CA2862057A1

CA2862057A1 - Method with register between print element and watermark

Info

Publication number: CA2862057A1
Application number: CA2862057A
Authority: CA
Inventors: Peter SCHIFFMAN; Harald Reiner; Angelika KECK-ANGERER; Jana Kisselova
Original assignee: Giesecke and Devrient GmbH
Current assignee: Giesecke and Devrient Currency Technology GmbH
Priority date: 2012-02-21
Filing date: 2013-02-20
Publication date: 2013-08-29
Also published as: ES2585279T3; WO2013124059A1; EP2817155A1; EP2817155B1; DE102012003601A1; PL2817155T3

Abstract

The invention relates to a method for producing a security element (1), wherein a substrate (2) is provided which has a front and a rear side (3, 4), is translucent at least for specific radiation (10) and has a water mark (5) which modulates the opacity of the substrate (2), a print element (9) is applied to the front side (3), wherein the print element (9) covers the water mark (5) at least partially on the front side (3), the print element (9) can be modified by means of the specific radiation (10), and the substrate (2) is penetrated by the specific radiation (10) from the rear side (4), wherein the water mark (5) is used as a mask with regard to the modification of the print element (9), and the print element (9) is modified in register with the water mark (5).

Description

Method with register between print element and watermark [0001] The invention relates to a method for manufacturing a security element, wherein a substrate is made available which has a front side and a back side, is translucent at least to certain radiation and has a watermark modulating the opacity of the substrate, and a print element is applied to the front side.

[0002] Watermarks are known to improve the falsification security of protected objects, for example of banknotes. They are usually incorporated during the manufacture of the substrate from which the security element is produced, usually a paper.
Watermarks are easily recognizable upon transmissive viewing of the security element, since they modify the opacity of the substrate. As a rule, during the manufacture of the paper, the thickness of the substrate is so varied that a reduction in thickness leads to a reduction in opacity.
However, also other approaches to producing a watermark are known, for example with the aid of so-called watermark inks, which also modify the opacity of the substrate locally.

[0003] Since watermarks are as a rule formed in the substrate during the manufacture of the start material for the security element, for example during the production of a banknote paper, it is fundamentally difficult in the later printing process to arrange print elements in an exact position to the watermark, thus to place them in register. This problem occurs independently of the concrete configuration of the print element, thus not only upon printing with different printing inks, but also upon coating with or applying print elements, upon applying transfer foils or lamination foils or upon applying a windowed thread.

[0004] Due to variations in the cutting process of a paper substrate, paper sheets usually have variations of the position of the watermark lying in the millimeter range. In a printing process further variations in register are added, due to variations of abutting positions, transfer variations in printing apparatus, variations of the paper's humidity and a rolling-out effect of the paper in the engraving procedure.

[0005] In order to apply a print element in register with the watermark, it would be conceivable to capture the position of the watermark in the substrate by means of a transmission sensor, and to adjust the subsequent printing procedure to the current position of the watermark, e.g. by a mechanical adjustment of a printing cylinder or correction of the printing data of a digital printing system. This results in an effort caused by the capture of the watermark's position, an image analysis to be carried out in real time to determine the position of the watermark, and a correction of the printing position to be carried out in real time. The effort increases in line with the printing speed.

[0006] It is consequently the object of the invention to specify a method for structuring a print element in register with a watermark, without having to precisely detect the position of the watermark and adjust the printing system to the position of the watermark beforehand.

[0007] This object is achieved according to the invention by a method for manufacturing a security element, wherein a substrate is made available which has a front side and a back side, is translucent at least to certain radiation and has a watermark modulating the opacity of the substrate, and to the front side a print element is applied, wherein on the front side the print element covers the watermark at least partially, the print element can be modified by means of the certain radiation, and the substrate is penetrated by the certain radiation from the back side, wherein the watermark is employed as a mask with reference to the modification of the print element, and the print element is thus modified in register with the watermark.

