EP4215377A2 - Verifying a security document based on droplet morphologies - Google Patents

Abstract

The invention relates to a method for checking a document. The method comprises: - receiving (302) a document (500, 700) having a document body (506) with an ink jet printing of an image (502, 600) by a verification unit (800). The image contains first regions (414.1, 414.2) containing inkjet droplets of a dye-based ink (216) and second regions (412.1, 412.2) containing a pigment-based ink (214). The first regions are free of the pigment-based ink and the second regions are free of the dye-based ink. The pigment-based ink is indistinguishable from the dye-based ink under visible light.- Recording (304) a test image that depicts at least the inkjet print (502);- Performing (306) an image analysis comprising:• acquiring (308) the morphology of the inkjet droplets; and• reconstructing (310) a digital graphic pattern (404) from the test image based on the droplet morphology;- comparing (312) the reconstructed graphic pattern with a reference value for checking the authenticity of the document.

Description

Area

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

background

A large number of documents are equipped with security features that are intended to make counterfeiting difficult or prevent them. These documents include, above all, value or security documents, which are used to verify the identity of a person, 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 the provision of a service.

Counterfeiting can be made more difficult or prevented by making such documents, such as banknotes, from a material that is not readily available. Additionally or alternatively, security features can be formed by special colors, for example luminescent or optically variable colors, optical elements such as holograms, tilting images, cinematic objects, lens or prism arrays, guilloches with visible colors/inks or luminescent colors/inks, mottled fibers, security threads and the like.

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

Summary

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

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

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

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

• Bereitstellung eines Tintenstrahldruckers, welcher zumindest einen ersten Tank mit einer farbstoffbasierten Tinte und einen zweiten Tank mit einer pigmentbasierten Tinte beinhaltet, wobei das Absorptionsspektrum der farbstoffbasierten Tinte und das Absorptionsspektrum der pigmentbasierten Tinte einander so ähnlich sind, dass ein Tintenstrahldruck mit der farbstoffbasierten Tinte und ein Tintenstrahldruck mit der pigmentbasierten Tinte (des gleichen Bildmotivs) bei einer Beleuchtung mit Licht im sichtbaren Spektralbereich durch das menschliche Auge ununterscheidbar sind;
• Bereitstellen eines digitalen graphischen Musters das gebildet wird aus ein oder mehreren ersten Bereichen und ein oder mehreren zweiten Bereichen; und
• Bereitstellen eines mit dem Dokument zu verbindenden digitalen Bildes;
• Überlagerung des graphischen Musters und des digitalen Bildes;
• Drucken des digitalen Bildes auf den oder in den Dokumentenkörper mit dem Tintenstrahldrucker derart, dass die farbstoffbasierte Tinte aber nicht die pigmentbasierte Tinte zum Druck der mit den ersten Bereichen überlagerten Regionen des digitalen Bildes verwendet wird und dass die pigmentbasierte Tinte aber nicht die farbstoffbasierte Tinte zum Druck der mit den zweiten Bereichen überlagerten Regionen des digitalen Bildes verwendet wird, wobei die Verteilung der farbstoffbasierten Tinte und der pigmentbasierte Tinte in dem gedruckten Bild das Sicherheitsmerkmal ist.

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

• Provision of an inkjet printer, which includes at least a first tank with a dye-based ink and a second tank with a pigment-based ink, wherein the absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar that an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink (of the same image motif) are indistinguishable by the human eye when illuminated with light in the visible spectral range;
• providing a digital graphic pattern formed from one or more first areas and one or more second areas; and
• providing a digital image to be associated with the document;
• overlay of the graphic pattern and the digital image;
• Printing the digital image on or in the document body with the inkjet printer 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 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 being the security feature.

The introduction of a security feature that is not visible to the human eye by using different types of ink of the same color under visible light, which are printed in such a way that several areas of the inkjet print that are printed with the different types of ink, form the security feature in the form of a pattern that is not visible to the human eye can be advantageous, since a counterfeiter cannot initially see that a security feature is hidden in the inkjet print. Under visible light, the areas printed with the different types of ink appear the same in the sense that these areas together make up the image without a human observer being able to discern any differences at the boundaries of the first and second image areas. A potential counterfeiter therefore does not have the idea of forging this security feature from the outset, because it cannot be seen in the document with the naked eye.

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

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

Even a component-related analysis of the testing device does not enable the counterfeiter to identify 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 check the document is merely comparing a picture of a person printed on the document with an image database of profile pictures or, as is intended 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 from the composition of the components of the checking device. Thus, neither the document itself nor any checking unit used gives a potential counterfeiter any indication that the document contains said security feature, where it is contained in the document, and how it can be checked or produced, if necessary.

A further advantage of documents according to embodiments of the invention can consist in the fact that the examination is very undemanding in terms of equipment. As will be explained in more detail later, in principle only a high-resolution image acquisition unit and appropriate software are required to check this security feature. A white light source can be used as an option, but is not required if there is sufficient daylight or another external white light source. It is therefore not necessary to use a special light source, for example an IR light source and/or a UV light source, to check the security feature. This can however, optionally also be present in the test 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.

Since the check is based on the droplet morphology and/or topology or since the security feature consists in the spatial distribution of inkjet droplets of different morphologies and/or topologies in or on the document, a document is provided whose check is also comparatively robust against interfering radiation. Droplet properties such as diameter, edge sharpness, roundness or "raggedness" of the droplet perimeter, and brightness gradients within the droplet are largely unaffected by ambient light intensity. Thus, as long as the document is exposed to light of sufficient intensity to allow the droplet morphology to be detected, the security feature can be checked at various light intensities, such as is often encountered in daylight, for example. Also, no special lamp with a narrow emission spectrum is required to check the security feature.

According to embodiments, providing the digital graphic pattern includes generating a bar or matrix code.

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

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

For example, polycarbonate-based documents can be created using methods that are described, for example, in the German patent applications DE102007052947 A1 and DE 102008012419A1 is described, can be provided with a color image, for example in order to personalize the document in color with a photo and/or signature of the owner. With this method, the colored printed image is created inside the card. This has the advantage that the printed image cannot be manipulated without destroying the laminated film structure. The cards produced in this way, such as ID cards and passports, are therefore particularly forgery-proof. For example, the document consists of multiple layers ("slides") of polycarbonate (PC) and PC-based inks. Some of these inks used to generate of color ink jet printing in or on a document are also disclosed in the patent applications DE102007052947 A1 and DE 102008012419A1 described.

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

In addition, the ink jet printer includes two tanks, both of which contain ink of black color, the black color of which cannot be distinguished from each other by the human eye under visible light. However, one of the black inks is a pigment-based ink and the other black ink is a dye-based ink. The two different black ink values are printed on the document according to an either/or principle such that first regions of the digital image are printed exclusively with the black dye-based ink and the second regions of the image are exclusively printed with the black pigment-based ink.

The first and second regions of the image also typically contain ink droplets from the other three tanks of magenta, yellow, and cyan ink for color images. These 3 inks can be 3 dye-based inks, for example, or 3 pigment-based inks.

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

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

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

According to embodiments, the document body material on or into which the image is printed has a color that functions as the background color of the printed image. For example, the background color can be light and, in particular, can be white. A "bright" background color is understood here as 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 over 50% of the theoretically possible value in this brightness space, preferably over 70% of this value. For example, if a monochromatic lightness space is defined between 0 (black) and 100 (maximum light) units, a "light" background color would be one that has at least 50 of those units, preferably at least 70 of those units.

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

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

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

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

As "Solvent Black 29" is a substance with the CAS number 61901-87-9 designated.

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

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

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

The substance known as "Solvent Blue 78" is also known under the name "1,4-bis(methylamino)antraquinone" and has the CAS number 2475-44-7 .

According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a magenta hue when illuminated with light in the visible 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.

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

The substance known as "Sudan Red" is also known under the names "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 .

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

According to embodiments, the dye-based ink in the first tank and the pigment-based ink in the second tank have a yellow hue when illuminated with light in the visible spectral range. For example, the dye-based ink may be Solvent Yellow 124. Additionally or alternatively, the pigment-based ink is Brilliant Yellow or Pigment Yellow 151.

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

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

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

• Bereitstellen eines weiteren digitalen graphischen Musters beinhaltend ein oder mehrere dritte Bereiche und ein oder mehrere vierte Bereiche;
• Überlagerung des weiteren graphischen Musters und des digitalen Bildes;
• Durchführen des Druckens des digitalen Bildes auf den oder in den Dokumentenkörper derart, dass die farbstoffbasierte Tinte aber nicht die pigmentbasierte Tinte zum Druck der mit den ersten Bereichen überlagerten Regionen des digitalen Bildes verwendet wird, dass die pigmentbasierte Tinte aber nicht die farbstoffbasierte Tinte zum Druck der mit den zweiten Bereichen überlagerten Regionen des digitalen Bildes verwendet werden, dass die weitere farbstoffbasierte Tinte aber nicht die weitere pigmentbasierte Tinte zum Druck der mit den dritten Bereichen überlagerten Regionen des digitalen Bildes verwendet wird, dass die weitere pigmentbasierte Tinte aber nicht die weitere farbstoffbasierte Tinte zum Druck der mit den vierten Bereichen überlagerten Regionen des digitalen Bildes verwendet werden, wobei die Verteilung der farbstoffbasierten Tinte, der pigmentbasierten Tinte, der weiteren farbstoffbasierten Tinte und der weiteren pigmentbasierten Tinte in dem gedruckten Bild das Sicherheitsmerkmal ist.

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, the absorption spectrum of the further dye-based ink being so similar to the absorption spectrum of the further pigment-based ink that this further dye-based ink is indistinguishable in color from the further pigment-based ink by the human eye when illuminated with light in the visible spectral range, but is distinguishable in color from the inks in the first and second tank. The procedure also includes:

• providing a further digital graphic pattern including one or more third areas and one or more fourth areas;
• superimposition of the further graphic pattern and the digital image;
• performing the printing of the digital image on or in 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 areas, 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, that 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 areas, 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, the distribution of the dye-based ink, the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image being the security feature.

For example, the third areas can be congruent with the first areas and the fourth areas can be congruent with the second areas. In one embodiment, some areas of the printed image consist solely of inkjet droplets of multiple different dye-based inks, while other areas of the printed image consist solely of inkjet droplets of multiple different pigment-based inks. In other embodiments, the third areas are not registered with the first areas and/or the fourth areas are not registered with the second areas such that the printed image consists of a complex pattern of different dye-based and pigment-based inks.

• Ermitteln einer Fläche innerhalb des Bildes, welches eine Mindesthomogenität und Mindestintensität hinsichtlich des Absorptionsspektrums des Farbtons aufweist, der mit der farbstoffbasierten Tinte oder der pigmentbasierten Tinte gedruckt werden soll; Das Ergebnis ist für beide Tintentypen identisch, da diese ja den gleichen Farbton repräsentieren;
• Durchführung der Überlagerung derart, dass das graphische Muster vollständig auf diese Fläche abgebildet wird.

According to embodiments, the method further includes:

• determining an area within the image that has a minimum homogeneity and minimum intensity with respect to the absorption spectrum of the hue to be printed with the dye-based ink or the pigment-based ink; The result is identical for both types of ink, since they represent the same hue;
• Carrying out the overlay in such a way that the graphic pattern is completely mapped onto this surface.

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

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

According to preferred embodiments, the inkjet imprint is printed on one or more layers which, after the layers have been joined together, lie inside the body of the document.

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

The layers of material, which can also be in the form of foils, can consist of PC, for example. The image is printed by the inkjet printer onto a core film, for example. The inks, which, like the individual layers, can contain PC, bond and thus the printed inkjet image firmly to the film surface. The core foil is then laminated with cover foils. This can be done with or preferably without the use of adhesive. During lamination, the individual layers of film form a continuous piece of polycarbonate. They cannot be separated from one another without destroying the entire document. As a result, counterfeiters cannot access the information-carrying inner layer. According to embodiments, chips and antennas, such as are used in the case of contactlessly read ID cards, are also integrated into the film structure and cannot be broken out of it without destroying the document or the chips or antennas.

According to embodiments, the dye-based ink and/or the pigment-based ink is designed to penetrate into one or more further material layers in addition to the material layer on which the ink was printed. According to embodiments, the dye-based ink penetrates the material of one or more of the layers further and/or to form a different droplet topology than the pigment-based ink upon and/or after the bonding of the multiple layers of material.

According to embodiments, the graphic template contains data in encoded form. In particular, the data may include data unique to the document and/or its owner. Optionally, this data can be contained in a further representation on or in the document, e.g. in the form of a further imprint, in the form of an engraving and/or as a data value electronically stored in a data memory of the document. The data storage can be, for example, a magnetic stripe or a memory of a chip.

