CN110050098B

CN110050098B - Method for manufacturing watermark paper, mask, method for manufacturing mask, and method for verifying watermark paper

Info

Publication number: CN110050098B
Application number: CN201780074963.8A
Authority: CN
Inventors: 伊万·蒂里; 史蒂芬·马洛尔
Original assignee: ArjoWiggins Security SAS
Current assignee: ArjoWiggins Security SAS
Priority date: 2016-12-02
Filing date: 2017-12-01
Publication date: 2022-02-11
Anticipated expiration: 2037-12-01
Also published as: KR102556073B1; BR112019010857B1; RU2019115980A3; RU2748464C2; RU2019115980A; WO2018100150A1; KR20190089898A; FR3059682B1; CN110050098A; BR112019010857A2; EP3548663A1; FR3059682A1

Abstract

A method for manufacturing security paper, comprising the steps of: a watermark is made using a mask (10) attached to the wire section of a paper machine, the mask comprising a network of elements (18) for forming bright areas on the watermark, the elements (18) being connected by connecting elements (15), the bright areas being arranged as pixels of a raster image, the connecting elements being sufficiently thin that at least some of the elements (18) for forming bright areas cause the formation of pixels of the raster image which appear in transmitted light to the naked eye as isolated pixels.

Description

Method for manufacturing watermark paper, mask, method for manufacturing mask, and method for verifying watermark paper

Technical Field

The present invention relates to a method for manufacturing watermarked security paper and to the paper and documents obtained thereby.

Background

Watermarks have been used for a long time in the field of security documents and are formed during the manufacture of paper substrates by varying the thickness of the paper substrate, by means of a fibre deposition process and/or by dewatering.

Many improvements have been made to the manufacture of watermarks.

The invention relates more particularly to watermarking for reproducing halftone images.

The applicant's patents EP 1122360B 1 and EP 1252389B 1 describe methods for making such watermarks, which are sold by the applicant under the trade name Pixel Watermark (Pixel Watermark). The mask used to make such watermarks is typically obtained by machining or chemically etching a metal plate. This process makes it difficult to fabricate a mask with high precision because, empirically, the spatial resolution of the details is at least 0.8 times the thickness. Thus, for a 0.5 millimeter (mm) thick mask, the size of the holes that can be created therein must be at least 0.4 millimeters (mm).

Other publications, namely WO2010/036104 a1, WO2011/117828 a1 or even WO2011/081561 a1, also relate to the manufacture of this type of watermark.

Patent application WO2013/140126 a1 discloses a watermark forming mask formed by a grid to which elements for forming a watermark pattern are attached. The areas formed by the elements in the watermark appear to be disconnected from each other, which is advantageous when reproducing the arabic letters. There is no problem in reproducing the raster image in this publication.

There is a need for further improvements in watermarks at least partially reproducing a raster image and corresponding methods of manufacture, in particular to improve the security provided by such watermarks.

Disclosure of Invention

The invention achieves this object by means of a method for producing security paper, comprising the steps consisting in:

-making a watermark using a mask fixed to a wire section of a paper machine, the mask comprising a network of elements for forming bright areas in the watermark, the elements being connected by connecting elements, the bright areas being arranged as pixels of a raster image, the connecting elements being sufficiently thin that at least some of the elements for forming bright areas cause the formation of pixels of the raster image, which pixels of the raster image appear in transmitted light as isolated pixels with the naked eye.

The expression "look with the naked eye" or "with the naked eye" refers to observation by a normal-sighted human eye in the absence of an instrument from a distance of 25 centimeters (cm) in sunlight.

The invention allows the production of visually attractive watermarks and allows increased security against counterfeiting, since such watermarks are difficult to imitate due to their potentially high definition.

Another advantage is to allow better dewatering of the paper during its formation.

