Optical device comprising an encapsulated lens array for a security document
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- IN SMART IDENTITY FRANCE
- Filing Date
- 2026-01-29
- Publication Date
- 2026-08-06
Smart Images
Figure FR2026050096_06082026_PF_FP_ABST
Abstract
Description
[0001] Encapsulated optical lens array device for secure documents
[0002] TECHNICAL FIELD OF THE INVENTION
[0003]
[0001] The invention relates to an optical device for a secure document and a secure document comprising such a secure device.
[0004]
[0002] The invention also relates to a method for manufacturing such an optical device.
[0005] STATE OF THE ART
[0006]
[0003] The identity market today requires increasingly secure identity documents. These documents must be easily authenticated and difficult to counterfeit (ideally unforgeable). This market encompasses a wide variety of documents, such as identity cards, passports, access badges, driver's licenses, etc., which can come in different formats (cards, booklets, etc.).
[0007]
[0004] Various types of secure documents containing images have thus been developed over time, particularly for the secure identification of individuals. Passports, identity cards, and various other official documents now generally include optical devices that allow the secure document to be authenticated and limit the risks of fraud, falsification, or counterfeiting.
[0008]
[0005] In particular, there is a type of optical device which combines data from the bearer of the secure document, which are for example in the form of engravings, images or other, with a lens array of the secure document to create visual effects such as multi-image animations or stereoscopic effects, depending on the viewing angle adopted.
[0009] DESCRIPTION OF THE INVENTION
[0010]
[0006] The present invention aims to provide an optical device of the same type which is safe and durable, while being simple, convenient and economical.
[0011]
[0007] To this end, the invention relates, according to a first aspect, to an optical device for secure documents, comprising a stack of layers including:
[0012] - a transparent support layer with refractive index nO having a first face and a second face, opposite the first face, said support layer comprising a first lens array forming the first face of said support layer, and
[0013] - a first transparent covering layer of refractive index n1 encapsulating the first lens array and having a first face and a second face, opposite to the first face, the first face of the first covering layer being in contact with the first lens array, and the refractive index n0 being different from the refractive index n1.
[0014]
[0008] In the optical device according to the invention, the first lens array is formed on the surface of the support layer and is encapsulated by the first covering layer.
[0015]
[0009] By encapsulation, it is understood that the first covering layer envelops and conforms to the shape of the first lens array, coming into direct contact with its surface.
[0016]
[0010] On the one hand, the first coating layer protects the first lens array against wear and the accumulation of dirt, thus preserving the quality of the optical effects over time.
[0017]
[0011] On the other hand, the first coating layer conceals the first lens array, which prevents direct observation of the first lens array and thus complicates its reproduction.
[0018]
[0012] In addition, the first cover layer influences the path of light rays passing through the stack of layers due to the variation in refractive index between the first cover layer and the support layer.
[0019]
[0013] It is then possible to create various optical effects by adjusting the refractive index of the first coating layer, the refractive index of the support layer and / or the shape of the lenses of the first lens array.
[0020]
[0014] In other words, the first overlay layer is configured to produce, in combination with the first lens array, a specific optical effect, such as multi-image animations or stereoscopic effects. The specific optical effect is obtained by combining the first lens array and the first overlay layer encapsulating said lens array. Without the first overlay layer, the first lens array cannot generate the specific optical effect.
[0021]
[0015] Thus, in order to imitate the optical effects produced by the optical device according to the invention, it would be necessary to recreate precisely the path of the light rays through this device, which implies reproducing not only the refractive indices of the layers, but also the shape of the lenses which, as explained previously, is difficult to identify due to encapsulation by the first covering layer.
[0022]
[0016] Thus, the optical device according to the invention combines a high level of security and great durability, while being simple, convenient and economical.
[0023]
[0017] Preferred, simple, convenient and economical characteristics of the optical device according to the invention are presented below.
[0018] The difference between the refractive index n0 and the refractive index n1 can be between 0.5 and 4, for example between 0.5 and 2.5.
[0024]
[0019] The support layer can be made of a first material. The first material can be configured to retain its initial shape when subjected to a determined temperature and / or a determined mechanical pressure.
[0025]
[0020] The first covering layer may be made of a second material. The second material may be configured to deform plastically when subjected to said specified temperature and / or said specified mechanical pressure.
[0026]
[0021] Thus, when the support layer and the first cover layer are laminated at said determined temperature and / or at said determined mechanical pressure, the first cover layer is deformed until it encapsulates the first lens array, while the first lens array retains its initial shape.
