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

Abstract

The invention relates to a value document having a security marking in the form of at least two luminescent substances, which are present in a defined relative quantity proportion and can be excited together by an excitation pulse. The time curves of the intensities are different and at least one luminescent substance has a non-monoexponential time curve. In a method for identifying the security marking, the time curve of the common intensity is detected and a linear combination of formula (I) is adapted, wherein Ii(t) are time curves of the intensities of the luminescent substances. The security marking is identified based on the linear coefficients Ci.

Description

[0001] The present invention lies in the technical field of the manufacture and verification of security documents and relates to a method for identifying a security mark.

[0002] Valuable documents are generally protected against unauthorized and often illegal duplication by special markings. It has long been known to treat valuable documents for this purpose with luminescent substances that exhibit specific emission characteristics.

[0003] Document WO 916009 A1 describes the authentication of a security document by determining the luminescence decay time of a security mark. The security mark is pulsed and the time elapsed after the end of the excitation pulse until a predefined luminescence intensity is reached is measured.

[0004] Another method for verifying the authenticity of a security document by determining the luminescence decay time of a security mark is disclosed in publication WO 0188846A1. In this method, the luminescence intensity is measured at several times after switching off an excitation pulse in order to determine the decay curve and compare it with a target curve.

[0005] Document US 7762468 B2 describes an authentication method that uses a combination of two luminescent substances with different decay times. In this method, the second, slower-decaying luminescent substance is only detected after the luminescence of the first luminescent substance has already faded.

[0006] The evaluation of a security marking with luminescent substances exhibiting different decay behavior and overlapping emission spectra can also be found in publication DE 102006047851 A1. This method involves measuring the temporal profile of the luminescence intensities and verifying the shape of the curve for authentication purposes, comparing it to target values.

[0007] US patent 9046486 B2 discloses a security marking and a method for its identification based on combinations of quasi-resonant luminescent materials with different exponential decay behavior. Both the amplitudes and decay times are determined using non-linear fitting. The described method is not suitable for marking materials with strongly non-exponential decay behavior, thus limiting the number of available marking materials. Furthermore, the analysis using non-linear fitting proves to be time-consuming and susceptible to noise, resulting in low speed and poor evaluation quality.

[0008] US patent 2014 / 0001351 A1 discloses two luminescent materials mixed in a security feature.

[0009] While existing luminescent materials and evaluation methods can achieve a satisfactory solution for the counterfeit-proof marking of security documents, a disadvantage, particularly with luminescent materials exhibiting non-exponential decay, is the limited combinatorial diversity of available materials. This results in restricted marking variability, which can, among other things, reduce counterfeit protection. Furthermore, when luminescent materials with complex time-dependent behavior are used as base materials for encoding mixtures with varying decay behavior, existing evaluation methods, such as those described in US 9046486B2, are unsuitable for reliably evaluating such security features in time-critical situations, such as on high-speed banknote processing machines.

[0010] In contrast, the object of the present invention is to enable reliable, secure, and rapid identification of the marking of a valuable document using luminescent substances with complex time behavior. Furthermore, the invention aims to allow the use of a variety of different luminescent substances with non-exponential time behavior.

[0011] These and other problems are solved according to the invention by a method for identifying a security marking with the features of claim 1. Advantageous embodiments of the invention are specified by the features of the dependent claims.

[0012] A document of value bearing a security mark (marking) is shown. For the purposes of this invention, the term "document of value" refers to any item requiring protection against unauthorized or unlawful duplication, such as banknotes, checks, shares, tokens, identity cards, credit cards, and passports, as well as labels, seals, packaging, or other items used for security purposes. The security mark of the document of value can be assigned to at least one freely definable (binary) property of the document of value, wherein the property is present upon identification (presence of the security mark) and not present upon non-identification (absence of the security mark). For example, the security mark can be assigned to the property of "authenticity" as an authenticity mark or feature to distinguish documents of value from counterfeits.It is also conceivable that valuable documents could be assigned to a specific class or group through the security marking, such as the note value or country of origin of banknotes.

[0013] According to the invention, the security marking is designed in the form of at least two luminescent substances (hereinafter also referred to as luminescent materials). The luminescent materials can be incorporated into or applied to the security document in a variety of ways. For example, the luminescent materials can be added to a paper or plastic material used to manufacture the security document or to a printing ink used to print on the security document. It is also conceivable to provide the luminescent materials as, for example, an invisible coating on the security document. The luminescent materials can also be provided on or in a carrier material, for example, made of plastic, which is embedded in a paper or plastic material used to manufacture the security document. The carrier material can, for example, be in the form of a security or identification thread, a reflective fiber, or a planchette.The carrier material can also be attached to the security document, for example in the form of a plaque, to implement a product security measure. In principle, any shape of carrier material is possible.

