Radar-transparent reflective module, associated system and production method

Abstract

The invention relates to a radar-transparent reflective module (1) for a motor vehicle part, the module comprising a substrate (10), a stack (100) comprising at least one first sub-layer (110) having a first refractive index nL, arranged between two second sub-layers (120) having a second refractive index nH such that nH> nL, the stack being configured to receive at least one incident light beam (2) and to form, by the Fabry-Pérot effect, a reflected light beam (2'), the module being characterised in that it further comprises a waveguide (200) which is positioned on top of the stack and is configured to transmit, to the stack, the incident light beam from a light source and to allow the reflected light beam to pass. The invention also relates to a system comprising the reflective module, to a vehicle part comprising the module or the system, and to a method for producing the reflective module.

Description

Radar-transparent reflective module, associated system and manufacturing method

[0001] The present invention relates to the field of reflective multilayer modules. It finds particularly advantageous application in the field of motor vehicle cladding, illumination or signaling, in particular for front parts of vehicles equipped with radar. STATE OF THE ART

[0002] Today, the front fascias of motor vehicles are designed with multiple aspects in mind, from safety and performance to aesthetics and energy efficiency. For example, especially for electric vehicles, the air intakes conventionally integrated into the front fascia of the vehicle to cool the engine are no longer crucial. This area of ​​the front fascia is currently evolving and can be used for lighting or to integrate sensors such as radars.

[0003] From an aesthetic point of view, the decorative finishes of the vehicle's front fascia, frequently requested by customers, generally include a chrome coating. However, due to its incompatibility with radars and its environmental impact, chrome is increasingly less used. An alternative exists and consists of using reflective multi-layer modules, exploiting the effect of Fabry-Pérot cavities.

[0004] In a Fabry-Pérot cavity, a reflected color called "structural color" appears when light is confined in a nanometric cavity delimited by two substantially parallel surfaces. These modules comprise a stack comprising at least one layer having a low reflection index, arranged between two layers having a higher reflection index. The structure of these layers determines the wavelengths of the reflected light beam that will exit the cavity by an interference phenomenon. These specific wavelengths correspond to a color in the visible spectrum and reach the eyes of an observer. The latter therefore has the impression that the layer of material has a particular color, for example a chrome color. Document US20140049427 A1 describes, for example, a radar-transparent component comprising a Fabry-Pérot cavity. This solution remains limited in practice in terms of optical properties.

[0005] An object of the present invention is therefore to optimize the optical properties, in particular the reflectivity, of a radar-transparent reflective module for a motor vehicle. The invention proposes in particular a solution improving a reflective multilayer module compared to existing solutions, and in particular to make it compatible with an illumination and / or signaling application in a motor vehicle.

[0006] Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0007] To achieve this objective, according to a first aspect of the invention, a radar-transparent reflective module is provided for an automobile part, the module comprising, at least in part superimposed in a z direction: a substrate, a stack comprising at least a first sub-layer having a first refractive index n L , arranged between two second sub-layers having a second refractive index n H such that n H > n L , the stack being configured to receive at least one incident light beam and form by Fabry-Pérot effect a reflected light beam, characterized in that it further comprises a waveguide surmounting the stack in the z direction, the waveguide being configured to transmit to the stack, the incident light beam from a light source, and to allow the reflected light beam to pass.

[0008] The reflective module described above, makes it possible to ensure on the one hand an illumination and / or signaling function thanks to the waveguide which is arranged on a front face of the reflective module, while guaranteeing transparency to the radar, and also to ensure a decorative appearance similar to a chrome finish thanks to the first stack forming a Fabry-Pérot cavity. Indeed, the location of the waveguide in front of the reflective module, makes it possible to effectively illuminate the stack by guiding and transmitting the light beams to the stack coming from a light source, in particular at night in the absence of ambient light. In addition, this waveguide is transparent to the radar and does not disturb the reflected beams produced by the interference at the level of the Fabry-Pérot cavity, which makes it possible to guarantee the decorative optical effect obtained thanks to its interferences.

[0009] A second aspect of the invention relates to a reflective system for an automotive part, the system comprising at least one reflective module.

[0010] A third aspect of the invention relates to a method for manufacturing the reflective module comprising: providing the substrate, providing the waveguide, forming the stack having, in the z direction, an upper surface and a lower surface, the formation of the stack comprising depositing the second sub-layers alternating with the first sub-layer in the z direction on one of the waveguide and the substrate, such that the first sub-layer is arranged between the second sub-layers, and assembling the waveguide, the stack and the substrate.

