EP3828000B1 - Security document and manufacturing process of a security document involving a personalised image with a metallic hologram - Google Patents

Description

Technical area

The invention relates to a technique for forming color images and relates more particularly to a secure document and its method of manufacture comprising a holographic structure forming an arrangement of pixels from which a color image is formed.

Prior technique

The identity market today requires increasingly secure identity documents (also known as identity documents). These documents must be easily authenticated and difficult to counterfeit (if possible tamper-proof). This market concerns a wide variety of documents, such as identity cards, passports, access badges, driving licenses, etc., which can be in different formats (cards, booklets, etc.).

Various types of secure documents comprising images have thus been developed over time, in particular for securely identifying persons. More and more passports, identity cards or other official documents now include security features that authenticate the document and limit the risk of fraud, falsification or counterfeiting. Electronic identity documents comprising a chip card, such as electronic passports for example, have thus experienced significant growth in recent years.

Various printing techniques have been developed over time to make color prints. The production in particular of identity documents such as those mentioned above requires the production of color images in a secure manner in order to limit the risks of falsification by malicious individuals. The production of such documents, in particular at the level of the bearer's identity image, needs to be sufficiently complex to make reproduction or falsification by an unauthorized individual difficult.

Thus, a known solution consists in printing on a support a matrix of pixels composed of color sub-pixels and in forming gray levels by laser carbonization in a laserable layer located opposite the matrix of pixels, so as to reveal a custom color image that is difficult to forge or reproduce. Examples of embodiments of this technique are described for example in the documents EP 2 580 065 B1 (dated August 6, 2014 ) and EP 2 681 053 B1 (dated April 8, 2015 ).

Although this known technique offers good results, improvements are still possible in terms in particular of the quality of the visual rendering of the image thus formed. From this image formation technique, it is indeed difficult to achieve high levels of color saturation. In other words, the color gamut (ability to reproduce a range of colors) of this known technique may prove to be limited, which may pose a problem in certain use cases. This results in particular from the fact that the color sub-pixels are formed by a conventional printing method, by "offset" type printing for example, which does not make it possible to form sufficiently rectilinear and continuous lines of sub-pixels, this which generates homogeneity defects when printing sub-pixels (breaks in pixel lines, irregular outlines, etc.) and degraded colorimetric rendering.

Current printing techniques also offer limited positioning accuracy due to the inaccuracy of printing machines, which also reduces the quality of the final image due to poor positioning of the pixels and sub-pixels. to each other (problems with overlapping sub-pixels, misalignments...) or due to the presence of a non-printing tolerance gap between sub-pixels.

Another example of a conventional security document is provided in EP1997 643 A2 . This document describes a security document having a carrier substrate to which a security sheet is adhered. The security sheet comprises two layers, each layer having a security identification, for example a halftone image, which is formed by a group of micro-perforations. The security identification is visually detectable when the document is viewed in transmitted light.

The figure 1 represents an example of printing 2 by offset of pixels 4 taking the form of rows 6 of sub-pixels of distinct colors. As shown, the contours of each line 6 of sub-pixels have irregularities. A tolerance must be taken into account for the positioning of these lines due to positioning inaccuracies during printing.

As illustrated in figure 1 , to compensate for these defects in homogeneity and positioning of the sub-pixels of each pixel (and thus avoid possible overlapping of neighboring sub-pixels and the degradation of the desired colors), it is possible to print the sub-pixels so as to keep a white area 8 between each of them. However, this technique of adding white areas has a drawback in that it limits the level of saturation that it is possible to obtain for a given color, which prevents a satisfactory color gamut from being obtained.

Today there is a need to form personalized images (in color or black and white) in a secure manner, in particular in documents such as identity documents, official documents or others. A need exists in particular to allow flexible and secure personalization of color images, so that the image thus produced is difficult to falsify or reproduce and can be easily authenticated.

Moreover, no solution capable of offering an appropriate level of security and flexibility today makes it possible to obtain a good level of image brightness as well as a sufficient color gamut, in particular to obtain the shades of color. necessary for the formation of certain high quality color images, for example when areas of an image must have a highly saturated level in a given color.

Disclosure of Invention

In view in particular of the problems and shortcomings mentioned above, it has been envisaged to form a color image by arranging a holographic structure forming an arrangement of colored pixels on a laserable layer, and by producing levels of gray in the arrangement of pixels by forming laser-opaque areas in the lasérisable layer.

The figure 2 thus represents, according to a particular example, a structure 2 comprising a stack formed by a holographic layer 6 interposed between a first transparent laserizable layer 4 and a second transparent laserizable layer 8. As a variant, the structure 2 may comprise only any among the two laserizable layers 4 and 8.

In this example, the holographic layer 4 comprises a metallic holographic structure forming by holographic effect an arrangement of colored pixels. Furthermore, the transparent layers 4 and 8 are laser sensitive in the sense that they can be locally opacified by carbonization by means of laser radiation 12 in order to at least partially block the passage of light. The laserable layers 4 and 8 thus comprise areas (or volumes) 14, called “opaque areas”, which are locally opacified by the laser radiation 12, these opaque areas being positioned opposite the holographic structure so as to mask certain parts of the pixels and thus produce grayscale to reveal a custom color image 10.

By acting in particular on the power delivered by the laser 12, it is therefore possible to form opaque zones 14 of the desired size at particular positions in the arrangement of pixels in order to create the personalized image 10.

This technique advantageously makes it possible to create shades of colors so as to form a secure color image by the interaction between the opaque zones and the arrangement of pixels formed by the holographic layer. It is thus possible to form color images having a satisfactory image quality while being secure and therefore resistant to falsifications and fraudulent reproductions.

However, it has been observed that structural defects occur during the manufacture of such structures comprising a metallic holographic layer facing a locally opacified laserable layer. Indeed, air bubbles form within the structure during the laser carbonization of the lasérizable layer, causing separations in the stack and destruction of the holographic structure in the surrounding area.

For example, the picture 3 is a cross-sectional view of a structure 15 comprising a metallic holographic layer 16 positioned opposite a transparent laserizable layer 17 (made of polycarbonate). As can be seen, an air bubble 18 formed within the structure 15 during its manufacture, causing irreversible damage.

Further study has determined that the formation of these air bubbles (known as the "blistering" effect) is caused by the projection of the laser to form the opaque areas in the laserable layer. The power delivered by the laser radiation in fact generates heating in the metallic holographic structure giving rise to these air bubbles and thus leading to irreversible destruction of the holographic structure.

In order to form a secure color image exhibiting good contrast and good image quality while overcoming the problems and deficiencies mentioned above, a new image formation technique has therefore been developed.

• une première couche comprenant une structure holographique métallique formant un arrangement de pixels comportant chacun une pluralité de sous-pixels de couleurs distinctes ;
• une deuxième couche positionnée en regard de la première couche, ladite deuxième couche étant opaque vis-à-vis au moins du spectre de longueurs d'onde du visible ;

dans lequel la première couche comprend des premières perforations formées par un premier rayonnement laser, au moins une première partie des premières perforations révélant localement au travers de la structure holographique des zones sombres dans les sous-pixels causées par des régions sous-jacentes de la deuxième couche opaque situées en regard de ladite au moins une première partie des premières perforations, de sorte à former une image personnalisée à partir de l'arrangement de pixels combinées aux zones sombres.To this end, the invention is aimed at a secure document comprising:

• a first layer comprising a metallic holographic structure forming an arrangement of pixels each comprising a plurality of sub-pixels of distinct colors;
• a second layer positioned opposite the first layer, said second layer being opaque with respect to at least the spectrum of visible wavelengths;

wherein the first layer comprises first perforations formed by first laser radiation, at least a first portion of the first perforations locally revealing through the holographic structure dark areas in the sub-pixels caused by underlying regions of the second opaque layer located opposite said at least a first part of the first perforations, so as to form a personalized image from the arrangement of pixels combined with the dark areas.

The invention advantageously makes it possible to form a personalized image, in color or black and white, of good quality (in particular with good contrast), easy to authenticate, robust with respect to the risks of fraud, falsification or counterfeiting. This is possible in particular it the invention makes it possible to avoid using a laserable layer which requires laser carbonization which, as already described, can generate air bubbles (blistering) and therefore cause the destruction or irreversible damage of the structure. By forming a personalized image without a laserable layer, one can avoid applying a powerful laser into the structure and thus preserve its integrity.

