Optical device equipped with a liquid-crystal display

Abstract

Disclosed is a luminous device (1) for a motor vehicle, characterized in that it comprises a light source (2) and an optical device (3), the optical device being arranged so as to receive light rays generated by the light source, the luminous device 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, the luminous device further comprising a second electrical current source (10) connected to the second positive terminal and the second negative terminal of the second layer so as to make a periodic electrical current flow through this layer.

Description

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

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

[0002] A motor vehicle is equipped with lighting devices to illuminate the vehicle's surroundings and make the vehicle clearly visible to other road users. A vehicle generally includes at least main beam headlights, dipped beam headlights, and position lights. Lighting devices also contribute to the aesthetic appearance of the vehicle by providing an original light signature. This light signature must remain clearly visible without being dazzling regardless of the ambient light conditions. Lighting devices must therefore produce light beams whose intensity varies according to the light conditions and according to the needs of a vehicle user. For example, vehicles are known to be equipped with position lights whose light intensity varies depending on whether the vehicle is used during the day or at night.

[0003] This diversity of needs encourages automobile manufacturers to design lighting systems equipped with numerous lighting devices, each designed to perform a specific lighting function. Vehicles therefore have complex, heavy and bulky lighting systems.

[0004] Furthermore, a motor vehicle must operate within very wide temperature ranges. In particular, the lighting devices of motor vehicles must remain fully functional at low temperatures, including negative temperatures, or even at -20°C, or even -40°C. Presentation of the invention

[0005] The aim of the invention is to provide a lighting device for a motor vehicle, overcoming the above drawbacks and improving the lighting devices known from the prior art.

[0006] More specifically, a first object of the invention is a light device capable of producing a light beam whose intensity is easily controllable and capable of operating at low temperature. Summary of the invention

[0007] The invention relates to a light device for a motor vehicle, the light device comprising a light source and an optical device, the optical device being arranged to receive light rays from the light source, the light device 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 light device further comprising a first source of electric current connected to the first positive terminal and the first negative terminal so as to produce an alternating electric field passing through ... liquid crystals, at least one layer among said first layer and said second layer comprising a second positive terminal and a second negative terminal, the light device further comprising a second electric current source connected to the second positive terminal and the second negative terminal so as to circulate a periodic electric current in this layer, the light device further comprising a synchronization means configured to synchronize a phase of the electric current generated by the second current source with a phase of the electric current generated by the first current source.

[0008] According to one embodiment, a frequency of the second current source is equal to a frequency of the first current source, or a frequency of the second current source is equal to a multiple of a frequency of the first current source.

[0009] According to one embodiment: - for each positive phase of the electric current generated by the first current source, the electric current generated by the second current source comprises at least one positive phase and at least one negative phase, and / or: - for each negative phase of the electric current generated by the first current source, the electric current generated by the second current source comprises at least one positive phase and at least one negative phase.

[0010] According to one embodiment, a frequency of the second current source is equal to a frequency of the first current source, and: - for each positive phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a positive phase and for each negative phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a negative phase, or: - for each positive phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a negative phase and for each negative phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a positive phase.

[0011] According to one embodiment, the second current source is configured to flow an alternating electric current between the second positive terminal and the second negative terminal.

[0012] According to one embodiment, the second current source is switched off for a negative phase of the first current source or for a positive phase of the first current source.

[0013] According to one embodiment, the light device comprises an electrical circuit comprising a first direct current generator, a second direct current generator, a set of switching elements, and an electronic control unit configured to control a state of each switching element so as to convert a direct current generated by the first direct current generator into an alternating current between the first positive terminal and the first negative terminal, and so as to convert a direct current generated by the second direct current generator into a periodic current between the second positive terminal and the second negative terminal.

[0014] According to one embodiment, the first current source is configured to produce an alternating electric field passing through the layer comprising liquid crystals whose amplitude is between 50 volts and 100 volts, and / or whose frequency is between 25 Hz and 100 Hz.

[0015] The invention also relates to a method for controlling a light device as defined above, the control method comprising: - the detection of a temperature lower 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.

[0016] According to one embodiment, the control method comprises: - activating the first current source so as to produce an alternating electric field passing through the layer comprising liquid crystals, and simultaneously: - activating the second current source so as to cause a periodic electric current to flow in the layer comprising the second positive terminal and the second negative terminal. Presentation of figures

[0017] 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:

[0018] Figure 1 is a schematic sectional view of a light device equipped with an optical device according to one embodiment of the invention.