[0014] The at least two luminescent substances of the security marking can be excited together by a (same) excitation pulse (e.g., a flash of light). Crucially, the time courses of the intensities of the emitted radiation from the luminescent substances excited by the excitation pulse are different from each other, with at least one luminescent substance exhibiting a non-monoexponential time course of the intensity of the emitted radiation.

[0015] The safety marking contains at least two luminescent substances in a definable or defined ratio in combination (preferably in the form of a mixture). This means that each luminescent substance is present in the safety marking in a definable or defined relative proportion, based on the total amount of luminescent substances. The safety marking can therefore be uniquely identified based on the ratio (mixing ratio) of the luminescent substances.

[0016] Depending on its relative proportion, each luminescent substance contributes the intensity of its emitted luminescence radiation to the total intensity of the radiation emitted simultaneously by the excited luminescent substances of the security marking. The term "total intensity" here and subsequently refers to the summed intensity of the luminescence radiation from the combined luminescent substances contained in the security marking, all excited by the same excitation pulse and detected at the same time.

[0017] The security marking is designed in such a way that, for identification of the security marking, the quantity ratio (mixing ratio) of the luminescent substances can be determined by an analysis of the time course of the total intensity of the emitted luminescence radiation (excited by an excitation pulse) based on the time courses of the intensities of the luminescence radiation of the luminescent substances (excited by the same excitation pulse).

[0018] The use of at least one luminescent substance with a non-mono-exponential time course of the emitted radiation intensity has the particular advantage that a large number of potentially suitable substances are available, and improved counterfeit protection can be achieved through specific selection. Furthermore, a relatively large difference in the rise and / or fall-off behavior of the luminescent substances can be achieved, allowing for reliable and secure identification of the security mark. If the excitation light is re-emitted with an (anti-)Stokes shifted wavelength due to intrinsic conversion processes, a clear separation of the excitation and emission radiation is easily possible using suitable filtering techniques.

[0019] Advantageously, the at least two luminescent substances are selected such that the intensity of the emitted radiation of each luminescent substance lies within a range of 5% to 95%, preferably 10% to 90%, and particularly preferably 15% to 85%, of the total intensity of the luminescent substances. This allows for a particularly accurate analysis of the time course of the total intensity of the security marking based on the time courses of the intensities of the luminescent radiation emitted by the respective luminescent substances, which contributes to an improvement in the reliability of the identification of the security feature.

[0020] Preferably, the at least two luminescent substances are each selected such that the decay time, i.e., in particular the time between the end of the excitation pulse and reaching an intensity of 1 / e of the intensity at the end of the excitation pulse, lies in the range of 100 ns to 100 ms, preferably 10 µs to 5 ms. This is advantageous for an accurate analysis of the time course of the total intensity of the emitted luminescence radiation from the luminescent substances based on the time courses of the intensities of the luminescence radiation emitted by the respective luminescent substances, which contributes to a further improvement in the reliability of the identification of the security feature.

[0021] Preferably, but not necessarily, the at least two luminescent substances have overlapping, and in particular identical, excitation spectra, which enables targeted and relatively strong excitation of the luminescent substances by a comparatively narrowband excitation pulse (flash of light). Particularly preferably, the at least two luminescent substances have overlapping emission spectra, which advantageously further improves the counterfeit protection of the security feature due to the significantly more difficult analysis of the emitted radiation.

[0022] In a further advantageous embodiment of the security document, the at least two luminescent substances are configured such that the time profiles of the intensities of the emitted radiation have a Bray-Curtis distance greater than 0.10, preferably greater than 0.20, and particularly preferably greater than 0.25. The Bray-Curtis distance of two vectors (v₁, ..., vₙ) and (w₁, ..., wₙ) is defined here as ∑ i = 1 n v i − w i v i + w i .

[0023] This measure can also increase the accuracy of the analysis of the time course of the total intensity of the emitted luminescence radiation of the luminescent substances of the security marking based on the time courses of the intensities of the luminescence radiation emitted by the luminescent substances, which contributes to a further improvement in the reliability of the identification of the security feature.

[0024] The luminescent substances used in the security marking of the valuable document can generally be freely selected, provided that they can be excited together by a single excitation pulse and that the time courses of the emitted radiation from the luminescent substances are different from one another, with at least one luminescent substance exhibiting a non-monoexponential time course of the intensity of the emitted radiation. Excitation and emission of the luminescent substances can occur in the UV, visible, and / or infrared range. For example, luminescent substances can be used that are excited in the UV range and emit in the UV or visible spectral range. Furthermore, it is possible to use luminescent substances that are excited in the visible spectral range and emit in the visible or infrared spectral range.Furthermore, luminescent substances can be used that are excited in the IR range and emit in the IR range or emit in the visible range (up-converters).

