Optical device equipped with a liquid crystal display

Abstract

Optical device equipped with a liquid crystal display. Lighting 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 lighting device comprising a layer of liquid crystals (4), a first layer (5) of an electrically conductive coating, and a second layer (6) of an electrically conductive coating, the lighting 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 circulate a periodic electric current in this layer. Figure for the abbreviation: Figure 2

Description

Title of the invention: 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 equipped with a liquid crystal display, in particular of the PDLC type (acronym for the English term "Polymer Dispersed Liquid Crystal"). The invention also relates to a method for controlling such a lighting device. 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 high beams, low beams, and position lights. The lighting devices also contribute to the vehicle's aesthetic appeal by providing a distinctive light signature. This light signature must remain clearly visible without being dazzling, regardless of ambient lighting conditions. The lighting devices must therefore produce light beams whose intensity varies according to lighting conditions and the needs of a vehicle user. For example, vehicles equipped with position lights whose light intensity varies depending on whether the vehicle is being used during the day or at night are known.

[0003] This diversity of needs encourages motor vehicle manufacturers to design lighting systems equipped with numerous lighting devices, each intended to perform a specific lighting function. Vehicles thus incorporate complex, heavy, and bulky lighting systems.

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

[0005] The object of the invention is to provide a lighting device for a motor vehicle, remedying the above disadvantages and improving the lighting devices known in the prior art.

[0006] More specifically, a first object of the invention is a lighting 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 lighting device for a motor vehicle, the lighting device comprising a light source and an optical device, the optical device being arranged to receive light rays from the light source, the lighting 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 distinct 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 lighting device further comprising a first electric current source 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 and a second negative terminal, the lighting 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 lighting 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 includes 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 includes 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 includes 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 includes 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.

[0011] According to one embodiment, the second current source is configured to circulate 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 lighting device comprises an electrical circuit including a first DC generator, a second DC 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 DC current generated by the first DC generator into an alternating current between the first positive terminal and the first negative terminal, and so as to convert a DC current generated by the second DC 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, the amplitude of which is between 50 volts and 100 volts, and / or the frequency of which is between 25Hz and 100Hz

[0015] The invention also relates to a method for controlling a lighting device as defined above, the control method comprising: - the detection of a temperature below or equal to a threshold, then - the flow 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: - the activation of the first current source so as to produce an alternating electric field passing through the layer comprising liquid crystals, and simultaneously: - the activation of the second current source so as to circulate a periodic electric current in the layer comprising the second positive terminal and the second negative terminal. Presentation of the figures

[0017] These objects, features and advantages of the present invention will be described in detail in the following description of a particular embodiment, given by way of non-limiting example, with reference to the accompanying figures, among which:

[0018] The [Fig. 1] is a schematic cross-sectional view of a lighting device equipped with an optical device according to an embodiment of the invention.

[0019] Fig. 2 is a schematic view of an electrical control circuit for the optical device of the light-emitting device.

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

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

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

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

[0024] Fig. 7 is a schematic view of a basic electrical control circuit for the optical device of the light-emitting device.

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

[0026] Figure 1 schematically illustrates a lighting device 1 for a motor vehicle according to an embodiment of the invention. The lighting device 1 can be positioned anywhere on the vehicle, for example at the front or rear of the vehicle, or even inside the passenger compartment. The lighting device may, in particular, be intended to illuminate the vehicle's surroundings or to make the vehicle clearly visible to other road users. The lighting device 1 may contribute to the vehicle's light signature. The lighting device 1 comprises a light source 2 and an optical device 3. The light source 2 may, for example, comprise one or more light-emitting diodes and / or one or more lamps. incandescence. The optical device 3 is arranged 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 region 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, depend on an electrical parameter, in particular an electric field, applied to it. More specifically, the optical device 3 comprises a layer of liquid crystals 4, a first electrically conductive layer 5, and a second electrically conductive layer 6. In one embodiment, the various layers 4, 5, 6 of the optical device 3 may extend in a plane substantially perpendicular to the optical axis X. In other embodiments, the various 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 distinct 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 first layer 5 and / or the second layer 6 can extend to the edges of each of the faces of the liquid crystal layer 4. The first layer 5 can face the light source 2 and the second layer 6 can be arranged on the opposite side from the light source 2. Alternatively, the positioning of the first and second layers could be reversed.

