65, comprising an at least semi-transparent window through which the security device or security article may be observed, the at least semi-transparent window being formed within an otherwise substantially opaque layer.

Claims

CLAIMS1. A security device comprising: an at least semi-transparent substrate comprising opposed first and second sides, the substrate defining an image display area on or in which are provided: a plurality of first image regions; and a plurality of second image regions which are relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the first image regions and the second image regions is sized such that said image region is not individually perceptible to the human eye; the first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions; and wherein the first image and second image are different.
2. A security device according to any preceding claim wherein the first image and the second image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.
3. A security device according to any preceding claim wherein the first image is of a constant first tint and first hue, and the second image is of a constant second tint and second hue.
4. A security device according to any preceding claim, wherein the first image regions comprise an at least semi-transparent material provided on the substrate.
5. A security device according to any preceding claim wherein the second image regions comprise an at least semi-opaque material provided on the substrate.
6. A security device according to any preceding claim, wherein the first image regions and second image regions are provided on the same side of the substrate.
7. A security device according to any of claims 1 to 5, wherein the first image regions and/or second image regions are provided in the substrate.
8. A security device according to claim 7, wherein the first image regions and/or second image regions comprise one or more of an integral marking, ink, pigment, dye or colorant in the substrate.
9. A security device according to any preceding claim, wherein the first image regions and second image regions are regions of a single layer.
10. A security device according to any of claims 1 to 8, comprising a continuous layer which is a least semi-transparent and extends across at least 75% of the image display area, wherein the second image regions comprise portions of an at least semi-opaque material which overlap the continuous layer, and wherein the first image regions are defined by the spacing between the portions of at least semi-opaque material.
11. A security device according to any preceding claims, wherein the first and second image regions are configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions.
12. A security device according to any preceding claims, wherein the first and second image regions are configured such that when the security device is viewed from the second side of the substrate in reflection the image display area exhibits the second image formed by the second image regions.
13. A security device according to any preceding claim, wherein one or more of the second image regions comprises a second image sub-layer and a reflection enhancing sub-layer; wherein the second image sub-layers are configured to form the second image; and wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective reflection enhancing sub-layers and a 10 viewer.
14. A security device according to any of claims 1 to 11, wherein one or more of the second image regions comprise a second image sub-layer, and a third image sub-layer; wherein the second image sub-layers are configured to form the second image; wherein the sub-layers are provided such that, when the security device is viewed from the first side of the substrate, the second image sub-layers are positioned between the respective third image sub-layers and the viewer; the sub-layers being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device viewed from the second side of the substrate in reflection the image display area exhibits the third image formed by the third image sub-layers; and wherein the first image and third image are different.
15. A security device according to claim 14, wherein one or more of the second image regions further comprise a reflection enhancing sub-layer provided between the second image sub-layer and the third image sub-layer.
16. A security device according to either claim 13 or 15, wherein the reflection enhancing sub-layer comprises: a metal; a metal alloy; a metallic ink; a binder comprising reflective particles; an optical interference thin film structure; a photonic crystal layer; or a liquid crystal layer.
17. A security device according to any of claims 14 to 16, wherein the second image sub-layers and/or the third image sub-layers are at least semitransparent
18. A security device according to any of claims 1 to 13, further comprising: a plurality of third image regions provided on or in the image display area, the third image regions being relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the third image regions is sized such that said third image region is not individually perceptible to the human eye; wherein the image regions are arranged such that, when viewed from the second side of the substrate, the third image regions are between the second image regions and the viewer; the first, second and third image regions being configured such that when the security device is viewed from the second side of the substrate in transmission the image display area exhibits the first image formed by the first image regions, and when the security device is viewed from the second side of the substrate in reflection the image display area exhibits a third image formed by the third image regions; and wherein the first image and third image are different.
19. A security device according to any of claims 14 to 18, wherein the first image and third image provide different visual appearances, and preferably differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.
20. A security device according to any of claims 14 to 19, wherein the second image and third image are different, and preferably provide different visual appearances, wherein more preferably the second image and third image differ by at least one: hue; tint; logo; symbol; letter; number; alphanumeric text; indicia; security pattern; portrait; graphic; or photograph.
21. A security device according to any preceding claim wherein one or more of the first image regions, second image regions and third image regions are printed regions.
22. A security device according to any preceding claim wherein each image region has a dimension of less than 100 pm in at least one direction in the plane of the security device, preferably less than 80 pm, more preferably less than 50 pm, more preferably still less than 25 pm.
23. A security device according to any preceding claim, wherein the reflectance of the second and/or third image regions is on average at least 20% greater than the reflectance of the first image regions for light with wavelengths between 400nm and 700nm, more preferably at least 30% greater; more preferably still at least 50% greater.
24. A security device according to any preceding claim, wherein the transmittance of the first image regions is on average at least 50% greater than the transmittance of the second and/or third image regions for light with wavelengths between 400nm and 700nm; more preferably at least 100%greater; more preferably still at least 120% greater.
25. A security device according to any preceding claim, wherein one or more of the first image regions, second image regions and third image regions are line regions, square regions or circular regions.
26. A security device according to any preceding claim, wherein the first image regions are interlaced with the second image regions and/or the third image regions
27. A security device according to claim 18 or any of claims 19 to 24 when dependent on at least claim 18, wherein the first image regions; second image regions and third image regions are line regions and wherein the first and second image regions are interlaced, wherein the third image regions extend in a direction which is substantially perpendicular to the direction which the first and second line regions extend.
28. A security article comprising a security device according to any preceding claim.
29. A security article according to claim 28, wherein the security article is a thread, stripe, patch, foil, transfer foil or insert.
30. A security document comprising a security device according to any of claims 1 to 27 or a security article according to either of claims 28 or 29.
31. A security document according to claim 30, wherein the security document is a banknote, cheque, identification document, certificate, share, stamp, visa, passport, licence, bank card, or ID card.
32. A security document according to either of claims 30 or 31, comprising an at least semi-transparent window through which the security device or security article may be observed, the at least semi-transparent window being formed within an otherwise substantially opaque layer.
33. A method of manufacturing a security device comprising: providing an at least semi-transparent substrate comprising opposed first and second sides, the substrate defining an image display area; providing in or on the image display area: a plurality of first image regions; and a plurality of second image regions which are relatively less transmissive and relatively more reflective than the first image regions; wherein each image region of the first image regions and the second image regions is sized such said image region is not individually perceptible to the human eye; and the first and second image regions being configured such that when the security device is viewed from the first side of the substrate in transmission the image display area exhibits a first image formed by the first image regions, and when the security device is viewed from the first side of the substrate in reflection the image display area exhibits a second image formed by the second image regions; and wherein the first image and second image are different.
34. A method according to claim 33, wherein the security device is a security device according to any of claims 1 to 27.