ELEMENT DE SECURITE POUR DOCUMENTS ET SA METHODE DE FABRICATION

A SECURITY ELEMENT FOR DOCUMENTS AND A METHOD FOR ITS FABRICATION

Abrégé français

Document constitué d'un substrat (1) et d'un élément de sécurité qui comprend un treillis optique. Afin de renforcer la sécurité contre la copie, le treillis optique est un hologramme volumique (2) au sein duquel un effet cinétique a été introduit (figure 2).

Abrégé anglais

A document consists of a substrate (1) and a security element which comprises an optical grating. In order to enhance the copying security, the optical grating is a volume hologram (2) into which a kinetic effect has been introduced (Fig. 2).

Revendications

The embodiments of the invention in which an exclusive property or privilege
is claimed are
defined as follows.
1. A method of producing a security element for a document comprising the
steps of:
(a) recording a volume hologram onto a substrate;
(b) introducing a kinetic effect into the volume hologram;
(c) performing an operation selected from shrinking local areas of the volume
hologram
to change Bragg network planes in the local areas and swelling local areas of
the volume
hologram to change Bragg network planes in the local areas.
2. A method according to claim 1, wherein the volume hologram is recorded onto
the substrate
from a surface relief by an optical copying process.
3. A method according to claim 2, wherein the volume hologram is recorded
using direct light
exposure.
4. A method according to claim 3, wherein the volume hologram is recorded
using direct light
with two or more wavelengths.
5. A document comprising a substrate and a security element produced by the
method of any
one of claims 1 to 4.

