Optical security element visible in transmission, production of such an optical security element and high security object equipped with such an optical security element - Patents.com

Abstract

The present invention relates to an optical security element for authentication in transmission, comprising a first layer (113) of a dielectric material transparent to visible light, a first diffractive structure (S) patterned in the first layer, and a metal layer (114) at least partially covering the first diffractive structure. The first diffractive structure comprises a first pattern (M1) forming a symmetrical first one-dimensional periodic waveform, the first one-dimensional periodic waveform being configured to form a diffraction grating generating a diffraction effect in 1st and -1st order reflection. The thickness of the metal layer, defined in a direction perpendicular to the plane of the optical security element, for each period of the first pattern, defined between two extrema of the first one-dimensional periodic waveform, varies between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, resulting in an asymmetric distribution of the metal and generating a color effect visible in transmission.

Description

[Technical Field]

[0001] The present description relates to the field of security marking, more particularly to optical security elements visible in transmission for verifying the authenticity of objects, such as valuable products or documents, for example identity cards or banknotes, a process for manufacturing such optical security elements, and high-security objects equipped with such optical security elements. [Background technology]

[0002] Many techniques are known for authenticating documents or products, in particular for securing documents such as valuable documents, banknotes, passports or other identity documents. These techniques aim to create optical security elements whose optical effects adopt a distinctive and verifiable configuration depending on the observation parameters (orientation of the optical security element relative to the observation axis, position and dimensions of the light source, etc.). The general aim of these optical security elements is to generate novel and distinctive optical effects using physical configurations that are difficult to reproduce. Among these optical security elements, DOVID (DOVID stands for Diffractive Optical Variable Image Device) is an optical element that generates a diffractive variable image, commonly called a hologram.

[0003] More precisely, this document focuses on optical security components that exhibit significant optical color effects in transmission that are distinct from the optical effects in reflection.

[0004] Inspection in transmission is particularly used to verify important documents such as banknotes or passports. Banknotes for this purpose have blank and / or partially transparent or scattering areas, while passports have transparent windows on the pages containing data about the holder. Optical security elements can take the form of, for example, security threads, security strips or patches, are intended to be viewed from above, can be at least partially overlapped on the see-through area, and can be located on the surface or within the thickness of the document.

[0005] Optical security components based on plasmon resonance are known that exhibit significant optical color effects in transmission. Such an optical security component is described, for example, in Patent Document 1. The optical security component described in Patent Document 1 comprises a metal structure on a transparent substrate that forms a periodic pattern with a subwavelength period. This pattern has a rectangular profile and is embedded between an embossed lacquer layer and a protective lacquer layer. The metal structure is obtained by evaporating a thin metal layer onto the lacquer layer after embossing. By varying the metal deposition angle, a subwavelength grating with an asymmetric profile can be obtained. The spectral characteristics in transmission and reflection of the resulting plasmon resonance-based optical security component depend on the metal deposition angle, making it possible to design optical security components with different color effects in transmission and reflection.

[0006] Plasmonic optical security components that exhibit significant color effects in transmission are also known (see Patent Document 2). The optical component described in Patent Document 2 comprises two transparent dielectric material layers and a metal layer, the metal layer being disposed between the dielectric material layers to form two dielectric-metal interfaces, and the metal layer being configured to form corrugations on at least a portion of its surface that are capable of coupling a surface plasmon mode supported by the dielectric-metal interfaces to an incident light wave. The corrugations are disposed in a first principal direction in a first coupling zone and in a second principal direction substantially perpendicular to the first principal direction in at least a second coupling zone different from the first coupling zone, the metal layer being continuous in each of the coupling zones. Such components exhibit significant transmission effects in a spectral band centered on a wavelength defined by the characteristics of the corrugations of these coupling zones. To an observer, the color change with the viewing angle of the optical security component differs depending on the coupling zone, allowing for easy and reliable authentication of the optical security component.

[0007] In the two aforementioned references, the physical mechanism involved is a plasmon resonance mechanism, which is highly sensitive to polarization. In particular, only the TM component of the incident electromagnetic field is coupled to the plasmon mode. Therefore, the color effects observed in transmission when illuminating these optical security components with unpolarized light are not very strong.

[0008] Furthermore, optical security elements observable in transmission are also known, which employ a resonant mechanism in a dielectric material layer rather than in a metal layer. Such optical security elements are described, for example, in patent document 3 or non-patent document 1, and are known as zero-order diffraction filters (ZOFs) or guided-mode resonant filters. The physical mechanism relies on the resonant reflection of a guided mode in a high-refractive-index dielectric layer. In fact, the transmission spectrum of such a resonant filter is the complement of the reflection spectrum. However, the transmission in such optical security elements remains high at all wavelengths, and therefore the optical effect in transmission does not appear to the observer as a strong color.

[0009] The present application describes an optical security element with a unique structure that makes it possible to obtain both remarkable optical color effects and very good luminosity in transmission. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US Patent No. 2010 / 0307705 [Patent Document 2] International Publication No. 2012136777 [Patent Document 3] European Patent Application Publication No. 2264491 [Non-patent literature]

[0011] [Non-Patent Document 1] MT Gale, "Zero-Order Grating Microstructures," in RL van Renesse, Optical Document Security, 2nd ed., pp. 267-287 Summary of the Invention [Means for solving the problem]

[0012] As used herein, "comprise" means the same as "include" or "contain," is inclusive or open, and does not exclude other elements not described or shown. Furthermore, as used herein, "about" or "approximately" means the same as "with at least one of an error of less than 10% and an error of more than 10%, for example, an error of 5%" of the respective value.

[0013] According to a first aspect, the invention relates to an optical security element for securing an object, such as an important document, for example an identity card or a banknote, that is adapted to be authenticated in transmission by the naked eye.

[0014] The optical security component according to the first aspect comprises: a first layer of dielectric material that is transparent in the visible light; - at least a first diffractive structure patterned in said first layer; a metal layer at least partially covering said first diffractive structure and having a reflection spectral band in the visible light; Including, the first diffractive structure comprises at least a first pattern forming, in a first region, a symmetrical first one-dimensional periodic waveform having a first pitch between about 400 nm and about 900 nm and a first depth, the first one-dimensional periodic waveform being configured to form, after deposition of the metal layer, a diffraction grating that generates a diffraction effect in 1st and -1st order reflections in a wavelength range between 400 nm and 700 nm; the thickness of the metal layer, defined in a direction perpendicular to the plane of the optical security element, for each period defined between two extrema of the first one-dimensional periodic waveform varies between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, resulting in an asymmetric distribution of metal and in the generation of a color effect that is visible in transmission.

