EP3781408B1 - Smartphone-verifiable, luminescent-material-based security feature and assembly for verification - Google Patents

Description

The present invention relates to a security feature with a phosphor that can be verified using a commercially available smartphone. The invention also relates to an arrangement for verifying a security document with such a security feature.

It has long been known from the prior art to use security features equipped with luminescent substances (phosphors) for the protection and for the authentication of value and security documents. They are mostly used as so-called Level 2 characteristics. Their presence can be detected via the emission of the phosphors, which can be excited with simple hand-held devices (UV or IR radiation sources) and mostly occurs in the visible spectral range. They also serve as copy protection. On the other hand, luminescent security features equipped with a particularly high degree of security against forgery are also used as Level 3 features that can be read by machines. The authenticity verification of such features is usually associated with a high level of technical effort.

Both in the area of security and value documents and in the area of product protection, there is increasing interest in the use of authenticity features that have a high level of security (Level ²⁺ or Level 3 characteristics) but are not very technical have the effort checked.

That's it WO 2012/083469 A1 a device for authentication of marked with photochromic systems documents known. The photochromic security feature shows a color change and/or a change in shape under the action of a flash light excitation. It is also described that the security feature is based on a retinal protein.

From the DE 10 2015 219 395 A1 an identification feature with at least two identification elements arranged in a defined, limited area for identifying an object is known. Upon exposure of the surface to visible light, a first identification element made of ink becomes visually visible and a second identification element is not visually visible.

the EP 0 091 184 A1 names a sulfide phosphor that emits green to orange light with a long afterglow time. This phosphor is described by the general formula (Zn _1-x , Cd _x ) S:eM ^I' , fM ^III' , gX', where M ^I' is selected from copper and gold, M ^III' is selected from gallium and indium , and X' is selected from chloro, bromo, iodo, fluoro and aluminum.

the WO 02/071347 A1 describes a method and a device for the authentication of security documents or objects which have a luminescent compound. The compound is excitable by excitation light.

From the U.S. 7,079,230 B1 a method and an authentication device for detecting an authentication mark are known.

the EP 1 672 568 A1 describes counterfeit-proof security labels and an optical scanner for Illuminating, detecting and classifying a sample or the security label.

In the U.S. 2010/0102250 A1 describes a method and a device for the authentication of a photoluminescent security feature.

the U.S. 2016/0314374 A1 describes a device for a portable smart device and a method for authenticating an object to be authenticated.

From the US 2013/0153789 A1 is a luminescent material and articles with this material known. Furthermore, a method for authenticating the objects with the luminescent material is described. The material consists of a first particle of a first inorganic host lattice with at least one first substance and a second particle of a second inorganic host lattice with at least one second substance.

the WO 2018/007444 A1 describes a method for authenticating a security marking, as well as the security marking and a reader capable of carrying out the method. The security marking can emit a long-lasting luminescent light. The reading device with CPU unit and memory unit, for example a smartphone, is used to generate and record the emission.

the WO 2013/012656 A1 discloses various phosphor compositions comprising one or more emissive ions and one or more impurity ions. An emitting ion is defined by a first decay time constant im undisturbed state marked. The impurity ions result in a predetermined decay behavior with a modified decay time that is greater than zero and less than the unperturbed decay time. An authentication system is configured to measure the decay time when this phosphor composition is attached to an article.

the WO 2013/034471 A1 as well as the DE 10 2011 082 174 A1 describe an apparatus for recognizing a document having a phosphor-based security feature with so-called wavelength conversion properties. For this purpose, a light generating device (e.g. an LED flash unit) is provided, which irradiates the security feature with excitation light, and an image recording device (e.g. a digital camera of a mobile communication device), which is intended to record the light emitted by the security feature. However, it has been shown that the disclosed phosphors regularly have decay times that do not allow the emission to be evaluated using widely used devices, in particular an authenticity check using commercially available smartphones.

the WO 2013/034603 A1 describes a method for verifying a security document with a security feature in the form of a fluorescent printing element. The method provides that the printing element by means of a light source is excited and it emits electromagnetic radiation as a result of this excitation, which can be detected in a further step by means of a sensor. The recorded data is evaluated by comparing it with specified data. In a further step, the verification result is output depending on the result of the comparison. In particular, the method is to be carried out with a smartphone, the flash module of the smartphone being used as the excitation source and the photo sensor of the camera of the smartphone being used as the detection unit. Inorganic phosphors, namely nitride phosphors, are mentioned as phosphors for the pigment-like fluorescent printing element; europium-doped alkaline earth orthosilicate and alkaline earth oxyorthosilicate phosphors; cerium-doped rare earth aluminum gallium garnet phosphors; red light emitting (Ca,Sr)S:Eu ²⁺ ; and green light emitting SrGa ₂ S ₄ :Eu ²⁺ . The proposed phosphors are so-called LED conversion phosphors which decay extremely quickly. However, it has been shown that it is practically not possible to reliably detect the luminescence signals of these rapidly decaying phosphors directly during the flash light excitation, because the luminescence signals have a much too low intensity compared to the excitation light and are covered by the high-intensity stimulating flash light of the smartphone .

In the practical application of the phosphors described in the aforementioned prior art, two hitherto unsolved problems have emerged. These previously known phosphors are regularly used as so-called conversion phosphors for the production of white LEDs, so that these phosphors are also regularly contained as radiation-converting components in the flashing LEDs of commercially available smartphones.

This means that the excitation radiation from the smartphone flash units used as an excitation source has a high probability of having the same luminescence signals as are expected from the security feature to be examined. A reliable proof of authenticity of the value and security documents equipped with such security features is excluded for this reason alone.

