Diffractive element, security document and method for manufacturing a security document comprising a diffractive element

Abstract

The invention relates to a diffractive element comprising n diffractive structures configured to form a sub-pixel matrix, the sub-pixel matrix consisting of a repeated arrangement of the n diffractive structures, n being an integer greater than or equal to 2, the diffractive element being intended to be visible in an angular range, each diffractive structure allowing light to be diffracted at wavelengths varying in a range of wavelength values when the diffractive element is tilted in the angular range, the diffractive element being such that, when the diffractive element is observed in the angular range, n ranges of wavelengths respectively associated with the n diffractive structures are located in the visible spectrum, the n ranges of wavelengths being separate.

Description

Technical Field

[0001] This invention relates to diffraction elements, which are primarily used in secure documents, for example, for encoding security information in secure documents. Background Technology

[0002] A secure or anti-counterfeiting document using a color matrix obtained by utilizing diffraction elements allows for the observation of variable colors of the document pattern under given illumination conditions, depending on the document's tilt angle. The color matrix can, for example, consist of multiple sub-pixels, typically three sub-pixels per pixel. The desired color of a pixel is then obtained by perforating a metal layer at certain sub-pixel locations to modulate the relative contribution of each sub-pixel and thus generate pixels with the desired color. Therefore, by carefully selecting the sub-pixels, a variety of colors can be produced within a relatively wide color gamut. It can be observed that, starting from a certain tilt angle of the document, some sub-pixels become saturated because the wavelengths emitted by these diffraction elements are no longer within the visible light range. This saturation of one or more sub-pixels causes discontinuities in the pixel's color evolution. If the pattern represented in the diffraction matrix is ​​a color barcode, this saturation of one or more sub-pixels no longer allows for three-color reading of the individual elements of the color barcode. Thus, for these tilt angles that cause saturation of all or some sub-pixels, the color is uncontrolled. Furthermore, since a pixel's color is obtained by merging the colors of several sub-pixels, the pixel's color may appear the same even if the sub-pixel colors differ, regardless of the tilt angle. This is undesirable when ensuring the uniqueness of pattern colors is crucial. In practice, some patterns can represent, for example, color barcodes, which can be scanned by mobile phones to retrieve security information.

[0003] Therefore, it is necessary to improve the diffraction elements used to generate color pixels (especially in secure files). Summary of the Invention

[0004] This invention aims to overcome at least one drawback of the prior art and relates to a diffraction element comprising n diffraction structures configured to form a sub-pixel matrix consisting of repeating arrangements of the n diffraction structures, where n is an integer greater than or equal to 2. The diffraction element is intended to be visible within an angular range, and during tilting of the diffraction element within the angular range, each diffraction structure allows light to diffract at wavelengths varying within a wavelength value range. The diffraction element is such that: - During observation of the diffraction element within the stated angular range, the ranges of n wavelength values ​​associated with the n diffraction structures are located within the visible spectrum. - The n wavelength ranges are separate.

[0005] Within the stated angular range, this advantageously allows for the avoidance of discontinuities in the image created by diffraction elements or even by perforations or other means affecting color during observation within that range. In fact, color recalibration calculations encounter discontinuities when a subpixel becomes color saturated, as the saturated subpixel no longer provides information. Such discontinuities are difficult to handle mathematically. This invention makes it possible to avoid these discontinuities, thereby simplifying image calibration when data access is triggered during image reading and improving image quality and file security.

[0006] The lower end of the visible spectrum is generally considered to be 380 nm, and the upper end is considered to be 780 nm. According to some embodiments, - The wavelength range of the first diffraction structure is between 380 nm and λ. a Between nm - The wavelength range of the second diffraction structure is between λ b nm and λ c Between nm - The wavelength range of the third diffraction structure is between λ d Between 780 nm and 780 nm.

[0007] λ a , λ b , λ c , λ d The value of λ makes a <λ b <λ c <λ d .

[0008] According to some embodiments, for a defined observation angle of the diffraction element, the wavelength value emitted by each diffraction structure at that defined angle corresponds to the wavelength value in the middle of the range of wavelength values ​​emitted by the diffraction structure.

[0009] For example, the defined observation angle corresponds to the center angle of the angle range in which the diffraction element is intended to be visible, i.e., the angle located in the middle of the angle range.