[0027]
[0022] For example, the first material and the second material can be amorphous polymers.
[0028]
[0023] In this case, the first material has a glass transition temperature (Tg) higher than that of the second material.
[0029]
[0024] The glass transition temperature (Tg) is the temperature at which an amorphous material, such as a polymer, transitions from a rigid, glassy state to a more flexible, rubbery state when heated.
[0030]
[0025] The glass transition temperature is typically measured by differential scanning calorimetry (DSC), in accordance with ASTM E1356 or ISO 11357-2.
[0031]
[0026] For example, the first material and the second material can be thermoplastic polymers.
[0032]
[0027] In this case, the first material has a softening temperature higher than that of the second material.
[0033]
[0028] The softening temperature is the temperature at which a thermoplastic polymer undergoes significant deformation under the effect of heat. At this temperature, the material becomes sufficiently malleable to be deformed under the application of low pressure without melting.
[0034]
[0029] The most common method for measuring the softening temperature of a thermoplastic polymer is the Vicat softening temperature determination (VST) method, in accordance with ISO 306.
[0035]
[0030] The support layer can be made of thermoplastic polymer, thermosetting polymer, or transparent photo-polymerizable resin, such as an acrylic monomer or oligomer crosslinkable under ultraviolet radiation, an epoxy resin or a polyurethane resin.
[0031] The thermoplastic polymer is, for example, polycarbonate (PC) or polymethyl methacrylate (PMMA).
[0036]
[0032] The thermosetting polymer is, for example, an acrylate.
[0037]
[0033] The transparent photo-polymerizable resin is, for example, an acrylic monomer or oligomer that is crosslinkable under ultraviolet radiation, an epoxy resin or a polyurethane resin.
[0038]
[0034] The first coating layer can be made of zinc sulfide (ZnS) or titanium dioxide (TiO2), for example in its crystalline form.
[0039]
[0035] The first lens array can be configured to modify the direction of incident light rays propagating in a direction from the cover layer to the support layer so as to direct them towards a focal point, depending on the variation of refractive index along said direction.
[0040]
[0036] In other words, the first lens array is configured, in combination with the variation of the refractive index of the layers, to focus (converge) the incident light rays towards a point, also called focal, located downstream of the lens array, according to the direction of propagation of the light rays.
[0041]
[0037] The first lens array can be configured so that the focal point is located in the support layer, for example between the first face and the second face of the support layer, or beyond the support layer (downstream of the support layer, according to the direction of propagation of the light rays).
[0042]
[0038] The refractive index n0 can be greater than the refractive index n1 and each lens of the first lens array is convex when viewed from the first cover layer.
[0043]
[0039] Alternatively, the refractive index n0 may be less than the refractive index n1 and each lens of the first lens array is concave when viewed from the first overlay layer.
[0044]
[0040] The second face of the first covering layer may have a different topography from that of the first lens array.
[0045]
[0041] In other words, the second face of the first covering layer may have a surface whose shape differs from that of the surface of the first lens array.
[0046]
[0042] For example, the second face of the first covering layer may be flat.
[0047]
[0043] The support layer may include a second lens array forming the second face of said support layer, each lens of the first lens array being aligned with a corresponding lens of the second lens array to form a lens doublet.
[0044] Thus, the second lens array is configured to reduce potential optical aberrations (chromatic and spherical) and improve focusing accuracy. The visual effects obtained through the combination of the first and second lens arrays then exhibit greater sharpness and precision.
[0048]
[0045] The stack of layers may include a second transparent covering layer of refractive index n2 encapsulating the second lens array and having a first face and a second face, opposite to said first face, the first face of said second covering layer being in contact with the second lens array, and with the refractive index n2 which is different from the refractive index n0.
[0049]
[0046] The second face of the second covering layer may have a different topography from that of the second lens array.
[0050]
[0047] In other words, the second face of the second covering layer may have a surface whose shape differs from that of the surface of the second lens array.
[0051]
[0048] For example, the second face of the second covering layer may be flat.
[0052]
[0049] The refractive index n2 can be equal to the refractive index n1.
[0053]
[0050] The invention also relates, according to a second aspect, to a secure document comprising a main body provided with a window and an optical device, in accordance with the first aspect of the invention, inserted in said window.
