EP1548657A1

EP1548657A1 - Security document comprising means for authentication and method for authentication of security document

Info

Publication number: EP1548657A1
Application number: EP03029829A
Authority: EP
Inventors: Javier Prof. Dep. Fisica Fonda- Tejada Palacios; Antonio J. Arieta
Original assignee: European Central Bank
Current assignee: European Central Bank
Priority date: 2003-12-24
Filing date: 2003-12-24
Publication date: 2005-06-29
Anticipated expiration: 2023-12-24
Also published as: EP1548657B1; WO2005064551A1; DE60315049T2; DE60315049D1; ES2287405T3; ATE367626T1

Abstract

A security document according to the invention comprises substrate means 2 and security means 5 for authentication provided on a predetermined area 4 of said substrate means. The security means comprises means selectively reflecting visible light depending on the temperature and means changing the temperature depending on the application of a magnetic field. The means selectively reflecting light and the means changing the temperature are arranged as to form said security means exhibiting different colors depending on a magnetic field applied to the substrate means. In a preferred embodiment of the invention, the means selectively reflecting visible light comprises liquid crystals and the means changing the temperature comprises gadolinium.

Description

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.
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.
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.
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.
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.
A further object of the present invention is to provide a method which allows an authentication of a security document with high security.
The above objects are solved with the features of claims 1 and 8, respectively. Preferred embodiments are the subject matter of independent claims.
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.
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.
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.
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.
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.
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.
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.
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.
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.
According to a specific embodiment, the security document is a banknote.
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.
The functional principle of the means changing the temperature dependent on a magnetic field is based on the magnetocaloric effect.
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 ΔSM (T,H) = ∫(dM/dT) HdH.
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.
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.
Fig. 1 shows the magnetic moment of gadolinium as a function of the temperature when a magnetic field µ₀ 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).
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 T _i and T _i ₊ ₁ temperatures respectively, under an applied magnetic field of intensity µ₀ H = 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.
The combination of the temperature induced colour change of liquid crystals and the magnetocaloric effect of gadolinium is illustrated with the following experiment.
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.
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.
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 2cm² 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 2.5cm³ in size was used to magnetize the sample.
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.
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.
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.
The graphical design printing 3 on the front side comprises a rectangular area 4 on which a visible security pattern 5 is provided.
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.
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.
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.

Claims

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.
Security document of claim 1, characterized in that said means selectively reflecting visible light comprises liquid crystals.
Security document of claim 1 or 2, characterized in that said means changing the temperature comprises gadolinium.
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.
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.
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.
Security document of one of claims 1 to 6, characterized in that said security document is a bank note.
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.
Method of claim 8, characterized in that said means selectively reflecting visible light comprises liquid crystals.
Method of claim 8 or 9, characterized in that said means changing the temperature comprises gadolinium.
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.
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.
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.
Method of one of claims 8 to 13, characterized in that said security document is a bank note.