[0001] The present invention relates to a security document comprising security means for
the authentication of the security document. Furthermore, the invention refers to
a method for authentication of a security document.
[0002] The identification and authentication of security documents, e.g. banknotes, passports,
identity cards, checks, stocks, bonds etc., is a long standing problem. In order to
achieve this goal, security means have been developed to allow users and/or machines
to discriminate between real and forged banknotes and/or discriminate between different
values of banknotes. Among the techniques adopted are the use of special papers, special
inks, special patterns, the inclusion of security threads and watermarks as well.
[0003] With respect to the decreasing price and thus increasing availability of advanced
reproduction equipment, it may be feared that printed reproductions of security documents
are produced. Depending on the degree of expertise of criminals, some reproductions
could cheat not only the general public using the security documents but also validating
machines used for security document assessment.
[0004] A method for a non-reproducable printed image on a security document is described
in WO 01/69915 A2. The ink to be used is reflective of light of predetermined wave
lengths.
[0005] It is an object of the present invention to provide a security document, particularly
a banknote, which allows the authentication of the document and is not possible to
be reproduced by making a photocopy or by scanning and printing or other unauthorized
reproduction.
[0006] A further object of the present invention is to provide a method which allows an
authentication of a security document with high security.
[0007] The above objects are solved with the features of claims 1 and 8, respectively. Preferred
embodiments are the subject matter of independent claims.
[0008] The security document and the method according to the invention are based on a colour
change of a material depending on the temperature and the change of the temperature
of another material depending on a magnetic field. Both physical or chemical effects
as such are well-known in other technical fields for the person skilled in the art.
[0009] The combination of the above physical or chemical effects allows to change the colour
of a security means depending on the strength of a magnetic field applied to a substrate
in which or on which the security means is provided.
[0010] The main advantage of the security document and the method according to the invention
is that the security means is not reproducable by making a photocopy of the document
or by scanning and printing, even if the copy or print is of highest quality. Furthermore,
the security document and the method allow authentication of the document simply by
applying a magnetic field and holding the document against light. The change of colour
should be seen by the naked eye.
[0011] For the general public, the recognition of the genuineness of a document will be
very easy. The colour change effect will not occur on any copy, reproduction, and,
in general, unauthorised printing of the genuine document is not possible since only
a fixed aspect can be reproduced.
[0012] In a preferred embodiment of the invention, the means selectively reflecting visible
light dependent on the temperature comprises liquid crystals (LC). The ability to
exhibit colours is a well-known attribute of liquid crystals. It is an aspect of the
invention that the colour change of the liquid crystals occurs in a temperature range
of -50 to +60 °C depending on the specific LC used, preferably -20 to +40°C, more
preferably 15 to 25°C. If the liquid crystal changes its colour at room temperature,
an authentication is possible without heating or cooling the security document.
[0013] In a further preferred embodiment, the means changing the temperature dependent on
a magnetic field comprises gadolinium (Gd). Gadolinium is paramagnetic at room temperature
and rapidly becomes ferromagnetic with a huge magnetic moment at lower temperatures.
This transition occurs in a narrow interval around the so-called Curie temperature,
and involves therefore a very large variation of the magnetisation close to the transition
point. If a magnetic field is applied adiabatically to a Gd sample, its temperature
increases a few degrees. In the same way, if the applied magnetic field is switched
off, then the temperature of the sample decreases.
[0014] It is an important advantage of gadolinium that the magnetocaloric effect occurs
at the preferred temperature range, i.e. room temperature, and the temperature of
gadolinium changes under the variation of relatively small magnetic fields.
[0015] The means selectively reflecting light and the means changing the temperature may
be formed as a mixture of gadolinium particles and liquid crystals in good thermic
contact. In a preferred embodiment, the means selectively reflecting light and the
means changing the temperature are embedded in an ink suitable for intaglio, offset,
silk-screen, ink-jet, flexography, rotogravure printing or any other printing technique.
For example, liquid crystals and gadolinium may be embedded as small particles (micro
or nanoparticles) in the ink. The embedding of this active elements in an ink allows
the production of the security documents with relatively low costs incurred.
[0016] A further aspect of the invention is to provide an ink for a multi-colour printing,
e.g. four-colour printing by using four different inks, each ink having a different
colour, e.g. yellow, red, blue and black at the same preferred temperature. Therefore,
a picture printed by using the different inks may appear at the preferred temperature
and disappears at other temperatures.
[0017] According to a specific embodiment, the security document is a banknote.
[0018] For the purpose of illustrating the present invention, there is shown in the accompanying
figures an embodiment which is presently preferred; it being understood that the invention
is not limited to the precise arrangement and instrumentalities shown.
