EP1265198B1 - Device and method for investigating documents - Google Patents

Description

The invention relates to a device for examining documents, in particular value, identity or security documents, with at least one excitation device for excitation of luminescent light in or on a document to be examined and at least two detector units for detecting at least a portion of the emitted from the document luminescence. The invention also relates to a corresponding method.

To increase anti-counterfeiting security, identity, security or value documents, such as Banknotes, provided with features or printed with suitable security inks containing luminescent substances. These are substances which are e.g. be excited by light, electric fields, radiation or sound to emit light. In the authenticity check, the documents to be checked are usually irradiated with light of a specific spectral range and the luminescent light emitted by the luminescent substances of the document is detected. Based on the intensity and / or spectral characteristics of the emitted luminescent light can then be determined whether the document is genuine or fake.

The reliability of statements about the authenticity of the tested documents is particularly dependent on the accuracy with which the spectral characteristic, ie the color of the luminescence is analyzed. Such an analysis can be done for example by spectrometers, which, however, require a relatively high technical complexity and high production costs. A simpler solution therefore represent individual detector units, such as photodiodes or photomultipliers, with different spectral sensitivity. Depending on the spectral characteristics of the luminescence, the detector units provide different Detector signals, which can then be used for the spectral analysis of the luminescence.

From the EP0083062 A2 a detector is known in which the detection beam path is guided by means of beam splitters on three detectors with different spectral sensitivity, which is achieved by different color filters.

However, devices of this type have the disadvantage that the luminescence light respectively detected by the individual detector units generally does not originate from exactly the same spatial subarea of the document due to parallax errors. This makes it impossible to reliably assess the color properties of the luminescent light emanating from a subregion of the document. This is disadvantageous in particular if partial regions with small dimensions are to be examined for their luminescence properties, since even slight parallax errors can lead to particularly great inaccuracies in the spectral analysis of the luminescent light.

Furthermore, from the US5965875 A a photodetector is known in which various detector units are integrated on the same substrate and arranged one behind the other, the spectral sensitivity of which is based on different levels of penetration of the light as a function of its wavelength. The disadvantage here is that the position of the pn junctions must be selected at a certain depth in the substrate and that one is spectrally limited to the sensitivity range of a single semiconductor material (silicon).

Furthermore, from the WO01 / 61654 A2 a device for the examination of value documents known in which two photodiodes with different Absorbent edge can be used to detect remission or transmission light of a value document. These photodiodes of different absorption edge are arranged one behind the other but not integrated on the same component.

It is an object of the invention to provide a device and a corresponding method which, with a simple construction, a higher reliability in the investigation of the luminescence of documents, in particular value, identity or security documents allow.

This object is achieved by the device according to claim 1 and the method according to claim 16. The invention is based on the idea that the detector units are arranged one behind another with respect to the direction of the luminescent light emitted by the document and striking the detector units. As a result, the luminescence light strikes one after the other on the successively arranged detector units and is thereby detected by them.

The inventive arrangement of the detector units ensures that all detector units arranged directly behind one another can detect the luminescence light emitted by a common spatial subarea of the document. Any parallax errors that would occur in a laterally staggered arrangement of detector units are greatly reduced by the inventive arrangement of the detector units in a row. Statements about the luminescence properties of the document can then be derived with high reliability from the spectral components of the luminescence light detected by the individual detector units.

In a preferred embodiment of the invention, it is provided that at least one first detector unit is permeable to that spectral subregion of the luminescence light which is to be detected by at least one second detector unit arranged behind the first detector unit. A first spectral subregion of the luminescent light is then detected by the first detector unit, while a second spectral subregion of the luminescence light can pass through the first detector unit and is detected by the second detector unit arranged behind it. In this case, the first detector unit acts as an optical filter in front of the second detector unit located behind it. For certain applications, therefore, it is usually possible to dispense with additional optical filters.

The detector units are preferably photodiodes, which are arranged one above the other in layers and in this case form a so-called sandwich diode. As a result, a very compact arrangement of the detector units is achieved.

In principle, the detector units may also be elements which emit light by means of other physical detection principles, e.g. can detect by means of avalanche effect.

In a further preferred embodiment of the invention, it is provided that the individual detector units are integrated on a common component, in particular a semiconductor component, which comprises at least two photosensitive layers, in particular pn junctions, wherein each layer, in particular each pn junction, each corresponds to a detector unit. Due to the small distance between the detector units, a particularly strong reduction of parallax errors is achieved in this embodiment.

The photodiodes or pn junctions preferably have different absorption edges, wherein the absorption edge of at least one first photodiode or of a first pn junction lies at smaller wavelengths than the absorption edge of at least one second photodiode arranged behind the first photodiode or one behind the first pn junction. Transition arranged second pn junction.

