EP2710564B1 - Method and apparatus for verifying security documents using white light interferometry - Google Patents

Description

The invention relates to a method for the verification of security documents and a verification device for security documents which carry out and use white light interferometry in order to verify security features present in a security document.

A large number of features are known in the prior art which are used to secure documents in such a way that they cannot be produced, falsified or manipulated in an unauthorized manner. Security features can thus be all features which make duplication, unauthorized production, falsification or other types of manipulation of a document or object difficult, impossible or at least make these undesirable actions detectable during a precise examination.

Security documents are all documents that have at least one feature that makes duplication, imitation, falsification or other manipulation difficult or impossible. Security documents include, for example, passports, ID cards, driver's licenses, access cards, visas, but also labels for high-quality products, such as software, entrance tickets, but also bank cards, credit cards, phone cards or the like, as well as documents that embody a value, such as stocks, securities, banknotes, Postage stamps, customs stamps and more, to name just a few examples.

The most varied of features can be used as security features. For example, the material composition of a document can serve as a security feature. As an example, reference should be made here to security papers, such as those used, for example, in ID or banknote printing. The different printing processes, which in part or in combination are difficult to imitate or rework for a forger or manipulator, can also serve as security features. In the production of security documents, almost all common printing processes, for example intaglio printing processes, letterpress printing processes, but also inkjet printing processes or offset printing processes, are used in their various forms. These usually show one Typical characteristic print image, which can be distinguished from the print images generated by other printing processes, at least during a microscopic examination.

Diffractive structures, relief structures, special colors and the like are used as further security features. In particular in the case of modern security documents which comprise a document body made from one or more plastic materials, a large number of different security features can be integrated into a security document. With technical progress, which makes it possible to integrate new and more complex security features that are more difficult to forge, especially in security documents produced in large series, such as banknotes or identity papers, the ability of counterfeiters to imitate and / or falsify security elements also increases. It is therefore necessary to create ever more sophisticated methods and devices for verifying security features which enable a reliable differentiation between genuine security documents and counterfeit or falsified security documents. Such a procedure is called verification.

From the WO 2010/142392 A2 a method for identifying and / or authenticating objects on the basis of their surface properties and a sensor for scanning a surface are known.

From the EP 0 892 971 A1 a device for validating sheets, in particular bank notes, is known. A self-service payment terminal is described. Each bank note is individually guided along an interferometer, which is connected to a control unit. The output of the control unit is used as input for a data processing device which generates a value which indicates the surface roughness of the bank note. The value of the roughness is compared with at least one stored reference value for real bank notes. The data processing device decides exclusively on the basis of this comparison or in connection with the output of other validity checking means whether the examined bank note is invalid.

From the WO 2010/001165 A1 For example, an authentication device and an authentication method are known which authenticate an object as a function of determining whether a section of the object has one or more predetermined features. The predetermined features include either the thickness of the portion of the object or the thickness of one or more layers within the portion of the object or both thicknesses, which are determined by means of an optically based measuring apparatus. The examined objects can, for example, be a security document or a page of a security document.

From the WO 2006/123341 A1 a so-called hyper-spectral imaging and analysis system for authenticating authentication objects is described. Such a system comprises (a) a lighting unit for generating and sending electromagnetic radiation onto authentication tokens of the authentication object to form a number of illuminated authentication tokens; (b) a hyper-spectral imaging unit for optically detecting the affected energy or the emitted beam emanating from the illuminated authentication tokens and for generating optical shapes of hyper-spectral images of the illuminated authentication tokens; (c) a hyper-spectral image conversion unit for converting the optical forms of the hyper-spectral images into corresponding electronic forms of hyper-spectral images; and (d) a central programming and control / data information signal processing unit.

Shoude Chang et al. describe in the article "Full-field optical coherence tomography used for security and document identity", in Optics and Photonics for Counter-Terrorism and Crime Fighting II, edited by Colin Leweis and Gari P. Owen, Proc of SPIE Vol. 6402, 28 September 2006 (2006-09-27), page 64020Q, DOI: 10.1117 / 12.692733 the application of full-field optical coherence tomography in the examination of security documents.

Stifter describes in " Beyond biomedicine: a review of alternative applications and developments for optical coherence tomography ", Applied Physics B; Lasers And Optics, Springer, Berlin, Vol. 88, No. 3, August 10, 2007 (2007-08-10), pages 337 -357 , the Development and alternative application of optical coherence tomography outside of biomedicine.

The US 6,584,214 B1 describes the verification of objects using three-dimensional structural features as the basis for a physical hash function.

Different verification methods and verification devices are known from the prior art, which are usually adapted to special security features and are provided for their verification. It is desirable to create new verification methods and verification devices that can be used for a large number of security features and / or security documents in order to verify these with regard to their authenticity and authenticity.

