Security feature

Abstract

A security feature for protecting valuable documents, in particular for ensuring the authenticity of valuable documents, comprises a luminescent pigment which has a host lattice doped with a first luminophore and a second luminophore, with an excitation energy of the first luminophore being transferable to the second luminophore. However, in the case of the luminescent pigment according to the invention, the excitation energy is not transferred completely from the first luminophore to the second, but rather only partially. The incomplete transfer of the excitation energy is achieved by selecting suitable amount-of-substance fractions of the first and the second luminophores on the luminescent pigment. As a result of the incomplete transfer of the excitation energy, the luminescent light that is emitted by the luminescent pigment also has, in addition to a luminescence peak of the second luminophore, a luminescence peak of the first luminophore.

Claims

1. A security feature for safeguarding value documents, comprising: a luminescence pigment that has a host lattice doped with a first luminophore and a second luminophore and that is optically excitable to emit luminescence light, wherein the luminescence pigment is configured that an excitation energy of the first luminophore generated through optical excitation of the luminescence pigment is transferable to the second luminophore through an interaction between the first luminophore and the second luminophore, wherein the substance amount fraction of the first luminophore in the luminescence pigment and the substance amount fraction of the second luminophore in the luminescence pigment are chosen such that the luminescence light of the luminescence pigment has a luminescence spectrum with a first luminescence peak emitted by the first luminophore and a second luminescence peak emitted by the second luminophore, wherein the share of the peak intensity of the second luminescence peak is at least 20% and at most 80% in the sum of the peak intensities of the first and of the second luminescence peak, wherein the substance amount fractions of the first and of the second luminophores in the luminescence pigment are further chosen such that an incomplete transfer of the excitation energy from the first to the second luminophore is achieved, and wherein the substance amount fraction of the second luminophore in the luminescence pigment lies between 50 ppm and 2000 ppm.

2. The security feature according to claim 1, wherein the peak intensity of the first luminescence peak and the peak intensity of the second luminescence peak have an intensity ratio to each other that is intrinsically defined by the composition of the luminescence pigment.

3. The security feature according to claim 1, wherein the first and second luminophores are respectively substantially homogeneously distributed in a volume region of the host lattice, which volume region is both doped with the first luminophore and with the second luminophore.

4. The security feature according to claim 1, wherein through a change of the substance amount fraction of the second luminophore, the peak intensities of the first and second luminescence peaks are changeable in mutually opposite fashion.

5. The security feature according to claim 1, wherein the peak wavelengths of the first and second luminescence peaks are spectrally spaced apart from each other by at least 20 nm.

6. The security feature according to claim 1, wherein the interaction through which the excitation energy is transferable from the first luminophore to the second luminophore and takes place within a volume region of the host lattice, which volume region is both doped with the first luminophore and with the second luminophore.

7. The security feature according to claim 1, wherein the excitation energy of the first luminophore is transferable from the first luminophore to the second luminophore through a radiationless interaction.

8. The security feature according to claim 1, wherein the peak wavelengths of the first and second luminescence peaks lie in the near infrared spectral region.

9. The security feature according to claim 1, wherein the first and/or the second luminophores are chosen from the rare earth ions.

10. The security feature according to claim 1, wherein the host lattice is configured as an inorganic host lattice, wherein the host lattice is a host lattice with a garnet structure or with a perovskite structure or an oxide or a mixed lattice with oxide ions.

11. The security feature according to claim 1, wherein a peak wavelength of the first luminescence peak lies at a smaller wavelength than a peak wavelength of the second luminescent peak.

12. The security feature according to claim 11, wherein the peak wavelength of the first luminescent peak lies at a greater wavelength than a wavelength of the optical excitation.