This can be advantageous since this provides additional protection against forgery of the document. Because, for example, a specific, secret data value is stored both in digital form in the data memory and in coded form in the form of the invisible pattern in the printed image, a test unit can check whether, firstly, the pattern is contained in the inkjet print and, secondly, whether the data memory contains the value coded in the pattern as well. 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 manipulation in various ways, for example by applying a protective film or by printing the inkjet print on a material layer inside the document. If the various layers of material are irreversibly bonded together into a single document body by pressure, temperature, adhesive or other means, it is no longer possible to change the inkjet printing inside the document without physically destroying or damaging the document. In addition or as an alternative to this, the value stored in the data memory can be cryptographically protected against manipulation by unauthorized persons. A manipulation of both the imprint and the protected stored data value and thus a falsification of the document can thus be effectively ruled out.

According to embodiments, the method further comprises providing the document body with an electronic circuit. Another representation of the unique data is stored in this circuit. In addition or as an alternative to this, further data is stored in the electronic circuit, which can only be accessed after successful authentication of a reading device and/or a user with respect to the document. Said data unique to the document or its owner may also belong to this data, which can only be accessed after a successful authentication.

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.

As an alternative or in addition, the electronic circuit can store additional data that is particularly protected against unauthorized access. In order to be able to access this data, the template can contain access data which are supplied to the electronic circuit in an authentication or authorization phase. The electronic circuit can be set up, for example, to compare the data from the template with access data stored in the electronic circuit. If the comparison shows that the access data match, the electronic circuit can allow access to the other stored data. Correspondingly, matching access data are stored in the template and in the electronic circuit when the document is produced.

The electronic circuit can be a semiconductor circuit, e.g. B. is 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. B are applied to the document layer by printing.

For example, the other data can include sensitive personal data, such as age, place of birth, birthday, address, health data, insurance data, etc. The document can be designed to only grant a user or a reader access to the personal data if the reader is able to transmit a data value to the document that is identical to a reference value that is stored in the digital memory of the document. The reference value is an identical copy/further representation of a data value encoded in the pattern in the inkjet print of the document. In this embodiment, the reading device is not permitted, at least for the time being, to read this data from the data memory. The reading device must determine the data stored as a reference value in the document by being able to correctly recognize and decode the pattern in the inkjet print in or on the document. The reader can use the data value obtained by decoding to authenticate itself to the document. The reader can thereby prove, for example, that it was able to optically detect the inkjet print, which implies that the user presented the document to the reader and the reader cannot access the memory without the user's knowledge and will.

In a further aspect, the invention relates to a document produced by the method according to one of the embodiments of the production method according to the invention described here.

In a further aspect, the invention relates to a document with a document body, wherein an inkjet print is located on or in the document body. Inkjet printing includes first inkjet droplets that have a first morphology and are made of a dye-based ink. Inkjet printing also includes second inkjet droplets composed of a pigment-based ink. The second ink jet droplets have a second morphology that differs from the first morphology. The absorption spectra of the dye-based ink and the pigment-based ink are so similar that the first and second ink jet droplets are indistinguishable in color by the human eye when illuminated with light in the visible spectral range.

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

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

• Empfang eines Dokuments mit einem Dokumentenkörper durch eine Prüfeinheit, die eine Bilderfassungseinheit beinhaltet. In oder auf dem Dokumentenkörper ist ein Tintenstrahldruck eines Bildes enthalten. Ein oder mehrere erste Regionen des Bildes sind mit einer farbstoffbasierten Tinte gedruckt. Ein oder mehrere zweite Regionen des Bildes sind mit einer pigmentbasierten Tinte gedruckt. Die ein oder mehreren ersten Regionen sind frei von der pigmentbasierten Tinte und die ein oder mehreren zweiten Regionen sind frei von der farbstoffbasierten Tinte. Das Absorptionsspektrum der farbstoffbasierten Tinte und das Absorptionsspektrum der pigmentbasierten Tinte sind einander so ähnlich, dass diese bei einer Beleuchtung des Tintenstrahldrucks mit Licht im sichtbaren Spektralbereich farblich für das menschliche Auge ununterscheidbar sind;
• Aufnahme eines Prüfbildes durch die Bilderfassungseinheit, wobei das Prüfbild ein digitales Bild ist, das zumindest den Tintenstrahldruck des Dokuments abbildet;
• Durchführung einer Bildanalyse des Prüfbilds durch die Prüfeinheit, wobei die Bildanalyse umfasst:
  • Automatische Erfassung der Morphologie der im Prüfbild abgebildeten Tintenstrahltröpfchen, aus welchen der Tintenstrahldruck besteht, wobei die Morphologie eines Tintenstrahltröpfchens dessen räumliche Erstreckung innerhalb des Prüfbildes beschreibt; und
  • Automatische Rekonstruktion eines digitalen graphischen Musters aus dem Prüfbild, wobei das Muster ein oder mehrere erste Bereiche und ein oder mehrere zweite Bereiche beinhaltet, wobei die ersten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine erste Morphologie aufweisen, wobei die zweiten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine zweite Morphologie aufweisen, wobei die zweite von der ersten Morphologie abweicht;
• Vergleichen des rekonstruierten graphischen Musters oder eines daraus abgeleiteten Wertes mit einem Referenzwert zur Prüfung der Echtheit des Dokuments.

In a further aspect, the invention relates to a method for checking a document. The procedure includes:

• Receiving a document with a document body by a verification unit that includes an image capture unit. An inkjet print of an image is contained in or on the document body. One or more first regions of the image are printed with a dye-based ink. One or more second regions of the image are printed with a pigment-based ink. The one or more first regions are void of the pigment-based ink and the one or more second regions are void of the dye-based ink. The absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar to each other that they are indistinguishable to the human eye when the inkjet print is illuminated with light in the visible spectral range;
• the image capture unit capturing a proof image, the proof image being a digital image that at least depicts the inkjet print of the document;
• Carrying out an image analysis of the test image by the test unit, the image analysis comprising:
  • Automatic detection of the morphology of the ink jet droplets depicted in the test image, which make up the ink jet print, the morphology of an ink jet droplet describing its spatial extent within the test image; and
  • Automatic reconstruction of a digital graphic pattern from the proof image, the pattern including one or more first areas and one or more second areas, the first areas being those proof image regions whose inkjet droplets have a first morphology, the second areas being those proof image regions whose inkjet droplets have a second morphology, the second differing from the first morphology;
• Comparing the reconstructed graphic pattern or a value derived therefrom with a reference value to check the authenticity of the document.

For example, the image analysis can be performed by software operating under explicit rules given by the programmer. These rules include various criteria and limits with regard to, for example, the droplet diameter in the X, Y and/or Z direction, with regard to intensity differences within the droplet, with regard to 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 imaged in the test image to 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). For the training of the software, a training data set was used that includes a plurality of training templates, each of the training templates being a digital image of an inkjet print, the inkjet print being printed with the same inks as the currently captured template and also containing first and second regions that form a pattern. The first regions of the training templates map regions of the inkjet print that were printed with the same or very similar dye-based ink as the first in terms of droplet morphology Inkjet print regions of the currently checked document. The second regions of the training test images map regions of the inkjet print printed with the same or very similar pigment-based ink in terms of droplet morphology as the second regions of the inkjet print of the current document under test. The first and second regions in the training probe images are labeled ("annotated") as such, and various properties of the droplets in the first and second regions of the training probe 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.

• Decodierung des Strich- oder Matrixcodes durch die Prüfeinheit, um einen decodierten Wert zu erhalten; und
• Verwendung des decodierten Wert als den aus dem graphischen Muster abgeleiteten Wertes bei dem Vergleich mit dem Referenzwert.

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

• the verification unit decoding the bar or matrix code to obtain a decoded value; and
• using the decoded value as the value derived from the graphic template in the comparison with the reference value.

According to embodiments, the resolution of the image acquisition unit is sufficiently high to be able to distinguish at least the ink jet droplets of the pigment-based ink from one another, that is, to be able to resolve them spatially. 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, can also be used.

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

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

• Aufnahme mehrerer Prüfbilder in mehreren Ebenen entlang der Dimension z durch die Bilderfassungseinheit; beispielsweise können die Bilder in Abständen von 10µm oder 20µm oder einem beliebigen anderen Abstand, vorzugsweise kleiner als 80µm, aufgenommen werden; jedes der mehreren Prüfbilder ist ein digitales Bild, das zumindest den Tintenstrahldruck des Dokuments in einer z-Ebene des Dokuments abbildet;
• Durchführung der Bildanalyse des Prüfbilds durch die Prüfeinheit derart, dass die Bildanalyse umfasst:
  • Automatische Erfassung der Morphologie der in den mehreren Prüfbildern jeweils abgebildeten Tintenstrahltröpfchen;
  • Automatisches Erfassen der Topologie der in den mehreren Prüfbildern jeweils abgebildeten Tintenstrahltröpfchen, wobei die Topologie eines Tintenstrahltröpfchens eine räumliche Erstreckung des Tintenstrahltröpfchens über eine oder mehrere der z-Ebenen entlang der z-Dimension beschreibt, wobei die ersten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine erste Morphologie in Verbindung mit einer ersten Topologie aufweisen, wobei die zweiten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine zweite Morphologie in Verbindung mit einer zweiten Topologie aufweisen, wobei die zweite von der ersten Topologie abweicht.

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

• acquisition of a plurality of inspection images in a plurality of planes along the dimension z by the image acquisition unit; for example, the images can be taken at a spacing of 10 µm or 20 µm or any other spacing, preferably less than 80 µm; each of the plurality of proof images is a digital image that maps at least the inkjet print of the document in a z-plane of the document;
• Carrying out the image analysis of the test image by the test unit in such a way that the image analysis includes:
  • Automatic detection of the morphology of the ink jet droplets respectively imaged in the plurality of test images;
  • Automatic detection of the topology of the inkjet droplets respectively imaged in the plurality of test images, the topology of an inkjet droplet describing a spatial extent of the inkjet droplet over one or more of the z-planes along the z-dimension, the first areas being those test image regions whose inkjet droplets have a first morphology in connection with a first topology, the second areas being those test image regions whose inkjet droplets have a second morphology in connection with a have a second topology, the second differing from the first topology.

• ein durchschnittlicher Tröpfchendurchmesser in z- Richtung von über 15µm;
• ein durchschnittlicher Tröpfchendurchmesser in derjenigen z-Ebene des Dokuments, auf welchen der Aufdruck erfolgte, im Bereich 15-100 µm;
• ein Intensitätsverlauf vom Zentrum nach außen in derjenigen z-Ebene des Dokuments, auf welchen der Aufdruck erfolgte, derart, dass der Intensitätsverlauf homogen ist oder das Tröpfchenzentrum dunkler ist als der Rand;
• unscharfe Tröpfchenränder in allen z-Ebenen des Dokuments, in welchen eines der Prüfbilder erfasst wurde.

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

• an average droplet diameter in z-direction of more than 15 µm;
• an average droplet diameter in the z-plane of the document on which the printing took place in the range 15-100 µm;
• an intensity profile from the center outwards in that z-plane of the document on which the print was made, such that the intensity profile is homogeneous or the center of the droplet is darker than the edge;
• blurred droplet edges in all z-planes of the document in which one of the test images was captured.

• ein durchschnittlicher Tröpfchendurchmesser in z- Richtung im Bereich von maximal 15 µm;
• ein durchschnittlicher Tröpfchendurchmesser in derjenigen z-Ebene des Dokuments, auf welchen der Aufdruck erfolgte, im Bereich 15-100 µm;
• ein radialer Intensitätsverlauf vom Zentrum nach außen in derjenigen z-Ebene des Dokuments, auf welchen der Aufdruck erfolgte, derart, derart, dass der Tröpchenumriss dunkler ist als das Zentrum und einen dunklen Rand um das Tröpfchen bildet;
• scharfe Tröpfchenränder in zumindest einer der mehreren z-Ebenen des Dokuments, in welchen eines der Prüfbilder erfasst wurde.

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

• an average droplet diameter in the z-direction in the range of at most 15 µm;
• an average droplet diameter in the z-plane of the document on which the printing took place in the range 15-100 µm;
• a radial intensity gradient from the center outwards in the z-plane of the document on which the printing was made, such that the droplet outline is darker than the center and forms a dark border around the droplet;
• sharp droplet edges in at least one of the multiple z-planes of the document in which one of the test images was captured.

According to embodiments of the invention, the image is a face or iris image of a person to whom the document is associated.

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

• Aktivieren der Infrarotlichtquelle;
• Aufnahme eines IR-Prüfbildes durch eine IR-Bilderfassungseinheit, wobei das IR-Prüfbild ein digitales Bild ist, das zumindest den Tintenstrahldruck des Dokuments abbildet;
• Durchführung einer Bildanalyse des IR-Prüfbilds durch die Prüfeinheit, wobei die Bildanalyse umfasst:
  • Automatische Erfassung dunkler Bereiche des IR-Prüfbildes, wobei ein dunkler Bereich eine durchschnittliche Intensität unterhalb eines vordefinierten Grenzwertes besitzt; und
  • Automatische Rekonstruktion eines digitalen graphischen Musters aus den dunklen Bereichen, wobei das Muster als "IR-Muster" bezeichnet wird;
• Vergleichen des IR-Musters oder eines daraus abgeleiteten Wertes mit einem IR-Muster-Referenzwert zur Prüfung der Echtheit des Dokuments.