The technical problem is solved by a verification method that uses white light interferometry to examine a surface structure and an internal structure of documents non-destructively. A verification can be carried out with a verification device which comprises a white light interferometer which is designed to create a depth profile of the security document for at least several positions on a surface of a security document. This does not necessarily have to extend over an entire extent along an examination direction of the security document. This means that a depth profile does not have to penetrate the entire document. This is for example, in the case of documents which comprise opaque layers or metallically reflective layers, it is not possible using white light interferometry.

Following this basic idea, a method for verifying a security document according to claim 1 is provided.

Such a method can be implemented with a verification device according to claim 7.

Definitions

Any optical interferometric device which interferes with broadband light with spatial coherence and evaluates this interference is regarded as a white light interferometer. It is irrelevant here whether this light spectrum is completely or partially arranged in the visible wavelength range, infrared wavelength range or UV wavelength range.

Measured values of a physical variable that are assigned to different depths along an examination direction, the examination direction extending into the interior, ie the volume, of a body, for example a document, are regarded as depth profiles. As a rule, a depth profile begins outside or on the surface of the object into which the examination direction or route extends. A depth profile is a position on a surface of the examined object, for example a security document, assigned. The position is that on the surface at which a straight line indicating the direction of examination intersects the surface.

Depth values are the distances along an examination direction along which a depth profile is created, based on a reference value. The point of intersection of a straight line indicating the direction of examination with the surface of the document support or any other surface perpendicular to the direction of light propagation in the measuring arm can be used as the reference value.

The direction of examination is the direction along which the light strikes the surface of the object to be examined. Local unevenness of the surface is not taken into account.

The measurement information derived from the interference values assigned to a depth value is referred to as the intensity value.

Deriving a feature is understood to mean deriving a quantity or any abstract or mathematical object or construct, for example a vector of intensity values or a group of depth value-intensity value tuples. The totality of depth values and intensity values or a selection thereof, like a value resulting from a statistical evaluation, represents a derivation of a feature.

A change in intensity value is a value that is assigned to a depth value of a depth profile and which is determined via a comparison with one or more intensity values that correspond to spatial areas of the examined object that are adjacent to the area determined by the position and the depth value (as well as the uniform examination direction) is defined for all jointly recorded depth profiles. In the simplest embodiment, the change in intensity value is determined by a comparison with the intensity value which is or is specified for the adjacent depth value or the adjacent depth values in the same depth profile. In other embodiments, the intensity values of neighboring surface positions and the same or neighboring depth values can also be taken into account.

Preferred Embodiments

The invention according to its basic idea offers the advantage that structures occurring on the surface or in the interior of a security document, which can occur transversely to the surface at different depths of the document, can be verified. For example, it can be determined in which plane of a document body made of transparent plastic material information that is perceptible as blackening for a human observer is stored. Here, different methods by which the information can be stored in the document body differ with regard to the spatial configuration of the information-bearing components of the security document. If the information is, for example, printed on a substrate layer which is then laminated with further substrate layers to form a document body, the information is located in the vicinity of an at least originally existing layer boundary. If, on the other hand, the information is introduced via a laser marking process, a larger volume area is usually colored via a partial carbonization of the plastic material. The depth profile shows the depth at which the information is marked, so that a printing process can be distinguished from a laser marking process. Laser marking processes are sometimes also referred to as laser engraving processes.

According to the invention, the depth values at which intensity values or changes in intensity values, which are each determined by comparing an intensity value with the intensity value of an adjacent spatial area in the security document or by comparing the intensity value with the intensity values of adjacent spatial positions in the security document, are above a threshold value or within a an upper threshold value and a lower threshold value of a limited range of values occur, evaluated with regard to their frequency in order to determine deviations and / or out-of-match results from one or more expected statistical distributions and to derive the verification decision therefrom. If, for example, the intensity values that characterize a certain layer transition or are typical for a certain printing process are evaluated in this way, an indication is obtained of the depths at which this type of transition or information introduced in this way by printing technology can be found. In the case of a manipulated document, such a depth value distribution is for one Layer transition, for example, broadened because the layer transition occurs at different depths due to the manipulation.

In order to minimize any diffraction effects that may occur on the document surface, particularly in the case of security documents that have a transparent or partially transparent document body produced on a plastic basis, the security document is preferably oriented so that the direction of examination, which is determined by the white light of the white light interferometer impinging on the security document , is oriented perpendicular to a surface of the security document. The surface here is the examination surface, which is generally the surface of the security document that has the largest two-dimensional extent. In the case of a verification device, such an orientation can be achieved simply in that the document receptacle is designed in such a way that a security document arranged on or in it is automatically oriented with its surface perpendicular to the direction of examination. For example, the document receptacle can be designed as a support surface which, for example, has almost no Absorption in the wavelength range having, transparent, plane-parallel plate is formed, on which the security document is placed flat with its surface to be examined or pressed. Pressing on ensures that the security document rests optimally on the support surface along its entire or a larger partial area of its surface. This is particularly advantageous for security documents which are deformed during use.

