TESTING A SAFETY DOCUMENT BASED ON DROPLET MORPHOLOGIES

Description

Area

[0001] The invention relates to a method for producing a document, in particular a valuable or security document, for example an identity document, as well as a method and a system for verifying the document produced using the method according to the invention. background

[0002] A variety of documents are equipped with security features designed to make counterfeiting more difficult or even impossible. These documents include, in particular, valuable or security documents, which serve to verify a person's identity, for example, when crossing a national border, the origin or originality of an item, or a claim, for example, for payment of a sum of money or for the release of a product or provision of a service.

[0003] Counterfeiting can be made more difficult or even prevented by manufacturing such documents, such as banknotes, from a material that is not readily available. Additionally or alternatively, security features can be formed by special inks, such as luminescent or optically variable inks, optical elements such as holograms, tilting images, cinematic objects, lens or prism arrays, guilloche patterns with visible inks or luminescent inks, mottled fibers, security threads, and the like.

[0004] The security features used in valuables or security products can serve exclusively to prove the authenticity of the documents, regardless of their type or user. Individualizing, for example personalizing, security features also contain information, in coded form or in plain text, about the type of document, the user of this document, and / or an object to which the document is uniquely assigned. Such information can be a portrait or photo of the user, their personal data such as their name, date of birth, place of birth, address, or a personal identifier such as a membership number, or even their signature. Another security feature that individualizes the valuables 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.

[0005] International patent application WO 2017 / 134130 A1 describes the provision of a valuable or security product formed with a product carrier for creating a complex security feature, wherein a security print 200 is located on and / or in the product carrier. The security print is formed with at least two printing inks C, M, Y that produce different color impressions and with at least one printing ink K, K+ that appears black. At least one of the at least two printing inks that produce different color impressions and the at least one printing ink K+ that appears black each contains an IR absorber.

[0006] US patent application US 2008 / 0305444 A1 describes a watermark embedded in an image that has the property of being relatively undecipherable under normal light, but decipherable under infrared illumination when viewed by a suitable infrared-sensitive device. This infrared marking comprises, in combination with at least one deflection pattern, an infrared-reflective substrate, and a first dye mixture and a second dye mixture printed as an image on the substrate.The first dye mixture layer in conjunction with the substrate has the property of highly reflecting infrared radiation and the property of having a low contrast under normal illumination with a second dye mixture printed in close spatial proximity to the first dye mixture pattern, so that the resulting image on the substrate, suitably exposed to infrared radiation, provides a recognizable image recognizable as an infrared mark by a suitable infrared-sensitive device.

[0007] US patent application US 2007 / 0152067 A1 describes a method for producing an identification document. The method comprises: providing a substrate constructed and arranged to be printable; printing a first variable indicia onto the substrate, wherein the first variable indicia is not visible to the human eye in ambient light but is visible to the human eye when viewed using a first type of light; and printing a second variable indicia onto the substrate, wherein the second variable indicia is visible when viewed using ambient light.

[0008] The German patent application DE 10 2017 004 055 A1 describes a security insert with optically recognizable characters for an identification document, comprising a first transparent layer and a second transparent layer. An ink coating is applied to the first transparent layer. Furthermore, a first UV ink coating, which reflects at least UV light in at least a first wavelength range, is applied to the first layer and / or the ink coating. The first transparent layer and the second transparent layer are connected to one another. At least one of the transparent layers has blackened areas. A first part of the optically recognizable characters is formed by the blackened areas in at least one of the layers. A second part of the optically recognizable characters is formed by the ink coating. A third part of the optically recognizable characters is formed by the UV ink coating.The first and second parts of the optically recognizable characters reflect visible light. The third part of the optically recognizable characters reflects UV light at least in a first wavelength range.

[0009] The international patent application WO 2009 / 056355 A1 relates to a method for producing a polymer layer composite and a polymer layer composite which is designed in particular as a security and / or value document.The method comprises the following steps: providing a plurality of polymer layers as the substrate layers; printing the first information on at least one substrate layer, assembling the substrate layers to form a substrate layer stack and laminating the substrate layers to form the poly multi-layer composite, wherein the first information is broken down into at least two print extracts, each of which comprises a partial information item of the first information, and the printing of the first information is carried out by printing the at least two print extracts onto at least two different substrate layer surfaces in a coordinated manner, so that the printed print extracts lie precisely on top of one another in the monolithic polymer layer composite and together reproduce the first information at one viewing angle and at least one color change of the reproduced first information occurs at at least a second viewing angle. Summary

[0010] The invention is based on the object of proposing an improved document which makes manipulation more difficult.

[0011] Furthermore, the invention is based on the object of proposing a method for producing such a document which offers increased security against manipulation.

[0012] Furthermore, 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 the check.

[0013] The objects underlying the invention are achieved with the features of the independent patent claims. Embodiments of the invention are specified in the dependent claims. The embodiments listed below can be freely combined with one another, provided they are not mutually exclusive.

[0014] In one aspect, the invention relates to a method for producing a document as defined in claim 1.

[0015] Incorporating a security feature that is invisible to the human eye by using different types of inks that are the same color under visible light and are printed in such a way that multiple areas of the inkjet print printed with the different types of ink form the security feature in the form of a pattern that is invisible to the human eye can be advantageous, as it is initially not even apparent to a counterfeiter 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 as a whole form the image, without a human observer being able to detect any differences at the boundaries between the first and second image areas.Thus, a potential counterfeiter would never even consider counterfeiting this security feature, as it is not visible to the naked eye in the document. Embodiments of the invention are based on the recognition that the inkjet droplets of pigment-based and dye-based inks have different morphologies and / or topologies, and that these differences can be used to create a security feature whose presence is not, or at least not easily, detectable by a counterfeiter.

[0016] In some embodiments, the two different ink types (pigment-based and dye-based) also have identical absorption spectra in other spectral ranges, such as infrared or UV light. In these embodiments, a counterfeiter will not become aware of this security feature even if they hold the document under UV or IR light.

[0017] Even a component-specific analysis of the testing device does not allow the forger to determine what 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 used to test the document simply compares a person's image printed on the document with an image database of profile pictures or, as is provided for embodiments of the invention, determines and analyzes the morphology and / or topology of the individual inkjet droplets, cannot be determined from the outside or based on the composition of the testing device's components. Thus, neither the document itself nor any testing unit used provides a potential forger with any indication that the document contains the security feature in question, where it is contained in the document, and how it can be tested or produced.

[0018] A further advantage of documents according to embodiments of the invention can be that the testing is very undemanding in terms of equipment. As will be explained in more detail later, in principle only a high-resolution image capture unit and corresponding software are required to test this security feature. Optionally, a white light source can be used, but this is not required if sufficient daylight or another external white light source is available. To test 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 be additionally present in the testing unit, e.g. to test further security features and / or if the dye-based ink and the pigment-based ink have a different absorption spectrum under IR light or UV light.

[0019] Since the inspection is based on 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 inspection is also comparatively robust against interference from radiation. Droplet properties such as diameter, edge sharpness, roundness or "frayedness" of the droplet circumference, and brightness gradients within the droplet are largely unaffected by the intensity of the ambient light. As long as the document is irradiated with light of sufficient intensity to allow detection of the droplet morphology, the security feature can be inspected at different light intensities, as is often the case in daylight, for example. Furthermore, no special lamp with a narrow emission spectrum is required to inspect the security feature.

[0020] According to embodiments, providing the digital graphic pattern comprises generating a barcode or matrix code.

[0021] According to embodiments, the digital image is a monochrome digital image.

[0022] According to other embodiments, the digital image is a multi-color digital image, e.g. a CMYK or RGB image.

[0023] For example, polycarbonate-based documents can be provided with a color image using processes described, for example, in German patent applications DE102007052947 A1 and DE 102008012419A1, e.g., to personalize the document with a photo and / or signature of the holder. The color print image is generated inside the card using this process. This has the advantage that the printed image cannot be tampered with without destroying the laminated film structure. Therefore, cards produced in this way, such as identity cards and passports, are particularly forgery-proof. For example, the document consists of several layers ("films") of polycarbonate (PC) and PC-based inks. Some of these inks, which can be used to produce a color inkjet print in or on a document, are also described in patent applications DE102007052947 A1 and DE 102008012419A1.

[0024] For example, a document can be personalized with an image that is a portrait of the document owner. The inkjet printer can, for example, be a printer with five ink tanks, with three of the tanks filled with magenta, yellow, and cyan inks. The inkjet printer prints the three inks onto the document according to their color contribution to each pixel of the digital image.

[0025] The inkjet printer also contains two tanks, each containing black ink, whose black colors are indistinguishable by the human eye under visible light. However, one of the black inks is a pigment-based ink, and the other is a dye-based ink. The two different black ink values are printed on the document according to an either / or principle, so that the first regions of the digital image are printed exclusively with the black dye-based ink, and the second regions of the image are printed exclusively with the black pigment-based ink.

[0026] In color images, the first and second regions of the image typically also contain ink droplets from the three other tanks containing magenta, yellow, and cyan inks. These three inks can be, for example, three dye-based inks or three pigment-based inks.

[0027] If the 3 inks are dye-based inks, the second regions can still be detected and distinguished from the first regions based on the droplet morphology, because the second regions of the inkjet print are the only areas whose droplets have a morphology and / or topology typical of pigment-based inks.

[0028] If the 3 inks are pigment-based inks, the first regions can still be recognized and distinguished from the second regions based on the droplet morphology, because the first regions of the inkjet print are the only areas whose droplets have a morphology and / or topology typical of dye-based inks.

[0029] 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 three colors magenta, yellow, and cyan if the morphologies and / or topologies of these three colors are clearly distinguishable from the morphologies and / or topologies of the black dye-based ink and / or the black pigment-based ink. In some cases, this can be achieved, for example, by additives that influence the viscosity and thus the droplet trajectory.

[0030] 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, for example, be light and can, in particular, be white. A "light" background color is understood here to be a color whose brightness value is encoded in a one-dimensional or multi-dimensional brightness space and whose average brightness value across all dimensions of this brightness space is more than 50% of the theoretically possible value in this brightness space, preferably more than 70% of this value. For example, if a monochromatic brightness space is defined between 0 (black) and 100 (maximum brightness) units, a "light" background color would be one that has at least 50 of these units, preferably at least 70 of these units.

[0031] 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 creates a transparent color impression for the human eye, so that the one or more first regions 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 creates an opaque color impression for the human eye, so that the one or more second regions have a different color than the background color. In the case of a monochrome image, a "different color" of the second regions compared to the first regions means a "different brightness value."

