WO2021083951A1

WO2021083951A1 - Verifying a security document on the basis of droplet morphologies

Info

Publication number: WO2021083951A1
Application number: PCT/EP2020/080268
Authority: WO
Inventors: Oliver Muth; Franziska Peinze; Arthur Mathea
Original assignee: Bundesdruckerei Gmbh
Priority date: 2019-10-31
Filing date: 2020-10-28
Publication date: 2021-05-06
Also published as: EP4215378A3; EP4215377A2; EP4051509A1; EP4215377A3; DE102019129491A1; EP4215378A2

Abstract

The invention concerns a method for verifying a document. The method comprises: - receiving (302) a document (500, 700) having a document body (506) with an inkjet print of an image (502, 600) by means of a testing unit (800). The image contains first regions (414.1, 414.2) that contain inkjet droplets of a dye-based ink (216) and second regions (412.1, 412.2) that contain a pigment-based ink (214). The first regions are free of the pigment-based ink and the second regions are free of the dye-based ink. The pigment-based ink is not distinguishable from the dye-based ink under visible light. - recording (304) a test image portraying at least the inkjet print (502); - performing (306) an image analysis comprising: · capturing (308) the morphology of the inkjet droplets; and · reconstructing (310) a digital graphical pattern (404) from the test image on the basis of the droplet morphology; - comparing (312) the reconstructed graphical pattern with a reference value in order to verify the authenticity of the document.

Description

Examination of a security document based on droplet morphologies

description

area

The invention relates to a method for producing a document, in particular a value or security document, for example an identity document, as well as a method and a system for checking the document produced using the method according to the invention. background

A large number of documents are equipped with security features that are intended to make forgery difficult or to prevent. These documents also include, above all, value or security documents that serve to identify the identity of a person, for example when crossing a national border, the origin or originality of a thing, or a claim, for example for payment of a sum of money or for the surrender of a Product or service provision.

Counterfeiting can be made more difficult or prevented in that such documents, such as bank notes, for example, are produced from a material that is not readily available. Additionally or alternatively, security features can be implemented using special colors, for example luminescent or optically variable colors, optical elements such as holograms, tilt images, cinegraphic objects, lens or prism arrays, guilloches with visible colors / inks or luminescent colors / inks, mottled fibers, security threads and the like, be formed.

The security features used in the valuable or security products can only serve to prove the authenticity of the documents regardless of their type or of their user. Individualizing, for example personalizing, security features also contain, in coded form or in plain text, information about the type of document, about the user of this document and / or about an object to which the document is clearly assigned. Such information can be a portrait picture or portrait photo of the user, his personal data, such as his name, birthday, place of birth, address, or a personal identifier, such as a membership number, or his signature. Another security feature that individualizes the valuable or security product can be, for example, a serial number of the product or the chassis number of a motor vehicle to which the product is assigned.

Summary

The invention is based on the object of proposing an improved document which makes manipulation difficult. Furthermore, the invention is based on the object of proposing a method for producing such a document which offers increased security against manipulation.

In addition, the invention is based on the object of proposing a method for checking a document according to the invention, as well as a system set up for checking.

The objects on which the invention is based are each achieved with the features of the independent patent claims. Embodiments of the invention are given in the dependent claims. The embodiments listed below can be freely combined with one another, provided they are not mutually exclusive.

In one aspect, the invention relates to a method for producing a document with a document body and a visually imperceptible security feature connected to the document body. The procedure includes:

Provision of an inkjet printer which includes at least a first tank with a dye-based ink and a second tank with a pigment-based ink, the absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink being so similar to one another that inkjet printing with the dye-based ink and an inkjet printing with the pigment-based ink (of the same image motif) when illuminated with light in the visible spectral range are indistinguishable by the human eye;

- Providing a digital graphic pattern that is formed from one or more first areas and one or more second areas; and

- providing a digital image to be combined with the document;

- superimposition of the graphic pattern and the digital image;

- Printing the digital image on or in the document body with the inkjet printer in such a way that the dye-based ink but not the pigment-based ink is used for printing the regions of the digital image overlaid with the first areas and that the pigment-based ink but not the dye-based Ink for printing the regions of the digital overlaid with the second areas Image is used, with the distribution of the dye-based ink and the pigment-based ink in the printed image being the security feature.

The introduction of a security feature that is not visible to the human eye by using inks of different types of the same color under visible light, which are printed in such a way that several areas of the inkjet printing that are printed with the different types of inks do not have a security feature in the form of one for the human eye Forming a visible pattern can be advantageous since it is initially not at all recognizable to a forger that a security feature is hidden in the inkjet print. Under visible light, the areas printed with the different types of ink look the same in the sense that these areas form the picture in their entirety, without a human observer being able to recognize any differences at the boundaries of the first and second image areas. Thus, a potential forger does not get the idea from the outset to forge this security feature, because it cannot be seen in the document with the naked eye.

Embodiments of the invention are based on the knowledge that the inkjet droplets of pigment-based and dye-based inks have a different morphology and / or topology and that these differences can be used to produce a security feature whose presence is not or at least not easy for a counterfeiter to recognize is.

In some embodiments, the two different types of inks (pigment-based and dye-based) also have an identical absorption spectrum in other spectral ranges such as, for example, infrared or UV light. In these embodiments, a forger will not be aware of this security feature even if he holds the document under UV light or IR light.

Even a component-related analysis of the testing device does not enable the forger to know which type of security feature is being tested: in the simplest case, the testing device only contains a high-resolution camera. Whether the image analysis software that is used to check the document only compares a picture of a person printed on the document with an image database of profile pictures, or how this is Embodiments of the invention is provided, the morphology and / or topology of the individual inkjet droplets is determined and analyzed, cannot be determined from the outside or on the basis of the composition of the components of the test device. Thus, neither the document itself nor a possibly used checking unit gives a potential forger an indication that the document contains the said security feature, where it is contained in the document, and how it can possibly be checked or produced.

Another advantage of documents according to embodiments of the invention can be that the examination is very undemanding in terms of apparatus. As will be explained in more detail at a later point, in principle only a high-resolution image acquisition unit and corresponding software are required to check this security feature. Optionally, a white light source can be used, but this is not necessary as long as there is sufficient daylight or another external white light source. To check the security feature, it is therefore not necessary to use a special light source, for example an IR light source and / or a UV light source. However, this can optionally also be present in the test unit, e.g. to test further safety features and / or if the dye-based ink and the pigment-based ink have a different absorption spectrum under IR light or UV light.

Since the check takes place on the basis of the droplet morphology and / or topology or since the security feature consists of the spatial distribution of inkjet droplets of different morphologies and / or topologies in or on the document, a document is provided whose check is also comparatively robust against interference radiation. Droplet properties such as diameter, edge sharpness, roundness or "fraying" of the droplet circumference and brightness gradients within the droplet are largely unaffected by the strength of the ambient light Security features can therefore be checked at different light intensities, as is often the case in daylight, for example, and no special lamp with a narrow emission spectrum is required to check the security feature. According to embodiments, the provision of the digital graphic pattern comprises the generation of a bar code or matrix code.

According to embodiments, the digital image is a monochrome digital image.

In other embodiments, the digital image is a multicolor digital image such as a CMYK or RGB image.

For example, polycarbonate-based documents can be provided with a color image using methods that are described, for example, in German patent applications DE102007052947 A1 and DE 102008012419A1, e.g. to personalize the document in color with a photo and / or signature of the owner. In this process, the colored print image is created inside the card. This has the advantage that the printed image cannot be manipulated without destroying the laminated film structure. That is why the cards produced in this way, such as identity cards and passports, are particularly forgery-proof. For example, the document consists of several layers ("films") made of polycarbonate (PC) and PC-based inks. Some of these inks, which can be used to produce a colored inkjet print in or on a document, are also described in patent applications DE102007052947 A1 and DE 102008012419A1.

For example, a document can be personalized with an image that is a portrait of the document holder. The inkjet printer can, for example, be a printer with 5 ink tanks, 3 of the tanks being filled with one ink each for magenta, yellow and cyan. The three inks are printed on the document by the inkjet printer according to their proportion of color in each of the pixels of the digital image.

In addition, the ink jet printer includes two tanks, each of which contains black ink, the black of which is indistinguishable from one another in terms of color by the human eye under visible light. However, one of the black inks is pigment-based and the other black ink is dye-based. The two different black ink values are printed out on the document according to an either / or principle, so that the first regions of the digital image exclusively with the black dye-based ink and the second Regions of the image are printed exclusively with the black pigment-based ink.

In the case of color images, ink droplets from the three other tanks with the magenta, yellow and cyan inks are usually also contained in the first and second regions of the image. These 3 inks can be, for example, 3 dye-based inks or 3 pigment-based inks.

If the 3 inks are dye-based inks, the second regions can still be recognized and distinguished from the first regions on the basis of the droplet morphology, because the second regions of inkjet printing are the only areas whose droplets have a morphology and typical for pigment-based inks / or have topology.

If the 3 inks are pigment-based inks, the first regions can still be recognized on the basis of the droplet morphology and differentiated from the second regions, because the first regions of inkjet printing are the only areas whose droplets are for dye-based inks have typical morphology and / or topology.

A combination of one dye-based ink and two pigment-based inks or a combination of two dye-based inks and one pigment-based ink is only possible for the 3 colors magenta, yellow and cyan if the morphologies and / or topologies of these 3 colors differ from the morphologies and / or the topology of the black dye-based ink and / or the black pigment-based ink are clearly distinguishable. In some cases this can be achieved, for example, by additives that influence the viscosity and thus the droplet flow.

According to embodiments, the document body material on or in which the image is printed has a color that functions as the background color of the printed image. The background color can be light, for example, and can in particular be white. A "light" background color is understood here as a color whose brightness value is encoded in a one-dimensional or multidimensional brightness space and whose average brightness value over all dimensions of this brightness space is over 50% of the theoretical table in this brightness range of possible value, preferably at over 70% of this value. If, for example, a monochromatic brightness space between 0 (black) and 100 (maximum light) units is defined, a “light” background color would be one that has at least 50 of these units, preferably at least 70 of these units.

According to one embodiment, the dye-based ink is such that an image printed with this ink and illuminated with light in the infrared spectral range produces a transparent color impression for the human eye, so that the one or more first areas have the background color. The pigment-based ink is such that an image printed with this ink and illuminated with light in the infrared spectral range produces an opaque color impression for the human eye, so that the one or more second areas have a different color than the background color. In the case of a monochrome image, a “different color” of the second in comparison to the first areas means a “different brightness value”.

This can be advantageous because a test device that has an IR light source and an image acquisition unit sensitive in the IR range can very quickly and unambiguously identify the pattern hidden in the image without having to carry out a more computationally complex analysis of the droplet morphologies . For example, the use of such inks can make it possible to use a test device that, in normal operation, recognizes the pattern contained in the inkjet printing by illuminating the inkjet printing of the document with an IR light source and a digital one with an IR image capture unit IR test image of the inkjet print recorded. The IR test image preferably has a resolution which is smaller than the test image created in visible light. A smaller resolution, which is no longer able to resolve individual ink jet droplets from one another, is completely sufficient here to make the pattern recognizable, since the pattern recognition is not based on an analysis of the droplet morphology. Since the dye-based ink is transparent in IR light, the first regions of the inkjet print have the background color of the document body and the second regions of the inkjet print have the color (or the lightness value) of the pigment-based color under IR light. The pattern is therefore comparatively fast and can be carried out on the basis of an image analysis of an IR test image of lower resolution, so that the computing time and CPU load are reduced. However, if the IR light source is defective, too much visible Res interference light is present and / or if an additional analysis of the security feature is to be carried out, an examination of the droplet morphology and / or topology can be used.

According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a black hue when illuminated with light in the visible spectrum. For example, the dye-based ink can be Solvent Black 27 or Solvent Black 29. Additionally or alternatively, the pigment-based ink is Carbon Black or Pigment Black 28.

"Solvent Black 27" is a substance (azo metal complex) with the CAS number 12237-22-8.

A substance with the CAS number 61901-87-9 is referred to as "Solvent Black 29".

"Carbon black" is understood here to mean soot, a black, powdery solid, which consists of more than 80%, in some cases more than 98% carbon. In particular, industrial soot (CAS-No .: 1333-86-4) , a soot specifically produced as an industrial base material, can be used as carbon black.

Pigment Black 28 is a substance with the CAS number 68186-91-4. Pigment Black 28 is an inorganic pigment that is obtained as a reaction product of high temperature calcination, in which copper (II) oxide and chromium (III) oxide are homogeneously and ionically interdiffused in different amounts to form a crystalline matrix.