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

• activating the infrared light source;
• capturing an IR test image by an IR image acquisition unit, the IR test image being a digital image that depicts at least the inkjet print of the document;
• Performing an image analysis of the IR test image by the test unit, the image analysis comprising:
  • Automatic detection of dark areas of the IR test image, where a dark area has an average intensity below a predefined threshold; and
  • Automatic reconstruction of a digital graphic pattern from the dark areas, the pattern being referred to as "IR pattern";
• Comparing the IR pattern or a value derived from it with an IR pattern reference value to check the authenticity of the document.

• Aufnahme eines Prüfbildes durch die Bilderfassungseinheit, wobei das Prüfbild ein digitales Bild ist, das zumindest den Tintenstrahldruck des Dokuments abbildet;
• Durchführung einer Bildanalyse des Prüfbilds durch den zumindest einen Prozessor der Prüfeinheit, wobei die Bildanalyse umfasst:
  • Automatische Erfassung der Morphologie der im Prüfbild abgebildeten Tintenstrahltröpfchen, aus welchen der Tintenstrahldruck besteht, wobei die Morphologie eines Tintenstrahltröpfchens dessen räumliche Erstreckung innerhalb des Prüfbildes beschreibt; und
  • Automatische Rekonstruktion eines digitalen graphischen Musters aus dem Prüfbild, wobei das Muster ein oder mehrere erste Bereiche und ein oder mehrere zweite Bereiche beinhaltet, wobei die ersten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine erste Morphologie aufweisen, wobei die zweiten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine zweite Morphologie aufweisen, wobei die zweite von der ersten Morphologie abweicht;
• Vergleichen des rekonstruierten graphischen Musters oder eines daraus abgeleiteten Wertes mit einem Referenzwert zur Prüfung der Echtheit des Dokuments durch den zumindest einen Prozessor der Prüfeinheit.

In a further aspect, the invention relates to a verification unit with an opening for receiving a document. The document has a document body in or on which is contained a printed area, also referred to herein as "inkjet printing". The inspection unit also includes an image acquisition unit with sufficient resolution to be able to spatially resolve ink jet droplets in the printed area. The testing unit further includes at least one processor and non-transitory memory having computer-interpretable instructions that, when executed by the at least one processor, cause testing methods according to any of the embodiments described herein to be performed. In particular, the following steps are carried out by the components of the verification unit after receipt of the document:

• the image capture unit capturing a proof image, the proof image being a digital image that at least depicts the inkjet print of the document;
• Carrying out an image analysis of the test image by the at least one processor of the test unit, the image analysis comprising:
  • Automatic detection of the morphology of the ink jet droplets depicted in the test image, which make up the ink jet print, the morphology of an ink jet droplet describing its spatial extent within the test image; and
  • Automatic reconstruction of a digital graphic pattern from the proof image, the pattern including one or more first areas and one or more second areas, the first areas being those proof image regions whose inkjet droplets have a first morphology, the second areas being those proof image regions whose inkjet droplets have a second morphology, the second differing from the first morphology;
• Comparing the reconstructed graphic pattern or a value derived therefrom with a reference value for checking the authenticity of the document by the at least one processor of the checking unit.

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

• eine IR-Lichtquelle; und/oder
• eine im IR-Spektrum empfindliche Bilderfassungseinheit, z.B. eine IR-sensitive Kamera; und/oder
• eine Lichtquelle für Licht im sichtbaren Spektralbereich; und/oder
• einen automatischen Dokumenteneinzug; und/oder
• eine Kontrollvorrichtung zur Kontrolle des Zugriffs auf oder Zutritts zu: geschützte Daten, Softwarefunktionen, Hardwarefunktionen oder räumliche Bereiche, wobei die Kontrollvorrichtung dazu konfiguriert ist, den Zugriff oder Zutritt nur dann zu gewähren, falls das Prüfverfahren ergibt, dass das Dokument valide ist.

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

• an IR light source; and or
• an image acquisition unit that is sensitive in the IR spectrum, for example an IR-sensitive camera; and or
• a light source for light in the visible spectral range; and or
• an automatic document feeder; and or
• a control device for controlling access to or access to: protected data, software functions, hardware functions or spatial areas, where the control device is configured to grant access or entry only if the verification procedure shows that the document is valid.

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

A light source in the visible range can be advantageous since the test device now has its own light source so that the test image can also be used at night, for example, i.e. in the absence of daylight and another external white light source.

The document feeder can be beneficial as 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.

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

In one embodiment of the testing device, the testing device only produces IR test images in “IR mode”, which is used as “normal mode”, since these can be analyzed very quickly and a large number of people can be tested in a short time. In this way, queues in front of the terminal can be avoided. However, on a random basis or if the IR light source fails, the tester performs the test in "white light mode". This means that a test image is captured in high resolution (e.g. at least 15 µm, preferably at least 5 µm) under white light and the inkjet droplets depicted therein are analyzed by image analysis software in order to recognize the pattern based on different droplet morphologies and/or topologies.

Definitions:

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

Insofar as the term "document", "product", "value or security product" and in particular the term "value or security document" are mentioned in the description and in the claims of the present application, these are to be understood synonymously and include, for example, a passport, identity card, driver's license or another ID card or an access control ID, a vehicle registration document, vehicle registration document, visa, cheque, means of payment, in particular a banknote, a cheque, bank, credit or cash payment card, customer card, health card, chip card, a company ID card, proof of entitlement , membership card, gift or shopping voucher, bill of lading or other proof of entitlement, revenue stamps, postage stamps, tickets, (game) chips, adhesive labels (e.g. for product security) or another ID document. The product can be a smart card, for example. The security or value document may be in ID 1, ID 2, ID 3 or any other format, for example in booklet form, like a passport-like item.

A document can be a laminate of several document layers, which are connected to one another, e.g. fused, with a precise fit under the action of heat and increased pressure. These products should meet the standardized requirements, for example the standards ISO 10373, ISO/IEC 7810, ISO 14443. The product layers consist, for example, of a carrier material that is suitable for lamination. In addition to value or security documents, the term “valuable or security product” also includes patches, labels and the like, which as security elements are components of documents and are or will be permanently connected to the document carrier and form the security feature there.

The document can be formed from a polymer selected from a group comprising polycarbonate (PC), in particular bisphenol A polycarbonate, polyethylene terephthalate (PET), their derivatives such as glycol-modified PET (PETG), polyethylene naphthalate (PEN), polyvinyl chloride (PVC), polyvinyl butyral (PVB), polymethyl methacrylate (PMMA), polyimide (PI), polyvinyl alcohol (PVA), polystyrene (PS), 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 ceramics. In addition, the product can also be made of several of these materials. It preferably consists of PC, PET and/or PVC. The polymers can be either filled or unfilled. In the latter case, they are preferably transparent or translucent. If the polymers are filled, they are opaque. The above information relates both to films to be bonded together and to liquid formulations that are applied to a preliminary product, such as a protective or top coat.

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 the action of pressure and heat. The individual foils can consist of the same material or of different materials. Overlay layers formed in this way protect a security feature arranged underneath and/or give the document the required abrasion resistance.

Insofar as the terms “individualized” and “individualizing” are used below, this is to be understood as meaning a property of a value or security document according to which the document can be assigned to a specific subject (person, organization, animal, object). Insofar as the terms “personalized” and “personalizing” are used below, this is to be understood as meaning the property of a value or security document, according to which the document can be assigned to a person. This property results from specified security features of the value or security products, preferably from the security print according to the invention.

Insofar as the term "color impression" or "color" is used in the description and in the claims of the present application, this is to be understood as meaning the optical impression on a human observer with regard to the color effect, whereby 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 hues that evoke a color impression on a human observer that differs from each of the closely spaced hues.

By an "ink" is meant herein a liquid of any viscosity permitting use by an ink jet printer and in which one or more colorants are incorporated. Preferably the ink has a low viscosity. The term "colorant" is to be understood as meaning substances with specific properties which absorb at least in the visual spectral range and optionally in other spectral ranges. Colorants are substances that give the substance (eg ink or solvent) in which they are contained the corresponding color

A “dye-based ink” is understood here to mean an ink that consists of a liquid—typically water—and colorants dissolved therein. Solubilizers may be used with some dye-based inks. The colorants dissolved in a dye-based ink are referred to as "dye". A "dye" is understood here to mean a substance or a mixture of substances which has light absorption in the visible spectral range and, in contrast to a color pigment, chemically dissolves in a liquid, in particular water.

A " pigment-based ink" is understood here to mean an ink which consists of 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 mean a substance or a mixture of substances which has light absorption at least in the visible spectral range.

A " printing color", also called "hue" , is understood here to mean the color that the human eye perceives when looking at a printout that was produced with a specific ink when the printout is illuminated with light of the visible spectral range. In embodiments in which the test image is captured as a monochromatic digital image, the term "ink" preferably refers to the intensity value of that color in one color channel of the monochromatic digital test image.

A "security feature" is a characteristic property of a document that proves the authenticity (authenticity) of a document and makes 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, multicolored processed guilloches, printing elements with tilting effect, lettering elements with microlettering or the like can serve as a security feature. A document according to an embodiment of the invention includes at least one security feature in the form of an inkjet printed image in or on the document containing a pattern which is not visible to the human eye in visible light but which can be recognized by image analysis of the droplets of this inkjet print.

An “ inkjet printer ” is a printer, in particular a dot matrix printer, in which a print image is generated by the targeted firing and/or deflection of small ink droplets.

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

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

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

" Light in the infrared (IR) range", "infrared radiation" or "IR radiation" is understood here to mean electromagnetic radiation in the spectral range between visible light and longer-wave terahertz radiation. In particular, this means light with a wavelength between 780 nm and 1 mm. This corresponds to a frequency range from 300 GHz to 400 THz or a wave number range from 10 cm ^-1 to 12800 cm ^-1 .

A "digital image" is a data set in which image content is represented and stored. In particular, the digital image can be a data record in which the content of an image is represented by whole numbers. In particular, the digital image can be a raster graphic.

A " proof image " is a digital image that depicts at least that part of the document that contains the inkjet printing and that was generated by an image capture unit digitally capturing that part. The detection preferably takes place with white light and with high resolution.

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

A “ verification unit” refers here to a device or a device component that is designed to verify the authenticity of a document using at least one security feature of the document.

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

An " IR image capture unit " is used here to refer to a device or a device component that contains at least one device for capturing a digital image of a physical object or is itself this device, the device being sensitive in the IR frequency range. The apparatus can be a camera or a microscope, for example.

A "camera" is used here to describe a photographic apparatus that can record static or moving images on a photographic film or electronically on a digital storage medium or transmit them to a recipient via an interface as a digital image. The camera can have a single-lens or multi-lens (comprising the eyepiece and lens) optics. The image is created by a lens on film (analogue camera) or on an electronic sensor (digital camera) on the opposite camera wall. Some cameras include a shutter button and a shutter (iris). The shutter release opens the shutter on the lens for a very short time, allowing light to pass through the lens and a picture to be taken.

A "microscope" is understood here to mean a light microscope, i.e. an optical apparatus for image acquisition that can produce greatly enlarged images of small structures or objects with the aid of light. The enlargement takes place according to the laws of optics using the refraction of light on glass lenses. In particular, the microscope can be a reflected-light microscope, but in some cases bright-field microscopes can also be used Embodiments are used, for example, in documents with a transparent document body. The microscope can be a "simple" or "compound" microscope. Simple microscopes only have a single optical system for magnification and work like a magnifying glass (see there for the principle of magnification). A single glass lens or a combination of several single 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, the lens, creates an enlarged real image, the intermediate image, which is enlarged a second time by the eyepiece. The eyepiece works like a magnifying glass and creates a virtual image of the intermediate image. The total magnification of the microscope is the product of objective magnification and eyepiece magnification. With a 20x objective and a 10x eyepiece, the total magnification is 200x. In some microscopes, several lenses are rotatably mounted, so that one lens and any one of the eyepieces can be freely combined with one another by mechanically rotating the bearing axis in order to achieve the desired resolution. Some cameras with high resolution can also be used as a microscope and referred to as a "microscope in the broader sense", but unlike microscopes in the narrower sense, cameras usually do not have the multiple rotatable lenses.

The "morphology" of an inkjet droplet is understood here to mean a specification of one or more characteristics of the inkjet droplet with regard 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. For example, the length of the document can be specified in the "x-direction" and the depth in the "y-direction". The 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 pixels of the droplet, e.g. the intensity curve from the center of the droplet to the edges when capturing an image of a cross section of the droplet in an xy plane. The morphology does not include the spatial extent of the droplet in the z-direction. The morphology of a droplet is therefore a description of the spatial extension of the droplet and optionally other properties in two-dimensional space, i.e. a plane. This level is preferable the plane on which at least part of the image was printed on the document, or a plane parallel to it.

The "topology" of an inkjet droplet is understood here as a specification of the morphology of the droplet in combination with a specification of one or more features of the inkjet droplet with regard to its spatial extension in the z-direction. The spatial extent of the document in the "z-direction" corresponds to the thickness of the document. For example, the length of the document may be specified in an "x-direction" and the depth in a "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 over one or more material planes in the z-direction. The topology can therefore include in particular the droplet diameter in the z-direction, but also the outline size and shape and/or the intensity distribution of the pixels of the droplet, e.g. the intensity curve 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 therefore a description of the spatial extent of the droplet and optionally other properties in all three spatial directions.

