Diffractive element, secure document and method of manufacturing a secure document comprising a diffractive element.

Abstract

The present invention relates to a diffractive element comprising n diffractive structures, configured to form a matrix of sub-pixels constituted by the repetitive arrangement of the n diffractive structures, n being greater than or equal to 2, said diffractive element being intended to be observable in an angular range, each diffractive structure allowing diffraction of light according to a wavelength evolving in a range of wavelength values ​​when said diffractive element is tilted in said angular range, said diffractive element being such that: - the n wavelength ranges associated respectively with the n diffractive structures are located in the visible spectrum, when said diffractive element is observed in said angular range, - the n wavelength ranges are disjoint. Figure for the abstract: Fig. 1.

Description

Title of the invention: Diffractive element, secure document and method of manufacturing a secure document comprising a diffractive element. Technical field

[0001] The present invention relates to diffractive elements, mainly used in secure documents, for example to encode security information therein. Prior art

[0002] Secure or security documents using color matrices obtained using diffractive elements make it possible to observe variable colors of the document patterns depending on the inclination of the document for a given lighting. The color matrices can for example be composed of a plurality of sub-pixels, typically three sub-pixels make up a pixel. The desired color can then be obtained for a pixel by perforating a metal layer at the location of certain sub-pixels to modulate the relative contribution of each sub-pixel and thus generate a pixel of the desired color. A plurality of colors is thus created, in a relatively wide color gamut provided that the sub-pixels are chosen judiciously. It is noted that from a certain inclination of the document, certain sub-pixels saturate because the wavelength emitted by these diffractive elements is no longer in the visible range.This saturation phenomenon of one or more sub-pixels constitutes a discontinuity in the evolution of the color of the pixel. If the pattern represented in this diffractive matrix is ​​a color barcode, then this saturation phenomenon of one or more sub-pixels no longer allows a tri-chromatic reading of the elements of this color barcode. The color is then not controlled for these inclination angles which lead to a saturation of all or part of the sub-pixels. In addition, the color of a pixel being obtained by adding the color of several sub-pixels, it can be identical for different inclination angles with different sub-pixel colors. This is not desirable when it is important to guarantee the uniqueness of the color of the pattern.Indeed, some patterns can, for example, represent color barcodes, which, when scanned, for example by a mobile phone, provide access to security information.

[0003] There is therefore a need to improve the diffractive elements used to generate color pixels, particularly in secure documents. Statement of the invention

[0004] The present invention proposes to overcome at least one of the drawbacks of the art prior art and relates to a diffractive element comprising n diffractive structures configured to form a matrix of sub-pixels constituted by the repetitive arrangement of the n diffractive structures, n being greater than or equal to 2, said diffractive element being intended to be observable in an angular range, each diffractive structure allowing diffraction of light according to a wavelength evolving in a range of wavelength values ​​when tilting said diffractive element in said angular range, said diffractive element being such that: - the n ranges of wavelength values ​​associated respectively with the n diffractive structures are located in the visible spectrum, when observing said diffractive element in said angular range, - the n wavelength ranges are disjoint.

[0005] Within said angular range, this advantageously makes it possible to avoid discontinuities in the images formed by the diffractive elements (and the perforations or other means of acting on the colors) during observation in the angular range. Indeed, when one of the sub-pixels saturates in color, a color recalibration calculation comes across a discontinuity since this saturating sub-pixel ceases to provide information. Such discontinuities are difficult to process mathematically. The present invention makes it possible to avoid this discontinuity and therefore simplifies the calibration of the images and improves the quality of the images and the security of the documents when the reading of the images triggers access to data.

[0006] The low end of the visible spectrum is commonly considered to be 380 nm and the high end of the visible spectrum is considered to be 780 nm. According to some embodiments, - the range of wavelength values ​​for a first diffractive structure is between 380 and nm, - the range of wavelength values ​​for a second diffractive structure is between Xb and Xc nm, - the wavelength value range for a third diffractive structure is between Xd and 780 nm.

[0007] The values ​​Xa, Xb, Xc, Xd are such that Xa <Xb<Xc<Xd.

