CA2645578A1 - Data carrier and method for manufacturing the same - Google Patents

Abstract

The invention relates to a method for manufacturing a data storage medium (10) having labelling (12) which can be detected visually and/or by machine, in the form of patterns, letters, numbers or images. In the method, a data storage medium (10) having a data storage medium substrate (14) is provided and a marking layer (16) is applied to the data storage medium substrate. According to the invention, the labellings (12) are made in the marking layer (16) by means of short laser impulses.

Description

Data Carrier and Method for Manufacturing the Same The present invention relates to a data carrier having a visually and/or machine-perceptible identifier in the form of patterns, letters, numbers or images, and a method for manufacturing such a data carrier.

Identification cards, such as credit cards or personal identity cards, have long been provided with an individual identifier by means of laser engraving. In marking by laser engraving, through suitable guidance of a laser beam, the optical properties of the card material are irreversibly changed in the form of a desired marking. For example, in publication DE 30 48 733 Al is described an identification card having applied information and exhibiting, on one surface, different colored layer regions that are stacked and that are at least partially interrupted by visually perceptible personalization data.

In addition to identification cards, other value documents that are at risk of counterfeiting, such as banknotes, stocks, bonds, certificates, vouchers, checks, admission tickets and the like, are often provided with laser-generated, individualizing marks, such as a serial number.

If an ink layer on a data carrier substrate is to be removed or modified with the aid of a laser, then this ink layer must exhibit, at least in part, a high absorption at the wavelength of the marking laser. The lower the absorption is, namely, the higher the energy input of the laser must be chosen to be in order to achieve the desired effect. However, due to the high energy input, usually undesired side-effects are produced in the substrate or in other layers lying above or below the ink layer to be marked.

If multiple layers are stacked, of which a lower lying one is to be marked with the laser, then it must be assumed that, in the irradiated regions, also

-2-even non-absorbent overprints of the laser-absorbent layer will be removed or modified with it. This means that, in the design of the data carrier, a certain space must be reserved for a laser identifier to ensure that other printing components are not destroyed by the laser identifier.

Based on that, the object of the present invention is to propose a data carrier of the kind cited above that exhibits a laser-generated individual identifier of high counterfeit security. In particular, the identifier should require little space on the data carrier and be easy to integrate into existing designs or print images. The present invention is also intended to provide a method for manufacturing such a data carrier.

This object is solved by the manufacturing method and the data carriers having the features of the independent claims. Developments of the present invention are the subject of the dependent claims.

According to the present invention, in a method for manufacturing a data carrier having a visually and/or machine-perceptible identifier in the form of patterns, letters, numbers or images, a data carrier having a data carrier substrate is provided and a marking layer applied to the data carrier substrate. According to the present invention, the identifiers are introduced into the marking layer by means of short laser pulses. Compared with conventional marking methods, such a laser marking with short laser pulses offers a range of advantages that are explained in detail below.

In a preferred variant of the present invention, identifiers are introduced into the marking layer that are not perceptible in the visible spectral range.
Rather, the identifiers are preferably perceptible only in the infrared spectral range, especially in the near infrared in a wavelength range between 780 nm

-3-and 1000 nm. They then constitute visually non-visible authenticity features that, however, can be machine-read with conventional silicon-based detectors with no problem. In alternative embodiments, the identifiers can also be readable in the ultraviolet spectral range.

The identifiers are advantageously produced by laser modification of a laser-modifiable feature substance in the marking layer. In a preferred embodiment, the feature substance is an infrared absorber, especially an infrared absorber having an absorption maximum in the near infrared.

The identifiers are advantageously introduced with a marking laser at a wavelength that does not correspond to the absorption maximum of the laser-modifiable feature substance. It is even possible to choose for the marking a wavelength at which the laser-modifiable feature substance exhibits substantially no or only a very low absorption. This decoupling of the marking wavelength and the absorption maximum permits a substantially greater freedom both in the choice of the feature substances used and in the choice of the marking laser used.

