HEAD-UP DISPLAY DEVICE

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention generally relates to the technical field of head-up display.

[0002] It relates more particularly to a head-up display device, for example for a motor vehicle. TECHNOLOGICAL BACKGROUND

[0003] Head-up display devices, more commonly referred to as HUDs (from the English "head-up displays"), are known, comprising an image-generating device and an optical system, generally arranged inside a housing. The image-generating device is designed to generate a light beam. The optical system is configured to project the light beam towards a partially transparent blade through a window formed in the housing.

[0004] After partial reflection on the partially transparent blade, the light beam reaches the driver's eyes so as to form a virtual image seen by the driver beyond the partially transparent blade, that is to say, in usual applications, at the front of the vehicle.

[0005] The intensity of the light beam generated by the image generating device is usually adjusted according to the ambient brightness: the higher the ambient brightness, the higher the intensity of the light beam must be.

[0006] However, in certain solar illumination configurations (in particular for certain relative positions of the sun, the housing window and the optical system), the solar flux follows the opposite path of the light beam forming the virtual image. The solar flux is then concentrated on the image generation device, which causes it to heat up. Such heating is detrimental to the operation of the image generation device and can even lead to its deterioration.

[0007] There are head-up display protection strategies based on temperature measurements inside the head-up display. For example, when the temperature inside the head-up display exceeds a threshold value, the image generating device of the head-up display may shut down or a moving part may block the solar flux.

[0008] One drawback of this strategy is that to trigger the shutdown of the head-up display, a temperature rise of the head-up display must necessarily occur. It can be difficult to stop this temperature rise at the most opportune moment. Indeed, the shutdown of the head-up display does not necessarily mean an immediate reduction in temperature. PRESENTATION OF THE INVENTION

[0009] In this context, the present invention provides a head-up display device for a vehicle comprising an image generating device designed to generate a light beam; an optical system configured to project said light beam towards a partially transparent blade and comprising a first mirror arranged to reflect said light beam; an optical sensor designed to determine a luminous flux; and a protection module capable of controlling a reduction in intensity of said light beam on the basis of said luminous flux; a field of vision of said optical sensor is oriented towards a reflection surface of said first mirror and the optical sensor comprises a matrix of several photoelectric cells.

[0010] Thus, thanks to the invention, the optical sensor makes it possible to determine the solar flux which penetrates into the head-up display device and which could damage the image generation device. Indeed, the solar flux reflected by the first mirror is responsible for the heating of the image generation device.

[0011] Therefore, by knowing the intrinsic properties of the image generating device, or by calibration, it is possible to anticipate the heating of the image generating device subjected to the solar flux. It is therefore possible to propose a thermal protection strategy, for example here by reducing the intensity of the virtual image, which is safe and temporally relevant, i.e. which can anticipate the rise in temperature. In other words, this strategy makes it possible to determine an effect, here the solar flux penetrating into the head-up display device, so as to prevent the cause which is the heating of the image generating device.

[0012] Other non-limiting and advantageous characteristics of the head-up display device according to the invention, taken individually or in all technically possible combinations, are as follows: said optical sensor comprises at least one photoelectric cell and said field of vision is oriented towards the center of the reflection surface of said light beam on said first mirror; said optical sensor is arranged near said image generating device such that the angle formed between a path of said light beam between said image generating device and said first mirror and a main observation axis of said optical sensor is less than 45 degrees; said first mirror comprises a first optical filter arranged on its reflecting surface and having a reflection coefficient of less than 5% for radiation included in a given infrared range; said first mirror comprises a second optical filter arranged on its reflecting surface and having a reflection coefficient of less than 5% for a given rectilinear polarization of a light beam;said optical sensor is capable of determining a luminous flux from radiation in the visible range; said optical system comprises a second mirror which is concave, on which said light beam is reflected and which is located between said first mirror and said partially transparent plate with respect to the path of said light beam; said second mirror is movable and can be moved to a protection position where the path of the light beam between the optical system and the partially transparent plate is interrupted; said protection module further controls said reduction in intensity on the basis of at least one of the following parameters: an ambient temperature of said image generating device; an external brightness; and a junction temperature of a semiconductor element.

