U.S. patent application number 13/580995 was filed with the patent office on 2013-02-21 for security device.
This patent application is currently assigned to DE LA RUE INTERNATIONAL LIMITED. The applicant listed for this patent is Brian William Holmes. Invention is credited to Brian William Holmes.
Application Number | 20130043670 13/580995 |
Document ID | / |
Family ID | 42125582 |
Filed Date | 2013-02-21 |
United States Patent
Application |
20130043670 |
Kind Code |
A1 |
Holmes; Brian William |
February 21, 2013 |
SECURITY DEVICE
Abstract
A security device includes a transparent, coloured element in a
first region of the device and in a surface of which a first
optically variable effect generating relief structure is formed. A
reflection enhancing layer extends over the first optically
variable effect generating relief microstructure and follows the
contour of the relief, the reflection enhancing layer also being
provided in a second region of the device laterally offset from the
first region.
Inventors: |
Holmes; Brian William;
(Fleet, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Holmes; Brian William |
Fleet |
|
GB |
|
|
Assignee: |
DE LA RUE INTERNATIONAL
LIMITED
Basingstoke, Hampshire
GB
|
Family ID: |
42125582 |
Appl. No.: |
13/580995 |
Filed: |
February 24, 2011 |
PCT Filed: |
February 24, 2011 |
PCT NO: |
PCT/GB2011/050362 |
371 Date: |
October 26, 2012 |
Current U.S.
Class: |
283/85 ;
29/527.1 |
Current CPC
Class: |
G07D 7/207 20170501;
B42D 25/328 20141001; B42D 25/29 20141001; G03H 1/0256 20130101;
G03H 1/30 20130101; G07D 7/0032 20170501; G03H 2250/34 20130101;
B42D 2033/18 20130101; B42D 25/324 20141001; B42D 25/40 20141001;
Y10T 29/4998 20150115; G03H 1/0011 20130101; B42D 2033/20
20130101 |
Class at
Publication: |
283/85 ;
29/527.1 |
International
Class: |
B42D 15/00 20060101
B42D015/00; B23P 25/00 20060101 B23P025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 24, 2010 |
GB |
1003136.7 |
Claims
1.-35. (canceled)
36. A security device comprising a transparent, coloured element in
a first region of the device and in a surface of which a first
optically variable effect generating relief structure is formed;
and a reflection enhancing layer extending over the first optically
variable effect generating relief microstructure and following the
contour of the relief, the reflection enhancing layer also being
provided in a second region of the device laterally offset from the
first region.
37. A device according to claim 36, wherein the reflection
enhancing layer extends continuously from the first region to the
second region.
38. A device according to claim 36, wherein the reflection
enhancing layer is provided in discrete portions in the first
region and second region respectively.
39. A device according to claim 36, wherein the second region
surrounds the first region.
40. A device according to claim 36, wherein the second region
includes a second, transparent coloured element having a colour
different from the colour of the first transparent coloured
element, a surface of the second transparent, coloured element
being provided with a second optically variable effect generating
relief microstructure, the reflection enhancing layer extending
over the second optically variable effect generating relief
microstructure and following the contour of the relief.
41. A device according to claim 40, wherein the first and second
optically variable effect generating relief microstructures are
different.
42. A device according to claim 36, wherein the reflection
enhancing layer is one of a metal layer or high refractive index
layer.
43. A device according to claim 36, wherein one or more parts of
the reflection enhancing layer are formed as one or more symbols,
characters, alphanumeric figures or other graphical shapes.
44. A device according to claim 40, wherein the first and second
elements have different shapes.
45. A device according to claim 36, further comprising an adhesive
layer, for example a photosensitive or heat sensitive adhesive
layer, over the transparent substrate.
46. A device according to claim 36, wherein the or each optically
variable effect generating surface relief microstructure comprises
one or more of a hologram, diffraction grating, prismatic
structure, and microlens array.
47. A stripe assembly having a sequence of security devices
according to claim 36 spaced along a support, with the colours of
successive transparent coloured elements in the first regions being
different.
48. A transfer structure comprising a security device according to
claim 36, supported on a carrier via a release layer and having an
adhesive on its surface furthest from the carrier.