[0011] A fourth aspect of the invention relates to a motor vehicle part comprising a reflective module or a reflective system. BRIEF DESCRIPTION OF THE FIGURES

[0012] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0013] Schematically illustrates a reflective module according to a first embodiment.

[0014] Schematically illustrates a reflective module according to a second embodiment.

[0015] It represents a reflective system according to an example of realization.

[0016] It represents a motor vehicle part according to an example of embodiment.

[0017] Figures 5A to 5E schematically illustrate steps of the module manufacturing process, according to the first embodiment.

[0018] Figures 6A to 6D schematically illustrate steps of the method of manufacturing the module, according to the second embodiment.

[0019] Schematically illustrates a waveguide according to an example of realization.

[0020] Illustrates a graph representing the transmittance of the reflective module according to different embodiment examples, in comparison with a Fabry-Pérot cavity alone.

[0021] Figures 9A to 9D illustrate graphs representing the reflectance and absorbance of the reflective module as a function of the wavelengths of the visible spectrum for different exemplary embodiments.

[0022] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular, on the schematic diagrams, the thicknesses and / or dimensions of the different layers, patterns and reliefs are not representative of reality. DETAILED DESCRIPTION

[0023] Before commencing a detailed review of embodiments of the invention, optional features which may optionally be used in combination or alternatively are set out below:

[0024] According to one example, the first sub-layer has a thickness e1 between 80 nm and 120 nm.

[0025] This thickness range of the first sub-layer is optimized to enable an improvement in the reflectance of the reflective module over all wavelengths of the visible spectrum. Indeed, depending on the nature of the materials present in the path of the light beams incident on the Fabry-Pérot stack and reflected by the latter, certain visible wavelengths may be partially absorbed. For example, silicon, which is commonly used in waveguides, absorbs blue wavelengths, which gives a yellowish appearance to the module. Consequently, the structure of the stack, and in particular the thickness e1 of the first sub-layer, is optimized in synergy with the presence of the waveguide, to enable an illumination and / or decoration function while obtaining a chrome finish.This thickness range therefore makes it possible to obtain, with such an architecture of the reflective module comprising a waveguide superimposed on the Fabry-Pérot cavity, a reflectance greater than 80% for wavelengths ranging from 400 nm to 650 nm, which makes it possible to obtain a neutral decorative appearance, similar to a chrome finish, sought after for an automotive application.

[0026] According to one example, the waveguide has a thickness e2 between 100 µm and 800 µm.

[0027] Since the thickness of the waveguide is greater than the coherence length of the light beams passing through it, the waveguide does not produce additional interference. Therefore, the location of the waveguide on the stack and on the path of the light beams reflected by the stack does not disturb the decorative optical effect produced by the Fabry-Pérot cavity. In addition, the waveguide thus has a thickness allowing transparency and therefore sufficient transmission of light.

[0028] In one example, the waveguide is configured to have a minimum transparency of around 60% across all wavelengths of the visible spectrum.

[0029] In one example, the first sub-layer is based on SiO2 or MgF2.

[0030] In one example, the first sub-layer is made of SiO2 or MgF2.

[0031] In one example, the second sub-layers are based on or made of a semiconductor material.

[0032] In one example, the second sub-layers are based on Si or Ge.

[0033] In one example, the second sub-layers are made of Si or Ge.

[0034] According to one example, the second sub-layers of the stack each have a thickness e3 of between 15 nm and 40 nm, preferably equal to 25 nm.

[0035] The materials of the sub-layers of the Fabry-Pérot stack are transparent to radar for the indicated thickness ranges. In addition, the SiO2(n L =1.46) and MgF2(n L =1.38) have low refractive indices compared to those of Ge (n H =4.2) and Si (n H=4.5). By placing the lower index sub-layer in the center of the cavity between the higher index sub-layers, an index contrast is created that promotes the multiple reflections and optical interference needed to generate the Fabry-Pérot effect.

[0036] According to an example, the difference between the index n L of the first sub-layer and the index n H of the second sub-layers is between 2 and 6.

[0037] In one example, the stack is arranged between the substrate and the waveguide.

[0038] In one example, the substrate is disposed between the stack and the waveguide, the substrate being configured to pass the incident beam from the waveguide to the stack and the reflected beam from the stack to the waveguide.

[0039] In one example, the stack has, along the z direction, a top surface and a bottom surface. In one example, the reflective module further comprises an opacifying coating covering at least a portion of the bottom surface of the stack.

[0040] The opacifying coating improves the chrome finish under the multi-layer stack by masking the elements underlying the Fabry-Pérot cavity.

[0041] In one example, the opacifying coating is configured to form at least one opacified area at the at least a portion of the bottom surface of the stack, and at least one uncoated area.