A “ pattern” is understood here as a structure on or in a physical object that can be detected optically (by the human eye or by an optical sensor). The pattern contains a graphic element that is repeated with a certain regularity. For example, the pattern can be a code, such as a barcode that contains multiple bars, or a 2D code that has multiple squares arranged in a grid. In the course of pattern recognition, an input image is analyzed in order to recognize the regularly repeated graphic elements and the pattern that is formed from these elements.

Brief description of the drawing

Fig. 1

ein exemplarisches Flussdiagramm einer Ausgestaltung des erfindungsgemäßen Verfahrens zur Herstellung eines Dokuments mit einem Dokumentenkörper und einem mit dem Dokumentenkörper verbundenen, visuell nicht wahrnehmbaren Sicherheitsmerkmal;

Fig. 2

ein Blockdiagramm eines Tintenstrahldruckers, der zur Herstellung des Sicherheitsmerkmals verwendet wird;

Fig. 3

ein exemplarisches Flussdiagramm von Teilschritten eines Verfahrens zur Prüfung eines Dokuments anhand dessen Sicherheitsmerkmal;

Fig. 4

eine Illustration von Teilschritten der Erzeugung des Sicherheitsmerkmals aus einem Bild und einem Muster;

Fig. 5

eine beispielhafte Ausführung einer Variante eines erfindungsgemäßen Dokuments;

Fig. 6

verschiedene Ausschnitte des durch Tintenstrahldruck erzeugten Sicherheitsmerkmals in unterschiedlichen Vergrößerungen;

Fig. 7

eine dreidimensionale Illustration unterschiedlicherTröpfchenmorphologien und Topologien in einem mehrschichtigen Dokumentenkörper;

Fig. 8

ein Blockdiagramm einer Prüfeinheit;

Fig. 9

ein Blockdiagramm eines Systems mit der Prüfeinheit und mehreren Lichtquellen und Bilderfassungseinheiten; und

Embodiments of the invention are described below with reference to the drawings. In the drawing shows

1

an exemplary flowchart of an embodiment of the method according to the invention for producing a document with a document body and a visually imperceptible security feature associated with the body of the document;

2

Figure 12 is a block diagram of an inkjet printer used to produce the security feature;

3

an exemplary flowchart of sub-steps of a method for checking a document based on its security feature;

4

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

figure 5

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

6

various sections of the security feature produced by inkjet printing in different enlargements;

7

a three-dimensional illustration of different droplet morphologies and topologies in a multi-layer document body;

8

a block diagram of a test unit;

9

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

figure 1 Figure 11 illustrates some steps of a method for producing a document having a security feature associated with the document body of the document which is not visually perceptible under daylight/"white light" according to an embodiment of the invention. The document can be any value or security document, for example an ID card, a passport, a bank note, 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 made up of one or more layers. The document body or the individual layers can consist of different materials, for example cardboard, plastic, in particular polycarbonate, metal, wood or combinations thereof, in particular composite materials made of metal, plastic and/or cardboard. In some cases, the document can contain other elements, for example a chip card, a magnetic stripe, a data memory, an RFID antenna or the like. Security features can be attached to or in the document body, for example holograms, complex imprints, etc. The application of a further security feature in or on the document or its document body is described below, which is not visible to the human eye when the document is illuminated in visible light.

First, in step 102, a special inkjet printer 200 is provided, as illustrated by way of example in FIG. The inkjet printer includes at least a first tank containing a dye-based ink and a second tank containing a pigment-based ink. The absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar that an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink (of the same object or motif) are indistinguishable by the human eye when illuminated with light in the visible spectral range.

For example, both inks can be black in visible light. For example, the dye-based ink may be Solvent Black 27 ink and the pigment-based ink may be Carbon Black ink.

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

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

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

Optionally, the inkjet printer can contain additional tanks for additional pigment-based or dye-based inks.

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 areas and one or more second areas. For example, the one or more first areas can be a plurality of first squares and the one or more second areas can be a plurality of second squares, with the first and second squares being positioned on a rectangular area in such a way that they form a matrix code, e.g. a QR code.

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 graphics program or printer program mentioned above and optionally also edited. The image can be any image, such as photos of faces, buildings, other physical objects, and/or digital images of numbers, letters, or symbols. The images are preferably related to a person or institution for which or by which the document is issued. For example, the image can be the facial image of the person for whom the document (e.g. identity card or passport) is issued. However, the image can also be the logo of a bank or a company that commissions the creation of the document, for example to serve as a credit card for customers or as an employee ID card for employees. Alternatively, the image can be, for example, a number representing, for example, the value represented by the document (document-based bill/payment card). The image can be a monochromatic image or a multicolor image, for example in the RGB or CMYK color spaces. Preferably, the image contains as many channels as there are ink tanks in the ink jet printer.

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

For example, the overlay can be performed by the image editing program mentioned above or the printer program. For example, the overlay means that the pattern and thus also its first and second areas are mapped onto the digital image in a specific way. For example, the overlay can include first searching in the digital image for the largest possible areas whose pixels have a particularly high and preferably homogeneous intensity in that color channel whose color corresponds to the color of the dye-based ink in the first tank and the pigment-based ink in the second tank.

If the "color" of the printing inks in the first and second tank is "black", the digital image would be searched for those areas that contain 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. Thus, according to embodiments of the invention, the pattern that does not occupy the entire area of the image is superimposed on the region of the image that has the smallest variations in the proportion of black ink. In one embodiment, it is sufficient if the fluctuations in the proportion of black printing ink are minimized in the areas that are printed with the pigment-based ink. The information about the black components can already be available when the photo is rendered into the individual printing colors or can be generated by a corresponding analysis. The pattern can then be positioned at different locations in the image and the variation in black ink levels can be determined.

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

The method includes a further step 110 in which the ink jet printer prints the digital image on or in the document body. The printing process is 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 areas, and that the pigment-based ink, but not the dye-based ink, is used to print the regions with regions of the digital image superimposed on 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 that an inkjet print with the dye-based ink and an inkjet print with the pigment-based ink (if the printed object or image motif is identical) 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 produced by the overlay is also invisible in the printed image under white light.

In some embodiments, the pattern is code that encodes data directly or indirectly related to the document. For example, the data may include information specific to and/or known by the issuer of the document, the person to whom the document is assigned. In particular, the data can be data that is unique to the document and/or the person. For example, the data can be a combination of account number and sort code, a credit card number, an identity card number, a serial number of a document-based bill, or the like.

In some embodiments, this data can also be contained in or on the document or its document body in another way. For example, the document may include data storage, such as a magnetic stripe or a memory chip, in which the data encoded in the pattern is also stored. In the course of the document check, these can be read out by the checking unit and compared with the decoded data of the printed code that has also been read out. If the compared values do not match, the document is assumed to be forged.

For example, in some embodiments, in the course of creating or personalizing the document, a cryptographic key pair may be created that is specific to that document and/or a person for whom the document is created. The cryptographic key pair includes a private key and a corresponding public key. With the public key, those associated with the document related data unique to the document. The data unique to the document is encrypted with the public key of the key pair and encoded in a further step in a graphic code, for example a barcode or a matrix code. Thereafter, this matrix code is overlaid with a photograph to be printed on or in the document and the overlaid image is printed as described above.

In one embodiment of the production method, information about the material of the document body, the printing process, the presence of surface coatings, and/or the inks used is applied to or in the document in machine-readable form, e.g. as an additional imprint, as an engraving or as a data record in a data memory of the document or in an external database in connection with an ID of the document. The verifier can be configured to read this information from the document or database during verification and use it in image analysis to improve recognition of the pattern.

figure 2 shows a block diagram of an inkjet printer 200 with a plurality of ink tanks 218, which are used to carry out the in figure 1 described manufacturing method of a document can be used. The color tanks include at least a first tank 228 with a dye-based ink 216 and a second tank 226 with a pigment-based ink 214. The dye-based ink 216 and the pigment-based ink 214 have a color that is perceived as identical by the human eye under white light. For example, this color can be "black". In some embodiments, it is possible for the dye-based ink 216 and the pigment-based ink 214 to have significantly different absorption properties, at least under light of different wavelengths, for example under infrared light, or under UV light, such that ink and pigment-based ink and corresponding printouts are clearly distinguishable to the human eye under this light.

In some embodiments, the color tanks can comprise one or more further tanks, so that printing in CMYK color mode, for example, is possible. 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.

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 into a polycarbonate derivative. The polycarbonate derivative exhibited a relative solution viscosity of 1.263. A liquid preparation was made from 17, 5 parts by weight of the polycarbonate derivative and 82.5 parts by weight of a solvent mixture with the following components: • mesitylene: 2.4 • 1-Methoxy-2-propanol acetate: 34.95 • 1,2,4-trimethylbenzene 10.75 • Ethyl 3-ethoxypropionate 33.35 • cumene 0.105 • Solvent Naphtha 18.45

A colourless, highly viscous solution with a solution viscosity at room temperature of 800 mPas is obtained.

In a next step, 4 g of the polycarbonate solution from Example 2 and 30 g of the solvent mixture from Example 2 are homogenized using a magnetic stirrer in a 50 mL wide-neck threaded jar. A colourless, low-viscosity solution with a solution viscosity at room temperature of 1.67 mPas was obtained. The surface tension of this base ink was determined to be 21.4±1.9 mN/m using an OEG Surftens measuring system using the pendant drop method. In the next step, the desired pigment(s) or the desired dye(s) is/are added in the amounts required to produce a specific shade. Alternative compositions for ink jet printing inks in or on document bodies DE 10 2007 052 947 A1 described.

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

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.

The test can be carried out, for example, using a test unit 800 as in figure 8 shown. The test can be carried out, for example, at a national border, at an airport, at the gate to a company site, at a building entrance to a corresponding terminal. It is also possible to use a mobile version of the checking unit, for example for the purpose of mobile personal checks or to check the authenticity of documents of value using a cash register terminal.

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

The security feature is an image overlaid with a pattern that has been inkjet printed in or on the document, such as with regard to figure 1 described. An inkjet print of the image and a pattern not visible under white light is thus contained in or on the body of the document. 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 void of the pigment-based ink and the one or more second regions are void of the dye-based ink. The security feature thus comprises a multiplicity of first inkjet droplets based on the dye-based ink and a multiplicity of second inkjet droplets based on the pigment-based ink. The absorption spectrum of the dye-based ink and that of the pigment-based ink are so similar that when the inkjet print is illuminated with light in the visible spectral range, they are indistinguishable to the human eye in terms of color and therefore the pattern of the image regions printed with the dye-based ink and the image regions printed with the pigment-based ink is not visible under white light.

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

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

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

In the course of the image analysis, at least the morphology of the inkjet droplets, which are imaged in the test image and make up 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 defined by its length and width of the document or the length and width of the part of the document depicted in the test image is formed. Morphology includes, but is not limited to, the cross-section of the droplet, the nature of the outline (defined or diffuse edge, substantially circular or severely jagged or star-shaped), the presence, strength, and/or orientation of an intensity gradient within the droplet, and/or the brightness of the droplet, all in 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 if the document consists of several material layers, the imprint was applied to one of the inner layers and the ink penetrated into one or more of the other layers during or after the combination of these material layers with other material layers. For example, it has been observed that when several layers of material are pressed and/or glued together, dye-based ink 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, the topology of the individual ink jet droplets can be determined by analyzing a plurality of test images, and it is possible to distinguish first image regions, which selectively contain droplets of the dye-based ink, from second image regions, which selectively contain droplets of the pigment-based ink, and thereby recognize a pattern formed from the first and second image areas.

In step 310 of the image analysis, a digital graphic pattern is thus automatically reconstructed from the test image based on the detected droplet morphologies or from a plurality of test images based on the detected droplet morphologies in combination with the detected droplet topologies. The recognized pattern is formed from one or more first areas and one or more second areas. The first areas consist of those test image regions whose ink jet droplets have a first morphology (and optionally also topology). The second areas consist of those test image regions whose ink jet droplets have a second morphology (and optionally also topology). The first and second morphologies (or topologies, if determined) differ sufficiently so that the first and second regions can be reliably recognized and distinguished by the image analysis program.

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

In a further step 312 of the checking method, the checking unit compares the graphic pattern or a value derived therefrom with a reference value. For example, the graphic pattern can represent a simple checkerboard pattern made up of first and second areas, which is stored as a reference pattern in a memory of the test unit. A comparison of the pattern reconstructed from the reference pattern reveals whether the reconstructed pattern must be identical or sufficiently similar to the reference pattern. In this case, the security feature formed 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 takes place, in which a specific value is reconstructed from the code. This value can be, for example, a document-specific or person-specific number, for example a document ID, a credit card number, an ID card number, or a combination of one or more document-related or personal data such as personal name, date of birth and place of birth. The "derived" value obtained with the aid of the decoding method is stored with a reference value stored in the test unit or in a data memory accessible to the test unit compared and if they are identical or sufficiently similar, the validity of the security feature is determined.

