WO2024038086A1

WO2024038086A1 - A method for generating at least one security feature on a banknote or on a tax stamp using laser writing

Info

Publication number: WO2024038086A1
Application number: PCT/EP2023/072558
Authority: WO
Inventors: Roland Gutmann; Tobias Fischer
Original assignee: Gleitsmann Security Inks Gmbh
Priority date: 2022-08-19
Filing date: 2023-08-16
Publication date: 2024-02-22
Also published as: EP4324658A1

Abstract

A method for generating at least one security feature on a banknote or a tax stamp, comprising: i) providing a substrate sheet in form of a sheet of paper or of a polymer foil or a composite, ii) printing onto at least one of the two surfaces of the substrate sheet at least one printing ink layer, extending over at least part of the substrate sheet, and wherein the printing ink comprises: a) at least one pigment and/or b) one dye, which changes its luminescence and/or its infrared absorption intensity upon exposition to laser radiation so that the integral of the emission curve obtained after excitation of the at least such pigment /dye after step iv) is at least 10% lower than the the corresponding one before step iv) iii) curing the printing ink layers, iv) exposing the printed substrate before, during and/or after the curing step iii) to laser radiation so as to create at least one security feature on the at least one banknote of the printed substrate sheet.

Description

A method for generating at least one security feature on a banknote or on a tax stamp using laser writing

The present invention relates to a method for generating at least one security feature, such as in particular an individual security feature, such as a number, a barcode, a QR code or any other pattern, on a banknote or on a tax stamp and in particular to a respective method being performed continuously, semi-continuously or post-printing.

Banknotes and tax stamps must comprise several security features, so that any skilled person is able to reliably determine, whether it is genuine or forged. This is a prerequisite for allowing a recipient of for instance a banknote, such as a cashier at a supermarket, to decide, whether he accepts it or not. In addition, it allows employees of a bank to evaluate the genuineness of banknotes, before filling it into a cash machine or transfer it into the cash desk. Moreover, the existence of such security features avoids or at least significantly hinders unauthorized persons to copy or reengineering the respective article. Furthermore, security features even allow a cash machine or a banknote counting machine to assure that the banknotes are properly aligned during their transport through the machine, which is a prerequisite for reliably counting the number of banknotes. Various sensor systems for detecting fluorescence effects are known, most of which are based on an UV excitation source and a photodiode or image camera that detects and checks the visible emission light, such as described in CA 2 349 681 A1 . Examples for security features are security images, which comprise an image being invisible or otherwise undetectable under ambient conditions, but which are rendered visible or detectable upon application of e.g. UV radiation. Alternatively, the image on the banknote may have a first color under ambient conditions, which is changed into a second color being different from the first color upon application of e.g. UV radia- tion. All these security features are applied onto the surface(s) of the banknotes or tax stamps during the printing process. This printing process is a continuous process, during which for instance a plurality of banknotes is printed simultaneously onto a substrate sheet made of paper or, sometimes, of polymer. Usually up to 10,000 substrate sheets may be printed within one hour, each of these sheets comprising 32 to 80 banknotes, e.g. 54 for 6 x 9 notes on a single sheet, which are later separated from each other by cutting.

One further very important component of a banknote and a tax stamp is an individual security feature allowing to identify each exemplary of the banknotes and tax stamps. A prominent example for such an individual security feature is the serial number provided on banknotes and tax stamps, which may be a serial number or an alphanumerical code. Examples for other respective individual security features are QR codes and barcodes. The serial number of a banknote for instance consisting of several numerals or digits, respectively, optionally together with one or more characters is not printed together with the printing inks comprising the security features onto the substrate sheets, but afterwards in a separate step. More specifically, the printed substrate sheet is first cured or dried, respectively, and then transferred to a different printing machine, in which the serial numbers are printed onto the sheets with mechanical numbering devices. With cured or dried printed substrate sheet a sheet is meant, in which the printing ink is cured or dried. Each of these mechanical numbering devices comprises - similar to a date stamp or combination lock - for every numeral and, if present, for every character of the serial number an own cylinder with ten numerals or a plurality of characters so that by respectfully (electronically controlled) rotating the single cylinders the serial number may be adjusted. The so adjusted numerals and optionally characters of the mechanical numbering devices are covered by coating them with printing ink from a drum, whereafter the printing ink is transferred from the mechanical numbering devices onto the sheets. More specifically, the mechanical numbering devices are arranged on a rotating drum, with a plurality of mechanical numbering devices being arranged with regular distance to each other in circumferential direction as well as in the direction perpendicular thereto. The number and arrangement of the mechanical numbering devices on the drum correspond to the number of banknotes printed onto the substrate sheet so that per one rotation of the drum with the mechanical numbering devices, all banknotes printed on one sheet are provided with serial numbers. Thus, if a sheet comprises 50 banknotes, 50 logistically controlled mechanical numbering devices have to be arranged on the drum surface.

However, this process has many drawbacks. First of all, printing inks are penetrating within the open mechanism of the mechanical numbering devices so that the numbering process must be stopped from time to time so as to clean the mechanical parts of the mechanical numbering devices. Such a cleaning step requires several hours, since the single mechanical numbering devices have be taken out of the holders being mounted on the drum, must be dismantled, carefully cleaned, oiled and mounted back into the holders. On account of this, several such numbering systems are required per one banknote printing machine, which outputs continuously up to 10,000 sheets per hour. Therefore, the whole numbering process is time and cost consuming. Moreover, the mechanical numbering devices have to be electrically connected with a control unit so as to allow to electronically adjust the single numbers and optionally characters of the single mechanical numbering devices. This makes the whole numbering apparatus not only very complex, but also accident-sensitive and maintenance-intensive.

In view of this, the object underlying the present invention is to provide a method for generating at least one security feature and preferably an individual security feature, such as a (serial) number, a barcode, a QR code or any other pattern, on a banknote and on a tax stamp, which may be performed continuously, semi- continuously or post-printing, which overcomes the aforementioned disadvantages and which allows to produce security features having a very high protection against forgery.

In accordance with the present invention, this objective is achieved by providing a method for generating at least one security feature and preferably at least one individual security feature on a banknote or a tax stamp, wherein the method comprises the following steps: i) providing a substrate sheet in form of a sheet of paper or of a polymer foil or a composite comprising at least one paper layer and at least one polymer foil, ii) printing onto at least one of the two surfaces of the substrate sheet each one or more printing ink layers so as to generate at least one banknote or at least one tax stamp on the substrate sheet, wherein each printing ink layer extends over a part or the whole of the surface area of the substrate sheet, and wherein the printing ink of at least one of the printing ink layers comprises a) at least one pigment and/or at least one dye, which changes its luminescence intensity upon exposition to laser radiation so that the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 10% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv) and/or b) at least one pigment and/or at least one dye, which changes its infrared absorption intensity upon exposition to laser radiation so that the integral of the emission curve of the at least one pigment and/or at least one dye after step iv) is at least 10% lower or at least 10% higher than the integral of the emission curve of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv), iii) curing the printing ink layers, iv) exposing the printed substrate sheet obtained in step ii) before, during and/or after the curing step iii) to laser radiation so as to create at least one security feature on the at least one banknote or on the at least one tax stamp of the printed substrate sheet.

