AU2016330007B2

AU2016330007B2 - Security print media and method of manufacture thereof

Info

Publication number: AU2016330007B2
Application number: AU2016330007A
Authority: AU
Inventors: Malcolm Baker; Alan KEEN
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2015-09-29
Filing date: 2016-09-27
Publication date: 2021-10-28
Anticipated expiration: 2036-09-27
Also published as: EP3356154A1; MA43351A; CA2999833A1; WO2017055825A1; GB2542785B; GB2542785A; AU2016330007A1; EP3356154B1; GB201517152D0

Abstract

A security print medium is disclosed for forming security documents therefrom. The security print medium comprises a transparent or translucent polymer substrate having first and second opposing surfaces, and at least one opacifying layer disposed on the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material. The security print medium further comprises a first print of a multi-tonal image disposed on the first and/or second surface of the polymer substrate, the first print being covered from the point of view of an observer on at least a first side of the security print medium by at least one of the opacifying layers which is disposed continuously across the multi-tonal image, and a second print of the multi-tonal image formed as a screened working comprising an array of screen elements in a colour which contrasts with that of the continuous opacifying layer. The second print is disposed on top of the continuous opacifying layer from the point of view of the observer and in alignment with the first print. When the security print medium is viewed by the observer in reflected light, the second print dominates the appearance of the multi-tonal image and when the security print medium is viewed by the observer in transmitted light, the first print dominates the appearance of the multi-tonal image.

Description

SECURITY PRINT MEDIA AND METHOD OF MANUFACTURE THEREOF

The present invention relates to security print media suitable for use in making security documents such as banknotes, identity documents, passports, certificates and the like, as well as methods for manufacturing such security print media, and security documents made from the security print media.

To prevent counterfeiting and enable authenticity to be checked, security documents are typically provided with one or more security elements which are difficult or impossible to replicate accurately with commonly available means, particularly photocopiers, scanners or commercial printers. Some types of security element are formed on the surface of a document substrate, for example by printing onto and/or embossing into a substrate such as to create fine-line patterns or latent images revealed upon tilting, whilst others including diffractive optical elements and the like are typically formed on an article such as a security thread or a transfer foil, which is then applied to or incorporated into the document substrate. A still further category of security element is that in which the security element is integrally formed in the document substrate itself. A well-known example of such a feature is the conventional watermark, formed in paper document substrates by controlling the papermaking process to as to vary the density of the paper fibres as they are laid down in accordance with a desired image. Techniques have been developed which can achieve highly intricate, multi-tonal watermarks which become visible when the substrate is viewed in transmitted light. Security elements such as watermarks which are integral to the document substrate have the significant benefit that they cannot be detached from the security document without destroying the integrity of the document.

Polymer document substrates, comprising typically a transparent or translucent polymer substrate with at least one opacifying layer coated on each side to receive print, have a number of benefits over conventional paper document substrates including increased lifetime due to their more robust nature and resistance to soiling. Polymer document substrates also lend themselves well to certain types of security features such as transparent windows which are more difficult to incorporate in paper-based documents. However, due to the non-fibrous construction of polymer substrates, conventional watermarking techniques are not available and as such the potential for forming security elements integrally in the substrate itself is limited. Instead, for polymer security documents, security elements are typically applied after the document substrate has been manufactured, for example as part of a subsequent security printing process line, or by the application of a foil.

It would be desirable to provide a polymer document substrate - i.e. a security print medium, which can then be printed upon and otherwise processed into a security document - with an integral security feature, to enhance the security of the document substrate itself, and ultimately of security documents formed from it.

In accordance with the present invention a security print medium for forming security documents therefrom comprises a transparent or translucent polymer substrate having first and second opposing surfaces, and at least one opacifying layer disposed on the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material, the security print medium further comprising a first print of a multi-tonal image disposed on the first and/or second surface of the polymer substrate, the first print being covered from the point of view of an observer on at least a first side of the security print medium by at least one of the opacifying layers which is disposed continuously across the multi-tonal image, and a second print of the multi-tonal image formed as a screened working comprising an array of screen elements in a colour which contrasts with that of the continuous opacifying layer, the second print being disposed on top of the continuous opacifying layer from the point of view of the observer and in alignment with the first print, whereby when the security print medium is viewed by the observer in reflected light, the second print dominates the appearance of the multi- tonal image and when the security print medium is viewed by the observer in transmitted light, the first print dominates the appearance of the multi-tonal image. The present invention also provides a method of making a security print medium comprising: providing a transparent or translucent polymer substrate having first and second opposing surfaces; applying a first print of a multi-tonal image onto the first and/or second surface of the polymer substrate; applying at least one opacifying layer onto the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material, wherein the first print is covered from the point of view of an observer on a first side of the security print medium by at least one of the opacifying layers which is disposed continuously across the multi-tonal image; and applying a second print of the multi-tonal image on top of the continuous opacifying layer from the point of view of the observer and in alignment with the first print, wherein the second print is formed as a screened working comprising an array of screen elements in a colour which contrasts with that of the continuous opacifying layer, whereby when the security print medium is viewed by the observer in reflected light, the second print dominates the appearance of the multi-tonal image and when the security print medium is viewed by the observer in transmitted light, the first print dominates the appearance of the multi- tonal image.