[0025] According to the invention, luminescent substances are advantageous if they exhibit a particularly strong non-monoexponential decay behavior after excitation. Particularly preferred are luminescent substances that each comprise a host lattice doped with at least one dopant selected from the rare-earth metals and transition metals (or their ions).

[0026] Suitable inorganic host lattices include, for example, oxides, borates, gallates, phosphates, garnets, perovskites, sulfides, oxysulfides, apatites, vanadates, tungstates, glasses, tantalates, niobates, halides, oxyhalides, especially fluorides, silicates or aluminates.

[0027] In particular, host lattices such as YAG, ZnS, YGG, YAM, YAP, AlPO 5 , zeolites, Zn 2 SiO 4 , YVO 4 , CaSiO 3 , KMgF 3 , Y 2 O 2 S, La 2 O 2 S, Ba 2 P 2 O 7 , Gd 2 O 2 S, NaYW 2 O 6 can be used as host lattices , SrMoO 4 , MgF 2 , MgO, CaF 2 , Y 3 Ga 5 O 12 , KY(WO 4 ) 2 , SrAl 12 O 19 , ZBLAN, LiYF 4 , YPO 4 , GdBO 3 , BaSi 2 O 5 or SrBeO 7 can be used. Suitable dopants include, for example, the rare earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or Bi, Pb, Ni, Sn, Sb, W, Tl, Ag, Cu, Zn, Ti, Mn, Cr and V (or their ions).

[0028] A specific selection of suitable host grid / doping ion combinations is described, for example, in publication EP 1632908 A1, the contents of which are fully referenced in this regard.

[0029] Luminescent substances with a strongly non-monoexponential time response of the emitted radiation intensity can be realized through various mechanisms. In particular, for luminescent substances with complex, possibly multi-stage, energy transfer processes between different dopants, especially rare-earth dopants, multiple time constants can intrinsically occur in both the rise and fall phases. Such energy transfer processes are known, for example, for the dopant ion combinations Yb / Er, Nd / Yb, Yb / Tm, Cr / Tm, Tm / Ho, Er / Tm, Er / Ho, Yb / Ho, Cr / Ho, Fe / Tm, Mn / Tm, Cr / Er, Fe / Er, Cr / Nd, Cr / Nd, Cr / Yb, and especially in combination with other dopants. According to the invention, the use of such dopant ion combinations is preferred.The precise time response of these substances is highly sensitive to both the host lattice used (due to the fine splitting of the energy states involved) and the respective dopant ion concentrations. This is caused by a relative change in the coupling rates compared to competing loss processes, such as non-radiative recombination of the ions involved.

[0030] In particular, luminescent substances with complex intrinsic energy transfer processes can exhibit intensity profiles with strongly non-monoexponential time behavior, whereby the luminescence intensity can continue to increase even after the excitation phase has ended. The combination of such substances with classical substances that exhibit a monotonically decreasing time behavior after excitation allows for the targeted adjustment of the overall intensity of the luminescent substances over time. This can include not only decreasing emission phases but also increases, plateaus, local maxima, and / or minima. According to the invention, it can be advantageous if the safety marking comprises a combination of at least one luminescent substance with a non-monoexponential time behavior and at least one luminescent substance with a monoexponential time behavior of the emitted luminescence radiation intensity.

[0031] In a further preferred embodiment, the security marking can comprise a combination of at least two luminescent substances, each with different non-monoexponential time behavior of the intensity of the emitted luminescent radiation.

[0032] Furthermore, there are luminescent materials in which several different transitions of a dopant ion, which are energetically very similar but have different lifetimes, contribute to emission in a narrow wavelength range. These luminescent materials also often exhibit non-monoexponential time behavior. Examples include Pr and Er.

[0033] Furthermore, luminescent materials can exhibit non-monoexponential behavior due to randomly occurring or intentionally induced inhomogeneities during production, such as an inhomogeneous grain size distribution or an inhomogeneous distribution of dopants. This can occur, for example, when grains with faster time responses (i.e., faster decay and / or faster rise times) and grains with slower time responses (i.e., slower decay and / or slower rise times) are formed. Their differing properties are averaged during the relevant macroscopic measurement, in which a relatively large number of individual grains are generally excited and measured simultaneously. This results in the individual emission time structures of the individual grains overlapping in such a way that an overall non-monoexponential time response can emerge.

