Patent Publication Number: US-2011056638-A1

Title: method of fabricating a sheet comprising a region of reduced thickness or of increased thickness in register with a ribbon, and an associated sheet

Description:
The present invention relates to the field of security documents and among others it relates to a method of fabricating a security sheet, e.g. made mostly of paper. 
     In this description, the term “paper” means any sheet obtained by a wet method from a suspension of fibers of natural cellulose and/or mineral, vegetable, or organic fibers other than cellulose fibers, possibly synthetic fibers, and possibly containing a variety of fillers and a variety of additives as commonly used in papermaking. 
     BACKGROUND 
     When fabricating security documents, and in particular security sheets, it is common practice to seek to incorporate one or more security element, e.g. by applying a security film or thread on a security sheet or by coating it with a strip of iridescent coating, or indeed by creating windows in the security sheet and applying a transparent film over the windows. This generally leads to regions of increased thickness being created on security documents that may give rise to problems of flatness of security documents or indeed may make printing steps difficult, for example. 
     Various solutions have been proposed seeking to reduce the thickness of a security sheet locally where the security elements are to be placed, during fabrication of the sheet on the vat former of a papermaking machine. In most cases, the solution proposed consists in closing off a portion of the forming fabric used during the method of fabricating the security sheet on a vat former. 
     International application WO 99/06629 describes a method of fabricating a sheet using a flat bed (Fourdrinier) papermaking machine fitted with a textile fabric that enables zones of low and high weight to be obtained. 
     International application WO 93/00474 also describes a method of fabricating a security sheet using a flat bed (Fourdrinier) papermaking machine fitted with a fabric having pellets added thereto that reduce de-watering, thereby enabling zones of different weights to be formed. 
     European patent application EP 0 625 431 describes a method of fabricating a security paper including a security thread fabricated from a film and incorporated between two plies. 
     European patent application EP 0 549 384 describes a method of fabricating a paper that comprises zones of reduced thickness obtained by securing pellets to the forming fabric of the vat former. 
     European patent application EP 1 122 360 describes a method of fabricating a paper in which a watermark is made from modifications made on the forming fabric of the vat former. 
     British patent application GB 2 433 469 describes fabricating a fibrous security substrate having incorporated therein a fiber ribbon including a plurality of openings. 
     European patent application EP 0 070 172 describes a method of fabricating a security paper including a ribbon that presents alternating zones enabling papermaking fibers to be deposited and zones preventing papermaking fibers being deposited. 
     European patent application EP 0 773 320 describes inserting a strip between two papermaking plies, one of which comprises a watermark. 
     British patent application GB 2 395 724 describes incorporating a polyester strip including an electronic chip in a fiber substrate. 
     The solutions proposed in the prior art do not enable zones of reduced thickness to be obtained in a security document by means of a method that is relatively simple. The solutions proposed require modifications and significant preparation of the forming fabrics used on vat formers and they generally do not make it possible to obtain thickness reductions of dimensions that are sufficient for incorporating the above-mentioned security elements. 
     SUMMARY 
     Consequently there exists a need to remedy at least some of the above-mentioned drawbacks. 
     In particular, there exists a need to improve the fabrication of security documents, in particular security sheets, in particular for making zones of varying thickness in the documents. 
     The invention seeks to satisfy these needs in full or in part by incorporating a ribbon, optionally a fiber ribbon, in a security sheet while it is being fabricated, the ribbon advantageously being suitable for being located at or close to a location where security elements are to be applied. 
     Thus, in one of its aspects, the invention provides a method of fabricating a sheet by a papermaking technique, the sheet comprising at least one ply of a fiber substrate and at least one fiber ribbon, where the ribbon is introduced into a fiber suspension of a vat former papermaking machine enabling the ply to be formed or is introduced directly into contact with the vat former, or in contract with the formed ply, in particular prior to the ply leaving the lifting cylinder. 
     The point at which the ribbon is introduced may be selected so as to create at least one region of reduced thickness or of increased thickness in the sheet in register with the ribbon, e.g. when it emerges from the fiber suspension or subsequently, the thickness reduction being due to a reduction in the de-watering of the fiber suspension caused by the ribbon. 
     By means of the invention, it is possible to obtain a security sheet that presents at least one region of reduced thickness or of increased thickness that is suitable for enabling security elements to be incorporated without requiring the use of a forming fabric that is modified or specific, in particular as a result of using a fiber ribbon that is incorporated in the fiber substrate during fabrication of the security sheet. As a result, it is possible to use the forming fabrics that are normally fitted to vat former papermaking machines that are used for fabricating security sheets. 
     Furthermore, the use of a ribbon presenting varying width and/or thickness may enable regions of reduced thickness or increased thickness to be obtained that are likewise of varying dimensions, for example. 
     The resulting regions of reduced thickness or of increased thickness may extend all along the sheet. 
     The ply carrying the ribbon need not partially overlie the ribbon, i.e. the fiber substrate does not cover the side portions of the ribbon on its main faces on either side thereof over a substantial width, e.g. greater than or equal to 1 mm. 
     By means of the invention, it is possible to incorporate one or more security elements of any type in the security sheet close to or in register with the regions of reduced thickness or increased thickness without that giving rise locally to regions of increased thickness associated with the presence of the security element(s), for example. A region of reduced thickness may for example make it possible to incorporate in register therewith at least one security element of thickness that may be compensated, at least in part, by the reduced thickness. A region of increased thickness may, for example, make it possible to juxtapose at least one security element with the ribbon, with the thickness thereof being compensated at least in part by the increased thickness. 
     The invention also makes it possible to provide novel security structures, independently of any thickness compensation as mentioned above. 
     The ribbon may be introduced into the fiber suspension or it may be put into direct contact with the vat former in various ways. 
     The term “direct contact” is used to mean that the point where the ribbon is introduced is in contact with the vat former where it is not covered in fibers of the fiber suspension at the point where the ribbon is introduced. In particular, the point of introduction may be located in a zone of the vat former that is not immersed in the fiber suspension. Preferably, and where appropriate, the cohesion of the sheet that is formed may be provided by a second fiber ply suitable for being placed adjacent to the first ply to be formed and in particular on the face that was in contact with the forming fabric during formation (“underside”). 
     The fiber ribbon may be introduced with one of its faces in direct contact with the vat former, prior to penetrating into the fiber suspension. This ensures that none of the fibers of the fiber suspension can be deposited on the face of the ribbon that is in contact with the vat former. This serves to obtain a paper ply having the face of the ribbon that was in contact with the vat former visible on one of its faces. Under such circumstances, a region of reduced thickness may be made in register with the other face of the ribbon, the face facing away from the vat former. 
     The ribbon may also be introduced into the fiber suspension at a ribbon introduction point where fibers are already deposited on the vat former. The fibers may already be deposited on the vat former over the entire surface thereof or over part of its surface. As a result, it is possible that no face of the ribbon comes into direct contact with the fabric of the vat former. The ribbon may on the contrary have one of its faces in contact with fibers that have already been deposited on the vat former prior to the ribbon being introduced into the fiber suspension. Thus, the face of the ribbon in contact with the fibers already deposited on the vat former need not be observable on the security sheet after fabrication. Under such circumstances, a region of thickness that is reduced or increased relative to the thickness of the vellum, i.e. the thickness of the zone of the sheet that does not comprise the ribbon, in particular the thickness of the substrate, may be made in register with the other face of the ribbon, the face facing away from the vat former. 
     The face of the ribbon that is in direct contact with the vat former or with fibers in the fiber suspension that have already been deposited on the vat former may obstruct holes in the forming fabric of the vat former and may thus prevent fibers being sucked against the fabric from the fiber suspension. 
     The ribbon may be introduced into a two-ply papermaking machine. Under such circumstances, the papermaking machine may, for example, have two vat formers placed in line, or one vat former and a flat bed, or indeed one vat former and a short former. In particular, the ply of paper formed on the vat former and comprising the ribbon may be united in the wet portion of the papermaking machine with a second ply of paper that comes to overlie the ribbon. The second ply may then serve to consolidate the first ply of paper in the zone where the ribbon is introduced. It may also constitute protection for elements present on or in the ribbon, e.g. an electronic device, and in particular an integrated microcircuit device, e.g. a radiofrequency identification (RFID) device. 