[0015] A layer that is transparent to visible light is defined herein as a layer that has a transmittance of at least 70%, preferably at least 80%, at wavelengths in the visible range, i.e., between about 400 nm and about 700 nm. Thus, a transparent layer allows the layer underneath to be observed with the naked eye.

[0016] In relation to the corrugations, "depth" is understood to mean the distance between the lowest and highest levels of the structures forming the corrugations, measured along an axis perpendicular to the plane of the part.

[0017] In this document, a one-dimensional periodic waveform is a periodic structure with a profile that is continuously variable in a single direction, called the direction of variation of the profile, i.e. a structure with a height that varies according to a continuously variable and periodic function.A symmetric one-dimensional periodic waveform is a one-dimensional periodic waveform that includes at least one plane of symmetry per period of the waveform, defined between two extrema of the waveform, that is perpendicular to the plane of the optical security element and perpendicular to the direction of variation of the profile.

[0018] According to one or more example embodiments, the profile of the symmetric one-dimensional periodic waveform is a sinusoidal or quasi-sinusoidal profile, i.e., a profile comprising a sum of sinusoids of different periods. These examples are non-limiting.

[0019] According to the present specification, the first one-dimensional periodic waveform has a first pitch and a first depth, the first pitch and the first depth being configured to form, after deposition of the metal layer, a diffraction grating defined to generate a diffraction effect in at least 1st and -1st order reflections in a wavelength range of 400 nm to 700 nm. The symmetry of the periodic waveform allows for increased efficiency of the 1st and -1st order diffraction effects. The first pitch is between about 400 nm and about 900 nm, advantageously between about 450 nm and about 750 nm, and advantageously between about 500 nm and about 700 nm.

[0020] Furthermore, according to the present specification, for each period defined between two extremes of the first waveform, the thickness of the metal layer defined in a direction perpendicular to the plane of the component is variable between a minimum value strictly less than 10 nm and a maximum value between about 10 nm and about 100 nm, with the metal being distributed asymmetrically to produce a color effect that is visible in transmission.

[0021] Asymmetric distribution of metal means that for each period defined between two extremes, there is no plane of symmetry in the metal layer perpendicular to the plane of the component and perpendicular to the direction of change in the corrugation profile.

[0022] The Applicant has shown that it is possible to obtain striking color effects with optical security elements in which the profile of the diffractive structure is symmetrical, but the metal distribution is asymmetrical, with regions having little or no metal and regions with a greater thickness of metal within a given period defined between two extremes. Such optical security elements exhibit striking and vivid color effects in transmission.

[0023] Furthermore, the Applicant has shown that, during observation in transmission, when the angle of observation is changed by tilting the component about an axis perpendicular to the direction of change in the profile of the corrugations, the observer can observe a color change that is asymmetric with respect to the normal to the plane of the optical security component. Such a pronounced and distinctive effect allows for more secure authentication, which has strong technological barriers due to the challenging nature of the component design and the mass production process required to obtain the above-mentioned visual effect.

[0024] Generally, rotation of a component about an axis contained in the plane of the component is referred to as the "tilting" or "rocking" of the optical security component.

[0025] Such an effect in transmission, combining intense color and luminosity, is unique in the security field and can be explained in particular, but not exclusively, by resonant transmission resulting from diffraction effects combined with the excitation of surface modes. This luminous efficiency is the result of effects visible in both TE and TM modes due to diffractive subwavelength structures, typically with periods between 200 nm and 400 nm, in contrast to plasmon resonances generated in known prior art optical security components.

[0026] According to one or more example embodiments, the ratio of the maximum metal layer thickness to the minimum metal layer thickness is greater than or equal to about 5, advantageously greater than or equal to about 10, and advantageously greater than or equal to about 15. The higher the ratio of the maximum metal layer thickness to the minimum metal layer thickness, the greater the asymmetry of the metal layer distribution, and the more pronounced the optical color effect.

[0027] According to one or more example embodiments, the metal layer results from metal deposition onto the first structure at a non-zero deposition angle, for example, at a deposition angle between about 25° and about 70°. The metal deposition angle can be selected relative to a known wave profile, for example, a sinusoidal or quasi-sinusoidal profile, to obtain a desired asymmetry in the distribution of the metal layer.

[0028] According to one or more examples of embodiment, the first diffractive structure includes at least a second pattern in a second region forming a symmetrical second one-dimensional periodic waveform, the second one-dimensional periodic waveform having a profile change direction parallel to the profile change direction of the first one-dimensional periodic waveform, having the same depth as the first depth of the first one-dimensional periodic waveform, and having a pitch different from the first pitch of the first one-dimensional periodic waveform, and the first pattern and the second pattern have contours that are recognizable when observed in transmission with the naked eye.

[0029] In such components, the different pitches of the corrugations will result in different colors being observed for each pattern during the next observation in transmission, and furthermore, by tilting the component about an axis perpendicular to the direction of change in the corrugation profile, a color variation for each pattern will be observed, with a notable asymmetry in the observed colors on either side of the position corresponding to the observation in the normal direction to the optical security component.

[0030] According to one or more examples of embodiments, the first diffraction structure includes at least a second pattern in a second region forming a symmetrical second one-dimensional periodic waveform, the second one-dimensional periodic waveform having a profile change direction parallel to the profile change direction of the first one-dimensional periodic waveform, a pitch identical to the first pitch of the first one-dimensional periodic waveform, and a second depth that is strictly less than the first depth of the first one-dimensional periodic waveform, and the first pattern and the second pattern have contours that are recognizable when observed in transmission with the naked eye.

[0031] In such components, the different depths of the corrugations make it possible to produce colored and non-colored patterns in transmission. Specifically, the asymmetry of the metal layer is less pronounced in areas where the corrugations have a smaller thickness, especially if the metal layer results from deposition at a non-zero deposition angle under vacuum.

[0032] According to one or more exemplary embodiments, the first diffractive structure further includes a micropattern including a set of facets with a variable slope in a fine dimension, and the first pattern modulates the micropattern.

[0033] According to one or more example embodiments, the optical element further includes at least a second diffractive structure patterned in the first layer, the second diffractive structure being selected from, for example, a scattering structure, a holographic structure, or an Alphagram (registered trademark) type diffractive structure.