[0032] This can be advantageous because a testing device equipped with an IR light source and an image capture unit sensitive to the IR range can very quickly and unambiguously detect the pattern hidden in the image without having to perform a computationally complex analysis of the droplet morphologies. For example, the use of such inks can enable the use of a testing device that, during normal operation, detects the pattern contained in the inkjet print by illuminating the inkjet print of the document with an IR light source and then capturing a digital IR test image of the inkjet print with an IR image capture unit. The IR test image preferably has a lower resolution than the test image created in visible light.A lower resolution, which, for example, is no longer capable of resolving individual inkjet droplets from one another, is completely sufficient to make the pattern recognizable here, since pattern recognition is not based on an analysis of 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 brightness value) of the pigment-based ink under IR light. The pattern is therefore comparatively fast and can be implemented based on image analysis of a lower-resolution IR test image, thus reducing computing time and CPU load.However, if the IR light source is defective, there is too much visible stray light and / or if an additional analysis of the security feature is to be performed, an examination of the droplet morphology and / or topology can be used.

[0033] 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 spectral range. For example, the dye-based ink may be Solvent Black 27 or Solvent Black 29. Additionally or alternatively, the pigment-based ink may be Carbon Black or Pigment Black 28.

[0034] "Solvent Black 27" is a substance (azo metal complex) with the CAS number 12237-22-8.

[0035] "Solvent Black 29" is a substance with the CAS number 61901-87-9.

[0036] "Carbon black" refers to soot, a black, powdery solid that consists of more than 80%, in some cases more than 98%, carbon. In particular, carbon black (CAS No. 1333-86-4), a soot specifically produced as an industrial raw material, can be used as carbon black.

[0037] Pigment Black 28 refers to a substance with the CAS number 68186-91-4. Pigment Black 28 is an inorganic pigment obtained as a reaction product of high-temperature calcination, in which copper(II) oxide and chromium(III) oxide in varying amounts are homogeneously and ionically interdiffused to form a crystalline matrix.

[0038] 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 spectral range. For example, the dye-based ink may be Solvent Blue 78. Additionally or alternatively, the pigment-based ink is Cu phthalocyanine.

[0039] The substance known as "Solvent Blue 78" is also known as "1,4-bis(methylamino)anthraquinone" and has the CAS number 2475-44-7.

[0040] 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 spectral range. For example, the dye-based ink can be Solvent Red 26 or Sudan Red. Additionally or alternatively, the pigment-based ink is quinacridone.

[0041] The substance known as "Solvent Red 26" is also known as "Oil Red EGN" or "CI 26120." It is a violet-red synthetic azo dye with the CAS number 4477-79-6.

[0042] The substance known as "Sudan Red" is also known as "Sudan III" or "1-[4-(phenylazo)phenylazo]-2-naphthol." It is a synthetically produced chemical compound from the group of azo and Sudan dyes with a red color and has the CAS number 85-86-9.

[0043] The substance known as "quinacridone" is an organic pigment and organic semiconductor with red to violet hues. It has the CAS number 1047-16-1.

[0044] According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a yellowColor tone. For example, the dye-based ink may be Solvent Yellow 124. Additionally or alternatively, the pigment-based ink may be Brilliant Yellow or Pigment Yellow 151.

[0045] The substance known as "Solvent Yellow 124" is an azo dye and has the CAS number 34432-92-3.

[0046] The substance known as "Brilliant Yellow" is also known as "Pigment Yellow 74" and has the CAS number 6358-31-2.

[0047] The substance known as "Pigment Yellow 151" is a benzimidazolone pigment with the CAS number 31837-42-0.

[0048] According to embodiments, 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 is so similar to the absorption spectrum of the further pigment-based ink that this further dye-based ink is indistinguishable from the further pigment-based ink by the human eye when illuminated with light in the visible spectral range, but is distinguishable from the inks in the first and second tanks. The method further comprises: Providing a further digital graphic pattern comprising one or more third regions and one or more fourth regions; overlaying the further graphic pattern and the digital image; printing the digital image onto or into the document body such 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 regions, the pigment-based ink but not the dye-based ink is used to print the regions of the digital image overlaid with the second regions, and the further dye-based ink but not the further pigment-based ink is used to print the regions of the digital image overlaid with the third regions.that 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, wherein 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 is the security feature.

[0049] For example, the third regions may be congruent with the first regions, and the fourth regions may be congruent with the second regions. According to one embodiment, some regions of the printed image consist exclusively of inkjet droplets from a plurality of different dye-based inks, while other regions of the printed image consist exclusively of inkjet droplets from a plurality of different pigment-based inks. In other embodiments, the third regions are not congruent with the first regions and / or the fourth regions are not congruent with the second regions, such that the printed image consists of a complex pattern of different dye-based and pigment-based inks.

[0050] 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 colour tone to be printed with the dye-based ink or the pigment-based ink; the result is identical for both ink types, since they represent the same colour tone; carrying out the overlay in such a way that the graphic pattern is completely mapped onto this area.

[0051] According to embodiments, the document body is constructed from multiple material layers. The digital image is printed onto or into the document body by applying the digital image to one or more of the material layers, and then the one or more printed layers, optionally together with additional material layers, are inseparably bonded to form the document body. The bonding can be achieved, for example, chemically, thermally by fusing the material layers, mechanically (e.g., by pressure), or by gluing the material layers.

[0052] When the material layers are glued or fused, printing inks can penetrate into the document body and bond with the document body, so that even if the layers are removed layer by layer by machining, a surface can never be obtained that shows the complete image and the pattern embedded in it.

[0053] According to preferred embodiments, the inkjet print is printed on one or more layers which, after the layers have been bonded, lie inside the document body.

[0054] By printing over several layers, it is possible to avoid the situation where some layers are first removed and then new layers containing incorrect information are applied to the rest of the original document body, without at least destroying the pattern or making it unreadable.

[0055] The material layers, which can also be in the form of films, can be made of PC, for example. The image is printed by the inkjet printer onto a core film, for example. The inks, which can contain PC, like the individual layers, bond firmly to the film surface, thus bonding 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 one another without destroying the entire document. This prevents counterfeiters from accessing the inner layer containing the information.Chips and antennas, such as those used in contactless ID cards, are additionally integrated into the foil structure according to certain designs and cannot be broken out of it without destroying the document or the chips or antennas.

[0056] According to embodiments, the dye-based ink and / or the pigment-based ink is configured to penetrate into one or more further material layers upon joining the multiple material layers, in addition to the material layer onto which the ink was printed. According to embodiments, the dye-based ink penetrates further and / or with a different drop topology into the material of one or more of the layers than the pigment-based ink during and / or after joining the multiple material layers.

[0057] According to embodiments, the graphic pattern contains data in coded form. The data may, in particular, contain data that is unique to the document and / or its owner. Optionally, this data may 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 stored electronically in a data storage device of the document. The data storage device may, for example, be a magnetic stripe or a chip memory.

[0058] This can be advantageous because it provides additional protection against document forgery. For example, if a specific secret data value is stored both in digital form in the data memory and in coded form in the form of an invisible pattern in the printed image, a verification unit can check whether, firstly, the pattern is contained in the inkjet print and, secondly, whether the value encoded in the pattern is also contained in the data memory. A document forger would therefore have to manipulate both the inkjet print and the value stored in the data memory in the same way. However, both can be prevented: the inkjet print can be protected against tampering in various ways, for example by applying a protective film or by printing the inkjet print onto a layer of material inside the document.When the various material layers are irreversibly bonded together to form a single document body through pressure, temperature, adhesive, or other means, it is no longer possible to alter the inkjet print inside the document without physically destroying or damaging the document. Additionally or alternatively, the value stored in the data storage device can be cryptographically protected against unauthorized manipulation. Manipulation of both the print and the securely stored data value, and thus document forgery, can thus be effectively ruled out.

[0059] According to embodiments, the method further comprises providing the document body with an electronic circuit. A further representation of the unique data is stored in this circuit. Additionally or alternatively, further data is stored in the electronic circuit, which can only be accessed after successful authentication of a reader and / or a user to the document. The said data unique to the document or its owner can also be included in this data, which can only be accessed after successful authentication.

[0060] The electronic circuit, e.g. a microchip with an interface for contact 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.

[0061] Alternatively or additionally, the electronic circuit can store additional data that is specially protected against unauthorized access. To access this data, the template can contain access data that is fed to the electronic circuit during an authentication or authorization phase. The electronic circuit can, for example, be configured to compare the data from the template with access data stored in the electronic circuit. If the comparison shows that the access data matches, the electronic circuit can allow access to the additional stored data. Accordingly, matching access data is stored in the template and in the electronic circuit during document production.

[0062] The electronic circuit can be a semiconductor circuit, e.g., silicon-based or polymer electronic. For example, the electronic circuit can be implemented by a chip located on or in a document layer. The electronic circuit can also be formed from polymer electronic components, e.g., applied to the document layer by printing.

[0063] For example, the additional data may include sensitive personal data, such as age, place of birth, date of birth, address, health data, insurance data, etc. The document may be configured to grant a user or a reading device access to the personal data only if the reading device is capable of transmitting a data value to the document that is identical to a reference value stored in the document's digital memory. 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 permitted, at least not initially, to read this data from the data memory. The reading device must determine the data stored in the document as a reference value by correctly recognizing and decoding the pattern in the inkjet print in or on the document.The reader can use the data value obtained through decoding to authenticate itself to the document. For example, the reader can use this to prove that it was able to optically capture the inkjet print, which implies that the user presented the document to the reader and that the reader cannot access the memory without the user's knowledge and consent.

[0064] In a further aspect, the invention relates to a document defined in claim 12.

[0065] The inkjet print includes one or more first regions containing the first inkjet droplets and free of the second inkjet droplets. The inkjet print includes one or more second regions containing the second inkjet droplets and free of the first inkjet droplets. The first and second regions form a pattern that is not recognizable when viewed by the human eye under illumination in the visible spectral range.

[0066] According to the invention, the document body consists of multiple material layers. The thickness of the document is its extension in a dimension "z." At least some of the inkjet droplets in the inkjet print extend across multiple material layers. The topology of the first inkjet droplets differs from the topology of the second inkjet droplets. The topology of an inkjet droplet describes its spatial extension along the dimension z across one or more of the material planes.

[0067] In a further aspect, the invention relates to a method for checking a document as defined in claim 13.