     It is also possible to use a papermaking machine fitted with a single vat former. 
     The ribbon may be introduced via a feeder device, e.g. such as an unreeling system placed along the width of the vat and leading into the vat close to the vat former, prior to the beginning of de-watering the fiber suspension on the fabric of the vat former. After de-watering, a security sheet is obtained that may subsequently be pressed and then dried using the conventional papermaking method. 
     The ply that is obtained may optionally be married in the wet state, i.e. before drying, with other plies having fiber compositions that may be identical or different and that may be formed on flat bed (Fourdrinier), short former, or vat former papermaking machines. 
     The ply that is obtained may be united by pasting with another ply of paper that comes to overlie the ribbon. 
     The sheet may be fiber constituted by two plies of paper, each comprising a ribbon with one or both of the sheets presenting at least one region of reduced thickness or of increased thickness in register with the ribbon. The two ribbons of the two plies may optionally be in contact with each other after the plies have been assembled together. 
     Advantageously, the ribbon may be introduced in the forward direction of the sheet, thus making it easy to introduce any security elements, where such elements are typically introduced in the same direction during a method of fabricating a security sheet. 
     The ribbon may have at least one of its faces that is not entirely covered by fibers of the fiber suspension on leaving the vat former papermaking machine. 
     The creation of a region of reduced thickness or of increased thickness may result in fibers from the fiber suspension being deposited in part only or not at all on at least one of the faces of the ribbon. This may depend in particular on the angle at which the ribbon is introduced, on its thickness, on a coating present on its surface, and/or on a portion in relief present on the vat former. 
     The thickness of the ribbon may be greater than or less than the thickness of the vellum of the security sheet. 
     When the ribbon has thickness that is less than the thickness of the vellum of the sheet in which the ribbon appears, the sheet may present reduced thickness in register with the ribbon, in particular in register with the face of the ribbon that faces away from the vat former, in particular when the ribbon is inserted in direct contact with the vat former or when the ribbon is inserted into the fiber suspension where the thickness of the fibers already present on the vat former is less than the difference in thickness between the thickness of the vellum of the sheet and the thickness of the ribbon. 
     In a variant, the sheet may present a region of increased thickness in register with the ribbon, e.g. when the ribbon is inserted into the fiber suspension at a point where the thickness of fibers already present on the vat former is greater than the difference in thickness between the thickness of the vellum and the thickness of the sheet. 
     When the ribbon presents a thickness that is greater than the thickness of the vellum of the sheet, the sheet may present increased thickness in register with the ribbon, in particular in register with the face of the ribbon that faces away from the vat former. 
     As mentioned above, the deposit of fibers on one of the faces of the ribbon, in particular its face facing away from the vat former, may be non-existent because the presence of the ribbon against the fabric of the vat former prevents de-watering, i.e. prevents from the fiber suspension becoming deposited against the fabric. 
     The deposit of fiber on one of the faces of the ribbon, in particular the face facing away from the vat former, may also be a partial deposit only, in particular because the ribbon may have been previously treated, e.g. with a coating so as to reduce its porosity, thereby preventing or slowing down the deposition of fibers on the treated zone. By way of example, the treated zone may correspond to the central zone of the ribbon, in particular on its face facing away from the vat former. 
     The treatment of the zone may, for example, be the result of applying a coating such as a composition comprising a hydrophobic and/or pore-filling compound. By way of example, this application may consist in a taking of latex or of a hydrophobic varnish. 
     The vat former need not have any portions in relief on which the ribbon is placed, or in a variant it may comprise some. Where appropriate, a portion in relief may be formed by embossing the forming fabric. For example, the ribbon may be introduced in such a manner that its center coincides with the center of the portion in relief on the vat former. 
     By way of example, relief on the vat former may present a shape in face view that is square, round, oval, or polygonal, with outlines that are rectilinear or curvilinear, and it is preferably rectangular. The portion in relief may be continuous or discontinuous along the circumference of the vat former. The ribbon may be of a width that is greater than that of the portion in relief. As a result, it is possible for example to have at least a portion of the ribbon, in particular both edges of the ribbon, not overlying the portion in relief. 
     The ribbon may be introduced where fibers from the fiber suspension are already deposited on the portion in relief of the vat former. As a result, the ribbon need not come directly into contact with the portion in relief of the vat former, since one of its faces comes into contact with fibers that have already been deposited on the vat former. 
     In a variant, the ribbon may be introduced where no fibers from the fiber suspension have been deposited on the portion in relief of the ribbon, in particular at the point where the ribbon is introduced. As a result, one of the faces of the ribbon may come directly into contact with the portion in relief on the vat former, which face may be visible on the security sheet after it has been fabricated. 
     When the vat former comprises a portion in relief and the ribbon is wider than the portion in relief, then the fibers from the fiber suspension may be deposited between the vat former and the portion(s) of the ribbon that do not overlie the portion in relief. As a result, it may be possible to create one or more regions of reduced thickness in the sheet in register with the face of the ribbon that faces the vat former. Under such circumstances, it may also be possible to create one or more regions of reduced thickness or of increased thickness in register with the face of the ribbon facing away from the vat former, in particular as a function of the thickness of the ribbon compared with the thickness of the vellum of the sheet. 
     The presence of one or more regions of reduced thickness or of increased thickness may make it possible to receive one or more security elements, at least in part, either overlying said regions or juxtaposed therewith. In particular, the security elements may be placed between two regions of increased thickness, thereby providing them with mechanical protection, e.g. against impacts, printing such as copper-plate printing, or subsequent pressing or lamination steps. 
     When the thickness of the security elements is substantially identical to the difference in thickness between the thickness of the vellum of the sheet and the thickness of the ribbon, it is possible to obtain a security sheet that presents substantially no regions of reduced or increased thickness. 
     In particular, the security sheet may comprise at least one security element overlying the ribbon on the reduced thickness side and presenting a thickness that is compensated at least in part by the reduced thickness. The reduced thickness may completely compensate the thickness of the security element. 
     Also in particular, the security sheet may comprise at least one security element juxtaposed with the ribbon and of thickness that is compensated at least in part by a region of increased thickness. The increased thickness may compensate the thickness of the security element completely. 
     The regions of increased thickness may for example serve to obtain particular security effects as a result of applying treatment to the fiber ribbon, e.g. embossing in register with the regions of increased thickness. 
     The ribbon may be treated in the regions of increased thickness to obtain a security sheet that presents substantially no reduced thickness or increased thickness. 
     The ribbon may for example be subjected to embossing, in particular hot embossing, in particular in register with a region of increased thickness. Under such circumstances, the ribbon may preferably comprise polyethylene fibers. Introducing a ribbon comprising polyethylene fibers makes it possible in particular subsequently to create particular portions in relief by hot embossing while imparting better cohesion to the sheet since the cohesion of a cellulose substrate is better than that of a substrate made up of polyethylene fibers, for example. 
     The ribbon may present a region of increased thickness, in particular prior to optional embossing of the ribbon, that extends over at least 50%, better 80% or even more of the surface of the face of the ribbon where the region of increased thickness appears. The region of increased thickness may extend over all of one face of the ribbon. 
     After embossing, the sheet need not have any region of increased thickness. The surface of the face of the ribbon that presented increased thickness prior to embossing may be substantially plane. 
     The ribbon may present portions in micro-relief, e.g. indentations or projections, in particular after it has been embossed, and these portions may be of small dimensions, e.g. lying in the range 1 micrometer (μm) to 3 μm. These portions in micro-relief do not form a region of increased thickness or reduced thickness in the meaning of the invention. 
     The reduced thickness or increased thickness may make it possible to obtain respectively a setback or a projecting portion in relief of the security sheet presenting a dimension of the order of several micrometers. Where appropriate, the reduced thicknesses or the increased thicknesses may be of values that are determined depending on the thicknesses of the security elements that are to be associated therewith. For example, printing or a security film may be deposited in a region of reduced thickness or between two regions of increased thickness. The looked-for purpose may in particular be obtaining a finished sheet that is flat, i.e. that comprises no regions of reduced thickness or of increased thickness. 
