Abstract:
A method for assisting the daytime driving of a motor vehicle, including the steps of: placing at least one variable-transmission screen between the driver of the motor vehicle and the road scene in front of the vehicle, measuring, by means of at least one sensor, the brightness of the road scene in front of the vehicle, and processing the brightness measurement signal to convert it into a signal for controlling the coefficient of transmission of the variable-transmission screen. In one embodiment, the method includes the further step of transmitting the control signal by remote control waves to a receiver fitted to the variable-transmission screen for modulating the coefficient of transmission of the variable-transmission screen as a function of the brightness measured by the sensor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase application of PCT/EP2013/056041 filed Mar. 22, 2013, which claims priority to French Application No. 1252674 filed Mar. 26, 2012, which applications are incorporated herein by reference and made a part hereof. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods and devices intended to provide assistance to the driving of motor vehicles, particularly in daytime driving, when the external brightness is variable to a considerable degree, for example in sunny weather, when various events may alternatively illuminate the road scene strongly or darken it. 
     2. Description of the Related Art 
     Various solutions have been tested for the mitigation of this situation. 
     In a conventional solution, the driver wears sunglasses in order to avoid being dazzled by the sun and to distinguish the details of the road scene in front of the vehicle more clearly. 
     The drawback of these sunglasses is that the driver must remove them whenever the vehicle travels through a shaded area or a tunnel, or when the sun is obscured by dense clouds. This is because the attenuation provided by sunglasses, which is intrinsically constant, has the same effects in the absence of any inconvenience due to the sun, making it harder for the driver to perceive the details of the road scene. 
     An improvement of these conventional sunglasses has been the provision of photochromic glasses lenses which are sensitive to the ambient light level. These lenses have the property of changing color according to the amount of ultraviolet radiation to which they are subjected. The well-known drawback of these photochromic lenses is that they only return very gradually to their state of greater clarity in the absence of ultraviolet radiation, the time taken for the return to the clear state being much longer than the time taken for coloration. 
     A further drawback is that, since they only react to the presence of ultraviolet rays, their use for driving motor vehicles generally yields no benefit, as the windshields of most modern vehicles block the transmission of these ultraviolet rays. Photochromic lenses are therefore ineffective for preventing the dazzling of drivers of motor vehicles due to large variations of external brightness. 
     Other solutions, more complicated than simple sunglasses or photochromic sunglasses, have been proposed. For example, U.S. Pat. No. 3,961,181 discloses a screen for vehicle drivers, which protects both eyes separately but simultaneously from the effect of daytime dazzling due to sunlight and nighttime dazzling due to headlights of approaching vehicles. The screen includes a series of vertical, juxtaposed, contiguous cells which can be controlled electrically, for example liquid crystals, these cells being transparent in the absence of an applied voltage, and darkening as a function of the voltage applied. An array of photosensitive sensors whose electrical resistance increases with brightness is associated with the series of cells. Shields are placed in front of the sensors to create shadows on these sensors. Electrical control means are interposed between the sensors and the cells of the screen to control the transmission factor of the cells as a function of the signals received from the various sensors. 
     This structure is relatively difficult to create and adjust, the aim being to darken only those cells located between the source of dazzling and the driver&#39;s eyes. 
     Additionally, U.S. Pat. No. 4,848,890 discloses glasses whose lenses are formed by a matrix of liquid crystal cells, and whose frame is provided with a directional photosensitive sensor. Depending on the direction from which the solar rays arrive, cells are switched to an opaque state to prevent the wearer of the glasses from being dazzled by the sun. The major drawback of this arrangement relates to the fact that a large part of the glasses lenses, practically a quarter of the surface of each lens, is darkened, causing a reduction in the field of view that is incompatible with driving a motor vehicle. 
     EP 0 498 143 which is equivalent to U.S. Pat. No. 5,258,607, also discloses also discloses an active anti-dazzle screen for motor vehicle drivers. This screen, which is fixed to the windshield of the vehicle or may take the form of glasses lenses, includes an ambient brightness sensor, while a measurement circuit which also compares the measured value with a threshold value controls the state of transparency of the liquid crystals forming the glasses lenses. The lenses are totally transparent in the absence of a measurement signal. The drawback of this arrangement is that it operates in binary mode, in an on-off way, the lenses being in a state of maximum or minimum transparency depending on whether the brightness is below or above a predetermined threshold. 
     Additionally, U.S. Pat. No. 4,286,308 discloses an anti-dazzle device for nighttime driving, comprising a variable-transmission screen located in front of the driver, headlights capable of being switched rapidly from an illuminated state to an extinguished state, a sensor of the brightness of the road scene in front of the vehicle, and a control unit. When the exterior brightness level is below a predetermined limit, the variable-transmission screen is in its state of maximum transmission. 
     On the approach of vehicles traveling in the opposite direction, the control unit automatically controls the transmission/absorption ratio of the variable-transmission screen, in accordance with the brightness detected by the sensor, in order to increase the apparent density of the screen. If the exterior brightness exceeds a predetermined value, the duration of the periods for which the variable-transmission screen is opaque is at a maximum, so that the screen is transmissive only when the headlights are illuminated, thus providing maximum reduction of dazzling. 
     Document WO 9620846 discloses a method and a device for attenuating the light from the headlights of approaching vehicles, comprising the emission of light pulses by the vehicle headlights and the control of the transparency of filters in front of the driver&#39;s eyes in synchronization with the light pulses emitted by the headlights of the vehicle, the transparency of the filters being at a maximum for a duration exceeding that of the light pulses. In this way, the brightness of oncoming headlights is reduced. 
     The last two of these documents describe systems which are cumbersome and difficult to use, which are relatively slow in operation with relatively long response times, and in which the transparency of the filters or screens used is always less than 50%; that is to say these systems, using liquid crystals, cause a decrease in the perceived luminous intensity, even when their transmission is maximal. 
     What is needed, therefore, is a method and device that overcomes one or more of the problems in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention lies within this context and has the object of providing driving motor vehicle driving assistance, by providing the driver of the vehicle with a method for:
         attenuating the brightness of a strongly sunlit road scene, and preventing the dazzling of the driver and passengers, and   providing a view of the road scene with an attenuation progressively decreasing as a function of the reduction of the brightness of the road scene, to a point of maximum transparency when the brightness of the road scene is low,   the variation of the attenuation taking place automatically,   without thereby impeding the driver&#39;s movements or restricting his field of view.       