[0019] Figure 2 is a schematic view of an electrical circuit for controlling the optical device of the light device.

[0020] Figure 3 is a graph representing the time evolution of three voltages between different terminals of the electrical circuit according to a first variant embodiment of the invention.

[0021] Figure 4 is a graph representing the time evolution of three voltages between different terminals of the electrical circuit according to a second variant embodiment of the invention.

[0022] Figure 5 is a graph representing the time evolution of three voltages between different terminals of the electrical circuit according to a third variant embodiment of the invention.

[0023] Figure 6 is a graph representing the time evolution of three voltages between different terminals of the electrical circuit according to a fourth variant embodiment of the invention.

[0024] Figure 7 is a schematic view of a basic electrical circuit for controlling the optical device of the light device.

[0025] Figure 8 is a graph representing the time evolution of two voltages between different terminals of the electrical circuit of Figure 7. Detailed description

[0026] Figure 1 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, or even inside the passenger compartment thereof. The lighting device can in particular be intended to illuminate the environment of the vehicle or to make the vehicle clearly visible to other road users. The lighting device 1 can contribute to the light signature of the vehicle. The lighting device 1 comprises a light source 2 and an optical device 3. The light source 2 may comprise, for example, one or more light-emitting diodes and / or one or more incandescent lamps. The optical device 3 is arranged to receive light rays R from the light source 2. These light rays R may form a light beam centered on an optical axis X. The light rays R may be emitted in any area of ​​the visible spectrum.

[0027] The optical device 3 is in particular an electro-optical device. The optical properties of the optical device 3, in particular its transparency, are a function of an electrical parameter, in particular an electric field, which is applied to it. More particularly, the optical device 3 comprises a layer comprising liquid crystals 4, a first electrically conductive layer 5, and a second electrically conductive layer 6. According to one embodiment, the different layers 4, 5, 6 of the optical device 3 may extend in a plane substantially perpendicular to the optical axis X. According to other embodiments, the different layers 4, 5, 6 of the optical device 3 may extend in a plane not perpendicular to the optical axis X or along a curve.

[0028] The first layer 5 and the second layer 6 are two separate layers arranged on either side of the layer comprising liquid crystals 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 first layer 5 and / or the second layer 6 may extend to the edges of each of the faces of the liquid crystal layer 4. 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.

[0029] The layer comprising liquid crystals 4, hereinafter referred to as 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. Thus, 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 molecules M 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 M may orient themselves according to a nematic, cholesteric or smectic state, 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.

[0030] 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 droplets can be randomly oriented within the liquid crystal layer 4.

[0031] 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, like any electrical conductor, 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 coloration. In particular, the first layer 5 and the second layer 6 may be layers of indium tin oxide, commonly referred to as ITO (acronym for the English term "indium tin oxide"). Alternatively, other materials could be considered. The first layer 5 and the second layer 6 may be of identical design, but not necessarily. These two layers may, for example, have different thicknesses and / or conductive properties.

[0032] The first layer 5 and the second layer 6 may be covered on their second face respectively with a substrate layer 7, 8 made of transparent and electrically insulating material. The substrate layers 7 and 8 may be made of thermoplastic saturated polyester, in particular a layer of poly(ethylene terephthalate), commonly referred to as PET. These layers may be obtained by a thermally induced phase separation process or by polymerization (process commonly referred to by the acronym PIPS meaning "polymerization induced phase separation" in English). Alternatively, the substrate layers 7 and 8 could also be made of poly(methyl methacrylate), commonly referred to as PMMA, poly(ethylene naphthalate) commonly referred to as PEN, polycarbonate, commonly referred to as PC, polyethylene terephthalate (PET), polycaprolactone (PCL), or any other equivalent material.The substrate layers 7, 8 can form a protective envelope enveloping both the liquid crystal layer 4, the first layer 5 and the second layer 6. The substrate layers 7 and 8 can also constitute a support on which the first layer 5 and the second layer 6 are respectively deposited.

[0033] As a note, the different layers shown in Figure 1 are represented schematically and their apparent thickness may be disproportionate.

[0034] 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 M 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 FIG. 1. The control of the first current source 9 thus makes it possible to control the level of transparency of the liquid crystal layer 4.