[0008] The invention is based on the finding that the energy of radiation is weakened by the substrate. The more opaque the substrate is, the stronger is the weakening. Therefore advantageous use is made of the property of the watermark of modulating the opacity of the substrate. The watermark is used as a mask for a modification of the print element by employing a print element that is modifiable by certain radiation to which the substrate is translucent, and that certain radiation is radiated from the back side of the substrate to the front side, whereto the print element is applied so as to cover the watermark.
Masked by the watermark, thereby a modification of the print element takes place, which is automatically disposed in perfect register with the watermark. As modification of the print element here in particular an ablation comes into question, when a print element is employed that can be ablated. However, fundamentally any suitable modification of a print element can be used.

[0009] This modification can take place directly, thus by direct action of the certain radiation on the print element, or indirectly. In the latter case, the certain radiation modifies an element that is in turn used to modify the print element perceivable on the front side. For example by means of the certain radiation a layer can be modified that influences the adhesion of the print element to the front side. After the modification of this layer influencing the adhesion, in a further step then the print element is removed in the regions where the layer was modified (or not modified in the inverse case). The procedure is similar to the principle to the so-called washing ink. A further possibility of indirect modification is the modification of a developer layer which, in interaction with a developing agent or fixing agent, modifies the print element in those places where the developer layer was suitably exposed. An indirect influencing of the print element is possible also in a two-step procedure, when the print element has an adhesive layer that is applied to the front side and exposed from the back side with the cooperation of the watermark as mask. In a second step then an ink layer of the print element can be applied that adheres only to those places where the exposure has taken place.
Possibly, an action on the exposed adhesive layer can also take place in an intermediate step here, so that said layer remains only in those regions where a suitable exposure has taken place (positive effect) or in those regions where no suitable exposure has taken place (negative effect).

[0010] Within the framework of the present description, the term "print element" is understood to mean an element applied to a substrate, realizing possibly after further processing an element that can be evaluated visually or by machine. Of course, print elements on the basis of printing inks are the most widespread. When mention is made of a printing ink in the present description, this is to be understood exemplarily for a print element.

[0011] Thus, when mention is made in the present description and in particular in the claims of a "print element" or a "printing ink", and the application of said print element to the front side above the watermark is mentioned, this also encompasses that in a print element layer system only one part of the layer system is applied and exposed with the certain irradiation; the other part is then applied subsequently to these steps and interacts suitably with the exposed part of the layer system in order to produce a printed image in register with the watermark.

[0012] A multilayer system is possible e.g. in such a sense that the print element interacts with an additional layer whose visual effect depends on the prior modification of the print element. Thus, it is for example known for optically variable layers that they are particularly easily recognizable in incident light on a dark or black background. An embodiment of the invention therefore provides that the print element is equipped with a contrast structure by the certain radiation. Onto the print element thus modified in register with the watermark, subsequently an optically variable layer is applied areally, i.e. without register requirements, whose visual effect is then automatically disposed in register with the watermark due to the precise placing in register of the print element acting as background, although the optically variable layer itself was applied without register requirements. An example for such an optically variable layer is a liquid crystal layer. With regard to this, reference is made to EP 0 435 029 or WO 97/19818, which disclose examples of optically variable layers employing liquid crystals. The disclosure of these publication prints is included in its entirety here with regard to the optically variable layers described there.

[0013] The term "print element" is thus also understood as a multilayer system, wherein a part of the layer system is applied to the front area prior to exposure and then illuminated with the certain irradiation. A further part of the layer system is then applied only after the exposure.

[0014] Particularly for use in banknotes such watermarks are usual which modulate the opacity of the substrate by varying a thickness of the substrate, while the substrate's density remains constant, since such watermarks are as a rule hard to imitate.
These watermarks are well suitable for the printing method according to the invention, which then complements the appearance of the watermark by further image information disposed in register with the watermark. It is thereby made possible for example to complement in color the image information made available by the watermark. The image information of the watermark, which per se appears only in transmitted light, can become recognizable also upon viewing in incident light by means of the printed image produced in register according to the invention.