[0010] According to some embodiments, in order to observe the diffraction element within the said angular range

[0011] - The wavelength values ​​of the first diffraction structure are between 440 nm and 491 nm. - The wavelength range of the second diffraction structure is between 498 nm and 575 nm. - The wavelength range of the third diffraction structure is between 582 nm and 630 nm.

[0012] According to some embodiments, for a defined observation angle of the diffraction element, the wavelength value emitted by each diffraction structure at that defined angle corresponds to the wavelength value in the middle of the range of wavelength values ​​emitted by the diffraction structure.

[0013] Therefore, instead of using a red, green, and blue diffraction element triplet at the preferred viewing angle (which would quickly lead to saturation in the red or blue range), a diffraction element triplet that emits over a range is used, so that for any color, and more particularly for red and blue that saturate rapidly when deviating from the middle position in the prior art configuration, saturation will not occur.

[0014] In some embodiments, the number of diffraction structures constituting the diffraction element is equal to 3, and for a defined observation angle of the diffraction element, the wavelength value emitted by each diffraction structure corresponds to...

[0015] - The wavelength values ​​of the first diffraction structure are between 474 nm and 478 nm. - The wavelength values ​​of the second diffraction structure are between 498 nm and 502 nm. - The wavelength values ​​of the third diffraction structure are between 595 nm and 600 nm.

[0016] Therefore, advantageously, instead of selecting the red, green, and blue subpixel triplet at the preferred observation angle, a yellow-orange, cyan-green, and blue triplet is chosen. By adjusting the angle range of the security file tilted around this observation position, the diffraction structure emits wavelengths within the visible light range.

[0017] According to some embodiments, - The defined angle corresponds to the normal of the plane including the diffraction element under zenith illumination. - The angle range is between -6 degrees and +6 degrees relative to the defined angle.

[0018] According to some embodiments, the range of visible light wavelength values ​​can be different for each diffraction structure.

[0019] According to some embodiments, the defined observation angle corresponds to the observation angle of the capturing device on the diffraction element, and this angle is different from the normal of the plane containing the diffraction element or the normal of the plane where the diffraction element is located.

[0020] According to some embodiments, the defined observation angle corresponds to the observation angle of the capturing device on the diffraction element, and this angle is different from the normal of the plane containing the diffraction element.

[0021] In fact, the secure files described in this article can be access passes or other files that allow access to sensitive or confidential data. These files can be read by an external reader that captures images and accesses data encoded by those images. When reading a file, it is typically not positioned perpendicular to the external reader.

[0022] According to some embodiments, the diffraction structure is configured in the form of a holographic structure, the orientation angle of which is determined based on the observation angle.

[0023] The orientation angle of the diffraction structure represents a specific angle of the diffraction structure within the diffraction element.

[0024] According to some embodiments, the length of the range of wavelength values ​​emitted by each diffraction structure depends on the pattern specific to each diffraction structure at the wavelength emitted within the scanning angle range.

[0025] In some embodiments, the diffraction structure is an optical diffraction grating.

[0026] The present invention also relates to a security document comprising at least one diffraction element according to one embodiment of the present invention.

[0027] The present invention also relates to a method for generating security documents, comprising: - A metal layer is formed on a substrate layer, the metal layer comprising a plurality of diffraction elements according to one embodiment of the present invention. - By focusing laser radiation onto the metal layer to form perforations, the perforations include at least one set of perforations made by focusing the laser radiation at a corresponding incident angle so as to reveal a corresponding personalized image when the security document is viewed at the incident angle.

[0028] Other features and advantages of the invention will become apparent from the following description, taking into account the accompanying drawings illustrating non-limiting examples of embodiments. Attached Figure Description

[0029] [ Figure 1 ] Figure 1 An example breakdown of a document implementing a set of diffraction elements according to the present invention is shown. [ Figure 2a ]、[ Figure 2b ]、[ Figure 2c ]picture, Figure 2a Figure 2c schematically illustrates the tilt angle of the diffraction element and the wavelength of the observed color. [ Figure 3 ] Figure 3 Examples of wavelength range values ​​in the CIE color space are shown. [ Figure 4a]、[ Figure 4b ]、[ Figure 4c ] Figure 4a , Figure 4b and Figure 4c According to a specific embodiment of the present invention, the arrangement of pixels and subpixels is schematically illustrated. [ Figure 5 ] Figure 5 A method for manufacturing security documents according to an embodiment of the present invention is shown. Detailed Implementation

[0030] This invention relates to the creation of colored patterns on, for example, security documents.

