Abstract:
The invention relates to an optical filter comprising: at least one optically transparent liquid-crystal shutter, the shutter having a polarization voltage ULCD that marks a threshold between two polarization states, and being designed to switch between at least two opacities OP 1  and OP 2,  OP 2  being strictly higher than OP 1,  when it receives a voltage that is above or below, respectively, the polarization voltage ULCD; and an electronic system comprising: an electronic module for controlling the liquid-crystal shutter, designed to control the voltage applied to the liquid crystal shutter; a photosensitive sensor, designed to deliver, to the electronic control module, a continuous voltage Ucs that varies depending on the luminous intensity that it receives, the photosensitive sensor being the only power supply of the electronic control module and the liquid-crystal shutter, the optical filter being characterized in that, if the photosensitive sensor receives a luminous intensity below a dazzle threshold le, the liquid-crystal shutter has an opacity OP 1 , and in that the electronic control module is designed, when the photosensitive sensor receives a luminous intensity above the dazzle threshold le, to deliver, to the liquid-crystal shutter, a continuous voltage Ue that is strictly above the polarization voltage ULCD of the shutter, so that the latter switches from opacity OP 1  to opacity OP 2.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of optical filters used to decrease luminous intensity, and more particularly the field of optical filters comprising crystal-liquid optical shutters which can adapt to ambient luminosity. 
         [0002]    The invention can be implemented especially in devices for protection against light used daily as sunglasses. 
       PRIOR ART 
       [0003]    Numerous devices for decreasing luminous intensity are already known, some of which comprise liquid-crystal shutters and can adapt to ambient luminosity. 
         [0004]    Of particular interest is document FR 2,693,562 which describes sunglasses comprising a photosensitive sensor, transmitting a continuous signal, which is an increasing function of the luminous intensity which reaches it, and an electronic circuit connected to crystal-liquid screens, such that the transmittance of screens diminishes, that is, the opacity increases when the luminous intensity received by the photosensitive sensor increases. 
         [0005]    Document FR 2,781,289 is also known and includes all the characteristics of document FR 2,693,562 and in which the darkening of the glasses is also proportional to the luminous intensity received by the photosensitive sensor. 
         [0006]    Document PCT/US2007/019631 is also known, which proposes a solution in which a battery feeds an electronic control circuit of a liquid-crystal shutter. This electronic circuit is highly complex and therefore has a lengthy response time. 
         [0007]    These devices however have numerous disadvantages. 
         [0008]    First, the darkening of glasses of these devices requires an adaptation time of a few tens of seconds, or even a few seconds, which makes them poorly adapted to some situations during which darkening must occur instantaneously. 
         [0009]    Examples of these situations especially are displacement situations in sunny weather, in a car or on a motorbike for example, during which the driver passes through tunnels, or under a canopy of trees, etc. Under normal circumstances, the driver can wear sunglasses or a helmet fitted with a tinted visor so as not to be dazzled. When he reaches one of these passages where rapid transitions of luminosity occur, the driver can remove his glasses or raise his visor, but this puts him in danger as this transition costs him a few seconds of inattention when he is not in possession of all his faculties. 
         [0010]    Also, the driver is taking risks if he does not remove his glasses or its visor, as he then has reduced visibility. 
         [0011]    None of the adaptable devices mentioned hereinabove resolves these problems because, over the abovementioned transition time, the user has perturbed or reduced visibility, which represents a risk both for him and for other users. 
         [0012]    Also, for devices using liquid crystals, for example of nematic type, the feed of which is continuous, proposing darkening proportioned to incident luminosity, it sometimes happens that the molecules of the liquid crystals do not align uniformly according to the orientation adapted to the required blocking. The result is optical phenomena such as variegations or sheens on the lenses of the glasses, which can be annoying for the user. 