[0013] The invention also provides a method for protecting a head-up display device comprising: an image generation device designed to generate a light beam; an optical system configured to project said light beam towards a partially transparent blade and comprising a first mirror arranged to reflect said light beam; an optical sensor comprising a matrix of several photoelectric cells and having a field of vision oriented towards the reflection surface of said light beam on said first mirror; and a protection module; said method comprising the following steps: determining, by said optical sensor, a luminous flux; and reducing, by said protection module, the intensity of said light beam on the basis of said luminous flux determined by said optical sensor.

[0014] Other non-limiting and advantageous characteristics of the protection method according to the invention, taken individually or in all technically possible combinations, are as follows: the method comprises at least one of the following steps: determining, by a temperature sensor, an ambient temperature of said head-up display device; determining, by a measuring circuit, a junction temperature of a semiconductor element; and determining, by an external brightness sensor, an external brightness; and comprising a step of reducing, by said protection module, the intensity of said light beam on the basis of at least one of the following parameters: said ambient temperature, said junction temperature and said external brightness; the method comprises a step of moving, controlled by said protection module on the basis of said luminous flux, a second movable mirror to a protection position where the path of the light beam between the optical system and the partially transparent plate is interrupted;said optical sensor determines a luminous flux from radiation in the visible range.; DETAILED DESCRIPTION OF THE INVENTION

[0015] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0016] On the attached drawings: [ Fig. 1 ] schematically represents in section a head-up display device according to the invention; [ Fig. 2 ] represents a reduction curve based on a temperature measurement; and [ Fig. 3 ] represents a block diagram of a sequence of steps allowing the implementation of a thermal protection process.

[0017] Of course, various other modifications may be made to the invention within the scope of the appended claims.

[0018] Here, a head-up display device 1 is described in the case where it is used within a motor vehicle.

[0019] As shown in the figure 1 , the head-up display device 1 comprises an image generation device 10, an optical system 20, an optical sensor 60 and a protection module 30.

[0020] The head-up display device 1 is controlled by a computer. The computer is programmed to control and / or connect the various elements of the head-up display device 1. Here, the computer is the vehicle's computer. Alternatively, the head-up display device could comprise a dedicated computer.

[0021] The image generating device 10 is designed to generate a light beam called a modulated light beam L. For this, the image generating device 10 comprises a light source 12 and a modulator 11.

[0022] The modulator 11 consists of a matrix of elements whose transmittance varies over time. The modulator 11 receives a source light beam generated by the light source 12 and transmits the modulated light beam L. Here, the light source 12 is polychromatic to form color images. Thanks to its matrix of elements, the modulator 11 spatially modulates the source light beam so as to form the modulated light beam L.

[0023] Here, the modulator 11 is a TFT (thin film transistor) type liquid crystal screen, that is to say a matrix of liquid crystal cells each controlled by a thin film transistor (hence the name of the screen called TFT). For each element of the matrix, three cells, each associated with a color filter, for example one for blue, one for green and one for red, make it possible to control the transmittance, by orienting the liquid crystals, in a stable manner and with a low response time.

[0024] Alternatively, to form the modulated light beam L, a rear face of a diffuser is scanned by a laser beam generated by a set of laser diodes, the scanning being carried out for example by a moving mirror. Then, a front face of the diffuser generates the modulated light beam.

[0025] The optical system 20 is arranged to project the modulated light beam L towards a partially transparent blade 70 along a determined path. The modulated light beam L is reflected by the partially transparent blade 70 towards an observation zone where the driver's eyes are located. The path of the modulated light beam L between the image generation device 10 and the partially transparent blade 70 defines an optical path.