[0034] A person skilled in the art can determine whether a luminescent substance exhibits monoexponential time behavior by simply measuring its time-dependent luminescence. This involves measuring the intensity over time during the decay phase and fitting an exponential curve to the decay curve. The coefficient of determination (R2) can be used as a measure of the goodness of fit, with the decay curve being considered "non-exponential" if R2 ≤ 0.98. During the measurement, the signal-to-noise ratio at the beginning of the decay curve should be at least 50 to prevent a fit with a goodness of R2 ≤ 0.98 being obtained by chance for a monoexponential decay curve.

[0035] The invention relates to a method for identifying (i.e., detecting the presence or absence of) the security marking of a security document designed as described above. The method comprises the following steps: Step i)

[0036] Simultaneous excitation of the luminescent substances of the security marking with a (same) excitation pulse. Step ii)

[0037] Detecting the time course of the total intensity of the radiation emitted simultaneously by the luminescent substances, excited by the excitation pulse. Step iii)

[0038] Fitting a linear combination I(t) according to the following formula I t = ∑ i = 1 n c i ⋅ I i t The time course of the total intensity of the emitted radiation is determined, where Ii(t) represents definable or defined time courses of the intensities of the luminescence radiation emitted by the luminescent substances (excited by the same excitation pulse), and ci represents linear coefficients to be fitted. The running index i refers to the luminescent substances, n indicates the number of luminescent substances, and t represents time. The time courses Ii(t) of the intensities of the luminescent substances can be determined for each luminescent substance by excitation with the same excitation pulse and detection of the luminescence radiation (in advance).

[0039] To fit the linear combination I(t) to the time course of the total intensity I(t), the linear coefficients ci are determined. The linear coefficients ci each represent the relative contribution of a single luminescent substance to the linear combination I(t) over a given time course Ii(t). From the linear coefficients ci, the relative proportion of each luminescent substance, relative to the total amount of luminescent substances in the security marking, and thus the ratio (e.g., mixing ratio) of the luminescent substances in the security marking, can be determined. Step iv)

[0040] Identifying (i.e., recognizing the presence or absence) of the security marking based on the linear coefficients ci .

[0041] The adaptation of the linear combination I(t), consisting of a sum of the previously known time profiles I i (t) weighted with the linear coefficients ci, to the total intensity I(t) of the simultaneously emitted luminescence radiation advantageously enables a particularly simple, reliable and very fast determination of the quantity ratio (e.g. mixture ratio) of the luminescent substances in the security marking, thereby enabling a reliable identification of the security marking.

[0042] In an advantageous embodiment of the method according to the invention, in step iii) the linear coefficients ci are determined such that absolute deviations of the linear combination I(t) from data points of the detected time course of the total intensity are minimized. Preferably, the linear coefficients ci are determined by the least squares method such that the sum of the squared deviations of the linear combination I(t) from data points of the detected total intensity is minimized. The least squares method is familiar to those skilled in the art in the field of statistical analysis of data sets, so further explanation is unnecessary here. It should merely be added that this is a standard statistical method for determining a regression curve for a data set with the smallest possible deviation of the data points from the regression curve.

[0043] In a further advantageous embodiment of the method according to the invention, step iv) comprises the following sub-steps: Substep iv-1)

[0044] For n-1 linear coefficients ci: Determine a ratio Mi for each linear coefficient ci, which results from the ratio of the linear coefficient ci to at least one other linear coefficient ci (e.g. c 1 / c 2 ).

[0045] Advantageously, the ratio Mi is determined by the ratio of the linear coefficient ci to the sum of at least one, preferably all, linear coefficients ci (e.g., c₁ / (c₁ + c₂)). For the nth linear coefficient, the ratio Mₙ is obtained from Mₙ = 1 - (M₁ + ... + Mₙ₋₁), i.e., the difference between 1 and the sum of the other ratios M₭. The ratios Mi indicate the quantitative ratio (e.g., mixing ratio) of the luminescent substances in the safety marking. Substep iv-2)

[0046] For each ratio Mi: Check whether the ratio Mi lies within an associated, definable or defined range of values ​​W i, which advantageously corresponds to a scattering range around the previously known relative quantity of the luminescent substance in the security marking. Substep iv-3)

[0047] For each ratio M i : Assign the attribute "ratio accepted" if the ratio M i is within the associated range of values ​​W i, or the attribute "ratio not accepted" if the ratio M i is outside the associated range of values ​​W i. Substep iv-4)

[0048] Identifying (i.e., recognizing the presence of) the security mark if all ratios M i have been assigned the attribute "ratio accepted", or not identifying (i.e., recognizing the absence of) the security mark if at least one ratio M i has been assigned the attribute "ratio not accepted".

[0049] The partial steps iv-1) to iv-4) advantageously enable simple and reliable identification of the security marking based on the linear coefficients ci.