     The thickness reductions that are obtained may for example have values that are greater than or equal to 15 μm, preferably greater than or equal to 30 μm, more preferably greater than or equal to 60 μm. For example, it is possible to obtain thickness reductions that are greater than or equal to 44 μm for a substrate presenting thickness that is greater than or equal to 115 μm. 
     The regions of increased thickness may be a function of the point of introduction of the ribbon and of its thickness. For reasons of machineability, it may be advantageous to limit regions of increased thickness to less than 20 μm even though the protection of certain security elements may require thicker regions of increased thickness. 
     The ribbon may also be introduced by bringing the ribbon into contact with the papermaking ply present on the fabric of the vat former at the outlet from the forming vat. By way of example, the ribbon may be inserted between the ply leaving the forming vat and a second ply that is assembled with the first between the lifting cylinder and the vat former. 
     Ribbon 
     The term “ribbon” designates a ribbon made as a single piece or made as an assembly, a stack, or a juxtaposition of strips or of individual parts. 
     Advantageously, the fiber ribbon is a paper ribbon. Introducing a paper ribbon, as contrasted with introducing a plastics ribbon, for example, may enable a sheet to be obtained that presents good cohesion because of the bonds that may develop between the paper ribbon and the substrate. In particular, when a second ply is applied to the ribbon, the fact that the ribbon is made of paper, and not of a plastics material, for example, may provide strong cohesion between the ribbon and the second ply. 
     The ribbon may be a fiber ribbon and it may comprise non-fiber elements. 
     The ribbon may present weight lying in the range 5 grams per square meter (g/m 2 ) to 100 g/m 2 , preferably in the range 15 g/m 2  to 55 g/m 2 , and more preferably in the range 20 g/m 2  to 30 g/m 2 . 
     The width of the ribbon may lie in the range 2 millimeters (mm) to 60 mm, preferably in the range 4 mm to 30 mm, and more preferably in the range 10 mm to 20 mm. 
     The ribbon may present a width that is greater than the width of a portion in relief on the vat former, where appropriate. 
     The ribbon may present thickness that is greater than 10 μm, preferably lying in the range 20 μm to 120 μm, and more preferably in the range 30 μm to 80 μm. 
     The thickness of the ribbon may be different from the thickness of the vellum of the sheet. For example, the ribbon may present thickness that is greater than that of the vellum of the sheet so as to enable regions of increased thickness to be created. The ribbon may also have a thickness that is less than the thickness of the vellum of the sheet so as to enable regions of reduced thickness to be created. 
     The ribbon may also comprise at least one electronic device, in particular an integrated microcircuit device. The electronic device may be a detector. The electronic device may be a radiofrequency identification device, commonly referred to as an RFID device, and in particular it may be an electronic chip and/or an antenna that may be printed on the ribbon, in particular. By way of example, the electronic device may be deposited and fastened, e.g. by means of adhesive, on one of the faces of the ribbon. The electronic device may also be incorporated inside the ribbon, in particular prior to the ribbon being introduced close to the vat former, either in direct contact therewith or in the fiber suspension. 
     The ribbon may be fabricated from a material, in particular a paper, that has previously been subjected to wet strength (WS) treatment. 
     The ribbon may present wet strength (WS) in traction that is greater than 30%, in particular so as to avoid problems when inserting the ribbon in the sheet. Wet strength in traction is measured by dividing the wet strength value in traction as measured using the standard NF Q 03.056 by the dry strength value in traction as measured using the standard NF EN ISO 1924. This wet strength may be provided in particular in conventional manner by adding a wet strength agent, e.g. such as a polyamine amide epichlorhydrine (PAAE) resin, a melamine formol resin, etc. . . . 
     In a variant, additional wet strength (WS) may be obtained by adding a fraction of the WS agent(s) in fibrous sheet surface treatment baths, e.g. used in fabricating the ribbon. 
     The security elements that may be incorporated in the ribbon or in the proximity of the ribbon may be of various types. 
     The security elements may show various patterns, e.g. one or more alphanumeric characters, drawings, or logos, e.g. as present elsewhere on the sheet. 
     The sheet may present at least two regions of reduced thickness or two regions of increased thickness or one of each facing each other and at least two security elements may be incorporated in or in the proximity of each of these regions of reduced thickness or of increased thickness, the security elements possibly facing each other. 
     The security elements may be applied, for example by printing in register. The term “printing in register” is used to mean that the positions of the security elements on the ribbon are determined. 
     The two security elements of the ribbon may enable differing effects to be obtained, in particular optical effects, by virtue of being complementary and observable in show-through. 
     The term “observable in show-through” is used to mean that observation is performed using light that passes through the ribbon in its thickness direction, e.g. light that is visible, ultraviolet, or infrared. 
     By way of example, the two security elements may serve to obtain a “moiré” effect in show-through. 
     By way of example, a moiré effect may reveal a pattern produced by superposing two security elements, e.g. by combining lines of both of the security elements. For example, a first printing may be provided on the recto face of the ribbon and a second print on the verso face of the same ribbon. The prints may be performed in or in the proximity of regions of reduced thickness or increased thickness. Then, when the ribbon is observed in show-through, a complementary pattern may appear by the moiré effect between the first and second print. 
     The two security elements may also enable “combinatorial” effects to be obtained in show-through. 
     A “combinatorial” effect results from observing a particular pattern in show-through, which pattern results from combining two patterns, each appearing on the recto and the verso of the ribbon. For example, a first pattern may be printed on the recto of the ribbon and a second pattern may be printed on the verso of the ribbon. When observed in show-through, a third pattern may then be revealed, as a result of combining the first and second patterns. 
     The security elements may be printing, in particular printed ink. The inks may comprise additional security elements, such as specific pigments. 
     The security elements may also be films, e.g. plastic films that are optionally transparent or that present reduced opaqueness, e.g. less than or equal to 40%, preferably less than or equal to 30%, with measurement being performed in compliance with the ISO 2469 standard. 
     Whether made of fibers or otherwise, the ribbon may comprise one or more openings, e.g. made in part or in full in the ribbon. 
     These openings may be of various shapes, e.g. they may be square, oval, round, rectangular, polygonal, or star shaped, among other possibilities. The openings may correspond to holes, to slots, to designs, to optionally through cavities, e.g. made by embossing, cutouts in the form of patterns that are alphanumeric, and of a variety of shapes and sizes. The openings may be through openings. The openings may be made after the security sheet has been formed. In a variant, the openings may be made in the ribbon before it is introduced. 
     The openings may comprise one or more security elements, in particular an electronic device, in particular an integrated microcircuit device, e.g. an RFID device. 
     Where appropriate, the security elements may extend fully or partially in the openings. 
     Security elements, in particular films, may be placed in or in the proximity of regions of reduced thickness or of increased thickness, in particular mutually facing regions of reduced thickness or of increased thickness. In particular, the security elements may be juxtaposed on either side of regions of increased thickness or they may overlie regions of reduced thickness, with the security elements facing one another. 
     The regions of reduced and/or increased thickness may then compensate the thickness of the security elements, at least in part. 
     The security elements, in particular films, may overlie regions of reduced thickness in register with holes made in the ribbon. 
     At least two security elements, in particular films, may overlie at least two facing regions of reduced thickness, with at least one opening being made in the ribbon in register with the two regions of reduced thickness. 
     The sheet may in particular comprise regions of reduced thickness on either side of the ribbon with at least part of at least one security element extending therein, in particular a film that extends parallel to the ribbon in contact therewith. A security element, in particular a film, may extend at least in part in each of the regions of reduced thickness of the sheet. 
     The security elements, in particular films, may be complementary, i.e. when two elements are observed in show-through, it is possible to obtain various optical effects, e.g. moiré effects and/or combinatorial effects, e.g. a pattern may appear that is the result of superposing patterns present in the two security elements. 
     By way of example, the security elements may present opaqueness suitable for enabling particular optical effects to be observed in show-through. 
     The security elements, in particular films, may be introduced in the proximity of the vat former during formation of the sheet. 