         [0013]    An alternative solution, presented in document WO 2009/069166, proposes an optical filter in which a photosensitive sensor delivers a voltage to a crystal-liquid screen to control blocking of the latter. The electronic circuit also comprises a resistor for rapidly discharging residual charges in the crystal-liquid screen so that the latter does not have the sheens mentioned hereinabove. 
         [0014]    Also, solutions in which an extra battery outputs a voltage tailored to the shutter have been proposed, but there is still the risk that the battery fails the user. 
         [0015]    As a consequence, an aim of the present invention is to provide an alternative to the previous solution, and in particular a secure optical filter for the user, allowing quasi-instantaneous adaptation to the incident luminous intensity on the filter, and avoiding any optical phenomenon awkward for the user. 
         [0016]    Another aim of the invention is to be able to be easily used on a support such as glasses, a helmet, or even a window. 
         [0017]    For this purpose, the invention proposes an optical filter comprising:
       at least one optically transparent liquid-crystal shutter, having a polarisation voltage U LCD  forming a threshold between two polarisation states, and adapted to switch between at least two opacities OP 1  and OP 2 , OP 2  being strictly greater than OP 1 , when the voltage which it receives is respectively less or greater than the polarisation voltage U LCD  and   an electronics system comprising
           an electronic control module of the liquid-crystal shutter, adapted to control the voltage of the liquid-crystal shutter,   a photosensitive sensor adapted to deliver to the electronic control module a continuous voltage U CS  variable as a function of the luminous intensity which it receives, the photosensitive sensor being the sole source of supply of the electronic control module and of the liquid-crystal shutter,
 
and such that:
   
           if the photosensitive sensor receives a luminous intensity less than a dazzle threshold l e , the liquid-crystal shutter has an opacity OP 1 , and   the electronic control module is adapted, when the photosensitive sensor receives a luminous intensity greater than the dazzle threshold l er  to deliver to the liquid-crystal shutter a continuous voltage U e  strictly greater than the polarisation voltage U LCD  of the shutter, such that the latter switches from the opacity OP 1  to the opacity OP 2 .       
 
         [0024]    Advantageously, but optionally, the optical filter proposed by the invention comprises at least one of the following characteristics:
       when the photosensitive sensor receives a luminous intensity greater than the dazzle threshold l e , the electronic control module delivers to the shutter a voltage U e  greater than the polarisation voltage U LCD  of the shutter to which some tens of volts are added,   when the photosensitive sensor receives a luminous intensity less than the dazzle threshold l e , the electronic control module delivers no voltage to the liquid-crystal shutter   the electronic control module comprises a voltage comparator and an interrupter,   the voltage comparator compares the voltage U CS  delivered by the photosensitive sensor to a threshold voltage by excess U ref1  or to a threshold voltage by default U ref2 , the voltages U ref1  and U ref2  being such that U ref1  is greater than U ref2  to which some tens of volts are added, and U ref2  is greater than the polarisation voltage U LCD  of the shutter to which some tens of volts are added,   if the voltage U CS  delivered by the sensor exceeds the threshold voltage by excess U ref1 , the interrupter closes and provides the liquid-crystal shutter with a voltage U e  equal to the voltage U CS , and the opacity of the liquid-crystal shutter becomes equal to OP 2 , and if the voltage delivered by the photosensitive sensor U CS  becomes less than the threshold voltage by default U ref2 , the interrupter opens, the liquid-crystal shutter ceases to be supplied and its opacity becomes equal to the opacity OP 1 ,   the opacity OP 2  increases with the voltage U e  at the terminals of the shutter, and the optical filter also comprises a voltage regulator, positioned downstream of the interrupter, and adapted to deliver a voltage U e  which is constant and strictly greater than the polarisation voltage U LCD  of the shutter, or else the opacity OP 2  is constant for all the voltage U e  strictly greater than the polarisation voltage U LCD ,   the optical filter also comprises a device for manual regulation of the opacity OP 2  of the liquid-crystal shutter,   the optical filter also comprises a device of permanent manual deactivation of the electronic control module,   the photosensitive sensor is integrated into at least one support surface, and the electronic control module is made on a printed circuit by silkscreen printing on at least one glass,   the liquid-crystal shutter is integrated into a partial zone of a glass.       