[0054]
[0051] The optical device can thus be used as a security element to enhance the authenticity and security of the secure document
[0055]
[0052] The first lens array of the optical device can be configured so that the focal point is beyond the support layer but within the body of the secure document, or beyond the body of the secure document, for example to produce an optical effect when checking another secure document.
[0056]
[0053] The invention also relates, according to a third aspect, to a method for manufacturing an optical device comprising the following steps:
[0057] - provision of a transparent support layer with refractive index n0 having a first face and a second face, opposite the second face, said support layer comprising a first lens array forming the first face of said support layer, - provision of a first transparent cover layer with refractive index n1 having a first face and a second face, opposite the first face, - positioning of the first face of the first cover layer opposite the first lens array of the support layer,
[0058] - lamination of the first cover layer on the support layer, configured so that the first face of the first cover layer is deformed by the first lens array and comes into contact with the first lens array.
[0059]
[0054] In other words, the rolling step includes a deformation step of the first face of the first covering layer around the lens array.
[0060]
[0055] Thus, following the lamination step, the lens array is encapsulated in the first covering layer.
[0061]
[0056] The manufacturing process can be configured to manufacture an optical device conforming to that of the first aspect of the invention.
[0062] BRIEF DESCRIPTION OF THE FIGURES
[0063]
[0057] The invention, according to an example of embodiment, will be well understood and its advantages will become more apparent upon reading the detailed description that follows, given by way of example and in no way limiting, with reference to the attached drawings.
[0064]
[0058] Figure 1 schematically and partially represents, in cross-section, an optical device according to a first embodiment of the invention.
[0065]
[0059] Figure 2 and Figure 3 schematically show the propagation of an incident light ray in the optical device of Figure 1, as a function of the shape of a lens and the refractive index of the layers comprising the optical device.
[0066]
[0060] Figure 4 is a view similar to that of Figure 1, showing the optical device according to a second embodiment of the invention.
[0067]
[0061] Figure 5 and Figure 6 schematically show the propagation of an incident light ray in the optical device of Figure 4, as a function of the shape of a lens and the refractive index of the layers comprising the optical device.
[0068]
[0062] Figure 7 represents, from a front view, a secure document comprising an optical device according to the invention.
[0069]
[0063] Figure 8 is a block diagram showing steps of a manufacturing process for an optical device according to the invention.
[0070]
[0064] Figure 9 schematically represents the formation of lens arrays on a layer during a manufacturing process of the optical device according to one embodiment of the invention.
[0071] DETAILED DESCRIPTION OF THE INVENTION
[0072]
[0065] Figure 1 represents an optical device 30 according to a first embodiment of the invention.
[0073]
[0066] The optical device 30 comprises a stack of layers including a transparent support layer 31 and a transparent cover layer 32.
[0067] The support layer 31 has a first face 33 and a second face 34, opposite the first face 33.
[0074]
[0068] The covering layer 32 has a first face 35 and a second face 36, opposite to the first face 35.
[0075]
[0069] The first face 33 of the support layer 31 is in contact with the first face 35 of the cover layer 32.
[0076]
[0070] Here, the second face 36 of the covering layer 32 is flat.
[0077]
[0071] In particular, the second face 36 has a surface which is relatively flat compared to the surface of the lens array 40.
[0078]
[0072] The support layer 31 comprises a lens array 40 formed on its first face 33.
[0079]
[0073] A lens array, also called a lenticular array, refers to an organized set of optical lenses arranged in a repetitive manner, for example according to a regular configuration, and configured to modulate or direct light rays in such a way as to produce specific visual effects, such as enlargements, animations or three-dimensional images.
[0080]
[0074] The support layer 31 has a refractive index n0, while the cover layer 32 has a refractive index n1 which is different from the refractive index n0.
[0081]
[0075] This difference in refractive index causes the refraction of incident light rays, thus generating a lens effect.
[0082]
[0076] Here, the lens array 40 comprises a plurality of lenses 41 having a substantially hemicylindrical profile, arranged parallel to each other in a regular manner. This is referred to as a cylindrical lens array 40.
[0083]
[0077] The lenses 41 have a diameter on the order of a micrometer, for example about 150 pm. They are called microlenses.
[0084]
[0078] The covering layer 32 covers the lens array 40.
[0085]
[0079] In particular, the first face 35 of the covering layer 32 encases the lens array 40 and thus has a shape complementary to that of the lens array 40.
[0086]
[0080] In other words, the lens array 40 is encapsulated in the cover layer 32.
