Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF MANUFACTURE AND OF VERIFICATION OF A SECURITY PAPER
The invention is concerned with security articles
such as cheque guarantee cards, identification cards
and the like and such as security paper including bank
notes, cheques and the like. The present invention
also relates to a method of manufacture of security
articles and a method of verifying the authenticity of
the security article.
It is widely known to use in banknotes security
threads which are made from a transparent film
provided with a continuous reflective metal layer,
vacuum deposited aluminium on polyester film being the
commonest example. Banknotes made from such paper
have been in general circulation in many countries for
many years. British patent specification nos.
GB-A-1552853 and GB-A-1604463 describe use of a
security thread in a security paper with the thread
exposed on one side of the security paper at intervals
along the length of the thread, the regions of
exposure being referred to as windows. The original
purpose of providing windows was to produce a strong
public security feature (i.e., a security feature
readily identified by the general public) which
presented a sharp contrast in appearance when viewed
in reflected light as opposed to when viewed in
transmitted light. In reflected light the security
thread appeared as a continuous dark line. In
contrast, in reflected light, the portions of the
thread visible at the windows appeared silver.
Furthermore, the windowed thread provided good
protection against photocopying because when a bank
note with windowed thread is photocopied, the
reflective silver surfaces appearing in the windows
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appear black in the photocopy, because of the
reflected light. -
Further to the original window thread design,
improved more complex versions have been produced.
For instance, threads have been produced which are
laminates of two metallised polyester plies with
magnetic material incorporated between them. Some
threads have been produced which are in parts
demetallised, so that alphanumeric characters, for
instance, can be viewed on the thread in transmitted
light. Some threads have been produced which change
colour with temperature. Some security threads have
been produced which have holographic images. Some
security threads have been produced which fluoresce
under ultra-violet or infra-red light and some of
these threads have been produced with alphanumeric
characters shown by demetallised portions.
With all of the windowed thread designs produced
to date the aim of incorporating the windowed thread
in the bank note was to present a highly reflective
image in regions on one face of the bank note, this
image being a very noticeable public security feature
and also a feature to prevent photocopying.
The applicants have appreciated that the strong
public perception of windowed security threads can in
some cases be a problem. As bank note sizes are
reduced on cost grounds there is an increasing
pressure on space. Bank note printers often wish to
incorporate complex print designs on a note surface
but are limited by the strong reflected light
appearance of the windowed security thread. Often
notes now have holograms/foils applied to their
surfaces and designers of bank notes are increasingly
reluctant to make provision for a second reflective
element on the surface, i.e., the windowed security
thread.
The present invention provides a security article
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having first and second surfaces and a security thread
having a first part located between the first and -
second surfaces and a second part exposed in a window
provided in the first surface, the security thread
comprising means providing at least one non- public
security feature which enables verification of
authenticity of the security article and which is not
visually detectable when the security thread is
exposed only to electro-magnetic radiation in the
visible spectrum, wherein the exposure of the second
part of the security thread in the window on the first
surface enhances detection of the non-public security
feature by providing a readily accessible portion of
the security thread, characterised in that the whole
of the part of the security thread exposed in the
window in the first surface has a matt non-reflective
appearance and closely matches in colour a portion of
the first surface surrounding the window, whereby when
the first surface is viewed in reflected light in the
visible spectrum the exposed part of the security
thread is unobtrusive and does not form a visually
striking feature of the appearance of the security
article in reflected light in the visible spectrum.
The present invention has departed from accepted
teaching by providing a windowed thread which is not a
readily apparent public security feature in reflected
light. Since the windowed thread is not readily
apparent in reflected light, it does not provide the
protection against photocopying which was a prime aim
of the previous windowed threads. The non-public
security feature could be a feature for recognition by
a trained inspector or cashier, e.g. using ultra-
violet light.
Preferably the non-public security feature is a
machine-measurable security feature and the exposure
of the second part of the security thread in the
window enhances measurement of the machine-measurable
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security feature.
