PROCEDURE FOR DETERMINING THE AUTHENTICITY OF AN OBJECT

Description

[0001] The invention relates to a method for determining the authenticity of an object, comprising the following method steps: marking the object with a first phosphor having a first temporal decay behavior, marking the object with a second phosphor having a second decay behavior that differs from the first decay behavior, exciting the phosphors with a light pulse, measuring the afterglow intensities of both phosphors after the excitation with the light pulse.

[0002] For the counterfeit-proof marking of objects with an authenticity feature, it is known to use phosphors that glow after excitation and to numerically characterize their decay time through time-resolved spectral measurement. The mere production of a phosphor with a specific, characteristic afterglow time makes it economically unattractive for a counterfeiter to synthesize the authenticity feature, and setting a specific afterglow time as a characteristic material feature is no trivial task.

[0003] The German patent application DE 10 2004 016 249 A1 presents a method for determining the authenticity of objects that is based precisely on the characterization of the decay behavior of a phosphor.

[0004] US patent US 10,900,898B2 presents a method for testing an authenticity feature. In this method, phosphors are excited with a modulation signal, and the afterglow is monitored with a lock-in amplifier. Because the phosphors exhibit a fixed afterglow, the exponential constant can be derived from the phase shift of the afterglow. A high modulation frequency can significantly reduce the time required for authenticity determination.

[0005] To further complicate the counterfeiting of such an authenticity feature, the German patent application DE 10 2017 130 027 A1 proposes irreproducibly altering known phosphors. The alteration takes place through thermal, chemical, and / or purely mechanical treatment, resulting in degradation of the phosphor. As a result of this degradation step, the resulting phosphor no longer exhibits the typical decay behavior of a first-order exponential process, but can rather be described by a sum of first-order processes. The intensity curve of the afterglow thus no longer corresponds to a first-order process. To determine the authenticity of an object marked with such a phosphor, highly precise measurement technology is required to precisely characterize the decay behavior and to adequately measure the curve that determines the afterglow as a function of time.While the high demands placed on the measurement technology make the marking process highly resistant to counterfeiting, widespread use of corresponding testing devices is either not cost-effective or difficult to achieve due to the high demands placed on the measurement system. For use with a high throughput of test items, the required measurement time is counteracting the high throughput.

[0006] For example, banknote processing systems can transport a banknote at a speed of approximately 12 m / s in order to process the sheer number of banknotes in a reasonable amount of time. This leaves only a few µs for authenticity verification in a small area of ​​the banknote. Numerically characterizing a signal curve within a few µs requires a shorter decay time in the ns range to statistically refine the afterglow through multiple measurements within the short measurement window. Phosphors with such a short decay behavior in the ns range are known. These also exhibit such a stable decay behavior as an intrinsic material property that authenticity verification is possible.A spectral measurement of decay processes in the nanosecond range at light intensities with irradiances at which banknotes do not bleach inevitably results in a single-photon measurement. Deriving a continuous decay behavior from a discrete single-photon measurement and characterizing it numerically requires a large number of measurements to perform the characterization purely statistically. The method is relatively forgery-proof, or at least counterfeiting is not economically feasible. However, the measurement effort required for authenticity testing is also quite high, which hinders widespread use of this method. Highly accurate, artifact-free, and calibrated measurement systems are required for the measurement. These are based on equally stable sensors with high linearity.

[0007] It would be desirable to have a method for determining the authenticity of objects that works on the basis of a decay behavior that is individual for a phosphor, but that can be operated with simpler measuring systems.

[0008] US 2019 / 0162864 A1 discloses a method for authenticity testing based on the decay behavior of phosphors. It proposes determining the decay time, the wavelength of maximum emission, or the ratio of the phosphor concentrations.

[0009] The object of the invention is therefore to provide a method for determining the authenticity of objects, which can be carried out on the basis of widely available standard components.

[0010] The object of the invention is achieved by a method having the features of claim 1. Further advantageous embodiments of the method are specified in the subclaims to claim 1.