[0008] According to certain embodiments, for a determined observation angle of said diffractive element, the value of the wavelength emitted by each diffractive structure, according to this determined angle, corresponds to a value of the wavelength centered on the range of wavelength values ​​emitted by said diffractive structure.

[0009] According to certain embodiments, for an observation of the diffractive element in said angular range - the range of wavelength values ​​for a first diffractive structure is between 440 and 498 nm, - the wavelength value range for a second diffractive structure is between 491 and 575 nm, - the wavelength value range for a third diffractive structure is between 582 and 630 nm.

[0010] According to certain embodiments, for a determined observation angle of said diffractive element, the value of the wavelength emitted by each diffractive structure, according to this determined angle, corresponds to a value of the wavelength centered on the range of wavelength values ​​emitted by said diffractive structure.

[0011] Thus, rather than taking a triplet of red, green, blue diffractive elements, in the preferred observation angle, which would very quickly give saturation in the red or blue domain, we take a triplet of diffractive elements emitting in ranges such that we do not obtain saturation, for any of the colors and therefore more particularly for red and blue which saturate quickly when we move away from the median position, in the configurations of the prior art.

[0012] According to certain embodiments, the number of diffractive structures composing a diffractive element is equal to three, the value of the wavelength emitted by each diffractive structure for the determined observation angle of said diffractive element corresponds to - a wavelength value between 474 and 478 nm for a first diffractive structure, - a wavelength value between 498 and 502 nm for a second diffractive structure, - a wavelength value between 595 and 600 nm for a third diffractive structure.

[0013] Thus, advantageously, a triplet of red, green, blue sub-pixels is not chosen in the preferred observation angle but a yellow-orange, cyan-green, blue triplet. By adapting the length of the angular range in which the secure document is tilted around this observation position, the diffractive structures emit a wavelength in the visible range.

[0014] According to certain embodiments, - said determined angle corresponds to the normal to a plane comprising the diffractive element under zenithal lighting, - said angular range is between minus 6 degrees and plus 6 degrees relative to said determined angle.

[0015] According to some embodiments, the length of the range of visible wavelength values ​​may be different for each diffractive structure.

[0016] According to certain embodiments, said determined observation angle corresponds to an observation angle of said diffractive element by a capture device and this angle is different from the normal to a plane containing (or in which is located) the diffractive element.

[0017] According to certain embodiments, said determined observation angle corresponds to an observation angle of said diffractive element by a capture device and this angle is different from the normal to a plane containing the diffractive element.

[0018] Indeed, the security documents as described may be access badges or other documents allowing access to sensitive or private data. These documents may be read by external readers which capture the image and access data encoded by these images. The documents are generally not placed perpendicular to the external readers when the document is read.

[0019] According to certain embodiments, the diffractive structures are configured in the form of holographic structures whose orientation angle is determined as a function of said observation angle.

[0020] According to certain embodiments, the length of the ranges of wavelength values ​​emitted by each diffractive structure depends on an evolution specific to each diffractive structure, of the wavelength emitted by scanning the angular range.

[0021] According to certain embodiments, said diffractive structures are diffractive optical gratings.

[0022] The present invention also relates to a secure document comprising at least one diffractive element according to one of the embodiments of the present invention.

[0023] The present invention also relates to a method of manufacturing a secure document comprising - the formation of a metal layer on a support layer, the metal layer comprising a plurality of diffractive elements according to one of the embodiments of the present invention, - forming perforations by focusing laser radiation on the metal layer, the perforations comprising at least one group of perforations made by focusing the laser radiation at a respective angle of incidence so as to reveal a corresponding personalized image when the secure document is observed at said angle of incidence.

[0024] Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the accompanying drawings which illustrate an exemplary embodiment thereof without any limiting character. Brief description of the drawings

[0025] [Fig.l] [Fig.l] represents an example of a decomposition of a document implementing a set of diffractive elements according to the present invention,

[0026] [Fig.2a], [Fig.2b], [Fig.2c] Figures 2a to 2c schematically represent the inclination of a diffractive element and the wavelengths of the colors visualized,

[0027] [Fig.3] [Fig.3] represents an example of values ​​of wavelength ranges in the CIE color space,

[0028] [Fig.4a], [Fig.4b], [Fig.4c] Figures 4a, 4b and 4c schematically represent arrangements of pixels and sub-pixels, according to particular embodiments of the invention,

[0029] [Fig.5] [Fig.5] represents a method of manufacturing a security document according to an embodiment of the present invention. Description of the embodiments

[0030] The present invention relates to the production of patterns, in color, for example on security documents.