According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a cyan hue when illuminated with light in the visible spectrum. For example, the dye-based ink can be Solvent Blue 78. Additionally or alternatively, the pigment-based ink is Cu-phthalocyanine.

The substance known as "Solvent Blue 78" is also known under the name "1,4-bis (Metylamino) antrachinon" and has the CAS number 2475-44-7. According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a magenta hue when illuminated with light in the visible spectrum. For example, the dye-based ink can be Solvent Red 26 or Sudan Red. Additionally or alternatively, the pigment-based ink is quinacridone.

The substance referred to as "Solvent Red 26" is also available under the names "Ölrot EGN" or "C.I. 26120 ". It is a purple-red synthetic azo paint with the CAS number 4477-79-6.

The substance known as "Sudan Red" is also known under the name "Sudan III" or "1- [4- (Phenylazo) phenylazo] -2-naphthol". It is a synthetically produced chemical compound from the group of the azo and Sudan dyes with red color and has CAS number 85-86-9.

The substance known as "quinacridone" is an organic pigment and organic semiconductor with red to purple hues. It has the CAS number 1047-16-1.

According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a yellow hue when illuminated with light in the visible spectrum. For example, the dye-based ink can be solvent yellow 124. Additionally or alternatively, the pigment-based ink is brilliant yellow or pigment yellow 151.

The substance known as "Solvent Yellow 124" is an azo dye and has CAS number 34432-92-3.

The substance known as "Brilliant Yellow" is also known under the name "Pigment Yellow 74" and has the CAS number 6358-31-2.

The substance referred to as "Pigment Yellow 151" is a benzimidazolone pigment with the CAS number 31837-42-0.

According to embodiments, the inkjet printer also includes at least a third tank with a further dye-based ink and a fourth tank with a further one pigment-based ink, whereby the absorption spectrum of the additional dye-based ink is so similar to the absorption spectrum of the additional pigment-based ink that this additional dye-based ink is indistinguishable in color from the other pigment-based ink when illuminated with light in the visible spectral range, but from the Inks in the first and second tanks can be distinguished in terms of color. The method also includes:

Providing a further digital graphic pattern containing one or more third areas and one or more fourth areas;

- Superposition of the further graphic pattern and the digital image;

Performing the printing of the digital image onto or into the document body in such a way that the dye-based ink but not the pigment-based ink is used for printing the regions of the digital image superimposed with the first areas, and that the pigment-based ink but not the dye-based ink is used for printing of the regions of the digital image superimposed with the second areas are used, that the further dye-based ink but not the further pigment-based ink is used for printing the regions of the digital image superimposed with the third areas, that the further pigment-based ink but not the further dye-based ink can be used for printing the regions of the digital image superimposed with the fourth areas, the distribution of the dye-based ink, the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image being the security ts feature is.

For example, the third areas can be congruent with the first areas and the fourth areas can be congruent with the second areas. In one embodiment, some areas of the printed image consist exclusively of inkjet droplets from several different dye-based inks, while other areas of the printed image consist exclusively of inkjet droplets from several different pigment-based inks. In other embodiments, the third areas are not congruent with the first areas and / or the fourth areas are not congruent with the second areas, so that the printed image consists of a complex pattern of different dye-based and pigment-based inks. According to embodiments, the method further comprises:

- Determining an area within the image which has a minimum homogeneity and minimum intensity with regard to the absorption spectrum of the hue that is to be printed with the dye-based ink or the pigment-based ink; The result is identical for both types of ink, since they represent the same color tone;

Implementation of the superimposition in such a way that the graphic pattern is completely mapped onto this surface.

According to embodiments, the document body is built up from several layers of material. The digital image is printed on or in the document body in such a way that the digital image is printed on one or more of the material layers, and then the one or more printed layers, optionally together with further material layers, are inseparably connected to form the document body. The connection can be made e.g. chemically, thermally by fusing the material layers, mechanically (e.g. by pressure) or by gluing the material layers.

When the layers of material are glued or fused, printing inks can penetrate the document body and bond with the document body, so that even if the layers are removed layer by layer, a surface that shows the complete image and the pattern embedded in it cannot be obtained at any time.

According to preferred embodiments, the ink jet print is printed on one or more layers which, after the layers have been connected, lie in the interior of the document body.

By printing distributed over several layers, it can be avoided that some layers are removed first and then new layers with incorrect information are applied to the rest of the original document body without at least destroying the pattern or making it illegible.

The material layers, which can also be designed as foils, can be made of PC, for example. The inkjet printer transfers the image to a core film, for example. prints. The inks, which, like the individual layers, can contain PC, bond firmly to the surface of the film and thus the printed inkjet image. The core film is then laminated with cover films. This can be done with or preferably without the use of adhesive. During lamination, the individual film layers form a continuous piece of polycarbonate. They cannot be separated from each other without destroying the entire document. This means that counterfeiters cannot access the information-bearing inner layer. Chips and antennas, as they are used in the case of contactless read ID cards, are additionally integrated into the film structure according to embodiments and cannot be broken out of it without destroying the document or the chips or antennas.

According to embodiments, the dye-based ink and / or the pigment-based ink is designed to penetrate into one or more further material layers in addition to the material layer on which the ink was printed when the multiple material layers are connected. According to embodiments, the dye-based ink penetrates further into the material of one or more of the layers during and / or after the joining of the plurality of material layers and / or with the generation of a different drop topology than the pigment-based ink.

According to embodiments, the graphic pattern contains data in coded form. The data can in particular contain data that are unique to the document and / or its owner. Optionally, these data can be contained in a further representation on or in the document, e.g. in the form of a further imprint, in the form of an engraving and / or as a data value electronically stored in a data memory of the document. The data memory can be, for example, a magnetic strip or a memory on a chip.

This can be advantageous because it provides additional protection against forgery of the document. Because, for example, a specific, secret data value is stored both in the data memory in digital form and in coded form in the form of the invisible pattern in the printed image, a checking unit can check whether, firstly, the pattern is contained in the inkjet print and, secondly, whether in the data memory is also contained in the pattern-coded value. A document forgery Scher would therefore have to manipulate both the inkjet print and the value stored in the data memory in the same way. Both can, however, be prevented: the inkjet print can be protected from manipulation in various ways, for example by applying a protective film or by printing the inkjet print on a material layer inside the document. If the various mate rialschichten are irreversibly connected to one another to form a single document body by pressure, temperature, glue or other measures, it is no longer possible to change the inkjet pressure inside the document without physically destroying or damaging the document. Additionally or alternatively, the value stored in the data memory can be cryptographically protected against manipulation by unauthorized persons. A manipulation of both the imprint and the protected stored data value and thus a forgery of the document can thus be effectively ruled out.

According to embodiments, the method further comprises providing the document body with an electronic circuit. Another representation of the unique data is stored in this circuit. Additionally or alternatively, further data are stored in the electronic circuit which can only be accessed after a successful authentication of a reading device and / or a user with respect to the document. The said data, which are unique for the document or its owner, can also belong to this data, which can only be accessed after successful authentication.

The electronic circuit, for example a microchip with an interface for contact-based or contactless communication with the electronic circuit and a memory can store a representation of the data unique to the document, which can be compared with data encoded in the pattern.

As an alternative or in addition, the electronic circuit can store further data that are particularly protected against unauthorized access. In order to be able to access this data, the pattern can contain access data which are fed to the electronic circuit in an authentication or authorization phase. The electronic circuit can be set up, for example, to include the data from the pattern in the electronic Circuit to compare stored credentials. If the comparison shows that the access data match, the electronic circuit can allow access to the further stored data. Correspondingly, matching access data are stored in the pattern and in the electronic circuit during the production of the document.

The electronic circuit can be a semiconductor circuit which, for. B. is silicon-based or polymer-electronic. For example, the electronic circuit can be implemented by a chip that is located on or in a document layer. The electronic circuit can also be formed from polymerelectroni's components which, for. B printed onto the document layer.

For example, the further data can include sensitive personal data, for example age, place of birth, birthday, address, health data, insurance data, etc. The document can be designed to grant a user or a reader access to the personal data only if that Reading device is able to transmit a data value to the document which is identical to a reference value which is stored in the digital memory of the document. The reference value is an identical copy / further representation of a data value encoded in the pattern in the inkjet print of the document. In this embodiment, the reading device is not allowed, at least for the time being, to read this data from the data memory. The reading device must determine the data stored as a reference value in the document in that it can correctly recognize and decode the pattern in the inkjet print in or on the document. The reading device can use the data value obtained through the decoding to authenticate itself to the document. The reading device can thereby prove, for example, that it was able to optically capture the inkjet print, which implies that the user has presented the document to the reading device and that the reading device cannot access the memory without the knowledge and willingness of the user . In a further aspect, the invention relates to a document produced by the method according to one of the embodiments of the production method according to the invention described here.

In a further aspect, the invention relates to a document with a document body, an ink jet print being located on or in the document body. Inkjet printing includes first inkjet droplets that have a first morphology and consist of a dye-based ink. Inkjet printing also contains second inkjet droplets, which consist of a pigment-based ink. The second inkjet droplets have a second morphology that is different from the first morphology. The absorption spectrum of the dye-based ink and the pigment-based ink are so similar that the color of the first and second inkjet droplets cannot be distinguished by the human eye when illuminated with light in the visible spectral range.

The inkjet printing includes one or more first regions that contain the first inkjet droplets and are devoid of the second inkjet droplets. The inkjet printing includes one or more second regions that contain the second inkjet droplets and are free of the first inkjet droplets. The first and second areas form a pattern which, when viewed by the human eye, cannot be recognized when illuminated with light in the visible spectral range.

According to embodiments of the invention, the document body consists of several material layers. The thickness of the document is its extension in a dimension "z". At least some of the inkjet droplets in inkjet printing extend over several layers of material. The topology of the first inkjet droplets differs from the topology of the second inkjet droplets. A topology of an inkjet droplet describes its spatial structure Extension along the dimension z over one or more of the material planes.

In a further aspect, the invention relates to a method for checking a document. The procedure includes: - Reception of a document with a document body by a checking unit that includes an image capture unit. Ink jet printing of an image is contained in or on the document body. One or more first regions of the image are printed with a dye-based ink. One or more second regions of the image are printed with a pigment-based ink. The one or more first regions are free of the pigment-based ink and the one or more second regions are free of the dye-based ink. The absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar that they are indistinguishable in color to the human eye when the inkjet printing is illuminated with light in the visible spectral range;

- Recording of a test image by the image acquisition unit, wherein the test image is a digi tal image that depicts at least the inkjet print of the document;

- Carrying out an image analysis of the test image by the inspection unit, the image analysis comprising:

• Automatic detection of the morphology of the ink jet droplets depicted in the test pattern, of which the ink jet print consists, the morphology of an ink jet droplet describing its spatial extent within the test pattern; and

Automatic reconstruction of a digital graphic pattern from the test image, the pattern including one or more first areas and one or more second areas, the first areas being those test image regions whose inkjet droplets have a first morphology, the second areas being those test image regions the inkjet droplets of which have a second morphology, the second differing from the first morphology;

- Comparing the reconstructed graphic pattern or a value derived therefrom with a reference value in order to check the authenticity of the document.

For example, the image analysis can be carried out by software that works with explicit rules that have been specified by the programmer. These rules contain various criteria and limit values with regard to, for example, the droplet diameter in the X, Y and / or Z direction, with regard to intensity differences in within the droplet, in terms of the presence, thickness and sharpness of a droplet outline and similar criteria. The image analysis software compares at least some or all of the inkjet droplets that are formed in the test image with these rules and classifies the droplets into first and second inkjet droplets based on the result of this comparison. In other implementation examples, the image analysis software is a trained machine learning (ML) software, for example a trained support vector machine (SVN), or a trained neural network (NN). For the training of the software, a training data set was used which contains a large number of training test images, each of the training test images being a digital image of an inkjet print, the inkjet print being printed with the same inks as the currently recorded test image and also first and contains second regions that form a pattern. The first regions of the training test images depict regions of the inkjet printing which were printed with the same or in terms of droplet morphology very similar dye-based ink as the first regions of the inkjet printing of the currently checked document. The second regions of the training test images depict regions of the inkjet printing that were printed with the same or very similar pigment-based ink in terms of droplet morphology as the second regions of the inkjet printing of the currently checked document. The first and second regions in the training test images are identified as such ("annotated") and various properties of the droplets in the first and second regions of the training test images are transferred to the ML software as input parameter values, so that during the Trainings the ML software learns to recognize first and second regions on the basis of the droplet morphology and / or topology.

