EP1122360A1 - Paper with multitone watermark and screen for manufacturing the paper - Google Patents

Abstract

Paper based on a fibrous composition, comprising at least one watermark with a multiton effect. This watermark comprises, when observed in transmitted light, a set of bright areas arranged in the manner of a raster image. <IMAGE>

Description

The present invention relates to a paper having an effect watermark multiton and a canvas to make such paper.

In the present invention, "paper" means sheet obtained by the wet process using a suspension of natural cellulose fibers and / or mineral fibers or organic other than cellulose, which may contain various fillers and various additives commonly used in stationery.

It is known to use the watermark technique to make paper security, i.e. documents that must not be falsified, such as banknotes, means of payment, identity documents, securities transport, entrance tickets to cultural or sporting events.

The presence of the watermark aims to make it impossible to reproduce the document by optical means such as photocopying, photography or scanning, since the copy medium would not carry the watermark of the original document.

Watermarked papers are also used for decorative purposes, especially for prestigious printing-writing papers.

Watermarks are conventionally obtained by a molding operation or embossing of the wet sheet from the aqueous suspension of cellulosic fibers when making paper. At this stage of manufacturing, the cellulosic fibers easily migrate in the aqueous suspension so that the aforementioned operation has for effect of concentrating the fibers in the thicker areas of the leaf and dispersing them in thinner areas, the result after drying the sheet is that this last one, when viewed in transmitted light, clear, poor areas in fibers, and dark areas, with high fiber density.

Bright areas are called low optical density areas, less than that of vellum, that is to say within the meaning of the present application, the non-watermarked paper, and the dark areas are called areas with high optical density, greater than that of paper.

Depending on the local thickness of the watermarked area, the fiber density is obtained desired, corresponding to the shade of tone sought. The pattern rendered thus includes a whole gradation of tones, which justifies the designation of shaded watermarks of these watermarks classics.

Classic shaded watermarks require means of production expensive because for each motif produced, a canvas or a roller must be used specific watermarks, which are also delicate to manufacture.

In addition, the fact that conventional watermarked papers include extra thickness poses different problems.

First of all, it slows down the production rates because it takes regularly stop the machines which are intended to handle flat paper, especially processing and printing machines. Cutting in particular is made difficult by the fact that the excess thicknesses of the watermark cause deformations paper.

Another difficulty arises at the time of printing, as the manner in which the inks are printed on the areas in excess thickness is difficult to control, so that we generally prefer to give it up by printing only outside watermarked areas.

To compensate for these excess thicknesses, a person skilled in the art positions shims between the leaves. Such compensation is necessary to allow processing correct sheets, but requires additional and tedious work which hinders a fast sheet processing, especially when printing.

In order to avoid these problems, we have already tried to make pseudo-watermarks using less expensive techniques, which retain their initial thickness at paper and make a visual appearance close to that of a classic watermark.

A known technique for producing pseudo-watermarks consists in making penetrate or print in certain areas of the paper a composition which modifies transparency of the paper in order to achieve light and dark areas, at the same time like a watermark.

The disadvantage attached to this technique is that the compositions used affect the surface properties of the paper, including its ability to receive inks printing, and that the result obtained does not allow to obtain the finesse and the variants brightness of a classic watermark.

In addition, such pseudo-watermarks are very easy to counterfeit because the impregnation of the sheet of paper with the appropriate composition is within the reach of many forgers.

The present invention aims to provide a new watermarked paper which can be manufactured under particularly simple and economical conditions.

The watermark according to the invention reconstructs the visual appearance of a shaded watermark classic and offers all the usual security features specific to conventional shaded watermarks.

The present invention relates in particular to a paper based on a fibrous composition, comprising at least one watermark with a multiton effect, paper characterized in that the watermark comprises, when observed in transmitted light, a set of clear areas arranged in the manner of a raster image.

Preferably, these clear areas have a strictly areal mass lower than the rest of the paper or vellum.