The applicants have realised that there is merit -
in providing a windowed thread, even when the windowed
thread is not intended to provide a public security
feature in reflected light. The applicants have
appreciated that the use of a windowed thread is
advantageous in presenting a machine-readable element
on the surface of, for instance, a bank note, where
the security thread is readily accessible to an
l0 appropriate detector. In the past, where security
threads have not been visible in reflected light, they
have been embedded totally within a bank note and then
machine-readable features included in the security
thread. For instance, use of a machine-readable
luminescent layer on a magnetic thread is disclosed in
GB-A-1585533, on fully embedded thread. However, the
full embedding of the threads means that the embedded
threads are covered with fibres and this makes it more
difficult for machine authentication of the
luminescent contents to occur. There is a great
practical advantage in using a windowed thread to
provide an area on the surface of, for instance, a
bank note which can be readily accessed by a machine
detector.
Preferably the security article comprises
material which allows transmission of light between
the first and second surfaces and the security thread
is visible in such transmitted light.
Thus, the thread of the present invention can be
seen in the security article as an immediately
apparent strong continuous line in transmitted light.
In an alternative embodiment the security article
again comprises material which allows transmission of
light between the first and second surfaces, but the
security thread is nearly transparent and is not
readily visible in transmitted light.
The non-public security feature could be detected
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by exposing the second exposed part of the security
thread to electromagnetic radiation outside of the
visible spectrum. The detection could be facilitated
by the security thread emitting electromagnetic
radiation on exposure to the non-visible
electromagnetic radiation. The emitted radiation
could be electromagnetic radiation in the visible
region of the electromagnetic spectrum.
Preferably the surface of the security thread
exposed in the window has a gloss of 50 units or less
as measured by a Novo-Gloss 60° glossmeter.
Additionally, or alternatively, a multi-angle Novo-
Gloss glossmeter could be used to measure the gloss,
in which case the measured gloss would preferably be
10 units or less at 20°, 50 units or less at 60° and
120 units or less at 75°.
Preferably the surface of the security thread
exposed in the window has a specular reflectance of
5.0% or less as measured by a Shimadzu UV3101-PC
spectrophotometer operating in a range of 400-700nm of
electromagnetic radiation.
Preferably the security article of the present
invention has a security thread which comprises a
metallised polymeric substrate coated at least in the
exposed portion with a matt coating obscuring the
metal in the thread. The matt coating renders the
thread substantially non-reflective. Preferably the
metallised polymeric substrate is opaque.
In one embodiment the matt coating on the thread
is luminescent and provides the non-public security
feature of the security thread by emitting measurable
light when exposed to ultra-violet radiation. In this
embodiment a metallised polyester thread could be
covered with a matt pale yellow coating on the top
surface (in practice on both surfaces so the thread
need not be oriented during manufacture, although use
of a top coating only is an option). The thread
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presents a machine-readable element on the surface
of the note which is readily accessible to an -
appropriate detector. The coating is luminescent and
the machine authentication involves exciting the
luminescent coating with appropriate wavelengths of
ultra-violet light and measuring the subsequent
emitted light. Although in principle such detection
would be possible with an embedded thread, there is a
great practical advantage in using a windowed thread
in that the intensity of stimulating light reaching
the target and the intensity of emitted light from the
target is much greater without the overlying absorbing
region of fibre.
In reflected light the thread with its matt pale
yellow coating is relatively unobtrusive and this is
particularly true once the paper has been over-
printed. The coating matches fairly closely the
colour of the bank note paper and once the bank note
is printed the windows in the paper become quite
difficult to see except at certain angles where some
specular reflection occurs from the surface of the
thread.
In one embodiment the luminescent material in the
security thread emits light in response to infra-red
radiation. For instance, the luminescent.material can
provide infra-red radiation stimulated Anti-Stokes
luminescence.
In a further embodiment of the invention the
means providing at least one non-public security
feature comprises an infra-red absorbent material
which is provided in a chosen pattern on the security
thread on at least the second part of the security
thread exposed in the window, the chosen pattern being
machine detectable when the security article is
exposed to infra-red radiation. Preferably the infra-
red absorbent material is coated with a layer of
infra-red transparent material, the infra-red
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transparent material having a colour which matches the
portion of the first surface surrounding the window.
In one embodiment the security thread comprises
thermoluminescent material at least in the second
exposed part and the thermoluminescent material
provides the non-public security feature.
In a further embodiment the security thread
comprises triboluminescent material at least in the
second exposed part and the triboluminescent material
provides the non-public security feature.
In one embodiment the security article has a
thread which has first and second luminescent
materials with different characteristics provided on
the security thread which together form the means
providing at least one non-public security feature,
the first and second luminescent materials having a
similar appearance and colour when exposed solely to
visible light.
