CA3000613A1

CA3000613A1 - Security print media and method of manufacture thereof

Info

Publication number: CA3000613A1
Application number: CA3000613A
Authority: CA
Inventors: Alan KEEN; Malcolm Baker
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: 2017-04-06
Also published as: EP3356152B1; AU2016330005B2; GB2542783A; WO2017055823A1; MX2018003717A; CN108290437B; AU2016330005A1; GB2542783B; AU2016330005C1; CN108290437A; GB201517150D0; EP3356152A1

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 a plurality of overlapping opacifying layers disposed on the first and/or second surfaces of the polymer substrate, each of the opacifying layers being a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate. In at least a region of the substrate, a multi-tonal image is exhibited by the plurality of overlapping opacifying layers in combination with one another, at least when the security print medium is viewed in transmitted light. Each of the plurality of overlapping opacifying layers has gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of each layer being defined in accordance with a different respective sub-image, the sub-images in combination defining the multi- tonal image, wherein either all the sub-images are different negative image versions of the multi-tonal image or all the sub-images are different positive image versions of the multi-tonal image. As a result, the number of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of overlapping opacifying layers in combination with one another giving rise to the multiple tones 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

2 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 a plurality of overlapping opacifying layers disposed on the first and/or second surfaces of the polymer substrate, each of the opacifying layers being a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, wherein in at least a region of the substrate a multi-tonal image is exhibited by the plurality of overlapping opacifying layers in combination with one another, at least when the security print medium is viewed in transmitted light, each of the plurality of overlapping opacifying layers having gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of each layer being defined in accordance with a different respective sub-image, the sub-images in combination defining the multi-tonal image, wherein either all the sub-images are different negative image versions of the multi-tonal image or all the sub-images are different positive image versions of the multi-tonal image, whereby the number of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of overlapping opacifying layers in combination with one another giving rise to the multiple tones 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 plurality of overlapping opacifying layers onto

3 PCT/GB2016/052996 the first and/or second surfaces of the polymer substrate, each of the opacifying layers being a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, each opacifying layer being applied in accordance with a different respective sub-image across at least a region of the substrate;
whereby each of the plurality of overlapping opacifying layers has gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of each layer being defined in accordance with a different respective sub-image, the sub-images in combination defining a multi-tonal image which is exhibited by the plurality of overlapping opacifying layers in combination with one another, at least when the security print medium is viewed in transmitted light, wherein either all the sub-images are different negative image versions of the multi-tonal image or all the sub-images are different positive image versions of the multi-tonal image, whereby the number of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of overlapping opacifying layers in combination with one another giving rise to the multiple tones 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, in the presently disclosed security print media, a plurality of the opacifying layers additionally provide a security feature in the form of a multi-tone image which is visible at least when the media is viewed in transmitted light (and, in some embodiments, also when viewed in reflected light). Like a conventional watermark formed in paper-based documents, the multi-tone image formed by the opacifying layers has a monochromatic "greyscale" appearance defined by relatively bright and relatively dark areas (and optionally one or more intermediate tones). However, as described below, in some embodiments additional layers can be provided to achieve a multi-coloured appearance.
It will be appreciated that the opacifying layers need not be in direct contact with the surface of the polymer substrate. Rather, one or more additional (transparent or

4 translucent) layers could be present between the polymer substrate and the opacifying layers, such as a primer layer and/or the additional coloured layer(s) mentioned above, the opacifying layers still being considered disposed "on"
the substrate surface.
The multi-tone image is achieved by inserting one or more gaps in each of a plurality of the opacifying layers (which otherwise cover substantially all of the polymer substrate, that is, preferably at least 50% of the substrate, more preferably at least 80% and most preferably all of the remaining substrate). In each opacifying layer, the gap(s) are arranged according to a different respective sub-image, the cumulative effect of which is a variation in the optical density of the security print media across the region of the substrate, depending on the number of opacifying layers present at each point, resulting in the displayed multi-tone image.
Locations in which fewer of the opacifying layers are present (i.e. where more of the opacifying layers have aligned gaps) will have a lower optical density, thereby appearing brighter when the substrate is viewed in transmission, or darker when the substrate is viewed against a dark surface, than other locations. Since relatively bright locations typically give the impression of being closer to the viewer, the resulting multi-tone image can provide a strong three-dimensional effect, especially where the sub-images are arranged to achieve a gradual change in optical density across the image (on a scale when viewed by the naked eye).
There may be one or more additional opacifying layers present which do not contribute to the multi-tone image, e.g. being entirely absent across the relevant region of the substrate or being provided uniformly across the region of the substrate. In addition, there could be more than one opacifying layer having gaps disposed in accordance with the same sub-image (provided there are at least two opacifying layers each having gaps disposed in accordance with different sub-images).
It will be noted that the sub-images will either all be negative image versions of the multi-tone image, or all positive image versions of the multi-tone image, and not a mixture of both. A "negative image version" of an image is one in which elements of the image are defined by the absence of colour (in this case, the absence of the opacifying material) against a surrounding background of colour (i.e. the presence of the opacifying material), whereas a "positive image version" of an image is the reverse: elements of the image are defined by the presence of colour (i.e. the opacifying material) against empty surroundings (i.e. the absence of the opacifying

5 material). It should be noted that here the fact that a sub-image is a "negative image version" of the multi-tonal image does not mean that it is the reverse of the multi-tonal image. Rather, if the multi-tonal image is a negative image then typically each of the sub-images will also be negative image versions of the same image, and if the multi-tonal image is a positive image then typically each of the sub-images will be positive image versions of that image.
The sub-images will each be "versions" of the multi-tone image in the sense that each will contribute to the definition of the same image, but any one of the sub-images by itself need not display all the elements which will be visible in the final multi-tone image. Rather, each portion of the multi-tone image will have a desired tone (or, analogously, optical density) relative to other portions of the multi-tone image and each portion will be present (i.e. correspond to an area of opacifying material) or absent (i.e. correspond to a gap) in each sub-image in dependence on the desired tone of that portion. Hence, each sub-image shows selected portions of the multi-tone image depending on their desired tone. In this way, ultimately, each element of the multi-tone image is built up by the presence or absence of each opacifying layer in the portion corresponding to the image element, the tone of the element resulting from the number of opacifying layers present. All of the sub-images are aligned with one another so that each portion of the multi-tone image has the same location in each sub-image.
Preferably, each sub-image defines portions of the multi-tonal image which have a (desired) tonal value falling within a respective tonal value range, the size of each respective tonal value range being different. That is, each sub-image is based on a different respective tonal value range. The tonal value of each point of the multi-tone image can be defined on an arbitrary scale relative to the darkest tone and lightest tone present in the multi-tone image (e.g. corresponding to tonal values of 100% and 0% respectively), or on an absolute scale as may be measured for example using a transmission densitometer such as the MacBeth TD932 (e.g.