According to embodiments, the digital graphic pattern is a bar or matrix code. The method also includes:

Decoding of the bar code or matrix code by the checking unit in order to obtain a decoded value; and

Use of the decoded value as the value derived from the graphic pattern in the comparison with the reference value.

According to embodiments, the resolution of the image acquisition unit is sufficiently high to distinguish at least the inkjet droplets of the pigment-based ink from one another. that, that is, to be able to dissolve spatially. According to embodiments, the ability to be able to spatially resolve a certain minimum proportion of the droplets, that is to say for example at least 20% or at least 50% of the inkjet droplets with the pigment-based ink, can also be achieved.

According to embodiments, the resolution of the image acquisition unit is at least 15 μm, preferably at least 5 μm. This can be advantageous since inkjet droplets according to embodiments of the invention have a diameter of 40 µm- 60 µm, e.g. 50 µm.

According to embodiments, the image acquisition unit is a camera or a microscope. For example, the image acquisition unit can be designed to record one or more digital color images from at least the inkjet print in or on the document, which are referred to as “test image (s)”. For example, the test images can be RGB images and, for example, as .jpg , .png image can be saved.

According to embodiments of the invention, the document has a length in the dimension x, a width in the dimension y and a thickness in the dimension z. The procedure for reviewing the document also includes:

- Recording of several test images in several planes along the dimension z by the image acquisition unit; for example, the images can be taken at intervals of 10pm or 20pm or any other interval, preferably less than 80pm; each of the plurality of test images is a digital image that depicts at least the inkjet print of the document in a z-plane of the document;

- Implementation of the image analysis of the test image by the test unit in such a way that the

Image analysis includes:

• Automatic detection of the morphology of the inkjet droplets depicted in each of the several test images;

• Automatic detection of the topology of the inkjet droplets each depicted in the multiple test images, the topology of an inkjet droplet describing a spatial extension of the inkjet droplet over one or more of the z-planes along the z-dimension, with the first areas are those test image regions whose inkjet droplets have a first morphology in connection with a first topology, the second areas being those test image regions whose inkjet droplets have a second morphology in connection with a second topology, the second of the first topology deviates.

According to embodiments of the invention, the morphology and / or topology of ink jet droplets of the dye-based ink is characterized by one or more features selected from a group comprising:

• an average droplet diameter in the z direction of over 15 pm;

• an average droplet diameter in the z-plane of the document on which the imprint was made, in the range 15-100 μm;

• an intensity profile from the center outwards in that z-plane of the document on which the imprint was made, such that the intensity profile is homogeneous or the droplet center is darker than the edge;

• Blurred droplet edges in all z-planes of the document in which one of the test images was recorded.

According to embodiments of the invention, the morphology and / or topology of ink jet droplets of the pigment-based ink is characterized by one or more features selected from a group comprising:

• an average droplet diameter in the z direction in the range of a maximum of 15 μm;

A radial intensity profile from the center outwards in that z-plane of the document on which the imprint was made, in such a way that the droplet outline is darker than the center and forms a dark edge around the droplet;

• sharp droplet edges in at least one of the multiple z-planes of the document in which one of the test images was recorded. According to embodiments of the invention, the image is a face or iris image of a person to whom the document is assigned.

According to embodiments of the invention, the checking unit additionally contains a light source for light in the visible spectral range for illuminating at least one area of the document to be recorded by the image capturing unit. The word “contain” also includes embodiments in which the light source is mechanically coupled to the test unit and is attached, for example, to the housing of the test unit. The method further includes: Before recording the test image, activating the light source so that at least the Inkjet printing is illuminated on or in the document by the light source.

According to embodiments of the invention, the checking unit additionally contains an infrared light source for illuminating at least one area of the document to be recorded by the image capturing unit. The method also includes:

- activating the infrared light source;

Recording of an IR test image by an IR image acquisition unit, the IR test image being a digital image which depicts at least the inkjet print of the document;

- Carrying out an image analysis of the IR test image by the inspection unit, the image analysis comprising:

• Automatic detection of dark areas of the IR test image, with a dark area having an average intensity below a predefined limit value; and

• Automatic reconstruction of a digital graphic pattern from the dark areas, the pattern being referred to as "IR pattern";

- Comparing the IR pattern or a value derived therefrom with an IR pattern reference value to check the authenticity of the document.

In a further aspect, the invention relates to a checking unit with an opening for receiving a document. The document has a document body in which or on which a printed area, also referred to here as "inkjet printing", is contained. The checking unit further comprises an image acquisition unit with sufficient resolution to To be able to spatially resolve inkjet droplets in the printed area. The checking unit further comprises at least one processor and a non-volatile memory with computer-interpretable instructions which, when they are executed by the at least one processor, cause the checking method to be carried out in accordance with one of the embodiments described here. In particular, the following steps are carried out by the components of the checking unit after the document has been received:

- Carrying out an image analysis of the test image by the at least one processor of the test unit, the image analysis comprising:

- Comparing the reconstructed graphic pattern or a value derived therefrom with a reference value for checking the authenticity of the document by the at least one processor of the checking unit.

For example, the image acquisition unit can be designed to be able to spatially separate at least 20% of the droplets printed with the pigment-based ink from one another.

In a further aspect, the invention relates to a system with a test unit according to embodiments of the invention. The system also includes:

- an IR light source; and or - An image capturing unit that is sensitive in the IR spectrum, for example an IR-sensitive camera; and or

- a light source for light in the visible spectral range; and or

- an automatic document feeder; and or

- A control device for controlling access to: protected data, software functions, hardware functions or spatial areas, the control device being configured to only grant access if the test procedure shows that the document is valid .

An additional IR light source and / or an IR-sensitive camera can be useful, for example, since these allow an additional operating mode that allows the pattern to be recognized in the inkjet print with less computational effort. If the dye-based ink and the pigment-based ink have a different absorption spectrum in the IR range, the pattern can also be used in an IR test image recorded under IR light with a significantly lower resolution (which does not allow the individual ink droplets to dissolve) be carried out, since the pattern can be recognized over a large area under IR light by an IR-sensitive sensor.

A light source in the visible range can be advantageous because the test device now has its own light source in order to be able to use the test image, for example, at night, i.e. when there is no daylight and another external white light source.

The document feeder can be advantageous because it can ensure that the document is automatically positioned so that the image capture unit can capture the inkjet print at the correct position and at the correct distance from the capture unit.

The control unit can be advantageous because it can ensure that a user only has access to a protected area, protected data or other protected entities who has authenticated himself as authorized by means of the document. For example, the test device with this control unit can be integrated into a terminal or functionally connected to the terminal. For example, the terminal can be used at Border controls, airport controls, building entrances and similar application scenarios are used.

In one embodiment of the test device, the test device produces only IR test images in "IR mode", which is used as "normal mode", since these can be analyzed very quickly and a large number of people can be tested in a short time . In this way, queues in front of the terminal can be avoided. Randomly or if the IR light source fails, the test device carries out the test in "white light mode". This means that a test image in high resolution (e.g. at least 15 miti, preferably at least 5 pm) is recorded under white light and the inks shown in it Jet droplets are analyzed by image analysis software in order to recognize the pattern on the basis of different droplet morphologies and / or topologies.

Definitions:

A document is understood here to mean a physical object which is intended to be used as a trustworthy information carrier. In particular, the document can be a value and / or security document.

Insofar as the term "document", "product", "valuable or security product" and in particular the term "valuable or security document" are mentioned in the description and in the claims of the present application, these are to be understood synonymously and include, for example, a passport, Identity card, driver's license or another ID card or an access control card, a vehicle registration document, vehicle registration document, visa, check, means of payment, in particular a banknote, a check, bank, credit or cash payment card, customer card, health card, chip card, company ID, Proof of eligibility, membership card, gift or shopping voucher, bill of lading or any other proof of eligibility, tax stamps, postage stamps, tickets, (game) tokens, adhesive labels (e.g. for product security) or another ID document. For example, the product can be a smart card. The security or value document can be in ID 1, ID 2, ID 3 or in any other format, for example in booklet form, as in the case of a passport-like object. A document can be a laminate of several document layers that are precisely connected to one another, for example fused, under the action of heat and under increased pressure. These products should meet the standardized requirements, for example the standards ISO 10373, ISO / I EC 7810, ISO 14443. The product layers consist, for example, of a carrier material that is suitable for lamination. In addition to value or security documents, the term "valuable or security product" also includes patches, labels and the like, which as security elements are components of documents and for this purpose are or are inextricably linked to the document carrier and form the security feature there.

The document can be formed from a polymer selected from a group comprising polycarbonate (PC), in particular bisphenol A polycarbonate, polyethylene terephthalate (PET), their derivatives, such as glycol-modified PET (PETG), polyethylene naphthalate (PEN ), Polyvinyl chloride (PVC), polyvinyl butyral (PVB), polymethyl methacrylate (PMMA), polyimide (PI), polyvinyl alcohol (PVA), polystyrene (PS), polyvinyl phenol (PVP), polypropylene (PP), polyethylene (PE), thermoplastic elastomers (TPE), in particular thermoplastic polyurethane (TPU), acrylonitrile-butadiene-styrene copolymer (ABS) and their derivatives, and / or paper and / or cardboard and / or glass and / or metal and / or ceramic. In addition, the product can also be made from several of these materials. It is preferably made of PC, PET and / or PVC. The polymers can be either filled or unfilled. In the latter case, they are preferably transparent or translucent. If the polymers are filled, they are opaque. The above information relates both to films to be bonded together and to liquid formulations that are applied to a preproduct, such as a protective or topcoat.

The document is preferably made from 3 to 12, preferably 4 to 10, films, preferably with a lamination process in which the films are fused together under the action of pressure and heat. The individual foils can consist of the same material or of different materials. Overlay layers formed in this way protect a security feature arranged underneath and / or give the document the required abrasion resistance. Insofar as the terms "individualized" and "individualizing" are used below, this is to be understood as a property of a value or security document according to which the document can be assigned to a specific subject (person, organization, animal, object). Insofar as the terms “personalized” and “personalizing” are used below, this is to be understood as the property of a value or security document, according to which the document can be assigned to a person. This property results from predetermined security features of the valuable or security products, preferably from the security printing according to the invention.

Insofar as the term "color impression" or "color" is used in the description and in the claims of the present application, this is to be understood as the visual impression acting on a human observer with regard to the color effect, these different impressions being made up of different shades of color , ie spectrally different absorptions and / or brightnesses of a print. The color impression can also result from interacting, closely spaced discrete color tones, which produce a color impression in a human viewer which differs from each of the closely spaced color tones.

An "ink" here denotes a liquid of any viscosity which allows use by an inkjet printer and which contains one or more colorants. The ink preferably has a low viscosity. The term "colorant" includes substances to understand certain absorbing properties, at least in the visual spectral range and optionally in other spectral ranges. Colorants are substances that give the substance (e.g. ink or solvent) in which they are contained the appropriate color

A “dye-based ink” is understood here to mean an ink that consists of a liquid - typically water - and colorants dissolved in it. Solubilizers can be used in some dye-based inks. The colorants that are dissolved in a dye-based ink are used referred to as "dye". A “dye” is understood here to mean a substance or a mixture of substances which has light absorption in the visible spectral range and, in contrast to a color pigment, dissolves chemically in a liquid, in particular water. A “pigment-based ink” is understood here to mean an ink that consists of a liquid with pigments dispersed therein. In contrast to dyes, pigments are insoluble. They are dispersed in the ink liquid - typically water - in particulate form. Pigments can in particular inorganic pigments or carbon black particles. A “pigment” is understood here to mean a substance or a mixture of substances which has light absorption at least in the visible spectral range.

A “printing color”, also called “hue”, is understood here to mean the color that the human eye perceives when viewing a printout that was created with a certain ink, when the printout is illuminated with light of the visible spectral range. In embodiments in which the test image is captured as a monochromatic digital image, the term “printing color” preferably relates to the intensity value of this color in the one color channel of the monochromatic digital test image.

A "security feature" is a characteristic property of a document that is intended to prove the authenticity of a document and to make counterfeiting impossible or at least considerably more difficult. For example, holographic kinematic features, surface embossing, and watermarks, watermarks, UV watermarks, special paper quality or card body texture, for example with integrated fibers, multicolored processed guilloches, printing elements with tilting effect, writing elements with microscript or the like serve as a security feature. A document according to one embodiment of the invention contains at least one security feature in the form of an inkjet printed in or on the document Image that contains a pattern that cannot be recognized by the human eye in visible light, but that can be recognized by means of an image analysis of the droplets of this inkjet print.