[0035] The first current source 9 is an alternating current source. That is, the first current source 9 produces a periodic electric current that changes direction twice per period and that carries alternately equal amounts of electric charge in one direction and the other. An alternating current therefore has a zero average value. The electric current produced by the first current source 9 therefore comprises an alternation of positive phases and negative phases. This electric current may have a sinusoidal profile, or a square-wave profile as illustrated in Figures 3 to 6, or any other profile shape.

[0036] The electric field passing through the liquid crystal layer 4 may, for example, have a voltage whose amplitude is between 50 volts and 100 volts and / or whose frequency is between 25 Hz and 100 Hz. According to the illustrated embodiments, it is assumed that the first current source 9 produces an alternating electric current whose frequency is equal to 50 Hz, and whose voltage has an amplitude of 70 volts. The period of the electric current is therefore equal to 20 ms. The voltage of the electric field crossing the liquid crystal layer 4 therefore varies between +70 volts and -70 volts.

[0037] In order to preserve the liquid crystal layer 4, it is very important that the electric field passing through it be on average equal to zero volts at any point of the liquid crystal layer over a given period. Indeed, if the electric field passing through the layer of the liquid crystal layer 4 were non-zero locally over a sufficiently long period, this electric field would lead to damage to the liquid crystal layer.

[0038] Furthermore, the second layer 6 comprises a second positive terminal P2 and a second negative terminal N2. According to one embodiment, the first positive terminal P1 and the second positive terminal P2 may form a single positive terminal. Alternatively, the first positive terminal P1 and the second positive terminal P2 may be two positive terminals distinct from each other.

[0039] 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 FIG. 1.

[0040] The second current source 10 is a periodic current source. That is, the electric current produced by the second current source exhibits periodic variations. As will be seen in more detail later, the second current source 10 may be an alternating current source but is not necessarily an alternating current source. The electric current produced by the second current source 10 may possibly carry different amounts of electric charge in one direction and the other. The electric current produced by the second current source 10 may therefore comprise an alternation of positive phases and negative phases or only positive phases or only negative phases. This electric current may have a sinusoidal profile, a square wave profile as illustrated in FIGS. 3 to 6, or any other profile shape.

[0041] The electric current produced by the second current source 10 may, for example, have a periodic voltage with an amplitude of between 5 volts and 30 volts. According to the illustrated embodiments, it is assumed that the second current source 10 produces an electric current with a voltage having an amplitude of 12 volts. The electric voltage between the second positive terminal P2 and the second negative terminal N2 may thus vary between +12 volts and -12 volts.

[0042] According to an alternative embodiment, the second positive terminal P2 and the second negative terminal N2 could be arranged on the first layer 5. Advantageously, only one layer among the first layer and the second layer comprises a second positive terminal P2 and a second negative terminal N2. This facilitates the design and manufacture of the optical device 3.

[0043] The positive terminals P1 and P2 and the negative terminals N1 and N2 are therefore 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.

[0044] As a note, in Figure 1, the terminals P1, N1, P2, N2 are represented on a particular face of the first layer 5 or respectively of the second layer 6. Alternatively, each of these terminals could be indifferently arranged on the other face of the first layer 5 or respectively on the other face of the second layer 6.

[0045] Due to the intrinsic electrical resistance of the second layer 6, the electric current produced by the current source 10 heats the second layer 6 by Joule effect. 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 an electrically conductive layer whose presence is already required to produce an electric field in the thickness of the liquid crystal layer 4. It is therefore simply necessary to provide at least one additional electrical terminal and an electric current source to form this heating means. The control of the second current source 10 thus makes it possible to control the temperature of the liquid crystal layer 4.

[0046] Preferably, 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 spaced apart 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 heat production is well distributed over the entire length of the second layer 6. The axis passing through the second positive terminal P2 and through the second negative terminal is called the Y axis. The Y axis may be substantially perpendicular to the optical axis X. By convention, it is defined that the second negative terminal N2 is positioned at point Y = 0, and that the second positive terminal P2 is positioned at point Y = Y1.

[0047] 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 optionally 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. The electronic control unit 15 comprises a memory containing a computer program and a microprocessor capable of executing said computer program.