If, for example, a feature substance having an absorption maximum in the near infrared is used, for example at a wavelength of 850 nm, then a marking wavelength that is substantially further removed from the visible spectral range, for example, 1.06 m, can be used for the identifier. In this way, the threat of undesired impact on other, visually more visible ink layers due to the laser impingement can be greatly reduced.

The decoupling of the marking wavelength and the absorption maximum is possible through the inventive use of short laser pulses for the marking that can modify the feature substance surprisingly also far beyond its absorption

-4-maximum. It is even possible and, in some embodiments, advantageous, if the feature substance exhibits substantially no or only a very low absorption at the marking wavelength. How the short laser pulses produce the modification at the marking wavelength despite a low or even zero absorption of the feature substance is currently not understood. Without wanting to be bound to a certain explanation, it is surmised that the modification occurs due to non-linear effects at high laser intensities, as explained in greater detail below.

The identifiers are advantageously introduced into the marking layer with a marking laser in the infrared spectral range, marking lasers particularly preferably being of a wavelength of about 1.06 Rm. Here, for example Nd:YAG lasers, Nd:YVO lasers, Nd:glass lasers, Yb:glass lasers and the like may be used.

The pulse length of the short laser pulses is expediently chosen to be less than the characteristic time of the heat diffusion in the marking layer. This characteristic time of the heat diffusion can be estimated with the heat diffusion equation 6T/6t = D 62T/6x2, wherein T denotes the temperature, D
the diffusion constant of the marking layer, and x a locus coordinate.
According to this, a characteristic time T on the order of i- r2/D is associated with the heat flux across a characteristic expanse r. If the pulse length of a laser pulse is shorter than this characteristic time z, then the heat produced upon absorption can be distributed in the marking layer for the duration of the laser pulse only within the characteristic expanse.

In an advantageous embodiment of the method, the identifiers are introduced into the marking layer with laser pulses of a pulse length of less than 100 ns, preferably of less than 50 ns, particularly preferably of less than

-5-30 ns. Pulse lengths of less than 10 ns or even of less than 1 ns may likewise be used.

In addition to the pulse length, also the pulse rate of the marking laser and the average output of the laser are important. To produce complex markings with high production speed, a high pulse rate (> 20 kHz, preferably > 30 kHz), a high average output (at least a few watts, preferably a few 10 W), and a short pulse duration of the individual pulses is required. These to some extent opposing requirements can be satisfied simultaneously given careful choice and optimization of the marking lasers used.

In further variants of the present invention, the so-called "first peak"-phenomenon is used. "First peak" denotes the peak power of a laser at the start of lasing.

In the laser medium, through suitable energy supply ("pumping"), a population inversion of the energy level is produced; that is, higher energy states are more populated than lower energy states. During lasing, the reduction of this energy level is stimulated, and energy is emitted in the form of laser light (Light Amplification by Stimulated Emission of Radiation).
Solid-state lasers are operated in such a way that the pump light is continuously on and the laser radiation is released by opening a shutter or the Q-switch in the resonator. Before this, a high population inversion has built up that is instantaneously depleted and leads to a high peak power (first peak) as soon as the shutter or Q-switch is opened. Thereafter, the further pulses or the cw (continuous wave) operation occur at a lower balanced power level, since there is never sufficient time between two pulses to build up such a complete population inversion as at the start. The larger

-6-the interval between the individual pulses or the lower the frequency of the pulses, the smaller the difference between the first and the subsequent pulses is.

In general, this first peak is also perceptible in laser markings through a stronger marking, a stronger effect or a divergent effect at the start of a marking vector. In this way, it can be characterized e.g. for a marking in a metallized foil at the start of an identifier by a larger dot, or for a marking in a color in which the color is removed, by a darker spot in the paper that can also be tangible.

At least portions of the identifier according to the present invention can be emphasized through the "first peak" and serve as a verification feature for a lasered identifier. Depending on the design of this thus-modified identifier, the change also can no longer be perceived with the naked eye, but rather now only with a magnifying glass or an image processing sensor, such that a higher security level is achievable.