[0029] The layer comprising liquid crystals 4, hereinafter referred to as the liquid crystal layer 4, is capable of modifying the path of light rays passing through it according to an applied electric field. Thus, the liquid crystal layer 4 is capable of changing its level of transparency according to an applied electric field. The liquid crystal layer 4 may comprise a crystalline liquid whose molecules M assume an ordered orientation when the liquid crystal layer is subjected to a given electric field. The ordered orientation of the molecules renders the crystalline liquid transparent. Conversely, when the liquid crystal layer is not subjected to an electric field, the molecules M may orient themselves into a nematic, cholesteric, or smectic state, which renders the crystalline liquid opaque or translucent.These properties of the liquid crystal layer 4 can only be obtained 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 all electrical conductors, still possess an intrinsic electrical resistance. The first layer 5 and the second layer 6 can be generally transparent. These layers may optionally include a slight coloration. In particular, the first layer 5 and the second layer 6 can 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 so. These two layers may, for example, have different thicknesses and / or conductive properties.

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

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

[0034] The second layer 6 comprises a first positive terminal PI and the first layer 5 comprises a first negative terminal NI. The first positive terminal PI and the first negative terminal NI are connected to a first electric current source 9, so as to produce an electric field passing through the crystal layer Liquids 4, in particular to orient the molecules M that make up the liquid crystal layer. The resulting electric field can be oriented approximately parallel to the optical axis X. This electric field is represented by arrows Fl in [Fig. 1]. Controlling the first current source 9 thus allows control of the transparency level of the liquid crystal layer 4.

[0035] The first current source 9 is an alternating current source. That is to say, the first current source 9 produces a periodic electric current that changes direction twice per period and carries alternately equal quantities of electric charge in one direction and then the other. An alternating current therefore has a zero average value. The electric current produced by the first current source 9 thus comprises an alternation of positive 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 type of profile.

[0036] The electric field passing through the liquid crystal layer 4 can, for example, have a voltage with an amplitude between 50 volts and 100 volts and / or a frequency between 25 Hz and 100 Hz. According to the illustrated embodiments, the first current source 9 is assumed to produce an alternating electric current with a frequency of 50 Hz and a voltage with an amplitude of 70 volts. The period of the electric current is therefore 20 ms. The voltage of the electric field passing through the liquid crystal layer 4 thus 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 every point of the liquid crystal layer over a given period. Indeed, if the electric field passing through the liquid crystal layer 4 were locally non-zero 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. In 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 separate positive terminals.

[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 make an electric current flow 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 on [Fig. 1].

[0040] The second current source 10 is a periodic current source. That is to say The electric current produced by the second current source exhibits periodic variations. As we will see 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 carry different amounts of electric charge in one direction and the other. Therefore, the electric current produced by the second current source 10 may consist of alternating positive 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 Figures 3 to 6, or any other type of profile.

[0041] The electric current produced by the second current source 10 can, for example, have a periodic voltage with an amplitude 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 amplitude of 12 volts. The voltage between the second positive terminal P2 and the second negative terminal N2 can thus vary between +12 volts and -12 volts.

[0042] According to one 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 and second layers comprises a second positive terminal P2 and a second negative terminal N2. This simplifies the design and manufacture of the optical device 3.

[0043] The positive terminals PI and P2 and the negative terminals NI 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 welding an electrical wire or by using an adhesive.

[0044] As a note, in [Fig.1], the terminals PI, NI, 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 heating. By thermal conduction, the liquid crystal layer 4 can thus be brought to a temperature sufficient to ensure its proper functioning. The second layer 6 thus forms a heating means for the liquid crystal layer 4. Advantageously, this heating means is formed by an electrically conductive layer, the presence of which is already required to produce an electric field within the thickness of the liquid crystal layer 4. It is therefore simply a matter of providing at least one An additional electrical terminal and a power source are used to create this heating method. Controlling the second power source 10 allows for temperature control 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 separated 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 along the entire length of the second layer 6. The axis passing through the second positive terminal P2 and the second negative terminal is called the Y-axis. The Y-axis may be substantially perpendicular to the optical axis X. By convention, the second negative terminal N2 is positioned at the point Y = 0, and the second positive terminal P2 is positioned at the 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 into a single housing. The electronic control unit 15 is configured to control the second current source 10 based on a temperature detected by the temperature sensor 14. The electronic control unit 15 includes a memory containing a computer program and a microprocessor capable of executing said computer program.