An "inkjet printer" is a printer, in particular a matrix printer, in which a print image is generated through the targeted shooting and / or the deflection of small ink droplets.

An "absorption spectrum" is a color or electromagnetic spectrum that arises when broadband (white) light shines through matter and light quanta (photons) certain wavelengths or wavelength ranges are absorbed (resonance absorption). The absorbed photons are absent in the light passing through, which is why the spectrum at the relevant wavelengths is dark or, in extreme cases, black. Anyone who absorbs the photons by exciting atoms is a question of sharply defined amounts of energy and thus wavelengths, and the dark areas are correspondingly narrow lines. In molecules, on the other hand, many absorbable energy values are often close together and form broader dark areas in the spectrum, so-called absorption bands. In any case, the observed absorption spectrum is characteristic of the type of matter that the radiation traverses.

Insofar as the term “visual” is used in the description and in the claims of the present application, this is to be understood as the visual perception ability of a person with normal vision, in particular with regard to the spectrum of the perceived wavelengths.

"White light", "visible light" or "light in the visible spectral range" is understood here to mean broadband light in the spectral range visible to the human eye, for example daylight or light from a lamp with an emission spectrum similar to daylight in the visible wavelength range of white light is continuous. In particular, white light comprises broadband light with wavelengths between approx. 380 and 780 nm.

"Light in the infrared (IR) range", "infrared radiation", or "IR radiation" is understood here to mean electromagnetic radiation in the spectral range between visible light and the longer-wave terahertz radiation nm and 1 mm, which corresponds to a frequency range from 300 GHz to 400 THz or a wave number range from 10 cm ^-1 to 12800 cm ^-1 .

A “digital image” is a data record in which image content is represented and stored. In particular, the digital image can be a data record in which the content of an image is represented by whole numbers Image can be a raster graphic. A "test image" is a digital image which depicts at least that part of the document which contains the inkjet print and which was generated by an image capture unit having captured this part digitally. The capture is preferably carried out with white light and with high resolution.

An "IR test image" is a digital image which depicts at least that part of the document which contains the inkjet print and which was generated by an image capture unit having digitally captured this part while at least this part of the document was irradiated with infrared light.

A “checking unit” here denotes a device or a device component which is designed to check the authenticity of a document on the basis of at least one security feature of the document.

An “image acquisition unit” or “white light image acquisition unit” refers to a device or a device component which contains at least one apparatus for acquiring a digital image of a physical object or is this apparatus itself, the apparatus being sensitive in the visible frequency range. The apparatus can be, for example, a camera or a microscope. Optionally, the image acquisition unit can contain a further apparatus for acquiring a digital image of a physical object, e.g. a camera or a microscope, this further apparatus being sensitive in a wavelength range outside of visible light, e.g. in the IR range. If the wavelength range outside of visible light is the IR range, the further apparatus can also be referred to as an "IR image acquisition unit". It is also possible that the same camera or microscope both in the visible frequency range and in this other Frequency range, for example the IR range, is sensitive and is used both as a "white light image acquisition unit" and as an "IR image acquisition unit", for example to acquire test images and IR test images with the same apparatus.

An "IR image acquisition unit" here denotes a device or a device component which contains at least one apparatus for acquiring a digital image of a physical object or is itself this apparatus, the apparatus being sensitive in the IR frequency range Be a camera or a microscope. A "camera" is used here to describe a photographic apparatus that can record static or moving images on photographic film or electronically on a digital storage medium or transmit them as a digital image to a receiver via an interface. The camera can be a single-lens or multi-lens ( The image is created by a lens on a film (analog camera) or on an electronic sensor (digital camera) on the opposite side of the camera. Some cameras have a shutter release and a shutter Shutter release, the shutter on the lens is opened for a very short time so that the light can pass through the lens and a picture can be taken.

A "microscope" is understood here as a light microscope, that is, an optical device for image acquisition that can generate greatly enlarged images of small structures or objects with the help of light. The magnification is carried out according to the laws of optics using light refraction on glass lenses. In particular, the microscope can be a reflected light microscope, but brightfield microscopes can also be used in some embodiments, for example for documents with a transparent document body. The microscope can be a “simple” or “composite” microscope. Simple microscopes have only a single optical system for magnification and function like a magnifying glass (for the principle of magnification, see there). A single glass lens or a combination of several individual lenses can be used. The microscope is preferably an existing composite microscope of at least two series-connected optical systems, each with its own magnification. The front lens, the objective, creates an enlarged real image, the intermediate image, which is enlarged a second time by the eyepiece. The eyepiece works like a magnifying glass and creates a virtual image of the intermediate image. The total magnification of the microscope is the product of the objective magnification and the eyepiece magnification. With a 20x objective and a 10x eyepiece, the total magnification is 200x. In some microscopes, several objectives are rotatably mounted so that an objective and any one of the eyepieces can be freely combined with one another by mechanical rotation of the bearing axis in order to achieve the desired resolution. Some high resolution cameras can also be used as a microscope and referred to as a "broader microscope," but cameras have the opposite to microscopes in the narrower sense usually not about the several rotatably mounted objectives.

The “morphology” of an inkjet droplet is understood here to be a specification of one or more features of the inkjet droplet with regard to its spatial extension in the x and y directions. The spatial extension of the document in the “z direction” corresponds to the thickness of the document. For example, the length of the document can be specified in the "x-direction" and the depth in the "y-direction". The morphology includes in particular the droplet diameter measured in the xy plane, but also the outline size and shape and / or the intensity distribution of the pixels of the droplet, e.g. the intensity profile from the center of the droplet to the edges if you have an image of a cross section of the droplet recorded in an xy plane. The morphology does not include the spatial extent of the droplet in the z-direction. The morphology of a droplet is therefore a description of the spatial extent of the droplet and optionally further properties in two-dimensional space, that is, a plane. This plane is preferably the plane on which at least parts of the image were printed on the document, or a plane parallel to this.

The "topology" of an inkjet droplet is understood here to be a specification of the morphology of the droplet in combination with a specification of one or more features of the inkjet droplet with regard to its spatial extent in the z-direction. The spatial extent of the document in the "z-direction" corresponds the thickness of the document. For example, the length of the document can be specified in an "x-direction" and the depth in "y-direction". If the document contains several material layers and / or was created from several material layers, the topology of the droplet specifies its extension over one or more material planes in the z-direction.

The topology can therefore include, in particular, the droplet diameter in the z-direction, but also the outline size and shape and / or the intensity distribution of the pixels of the droplet, e.g. the intensity profile from the center of the droplet to the edges if you have an image of a cross-section of the droplet recorded in an xz or yz plane. The topology of a droplet is therefore a description of the spatial extent of the droplet and optionally other properties in all three spatial directions. A "pattern" is understood here as an optically detectable structure (by the human eye or by an optical sensor) on or in a physical object. The pattern contains a graphic element that is repeated with a certain regularity. For example, the pattern can be a code, For example, a barcode that contains several bars or a 2D code that has several squares arranged in a grid pattern. In the course of pattern recognition, an input image is analyzed in order to identify the regularly repeated graphic elements and the pattern that is formed from these elements .

Brief description of the drawing

Embodiments of the invention will be described with reference to the drawings. In the drawing shows

1 shows an exemplary flow diagram of an embodiment of the method according to the invention for producing a document with a document body and a visually imperceptible security feature connected to the document body;

Fig. 2 is a block diagram of an ink jet printer used to produce the security feature;

3 shows an exemplary flow chart of partial steps of a method for checking a document on the basis of its security feature;

4 shows an illustration of partial steps in the generation of the security feature from an image and a pattern;

5 shows an exemplary embodiment of a variant of a document according to the invention;

6 shows different sections of the security feature produced by inkjet printing in different magnifications;

Figure 7 is a three-dimensional illustration of different droplet morphologies and topologies in a multilayer document body; Fig. 8 is a block diagram of a test unit;

9 shows a block diagram of a system with the inspection unit and a plurality of light sources and image acquisition units; and

FIG. 1 illustrates some steps of a method for producing a document with a security feature connected to the document body of the document, visually imperceptible under daylight / "white light" according to an embodiment of the invention Act as a security document, for example an identity card, a passport, a bill, an employee ID, a membership card, a credit card, a driver's license, etc. The document comprises a document body which can be made up of one or more layers. The document body or the individual layers can consist of different materials, for example cardboard, plastic, in particular polycarbonate, metal, wood or combinations thereof, in particular composites made of metal, plastic and / or cardboard , for example a chip card, an M magnetic strips, a data memory, an RFID antenna or the like. Security features can be attached to or in the document body, for example holograms, complexly designed imprints, etc. The following is to describe the application of a further security feature in or on the document or its document body, which the human eye can see when the Document is not visible in visible light.

First, in step 102, a special inkjet printer 200 is provided, as illustrated by way of example in FIG. Two. The inkjet printer includes at least a first tank with a dye-based ink and a second tank with a pigment-based ink. The absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar that an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink (of the same object or image motif) when illuminated with light in the visible spectral range through the human eyes are indistinguishable. For example, both inks can be black in visible light. The dye-based ink can be, for example, ink with solvent black 27 and the pigment-based ink can be ink with carbon black.

Alternatively, both inks can be magenta in visible light. The dye-based ink can be, for example, Solvent Red 26 and the pigment-based ink can be an ink with quinacridone.

Alternatively, both inks can be cyan in visible light. The dye-based ink can be, for example, Solvent Blue 78 and the pigment-based ink can be ink with Cu phthalocyanine.

Alternatively, both inks can be yellow in visible light. The dye-based ink can be, for example, ink with solvent yellow 124 and the pigment-based ink can be ink with brilliant yellow.

The inkjet printer can optionally contain further tanks for further pigment-based or dye-based inks.

In a further step 104, a digital graphic pattern is provided. For example, the pattern can be created in an image processing program and / or in a program for operating the inkjet printer automatically, semi-automatically and / or manually by a user of this image processing program or inkjet printer program. The pattern is formed from one or more first areas and one or more second areas. For example, the one or more first areas can be a multiplicity of first squares and the one or more second areas can be a multiplicity of second squares, the first and second squares being positioned on a rectangular surface area in such a way that they form one Form a matrix code, e.g. a QR code.

In step 106, a digital image to be linked to the document is provided. For example, the image can also be imported or generated in the above-mentioned graphics program or printer program and optionally also edited. The image can be any image, for example photos of faces, Buildings, other physical objects, and / or digital images of numbers, letters or symbols. The images preferably relate to a person or institution for or by whom the document is issued. For example, the image can be the facial image of the person for whom the document (for example identity card or passport) is issued. The image can also be the logo of a bank or company that orders the creation of the document, for example to serve as a credit card for customers or as an employee ID card for employees. As an alternative to this, the image can be, for example, a number which, for example, represents the value represented by the document (document-based bank note / payment card). The image can be a monochromatic image or a multicolor image, for example in the RGB or CMYK color spaces. Preferably the image contains as many channels as there are ink tanks in the ink jet printer.

In a further step 108, the graphic pattern and the digital image are superimposed.

For example, the overlay can be carried out by the above-mentioned image processing program or the printer program. For example, the superposition means that the pattern and thus also its first and second areas are mapped onto the digital image in a certain way. For example, the overlay can include that the digital image is initially searched for areas that are as large as possible, the pixels of which have a particularly high and preferably homogeneous intensity in the color channel whose color corresponds to the color of the dye-based ink in the first tank and the pigment-based ink in the second tank.

If the "color" of the printing inks in the first and second tanks is therefore "black", the digital image would be searched for those areas that have a particularly high and homogeneous proportion of black pixels (or RGB pixels with very low intensity value). The pattern would then be mapped onto this dynamically identified area. According to embodiments of the invention, the pattern which does not take up the entire area of the image is superimposed on the region of the image which has the smallest fluctuations in the proportion of black printing ink. In one embodiment it is sufficient if the fluctuations in the proportion of black Ink can be minimized in the areas that are printed with the pigment-based ink. The information about the black components can already be available when the photo is rendered into the individual printing colors or can be generated by a corresponding analysis. The pattern can then be positioned at different points in the image, and the respective fluctuation in the proportions of black printing ink can be determined.

In other embodiments, however, the superposition can also take place in such a way that the pattern is mapped onto the digital image according to a fixed, predefined scheme, regardless of the content of the digital image.

