Note: Descriptions are shown in the official language in which they were submitted.
PCT/AU00/00723
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CORRECTED VERSION
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SECURITY ARTICLE INCLUDING DIFFRACTIVE OPTIC FILTER
The present invention relates generally to devices for authenticating
various items which are subject to counterfeiting, and in particular to
authenticating devices of this type which act to diffract incident light. The
invention is suitable for use with banknotes and other security documents such
as credit cards, passports, security passes, etc., and it will be convenient
to
hereinafter describe the invention in relation to that exemplary application.
It is
to be appreciated, however, that the invention is not limited to use in this
application.
A wide variety of security devices or features for security documents,
such as banknotes, travellers' cheques or the like have been proposed
previously. Examples of such security devices and features include optically
variable devices such as holograms and diffraction gratings, security threads
or
strips, micro-print, fine line or "filigree" patterns, moire inducing patterns
and
optically variable inks.
However, recently considerable advances have been made in
photocopying and like machines for the colour reproduction of documents. The
advances in such machines, and in the technology associated with like
equipment enabling the reproduction of banknotes and other security
documents, increasingly diminishes the effectiveness of known security
devices.
Accordingly, there is a need to provide security devices or features for
incorporation in security articles or documents of increasing sophistication
in
order to make unauthorised reproduction of such objects increasingly
difficult.
A security article comprising a transparent substrate bearing a diffractive
optic filter comprising:
at least a first zone and a second zone, and
a first and second diffractive structure respectively formed in the first and
second zones,
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the first diffractive structure being operative to discriminatingly project a
first selected colour at a viewing angle by diffracting at least a first
spectral
component of polychromatic light transmitted through the first zone by an 8
from the viewing angle,
the second diffractive structure being operative to discriminatingly
project a second selected colour at the viewing angle by diffracting at least
a
second spectral component of polychromatic light transmitted through the
second zone by the angle A from the viewing angle,
the first and second colours together form at least part of a projected
security image when the security article is viewed from the viewing angle.
In one embodiment, the substrate is formed of transparent plastics
material. The transparent plastic substrate may comprise at least one film of
transparent polymeric material.
An opacifying layer may be applied to at least a first surface of a
substrate, said opacifying layer only partly covering said first surface of
the
substrate to leave at least said diffractive optic filter essentially
uncovered. The
substrate may include an uncovered region at least partially surrounding said
diffractive optic filter.
The security article may be a security document, such as a banknote.
The following description refers in more detail to the various features of
the present invention. To facilitate an understanding of the invention,
reference
is made in the description to the accompanying drawings where the security
article is illustrated in a preferred embodiment. It is to be understood that
the
security article of the present invention is not, however, limited to that
preferred
embodiment.
In the drawings:
Figure 1 is a plan view of a banknote in accordance with one
embodiment of the invention;
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Figure 2 is a schematic diagram illustrating the operation of the
diffractive optic filter of the banknote of Figure 1 in the presence of
incident
polychromatic light; and
Figure 3 is a schematic diagram illustrating the projection of a security
image by the diffractive optic filter of the banknote of Figure 1.
Referring now to Figure 1, there is shown generally a banknote 1, being
one example of a security article according to the present invention. The
banknote 1 has a substantially planar structure and comprises a flexible sheet-
like substrate 2 of transparent plastics material. The substrate 2 is covered
over
most of its upper and
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lower surfaces by opacifying layers 3 and 4. As shown in Figure 1, the
opacifying
layers are not applied over the entire surfaces of the sheet-like substrate 2,
and thus
leave a transparent portion 5 of the substrate which is at least partially
uncovered by
the opacifying layers 3 and 4. This transparent, essentially uncovered portion
5
constitutes a "window" in the banknote through which light may be transmitted.
The substrate 2 of transparent plastics material is preferably formed from a
transparent polymeric material which may be made from at least one biaxially
oriented film. The substrate may comprise a single layer film of polymeric
material. Alternatively, the substrate may comprise a laminate of two or more
layers of transparent biaxially oriented polymeric film.
The opacifying layers 3 and 4 may comprise in one or more of a variety of
opacifying inks which can be used in the printing of banknotes or security
documents. For example, the layers of opacifying ink may comprise pigmented
coatings comprising a pigment, such as titanium dioxide dispersed within a
binder
or carrier of heat activated cross-linkable polymeric material. Alternatively,
a
substrate of transparent plastics material 2 may be sandwiched between
opacifying
layers of paper to which indicia is printed or otherwise applied.
Whilst the banknote in Figure 1 is shown as having opacifying layers applied
to both upper and lower surfaces, in other embodiments of the invention, only
one
of the surfaces of the banknote may have an opacifying layer applied thereto.