[0048] The light device 1 also comprises a synchronization means configured to synchronize a phase of the electric current generated by the second current source 10 with a phase of the electric current generated by the first current source 9. The phase of the electric current generated by the second current source can thus be synchronized with the phase of the electric current generated by the first current source so that the alternating electric field passing through the liquid crystal layer 4 has a zero time average at any point of this layer. This avoids damaging the liquid crystal layer 4. This makes it possible to control the level of transparency of the liquid crystal layer 4 with the first current source while simultaneously producing heat by the Joule effect with the second current source 10.

[0049] By "synchronize" is meant that the phase changes of the electric current generated by the first current source 9 are concomitant with phase changes of the electric current generated by the second current source 10. In other words, the phase changes of the electric current generated by the first current source 9 occur at the same time as phase changes of the electric current generated by the second current source 10.

[0050] Figure 2 schematically illustrates an embodiment of an electrical circuit 16 for controlling the optical device 3 of the light device 1. The electrical circuit 16 is integrated into the electronic control unit 15. The electrical circuit 16 comprises a first direct current generator 21, a second direct current generator 22, and a set of switching elements S1, S2, S3, S4, S5, S6, S7, S8. On the one hand, the switching elements are configured to convert the direct current generated by the first direct current generator 21 into an alternating current to generate an alternating electric field passing through the liquid crystal layer 4. On the other hand, the switching elements are also configured to convert the direct current generated by the second direct current generator 22 into a periodic current flowing in the second layer 6.The first direct current generator 21 can provide a nominal voltage VBOOST, for example, equal to approximately 70 volts. The second direct current generator 22 can provide a nominal voltage VHEAT, for example, equal to approximately 12 volts.

[0051] In Figure 2 the intrinsic resistance of the second layer 6 between the second negative terminal N2 and the second positive terminal P2 is represented by a resistance R.

[0052] The switching elements S1 to S8 are controlled switches. Each switching element can be controlled so that it is in a closed state, in which it allows an electric current to pass, or, conversely, in an open state, in which it interrupts the flow. of the electric current. The switching elements are thus controlled by the microprocessor integrated into the electronic control unit 15.

[0053] The switching elements S1, S2, S3 and S4 can be arranged in an H-bridge between the first direct current generator 21 and ground. More specifically, the switching element S1 is arranged between the voltage generator 21 and the first negative terminal N1. The switching element S2 is arranged between the first negative terminal N1 and ground. The switching element S3 is arranged between the voltage generator 21 and the first negative terminal N1. The switching element S4 is arranged between the second negative terminal N2 and ground. The switching element S5 is arranged between the voltage generator 21 and the first positive terminal P1, which coincides with the second positive terminal P2. The switching element S6 is arranged between the first positive terminal P1 and ground. The switching element S7 is arranged between the voltage generator 22 and the second negative terminal N2.The switching element S8 is arranged between the voltage generator 22 and the second positive terminal P2. Alternatively, other configurations could be proposed.

[0054] In addition, the electrical circuit 16 is here equipped with three differential probes SD1, SD2 and SD3. These differential probes are not useful for the proper functioning of the electrical circuit but allow voltages to be visualized on the graphs of figures 3 to 6. The first differential probe SD1 is arranged between the first negative terminal N1 and the second negative terminal N2. The first differential probe SD1 therefore measures the potential difference between the first layer 5 and the second layer 6 at the point Y = 0. The second differential probe SD2 is arranged between the first negative terminal N1 and the first positive terminal P1. The second differential probe SD2 therefore measures the potential difference between the first layer 5 and the second layer 6 at point Y = Y1. The third differential probe SD3 measures the potential difference between the second negative terminal N2 and the second positive terminal P2. The third differential probe SD3 therefore measures the voltage across the resistor R.

[0055] As a note, in the electrical circuit as shown in Figure 2, the voltage generators 21 and 22 are shown several times, this in order not to increase the number of electrical connection wires shown and therefore in order to simplify the understanding of this figure. In the graph of Figure 3, as well as in the graphs of Figures 4, 5, 6 and 8, the abscissa axis represents an elapsed time in milliseconds and the ordinate axis represents a voltage in volts.

[0056] It is assumed that current generators 21 and 22 deliver an electric current with a positive electric potential.

[0057] In a first configuration C1, the switching elements S1, S4 and S8 are closed and the switching elements S2, S3, S5, S6 and S7 are open. In this first configuration, the voltages measured by the three differential probes SD1, SD2 and SD3 are positive. The potential difference between the first layer 5 and the second layer 6 at point Y = 0 is then equal to the voltage +VBOOST. The potential difference between the first layer 5 and the second layer 6 at point Y = Y1 is then equal to the voltage +VBOOST - VHEAT. The voltage across the resistor R is then equal to +VHEAT.