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

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

The image 402 can be, for example, a digital portrait image, captured in visible light, of a person for whom a new document is to be issued and/or personalized. For example, the image 402 may be captured as an RGB image and converted to a CMYKK+ for inkjet printing. A digital pattern 404 is also provided. In some embodiments, the pattern is very simple. For example, this consists of figure 4 imaged patterns from two first areas 408.1, 408.2 and two second areas 406.1, 406.2. The first areas are areas intended to be printed with the dye-based ink but not the pigment-based ink. The second areas are areas intended to be printed with the pigment-based ink but not with the dye-based ink. In other embodiments, the pattern can also be much more complex and in particular 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 encoded form in the pattern. Before printing, the digital image 402 and the pattern 404 are overlaid. In the example shown here, the image and pattern are the same size and are overlaid at a 1 to 1 scale. In other embodiments, however, it is also possible for the superimposition to take place in such a way that the pattern superimposes only part of the image and/or that the pattern is superimposed on the image at positions determined dynamically and as a function of 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 the second regions 412.1, 412.2 of the document body correspond to the second image areas 406.1, 46.2 and are printed with the pigment-based ink are printed. In visible light, the first 414 and second 412 regions of the resulting inkjet printed image are indistinguishable to the naked eye. With a microscope or a high-resolution camera, however, it is possible to recognize the first and second regions and the pattern formed by them based on the different droplet morphology or droplet topology.

In some embodiments like that in figure 4 shown, the dye-based ink and the pigment-based ink have the additional property that under a special light, for example infrared light, only the pigment-based ink is opaque and absorbs light, while 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 discernible, but the background color of the card body appears. In contrast, in the second regions 412.1, 412.2, which were printed with the pigment-based ink that is also opaque in infrared light, the corresponding regions of the portrait image are visible in a manner similar to that in white light. In embodiments in which the dye-based ink and the pigment-based ink cause different color values or intensities in the visible wavelength range under a special light such as infrared light or UV light, the document or the imprint can 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.

figure 5 Figure 5 shows 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 containing a visible light invisible security feature in the form of a visible light invisible pattern of first and second regions of ink jet droplets of different morphology or topology. The document can optionally have a large number of other security features, such as watermarks, embossed marks, holograms or tilting images, which serve to make counterfeiting more difficult and are not shown here. The image 502 can in particular be a personalized image, for example a portrait image of the person to whom this document is assigned. The document body can be a substantially transparent to visible light be plastic card such as in EP 1222620 B1 or WO 02/45008 82 described. However, the document body can also be opaque.

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

An image 502 is ink jet printed on or in the document. The image contains first areas or regions that are only printed 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 only printed 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. For example, the image may occupy 5-95% of the surface area of the document.

Optionally, the document may include other elements such as a microchip 504 and/or a magnetic stripe (not shown). In addition or as an alternative to this, the document can contain further optically detectable data 508, for example a printed document ID or personal ID. The optically detectable data 508 can also be printed or applied using other methods, for example laser engraving. The pattern contained invisibly in the image 502 can be a QR code, for example, which contains the further optically detectable data 508 in encoded form.

Figure 6A shows a detail 600 of the face of the person depicted in image 502 under white light at 20x magnification. Although the right eye is printed with dye-based ink 216 and the left eye is printed with pigment-based ink 214, no color or shadow is visible at this resolution under white light. Intensity difference and thus no pattern from the first and second image areas recognizable.

Figure 6B 12 shows a sub-section 602 of the image 502 with the person's right eye printed with the dye-based ink and magnified 100 times. It is recognizable that the droplet edges are blurred. It is not possible for the image acquisition unit to find a plane in which the droplets are sharply focused.

Figure 6C FIG. 6 shows a sub-section 604 of the image 502 with the person's left eye printed with the pigment-based ink and magnified 100 times. You can see the compared to the edges of the in Figure 6B shown droplets 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 includes modifying the zooming of the image capture unit to attempt to focus in multiple planes along the z-dimension of the document. It can be determined whether it is possible to find at least one plane per droplet in which its edges appear sharp. The different focusability or edge sharpness of the droplets allows an inspection unit to detect areas that only contain droplets of the pigment-based ink or only droplets of the dye-based ink.

Figure 6D 12 shows a sub-section 606 of the image 502 with the person's right eye printed with the dye-based ink at a magnification of 200X. 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 distributed largely homogeneously within the respective droplets.

Figure 6E FIG. 6 shows a sub-section 608 of the image 502 with the person's left eye printed with the pigment-based ink and magnified 200 times. Even with 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 droplet, in particular the droplet edges are darker than the interior areas of the droplets.

figure 7 a three-dimensional illustration of different droplet morphologies and topologies in a multi-layer document body of a document 700. The document comprises several material layers 702-714 made of a polymer material, in particular polycarbonate. The document has a length (dimension x), a width (dimension y) and a height (z dimension). The multiple layers are stacked on top of each other along the dimension z before being processed into a laminate.

During the production of the document, the image with the non-visible security feature is printed on one of the layers, here layer 708, by inkjet printing, as has already been described here for embodiments of the invention. Preferably, the printing is done on one of the layers of material that will later be inside the finished document. Immediately after printing onto the layer 708, the individual ink jet droplets 716, 718, 720, 722 travel on the surface of the layer of material 708 defined by the X and Y dimensions. The droplets 716, 718 consist of the pigment-based ink 214, e.g. an ink with carbon black and the droplets 720, 722 consist of the dye-based ink 216, e.g. an ink with solvent black 27. The two-dimensional progression of the droplets onto this surface is referred to as the morphology of the droplets. Applicant has found that the morphology of the droplet depends critically on whether the droplet is 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 center of the droplet, whereas the edges of dye-based ink droplets are blurry and fuzzy and usually have the same brightness as the center of the droplet, sometimes the edges also appear lighter than the center of the droplet. In addition, it was found that the depth of penetration of the color across multiple layers differs between the pigment-based ink and the dye-based ink. The depth of penetration of the dye-based ink is greater and thus also the extension of the droplets 722, 720 along the z-axis, here represented 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 test images are captured by the image capturing unit along the Z-plane, preferably in such a way that at least one test image is captured per plane 702-714. For example, the focus of the image capturing unit can be changed in such a way that different levels of the document are "traversed through", the test images being captured in the process.

The multi-layer document is made according to one embodiment by providing two or more polymer layers and at least one surface on at least one 708 of these polymeric layers is printed with the dye-based ink and the pigment-based ink to form an image 502 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, e.g. as described with reference to figures 4 and 5 was described. For example, the image may occupy 5-95% of the surface area of the document. After the printing step, the several layers of polymer material are stacked on top of one another and connected in a materially bonded manner, for example by means of printing and/or adhesive methods. The printed polymer layer surface can form the top or bottom of the layered composite. The printed polymer layer is preferably surrounded on the bottom and/or top by at least one further polymer layer 706, 710, so that the printed surface is inside the layered composite. Depending on the type of material used in the adjacent layers, it may be that the depth of penetration of the droplets 716, 718, 720, 722 into the adjacent layers is different.

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

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 that the first image areas printed with the dye-based ink are continuously printed on different layers of material than the second image areas printed with the pigment-based ink. In other embodiments, however, the distribution of the pressure on different layers is independent of the ink used, so that each layer is printed with both the pigment-based ink and the dye-based ink, albeit on different subareas of the respective layer. Associated with each of the layers may be part or part of an image 502 having a visually imperceptible security feature embedded therein. Only in the connection are all parts of the image and the security feature embedded in it arranged together in such a way that an observer can recognize the image.

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

The test unit also includes an image acquisition 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 quantity of the ink jet droplets (e.g. at least 20%) can be spatially resolved from one another on an imprint 502 of the correctly positioned document. The image acquisition unit is designed to have one or more capture test images. For example, the image acquisition unit has an automatic and/or manually operable focus. The focus is changed stepwise or continuously so that at least some of the inkjet droplets (namely those printed with pigment-based ink) appear sharp. In at least this plane and optionally in further planes in the Z-direction, a digital image, the so-called test image, is recorded. The test image depicts at least the printed image 502 and optionally other areas of the document body. The captured image can be a monochrome image or an RGB image, for example. It is preferably the same type of image from which a training data set of images also consisted, on the basis of which image analysis software 806 was trained. However, it is also possible for a captured monochrome image or RGB image to be subsequently converted into such an image format. The test images are captured under white light. That is, at the time of image capture, the document within the inspection unit is either illuminated with ambient white light through aperture 808 and/or the document is illuminated using a white light source located within or near the inspection unit.

In addition, the test unit includes an image analysis unit 804 with one or more processors 805 and image analysis software 806. The image analysis software can be a trained neural network, for example. 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 from the image acquisition unit have been stored. To be analyzed to determine the position, morphology and/or topology of the inkjet droplets making up the print 502. In addition, the image analysis software is designed to use the droplet morphology and/or topology to recognize first image regions 414.1, 414.2 that contain droplets of dye-based ink and that are free of droplets of pigment-based ink, and second image regions 412.1, 412.2 that contain droplets of pigment-based ink and that 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 droplet morphology and/or topology. The checking unit 800 compares the recognized pattern with a reference value. If the comparison shows that the sample is identical or sufficiently similar to the reference value, the test unit determines as a result of the test that the security feature is valid and that the document, unless other security features of the document are invalid, is to be regarded as valid (and thus authentic). The check can also be more complex, for example when the pattern is a two-dimensional code. In this case, the test unit uses a decoding method on the recognized pattern in order to obtain the value encoded in the pattern and to compare this with a corresponding reference value that is stored in a memory accessible to the test unit. If they are identical or sufficiently similar to the reference value, the validity of the security feature or the document is determined.

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

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

Alternatively, the decoded but still encrypted data value can be transmitted from the test unit to a decryption device. In this case, the verification unit does not have 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 data that has been provided by a large number of test units but has been decoded but is still encrypted. The decrypted data can then be transmitted back from the decryption device to the checking unit. This is preferably done via a protected data communication channel, for example end-to-end encryption. For example, the decrypted data can be a serial number or other identifier of the document or of the user to whom the document is assigned. The serial number or other identifier is stored in the test unit in the form of a reference value and/or can be read from the document by the test unit via another transmission channel. For example, the serial number or identifier can be printed or engraved on the body of the document and can be captured by the checking unit via a camera. The authenticity of the security feature can be established by the checking unit by comparing the decrypted data value with the reference value and/or with the value received via the other transmission channel, with values being identical or sufficiently similar in value implying authenticity.

figure 9 FIG. 8 shows a block diagram of a system 900 with the inspection unit 800 and several light sources and image capturing units. The system 900 includes a checking unit, such as those for embodiments of the invention and, for example, with reference to FIG figure 8 as well as a white light source 906 and an infrared light source 904. Both light sources are positioned to illuminate a document 500 properly placed in the aperture or inspection unit, or at least to illuminate an image 502 formed in or on that document. The light sources 904, 906 can be installed as an integral part of the test unit or installed or positioned on or next to the test unit be. For example, the opening 808 can be large enough for light from light sources external to the test unit to hit the document with sufficient intensity.

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

The verification unit can be operated in at least two different operating modes: in "white light mode" the document 500 located in the verification unit 800 is illuminated with white light and the white light camera 910 makes one or more high-resolution verification images under white light irradiation. For example, the test images can be captured as RGB images and stored as monochrome images, where the pixel intensities of the monochrome channel result as a sum or an average value of the intensities of the 3 R, B, G color channels. The high-resolution test images 912 are transmitted to the image analysis software 806 and this automatically recognizes a pattern 914 within the image based on the droplet morphology and/or topology. The pattern represents the security feature that is invisible to the human eye and is used by the test unit as described in order 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 recognized here, which corresponds, for example, to a certain expected type of code (e.g. barcode or matrix codes).

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

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

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

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

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

If the document 500, 700 is to support an examination in IR mode in addition to the white light mode, some additional steps can already be carried out in the method for producing or personalizing the document according to embodiments of the invention. 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 to the IR spectrum on the other hand for a large number of color tones produced 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, take the form of an allocation table or in the form of one or more analytical descriptions. The relationship can be made, for example, by creating a color image of a color reference table and a corresponding image with the camera, which is sensitive in the IR spectrum, when illuminated with light in the IR spectrum, The signals from the color camera and the camera, which is sensitive in the IR spectrum, are assigned to one another for identical image areas in the recordings.

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

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

The assignment can be created as follows: First, a color reference table is printed using the inks provided for the production of the document and, if necessary, a carrier material provided for the production of the document. The color reference table includes a plurality of inks representing a plurality of hues present in one or more images to be associated with the document, the plurality of hues including both pigment-based ink hues and dye-based ink hues. A color image (i.e., an RGB image under white light) is captured from the color reference panel, and a monochrome image is captured with an IR spectrum-sensitive camera illuminating the color reference panel with IR light. These steps can be done in any order. The hues of the RGB color image arranged at specific positions in the color reference table are then assigned to brightness values of the monochrome image at corresponding positions. The mapping can be provided as a mapping table, or in the form of an analytical description.