In a preferred embodiment, a white light interferometer comprises a light source, a beam splitter, a detector, a reflector mounted on a controllable linear actuator and a control and data acquisition device, the light source generating broadband light having spatial coherence and being arranged with respect to the beam splitter in such a way that the beam splitter guides part of the light into a measuring arm in which the object holder is located, and guides part of the light into a reference arm in which the reflector is arranged in such a way that it reflects the light back onto the beam splitter and is superimposed there with the light which is reflected back to the beam splitter on a security document arranged in or on the document receptacle, the detector being arranged in such a way that it interferes with the reflected light of the reference arm when the reflected light of the measuring arm is superimposed can detect nzsignal, wherein the control and detection device is coupled to the linear actuator in order to vary a reference arm length via a linear displacement of the reflector during the detection of the interference signal, the reference arm lengths corresponding to measuring arm lengths corresponding to the distances from the beam splitter along the examination direction which correspond at least to the surface of the security document on and in the document receptacle or into the interior of the document receptacle. A reference arm length of the interferometer is thus varied by varying the reflector position. The light in the measuring arm strikes the surface of the object to be examined, ie the security document, and also at least partially penetrates the security document. Both on the surface and along the path of propagation of the light along the direction of examination in the security document, portions of the examination light in the measuring arm are reflected back into the interferometer, depending on the nature of the security document or the features contained therein. Depending on the length of the reference arm, which is determined by the position of the reflector (for example a reference mirror), it is determined which portions of the light that are along the Examination direction in the security document are reflected back into the interferometer, lead to a constructive interference at the detector. The reflector position in the reference arm thus determines the depth that is scanned by the resulting interference. However, since it is not monochromatic light, but rather broadband light with spatial coherence that is used very specifically, the resulting interference pattern is much more complex. However, computing algorithms are known from the prior art which make it possible to derive a depth profile along the examination direction from the interference signals obtained one after the other, which are recorded during the variation of the reference arm length. Such algorithms are also used in optical coherence tomography (OCT). Optical coherence tomography (OTC) is known in particular from the field of ophthalmology and is used there, for example, to examine the retina.

The white light interferometer is thus preferably designed as an optical coherence tomograph.

According to the invention, very much more precise information or more complex verifications are possible than when evaluating only one depth profile, since depth profiles are determined for several locations on the surface of the security document. In a preferred embodiment, the depth profiles are detected at the same time for the multiple locations that are arranged together along a route on the surface of the security document, or for the multiple locations that are arranged in a surface area of the surface of the security document. With a suitable configuration, it is possible to achieve that a light beam of broadband, spatial coherence having light that is widened along one direction is sent to the beam splitter and illuminates the security document along a line, ie a path, on the surface of the security document and thus across the Surface, preferably perpendicular to the surface, a depth profile is recorded at the same time at the illuminated locations. For this it is necessary that the detector comprises a light-sensitive detection element for each of the locations to be detected in order to be able to detect the interferences assigned to the individual locations and to be able to derive the corresponding depth profiles for the individual locations. If the light is expanded along two spatial directions, it is possible to record the depth profiles for locations in an area at the same time. In this case, the detector doesn't just need one linear array of photosensitive sensors, but a flat arrangement of photosensitive detection elements, which can each detect the interference signal for the corresponding locations in the area during the variation of the reflector position.

In order to be able to carry out a verification, it is advantageous if at least one cross-sectional area of the security document determined from the detected depth profiles is displayed on the display device, which is spanned by the examination direction and the route or a contour lying in the surface area. In principle, it is possible to form one axis of the cross-sectional area by any arrangement of positions on the surface of the security document and to arrange the corresponding depth profiles next to one another in order to form a cross-sectional area. As a rule, however, it makes sense that the depth profiles shown next to one another in the cross-sectional area also correspond to points that are arranged spatially adjacent to one another on the surface. One advantage of such a cross-sectional area representation, which for example shows a cross-section through a volume area inside the document transversely to the surface of the document, is that layers or layer transitions present in the document can be recognized, for example. Modern security documents are often assembled from several substrate layers in a lamination process. It is not possible in all cases to carry out this lamination process in such a way that there is a monolithic document body in which the original layer boundaries in the solid body are no longer detectable as phase transitions, even by measurement.

