EP1182048B1 - Method for authentification of sensitive documents - Google Patents

Abstract

The note has luminescent substances, where each substance emits a light along a determined response frequency spectrum when the substance is exposed to excitation light ray i.e. laser ray. The frequency spectrum is composed of lines. The substances have same dopant ion e.g. neodymium, in different crystalline host matrices e.g. yttrium lithium fluoride and yttrium aluminum garnet type matrices, so that the response lines on the different matrices are moved from a width greater than thinness of the lines when the note is exposed to the excitation light ray. An independent claim is also included for a method for authenticating a bank note.

Description

The present invention relates to a method for authenticating sensitive documents such as fiduciary documents, as well as documents and means for implementing such a method.

Many ways are known for securing and authenticating sensitive documents.

Such means may allow authentication of a document by any user; this is the case of authentication means known as "general public", which do not require any machine or complex device to enable the authentication of the document.

It is thus known to incorporate in banknotes security threads or watermarks, or even luminescent substances producing a particular light emission when the bill is exposed to a given excitation radiation (from a lamp to ultra-violet for example).

It is also known to provide security signs that are not legible to the general public; the detection of these signs requires the implementation of a particular machine, such as a magnetic detector for example.

Signs of the latter type provide a higher level of security because they are not directly accessible to the general public.

And many safety signs - general or not - use optical properties. Indeed the optical properties are easily perceptible to the human eye, or to suitable sensors.

Generally, these optical signs use the luminescence properties of suitable substances which, subjected to a given excitation radiation, in return produce an emission according to a determined spectrum. And the authentication of the document in which these substances are integrated is done by comparing the spectrum of the transmitted response with an expected spectrum, this comparison may involve electro-optical machines or not.

Attempts are also made to produce a document comprising a coding made from luminescent substances. The document FR 1 471 367 disclose an example of such an attempt.

The fact of thus providing luminescent substances intended not only to be detected, but also to form a specific code associated with the document, should make it possible to significantly increase the security level of the document.

But such attempts do not allow efficient coding of the document, in particular because of the large width of the emission peaks of the substances used; it would indeed be necessary to obtain a response with fine and distinct lines to achieve a true coding.

The document WO 98/36376 A1 discloses a method of authenticating a document that incorporates a security sign including layers including dye molecules and liquid crystal molecules that function as diffusion sites to produce a laser emission when the layers are exposed to radiation laser excitation.

The document WO 00/46742 A1 discloses a method of authenticating a document which incorporates a security sign comprising a guide structure such as fibers and an amplifying medium, such as a dye or a rare earth ion, capable of producing a laser emission. When the structure is subjected to laser excitation radiation, it emits a response in the form of fine lines at predetermined wavelengths.

The document US 5,903,340 discloses a method of authenticating a document which incorporates a security sign comprising an amplifying medium, such as fibers, coated or impregnated with a dye or other material capable of emitting light in combination with scattering particles. When the structure is subjected to laser excitation radiation, it emits a characteristic response in the form of fine lines at predetermined wavelengths. The document is declared authentic only if a laser emission is detected.

An object of the invention is to enable such a luminescent coding of sensitive documents to be carried out.

Another object of the invention is to avoid in addition to resorting to this for expensive substances to manufacture and / or integrate the document, so as not to excessively overburden the cost of the document.

• L'intégration au document d'au moins une substance luminescente apte à émettre de la lumière selon un spectre fréquentiel de réponse déterminé lorsque la substance est exposée à un rayonnement lumineux d'excitation,
• L'exposition du document à un rayonnement lumineux d'excitation, et
• Le recueil de la réponse du document audit rayonnement d'excitation,
• le rayonnement d'excitation est un rayonnement laser,
• chaque spectre fréquentiel de réponse est essentiellement composé d'au moins une raie, et
• le procédé comporte également le codage de la réponse lumineuse du document au rayonnement d'excitation, et la comparaison de cette réponse à une réponse attendue mémorisée.