According to a particular embodiment, each pixel of said arrangement of pixels is configured so that each sub-pixel presents a unique color in said pixel.

• une sous-couche de vernis formant les reliefs d'un réseau holographique ; et
• une sous-couche métallique déposée sur les reliefs de la sous-couche de vernis, ladite sous-couche métallique présentant un indice de réfraction supérieur à celui de la sous-couche de vernis.

According to a particular embodiment, the first layer comprises:

• an undercoat of varnish forming the reliefs of a holographic network; and
• a metal underlayer deposited on the reliefs of the varnish underlayer, said metal underlayer having a refractive index greater than that of the varnish underlayer.

According to a particular embodiment, the second opaque layer comprises an opaque black surface facing the first layer or comprises opacifying black pigments in its mass.

According to a particular embodiment, the first laser radiation is at a first spectrum of wavelengths different from the spectrum of visible wavelengths.

According to a particular embodiment, said at least a first part of the first perforations are through perforations which extend through the thickness of the holographic structure so as to reveal said underlying regions of the second opaque layer.

• ladite troisième couche étant transparente ou de couleur plus claire que la deuxième couche opaque, et formant un arrière-plan vis-à-vis de l'image personnalisée,

dans lequel la deuxième couche comprend des deuxièmes perforations formées par un deuxième rayonnement laser différent du premier rayonnement laser, les deuxièmes perforations étant positionnées dans le prolongement d'une deuxième partie des premières perforations de sorte que les premières et deuxièmes perforations situées en vis-à-vis révèlent localement au travers de la structure holographique et de la deuxième couche opaque des zones éclaircies dans les sous-pixels causées par des régions sous-jacentes de la troisième couche situées en regard desdites deuxièmes perforations, formant ainsi une image personnalisée à partir de l'arrangement de pixels combiné aux zones sombres et aux zones éclaircies.According to a particular embodiment, the secure document comprises a third layer located opposite the second layer so that said second layer is interposed between the first layer and the third layer,

• said third layer being transparent or of a lighter color than the second opaque layer, and forming a background vis-à-vis the personalized image,

in which the second layer comprises second perforations formed by a second laser radiation different from the first laser radiation, the second perforations being positioned in the extension of a second part of the first perforations so that the first and second perforations located opposite -vis locally reveal through the holographic structure and the second opaque layer brightened areas in the sub-pixels caused by regions of the third layer located opposite said second perforations, thus forming a personalized image from the arrangement of pixels combined with the dark areas and the lightened areas.

According to a particular embodiment, the second perforations are through perforations which extend through the thickness of the second layer so as to reveal, together with the second part of the first perforations located opposite, said underlying regions of the third opaque layer through the first and second layers.

According to a particular embodiment, the lightened areas are areas that are brighter than the dark areas.

• fourniture d'une première couche comprenant une structure holographique métallique formant un arrangement de pixels comportant chacun une pluralité de sous-pixels de couleurs distinctes ;
• positionnement d'une deuxième couche en regard de la première couche, ladite deuxième couche étant opaque vis-à-vis au moins du spectre de longueurs d'onde du visible ; et
• formation dans la première couche de premières perforations par un premier rayonnement laser, au moins une première partie des premières perforations révélant localement au travers de la structure holographique des zones sombres dans les sous-pixels causées par des régions sous-jacentes de la deuxième couche opaque situées en regard de ladite au moins une première partie des premières perforations, de sorte à former une image personnalisée à partir de l'arrangement de pixels combiné aux zones sombres.

The invention also relates to a corresponding method of manufacture. More particularly, the invention relates to a process for producing a document, comprising:

• providing a first layer comprising a metallic holographic structure forming an arrangement of pixels each comprising a plurality of sub-pixels of distinct colors;
• positioning a second layer facing the first layer, said second layer being opaque with respect to at least the visible wavelength spectrum; and
• formation in the first layer of first perforations by first laser radiation, at least a first part of the first perforations locally revealing through the holographic structure dark areas in the sub-pixels caused by underlying regions of the second opaque layer located opposite said at least a first part of the first perforations, so as to form a personalized image from the arrangement of pixels combined with the dark areas.

According to a particular embodiment, the first laser radiation is at a first spectrum of wavelengths different from the spectrum of visible wavelengths.

• positionnement d'une troisième couche en regard de la deuxième couche de sorte que ladite deuxième couche soit interposée entre la première couche et la troisième couche, ladite troisième couche étant transparente ou de couleur plus claire que la deuxième couche opaque, et formant un arrière-plan vis-à-vis de l'image personnalisée,
• formation dans la deuxième couche de deuxièmes perforations par un deuxième rayonnement laser différent du premier rayonnement laser, les deuxièmes perforations étant positionnées dans le prolongement d'une deuxième partie des premières perforations de sorte que les premières et deuxièmes perforations situées en vis-à-vis révèlent localement au travers de la structure holographique et de la deuxième couche opaque des zones éclaircies dans les sous-pixels causées par des régions sous-jacentes de la troisième couche situées en regard desdites deuxièmes perforations, formant ainsi une image personnalisée à partir de l'arrangement de pixels combinées aux zones sombres et aux zones éclaircies.

According to a particular embodiment, the manufacturing method comprises:

• positioning of a third layer facing the second layer so that said second layer is interposed between the first layer and the third layer, said third layer being transparent or of a lighter color than the second opaque layer, and forming a background plan vis-à-vis the custom image,
• formation in the second layer of second perforations by a second laser radiation different from the first laser radiation, the second perforations being positioned in the extension of a second part of the first perforations so that the first and second perforations located opposite each other locally reveal through the holographic structure and the second opaque layer lightened areas in the sub-pixels caused by underlying regions of the third layer located facing said second perforations, thus forming a personalized image from the arrangement of pixels combined with dark areas and lightened areas.

According to a particular embodiment, the third layer is transparent with respect to the first and second laser radiation.

Brief description of the drawings

• [ Fig. 1 ] The figure 1 , already described above, schematically represents the printing of lines of colored sub-pixels on a support.
• [ Fig. 2 ] The picture 2 , already described above, schematically represents a known structure for forming a personalized image;
• [ Fig. 3 ] The picture 3 , already described above, defects arising from known structures during the production of an image;
• [ Fig. 4 ] The figure 4 schematically represents a secure document comprising a personalized image, according to a particular embodiment of the invention;
• [ Fig. 5 ] The figure 5 is a sectional view schematically representing a multilayer structure in an initial state, according to a particular embodiment of the invention;
• [ Fig. 6 ] The figure 6 is a sectional view schematically representing a multilayer structure forming a personalized image, according to a particular embodiment of the invention;
• [ Fig. 7 ] The figure 7 represents first perforations made in the holographic layer of a multilayer structure, according to a particular embodiment of the invention;
• [ Fig. 8 ] The figure 8 schematically represents a multilayer structure before personalization and after personalization, according to a particular embodiment of the invention;
• [ Fig. 9A-9B ] The figures 9A and 9B represent respectively an image formed by a multilayer structure without an opaque layer and an image formed by a multilayer structure equipped with an opaque layer, according to a particular embodiment of the invention;
• [ Fig. 10 ] The figure 10 schematically represents the reliefs of a holographic structure, according to a particular embodiment of the invention;
• [ Fig. 11A-11B ] The Figures 11A and 11b schematically represent an arrangement of pixels and sub-pixels, according to a particular embodiment of the invention;
• [ Fig. 12A-12B-12C ] The figures 12A, 12B and 12C schematically represent arrangements of pixels and sub-pixels, according to particular embodiments of the invention;
• [ Fig. 13 ] The figure 13 is a sectional view schematically representing a multilayer structure forming a personalized image, according to a particular embodiment of the invention; and
• [ Fig. 14 ] The figure 14 schematically represents a manufacturing method according to a particular embodiment of the invention.

Description of embodiments

As indicated previously, the invention generally relates to the formation of a color image and relates in particular to a secure document comprising such an image.

