EP4223548A1

EP4223548A1 - Method for producing of marked paper substrate with the mark in the form of a watermark and a marked security paper substrate, in particular a security paper, obtained by such a method

Info

Publication number: EP4223548A1
Application number: EP22217014.4A
Authority: EP
Inventors: Kamil GAWEL; Magdalena Lula
Original assignee: Polska Wytwornia Papierow Wartosciowych SA
Current assignee: Polska Wytwornia Papierow Wartosciowych SA
Priority date: 2021-12-29
Filing date: 2022-12-28
Publication date: 2023-08-09
Also published as: PL440024A1

Abstract

Method for manufacturing a paper substrate with a marking, with the steps: providing a paper substrate which can be fibrous paper; applying a mark to the substrate by irradiating it with an ultra-fast laser which is a femtosecond laser with a pulse below 400 fs, whose beam modifies the paper substrate, the mark being obtained by selective, local removal of paper substrate layers in the area defined on the basis of the desired image, and wherein the thinning of the paper substrate obtained by selectively locally removing layers of the paper substrate to produce the watermark in irradiating step is less than the thickness of the paper substrate, and the area over which it is applied does not differ in colour from the area not processed with an ultrafast laser. The resolution can be 900-1500 DPI. The paper can be uncoated and have 40-700 g.

Description

The present invention relates to a method for the manufacture of paper substrate provided with a marking in the form of a watermark, in particular a data carrier for documents with security features, in particular security documents with security features, the marking preferably comprising personal data and/or individualizing markings, and a secured paper substrate, in particular secured paper, obtained by such a method.