In order to achieve these objects, the invention proposes a method of authenticating sensitive documents such as bank notes according to one of claims 1 or 5. The method comprises:

• Integrating into the document at least one luminescent substance able to emit light according to a frequency spectrum of determined response when the substance is exposed to excitation light radiation,
• The exposure of the document to a bright light of excitation, and
• The collection of the response of the document to said excitation radiation,
• the excitation radiation is a laser radiation,
• each frequency spectrum of response is essentially composed of at least one line, and
• the method also includes coding the light response of the document to the excitation radiation, and comparing this response to a stored expected response.

• chaque raie du ou des spectre(s) de réponse a une largeur à mi-hauteur inférieure à une valeur de l'ordre de 10nm,
• le codage est réalisé selon au moins une des manières suivantes :
  • o codage spatial,
  • o codage sur les fréquences de réponse,
  • o codage en intensité de réponse.
• le rayonnement d'excitation laser est obtenu par un laser de type YAG présentant un pic d'émission vers 532 nm,
• au moins une substance luminescente comprend un milieu ne présentant pas d'effet laser,
• ledit milieu comprend un mélange de complexes d'Europium avec des polymères du type CN-PPP (poly(2-(6'-cyano-6'-méthyl-heptyloxy)-1,4-phénylène)),
• les ions d'Europium sont incorporés au polymère en synthétisant une famille de complexes d'Europium solubles avec des ligands □-dicétonates, et en dissolvant ces complexes dans un solvant en conjugaison avec le polymère,
• le niveau d'énergie de triplets du ligand est supérieur au niveau d'émission de l'Europium,
• le spectre d'émission du polymère et le spectre d'absorption du complexe se recouvrent au moins partiellement,
• au moins une substance luminescente comprend une matrice-hôte dopée avec au moins un ion de terres rares,
• le codage est obtenu par au moins une des manières suivantes :
  • o en utilisant des dopants différents sur une même matrice,
  • o et/ou en mettant en oeuvre un même ion dopant sur des matrices de caractéristiques différentes.

Other aspects of the process according to the invention are the following:

• each line of the spectrum (s) of response has a width at half height less than a value of the order of 10 nm,
• the coding is performed according to at least one of the following ways:
  • o spatial coding,
  • o coding on the response frequencies,
  • o coding in response intensity.
• the laser excitation radiation is obtained by a YAG laser having an emission peak around 532 nm,
• at least one luminescent substance comprises a medium which does not exhibit a laser effect,
• said medium comprises a mixture of Europium complexes with polymers of CN-PPP type (poly (2- (6'-cyano-6'-methyl-heptyloxy) -1,4-phenylene)),
• the Europium ions are incorporated into the polymer by synthesizing a family of soluble Europium complexes with β-diketonate ligands, and dissolving these complexes in a solvent in conjugation with the polymer,
• the triplet energy level of the ligand is higher than the emission level of Europium,
• the emission spectrum of the polymer and the absorption spectrum of the complex overlap at least partially,
• at least one luminescent substance comprises a host matrix doped with at least one rare earth ion,
• the coding is obtained by at least one of the following ways:
  • o using different dopants on the same matrix,
  • and / or by implementing the same doping ion on matrices with different characteristics.

Other aspects, objects and advantages of the invention will become more apparent upon reading the following description of preferred embodiments of the invention.

In a first embodiment which does not form part of the invention, a bank note is produced by integrating in said banknote a security marking made from silica fibers doped with a laser dye.

Such doped fibers constitute a material whose pores have a size of the order of a few tens to a few hundred angstroms, thus constituting a mesoscopic structure, and are ordered, so that the structure formed by the pores is homogeneous.

• 100 H2O,
• 0.0246 CTAB (CTAB étant un agent de surface cationique, par exemple de formule CH3(CH2)15N(CH3)3Br),
• 2.92 HCI,
• 0.00017 Rh6G (Rh6G étant un colorant cationique tel que la rhodamine).