The invention proposes to form a color image in a secure manner from a metallic holographic layer forming an arrangement of pixels and an opaque layer located opposite the metallic holographic layer. The metallic holographic layer includes perforations (or holes) locally revealing dark (opaque, non-reflective) areas in the pixel arrangement caused by underlying (corresponding) regions of the opaque layer located opposite the perforations, so forming a personalized image from the arrangement of pixels combined with the dark areas.

The invention relates in particular to a secure document comprising a first layer comprising a metallic holographic structure forming an arrangement of pixels each comprising a plurality of sub-pixels of distinct colors; and a second layer positioned opposite the first layer. This second layer is opaque with respect to at least the visible wavelength spectrum. The first layer comprises perforations formed by a first laser radiation (or laser etching), these perforations (or at least a part of them) locally revealing through the holographic structure dark (or black) areas in the undersides. pixels caused by underlying (corresponding) regions of the second opaque layer located opposite the perforations, so as to form a personalized image from the arrangement of pixels combined with the dark areas.

As explained below, it is thus possible to form a personalized image, in color or black and white, which is of good quality (in particular with good contrast), easy to authenticate, robust with respect to the risks of fraud, falsification or counterfeiting, while avoiding the use of a laserable layer which requires laser carbonization which, as already described, can generate air bubbles (blistering) and therefore cause destruction or irreversible damage to the structure. By forming a personalized image without a laserable layer, one can avoid applying a powerful laser into the structure and thus preserve its integrity.

The invention also relates to a method of forming such a personalized image.

Other aspects and advantages of the present invention will emerge from the embodiments described below with reference to the drawings mentioned above.

In the remainder of this document, examples of implementations of the invention are described in the case of a document comprising a color image according to the principle of the invention. This document can be any document, called a secure document, of the booklet, card or other type. The invention finds particular applications in the formation of identity images in identity documents such as: identity cards, credit cards, passports, driving licenses, secure entry badges, etc. The invention also applies to security documents (banknotes, notarized documents, official certificates, etc.) comprising at least one color image.

Generally, the image according to the invention can be formed on any suitable support.

Similarly, the exemplary embodiments described below aim to form an identity image. It is however understood that the color image considered can be arbitrary. It may for example be an image representing the portrait of the holder of the document concerned, other implementations being however possible.

Unless otherwise indicated, the elements common or similar to several figures bear the same reference signs and have identical or similar characteristics, so that these common elements are generally not described again for the sake of simplicity.

As already indicated, the color image IG can be formed on any support. The figure 4 represents, according to a particular embodiment, a secure document 20 comprising a document body 21 in or on which is formed a secure image IG according to the concept of the invention.

It is assumed in the embodiments which follow that the secure document 20 is an identity document, for example in the form of a card, such as an identity card, identification badge or other. In these examples, the IG image is a color image whose pattern matches the document holder's portrait. As already indicated, however, other examples are possible.

The figure 5 represents a multi-layered structure 22 in an initial (virgin) state, from which can be formed a personalized IG color image as represented in figure 4 . As explained later with reference to figure 6 , this structure 22 can be personalized in order to form a personalized image IG.

As illustrated in figure 5 , the structure 22 comprises a holographic layer 24 (also called "first layer") and an opaque layer 34 (also called "second layer") positioned opposite the holographic layer 24. In this example, the holographic layer 24 is placed on the opaque layer 34, although variants are possible in which one or more intermediate layers are present at the interface between the holographic layer 24 and the opaque layer 34.

According to a variant, the opaque layer 34 is spaced from the holographic layer by a transparent layer. The establishment of a spacing between the opaque layer and the holographic layer can make it possible in particular to obtain a color variation effect in the final image in the particular case where the opaque layer is also perforated or laser-engraved as described later. ( Figures 13-14 ).

The holographic layer 24 comprises a metallic holographic structure 32 forming an arrangement 29 of pixels 30, each of these pixels 30 comprising a plurality of sub-pixels 31 of distinct colors.

More particularly, the holographic structure 32 intrinsically forms an arrangement 29 of pixels which is blank, in the sense that the pixels 30 do not include the information defining the pattern of the color image IG that it is desired to form. As described later, it is by combining this arrangement 29 of pixels with dark areas (illustrated in figure 6 ) revealing a pattern of the IG custom color image.

The holographic structure 32 produces the arrangement 29 of pixels 30 in the form of a hologram by diffraction, refraction and/or reflection of incident light. The principle of the hologram is well known to those skilled in the art. Certain elements are listed below for reference. Examples of embodiments of holographic structures are described for example in the document EP 2 567 270 B1 .

As depicted in figure 5 , the holographic layer 24 comprises a layer (or sub-layer) 26 as well as reliefs (or structures in relief) 30, containing three-dimensional information, which are formed from the layer 26 serving as a support. These reliefs 30 form projecting portions (also called “mountains”) separated by recesses (also called “valleys”).

The holographic layer 22 further comprises a layer (or sub-layer) 28, called "high refractive index layer", which has a refractive index n2 greater than the refractive index n1 of the reliefs 30 (it is assumed here that the reliefs 30 form an integral part of the layer 26 serving as a support, so that the reliefs 30 and the layer 26 have the same refractive index n1). It is considered here that this high refractive index layer 28 is a metal layer covering the reliefs 30 of the holographic layer 24. As understood by those skilled in the art, the reliefs 30 form in combination with the layer 28 a holographic structure 32 which produces a hologram (a holographic effect).

The reliefs 30 of the holographic structure 32 can be formed for example by embossing a layer of stamping varnish (included in the layer 26 in this example) in a known manner for producing diffractive structures. The stamped surface of the reliefs 30 thus has the form of a periodic network whose depth and period can be respectively of the order of a hundred to a few hundred nanometers for example. This stamped surface is coated with layer 34, for example by means of vacuum deposition of a metallic material. The holographic effect results from the combination of the reliefs 30 and the layer 28 forming the holographic structure 32.

The holographic layer 24 may optionally comprise other sub-layers (not shown) necessary for maintaining the optical characteristics of the hologram and/or making it possible to ensure mechanical and chemical resistance of the assembly.

The metallic layer 28 with a high refractive index ( figure 5 ) may comprise at least one of the following materials: aluminum, silver, copper, zinc sulphide, titanium oxide, etc.

In the embodiments described in this document, the holographic layer 24 is transparent, so that the holographic effect producing the arrangement 29 of pixels 30 is visible by diffraction, reflection and refraction.

The holographic structure 32 is produced by any suitable method known to those skilled in the art.

The reliefs 30 have a refractive index denoted n1, of the order of 1.56 at a wavelength λ=656 nm for example.

In the example considered here ( figure 5 ), layer 26 is a transparent varnish layer. The holographic structure 32 is coated with a thin layer 28, for example of aluminum or zinc sulphide, having a high refractive index n2 (compared to n1), for example of 2.346 at a wavelength λ=660 nm for zinc sulfide. The thin layer 28 has for example a thickness of between 30 and 200 nm.

Layer 26 may be a heat-formable layer, thus allowing the reliefs 30 of holographic structure 32 to be formed by embossing on layer 26 serving as a support. As a variant, the reliefs 30 of the holographic structure 32 can be produced using an ultraviolet (UV) crosslinking technique. These manufacturing techniques being known to those skilled in the art, they are not described in more detail for the sake of simplicity.

Still with reference to the figure 5 , the second layer 34 positioned opposite the holographic layer 24 is opaque (non-reflective) with respect to at least the visible wavelength spectrum. In other words, the second layer 34 absorbs at least the wavelengths in the visible spectrum. It is for example a dark layer (of black color for example). It is considered in this document that the visible wavelength spectrum is approximately between 400 and 800 nanometers (nm), or more precisely between 380 and 780 nm in a vacuum. It should be noted that this second layer 34 can on the other hand be transparent to other wavelengths, in particular to infrared.

According to a particular example, the opaque layer 34 is such that the black density of the secure image IG formed in the secure document 20 ( figure 4 ) from said opaque layer in particular is greater than the intrinsic black density of the holographic layer 24 without (independently of) the opaque layer 34. As well known to those skilled in the art, the black density can be measured by means of a suitable measuring device (for example, a colorimeter or a spectrometer) .

According to a particular example, the opaque layer 34 comprises an opaque black surface opposite the holographic layer 24 and/or comprises black or black opacifying (or dark) pigments in its mass. The opaque layer 34 can notably comprise a black ink, or even a material tinted in its mass by black or opacifying (or dark) pigments.

