Optical device provided with a liquid crystal screen

Abstract

The invention relates to an optical device (3) for a motor vehicle, the device comprising a liquid crystal layer (4), a first layer (5) of an electrically conductive coating, and a second layer (6) of an electrically conductive coating, the first layer and the second layer being separate layers arranged on either side of the liquid crystal layer, the first layer comprising a first positive terminal (P1) and the second layer comprising a first negative terminal (N1), the first positive terminal and the first negative terminal being intended to be connected to a first electric current source (9) so as to generate an electric field through the liquid crystal layer, at least one layer from between the first layer and the second layer comprising a second positive terminal (P2) and a second negative terminal (N2).

Description

Optical device equipped with a liquid crystal display Technical field of the invention

[0001] The invention relates to an optical device for a motor vehicle, the optical device comprising a liquid crystal screen, in particular of the PDLC (acronym for "Polymer Dispersed Liquid Crystal") type. The invention also relates to a luminous device for a motor vehicle comprising such an optical device. The invention also relates to a method of using such an optical device. State of the prior art

[0002] Liquid crystal displays are optical devices whose transparency varies depending on an electric field passing through them. Liquid crystal displays can perform a wide variety of optical functions and are therefore of interest to players in the automotive industry. For example, we know of motor vehicles equipped with a panoramic roof fitted with a PDLC liquid crystal display. The transparency level of the panoramic roof can be easily controlled, which allows different atmospheres to be created within the passenger compartment.

[0003] However, state-of-the-art liquid crystal displays do not operate at low temperatures. This constraint limits the deployment of liquid crystal displays in the automotive industry, since motor vehicles must retain all their functionality at low temperatures, particularly at negative temperatures, or even at -20°C or -40°C.

[0004] In addition, automotive liquid crystal displays must remain simple, compact and easy to manufacture. Presentation of the invention

[0005] The aim of the invention is to provide an optical device provided with a liquid crystal screen, for a motor vehicle, overcoming the above drawbacks and improving the optical devices known from the prior art.

[0006] More specifically, a first object of the invention is a simple, compact optical device, easy to manufacture and capable of operating at low temperature.

[0007] The invention relates to an optical device for a motor vehicle, comprising a layer comprising liquid crystals, a first layer of an electrically conductive coating, and a second layer of an electrically conductive coating, said first layer and said second layer being two separate layers arranged on either side of the layer comprising liquid crystals, said first layer comprising a first positive terminal and said second layer comprising a first negative terminal, the first positive terminal and the first negative terminal being intended to be connected to a first source of electric current so as to produce an electric field passing through the layer comprising liquid crystals,at least one layer among said first layer and said second layer comprising a second positive terminal and a second negative terminal intended to be connected to a second source of electric current so as to cause an electric current to flow in this layer, between the second positive terminal and the second negative terminal.,

[0008] The first positive terminal and the second positive terminal may form a single positive terminal, or the first negative terminal and the second negative terminal may form a single negative terminal.

[0009] The first positive terminal, the second positive terminal, the first negative terminal and the second negative terminal can be four separate terminals.

[0010] The layer comprising liquid crystals may be a layer comprising liquid crystals dispersed in a polymer, and / or said first layer and / or said second layer may be a layer of indium tin oxide.

[0011] The optical device may include a first electrical current source electrically connected to the first positive terminal and the first negative terminal, and a second electrical current source electrically connected to the second positive terminal and the second negative terminal.

[0012] The first current source can be an alternating current source

[0013] The second current source can be a direct current source or alternatively an alternating current source.

[0014] The second positive terminal and the second negative terminal may be spaced apart from each other by a distance greater than or equal to 50% of a largest dimension of the electrically conductive coating layer comprising them.

[0015] The layer comprising the second positive terminal and the second negative terminal may comprise a rectangular shape, wherein the second positive terminal and / or the second negative terminal extends along at least 50% of a short side of the rectangular shape.

[0016] The invention also relates to a lighting device for a motor vehicle, the lighting device comprising a light source and an optical device as defined previously, the optical device being arranged so as to receive light rays from the light source.

[0017] The invention also relates to a method for controlling an optical device as defined above or a light device as defined above, the control method comprising: - the detection of a temperature less than or equal to a threshold, then - the circulation of an electric current between the second positive terminal and the second negative terminal to heat the electrically conductive coating layer comprising the second positive terminal and the second negative terminal. Presentation of figures

[0018] These objects, characteristics and advantages of the present invention will be explained in detail in the following description of a particular embodiment made without limitation in relation to the attached figures among which: La is a schematic sectional view of a light device according to an embodiment of the invention. La is a schematic sectional view of an optical device of the light device of the. La is a schematic top view of the optical device of the. La is a schematic sectional view of an optical device according to an alternative embodiment of the invention. La is a schematic top view of the optical device of the. Detailed description

[0019] Schematically illustrates a lighting device 1 for a motor vehicle according to one embodiment of the invention. The lighting device 1 can be positioned anywhere in the vehicle, for example at the front or rear of the vehicle. The lighting device 1 can participate in the light signature of the vehicle. The lighting device 1 comprises a light source 2 and an optical device 3 according to one embodiment of the invention. The light source 2 can comprise, for example, one or more light-emitting diodes and / or one or more incandescent bulbs. The optical device 3 is arranged so as to receive light rays R from the light source 2. These light rays R can form a light beam centered on an optical axis X. The light rays R can be emitted in any area of ​​the visible spectrum.