In order to print a specific image of a person or other physical object on or in the document body, an RGB color image of the object or person that is to appear on the document body is first captured with a color camera recorded under white light, or an existing RGB image recorded under white light is provided. A color print is made from this image and then a monochrome image of this print is taken with an IR spectrum sensitive camera under illumination with IR light. The order of the recordings is irrelevant. An image is calculated from the color image which corresponds to its brightness level image as captured by an IR spectrum sensitive camera when illuminated with infrared light, using only one opaque variant (a pigment-based ink) of two variant inks (pigment-based ink and dye-based ink of substantially the same color in visible light), the other variant being transparent when illuminated with infrared light. An assignment can be provided for this purpose, for example an assignment produced in the manner described. In other words, the color camera image is computed using a mapping to match its lightness level image as captured by an IR spectrum sensitive camera when illuminated with infrared light, using only one opaque variant (pigment-based ink) of a two-variant ink whose other variant (dye-based ink) is transparent when illuminated with infrared light.

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

According to one embodiment, a pattern embedded in the image captured by the color camera is reconstructed from the difference image, the content of which is decoded. Supplied coding parameters that were used when generating the pattern or the graphic code can be used for this purpose.

The document 500 can be, for example, an identity card that contains one or 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 inkjet regions with a dye-based ink and in second inkjet regions with a pigment-based ink. The first and second regions cannot be distinguished by the human eye in visible light, so that formed from the first and second regions pattern is a security feature that is invisible to the human eye in visible light.

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

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

In particular, the black ink, K, can come in two different versions, K and K+, which are visually indistinguishable, one version of which 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 hue, each area of a color image has a greater or lesser proportion of black ink. The pattern is embedded in the image (i.e. in the photo) during printing 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 only printed with the pigment-based black ink. Printing of the remaining C, M, and Y inks is unaffected by overlaying the pattern on the image. Because of the visual indistinguishability of the two black inks, a human viewer sees a normal color photograph. The visual indistinguishability also applies to a picture taken with the color camera, ie the signals from the color camera for the different versions of the printing colors are identical. When illuminated with IR light and photographed with a camera sensitive to the IR spectrum, pattern 914 becomes visible, as shown in figure 9 is indicated. In particular, the elements of the pattern printed with the IR light absorbing version of the black ink can be within an element or from element to element have strongly fluctuating degrees of absorption, which depend on the image content of the color image.

1. 1. Verfahren zur Herstellung eines Dokuments (500,700) mit einem Dokumentenkörper (506) und einem mit dem Dokumentenkörper verbundenen, visuell nicht wahrnehmbaren Sicherheitsmerkmal, umfassend:
   • Bereitstellung (102) eines Tintenstrahldruckers (200), welcher zumindest einen ersten Tank (228) mit einer farbstoffbasierten Tinte (216) und einen zweiten Tank (226) mit einer pigmentbasierten Tinte (214) beinhaltet, wobei das Absorptionsspektrum der farbstoffbasierten Tinte und das Absorptionsspektrum der pigmentbasierten Tinte einander so ähnlich sind, dass ein Tintenstrahldruck mit der farbstoffbasierten Tinte und ein Tintenstrahldruck mit der pigmentbasierten Tinte bei einer Beleuchtung mit Licht im sichtbaren Spektralbereich durch das menschliche Auge ununterscheidbar sind;
   • Bereitstellen (104) eines digitalen graphischen Musters (404) das gebildet wird aus ein oder mehreren ersten Bereichen (408.1, 408.2) und ein oder mehreren zweiten Bereichen (406.1, 406.2); und
   • Bereitstellen eines mit dem Dokument zu verbindenden digitalen Bildes (402);
   • Überlagerung (106) des graphischen Musters und des digitalen Bildes;
   • Drucken (108) des digitalen Bildes auf den oder in den Dokumentenkörper mit dem Tintenstrahldrucker derart, dass die farbstoffbasierte Tinte aber nicht die pigmentbasierte Tinte zum Druck der mit den ersten Bereichen überlagerten Regionen (414.1, 414.2) des digitalen Bildes verwendet wird und dass die pigmentbasierte Tinte aber nicht die farbstoffbasierte Tinte zum Druck der mit den zweiten Bereichen überlagerten Regionen (412.1, 412.2) des digitalen Bildes verwendet wird, wobei die Verteilung der farbstoffbasierten Tinte und der pigmentbasierten Tinte in dem gedruckten Bild das Sicherheitsmerkmal ist.
2. 2. Verfahren nach Anspruch 1, wobei das Muster einen Strich- oder Matrixcodes umfasst.
3. 3. Verfahren nach einem der vorigen Ansprüche, wobei das digitale Bild ein monochromes digitales Bild ist oder wobei das digitale Bild ein polychromatisches Bild, insbesondere ein RGB Bild oder CMYK Bild ist.
4. 4. Verfahren nach einem der vorigen Ansprüche, wobei das Dokumentenkörpermaterial, auf welchen das Bild aufgedruckt ist, eine Hintergrundfarbe hat, wobei die Hintergrundfarbe hell und insbesondere weiß ist, wobei bei einer Beleuchtung mit Licht im infraroten Spektralbereich die farbstoffbasierte Tinte für das menschliche Auge einen transparenten Farbeindruck bewirkt, sodass die ein oder mehreren ersten Bereiche die Hintergrundfarbe haben, und die pigmentbasierte Tinte für das menschliche Auge einen opaken Farbeindruck bewirkt, sodass die ein oder mehreren zweiten Bereiche eine andere Farbe als die Hintergrundfarbe haben.
5. 5. Verfahren nach einem der vorigen Ansprüche, wobei die Tinten im ersten und zweiten Tank bei einer Beleuchtung mit Licht im sichtbaren Spektralbereich beide:
   • einen schwarzen Farbton haben,
     • wobei die farbstoffbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: Solvent Black 27, Solvent Black 29; und/oder
     • wobei die pigmentbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: Carbon Black, Pigment Black 28;
   • oder beide einen cyanfarbenen Farbton haben,
     • wobei die farbstoffbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: Solvent Blue 78, Sudanblau; und/oder
     • wobei die pigmentbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: Cu-Phthalocyanin;
   • oder beide einen magenta Farbton haben,
     • wobei die farbstoffbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: rote Azofarbstoffe, z.B. Solvent Red 26, Sudan Rot; und/oder
     • wobei die pigmentbasierte Tinte Quinacridon ist;
   • oder beide einen gelben Farbton haben,
     • wobei die farbstoffbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: gelbe Azofarbstoffe, z.B. Solvent Yellow 124; und/oder
     • wobei die pigmentbasierte Tinte vorzugsweise ein oder mehrere der folgenden Substanzen umfasst: Brilliantgelb, Pigment Gelb 151.
6. 6. Verfahren nach einem der vorigen Ansprüche, wobei der Tintenstrahldrucker zumindest noch einen dritten Tank mit einer weiteren farbstoffbasierte Tinte und einen vierten Tank mit einer weiteren pigmentbasierte Tinte beinhaltet, wobei das Absorptionsspektrum der weiteren farbstoffbasierten Tinte und das Absorptionsspektrum der weiteren pigmentbasierten Tinte einander so ähnlich sind, dass diese weitere farbstoffbasierte Tinte bei einer Beleuchtung mit Licht im sichtbaren Spektralbereich farblich von der weiteren pigmentbasierten Tinte durch das menschliche Auge ununterscheidbar ist, aber von den Tinten im ersten und zweiten Tank farblich unterscheidbar ist, ferner umfassend:
   • Bereitstellen eines weiteren digitalen graphischen Musters beinhaltend ein oder mehrere dritte Bereiche und ein oder mehrere vierte Bereiche; und
   • Überlagerung des weiteren graphischen Musters und des digitalen Bildes;
   • Durchführen des Druckens des digitalen Bildes auf den oder in den Dokumentenkörper derart, dass die farbstoffbasierte Tinte aber nicht die pigmentbasierte Tinte zum Druck der mit den ersten Bereichen überlagerten Regionen des digitalen Bildes verwendet wird, dass die pigmentbasierte Tinte aber nicht die farbstoffbasierte Tinte zum Druck der mit den zweiten Bereichen überlagerten Regionen des digitalen Bildes verwendet werden, dass die weitere farbstoffbasierte Tinte aber nicht die weitere pigmentbasierte Tinte zum Druck der mit den dritten Bereichen überlagerten Regionen des digitalen Bildes verwendet wird, dass die weitere pigmentbasierte Tinte aber nicht die weitere farbstoffbasierte Tinte zum Druck der mit den vierten Bereichen überlagerten Regionen des digitalen Bildes verwendet werden, wobei die Verteilung der farbstoffbasierten Tinte, der pigmentbasierten Tinte, der weiteren farbstoffbasierten Tinte und der weiteren pigmentbasierten Tinte in dem gedruckten Bild das Sicherheitsmerkmal ist.
7. 7. Verfahren nach einem der vorigen Ansprüche, umfassend:
   • Ermitteln einer Fläche innerhalb des Bildes, welches eine Mindesthomogenität und Mindestintensität hinsichtlich des Absorptionsspektrums des Farbtons aufweist, der mit der farbstoffbasierten Tinte oder der pigmentbasierten Tinte gedruckt werden soll;
   • Durchführung der Überlagerung derart, dass das graphische Muster vollständig auf diese Fläche abgebildet wird.
8. 8. Verfahren nach einem der vorigen Ansprüche, wobei der Dokumentenkörper aus mehreren Materialschichten (702-714) aufgebaut ist, wobei das Drucken des digitalen Bildes in den Dokumentenkörper so erfolgt, dass das digitale Bild auf eine oder mehrere der Materialschichten aufgedruckt wird, und danach die ein oder mehreren bedruckten Schichten, optional zusammen mit weiteren Materialschichten, zu dem Dokumentenkörper untrennbar verbunden werden.
9. 9. Verfahren nach Anspruch 8, wobei die farbstoffbasierte Tinte bei dem Verbinden der mehreren Materialschichten weiter und/oder unter Erzeugung einer anderen Tropfentopologie in das Material einer oder mehrerer der Schichten eindringt als die pigmentbasierte Tinte.
10. 10. Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, wobei das graphische Muster Daten (508) in codierter Form enthält, wobei die Daten für das Dokument und/oder dessen Besitzer einzigartig sind.
11. 11. Verfahren nach Anspruch 10, außerdem umfassend:
    • Versehen des Dokumentenkörpers mit einem elektronischen Schaltkreis (504), in dem eine weitere Repräsentation der einzigartigen Daten (508) gespeichert sind, und/oder wobei der elektronische Schaltkreis weitere Daten speichert, wobei auf die in dem Schaltkreis gespeicherten Daten nur nach einer erfolgreichen Authentifizierung und/oder Autorisierung zugegriffen werden kann.
12. 12. Dokument (500, 700) hergestellt nach dem Verfahren gemäß einem der Ansprüche 1-11.
13. 13. Dokument (500, 700) mit einem Dokumentenkörper (506),
    • wobei sich auf dem oder in dem Dokumentenkörper ein Tintenstrahlaufdruck (502, 600) befindet,
    • wobei der Tintenstrahlaufdruck erste Tintenstrahltröpfchen (720, 722) beinhaltet, die eine erste Morphologie haben und aus einer farbstoffbasierten Tinte (216) bestehen;
    • wobei der Tintenstrahlaufdruck zweite Tintenstrahltröpfchen (716, 718) beinhaltet, die eine zweite Morphologie haben die sich von der ersten Morphologie unterscheidet und aus einer pigmentbasierten Tinte (214) bestehen;
    • wobei das Absorptionsspektrum der farbstoffbasierten Tinte und das Absorptionsspektrum der pigmentbasierte Tinte einander so ähnlich sind, dass die ersten und zweiten Tintenstrahltröpfchen bei einer Beleuchtung mit Licht im sichtbaren Spektralbereich durch das menschliche Auge farblich ununterscheidbar sind;
    • wobei der Tintenstrahlaufdruck ein oder mehrere erste Bereiche (414.1, 414.2) beinhaltet, die die ersten Tintenstrahltröpfchen beinhalten und frei sind von den zweiten Tintenstrahltröpfchen;
    • wobei der Tintenstrahlaufdruck ein oder mehrere zweite Bereiche (412.1, 412.2) beinhaltet, die die zweiten Tintenstrahltröpfchen beinhalten und frei sind von den ersten Tintenstrahltröpfchen;
    • wobei die ersten und zweiten Bereiche ein Muster (404) bilden, welches bei Betrachtung durch das menschliche Auge bei einer Beleuchtung mit Licht im sichtbaren Spektralbereich nicht erkennbar ist.
14. 14. Dokument (500, 700) nach Anspruch 13,
    • wobei die Dicke des Dokuments dessen räumliche Erstreckung in einer Dimension z spezifiziert ist;
    • wobei der Dokumentenkörper aus mehreren Materialschichten (702-714) besteht, wobei zumindest einige der Tintenstrahltröpfchen sich über mehrere Materialschichten erstrecken;
    • wobei eine Topologie eines Tintenstrahltröpfchens dessen räumliche Erstreckung über ein oder mehrere der Materialebenen in der Dimension z beschreibt; und
    • wobei die Topologie der ersten Tintenstrahltröpfchen sich von der Topologie der zweiten Tintenstrahltröpfchen unterscheidet.
15. 15. Verfahren zur Prüfung eines Dokuments, umfassend:
    • Empfang (302) eines Dokuments (500, 700) mit einem Dokumentenkörper (506) durch eine Prüfeinheit (800), die eine Bilderfassungseinheit (802) beinhaltet, wobei in oder auf dem Dokumentenkörper ein Tintenstrahldruck eines Bildes (502, 600) enthalten ist, wobei ein oder mehrere erste Regionen (414.1, 414.2) des Bildes mit einer farbstoffbasierten Tinte (216) gedruckt sind, wobei ein oder mehrere zweite Regionen (412.1, 412.2) des Bildes mit einer pigmentbasierten Tinte (214) gedruckt sind, wobei die ein oder mehrere ersten Regionen frei sind von der pigmentbasierten Tinte und wobei die ein oder mehreren zweiten Regionen frei sind von der farbstoffbasierten Tinte, wobei das Absorptionsspektrum der farbstoffbasierten Tinte und der pigmentbasierten Tinte so ähnlich ist, dass diese bei einer Beleuchtung des Tintenstrahldrucks mit Licht im sichtbaren Spektralbereich farblich ununterscheidbar sind;
    • Aufnahme (304) eines Prüfbildes durch die Bilderfassungseinheit, wobei das Prüfbild ein digitales Bild ist, das zumindest den Tintenstrahldruck (502) des Dokuments abbildet;
    • Durchführung (306) einer Bildanalyse des Prüfbilds durch die Prüfeinheit, wobei die Bildanalyse umfasst:
      • Automatische Erfassung (308) der Morphologie der im Prüfbild abgebildeten Tintenstrahltröpfchen (716, 718, 720, 722), aus welchen der Tintenstrahldruck besteht, wobei die Morphologie eines Tintenstrahltröpfchens dessen räumliche Erstreckung innerhalb des Prüfbildes beschreibt; und
      • Automatische Rekonstruktion (310) eines digitalen graphischen Musters (404) aus dem Prüfbild, wobei das Muster ein oder mehrere erste Bereiche (408.1, 408.2, 414.1, 414.2) und ein oder mehrere zweite Bereiche (406.1, 406.2, 412.1, 412.2, beinhaltet, wobei die ersten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine erste Morphologie aufweisen, wobei die zweiten Bereiche diejenigen Prüfbildregionen sind, deren Tintenstrahltröpfchen eine zweite Morphologie aufweisen, wobei die zweite von der ersten Morphologie abweicht;
    • Vergleichen (312) des rekonstruierten graphischen Musters oder eines daraus abgeleiteten Wertes mit einem Referenzwert zur Prüfung der Echtheit des Dokuments.
16. 16. Verfahren nach Anspruch 15, wobei das digitale graphische Musters ein Strich- oder Matrixcode ist, ferner umfassend:
    • Decodierung des Strich- oder Matrixcodes durch die Prüfeinheit, um einen decodierten Wert (508) zu erhalten; und
    • Verwendung des decodierten Wert als den aus dem graphischen Muster abgeleiteten Wertes bei dem Vergleich mit dem Referenzwert.
17. 17. Verfahren nach einem der vorigen Ansprüche 15-16, wobei die Prüfeinheit zusätzlich eine Infrarotlichtquelle (904) zur Beleuchtung zumindest eines von der Bilderfassungseinheit aufzunehmenden Bereichs des Dokuments beinhaltet, ferner umfassend:
    • Aktivieren der Infrarotlichtquelle;
    • Aufnahme eines IR-Prüfbildes durch eine IR-Bilderfassungseinheit, wobei das IR-Prüfbild ein digitales Bild ist, das zumindest den Tintenstrahldruck des Dokuments abbildet;
    • Durchführung einer Bildanalyse des IR-Prüfbilds durch die Prüfeinheit, wobei die Bildanalyse umfasst:
      • Automatische Erfassung dunkler Bereiche des IR-Prüfbildes, wobei ein dunkler Bereich eine durchschnittliche Intensität unterhalb eines vordefinierten Grenzwertes besitzt; und
      • Automatische Rekonstruktion eines digitalen graphischen Musters aus den dunklen Bereichen, wobei das Muster als "IR-Muster" bezeichnet wird;
    • Vergleichen des IR-Musters oder eines daraus abgeleiteten Wertes mit einem IR-Muster-Referenzwert zur Prüfung der Echtheit des Dokuments.
18. 18. Prüfeinheit (800) mit:
    • einer Öffnung (808) für den Empfang eines Dokuments (500, 700) mit einem Dokumentenkörper (506), der einen gedruckten Bereich beinhaltet;
    • einer Bilderfassungseinheit (802) mit hinreichender Auflösung, um mindestens einen Teil der Tintenstrahltröpfchen (716, 718, 27, 722) in dem bedruckten Bereich räumlich auflösen zu können;
    • zumindest einem Prozessor (805);
    • einem nicht-flüchtigen Speicher mit computerinterpretierbaren Instruktionen (806), die, wenn sie von dem zumindest einen Prozessor ausgeführt werden, bewirken, dass Prüfverfahren gemäß einem der Ansprüche 15-17 durchführt.
19. 19. Ein System (900) umfassend:
    • die Prüfeinheit (800) von Anspruch 18; und
    • eine IR-Lichtquelle (904); und/oder
    • eine im IR-Spektrum empfindliche Bilderfassungseinheit; und/oder
    • eine Lichtquelle (906) für Licht im sichtbaren Spektralbereich; und/oder
    • einen automatischen Dokumenteneinzug; und/oder
    • eine Kontrollvorrichtung (902) zur Kontrolle des Zugriffs auf oder Zutritts zu: geschützte Daten, Softwarefunktionen, Hardwarefunktionen oder räumliche Bereiche, wobei die Kontrollvorrichtung dazu konfiguriert ist, den Zugriff oder Zutritt nur dann zu gewähren, falls das Prüfverfahren ergibt, dass das Dokument valide ist.