[0048] The lighting device 1 also includes 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 every point in this layer. This prevents damage to the liquid crystal layer 4. This makes it possible to control the transparency level of the liquid crystal layer 4 with the first current source while simultaneously generating heat by Joule heating with the second current source 10.

[0049] By "synchronize," it is understood 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 instant as the phase changes of the current electrical 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 lighting device 1. The electrical circuit 16 is integrated into the electronic control unit 15. The electrical circuit 16 comprises a first DC current generator 21, a second DC 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 DC current generated by the first DC 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 DC current generated by the second DC current generator 22 into a periodic current flowing in the second layer 6.The first DC generator 21 can supply a nominal voltage VBOOST, for example, equal to approximately 70 volts. The second DC generator 22 can supply a nominal voltage VHEAT, for example, equal to approximately 12 volts.

[0051] On [Fig.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 SI to S8 are controlled switches. Each switching element can be controlled to be in a closed state, in which it allows an electric current to flow, or, conversely, in an open state, in which it interrupts the flow of 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 configuration between the first DC current source 21 and ground. More specifically, switching element S1 is arranged between the voltage source 21 and the first negative terminal N1. Switching element S2 is arranged between the first negative terminal N1 and ground. Switching element S3 is arranged between the voltage source 21 and the first negative terminal N1. Switching element S4 is arranged between the second negative terminal N2 and ground. Switching element S5 is arranged between the voltage source 21 and the first positive terminal P1, which coincides with the second positive terminal P2. Switching element S6 is arranged between the first positive terminal P1 and ground. Switching element S7 is arranged between the voltage source 22 and the second negative terminal N2.The switching element S8 is arranged between the voltage generator 22 and the second positive terminal P2. Alternative 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 essential for the proper functioning of the electrical circuit but allow us to visualize voltages on the graphs in Figures 3 to 6. The first differential probe, SD1, is connected between the first negative terminal NI and the second negative terminal N2. SD1 therefore measures the potential difference between the first layer 5 and the second layer 6 at point Y = 0. The second differential probe, SD2, is connected between the first negative terminal NI and the first positive terminal PL. 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. SD3 therefore measures the voltage across resistor R.

[0055] As a note, in the electrical circuit as shown in [Fig. 2], the voltage generators 21 and 22 are shown several times in order to avoid increasing the number of electrical connecting wires depicted and thus to simplify the understanding of this figure. In the graph in [Fig. 3], as well as in the graphs of Figures 4, 5, 6, and 8, the x-axis represents elapsed time in milliseconds and the y-axis represents voltage in volts.

[0056] It is assumed that the 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 switching elements S1, S6, and S7 are closed and switching elements S2, S3, S4, S5, and S8 are open, the voltages measured by differential probes SD1 and SD2 are positive, and the voltage measured by 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 resistor R is then equal to -VHEAT.

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

[0060] The various switching elements can be controlled so that the electrical circuit 16 passes successively through the first configuration Cl, then through the second configuration C2, then through the third configuration C3, according to the graph in [Fig. 3]. More precisely, the first configuration Cl is maintained for the first quarter of a period, the second configuration C2 is maintained for the second quarter of a period, and the third configuration is maintained for half a period.

[0061] Thus, at point Y = 0, the voltage varies in a balanced way between +VBOOST and -VBOOST. At point Y = Yl, 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 and negative phases at point Y = YL. Thus, the electric field passing through the liquid crystal layer 4 is on average equal to zero volts at every point of the liquid crystal layer over a given period. The liquid crystal layer 4 can thus be preserved. It is therefore possible to consider heating the liquid crystal layer 4 by Joule heating and simultaneously controlling the transparency level of the liquid crystal layer 4.

[0062] According to this first embodiment, the electric current flows in the second layer 6 only half the time, during the positive phases of the electric current generated by the first current source 9. This is nevertheless sufficient to efficiently heat the liquid crystal layer 4 and allows the consumption of electric current to be limited.

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

[0064] As a note, according to the embodiment illustrated in [Fig. 3], during a positive phase of the electric current generated by the first current source, the electrical 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 period quarter of a period. According to an alternative embodiment, a greater number of alternations between the first configuration Cl and the second configuration C2 could be provided during the same positive phase. Thus, two alternations of the Cl and C2 configurations could be provided, each configuration Cl and C2 being maintained successively for one-eighth of a period. Any other number of alternations between the Cl and C2 configurations greater than or equal to 3 could be provided.

