EP2591067A1

EP2591067A1 - Security feature

Info

Publication number: EP2591067A1
Application number: EP10803059.4A
Authority: EP
Inventors: Johann Kecht; Kai Uwe Stock; Stephan Steinlein
Original assignee: Giesecke and Devrient GmbH
Current assignee: Giesecke and Devrient Currency Technology GmbH
Priority date: 2010-07-09
Filing date: 2010-12-23
Publication date: 2013-05-15
Also published as: US9409434B2; CN102971397B; RU2013105373A; WO2012003854A1; US20130106090A1; RU2570670C2; CN102971397A; HK1181417A1

Abstract

The invention relates to a security feature comprising a luminescence pigment which has a host grating doped with a luminophore and is optically excited in order to emit luminescence light. The luminescence light of the luminescence pigment has a luminescence spectrum with a first luminescence peak and a second luminescence peak, the intensity of each peak being dependent on a substance constituent amount x of the luminophore in the luminescence pigment. With the luminescence pigment according to the invention, the host grating, the luminophore and the substance constituent amount x of the luminophore are selected such that even a slight increase or decrease in the substance constituent amount x of the luminophore brings about an intense relative variation in the peak intensities I_Aand I_B. As a result thereof, the security against falsification provided by the luminescence pigment according to the invention is increased.

Description

safety feature

The invention relates to a security feature and a method for testing the security feature. Such security features are used to secure value documents, in particular to authenticate the value documents.

To safeguard value documents, these are equipped with security features and / or security elements having security features in order to enable verification of the authenticity of the value document. The security features and security elements serve to protect against unauthorized reproduction of the value documents. As security elements, for example, security threads or film elements are used, which are connected to a value document. The security features may be connected to the substrate of the security element or to the substrate of the value document itself.

As a security feature, for example, luminescent pigments are used, which consist of a doped with a luminophore host lattice. Optical transitions of the luminophor lead to luminescence of the luminescent pigment. For checking the authenticity of a value document which has the luminescence pigment, it is checked, for example, whether the value document has the desired luminescence and, depending on this, decides whether the value document is genuine or not.

To make it difficult to imitate the value documents, they are provided with a security feature of a luminescent pigment, which, with appropriate optical excitation, shows a characteristic luminescence spectrum with several luminescence peaks. However, in some cases, the luminescence spectrum of such a security feature can be readjusted by another luminescent pigment which, although not the identical but a similar luminescence spectrum provides as the security feature. In order to obtain a sufficiently similar luminescence spectrum, it is not necessary to strike the identical chemical composition of the luminescence pigment. For slight deviations from the chemical composition of the security feature usually do not have a strong effect on the luminescence spectrum obtained.

An object of the present invention is therefore to provide a security feature with a luminescent pigment whose luminescence spectrum is difficult to imitate.

This object is solved by the subject matters of the independent claims. In dependent claims advantageous developments and refinements of the invention are given.

The security feature includes a luminescent pigment which has a host lattice doped with a luminophore and which is optically excitable to emit luminescent light. The luminescent light of the luminescent pigments has a luminescence spectrum with at least one first luminescence peak and at least one second luminescence peak, the peak intensities of which each depend on a mole fraction x of the luminophore on the luminescence pigment. Hereinafter, the peak intensity of the first luminescent peak A will be referred to as IA (X) and that of the second luminescence peak B as IB (X). The first and second peak intensities are both changeable by changing the mole fraction x of the luminophore. In the case of the luminescence pigment according to the invention, the host lattice and the luminophore and the molar fraction x of the luminophore are chosen such that the peak intensity I A (X) of the first luminescence pigment is selected. peaks and the peak intensity IB (X) of the second luminescence peak as a function of the mole fraction x of the luminophore, in which molar fraction x which the luminophore has on the luminescence pigment has the following relationship:

where the parameter F = 10. The peak intensity IA of the first luminescence peak and the peak intensity IB of the second luminescence peak can be changed by changing the molar fraction x of the luminophore such that the peak intensities IA (X) and IB (X) as a function of the mole fraction x of the luminophore, in the molar fraction x that the luminophore has on the luminescent pigment, the above-mentioned relationship (1) holds.

In the following, the quotient of the difference of the first and second peak intensity and the sum of the first and second peak intensity as

Luminescence peak contrast K denotes, i. E.