As in conventional polymer document substrates (security print media), the primary function of the opacifying layers (which are typically formed of a polymeric, non- fibrous, light-scattering material) is to render the majority of the document non- transparent and to provide a suitable background on which to print graphics, security patterns and other information as may be required on the finished security document. However, the presently disclosed security print media also provides a security feature which resembles the integral security features, such as watermarks, that are common in fibrous document substrates. Specifically, the presently disclosed security print media provides that, when viewed in reflection, the second print, i.e. a screened working of a multi-tonal image, is visible and dominates the appearance of the security print media, but when held up to the light so as to be viewed in transmission, provides that the multi-tonal image of the first print, which was obscured in reflection by the opacifying layer(s) and the second print, is revealed and displayed to the viewer by light passing through the security print media.

By "screened working", it is meant a layer formed of an array of screen elements whose characteristics may be spatially modulated across the layer so as to provide regions of visual contrast. For example, the screen elements (or analogously the background surrounding the elements) may vary in their size, thickness, spacing, ink density, colour, tone, hue and/or saturation. In the present case, the screened working of the multi-tonal image is a version of the image formed of an array of screen elements, whose shape, and placement form the version of the image. Specifically, the second print is a screened working, and comprises an array of print elements which together form a version of the image.

In contrast to the screened working of the multi-tonal image, the first print of the multi-tone image is a print located on the surface of the substrate and covered by the opacifying layers. The print is a layer of at least semi-transparent or semi- translucent material which, in its most basic form, affects the intensity of light passing through it by different amounts for all or a selection of the wavelengths of visible light in different lateral locations to produce the appearance of a multi-tonal image.

It will be appreciated that the term "print" is intended to cover an image formed of a composition such as ink applied by any technique including conventional printing methods such as gravure, flexographic printing, lithography etc., but also ablation methods in which an all-over ink layer is applied and then selectively removed to leave an image, e.g. using a laser. It will also be appreciated that the first print and/or the at least one opacifying layer need not be in direct contact with the surface of the polymer substrate. Rather, one or more additional (transparent or translucent) layers could be present between the polymer substrate and the first print and/or opacifying layers, such as a primer layer, the opacifying layers still being considered disposed "on" the substrate surface. Similarly, the second print need not be directly in contact with the continuous opacifying layer, and, again, one or more additional (transparent or translucent) layers could be present between them.

In some embodiments, the first print of the multi-tonal image is a multi-coloured print, preferably comprising multiple workings and preferably comprising multiple workings in different colours. By providing a multi-coloured multi-tonal image as the print which is covered by the screened working, a very striking transition can be achieved when the viewer switches from viewing in reflection to viewing in transmission and vice versa, with the image displayed changing between that presented by the single screened working in reflection and the multi-coloured, multi- toned version in transmission. While preferable, a monochromatic, multi-toned print may instead be used. As mentioned, it is preferable that the multi-colour print is formed by multiple print workings applied to the substrate. This maybe in the form of a first half of the image being formed of a first working in a first colour and a second half of the working formed in a second colour, or of a second, and optionally a third, monochromatic print of the multi-tone image being applied over and in register with a first monochromatic print of the multi-tone image, in different colours, to produce a single multi-colour depiction of the multi-tone image. Preferably, the at least one opacifying layer further includes one or more opacifying layers disposed continuously across the first print of the multi-tone image so as to additionally cover the first print from the point of view of an observer on the second side of the security print medium. In such embodiments, the image is concealed from the side of the medium not intended for viewing, which results in a more striking appearance of the image when viewed as intended. While preferable, in alternative embodiments, the first print may also be uncovered from the point of view of an observer on the second side of the security print medium. In many embodiments, it is desirable to provide at least one opacifying layer on each side of the substrate, for example, to enable later printing of both sides of the document. Further, it is generally preferable to provide both sides of the document with approximately the same number of opacifying layers so as to give a consistent appearance between the two sides of the document. However, these are not essential features of the invention. In other embodiments, all opacifying layers may be provided on one side of the document or more opacifying layers may be provided on one side than the other..

As mentioned above, the term 'print' is intended to be construed broadly, and there are many types of print, and corresponding methods for applying said prints, which would be suitable for forming the second print; however, in particularly preferable embodiments, the second print is an intaglio print, i.e. formed using an intaglio printing method. While intaglio is preferable for reasons including that it enables the printed screen working to be provided with a raised tactile feel, and that there is a feathering effect of the lines which is characteristic of intaglio printing and provides an additional authentication over more commercial printing techniques used in the packaging industry, other types of print and corresponding printing methods may alternatively be used.

While embodiments require that the first print is covered by at least one opacifying layer which, typically this opacifying layer will extend across substantially the entire surface of the substrate, since opacifying layers are normally used to provide a suitable surface across the entire medium for printing and applicating of security document features. Generally, each surface of the security document will be coated in multiple layers to give the medium a high opacity. However, if the multiple opacifying layers cover the first print, they will act to reduce the intensity of the image when viewed in transmission. It is therefore preferable that the at least one opacifying layer further includes one or more opacifying layers each defining at least one gap, laterally inside which the multi-tonal image is located. By providing that an additional opacifying layer has a gap, in which the multi-tonal image is located, the opacifying layer can be used to increase the opacity of only regions away from the multi-tonal image, thereby maintaining a high intensity appearance of the multi-tonal image in transmission. These layers may be applied to either or both sides of the substrate in any number. In some embodiments, one or more such opacifying layers are provided defining a gap that reveals the first print of the multi-tone image from the point of view of an observer on a second side of the security print medium.

In some embodiments, a raised pattern layer is applied to the outermost opacifying layer on one or both sides of the substrate, the raised pattern layer comprising an array of screen elements which are sufficiently large to be individually discernible to the naked eye, the raised pattern layer preferably being tactile and/or of varying visibility depending on the viewing angle. The raised pattern layer may be applied to any portion of the upper and/or lower surface of the substrate, however preferably it is applied in the region of the multi-tonal image. The raised pattern layer may cooperate with the second print in either or both of a visual and tactile manner. In particularly preferable embodiments, the second print is the raised pattern layer, which is to say that second print is raised with respect to the surface onto which it is applied, and is preferably tactile.