[0020] The optical device 3 comprises a layer comprising liquid crystals 4, a first layer 5 of an electrically conductive coating, and a second layer 6 of an electrically conductive coating. The layer comprising liquid crystals 4, hereinafter referred to as the liquid crystal layer 4, is a screen extending generally in a plane. This plane may be substantially perpendicular to the optical axis X. The first layer 5 and the second layer 6 are two separate layers arranged on either side of the liquid crystal layer 4. The first layer 5 and the second layer 6 therefore extend respectively over two opposite faces of the liquid crystal layer 4. In particular, the two opposite faces of the liquid crystal layer 4 are completely covered respectively by the first layer 5 and by the second layer 6.The first layer 5 may face the light source 2 and the second layer 6 may be arranged on the side opposite the light source 2. Alternatively, the positioning of the first layer and the second layer could be reversed.

[0021] The liquid crystal layer 4 is capable of modifying the trajectory of light rays passing through it as a function of an electric field applied to it. In particular, the liquid crystal layer 4 is capable of modifying its level of transparency as a function of an electric field applied to it. The liquid crystal layer 4 may comprise a crystalline liquid whose elongated molecules take an ordered orientation when the liquid crystal layer is subjected to a given electric field. The ordered orientation of the molecules makes the crystalline liquid transparent. Conversely, when the liquid crystal layer is not subjected to an electric field, the molecules may orient themselves according to a nematic, cholesteric or smectic configuration which makes the crystalline liquid opaque or translucent.These properties of the liquid crystal layer 4 can be obtained only from a given temperature, for example when the liquid crystal layer 4 is at a temperature greater than or equal to 0°C. According to a preferred embodiment, the liquid crystal layer 4 is a layer of liquid crystals, possibly in the form of birefringent droplets, dispersed in a polymer matrix, commonly called PDLC (acronym for the English term "Polymer Dispersed Liquid Crystal"). The thickness of the liquid crystal layer 4 can typically be between 15 µm and 30 µm.

[0022] The first layer 5 and the second layer 6 are configured to apply an electric field to the liquid crystal layer 4. The first layer 5 and the second layer 6 are capable of conducting electricity but still have an intrinsic electrical resistance. The first layer 5 and the second layer 6 may be generally transparent. These layers may optionally include a slight coloring. In particular, the first layer 5 and the second layer 6 may be layers of indium tin oxide ITO, commonly referred to as ITO (acronym for the English term "indium tin oxide"). The thickness of the first layer 5 and / or the second layer 6 may typically be between 30nm and 50nm.

[0023] The first layer 5 and the second layer 6 may be covered on their free face respectively with a layer 7, 8 of transparent and electrically insulating material, in particular a layer of thermoplastic saturated polyester, in particular a layer of poly(ethylene terephthalate), commonly called PET. These layers may be obtained by a thermally induced phase separation process or by polymerization (process commonly called by the acronym PIPS meaning "polymerization induced phase separation"). The layers 7, 8 of thermoplastic saturated polyester may form a protective envelope enveloping both the liquid crystal layer 4, the first layer 5 and the second layer 6. The layers 7 and 8 may also constitute a support on which the first layer 5 and the second layer 6 are deposited respectively.As a remark, the different layers represented in figures 1, 2 and 4 are represented schematically and their apparent thickness may be disproportionate.

[0024] The second layer 6 comprises a first positive terminal P1 and the first layer 5 comprises a first negative terminal N1. The first positive terminal P1 and the first negative terminal N1 are connected to a first electric current source 9, so as to produce an electric field passing through the liquid crystal layer 4, in particular in order to orient the molecules which compose the liquid crystal layer. The electric field thus obtained can be oriented substantially parallel to the optical axis X. This electric field is represented by arrows F1 in Figures 1, 2, and 4. Preferably, the first current source 9 can be an alternating current source. The potential difference between the first positive terminal P1 and the first negative terminal N1 can be between 70V and 120V peak-to-peak, when an electric field is established.