[0058] In a second configuration C2, when the switching elements S1, S6 and S7 are closed and the switching elements S2, S3, S4, S5 and S8 are open, the voltages detected by the differential probes SD1, SD2 are positive and the voltage detected by the differential probe SD3 is negative. The potential difference between the first layer 5 and the second layer 6 at point Y = 0 is then equal to the voltage +VBOOST. The potential difference between the first layer 5 and the second layer 6 at point Y = Y1 is then equal to the voltage +VBOOST + VHEAT. The voltage across the resistor R is then equal to -VHEAT.

[0059] In a third configuration C3, when the switching elements S2 and S3 are closed and the switching elements S1, S4, S5, S6, S7 and S8 are open, the voltages detected by the differential probes SD1, SD2 are negative and the voltage detected by the differential probe SD3 is zero. The potential difference between the first layer 5 and the second layer 6 at the point Y = 0 and at the point Y = Y1 is then equal to the voltage - VBOOST.

[0060] The different switching elements can be controlled so that the electrical circuit 16 passes successively into the first configuration C1, then into the second configuration C2, then into the third configuration C3, in accordance with the graph in figure 3. More precisely, the first configuration C1 is maintained over a first quarter period, the second configuration C2 is maintained for a second quarter period, and the third configuration is maintained over a half period.

[0061] Thus, at point Y = 0, the voltage varies in a balanced manner between +VBOOST and -VBOOST. At point Y = Y1, during the positive phases, the voltage varies between +VBOOST - VHEAT and +VBOOST + VHEAT. This voltage is therefore on average equal to +VBOOST during each positive phase. During the negative phases, the voltage is equal to -VBOOST. The voltage is therefore also balanced between the positive phases and the negative phases at point Y = Y1. Thus, the electric field which crosses the liquid crystal layer 4 is on average equal to zero volts in all point of the liquid crystal layer over a given period. This allows the liquid crystal layer 4 to be preserved. It is thus possible to consider heating the liquid crystal layer 4 by the Joule effect and simultaneously controlling the level of transparency of the liquid crystal layer 4.

[0062] According to this first embodiment, the electric current only flows in the second layer 6 half the time, during the positive phases of the electric current generated by the first current source 9. This nevertheless remains sufficient to effectively heat the liquid crystal layer 4 and makes it possible to limit the consumption of electric current.

[0063] As a side note, the electric field variations at point Y = Y1 may cause slight local variations in the transparency level of the liquid crystal layer. This is not a problem, however, because these variations are not perceptible to the naked eye due to their very high variation frequency. Advantageously, the VBOOST voltage is significantly higher than the VHEAT voltage so as to make these variations as small as possible. The VBOOST voltage may, for example, be at least five times higher than the VHEAT voltage.

[0064] As a remark, in accordance with the embodiment illustrated in Figure 3, during a positive phase of the electric current generated by the first current source, the electric circuit 16 is in its first configuration C1 only once for a quarter of a period, then in its second configuration C2 only once for a quarter of a period. According to an alternative embodiment, a greater number of alternations between the first configuration C1 and the second configuration C2 could be provided during the same positive phase. Two alternations of the configurations C1 and C2 could thus be provided, each configuration C1 and C2 being thus maintained successively for one eighth of a period. Any other number of alternations between configurations C1 and C2 greater than or equal to 3 could be provided.

[0065] According to another variant embodiment (not shown), the electrical circuit 16 could be adapted so that the electric current only flows in the second layer 6 during the negative phases of the electric current generated by the first current source 9.

[0066] According to yet another alternative embodiment, shown in Figure 4, the electrical circuit 16 could be adapted so that the electric current flows in the second layer 6 during the negative phases and during the negative phases of the first current source 9. According to this alternative, the electric current generated by the second current source 10 could then be an alternating current whose frequency would be a multiple of the frequency of the electric current generated by the first current source 9. As can be seen in Figure 2, the frequency of the second current source 10 can be twice the frequency of the first current source 9. Thus, at the point Y = 0, the voltage varies in a balanced manner between +VBOOST and - VBOOST. At the point Y = Y1, during the positive phases, the voltage varies between +VBOOST - VHEAT and +VBOOST + VHEAT. This voltage is therefore on average equal to +VBOOST during each positive phase.During the negative phases, the voltage varies between -VBOOST - VHEAT and -VBOOST + VHEAT. This voltage is therefore on average equal to -VBOOST during each positive phase. The electric field which passes through the liquid crystal layer 4 is on average equal to zero volts at any point of the liquid crystal layer over a given period. According to this embodiment, an electric current flows continuously between the second negative electrode N2 and the second positive electrode P2, the liquid crystal layer 4 is therefore heated more quickly.