In an advantageous variant of the present invention, a mixture composed of a laser-transparent mixture component and a mixture component that is modifiable by the laser radiation is applied as the marking layer.
Alternatively, as the marking layer, also a sequence of layers composed of two or more layers can be applied, at least a first layer being transparent to the laser radiation and at least a second layer being modifiable by the laser radiation.

In both variants, the marking layer can be applied over a visually visible printing layer. The marking layer itself can be applied, especially imprinted, by means of intaglio printing or also contiguously. If the marking layer is

-7-executed in intaglio printing, it expediently includes, in addition to the feature substance, an etching ink. In all embodiments, the marking layer can include, in addition to the feature substance, further color and/or security pigments, such as optically variable color pigments or magnetic pigments.

According to a further advantageous embodiment of the present invention, at least one layer that masks the marking layer and is transparent to the laser radiation is applied over the marking layer. As the masking layer, especially a printing layer, especially a contiguous printing layer or an intaglio printing layer, can be used.

The present invention also includes a data carrier, especially a value document or security paper, having a substrate and a marking layer applied on the substrate, into which, by the action of laser radiation, visually and/or machine-perceptible identifiers are introduced in the form of patterns, letters, numbers, graphic codes (e.g. barcodes, matrix codes) or images.
According to the present invention, the marking layer of the data carrier includes at least one laser-transparent substance and one laser-modified feature substance that exhibits substantially no or only a low absorption at the laser wavelength. In particular, the absorption maximum of the laser-modified feature substance is advantageously beyond the laser wavelength, as explained above.

Here, the marking layer can exhibit a mixture composed of a laser-transparent mixture component and a mixture component modified by the laser radiation, or it can constitute a sequence of layers composed of two or more layers, at least a first layer being transparent to the laser radiation and at least a second layer being modified by the laser radiation.

-8-In a further aspect of the present invention, the invention includes a data carrier of the kind cited above in which the marking layer constitutes a sequence of layers composed of two or more layers, a first layer that is transparent to the laser radiation being disposed over a second layer modified by the laser radiation.

In all cited embodiments, the marking layer of the data carrier can be disposed over a visually visible printing layer. The marking layer itself can be formed by an intaglio printing layer or by a contiguous printing layer.
Over the marking layer can be disposed at least one layer that masks the marking layer and is transparent to the laser radiation, this masking layer being formed, for example, by a printing layer, especially a contiguous printing layer or an intaglio printing layer.

The identifiers of the marking layer are preferably not perceptible in the visible spectral range, but rather are machine-perceptible only in the UV-spectral range or preferably in the infrared spectral range, especially in a wavelength range between 780 nm and 1000 nm.

The substrate of the data carrier can be formed from paper, a foil or a paper-foil laminate. The data carrier itself constitutes, for example, a security element, a sheet-type value document or the card body of an identification card, credit card or the like.

Further exemplary embodiments and advantages of the present invention are explained below by reference to the drawings, in which a depiction to scale and proportion was omitted in order to improve their clarity.

Shown are:

-9-Fig. 1 a schematic diagram of a marked banknote according to an exemplary embodiment of the present invention, Fig. 2 a cross section through the banknote in fig. 1 along the line II-II in the region of the inscribed identifier, Fig. 3 schematically, the reflection spectrum of an infrared-absorbing feature substance prior to laser impingement (solid curve) and after laser impingement (dotted curve), and Fig. 4 to 8 cross sections as in fig. 2 through data carriers according to further exemplary embodiments of the present invention.
The present invention will now be explained in greater detail first with reference to figures 1 and 2 using a banknote as an example. For this, fig. 1 shows a schematic diagram of a banknote 10 that is provided with an identifier 12 that is machine-perceptible only in the infrared spectral range, such as the two-dimensional matrix code indicated in the figure. Fig. 2 shows a cross section through the banknote 10 along the line II-11 in fig. 1 in the region of the identifier 12.

As can be perceived when looking at figures 1 and 2 together, the banknote substrate 14 exhibits a marking layer 16 and a printing layer 18 that is transparent to the laser radiation of the marking laser. The marking layer 16 includes an infrared-absorbing feature substance whose absorption maximum is, for example, about 850 nm. In the laser-modified regions 20, the absorption of the feature substance, at 850 nm, is significantly reduced such that the inscribed identifier 12 can easily be read out with conventional

-10-silicon-based image processing sensors. In the visible spectral range, in contrast, the laser-modified regions 20 and the unmodified regions appear having a substantially identical color impression and identical brightness such that the identifier 12 is not visible with the naked eye.