A second problem results from the fact that the luminescent materials mentioned in the prior art for use as security features usually have just as short decay times in the ns to µs range as they also apply to flashing LEDs for the reasons mentioned . In the case of excitation with the flashlight of a smartphone, the emissions originating from the security feature are either completely superimposed by the flashlight or they have already decayed before the image is recorded.

Based on the prior art, one object of the present invention is to provide an improved luminescent-based security feature that can be verified solely using a smartphone or a comparable multifunctional, widely used data processing device. A further object of the present invention is to provide an arrangement for verifying such a security feature.

According to the invention, the object is achieved by a luminescent-based security feature that can be verified with a smartphone according to appended claim 1 and by an arrangement for verifying such a security feature according to appended independent claim 9.

A general idea for the solution to the stated task, which the invention implements, consists first of all in that a security feature is equipped with a specific phosphor that avoids the problems described above. For this purpose, this phosphor must be configured in such a way that it can be excited with a light source of a smartphone or a similar mobile data processing device, ie in particular a flashing LED of a smartphone. At the same time, the luminescent material must have such a luminescence characteristic that it is still possible to detect the luminescence signals with a high degree of certainty using the image acquisition unit of the same smartphone (mobile data processing device) even after the excitation process has ended. In addition to a high efficiency of the spectral excitability and a high luminescence yield, this primarily requires a decay time of the phosphor according to the invention that is adapted to the exceptional speed of the image acquisition unit of the smartphone.

The invention provides a security feature that can be reliably evaluated, which allows exclusive luminescence properties such as the spectral emission and decay characteristics of the special phosphors used to create the security feature to be included in the test as authenticity criteria. In addition, it is of great advantage that the decaying luminescence signals of the security feature according to the invention are not visible to the human eye either during or after the end of the excitation. It has been shown that the options for providing suitable phosphors for realizing the inventive solution shown are severely limited. This applies in particular to the required decay characteristics.

The invention provides a level 3 feature or at least a feature with level 2+ functionality, as can be used in security and value documents for authenticity verification. Such features are generally invisible to the human eye, for example even after excitation with UV or IR light sources. Until now, their characteristics could only be tested with great technical effort, for example with the help of high-speed sorting machines. The present invention makes it possible for the first time to check the authenticity of such exclusive features using commercially available smartphones.

The security feature according to the invention can be applied to or in a valuable or security document and comprises a luminescent material that can be excited to luminescence with electromagnetic radiation of a predetermined wavelength, such as can be generated by a lighting unit of a smartphone, whereupon the luminescent material is emitted by the camera device a smartphone emits detectable radiation. The emission of the phosphor has a decay time in the ms range. According to the invention, the decay times are selected in the range between 1 and 100 ms, preferably in the range between 5 and 50 ms, again preferably between 10 and 30 ms.

The decay process (decay process) basically characterizes the time-dependent decrease in the intensity of the radiation emitted by a phosphor. The decay curve can often be described with a simple exponential equation of the form I = I ₀ ^e-(t/τ) . The decay constant τ contained therein denotes the length of time up to which the intensity of the emission has fallen by 36.79% (=1/e-th) of the output intensity after the excitation source has been switched off is. However, it has been shown that not all luminescent substances exhibit a mono-exponential decay. Rather, multi-exponential (e.g. bi- or tri-exponential) decay curves may also result from the superimposition of different relaxation processes.

Ce ³⁺ - and Mn ²⁺ - co-doped silicate garnet phosphors (CSS) have proven to be a particularly suitable class of phosphors for the security feature, with the formula: Ca ₃ Sc ₂ Si ₃ O ₁₂ : Ce ³⁺ , Mn ^{2 +}
can be described. Such phosphors are characterized by a high absorption strength at 450 nm, a high luminescence intensity and an efficient energy transfer between the Ce ³⁺ and the Mn ²⁺ ions.

According to an alternative notation, the phosphor can be written with the formula:

(Ca _1-x Ce _x ) ₃ (Sc _1-z Mn _z ) ₂ Si ₃ O ₁₂

be described, whereby on the basis of the known ionic radii in the technical literature it is often assumed that the Ce ³⁺ ions are preferentially installed on Ca ²⁺ and the Mn ²⁺ ions preferentially on Sc ³⁺ lattice positions.

In experimental investigations, it was surprisingly found that phase-pure, highly efficient and particularly stable Ca ₃ Sc ₂ Si ₃ O ₁₂ :Ce ³⁺ , Mn ²⁺ phosphors are formed above all when it is assumed when calculating the starting material weights that the Mn ²⁺ ions are incorporated into the lattice at both Ca ²⁺ and Sc ³⁺ sites. Particularly good results are obtained when the weight-in calculations are based on the assumption that about 75% of the Mn ²⁺ coactivators replace the Ca ²⁺ and about 25% the Sc ³⁺ ions as lattice components.

• mit 0<x≤0,1; 0<y≤0,8; 0<z≤0,8
• wobei ein y/z Verhältnis von ≈ 2 bevorzugt ist.

The phosphor according to the invention is particularly preferred by the following
general chemical formula can be described:

(Ca _1-xy Ce _x Mn _y ) ₃ (Sc _1-z Mn _z ) ₂ Si ₃ O ₁₂

• with 0<x≤0.1;0<y≤0.8;0<z≤0.8
• with a y/z ratio of ≈ 2 being preferred.

Taking the stoichiometric factors into account, this corresponds to the specified ratio for the occupation of the Ca ²⁺ or Sc ³⁺ lattice sites by Mn ²⁺ coactivator ions.

With the Ca ₃ Sc ₂ Si ₃ O ₁₂ :Ce ³⁺ ,Mn ²⁺ materials described, particularly advantageous phosphors are provided according to the invention which have emissions with decay times between 5 and 30 ms and whose luminescence signals are highly reliable even after the flash light excitation has ended can be detected using the camera modules of commercially available smartphones.