6 darkest tone portions having an optical density of 0.9, and lightest tone portions having an optical density of 0). The size of the different tonal value ranges may increase in constant steps, e.g. by 10% or by 20% where the scale is relative, or by 0.1 or 0.2 where the scale is absolute) from one sub-image to another. It should be noted that the sub-images do not need to be physically arranged on or applied to the substrate in the same order as that denoted by their respective tonal value ranges.
The order in which the opacifying layers (and their respective sub-images) are arranged on the substrate ¨ and to which side(s) of the substrate each is applied ¨
is generally unimportant since it is the cumulative effect of the layers which produces the desired image.
Advantageously, when the tonal value ranges of the sub-images are ordered according to increasing size, each tonal value range falls within the tonal value range next in the sequence. For example, a first sub-image may define portions of the multi-tone image having a tonal value in the range 0% to 10%, a second sub-image may define portions having a tonal value in the range 0% to 20% (thereby including all the same portions as in the first sub-image, plus more), a third sub-image may define portions having a tonal value in the range 0% to 30%, and so on.
In this way, the desired tone of each image portion will be provided by the cumulative effect of the sub-layers which define that portion. The smaller the difference in tonal value range from one sub-image to the next (and the greater the number of opacifying layers), the more different tones can be displayed in the final image. As in the above example it is particularly preferred that all of the tonal value ranges share substantially the same first end value and differ in their second end values, but this is not essential.
In some preferred embodiments each or at least one of the sub-images will be a binary or "flat" image with no tonal variation: the opacifying material is either present or absent on a scale visible to the naked eye, with no intermediate areas.
However, in more preferred embodiments, at least some of the sub-images are multi-tonal sub-images, preferably half-tone sub-images. In this way, multiple tones can be introduced within the sub-image itself, e.g. allowing for a gradual change from a region of 100% opacifying material though a region in which the opacifying material is applied to a gradually decreasing proportion of the surface (on a scale too small to

7 be appreciated by the naked eye) to a region in which the opacifying material is absent (i.e. a gap). This can be used to create a smoother transition between tones in the final multi-tone image, and more complex effects. For example, this allows for the creation of even more different tones in the final image than the number of different opacifying layers would itself permit.
The opacifying layers each preferably comprise a non-fibrous, polymeric material which will scatter light (as opposed to allowing clear light transmission therethrough), and will be translucent to a degree. In preferred examples, each individual opacifying layer may have an optical density in the range 0.1 to 0.5, more preferably 0.1 to 0.4, most preferably 0.1 to 0.3 (as measured on a transmission densitometer, with an aperture area equivalent to that of a circle with a 1mm diameter ¨ a suitable transmission densitometer is the MacBeth TD932). The individual opacifying layers may or may not be of the same composition as one another ¨ for example, 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 ¨ but nonetheless, preferably, 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. In preferred implementations, 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.
A multi-tone image formed solely of the opacifying layers in the manner so-far described will appear monochromatic with different portions having different darkness levels (tones) but all of the same hue, e.g. different levels of grey. To further increase the visual impact and security level of the feature, in preferred embodiments the security print medium further comprises a mono-tone or multi-tone print of at least part of the multi-tonal image in one or multiple colours which contrast visually with the opacifying layers, located between at least one, preferably all, of the opacifying layers and the polymer substrate on the first and/or second surfaces thereof, the print of the multi-tonal image being in alignment with the sub-images in the opacifying layers. The print is most preferably located on the surface of the polymer substrate (optionally on top of an additional layer such as a primer), underneath all the opacifying layers, although it is also possible to locate it between

8 any two of the opacifying layers. 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 should be noted that this print of the image could be a negative image version or a positive image version of the multi-tone image, irrespective of the nature of the sub-images. Indeed, it is preferred that if the sub-images are negative image versions, the print is a positive image version (so as to "colour in" the gaps in the opacifying layers), and vice versa. The print could be a flat, binary image. However, in preferred examples, the print is itself a multi-tone print and comprises at least one multi-tone, preferably half-tone, print working. This can be used for example to add additional shading, e.g. using various different spatial densities of a dark-coloured ink such as black, to further enhance the multi-tonal nature of the overall image. In particularly preferred examples, the print is multi-coloured and may comprise at least two print workings in different colours. In this way the end result is a multi-coloured, multi-tonal image of which the different colours are provided by the print whilst the shading is provided primarily by the opacifying layers (and optionally the print).
The multi-tone image can be designed for intended viewing in either reflected or transmitted light (although, irrespective of the designed intention it will still be visible in at least transmitted light and in some cases both). Thus, in some preferred embodiments, the sub-images are configured such that a greater number of the opacifying layers overlap one another at locations across the substrate corresponding to darker tones in the multi-tone image, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image, the multi-tone image being configured for viewing in transmitted light. If the same image is viewed in reflected light against a dark background, the multi-tone image may still be visible but will appear reversed, with those regions intended to be darkest appearing lightest and vice-versa.

9 In other preferred embodiments, the sub-images are configured such that a smaller number of the opacifying layers overlap one another at locations across the substrate corresponding to darker tones in the multi-tone image, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image, the multi-tone image being configured for viewing in reflected light. In this case, it is a dark background apparent through the opacifying layers which provides the darkness to the image, areas with more opacifying layers obscuring the dark background and reflecting light so as to appear brighter than areas with fewer opacifying layers. If the same image is viewed in transmitted light, the multi-tone image will still be visible but again will appear reversed as compared with the intended image.
To make best use of the ability of the multi-tone image to display distinct light and dark portions, and preferably different intermediate tones as well, it is particularly advantageous if 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. Shadows in the image can be denoted by darker tones created by the multiple opacifying layers, and highlights by lighter tones.
As already mentioned, the greater the number of opacifying layers (and corresponding different sub-images), the more different tones can be achieved in the final image. Hence, preferably, the plurality of overlapping opacifying layers includes at least three overlapping opacifying layers each having gap(s) defined in accordance with a different respective sub-image, preferably at least four, more preferably at least six. The thickness of each layer may be reduced to avoid an overly thick document substrate.
All of the opacifying layers could be located on the same surface of the polymer substrate. However it is generally preferred to distribute the opacifying layers on both surfaces so that both sides of the document can later be printed on.
Hence, advantageously, at least one of the plurality of overlapping opacifying layers is provided on each of the first and the second surfaces of the polymer substrate, preferably half of the plurality of overlapping opacifying layers being provided on each of the first and second surfaces. It should be noted that the opacifying layers do not need to be applied to the substrate in any particular order related to their sub-images although this may be preferred in some cases. The substrate can also be located at any position within, or on one side of, the set of layers as its location will 5 not affect the end image displayed by the cumulative effect of the layers.
Nonetheless for other reasons it may be desirable to provide at least one opacyifying layer on each surface of the substrate, e.g. to enable layer printing thereon and/or to protect the substrate or control its surface texture.