     To this end, the present invention proposes a device for daytime motor vehicle driving assistance, including the steps of:
         placing at least one variable-transmission screen between the driver of the motor vehicle and the road scene in front of the vehicle,   measuring, by means of at least one sensor, the brightness of the road scene in front of the vehicle,   processing the brightness measurement signal to convert it into a signal for controlling the coefficient of transmission of the variable-transmission screen.       

     According to the invention, the method includes the further step of transmitting the control signal by remote control waves to a receiver fitted to the variable-transmission screen for modulating the coefficient of transmission of the variable-transmission screen as a function of the brightness measured by the sensor. 
     According to other characteristics of the invention, considered separately or in combination:
         the coefficient of transmission of the variable-transmission screen is modulated as a function of the brightness measured by the sensor, between a maximum value for a maximum transparency for a first duration and a minimum value for a minimum transparency for a second duration;   the first duration with a maximum coefficient and the second duration with a minimum coefficient of transmission follow one another according to a pulse width modulation (PWM) cycle;   the coefficient of transmission of the variable-transmission screen is a function of the measurement of the brightness of the road scene in front of the vehicle.       

     The invention also proposes a device for daytime motor vehicle driving assistance, including:
         a variable-transmission screen between the driver of the motor vehicle and the road scene in front of the vehicle,   a sensor which measures the brightness of the road scene in front of the vehicle,   a device for processing the brightness measurement signal to convert it into a signal for controlling the coefficient of transmission of the variable-transmission screen.       