     By way of example, the films are of thickness that is greater than or equal to 10 μm, preferably greater than or equal 30 μm. 
     The thickness of the films may be compensated at least in part by the regions of reduced or increased thickness. For example, the total thickness of a film plus a ribbon may be equal to the thickness of the vellum of the sheet to within plus or minus 5 μm, better to within plus or minus 3 μm. That constitutes an advantageous embodiment of the invention enabling a sheet to be obtained that is essentially flat, thereby reducing problems that can be caused by differences in thickness and that may occur during possible subsequent steps: for example steps of rolling, unrolling or cutting the sheet, or steps of stacking and printing sheets. 
     The films may comprise a specific material that collects light. Such luminescent light-collecting materials that may be suitable are referred to as “wave guides”. By way of example, they may be polycarbonate-based polymer films called Lisa® and sold by the supplier Bayer. 
     The films may be inks, varnishes, resins, or cutout strips, that are pasted or laminated, in particular hot-laminated, or extruded. 
     The films may be colored, in particular they may have different colors. 
     The ribbon may comprise at least one security thread, in particular a security thread on which an electronic device appears, in particular an integrated microcircuit device, e.g. an RFID device. 
     An electronic device, in particular an integrated microcircuit device, e.g. an RFID device, may be housed at least in part in a region of reduced thickness, the thickness reduction compensating for the thickness of the electronic device, at least in part. 
     The ribbon may be perforated and fibers of the substrate may appear in the perforations in the ribbon. In particular, the fibers of the substrate that appear in the perforations of the ribbon may have a color that is different from that of the ribbon. 
     The ribbon and/or the substrate may comprise a plurality of particular characteristics as mentioned below. These characteristics may apply to the entire ribbon and/or to the entire substrate, or else to a portion only of the ribbon and/or of the substrate, and more preferably to one or more portions of the ribbon and/or the substrate where one or more regions of reduced thickness or increased thickness appear. 
     By way of example, the ribbon may be combined with a strip of thermoplastic and/or metal material, e.g. of polyethylene terephthalate (PET), pasted or hot-laminated to the ribbon before it is introduced in the proximity of the vat former. 
     The ribbon and/or the substrate may be deprived of any watermark. 
     In a variant, the ribbon and/or the substrate may comprise at least one watermark. 
     The watermark of the ribbon may be situated in a region of reduced thickness or of increased thickness of the security sheet. 
     When the ribbon and the substrate comprise a watermark, causing the watermark of the substrate to be superposed relative to the watermark of the ribbon may enable a pattern to be observed that is the result of them being associated. 
     The ribbon watermark may also be juxtaposed with the substrate watermark so that the juxtaposition creates a new pattern. 
     The optional watermark of the ribbon and/or of the substrate may be a pale watermark. Under such circumstances, the pale zones of the watermark present a weight per unit area that is strictly less than that of the vellum. In a variant, the watermark may be a dark watermark. The watermark may also be a multi-tone effect watermark comprising a set of pale zones arranged to form a screened image presenting pale and dark tones. The screened image may comprise screened patterns, e.g. made up of lines. Patent application EP 1 122 360 discloses how such a watermark can be made. 
     The optional watermarks of the ribbon and of the substrate may also be superposed so as to obtain a moiré effect. 
     The ribbon may present optical properties that are different from those of the substrate, in particular in a region of reduced thickness and/or a region of increased thickness, in particular properties concerning color, fluorescence, phosphorescence, magneto-optics, photochromy, thermochromy, piezochromy, iridescence, or other properties. 
     For example, the substrate and the ribbon may have different colors. Any color is possible. The ribbon and the substrate may present a color or colorimetric difference ΔE (using the CIE Lab 1976 color space) that is greater than 0.2 and preferably greater than 1. Nevertheless, the human eye is more sensitive to a color difference on hues that are not saturated, and the person skilled in the art may adapt the color difference so as to obtain the desired level of “contrast”. 
     The ribbon may comprise a conventional ink or a security ink, in particular a fluorescent, phosphorescent, magnetic, photochromic, thermochromic, piezochromic, iridescent, transparentizing, or other ink. The ink may be applied in part or completely to the ribbon, i.e. on only certain zones of the surface of the ribbon or over the entire surface of the ribbon. The ink may be applied to the ribbon by print means, e.g. offset, photogravure, silkscreen, or ink-jet print means. 
     One or more security elements may be selected from the above-mentioned inks and may be applied to the region of reduced thickness or juxtaposed with the region of increased thickness. 
     The ribbon may also be coated in a varnish that is transparent or colored. 
     The ribbon may include a fluorescent agent. The fluorescent agent may optionally be colored, and may be visible or invisible in visible light. By way of example the fluorescent agent may be incorporated in the material of the ribbon. By way of example the fluorescent agent may be visible under ultraviolet (UV) light and/or infrared (IR) light. 
     Preferably, the ribbon may extend between two opposite edges of the sheet. 
     The ribbon may present edges of irregular shape, e.g. in the form of broken or undulating, crenellated, zigzag, or other lines. The edges of the ribbon may be of shapes that are the same or different, that are regular or irregular. 
     The ribbon may also present characteristics that are perceptible, e.g. on sight, or to the touch. 
     By way of example, the ribbon may be embossed, thereby serving to create a particular effect, e.g. a touch effect or a surface relief effect. The effect may serve to form microportions in relief on the ribbon, that are distinct from the regions of reduced or increased thickness. 
     The ribbon may be coated in a thermoplastic polymer and then optionally embossed after it has been coated. 
     The ribbon may be printed with drops of varnish, of resin, or of hot-puff ink, among others. 
     When the ribbon comprises printed marks, this may make it possible to obtain variable optical effects, for example. 
     The ribbon may comprise a hot-puff ink serving to create at least one portion in relief, the ink being activated prior to introducing the ribbon. Activation prior to incorporation serves to reduce the risk of the sheet being destroyed as the ink swells. 
     The ribbon may comprise particles, e.g. detectable to the naked eye or to the touch, that are introduced into the material of the ribbon, e.g. in the form of beads, e.g. beads of synthetic material, e.g. polyurethane, or of inorganic material, e.g. glass, the beads presenting a size of less than 300 μm, for example, or being provided by silkscreen or photogravure deposition, e.g. in the form of beads, e.g. presenting a size of less than 100 μm. 
     The ribbon may comprise a textile strip, e.g. a knitted strip, that is incorporated in the ribbon or is pasted thereto. The ribbon may comprise textile fibers. The ribbon may be a woven fabric or a non-woven fabric. 
     The ribbon may comprise printing in relief, e.g. silkscreen printing. 
     The ribbon may present a greasy, rough, smooth, silky, smooth, or other feel. 
     The ribbon may in particular comprise deposits of polymers, in particular in the form of beads, e.g. beads of polyurethane (PU), beads of glass, polyamide 6 or polyamide 12, styrene-acrylic pigments, waxes, or beads of polypropylene (PP), of polyethylene (PE), of polyvinyl chloride (PVC), of polymers based on methyl methacrylate. 
     The ribbon may advantageously conserve its own cohesion after being introduced into the fiber substrate. After it has been introduced into the fiber substrate, the structure of the ribbon may be observable under certain conditions, e.g. in section or by viewing in reflection or in transmission. Thus, it is possible to observe the ribbon by differences in optical properties, e.g. differences in contrast, hue, saturation, paleness, or opaqueness, that make the frontier between the ribbon and the substrate visible or that characterize the presence of the ribbon. 
     The ribbon may also be observable topographically in the sheet. 
     Where appropriate, the ribbon may also be observable in transparency through the sheet, in particular to the naked eye. 
     In order to obtain the ribbon, it is possible for example to fabricate a fiber sheet, preferably of low weight, e.g. of weight lying in the range 15 g/m 2  to 90 g/m 2 , using a conventional papermaking method, e.g. by de-watering a suspension of fibers possibly comprising fillers and conventional papermaking additives, optional specific security elements, then pressing the fiber mass that is obtained, and then drying it. The sheet is then optionally calendared serving in particular to reduce its thickness, and then cut to form the ribbons that are then wound onto reels. 