[0031] In the remainder of this disclosure, examples of implementations of the invention are described in the case of a document comprising at least one personalized image according to the principle of the present disclosure. This document may be any document, called a secure document, of the booklet, card or other type. The invention finds particular applications in the formation of color barcodes 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 personalized image. Other implementations are, however, possible.Such barcodes are often read by devices such as cameras, embedded in mobile phones, and can encode information such as links to websites, specific pages, videos, contact information (telephone, email address or sensitive personal information), serial numbers, unique identifiers.

[0032] Unless otherwise indicated, 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.

[0033] [Fig.l] shows an example of the layers that can be assembled to form a secure document component 100 as provided by the present disclosure. The secure document 100 is provided as an example to illustrate the composition of a pixel into sub-pixels as provided in the present disclosure. In addition to a metal layer configured to include a plurality of diffractive elements, the document may include one or more layers.

[0034] The present disclosure relates to a secure document comprising diffractive elements as described below according to any one of their embodiments and / or manufactured by the method according to any one of its embodiments described below.

[0035] A metal or metal oxide layer 101 is positioned between two layers 102a and 102b. The layer 102b is for example a transparent protective layer which may be a high refractive index varnish making it possible to view through a colored pattern formed by the matrix of diffractive structures SDi, also called sub-pixels. The layer 102a may be an opaque layer with respect to at least the visible wavelength spectrum. The metal layer 101 comprises a plurality of diffractive elements EDi, forming pixels of the formed image, each formed by several diffractive structures SDi, the whole forming an arrangement of pixels visible to an observer OB.

[0036] As illustrated in [Fig.l], the metal layer 101 comprises perforations Pi formed by laser radiation which has ablated the metal layer. These through perforations locally reveal dark areas in the diffractive structures through the metal structure 101, these dark areas being formed by underlying regions of the opaque layer 102a located opposite the perforations Pi, so as to form a personalized image from the arrangement of pixels combined with the dark areas 142. The contribution of each sub-pixel is thus modulated to determine the hue, saturation and brightness of the pixel formed by the sub-pixels.

[0037] The colorimetry of the reflected light is then a function of the observation angle, the illuminant and the combination of light-matter interaction phenomena (diffraction, diffusion, absorption, etc.) occurring at the level of the arrangement of the diffractive structures previously perforated by laser.

[0038] This technique makes it possible in particular to form a personalized image by ablation, as opposed to carbonization, which is secure, without resorting to powerful laser radiation (necessary in the case of carbonization) likely to generate air bubbles by heating in the metal structure 101 which would lead to irreversible destruction of the holographic structure. Indeed, the ablation of the thin metal layer requires only low energy compared to carbonization.

[0039] The metal layer 101 comprises an arrangement of diffractive structures. Various types (shapes, sizes, etc.) of diffractive structures may be envisaged within the scope of the invention. Generally, according to the present disclosure, the diffractive structures present in the metal layer 101 are configured to diffract light in the visible wavelength spectrum, within a defined angular range. Each pixel in the image is associated with a diffractive element, a diffractive element being a set of diffractive structures comprising at least two diffractive structures, and preferably three diffractive structures. The diffractive structures are all in the same plane, which is the one defined by the metal layer.

[0040] The diffractive elements may be composed of diffractive structures in the form of diffractive optical gratings. The gratings are for example inclined in different directions for each of the diffractive structures or any other equivalent solution. This may allow each of the diffractive structures to emit a spectrum whose wavelength evolves in a determined angular range.