However, in the prior art, the bridges or even the frames hold together the various elements placed and welded onto the forming mesh (which has a relative positioning corresponding to the pattern to be reproduced) and still visible when the watermark is observed with the naked eye, in the present invention at least some of the elements of the watermark design appear to be disconnected from each other with the naked eye, which allows to obtain a new visual effect that can be perceived by a normal person and extends the range of patterns that can be produced.

The visual impression of fluidity created by the constituent elements of the pattern may be used in particular to improve the aesthetic appearance. The watermark can be more easily recognized by a regular person. Thus, a new type of watermark can be obtained, no longer in the form of a raster of connected points or a raster of connected lines at least partially as in the case of current pixel-type watermarks, but in the form of a point cloud or line cloud, the connection points of which are no longer visible or hardly visible.

A further subject of the invention is a watermark-forming mask for implementing the method according to the invention, said watermark-forming mask comprising a set of elements for forming bright areas, which are connected by connecting elements and are arranged, for example, in the form of lines (such as lines analogous to those known in the banknote field as "intaglio" lines) as pixels of a raster image, the connecting elements being sufficiently thin not to be visible in the watermark, said elements for forming bright areas appearing, in transmitted light, with the naked eye as isolated pixels in said image.

Grating image

By "raster image" it must be understood an image formed by pixels arranged in a raster with variable pitch and/or size in order to reproduce a halftone image. Multiple "grey levels" may be reproduced depending on the density of the pixels, the size of the pixels, the height of the pixels and/or the amplitude of the pixels, or even depending on the local opacity of the paper in each pixel. In this case, the plurality of grey levels preferably produce a three-dimensional effect, such as a relief effect.

The raster image preferably represents a monument, a landscape, an animal, a plant or a portrait. In particular, the raster image may represent only a portion of a monument, a landscape, an animal, a plant, or a portrait.

As a variant, the raster image may represent any geometric shape, in particular a background or a part of a background forming another image, such as a monument, a landscape, an animal, a plant or a portrait. According to this variant, the raster image allows to emphasize the other image.

In particular, when the raster image is observed with the naked eye in transmitted light, the pixels of the raster image cannot be distinguished from each other. In particular, when the raster image is viewed under magnification (e.g., at least 10 times, preferably at least 20 times magnified) in transmitted light, the pixels of the raster image can be distinguished from each other. Thus, the pixels of the raster image form a secondary security feature.

According to one variation, the pixels of the raster image are distinguishable from one another when viewed with the naked eye in transmitted light, thereby forming a primary security feature.

Pixel

By "pixel" is meant a single element in the watermark pattern, similar to a pixel of a digital image.

The pixels may have any shape, but preferably take the form of dots (in particular dots having a circular, elliptical or polygonal shape), or lines, or letters, alphanumeric characters, ideograms or symbols. The raster image may include pixels of various shapes.

The pixels form the raster elements of the raster image.

The raster image may comprise pixels arranged at a raster angle, i.e. in rows at an angle to the horizontal.

The thickness of the grating elements may be between 200 micrometers (μm) and 1500 micrometers (μm).

In case the pixels take the form of lines, the width of the grating elements (i.e. the width of the lines) may be larger than 50 μm, in particular larger than 70 μm, preferably larger than 100 μm.

In case the pixels are dots, letters, alphanumeric characters or ideograms, the maximum dimension (excluding the thickness) of the raster elements (i.e. pixels) may be larger than 50 μm, in particular larger than 70 μm, preferably larger than 100 μm.

The grating may be amplitude modulated (i.e. the pixels are evenly distributed and the size of the pixels varies), or frequency modulated, or even frequency and amplitude modulated. In particular, the grating may be amplitude modulated and subsequently frequency modulated from a particular pixel size.

The raster may be random, e.g. with a fixed pixel size, the statistical frequency of which is varied in order to reproduce various grey levels. The pixels may have the same geometry depending on the amplitude, or as a variant, the geometry of the pixels may vary with the amplitude.

The arrangement of pixels in the image is preferably automatically generated by a software package.

Mask and method for manufacturing the same

The mask may be made of any suitable material, in particular metal and/or plastic.