     The ribbon may comprise cellulose fibers (in particular cotton fibers) and/or natural organic fibers other than cellulose fibers and/or synthetic fibers, e.g. such as polyester or polyimide fibers and/or inorganic fibers, e.g. such as glass fibers. The ribbon may be deprived of fibers of polyvinyl alcohol (PVA). 
     The fibers may be hydrophilic, in particular so as to develop chemical bonds, mainly hydrogen bonds, with the fiber substrate. 
     The ribbon may have at least 50% cellulose fibers in dry weight. 
     The ribbon may comprise perforations, in particular to enhance de-watering during fabrication of the sheet or indeed to add an additional optical effect, in particular in combination with at least one watermark. For example, these perforations may be located at the border of said ribbon or in such a manner as to form a pattern or a code. 
     The ribbon may comprise a watermark and perforations around the watermark. 
     The ribbon may comprise at least one security element on one of its faces, or indeed on each of its faces. The ribbon may comprise different security elements on each of its faces. 
     The ribbon may comprise one or more security elements superposed on the ribbon in a region of reduced thickness or juxtaposed with a region of increased thickness of the ribbon and/or security elements incorporated in the ribbon or placed on the ribbon independently of the regions of reduced or increased thickness. These security elements may be as described below. 
     Among security elements, some are detectable to the naked eye, in visible light, without using any particular appliance. By way of example, these security elements include a watermark, colored fibers or flakes, printed, metal-plated or holographic threads, holographic foils, or variable optical effects printed marks. 
     Such security elements are said to be “first-level” elements. 
     Other types of security element may be detected solely with the help of an appliance that is relatively simple such as a lamp emitting ultraviolet or infrared light. By way of example such security elements comprise fibers, flakes, strips, threads, or particles. These security elements may optionally be visible to the naked eye, e.g. luminescing when illuminated with a Wood lamp emitting at a wavelength of 365 nanometers (nm). 
     These security elements are said to be “second level” elements. 
     Still further types of security elements require a more sophisticated detector appliance in order to be detected. By way of example, these security elements are capable of generating a specific signal on being subjected simultaneously or otherwise to one or more external excitation sources. Automatic detection of the signal enables a document to be authenticated, where appropriate. 
     By way of example, these security elements comprise tracers that are present in the form of active material, particles, or fibers capable of generating a specific signal when the tracers are subjected to optronic, electric, magnetic, or electromagnetic excitation. 
     These security elements are said to be “third level” elements. 
     The security elements carried by the ribbon, whether fibrous or otherwise, may be first, second, or third level. 
     The security elements may in particular be selected from tracers, in particular nanometric tracers, security fibers, in particular fibers that are metallic, magnetic (with soft and/or hard magnetism), or absorbent or excitable in the ultraviolet (UV), the visible or the infrared (IR), and in particular in the near infrared (NIR), security elements that are flat and of relatively small format such as flakes, pigments or agglomerations of pigments in particular those that are absorbent or excitable under laser illumination or illumination in the ultraviolet (UV), visible, or infrared (IR), and in particular in the near infrared (NIR), and security threads (based on plastics, in particular polyester) comprising in particular an at least partial coating, being metallic, metal-plated, iridescent, or magnetic (with soft and/or hard magnetism), the coating potentially including positive or negative patterns, and said patterns being obtainable in particular by removing metal, chemical or biochemical reagents for countering falsification and/or for authentication, in particular being suitable for reacting with at least one falsification and/or authentication and/or identification agent, respectively, and optically variable elements, in particular holograms, liquid crystals, iridescent pigments, or mirror effect structures, in particular dielectric structures, and combinations thereof. 
     The ribbon may comprise a series of parallel security threads, and more particularly the succession of spaces between the threads and/or the series of different widths of security thread may constitute a code, in particular of the bar code type. 
     The ribbon may also comprise an antitheft thread with soft magnetism. 
     The ribbon may also comprise at least one chemical or biochemical reagent for countering falsification and/or for authentication and/or for identification that reacts in colored manner with at least one falsification and/or authentication and/or identification agent. 
     The ribbon may comprise perforations in a pattern or a code. Depending on differences in opaqueness and color between the substrate and the ribbon, the pattern or code is observable or is observable only in transmitted light or is observable both in transmitted light and in reflected light. If the ribbon possesses invisible fluorescent printing, the pattern may also be visible only under UV illumination. 
     In particular, the pattern made by perforation may comprise at least one alphanumeric character or ideogram. 
     When the ribbon comprises at least one opening, the opening may overlie at least in part a corresponding opening formed in a second ply of paper placed against the first ply in which the ribbon is incorporated. The opening in the ribbon may correspond, for example, exactly with the opening in the second ply. At least one security element may overlie the opening, with the security element being received in a region of reduced thickness created in the first ply of paper by the ribbon, for example. The presence of the opening in the second ply may make it possible, for example, to observe the inside face of the security element, facing towards the ribbon. The ribbon may optionally be a fiber ribbon. 
     The invention enables a security sheet to be made that comprises two plies of paper each provided with a respective ribbon that has served to create a region of reduced thickness. These two plies of paper may be united in such a manner that the ribbons overlie each other at least in part, and preferably substantially exactly. Each region of reduced thickness may receive a security element. Each of the ribbons may comprise at least one zone of reduced opaqueness, e.g. defined by an opening, and the two security elements disposed on either side of the set of ribbons may be observed by looking through zones of reduced opaqueness of the ribbons. By way of example, this may enable additional patterns to be observed. The ribbons may be made of fiber, or be deprived of fibers. 
     The invention may enable a security sheet to be made by uniting two papermaking plies, each provided with a ribbon defining a region of reduced thickness. The two plies may be adjacent via faces that present regions of reduced thickness, and these regions of reduced thickness may receive a third ribbon introduced between the plies. The third ribbon is thus situated between the ribbons that are incorporated within each of the plies. 
     The invention may also enable a security sheet to be made that comprises two ribbons that are disposed in such a manner as to leave a space between them, these ribbons forming regions in one face of the sheet that are of thickness that is increased, relative to the fiber substrate, with a security element being located between the projecting portions of the ribbons. The presence of the ribbons serves to protect the security element from a risk of being flattened, for example. 
     The ribbon need not have a surface coating, in particular it need not have a pigmented layer. Good affinity between the fiber substrate and the ribbon is thus enhanced. 
     The ribbon may present wet strength (WS) in traction that is greater than 30%, in particular so as to avoid giving rise to problems while being introduced in the sheet. 
     The ribbon may comprise an adhesive, e.g. a heat-sealing agent, in order to improve its cohesion within the fiber substrate. 
     When the ribbon comprises an adhesive, the adhesive may serve to enhance adhesion of the ribbon in the sheet. 
     By way of example, the adhesive may be a heat-sealing coating, e.g. a heat-sealing varnish, a UV-curable agent, an irradiation-sensitive adhesive, a pressure-sensitive adhesive (PSA), a solvent-based varnish, e.g. of the polyester type, an aqueous phase adhesive, etc. . . . 
     As aqueous phase adhesive, mention may be made in particular of the following trademarks: Mowilith DC (an aqueous dispersion of vinyl acetate homopolymer with particles having a size lying in the range 0.3 μm to 2 μm and a glass transition temperature T g  of about 38° C., and a solid matter dry content lying in the range 55% to 57%), and Vinamul 3265 from the supplier Celanese; DH9004, DH9017, DH9044, and DL5001 from the supplier Collano; Primal NW1845, Primal LC40, Primal P308M, and Primal EP6000 from the supplier Rohm &amp; Haas; 006SDW078-2 from the supplier BASF. 
     The adhesive may advantageously be an adhesive based on polyvinyl acetate, such as Mowilith DC. 
     The Applicant has observed that among the above-mentioned adhesives, Vinamul 3265, Mowilith DC, Collano DL5001, Primal NW1845, and Primal P308M present very good results in the dry crease test, i.e. the adhesion of the ribbon in the paper is not excessively affected by creasing. 
     Furthermore, Vinamul 3265 and Mowilith DC also present very good results in the wet crease test. Mowilith DC presents very good results in the washing test. 
