Note: Descriptions are shown in the official language in which they were submitted.
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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
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paper-based documents. However, due to the non-fibrous construction of polymer
substrates, conventional watermarking techniques are not available and as such
the
potential for forming security elements integrally in the substrate itself is
limited.
Instead, for polymer security documents, security elements are typically
applied
after the document substrate has been manufactured, for example as part of a
subsequent security printing process line, or by the application of a foil.
It would be desirable to provide a polymer document substrate ¨ i.e. a
security print
medium, which can then be printed upon and otherwise processed into a security
document ¨ with an integral security feature, to enhance the security of the
document substrate itself, and ultimately of security documents formed from
it.
In accordance with the present invention, a security print medium for forming
security documents therefrom comprises a transparent or translucent polymer
substrate having first and second opposing surfaces, and a plurality of
overlapping
opacifying layers disposed on the first and/or second surfaces of the polymer
substrate, each of the opacifying layers being a layer of semi-opaque material
disposed over substantially the whole area of the polymer substrate, wherein
in at
least a region of the substrate a multi-tonal image is exhibited by the
plurality of
overlapping opacifying layers in combination with one another, at least when
the
security print medium is viewed in transmitted light, each of the plurality of
overlapping opacifying layers having gap(s) in which the semi-opaque material
of
the layer is absent, the gap(s) of each layer being defined in accordance with
a
different respective sub-image, the sub-images in combination defining the
multi-
tonal image, wherein either all the sub-images are different negative image
versions
of the multi-tonal image or all the sub-images are different positive image
versions of
the multi-tonal image, whereby the number of opacifying layers overlapping one
another at any one location varies across the substrate, the resulting
variation in
optical density of the plurality of overlapping opacifying layers in
combination with
one another giving rise to the multiple tones of the multi-tonal image.
The present invention also provides a method of making a security print
medium,
comprising: providing a transparent or translucent polymer substrate having
first and
second opposing surfaces; applying a plurality of overlapping opacifying
layers onto
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the first and/or second surfaces of the polymer substrate, each of the
opacifying
layers being a layer of semi-opaque material disposed over substantially the
whole
area of the polymer substrate, each opacifying layer being applied in
accordance
with a different respective sub-image across at least a region of the
substrate;
whereby each of the plurality of overlapping opacifying layers has gap(s) in
which
the semi-opaque material of the layer is absent, the gap(s) of each layer
being
defined in accordance with a different respective sub-image, the sub-images in
combination defining a multi-tonal image which is exhibited by the plurality
of
overlapping opacifying layers in combination with one another, at least when
the
security print medium is viewed in transmitted light, wherein either all the
sub-
images are different negative image versions of the multi-tonal image or all
the sub-
images are different positive image versions of the multi-tonal image, whereby
the
number of opacifying layers overlapping one another at any one location varies
across the substrate, the resulting variation in optical density of the
plurality of
overlapping opacifying layers in combination with one another giving rise to
the
multiple tones of the multi-tonal image.
As in conventional polymer document substrates (security print media), the
primary
function of the opacifying layers (which are typically formed of a polymeric,
non-
fibrous, light-scattering material) is to render the majority of the document
non-
transparent and to provide a suitable background on which to print graphics,
security
patterns and other information as may be required on the finished security
document. However, in the presently disclosed security print media, a
plurality of
the opacifying layers additionally provide a security feature in the form of a
multi-
tone image which is visible at least when the media is viewed in transmitted
light
(and, in some embodiments, also when viewed in reflected light). Like
a
conventional watermark formed in paper-based documents, the multi-tone image
formed by the opacifying layers has a monochromatic "greyscale" appearance
defined by relatively bright and relatively dark areas (and optionally one or
more
intermediate tones). However, as described below, in some embodiments
additional
layers can be provided to achieve a multi-coloured appearance.
It will be appreciated that the opacifying layers need not be in direct
contact with the
surface of the polymer substrate. Rather, one or more additional (transparent
or
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translucent) layers could be present between the polymer substrate and the
opacifying layers, such as a primer layer and/or the additional coloured
layer(s)
mentioned above, the opacifying layers still being considered disposed "on"
the
substrate surface.
The multi-tone image is achieved by inserting one or more gaps in each of a
plurality
of the opacifying layers (which otherwise cover substantially all of the
polymer
substrate, that is, preferably at least 50% of the substrate, more preferably
at least
80% and most preferably all of the remaining substrate). In each opacifying
layer,
the gap(s) are arranged according to a different respective sub-image, the
cumulative effect of which is a variation in the optical density of the
security print
media across the region of the substrate, depending on the number of
opacifying
layers present at each point, resulting in the displayed multi-tone image.
Locations
in which fewer of the opacifying layers are present (i.e. where more of the
opacifying
layers have aligned gaps) will have a lower optical density, thereby appearing
brighter when the substrate is viewed in transmission, or darker when the
substrate
is viewed against a dark surface, than other locations. Since relatively
bright
locations typically give the impression of being closer to the viewer, the
resulting
multi-tone image can provide a strong three-dimensional effect, especially
where the
sub-images are arranged to achieve a gradual change in optical density across
the
image (on a scale when viewed by the naked eye).
There may be one or more additional opacifying layers present which do not
contribute to the multi-tone image, e.g. being entirely absent across the
relevant
region of the substrate or being provided uniformly across the region of the
substrate. In addition, there could be more than one opacifying layer having
gaps
disposed in accordance with the same sub-image (provided there are at least
two
opacifying layers each having gaps disposed in accordance with different sub-
images).
It will be noted that the sub-images will either all be negative image
versions of the
multi-tone image, or all positive image versions of the multi-tone image, and
not a
mixture of both. A "negative image version" of an image is one in which
elements of
the image are defined by the absence of colour (in this case, the absence of
the
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opacifying material) against a surrounding background of colour (i.e. the
presence of
the opacifying material), whereas a "positive image version" of an image is
the
reverse: elements of the image are defined by the presence of colour (i.e. the
opacifying material) against empty surroundings (i.e. the absence of the
opacifying
5 material). It should be noted that here the fact that a sub-image is a
"negative
image version" of the multi-tonal image does not mean that it is the reverse
of the
multi-tonal image. Rather, if the multi-tonal image is a negative image then
typically
each of the sub-images will also be negative image versions of the same image,
and
if the multi-tonal image is a positive image then typically each of the sub-
images will
be positive image versions of that image.
The sub-images will each be "versions" of the multi-tone image in the sense
that
each will contribute to the definition of the same image, but any one of the
sub-
images by itself need not display all the elements which will be visible in
the final
multi-tone image. Rather, each portion of the multi-tone image will have a
desired
tone (or, analogously, optical density) relative to other portions of the
multi-tone
image and each portion will be present (i.e. correspond to an area of
opacifying
material) or absent (i.e. correspond to a gap) in each sub-image in dependence
on
the desired tone of that portion. Hence, each sub-image shows selected
portions of
the multi-tone image depending on their desired tone. In this way, ultimately,
each
element of the multi-tone image is built up by the presence or absence of each
opacifying layer in the portion corresponding to the image element, the tone
of the
element resulting from the number of opacifying layers present. All of the sub-
images are aligned with one another so that each portion of the multi-tone
image
has the same location in each sub-image.