[0025] Furthermore, as illustrated in the, the second layer 6 comprises a second positive terminal P2 and a second negative terminal N2. In this case, the first positive terminal P1 and the second positive terminal P2 form a single positive terminal. The second positive terminal P2 and the second negative terminal N2 are connected to a second electric current source 10, configured so as to circulate an electric current in this second layer 6, between the second positive terminal P2 and the second negative terminal N2. This electric current is represented by an arrow F2 in FIGS. 1, 2 and 4. Preferably, the second current source 10 may be a direct current source. The potential difference between the second positive terminal P2 and the second negative terminal N2 may be determined as a function of the dimensions of the liquid crystal layer.It can reach, for example, around ten volts, or even more, when an electric current flows between the second positive terminal P2 and the second negative terminal N2.

[0026] The positive terminals P1 and P2 and the negative terminals N1 and N2 are therefore three separate electrical terminals electrically connected to the first layer 5 or to the second layer 6. These electrical terminals can be obtained, for example, by soldering an electrical wire or by using an adhesive.

[0027] The dimensions along at least one axis perpendicular to the optical axis X of the first layer 5 and / or of the second layer 6 may be strictly greater than a dimension of the liquid crystal layer 4 along this same axis. This results in portions 11, 12 of the first layer 5 and respectively of the second layer 6 which are offset relative to the liquid crystal layer 4. These portions 11, 12 provide contact surfaces on which the electrical terminals P1, P2, N1, N2 may be arranged. Alternatively or additionally, the layer 7 and / or 8 of transparent and electrically insulating material may be locally perforated so as to reveal a portion 13 of the first layer 5 and / or of the second layer 6 on which an electrical terminal is arranged.

[0028] By a surprising effect, the electric current produced by the second current source 10 heats the second layer 6. By thermal conduction, the liquid crystal layer 4 can thus be brought to a sufficient temperature to ensure its proper functioning. The second layer 6 thus forms a means of heating the liquid crystal layer 4. Advantageously, this heating means is formed by a layer of an electrically conductive coating whose presence is already required to produce an electric field in the thickness of the liquid crystal layer 4. It is therefore simply sufficient to provide at least one additional electrical terminal and a second source of electric current to form this heating means.

[0029] The first current source 9 and the second current source 10, as well as a temperature sensor 14 are electrically connected to an electronic control unit 15. As a note, the first current source 9, the second current source 10 and the electronic control unit 15 may possibly be integrated in the same housing. The electronic control unit 15 is configured to control the second current source 10 as a function of a temperature detected by the temperature sensor 14. A method for controlling the optical device 3 may be implemented in the following manner: Firstly, a temperature less than or equal to a threshold is detected with the temperature sensor 14, for example a threshold of 0°C, -10°C or even -20°C. Then, the electronic control unit 15 controls the second current source 10 so that it delivers an electric current.This electric current flows between the second positive terminal P2 and the second negative terminal N2 and allows the second layer 6 to be heated.

[0030] Advantageously, the second positive terminal P2 and the second negative terminal N2 are positioned substantially at two opposite ends of the second layer 6. They may be separated from each other by a distance greater than or equal to at least 50% of the largest dimension of the second layer, or even at least 75% of the largest dimension of the second layer. Thus, the production of heat is well distributed over the entire length of the second layer 6.

[0031] As can be seen in Figures 2 and 3 illustrating the first embodiment of the optical device 3, the second layer 6 comprises the first positive terminal P1, the second positive terminal P2 and the second negative terminal N2. The first positive terminal P1 and the second positive terminal P2 form a single positive terminal. The first layer 5 comprises only the first negative terminal N1. The first layer and the second layer 6 may comprise a generally rectangular shape. Advantageously, the second positive terminal P2 and the second negative terminal N2 extend respectively along the two opposite short sides of the rectangular shape. Thus, the distance between the terminals P2 and N2 is maximum and the heating obtained is optimized.Preferably, the second positive terminal and / or the second negative terminal extend respectively along at least 50% of the short side of the rectangular shape, preferably at least 75% of the short side of the rectangular shape, or even the entire short side of the rectangular shape. By increasing the surface area of ​​the second positive terminal P2 and / or the second negative terminal N2, a better distribution of the electron flow in the second layer 6 and thus more powerful heating is obtained.

[0032] According to an alternative embodiment not shown, the positive terminals would be transformed into negative terminals and the negative terminals would be transformed into positive terminals. This would result in an optical device in which the first negative terminal and the second negative terminal form a single electrical terminal. According to another alternative embodiment, the second positive terminal and the second negative terminal could be arranged on the first layer 5.