It is preferable that at least some of the screen elements of the screened working have dimensions which render them individually discernible to the naked eye. It is generally considered that the naked eye can resolve individual objects down to a scale of about 40 μηι when viewed from a distance of about 15 cm. However, it will be appreciated that it is preferable that at least some of the screen elements of the screened working have a smallest dimension in a direction along the first or second surface of the polymer substrate of 100 micrometres or more, more preferably 200 micrometres or more, such that the screen elements are comfortably visible to the naked eye.

In particularly preferable embodiments, the screened working is a multi-tonal screened working. There are a number of ways that this can be achieved, but preferably, the multiple tones of the multi-tonal screened working are provided by variations in the dimensions and/or frequency of the screen elements. For example, the size of the screen elements may be increased without altering their spacing, thereby increasing the surface area of the elements of the second print and providing a darker tone to the region. Alternatively, the frequency, i.e. the number of equal sized screen elements per unit area, may be increased to achieve the same effect. Further, a combination of these two effects may be used to provide regions of different tone. It will be appreciated that regions of different tone need not be discrete, and the tone could change according to a gradient, e.g. the screen elements could be provided to produce a half-tone effect.

It is preferable, that the screen elements of the screened working comprise one or more of lines, dots, squares and indicia. For example, the screened working may be a line screen comprising an array of lines whose delineations produce a version of the multi-tonal image, with the thickness of the lines varying to provide multiple tones to a multi-tonal screened working.

So that the screened working is distinct from elements of any other layer, it is preferable that the screen elements have an average area in a plane parallel to the first or second surface of the polymer substrate greater than the average area in a plane parallel to the first or second surface of the polymer substrate of elements of any element structure in any other layer.

It may also be preferable to apply at least one opacifying layer across the substrate in the form of an array of screen elements which are too small to be individually discernible to the naked eye. This will typically be the case where the layer is applied by gravure printing with a cell size too small for individual recognition by the naked eye. Alternatively, all opacifying layers may be substantially continuous layers of opacifying material.

Preferably, at least one opacifying layer is a printed opacifying layer, preferably applied to the substrate by gravure printing. While it is preferable that the opacifying layers are printed, any method of application may be used. As mentioned above, polymer document substrates have a number of advantages over conventional paper substrates, and it is preferable that these advantages are also be exhibited by the opacifying layer. Accordingly, preferably the or each opacifying layer is polymeric and non-fibrous, and will scatter light (as opposed to allowing clear light transmission therethrough), and will be translucent to a degree. In some preferred cases at least one of the opacifying layers will contain electrically conductive particles (desirable to reduce the effects of static charge), whereas others will not.

It is also preferable that all of the opacifying layers are substantially the same colour as one another, most preferably a light and bright colour such as white (including off-white) or grey to provide an ideal surface on which additional printing or applicating can be performed in the manufacturing of a security document. While preferable, opacifying layers of different colours may be used. In some embodiments, the screen elements of the screened working are substantially all of the same colour. This will provide that a monochromatic image is visible in reflection, which can lead to visually striking transitions, in particular if the multi-tonal image seen in transmission is a multi-coloured, multi-tonal image. In some embodiments the second print is registered to the first print. This is achieved during the making of the security print media, by using registration techniques when applying the second print to the substrate such that it is in precise alignment with the first print of the multi-tonal image. In all of the above embodiments, the multi-tonal image can depict anything, however, the effect is most visually striking when the multi-tonal image comprises an image of a three-dimensional object, preferably a geometrical solid or wireframe model, a person, an animal, a building or other architectural structure or a three- dimensional logo.

The method of making a security print medium already introduced above can be adapted to make any of the preferred features described above. The invention further provides a security document comprising a security print medium as described above, and at least one graphics layer applied on the outermost opacifying layer(s) on the first and/or second surfaces of the polymer substrate. The security document could be for example any of: a bank note, an identification document, a passport, a licence, a cheque, a visa, a stamp or a certificate. A corresponding method of manufacturing a security document comprises making a security print medium in accordance with the above-described method; and applying at least one graphics layer to the outermost opacifying layer(s) on the first and/or second surfaces of the polymer substrate. Typically the step of applying at least one graphics layer to the outermost opacifying layers will be carried out in a separate manufacturing process (e.g. at a different manufacturing facility and possibly by a different entity) from the manufacture of the security print media itself. However, the at least one graphics layer may preferably be applied in register with the multi-tone image and or the screened working in the opacifying layers so as to achieve a visual co-operation between the graphics layer and the multi-tone image and/or screened working. This can be achieved by using a sensor such as a camera system to detect the location of the multi-tone image and/or screened working and adjust the position of the applied graphics layer accordingly. The graphics layer can be applied using any available printing process such as gravure, flexographic, lithographic or intaglio printing, for example. The graphics layer may typically include security patterns such as fine line patterns or guilloches, information as to the nature of the security document such as denomination and currency identifiers for a banknote, and/or personalisation information such as a serial number on a banknote or bibliographic data of the holder on a passport. Examples of security print media in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a first embodiment of a security print medium (a) in plan view, before application of any opacifying layers, (b) in cross-section, before application of any opacifying layers, (c) in plan view, after the application of opacifying layers, and (d) in cross-section, after the application of opacifying layers, layers of the security print medium being shown spaced apart for clarity;

Figure 2 shows schematically a second embodiment of a security print medium, with each layer applied to the substrate being depicted individually in plan view;

Figure 3 shows a third embodiment of a security print medium (a) in plan view, before application of any opacifying layers, (b) in cross-section, before application of any opacifying layers, (c) in plan view, after the application of opacifying layers, and

(d) in cross-section, after the application of opacifying layers;

Figure 4(a) shows an example of a raised pattern layer, and Figure 4(b) shows a cross-section through a security print medium according to the embodiment first embodiment featuring the raised pattern layer; and

Figure 5 shows a first embodiment of a security document (a) in plan view, and (b) in cross-section, layers of the security document being shown spaced apart for clarity.