Fig. 1 shows the thermal variation of the saturation magnetization of a gadolinium
sample for illustrating purposes;
Fig. 2 shows the thermal dependence of the derivative dM/dT of gadolinium for illustrating
purposes;
Fig. 3 shows the isothermal demagnetization curves of the gadolinium sample for illustrating
purposes,
Fig. 4 shows the magnetic entropy change for illustrating purposes,
Fig. 5 shows the transmission coefficient of a liquid crystal versus the wavelength
of the light at several temperatures for illustrating purposes, and
Fig. 6 shows a schematic view of a banknote comprising substrate means and security
means according to the invention.
[0019] The functional principle of the means changing the temperature dependent on a magnetic
field is based on the magnetocaloric effect.
[0020] The magnetocaloric effect is the temperature change produced when a magnetic field
variation is applied to a magnetic material. It can be measured as the adiabatic temperature
change or as the isothermal magnetic entropy change ΔS
M. The magnetic entropy change of the system is related to the change of the magnetization
M as a function of temperature,
T, and magnetic field,
H, and can be calculated from magnetization data using the well known Maxwell relationship

[0021] The integral of the above equation should be performed in the interval between H
max and H
min. A high magnetocaloric effect will be observed when the magnetization varies sharply
in a constant field. Thus, a high entropy change can be observed in the ferro to paramagnetic
phase transition when magnetization decays sharply at the Curie temperature.
[0022] Gadolinium is paramagnetic at room temperature and rapidly becomes ferromagnetic
with a huge magnetic moment at lower temperatures. This transition occurs in a narrow
interval around the so-called Curie temperature and involves a very large variation
of the magnetic moment.
[0023] Fig. 1 shows the magnetic moment of gadolinium as a function of the temperature when
a magnetic field µ
0 H = 0.3 T was applied. A 0,25 mm thick 99,9% pure foil of gadolinium with a mass
of 16 mg was used. The maximum negative variation of the magnetic moment occurs at
293 K corresponding to the Curie temperature above gadolinium behaves as a paramagnet.
Fig. 2 shows the thermal dependance of the derivative dM/dT of gadolinium to illustrate
the rapid variation of the magnetization around the Curie temperature (293 K).
[0024] Fig 3 shows the isothermal demagnetization curves in a temperature range from 285
K to 305 K with a temperature step of 1 K. The magnetic entropy change was calculated
from these values of magnetization at
Ti and
Ti+1 temperatures respectively, under an applied magnetic field of intensity µ
0H = 1T. Fig 4 shows the experimental values of the magnetic entropy change obtained
from the experimental isothermal magnetization curves under a magnetic field variation
of 1 T from saturation to zero magnetic field. It is observed that the value of maximum
entropy change peaks around the Curie temperature.
[0025] The combination of the temperature induced colour change of liquid crystals and the
magnetocaloric effect of gadolinium is illustrated with the following experiment.
[0026] The gadolinium sample was surrounded by a liquid crystal with a well known colour
change in the temperature interval comprised between 292.9 and 294.6 K. Fig. 5 shows
the transmission coefficient of the liquid crystal versus the wavelength of the light
at several temperatures ranging from 19.8 to 21.6 °C.
[0027] The temperature change produced in the Gd sample at the maximum entropy change is
about 1.5 K resulting also in the cooling of the liquid crystal surrounding the Gd
sample. As a consequence of this process a colour change in the liquid crystal was
detected. This process is reversible, shows neither hysteresis phenomena nor memory
effects and is very fast.
[0028] In the experiments on colour change induced by magnetocaloric effect the gadolinium
and the liquid crystal are put together. The sample was made of a 1.5 x 2cm
2 foil of gadolinium with a thickness of 0.25mm. One side of this foil was covered
with a thin film of liquid crystal. The temperature of the sample was controlled by
putting it in thermal contact with a Peltier plate. The Peltier effect consists on
the production or absorption of heat as a consequence of the application of an electrical
current through a junction of two different semiconductors. The direction of the current
determines which side of the plate is heated or cooled. The Peltier effect is the
opposite of the thermoelectric (or Seebeck) effect. The current applied to the Peltier
plate (about 0.3 amperes in the experiments) was chosen in order to obtain a blue-green
colour of the liquid crystal. The Peltier plate used in the experiments presents a
very small temperature gradient (about 0.2°C) along its surface. This did not allow
to observe a homogenius colour in the liquid crystal. A commercial 1 tesla NdFeB magnet
of 5 x 5 x 2.5cm
3 in size was used to magnetize the sample.