The method includes a further step 110 in which the inkjet printer prints the digital image on or in the document body. The printing process is carried out in such a way that the dye-based ink, but not the pigment-based ink, is used to print the regions of the digital image overlaid with the first areas and that the pigment-based ink, but not the dye-based ink, is used to print the regions of the overlaid with the second areas digital image is used. The distribution of the dye-based ink and the pigment-based ink in the printed image forms the security feature. Since the absorption spectra of the dye-based ink and the pigment-based ink are so similar to each other that an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink (if the printed object or image motif is identical) when illuminated with light in the visible spectral range through the are indistinguishable by the human eye, the printed pattern of the dye-based ink and the pigment-based ink produced by the overlay is also invisible in the printed image under white light.

In some embodiments, the pattern is code that encodes data directly or indirectly related to the document. For example, the data can contain information that specifically belongs to and / or is known to the publisher of the document, the person to whom the document is assigned. The data can in particular be data that are unique to the document and / or the person. For example, the data can be a combination of an account number and sort code, a credit card number, an identity card number, a serial number of a document-based bank note or the like.

In some embodiments, this data can also be contained in or on the document or its document body in some other way. For example, the document can contain a data memory, e.g. a magnetic strip or a chip with an integrated memory, in which the data encoded in the pattern are also stored. In the course of the document check, these can be read by the checking unit and compared with the decoded data of the printed code that has also been read out. If the compared values do not match, a forgery of the document is assumed.

For example, in some embodiments, in the course of the generation or personalization of the document, a cryptographic key pair can be generated which is specific for this document and / or a person for whom the document is created. The cryptographic key pair comprises a private and a corresponding public key. The data relating to the document and unique to the document are encrypted with the public key. The data unique to the document are encrypted with the public key of the key pair and, in a further step, encoded in a graphic code, for example a barcode or a matrix code. A photo to be printed on or in the document is then overlaid on this matrix code and the overlaid image is printed as described above.

In one embodiment of the manufacturing process, information about the mate rial of the document body, the printing process, the presence of surface coatings, and / or the inks used are applied in machine-readable form on or in the document, e.g. as a further imprint, as an engraving or as a data record stored in a data store of the document or in an external database in connection with an ID of the document. The test device can be configured to read this information from the document or the database in the course of the test and to use it in the image analysis in order to improve the recognition of the pattern. FIG. 2 shows a block diagram of an inkjet printer 200 with a plurality of ink tanks 218, which can be used to carry out the method for producing a document described in FIG. The color tanks include at least a first tank 228 with a dye-based ink 216 and a second tank 226 with a pigment-based ink 214. The dye-based ink 216 and the pigment-based ink 214 have a color that is perceived as identical by the human eye under white light. For example, this color can be “black.” In some embodiments, it is possible that the dye-based ink 216 and the pigment-based ink 214 have significantly different absorption properties at least under light of other wavelengths, for example under infrared light or under UV light, so that Ink- and pigment-based inks and corresponding printouts are clearly distinguishable for the human eye under this light.

In some embodiments, the ink tanks can comprise one or more further tanks, so that printing is possible, for example, in the CMYK color mode. In one embodiment, the tanks 218 include a cyan tank 220 208, a magenta ink tank 222 210, and another yellow ink tank 224 212.

For example, the inks can be prepared as follows: First, a mixture of 149.0 g (0.65 mol) of bisphenol A (2,2-bis- (4-hydroxyphenyl) propane and 107.9 g (0.35 mol) is added Mol) 1,1-bis- (4-hydroxyphenyl) -3,3,5-trimethylcyclohexane can be converted to a polycarbonate derivative. The polycarbonate derivative had a relative solution viscosity of 1.263. A liquid preparation was made from 17.5 parts by weight of the Polycarbonate derivative and 82.5 parts by weight of a solvent mixture prepared with the following components:

• Mesitylene: 2.4

• 1-methoxy-2-propanol acetate: 34.95

• 1,2,4-trimethylbenzene 10.75

• ethyl 3-ethoxypropionate 33.35

• cumene 0.105

• Solvent naphtha 18.45

A colorless, highly viscous solution with a solution viscosity at room temperature of 800 m Pas is obtained. In a next step, 4 g of polycarbonate solution from Example 2 and BO g of the solvent mixture from Example 2 are homogenized using a magnetic stirrer in a 50 mL wide-necked threaded glass. A colorless, low-viscosity solution with a solution viscosity at room temperature of 1.67 mPa s was obtained. The surface tension of this base ink was determined with an OEG Surftens measuring system according to the pendant drop method to be 21.4 ± 1.9 mN / m. In the next step, the desired pigment (s) or the desired dye (s) are added in amounts that are necessary to produce a certain color shade. Alternative compositions for inks for inkjet printing in or on document bodies are described in DE 10 2007 052 947 A1.

Optionally, the printer can include a document feeder 206, which is preferably designed to feed a large number of documents in a short time and to feed them to the print head unit 204. The document feeder and / or the print head unit 204 are preferably movable relative to one another, so that the print head unit can print the image overlaid with the pattern at a predefined location on the document body. The inkjet printer may include a control unit 202 which coordinates the alignment of the printhead unit and the document relative to one another and optionally also the provision and superimposition of the digital image and the pattern. The control unit 202 then contain, for example, a print program which includes functions for importing and optionally for processing images and / or patterns. The program can also include program functions for generating cryptographic keys and / or for generating a pattern which encodes a specific data value assigned to the document and possibly unique. According to one embodiment, the print head unit 204 comprises a single print head, which is charged sequentially with the colors of the individual tanks 218. Preferably, however, the printhead assembly 204 contains multiple printheads, preferably one printhead per tank 220-228.

FIG. 3 shows an exemplary flow diagram of a method according to the invention for checking a document with a document body and a visually imperceptible security feature connected to the document body. The test can be carried out, for example, with the aid of a test unit 800 as shown in FIG. The test can be carried out, for example, at a national border, at an airport, at the gate to a company premises, at a building entrance to a corresponding terminal. It is also possible to use a mobile version of the checking unit, for example for the purpose of mobile personal control or for checking the authenticity of documents of value by means of a point of sale terminal.

In a first step 302, a checking unit, for example a mobile or mobile terminal, receives a document with a document body on or in which said security feature is contained. For example, the terminal can contain an automatic document feeder or a surface or a carrier on which the document is placed manually.

The security feature is an image overlaid with a pattern, which was printed in or on the document by inkjet printing, as described, for example, with regard to FIG. An inkjet print of the image and a pattern that is not visible under white light are therefore contained in or on the document body. One or more first regions of the image contain a dye-based ink. One or more second regions of the image are printed with a pigment-based ink. The one or more first regions are free of the pigment-based ink and the one or more second regions are free of the dye-based ink. The security feature thus comprises a multiplicity of first inkjet droplets based on the dye-based ink and a multiplicity of second inkjet droplets based on the pigment-based ink. The absorption spectrum of the dye-based ink and that of the pigment-based ink are so similar that they are indistinguishable in color to the human eye when the inkjet printing is illuminated with light in the visible spectral range, and therefore the pattern of the dye-based ink and the pigment-based ink Ink-printed image regions are not visible under white light.

The checking unit contains an image capturing unit, the resolution of which is sufficiently high to detect at least a certain proportion of the inkjet droplets (for example min- at least 20%, preferably at least 50%) spatially resolved from one another. For example, the image acquisition unit can be a microscope or a high-resolution camera.

In step 304, the image frame unit takes a test image. The test image is a digital image that depicts at least the inkjet print of the document and optionally other regions of the document. It is also possible for the test image to cover the entire document. The test image is, for example, an RGB image or a monochromatic image that is recorded under white light.

In a next step 306, the checking unit carries out an image analysis of the checking image in order to check the authenticity of the document using the inkjet printer.

In the course of the image analysis, at least the morphology of the inkjet droplets depicted in the test image, of which the inkjet print consists, is automatically recorded in step 308. The morphology of an inkjet droplet describes its spatial extent within the test image, i.e. on a two-dimensional plane that is formed by the length and width of the document or the length and width of the part of the documents shown in the test image. The morphology includes in particular the cross-section of the droplet, the nature of the outline (clearly delimited or diffuse edge, essentially circular or heavily frayed or star-shaped), the presence, strength and / or orientation of an intensity gradient within the droplet and / or the brightness of the Droplets in each case with white light. In some embodiments, the image acquisition unit acquires multiple test images from different z-planes of the document, for example by modulating the focus settings of the image acquisition unit. This can be particularly advantageous if the document consists of several layers of material, the print was applied to one of the inner layers and the ink penetrated into one or more of the further layers during or after the combination of these material layers with further material layers . For example, it has been observed that when several layers of material are pressed and / or glued together, dye-based inks and pigment-based inks penetrate adjoining material layers to different depths and that inkjet droplets from pigment-based inks and dye-based inks therefore have different droplet morphologies. In this case it can Thus, by analyzing a plurality of test images, the topology of the individual ink jet droplets can be determined and it is possible to differentiate first image regions, which selectively contain droplets of the dye-based ink, from the second image regions, which selectively contain droplets of the pigment-based ink, and thereby differentiate one from the first and to recognize patterns formed in second image areas.

In the BIO step of the image analysis, a digital graphic pattern is automatically reconstructed from the test image based on the detected droplet morphologies or from a plurality of test images based on the detected droplet morphologies in combination with the detected droplet topologies. The recognized pattern is formed from one or more first areas and one or more second areas. The first areas consist of those test image regions whose inkjet droplets have a first morphology (and optionally also topology). The second areas consist of those test image regions whose inkjet droplets have a second morphology (and optionally also topology). The first and second morphologies (or topologies, if these have been determined) differ from one another sufficiently so that the first and second regions can be reliably recognized and differentiated by the image analysis program.

According to embodiments, the first and second regions are recognized on the basis of the droplet morphology or topology by a trained machine learning (ML) program, in particular a neural network or a support vector machine. The ML program can be part of the image analysis program. For example, the ML program can be trained on a training data set which contains a plurality of images with first and second image regions noted on, where the first image regions only contain inkjet droplets of the dye-based ink and the second image regions contain only the inkjet droplets of the pigment-based ink.

In a further step 312 of the test method, the test unit compares the graphic pattern or a value derived therefrom with a reference value. For example, the graphic pattern can represent a simple checkerboard pattern made up of first and second areas, which is stored as a reference pattern in a memory of the test unit is. A comparison of the reconstructed pattern with the reference pattern shows whether the reconstructed had to be identical or sufficiently similar to the reference pattern. In this case, the security feature created by the inkjet printing and also the document that contains it are considered valid. It is also possible that the reconstructed pattern is not compared directly with a reference pattern, but is interpreted and processed as a code. For example, the pattern can be interpreted as a barcode or matrix code, in particular as a QR code. In this case there is a decoding step in which a specific value is reconstructed from the code. This value can be, for example, a document-specific or person-specific number, for example a document ID, a credit card number, an ID number, or a combination of one or more document-related or personal data such as a person's name , Birthday and place of birth. The "derived" value obtained with the aid of the decoding method is compared with a reference value stored in the test unit or in a data memory accessible to the test unit, and if there is identity or sufficient similarity, the validity of the security feature is determined.

Optionally, the check can also include a check of further security features known in the prior art, the document being recognized as genuine or valid only if these further security features are also recognized as valid.

FIG. 4 shows an illustration of partial steps in the generation of the security feature from an image 402 and a digital pattern 404.

The image 402 can be, for example, a digital portrait image captured in visible light of a person for whom a new document is to be issued and / or personalized. The image 402 can be captured as an RGB image, for example, and converted into a CMYKK + for the purpose of inkjet printing. A digital pattern 404 is also provided. In some embodiments the pattern is very simple. For example, the pattern shown in FIG. 4 consists of two first areas 408.1, 408.2 and two second areas 406.1, 406.2. The first areas are areas that are designed to be printed with the dye-based inks but not the pigment-based inks. The second areas are areas that are meant to be with of the pigment-based ink but not to be printed with the dye-based ink. In other embodiments, the pattern can also be significantly more complex and in particular a barcode or matrix code, the complexity of which allows one or more data values such as person IDs, document IDs and other values to be stored in coded form in the pattern . The digital image 402 and pattern 404 are overlaid prior to printing. In the example shown here, the image and pattern have the same size and are superimposed on a 1 to 1 scale. In other embodiments, however, it is also possible for the overlay to take place in such a way that the pattern overlays only a part of the image and / or that the pattern is overlaid with the image at positions determined dynamically and depending on the image content. Finally, the digital image 402 is printed on or in the document body in such a way that first regions 414.1, 414.2 of the document body correspond to the first image areas 408.1, 408.2 and are printed with the dye-based ink and the second regions 412.1, 412.2 of the document body correspond to the second image areas 406.1, 46.2 and are printed with the pigment-based ink. In visible light, the first 414 and second 412 regions of the resulting inkjet printed image are not distinguishable with the naked eye. With a microscope or a high-resolution camera, however, it is possible to recognize the first and second regions and the pattern formed by them on the basis of the different droplet morphology or droplet topology.