Preferably, each sub-image defines portions of the multi-tonal image which
have a
(desired) tonal value falling within a respective tonal value range, the size
of each
respective tonal value range being different. That is, each sub-image is based
on a
different respective tonal value range. The tonal value of each point of the
multi-
tone image can be defined on an arbitrary scale relative to the darkest tone
and
lightest tone present in the multi-tone image (e.g. corresponding to tonal
values of
100% and 0% respectively), or on an absolute scale as may be measured for
example using a transmission densitometer such as the MacBeth TD932 (e.g.
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darkest tone portions having an optical density of 0.9, and lightest tone
portions
having an optical density of 0). The size of the different tonal value ranges
may
increase in constant steps, e.g. by 10% or by 20% where the scale is relative,
or by
0.1 or 0.2 where the scale is absolute) from one sub-image to another. It
should be
noted that the sub-images do not need to be physically arranged on or applied
to the
substrate in the same order as that denoted by their respective tonal value
ranges.
The order in which the opacifying layers (and their respective sub-images) are
arranged on the substrate ¨ and to which side(s) of the substrate each is
applied ¨
is generally unimportant since it is the cumulative effect of the layers which
produces the desired image.
Advantageously, when the tonal value ranges of the sub-images are ordered
according to increasing size, each tonal value range falls within the tonal
value
range next in the sequence. For example, a first sub-image may define portions
of
the multi-tone image having a tonal value in the range 0% to 10%, a second sub-
image may define portions having a tonal value in the range 0% to 20% (thereby
including all the same portions as in the first sub-image, plus more), a third
sub-
image may define portions having a tonal value in the range 0% to 30%, and so
on.
In this way, the desired tone of each image portion will be provided by the
cumulative effect of the sub-layers which define that portion. The smaller the
difference in tonal value range from one sub-image to the next (and the
greater the
number of opacifying layers), the more different tones can be displayed in the
final
image. As in the above example it is particularly preferred that all of the
tonal value
ranges share substantially the same first end value and differ in their second
end
values, but this is not essential.
In some preferred embodiments each or at least one of the sub-images will be a
binary or "flat" image with no tonal variation: the opacifying material is
either present
or absent on a scale visible to the naked eye, with no intermediate areas.
However,
in more preferred embodiments, at least some of the sub-images are multi-tonal
sub-images, preferably half-tone sub-images. In this way, multiple tones can
be
introduced within the sub-image itself, e.g. allowing for a gradual change
from a
region of 100% opacifying material though a region in which the opacifying
material
is applied to a gradually decreasing proportion of the surface (on a scale too
small to
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be appreciated by the naked eye) to a region in which the opacifying material
is
absent (i.e. a gap). This can be used to create a smoother transition between
tones
in the final multi-tone image, and more complex effects. For example, this
allows for
the creation of even more different tones in the final image than the number
of
different opacifying layers would itself permit.
The opacifying layers each preferably comprise a non-fibrous, polymeric
material
which will scatter light (as opposed to allowing clear light transmission
therethrough), and will be translucent to a degree. In preferred examples,
each
individual opacifying layer may have an optical density in the range 0.1 to
0.5, more
preferably 0.1 to 0.4, most preferably 0.1 to 0.3 (as measured on a
transmission
densitometer, with an aperture area equivalent to that of a circle with a 1mm
diameter ¨ a suitable transmission densitometer is the MacBeth TD932). The
individual opacifying layers may or may not be of the same composition as one
another ¨ for example, in some preferred cases at least one of the opacifying
layers
will contain electrically conductive particles (desirable to reduce the
effects of static
charge), whereas others will not ¨ but nonetheless, preferably, all of the
opacifying
layers are substantially the same colour as one another, most preferably a
light and
bright colour such as white (including off-white) or grey. In preferred
implementations, the opacifying layers each have a brightness L* in CIE L*a*b*
colour space of at least 70, preferably at least 80 and more preferably at
least 90.
A multi-tone image formed solely of the opacifying layers in the manner so-far
described will appear monochromatic with different portions having different
darkness levels (tones) but all of the same hue, e.g. different levels of
grey. To
further increase the visual impact and security level of the feature, in
preferred
embodiments the security print medium further comprises a mono-tone or multi-
tone
print of at least part of the multi-tonal image in one or multiple colours
which contrast
visually with the opacifying layers, located between at least one, preferably
all, of the
opacifying layers and the polymer substrate on the first and/or second
surfaces
thereof, the print of the multi-tonal image being in alignment with the sub-
images in
the opacifying layers. The print is most preferably located on the surface of
the
polymer substrate (optionally on top of an additional layer such as a primer),
underneath all the opacifying layers, although it is also possible to locate
it between
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any two of the opacifying layers. The term "print" is intended to cover an
image
formed of a composition such as ink applied by any technique including
conventional printing methods such as gravure, flexographic printing,
lithography
etc., but also ablation methods in which an all-over ink layer is applied and
then
selectively removed to leave an image, e.g. using a laser.
It should be noted that this print of the image could be a negative image
version or a
positive image version of the multi-tone image, irrespective of the nature of
the sub-
images. Indeed, it is preferred that if the sub-images are negative image
versions,
the print is a positive image version (so as to "colour in" the gaps in the
opacifying
layers), and vice versa. The print could be a flat, binary image. However, in
preferred examples, the print is itself a multi-tone print and comprises at
least one
multi-tone, preferably half-tone, print working. This can be used for example
to add
additional shading, e.g. using various different spatial densities of a dark-
coloured
ink such as black, to further enhance the multi-tonal nature of the overall
image. In
particularly preferred examples, the print is multi-coloured and may comprise
at
least two print workings in different colours. In this way the end result is a
multi-
coloured, multi-tonal image of which the different colours are provided by the
print
whilst the shading is provided primarily by the opacifying layers (and
optionally the
print).
The multi-tone image can be designed for intended viewing in either reflected
or
transmitted light (although, irrespective of the designed intention it will
still be visible
in at least transmitted light and in some cases both). Thus, in some preferred
embodiments, the sub-images are configured such that a greater number of the
opacifying layers overlap one another at locations across the substrate
corresponding to darker tones in the multi-tone image, relative to the number
of
opacifying layers which overlap one another at locations corresponding to
lighter
tones in the multi-tone image, the multi-tone image being configured for
viewing in
transmitted light. If the same image is viewed in reflected light against a
dark
background, the multi-tone image may still be visible but will appear
reversed, with
those regions intended to be darkest appearing lightest and vice-versa.
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In other preferred embodiments, the sub-images are configured such that a
smaller
number of the opacifying layers overlap one another at locations across the
substrate corresponding to darker tones in the multi-tone image, relative to
the
number of opacifying layers which overlap one another at locations
corresponding to
lighter tones in the multi-tone image, the multi-tone image being configured
for
viewing in reflected light. In this case, it is a dark background apparent
through the
opacifying layers which provides the darkness to the image, areas with more
opacifying layers obscuring the dark background and reflecting light so as to
appear
brighter than areas with fewer opacifying layers. If the same image is viewed
in
transmitted light, the multi-tone image will still be visible but again will
appear
reversed as compared with the intended image.