These fibers can be synthesized by spontaneous growth of fibers in an aqueous solution whose molar composition is as follows:

• 100 H2O,
• 0.0246 CTAB (CTAB being a cationic surfactant, for example of formula CH3 (CH2) 15N (CH3) 3Br),
• 2.92 HCI,
• 0.00017 Rh6G (Rh6G being a cationic dye such as rhodamine).

This first aqueous solution is then stirred, then 0.05 mol of TBOS of formula (C4H9O) 4Si is added without stirring the solution, so as to cover the surface with a thin layer of TBOS.

As explained in the article of Marlow et al. "Doped mesoporous silica fibers: a new laser material" (Advanced Materials, 1999, 11, No. 8 ), after two days, a spontaneous growth of rhodamine-doped silica fibers is observed, which constitutes a laser dye.

After taking these fibers for about five days, they are then dried.

In one embodiment, the percentage by weight of dye in the fibers is thus of the order of 15%.

And in this way fibers are obtained whose physical characteristics (dimensions, the diameter of fibers obtained by this process being of the order of 22 angstroms) and optical characteristics (absorption coefficient and birefringence coefficient) are homogeneous; these fibers constitute efficient waveguides.

These fibers can then be integrated into a banknote (or any sensitive document) by any known means, for example by embedding the fibers in the paper weft, by constituting a security thread in which doped fibers are integrated, and it even embedded in the thickness of the paper, by diluting these fibers in an ink or a printing varnish of the bill, etc.

The fibers incorporated in the ticket are a security sign that is invisible to an observer, but recognizable by machine.

More precisely, the note is exposed to which the fibers have been integrated into the radiation of a laser source.

The energy of such incident radiation is concentrated in a very fine frequency line. In a preferred embodiment of the invention, the laser is Nd: YAG type with a peak emission at 532 nm, a frequency of 10Hz.

By exciting the fibers doped with such a laser radiation, a laser-like light emission is observed by the fibers themselves, the rhodamine emitting at a much higher peak than in the case of the exposure of a simple solution of rhodamine at the same radiation.

This laser effect results from the internal reflections of the photons inside the fibers of which they are trapped throughout their path in the fiber.

And the exploitation of this laser effect, combined with an exposure to laser excitation radiation, makes it possible to obtain responses from the fibers in the form of extremely fine lines, the characteristic width of these lines (half width). height) being less than 10 nm.

• codage spatial : les fibres sont intégrées en certains endroits choisis du billet, de manière à réaliser un code géométrique (code à barres par exemple, ou figure graphique prédéterminée),
• codage sur les fréquences de réponse : on intègre dans ce cas plusieurs types de fibres dans le billet, chaque fibre ayant un spectre de réponse différent pour une excitation laser donnée. On pourra pour cela constituer des fibres à partir de colorants laser différents, comprenant ou non la rhodamine. De la sorte, on obtient en illuminant le billet avec des flash laser une réponse lumineuse selon certaines raies discrètes d'émission qui correspondent aux longueurs d'onde des raies d'émission des différents colorants,
• codage en intensité par variation de la quantité de rhodamine (ou de tout colorant) que comprennent différentes fibres intégrées dans le même billet. Dans ce cas, les différences de concentration de colorant dans les différentes fibres entraînent des différences dans les intensités de réponse de ces différentes fibres, qui sont par ailleurs centrées sur la même longueur d'onde,

And we have the fibers in the ticket so as to perform a coding.
This coding can be carried out according to one or more of the following dimensions:

• spatial coding: the fibers are integrated in certain selected places of the ticket, so as to produce a geometrical code (bar code for example, or predetermined graphic figure),
• coding on the response frequencies: in this case, we integrate several types of fibers in the ticket, each fiber having a different response spectrum for a given laser excitation. It may be for that to constitute fibers from different laser dyes, including or not rhodamine. In this way, a luminous response is obtained by illuminating the ticket with laser flashes according to certain discrete emission lines which correspond to the wavelengths of the emission lines of the various dyes.
• intensity coding by varying the amount of rhodamine (or any dye) that comprises different fibers integrated in the same note. In this case, differences in dye concentration in the different fibers cause differences in the response intensities of these different fibers, which are also centered on the same wavelength,

Such a code is recognized by reading the light signal emitted by the bill, thanks to a set of sensors adapted to collect and quantify the light emitted by the bill in response to the excitation of the laser, according to the expected frequency lines.