The emission spectra of the phosphors according to the invention each consist of three bands, the direct luminescence of the Ce ³⁺ activator ions (band with a λ _max of about 505 nm), and the Ce ³⁺ - Mn ²⁺ energy transfer enabled emissions of the different lattice site positioned Mn ²⁺ coactivators. The maxima of the last-mentioned emission bands are around 570 nm (Mn ²⁺ on Ca ²⁺ place) and around 700 nm (Mn ²⁺ on Sc ³⁺ place).

The relative intensities of the different emission bands can be determined via the concentrations of the activator and coactivator ions and via the respective concentration ratios be varied and adjusted. In addition, the individual emissions have different spectral decay times. While the decay time of the quantum-mechanically permitted Ce ³⁺ emission is in the nanosecond range, decay times for the two Mn ²⁺ emission bands resulting from quantum-mechanically forbidden optical transitions are in the single-digit (Mn ²⁺ on Ca ²⁺ place) or in the two-digit millisecond range (Mn ²⁺ on Sc ³⁺ place) achieved.

The fact that the emission bands found in the green spectral range with maxima at around 505 and 570 nm overlap to a pronounced extent due to their comparatively large half-widths, also leads to an overlapping of the decay curves of these emissions. Nevertheless, the modified CSS phosphors are characterized by different spectral decay curves with different decay times. On the other hand, this constellation means that if the entire visible spectral range is detected in the decay measurements, a significant color shift of the decaying luminescence of the phosphors results. Furthermore, it has been shown that the individual emission bands do not exhibit any mono-exponential decay curves due to their characteristic superimpositions. Rather, bi- or tri-exponential decay curves are characteristic.

The special decay behavior described makes a major contribution to the exclusivity of the Ca ₃ Sc ₂ Si ₃ O ₁₂ :Ce ³⁺ ,Mn ²⁺ phosphors according to the invention. There are also other properties that recommend these phosphors for use in luminescent security features, the presence and authenticity of which can be verified using commercially available smartphones. On the one hand are the named phosphors practically not excitable in the ultraviolet spectral range and on the other hand the body color of the corresponding luminescent pigments is such that it can be easily matched to the color design of the security and valuable documents to be protected (banknotes, identity cards, passports, driver's licenses, etc.) or adapted to the production of these Documents used inks can be covered. This means that the presence of the CSS:Ce ³⁺ ,Mn ²⁺ phosphors incorporated into the security documents as a security feature cannot be recognized by the viewer either with the naked eye or with the aid of conventional UV excitation sources. On the other hand, it is a special embodiment of the invention that, with UV excitation, effectively luminescent, rapidly decaying phosphors are added to the phosphors with delayed decay behavior that emit almost exclusively in the visible spectral range and can preferably be excited at 450 nm. The stationary photoluminescence of the corresponding additional components, which is clearly perceptible with UV excitation, can serve as a security-enhancing masking of the security features integrated into the security documents.

Even if the options for providing the phosphors required for the realization of the inventive solution shown are severely limited, there are a few other materials in addition to the Ca ₃ Sc ₂ Si ₃ O ₁₂ :Ce ³⁺ ,Mn ²⁺ phosphors that are due to their decay behavior could be used to produce security features, but do not fall within the scope of the claims. The table below lists some of the phosphor compositions tested for their suitability according to the invention and those not according to the invention, including those for the inventive Application relevant luminescence properties compiled. connection _λem [nm] Decay time τ [ms] luminescence yield Excitability at 450 nm Ce ³⁺ /Eu ²⁺ -,Mn ²⁺ - coactivated compounds high Ca ₃ SC ₂ Si ₃ O ₁₂ : Ce ³⁺ , Mn ²⁺ 470 - 900 5-30 strong _Ca3Sc2Ge3O12 ^: _{Ce3 +} , _Mn2 ⁺ 470 - 850 5-15 weak Sr ₃ Sc ₂ Ge ₃ O ₁₂ : Ce ³⁺ , Mn ²⁺ 470-800 3-15 moderate Ca ₉ MgNa(PO ₄ ) ₇ : Eu ²⁺ ,Mn ²⁺ 500-750 ~ 26 weak - moderate _BaMg2Si2O7 : _{Ce3 +} , ^Mn2 + , ^{Dy3 +} 570 - 800 5-10 weak - moderate _BaMg2Si2O7 ^: _{Mn2 +} , ^Dy3 ₊ 570 - 800 ~ 5 weak _CaSiO3 : ^Eu2 + , ^Mn2 + 550 - 700 10-20 weak - moderate _Ca2P2O7 : _{Eu2 +} , ^{Mn2 +} 400 - 660 10-20 weak Cr ³⁺ - activated compounds high _ZnGa2O4 : _Cr3 + , ^{Bi3 +} 650 - 770 5-10 moderate _Zn3Ga2SnO8 : _{Cr3 +} , ^{Bi3 +} 660 - 820 1-5 moderate Mn ⁴⁺ activated compounds high BaGeF ₆ : Mn ⁴⁺ ~ 634 ~ 6 moderate _K2SiF6 : _Mn4 ⁺ ~ 631 ~8 moderate - strong

In particular, the table contains information on the measured maxima of the respective emission bands and on the decay times. Verbal scaling was used to assess luminescence yield and spectral excitability at 450 nm.

The phosphors listed are essentially Ce ³⁺ - and Mn ²⁺ - co-doped silicate garnets or germanate garnets, activated with Mn ²⁺ ions and possibly additionally with certain rare earth ions (Ce ³⁺ , Eu ^{2 +} , Dy ³⁺ ) co-activated complex silicate or phosphate basic lattices, around Cr ³⁺ - activated gallate compounds and around the Mn ⁴⁺ - activated phosphors BaGeF ₆ :Mn ⁴⁺ and K ₂ SiF ₆ :Mn ⁴⁺ .