The first and second luminescent materials can
emit light of two different wavelengths and/or the two
different materials can have phosphorescent decay half
lives which are different. These parameters can be
measured by machine.
In one embodiment the first and second
luminescent materials are provided in a chosen pattern
in the security thread.
In a further embodiment the security thread
comprises a material provided in the security thread
which reflects infra-red radiation, the infra-red
reflecting material being present in the security
thread in the exposed portion thereof and thereby
forming the means providing at least one non-public
security feature.
Alternatively, the means providing the at least
one non-public security feature can comprise infra-red
absorbent material provided in the security thread at
least in the exposed portion thereof.
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In an additional embodiment the means providing
at least one non-public security feature in the
security thread comprises ultra-violet reflecting
material providing in the security thread at least in
the exposed portion thereof. Alternatively the means
providing at least one non-public security feature
could comprise ultra-violet absorbent material
provided in the security thread at least in the
exposed portion thereof.
Preferably, in all embodiments of the security
article a graphic design is applied to the first
surface of the security article, the graphic design
being applied to extend over and at least partially
obscure the exposed part of the security thread. The
present invention has as one of its advantages the
fact that it frees up space on the surface of, for
instance, a bank note for print designs, foils and
holograms. Thus, whilst in the past the designer of a
bank note had to consider the windows in the bank note
when making his design, he can now consider the
windows in the bank note surface as being continuous
with the bank note surface and can create graphic
designs which extend over the window, provided that
the design does not completely obscure the window with
an ink which blocks the absorption, stimulation or
emission of radiation required to determine the
presence of the security thread. Once these designs
are printed on the bank note then the windows in the
bank note are further obscured and are made less
detectable. Thus, for instance, where the security
article is a security paper such as a bank note, the
graphic design is printed on the first surface of the
security paper with the exposed part of the security
thread being at least partly over- printed. In this
case, the exposed part of the security thread would
closely match in colour with the surrounding paper.
Alternatively, the exposed part of the security thread
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could closely match the colour of print on the _
surrounding paper.
In a second aspect the present invention provides
a method of manufacture of the security article
described above comprising, in any order, the steps
of
fabricating the security article with the first
and second surfaces, whilst defining the window in the
first surface and locating the security thread in the
security article with the first part of the security
thread lying between the first and second surfaces and
the second part of the security thread exposed in the
window;
providing a matt non-reflective surface finish on
at least the exposed portion of the security thread;
and
selecting the colour of the exposed portion of
the security thread to match the colour of the portion
of the security article surrounding the window.
When the security thread comprises a polymeric
substrate then the step of selecting the colour of the
exposed portion comprises selecting a coloured coating
for the substrate, which colour coating is applied to
the substrate to provide the colour of the surface of
the exposed part of the security thread.
In a third aspect the present invention provides
various methods of verifying the authenticity of the
security articles mentioned above. These methods
variously include:
irradiating the security article with ultra-
violet light and detecting the light emitted by the
luminescent material on the security thread;
irradiating the security article with infra-red
radiation and measuring reflection of the infra-red
radiation;
irradiating the security article with infra-red
radiation and measuring absorption of the infra-red
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radiation;
irradiating the security article with ultra- -
violet radiation and measuring reflection of the
ultra-violet radiation;
irradiating the security article with ultra-
violet radiation and measuring absorption of the
ultra-violet radiation;
irradiating the security article with infra-red
radiation and detecting light emitted by luminescent
material;
irradiating the security article with infra-red
radiation and detecting a pattern of absorption of the
infra-red radiation;
irradiating the security article with ultra-
violet radiation and measuring the wavelengths of
light emitted by first and second luminescent
materials on the security thread (indeed there may be
more than two luminescent materials on the security
thread and authentication could involve the
measurement of the wavelengths of each light emitted
by each material);
irradiating the security article with ultra-
violet radiation and measuring the phosphorescent
decay half-lives of first and second luminescent
materials of the security threads (indeed there may be
more than two luminescent materials on the security
thread and authentication could involve the
measurement of the half-life of each material);
heating the security article by exposing the
security article to infra-red radiation and detecting
visible light emitted by thermoluminescent material;
and
applying a mechanical force to the security
thread and detecting visible light emitted by
triboluminescent material.