_K _ I _A {x) - I _B {x)

I _A (x) + I _B (x) ^'

In the case of the security feature according to the invention, the absolute value of the first derivative of the luminescence peak contrast K results according to the substance content fraction x of the luminophore of at least 10. This leads to a slight increase or reduction of the mole fraction x of the luminophore leading to a strong relative change in the peak intensities IA and IB leads. The parameter F forms a minimum value for the absolute value of the derivative of the luminescence peak contrast K according to the amount of substance x of the luminophore which the safety features according to the invention achieve or exceed. To check whether a considered luminescent pigment satisfies the above-mentioned relationship, several further luminescent pigments can be used, which differ from the considered luminescent pigment exclusively with respect to the mole fraction x of the luminophore, while all other properties of the further luminescent pigments and of the considered luminescent pigment are the same , The further luminescent pigments contain proportions of substance quantities x1, x2, etc. that deviate slightly from the luminescent pigment under consideration, with suitably large deviations from the molar fraction x, which may be, for example, in the percent range. From the respective luminescence spectra, which result with identical optical excitation of these luminescent pigments, the peak intensities IA and IB can be determined for the different mole fraction xl, x2, etc., respectively. For several slightly different molar proportions, this results in a continuous dependence of the two peak intensities IA (X) and IB (X) on the molar fraction x. Subsequently, the difference and the sum of the two peak intensities IA (X) and IB (X) and their quotient are formed to determine the luminescence peak contrast K. The absolute value of the first derivative of the luminescent peak contrast K according to the mole fraction x of the luminescent substance is compared with the parameter F. In this way it can be checked whether the luminescence of a luminescent pigment has the inventive strong dependence on the molar fraction x of the luminophore or not. The molar fraction x of the luminophore is the quantitative relative proportion of the luminophore to the luminescent pigment. The mole fraction x of a luminophore is the relative number of lumi- nophor particles (atoms, ions) in the total particle number exhibited by the luminescent pigment according to its ratio formula. From the concentra- tion parameter z, by which the amount of luminophore is given in the ratio formula of the luminescent pigment, the mole fraction x of the luminophore is therefore calculated by division by the total number of particles (atoms, ions) given in the ratio formula.

In the case of the luminescent pigment of the security element, even a slight increase or reduction of the mole fraction x of the luminophore causes a large change in the luminescence peak contrast K. This strong dependence has the advantage that the anti-counterfeiting security of the luminescent pigment is increased. Because even if the right constituents of the true luminescent pigment were to be found for imitation, the molar fraction of the luminophore must be taken very precisely in order to obtain a luminescence spectrum which is sufficiently close to that of the security feature.

The first and second luminescence peak are emitted in the security feature according to the invention by a single luminescence pigment. The first and second luminescence peaks are therefore contained in the luminescence spectrum of the luminescence light, which emits the luminescence pigment on the basis of its optical excitation. For example, the first and second luminescent peaks may result from different electronic transitions of the same luminescent pigment. Thus, in the luminescent pigment of the invention, the first and second luminescent peaks are not emitted by two different luminescent pigments, e.g. as a mixture of luminescent pigments or spatially separated, but by a single luminescent.

Compared with the previously known security features, which consist of a mixture of two luminescent pigments, has the inventive Security feature preferably an intrinsically determined by the composition of the luminescent pigment luminescence spectrum. For with a mixture of two luminescent pigments, each having a luminophore with a characteristic luminescence peak, the intensity ratio of the luminescence peaks of the two luminescent phosphors depends on the proportion of the two luminescent pigments in the pigment mixture. The mixing of different luminescent pigments, however, has the disadvantage that the various luminescent pigments of the mixture can separate again, for example because of different particle sizes or different densities. Such segregation of the luminescent pigments of a security feature can take place in particular due to vibrations during transport of the security feature or also during processing of the security feature for application to the value document. The demixing results in an inhomogeneity of the pigment mixture, which can lead to unintended spatial variations of the luminescence of the security feature on the value document. Such variations can lead to a misjudgment of genuine value documents or to a "softening" of the authenticity requirements, which impairs the detection of counterfeits.Thus, in the case of the previous security features, which consists of a mixture of several luminescent pigments, then the homogeneity of the pigment mixture usually has to be carried out with great difficulty This is not necessary in the case of the security feature according to the invention.The absolute value of the first derivative of the luminescent peak contrast K according to the molar fraction x of the luminophore is preferably at least 10, preferably at least 40, particularly preferably at least 150, corresponding to the parameter F = 10 or F = 40 or F = 150. The larger absolute value of the first derivative is advantageous since it has a larger empirical value. sensitivity of the luminescence peak contrast and thus the luminescence spectrum to variations in the amount of substance x of the luminophore means. The above-mentioned advantages of the security feature according to the invention are then greater.