A distinctive feature of the present invention lies in the fact that the laser modification of the feature substance occurs with short laser pulses and at a different wavelength than the wavelength of its maximum absorption. For example, in the exemplary embodiment, the identifier 12 is introduced into the feature layer 16 at a wavelength of about 1.06 m and with laser pulses of a pulse length between 6 ns to 30 ns.

For this, fig. 3 shows, schematically, the reflection spectrum of an infrared-absorbing feature substance that is suitable for the present invention, before and after the laser impingement with the marking laser. Here, the reflectivity, indicated in arbitrary units, is shown in each case in the spectral range from 400 nm to 1200 nm. The reflection spectrum of the feature substance prior to the laser impingement or outside the impinged-on regions is described by the solid curve 30. As evident from fig. 3, in these regions, the feature substance exhibits a pronounced absorption maximum at about 850 nm that appears in the depicted reflection spectrum as the xninimum of the reflectivity R, indicated by the arrow 32.

The laser impingement occurs with short pulses at a wavelength of 1.064 ~tm, indicated by the arrow 34, so clearly outside the absorption maximum of the feature substance. The reflectivity R of the marking layer 16 with the feature substance is already about 95% at this wavelength. Despite the low absorption, due to the laser impingement, surprisingly, the absorption and reflection of the feature substance change significantly in the absorption

-11-maximum at 850 nm. The reflection spectrum of the feature substance after the laser impingement is depicted in fig. 3 by the dotted curve 36.

As can be seen from fig. 3, the modified regions 20 and the unmodified regions can easily be distinguished based on their different reflection in the near infrared, for example at a wavelength of 850 nm. The differences in the visible spectral range, in contrast, are small and, moreover, in the exemplary embodiment in figures 1 and 2, masked by the overprint layer 18.

The inventive measure offers a range of advantages. For example, the choice of Iaser-modifiable feature substances is no longer limited to substances that exhibit a high absorption at the laser wavelength. Also, due to the short pulses, only a low thermal load on the layer structure and on the substrate upon lasing is facilitated. Further, the wavelength of the marking laser can be chosen such that other elements of the layer structure are not affected by the laser radiation.

For example, through the greater distance of the wavelength of the marking laser at 1.06 m from the visible spectral range, it can be ensured that ink layers that are to be maintained unchanged are not impacted by the lasing.
The risk of such an impact is substantially higher when a marking laser at the absorption maximum of the feature substance (850 nm) is used, due to the greater proximity to the visible spectral range.

In the context of the present invention, short pulses denote pulses for which the pulse length is less than the characteristic time for the heat diffusion in the marking layer, as described in greater detail above.

-12-The precise mechanism with which the short laser pulses produce the described modification of the reflection spectrum beyond the laser wavelength is not known. Without being bound to a certain explanation, it is currently surmised that non-Iinear effects that occur due to the high laser intensity of the short pulses play a substantial role here. The non-Iinear effects may be, for example, 2-photon processes, or the production of free electrons in the initial phase of the laser pulse, associated with a subsequently increased absorption by the ionized surface layer.

The feature substance can be removed or ablated by the short laser pulses or, due to the deposited energy, for example also be so changed in its chemical bonds that the observed changed in the reflection properties results.

Good results were achieved with marking lasers having wavelengths at about 1.06 m, such as Nd:YAG lasers, Nd:YVO lasers and Yb:glass-fiber lasers, wherein the preferred laser pulse lengths were consistently below 100 ns. Particularly good results were achieved with laser pulse lengths between about 6 ns and 30 ns, but also shorter pulse lengths in the single-digit nanosecond range or even in the picosecond or femtosecond range may be used. Here, the pulse length is, as usual, defined as the width of the pulse at half the maximum intensity.

In the following are described, with reference to figures 4 to 8, further embodiments of the present invention with different marking layers and different layer structures.