[0026] The partially transparent blade 70 is oriented so as to reflect a portion of the modulated light beam L towards an observation zone in which the driver's eyes are located, so as to form a virtual image. The partially transparent blade 70 may be a portion of a windshield of the vehicle or a combiner, i.e., a partially transparent blade dedicated to the head-up display. Such a combiner would be placed between the windshield and the driver's eyes.

[0027] The virtual image contains indications or information intended for the driver of the vehicle, for example in the form of regulatory symbols and / or a speed indicator and / or an engine speed indicator and / or a fault indicator and / or a navigation instruction.

[0028] As shown in the figure 1 , the optical system 20 here comprises a first mirror 21 (sometimes called a "folding mirror") which is a plane mirror. The optical system 20 also comprises a second mirror 25 which is a concave mirror. The second concave mirror 25 allows for example the enlargement of small images generated by the image generation device 10 to form virtual images of adequate size. Here, the second mirror 25 is movable, for example in rotation, so as to adjust the position of the virtual image to the observation zone, which depends on the size of the driver. The first mirror 21 directs the modulated light beam L produced by the image generation device 10 towards the second mirror 25.

[0029] Conventionally, the head-up display device 1 is here included in a protective housing 50 or a casing. An opening 51 is made in the housing 50 to allow the modulated light beam L to propagate towards the partially transparent blade 70. The opening 51 can be covered by a transparent material such as a glass blade.

[0030] In the following, the adjective “interior” refers to the interior of the head-up display device 1, therefore here to the interior of the housing 50. The adjective “exterior” refers to the exterior of the head-up display device 1, therefore here to the exterior of the housing 50. The adjective “exterior” refers, for example, to the passenger compartment of the vehicle.

[0031] As shown in the figure 1 , in certain solar illumination configurations (in particular for certain relative positions of the sun, the window of the housing and the optical system), a solar light beam LS can penetrate inside the head-up display device 1, here through the opening 51. This happens in particular when the sun, the partially transparent blade 70 and the second mirror 25 are substantially aligned, as shown in the figure 1 .

[0032] There figure 1 represents a particular example of propagation of the solar light beam LS. In this example the solar light beam LS travels the optical path in the opposite direction. This scenario corresponds to the most damaging situation for the head-up display device 1 since the second mirror 25 focuses, after reflection on the first mirror 21, the solar light beam LS at the center of the modulator 11. In other cases, the solar light beam LS can illuminate other areas of the modulator 11 but is less focused.

[0033] The modulator 11 is relatively absorbent. Thus, when the solar light beam LS reaches the modulator 11, the solar light beam LS quickly causes the modulator 11 and more generally the image generation device 10 to heat up. Such heating is detrimental to the operation of the image generation device 10 and can lead to its deterioration. For example, here, from a certain temperature, the liquid crystals of the modulator 11 can lose their polarization.

[0034] To limit the heating of the image generation device 10, the first mirror 21 here comprises a first optical filter 22 arranged on its reflecting surface and having a reflection coefficient of less than 5% for radiation included in a given infrared range. The solar light beam LS is therefore filtered during its reflection on the first mirror 21. In other words, radiation included in a given infrared range, here this given range corresponds to wavelengths less than 700 nm, is only very weakly reflected, at less than 5%, on the first mirror 21.

[0035] Thanks to the first optical filter 22, the infrared part of the solar light beam LS does not reach the image generation device 20. This first optical filter 22 does not disturb the operation of the head-up display device 1 because the latter operates in the visible range.

[0036] To limit the heating of the image generation device 10, the first mirror 21 here comprises a second optical filter 23 arranged on its reflecting surface and having a reflection coefficient of less than 5% for a given rectilinear polarization of a light beam. Indeed, the modulator 11 polarizes the modulated light beam L in a rectilinear manner in a given direction. The second optical filter 23 is arranged so as to almost completely reflect a light beam in this given direction but has a reflection coefficient of less than 5% for a rectilinear polarization perpendicular to this given direction. Thus only a portion of the solar light beam LS, that which has the same polarity as the modulated light beam L, is reflected towards the image generation device 20, which limits its heating without disturbing its operation.

