Lighting device comprising a liquid crystal film, and corresponding light emission method

Abstract

The present invention relates to a vehicle lighting device (1) comprising: - at least one photon emitter (3); - a control unit (2) for controlling the photon emitter (3), the control unit being capable of applying, at a first frequency, voltage pulses that bias the photon emitter (3); - a liquid crystal film (4) capable of transmitting or blocking light emitted by the photon emitter (3); and - control means (5) capable of applying an AC voltage (VAC) at a second frequency to the film (4), the lighting device (1) being characterised in that it comprises means for synchronising the AC voltage (VAC) applied to the film (4) with the voltage pulses applied to the photon emitter (3).

Description

DESCRIPTION Title of the invention ■ Lighting device comprising a liquid crystal film and corresponding light emission method

[0001] The present invention relates to the fields of electronics and automobiles, and more specifically concerns a vehicle lighting device using a liquid crystal film, for example a polymer-dispersed liquid crystal film, also called a PDLC (Polymer Dispersed Liquid Crystal) film. Liquid crystal films find applications in the automotive industry, in particular for producing smart glazing, displaying content, in particular on a roof window, or modifying the appearance of lighting elements to create various lighting effects including signaling or active light concealment.

[0002] Applying an electric field to such a film makes it possible to orient the liquid crystals that the film contains, so as to allow the light arriving on the film to pass through, the latter then having a transparent or almost transparent appearance, whereas in the absence of such an electric field, the molecules do not have a coherent orientation and generate within the film a multitude of reflections in multiple directions, which generates at the output an opaque effect or at the very least a diffusing effect on the light having succeeded in passing through the film.

[0003] Liquid crystal films are similar to capacitive loads and must be powered by an alternating voltage with a zero average voltage. Applying a direct voltage to the film eventually damages it.

[0004] Figure 1 represents as a function of time an alternating voltage VAC applied to a PDLC film in volts (V), as well as a current 1A in milliamperes (mA) passing through the film subjected to this alternating voltage VAC. The alternating voltage VAC here has a frequency of 100Hz (Hertz) or an electrical period of 10ms (milliseconds). We note that the current 1A is zero outside the change phases of polarity of the alternating voltage VAC, phases during which the 1A current passing through the film reaches a peak of negative amplitude when the alternating voltage VAC decreases, or a peak of positive amplitude when the alternating voltage VAC increases.

[0005] The last curve in Figure 1 represents, as a function of time and in percentages, the transmission rate TT of a luminous flux passing through the PDLC film subjected to the alternating voltage VAC. It can be seen that this transmission rate TT of the PDLC film is fixed and of the order of 70% when the alternating voltage VAC is maximum in absolute value, while this transmission rate TT decreases during the phases of change of polarity of the alternating voltage VAC, until reaching 50% when the alternating voltage VAC reaches the zero value in the middle of such a phase. Due to the decrease in the transmission rate TT during the phases of change of polarity of the alternating voltage VAC, the average transmission rate of the PDLC film is only 68%.Similarly, the transmission of other types of liquid crystal films is decreased during polarity change phases, during which the AC voltage is lower in absolute value than a threshold voltage.

[0006] Figure 2 represents, as a function of time, the relative value 4>LR of an incident luminous flux arriving on the PDLC film subjected to the alternating voltage VAC. This value has no unit, being a ratio between the value of the luminous flux in candela, on the peak value of this flux in candela. This relative value 4>LR is here 1, the incident luminous flux being produced by light-emitting diodes powered by a direct voltage. The transmission rate TT of the PDLC film, reproduced under the curve of relative value 4>LR of the luminous flux, is identical to that represented in figure 1. The average transmission rate of the luminous flux emitted by light-emitting diodes is therefore 68%.

[0007] The inventors considered using PDLC film technology to hide lighting or signaling means of a vehicle, particularly when the vehicle is stationary, or when some functions are not used (such as dipped headlights during the day), for example by placing such a PDLC film opposite the closing glass of each headlight of the vehicle and therefore across the light beams likely to be emitted by the lighting or signaling means, or across the daylight arriving on these lighting or signaling means. It is then necessary to adapt the supply voltage of the light-emitting diodes fulfilling the lighting or signaling functions of the vehicle, so that they provide the same light intensity with the PDLC film as before without the use of such a film, to meet the regulatory constraints associated with these functions. However, with the average transmission rate of 68% mentioned above, the supply voltage of the diodes must be significantly increased, which is energy-consuming.Furthermore, depending on the lighting or signalling function performed by the light-emitting diodes, their supply voltage may be variable and produce flickering, the coupling of which with the alternating voltage applied to the film may increase discomfort for a road user, whether the driver of the vehicle or not.