Prior art

Secured documents, such as securities or identification documents, confirming the existence of certain rights, legal relationships, or legal events expressed on specific information carriers, are currently an indispensable element of life in developed countries. Their importance is constantly increasing, as such documents find many applications and their number is constantly growing.
Moreover, despite the constant digitization of all spheres of human life, including in particular the popularization of non-cash payments in circulation, such secured documents as banknotes still remain and constitute an important means of payment.
Many of the identification or value documents comprise a paper substrate bearing at least one security marking selected from a wide variety of security features. Known examples of such security features include watermarks, built-in and windowed security threads, fluorescent pigments, and the like, of which a particularly useful feature used in paper substrates for hundreds of years to provide protection against counterfeiting of secured documents or banknotes is the watermark.
Watermarks, which are recognizable images or patterns on paper when viewed in transmitted light or reflected light against a dark background, appear as varying shades of bright/dark. In fact, watermarks and their interesting designs are the most easily identifiable security features used to authenticate any secured documents such as banknotes, available to the general public.
The methods of creating a watermark are also widely known and used in practice. For example, in the case of the most traditional manufacturing methods, in the first step of the method, it is necessary to develop a graphic design of such a sign, based on which a die containing this desired sign in the form of embossing (a relief) is produced. Such a die consists of two parts - a die and a counter-die with mutual fitting. The die is used to press the screen on the press, during which the wire is placed between the die and the counter-die and the mark is embossed by pressure. The wire with embossed characters finally forms a cylinder which, on rotating in the paper machine, forms a paper web, while the wire drains the water and retains the cotton/cellulose fibre fraction constituting the paper pulp. Then, if an embossing/relief is formed on the wire, a smaller layer of pulp is retained on the wire than in the rest of the area. Similarly, a smaller layer of pulp is retained in the depressions than in the rest of the area. This causes a local increase and reduction in basis weight in specific areas of the paper web produced. Such a change is observed in transmitted light as a watermark. The formed paper pulp is removed from the wire with a felt, due to which the paper obtained in this way has two sides, differing in structure - the screen side and the felt side. The watermark obtained in this standard way differs in appearance depending on the side from which it is observed. While it is clear when viewed from the wire side, when viewed from the felt side, it provides a less structured, non-sharp-edged watermark image. It should be emphasized that the structure of the paper obtained by this method may be different, depending on the composition of the fibrous material used for paper production and the technology of further processing of the raw paper, e.g. its coating, smoothing, etc. The thickness of the paper in the area of the created watermark depends inter alia on the thickness/basis weight around the mark (so-called vellum) and the amplitude of the watermark itself. The tonal range is considerable. The light tones of the watermark marking correspond to the areas of the resulting paper that are as thin as possible, while the dark colours correspond to the areas of the marking where the thickness of the paper is increased. However, it should be borne in mind that in practical applications, paper webs with areas of significantly different thickness are not of essential interest due to the numerous problems that arise during their overprinting.
Such standard methods for the manufacture of a paper substrate for security documents are described, for example, in WO9420679 . In such traditional watermarks, the bright areas are due to the fact that the sheet thickness and fibre density are less than the sheet thickness and fibre density in the areas where the watermark has not been printed. In contrast, the dark areas are due to the fact that the thickness of the sheet and the density of the fibres are greater. Conventional watermarks have transparent portions that typically have areas of a few mm², and there is little difference in opacity between the bright portion of the watermark and the non-watermarked paper. In such traditional solutions, it is very difficult to print on the bright areas of these watermarks, it is very difficult to print on the thinner, bright areas, and even with very precise identification of watermarks produced by the classical method, it is difficult to obtain a good match.
A classic watermark can also be obtained by applying thin plates of various shapes to the wire, the so-called electrotypes/filigrees. For example, they can take the form of a denomination applied to banknotes, but also they can be much larger markings, such as PixelMark or MultiHexa graphic symbols.
Due to the different thickness of the paper within a single watermark, depending on the number of areas that can be distinguished within such a marking as differing in this parameter, the supplied marking can be a one-tone, two-tone or multi-tone watermark.
For example, a high-resolution, high-contrast multi-tone watermark having areas of distinct brightness is disclosed in US 2013/0255896 A1 . Such a watermark is obtained in a manner analogous to that described above by means of a device provided with a surface having a raised area and perforations. The perforations are such that making the cross section thereof, at a given distance from the draining surface, directly depends on said distance.
In turn, the application EP1743778A2 describes a method for the manufacture of colour watermarks comprising the use of a watermark applicator to which auxiliary colour material is supplied. The watermark applicator contains a cylinder with an open design (similar to the open body of a dandy roll) and covered with an embossed wire mesh.
A watermark can also be a so-called dark watermark. Such a watermark is obtained during the manufacture of the fibre sheet by round molds or watermark rollers for the Fourdrinier paper machine, including the intaglio print of the watermark to be obtained. A watermarked sheet is then obtained which appears dark when the sheet is examined in transmitted light. As mentioned above, the dark areas correspond to the accumulation of fibres during sheet formation in the recessed portions of the watermarking means; thus the thickness of the sheet and the density of the fibres are greater in these areas than in the rest of the sheet.
So-called clear watermarks can be produced on a Fourdrinier machine by pressing a watermarking roller against a wet stream. The watermark is then obtained by lifting part of the watermarking roller or mould, by simply displacing the fibres and reducing the density of the material locally.
However, the production of classic watermarks requires the use of specific equipment already at the initial stage of production of the paper substrate for secured documents. Moreover, it limits the possibility of variability of the applied watermark, because obtaining a different mark requires the use of different masks, rollers, etc. However, due to high legibility of such security feature and possibility of its verification without any additional means by a naked eye, for many years a search is on the way for simplified means to ensure watermark type markings, at least by making the method independent of the initial stage of manufacturing properly prepared masks, rollers or other forms, characteristic for a strictly defined marking to take the form of a watermark.