10 As already mentioned, the security print medium could additionally include one or more opacifying layers which do not take part in the formation of the multi-tone image. Hence in some preferred embodiments the security print medium further comprises one or more additional opacifying layers each comprising a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, the one or more additional opacifying layers each either extending continuously across the region of the substrate containing the multi-tonal image or comprising a gap substantially across the region.
The security print medium could be configured such that at least one opacifying layer is present at every point across the region, so that the document substrate does not appear transparent. However, to increase the visibility of the security feature, and to add an additional level of security, in preferred embodiments, at least one transparent window region is formed by aligned gaps in each of the opacifying layers, the at least one transparent window region preferably substantially surrounding the multi-tonal image.
The or each opacifying layer may be laid down via an application technique which results in no additional visible sub-structure to the layer beyond that defined by the sub-image, i.e. the opacifying material being present in a macroscopically uniform, homogenous layer across all regions outside the gaps defined by the sub-image.
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. Thus, at least some of the opacifying layers are applied in the form of an array of screen elements which are too small to be individually discernible to the naked eye. However, in other

11 preferred cases one or more of the opacifying layers may be laid down in the form of a visible screen. Hence, at least one of the sub-images is formed of an array of screen elements which are sufficiently large to be individually discernible to the naked eye, the size of the screen elements varying across the array to define the sub-image. For example the sub-image may be defined by line screen elements or dot screen elements, e.g. to give the appearance of an intaglio-printed pattern.
The security print medium may advantageously further comprises a raised pattern layer (e.g. of transparent or coloured ink) 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. For example, the pattern layer could be applied by intaglio printing. In a particularly preferred embodiment, such a raised pattern layer is provided in combination with a visibly-screened opacifying layer and the array of screen elements forming the at least one of the sub-images is arranged to visually cooperate with the array of screen elements forming the raised pattern layer.
For example, the raised pattern layer could be provided across one area of the region containing the multi-tone image and the screened opacifying layer across a second, different area, the two areas merging into one another. This gives the impression of a continuous screened pattern at some viewing angles and not others.
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

12 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 in the opacifying layers so as to achieve a visual co-operation between the graphics layer and the multi-tone image. This can be achieved by using a sensor such as a camera system to detect the location of the multi-tone image 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, and (b) in cross-section, layers of the security print medium being shown spaced apart for clarity;
Figures 2(a) to (c) show portions of different opacifying layers of the security print medium of Figure 1;
Figures 3(a) to (c) show portions of different opacifying layers of the security print medium of Figure 1 in a variant thereof;
Figure 4 shows a second embodiment of a security print medium (a) in plan view, and (b) in cross-section, layers of the security print medium being shown spaced apart for clarity;
Figures 5(a) to (d) show portions of different opacifying layers of the security print medium of Figure 4;
Figure 6(a) shows an example of a raised pattern layer, and Figure 6(b) shows an example of an opacifying layer which may be provided to the security print medium of Figure 4 according to a variant thereof;

13 Figure 7 shows a third embodiment of a security print medium (a) in plan view, and (b) in cross-section, layers of the security print medium being shown spaced apart for clarity;
Figure 8 shows schematically a fourth embodiment of a security print medium, each layer of the security print medium being depicted individually in plan view;
Figure 9 shows schematically a fifth embodiment of a security print medium, each layer of the security print medium being depicted individually in plan view;
Figure 10 shows three additional layers which may be provided to the security print medium of Figure 9 in a variant thereof;
Figure 11 shows schematically a sixth embodiment of a security print medium, each layer of the security print medium being depicted individually in plan view;
and Figure 12 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

14 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, Figure 1(a) showing a plan view and Figure 1(b) showing a cross-section along the line X-X'. It will be appreciated that in Figure 1(b) the various 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 Figure 1(a), 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. At least some of the opacifying layers forming coating 6 also have gaps in a region 9 of the medium, which are configured to form a multi-tone image 10 as will be detailed below. Nonetheless, in preferred examples each opacifying layer covers at least 50% of the area of the security print medium corresponding to one security document, more preferably at least 80%.
As shown in the cross-section of Figure 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 opacifying coating 6 can be applied on one or both surfaces 5a, 5b of the polymer substrate 5 and in this case comprises four opacifying layers 6a, 6b, 6c and 7. 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 5 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.
In this example, three of the opacifying layers 6a, 6b, 6c contribute to the formation 10 of multi-tone image 10 whilst the fourth opacifying layer 7 (which is optional) is continuous across region 9 and hence does not contribute to the multi-tone image other than to increase its optical density uniformly throughout. Each of the opacifying layers 6a, 6b and 6c includes a gap in the region 9 which is defined in accordance with a different respective sub-image. The sub-images are shown in

15 plan view in Figures 3(a), (b) and (c) for layers 6a, 6b and 6c, respectively (each of Figures 3(a) to (c) showing only a section of the respective opacifying layer including and surrounding region 9, and omitting the remainder of the layer). The different sub-images are configured such that once the opacifying layers are arranged on top of one another, as shown in Figure 1(b), the cumulative effect of the different sub-images is a variation in the optical density of the security print medium across region 9 which appears as the multi-tone image 10, at least when the medium is viewed in transmitted light. It should be noted that the order in which the opacifying layers 6a, 6b, 6c and 7 are arranged on the substrate is unimportant since it is their cumulative effect, when all are viewed in combination, which creates the desired image.
Similarly the position of the substrate within the stack of opacifying layers will not affect the image exhibited by the end product and so can be freely selected.
However it may be desirable to apply at least one opacifying layer to each surface of the substrate for other reasons, e.g. to enable later printing of both sides of the document. These considerations apply to all embodiments.
The multi-tone image 10 in the present embodiment depicts a three-dimensional hemisphere, and is made up of four different tones. The innermost circular portion 10a has the lowest optical density (or tone), achieved by providing corresponding gaps in all of the opacifying layers except for layer 7 such that a single opacifying