[0042] In one example, the opacifying coating is opaque in the visible range. This blocks the transmission of light beams in the visible range.

[0043] In one example, the substrate is based on polycarbonate.

[0044] In one example, the substrate is made of polycarbonate.

[0045] According to one example, the waveguide is multi-layered and comprises third sub-layers alternating in superposition with fourth sub-layers along the z direction.

[0046] In one example, the third sub-layers are based on or made of polycarbonate and the fourth sub-layers are based on or made of silicon (Si).

[0047] According to one example, the waveguide comprises a protective layer overlying the alternation of third and fourth sub-layers along the z direction.

[0048] In one example, the protective layer is based on a polymer.

[0049] In one example, the protective layer is based on polyurethane (PUR).

[0050] For example, the waveguide is equipped with decoupling elements, such as prisms or suspended particles, making it possible to return the light rays propagating within it to at least one of the reflective modules.

[0051] In one example, the stack may include a plurality of first sub-layers alternating with second sub-layers.

[0052] According to one example, the reflective system comprises a plurality of reflective modules juxtaposed in at least one direction parallel to a main extension direction of the reflective modules.

[0053] According to one example, the system further comprises a side light source emitting the incident light beam transmitted by the waveguide to the stack.

[0054] When ambient light is not sufficient to obtain a visible reflection, for example at night, the system is equipped with its own light source to inject a beam into the reflective module and emit a reflected beam. Thus, the system has reduced consumption compared to existing systems using active lighting modules, while allowing good visibility at night.

[0055] In one example, the method further comprises depositing an opacifying coating on at least a portion of the bottom surface of the stack.

[0056] According to one example, the deposition of the second sub-layers alternating with the first sub-layer is carried out on a lower face of the waveguide in the z direction, and the assembly of the waveguide, the stack and the substrate comprises overmolding the substrate on the stack, so that the stack is arranged between the substrate and the waveguide.

[0057] In one example, the method further comprises depositing a protective intermediate layer on the lower surface of the stack or on the opacifying coating.

[0058] In one example, the assembly of the waveguide, stack, and substrate includes overmolding the substrate directly onto the bottom surface of the stack.

[0059] In one example, the assembly of the waveguide, the stack, and the substrate includes overmolding the substrate onto the stack via the opacification coating, the opacification coating being interposed between the stack and the substrate.

[0060] According to one example, the assembly of the waveguide, the stack and the substrate comprises an overmolding of the substrate on the waveguide, and the stack is then assembled on a lower face of the substrate or the deposition of the second sub-layers alternating with the first sub-layer is carried out on said lower face of the substrate, so that the substrate is arranged between the stack and the waveguide.

[0061] In one example, the assembly of the waveguide, stack, and substrate is achieved by directly overmolding the substrate onto the waveguide.

[0062] A substrate or layer "based on" a species A means a substrate or layer comprising this species A only or this species A and possibly other species.

[0063] Several embodiments of the invention implementing successive steps of the manufacturing process are described below. Unless explicitly stated, the adjective “successive” does not necessarily imply, even if this is generally preferred, that the steps follow one another immediately, intermediate steps being able to separate them.

[0064] Furthermore, the term "step" means the carrying out of a part of the process, and can designate a set of sub-steps.

[0065] Furthermore, the term "step" does not necessarily mean that the actions carried out during a step are simultaneous or immediately successive. Certain actions of a first step may in particular be followed by actions linked to a different step, and other actions of the first step may be repeated subsequently. Thus, the term "step" does not necessarily mean actions that are unitary and inseparable in time and in the sequence of phases of the process.

[0066] It is specified that in the context of the present invention, the thickness of a layer or substrate is measured in a direction perpendicular to the surface along which this layer or substrate has its maximum extension. The thickness is thus taken in a direction perpendicular to the main faces of the substrate on which the different layers rest.

[0067] It is specified that, in the context of the present invention, the terms "on", "overcomes", "covers", "underlying", "facing" and their equivalents do not necessarily mean "in contact with". Thus, for example, the deposition, transfer, bonding, assembly or application of a first layer on a second layer does not necessarily mean that the two layers are in direct contact with each other, but means that the first layer at least partially covers the second layer by being either directly in contact with it or by being separated from it by at least one other layer or at least one other element.

[0068] Unless explicitly stated, it is specified that, within the framework of the present invention, the relative arrangement of a third layer or element interposed between a first layer or first element and a second layer or second element does not necessarily mean that these layers are in direct contact with each other, but means that the third layer is either directly in contact with the first and second layers, or separated from them by at least one other layer or at least one other element.