The method in accordance with the present invention has the advantage that it does not necessarily comprise two completely separate printing steps for generating security feature(s) and in particular individual security feature(s) on banknotes and tax stamps, wherein first all printing ink layers except the security features, such as serial numbers, QR codes, barcodes and other patterns, are written onto the substrate sheet, before the so obtained printed sheet is cured (meaning that the printing ink on the sheet is cured, either by exposure to UV radiation in case of an UV curing printing ink or by drying in case of a non-UV curing printing ink) and then in a second printing process step the security feature(s), such as in particular serial numbers, QR codes and/or barcodes, are written onto the printed substrate sheet. On the contrary, all printing ink layers including that or those printing ink layer(s) including the pigment(s)/dye(es) changing its/their luminescence intensity and/or infrared absorption intensity upon exposition to laser radiation for creating the security feature(s), such as in particular serial numbers, QR codes and/or barcode, which are later activated by the laser exposition step, may be printed simultaneously or one after the other in one continuous or semi-continuous printing process onto the substrate sheet, before the printed substrate sheet is, in any order, cured and exposed to laser radiation so as to create - by laser writing - one or more security features and in particular individual security features, such as individual (serial) numbers each comprising at least two numerals and optionally also one or more characters, such as typically 10 to 14 numerals and characters in total, one or more QR codes and/or one or more barcodes on each banknote or tax stamp of the printed substrate sheet. Consequently, the speed for laser writing the (individual) security feature(s) onto the substrate sheets has to be exactly the speed of printing the other printing layers onto the substrate sheets. In addition, the pigment(s) and/or dye(s) changing its/their luminescence intensity and/or infrared absorption intensity upon exposition to laser radiation may be incorporated into one of the printing ink(s) required for applying one or more other security features onto the substrate sheet, so that in fact one printing step may be saved. Semi- continuous means in accordance with the present invention that at least a part of the process is performed continuously. Another advantage of the method in accordance with the present invention is that it allows to perform the laser writing with laser radiation having a comparably low wavelength of 200 to 800 nm, which allows to generate security features and in particular individual security features, such as serial numbers, QR codes and barcodes, with a high resolution, which are thus reliably machine detectable and readable with standard software. Thereby, even very small QR codes and barcodes of e.g. 5 x 5 mm or even smaller may be generated in high resolution. In advantage to, for instance, the usual banknote numbering procedure, the method in accordance with the present invention does not make use of open mechanical numbering devices for printing the serial numbers or other individual security feature onto the banknote sheets after having printed the other printing ink layers and is therefore not connected with the aforementioned disadvantages. On the contrary, all of the serial numbers and/or other individual security feature(s) are created in the method in accordance with the present invention by laser writing, i.e. by exposing the printed and cured substrate sheet to laser radiation, wherein the laser beam is moved so that on the printed and cured substrate sheet one or more serial numbers, one or more QR codes and/or one or more barcodes and optionally further individual security features are generated. Advantageously, the laser writing step is much less accident-sensitive as well as much less maintenance-intensive than the use of open mechanical numbering devices. Moreover, a time-consuming cleaning of open mechanical numbering devices is dispensed. All in all, the method in accordance with the present invention allows to generate at least one security feature and preferably an individual security feature on a banknote or on a tax stamp, wherein the method may be performed continuously, semi-continuously or post-printing, wherein the method overcomes the aforementioned disadvantages, and wherein the method allows to produce security features having a very high protection against forgery.

In accordance with the present invention, the term printing ink means any printable composition comprising a dye and/or pigment, so that even in the case that the dye and/or pigment is transparent or colorless, the composition is considered as printing ink and not as printing varnish. The same applies if the included pigment or dye is only detectable in the ultraviolet or infrared region.

Number means in accordance with the present invention any number consisting solely of numerals as well as alphanumeric numbers comprising one or more numerals and one or more characters. Thus, creating one or more individual (serial) numbers means comprising at least two numerals and optionally one or more characters on each banknote of the printed substrate sheet means in accordance with the present invention any (alphanumeric) number comprising two or more numerals (i.e. two or more numerals form 0 to 9) and optionally one or more characters (i.e. A, B, C or any other), such as typically in sum of numerals and characters 8 to 12.

In accordance with the present invention, preferably all of the one or more preferred individual (serial) security features, such as serial numbers, one or more QR codes and one or more barcodes, are generated on the substrate sheet by the exposure to the laser radiation. Also, preferably all of the optional non-individual security features are generated on the substrate sheet by the exposure to the laser radiation. This means that the method does not comprise any mechanical (typography) provision of a security feature, such as any mechanical (typography) numbering or barcoding, i.e. the process is performed without any mechanical security feature producing device and this without any mechanical numbering device, without any mechanical QR coding device and without any mechanical barcoding device, and that the all of the security feature and preferably individual security features are generated by the exposure to the laser radiation, but not generated by any other technique than laser writing, such as in particular not generated by an inkjet process, an embossing process or the like. Usually, banknotes comprise one serial number on one of the two banknote sides. In addition, the banknote may comprise a second serial number on the other side of the banknote, one or more QR codes and/or one or more barcodes on one or both sides of the banknote. Still alternatively, the banknote may comprise no serial number, but one or more QR codes and/or one or more barcodes on one or both sides of the banknote. In any of these cases, in accordance with the present invention all serial numbers, if any, all QR codes, if any, and all barcodes, if any, are generated on the banknote by means of the laser writing step.

According to the present invention, the printing ink of at least one of the printing ink layers comprises a) at least one pigment and/or at least one dye, which changes its luminescence intensity upon exposition to laser radiation so that the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 10% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv) and/or b) at least one pigment and/or at least one dye, which changes its infrared absorption intensity upon exposition to laser radiation so that the integral of the emission curve of the at least one pigment and/or at least one dye after step iv) is at least 10% lower or at least 10% higher than the integral of the emission curve of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv). The pigment and/or at least one dye a), which changes its luminescence intensity upon exposition to laser radiation, may have additional optical properties, such as that it also changes its infrared absorption intensity upon exposition to laser radiation. Likewise thereto, the pigment and/or at least one dye b), which changes its infrared absorption intensity upon exposition to laser radiation, may have additional optical properties, such as that it also changes its luminescence intensity upon exposition to laser radiation. In other words, the at least one pigment and/or at least one dye a) as well as the at least one pigment and/or at least one dye a) may be a pigment and/or at least one dye, which changes its luminescence intensity upon exposition as well as its infrared absorption intensity upon exposition to laser radiation.

Furthermore, a pigment or dye changing its luminescence intensity and/or infrared absorption intensity upon exposition to laser radiation is in accordance with the present invention a pigment or dye changing its luminescence intensity and/or infrared absorption intensity upon exposition to laser radiation without participation of any further compound or reactant, respectively, due to the molecule specific energy of the pigment or dye, for instance by a changing its optical properties as direct consequence of the irradiation with laser radiation. Alternatively, the pigment or dye changing its luminescence intensity and/or infrared absorption intensity upon exposition to laser radiation may change its luminescence intensity and/or infrared absorption intensity upon exposition to laser radiation with participation of one or more further compounds or reactants, respectively. For instance, a further compound included in the printing ink may work as activator, when exposed to laser radiation, so that the pigment or dye may react upon exposure to the laser radiation with another reactant, thereby forming reaction products having another luminescence intensity and/or infrared absorption intensity than the educt(s). It is preferred that the the pigment or dye is not decomposed upon the exposition to laser radiation, i.e. that the luminescence intensity change and/or infrared absorption intensity change upon the exposition to laser radiation is not the result of a decomposition reaction of the pigment or dye.

In accordance with a particular preferred embodiment of the present invention, in step ii) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which changes its luminescence intensity upon exposition to laser radiation. The term luminescence comprises in accordance with the present invention all kinds of luminescence, such as thermoluminescence, electroluminescence, chemiluminescence and others. However, it is preferred that the in step ii) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which changes its photoluminescence intensity upon exposition to laser radiation, such as its phosphorescence intensity, and most preferably, changes its fluorescence intensity upon exposition to laser radiation.

Preferably, the pigment and/or dye, which changes its photoluminescence intensity, such as its phosphorescence intensity or preferably fluorescence intensity, upon exposition to laser radiation has an emission spectrum being shifted to longer wavelengths in comparison to its excitation spectrum. Furthermore, it is preferred that the pigment and/or dye, which changes its photoluminescence intensity, such as its phosphorescence intensity or preferably fluorescence intensity upon exposition to laser radiation, is excitable in the UV- or VIS-region and emits in the VIS- or IR region. Exciting or emitting in the UV-, VIS- or IR-region means that more than 50% of the integral of the excitation curve or emission curve, respectively, of the pigment/dye lies in the UV-, VIS- or IR-region, respectively, irrespective in which region the excitation maximum or emission maximum, respectively, lies. UV-region means in accordance with the present invention the range between 100 and less than 380 nm, whereas the VIS-region means the range between 380 and less than 780 nm and the IR-region means the range between 780 and 1 ,700 nm.

Changing its fluorescence intensity upon exposition to laser radiation means that the pigment/dye has a first fluorescence intensity before the exposition to laser radiation and a second fluorescence intensity after the exposition to laser radiation, wherein the first fluorescence intensity is different to the second fluorescence intensity. Fluorescence intensity is in this connection the integral of the fluorescence emission curve. Both, the first and second fluorescence intensities refer to the fluorescence obtained after excitation with the same excitation wavelength and the same excitation intensity. Thus, if the laser writes in step iv) a digit, such as 8, on the printed substrate sheet, i.e. if the laser contacts in step iv) a portion of the surface of the printed substrate sheet comprising printing ink including the pig- ment/dye changing its fluorescence intensity upon exposition to laser radiation in the form of the digit 8, the area of this portion of the printed substrate sheet having been contacted with the laser radiation contains pigment/dye with the second, usually lower fluorescence intensity, whereas the surrounding area of this portion having been not in contact with the laser radiation contains pigment/dye with the first, usually higher fluorescence intensity. Therefore, as long as the respective portion of the banknote or tax stamp, respectively, is not irradiated with appropriate radiation exciting the pigment/dye changing its fluorescence intensity upon exposition to laser radiation, the digit 8 is not detectable, since the respective pigment/dye does not emit fluorescence. However, when the respective portion of the banknote or tax stamp, respectively, is irradiated with appropriate radiation exciting the pigment/dye changing its fluorescence intensity upon exposition to laser radiation, the digit 8 is detectable, because the fluorescence emission spectrum of the (lasered) pigment/dye in the area of the digit 8 differs from that of the surrounding (non-lasered) pigment/dye.