16 layer is present across portion 10a, and the outermost annular portion 10d has the highest optical density (or tone), achieved by providing gaps in none of the opacifying layers in this portion, such that all four are present here.
Intermediate annular portions 10b and 10c are provided with respective intermediate optical density / tonal values, achieved by locating gaps in these portions in one of the four opacifying layers and in two of the four opacifying layers, respectively.
Thus, taking the optical density of innermost portion 10a to be 0% on an arbitrary relative scale, and that of outermost portion 10d to be 100%, portion 10b has an optical density of 33% and portion 10c an optical density of 66%. Alternatively, on an absolute scale, if each opacifying layer 6a,b,c and 7 has an optical density of 0.2 (as measured on a transmission densitometer such as the MacBeth TD932, with an aperture area equivalent to that of a circle with a 1mm diameter), portion 10a will have an optical density of 0.2, portion 10b an optical density of 0.4, portion 10c an optical density of 0.6 and portion 10d an optical density of 0.8. These different optical densities appear as a variation in tone across the image, resulting in a three-dimensional effect.
When the medium 1 is viewed in transmitted light (i.e. against a backlight), the innermost portion 10a will appear brightest since its low optical density allows the greatest transmission of light, whilst the outermost portion 10d will appear darkest due to its high optical density. As a result, the centre of the hemisphere appears to protrude out of the plane of the medium 1, towards the viewer, relative to the periphery of the hemisphere which appear farther behind. The multi-tone image may or may not be visible in reflected light depending on the optical density of the opacifying layers. However, if the layers are sufficiently translucent, when the medium 1 is viewed in reflected light against a dark background, now the inner portion 10a will appear darkest, since it reflects the least light and obstructs the view of the dark background to the smallest degree, whilst the outer portion 10d will appear lightest, since it reflects the most light and largely conceals the underlying dark background. Hence the hemisphere appears reversed relative to its appearance in transmitted light, with its centre portion 10a appearing farthest from the viewer and the edge portion 10a appearing nearest.

17 Referring now to Figure 2, the sub-images according to which each opacifying layer 6a, b, c is arranged will be described. The sub-image defined in opacifying layer 6a is shown in Figure 2(a) and it will be seen that this comprises a circular gap extending across the portions 10a,b,c of the multi-tone image, wholly surrounded by the opacifying material of layer 6a. The periphery of the circular gap therefore corresponds to the boundary between portion 10c and portion 10d in the multi-tone image 10. The opacifying material of layer 6a is present across portion 10d of the multi-tone image (the outer edge of which corresponds to the periphery of region 9, shown for reference). Hence the sub-image is defining portions of the desired multi-tone image according to their intended tone (or analogously their optical density): in this sub-image, portions of the multi-tone image having a desired tone of more than 66% up to and including 100% (in this case, portion 10d) are denoted by the presence of opacifying material whilst portions having a desired tone of 66%
or less correspond to a gap in the layer.
Likewise, the sub-image defined in opacifying layer 6b (Figure 2(b)) defines portions of the desired multi-tone image according to a different, larger, tonal range:
here, portions of the multi-tone image having a desired tone of more than 33% up to and including 100% (in this case, portions 10c and 10d) are denoted by the presence of opacifying material whilst portions having a desired tone of 33% or less correspond to a gap in the layer. Hence, the sub-image comprises a circular gap extending across portions 10a and 10b of the multi-tone image, its periphery lying on the boundary between portions 10b and 10c.
Finally, the sub-image defined in opacifying layer 6c (Figure 2(c)) defines portions of the desired multi-tone image according to a still larger tonal range: portions of the multi-tone image having a desired tone of more than 0% up to and including 100%
(in this case, portions 10c and 10d) are denoted by the presence of opacifying material whilst portions having a desired tone of 0% correspond to a gap in the layer. Hence, the sub-image comprises a circular gap extending across portion 10a only of the multi-tone image, its periphery lying on the boundary between portions 10a and 10b.

18 It will be noted that the size of the tonal range defined in each sub-image is different and that each range falls wholly within that of the next sub-image (if they are placed in sequence according to the size of their respective tonal ranges). One end point (100%) is the same for each tonal range whilst the other end value varies.
It will also be noted that, in this example, all of the sub-images are negative images ¨ that is, they each define elements of the desired multi-tone image by the absence of opacifying material against surroundings of that material, rather than vice versa.
In the above example, all of the sub-images are binary or "flat" images meaning that the opacifying material is either present or absent across each part of the image (on a scale large enough to be appreciated by the naked eye), and there are no intermediate levels. This will be desirable in many cases, especially where a sharp "step-change" in tone is required in the final multi-tone image, e.g. to define a straight edge in the image of an object. If this is not desired, one option to achieve a more gradual change in tone from one portion of the image to the next would be to utilise a greater number of opacifying layers, possibly of lower individual optical density, and a corresponding number of sub-images, arranged so as to achieve a more closely-spaced series of smaller changes in tone. However, this may result in an undesirably thick construction of the security print medium 1 and would also require a corresponding increase in the number of processing steps.
Figures 3(a), (b) and (c) show alternative sub-images for each of the opacifying layers 6a, b, c respectively in the Figure 1 embodiment, which address this.
The sub-image for each layer is substantially the same as in the Figure 2 example, defining portions of the multi-tone image according to their desired tone, based on the same different tonal range for each sub-image as previously described, but in this case each of the sub-images itself is multi-tonal, i.e. defining at least one intermediate tone beyond the binary options of "present" or "absent", on a scale visible to the naked eye. For example, each sub-image may be formed as a half tone image in which elements of the image are laid down with varying size and/or ink weight to give rise to the required variation in tone. In the present example, this multi-tonal nature of the sub-image is used to replace the sharp periphery of the gap in each opacifying layer with a boundary region 11 in which the tone of the sub-

19 image is intermediate (e.g. 50% fill factor) or gradually increases from zero on the side of the gap to 100% on the other wise. This visually softens the edge of each opacifying layer resulting in a more gradual change between tones in the final multi-tone image.
In the present example, all of the sub-images are formed as multi-tone images in this way but this is not essential. In other cases, just one of the sub-images, or a sub-set of the sub-images, may be multi-tonal whilst the remaining one or more sub-images may be binary images.
Multi-tonal sub-images can also be used for purposes other than smoothing transitions between gaps and non-gap portions of a sub-image. More generally, the use of multiple tones in one or more of the sub-images allows for the creation of more complex multi-tonal images once the sub-images are combined since the number of available tones is no longer limited to the number of opacifying layers applied. Rather, by varying the tone across any of the individual sub-images and layering them with further sub-images as necessary, a much larger number of different tones can be created thereby allowing for the formation of a more complex multi-tone image.
It should be appreciated that this can be applied to all embodiments described below, in which any one or more of the described sub-images could be implemented as a multi-tonal sub-image to obtain the above-mentioned advantages.
Figure 4 shows a second embodiment of a security print medium 1 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. Again, a multi-tone image 10 is formed within a region 9 of the medium 1 and here this again takes the form of a hemisphere.
However, due to the different construction of the multi-tone image 10, described below, in this case the appearance of the hemisphere in reflected and transmitted light will be opposite to that in the Figure 1 embodiment. In addition, in the Figure 2 embodiment, a transparent window region 12 is provided to further enhance the security level of the medium 1. It is especially preferred that where a window region 12 is provided, this is arranged to surround the multi-tone image 10 (as in the present example), to assist in delimiting the multi-tone image from the rest of the medium 1. A transparent window region 12 of this sort can be provided in any of the 5 other embodiments disclosed herein.
As shown in the cross-section of Figure 4(b), in this embodiment four opacifying layers 6a, 6b, 6c and 6d contribute to the definition of the multi-tone image 10. The sub-images according to which each respect layer is arranged in the vicinity of 10 region 9 are shown in Figures 5(a) to (d) respectively. It will be noted that each of the sub-images are now positive images rather than negative images (as in the previous embodiment). That is, inside region 9, each sub-image defines features of the image by the presence of opacifying material against empty surroundings.
15 The multi-tone image 10 again has four different tonal levels: on a relative scale this time taking the transparent window region 12 to have an optical density of 0%, the innermost portion 10a has the highest optical density of 100%, and the surrounding annular portions 10b, 10c and 10d have respective optical densities of 75%, 50%
and 25%. This is achieved by applying each opacifying layer 6a to 6d according to