To make best use of the ability of the multi-tone image to display distinct
light and
dark portions, and preferably different intermediate tones as well, it is
particularly
advantageous if the multi-tonal image comprises an image of a three-
dimensional
object, preferably a geometrical solid or wireframe model, a person, an
animal, a
building or other architectural structure or a three-dimensional logo. Shadows
in the
image can be denoted by darker tones created by the multiple opacifying
layers, and
highlights by lighter tones.
As already mentioned, the greater the number of opacifying layers (and
corresponding different sub-images), the more different tones can be achieved
in
the final image. Hence, preferably, the plurality of overlapping opacifying
layers
includes at least three overlapping opacifying layers each having gap(s)
defined in
accordance with a different respective sub-image, preferably at least four,
more
preferably at least six. The thickness of each layer may be reduced to avoid
an
overly thick document substrate.
All of the opacifying layers could be located on the same surface of the
polymer
substrate. However it is generally preferred to distribute the opacifying
layers on
both surfaces so that both sides of the document can later be printed on.
Hence,
advantageously, at least one of the plurality of overlapping opacifying layers
is
provided on each of the first and the second surfaces of the polymer
substrate,
preferably half of the plurality of overlapping opacifying layers being
provided on
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each of the first and second surfaces. It should be noted that the opacifying
layers
do not need to be applied to the substrate in any particular order related to
their sub-
images although this may be preferred in some cases. The substrate can also be
located at any position within, or on one side of, the set of layers as its
location will
5 not affect the end image displayed by the cumulative effect of the
layers.
Nonetheless for other reasons it may be desirable to provide at least one
opacyifying layer on each surface of the substrate, e.g. to enable layer
printing
thereon and/or to protect the substrate or control its surface texture.
10 As already mentioned, the security print medium could additionally
include one or
more opacifying layers which do not take part in the formation of the multi-
tone
image. Hence in some preferred embodiments the security print medium further
comprises one or more additional opacifying layers each comprising a layer of
semi-
opaque material disposed over substantially the whole area of the polymer
substrate, the one or more additional opacifying layers each either extending
continuously across the region of the substrate containing the multi-tonal
image or
comprising a gap substantially across the region.
The security print medium could be configured such that at least one
opacifying
layer is present at every point across the region, so that the document
substrate
does not appear transparent. However, to increase the visibility of the
security
feature, and to add an additional level of security, in preferred embodiments,
at least
one transparent window region is formed by aligned gaps in each of the
opacifying
layers, the at least one transparent window region preferably substantially
surrounding the multi-tonal image.
The or each opacifying layer may be laid down via an application technique
which
results in no additional visible sub-structure to the layer beyond that
defined by the
sub-image, i.e. the opacifying material being present in a macroscopically
uniform,
homogenous layer across all regions outside the gaps defined by the sub-image.
This will typically be the case where the layer is applied by gravure printing
with a
cell size too small for individual recognition by the naked eye. Thus, at
least some
of the opacifying layers are applied in the form of an array of screen
elements which
are too small to be individually discernible to the naked eye. However, in
other
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preferred cases one or more of the opacifying layers may be laid down in the
form of
a visible screen. Hence, at least one of the sub-images is formed of an array
of
screen elements which are sufficiently large to be individually discernible to
the
naked eye, the size of the screen elements varying across the array to define
the
sub-image. For example the sub-image may be defined by line screen elements or
dot screen elements, e.g. to give the appearance of an intaglio-printed
pattern.
The security print medium may advantageously further comprises a raised
pattern
layer (e.g. of transparent or coloured ink) applied to the outermost
opacifying layer
on one or both sides of the substrate, the raised pattern layer comprising an
array of
screen elements which are sufficiently large to be individually discernible to
the
naked eye, the raised pattern layer preferably being tactile and/or of varying
visibility
depending on the viewing angle. For example, the pattern layer could be
applied by
intaglio printing. In a particularly preferred embodiment, such a raised
pattern layer
is provided in combination with a visibly-screened opacifying layer and the
array of
screen elements forming the at least one of the sub-images is arranged to
visually
cooperate with the array of screen elements forming the raised pattern layer.
For
example, the raised pattern layer could be provided across one area of the
region
containing the multi-tone image and the screened opacifying layer across a
second,
different area, the two areas merging into one another. This gives the
impression of
a continuous screened pattern at some viewing angles and not others.
The method of making a security print medium already introduced above can be
adapted to make any of the preferred features described above.
The invention further provides a security document comprising a security print
medium as described above, and at least one graphics layer applied on the
outermost opacifying layer(s) on the first and/or second surfaces of the
polymer
substrate. The security document could be for example any of: a bank note, an
identification document, a passport, a licence, a cheque, a visa, a stamp or a
certificate. A corresponding method of manufacturing a security document
comprises making a security print medium in accordance with the above-
described
method; and applying at least one graphics layer to the outermost opacifying
layer(s) on the first and/or second surfaces of the polymer substrate.
Typically the
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step of applying at least one graphics layer to the outermost opacifying
layers will be
carried out in a separate manufacturing process (e.g. at a different
manufacturing
facility and possibly by a different entity) from the manufacture of the
security print
media itself. However, the at least one graphics layer may preferably be
applied in
register with the multi-tone image in the opacifying layers so as to achieve a
visual
co-operation between the graphics layer and the multi-tone image. This can be
achieved by using a sensor such as a camera system to detect the location of
the
multi-tone image and adjust the position of the applied graphics layer
accordingly.
The graphics layer can be applied using any available printing process such as
gravure, flexographic, lithographic or intaglio printing, for example. The
graphics
layer may typically include security patterns such as fine line patterns or
guilloches,
information as to the nature of the security document such as denomination and
currency identifiers for a banknote, and/or personalisation information such
as a
serial number on a banknote or bibliographic data of the holder on a passport.
Examples of security print media in accordance with the present invention will
now
be described with reference to the accompanying drawings, in which:
Figure 1 shows a first embodiment of a security print medium (a) in plan view,
and
(b) in cross-section, layers of the security print medium being shown spaced
apart
for clarity;
Figures 2(a) to (c) show portions of different opacifying layers of the
security print
medium of Figure 1;
Figures 3(a) to (c) show portions of different opacifying layers of the
security print
medium of Figure 1 in a variant thereof;
Figure 4 shows a second embodiment of a security print medium (a) in plan
view,
and (b) in cross-section, layers of the security print medium being shown
spaced
apart for clarity;
Figures 5(a) to (d) show portions of different opacifying layers of the
security print
medium of Figure 4;
Figure 6(a) shows an example of a raised pattern layer, and Figure 6(b) shows
an
example of an opacifying layer which may be provided to the security print
medium
of Figure 4 according to a variant thereof;
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Figure 7 shows a third embodiment of a security print medium (a) in plan view,
and
(b) in cross-section, layers of the security print medium being shown spaced
apart
for clarity;
Figure 8 shows schematically a fourth embodiment of a security print medium,
each
layer of the security print medium being depicted individually in plan view;
Figure 9 shows schematically a fifth embodiment of a security print medium,
each
layer of the security print medium being depicted individually in plan view;
Figure 10 shows three additional layers which may be provided to the security
print
medium of Figure 9 in a variant thereof;
Figure 11 shows schematically a sixth embodiment of a security print medium,
each
layer of the security print medium being depicted individually in plan view;
and
Figure 12 shows a first embodiment of a security document (a) in plan view,
and (b)
in cross-section, layers of the security document being shown spaced apart for
clarity.