For example, imitations of true clear watermarks can be obtained by chemical methods, based on ensuring local transparency of the paper carrier. By adding a binder or resin to the fibrous material to fill the micropores in the paper structure, light can pass through the medium more easily, giving the appearance of a pale watermark, as seen through transparency. The disadvantage of this technique is that the applied chemicals that modify the transparency of the paper change its surface properties, in particular, they affect the ability of such a paper substrate to accept printing inks.
Other techniques for changing the transparency of the paper involve locally compressing the fibrous material. However, the markings obtained by these methods are only a weak imitation of a watermark, and can be easily distinguished by the naked, unaided eye from the classic true pale watermark obtained by displacing the fibres of the paper stock. Moreover, they often affect the quality of the paper substrate and its ability to print over the entire modified area.
Therefore, other techniques were sought to ensure the desired effect, illusively similar to a traditional watermark.
For example, the application EP1122360 discloses paper comprising at least one multitone effect watermark comprising, when viewed in transmitted light, a set of bright areas arranged in a halftone pattern. A watermark is obtained due to the presence of areas of a paper substrate with different optical densities within it. Then, the bright areas of the marking applied to the paper substrate in the form of a watermark correspond to the areas of low optical density, in particular lower in relation to the areas not modified by the marking, while the dark areas are areas of high optical density, in particular higher in relation to the areas not modified by the marking. For example, the depth of the faded areas of the watermark is between 50 and 90 % of the total thickness of the paper. Depending on the local thickness of the watermarked area, the desired fibre density is obtained, corresponding to the desired tone shade. The rendered pattern forming a watermark marking thus contains a range of shades of a given tone. However, the mark imitating a watermark obtained in this way is also distinguishable with the naked, unaided eye of the observer from a classic mark, because its structure is based on a change in the density of the marking and does not represent a smooth change in the appearance of the marking depending on the angle of observation associated with a change in the angle of illumination of such a watermark. Moreover, as with the traditional watermarks described above, it is very difficult with this solution to perfectly match the bright and dark areas of the watermark to the print.
Since there is still a need for secured papers, such as those used to produce banknotes or other security documents, with security features that are easily recognizable to the general public by the naked, unaided eye of an observer, and which at the same time are difficult to counterfeit, such as, for example, identification documents (passports) or certificates, new watermarks are still being sought, characterized by a higher degree of security and at the same time easy to produce. Despite numerous approaches to providing such a simple method for the manufacture a watermark, as well as a watermark produced in this way, preserving the full characteristics of this security feature, including a changeable appearance depending on the viewing angle associated with a change in the illumination angle of the marking, such a need has not been satisfied so far.
Some attempts have been made in the art to provide a watermark that retains the nature of the watermark and is produced by standard techniques. For example, the patent publication EP0918649B1 discloses a document with a security feature in the form of an identification number of such a document, which number is produced by laser etching of a paper substrate leading to the removal of a part of the substrate material by means of a laser. However, as emphasized in the publication, the part of the surface that has been marked by removing some of the structure material of the paper substrate tends to turn brown, due to the fact that the laser beam burns the paper substrate slightly. Thus, such marking has no "watermark" character.
An analogous solution is known from the document WO2004009371 , which discloses a security document, such as a banknote, comprising a secured paper bearing at least one tactile marking in the form of a relief structure which is generated by a laser, and in which a visually recognizable colour change is observed, in particular blackening of the paper in the tactile marking area. Thus, also this solution is not a watermark, in particular it is not suitable for applying multi- and single-tone watermarks in which at least part of the watermark would be a clear watermark.
In order to obtain a mark that resembles a watermark, the publication EP0918649B1 proposes to subject a part of the paper substrate, or even a document on which the mark produced by laser etching was applied, leading to local removal of part of the thickness of the paper, to a bleaching or de-inking process to remove the brown (dark) colour. However, such pre-treatment with bleaching or de-inking with a suitable chemical agent leads to etching the entire paper substrate and thus modifies the substrate, changing its parameters and characteristics. Moreover, it requires an additional step, which can only be carried out before other elements, including security markings, are applied to the paper substrate, or before it is overprinted. In addition, chemically modified surfaces, as in the case of watermark imitation markings obtained with the use of reagents that change the transparency of the paper substrate, are only a weak imitation of a watermark, and can be easily distinguished from a classic true, pale watermark. Moreover, the bleaching and/or decolorizing substances used affect the quality of the paper substrate and thus change its ability to print over the entire modified area.
As can be seen from the above, attempts to increase the functionality and the degree of security of a document protected with a watermark have been aimed, inter alia, at increasing the character of the individualizing and personalizing marking applied in the form of a watermark. However, none of the above-mentioned standard methods for the manufacture of a watermark preserving its classic characteristics, based on the need to use a mask and a counter-mask during the watermark production, allowed for easy personalization of the marking applied in the form of a watermark, because the individualization of the marking required changing the elements of the device used for paper production each time. It is true that this particularly desirable feature is provided by the use of a laser to modify the paper surface, however, as indicated in the prior art, the marking obtained by such a method is not a watermark. Obtained as a result of laser etching, in accordance with the description of EP0918649B1 , the marking/security feature on a paper substrate is characterized by locally reduced thickness, however, the mark has a dark (brown) colour regardless of the degree of etching of this surface. Removal of such undesirable staining requires an additional staining/bleaching step that significantly affects the characteristics of the entire modified area, in particular the markings obtained by laser engraving. Unlike such an imitated watermark, in practical applications it is desirable to retain the original characteristics of such a marking obtained by standard methods, thus providing a marking whose perception changes smoothly depending on its viewing angle and illumination while providing impression of depth and three-dimensionality, the marking being colourless when viewed in light transmitted at an angle perpendicular to the plane of the sign.