49. A method of manufacturing a security device, the method
comprising providing a curable material on a carrier to define a
transparent, coloured element in a first region; forming a surface
of the element with an optically variable effect surface relief
microstructure; curing the material so that the microstructure is
retained by the cured material; and providing a reflection
enhancing layer extending over the first optically variable effect
surface relief microstructure following the contour of the relief,
the reflection enhancing layer also being provided in a second
region laterally offset from the first region.
50. A method according to claim 49, wherein the reflection
enhancing layer extends continuously from the first region to the
second region.
51. A method according to claim 49, wherein the reflection
enhancing layer is provided in discrete portions in the first and
second regions respectively.
52. A method according to claim 49, wherein the second region
surrounds the first region.
53. A method according to claim 49, further comprising providing a
second, transparent coloured element in the second region on the
carrier, the second element having a colour different from the
colour of the first transparent, coloured element, providing a
surface of the second transparent, coloured element with a second
optically variable effect generating relief microstructure, and
wherein the reflection enhancing layer is provided over the second,
transparent coloured element following the contour of the
relief.
54. A method according to claim 53, wherein the first and second
optically variable effect generating relief structures are
different.
55. A method according to claim 49, wherein the reflection
enhancing layer comprises a metal or high refractive index
layer.
56. A method according to claim 49, wherein the optically variable
effect generating relief microstructures are cast or embossed into
the curable materials.
57. A method according to claim 49, wherein the or each element of
curable material is printed onto the carrier.
58. A method according to claim 49, wherein the reflection
enhancing layer is provided over both the cured regions and areas
where there is no cured or curable material.
59. A method according to claim 49, wherein the reflection
enhancing layer is vacuum deposited or printed.
60. A method according to claim 49, wherein the reflection
enhancing layer is printed using a metallic ink including one or
more colourants.
61. A method according to claim 49, further comprising forming
parts of the reflection enhancing layer as one or more symbols,
characters, alphanumeric figures or other graphical shapes.
62. A method according to claim 61, wherein the forming step
comprises selectively demetallising a metallic layer.
63. A method according to claim 49, further comprising providing an
adhesive layer, for example a photosensitive or heat sensitive
adhesive layer, over the cured material and after the provision of
the reflection enhancing layer.
64. A method according to claim 49, wherein the or each optically
variable effect generating relief microstructure comprises one or
more of a hologram, diffraction grating, prismatic structure, and
microlens array.
65. A security device manufactured according to claim 49.
66. A security device according to claim 65, the device comprising
a label or stripe.
67. A method of transferring a security device on a carrier web
manufactured according to claim 49 onto an article to which it is
secured by adhesive, the method comprising hot stamping the
security device onto the article with a heat sensitive adhesive
between the security device and the article.
68. An article provided with a security device according to claim
36.
69. An article according to claim 68, wherein the article is a
security article such as a security document, for example one of
banknotes, cheques, passports, identity cards, certificates of
authenticity, fiscal stamps and other documents for securing value
or personal identity.
70. A security device according to claim 36, wherein the metallic
layer is a metal such as aluminium, copper or gold.
Description
[0001] The invention relates to a security device, in particular
incorporating an optically variable effect generating structure,
and a method for its manufacture. Optically variable effect
generating structures such as holograms and diffraction gratings
have been used widely over the last few years to impart security to
documents of value such as banknotes, credit cards and the like.
Conventionally, the structure is provided on a transfer foil and is
then hot stamped from the transfer foil onto the final substrate.
An early example of this approach is described in U.S. Pat. No.
4,728,377.
[0002] There is a need to increase the security of such devices and
one approach is described in EP-A-1294576. In this case, the
hologram or diffraction grating is spatially modulated by two or
more metals provided in intimate contact with the surface relief.
This varies the intensity of the diffractive light in an
unconventional and difficult to replicate manner. However, the use
of two metals is difficult in practice and expensive and there is a
need to provide a simpler approach. Furthermore vapour deposited
metals provide a very limited choice of colours.
[0003] In accordance with the first aspect of the present
invention, a security device comprises a transparent, coloured
element in a first region of the device and in a surface of which a
first optically variable effect generating relief structure is
formed; and a reflection enhancing layer extending over the first
optically variable effect generating relief microstructure and
following the contour of the relief, the reflection enhancing layer
also being provided in a second region of the device laterally
offset from the first region.