[0015] A watermark modulating the opacity of a substrate can also be produced by a so-called watermark ink instead of by varying the thickness of the substrate. The watermark ink herein effects a reduction of the scattering of light in the substrate core.
Fundamentally, scattering is based on a large number of interfaces of fibers, fillers, adhesive, etc. with the ambient air. These interfaces with the ambient air are clearly reduced by suitable binding agent components in the watermark ink, so that the opacity is reduced in these regions. Such a binding agent component is glycerin for example, which penetrates into the substrate and remains in the pores of the substrate.

[0016] Watermarks produced by watermark ink are also referred to as "false"
watermarks. Watermarks produced by varying the thickness of the substrate while the density of the substrate remains constant, are also referred to as "true"
watermarks.

[0017] This is applicable analogously when the watermark is effected by a dyeing that modulates the opacity of the substrate.

[0018] A watermark can modulate the opacity of a substrate binarily, i.e. it produces a structure that is recognizable in transmission by the substrate being less opaque or more translucent in individual places. However, also watermarks are known in the sense of a grayscale modulation that modulates the opacity or translucence between two maximum values. Upon transmissive viewing, a grayscale image is then recognizable.
Either type of watermark is equally suitable for the present printing method. In a watermark that is configured in the fashion of a grayscale image it is of course particularly advantageous to employ a printing ink whose modifiability goes beyond a simple threshold value modification, whose modification thus has more than two states. The multistage modulation by the watermark is then converted into a corresponding multistage modification of the print element.

[0019] A particularly finely graded modulation of the opacity can be achieved by a combination of a paper-thickness modulating "true" watermark with a "false"
watermark produced by watermark ink.

[0020] Print elements usually consist of binding agents, accessory agents, drying agents or drying accelerators and chromophore components, such as dyes or pigments.
When a radiation is irradiated that is of a wavelength which is absorbed to a certain degree by the chromophore components, it is achieved that the certain radiation ablates the print element for modification. In the case of ablation thus the print element and the certain radiation are chosen to so match each other that the certain radiation is absorbed by the print element in such a fashion that an ablation is possible.

[0021] In order to prevent influences of the certain radiation on other components of the substrate or coatings applied to the substrate as far as possible, it is preferable to irradiate the certain radiation in an as narrow-band-fashion as possible in the range of an absorption band of the print element. This can be realized particularly easily with laser radiation.

[0022] When a print element per se does not have a component making it suitably absorbing for an ablation, it is preferable for the purpose of ablation to add a certain substance to the print element that absorbs the certain radiation, for example carbon black. The employment of an added substance permits in particular to so choose the certain radiation that it is in the invisible spectral range, for example IR
radiation, when the added substance increases the degree of absorption of the print element in the corresponding spectral range.

[0023] Instead of an ablation usually leading to a brightening when dark colors are employed, it is of course also possible to so design the modification that it leads to a color alteration, for example by thermal activation of an ink. This is valid in particular when special-effect inks are employed on the basis of inks which show a viewing-angle dependent color impression (so-called optically variable inks), and inks that can be structured by a magnetic field before drying.

[0024] It is therefore particularly preferred that the printing ink has one or several of the following substances: an optically variable ink, a magnetically orientable ink, a metallic ink, an ink effective in the invisible spectral range.

[0025] As already mentioned, the principle of the invention provides that the print element and the certain radiation are so matched that the certain radiation is capable of modifying the print element, wherein the watermark modulating the opacity of the substrate is employed as a mask. The term "certain radiation" employed in the present description therefore expresses that a print element is used that can be modified by means of this radiation. The modification, for example in the form of an ablation, is achieved when the certain radiation on the front side has a suitable intensity. The modulation of opacity effected by the watermark reduces the intensity of the radiation on the front side where the print element is disposed. The radiation intensity on the back side is consequently adjusted in such a fashion that the opacity modulation effected by the watermark leads to a radiation modulation on the front side, which modulates the modification effect on the print element. In other words, the radiation intensity on the back side is so chosen that in the places where the watermark effects the lowest opacity in the substrate, a modification of the print element occurs, whereas no or only a very small modification is achieved in those places where the watermark effects the highest opacity of the substrate. The watermark then serves as mask when the substrate is penetrated by the certain radiation. Since the modification of the print element as a rule requires a comparatively high radiation intensity, the use of a beam is advantageous that is applied in gridded fashion over the back side of the substrate. The beam can be present for example in the form of a laser beam. It is then not only easy to make available the required radiation intensity, but also the intensity adjustment can take place in a simple fashion, since less effort is required to adjust the intensity of a beam than to effect a homogeneous intensity adjustment of a fanned out light beam.