     According to the invention, a control unit receives the control signal and transmits it by remote control waves to a receiver fitted to the variable-transmission screen in order to control the coefficient of transmission of the screen. 
     According to other characteristics of the invention, considered separately or in combination:
         the coefficient of transmission of the variable-transmission screen is modulated as a function of the brightness measured by the sensor, between a maximum value for a maximum transparency for a first duration and a minimum value for a minimum transparency for a second duration;   the coefficient of transmission of the variable-transmission screen is modulated according to a pulse width modulation (PWM) cycle;   the duty cycle of the pulse width modulation (PWM) cycle is a function of the measurement of the brightness of the road scene in front of the vehicle;   the variable-transmission screen between the driver of the motor vehicle and the road scene in front of the vehicle is formed by:
           the windshield of the vehicle,   a screen placed between the windshield of the vehicle and the driver of the vehicle, or   glasses worn by the driver of the vehicle.   
           the coefficient of transmission of the variable-transmission screen is controlled as a function of the measured brightness according to a wireless communication protocol;   the coefficient of transmission of the variable-transmission screen is controlled by a control unit receiving the signals from a sensor for measuring the brightness of the road scene in front of the vehicle, and the signals from a sensor for measuring the amount of light transmitted by the variable-transmission screen;   the duty cycle of the pulse width modulation (PWM) cycle is a function of the measurement of the brightness of the road scene in front of the vehicle.       

     The invention also proposes a variable-transmission screen adapted to be placed between the driver of a motor vehicle and the road scene in front of the vehicle. 
     According to the invention, the coefficient of transmission of the screen is variable according to a pulse width modulation (PWM) mode between a maximum value and a minimum value, the coefficient of transmission of the variable-transmission screen being controlled by a control unit according to a wireless communication protocol, by means of remote control waves transmitted by a transmitter of the control unit and received by a receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
       Other objects, characteristics and advantages of the present invention will be apparent from the following description of an exemplary embodiment, provided on a non-limiting basis with reference to the attached drawings, in which 
         FIG. 1  shows schematically, in partial section, a vehicle fitted with a device according to the present invention; 
         FIG. 2  shows schematically a diagram of the operation of the device according to the present invention; and 
         FIGS. 3A to 3C  show diagrams of the temporal variation of the various signals used in the circuits of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a partial section through a vehicle indicated as a whole by the reference  20 , traveling on a road  10  and controlled by a driver  24 , indicated symbolically by an eye. 
     As shown in this  FIG. 1 , in sunny weather, notably at the end of the day when the sun S is low on the horizon, the road scene SR in front of the vehicle  20  is strongly illuminated, and the driver  24  may not only be dazzled, but may also fail to distinguish details of this road scene SR that are important for his safety, for example traffic signs warning of the proximity of a hazard, or the state of the roadway on which he is traveling. 
     The invention therefore proposes to place between the driver  24  and the road scene SR a variable-transmission screen to modulate the amount of light reaching the driver&#39;s eye  24 . 
     According to the embodiments of the invention, the variable-transmission screen may be formed by:
         a screen E properly so-called, which is placed between the driver  24  and the windshield  26 , and which can be folded back like a sun visor,   the windshield  26  itself, or   a pair of glasses  28 , worn by the driver  24  like sunglasses or corrective spectacles, only one glasses lens  28  being shown in  FIG. 1 .       

     These three embodiments have been shown simultaneously in  FIG. 1  for ease of description. However, they are only variant embodiments of the invention, each of them tending to provide the same result. 
     In the rest of the description, the term “variable-transmission screen” will be used to denote any of:
         the screen E, regardless of whether it is fixed or of the foldable sun visor type;   the windshield  26 , or   a pair of glasses  28 .       