     Also, Mowilith DC provides very good results in terms of adhesive power, but the invention is not limited to any particular adhesive. 
     In an embodiment of the invention the adhesive is not present in the form of fibers nor in the form of particles. 
     The adhesive may be added to the security ribbon in particular by a coating method, by a surface-application method, e.g. coating, or by methods derived from techniques for printing a paper base after it has been fabricated, e.g. air-knife coating, photogravure, silkscreen-printing, curtain coating, flexography, among others. 
     The adhesive may be applied, e.g. on a fiber sheet used for fabricating the ribbon, e.g. at a rate of 2 g/m 2  to 15 g/m 2  per face, preferably at a rate of 3 g/m 2  to 8 g/m 2  per face. 
     The sealing temperature may lie in the range 70° C. to 135° C., for example, depending on the adhesive used, and the sealing may take place while the security sheet is being dried. 
     The surface application may be performed on the surface, or at least in part to the core, i.e. it may penetrate deep, possibly over the entire ribbon. 
     Preferably, the surface application is performed on the medium that is used for fabricating the ribbon prior to cutting up said medium. In a variant, the surface application may be performed while fabricating the medium that is used for fabricating the ribbon. 
     The surface application may be performed on a single face of the medium or on both faces thereof. The adhesive may cover completely the face on which it is deposited. 
     The adhesive may also be incorporated in the ribbon by impregnation, by immersing the ribbon or the medium used for fabricating it in a bath. 
     In an embodiment of the invention, a 20 g/m 2  fiber sheet that has been subjected to wet strength (WS) treatment is coated with an aqueous phase adhesive at 5 g/m 2  per face, using a photogravure method, for example. This obtains a satisfactory compromise between adhesive power and the quantity of coated material. 
     In order to detect the presence of adhesive in a security sheet of the invention, it is possible to observe the adhesive, e.g. the heat-sealing varnish, in the form of bubbles, e.g. using a scanning electron microscope (SEM). 
     Substrate 
     The term “substrate” designates a fiber sheet that may in particular comprise one or more fiber layers. In particular, the term “ply” is used to designate layers that are produced continuously on a given papermaking machine and then associated while in the wet state. 
     The substrate may comprise at least one security element as described above, for example selected from tracers, in particular nanometric tracers, security fibers, in particular fibers that are metallic, magnetic (with soft and/or hard magnetism), or absorbent or excitable in the ultraviolet (UV), the visible or the infrared (IR), and in particular in the near infrared (NIR), security elements that are flat and of relatively small format such as flakes, pigments or agglomerations of pigments in particular those that are absorbent or excitable under laser illumination or illumination in the ultraviolet (UV), visible, or infrared (IR), and in particular in the near infrared (NIR), and security threads (based on plastics, in particular polyester) including in particular an at least partial coating, being metallic, metal-plated, iridescent, or magnetic (with soft and/or hard magnetism), the coating potentially including positive or negative patterns, and said patterns being obtainable in particular by removing metal, chemical or biochemical reagents for countering falsification and/or authentication, in particular being suitable for reacting with at least one falsification and/or for authentication and/or identification agent, respectively, and optically variable elements, in particular holograms, liquid crystals, iridescent pigments, or mirror effect structures, in particular dielectric structures, and combinations thereof. 
     The ribbon substrate may in particular comprise security elements that are visible to the naked eye, but it may also comprise tracers that are in the form of active material, particles, or fibers, capable of generating a specific signal when the tracers are subjected to an optronic, electric, magnetic, or electromagnetic excitation. Such “tracers” constitute a substance that is identifiable by means of a distinctive property and that is used to mark an element (here a security document) and to enable it to be tracked, its evolution to be tracked, or enabling it to be recognized, authenticated, or identified. 
     Mirror dielectric structures are constituted by alternating layers of high and low refractive index, e.g. respective layers of hafnium dioxide and of silicon dioxide, and may in particular be obtained by ion etching. 
     In a particular embodiment of the invention, the fiber substrate comprises at least one chemical or biochemical reagent for countering falsification and/or for authentication and/or for identification that reacts in colored manner with at least one agent respectively for falsification and/or authentication and/or identification. 
     The fiber substrate may be based on fibers such as those described above for the ribbon. 
     In particular embodiment of the invention, the fiber substrate and the ribbon are based on the same fibers, i.e. their fiber composition is the same in terms of nature and preferably in terms of proportions, thus ensuring good affinity between the components and possibly also constituting additional authentication means. 
     Security Sheet 
     In another of its aspects, the invention also provides a sheet comprising a fiber ribbon incorporated in part in a fiber substrate made using a single ply papermaking machine, the ribbon having at least one face situated set back or projecting relative to the vellum of the sheet and that is not entirely covered by the substrate. 
     In another of its aspects, the invention also provides a sheet comprising a fiber ribbon and a fiber substrate, at least one security element being placed in register with the ribbon, in particular overlying the ribbon and having a thickness that is compensated at least in part by a thickness difference between the ribbon and the vellum of the sheet and/or by an opening in the ribbon. 
     In another of its aspects, the invention also provides a sheet comprising first and second fiber plies adjacent to each other, the first ply comprising an optionally fiber ribbon forming at least one region of reduced thickness in the face of the first ply facing away from the second ply, and the second ply comprising an opening that overlies an opening in the ribbon at least in part and preferably exactly. 
     In another of its aspects, the invention also provides a sheet comprising two mutually adjacent fiber plies, each of the plies comprising an optionally fiber ribbon, each ribbon forming a region of reduced thickness in each of the faces of the plies that are not adjacent to each other, each ribbon comprising an opening, the openings overlying at least in part and preferably exactly. 
     In another of its aspects, the invention also provides a sheet comprising two mutually adjacent fiber plies, each including an optionally fiber ribbon, each ribbon creating a region of reduced thickness, the regions of reduced thickness overlying each other at least in part and preferably exactly so as to leave between the plies a cavity suitable for receiving another optionally fiber ribbon. 
     In another of its aspects, the invention also provides a sheet comprising a fiber substrate with two optionally fiber ribbons forming two regions of increased thickness on the surface of the sheet, the ribbons being spaced apart from each other in such a manner as to enable a security element to be placed between the two ribbons. 
     The sheet may be obtained by the above-described method. In particular, the sheet may comprise any one of the above-mentioned characteristics. 
     Security Document 
     The invention also provides a security document including a sheet as defined above. 
     The document may be fabricated by cutting, associating, binding, laminating, adhesively bonding, and/or pasting said sheet. 
     More particularly, the security document may constitute payment means, such as a bank note, a check, or a meal ticket, an identity document such as an identity card or a visa or a passport or a driver&#39;s license, a lottery ticket, a transport ticket, or indeed a ticket for entry to cultural or sporting events. 
     Article to be Authenticated 
     The invention also provides an article to be authenticated that comprises the sheet as defined or obtained above, the article being selected from a security label, packaging, in particular packaging for medication or for food or for cosmetics or for perfume or for electronic components or for spare parts, a sheet used in the medical or hospital field, in particular a sheet used for making sterilization packages, or indeed art paper. 
    
    
     
       The invention can be better understood on reading the following description of non-limiting embodiments of the invention and on examining the diagrammatic and fragmentary figures of the drawings, in which: 
         FIG. 1  is a section view showing an example of a ribbon of the invention being incorporated; 
         FIGS. 2 and 3  are section views showing other examples of the ribbon of the invention being incorporated; 
         FIGS. 4 to 10  are section views showing examples of sheets of the invention; 
         FIG. 11  is a view from above the  FIG. 10  sheet; 
         FIG. 12  is a view from below of the  FIG. 10  sheet; and 
         FIGS. 13 to 15  are section views showing various sheets of the invention. 
     
    
    
       FIG. 1  is a section view showing an example of a ribbon  3  being incorporated in a method of fabricating a sheet  1  in accordance with the invention. 
     The method is implemented on a papermaking machine, shown in part, fitted with a lifting cylinder  40  and a vat former  4  that is partially immersed in a fiber suspension  5  and that serves to form a sheet  1  comprising a ply of a fiber substrate  2  and a fiber ribbon  3 . 
