GB2542784A

GB2542784A - Security print media and method of manufacture thereof

Info

Publication number: GB2542784A
Application number: GB1517151.5A
Authority: GB
Inventors: Keen Alan; Baker Malcolm
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2015-09-29
Filing date: 2015-09-29
Publication date: 2017-04-05
Anticipated expiration: 2035-09-29
Also published as: EP3356155B1; GB2542784B; CA2999802A1; GB201517151D0; AU2016330008A1; WO2017055826A1; MX2018003881A; EP3356155A1; AU2016330008B2

Abstract

A security print medium 1 is disclosed for forming security documents therefrom. The security print medium comprises a transparent or translucent polymer substrate 5 having first and second opposing surfaces 5a, 5b and at least one opacifying layer 6a, 6b, 7 disposed on the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material. The security print medium further comprises a first print of a multi-tonal image 10a disposed on the first and/or second surface of the polymer substrate, the print being covered from the point of view of an observer on a first side of the security print medium by at least one of the opacifying layers 7 which is disposed across the substrate in accordance with a screened working (7a, Fig.1c) of the multi-tonal image in alignment with the first print of the multi-tonal image. The screened working comprises of an array of screen elements. When the security print medium is viewed in reflected light, the screen elements dominate the appearance of the multi-tonal image and when the security print medium is viewed in transmitted light, the first print dominates the appearance of the multi-tonal image.

Description

SECURITY PRINT MEDIA AND METHOD OF MANUFACTURE THEREOF

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

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

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

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

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

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

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

It will be appreciated that the first print and/or the at least one opacifying layer need not be in direct contact with the surface of the polymer substrate. Rather, one or more additional (transparent or translucent) layers could be present between the polymer substrate and the first print and/or opacifying layers, such as a primer layer, the opacifying layers still being considered disposed “on” the substrate surface.

By “screened working”, it is meant a layer formed of an array of screen elements whose characteristics may be spatially modulated across the layer so as to provide regions of visual contrast. For example, the screen elements (or analogously the background surrounding the elements) may vary in their size, thickness, spacing, ink density, colour, tone, hue and/or saturation.

In the present case, the screened working of the multi-tonal image is a version of the image formed of an array of screen elements, whose shape, and placement form a version of the image. The screen elements are delimited by the presence and absence of the opacifying layer across the surface of the polymeric substrate. Typically, an opacifying layer will be a consistent white or grey in appearance, and therefore the screen elements will typically be white or grey elements thereby giving the screened working of the multi-tonal image a monochromatic appearance. A striking and difficult to counterfeit transition will be visible between the monochromatic screened working of the multi-tonal image, which is visible in reflection, and the image formed by the multi-tonal print beneath, which becomes visible when the security print media is viewed in transmission.

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

It will be appreciated that the term “print” is intended to cover an image formed of a composition such as ink applied by any technique including conventional printing methods such as gravure, flexographic printing, lithography etc., but also ablation methods in which an all-over ink layer is applied and then selectively removed to leave an image, e.g. using a laser.

In some embodiments, the first print of the multi-tonal image is a multi-coloured print, preferably comprising multiple workings and preferably comprising multiple workings in different colours. By providing a multi-coloured, multi-tonal image as the print which is covered by the screened working, a very striking transition can be achieved when the viewer switches from viewing in reflection to viewing in transmission and vice versa, with the image displayed changing between a single colour version of the image in reflection and a multi-coloured, multi-toned version in transmission. While preferable, a monochromatic, multi-toned print may instead be used. As mentioned, it is preferable that the multi-colour print is formed by multiple print workings applied to the substrate. This maybe in the form of a first half of the image being formed of a first working in a first colour and a second half of the working formed in a second colour, or of a second, and optionally a third, monochromatic print of the multi-tone image being applied over and in register with a first monochromatic print of the multi-tone image, in different colours, to produce a single multi-colour depiction of the multi-tone image.

Preferably, the at least one opacifying layer further includes one or more opacifying layers disposed continuously across the first print of the multi-tonal image so as to cover the first print from the point of view of an observer on the second side of the security print medium. In such embodiments, the image is concealed from the side of the medium not intended for viewing, which results in a more striking appearance of the image when viewed as intended. While preferable, in alternative embodiments, the first print may also be uncovered from the point of view of an observer on the second side of the security print medium.

In some embodiments, the at least one opacifying layer further includes one or more opacifying layers each defining at least one gap, laterally inside which the multi-tonal image is located. These additional one or more opacifying layers may thereby, for example, present the optical effect as part of a transparent window feature in the substrate.

In preferable embodiments, the at least one opacifying layer further includes one or more opacifying layers having gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of the one or more opacifying layers being defined in accordance with at least one sub-image, wherein either the or each sub-image is a positive image version of the multi-tonal image, or the or each sub-image is a negative image version of the multi-tonal image, and wherein the or each sub-image is in alignment with the first print of the multi-tonal image. 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 material). Whether a positive or negative version of the multi-tonal image is used for the subimages will be determined by whether the opacifying layers formed in accordance with the sub-images are intended to enhance (e.g. contribute tone) to the image as seen by an observer in reflection or in transmission, that is, to complement the screened working or the first print. Typically, the image used for the opacifying layer formed as a screened working will be a negative image version as the elements of the image formed by the screened working are produced by areas in which the opacifying material is removed from an otherwise continuous opacifying layer to provide regions of darker tone. In contrast, the image used for the print of the multi-tonal image is a positive image version, as the print is applied only in areas in which the image is to be defined, to provide the darker toned elements of the image. The opacifying layers formed in accordance with the sub-images may then be configured to contribute more subtle tonal variations to either the image as seen in reflection or transmission. Negative image versions will be used when the opacifying layers formed in accordance with sub-images are intended for contributions in reflection, whereas positive image versions will be used when the opacifying layers formed in accordance with sub-images are intended for contributions in transmission.