[0041] It is known in the state of the art to choose diffractive structures configured to emit respectively red, green and blue light at the same angle of incidence of the light on the metal layer. This angle of incidence is generally the angle of incidence corresponding to the normal to the document or a preferred angle of observation of the document. The applicant has noticed that when the metal layer (or more generally the document comprising this metal structure) is tilted by an increasingly large angle, color saturation is quickly reached, in particular for the red pixel but also for the blue pixel. This phenomenon is explained by the fact that the wavelengths of the visible spectrum are between approximately 380 and 780 nm and that the wavelength of red is between approximately 620 and 750 nm, that of blue approximately between 450 and 495 nm and that of green between 495 and 570 nm.Green is therefore located in the middle of the visible spectrum, red at one end of the spectrum (infrared side) and blue relatively close to the other end of the spectrum (violet separates it from the ultraviolet end of the visible spectrum).

[0042] Achieving color saturation for at least one of the sub-pixels is not desirable because the rendering obtained by the combination of the three sub-pixels is reduced by the contribution of one of the sub-pixels. This has the consequence in particular that when observing a multi-colored image on either side of its optimal observation position, the component of the red sub-pixel is quickly lost and quite quickly the component of the blue sub-pixel. Note that when one deviates from the optimal observation position, each sub-pixel emits a color offset from its nominal position. For example, the green sub-pixel can emit a cyan, the blue an indigo and the red an orange. Or conversely, the blue sub-pixel can emit a cyan, the green a yellow-green and the red a dark red.

[0043] The present disclosure makes it possible to advantageously configure such diffractive structures so that they all reflect light along a length wave remaining in the visible spectrum when the document 100 is tilted within a chosen or determined angular range.

[0044] For this purpose, the present disclosure proposes a diffractive element comprising n diffractive structures, n being greater than or equal to 2, said diffractive element being intended to be observable in an angular range, each diffractive structure allowing diffraction of light according to a wavelength evolving in a range of wavelength values ​​when said diffractive element is tilted in said angular range, said diffractive element being such that: - the n wavelength ranges associated respectively with the n diffractive structures are located in the visible spectrum, when observing said diffractive element in said angular range, - the n wavelength ranges are disjoint when the observation remains in the said angular range.

[0045] In the remainder of the disclosure, we will take as an example a diffractive element composed of three diffractive structures but those skilled in the art will easily understand how to transpose the present disclosure to a diffractive element comprising two or more than three diffractive structures.

[0046] The diffractive elements are made in the metal layer. Several arrangements can be made. Different examples of arrangements that can be used are illustrated in Figures 5 to 7.

[0047] The diffractive element may be configured to widen its diffraction angle. This may, for example, be achieved by spacing grooves in the diffractive grating or by adding a high diffraction index varnish. This may advantageously make it possible to reach a saturation zone less quickly when the document is not viewed from a specific viewing angle.

[0048] The characteristics of the spectra of each sub-pixel, or more precisely of each diffractive grating, can advantageously be taken into account. Indeed, each diffractive structure has differences. One diffractive structure may, for example, have a larger range of greens than another, which may, for example, have a larger range of blues. Thus, by characteristic, one can understand an evolution (or a speed of evolution) of the wavelength as a function of the observation angle under a determined illuminant, this within said angular range. In other words, for an observation angle which increases, certain diffractive structures can reach saturation while others reach it for a larger or smaller but different observation angle.Thus, the length of the wavelength ranges emitted by each diffractive structure depends on an evolution specific to each diffractive structure, of the wavelength emitted by scanning the angular range.

[0049] Each diffractive structure is such that the value of the wavelength associated with the spectrum emitted by each diffractive structure evolves in a range of wavelength values ​​located in the visible spectrum, when observing the diffractive element in said angular range, for example when tilting the document comprising the diffractive structures. In addition, the ranges of wavelength values ​​are disjoint for each diffractive structure within said angular range. Since none of the wavelength values ​​are outside the visible spectrum, no diffractive structure saturates within said angular range.

[0050] According to certain embodiments, the diffractive element is equivalent to a prism whose shape is similar for the three colors, but whose orientation is different for each of the diffractive structures, in order to emit a wavelength in the determined angle, different for each and making it possible to avoid saturation in the determined angular range.