20 the sub-images shown in Figures 5(a) to (d) respectively. Layer 6a is laid down according to the sub-image shown in Figure 5(a) which comprises a circular element extending across the portions 10a to 10d of the multi-tone image 10, being absent only in an annular region corresponding to window 12. Hence the sub-image differentiates between portions of the multi-tone image 10 having a relative tonal value (or optical density) in the range of 25% to 100% (in which the opacifying material is present) and portions in the range 0 to less than 25% (in which the opacifying material is absent).
Similarly, the sub-image according to which layer 6b is arranged (Figure 5(b)) defines portions of the desired multi-tone image according to a different, smaller, tonal range: here, portions of the multi-tone image having a desired tone of 50% up to and including 100% (in this case, portions 10a, 10b and 10c) are denoted by the presence of opacifying material whilst portions having a desired tone of less than 50% correspond to a gap in the layer. Hence, the sub-image comprises a circular

21 element extending across portions 10a, 10b and 10c of the multi-tone image, surrounding by an annular gap encompassing portion 10d and window region 12.
Likewise, the sub-image according to which layer 6c is arranged (Figure 5(c)) defines portions of the desired multi-tone image according to a different, still smaller, tonal range: here, portions of the multi-tone image having a desired tone of 75% up to and including 100% (in this case, portions 10a and 10b) are denoted by the presence of opacifying material whilst portions having a desired tone of less than 75% correspond to a gap in the layer. Hence, the sub-image comprises a circular element extending across portions 10a and 10b of the multi-tone image, surrounding by an annular gap encompassing portions 10c and 10d and window region 12.
Finally, the sub-image according to which layer 6d is arranged (Figure 5(d)) defines portions of the desired multi-tone image according to an even smaller tonal range:
here, portions of the multi-tone image having a desired tone of 75% up to and including 100% (in this case, portions 10a only) are denoted by the presence of opacifying material whilst portions having a desired tone of less than 75%
correspond to a gap in the layer. Hence, the sub-image comprises a circular element extending across portion 10a of the multi-tone image, surrounding by an annular gap encompassing portions 10b, 10c and 10d and window region 12.
The result is that the innermost portion 10a of the multi-tone image 10 now has the highest optical density whilst the outmost portion 10d has the lowest and window region 12 has a still lower optical density. Hence when the medium 1 is viewed in transmitted light, the centre of the hemisphere will appear darkest and therefore furthest from the viewer, giving the impression that the hemisphere is depressed into the plane of the medium 1. When the medium is viewed in reflected light against a dark background, provided the optical density of the layers 6 is sufficiently low, the innermost portion 10a will now appear lightest and the outermost portions 10d darkest, with the window region 12 taking on the dark colour of the background unmoderated. Hence the hemisphere will now appear reversed, having its centre protruding towards the viewer.

22 Again, any one or more of the sub-images could be formed as a multi-tonal sub-image in the manner previously described with respect to Figure 3 in order to achieve a more gradual variation in tone.
A further optional but beneficial feature will now be described with reference to Figure 6. Figure 6(a) shows an exemplary raised pattern layer 13 which may be applied over the outermost opacifying layer(s) across the region 9 of the medium 1.
For instance, in the Figure 1/2 embodiment, the raised pattern layer 13 may be applied over the opacifying layer 6d on the first surface of the substrate 5, and/or over the opacifying layer 6c on the second surface 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). For example, the raised pattern layer 13 may be applied in the form of an array of line or dot screen elements. In this case, the raised pattern layer is in the form of a grid of lines as shown. 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 same region 9 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 region 9. Preferably the raised pattern layer is tactile (i.e. can be detected by human touch), but this is not essential.
A raised pattern layer of the sort described above can be used on its own to add complexity to the multi-tone image feature. However it is preferred to integrate the pattern with the multi-tone image by arranging one of the opacifying layers 6 in accordance with a similar screened pattern.

23 PCT/GB2016/052996 In previous embodiments, each opacifying layer 6a to 6d has been 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.
However in the present example, one or more of the opacifying layers is formed in accordance with an array of screen elements, such as dots or lines, which are sufficiently large that the screen structure is visible to the naked eye. An example of such an opacifying layer 6a' is shown in Figure 6(b). This could be provided in place of layer 6a or in addition to layer 6a. In this example, the circular element of the sub-image covering portions 10a to d of the multi-tone image is now applied in accordance with a screen of dots arranged on an orthogonal grid. The size of the dot elements varies across the region from small on the left side to large on the right side. This results in a small-scale structure to the tones visible in the multi-tone pattern which interacts with that of the raised pattern layer 13. The two screen patterns are selected to be of similar sizes and element shapes, that of raised pattern layer 13 being dominant on the left hand side of the region 9, and that that of opacifying layer 6a' being dominant on the right hand side.
In the above examples, the multi-tone image will be monochromatic, i.e.
displaying multiple shades of the same colour with different darkness levels. Typically where the opacifying layers are white, off-white or grey, the multi-tone image 10 will appear in greyscale, with tones varying from white to dark grey or black, with various intermediate grey tones inbetween. However, in some cases it will be desirable to colour the multi-tone image 10, either with a single colour different from that of the opacifying layers, or with multiple colours.
Figure 7 shows a third embodiment in which the multi-tone image 10 is coloured by the addition of a print 8 of the same image, in this case formed of two print workings 8a and 8b, located on respective surfaces of the polymer substrate 5, under the opacifying layers 6a to 6d on each side. In this example, the multi-tone image 10 is of a three-dimensional cube and is located in a transparent window region 12 inside