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

[0066] According to yet another embodiment, shown in [Fig. 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 embodiment, the electric current generated by the second current source 10 could then be an alternating current whose frequency is a multiple of the frequency of the electric current generated by the first current source 9. As can be seen in [Fig. 2], the frequency of the second current source 10 can be twice the frequency of the first current source 9. Thus, at point Y = 0, the voltage varies in a balanced way 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 varies between -VBOOST - VHEAT and -VBOOST + VHEAT. This voltage is therefore on average equal to -VBOOST during each positive phase. The electric field passing through the liquid crystal layer 4 is on average equal to zero volts at every 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, so the liquid crystal layer 4 is heated more rapidly.

[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 such 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, and the liquid crystal layer is therefore heated more rapidly. At point Y = 0, the voltage varies from The voltage varies in a balanced way between +VBOOST and -VBOOST. At point Y = Yl, the voltage varies in a balanced way between +VBOOST + VHEAT and -VBOOST - VHEAT. The electric field passing through liquid crystal layer 4 is on average equal to zero volts at every point of the liquid crystal layer over a given period. Liquid crystal layer 4 is therefore not damaged. The transparency level of liquid crystal layer 4 between points Y = 0 and Y = Yl may nevertheless 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 such 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, thus heating the liquid crystal layer more rapidly. At point Y = 0, the voltage varies in a 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 liquid crystal layer 4 averages zero volts at every point in the liquid crystal layer over a given period. Therefore, liquid crystal layer 4 is not damaged. However, the transparency level of liquid crystal layer 4 between points Y = 0 and Y = Yl may be slightly different.

[0069] A method for controlling the optical device 3 can be implemented as follows. First, a desired level of transparency for the optical device 3 is determined. This level of transparency can be chosen, for example, based on ambient light and / or to perform a given optical function. Next, the first current source 9 is controlled so that the liquid crystal layer 4 reaches the desired level of transparency. An alternating electric current therefore flows between the first positive terminal PI and the first negative terminal NI. In parallel, a temperature less than or equal to a threshold is detected using the temperature sensor 14, for example, a threshold of 0°C, -10°C, or -20°C.Next, the electronic control unit 15 commands 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 Joule effect.

[0070] The lighting device 1 can be used to adapt the light signature of a vehicle. In particular, by controlling the electrical current delivered by the voltage source 9, the liquid crystal layer 4 can be made more or less transparent depending on the ambient light. This makes it possible to achieve a highly visible and consistent-appearing light signature regardless of external lighting conditions, especially during daytime or nighttime use of the vehicle. Simultaneously, by controlling the electrical current delivered by the current source 10, the optical device 3 can be heated, allowing it to operate even in low temperatures.

[0071] To illustrate the advantages of the invention, a basic electrical circuit 16' for controlling an optical device 3' is shown in [Fig. 7]. The optical device 3' is similar to the optical device 3 described previously. In particular, it comprises a layer of liquid crystals 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' includes a first electric current source 9' configured to produce an alternating electric field passing through the liquid crystal layer 4'. The electrical circuit 16' includes 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 Joule heating. 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 point Y = Yl. The graph in [Fig. 7] represents the time evolution of the voltages measured by the differential probes SD1' and SD2'. At point Y = 0, the voltage varies between +VBOOST and -VBOOST. At point Y = Yl, the voltage varies between +VBOOST - VHEAT and -VBOOST - VHEAT. At point Y = Yl, the average voltage of the electric field passing through the liquid crystal layer is therefore not zero but is equal to -VHEAT. With such an electrical circuit 16', there is a risk of damaging the liquid crystal layer 4', at least locally at point Y = Yl. It is therefore impossible to control the transparency of the liquid crystal layer 4' while simultaneously heating it by Joule heating using an electric current flowing through 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 (PI) and said second layer comprising a first negative terminal (NI),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. A light 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 (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 comprises a positive phase.

5. Lighting 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 (SI, 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 (PI) and the first negative terminal (NI), 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) whose 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 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 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 circulate a periodic electric current in the layer comprising the second positive terminal (P2) and the second negative terminal (N2).

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