[0008] There is therefore a need to conceal the lighting or signalling means of a vehicle, for aesthetic reasons, which is energy-efficient and does not present the risk of generating visual discomfort. This includes standardising the style of the vehicle.

[0009] The present invention aims to remedy at least in part the drawbacks of the prior art by providing a vehicle lighting device provided with a liquid crystal film, an optical unit comprising such a lighting device and a light emission method implemented by such a lighting device or by such an optical unit, which limit energy consumption and flicker compared to the prior art.

[0010] To this end, the invention proposes a lighting device for a vehicle, comprising: - at least one photonic transmitter, - a photonic emitter control unit, capable of applying voltage pulses polarizing the photonic emitter so that the photonic emitter emits light, - a liquid crystal film, capable of transmitting or blocking at least part of light emitted by the photonic emitter, and - control means capable of applying an alternating voltage to said film, the lighting device being characterized in that it comprises means for synchronizing the alternating voltage applied to the film, with the voltage pulses applied to the photonic emitter.

[0011] The photonic emitter is, for example, a light-emitting diode. Preferably, the lighting device comprises several photonic emitters, for example light-emitting diode matrices, capable of producing a regulatory light through the liquid crystal film. This is, for example, a PDLC film, but other types of liquid crystal films can of course be used.

[0012] It should be noted that since the liquid crystal film is capable of blocking the light that can be emitted by the photonic emitters, it is also capable of blocking daylight reaching the photonic emitters and therefore of fulfilling the function of obscuring these lighting or signaling elements when they are not in use.

[0013] The control unit and / or the control means are configured to use the synchronization means so that none of the voltage pulses are applied during a change in polarity of the AC voltage. The voltage pulses and the AC voltage may not have a particular frequency. However, the synchronization means allow the control means to inhibit the emission of light during decreases in the transmission rate of the film, or the control means to change the polarization of the AC voltage between two voltage pulses.

[0014] In one embodiment of the invention, the control unit is capable of applying the voltage pulses at a first frequency, and the control means are capable of applying to said film the alternating voltage with a second frequency.

[0015] The control unit comprises a voltage pulse source, in the form of square waves for example, and analog means for modifying the first frequency of these square waves. Such a voltage pulse source is for example a pulse width modulation source, used to avoid an excessive temperature rise of the diodes but also to adapt the light intensity emitted on the same output surface to carry out various lighting or signaling functions with the same diodes, and / or to adapt the light emission in the case where the light modules of the vehicle are segmented.

[0016] The control means comprise an alternating voltage source whose electrical period preferably comprises a first phase during which the voltage is constant and positive, followed by a second phase of polarity change and a third phase during which the voltage is constant and of opposite value to that of the voltage during the first phase.

[0017] By virtue of the invention, the lighting device comprises means for synchronizing the voltage pulses applied to the photonic emitters and the alternating voltage applied to the film, which makes it possible to adapt the second frequency to the first frequency or vice versa, so as to avoid a disturbing flickering phenomenon, also called "flickering". This phenomenon appears in particular when the extinction phases of the photonic emitters are offset in relation to the polarity change phases of the alternating voltage, during which the transmission rate of the film decreases. This synchronization can also allow energy savings compared to a lighting device of the prior art having the same transmission rate, since the durations during which the emitters photonics are not powered correspond to phases of low transmission rate of the film.

[0018] According to an optional feature of the invention, the synchronization means comprise means for sending a synchronization signal to the control unit. In this case, the control unit is for example capable of producing, from the synchronization signal, the voltage pulses whose first frequency is equal to the second frequency or to an integer multiple of the second frequency. These sending means can for example be in the control means. The first frequency is preferably chosen as twice the second frequency, or more. In particular when the photonic emitters emit a signal light, the first frequency is much higher than the second frequency, for example equal to four times the second frequency, because the intensity emitted by the photonic emitters is lower than in the case of a lighting light.