24 the region 9. The construction of the medium 1 is substantially the same as in the previous embodiments with a polymer substrate 5 having opacifying layers 6a to 6d applied to each side in accordance with respective sub-images to define the different tones desired in the multi-tone image 10. In this example, two opacifying layers 6a, 6b are applied to the first surface of the substrate 5, layer 6a defining portions 10a and 10b of the cube, surrounded by a gap corresponding to window 12, and layer 6b being present only in portion 10b of the cube, resulting in that portion having a higher optical density than portion 10a. Two further opacifying layers 6c, 6d are provided on the second surface of the substrate 5, layer 6c being defined according to the same sub-image as layer 6b and layer 6d being defined according to the same sub-image as layer 6a. Of course, more opacifying layers could be provided in accordance with still further different sub-images to increase the complexity of the multi-tone image if desired.
The print 8 of the multi-tone image 10 comprises two workings 8a, 8b preferably in different colours. For example, one of the workings 8a may provide an overall, solid region of one colour (e.g. red) across the whole of the area corresponding to the multi-tone image (i.e. both regions 10a and 10b). This combined with the shading achieved by the opacifying layers 6a to 6d will result in an image 10 of a red, three-dimensional cube with the different portions of the image having relatively light or dark shades of red resulting from the different optical densities of the opacifying layers in combination with one another. The other working 8b could be identical to the first working 8a, and in the same colour, to increase the intensity of the colour.
Alternatively, the second working could be in a different colour and configured to provide different elements of the multi-tone image ¨ e.g. the first working could be provided only in portion 10a of the image and the second working only in portion 10b so that two facets of the cube appear 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, one of the workings could be provided in a dark colour such as black and used to provide 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. Any one or more of the print workings may advantageously itself be multi-tonal, e.g. formed as a half-tone image, to introduce further complexity to the feature.

The print 8 can be applied using any available application technique and may comprise a single working or a plurality of workings. For example, the print 8 could be applied by gravure, flexographic, lithographic or any other available printing 5 technique, or by applying an all-over layer of ink and then selectively removing parts of it to define an image, e.g. by laser ablation or etching. The composition forming the image should preferably be at least semi-transparent so that it does not negate the variation in optical density created in the opacifying layers 6.
10 It is generally preferred that the print 8 is located under the opacifying layers ¨ i.e.
between the opacifying layers and the polymer substrate 5 (optionally on top of a primer layer), as shown in Figure 7(b). However this is not essential and the print could be located between any of the opacifying layers. It is less preferred that the print 8 be located on the outer surface of the outermost opacifying layers 6b, 6d 15 since in this case its reflective colour may overwhelm the multi-tonal effect of the feature, especially when viewed in reflection.
A print 8 of this sort can be incorporated into any of the presently disclosed embodiments.
Figure 8 illustrates a fourth embodiment of a security print medium, showing each layer applied to a polymer substrate 5 separately, in plan view. Layers 8a, 6b, 6d and 6f are applied in that order to a first surface of substrate 5, and layers 8b, 6a, 6c and 6e are applied in that order to a second surface of substrate 5 (although as mentioned previously the order of the layers, and on which surface of the substrate each is applied, is unimportant). As before, the substrate 5 may carry additional layers on either of its surfaces such as a primer layer, which are not shown here.
The security print medium comprises six opacifying layers 6a to 6f, each applied according to a different sub-image as illustrated in the Figure. It should be appreciated that whilst in practice the opacifying layers will typically be white, here the opacifying material is illustrated in each of layers 6a to 6f as black in order to be visible in the Figure. Thus the white portions surrounded by black in each of the sub-images in fact correspond to gaps in the sub-images, and the black portions represent the areas where opacifying material is present. Layers 8a and 8b are two workings forming a print 8 in colours contrasting with that of the opacifying layers 6a to 6f, in the same manner as described in connection with Figure 7 above. In this example, working 8a is a flat (binary) print in purple, and working 8b is a multi-tonal print in black.
Collectively, the layers depicted in Figure 8 form a multi-tone image 10 of a three-dimensional twisted loop structure. With reference to working 8b, the portion marked F of the loop appears as the front-most part of the object (i.e.
projecting towards the viewer), and the portion marked R appears as the rear-most part of the object (i.e. projecting away from the viewer), when the medium 1 is viewed in transmitted light. This is achieved by arranging portion F of the image to have the lowest optical density, by providing that portion with corresponding gaps in the greatest number of opacifying layers 6a to 6f: indeed, as can be appreciated by comparing the location marked F in layer 6a with the same location in each of layers 6b to 6f, it will be seen that all of the opacifying layers 6a to 6f have an aligned gap across portion F of the image and hence there is no opacifing material present here, only the ink of print workings 8a and 8b. Hence, when viewed in transmitted light, the portion F will appear light and a bright shade of purple.
In contrast, the region R is provided with a high optical density by arranging none of the opacifying layers 6a to 6f to have a gap at this location. This can be appreciated by comparing the portion marked R in layer 6f with the same location in each of layers 6a to 6e.
Other portions of the loop joining portion R to portion F have intermediate tones by virtue of the different number of opacifying layers present in each portion, resulting in a substantially continuous variation in the shade of the image and giving rise to a strong three-dimensional effect.
As in previous embodiments, each of the sub-images effectively defines portions of the multi-tone image in accordance with different tonal ranges. In this case, layer 6a defines the portions of the image having the lightest tones (or optical densities), e.g.
from 0% to 16%, as a gap, whilst the respective gaps in each of layers 6b, 6c, 6d, 6e and 6f correspond to different tonal value ranges of increasing size ¨ e.g.
0% to 33%; 0% to 50%; 0% to 66%; 0 to 82% and 0 to 95% respectively. It will be noted that all of the sub-images 6a to 6f in this example are negative images, whilst the print workings 8a and 8b are both positive images.
In this embodiment, each of the sub-images is a multi-tonal image as described with reference to Figure 3 above, the edges of the gap in each sub-image being "softened" by a boundary region of intermediate tone so that in the final multi-tone image the transition from one tone to the next appears gradual. However this is not essential.
The multi-tonal nature of the final image in this example is further enhanced by print working 8b which is a multi-tonal (e.g. half-toned) working in a dark colour configured to provide additional definition and shading to the twisted loop structure.
A fifth embodiment of a security print medium is shown in Figure 9, the various layers being depicted individually in plan view in the same manner as in Figure 8. In this case, three opacifying layers 6b, 6d and 6f are provided on a first surface of a polymer substrate 5 and three opacifying layers 6a, 6c and 6e are provided on its second surface. Again, in the Figures, the white portions surrounded by black in each of the sub-images in fact correspond to gaps in the sub-images, and the black portions represent the areas where opacifying material is present. Here, the multi-tonal image comprises a portrait P of a person, a drawing of a building B and a stripe element S, selected parts of which are visible in each sub-image (labelled only in layer 6a for clarity). A transparent window 12 surrounds at least part of the portrait P.
This multi-tone image is designed for viewing in reflected light against a dark background, although it will also be visible in transmitted light (the portrait appearing reversed). Hence portions of the image which are intended to appear brightest, such as the bridge of the person's nose and his cheekbones, require the highest optical density, and the portions which are to appear darkest, such as the shadows under his eyebrows and under his fingers, require the lowest. In this case, the darkest portions of the image have an optical density of 0%, i.e. the opacifying material is absent in all six layers, meaning that the dark colour of the underlying background is unobscured as can be appreciated by noting that these portions correspond to gaps (i.e. white regions, in the Figures) in every one of the sub-images. In contrast, the brightest portions of the portrait are provided with opacifying material in every one of the sub-images (i.e. black regions, in the Figures).
Again, the sub-images define portions of the multi-tone image in accordance with their desired tone. However, in this example, sub-images 6a and 6b are the same as one another, as are sub-images 6c and 6d, and 6e and 6f. Hence sub-images 6a and b define portions of the multi-tone image with optical densities in the range 0%
to less than 33% as gaps; sub-images 6c and d define portions with optical densities in the range 0% to less than 66% as gaps, and sub-images 6e and f define portions with optical densities in the range 0% to 95% as gaps. In this case, each of the sub-images is a positive image. As in previous embodiments the order in which the layers are applied to the substrate is unimportant.
The construction shown in Figure 9 will result in a monochromatic multi-tone image.
To provide additional colour to the image, one or more print workings may be added and examples of these are shown in Figure 10. Here, three print workings 8a, 8b and 8c are shown. Working 8a is located on the first surface of substrate 5, underneath opacifying layer 6b shown in Figure 9, and workings 8b and c are located on the second surface of substrate 5, underneath opacifying layer 6a.
In this case, each of the print workings 8a, 8b and 8c is a multi-tonal working in a different colour. For example, working 8a may be red, working 8b brown and working 8c blue. The print provides multiple colours and additional shading to the multi-tonal image.
Figure 11 shows a sixth embodiment of a security print medium. Again, the various layers forming the security print medium are shown individually in plan view.
Three opacifying layers 6a,b,c are applied to a first side of a substrate 5, and three additional, optional opacifying layers 7a,b,c to the other. In this case the three opacifying layers 7a,b,c are applied uniformly across the substrate 5 and hence do not contribute to the multi-tone image other than to increase its optical density uniformly throughout. Each of layers 6a, 6b and 6c is defined according to a different sub-image in the same manner as previously described. In this case the resulting multi-tonal image is a complex pattern of interlocking geometrical elements. Different parts of the pattern are provided with different tones by virtue of the number of opacifying layers present at any one location. As in Figures 8 and 9, the white portions illustrated in each of the sub-images in fact correspond to gaps in the sub-images, and the black portions represent the areas where opacifying material is present. Hence in this case, the fine lines visible in layer 6c, which align with the centre of the thicker black lines in Figure 6b and the thicker-still black regions in Figure 6c, will ultimately have the highest optical density, whilst the intervening areas will have the lowest optical density. As a result, in the finished image, different portions of the pattern will appear to protrude towards and away from the viewed, resulting in a three-dimensional effect. As in all previous embodiments, the order in which the opacifying layers are applied to the substrate is unimportant and the position of the substrate within the stack of layers could also be chosen at will ¨ for instance, the substrate S could be located between layers 6c and 6b, or between layers 7a and 7b, or any other two adjacent layers in the stack.
Again, each of the sub-images defines portions of the pattern according to their optical density and in this case all of the sub-images are positive images.
In all of the above embodiments, it is preferred that each opacifying layer has an optical density in the range 0.1 to 0.5 (as measured on a transmission densitometer such as the MacBeth TD932, with an aperture area equivalent to that of a circle with a 1mm diameter), more preferably 0.1 to 0.3. Advantageously, 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. Preferably, the opacifying layers should be white, off-white or grey. 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 7 and 6a in Figure 1, layers 6d and 6c in Figure 8, layers 6d and 6c in Figure 9 and layers 6b and 7b in Figure 11.