[0026] The employment of a gridded beam further has the advantage that the intensity of the certain radiation can be varied in addition to the employment of the watermark as a mask, in order to effect an additional structuring of the modification of the print element.
This additional modification is then of course not in register with the watermark.

[0027] Additionally, it is possible to apply an additional masking to the back side that weakens or absorbs the certain radiation. Thereby a further structuring of the modification of the print element is achieved.

[0028] Examinations by the inventors have shown that usually no visually visible marking occurs on the back side, when the printing method according to the invention is used to expose and modify, for example ablate, the print element masked by the watermark on the front side.

[0029] It is also possible to utilize the incorporation of the modified radiation, in particular of laser radiation, to effect a visible modification of the substrate on the entry side of the laser radiation, i.e. before the onset of the masking effect of the watermark, possibly after prior application of a radiation-sensitive layer. Thus for example pulsed laser radiation can be employed that modifies the substrate, for example paper, already on the entry side, e.g. by a blackening or also a foaming of the substrate.
This effect is then easily visible in transmission. Thus, there advantageously results a combination of the watermark, the print element modified in register with the watermark, and a visual modification of the surface of the substrate on the entry side of the modifying radiation.
This modification on the entry side of the radiation of course depends exclusively on the application of the radiation. It is thus possible for example to create a window in which the watermark and the print element modified in register therewith are visible.

[0030] Fundamentally, the print element to be modulated can contain both substances that are ablatable by means of radiation and substances that are developable by means of radiation (for example thermochromic substances preferably having an irreversible color change or opacity change), and additionally also substances that are not ablatable or developable by means of radiation. Depending on the ink formulation, the last-mentioned substances remain on the substrate in an unchanged state or are removed only partly.
Further, the print element can contain a mixture of substances whose individual components are modified in stages in dependence on the impinging energy. For example a first component is ablated at a very low opacity, and a second component is ablated already at medium opacity.

[0031] According to a preferred embodiment, the printing ink contains a laser-markable feature substance with core-shell particles known from EP 1 826 728 A2, wherein one of the materials of core and shell absorbs the radiation of a marking laser, and the other of the materials of core and shell does not absorb the radiation of the marking laser.
According to a further preferred embodiment, the core-shell particle contains a UV-excitable organic luminescent substance known from EP 1 826 730 A2, with the organic luminescent substance forming the core that is coated with a shell increasing the chemical and/or physical resistance of the luminescent substance.

[0032] According to a further preferred embodiment, the printing ink contains a modified iron blue pigment known from EP 1 826 246 A2, coated with an alkali-resistant and/or acid-resistant shell.

[0033] The term printing ink employed here is intended to express the printing effect achieved by the watermark-masked and radiation-based modification. The printing ink itself can of course comprise also such structures which exceed conventional ink systems, for example a foil coating or a metallization.

[0034] Objects to be protected within the framework of the present description can be for example security papers, identity documents and value documents (such as e.g.
banknotes, chip cards, passports, cards, identification cards, identity cards, shares, bonds, deeds, vouchers, checks, admission tickets, credit cards, health cards, ...), as well as product securing elements, such as e.g. labels, seals, packages.

[0035] The term security paper is understood here to be in particular the not yet circulable precursor to a value document (e.g. a card), which can have besides the print manufactured according to the invention for example also further authentication features (such as e.g. luminescent substances provided within the volume). Value documents here are understood to be documents manufactured from security papers, e.g.
banknotes, on the one hand. On the other hand, value documents can also be other documents and objects that are processed using the printing method according to the invention in order for the value documents to have uncopiable authenticity features, thereby making it possible to check authenticity and at the same time preventing unwanted copies.

