Value document having security marking and method for identifying the security marking

Abstract

A value document has a security marking in the form of at least two luminescing substances which are present in a defined relative quantitative share and are jointly excitable by one excitation pulse. The time courses of the intensities are different and at least one luminescing substance has a non-monoexponential time course. In a method for identifying the security marking, the time course of the total intensity is detected and a linear combination of a formula is adapted including time courses of the intensities of the luminescing substances. The security marking is identified on the basis of the linear coefficients.

Claims

1. A value document having a security marking in the form of at least two luminescing substances, wherein the at least two luminescing substances are respectively present in a defined relative quantitative share, based on the total quantity of the at least two luminescing substances, the at least two luminescing substances are jointly excitable by one excitation pulse, time courses of intensities of emitted radiations of the at least two luminescing substances are different from each other, and at least one luminescing substance of the at least two luminescing substances has a non-monoexponential time course of the intensity of the emitted radiation of the at least one luminescing substance.

2. The value document according to claim 1, in which the at least two luminescing substances have overlapping, in particular identical, excitation spectra.

3. The value document according to claim 1, in which the at least two luminescing substances have overlapping emission spectra.

4. The value document according to claim 1, in which the security marking has luminescing substances whose time courses of the intensities of the emitted radiations have a Bray-Curtis distance of greater than 0.10.

5. The value document according to claim 1, in which the at least two luminescing substances respectively have an intensity of the emitted radiation which is in the region of 5% to 95%, of the total intensity of the emitted radiations of the luminescing substances.

6. The value document according to claim 1, in which the at least two luminescing substances respectively have a decay time in the region of 100 ns to 100 ms.

7. The value document according to claim 1, in which at least one luminescing substance comprises a host lattice doped with at least one rare-earth metal and/or at least one transition metal.

8. A method for identifying the security marking of a value document according to claim 1, which comprises the following steps: i) jointly exciting the luminescing substances with one excitation pulse, ii) detecting the time course of a total intensity of the emitted radiations of the luminescing substances, iii) adapting a linear combination I(t) of the formula $I\left(t\right)=\underset{i=1}{\overset{n}{.Math.}}{c}_i.Math.I_i\left(t\right)$ to the time course of the total intensity of the emitted radiations, wherein I.sub.i(t) are time courses of the intensities of the radiations emitted by the luminescing substances and c.sub.i are linear coefficients, wherein the index i relates to the luminescing substances and n indicates the number of luminescing substances, wherein the linear coefficients c.sub.i are ascertained, and iv) identifying the security marking on the basis of the linear coefficients c.sub.i.

9. The method according to claim 8, in which in step iii) the linear coefficients c.sub.i are determined such that absolute deviations of the linear combination I(t) from data points of the time course of the detected total intensity are minimized.

10. The method according to claim 9, in which the linear coefficients c.sub.i are determined by the method of least squares such that the sum of the square deviations of the linear combination I(t) from data points of the time course of the detected total intensity of the emitted radiations are minimized.

11. The method according to claim 8, in which step iv) comprises the following substeps: iv-1) for n1 linear coefficients c.sub.i: respectively ascertaining a ratio value M.sub.i for each linear coefficient c.sub.i, which results from the ratio of the linear coefficient c.sub.i to at least one further linear coefficient c.sub.i or to a sum of c.sub.i and at least one further linear coefficient c.sub.i, iv-2) for each ratio value M.sub.i: checking whether the ratio value M.sub.i is within an associated, definable or defined values range W.sub.i, iv-3) for each ratio value M.sub.i: assigning the attribute ratio value accepted, if the ratio value M.sub.i is within the associated values range W.sub.i, or the attribute ratio value not accepted, if the ratio value M.sub.i is outside the associated values range W.sub.i, iv-4) identifying the security marking, if all ratio values M.sub.i have been assigned the attribute ratio value accepted.

12. The method according to 11, in which in step iv-1) the ratio M.sub.i is ascertained by the ratio of the associated linear coefficient c.sub.i to the sum of all linear coefficients c.sub.i.

13. The method according to claim 8, which has a further step v) which comprises the following substeps: v-1) ascertaining a measure value G characterizing the goodness of the adaptation of the linear combination I(t) to the time course of the total intensity of the luminescing substances, v-2) comparing the measure value G with a threshold value, v-3) assigning the attribute measure value accepted to the measure value G, if the measure value G is greater than the threshold value, or the attribute measure value not accepted, if the measure value G is smaller than or equal to the threshold value, and v-4) identifying the security marking, if the measure value G has been evaluated with the attribute measure value accepted.

14. The method according to claim 13, in which the measure value G is the coefficient of determination R.sup.2, wherein the threshold value is 0.9.