[0068] For example, the image analysis can be performed by software that operates with explicit rules specified by the programmer. These rules include various criteria and thresholds regarding, for example, the droplet diameter in the X, Y, and / or Z directions, intensity differences within the droplet, the presence, thickness, and sharpness or focusability of a droplet outline, and similar criteria. The image analysis software compares at least some or all of the inkjet droplets depicted 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 trained machine learning (ML) software, such as a trained support vector machine (SVN) or a trained neural network (NN).To train the software, a training dataset was used that 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 acquired test image and also containing first and second regions that form a pattern. The first regions of the training test images represent regions of the inkjet print that were printed with the same or, in terms of droplet morphology, very similar dye-based ink as the first regions of the inkjet print of the currently tested document. The second regions of the training test images represent regions of the inkjet print that were printed with the same or, in terms of droplet morphology, very similar pigment-based ink as the second regions of the inkjet print of the currently tested document.The first and second regions in the training test images are labeled ("annotated") as such, and various properties of the droplets in the first and second regions of the training test images are passed to the ML software as input parameter values, so that during training, the ML software learns to recognize first and second regions based on droplet morphology and / or topology.

[0069] According to embodiments, the digital graphic pattern is a barcode or matrix code. The method further comprises: Decoding the bar code or matrix code by the verification unit to obtain a decoded value; and using the decoded value as the value derived from the graphic pattern in the comparison with the reference value.

[0070] According to embodiments, the resolution of the image capture unit is sufficiently high to at least distinguish the inkjet droplets of the pigment-based ink from one another, i.e., to spatially resolve them. According to embodiments, the ability to spatially resolve a certain minimum proportion of the droplets, for example, at least 20% or at least 50% of the inkjet droplets with the pigment-based ink, may also be sufficient.

[0071] According to embodiments, the resolution of the image capture 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.

[0072] According to embodiments, the image capture unit is a camera or a microscope. For example, the image capture unit can be configured to capture one or more digital color images of at least the inkjet print in or on the document, referred to as "proof image(s)." For example, the proof images can be RGB images and saved, for example, as .jpg or .png images.

[0073] According to embodiments of the invention, the document has a length in the x dimension, a width in the y dimension, and a thickness in the z dimension. The method for checking the document further comprises: Acquisition of a plurality of test images in a plurality of planes along the z dimension by the image acquisition unit; for example, the images may be acquired at intervals of 10µm or 20µm or any other interval, preferably less than 80µm; each of the plurality of test images is a digital image depicting at least the inkjet print of the document in a z-plane of the document; performance of the image analysis of the test image by the inspection unit such that the image analysis comprises: automatic detection of the morphology of the inkjet droplets depicted in the plurality of test images;Automatically detecting the topology of the inkjet droplets respectively imaged in the plurality of test images, wherein the topology of an inkjet droplet describes a spatial extension of the inkjet droplet over one or more of the z-planes along the z-dimension, wherein the first areas are those test image regions whose inkjet droplets have a first morphology in conjunction with a first topology, wherein the second areas are those test image regions whose inkjet droplets have a second morphology in conjunction with a second topology, wherein the second topology deviates from the first topology;

[0074] According to embodiments of the invention, the morphology and / or topology of inkjet droplets of the dye-based ink characterized by one or more features selected from a group comprising: an average droplet diameter in the z-direction of over 15µm; an average droplet diameter in the z-plane of the document on which the print was made in the range 15-100µm; an intensity gradient from the center outwards in the z-plane of the document on which the print was made, such that the intensity gradient 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 captured.

[0075] According to embodiments of the invention, the morphology and / or topology of inkjet 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; an average droplet diameter in the z-plane of the document on which the print was made in the range 15-100 µm; a radial intensity gradient from the center outwards in the z-plane of the document on which the print was made, such 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 several z-planes of the document in which one of the test images was captured.

[0076] According to embodiments of the invention, the image is a facial or iris image of a person to whom the document is assigned.

[0077] According to embodiments of the invention, the inspection unit additionally includes 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 capture unit. The word "include" also encompasses embodiments in which the light source is mechanically coupled to the inspection unit and, for example, is attached to the housing of the inspection unit. The method further comprises: prior to recording the inspection image, activating the light source so that during recording, at least the inkjet print on or in the document is illuminated by the light source.

[0078] According to embodiments of the invention, the inspection unit additionally includes an infrared light source for illuminating at least one area of the document to be captured by the image capture unit. The method further comprises: Activating the infrared light source; capturing an IR inspection image by an IR image capture unit, the IR inspection image being a digital image depicting at least the inkjet print of the document; performing an image analysis of the IR inspection image by the inspection unit, the image analysis comprising: automatically capturing dark areas of the IR inspection image, a dark area having an average intensity below a predefined threshold; and automatically reconstructing a digital graphic pattern from the dark areas, the pattern being referred to as an "IR pattern"; comparing the IR pattern or a value derived therefrom with an IR pattern reference value to verify the authenticity of the document.

[0079] In a further aspect, the invention relates to a verification unit as defined in claim 16. In particular, the following steps are performed by the components of the verification unit after receipt of the document:Capturing a test image by the image capture unit, wherein the test image is a digital image depicting at least the inkjet print of the document; performing an image analysis of the test image by the at least one processor of the test unit, wherein the image analysis comprises: automatically capturing the morphology of the inkjet droplets depicted in the test image, which make up the inkjet print, wherein the morphology of an inkjet droplet describes its spatial extent within the test image;and Automatically reconstructing a digital graphic pattern from the test image, wherein the pattern includes one or more first regions and one or more second regions, wherein the first regions are those test image regions whose inkjet droplets have a first morphology, wherein the second regions are those test image regions whose inkjet droplets have a second morphology, wherein the second morphology differs from the first morphology; Comparing the reconstructed graphic pattern or a value derived therefrom with a reference value for verifying the authenticity of the document by the at least one processor of the verification unit. ;

[0080] For example, the image capture unit can be designed to spatially separate at least 20% of the droplets printed with the pigment-based ink.

[0081] In a further aspect, the invention relates to a system as defined in claim 17.

[0082] An additional IR light source and / or an IR-sensitive camera, for example, can be useful, as they allow an additional operating mode that allows for pattern recognition in the inkjet print with less computational effort. If the dye-based ink and the pigment-based ink have different absorption spectra in the IR range, the pattern can also be detected in an IR test image captured under IR light with significantly lower resolution (which does not allow for the resolution of individual inkjet droplets), since the pattern can be recognized over a large area by an IR-sensitive sensor under IR light.

[0083] A light source in the visible range can be advantageous because the test device now contains its own light source so that the test image can also be used at night, for example, in the absence of daylight and another external white light source.

[0084] The document feeder can be beneficial 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 distance from the capture unit.

[0085] 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 if they have authenticated themselves as authorized using the document. For example, the verification device with this control unit can be integrated into a terminal or functionally connected to the terminal. The terminal can be used, for example, at border controls, airport checkpoints, building entrances, and similar application scenarios.

[0086] In one embodiment of the test device, the "IR mode," which is used as "normal mode," creates only IR test images, as these can be analyzed very quickly, allowing a large number of people to be tested in a short period of time. This helps avoid queues in front of the terminal. However, for random checks or if the IR light source fails, the test device performs the test in "white light mode." This means that a high-resolution test image (e.g., at least 15 µm, preferably at least 5 µm) is captured under white light, and the inkjet droplets depicted therein are analyzed using image analysis software to identify the pattern based on different droplet morphologies and / or topologies. Definitions:

[0087] A document is defined here as a physical object intended for use as a trusted information carrier. In particular, the document can be a valuable and / or security document.

[0088] Where the terms "document," "product," "value or security product," and in particular the term "value or security document" are used in the description and claims of this application, these are to be understood synonymously and include, for example, a passport, identity card, driver's license or other ID card or access control card, a vehicle registration document, vehicle registration certificate, 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 authorization, membership card, gift or shopping voucher, waybill or other proof of authorization, tax stamps, postage stamps, tickets, (gaming) tokens, adhesive labels (for example, for product security), or another ID document. The product can be, for example, a smart card.The security or valuable document can be in ID 1, ID 2, ID 3 or any other format, for example in booklet form, like a passport-like item.

[0089] A document can be a laminate consisting of multiple document layers that are precisely bonded, for example, fused, under heat and increased pressure. These products must meet standardized requirements, such as ISO 10373, ISO / IEC 7810, and ISO 14443. The product layers consist, for example, of a carrier material suitable for lamination. In addition to valuable or security documents, the term "value or security product" also includes patches, labels, and the like, which are components of documents as security elements and are or become permanently bonded to the document carrier, forming the security feature there.

[0090] The document can be formed from a polymer selected from a group comprising polycarbonate (PC), in particular bisphenol A polycarbonate, polyethylene terephthalate (PET), and derivatives thereof, 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), polyvinylphenol (PVP), polypropylene (PP), polyethylene (PE), thermoplastic elastomers (TPE), in particular thermoplastic polyurethane (TPU), acrylonitrile-butadiene-styrene copolymer (ABS), and derivatives thereof, and / or paper and / or cardboard and / or glass and / or metal and / or ceramic. Furthermore, 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 applies both to films that are to be bonded together and to liquid formulations that are applied to a precursor, such as a protective or topcoat.

[0091] The document is preferably produced from 3 to 12, preferably 4 to 10, foils, preferably using a lamination process in which the foils are fused together under pressure and heat. The individual foils can be made of the same material or of different materials. Overlay layers formed in this way protect a security feature arranged underneath and / or provide the document with the required abrasion resistance.

[0092] Where the terms "individualized" and "individualizing" are used below, they refer to a property of a valuable or security document that allows the document to be assigned to a specific subject (person, organization, animal, object). Where the terms "personalized" and "personalizing" are used below, they refer to the property of a valuable or security document that allows the document to be assigned to a person. This property results from predefined security features of the valuable or security products, preferably from the security printing according to the invention.

[0093] Where the term "color impression" or "color" is used in the description and claims of this application, it is to be understood as the optical impression on a human observer with regard to the color effect. These different impressions can result from different color tones, i.e., spectrally different absorptions, and / or brightnesses of a print. The color impression can also result from interacting, closely spaced discrete color tones, which evoke a color impression in a human observer that differs from each of the closely spaced color tones.

[0094] Under a " ink" is defined here as a liquid of any viscosity that allows use by an inkjet printer and in which one or more colorants are contained. Preferably, the ink has a low viscosity. The term " Colorants " are substances with certain absorbing properties, at least in the visual spectral range and optionally in other spectral ranges. Colorants are substances that impart the corresponding color to the substance (e.g. the ink or the solvent) in which they are contained.

[0095] Under a " dye-based ink"Here, an ink is understood to consist of a liquid—typically water—with colorants dissolved in it. Solubilizers may be used with some dye-based inks. The colorants dissolved in a dye-based ink are referred to as "dyes." A "dye" is understood here to be a substance or mixture of substances that exhibits light absorption in the visible spectral range and, unlike a color pigment, is chemically soluble in a liquid, especially water.