Preferred embodiments of the present invention
will now be described with reference to the
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accompanying drawings, in which:
Figure 1 is a schematic representation of a bank
note according to one embodiment of the present
invention, viewed in reflected light;
Figure 2 is a schematic view of the bank note of
Figure 1, viewed in transmitted light;
Figure 3 is an illustration depicting windows in
the surface of the bank note of Figures 1 and 2, which
windows are not normally visible in reflected light;
Figure 4 is a cross-section through the bank note
illustrated in Figures 1, 2 and 3, taken along the
line A-A in Figure 3 in the direction of the arrows;
Figure 5 is a detail view of one embodiment of
the invention, showing one window in a bank note;
Figure 6 is a schematic cross-section through a
first embodiment of a security thread which can be
used in the bank note of Figures 1 - 4;
Figure 7 is a cross-section through a second
embodiment of security thread which can be used in the
bank notes of Figures 1 - 4; and
Figure 8 is a schematic representation of
apparatus used to test the authenticity of the bank
note illustrated in Figures 1 - 4.
In Figure 1 there can be seen a bank note 10
according to a first embodiment of the present
invention. The Figure shows the bank note 10 viewed
in reflected light. The bank note 10 is printed over
the whole of the surface of the bank note shown with a
graphic design which comprises the numeral 2,000, two
stripes and a human head in profile.
The bank note 10 comprises a security thread 11
which can be seen as a striking, immediately apparent,
strong continuous line when the bank note 10 is viewed
in transmitted light as seen in Figure 2.
The surface of the bank note 10 illustrated in
Figures 1, 2 and 3 in fact comprises four windows 12,
13, 14 and 15. In these windows the thread 11 is
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exposed, but between these windows the thread 11 lies
between the top surface 16 of the bank note 10 and a
bottom surface 17 of the bank note 10 (see Figure 4).
In Figure 4 it can be seen that the security thread 11
has the four windows 12, 13, 14 and 15 in which the
security thread 11 is exposed.
The security thread 11 comprises in one
embodiment (see Figure 5) a polyester substrate 20
which is metallised with a metallic layer 21.
Covering the metallic layer 21 is a matt pale yellow
coating 22. A final external coating of transparent
colourless adhesive 23 is applied over the top of the
matt pale yellow coating to provide some protection to
the thread and to aid and ease manufacture. A coating
24 of transparent colourless adhesive 23 is also
applied to the bottom surface of the security thread
11. The matt pale yellow coating 22 matches fairly
closely the appearance of the bank note paper 18
surrounding the security thread 11. Thus, the parts
of the security thread visible in the windows 12, 13,
14 and 15 become quite difficult to see in reflected
light, except at certain angles when some specular
reflection occurs from the adhesive external coating
of the security thread. For this reason, in Figure 1
the illustration of the bank note 10 shows that the
windows 12, 13, 14 and 15 are not visible in reflected
light. The portions of the thread 11 visible in the
windows 12, 13, 14 and 15 are even less visible when
the bank note paper 18 is over printed with a graphic
design, as illustrated in Figure 1. Thus, the bank
note is provided with a surprising optical effect in
that the general public will be generally unaware of
the presence of a security thread in the bank note l0
until the bank note 10 is held up to a light, when the
security thread becomes visible as a dark continuous
line as can be seen in Figure 2.
The security thread 11 presents a machine
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readable element on the surface of the bank note 10
where it is exposed by the windows 12, 13, 14 and 15.
The security thread 11 is readily accessible to an
appropriate machine detector at the windows 12, 13, 14
and 15.
In the preferred embodiment the matt pale yellow
coating comprises luminescent material and the machine
authentication involves exciting the luminescent
coating with appropriate wavelengths of ultra-violet
light and then measuring the subsequent emitted light.
Although in principle such detection would be possible
with a fully embedded thread, there is a great
practical advantage in using a windowed thread in that
the intensity of the stimulating light reaching the
target and the intensity of the emitted light from the
target is much greater without an absorbing region of
fibre overlying the security thread.
In essence the bank note 10 provides a new way of
utilising windowed thread technology. Whereas in the
past the windowed thread technology was used for
public identification purposes primarily, now the
windowed thread is used primarily for machine
authentication purposes and the public effect in
reflected light thought so important with existing
windowed thread technology is deliberately avoided.
Since the exposed portions of the security thread
match the surrounding paper, the bank note of the
present invention frees up space on the note surface
for the printing of designs and for the affixing of
foils and holograms. The bank note at the same time
also maximises the signal available from a machine
readable layer in or on the security thread 11. The
bank note 10 also includes what becomes an unexpected
effect for the general public in that the security
thread 11 can be readily perceived in transmitted
light.