[0004] A novel security device has been developed which provides an
optically variable effect from a coloured element and makes use of
the reflection enhancing layer used to enhance the optically
variable effect by providing that reflection enhancing layer in the
second region. In the second region, the reflection enhancing layer
can simply be a plain layer which will present a distinctive, plain
appearance to the observer or it could be formed with patterns
defining symbols, characters and the like. In a particularly
preferred embodiment, the second region includes a second,
transparent coloured element having a colour different from the
colour of the first transparent coloured element, a surface of the
second transparent, coloured element being provided with a second
optically variable effect generating relief microstructure, the
reflection enhancing layer extending over the second optically
variable effect generating relief microstructure and following the
contour of the relief. This provides a multi-coloured device in
which both colours are enhanced by the reflection enhancing
layer.
[0005] The reflection enhancing layer can be a metal layer, which
could be a pure metal or a metal containing layer, or a high
refractive index layer such as ZnS or the like.
[0006] Where a metal or metallic layer is provided, this combines
with at least the transparent, coloured element in the first region
to present an unusual coloured, metallic effect to the viewer.
[0007] As mentioned above, the reflection enhancing layer follows
the contour of the surface relief and typically, the reflection
enhancing metallic layer will be in contact with the surface of the
element in which the microstructure is formed. However, it could be
spaced from that element by an intermediate transparent layer or
the like, provided that intermediate layer was sufficiently thin so
that the reflection enhancing layer again followed the surface
relief contour.
[0008] Typically, the second region surrounds or fully encloses the
first region although in some cases it could be laterally offset in
just one dimension.
[0009] It is particularly convenient if one or more parts of the
reflection enhancing layer are formed as one or more symbols,
characters, alphanumeric figures or other graphical shapes. This
enables a variety of characters to be provided in one or both
regions and these can be related to the article on which the
security device is provided.
[0010] The first and second regions could be spaced apart or abut
one another. In the former case, where first and second elements
are provided, different optically variable effect generating
surface relief microstructures could be provided on each element
while in the latter case typically the surfaces of the elements are
formed of different parts of the same optically variable effect
surface relief microstructure.
[0011] The device can be constructed in a variety of different
ways, for example directly on an article to be protected but
typically will be provided initially in the form of a transfer
structure. In other embodiments, the security device can be used to
form a stripe assembly.
[0012] In order to adhere the device to an article, the device
preferably further comprises an adhesive layer, for example a
photosensitive or heat sensitive adhesive layer, over the
transparent substrate. Alternatively, adhesive could be provided on
the article itself.
[0013] The or each optically variable effect generating surface
relief microstructure can have any conventional form but typically
comprises one or more of a hologram, diffraction grating, prismatic
structure, and microlens array.
[0014] In accordance with a second aspect of the present invention,
a method of manufacturing a security device comprises providing a
curable material on a carrier to define a transparent, coloured
element in a first region; forming a surface of the element with an
optically variable effect surface relief microstructure; curing the
material so that the microstructure is retained by the cured
material; and providing a reflection enhancing layer extending over
the first optically variable effect surface relief microstructure
following the contour of the relief, the reflection enhancing layer
also being provided in a second region laterally offset from the
first region.
[0015] Techniques which can be used in this method include cast
curing, hot embossing and in-situ polymerisation replication
(ISPR). An example of this latter technique is UV casting.
[0016] U.S. Pat. No. 4,758,296 describes the production of a
holographic foil, generated by UV casting, which can be transferred
to a substrate as a patch using the hot stamping process. In order
to facilitate the hot stamping process a UV curable polymer is
selected which is brittle enabling it to fracture at the edges of
the region contacted by the stamping die. This solution is not
ideal for applications concerning flexible documents as the use of
a brittle material will reduce the durability of the final device
especially if, as is the case with a banknote, the document is
repeatedly folded or crumpled during circulation. The use of a
brittle material also becomes more problematic the thicker the
device becomes, making the prior art solution even less suitable
for the replication of the non-holographic micro-optical
devices.