[0037] Alternatively, provision could be made for the first optical filter and / or the second optical filter to be arranged elsewhere, for example on the surface of the second mirror or on the surface of the modulator. Alternatively, provision could be made for the first optical filter and the second optical filter to be a single optical filter.

[0038] As shown in the figure 1 , the head-up display device 1 also comprises a protection module 30 capable of controlling a reduction in the intensity of the modulated light beam L. The protection module 30 can for example control a reduction in the intensity of the source light beam emitted by the light source 12, up to its total stopping if necessary. For this, the protection module 30 can for example control a reduction in the electric current supplied to the light source 12. The actions carried out by the protection module 30 are made possible thanks to a computer internal to the protection module 30 or via the computer of the head-up display device.

[0039] To determine the magnitude of the intensity reduction, the protection module 30 can for example be connected to a temperature sensor 40 located near the modulator 11 or on the electronic board of the light source 12. The temperature sensor 40 is designed to measure the ambient temperature TA of the image generation device 10. The temperature sensor 40 is for example a thermistor with a negative temperature coefficient.

[0040] Here, the protection module 30 is designed to control the reduction of intensity of the modulated light beam L when the ambient temperature TA is higher than an ambient threshold. The ambient threshold is for example between 60°C and 100°C, the ambient threshold is preferably between 70°C and 100°C.

[0041] This measurement of ambient temperature TA makes it possible to implement a reduction strategy called “derating”, characterized by a reduction in the performance of the image generation device 10. This strategy consists of defining the maximum intensity of the modulated light beam L generated by the image generation device 10, i.e. the maximum luminance of the virtual image, as a function of the ambient temperature TA.

[0042] As shown in the figure 2 , the amplitude of the intensity reduction, represented by a first coefficient C1 of reduction of the intensity of the modulated beam L, can be given by a reduction curve called “derating curve” defined in three intervals: i) an interval where the ambient temperature TA is less than or equal to a threshold value TA1, the image generation device 10 can then operate at its maximum capacity, the first coefficient C1 is equal to 1; ii) an interval where the ambient temperature TA is greater than the threshold value TA1 and less than a maximum value TA2, the protection module 30 can reduce the intensity of the modulated light beam L, the amplitude of the reduction is obtained by a projection on a decreasing linear curve, the first coefficient C1 is between 0 and 1; iii) an interval where the ambient temperature TA is greater than the maximum value TA2, the protection module 30 can command the stopping of the image generation device 10, the first coefficient is equal to 0.

[0043] Here, the head-up display device 1 also comprises a measuring circuit designed to measure an internal temperature at the image generating device 10, for example at the light source 12.

[0044] Thus, when the image generating device 10 comprises a semiconductor element, for example a semiconductor element of the TFT screen or a light-emitting diode of the light source 12, the measuring circuit can be designed to measure a junction temperature TJ of the semiconductor element. If the limit temperature of the semiconductor element is reached, the protection module 30 controls the stopping of the image generating device 10.

[0045] The protection module 30 can then control a reduction in the intensity of the light beam L when the junction temperature TJ is higher than a junction threshold. The junction threshold is preferably between 95°C and 110°C, which typically corresponds to the thermal limits of a TFT screen. The junction threshold may, for example, be equal to 110°C.

[0046] It is then possible to provide for the implementation of a second reduction strategy associated with the operation of the semiconductor element(s) and based on the junction temperature TJ. This means that the protection module 30 can reduce the intensity of the modulated light beam L to regulate the junction temperature. As previously, the amplitude of the reduction, represented by a first coefficient C2, of intensity can be given by a second reduction curve defined in intervals.