In other words, in the paper according to the invention, the watermark appears locally in transmitted light as a juxtaposition of bright areas and areas dark, distinguishable from each other, the latter having in fact the same mass area of fibrous composition than vellum, but appearing dark in comparison with neighboring bright areas thanks to a contrast effect.

The light areas within the meaning of the invention have a low optical density in because of their lower areal mass.

In the watermark according to the invention, the light areas, with low optical density, contrast with the higher optical density paper that surrounds them, and you can't find that a finite number of optical densities, namely that (s) of the light areas and that of the vellum.

The presence of clear areas in the vellum, with variable distributions depending on the locations of the pattern produced, provides a macroscopic visual effect which reproduces the shadows of the classic watermark.

In other words, in a given region of the watermark, the light areas are more or less numerous and occupy a greater or lesser proportion of the surface total of this region, which provides a more or less shaded effect.

Preferably, the light areas all have the same basis weight, which results in the same optical density for all light areas, with irregularities in the paper close. In this case, there are only two optical densities, namely that of the zones clear and that of vellum.

To arrive at a desired image, one can play on the individual size of the clear areas and their number and distribution.

It is necessary to give to the expression "raster image", within the meaning of the present invention, a broad meaning which covers all types of frames or pseudo-frames, the shape of the zones light and that of dark areas is not limited to a particular shape but any.

In a particular embodiment of the invention, the paper is colored, fluorescent, iridescent or has any other shade or optical effect known to the non watermarked papers.

In a preferred embodiment, the watermark appears in light transmitted as a raster image, the halftone dots are in the majority formed by lines.

These lines are inclined, for example at 45 °, but regions different from the raster image may have differently slanted lines.

In a particular embodiment, the frame of the raster image is modulated amplitude, with a constant linearity, for example between 5 and 20 1 / cm.

By amplitude modulation, it should be understood that the light areas are more or less depending on the optical density to be reproduced, being more extensive in the highlights only in low lights.

In another particular embodiment, the screen of the raster image is frequency modulation.

By frequency modulation, it should be understood that the light areas are more or less numerous depending on the optical density to be reproduced, being more numerous in the highlights than in the shadows.

We can also propose other screening methods to take into account the fact for example problems with the reproduction of halftone dots in regions of high and low optical densities.

In particular, due to the papermaking process, training of a clear area having a size smaller than a predetermined size may pose difficulties.

Likewise, the formation of a dark area smaller than a size predetermined, located between two bright areas, can be difficult.

To restore a shaded effect given in low light, we will choose to preferably clear areas whose size does not pose a training problem on the paper and we will choose the number of these light areas according to the optical density, the higher the optical density, the fewer the bright areas.

In the highlights, we will preferably choose dark areas having sufficient in size to allow them to be easily formed on paper between the bright areas, the number of dark areas being more or less important depending on the density optical to reproduce, the number of dark areas being lower as the density optical is weak.

For regions of the watermark whose brightness in transmitted light is between highlights and shadows and for which the size of the areas dark and that of the light areas does not pose a problem of formation on the paper, one can advantageously modulate the size of the light areas and that of the dark areas by function of the optical density to be reproduced.

The present invention also relates to a canvas used during the wet phase of papermaking, which is characterized by the fact that it is provided a set of masks which corresponds to a pattern to be produced as an effect watermark multiton on paper, the set of masks being denser in regions corresponding to the light parts of the required watermark and less dense in regions corresponding to the dark parts, when the watermark is observed in transmitted light.

The set of masks makes it possible to obtain two local optical densities in the paper obtained, namely a low optical density at the level of the masks, this density weak optics resulting from the fact that the masks limited the accumulation of fibers during of the formation of the sheet of paper, and a high optical density at the level of the parts unmasked from the set of masks, high optical density which is that observed throughout the rest of the paper.

In a particular embodiment of the invention, the masks have different thicknesses, which creates, in the watermark, clear areas of masses different surfaces. The watermark then has more than two optical densities, which improves its multiton effect.