[0033] Figures 4 and 5 illustrate another alternative embodiment of the optical device 3. For the description of this embodiment, the same references are used as for the description of the first embodiment in order to designate the same objects. In addition, the aim is to describe only the differences compared to the first embodiment. This second embodiment differs from the first embodiment described mainly in that the first positive terminal P1 is distinct from the second positive terminal P2. The optical device thus comprises four distinct electrical terminals P1, P2, N1 and N2. An advantage of this second embodiment could be to make it possible to establish an electric field between the first positive terminal P1 and the first negative terminal N1 simultaneously with the circulation of an electric current between the second positive terminal P2 and the second negative terminal N2.It would thus be possible to heat the liquid crystal layer while producing an electric field suitable for making the liquid crystal layer transparent. However, this embodiment requires the use of an additional electrical terminal compared to the first embodiment. Advantageously, the first positive terminal P1 is not positioned between the second positive terminal P2 and the second negative terminal N2. Thus, the circulation of an electric current between the second positive terminal P2 and the second negative terminal N2 does not disturb the electric field between the first layer 5 and the second layer 6. As previously, and according to another variant embodiment not shown, the positive terminals and the negative terminals could be interchanged.

[0034] According to yet another variant (not shown), the first layer could comprise a first positive terminal and a first negative terminal, and the second layer could comprise a second positive terminal and a second negative terminal. The first layer could form a first heating means by circulating an electric current between the first positive terminal and the first negative terminal. The second layer could form a second heating means by circulating an electric current between the second positive terminal and the second negative terminal. The electric field passing through the thickness of the liquid crystal layer 4 could be obtained by means of a current source connected on the one hand to the first positive terminal and / or to the first negative terminal, and connected on the other hand to the second positive terminal and / or to the second negative terminal.

[0035] Finally, with each of the embodiments presented, an optical device 3 is obtained that is capable of operating at low temperatures, in particular at temperatures lower than or equal to -20°C. The heating means integrated into the optical device is partly based on existing components. The optical device 3 thus remains simple, compact, and easy to manufacture. When integrated into a lighting device, the optical device makes it possible to vary the light signature of a vehicle according to different circumstances, in particular according to an external brightness level. The optical device 3 that has just been described can also be integrated into a motor vehicle independently of any light source. The optical device 3 can, for example, be used to opacify or to make transparent a glazed surface of the vehicle.

Claims

Optical device (3) for a motor vehicle, comprising a layer comprising liquid crystals (4), a first layer (5) of an electrically conductive coating, and a second layer (6) of an electrically conductive coating, said first layer and said second layer being two distinct layers arranged on either side of the layer comprising liquid crystals, said first layer comprising a first positive terminal (P1) and said second layer comprising a first negative terminal (N1), the first positive terminal and the first negative terminal being intended to be connected to a first source of electric current (9) so as to produce an electric field passing through the layer comprising liquid crystals,characterized in that at least one layer among said first layer and said second layer comprises a second positive terminal (P2) and a second negative terminal (N2) intended to be connected to a second electric current source (10) so as to make an electric current flow in this layer, between the second positive terminal and the second negative terminal. Optical device (3) according to the preceding claim, characterized in that the first positive terminal (P1) and the second positive terminal (P2) form a single positive terminal, or in that the first negative terminal (N1) and the second negative terminal (N2) form a single negative terminal. Optical device according to claim 1, characterized in that the first positive terminal (P1), the second positive terminal (P2), the first negative terminal (N1) and the second negative terminal (N2) are four distinct terminals. Optical device (3) according to any one of the preceding claims, characterized in that the layer comprising liquid crystals (4) is a layer comprising liquid crystals dispersed in a polymer, and / or in that said first layer (5) and / or said second layer (6) is an indium tin oxide layer. Optical device according to one of the preceding claims, characterized in that it comprises a first electric current source (9) electrically connected to the first positive terminal (P1) and to the first negative terminal (N1), and a second electric current source (10) electrically connected to the second positive terminal (P2) and to the second negative terminal (N2). Optical device (3) according to the preceding claim, characterized in that the first current source (9) is an alternating current source, and / or in that the second current source (10) is a direct current source. Optical device (3) according to any one of the preceding claims, characterized in that the second positive terminal (P2) and the second negative terminal (N2) are separated from each other by a distance greater than or equal to 50% of a larger dimension of the electrically conductive coating layer which comprises them. Optical device (3) according to any one of the preceding claims, characterized in that the layer comprising the second positive terminal (P2) and the second negative terminal (N2) comprises a rectangular shape, the second positive terminal (P2) and / or the second negative terminal (N2) extending along at least 50% of a short side of the rectangular shape. Lighting device (1) for a motor vehicle, characterized in that it comprises a light source (2) and an optical device (3) according to any one of the preceding claims, the optical device being arranged so as to receive light rays (R) from the light source. Method of controlling an optical device (3) according to any one of claims 1 to 8 or a light device (1) according to the preceding claim, characterized in that it comprises:- the detection of a temperature less than or equal to a threshold, then- the circulation of an electric current between the second positive terminal (P2) and the second negative terminal (N2) to heat the electrically conductive coating layer comprising the second positive terminal and the second negative terminal.

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