The inventors of the present invention surprisingly found that by using an ultra-fast laser, a femtosecond laser in particular, in a single-step method of modifying the paper surface for secured documents, a watermark with the same characteristics is obtained, and therefore difficult to distinguish with the naked eye, in relation to the characteristics of watermarks obtained by standard methods, and this method additionally enables a significant improvement in the degree of security of the document provided with such a security feature due to the possibility of easy personalization of the watermark obtained by the method according to the invention.
The use of lasers to process various surfaces has been known in the art. However, as discussed above, attempts to use a laser to treat unprotected and unmodified paper surfaces resulted in blackening of the treated areas. The laser has been successfully used to modify paper surfaces covered with additional layers, such as, for example, varnish layers ("Laser Engraving of Paper and Polymeric Materials for Tactile Detection", Soriano C. et al., Proceedings of SPIE - The International Society for Optical Engineering, conference materials, April 2009), however, also in such applications, a change in colour in the area of the relief created with a laser was observed. Finally, ultrafast laser ablation has been used to remove fine surface paint soiling from a paper surface in preservation techniques ("Scanning the laser beam for ultrafast pulse laser cleaning of paint", Rode A.V. et al., Appl Phys A (2008) 93: 135-139). However, not only none of these publications disclose, but even suggest the potential applicability of ultra-fast lasers, femtosecond lasers in particular, for ablation of a paper substrate uncoated with an additional outer layer, in particular leading to the manufacture of a marking with a watermark character, with characteristics identical to the watermark obtained using standard methods, i.e. an uncolored watermark with a clearly outlined relief structure in grey scale, created, for example, on the basis of raster or vector graphics, ensuring, due to the different levels of thinning of the paper substrate on which the watermark is applied, the impression of depth identical to the observed for standard watermarks. Moreover, the method proposed herein guarantees the possibility for the manufacture a multi- and single-tone watermark, including in particular a clear watermark, and provides the possibility of personalizing the marking in the form of a watermark manufactured by the method according to the invention.
The laser "watermark" obtained by the method according to the invention does not show any differences from the classic watermark obtained by standard methods when observing the marking applied as a watermark in the transmitted light with the unaided, naked eye. However, a person skilled in the art will notice subtle differences between a conventional and a laser watermark when viewed under magnification. The edges of the laser-applied marking and its surface have a "hair-like" structure, in which it is possible to distinguish the places where the paper fibres have been broken. In a classic watermark, which is created at the stage of paper production, the thin structure of the watermark is free of visible "hairiness", and the surface of such a substrate is similar in the place of the applied marking in the form of a watermark and beyond. In the case of a laser watermark, the paper surface in the area where it has been modified with a femtosecond laser is more hairy and has different surface morphology from the rest of the non-laser-processed area. However, it should be emphasized that in the case of a laser watermark, the thinning obtained is as smooth as in the case of classic watermarks, and thus provides the same impression on the recipient of such marking. The only limitation for laser watermarks is the inability to produce dark watermarks where the paper substrate is thickened where the marking occurs. In the case of a laser, the tones of the character can only be provided in the direction of thinning the paper, namely obtaining bright tones in transmitted light.
The invention aims to meet this need for a relatively inexpensive and easily naked eye-identifiable security-enhanced paper authentication means that enables the personalization of security-provided documents in a simple, one-step process.
The watermark obtained by the method of the invention preferably has a repeatable pattern well recognized by the general public, which is, for example, a pattern of a simple geometric shape, such as a circle, a polygon, a star, a logo, text, for example, at least one letter or number. However, due to the method of the invention allowing very precise, selective removal of paper layers, in other preferred embodiments the watermark can adopt anything from simple one-tone images to vector graphics to grey-scale raster graphics. Due to such applicability of the method, it is possible to highly personalize the secured document by providing it with a security feature in the form of a watermark reproducing a photograph, which reflects properly small details.
It should be emphasized that applying a watermark by the method according to the invention is not only extremely simple and effective, but also does not lead to any modification, in particular damage, to the paper surface of the document with security features. Thus, it is not only possible, but also in no way limited, to apply security features other than the marking in the form of a watermark to the paper surface, including the area where the watermark has been created. Moreover, the watermark obtained by the method according to the invention has a depth less than the thickness of the paper substrate on which it is applied and none of the surfaces forming the marking, in particular none of the edges of the marking in the form of a watermark obtained by selectively removing layers of paper, protrudes above the top surface, nor over the opposite bottom surface of the paper substrate. Thus, the paper substrate obtained in this way with the security feature applied to it can be easily additionally protected with an additional protective coating/coatings, for example, by laminating or varnishing, perfectly adhering to the surface of the paper substrate, due to which a longer life of such a document can be easily ensured and the potential manipulation of such a secured document will be hindered.
In addition, it should be emphasized that due to the simplicity of the proposed solution, the security feature in the form of a watermark can be applied at any stage of the production of a document with security features, i.e. not only at the stage for the manufacture the paper substrate, but also at later stages, for example after the substrate is initially provided with other elements, which in turn allows for both centralized and decentralized personalization. None of the prior art watermarking methods provided the possibility of introducing such a security feature on a substrate already pre-protected with security features of another type. Thus, the present invention provides an effective, accessible and inexpensive security feature that is easy to process and incorporate with other security features.
According to one of its aspects, the invention relates to a method for the manufacture of a paper substrate provided with a marking in the form of a watermark, in particular a data carrier for documents with security features, in particular security documents with security features, preferably provided with a watermark containing personal data and/or individualizing markings, which method includes the following steps:

a) designing the source image corresponding to the marking applied to the paper substrate in the form of a watermark, at which step, after developing, preferably by means of a computer configured for graphic image design, the graphic image to be ultimately applied to the paper substrate in the form of a watermark, a set of input data defining a desired image is determined, and then, such determined input data is converted by a computer configured to perform such a conversion into a graphic code file comprising the source image, describing the marking to be applied;
b) providing a paper substrate;
c) securing the paper substrate from step b) by applying a marking in the form of a watermark to the substrate by irradiating the paper substrate with a computer-controlled ultra-fast laser whose beam modifies the paper substrate at least in a part of the area, preferably in the entire transparent area, the provided watermark being obtained by selective, local removal of paper substrate layers in the area defined on the basis of the source code graphic file describing the marking to be applied in the form of a watermark,

Preferably, the watermark applied in step c) has a relief structure.
Also preferably, irradiation of the paper substrate in step c) is performed by using a femtosecond laser. In a particularly preferred embodiment of the method according to the invention, the irradiation of the paper substrate in step c) is carried out using a femtosecond laser with the following parameters: laser operating frequency: 50 kHz; laser pulse energy: 44 µJ; single pixel exposure time: 0.06 ms.
In preferred embodiments of the method, the marking in the form of a watermark is applied based on the designed source image in the form of raster graphics or vector graphics, more preferably, the watermark is applied in the form of raster graphics with a resolution of 900 - 1500 DPI, preferably 950-1050 DPI, most preferably 953 DPI.
In preferred embodiments of the method of the present invention, the marking in the form of a watermark is applied to a paper substrate which is unmodified fibre paper, preferably offset paper or banknote paper. Particularly preferably, the paper is an uncoated offset paper with a basis weight in the range of 40 g to 700 g, more preferably in the range of 50 g to 450 g, most preferably in the range of 70 g do 250 g. Even more preferably, the thickness of the offset paper is 0.08 - 0.10 mm.
The invention also relates to a secured paper substrate provided with the marking in the form of a watermark, in particular comprising a data carrier for documents with security features, in particular security documents with security features, obtained by a method as defined in any one of claims 1-9. Preferably, the marking in the form of a watermark is a marking containing personal data and/or individualizing markings, and particularly preferably the marking in the form of a watermark is a photograph, in particular a photograph of the holder of a document such as a passport.
At least a portion of the substrate to which the marking in the form of a watermark is applied by the method of the present invention corresponds to that portion of the substrate on which it is desired to obtain such a security feature. A portion of the substrate is not limited, and it may correspond to either the entire paper substrate or a small portion(s) thereof, and when the watermark is located in different areas of the provided system including the watermark on the substrate, the portion of the substrate may correspond to several areas of the substrate on which the watermark is applied according to the invention. The areas of the substrate forming such a security feature may constitute only a part of the substrate, as well as collectively correspond to the entire substrate of the system. Different areas of the security feature, both adjacent to each other or so far apart that it is not possible to recognize them as forming a common image, may be covered with one or more watermarks obtained by the method of the invention to provide specific patterns.
As used herein, the terms "light" and "radiation" may be used interchangeably and mean flux of particles emitted by a material.
It is to be understood that as used herein, the terms "hue" and "shade" and "colour" are used interchangeably herein to denote different lengths of reflected light as perceived by an observer. In summary, with regard to the phenomenon of colour perception, it should be assumed that two different shades/hues/colours are different from each other because light is emitted or absorbed by one shade at a specific wavelength that is different from the wavelength of the other shade.
In accordance with the present invention, the term "tone" is to be understood as referring to the different intensity of the same colour. In summary, regarding the phenomenon of perceiving a change in tone, it must be assumed that two different tones are different from each other because the light is emitted or absorbed with different intensities.
As used herein, the term "substrate" should be interpreted broadly. Thus, for example, the substrate may be a blank for the manufacture of the final product, such as blank sheets of paper which, after marking, provide the secured paper, as well as the final product itself, such as an identification document, containing at least one marking intended to convey information, such as, for example, a passport.
The watermarked paper substrate is in particular a secured paper, especially in the form of a sheet of such paper, in particular a secured paper which is watermarked over the entire surface or only in a part.
As used herein, whenever an "image" or "marking" is referred to, it is generally understood to mean any visually and/or machine readable pattern, often of specific information, that can be captured by an observer in a normal, single act of cognition. In particular, such patterns may be simple geometric patterns or textual and numerical information. Such images or markings include realistic motifs as well as abstract motifs. Examples of realistic motifs are, in particular, photographs, portraits, landscape, plant or animal motifs. Realistic motifs can also be images of coats of arms, buildings, flags. Abstract motifs include all other graphically presentable markings, including in particular those that are specific patterns, signs, including trademarks or codes. Other specific examples of abstract motifs are text or individual letters or numbers. However, the list of the above-indicated images is not exhaustive as such images or markings within the security feature can be laser engraved into any shape.
The present invention is illustrated in the drawing, in which:

Fig. 1 shows a marking in the form of a watermark obtained as a raster graphics by removing the outer paper layers with a femtosecond laser on a) 90 g/cm² basis weight printer paper and b) banknote paper.
Fig. 2 shows a fragment of the graphics of the marking in the form of a watermark obtained as a raster graphics by removing the outer layers of paper with a femtosecond laser on banknote paper a) in side light and b) in transmitted light.
Fig. 3 shows a marking in the form of a watermark obtained as a raster graphics by removing the outer layers of paper with a femtosecond laser on banknote paper a) in right side light, b) in transmitted light and c) in illumination from above.