More specifically, the opacifying layer(s) formed as sub-images can be used to provide regions of high optical density, corresponding to locations in which the one or more opacifying layer is present, which may then appear as bright portions of the multi-tonal image when viewed in reflection against a dark surface, or as dark portions of the multi-tonal image when viewed in transmission.

It will be appreciated that opacifying layers formed as sub-images which darken tone of the image as seen in transmission will tend to lighten the tone of the image as seen in reflection and vice versa. In some cases the print and the screened working may be configured to take advantage of this; however in other embodiments this effect may be undesirable. In such embodiments, where the opacifying layers formed as sub-images would tend to detract from the desired appearance of the image in either transmission or reflection, the respective one of the print and the screened working may be configured to compensate for this effect. More generally, the contribution to the appearance of the multi-tonal image (both when viewed in reflection and in transmission) from each of the sub-images, the screened working and the print may need to be balanced to achieve an optimum visual effect under each viewing condition. The designer will be able to achieve this by appropriate adjustment of each layer’s coverage, optical density, colour or other parameters.

In particularly preferable embodiments, the at least one opacifying layer includes a plurality of 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, wherein either all the sub-images are different positive image versions of the multi-tonal image, or all the sub-images are different negative image versions of the multi-tonal image, whereby the number of the plurality of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of opacifying layers contributing to the multiple tones of the multi-tonal image as viewed by the observer in reflected light and/or transmitted light, and wherein each sub-image is in alignment with the first print of the multi-tonal image. 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. The plurality of opacifying layers formed in accordance with sub-images may contribute multiple levels of tonal variation to the image seen in either transmission or reflection depending on the number of opacifying layers present in any one region. Those regions with the greatest number of opacifying layers present will have the highest optical density, and therefore appear brightest when viewed in reflection on a dark background, and darkest when viewed in transmission, while those regions with the fewest opacifying layers will appear less bright in reflection and more bright in transmissions 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 and screened working or print working are arranged to achieve a gradual change in optical density across the image (on a scale when viewed by the naked eye).

In some embodiments featuring these opacifying layers formed as sub-images, the or each sub-image defines portions of the multi-tonal image which have a (desired) tonal value falling within a 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. lightest tone portions having an optical density of 0.9, and darkest 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.

Preferably the size of each respective tonal value range is different, and 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 subimage 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 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.

As mentioned above, in some embodiments, the sub-images may be 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 as viewed by the observer in reflected light, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image as viewed by the observer in reflected light. In such embodiments it is a dark background apparent through the opacifying layers which provides the darkness to the image, and areas with more opacifying layers obscuring the dark background and reflecting light so as to appear brighter than areas with fewer opacifying layers which provide the tonal variations.

Alternatively, in some 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 as viewed by the observer in transmitted light, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image as viewed by the observer in transmitted light. In such embodiments it is the attenuation of transmitted light according to the optical density of the opacifying layers which provides the tonal variation, with those areas having more overlapping opacifying layers attenuating more light and therefore appearing darker in transmission.

So that the visibility of the screened working of the multi-tonal image is maximised, it is preferable that the opacifying layer which is disposed across the substrate in accordance with the screened working of the multi-tonal image is the outermost opacifying layer. While preferable, alternative embodiments exist in which the screened working is covered by one or more opacifying layers, such as the opacifying layers formed in accordance with a number of sub-images. It will be appreciated that in this context outermost opacifying layer means, for opacifying layers on a first surface of the substrate, that the opacifying layer formed as a screened working is arranged on top of all other opacifying layers from the point of view of an observer viewing the first surface of the substrate. In embodiments featuring a screened working on each side of the substrate, this may mean that the screened working applied to the first side of the substrate is on top of all other opacifying layers of the first side of the substrate, and the screened working applied to the second side of the substrate is on top of all other opacifying layers on the second side of the substrate.

In some embodiments, the security print medium further comprises a second print located on top of the opacifying layer which is disposed in accordance with the screened working of the multi-tonal image from the point of view of the observer on the first side of the security print medium, the second print being a second screened working of the multi-tonal image in a colour which contrasts with that of the opacifying layer(s), wherein the second print is preferably an intaglio print. The screened working of the second print and the screened working of the opacifying layer may be used to demonstrate the high levels of registration achieved in production of the medium, and thereby present a further obstacle to producing a convincing counterfeit. Typically the second print is on top of all the opacifiying layers, and, preferably the opacifying layer which is disposed across the substrate in accordance with the screened working is the outermost opacifying layer. In other embodiments, however, the second print may be disposed either on top or beneath any additional opacifying layers, such as the opacifying layers formed in accordance with one or more sub-images.

In some embodiments there is provided 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. In some particularly preferable embodiments this raised pattern layer is the abovementioned second print.

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

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

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

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

It may also be preferable to apply at least one opacifying layer, which is not the at least one opacifying layer disposed across the substrate in accordance with a screened working of the multi-tonal image, in the form of an array of screen elements which are too small to be individually discernible to the naked eye. This will typically be the case where the layer is applied by gravure printing with a cell size too small for individual recognition by the naked eye. Alternatively, all opacifying layers which are not the screened working may be substantially continuous layers of opacifying material.

Preferably, at least one opacifying layer is a printed opacifying layer, preferably applied to the substrate by gravure printing. While it is preferable that the opacifying layers are printed, any method of application may be used.