     In this example, the fiber ribbon  3  is introduced in direct contact with the vat former  4  via one of its faces  9  prior to penetrating into the fiber suspension  5  used for forming the substrate  2 . 
     In this way, no fiber of the fiber suspension  5  can become deposited on the face  9  of the ribbon  3  that is in contact with the vat former  4 , in particular because the face  9  obstructs the holes in the forming fabric of the vat former  4 . The face  9  of the ribbon  3  can then be visible on the sheet  1  after fabrication. 
     Thus, only the face  6  of the ribbon  3 , i.e. the face facing away from the vat former  4 , may optionally be covered by the fibers of the fiber suspension  5 . 
     The ribbon  3  may have been subjected to treatment on at least part of its face  6  that prevents any deposition of fibers in the fiber suspension  5 . 
     One or more regions of reduced or increased thickness may be formed in the ribbon  3  and one or more security elements may be applied to the ribbon  3 , in particular elements that are superposed on regions of reduced thickness or juxtaposed with regions of increased thickness. 
     After the sheet  1  has been formed on the vat former  4  and after the fiber suspension  5  has been de-watered on the fabric of the vat former  4 , the sheet  1  may be pressed and then dried by the usual papermaking method. 
     In an advantageous embodiment of the invention, the sheet  1  obtained prior to drying is married while in the wet state with other plies of ribbon compositions that may be identical or different and that may be formed on flat bed (Fourdrinier), short former, or vat former papermaking machines. 
       FIG. 2  shows another example of a ribbon  3  being incorporated in a method of fabricating a sheet  1  of the invention. 
     By way of example, the method may be implemented on the same papermaking machine as that shown in  FIG. 1 . 
     In this example, the ribbon  3  is introduced into the fiber suspension  5  close to the vat former  4  but after fibers have already been deposited on the vat former  4 . 
     As a result, neither of the faces  6  and  9  of the ribbon  3  comes into contact with the vat former  4 . The ribbon  3  is in contact with fibers that have already become deposited on the vat former  4  prior to the ribbon  3  being introduced in the fiber suspension  5 . The face  9  of the ribbon  3  then need not be visible on the sheet  1  once it has been fabricated. 
     Thus, both faces  6  and  9  of the ribbon  3  may be covered at least in part by fibers of the fiber suspension  5 . 
     It is possible to obtain one or more regions of reduced or increased thickness in the ribbon  3  by any of the techniques described above. 
     One or more security elements may be incorporated at or close to the regions of reduced or increased thickness. 
     After being formed, the sheet  1  may be processed in the same manner as described above. 
       FIG. 3  shows the possibility of incorporating the ribbon  3  in the sheet  1  by bringing it into contact with the ply  2   a  formed on the vat former  4  at the outlet from the cylinder vat. 
     The ribbon  3  may be inserted between the ply  2   a  leaving the cylinder vat and a ply  2   b  that may be made using any papermaking means. 
     Under such circumstances, a region of increased thickness may be formed on the sheet  1  in register with the ribbon  3 , this increased thickness projecting, e.g. from the outside face of the ply  2   b  and/or of the ply  2   a.    
       FIGS. 4 and 5  are section views of examples of each one obtained by a method of the invention. 
       FIG. 4  shows a sheet  1  that may be obtained by the method shown in  FIG. 1 . 
     The sheet  1  comprises a ribbon  3  with its face  9  not covered by fibers of the fiber suspension  5 , thus remaining visible on the sheet  1 . 
     The ribbon  3  may have a thickness e R  that is less than the thickness e of the vellum of the sheet  1  or it may have been subjected to prior special treatment. As a result, a region of reduced thickness  7  is created in the sheet  1  in register with the ribbon  3 . 
     In this example in particular, the thickness e of the vellum of the sheet  1  is greater than the total thickness e 2  of the sheet  1  in the region of the sheet  1  where the ribbon  3  appears, this thickness e 2  corresponding, for example, to the thickness e R  of the ribbon  3 , for example. The thickness e of the vellum of the sheet  1 , in particular of the substrate  2 , corresponds to the sum of the thickness e 2  plus the value Δe −  of the reduction in thickness  7 . 
       FIG. 5  shows a sheet  1  that may be obtained by the method shown in  FIG. 2 . 
     The sheet  1  comprises a ribbon  3  having its face  9  completely covered in fibers of the fiber suspension  5 , and is thus not visible on the sheet  1 . 
     The ribbon  3  may have a thickness e R  that is less than the thickness e of the vellum of the sheet  1  or it may have been subjected to prior special treatment. As a result, a region of reduced thickness  7  is created in the sheet  1  in register with the ribbon  3 . 
     In this example in particular, the thickness e of the sheet  1 , in particular of the substrate  2 , is greater than the thickness e 2  of the sheet  1  where the ribbon  3  appears. The thickness e is in particular equal to the sum of the value Δe −  of the region of reduced thickness  7  plus the thickness e 2  of the sheet  1  where the ribbon  3  appears, this thickness itself being equal to the sum of the thickness e R  of the ribbon  3  plus the thickness e 3  of the fibers of the fiber suspension  5  in contact with the face  9  of the ribbon  3 . 
     The resulting regions  7  of reduced thickness may enable one or more security elements to be incorporated on the sheet  1  of  FIGS. 4 and 5 . 
     In the examples described above and below, the various thicknesses may be equal to the above-mentioned value. 
     In particular, the thickness e R  of the ribbon  3  may be greater than 10 μm, preferably lying in the range 20 μm to 120 μm, and more preferably in the range 30 μm to 80 μm. 
     The value Δe −  of the region  7  of reduced thickness may be greater than or equal to 15 μm, preferably 30 μm, more preferably 60 μm. 
       FIG. 6  shows an example of a sheet  1  that may be obtained for example by the method shown in  FIG. 1 . 
     The sheet  1  comprises a ribbon  3  in register with a region  7  of reduced thickness. 
     The ribbon  3  comprises an electronic device  20 , e.g. in the form of an RFID device, e.g. incorporated in the ribbon  3  during fabrication of the ribbon  3 . 
     One or more security elements may be incorporated in the sheet  1  in register with the face  6  of the ribbon  3  particularly in order to protect the electronic device  20 . 
     In addition, a second ply or a film may overlie the substrate  2  on the side of the face  6  of the ribbon  3  in order to protect the electronic device  20 . 
     In this example, the thickness e 2  of the sheet  1  where the ribbon  3  appears corresponds to the thickness e R  of the ribbon  3 . 
       FIG. 7  shows another example of a sheet  1 , in which the ribbon  3  comprises an electronic device  20 , in particular in the form of an RFID device, which device is fastened to the face  6  of the ribbon  3 , e.g. by adhesive. 
     The electronic device  20  has a width that is substantially equal to the width of the ribbon  3 , for example. 
     The value Δe −  of the region  7  of reduced thickness may for example compensate the thickness e D , of the electronic device  20  completely. 
     The thickness e 2  of the sheet  1  where the ribbon  3  appears may be equal to the sum of the thickness e D , of the electronic device  20  plus the thickness e R  of the ribbon  3 . 
       FIG. 8  shows an example of a sheet  1  that may be obtained by the method shown in  FIG. 1 . 
     In this example, the ribbon  3  has a thickness e R  that is greater than the thickness e of the vellum of the sheet  1  such that a region of increased thickness  8  is created in the face  6  of the ribbon  3 . The increased thickness  8  may have a value Δe +  that corresponds to the difference between the thickness e R  of the ribbon  3  and the thickness e of the vellum of the sheet  1 . The thickness e 2  of the sheet  1  where the ribbon  3  appears corresponds to the thickness e R  of the ribbon  3 . 
     The value Δe +  of the increased thickness  8  may be less than 20 μm. 
     Advantageously, the ribbon  3  may comprise polyethylene fibers. 
     As a result of the face  6  of the ribbon  3  being embossed hot, the polyethylene fibers may melt and be made transparent. 
     As a result, it is possible to create a particular portion in relief  11  at the surface of the face  6  of the ribbon  3  that may eliminate the previously-existing increased thickness  8 . By way of example, the embossing may enable a face  6  of the ribbon  3  to be obtained that is substantially plane and substantially flush with the substrate  2 , or indeed to form a window or a strip of reduced opaqueness without obtaining any difference in thickness relative to the substrate. 