Luminescence pigments are therefore preferably used for the protection in which the molar fraction is in a range in which at least the parameter F is achieved according to the relationship (1), since even a slight deviation of the stoichiometry causes a marked change in the luminescence. In regions of the molar fraction in which the absolute value of the first derivative of the luminescence peak contrast K falls below the value F, there is little or no change in the luminescence peak contrast with the mole fraction. It is thus possible in these areas to obtain luminescent pigments with a similar luminescence spectrum without knowing the exact stoichiometry of the true security feature. A forgery of security features by imitation of Lumineszenzpigments is therefore much easier in the previous security features.

A change in the mole fraction x can cause both peak intensities to increase or decrease. It is preferred that the peak intensity of one of the two luminescence peaks decreases with increasing mole fraction x of the luminophore, since such a behavior is unusual in comparison with previous luminescent pigments. A decreasing with increasing mole fraction of peak intensity has in particular that of the two luminescence peaks with the smaller peak wavelength on. In addition, the peak intensity IA or IB of one of the two luminescence peaks can increase with increasing mole fraction x. An increasing peak intensity with increasing molar fraction especially that of the two luminescence peaks with the larger peak wavelength.

The luminophore may be distributed over a portion of the host lattice. However, luminescent pigments in which the luminophore is distributed over the entire host lattice of the luminescent pigment are preferred, since in this case the production costs for the luminescence pigment are low. In the luminescence pigments according to the invention, the luminescence spectrum varies greatly depending on the exact mole fraction of the luminophore. Local variations of the luminophore content within the

Host lattices can then lead to slightly different peak intensities locally. Therefore, it is preferable that the luminophore is homogeneously distributed in the host lattice of the luminescent pigment. In a first embodiment, the first and second luminescent peaks are emitted by the same luminophore. For example, the first and second luminescent peaks result from electronic transitions of the same luminophore. The host lattice of the luminescent pigment may, in addition to the luminophore emitting the two luminescence peaks, be doped with one or more further dopants. The further dopants may be further luminophores or other dopants.

In a second embodiment, the first and second luminescence peaks are emitted by two different luminophores, with which the host lattice of the luminescent pigment is doped. In particular, the first and second luminescence peaks can result from electronic transitions of two different luminophores, with which the host lattice of the luminescence pigment is doped. The first luminescence peak of the luminescence spectrum is then passed through a first luminophore and the second Luminescence peak emitted by a second luminophore, with which the host lattice of the luminescent pigment is doped. The mole fractions of the two luminophores may be the same or different. If the mole fractions of the first luminophore and the second luminophore are different, the first and second luminophore will be referred to as the first luminophore whose molar fraction of the luminescent pigment is lower and the molar fraction x of the luminophore will be that lower mole fraction of the first luminophore. The mole fraction of the second luminophore is denoted by y. To determine the peak intensities IA (X) and IB (X) as a function of the mole fraction x of the first luminophore and to determine the first derivative according to the molar fraction x, the ratio of the mole fractions (x: y) of the first and the second luminophore becomes constant held. The first and the second luminophore are contained in the same volume range of the luminescent pigment. Preferably, both the first and the second luminophore are substantially homogeneously distributed in this volume region of the luminescent pigment. The first and second luminophore may be distributed over a portion of the host lattice. In this case, the spatial distributions of the first and the second luminophor can completely or partially overlap. However, luminescent pigments in which the first and second luminophore are distributed over the entire host lattice of the luminescent pigment are preferred, since in this case the production costs for the luminescence pigment are low.

In both exemplary embodiments, it is also possible for the particular luminescence pigment additionally to have one or more further luminophores which likewise emit two luminescence peaks which behave in accordance with the relationship (1) according to the invention. The luminophores of Both embodiments can also be used together, within the same luminescence pigment. In both cases, it is then preferred that the luminescence peaks of the different luminophores do not overlap spectrally with one another.