As indicated above, the holographic structure 32 intrinsically forms an arrangement 29 of pixels which is blank, in the sense that the pixels 30 do not include the information defining the pattern of the color image IG that it is desired to form. In the initial state (before customization) shown in figure 5 , the structure 22 therefore forms no personalized image IG. As depicted in figure 6 in a particular embodiment, the multilayer structure can be personalized by combining the arrangement 29 of pixels with dark areas so as to reveal a pattern of the personalized image IG that one wishes to create.

More precisely, as represented in FIG. 6, the holographic layer 24 of the multilayer structure 22 also comprises perforations (or holes) 40 formed by a first laser radiation LS1 (or laser engraving). The perforations 40 constitute “first perforations” within the meaning of the invention. As explained below, other types of perforations can also be made according to a particular embodiment.

The first perforations 40 constitute regions in which the holographic layer 24 is destroyed or eliminated by the perforation effect of the laser.

These perforations 40 (or at least part of them as explained later) locally reveal through the holographic structure 32 dark (or opaque, non-reflecting) areas 42 in the sub-pixels 31 caused by underlying regions (corresponding) 41 of the opaque layer 34 located opposite the perforations 40, so as to form a personalized color image IG from the arrangement 29 of pixels 30 combined with the dark areas 42.

In the example shown in figure 6 , the perforations 40 are through-perforations which extend through the thickness of the holographic structure 32 (and more generally through the thickness of the holographic layer 24) so as to reveal underlying regions 40 of the opaque layer 34 at the level of the arrangement 29 of pixels 30. so as to produce dark (or opaque) areas 42 in all or parts of sub-pixels 31.

Thus, the perforations 40 occupy all or part of a plurality of sub-pixels 31 of the holographic structure 32. The opaque nature of the second layer 34 then generates dark (or opaque) zones 42 in the perforated parts of the sub-pixels 31.

To do this, the perforations 40 may have various shapes and dimensions which may vary depending on the case.

More particularly, the perforations 40 are arranged so as to select the color of the pixels 30 by modifying the colorimetric contribution of the sub-pixels 31 relative to each other in at least part of the pixels 30 formed by the holographic layer 24, so to reveal the custom IG image from the arrangement 29 pixels combined dark areas 42.

The laser perforation in the holographic layer 24 leads to a local elimination (or deformation) of the geometry of the holographic structure 32, and more particularly of the reliefs 30 and/or of the layer 28 covering said reliefs. These local destructions lead to a modification of the behavior of light (i.e. reflection, diffraction, transmission and/or refraction of light) in the corresponding pixels and sub-pixels.

By locally destroying all or part of the sub-pixels 31 by perforation and by revealing, instead, dark or opaque parts of the opaque layer 34, levels of gray (or shades of colors) are thus generated in the pixels 30 in modifying the colorimetric contribution of certain sub-pixels, relative to each other, in the visual rendering of the final IG image. The creation of the dark zones 42 makes it possible in particular to modulate the passage of light so that, for at least part of the pixels 30, one sub-pixel or more has a contribution (or a colorimetric weight) increased or decreased relative to that of at least one other sub-pixel close to the pixel concerned.

In particular, the selective description, partial or total, of one or a plurality of sub-pixels 31 in at least part of the pixels 30, generates a modification of the holographic effect in the regions concerned. The holographic effect is eliminated, or reduced, in the perforated regions of the holographic structure 27, which decreases (or even completely eliminates) the relative color contribution of the at least partially perforated sub-pixels 31 compared to at least one other neighboring sub-pixel 31 of the pixels 30 concerned.

It is assumed here that the image IG thus created is a color image resulting from a selective modulation of the colorimetric contributions of sub-pixels 31 of color. It should be noted, however, that a personalized image IG in shades of gray can be produced in the same way, for example by adapting the colors of the sub-pixels 31 accordingly.

The laser radiation LS1 (also called "first laser radiation") used to form the perforations (or holes) 40 in the holographic structure 32 is preferably at a first spectrum of wavelengths SP1 different from the spectrum of wavelengths of the visible. To do this, it is possible for example to use a YAG laser (for example at a wavelength of 1064 nm), a blue laser, a UV laser, etc. It is also possible to apply, for example, a pulse frequency of between 1 kHz and 100 kHz, although other configurations can be envisaged. It is up to the person skilled in the art to choose the configuration of the laser radiation LS1 according to the specific case.

In addition, it is necessary for the holographic layer 24 (and more particularly the holographic structure 32) to at least partially absorb the energy delivered by the laser radiation LS1 to create the perforations 40 previously described. In other words, the first laser radiation LS1 is characterized by a spectrum of wavelengths SP1 which is absorbed at least partially by the holographic structure 32. The materials of the holographic layer 24 are therefore chosen accordingly.

According to a particular example, the materials forming the holographic structure 32 are selected so that they do not absorb light in the visible. Of In this way, it is possible to create perforations 40 by means of laser radiation emitting outside the visible spectrum and to generate a personalized image IG which is visible to the human eye by holographic effect. Examples of materials are described later (transparent polycarbonate, PVC, transparent glue, etc.).

On the other hand, the SP1 spectrum is preferably chosen so that the LS1 radiation is not absorbed by the opaque layer 34.

Additional layers (not shown), made of polycarbonate or any other appropriate material, can also be applied on either side of the multilayer structure 22, in particular to protect the assembly. In particular, a transparent layer can thus be applied to the upper face of the holographic layer 24.

In general, the invention advantageously makes it possible to create shades of colors so as to form a secure color image by the interaction between the uncovered opaque zones of the opaque layer and the arrangement of pixels formed by the holographic layer. Without the appearance of these opaque zones by perforation as described above to judiciously direct or select the passage of the incident light, the pixels only form a blank arrangement insofar as this set is devoid of the information characterizing the color picture. It is the perforations 40 which are configured, depending on the arrangement of sub-pixels chosen, to personalize the visual appearance of the pixels and thus reveal the final color image.

By thus using an opaque layer to generate shades of gray or color, it is possible to form a personalized image which is secure and which has good image quality (in particular good contrast), while avoiding the use of a laserable layer which, as explained previously, is a source of structural defects (“blistering” problems) when customizing the structure. This technique thus makes it possible to dispense with the use of one or more laserable layers.

As previously described, the laser carbonization of a lasérisable layer in a multilayer structure to create opacified zones requires to deliver a significant power in the structure, consequently causing a significant heating and the formation of air bubbles which are destructive in particular for the metallic holographic structure. Thanks to the invention, it is possible to use laser radiation of lower power, or at least to apply a laser power lower than what would risk generating such air bubbles. By working at reduced laser power, the physical integrity of the metallic holographic structure is preserved.

According to a particular example, the perforations 40 are formed by projecting the first laser radiation LS1 onto the holographic layer 24 at a power less than or equal to a first threshold value beyond which the "blistering" effect previously described is likely to occur, which makes it possible to ensure that no air bubbles are generated liable to damage the structure 22. This first laser power threshold value is however variable and depends on each case of use (depends including types of hologram and characteristics of the laser used). This first threshold value can be determined by a person skilled in the art, in particular by means of an appropriate experimental design which makes it possible to determine the laser power beyond which the laser causes destruction of the structure (appearance of bubbles).

Advantageously, it is possible to finely parameterize the size of the laser holes 40 in the hologram in order to produce a personalized image IG of good quality.

In addition, the use of reduced laser power makes it possible to increase the lifetime of the lasers used and therefore to reduce manufacturing costs. The use of materials which are not sensitive to the laser (ie which does not have the capacity to become opacified locally under the effect of a laser) also makes it possible to limit the manufacturing costs.

The use of a holographic layer makes it possible to obtain an increased image quality, namely a better overall luminosity of the final image (more brilliance, more vivid colors) and a better capacity for color saturation. It is thus possible to form a high quality color image with an improved colorimetric gamut compared to a printed image for example.

The use of a holographic structure to form the pixel arrangement is advantageous in that this technique offers high positioning accuracy. pixels and sub-pixels thus formed. This technique makes it possible in particular to avoid overlaps or misalignments between sub-pixels, which improves the overall visual rendering.