13. The security feature according to claim 1, wherein the host lattice of the luminescence pigment is doped with at least a third luminophore wherein the luminescence pigment is configured that an excitation energy of the second luminophore generated through the interaction between the first luminophore and the second luminophore is transferable to the third luminophore through an interaction between the second luminophore and the third luminophore wherein the substance amount fraction of the first luminophore in the luminescence pigment, the substance amount fraction of the second luminophore in the luminescence pigment and a substance amount fraction of the third luminophore are chosen such that the luminescence light of the luminescence pigment has a luminescence spectrum with a first luminescence peak emitted by the first luminophore, a second luminescence peak emitted by the second luminophore and a second luminescence peak emitted by the second luminophore, and wherein the substance amount fractions of the first, of the second and of the third luminophores in the luminescence pigment are further chosen such that an incomplete transfer of the excitation energy from the first to the second luminophore and from the second to the third luminophore is achieved.

14. The security feature according to claim 1, wherein the first and/or the second luminophores are chosen from the rare earth ions erbium, holmium, neodymium, thulium, and ytterbium.

15. The security feature according to claim 1, wherein the components of the luminescence pigment are chosen such that an increase in the substance amount fraction of the second luminophore would result in an increased transfer of excitation energy from the first to the second luminophore, until the substance amount fraction is such that a complete transfer of excitation energy from the first to the second luminophore occurs.

16. A security element or printing ink, which has a security feature according to claim 1.

17. A value document or security paper, which has a security element and/or a printing ink according to claim 16.

18. A method for proving a security feature according to claim 1, comprising irradiating the security feature with light of a spectral region in which the luminescence pigment of the security feature absorbs, in order to optically excite the luminescence pigment to emit the luminescence light, and detecting intensities of the first and second luminescence peak contained in the luminescence spectrum of the luminescence light, and evaluating the detected intensities of the first and second luminescence peak to prove the security feature.

19. The method according to claim 18, wherein through the irradiating of the security feature an excitation energy of the first luminophore is generated, which is partly transferred from the first luminophore to the second luminophore, the excitation energy of the first luminophore being generated directly through selective optical excitation of the first luminophore, and/or being generated through optical excitation of the host lattice and subsequent transfer of the excitation energy from the host lattice to the first luminophore.

20. A security feature for safeguarding value documents, comprising: a luminescence pigment that has a host lattice doped with a first luminophore and a second luminophore and that is optically excitable to emit luminescence light, wherein the luminescence pigment is configured that an excitation energy of the first luminophore generated through optical excitation of the luminescence pigment is transferable to the second luminophore through an interaction between the first luminophore and the second luminophore, wherein the substance amount fraction of the first luminophore in the luminescence pigment and the substance amount fraction of the second luminophore in the luminescence pigment are chosen such that the luminescence light of the luminescence pigment has a luminescence spectrum with a first luminescence peak emitted by the first luminophore and a second luminescence peak emitted by the second luminophore, wherein the share of the peak intensity of the second luminescence peak is at least 20% and at most 80% in the sum of the peak intensities of the first and of the second luminescence peak, wherein the substance amount fractions of the first and of the second luminophores in the luminescence pigment are further chosen such that an incomplete transfer of the excitation energy from the first to the second luminophore is achieved, wherein the peak wavelengths of the first and second luminescence peaks lie in the near infrared spectral region, the peak wavelength of the first luminescence peak lies at a smaller wavelength than the peak wavelength of the second luminescent peak, and the peak wavelength of the first luminescent peak lies at a greater wavelength than a wavelength of the optical excitation, and wherein the first and/or the second luminophores are chosen from the rare earth ions erbium, holmium, neodymium, thulium, and ytterbium.

21. The security feature according to claim 20, wherein the substance amount fraction of the second luminophore in the luminescence pigment lies between 50 ppm and 2000 ppm.