In contrast to previous luminescence pigments, its luminescence spectrum can be qualitatively changed by a change in the molar fraction x of the luminophore on the luminescence pigment, ie the peak intensities do not scale uniformly in the luminescence pigment according to the invention, but the ratio of the peak intensities of the first and second luminescence peaks changes as the luminescence changes Mole fraction x of the luminophore. Particularly suitable are luminescent pigments with a mole fraction x of the luminophore, in which, by changing the molar fraction x, the peak intensities of the first and second luminescent peaks can be changed in opposite directions. In this case, by changing the molar fraction x of the luminophore, either the peak intensity of the first luminescence peak can be increased and the peak intensity of the second luminescence peak can be reduced or the peak intensity of the first luminescence peak can be reduced and, at the same time, the peak intensity of the second luminescence peak can be increased. In the first exemplary embodiment, the opposite change in the peak intensities results exclusively from the change in the molar fraction x of the luminophore, the luminescence pigment otherwise remaining unchanged. However, in certain cases of the first embodiment, the luminophore may not be incorporated into the lattice of the luminescent pigment without charge, without producing undesirable flaws. In order to produce such a defect in a change in the mole fraction of the luminophore, in such cases, an additional dopant in introduced the luminescent, which itself is not a luminophore to compensate for the charge of the luminophore. When the mole fraction of the luminophore is changed, the amount of compensating dopant is adjusted relative to the changed mole fraction x of the luminophore in order to prevent a change in the luminescence pigment due to otherwise resulting defects. In such a case, to determine the peak intensities IA (X) and IB (X) as a function of the mole fraction x of the luminophore and to determine the first derivative according to the mole fraction x, the ratio of the molar proportions x to the amount of material content of this dopant kept constant. The opposite change in the two peak intensities results in these cases from the change in the mole fraction x of the luminophore, wherein at most the mole fraction of such an additional dopant is adjusted accordingly and the luminescence pigment otherwise remains unchanged.

In the second exemplary embodiment, the opposite change in the peak intensities results exclusively from the change in the mole fraction x, y of the first and second luminophore, but the ratio of the two mole fractions x and y is kept constant and the luminescence pigment otherwise remains unchanged. In the second embodiment, therefore, the peak intensities of the first and second luminescence peaks by a change in the mole fraction x of the first luminophore, in which the ratio of the molar ratio (x: y) of the first and the second Lurrtinophors is kept constant on the luminescent, in opposite directions to each other.

The peak wavelength of the first luminescence peak and the peak wavelength of the second luminescence peak are preferably spectrally spaced apart by at least 20 nm, more preferably at least 30 nm. The Both luminescence peaks can therefore be distinguished spectrally from one another when checking the security feature. The peak wavelengths of the first and second luminescence peaks are preferably in the near-infrared spectral range, in particular in the spectral range between 750 nm and 2900 nm, preferably between 800 nm and 2200 nm. Especially the near-infrared spectral range is preferred since these wavelengths lie outside the visible spectral range that an inconspicuous use of the security feature is possible. Depending on which host lattice and which first and second luminophore are used, the luminescence pigment for emitting the luminescence light can be optically excited by irradiation with light in the ultraviolet or in the visible spectral range or in the near-infrared spectral range. Depending on the type of luminophore (s) selected, and depending on the optical excitation, further luminescence peaks can be emitted in addition to the two luminescence peaks. The peak wavelengths of the first and second luminescence peaks are preferably shifted to longer wavelengths (Stokes emission) compared to the optical excitation of the luminescent pigment. Compared to the opposite case, when the optical excitation is at a greater wavelength than the luminescence peaks (anti-Stokes emission, as they have, for example, Upconverter luminescence), this is advantageous since higher luminescence intensities can be achieved in Stokes emission than at an Antistokes emission. In contrast to upconverter luminescence pigments, therefore, a small amount of the luminescent pigment is already sufficient in the case of the luminescence pigments according to the invention in order to obtain well detectable peak intensities.

The luminescent pigment consists for example of a doped host lattice which is doped at least with a luminophore. The molar fraction x of the luminophore on the luminescent pigment is, for example, between at least 50 ppm and at most 10,000 ppm, in particular between at least 50 ppm and at most 5000 ppm. In addition, the host lattice may also be doped with other dopants which do not luminesce, for example with dopants which are required for crystal formation or for the prevention of defects in the crystal. The luminescence pigment can be formed, for example, as a powder whose particles consist of the doped host lattice. The particles may, for example, have a particle size in the range from 1 to 20 μm, preferably <6 μm. The luminophore with which the host lattice is doped or the first and / or the second luminophore with which the host lattice is doped is / are preferably selected from the rare earth ions, in particular from the rare earth ions erbium, holmium, neodymium, thulium, ytterbium , The host lattice is preferably designed as an inorganic host lattice. In particular, the host lattice may have a perovskite structure or a garnet structure. For example, the host lattice is an yttrium aluminum garnet or a mixed garnet derived therefrom. If the host lattice has a garnet structure or a perovskite structure, this preferably also contains one or more of the elements vanadium, chromium, manganese, iron, cobalt or nickel as the absorbing element. The host lattice may also be an oxide or a mixed lattice with oxide ions, for example a tungstate, phosphate, niobate, tantalate, silicate or aluminate.