[0036] The substrate consists particularly preferably of paper of cotton fibers such as is employed for example for banknotes. Preferably the substrate can also consist of paper of other natural fibers, preferably likewise of synthetic fibers, i.e. a mixture of natural and synthetic fibers. Further preferably, the substrate consists of a combination of at least two different substrates arranged above each other and connected to each other, a so-called hybrid. It can also be for example a combination of plastic foil and paper or also a three-layer composite, such as plastic foil - paper - plastic foil, i.e. a paper substrate is covered on both of its two sides by a plastic foil, or paper - plastic foil -paper, i.e. a plastic-foil substrate is covered on both of its two sides by a substrate of paper.

[0037] The substrate preferably consists of a plastic foil that is at least partially transparent. In this case the watermark is produced by an ink applied to one side of the substrate and whose color tone is at least similar to the color tone of the substrate. Such a watermark is known for example from DE 10 2009 056 462 Al.

[0038] Translucence or translucent is understood in this description as the partial light transmission of a body, thus the property of letting light pass in scattered fashion.
Translucence has to be seen i.a. delimited from transparency (= image permeability or visual permeability). The reciprocal property of translucence is opacity. To the extent that it is stated here that a watermark modulates the opacity, it can likewise be stated that it modulates translucence inversely thereto.

[0039] It is evident that the features mentioned above and those to be explained hereinafter are usable not only in the stated combinations, but also in other combinations or in isolation, without going beyond the scope of the present invention.

[0040] In the following the invention is explained in more detail by way of example with reference to the accompanying figures that also disclose essential characteristics of the invention. The figures are described as follows:

[0041] Fig. la and lb A schematic representation of a substrate in a sectional view (Fig.
la) and in plan view (Fig. lb) to illustrate the opacity modulation by the watermark,

[0042] Fig. 2 a schematic representation of the substrate of Fig. 1 for producing a printed image placed in register with the watermark,

[0043] Fig. 3a and 3b a schematic representation similar to Fig. la and lb, concerning an embodiment wherein the watermark is produced by a watermark ink,

[0044] Fig. 4 a representation similar to Fig. 2 for the substrate of Fig.
3a and 3b,

[0045] Fig. 5a a schematic representation similar to Fig. 2 for an exemplary embodiment wherein on the front side of the substrate a non-modifiable coating is provided,

[0046] Fig. 5b and 5c views of the substrate of Fig. 5a from the back side and the front side after a modification, and

[0047] Fig. 6 to 8 plan views (Fig. 6a, 6b, 7, 8) and sectional views (Fig.
6c, d) of an exemplary embodiment having a windowed thread.

[0048] Fig. la shows a security element 1 in a sectional view, with only one substrate 2 drawn for the sake of simplification. The substrate 2 has a front side 3 (which is arranged on the bottom in the figures without any further limitation), as well as a back side 4. Into the substrate 2 a watermark 5 is integrated, modulating the thickness of the substrate 2 and thereby its opacity. This opacity modulation is represented in Fig. lb, where a plan view is schematically shown of the front side 3 of the substrate 2. There are regions 6 of high opacity in places where the substrate 2 is of the maximal (unreduced) thickness. In the regions 7 where the thickness is reduced by a medium degree, the substrate 2 is of medium opacity. In a region 8 where the thickness is strongly reduced, the substrate 2 is of low opacity or high translucence.

[0049] To produce a printed image placed in register with the watermark, a printing ink that is ablatable by means of laser radiation is applied to the front side 3.
Fig. 2 shows the substrate 2 with printing ink 9 applied to the front side 3. The printing ink 9 is not (yet) further structured, but covers at least partially the region where the watermark 5 is present in the substrate 2.

[0050] From the front side 3 now a laser beam 10 is guided over the substrate.
It penetrates the substrate 2. The wavelength of the laser beam is such that the printing ink 9 absorbs the radiation. The laser beam 10 is weakened in dependence on the opacity of the substrate 2 modulated by the watermark 5. In a position 10a, where the substrate is of high opacity due to the watermark, the laser beam 10 is greatly weakened. In a position 10b, where the substrate is of the lowest opacity, the laser beam is the least weakened. In Fig. 2 this is schematically illustrated by the thickness of the laser beam 10. In fact, of course only the intensity of the laser beam 10 is weakened upon passing through the substrate 2, but not its beam cross section.