[0017] In a preferred embodiment, the method further comprises
providing a second, transparent coloured element in the second
region on the carrier, the second element having a colour different
from the colour of the first transparent, coloured element,
providing a surface of the second transparent, coloured element
with a second optically variable effect generating relief
microstructure, and wherein the reflection enhancing layer is
provided over the second, transparent coloured element following
the contour of the relief. Preferably, the two regions are spaced
apart and formed with respective reflection enhancing layers. The
advantage of this is that the cured material does not have to be
fractured during the hot stamping process and thus can easily
transfer onto an article during hot stamping or as a label. A
similar approach is described in U.S. Pat. No. 6,302,989.
[0018] Preferably, the reflection enhancing layer also extends
across areas between and/or laterally outside the cured
elements.
[0019] The optically variable effect generating relief
microstructures may be embossed into the cured or curable materials
or provided in other known ways such as by cast curing.
[0020] Typically, the discrete regions of the curable materials are
printed onto the carrier although other known forms of deposit
could be used.
[0021] The reflection enhancing layer may be provided over both the
elements and areas where there is no cured or curable material.
[0022] The reflection enhancing layer may be formed from a pure
metal such as aluminium, copper or gold or alternatively may
include one or more colourants.
[0023] The method may further comprise forming parts of the
reflection enhancing layer as one or more symbols, characters,
alphanumeric figures or other graphical shapes. In this case, the
forming step may include selectively demetallising the metallic
layer.
[0024] In order to fix the security device onto an article, it is
necessary to provide an adhesive layer. Although this could be
provided on the article and then the security device brought into
contact with the adhesive, preferably the method further comprises
providing an adhesive layer, for example a photosensitive or heat
sensitive adhesive layer, over the cured, embossed material and
after the provision of a reflection enhancing layer if
provided.
[0025] Examples to which security devices according to the
invention can be applied include banknotes, cheques, passports,
identity cards, certificates of authenticity, fiscal stamps and
other documents for securing value or personal identity.
[0026] Some examples of security devices and methods of manufacture
according to the invention will now be described and contrasted
with a known example with reference to the accompanying drawings,
in which:--
[0027] FIG. 1 is a schematic view of a known manufacturing
apparatus;
[0028] FIG. 1A is a view similar to FIG. 1 but of apparatus for use
in a method according to the invention;
[0029] FIG. 2 illustrates a comparative example;
[0030] FIGS. 3A and 3B are a plan and cross-section respectively
and not to scale of a security device made according to an example
of the invention;
[0031] FIGS. 4A, 4B; 5A, 5B; 6A, 6B; and 7A and 7B are views
similar to FIGS. 3A and 3B respectively but of further examples of
security devices made according to different methods according to
the invention.
[0032] FIG. 1 illustrates schematically a continuous in-situ
polymerisation replication process as is currently known in the
art.
[0033] A web 2 of polymer material such as polyethylene terephalate
(PET) or biaxially oriented polypropylene (BOPP) is unwound from a
reel 4 and coated with a UV curable resin 6 in a coating unit 8. An
optional drying chamber 10 can be used to remove solvent from the
resin. The resin 6 on the web 2 is then held in contact with an
embossing roller 12 in order to replicate the optically variable
microstructure, embossed into the roller, in the resin. The
embossed resin 6 is then cured and hardened while in contact with
the embossing roller 12 using appropriate radiation such as ultra
violet light 14. The final web comprising the optically variable
microstructure is then rewound onto a reel 16.
[0034] The resin 6 is typically applied to the substrate using one
of precision bead coating, direct and indirect gravure coating,
meyer bar coating or slot coating. The radiation curable material
preferably comprises a resin which may typically be of two types:
[0035] a) Free radical cure resins which are unsaturated resins or
monomers, prepolymers, oligomers etc, containing vinyl or acrylate
unsaturation for example and which cross-link through use of a
photo initiator activated by the radiation source employed e.g. UV.
[0036] b) Cationic cure resins in which ring opening (e.g. epoxy
types) is effected using photo initiators or catalysts which
generate ionic entities under the radiation source employed e.g.
UV. The ring opening is followed by intermolecular
cross-linking.