The properties according to the invention are achieved only in certain compositions of the luminescent pigment, ie certain lumi- nophores, certain mole fractions of the luminophore, certain combinations of luminophore and host lattice and - in the second embodiment - certain combinations of the two luminophores. The choice of a different luminophore, a different mole fraction or another host lattice generally leads to a luminescent pigment which does not have the properties according to the invention.

Several of the luminescent pigments of the present invention having different peak intensities of the first and second luminescent peaks may be used to produce security features having different encodings, e.g. to provide different types of value documents with different encodings. While a luminescence pigment is used for a first security feature, which has a specific ratio of the first and second peak intensity, further security features each receive a luminescent pigment with a different ratio of the first and second peak intensities, the spectral position of the luminescence peaks being the same as in the first security feature , Of course, to encode various value documents, it is also possible to use security features which simultaneously contain various or even several of the luminescent pigments according to the invention. For example, security features can be encoded with different types of luminescent pigments of the invention whose first and second luminescence peaks are each at different wavelengths.

The invention also relates to a security element having a security feature according to the invention. The security element is intended to be applied to a value document or incorporated into a value document. The security feature is eg a security strip, a security thread, a security tape or a transfer element for application to a document of value. Furthermore, the security feature can be admixed with a printing ink, which is provided, for example, for application to a value document. The printing ink containing the security feature can, for example, be limited to the value document in one or more specific areas. ment be printed. The security feature can also be introduced into the value document, for example by being mixed with the substrate material of the value document, in particular a paper or plastic substrate, during its production.

Moreover, the invention relates to a security paper and a value document in which a security feature according to the invention is applied or incorporated and / or into which a security feature provided with the security element and / or a printing ink with the security feature. The security feature can be added to the security paper during the production of the security paper. The security feature may be over the entire area or over part of the area, e.g. in the form of characters or patterns, be applied to a surface of the value document or the security paper or the security element. Different sections of the value document or the security paper or the security element can be provided with security features with different coding.

The invention also relates to a method for detecting the security feature, in which an optical excitation of the luminescent pigment is carried out in order to optically excite the luminescence pigment for emission of the luminescence light, and in which the intensities of the first and second luminescence peaks contained in the luminescence spectrum of the luminescence pigment are detected. These detected intensities of the luminescence peaks may be the peak intensities or the intensity integrated spectrally over the respective luminescence peak. The optical excitation of the luminescent pigment is effected by irradiating the security feature with light in which the luminescence pigment of the security feature absorbs, eg with light of the near-infrared spectral range. To detect the security feature, the detected intensities are the first and the second luminescence peak were evaluated in order to check the authenticity of the security feature or the security element, the printing ink or the value document. The irradiation of the security feature with light and the detection of the intensities as well as optionally also the evaluation is performed by a sensor designed for this purpose.

The security documents to be secured are, for example, banknotes, checks, identity cards, passports, credit cards, check cards, tickets, vouchers, shares, certificates, tokens etc.

The invention will be explained by way of example with reference to the following figures. Show it:

FIGS. 1 a, b luminescence spectra of conventional luminescent pigments:

a) a luminescence pigment having a first molar fraction of the luminophore, b) a luminescent pigment having a second molar fraction of the luminophore,

FIGS. 2 a, b show luminescence spectra of luminescent pigments according to the invention: a) a luminescence pigment with a first substance fraction x 1 of the luminophore, b) a luminescence pigment with a second molar fraction x 2 of the luminophore,

FIG. 3 a shows the course of the peak intensity of the two luminescence peaks of FIG

Luminescence pigments of a first example as a function of the mole fraction x of the luminophore,

FIG. 3b luminescence peak contrast K as a function of the mole fraction of the luminophore x for the first example,

Figure 3c First derivative of the luminescence peak contrast K after

Mole fraction x of the luminophore for the first example. FIGS. 1 a and 1 b each show the luminescence spectrum of a conventional luminescence pigment which consists of two luminescence peaks A *, B *. The luminescence peaks A *, B * result, for example, from electronic transitions of a luminophore used for doping the luminescence pigment. The luminescent pigments of FIGS. 1 a and 1 b are composed of the same host lattice and luminophore and differ solely in terms of the mole fraction of the luminophore. In this example, the luminescent pigment from FIG. 1 a has a higher substance content fraction of the luminophore than the luminescence pigment from FIG. The different mole fraction of the luminophore leads to a change in the luminescence spectrum. The peak intensity I of the luminescence peaks A *, B * usually increases proportionally with the mole fraction of the luminophore. Thus, in the case of the luminescence pigment in FIG. 1a, both the peak intensity of the luminescence peak A * and the luminescence peak B * are about twice as large as in the luminescence pigment in FIG. 1b. Despite the changed mole fraction of the luminophore, the intensity ratio of the two luminescence peaks A and B thus remains approximately the same. In this behavior, the luminescence peak contrast K remains constant as a function of the mole fraction and the first derivative according to the relationship (1) is vanishingly small.