[0067] Figure 5 illustrates another embodiment of the invention. According to this embodiment, the second current source 10 is an alternating current source of the same frequency as the first current source 9. The second current source 10 is synchronized with the first current source so that a positive phase of the electric current generated by the second current source coincides with a positive phase of the electric current generated by the first current source 9. According to this embodiment, an electric current flows continuously between the second negative electrode N2 and the second positive electrode P2, the liquid crystal layer is therefore heated more quickly. At the point Y = 0, the voltage varies in a balanced manner between +VBOOST and -VBOOST. At the point Y = Y1, the voltage varies in a balanced manner between +VBOOST + VHEAT and -VBOOST - VHEAT.The electric field passing through the liquid crystal layer 4 is on average equal to zero volts at any point in the liquid crystal layer over a given period. The liquid crystal layer 4 is therefore not damaged. The level of transparency of the liquid crystal layer 4 between the points Y = 0 and Y = Y1 may, however, be slightly different.

[0068] Figure 6 illustrates yet another embodiment of the invention. According to this embodiment, the second current source 10 is also an alternating current source of the same frequency as the first current source 9. The second current source 10 is synchronized with the first current source so that a positive phase of the electric current generated by the second current source coincides with a negative phase of the electric current generated by the first current source 9. According to this embodiment, an electric current flows continuously between the second negative electrode N2 and the second positive electrode P2, the liquid crystal layer is therefore heated more quickly. At the point Y = 0, the voltage varies from balanced manner between +VBOOST and -VBOOST. At point Y = Y1, the voltage varies in a balanced manner between +VBOOST - VHEAT and - VBOOST + VHEAT. The electric field passing through the liquid crystal layer 4 is on average equal to zero volts at any point in the liquid crystal layer over a given period. The liquid crystal layer 4 is therefore not damaged. The level of transparency of the liquid crystal layer 4 between points Y = 0 and Y = Y1 may nevertheless be slightly different.

[0069] A method for controlling the optical device 3 can be implemented in the following manner. First, a desired transparency level for the optical device 3 is determined. This transparency level can be chosen, for example, according to ambient brightness and / or to perform a given optical function. Then, the first current source 9 is controlled so that the liquid crystal layer 4 reaches the desired transparency level. An alternating electric current therefore flows between the first positive terminal P1 and the first negative terminal N1. In parallel, a temperature less than or equal to a threshold, for example a threshold of 0°C, -10°C or -20°C, is detected with the temperature sensor 14. Then, the electronic control unit 15 controls the second current source 10 so that it delivers a periodic electric current synchronized with the alternating electric current generated by the first current source 9.This periodic electric current flows between the second positive terminal P2 and the second negative terminal N2 and allows the second layer 6 to be heated by the Joule effect.

[0070] The light device 1 can be used to adapt the light signature of a vehicle. In particular, by controlling the electric current delivered by the voltage source 9, the liquid crystal layer 4 can be made more or less transparent depending on the brightness ambient. This makes it possible to produce a clearly visible light signature of constant appearance regardless of the external light conditions, particularly during daytime or nighttime use of the vehicle. At the same time, by controlling the electric current delivered by the current source 10, it is possible to generate heating of the optical device 3 which allows it to operate even at low temperatures.