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

It is also preferable that all of the opacifying layers are substantially the same colour as one another, most preferably a light and bright colour such as white (including off-white) or grey, to give a seamless appearance to the surface of the security print media, and to provide an ideal surface on which additional printing or applicating can be performed in the manufacturing of a security document. While preferable, opacifying layers of different colours may be used, and in particular, the opacifying layer providing the screened working may be provided in a different colour to the remaining opacifying layers. Whether or not all layers are the same colour, it is preferable that the screen elements of the screened working are substantially all of the same colour. This will be the case where the opacifying layer forming the screened working has a consistent colour across its surface.

In some embodiments the screened working is registered to the first print. This is achieved during the making of the security print media, by using registration techniques when applying the opacifying layer which is disposed across the substrate in accordance with a screened working of the multi-tonal image to the substrate such that it is in precise alignment with the first print of the multi-tonal image.

In all of the above embodiments, the multi-tonal image can depict anything, however, the effect is most visually striking when the multi-tonal image comprises an image of a three-dimensional object, preferably a geometrical solid or wireframe model, a person, an animal, a building or other architectural structure or a three-dimensional logo.

The method of making a security print medium already introduced above can be adapted to make any of the preferred features described above.

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

Examples of security print media in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

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

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

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

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

Figure 5 shows fourth and fifth embodiments of a security print medium, Figure 5(a) shows a fourth embodiment in cross-section, and Figures 5(b) to 5(d) show portions of different opacifying layers of the security print medium of Figure 5(a), while Figure 5(e) shows a fifth embodiment in cross-section, and Figures 5(f) to 5(h) show portions of different opacifying layers of the security print medium of Figure 5(e); Figure 6 shows schematically a sixth embodiment of a security print medium, with each layer applied to the substrate being depicted individually in plan view;

Figure 7 shows schematically a seventh embodiment of a security print medium, with each layer applied to the substrate being depicted individually in plan view; Figure 8 shows schematically a eighth embodiment of a security print medium, with each layer applied to the substrate being depicted individually in plan view; and Figure 9 shows a first embodiment of a security document (a) in plan view, and (b) in cross-section, layers of the security document being shown spaced apart for clarity.

The description below will focus on examples security print media used in the production of banknotes. However, as mentioned above, the security print media could be used to form any type of security document, including passports (or individual pages thereof), identification cards, certificates, cheques and the like. Throughout this disclosure, the term “security print media” is used synonymously with the term “document substrate”, meaning a medium which can then be printed upon and otherwise processed to form the desired security document, in a manner analogous to the printing and subsequent processing of a conventional paper substrate (albeit with processes adapted for use on polymer). Hence a “security print medium” does not encompass graphics layers and the like, which are later printed onto the security print medium to provide security patterns, indicia, denomination identifiers, currency identifiers etc. The combination of such a graphics layer and a “security print medium” (and optionally additional features such as applied foils, strips, patches etc.) is the “security document”.

Throughout the following examples, the security print medium will be illustrated as having the same size and shape as a security document into which it is later formed. However, typically the security print medium will be formed as a web or sheet large enough to carry multiple repeats of the desired security document, and will then be cut into individual document either before, but more usually after, printing of the graphics layer and any other required processing steps.

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

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

The polymer substrate 5 has opposing upper and lower surfaces 5a, 5b. In a region, on the upper surface 5a, there is disposed a multi-tonal image 10 in the form of a single print working 10a. In this example, the multi-tone image 10 is of a hemisphere; the circular print working 10a is a halftone print working, and is provided with a lighter tone towards its centre, and a darker tone towards its edges to give the impression to the viewer that the centre is closer than the edges. The print working 10a may be formed by a conventional colour ink using a process such as gravure printing or flexographic printing. The ink may be selected so as to be translucent to allow the colour to be visible in transmitted light.

As shown in Figure 1(c), substantially all of the medium 1 carries a coating 6 formed of a plurality of opacifying layers as described further below. This renders the medium non-transparent across the whole of the coated area and provides a suitable background for printing thereon. The coating 6 may optionally be omitted in certain areas of the medium to form features such as strip 2 and window 3, which are transparent or translucent (relative to the coated areas). Such transparent areas may be provided as security features in their own right or may be later equipped with additional security devices during the manufacture of a security document using the medium 1, as described further below.

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

The opacifying layer 6a on the upper side 5a of the substrate 5 defines a circular gap 12, laterally inside which the multi-tonal image 10 is located, thereby presenting the multi-tonal image as a window feature. 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 other embodiments disclosed herein. In contrast, the opacifying layer 6b on the lower side 5b of the substrate 5 is disposed continuously across the region of the multi-tonal image 10 so as to cover the multi-tonal image from the point of view of an observer viewing the lower surface 5b of the substrate 5. Alternatively, the opacifying layer on the lower side may also define a gap which surrounds the multi-tonal image 10, thereby presenting it as a full window feature rather than a half window feature.

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

As further shown in Figures 1(c) and 1(d), the multi-tonal image 10 is covered from the point of view of an observer viewing the upper side 5a of the security print medium by an opacifying layer 7. The opacifying layer 7 is disposed across the substrate 5 in accordance with a screened working 7a of the multi-tonal image 10. In this example, the screened working 7a is a line screen, and comprises a series of screen elements which are separate straight lines of opacifying material that run along the surface of the substrate. Each screen element has a width of approximately 200 pm, and is separated from the adjacent screen element by a gap of approximately 150 pm, thereby rendering the individual screen elements individually discernible to the naked eye. The elements of the line screen 7a are present within an area directly over the multi-tonal image 10, i.e. the elements are arranged in the form of a circle, matching the outline of the multi-tonal image 10, with its placement being registered to that of the print working 10a and the other opacifying layers, as described above. In this embodiment, the opacifying layer 7 ceases where the screened working 7a of the multi-tonal image ceases; however, in some embodiments, the opacifying layer 7 may continue across substantially all of the rest of the surface 5a of the substrate 5 as a homogenous opacifying layer, similar to the opacifying layers 6a, 6b.