[0050] In a further advantageous embodiment of the method according to the invention, it has a further step v) which comprises the following sub-steps: Substep v-1)

[0051] Determining a measure G that characterizes the goodness of fit of the linear combination I(t) to the time course of the total intensity. Preferably, the coefficient of determination R2 is used as the measure G. The coefficient of determination R2 is familiar to those skilled in the art in the field of statistical analysis of data sets, so further explanation is unnecessary here. It should merely be added that the coefficient of determination R2 is a standard statistical method with which the quality of a linear approximation can be determined. Substep v-2)

[0052] Comparing the measure G with a definable or defined threshold value. If the coefficient of determination R2 is used as the measure G, a lower threshold value of preferably 0.9, particularly preferably 0.95, is used, thereby achieving high reliability in the identification of the security marking. Substep v-3)

[0053] Assigning the attribute "Measure accepted" to the measure G if the measure G is greater than the threshold, or the attribute "Measure not accepted" if the measure G is less than or equal to the threshold. Substep v-4)

[0054] Identifying (i.e., recognizing the absence of) the security mark if the measure G was rated with the attribute "measure accepted", or not identifying (i.e., recognizing the absence of) the security mark if the measure G was rated with the attribute "measure not accepted".

[0055] In the event that sub-steps iv-1) to iv-4) are carried out in step iv), the following applies to sub-step v-4): Identify the security mark if all ratios M i have been rated with the attribute "ratio accepted" and in addition the measure G has been rated with the attribute "measure accepted", or do not identify (i.e. detect the absence of) the security mark if at least one ratio M i has been rated with the attribute "ratio not accepted" and / or the measure G has been rated with the attribute "measure not accepted".

[0056] Step v), in particular in conjunction with sub-steps iv-1) to iv-4), can further improve the reliability of the identification of the security marking in a particularly advantageous way.

[0057] In a further advantageous embodiment of the method according to the invention, more data points for detecting the overall intensity are acquired in a first period immediately following the switching off of the excitation pulse than in a second period immediately following the first period, wherein the first and second periods are of equal length. This measure advantageously enables high reliability in the identification of the security mark with limited memory resources.

[0058] To select luminescent substances and define their relative proportions for a security document, for example as shown above, an overall intensity as a function of time (i.e., the linear combination I(t)) can be defined and assigned to a piece of information (e.g., authenticity). The linear combination I(t) is a combination of the time-dependent intensities Ii(t) of the luminescent substances with the linear coefficients ci of the luminescent substances. Based on the defined linear combination I(t), the proportions of the luminescent substances are determined. Thus, the given, desired linear combination I(t) results in a defined ratio and defined proportions of the luminescent substances.To determine and / or select the luminescent substances and their defined proportions, the respective time profiles of the intensities Ii(t) of the luminescent substances and, if applicable, their respective linear coefficients ci are considered and / or evaluated. Using a database containing the time profiles of the intensities Ii(t), a combination of luminescent substances can be defined. Subsequently, the relative proportion of each luminescent substance can be defined using the linear coefficients ci. It can be taken into account that the luminescent substance is masked with so-called camouflage agents to adjust its intensity Ii(t).The camouflage substances reduce the luminescence intensity of the luminescent substance, in particular by a time-constant factor, so that depending on the amount of camouflage substance, a different relative quantity fraction for the respective luminescent substance results from the linear coefficient ci. Brief description of the drawings

[0059] The invention will now be explained in more detail with reference to the accompanying figures. These show: Fig. 1: Time course of the luminescence intensities of two luminescent substances A, B with different, non-monoexponential emission behavior; Fig. 2: Time course of the total intensity of the luminescent radiation of a combination of the two luminescent substances A, B. Fig. 1, with fitting curve; Fig. 3: Time courses of the luminescence intensities of three luminescent substances A, B, C with different, partially non-mono-exponential emission behavior; Fig. 4: A time course of the total intensity of the luminescent radiation of a combination of the luminescent substances A, B, C of Fig. 3 , with fitting curve; Fig. 5 : a diagram illustrating a mixture tuple (a,b) with scattering range for the mixture of three luminescent substances of Fig. 4Fig. 6: upper figure: simulated time course of the luminescence intensity of a combination of luminescent substances with a defined noise component during the decay phase; lower figure: dependence of the relative mixing ratio on the magnitude of the noise component; Fig. 7: time course of the total intensity of the emitted radiation of a mixture of two luminescent substances with different monoexponential emission behavior to illustrate a forgery attempt, with a fitting curve; Fig. 8: a security document with a security thread bearing a security mark. Detailed description of the figures

[0060] First, be Fig. 1This figure illustrates, by way of example, the measured time profiles of the intensities of the emitted luminescence radiation from two different luminescent substances A and B. The intensity I is plotted against time t (in arbitrary time and intensity units). The measured data points are connected by a solid data line.