[0096] Under a "pigment-based ink"Here, an ink is understood to be a liquid with pigments dispersed therein. Unlike dyes, pigments are not soluble. They are dispersed in the ink liquid – typically water – in particulate form. Pigments can include, in particular, inorganic pigments or carbon black particles. A "pigment" is understood here to be a substance or mixture of substances that exhibits light absorption at least in the visible spectral range.

[0097] Under a "Printing ink", also "Shade""Print color" refers here to the color perceived by the human eye when viewing a printout produced with a specific ink when the printout is illuminated with light from the visible spectral range. In embodiments in which the proof image is captured as a monochromatic digital image, the term "printing color" preferably refers to the intensity value of this color in one color channel of the monochromatic digital proof image.

[0098] A "Security feature"is a characteristic property of a document that proves its authenticity and is intended to make forgery impossible or at least significantly more difficult. For example, holographic-kinematic features, surface embossing, and watermarks, watermarks, UV watermarks, special paper quality or card body properties, for example with integrated fibers, multi-colored guilloches, printing elements with a tilt effect, writing elements with microprinting, or the like can serve as security features. A document according to one embodiment of the invention contains at least one security feature in the form of an image printed into or on the document by inkjet printing, which contains a pattern that is not visible to the human eye in visible light, but can be detected by image analysis of the droplets of this inkjet print.

[0099] A "Inkjet printer"is a printer, especially a matrix printer, in which a print image is created by the targeted firing and / or deflection of small ink droplets.

[0100] A "Absorption spectrum"is a color or electromagnetic spectrum that is created when broadband (white) light shines through matter and light quanta (photons) of certain wavelengths or wavelength ranges are absorbed (resonance absorption). The absorbed photons are missing from the passing light, which is why the spectrum at the relevant wavelengths is dark or, in extreme cases, black. If the photons are absorbed by exciting atoms, they involve sharply defined amounts of energy and therefore wavelengths, and the dark regions are correspondingly narrow lines. In molecules, on the other hand, many absorbable energy values are often close together and form broader dark regions in the spectrum, so-called absorption bands. In each case, the observed absorption spectrum is characteristic of the type of matter through which the radiation passes.

[0101] As far as in the description and in the claims of the present application the term "visually" is used, it is understood to mean the visual perception ability of a person with normal vision, in particular with regard to the spectrum of perceived wavelengths.

[0102] Under "White light", "Visible light" or "light in the visible spectral range" refers here to broadband light in the spectral range visible to the human eye, e.g., daylight or light from a lamp with an emission spectrum similar to daylight in the visible wavelength range. The wavelength distribution of white light is continuous. In particular, white light includes broadband light with wavelengths between approximately 380 and 780 nm.

[0103] Under "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-wavelength terahertz radiation. Specifically, this refers to light with a wavelength between 780 nm and 1 mm. This corresponds to a frequency range of 300 GHz to 400 THz, or a wavenumber range of 10 cm -1 to 12800 cm -1 .

[0104] A "digital image" is a data set in which image content is represented and stored. In particular, the digital image can be a data set in which the content of an image is represented by integers. In particular, the digital image can be a raster graphic.

[0105] A "Test image"is a digital image that depicts at least that portion of the document containing the inkjet print, and which was created by an image capture unit digitally capturing this portion. Preferably, the capture occurs under white light and at high resolution.

[0106] A "IR test image" is a digital image which depicts at least that part of the document which contains the inkjet print and which was created by an image capture unit digitally capturing that part while at least that part of the document was irradiated with infrared light.

[0107] Under a "Test unit" refers here to a device or a device component which is designed to check the authenticity of a document based on at least one security feature of the document.

[0108] Under a "Image acquisition unit"or "white light image capture unit" refers here to a device or device component that includes at least one apparatus for capturing a digital image of a physical object or is itself this apparatus, wherein the apparatus is sensitive in the visible frequency range. The apparatus can be, for example, a camera or a microscope. Optionally, the image capture unit can include another apparatus for capturing a digital image of a physical object, e.g., a camera or a microscope, wherein this further apparatus is 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 capture unit." It is also possible for the same camera or microscope to be capable of capturing images in both the visible frequency range and this other frequency range, e.g.,the IR range, and is used both as a "white light image acquisition unit" and as an "IR image acquisition unit", e.g. to capture test images and IR test images with the same apparatus.

[0109] Under a "IR image acquisition unit" "Digital image processing" refers to a device or device component that includes at least one apparatus for capturing a digital image of a physical object, or is itself this apparatus, wherein the apparatus is sensitive in the IR frequency range. The apparatus can be, for example, a camera or a microscope.

[0110] Under a "Camera"is a photographic device that can record static or moving images on photographic film or electronically on a digital storage medium, or transmit them to a receiver as a digital image via an interface. The camera can have a single-lens or multi-lens (including an eyepiece and objective) lens. The image is created by a lens on a film (analog camera) or on an electronic sensor (digital camera) on the opposite wall of the camera. Some cameras contain a shutter button and a shutter (aperture). The shutter button opens the shutter on the lens for a very short time, allowing light to pass through the lens and an image to be taken.

[0111] Under a "Microscope"A light microscope is understood here as an optical device for image capture that can produce highly magnified images of small structures or objects using light. Magnification occurs according to the laws of optics by utilizing the refraction of light by glass lenses. In particular, the microscope can be a reflected-light microscope, but bright-field microscopes can also be used in some embodiments, e.g., for documents with transparent document bodies. The microscope can be a "simple" or "compound" 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 a compound microscope consisting of at least two optical systems connected in series, each with its own magnification. The front lens, the objective, creates a magnified real image, the intermediate image, which is magnified a second time by the eyepiece. The eyepiece functions 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. For a 20x objective and a 10x eyepiece, the total magnification is 200x. In some microscopes, several objectives are mounted on rotatable bearings, so that one objective and any one of the eyepieces can be freely combined with one another by mechanically rotating the bearing axis to achieve the desired resolution.Some high-resolution cameras can also be used as a microscope and can be called a "microscope in the broad sense", but unlike microscopes in the narrower sense, cameras usually do not have multiple rotatable lenses.

[0112] Under the "Morphology"The morphology of an inkjet droplet is understood here as a specification of one or more characteristics of the inkjet droplet with respect to its spatial extent in the x- and y-directions. The spatial extent of the document in the "z-direction" corresponds to the thickness of the document. The length of the document can be specified, for example, in the "x-direction" and the depth in the "y-direction." Morphology therefore 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 droplet's pixels, e.g., the intensity progression from the center of the droplet to the edges when capturing an image of a cross-section of the droplet in an xy-plane. 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 other properties in two-dimensional space, i.e., 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 it.

[0113] Under the "Topology"of an inkjet droplet is understood here as a specification of the droplet's morphology in combination with a specification of one or more features of the inkjet droplet regarding its spatial extension in the z-direction. The spatial extension of the document in the "z-direction" corresponds to the thickness of the document. The length of the document can, for example, be specified in an "x-direction" and the depth in the "y-direction." If the document contains multiple material layers and / or was created from multiple material layers, the topology of the droplet specifies its extension across 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 droplet's pixels, e.g.The intensity distribution from the center of the droplet to the edges when capturing an image of a cross-section of the droplet in an xz or yz plane. The topology of a droplet is thus a description of the spatial extent of the droplet and optionally other properties in all three spatial directions.

[0114] Under a " Pattern " is understood here to be 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 repeats with a certain regularity. For example, the pattern can be a code, e.g. a barcode that contains several bars, or a 2D code that has several squares arranged in a grid. In the course of pattern recognition, an input image is analyzed in order to recognize the regularly repeating graphic elements as well as the pattern that is formed from these elements. Short description of the drawing

[0115] Embodiments of the invention are described below with reference to the drawing. In the drawing, Fig. 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 shows a block diagram of an inkjet printer used to produce the security feature; Fig. 3 shows an exemplary flow diagram of substeps of a method for checking a document based on its security feature; Fig. 4 shows an illustration of substeps of generating the security feature from an image and a pattern; Fig. 5 shows an exemplary embodiment of a variant of a document according to the invention; Fig. 6 shows various sections of the security feature generated by inkjet printing at different magnifications; Fig. 7 shows a three-dimensional illustration of different droplet morphologies and topologies in a multi-layer document body;Fig. 8 shows a block diagram of a test unit; Fig. 9 shows a block diagram of a system with the test unit and several light sources and image acquisition units; and .

[0116] Figure 1illustrates some steps of a method for producing a document with a security feature connected to the document body and visually imperceptible under daylight / "white light," according to one embodiment of the invention. The document can be any valuable or security document, for example, an identity card, a passport, a banknote, an employee ID card, a membership card, a credit card, a driver's license, or the like. The document comprises a document body, which can be constructed from one or more layers. The document body or the individual layers can consist of various materials, for example, cardboard, plastic, in particular polycarbonate, metal, wood, or combinations thereof, in particular composite materials made of metal, plastic, and / or cardboard.In some cases, the document may contain additional elements, such as a chip card, a magnetic stripe, a data storage device, an RFID antenna, or the like. Security features may be applied to or in the document body, such as holograms, complex prints, etc. The following describes the application of an additional security feature in or on the document or its body, which is invisible to the human eye when the document is illuminated with visible light.

[0117] First, in step 102, a special inkjet printer 200 is provided, as illustrated by way of example in Figure 2. 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 to each other 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) are indistinguishable by the human eye when illuminated with light in the visible spectral range.

[0118] For example, both inks can appear black in visible light. The dye-based ink could be Solvent Black 27, for example, and the pigment-based ink could be Carbon Black.

[0119] Alternatively, both inks can appear magenta in visible light. For example, the dye-based ink could be Solvent Red 26, and the pigment-based ink could be an ink containing quinacridone.

[0120] Alternatively, both inks can appear cyan in visible light. For example, the dye-based ink could be Solvent Blue 78, and the pigment-based ink could be Cu phthalocyanine ink.

[0121] Alternatively, both inks can appear yellow in visible light. For example, the dye-based ink could be Solvent Yellow 124, and the pigment-based ink could be Brilliant Yellow.

[0122] Optionally, the inkjet printer can include additional tanks for additional pigment-based or dye-based inks.

[0123] In a further step 104, a digital graphic pattern is provided. For example, the pattern can be created automatically, semi-automatically and / or manually in an image processing program and / or in a program for operating the inkjet printer by a user of this image processing program or inkjet printer program. The pattern is formed from one or more first regions and one or more second regions. For example, the one or more first regions can be a plurality of first squares and the one or more second regions can be a plurality of second squares, wherein the first and second squares are positioned on a rectangular surface area such that they form a matrix code, e.g. a QR code.