[0037] The radiation 14 used to effect curing will typically be UV
radiation but could comprise electron beam, visible, or even
infra-red or higher wavelength radiation, depending upon the
material, its absorbance and the process used. The web 2 is
preferably a polymeric film and will be substantially transparent
so that the optically variable effect structure can be provided on
a surface of the transparent resin 6 on the web which will not be
externally exposed in use, while permitting the optically variable
effect to be viewed through the substrate. Flexible polymeric films
suitable for the invention include polyethylene teraphthalate
(PET), polyethylene, polyamide, polycarbonate, poly(vinylchloride)
(PVC), poly(vinylidenechloride) (PVdC), polymethylmethacrylate
(PMMA), polyethylene naphthalate (PEN), and polypropylene.
[0038] The optically variable structure may comprise a hologram or
diffraction grating or a non-holographic micro-optical structure.
Prismatic structures are a preferred type of a micro-optical
structure. Examples of prismatic structures suitable for the
current invention include, but are not limited to, a series of
parallel linear prisms with planar facets arranged to form a
grooved surface, a ruled array of tetrahedra, an array of square
pyramids, an array of corner-cube structures, and an array of
hexagonal-faced corner-cubes. A second preferred type of
micro-optical structure is one which functions as a microlens
including those that refract light at a suitably curved surface of
a homogenous material such as plano-convex lenslets, double convex
lenslets, plano-concave lenslets, and double concave lenslets.
Other suitable micro-optical structures include geometric shapes
based on domes, hemispheres, hexagons, squares, cones, stepped
structures, cubes, or combinations thereof.
[0039] In the new process, the known process has been modified to
enable more complex, secure and aesthetically pleasing security
devices comprising optically variable structures to be created.
[0040] In the new process (FIGS. 1A and 2), the radiation curable
resin is applied, typically by printing at a printing unit 7 in
place of the coating unit 8 only partially onto a release layer 3
on the flexible polymeric substrate web 2 and typically in register
with the optically variable microstructures 22 on the embossing
roller 12. At least one coloured radiation curable resin is used
each in register with one or more optically variable
microstructure. Suitable printing methods include direct and
indirect gravure printing, flexographic printing, lithographic
printing and screen printing.
[0041] Thus, in FIGS. 1A and 2 the coating unit 8 in FIG. 1 is
modified such that it is capable of applying, typically printing,
the radiation curable resin in localised regions or patches 20.
This has the benefit that the resin 6 only needs to be applied in
the regions 20 on the web 2 that will ultimately form the security
device. The locally applied regions 20 of the radiation curable
resin 6 are preferably in register with the optically variable
microstructures 22 on the embossing roller 12 as shown
schematically in FIG. 2 where the regions of resin 20 are aligned
with respective regions of microstructures 22 on the embossing
roller 12. The result of this is a series of patches or discrete
devices across the polymeric carrier web 2 which exhibit the
desired optically variable effect.
[0042] The patches or discrete devices 20 after curing can then be
easily transferred to a secure document such as a banknote using a
conventional hot stamping transfer process because the stamping
tool does not need to cut through the inherently strong radiation
cured resin and instead just needs to penetrate the release layer
on the polymeric carrier substrate. Spaces 24 between the devices
20 define the boundaries of the devices.
[0043] If the optically variable effect is a holographic generating
structure, and by this we mean structures that generate graphical
images by the mechanism of diffraction of light, then a reflection
enhancing layer is usually provided on the optically variable
microstructure. The holographic generating structures include those
formed by the following non-exhaustive list of techniques optical
interferometry dot-matrix interferometry, lithographic
interferometry or e-beam lithography. The reflection enhancing
layer can be a vacuum deposited metallic layer, a printed metallic
layer or a substantially transparent high refractive index layer.
If a vapour deposited metallic layer is used this may be
selectively demetallised by etching or the like to enable
underlying information to be visible when the device is secured to
an article or document.
[0044] The secure nature of the security device generated in FIG. 2
is increased by the use of coloured radiation curable resins. The
resin 6 can be tinted by using dyes or pigments. The resin will
still need to be substantially transparent for the final
diffractive element to replay and therefore dyes are the preferred
tinting method rather than pigments. This enables the creation of
coloured holographic or diffractive security devices. Previously
different background colour variations have been achieved by using
different coloured reflection enhancing layers for example
replacing vapour deposited aluminium with vapour deposited gold or
copper but in this case the choice of colours available are limited
by the relatively small number of metals suitable for cost
effective vapour deposition.