This list is not final.

In this context, it is to be regarded as extremely advantageous to modify phosphors that appear to be suitable by deliberately changing their chemical composition, i.e. by deliberately made substitutions in the cation and/or anion partial lattice, in such a way that their luminescence properties, in particular their characteristic decay times, change significantly differ from the data described in the technical literature. In this way, the exclusivity of the luminescent materials with a delayed decay and the counterfeit security of the corresponding security features can be significantly increased.

A further embodiment of the invention is characterized in that phosphor mixtures are used to create the security features, the individual, preferably exclusive components of which have different and sensorically distinguishable decay times. In this case, too, there is an increase in the protection against forgery of the security features according to the invention.

The phosphor preferably has a decay time in the one-digit or two-digit ms range, so that the emission of the phosphor can be detected with an image acquisition unit, in particular with a camera of a smartphone. Currently known smartphone cameras have an image frequency in the range of 240 fps (frames per second) up to 960 fps. Higher frame rates are conceivable, above all, in future devices, but this makes it possible to use the invention described here not opposed. With the frame rates currently known, the smartphone camera can record the first image after around 4.2 ms or, in exceptional cases, after 1 ms.

The image frequency of the image sensor used (especially a smartphone camera) determines a lower limit for the decay time of the phosphors that can be used within the meaning of the invention. In the event that the security feature should not be recognized by the human eye in terms of realizing a particularly high security level, an upper limit is specified by the physiology of human vision. In particular, the decay time of the phosphor should be less than 1 s in this case, since an afterglow of the phosphor that lasts longer than 1 s can be perceived by a normal human observer.

• 505 nm (zuordenbar der Emission von Ce³⁺-Ionen auf dodekaedrischen Ca²⁺-Plätzen),
• 570 nm (zuordenbar der sensibilisierten Emission von Mn²⁺-Ionen auf dodekaedrischen Ca²⁺-Plätzen),
• 700 nm (zuordenbar der sensibilisierten Emission von Mn²⁺-Ionen auf oktaedrischen Sc³⁺-Plätzen).

Die spektralen Abklingzeiten der unterschiedlichen Emissionsbanden liegen in der aufgeführten Reihenfolge im ns-, im einstelligen bzw. im zweistelligen ms-Bereich.According to the invention, the phosphor is a Ce ³⁺ or Mn ²⁺ co-doped silicate garnet phosphor. When excited with the light of white-emitting LEDs, preferably at a maximum wavelength of 450 nm, the stationary luminescence of the phosphor has a broadband emission spectrum with several emission maxima in the visible spectral range. These maximums are around

• 505 nm (assignable to the emission of Ce ³⁺ ions on dodecahedral Ca ²⁺ sites),
• 570 nm (assignable to the sensitized emission of Mn ²⁺ ions on dodecahedral Ca ²⁺ sites),
• 700 nm (assignable to the sensitized emission of Mn ²⁺ ions on Sc ³⁺ octahedral sites).

The spectral decay times of the different emission bands are in the listed order in the ns, in the one-digit or in the two-digit ms range.

The decay time of the phosphor of the security feature is preferably in the range from 1 ms to 50 ms. The phosphor of the security feature particularly preferably has a decay time of 10 ms to 30 ms.

So that the security feature can only be detected using a smartphone, the phosphor is configured in such a way that it can be excited in the visible spectral range, in particular in the blue spectral range, so that the flash light source of the smartphone can deliver this excitation radiation. Furthermore, the phosphor is configured in such a way that it emits in the visible spectral range in order to ensure that it can be detected with the camera module of a commercially available smartphone. In addition, the phosphor is configured in such a way that its luminescence decays in the ms range after the flash excitation is completed, so that reliable verification is possible after the excitation has ended.

The white light of a lighting unit of a smartphone is generated by an LED, which consists of an LED semiconductor chip emitting at approximately 450 nm, for example, and one or more LED conversion phosphors placed above the LED semiconductor chip. These conversion phosphors are capable of converting the emission of the blue LED proportionately into longer-wave visible luminescence radiation (broadband emissions in the green, yellow and red spectral range) with an emission maximum of around 560 nm, for example. The white light of the LED available as the lighting unit of commercially available smartphones results from the additive color mixture of the individual luminescence components described, with the blue spectral component having a significantly higher intensity. That means that the The phosphor that can be used to provide the security feature according to the invention must preferably be configured in such a way that it has a high efficiency of spectral excitability, particularly in the range between 420 nm and 470 nm. The maximum of the spectral excitability of the phosphor is particularly preferably at about 450 nm.

The smartphone camera is available as an image acquisition unit for detecting the luminescence signals of the phosphor. The image acquisition unit is preferably equipped with a CMOS sensor and an IR filter. It therefore has a spectral sensitivity that covers the entire visible spectral range up to about 750 nm. Single images, image series or video recordings can be recorded by means of the image acquisition unit. For the phosphor used to create the security feature, this means that it must be configured in such a way that, after the excitation has taken place, it emits with the highest possible intensity, preferably in the spectral range between 480 nm and 750 nm.

The mobile terminal device used according to the invention for verifying the security feature is preferably a conventional smartphone. It is understandable for the person skilled in the art that the same functionality can also be integrated into a tablet or a similar multifunctional data processing device, for which purpose it must be equipped with a camera with an image acquisition unit and/or lighting unit and a data processing unit. Devices of this type that have the same effect should also be included in the invention. The data processing unit is preferably a processor, in particular a microprocessor.