[0051] The intensity of the laser beam 10 is so adjusted that there results a modulation of the ablation effect exerted by the laser beam 10 on the printing ink 9 applied to the front side 3: in the region 8 of low opacity, the printing ink 9 is maximally ablated, in regions 7 of medium opacity the printing ink is ablated to a lesser degree, and in regions 6 of maximal opacity, the least ablation takes place, or possibly even no ablation.
This is of course only valid to the degree that the printing ink 9 on the front side 3 covers the region where the watermark 5 is formed in the substrate 2.

[0052] The described embodiment can be modified so that instead of a watermark modulating the opacity through a thickness variation of the substrate, a so-called "false"

or printed watermark is used, thus a watermark produced on the substrate 2 by so-called watermark ink.

[0053] Fig. 3a and 3b schematically show the making available of such a printed watermark. On the back side 4 of the substrate 2 a watermark ink ills printed.
The watermark ink 11 according to Fig. 3b penetrates into the substrate 2, after drying or other suitable processing effecting an increase in translucence 12 in the substrate 2 in the places where the watermark ink 11 was printed.

[0054] When the substrate 2 with the printed watermark 12 is now exposed to the laser beam 10, as represented in Fig. 4, again the effect results that the intensity of the laser beam 10 is weakened less in positions 10a, where the watermark or the watermark ink increases the translucence of the substrate 2, than in positions 10b, where the watermark does not have this effect. Again, an ablation of the printing ink 9 is obtained in exact register with the watermark, without adjustment of said register upon application of the printing ink 9.

[0055] The Figures 5a to 5c relate to a third embodiment, wherein to the front side 3 additionally a coating 13 not modifiable by the laser radiation 10 was applied, which is transparent to the laser radiation 10, thus not weakening it.

[0056] Fig. 5a shows a sectional view similar to Fig. 2, wherein in some regions the coating 13 is applied now that is not modifiable by the laser radiation. Fig.
5b shows a plan view of the back side 4 with the coating 13.

[0057] Fig. Sc shows a plan view of the front side 3 after irradiation with the laser radiation 10. Due to the different opacity of the substrate 2 in the region of the watermark, the printing ink 9 is influenced by the laser radiation 10 to different degrees, so that the different regions 14, 15, 16 result. In the region 16 the printing ink 9 is almost uninfluenced, since the energy of the laser beam is greatly weakened due to the opacity of the substrate 2. In the region 14 the printing ink 9 is modified to a medium degree, for example ablated, due to a medium laser-energy weakening by the substrate due to the medium opacity of the substrate 2. The laser beam had a strong effect on the printing ink 9 in the region 15, since the substrate 2 is of low opacity there and weakened the laser beam only slightly.

[0058] The exemplary embodiment described by means of the Figures 5a to 5c can of course also be realized with a watermark that was produced by a watermark ink and not by thickness modulation.

[0059] Of course it is fundamentally possible for all embodiments to employ a combination of watermark ink and thickness modulation. A watermark ink is available for example from Sun Chemical under the designation 669440 Vernes UVSP (art.
no. JV
40000009).

[0060] Of course the procedures described here can also be executed on both sides of a substrate, with the printing ink 9 then being applied alternatingly to the front side and the back side of the substrate, so that it does not overlap.

[0061] Moreover, also two different printing inks can be used that can be modified by radiation of different spectral ranges. It is thus allowed to work with different intensity profiles for these two radiations or to print the same region on the front side and on the back side with radiation-modified printing inks in register with the watermark. For this purpose a first printing ink is applied to the front side and a second printing ink is applied to the back side. Subsequently, these printing inks are correspondingly exposed and modified by laser radiation applied from the back side (for the printing ink disposed on the front side) or from the back side (for the printing ink disposed on the front side).