The description below will focus on examples security print media used in the
production of banknotes. However, as mentioned above, the security print media
could be used to form any type of security document, including passports (or
individual pages thereof), identification cards, certificates, cheques and the
like.
Throughout this disclosure, the term "security print media" is used
synonymously
with the term "document substrate", meaning a medium which can then be printed
upon and otherwise processed to form the desired security document, in a
manner
analogous to the printing and subsequent processing of a conventional paper
substrate (albeit with processes adapted for use on polymer). Hence a
"security
print medium" does not encompass graphics layers and the like, which are later
printed onto the security print medium to provide security patterns, indicia,
denomination identifiers, currency identifiers etc. The combination of such a
graphics layer and a "security print medium" (and optionally additional
features such
as applied foils, strips, patches etc.) is the "security document".
Throughout the following examples, the security print medium will be
illustrated as
having the same size and shape as a security document into which it is later
formed.
However, typically the security print medium will be formed as a web or sheet
large
enough to carry multiple repeats of the desired security document, and will
then be
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cut into individual document either before, but more usually after, printing
of the
graphics layer and any other required processing steps.
Figure 1 shows a first embodiment of a security print medium 1, Figure 1(a)
showing
a plan view and Figure 1(b) showing a cross-section along the line X-X'. It
will be
appreciated that in Figure 1(b) the various layers forming the security print
medium
1 are shown spaced apart for clarity whereas in practice all of the layers
will contact
one another and form a cohesive unit. The same applies to all other cross-
sections
shown in other Figures.
As shown in Figure 1(a), substantially all of the medium 1 carries a coating 6
formed
of a plurality of opacifying layers as described further below. This renders
the
medium non-transparent across the whole of the coated area and provides a
suitable background for printing thereon. The coating 6 may optionally be
omitted in
certain areas of the medium to form features such as strip 2 and window 3,
which
are transparent or translucent (relative to the coated areas). Such
transparent areas
may be provided as security features in their own right or may be later
equipped with
additional security devices during the manufacture of a security document
using the
medium 1, as described further below. At least some of the opacifying layers
forming coating 6 also have gaps in a region 9 of the medium, which are
configured
to form a multi-tone image 10 as will be detailed below. Nonetheless, in
preferred
examples each opacifying layer covers at least 50% of the area of the security
print
medium corresponding to one security document, more preferably at least 80%.
As shown in the cross-section of Figure 1(b), the security print medium 1
comprises
a polymer substrate 5, which is transparent (i.e. optically clear, but may be
tinted) or
translucent (i.e. optically scattering, but non-opaque). The polymer substrate
5 may
be monolithic or could be multi-layered and may carry additional layers on its
first
and/or second surfaces 5a, 5b such as a primer layer for improving the
adhesion of
outer layers. The polymeric substrate may comprise BOPP or polycarbonate, for
example.
The opacifying coating 6 can be applied on one or both surfaces 5a, 5b of the
polymer substrate 5 and in this case comprises four opacifying layers 6a, 6b,
6c and
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7. Each opacifying layer comprises a translucent, semi-opaque material which
is
preferably polymeric and non-fibrous, e.g. white ink. The opacifying layers
are each
preferably substantially the same colour as one another (and are spatially
uniform in
colour), most preferably white or another light colour such as off-white or
grey so
5 that a
later-applied graphics layer will contrast well against it. In preferred
examples, the opacifying layers each have a brightness L* in CIE L*a*b* colour
space of at least 70, preferably at least 80 and more preferably at least 90.
In this example, three of the opacifying layers 6a, 6b, 6c contribute to the
formation
10 of multi-tone image 10 whilst the fourth opacifying layer 7 (which is
optional) is
continuous across region 9 and hence does not contribute to the multi-tone
image
other than to increase its optical density uniformly throughout. Each
of the
opacifying layers 6a, 6b and 6c includes a gap in the region 9 which is
defined in
accordance with a different respective sub-image. The sub-images are shown in
15 plan view in Figures 3(a), (b) and (c) for layers 6a, 6b and 6c,
respectively (each of
Figures 3(a) to (c) showing only a section of the respective opacifying layer
including
and surrounding region 9, and omitting the remainder of the layer). The
different
sub-images are configured such that once the opacifying layers are arranged on
top
of one another, as shown in Figure 1(b), the cumulative effect of the
different sub-
images is a variation in the optical density of the security print medium
across region
9 which appears as the multi-tone image 10, at least when the medium is viewed
in
transmitted light. It should be noted that the order in which the opacifying
layers 6a,
6b, 6c and 7 are arranged on the substrate is unimportant since it is their
cumulative
effect, when all are viewed in combination, which creates the desired image.
Similarly the position of the substrate within the stack of opacifying layers
will not
affect the image exhibited by the end product and so can be freely selected.
However it may be desirable to apply at least one opacifying layer to each
surface of
the substrate for other reasons, e.g. to enable later printing of both sides
of the
document. These considerations apply to all embodiments.
The multi-tone image 10 in the present embodiment depicts a three-dimensional
hemisphere, and is made up of four different tones. The innermost circular
portion
10a has the lowest optical density (or tone), achieved by providing
corresponding
gaps in all of the opacifying layers except for layer 7 such that a single
opacifying
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layer is present across portion 10a, and the outermost annular portion 10d has
the
highest optical density (or tone), achieved by providing gaps in none of the
opacifying layers in this portion, such that all four are present here.
Intermediate
annular portions 10b and 10c are provided with respective intermediate optical
density / tonal values, achieved by locating gaps in these portions in one of
the four
opacifying layers and in two of the four opacifying layers, respectively.
Thus, taking
the optical density of innermost portion 10a to be 0% on an arbitrary relative
scale,
and that of outermost portion 10d to be 100%, portion 10b has an optical
density of
33% and portion 10c an optical density of 66%. Alternatively, on an absolute
scale,
if each opacifying layer 6a,b,c and 7 has an optical density of 0.2 (as
measured on a
transmission densitometer such as the MacBeth TD932, with an aperture area
equivalent to that of a circle with a 1mm diameter), portion 10a will have an
optical
density of 0.2, portion 10b an optical density of 0.4, portion 10c an optical
density of
0.6 and portion 10d an optical density of 0.8. These different optical
densities
appear as a variation in tone across the image, resulting in a three-
dimensional
effect.
When the medium 1 is viewed in transmitted light (i.e. against a backlight),
the
innermost portion 10a will appear brightest since its low optical density
allows the
greatest transmission of light, whilst the outermost portion 10d will appear
darkest
due to its high optical density. As a result, the centre of the hemisphere
appears to
protrude out of the plane of the medium 1, towards the viewer, relative to the
periphery of the hemisphere which appear farther behind. The multi-tone image
10
may or may not be visible in reflected light depending on the optical density
of the
opacifying layers. However, if the layers are sufficiently translucent, when
the
medium 1 is viewed in reflected light against a dark background, now the inner
portion 10a will appear darkest, since it reflects the least light and
obstructs the view
of the dark background to the smallest degree, whilst the outer portion 10d
will
appear lightest, since it reflects the most light and largely conceals the
underlying
dark background. Hence the hemisphere appears reversed relative to its
appearance in transmitted light, with its centre portion 10a appearing
farthest from
the viewer and the edge portion 10a appearing nearest.