15. The method according to claim 8, in which in step ii) more data points for detecting the total intensity are captured in a first time period immediately following the switching-off of the excitation pulse than in a second time period immediately following the first time period, wherein the first time period and the second time period are of equal length.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained in more detail on the basis of embodiment examples, reference is made to the enclosed figures. There are shown:

(2) FIG. 1 time courses of the luminescence intensities of two luminescent substances A, B with a different, non-monoexponential emission behaviour;

(3) FIG. 2 a time course of the total intensity of the luminescent radiations of a combination of the two luminescent substances A, B of FIG. 1, with adaptation curve;

(4) FIG. 3 time courses of the luminescence intensities of three luminescent substances A, B, C with a different, partially non-monoexponential emission behaviour;

(5) FIG. 4 a time course of the total intensity of the luminescent radiations of a combination of the luminescent substances A, B, C of FIG. 3, with adaptation curve;

(6) FIG. 5 a diagram for illustrating a mixture tuple (a, b) with scatter region for the mixture of three luminescent substances of FIG. 4;

(7) FIG. 6 upper image: simulated time course of the luminescence intensity of a combination of luminescent substances with a defined noise component during the decay phase; lower image: dependence of the relative mixture share on the size of the noise component;

(8) FIG. 7 time course of the total intensity of the emitted radiation of a mixture of two luminescent substances with a different, monoexponential emission behaviour for illustrating a forgery attempt, with adaptation curve;

(9) FIG. 8 a value document with a tracer thread which has a security marking.

DETAILED DESCRIPTION OF THE IMAGES

(10) Referring first to FIG. 1, where by way of example the measured time courses of the intensities of the emitted luminescence radiations of two different luminescent substances A, B are illustrated. The intensity I is plotted against time t (in arbitrary time and intensity units). The measured data points are respectively connected to each other by a continuous data line.

(11) The luminescence radiations of the two luminescent substances A, B are jointly excited by one single or same excitation pulse (light flash). The excitation pulse is switched on at the point in time t=0 and switched off at the point in time t=t.sub.p. Time duration and intensity of the excitation pulse are illustrated by the dashed lines. The duration of the light flash is preferably in the region of 10 s to 10 ms and is for example 40 s.

(12) The time courses of the intensities of the two luminescent substances A, B respectively have a rise phase in which the intensity increases from zero up to a maximum value, as well as a decay phase in which the intensity decreases from the maximum value. The intensity of the luminescent substance A recognizably reaches a maximum value at the point in time t=t.sub.p, so that the rise phase ends when the excitation pulse is switched off. Unlike the luminescent substance B whose intensity reaches a maximum value only after the switching off of the excitation pulse. The time courses of the intensities of the two luminescent substances differ strongly from each other, both luminescent substances showing a non-monoexponential emission behaviour. The time courses of the intensities of the two luminescent substances have a Bray-Curtis distance of 0.25 which reflects a low and thus preferred correlation behaviour of the two emission courses.

(13) FIG. 2 shows the measured time course of the total intensity of the simultaneously emitted radiations of a mixture of the two luminescent substances A, B in the I-t diagram. The combination of the two luminescent substances A, B can be employed as a security marking for a value document. Furthermore, there are represented the excitation pulse for the joint excitation of the two luminescent substances A, B (which is equal to the excitation pulse in FIG. 1) as well as an adaptation curve drawn with a solid line. In the mixture of the luminescent substances, the luminescent substance A is present with a mixture share of 30% and the luminescent substance B with a mixture share of 70%, in each case based on the total quantity of the luminescent substances A, B. The (preknown) quantity ratio (mixing ratio) of the luminescent substances A, B therefore is 30%/70%. The rise phase of the total intensity of the emitted radiations continues until after t=t.sub.p; a maximum value of the total intensity is achieved only after the switching off of the excitation pulse.

(14) The measurements of the total intensity take place at defined points in time. The measurements can be effected at equidistant points in time, but also at non-equidistant points in time, the latter offering the advantage that for example in the case of limited memory resources in the proof sensor there can be selected a reduced amount of data without significantly impairing the adaptation goodness. For this, preferably in time segments in which the intensity patterns of the base substances strongly differ, more measuring points are taken, whereas during the decay phase long after the excitation, when the luminescence has decayed quite strongly, fewer measuring points are taken.

(15) The measured time course of the total intensity I(t) is evaluated by adapting a linear combination of the general formula
I(t)=.sub.i=1.sup.nc.sub.i.Math.I.sub.i(t)(A).

(16) The formula (A) used for the linear adaptation is a linear combination of (sampled) base vectors I.sub.i(t). The running index i characterizes the luminescent substances. In the present case, n=2, i.e. i=1 and i=2, corresponding to the two luminescent substances A, B. The base vectors I.sub.i(t) are definable or defined (preknown) time courses of the luminescent substances used and result preferably from temporal intensity measurements of the luminescent substances used ascertained in advance. The base vectors I.sub.i(t) are to be respectively weighted with the associated linear coefficients c.sub.i. In the present embodiment example, the base vectors I.sub.i(t) correspond to the preknown time courses I.sub.A(t), I.sub.B(t) of the two luminescent substances A, B, as they are shown in FIG. 1.