The opacifying coating for any of the above embodiments 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 opacifying layers can each be applied by any suitable application process which allows their selective application in accordance with the respective sub-images.
Typically, each opacifying layer will be applied by gravure printing.
Alternatively, any of flexographic printing, screen printing or lithographic printing may be used. The 10 opacifying layers and any print workings should preferably be applied in register with one another, as may be achieved by applying all of them 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 15 protective layer (preferably transparent) or a print-receptive coating.
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 20 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 12, (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, are as previously described in relation to Figures 1, 2 and 3 and hence will not be described again.
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 6b and 6c. In other cases the graphics layer 20 may be applied only to one or other 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 12 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 a plurality of overlapping opacifying layers disposed on the first and/or second surfaces of the polymer substrate, each of the opacifying layers being a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, wherein in at least a region of the substrate a multi-tonal image is exhibited by the plurality of overlapping opacifying layers in combination with one another, at least when the security print medium is viewed in transmitted light, each of the plurality of overlapping opacifying layers having gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of each layer being defined in accordance with a different respective sub-image, the sub-images in combination defining the multi-tonal image, wherein either all the sub-images are different negative image versions of the multi-tonal image or all the sub-images are different positive image versions of the multi-tonal image, whereby the number of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of overlapping opacifying layers in combination with one another giving rise to the multiple tones of the multi-tonal image.

2. A security print medium according to claim 1, wherein each sub-image defines portions of the multi-tonal image which have a tonal value falling within a respective tonal value range, the size of each respective tonal value range being different.

3. A security print medium according to claim 2, wherein when the tonal value ranges of the sub-images are ordered according to increasing size, each tonal value range falls within the tonal value range next in the sequence.

4. A security print medium according to claim 3, wherein all of the tonal value ranges share substantially the same first end value and differ in their second end values.

5. A security print medium according to any of the preceding claims, wherein at least some of the sub-images are multi-tonal sub-images, preferably half-tone sub-images.

6. 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.

7. A security print medium according to any of the preceding claims, further comprising a mono-tone or multi-tone print of at least part of the multi-tonal image in one or multiple colours which contrast visually with the opacifying layers, located between at least one, preferably all, of the opacifying layers and the polymer substrate on the first and/or second surfaces thereof, the print of the multi-tonal image being in alignment with the sub-images in the opacifying layers.

8. A security print medium according to claim 7, wherein the print is a multi-tone print and comprises at least one multi-tone, preferably half-tone, print working.

9. A security print medium according to claim 7 or claim 8, wherein the print comprises at least two print workings in different colours.

10. A security print medium according to any of the preceding claims, wherein the sub-images are configured such that a greater number of the opacifying layers overlap one another at locations across the substrate corresponding to darker tones in the multi-tone image, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image, the multi-tone image being configured for viewing in transmitted light.

11. A security print medium according to any of claims 1 to 9, wherein the sub-images are configured such that a smaller number of the opacifying layers overlap one another at locations across the substrate corresponding to darker tones in the multi-tone image, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image, the multi-tone image being configured for viewing in reflected light.