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Referring now to Figure 2, the sub-images according to which each opacifying
layer
6a, b, c is arranged will be described. The sub-image defined in opacifying
layer 6a
is shown in Figure 2(a) and it will be seen that this comprises a circular gap
extending across the portions 10a,b,c of the multi-tone image, wholly
surrounded by
the opacifying material of layer 6a. The periphery of the circular gap
therefore
corresponds to the boundary between portion 10c and portion 10d in the multi-
tone
image 10. The opacifying material of layer 6a is present across portion 10d of
the
multi-tone image (the outer edge of which corresponds to the periphery of
region 9,
shown for reference). Hence the sub-image is defining portions of the desired
multi-
tone image according to their intended tone (or analogously their optical
density): in
this sub-image, portions of the multi-tone image having a desired tone of more
than
66% up to and including 100% (in this case, portion 10d) are denoted by the
presence of opacifying material whilst portions having a desired tone of 66%
or less
correspond to a gap in the layer.
Likewise, the sub-image defined in opacifying layer 6b (Figure 2(b)) defines
portions
of the desired multi-tone image according to a different, larger, tonal range:
here,
portions of the multi-tone image having a desired tone of more than 33% up to
and
including 100% (in this case, portions 10c and 10d) are denoted by the
presence of
opacifying material whilst portions having a desired tone of 33% or less
correspond
to a gap in the layer. Hence, the sub-image comprises a circular gap extending
across portions 10a and 10b of the multi-tone image, its periphery lying on
the
boundary between portions 10b and 10c.
Finally, the sub-image defined in opacifying layer 6c (Figure 2(c)) defines
portions of
the desired multi-tone image according to a still larger tonal range: portions
of the
multi-tone image having a desired tone of more than 0% up to and including
100%
(in this case, portions 10c and 10d) are denoted by the presence of opacifying
material whilst portions having a desired tone of 0% correspond to a gap in
the
layer. Hence, the sub-image comprises a circular gap extending across portion
10a
only of the multi-tone image, its periphery lying on the boundary between
portions
10a and 10b.
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It will be noted that the size of the tonal range defined in each sub-image is
different
and that each range falls wholly within that of the next sub-image (if they
are placed
in sequence according to the size of their respective tonal ranges). One end
point
(100%) is the same for each tonal range whilst the other end value varies.
It will also be noted that, in this example, all of the sub-images are
negative images
¨ that is, they each define elements of the desired multi-tone image by the
absence
of opacifying material against surroundings of that material, rather than vice
versa.
In the above example, all of the sub-images are binary or "flat" images
meaning that
the opacifying material is either present or absent across each part of the
image (on
a scale large enough to be appreciated by the naked eye), and there are no
intermediate levels. This will be desirable in many cases, especially where a
sharp
"step-change" in tone is required in the final multi-tone image, e.g. to
define a
straight edge in the image of an object. If this is not desired, one option to
achieve a
more gradual change in tone from one portion of the image to the next would be
to
utilise a greater number of opacifying layers, possibly of lower individual
optical
density, and a corresponding number of sub-images, arranged so as to achieve a
more closely-spaced series of smaller changes in tone. However, this may
result in
an undesirably thick construction of the security print medium 1 and would
also
require a corresponding increase in the number of processing steps.
Figures 3(a), (b) and (c) show alternative sub-images for each of the
opacifying
layers 6a, b, c respectively in the Figure 1 embodiment, which address this.
The
sub-image for each layer is substantially the same as in the Figure 2 example,
defining portions of the multi-tone image according to their desired tone,
based on
the same different tonal range for each sub-image as previously described, but
in
this case each of the sub-images itself is multi-tonal, i.e. defining at least
one
intermediate tone beyond the binary options of "present" or "absent", on a
scale
visible to the naked eye. For example, each sub-image may be formed as a half
tone image in which elements of the image are laid down with varying size
and/or
ink weight to give rise to the required variation in tone. In the present
example, this
multi-tonal nature of the sub-image is used to replace the sharp periphery of
the gap
in each opacifying layer with a boundary region 11 in which the tone of the
sub-
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image is intermediate (e.g. 50% fill factor) or gradually increases from zero
on the
side of the gap to 100% on the other wise. This visually softens the edge of
each
opacifying layer resulting in a more gradual change between tones in the final
multi-
tone image.
In the present example, all of the sub-images are formed as multi-tone images
in
this way but this is not essential. In other cases, just one of the sub-
images, or a
sub-set of the sub-images, may be multi-tonal whilst the remaining one or more
sub-
images may be binary images.
Multi-tonal sub-images can also be used for purposes other than smoothing
transitions between gaps and non-gap portions of a sub-image. More generally,
the
use of multiple tones in one or more of the sub-images allows for the creation
of
more complex multi-tonal images once the sub-images are combined since the
number of available tones is no longer limited to the number of opacifying
layers
applied. Rather, by varying the tone across any of the individual sub-images
and
layering them with further sub-images as necessary, a much larger number of
different tones can be created thereby allowing for the formation of a more
complex
multi-tone image.
It should be appreciated that this can be applied to all embodiments described
below, in which any one or more of the described sub-images could be
implemented
as a multi-tonal sub-image to obtain the above-mentioned advantages.
Figure 4 shows a second embodiment of a security print medium 1 formed based
on
the same principles as described in relation to the first embodiment. The
construction of the security print medium 1 is largely the same as previously
described, common components being denoted in the Figures using the same
reference numerals as used above. Again, a multi-tone image 10 is formed
within a
region 9 of the medium 1 and here this again takes the form of a hemisphere.
However, due to the different construction of the multi-tone image 10,
described
below, in this case the appearance of the hemisphere in reflected and
transmitted
light will be opposite to that in the Figure 1 embodiment. In addition, in the
Figure 2
embodiment, a transparent window region 12 is provided to further enhance the
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security level of the medium 1. It is especially preferred that where a window
region
12 is provided, this is arranged to surround the multi-tone image 10 (as in
the
present example), to assist in delimiting the multi-tone image from the rest
of the
medium 1. A transparent window region 12 of this sort can be provided in any
of the
5 other embodiments disclosed herein.
As shown in the cross-section of Figure 4(b), in this embodiment four
opacifying
layers 6a, 6b, 6c and 6d contribute to the definition of the multi-tone image
10. The
sub-images according to which each respect layer is arranged in the vicinity
of
10 region 9 are shown in Figures 5(a) to (d) respectively. It will be noted
that each of
the sub-images are now positive images rather than negative images (as in the
previous embodiment). That is, inside region 9, each sub-image defines
features of
the image by the presence of opacifying material against empty surroundings.
15 The multi-tone image 10 again has four different tonal levels: on a
relative scale this
time taking the transparent window region 12 to have an optical density of 0%,
the
innermost portion 10a has the highest optical density of 100%, and the
surrounding
annular portions 10b, 10c and 10d have respective optical densities of 75%,
50%
and 25%. This is achieved by applying each opacifying layer 6a to 6d according
to
20 the sub-images shown in Figures 5(a) to (d) respectively. Layer 6a is
laid down
according to the sub-image shown in Figure 5(a) which comprises a circular
element
extending across the portions 10a to 10d of the multi-tone image 10, being
absent
only in an annular region corresponding to window 12. Hence the sub-image
differentiates between portions of the multi-tone image 10 having a relative
tonal
value (or optical density) in the range of 25% to 100% (in which the
opacifying
material is present) and portions in the range 0 to less than 25% (in which
the
opacifying material is absent).