The simplest layer structure 40 that may be used according to the present invention is depicted in fig. 4. Here, to a substrate 42, which can be, for example, a banknote paper, a foil, a card body or also a paper-foil laminate, a

-13-marking layer 44 is applied, preferably imprinted. The marking layer 44 includes a laser-modifiable feature substance that, through impingement of a region 46 of the marking layer with short laser pulses, is changed locally in its absorption or reflection properties. The change in the region 46 can then be perceived visually and/or by machine. In addition to the infrared-absorbing feature substances already mentioned, of course also feature substances having laser-induced modifications in the visible or ultraviolet spectral range may be used.

A further exemplary embodiment of the present invention is depicted in fig.
5. The layer structure 50 shown there comprises a substrate 52, a printing layer 54, a marking layer 56 and an optional protective layer 58. The printing layer 54 can be, for example, an offset or indirect layer that does not react with the laser radiation. In the exemplary embodiment, the marking layer 56 consists of a mixture of an etching ink 60 with a suitable feature substance 62, for example the laser-modifiable infrared absorber in fig. 3.

By impingement of regions 64 of the marking layer 56, imprinted in intaglio printing, with the pulsed radiation of an infrared laser, in the exemplary embodiment a diode-pumped Yb:glass-fiber laser of a wavelength A of 1.06 m, a pulse length between 20 and 30 ns, a pulse frequency of 30 kHz or more, and an average output of 10 to 20 W, the feature substance 62 is already sufficiently changed in its absorption properties by a single laser pulse to subsequently permit a machine read-out of the introduced identifier.

Since, in this way, only a single laser pulse is required for each written bit of information, very high marking speeds can be achieved, such as those required for the production of complex identifiers, such as cryptographically

-14-securely encrypted matrix codes, at the speed of modern banknote printing lines (10,000 sheets/hour).

Primarily when the printing layer 54 includes a high proportion of dark ink, it can happen that the inscribed identifier becomes weakly visible due to the surface change of the marking layer 56. However, this can be prevented in that the marking layer is provided with a protective layer 58 that lends the entire surface a uniform gloss impression. Moreover, through such a protective layer, the identifier is well protected against soiling, such that its machine-readability is ensured for a long time.

Another possibility to prevent the inscribed identifier from becoming weakly visible due to the surface change consists in briefly heating up the sequence of layers after the laser impingement such that the binder of the marking layer softens at the surface, flows easily and compensates potential surface changes.

The marking layer can also be disposed below further masking printing layers, as illustrated by the sequence of layers 70 in fig. 6. There, on a substrate 72, a feature substance is printed as a contiguous area 74 that is chosen to be sufficiently large in order to receive the laser identifier. Then the marking layer 74 produced in this way is overprinted with at least one printing method, for example, with indirect printing, and with an ink that does not react with the radiation of the marking laser (reference number 76).
Subsequently, through laser impingement, the infrared absorption of the feature substance was changed in a sub-region 78 of the marking layer 74 without impacting the overprint layer 76 lying above it. The overprint layer 76 likewise ensures that the identifier cannot be visually perceived, also given any slight surface changes in the marking layer 74.

-15-The overprint 76 can consist, for example, of guilloches. Also, one or multiple further layers can be applied over the overprint layer 76, such as an intaglio print portrait. The contiguous marking layer 74 can also include, in addition to the feature substance, further color and/ or security pigments, such as optically variable color pigments or magnetic pigments.

If the marking layer is applied, as in fig. 5, in intaglio printing, the infrared absorption of the feature substance is normally not areally constant due to the structural width of the intaglio printing, which can make the machine-readout of the inscribed identifier more difficult. To remedy this, in addition to the intaglio printing layer, a contiguous printing layer can be applied with the feature substance, as illustrated in the exemplary embodiment in fig. 7.