Embodiments of the invention include, for example, the following features:

1. 1. A method for producing a document (500,700) with a document body (506) and a visually imperceptible security feature connected to the document body, comprising:
   • Provision (102) of an inkjet printer (200), which includes at least a first tank (228) with a dye-based ink (216) and a second tank (226) with a pigment-based ink (214), wherein the absorption spectrum of the dye-based ink and the absorption spectrum of the pigment-based ink are so similar to each other that inkjet printing with the dye-based ink and inkjet printing with the pigment-based ink when illuminated with light in the visible spectral range by the human eye are indistinguishable;
   • providing (104) a digital graphic pattern (404) formed from one or more first areas (408.1, 408.2) and one or more second areas (406.1, 406.2); and
   • providing a digital image (402) to be associated with the document;
   • superimposing (106) the graphic pattern and the digital image;
   • Printing (108) the digital image on or in the document body with the inkjet printer such that the dye-based ink but not the pigment-based ink is used to print the regions (414.1, 414.2) 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 (412.1, 412.2) 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 is the security feature.
2. 2. The method according to claim 1, wherein the pattern comprises a bar or matrix code.
3. 3. The method according to any one of the preceding claims, wherein the digital image is a monochrome digital image or wherein the digital image is a polychromatic image, in particular an RGB image or CMYK image.
4. 4. The method according to any one of the preceding claims, wherein the document body material on which the image is printed has a background color, the background color being light and in particular white, wherein when illuminated with light in the infrared spectral range, the dye-based ink causes a transparent color impression for the human eye, so that the one or more first areas have the background color, and the pigment-based ink causes an opaque color impression for the human eye, so that the one or more second areas have a different color than the background color.
5. 5. The method according to any one of the preceding claims, wherein the inks in the first and second tank when illuminated with light in the visible spectral range both:
   • have a black tint
     • 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: carbon black, pigment black 28;
   • or both have a cyan hue,
     • preferably wherein the dye-based ink comprises one or more of the following: Solvent Blue 78, Sudan Blue; and or
     • wherein the pigment-based ink preferably comprises one or more of the following substances: Cu-phthalocyanine;
   • or both have a magenta hue,
     • wherein the dye-based ink preferably comprises one or more of the following substances: red azo dyes, eg Solvent Red 26, Sudan Red; and or
     • wherein the pigment-based ink is quinacridone;
   • or both have a yellow tint,
     • wherein the dye-based ink preferably comprises one or more of the following substances: yellow azo dyes, eg Solvent Yellow 124; and or
     • wherein the pigment-based ink preferably comprises one or more of the following: Brilliant Yellow, Pigment Yellow 151.
6. 6. The method according to any one of the preceding claims, wherein the inkjet printer includes at least a third tank with a further dye-based ink and a fourth tank with a further pigment-based ink, the absorption spectrum of the further dye-based ink and the absorption spectrum of the further pigment-based ink being so similar to one another that this further dye-based ink is indistinguishable in terms of color from the further pigment-based ink by the human eye when illuminated with light in the visible spectral range, but is different in color from the inks in the first and second tank is divisible, further comprising:
   • providing a further digital graphic pattern including one or more third areas and one or more fourth areas; and
   • superimposition of the further graphic pattern and the digital image;
   • performing the printing of the digital image on or in the document body such that the dye-based ink but not the pigment-based ink is used for printing the regions of the digital image overlaid with the first areas that the pigment-based ink but not the dye-based ink are used to print the regions of the digital image overlaid with the second areas, that 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 areas, that the further pigment-based ink but not the further dye-based ink are used to print the regions of the digital image overlaid with the fourth areas, the distribution of the dye-based ink, the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image is the security feature.
7. 7. The method according to any one of the preceding claims, comprising:
   • determining an area within the image that has a minimum homogeneity and minimum intensity with respect to the absorption spectrum of the hue to be printed with the dye-based ink or the pigment-based ink;
   • Carrying out the overlay in such a way that the graphic pattern is completely mapped onto this surface.
8. 8. The method according to any one of the preceding claims, wherein the document body is made up of a plurality of material layers (702-714), the digital image being printed in the document body in such a way that the digital image is printed on one or more of the material layers, and then the one or more printed layers, optionally together with other material layers, are inseparably connected to form the document body.
9. 9. The method of claim 8, wherein the dye-based ink penetrates further into the material of one or more of the layers than the pigment-based ink during the bonding of the plurality of material layers and/or to produce a different droplet topology.
10. A method according to any one or more of the preceding claims, wherein the graphic pattern includes data (508) in encoded form, the data being unique to the document and/or its owner.
11. 11. The method of claim 10, further comprising:
    • Providing the document body with an electronic circuit (504) in which a further representation of the unique data (508) is stored, and/or wherein the electronic circuit stores further data, the data stored in the circuit being only accessible after successful authentication and/or authorization.
12. 12. Document (500, 700) produced by the method according to any one of claims 1-11.
13. 13. Document (500, 700) with a document body (506),
    • inkjet printing (502, 600) on or in the body of the document,
    • wherein the inkjet print includes first inkjet droplets (720, 722) having a first morphology and composed of a dye-based ink (216);
    • wherein the inkjet imprint includes second inkjet droplets (716,718) having a second morphology different from the first morphology and made 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 sufficiently similar that the first and second ink jet droplets are color indistinguishable by the human eye when illuminated with light in the visible spectral range;
    • wherein the inkjet print includes one or more first areas (414.1, 414.2) containing the first inkjet droplets and devoid of the second inkjet droplets;
    • wherein the inkjet print includes one or more second areas (412.1, 412.2) containing the second inkjet droplets and devoid of the first inkjet droplets;
    • wherein the first and second areas form a pattern (404) which is not recognizable when viewed by the human eye when illuminated with light in the visible spectral range.
14. 14. Document (500, 700) according to claim 13,
    • wherein the thickness of the document is specified as its spatial extent in one dimension z;
    • wherein the document body consists of multiple layers of material (702-714), at least some of the ink jet droplets extending across multiple layers of material;
    • wherein a topology of an inkjet droplet describes its spatial extension over one or more of the material planes in dimension z; and
    • wherein the topology of the first ink jet droplets differs from the topology of the second ink jet droplets.
15. 15. A method of examining a document, comprising:
    • Reception (302) of a document (500, 700) with a document body (506) by a verification unit (800) that includes an image capture unit (802), wherein an inkjet print of an image (502, 600) is contained in or on the document body, wherein one or more first regions (414.1, 414.2) of the image are printed with a dye-based ink (216), wherein one or more second regions (4 12.1, 412.2) of the image are printed with a pigment-based ink (214), wherein the one or more first regions are free of the pigment-based ink and wherein the one or more second regions are free of the dye-based ink, wherein the absorption spectrum of the dye-based ink and the pigment-based ink is so similar that when illuminated of inkjet printing with light in the visible spectral range are color indistinguishable;
    • capturing (304), by the image capture unit, a proof image, the proof image being a digital image representing at least the inkjet print (502) of the document;
    • Performing (306) an image analysis of the test image by the test unit, the image analysis comprising:
      • Automatic detection (308) of the morphology of the ink jet droplets (716, 718, 720, 722) imaged in the test image and constituting the ink jet print, the morphology of an ink jet droplet describing its spatial extent within the test image; and
      • Automatic reconstruction (310) of a digital graphic pattern (404) from the proof image, the pattern including 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, the first areas being those proof image regions whose inkjet droplets have a first morphology, wherein the second areas are those test image regions whose ink jet droplets have a second morphology, the second differing from the first morphology;
    • Comparing (312) the reconstructed graphic pattern or a value derived therefrom with a reference value for checking the authenticity of the document.
16. 16. The method of claim 15, wherein the digital graphic pattern is a bar or matrix code, further comprising:
    • the verification unit decoding the bar or matrix code to obtain a decoded value (508); and
    • using the decoded value as the value derived from the graphic template in the comparison with the reference value.
17. 17. The method according to any one of the preceding claims 15-16, wherein the checking unit additionally includes an infrared light source (904) for illuminating at least one area of the document to be recorded by the image capturing unit, further comprising:
    • activating the infrared light source;
    • capturing an IR test image by an IR image acquisition unit, the IR test image being a digital image that depicts at least the inkjet print of the document;
    • Performing an image analysis of the IR test image by the test unit, the image analysis comprising:
      • Automatic detection of dark areas of the IR test image, where a dark area has an average intensity below a predefined threshold; and
      • Automatic reconstruction of a digital graphic pattern from the dark areas, the pattern being referred to as "IR pattern";
    • Comparing the IR pattern or a value derived from it with an IR pattern reference value to check the authenticity of the document.
18. 18. Test unit (800) with:
    • an opening (808) for receiving a document (500, 700) having a document body (506) including a printed area;
    • an image capture unit (802) having sufficient resolution to spatially resolve at least a portion of the ink jet droplets (716, 718, 27, 722) in the printed area;
    • at least one processor (805);
    • a non-volatile memory containing computer-interpretable instructions (806) which, when executed by the at least one processor, cause the testing method according to any one of claims 15-17 to be performed.
19. 19. A system (900) comprising:
    • the checking unit (800) of claim 18; and
    • an IR light source (904); and or
    • an IR spectrum sensitive imaging unit; and or
    • a light source (906) for light in the visible spectral range; and or
    • an automatic document feeder; and or
    • a control device (902) for controlling access to or access to: protected data, software functions, hardware functions or spatial areas, the control device being configured to grant access or access only if the verification procedure shows that the document is valid.