All markings presented in the figure were made with the Origami Fiveone laser integrated with the Sino-Galvo SG2203 scanning head with a pulse below <400 fs, frequency in the range of 50 kHz - 1 MHz. The maximum laser power was 4 W at a pulse of 44 µJ.
Banknote paper without additives and plain printer paper with a basis weight of 90 g/cm² were tested. In both cases, by selectively removing the layers of the marked paper substrate, excellent marking results were obtained, providing a marking in the form of a watermark with identical characteristics to the watermark obtained by standard methods. Due to the use of a femtosecond pulse, the marked paper substrate is not darkened (browned) even in the area where the marking is applied. The effect of marking is only the selective removal of a part of the paper structure in the area where the marking is applied, and thus local thinning of the paper substrate, which gives the effect identical to that observed for classic watermarks obtained by standard manufacture methods.
For comparison, analogous tests were performed using a picosecond laser. Depending on the choice of parameters, the thinning effect of the paper substrate may be more or less visible, especially in transmitted light. Using the effect of selective removal of layers of the paper substrate, any image can be obtained, including, in particular, raster graphics in "grey scale".
The graphics shown in Fig. 1-3 were made at 953 DPI. The image obtained was computer-designed using the Floyd-Steinberg algorithm, ensuring excellent quality and the desired repeatability of the marking obtained and containing, to the extent desired, data individualizing each document, without the need to use a mask during its manufacture. The effect obtained is identical to that obtained in the case of classic grey-scale watermarks. The intensity of the colour representation was ensured by the selection of the distance between the pixels forming the graphics, in particular the pixels adjacent to each other. With a high resolution of the applied image comprising the marking in the form of a watermark, individual pixels may overlap, which in effect, in the context of removing layers of paper, provides thinner or thicker layers.
The initial graphic marking was each time converted into a graphic file reflecting the marking to be applied using a computer configured for such transformations. Any digital machine/computer equipped with, for example, computer software capable of converting a graphic file with a graphic design into a graphic file containing a binary record of such a marking may be suitable for carrying out this process. Suitable mathematical transformations are available in many freely available software packages known in the art. A person skilled in the art will, without undue experimentation, be able to select software from those available on the market suitable for computer-aided design of an image, in particular a graphic image in raster graphics.
The watermark was applied to the paper substrate by irradiating the paper substrate with a computer-controlled femtosecond laser, the beam of which selectively removes the layers of the paper substrate at least in a part of the area where the security marking in the form of a watermark is to be applied, such marking corresponding each time to the computer-designed source image in the form of raster graphics using a computer adapted to perform appropriate transformations of the image into raster graphics. In a similar way, markings in vector graphics are obtained using software that allows the transformation of the target image, which is a marking in the form of a watermark to be applied to a paper substrate, into the source data of the image in vector graphics.
The most preferable markings, in which the applied images retained full depth and high mapping accuracy, were obtained with the following operating parameters of the femtosecond laser: laser operating frequency: 50 kHz; laser pulse energy: 44 µJ; single pixel exposure time: 0.06 ms, resolution: 953 DPI.
With a longer exposure time of a single pixel or using a much higher resolution, for example above 1500 DPI, and even above 1000 DPI in the practical embodiments of the method according to the invention, even with short exposure times of a single pixel, complete local removal of paper layers was observed, which resulted in perforation of the paper substrate and loss of its properties as a result of (due to) disintegration of the substrate structure at the marking site. This was caused by too great local laser power in a given place, because longer exposure in a given place results in stronger thinning of the paper until all layers of the paper substrate are removed. A similar phenomenon was also observed when increasing the marking resolution. This phenomenon is a local increase in laser power used for selective removal of layers of paper substrate. A person skilled in the art will easily select the appropriate parameters so that the total energy acting at a given point is constant for a given paper substrate. This means that by slightly reducing the exposure time of a single pixel in order to maintain constant energy acting pointwise on the substrate in time, the resolution parameter of the marking applied to the paper substrate by means of an ultra-fast laser, in particular an image such as a photograph, can be increased accordingly. In contrast, by slightly increasing the exposure time of a single pixel in order to maintain a constant energy acting pointwise on the substrate in time, the resolution parameter of the marking applied to the paper substrate can be reduced accordingly. In practice, the exposure time of a single pixel is selected according to the resolution of the marking to be applied to the paper substrate. This time, although to a small extent, is dependent on the structure and properties of the paper - such as its basis weight, thickness, composition and hairiness - it is primarily dependent on the resolution of the applied marking. In addition, to ensure the continuity and uniformity of the perceived watermark, pixel sizes should be chosen such that adjacent pixels, corresponding to separate pulses used to selectively remove layers of paper, are close enough to each other that there is no interruption in the removal of material, and at the same time are apart enough from each other so that there is no local removal of additional layers of the paper substrate in areas where the pixels of the applied image overlap. Optimally, the distance of two adjacent pixels determining the location of the laser pulse should be equal to or slightly smaller, e.g. by 5 %, than the average diameter of a single coding pixel applied to the paper surface of the marking.