[0047] The intensity of the modulated light beam L at a given instant also depends on external factors such as an exterior brightness LEX. Indeed, it is important to consider the exterior brightness LEX around the driver to adapt the luminance of the virtual image, for example to adapt to passing through a tunnel, exiting a tunnel or a glare situation. Thus, it is possible to implement a so-called "dimming" attenuation strategy to adapt the luminance of the virtual image according to the exterior brightness LEX. This attenuation strategy can cause the image generation device 10 to increase or decrease the intensity of the modulated light beam L by a third coefficient C3. To implement the attenuation strategy, the protection module 30 can for example be connected to an external brightness sensor located in the passenger compartment of the vehicle.

[0048] Thus, during a thermal protection process represented in figure 3 , the corrected intensity IC of the modulated light beam at a given instant may correspond to a setpoint intensity, for example previously chosen by the driver to obtain a desired luminance of the virtual image, weighted by the first coefficient C1 and / or the second coefficient C2 and / or the third coefficient C3. This means that the luminance of the virtual image at a given instant may correspond to an original luminance weighted by the first coefficient C1 and / or the second coefficient C2 and / or the third coefficient C3, called corrected luminance.

[0049] Thus, the protection process may include: a step e3 in which the ambient temperature TA is determined by the temperature sensor 40 and in which the protection module 30 determines a first coefficient C1 for reducing the intensity of the modulated light beam L on the basis of the ambient temperature TA; a step e4 in which the junction temperature TJ is determined by the measuring circuit and in which the protection module 30 determines a second coefficient C2 for reducing the intensity of the modulated light beam L on the basis of the junction temperature TJ; a step e5 in which the external brightness LEX is determined by the external brightness sensor and in which the protection module 30 determines a third coefficient C3 for adapting the intensity of the modulated light beam L;a step e6 in which the corrected intensity IC is determined by the protection module 30 on the basis of the first coefficient C1 and / or the second coefficient C2 and / or the third coefficient C3.;

[0050] The protection module 30 can therefore be based on the following parameters to control a reduction in the intensity of the modulated light beam L: ambient temperature TA; external brightness LEX; junction temperature TJ.

[0051] Although they may be sufficient to protect the head-up display device 1 against internal thermal stresses due to its operation, strategies based on temperature measurements, such as reduction strategies, have limitations in protecting the head-up display device 1 from the external thermal stress that is solar radiation.

[0052] Indeed, the solar light beam LS is an external thermal stress which can be three to four times greater than the internal thermal stresses and which can generate rapid local heating, it is therefore appropriate to anticipate this heating. Anticipating this heating means for example that the protection module 30 can reduce the intensity of the modulated light beam L in a preventive manner to prevent deterioration of the image generation device 10 and in particular of the modulator 11.

[0053] Furthermore, it can also be provided that the protection module 30 can control the closing of the opening 51 by means of a movable part so as to prevent the solar light beam LS from penetrating inside the head-up display device 1. It can also be provided that the second mirror 25, which here is movable, is moved to a protection position where the path of the modulated light beam L between the optical system 10 and the partially transparent blade 70 is interrupted, that is to say to a position where the solar light beam LS cannot reach the image generation device 10.

[0054] In this context, the head-up display device 1 comprises an optical sensor 60 designed to determine a luminous flux FL. The optical sensor 60 has a field of view C that can be defined by a solid angle through which the optical sensor 60 is sensitive to electromagnetic radiation.

[0055] As shown in the figure 1 , the field of vision C of the optical sensor 60 is oriented towards the first mirror 21. Thus, the optical sensor 60 can determine the luminous flux FL coming from the first mirror 21.

[0056] In the case where, as in the figure 1 , a solar light beam LS enters the head-up display device 1, the light flux FL coming from the first mirror 21 is notably due to the solar light beam LS. Here, more specifically, the arrangement and orientation of the optical sensor 60 mean that the light flux FL, determined by the optical sensor 60, coming from the first mirror 21 is mainly due to the solar light beam LS compared to the modulated light beam L. Indeed, the light flux FL determined by the optical sensor 60 is a fraction of the light flux incident on the image generation device 10.

[0057] Based on the luminous flux FL determined by the optical sensor 60, correction factors make it possible to determine a luminous flux incident on the image generation device 10. These correction factors, which can be determined by calibration, depend for example on the position and orientation of the optical sensor 60.