Such a sensor assembly may comprise a series of separating mirrors each of which directs a portion of the light emitted by the bill to a sensor associated with a specific wavelength (for example a photoelectric diode in series behind a colored filter), or still a matrix sensor of the CCD type.

• pour chaque intervalle de longueur d'onde d'un spectre de réponse qui englobe les raies d'émission des différentes substances intégrées au billet - et qui comprend donc typiquement le visible et/ou l'infra-rouge ; chaque intervalle peut ainsi avoir une largeur de l'ordre de 5 à 10 nm,
• et pour chaque endroit du billet - l'ensemble de capteur ayant des moyens de repérage de sa position par rapport à la surface du billet, par exemple en utilisant une origine liée aux bords du billet ou à un signe-origine intégré au billet - la surface du billet peut ainsi être divisée en parcelles dont les dimensions sont adaptées à la résolution de l'ensemble de capteur et aux contraintes de fabrication.

In all cases, the sensor assembly collects the light emitted by the bill and produces electrical signals that reflect the presence and intensity of light emission:

• for each wavelength range of a response spectrum which includes the emission lines of the different substances integrated in the ticket - and which therefore typically comprises the visible and / or infra-red; each interval can thus have a width of the order of 5 to 10 nm,
• and for each location of the ticket - the sensor assembly having means for locating its position with respect to the ticket surface, for example using an origin related to the edges of the ticket or a sign-origin integrated in the ticket - the The ticket surface can thus be divided into parcels whose dimensions are adapted to the resolution of the sensor assembly and the manufacturing constraints.

To recognize the code, a memory containing the expected response of the note is connected to the sensor assembly, and a comparator compares the signal picked up and the expected signal.

The high fineness of the spectra of the response spectra makes it possible to perform efficient coding, which was not really the case with light-emitting substances with a broad spectrum of response.

It is also possible to substitute rhodamine for other materials whose response to a laser excitation is focused around a discrete line, or a finite number of emission lines; in particular rhodamine may be replaced by nanocrystals doped with one or more rare earth ions, or by conjugated polymers.

It is still possible to replace in the ticket the doped fibers by doping the paper or a paper printing ink with pigments consisting of the assembly of semiconductor organic elements with a solid state laser effect. .

In this second embodiment which is not part of the invention and which also provides a laser effect for the response to incident radiation, it is these organic elements that provide the laser effect. A more complete description of such organic elements can be found in the article Tessler et al. "Lasers based on semiconducting organic materials" (Advanced Materials, 1999, 11, No. 5 ).

These semiconductor organic elements can be made from a polymer of the MEH-PPV type (poly (2-methoxy-5- (2'-ethylhexyloxy) -1,4-phenylenevinylene)) or PPV (poly (p) phenylenevinylene)), and may be in the solid state, preferably dispersed in a glass-type host-support for example. It is also possible to further stimulate the laser effect by adding in the host support scattering particles, for example TiO2.

To define the polarization properties of such materials, the polymer chains can be further aligned.

And these assemblies of organic elements can take the form of tubular structures that constitute waveguides, or stacks that can form microcavities.

Finally, the invention is implemented without exploiting a real "laser effect", ie a jump of the energy emitted in response to an excitation to concentrate this energy in discrete lines, the important thing is to be able to get the ticket a response in the form of discrete lines of a width typically less than about 10nm.

• toujours par recueil et analyse de la réponse codée des billets à un rayonnement d'une source laser (ce point étant important pour ne stimuler les substances intégrées au billet que selon une ou des raie(s) spectrale(s) bien déterminée(s)),
• mais sans exploiter un effet laser fourni par ces substances.