     The variable-transmission screen E is designed to have a very short response time and to allow a fast variation of its coefficient of transmission. 
     The liquid crystals make it possible to produce rapid reaction screens of this type. The same results can also be achieved by using other technologies, for example microelectromechanical systems, such as those of the type described in U.S. Pat. No. 7,684,105. 
     If the variable-transmission screen is fixed, as in the case of the windshield  26 , it can be connected directly to the vehicle&#39;s  20  wiring harness to provide its power supply (by a link which is not shown). 
     If the variable-transmission screen is movable, as in the case of the screen E of the sun visor type or the pair of glasses  28 , it may comprise its own independent power supply (not shown). In fact, the control of the state of transparency, or of the coefficient of transmission, of a liquid crystal screen or a microelectromechanical system requires only minimal electrical power, so that batteries such as button cells are sufficient to provide correct operation of the liquid crystal screen or of the microelectromechanical system for a considerable length of time, especially in the case of the glasses  28 . 
     The term “microelectromechanical” is used in the present description to describe the concept known from the English acronym MEMS (standing for “MicroElectroMechanical System”). 
     For controlling the coefficient of transmission of the variable-transmission screen E, the invention provides for the use of a photosensitive sensor  31  for measuring the brightness of the road scene SR in front of the vehicle  20 . 
     Advantageously, the photosensitive sensor  31  is located on the inner face of the windshield  26 , at the level of the interior rear view mirror (not shown), that is to say in the middle of the upper part of the windshield  26 . This position makes it possible to collect information particularly representative of the brightness outside the vehicle  20 , originating from the road scene SR. Also advantageously, the measurement signals of the photosensitive sensor  31  may also be used to cause the low beam headlights to be switched on when the brightness of the road scene SR falls below a predetermined threshold, as in most modern vehicles. 
     As is shown in greater detail in  FIG. 2 , the output signal S L  of this photosensitive sensor  31  is received and processed by a circuit  33  adapted to convert this output signal S L  into a signal S C  for controlling the coefficient of transmission of the variable-transmission screen E, this signal S C  in turn being received by a control unit  30  for controlling the coefficient of transmission of the variable-transmission screen E. 
     If the variable-transmission screen is fixed, as in the case of the windshield  26 , the control unit  30  can be connected directly to it, for example by a wire link  37 , as shown in dotted lines in  FIG. 2 . 
     If the variable-transmission screen is movable, as in the example of the screen E of the sun visor type or the pair of glasses  28 , the control unit  30  may comprise a transmitter, for transmitting radio, infrared or ultrasonic waves for example, according to a wireless communication protocol, for example according to Bluetooth or Wi-Fi (registered trademarks) standards. These waves are referred to in the rest of the description as “remote control waves” OT. In this case, the variable-transmission screen E or the glasses  28  are provided with a receiver  40  of these remote control waves OT. 
     According to the present invention, the modulation of the coefficient of transmission of the variable-transmission screen E takes place in real time, as a function of the brightness of the road scene SR measured by the photosensitive sensor  31 . 
     For this purpose, in response to the output signal S L  of the photosensitive sensor  31 , representing the brightness of the road scene SR in front of the vehicle  20 , the circuit  33  generates a control signal S C  which is a function of the signal S L . The control signal S C  is then transmitted, either directly by the control unit  30  and the wire link  37 , or by the transmitter of the control unit  30 , the waves OT and the receiver  40 , to the variable-transmission screen E. 
     The coefficient of transmission of the variable-transmission screen E is thus modulated as a function of the received signal S C , that is to say as a function of the brightness measured by the sensor  31 , according to a well-known principle. 
     Thus, as the brightness measured by the sensor  31  increases, the signal S L  becomes stronger and the coefficient of transmission of the variable-transmission screen E becomes smaller, or, in other words, the transmission by the variable-transmission screen E decreases. 
     Thus, according to the invention, a variable-transmission screen E has been provided, the coefficient of transmission of this screen E being adjusted in real time as a function of the brightness of the road scene SR observed by the driver  24 : as the brightness increases, the variable-transmission screen E becomes darker, and vice versa. 
     The driver  24  of the vehicle  20  traveling in an environment in which the brightness is highly variable can thus observe the road scene SR in front of his vehicle  20  through the variable-transmission screen E described above, this screen E:
         attenuating the brightness of a strongly sunlit road scene SR, and preventing the dazzling of the driver  24 ,   providing a view of the road scene SR with an attenuation progressively decreasing as a function of the reduction of the brightness of the road scene SR, to a point of maximum transparency when the brightness of the road scene SR is low,   the variation of the attenuation taking place automatically.       