[0011] According to the concept of the invention, an authenticity feature comprises at least two differently afterglow phosphors. The object to be marked for authenticity is thus marked with these two phosphors. Both upconversion phosphors and downconversion phosphors can be used as phosphors, as well as a combination of upconversion phosphors and downconversion phosphors. It is immaterial and therefore advantageous whether the marking is carried out with the different phosphors at different locations or at the same location. The different phosphors have emission bands in different regions of the spectrum. This makes it easy to separate the afterglow signals of both phosphors by having a broadband detector track the signal of a first phosphor via a first filter, and another detector track the signal of a further phosphor via a further filter.Instead of numerically characterizing the decay behavior, which may no longer be describable by a first-order physical process, the idea of ​​the invention is to calculate the difference between the signals. If the signal of two signals combined to form a difference signal is based on phosphors with different saturation emissions and different temporal profiles, a zero crossing inevitably occurs. A zero crossing is easily detectable by measurement, even for very short or high-frequency signals. The advantage of calculating the difference is that it is artifact-free or at least very low in artifacts. This is because both detectors can be constructed in such a way that they are subject to the same artifacts. This allows the use of a nonlinear detector or a detector with a lower linearity quality.Even if simple detectors exhibit sufficient linearity, the high-frequency electronics used to track signals in the nanosecond range are susceptible to systematically nonlinear signal imaging at many points. By forming the difference, these artifacts cancel each other out. The actual measured zero crossing of the difference between the two signals depends heavily on the actual afterglow of both signals and less on the signal processing in the electronics. This makes it possible to perform authenticity verification using widely available tools. The difference between the signals is used here to detect a signal of equal magnitude between two signal waveforms at a specific time t. This detection can be performed by forming the difference between the two signals and determining the zero crossing, or by comparing the two signals.From the perspective of the electronics being used, there is a slight difference between actually subtracting a first signal from a second signal and detecting the zero crossing, or determining the identical signal level using an identity signal from a comparator. Both techniques, the signal difference circuit and the comparator circuit, are well-known in electronics and are considered equivalent when it comes to detecting the zero crossing of a differential signal from two signals or detecting the identical signal level of two signals using an identity signal from a comparator.

[0012] If an irreversibly modified phosphor is used, it is not "lost" even if the time of the zero crossing / identity signal is simulated by a combination of other phosphors in a counterfeit. It is possible to create a new authenticity feature by combining two otherwise known irreversibly modified phosphors. What matters is the combination and the resulting zero crossing / identity signal. The original phosphor does not immediately lose its relevance even if a counterfeit is successfully made. It is conceivable that an authenticity test is carried out with two or more phosphors, number n, where n over two combinations result in a zero crossing / identity signal. With three phosphors, there are three zero crossings, with four phosphors, there are six zero crossings, and with five phosphors, there are already ten zero crossings.With a manageable number of different phosphors, the number of zero crossings of different afterglow signals is very difficult to simulate.

[0013] In order to make reverse engineering of the phosphors used more difficult, it can advantageously be provided that at least one of the phosphors used shows a decay behavior which can be described by a linear combination of various exponential decay behaviors of the first order, according to the following afterglow behavior I t = ∑ i = 0 n I 0 , i e − k i t with I ( t ) Intensity I at the time t i Index over a number n different processes n number of processes I 0 Saturation intensity at time t = 0 ki Decay constant of process i from n t Time e base of the natural logarithm whereby all ki are different from each other. I t the afterglow intensity after excitation, I 0,i corresponds to the saturation intensity or the initial intensity, e the base of the natural logarithm, k i a constant and t the time.

[0014] It is possible to work with phosphors that strongly overlap in the emission range, although different peaks (emission maxima) exhibit different decay behavior. For optimal signal separation, the emission bands of the phosphors used can be designed to overlap as little as possible, with an overlap of the emission bands of different phosphors of less than 20% being preferred, and an overlap of less than 5% being particularly preferred, based on the area under the respective normalized emission band when plotting the emission band against the wavenumber v. The method presented here can be used for phosphors with half-lives of the decay behavior in the ms range, the µs range, and the ns range. The half-lives can thus be in the range between 1 ms and 1,000 ms, between 1 µs and 1,000 µs, or between 1 ns and 1,000 ns.