[0045] FIG. 3 shows a first embodiment of a security device
according to the invention in cross-section (3B) and plan-view (3A)
where the radiation curable resin has been tinted to create a
device with a dual coloured background. In this example, a
radiation curable resin patch or element 20 which has been tinted
green using either a dye or pigment is coated or printed onto the
polymeric carrier web 2. If required an optional release layer 3 is
used directly on the surface of the polymeric carrier substrate and
a further protective/supporting layer 5 may be provided on the
release layer 3. The protective layer 5 is preferably a
conventional thermal/chemical cross linked layer of the type
normally employed as scuff coats within conventional hot stamped
foils. Such layers unlike the UV cured layer are easier to break
during hot stamping process. In this example, the radiation curable
resin is applied in a discrete area 20 which is registered to a
holographic optically variable microstructure on the embossing
roller (not shown). This is then embossed or cast into the resin
element 20 as shown at 21. FIG. 3 illustrates exact registration
between the holographic optically variable microstructure 21 and
the radiation curable resin element 20 but in practice the
holographic variable microstructure may be slightly inset to allow
for production tolerances. The radiation curable resin does not
necessarily need to follow the same outline as the holographic
optically variable microstructure with the main requirement being
that the optically variable microstructure is registered such that
it is surrounded by the radiation curable resin. A reflection
enhancing layer 30 is then applied as a continuous layer over the
security device, in this case a vapour deposited aluminium layer is
used. Although not shown in the drawings, this metal layer follows
the contour of the surface relief microstructure. (The same is true
for all other embodiments). Preferably the reflection enhancing
layer is a pure metal such as AL, Cu or Au but this is not
essential. Alternatively the reflection enhancing layer could be a
substantially transparent high refractive index (hri) layer. Such
materials, typically inorganic, are well known in the art and
described in U.S. Pat. No. 4,856,857. Typical examples of materials
suitable for the high refractive index layer include zinc sulphide,
titanium dioxide and zirconium dioxide. Replacing the vapour
deposited metal reflection enhancing layer with a transparent hri
layer is particularly beneficial when the security device of the
current invention is applied over transparent regions (typically
known as apertures or windows) of secure documents. For example if
the reflection enhancing layer for the security device illustrated
in FIGS. 3A and 3B was replaced with a transparent hri layer than
when viewed in reflection the holographic image in zone A will be
viewed against a reflective green background and zone B will appear
colourless. When viewed in transmission zone A will appear green
against a transparent background. The holographic image will not be
readily apparent when viewed in transmission and therefore the
contrast between the reflection and transmission viewing conditions
can be used as a method of authentication.
[0046] Finally an adhesive layer 32 is applied to the reflection
enhancing layer 30 to enable the device to be applied to a document
of value. The security device is then transferred to a secure
document by hot stamping. After transfer the carrier web 1 may be
removed, leaving the security device as the exposed layer.
[0047] On viewing the security device in FIG. 3A, once applied to
the secure document, a holographic image defined by the optically
variable microstructure 21 is observed in zone A viewed against a
green metallic background as a result of the combination of the
metallic reflection enhancing layer 30 and the tinted radiation
curable resin 20. The rest of the device (Zone B) appears metallic
silver due to the vapour deposited aluminium layer 30 and therefore
a dual-coloured holographic optically variable device is
created.
[0048] FIGS. 4A and 4B show a further modification of the security
device in FIGS. 3A and 3B where the vapour deposited aluminium
layer 30 has been demetallised to create discrete star-shaped
metallic images 40. Although not illustrated the aluminium layer in
contact with the holographic optically variable microstructure
could also be demetallised to create sub-regions within the
holographic image.