The canvas provided with a set of masks according to the invention makes it possible to obtain a watermark imitating a classic multiton effect watermark, reproducing its different tone nuances, which are traditionally obtained by varying the depth paper embossing, replacing this embossing with optical effects resulting from the arrangement of the masks, the number, the distribution and the form of these masks being chosen according to the desired effect, the masks occupying a relatively large area important canvas in its regions to provide clear areas of the watermark and a smaller area in its regions to provide darker areas of the watermark.

The invention makes it possible to avoid playing on the embossing depth, which in particular eliminates the aforementioned drawbacks of conventional watermarks linked to extra thicknesses.

In other words, an advantage of the fabric according to the invention is that it provides a paper without extra thickness, since the nuances of tone do not result from a more or less marked embossing of the paper.

Preferably, the raster image reproduced by the watermark is dotted with halftone formed by lines, as this facilitates the production of the set of masks.

In a first embodiment of the invention, the canvas used for the formation of the watermark constitutes the canvas for forming the paper.

In a second embodiment, the canvas is used for lifting the sheet formed.

In a third embodiment of the invention, the canvas is fixed to a wet press which works the sheet still wet after lifting.

In a fourth embodiment of the invention, the canvas is fixed to a watermark roll.

In a fifth embodiment of the invention, the canvas is fixed to a grainer element located outside the leaf formation area.

The canvas can, for example, be on a round-shaped paper machine or on a flat table paper machine.

In each of the embodiments described above, the masks can be on the internal face of the fabric, in its thickness or on the external face of the canvas, several canvases can be combined.

Preferably, the masks are arranged on the side of the canvas located in contact with the aqueous suspension containing the paper fibers.

The invention also relates to a stack of fabrics, at least one of which is a canvas as described above.

Of course, any combination of the embodiments described above is also possible.

• réaliser par un procédé de tramage connu, une image tramée à partir d'une image numérisée,
• réaliser à partir de cette image tramée un élément ajouré pouvant être utilisé lors de la phase aqueuse de formation du papier, cet élément ajouré comportant des régions pleines disposées comme les parties claires du filigrane,
• réaliser au moyen dudit élément ajouré un filigrane en disposant l'élément ajouré de manière à ce qu'il limite l'accumulation des fibres au droit des régions pleines de l'élément ajouré lors de la phase aqueuse de formation du papier.

Another subject of the invention is a method for producing a raster image intended for the formation of a watermark, this method comprising the following steps:

• produce, by a known screening process, a screened image from a digitized image,
• make from this raster image an openwork element which can be used during the aqueous phase of paper formation, this openwork element comprising solid regions arranged as the clear parts of the watermark,
• achieve by means of said openwork element a watermark by arranging the openwork element so that it limits the accumulation of fibers in line with the solid regions of the openwork element during the aqueous phase of paper formation.

Advantageously, the aforementioned raster image is a frame dot image formed by lines.

Advantageously again, the raster image is retouched before the realization of the openwork element to ensure that there is no isolated clear area on the watermark.

In this case, the perforated element can be easily produced in the form of a monobloc plate with openings and the solid parts of which are arranged as the light areas of the watermark.

The watermark according to the invention can be recognized by means of software image analysis.

We will now describe different realizations of all the masks used in the invention.

In a first embodiment, all of the masks are made up by a grid, coming from a metal or thin plastic plate, in which openings have been made. The solid parts of the grid remaining between the openwork constitute the masks within the meaning of the invention.

The openwork can be made by chemical attack, laser cutting, water jet cutting or mechanical drilling of the grid.

The openwork can be in the form of hatches which leave between they also mask in the form of hatching.

In this case, the presence and density of the hatching determines the shade of tone obtained macroscopically by juxtaposition of light areas and dark areas in the watermark.

Hatching can all be parallel to a given direction or oriented in different directions. Likewise, they can provide different weft, in order to give the watermark obtained a personalized visual effect.

In a second embodiment, the set of masks consists of a juxtaposition of pieces individually attached to the canvas.