     In order to find the coefficient of transmission which exactly yields the desired darkening, a feedback loop may be included in the control of this coefficient, as shown in  FIG. 2 . 
     In this loop, an ocular sensor  50  measures the amount of light reflected by the cornea of the driver&#39;s eye  24 . It is assumed that the light received by the driver&#39;s eye  24  has already passed through the variable-transmission screen E. 
     If the variable-transmission screen is formed by the windshield  26  or a screen E of the sun visor type, the sensor  50  may, for example, be formed by a miniature video camera, placed in the proximity of the sensor  31 , and pointing toward the driver&#39;s  24  face. The processing of the image supplied by this sensor  50  enables the driver&#39;s eyes  24  to be isolated. Sensors of this type are already known and are used to detect any sleepiness in the driver  24  and to warn him. These sensors  50  may also measure the amount of light reflected by the cornea of the driver&#39;s eye  24 , and may therefore deduce from this the amount of light transmitted by the variable-transmission screen E. 
     If the variable-transmission screen is formed by glasses  28  worn by the driver  24 , the sensor  50  may be integrated into the frame of these glasses  28 . 
     The measurement by the sensor  50  of the amount of light reflected by the cornea of the driver&#39;s eye  24  provides a measurement of the amount of light reaching this driver&#39;s eye  24 , after preliminary calibration or graduation if necessary, and therefore provides an indirect measurement of the amount of light transmitted by the variable-transmission screen E. 
     This measurement L I  of light transmitted by the variable-transmission screen E and striking the driver&#39;s eye  24  is transmitted by a link  52  to the control unit  30 . 
     The link  52  may be a wire link if the variable-transmission screen is formed by the windshield  26  or by the movable screen E. If the variable-transmission screen is formed by glasses  28 , the link  52  will advantageously be formed by a wireless link, for example by using radio, infrared or ultrasonic waves according to a wireless communication protocol. 
     Consequently the control unit  30  can simultaneously access:
         the direct measurement of the brightness of the road scene SR, provided by the sensor  31 , and   the brightness of the road scene SR attenuated by the variable-transmission screen E, provided by the sensor  50 .       

     The control unit  30  contains a comparator which compares the measured value L I  of the light striking the driver&#39;s eye  24 , after its passage through the variable-transmission screen E, with a set point value V C , contained in a memory  54 . As a function of the difference between the set point value V C  and the measured value L I , and as a function of the signal S C , which is itself a function of the value of the brightness outside the vehicle, the control unit  30  constantly adjusts the control signal transmitted by the wire link  37  or by the waves O T , so as to make the measured value L I  equal to the set point value V C . 
     This set point value V C  may be fixed in the memory  54 , or, preferably, may be adjustable, for example by being regulated on the dashboard of the vehicle  20 , as shown in  FIG. 1 . 
     Thus the driver  24  can set the degree of darkening to any desired value in order to observe the road scene SR in front of his vehicle  20  in the best possible conditions, the amount of light reaching his driver&#39;s eye  24  remaining constant and equal to a predetermined value, as assigned by the driver  24  to the memory  54 . 
     According to a preferred embodiment of the invention, the transmission of the signals could be made to take place not in a continuous, analog way as described above, but rather in a digital way, that is to say in an alternating way, preferably in PWM (standing for the English expression “pulse width modulation”) mode, at a predetermined frequency, according to the diagrams of  FIG. 3 . 
     According to this embodiment, the photosensitive sensor  31  sends an analog signal whose value is a function of the luminous intensity that it receives from the road scene SR in front of the vehicle  20 . The sensor  31  is associated with a circuit that converts this analog signal into a PWM coded digital signal S L . 
     As shown in  FIG. 3A , this signal S L  varies between a value of S Lmin  for a duration (t1) and a value of S LMAX  for a duration t 2 , the sum of the durations t 1  and t 2  defining the period T of the alternating signal S L . 
     It should be noted that the duty cycle β of the signal S L  is determined by the ratio between the duration t 2  for which the signal is at a maximum and the duration T of the period, and thus varies from 0 to 100%: 
     
       
         
           
             β 
             = 
             
               
                 t 
                 2 
               
               T 
             
           
         
       
     
     The duty cycle β thus appears as a direct function of the luminous intensity received by the sensor  31   
     This signal S L  is received by the circuit  33 , which converts it into a control signal S C  shown in  FIG. 3B . 
     The signal S C  varies between a maximum value S CMAX  for the duration t 1  and a minimum value S Cmin  for the duration t 2 , with the same period T=t 1 +t 2  as the signal S L  and with a duty cycle α such that 
     
       
         
           
             α 
             = 
             
               
                 
                   t 
                   1 
                 
                 T 
               
               = 
               
                 1 
                 - 
                 β 
               
             
           
         
       
     
     To summarize, in response to the signal S L  supplied by the photosensitive sensor  31 , which is a function of the value of the brightness of the road scene SR in front of the vehicle  20 , the circuit  33  generates an alternating signal S C  
         in pulse width modulation (PWM) mode,   at a predetermined frequency       

             v   =     1   T           
and
 
with a predetermined duty cycle α, according to the diagram of  FIG. 3B , showing the brightness of the road scene in front of the vehicle as measured by the sensor  31 .
 