In the case of security documents in which such a monolithic configuration in the document body does not succeed, these layer boundaries, which are not perceptible to a human user when viewed optically, can nevertheless be detected by different intensity values in the depth profiles. Such a cross-sectional area thus shows deviating intensity values or intensity value changes at the layer boundaries, so that these layer boundaries can be clearly recognized in the cross-sectional representation. If such a document has now been split for a manipulation in order to change, for example, passport photo information or other information which, for example, identify a person to whom the security document is assigned, then the split Layers usually show significant deviations in the layer structure, which is visible in the cross-sectional area derived by white light interferometry. A visual check of such a cross-sectional area representation thus enables verification personnel to easily find documents on which manipulations have been made. Different printing processes can be identified in a similar way. If, for example, a letterpress process is used, pinches occurring during printing can be seen on the edge of the embossed letters or characters. A pressure layer produced by means of high pressure thus shows characteristic height profile properties that can be derived from the cross-sectional profile. The same applies to printing elements produced by intaglio printing.

In modern security documents which are produced on a plastic basis, for example, inkjet printing processes are used, the ink of which is produced on the basis of the plastic material from which the substrate layers are made to which the inkjet print is applied. These printing layers have the advantage that they can also be implemented over a large area without creating a possibility of delamination on the printed area, since the ink material optimally bonds with that of the adjacent substrate layers during the lamination step. However, the colorants used do not necessarily remain on the printed surface, depending on the specific composition of the printing ink used, but instead diffuse in a targeted manner into the substrate layer to which the print is applied. When examining the depth profile, printing areas generated by means of such an inkjet printing method can thus be clearly distinguished from other printing areas.

Other manipulations associated with an interim split or delamination of a document body and / or possibly a planing off and / or new insertion of material, insertion of changed or falsified other security elements, such as metallized foils or patches of a different type or holograms, in connection are recognizable with the proposed verification method, since when joining and / or inserting, as a rule, the same method parameters cannot be used that were used in the original document production to connect the different layers to one another. Therefore occur in the places where Manipulations have been made, changes in the cross-sectional profile that are clearly perceptible, for example in the form of cracks in otherwise parallel layer transitions.

In order to enable a machine evaluation, depth profiles or cross-sectional areas formed from these can be stored as specifications in a database or a memory as specifications. A verification is possible by comparing the determined depth profiles and cross-sectional profiles with the specifications.

According to the invention it is provided that the intensity values and / or intensity value changes corresponding to individual depth values or depth ranges are statistically evaluated with regard to frequency in order to determine a deviation from an expected statistical distribution as an indication of manipulation or forgery. For example, a region adjacent to a layer transition in the document can be selected as the depth region in which, in the case of a security document that has not been manipulated, no intensity values characterizing the transition and / or intensity value changes occur. Since the layer transition or associated characteristic intensity values or changes in intensity values also occur in the case of manipulated security documents with depth values that do not exactly correspond to the depth of the layer transition, in such an evaluation, for example in a forged security document, intensity values that indicate a layer transition are generated with a greater frequency than in Original documents found. It is also possible to evaluate the depth region of the layer transition and to find an accumulation of intensity values or changes in intensity values in the region of the layer transition in a manipulated security document that do not correspond to a layer transition.

The various statistical evaluation methods mentioned can also be evaluated and / or used in combination.

In a further embodiment it is provided that in at least one section, for example a depth value range, of the at least one cross-sectional area, the depth values to which intensity values of a value range are assigned, or depth values to which intensity value changes of a value range are assigned, with respect to their assigned positions along the at least one route or curve can be approximated by a predetermined parameterized function of the position and the verification decision is made on the basis of the parameters derived during the approximation. If recesses are made in a security document as a security feature, for example in a boundary layer arranged in the interior of the finished document, before they are joined, which modify the surface profile in a cutting plane and are then filled in such a way that the finished security document does not contain any hollow volumes Cross-sectional area has a characteristic line that follows the contours of the cutouts and corresponds to the boundary edge. A function can then be adapted to this line, which can optimally approximate this cutting line course. In the prior art, approximation methods are well known which vary the parameters of a function in such a way that the best possible correspondence is achieved between the experimentally determined values, here the depth values, the given layer line course and the parameterized function. A verification can then be carried out in a simple manner on the basis of the parameters, which then characterize, for example, a position, shape and depth of the recesses made.

For a human perception of the different intensity values, it is advantageous if these are represented on the display surface using different color values, for example. A display device, which is coupled to the evaluation device, on which a cross-sectional area formed from the at least one depth profile and further depth profiles for further positions on the surface of the security document can be displayed, is preferably designed so that it offers such a colored representation. It is noted that the colors are not correlated with the colors with which the security features are possibly stored in the security document. They only serve to make it easier to distinguish between the different intensity values determined.