[0062] Likewise, it is possible to employ = at least two different printing inks requiring different radiation intensities for modification, or = at least one printing ink containing substances requiring different radiation intensities for modification, or = at least one printing ink containing a mixture of substances that are modifiable and that are not modifiable.

[0063] For the printing method according to the invention, three examples will be described hereinafter:

[0064] Example 1:

[0065] A black offset ink on the basis of carbon black was printed over the region of a watermark. The ink was dried for several days and then ablated by means of an ND
vanadate laser from the back side to the ink disposed on the front side. In regions of low paper thickness, i.e. with maximally translucent watermark, 80% to 95% of the ink were removed thereby. In regions of maximal paper thickness, i.e. with minimal translucence of the watermark, only 10% to 30% of the ink were ablated.

[0066] Example 2:

[0067] The region of a watermark was printed on the front side by means of an OVI
screen printing ink WP of the producer SICPA. The ink dried for several days.
Subsequently the ink disposed on the front side was ablated from the back side by means of a Nd:YAG laser. In regions of low paper thickness 80% to 95% of the ink were removed. In the regions of maximal paper thickness 10% to 30% of the ink were ablated.

[0068] Example 3:

[0069] The region of a watermark was printed on the front side by means of a magnetic OVMI screen printing ink of the producer SICPA. The ink dried for several days.
Subsequently the ink disposed on the front side was ablated from the back side by means of a Nd:YAG laser. In regions of low paper thickness 80% to 95% of the ink were removed, by the exception of a yellow colorant. In the regions of maximal paper thickness 10% to 30% of the ink were ablated.

[0070] Generally, the certain radiation is to be chosen so that no absorption of the radiation occurs in the substrate that would lead to an alteration of the substrate. For this purpose both continuous-wave lasers and lasers operated in pulsed mode are possible. An example for a suitable continuous-wave laser is the model Innoslab, 1S81-E of the producer EdgeWave GmbH, which is a Nd:YV04-based laser at a wavelength of 1.064 nm. It provides a power of 100 Watt in continuous-wave operation. The ablation of an ink on carbon-black basis can take place at a scanning speed of 2.5 m/s to 5 m/s.

[0071] Employing an IR-modifiable ink as printing ink results in the advantage that the printed image and the check of the register is possible by machine-based infrared measuring. This ink can be both visually visible and invisible.

[0072] The printing method according to the invention is applicable also to a watermark that is provided in connection with a so-called windowed thread in a substrate of a security element. Fig. 6a shows a plan view in transmitted light of a front side 3 of a substrate 2 having a windowed thread 17. In the substrate 2 here a sequence of strip-shaped regions 18 and 19 is provided, forming the watermark: in the regions 18 the thickness of the substrate is unchanged, so that the watermark is of high opacity, and in the regions 19 the thickness of the substrate is reduced, so that the watermark is of low opacity (in the exemplary embodiment of the Figures 6 to 8 the watermark is disposed on the front side 3 of the substrate, in contrast to the exemplary embodiments of the Figures 1 to 5). In incident light, this has the effect that the windowed thread 17 has visible portions 20, where one side of the windowed thread is not covered by the substrate in accordance with Fig. 6d, and invisible portions 21, since these portions are completely embedded in the substrate 2 in accordance with Fig. 6c. The effect in incident light is shown in Fig. 6b: only the visible portions 20 are bare on the front side 3 and are visible.

[0073] Transmitted light is given within the meaning of the present invention when an illumination of the substrate takes place from the side opposite to the viewer, i.e. the illumination takes place through the substrate. Incident light is given within the meaning of the present invention when an illumination of the substrate takes place from the side of the viewer.

[0074] In the region of the watermark 5 formed of the regions 18 and 19, now printing ink 9 is applied to the front side 3.

[0075] The Figures 6c and 6d show a section through the substrate 2 of Fig.
6a, wherein Fig. 6c shows a section disposed in a region 18 and Fig. 6d a section in a region 19. In Fig. 6c the thread 17 is disposed below the surface of the front side 3, for which reason one of the invisible portions 21 is disposed here. In the region 19, in contrast, according to Fig. 6d the windowed thread 17 is disposed on the surface of the front side 3, so that a visible portion 21 is formed.