In another example of this embodiment, a QR code or barcode may be generated on a portion of the banknote/tax stamp comprising the printing ink comprising the pigment/dye changing its fluorescence intensity upon exposition to laser radiation. For instance, the black squares of the QR code or the lines of the barcode are generated by laser writing, i.e. by exposing these areas of the portion of the banknote/tax stamp comprising the printing ink comprising the pigment/dye changing its fluorescence intensity upon exposition to laser radiation with laser radiation, whereas the other areas of the portion are not exposed to laser radiation. While the QR code or barcode, respectively, is not visible, if the banknote/tax stamp is not excited with appropriate excitation radiation, the QR code or barcode, respec- tively, becomes detectable in the UV-, VIS- or IR- region (depending in which of the regions the pigment/dye emits fluorescence), because the fluorescence emission spectrum of the squares of the QR code or the lines of the barcode (which have been generated by the laser writing) have a different, usually lower, fluorescence intensity than the surrounding portions comprising the pigment/dye, but having been not contacted with the laser radiation in step iv).

In accordance with the present invention, the printing ink of at least one of the printing ink layers comprises in this embodiment at least one pigment and/or at least one dye, which reduces its fluorescence intensity upon exposition to laser radiation. Reducing its fluorescence intensity upon exposition to laser radiation means that the integral of the fluorescence emission curve after the laser radiation exposition is less than the integral of the fluorescence emission curve before the laser radiation exposition, both again being determined by excitation of the pigment/dye with the same excitation wavelength and the same excitation intensity. In accordance with the present invention the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 10%, preferably at least 20%, more preferably at least 30% and most preferably at least 40% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv). This allows to easily detect the difference between the lasered and the non-lasered portions of the printing ink containing the at least one pigment and/or at least one dye.

In accordance with another particular preferred embodiment of the present invention, in step ii) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which reduces its phosphorescence intensity upon exposition to laser radiation. Reducing its phosphorescence intensity upon exposition to laser radiation means that the integral of the phosphorescence emission curve after the laser radiation exposition is less than the integral of the phosphorescence emission curve before the laser radiation exposition, both again being determined by excitation of the pigment/dye with the same excitation wavelength and the same excitation intensity. In accordance with the present invention the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 10%, preferably at least 20%, more preferably at least 30% and most preferably at least 40% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv). This allows to easily detect the difference between the lasered and the non-lasered portions of the printing ink containing the at least one pigment and/or at least one dye.

In accordance with another particular preferred embodiment of the present invention, in step ii) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which reduces or increases its infrared absorption intensity upon exposition to laser radiation. Reducing its infrared absorption intensity upon exposition to laser radiation means that the integral of the infrared absorption curve after the laser radiation treatment is less than the integral of the infrared absorption curve before the laser radiation treatment, both again being determined under the same conditions, whereas increasing its infrared absorption intensity upon exposition to laser radiation means that the integral of the infrared absorption curve after the laser radiation treatment is higher than the integral of the infrared absorption curve before the laser radiation treatment, both again being determined under the same conditions. In accordance with the present invention the integral of the infrared absorption curve of the at least one pigment and/or at least one dye after step iv) is at least 10%, preferably at least 20%, more preferably at least 30% and most preferably at least 40% lower or higher than the integral of the infrared absorption curve of the at least one pigment and/or at least one dye before step iv). This allows to easily detect the difference between the lasered and the non-lasered portions of the printing ink containing the at least one pigment and/or at least one dye.

Preferably, the method in accordance with the present invention is performed continuously, semi-continuously or post-printing. Therefore, it is preferred that in step b) onto at least one of the two surfaces of the substrate sheet each one or more printing ink layers is printed so as to generate a plurality of banknotes or tax stamps on the substrate sheet, wherein in step iv) the printed and optionally cured (dried) substrate sheet is exposed to laser radiation so as to create at least one security feature on each banknote or each tax stamp, and wherein the method further comprises as step v) cutting the printed substrate sheet obtained in step iv) to individual banknotes or to individual tax stamps. Alternatively, the cutting step v) may be performed before the laser radiation step iv).

In a further development of the idea of the present invention, it is proposed that in step iv) two or more individual security features are generated, wherein all of the individual security features on each banknote or each tax stamp are generated by the exposition to laser radiation. Again, individual security feature means a security feature allowing to identify a specific banknote or tax stamp from all others, such as s serial number, a barcode or a QR code.

Thus, it is preferred that at least one security feature generated by the laser radiation in step iv) and preferably all of the at least one security feature on each banknote or on each tax stamps are selected from the group consisting of individual numbers each comprising at least two numerals and optionally one or more characters, of individual QR codes, of individual barcodes and arbitrary combinations of two or more of the aforementioned security features, wherein all of these security features are generated by the exposition to the laser radiation. However, it is also possible that one or more non-individual security features, such as a specific pattern or picture, are generated by the laser exposition in step iv) in the banknote or tax stamp. It is also possible to generated by the laser exposition in step iv) in the banknote or tax stamp both, one or more individual security features as well as one or more non-individual security features.

In order to further improve the protection against forgery, it is suggested in a further particularly preferred embodiment of the present invention that in step b) the printing ink(s) comprise(s) at least two different pigment(s) and/or dye(s), from which one is at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation and another one is at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity and/or its color upon exposition to laser radiation. The at least two different pigments/dyes may be contained in the same printing ink layer or in at least two different printing ink layers. For instance, two different pigments/dyes changing their luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation may be contained in one or different printing ink layers in step ii). Alternatively, two different pigments/dyes changing their infrared absorption intensity upon exposition to laser radiation may be contained in one or different printing ink layers, or one pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation and one pigment/dye changing its infrared absorption intensity upon exposition to laser radiation may be contained in one or different printing ink layers in step ii). This embodiment allows to generate two different security features being spatially separated on the banknote or tax stamp, respectively. Alternatively, two different security features may be generated, which overlap on the banknote or tax stamp, respectively. Still alternatively, one security feature may be generated, a portion of which being made by one of the two different pigments/dyes and anoth- er portion of which being made by the other one of the two different pig- ments/dyes.

In accordance with another particularly preferred embodiment of the present invention, at least one printing ink layer comprises in step b) at least one pigment and/or at least one dye, which changes its color upon exposition to laser radiation. Changing its color upon exposition to laser radiation means that the pigment/dye has a first color before the exposition to laser radiation and a second color after the exposition to laser radiation, wherein both colors are different to each other. Preferably, both colors differ by a delta E-value of more than 0.5, preferably by a AEoo-value of more than 0.5, more preferably by a AEoo-value of more than 1 , still more preferably by a AEoo-value of more than 2 and most more preferably by a AEoo-value of more than 5 so as to allow to easily detect the color change for instance with human eyes. This embodiment allows to even further improve the protection against forgery, since at least three different pigments/dyes are included, which allows several combinations. In accordance with a further preferred embodiment of the present invention, at least one printing ink layer comprises in step b) at least two different pigments and/or at least one dyes, which change their color upon exposition to laser radiation, wherein each of the two or more of pigment(s) and/or dye(s) forms a different color upon exposition to laser radiation with a given intensity and wavelength and/or signal shape (continuous or pulsed laser beam). This allows to generate during the laser writing step iv) security features with different colors. For instance, the first two characters of a serial number may have a different color than the numerals of the serial number or the second and fourth numeral of the serial number may have a different color than the numerals and optional characters of the serial number.

For instance, in step b), in one or in different printing ink layers, the following combinations of pigments and dyes, respectively, may be included: i) a pigment/dye changing its color upon exposition to laser radiation and a pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation, or ii) a pigment/dye changing its color upon exposition to laser radiation and a pigment/dye changing its infrared absorption intensity upon exposition to laser radiation, or iii) a pigment/dye changing its color upon exposition to laser radiation and two different pigments/dyes changing their luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation, or iv) a pigment/dye changing its color upon exposition to laser radiation and two different pigments/dyes changing their infrared absorption intensity upon exposition to laser radiation, or v) a pigment/dye changing its color upon exposition to laser radiation, a pigment/dye changing its infrared absorption intensity upon exposition to laser radiation and a pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation.