[0049] A preferred embodiment of the current invention is a
security device comprising an optically variable microstructure in
the form of one or more holographic generating microstructures
where the one or more holographic generating microstructures are
formed in at least one region of a layer of a radiation curable
resin where the layer is provided by registered printing of at
least two differently coloured resins. Preferably the differently
coloured resins are registered to one or more of the holographic
generating microstructures. FIGS. 5A and 5B show an example of such
a device in plan-view and cross-section respectively. As can be
seen from FIG. 5, a first radiation curable resin element 50 tinted
green is in register with the first holographic microstructure 52
to define a first holographic image in zone A. Second radiation
curable resin elements 54 tinted blue and laterally offset from the
element 52 are in register with a second holographic microstructure
56 to define second holographic images in zone B. A reflection
enhancing layer 30 is then applied as a continuous layer over the
security device, in this case a vapour deposited aluminium layer is
used. Finally, an adhesive layer 32 is applied to the reflection
enhancing layer to enable the device to be applied to a document of
value. On viewing the secure device the holographic image in zone A
is viewed against a metallic green background, and the holographic
images in zone B are viewed against a metallic blue background
whereas the rest of the device (Zone C) appears metallic silver due
to the vapour deposited aluminium layer 30. An optically variable
security device is created which appears to have three different
reflection enhancing layers but has been produced by the registered
printing of two different coloured radiation curable resins as a
first stage in an ISPR process.
[0050] The secure nature of the security device in FIGS. 5A and 5B
can be enhanced further by increasing the number of differently
coloured radiation curable resin elements and preferably these are
printed in register with further holographic generating
microstructures. FIGS. 6A and 6B shows an example of such a
structure. A first radiation curable resin element tinted green 50
and a second radiation curable resin element tinted yellow 54 are
printed in register with the first holographic microstructure 52 to
define a first holographic image with a multicoloured background in
zone A. A third radiation curable resin element tinted blue 60 is
printed in register with a second holographic microstructure 56 to
define a second holographic image in zone B. Finally the vapour
deposited aluminium layer 30 is then demetallised to remove the
metal in the non holographic image areas 40 except in zone C where
the plain metallic regions forms an identifying symbol 62 which in
this case is the denomination of the secure document being
protected. This example illustrates that differently coloured
radiation curable resin elements can be registered to a single
holographic microstructure such that different regions of the
resultant holographic image have different background colours. In a
modification (not shown), the plain metallic regions 62 could be
omitted.
[0051] FIGS. 7A and 7B illustrate a plan and cross-sectional view
of a coloured holographic stripe with two differently coloured
radiation curable resin elements, for example one 70 tinted green
and one 72 tinted blue, registered to a repeating holographic
microstructure 74. The structure is supported on carrier webs by
release and protective layers 3,5. A vapour deposited aluminium
layer 30 is applied over the device and then demetallised in
register with the differently coloured radiation curable resins
70,72. The final stripe has a repeating design of a holographic
image which can be seen on an alternating green and blue metallic
backgrounds (Zone A and B respectively). One advantage of this
technique is that it can be used to provide the illusion of a
series of differently coloured discrete holographic patches along a
continuous stripe.
[0052] In all the previous examples the reflection enhancing layer
has been a vapour deposited aluminium layer 30. It is of course
also possible to use different vapour deposited metals for example
copper and gold and of course more than one metal can be used on a
single device to generate even more secure devices.
[0053] In a further embodiment the vapour deposited metal layer can
be replaced by a printed metallic layer. One of the advantages of
using a metallic ink compared to a vapour deposited metallic layer
is the ability to add colourants to the metallic ink, for example
by using pigments or dyestuffs. This enables the creation of
multicoloured holograms because the reflective layer can be formed
by the registered printing of multicoloured metallic inks.
Furthermore, the metallic flakes in the ink can be varied typically
from aluminium (silver effect), bronze (gold effect), iron or zinc
to give different coloured effects.
[0054] The brightness of the printed metallic ink layer can be
enhanced by incorporating an additional visually transparent, high
refractive index layer into the structure, as described in
PCT/GB2008/003634. By high refractive index, we mean an index of
refraction which exceeds that of the embossed base layer by a
numerical value of 0.5 or more. Since the refractive index of the
embossed base layer will typically fall in the range of 1.45-1.55
then a high refractive index material will be one with an index of
2.0 or more. In practice high refractive index materials with good
visual transparency transparent will have an index in the range
2.0-2.5.