[0061] The luminescence radiation of the two luminescent materials A and B is simultaneously excited by a single, identical excitation pulse (flash of light). The excitation pulse is switched on at time t=0 and switched off at time t=tp. The duration and intensity of the excitation pulse are illustrated by the dashed lines. Preferably, the duration of the light flash is in the range of 10 µs to 10 ms and is, for example, 40 µs.

[0062] The time-dependent intensity profiles of both luminescent substances, A and B, each exhibit an accelerating phase, in which the intensity rises from zero to a maximum value, and a decay phase, in which the intensity falls from the maximum value. It is evident that the intensity of luminescent substance A reaches a maximum value at time t=tp, so that the accelerating phase ends when the excitation pulse is switched off. In contrast, the intensity of luminescent substance B only reaches a maximum value after the excitation pulse is switched off. The time-dependent intensity profiles of the two luminescent substances differ significantly, with both exhibiting non-monoexponential emission behavior. The Bray-Curtis difference between the time-dependent intensity profiles of the two luminescent substances is 0.25, which reflects a low and therefore favored correlation between the two emission profiles.

[0063] Fig. 2The diagram shows the measured time course of the total intensity of the simultaneously emitted radiation from a mixture of the two luminescent substances A and B. The combination of these two luminescent substances can be used as a security mark for a valuable document. Also shown is the excitation pulse for the simultaneous excitation of the two luminescent substances A and B (which is identical to the excitation pulse in [reference missing]). Fig. 1) and an adjustment curve shown with a solid line. In the mixture of luminescent substances, luminescent substance A is present at a proportion of 30% and luminescent substance B at a proportion of 70%, each relative to the total amount of luminescent substances A and B. The (previously known) ratio (mixing ratio) of luminescent substances A and B is therefore 30% / 70%. The decay phase of the total intensity of the emitted radiation lasts until t=tp; a maximum value of the total intensity is only reached after the excitation pulse is switched off.

[0064] The total intensity measurements are taken at defined time points. These measurements can be taken at equidistant or non-equidistant times, the latter being advantageous because, for example, with limited storage resources in the detection sensor, a reduced data set can be selected without significantly degrading the accuracy. For this purpose, more measurement points are preferably taken during time periods in which the intensity profiles of the base substances differ significantly, whereas fewer measurement points are taken during the decay phase long after excitation, when the luminescence has already largely subsided.

[0065] The measured time course of the total intensity I(t) is obtained by fitting a linear combination of the general formula I t = ∑ i = 1 n c i ⋅ I i t evaluated.

[0066] The formula (A) used for linear fitting is a linear combination of (sampled) basis vectors Ii(t). The running index i identifies the luminescent substances. In this case, n=2, i=1 and i=2, corresponding to the two luminescent substances A and B. The basis vectors Ii(t) are definable or defined (previously known) time profiles of the luminescent substances used and preferably result from previously determined temporal intensity measurements of the luminescent substances. The basis vectors Ii(t) are each weighted by the associated linear coefficients ci. The basis vectors Ii(t) correspond to the previously known time profiles IA(t) and IB(t) of the two luminescent substances A and B, as described in Fig. 1 shown.

[0067] Fitting the linear combination I(t) to the data points of the measured total intensity requires determining the linear coefficients ci, which is done here using the least-squares fit method. This allows the linear coefficients ci to be determined efficiently with a good fit of the regression curve. From the linear coefficients ci, the relative proportions of the luminescent substances used in the security marking are derived, each relative to the total amount of luminescent substances. The evaluation yields a proportion of 28.8% for luminescent substance A and a proportion of 71.2% for luminescent substance B, corresponding to a ratio (mixture ratio) A / B = 28.8% / 71.2%.

[0068] To identify the security mark, the determined linear coefficients ci are combined as a 2-tuple (c₁, c₂) and converted into a scale-independent value, a ratio M₭. The ratio M₁ is derived from the linear coefficients c₁ and c₂ as follows: M₁ = c₁ / (c₁ + c₂). Thus, the ratio of the first linear coefficient c₁ to the sum of the two linear coefficients c₁ and c₂ is calculated. For the second linear coefficient c₂, the corresponding ratio M₂ is derived from M₂ = 1 - M₁. Subsequently, it is checked whether the ratio M₁ or M₂ lies within a corresponding, definable or defined (predetermined) range of values ​​W₁ or W₂, respectively. The value ranges W 1 , W 2 each indicate a scattering range around the known proportions of the luminescent substances A, B in the security marking.Then, for the tested ratio M1 or M2, the attribute "ratio accepted" is assigned if the ratio lies within the corresponding value range, or the attribute "ratio not accepted" if the ratio lies outside the corresponding value range. In the present case, the ratios M1 and M2 lie within the corresponding value ranges W1 and W2, meaning that, within the range of variation, the correct, i.e., previously known, proportions of the two luminescent substances A and B were determined, respectively, in relation to the total amount of the luminescent substances A and B, or the previously known ratio (mixing ratio) A / B.