A particularly simple and reliable derivation of statements about the spectral properties of the detected luminescence light from the detector signals generated by the individual detector units can be carried out on the basis of a division of two detector signals and / or the difference of two logarithmic detector signals.

Fig. 1

einen bevorzugten Aufbau der erfindungsgemäßen Vorrichtung;

Fig. 2

eine erste Ausführungsform der erfindungsgemäß angeordneten Detektoreinheiten;

Fig. 3

a) und b) jeweils eine zweite Ausführungsform der erfindungsgemäß angeordneten Detektoreinheiten;

Fig. 4

Beispiele für spektrale Empfindlichkeiten der in Fig. 2 bzw. Fig. 3 dargestellten Detektoreinheiten; und

Fig. 5

ein Schaltbild der in Fig. 3 dargestellten zweiten Ausführungsform der erfindungsgemäß angeordneten Detektoreinheiten.

The invention will be explained in more detail with reference to embodiments shown in FIGS. Show it:

Fig. 1

a preferred construction of the device according to the invention;

Fig. 2

a first embodiment of the present invention arranged detector units;

Fig. 3

a) and b) each show a second embodiment of the present invention arranged detector units;

Fig. 4

Examples of spectral sensitivities of Fig. 2 respectively. Fig. 3 illustrated detector units; and

Fig. 5

a schematic diagram of in Fig. 3 illustrated second embodiment of the present invention arranged detector units.

Fig. 1 shows a preferred construction of the device according to the invention. A document to be examined, in the example shown a banknote 10, is transported past the sensor system 7 by means of a transport device indicated by transport rollers 40 and transport belts 41. In this case, the banknote 10 is irradiated with the excitation light 15 of the light sources 12. The light sources 12 are, for example, fluorescent tubes, incandescent lamps, lasers or LEDs which in each case emit light which is suitable for exciting luminescent light in or on the banknote 10. Preferably, the excitation light 15 is ultraviolet (UV) light. To eliminate spectral components at higher wavelengths, ie for example in the visible or infrared spectral range, corresponding filters (not shown) can be arranged in front of the light sources 12.

In the illustrated example, the excitation of luminescent light 16 in or on the document is effected by the light 15 of the light sources 12. A corresponding luminescence phenomenon is therefore referred to as photoluminescence. Alternatively or additionally, other types of luminescence phenomena, such as electromagnetic or electric fields, radiation or sound, may also be used. Electro-, radio- or sonoluminescence, are excited in or on the document. The excitation is effected by appropriate excitation means, e.g. electrical contacts or field plates, radiation sources for cathode, ion or X-rays, ultrasound sources or antennas.

In an alternative embodiment of the invention, it is provided that the excitation light 15 emitted by the respective light sources 12 lies at different wavelengths or wavelength ranges. The luminescent light 16 excited at different wavelengths or wavelength ranges permits even more precise statements about the luminescence properties of the banknote 10. In this case, provision may be made, in particular, for the light sources 12 to illuminate the banknote 10 either individually or in combination and to evaluate the luminescent light 16 detected in each case individually or in combination with the illuminated banknote 10. In the example shown, the FIG. 1 initially illuminated with only one light source 12, then detect the two detector units 1 and 2, a first intensity value pair. Upon subsequent illumination with the other light source 12, a second intensity value pair is generated. With simultaneous illumination with both light sources 12, a third intensity value pair is finally obtained. By comparison and / or computational linkage of the resulting, generally different, intensity values, a particularly accurate investigation of the luminescence properties of the banknote 10 under investigation is achieved.

Depending on the time decay behavior, it is possible to distinguish between phosphorescence or fluorescent light in the case of luminescence light. The device or the method according to the invention is equally suitable for the examination of phosphorescence and fluorescent light.

The luminescent light 16 excited in or on the banknote 10 is emitted by the banknote 10 and impinges on two detector units 1 and 2, which according to the invention are arranged one behind the other in such a way that the luminescent light 16 emanating from the banknote 10 is successively applied to the individual detector units 1 and 2, respectively meets and can be detected by them. The two detector units 1 and 2 each have different spectral sensitivities, so that in each case a different spectral component of the luminescent light 16 is detected. Accordingly, the detector signals S generated by the detector units 1 and 2 differ, which are supplied to an evaluation device 9 for evaluation and analysis.