[0034] According to a second aspect, the present description relates to a high-security object, such as an important security document, comprising a substrate and an optical security element according to any one of the preceding claims deposited on said substrate.

[0035] According to a third aspect, the present description relates to a process for manufacturing an optical security element according to any of the above-mentioned embodiments.

[0036] Generally, the present description describes a process for manufacturing an optical security component for securing an object adapted to be authenticated in transmission by the naked eye, such as an important document, e.g. an identity card or a banknote, comprising: - depositing a first layer of a dielectric material, which is transparent in visible light, on a carrier film; - forming at least a first diffractive structure on said first layer; depositing a metal layer at least partially covering said first diffractive structure and having a reflection spectral band in the visible light; Including, the first diffractive structure comprises at least a first pattern forming, in a first region, a symmetrical first one-dimensional periodic waveform having a first pitch between about 400 nm and about 900 nm and a first depth, the first one-dimensional periodic waveform being configured to form, after deposition of the metal layer, a diffraction grating that generates a diffraction effect in 1st and -1st order reflections in a wavelength range between 400 nm and 700 nm; - for each period of the first pattern, defined between two extreme values ​​of the first one-dimensional periodic waveform, the thickness of the metal layer, defined in a direction perpendicular to the plane of the optical security element, varies between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, resulting in an asymmetric distribution of the metal and in the generation of a color effect that is visible in transmission.

[0037] According to one or more example embodiments, the metal layer results from metal deposition onto the first diffractive structure at a non-zero deposition angle, for example, at a deposition angle between about 25° and about 70°.

[0038] Other characteristics and advantages of the invention will become apparent from the following description and the accompanying drawings in which: [Brief explanation of the drawings]

[0039] [Figure 1A] 1 shows a schematic (partial) cross-sectional view of an example of an embodiment of a component according to the present disclosure; [Figure 1B] 10 shows a (partial) cross-sectional view of another example of an embodiment of a component according to the present disclosure. [Figure 2] 2 shows some examples of first diffractive structures according to the present description in a security element according to the present description, with a first pattern M1 and coated with asymmetrically arranged metal layers; [Figure 3] 1 shows by way of example an animation visible in the reflection during a tilting movement of a high-security object equipped with an optical security element according to an example herein. [Figure 4]4 shows by way of example an animation visible in transmission during a tilting movement of a high-security object equipped with an optical security element similar to that shown in FIG. 3. [Figure 5A] 1 shows the parameters of an optical security element according to the present description; [Figure 5B] 5B shows the calculated transmission curve as a function of wavelength for an example of an optical security element as shown in FIG. 5A illuminated at normal incidence with TE polarized light, as well as a comparative transmission curve. [Figure 5C] 5B shows the calculated transmission curve as a function of wavelength for an example of an optical security element as shown in FIG. 5A illuminated at normal incidence with TM polarized light, as well as a comparative transmission curve. [Figure 5D] 5B shows the calculated transmission curve as a function of wavelength for an example of an optical security element as shown in FIG. 5A, illuminated at normal incidence with unpolarized light, and a comparative transmission curve. [Figure 6A] 1 shows a schematic (partial) cross-section of an optical security element with a diffractive structure comprising in first areas a first pattern according to the present disclosure and unstructured areas between the first areas; [Figure 6B] 6B shows the optical effect visible in transmission of a high resolution color image with variable color depending on tilt obtained with an optical security element as shown diagrammatically in FIG. 6A. [Figure 7] 3 shows the calculated transmission curves as a function of wavelength for an example of an optical security element according to the present description for various slope values. [Figure 8A] 1 shows a schematic (partial) cross-section of an optical security element with a diffractive structure comprising in a first region a first pattern according to the present disclosure and in a second region a second pattern formed from one-dimensional periodic corrugations identical to the first pattern but shallower, and shows the visual effect in reflection of such an element. [Figure 8B] 8B shows the visual effect in transmission of the optical security element shown in FIG. 8A. [Figure 8C]8B shows the optical effect visible in transmission of a high resolution color image with variable color depending on the tilt obtained with an optical security element as shown diagrammatically in FIG. 8A. [Figure 9A] 1 shows a schematic (partial) 3D view of an example of an optical security element according to the present description, in which a first pattern modulates a second pattern formed by fine-scale facets; FIG. [Figure 9B] 9B shows the optical effects visible in transmission in an optical security element of the type shown in FIG. 9A. [Figure 9C] 9B shows a schematic diagram illustrating a visual animation visible in transmission obtained with an optical security element of the type shown in FIG. 9A. DETAILED DESCRIPTION OF THE INVENTION

[0040] In the figures, elements are not shown to scale for ease of reading.

[0041] 1A and 1B show, schematically and via (partial) cross-section, two examples of optical security elements according to the present invention.

[0042] The optical security element 100 shown in FIG. 1A A is an optical security element intended to be transferred to, for example, a document or product in order to enhance its security. In this example, the optical security element comprises a carrier film 111, for example a polymer film, for example a film made of polyethylene terephthalate (PET) having a thickness of a few tens of microns, more typically between about 10 μm and about 50 μm, and a release layer 112, for example made of natural or synthetic wax. The release layer makes it possible to remove the polymeric carrier film 111 after the optical element has been transferred to the product or document whose security is to be enhanced. Optical security element 100 Afurther comprises a first layer 113 of a dielectric material having a first refractive index n1 and at least a first diffractive structure S comprising at least a first pattern M1 forming a one-dimensional periodic waveform, the first diffractive structure S being embossed in said first layer 113, as will be described in more detail below.

[0043] In the example of FIG. 1A, an optical security element 100 A also includes a metal layer 114 that covers the first structure S and has a reflection spectral band in visible light. The metal layer includes, for example, a metal selected from the group including aluminum, silver, copper, chromium, and alloys of the aforementioned metals.

[0044] The optical security element may further comprise one or more optional layers that are not optically functional but are suitable for the application.

[0045] For example, in the example of FIG. 1A, the optical security element further comprises an adhesive layer 117, for example a heat-reactivatable adhesive layer for transferring the optical security element to a product or document.

[0046] As will be explained in more detail below, in practice the optical security element may be manufactured by stacking layers on a carrier film 111, which is then transferred to the document / product whose security is to be enhanced by means of an adhesive layer 117. Optionally, the carrier film 111 can then be peeled off, for example by means of a release layer 112.