[0031] In the remainder of this disclosure, examples of implementations of the invention will be described with respect to documents comprising at least one personalized image in accordance with the principles of this disclosure. This document can be any type of document, referred to as a security document, such as a brochure, card, or other type. The invention is particularly applicable to the generation of color barcodes in identification documents such as ID cards, credit cards, passports, driver's licenses, and secure access cards. The invention is also applicable to security documents comprising at least one personalized image, such as banknotes, notarized documents, official certificates, etc. However, other implementations are also possible. Such barcodes are typically read by devices such as cameras built into mobile phones and can encode information such as website links, specific pages, videos, contact information (such as phone numbers, email addresses, or sensitive personal information), serial numbers, and unique identifiers.

[0032] Unless otherwise stated, elements that are common or similar in multiple figures have the same reference numerals and have the same or similar characteristics, and for the sake of simplicity, these common elements are generally not described again.

[0033] Figure 1 Examples of layers that can be assembled to form the components of the security file 100 proposed in this disclosure are shown. The security file 100 is given by way of example to illustrate the composition of the pixels presented in this disclosure in terms of subpixels. In addition to a metal layer configured to include multiple diffraction elements, the file may also include one or more layers.

[0034] This disclosure relates to a security document comprising a diffraction element according to any diffraction element embodiment described below and / or a diffraction element manufactured by a method according to any method embodiment described below.

[0035] A metal or metal oxide layer 101 is located between two layers 102a and 102b. Layer 102b is, for example, a transparent protective layer, which may be a varnish with a high refractive index, allowing observation through a color pattern formed by a matrix of diffractive structures SDi (also called subpixels). Layer 102a may be opaque at least relative to the visible wavelength spectrum. The metal layer 101 includes a plurality of diffractive elements EDi forming pixels of the formed image, each diffractive element being formed by several diffractive structures SDi, all of which form a pixel arrangement visible to the observer OB.

[0036] like Figure 1 As illustrated in the diagram, the metal layer 101 includes perforations Pi formed by laser radiation that ablates the metal layer. These through-holes partially expose dark areas in the diffraction structure through the metal structure 101. These dark areas are formed by the underlying region of the opaque layer 102a opposite the perforations Pi, thereby forming a personalized image based on the pixel arrangement combined with the dark areas 142. Therefore, the contribution of each sub-pixel is modulated to determine the hue, saturation, and brightness of the pixel formed by the sub-pixels.

[0037] The chromaticity of reflected light depends on a combination of the viewing angle, the light source, and light-matter interaction phenomena (such as diffraction, diffusion, absorption, etc.) that occur at the arrangement of the previously laser-perforated diffraction structure.

[0038] In particular, this technology enables the formation of personalized images through ablation (rather than carbonization), which is safe and eliminates the need for the high-power laser radiation required in carbonization, which can generate bubbles in the metal structure 101 upon heating, leading to irreversible damage to the holographic structure. In fact, ablation of a thin metal layer requires significantly less energy compared to carbonization.

[0039] Metal layer 101 comprises an arrangement of diffractive structures. Various types of diffractive structures, particularly those of various shapes or sizes, can be considered within the scope of this invention. Typically, according to this disclosure, the diffractive structures present in metal layer 101 are configured to diffract light within the visible wavelength spectrum over a defined angular range. Each pixel of the image is associated with a diffractive element, wherein the diffractive element is a group of diffractive structures comprising at least two, preferably three, diffractive structures. All diffractive structures are located in the same plane defined by the metal layer.

[0040] Diffraction elements can be constructed from diffraction structures in the form of optical diffraction gratings. For example, the grating can be tilted in different directions for each diffraction structure, or any other equivalent solution can be used. This allows each diffraction structure to emit a spectrum whose wavelength varies within a given angular range.