[0015] In addition to the individual measurement of the times for zero crossings, there are basically two different methods available for measurement.

[0016] A first method involves exciting the phosphors with a sequence of rectangular light pulses, whereby the time interval between two consecutive light pulses, measured between the half-life of the falling edge of a preceding light pulse and the half-life of the rising edge of a subsequent light pulse, is greater than the time of the zero crossing / identity signal expected based on the decay behavior of the phosphors used after excitation with the preceding light pulse. The time interval between any two consecutive light pulses is random or pseudorandom. The randomly modulated signal helps smooth out the noise that inevitably accompanies a measurement during the evaluation of the high-frequency signal and thus statistically refine the result.A light pulse for excitation can be a narrowband light pulse from an ultrafast laser or a light pulse from a light-emitting diode. The spectral bandwidth of the light pulse should be as narrowband as possible. The light pulse from a laser is considered monochromatic, although a laser also has a physically determined bandwidth which, when the light intensity is plotted against the wavenumber (frequency), is almost Gaussian, i.e., can be described by a Gaussian distribution function, and can have a half-width of 10 nm down to 2 nm. Deviations arise from the Boltzmann distribution and design-related artifacts of the laser. Light-emitting diodes have a wider bandwidth. Here, too, the distribution of light intensity is roughly Gaussian when plotted against the wavenumber.With a Gaussian approximation of the actual wavelength distribution with an RMS error of less than 5%, the bandwidth of an LED ranges from 10 nm half-width of the approximate Gaussian function to 50 nm half-width of the approximate Gaussian function. Particularly suitable excitation wavelengths for LEDs or laser diodes are 640 nm (red), 530 nm (green), 460 nm to 480 nm (blue) in the visual range, and 940 nm and 980 nm, the latter two in the NIR range. These wavelengths are derived from well-known LEDs / laser diodes, which exhibit particularly long-term stability. For excitation, it is possible to use a narrowband light pulse at a central wavelength with a previously specified bandwidth, or a combination of at least two or more narrowband light pulses, each with a previously specified bandwidth.

[0017] A second method involves exciting the phosphors with a regular, sinusoidal excitation signal whose lower peak of the sinusoidal waveform is approximately zero, and determining the phase shift between the upper peak of the excitation signal and the zero crossing of the difference signal or the identity signal. Such measurement techniques can be performed using lock-in amplifiers, where the phase shift, in conjunction with the frequency of the lock-in amplifier, allows for the determination of the zero crossing or the identity signal.

[0018] The invention is explained in more detail with reference to the following figures. They show: Fig. 1 Emission spectra of three exemplary phosphors with a narrow emission band. Fig. 2 A signal diagram illustrating the signal formation from the different decay behaviors of the phosphors used. Fig. 3 A signal diagram illustrating random modulation. Fig. 4 A signal diagram illustrating sine modulation. Fig. 5 A signal diagram illustrating the difference in the decay behavior of a first-order process and a phosphor that has undergone irreversible degradation. Fig. 6 A first exemplary object with a marking according to the concept of the invention. Fig. 7 A second exemplary object with a marking according to the concept of the invention. Fig. 8 A third exemplary object with a marking according to the concept of the invention. Fig. 9 A fourth exemplary object with a marking according to the concept of the invention.

[0019] In Figure 1Three emission spectra of three example phosphors L1, L2 and L3 are shown, each with a narrow emission band I(λ 1 ), I(λ 2 ) and I(λ 3 ). These phosphors can either be upconversion phosphors that can be excited in the IR range up to the VIS range and that emit in the VIS range up to the UV range after excitation. Due to the upconversion from lower excitation energy to higher emission energy per photon, upconversion phosphors generally have a low conversion rate in the range of 1 to 5%. However, this conversion rate is sufficient to carry out an authenticity check. For optimal signal separation and to avoid a cascade of excitation, emission and re-excitation of another phosphor by the emission of the original phosphor, it can be provided that the emission bands overlap as little as possible.The overlap is best defined by plotting the emission band against the wavenumber v, since plotting it against the wavelength distorts and overestimates the long-wavelength components in relation to the energy distribution of the emitted photons. In the example shown here, the overlap of the emission bands is less than 20% relative to the standardized emission bands. According to the invention, it is particularly preferred to have 5% relative to the standardized emission bands.