The phosphor is preferably arranged in the security feature in such a way that it forms a pattern. The phosphor pigments are preferably applied to a carrier as a defined pattern. The pattern can be arranged as a shape such as a triangle or a star. Alternatively, the pattern of the security feature formed by the phosphor itself can contain data and be arranged as a code, for example a QR code. The luminescent pigments are printed as a security feature, for example on a security document. The printing or application can be done using known printing methods such as gravure printing, flexographic printing, offset printing or screen printing. Furthermore, the phosphor can be applied to the security document or introduced into the security document by coating processes or lamination processes. The grain size distribution of the phosphor pigments is preferably adapted to the respective printing and application process.

The security feature, in particular the phosphor, preferably has high processing stability. In particular, the phosphor has high thermal and mechanical stability. The phosphor preferably has high aging resistance to environmental influences. Stability and resistance to aging are required in order to ensure that the security feature can be reliably verified over the entire life cycle of the security document.

An advantage of the security feature according to the invention, which comprises a phosphor, can be seen in the fact that the security feature can be excited by means of a smartphone flash light due to the specially configured luminescence characteristics of the phosphor and its emission can be detected by the smartphone camera, which is a simple, enables quick and user-friendly verification of the security feature. An authenticity check and/or integrity check can be carried out. It has proven to be advantageous to select a special phosphor with decay times in the ms range for providing the security feature, the luminescence signals of which can still be reliably measured even after the end of the excitation process. The verification advantageously does not relate exclusively to the proof of the presence of the security feature, it also includes the emission spectrum, the specific form of the decay curve (decay characteristic) and the pattern formed by the luminescent pigments in the authenticity test as authenticity criteria. Another advantage of the security feature is that it cannot be visually perceived by humans.

The arrangement according to the invention comprises a security feature according to the invention in one of the embodiments described above, which is attached to a valuable or security document or incorporated into a valuable or security document. In addition, the arrangement includes a smartphone, which includes an illumination unit, an image acquisition unit and a data processing unit.

It has been found that an advantageous solution for being able to reliably measure the decaying luminescence of the phosphors after the end of the excitation process is to use a combination of individual flashes and serial or video recordings as the detection method, with the duration of the serial or video recordings must clearly surpass that of the stimulating flashlight.

At the same time, the exposure time must be adapted to the decay time of the phosphor used. In the last recording, ie in the last frame, the emission intensity of the phosphor should be zero, just as it was before the flash light was excited. This frame can then be used as a reference for calculating the image differences (B ₁ -R; B ₂ -R;... B _n -R). The analysis of the image differences, the contrast adjustment to be made and the consideration and inclusion of other methods for image analysis (histogram analysis of the different color channels) can be regarded as an essential prerequisite for not only proving the presence of a selected inventive phosphor with the help of the smartphone, but also at the same time verify spectral emission and exclusive decay characteristics.

Furthermore, it has been shown that it is extremely advantageous to keep the distance between the smartphone and the security feature to be checked as small as possible when verifying the authenticity of the security document. In this way, the intensity of flash light excitation can be increased and the disturbing influence of extraneous light can be significantly reduced. In particular, the distance between the detection device and the security document can be selected to be smaller than the shaft adjustment range of the smartphone; sharp images are not required for the exception and verification of the diffuse luminescence signals.

For example, the smartphone must be configured with an app in such a way that at least the following steps are carried out to verify the security feature:
In a first method step, the security feature is excited to luminescence by means of the lighting unit of the smartphone, preferably by triggering a single flash of the LED flash light module, so that the security feature emits electromagnetic radiation in the visible spectral range.

In a second method step, parallel to the single flash excitation, the decaying luminescence signals of the phosphor of the security feature according to the invention occurring after the end of the excitation are detected by means of the image acquisition unit, ie with the aid of the camera module of the smartphone.

In a further method step, the luminescence characteristics in the captured images are evaluated by the data processing unit and compared with reference data in order to verify the security feature and to confirm the authenticity of the security document.

Fig. 1

eine schematische Darstellung einer Ausführungsform eines erfindungsgemäßen Sicherheitsmerkmals auf einem Sicherheitsdokument in Gestalt einer Banknote;

Fig. 2

eine schematische Darstellung von Komponenten einer erfindungsgemäßen Anordnung zur Verifikation des Sicherheitsmerkmals;

Fig. 3

eine schematische Darstellung des An- und Abklingverhaltens eines Leuchtstoffes des Sicherheitsmerkmals bei Blitzlichtanregung;

Fig. 4

ein Ablaufplan zur Durchführung der Verifikation des Sicherheitsmerkmals mit der erfindungsgemäßen Anordnung;

Fig. 5

ein Anregungsspektrum der 700 nm Emissionsbande eines erfindungsgemäßen Leuchtstoffes gemäß Ausführungsbeispiel 1;

Fig. 6

ein Emissionsspektrum der bei 450 nm angeregten stationären Photolumineszenz des Leuchtstoffes gemäß Ausführungsbeispiel 1;

Fig. 7

spektrale Abklingkurven der unterschiedlichen Emissionsbanden eines im Ausführungsbeispiel 1 beschriebenen erfinderischen Leuchtstoffes;

Fig. 8

ein Farbshift der über den gesamten sichtbaren Spektralbereich detektierten, abklingenden Lumineszenz des Leuchtstoffes gemäß Ausführungsbeispiel 1, dargestellt anhand des zeitlichen Verlaufes der x-y-Farbkoordinaten in der CIE- Normfarbtafel;

Fig. 9

Emissionsspektren der stationären Photolumineszenz der bei 450 nm angeregten Leuchtstoffe gemäß der Ausführungsbeispiele 2 und 3;