The mask comprises thin connecting elements.

These connecting elements, because they are thin, leave no or few visible traces in the watermark and connect and ensure the strength of the elements used to form the visual pattern of the watermark. The thickness of the connecting element is preferably comprised between 20 μm and 200 μm, more preferably between 30 μm and 100 μm. The thickness of the connecting element is preferably constant.

The width of the connection element is preferably comprised between 20 μm and 200 μm, more preferably between 30 μm and 100 μm. The width of the connecting element is preferably constant.

The width and thickness of the connecting element are preferably comprised between 20 μm and 200 μm, more preferably between 30 μm and 100 μm. The thickness and width of the connecting element are preferably constant.

The connecting element is preferably an elongated element.

The connecting elements are preferably arranged in a grid. In this case, the smallest dimension l of the cells of the grid may be greater than 300 μm, in particular comprised between 300 μm and 5mm, preferably between 500 μm and 3 mm.

The connecting elements may in particular be arranged between the base and the top of the elements for forming the bright zone to which they are connected. The connecting elements are preferably arranged between the bases or between the tops of the elements for forming the bright zone to which they are connected.

The height of the elements used to form the bright areas may be constant across the mask. As a variant, the height of the elements for forming the bright areas is variable on the mask, so as to influence the formation of the watermark and obtain pixels that are more or less dark in transmitted light.

The elements for forming the bright areas are preferably made of the same material as the connecting elements. In particular, the elements for forming the bright areas and the connecting elements preferably form an integral part, which provides greater robustness to the mask.

The mask may comprise in at least one area elements for forming bright areas (arranged as pixels of a raster image) and in particular no elements for forming bright areas (arranged as pixels of a raster image) in at least one area for forming another type of watermark. Thus, a watermark may be obtained which in some aspects is a mixture resulting from the superposition and/or juxtaposition of at least one fully rasterized watermark and at least one conventional watermark (such as a multi-tonal watermark obtained by embossing, e.g. an embossed watermark or an electrotype watermark). Such "hybrid" watermarks are particularly attractive from a visual point of view. Thus, the raster image of the watermark according to the invention may form only a part of the pattern, the other part, in particular the complementary part, being another type of watermark, in particular a multi-tone watermark obtained by imprinting. This is particularly advantageous when the watermark forms an image, preferably a monument, a landscape, an animal, a plant or a portrait, since according to a first variant the watermark according to the invention may form only a part of said image, while another part, in particular a complementary part, is formed by another type of watermark, in particular a multi-tonal watermark obtained by imprinting; and according to a second variant, a multi-tonal watermark, in particular obtained by imprinting, may form the image, while another part, in particular a background of the image or a part of the background, is formed by a watermark according to the invention. The part formed by the watermark according to the invention is then emphasized, or the part formed by the watermark according to the invention is otherwise used to emphasize and highlight part or all of another part of the watermark.

The mask may comprise a plurality of elements for forming bright areas, each element appearing as an isolated pixel in the image, preferably the density of elements in at least one region of the mask is per square centimeter (cm)²) Containing between 30 and 80 elements, for example as many isolated pixels are produced in the acquired image.

The mask may comprise in one area elements for forming bright areas (arranged as pixels of the raster image) and in another area apertures for forming areas that appear darker than said bright areas, which apertures are also arranged as pixels of the raster image. These two regions correspond, for example, to bright and dark regions of the image, respectively. Between these two regions, the watermark may comprise regions to which the pixels corresponding to the bright regions are connected and which are no longer isolated in the image. The darker areas may have the same opacity as the areas of the paper cover and may even be considered as micro-areas of the paper cover.

The at least one element for forming the bright zone may have a maximum dimension less than or equal to 0.3mm, in particular comprised between 0.1mm and 0.3 mm.

Such a level of detail is particularly advantageous as it makes counterfeiting of the watermark thus obtained more difficult.