Generally, masks can be individually shaped into a particular elementary pattern intended to personalize the sheet of paper, by creating in the thickness of this one clear zones taking again the elementary pattern of masks. For example, each elementary pattern can form a letter or a set of letters, for example characteristics of the issuer of the security document drawn up at starting from the sheet according to the invention.

• la figure 1 représente de manière schématique un papier comprenant un filigrane selon l'invention,
• la figure 2 représente de façon schématique une grille utilisée pour réaliser un filigrane conforme à l'invention,
• la figure 3 est une section partielle et schématique du filigrane et illustre le positionnement des masques sur la toile utilisée pour la formation du papier, et
• la figure 4 représente un détail de la grille de la figure 2.

In order to better understand the invention, we will now describe an embodiment given by way of non-limiting example, with reference to the accompanying drawing, in which:

• FIG. 1 schematically represents a paper comprising a watermark according to the invention,
• FIG. 2 schematically represents a grid used to produce a watermark according to the invention,
• FIG. 3 is a partial and schematic section of the watermark and illustrates the positioning of the masks on the canvas used for the formation of paper, and
• FIG. 4 represents a detail of the grid of FIG. 2.

FIG. 1 shows a security document 1 according to the invention, such than a bank note, comprising a watermark 2 represented very schematically and reproducing an image, for example portait of a famous person.

To make the watermark 2 in Figure 1, we use a set of masks consisting of a thin grid, the openings of which give a negative image watermark.

FIG. 2 shows a grid forming a set of masks 3, can be used to make a watermark according to the invention.

FIG. 3 shows a section, taken perpendicular to the orientation of the frame, the watermark 2.

In this figure, the light areas 4 of the watermark correspond to regions in which the paper has a lower basis weight and the areas more dark 5 correspond to regions of the paper which have a basis weight equal to that of paper outside the watermark.

In the example of embodiment described, the light areas correspond to hollow on the surface of the paper, while the darker areas correspond to regions of constant thickness e of the paper.

To make the light areas, that is to say the recesses on the surface of the paper, we has the set of masks 3 on a canvas 6 used for example during training wet paper.

The fabric 6 is provided with the set of masks 3 on one side, preferably its face which is in contact with the aqueous suspension of fibers.

Each elementary mask 7 of the set of masks 3 limits the accumulation of fibers on the surface of the fabric 6 during the process of forming the paper, as shown in Figure 3.

The depth of the hollow parts corresponding to the light areas of the watermark is for example between 50 and 90% of the total thickness e of the paper.

To make the set of masks 3, it is possible, for example, to next way.

From a digitized image of a portrait or any other subject serving base with the watermark, a halftone image is produced in a conventional manner, by means of example of software such as that marketed by ADOBE under the brand PHOTOSHOP.

Such software makes it possible in particular to choose the shape of the halftone dots, the linearity and the screening process used to produce the screened image.

The simplest raster image to obtain is a halftone image, i.e. that the shades of gray of the scanned image are translated on the screened image by black halftone dots on a white background.

The screening process used can be a screening process known as amplitude modulation, i.e. the size of the halftone dots varies according to the optical density to reproduce.

It is also possible to use a screening process called modulation modulation. frequency, i.e. the size of the halftone dots is constant but the spacing varies according to the optical density to be restored.

It is also possible to use any other known screening process, for example a screening process in which we would change the way of constructing the grid points according to the optical density to be restored, with a view in particular to taking account of the problems of reproduction of the raster image during the formation of the watermark.

We can also screen different regions of the image differently digitized and in particular, when we use dots consisting of lines, some parts of the scanned image can be reproduced with lines having a given tilt and other parts of the scanned image with lines having a different inclination.

We can thus have, within the same raster image, several regions, by example three or four, the halftone points of which consist of lines having different orientations.

To take an example, the beard of the character shown in Figure 1 could be screened with halftone dots formed by lines having a orientation different from that of the lines used to reproduce the rest of the character.