Fig. 1

eine schematische Darstellung einer Verifikationsvorrichtung;

Fig. 2

eine schematische Darstellung eines Ausschnitts eines Schichtaufbaus eines Sicherheitsdokument, in welches Informationen mittels eines Tintenstrahldruckverfahrens eingebracht sind;

Fig. 3a, 3b

schematische Darstellungen zur Erläuterung eines Sicherheitsmerkmals, welches über eine eingebrachte und anschließend aufgefüllte Ausnehmung ausgebildet ist;

Fig. 4

eine schematische Darstellung einer aus Tiefenprofilen abgeleiteten Querschnittsfläche für ein Sicherheitsdokument mit einer Struktur nach Fig. 3b;

Fig. 5

eine schematische Darstellung einer aus Tiefenprofilen eines Sicherheitsdokuments abgeleiteten Querschnittsfläche, welches das Merkmal nach Fig. 3b nicht aufweist;

Fig. 6

eine schematische Darstellung einer statistischen Auswertung für ein Dokument mit nicht modifizierten parallelen Schichten, bei der die Häufigkeiten der Intensitätswerte eines Intensitätswertebereichs gegenüber ihren zugeordneten Tiefenwerten darstellt sind;

Fig. 7

grafische Darstellung einer entsprechenden statistischen Auswertung analog zu der nach Fig. 6 für ein Sicherheitsdokument mit einer Ausgestaltung analog zu der nach Fig. 3b;

Fig. 8

Darstellung einer anderen statistischen Auswertung eines nicht gefälschten Sicherheitsdokuments;

Fig. 9

eine schematische Darstellung einer statistischen Auswertung analog zu der nach Fig.8 für ein gefälschtes Sicherheitsdokument; und

Fig. 10

schematische grafische Darstellung einer Auswertung, bei der die Tiefenwerte, eines Tiefenwertebereichs gegenüber den zugeordneten Positionen auf der Oberfläche aufgetragen sind und eine parametrisierte Funktion der Position an die Tiefenwerte angepasst ist.

The invention is explained in more detail below on the basis of preferred exemplary embodiments with reference to a drawing. Here show:

Fig. 1

a schematic representation of a verification device;

Fig. 2

a schematic representation of a section of a layer structure of a security document into which information is introduced by means of an inkjet printing process;

Figures 3a, 3b

schematic representations to explain a security feature which is formed via an introduced and then filled recess;

Fig. 4

a schematic representation of a cross-sectional area derived from depth profiles for a security document with a structure according to FIG Figure 3b ;

Fig. 5

a schematic representation of a cross-sectional area derived from depth profiles of a security document, which the feature according to Figure 3b does not have;

Fig. 6

a schematic representation of a statistical evaluation for a document with unmodified parallel layers, in which the Frequencies of the intensity values of an intensity value range are shown in relation to their assigned depth values;

Fig. 7

graphic representation of a corresponding statistical evaluation analogous to that according to Fig. 6 for a security document with an embodiment analogous to that according to Figure 3b ;

Fig. 8

Representation of another statistical evaluation of a non-forged security document;

Fig. 9

a schematic representation of a statistical evaluation analogous to that according to Fig. 8 for a forged security document; and

Fig. 10

schematic graphical representation of an evaluation in which the depth values, a depth value range are plotted against the assigned positions on the surface and a parameterized function of the position is adapted to the depth values.

In Fig. 1 a verification device 1 is shown schematically. This includes a white light interferometer 2. The white light interferometer 2 includes a light source 3 which emits broadband light 4 with spatial coherence. In the embodiment shown, the light 4 is first expanded via an optical element 5. This then strikes a beam splitter 6. The beam splitter allows part of the light 4r to enter a reference arm 7, at the end 8 of which a mirror 9 is located as a reflector. This is movably mounted on an actuator 10. The actuator 10 can displace the mirror 9 linearly so that a length I _{r of} the reference arm can be varied.

At the beam splitter 6, a further part of the light 4m is directed into a measuring arm 11. In the measuring arm 11, a security document 13 is arranged as a measuring object on a document receptacle 12 designed, for example, as a glass plate. The light 4mR reflected back into the measuring arm 11 on the surface 15 and in the volume 16 inside the security document 13 is superimposed on the beam splitter 6 with the light 4rR reflected back by the mirror 9 from the reference arm 7 and guided to a detector 14. In the embodiment shown, the detector 14 has a plurality of light-sensitive sensor elements (not shown).