[0041] In the prior art, it is known to select diffraction structures configured to emit red, green, and blue light respectively at the same incident angle on the metal layer. This incident angle is typically the incident angle corresponding to the normal to the document, or the preferred viewing angle of the document. The applicant observed that color saturation is rapidly achieved when the metal layer, or more generally the document including the metal structure, is tilted at increasingly larger angles, particularly for red pixels, but also for blue pixels. This phenomenon can be explained by the fact that wavelengths in the visible spectrum are between approximately 380 nm and 780 nm, with red wavelengths between approximately 620 nm and 750 nm, blue wavelengths between approximately 450 nm and 495 nm, and green wavelengths between 495 nm and 570 nm. Thus, green is closer to the middle of the visible spectrum, red is at one end of the spectrum (the red end), and blue is relatively closer to the other end of the spectrum (violet separates it from the ultraviolet end of the visible spectrum).

[0042] It is undesirable for at least one subpixel to reach color saturation because the resulting image obtained by combining three subpixels deprives it of the contribution of one of the subpixels. A particular consequence is that when viewing a multicolor image from either side of its optimal viewing position, the red subpixel component is rapidly lost, and the blue subpixel component is also lost fairly quickly. It should be noted that when far from the optimal viewing position, the color emitted by each subpixel deviates from its nominal position, i.e., from the color observed at the optimal viewing position. For example, a green subpixel might emit cyan, a blue subpixel might emit indigo, and a red subpixel might emit orange. Or conversely, a blue subpixel might emit cyan, a green subpixel might emit yellowish-green, and a red subpixel might emit dark red.

[0043] This disclosure advantageously makes it possible to configure such a diffraction structure so that when the documents 100 are tilted within a selected or defined angular range, they all reflect light at wavelengths within the visible spectrum.

[0044] To this end, this disclosure proposes a diffraction element comprising n diffraction structures, where n is an integer greater than or equal to 2, said diffraction element being intended to be observed within an angular range, each diffraction structure allowing light to diffract at wavelengths varying within a wavelength range when said diffraction element is tilted within said angular range, said diffraction unit being such that: - When the diffraction element is observed within the stated angular range, the n wavelength ranges associated with the n diffraction structures are located within the visible spectrum. - The n wavelength ranges are separated when the observation is kept within the angular range.

[0045] In the remainder of this disclosure, we will use a diffraction element consisting of three diffraction structures as an example, but those skilled in the art will readily understand how this disclosure can be applied to diffraction elements comprising two or more than three diffraction structures.

[0046] The diffraction element is formed in the metal layer. Several possible arrangements exist. Figure 5 Figure 7 illustrates different permutation examples that can be used.

[0047] The diffraction element can be configured to widen its diffraction angle. This can be achieved, for example, by increasing the spacing of the grooves in the diffraction grating or by adding a varnish with a high diffraction index. This can advantageously allow the saturation region to be reached less quickly when the document is not viewed from a defined viewing angle.

[0048] It is advantageous to consider the spectral characteristics of each sub-pixel (or more precisely, each diffraction grating). In reality, each diffraction structure differs. For example, one diffraction structure may have a larger green range than another, while the other may have, for example, a larger blue range. Therefore, the characteristics can be understood as the variation or rate of change of wavelength with respect to the observation angle within the defined angular range, under a defined light source. In other words, as the observation angle increases, some diffraction structures may reach saturation, while others will reach saturation at different (larger or smaller) observation angles. Therefore, the length of the wavelength range emitted by each diffraction structure depends on the pattern specific to each diffraction structure at the wavelengths emitted within the scanning angular range.

[0049] Each diffraction structure is configured such that, during observation of the diffraction element within the stated angular range, for example when the document including the diffraction structure is tilted, the wavelength values ​​associated with the spectrum emitted by each diffraction structure vary within a range of wavelengths located in the visible spectrum. Furthermore, within the stated angular range, the wavelength value ranges are separate for each diffraction structure. Since no wavelength value falls outside the visible spectrum, no diffraction structure reaches saturation within the stated angular range.

[0050] According to some embodiments, the diffraction element is equivalent to a prism, the shape of which is similar for the three colors, but its orientation is different for each diffraction structure, so as to emit a different wavelength for each diffraction structure within a defined angle, and to prevent saturation within the defined angle range.

[0051] In other words, diffraction structures can be configured such that they allow the maximum possible angular observation range for each diffraction structure, while having wavelengths within ranges that are separated from each other.