[0019] According to an optional characteristic of the invention, the lighting device according to the invention comprises, for example, means for detecting the passage of the absolute value of a slope of the alternating voltage above a minimum slope threshold, capable of forming the synchronization signal, and the control unit is capable of forming a falling edge of one of the voltage pulses upon receipt of a pattern of the synchronization signal corresponding to said passage. Thus, each change of sign of the alternating voltage takes place between two voltage pulses, i.e. when the photonic emitter(s) are not polarized. As a result, during the phases of change of polarity of the alternating voltage, corresponding to a lower transmission rate of the film, the photonic emitter(s) do not emit light, which makes it possible to achieve an average transmission rate of 70% and not 68%, with the same film as that used in relation to Figures 1 and 2.In other words, this saves, for the same transmission rate, the energy used to power the photonic transmitters.

[0020] It should be noted that the transmission rate of the film depends on the relaxation times of the liquid crystals. For some applications such as displays, a higher polarity change rate may be advantageous. In this case, the transmission gain provided by the invention is even better.

[0021] According to an optional feature, in an alternative of the invention, the synchronization means comprise means for sending a synchronization signal to the control means. In this case, the control means are for example capable of making the second frequency of the alternating voltage equal to the first frequency or to an integer sub-multiple of the first frequency. These sending means are then for example in the control unit, and the control means comprise analog means for modifying the second frequency of the alternating voltage applied to the film. The second frequency is for example chosen as half of the first frequency, or as a smaller fraction in particular when the photonic emitter(s) emit a traffic light, or more generally when pulse width modulation is used to power the photonic emitters.

[0022] According to an optional characteristic of the invention, in this alternative of the invention, the control means are capable, from the synchronization signal, of timing each change of sign of the alternating voltage between a falling edge and a rising edge of two successive pulses among said voltage pulses. Thus each change of sign of the alternating voltage takes place between two voltage pulses, which makes it possible to save, for the same transmission rate, the energy used to power the photonic transmitters.

[0023] According to an optional characteristic of the invention, the control unit comprises means for determining a peak voltage value of the voltage pulses, as a function of a duration of a phase of change of polarity of the alternating voltage and / or of the second frequency and / or of a response time of the film. These determination means make it possible to compensate for the loss of brightness due to the extinction of the photonic emitters during the phases of change of polarity of the alternating voltage. Thus, for example, when a lighting function is implemented by the lighting device, normally requiring a continuous supply of the photonic emitters, the peak voltage value is increased compared to this initially required continuous supply mode, so as to send on average over a period of the supply signal of the photonic emitters, the same quantity of light as in this continuous supply mode, this quantity of light being emitted only on phases where the voltage applied to the film is constant.The duration of the polarity change phases depends in particular on the film response time and the film frequency, hence the interest in using a map when these characteristics can vary.

[0024] The invention also relates to a front left or front right optical unit for a vehicle, comprising a lighting device according to the invention, and capable of fulfilling a lighting or signaling function, the optical unit comprising a closing glass, the film being arranged between on the one hand the control unit, the control means, the photonic emitter and on the other hand the closing glass, so as to conceal the control unit, the control means and the photonic emitter when the lighting or signaling function is not activated.

[0025] The invention also relates to a method of emitting light implemented by the lighting device according to the invention or by the optical unit according to the invention, comprising the steps of: - application to the film by the control means, of the alternating voltage at the second frequency, - reception by the control unit of the synchronization signal, - application to the photonic transmitter, by the control unit, of voltage pulses at the first frequency, the voltage pulses being synchronized with the AC voltage by the control unit.

[0026] The invention also relates to a method of emitting light implemented by the lighting device according to the invention or by the optical unit according to the invention, comprising the steps of: - application to the photonic transmitter by the control unit, of voltage pulses at the first frequency, - reception of the synchronization signal by the control means, - application to the film by the control means of the alternating voltage at the second frequency, the alternating voltage being synchronized with the voltage pulses by the control means.

[0027] According to an optional characteristic of the invention, in the light emission methods according to the invention, each change of sign of the alternating voltage occurs between a falling edge and a rising edge of two successive pulses among said voltage pulses.

[0028] According to an optional characteristic of the invention, the light emission methods according to the invention may comprise a step of determining a peak voltage value of the voltage pulses, as a function of a duration of a phase of change of polarity of the alternating voltage and / or of the second frequency and / or of a response time of the film.

[0029] The optical block according to the invention as well as the emission methods according to the invention have advantages similar to those of the lighting device according to the invention.