The invention makes it possible to produce personalized images that are easily authenticated and resistant to falsifications and fraudulent reproductions. The level of complexity and security of the image which is achieved thanks to the invention is not at the expense of the quality of the visual rendering of the image.

Furthermore, the present invention makes it possible to limit the appearance of a color variation effect when the observation or illumination angle is varied. In particular, the attenuation of this color variation effect can be obtained if the spacing of the opaque black layer with the hologram is relatively small (for example a spacing less than or equal to 100 μm, preferably in a range comprised in 0 μm and 250 μm) and/or if the low thickness of the black layer in certain cases of implementation limits this effect. If the spacing between the opaque black layer and the hologram exceeds the value of 250 µm, it may be necessary to significantly increase the size of the pixels of the holographic layer to limit the color variations in the hologram, which has result in reducing the resolution of the final image.

Note that in the embodiment described above with reference to the figures 5 and 6 , the opaque layer 34 is arranged in the multilayer structure 22 so as to face the holographic layer 24 which is also part of this multilayer structure 22. As already indicated, the opaque layer 22 can be fixed or formed directly on or under the holographic layer 24, or optionally at least one transparent layer, can separate the opaque layer 22 from the holographic layer 22.

More generally, the production of the secure document 20 ( figure 4 ) requires that the opaque layer 34 can be positioned facing the holographic layer 24 to reveal in particular the dark areas 42 as previously described. On the other hand, it is not compulsory for the opaque layer 34 and the holographic layer 24 to be part of the same multilayer structure.

Thus, according to a variant of the embodiment of the figures 5 and 6 , the holographic layer 24 and the opaque layer 34 are positioned at different parts secure document 20, these parts being movable so that the opaque layer 34 can be positioned opposite the holographic layer 24 in order to reveal the dark zones 42 and thus form the personalized image IG.

Thus, the secure document 20 can for example take the form of a booklet (a passport for example), of which a first page comprises the holographic layer 24 and another page comprises the opaque layer 34, the two pages being mobile so that it is possible to position the opaque layer 34 opposite the holographic layer 24 in order to reveal the personalized image IG. According to a particular example, the first page comprises a transparent window in which the holographic layer 24 is placed and the opaque layer 34 is positioned on the page adjoining this first page. In this way, the personalized IG image can be read in reflection with the opaque layer positioned at the back, and also in transmission without the use of the black layer. This variant makes it possible in particular, in the case where laser perforations are made in the holographic layer and in the opaque layer (see below with reference to the figures 13- 14 ), to make these perforations at different stages which limits the risk of interference (disturbances) between the two laser engravings (so that the laser perforation of the holographic layer does not affect the opaque layer, and Conversely). In particular, the physical separation of the holographic layer and the opaque layer can be advantageous if it is desired to carry out these two laser engravings separately because it is possible in particular to use the same laser to engrave the opaque layer and the holographic layer while avoiding the cross interference problems mentioned above.

The figure 7 is a view representing perforations 40 produced by means of laser radiation LS1 in the holographic structure 32 as previously described with reference to the figures 5-6 . In this example, the perforations have variable sizes, with diameters comprised approximately between 9 and 35 micrometers (μm).

It should be noted that the perforations 40 can be arranged in various ways in the holographic layer 24. According to a particular example, it is possible to play on the size of the perforations 40 and/or on the number of perforations in order to obtain a hole density required in certain zones of the arrangement 29 of pixels where one wishes to reveal (or uncover) underlying regions 41 of the opaque layer 34. In particular, the perforations 40 can for example be arranged in a matrix (orthogonal or not) of rows and columns. According to a particular example, the perforations 40 have a constant diameter. It is by varying the number and position of the holes 40 that the desired shades of color are obtained.

The figure 8 schematically illustrates the arrangement 29 of pixels 30 in the virgin state as described with reference to the figure 5 (that is to say without the perforations 40), as well as the arrangement 29 of pixels 30 once personalized by the dark or opaque areas 42 so as to reveal the personalized image IG as described with reference to figure 6 .

The figures 9A and 9B illustrate the contribution of the opaque layer 34 present under the arrangement 29 of pixels, in the multilayer structure 22, to produce a personalized image IG.

More specifically, the figure 9A represents an example of a personalized image produced according to the concept of the invention. In this example, the custom image is a black and white face of an individual. The Figure 9B represents the image obtained this time without the opaque layer 34 under the arrangement 29 of pixels. As can be seen, the opaque layer 34 makes it possible to provide strong contrast in the final IG image and thus to significantly improve the quality of the image.

The figure 10 shows examples of reliefs 30 of a holographic structure 32, comprising protruding portions and recesses. Various shapes and dimensions of the holographic structure are possible within the scope of the present invention.

Always with reference to figures 5-6 , the holographic layer 24 may be encapsulated or assembled with various other layers. Moreover, as already indicated, the holographic layer 24 forms an arrangement 29 of pixels 30. Each pixel 30 comprises a plurality of sub-pixels 31 of color.

The figures 11A and 11B represent a particular example according to which each pixel 30 comprises 3 sub-pixels 31. The number, the shape and more generally the configuration of the pixels and sub-pixels can however vary depending on the case.

An external observer OB can thus visualize according to a particular direction of observation the arrangement 29 of pixels from light refracted, reflected and/or diffracted from the holographic structure 32 of the holographic layer 24.

More precisely, each pixel 30 is formed by a region of the holographic structure 32 present in the holographic layer 12. It is considered here that the reliefs 30 of the holographic structure 32 ( figures 5-6 ) form parallel lines 34 of sub-pixels, other implementations being however possible. For each pixel 30, its constituent sub-pixels 31 are thus formed by a portion of a respective line 34, this portion constituting a respective holographic grating (or holographic grating portion) configured to generate by diffraction and/or reflection a corresponding color said sub-pixel.

In the example considered here, the pixels 30 thus comprise 3 sub-pixels of distinct colors, other examples being however possible. It is assumed that each sub-pixel 31 is monochromatic. Each holographic grating is configured to generate a color in each sub-pixel 31 corresponding to a predetermined viewing angle, this color being modified under a different viewing angle. It is assumed for example that the sub-pixels 31 of each pixel 30 respectively present a distinct fundamental color (for example green/red/blue or cyan/yellow/magenta) according to a predetermined viewing angle.

As depicted in figures 11A and 11B , the holographic networks corresponding to the three lines 34, which form the sub-pixels 31 of the same pixel 30, have particular geometric specifications so as to generate a desired distinct color. In particular, the holographic gratings forming the 3 sub-pixels 31 in this example have a width denoted I and a pitch between each holographic grating denoted p.

Thus, according to a particular example where each pixel 30 is composed of 4 sub-pixels 31, the maximum theoretical saturation capacity S in one of the colors of the sub-pixels in the same pixel can be stated as follows: $$S=\frac{25}{100}\times \frac{I}{I+p}$$

By way of example, it can be considered that I=60 μm and p=10 μm, which leads to a maximum theoretical saturation capacity S=0.21.

It is possible to form the holographic gratings forming the sub-pixels 31 so that the pitch p tends towards zero, which makes it possible to increase the maximum theoretical saturation capacity in a color of a sub-pixel (S then tending towards 0.25).

According to a particular example, the pitch is fixed at p=0, which makes it possible to reach a maximum theoretical saturation capacity S equal to 0.25. In this case, the lines 34 of sub-pixels as represented in figures 11A and 11B are contiguous (no space or white area being present between the lines of sub-pixels).

The invention thus makes it possible to form rows of sub-pixels which are contiguous, that is to say adjacent to each other without it being necessary to leave separating white zones between each row, or possibly by keeping Separating white zones but of limited size between the lines of sub-pixels (with a small pitch p). This particular configuration of the holographic gratings makes it possible to substantially improve the quality of the final image IG (better color saturation) compared to conventional image formation techniques which do not make use of a holographic structure. This is possible in particular because the formation of holographic structures makes it possible to achieve better positioning precision of the sub-pixels and better homogeneity than by conventional printing of the sub-pixels (by offset or other).

As already indicated, the arrangement 29 of pixels 30 formed by the holographic layer 24 ( figures 5-6 ) can occur in various forms. Examples of embodiments are described below.