If the elements used to form the bright areas have a minimum size (excluding thickness), for example a width greater than 1mm, it is possible to produce holes with very small diameters (for example holes less than or equal to 0.3 mm) in order to ensure a level of dewatering during the manufacture of the paper.

The elements for forming the bright areas may have a cross section that is amplitude modulated along the raster lines of the image, which allows to reproduce raster points or raster lines of variable size; and/or the elements used to form the bright areas may have a height that is amplitude modulated along the raster lines of the image, which allows raster points or raster lines to be produced with variable opacity. In the latter case, it is thus possible to vary not only the distribution of the pixels but also the local opacity and thus the brightness of the pixels in transmitted light, which creates new possibilities in terms of the final appearance of the watermark.

The spatial frequency of the elements used to form the bright areas may be constant along at least one grating of the image. The spatial frequency of the elements used to form the bright areas may even vary along at least one grating of the image. For optimal image rendering, the amplitude modulation of the cross-section of the elements for forming the bright areas, the amplitude modulation of their height and/or the modulation of their spatial frequency may be combined.

Net part

The wire section is the part of the paper machine that is used to form a fibrous web by dewatering a suspension of papermaking fibers.

The wire section may be a wire section of a cylinder or fourdrinier machine.

Preferably, the papermaking machine is a cylinder machine.

Advantageously, the web section comprises a housing oriented towards the outside of the web in suspension, and the mask is arranged in the housing. This allows protection of the mask, facilitates positioning of the mask and optimizes the opacity change (distribution of dark/light areas) of the watermark. The housing may be formed by embossing the web portion. As a variant, the mask may be arranged on the surface of the web instead of in its grooves, or may even be placed on the "positive" relief of the web.

The mask may be secured to the web by any suitable means, for example welding, in particular brazing, or even bonded, crimped or otherwise mechanically secured to the web.

The web portion and mask may be manufactured such that the watermark comprises at least one grating region and one non-grating region. This involves, for example, the juxtaposition and/or superimposition of various types of watermarks, for example, the embossing and masking of web portions according to the invention as described above.

The watermark may be used with other security elements, such as security threads, security fibres or security flakes, introduced into the paper during manufacture of the watermark or subsequently. More generally, the watermark paper according to the invention may comprise any additional security element in addition to the watermark. In the additional security elements, some may be detectable under the naked eye, sunlight or artificial light, without the use of any special equipment. These security elements comprise, for example, coloured flakes or fibres. These security elements are referred to as primary security elements.

Other types of additional security elements may be detected using only relatively simple devices, such as Ultraviolet (UV) or Infrared (IR) emitting lamps or magnifying glasses. These security elements comprise, for example, fibers, flakes, particles or microtext. These security elements may be visible to the naked eye or may be invisible, for example luminescent when illuminated by a wood lamp emitting a wavelength of 365 nm. These security elements are referred to as secondary security elements.

Other types of additional security elements require more complex detection equipment in order to be detected. These security elements are for example capable of generating a specific signal when they are affected by one or more external stimuli, whether at the same time or not. Automatic detection of the signal allows the document to be authenticated under appropriate circumstances. These security elements comprise, for example, tracers in the form of active materials, particles or fibres, which are capable of generating a specific signal when subjected to optical, electrical, magnetic or electromagnetic excitation. These security elements are referred to as tertiary security elements.

The one or more additional security elements present in the paper according to the invention or in the security document according to the invention may have primary, secondary and/or tertiary security features.

A further subject of the invention is a method for manufacturing a mask according to the invention, wherein the elements for forming the bright areas and optionally the connecting elements are manufactured using an additive method. This allows to obtain a higher resolution than is possible by mechanical machining or chemical etching.

By "additive process" is meant a process in which material is added, as opposed to a process in which material is removed (e.g., machining or chemical etching).

Preferably, the elements for forming the bright areas and, where appropriate, the connecting elements are manufactured by electrodeposition, which may allow a higher resolution to be achieved than by chemical etching.