In the following embodiments of the present invention, computer-designed raster graphics with a single pixel size of about 25 µm are used. The optimized spacing between adjacent pixels was also 25 µm. A spacing slightly smaller than the average pixel diameter of 24 µm was also used, which was associated with an increase in resolution to about 1000 DPI. The marking obtained with such parameters was characterized by the desired characteristics corresponding to a standard watermark. Further reduction of the spacing between adjacent pixels, in particular below a dozen or so µm (for example, above 25 % of the average diameter of a single pixel), however, led to holes in the paper, and to some extent the effect was improved by reducing the power. By lowering the laser power (energy of a single pulse) - while maintaining constant spacing of adjacent pixels, the obtained image was less clear, the thinning of the paper was smaller, which resulted in poorer visual reception of the image and lower legibility. In this case, the pixel density or exposure time can be increased (to a certain extent) to ensure the same effect as that observed for classic watermarks obtained with standard methods. During the study, it was also found that the best results are obtained when working at low frequencies of the femtosecond laser, and increasing the frequency is associated with reducing the maximum power of a single pulse, which in turn resulted in the extension of the entire watermarking process with the femtosecond laser.
Markings applied in the form of a watermark were characterized each time by the maximum depth of the removed layers of the paper substrate, which was smaller than the thickness of the substrate itself. It has been observed, however, that particularly desirable effects are obtained when modification of the substrate does not lead to significant local thinning. Irrespective of the thickness of the paper substrate used, such modified paper substrates were found to be particularly useful, on which the maximum thinning of the substrate thickness after applying the marking in the form of a watermark did not exceed half the thickness of the substrate. However, while standard 90 g/cm² printer papers have an optimal depth of removal of layers not more than half the thickness of the substrate, for heavier papers such as 150 g/cm², the maximum thinning of the substrate thickness after having applied the marking in the form of a watermark, may be slightly more than half the thickness. The watermark-secured paper substrates obtained in this way showed high durability and resistance to mechanical damage during further processing, for example they were suitable for printing in the area of the watermark.
Exemplary markings obtained by the method of the present invention are shown in the accompanying drawing. Fig. 1 shows a marking in the form of a watermark obtained in raster graphics, which marking is a photograph personalizing the paper substrate provided with it, as observed in transmitted light. Fig 1 a) shows a watermark obtained on traditional 90 g/cm² printer paper commonly used for printing. Fig. 1 b) shows the same marking made on banknote paper. In both cases, the desired watermark effect was obtained, identical to the watermark obtained by standard methods. As shown in Fig. 1, the marking is perceived in the grey scale, however, within the areas of the surface subjected to modification by the laser beam, no change in colour of this paper surface, in particular any darkening, is visible.
This effect is even more evident in Fig. 2 showing a portion of the marking in the form of a watermark of Fig. 1 on banknote paper viewed with the naked eye in a) side light and b) transmitted light. As can be seen in Fig. 2 a), the watermark obtained by selectively removing layers of the paper substrate with a laser has the same characteristics as those observed for the classic markings in the form of a watermark obtained with standard methods. The marking in transmitted light does not show a dark colour, and the obtained watermark is in the form of a clear watermark characteristic of watermarks obtained as a result of thinning the paper substrate.
Fig. 3 shows a marking in the form of a watermark obtained in raster graphics on banknote paper in various lighting conditions. As can be easily seen in Fig. 3, the marking obtained by selective ablation of the paper substrate is a marking having the characteristics of a clear watermark with the marking area free of any dark coloration. In addition, it retains all the characteristics of such a marking.
To sum up, as shown in the exemplary embodiments of the invention discussed above, the method proposed by the present inventors provides the possibility of full personalization of the applied marking acting as a security feature made with high precision and high repeatability while maintaining the characteristics identical to classic watermarks obtained using standard methods. In addition, it should be emphasized that not only the area outside the area on which the watermark was applied, but also the surfaces exposed after selective removal of the outer layers of paper are either unmodified, fully retaining their original, starting structure (area outside the watermark), or slightly modified, only in terms of changing the local thickness of the substrate (areas of the watermark), and thus are perfect for further modification, including the adoption of additional security features and overprinting. The paper substrate obtained in accordance with the invention is an excellent pre-secured carrier of important information, in particular personalization elements. As described above, the implementation of the method according to the present invention is extremely simple, does not require the use of complex installations and/or devices, and the process of applying a watermark to a paper substrate is quick and easy to implement.
Moreover, as mentioned before and illustrated in the exemplary embodiments of the invention described above, due to use of the method according to the present invention, employing an ultra-fast, in particular a femtosecond laser, to apply a watermark to a paper substrate, it is possible to prepare a pre-protected substrate, either with a universal, or individualizing marking in the form of a watermark, which can be subjected to further modification and application of additional security features only later, at a separate stage. Moreover, due to the minimal interference of the laser in the area of the applied marking(s), even in areas with a watermark applied, additional elements can be introduced that can perform further functions, such as a security, individualizing, personalizing function, as well as overprinting the areas.
Thus, the solution provided by the present inventors, although relatively easy and inexpensive to implement, is characterized by a very universal character and many different applications, including in particular for the production of personalization elements in the area of the security features of security documents.
Although the present invention has been described with reference to specific preferred embodiments only, it will be apparent to those skilled in the art that various modifications, changes, omissions, and substitutions may be made in the described embodiments without departing from the common technical idea defined in the specification of the claimed solutions comprising the subject-matter of the patent claims defining the scope of patent protection.