The raster image produced by the software is advantageously retouched to take into account the specifics related to the reproduction of this image in the form of watermark.

One can obtain by means of the aforementioned software, a raster image in negative of the scanned image.

This negative image is also a raster image which is advantageously retouched so that there is no isolated dark area.

Such a retouched image may resemble, for example, the image of the figure 2.

When touching up, you can, as illustrated in Figure 4, join two lines dark adjacent by bridges of material 8, this in order to improve the aesthetics of the image possibly but above all to allow the production of an openwork element which the solid parts correspond to the black areas of the halftone negative image of the figure 2 for example.

Note that in Figure 2, there is no isolated black area, i.e. that the set of masks 3 can be produced in a single piece by etching.

An openwork plate is then obtained comprising solid shaped parts hatching which correspond to the black halftone dots of the halftone negative image of Figure 2, these solid hatched regions being connected by parts of solid links 8 extending between them, resulting from the retouching of the initial raster image.

Such an openwork plate gives during the formation of the watermark in the manner explained above a positive image whose light areas correspond to the areas black in the raster negative image of Figure 2 and the dark areas correspond to perforated areas of the plate.

The set of masks 3 used for the formation of the watermark is by example obtained by etching a photosensitive plate exposed through a film reproducing the screened negative image of Figure 2.

As a variant, the set of masks 3 can be produced by cutting out a thin metal or plastic plate.

We understand that it is advantageous to touch up the raster image obtained by the through software from a scanned image because it allows for a whole of one-piece masks as explained above.

However, the invention is not limited to this embodiment and we can very well use the raster image without retouching by coming to report on the canvas used to make the watermark a set of individual pieces.

Claims (14)

Paper based on a fibrous composition, comprising at least one watermark with multiton effect, characterized in that the watermark includes, when observed in transmitted light, a set of bright areas arranged in the manner of an image screened. Paper according to claim 1, characterized in that the light areas have a surface mass strictly lower than that of the rest of the paper. Paper according to either of Claims 1 and 2, characterized in that the watermark appears as a raster image with halftone dots formed by lines. Paper according to any one of the preceding claims, characterized by the fact that the light areas all have the same composition basis weight fibrous. Paper according to any one of the preceding claims, characterized by the fact that it is colored, fluorescent, iridescent or has any other shade or effect optical. Canvas used during the wet phase of papermaking, characterized by the fact that it is provided with a set (3) of masks (7) which represents a watermark motif with a multiton effect on the paper, the set of masks being denser in regions corresponding to the light parts of the watermark sought and less dense in regions corresponding to the other parts of the watermark sought. Canvas according to the preceding claim, characterized in that the masks are arranged on the side of the canvas located in contact with the aqueous suspension containing the paper fibers. Method for producing a raster image intended for the formation of a watermark, this method comprising the following steps: produce, by a known screening process, a screened image from a digitized image, make from this raster image an openwork element (3) which can be used during the aqueous phase of paper formation, this openwork element comprising solid regions arranged as the clear parts of the watermark, achieve by means of said openwork element a watermark by arranging the openwork element so that it limits the accumulation of fibers in line with the solid regions of the openwork element during the aqueous phase of paper formation. Method according to the preceding claim, characterized in that the raster image is a halftone dot image consisting of lines. Method according to one of the two preceding claims, characterized by the fact that the raster image is retouched before the perforated element is produced to make so that there is no isolated clear area on the watermark. Method according to the preceding claim, characterized in that the perforated element is produced in the form of a monobloc plate comprising perforations and the solid parts of which are arranged like the light areas of the watermark. Set of masks constituting an openwork element according to one any of the preceding process claims. Set of masks according to the preceding claim, characterized by the fact that the masks are individually shaped in a particular elementary pattern designed to personalize the sheet of paper, creating zones in its thickness clear with the elementary pattern of the masks. Set of masks according to the preceding claim, characterized by makes each elementary pattern form a letter or a set of letters.

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