The beam guidance for three positions P1, P2, P3 on a surface 15 of the security document 13 is shown as an example. The light 4mR reflected back at the surface positions P1, P2, P3 or inside the security document 13 along the directions of propagation of the light 4m of the measuring arm 11 impinging on the surface 15 is superimposed on a sensor element of the detector 14 with the corresponding light 4rR reflected back in the reference arm 7 and recorded as an interference signal. The length I _{r of} the reference arm 7 is varied during the measurement. Corresponding measuring arm lengths I _{m of} equal length extend from the surface 15 into the volume 16 of the security document 13. The time-resolved interference signals are fed to a control and data acquisition device 17 which, based on the measured values of each measuring element, creates a depth profile for the corresponding position P1, P2, P3 determined along examination directions 19-1 to 19-3. The examination directions 19-1 to 19-3 are determined by the direction of the light 4m at the corresponding positions P1 to P3. It goes without saying that, depending on the illumination of the document 13 and a resolution of the detector 14, depth profiles for more positions along a path 20 or, if the light 4 is widened over an area, for positions distributed over an area can be detected and evaluated at the same time. The control and data acquisition device 17 controls the actuator 10 during the acquisition of the measured data, with which the mirror 9 is linearly displaced in order to vary _{the reference arm length I r} and thus the corresponding measuring arm length I _m.

The acquired measurement data, which represent interference signals, are evaluated by the control and data acquisition device 17 in order to create the associated depth profiles for the individual positions P1 to P3. This is done according to algorithms that are known for optical coherence tomography. A depth profile for a position includes the associated intensity values for the depth values along the examination direction, which represent a measure for the reflection of the area of the examined security document which is defined by the position and the corresponding depth value. The depth profiles are then evaluated in an evaluation device 21. The control and data acquisition device 17 can be combined with the evaluation device 21 in one device. Both can be designed individually or together as a program-controlled device. Alternatively, at least the control and detection device 17 can be implemented purely in hardware. In the embodiment shown, the verification device comprises 1 In addition, a display device 23 which comprises a freely programmable display area 24 on which, for example, a cross-sectional area 25 derived from depth profiles is displayed. This is schematically in Fig. 1 shown. Different layer boundaries 44, 45 can be seen, at which striking intensity values occur. In addition, the verification device can include a storage device 26 in which default data, for example for certain depth profiles, statistical key figures or exemplary cross-sectional areas, are stored, which can be used for comparison with determined depth profiles, cross-sectional areas or static evaluations in order to verify the security document being examined. It is also possible for the evaluation device to include an interface 27, via which default values can be called up from a database. It is also possible to output measurement results and / or a verification decision via the interface 27, which can be designed as a wired interface or as a radio interface, etc.

In Fig. 2 a section of a security document 12 is shown schematically, which is formed from three different substrate layers 31, 32, 33. In the document, information is encoded by means of an inkjet printing. Three printing pixels 34, 35, 36 are printed on the middle substrate layer 32. Here, part of the printing ink has diffused into the middle substrate layer 32 and the substrate layer 33 located below it. Overall, the substrate layers 31 to 33 are joined together to form a document body 40, for example in a lamination process. When examining a depth profile, the clear expansion is perceptible not only in the lateral direction 37, but also in the perpendicular direction 38, which is oriented perpendicular to the substrate layer surface 39 of the substrate layer 32. The information stored in the document body 40 can thus be differentiated from information that is applied using a different printing method in which no diffusion or a less pronounced diffusion of the colorants into the substrate layers 32, 33 takes place.

In Figures 3a and 3b further details of a document body 40 are shown schematically. In Fig. 3a it is shown that a cone-like recess 41 is first made in the document body 40. The individual substrate layers 31 to 33 are preferably all made transparent and provided here by means of different hatchings merely for the sake of simplifying the illustration. The However, substrate layers can have different refractive indices. In Figure 3b the finished document body is shown in which the cone-like recess 41 is filled with a preferably likewise transparent filling material 42. A refractive index of the filler material 42 preferably deviates slightly from the refractive indices of the materials from which the substrate layers 31 to 33 are made. When creating a depth profile of the document body, the filled recess 41 can be detected on the basis of the changes in the originally flat boundary layers 44 and 45.

In Fig. 4 a cross-sectional area 25 formed on the basis of a plurality of depth profiles arranged next to one another is shown schematically. The different positions are plotted along the X axis and the different depths are plotted along the Y axis. The assigned intensity values are graded according to color or gray level. The intensity values assigned to a position for the different depth values represent a depth profile. In the example shown, for the sake of simplicity, only intensity values are shown which are characteristic of transitions from one material layer to the other material layer. An intensity value of zero is assigned here to the remaining depth values at which no boundary surface is recognized. The boundary layers 44 and 45 and the outer surface 15 can be clearly seen in the illustrated cross-sectional area 25 derived from the depth profiles. In addition, intensity values which correspond to characteristic substrate transitions can be seen at those edges which delimit the introduced and filled recess 41.