[0029] In the final experimental setup, a thin crystal plate (0.25mm of thickness) was placed
between the Peltier plate and the Gd sample in order to reduce the thermal contact.
This allowed to maintain the induced temperature difference during a few seconds.
The same effect can be obtained using, for example, a piece of paper.
[0030] The sample temperature was stabilised with that of the Peltier plate. The magnet
was placed near the sample. The temperature of the sample began to increase until
the sample achieved its maximum temperature so that the sample became blue-violett.
The temperature began to decrease slowly until it was stabilized with the Peltier
plate. The magnet was removed and the temperature of the sample began to decrease
so that the sample became red. The duration of this colour change was enough to be
observed by the naked eye.
[0031] Fig. 6 shows the front side of a security document 1 according to the invention,
which is a banknote in the preferred embodiment. The banknote 1 comprises a suitable
printing substrate 2, for example made of a sheet of paper. A graphical design printing
3 is provided on the front and reverse side of the substrate.
[0032] The graphical design printing 3 on the front side comprises a rectangular area 4
on which a visible security pattern 5 is provided.
[0033] In accordance with a first embodiment, the security pattern consists of a foil of
Gd with a thickness of 0.25 mm. One side of this foil is covered with a thin film
of liquid crystal with a colour variation centred on the Curie temperature. This sandwich
structure is applied on or embedded in the substrate of the banknote.
[0034] According to an alternative preferred embodiment, the security pattern is printed
on the substrate using an ink in which liquid crystals and gadolinium are embedded
as active elements (micro or nanoparticles). The security pattern is printed using
any one of the well-known printing techniques.
[0035] When the banknote is observed by naked eye, the security pattern 5 appears as a coloured
rectangular printing. If a magnet is placed near the substrate on which the security
pattern is printed, the temperature of the liquid crystals increases as a result of
the increasing temperature of gadolinium so that the colour of the security pattern
changes, e.g. the colour becomes blue-violet. When the magnet is removed, the temperature
of the liquid crystals decreases as a result of the removed, the temperature of the
liquid crystals decreases as a result of the decreasing temperature of the gadolinium
so that the colour of the security pattern changes again, e.g. the colour becomes
red.
1. Security document comprising substrate means (2) and security means (5) for authentication
provided on a predetermined area (4) of said substrate means, characterized in that means selectively reflecting visible light depending on the temperature and means
changing the temperature depending on the application of a magnetic field are provided,
said means selectively reflecting visible light and said means changing the temperature
are arranged as to form said security means (5) exhibiting different colours depending
on a magnetic field applied to the substrate means.
2. Security document of claim 1, characterized in that said means selectively reflecting visible light comprises liquid crystals.
3. Security document of claim 1 or 2, characterized in that said means changing the temperature comprises gadolinium.
4. Security document of one of claims 1 to 3, characterized in that said means selectively reflecting visible light and said means changing the temperature
are embedded in an ink deposited on said predetermined area of the surface of said
substrate means.
5. Security document of one of claims 1 to 4, characterized in that the temperature range in which the means selectively reflect visible light is in
the range of -50 to +60 °C, preferably -20 to +40°C, more preferably 15 to 25°C.
6. Security document of one of claims 1 to 5, characterized in that said substrate means (2) comprises one or more materials selected of the group consisting
of papers, plastics, polymers, metallic foils and metallic alloy foils.
7. Security document of one of claims 1 to 6, characterized in that said security document is a bank note.
8. Method for authentication of a security document comprising the following steps:
Providing means selectively reflecting visible light depending on the temperature
and means changing the temperature depending on the application of a magnetic field,
Arranging said means selectively reflecting visible light and said means changing
the temperature as to form said security means exhibiting different colors depending
on a magnetic field applied to the substrate means,
Providing said security means on or in said substrate means, and
Applying a magnetic field to said substrate means.
9. Method of claim 8, characterized in that said means selectively reflecting visible light comprises liquid crystals.
10. Method of claim 8 or 9, characterized in that said means changing the temperature comprises gadolinium.
11. Method of one of claims 8 to 10, characterized in that said means selectively reflecting visible light and said means changing the temperature
are embedded in an ink and said ink is deposited on said predetermined area of the
surface of said substrate means.
12. Method of one of claims 8 to 11, characterized in that the temperature range in which the means selectively reflect visible light is in
the range of -50 to +60 °C, preferably -20 to +40°C, more preferably 15 to 25°C.
13. Method of one of claims 8 to 12, characterized in that said substrate means comprises one or more materials selected of the group consisting
of papers, plastics, polymers, metallic foils and metallic alloy foils.
14. Method of one of claims 8 to 13, characterized in that said security document is a bank note.