[0030] Other characteristics and advantages of the invention will become apparent from the following description on the one hand, and from several examples of embodiment given for informational and non-limiting purposes with reference to the attached schematic drawings on the other hand, in which:

[0031] [fig 1] already commented in relation to the prior art, represents as a function of time, an alternating voltage applied to a PDLC film, the current passing through this PDLC film and the associated transmission rate of the PDLC film,

[0032] [fig 2] also already commented in relation to the prior art, represents as a function of time, a relative value of incident light flux arriving on the PDLC film on which the alternating voltage of figure 1 is applied, and the associated transmission rate of the PDLC film,

[0033] [fig 3] schematically represents elements of a lighting device according to the invention, in one embodiment of the invention,

[0034] [fig 4] represents steps of a light emission method according to the invention, implemented by the lighting device of figure 3,

[0035] [fig 5] represents, as a function of time, relative values ​​of incident light flux arriving on a PDLC film to which an alternating voltage is applied according to the light emission method of figure 4, and the associated transmission rate of the PDLC film,

[0036] [fig 6] represents, as a function of a response time of the PDLC film, relative values ​​of incident light flux arriving on the PDLC film, determined according to the light emission method of figure 4, and the associated light flux ratios,

[0037] [fig 7] represents, as a function of the frequency of the alternating voltage applied to the PDLC film, relative values ​​of incident light flux arriving on the PDLC film, determined according to the light emission method of figure 4, and the associated light flux ratios,

[0038] [fig 8] represents as a function of time, a relative value of incident light flux arriving on the PDLC film to which an alternating voltage is applied according to the light emission method of figure 4, and the associated transmission rate of the PDLC film, in a case of use of the lighting device of figure 3 corresponding to the emission of a traffic light, and

[0039] [fig 9] represents steps of another method of emitting light according to the invention, in another embodiment of the invention.

[0040] According to an embodiment of the invention shown in Figure 3, a lighting device 1 according to the invention, intended to equip a vehicle, comprises one or more photonic emitters 3, in particular light-emitting diodes, the power supply and therefore the brightness of which are controlled by a control unit 2.

[0041] The control unit 2 comprises a voltage pulse source, taking the form of voltage square pulses, and means for modifying the frequency of these voltage square pulses, called first frequency in this application. The voltage pulse source supplies the light-emitting diodes 3 with voltage square pulses, that is to say with a voltage signal taking the zero value between two voltage square pulses, and taking on each voltage square pulse a non-zero voltage value VL or peak value, greater than or equal to the bias voltage of the light-emitting diodes 3. As a result, between two voltage square pulses, the light-emitting diodes 3 do not emit light, and when they are subjected to the voltage VL of a voltage square pulse, they emit a light flux 4>LI.To modulate the light intensity emitted by the light-emitting diodes 3, the duration Ti of a voltage pulse (referenced in FIG. 5) can be chosen to be more or less short compared to the duration To of zero voltage (referenced in FIG. 5) between two successive voltage pulses. The control unit 2 comprises means for modifying these respective durations and therefore means for modulating the light intensity emitted by the light-emitting diodes 3, in particular as a function of a control signal for activating a lighting lamp, a dipped beam headlight, a signaling lamp, or a daytime running light (also called DRL). Indeed, in this embodiment of the invention, the light-emitting diodes 3 are capable of fulfilling distinct lighting or signaling functions as a function of time. The light flux 4>LI emitted by the light-emitting diodes 3 is sent towards a closing window so that it passes through a film 4 of liquid crystals dispersed in a polymer, called a PDLC film, to which an alternating voltage VAC is applied by control means 5, this alternating voltage having a frequency called a second frequency.

[0042] The control means 5 comprise an alternating voltage source whose electrical period comprises, as in the prior art, a first phase during which the voltage is constant and positive, followed by a second phase of change of polarity corresponding to a descending voltage slope, then a third phase during which the voltage is constant and of opposite value to that of the voltage during the first phase and finally a fourth phase of change of polarity corresponding to an ascending voltage slope.

[0043] According to the invention, the lighting device 1 comprises means 10 for synchronizing the alternating voltage VAC with the voltage pulses emitted by the control unit 2. In this embodiment of the invention, the synchronization means 10 comprise means for sending a synchronization signal h to the control unit 2, the sending means being integrated into the control means 5. The control unit 2 then uses this synchronization signal h to synchronize the voltage pulses with the alternating voltage VAC, as explained below in relation to FIG. 4.