This allows for instance in the aforementioned alternative i) to generate on the banknote or tax stamp, respectively, a serial number and a barcode. The alpha- numerical serial number may comprise red digits and characters being generated during the laser radiation step iv) by lasering the digits and characters on a portion of the banknote/tax stamp comprising printing ink comprising the pigment/dye changing its color upon exposition to laser radiation, wherein the remaining non- lasered, non-numbered part of this portion has the other color of the pigment/dye changing its color upon exposition to laser radiation (for example: green) so that the serial number is visible. The barcode may be generated in this embodiment on a portion of the banknote/tax stamp comprising printing ink comprising the pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation, wherein the lines of the barcode are generated by laser writing. Thus, the barcode is not detectable, unless the banknote/tax stamp is excited with appropriate excitation radiation, whereupon the barcode is detectable in the UV-, VIS- or IR- region, depending in which of these regions the pig- ment/dye emits luminescence intensity (such as fluorescence), because the luminescence intensity (such as fluorescence) of the lines of the barcode (which have been generated by the laser writing) have a different, usually lower, luminescence intensity (such as fluorescence intensity) than the surrounding portions comprising the pigment/dye, but having been not exposed to the laser radiation in step iv).

In addition, this embodiment allows for instance in the aforementioned alternative v) to generate on the banknote or tax stamp, respectively, a serial number and two barcodes. The serial number may comprise, as described above, red digits and characters being generated during the laser radiation step iv) by lasering the digits and characters on a green background. The first (fine) barcode may be generated in this embodiment on a portion of the banknote/tax stamp comprising printing ink comprising the pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation and the second (less fine) barcode may be generated in a different portion of the comprising printing ink comprising the pigment/dye changing its infrared absorption intensity upon exposition to laser radiation. It is also possible to overlap the first (fine) barcode and the second (less fine) barcode by laser writing both on a portion of the banknote/tax stamp comprising printing ink comprising both, a pigment/dye changing its infrared absorption intensity upon exposition to laser radiation and a pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation. Consequently, the barcodes are not detectable, as long as the pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation is not excited with radiation having an appropriate excitation wavelength and as long as no appropriate infrared radiation is irradiated onto the banknote/tax stamp, which may be absorbed in a sufficient extent by the pigment/dye changing its infrared absorption intensity upon exposition to laser radiation. However, if the pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation is excited with radiation having an appropriate excitation wavelength, the first (fine) barcode is detectable and if appropriate infrared radiation is irradiated onto the banknote/tax stamp the second (less fine) barcode is detectable.

As set out above, the printing ink of at least one of the printing ink layers in step b) may comprise, but must not comprise at least one pigment and/or at least one dye, which changes its color upon exposition to laser radiation. Thus, in a further embodiment of the present invention, in step b) none of the printing inks of the printing ink layers comprises a pigment and/or dye, which changes its color upon exposition to laser radiation, but at least one pigment and/dye, which changes its infrared absorption intensity upon exposition to laser radiation, and/or at least one pigment and/dye, which changes its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation.

As set out above, the security feature and in particular individual security feature may be visible under ambient light without irradiating any excitation radiation or other extra-radiation in addition to the ambient light. This is the case, if the pig- ment/dye changing its color upon exposition to laser radiation is used in the security feature. Alternatively, this is the case, if the security feature is at least partially made by the pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation, if the pigment/dye is excitable by visible light and if the pigment/dye emits luminescence radiation (such as fluorescence radiation) in the visible light region. However, the security feature and in particular individual security feature is not visible under ambient light without irradiating any excitation radiation or other extra-radiation in addition to the ambient light, if the security feature is made by the pigment/dye changing its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation, when the pigment/dye is not excitable by visible light and/or the pigment/dye does not emit luminescence radiation (such as fluorescence radiation) in the visible light region. Also, the security feature and in particular individual security feature is not visible under ambient light without irradiating any excitation radiation or other extra-radiation in addition to the ambient light, if the security feature is made by the pigment/dye changing its infrared absorption intensity upon exposition to laser radiation. By combining respective security features made of different pigments/dyes a plurality of security feature combinations being detectable under different conditions may be generated so that none, one or more security features are visible with human eyes under ambient light, whereas one or more security features are (only) detectable after irradiating UV- and/or IR-radiation.

Moreover, it is proposed in a further development of the idea of the present invention to print in step ii) above the uppermost layer comprising at least one pigment/dye changing upon exposition to laser radiation its color and/or its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity a printing ink layer comprising a printing ink covering the lowermost printing ink layers. This embodiment is particularly preferred, if at least one printing ink layer comprises at least one pigment and/or dye changing its color upon exposition to laser radiation so as to hide the security feature generated by the pigment/ dye changing its color upon exposition to laser radiation.

In accordance with a particular preferred embodiment of the present invention, all printing ink layers including that or those printing ink layer(s) including the pig- ment(s)/dye(es) changing its/their luminescence intensity (such as fluorescence intensity) and/or its/their infrared absorption intensity and/or its/their color upon exposition to laser radiation for creating the security feature(s), which are later activated by the laser exposition step iv), are printed simultaneously or one after the other in one continuous printing process onto at least one (i.e. either onto one or onto each) of the two surfaces of the substrate sheet, before the printed substrate sheet is, in any order, cured and exposed to laser radiation. In this embodiment, all printing ink layers are simultaneously cured in step iii). Preferably, the printed sub- strate sheet is first cured and then exposed to laser radiation or is simultaneously cured and exposed to laser radiation. Moreover, it is preferred that all printing ink layers including that or those printing ink layer(s) including the pigment(s)/dye(es) changing its/their luminescence intensity (such as fluorescence intensity) and/or its/their infrared absorption intensity and/or its/their color upon exposition to laser radiation for creating the security feature(s), which are later activated by the laser exposition step iv), are printed onto at least one of the two surfaces of the substrate sheet on one print roller, i.e. that the method uses only one print roller. This one-step method is not only time saving, but also particularly economical.

However, even if generally less preferred, it is possible to first print only those printing ink layers not including the pigment(s)/dye(es) changing its/their luminescence intensity (such as fluorescence intensity) and/or its/their infrared absorption intensity upon exposition to laser radiation. Afterwards, the substrate sheet being printed with these printing ink layers may be cured, before one or more printing ink layers comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, is/are printed onto the uppermost printing ink layer of the sheet having been cured before. Afterwards, the printed sheet is cured again, before the laser writing step iv) is performed, or the laser writing step iv) is performed, before the printed sheet is cured again, or the printed sheet is cured again simultaneously to performing the laser writing step iv). In this embodiment, step iii) is performed in two sub-steps. This embodiment may be advantageous, if one or more UV-curing printing ink layers are used in combination with one or more non-UV curing printing ink layers. In this case, the non-UV curing printing ink layers are separately applied to the substrate and dried, either before or after the UV curing printing ink layers are applied to the substrate and cured upon exposure to UV radiation. Moreover, it is preferred that in step iv) the laser radiation is radiated onto the one side of the printed and cured substrate sheet, onto which printing ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, is printed, if only one side of the printed and cured substrate sheet comprises ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation. However, if both sides of the printed and cured substrate sheet comprise ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, it is preferred that the laser radiation is radiated onto both sides of the printed and cured substrate sheet by using two lasers. Thereby, the laser radiation may be applied with the lowest possible energy intensity, because the laser radiation is applied directly onto the printing ink comprising at least one pigment and/or at least one dye, which changes its color upon exposition to laser radiation, and does not first have to pass through the substrate sheet, before reaching the printing ink comprising at least one pigment and/or at least one dye, which changes its color upon exposition to laser radiation. Such a pass of laser radiation through the substrate sheet would be necessary, if the laser radiation would be applied onto the side of the substrate sheet being opposite to that onto which the printing ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, is applied. If the laser is applied onto the side of the substrate sheet being opposite to that onto which the printing ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, is applied, higher laser energy intensities have to be applied, which may decompose the pigments and/or dyes of the printing ink(s) and/or may decompose the substrate sheet. In accordance with another particularly preferred embodiment of the present invention, the printed and cured substrate sheet is exposed in step iv) to laser radiation having a wavelength of 200 to 1 ,100 nm, preferably of 200 to less than 800 nm. Good results are for example obtained, when the printed and cured substrate sheet is exposed in step iv) to laser radiation having a wavelength of 350 to 790 nm, such as of 355 nm, of 405 nm, of 485 nm or of 785 nm. This allows to generate security features, such as serial numbers, QR codes and/or barcodes with a high resolution, which are thus reliably machine detectable and readable with standard software. Thereby, even very small QR codes and barcodes of 5 x 5 mm or even smaller may be generated in high resolution. Even more preferably, the printed and cured substrate sheet is exposed in step iv) to laser radiation having a wavelength of 200 to 500 nm, in order to even further increase the resolution of the security feature(s).