[0047] An optical security element according to the present specification generally comprises a first surface 101 for observing the optical security element in reflection, which in the example of FIG. 1A is located on the side of the first layer 113 opposite the patterned surface of the first layer 113, and a second surface 102 for observing the optical security element in transmission, which in the example of FIG. 1A is located on the same side as the patterned surface of the first layer 113.

[0048] The optical security element 100 shown in FIG. Bis an optical security element intended to enhance the security of, for example, a banknote. It may be, for example, part of a security thread intended to be integrated into the paper during the production of the banknote, or a laminated strip or patch covering a window in the paper. In this example, the optical security element 100 B As described above, the security thread includes a carrier film 111 (typically having a thickness between about 10 μm and about 50 μm) that also functions as a protective film for protecting the security thread, a first layer 113 of a dielectric material having a first refractive index n1, as in the example of FIG. 1A, at least a first diffractive structure S embossed in the first layer 113, and a metal layer 114 covering the first diffractive structure S and having a reflection spectral band in visible light.

[0049] In the example of Figure 1B, the first diffractive structure S includes a first pattern M1 and a second pattern M2 each forming a one-dimensional periodic wave, these waves having different depths, and the first and second patterns are embossed in the first layer 113 to form a particular visual effect, which will be described in more detail below.

[0050] In the example of FIG. 1B, the optical security element 100 B further comprises a set of optional layers 115, 116, 118. The (optional) layer 115 is, for example, a transparent layer 115 of a dielectric material. The (optional) layer 116 is, for example, a security layer 116, which is a discontinuous layer on which a specific pattern has been locally printed with UV ink, for example, to create an additional marking that can be inspected visually or mechanically. The (optional) layer 118 is, for example, a transparent protective layer, such as a second polymer film or varnish. In the case of a laminated strip, the layer 118 can be an adhesive layer. The manufacturing can be carried out by stacking the layers on the carrier film 111, as in the previous example. The dielectric layer 115 and the security layer 116 can form a single layer. The protective layer (or adhesive layer) 118 and the layer 115 can also form the same layer.

[0051] It will be apparent to one skilled in the art that in each of the examples shown in Figures 1A and 1B, other optically non-functional layers can be added as needed for the application, and variations of the embodiments shown in Figures 1A and 1B can be combined.

[0052] It should be noted that the additional optically non-functional layers, such as layer 117 or layers 115, 116, 118, are transparent in the visible spectrum, like the intended final carrier, so that the optical security element can be authenticated from both sides, i.e. by observing both of its faces 101, 102, in particular in transmission.

[0053] Figure 2 shows in more detail examples 201, 202, 203 of first diffractive structures according to the present description, with a first pattern M1 forming a symmetrical one-dimensional periodic wave. The first diffractive structure is coated with a metal layer 114. The schematic shows a partial cross-section of the component. The cross-section is in the (xz) plane perpendicular to the (xy) plane of the optical security element and includes the direction of change of the profile of the wave (x).

[0054] In the three examples shown in the schematic diagrams 201, 202 and 203, the first pattern consists of a one-dimensional periodic corrugation with a period (or pitch) d1 and a profile height h defined along the z-axis perpendicular to the (xy)-plane of the component. Such a periodic corrugation is a periodic structure whose profile, described by the height h, is continuously variable. Furthermore, according to this description, the profile is symmetrical, i.e., as shown by way of example in the schematic diagram 201, the profile comprises, for each period of the corrugation defined between two extrema, at least one plane of symmetry π perpendicular to the (xy)-plane of the optical security component and perpendicular to the (x) direction of variation of the profile of the corrugation.

[0055] In the example shown in diagrams 201 and 202, the profile of the waveform is approximately sinusoidal, that is, patterned with a sine function h(x) of the following form as a set point:

number

[0056] Of course, in practice, during the manufacture of the optical security element, for example during the embossing process, the profile may be deformed relative to a perfect sine wave.

[0057] Additionally, any other form of symmetrical periodic waveform is possible.

[0058] As shown in the example of diagram 203, the profile of the one-dimensional periodic waveform can be, for example, approximately quasi-sinusoidal, i.e., can be patterned with a function h(x) of the form: h(x) = (x) / ( ...

number

[0059] In either case, the symmetric periodic waveform has a pitch d1 of between about 400 nm and about 900 nm, advantageously between about 500 nm and about 700 nm, and a depth h1, to form a diffraction grating defined to produce a diffraction effect in 1st and -1st order reflections in the wavelength range between 400 nm and 700 nm after deposition of the metal layer. The symmetry of the periodic waveform allows for increased efficiency of the 1st and -1st order diffraction effect.

[0060] In example embodiments, the depth of the corrugations is between about 60 nm and about 400 nm.

[0061] Furthermore, according to this description and as shown in Examples 201 to 203, the thickness e of the metal layer, defined in the direction (z) perpendicular to the plane of the optical security element, varies over a period P1 of the first diffractive structure defined between two extreme values, between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, resulting in an asymmetric distribution of the metal, which means that for each period P1 defined between two extreme values, the metal layer does not have a plane of symmetry perpendicular to the (xy) plane of the element and perpendicular to the (x) direction of variation of the wave profile.

[0062] According to an example embodiment, such an asymmetric distribution of the metal used to form the metal layer 114 is obtained by depositing the metal onto the first structure at a predetermined deposition angle α, which depends on the corrugation profile and the desired minimum and maximum thickness of the metal layer. The deposition angle α is defined in the (xz) plane, i.e., the plane of FIG. 2, with respect to the (z) normal to the plane of the component. The (xz) plane is perpendicular to the (xy) plane of the component and contains the (x) direction of variation of the corrugation profile.

[0063] Thus, for example, in the case of a sinusoidal or quasi-sinusoidal profile with a period between about 400 nm and about 900 nm, advantageously between about 500 nm and about 700 nm, and a depth between about 60 nm and about 400 nm, the deposition angle may be between about 25° and about 70°. Of course, values ​​in this range can be adjusted depending on the waveform parameters to obtain the desired metal thickness and corresponding visual effect, as explained in more detail below.

[0064] To obtain an asymmetric distribution of metal on the first pattern, methods other than oblique deposition of metal can be used, for example a partial demetallization operation.

[0065] Figures 3 and 4 show diagrammatically the visual effects that can be obtained in reflection and transmission respectively with an optical security element according to the present invention.