[0087]
[0081] Thanks to the coating layer 32, the lens array 40 is protected against wear and dirt accumulation, which guarantees the durability and quality of the visual effects of the optical device 30.
[0082] This encapsulation by the coating layer 32 also makes it difficult to access the lens array 40, which complicates its reproduction, and more generally the reproduction of the optical device 30.
[0088]
[0083] The support layer 31 and the cover layer 32 can be assembled and held together by different means, for example by lamination or by bonding with a transparent adhesive disposed between these layers.
[0089]
[0084] Figures 2 and 3 illustrate the propagation of an incident light ray in the optical device 30, as a function of the shape of a lens 41 of the lens array, the refractive index of the support layer 31 and the refractive index of the cover layer 32.
[0090]
[0085] In the example shown, the lens array is configured to change the direction of the incident light rays propagating in a direction from the cover layer 32 to the support layer 31 so as to direct them towards a focal point, depending on the variation of the refractive index along this direction.
[0091]
[0086] In other words, the lens array is configured to cause the redirection of incident light rays towards a focal point located downstream of the lens array, according to the direction of propagation of said incident light rays.
[0092]
[0087] The lens array can be configured so that the focal point is located in the support layer 31 or beyond the support layer 31.
[0093]
[0088] In Figure 2, the refractive index n0 of the support layer 31 is greater than the refractive index n1 of the cover layer and each lens 41 of the lens array is convex as seen from the cover layer 32.
[0094]
[0089] As illustrated, when an incident light ray passes from the coating layer 32 to the support layer 31, the change in refractive index causes a deviation towards the normal to the surface at the point of incidence. This deviation, combined with the convex geometry of the lenses 41, directs the incident light ray towards a focal point located downstream of the lens array 40, thus creating a convergence effect.
[0095]
[0090] In Figure 3, the refractive index n0 of the support layer 31 is less than the refractive index n1 of the cover layer and each lens 41 of the lens array 40 is concave as seen from the cover layer 32.
[0096]
[0091] As illustrated, when an incident light ray passes from the coating layer 32 to the support layer 31, the change in refractive index causes a deviation away from the normal to the surface at the point of incidence. This deviation, combined with the concave geometry of the lenses 41, directs the incident light ray towards a focal point located downstream of the lens array, thus creating a convergence effect.
[0092] The difference between the refractive index n0 and the refractive index n1 is, for example, greater than or equal to 0.5. With a difference of at least 0.5, the generated visual effects appear sharper.
[0097]
[0093] By way of example, the support layer 31 is made of zinc sulfide (ZnS) and the cover layer 32 is made of polycarbonate (PC).
[0098]
[0094] Zinc sulfide has a refractive index of about 2.3, while polycarbonate has a refractive index of about 1.6, i.e. a difference in refractive indices of about 0.7.
[0099]
[0095] Other materials can of course be considered for the support layer 31, such as titanium dioxide (TiO2) which has a refractive index of about 2.7, and about 2.9 in its crystalline form.
[0100]
[0096] Figure 4 represents an optical device 130 according to a second embodiment of the invention.
[0101]
[0097] The optical device 130 is similar to that according to the first embodiment, except that the support layer 31 further comprises a second lens array 50 formed on its second face 34 and that the stack of layers further comprises a second covering layer 132 covering the second lens array 50.
[0102]
[0098] The second covering layer 132 has a first face 135 and a second face 136, opposite the first face 135.
[0103]
[0099] Here, the second face 136 of the second covering layer 132 is flat.
[0104]
[0100] In particular, the second face 136 has a surface which is relatively flat compared to the surface of the second lens array 50.
[0105]
[0101] The second face 34 of the support layer 31 is in contact with the first face 135 of the second covering layer 132.
[0106]
[0102] The second coating layer 132 has a refractive index n2 which is different from the refractive index n0 of the support layer 31.
[0107]
[0103] This difference in refractive index causes the refraction of incident light rays, thus generating a lens effect.
[0108]
[0104] In particular, the first face 135 of the second covering layer 132 encases the second lens array 50 and thus has a shape complementary to that of the second lens array 50.
[0109]
[0105] In other words, the second lens array 50 is encapsulated in the second overlay layer 132.
[0110]
[0106] Here, the second lens array 50 comprises lenses 51 having a substantially hemicylindrical profile, arranged parallel to each other in a regular manner.
[0107] The second lens array 50 is here identical to the first lens array 40.