[0069] Furthermore, the goodness of fit of the linear combination I(t) to the time course of the total intensity of the two luminescent substances A and B is determined. For this purpose, the coefficient of determination R2 is used, preferably if the coefficient of determination R2 is above the threshold of 0.9, preferably above the threshold of 0.95. In the present case, a coefficient of determination R2 = 0.977 is obtained.

[0070] The security mark is thus uniquely identified (i.e., present) because the ratios M1 and M2 have been assigned the attribute "ratio accepted" and the goodness of fit is above the desired threshold. The requirement of both conditions (attribute ratio, goodness of fit) allows for particularly high reliability in identifying the security mark.

[0071] With reference to the Figures 3 to 5Only the differences from the exemplary embodiment of the Figure 1 and 2 The details are explained, and otherwise reference is made to the explanations provided there. Accordingly, a security marking with three combined luminescent substances A, B, and C is considered, which are all excited together by the same excitation pulse. The luminescent substances A and B correspond to those of Fig. 1 The luminescent substance C is added as a supplement. As shown in the It diagram of Fig. 3 The time courses of the emitted luminescence radiation intensities differ significantly, with luminescent substance C, unlike luminescent substances A and B, exhibiting monoexponential emission behavior. The measured data points are connected by solid lines.

[0072] In the mixture of luminescent substances, the proportions of substances A, B, and C are, in that order, 20%, 50%, and 30%, respectively, based on the total amount of luminescent substances. The mixing ratio A / B / C is therefore 20% / 50% / 30%. The combined intensity was measured with a signal-to-noise ratio of approximately 20. The measurement data are in Fig. 4 The linear combination of the general formula A mentioned above with three basis vectors IA(t), IB(t), IC(t), as shown in Fig. 3The linear coefficients c₁, c₂, and c₃ are shown, fitted using the least squares method. A good fit of the fit curve to the data points is evident despite the visually noticeable noise component. The evaluation yields relative proportions of the luminescent substances A, B, and C, in that order, of 18.8%, 50.7%, and 30.5%, respectively, based on the total amount of luminescent substances. For identification of the security marking, the determined linear coefficients c₁, c₂, and c₃ are grouped as a 3-tuple (c₁, c₂, c₃) and converted into the scale-independent ratios M₁ = c₁ / (c₁ + c₂ + c₃) and M₂ = c₂ / (c₁ + c₂ + c₃). For the third linear coefficient c 3, the corresponding ratio M 3 is obtained from M 3 = 1 - (M 1 + M 2 ). Subsequently, for two ratios M 1 , M 2, it is checked whether the ratios are within a corresponding, definable orwithin the defined (predetermined) range of values ​​W 1 , W 2, corresponding to the scatter ranges of the known mixture proportions, i.e., the distance of the mixture tuple (c 1 / (c 1 +c 2 +c 3 ), c ​​2 / (c 1 +c 2 +c 3 )) to the reference coordinates formed from the original mixture composition is determined.

[0073] For a simple check of the position of the measured mixture tuple in relation to the previously known mixture tuple, a tolerance range, for example elliptically shaped, is defined in an ab-plane (see Fig. 5 This can vary in extent depending on the shape of the temporal intensity behavior in different directions. Fig. 5 The measured mixture tuple is represented by the filled circle, the target value (previously known mixture tuple) by the empty circle.

[0074] Then, for two ratios M 1 , M 2, the attribute "ratio accepted" is assigned if the ratio is within the corresponding range of values, or the attribute "ratio not accepted" is assigned if the ratio is outside the corresponding range of values.

[0075] In the present case, the two ratios M 1 , M 2 lie within their respective value ranges W 1 , W 2 , whereby, within the scope of the dispersion, the correct, i.e., previously known, relative proportions of the two luminescent substances A, B, each relating to the total amount of luminescent substances A, B, C, were determined.

[0076] Furthermore, the coefficient of determination R 2< was determined, which in the present case is R 2< =0.9989, showing that it is significantly above the preferred threshold values.

[0077] As a result, it can be stated that the security marking has the previously known composition, thus identifying the security marking.