[0066] These figures show a high-security object 300, for example a security document, on which an optical security element 301 is arranged. The optical security element may for example take the form of a security strip, for example as shown in Figure 1B, and may include at least a first structure according to the present description.

[0067] In this example, the first structure includes multiple patterns in the first region, the outlines of which are indicated in the figure by reference numerals 311-315. These patterns are formed from respective symmetrical one-dimensional periodic corrugations arranged in the same direction (parallel corrugations) but having, for example, different pitches. In each of the patterns, the corrugations are configured to form a diffraction grating defined to produce diffraction effects in the 1st and -1st order reflections in the wavelength range between 400 nm and 700 nm after deposition of the metal layer. Higher order diffraction effects can also be observed. However, because the pitch is different in each pattern, the observer will see different colors in the 1st order reflections for each pattern.

[0068] According to this specification, the first structure is coated with a metal layer (114, FIG. 1B) in which the metal is asymmetrically distributed.

[0069] As shown in Figure 3, when an observer observes the high security object 300 in reflection in a given observation direction and applies a tilting movement about an axis (Δ) contained in the plane of the optical security element and perpendicular to the direction of change of the corrugated profile ((y) direction, Figure 2), a colorful rainbow animation is seen. A and 30 BThis color animation, shown schematically between two tilt positions, each referenced as , contains the sequential appearance of different color patterns. Specifically, the different pitch of the corrugations in each of the patterns makes it possible for a given observation angle to see only one or more of the patterns via +1 or -1 order diffraction, and with a color that depends on the pitch. By varying the observation angle, other patterns reveal other colors. The color visible in reflection is the usual color of +1 or -1 order diffraction, resulting from the relationship governing the angular dispersion of gratings (Bragg's law).

[0070] Note that because the waveform is symmetric and the metal is nearly continuous on the first diffraction structure, the asymmetric metal distribution has little effect on the observation of the +1st or -1st diffraction orders.

[0071] Figure 4 shows a second animation, schematically, resulting from the observation of the same secure object 300, but in transmission. C , 30 D and 30 E shows three tilt positions of the high-security object 300 in the case where an observer is observing the high-security object in transmission. The observation direction coincides with the illumination direction indicated by the arrow in Fig. 4 (observation of the zeroth diffraction order).

[0072] As explained above, the intense and vivid colors are observed due to the asymmetric distribution of metal. The colors vary from pattern to pattern because the corrugation pitch is different for different patterns. Furthermore, the color of each pattern can be changed by tilting the part about an axis (Δ) perpendicular to the direction of change in the corrugation profile.

[0073] Furthermore, as shown in FIG. 4, when the safety object is tilted, the position (3 C ) (zero next observation position). In other words, a given pattern is observed at tilt positions 30 corresponding to symmetric tilts of the secure object on either side of the normal to the part.D and 30 E And they don't seem to have the same color.

[0074] In the optical effect observed in transmission, as shown by the schematic diagram in Figure 4, the angular dispersion associated with + / -1st diffraction orders, as was the case in reflection, is no longer an issue because the zeroth order is observed. Applicants attribute this unique optical effect to a redistribution of the spectral energy density coupled into the various diffraction orders as the angle changes. In other words, energy not coupled into the zeroth order is absorbed or emitted in other diffraction orders. This effect is dependent on the angle of incidence and is inherently asymmetric. As will be explained with reference to Figures 5A and 5B, this effect is polarization independent. This is an important distinction from plasmonic color.

[0075] 5B, 5C and 5D show, by way of example, the transmission curve as a function of wavelength calculated for an optical security element as shown in FIG. 5A (partial cross section) and a comparative transmission curve.

[0076] These curves were calculated using a known computer code that uses the RCWA method (RCWA stands for Rigorous Coupled Wave Approximation; see Popov, Evgeny, (2001), "Maxwell equations in Fourier space: fast-converging formulation for diffraction by arbitrary shaped, periodic, anisotropic media", Journal of the Optical Society of America A, 18(11):2886-94, Bibcode:2001 JOSAA, 18.2886P. doi:10.1364 / JOSAA.18.002886).

[0077] More precisely, FIG. 5B shows the calculated transmittance curve for the case of an optical security element illuminated with TE polarized light at normal incidence, FIG. 5C shows the calculated transmittance curve for the case of an optical security element illuminated with TM polarized light at normal incidence and FIG. 5D shows the calculated transmittance curve for the case of an optical security element illuminated with unpolarized light at normal incidence.

[0078] The optical security element C3 was according to the present specification, as shown in Fig. 5A, and included a first pattern with a one-dimensional corrugation of sinusoidal profile, with a pitch d1 of 500 nm and a depth h1 of 200 nm. The metal layer 114 resulted from the deposition of aluminum onto the structure at an angle α = 40° and with a deposition rate such that the equivalent thickness on a plane at normal incidence was 30 nm. The metal layer 114 thus formed had an asymmetric metal distribution, in particular, one part of the sinusoid had a considerable metal thickness, typically up to about 20 nm, and the other part of the sinusoid had very little metal, typically down to about 2 nm.

[0079] When such an optical security element is viewed at normal incidence under TE polarized light, a strong wavelength dependent transmission response is observed, as shown in curve 503 in FIG. 5B.

[0080] For comparison, curves 501 and 502 were calculated using optical security elements C1 and C2, respectively, which are identical to optical security element C3 according to the present description, except that for curve 501 the metal layer is uniform and has a thickness of 30 nm, and for curve 502 the metal layer has an asymmetric distribution resulting from aluminum deposition at a deposition angle α of 20°.

[0081] Curve 501 is the transmission curve of an optical security element as described in the prior art, for example as described in US Pat. No. 5,649,999. Due to the zero deposition angle, the transmission of the structure is very weak.

[0082] Curve 502 is the transmittance curve of an optical security element with a metal layer that has an asymmetric metal distribution due to a non-zero deposition angle, although a deposition angle of 20° is small for a structure such as that described with reference to Figure 5A, and the metal distribution over one period of the corrugation remains fairly uniform, so the effect on transmission is not very noticeable.

[0083] Curves 513, 511 and 512 shown in FIG. 5C are calculated transmission curves for TM polarized light at normal incidence for an optical security element C3 according to the present description and for the optical security elements C1 and C2 introduced above, respectively, for comparison.

[0084] It is noteworthy that for component C3 in accordance with the present invention (curve 513), a strong wavelength-dependent transmission response is again observed.