[0052] Preferably, in order to obtain the largest possible angular range, for a defined observation angle of the diffraction element, the wavelength value emitted by each diffraction structure at that defined angle corresponds to the wavelength value within the spectrum emitted by the diffraction structure at the center of the wavelength value range associated with the diffraction structure.

[0053] Preferably, the diffraction element emits a wavelength that is a combination of the wavelengths of the individual diffraction structures constituting the diffraction element. The wavelength value of the diffraction element is obtained through a unique combination of the wavelength values ​​of the individual diffraction structures constituting the diffraction element. Therefore, the color obtained by combining the wavelengths of the diffraction structures is obtained from a single combination of these values. This makes the document more secure.

[0054] In some embodiments, the viewing angle can be limited to the front of the document, and therefore 90 degrees relative to the diffraction structure. Thus, the parallelogram effect is minimized; that is, a square is observed as a square without any correction calculations.

[0055] According to some embodiments, the viewing angle corresponds to the viewing angle of the document by the device used to capture an image of the document. In practice, barcodes are typically designed to be read by an external optical device that is itself connected to a means for analyzing the scanned barcode. Barcodes can encode information such as website links, specific pages, videos, contact information (e.g., phone numbers, email addresses, or sensitive personal information), serial numbers, unique identifiers, etc. The viewing angle of the reading device on the document is typically tilted relative to the document's position relative to the human eye's viewing position. In the prior art, it is known to correct the image of the barcode using the position of markers typically placed at the corners of the barcode.

[0056] For each diffraction structure, the angle range is centered on the wavelength value in the spectrum emitted by the diffraction structure, so that the wavelength change corresponding to the angle change remains within the visible range and does not overlap with the wavelength values ​​associated with other diffraction structures.

[0057] Figure 2a , Figure 2b and Figure 2c The diagram illustrates an example of a diffraction element ED1 that includes three diffraction structures: SD1, SD2, and SD3.

[0058] Figure 2a The illustration depicts the first example, where the diffraction element, or the document including the diffraction element, is viewed at a 90-degree angle relative to the plane of the diffraction element, and the document is viewed from its front or the side visible in the resulting image. A human eye is shown here, but a device that reads documents including diffraction structures can be used instead.

[0059] Each diffraction element is configured to emit a spectrum encompassing angular ranges α1, α2, and α3, centered at wavelengths λ1, λ2, and λ3, respectively. For simplicity, the three angular ranges α1, α2, and α3 are equal. The three angular ranges α1, α2, and α3 are defined such that within these angular ranges, the associated diffraction structure emits in the visible spectrum, and the wavelengths of the spectra emitted by each diffraction structure do not overlap within these ranges.

[0060] Figure 2b The diagram illustrates the second example, where the diffraction element, i.e., the document including the diffraction element, is observed at an angle of α1 / 2, α2 / 2, or α3 / 2, all three angles being equal, i.e., observed at an angle corresponding to the upper bound of the angle range α1, α2, or α3. If these three angles are not equal, the length of the angle range defining the diffraction element is selected as the smallest angle range within the angle range constituting the diffraction structure of the diffraction element.

[0061] Therefore, it can be observed that when the maximum tilt angle of the document is less than or equal to that angle α1 / 2, or when the three angles are different, it is less than or equal to half of the smallest angle among the three angles, the observer's eye or reading device can observe the spectrum emitted by the diffraction element in the visible light range.

[0062] Figure 2c The diagram illustrates the second example, where the diffraction element, such as the file containing the diffraction element, is observed at an angle of -α1 / 2, -α2 / 2, or -α3 / 2, all three angles being equal, i.e., observed at an angle corresponding to the lower bound of the angle range α1, α2, or α3. If these three angles are not equal, the length of the angle range defining the diffraction element is selected as the smallest angle range within the angle range constituting the diffraction structure of the diffraction element.

[0063] Therefore, it can be observed that when the maximum tilt angle of the document is less than or equal to that angle α1 / 2, or when the three angles are different, it is less than or equal to half of the smallest angle among the three angles, the observer's eye or reading device can observe the spectrum emitted by the diffraction element in the visible light range.