The description below will focus on examples security print media used in the production of banknotes. However, as mentioned above, the security print media could be used to form any type of security document, including passports (or individual pages thereof), identification cards, certificates, cheques and the like. Throughout this disclosure, the term "security print media" is used synonymously with the term "document substrate", meaning a medium which can then be printed upon and otherwise processed to form the desired security document, in a manner analogous to the printing and subsequent processing of a conventional paper substrate (albeit with processes adapted for use on polymer). Hence a "security print medium" does not encompass graphics layers and the like, which are later printed onto the security print medium to provide security patterns, indicia, denomination identifiers, currency identifiers etc. The combination of such a graphics layer and a "security print medium" (and optionally additional features such as applied foils, strips, patches etc.) is the "security document". Throughout the following examples, the security print medium will be illustrated as having the same size and shape as a security document into which it is later formed. However, typically the security print medium will be formed as a web or sheet large enough to carry multiple repeats of the desired security document, and will then be cut into individual document either before, but more usually after, printing of the graphics layer and any other required processing steps.

Figure 1 shows a first embodiment of a security print medium 1. Figures 1 (a) and 1 (b) show the security print medium before the application of any opacifying layers and before application of the second print, with Figure 1 (a) showing a plan view of the security print medium, and Figure 1 (b) showing a cross-section along the line X- X'. It will be appreciated that in Figure 1 (b) the layers forming the security print medium 1 are shown spaced apart for clarity whereas in practice all of the layers will contact one another and form a cohesive unit. The same applies to all other cross- sections shown in other Figures.

As shown in Figures 1 (a) and 1 (b), the security print medium 1 comprises a polymer substrate 5, which is transparent (i.e. optically clear, but may be tinted) or translucent (i.e. optically scattering, but non-opaque). The polymer substrate 5 may be monolithic or could be multi-layered and may carry additional layers on its first and/or second surfaces 5a, 5b such as a primer layer for improving the adhesion of outer layers. The polymeric substrate may comprise BOPP or polycarbonate, for example.

The polymer substrate 5 has opposing upper and lower surfaces 5a, 5b. In a region, on the upper surface 5a, there is disposed a multi-tonal image 10 in the form of a single print working 10a (the first print). In this example, the multi-tone image 10 is of a hemisphere; the circular print working 10a is a halftone print working, and is provided with a lighter tone towards its centre, and a darker tone towards its edges to give the impression to the viewer that the centre is closer than the edges. The print working 10a may be formed by a conventional colour ink using a process such as gravure printing or flexographic printing. The ink may be selected so as to be translucent to allow the colour to be visible in transmitted light. As shown in Figure 1 (c), substantially all of the medium 1 carries a coating 6 formed of a plurality of opacifying layers as described further below. This renders the medium non-transparent across the whole of the coated area and provides a suitable background for printing thereon. The coating 6 may optionally be omitted in certain areas of the medium to form features such as strip 2 and window 3, which are transparent or translucent (relative to the coated areas). Such transparent areas may be provided as security features in their own right or may be later equipped with additional security devices during the manufacture of a security document using the medium 1 , as described further below.

As shown in Figure 1 (d), the opacifying coating 6 is formed by first and second opacifying layers 6a, 6b, disposed on the upper and lower surfaces 5a, 5b of the substrate 5 respectively. While one opacifying layer is provided on each side in this case, more than one opacifying layer may be used to increase the optical density of the coating 6. Each opacifying layer comprises a translucent, semi-opaque material which is preferably polymeric and non-fibrous, e.g. white ink. The opacifying layers are each preferably substantially the same colour as one another (and are spatially uniform in colour), most preferably white or another light colour such as off-white or grey so that a later-applied graphics layer will contrast well against it. In preferred examples, the opacifying layers each have a brightness L* in CIE L*a*b* colour space of at least 70, preferably at least 80 and more preferably at least 90.

The opacifying layer 6a on the upper side 5a of the substrate 5 entirely covers the multi-tone image from the point of view of an observer viewing the upper side 5a of the substrate 5. While in this embodiment, the opacifying layer covers substantially all of the upper side 5a of the substrate 5, with the only gaps being those for additional security features such as strip 2 and window 3, it will be appreciated that the opacifying layer need only cover the print working 10a of the multi-tonal image 10, outside of the area directly over the multi-tonal image, the opacifying layer may cover anywhere between none and all of the rest of the surface of the substrate 5.

The opacifying layer 6b on the lower side 5b of the substrate is identical to the upper opacifying layer 6a, and covers substantially all of the lower side 5b of the substrate, with the only gaps being those for additional security features such as strip 2 and window 3. However, it will be appreciated that this opacifying layer is optional, and may be completely absent, or may cover any amount of the lower side of the substrate 5 up to an entire coating of the lower side 5b.