[0111]
[0108] In particular, each lens 41 of the first lens array 40 is aligned with a lens 51 of the second lens array 50 to form a lens doublet.
[0112]
[0109] A lens doublet is a combination of two lenses aligned on the same optical axis, forming an optical unit configured to focus incident light rays by combining the optical properties of each lens.
[0113]
[0110] Adding an additional lens to form a lens doublet makes it possible in particular to reduce optical aberrations (chromatic and spherical) and to improve focusing accuracy. This leads to relatively sharp and precise visual effects.
[0114]
[0111] Figures 5 and 6 illustrate the propagation of an incident light ray in the optical device 130, as a function of the shape of the lenses 41, 51 of each lens array, the refractive index of the support layer 31, the refractive index of the first cover layer 32 and the refractive index of the second cover layer 132.
[0115]
[0112] In the example shown, the first lens array and the second lens array are configured to change the direction of the incident light rays propagating in a direction from the first cover layer 32 to the support layer 31 so as to direct them towards a focal point, depending on the variation of the refractive index along this direction.
[0116]
[0113] In other words, the first lens array and the second lens array are configured to cause the redirection of incident light rays to a focal point located downstream of the first lens array 40, according to the direction of propagation of said incident light rays.
[0117]
[0114] In the example shown, the first lens array and the second lens array are configured so that the focal point is located in the second overlap layer 132. Other configurations can of course be considered.
[0118]
[0115] In Figure 5, the refractive index n0 of the support layer 31 is greater than the refractive index n1 of the first cover layer 32 and each lens 41 of the first lens array is convex as seen from the first cover layer 32, while the refractive index n0 of the support layer 31 is greater than the refractive index n2 of the second cover layer 132 and each lens 51 of the second lens array is concave as seen from the support layer 31.
[0119]
[0116] As illustrated, when an incident light ray passes from the first coating layer 32 to the support layer 31, the change in refractive index causes the ray to deviate towards the normal to the surface at the point of incidence. This deviation, combined with the convex geometry of the lenses 41, directs the light ray towards an intermediate focal point located between the lens arrays.
[0117] Then, when the light ray passes from the support layer 31 to the second coating layer 132, the change in refractive index causes a deviation away from the normal. This deviation, combined with the concave geometry of the lenses 51, directs the light ray towards a final focal point located downstream of the second lens array 50, thus creating a convergence effect.
[0120]
[0118] In Figure 6, the refractive index n0 of the support layer 31 is less than the refractive index n1 of the first cover layer 32 and each lens 41 of the first lens array is concave as seen from the first cover layer 32, while the refractive index n0 of the support layer 31 is less than the refractive index n2 of the second cover layer 132 and each lens 51 of the second lens array is convex as seen from the support layer 31.
[0121]
[0119] As illustrated, when an incident light ray passes from the first coating layer 32 to the support layer 31, the change in refractive index causes the ray to be deviated away from the normal to the surface at the point of incidence. This deviation, combined with the concave geometry of the lenses 41, directs the light ray towards an intermediate focal point located between the lens arrays.
[0122]
[0120] Next, when the light ray passes from the support layer 31 to the second covering layer 132, the change in refractive index causes a deviation towards the normal. This deviation, combined with the convex geometry of the lenses 51, directs the light ray towards a final focal point located downstream of the second lens array, thus creating a convergence effect.
[0123]
[0121] The refractive index n2 is for example identical to the refractive index n1.
[0124]
[0122] More specifically, the material of the second cover layer 132 can be the same material as that of the first cover layer 32.
[0125]
[0123] Generally, in each of the optical devices 30, 130 illustrated in figures 1 to 6, each lens array 40, 50 is formed on the surface of the support layer 31 and is encapsulated by a respective cover layer 32, 132.
[0126]
[0124] By encapsulation, it is understood that each covering layer 32, 132 envelops and conforms to the shape of the associated lens network 40, 50, by coming into direct contact with its surface.
[0127]
[0125] On the one hand, each coating layer 32, 132 protects the associated lens array 40, 50 against wear and dirt accumulation, thus preserving the quality of the optical effects over time.
[0128]
[0126] On the other hand, each cover layer 32, 132 conceals the associated lens array 40, 50, which prevents direct observation of each lens array and thus complicates its reproduction.
[0127] Furthermore, each cover layer 32, 132 influences the path of light rays passing through the stack of layers due to the variation in refractive index between the cover layer 32, 132 and the support layer 31.