Between the banknote 10 and the detector devices 1 and 2, an optical device 13 is provided in the illustrated example, which directs the luminescent light 16 emitted by the banknote 10 onto the detector units 1 and 2, in particular focused. This is preferably an imaging optic which images a subarea 11 of the banknote 10 onto the detector units 1 and 2. Self-focusing lenses, so-called Selfoc lenses, are preferably used for this purpose. Self-focusing lenses are cylindrical optical elements made of a material which has a refractive index decreasing from the optical axis of the cylinder towards its cladding. By using Selfoc lenses, one of the distance between the Banknote 10 and the detector units 1 and 2 independent and adjustment-free 1: 1 mapping of the subsection 11 of the banknote 10 to be examined on the detector units 1 and 2 achieved.

In front of the detector units 1 and 2, a filter 14 is arranged in this example, which filter is permeable to those spectral subregions of the luminescent light 16 which are to be detected by the detection units 1 and 2.

In Fig. 2 a first embodiment of the inventively arranged detector units is shown. The individual detector units are formed as photodiodes 1 and 2, respectively, and arranged one behind the other with respect to the direction of the luminescent light 16 emitted by the document. The individual photodiodes 1 and 2 each have a pn junction 3/4 or 5/6 between in each case a p-doped 3 or 5 and an n-doped 4 or 6 semiconductor layer. The doping profile is here shown greatly simplified and generally does not reflect the actual size ratios of the layer thicknesses. Spacers 8 are provided between the photodiodes 1 and 2 in order to avoid electrical short circuits. In order to minimize any parallax errors, the height of the spacers 8 should not be too large and be about the same order of magnitude as the height of the photodiodes 1 and 2. Optionally, also spaced with corresponding spacers 8, in front of the photodiode 1, a filter 14 may be arranged. In addition, it is also possible to provide a corresponding filter (not shown) between the individual photodiodes 1 and 2. With the electrical connections 17, voltages between the differently doped semiconductor layers 3/4 or 5/6 are tapped and forwarded as detector signals S to an evaluation unit (not shown).

In the FIGS. 3a and 3b in each case a second embodiment of the arrangement according to the invention is shown. FIG. 3a shows a device 20 on which the detector units 1 and 2 are integrated together, wherein the device 20 has two PN junctions 22/21 and 23/21, respectively, which correspond to a detector unit 1 and 2 respectively. The n-doped semiconductor layer 21 forms the substrate on which the two pn junctions 22/21 and 23/21 are applied in layers. The doping profile is here also shown greatly simplified and generally does not reflect the actual size ratios of the layer thicknesses. Analogous to the in FIG. 2 shown here voltages are tapped with suitable terminals 17 and forwarded as detector signals S to an evaluation unit (not shown).

FIG. 3b shows a variant of the second embodiment of the inventive arrangement. The illustrated device 30 comprises two layered pn junctions 32/33 and 34/33, which are applied to a common substrate 31. The substrate 31 itself may be a semiconductor or ceramic substrate. With regard to the operation of this embodiment, the explanations apply to FIG. 3a analogous.

The in the Figures 2, 3a and 3b Detector units 1 and 2 shown are selected so that the first detector unit 1 is permeable to that spectral portion of the luminescent light 16, which is to be detected with the disposed behind the first detector unit 1 second detector unit 2. The detector units 1 and 2 designed in particular as photodiodes or pn junctions in this case have different absorption edges, wherein the absorption edge of the first photodiode 1 or the first pn junction 3/4, 32/33 and 22/21 at smaller wavelengths lies as the second absorption edge of the arranged behind the first photodiode 1 and the first pn junction 3 / 4,32 / 33 and 22/21 second photodiode 2 and the second pn junction 5/6, 34/33 and 23rd / 21st

In the in FIG. 2 represented sandwich-like arrangement of the individual detector units 1 and 2 on top of each other, the respective pn junctions 3/4 or 5/6 are preferably realized on different semiconductor materials. Thus, for example, a photodiode based on silicon (Si) is used for the first detector unit 1 and a photodiode based on germanium (Ge) is used for the second detector unit 2. Wavelengths below about one micrometer can then be detected by the silicon-based photodiode 1, while wavelengths above about one micrometer can pass through this photodiode 1 and be detected by the germanium-based photodiode 2 located behind it. Analogously, photodiodes based on silicon and indium gallium arsenide (InGaAs) or silicon and lead sulfide (PbS) can be combined to detect the luminescent light 16 in two different spectral subregions. In addition, of course, the combination of several corresponding photodiodes is possible, for. As silicon, indium gallium arsenide and lead sulfide.