The connecting element is preferably manufactured using the same additive method as the element for forming the bright zone, and is thus integrally formed with the element for forming the bright zone.

Electrodeposition allows the variation of the amplitude of the height of the elements used to form the bright areas to be easily achieved, if desired, and may be continuous or discontinuous to provide additional "grey" shading of the watermark.

Electrodeposition allows the elements of the mask to be manufactured with high precision. Details of an isolated pixel of about 0.1mm size can be obtained and the spacing between pixels can be varied to create degradation. A detail value of 0.2mm is particularly well suited to the resolution allowed by the paper substrate. The largest elements preferably have a size less than or equal to 0.8 mm. Beyond this value, the element is preferably made with very small drainage holes, for example with a diameter comprised between 0.2mm and 1 mm.

The finest details may be formed by light in transmitted light in the region of the watermark.

A further subject of the invention is a water-marking paper, in particular obtained by implementing the method as described above, comprising a watermark with bright areas arranged as pixels of a raster image comprising at least one bright area which, in transmitted light, appears to the naked eye as an isolated pixel in the image. The watermark may comprise a plurality of such bright regions.

The watermark may comprise regions where the pixels corresponding to at least one bright region are no longer isolated but connected. The watermark may even comprise at least one region with isolated pixels which are darker than the light areas, the darker pixels being arranged as pixels of the raster image.

The watermark is preferably formed only by bright areas arranged as pixels of the raster image.

WatermarkingMay comprise regions having a density of light areas each appearing in transmitted light to contain between 30 and 80 isolated pixels per square centimetre, preferably occupying more than 20 square millimetres (mm) of the watermark²)。

A further subject of the invention is a method for authenticating a watermark paper according to the invention, comprising checking the watermark in transmitted light in order to detect the presence of a raster image, in particular along grid lines of the image, the pixels of which appear to be bright areas with at least one isolated pixel in the image, and better with a plurality of isolated pixels in the image.

The invention can be used to manufacture any type of security document, such as banknotes, passports and other identity documents or value documents.

Drawings

The invention may be better understood by reading the following detailed description of non-limiting examples of embodiments of the invention, and by viewing the accompanying drawings, in which:

FIGS. 1A to 1E show details of a mask for forming a watermark according to the invention;

FIGS. 2a to 2c are cross-sections of examples of masks according to the invention;

FIG. 3 shows an example of a mask according to the present invention;

figure 4 shows the image produced when the watermark is viewed in transmitted light;

FIG. 5 shows a detail of FIG. 4;

figures 5a and 5b show another example of a mask according to the invention and the image produced when the watermark is viewed in transmitted light, respectively; and

figure 6 shows the placement of a mask according to the invention in the housing of a wire section.

Detailed Description

Fig. 3 shows an example of a mask 10 according to the invention. The mask comprises connecting elements 15 arranged in a grid, for example with grid cells of constant cross-section, each grid being square or rectangular in the example shown. As a variant, the connecting elements 15 are not arranged in a grid.

Fig. 1A shows an example of a mask.

The connection element 15 connects isolated elements 18 for forming bright areas, which are arranged as pixels of the raster image. These elements 18 for forming the bright areas have a maximum dimension w.

The elements 18 may be placed at the intersections of the grid formed by the connecting elements 15 or elsewhere.

Fig. 1B-1E show pixels of a picture element that may use non-circular contours.

The image elements of the non-circular outline may be polygonal elements, for example, rectangular elements as shown in FIG. 1B, linear elements as shown in FIG. 1C, or L-shaped elements as shown in FIG. 1D. As shown in fig. 1E, different geometries may be provided on the same retained grid element 18.

The smallest dimension l of the cells is for example greater than 300 μm, in particular comprised between 300 μm and 5mm, preferably between 500 μm and 3 mm.

With reference to fig. 2a to 2c, it can be seen that the thickness e of the connecting element 15_gLess than the thickness e of the elements 18 for forming the bright zone_e. The connecting elements 15 may form orthogonally intersecting lines. Some of the lines may be oriented parallel to the gratings of the image, and the spacing between the lines may correspond to the spacing between the gratings in the image.