     This signal S C  is transmitted to the variable-transmission screen E by a wire link  37  or by means of the waves O T , the signals L I  and V C  being taken into account if appropriate. 
     The coefficient of transmission of the variable-transmission screen E varies, in response to the signal S C , between a value of CT MAX  for the duration t 1  and a value (CT min ) for the duration t 2 , with the same duty cycle α as the signal S C  and the same frequency ν. 
     The value CTMAX is the value for which the glasses lenses  28  have their maximum transparency. In most cases, liquid crystal screens or microelectromechanical systems have this state in the absence of any electrical excitation, in other words in the rest state, and are opaque only under the effect of an electric field. In these cases, the value CT MAX  corresponds to a minimum excitation of the liquid crystals or the microelectromechanical systems forming the glasses lenses  28 . 
     In some cases, the rest state of a liquid crystal screen or of a microelectromechanical system may be that in which it has maximum opacity, becoming transparent only under the effect of an electric field. In this eventuality, the value CT MAX  corresponds to a maximum excitation of the liquid crystals or of the microelectromechanical systems forming the glasses lenses  28 . 
     The above explanations are applicable, mutatis mutandis, to the value CT min  of the coefficient of transmission of the glasses lenses  28 . 
     Thus the diagram of  FIG. 3C  shows the variation of the coefficient of transmission CT of the glasses lenses  28 , rather than the variation of the excitation signal of these glasses lenses  28 . 
     The driver  24  can therefore observe the road scene SR through the variable-transmission screen, regardless of whether this screen is the windshield  26 , the screen E, or glasses  28 , the coefficient of transmission of the screen E being adjusted in real time as a function of the brightness of the road scene SR: as the road scene SR becomes brighter, the variable-transmission screen E increasingly attenuates the light reaching the driver  24 . 
     The automatic adjustment of the coefficient of transmission of the variable-transmission screen E is achieved by a sequence of states of maximum and minimum transparency of this screen E, at a frequency ν and with a duty cycle α. The frequency ν is chosen to be high enough to prevent any scintillation phenomenon for the driver  24  of the vehicle  20 . The frequency ν will be, for example, greater than 100 Hz, to benefit fully from the phenomenon of persistence of vision. 
     Thus a method and a device for daytime motor vehicle driving assistance has indeed been devised, providing a vehicle driver  24  with:
         attenuation of the brightness of a strongly sunlit road scene SR, thus preventing the dazzling of the driver  24 ,   a view of the road scene SR with attenuation progressively decreasing as a function of the reduction of the brightness of the road scene SR, to a point of maximum transparency when the brightness of the road scene SR is low,   the variation of the attenuation taking place automatically and in real time, without any inconvenience caused to the driver  24  by the device, because the latter is integrated either into the windshield  26  of the vehicle  20  or into a movable screen of the sun visor type, or into a pair of glasses  28  whose coefficient of transmission is controlled by a wireless control system.       

     Clearly, the present invention is not limited to the embodiments which have been described; in fact, persons skilled in the art may make numerous modifications to the invention, all of which fall within its scope. Thus, for example, the invention is applicable to the passengers as well as to the driver  24 , particularly if the variable-transmission screen E is formed by the windshield  26  of the vehicle  20 . If the variable-transmission screen E is formed by glasses  28  for the driver  24  , as described more particularly above, then the passenger can obviously also wear glasses with a variable coefficient of transmission like those of the driver  24 . If the transmission of the control signal S C  to the driver&#39;s glasses  28  takes place by wireless communication, the passengers&#39; glasses will receive the same control signal and will react in the same way as the driver&#39;s glasses  28 , without any adaptation. 
     Similarly, it is possible to provide two successive variable-transmission screens, for example the windshield  26  and a movable screen E of the sun visor type, to provide the upper part of the driver&#39;s  24  field of view (seen via the screen E and the windshield  26 ) with greater attenuation than the lower part (seen via the windshield  26  alone). 
     It is also possible to provide two successive variable-transmission screens, formed by the windshield  26  and the glasses  28  worn by the driver  24 . With this arrangement, the driver  24  will benefit from the fixed attenuation provided by the windshield  26 , supplemented by an adjustable attenuation that he can control on the dashboard, while the passengers will benefit from the fixed attenuation provided by the windshield  26  alone. 
     While the system, apparatus, process and method herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise system, apparatus, process and method, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.