Thus, in a third embodiment according to the invention, ticket authentication is carried out:

• always by collecting and analyzing the coded response of the banknotes to radiation from a laser source (this point being important in order to stimulate the substances integrated in the banknote only according to one or more determined spectral lines) )
• but without exploiting a laser effect provided by these substances.

In this case, use will be made of synthesis molecules embedded in the paper of the note, or in an ink or varnish, or in an intermediate medium such as a security thread.

Examples of synthesis of such molecules can be found in the article Mc Gehee et al. "Narrow bandwidth luminescence from blends with energy transfer frop semiconducting conjugated polymers to Europium complexes" (Advanced Materials, 1999, 11, N ° 16 ).

Thus, a mixture of Europium complexes can be used with polymers of the CN-PPP type (poly (2- (6'-cyano-6'-methyl-heptyloxy) -1,4-phenylene)).

More specifically, the Europium ions are incorporated into the polymer by synthesizing a family of soluble Europium complexes with β-diketonate ligands, dissolving these complexes in a solvent in conjugation with the polymer, and then rolling a film from this double dissolution.

To obtain fluorescent Europium complexes, the triplet energy level of the ligand must be greater than the emission level of the Europium.

In addition, to allow the energy transfer of the conjugated polymer to the ligands of the Europium complex, the emission spectrum of the polymer and the absorption spectrum of the complex must overlap at least partially.

It is thus possible to synthesize different molecules from CN-PPP, as explained in the article by Mc Gehee et al. These molecules have the characteristic of emitting in very fine lines (whose width at half height is less than 10 nm), in response to an ultraviolet light excitation.

By way of example, it is possible under the conditions mentioned above to synthesize four Europium complexes: Eu (acac) 3phen, Eu (mppd) 3phen, Eu (dbm) 3phen, and Eu (dnm) 3phen. The structure of these complexes is specified on page 1350 of the article by Mc Gehee et al.

In this variant embodiment of the invention, the narrowness of the excitation lines of the laser source combined with the narrowness of the response lines of the molecules thus synthesized are used, in order to obtain an extremely well-marked response as a function of the complexes of Europium implemented in combination with CN-PPP; this makes it possible to implement a true coding, using in the same ticket different molecules whose response lines to a given excitation (for example a laser excitation close to the ultraviolet) are separated from a frequency band of the order of 5 nm.

Finally, it is also possible according to another variant of the invention to integrate in the banknote (according to one of the modes mentioned above) a substance comprising a host matrix which can be crystalline, doped with rare earth ions such as those commonly used in lasers.

It is known to use the luminescence properties of such substances. But it is not known to implement them for the authentication of products such as banknotes, with exposure to laser-type excitation radiation (which is an important element of the invention).

The fact of exciting such a substance by an extremely concentrated laser radiation on a thin line makes it possible to obtain an equally concentrated response on one or more lines (s).

• en utilisant des dopants différents sur une même matrice. On pourra ainsi intégrer au billet deux encres différentes, l'une dopée avec un premier ion de terres rares dont la réponse à une excitation laser (qui est elle-même concentrée autour d'une longueur d'onde du visible, du proche infra rouge ou de l'ultraviolet), l'autre dopée avec un deuxième ion de terres rares, différent du premier, et dont la longueur d'onde de réponse à la même excitation est différente,
• mais également en mettant en oeuvre un même ion dopant sur des matrices de caractéristiques différentes (type de matrice, stoechiométrie de la matrice...) de sorte que les raies de réponse sur les différentes matrices sont décalées d'une largeur supérieure à la finesse des raies, et de préférence à une largeur de l'ordre de 10 nm.

And it is possible according to the invention to proceed to a coding of the response of the ticket; this coding may in this last variant of the invention be realized in several ways, separately or in combination:

• using different dopants on the same matrix. It will thus be possible to integrate two different inks, one doped with a first rare earth ion, whose response to a laser excitation (which is itself concentrated around a wavelength of the visible, the near infra red or ultraviolet), the other doped with a second rare earth ion, different from the first, and whose wavelength of response to the same excitation is different,
• but also by implementing the same doping ion on matrices with different characteristics (type of matrix, stoichiometry of the matrix, etc.) so that the response lines on the different matrices are shifted by a width greater than the fineness lines, and preferably at a width of the order of 10 nm.