In some embodiments such as that shown in FIG. 4, the dye-based ink and the pigment-based ink additionally have the property that under a special light, for example infrared light, only the pigment-based ink is opaque and absorbs light, whereas the dye-based ink is almost completely transparent and therefore invisible is cash. In the first regions 414.1, 414.2 in which the portrait image is printed with the dye-based ink, no imprint can be seen, but the background color of the card body appears. In the second regions 412.1, 412.2, which were printed with the pigment-based ink that is also opaque in infrared light, the corresponding regions of the portrait image are visible in a manner similar to that in white light. In embodiments in which the dye-based ink and the pigment-based ink produce different color values or intensities in the visible wavelength range under a special light such as infrared light or UV light, the document can be illuminated. ments or the imprint can be carried out with this special light to test whether the pattern is visible under these conditions. If no pattern is visible, the document is considered to be false.

FIG. 5 shows a document 500 according to an embodiment of the invention. The document contains an edge-shaped card body 506 with at least one image 502, which contains a security feature that is not recognizable in visible light in the form of a pattern of first and second regions of ink jet droplets of different morphology or topology, which is not recognizable in visible light. The document can optionally have a large number of other security features such as watermarks, embossing marks, holograms or tilt images, which serve to make forgery more difficult and are not shown here. The image 502 can in particular be a personalized image, for example a portrait image of the person to whom this document is assigned. The document body can be a plastic card that is essentially transparent in visible light, as described, for example, in EP 1222620 B1 or WO 02/4500882. However, the document body can also be opaque.

The document body can in particular consist of several layers, as is illustrated in FIG. For example, the document can consist of several layers of polymer materials such as polyolefins, polyesters and / or polycarbonates. In particular, the document can be a polymer layer composite document, the production of which is described in DE10 2008012419 A1.

An image 502 is inkjet printed on or in the document. The image contains first areas or regions which are printed only with the dye-based ink and optionally further inks, but not with the pigment-based ink of the same printing color, and second areas or regions which are only printed with the pigment-based ink and optionally further inks , but are not printed with the dye-based ink of the same printing color. The first and second regions form a pattern that is invisible to the human eye. The image can, for example, have an area proportion of 5-95% on the surface of the document. Optionally, the document can contain further elements such as a microchip 504 and / or a magnetic strip (not shown). Additionally or alternatively, the document can contain further optically detectable data 508, for example a printed document ID or person ID. The optically detectable data 508 can likewise be printed on or applied by means of other methods, for example laser engraving. The invisible pattern contained in the image 502 can be, for example, a QR code which contains the further optically detectable data 508 in coded form.

FIG. 6A shows a section 600 of the face of the person depicted in image 502 under white light at 20 times magnification. Although the right eye is printed with the dye-based ink 216 and the left eye with the pigment-based ink 214, with this resolution under white light no color or color is possible. Difference in intensity and therefore no recognizable pattern from the first and second image areas.

FIG. 6B shows a sub-section 602 of the image 502 with the right eye of the person printed with the dye-based ink, magnified 100 times. It can be seen that the edges of the droplets are blurred. It is not possible for the image capture unit to find a plane in which the droplets are sharply focused.

FIG. 6C shows a sub-section 604 of the image 502 with the left eye of the person printed with the pigment-based ink, magnified 100 times. One recognizes the sharp, dark edges of the pigment-based ink droplets compared to the edges of the droplets shown in FIG. 6B, which enable the image acquisition unit to find at least one plane in which a sharp focus on the droplets is possible. According to embodiments, the document checking method comprises a modification of the zooming of the image acquisition unit in such a way that an attempt is made to bring the focus into focus in several planes along the z dimension of the document. It can be determined whether it is possible to find at least one plane per droplet in which its edges appear sharp. The different focusability or edge sharpness of the droplets allows an inspection unit to recognize areas that contain only droplets of the pigment-based ink or only droplets of the dye-based ink. FIG. 6D shows a sub-section 606 of the image 502 with the right eye of the person printed with the dye-based ink, magnified 200 times. Even at this magnification, the morphology of the droplets is characterized by a blurred outline, the color intensity or brightness of the droplets is largely homogeneously distributed within the respective droplets.

FIG. 6E shows a sub-section 608 of the image 502 with the left eye of the person printed with the pigment-based ink, magnified 200 times. Even with this enlargement, the morphology of the droplets is characterized by a sharp outline, the color intensity or brightness of the droplets is often inhomogeneously distributed within the respective droplets, in particular the droplet edges are darker than the inner regions of the droplets.

FIG. 7 shows a three-dimensional illustration of different droplet morphologies and topologies in a multilayer document body of a document 700. The document comprises several material layers 702-714 made of a polymer material, in particular polycarbonate. The document has a length (dimension x), a width (dimension y) and a height (dimension z). The multiple layers are stacked on top of one another along the dimension z before they are processed into a layer composite.

In the course of the production of the document, the image with the non-visible security feature is printed on one of the layers, here layer 708, by inkjet printing, as has already been described here for embodiments of the invention. The printing is preferably carried out on one of the material layers that will later lie inside the finished document. Immediately after the printing on the layer 708, the individual inkjet droplets 716, 718, 720, 722 run on the surface of the material layer 708, which is defined by the X and Y dimensions. The droplets 716, 718 consist of the pigment-based ink 214, for example an ink with carbon black, and the droplets 720, 722 consist of the dye-based ink 216, for example an ink with solvent black 27. The two-dimensional course of the droplets on this surface is called the morphology of the Called droplets. The applicant has found that the morphology of the droplets depends critically on whether the droplet consists of a pigment-based ink 214 or a dye-based ink 216. So are the edges of pigment-based Ink droplets are sharply focused and often darker than the inside of the droplet, whereas the edges of dye-based ink droplets are blurred and fuzzy and usually have the same brightness as the inside of the droplets, sometimes the edges also appear lighter than the center of the droplet. It was also found that the depth of penetration of the color over several layers is also different for pigment-based and dye-based inks. The depth of penetration of the dye-based ink is greater and thus also the extent of the droplets 722, 720 along the z-axis, represented here by the larger L2 compared to the smaller LI.

The edges of the droplets are also blurred in the z dimension. According to embodiments, several test images are recorded by the image acquisition unit along the Z plane, preferably in such a way that at least one test image is recorded per plane 702-714. For example, the focus of the image capturing unit can be changed in such a way that different levels of the document are “traversed”, the test images being captured.

According to one embodiment, the document consisting of several layers is produced in that two or more polymer layers are provided and at least one surface on at least one 708 of these polymer layers is printed with the dye-based ink and the pigment-based ink in such a way that an image 502 is formed in which the first and second areas of the image, which were printed with either the dye-based or the pigment-based inks, form an invisible pattern, as described, for example, with reference to FIGS. 4 and 5. The image can, for example, have an area proportion of 5-95% on the surface of the document. After the printing step, the multiple layers of polymer material are stacked on top of one another and firmly bonded, for example by means of printing and / or adhesive processes. The printed polymer layer surface can form the top or bottom of the layer composite. The printed polymer layer surface is preferably surrounded at the bottom and / or at the top by at least one further polymer layer 706, 710, so that the printed surface is located in the interior of the layer composite. Depending on the type of material used in the adjacent layers, the penetration depth of the droplets 716, 718, 720, 722 into the adjacent layers may differ.

In principle, all materials customary in the field of security and / or value documents can be used as materials for the polymer layers. Layers 702-714 are pre- preferably 50 miti to 300 miti thick. The polymer layers can, identical or different, based on a polymer material from the group comprising PC (polycarbonate, in particular special Eisphenol A polycarbonate), PET (polyethylene glycol terephthalate), PMMA (polymethyl methacrylate), TPU (thermoplastic polyurethane elastomers), PE (polyethylene ), PP (polypropylene), PI (polyimide or poly-trans-isoprene), PVC (polyvinyl chloride) and copolymers of such polymers. Furthermore, coextruded films of these materials can be used. The use of PC materials is preferred, with so-called low-Tg materials based on polycarbonate also being used, for example, but by no means necessarily, in particular for a polymer layer on which a printing layer is applied and / or for a polymer layer, which is connected to a polymer layer carrying a printing layer, namely on the side with the printing layer. Low-Tg materials are polymers with a glass transition temperature below 140 ° C. Further embodiments relating to a multilayer document, the printing of which and how the individual layers can be connected to one another to form a single form-fitting document are described in patent applications DE102007052947 A1 and DE 102008012419A1.

In some embodiments, the image 502 can also be printed onto the multilayer document body in such a way that different parts of the image are printed in different layers. It is possible that the first image areas with the dye-based ink are continuously printed on different material layers than the second image areas with the pigment-based ink. In other embodiments, however, the distribution of the pressure on different layers is independent of the ink used, so that each layer is printed with both the pigment-based ink and the dye-based ink, albeit on different partial areas of the respective layer. A part of an image 502 with a visually imperceptible security feature embedded therein or a part thereof can be associated with each of the layers. Only in the connection are all parts of the image and the security feature embedded therein arranged together in such a way that a viewer can recognize the image.

FIG. 8 shows a block diagram of a test unit 800, which is designed to ensure that security features that are invisibly hidden in a printed image in the case of white light and also to be checked in the absence of a special light source (e.g. infrared light, UV light). The test unit can be designed as a mobile test unit or as a mobile terminal. It contains an opening 808 for receiving a value or security document 500, 700. The opening can consist, for example, only of a container or a filing device for the document, so that the user has to position the document manually in or on this receptacle or the tray . Additionally or alternatively, the opening 808 can contain an automatic document feeder, which receives the document and positions it at a predefined location within the checking unit.

The inspection unit further comprises an image acquisition unit 802, for example a microscope or a high-resolution camera. The resolution of the image acquisition unit must be at least so high that a minimum amount of the inkjet droplets (for example at least 20%) on an imprint 502 of the correctly positioned document can be spatially resolved from one another. The image acquisition unit is designed to acquire one or more test images. For example, the image acquisition unit has an automatic and / or manually operable focus. The focus is changed gradually or continuously so that at least some of the inkjet droplets (namely those that were printed with pigment-based inks) appear sharp. A digital image, the so-called test image, is recorded in at least this plane and optionally in further planes in the Z direction. The test image depicts at least the printed image 502 and optionally other areas of the document body. The captured image can be, for example, a monochrome image or an RGB image. It is preferably the same type of image from which a training data set of images also consisted, on the basis of which image analysis software 806 was trained. However, it is also possible that a captured monochrome image or RGB image is subsequently converted into such an image format. The test images are recorded under white light. This means that at the time of the image acquisition, the document located in the checking unit is either illuminated with ambient white light through the opening 808 and / or that the document is illuminated with the aid of a white light source which is located inside or in the vicinity of the checking unit.

In addition, the checking unit comprises an image analysis unit 804 with one or more processors 805 and image analysis software 806. The image analysis software can be for example a trained neural network. The image analysis software is configured to receive the high-resolution test images from the image acquisition unit or to read them from a memory in which the test images were stored by the image acquisition unit. Analyze to determine the location and morphology and / or topology of the inkjet droplets making up the imprint 502. In addition, the image analysis software is designed to use the droplet morphology and / or topology to recognize first image regions 414.1, 414.2 which contain droplets of dye-based ink and which are free from droplets of pigment-based ink, as well as to recognize second image regions 412.1, 412.2, which contain pigment-based ink droplets and which are free of dye-based ink droplets. The image analysis software is thus configured to recognize a pattern made up of first and second regions on the basis of the droplet morphology and / or topology. The checking unit 800 compares the recognized pattern with a reference value. If the comparison shows that the pattern is identical or sufficiently similar to the reference value, the test unit determines as a result of the test that the security feature is valid and that the document, unless other security features of the document are invalid, is valid (and thus real) is to be seen. The test can also be more complex, for example if the pattern is a two-dimensional code. In this case, the test unit uses a decoding method on the recognized pattern in order to obtain the value encoded in the pattern and to compare this with a corresponding reference value which is stored in a memory accessible to the test unit. In the event of identity or sufficient similarity to the reference value, a validity of the security feature or the document is determined.