[0069] In the following detailed description, terms such as "longitudinal", "transverse", "upper", "lower" may be used. These terms must be interpreted relatively in relation to the position of the elements of the reflective module or of the system once assembled, by assimilating the direction normal to the main extension plane of the layers of the stack, to the vertical direction. A lateral or transverse dimension is understood as a dimension in a plane parallel or coincident with the main extension plane of the layers of the stack.

[0070] By "juxtaposed" elements we mean here that these elements are arranged side by side according to their main extension plane or arranged one above the other according to the direction of stacking, this direction being perpendicular to the main extension plane.

[0071] By "in contact" we mean that a fine interface may exist, for example caused by manufacturing variability.

[0072] A parameter that is "substantially equal to / greater than / less than" a given value means that this parameter is equal to / greater than / less than the given value, within plus or minus 10% of this value. A parameter that is "substantially between" two given values ​​means that this parameter is at least equal to the smallest given value, within plus or minus 10% of this value, and at most equal to the largest given value, within plus or minus 10% of this value.

[0073] By “nanometric”, and more particularly “nanometric thickness”, we mean a dimension, more particularly a thickness, greater than or equal to 1 nm and strictly less than 1 µm.

[0074] The term "visible spectrum" or "visible range" refers to the wavelength range between 350 and 900 nm.

[0075] The reflective module 1 and the reflective system 3 comprising it are now described according to several exemplary embodiments.

[0076] As illustrated for example, the reflective module 1 comprises a first substrate 10 extending in an xy plane defined by an x ​​direction and a y direction perpendicular to the x direction. The reflective module further comprises a stack 100 extending in a plane parallel to the xy plane.

[0077] The stack 100 is multilayer and comprises a first sublayer 110 arranged between two sublayers 120. The first and second sublayers 110, 120 are superimposed in a z direction perpendicular to the xy plane. The first sublayer 110 has a first refractive index n Lcalled “low index”, and the second sub-layers 120 have a second refractive index n H said "high index" such that n H > n L . This refractive index contrast between the first and second sub-layers 110, 120 makes it possible to form a Fabry-Pérot cavity within the stack 100. Thus, when the stack 100 receives an incident light beam 2, the latter is confined in the cavity because the refractive index contrast of the sub-layers 110, 120 generates multiple reflections on the interfaces between the sub-layers of different indices. After several internal reflections, a reflected light beam 2' emerges from the cavity.

[0078] Multiple reflections inside the Fabry-Pérot cavity give rise to constructive and destructive interference, creating resonance peaks in the reflection or transmission spectrum. When a spectrum of several wavelengths arrives at the Fabry-Pérot cavity, different wavelengths will be reflected and transmitted selectively depending on the cavity resonance conditions for each wavelength. Thus, the cavity structure can be optimized to obtain a colored or neutral rendering of the reflective module by an interference phenomenon. The refractive index contrast, as well as a thickness e1 of the first sub-layer of 110, determine the position and width of the resonance peaks in the reflection or transmission spectrum of the cavity.

[0079] The reflective module 1 further comprises a waveguide 200 which surmounts the stack 100 in the z direction. The waveguide 200 is configured to receive an incident light beam 2 and transmit it to the stack 100. This incident light beam 2 preferably comes from a light source. Indeed, the light beams incident on the stack 100 may come from ambient light, such as sunlight during the day for example, or from a light source, such as a light-emitting diode or “LED” (from the English Light-Emitting Diode) at night for example.

[0080] The waveguide 200 is configured to allow the incident light beam 2 from the ambient light to pass towards the stack 100 as well as the reflected light beam 2' produced by the interference within the stack 100. The location of the waveguide 200 relative to the stack 100 therefore makes it possible to effectively illuminate the stack by guiding and / or transmitting the incident light beams, without disturbing the transmission of the light beams reflected by the stack 100, and therefore without disturbing the decorative optical effect produced by the Fabry-Pérot effect.

[0081] As described previously, this decorative effect produced by the Fabry-Pérot effect depends on several parameters, in particular the thickness e1 of the first sub-layer 110. This thickness e1 can advantageously be between 80 nm and 120 nm, and preferably substantially equal to 90 nm. This thickness range makes it possible to improve the reflectance of the reflective module 1 over all the wavelengths of the visible spectrum. Indeed, although the waveguide 200 allows the transmission of the light beams 2, 2', certain wavelengths of the visible spectrum such as blue for example, can be partially absorbed depending on the nature of the materials present in the path of these light beams.By modifying the thickness e1 of the first sub-layer 110 in synergy with the presence of the waveguide 200, the absorption of these wavelengths can be compensated by the Fabry-Pérot cavity, which makes it possible to neutralize the apparent color of the reflective module and approach a chrome finish.