In practice, the first derivative of the luminescence peak contrast K can be determined on the basis of a sample series for the luminescence pigment, wherein the substance amount fractions of the luminophore x are varied within the sample series. If one determines from the luminescence spectrum of each sample of the sample series in each case the luminescence peak contrast K and carries this as a function of the mole fraction of the luminophore x of the respective sample. be, the first derivative which can be used for checking the relationship (1) results from the slope of the resulting function.

FIG. 2 a shows the luminescence spectrum of a first luminescence pigment P according to the invention on the basis of a host lattice doped with a luminophore L with a first mole fraction x 1 of the luminophore. FIG. 1b shows the luminescence spectrum of a second luminescent pigment P 'on the basis of the host lattice doped with the same luminophore L with a second mole fraction of the luminophore x2, which, depending on the type of luminescent pigment, is slightly larger or smaller than xl. The luminescent pigments P, P 'consist of the same host lattice and luminophore and differ only in the mole fraction of the luminophore L. In both spectra of FIGS. 2a, 2b, two luminescent peaks A, B of the luminophore L are contained, which vary in intensity and in intensity Differ intensity ratio. The shape of the spectra in FIGS. 2a, 2b is shown only schematically. In particular, the shape and width of the luminescence peaks may differ from this illustration.

The different mole fractions x1, x2 of the luminophore L lead to an uneven change in the luminescence spectrum. While the peak intensity of the luminescence peak A is greater in the molar fraction x2 than in the molar fraction xl, the peak intensity of the luminescent peak B in the molar fraction x2 is smaller than in molar fraction xl, cf. FIGS. 2a, 2b. The changed mole fraction of the Lumi- nophor L causes the intensity ratio of the two luminescence peaks A and B changes greatly. Depending on the mole fraction of the luminophore L thus results in a qualitative change in the luminescence spectrum. When the substance content of the luminophore L changes, the intensities of the luminescence peaks A, B change. in between. Thus, when the mole fraction of the luminophore changes from x1 to x2, the luminescence peak A becomes stronger at the expense of the luminescence peak B. The relative change in the peak intensities as a function of the mole fraction x of the luminophore is particularly great in the case of the security feature according to the invention.

Example V. LII _z Tm _z bi-2zTi2z03

For the preparation of the luminescent pigment, the corresponding amounts of lithium carbonate, thulium oxide, niobium oxide and titanium oxide are intimately mixed with one another in the agate mortar according to Table 1 below. The mixture is then calcined in a corundum crucible for 8 hours at 1150 ° C.

Table 1

FIG. 3 shows the course of the peak intensities IA and IB of two luminescent peaks A, B of the luminescent pigment Li ₁ -z Tm _z Nb -2zTi 2zO 3 as a function of the concentration parameter z, which is indicated in the diagram on the upper horizontal axis. On the lower horizontal axis the mole fraction x is given, which in this case, due to the number of 5 atoms in the formula Lii _-z Tm _z Nb _1- 2 _Z Ti2 _Z 03, from the concentration parameter z by division by the factor 5 results. A mole fraction x = 0.001 is equivalent to a lurninophore Proportion of the host lattice of 1000 ppm (parts per million). The two considered luminescence peaks A, B are emitted as a consequence of an optical excitation of the luminescence pigment at 780 nm through the lumi- nophor Tm. The luminescence peak A is at a wavelength λ _Α οη about 798 nm and the luminescence peak B at a wavelength λβ of about 1758 nm. In the considered range of the substance fraction x the peak intensity IA of the luminescent peak A falls with increasing mole fraction x of the luminophore, while the Peak intensity of the luminescence peak B increases. The luminescence peak contrast K = (IA-IB) / (IA + IB) falls accordingly from about +0.95 at a mole fraction x = 0.0005 to about -0.91 at a mole fraction x = 0.01, cf. FIG. 3b. The diagram of FIG. 3c shows the absolute value of the first derivative of the luminescent peak contrast K according to the substance fraction x. The derivative values, ie the change of the luminescence peak contrast K as a function of the mole fraction x, are above 40 for x = 0.001 to x = 0.006 and amount to up to about 600.