[0085] Components C1 and C2 exhibit very weak transmission responses to TM polarized light (curves 511 and 512, respectively).

[0086] FIG. 5D shows the calculated transmission curve for the optical security element described above, but now under unpolarized light.

[0087] It can be seen that the curve 523 calculated with the optical security element C3 according to the present description shows a strong transmission response that is also strongly dependent on wavelength. This is explained by the response of such a component to both TE and TM polarized light. In practice, this means that when the component is illuminated with white light, a strong and vivid color effect is obtained.

[0088] By way of comparison, curves 521 and 522 show the transmission curves calculated under the same conditions for the optical security elements C1 and C2 described above. A very weak transmission response is observed.

[0089] Also for comparison, curve 524 shows the transmittance curve calculated under the same conditions for an optical security element C4 that includes a first pattern identical to that of optical security element C3 (one-dimensional periodic waveform, period 500 nm, and depth 200 nm), except that the metal layer has been replaced by an 80 nm thick layer of high refractive index material. Such optical security elements are in accordance with the prior art, as described for example in patent document 3 or non-patent document 1 (guided mode resonant filters). In transmission, such elements behave like subtractive filters. As curve 524 shows, the transmittance is very high and therefore the resulting color effect is very vivid, although the color is not very noticeable because the transmittance remains high at all wavelengths.

[0090] These above mentioned figures therefore show that the optical security element according to the present invention exhibits a significant effect in transmission.

[0091] As will be explained with reference to the following figures, this remarkable effect can be exploited to create more complex visual effects or animations.

[0092] Figure 6A shows a (partial) cross-sectional view of an optical security element according to the present specification, in which a first diffractive structure S comprises in first regions ("zone 1") a first pattern M1 according to the present specification, for example as described with reference to Figure 2, and between these first regions comprises a region ("zone 2") that is said to be unstructured.

[0093] 6B shows an optical security element observed in transmission, in which, during the embossing stage of the embossing layer (113, FIGS. 1A and 1B), the diffractive structure is designed to generate high-resolution images, i.e., images with a resolution of more than 2500 dots per inch (dpi) (i.e., a pitch between two points of less than approximately 10.1 μm), preferably images with a resolution of more than 3000 dpi (i.e., a pitch between two points of less than approximately 8.5 μm). In the example of FIG. 6B, the diffractive structure comprises a structure of the type shown schematically in FIG. 6A, with a first region (Zone 1) comprising a first pattern M1, and unstructured regions (Zones 2, 3). In the region structured with pattern M1, the optical security element appears colored, with a color depending on the first pattern, the metal layer, and the tilt angle, as explained above. In contrast, in the unstructured regions, the metal layer is thick, there are no resonance effects, and the transmittance is almost zero. Thus, as shown in Figure 6B, the viewer sees a very good resolution color image in transmission with bright, vivid colors.

[0094] By varying the tilt angle around a direction Δ parallel to the direction of the one-dimensional periodic waveform, a change in color in transmission is observed.

[0095] 7 shows, by way of example, a calculated transmittance curve for an optical security element according to the present invention. More precisely, as a simulation, the first pattern consists of a one-dimensional periodic waveform of sinusoidal profile with a pitch of 500 nm and a depth of 200 nm. A metal layer results from aluminum deposition onto the diffractive structure at an angle α=60° and with a deposition rate such that the equivalent thickness on a flat surface at normal incidence is 30 nm.

[0096] Curves 701, 702, 703, 704, 705, 706, and 707 are calculated transmittance curves as a function of wavelength for angles of incidence θ=0° (normal incidence), θ=−5°, θ=−10°, θ=−15°, θ=+5°, θ=+10°, and θ=+15°, respectively. It is noteworthy that not only does the spectrum vary significantly with wavelength, resulting in a change in color with tilt, but the spectrum is not symmetrical on either side of normal incidence. In other words, an observer will not see the same colors on either side of normal incidence, which enhances the authenticity of the document.

[0097] Thus, returning to the example of an optical security element as shown in Figures 6A and 6B, an observer will be able to authenticate the optical security element in transmission by observing a strong and unusual visual effect of colour that is variable depending on the tilt angle and asymmetric on either side of normal incidence.

[0098] It should be noted that when an observer views the same optical security element in reflection, it is also possible to observe a color image in which the color resulting from the +1st or -1st diffraction order varies with the angle of inclination. When viewed in reflection, the color effect is symmetric on both sides of illumination at normal incidence. It should be noted that a zeroth order color effect can be observed in reflection, which is complementary to the color effect in transmission.

[0099] 8A and 8B show (partial) cross-sectional views of another example of an optical security element according to the present disclosure, exhibiting a striking visual effect. In this example, the optical security element comprises a first diffractive structure S, which in a first region ("Zone 1") comprises a first pattern M1 according to the present disclosure, for example as described with reference to FIG. 2, and in a second region ("Zone 2") comprises a second pattern M2 forming symmetrical one-dimensional periodic corrugations with a pitch between about 400 nm and about 900 nm, for example identical to the pitch of the corrugations forming the first pattern M1, but with a second depth shallower than the first depth of the first pattern M1. The corrugations of the second pattern, like the first pattern, are configured to form, after deposition of a metal layer, a diffraction grating defined to generate a diffraction effect in the 1st and -1st order reflections in the wavelength range between 400 nm and 700 nm. However, in this example, since the depth of the corrugation is shallower, even though the thickness of the metal layer remains variable over the period of the first diffraction structure defined between the two peaks, it has a minimum thickness strictly greater than 10 nm, and the asymmetry of the distribution of the metal layer is no longer sufficient to produce the significant effect in transmission described in relation to the first pattern M1.

[0100] Thus, similar color effects resulting from diffraction in the first region ("Zone 1") and the second region ("Zone 2") can be observed in reflection, as shown in Figure 8A. In contrast, in the second region ("Zone 2"), the transmittance is zero or nearly zero and wavelength-dependent in the first region ("Zone 1"), as shown in Figure 8B, because a significant effect results from the asymmetry of the metal layer in the first pattern.

[0101] FIG. 8C shows an optical security element observed in transmission, at the embossing stage of the embossing layer (113, FIGS. 1A and 1B), in which the diffractive structures are designed to produce a high-resolution image similar to that shown in FIG. 6B.