[0020] In Figure 2is a signal diagram to illustrate the signal formation from the different decay behaviors of the phosphors L1, L2 and L3 used. The left abscissa indicates the intensity I of the afterglow. The first phosphor L1 has a natural decay behavior according to a first-order process, with the decay behavior being shown by the curve for the afterglow intensity I(λ1,t). This curve is based on the highest saturation intensity I0 at t=0, with the half-life for the decay curve being the shortest. The second phosphor L2 has a natural decay behavior according to a first-order process, with the decay behavior being shown by the curve for the afterglow intensity I(λ2,t). This curve is based on an average saturation intensity I0 at t=0, with the half-life for the decay curve being in the middle range here.Finally, the third phosphor L3 also exhibits a natural decay behavior according to a first-order process, which is shown by the curve for the persistence intensity I(λ3,t). This third curve is based on the lowest saturation intensity I0 at t=0, with the half-life for the decay curve being the longest here.

[0021] The diagram shown here shows phosphors with short persistence times in the ns range. The persistence intensity curves I(λ1,t) and I(λ2,t) intersect at approximately t = 19 ns. A difference signal Δ(I(λ1,t),I(λ2,t)) can be derived from these two curves. The right abscissa indicates this difference value. The first difference signal Δ(I(λ1,t),I(λ2,t)) from the two persistence intensities I(λ1,t) and I(λ2,t) has a zero crossing at the same point, namely at t = 19 ns, which can be easily determined electronically. Analogous to the formation of the difference signal Δ(I(λ1,t),I(λ2,t)), two further difference signals Δ(I(λ1,t),I(λ3 ,t)) and Δ(I(λ2,t),I(λ3 ,t)) can be formed from the curves of the afterglow intensities I(λ1,t) and I(λ3,t) and from the curves of the afterglow intensities I(λ2,t) and I(λ3,t).With the three different phosphors L1, L2 and L3, three zero crossings of signals can be formed, which are fixed and unchangeable as physical or intrinsic material constants and which can be easily determined by simple means even at high frequencies because artifacts are canceled out during the measurement.

[0022] In Figure 3A signal diagram is shown to illustrate random modulation. In this diagram, the excitations are shown as randomly modulated rectangular signals with a white background. The excitation duration is always the same, and the excitation pulses with the white background have the same width. However, the time interval between a half-life time Htd of a falling edge of the excitation pulses with the white background and a half-life Htu of a subsequent rising edge of a white-backed excitation pulse varies randomly or at least pseudo-randomly. The times of the rising edges of the excitation pulses at t = (t1, t2, ...tn) are therefore randomly chosen. Each excitation signal is followed by a relaxation signal of the respective phosphor with an afterglow intensity I(λ1,t). This modulated random signal can be read out by evaluation electronics. This diagram only shows the relaxation of a first phosphor.An almost identical signal diagram would result for the relaxation of a second phosphor, although the relaxation times are different. Only a difference signal from I(λ1,t) and I(λ2,t) yields the difference signal Δ(I(λ1,t),I(λ2,t)), in which the zero crossing can be reliably determined.

[0023] In Figure 4A signal diagram is shown to illustrate sine modulation. In sine modulation, the excitation does not occur via rectangular pulses, which strictly separates excitation and relaxation. Instead, the excitation signal is modulated sinusoidally, with the lower peak Su of the sine modulation being approximately zero. Relaxation of the phosphor already occurs during excitation. With this modulation, a phase shift can be detected, for example, using a lock-in amplifier. The phase shift cannot be defined here, as is the case with the phase shift between two sine signals, which have a fixed, temporal relationship between their zero crossings.Rather, a phase shift Δφ can be described by the distance between the upper peak So and the zero crossing by forming the difference between two relaxation signals of two different phosphors, in which the difference φ λ1,λ2 is formed from the afterglow intensity I(λ1,t) for a first phosphor L1 and from the afterglow intensity I(λ2,t) for a second phosphor L2. The distance between the time t for φ 0 and the time for φ λ1,λ2 = 0 can be used as a characteristic phase shift because the time of the peak So is easily determined by a zero crossing of a differential signal, in this case a cosine signal. A differential signal can be achieved by a resonant coil or capacitor circuit. The reference to the differential signal So and the zero crossing makes it possible to ignore the inflection point of the excitation signal superimposed by the emission.