[0082] The waveguide 200 may be multilayer, and has a thickness e2 which is preferably greater than the coherence length of the light beams passing through the waveguide 200. The thickness e2 of the waveguide 200 may be between 100 µm and 800 µm. This makes it possible to avoid additional interference within the waveguide so as not to disturb the decorative optical effect produced by the stack 100.

[0083] This thickness e2 further allows the waveguide 200 to be transparent to a wide range of wavelengths in the visible spectrum, as well as to radars.

[0084] Figures 1 and 2 illustrate two different embodiments of the reflective module 1.

[0085] As illustrated in the, according to the first embodiment, the stack 100 can be arranged between the substrate 10 and the waveguide 200. The waveguide 200 thus transmits the incident light beams directly to the underlying stack 100.

[0086] As illustrated in, according to the second embodiment, the substrate 10 can be arranged between the stack 100 and the waveguide 200. The substrate 10 in this case is preferably configured to allow the incident light beams 2 from the waveguide 200 to pass through to the stack 100 as well as the reflected light beams 2' from the stack 100 to the waveguide 200. For this, the substrate 10 is at least partly transparent to light in the visible range. For example, the substrate has a transmission greater than 80%, preferably 85% of visible light.

[0087] The stack 100 has, along the z direction, an upper surface 100a and a lower surface 100b. The reflective module 1 may further comprise an opacifying coating 30 covering at least a portion 31 of the lower surface 100b of the stack 100. The opacifying coating 30 makes it possible to improve the chrome rendering under the multilayer stack 100 by masking the elements underlying the Fabry-Pérot cavity. In the two embodiments shown in FIGS. 1 and 2, the portion 31 extends over the entire lower surface 100b in the xy plane. According to an example not illustrated, the portion 31 may extend over only a portion of the lower surface 100b in the xy plane.

[0088] The opacifying coating 30 may be configured to form an opacified area at least on a portion of the lower surface of the stack, and at least one uncoated area. The opacifying coating 30 may be opaque to wavelengths of the visible spectrum, thereby blocking the transmission of light beams in the visible range. The maximum transmission in the visible spectrum of the opacifying coating may be of the order of 10%. The opacifying coating 30 is preferably based on a material that is not very conductive or even insulating, thereby advantageously making it transparent to radars.

[0089] A particular example of system 3 is now described with reference to the.

[0090] The system 3 may comprise at least one reflective module 1, and preferably several reflective modules 1. As illustrated in, according to one example, the system 3 may comprise a plurality of reflective modules 1 juxtaposed along at least one so-called "juxtaposition" direction, parallel to or coincident with a main extension direction of these modules 1. Preferably, the reflective modules 1 are juxtaposed along at least two so-called "juxtaposition" directions of a plane parallel to or coincident with a main extension plane of these modules 1. The system 3 may for example comprise at least five juxtaposed reflective modules 1, preferably at least five juxtaposed reflective modules 1 per juxtaposition direction, as illustrated in.

[0091] From the above description, it is understood that the reflective module 1 can reflect an incident beam 2 coming from the environment outside the stack 100, for example ambient light. In a dimly lit environment, for example at night, it may be advantageous to retain a display or signaling function by the reflective module(s) 1. For this, and as illustrated in, the reflective system 3 may comprise at least one light source 20, preferably lateral. The light source 20 is configured to emit a light beam 2. According to this example, it is therefore understood that the reflective module 1 can reflect an incident beam 2 coming from the environment and / or a light beam 2 coming from the light source 20. Each waveguide 200 is configured to transmit the light beam 2 coming from the light source 20 to the stack 100 of each of the corresponding reflective modules 1.For this, the waveguide 200 may comprise internal total reflection elements configured to conduct the light beam 2 from the source. Depending on the angle of reflection of the beam in the waveguide 200, the beam 2 from the source 20 may be transmitted to a stack 100 or continue its propagation towards the neighboring reflective module 1.

[0092] According to a non-illustrated example, a waveguide 200 may be common to the reflective modules 1 of the system 3.

[0093] For example, the waveguide 200 may comprise prisms configured to modify the optical path of a portion of the beam 2 coming from the source 20 to send it to the corresponding stack 100. A person skilled in the art is able to produce a waveguide in accordance with the arrangement of one or more reflective modules 1. The prisms may for example be arranged at regular intervals along the waveguide 200, in accordance with the juxtaposition of the reflective modules 1. Other structures may be provided as an alternative or in addition to the prisms by a person skilled in the art, for example suspended particles.

[0094] The reflective module 1 or the system 3 comprising it may be incorporated into parts 4 such as car parts. Illustrated by way of example is a front part 4 of a car front hood comprising the reflective system 3 in the center. Provision may be made for the reflective module or the reflective system 3 to be incorporated into other parts, for example inside the passenger compartment or on other parts of the bodywork.