As shown in fig. 2b and 2c, the height e of the elements 18 for forming the bright zone_eMay vary across the mask 10. This may allow for stronger or weaker luminance pixels to be obtained, which increases the number of parameters that may be varied for reproducing the image. Variations in the height of the elements 18 used to form the bright areas can make it more difficult to reproduce the watermark.

In the example shown in fig. 2a to 2c, the elements 18 for forming the bright areas have, seen from above, a cross-section that varies along the raster in accordance with the size of the raster points to be reproduced in the image.

The connecting element 15 connects the bases of the elements 18 for forming the bright zones, as shown in fig. 2a to 2 c. As a variant, the connecting element 15 connects the top of the element 18 for forming the bright zone.

The invention is not limited to the connecting element 15 connecting the base or the top of the element 18 for forming the bright zone. In particular, the connecting element 15 may connect the elements 18 for forming bright zones at points other than the base or top of the elements 18 for forming bright zones. As a variant, it is possible to combine the special cases that the connecting elements 15 connect the elements 18 via the base and/or the top of the elements 18 for forming the bright zones and/or other points.

It can be seen that the particular elements 18 used to form the bright areas of the mask are isolated and produce bright areas in the resulting watermark 30 that appear to be isolated pixels 20, such as shown in fig. 4 and 5. This is for example the case for the back of the hair.

The area 21 surrounding the pixel 20 may have the same thickness as the paper-covered area 31. Fig. 6 shows that the mesh portion T receiving the mask 10 may include a housing 100.

The mask 10 is arranged in a housing 100, the connecting elements 15 abutting against a flat bottom of the housing 100 (fig. 6) or at least some of the elements 18 for forming the bright areas abutting against a flat bottom of the housing 100.

According to a variant, all the elements 18 for forming the bright zones and/or all the connecting elements 15 rest against the flat bottom of the casing 100. In particular, when the elements 18 for forming the bright zones have a variable height, this configuration can be realized, for example, by means of a housing 100 whose bottom is not planar and/or by means of a connecting element (following the relief of the elements for forming the bright zones).

The invention is not limited to the examples already given.

In particular, one or more watermarks according to the invention may be arranged on the paper so as to interact with other security elements (such as lines or foils).

The paper may be based on natural and/or synthetic fibers. Preferably, the paper comprises natural fibers, such as flax fibers and/or cotton fibers, in particular cotton linters.

The paper may be single or multi-ply. In the case of multiple layers, the individual layers may be colored differently.

Claims

1. A method for manufacturing security paper, comprising the steps of:

-making a watermark using a mask (10) fixed to a wire section of a paper machine, the mask comprising a network of elements (18) for forming bright areas in the watermark, the elements being connected by connecting elements (15), the bright areas being arranged as pixels of a raster image, the connecting elements being sufficiently thin not to be visible in the watermark, and such that at least some of said elements (18) for forming bright areas result in the formation of pixels of said raster image which, in transmitted light, appear to the naked eye as isolated pixels, the element (18) for forming the bright zone is made of the same material as the connecting element (15), said element (18) for forming the bright zone and said connection element (15) forming an integral part, wherein the connection elements (15) are arranged in a grid and the size (l) of the cells of the grid is comprised between 500 μm and 3 mm.

2. The method of claim 1, the pixel being in the form of a dot.

3. The method of claim 2, the dots having a circular, elliptical, or polygonal shape.

4. The method of claim 1, the pixels being in the form of lines, or in the form of alphabetic or alphanumeric characters.

5. The method of claim 1, the pixel being in the form of an ideogram.

6. The method of claim 1, the pixel being in the form of a symbol.

7. The method of claim 1, wherein the wire section is a wire section of a cylinder mould machine.

8. The method of claim 1, the web portion (T) comprising a housing (100) and the mask (10) being arranged in the housing.