In a further development of the idea of the present invention, it is proposed to generate on the banknote or tax stamp at least two security features, preferably at least two individual security features and more preferably at least two individual security features of the same category, i.e. for instance two barcodes or two QR codes, having each a different resolution. Furthermore, it is preferred that one of the security features has a sufficient low resolution so that it may be detected with human eyes, whereas at least another security feature has a resolution, which is so high or fine, respectively, that it may be not detected with human eyes, but only with devices, such as a high-resolution camera or a microscope. For instance, two barcodes are generated, wherein the first barcode has lines being distant from each other by more than 100 pm so that it is easily detectable with human eyes, whereas the second barcode has lines being distant from each other by at most 100 pm, preferably at most 10 pm, more preferably at most 20 pm, still more preferably at most 1 pm and most preferably at most 500 nm so that it is not detectable with human eyes. For instance, the first barcode may be generated with laser radiation of more than 500 nm, whereas the second barcode is generated with laser radiation having a wavelength of 200 to 500 nm. Both barcodes may be spatially apart from each other. Alternatively, both barcodes may overlap each other, for instance by providing the second fine barcode within the first barcode with lower resolution. While the first barcode with lower resolution may be made by a pigment changing its color upon exposition to laser radiation, so that it is visible under ambient light, the second finer barcode may be made by a pigment changing infrared absorption intensity and/or its luminescence intensity (such as fluorescence intensity), but being not excitable under ambient light, upon exposition to laser radiation, so that the second finer barcode is even with a high-resolution camera or microscope not dateable under ambient light.

In principle, the method in accordance with the present invention is flexible concerning the location of the at least one of the printing ink layers comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation. Accordingly, the at least one of the printing ink layers comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, may be the lowermost printing ink layer placed directly on a surface of the substrate sheet, may be the uppermost printing ink layer of the banknote or may be any printing ink layer between the lowermost and uppermost printing ink layer. Moreover, the at least one of the printing ink layers comprising at least one pigment and/or at least one dye, which changes its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation, may or may not contain any further pigments and/or dyes not changing their luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity and/or color upon exposition to laser radiation. It is preferred that at least one of the printing ink layers comprising the at least one pigment and/or at least one dye changing its lumines- cence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation is the uppermost printing ink layer on one of the two surfaces or on both of the surfaces of the substrate sheet.

As set out above, one, two or more printing ink layers comprising the at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation may be printed onto the substrate. Preferably, only one of the printing ink layers - and preferably one of the two uppermost printing ink layers on both surfaces of the substrate sheet - comprises the at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation and the others not.

In a further development of the idea of the present patent application it is suggested that the printing ink of the at least one of the printing ink layers comprising at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or its infrared absorption intensity upon exposition to laser radiation is transparent or colorless. This embodiment is particularly preferred if the uppermost printing ink layer on one surface of the sheet substrate comprises at least one pigment and/or at least one dye changing its color upon exposition to laser radiation. In the case of being transparent, the respective printing ink layer does not at all hide color and optional security features being detectable under ambient light of the one or more lower printing ink layers and in the later laser writing step iv), the security feature is generated in this transparent field and thus with high resolution. In the case of being colorless, but not transparent, the respective printing ink layer does hide color and optional security features being detectable in ambient light, but also generates in the later laser writing step iv) a clearly visible and highly resolved security feature. In accordance with a particularly preferred embodiment of the present invention, the luminescence intensity (such as fluorescence intensity), which the at least one pigment and/or at least one dye a), which changes its fluorescence intensity upon exposition to laser radiation has after termination of the exposition to laser radiation and/or the infrared absorption intensity, which the at least one pigment and/or at least one dye b), which changes its infrared absorption intensity upon exposition to laser radiation, has after termination of the exposition to laser radiation depends on the wavelength and/or the intensity of the laser so that the at least one pigment and/or at least one dye a) may change its luminescence intensity (such as fluorescence intensity) upon exposition to laser radiation to one of a plurality of luminescence intensities (such as fluorescence intensities) and/or the at least one pigment and/or at least one dye b) may change its infrared absorption intensity upon exposition to laser radiation to one of a plurality of infrared absorption intensities. This allows to generate in the later laser writing step iv) security features having at least two different luminescence intensities (such as fluorescence intensities) and/or infrared absorption intensities, even if only one pigment or dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation is included in the respective printing ink layer.

In the aforementioned embodiment, it is preferred that the printed and cured substrate sheet is exposed in step iv) to at least two different laser radiation wavelengths and/or to at least two different laser radiation intensities so as to create one or more preferably individual security features serial numbers, each comprising two or more portions having different luminescence (such as fluorescence) and/or infrared absorption intensities.

The at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation may be any pigment or dye which changes its lumines- cence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation, independent from the underlying mechanism. For instance, even if least preferred, the pigment or dye may decompose upon exposition to laser radiation and thereby changes its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity. For example, the pigment or dye may carbonize during the exposure to laser radiation. However, it is more preferred that the pigment or dye changes its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity without decomposition.

Preferably, the pigment or dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation may be part of a reactive system, which forms upon exposition to laser radiation a fluorescence intensity and/or infrared absorption intensity change. In this embodiment, it is preferred that at least one of the printing ink layers comprising the at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation further comprises at least one sensitizer. The at least one sensitizer is preferably a light-to-heat convertor, which generates heat upon exposure to laser light. In this embodiment, the heat generated by the sensitizer may activate a decomposition of the pigment or dye, thus leading to a luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity change of the pigment or dye. Alternatively, heat generated by the sensitizer may activate a reaction between the pigment or dye with another reactant leading to reaction products having another luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity than the pigment or dye. The aforementioned reaction may be activated by the heat generated by the sensitizer upon exposition to laser radiation by selecting the dye/pigment and reactant so that a luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity changing chemical reaction is initiated at ambient temperature, but initiat- ed by the heat provided by the sensitizer upon exposure to laser radiation by providing the necessary activation energy.

Good results are in particular obtained, when the at least one sensitizer is an infrared absorbing dye. The at least one infrared absorbing dye is preferably selected from the group consisting of polymethyl indoliums, metal complex infrared dyes, indocyanine green, polymethine dyes, croconium dyes, cyanine dyes, merocya- nine dyes, squarylium dyes, chalcogeno- pyryloarylidene dyes, metal thiolate complex dyes, quinoline dyes, indolenine dyes, bis(chalcogenopyrylo)-polymethine dyes, oxyindolizine dyes, bis(aminoaryl)polymethine dyes, indolizine dyes, pyrylium dyes, quinoid dyes, quinone dyes, phthalocyanine dyes, naphthalo- cyanine dyes, azo dyes, (metalized) azomethine dyes, benzoxaquinone dyes, ox- inates, quinazolinoes, porphyrines, porphyra-zines, carbon black, graphite, indiumoxide, indium tin oxide, indium anti-mony tin oxide, iron oxides, iron phosphates, doped tungsten oxides, tungsten bronzes, tungsten oxide phosphate, dicopper hydroxy phospha-tes, doped zinc sulphides, rare earth doped ceramics and arbitrary combinations of two or more of the aforementioned compounds. Preferred examples for suitable sensitizers of the infrared absorbing dye-type are polymethine dyes and benzo[cd]indoline dyes, such as 5-[2,5-bis[2-[1-(1- methylbutyl)-benz[cd]indol-2(1 H)-ylidene]ethylidene]-cyclopentylidene]-1-butyl-3- (2-methoxy-1 -methylethyl)- 2, 4, 6(1 H,3H,5H)-pyrimidinetrione.

Good results are also obtained, when the at least one sensitizer is an infrared absorbing pigment. The at least one infrared absorbing pigment is preferably a carbon black, a cyanine pigment, a merocyanine pigment or a compound being selected from the group consisting of oxides, hydroxides, sulfides, sulfates and phosphates of metals such as copper, bismuth, iron, nickel, tin, zinc, manganese, zirconium, tungsten, lanthanum and antimony. More preferably, the infrared absorbing pigment is carbon black, such as acetylene black, channel black, furnace black, lamp black or thermal black. The dso-particle diameter of the infrared ab- sorbing pigment is preferably 0.02 to 5 pm, whereas the concentration of the infrared absorbing pigment is preferably 0.001 to 0.1% by weight based on the total weight of the (wet) printing ink of the respective printing layer.