[0051] In other words, the diffractive structures can be configured such that they allow the largest possible angular observation range for each diffractive structure, while having wavelengths included in ranges that are separate from each other.

[0052] Preferably, to obtain the largest possible angular ranges, for a determined observation angle of said diffractive element, the value of the wavelength emitted by each diffractive structure, according to this determined angle, corresponds to a value of the wavelength of the spectrum centered on the range of wavelength values ​​associated with said diffractive structure.

[0053] Preferably, the diffractive element emits a wavelength whose value is the combination of the wavelength of the diffractive structures composing it, the value of the wavelength of the diffractive element is obtained by a unique combination of the wave values ​​of the diffractive structures composing it. Thus, a color obtained by the combination of the wavelengths of the diffractive structures is obtained from a single combination of these values. This gives the document much better security.

[0054] According to certain embodiments, the observation angle can be determined as being 90 degrees relative to the front face of the document, therefore to the diffractive structure. Thus, the parallelogram effect is minimized, i.e. a square is observed as a square and does not require a rectification calculation.

[0055] According to certain embodiments, the observation angle corresponds to an angle of observation of said document by a device for capturing an image of said document. Indeed, very often the bar codes are intended to be read by an external optical device, itself connected to means for analyzing this read bar code. The bar code can encode information such as links to websites, specific pages, videos, contact information (phone, email address or sensitive personal information), serial numbers, unique identifiers. The angle at which a reading device views the document is often inclined relative to the position at which the document is viewed by the human eye. It is known in the state of the art to straighten the barcode image using the position of the registration markers most often placed in the corners of the barcode.

[0056] The angular range is centered on a wavelength value of the light spectrum such that the wavelength variation corresponding to the angular variation remains in the visible range and does not overlap a wavelength value associated with another diffractive structure, this for each of the diffractive structures.

[0057] Figures 2a, 2b and 2c illustrate an example of an EDI diffractive element comprising three diffractive structures SD1, SD2 and SD3.

[0058] [Fig.2a] illustrates a first example in which the diffractive element (or the document comprising this diffractive element) is observed at an angle of 90 degrees relative to the plane of the diffractive element (of the document) and viewing the document from its front face or face on which the image obtained is visible. A human eye has been shown but the human eye can be replaced by a device for reading the document comprising the diffractive structure.

[0059] Each diffractive element is configured to emit a light spectrum comprised respectively in an angular range ai, a2, a3, each being centered on a wavelength X2, X3. For reasons of simplification, the three angular deflections a1, a2, a3 are equal. The three angular ranges ab a2, a3 are determined as being the angular ranges in which the associated diffractive structure emits in the visible spectrum and such that the wavelengths of the spectra emitted by each of the diffractive structures do not overlap in the ranges.

[0060] [Fig.2b] illustrates a second example in which the diffractive element (the document comprising this diffractive element) is observed at an angle of ai / 2 or a2 / 2 or a3 / 2, the three angles being equal. In the case where the three angles are not equal, the length of the angular range determined for the diffractive element is selected as the smallest angular range among the angular ranges of the diffractive structures composing it.

[0061] It can thus be seen that the eye of the observer (or the reading device) visualizes the spectrum emitted by the diffractive element in the visible, when the maximum inclination of the document is less than or equal to this angle ai / 2 (or to half of the smallest of the three angles when they are different).

[0062] [Fig.2c] illustrates a second example in which the diffractive element (the document comprising this diffractive element) is observed at an angle of -ai / 2 or -a2 / 2 or -a3 / 2, the three angles being equal. In the case where the three angles are not equal, the length of the angular range determined for the diffractive element is selected as the smallest angular range among the angular ranges of the diffractive structures composing it.

[0063] It can thus be seen that the eye of the observer (or the reading device) visualizes the spectrum emitted by the diffractive element in the visible, when the maximum inclination of the document is less than or equal to this angle -cq / 2 (or to half of the smallest of the three angles when they are different).