[0124] In step 106, a digital image to be associated with the document is provided. For example, the image can also be imported or created in the aforementioned 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. Preferably, the images relate to a person or institution for whom or by which 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 could also be the logo of a bank or a company commissioning the document to be created, for example, to serve as a credit card for customers or as an employee ID card for employees. Alternatively, the image could be a number, for example, representing the value represented by the document (document-based banknote / payment card). The image can be a monochromatic image or a multi-color image, for example, in the RGB or CMYK color spaces. The image should preferably contain as many channels as there are ink tanks in the inkjet printer.

[0125] In a further step 108, the graphic pattern and the digital image are superimposed.

[0126] For example, the overlay can be performed by the aforementioned image processing program or the printer program. For example, the overlay means that the pattern, and thus also its first and second regions, are mapped onto the digital image in a specific way. For example, the overlay can involve first searching the digital image for the largest possible regions whose pixels 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.

[0127] If the "color" of the printing inks in the first and second tanks is "black," the digital image would be searched for those areas containing a particularly high and homogeneous proportion of black pixels (or RGB pixels with a 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 occupy the entire area of the image, is superimposed on the region of the image exhibiting the smallest fluctuations in the proportion of black printing ink. In one embodiment, it is sufficient if the fluctuations in the proportion of black printing ink are minimized in the areas printed with the pigment-based ink. The information about the black components can already be available during the so-called rendering of the photo into the individual printing colors or can be generated through a corresponding analysis.The pattern can then be positioned at different points in the image and the respective fluctuation in the proportions of black ink can be determined.

[0128] In other embodiments, however, the overlay may also be performed in such a way that the pattern is mapped onto the digital image according to a fixed, predefined scheme, independent of the content of the digital image.

[0129] The method includes a further step 110 in which the inkjet printer prints the digital image onto or into 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 digital image overlaid with the second areas. 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 one another that an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink (if the printed object orimage motif) are indistinguishable by the human eye when illuminated with light in the visible spectral range, the printed pattern of the dye-based ink and the pigment-based ink in the printed image created by the overlay is also invisible under white light.

[0130] In some embodiments, the pattern is a code that encodes data that is directly or indirectly related to the document. For example, the data may include information that specifically belongs to and / or is known to the issuer of the document, the person to whom the document is assigned. In particular, the data may be data that is unique to the document and / or the person. For example, the data may be a combination of account number and bank routing number, a credit card number, an ID number, a serial number of a document-based banknote, or the like.

[0131] In some embodiments, this data may be additionally contained in or on the document or its body in other ways. For example, the document may contain a data storage device, e.g., a magnetic stripe or a chip with integrated memory, in which the data encoded in the pattern is also stored. During the document verification, this data can be read by the verification unit and compared with the decoded data of the printed code, which is also read. If the compared values do not match, the document is assumed to be forged.

[0132] For example, in some embodiments, during the creation or personalization of the document, a cryptographic key pair can be generated that is specific to this document and / or a person for whom the document is created. The cryptographic key pair comprises a private key and a corresponding public key. The public key is used to encrypt the data related to the document that is unique to the document. The public key of the key pair is used to encrypt the data unique to the document and, in a further step, to encode it into a graphic code, e.g., a barcode or a matrix code. This matrix code is then overlaid with a photo that is to be printed on or in the document, and the overlaid image is printed as described above.

[0133] In one embodiment of the manufacturing method, information about the material of the document body, the printing process, the presence of surface coatings, and / or the inks used is applied in machine-readable form to or in the document, e.g., as an additional print, as an engraving, or as a data record stored in a data storage device of the document or in an external database in conjunction with a document ID. The inspection device can be configured to read this information from the document or database during the inspection and use it in image analysis to improve pattern recognition.

[0134] Figure 2 shows a block diagram of an inkjet printer 200 with several ink tanks 218, which are used to carry out the Figure 1The ink 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 for the dye-based ink 216 and the pigment-based ink 214 to 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 ink and corresponding printouts are clearly distinguishable to the human eye under this light.

[0135] In some embodiments, the ink tanks may include one or more additional tanks, enabling printing in CMYK color mode, for example. In one embodiment, the tanks 218 include a tank 220 with cyan ink 208, a tank 222 with magenta ink 210, and another tank 224 with yellow ink 212.

[0136] 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) of 1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane is converted to a polycarbonate derivative. The polycarbonate derivative exhibited a relative solution viscosity of 1.263. A liquid preparation was prepared from 17.5 parts by weight of the polycarbonate derivative and 82.5 parts by weight of a solvent mixture 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

[0137] A colorless, highly viscous solution with a solution viscosity at room temperature of 800 m Pas is obtained.

[0138] In the next step, 4 g of polycarbonate solution from Example 2 and 30 g of the solvent mixture from Example 2 are homogenized with a magnetic stirrer in a 50 mL wide-necked screw-bottom glass. This gives a colorless, low-viscosity solution with a solution viscosity at room temperature of 1.67 mPa s. The surface tension of this base ink was determined using an OEG Surftens measuring system and the pendant drop method at 21.4 ± 1.9 mN / m. In the next step, the desired pigment(s) or dye(s) are added in the amounts necessary to achieve a specific color tone. Alternative compositions for inks for inkjet printing in or on document bodies are described in DE 10 2007 052 947 A1.

[0139] Optionally, the printer may include a document feeder 206, which is preferably configured to feed a large number of documents in a short period of time and supply 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 superimposed 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 print head unit and the document relative to one another and, optionally, also the provision and overlay of the digital image and the pattern. The control unit 202 may then include, for example, a printing program, which includes functions for importing and optionally for editing images and / or patterns.The program may also include program functions for generating cryptographic keys and / or for generating a pattern that encodes a specific, possibly unique, data value associated with the document. According to one embodiment, the print head unit 204 comprises a single print head that is sequentially supplied with the inks of the individual tanks 218. Preferably, however, the print head unit 204 contains multiple print heads, preferably one print head per tank 220-228.

[0140] Figure 3 shows an exemplary flowchart 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.

[0141] The test can be carried out, for example, using a test unit 800 as in Figure 8The check can be performed, for example, at a national border, at an airport, at the gate to a company site, or at a building entrance at a corresponding terminal. It is also possible to use a mobile version of the verification unit, for example, for mobile personal checks or to verify the authenticity of valuable documents using a cash register terminal.

[0142] In a first step 302, a verification unit, for example, a mobile or portable terminal, receives a document with a document body on or in which the security feature is contained. For example, the terminal may include an automatic document feeder or a surface or carrier onto which the document is manually placed.

[0143] The security feature is an image overlaid with a pattern, which has been printed by inkjet in or on the document, such as with regard to Figure 1described. In or on the document body, an inkjet print of the image and a pattern that is not visible under white light are therefore included. 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 therefore comprises a plurality of first inkjet droplets based on the dye-based ink and a plurality 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 to each other that when the inkjet print is illuminated with light in the visible spectral range, they are indistinguishable in color to the human eye and therefore the pattern of the image regions printed with the dye-based ink and the pigment-based ink is not visible under white light.

[0144] The inspection unit includes an image capture unit with a resolution sufficiently high to spatially resolve at least a certain proportion of the inkjet droplets (e.g., at least 20%, preferably at least 50%) at the given distance between the image capture unit and the received document. For example, the image capture unit can be a microscope or a high-resolution camera.

[0145] In step 304, the image capture unit captures a proof image. The proof image is a digital image that captures at least the inkjet print of the document and optionally other regions of the document. It is also possible for the proof image to capture the entire document. The proof image is, for example, an RGB image or a monochromatic image captured under white light.

[0146] In a next step 306, the verification unit performs an image analysis of the verification image to verify the authenticity of the document using the inkjet printer.

[0147] During the image analysis, at least the morphology of the inkjet droplets depicted in the test image, which comprise the inkjet print, 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 formed by the length and width of the document or the length and width of the document part depicted in the test image. The morphology includes, in particular, the cross-section of the droplet, the nature of the outline (clearly defined or diffuse edge, essentially circular or strongly frayed or star-shaped), the presence, strength, and / or orientation of an intensity gradient within the droplet, and / or the brightness of the droplet, each under white light.In some embodiments, the image capture unit captures multiple test images from different z-planes of the document, for example, by modulating the focus settings of the image capture unit. This can be particularly advantageous when the document consists of multiple material layers, the print was applied to one of the inner layers, and the ink penetrated into one or more of the additional layers during or after the combination of these material layers with additional material layers. For example, it has been observed that when multiple material layers are pressed and / or bonded, dye-based inks and pigment-based inks penetrate to different depths into adjacent material layers, and as a result, inkjet droplets of pigment-based inks and dye-based inks have different droplet morphologies.In this case, by analyzing a plurality of test images, the topology of the individual inkjet droplets can be determined and it is possible to distinguish first image regions that selectively contain droplets of the dye-based ink from second image regions that selectively contain droplets of the pigment-based ink and thereby recognize a pattern formed from the first and second image areas.

[0148] In step 310 of the image analysis, an automatic reconstruction of a digital graphic pattern is carried out 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 detected 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 sufficiently from one another such that the first and second regions can be reliably detected and differentiated by the image analysis program.

[0149] According to embodiments, the detection of the first and second regions based on the droplet morphology and topology, respectively, is performed 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 dataset containing a plurality of images with noted first and second image regions, wherein the first image regions contain only inkjet droplets of the dye-based ink and the second image regions contain only inkjet droplets of the pigment-based ink.

[0150] In a further step 312 of the testing method, the testing 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 testing unit. A comparison of the reconstructed pattern with the reference pattern determines whether the reconstructed pattern is identical or sufficiently similar to the reference pattern. In this case, the security feature created by inkjet printing and the document containing 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, a decoding step is performed, in which a specific value is reconstructed from the code. This value can be, for example, a document-specific or person-specific number, such as 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, date of birth, and place of birth. The "derived" value obtained using the decoding process is compared with a reference value stored in the verification unit or a data storage device accessible to the verification unit. If the two values are identical or sufficiently similar, the validity of the security feature is determined.

[0151] Optionally, the verification may also include a verification of other security features known in the state of the art, whereby the document will only be recognized as genuine or valid if these other security features are also recognized as valid.

[0152] Figure 4 shows an illustration of sub-steps of generating the security feature from an image 402 and a digital pattern 404.