As set out above, the luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity change during the laser writing step iv) may be achieved by a reaction of the pigment and/or dye initiated upon exposure to the laser radiation with another reactant, thereby forming reaction products having another luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity than the educts. It is preferred in this embodiment that at least one of the printing ink layers comprising the at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation further comprises at least one developer compound and/or at least one thermal acid generating compound, wherein the developer compound and/or thermal acid generating compound chemically reacts with the pigment and/or dye so as to cause a luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity change of respective cured printing ink layer. This embodiment may in addition use in the printing ink of the respective printing ink layer - in addition to the luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity changing pigment and/or dye and in addition to the at least one developer compound and/or at least one thermal acid generating compound - one or more sensitizers as described in detail above. The one or more sensitizers convert upon exposure with laser radiation the laser radiation into heat, which activates the chemical reaction between i) the luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity changing pigment and/or dye and ii) the developer compound and/or thermal acid generating compound.

Preferably, the least one developer compound is a phenolic compound, an organic acidic compound, an inorganic acidic compound or an ester or salt thereof. Specif- ic examples for suitable developer compounds are bisphenol compounds, gallic acid, salicylic acid, salicylate compounds, monophenol compounds, catechol, catechol compounds, resorcin, hydroquinone, pyrogallol, fluoroglycine, fluoroglycine carboxylates, sulfone compounds, tartaric acid, oxalic acid, maleic acid, citric acid, succinic acid, stearic acid, 4-hydroxyphthalic acid, boric acid, thiourea compounds and arbitrary combinations of two or more of the aforementioned compounds. The concentration of the developer compound based on the total weight of the (wet) printing ink layer is preferably 0.1 to 10% by weight and more preferably 0.5 to 2% by weight.

Preferably, the least one thermal acid generating compound are a sulfonate esters, phosphonate esters, iodonium salts, sulfonium salts, ferrocenium salts, sulfonyl oximes, halomethyl triazines, halomethyl-arylsulfones, haloacetophenones, sulfonate esters, t-butyl esters, allyl substituted phenols, t-butyl carbonates and phosphate esters. The concentration of the thermal acid generating compound based on the total weight of the (wet) printing ink layer is preferably 0.1 to 5% by weight and more preferably 0.5 to 2 % by weight.

In a further development of the idea of the present patent application it is proposed that the at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation is selected from the group consisting of spirobenzopyrans, spironaphtooxazines, spirothiopyrans, quinone dyes, oxazines, diazines, thiazines, phenazine, triarylmethane phtalides, diarylmethane phthalides, monoarylmethane phthalides, heterocyclic substituted phthalides, alkenyl substituted phthalides, bridged phthalides, bisphthalides, fluoresceins, rhodamines, rhodols, crystal violet, ketazines and arbitrary combinations of two or more of the aforementioned compounds. Suitable examples for dyes changing their luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposure to laser radiation without needing to be encapsulated into capsules and not requiring a developer or thermal acid generating compound, are mixed carbonate esters of a quinophthalone and a tertiary alkanol containing not more than about 9 carbon atoms and thermally instable carbamate compounds. These compounds undergo a fragment reaction or an irreversible molecular change, respectively, upon heat, such as generated by laser radiation with or without use of a sensitizer compound. Suitable thermal acid generating compounds for these dyes are those mentioned above.

As set out above, the printing ink layer changing its their luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposure to laser radiation may contain at least two different dyes/pigments changing their luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposure to laser radiation. In this embodiment, the respective printing ink may contain, depending from the nature of the dyes/pigments, at least two different sensitizers as mentioned above and/or at least two different developer compounds as mentioned above and/or at least two different thermal acid generating compound as mentioned above.

Preferably, the printing ink of the at least one of the printing ink layers comprising at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation has a total concentration of pigments and dyes changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation of 0.01 to 10% by weight, preferably 0.5 to 5% by weight and more preferably 1 to 3%. The thickness of the at least one of the printing ink layers comprising at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation is preferably 0.5 to 25 pm and more preferably 1 to 10 pm.

The thickness of any optional further printing ink layer not containing at least one pigment and/or at least one dye changing its luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity upon exposition to laser radiation, is preferably 1 to 20 pm and more preferably 2 to 15 pm.

The present invention is not particularly limited concerning the printing technique, with which the (security) printing ink layers are applied onto the substrate sheet. In particular, the (security) printing ink(s) may be formulated as printing ink being selected from the group consisting of offset inks, intaglio inks, die embossing inks, flexographic inks and screen inks.

In addition to the pigment(s) and/or dye(s), the printing ink(s) used for the at least one printing ink layer in accordance with the present invention include one or more binders, wherein the binder(s) is/are preferably selected from the group consisting of polyesters, polyethers, polyurethanes, polyamides, polyacrylates, maleinate resins, collophonium resins, ketone resins, alkyd resins, collophonium modified phenolic resins, hydrocarbon resins, silicates, silicones, silanes, phenolic resins, urea resins, melamine resins, polyterpene resins, polyvinylalcohols, polyvinylacetates, polyvinylchloride, polyvinylethers, polyvinylpropionates, polymethacrylates, polystyrenes, polyolefines, coumarone-indene resins, aromatic formaldehyde resins, carbamide acid resins, sulfonamide resins, chlorinated resins, nitrocellulose, CAB (cellulose acetate butyrate), CAP (cellulose acetate propionate), cellulose compounds, rubbers, radiation curing resins and arbitrary combinations of two or more of the aforementioned binders. Moreover, it is preferred that at least one of and preferably all of the printing inks of the printing ink layers contain at least one solvent, which is preferably one or more vegetable oils.

In accordance with a further preferred embodiment of the present invention, at least one of and preferably each of the printing inks of the printing ink layers comprises: i) 0.01 to 50% by weight and preferably 0.5 to 30% by weight in sum all pigments and dyes, ii) 10 to 40% by weight and preferably 20 to 30% by weight of one or more binders, iii) 20 to 60% by weight and preferably 30 to 40% by weight of one or more solvents and iv) optionally 0.1 to 10% by weight of one or more additives being selected from the selected from the group consisting of rheological additives, adhesives, defoamers, slip additives, anti-corrosion additives, gloss additives, waxes, wetting agents, curing agents, chelating agents, photoinitiators, inhibitors, desiccants, stabilizers, emulsifiers, pH adjustment additives, abrasion resistance additives, plasticizers, antistatic additives, preservatives, light protection agents, matting agents and arbitrary combinations of two or more of the aforementioned additives.

A suitable offset ink may comprise:

Sum of pigments and dyes: 0.01 to 50% by weight

Filler: 0 - 5% by weight

Alkyd resin: 15 - 20% by weight

Vegetable oil: 5 - 15% by weight

Phenolic modified rosin resin: 10 - 20% by weight

Wax: 0.5 - 5% by weight

Hydrochinon stabilizer: 0 - 1 .5% by weight Oxidatively drying agent: 1 - 2% by weight

Suitable colored pigments not changing their luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity and/or color upon exposure to laser radiation are available from BASF SE, from Clariant Plastics & Coatings Ltd., from Heubach GmbH and from Ferro Performance Pigments, S.L.. Pigments in the form of titanium dioxide can be purchased from The Chemours Company TT, LLC, from The Kerala Minerals & Metals Ltd. and from Shandong Doguide Group Co., Ltd.

Suitable fillers are available e.g. from Evonik Industries AG, Krahn Chemie GmbH, BCD Chemie GmbH, Omya AG, Bassermann minerals GmbH & Co. KG, BYK- Chemie GmbH, Elementis pic and Solvay GmbH.

Alkyd resins can be purchased e.g. from Lawter, Inc., Allnex Resins Germany GmbH, Synthopol Chemie Germany and Dr. rer. pol. Koch GmbH & Co. KG.

Suitable vegetable oils are e.g. calendula, canola, castor, china wood, coconut, cottonseed, dehydrated castor, flaxseed, grape seed, linseed, palm, palm kernel, peanut, rapeseed, oiticica, safflower, soyabean, sunflower, tall and tung supplied e.g. by Alberdingk Boley GmbH and Mercur Handel GmbH.

Eligible phenolic modified rosin resins are available e.g. from Lawter, Inc., Respol Resinas, S.A. and Euro-Yser - Produtos Quimicos, S.A..

Waxes and stabilizers may also be contained in the offset inks. Suitable and suppliers for waxes are e.g. euroceras Sp. z o.o. and Eastman Chemical Company and Clariant Plastics & Coatings Ltd. Stabilizers are obtained from Eastman Chemical Company, Ratnagiri Chemicals Pvt. Ltd. and Merck KGaA. Oxidatively drying agent like metal carboxylates or metal soaps of e.g. cobalt, manganese, iron, vanadium, lead, zirconium, lithium or strontium, cerium, aluminium, potassium, calcium, barium or zink are available e.g. from OMG Borchers GmbH.