In general, the arrangement 29 of pixels can be configured so that the sub-pixels 31 are uniformly distributed in the holographic layer 24. The sub-pixels 31 can for example form parallel lines of sub-pixels or else a network in the shape of a hexagon (of the Bayer type), other examples being possible.

The sub-pixels 31 can for example form an orthogonal matrix.

Pixels 30 can be evenly distributed in array 29 so that the same pattern of sub-pixels 31 repeats periodically in holographic layer 24.

Furthermore, each pixel 30 of the arrangement 29 of pixels can be configured so that each sub-pixel 31 has a unique color in said pixel considered. According to a particular example, each pixel 30 in the arrangement 29 of pixels forms an identical pattern of color sub-pixels.

Specific examples of arrangements (or tiling) 29 of pixels that can be implemented in the secure document 20 ( figure 4 ) are now described with reference to figures 12A, 12B and 12C . It should be noted that these implementations are presented here only by way of non-limiting examples, many variants being possible in terms in particular of arrangement and shape of the pixels and sub-pixels, as well as the colors assigned to these sub-pixels.

According to a first example shown in Figure 12A , the pixels 30 of the arrangement 29 of pixels are rectangular (or square) in shape and comprise 3 sub-pixels 31a, 31b and 31c (collectively denoted 31) of distinct colors. As already described with reference to Figures 12A-12B , the sub-pixels 31 can each be formed by a portion of a line 34 of sub-pixels. In this example, the tiling 29 thus forms a matrix of rows and columns of pixels 30, orthogonal to one another.

The figure 12B is a top view representing another example of regular tiling in which each pixel 30 is composed of 3 sub-pixels 31, denoted 31a to 31c, each of a distinct color. The sub-pixels 31 are here hexagonal in shape.

The figure 12C is a top view representing another example of regular tiling in which each pixel 30 is composed of 4 sub-pixels 31, denoted 31a to 31d, each of a distinct color. The sub-pixels 31 are here triangular in shape.

For each of the arrangements of pixels considered, it is possible to adapt the shape and the dimensions of each pixel 30 and also the dimensions of the separating white zones present, if necessary, between the sub-pixels, so as to achieve the desired maximum color saturation level and desired brightness level.

A multilayer structure 23 is now described with reference to the figure 13 according to a particular embodiment. This multilayer structure 23 is made so as to form a personalized image IG.

The multilayer structure 23 is similar to the multilayer structure 22 previously described with reference to the figures 5-6 and differs mainly in that the multilayer structure 23 comprises a third layer 50 under the opaque layer 34 and in that the opaque layer 34 comprises perforations 52 as described below.

More precisely, the multilayer structure 23 comprises a third layer 50 located opposite the opaque layer 34 so that this opaque layer 34 is interposed between the holographic layer 26 and the third layer 50.

The third layer 50 is a transparent layer or a lighter color (or brighter, or brighter) than the opaque layer 34, so as to form a background vis-à-vis the final personalized IG image.

Furthermore, the opaque layer 34 comprises perforations (or holes) 52 formed by a second laser radiation LS2 (or laser engraving) different from the first laser radiation LS1 used to form the perforations 40 in the holographic structure 32. The perforations 52 formed in the opaque layer 34 constitute second perforations within the meaning of the invention.

It is considered here that the second perforations 52 constitute regions in which the opaque layer 34 is destroyed or eliminated by the perforation effect of the laser (formation of holes). According to a variant, these second laser perforations 52 do not form holes as such but constitute regions of the opaque layer 34 whose physicochemical properties are altered (so-called “photobleaching” technique) by a chemical reaction caused by the laser LS2 so as to modify the response to light of opacifying pigments (for example black opacifying pigments) present in said opaque layer 34. Thus, it is possible to use an opaque layer 34 which comprises opacifying pigments which lose (at least less partially) their black coloration under the effect of appropriate LS2 laser radiation (depending on the wavelength and/or energy density applied). In this way, it is possible to selectively create lightened areas in the opaque layer 34 by means of the laser radiation LS2.

These second perforations 54 are positioned in the extension of a part of the first perforations 40 so that the first and second perforations 40, 52 located opposite each other reveal locally through the holographic structure 32 and the opaque layer 34 of the lightened areas 56 in the sub-pixels 31, these lightened areas being caused by underlying (corresponding) regions 54 of the third layer 50 located opposite the second perforations 52, thus forming a personalized image IG to from the 29 arrangement of 30 pixels combined with 42 dark areas and 56 bright areas.

Thus, in this particular embodiment, only a part - called the first part - of the perforations 40 (namely one or a plurality of them) locally reveals through the holographic structure 32 dark (or opaque) areas 42 in the sub-pixels 31 caused by underlying regions 41 of the opaque layer 34 located opposite these first perforations 40. Furthermore, another part of the perforations 40 (namely one or a plurality of them) - called second part - is located opposite, or in alignment with, respective second perforations 54 formed in the third layer 50. The first and second perforations 40, 52 located opposite each other thus collectively form through perforations, in the layer holographic 22 and in the opaque layer 34, making it possible to collectively discover underlying regions 54 of the third background layer 50. These underlying regions 54 discovered in rega rd of the second perforations 52 thus produce lightened areas (also called bright areas, or bright areas) 56, from the point of view of an external observer OB, in the personalized image IG formed by the combination of the arrangement 29 of pixels 30, dark areas 42 and bright areas 56.

Note that the size and dimensions of the second perforations 52 may vary depending on the case. Although being located in the extension of first perforations 40, it is not necessary for the second perforations 52 to have a diameter identical to the first perforations 40 which they face. On the other hand, it is necessary that at least a part of each second perforation 52 be positioned opposite at least part of a corresponding first perforation 40 in order to reveal in the personalized image IG an underlying region 54 of the third layer 50.

In the example shown in figure 13 , the second perforations 52 are through perforations which extend through the thickness of the second opaque layer 34 (at the underlying regions 41) so as to reveal, together with the first perforations 40 located opposite opposite the underlying regions 54 of the third layer 50 at the level of the arrangement 29 of pixels 30. In other words, by producing these second perforations 52 with a laser in the thickness of the third layer 50, underlying regions 54 of the third layer 50 so as to produce, in all or parts of sub-pixels 31, lightened areas compared to dark areas 42.

According to a particular example, the lightened areas 56 are areas that are brighter (or brighter) than the dark areas 42.

According to a specific example, the IG color image thus produced comprises at least one dark or opaque zone 42 (revealed by a respective perforation 40) and at least one brightened zone 56 (jointly revealed by a perforation 40 and a perforation 52 located opposite one from the other).

According to a particular example, the first and second perforations 40, 52 are configured so that one or a plurality of first perforations 40 simultaneously reveal one (or more) dark zone 42 caused by an underlying region 41 of the layer opaque 34 and one (or more) lightened area 56 caused by an underlying region 54 of the third layer 50.

Thus, according to a principle analogous to the first perforations 40, the second perforations 52 are arranged so as to select the color of the pixels 30 by modifying the colorimetric contribution of the sub-pixels 31 relative to each other in at least part of the pixels 30 formed by the holographic layer 24, so as to reveal the personalized image IG from the arrangement 29 of pixels combined this time with the dark areas 42 and the lightened areas 56.

By revealing lightened areas 56 instead of dark areas 42, it is possible to adapt the levels of gray (or shades of colors) in the pixels 30 by modifying the colorimetric contribution of certain sub-pixels, relative to each other, in the visual rendering of the final IG image. The creation of lightened areas 56 makes it possible in particular to lighten at least part of certain sub-pixels 31.

As already indicated, it is assumed here that the image IG thus created is a color image resulting from a selective modulation of the colorimetric contributions of sub-pixels 31 of color. It should be noted, however, that a personalized image IG in shades of gray can be produced in the same way, for example by adapting the colors of the sub-pixels 31 accordingly.

As already indicated above, the LS2 laser radiation (also called "second laser radiation") used to form the second perforations (or holes) 52 in the opaque layer 34 is different from the first LS1 radiation used to form the first perforations 40 in the holographic structure 32. The first and second laser radiation LS1, LS2 preferably have distinct wavelength spectra. It is thus possible to selectively form perforations in one of the holographic structure 32 and the opaque layer 34 without perforating the other.