Description

CA 02247974 2006-05-25
A Security Element For Documents and A Method For Its Fabrication
Field of the Invention
The invention relates to a document with a substrate and a security element
which
comprises an optical grating and to a security element for such a document.
The
invention further relates to a method for the fabrication of such a document
or security
element.
Background of the Invention
With documents of the most varied type, for example with bank notes,
securities or
similar documents, a security element is often required which is intended to
provide
protection against copying. Security elements of this type which comprise an
optical
grating are well known in the art. In the solutions previously known, optical
gratings are
used which are designed as surface reliefs. However, the copy protection
obtained in this
way is not sufficient in all cases.
It is therefore the object of the invention to improve the copy protection in
documents
with a security element.
Summary of the Invention
The present invention relates to an optical grating comprising a volume
hologram into
which a kinetic effect is introduced. In this way, a substantially greater
copy protection
can be achieved.
In the fabrication of a volume hologram, a film or another grain-free,
translucent,
photosensitive medium is irradiated with coherent light, in particular with
laser light.
Due to interference in the film material, Bragg network planes are created
which embody
the information written in the hologram.
(Dne advantageous embodiment is characterised in that the volume hologram has
been
fabricated with two or more different wavelengths. In this way, a controlled
multi-colour
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CA 02247974 2006-05-25
effect can be achieved. The controlled multi-colour effect can be obtained
with the use of
only two different wavelengths. When at least three different wavelengths are
used
vvhich correspond to the colours in the colour triangle, a controlled fast-
colour effect can
be achieved. This cannot be obtained with the previously known surface relief
structures
as the colour dispersion is fixed by the physical principles of the
diffraction grating.
With a surface relief, therefore, a rainbow colour spectrum is always
generated in the
reconstruction with white light. In accordance with the advantageous
embodiment of the
method of the present invention, controlled colour effects are possible in
contrast to those
of the prior art methods.
The document or security element can be fabricated in that a volume hologram
is
fabricated from a surface relief by means of an optical copying process. The
surface
relied is used as the so-called master. The volume hologram is fabricated from
this by
means of an optical copying process. But it is also possible to fabricate the
volume
hologram by means of direct irradiation.
The improved copy protection of the volume holograms is based on the one hand
by such
volume hologram being able to be reconstructed only in monochrome form. If,
therefore,
an attempt is made to copy a volume hologram, this can only be successful if
exactly the
same wavelength of coherent light or laser light is used as in the recording
of the volume
liologram. Even the, the copying process will only be successful if there is
no topical
shrinking or swelling in the film material after the recording of the volume
hologram.
Based on this, an additional copy protection can be obtained by topical region
of the
volume hologram which are otherwise not recognisable being shrunk and/or
swollen after
the recording of the volume hologram so that the Bragg network planes in these
regions
change correspondingly. A copy of such a volume hologram could only be made
with
liolographic means by an adaptation of the wavelength, but this is not
possible as the
topical regions in which the Bragg network planes have been changed are not
recognisable to the outside. This means that with an only topical or
inhomogeneous
shrinking or swelling of the layers (network planes), the volume hologram can
only be
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CA 02247974 2006-05-25
copied point by point or pixel by pixel; but this requires such a great effort
that it is no
longer interesting for economic reasons.
On the other hand, the increased copy protection is achieved by the
diffraction gratings
being very angle-selective in the volume holograms. This means that the volume
hologram can only be copied if exactly the same irradiation angle is used as
in the
recording. Even then, the volume hologram can only be copied with the greatest
difficulty - if at all - if it has been applied to a surface which is not
exactly smooth; if,
fhr example, it has been applied to a document made from paper or to any other
not
exactly smooth surface. In this way, topical partial regions of the surface
possesses
angles which differ slightly from one another with only small angle
differences being
sufficient to make copying impossible. Due to the angle differences which are
created by
the surface rouglmess, the whole hologram cannot be reconstructed with a plane
wave. In
the event of attempted copying, this has the result that only parts of the
volume hologram
are copied and that at those points where no reconstruction of the volume
hologram was
performed the copied hologram remains unirradiated or that at least a clear
colour
difference or variation in brightness occurs.
A kinetic or kinematographic effect is introduced or written into the volume
hologram.
To generate such kinetic effects, the diffraction structure in the volume
hologram is such
that patterns which occur at a given direction of illumination and observation
move with
regard to a coordinate system in a fixed connection with the substrate along a
pre-
determined path at a predetermined speed when the substrate is moved
preferable in its
plane, for example is turned in a certain direction of rotation and at a
certain speed. By
the introduction of such kinetic effects into volume holograms, the copying of
these
holograms can be made more difficult. Furthermore, in this way a greater
recognition
value can be provided. In addition, by means of different single irradiations,
individual
data can be irradiated into the volume hologram as well whereby the security
value of the
security element in enhanced even further.
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CA 02247974 2006-12-20
In a first aspect, the present invention seeks to provide a method of
producing a security element
for a document comprising the steps of: recording a volume hologram onto a
substrate;
introducing a kinetic effect into the volume hologram; performing an operation
selected from
shrinking local areas of the volume hologram to change Bragg network planes in
the local areas,
or swelling local areas of the volume hologram to change Bragg network planes
in the local
areas, and combinations thereof.
According to a first broad aspect of an embodiment of the present invention,
there is disclosed a
method of producing a security element for a document comprising the steps of
(a) recording a volume hologram onto a substrate;
(b) introducing a kinetic effect into the volume hologram;
(c) performing an operation selected from shrinking local areas of the volume
hologram
to change Bragg network planes in the local areas and swelling local areas of
the volume
hologram to change Bragg network planes in the local areas.
Brief Description of the Drawings
Fig. 1 shows a hologram designed as a surface relief according to the present
invention;
Fig. 2 shows a section through a volume hologram of the present invention.
Detailed Description of the Drawings
The surface relief shown in Fig. 1 can be applied to a document as a security
element. The
information in the hologram is stored in the surface structure.
Fig. 2 shows a document with a substrate 1 onto which a volume hologram 2 has
been applied.
By means of irradiation, there are created in the film material Bragg network
planes nl, n2, etc.,
which are located at a distance of a/2 (where k is. the wavelength of the
light used) and which
contain the holographic information.
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