A suitable UV-curing water-based screen ink may comprise:

Sum of pigments and dyes except pearlecent pigments: 0.1 to 25% by weight Pearlescent pigment: 6 - 15% by weight

Urethane acrylate dispersion: 60 - 80% by weight

Wetting additive: 0.1 - 2.5% by weight

Substrate wetting additive: 0.1 - 2.5% by weight

Defoamer: 0.1 - 2.5% by weight

Photoinitiator: 2 - 10% by weight

Suitable urethane acrylate dispersions are available e.g. from Sartomer Europe - Arkema, Allnex Resins Germany GmbH or Alberdingk Boley GmbH.

Suitable wetting agents are available e.g. from Evonik Industries AG, BYK-Chemie GmbH, Munzing Chemie GmbH and Elementis pic.

Suitable defoamers for water-based formulations are available e.g. from Evonik Industries AG, BYK-Chemie GmbH, Munzing Chemie GmbH and Elementis pic.

Suitable photoinitiators for radically curing systems are supplied e.g. by IGM Resins B.V. or Lambson Ltd.

A suitable cationically radiation-curing screen ink may comprise:

Sum of pigments and dyes: 0.01 to 30% by weight

Pearlescent pigment: 0 - 25% by weight Cycloaliphatic epoxide resin: 60 - 80% by weight

Fumed silica: 0.5 - 10% by weight

Defoamer: 0.1 - 5% by weight

Photosensitizer: 0.1 - 2% by weight

Photoinitiator: 2 - 10% by weight

Cycloaliphatic epoxide resins are available e.g. form Dow Chemical Company, Gabriel Chemical or IGM Resins B.V. Fumed silica is supplied e.g. by Evonik Industries AG and Orisil Ltd.

Defoamers are available e.g. from Evonik Industries AG and BYK-Chemie GmbH.

Photosensitizers are supplied e.g. by Lambson Ltd, IGM Resins B.V., Merk KGaA and TCI Deutschland GmbH.

Suitable photoinitiators are available e.g. from Dow Chemical Company, Lambson Ltd, IGM Resins B.V. and BASF SE. A suitable intaglio ink may comprise:

Sum of pigments and dyes: 0.5 to 55% by weight

Filler 20 - 60% by weight

Alkyd resin 5 - 30% by weight

Vegetable oil 5 - 25% by weight

Phenolic modified rosin resin 5 - 10% by weight

Wax 3 - 15% by weight

Drier 0.1 - 2.5% by weight

Diluent 1 - 10% by weight Suitable colored pigments not changing their luminescence intensity (such as fluorescence intensity) and/or infrared absorption intensity and/or color upon exposure to laser radiation are available e.g. from BASF SE, Clariant Plastics & Coatings Ltd, Heubach GmbH and Ferro Performance Pigments, S.L. Pigments in the form of titanium dioxide are available e.g. from The Chemours Company TT, LLC, The Kerala Minerals & Metals Ltd. and Shandong Doguide Group Co., Ltd.

Alkyd resins can be purchased e.g. from Lawter, Inc., Allnex Resins Germany GmbH and Synthopol Chemie - Dr. rer. pol. Koch GmbH & Co. KG.

Suitable vegetable oils are e.g. calendula, canola, castor, china wood, coconut, cottonseed, dehydrated castor, flaxseed, grape seed, linseed, palm, palm kernel, peanut, rapeseed, oYticica, safflower, soyabean, sunflower, tall and tung supplied e.g. by Alberdingk Boley GmbH and Mercur Handel GmbH.

Eligible phenolic modified rosin resins are available e.g. from Lawter, Inc., Respol Resinas, S.A., Arizona Chemical Ltd. and Euro-Yser - Produtos Quimicos, S.A.

Waxes and stabilizers may also be contained in the intaglio inks. Suitable suppliers for waxes are e.g. from euroceras Sp. z o.o. and Clariant Plastics & Coatings Ltd and BYK-Chemie GmbH.

Oxidative driers like metal carboxylates or metal soaps of e.g. cobalt, manganese, iron, vanadium, lead, zirconium, lithium or strontium, cerium, aluminium, potassium, calcium, barium or zink are available e.g. from OMG Borchers GmbH. Suitable diluents or thinners are available e.g. from Shell Global Solutions International B.V., Total S.A. and Alberdingk Boley GmbH.

An effect pigment can also be incorporated.

The other components are preferably those mentioned above for the offset security ink composition.

The present invention is not particularly restricted concerning the kind of substrate, as long as it is a (optional security) paper sheet, a (optional security) polymer foil or a (optional security) composite comprising at least one paper layer sheet and at least one polymer foil. Sheet, such as paper sheet, may have the form and dimensions of a banknote or tax stamp, i.e. a banknote and a tax stamp are considered as a sheet. The polymer of the polymer foils may be in particular selected from the group consisting of polycarbonate foils, polyethylene terephthalate foils, composites of a lower security paper and an upper polymer foil, composites of a lower polymer foil and an upper (security) paper, composites of a lower security paper, an intermediate polymer foil and an upper security paper, composites of a lower polymer foil, an intermediate security paper and an upper polymer foil and hybrids of a security paper, in which a part of the security paper is replaced by a polymer foil. Security paper usually means a paper made of cotton wool and usually further containing a security feature, such as a watermarking.

The curing of the printing ink layers is preferably performed in case of non-UV curing printing inks by drying at a temperature between 23 and 130°C and preferably between 23 and 60°C, preferably in a heating room, in an oven or in any other heated space. In case of UV curing printing inks, the curing is performed by exposing the printing ink layers to UV radiation, for instance to UV radiation having an energy intensity of 200 to 400 J/cm². Principally, the present invention is not particularly limited concerning the kind, how the one or more security features and preferably individual security features are generated in step iv) by laser writing, i.e. by exposing the printed and cured substrate sheet to laser radiation. Preferably, in step iv) one or more laser beams are moved relatively to the moving or non-moving substrate sheet so that on the printed and cured substrate sheet one or more security features and preferably individual security features are generated. The number of laser beams required, depend on the width of the printed and cured substrate sheet and may be between 1 and 10 laser beams and preferably between 1 and 6 laser beams. The movement of the one or more laser beams relative to the substrate sheet may be effected by only moving the laser beam(s) or by moving the laser beam(s) as well as moving the substrate sheet, for instance by rotating the substrate sheet on a cylindrical drum or in the transport area of the sheets after the curing. The movement of the one or more laser beams may be achieved by an appropriate arrangement of movable mirrors and/or movable lenses, which are controlled by a computer software. Moreover, a commercially available three-dimensional multi-sensor positioning system may be used, such as NMM-1 distributed by Sios Messtechnik GmbH, Ilmenau, Germany. If more than one laser beam is used, the laser beams may have different wavelengths. For instance, a YAG laser may be used having a base wavelength of 1 ,064 nm which leads by frequency doubling to 532 nm and/or even by triplication to 355 nm.

Any appropriate laser may be used, such as a femtosecond laser, such as a titani- ur saphir-femtosecond laser, as it is distributed by Integral Pro, FEMTOLASERS Produktions GmbH, Wien, Austria. Preferably, in step iv) one or more pulsed laser beams are used, each of which preferably having a pulse duration of 1 fs to 1 ns and preferably of 100 fs to 1 ps. This results in a precise and highly space- resolved security features and preferably individual security features, such as serial number, QR codeor barcode. In a further development of the idea of the present invention it is proposed that in step iv) one or more pulsed laser beams are used, each of which having a wavelength of 200 nm to 2,000 nm and preferably of 250 to 1 ,100 nm. As set out further above, it is particularly preferred that the printed and cured substrate sheet is exposed in step iv) to laser radiation having a wavelength of 200 to 1 ,100 nm, preferably of 200 to less than 800 nm and more preferably of 350 to 790 nm, such as of 355 nm, of 405 nm, of 485 nm or of 785 nm. This allows to generate security features and preferably individual security features with a high resolution, which are thus reliably machine detectable and readable with standard software. Thereby, even very small QR codes and barcodes of 5 x 5 mm or even smaller may be generated in high resolution. Preferably, the wavelength can be varied so that two or more laser-sensitive pigments or dyes or sensitizers may be activated, if a security feature having two or even more different luminescence intensities (such as fluorescence and/or infrared absorption intensities) shall be generated.

In accordance with a further preferred embodiment of the present invention, in step iv) one or more pulsed laser beams are used, each of which having energies of 10 mW to 1 kW and preferably of 100 mW to 100 W. Preferably, the energy intensity can be varied so that two or more laser-sensitive pigments or dyes or sensitizers may be activated, if a security feature having two or even more different luminescence intensities (such as fluorescence and/or infrared absorption intensities) shall be generated.