[0064] [Fig.3] gives an example of values ​​of wavelengths and ranges an determined gulars. It represents a grayscale visualization of the color space according to the International Commission on Illumination (CIE) which is a means of linking the wavelengths of the visible spectrum and the colors perceived by the human visual system. For reasons of reproduction of this document, this visualization must be in grayscale but the skilled person can easily find a colored version on internet links and for example on the url: http s : / / fr. wikipedia.org / wiki / CIE_XYZ.

[0065] [Fig.3] shows an example in which the angular variation range observation angle is 12° for each diffractive structure. This angle is given for information purposes only, and other angles may be determined according to the structural parameters of each of the diffractive structures (such as the size of the grooves or particles of said structures). According to this example, each diffractive element comprises three diffractive structures configured to emit, according to an observation angle varying around a determined angle, - for a first diffractive structure, a spectrum whose wavelength is between 474 and 478 nm - for a second diffractive structure, a spectrum whose wavelength is between 498 and 502 nm, - for a third diffractive structure, a spectrum whose wavelength is between 595 and 600 nm.

[0066] According to this example, the angular range is between minus 6 and plus 6 degrees relative to the determined angle. Preferably, this determined angle is 90 degrees relative to the front face of the document.

[0067] Thus, according to certain embodiments, the diffractive structures, unlike the prior art, do not emit red, green, blue light when they are exposed to normal incident light but a light rather close to orange (wavelength located around 595 to 600 nm) for one, a light close to green (wavelength located around 498 to 502 nm) for the other and a light close to blue (wavelength located around 474 to 478 nm) for the third.

[0068] According to certain embodiments, the angular range in which the diffractive elements, and more particularly the document containing an arrangement of diffractive elements, may vary slightly around a central position during its observation. It may be assumed that the central position is, according to certain observation modes, the position along a normal to the document. However, as mentioned previously, it is common for the document to be inclined and according to the present disclosure, an angular movement around this normal in a determined angular range may make it possible not to emit a light wave which leaves the visible spectrum.

[0069] Thus, it can be observed, in [Fig.3], for an observation of the diffractive element in the angular range - a range of wavelength values ​​for a first diffractive structure between 440 and 498 nm, - a range of wavelength values ​​for a second diffractive structure between 491 and 575 nm - a range of wavelength values ​​for a third diffractive structure between 582 and 630 nm.

[0070] It can be noted that a margin is advantageously kept between each range of wavelength values. This can advantageously make it possible to overcome an uncertainty linked to the calculation of the color calibration. The margins make it possible to never risk confusing a color from the end of the useful spectrum of a sub-pixel with the beginning of another sub-pixel.

[0071] According to certain embodiments, the range of wavelength values ​​may be of different length for each diffractive structure. More particularly, the length of the range of wavelength values ​​may depend on the evolution of the wavelength as a function of the angular amplitude for a diffractive structure and / or the angular amplitude of the diffractive element.

[0072] Particular examples of arrangements (or tiling) of diffractive elements that can be implemented in the secure document 100 ([Fig. 1]) are now described with reference to FIGS. 4a-c. It should be noted that these implementations are presented here only as non-limiting examples, numerous variants being possible in terms in particular of arrangement and shape of the diffractive elements and diffractive structures, as well as the colors assigned to these diffractive structures.

[0073] According to a first example shown in [Fig.4a], the diffractive elements are rectangular (or square) in shape and comprise 3 diffractive structures SD1, SD2, SD3 generating distinct colors. The diffractive structures SD1, SD2, SD3 can each be formed by a portion of a line of sub-pixels. In this example, the tiling 30 thus forms a matrix of rows and columns of sub- pixels, orthogonal to each other.

[0074] [Fig.4b] is a top view showing another example of regular tiling in which each diffractive element Edi is composed of 3 diffractive structures SDi, each of a distinct color. The diffractive structures SDi are here hexagonal in shape.

[0075] [Fig.4c] is a top view showing another example of regular tiling in which each diffractive element EDi is composed of 4 diffractive structures SDi, each generating a distinct color. The diffractive structures SDi are here triangular in shape.

[0076] For each of the arrangements of diffractive elements considered, it is possible to adapt the shape and dimensions of each diffractive structure.