[0153] The image 402 may, for example, be a digital, visible-light portrait image of a person for whom a new document is to be issued and / or personalized. The image 402 may, for example, be captured as an RGB image and converted to 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 Figure 4The illustrated pattern consists of two first regions 408.1, 408.2 and two second regions 406.1, 406.2. The first regions are regions intended to be printed with the dye-based ink but not the pigment-based ink. The second regions are regions intended to be printed with the pigment-based ink but not the dye-based ink. In other embodiments, the pattern can also be significantly more complex and, in particular, can be a barcode or matrix code, the complexity of which allows one or more data values, such as personal IDs, document IDs, and other values, to be stored in coded form in the pattern. Before printing, the digital image 402 and the pattern 404 are superimposed. In the example shown here, the image and pattern have the same size and are superimposed at a scale of 1:1.In other embodiments, however, it is also possible for the overlay to occur such that the pattern overlays only part of the image, and / or for the pattern to be overlaid with the image at positions determined dynamically depending on the image content. Finally, the digital image 402 is printed on or in the document body such 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 second regions 412.1, 412.2 of the document body correspond to the second image areas 406.1, 406.2 and are printed with the pigment-based ink. In visible light, the first 414 and second 412 regions of the inkjet print image thus obtained are not distinguishable with the naked eye. Using a microscope orHowever, using a high-resolution camera, it is possible to identify the first and second regions and the pattern they form based on the different droplet morphology and droplet topology, respectively.

[0154] In some embodiments such as the one in Figure 4As shown, 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 thus invisible. In the first regions 414.1, 414.2, in which the portrait image is printed with the dye-based ink, no print is recognizable, but rather 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 opaque even in infrared light, the corresponding regions of the portrait image are visible in a similar way to how they are visible in white light. In embodiments in which the dye-based ink and the pigment-based ink have different color values orIntensities in the visible wavelength range can cause the document or print to be illuminated with this special light to test whether the pattern is visible under these conditions. If no pattern is visible, the document is considered fake.

[0155] Figure 5shows a document 500 according to an embodiment of the invention. The document includes an edge-shaped card body 506 with at least one image 502, which includes a security feature not detectable in visible light in the form of a pattern of first and second regions of inkjet droplets of different morphology or topology, not detectable in visible light. The document can optionally have a variety of further security features, such as watermarks, embossed marks, holograms, or tilted 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 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 / 45008 82. However, the document body can also be opaque.

[0156] In particular, the document body can consist of several layers, as in Figure 7 is illustrated. For example, the document may consist of multiple layers of polymer materials such as polyolefins, polyesters, and / or polycarbonates. In particular, the document may be a polymer layer composite document, the production of which is described in DE10 2008012419 A1.

[0157] An image 502 is printed on or in the document using inkjet printing. The image contains first areas or regions that are printed only with the dye-based ink and optionally other inks, but not with the pigment-based ink of the same printing color, and second areas or regions that are printed only with the pigment-based ink and optionally other inks, but not with the dye-based ink of the same printing color. The first and second regions form a pattern invisible to the human eye. The image can, for example, have an area share of 5-95% of the surface of the document.

[0158] Optionally, the document may include additional elements such as a microchip 504 and / or a magnetic stripe (not shown). Additionally or alternatively, the document may contain additional optically detectable data 508, for example, a printed document ID or personal ID. The optically detectable data 508 may also be printed or applied using other methods, such as laser engraving. The pattern invisibly contained in the image 502 may, for example, be a QR code containing the additional optically detectable data 508 in coded form.

[0159] Figure 6Ashows a section 600 of the face of the person depicted in image 502 under white light at 20x magnification. Although the right eye is printed with the dye-based ink 216 and the left eye with the pigment-based ink 214, at this resolution under white light, no color or intensity difference and thus no pattern between the first and second image areas is recognizable.

[0160] Figure 6B shows a subsection 602 of image 502 with the person's right eye printed with the dye-based ink at 100x magnification. It can be seen that the droplet edges are blurred. It is not possible for the image capture unit to find a plane in which the droplets are sharply focused.

[0161] Figure 6Cshows a subsection 604 of the image 502 with the person's left eye printed with the pigment-based ink at 100x magnification. One can see the contrast with the edges of the Figure 6BThe droplets shown have sharp, dark edges of the pigment-based ink droplets, which enable the image capture unit to find at least one plane in which a sharp focus on the droplets is possible. According to embodiments, the document inspection method comprises a modification of the zooming of the image capture unit such that an attempt is made to focus the focus in several planes along the z-dimension of the document. In this case, 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 detect areas that contain only droplets of the pigment-based ink or only droplets of the dye-based ink.

[0162] Figure 6Dshows a subsection 606 of image 502 with the person's right eye printed with the dye-based ink at 200x magnification. Even at this magnification, the morphology of the droplets is characterized by a blurred outline, and the color intensity or brightness of the droplets is largely homogeneously distributed within the respective droplets.

[0163] Figure 6E shows a subsection 608 of image 502 with the person's left eye printed with the pigment-based ink at 200x magnification. Even at this magnification, 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.

[0164] Figure 7A three-dimensional illustration of different droplet morphologies and topologies in a multilayer document body of a document 700. The document comprises multiple 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 z dimension before being processed into a layered composite.

[0165] During the production of the document, the image with the invisible security feature is printed onto one of the layers, here layer 708, by inkjet printing, as already described here for embodiments of the invention. Preferably, the printing takes place on one of the material layers that will later be located inside the finished document. Immediately after printing onto layer 708, the individual inkjet droplets 716, 718, 720, 722 run across the surface of material layer 708, which is defined by the dimensions X and Y. Droplets 716, 718 consist of pigment-based ink 214, e.g., an ink containing carbon black, and droplets 720, 722 consist of dye-based ink 216, e.g., an ink containing solvent black 27. The two-dimensional progression of the droplets onto this surface is referred to as the morphology of the droplets.The applicant has determined that the morphology of the droplets depends crucially on whether the droplet consists of a pigment-based ink 214 or a dye-based ink 216. For example, the edges of pigment-based ink droplets are sharply focused and often darker than the interior of the droplets, whereas the edges of dye-based ink droplets are blurred and unclear and generally have the same brightness as the interior of the droplets; in some cases, the edges even appear brighter than the droplet center. Furthermore, it was determined that the penetration depth of the ink across multiple layers also differs between pigment-based ink and dye-based ink. The penetration depth of the dye-based ink is greater and thus also the extension of the droplets 722, 720 along the z-axis, represented here by the larger L2 compared to the smaller L1.The edges of the droplets are also blurred in the z-dimension. According to embodiments, multiple inspection images are captured by the image capture unit along the z-plane, preferably such that at least one inspection image is captured per plane 702-714. For example, the focus of the image capture unit can be changed so that different planes of the document are "traversed," capturing the inspection images.

[0166] The multi-layer document is produced according to one embodiment by providing two or more polymer layers and printing at least one surface on at least one 708 of these polymer layers with the dye-based ink and the pigment-based ink such that an image 502 is formed in which the first and second areas of the image printed with either the dye-based or the pigment-based ink form an invisible pattern, as described, for example, with reference to Figures 4 and 5described. The image can, for example, have an area share of 5-95% of the surface of the document. After the printing step, the multiple layers of polymer material are stacked on top of one another and bonded together, for example by means of printing and / or adhesive processes. The printed polymer layer surface can form the top or bottom side of the layer composite. The printed polymer layer surface is preferably surrounded at the bottom and / or top by at least one further polymer layer 706, 710, so that the printed surface is located inside 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 can vary.

[0167] In principle, all materials commonly used in the field of security and / or valuable documents can be used for the polymer layers. Layers 702-714 are preferably 50 µm to 300 µm thick. The polymer layers can be identical or different and based on a polymer material from the group comprising PC (polycarbonate, in particular polyphenol 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. Coextruded films of these materials can also be used.The use of PC materials is preferred, although so-called low-Tg materials based on polycarbonate can also be used, for example, but by no means necessarily, in particular for a polymer layer to which a printing layer is applied and / or for a polymer layer bonded to a polymer layer bearing a printing layer, specifically on the side with the printing layer. Low-Tg materials are polymers whose glass transition temperature is below 140°C. Further embodiments regarding a multilayer document, its printing, and how the individual layers can be bonded to one another to form a single, form-fitting document are described in patent applications DE102007052947 A1 and DE 102008012419A1.

[0168] In some embodiments, the image 502 can also be printed onto the multi-layer document body such that different parts of the image are printed in different layers. It is possible for the first image areas to be printed with the dye-based ink continuously on different material layers than the second image areas with the pigment-based ink. In other embodiments, however, the printing is distributed across different layers independently of the ink used, so that each layer is printed with both the pigment-based ink and the dye-based ink, albeit on different sub-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 in it arranged together in such a way that a viewer can recognize the image.

[0169] Figure 8shows a block diagram of a verification unit 800, which is designed to verify the security feature invisibly hidden in a printed image in white light and also in the absence of a special light source (for example, infrared light, UV light). The verification unit can be designed as a mobile verification unit or as a mobile terminal. It includes an opening 808 for receiving a valuable or security document 500, 700. The opening can, for example, consist merely of a container or a storage device for the document, so that the user must manually position the document in or on this container or storage device. Additionally or alternatively, the opening 808 can include an automatic document feeder, which receives the document and positions it at a predefined location within the verification unit.

[0170] The inspection unit further comprises an image capture unit 802, for example, a microscope or a high-resolution camera. The resolution of the image capture unit must be at least high enough that a minimum number of the inkjet droplets (for example, at least 20%) on a print 502 of the correctly positioned document can be spatially resolved from one another. The image capture unit is designed to capture one or more inspection images. For example, the image capture unit has an automatic and / or manually operable focus. The focus is changed step by step or continuously such that at least some of the inkjet droplets (namely, those printed with pigment-based ink) appear sharp. A digital image, the so-called inspection image, is captured at least in this plane and optionally in further planes in the Z direction.The inspection 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. Preferably, it is the same type of image that comprised a training data set of images on the basis of which image analysis software 806 was trained. However, it is also possible for a captured monochrome image or RGB image to be subsequently converted into such an image format. The inspection images are captured under white light. This means that at the time of image capture, the document in the inspection unit is either illuminated with ambient white light through the opening 808 and / or that the document is illuminated using a white light source located inside or near the inspection unit.

[0171] The inspection unit also includes 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 inspection images from the image acquisition unit or to read them from a memory in which the inspection images were stored by the image acquisition unit. To analyze them in order to determine the position as well as the morphology and / or topology of the inkjet droplets that make up the imprint 502. Furthermore, the image analysis software is configured to use the droplet morphology and / or topology to identify first image regions 414.1, 414.2 that contain droplets of dye-based ink and are free of droplets of pigment-based ink, as well as second image regions 412.1, 412.2, which contain droplets of pigment-based ink and which are free of droplets of dye-based ink. The image analysis software is thus configured to recognize a pattern of first and second regions based on 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 checking unit determines as a result of the check that the security feature is valid and that the document, unless other security features of the document are invalid, is to be regarded as valid (and thus authentic). The check can also be more complex, for example if the pattern is a two-dimensional code.In this case, the verification unit applies a decoding process to the recognized pattern to obtain the value encoded in the pattern and compares it with a corresponding reference value stored in a memory accessible to the verification unit. If the security feature or document is identical or sufficiently similar to the reference value, it is determined to be valid.