[0044] The lighting device 1 is integrated into a front left or front right optical unit of a vehicle, capable of fulfilling a lighting or signaling function. The PDLC film 4 is placed opposite, for example glued to, a closing glass of the optical unit, inside the latter. Thus, the PDLC film 4 makes it possible to conceal the control unit 2, the control means 5 and the light-emitting diodes 3 when the lighting or signaling function is not activated and the PDLC film is in an opaque or almost opaque mode.

[0045] Figure 4 shows steps of a light emission method 100 implemented by the lighting device 1.

[0046] The first step 110 is the application to the PDLC film 4, by the control means 5, of the alternating voltage VAC at the second frequency, for example 100 Hz. During this step, detection means, integrated into the control means 5 and forming part of the synchronization means 10, detect the beginnings of the polarity change phases of the alternating voltage VAC and emit a voltage pulse at each beginning of a polarity change phase, thus forming the synchronization signal h, sent to the control unit 2.

[0047] These detection means detect, for example, more precisely a variation in the alternating voltage VAC, OR a non-zero slope of the alternating voltage VAC, this variation or this slope having to, in absolute value, be greater than or equal to a predetermined minimum threshold to trigger a detection.

[0048] These detection means optionally also detect the ends of the polarity change phases of the alternating voltage VAC, and emit a voltage pulse when such a polarity change phase ends. In this case, the synchronization signal h comprises two pulses per polarity change phase, one marking the start of such a phase and the other the end of such a phase.

[0049] The next step 120 is the reception by the control unit 2 of the synchronization signal h.

[0050] The next step 130 is the determination by the control unit 2 of the square wave voltage value VL to be applied to the light-emitting diodes 3, as a function of the duration of a polarity change phase of the alternating voltage VAC, this phase possibly depending on the second frequency of the PDLC film and / or a response time of the PDLC film. This duration, as well as the second frequency of the alternating voltage VAC, is for example determined by the control unit 2 using the synchronization signal h, when the latter indicates the beginnings and ends of each polarity change period. Alternatively, this duration and second frequency are predetermined.

[0051] During this step 130, the control unit 2 also determines, as a function of a desired brightness to fulfill the lighting or signaling function provided by the lighting device 1, an initial duration Ti of zero voltage between two voltage pulses, as well as a first frequency, taken equal to an integer multiple of the second frequency, for example twice the second frequency. The initial duration Ti of zero voltage is determined as a function of an initial DC supply voltage of the light-emitting diodes 3, so that by cutting this initial DC supply voltage into voltage pulses having the initial duration Ti of zero voltage between two voltage pulses, the luminous flux emitted by the light-emitting diodes 3 through the PDLC film 4 provides the desired brightness.

[0052] When the initial zero voltage duration Ti is less than the duration of a polarity change phase, the control unit 2 determines a zero voltage duration To value equal to the duration of a polarity change phase, otherwise the control unit 2 determines a zero voltage duration To value equal to the initial zero voltage duration Ti.

[0053] When the initial zero voltage duration Ti is less than the zero voltage duration value To thus determined, the control unit 2 optionally adjusts in this step 130 the square wave voltage value VL, so as to compensate for the loss of brightness due to the increase in the zero voltage duration To compared to the initial zero voltage duration Ti which would have been sufficient to fulfill the desired lighting or signaling function through the PDLC film 4, without optimizing the average transmission rate of the PDLC film 4. The determination of this square wave voltage value VL compensating for the loss of brightness due to the prolonged extinction of the light-emitting diodes 3 possibly uses, for example when the initial duration Ti of zero voltage is zero, a mapping giving the VL value of slot voltage to be applied as a function of the duration of a polarity change phase.

[0054] In an alternative embodiment in this step 130, when the synchronization signal h contains only the phase start indications, the control unit 2 deduces the second frequency therefrom, then uses for example a map to determine the duration of a polarity change period as a function of the second frequency, and / or to determine a square wave voltage value VL to be applied to compensate for the loss of brightness due to the extinction of the light-emitting diodes during this polarity change period, as a function of the second frequency. A different map, depending on the type of PDLC film used, is for example used by the control unit 2, in order to adapt this peak value VL as a function of the response time of the PDLC film.