Similarly, the sub-image according to which layer 6b is arranged (Figure 5(b))
defines portions of the desired multi-tone image according to a different,
smaller,
tonal range: here, portions of the multi-tone image having a desired tone of
50% up
to and including 100% (in this case, portions 10a, 10b and 10c) are denoted by
the
presence of opacifying material whilst portions having a desired tone of less
than
50% correspond to a gap in the layer. Hence, the sub-image comprises a
circular
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element extending across portions 10a, 10b and 10c of the multi-tone image,
surrounding by an annular gap encompassing portion 10d and window region 12.
Likewise, the sub-image according to which layer 6c is arranged (Figure 5(c))
defines portions of the desired multi-tone image according to a different,
still smaller,
tonal range: here, portions of the multi-tone image having a desired tone of
75% up
to and including 100% (in this case, portions 10a and 10b) are denoted by the
presence of opacifying material whilst portions having a desired tone of less
than
75% correspond to a gap in the layer. Hence, the sub-image comprises a
circular
element extending across portions 10a and 10b of the multi-tone image,
surrounding
by an annular gap encompassing portions 10c and 10d and window region 12.
Finally, the sub-image according to which layer 6d is arranged (Figure 5(d))
defines
portions of the desired multi-tone image according to an even smaller tonal
range:
here, portions of the multi-tone image having a desired tone of 75% up to and
including 100% (in this case, portions 10a only) are denoted by the presence
of
opacifying material whilst portions having a desired tone of less than 75%
correspond to a gap in the layer. Hence, the sub-image comprises a circular
element extending across portion 10a of the multi-tone image, surrounding by
an
annular gap encompassing portions 10b, 10c and 10d and window region 12.
The result is that the innermost portion 10a of the multi-tone image 10 now
has the
highest optical density whilst the outmost portion 10d has the lowest and
window
region 12 has a still lower optical density. Hence when the medium 1 is viewed
in
transmitted light, the centre of the hemisphere will appear darkest and
therefore
furthest from the viewer, giving the impression that the hemisphere is
depressed into
the plane of the medium 1. When the medium is viewed in reflected light
against a
dark background, provided the optical density of the layers 6 is sufficiently
low, the
innermost portion 10a will now appear lightest and the outermost portions 10d
darkest, with the window region 12 taking on the dark colour of the background
unmoderated. Hence the hemisphere will now appear reversed, having its centre
protruding towards the viewer.
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Again, any one or more of the sub-images could be formed as a multi-tonal sub-
image in the manner previously described with respect to Figure 3 in order to
achieve a more gradual variation in tone.
A further optional but beneficial feature will now be described with reference
to
Figure 6. Figure 6(a) shows an exemplary raised pattern layer 13 which may be
applied over the outermost opacifying layer(s) across the region 9 of the
medium 1.
For instance, in the Figure 1/2 embodiment, the raised pattern layer 13 may be
applied over the opacifying layer 6d on the first surface of the substrate 5,
and/or
over the opacifying layer 6c on the second surface of the substrate 5. The
raised
pattern layer may comprise for example a colourless, transparent ink which is
applied to the medium 1 in accordance with a screen pattern, the elements of
which
are large enough to be individual discernible to the naked eye (possibly only
under
close inspection). For example, the raised pattern layer 13 may be applied in
the
form of an array of line or dot screen elements. In this case, the raised
pattern layer
is in the form of a grid of lines as shown. The raised pattern layer may be
applied by
intaglio printing for example and preferably has a latent appearance in that
its
presence is less visible when the medium is viewed at some angles, relative to
others. At certain viewing angles, which depend on the location of the
illuminating
light source, the raised image pattern will reflect light more strongly to the
viewer,
and thus become more visible, than at other viewing angles. The pattern 13 may
or
may not be directly related to the content of the multi-tone image 10. In this
example, the raised pattern layer extends across the same region 9 but
otherwise
does not reflect the features of the multi-tone image, instead comprising a
grid
pattern, the line weight of which varies from left to right across the region
such that it
fades to absent on the right side of the region 9. Preferably the raised
pattern layer
is tactile (i.e. can be detected by human touch), but this is not essential.
A raised pattern layer of the sort described above can be used on its own to
add
complexity to the multi-tone image feature. However it is preferred to
integrate the
pattern with the multi-tone image by arranging one of the opacifying layers 6
in
accordance with a similar screened pattern.
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In previous embodiments, each opacifying layer 6a to 6d has been laid down in
a
substantially homogenous manner so as to uniformly cover the desired portions
of
the substrate 5, at least on a macroscopic scale which is visible to the naked
eye. In
practice such layers may be formed by gravure printing for example, which
involves
applying the opacifying material from an array of cells, the size of which is
typically
too small for any resulting pattern structure to be visible to the naked eye.
However in the present example, one or more of the opacifying layers is formed
in
accordance with an array of screen elements, such as dots or lines, which are
sufficiently large that the screen structure is visible to the naked eye. An
example of
such an opacifying layer 6a' is shown in Figure 6(b). This could be provided
in place
of layer 6a or in addition to layer 6a. In this example, the circular element
of the
sub-image covering portions 10a to d of the multi-tone image is now applied in
accordance with a screen of dots arranged on an orthogonal grid. The size of
the
dot elements varies across the region from small on the left side to large on
the right
side. This results in a small-scale structure to the tones visible in the
multi-tone
pattern which interacts with that of the raised pattern layer 13. The two
screen
patterns are selected to be of similar sizes and element shapes, that of
raised
pattern layer 13 being dominant on the left hand side of the region 9, and
that that of
opacifying layer 6a' being dominant on the right hand side.
In the above examples, the multi-tone image will be monochromatic, i.e.
displaying
multiple shades of the same colour with different darkness levels. Typically
where
the opacifying layers are white, off-white or grey, the multi-tone image 10
will appear
in greyscale, with tones varying from white to dark grey or black, with
various
intermediate grey tones inbetween. However, in some cases it will be desirable
to
colour the multi-tone image 10, either with a single colour different from
that of the
opacifying layers, or with multiple colours.
Figure 7 shows a third embodiment in which the multi-tone image 10 is coloured
by
the addition of a print 8 of the same image, in this case formed of two print
workings
8a and 8b, located on respective surfaces of the polymer substrate 5, under
the
opacifying layers 6a to 6d on each side. In this example, the multi-tone image
10 is
of a three-dimensional cube and is located in a transparent window region 12
inside
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the region 9. The construction of the medium 1 is substantially the same as in
the
previous embodiments with a polymer substrate 5 having opacifying layers 6a to
6d
applied to each side in accordance with respective sub-images to define the
different tones desired in the multi-tone image 10. In this example, two
opacifying
layers 6a, 6b are applied to the first surface of the substrate 5, layer 6a
defining
portions 10a and 10b of the cube, surrounded by a gap corresponding to window
12,
and layer 6b being present only in portion 10b of the cube, resulting in that
portion
having a higher optical density than portion 10a. Two further opacifying
layers 6c,
6d are provided on the second surface of the substrate 5, layer 6c being
defined
according to the same sub-image as layer 6b and layer 6d being defined
according
to the same sub-image as layer 6a. Of course, more opacifying layers could be
provided in accordance with still further different sub-images to increase the
complexity of the multi-tone image if desired.