[0129]
[0128] It is then possible to create various optical effects by adjusting the refractive index of each coating layer 32, 132, the refractive index of the support layer 31 and / or the shape of the lenses of each lens array 40, 50.
[0130]
[0129] In other words, the overlay layer(s) 32, 132 are configured to produce, in combination with the lens array(s) 40, 50, a specific optical effect, such as multi-image animations or stereoscopic effects. The specific optical effect is obtained by combining the lens array(s) 40, 50 and the overlay layer(s) 32, 132 encapsulating the lens array(s). Without the overlay layer(s) 32, 132, the lens array(s) 40, 50 cannot generate the specific optical effect.
[0131]
[0130] Thus, to imitate the optical effects produced by one of these optical devices 30, 130, it would be necessary to recreate precisely the path of the light rays through this device, which implies reproducing not only the refractive indices of the layers 31, 32, 132, but also the shape of the lenses 41, 51 which, as explained previously, is difficult to identify due to encapsulation by the cover layer(s) 32, 132.
[0132]
[0131] Each of these optical devices 30, 130 therefore combines a high level of security and great durability, while being simple, convenient and economical.
[0133]
[0132] Figure 7 represents a secure document 1 in the form of a card, such as a debit card, a credit card, an identification card, a loyalty card, a membership card, a health care card, a security card, etc.
[0134]
[0133] The card 1 comprises a body 2 in which a window 3 is provided, and an optical device 4 inserted in the window 3.
[0135]
[0134] The optical device 4 is for example in accordance with one of the optical devices according to the first and second embodiments.
[0136]
[0135] The body 2 of the card 1 is here opaque, while the optical device 4 is transparent.
[0137]
[0136] In particular, the body 2 has a light transmission (TL) of less than 10% (between 0% and 10%), while the optical device 4 has a light transmission greater than 10%, and for example greater than 60% (between 60% and about 100%), or even greater than 80% (between 80% and about 100%).
[0138]
[0137] Generally, in the present application, the term "transparent" refers to objects with a light transmission greater than 10%, for example greater than 60%, or even greater than 80%, while the term "opaque" refers to objects with a light transmission less than 10%.
[0139]
[0138] Light transmission is measured according to ISO 9050:2003 using illuminant D65, and corresponds to the total transmission (integrated in the visible range), taking into account both direct transmission and possible diffuse transmission, the measurement being made for example using a spectrophotometer equipped with an integrating sphere, the measurement at a given thickness then being converted where appropriate to the reference thickness of 4 mm according to ISO 9050:2003.
[0140]
[0139] The window 3 is through-hole and it is then possible for an observer to see through the card 1 by means of the optical device 4.
[0141]
[0140] The body 2 has a first face 6 (visible in figure 7) and a second face (not visible), opposite to the first face.
[0142]
[0141] In an example not shown, the card comprises a first protective layer and a second protective layer arranged on either side of the card body, respectively against the first face and against the second face, and enclosing the optical device in the window. The protective layers may be transparent and made of polycarbonate (PC).
[0143]
[0142] We will now describe, with reference to Figure 8, a manufacturing process 100 of an optical device such as that according to the first embodiment described with reference to Figures 2 to 4 or according to the second embodiment described with reference to Figures 5 to 7.
[0144]
[0143] The manufacturing process 100 includes a step of supplying 101 a transparent or semi-transparent support layer of refractive index nO having a first face and a second face, opposite to the first face.
[0145]
[0144] The manufacturing process 100 includes a step of supplying 102 with a transparent or semi-transparent coating layer of refractive index n1 having a first face and a second face, opposite to the first face.
[0146]
[0145] Here, the manufacturing process 100 further includes a step of forming 103 a lens array on the first face of the support layer.
[0147]
[0146] The manufacturing process 100 includes a positioning step 104 of the first face of the covering layer opposite the lens array of the support layer.
[0148]
[0147] The manufacturing process 100 further includes a step of rolling the covering layer onto the support layer configured so that the first face of the covering layer is deformed by the lens array and comes into contact with the first face of the support layer.
[0148] Rolling is a mechanical process during which mechanical pressure is applied for an appropriate duration, with or without the addition of heat, so as to form a rolled assembly.
[0149]
[0149] The implementation of the rolling step with heat input can, in some cases, lead to a softening of the coating layer and thus facilitate its deformation in contact with the lens network.