[0058] For example, for a given position of the sensor, the intensity of the modulated light beam L can be varied and a first mathematical correlation model can be constructed between the intensity of the modulated light beam L and the luminous flux values ​​FL measured by the optical sensor 60. The intensity of a light beam entering the head-up display device 1 can then be varied and a second mathematical correlation model can be constructed between the intensity of the solar light beam LS and the luminous flux values ​​FL measured by the optical sensor 60. Finally, a third mathematical correlation model can be constructed by varying both the intensity of the modulated light beam L and the intensity of a light beam entering the head-up display device 1.

[0059] According to a first possible embodiment, the optical sensor 60 can comprise a photoelectric cell and the field of vision C is then oriented towards a remarkable point R which is the center of the reflection surface of the modulated light beam L on the first mirror 21. Here, this remarkable point R also corresponds to the image of the center of the modulator 11 on the first mirror 21.

[0060] When the solar light beam LS enters the head-up display device 1, the solar light beam LS is focused towards the remarkable point R because it lies on the optical axis of the second concave mirror 25. The luminous flux FL from the first mirror 21, which here is mainly due to the solar light beam LS, can therefore be determined with high precision by orienting the field of view C of the optical sensor 60 towards this remarkable point R.

[0061] According to a second embodiment, the optical sensor 60 may comprise a matrix of several photoelectric cells and the field of vision C is then oriented towards the reflection surface of the modulated light beam L on the first mirror 21. A matrix of photoelectric cells makes it possible to improve the accuracy of the determination of the solar luminous flux FL by combining measurements at several points on the surface of the first mirror 21. A matrix of several photoelectric cells makes it possible to more easily detect hot spots which could form on the modulator 11 by focusing the solar light beam LS. A hot spot may be very local, which may damage the image generation device 10 without significantly raising the ambient temperature.

[0062] Remarkably, orienting the optical sensor 60 towards the first mirror 21 rather than towards the modulator 11 allows a better determination of the incident light flux on the image generation device 10. Indeed, since the modulator is relatively absorbent, only a small part of the solar light beam would be reflected towards an optical sensor oriented towards the modulator, which would make the determination of its intensity imprecise. In addition, this positioning of the optical sensor 60 facilitates its integration inside the head-up display device 1.

[0063] Here, the optical sensor 60 operates mainly in the visible range. Indeed, since the infrared part of the solar light beam LS is hardly reflected by the first mirror 21, only the visible part of the solar light beam LS can cause the image generation device 10 to heat up. The optical sensor 60 is therefore designed to measure a luminous flux FL of radiation in the visible range, for example for a wavelength range from 700 nm to 400 nm.

[0064] To best determine the incident light flux on the image generation device 10, the optical sensor 60 is here arranged close to the image generation device 10, that is to say it is closer to the image generation device 10 than to the first mirror 21 or the second mirror 25.

[0065] For example, the optical sensor 60 may be positioned such that the angle formed between a path of the modulated light beam L between the image generating device 10 and the first mirror 21 and a main observation axis of the optical sensor 60 is less than 45 degrees and preferably less than 20°. The path of the modulated light beam L between the image generating device 10 and the first mirror 21 corresponds to the section of the optical path going from the image generating device 10 to the first mirror 21.

[0066] The main axis of observation P of the optical sensor 60 is defined as the direction in which the optical sensor 60 is most sensitive to radiation. Here, the main axis of observation P corresponds to the average direction of the field of vision C of the optical sensor 60 passing through the center of the optical sensor 60.

[0067] As shown in the figure 1 , the main observation axis P is here oriented towards the reflection surface of the modulated light beam L on the first mirror 21. More specifically, the main observation axis P is oriented towards the remarkable point R. Here, so that the angle formed between the optical path and the main observation axis P of the optical sensor 60 is less than 20 degrees, the optical sensor 60 is arranged close to the modulator 11. It is nevertheless advisable not to obstruct the optical path to allow the formation of the virtual image.