Example 2: Y2-5 ₂ (NdiYb ₄ ) ₂ SiO ₅ As a second example, the luminescent pigment Y2-5z (NdiYb ₄ ) _z Si05 is considered. For its preparation, the corresponding amounts of urea, silicon dioxide, and yttrium nitrate hexahydrate are dissolved in 3 ml of water in accordance with Table 2 below. Subsequently, the appropriate amounts of neodymium nitrate hexahydrate and ytterbium nitrate pentahydrate are dissolved together in water, then added to the reaction mixture and mixed. The reaction mixture is evaporated on a hot plate at 500 ° C. Subsequently, the material obtained is transferred to a corundum crucible and annealed in the oven at 1500 ° C for 10h. Sample z XY (N0 ₃ ) ₃ HarnSi0 ₂ Nd (N0 ₃ ) ₃ Yb (N0 ₃ ) ₃

* 6H ₂ 0 substance * 6H ₂ 0 * 5H ₂ 0

2-1 0.0005 0.0000625 6.791 g 2.665 g 0.533 g 0.0019 g 0.0080 g

2-2 0.001 0.000125 6.781 g 2.665 g 0.533 g 0.0039 g 0.0159 g

2-3 0.0025 0.0003125 6.753 g 2.664 g 0.533 g 0.0097 g 0.0398 g

2-4 0.004 0.0005 6.725 g 2.663 g 0.533 g 0.0155 g 0.0637 g

2-5 0.005 0.000625 6.706 g 2.662 g 0.533 g 0.0194 g 0.0796 g

2-6 0.006 0.00075 6.687 g 2.661 g 0.532 g 0.0233 g 0.0955 g

2-7 0.007 0.000875 6.688 g 2.660 g 0.532 g 0.0272 g 0.1114 g

2-8 0.01 0.00125 6.612 g 2.658 g 0.532 g 0.0388 g 0.1590 g

Table 2

The luminescent pigment contains two luminophores LI, L2 in this example with Yb and Nd. By optically exciting the luminescent pigment with light having a wavelength of 532 nm, the luminescent pigment samples 2-1 to 2-8 respectively emit a first luminescent peak A at a wavelength XA of 1075 nm emitted by the first luminophore neodymium and a second luminescent peak B at nm, which is emitted by the second luminophore ytterbium. The eight luminescent pigment samples 2-1 to 2-8 differ only in terms of the mole fractions of the luminophores LI, L2. The amount of substance x is calculated in this case from the concentration parameter z by division by 8, corresponding to the total number of atoms in the ratio formula

According to the ratio formula, the mole fraction of the second luminophore L2 (Yb) is always four times the mole fraction x of the first luminophore s LI (Nd). If, in the case of a luminescence pigment, the substance proportions of the two luminophores are different, the two luminophores of the luminescence pigment are considered to be the first luminophore LI which has the lower molar fraction. Consequently, the luminescence peak contrast K = (IA-IB) / (IA + IB) as a function of the amount of substance genanteils x of the first luminophore LI (Nd) considered. The dependence of the luminescence peak contrast K on the molar fraction x is determined under the condition that the mole fraction x of the first luminophore LI (Nd) is in a constant ratio to the mole fraction of the second luminophore L2 (Yb). In Example 2, this ratio, corresponding to the ratio formula, constant 4. Also, the first derivative of the luminescence peak contrast K to the mole fraction x of the first luminophore L (Nd) is determined under the condition that the mole fraction x of the first luminophore LI (Nd ) in the constant ratio 4 to the substance content of the second luminophore L2 (Yb).

In the molar proportions x in accordance with Table 2, the luminescent peaks A, B show a strong intensity ratio depending on the mole fraction x of the first luminophore LI. Therefore, the luminescent pigment samples 2-1 to 2-8 are very well distinguishable from each other by the intensity ratio and the luminescent peak contrast K, respectively. The luminescence peak contrast K = (IA-IB) / (IA + IB) of the first luminescent peak A at 1075 nm and the second luminescent peak B at 978 nm is K = + 0.84 for the luminescent pigment sample 2-1, and K = + 0.75; K + = 0.50; K + = 0.33; K + = 0.17; K + = 0.10; K + = 0.02; K = -0.26 for the respective luminescent pigment samples 2-2 to 2-8. The absolute value of the first derivative of the luminescence peak contrast K according to the mole fraction x of the first luminophore LI (Nd) is more than 150 for all eight luminescent pigments.