[0024] In Figure 5A signal diagram is shown to illustrate the difference in the decay behavior of a first-order process and a phosphor, the latter having undergone irreversible degradation.

[0025] The signal of the first phosphor, which obeys a first-order process, is described by a function for the afterglow according to I t = I 0 e − kt with I ( t ) Intensity I at the time t I 0 Saturation intensity at time t = 0 k Decay constant t Time e base of the natural logarithm

[0026] The signal of the second phosphor, which does not obey a first-order process due to degradation, is described by a function for the afterglow according to I t = ∑ i = 1 n I 0 e k i t with I ( t ) Intensity I at the time t iIndex over a number n different processes n number of processes I 0 Saturation intensity at time t = 0 ki Decay constant of process i from n t Time e base of the natural logarithm where this is a sum of first-order processes. Depending on the type of degradation, the saturation intensity can also vary according to It=∑i=1nI0,iekit with I ( t ) Intensity I at the time t i Index over a number n different processes n number of processes I 0 ,i Saturation intensity of process i from n at time t = 0 ki Decay constant of process i from n t Time

[0027] The signal curve of the sum of first-order processes can no longer be described by a decaying e-function. To measure the exact curve, the measurement lacks a clear regularity. Consequently, the cumulative curve cannot be determined by regression calculations of the results from a large number of individual measurements. The measurement accuracy required to characterize the cumulative curve is therefore very significant, since statistical methods for curve determination and smoothing may be lacking. The method according to the invention reduces the measurement effort to the temporal determination of zero crossings, with the zero crossings being generated by calculating the difference between two signals from the phosphors themselves.

[0028] In Figure 6A use of the security feature by marking with at least two phosphors with different decay behavior on an exemplary banknote 500 is shown. The security feature 100 is arranged where, on many banknotes 500, a classic watermark is arranged, which is visible against the light from both sides of the banknote 500.

[0029] In Figure 7 A use of the marking with at least two phosphors with different decay behavior is shown on an exemplary concert ticket 600. The marking is arranged where, on many concert tickets 600, a hologram is arranged as a security feature that is visible from one side.

[0030] In Figure 8The security feature is used by marking with at least two phosphors with different decay behaviors on an example medication container. Since many medication containers are colored but clear, the marking can only be viewed either from the front or only through the medication container.

[0031] In Figure 9 Finally, the use of the security feature by marking with at least two phosphors with different decay behavior is shown on an example label for a product, in this case a label for a modern shoe. Instead of the shoe, high-priced goods such as jewelry or watches are also possible. However, high-priced food products can also be marked with a security feature according to the invention on a label. LIST OF REFERENCE SYMBOLS

[0032] 500Object 600Object 700Object 800Object Δ(I(λ1,t),I(λ2,t))Difference signal Δ(I(λ1,t),I(λ3,t))Difference signal Δ(I(λ2,t),I(λ3,t))Difference signal eBase of the natural logarithm I(λ1,t)Persistence intensity I(λ2,t)Persistence intensity I(λ 1 )Emission band I(λ 2 )Emission band I(λ 3 )Emission band I 0 Saturation intensity I sin excitation signal k Decay constant ki Decay constant of process i out of n λWavelength nNumber of processes L1Phosphor L2Phosphor L3Phosphor vWavenumber SuLower peak SoUpper peak tTime t Δ0 (I(λ1,t), I(λ2, t))Time of zero crossing t Htd Half-life of the falling edge t Htu Half-life of the rising edge