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

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

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

Over the multi-tonal image is provided an opacifying layer 7. The opacifying layer in this embodiment extends over substantially all of the upper surface 5a of the substrate 5, with the only gaps being those contributing to the screened working 7a of the multi-tonal image, which lays directly over the three print workings 10a, 10b, 10c, in a way similar to the first embodiment. In this embodiment, the screened working is comprised of an array of indicia arranged in accordance with the portrait to form a representation of that portrait. It should be appreciated that whilst in practice the opacifying layers will typically be white, here the opacifying material is illustrated as black in order to be visible in the Figure. Thus the white portions surrounded by black in fact correspond to gaps in the screened working 7a, and the black portions represent the areas where opacifying material is present.

In this example, the screened working 7a is a multi-tonal screened working. The multiple tones of the multi-tonal screened working 7a are provided by incremental changes in the weight and density of the indicia used as the screen elements; in areas intended to be lightest in tone, the surface area of each indicium is greater, and the indicia are positioned closer together to increase the covering of that area with opacifying material. The white or grey colour of the opacifying material used for the screened working results in the areas having the most opacifying material appearing lightest when viewed in reflection.

In this embodiment, the lower side 5b of the substrate is substantially completely coated by first 6b and second 6d layers of opacifying material applied on top of one another and extending across the entire surface of the substrate 5.

The embodiment of Figure 3 shows a third embodiment of the invention. This embodiment is similar to the embodiment of Figure 1, and the differences will now be described.

This embodiment features a second print working 10b on the lower surface 5b of the substrate 5, as shown in Figure 3(a). In this embodiment, the second print working 10b is identical to the first print working 10a, and is placed on the lower surface in register with the first print working 10a, as shown in Figure 3(b). The opacifying layer 6b on the lower surface 5b of the substrate now also features a gap 12, laterally inside which the multi-tonal image 10 of a hemisphere is located.

In this embodiment, the opacifying layer 7 is formed as a screened working 7a, which has screen elements that are a series of concentric rings of decreasing radius. The elements of the screened working 7a are present within an area directly over the multi-tonal image 10, i.e. the outermost ring substantially matches the periphery of the multi-tonal image 10, and the remaining rings complete the area within the outermost ring to give the screened working the appearance of a circle, with its placement being registered to that of the print working 10a and the other opacifying layers. The screened working 7a in this case is a multi-tonal screened working, with the multiple tones being provided by an increasing line width of the ring elements of the screened working towards the centre of the screened working. In this embodiment, the opacifying layer 7 further extends over substantially the entire upper surface 5a of substrate 5 in the region laterally outside of the region of the multi-tonal image 10.

An opacifying layer 7’, identical to the opacifying layer 7, is disposed on the lower surface 5b of the substrate 5 such that a second screened working 7’a, identical to the first 7a, is positioned in the gap 12 and covers the second print working 10b from the point of view of an observer viewing the lower side 5b of the security print medium 1.

In this embodiment, the transition effect described with relation to Figure 1 is visible from both sides of the substrate. A further optional but beneficial feature will now be described with reference to Figure 4. Figure 4(a) shows an exemplary raised pattern layer 13 which may be applied over the outermost opacifying layer(s) and/or across the screened working 7a. For instance, in the Figure 3 embodiment, the raised pattern layer 13 may be applied over the opacifying layer 7 on the first surface 5a of the substrate 5, and/or over the opacifying layer 7’ on the second surface 5b of the substrate 5. The raised pattern layer may comprise for example a colourless, transparent ink which is applied to the medium 1 in accordance with a screen pattern, the elements of which are large enough to be individual discernible to the naked eye (possibly only under close inspection). Similar to the opacifying layer, the raised pattern layer 13 may be applied in the form of an array of line or dot screen elements. In the present case, the raised pattern layer is in the form of a grid of lines as shown, whose height depends upon their position across the screened working, e.g. having a maximum height at the leftmost side of the screened working 7a and tapering down towards the right side of the screened working, as shown in Figure 4(b). The raised pattern layer may be applied by intaglio printing for example and preferably has a latent appearance in that its presence is less visible when the medium is viewed at some angles, relative to others. At certain viewing angles, which depend on the location of the illuminating light source, the raised image pattern will reflect light more strongly to the viewer, and thus become more visible, than at other viewing angles. The pattern 13 may or may not be directly related to the content of the multi-tone image 10. In this example, the raised pattern layer extends across the screened working 7a but otherwise does not reflect the features of the multi-tone image, instead comprising a grid pattern, the line weight of which varies from left to right across the region such that it fades to absent on the right side of the screened working 7a. 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 it to substantially match the screened working 7a.

Figure 5 shows two particularly preferable embodiments of the invention. These embodiments feature a substrate 5 having upper and lower sides 5a, 5b. On the upper side is a multi-tonal image 10 of a hemisphere formed by a single print working 10a, and a screened working 7a in the form of a series of concentric rings, as in the third embodiment, over the print working 10a. On the lower side 5b are opacifying layers 6b, 6d, 6f, each covering the entire lower side of the substrate 5.