[0068] Thus, thanks to the optical sensor 60, the protection module 30 can control the reduction of intensity of the modulated light beam L, i.e. a reduction of the performance of the image generation device 10, on the basis of a luminous flux FL coming from the first mirror 21. If the luminous flux FL is high, for example because of an intense solar light beam LS, the protection module 30 can control the reduction of intensity of the modulated light beam L.

[0069] As shown in the figure 3 , the method for protecting the head-up display device 1 comprises: a step e1 of determination by the optical sensor 60 of a luminous flux FL; and a step e2 of reduction, controlled by the protection module 30, of the intensity of the modulated light beam L on the basis of the luminous flux FL.

[0070] Thus, thanks to the optical sensor 60, the protection module 30 can control a solar protection strategy, consisting of the steps of determining a luminous flux e1 and of reducing the intensity e2, in order to anticipate the heating of the head-up display device 1 and in particular of the image generation device 10. Here, the solar protection strategy represents an additional level of security compared to the reduction strategies.

[0071] The corrected intensity IC of the modulated light beam L therefore depends on the intensity reduction controlled on the basis of the luminous flux FL determined by the optical sensor 60. Here, the corrected intensity IC corresponds to the setpoint intensity weighted, among other things, by a fourth coefficient C4 determined on the basis of the luminous flux FL.

[0072] If the intensity of the solar light beam LS is such that stopping the head-up display device 1 is not sufficient to protect it, the protection module can control the closing of the housing 50 or the movement of the second mirror 25. The movement of the second mirror 25 can therefore be controlled by the protection module 30 on the basis of the luminous flux FL.

[0073] In addition, it can be provided that the solar protection strategy takes into account the ambient temperature TA. Thus, here, the protection module 30 uses a correspondence table indicating the maximum intensity of the modulated light beam L as a function of the luminous flux FL and for a given ambient temperature TA. This correspondence table can be obtained by calibrations by subjecting the image generation device 10 to determined luminous fluxes and by measuring its heating. The correspondence table can be based on the mathematical correlations presented previously.

[0074] Here, the correspondence table provided to the protection module 30 the fourth coefficient C4 to control the reduction of intensity of the light beam L. The fourth coefficient C4 depends on the luminous flux FL. Here, the fourth coefficient C4 also depends on the ambient temperature TA. The stopping of the head-up display device 1 can correspond to a fourth coefficient C4 equal to 0.

[0075] Alternatively, it could be provided that to determine the fourth coefficient, the protection module uses a protection curve based on the luminous flux determined by the optical sensor and characterized by two luminous flux limit values.

[0076] Alternatively, it could be provided that the heating of the image generating device is estimated by simulation on the basis of the luminous flux incident on the image generating device and its intrinsic properties such as its absorbance and its thermal capacity.

[0077] As shown in the figure 3, the solar protection strategy can also take into account the corrected intensity IC of the modulated light beam L. Indeed, when the optical sensor 60 determines the luminous flux FL coming from the first mirror 21, it is possible to estimate the part of this luminous flux FL which is due to the modulated light beam L since the corrected intensity IC of the modulated light beam L at a given instant is known. Since the relative positions of the image generation device 10, the first mirror 21 and the optical sensor 60 are fixed, it is easy, by calibration, to determine the luminous flux due to the modulated light beam L at a given instant. Consequently, it is possible to precisely determine the part of the luminous flux FL which is due to the solar light beam LS.

[0078] In other words, the protection module 30 can take into account the corrected intensity IC to control the reduction of intensity in step e2, consequently forming a feedback loop.

[0079] Thus, the different strategies (reduction, attenuation and solar protection) interact optimally in order to maximize the luminance of the virtual image while protecting the head-up display device 1 from heating which could damage it.