(17) An adaptation of the linear combination I(t) to the data points of the measured total intensity requires a determination of the linear coefficients c.sub.i, which in the present case is effected with the method of least squares (least square fit method). This allows the linear coefficients c.sub.i to be ascertained efficiently with a good adaptation of the compensation curve. From the linear coefficients c.sub.i there result the relative mixture shares of the employed luminescent substances in the security marking, respectively based on the total quantity of luminescent substances. The evaluation yields a mixture share of 28.8% for the luminescent substance A and a mixture share of 71.2% for the luminescent substance B, corresponding to a quantity ratio (mixing ratio) A/B=28.8%/71.2%.

(18) For an identification of the security marking, the ascertained linear coefficients c.sub.i are combined as 2-tuple c) and are converted into a scaling-independent value, a ratio value M.sub.i. The ratio value M.sub.1 results from the linear coefficients c.sub.1, c.sub.2 as follows: M.sub.1=c.sub.1/(c.sub.1+c.sub.2). Accordingly, for the first linear coefficient c.sub.1, the ratio to the sum of the two linear coefficients c.sub.1 and c.sub.2 is formed. For the second linear coefficient c.sub.2, the associated ratio value M.sub.2 results from M.sub.2=1M.sub.1. Subsequently, for the ratio value M.sub.1 or M.sub.2 there is checked, whether the ratio value lies within an associated definable or defined (predetermined) values range W.sub.1 or W.sub.2. The values ranges W.sub.1, W.sub.2 respectively indicate a scatter region around the preknown mixture shares of the luminescent substances A, B in the security marking. For the checked ratio value M.sub.1 or M.sub.2 there is then effected an assignment of the attribute ratio value accepted, if the ratio value is within the associated values range, or the attribute ratio value not accepted, if the ratio value is outside the associated values range. In the present case, the ratio values M.sub.1, M.sub.2 are within the associated values ranges W.sub.1, W.sub.2, i.e. within the frame of the scatter there were ascertained the correct, i.e. preknown mixture shares of the two luminescent substances A, B, respectively based on the total quantity of the luminescent substances A, B, or the preknown quantity ratio (mixing ratio) A/B.

(19) Furthermore, the goodness of the adaptation of the linear combination I(t) to the time course of the total intensity of the two luminescing substances A, B is ascertained. For this purpose, the coefficient of determination R.sup.2 is used, it being preferred when the coefficient of determination R.sup.2 is above the threshold value 0.9, preferably above the threshold value 0.95. In the present case, the result is a coefficient of determination R.sup.2=0.977.

(20) The security marking is thus unambiguously identified (i.e. it is present), because the ratio values M.sub.1, M.sub.2 were assigned the attribute ratio value was accepted and the goodness of adaptation is above the desired threshold value. Due to the necessity of the presence of both conditions (attribute ratio value, goodness of adaptation), a particularly high reliability can be achieved upon identification of the security marking.

(21) Referring to FIGS. 3 to 5, another embodiment example is explained. In order to avoid unnecessary repetitions, merely the differences over the embodiment example of FIGS. 1 and 2 are explained and otherwise reference is made to the explanations there. Accordingly, a security marking having three combined luminescent substances A, B, C which are jointly excited by a same excitation pulse is viewed. The luminescent substances A, B correspond to those of FIG. 1, the luminescent substance C is additionally added. As recognizable in the I-t diagram of FIG. 3, the time courses of the intensities of the emitted luminescence radiations strongly differ from each other, the luminescent substance C showing, in contrast to the luminescent substances A, B, a monoexponential emission behaviour. The measured data points are respectively connected to each other by continuous data lines.