In the embodiment shown in Figures 5(a) to 5(d), three opacifying layers 6a, 6c, 6e are present on the upper surface 5a of the substrate 5, located over the print working 10a and below the screened working 7a and these three opacifying layers contribute to the multiple tones of the multi-tonal image, at least when viewed in reflection.

All three opacifying layers are present across substantially the entire upper surface of the substrate and define a circular gap 12 laterally inside of which is the multi-tonal image.

In addition to this, each of the opacifying layers 6a, 6c and 6e includes a region of opacifying material laterally inside the circular gap 12 which is defined in accordance with a different respective sub-image. The sub-images are shown in plan view in Figures 5(b), (c) and (d) for layers 6a, 6c and 6e, respectively (each of Figures 5(b) to (d) showing only a section of the respective opacifying layer including and surrounding the gap 12, and omitting the remainder of the layer). The different subimages are configured such that once the opacifying layers are arranged on top of one another, as shown in Figure 5(a), the cumulative effect of the different sub images is a variation in the optical density of the security print medium across the screened working 7a which contribute multiple tones to the screened working 7a when the medium is viewed in reflected light.

In this embodiment the opacifying layers 6a, 6c and 6e contribute by providing regions of increasing optical density of opacifying layers towards the centre of the screened working 7a. An outer annulus, immediately inside the gap 12, where none of the opacifying layers 6a, 6c and 6e are present will have the darkest tone when viewed in reflection, as light will reflect only off the opacifying layer 7 formed as the screened working 7a. Inside this outermost region, there is an annular region in which the opacifying layer 6a is disposed between the opacifying layer 7 of the screened working and the print working 10a, thereby giving the region a higher optical density than the previous region and therefore appearing lighter in tone in reflection. Inside this second annulus region is another annulus region in which the opacifying layer 6c and opacifying layer 6a are disposed between the opacifying layer 7 of the screened working and the print working 10a, providing an even higher optical density and an even lighter tone in reflection. Finally, a central circular region exists, inside of all of the annulus regions, in which the opacifying layers 6a, 6c, 6e are present between the opacifying layer 7 of the screened working and the print working 10a, providing the highest optical density, and hence the lightest tone when viewed in reflection.

To describe the variation in optical density more specifically, ignoring the contribution to optical density of the screened working 7a, taking the optical density of outermost annular region to be 0% on an arbitrary relative scale, and that of innermost portion to be 100%, the intermediate region having only opacifying layer 6a has an optical density of 33% and the intermediate region having opacifying layers 6a and 6c has an optical density of 66%. Alternatively, on an absolute scale, if each opacifying layer 6a,c,e 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), outermost portion will have an optical density of 0.0, the intermediate region having only opacifying layer 6a has an optical density of 0.2, the intermediate region having opacifying layers 6a and 6c has an optical density of 0.4 and the innermost region has an optical density of 0.6.

These different optical densities appear as a variation in tone across the image, resulting in a three-dimensional effect.

In this embodiment, just as in the previous embodiments, when viewed in reflection, the appearance will be dominated by the opacifying layer 7 formed as a screened working 7a, while the appearance is enhanced by the opacifying layers 6a, 6c, 6e, which contribute tone to the image seen in reflection. The viewer will see a circle which appears lighter in tone in its centre as a result of the increased size of screen elements in combination with the higher optical density provided by the greater number of layers of white opacifying material. However, when the viewer changes to viewing in transmission, the modulation of light passing through the security medium 1 by the print working 10a of the multi-tonal image 10 dominates the appearance of security medium, overwhelming the appearance of the opacifying layers. It will be noted that the opacifying layers 6a, 6c, 6e, which increase the optical density to provide a lighter tone in reflection, will act to reduce the intensity of light in transmission according to the optical density at any particular location. The print working 10a may be configured so as to compensate for this darkening in tone resulting from the opacifying layers 6a, 6c, 6e so that when viewed in transmission, the desired multi-tonal image is seen. In other words, the contribution to the appearance of the multi-tonal image (both when viewed in reflection and in transmission) from each of the opacifying layers 6a, 6c, 6e formed according to subimages, the screened working 7a and the print working 10a may need to be balanced to achieve an optimum visual effect under each viewing condition. This can be achieved by appropriate adjustment of each layer’s coverage, optical density, colour or other parameters.

Figures 5(e) to 5(h) show an alternative configuration of opacifying layers 6a, 6e, 6f. As in the embodiment of Figures 5(a) to 5(d), three opacifying layers 6a, 6c, 6e are present on the upper surface 5a of the substrate 5, located over the print working 10a and below the screened working 7a. However, in this embodiment, the opacifying layers 6a, 6c, 6e are configured to contribute tone to the image as seen in transmission.

All three opacifying layers 6a, 6c, 6e are present across substantially the entire upper surface of the substrate. In the region 12’, over the multi-tonal image 10, these opacifying layers are disposed in accordance with a respective sub-image, which is a positive image version of the multi-tonal image 10. The sub-images are shown in plan view in Figures 5(f), (g) and (h) for layers 6e, 6c and 6a, respectively (each of Figures 5(f) to (h) showing only a section of the respective opacifying layer, 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 5(e), the cumulative effect of the different sub-images is a variation in the optical density of the security print medium across the print working 10a which contribute multiple tones to the multi-tonal image 10 when the medium is viewed in transmitted light.