[0102] In the example of FIG. 8C, the diffractive structure includes a structure of the type shown schematically in FIGS. 8A and 8B, with a first region (Zone 1) containing a first pattern M1, a second region (Zone 2) containing a second pattern M2, and a third region (Zone 3), which is unstructured in this example. In the first region structured with pattern M1, the optical security element appears to display a color that depends on the first pattern, the metal layer, and the tilt angle, as explained above. In contrast, in the second region containing second pattern M2, the metal layer does not have sufficient asymmetry to produce a noticeable effect, and the transmittance is zero or near zero. The transmittance is also zero or near zero in the unstructured region (Zone 3). Therefore, as shown in FIG. 8C, the observer sees a color image in transmission with bright, vivid colors and very good resolution. By varying the tilt angle around a direction Δ parallel to the corrugation direction, a change in color in transmission is observed. As mentioned above, the color does not change symmetrically on both sides of normal incidence.

[0103] If the pitch of the one-dimensional periodic waveform of the first pattern is identical to that of the second pattern, no image will be visible in reflection, since the second region will diffract under the same conditions (wavelength depending on the slope) as the first region. Only the contour (zone 3) will stand out, since this region is unstructured and therefore does not diffract.

[0104] Rapid color changes in optical security elements can be advantageously utilized to generate unique color animations.

[0105] Figure 9A shows a (partial) three-dimensional view of an optical security element comprising a first diffractive structure S with a first pattern M1 according to the present description. The first pattern M1 modulates a fine pattern M3 made up of fine-sized facets 110, 120. The facets are characterized by a gradient (β1, β2 in the example of Figure 9A). The gradient of the facets is, for example, in a direction parallel to the direction of change of the corrugated profile of the first pattern.

[0106] For example, the facets have a dimension in the direction of their gradient (or "width") that is about four times or more, preferably about eight times or more, the period of the grating formed by the corrugations that make up the first pattern M1. Accordingly, the minimum dimension can be selected depending on the period of the corrugations. For example, the minimum dimension of the facet width is equal to about 2 μm. According to one or more examples, the facet width is between about 2 μm and about 100 μm, preferably between about 2 μm and about 80 μm, and preferably between about 4 μm and about 80 μm.

[0107] According to an example embodiment, the facets have a generally rectangular shape and a "length" measured in a direction perpendicular to the direction of their gradient, which length is, for example, less than about 100 μm.

[0108] According to one or more examples, all of the facets have substantially the same height, measured in a direction perpendicular to the plane of the part, e.g., less than 2 microns, and preferably less than 1 micron.

[0109] According to one or more examples, the facets of the group of facets have different heights, but in this case, all of the facets have a maximum height, for example, less than 2 microns, preferably less than 1 micron.

[0110] According to one or more example embodiments, the maximum angular value of the slope (in absolute value) is between about 7° and about 15°. By convention, the positive direction in measuring the angular value of the slope of a facet herein is the clockwise direction.

[0111] As shown in Figure 9B, in example embodiments, at least some of the facets in the facet group have different slopes, with the change in slope increasing or decreasing to simulate reflective elements with convex or concave regions, respectively. Herein, when the change in angle value of the slope of the facets is increasing or decreasing and the angle values ​​have the same sign, the visual effect resulting from such an arrangement of facets is said to be a "half-wave" type dynamic effect. When the change in angle value of the slope of the facets is increasing or decreasing and the arrangement is such that at least one sign change is observed, as shown in Figure 9B, the visual effect resulting from such an arrangement of facets will be said to be a "wave" type dynamic effect.

[0112] In reflection, the facets are modulated by the first pattern M1 according to the present description so that, when the optical security element is tilted, the observer sees a dynamic effect of the "wave" or "half-wave" type, i.e. an effect such as a continuous swirl of colored lines of light.

[0113] 9B, considering a set of facets 110, 120, 130, 140, 150, and 160 with different slopes (with slope angles β1, β2, β3, β4, β5, and β6, respectively), in transmission, for incident light at normal incidence (perpendicular to the plane of the part), it is possible to observe as many different colors as there are slopes. These different colors are represented diagrammatically by arrows 11, 12, 13, 14, 15, and 16, respectively. Specifically, from the perspective of the incident light beam, the angle of the facets equates to a non-zero angle of incidence onto the first pattern formed by the corrugations.

[0114] FIG. 9C shows a color animation visible in transmission for an optical security element in which the first pattern M1 modulates a set of facets M3 as described above.

[0115] When observing an optical security element in transmission, changing its tilt by tilting the element (rotating it around an axis parallel to the direction of the one-dimensional periodic waveform) will result in an observed change in the color of each facet, resulting in a movement of colored lines. Note that the color is asymmetric depending on the angle of incidence, so the color will be different depending on whether the optical security element is tilted (rocked) in one direction (position 121) or another (position 122).

[0116] It is to be noted that the optical security element according to the present description may in addition to the first structure according to the present description, in particular as described by way of example above, comprise other structures (not shown), which may for example be scattering structures, holographic structures or diffractive structures capable of producing the so-called Alphagram® effect.

[0117] Furthermore, partial demetallization (or local removal of the metal layer) is also possible using known methods in order to create macroscopic patterns visible to the naked eye and to provide an additional authentication method.

[0118] An example of a process for manufacturing an optical security element according to this specification will now be described.

[0119] The first step includes designing the at least one first diffractive structure according to the above-described embodiments, and any other diffractive structures.

[0120] The next step is to record an original, also called an optical master, which is, for example, an optical medium on which one or more structures are formed.

[0121] The optical master can be formed using electron lithography or optical lithography methods known in the art.

[0122] For example, according to a first embodiment, an optical master is produced by patterning a resist sensitive to electromagnetic radiation using an electron beam. In this example embodiment, a diffractive structure having a second pattern modulated by the first pattern can be patterned in a single step, when the first pattern modulates the second pattern.

[0123] According to another embodiment, optical lithography (or photolithography) techniques can be used. In this example, the optical master is a sheet of photoresist, and the step of generating it is carried out by one or more exposures by projecting light onto a mask, for example a phase mask and / or an amplitude mask, followed by development in a suitable chemical solution. For example, a first exposure is carried out by projecting light onto an amplitude mask, the transmission coefficient of which is configured so that after development, a relief corresponding to the first pattern is formed in the areas where the first pattern is required. Then, a comprehensive second exposure is carried out using interference photolithography methods known to those skilled in the art. Thus, at least in the first area where the first pattern is required, a waveform constituting the first pattern is recorded. Similar steps can be used to generate other reliefs, such as second patterns, in other areas. The order in which the patterns are formed is arbitrary and can be changed. A development step is then carried out. In this way, an optical master is obtained after development, which includes at least a first structure with a first pattern.