[0095] The method of manufacturing a reflective module 1 is now described with reference to FIGS. 5A to 6D according to different exemplary embodiments.

[0096] The method for manufacturing the reflective module 1 comprises providing the substrate 10 and providing the waveguide 200. The method further comprises forming the stack 100. Forming the stack 100 comprises depositing the second sub-layers 120 alternating with the first sub-layer 110 along the z direction so that the first sub-layer 110 is arranged between the second sub-layers 120. This deposition of the sub-layers 110, 120 may be carried out on the waveguide 200 according to the first embodiment, as illustrated in FIGS. 5A to 5E, or on the substrate 10 according to the second embodiment, as illustrated in FIGS. 6A to 6D.

[0097] The method further comprises an assembly of the waveguide 200, the stack 100 and the substrate 10. This assembly step is not necessarily a single step but may comprise assembly sub-steps, which may take place at separate steps of the method. For example, according to the first embodiment, the formation of the stack 100 may be carried out on the waveguide 200, then the entire waveguide and the stack 100 is assembled with the substrate 10. According to the second embodiment, the waveguide 200 and the substrate 100 are assembled first, then the stack 100 is formed on the entire waveguide 200 and the substrate 10.

[0098] As illustrated in the, according to the first embodiment, a waveguide 200 is provided. This waveguide 200 can be manufactured by methods known to those skilled in the art. As illustrated in the, the method comprises the deposition of the second sub-layers 120 alternating with the first sub-layer 110 on a lower face of the waveguide 200 in the z direction, to form the stack 100 having an upper surface 100a in contact with the lower face of the waveguide 200, and an exposed lower surface 100b. This deposition can for example be carried out by a physical vapor deposition (PVD) method, also known as vacuum deposition. This deposition technique is used to deposit thin layers of materials. According to one example, the lower surface 100b of the stack 100 can for example be structured by laser ablation. This helps improve the adhesion of the opacifying coating.

[0099] As illustrated in, according to one example, the opacification coating 30 can be deposited on at least a portion 31 of the lower surface 100b of the waveguide 200. According to one example, an intermediate layer 40 can be deposited on a lower face 31a of the opacification coating 30. This intermediate layer 40 makes it possible to protect the opacification coating 30 for the step of assembly with the substrate 10.

[0100] As illustrated in , the waveguide 200, the stack 100 deposited on the waveguide 200 and the substrate 10 are then assembled by overmolding the substrate 10 on the stack 100 so that the stack 100 is arranged between the substrate 10 and the waveguide 200. Preferably, the overmolding of the substrate 10 on the stack 100 is done on the intermediate layer 40 so that it is arranged between the stack 100 and the substrate 10. A reflective module 1 can thus be obtained according to the first embodiment, as illustrated in .

[0101] As illustrated in , according to the second embodiment, a waveguide 200 and a substrate 10 are provided. As illustrated in , the substrate 10 is overmolded on the lower face of the waveguide 200. As illustrated in , the stack 100 can then be assembled on a lower face of the substrate 10 for example.

[0102] According to another preferable example, the stack 100 may be formed by depositing the second sub-layers 120 alternately with the first sub-layer 110 on the lower face of the substrate 100, so that the substrate 10 is arranged between the stack 100 and the waveguide 200. This deposition may be carried out by the PVD method. According to this embodiment, the deposition of the thin sub-layers on the substrate 10 makes it possible to avoid mechanical stress within the waveguide 200. As described previously and as illustrated in , the opacification coating 30 may be deposited on at least a portion 31 of the lower surface 100b of the stack 100. A reflective module 1 may thus be obtained according to the second embodiment, as illustrated in .

[0103] According to one example, the stack 100 may comprise a plurality of first sub-layers 110 alternating with second sub-layers 120.

[0104] The first sub-layer 110 of the stack 100 is preferably based on a dielectric. The first sub-layer 110 may, for example, be based on SiO2 or MgF2. It may also be made of SiO2 or MgF2. The second sub-layers 120 may be based on Si or Ge, or may be made of Si or Ge.

[0105] SiO2(n L =1.46) and MgF2(n L =1.38) have low refractive indices compared to those of Ge (n H =4.2) and Si (n H =4.5). By placing the lower index 110 sub-layer in the center of the cavity between the higher index 120 sub-layers, an index contrast is created that promotes the multiple reflections and optical interference needed to generate the Fabry-Pérot effect.

[0106] According to one example, the second sub-layers of the stack each have a thickness e3 of between 15 nm and 40 nm, preferably equal to 25 nm.