According to the present invention, the transparent substrate 2 bears a
diffractive optic filter 6. The diffractive optic filter may be incorporated
into the
surface of the substrate 2, or may be formed within a transfer foil structure
that is
later applied to the substrate 2. The diffractive optic filter comprises a
first zone 7,
and, in this example, at least a second zone 8. A diffractive structure is
formed in
each of the first and second zones 7 and 8. More specifically, each region
incorporates a transmissive diffractive structure, such as a diffraction
grating,
formed as a microscopic 3-D relief pattern. The grating structures of both the
first
diffractive structure formed in the first zone 7 and the second diffractive
structure
formed in the second zone 8 have profiles and line spacings, or a periodicity,
chosen
so as to diffract at least a first spectral component of polychromatic light
transmitted
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through the transmissive diffractive structure by a given angle B. The
characteristics of each of the diffractive structures in the two zones 7 and 8
are
chosen so that different spectral components of the polychromatic light
transmitted
by each through the window in the banknote 1 are diffracted, so that the
remaining
undiffracted spectral component projected by each of the diffractive
structures
produces visually discernible colours which together form a projected security
image.
The characteristics of each of the diffractive structures in the first and
second
zones 7 and 8, such as the line spacing, profile and depth of the grating
structure,
may be chosen to precisely define that spectral component of the polychromatic
light which is to be diffracted.
Whilst the embodiment of the security article shown in Figure 1 includes a
diffractive optic filter having two zones, each of which includes a different
diffractive structure, in other embodiments of the invention only one such
zone
having a single diffractive structure may be used. Similarly, three or more
zones,
each having a separate diffractive structure, may be incorporated in other
embodiments.
Figures 2 and 3 shown one example in which the authenticity of the banknote
1 may be verified by a user. As seen in Figure 2, a user may place the
banknote 1
directly in front of an appropriate polychromatic light source. For example,
the
banknote 1, containing the diffractive optic filter in the clear window 5
thereof, may
be placed against a video display terminal (VDT) screen 10. The VDT screen
contains an array of red, green and blue (RGB) pixels, which respectively
transmit a
red, green and blue spectral component of the polychromatic light emitted from
the
screen.
In other embodiments, a different polychromatic light source may be used,
such as a broad flat white light source such as a fluorescent light box.
Preferably
the light source used will cover an area which is larger than the area of the
diffractive optic filter, and will produce light that can be separated into at
least two
spectral components.
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The banknote 1 itself may also act as the polychromatic light source. For
example, the banknote 1 may be folded so that a multi-coloured surface is
superposed with the clear window 5. Ambient light may then be reflected from
the
multi-coloured surface of the banknote 1 and transmitted through the
diffractive
5 optic filter 6 in the clear window 5.
In the configuration shown in Figure 2, the light emanating from the light
source 10 passes outwards towards a user 11 situated at a viewing angle from
the
banknote l, in this case perpendicular to the plane of the banknote.
In this example, the first diffractive structure formed in the first zone 7 of
the
diffractive optic filter 5 acts to diffract the red and green spectral
components of the
polychromatic light transmitted through the substrate 2 of the banknote 1, and
then
through the first zone 7. The characteristics of the first diffractive
structure are
chosen such that the red and green spectral components are diverted by at
least an
angle 8 similarly, the second diffractive structure formed in the second zone
8 of the
diffractive optic filter 6 acts to diffract the green and blue spectral
components of
the polychromatic light emitted from the VDT screen 10 by at least the angle
8.
Accordingly, only the blue spectral component of the light emitted from the
VDT screen is transmitted to the user 11 through the first zone 7, whilst only
the red
spectral component of the polychromatic from the VDT screen 10 is transmitted
by
the second zone 8 to the user 11.
The colours transmitted by each of the zones of the diffractive optic filter 6
may be chosen so that each transmits a colour which is visually discernible
from the
colours in each of the other zones. In this way, the colours from the various
zones
of the diffractive optic filter combine to form at least part of a projected
security
(schematically represented in Figures 1, 2 and 3 in the form of an "S").
As may be best appreciated from Figure 3, the angle 8 by which the different
spectral components of polychromatic light are diffracted by each of the first
and
second diffractive structures, should be chosen so that the diffracted
spectral
components are diverted outside of the users field of view 12.
Referring again to Figure 1, an uncovered zone 13 may be left around at least
part of the diffractive optic filter 6 in the window 5, this uncovered portion
13
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allowing all spectral components of the polychromatic light emitted from VDT
screen 10 to be transmitted to the user 11. This enables the user 11 to more
easily
distinguish between the unfiltered light transmitted through the uncovered
portion
13 and that transmitted through the diffractive optic filter 6.
Many other variations may be made to the above described embodiment
without departing from the spirit or ambit of the invention.