It can be clearly seen that such a cross-sectional area representation differs significantly from a cross-sectional area representation as shown in Fig. 5 is shown, in which a representation for a similar security document is shown by way of example, which does not include the filled cone-like recess. The surface 15 and the two layer boundaries 44, 45, which are not modified, can be clearly seen.

The invention provides for statistical evaluations to be carried out. If, for example, the frequency is plotted for a selected intensity value or an intensity value range as a function of the assigned depth value, then for the document without the filled recess one obtains, for example, a view as shown in FIG Fig. 6 is shown. You can see three sharply delimited local peaks 53-55, the all have approximately the same frequency and can be assigned to the surface 15 and the layer transitions 44, 45.

In Fig. 7 the same statistical evaluation is shown for the security document with the filled cone-like recess. The three elevations 53-55 associated with the surface 15 and the layer boundaries 44, 45 can again be seen, but a finite number 57 of intensity values can also be recognized over a wide depth value range 56, which is caused by the conical walls 43 running obliquely through the document are (cf. Figure 3b ).

In Figures 8 and 9 statistical evaluations are shown in which the intensity values occurring in a given depth range are plotted as a function of their frequency. While a verification of a non-manipulated layer boundary ( Fig. 8 ) almost exclusively the intensity values 61 characterizing a layer transition are observed, are in the distribution according to Fig. 9 , which corresponds to a document with a manipulated layer boundary, in the depth area around the layer boundary not only the intensity values 61 characterizing a layer transition, but also a large number of further intensity values are present which correspond to the reflection property of the volume material 62 or to other transitions 63 that exist between the substrate and an adhesive used, for example, or between the adhesive and the second substrate layer. It can be seen that a manipulation can also be clearly seen from the comparison of these two spectra.

Another evaluation can provide that, for example, the depth values of a depth value range 71 (cf. Fig. 4 ), to which intensity values or intensity value changes above a threshold value or within a value range are assigned, are plotted against the corresponding position. In Fig. 10 a resulting graph is shown. For the evaluation, a parameterized function 72 of the position f (x, t1, t2, a, b, c) is adapted to the depth values. The function 72 is shown as a solid line. If the function 72 is optimally adapted, the parameters t1, t2, a, b, c characterize the tip of the recess 41. b indicates the center position. Δt = t2-t1 indicates the depth of the tip and the difference ac indicates a width of the tip at the boundary layer at depth t1. A verification is thus possible by comparing the parameter values with specifications.

It goes without saying that only exemplary evaluations are described here. The individual evaluations can of course be carried out in combination and complex evaluations can be carried out in order to identify different security features in the examined documents or to prove their absence. For example, a surface relief can also be scanned and compared with a specification. The individual features described can be used in any combination to carry out the invention.

List of reference symbols

1

Verification device

2

White light interferometer

3

Light source

4th

light

4m

Proportion of the light sent into the measuring arm

4mR

Proportion of the light reflected back from the measuring arm

4r

Share of light sent into the reference arm

4rR

Proportion of the light reflected back from the reference arm

5

optical element

6th

Beam splitter

7th

Reference arm

8th

The End

9

mirror

10

Actuator

I _r

Reference arm length

I _m

Measuring arm length

11

Measuring arm

12

Document recording

13

Security document

14th

detector

15th

surface

P1, P2, P3

Positions

16

volume

17th

Control and data acquisition device

19-1-19-3

Direction of investigation

20th

route

21st

Evaluation device

23

Display device

24

Display area

25th

Cross sectional area

26th

Storage device

27

interface

31, 32, 33

Substrate layers

34, 35, 36

Print pixels

37

lateral direction

38

perpendicular direction

39

Substrate layer surface

40

Document body

41

Recess

42

filling material

43

Cone walls

44.45

Boundary layers

53-55

local superelevations

56

Depth range

57

number

61

Intensity values characterizing a layer transition

62

Intensity values characterizing volume material

63

intensity values characterizing other layer transitions

71

Depth range

72

function

t1, t2

Parameter values for depths

a, b, c

Parameter values for positions

Claims (9)