Reference List

102-110

Steps Printing Process

200

inkjet printer

202

Inkjet printer control unit

204

printhead assembly

206

Document feeder printer

208

Ink with the ink color cyan

210

Ink with the printing color red

212

Yellow ink

214

Black ink: pigment-based ink

216

Black ink: dye-based ink

218

paint tank container

220-228

paint tanks

302-312

Steps test procedure

402

digital RGB image

404

Pattern

406.1, 406.2

second areas of the pattern

408.1, 408.2

first areas of 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

Detail from picture 502: right eye, dye-based. ink, x100

604

Detail from picture 502: left eye, pigment-based. ink, x100

606

Detail from picture 502: right eye, dye-based. ink, 200x

608

Detail from picture 502: left eye, pigment-based. ink, 200x

700

Document made up of several layers of material

702-714

individual layers of material

716

Inkjet droplets of pigment-based ink

718

Inkjet droplets of pigment-based ink

720

Inkjet droplets of dye-based ink

722

Inkjet droplets of dye-based ink

800

testing unit

802

image capture unit

804

image analysis unit

805

processor

806

image analysis software

808

Opening/Document Feeder

810

Display/Screen

812

displayed test result

900

system

902

barrier/gate

904

Infrared light source

906

white light source

908

infrared sensitive camera

910

high resolution visible camera/microscope

912

test image

914

Pattern

Claims (16)

Method for producing a document (500,700) with a document body (506) and a visually imperceptible security feature connected to the document body, comprising: - Provision (102) of an inkjet printer (200), which at least includes a first tank (228) with a dye -based ink (216) and a second tank (226) with a pigment -based ink (214), whereby the absorption spectrum of the dye -based ink and the absorption spectrum of the pigment -based ink are so similar that an ink beam are so similar Pressure with the dye -based ink and an inkjet pressure with the pigment -based ink with lighting with light in the visible spectral area through the human eye are indistinguishable, with the inkjet printer at least a third tank with another dye -based ink and a fourth tank with another pigment -based ink, whereby the absorption spectrum of the other dyes Ailed ink and the absorption spectrum of the other pigment -based ink are so similar that this further color -based ink with light in the visible spectral area is indispensable by the other pigment -based ink through the human eye, but can be distinguished from the inks in the first and second tank; - providing (104) a digital graphic pattern (404) formed from one or more first areas (408.1, 408.2) and one or more second areas (406.1, 406.2); and - providing a further digital graphic pattern including one or more third areas and one or more fourth areas; - providing a digital image (402) to be associated with the document; - overlaying (106) the graphic pattern and the digital image; - superposition of the further graphic pattern and the digital image; - Printing (108) the digital image on or in the document body with the inkjet printer such that the dye-based ink but not the pigment-based ink is used to print the regions (414.1, 414.2) 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 (412.1, 412.2) of the digital image overlaid with the second areas, the printing being carried out in this way that 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 areas, 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, the distribution of the dye-based ink, the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image being the security feature. A method according to claim 1, wherein the pattern comprises a bar or matrix code. Method according to one of the preceding claims, in which the digital image is a monochrome digital image or in which the digital image is a polychromatic image, in particular an RGB image or CMYK image. Method according to one of the preceding claims, wherein the document body material on which the image is printed has a background colour, the background color being light and in particular white, with illumination by light in the infrared spectral range the dye-based ink giving the human eye a transparent color impression, so that the one or more first areas have the background colour, and the pigment-based ink giving the human eye an opaque color impression, so that the one or more second areas have a different color than the background colour. A method according to any one of the preceding claims, wherein the inks in the first and second tanks when illuminated with light in the visible spectral range both: - have a black tint, • wherein the dye-based ink preferably comprises one or more of the following substances: Solvent Black 27, Solvent Black 29; and or • wherein the pigment-based ink preferably comprises one or more of the following substances: carbon black, pigment black 28; - or both have a cyan hue, • wherein the dye-based ink preferably comprises one or more of the following: Solvent Blue 78, Sudan Blue; and or • wherein the pigment-based ink preferably comprises one or more of the following substances: Cu-phthalocyanine; - or both have a magenta hue, • wherein the dye-based ink preferably comprises one or more of the following substances: red azo dyes, eg Solvent Red 26, Sudan Red; and or • wherein the pigment-based ink is quinacridone; - or both have a yellow tint, • wherein the dye-based ink preferably comprises one or more of the following substances: yellow azo dyes, eg Solvent Yellow 124; and or • wherein the pigment-based ink preferably comprises one or more of the following: Brilliant Yellow, Pigment Yellow 151. Method according to one of the preceding claims, comprising: - Identify an area within the image that has a minimum homogeneity and minimum intensity with respect to the absorption spectrum of the hue to be printed with the dye-based ink or the pigment-based ink; - Carrying out the overlay in such a way that the graphic pattern is completely mapped onto this surface. Method according to one of the preceding claims, wherein the document body is constructed from a plurality of material layers (702-714), the digital image being printed in the document body in such a way that the digital image is printed on one or more of the material layers, and then the one or more printed layers, optionally together with further material layers, are inseparably connected to form the document body. 8. The method of claim 7, wherein the dye-based ink penetrates the material of one or more of the layers further and/or to form a different droplet topology than the pigment-based ink upon bonding the plurality of layers of material. A method according to one or more of the preceding claims, wherein the graphic pattern includes data (508) in encoded form, the data being unique to the document and/or its owner. The method of claim 9, further comprising: - providing the body of the document with an electronic circuit (504) in which a further representation of the unique data (508) is stored, and/or wherein the electronic circuit stores further data, the data stored in the circuit being only accessible after successful authentication and/or authorization. Document (500, 700) with a document body (506), - wherein there is an inkjet imprint (502, 600) on or in the body of the document, - wherein the inkjet print includes first inkjet droplets (720, 722) having a first morphology and consisting of a dye-based ink (216); - wherein the ink jet imprint includes second ink jet droplets (716, 718) having a second morphology different from the first morphology and consisting 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 that the first and second ink jet droplets are color indistinguishable by the human eye when illuminated with light in the visible spectral range; - wherein the inkjet print includes one or more first areas (414.1, 414.2) containing the first inkjet droplets and being free of the second inkjet droplets; - wherein the inkjet print includes one or more second areas (412.1, 412.2) containing the second inkjet droplets and being free of the first inkjet droplets; - wherein the first and second areas form a pattern (404) which is not recognizable when viewed by the human eye when illuminated with light in the visible spectral range; - wherein the pattern is overlaid with a digital image (402); - where the digital image is overlaid with another graphic pattern, - wherein the further graphic pattern includes one or more third areas and one or more fourth areas, wherein a further dye-based ink but not a further pigment-based ink was used to print the regions of the digital image superimposed with the third areas, wherein the further pigment-based ink but not the further dye-based ink was used for printing the regions of the digital image superimposed with the fourth areas, the distribution of the dye-based ink, the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image being the security feature is. A method for examining a document, comprising: - Reception (302) of a document (500, 700) having a document body (506) by a verification unit (800) that includes an image capture unit (802), wherein in or on the document body an inkjet print of an image (502, 600) is contained, wherein one or more first regions (414.1, 414.2) of the image are printed with a dye-based ink (216), wherein one or more second regions ( 412.1, 412.2) of the image are printed with a pigment-based ink (214), the one or more first regions being free of the pigment-based ink and the one or more second regions being free of the dye-based ink, the absorption spectrum of the dye-based ink and the pigment-based ink being so similar that they are indistinguishable in terms of color when the inkjet print is illuminated with light in the visible spectral range, the digital image being overlaid with a further graphic pattern is, wherein the further graphic pattern includes one or more third areas and one or more fourth areas, wherein a further dye-based ink but not a further pigment-based ink was used to print the regions of the digital image superimposed with the third areas, wherein the further pigment-based ink but not the further dye-based ink was used for printing the regions of the digital image superimposed with the fourth areas, the distribution of the dye-based ink, the pigment-based ink, the further dye-based ink and the further pigment-based ink in the printed image being that security feature is; - capturing (304) a test image by the image acquisition unit, the test image being a digital image that depicts at least the inkjet print (502) of the document; - performing (306) an image analysis of the test image by the test unit, the image analysis comprising: • Automatic detection (308) of the morphology of the ink jet droplets (716, 718, 720, 722) imaged in the test image, from which the ink jet print consists, the morphology of an ink jet droplet describing its spatial extent within the test image; and • Automatic reconstruction (310) of a digital graphic pattern (404) and a further graphic pattern from the test image, the pattern including 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, the first areas being those test image regions whose ink jet tr droplets have a first morphology, the second areas being those proof image regions whose inkjet droplets have a second morphology, the second differing from the first morphology, the further pattern including one or more third areas and one or more fourth areas, the third areas being those proof image regions that were printed with the further dye-based ink, the fourth areas being those proof image regions that were printed with the further pigment-based ink; - Comparing (312) the reconstructed graphic pattern and the reconstructed further graphic pattern or values derived from these with reference values for checking the authenticity of the document. The method of claim 12, wherein the digital graphic pattern is a bar or matrix code, further comprising: - decoding the bar or matrix code by the verification unit to obtain a decoded value (508); and - Using the decoded value as the value derived from the graphic template in the comparison with the reference value. Method according to any one of the preceding claims 12-13, wherein the inspection unit additionally includes an infrared light source (904) for illuminating at least one of the image acquisition unit includes the area of the document to be included, further comprising: - Activate the infrared light source; - Recording an IR test image by an IR image acquisition unit, wherein the IR test image is a digital image that depicts at least the inkjet print of the document; - Performing an image analysis of the IR test image by the test unit, the image analysis including: • Automatic detection of dark areas of the IR test image, where a dark area has an average intensity below a predefined threshold; and • Automatic reconstruction of a digital graphic pattern from the dark areas, the pattern being called "IR pattern"; - Compare the IR pattern or a value derived from it with an IR pattern reference value to check the authenticity of the document. Test unit (800) with: - an opening (808) for receiving a document (500, 700) having a document body (506) including a printed area; - an image capture unit (802) having sufficient resolution to spatially resolve at least a portion of the ink jet droplets (716, 718, 27, 722) in the printed area; - at least one processor (805); - a non-volatile memory containing computer-interpretable instructions (806) which, when executed by the at least one processor, cause the testing method according to any one of claims 12-14 to be performed. A system (900) comprising: - the test unit (800) of claim 15; and - an IR light source (904); and or - an image acquisition unit sensitive in the IR spectrum; and or - A light source (906) for light in the visible spectral range; and or - an automatic document feeder; and or - a control device (902) for controlling access to or access to: protected data, software functions, hardware functions or spatial areas, the control device being configured to grant access or access only if the verification procedure shows that the document is valid.

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