[0076] Fig. 7 shows a plan view of the front side 3 after the applied irradiation of the laser radiation from the back side 4. In the regions 18 of high opacity the printing ink 9 is not ablated, a region 16 like in Fig. 5c is present. In the regions 19 the printing ink 9 is ablated in contrast, a region 14 within the meaning of Fig. 2 is present.

[0077] Fig. 8 shows a variation wherein the laser radiation does not effect an ablation, but a modification inverse thereto, e.g. by development of thermochromic effect pigments. The regions 14 and 16 are exchanged exactly here.

[0078] It is fundamentally unessential for the printing procedure whether the printing ink 9 is applied to the front side or the back side of the substrate. What is decisive is merely that the substrate 2 is penetrated by the radiation from the side that is opposite the side on which the printing ink 9 is arranged. Of course the printing ink can be provided on the back side and the radiation can be coupled in from the front side in all exemplary embodiments.

List of reference numbers 1 security element 2 substrate 3 front side 4 back side 5 watermark 6, 7, 8 region 9 printing ink 10 laser beam 10a, 10b position 11 watermark ink 12 translucence increase 13 coating 14, 15, 16 region 17 windowed thread 18, 19 region 20, 21 portion

Claims

claims

1. A method for manufacturing a security element (1), wherein - a substrate (2) is made available that has a front side and a back side (3, 4), is translucent at least to certain radiation (10) and has a watermark (5) modulating the opacity of the substrate (2), - to the front side (3) a print element (9) is applied, characterized in that - on the front side (3) the print element (9) covers the watermark (5) at least partially, - the print element (9) can be modified by means of the certain radiation (10), and - the substrate (2) is penetrated by the certain radiation (10) from the back side (4), wherein the watermark (5) is employed as a mask with reference to the modification of the print element (9), and the print element (9) is modified in register with the watermark (5).

2. The method according to claim 1, characterized in that the watermark (5) of the substrate (2) made available modulates the opacity of the substrate (2) by modulation of a thickness of the substrate (2).

3. The method according to claim 1 or 2, characterized in that the watermark (5) of the substrate (2) made available has a dyeing modulating the opacity of the substrate (2).

4. The method according to any of the above claims, characterized in that the certain radiation (10) ablates the print element (9) for modification.

5. The method according to claim 4, characterized in that for ablation the print element (9) is equipped with an additional substance, e.g. carbon black, absorbing the certain radiation (10).

6. The method according to any of the above claims, characterized in that the print element is a printing ink (9) having in particular one or several of the following substances: an optically variable ink, a magnetically orientable, optically variable ink, a metallic ink, an ink effective in the invisible spectral range.

7. The method according to any of the claims 1 to 5, characterized in that the print element comprises a coating, transfer foil, laminating foil and/or a windowed thread.

8. The method according to any of the above claims, characterized in that the print element (9) additionally has substances that cannot be modified or ablated by means of the certain radiation (10) and absorb the radiation in the visual, infrared and/or ultraviolet wavelength range.

9. The method according to any of the above claims, characterized in that the certain radiation is applied in form, by guiding a beam, in particular a laser beam (10), over the back side (4).

10. The method according to claim 9, characterized in that in addition to employing the watermark (5) as a mask during the marking, also an intensity of the certain radiation (10) is varied in order to effect an additional structuring of the modification of the print element (9).

11. The method according to any of the above claims, characterized in that to the back side (4) an additional masking is applied that weakens or absorbs the certain radiation (10).

12. The method according to any of the above claims, characterized in that the print element (9) is so modified by means of the certain radiation (10) that it is provided with a light/ dark contrast structure, and that exclusively such an additional layer is applied whose visual impression depends on the brightness of the modified print element (9) forming the background of the additional layer.

13. The method according to claim 12, characterized in that as additional layer an optically variable layer, in particular a liquid crystal layer, is applied, which conveys a different color impression against a dark background than against a light background.