• d'une part sur une matrice de type YLF et obtenir ainsi, comme réponse à excitation laser de longueur d'onde 800 nm, une raie d'émission centrée sur 1054 nm,
• et d'autre part sur une matrice de type YAG et observer une raie centrée cette fois sur 1064 nm. La largeur à mi-hauteur des raies étant dans les deux cas de l'ordre de 5 nm, on comprend qu'il est aisé de distinguer les réponses sur les deux matrices.

For example, it is possible to use neodyne ions:

• on the one hand on a YLF type matrix and thus obtain, as a laser excitation response of wavelength 800 nm, an emission line centered on 1054 nm,
• and on the other hand on a matrix of YAG type and observe a line centered this time on 1064 nm. The half-height width of the lines being in both cases of the order of 5 nm, it is understood that it is easy to distinguish the responses on the two matrices.

One can thus combine one or more different dopants on one or more types of matrix, and integrate these different substances in the ticket mixed or separately, so as to perform a coding of the response of the note to a radiation exposure of a laser.

Finally it is specified that it is obviously possible to integrate in the same ticket several luminescent substances corresponding to any embodiment or any variant of the invention described above.

Claims (9)

1. A method for authentication of documents such as bank notes, the method comprising:

   • integration to the document of at least one luminescent substance capable of emitting light according to a response frequency spectrum determined when the substance is exposed to excitation light radiation,

   • exposure of the document to excitation light radiation, the excitation radiation being laser radiation,

   • collection of the response of the document to said excitation radiation, each response frequency spectrum essentially consisting of at least one line, and

   • encoding of the light response of the document to the excitation radiation, and comparison of this response to an expected response stored in memory

   characterized in that:

   • at least one luminescent substance comprises a medium which does not have a laser effect, said medium comprising a mixture of europium complexes with polymers of the CN-PPP (poly(2-(6'-cyano-6'-methyl-heptyloxy)-1,4-phenylene)) type.
2. The method according to claim 1, characterized in that the europium ions are incorporated to the polymer by synthesizing a family of soluble europium complexes with β-diketonate ligands, and by dissolving these complexes in a solvent in conjugation with the polymer.
3. The method according to claim 2, characterized in that the energy level of triplets of the ligand is above the emission level of europium.
4. The method according any of claims 1 to 3,
   characterized in that the emission spectrum of the polymer and the absorption spectrum of the complex at least partly overlap.
5. A method for authentication of documents such as bank notes, the method comprising:

   • integration to the document of at least a luminescent substance capable of emitting light according to a response frequency spectrum determined when the substance is exposed to excitation light radiation,

   • exposure of the document to excitation light radiation, the excitation radiation being laser radiation,

   • collection of the response of the document to said excitation radiation, each response frequency spectrum essentially consisting of at least one line, and

   • encoding of the light response of the document to the excitation radiation, and comparison of this response with the expected response stored in memory,

   characterized in that:

   • at least one luminescent substance comprises a medium which does not have any laser effect, said substance comprising a host matrix doped with at least one rare earth ion,

   and in that:

   • one proceeds with authenticating the documents without utilizing a laser effect provided by the substance.
6. The method according to claim 5, characterized in that the encoding is obtained by at least one of the following ways:

   • by using different dopants on a same matrix,

   • and / or by applying a same dopant ion on matrices of different characteristics.
7. The method according to any of the preceding claims, characterized in that each line of the response spectrum (a) has a width at half height less than a value of the order of 10 nm.
8. The method according to any of the preceding claims, characterized in that the encoding is achieved according to at least one of the following ways:

   • spatial encoding,

   • response frequency encoding,

   • response intensity encoding.
9. The method according to any of the preceding claims, characterized in that the laser excitation radiation is obtained with a YAG type laser having an emission peak around 532 nm.