12. A security print medium according to any of the preceding claims, wherein the plurality of overlapping opacifying layers includes at least three overlapping opacifying layers each having gap(s) defined in accordance with a different respective sub-image, preferably at least four, more preferably at least six.

13. A security print medium according to any of the preceding claims, wherein at least one of the plurality of overlapping opacifying layers is provided on each of the first and the second surfaces of the polymer substrate, preferably half of the plurality of overlapping opacifying layers being provided on each of the first and second surfaces.

14. A security print medium according to any of the preceding claims, further comprising one or more additional opacifying layers each comprising a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, the one or more additional opacifying layers each either extending continuously across the region of the substrate containing the multi-tonal image or comprising a gap substantially across the region.

15. A security print medium according to any of the preceding claims, wherein the plurality of opacifying layers are disposed across at least 50% of the substrate, preferably at least 80% of the substrate and more preferably all of the substrate outside the region.

16. A security print medium according to any of the preceding claims, further comprising at least one transparent window region formed by aligned gaps in each of the opacifying layers, the at least one transparent window region preferably substantially surrounding the multi-tonal image.

17. A security print medium according to any of the preceding claims, wherein at least one of the sub-images is formed of an array of screen elements which are sufficiently large to be individually discernible to the naked eye, the size of the screen elements varying across the array to define the sub-image.

18. 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.

19. A security print medium according to claims 17 and 18, wherein the array of screen elements forming the at least one of the sub-images is arranged to visually cooperate with the array of screen elements forming the raised pattern layer.

20. A security print medium according to any of the preceding claims, wherein the opacifying layers are printed opacifying layers, preferably applied to the substrate by gravure printing.

21. A security print medium according to any of the preceding claims, wherein at least some of the opacifying layers are applied in the form of an array of screen elements which are too small to be individually discernible to the naked eye.

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

23. 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.

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

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

26. A method of making a security print medium, comprising:
providing a transparent or translucent polymer substrate having first and second opposing surfaces;
applying a plurality of overlapping opacifying layers onto the first and/or second surfaces of the polymer substrate, each of the opacifying layers being a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, each opacifying layer being applied in accordance with a different respective sub-image across at least a region of the substrate;
whereby each of the plurality of overlapping opacifying layers has gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of each layer being defined in accordance with a different respective sub-image, the sub-images in combination defining a multi-tonal image which is exhibited by the plurality of overlapping opacifying layers in combination with one another, at least when the security print medium is viewed in transmitted light, wherein either all the sub-images are different negative image versions of the multi-tonal image or all the sub-images are different positive image versions of the multi-tonal image, whereby the number of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of overlapping opacifying layers in combination with one another giving rise to the multiple tones of the multi-tonal image.

27. A method of making a security print medium according to claim 26, wherein each sub-image defines portions of the multi-tonal image which have a tonal value falling within a respective tonal value range, the size of each respective tonal value range being different.

28. A method of making a security print medium according to claim 27, wherein when the tonal value ranges of the sub-images are ordered according to increasing size, each tonal value range falls within the tonal value range next in the sequence.

29. A method of making a security print medium according to claim 28, wherein all of the tonal value ranges share substantially the same first end value and differ in their second end values.

30. A method of making a security print medium according to any of claims 26 to 29, wherein at least some of the sub-images are multi-tonal sub-images, preferably half-tone sub-images.

31. A method of making a security print medium according to any of claims 26 to 30, wherein all of the opacifying layers are substantially the same colour as one another, preferably white or grey.

32. A method of making a security print medium according to any of claims 26 to 31, further comprising applying a mono-tone or multi-tone print of at least part of the multi-tonal image in one or multiple colours which contrast visually with the opacifying layers, the print of the multi-tonal image being in alignment with the sub-images in the opacifying layers, where in the print is applied to the substrate before at least one, preferably all, of the opacifying layers are applied.

33. A method of making a security print medium according to claim 32, wherein the print is a multi-tone print and comprises at least one multi-tone, preferably half-tone, print working.

34. A method of making a security print medium according to claim 32 or 33, wherein the print comprises at least two print workings in different colours.

35. A method of making a security print medium according to any of claims 26 to 34, wherein applying the plurality of overlapping opacifying layers comprises applying at least three overlapping opacifying layers each having gap(s) defined in accordance with a different respective sub-image, preferably at least four, more preferably at least six.

36. A method of making a security print medium according to any of claims 26 to 35, wherein applying the plurality of overlapping opacifying layers comprises applying at least one of the plurality of overlapping opacifying layers on the first surface of the polymer substrate and applying at least one of the plurality of overlapping opacifying layers on the second surface of the polymer substrate, preferably applying half of the plurality of overlapping opacifying layers on to each of the first and second surfaces.

37. A method of making a security print medium according to any of claims 26 to 36, further comprising applying one or more additional opacifying layers each comprising a layer of semi-opaque material disposed over substantially the whole area of the polymer substrate, the one or more additional opacifying layers each either extending continuously across the region of the substrate containing the multi-tonal image or comprising a gap substantially across the region.

38. A method of making a security print medium according to any of claims 26 to 37, wherein applying the plurality of overlapping opacifying layers comprises applying the opacifying layers across at least 50% of the substrate, preferably at least 80% of the substrate and more preferably all of the substrate outside the region.

39. A method of making a security print medium according to any of claims 26 to 38, wherein the plurality of opacifying layers are applied such that gaps in each of them align to form at least one transparent window region, the at least one transparent window region preferably substantially surrounding the multi-tonal image.

40. A method of making a security print medium according to any of claims 26 to 39, wherein at least one of the opacifying layers is applied in the form of an array of screen elements defining the respective sub-image, the screen elements being are sufficiently large to be individually discernible to the naked eye, the size of the screen elements varying across the array to define the sub-image.

41. A method of making a security print medium according to any of claims 26 to 40, further comprising applying a raised pattern layer 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.

42. A method of making a security print medium according to claims 40 and 41, wherein the array of screen elements forming the at least one of the sub-images is arranged to visually cooperate with the array of screen elements forming the raised pattern layer.

43. A method of making a security print medium according to any of claims 26 to 42, wherein the opacifying layers are applied by printing, preferably by gravure printing.

44. A method of making a security print medium according to any of claims 26 to 43, wherein at least some of the opacifying layers are applied in the form of an array of screen elements which are too small to be individually discernible to the naked eye.

45. A method of making a security print medium according to any of claims 26 to 44, wherein at least one of the opacifying layers comprises electrically conductive particles.

46. A method of making a security print medium according to any of claims 26 to 45, 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.

47. A method of making a security document comprising:
making a security print medium in accordance with the method of any of claims 26 to 46; 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.