 
         [0035]    The invention also relates to a pair of glasses and a helmet equipped with the optical filter according to the invention. 
     
    
     
       DESCRIPTION OF FIGURES 
         [0036]    Other characteristics, aims and advantages of the present invention will emerge from the following detailed description, with reference to the attached figures, given by way of non-limiting examples and in which: 
           [0037]      FIGS. 1   a  and  1   b  represent two examples of electronic structures of an optical filter according to embodiments of the invention, 
           [0038]      FIGS. 2   a  to  2   c  represent examples of working diagrams of elements of the optical filter in relation to the incident luminous intensity on the filter, 
           [0039]      FIGS. 3   a  to  3   c  schematically represent examples of physical structures of an optical filter according to the invention, 
           [0040]      FIGS. 4   a  and  4   b  represent examples of integration of an optical filter according to the invention, 
           [0041]      FIG. 5  represents an example of integration of an optical filter according to the invention in a spectacle lens with optical correction. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    In reference to  FIGS. 1   a  and  1   b , these show examples of electronic structures according to preferred embodiments of the optical filter according to the present invention. 
         [0043]    This optical filter  1  preferably comprises:
       a photosensitive sensor  10 , for example a photovoltaic cell, adapted to deliver a voltage U CS  variable as a function of the luminous intensity which it receives. The voltage U CS  can for example be continuous and increasing as a function of the luminous intensity received by the sensor  10 ,   an electronic control module  20 ,   at least one optical shutter  30 , preferably crystal-liquid.       
 
         [0047]    The optical shutter  30  is of type known from the state of the art. This is for example a liquid-crystal shutter of nematic type. Where appropriate, it preferably comprises electrodes  310 ,  311 , a film of liquid crystals  32 , optically transparent screens  330 ,  331  and polarisers  340 ,  341  (illustrated in  FIG. 5 ). 
         [0048]    Preferably, the film of liquid crystals  32  is positioned between two electrodes  310  and  311 , with the latter in turn being positioned between two optically transparent screens  330  and  331 . 
         [0049]    Finally, the optical shutter  30  advantageously comprises two polarisers  340  and  341 , the two polarisers being preferably positioned against the external surfaces of the optically transparent screens  330  and  331 , such that the latter are located between the polarisers. Alternatively, the polarisers  340  and  341  can be between an electrode  310  (resp.  311 ) and a corresponding screen  330  (resp.  331 ), or between a corresponding electrode and the film of liquid crystals. 
         [0050]    As is known to the expert, the two polarisers  340 ,  341  are typically oriented by 90° and the nematic molecules have a helicoidal orientation when no voltage is applied, allowing light to pass through without being blocked. The application of voltage to the terminals of these electrodes  310 ,  311  can cause, according to the value of the voltage, a particular orientation of the nematic molecules, causing partial or total clouding of the shutter  30 . So, if the shutter  30  is placed on the path of an incident light beam, the quantity of light transmitted varies as a function of its opacity. In fact, the opacity is defined here as the inverse of transmittance, the latter being the ratio between the luminous intensity transmitted through the shutter and the incident luminous intensity. The opacity is therefore the ratio between the incident luminous intensity and the transmitted luminous intensity. 
         [0051]    The optical shutter  30  preferably has a polarisation voltage U LCD  forming a threshold between two distinct states, for which the opacity of the optical shutter has two different levels. 