[0055] In the next step 140, the control unit 2 uses the synchronization signal h to construct the signal in voltage slots having the previously determined slot voltage VL and the previously determined zero voltage duration To, at the first frequency, taken equal to the integer multiple of the second frequency determined in the previous step.

[0056] For example, the control unit 2 forms a falling voltage edge upon each reception of a voltage pulse from the synchronization signal h marking the start of a polarity change phase, then a rising edge after a time interval equal to the duration To of zero voltage determined in step 130. The control unit 2 sends the voltage square waves thus formed to the light-emitting diodes 3.

[0057] Figure 5 illustrates the evolution of the luminous flux 4>LI emitted by the light-emitting diodes 3 when these are powered by a voltage square wave signal constructed according to step 140 of the light emission method 100, but in which the voltage value VL of slot has not been adjusted, in step 130, to compensate for a loss of brightness due to the diodes turning off during the polarity change phases. The corresponding voltage curve as a function of time is the relative luminous flux curve 4>LRI in bold in Figure 5, corresponding to the luminous flux 4>LI whose maximum value is reduced to one. In this example of application of the light emission method 100, the first frequency is chosen to be equal to twice the second frequency, which makes it possible not to power the light-emitting diodes 3 during each polarity change phase.

[0058] Figure 5 also reproduces the evolution as a function of time of the transmission rate TT of the PDLC film 4 subjected to the alternating voltage VAC.

[0059] It can be seen that each duration of extinction of the light-emitting diodes 3, corresponding to a zero relative luminous flux 4>LRI between two slots of relative luminous flux 4>LRI, corresponds to a zone 41 of low luminous flux transmission, that is to say to a phase of change of polarity of the alternating voltage VAC. Therefore, when the light-emitting diodes 3 emit light, this passes through the PDLC film 4 while benefiting from its best transmission rate, 70% in this embodiment of the invention. The average transmission rate of the PDLC film subjected to the luminous flux 4>LI is therefore 70% instead of 68% in the prior art. Of course, other types of PDLC films can be used, with transmission rates that may be different from 70%, for example whose transmission rate is 80% during constant voltage supply phases.

[0060] Another curve shows the evolution of a luminous flux 4>L2 emitted by the light-emitting diodes 3 when these are powered by a voltage square wave signal constructed according to step 140 of the light-emitting method 100, but in which the square wave voltage value VL has been adjusted, in step 130, to compensate for the loss of brightness due to the extinction of the diodes during the phases of change of polarity. The corresponding voltage versus time curve is the relative luminous flux curve 4>LR2 in thin line in Figure 5, corresponding to the value of the luminous flux 4>L2 divided by that of the luminous flux 4>LI.

[0061] Figure 6 shows the evolution of the value of the relative luminous flux 4>LR2 as a function of the second frequency, the corresponding luminous flux 4>L2 being emitted by the light-emitting diodes 3 powered by a voltage square wave signal whose square wave voltage value VL is adjusted as a function of the second frequency, to compensate for the loss of brightness due to the polarity change phases. As represented by the curve L2, the ratio between on the one hand the luminous flux transmitted by the film 4>L2 *TT and on the other hand a luminous flux which would be emitted by the light-emitting diodes 3 powered by the initial DC supply voltage, is kept constant at 70%, therefore at the maximum transmission rate of the PDLC film 4, whatever the value of the second frequency. To obtain this result, the value of the relative luminous flux 4>LR2, therefore of the square wave voltage VL adjusted in step 130, is all the higher as the second frequency is important.

[0062] Figure 6 shows that the value of the relative luminous flux 4>LRI does not vary as a function of the second frequency, since the VL value of the square wave voltage supplying the light-emitting diodes 3 to produce the luminous flux 4>LI is not adjusted to compensate for the loss of brightness due to the polarity change phases. Therefore, as represented by the curve Ll, the ratio between, on the one hand, the luminous flux transmitted by the film 4>LI*TT and, on the other hand, the light flux that would be emitted by the light-emitting diodes 3 supplied by the initial continuous supply voltage, drops drastically as a function of the second frequency.

[0063] The curve L represents the ratio between on the one hand the luminous flux transmitted by the film 4>L*TT in the prior art and on the other hand the flux of light that would be emitted by the light-emitting diodes 3 powered by the initial DC supply voltage. This ratio decreases with the second frequency, still providing more brightness than the light flux 4>LI.