22. The security feature according to claim 20, wherein the host lattice is configured as an inorganic host lattice, wherein the host lattice is a host lattice with a garnet structure or with a perovskite structure or an oxide or a mixed lattice with oxide ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Hereinafter the invention will be explained by way of example with reference to the following Figures. There are shown:

(2) FIG. 1a luminescence spectrum of a luminescence pigment with a luminescence peak B of a second luminophore L2 and disappearing first luminescence of a first luminophore L1,

(3) FIGS. 1b,c respectively the luminescence spectrum of a luminescence pigment according to the invention with a first luminescence peak A of a first luminophore L1 and a second luminescence peak B of a second luminophore L2,

(4) FIG. 2 course of the share P of the peak intensity of the luminescence peak B of the second luminophore L2 in the sum of the peak intensities of the first luminescence peak A and second luminescence peak B.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) In FIG. 1a there is shown the luminescence spectrum of a luminescence pigment that is doped with a first luminophore L1 and a second luminophore L2, between which a complete transfer of their excitation energy takes place. The luminescence spectrum consists of a luminescence peak B at a wavelength λ.sub.B, which results from the luminescence of the second luminophore L2. The first luminophore acts as an activator that transfers its excitation energy completely to the second luminophore, and thus shows no luminescence at the wavelength λ.sub.A, at which the first luminophore L1 in isolation usually luminesces. The luminescence pigment from the example of FIG. 1a has the second luminophore L2 with a substance amount fraction y0.

(6) The FIGS. 1b, 1c respectively show the luminescence spectrum of a luminescence pigment according to the invention, which likewise is doped with the first luminophore L1 and the second luminophore L2. The luminescence pigments of the examples of FIGS. 1b, c, compared to the luminescence pigment of FIG. 1a, are characterized by a particular substance amount fraction y with which the second luminophore L2 is contained in the luminescence pigment. The two luminescence pigments of FIGS. 1b, c differ from each other only by the substance amount fraction y of the second luminophore L2, which in the luminescence pigment of FIG. 1b is referred to as y1 and in the example of FIG. 1c as y2.

(7) Both luminescence spectra of the FIGS. 1b, 1c respectively have a luminescence peak A at a wavelength λ.sub.A, which results from the luminescence of the first luminophore L1, and a luminescence peak B at a wavelength λ.sub.B, which results from the luminescence of the second luminophore L2. In contrast to the example of FIG. 1a, in these examples therefore also the luminescence of the first luminophore L1 occurs, even though the first luminophore L1 tends to transfer its excitation energy to the second luminophore L2. In the example of FIG. 1b, the intensity ratio I.sub.A/I.sub.B of the two luminescence peaks A, B is about 0.43:1 corresponding to a share P=I.sub.B/(I.sub.A+I.sub.B) of the peak intensity I.sub.B in the sum of the two peak intensities (I.sub.A+I.sub.B) of 70%. In the example of FIG. 1c, the intensity ratio I.sub.A/I.sub.B of the two luminescence peaks A, B is about 1.9:1 corresponding to a share P=I.sub.B/(I.sub.A+I.sub.B) of the peak intensity I.sub.B in the sum of the two peak intensities (I.sub.A+I.sub.B) of 34%.

(8) In FIG. 2, the share P=I.sub.B/(I.sub.A+I.sub.B) of the second peak intensity is outlined as a function of the substance amount fraction y of the second luminophore L2. Over a wide range of the substance amount fraction y, the share P of the peak intensity I.sub.B lies at 100%, as with these substance amount fractions y of the second luminophore there takes place a complete transfer of the excitation energy from the first luminophore L1 to the second luminophore L2. Only with very low substance amount fractions y, the share P falls below 100% and is continually reduced to 0% with the substance amount fraction y=0, i.e. when the luminescence pigment has only the first luminophore but no second luminophore L2. With suitable substance amount fractions y of the second luminophore, e.g. y1 and y2, there thus results an incomplete transfer of the excitation energy to the second luminophore. y0, y1, y2 in FIG. 2 mark the substance amount fractions y of the second luminophore L2 in the luminescence pigment in the examples in FIGS. 1a, 1b, 1c.