The sequence of layers 80 shown there includes, in addition to the layers already described in connection with fig. 5, an additional layer 82 applied with a background printing method that likewise includes the feature substance 62. Since the printing layer 54 is transparent to the radiation of the marking laser, the feature substance 62 is modified by the laser impingement both in the marking layer 56 and in the additional contiguous printing layer 82. The appearance of the inscribed identifier is homogenized by the additional layer 82 and the machine readout reliability increased. It is understood that also the additional layer 82 can include, in addition to the feature substance, also further color and/or security pigments.

In a further variant of the present invention, different marking layers are combined in a sequence of layers. For illustration, fig. 8 shows a sequence of layers 90 having a first marking layer 92, a masking overprint layer 94 and a second marking layer 96. The two marking layers 92, 96 include different

-16-feature substances that can each be modified by impingement with laser pulses of different lengths.

For example, the feature substance of the first marking layer 92 can be chosen such that it already reacts with the laser radiation at longer pulse durations, for example about 50 ns, while shorter laser pulses, for example about 10 ns, are required to modify the feature substance of the second marking layer 96.

In such a sequence of layers 90, in a first step, first identifiers 93 can then be introduced into the marking layer 92 with longer laser pulses (50 ns). The second marking layer 96 is not changed in this step, since the intensity of the longer pulses is not sufficient to modify its feature substance. In a second step, a second identifier 97 is then introduced into the marking layer 96 with shorter pulses (10 ns). In this step, the first marking layer 92 can likewise be changed or remain unchanged.

Of the identifiers 93 and 97, one or both can remain non-visible in the visible spectral range. For example, the identifier 93 of the upper marking layer 92 can be perceptible in the visible spectral range, while the masked identifier 97 of the lower marking layer 96 can be read out only by machine in the near infrared. Also the reverse arrangement of the first and second marking layer is conceivable such that the upper marking layer is modified with short, the lower marking layer with longer laser pulses. Laser pulses of different lengths can often be achieved without great effort with a single laser system since, in many laser systems, an increase in the pulse rate involves an extension of the pulse duration.

-17-The perception of the inscribed identifiers can occur with the aid of a camera, for example with a CCD or CMOS detector. Suitable filters (long pass, short pass, band pass, especially narrow-band interference filters) can significantly increase the contrast between laser-modified and unmodified regions. Also, downstream image processing can further improve the perceived raw image prior to analysis.

To increase the read reliability, also different error correction algorithms known to the person of skill in the art can be used, in which, for example, through targeted addition of redundancy, potential later read errors can be corrected.

The reflection behavior of the feature substance can also be analyzed at more than one wavelength in order to be able to distinguish between different feature substances. As is evident from fig. 3, the difference in the reflectivities of the modified and unmodified regions changes sign multiple times between 400 nm and 1000 nm. The wavelengths at which these changes in sign occur, or the relationship between the reflectivities at certain, prechosen wavelengths, is characteristic for the feature substance used in each case. If detectors are used that consist of other materials than silicon, such as an InGaAs detector, the reflection behavior also permits analysis at wavelengths greater than 1000 nm.

To achieve a reliable marking of banknotes, the following procedure, for example, can be used:

First, at banknote production, the numbering of a banknote on the sheet is read or calculated with the aid of a camera and electronic image processing.
In addition, a batch code that is not perceptible in the visible and that varies

-18-for each up on the banknote sheet is read or calculated. In another possibility, for each sheet that enters the production unit, only one numeral and one batch code is read and the values for the remaining ups on the sheet are calculated from the read values. In a further variant, a control unit of the numbering machine transmits the numerals to the computing system for the laser identifier, and only the batch code is perceived and read with the camera.

From the banknote numeral and the established batch code, by means of a secret key and a suitable encryption algorithm, such as RSA, DSA, elliptic curves or the like, a signature is then calculated, converted to a two-dimensional code, and inscribed in the marking layer as an identifier in the above-described manner. The signature can exhibit, for example, 40 payload bytes or more in order to be considered secure according to the current state of cryptography.

To further increase the counterfeit security, also statistical data from the individual banknote can go into the signature, such as cutting tolerances, position tolerances between the different printing methods, a dispersion of mottled fibers and the like.