[0064] Similarly, Figure 7 shows the evolution of the value of the relative luminous flux 4>LR2 as a function of the response time of the PDLC film used, the luminous flux 4>L2 being emitted by the light-emitting diodes 3 powered by a voltage square wave signal whose square wave voltage value VL is adjusted as a function of this response time, to compensate for the loss of brightness due to the polarity change phases. As represented by the curve L20, the ratio between on the one hand the luminous flux 4>L2*TT transmitted by the film and on the other hand a light flux which would be emitted by the light-emitting diodes 3 powered by the initial DC supply voltage, is kept constant at 70%, therefore at the maximum transmission rate of the PDLC film 4, whatever the value of the response time of the PDLC film.To obtain this result, the value of the relative luminous flux 4>LR2, therefore of the slot voltage VL adjusted in step 130, is all the higher as the response time of the PDLC film is long.

[0065] Figure 7 shows that the value of the relative luminous flux 4>LRI does not vary as a function of the response time of the PDLC film, since the voltage value VL of the square waves supplying the light-emitting diodes 3 to produce the luminous flux 4>LI is not adjusted to compensate for the loss of brightness due to the polarity change phases. Therefore, as represented by the curve L10, the ratio between on the one hand the luminous flux 4>LI*TT transmitted by the film and on the other hand the light flux that would be emitted by the light-emitting diodes 3 supplied by the initial continuous supply voltage, drops drastically as a function of the response time of the PDLC film.

[0066] The LO curve represents the ratio between, on the one hand, the luminous flux 4>L*TT transmitted by the film in the prior art and, on the other hand, the light flux that would be emitted by the light-emitting diodes 3 powered by the initial DC supply voltage. This ratio decreases with the response time of the PDLC film, nevertheless providing more brightness than the light flux 4>LI.

[0067] Figure 8 illustrates the evolution of the luminous flux 4>LI emitted by the light-emitting diodes 3 when these are powered by a voltage square wave signal constructed according to step 140 of the light-emitting method 100, in which the square wave voltage value VL has not been adjusted, in step 130, to compensate for a loss of brightness due to the extinction of the diodes during the polarity change phases, and in which the first frequency is taken equal to eight times the second frequency. Such an adjustment is not necessary here to produce the desired brightness given that the duration To of zero voltage is greater than the duration of a polarity change phase. The corresponding relative luminous flux curve 4>LRI therefore makes very small square waves broadly framing each zone 41 of low luminous flux transmission.In this example of application of the light emission method 100, the lighting device 1 produces a daytime running light which requires little brightness.

[0068] Finally, Figure 9 represents steps of another method of emitting light 200 according to the invention, implemented by the lighting device 1.

[0069] The first step 210 is the determination, by the control unit 2, of a first frequency of a signal in voltage slots to be applied to the light-emitting diodes 3, and of a duration T o zero voltage between the slots in order to produce the desired brightness through the PDLC film 4, depending on the lighting or signaling function to be performed by the lighting device 1.

[0070] The second step 220 is the application to the light-emitting diodes 3 by the control unit 2 of a signal in voltage square waves at the first frequency determined in the previous step, and having between the square waves the voltage duration To determined previously. During this step 220, the control unit 2 duplicates the signal in voltage square waves and sends it to the control means 5, the signal in voltage square waves constituting a synchronization signal h.

[0071] The third step 230 is the reception of the synchronization signal h by the control means 5.

[0072] Finally, the fourth step 240 is the determination, by the control means 5, of the second frequency from the first frequency, and the application to the PDLC film, by the control means, of an alternating voltage at the second frequency, synchronized with the voltage pulses sent to the light-emitting diodes 3. The second frequency is chosen as a sub-multiple of the first frequency, for example as half of the first frequency. The synchronization is carried out by the control means 5 so as to carry out each change of sign of the alternating voltage between a falling edge and a rising edge of two successive pulses of the voltage square wave signal applied to the light-emitting diodes 3.

[0073] In this other emission method 200 according to the invention, the phases of change of polarity of the alternating voltage VAC may be of duration less than, equal to or greater than the duration To of zero voltage between two voltage pulses supplying the light-emitting diodes 3. Nevertheless, this other emission method 200 makes it possible to improve the average transmission rate of the PDLC film compared to the prior art.

[0074] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention. In particular, the characteristics of the different variant embodiments of the invention contemplated in this application, may be combined to carry out the invention, to the extent that these variants are not incompatible with each other.