In the in the FIGS. 3a and 3b illustrated embodiments of the arrangement according to the invention, the different permeability or sensitivity of the detector units 1 and 2 is achieved by the selection of suitable semiconductor materials and / or a corresponding doping of the respective material. A corresponding component 20 or 30 can be realized for example on the basis of silicon, wherein the first pn junction 22/21 or 32/33 by a lower penetration depth for short-wave Light is particularly sensitive. By contrast, long-wave light can penetrate deeper into the layer system and be detected by the second pn junction 23/21 or 34/33, which is more sensitive in the long-wave spectral range.

In principle it is also possible, individual components 20 and 30 according to the in Fig. 2 illustrated embodiment to arrange one behind the other. With a suitable selection of the semiconductor materials used, the luminescent light 16 can be detected in more than two spectral subareas in a simple manner.

FIG. 4 shows an example of different spectral sensitivities E in the FIGS. 2 and 3 As can be seen from the diagram, the spectral sensitivity E1 of the first detector unit 1 is greatest in the region of short wavelengths λ, while the spectral sensitivity E2 of the second detector unit 2 arranged behind the first detector unit 1 at longer wavelengths λ reached its maximum. The respective spectral transmittances of the detector units 1 and 2 are complementary thereto. The spectral transmittance of the detector unit 1 is therefore greatest at higher wavelengths λ, so that the luminescent light can penetrate the detector unit 1 in this subregion of the spectrum and finally from the detector unit 2 can be detected.

FIG. 5 shows a circuit diagram in the FIGS. 3a or 3b illustrated second embodiments. The detector units 1 and 2, ie the corresponding pn junctions 22/21 and 23/21 and 32/33 and 34/33, of the device 20 and 30 are shown as oppositely connected in series photodiodes whose cathodes at a common potential 18 lie. The signals S1 and S2 are fed to an evaluation device 9 via the anode outputs 19 of the photodiodes. In the evaluation device 9, the signals S1 and S2 are logarithmically amplified in each case in a logarithmic amplifier 28 and then applied to a differential amplifier 29. Since the difference between two logarithmic values corresponds to the logarithm of the quotient of both values, the output voltage Ua of the differential amplifier 29 is proportional to the logarithm of the quotient of the two detector signals S2 / S1 and thus independent of the absolute intensity of the luminescence light 16. Statements can then be made from the output voltage Ua via the spectral properties, in particular the color, of the detected luminescent light 16 are derived with particularly high reliability.

The spectral properties of the luminescent light 16, in particular the wavelength, e.g. the central wavelength, and / or the wavelength range and / or the color, according to the invention not only in the visible spectral range, but also in invisible spectral ranges, such. in the infrared or ultraviolet, can be detected and analyzed.

As an alternative or in addition to the analog analysis described, it is also possible first to digitize the detector signals S1 and S2 and then to derive statements about the luminescent light in a digital, in particular computer-aided, evaluation from the digitized signals.

Claims (21)

1. An apparatus for examining documents, in particular value documents, identification documents or security documents, having

   - at least one excitation device for exciting luminescence light (16) in or on a document (10) to be examined, and

   - at least two detector units (1, 2) formed as photo diodes for capturing at least part of the luminescence light (16) emitted by the document (10),
   wherein the detector units (1, 2) are disposed one behind the other with respect to the luminescence light (16) emitted by the document (10),
   characterized in that

   - the photo diodes have different absorption edges and are integrated on a common component (20, 30), wherein the component (20, 30) comprises at least two p-n junctions (22/21, 23/21, 32/33, 34/33), wherein to each p-n junction (22/21, 23/21, 32/33, 34/33) there corresponds respectively one detector unit (1, 2), and