Optionally, the test unit can contain a display 810, for example an LED display. The result of the test can be displayed to a user via the display 810. Optionally, the image analysis software can also make the recognized pattern visible to a human user in the form of a graphic representation, for example an overlay image, and output it via the display 810.

According to one embodiment, a public cryptographic key is stored in a data memory of the document 500, 700. The public key forms an asymmetrical cryptographic key with a private cryptographic key. selpaar. The private key is assigned to a specific organization, for example the issuer of the document, a company or an authority, in particular a state supervisory authority. The private key is stored in a central data memory to which the test unit has access. A copy of the public key can be stored on several documents from the same publisher and used to encrypt a secret value. For example, the pattern can contain a graphic code, for example a QR code, a secret data value being encoded in the QR code in encrypted form. The secret value can be personal data or cryptographic keys. The test unit is designed to decode the QR code and thereby receive the secret value in encrypted form. The verification unit now uses the private key to decrypt the encrypted secret value. The decrypted secret value can in turn be checked and further processed by the checking unit in various ways. For example, the value can be compared with a reference value.

Alternatively, the decoded but still encrypted data value can be transmitted from the checking unit to a decryption device. In this case, the checking unit has no access to the private cryptographic key, which increases the security of the key. Only the decryption device has access to the private key and uses it to decrypt the decoded but still encrypted data made available by a large number of test units. The decrypted data can then be transmitted back from the decryption device to the test unit. This is preferably done via a protected data communication channel, for example end-to-end encryption. For example, the decrypted data can be a serial number or other identifier of the document or of the user to whom the document is assigned. The serial number or other identifier is stored in the checking unit in the form of a reference value and or can be read out from the document by the checking unit via another transmission channel. For example, the serial number or identifier can be pressed or engraved on the document body and recorded by the checking unit via a camera. By comparing the decrypted data value with the reference value and / or with the value received via the other transmission channel, the authenticity of the security Security feature can be determined by the test unit, values being equal or sufficient similarity implying authenticity.

FIG. 9 shows a block diagram of a system 900 with the test unit 800 and a plurality of light sources and image acquisition units. The system 900 includes a test unit, as it is described for embodiments of the invention and, for example, with reference to FIG. 8, as well as a white light source 906 and an infrared light source 904. Both light sources are positioned so that they are correctly inserted in the opening or in illuminate the document 500 placed in the checking unit or at least illuminate an image 502 formed in or on this document. The light sources 904, 906 can be installed as an integral part of the test unit or installed or positioned on or next to the test unit. For example, the opening 808 can be large enough that light from light sources external to the test unit also strikes the document with sufficient intensity through it.

The image acquisition unit 802 comprises at least one high-resolution image acquisition unit 910, which is sensitive in the wavelength range of visible light and can record images. In addition, the image acquisition unit comprises an infrared light camera 908, the resolution of which can be lower than that of the white light camera 910. The infrared light camera 908 is sensitive at least in the wavelength range of the infrared light. In some embodiments, it is also possible for a high-resolution camera that is sensitive both in the white light range and in the infrared light range and thus corresponds in functional terms to a combination of the two image acquisition units 908, 910.

The checking unit can be operated in at least two different operating modes: in the "white light mode" the document 500 located in the checking unit 800 is illuminated with white light and the white light camera 910 makes one or more high-resolution test images under white light irradiation. The test images can be captured as RGB images, for example and are stored as monochrome images, with the pixel intensities of the monochrome channel resulting as a sum or an average value of the intensities of the 3 R, B, G color channels. The high-resolution test images 912 are transmitted to the image analysis software 806, which is automatically recognized using the Droplet morphology and / or topology a pattern within the image 914. The pattern represents the security feature that is invisible to the human eye and is used by the inspection unit such as is used to compare the pattern or a value derived from it with a reference value. In addition, the pattern 914 can also be displayed to a user via the display 810 as an overlay pattern of the image 912, so that the user can recognize that a code has actually been recognized here which corresponds, for example, to a certain expected type of code (for example barcode or Matrix codes).

In addition, the checking unit can be operated in an "IR mode". In the "IR mode", the document 500 located in the checking unit 800 is illuminated with infrared light from the infrared light source 904 and the IR camera 908 makes one or more IR test images Infrared light irradiation. The IR test images can be captured and saved as monochrome images, for example. The IR test images can have the same or a lower resolution than the white light test images. The IR inspection image or images can be displayed directly on the display 810 to a user. The IR mode can be particularly advantageous when the pigment-based ink 214 and dye-based ink 216 used, which have an identical or indistinguishably similar absorption spectrum under white light, but have a significantly different absorption spectrum under IR light. In this case, the pattern can be recognized even without image analysis simply by exposing the document to an infrared light source 904. In the IR mode, the IR test image recorded by the IR camera 908 under IR light is forwarded directly to the display 810, where it is displayed to the user. Even without the contribution of the image analysis, the user can recognize the pattern and thereby verify the security feature. Optionally, a decoding step can also take place in the IR operating mode in order to extract information encoded in the pattern and to compare it with a reference value.

In addition to or instead of the test image, for example a high-resolution RGB image recorded under white light, the test unit can also be configured to also record an IR image, which is referred to as the "IR test image". This is in particular a digital one monochrome image recorded under IR illumination of lower resolution than the test image recorded under white light. The IR test image corresponds to a brightness level image and can also be displayed as a grayscale image. The test method can also include the calculation of an image from the test image which is used as the test image. The color image that corresponds to its brightness level image if it had been recorded with a camera that is sensitive in the IR spectrum and illuminated with infrared light. The calculation can be done using a mapping that was provided when the document was produced. The assignment assigns color tones of an RGB color image, for example the image printed on or in the document, to brightness values at corresponding positions of a monochrome image recorded under IR light by the inkjet printing of this image. In embodiments of the invention, only an opaque variant of the black printing ink, ie the printing ink of the pigment-based ink, is taken into account when calculating the brightness level image, since the dye-based ink is transparent when illuminated with infrared light. After the calculated image is available, it is compared with the monochrome IR test image. The comparison comprises a comparison of the brightness levels of the calculated image and the monochrome image, with the same areas of the images being compared. The image brightness and / or the contrast can be adjusted for one or both images before the comparison. The comparison leads to a difference image in which the pattern embedded in the image can already be recognized. In a further step of the method, the pattern is reconstructed from the difference image. In this case, the coding parameters supplied to the device executing the method can be used which were used in generating the pattern, for example a number of regions arranged next to one another, their spatial extent, an error correction used in the coding, and the like. The coding parameters can, for example, be supplied from a database. Information can be present on the document which is fed to the database in order to select the coding parameters that are suitable for the document. This information is preferably designed to be machine-readable. The coding parameters can be transmitted to the test unit via a secure data connection. The reconstructed pattern, for example a graphic code, is then decoded.

According to embodiments of the invention, it has been found that there are advantageous combinations of a dye-based ink and a pigment-based ink which, although indistinguishable from one another under visible light, are optically clear from one another under light of other wavelengths, such as infrared or UV are distinguishable. Thus, the IR mode can be viewed as a faster and less computationally intensive variant of the white light mode.

According to embodiments of the invention, the test unit is configured in such a way that it normally operates in IR mode. However, under certain situations it can happen that the IR operation is not possible, and even fails for a short time and unpredictably. For example, the IR light source can fail due to a technical defect or due to the end of life of the light source 904. If the IR light source 904 is located inside the test unit 800, but the test unit is used outdoors and is exposed to strong sunlight, it may be that the sunlight penetrating through the opening 904, which represents interference light, is a detection of IR light. Makes test images under pure IR light impossible. In this case, the system switches to white light mode. This is robust against penetrating sunlight. According to some embodiments, the change between IR mode and white light mode is done manually. In other embodiments, the change takes place automatically when the test unit determines that the IR light source 904 is defective and / or that a white light component inside the test unit exceeds a maximum value. For example, the test unit can have a white light sensor to automatically determine whether the maximum value has been exceeded.

According to one embodiment, the system includes an access control device 902, for example a barrier or a gate, or is coupled to the access control device. The access control device is opened automatically depending on the result of the examination of the document. For example, the gate to a secured area or building is opened automatically and only when a user has successfully authenticated himself to the test unit with an ID document assigned to the user, the authentication including a test of the invisible security feature, as was done for Embodiments of the invention are described herein.

If the document 500, 700 is to support a check in the IR mode in addition to the white light mode, according to embodiments of the invention, some additional steps can already be carried out during the process for producing or personalizing the document. For example, an assignment is first made available that contains a relationship hung of color signals from a white-light-sensitive RGB camera on the one hand and monochrome signals from a camera sensitive in the IR spectrum on the other hand for a variety of color tones generated by mixing colorants intended for the production of the document. The monochrome signals correspond to the brightness levels of the respective color tones when illuminated with IR light. The relationship between the color signals and the monochrome signals can take place, for example, as an assignment table or in the form of one or more analytical descriptions. The relationship can take place, for example, by creating a color image of a color reference panel and a corresponding image with the camera sensitive in the IR spectrum when illuminated with light in the IR spectrum, the signals from the color camera and the camera sensitive in the IR spectrum for identical image areas can be assigned to each other in the recordings.

Finally, the image with the graphic pattern embedded therein is printed on or in the document body.

Printing can be done using one of two visually indistinguishable ink variants of one of the colors intended for the production of the document, one variant, the "pigment-based ink", opaque when illuminated with infrared light and the other variant, the "dye-based ink" , is transparent, and for which the color signals recorded by the white-light-sensitive RGB camera do not differ. The variant used in each case of the inks, which are present in two variants, is selected when the graphic pattern is embedded in the image and accordingly supplied to an inkjet printer as printing information.

The creation of the assignment can take place in detail as follows: first a color reference table is printed using the inks provided for the production of the document and, if necessary, a carrier material provided for the production of the document. The color reference panel includes a plurality of inks that depict a plurality of hues that occur in one or more images to be associated with the document, the plurality of hues including both pigment-based inks and dye-based inks. A color image (i.e. an RGB image under white light) is recorded from the color reference board, and a monochrome picture with a camera that is sensitive in the IR spectrum when the color reference board is illuminated with IR Light. These steps can be done in any order. The color tones of the RGB color image, which are arranged at certain positions in the color reference table, are then assigned to brightness values of the monochrome image that are located at corresponding positions. The mapping can be provided as a mapping table, or in the form of an analytical description.

In order to now print a certain image of a person or another physical object on or in the document body, an RGB color image of the object or the person who is to appear on the document body is first recorded with a color camera under white light, or a existing RGB image taken under white light provided. A color printout is made from this image and then a monochrome image of this printout is recorded with a camera sensitive in the IR spectrum under illumination with IR light. The order of the recordings is irrelevant. An image is calculated from the color image, which corresponds to its brightness level image when captured by a camera sensitive in the IR spectrum when illuminated with infrared light, if only one opaque variant (a pigment-based ink) one in two variants (pigment-based ink and dye-based ink one essentially the same color in visible light) is used, the other variant of which is transparent when illuminated with infrared light. For this purpose, an assignment can be provided, for example an assignment made in the manner described. In other words, an image is calculated from the image of the color camera using an assignment that corresponds to its brightness level image when recorded by a camera sensitive in the IR spectrum when illuminated with infrared light, if only one opaque variant (pigment-based ink) is one in two Variants present printing ink is used, the other variant (dye-based ink) is transparent when illuminated with infrared light.

The calculated image and the brightness level image recorded under illumination with infrared light from the camera, which is sensitive in the IR spectrum, are compared and a difference image is calculated that contains the pattern.

According to one embodiment, a pattern embedded in the image captured by the color camera is reconstructed from the difference image, the content of which is decoded. To do this, NEN supplied coding parameters are used, which were used in the generation of the pattern or the graphic code.

The document 500 can be, for example, an identity card that contains one of the portrait images of the person to whom it is assigned in the form of a colored inkjet print 502. The color inkjet print was created with CMYK printing, with the color black being printed with a dye-based ink in the first regions of the inkjet printing and with a pigment-based ink in the second regions of the inkjet printing. The first and second regions cannot be distinguished in visible light with the human eye, so that the pattern formed from the first and second regions is a security feature that is invisible to the human eye in visible light.

The portrait image of the woman depicted on the document can be captured by a camera as an RGB image, for example. The image is a pixel graphic that contains triples per pixel of values for the basic colors red, green and blue (RGB) or signals derived therefrom.