[0064] Figure 3 An example of values ​​for a defined wavelength and angular range is shown. It is a grayscale representation of a color space defined by the International Commission on Illumination (CIE), a means of associating wavelengths of the visible spectrum with colors perceived by the human visual system. For reproduction purposes, this visualization must be grayscale, but a color version can be readily found online by those skilled in the art, for example at the URL: https: / / fr.wikipedia.org / wiki / CIE_XYZ.

[0065] Figure 3 An example is shown where the viewing angle range is 12° for each diffraction structure. This angle is for reference only, and other angles may be defined based on the structural parameters of each diffraction structure (such as the size of the grooves or particles in the structure). According to this example, each diffraction element includes three diffraction structures configured to emit at viewing angles varying around the defined angle. - For the first diffraction structure, the spectrum has wavelengths between 474 nm and 478 nm. - For the second diffraction structure, the spectrum has wavelengths between 498 nm and 502 nm. - For the third diffraction structure, the wavelength is between 595 nm and 600 nm.

[0066] In this example, the angle range is between -6 degrees and +6 degrees relative to the defined angle. Preferably, the defined angle is 90 degrees relative to the front of the document.

[0067] Therefore, according to some embodiments, unlike the diffraction structures of the prior art, these diffraction structures do not emit red, green, or blue light when exposed to normal incident light. Instead, one diffraction structure emits light that is closer to orange (wavelength approximately 595 nm to 600 nm), another diffraction structure emits light that is closer to green (wavelength approximately 498 nm to 502 nm), and a third diffraction structure emits light that is closer to blue (wavelength approximately 474 nm to 478 nm).

[0068] According to some embodiments, the angular range of the diffraction elements (more particularly, the document comprising an arrangement of diffraction elements) can vary slightly around a central position when observed. Acceptably, according to certain observation modes, the central position is along the document's normal. However, as mentioned above, the document is typically tilted, and according to this disclosure, angular displacements from that normal within a defined angular range prevent the emission of light waves beyond the visible spectrum.

[0069] Therefore, in Figure 3 In order to observe the diffraction element within the stated angular range, the following can be observed:

[0070] - The wavelength range for the first diffraction structure is between 440 nm and 491 nm. - The wavelength range for the second diffraction structure is between 498 nm and 575 nm. - The wavelength range for the third diffraction structure is between 582 nm and 630 nm.

[0071] It is noteworthy that a margin is advantageously maintained between each wavelength value range. Advantageously, this makes it possible to account for uncertainties associated with color calibration calculations. These margins ensure that there is never a risk of color at the end of the available spectrum of one subpixel being confused with color at the beginning of the available spectrum of another subpixel.

[0072] In some embodiments, the wavelength range can have a different length for each diffraction structure. More specifically, the length of the wavelength range may depend on the evolution of the wavelength of the diffraction structure with respect to the angular amplitude, and / or on the angular amplitude of the diffraction element.

[0073] Now for reference Figures 4a-4c The description is able to be found in safe file 100 ( Figure 1 ) are specific examples of the arrangement (or tiling) of diffraction elements implemented in ). It should be noted that these implementations are presented here only as non-limiting examples, and many variations are possible, particularly in terms of the arrangement and shape of the diffraction elements and diffraction structures, as well as the colors assigned to these diffraction structures.

[0074] according to Figure 4a In the first example shown, the diffraction element is rectangular or square in shape and includes three diffraction structures SD1, SD2, and SD3 that produce different colors. The diffraction structures SD1, SD2, and SD3 can each be formed from a portion of a row of subpixels. In this example, tiling 30 thus forms a matrix of subpixel rows and columns that are orthogonal to each other.

[0075] Figure 4b This is a top view showing another example of regular tiling, where each diffraction element EDi consists of 3 diffraction structures SDi, each with a different color. The diffraction structures SDi here are hexagonal.

[0076] Figure 4c This is a top view showing another example of regular tiling, where each diffraction element EDi consists of four diffraction structures SDi, each producing a different color. The diffraction structures SDi are triangular here.

[0077] For each arrangement of diffractive elements considered, the shape and size of each diffraction structure can be adjusted.

[0078] According to some embodiments, the same diffraction grating used to encode the barcode is arranged on the security document in defined locations, such as corresponding to the positions of markers or patterns used to locate the barcode (or QR code). Advantageously, this allows available frames of video recorded by a capturing device (such as a mobile phone camera) to be matched with observations made within the stated angular range.