   - for the first detector unit a photo diode based on silicon is used and for the second detector unit a photo diode based on germanium is used or that photo diodes based on silicon and indium-germanium-arsenide (InGaAs) or on silicon and lead sulfide (PbS) are combined in order to detect the luminescence light in two different partial spectral ranges.
2. The apparatus according to claim 1, characterized in that the excitation device comprises at least one light source (12) for illuminating the document (10) with excitation light (15) that is suitable for exciting luminescence light (16) in or on the document (10).
3. The apparatus according to either of claims 1 to 2, characterized in that the detector units (1, 2) have different spectral sensitivities (E1, E2).
4. The apparatus according to any of claims 1 to 3, characterized in that at least one first detector unit (1) is permeable to at least a partial spectral range of the luminescence light (16) which partial range can be captured with at least one second detector unit (2) disposed behind the first detector unit (1).
5. The apparatus according to any of claims 1 to 4, characterized in that at least between two photo diodes at least one optical filter is disposed.
6. The apparatus according to any of claims 1 to 5, characterized in that the p-n junctions (22/21, 23/21, 32/33, 34/33) are formed in layers and applied to a common substrate (21, 31), in particular a semiconductor or ceramic substrate.
7. The apparatus according to claim 6, characterized in that the p-n junctions (22/21, 23/21, 32/33, 34/33) are disposed in layers one on the other with respect to the luminescence light (16) emitted by the document (10).
8. The apparatus according to any of claims 1 to 7, characterized in that the first absorption edge of at least one first photo diode or of at least one first p-n junction (22/21, 32/33) is at smaller wavelengths (λ) than the second absorption edge of at least one second photo diode disposed behind the first photo diode or of at least one second p-n junction (23/21, 34/33) disposed behind the first p-n junction (22/21, 32/33).
9. The apparatus according to any of claims 1 to 8, characterized in that an optical device (13) is provided for directing the luminescence light (16) emanating from the document (10) onto the detector units (1, 2).
10. The apparatus according to claim 9, characterized in that the optical device (13) comprises at least one lens, in particular a self-focusing lens, for focusing the luminescence light (16) emanating from the document (10) onto the detector units (1, 2).
11. The apparatus according to any of claims 1 to 10, characterized in that an evaluation device (9) is provided for deriving statements about the spectral properties, in particular the wavelength, such as, for example, the central wavelength, and/or the wavelength range and/or the color, of the detected luminescence light (16) from detector signals (S, S1, S2) generated by the detector units (1, 2).
12. The apparatus according to claim 11, characterized in that the evaluation device (9) comprises a logarithmic amplifier (28) for logarithmizing individual detector signals (S, S1, S2).
13. The apparatus according to either of claims 11 to 12, characterized in that the evaluation device (9) comprises a differential amplifier (29) for forming the difference between two detector signals (S, S1, S2) or between two logarithmized detector signals (S, S1, S2).
14. The apparatus according to any of claims 11 to 13, characterized in that the evaluation device (9) for deriving statements about the spectral properties, in particular the wavelength, such as, for example, the central wavelength, and/or the wavelength range and/or the color, of the detected luminescence light (16) is formed on the basis of

    - the division of two detector signals (S, S1, S2) and/or

    - the difference of two logarithmized detector signals (S, S1, S2).
15. The apparatus according to claim 2, characterized in that at least two light sources (12) are provided, wherein the excitation light (15) from the respective light sources (12) is at different wavelengths or in different wavelength ranges.
16. A method for examining documents, in particular documents of value, identification documents or security documents, wherein

    - a document (10) to be examined is excited to emit luminescence light (16), and

    - at least part of the luminescence light (16) emitted by the document (10) is captured by at least two detector units (1, 2) which are formed as photo diodes, disposed one behind the other with respect to the luminescence light (16) emitted by the document (10) and have different absorption edges and are integrated on a common component (20, 30), wherein the component (20, 30) comprises at least two p-n junctions (22/21, 23/21, 32/33, 34/33), wherein to each p-n junction (22/21, 23/21, 32/33, 34/33) there corresponds respectively one detector unit (1, 2), wherein for the first detector unit a photo diode based on silicon is used and for the second detector unit a photo diode based on germanium is used or photo diodes based on silicon and indium-germanium-arsenide (InGaAs) or on silicon and lead sulfide (PbS) are combined in order to detect the luminescence light in two different partial spectral ranges, wherein the luminescence light (16) impinges consecutively on the detector units (1, 2) disposed one behind the other and is captured by these thereby, and

    wherein the detector units (1, 2) generate detector signals (S, S1, S2) and from the detector signals (S, S1, S2) statements are derived about the spectral properties of the captured luminescence light (16).
17. The method according to claim 16, characterized in that the document (10) is illuminated with excitation light (15) that is suitable for exciting luminescence light (16) in or on the document (10).
18. The method according to either of claims 16 to 17, characterized in that the luminescence light (16) is captured by detector units (1, 2) with different spectral sensitivities (E1, E2).
19. The method according to any of claims 16 to 18, characterized in that at least a partial spectral range of the luminescence light (16) passes through at least one first detector unit (1) and is detected by at least one second detector unit (2) disposed behind the first detector unit (1).
20. The method according to any of claims 16 to 19, characterized in that statements about the wavelength, such as, for example, the central wavelength, and/or the wavelength range and/or the color, of the captured luminescence light (16) are derived from the detector signals (S, S1, S2).
21. The method according to claim 20, characterized in that the derivation of statements about the spectral properties, in particular the wavelength, such as, for example, the central wavelength, and/or the wavelength range and/or the color, of the captured luminescence light (16) is effected on the basis of

    - the division of two detector signals (S, S1, S2) and/or

    - the difference of two logarithmized detector signals (S, S1, S2).

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