Whilst in the embodiment described above the matt
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coating is applied over the entirety of one surface of
the security thread 11, in fact the matt coating could
be applied selectively only in those portions which
are to be exposed in the windows 12, 13, 14 and 15.
Also, whilst shown above the coating material 22 is
provided on only one side of the polyester substrate
20, the polyester substrate 20 could be provided with
the matt coating on both sides and with adhesive top
coatings on both sides, in order that the thread need
not be oriented to one side only before being embedded
in paper stock in the manufacture of the bank note.
The manufacture of the bank note 10 would follow
the steps usual in producing windowed thread bank
notes, except that the method would have the
additional steps of providing a matt non-reflective
surface finish on at least those portions of the
security thread which will be exposed in the final
bank note and the method includes the step of
selecting the matt coating with a colour which matches
the colour of the bank note paper.
Whilst above the colour of the matt coating is
chosen to match the colour of surrounding paper, the
colour of the coating could be chosen to match the
colour of ink printed on the regions of the paper
surrounding the windows in the paper.
Whilst above, the security thread 11 is provided
with a machine measurable parameter by the use of
luminescent material in the matt coating 22, other
machine measurable parameters could be used. For
instance, infra-red stimulated Anti-Stokes luminescent
material could be used in or on the security thread at
least in those portions exposed in the windows 12, 13,
l4 and 15, which luminescence could then be detected
by a suitable authentication machine. Alternatively,
the coating 22 could be provided with infra-red
reflecting or absorbing material, with the method of
authentication of the bank note then comprising
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analysis of the reflection/absorption of infra-red
radiation by the security thread in the bank note.
Similarly, ultra-violet reflecting/absorbing material
could be used in the security thread 11 and then the
bank note 10 irradiated with ultra-violet radiation
and the reflection/absorption characteristics
monitored for when verifying the authenticity of the
bank note. Furthermore, thermoluminescent material
could be used in the coating and this material would
emit light when heated (e.g. through exposure to
infra-red radiation). It is also possible to use
triboluminescent material and excite the material by
applying a mechanical force.
With all of these approaches, there is a benefit
in having the security thread 11 exposed in the
windows, since there would be no fibres of the bank
note 10 which overlie the security thread 11 in these
regions and thus interfere with the detection process.
Furthermore, the use of a matt layer permits the use
of much higher concentrations of luminescent material,
infra-red reflecting/absorbing material, ultra-violet
reflection/absorbing material, thermoluminescent
material and/or triboluminescent material (e. g.,
inorganic pigments) than would be acceptable in the
coating on the thread if the thread had to retain the
traditionally very reflective/shiny appearance.
The applicant has considered in one embodiment
printing an infra-red absorbent material in the
characteristic pattern on at least portions of the
security thread 11. In Figure 6 it can be seen that
the polyester substrate 20 is metallised and covered
with a metallic layer 21 and is then covered with the
infra-red absorbent material 30 which is printed in a
characteristic pattern, e.g., in alphanumeric
characters. The infra-red absorbent material 30 is
then covered with an overlying overcoat 31 of an
infra-red transparent material of a visible colour
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closely matching the colour of the surrounding bank
note paper or print on the paper. Finally the infra-
red transparent material would be covered with two
coats 23 and 24 of transparent colourless adhesive
for protection purposes. The infra-red absorbent
material could for instance be printed in the numerals
2,000 and in Figure 7 there can be seen an image of a
part of a bank note showing a window 12 in which the
exposed part of the security thread 11 is printed with
infra-red absorbent material in the numerals 2,000,
the image shown in Figure 7 then being obtainable by
use of suitable infra-red detecting apparatus.
In a further embodiment, a security thread can be
printed with a characteristic pattern by using two or
more inorganic luminescent materials of similar
appearance/colour in visible light, but which emit
light of different wavelengths and/or are excited by
different wavelengths of excitation light and/or have
different phosphorescent decay half-lives. The
appearance/ colour of the luminescent materials in
visible light will be chosen to match the parts of the
bank notes surrounding the windows. The materials
could be printed with a pattern showing alphanumeric
characters, e.g., the numerals 2,000 as shown in
Figure 7. The luminescent pattern is machine
detectable, but could also be observed by a human
observer.