9. The method of claim 8, the housing being formed by embossing the web portion.

10. The method of claim 1, the mask being welded or brazed to the web portion.

11. The method of claim 1, the web portion and the mask being fabricated such that the watermark includes at least one grating region and one non-grating region.

12. Method according to claim 1, the height (e) of the elements (18) for forming bright areas being such that_e) Is constant over the mask.

13. Method according to claim 1, the height (e) of the elements (18) for forming bright areas being such that_e) Is variable across the mask.

14. A watermark forming mask for implementing the method according to any one of claims 1 to 13, the watermark forming mask comprising a set of elements (18) for forming bright areas, the elements for forming bright areas being connected by connecting elements and the bright areas being arranged as pixels of a raster image, the connecting elements (15) being sufficiently thin not to be visible in the watermark, the elements for forming bright areas causing the formation of pixels of the raster image which appear in transmitted light to the naked eye as isolated pixels in the raster image.

15. Mask according to claim 14, the height of the elements (18) for forming the bright areas being constant over the mask.

16. A mask as claimed in claim 14, the height of the elements (18) for forming the bright areas being variable across the mask.

17. Mask according to claim 14, comprising in at least one area elements (18) for forming bright areas arranged as pixels of the raster image and in at least one other area elements for forming bright areas arranged as pixels of the raster image.

18. A mask according to claim 14, comprising a plurality of elements (18) for forming bright areas, each element for forming a bright area appearing as an isolated pixel in the raster image.

19. The mask according to claim 18, wherein in at least one area of the mask the density of elements (18) for forming the bright areas is comprised between 30 and 60 elements per square centimeter.

20. Mask according to claim 14, at least one of said elements (18) for forming bright areas having a dimension (w) less than or equal to 0.3 mm.

21. Mask according to claim 20, at least one of said elements (18) for forming the bright areas having a size (w) comprised between 0.1mm and 0.3 mm.

22. A mask according to claim 14, the elements (18) for forming the bright areas having a cross-section which is amplitude modulated along a grating line of the grating image.

23. Mask according to claim 14, the elements (18) for forming bright areas having a height (e) which is amplitude modulated along a grating line of the grating image_e)。

24. The mask of claim 14, the spatial frequency of the elements (18) for forming bright areas being constant along at least one grating of the grating image.

25. The mask of claim 14, the spatial frequency of the elements (18) for forming bright areas being variable along at least one grating of the grating image.

26. Mask according to claim 14, the connection elements (15) having a thickness (e) comprised between 30 and 200 microns_g)。

27. A method for manufacturing a mask according to any one of claims 14 to 26, wherein an additive method is used for the elements (18) forming bright areas, which elements for forming bright areas result in the formation of pixels of the raster image which appear in transmitted light to the naked eye as isolated pixels in the raster image.

28. A water marked paper obtained by implementing the method according to any one of claims 1 to 13, comprising a watermark (30) with bright areas arranged as pixels of a raster image comprising at least one bright area which appears to the naked eye in transmitted light as an isolated pixel in the raster image.

29. The watermark paper of claim 28, said watermark being formed only by bright regions arranged as pixels of a raster image.

30. The watermark paper of claim 28, said watermark comprising a plurality of bright regions, each bright region appearing as an isolated pixel in said raster image.

31. The watermark of claim 30, said watermark comprising regions having a density of highlights each appearing in transmitted light to contain between 30 and 80 isolated pixels per square centimeter, the regions occupying more than 20 square millimeters in said watermark.

32. A method for authenticating a water marked paper according to any one of claims 28 to 30, comprising inspecting the water mark in transmitted light in order to detect the presence of a raster image, the pixels (20) of which appear to the naked eye as bright areas with at least one isolated pixel in the raster image.

33. The method of claim 32, comprising inspecting the watermark in transmitted light to detect the presence of a raster image, the pixels (20) of the raster image appearing to the unaided eye as bright regions with isolated pixels in the raster image along raster lines of the raster image.