A simpler code, and one that is also more readable by fast-running banknote processing machines, can include, for example, only up to 10 payload bytes and be inscribed in the marking layer without elaborate encryption. Here, the serial number of the banknote can be, for example, entirely or partially stored in the visually not visible code, or also other logistical data.

A further application of the identifier according to the present invention consists in the use as an intelligent infrared precision color split. In principle,

-19-infrared color split is created in that, for example, in intaglio printing, a motif is printed with two inks that are identical in the visual spectral range and that differ in the infrared spectrum such that the motif is perceptible only through pictures in the infrared range. Conventional color splits are produced through overlapping color stencils such that the form and information of the color split can be only relatively crude. In addition, for the production of the conventional color split, two inking units are needed, which limits the number of colored inks available for the design.

With the above-described laser modification, it is now possible to introduce very finely structured information in the infrared range that becomes visible as a lightening when viewed in the infrared spectral range, for example, at 850 nm. For example, graphics and image motifs, optical codes, characters, etc. can be implemented. Both vector and pixel/bitmap inscription methods can be applied. The method is preferably applied to regions that were preprinted in intaglio printing, the intaglio printing inks including the infrared-absorbing feature substance as an addition. However, it is also conceivable to use all other printing methods for the preprinting of the color split area, such as simultaneous printing, indirect relief printing, screen printing, offset printing, gravure printing and the like.

Here, a dense pattern of the preprint or even a contiguous area (background print) is advantageous in order to to make detailed color split patterns perceptible. But also in an etched portrait, the possibility exists to depict fine color split details. Also, a banknote window, a metallic strip or a foil application can serve as the carrier of the marking layer. In this way, in a banknote window for example, a code that is readable from both sides can be produced.

-20-A particular advantage of such a laser-color-split method consists in that an additional inking unit becomes available for any colored ink, and in that, through the use of a special infrared absorber, also the application of the color split for light tones becomes feasible. A further advantage is the diverse inscription of identical banknotes in order to be able to separate them in later sorting.

In passport applications, with the described procedure, a hidden image of the owner can be produced in the infrared range - a repetition of the passport image that is visible only in the infrared. It is of course also conceivable to produce a combination of classical color split and laser precision color split.

A further application of the laser marking consists in securing products against counterfeiting. Various features of a product, such as the article number, serial number, date of manufacture, production facility and the like, can be encrypted as a signature and incorporated in the visually and/or machine-perceptible identifier. Here, too, the data can be linked with statistical features of the product. The identifier can be applied to a label, the packaging or, in individual cases, also to the product itself.

In ticketing applications, characteristic data, such as the name of the event, the event location, date, time, price, seat number, name of the owner and the like, can be stored encrypted on the admission ticket.

The present invention is advantageous also for automatically issued and accepted value coupons, such as are increasingly used by supermarket chains. For example, in a dispensing machine can be applied to a value coupon an identifier of the kind described that includes data such as the

-21-value of the coupon, a coupon number, the issue date, the issuing office, the issuing company and the like. After checking the identifier, the acceptance of counterfeited or falsified coupons can then be denied by a corresponding receiving machine.

Claims (29)

1. A method for manufacturing a data carrier having a visually and/or machine-perceptible identifier in the form of patterns, letters, numbers or images, in which - a data carrier having a data carrier substrate is provided and - a marking layer is applied to the data carrier substrate, characterized in that - the identifiers are introduced into the marking layer by laser modification of a laser-modifiable feature substance in the marking layer by means of short laser pulses, - the identifiers being introduced with a marking laser at a wavelength at which the laser-modifiable feature substance exhibits substantially no or only a low absorption.

2. The method according to claim 1, characterized in that identifiers are introduced into the marking layer that are not perceptible in the visible spectral range.

3. The method according to claim 1 or 2, characterized in that identifiers are introduced into the marking layer that are perceptible in the infrared spectral range, preferably in the near infrared in a wavelength range between 780 nm and 1000 nm.

4. The method according to at least one of claims 1 to 3, characterized in that the feature substance is an infrared absorber, especially an infrared absorber having an absorption maximum in the near infrared.

5. The method according to at least one of claims 1 to 4, characterized in that the identifiers are introduced with a marking laser at a wavelength that does not correspond to the absorption maximum of the laser-modifiable feature substance.