Fig. 10

Abklingkurven der Hauptemissionsbanden der bei 450 nm angeregten Leuchtstoffe gemäß der Ausführungsbeispiele 2 und 3;

Further details, advantages and developments of the invention result from the following description of preferred embodiments of the invention, with reference to the drawing. Show it:

1

a schematic representation of an embodiment of a security feature according to the invention on a security document in the form of a bank note;

2

a schematic representation of components of an inventive arrangement for verification of the security feature;

3

a schematic representation of the rise and fall behavior of a phosphor of the security feature when excited by a flash light;

4

a flow chart for carrying out the verification of the security feature with the arrangement according to the invention;

figure 5

an excitation spectrum of the 700 nm emission band of a phosphor according to the invention according to exemplary embodiment 1;

6

an emission spectrum of the excited at 450 nm stationary photoluminescence of the phosphor according to embodiment 1;

7

spectral decay curves of the different emission bands of an inventive phosphor described in example 1;

8

a color shift of the decaying luminescence of the phosphor detected over the entire visible spectral range according to exemplary embodiment 1, illustrated using the time profile of the xy color coordinates in the CIE standard color table;

9

Emission spectra of the stationary photoluminescence of the phosphors excited at 450 nm according to exemplary embodiments 2 and 3;

10

Decay curves of the main emission bands of the phosphors excited at 450 nm according to exemplary embodiments 2 and 3;

1 shows a security feature 01 according to the invention, which is symbolized on a value document, namely a illustrated security document 02 is applied in the form of a bank note. The security feature serves to prove the authenticity of the security document 02. The security feature 01 has the shape of a star here. It is positioned below a visible feature 03, in this case the denomination of the banknote. The security feature 01 consists of a phosphor that can be excited by the lighting unit of a smartphone, preferably in the blue spectral range, to luminescence that decays in the ms range, as disclosed above in the context of the description of the invention.

2 shows a schematic representation of an arrangement for verifying the security feature 01, the security feature being excited to luminescence by means of a lighting unit 04 of an image recording unit 06 of a mobile terminal device, namely a smartphone 07, in that the lighting unit 04 excites light, in particular white LED flashlight 08 with a spectral maximum of about 450 nm. The flashlight 08 has an intensity I _A . During the excitation, the phosphor of the security feature 01 emits stationary electromagnetic radiation in the visible spectral range, which decays in the ms range after the end of the excitation. The decaying emission I _E of the phosphor is detected with a camera 09 of the image recording unit 06 of the smartphone 07 by triggering a series or video recording. Furthermore, the camera 09 operating as a detector detects ambient radiation I ₀ of daylight or room light that impinges on the security feature 01 and the bank note 02 and is reflected there. The influence of the ambient radiation I ₀ can be kept small in the method according to the invention by keeping a distance d between the security feature 01 and the smartphone 07 small. Due to the small distance d, which is preferably below the focus range of the image recording unit 06, the smartphone 07 shields the ambient radiation I ₀ for the most part. This is because sharp images are not required for reliable verification of the diffuse luminescence signals of the security feature.

3 shows a schematic representation of the rise and fall behavior of the phosphor used in security feature 01. An emission curve 11 of the security feature 01 excited to luminescence is shown in the diagram along a time axis t. Furthermore, a flash light excitation curve 12 is plotted along the time axis. If the single flash is fired using the smartphone 07 ( 2 ) is generated, the LED flashlight excitation curve 12 rises steeply, holds its level for a short time and then falls to zero in the ns to μs range. The phosphor of the security feature 01 is excited to luminescence by the electromagnetic radiation of the flashlight, with its emission curve 11 rising almost simultaneously with the flashlight excitation curve 12 . After the excitation of the flashlight 12 has ended, the emission of the phosphor 11 decays significantly more slowly than the stimulating radiation of the smartphone's lighting unit, which is preferably equipped with white-emitting LEDs. According to the invention, the decay time of the phosphor is in the ms range.

Below the timeline are in 3 individual through the detector 09 of the smartphone 07 ( 2 ) captured images 13 of the security feature 01 are shown. The images 13 show the decaying emission intensity of the security feature 01 based on the brightness of the star pattern used as an example, which decreases over time. After the essentially complete decay of the emission of the phosphor a reference image 14b can be recorded as the last image of the recorded image sequence. Depending on the evaluation method, an additional reference image 14a (starting image) can also be recorded before the activation of the excitation radiation (triggering of the flash). To ensure the availability of a reference image required for calculating the image differences, a start image 14a can optionally be recorded as an additional reference image before the series or video recordings that are decisive for the detection of the decaying luminescence signals of the security feature are triggered.

4 shows in a simplified form the basic process of verifying the security feature 01 using the in 3 arrangement shown. In a positioning step 41, the secure document to be verified is positioned in such a way that it can be securely captured by the smartphone's image capture unit. In an optional reference check step 42, the start image 14a of the security feature is already generated before the triggering of the flash light excitation of the smartphone. In a detection step 43, a single flash is triggered with the help of the image recording unit of the lighting unit of the smartphone and a serial image or video recording is carried out in order to capture the luminescence signals of the phosphor used to create the security feature that are present after the end of the flash light excitation and decay in the ms range to record. Finally, in an emission analysis step 44, the recorded series of images and the reference recordings are compared using the data processing unit. In addition to the calculation of the image differences and their analysis, other methods of image processing such as contrast adjustment and histogram analysis are used of the different color channels is applied in order to verify in this way both the spectral emission and the exclusive decay characteristics of the phosphor used according to the invention. The authenticity of the checked security document can be confirmed in a release step 45 by comparing the calculated parameters with the authenticity parameters of the security feature that are preferably stored in the data memory of the smartphone. In particular, the authenticity and integrity of the security document can be confirmed by the verification of the security feature on the security document.