Claims

1. A method for determining the authenticity of an object (500, 600, 700, 800), comprising the following method steps - marking the object (500, 600, 700, 800) with a first phosphor (L1) having a first temporal decay behaviour, - marking the object (500, 600, 700, 800) with a second phosphor (L2) having a second decay behaviour which differs from the first decay behaviour, - exciting of the phosphors (L1, L2) with a light pulse, - measuring the afterglow intensities (I(λ1,t), I(λ2,t)) of both phosphors (L1, L2) after excitation with the light pulse, characterised by - forming a difference (Δ(I(λ1,t), I(λ2,t))) or identity signal from the afterglow intensities (I(λ1,t), I(λ2,t)) measured over the elapsed time (t), - determining the time (tΔ0 (I(λ1,t), I(λ2, t))) of a zero crossing of the difference signal (Δ(I(λ1,t), I(λ2,t))) or the time (tΔ0 (I(λ1,t), I(λ2, t))) of the identity signal of a comparator, wherein in the latter case an identical signal level is determined by an identity signal of a comparator, - comparing the time (tΔ0 (I(λ1,t), I(λ2, t))) determined by the zero crossing or by the identity signal with a target value.

2. The method according to claim 1, characterised by marking the object (500, 600, 700, 800) with at least one additional phosphor (L3) having an additional decay behaviour which differs from the decay behaviours of the other phosphors.

3. The method according to claim 1 or 2, characterised by using at least one phosphor (L1, L2, L3), the decay behaviour of which corresponds by a linear combination of different first-order exponential decay behaviours according to I t = ∑ i = 0 n I 0 , i e − k i t wherein all ki are different from one another, with It afterglow intensity after excitation I0,i saturation intensity or of the initial intensity of process i among n processes. Ki decay constant of process i among n processes n number of processes t time e base of the natural logarithm.

4. The method according to one of the claims 1 to 3, characterised by an overlap of the emission bands (I(λ1), I(λ2), I(λ3)) of different phosphors (L1, L2, L3) of less than 20%, preferably of less than 5%, relative to the area under the respective normalised emission band (I(λ1), I(λ2), I(λ3)) when the emission band (I(λ1), I(λ2), I(λ3)) is plotted over the wave number v.

5. The method according to one of the claims 1 to 4, characterised by exciting the phosphors (L1, L2, L3) with a sequence of rectangular light pulses, wherein - the time interval (t1, t2, t3...tn) between two successive light pulses, measured between the half-value time (tHtd) of the falling edge of a previous light pulse and the half-value time (Htu) of an ascending edge of a subsequent light pulse, is greater than the time (tΔ0 (I(λ1,t), I(λ2, t))) of the zero crossing / identity signal to be expected from the decay behaviour of the used phosphors (L1, L2, L3) after excitation with the preceding light pulse, wherein the time interval between two successive light pulses is random or pseudorandom.

6. The method according to one of the claims 1 to 5, characterised by - exciting the phosphors (L1, L2, L3) with a regular, sinusoidal excitation signal (Isin), the sinusoidal curve of which has a lower peak (Su) of approximately zero, - determining the phase offset (Δφ) between the upper peak (So) of the excitation signal (φ0) and the zero crossing of the difference signal (tΔ0 (I(λ1,t), I(λ2, t))) or the identity signal of a comparator, with φλ1, λ2.

7. The method according to one of the claims 1 to 6, characterised by using phosphors (L1, L2, L3) whose half-life (t1 / 2) of all phosphors in relation to the decrease in luminous intensity after excitation with a light pulse is in the range of 1 ms to 1,000 ms, in the range of 1 µs to 1,000 µs or in the range of 1 ns to 1,000 ns.

8. The method according to one of the claims 1 to 7, characterised by using phosphors (L1, L2, L3) whose saturation intensity I0 is different.

9. The method according to one of the claims 1 to 8, characterised by using upconversion phosphors as phosphors (L1, L2, L3) which can be excited in the UV, VIS and / or IR range of the light spectrum and which luminesce in the UV range, in the VIS range and into the UV range.

10. The method according to one of the claims 1 to 7, characterised by using downconversion phosphors (L1, L2, L3) which can be excited in the IR range, in the VIS range and into the UV range and luminesce in the UV range, in the VIS range and / or in the IR range.