[0071] To illustrate the advantages of the invention, Figure 7 shows a basic electrical circuit 16' for controlling an optical device 3'. The optical device 3' is similar to the optical device 3 described previously. It notably comprises a liquid crystal layer 4' arranged between a first layer 5' of an electrically conductive coating, and a second layer 6' of an electrically conductive coating. The electrical circuit 16' comprises a first electric current source 9' configured to produce an alternating electric field passing through the liquid crystal layer 4'. The electrical circuit 16' comprises a second current source 10' configured to supply a direct electric current passing through the first layer 6' and thus produce heating of this layer by the Joule effect. A first differential probe SD1' measures the potential difference between the first layer 5' and the second layer 6' at the point Y = 0.A second differential probe SD2' measures the potential difference between the first layer 5' and the second layer 6' at the point Y = Y1 . The graph in Figure 7 represents the time evolution of the voltages measured by the differential probes SD1' and SD2'. At the point Y = 0, the voltage varies between +VBOOST and -VBOOST. At the point Y = Y1 , the voltage varies between +VBOOST - VHEAT and - VBOOST - VHEAT. At the point Y = Y1 , the average voltage of the electric field crossing the liquid crystal layer is therefore not zero but is equal to - VHEAT. With such an electrical circuit 16', there is therefore a risk of damaging the liquid crystal layer 4', at least locally at the point Y = Y1. This cannot be done. therefore not controlling the transparency of the 4' liquid crystal layer while heating it by Joule effect by means of an electric current circulating in the second layer.

Claims

CLAIMS

1. A light device (1) for a motor vehicle, characterized in that it comprises a light source (2) and an optical device (3), the optical device being arranged to receive light rays from the light source, the light device 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 separate 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 light device further comprising a first electric current source (9) connected to the first positive terminal and the first negative terminal so as to produce an alternating 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 (P2) and a second negative terminal (N2), the light device further comprising a second electric current source (10) connected to the second positive terminal and the second negative terminal so as to circulate a periodic electric current in this layer, the light device further comprising a synchronization means configured to synchronize a phase of the electric current generated by the second current source with a phase of the electric current generated by the first current source.,

2. Luminous device (1) according to the preceding claim, characterized in that a frequency of the second current source (10) is equal to a frequency of the first current source (10). current (9), or in that a frequency of the second current source (10) is equal to a multiple of a frequency of the first current source (9).

3. Luminous device (1) according to one of the preceding claims, characterized in that: - for each positive phase of the electric current generated by the first current source (9), the electric current generated by the second current source (10) comprises at least one positive phase and at least one negative phase, and / or in that: - for each negative phase of the electric current generated by the first current source (9), the electric current generated by the second current source (10) comprises at least one positive phase and at least one negative phase.

4. A light device (1) according to one of claims 1 or 2, characterized in that a frequency of the second current source (10) is equal to a frequency of the first current source (9), and in that: - for each positive phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a positive phase and for each negative phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a negative phase, or: - for each positive phase of the electric current generated by the first current source, the electric current generated by the second current source comprises a negative phase and for each negative phase of the electric current generated by the first current source, the electric current generated by the second current source includes a positive phase.

5. Luminous device (1) according to one of the preceding claims, characterized in that the second current source (10) is configured so as to circulate an alternating electric current between the second positive terminal (P2) and the second negative terminal (N2).

6. Lighting device (1) according to one of claims 1 to 4, characterized in that the second current source (10) is switched off for a negative phase of the first current source or for a positive phase of the first current source (9).

7. A lighting device (1) according to one of the preceding claims, characterized in that it comprises an electrical circuit (16) comprising a first direct current generator (21), a second direct current generator (22), a set of switching elements (S1, S2, S3, S4, S5, S6, S7, S8), and an electronic control unit (15) configured to control a state of each switching element so as to convert a direct current generated by the first direct current generator into an alternating current between the first positive terminal (P1) and the first negative terminal (N1), and so as to convert a direct current generated by the second direct current generator into a periodic current between the second positive terminal (P2) and the second negative terminal (N2).

8. Luminous device (1) according to one of the preceding claims, characterized in that the first current source (9) is configured to produce an alternating electric field passing through the layer comprising liquid crystals (4) of which the amplitude is between 50 volts and 100 volts, and / or whose frequency is between 25Hz and 100Hz

9. Method for controlling a light device (1) according to one of claims 1 to 8, characterized in that it comprises: - the detection of a temperature lower 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 layer (6) of electrically conductive coating comprising the second positive terminal and the second negative terminal.

10. Method for controlling a light device (1) according to one of claims 1 to 8, characterized in that it comprises: - activating the first current source (9) so as to produce an alternating electric field passing through the layer comprising liquid crystals (4), and simultaneously: - activating the second current source (10) so as to cause a periodic electric current to flow in the layer comprising the second positive terminal (P2) and the second negative terminal (N2).

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