1. Method for verifying a security document (13), comprising the steps:

   orientating the security document (13) relative to a white light interferometer (2), carrying out an examination by means of white light interferometry at several locations (P1-P3) on a surface (15) of a security document (13),

   wherein at each of the locations (P1-P3) a depth profile is produced along a direction of examination (19-1 to 19-3), which is oriented transverse to the surface (15) of the security document (13), wherein, as the depth profile, the measured values of a physical quantity are regarded, which are assigned to different depths along the examination direction, wherein the examination direction extends into an interior of the security document (13), and the depth profile is assigned to the location (P1-P3) of the surface (15) of the security document (13) at which the examination direction intersects the surface (15) of the security document (13),

   deriving a feature from the depth profiles, and comparing the at least one derived feature with one or more predetermined specifications in order to derive a verification decision,

   characterised in that

   the intensity values corresponding to the individual depth values or depth regions and/or intensity value changes of several depth profiles are statistically evaluated in respect of their frequency, in order to determine any deviation from an expected statistical distribution as an indicator of manipulation or falsification.
2. Method according to claim 1, characterised in that the security document (13) is oriented in such a way that the direction of examination (19-1 to 19-3) is oriented perpendicular to a surface (15) of the security document (13).
3. Method according to claim 1 or 2, characterised in that for several locations (P1 to P3), which are arranged in common along a path (20) on the surface (15) of the security document (13), or for several locations (P1 to P3), which are arranged in a surface region of the surface (15) of the security document (13), which comprises at least one location (P1), depth profiles are detected at the same time as the depth profile for the at least one location (P1).
4. Method according to claim 3, characterised in that at least one cross-section surface (25) of the security document (13), determined from the depth profiles which have been detected, is displayed on a display apparatus (23), this surface being spanned by the direction of examination (19-1 to 19-3) and the path (20) or by a contour lying in the surface region.
5. Method according to claim 4, characterised in that, in at least one section or depth region of the at least one cross-section surface (25), the depth values to which are assigned intensity values from a value range, or the depth values to which are assigned intensity value changes from a value range, are approximated in relation to their assigned positions (P1 to P3) along the at least one path or curve by means of a predetermined parameterised function of the position, and, on the basis of the parameters (t1, t2, a, b, c) derived from the approximation, the verification decision is reached.
6. Method according to any one of the preceding claims, characterised in that depth values in which intensity values or intensity value changes arise, which in each case are determined by a comparison of an intensity value with the intensity value of an adjacent spatial position in the security document (13) or with the intensity values of adjacent spatial positions in the security document (13), above a threshold value or within a value range delimited by an upper threshold value and a lower threshold value, are then evaluated in respect of their frequency in order to derive deviations or concordances in relation to one or more expected statistical distributions, and from this the verification decision is derived.
7. Verification apparatus (1) for carrying out the method according to any one of claims 1 to 6, comprising a document receiver (12) and a white light interferometer (2), which is configured such as to produce in each case a depth profile for at least several positions (P1-P3) on a surface (15) of a security document (13) arranged in the document receiver (12), along a direction of examination (19-1) oriented transversely to the surface (15), wherein, as a depth profile, measured values of a physical quantity are regarded, the different depths are arranged along the direction of examination, wherein the direction of examination extends into an interior of the security document (13) and the depth profile is assigned to the location (P1-P3) on the surface (15) of the security document (13) at which the direction of examination intersects the surface (15) of the security document (13), and an evaluation device (21), which is configured such as to evaluate several depth profiles, in that the intensity values and/or intensity value changes of several depth profiles, corresponding to individual depth values or depth ranges, are statistically evaluated in respect of their frequency in order to determine a deviation from an expected statistical distribution as an indicator of manipulation or falsification.
8. Verification apparatus (1) according to claim 7, characterised in that the white light interferometer (2) is configured as an optical coherence tomograph.
9. Verification apparatus (1) according to any one of claims 7 or 8, characterised in that the white light interferometer (2) comprises a light source (3), a beam divider (6), a detector (14), a reflector (9) secured to a controllable linear adjustment element (10), and a control and data acquisition device (17), wherein the light source (3) produces broad-band light (4) exhibiting spatial coherence and is arranged in relation to the beam divider (6) in such a way that the beam divider (6) conveys a portion of the light (4m) into a measuring arm (11) in which the document receiver (12) is located, and a portion of the light (4r) to a reference arm (7), in which the reflector (9) is arranged in such a way that this reflects the light (4rR) back onto the beam divider (7) and there overlays it with the light (4mR), which, at a security document (13) arranged in or on the document receiver (12), is reflected back to the beam divider (6), wherein the detector (14) is arranged in such a way that, at the overlayering of the reflected light (4rR) from the reference arm (7) with the reflected light (4mR) from the measuring arm (11), it can detect an interference signal which occurs, wherein the control and detection device (17) is coupled to the linear adjustment element (10), in order, during the detection of the interference signal, to vary a reference arm length (I_r) by a linear displacement of the reflector (9), wherein the reference arm lengths (I_r) correspond to measurement arm lengths (I_m) which accord with the distances from the beam divider (6) along the direction of examination (19-1 to 19-3), which extend at least to the surface (15) of the security document (13) on or in the document receiver (12) and then into the interior of the security document (13).

Images