(9) The luminescence pigment according to the invention shows in dependence of the substance amount fraction y of the second luminophore a qualitative change of the luminescence spectrum, i.e. with the luminescence pigment according to the invention the peak intensities do not scale uniformly, but the ratio of the peak intensities of the first and second luminescence peaks changes upon a change of the substance amount fraction y of the second luminophore. The particular substance amount fraction y of the examples of FIGS. 1b and 1c leads to the fact that the intensity ratio of the two luminescence peaks A and B strongly changes. Upon a change of the substance amount fraction y, an opposite change of the two peak intensities arises in this example. While the peak intensity I.sub.A of the first luminescence peak A is smaller with the substance amount fraction y1 than with the substance amount fraction y2, the peak intensity I.sub.B of the second luminescence peak B is larger with the substance amount fraction y1 than with the substance amount fraction y2. Upon a change of the substance amount fraction y from y1 to y2, the luminescence peak A becomes stronger at the expense of the luminescence peak B.

(10) In the following two concrete examples of luminescence pigments according to the invention are stated for the employment in security features. The luminescence pigments respectively have a host lattice doped with a first and a second luminophore. As in the example shown above, the first and second luminophores are chosen such that the first luminophore L1 acts as an activator and tends to transfer its excitation energy to the second luminophore L2. With substance amount fractions y of the second luminescence pigment L2, according to the invention, there arises an incomplete transfer of the excitation energy, however, so that the luminescence spectrum of the respective luminescence pigment has both the luminescence peak B of the second luminophore L2 as well as the luminescence peak A of the first luminophore L1.

Example 1: Y.SUB.2.68-y.Ho.SUB.y.Yb.SUB.0.32.Al.SUB.5.O.SUB.12

(11) As a luminescence pigment there is employed a rare-earth-doped yttrium-aluminum-garnet. This is doped with the first luminophore Yb and the second luminophore Ho. The luminescence spectrum of the luminescence pigment has, upon an excitation with light of a wavelength of 941 nm, a first luminescence peak A at λ.sub.A=1027 nm, which results from the luminescence of the first luminophore Yb, and a second luminescence peak B at λ.sub.B=2086 nm, which results from the luminescence of the second luminophore Ho. The dependence shown by the share P of the Ho-peak-intensity I.sub.B in the sum I.sub.A+I.sub.B of the two peak intensities I.sub.A, I.sub.B on the substance amount fraction y of the second luminophore, qualitatively corresponds to the course shown in FIG. 2.

Example 1a: Y.SUB.2.64.Ho.SUB.0.04.Yb.SUB.0.32.Al.SUB.5.O.SUB.12

(12) For manufacturing this rare earth-doped yttrium-aluminum-garnet 2.308 g Y(NO.sub.3).sub.3.6H.sub.2O, 4.282 g Al(NO.sub.3).sub.3.9H.sub.2O, 0.328 g Yb(NO.sub.3).sub.3.5H.sub.2O, 0.040 g Ho(NO.sub.3).sub.3.5H.sub.2O and 2.742 g urea are dissolved at 60° C. in 15 g water and subsequently evaporated at 675° C. The Yb-concentration parameter of 0.32 and the Ho-concentration parameter of 0.04, due to the 20-atom stoichiometric formula, correspond to an Yb-substance amount fraction of x=0.016 and an Ho-substance amount fraction of y=0.002. With these substance amount fractions x, y of the first and second luminophores there arises a share P=I.sub.B/(I.sub.A+I.sub.B) of the second peak intensity I.sub.B of about 46%. In this case, the peak intensity I.sub.A of the luminescence peak A and the peak intensity I.sub.B of the luminescence peak B are therefore comparable.