Claims

Patent claims

A security feature for securing value documents comprising: a luminescent pigment having a host lattice doped with a luminophore (L, LI) and being optically excitable to emit luminescent light, the luminescent light of the luminescent pigment having a luminescence spectrum with at least a first luminescence peak (A ) and at least one second luminescence peak (B) whose peak intensities (IA (X), IB (X)) each depend on a molar fraction x of the luminescent substance (L, LI) on the luminescence pigment, characterized in that the Host lattice and the luminophore (L, LI) and the molar fraction x of the luminophore s (L, LI) are selected such that for the peak intensity IA (X) of the first luminescence peak (A) and the peak intensity IB (X) of the second luminescence peak ( B) as a function of the mole fraction x of the luminophore s (L, LI) the relationship applies, where the parameter F = 10.

2. Security feature according to claim 1, characterized in that the parameter F = 40, preferably F = 150.

3. Security feature according to one of the preceding claims, characterized in that the peak intensity (IA) of the first luminescence peak (A) and the peak intensity (IB) of the second luminescence peak (B) have an intensity ratio to each other, which is determined intrinsically by the composition of the luminescent ,

4. Security feature according to one of the preceding claims, characterized in that the peak intensity (IA or IB) of one of the two luminescence peaks (A or B) decreases with increasing mole fraction x.

5. Security feature according to one of the preceding claims, characterized in that, by a change in the molar fraction x of the luminophore (L, LI), the peak intensities (IA, IB) of the first and second luminescence peaks (A, B) are opposite to each other changeable.

6. Security feature according to one of the preceding claims, characterized in that the luminophore (L, LI) is distributed substantially homogeneously in the host lattice.

7. Security feature according to one of the preceding claims, characterized in that the first and second luminescence peak (A, B) by the same luminophore (L) are emitted.

8. Security feature according to one of claims 1 to 6, characterized in that the luminophore of claim 1 is a first luminophore (LI), with which the host lattice is doped, and the mole fraction x of claim 1, the amount of substance of the first luminophore (LI) on the luminescent pigment and that the host lattice is additionally doped with a second luminophore (L2) whose molar fraction y of the luminescent pigment is equal to or greater than the molar fraction x of the first luminophore (LI), and in that the first luminescence peak (A) of the luminescence spectrum by the first luminophore (LI) and the second luminescence peak (B) of the luminescence spectrum by the second luminophore (L2) is emitted, and that, for determining the peak intensities (IA (X), IB (X)) as a function of the mole fraction x of the first luminophore and to determine the first derivative

according to the amount of substance x of the first luminophore (LI), the ratio (x: y) of the mole fraction (x, y) of the first and the second luminophore (LI, L2) is kept constant on the luminescent pigment.

9. Security feature according to one of the preceding claims, characterized in that the first and second luminescence peak (A, B) peak wavelengths (λΑ, B), the spectrally at least 20 nm, preferably at least 30 nm, are spaced apart.

10. Security feature according to one of the preceding claims, characterized in that the peak wavelengths XA, XB) of the first and second luminescence peaks (A, B) are in the near-infrared spectral range, in particular in the spectral range between 750 nm and 2900 nm, preferably between 800 nm and 2200 nm.

11. Security feature according to one of the preceding claims, characterized in that the luminophore is selected from the rare earth ions, preferably from the rare earth ions erbium, holmium, neodymium, thulium, ytterbium.

12. Security feature according to one of the preceding claims, characterized in that the host lattice is formed as an inorganic host lattice, wherein the host lattice is in particular a host lattice with a garnet structure or with a perovskite structure or an oxide or a A mixed grating with oxide ions is, for example, a tungstate or phosphate or niobate or tantalate or silicate or aluminate.

13. A security element or printing ink having one or more security features according to one of claims 1 to 12.

14. security document or security paper having one or more security features according to one of claims 1 to 12 and / or a security element according to claim 13 and / or a printing ink according to claim 13.

15. A method for detecting a security feature according to any one of claims 1-12, wherein

the security feature is irradiated with light of a spectral region in which the luminescent pigment of the security feature absorbs the light

To stimulate luminescence pigment to emit the luminescence light, and

Intensities of the first and second luminescence peaks (A, B) contained in the luminescence spectrum of the luminescence light are detected, and

- For detecting the security feature, the detected intensities of the first and second luminescence peaks (A, B) are evaluated.