In this embodiment the opacifying layers 6a, 6c and 6e enhance the image as seen in transmission by providing regions of decreasing optical density of opacifying layers towards the centre of the multi-tonal image, thereby contributing additional variation in tone. An outer annulus, where all of the opacifying layers 6a, 6c and 6e are present beneath the screened working, will have the darkest tone when viewed in transmission owing to the high optical density of the opacifying coating in this region. Inside this outermost region, there is an annular region in which only the opacifying layers 6a and 6c are disposed between the opacifying layer 7 of the screened working and the print working 10a, thereby giving the region a lower optical density than the previous region and therefore appearing lighter in tone in transmission. Inside this second annulus region is another annulus region in which only the opacifying layer 6a is disposed between the opacifying layer 7 of the screened working and the print working 10a, providing an even lower optical density and an even lighter tone in transmission. Finally, a central circular region exists, inside of all of the annulus regions, in which none of the opacifying layers 6a, 6c, 6e are present between the opacifying layer 7 of the screened working and the print working 10a, providing the lowest optical density, and hence the lightest tone when viewed in transmission.

When this embodiment is viewed in transmission, the viewer will see a circle which appears lighter in tone in its centre as a result of a combination of the print working 10a and the lower optical density provided by the smaller number of layers of opacifying material. When the viewer changes to viewing in reflection, the appearance will be dominated by the screen elements of the screened working 7a, which is disposed over the opacifying layers 6a, 6c, 6e. It will be noted that the opacifying layers 6a, 6c, 6e, which increase the optical density to provide a darker tone in transmission, will act to produce a lighter tone in reflection according to the optical density at any particular location. The screened working 7a, if multi-tonal, may be configured so as to compensate for this effect, or the size and/or frequency of the screen elements selected so as to otherwise dominate the appearance in reflection. Again, for this embodiment, the contributions of the various layers may need to be balanced to achieve an optimum visual effect under each viewing condition

While the embodiment of Figure 5 depicts a simple example of the use of additional opacifying layers to contribute tone to the image of a hemisphere seen in reflection, it will be appreciated that the general principle can be applied to more complex images, such as that depicted in Figure 6.

The embodiment of Figure 6 comprises the same multi-coloured multi-tonal image 10, depicting a portrait, formed of print workings 10a, 10b, 10c as the embodiment of Figure 2. This embodiment also includes the same opacifying layer 7 formed as a multi-tonal screened working 7a of that portrait. This embodiment, however, features additional opacifying layers 6a and 6c, applied over the opacifying layer 7, substantially across the entire upper surface 5a of the substrate 5. Gaps in the opacifying layers 6a, 6c are formed in accordance with respective sub-images of the multi-coloured multi-tonal image 10. The sub-images are negative image versions of the multi-tonal image, and are configured such that the opacifying layers 6a, 6c contribute additional tone to the multi-tonal screened working 7a by providing regions of opacifying material in those areas intended to appear lightest in reflection. In this embodiment, the sub-images are also halftone sub-images. This means that the opacifying layers 6a, 6c, as well as being uniformly present in regions corresponding to the lightest tone (e.g. in a region corresponding to the white shirt collar of the portrait), and completely absent in regions corresponding to the darkest tones (e.g. in a region corresponding to the black suit jacket of the portrait), may also have intermediate regions, corresponding to areas between lightest and darkest tone, where the opacifying material is partially present to produce a gradient effect between the lightest and darkest tone regions of the sub-image.

Another example is shown in the embodiment of Figure 7. This embodiment is as in the embodiment of Figure 6; however, the opacifying layer 7 formed as a screened working is the uppermost opacifying layer on the upper surface 5a of the substrate 5 to maximise visibility of the screened working, and the opacifying layers 6a, 6c have gaps formed in accordance with respective sub-images, which are positive image versions of the multi-tonal image 10. The opacifying layers 6a, 6c thereby contribute additional tone to the multi-tonal image 10 as seen in transmission by providing regions of opacifying material overlapping those areas intended to appear darkest in transmission.

The embodiment of Figure 8 comprises the same multi-coloured multi-tonal image 10, depicting a portrait, formed of print workings 10a, 10b, 10c as the embodiment of Figure 2. This embodiment also includes the same opacifying layer 7 formed as a multi-tonal screened working 7a of that portrait. This embodiment differs from that of the embodiment of Figure 2 in that it additionally includes a second print 11 located on top of the screened working 7a from the point of view of an observer viewing the upper side 5a of the security print medium. This second print is also formed as a screened working of the multi-tonal image, and is applied registered to the screened working 7a in the opacifying layer 7, but in a colour (in this case black) which contrasts with that of the opacifying layer 7. In this case, the second print 11 is applied using an intaglio process, using a conventional intaglio ink. It should be noted that, depending upon the thickness of the second print 11, it may also provide a tactile effect to the upper surface 5a of the medium 1, and may cooperate with or act as the raised pattern layer 13 described in relation to Figure 4.

The above described arrangement results in an appearance in reflection of a black and white version of the multi-tonal image. It will be appreciated that whereas in previous embodiments, dark tones in the screened working were provided by reducing the amount of opacifying material in that region, in this embodiment, the black of the second print 11 may be used to positively provide those darker tones, while the white opacifying layer continues to provide the lighter tones in regions in which the second print will be mostly absent. When the security print medium 1 of this embodiment is viewed in transmission, the multi-coloured, multi-tonal image 10 provided by the print workings 10a, 10b, 10c will become visible through the opacifying layer, and will cooperate with the dark second print 11 to display the multi-tonal image in full colour to the viewer.