[0150]
[0150] To prevent the lenses from being deformed during the lamination step, a material configured to retain its initial shape at a determined temperature and / or mechanical pressure is chosen for the support layer, and a second material configured to deform plastically at said determined temperature and / or mechanical pressure is chosen for the cover layer.
[0151]
[0151] In this way, when the support layer and the first cover layer are laminated at said determined temperature and / or at said determined mechanical pressure, the first face of the cover layer is deformed until it encapsulates the first lens array, while the first lens array retains its initial shape.
[0152]
[0152] Thus, the first face of the coating layer perfectly matches the shape of the lens array without altering it, which preserves the quality of the optical effects.
[0153]
[0153] In this regard, it has been observed that by laminating a polycarbonate support layer and a zinc sulfide coating layer at a determined temperature slightly above the softening temperature of polycarbonate (approximately 147°C), the lens array retains its initial shape while the first face of the coating layer is deformed in contact with the lens array.
[0154]
[0154] Thus, the use of polycarbonate for the support layer and zinc sulfide for the coating layer makes it possible to coat the lens array without altering its shape, because zinc sulfide deforms at a higher temperature than polycarbonate, while offering sharp and precise optical effects thanks to a refractive index difference greater than 0.5.
[0155]
[0155] It should be noted that it is not always possible to compare the materials of the support layer and the cover layer on the basis of parameters such as the glass transition temperature or the softening temperature, for example.
[0156]
[0156] Indeed, some materials, including zinc sulfide, do not have a clearly defined glass transition or softening temperature.
[0157]
[0157] In an example not shown, the manufacturing process may further include a step of encapsulating the stack of laminated layers in a transparent or semi-transparent substrate, for example polycarbonate.
[0158] Furthermore, various manufacturing techniques are possible in the manufacturing process of Figure 8 to form the lens array on the support layer.
[0158]
[0159] Figure 9 represents a relief rolling device 200 configured to form lens arrays on two opposite faces of a support layer.
[0159]
[0160] Device 200 is thus configured to manufacture a support layer such as that of the optical device according to the second embodiment of the invention.
[0160]
[0161] Device 200 comprises two rollers 202, each having a textured external surface 203.
[0161]
[0162] In an example not shown, the device may include a roller with a textured outer surface and a roller with a smooth outer surface. Such a device is configured to form a lens array on only one face of a support layer, such as that of the optical device according to the first embodiment.
[0162]
[0163] The textured external surface 203 has on its entire periphery a series of ridges 205 and troughs 206 defining a periodic cycloidal profile.
[0163]
[0164] The 205 crests have a convex shape, corresponding to the peaks of the cycloidal profile, while the 206 troughs have a concave shape, corresponding to the low points of the cycloidal profile.
[0164]
[0165] The 202 rollers are configured to rotate in opposite directions (contrarotating) and at synchronized speeds to ensure alignment of the recessed and raised patterns.
[0165]
[0166] The textured external surfaces 203 of the rollers 202 are configured to exert pressure on a material, such as zinc sulfide, as it passes between them and thus imprint a raised or recessed pattern on the material.
[0166]
[0167] The patterns of the rollers (ridges and hollows) interact directly with the material, generating variations in thickness according to their geometry.
[0167]
[0168] In the example shown, the textured external surfaces 203 of the rollers 202 are configured to form thin areas 208 and thick areas 209 on the material.
[0168]
[0169] The thin areas 208 correspond to the areas of the material that pass between the ridges 205, while the thick areas 209 correspond to the areas of the material that pass between the hollows 206 and here form doublets of biconvex lenses.
[0169]
[0170] To do this, the device 200 includes a drive means (not shown), such as an electric motor, and a transmission mechanism (not shown), such as a set of gears or a drive chain, coupled to the drive means.
[0170]
[0171] The alignment of the patterns allows the lenses of the first lens array and the lenses of the second lens array to be precisely aligned to form the lens doublets.
[0172] Each roller 202 may include a heating means configured to heat the textured external surface 203. This may soften the material, and thus facilitate the formation of patterns.
[0171]
[0173] Unillustrated variants are shown below.
[0172]
[0174] The secure document may take a different form, for example in the form of a booklet (in the case of a passport for example).
[0173]
[0175] The lens array can differ from a cylindrical array. For example, the lens array can include lenses with pyramidal, polygonal, spherical, or polyhedral profiles. Furthermore, the lenses can be arranged in various geometric configurations or patterns, such as regular or irregular arrays.
[0174]
[0176] The topcoat can be made of thermosetting polymer, or of photo-polymerizable resin.