[0172] Optionally, the inspection unit can include a display 810, for example, an LED display. The test result can be displayed to a user via the display 810. Optionally, the image analysis software can also make the detected pattern visible to a human user in the form of a graphical representation, for example, an overlay image, and output it via the display 810.

[0173] According to one embodiment, a public cryptographic key is stored in a data storage of the document 500, 700. The public key forms an asymmetric cryptographic key pair with a private cryptographic key. The private key is assigned to a specific organization, for example, the publisher of the document, a company, or an authority, in particular a government regulatory authority. The private key is stored in a central data storage to which the verification unit has access. The public key can be stored in copies on multiple 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, wherein a secret data value is encoded in encrypted form in the QR code.The secret value can be personal data or cryptographic keys. The verification unit is designed to decode the QR code and thereby obtain the secret value in encrypted form. The verification unit then uses the private key to decrypt the encrypted secret value. The decrypted secret value can, in turn, be verified and further processed by the verification unit in various ways. For example, the value can be compared with a reference value.

[0174] Alternatively, the decoded but still encrypted data value can be transmitted from the verification unit to a decryption device. In this case, the verification 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 provided by a plurality of verification units. The decrypted data can then be transmitted back from the decryption device to the verification unit. This preferably occurs 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 the user to whom the document is assigned.The serial number or other identifier is stored in the verification unit in the form of a reference value and / or can be read from the document by the verification unit via another transmission channel. For example, the serial number or identifier can be printed or engraved on the document body and captured by the verification 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 verification unit can determine the authenticity of the security feature. Equality or sufficient similarity of the values implies authenticity.

[0175] Figure 9 shows a block diagram of a system 900 with the inspection unit 800 and several light sources and image acquisition units. The system 900 includes a inspection unit as described for embodiments of the invention and, for example, with reference to Figure 8described, as well as a white light source 906 and an infrared light source 904. Both light sources are positioned to illuminate a document 500 correctly placed in the opening or in the inspection unit, or at least to illuminate an image 502 depicted in or on this document. The light sources 904, 906 can be installed as an integral part of the inspection unit or installed or positioned on or next to the inspection unit. For example, the opening 808 can be large enough that light from light sources external to the inspection unit also hits the document with sufficient intensity.

[0176] The image capture unit 802 comprises at least one high-resolution image capture unit 910, which is sensitive in the visible light wavelength range and can capture images. The image capture unit also comprises an infrared camera 908, whose resolution may be lower than that of the white light camera 910. The infrared camera 908 is sensitive at least in the infrared light wavelength range. In some embodiments, it is also possible to have a high-resolution camera that is sensitive in both the white light and infrared light ranges and thus functionally corresponds to a combination of both image capture units 908, 910.

[0177] The inspection unit can be operated in at least two different modes: in "white light mode," the document 500 located in the inspection unit 800 is illuminated with white light, and the white light camera 910 takes one or more high-resolution inspection images under white light irradiation. The inspection images can, for example, be captured as RGB images and saved as monochrome images, with the pixel intensities of the monochrome channel resulting from a sum or average of the intensities of the three R, B, and G color channels. The high-resolution inspection images 912 are transmitted to the image analysis software 806, which automatically recognizes a pattern 914 within the image based on the droplet morphology and / or topology. The pattern represents the security feature invisible to the human eye and is used by the inspection unit as described 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 see that a code has actually been detected that corresponds, for example, to a certain, expected type of code (for example, barcode or matrix codes).

[0178] Furthermore, the inspection unit can be operated in an "IR mode." In "IR mode," the document 500 located in the inspection unit 800 is illuminated with infrared light from the infrared light source 904, and the IR camera 908 takes one or more IR inspection images under infrared light irradiation. The IR inspection images can be captured and stored, for example, as monochrome images. The IR inspection images can have the same or lower resolution as the white light inspection images. The IR inspection image or images can be displayed directly to a user on the display 810. The IR mode can be particularly advantageous when the pigment-based ink 214 and dye-based ink 216 used have an identical or indistinguishably similar absorption spectrum under white light, but a significantly different absorption spectrum under IR light.In this case, the pattern is recognizable even without image analysis simply by exposing the document to an infrared light source 904. In IR mode, the IR inspection image captured by the IR camera 908 under IR light is forwarded directly to the display 810, where it is shown to the user. Even without the contribution of image analysis, the user can recognize the pattern and thereby verify the security feature. Optionally, a decoding step can also be performed in IR operating mode to extract information encoded in the pattern and compare it with a reference value.

[0179] In addition to or instead of the test image, e.g., a high-resolution RGB image captured under white light, the inspection unit can also be configured to capture an IR image, referred to as an "IR test image." This is, in particular, a digital monochrome image captured under IR illumination with a lower resolution than the test image captured under white light. The IR test image corresponds to a brightness level image and can also be represented as a grayscale image. The inspection method can further comprise calculating an image from the color image serving as the test image that corresponds to its brightness level image if it had been captured with a camera sensitive in the IR spectrum under infrared light illumination. The calculation can be performed using a mapping provided during document production. The mapping assigns color tones of an RGB color image, e.g.,of the image printed on or in the document, at corresponding positions of a monochrome image taken under IR light from the inkjet print of this image. In embodiments of the invention, only an opaque variant of the black printing ink, i.e. 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. Once 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 identical areas of the images being compared. An adjustment of the image brightness and / or contrast can be made 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. Coding parameters supplied to the device executing the method, which were used to generate the pattern, can be used, for example, a number of adjacent regions, their spatial extent, an error correction used in the coding, and the like. The coding parameters can be supplied, for example, from a database. Information can be present on the document, which is supplied to the database in order to select the coding parameters suitable for the document. This information is preferably designed to be machine-readable. The coding parameters can be transmitted to the testing unit via a secure data connection. The reconstructed pattern, e.g., a graphic code, is then decoded.

[0180] 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 that are indistinguishable from each other under visible light, but are clearly optically distinguishable from each other under light of other wavelengths, such as infrared or UV. Thus, the IR mode can be considered a faster and less computationally intensive variant of the white light mode.

[0181] According to embodiments of the invention, the inspection unit is configured to normally operate in IR mode. However, under certain situations, IR operation may not be possible, or even fail unexpectedly for a short time. For example, the IR light source may fail due to a technical defect or due to the end of the life of the light source 904. If the IR light source 904 is located inside the inspection unit 800, but the inspection unit is used outdoors and exposed to strong sunlight, the sunlight penetrating through the opening 904, which represents stray light, may make it impossible to capture IR inspection images under pure IR light. In this case, the system switches to white light mode. This is robust against penetrating sunlight. According to some embodiments, the switch between IR mode and white light mode is performed manually.In other embodiments, the switch occurs automatically when the test unit detects 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 may have a white light sensor to automatically determine whether the maximum value has been exceeded.

[0182] According to one embodiment, the system includes an access control device 902, for example, a barrier or gate, or is coupled to the access control device. The access control device is automatically opened depending on the result of the document verification. For example, the gate to a secured area or building is opened automatically and only when a user has successfully authenticated themselves to the verification unit using an ID document assigned to the user, wherein the authentication includes a verification of the invisible security feature as described for embodiments of the invention herein.

[0183] If the document 500, 700 is intended to support an inspection in 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 provided that maps a relationship between 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 plurality of color tones generated by mixing colorants intended for the production of the document. The monochrome signals correspond to brightness levels of the respective color tones when illuminated with IR light. The relationship between the color signals and the monochrome signals can, for example, be in the form of an assignment table or in the form of one or more analytical descriptions. The relationship can, for example,by creating a color image of a color reference chart and a corresponding image with the camera sensitive in the IR spectrum when illuminated with light in the IR spectrum, whereby the signals of the color camera and the camera sensitive in the IR spectrum are assigned to each other for identical image areas in the images.

[0184] Finally, the image with the graphic pattern embedded in it is printed on or in the document body.

[0185] Printing can be performed using one of two visually indistinguishable ink variants of one of the colors intended for the document. One variant, the "pigment-based ink," is opaque when illuminated with infrared light, and the other variant, the "dye-based ink," is transparent, and the color signals recorded by the white-light-sensitive RGB camera are identical. The respective variant of the two inks used is selected when the graphic pattern is embedded in the image and fed to an inkjet printer as printing information.

[0186] The assignment can be created as follows: first, a color reference chart is printed using the inks intended for document production and, if applicable, a substrate intended for document production. The color reference chart contains a plurality of inks that represent a plurality of color tones that occur in one or more images to be associated with the document, whereby the plurality of color tones includes color tones from both pigment-based inks and dye-based inks. A color image (i.e., an RGB image under white light) is taken of the color reference chart, and a monochrome image is taken with a camera sensitive to the IR spectrum when the color reference chart is illuminated with IR light. These steps can be performed in any order.The hues of the RGB color image located at specific positions in the color reference table are then assigned to brightness values of the monochrome image at corresponding positions. The assignment can be provided as a mapping table or in the form of an analytical description.

[0187] To print a specific image of a person or other physical object on or into the document body, an RGB color image of the object or person to be printed on the document body is first captured with a color camera under white light, or an existing RGB image captured under white light is provided. A color print is made of this image, and then a monochrome image of this print is captured with a camera sensitive to the IR spectrum under IR light. The order of the captures is irrelevant.An image is calculated from the color image that corresponds to its brightness level image when captured by a camera sensitive to the IR spectrum under infrared light illumination, using only one opaque variant (a pigment-based ink) of two inks available (a pigment-based ink and a dye-based ink of essentially the same color in visible light), the other of which is transparent when illuminated with infrared light. A mapping can be provided for this purpose, e.g., a mapping created as described above.In other words, an image is calculated from the color camera image using a mapping that corresponds to its brightness level image when captured by a camera sensitive in the IR spectrum when illuminated with infrared light, when only one opaque variant (pigment-based ink) of a printing ink available in two variants is used, the other variant (dye-based ink) of which is transparent when illuminated with infrared light.

[0188] The calculated image and the brightness level image taken under infrared illumination by the camera sensitive in the IR spectrum are compared and a difference image containing the pattern is calculated.

[0189] According to one embodiment, a pattern embedded in the image captured by the color camera is reconstructed from the difference image, and its content is decoded. For this purpose, supplied coding parameters that were used in generating the pattern or graphic code can be used.