figure 5 shows an excitation spectrum 121 of the 700 nm emission band of a phosphor according to embodiment 1. To produce this phosphor, 0.2822 g CaCO ₃ , 0.5335 g Sc ₂ (C ₂ O ₄ ) ₃ ·10.723H ₂ O, 0.1803 g SiO ₂ , 0.0052 g CeO ₂ , and 0.0358 g MnC ₂ O ₄ .2H ₂ O are completely homogenized using a mortar with the addition of acetone. After evaporating the solvent, the dry powder mixture is transferred to a corundum crucible. The sample is first precalcined in a chamber furnace at 500° C. for 2 h in an air atmosphere and then annealed in a tube furnace at 1400° C. for 4 h in a 5% H ₂ /95% N ₂ atmosphere. The resulting product is then sieved. This phosphor has the formula (Ca _2.82 Ce _0.03 Mn _0.15 )(Sc _1.95 Mn _0.05 )Si ₃ O ₁₂ . The excitation spectrum makes it clear that the exemplary inventive phosphor has a maximum spectral excitability in the range from 440 to 450 nm.

6 shows a corresponding emission spectrum 111 of the phosphor according to exemplary embodiment 1 at 450 nm excitation. It is found that the phosphor specially configured via the phosphor composition and the selected preparation conditions has broadband emissions over the has the entire visible spectral range. Three emission bands with maxima at about 505 nm, 570 nm and about 700 nm are visible, the band with a maximum of about 700 nm having the highest relative intensity. As already described, these bands can be attributed to the direct luminescence of the Ce ³⁺ activator ions (Ce ³⁺ on Ca ²⁺ site), as well as the emissions made possible by the Ce ³⁺ - Mn ²⁺ energy transfer of the Mn ^{2 +} Coactivators (Mn ²⁺ on Ca ²⁺ -place or Mn ²⁺ on
Assign Sc ³⁺ place).

7 shows the spectral decay curves of the individual emission bands. Curve 1311 is the decay curve for the 505 nm emission, curve 1312 is the decay curve for the 570 nm emission, and curve 1313 is the decay curve for the 700 nm emission. It can be clearly seen that the spectral decay curves for the individual emissions differ significantly. As already explained, a decay in the nanosecond range is determined for the emission with a maximum of approximately 505 nm, while the luminescence bands with maxima of approximately 570 and approximately 700 nm have decay times in the single-digit and double-digit millisecond range. In addition, it is apparent to the person skilled in the art that the individual decay curves are very unlikely to run exponentially. Rather, the measured curves appear to have multi-exponential decay characteristics.

8 illustrates the color shift that results when the decaying luminescence is detected over the entire visible spectral range. The 8 first a schematic representation of a CIE standard color table 15 of the CIE standard valence system. The CIE standard valence system was Defined in 1931 in order to establish a relation between human color perception and the physical causes of the color stimulus and typically records all perceptible colors, whereby the color perception refers to that of a defined normal observer. Each color or emission spectrum of a self-luminous object is represented by a single xy coordinate in the CIE table of norms. The color coordinates of the luminescence signals measured integrally as a function of the decay time are shown in FIG 8 140 to 147 elements shown with the reference numerals. At the same time, the data determined for a phosphor according to exemplary embodiment 1 can be taken from the following table. Reference sign Cooldown in ms color coordinates x y 140 0 0.385 0.528 141 1 0.515 0.472 142 10 0.521 0.465 143 20 0.534 0.453 144 30 0.554 0.432 145 40 0.577 0.407 146 50 0.595 0.385 147 60 0.603 0.374

The color shift, which tends to lead from the green to the red spectral range, results from the superimposition of the 6 emission bands shown and from the differences and the superimposition of the corresponding in the 7 illustrated decay curves of the phosphor according to the invention according to embodiment 1. The special decay behavior described contributes to a large extent exclusivity of the Ca ₃ Sc ₂ Si ₃ O ₁₂ :Ce ³⁺ , Mn ²⁺ phosphor according to the invention.

9 shows the emission spectra 1123, 113 of the stationary photoluminescence of the phosphors excited at 450 nm according to exemplary embodiments 2 and 3. 10 shows the associated decay curves 132, 133 of the main emission bands of the phosphors excited at 450 nm according to exemplary embodiments 2 and 3.

To produce the phosphor according to Example 2, 0.2898 g CaCO ₃ , 0.1362 g Sc ₂ O ₃ , 0.1803 g SiO ₂ , 0.0130 g Ce(NO ₃ ) ₃ .6H ₂ O, 0.0179 g MnC ₂ O ₄ .2H ₂ O and 1.8170 g of tris(hydroxymethyl)aminomethane are completely dissolved in a mixture of 10 ml of nitric acid and 100 ml of water with stirring and heating on a hot plate. The liquid is then evaporated until the remaining gel ignites and a black foam forms. This foam is first dried at 150 °C in a drying cabinet, then finely ground and transferred to a porcelain crucible. In a first heating step, the mixture is annealed for 2 h at 1000° C. in the air atmosphere of a chamber furnace in order to decompose the remaining organic residues. The annealed material, which now has a white body colour, is then mixed with two percent by mass of boric acid and annealed again this time for 4 hours at 1300° C. in a 5% forming gas atmosphere. The resulting phosphor has the composition (Ca _2.895 Ce _0.03 Mn _0.075 ) (Sc _1.975 Mn _0.025 )Si ₃ O ₁₂ . The curve 112 in the 9 shows the emission spectrum of this phosphor. In the 10 the curve 132 designates the decay curve for this phosphor, which preferably emits in the green spectral range.