[0055] An optimum brightness can be achieved by carefully
determining the thickness of the high refractive index layer needed
to ensure constructive interference between the two partial
amplitudes diffracted off the first and second surfaces of the high
refractive index layer. The first surface being that which forms
the interface with surface relief of the embossed base layer,
whilst the second surface being that which forms the interface with
the metallic ink. The thickness of the high refractive layer
required to ensure constructive interference between the partial
diffracted amplitudes differs from that needed to ensure
constructive interference between partial amplitudes reflected off
two strictly planar interfaces and is best determined empirically
by practical methods as its precise value depends on the
periodicities and amplitudes present in the optically variable
microstructure.
[0056] A modified version (FIG. 1A) of the ISPR method illustrated
in FIG. 1 can be used to form the security device using the new
method.
[0057] The first step is to print one or more radiation curable
resin elements on a polymeric carrier film where each resin element
has been preferably tinted by dyes or pigments such that they
provide the final device with more than one base colour. The resin
will still need to be substantially transparent for the final
diffractive/holographic element to replay and therefore dyes are
the preferred tinting method rather than pigments. An optically
variable microstructure is then cast, for example by using an
embossing roller as illustrated in FIG. 1, into the one or more
resin elements to generate a design which is preferably in register
to the coloured pattern of the base layer.
[0058] A reflection enhancing layer is then applied over the
radiation curable resin elements either as continuous layer or as
partial layer preferably in register with either one or more of the
radiation curable resins or the optically variable microstructure.
The reflection enhancing layer can be applied by vapour deposition
or printing and more than one reflection enhancing layer can be
applied to create further coloured effects.
[0059] The finished device can be applied to an article or document
in a variety of different ways, some of which are set out below.
The security device could be arranged either wholly on the surface
of the document, as in the case of a stripe or patch, or may be
visible only partly on the surface of the document in the form of a
windowed security thread.
[0060] Security threads are now present in many of the world's
currencies as well as vouchers, passports, travellers' cheques and
other documents. In many cases the thread is provided in a
partially embedded or windowed fashion where the thread appears to
weave in and out of the paper. One method for producing paper with
so-called windowed threads can be found in EP0059056. EP0860298 and
WO03095188 describe different approaches for the embedding of wider
partially exposed threads into a paper substrate. Wide threads,
typically with a width of 2-6 mm, are particularly useful as the
additional exposed area allows for better use of optically variable
devices such as the current invention.
[0061] The device could be incorporated into the document such that
regions of the device are viewable from both sides of the document.
Techniques are known in the art for forming transparent regions in
both paper and polymer substrates. For example, WO8300659 describes
a polymer banknote formed from a transparent substrate comprising
an opacifying coating on both sides of the substrate. The
opacifying coating is omitted in localised regions on both sides of
the substrate to form a transparent region. In one embodiment the
transparent substrate of the polymer banknote also forms the
carrier substrate of the security device.
[0062] Alternatively the security device of the current invention
could be incorporated in a polymer banknote such that it is only
visible from one side of the substrate. In this case, the security
device is applied to the transparent polymeric substrate and on one
side of the substrate the opacifying coating is omitted to enable
the security device to be viewed while on the other side of the
substrate the opacifying coating is applied over the security
device such that it conceals the security device.
[0063] Methods for incorporating a security device such that it is
viewable from both sides of a paper document are described in
EP1141480 and WO03054297. In the method described in EP1141480, one
side of the device is wholly exposed at one surface of the document
in which it is partially embedded, and partially exposed in windows
at the other surface of the substrate.
[0064] In the case of a stripe or patch the security device is
formed on a carrier substrate and transferred to the security
substrate in a subsequent working step. The device can be applied
to the security substrate using an adhesive layer. The adhesive
layer is applied either to the device, or the surface of the
security substrate to which the device is to be applied. After
transfer, the carrier substrate may be removed, leaving the
security device as the exposed layer.
[0065] Following the application of the security device, the
security substrate undergoes further standard security printing
processes to create a secure document, including one or all of the
following; wet or dry lithographic printing, intaglio printing,
letterpress printing, flexographic printing, screen printing,
and/or gravure printing.
* * * * *