     It is also possible to make the face  6  of the ribbon  3  transparent, thereby enabling visual effects to be created that can be observed both in reflection and in transmission. 
     In addition, the portion in relief  11  that is created also serves to obtain touch effects on the face  6  of the ribbon  3 . 
       FIG. 9  shows a sheet  1  comprising two plies  2   a  and  2   b.    
     The ply  2   a  carries the ribbon  3 , which is incorporated in the ply  2   a , e.g. by using the method described above with reference to  FIG. 1 . 
     The second ply  2   b  is adjacent to the first ply  2   a , e.g. on the side thereof that is opposite from the region  7  of reduced thickness. As shown, the second ply  2   b  may include an opening, e.g. a window  22   b  for coming into register with an opening, e.g. a window  22   a  in the ribbon  3 . 
     The region  7  of reduced thickness may receive a security element  21  and the presence of the window  22   b  in register with the window  22   a  may enable the inside face  21   a  of the security element  21  to be observed from outside the ply  2   b.    
     In order to fabricate the sheet  1  shown in  FIG. 9 , it is possible to fabricate the first ply  2   a  with the ribbon  3  and to marry the first and second plies  2   a  and  2   b  while wet. The assembly made up of the two plies  2   a  and  2   b  and the ribbon  3  may be perforated so as to create the openings  22   a  and  22   b , and the security element  21  may be laminated with the first ply  2   a  so as to be in register with the ribbon  3 , e.g. so as to be assembled therewith. 
     In a variant that is not shown, the second ply  2   b  does not comprise the opening  22   b , but may for example include an imprint of ink seeking to make it transparent. 
     By way of example, the security element is a transparent film  21  that is thicker than the transparent films usually used in the prior art. 
     The transparent film  21  may be introduced in line, e.g. between two papermaking plies or on a single ply when it is sufficiently formed. In a variant it may be introduced off-line, e.g. by a method of lamination and/or pasting. 
     By way of example, the transparent film  21  may be made of stretched polyester or of stretched polycarbonate, and it may have a thickness e S  that is greater than or equal to 30 μm, preferably greater than or equal to 40 μm. The transparent film  21  may also include patterns, e.g. obtained by laser etching or by copperplate printing on the transparent film  21 . 
     The transparent film  21  may also be an ink, a varnish, or a resin. 
     In the configuration (not shown) in which the ribbon comprises an electronic device, the transparent film  21  may serve to protect the electronic device. 
     The transparent film  21  may also be made of a textile that is stuck or fastened in the region  7  of reduced thickness. 
     In this example, the thickness e S  of the transparent film  21  corresponds substantially to the value Δe −  of the reduced thickness  7 , e.g. being equal to 40 μm. 
     The ply  2   a  may have a thickness e of 115 μm. 
       FIG. 10  shows a variant embodiment of a sheet  1  of the invention comprising two paper plies  2   a  and  2   b , each including a corresponding ribbon  3   a  or  3   b , the ribbons  3   a  and  3   b  being inserted in the corresponding plies using the method described with reference to  FIG. 1 , for example. 
     Each ply  2   a  and  2   b  comprises a respective corresponding region  7   a  or  7   b  of reduced thickness associated with the presence of the respective ribbon  3   a  or  3   b.    
     In the example shown, the two plies  2   a  and  2   b  are assembled together so that the regions  7   a  and  7   b  of reduced thickness formed in the respective substrates are directed towards the opposite faces of the sheet  1 . 
     Each region  7   a  or  7   b  of reduced thickness receives a corresponding security element  21  or  23 . The respective thicknesses e S  and e S ′ of the security elements  21  and  23  correspond substantially to the reduced thicknesses  7   a  and  7   b , for example. 
     Each of the ribbons  3   a  and  3   b  may comprise a corresponding opening  22   a  or  22   b , these openings possibly being at least partially in register, and preferably exactly in register. 
     The security elements  21  and  23  are preferably not entirely opaque so as to make it possible to observe special show-through effects via the openings  22   a  and  22   b.    
     By way of example, the transparent film  21  may have a thickness e S  of 20 μm. By way of example, the transparent film  23  may have a thickness e S ′ of 15 μm. By way of example, the ribbon  3  may have a thickness e R  of 60 μm. The values Δe −  and Δe − ′ of the reduced thicknesses  7  may respectively compensate the thicknesses e S  and e S ′ of the films  21  and  23  completely. 
     The thickness e of the vellum of the sheet  1  may correspond to the sum of the thicknesses e S  and e S ′ of the films  21  and  23  plus the thickness e Ra  and e Rb  of the ribbons  3   a  and  3   b.    
     In a particular embodiment, the transparent films  21  and  23  may be fluorescent with different colors. 
     The transparent films  21  and  23  may make it possible to obtain the above-described effects. For example, they may include patterns enabling other patterns to be observed by moiré and/or combinatorial effects. 
     Although not shown, in a variant, in the examples of  FIGS. 9 and 10 , one or more security elements  21  and  23  may extend at least in part into one or more of the openings  22   a  and  22   b.    
       FIGS. 11 and 12  are respectively a view from above and a view from beneath of the sheet  1  of  FIG. 10 . 
     From these two figures, it can be seen that a show-through color appears in the openings  22   a  and  22   b , which color corresponds to the sum of the colors of the two transparent films  21  and  23 . 
     In contrast, away from the openings  22   a  and  22   b , but still in the ribbon  3 , it is the color of the transparent film  21  that is observed in  FIG. 11  and the color of the transparent film  23  that is observed in  FIG. 12 . 
       FIG. 13  shows a variant embodiment in which the sheet  1  comprises two plies  2   a  and  2   b , each including a corresponding ribbon  3   a  or  3   b  that has been used to create a corresponding region  7   a  or  7   b  of reduced thickness. 
     In the example of this figure, the two plies  2   a  and  2   b  are assembled so that the regions  7   a  and  7   b  of reduced thickness are in register with each other so as to leave a cavity between the two plies suitable for receiving a third ribbon  3   c , as shown. 
     Each ply  2   a  and  2   b  may be made with its corresponding ribbon  3   a  and  3   b , and the third ribbon  3   c  may be introduced between the plies when they are assembled together. 
     The third ribbon  3   c  may be constituted by any security element. 
       FIG. 14  shows a variant embodiment in which the substrate ply  2  is made in contact with a vat former in which the fabric T includes a portion in relief R that enables a recess  50  to be made in the substrate  2 . 
     The ribbon  3  is incorporated in the fiber suspension after a certain thickness e 3  has accumulated on the portion in relief R of the fabric T. This makes it possible to obtain a paper ply that includes a region  7  of reduced thickness beside the ribbon  3 , and on the opposite side another region  50  of reduced thickness, associated with the presence of the portion in relief R. 
     The ply as fabricated in this way may receive any security element in the region  7  of reduced thickness, as in the above-described examples. 
       FIG. 15  shows a sheet  1  in which the substrate  2  carries two ribbons  3  that are spaced apart by a distance d that may lie in the range 3 m to 20 mm, for example. 
     The thickness e 3  of the substrate under each ribbon  3  is identical, as shown, but in a variant it could be different, if the ribbons are introduced in contact with ribbons that have accumulated on the vat former at different locations and/or if the thicknesses of the ribbons  3  are different. 
     In the example described, each of the ribbons  3  forms a region of increased thickness Δe +  and the space  60  formed between the regions of increased thickness created by the ribbons  3  receives a security element  21 , which may be of any kind, and in particular one of those described above. 
     The security element  21  presents a thickness e S  that may for example be less than Δe +  or equal thereto, so as to be protected from any risk of flattening by the presence of the ribbons  3 . 
     Naturally, the invention is not limited to the embodiments described above. 
     In the above-described embodiments, the ribbon(s) of the sheet of the invention is/are made of fiber. In a variant, such a ribbon need not be made of fiber, for example it could be a synthetic ribbon, e.g. by being cut out from a thermoplastic film. 
     The term “comprising a” is synonymous with “comprising at least one”.