[0078] Reference will now be made to Fig. 6 The upper figure shows the following. To investigate the noise sensitivity of the method according to the invention, differently normally distributed noise components were added to the measurement points of a decay curve of a mixture of two luminescent substances with monoexponential decay behavior. An evaluation was performed using a linear fitting method according to the present invention and a non-linear fitting method known in the prior art. Fig. 6The lower figure illustrates the evaluation in a diagram where the relative proportion of a luminescent substance is plotted against the noise level. The determination of the relative proportion according to the invention shows a lower susceptibility to noise compared to the prior art method. For the non-linear method, there is an approximately linear relationship between the spread of the determined proportion and the noise level within the considered noise level interval. In contrast, the linear fitting method shows stability with a spread of 0.05 (absolute) of the proportion. These results suggest that even at low noise levels, non-linear fitting methods no longer provide reliable results, while the linear fitting method according to the invention functions with sufficient reliability within the considered intensity interval.

[0079] Fig. 7Figure 1 shows the time behavior of a monoexponentially decaying luminescent substance, which could, for example, be used for counterfeiting. The matching curve determined by the method according to the invention is represented by a solid line. Assuming that the security marking contains the two luminescent substances A and B, the mixture proportions are 61.2% and 37.8%, respectively, and the matching ratio is R ≤ 0.793. Because the matching ratio is far below the threshold value of preferably 0.9, the security marking is not identified.

[0080] Fig. 8 Figure 1 shows a security document 1, for example in the form of a banknote, which has an identification thread 2 with a security feature 3. The security feature 3 can be designed as described above.

[0081] As can be seen from the above description, the invention offers significant advantages over prior art evaluation methods with non-linear fitting, in which decay times are used as model parameters in addition to the amplitudes of the temporal intensity spectra. In particular, the method according to the invention, with its predetermined temporal behavior (especially decay curves) for the luminescent substances used in combination, enables a much faster and more stable evaluation (i.e., faster convergence behavior of the fitting routine) for both clean and noisy intensity measurements. Quantitative evaluation results in a computation time reduction of approximately three orders of magnitude compared to prior art non-linear fitting, which illustrates the increase in efficiency with regard to evaluation speed.In time-critical applications, a fast evaluation method is essential, for example for analysis on high-speed banknote processing machines with banknotes moving at up to 12 m / s, as these essentially determine the processing speed. Reference symbol list

[0082] 1. Valuable document 2. Identification thread 3. Security marker

Claims

1. Method for identifying the security marking of a value document, wherein the security marking is in the form of at least two luminescing substances and wherein - the luminescing substances (A, B) are respectively present in a defined relative quantitative share, based on the total quantity of the luminescing substances (A, B), - the luminescing substances (A, B) are jointly excitable by one excitation pulse, - the time courses of the intensities of the emitted radiations of the luminescing substances (A, B) are different from each other, - wherein at least one luminescing substance (A, B) has a non-monoexponential time course of the intensity of the emitted radiation, wherein the method 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 t = ∑ i = 1 n c i ⋅ I i t to the time course of the total intensity of the emitted radiations, wherein Ii(t) are time courses of the intensities of the radiations emitted by the luminescing substances and ci are linear coefficients, wherein the index i relates to the luminescing substances and n indicates the number of luminescing substances, wherein the linear coefficients ci are ascertained, iv) identifying the security marking on the basis of the linear coefficients ci.

2. Method according to claim 1, in which in step iii) the linear coefficients ci 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.

3. Method according to claim 2, in which the linear coefficients ci 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.

4. Method according to one of the claims 1 to 3, in which step iv) comprises the following substeps: iv-1) for n-1 linear coefficients ci: respectively ascertaining a ratio value Mi for each linear coefficient ci, which results from the ratio of the linear coefficient ci to at least one further linear coefficient ci or to a sum of ci and at least one further linear coefficient ci, iv-2) for each ratio value Mi: checking whether the ratio value Mi is within an associated, definable or defined values range Wi, iv-3) for each ratio value Mi: assigning the attribute "ratio value accepted", if the ratio value Mi is within the associated values range Wi, or the attribute "ratio value not accepted", if the ratio value Mi is outside the associated values range Wi, iv-4) identifying the security marking, if all ratio values Mi have been assigned the attribute "ratio value accepted".

5. Method according to 4, in which in step iv-1) the ratio Mi is ascertained by the ratio of the associated linear coefficient ci to the sum of all linear coefficients ci.

6. Method according to one of the claims 1 to 5, 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, v-4) identifying the security marking, if the measure value G has been evaluated with the attribute "measure value accepted".

7. Method according to claim 6, in which the measure value G is the coefficient of determination R2, wherein the threshold value is 0.9, preferably 0.95.

8. Method according to one of the claims 1 to 7, 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.

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