In the printing method according to one embodiment, a large number of color tones for the display of color images are printed with a few printing colors, for example by printing very small, closely spaced areas or dots with the colors cyan, magenta, yellow and black. These inks are also referred to by the abbreviations C, M, Y and K. Viewed from a distance from which the areas printed with a single color are no longer individually perceptible, these areas appear in the most varied of color tones depending on the area ratio of the printing inks.

In particular, the black ink, K, can exist in two different versions, K and K +, which do not differ visually, one version of which is a dye-based ink, which in some embodiments is transparent to IR light and of which the other Version is a pigment-based ink, which in some embodiments is opaque under IR light. Depending on the hue, each area of a color image has a greater or lesser proportion of black printing ink. The embedding of the pattern in the image (i.e. in the photo) during printing takes place in such a way that the image areas overlaid with the first areas of the pattern are only printed with the black, dye-based ink and the image areas overlaid with the second areas of the pattern only with the pigment-based black ink. The printing of the remaining C, M and Y inks is not affected by the overlay of the image with the pattern. Because of the visual indistinguishability of the two black inks, a human viewer sees a normal color photo. The visual indistinguishability also applies to a recording with the color camera, ie the signals from the color camera for the different versions of the printing inks are identical. When illuminated with IR light and recording with a camera sensitive in the IR spectrum, the pattern 914 becomes visible, as is indicated in FIG. In particular, the elements of the pattern that are printed with the IR light absorbing version of the black printing ink within an element or from element to element can have strongly fluctuating degrees of absorption which are dependent on the image content of the color image.

LIST OF SIGNS

102-110 steps printing process

200 inkjet printers

202 control unit inkjet printer

204 printhead assembly

206 Document feeder printer

208 Cyan ink

210 Red ink

212 Ink with the printing color yellow

214 Black ink: pigment-based ink

216 Black ink: dye-based ink

218 paint tank container

220-228 paint tanks

302-312 steps test procedure

402 digital RGB image

404 pattern

406.1, 406.2 second areas of the pattern

408.1, 408.2 first areas of pattern 410 printed image with first and second regions in IR light

414.1, 414.2 first regions: printed with dye-based ink

412.1, 412.2 second regions: printed with pigment-based ink 500 document

502 imprint (in visible light)

504 chip

506 document body

508 card-specific value

600 Section of the printed image 502

602 Detail from Fig. 502: eye on the right, dye-based. Ink, 100 x

604 Detail from Fig. 502: eye on the left, pigment-based. Ink, 100 x

606 Detail from Fig. 502: eye on the right, dye-based. Ink, 200 x 608 Detail from Fig. 502: eye on the left, pigment-based. Ink, 200 x

700 document made up of several layers of material

702-714 individual layers of material

716 inkjet droplets pigment-based ink

718 inkjet droplets pigment-based ink

720 inkjet droplets of dye-based ink

722 inkjet droplets dye-based ink

800 test unit

802 image capture unit

804 image analysis unit

805 processor

806 Bi Idana lysesoftwa re 808 Opening / document feed 810 Display / screen 812 Displayed test result 900 System 902 Barrier / gate 904 Infrared light source 906 White light source 908 Infrared-sensitive camera 910 High-resolution camera in the visible range / microscope 912 Test image 914 Pattern

Claims

Patent claims

1. A method for producing a document (500,700) with a document body (506) and a visually imperceptible security feature connected to the document body, comprising:

- Providing (102) an inkjet printer (200), which includes at least a first tank (228) with a dye-based ink (216) and a second tank (226) with a pigment-based ink (214), the absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar that an inkjet printing with the dye-based ink and an inkjet printing with the pigment-based ink are indistinguishable by the human eye when illuminated with light in the visible spectral range;

- providing (104) a digital graphic pattern (404) which is formed from one or more first areas (408.1, 408.2) and one or more second areas (406.1, 406.2); and

- providing a digital image (402) to be associated with the document;

- superimposition (106) of the graphic pattern and the digital image;

- Printing (108) the digital image on or in the document body with the inkjet printer in such a way that the dye-based ink but not the pigment-based ink is used for printing the regions (414.1, 414.2) of the digital image superimposed with the first areas and that the pigment-based ink but not the dye-based ink is used for printing the regions (412.1, 412.2) of the digital image superimposed with the second areas, the distribution of the dye-based ink and the pigment-based ink in the printed image being the security feature.

2. The method of claim 1, wherein the pattern comprises a bar code or matrix code.

3. The method according to any one of the preceding claims, wherein the digital image is a monochrome digital image or wherein the digital image is a polychromatic image, in particular a special RGB image or CMYK image.

4. The method according to any one of the preceding claims, wherein the document body material on which the image is printed has a background color, the background color is light and in particular white, the dye-based ink for the human eye when illuminated with light in the infrared spectral range causes a transparent color impression, so that the one or more first areas have the background color, and the pigment-based ink causes an opaque color impression for the human eye, so that the one or more second areas have a different color than the background color.

5. The method according to any one of the preceding claims, wherein the inks in the first and two th tank when illuminated with light in the visible spectral range both:

- have a black shade,

• wherein the dye-based ink preferably comprises one or more of the following substances: Solvent Black 27, Solvent Black 29; and or

• wherein the pigment-based ink preferably comprises one or more of the following substances: carbon black, pigment black 28;

- or both have a cyan hue,

• wherein the dye-based ink preferably comprises one or more of the following substances: Solvent Blue 78, Sudan Blue; and or

• wherein the pigment-based ink preferably comprises one or more of the following substances: Cu phthalocyanine;

- or both are magenta, • wherein the dye-based ink preferably comprises one or more of the following substances: red azo dyes, for example Solvent Red 26, Sudan Red; and or

• where the pigment-based ink is quinacridone;

- or both are yellow in color,

• wherein the dye-based ink preferably comprises one or more of the following substances: yellow azo dyes, e.g. Solvent Yellow 124; and or

The pigment-based ink preferably comprising one or more of the following substances: brilliant yellow, pigment yellow 151.

6. The method according to any one of the preceding claims, wherein the inkjet printer includes at least a third tank with a further dye-based ink and a fourth tank with a further pigment-based ink, wherein the absorption spectrum of the further dye-based ink and the absorption spectrum of the further pigment-based ink are mutually exclusive are so similar that this further dye-based ink is indistinguishable in color from the further pigment-based ink by the human eye when illuminated with light in the visible spectral range, but is color-distinguishable from the inks in the first and second tanks, further comprising:

Providing a further digital graphic pattern containing one or more third areas and one or more fourth areas; and

- Superposition of the further graphic pattern and the digital image;

- Carrying out the printing of the digital image on or in the document body in such a way that the dye-based ink but not the pigment-based ink is used for printing the regions of the digital image superimposed with the first areas, but that the pigment-based ink is not used Dye-based ink can be used to print the regions of the digital image that are superimposed with the second Be rich, that the further dye-based ink but not the further pigment-based ink Ink is used to print the regions of the digital image overlaid with the third areas, the further pigment-based ink but not the further dye-based ink is used to print the regions of the digital image overlaid with the fourth areas, where the distribution of the dye-based ink is used , the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image is the security feature.

7. The method according to any one of the preceding claims, comprising:

- Determining an area within the image, which has a minimum homogeneity and minimum intensity with regard to the absorption spectrum of the color tone that is to be printed with the dye-based ink or the pigment-based ink;

- Implementation of the overlay in such a way that the graphic pattern is completely mapped onto this surface.

8. The method according to any one of the preceding claims, wherein the document body is composed of a plurality of material layers (702-714), wherein the printing of the digital image in the document body takes place in such a way that the digital image is printed on one or more of the material layers, and then the one or more printed layers, optionally together with further material layers, are inseparably connected to form the document body.

9. The method according to claim 8, wherein the dye-based ink penetrates the material of one or more of the layers when connecting the plurality of material layers further and / or while generating a different drop topology than the pigment-based ink.

10. The method according to one or more of the preceding claims, wherein the graphic pattern contains data (508) in coded form, the data being unique to the document and / or its owner.

11. The method of claim 10 further comprising:

- Providing the document body with an electronic circuit (504) in which a further representation of the unique data (508) is stored, and / or wherein the electronic circuit stores further data, the data stored in the circuit only after a successful authentication - fication and / or authorization can be accessed.

12. Document (500, 700) produced by the method according to one of claims 1-11.

13. Document (500, 700) with a document body (506),

- an ink jet print (502,

600) is located,

- wherein the inkjet print includes first inkjet droplets (720, 722) which have a first morphology and are composed of a dye-based ink (216);

- wherein the inkjet print includes second inkjet droplets (716, 718) which have a second morphology that differs from the first morphology and consist of a pigment-based ink (214);

wherein the absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar to one another that the first and second inkjet droplets are indistinguishable in color when illuminated with light in the visible spectral range by the human eye;

- wherein the inkjet print includes one or more first areas (414.1, 414.2) which contain the first inkjet droplets and are free of the second inkjet droplets;

- wherein the inkjet print includes one or more second areas (412.1, 412.2) which contain the second inkjet droplets and are free of the first inkjet droplets; - The first and second areas form a pattern (404) which, when viewed by the human eye, cannot be recognized when illuminated with light in the visible spectral range.

14. Document (500, 700) according to claim 13,

- The thickness of the document, its spatial extension in a dimension z is specified;

- wherein the document body consists of a plurality of layers of material (702-714), at least some of the inkjet droplets extending over a plurality of layers of material;

wherein a topology of an inkjet droplet describes its spatial extent over one or more of the material planes in the dimension z; and

- wherein the topology of the first inkjet droplets differs from the topology of the second inkjet droplets.

15. A method for reviewing a document, comprising:

- Reception (302) of a document (500, 700) with a document body (506) by a checking unit (800) which contains an image acquisition unit (802), where an inkjet print of an image (502, 600) is contained in or on the document body wherein one or more first regions (414.1, 414.2) of the image are printed with a dye-based ink (216), wherein one or more second regions (412.1, 412.2) of the image are printed with a pigment-based ink (214), wherein the one or more first regions are free from the pigment-based ink and wherein the one or more second regions are free from the dye-based ink, the absorption spectrum of the dye-based ink and the pigment-based ink being so similar that they are illuminated when the Inkjet printing with light in the visible spectral range are indistinguishable in color; - Recording (304) of a test image by the image acquisition unit, the test image being a digital image which depicts at least the inkjet print (502) of the document;

- Implementation (306) of an image analysis of the test image by the test unit, the

Image analysis includes:

• Automatic detection (308) of the morphology of the inkjet droplets (716, 718, 720, 722) depicted in the test image, which make up the inkjet print, the morphology of an inkjet droplet describing its spatial extent within the test image; and

• Automatic reconstruction (310) of a digital graphic pattern (404) from the test image, the pattern containing one or more first areas (408.1, 408.2, 414.1, 414.2) and one or more second areas (406.1, 406.2, 412.1, 412.2) wherein the first areas are those test image regions whose inkjet droplets have a first morphology, the second areas being those test image regions whose inkjet droplets have a second morphology, the second deviating from the first morphology;

- Comparing (312) the reconstructed graphic pattern or a value derived therefrom with a reference value for checking the authenticity of the document.

16. The method of claim 15, wherein the digital graphic pattern is a bar or matrix code, further comprising:

- Decoding of the bar or matrix code by the checking unit in order to obtain a decoded value (508); and

17. The method according to any one of the preceding claims 15-16, wherein the test unit additionally includes an infrared light source (904) for illuminating at least one of the image The area of the document to be included in the solution unit, also includes:

- activating the infrared light source;

- Recording of an IR test image by an IR image acquisition unit, whereby the IR

Proof image is a digital image that depicts at least the inkjet print of the document;

- Carrying out an image analysis of the IR test image by the test unit, wherein the

Image analysis includes:

- Compare the IR pattern or a value derived from it with an IR

Sample reference value for checking the authenticity of the document.

18. Test unit (800) with:

- an opening (808) for receiving a document (500, 700) with a document body (506) containing a printed area;

- An image acquisition unit (802) with sufficient resolution to be able to spatially resolve at least some of the inkjet droplets (716, 718, 27, 722) in the printed area;

- at least one processor (805);

- a non-volatile memory with computer-interpretable instructions (806) which, when they are executed by the at least one processor, cause the test method to be carried out according to one of claims 15-17.

19. A system (900) comprising:

- The test unit (800) of claim 18; and - an IR light source (904); and or

- an image acquisition unit sensitive in the IR spectrum; and or

- A light source (906) for light in the visible spectral range; and or

- an automatic document feeder; and / or a control device (902) for controlling access to: protected data, software functions, hardware functions or spatial areas, the control device being configured to only grant access if the test method results that the document is valid.