To produce the phosphor according to Example 3 with the composition (Ca _2.745 Ce _0.03 Mn _0.225 ) (Sc _1.925 Mn _0.075 )Si ₃ O ₁₂ 0.2747 g CaCO ₃ , 0.1327 g Sc ₂ O ₃ , 0.1803 g SiO ₂ , 0.0130 g Ce(NO ₃ ) ₃ .6H ₂ O, 0.0537 g MnC ₂ O ₄ .2H ₂ O and 1.8170 g tris(hydroxymethyl)aminomethane with stirring and heating in a mixture of 10 ml of nitric acid and 100 ml of water. The liquid is then evaporated until the resulting gel ignites. The resulting black foam is dried in a drying cabinet at 150 °C, then finely ground and transferred to a porcelain crucible. After an initial two-hour annealing at 1000 °C in the air atmosphere of a chamber furnace and the subsequent addition of two percent by mass of boric acid to the cooled annealed material, another four-hour thermal treatment takes place at 1100 °C in a 5% forming gas atmosphere. The emission spectrum of the phosphor obtained, measured at 450 nm excitation, is shown in curve 113 of FIG 9 shown, the associated decay curve is the curve 133 of 10 refer to.

The two exemplary embodiments and the associated figures once again clearly show that the Ca ₃ Sc ₂ Si ₃ O ₁₂ :Ce ³⁺ , Mn ²⁺ phosphors are a particularly suitable class of phosphors for forming a security feature according to the invention. By varying the phosphor composition and the preparation conditions, numerous exclusive phosphor compositions can be created with different decay behavior and distinguishable emission spectra and, for this reason, with a markedly high level of security and authenticity. The exclusive properties of the phosphors that can be used to protect valuable and security documents in the form of security features can be reliably verified with the help of commercially available smartphones.

Reference List

01

security feature

02

Security document / banknote

03

face value

04

lighting unit

05

-

06

image acquisition unit

07

smartphone

08

flashlight

09

camera / detector

10

-

11

emission curve

12

Flash excitation curve

13

Image recording of the security feature 01

14a

splash screen

14b

reference image

15

CIE standard color chart

41 - 45

process steps

111

Emission spectrum of the phosphor according to Example 1

112

Emission spectrum of the phosphor according to embodiment 2

113

Emission spectrum of the phosphor according to embodiment 3

121

Excitation spectrum of the phosphor according to embodiment 1

1311

Decay curve for the 505 nm emission of the phosphor according to embodiment 1

1312

Decay curve for the 570 nm emission of the phosphor according to embodiment 1

1313

Decay curve for the 700 nm emission of the phosphor according to embodiment 1

132

Decay curve of the predominantly green emission of the phosphor according to embodiment 2

133

Decay curve of the predominantly green emission of the phosphor according to embodiment 3

140-147

x-y color coordinates of the decaying integral luminescence of the phosphor according to embodiment 1

Claims (11)

1. A security feature that can be verified by a smartphone, the security feature having a luminescent material which can be excited to luminescence by visible electromagnetic radiation generated by the smartphone, and which exhibits radiation that can be detected by an image detection unit of the smartphone, wherein

   the luminescent material is selected from the following group:
   
           - (Ca_1-x-yCe_xMn_y)₃(Sc_1-z/Mn_z)₂Si₃O₁₂;
   
   with 0<x≤0.1; 0<y≤0.8; and 0<z≤0.8; and y/z≈2;
   
           - Ca₃Sc₂Si₃O₁₂:Ce³⁺, Mn²⁺;
   
   and wherein

   after the excitation is over, the luminescent material exhibits radiation over a decay time in the range of 1 ms to 100 ms.
2. Security feature according to claim 1, characterized in that after the excitation ends, the luminescent material has a decay time of 1 ms to 50 ms, preferably a decay time of 10 ms to 30 ms, in which the emission of the luminescent material in the visible spectrum has a luminescence characteristic that can be detected by the image detection unit of the smartphone.
3. Security feature according to Claim 1 to 2, characterized in that the luminescent material can be excited to luminescence by a flashing LED of the smartphone that emits white light.
4. Security feature according to any one of Claims 1 to 3, characterized in that during the decay time, the luminescent material exhibits radiation in the spectral range between 480 nm and 750 nm.
5. Security feature according to any one of Claims 1 to 4, characterized in that the luminescent material is configured as a luminescent material mixture, the luminescent material components of which have different decay times after the excitation ends, which can be distinguished by sensors.
6. Security feature according to Claim 5, characterized in that the luminescent material mixture comprises a luminescent material that luminesces under UV excitation and decays quickly, the stationary, visually perceptible photoluminescence of which acts as a mask for the emission of the luminescent material serving as a security feature.
7. Security feature according to any one of Claims 1 to 6, characterized in that the luminescent material is formed as luminescent material pigments that can be processed in a printing method.
8. Security feature according to claim 7, characterized in that the luminescent material pigments represent a predetermined pattern in the security feature.
9. Arrangement for verifying a security document (02), comprising:

   - a security feature (01), which is applied to the security document (02), contains a luminescent material that can be excited to emission, and is designed according to any of Claims 1 to 8,

   - a smartphone (07) having an illumination unit (04) for exciting the luminescent material of the security feature, having a camera (09) for detecting the emission of the luminescent material during a predetermined decay time after the excitation ends by recording an image sequence, and having a data processing unit for evaluating the image sequence, wherein the radiation detected during the decay time is compared with stored reference values in order to verify the security document.
10. Arrangement according to Claim 9, characterized in that an application (App) is installed on the smartphone (07), which application controls the illumination unit (04), the camera (09), and the data processing unit.
11. Arrangement according to either of Claims 9 or 10, characterized in that the distance between the smartphone and the security document to be checked is selected to be less than or equal to the focus range of the smartphone.

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