         [0052]    According to a preferred embodiment of the optical shutter, the polarisation voltage U LCD  is such that, when it receives at its terminals a voltage less than this polarisation voltage, or even a zero voltage or an absence of voltage, the nematic molecules are oriented so as to give to the shutter a particular opacity, for example less than or equal to a first opacity OP 1 . Preferably, for an input voltage to the terminals of the shutter  30  of less than U LCD , the opacity of the shutter is constant and equal to the opacity OP 1 . The opacity OP 1  is for example very low, that is, of the order of magnitude of the opacity of transparent glass traditionally used in the eyewear field. 
         [0053]    Inversely, when the shutter  30  receives at its terminals a voltage greater than this polarisation voltage U LCD,  the nematic molecules are oriented so that the shutter has at least one opacity OP 2  strictly greater than the opacity OP 1 . 
         [0054]    According to a particular embodiment of the shutter, the latter can be a shutter of “all-or-nothing” type having an opacity OP 1  for an input voltage of less than the polarisation voltage U LCD , or zero or for no voltage applied, and a constant opacity OP 2  strictly greater than the opacity OP 1  for an input voltage greater than the polarisation voltage U LCD , 
         [0055]    According to an alternative embodiment, the liquid-crystal shutter  30  can have a variable opacity OP 2 , for example increasing as a function of the input voltage at the terminals of the shutter  30 . 
         [0056]    According to the invention, the electronic control module  20  delivers a voltage U e  to the terminals  31  of the electrodes  310 ,  311  of the shutter  30 . 
         [0057]    The electronic control module is itself electrically connected to the photosensitive sensor  10  which constitutes its sole source of supply. Therefore the photosensitive sensor  10  provides at output a voltage U CS , this voltage being the input voltage at the terminals of the electronic control module  20 . 
         [0058]    The electronic control module  20  preferably comprises at least one comparator  21  and an interrupter  22 . The comparator  21  receives the voltage U CS  delivered by the photosensitive sensor  10  directly, and controls the opening and the closing of the interrupter  22 , this interrupter being connected in series to the terminals of the electrodes  310 ,  311  of the optical shutter  30 . 
         [0059]    In this way, the sole source of supply of the optical shutter  30  is the photosensitive sensor  10 . In particular, when the comparator  21  controls the opening of the interrupter  22 , the shutter is not fed, and its opacity is therefore less than or equal to OP 1 , and preferably equal to OP 1 . 
         [0060]    If the optical filter comprises a plurality of liquid-crystal shutters, they are connected in parallel on the output of the electronic control module  20  to receive at the terminals of their electrodes the same input voltage. 
         [0061]    The operating principle of the optical filter consists of a threshold operation on the incident luminous intensity on the liquid-crystal shutters  30 . If the luminous intensity exceeds a certain threshold, for example a dazzle threshold l e , then the electronic control module  20  delivers to the shutter  30  a voltage strictly greater than the polarisation voltage U LCD  so that the opacity of the shutter is equal to at least one opacity OP 2  strictly greater than the opacity OP 1 . 
         [0062]    A preferred embodiment of the invention is described hereinbelow. 
         [0063]    In operation, the comparator  21  compares the voltage U CS  delivered by the photosensitive sensor  10  to a threshold voltage U ref . If the voltage U CS  is strictly greater than the voltage U ref , the comparator  21  controls the closing of the interrupter  22 . So the voltage U e  at the terminals  31  of the shutter or shutters  30  is equal to the voltage U CS . 
         [0064]    In the event where the voltage U CS  is less than or equal to the voltage U ref , the comparator  21  controls the opening of the interrupter  22 , and the shutter  30  is no longer fed. 
         [0065]    In reference to  FIG. 1   a , a particular embodiment of the electronic assembly of the optical filter  1  is shown. 
         [0066]    In this embodiment, the voltage U e  delivered to the liquid-crystal shutter  30  is equal to the voltage U CS  when the interrupter  22  is closed. Now, in the case of a liquid-crystal shutter, for example of nematic type, there is a minimum polarisation voltage U LCD  to be applied to the shutter so that the orientation of the molecules changes and darkening is effective on the glass. 