Counter-Example (not According to the Invention) to Example 1: Y.SUB.2.58.Ho.SUB.0.1.Yb.SUB.0.32.Al.SUB.5.O.SUB.12

(13) For manufacturing this rare earth-doped yttrium-aluminum-garnet 2.256 g Y(NO.sub.3).sub.3.6H.sub.2O, 4.282 g Al(NO.sub.3).sub.3.9H.sub.2O, 0.328 g Yb(NO.sub.3).sub.3.5H.sub.2O, 0.101 g Ho(NO.sub.3).sub.3.5H.sub.2O and 2.742 g urea are dissolved at 60° C. in 15 g water and subsequently evaporated at 675° C. The Yb-concentration parameter of 0.32 and the Ho-concentration parameter of 0.1, due to the 20-atom stoichiometric formula, correspond to an Yb-substance amount fraction of x=0.016 and an Ho-substance amount fraction of y=0.005. With this luminescence pigment there arises a share P=I.sub.B/(I.sub.A+I.sub.B) of the second peak intensity I.sub.B of about 86%, corresponding to a ratio of the peak intensity of the Ho-luminescence peak to the peak intensity of the Yb-luminescence peak of 6:1.

Example 2: Na.SUB.0.9875.Er.SUB.0.0025.Ho.SUB.0.01.Ti.SUB.0.025.Nb.SUB.0.975.O.SUB.3

(14) As a luminescence pigment there is employed a rare-earth-doped sodium niobate, which is doped with the first luminophore Er and the second luminophore Ho. For manufacturing this rare-earth-doped niobate 2.808 g Na.sub.2CO.sub.3, 6.956 g Nb.sub.2O.sub.5, 0.107 g TiO.sub.2, 0.0257 g Er.sub.2O.sub.3 and 0.101 g Ho.sub.2O.sub.3 are homogenized in an agate mortar and ignited in a corundum crucible for 8 h at 1150° C. The luminescence spectrum of the luminescence pigment has, upon an excitation with light of a wavelength of 650 nm, a first luminescence peak A at λ.sub.A=982 nm, which results from the luminescence of the first luminophore Er, and a second luminescence peak B at λ.sub.B=1200 nm, which results from the luminescence of the second luminophore Ho. The Er-concentration parameter of 0.0025 and the Ho-concentration parameter of 0.01, due to the 5-atom stoichiometric formula, correspond to an Er-substance amount fraction of x=0.0005 and a Ho-substance amount fraction of y=0.002. With these substance amount fractions x of the first luminophore Er and of the second luminophore Ho there arises a share P=I.sub.B/(I.sub.A+I.sub.B) of the second peak intensity I.sub.B of about 40%, corresponding to a ratio of the peak intensity of the Ho-luminescence peak to the peak intensity of the Er-luminescence peak of 2:3.

Counter-Example (not According to the Invention) to Example 2: Na.SUB.0.8975.Er.SUB.0.0025.Ho.SUB.0.1.Ti.SUB.0.205.Nb.SUB.0.795.O.SUB.3

(15) As in example 2, a rare-earth-doped sodium niobate is considered a luminescence pigment. For manufacturing this rare-earth-doped niobate 2.517 g Na.sub.2CO.sub.3, 5.591 g Nb.sub.2O.sub.5, 0.866 g TiO.sub.2, 0.0252 g Er.sub.2O.sub.3 and 0.999 g Ho.sub.2O.sub.3 are homogenized in an agate mortar and ignited in a corundum crucible for 8 h at 1150° C. The Er-concentration parameter of 0.0025 and the Ho-concentration parameter of 0.1, due to the 5-atom stoichiometric formula, correspond to an Er-substance amount fraction of x=0.0005 and a Ho-substance amount fraction of y=0.02. With this luminescence pigment there arises a share P=I.sub.B/(I.sub.A+I.sub.B) of the second peak intensity I.sub.B of about 93%, corresponding to a ratio of the peak intensity of the Ho-luminescence peak to the peak intensity of the Er-luminescence peak of 13.3:1.