[0079] Therefore, the present invention relates to a method for reading an image encoded on a secure file, the secure file comprising a plurality of diffraction elements as described above, the diffraction elements comprising: - The first set of diffractive elements used to encode the barcode. - A second set of diffraction elements that encode the markers. The first and second groups of diffraction elements have the same structure, but the size of the first group of diffraction elements is smaller than that of the second group of diffraction elements. The method includes: - Capture the file at different tilt angles relative to the capture device. - For each tilt angle, check whether the wavelength emitted by each diffraction structure is within the visible light range. - If at least one of the second set of diffraction structures fails to emit wavelengths in the visible spectrum, the tilt angle is determined to be too large.

[0080] According to some embodiments, the size of the diffraction element encoding the mark is approximately 100 times the size of the diffraction element encoding the barcode.

[0081] According to some embodiments, the size of the diffraction element encoding the mark is a few micrometers, for example, 35 µm, while the size of the diffraction element encoding the barcode is about a few millimeters.

[0082] When the diffraction elements have the same structure, under given lighting and observation conditions, both barcodes and tags can obtain the same color.

[0083] When the observation or reading device is a smartphone camera, the camera's pixels can clearly see the color of the marker, but it is difficult for the same camera's pixels to meaningfully sample the width of a single subpixel.

[0084] For example, if the color bits of a 2D barcode are formed by 10 pixels × 10 pixels, and therefore by 30 subpixels × 30 subpixels, and the size of each subpixel is 35 µm, then the pixel size is 35 micrometers on each side. A square with a diameter of 30 = 1.05 mm. A mobile phone camera's pixels record the average color, but cannot distinguish the details of color at the molecular pixel level.

[0085] Following the same example, if the color bits of the marker are formed by 10 pixels × 10 pixels, and therefore by 30 subpixels × 30 subpixels, and the size of each subpixel is 1 mm, then the size of the pixel is a side length of 1 mm. A 30 = 30 mm square. The pixels of a mobile phone camera can distinguish the color of each sub-pixel, therefore, for each tilt angle, it is possible to verify whether the wavelength emitted by each diffraction structure or sub-pixel is within the visible light range. If at least one of the marked diffraction structures does not emit wavelengths within the visible spectrum, the tilt angle is determined to be too large.

[0086] Now for reference Figure 5 A method for manufacturing a security document comprising a plurality of diffraction elements as described above is described according to a specific embodiment. For example, assuming that in... Figure 1 At least one barcode is formed in the security document 100 shown in the diagram.

[0087] During formation step S2, a metal layer 101 is formed on the substrate layer 102a. The metal layer 101 and the substrate layer 102a are as described in the above embodiments. Specifically, the metal layer 101 includes an arrangement of diffractive elements, each diffractive element comprising n diffractive structures SDi, or a matrix of diffractive structures SDi as described above. As previously stated, the arrangement of these diffractive elements is configured to diffract light at least within the visible wavelength spectrum. These diffractive nanostructures can be arranged periodically to form, for example, diffractive holographic structures, or aperiodically (non-periodicly) to control or change the chromaticity of the reflected light according to the angle of incidence of light on the metal layer 101, as already described above. The diffractive nanostructures are configured as described above. Additionally, the substrate layer 102a can be at least opaque to the visible wavelength spectrum or at least transparent to the visible wavelength spectrum, depending on the visual effect sought in one or more personalized barcodes.

[0088] Adhesives and / or adhesive layers (not shown) may be used to ensure adhesion between the metal layer 101 and the base layer 102a.

[0089] In step S4, a through hole or aperture P is formed in the metal layer by focused laser radiation. i Therefore, these perforated P i Includes at least one set of perforations produced by focused laser radiation.

[0090] Perforated P i These are fabricated to occupy all or part of multiple sub-pixels within a metal layer. These perforated P... i Locally revealing dark areas (if layer 102a is opaque) or bright areas (if layer 102a is transparent) within subpixels, these areas are formed by the base layer 102a located at the perforation P. i The opposite lower region causes or generates this. Therefore, the perforation P here... iThese are through-holes that extend through the thickness of the metal layer to expose the underlying region of the substrate 102a at the pixel arrangement level. In other words, the underlying region modifies the contribution of sub-pixels to form the final barcode. Therefore, based on the arrangement of pixels combined with the dark or bright areas, one or more personalized images, particularly barcodes, can be formed.