In Figure 8 there can be seen a schematic drawing
showing apparatus for use in the methods of
authenticating bank note 10. An irradiator 40 is used
to irradiate bank note 10 with, for instance, ultra-
violet or infra-red radiation and the detector 41 then
detects what light is emitted from and/or ultra-violet
radiation and/ or infra-red radiation is reflected
from the surface of the bank note 10. The detector 41
will then enable determination of either the
luminescent characteristics of material in the
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security thread in the bank note 10, the infra-red or
ultra-violet reflecting characteristics of the
security thread in the bank note 10 and/or the infra-
red or ultra-violet absorbing characteristics of the
security thread in the bank note 10.
Whilst above all embodiments have described use
of the invention for a bank note 10, the bank note 10
is only one example of a security article for which
the present invention is applicable. For instance,
the present invention could be used for credit cards
and debit cards, with the security thread (which terms
for the purpose of this specification and claims will
be interpreted as encompassing the term security
strip, commonly used for cards as opposed to bank
notes) partially embedded in a plastic material
typically, with portions exposed at one surface. The
term security article can also include any form of
security paper, for instance cheques and travellers'
cheques, bond documents, mortgage documents, in
addition to standard bank notes used in currency.
The exposed portions of the security thread 11
could have a colour which matches the colour at the
surrounding material of the security article (e. g.
paper, plastics} or the colour of the printing on the
portions of surface of the security article
surrounding the exposed portions.
Examples of security articles incorporating the
present invention will now be given, as follows:
Example 1
A roll of polyester was vacuum-metallised with
aluminium to a metal thickness of approximately 30 nm.
A coating comprising an inorganic phosphor, e.g.
copper-doped zinc sulphide (copper: zinc sulphide ratio
between 50 and 100 parts per million) dispersed in an
organic binder at a proportion of 15% phosphor: binder
by weight was applied to both surfaces of the
metallised polyester to a dry coating thickness of 5
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microns. A further organic protective coat and/or
adhesive layer was applied over one or both sides of
the phosphor coated metallised film. The film was then
mechanically reduced by known means to form security
threads in the width range typically 0.5 -4.0 mm. The
security threads were then incorporated into banknote
paper by the known technique described in EP-A-0059056
to form a windowed security thread. The paper was then
printed, cut and issued as banknotes.
The coating has a pale yellow colour in daylight
which closely matches that of the paper into which the
thread is incorporated. The threads and the coated
film from which the threads are cut have a matt
appearance. The matt appearance of the coated film was
characterised by one of several techniques as
follows:-
(i) The gloss of the coated film was measured on
a statistical Novo-Gloss 60° glossmeter with
an upper limit of 1000 gloss units
(theoretical perfect mirror). The coated
film had a gloss measurement of 31 units;
this compares with vacuum-metallised film
with an identical adhesive coat which, on
the same instrument, measured 497 gloss
units.
(ii) The gloss of the same film was measured on a
mufti-angle Novo-Gloss glossmeter at 20°,
60° and 75° angle. This unit has an upper
limit of 199 gloss units. The measurements
were as shown in the following table:
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Sample Angle Head
Average gloss
Metallised film with 20 5.8
luminescent coating and 60 31.0
.. adhesive coating 75 73.0
Adhesive coated metallised 20 >199
polyester 60 >199
75 >199
(iii) The specular reflectance was measured on a
Shimadzu UV3101-PC spectrophotometer. The
sample comprising a luminescent coating plus
adhesive coating on metallised polyester
produced measurements ranging from 0.8% at
400nm to 1.6 at 700nm; the adhesive-coated
metallised polyester produced a value
ranging from 50% at 400nm to 58% at 700 nm.
Under stimulation by UV light at a wavelength of
366 nm, the coating emitted green light. Other
phosphors may be used, e.g. manganese-doped zinc
sulphide, which emits orange light and silver-doped
zinc sulphide which emits blue light.
During subsequent used note sorting operations,
the banknotes were carried by a transport path through
a detection unit comprising a UV light source and
optical filter tuned to the emission wavelength of the
doped zinc sulphide and an appropriate photodetector.