In this embodiment, each opacifying layer 6a, 6b is laid down in a substantially homogenous manner so as to uniformly cover the desired portions of the substrate 5, at least on a macroscopic scale which is visible to the naked eye. In practice such layers may be formed by gravure printing for example, which involves applying the opacifying material from an array of cells, the size of which is typically too small for any resulting pattern structure to be visible to the naked eye. The opacifying layers 6a, 6b and print working 10a should preferably be applied in register with one another, as may be achieved by applying all in the same in-line process. As already mentioned, additional layers such as a primer could be applied to the substrate before the opacifying layers (and any optional print workings). Further layers could be applied to the outside of the opacifying layers, such as a protective layer (preferably transparent) or a print-receptive coating.

As further shown in Figures 1 (c) and 1 (d), a print in the form of a screened working 7 (second print) of the multi-tonal image is disposed on top of the opacifying layer 6a from the point of view of an observer viewing the upper side 5a of the substrate 5, the screened working of the multi-tonal image being in registered alignment with the print working 10a of the multi-tonal image. The screened working is an array of screen elements in a colour which contrasts with that of the opacifying layer 6a, and which form a version of the multi-tonal image 10. Specifically, the screened working is a line screen comprising straight lines of print material that run along the surface of the opacifying layer 6a. Each screen element has a width of approximately 200 μηι, and is separated from the adjacent screen element by a gap of approximately 150 μηι, thereby rendering the individual screen elements individually discernible to the naked eye. As mentioned above, the line screen 7 is in registered alignment with the multi-tonal image, in an area directly over the multi-tonal image 10, which means that the line elements are arranged in the form of a circle, matching the outline of the multi-tonal image 10, with placement being registered to that of the print working 10a.

When the upper side of the security medium 1 is viewed in reflection, the opacifying coating 6 appears as a substantially continuous white or grey surface. In the region of the multi-tonal image 10, the opacifying layer 6a and screened working obscure the multi-tonal image 10, and the appearance is dominated by light reflecting off the line elements of the screened working 7. Since the screen elements are visible to the naked eye, the viewer sees, in the region of the multi-tonal image 10, a version of the multi-tonal image formed by the screen elements of the screen working 7, i.e. they see a circle formed of a series of line elements. When the upper side of the security medium 1 is viewed in transmission (i.e. against a backlight), light passes through the security medium 1 , and in particular, through the print working 10a of the multi-tonal image 10 and through the opacifying layer 6a. When the viewer looks in the region of the multi-tonal image, a bright central region is visible, corresponding to the lighter areas of the multi-tonal image 10, contrasted against a darker outer region, corresponding to the darker areas of the multi-tonal image. The modulation of light passing through the security medium 1 by the print working 10a of the multi-tonal image 10 dominates the appearance of security medium, overwhelming the visibility of the screened working 7.

Figure 2 shows a second embodiment of the invention formed based on the same principles as described in relation to the first embodiment. The construction of the security print medium 1 is largely the same as previously described, common components being denoted in the Figures using the same reference numerals as used above.

In this embodiment, the multi-tonal image is formed by three print workings 10a, 10b, 10c applied one on top of the other. In this example, the multi-tonal image 10 is a multi-coloured portrait of a person. Each print working 10a, 10b, 10c is a halftone image in a different component colour, e.g. print working 10a is in yellow, print working 10b is in cyan, and print working 10c is in magenta, which together form a multi-colour, multi-tonal image. While yellow, cyan and magenta are used in this embodiment, many combinations of print workings may be used to achieve different appearances of the multi-tonal image. For example, a black halftone print working may be used in combination with one or more print workings of other colours, with the black print working providing additional shading to the multi-tone image, e.g. being provided only in the portions of the image which are intended to have the darkest tone. In another example, a second working could be in a different colour to a first, and configured to provide different elements of the multi-tone image - e.g. the first working could be provided only in a first half on the image and the second working in a second half to provide two halves in different colours - or could overlap with the first to provide an intermediate colour such as orange where the first working is red and the second yellow. Alternatively still, the workings 10a, 10b, 10c could be identical and in the same colour, to increase the intensity of the colour.

Over the multi-tonal image, on the upper side 5a of the substrate 5, is provided a series of opacifying layers 6a, 6c, 6e, each extending over the entire upper side of the substrate. Similarly, on the lower side 5b of the substrate 5, there is provided a series of opacifying layers 6b, 6d, 6f each extending over the entire lower side of the substrate. It should be appreciated that whilst in practice the opacifying layers will typically be white, here the opacifying material is illustrated as black in order to be visible in the Figure. On the outermost opacifying layer on the upper side 5a of the substrate 5, there is disposed a print in the form of a screened working 7 of the portrait which is the multi- tonal image 10. The screened working is comprised of an array of indicia arranged in accordance with the portrait to form a representation of that portrait. The screened working being positioned directly over and in registered alignment with the multi-tonal image 10 beneath the opacifying layers 6a, 6c, 6e. In this example, the screened working 7 is a multi-tonal screened working. The multiple tones of the multi-tonal screened working 7 are provided by incremental changes in the weight and density of the indicia used as the screen elements; in areas intended to be darkest in tone, the surface area of each indicium is greater, and the indicia are positioned closer together to increase the covering of that area with print material. The black colour of the print material used for the screened working results in the areas having the most print material appearing darkest when viewed in reflection.

As in the embodiment of Figure 1 , when viewed in reflection, the screened working 7 of the multi-tonal image 10 dominates the appearance of the medium. In this embodiment the screened working is multi-tonal, and displays a monochromatic version of the portrait which is the multi-tonal image 10 to the viewer. When the medium is switched to being viewed in transmission the multi-coloured, multi-tonal image 10 is displayed to the viewer as light passes through the medium 1 and, in particular, through the print workings 10a, 10b, 10c. In this embodiment, a very striking transition is observed between a monochromatic, multi-tonal image made up of individually visible elements in reflection, and a multi-coloured version of the same image in the same location in transmission. The embodiment of Figure 3 shows a third embodiment of the invention. This embodiment is similar to the embodiment of Figure 1 , and the differences will now be described.