[0124] As mentioned above, a step of transferring the optical master into metal, for example by electroplating, can then be carried out in order to obtain a metal master. According to one variant, a step of replicating the metal master can be carried out in order to obtain a large-scale production tool suitable for replicating structures on an industrial scale.

[0125] The production of the optical security element then includes a replication step. For example, replication can be achieved by hot embossing a first layer 113 (FIGS. 1A and 1B) made of a dielectric material with a refractive index n1, e.g., a low-refractive index layer, typically an embossed lacquer with a thickness of a few microns. The first layer 113 is advantageously supported by a carrier film 111, e.g., a film made of a polymer material, e.g., PET (polyethylene terephthalate), with a thickness of 10 μm to 50 μm. Replication can also be achieved by casting and drying the embossed lacquer layer, followed by UV casting. Replication by UV casting makes it possible, in particular, to reproduce structures with large amplitude depths, thereby obtaining replications with higher fidelity. In general, any other high-resolution replication method known in the art can be used in the replication step.

[0126] The next step is to deposit, via a coating process, on top of the thus embossed layer all the other layers, in particular the metal layer 114, then the (optional) layer 115 of dielectric material, the (optional) security layer 116 (which can be deposited uniformly or selectively to form new patterns), and the adhesive or lacquer layers 117, 118.

[0127] As explained above, the metal layer is advantageously deposited by vacuum evaporation of the metal at a non-zero angle to produce the desired asymmetry in the metal distribution. Alternatively, a partial demetallization operation can be performed to obtain the desired asymmetry.

[0128] Additionally, any steps known to those skilled in the art are possible, such as partial demetallization of the reflective layer 114 to form transparent areas with macroscopically sized contours that are visible to the naked eye.

[0129] Although described through a certain number of example embodiments, it will be understood that the optical security element according to the invention, and the process for manufacturing said optical security element, encompass various variations, modifications and improvements that will be apparent to those skilled in the art, and that these various variations, modifications and improvements are included within the scope of the invention as defined by the claims that follow.

Claims

1. Optical security component (100) for securing an object adapted to be authenticated in transmission by the naked eye Ａ , 100 Ｂ ) where a first layer (113) of dielectric material that is transparent in visible light; - at least a first diffractive structure (S) patterned in said first layer, a metal layer (114) at least partially covering said first diffractive structure and having a reflection spectral band in the visible light; Including, said first diffractive structure has a first pitch (d １ ), and the first depth (h １ ) forming a symmetrical first one-dimensional periodic waveform having at least a first pattern (M １ the first one-dimensional periodic waveform is configured to form, after deposition of the metal layer, a diffraction grating that produces a diffraction effect in 1st and −1st order reflections in a wavelength range between 400 nm and 700 nm; - for each period of said first pattern, defined between two extrema of said first one-dimensional periodic waveform, the thickness of said metal layer, defined in the direction perpendicular to the plane of said optical security element, has a minimum value (e ｍｉｎ ) and a maximum value (e ｍａｘ ) and the metal of said metal layer is asymmetrically distributed to produce a color effect that is visible in transmission.

2. 2. An optical security element according to claim 1, The first pitch (d １ ) is between about 500 nm and about 700 nm.

3. 2. An optical security element according to claim 1, The first diffractive structure has at least a second pattern (M ２ 1. An optical security element comprising: a second one-dimensional periodic waveform having a direction of change of profile parallel to the direction of change of profile of the first one-dimensional periodic waveform, having the same depth as the first depth of the first one-dimensional periodic waveform, and having a pitch different from the first pitch of the first one-dimensional periodic waveform, wherein the first pattern and the second pattern have contours that are recognizable when observed in transmission with the naked eye.

4. 2. An optical security element according to claim 1, The first diffractive structure has at least a second pattern (M) that forms a symmetric second one-dimensional periodic waveform in a second region. ２ ), wherein the second one-dimensional periodic waveform has a direction of change of profile that is parallel to the direction of change of profile of the first one-dimensional periodic waveform, has a pitch that is the same as the first pitch of the first one-dimensional periodic waveform, and has a second depth that is strictly less than the first depth of the first one-dimensional periodic waveform, and wherein the first pattern and the second pattern have contours that are recognizable when observed in transmission with the naked eye.

5. 2. An optical security element according to claim 1, An optical security element, wherein said metal layer results from metal deposition onto said first diffractive structure at a non-zero deposition angle.

6. 2. An optical security element according to claim 1, An optical security element, wherein said first diffractive structure further comprises a micropattern comprising a set of facets with variable slope at fine dimensions, said first pattern modulating said micropattern.

7. 2. An optical security element according to claim 1, 1. An optical security element further comprising at least a second diffractive structure patterned in said first layer, said second diffractive structure being selected from a scattering structure, a holographic structure, a diffractive structure of Alphagram® type.

8. A high-security object comprising a substrate and an optical security element according to claim 1 deposited on said substrate.

9. 1. A process for manufacturing an optical security component for securing an object adapted to be authenticated in transmission by the naked eye, comprising: - depositing on a carrier film a first layer (113) of a dielectric material that is transparent in visible light; - forming at least a first diffractive structure (S) on said first layer; depositing a metal layer (114) at least partially covering said first diffractive structure and having a reflection spectral band in the visible light; Including, said first diffractive structure has a first pitch (d １ ), and the first depth (h １ ) forming a symmetrical first one-dimensional periodic waveform having at least a first pattern (M １ the first one-dimensional periodic waveform is configured to form, after deposition of the metal layer, a diffraction grating that produces a diffraction effect in 1st and −1st order reflections in a wavelength range between 400 nm and 700 nm; - for each period of said first pattern, defined between two extrema of said first one-dimensional periodic waveform, the thickness of said metal layer, defined in the direction perpendicular to the plane of said optical security element, has a minimum value (e ｍｉｎ ) and a maximum value (e ｍａｘ ) wherein the metal of the metal layer is asymmetrically distributed to produce a color effect that is visible in transmission.

10. 10. A process for manufacturing an optical security element according to claim 9, comprising the steps of: The metal layer results from metal deposition onto the first diffractive structure at a non-zero deposition angle.

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