Example 2
A dispersion of magnetic material (gamma ferric
oxide) in an organic binder was coated to a dry film
thickness of 5~.m onto the metallised surface of a
vacuum aluminised 12~m thick polyester. A second ply
of,vacuum aluminised l2~cm polyester was laminated to
the first ply such that both aluminium layers and the
magnetic layer were internal to the laminate. A
coating of luminescent material comprising copper-
doped zinc sulphide in an organic binder was applied
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to each side of the laminate, to a dry thickness of
5~m. The film was then adhesive-coated on both sides
and processed as described in Example 1. During
subsequent used note sorting operations, both the
phosphorescent emission and magnetic content of the
security thread were measured according to known
techniques by appropriate detectors fitted to the
sorting machine transport path.
Example 3
A layer of copper-doped zinc sulphide pigment
incorporated into an organic binder at a proportion of
5% by weight was coated to a dry thickness of 2um onto
transparent 12~.m polyester. A layer of transparent
adhesive was applied to each side to a dry thickness
of 4~m and the film converted to security threads as
described in Example 1. In contrast to the devices
described in Examples 1 and 2, this thread is semi-
transparent and not readily apparent in the finished
article when viewed in transmitted light. In use, the
presence of the luminescent coating may be determined
by a machine, as described in Example 1, or by a human
observer when the luminescent component is excited by
placing the article under a source of UV light or
between a source of Uv light and the observer.
Example 4
As Example 1, except that two different doped
zinc sulphide phosphors were incorporated into the
organic binder in equal proportions to produce a
combined weight of 15% pigment:binder. In use, the
different phosphors emitted different wavelengths of
light which were detected by photo-detectors fitted
with appropriate narrow band optical filters tuned to
the emission wavelengths of the two phosphors.
Exam-p 1 a 5
A security thread was prepared as described in
Example 4 except that in this instance phosphors with
different half-life decay times were used. To
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authenticate banknotes incorporating the thread,
measurements were made of the different decay
properties of the two phosphors.
Example 6
As Example 1, except that a different zinc
phosphor was used with a pale blue colour in visible
light, chosen to match the colour of the surrounding
ink in the finished banknote prepared from the paper
incorporating the security thread.
Example 7
As Example 1, except that a fluorophor was used
instead of an inorganic phosphor as the luminescent
pigment. A fluorescence rather than phosphorescence
detector was then used to authenticate the banknote on
the sorting machine transport system.
Example 8
As Example 3, except that a lightly coloured IR
absorbing pigment, e.g. substituted chloro copper pH
Halo cyanine, trade name PROJECT 900NP from the Zeneca
company, was incorporated into an organic binder at
proportion of 5% pigment binder and used for the
coating over the transparent polyester to a dry
coating thickness of 2 microns. In use on used note
sorting machines, measurement was made of the IR peak
of 890nm absorption due to the pigment which
contrasted sharply with the IR transmission of the
surrounding areas of the security article.
Example 9
As Example 1, except that an Anti-Stokes pigment
such as yttrium oxysulphide was incorporated into the
binder in place of the zinc sulphide phosphor at a
concentration of 30% pigment:binder and applied at a
dry coating thickness of 2 microns. To authenticate
the finished banknote, it was placed under a source of
IR radiation at 970nm which excited the Anti-Stokes
compound which then emitted green light at a
wavelength of 540nm discernible to a human observer.
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Example 10
As Example 1, except that a thermoluminescent
pigment was incorporated into the binder instead of
the zinc sulphide phosphor. In use, the security
article was heated by an IR source or other heat
source and the thermoluminescent material emitted
visible light which was detected by a photo-detector
equipped with a suitable narrow band optical filter.
Example 11
A manganese-doped zinc sulphide phosphor
(manganese content 3000 parts per million) was
dispersed in an organic binder at a proportion of 30°s
pigment: binder and coated onto one side of a 12 micron
metallised polyester film to produce a
triboluminescent coating. The other side of the film
was coated with an adhesive and the film reduced by
mechanical means to dimensions suitable for a banknote
security thread, e.g. 1-4mm. The thread was inserted
into paper according to the process described in EP
0059056 such that the side of the film coated with the
phosphor and binder was exposed in the window regions.
The paper was then printed, cut and issued as
banknotes. The colour of the ink in the window region
of the banknote was selected to match closely that of
the phosphor/binder coating such that the presence of
the windowed security thread was not readily
discernible in reflected light.
In use, the phosphor coating was stimulated by
mechanical action such as rubbing or pressing the
surface with a hard transparent plastic rod. The
phosphor exhibited triboluminescent properties and
emitted visible light which was discernible to the
human eye.
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