This embodiment features a second print working 10b on the lower surface 5b of the substrate 5, as shown in Figure 3(a). In this embodiment, the second print working 10b is identical to the first print working 10a, and is placed on the lower surface in register with the first print working 10a, as shown in Figure 3(b). The opacifying layer 6b on the lower surface 5b of the substrate now also covers the second print working 10b from the point of view of an observer viewing the lower surface 5b of the substrate 5.

In this embodiment, the screened working 7, which is applied to the opacifying layer 6a, has screen elements that are a series of concentric rings of decreasing radius. The elements of the screened working 7 are present within an area directly over the multi-tonal image 10, i.e. the outermost ring substantially matches the periphery of the multi-tonal image 10, and the remaining rings complete the area within the outermost ring to give the screened working the appearance of a circle, with its placement being registered to that of the print working 10a. The screened working 7 in this case is a multi-tonal screened working, with the multiple tones being provided by an decreasing line width of the ring elements of the screened working towards the centre of the screened working.

In addition to the first screened working 7, there is now provided a second screened working T on the lower surface 5b of the substrate 5. The second screened working 7' is identical to the first screened working 7 and sits on the opacifying layer 6b that overlaps the second print working 10b, with its placement being registered to that of the second print working 10b.

This embodiment further includes additional opacifying layers 6c, 6e on the upper side 5a of the substrate 5, and 6d, 6f on the lower side 5b of the substrate. These additional opacifying layers cover substantially all of their respective surface of the substrate 5, including gaps in certain areas of the medium to align with the gaps for the strip 2 and window 3 in the first opacifying layers 6a, 6b. The opacifying layers 6c, 6d, 6e, 6f differ from the first opacifying layers 6a, 6b in that they each additionally include a circular gap 12 laterally inside of which is the multi-tonal image 10 and the screened workings 7, 7'. In this embodiment, the circular gap 12 closely matches the periphery of the multi-tonal image 10 and the screened workings 7, 7'; however, in other embodiments, they may present these features in a larger 'window' region.

A further optional but beneficial feature will now be described with reference to Figure 4. Figure 4(a) shows an exemplary raised pattern layer 13 which may be applied over the outermost opacifying layer and/or across the screened working 7. For instance, in the Figure 1 embodiment, the raised pattern layer 13 may be applied over the screened working 7 on the first surface 5a of the substrate 5. The raised pattern layer may comprise for example a colourless, transparent ink which is applied to the medium 1 in accordance with a screen pattern, the elements of which are large enough to be individual discernible to the naked eye (possibly only under close inspection). Similar to the screened working, the raised pattern layer 13 may be applied in the form of an array of line or dot screen elements. In the present case, the raised pattern layer is in the form of a grid of lines as shown, whose height depends upon their position across the screened working, e.g. having a maximum height at the leftmost side of the screened working 7 and tapering down towards the right side of the screened working, as shown in Figure 4(b). The raised pattern layer may be applied by intaglio printing for example and preferably has a latent appearance in that its presence is less visible when the medium is viewed at some angles, relative to others. At certain viewing angles, which depend on the location of the illuminating light source, the raised image pattern will reflect light more strongly to the viewer, and thus become more visible, than at other viewing angles. The pattern 13 may or may not be directly related to the content of the multi-tone image 10. In this example, the raised pattern layer extends across the screened working 7 but otherwise does not reflect the features of the multi-tone image, instead comprising a grid pattern, the line weight of which varies from left to right across the region such that it fades to absent on the right side of the screened working 7. Preferably the raised pattern layer is tactile (i.e. can be detected by human touch), but this is not essential.

While in this embodiment, the raised pattern layer 13 is its own discrete layer, in some embodiment, the screened working 7 is configured such that it is visibly raised with respect to the surface to which it is applied, thereby providing the same or similar visual and/or tactile effects as a separate raised pattern layer 13, without the need for application of the additional raised pattern layer. It will be appreciated that a raised screened layer 7 could also be used together with a separate raised pattern layer 13, and in particular, a cooperation between the two, e.g. the raised pattern layer providing a tactile background to the screened working 7, may be particularly preferable.

In all of the above embodiments, it is preferred that the opacifying layers should be white, off-white or grey; however, other colours could be used. The opacifying coating will typically comprise a resin such as a polyurethane based resin, polyester based resin or an epoxy based resin and an opacifying pigment such as titanium dioxide (Ti02), silica, zinc oxide, tin oxide, clays or calcium carbonate. The composition of each opacifying layer may be the same or different to one another. In preferred examples, one of the opacifying layers on each side of the substrate may comprise electrically conductive particles to reduce the effect of static charge. Preferably this is the penultimate layer on each side: for example, layers 6b and 6c in Figure 2. In all of the above embodiments, it is preferred that the screened working is black in colour, to contrast the typically white colour of the opacifying layer; however any colour may be used provided it contrasts the selected opacifying material. It is preferable that the screened working is applied in an intaglio printing process using a conventional intaglio ink, as is well known in the art, however other processes, such as gravure printing, flexographic printing, lithographic printing etc, may alternatively be used.