Claims

A method for the manufacture of a paper substrate provided with a marking in the form of a watermark, in particular a data carrier for documents with security features, in particular security documents with security features, preferably provided with a watermark containing personal data and/or individualizing markings, which method includes the following steps:
a) designing the source image corresponding to the marking in the form of a watermark applied to the paper substrate, at which step, after developing, preferably by means of a computer configured for graphic image design, the graphic image to be ultimately applied to the paper substrate in the form of a watermark, a set of input data defining a desired image is determined, and then, such determined input data is converted by a computer configured to perform such a conversion into a graphic code file comprising the source image describing the marking to be applied;

b) providing a paper substrate;

c) securing the paper substrate from step b) by applying a mark in the form of a watermark to the substrate by irradiating the paper substrate with a computer-controlled ultra-fast laser whose beam modifies the paper substrate at least in a part of the area, preferably in the entire transparent area, the provider watermark being obtained by selective, local removal of paper substrate layers in the area defined on the basis of the source code graphic file describing the marking to be applied in the form of a watermark,
and wherein the thinning of the paper substrate obtained by selectively locally removing layers of the paper substrate to produce the watermark in step c) is less than the thickness of the paper substrate, preferably less than half the thickness of the paper substrate, and the area to which the marking is applied in step c) in the form of a watermark does not differ in colour from the area not processed with an ultrafast laser.
The method according to claim 1, characterized in that the marking in the form of a watermark applied in step c) has a relief structure.
The method according to claim 1 or 2, characterized in that the exposure of the paper substrate in step c) is carried out using a femtosecond laser.
The method according to claim 3, characterized in that the exposure of the paper substrate in step c) is carried out using a femtosecond laser with the following parameters: laser operating frequency: 50 kHz; laser pulse energy: 44 µJ; single pixel exposure time: 0.06 ms.
A method according to any one of claims 1 to 4, characterized in that the watermark is applied based on the designed source image in the form of raster graphics or vector graphics.
The method according to claim 5, characterized in that the marking in the form of a watermark is applied in the form of raster graphics with a resolution of 900 - 1500 DPI, preferably 950-1050 DPI, most preferably 953 DPI.
A method according to any one of claims 1 to 6, characterized in that the marking in the form of a watermark is applied to a paper substrate which is unmodified fibrous paper, preferably offset paper or banknote paper.
The method according to claim 7, characterized in that the paper is uncoated offset paper with a basis weight in the range of 40 g to 700 g, more preferably in the range of 50 g to 450 g, most preferably in the range of 70 g to 250 g.
The method according to claim 8, characterized in that the thickness of the offset paper is 0.08 - 0.10 mm.
A secured paper substrate provided by the marking in the form of a watermark, in particular constituting a data carrier for documents with security features, in particular security documents with security features, obtained by a method as defined in any one of claims 1-9.
The secured paper substrate of claim 11, characterized in that the marking in the form of a watermark is a marking containing personal data and/or individualizing markings.
The secured paper substrate of claim 11, characterized in that the marking in the form of a watermark is a photograph, preferably a photograph of the holder of a document, such as a passport.