Claims

CLAIMS

1. Lighting device (1) for a vehicle, comprising: - at least one photonic transmitter (3), - a control unit (2) of the photonic emitter (3), capable of applying voltage pulses polarizing the photonic emitter (3) so that the photonic emitter emits light, - a liquid crystal film (4), in particular of the PDLC type, capable of transmitting or blocking at least part of the light emitted by the photonic emitter (3), and - control means (5) capable of applying an alternating voltage (VAC) to said film (4), the lighting device (1) being characterized in that it comprises means (10) for synchronizing the alternating voltage (VAC) applied to the film (4), with the voltage pulses applied to the photonic emitter (3).

2. Lighting device (1) for vehicle according to claim 1, wherein the control unit (2) and / or the control means (5) are configured to use the synchronization means (10) so that none of the voltage pulses are applied during a change of polarity of the alternating voltage (VAC).

3. Lighting device (1) for vehicle according to claim 1 or 2, wherein the control unit (2) is adapted to apply the voltage pulses at a first frequency, and wherein the control means (5) are adapted to apply to said film (4) the alternating voltage (VAC) with a second frequency.

4. Lighting device (1) for a vehicle according to any one of claims 1 to 3, in which the synchronization means (10) comprise means for sending a synchronization signal (h) to the control unit (2).

5. Lighting device (1) for a vehicle according to claims 3 and 4, in which the control unit (2) is capable of producing, from the synchronization signal (h), the voltage pulses whose first frequency is equal to the second frequency or to an integer multiple of the second frequency.

6. Lighting device (1) for a vehicle according to any one of claims 1 to 3, in which the synchronization means comprise means for sending a synchronization signal to the control means (5).

7. Lighting device (1) for a vehicle according to claim 6, in which the control means (5) are capable, from the synchronization signal, of timing each change of sign of the alternating voltage (VAC) between a falling edge and a rising edge of two successive pulses among said voltage pulses.

8. Lighting device (1) for a vehicle according to claim 3 and claim 6 or 7, wherein the control means (5) are capable of making the second frequency of the alternating voltage (VAC) equal to the first frequency or to an integer sub-multiple of the first frequency.

9. Lighting device (1) for a vehicle according to any one of claims 3 to 8, in which the control unit (2) comprises means for determining a peak voltage value (VL) of the voltage pulses, as a function of a duration of a phase of change of polarity of the alternating voltage (VAC) and / or of the second frequency and / or of a response time of the film (4).

10. Front left or front right optical unit for a vehicle, comprising a lighting device (1) according to any one of claims 1 to 9, and capable of fulfilling a lighting or signaling function, the optical unit comprising a closing glass, the film (4) being arranged between on the one hand the control unit (2), the means of control (5), the photonic transmitter (3) and on the other hand the closing glass, so as to conceal the control unit (2), the control means (5) and the photonic transmitter (3) when the lighting or signaling function is not activated.

11. A method of emitting light implemented by the lighting device (1) according to claim 3 and claim 4 or 5, or by the optical unit according to claim 10 taken in dependence on claim 3 and claim 4 or 5, comprising the steps of: - application to the film (4) by the control means (5), of the alternating voltage (VAC) at the second frequency, - reception by the control unit (2) of the synchronization signal (h), - application to the photonic transmitter (3), by the control unit (2), of voltage pulses at the first frequency, the voltage pulses being synchronized with the alternating voltage (VAC) by the control unit (2).

12. Method of emitting light implemented by the lighting device (1) according to claim 3 and one of claims 6 to 8, or by the optical unit according to claim 10 taken in dependence on claim 3 and one of claims 6 to 8, comprising the steps of: - application to the photonic transmitter (3) by the control unit (2), of voltage pulses at the first frequency, - reception of the synchronization signal by the control means (5), - application to the film (4) by the control means (5) of the alternating voltage (VAC) at the second frequency, the alternating voltage (VAC) being synchronized with the voltage pulses by the control means (5).

13. A light emitting method according to claim 11 or 12, wherein each change of sign of the alternating voltage (VAC) occurs between a falling edge and a rising edge of two successive ones of said voltage pulses.

14. A light emitting method according to claim 13, further comprising a step of determining a peak voltage value (VL) of the voltage pulses, as a function of a duration of a polarity change phase of the alternating voltage (VAC) and / or of the second frequency and / or of a response time of the film (4).

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