The above-described security print media can then be processed into security documents. The processing steps involved in doing so may be carried out on a separate processing line, typically at a different manufacturing site and optionally by a different entity. An example of a security document 100 formed using the security print medium 1 described above in relation to Figure 1 is shown in Figure 5, (a) in plan view and (b) in cross-section. All of the components already provided as part of the security print medium 1 , including multi-tone image 10 and screened working 7, are as previously described in relation to Figure 1.

The security document comprises a graphics layer 20 applied in this example to the outer surfaces of the security print medium 1 , i.e. to the surface of outermost opacifying layers 6a and 6b. In other cases the graphics layer 20 may be applied only to one of the surfaces. As mentioned previously there could be intermediate layers between the opacifying layers and the graphics layer, such as a protective layer or primer. In this example, the security document is a banknote and hence the graphics layer comprises background security patterns 20a (such as guilloches) as well as identifiers such as denomination information 20b. The graphics layer 20 could be applied in a single working or in multiple workings, optionally using more than one printing technique. Any available printing techniques can be utilised for forming the graphics layer as would be applied to a conventional polymer document substrate, e.g. intaglio printing, gravure printing, flexographic printing, lithographic printing etc.

Figure 5 also illustrates examples of other security devices which may optionally be applied to the security print media to form the security document, such as an optically variable device 21 in window 3, e.g. a moire magnification device, a lenticular device or an integral imaging device as may be formed by cast-curing or laminating a lens array on one side of the polymer substrate 5 and forming image elements on the other. Also depicted is a security device 22 in the form of a patch which has been applied to the surface of the security print media, e.g. by lamination or hot stamping. The security device 22 may comprise a diffractive optical element such as a hologram, for example.

The security documents and security devices of the current invention can optionally 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.

Claims

1. A security print medium for forming security documents therefrom, comprising a transparent or translucent polymer substrate having first and second opposing surfaces, and at least one opacifying layer disposed on the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material, the security print medium further comprising a first print of a multi-tonal image disposed on the first and/or second surface of the polymer substrate, the first print being covered from the point of view of an observer on at least a first side of the security print medium by at least one of the opacifying layers which is disposed continuously across the multi-tonal image, and a second print of the multi-tonal image formed as a screened working comprising an array of screen elements in a colour which contrasts with that of the continuous opacifying layer, the second print being disposed on top of the continuous opacifying layer from the point of view of the observer and in alignment with the first print, whereby when the security print medium is viewed by the observer in reflected light, the second print dominates the appearance of the multi-tonal image and when the security print medium is viewed by the observer in transmitted light, the first print dominates the appearance of the multi-tonal image.

2. A security print medium according to claim 1 , wherein the first print of the multi-tonal image is a multi-coloured print, preferably comprising multiple workings and preferably comprising multiple workings in different colours.

3. A security print medium according to claim 1 or 2, wherein the at least one opacifying layer further includes one or more opacifying layers disposed continuously across the first print of the multi-tone image so as to additionally cover the first print from the point of view of an observer on the second side of the security print medium.

4. A security print medium according to any of claims 1 to 3, wherein the second print is an intaglio print.

5. A security print medium according to any of the preceding claims, wherein the at least one opacifying layer further includes one or more opacifying layers each defining at least one gap, laterally inside which the multi-tonal image is located.

6. A security print medium according to any of the preceding claims, further comprising a raised pattern layer applied to the outermost opacifying layer on one or both sides of the substrate, the raised pattern layer comprising an array of screen elements which are sufficiently large to be individually discernible to the naked eye, the raised pattern layer preferably being tactile and/or of varying visibility depending on the viewing angle.

7. A security print medium according to claim 6, wherein the second print is the raised pattern layer.

8. A security print medium according to any of the preceding claims, wherein at least some of the screen elements of the screened working have dimensions which render them individually discernible to the naked eye.

9. A security print medium according to any of the preceding claims, wherein at least some of the screen elements of the screened working have a smallest dimension in a direction along the first or second surface of the polymer substrate of 100 micrometres or more, more preferably 300 micrometres or more.

10. A security print medium according to any of the preceding claims, wherein the screened working is a multi-tonal screened working.

1 1. A security print medium according to claim 10, wherein the multiple tones of the multi-tonal screened working are provided by variations in the dimensions and/or frequency of the screen elements.

12. A security print medium according to any of the preceding claims, wherein the screen elements of the screened working comprise one or more of lines, dots, squares and indicia.

13. A security print medium according to any of the preceding claims, wherein the screen elements have an average area in a plane parallel to the first or second surface of the polymer substrate greater than the average area in a plane parallel to the first or second surface of the polymer substrate of elements of any element structure in any other layer.

14. A security print medium according to any of the preceding claims, wherein at least one opacifying layer is disposed across the substrate in the form of an array of screen elements which are too small to be individually discernible to the naked eye.

15. A security print medium according to any of the preceding claims, wherein at least one opacifying layer is a printed opacifying layer, preferably applied to the substrate by gravure printing.

16. A security print medium according to any of the preceding claims, wherein the or each opacifying layer is polymeric and non-fibrous.

17. A security print medium according to any of the preceding claims, wherein all of the opacifying layers are substantially the same colour as one another, preferably white or grey.

18. A security print medium according to any of the preceding claims, wherein at least one opacifying layer comprises electrically conductive particles.

19. A security print medium according to any of the preceding claims, wherein the multi-tonal image comprises an image of a three-dimensional object, preferably a geometrical solid or wireframe model, a person, an animal, a building or other architectural structure or a three-dimensional logo.

20. A security print medium according to any of the preceding claims, wherein the second print is registered to the first print.

21. A security print medium according to any of the preceding claims, wherein the screen elements of the screened working are substantially all of the same colour.