[0077] According to certain embodiments, diffractive gratings identical to those coding the bar code are placed on the secure document at a specific location, corresponding for example to a position of location markers or bar code (or QR code) location patterns. This can advantageously allow the useful frames of a video recorded by a capture device, such as a telephone camera, to correspond to observations made within said angular range.

[0078] To this end, the present invention relates to a method for reading coded images on a secure document comprising a plurality of diffractive elements as described previously, said diffractive elements comprising: - a first group of diffractive elements coding a bar code, - a second group of diffractive elements encoding a location marker, the structure of the diffractive elements of said first group and said second group being identical, said diffractive elements of said first group being of a size smaller than that of the diffractive elements of said second group, said method comprises: - a capture of said document at different angles of inclination of said document relative to a capture device, - a verification, for each tilt angle, that the wavelength emitted by each diffractive structure is in the visible range, - a determination that the tilt angle is too large when at least one of the diffractive structures of said second group does not emit a wavelength in the visible spectrum.

[0079] According to certain embodiments, the size of the diffractive elements encoding said identification marker are of the order of 100 times larger than the size of the diffractive elements encoding said bar code.

[0080] According to certain embodiments, the size of the diffractive elements encoding said location marker is a few microns, for example 35 pm and the size of the diffractive elements encoding the bar code are of the order of a few millimeters.

[0081] When the structure of the diffractive elements is the same, it is then possible to obtain, for given lighting and observation conditions, the same color for the bar code and the marker.

[0082] When the observation (reading) device is a smartphone camera, the camera pixels can distinctly see the color of a tracking marker while a pixel of this same camera has difficulty being significant for sampling the width of a single sub-pixel.

[0083] For example, if the color bit of the 2D barcode is formed of 10 pixels by 10 pixels, so 30 sub-pixels by 30 sub-pixels, and the size of a sub-pixel is 35 pm, the size of a pixel is a square of 35 microns * 30 = 1.05 mm on each side. The pixels of the phone's camera record an average color, without being able to distinguish the detail of the colors of the sub-pixels.

[0084] According to the same example, if the color bit of the marker is formed of 10 pixels by 10 pixels, therefore 30 sub-pixels by 30 sub-pixels, and the size of a sub-pixel is 1 mm, the size of a pixel is a square of lmm*30=30mm on each side. The pixels of the phone's camera can distinguish the color of each sub-pixel and thus it is possible to carry out a verification, for each tilt angle, that the wavelength emitted by each diffractive structure or sub-pixel is in the visible range, and to carry out a determination that the tilt angle is too large when at least one of the diffractive structures of said marker does not emit a wavelength in the visible spectrum.

[0085] A method for manufacturing a secure document comprising a plurality of diffractive elements as described previously is now described with reference to [Fig. 5], according to a particular embodiment. It is assumed for example that at least one barcode is formed in a secure document 100 as illustrated in [Fig. 1].

[0086] During a formation step S2, a metal layer 101 is formed on a support layer 102a. The metal layer 101 and the support layer 102a are as already described in the embodiments above. In particular, the metal layer 101 comprises an arrangement of diffractive elements each comprising n (or a matrix of) diffractive structures SDi as described above. As already indicated, this arrangement of diffractive elements is configured to diffract light at least in the visible wavelength spectrum. These diffractive nanostructures can be arranged periodically (to form for example a diffractive holographic structure) or aperiodically (non-periodically) so as to control (or modify) the colorimetry of the reflected light as a function of the angle of incidence of the light on the metal layer 101, as already described previously. The diffractive nanostructures are configured as described previously. In addition, the support layer 102a may be opaque to at least the visible wavelength spectrum or transparent to at least the visible wavelength spectrum, depending on the visual effect that one wishes to create in one or more personalized barcodes.

[0087] An adhesive and / or glue layer (not shown) may be used to ensure adhesion of the metal layer 101 to the support layer 102a.

[0088] During a step S4, perforations (or holes) P; are formed in the metal layer by focusing laser radiation. These perforations P; thus comprise at least one group of perforations made by focusing laser radiation.