[0107] These materials of the sub-layers 110, 120, of the stack 100 are preferably transparent to radar for the indicated thickness ranges.

[0108] As illustrated in the, the waveguide 200 may be multi-layered, and may comprise third sub-layers 210 alternating in superposition along the z direction, with fourth sub-layers 220. The waveguide 200 may further comprise a protective layer 230 overlying the alternation of third and fourth sub-layers 210, 220, along the z direction. This protective layer 230 makes it possible to protect the reflective module 1 from the external environment.

[0109] The third sub-layers 210 may, for example, be based on or made of polycarbonate, and the fourth sub-layers 220 may be based on or made of silicon. According to one example, the protective layer 230 may be based on a polymer, such as polyurethane (PUR).

[0110] All of the materials of the reflective module 1 are advantageously non-conductive or even dielectric, which makes it possible to guarantee transparency of the reflective module 1 to radar, the conductive materials not being transparent to radar waves.

[0111] Silicon is not an electrically conductive material; it has an intrinsic dielectric permittivity, unlike elements in the metalloid family, which are not intrinsically compatible with radar waves. As long as the thickness of silicon is ten times less than the radar wavelength (between 76 GHz and 81 GHz), its effect on the radar wave can be neglected. The permittivity of silicon at 77 GHz is about 11.75. This is about 0.11 mm of maximum thickness for silicon.

[0112] The silicon present in the reflective module 1, in the waveguide 200 or in the stack 100, can furthermore absorb at least partially the blue wavelength of the reflected light beams, which can give a yellowish appearance to the reflective module 1. This yellowish appearance can occur in both embodiments.

[0113] As described previously, to avoid this yellowish appearance and compensate for the absorption of blue radiation, the thickness e1 of the first sub-layer 110 is chosen between 80 nm and 120 nm. This thickness range makes it possible, in synergy with the presence of the waveguide, to substantially flatten the reflectivity curve as a function of the wavelengths of the visible spectrum. Illustrates a graph representing the transmittance 51 calculated as a function of the wavelengths 52 of the visible spectrum, for a reflective module 1 according to the two embodiments and for a stack 100 without a waveguide 200.As shown, the comparison of the transmittance curve in the visible spectrum of a stack 100 alone (T1), with that of the reflective module 1 according to the first embodiment (T2) and that of the reflective module 1 according to the second embodiment (T3), shows that by superimposing the waveguide 200 on the stack 100, the transmission curves T2, T3, are more flattened between 400 nm and 650 nm. This comes from the linear absorption coefficient of materials such as PC, PMMA or even optical silicone which preferentially absorb blue. The difference between the reflectance at 400 nm and at 650 nm for the stack 100 alone is of the order of 16%, for a reflective module 1 according to the first embodiment is of the order of 10% and for a reflective module 1 according to the second embodiment is of the order of 8%.

[0114] According to the first embodiment, the light beams pass through only the waveguide 200 to reach the stack 100. On the other hand, according to the second embodiment, the light beams pass through the substrate 10 in addition to the waveguide 200 to reach the stack 100. The substrate 10 may have a thickness of the order of millimeters, and may be based on polycarbonate. This substrate 10 having a thickness greater than the coherence length of the light beams, does not produce additional interference, but can absorb an additional portion of the blue light beams passing through it.

[0115] To avoid this additional absorption of blue radiation, the thickness e1 of the first sub-layer 100 can be modified in the range described, in order to compensate for the absorption of blue radiation by the substrate 10 of the reflective module according to the second embodiment. A reflectance greater than 80%, or even greater than 85%, can thus be obtained for a wavelength spectrum between 400 nm and 650 nm regardless of the angle of incidence of the incident light beams. The reflective module 1 thus has an optical appearance similar to a chrome finish with a slight blue shade according to one example.

[0116] As illustrated in the graphs of Figures 9A to 9D, calculations of the absorbance, and in particular of the reflectance of the reflective module 1, expressed as a percentage 53 as a function of the wavelengths 52 of the visible spectrum for different embodiments, are represented by the absorbance curves A1 to A4 and reflectance curves R1 to R4. These calculations are carried out for second sub-layers 120 made of silicon and having a thickness e3 equal to 25 nm and for a first sub-layer 110 made of SiO2 and having thicknesses e1 ranging from 30 nm to 120 nm. For a thickness e1 equal to 30 nm (), a reflectance curve R1 having significant variations is obtained. For a thickness e1 equal to 60 nm (), a flatter reflectance curve R2 is obtained. For a thickness e1 equal to 90 nm () and 120 nm (), increasingly flattened R3 and R4 curves are respectively obtained.Note that from 120 nm, the R4 reflectance curve begins to show considerable variations around 400 nm.