In the example considered here, the second laser radiation LS2 is at a second spectrum of wavelengths SP2 which is absorbed at least partially by the second opaque layer 34 to be able to create the second perforations 52. In other words, the second laser radiation LS2 is characterized by a spectrum of wavelengths SP2 which is absorbed at least partially by the second layer 34. The materials of the third layer 50 are therefore chosen accordingly. In particular, the third layer 50 serving as a support layer for the opaque layer 34, its characteristics must be chosen so that this third layer 50 retains its physical or mechanical properties during the etching by means of the lasers LS1 and/or LS2. The composition of the third layer 50 therefore depends on the types and materials of the holographic layer and of the opaque layer as well as the characteristics of the SP1 and SP2 lasers used.

On the other hand, the second spectrum SP2 is preferably chosen so that the second radiation LS2 is not absorbed by the holographic structure 32 (although this variant is possible).

Furthermore, it is considered in this example that the third layer 50 is transparent with respect to the second and third laser radiation LS1, LS2. In other words, the third layer 50 does not absorb the laser radiation LS1 and LS2 which makes it possible not to affect this background layer when the perforations 40 and 52 are formed.

To form the second perforations 52, it is possible for example to use a YAG type LS2 laser, a blue laser, a UV laser, etc. It is also possible to apply, for example, a pulse frequency of between 1 kHz and 100 kHz, although other configurations can be envisaged. It is up to the person skilled in the art to choose the configuration of the laser radiation LS1 according to the specific case.

By thus using an opaque layer and a background layer of a lighter (or brighter) color than the opaque layer, it is advantageous to further increase the gamut as well as the fineness of the personalized image due to the gray levels thus obtained. It is also possible to obtain a higher level of safety thanks to the increased complexity of the structure as a whole, and this while avoiding the use of a laserable layer which, as already explained, generates structural defects (problems of "blistering") .

According to a particular example, the second perforations 52 are formed by projecting the second laser radiation LS2 onto the opaque layer 34 at a power less than or equal to a second threshold value beyond which the "blistering" effect previously described is likely to occur, which makes it possible to ensure that no air bubbles are generated liable to damage the structure 23. Analogously to the first laser radiation LS1, this second laser power threshold value is variable and depends on each use case (depends in particular on the type of hologram and the opaque layer, and the characteristics of the laser used). This second threshold value can be determined by a person skilled in the art, in particular by means of an appropriate experimental design which makes it possible to determine the laser power beyond which the laser causes destruction of the structure (appearance of bubbles).

Furthermore, the present invention also relates to a manufacturing method for manufacturing a personalized image IG according to any one of the preceding embodiments described. Also, the various variants and technical advantages described above with reference to the multilayer structures 22 and 23, and more generally to a personalized image in accordance with the concept of the invention, apply analogously to the manufacturing method of the invention to obtain such an image or structure.

A method of making an IG color image as previously described is now described with reference to the figure 14 , according to a particular embodiment. It is assumed for example that a color image IG is formed in a document 20 as illustrated in figure 4 .

During a supply step S2, a first holographic layer 22 is thus supplied as already described above. This holographic layer 32 therefore comprises a metallic holographic structure 32 forming an arrangement 29 of pixels 30 each comprising a plurality of sub-pixels 31 of distinct colors. The different characteristics and variants of the holographic layer 22 (including the arrangement 29 of pixels) described above with particular reference to the figures 5-6 apply analogously to the manufacturing process.

According to a particular example, the supply step S2 comprises the supply of an under-layer of varnish 26 forming the reliefs 30 of a holographic network; and the formation of a metal underlayer 28 on the reliefs 30 of the varnish underlayer 26, the metal underlayer 28 having a refractive index greater than that of the varnish underlayer ( figures 5-6 ).

Layer 26 ( figure 4 ) may for example be a thermo-formable layer thus allowing the reliefs 30 of the holographic structure 32 to be formed by embossing on the layer 26 serving as a support. As a variant, the reliefs 30 of the holographic structure 32 can be made using a UV crosslinking technique, as already indicated. These manufacturing techniques being known to those skilled in the art, they are not described in more detail for the sake of simplicity.

A layer of adhesive and/or glue (not shown) can also be used to ensure adhesion of the holographic layer 24 to a support (not shown).

During a positioning step S4, a second layer 34 is positioned (or deposited, or formed) facing the first holographic layer 22, this second layer 34 being opaque with respect to at least the length spectrum d wave of the visible as already explained. The different characteristics and variants of the opaque layer 24 described above with particular reference to the figures 5-6 apply analogously to the manufacturing process.

During a perforation step S6, first perforations (or holes) 40 are formed in the first holographic layer 22 by a first laser radiation LS1 ( figure 6 ). The first perforations 40 thus occupy all or part of a plurality of sub-pixels 31 of the holographic structure 32. At least a first part of the first perforations 40 locally reveals through the holographic structure dark (or opaque) areas 42 in the sub-pixels 31, these dark areas being caused (or produced) by underlying regions 41 of the second opaque layer 34 located facing said at least a first part of the first perforations 40, so as to form a personalized image IG from 29 pixels arrangement combined with 42 dark areas.

Once step S6 has been completed, a multilayer structure 22 is thus obtained as previously described with reference to the figure 6 .

The different characteristics and variants of the first perforations 40 described above with particular reference to the figures 5-6 apply analogously to the manufacturing process.

According to a particular example, each first perforation 40 opens onto an underlying region 41 of the opaque layer 34 so as to reveal corresponding dark zones in the final image IG. Variants are however possible as described above, in which a non-zero part of the first perforations 40 are located opposite second perforations 52 made in the opaque layer 34 so as to reveal lightened areas 56 in the arrangement 29 of pixels 30.

As already described, the perforations 40 are here through perforations which extend through the thickness of the holographic structure 32 (and more generally through the thickness of the holographic layer 24) so as to reveal regions under -jacent 40 of the opaque layer 34 at the level of the arrangement 29 of pixels 30. 41 of the opaque layer 34 so as to produce dark (or opaque) areas 42 in all or parts of sub-pixels 31.

The personalized image IG thus created is a color image resulting from a selective modulation of the colorimetric contributions of sub-pixels 31 of color. It should be noted, however, that a personalized image IG in shades of gray can be produced in the same way, for example by adapting the colors of the sub-pixels 31 accordingly.

The first laser radiation LS1 used in S6 to form the perforations 40 in the holographic structure 32 is preferably at a first spectrum of wavelengths SP1 different from the spectrum of wavelengths of the visible. To do this, one can for example use a YAG laser (1064 nm), a blue laser, a UV laser, etc. It is also possible to apply, for example, a pulse frequency of between 1 kHz and 100 kHz, although other configurations can be envisaged. As already indicated, it is up to the person skilled in the art to choose the configuration of the laser radiation LS1 according to the specific case.

In addition, it is necessary for the holographic layer 24 (and more particularly the holographic structure 32) to absorb, at least partially, the energy delivered by the laser radiation LS1 to create the perforations 40 previously described. In other words, the first laser radiation LS1 is characterized by a spectrum of wavelengths SP1 which is absorbed at least partially by the holographic structure 32. The materials of the holographic layer 24 are therefore chosen accordingly.

According to a particular example, the materials forming the holographic structure 32 are selected so that they do not absorb light in the visible. These may be transparent materials such as those used in particular in identity documents. Thus the holographic structure 32 is formed from at least one of the following materials: transparent polycarbonate, PVC, transparent glue, etc. In this way, it is possible to create perforations 40 by means of laser radiation LS1 emitting outside the visible spectrum and to generate a personalized image IG which is visible to the human eye by holographic effect.

On the other hand, the SP1 spectrum is preferably chosen so that the LS1 radiation is not absorbed by the opaque layer 34.

Additional layers (not shown) of polycarbonate or any other suitable material can also be applied on either side of the multilayer structure 22 thus obtained ( figure 6 ), in particular to protect the whole. In particular, a transparent layer can thus be applied to the upper face of the holographic layer 24.

As already indicated, the invention makes it possible to work at moderate laser power and thus to form a secure personalized image of good quality while avoiding generating heating which would risk producing destructive air bubbles in the structure.

Furthermore, as described above, it is possible to continue the manufacturing process represented in figure 14 by carrying out the steps S10 and S12 described below so as to manufacture, from the multilayer structure 22 represented in figure 6 , the multilayer structure 23 shown in figure 13 . It is thus possible to form one or more lightened areas 56 in the arrangement 29 of pixels instead of dark areas 42, in particular in order to further improve the quality and security of the personalized image IG thus produced.