Preferably, in step iv) on every banknote printed on the substrate sheet each one or more serial numbers and optionally one or more QR codes and/or one or more barcodes and optionally further individual security features are generated.

A further aspect of the present invention is a banknote obtainable with the aforementioned method.

Claims

1 . A method for generating at least one security feature on a banknote or a tax stamp, wherein the method comprises the following steps: i) providing a substrate sheet in form of a sheet of paper or of a polymer foil or a composite comprising at least one paper layer and at least one polymer foil, ii) printing onto at least one of the two surfaces of the substrate sheet each one or more printing ink layers so as to generate at least one banknote or at least one tax stamp on the substrate sheet, wherein each printing ink layer extends over a part or the whole of the surface area of the substrate sheet, and wherein the printing ink of at least one of the printing ink layers comprises a) at least one pigment and/or at least one dye, which changes its luminescence intensity upon exposition to laser radiation so that the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 10% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv) and/or b) at least one pigment and/or at least one dye, which changes its infrared absorption intensity upon exposition to laser radiation so that the integral of the emission curve of the at least one pigment and/or at least one dye after step iv) is at least 10% lower or at least 10% higher than the integral of the emission curve of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv), iii) curing the printing ink layers, iv) exposing the printed substrate sheet obtained in step ii) before, during and/or after the curing step iii) to laser radiation so as to create at least one security feature on the at least one banknote or on the at least one tax stamp of the printed substrate sheet. The method in accordance with claim 1 , wherein i) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which reduces its fluorescence intensity upon exposition to laser radiation so that the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 20%, preferably at least 30% and most preferably at least 40% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv), and/or wherein ii) the printing ink of at least one of the printing ink layers comprises a) at least one pigment and/or at least one dye, which reduces its phosphorescence intensity upon exposition to laser radiation so that the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye after step iv) is at least 20%, preferably at least 30% and most preferably at least 40% lower than the integral of the emission curve obtained after excitation of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv) and/or wherein iii) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which reduces or increases its infrared absorption intensity upon exposition to laser radiation so that the integral of the emission curve of the at least one pigment and/or at least one dye after step iv) is at least 20%, more preferably at least 30% and most preferably at least 40% lower or higher than the integral of the emission curve of the at least one pigment and/or at least one dye with the same excitation wavelength and the same excitation intensity before step iv). The method in accordance with claim 1 or 2, wherein in step b) onto at least one and preferably onto each of the two surfaces of the substrate sheet each one or more printing ink layers is printed so as to generate a plurality of banknotes or tax stamps on the substrate sheet, wherein in step iv) the printed and optionally cured (dried) substrate sheet is exposed to laser radiation so as to create at least one security feature on each banknote or each tax stamp, and wherein the method further comprises as step v) cutting the printed substrate sheet obtained in step iv) to individual banknotes or to individual tax stamps. The method in accordance with any of the preceding claims, wherein in step iv) two or more individual security features are generated, wherein all of the individual security features on each banknote or each tax stamp are generated by the exposure to laser radiation in step iv). The method in accordance with any of the preceding claims, wherein at least one security feature generated by the laser radiation in step iv) and preferably all of the at least one security feature on each banknote or on each tax stamps are selected from the group consisting of individual numbers each comprising at least two numerals and optionally one or more characters, of individual QR codes, of individual barcodes and arbitrary combinations of two or more of the aforementioned security features, wherein all of these security features are generated by the exposure to the laser radiation. . The method in accordance with any of the preceding claims, wherein in step b) the printing ink(s) comprise(s) at least two different pigment(s) and/or dye(s), from which one is at least one pigment and/or at least one dye changing its luminescence intensity, preferably fluorescence intensity, and/or its infrared absorption intensity upon exposition to laser radiation and another one is at least one pigment and/or at least one dye changing its luminescence intensity, preferably fluorescence intensity, and/or its infrared absorption intensity and/or its color upon exposition to laser radiation.

7. The method in accordance with any of the preceding claims, wherein in step b) the printing ink of at least one of the printing ink layers comprises at least one pigment and/or at least one dye, which changes its color upon exposition to laser radiation.

8. The method in accordance with claim 7, wherein in step b) in one or in different printing ink layers the following combinations of pigments and dyes, respectively are included: i) a pigment/dye changing its color upon exposition to laser radiation and a pigment/dye changing its luminescence intensity, preferably fluorescence intensity upon exposition to laser radiation, or ii) a pigment/dye changing its color upon exposition to laser radiation and a pigment/dye changing its infrared absorption intensity upon exposition to laser radiation, or iii) a pigment/dye changing its color upon exposition to laser radiation and two different pigments/dyes changing their luminescence intensity, preferably fluorescence intensity upon exposition to laser radiation, or iv) a pigment/dye changing its color upon exposition to laser radiation and two different pigments/dyes changing their infrared absorption intensity upon exposition to laser radiation, or v) a pigment/dye changing its color upon exposition to laser radiation, a pigment/dye changing its infrared absorption intensity upon exposition to laser radiation and a pigment/dye changing its luminescence intensity, preferably fluorescence intensity upon exposition to laser radiation. The method in accordance with any of the preceding claims, wherein in step ii) all printing ink layers including that or those printing ink layer(s) including the pigment(s)/dye(es) changing its/their luminescence intensity, preferably fluorescence intensity, and/or infrared absorption intensity upon exposition to laser radiation are printed simultaneously or one after the other in one continuous printing process onto at least one of the two surfaces of the substrate sheet, preferably using only one print roller, and, wherein in step iv) the laser radiation is radiated onto the one side of the printed and cured substrate sheet, onto which printing ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity, preferably fluorescence intensity, and/or infrared absorption intensity upon exposition to laser radiation, is printed, if only one side of the printed and cured substrate sheet comprises printing ink comprising at least one pigment and/or at least one dye, which changes its luminescence intensity, preferably fluorescence intensity, and/or infrared absorption intensity upon exposition to laser radiation, or the laser radiation is radiated onto both sides of the printed and cured substrate sheet, if both sides of the printed and cured substrate sheet comprise printing ink comprising at least one pigment and/or at least one dye, which changes its color upon exposition to laser radiation. The method in accordance with any of the preceding claims, wherein the printed and optionally cured substrate sheet is exposed in step iv) to laser radiation having a wavelength of 200 to 800 nm, preferably of 200 to less than 800 nm and more preferably of 200 to 500 nm or of 350 to 790 nm. The method in accordance with any of the preceding claims, wherein the luminescence intensity, preferably fluorescence intensity, which the at least one pigment and/or at least one dye a), which changes its luminescence intensity, preferably fluorescence intensity upon exposition to laser radiation has after termination of the exposition to laser radiation, and/or the infrared absorption intensity, which the at least one pigment and/or at least one dye b), which changes its infrared absorption intensity upon exposition to laser radiation, has after termination of the exposition to laser radiation depends on the wavelength and/or the intensity of the laser so that the at least one pigment and/or at least one dye a) may change its luminescence intensity, preferably fluorescence intensity, upon exposition to laser radiation to one of a plurality of luminescence intensities, preferably fluorescence intensities, and/or the at least one pigment and/or at least one dye b) may change its infrared absorption intensity upon exposition to laser radiation to one of a plurality of infrared absorption intensities. The method in accordance with any of the preceding claims, wherein the at least one pigment and/or at least one dye changing its luminescence intensity, preferably fluorescence intensity, and/or infrared absorption intensity upon exposition to laser radiation is selected from the group consisting of spirobenzopyrans, spironaphtooxazines, spirothiopyrans, quinone dyes, oxazines, diazines, thiazines, phenazine, triarylmethane phtalides, diarylmethane phthalides, monoarylmethane phthalides, heterocyclic substituted phthalides, alkenyl substituted phthalides, bridged phthalides, bisphthalides, fluoresceins, rhodamines, rhodols, crystal violet, ketazines and arbitrary combinations of two or more of the aforementioned compounds. The method in accordance with any of the preceding claims, wherein in step iv) one or more pulsed laser beams are used, each of which preferably having a pulse duration of 1 fs to 1 ns and preferably of 100 fs to 1 ps and/or each of which having a wavelength of 200 nm to 2,000 nm, preferably of 250 to 1 ,100 nm, more preferably of 200 to less than 800 nm and yet more preferably of 350 to 790 nm. The method in accordance with any of the preceding claims, wherein in step iv) one or more pulsed laser beams are used, each of which having an en- ergy intensity of 10 mW to 1 kW and preferably of 100 mW to 100 W. A banknote or tax stamp obtainable with a method in accordance with any of the preceding claims.