[0117] Curves R1 to R4 clearly show the evolution of the reflectance in the visible range as a function of the thickness e1 of the first sub-layer 110, and show that for the reflective module 1 described above, the thickness range e1 described (between 80 nm and 120 nm), gives rise to a more flattened reflectance curve. This makes it possible to obtain a reflective module 1 which provides an illumination function and in particular signaling during the day and during the night, while guaranteeing a decorative effect similar to a chrome finish.

[0118] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention. Various particular examples of manufacturing methods as well as configurations of reflective modules for a vehicle have been described. Many other variant embodiments are possible, for example by combining previously described features, without departing from the scope of the invention. Furthermore, the features described in relation to one aspect of the invention may be combined with another aspect of the invention.

Claims

Radar-transparent reflective module (1) for an automotive part, the module comprising, at least partly superimposed in a z direction: a substrate (10), a stack (100) comprising at least a first sub-layer (110) having a first refractive index n L , arranged between two second sub-layers (120) having a second refractive index n H such that n H > n L , the stack (100) being configured to receive at least one incident light beam (2) and form by Fabry-Pérot effect a reflected light beam (2'), characterized in that it further comprises a waveguide (200) surmounting the stack (100) in the z direction, the waveguide (200) being configured to transmit to the stack (100), the incident light beam (2) coming from a light source, and to allow the reflected light beam (2') to pass. Reflective module (1) according to the preceding claim, in which the first sub-layer (110) has a thickness e1 of between 80 nm and 120 nm. Reflective module (1) according to any one of the preceding claims, in which the waveguide (200) has a thickness e2 of between 100 µm and 800 µm. Reflective module (1) according to any one of the preceding claims, wherein the first sub-layer (110) is based on SiO2 or MgF2. Reflective module (1) according to any one of the preceding claims, wherein the second sub-layers (120) are based on Si or Ge. Reflective module (1) according to any one of the preceding claims, wherein the stack (100) is arranged between the substrate (10) and the waveguide (200). Reflective module (1) according to any one of claims 1 to 5, wherein the substrate (10) is arranged between the stack (100) and the waveguide (200), the substrate (10) being configured to allow the incident beam (2) to pass from the waveguide (200) to the stack (100) and the reflected beam (2') from the stack (100) to the waveguide (200). Reflective module (1) according to any one of the preceding claims, wherein the stack (100) having, in the z direction, an upper surface (100a) and a lower surface (100b), the reflective module (1) further comprising an opacifying coating (30) covering at least a portion (31) of the lower surface (100b) of the stack (100). Reflective module (1) according to any one of the preceding claims, wherein the substrate (10) is based on polycarbonate. Reflective system (3) for an automotive part, the system comprising at least one reflective module (1) according to any one of the preceding claims. System (3) according to the preceding claim, comprising a plurality of said reflective modules (1) juxtaposed in at least one direction parallel to a main direction of extension of said reflective modules (1). System (3) according to any one of the two preceding claims, further comprising a lateral light source (20) emitting the incident light beam (2) transmitted by the waveguide (200) to the stack (100). A method of manufacturing the reflective module (1) according to any one of claims 1 to 9, comprising: providing the substrate (10), providing the waveguide (200), forming the stack (100) having, in the z direction, an upper surface (100a) and a lower surface (100b), the formation of the stack (100) comprising depositing the second sub-layers (120) alternating with the first sub-layer (110) in the z direction on one of the waveguide (200) and the substrate (10), such that the first sub-layer (110) is arranged between the second sub-layers (120), and assembling the waveguide (200), the stack (100) and the substrate (10). Method for manufacturing a reflective module (1) according to the preceding claim, in which the deposition of the second sub-layers (120) alternating with the first sub-layer (110) is carried out on a lower face of the waveguide (200) in the z direction, and the assembly of the waveguide (200), the stack and the substrate (10) comprises overmolding the substrate (10) on the stack (100), so that the stack (100) is arranged between the substrate (10) and the waveguide (200). A method of manufacturing a reflective module (1) according to claim 13, wherein the assembly of the waveguide (200), the stack (100) and the substrate (10) comprises overmolding the substrate (10) onto the waveguide (200), and the stack (100) is then assembled on a lower face of the substrate (10) or the deposition of the second sub-layers (120) alternating with the first sub-layer (110) is carried out on said lower face of the substrate (100), so that the substrate (10) is arranged between the stack (100) and the waveguide (200). Motor vehicle part (4) comprising a reflective module (1) according to any one of claims 1 to 9 or a reflective system (3) according to any one of claims 10 to 12.

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