[0190] The document 500 can, for example, be an identity card containing a portrait image of the person to whom it is assigned in the form of a color inkjet print 502. The color inkjet print was created using CMYK printing, with the color black being printed in first regions of the inkjet print using a dye-based ink and in second regions of the inkjet print using a pigment-based ink. The first and second regions cannot be distinguished by the human eye in visible light, 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.

[0191] For example, the portrait image of the woman depicted in the document can be captured by a camera as an RGB image. The image is a pixel graphic containing, per pixel, triples of values for the primary colors red, green, and blue (RGB) or signals derived from them.

[0192] In the printing process according to one embodiment, a multitude of color tones for the representation of color images are printed using a few printing inks, for example, by printing very small, closely spaced areas or dots with the colors cyan, magenta, yellow, and black. These printing inks are also referred to by the abbreviations C, M, Y, and K. Viewed from a distance at which the areas printed with a single color are no longer individually perceptible, these areas appear in a wide variety of color tones depending on the area ratio of the printing inks.

[0193] In particular, the black printing ink, K, can be present in two different versions, K and K+, which are visually indistinguishable. One version is a dye-based ink that is transparent to IR light in some embodiments, and the other version is a pigment-based ink that is opaque under IR light in some embodiments. Depending on the color tone, each area of a color image has a greater or lesser proportion of black printing ink. The embedding of the pattern into the image (i.e., into the photo) during printing occurs in such a way that the image areas overlaid with the first areas of the pattern are printed only with the black dye-based ink, and the image areas overlaid with the second areas of the pattern are printed 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 observer sees a normal color photograph. This visual indistinguishability also applies to a color camera image, meaning the color camera signals for the different versions of the printing inks are identical. When illuminated with IR light and captured with a camera sensitive to the IR spectrum, pattern 914 becomes visible, as shown in . Figure 9 In particular, the elements of the pattern printed with the IR-absorbing version of the black printing ink can exhibit strongly fluctuating absorption levels within an element or from element to element, depending on the image content of the color image. List of reference symbols

[0194] 102-110 Steps Printing Procedure 200 Inkjet Printer 202 Inkjet Printer Control Unit 204 Print Head Unit 206 Printer Document Feeder 208 Cyan Ink 210 Red Ink 212 Yellow Ink 214 Black Ink: Pigment-Based Ink 216 Black Ink: Dye-Based Ink 218 Ink Tank Container 220-228 Ink 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 the 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 Section of image 502: right eye, dye-based ink, 100 x 604 Section of image 502: left eye, pigment-basedInk, 100 x 606 Detail of image 502: right eye, dye-based. Ink, 200 x 608 Detail of image 502: left eye, pigment-based. Ink, 200 x 700 Document made up of multiple material layers 702-714 Individual material layers 716 Inkjet droplets, pigment-based ink 718 Inkjet droplets, pigment-based ink 720 Inkjet droplets, dye-based ink 722 Inkjet droplets, dye-based ink 800 Inspection unit 802 Image acquisition unit 804 Image analysis unit 805 Processor 806 Image analysis software 808 Opening / document feeder 810 Display / screen 812 Displayed inspection 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 Inspection image 914 Sample.

Claims

1. A method for producing a document (500, 700) having a document body (506) and a visually imperceptible security feature connected to the document body, said method comprising: - providing (102) an inkjet printer (200) which includes at least a first tank (228) containing a dye-based ink (216) and a second tank (226) containing 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 an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink are indistinguishable to the human eye under illumination by light in the visible spectral range; - providing (104) a digital graphical 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 connected to the document; - superimposing (106) the graphical pattern and the digital image; - providing the document, wherein the thickness of the document specifies its spatial extent in a dimension z, wherein the document body consists of a plurality of material layers (702-714); - printing (108) the digital image onto the or into the document body by means of 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 in such a way 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, wherein the distribution of the dye-based ink and the pigment-based ink in the printed image is the security feature, wherein the one or more regions (414.1, 414.2) superimposed with the first areas (408.1, 408.2) contain first inkjet droplets and are free of the second inkjet droplets, wherein the regions (412.1, 412.2) superimposed with the second areas (406.1, 406.2) include second inkjet droplets and are free of the first inkjet droplets, wherein the first inkjet droplets (720, 722) have a first morphology and consist of the dye-based ink (216), wherein the second inkjet droplets (716, 718) have a second morphology, which differs from the first morphology, and consist of the pigment-based ink (214), wherein the first and second areas form the pattern (404), which, under illumination by light in the visible spectral range, is not perceptible to the human eye, - wherein at least some of the inkjet droplets extend over a plurality of the material layers, 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.

2. The method according to claim 1, wherein the pattern comprises a barcode or matrix code.

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

4. The method according to any one of the preceding claims, wherein the document body material onto which the image is printed has a background colour, wherein the background colour is light and in particular white, wherein, under illumination by light in the infrared spectral range, the dye-based ink provides a transparent colour impression to the human eye, so that the one or more first areas have the background colour, and the pigment-based ink provides an opaque colour impression to the human eye, so that the one or more second areas have a different colour from the background colour.

5. The method according to any one of the preceding claims, wherein the inks in the first and second tank, under illumination by light in the visible spectral range, both: - have a black tone, • 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-coloured tone, • 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: copper phthalocyanine; - or both have a magenta tone, • 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 • wherein the pigment-based ink is quinacridone; - or both have a yellow tone, • wherein the dye-based ink preferably comprises one or more of the following substances: yellow azo dyes, for example Solvent Yellow 124; and / or • wherein the pigment-based ink preferably comprises 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 also a third tank containing a further dye-based ink and a fourth tank containing 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 so similar to one another that this further dye-based ink, under illumination by light in the visible spectral range, is chromatically indistinguishable by the human eye from the further pigment-based ink but is chromatically distinguishable from the inks in the first and second tank, the method further comprising: - providing a further digital graphical pattern including one or more third areas and one or more fourth areas; and - superimposing the further graphical pattern and the digital image; - performing the printing of the digital image onto the 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, in such a way that the pigment-based ink, but not the dye-based ink is used for printing the regions of the digital image superimposed with the second areas, in such a way 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, and in such a way that the further pigment-based ink, but not the further dye-based ink is used for printing the regions of the digital image superimposed with the fourth areas, wherein 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 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 in respect of the absorption spectrum of the tone that is to be printed with the dye-based ink or the pigment-based ink; - performing the superimposition in such a way that the graphical pattern is imaged fully onto this area.

8. The method according to any one of the preceding claims, wherein the document body is constructed from a plurality of material layers (702-714), wherein the digital image is printed into the document body such that the digital image is printed onto one or more of the material layers, and then the one or more printed layers, optionally together with further material layers, are connected inseparably to form the document body.

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

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

11. The method according to claim 10, additionally comprising: - providing the document body with an electronic circuit (504), in which a further representation of the unique data (508) are stored, and / or wherein the electronic circuit stores further data, wherein the data stored in the circuit may only be accessed following successful authentication and / or authorisation.

12. A document (500, 700) having a document body (506), - wherein the thickness of the document specifies its spatial extent in a dimension z; - wherein the document body consists of a plurality of material layers (702-714); - wherein an inkjet imprint (502, 600) is located on the or in the document body, - wherein the inkjet imprint includes first inkjet droplets (720, 722), which have a first morphology and consist of a dye-based ink (216); - wherein the inkjet imprint includes second inkjet droplets (716, 718), which have a second morphology different 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, under illumination by light in the visible spectral range, are chromatically indistinguishable by the human eye; - wherein the inkjet imprint includes one or more first areas (414.1, 414.2) which include the first inkjet droplets and are free from the second inkjet droplets; - wherein the inkjet imprint includes one or more second areas (412.1, 412.2), which include the second inkjet droplets and are free from the first inkjet droplets; - wherein the first and second areas form a pattern (404), which, under illumination by light in the visible spectral range, is not perceptible to the human eye; and - wherein at least some of the inkjet droplets extend over a plurality of material layers; - 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.

13. A method for verifying a document, comprising: - receiving (302) a document (500, 700) according to claim 12 by means of a testing unit (800), which includes an image detection unit (802), - recording (304) a plurality of test images by the image detection unit along the z-dimension of the document, wherein the test image is a digital image that portrays at least the inkjet print (502) of the document; - carrying out (306) an image analysis of the test images by the testing unit, wherein the image analysis comprises: • automatically detecting (308) the morphology and topology of the inkjet droplets (716, 718, 720, 722) which are imaged in the test image and of which the inkjet print consists, wherein the morphology of an inkjet droplet describes its spatial extent within the test image, wherein the topology of an inkjet droplet describes its spatial extent over one or more material planes in the dimension z; and • automatically reconstructing (310) a digital graphical pattern (404) from the test images, wherein the pattern includes 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 of which the inkjet droplets have a first morphology, wherein the second areas are those test image regions of which the inkjet droplets have a second morphology and second topology, wherein the second morphology deviates from the first, wherein the second topology deviates from the first; - comparing (312) the reconstructed graphical pattern or a value derived therefrom with a reference value for verifying the authenticity of the document.

14. The method according to claim 13, wherein the digital graphical pattern is a barcode or matrix code, further comprising: - decoding the barcode or matrix code by the testing unit in order to obtain a decoded value (508); and - using the decoded value as the value derived from the graphical pattern in the comparison with the reference value.

15. The method according to any one of preceding claims 13-14, wherein the testing unit additionally includes an infrared light source (904) for illuminating at least an area of the document to be received by the image detection unit, the method further comprising: - activating the infrared light source; - recording an IR test image by an IR image detection unit, wherein the IR test image is a digital image which portrays at least the inkjet print of the document; - carrying out an image analysis of the IR test image by the testing unit, wherein the image analysis comprises: • automatically detecting dark areas of the IR test image, wherein a dark area has an average intensity below a predefined limit value; and • automatically reconstructing a digital graphical pattern from the dark areas, wherein the pattern is referred to as an "IR pattern"; - comparing the IR pattern or a value derived therefrom with an IR pattern reference value for verifying the authenticity of the document.

16. A testing unit (800) comprising: - an opening (808) for receiving a document (500, 700) having a document body (506) which includes a printed area; - an image detection unit (802) with sufficient resolution to be able to spatially resolve at least part 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 executed by the at least one processor, cause the test method according to any one of clams 13-15 to be carried out.

17. A system (900) comprising: - the testing unit (800) from claim 16; and - an IR light source (904); and / or - an image detection unit sensitive in the IR spectrum; and / or - a light source (906) for light in the visible spectral range; and / or - an automatic document feed; and / or - a monitoring device (902) for monitoring access or entry to: protected data, software functions, hardware functions or spatial areas, wherein the monitoring device is configured to grant access or entry only if the verification method shows that the document is valid.