[0089] The perforations P; are made so as to occupy all or part of a plurality of sub-pixels of the metal layer. These perforations P; locally reveal dark areas if the layer 102a is opaque) or light areas (if the layer 102a is transparent) in the sub-pixels, these areas being caused (or produced) by underlying regions of the support layer 102a located opposite the perforations P;. To do this, the perforations P; are here through perforations which extend through the thickness of the metal layer so as to reveal underlying regions of the support layer 102a at the level of the pixel arrangement. In other words, the underlying regions modify the contribution of the sub-pixels so as to form the final barcode. It is thus possible to form one or more personalized images, and in particular barcodes, from the pixel arrangement combined with said dark or light areas.

[0090] As mentioned previously, a layer 102b may further be added to protect the document or solidify it. The layer 102b is for example a transparent protective layer which may be a high refractive index varnish allowing a colored pattern formed by the matrix of diffractive structures SDi to be viewed through.

[0091] A person skilled in the art will understand that the embodiments and variants described above constitute only non-limiting examples of implementation of the invention. In particular, a person skilled in the art may envisage any adaptation or combination of the embodiments and variants described above, in order to meet a very specific need in accordance with the claims presented below.

Claims

Claims

1. Diffractive element comprising n diffractive structures configured to form a matrix of sub-pixels constituted by the repetitive arrangement of the n diffractive structures, n being greater than or equal to 2, said diffractive element being intended to be observable in an angular range, each diffractive structure allowing diffraction of light according to a wavelength evolving in a range of wavelength values ​​when said diffractive element is tilted in said angular range, said diffractive element being such that: - the n ranges of wavelength values ​​associated respectively with the n diffractive structures are located in the visible spectrum, when observing said diffractive element in said angular range, - the n wavelength ranges are disjoint.

2. Diffractive element according to claim 1 characterized in that, - for a determined observation angle of said diffractive element, the value of the wavelength emitted by each diffractive structure, according to this determined angle, corresponds to a value of the wavelength centered on the range of wavelength values ​​emitted by said diffractive structure.

3. Diffractive element according to claim 2 characterized in that, the number of diffractive structures composing a diffractive element is equal to three, the value of the wavelength emitted by each diffractive structure for the determined observation angle of said diffractive element corresponds to - a wavelength value between 474 and 478 nm for a first diffractive structure, - a wavelength value between 498 and 502 nm for a second diffractive structure, - a wavelength value between 595 and 600 nm for a third diffractive structure.

4. Diffractive element according to claim 3 characterized in that, for an observation of the diffractive element in said angular range - the range of wavelength values ​​for a first diffractive structure is between 380 and nm, - the range of wavelength values ​​for a second diffractive structure is between Xb and Xc nm, - the range of wavelength values ​​for a third structure diffractive is between Xd and 780 nm with Xa <Xb<Xc<Xd.

5. Diffractive element according to one of claims 2 to 4 wherein - said determined observation angle corresponds to the normal to a plane containing the diffractive element under zenithal lighting, - said angular range is between minus 6 degrees and plus 6 degrees relative to said determined angle.

6. Diffractive element according to one of claims 2 to 4 wherein - said determined observation angle corresponds to an observation angle of said diffractive element by a capture device and this angle is different from the normal to a plane containing the diffractive element.

7. Diffractive element according to one of the preceding claims such that the diffractive structures are configured in the form of holographic structures, the orientation angle of which is determined as a function of said observation angle.

8. Diffractive element according to one of the preceding claims in which the length of the ranges of wavelength values ​​emitted by each diffractive structure depends on an evolution specific to each diffractive structure, of the wavelength emitted by scanning the angular range.

9. Diffractive element according to claim 9 such that said diffractive structures are diffractive optical gratings.

10. Secure document comprising at least one diffractive element according to one of claims 1 to 9.

11. A method of manufacturing a secure document (100) comprising - forming a metal layer on a support layer, the metal layer comprising a plurality of diffractive elements according to one of claims 1 to 8, - forming perforations (20) by focusing laser radiation (RY) on the metal layer, the perforations comprising at least one group of perforations made by focusing the laser radiation at a respective angle of incidence so as to reveal a corresponding personalized image (IG) when the secure document is observed at said angle of incidence.

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