In all of the above embodiments, it is preferred that the opacifying layers should be white, off-white or grey; however, other colours could be used. The opacifying coating will typically comprise a resin such as a polyurethane based resin, polyester based resin or an epoxy based resin and an opacifying pigment such as titanium dioxide (Ti02), silica, zinc oxide, tin oxide, clays or calcium carbonate. The composition of each opacifying layer may be the same or different to one another. In preferred examples, one of the opacifying layers on each side of the substrate may comprise electrically conductive particles to reduce the effect of static charge. Preferably this is the penultimate layer on each side: for example, layers 6b and 6c in Figure 5a.

The above-described security print media can then be processed into security documents. The processing steps involved in doing so may be carried out on a separate processing line, typically at a different manufacturing site and optionally by a different entity. An example of a security document 100 formed using the security print medium 1 described above in relation to Figure 1 is shown in Figure 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 and screened working 7a, are as previously described in relation to Figure 1.

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

Figure 9 also illustrates examples of other security devices which may optionally be applied to the security print media to form the security document, such as an optically variable device 21 in window 3, e.g. a moire magnification device, a lenticular device or an integral imaging device as may be formed by cast-curing or laminating a lens array on one side of the polymer substrate 5 and forming image elements on the other. Also depicted is a security device 22 in the form of a patch which has been applied to the surface of the security print media, e.g. by lamination or hot stamping. The security device 22 may comprise a diffractive optical element such as a hologram, for example.

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

Claims

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

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

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

4. A security print medium according to any of claims 1 to 3, wherein the at least one opacifying layer further includes one or more opacifying layers each defining at least one gap, laterally inside which the multi-tonal image is located.

5. A security print medium according to any of the preceding claims, wherein the at least one opacifying layer further includes one or more opacifying layers having gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of the one or more opacifying layers being defined in accordance with at least one sub- image, wherein either the or each sub-image is a positive image version of the multi-tonal image, or the or each sub-image is a negative image version of the multi-tonal image, and wherein the or each sub-image is in alignment with the first print of the multi-tonal image.

6. A security print medium according to claim 5, wherein the at least one opacifying layer further includes a plurality of 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, wherein etiher all the sub-images are different positive image versions of the multi-tonal image, or all the sub-images are different negative image versions of the multi-tonal image, whereby the number of the plurality of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of opacifying layers contributing to the multiple tones of the multi-tonal image as viewed by the observer in reflected light and/or transmitted light, and wherein each sub-image is in alignment with the first print of the multi-tonal image.

7. A security print medium according to claim 5 or claim 6, wherein the or each sub-image defines portions of the multi-tonal image which have a tonal value falling within a respective tonal value range.

8. A security print medium according to claim 7 when dependent on claim 6, wherein the size of each respective tonal value range is different, and 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.

9. A security print medium according to claim 8, wherein all of the tonal value ranges share substantially the same first end value and differ in their second end values.

10. A security print medium according to any of claims 5 to 9, wherein at least one of the at least one sub-image is a multi-tonal sub-image, preferably a half-tone sub-image.

11. A security print medium according to at least claim 6, wherein the subimages 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 as viewed by the observer in reflected light, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image as viewed by the observer in reflected light.

12. A security print medium according to at least claim 6, wherein the subimages 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 as viewed by the observer in transmitted light, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image as viewed by the observer in transmitted light.

13. A security print medium according to any of the preceding claims, wherein the opacifying layer which is disposed across the substrate in accordance with the screened working of the multi-tonal image is the outermost opacifying layer.

14. A security print medium according to any of the preceding claims, further comprising a second print located on top of the opacifying layer which is disposed in accordance with the screened working of the multi-tonal image from the point of view of the observer on the first side of the security print medium, the second print being a second screened working of the multi-tonal image in a colour which contrasts with that of the opacifying layer(s), wherein the second print is preferably an intaglio print.

15. 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.

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

17. A security print medium according to any of the preceding claims, wherein at least some of the screen elements of the screened working have a smallest dimension in a direction along the first or second surface of the polymer substrate of 100 micrometres or more, more preferably 200 micrometres or more.

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

19. A security print medium according to claim 18, wherein the multiple tones of the multi-tonal screened working are provided by variations in the dimensions and/or frequency of the screen elements.

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

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

22. A security print medium according to any of the preceding claims, wherein at least one opacifying layer, which is not the at least one opacifying layer disposed across the substrate in accordance with a screened working of the multi-tonal image, is disposed across the substrate in the form of an array of screen elements which are too small to be individually discernible to the naked eye.

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

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

25. 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.

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

27. 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.

28. A security print medium according to any of the preceding claims, wherein the screened working is registered to the first print.

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

30. A security document comprising a security print medium according to any of claims 1 to 29, and at least one graphics layer applied on the outermost opacifying layer(s) on the first and/or second surfaces of the polymer substrate.

31. A security document according to claim 30, wherein the security document is a bank note, an identification document, a passport, a licence, a cheque, a visa, a stamp or a certificate.

32. A method of making a security print medium, comprising: providing a transparent or translucent polymer substrate having first and second opposing surfaces; applying a first print of a multi-tonal image onto the first and/or second surface of the polymer substrate; and applying at least one opacifying layer onto the first and/or second surfaces of the polymer substrate, the or each opacifying layer being a layer of semi-opaque material, wherein the first print is covered from the point of view of an observer on a first side of the security print medium by at least one of the opacifying layers which is disposed across the substrate in accordance with a screened working of the multi-tonal image in alignment with the first print of the multi-tonal image, the screened working comprising an array of screen elements, whereby when the security print medium is viewed by the observer in reflected light, the screen elements dominate the appearance of the multi-tonal image and when the security print medium is viewed by the observer in transmitted light, the first print dominates the appearance of the multi-tonal image.