According to a particular embodiment, once the formation step S6 has been completed, a third layer 50 is thus positioned (or deposited) facing the second opaque layer 34 during a step S10 ( figure 14 ) so that this second opaque layer 34 is interposed between the first holographic layer 22 and the third layer 50. This third layer 50, which is transparent or lighter in color (or brighter) than the second opaque layer 34, forms a background against the custom IG image you want to form.

The different characteristics and variants of the background layer 50 described with reference to the figure 13 apply analogously to the manufacturing process.

During a training step S12 ( figure 14 ), second perforations 52 are formed in the second opaque layer 34 by a second laser radiation LS2 different from the first laser radiation LS1 used in S6 to form the first perforations 40. The second perforations 40 are positioned in the extension of one or a plurality of first perforations 40 formed in S6 so that the first and second perforations 40, 52 located facing each other locally reveal through the holographic structure 32 and the second opaque layer 34 brightened areas 56 in the sub-pixels 31 caused by sub-pixel regions adjacent 54 of the third background layer 50 located opposite the second perforations 52, thus forming a personalized image IG from the arrangement 29 of pixels 30 combined with the dark areas 42 and the lightened areas 56.

The different characteristics and variants of the second perforations 52 described above with particular reference to the figure 14 apply analogously to the manufacturing process.

It is thus considered in this variant that a non-zero part of the first perforations 40 (for example a first group of first perforations 40) formed at S6 opens onto a respective underlying region 41 of the opaque layer 34 so as to reveal areas dark 42 corresponding in the final image IG, and that another, called second non-zero part of the first perforations 52 (for example a second group of first perforations 40) formed in S6 is positioned facing the second perforations 52 so as to revealing, together with the second perforations 52, corresponding brightened areas 56 in the final image IG.

As already indicated above, the second laser radiation LS2 used in S12 to form the second perforations (or holes) 52 in the opaque layer 34 is different from the first radiation LS1 used in S6 to form the first perforations 40 in the holographic structure 32 The first and second laser radiation LS1, LS2 preferably have distinct wavelength spectra. It is thus possible to selectively form perforations in one of the holographic structure 32 and the opaque layer 34 without affecting the other.

In the example considered here, the second laser radiation LS2 is at a second spectrum of wavelengths SP2 which is absorbed at least partially by the second opaque layer 34 in order to be able to create the second perforations 52. In other words, the second laser radiation LS2 is characterized by an SP2 wavelength spectrum which is at least partially absorbed by the second layer 34. As already described, the materials of the third layer 50 are therefore chosen accordingly.

On the other hand, the second spectrum SP2 is preferably chosen so that the second radiation LS2 is not absorbed by the holographic structure 32 (although this variant is possible).

Furthermore, it is considered in this example that the third layer 50 is transparent with respect to the second and third laser radiation LS1, LS2. In other words, the third layer 50 does not absorb the laser radiation LS1 and LS2 which makes it possible not to affect this background layer when the perforations 40 and 52 are formed. Variants are however possible. Thus, the third layer 50 is not necessarily transparent to the laser LS1 and LS2 but the absorption of the radiation LS1 and LS2 by this third layer 50 must be low so that its physical integrity (mechanical resistance and color) 50 is preserved.

To form the second perforations 52, it is possible for example to use a YAG type LS2 laser, a blue laser, a UV laser, etc. It is also possible to apply, for example, a pulse frequency of between 1 kHz and 100 kHz, although other configurations can be envisaged. It is up to the person skilled in the art to choose the configuration of the laser radiation LS1 according to the specific case.

Note that the order in which the steps of the manufacturing process shown in figure 14 are carried out may vary depending on the case. Thus, it is for example possible to make the perforations 40 and 52 (steps S6 and S12; figure 14 ) after performing steps S2, S4, S6 and S10. Similarly, the perforations 40 and 52 can be made (S6, S12) simultaneously or in any order.

Claims (13)

1. A secure document (2), comprising:

   - a first layer (24) comprising a metallic holographic structure (32) forming an arrangement (29) of pixels (30) each comprising a plurality of sub-pixels (31) of discrete colors; and

   - a second layer (34) positioned facing the first layer, said second layer being opaque with respect at least to the visible wavelength spectrum;

   - wherein the first layer comprises first perforations (40) formed by a first laser radiation (LS1), at least a first part of the first perforations (40) revealing locally through the holographic structure, dark areas (42) in the sub-pixels caused by underlying regions (41) of the second opaque layer which are located facing said at least one first part of the first perforations (40), so as to form a customized image (IG) from the arrangement of pixels (30) combined with the dark areas (42).
2. The document according to claim 1, wherein each pixel of said arrangement of pixels is configured such that each sub-pixel has a single color in said pixel.
3. The document according to one of claims 1 or 2, wherein the first layer comprises:

   - a varnish underlayer forming the reliefs of a holographic grating; and

   - a metallic underlayer deposited on the reliefs of the varnish underlayer, said metallic underlayer having a refractive index greater than that of the varnish underlayer.
4. The document according to any one of claims 1 to 3, wherein the second opaque layer comprises a black opaque surface facing the first layer or comprises black opacifying pigments in its mass.
5. The document according to any one of claims 1 to 4, wherein the first laser radiation is at a first wavelength spectrum different from the visible wavelength spectrum.
6. The document according to claim 5, wherein said at least one first part of the first perforations are through-perforations which extend through the thickness of the holographic structure so as to reveal said underlying regions of the second opaque layer.
7. The document according to any one of claims 1 to 6, comprising a third layer (50) located facing the second layer (34) such that said second layer is interposed between the first layer (24) and the third layer (50),

   - said third layer being transparent or having a lighter color than the second opaque layer, and forming a background with respect to the customized image (IG),

   - wherein the second layer (34) comprises second perforations (52) formed by a second laser radiation (LS2) different from the first laser radiation, the second perforations being positioned in the extension of a second part of the first perforations such that the first and second perforations located facing one another reveal, locally through the holographic structure and through the second opaque layer, lightened areas in the sub-pixels caused by underlying regions of the third layer which are located facing said second perforations, thereby forming a customized image using the arrangement of pixels combined with the dark areas and the lightened areas.
8. The document according to claim 7, wherein the second perforations are through-perforations which extend through the thickness of the second layer so as to reveal, together with the second part of the first perforations located facing same, said underlying regions of the third opaque layer through the first and second layers.
9. The document according to claim 7 or 8, wherein the lightened areas are brighter areas than the dark areas.
10. A method for manufacturing a secure document, comprising:

    - providing (S2) a first layer (24) comprising a metallic holographic structure (32) forming an arrangement (29) of pixels (30) each comprising a plurality of sub-pixels (31) of discrete colors;

    - positioning (S4) a second layer (34) facing the first layer (24), said second layer (34) being opaque with respect at least to the visible wavelength spectrum; and

    - forming, in the first layer (24), first perforations (40) by a first laser radiation, at least a first part of the first perforations (40) revealing, locally through the holographic structure, dark areas (42) in the sub-pixels caused by underlying regions of the second opaque layer (34) which are located facing said at least one first part of the first perforations (40), so as to form a customized image (IG) from the arrangement of pixels (30) combined with the dark areas (42).
11. The method according to claim 10, wherein the first laser radiation (LS1) is at a first wavelength spectrum (SP1) different from the visible wavelength spectrum.
12. The method according to claim 10 or 11, comprising:

    - positioning (S10) a third layer facing the second layer such that said second layer is interposed between the first layer and the third layer, said third layer being transparent or of a lighter color than the second opaque layer, and forming a background with respect to the customized image,

    - forming (S12), in the second layer, second perforations by a second laser radiation different from the first laser radiation, the second perforations being positioned in the extension of a second part of the first perforations such that the first and second perforations located facing one another reveal, locally through the holographic structure and through the second opaque layer, lightened areas in the sub-pixels caused by underlying regions of the third layer which are located facing said second perforations, thereby forming a customized image from the arrangement of pixels combined with the dark areas and the lightened areas.
13. The method according to claim 12, wherein the third layer is transparent with respect to the first and second laser radiations.

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