The print 8 of the multi-tone image 10 comprises two workings 8a, 8b
preferably in
different colours. For example, one of the workings 8a may provide an overall,
solid
region of one colour (e.g. red) across the whole of the area corresponding to
the
multi-tone image (i.e. both regions 10a and 10b). This combined with the
shading
achieved by the opacifying layers 6a to 6d will result in an image 10 of a
red, three-
dimensional cube with the different portions of the image having relatively
light or
dark shades of red resulting from the different optical densities of the
opacifying
layers in combination with one another. The other working 8b could be
identical to
the first working 8a, and in the same colour, to increase the intensity of the
colour.
Alternatively, the second working could be in a different colour and
configured to
provide different elements of the multi-tone image ¨ e.g. the first working
could be
provided only in portion 10a of the image and the second working only in
portion 10b
so that two facets of the cube appear in different colours ¨ or could overlap
with the
first to provide an intermediate colour such as orange where the first working
is red
and the second yellow. Alternatively still, one of the workings could be
provided in a
dark colour such as black and used to provide additional shading to the multi-
tone
image, e.g. being provided only in the portions of the image which are
intended to
have the darkest tone. Any one or more of the print workings may
advantageously
itself be multi-tonal, e.g. formed as a half-tone image, to introduce further
complexity
to the feature.
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The print 8 can be applied using any available application technique and may
comprise a single working or a plurality of workings. For example, the print 8
could
be applied by gravure, flexographic, lithographic or any other available
printing
5 technique, or by applying an all-over layer of ink and then selectively
removing parts
of it to define an image, e.g. by laser ablation or etching. The composition
forming
the image should preferably be at least semi-transparent so that it does not
negate
the variation in optical density created in the opacifying layers 6.
10 It is generally preferred that the print 8 is located under the
opacifying layers ¨ i.e.
between the opacifying layers and the polymer substrate 5 (optionally on top
of a
primer layer), as shown in Figure 7(b). However this is not essential and the
print
could be located between any of the opacifying layers. It is less preferred
that the
print 8 be located on the outer surface of the outermost opacifying layers 6b,
6d
15 since in this case its reflective colour may overwhelm the multi-tonal
effect of the
feature, especially when viewed in reflection.
A print 8 of this sort can be incorporated into any of the presently disclosed
embodiments.
Figure 8 illustrates a fourth embodiment of a security print medium, showing
each
layer applied to a polymer substrate 5 separately, in plan view. Layers 8a,
6b, 6d
and 6f are applied in that order to a first surface of substrate 5, and layers
8b, 6a, 6c
and 6e are applied in that order to a second surface of substrate 5 (although
as
mentioned previously the order of the layers, and on which surface of the
substrate
each is applied, is unimportant). As before, the substrate 5 may carry
additional
layers on either of its surfaces such as a primer layer, which are not shown
here.
The security print medium comprises six opacifying layers 6a to 6f, each
applied
according to a different sub-image as illustrated in the Figure. It
should be
appreciated that whilst in practice the opacifying layers will typically be
white, here
the opacifying material is illustrated in each of layers 6a to 6f as black in
order to be
visible in the Figure. Thus the white portions surrounded by black in each of
the
sub-images in fact correspond to gaps in the sub-images, and the black
portions
represent the areas where opacifying material is present. Layers 8a and 8b are
two
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workings forming a print 8 in colours contrasting with that of the opacifying
layers 6a
to 6f, in the same manner as described in connection with Figure 7 above. In
this
example, working 8a is a flat (binary) print in purple, and working 8b is a
multi-tonal
print in black.
Collectively, the layers depicted in Figure 8 form a multi-tone image 10 of a
three-
dimensional twisted loop structure. With reference to working 8b, the portion
marked F of the loop appears as the front-most part of the object (i.e.
projecting
towards the viewer), and the portion marked R appears as the rear-most part of
the
object (i.e. projecting away from the viewer), when the medium 1 is viewed in
transmitted light. This is achieved by arranging portion F of the image to
have the
lowest optical density, by providing that portion with corresponding gaps in
the
greatest number of opacifying layers 6a to 6f: indeed, as can be appreciated
by
comparing the location marked F in layer 6a with the same location in each of
layers
6b to 6f, it will be seen that all of the opacifying layers 6a to 6f have an
aligned gap
across portion F of the image and hence there is no opacifing material present
here,
only the ink of print workings 8a and 8b. Hence, when viewed in transmitted
light,
the portion F will appear light and a bright shade of purple.
In contrast, the region R is provided with a high optical density by arranging
none of
the opacifying layers 6a to 6f to have a gap at this location. This can be
appreciated
by comparing the portion marked R in layer 6f with the same location in each
of
layers 6a to 6e.
Other portions of the loop joining portion R to portion F have intermediate
tones by
virtue of the different number of opacifying layers present in each portion,
resulting
in a substantially continuous variation in the shade of the image and giving
rise to a
strong three-dimensional effect.
As in previous embodiments, each of the sub-images effectively defines
portions of
the multi-tone image in accordance with different tonal ranges. In this case,
layer 6a
defines the portions of the image having the lightest tones (or optical
densities), e.g.
from 0% to 16%, as a gap, whilst the respective gaps in each of layers 6b, 6c,
6d,
6e and 6f correspond to different tonal value ranges of increasing size ¨ e.g.
0% to
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33%; 0% to 50%; 0% to 66%; 0 to 82% and 0 to 95% respectively. It will be
noted
that all of the sub-images 6a to 6f in this example are negative images,
whilst the
print workings 8a and 8b are both positive images.
In this embodiment, each of the sub-images is a multi-tonal image as described
with
reference to Figure 3 above, the edges of the gap in each sub-image being
"softened" by a boundary region of intermediate tone so that in the final
multi-tone
image the transition from one tone to the next appears gradual. However this
is not
essential.
The multi-tonal nature of the final image in this example is further enhanced
by print
working 8b which is a multi-tonal (e.g. half-toned) working in a dark colour
configured to provide additional definition and shading to the twisted loop
structure.
A fifth embodiment of a security print medium is shown in Figure 9, the
various
layers being depicted individually in plan view in the same manner as in
Figure 8. In
this case, three opacifying layers 6b, 6d and 6f are provided on a first
surface of a
polymer substrate 5 and three opacifying layers 6a, 6c and 6e are provided on
its
second surface. Again, in the Figures, the white portions surrounded by black
in
each of the sub-images in fact correspond to gaps in the sub-images, and the
black
portions represent the areas where opacifying material is present. Here, the
multi-
tonal image comprises a portrait P of a person, a drawing of a building B and
a
stripe element S, selected parts of which are visible in each sub-image
(labelled only
in layer 6a for clarity). A transparent window 12 surrounds at least part of
the
portrait P.
This multi-tone image is designed for viewing in reflected light against a
dark
background, although it will also be visible in transmitted light (the
portrait appearing
reversed). Hence portions of the image which are intended to appear brightest,
such as the bridge of the person's nose and his cheekbones, require the
highest
optical density, and the portions which are to appear darkest, such as the
shadows
under his eyebrows and under his fingers, require the lowest. In this case,
the
darkest portions of the image have an optical density of 0%, i.e. the
opacifying
material is absent in all six layers, meaning that the dark colour of the
underlying
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background is unobscured as can be appreciated by noting that these portions
correspond to gaps (i.e. white regions, in the Figures) in every one of the
sub-
images. In contrast, the brightest portions of the portrait are provided
with
opacifying material in every one of the sub-images (i.e. black regions, in the
Figures).