Claims

1. Head-up display device (1) for a vehicle comprising: - an image generation device (10) designed to generate a light beam (L); - an optical system (20) configured to project said light beam (L) towards a partially transparent plate (70) and comprising a first mirror (21) arranged to reflect said light beam (L); - an optical sensor (60) designed to determine a light flux (FL), a field of view (C) of said optical sensor (60) being oriented towards a reflection surface of said first mirror (21); and - a protective module (30) capable of controlling a reduction in intensity of said light beam (L) on the basis of said light flux (FL); characterized in that said optical sensor (60) comprises a matrix of a plurality of photoelectric cells.

2. Head-up display device (1) according to Claim 1, wherein said optical sensor (60) comprises at least one photoelectric cell and said field of view (C) is oriented towards the centre (R) of the reflection surface of said light beam (L) on said first mirror (21).

3. Head-up display device (1) according to any of Claims 1 to 2, wherein said optical sensor (60) is disposed in the vicinity of said image generation device (10) such that the angle formed between a path of said light beam (L) between said image generation device (10) and said first mirror (21) and a main observation axis (P) of said optical sensor (60) is less than 45 degrees.

4. Head-up display device (1) according to any of Claims 1 to 3, wherein said first mirror (21) comprises a first optical filter (22) disposed on its reflecting surface and having a reflection coefficient of less than 5% for radiation within a given infrared range.

5. Head-up display device (1) according to any of Claims 1 to 4, wherein said first mirror (21) comprises a second optical filter (23) disposed on its reflecting surface and having a reflection coefficient of less than 5% for a given linear polarization of a light beam.

6. Head-up display device (1) according to any of Claims 1 to 5, wherein said optical sensor (60) is capable of determining a light flux (FL) from radiation in the visible domain.

7. Head-up display device (1) according to any of Claims 1 to 6, wherein said optical system (20) comprises a second mirror (25), which is concave and on which said light beam (L) reflects and which is located between said first mirror (21) and said partially transparent plate (70) with respect to the path of said light beam (L).

8. Head-up display device (1) according to Claim 7, wherein said second mirror (25) is movable and can be moved to a protective position where the path of the light beam (L) between the optical system (20) and the partially transparent plate (70) is interrupted.

9. Head-up display device (1) according to any of Claims 1 to 8, wherein said protective module (30) also controls said reduction in intensity on the basis of at least one of the following parameters: - an ambient temperature (TA) of said head-up display device (1); - an external brightness (LEX); and - a junction temperature (TJ) of a semiconductor element.

10. Method for protecting a head-up display device (1) comprising: - an image generation device (10) designed to generate a light beam (L); - an optical system (20) configured to project said light beam (L) towards a partially transparent plate (70) and comprising a first mirror (21) arranged to reflect said light beam (L); - an optical sensor (60) comprising a matrix of a plurality of photoelectric cells and having a field of view (C) oriented towards the reflection surface of said light beam (L) on said first mirror (21); and - a protective module (30); said method comprising the following steps: - said optical sensor (60) determining (e1) a light flux (FL); and - said protective module (30) reducing (e2) the intensity of said light beam (L) on the basis of said light flux (FL) determined by said optical sensor (60).

11. Protection method according to Claim 10, comprising at least one of the following steps: - a temperature sensor (40) determining (e3) an ambient temperature (TA) of said head-up display device (1); - a measurement circuit determining (e4) a junction temperature (TJ) of a semiconductor element; and - an external brightness sensor determining (e5) an external brightness (LEX); and comprising a step of said protective module (30) reducing (e6) the intensity of said light beam (L) on the basis of at least one of the following parameters: said ambient temperature (TA), said junction temperature (TJ) and said external brightness (LEX).

12. Protection method according to Claim 10 or 11, comprising a step of moving, controlled by said protective module (30) on the basis of said light flux (FL), a second movable mirror (25) to a protection position where the path of the light beam (L) between the optical system (20) and the partially transparent plate (70) is interrupted.

13. Protection method according to any of Claims 10 to 12, wherein said optical sensor (60) determines a light flux (FL) from radiation in the visible domain.