33. A method of making a security print medium according to claim 32, wherein the first print of the multi-tonal image is a multi-coloured print, and wherein preferably the step of applying the first print comprises the steps of applying a first working onto the first and/or second surface of the polymer substrate and applying a second working onto the first and/or second surface of the polymer substrate, and wherein preferably the first and second workings are in different colours.

34. A method of making a security print medium according to claim 32 or claim 33, wherein applying at least one opacifying layer further includes applying one or more opacifying layers continuously across the first print of the multi-tonal image so as to cover the first print from the point of view of an observer on the second side of the security print medium.

35. A method of making a security print medium according to any of claims 32 to 34, wherein applying at least one opacifying layer further includes applying one or more opacifying layers each defining at least one gap, laterally inside which the multi-tonal image is located.

36. A method according to any of claims 32 to 35, wherein applying at least one opacifying layer further includes applying one or more opacifying layers having gap(s) in which the semi-opaque material of the layer is absent, the gap(s) of the one or more opacifying layers being defined in accordance with at least one subimage, wherein either the or each sub-image is a positive image version of the multi-tonal image, or the or each sub-image is a negative image version of the multi-tonal image, and wherein the or each sub-image is in alignment with the first print of the multi-tonal image.

37. A method according to claim 36, wherein applying at least one opacifying layer further includes applying a plurality of 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, wherein either all the sub-images are different positive 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 the plurality of opacifying layers overlapping one another at any one location varies across the substrate, the resulting variation in optical density of the plurality of opacifying layers contributing to the multiple tones of the multi-tonal image as viewed by the observer in reflected light and/or transmitted light, and wherein each sub-image is in alignment with the first print of the multi-tonal image.

38. A method according to claim 36 or 37, wherein the or each sub-image defines portions of the multi-tonal image which have a tonal value falling within a respective tonal value range.

39. A method according to claim 38 when dependent on claim 37, wherein the size of each respective tonal value range is different, and 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.

40. A method according to claim 39, wherein all of the tonal value ranges share substantially the same first end value and differ in their second end values.

41. A method according to any of claims 36 to 40 wherein at least one of the at least one sub-image is a multi-tonal sub-image, preferably a half-tone sub-image.

42. A method according to at least claim 37, 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 as viewed by the observer in reflected light, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image as viewed by the observer in reflected light.

43. A method according to at least claim 37, 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 as viewed by the observer in transmitted light, relative to the number of opacifying layers which overlap one another at locations corresponding to lighter tones in the multi-tone image as viewed by the observer in transmitted light

44. A method according to any of claims 32 to 43 wherein the opacifying layer which is disposed across the substrate in accordance with the screened working of the multi-tonal image is the outermost opacifying layer.

45. A method according to any of claims 32 to 44, further comprising the step of applying a second print on top of the opacifying layer which is disposed in accordance with the screened working of the multi-tonal image from the point of view of the observer on the first side of the security print medium, the second print being a second screened working of the multi-tonal image in a colour which contrasts with that of the opacifying layer(s), wherein the second print is preferably an intaglio print.

46. A method according to any of claims 32 to 45, further comprising the step of 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.

47. A method according to any of claims 32 to 46, wherein at least some of the screen elements of the screened working have dimensions which render them individually discernible to the naked eye.

48. A method according to any of claims 32 to 47, wherein at least some of the screen elements of the screened working have a smallest dimension in a direction along the first or second surface of the polymer substrate of 100 micrometres or more, more preferably 200 micrometres or more.

49. A method according to any of claims 32 to 48, wherein the screened working is a multi-tonal screened working.

50. A method according to claims 49, wherein the multiple tones of the multi-tonal screened working are provided by variations in the dimensions and/or frequency of the screen elements.

51. A method according to any of claims 32 to 50, wherein the screen elements of the screened working comprise one or more of lines, dots, squares and indicia.

52. A method according to any of claims 32 to 51, wherein the screen elements have an average area in a plane parallel to the first or second surface of the polymer substrate greater than the average area in a plane parallel to the first or second surface of the polymer substrate of elements of any element structure in any other opacifying layer.

53. A method according to any of claims 32 to 52, wherein at least one opacifying layer, which is not the at least one opacifying layer disposed across the substrate in accordance with a screened working of the multi-tonal image, is applied to the substrate in the form of an array of screen elements which are too small to be individually discernible to the naked eye.

54. A method according to any of claims 32 to 53, wherein at least one opacifying layer is applied by printing, preferably by gravure printing.

55. A method according to any of claims 32 to 54, wherein the or each opacifying layer is polymeric and non-fibrous.

56. A method according to any of claims 32 to 55, wherein all of the opacifying layers are substantially the same colour as one another, preferably white or grey.

57. A method according to any of claims 32 to 56, wherein at least one opacifying layer comprises electrically conductive particles.

58. A method according to any of claims 32 to 57, 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.

59. A method according to any of claims 32 to 58, wherein the step of applying the at least one opacifying layer comprises applying the at least one of the opacifying layers which is disposed across the substrate in accordance with a screened working such that the screened working is registered to the first print.

60. A method according to any of claims 32 to 59, wherein the screen elements of the screened working are substantially all of the same colour.

61. A method of making a security document comprising: making a security print medium in accordance with the method of any of claims 32 to 60; and applying at least one graphics layer to the outermost opacifying layer(s) on the first and/or second surfaces of the polymer substrate.

62. A method of making a security document according to claim 61, wherein the security document is a bank note, an identification document, a passport, a licence, a cheque, a visa, a stamp or a certificate.