(22) In the mixture of the luminescent substances, the mixture shares of the luminescent substances A, B, C are, in this order, 20%, 50%, 30%, in each case based on the total quantity of luminescent substances. The mixing ratio A/B/C thus is 20%/50%/30%. The combined intensity behaviour was measured with a signal-to-noise ratio of approx. 20. The measurement data are represented in FIG. 4. Subsequently, the above-mentioned linear combination is adapted to the general formula A having three base vectors I.sub.A(t), I.sub.B(t), I.sub.C(t), as shown in FIG. 3, the linear coefficients c.sub.1, c.sub.2, c.sub.3 being determined by the method of least squares. One recognizes a good adaptation of the adaptation curve to the data points in spite of the visually clearly recognizable noise component. The evaluation yields relative mixture shares of the luminescent substances A, B, C, in this order, of 18.8%, 50.7%, 30.5%, in each case based on the total quantity of luminescent substances. For an identification of the security marking, the ascertained linear coefficients c.sub.1, c.sub.2, c.sub.3 are combined as 3-tuple (c.sub.1, c.sub.2, c.sub.3) and converted into the scaling-independent ratio values M.sub.1=c.sub.1/(c.sub.1+c.sub.2+c.sub.3), M.sub.2=c.sub.2/(c.sub.1+c.sub.2+c.sub.3). For the third linear coefficient c.sub.3, the associated ratio value M.sub.3 results from M.sub.3=1(M.sub.1+M.sub.2). Subsequently, it is checked for two ratio values M.sub.1, M.sub.2 whether the ratio values are within an associated, definable or defined (predetermined) values range W.sub.1, W.sub.2, corresponding to scatter regions of the preknown mixture shares, i.e. the distance of the mixture tuple (c.sub.1/(c.sub.1+c.sub.2+c.sub.3), c.sub.2/(c.sub.1+c.sub.2+c.sub.3)) from the reference coordinates formed from the original mixture composition is ascertained.

(23) For an easy check of the position of the measured mixture tuple relative to the preknown mixture tuple, in an a-b plane there is defined a, for example, elliptically formed tolerance region (see FIG. 5). This can be extended differently in different directions, which is due to the shape of the temporal intensity behaviour. In FIG. 5, the measured mixture tuple is represented by the filled circle, the target value (preknown mixture tuple) by the empty circle.

(24) For two ratio values M.sub.1, M.sub.2 there is then effected an assignment of the attribute ratio value accepted, if the ratio value is within the associated values range, or the attribute ratio value not accepted, if the ratio value is outside the associated values range.

(25) In the present case, the two ratio values M.sub.1, M.sub.2 are within associated values ranges W.sub.1, W.sub.2, whereby, within the frame of the scatter, the correct, i.e. preknown relative mixture shares of the two luminescent substances A, B were ascertained respectively based on the total quantity of the luminescent substances A, B, C.

(26) Furthermore, the coefficient of determination R.sup.2 was ascertained which in the present case is R.sup.2=0.9989, whereby it was shown that it is clearly above the preferred threshold values.

(27) As a result, it can be established that the security marking has the preknown composition, thus the security marking having been identified.

(28) Now, reference is made to FIG. 6, upper image. For examining the noise sensitivity of the method according to the invention, at one decay curve of a mixture from two luminescent substances with monoexponential decay behaviour differently normal-distributed noise components were added to the measuring points. There was effected an evaluation with a linear adaptation method according to the present invention and with a non-linear adaptation method known in the prior art. In FIG. 6, lower image, the evaluation is illustrated in a diagram in which the relative mixture share of a luminescent substance is plotted against the noise level. Upon determining the relative mixture share according to the invention there recognizably shows up a lower noise susceptibility in comparison to the method known in the prior art. For the non-linear method in the viewed interval of the noise level, there recognizably exists an approximately linear relation between the scatter range of the ascertained mixture share and the noise level. However, the linear adaptation method shows up stably with a scatter range of 0.05 (absolute) of the mixture share. These results suggest that already with low noise components non-linear adaptation methods do no longer deliver reliable results, while the linear adaptation method according to the invention functions sufficiently reliably in the viewed intensity interval.

(29) FIG. 7 shows the time behaviour of a monoexponentially decaying luminescent substance which, for example, could be used for a forgery attack. The adaptation curve ascertained by the method according to the invention is represented with a solid line. If it is assumed, that the security marking includes the two luminescent substances A, B, there then result the mixture shares 61.2% and 37.8% as well as an adaptation goodness of R.sup.2=0.793. Because of the adaptation goodness being far below the threshold value of preferably 0.9, the security marking is not identified.

(30) FIG. 8 shows a value document 1 configured, for example, in the form of a bank note which has a tracer thread 2 with a security marking 3. The security marking 3 can be configured as described above.

(31) As results from the above description, the invention offers great advantages over the evaluating methods with non-linear adaptation known in the prior art in which besides the amplitudes of the temporal intensity spectra also the decay times are used as model parameters. In particular there can be obtained by the method according to the invention with given time behaviour (in particular decay curves) a much faster and more stable evaluation (i.e. faster convergence behaviour of the adaptation routine) for the luminescent substances employed in combination, both for clean intensity measurements and for intensity measurement exhibiting noise. A quantitative evaluation results in a computing time reduced by approx. 3 orders of magnitude in comparison to the non-linear adaptation known in the prior art what makes clear the efficiency increase with respect to the evaluation speed. In time-critical cases of application, a fast evaluating method is essential, for example for the analysis in high-speed bank note processing machines with bank notes moved with up to 12 m/s, because these substantially determine the processing speed.

LIST OF REFERENCE SIGNS

(32) 1 value document 2 tracer thread 3 security marking