22. A security print medium according to any of the preceding claims, wherein the at least one opacifying layer is disposed across at least 50% of the substrate, preferably at least 80% of the substrate, more preferably all of the substrate.

23. A security document comprising a security print medium according to any of claims 1 to 22, and at least one graphics layer applied on the outermost opacifying layer(s) on the first and/or second surfaces of the polymer substrate.

24. A security document according to claim 23, wherein the security document is a bank note, an identification document, a passport, a licence, a cheque, a visa, a stamp or a certificate.

25. A method of making a security print medium, comprising:

providing a transparent or translucent polymer substrate having first and second opposing surfaces;

applying a first print of a multi-tonal image onto the first and/or second surface of the polymer substrate;

applying at least one opacifying layer onto the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material, wherein the first print is covered from the point of view of an observer on a first side of the security print medium by at least one of the opacifying layers which is disposed continuously across the multi-tonal image; and

applying a second print of the multi-tonal image on top of the continuous opacifying layer from the point of view of the observer and in alignment with the first print, wherein the second print is formed as a screened working comprising an array of screen elements in a colour which contrasts with that of the continuous opacifying layer, whereby when the security print medium is viewed by the observer in reflected light, the second print dominates the appearance of the multi-tonal image and when the security print medium is viewed by the observer in transmitted light, the first print dominates the appearance of the multi-tonal image.

26. A method of making a security print medium according to claim 25, wherein the first print of the multi-tonal image is a multi-coloured print, and wherein preferably the step of applying the first print comprises the steps of applying a first working onto the first and/or second surface of the polymer substrate and applying a second working onto the first and/or second surface of the polymer substrate, and wherein preferably the first and second workings are in different colours.

27. A method of making a security print medium according to claim 25 or 26, wherein applying at least one opacifying layer further includes applying one or more opacifying layers continuously across the first print of the multi-tonal image so as to cover the first print from the point of view of an observer on the second side of the security print medium.

28. A method of making a security print medium according to any of claims 25 to

27, wherein the second print is an intaglio print.

29. A method of making a security print medium according to any of claims 25 to

28, wherein applying at least one opacifying layer further includes applying one or more opacifying layers each defining at least one gap, laterally inside which the multi-tonal image is located.

30. A method of making a security print medium according to any of claims 25 to

29, further comprising the step of applying a raised pattern layer to the outermost opacifying layer and/or the second print on one or both sides of the substrate, the raised pattern layer comprising an array of screen elements which are sufficiently large to be individually discernible to the naked eye, the raised pattern layer preferably being tactile and/or of varying visibility depending on the viewing angle.

31. A method of making a security print medium according to claim 30, wherein the second print is the raised pattern layer.

32. A method of making a security print medium according to any of claims 25 to 31 , wherein at least some of the screen elements of the screened working have dimensions which render them individually discernible to the naked eye.

33. A method of making a security print medium according to any of claims 25 to 32, wherein at least some of the screen elements of the screened working have a smallest dimension in a direction along the first or second surface of the polymer substrate of 100 micrometres or more, more preferably 300 micrometres or more.

34. A method of making a security print medium according to any of claims 25 to 33, wherein the screened working is a multi-tonal screened working.

35. A method of making a security print medium according to claim 34, wherein the multiple tones of the multi-tonal screened working are provided by variations in the dimensions and/or frequency of the screen elements.

36. A method of making a security print medium according to any of claims 25 to 35, wherein the screen elements of the screened working comprise one or more of lines, dots, squares and indicia.

37. A method of making a security print medium according to any of claims 25 to

36, wherein the screen elements have an average area in a plane parallel to the first or second surface of the polymer substrate greater than the average area in a plane parallel to the first or second surface of the polymer substrate of elements of any element structure in any other layer.

38. A method of making a security print medium according to any of claims 25 to

37, wherein the step of applying at least one opacifying layer comprises applying at least one opacifying layer across the substrate in the form of an array of screen elements which are too small to be individually discernible to the naked eye.

39. A method of making a security print medium according to any of claims 25 to

38, wherein at least one opacifying layer is applied by printing, preferably by gravu re printing.

40. A method of making a security print medium according to any of claims 25 to

39, wherein the or each opacifying layer is polymeric and non-fibrous.

41. A method of making a security print medium according to any of claims 25 to

40, wherein all of the opacifying layers are substantially the same colour as one another, preferably white or grey.

42. A method of making a security print medium according to any of claims 25 to 41 , wherein at least one opacifying layer comprises electrically conductive particles.

43. A method of making a security print medium according to any of claims 25 to

42, wherein the multi-tonal image comprises an image of a three-dimensional object, preferably a geometrical solid or wireframe model, a person, an animal, a building or other architectural structure or a three-dimensional logo.

44. A method of making a security print medium according to any of claims 25 to

43, wherein the step of applying the second print comprises applying the at second print such that the it is registered to the first print.

45. A method of making a security print medium according to any of claims 25 to

44, wherein the screen elements of the screened working are substantially all of the same colour.

46. A method of making a security print medium according to any of claims 25 to 45 wherein applying at least one opacifying layer onto the first and/or second surfaces of the polymer substrate comprises applying the at least one opacifying layer across at least 50% of the substrate, preferably at least 80% of the substrate, more preferably all of the substrate.

47. A method of making a security document comprising:

making a security print medium in accordance with the method of any of claims 25 to 45; and

applying at least one graphics layer to the outermost opacifying layer(s) on the first and/or second surfaces of the polymer substrate.

48. A method of making a security document according to claim 47, wherein the security document is a bank note, an identification document, a passport, a licence, a cheque, a visa, a stamp or a certificate.