[0091] As described above, layer 102b can also be added to protect or enhance the document. Layer 102b is, for example, a transparent protective layer, which can be a varnish with a high refractive index, allowing observation through the colored pattern formed by the matrix of the diffraction structure SDi.

[0092] Those skilled in the art will understand that the above embodiments and variations are merely non-limiting examples of the present invention. In particular, those skilled in the art can consider any adjustments or combinations of the above embodiments and variations to meet very specific needs according to the claims set forth below.

Claims

1. A diffraction element (EDi) comprising n diffraction structures (SDi) configured to form a subpixel matrix, the subpixel matrix consisting of repeated arrangements of the n diffraction structures (SDi), where n is an integer greater than or equal to 2, the diffraction element (EDi) being intended to be visible over an angular range, each diffraction structure (SDi) allowing light to diffract at wavelengths varying over a range of wavelength values ​​when the diffraction element (EDi) is tilted over the angular range, the diffraction element such that: - When the diffraction element (SDi) is observed within the stated angular range, the ranges of n wavelength values ​​associated with each of the n diffraction structures (SDi) lie within the visible spectrum. - The n wavelength ranges are separate.

2. The diffraction element (EDi) according to claim 1, characterized in that, - For a defined viewing angle of the diffraction element (EDi), the wavelength value emitted by each diffraction structure (SDi) at that defined angle corresponds to the center wavelength value within the range of wavelength values ​​emitted by the diffraction structure (SDi).

3. The diffraction element (EDi) according to claim 2, characterized in that... The number of diffraction structures (SDi) constituting the diffraction element (EDi) is equal to 3. For a defined observation angle of the diffraction element, the wavelength value emitted by each diffraction structure corresponds to... - The wavelength values ​​of the first diffraction structure are between 474 nm and 478 nm. - The wavelength values ​​of the second diffraction structure are between 498 nm and 502 nm. - The wavelength values ​​of the third diffraction structure are between 595 nm and 600 nm.

4. The diffraction element (EDi) according to claim 3, characterized in that... In order to observe the diffraction element (EDi) within the stated angular range, - The wavelength range of the first diffraction structure is between 380 nm and λ. a Between nm - The wavelength range of the second diffraction structure is between λ b nm and λ c Between nm - The wavelength range of the third diffraction structure is between λ d Between 780 nm and 780 nm Where λ a <λ b <λ c <λ d .

5. The diffraction element (EDi) according to any one of claims 2 to 4, wherein - The defined observation angle corresponds to the normal of the plane containing the diffraction element (EDi) under zenith illumination. - The angle range is between -6 degrees and +6 degrees relative to the defined angle.

6. The diffraction element (EDi) according to any one of claims 2 to 4, wherein - The defined observation angle corresponds to the observation angle of the capturing device on the diffraction element (EDi), and this angle is different from the normal of the plane including the diffraction element (EDi).

7. The diffraction element (EDi) according to any one of the preceding claims, wherein the diffraction structure (SDi) is configured in the form of a holographic structure, the orientation angle of the holographic structure being defined based on the observation angle.

8. The diffraction element (EDi) according to any one of the preceding claims, wherein the length of the range of wavelength values ​​emitted by each diffraction structure (SDi) depends on a pattern specific to each diffraction structure (SDi) of the emitted wavelengths when scanning the angular range.

9. The diffraction element (EDi) according to claim 8, such that the diffraction structure (SDi) is an optical diffraction grating.

10. A security document (100) comprising at least one diffraction element (EDi) according to any one of claims 1 to 9.

11. A method for creating a security document (100), comprising: - A metal layer (101) is formed on a substrate, the metal layer (101) comprising a plurality of diffraction elements (EDi) according to any one of claims 1 to 9. - By focusing laser radiation onto the metal layer (101), a perforation (P) is formed. i The perforation (P) i The security document (100) includes at least one set of perforations made by focusing the laser radiation at a corresponding incident angle so as to reveal a corresponding personalized image when the security document (100) is viewed at the incident angle.

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