[0175]
[0177] The optical device can present an image on which the lens array is configured to converge incident light rays. The image can be formed on the second face of the support layer in an optical device according to the first embodiment, or on the second face of the second covering layer in an optical device according to the second embodiment.
[0176]
[0178] The lens array(s) can be configured so that the focal point is located beyond the optical device, for example to produce an effect when checking another secure document.
[0177]
[0179] The lens array can be formed by laser ablation or by additive manufacturing.
[0178]
[0180] The face of the covering layer could be preformed to match (by complementary shapes) with the lens array, rather than being deformed directly by the lens array.
[0179]
[0181] The coating layer can be poured in liquid form, so as to coat the lens array, and then solidified, for example by UV curing.
[0180]
[0182] In other words, the manufacturing process may include a step of pouring the coating layer in liquid form and a step of solidifying the coating layer by UV cross-linking.
[0181]
[0183] More generally, the invention is not limited to the examples described and shown.
Claims
Demands 1. Optical device for secure document, comprising a stack of layers including: - a transparent support layer (31) with refractive index n0 having a first face (33) and a second face (34), opposite the first face (33), said support layer (31) comprising a first lens array (40) forming the first face (33) of said support layer (31), and - a first transparent covering layer (32) with refractive index n1 encapsulating the first lens array (40) and having a first face (35) and a second face (36), opposite to the first face (35), the first face of the first coating layer being in contact with the first lens array and the refractive index n0 being different from the refractive index n1.
2. Optical device according to claim 1, wherein the difference between the refractive index n0 and the refractive index n1 is between 0.5 and 4.
3. Optical device according to claim 1 or 2, wherein the support layer (31) is made of a first material configured to retain its initial shape when subjected to a determined temperature and / or a determined mechanical pressure, while the first cover layer (32) is made of a second material configured to deform plastically when subjected to said determined temperature and / or said determined mechanical pressure, whereby when the support layer (31) and the first cover layer (32) are laminated at said determined temperature and / or said determined mechanical pressure, the first cover layer (32) is deformed until it encapsulates the first lens array (40), while the first lens array (40) retains its initial shape.
4. Optical device according to any one of claims 1 to 3, wherein the support layer (31) is made of thermoplastic polymer, thermosetting polymer, or photopolymerizable resin.
5. Optical device according to any one of claims 1 to 4, wherein the first coating layer (32) is made of zinc sulfide (ZnS).
6. Optical device according to any one of claims 1 to 5, wherein the first lens array (40) is configured to modify the direction of incident light rays propagating in a direction from the first cover layer (32) to the support layer (31) so as to direct them towards a focal point, depending on the variation of refractive index along said direction.
7. Optical device according to any one of claims 1 to 6, wherein the refractive index n0 is greater than the refractive index n1 and each lens (41) of the first lens array (40) is convex as seen from the first cover layer (32), or the refractive index n0 is less than the refractive index n1 and each lens (41) of the first lens array (40) is concave as seen from the first cover layer (32).
8. Optical device according to any one of claims 1 to 7, wherein the second face (36) of the first covering layer (32) has a different topography from that of the first lens array (40).
9. Optical device according to any one of claims 1 to 8, wherein the support layer (31) comprises a second lens array (50) forming the second face (34) of said support layer (31), each lens (41) of the first lens array (40) being aligned with a respective lens (51) of the second lens array (50) to form a lens doublet.
10. Optical device according to claim 9, wherein the stack of layers comprises a second transparent overlay layer (132) of refractive index n2 encapsulating the second lens array (50) and having a first face (135) and a second face (136), opposite said first face (135), the first face (135) of the second overlay layer (132) being in contact with the second lens array (50), and with the refractive index n2 which is different from the refractive index n0.
11. Secure document comprising a body (2) provided with a window (3) and an optical device (4, 30, 130) according to any one of claims 1 to 10 inserted in said window (3).
12. A method for manufacturing an optical device comprising the following steps: - supplying (101) a transparent support layer of refractive index n0 having a first face and a second face, opposite said second face, said support layer having a first lens array forming the first face of said support layer, - supplying (102) a first transparent coating layer of refractive index n1 having a first face and a second face, opposite said first face, - positioning (104) the first face of the first coating layer opposite the first lens array of the support layer, and - lamination (105) of the first cover layer on the support layer, configured so that the first face of the first cover layer is deformed by the first lens array and comes into contact with the first lens array.