Again, the sub-images define portions of the multi-tone image in accordance
with
their desired tone. However, in this example, sub-images 6a and 6b are the
same
as one another, as are sub-images 6c and 6d, and 6e and 6f. Hence sub-images
6a
and b define portions of the multi-tone image with optical densities in the
range 0%
to less than 33% as gaps; sub-images 6c and d define portions with optical
densities
in the range 0% to less than 66% as gaps, and sub-images 6e and f define
portions
with optical densities in the range 0% to 95% as gaps. In this case, each of
the sub-
images is a positive image. As in previous embodiments the order in which the
layers are applied to the substrate is unimportant.
The construction shown in Figure 9 will result in a monochromatic multi-tone
image.
To provide additional colour to the image, one or more print workings may be
added
and examples of these are shown in Figure 10. Here, three print workings 8a,
8b
and 8c are shown. Working 8a is located on the first surface of substrate 5,
underneath opacifying layer 6b shown in Figure 9, and workings 8b and c are
located on the second surface of substrate 5, underneath opacifying layer 6a.
In
this case, each of the print workings 8a, 8b and 8c is a multi-tonal working
in a
different colour. For example, working 8a may be red, working 8b brown and
working 8c blue. The print provides multiple colours and additional shading to
the
multi-tonal image.
Figure 11 shows a sixth embodiment of a security print medium. Again, the
various
layers forming the security print medium are shown individually in plan view.
Three
opacifying layers 6a,b,c are applied to a first side of a substrate 5, and
three
additional, optional opacifying layers 7a,b,c to the other. In this case the
three
opacifying layers 7a,b,c are applied uniformly across the substrate 5 and
hence do
not contribute to the multi-tone image other than to increase its optical
density
uniformly throughout. Each of layers 6a, 6b and 6c is defined according to a
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different sub-image in the same manner as previously described. In this case
the
resulting multi-tonal image is a complex pattern of interlocking geometrical
elements. Different parts of the pattern are provided with different tones by
virtue of
the number of opacifying layers present at any one location. As in Figures 8
and 9,
the white portions illustrated in each of the sub-images in fact correspond to
gaps in
the sub-images, and the black portions represent the areas where opacifying
material is present. Hence in this case, the fine lines visible in layer 6c,
which align
with the centre of the thicker black lines in Figure 6b and the thicker-still
black
regions in Figure 6c, will ultimately have the highest optical density, whilst
the
intervening areas will have the lowest optical density. As a result, in the
finished
image, different portions of the pattern will appear to protrude towards and
away
from the viewed, resulting in a three-dimensional effect. As in all
previous
embodiments, the order in which the opacifying layers are applied to the
substrate is
unimportant and the position of the substrate within the stack of layers could
also be
chosen at will ¨ for instance, the substrate S could be located between layers
6c
and 6b, or between layers 7a and 7b, or any other two adjacent layers in the
stack.
Again, each of the sub-images defines portions of the pattern according to
their
optical density and in this case all of the sub-images are positive images.
In all of the above embodiments, it is preferred that each opacifying layer
has an
optical density in the range 0.1 to 0.5 (as measured on a transmission
densitometer
such as the MacBeth TD932, with an aperture area equivalent to that of a
circle with
a 1mm diameter), more preferably 0.1 to 0.3. Advantageously, the opacifying
layers
each have a brightness L* in CIE L*a*b* colour space of at least 70,
preferably at
least 80 and more preferably at least 90. Preferably, the opacifying layers
should be
white, off-white or grey. The composition of each opacifying layer may be the
same
or different to one another. In preferred examples, one of the opacifying
layers on
each side of the substrate may comprise electrically conductive particles to
reduce
the effect of static charge. Preferably this is the penultimate layer on each
side: for
example, layers 7 and 6a in Figure 1, layers 6d and 6c in Figure 8, layers 6d
and 6c
in Figure 9 and layers 6b and 7b in Figure 11.
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The opacifying coating for any of the above embodiments will typically
comprise a
resin such as a polyurethane based resin, polyester based resin or an epoxy
based
resin and an opacifying pigment such as titanium dioxide (Ti02), silica, zinc
oxide,
tin oxide, clays or calcium carbonate.
5
The opacifying layers can each be applied by any suitable application process
which
allows their selective application in accordance with the respective sub-
images.
Typically, each opacifying layer will be applied by gravure printing.
Alternatively, any
of flexographic printing, screen printing or lithographic printing may be
used. The
10 opacifying layers and any print workings should preferably be applied in
register with
one another, as may be achieved by applying all of them in the same in-line
process. As already mentioned, additional layers such as a primer could be
applied
to the substrate before the opacifying layers (and any optional print
workings).
Further layers could be applied to the outside of the opacifying layers, such
as a
15 protective layer (preferably transparent) or a print-receptive coating.
The above-described security print media can then be processed into security
documents. The processing steps involved in doing so may be carried out on a
separate processing line, typically at a different manufacturing site and
optionally by
20 a different entity. An example of a security document 100 formed using
the security
print medium 1 described above in relation to Figure 1 is shown in Figure 12,
(a) in
plan view and (b) in cross-section. All of the components already provided as
part
of the security print medium 1, including multi-tone image 10, are as
previously
described in relation to Figures 1, 2 and 3 and hence will not be described
again.
The security document comprises a graphics layer 20 applied in this example to
the
outer surfaces of the security print medium 1, i.e. to the surface of
outermost
opacifying layers 6b and 6c. In other cases the graphics layer 20 may be
applied
only to one or other of the surfaces. As mentioned previously there could be
intermediate layers between the opacifying layers and the graphics layer, such
as a
protective layer or primer. In this example, the security document is a
banknote and
hence the graphics layer comprises background security patterns 20a (such as
guilloches) as well as identifiers such as denomination information 20b. The
graphics layer 20 could be applied in a single working or in multiple
workings,
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optionally using more than one printing technique. Any available printing
techniques
can be utilised for forming the graphics layer as would be applied to a
conventional
polymer document substrate, e.g. intaglio printing, gravure printing,
flexographic
printing, lithographic printing etc.
Figure 12 also illustrates examples of other security devices which may
optionally be
applied to the security print media to form the security document, such as an
optically variable device 21 in window 3, e.g. a moire magnification device, a
lenticular device or an integral imaging device as may be formed by cast-
curing or
laminating a lens array on one side of the polymer substrate 5 and forming
image
elements on the other. Also depicted is a security device 22 in the form of a
patch
which has been applied to the surface of the security print media, e.g. by
lamination
or hot stamping. The security device 22 may comprise a diffractive optical
element
such as a hologram, for example.
The security documents and security devices of the current invention can
optionally be made machine readable by the introduction of detectable
materials
in any of the layers or by the introduction of separate machine-readable
layers.
Detectable materials that react to an external stimulus include but are not
limited
to fluorescent, phosphorescent, infrared absorbing, thermochromic,
photochromic, magnetic, electrochromic, conductive and piezochromic materials.