6. The method according to at least one of claims 1 to 5, characterized in that the identifiers are introduced with a marking laser in the infrared spectral range, preferably with a marking laser of a wavelength of about 1.06 µm.

7. The method according to at least one of claims 1 to 6, characterized in that the pulse length of the short laser pulses is chosen to be less than the characteristic time of the heat diffusion in the marking layer.

8. The method according to at least one of claims 1 to 7, characterized in that the identifiers are introduced into the marking layer with laser pulses of a pulse length of less than 100 ns, preferably of less than 50 ns, particularly preferably of less than 30 ns.

9. The method according to at least one of claims 1 to 8, characterized in that the identifiers are introduced into the marking layer with laser pulses of a pulse length of less than 10 ns, especially of less than 1 ns.

10. The method according to at least one of claims 1 to 9, characterized in that, as the marking layer, a mixture composed of a laser-transparent mixture component and a mixture component that is modifiable by the laser radiation is applied.

11. The method according to at least one of claims 1 to 9, characterized in that, as the marking layer, a sequence of layers composed of two or more layers is applied, at least a first layer being transparent to the laser radiation and at least a second layer being modifiable by the laser radiation.

12. The method according to at least one of claims 1 to 11, characterized in that the marking layer is applied over a visually visible printing layer.

13. The method according to at least one of claims 1 to 12, characterized in that the marking layer is applied by means of intaglio printing.

14. The method according to at least one of claims 1 to 13, characterized in that the marking layer is applied, especially imprinted, contiguously.

15. The method according to at least one of claims 1 to 14, characterized in that, over the marking layer, at least one layer that masks the marking layer and is transparent to the laser radiation is applied.

16. The method according to claim 15, characterized in that, as the masking layer, a printing layer, especially a contiguous printing layer or an intaglio printing layer, is applied.

17. A data carrier, especially a value document or security paper, having a substrate and a marking layer applied on the substrate, into which visually and/or machine-perceptible identifiers in the form of patterns, letters, numbers or images are introduced by the action of laser radiation, characterized in that the marking layer includes at least one laser-transparent substance and one laser-modified feature substance, and in that the laser-modified feature substance exhibits substantially no or only a low absorption at the laser wavelength.

18. The data carrier according to claim 17, characterized in that the marking layer exhibits a mixture composed of a laser-transparent mixture component and a mixture component modified by the laser radiation.

19. The data carrier according to claim 17, characterized in that the marking layer constitutes a sequence of layers composed of two or more layers, at least a first layer being transparent to the laser radiation and at least a second layer being modified by the laser radiation.

20. The data carrier according to claim 19, characterized in that the marking layer constitutes a sequence of layers composed of two or more layers, a first layer that is transparent to the laser radiation being disposed over a second layer that is modified by the laser radiation.

21. The data carrier according to at least one of claims 17 to 20, characterized in that the marking layer is disposed over a visually visible printing layer.

22. The data carrier according to at least one of claims 17 to 21, characterized in that the marking layer is formed by an intaglio printing layer.

23. The data carrier according to at least one of claims 17 to 22, characterized in that the marking layer is formed by a contiguous printing layer.

24. The data carrier according to at least one of claims 17 to 23, characterized in that over the marking layer is disposed at least one layer that masks the marking layer and is transparent to the laser radiation.

25. The data carrier according to claim 24, characterized in that the masking layer is formed by a printing layer, especially a contiguous printing layer or an intaglio printing layer.

26. The data carrier according to at least one of claims 17 to 25, characterized in that the identifiers of the marking layer are not perceptible in the visible spectral range.

27. The data carrier according to at least one of claims 17 to 26, characterized in that the identifiers of the marking layer are perceptible in the infrared spectral range, preferably in a wavelength range between 780 nm and 1000 nm.

28. The data carrier according to at least one of claims 17 to 27, characterized in that the substrate is formed from paper, a foil or a paper-foil laminate.

29. The data carrier according to at least one of claims 17 to 28, characterized in that the data carrier is a security element, a sheet-type value document or a card body.