         [0067]    Also, so that the orientation of the molecules of the liquid crystals changes entirely and evenly, the applied voltage must be greater than this polarisation voltage, with a few tens of volts added. In fact, if this is not the case, the molecules of the liquid crystals do not all orient uniformly, which is the origin of sheens or variegations which can appear on the glass and annoy the user. 
         [0068]    Consequently, the voltage U CS , which is transmitted to the shutters  30  when the interrupter  22  is closed, must be strictly greater than the polarisation voltage U LCD , that is, equal to the polarisation voltage U LCD , with a few tens of volts added, to be noted U LCD +ε. For this, the reference voltage U ref  of the comparator is selected equal to U LCD +ε. For example, for a voltage U LCD  of 3V, the voltage U ref  is selected equal to 3.3V. 
         [0069]    According to another embodiment of the invention, shown in  FIG. 1   b , the electronic control module also comprises a voltage regulator  23 , positioned between the interrupter  22  and the shutter or optical-crystal shutters  30 . 
         [0070]    In the event where the opacity of the shutter  30  increases with the applied voltage, and particularly when the latter is greater than the polarisation voltage U LCD , this voltage regulator  23  limits the value of the voltage U e  at the terminals of the electrodes  310 ,  311  of the shutter  30  to limit clouding of the shutter. In fact, if the optical filter according to the invention is utilised for example in a form integrated into sunglasses lenses, it is preferable not to exceed an opacity threshold. If not, the user of the glasses would find this awkward or even could be put in danger. 
         [0071]    According to an alternative embodiment of the invention such as illustrated in  FIG. 1   b , the voltage regulator  23  can be a voltage boost regulator. Therefore, the voltage U ref  can be selected independently of the value of the voltage U LCD . In particular, it can be selected less than the voltage U LCD . In this case, the interrupter is closed for all voltage U CS  greater than the voltage U ref,  which can comprise voltages less than U LCD +ε. The voltage regulator  23  therefore receives this voltage U CS  and delivers at output a voltage U e , preferably strictly greater than U LCD , and even greater than or equal to U LCD +ε so that the molecules of the liquid crystals can adopt uniform orientation. 
         [0072]    In reference to  FIG. 2 , this shows working diagrams of the optical filter according to the invention. 
         [0073]    The first diagram  2   a  illustrates a variation in the luminous intensity on a given time window. This luminous intensity varies from a state of total obscurity to a highly luminous state, similar to ambient luminosity in the sun, by crossing over a level l e , which can be dazzling. 
         [0074]    The second diagram  2   b  shows the voltage U CS  delivered by the photosensitive sensor  10  over the evolution of the luminosity. The voltage U CS  delivered as a function of the luminosity is adjusted by the manufacturer and it can be selected such that the voltage U CS  delivered at the level of the dazzle threshold l e  is strictly greater than the threshold voltage U ref . 
         [0075]    It is evident that the voltage U CS  evolves at the same time as the luminous intensity. 
         [0076]    Finally, the third diagram  2   c  illustrates the voltage at the electrodes  310 ,  311  of the liquid-crystal shutter  30 . At the moment when the luminosity reaches the dazzling level l e , the voltage U CS  reaches the threshold greater than U ref , as described hereinabove, and the transition to the level of the electrodes  310 ,  311  avoids the risk of sheen. 
         [0077]    Again in  FIG. 2   b , it is clear that in reality the electronic control module  20  has hysteresis, causing the threshold voltage by excess to be different to the threshold voltage by default. These two voltages will be referred to as threshold U ref1  and U ref2  hereinbelow. This hysteresis stabilises the circuit. Electronic components ensuring a comparator function relative to two different thresholds respectively by excess and by default exist and are known to the expert. They will therefore not be described in greater detail hereinbelow. 
         [0078]    In particular, according to the preferred embodiment for which the voltage U CS  delivered by the photosensitive sensor  10  grows with luminous intensity, the activation voltage threshold when the luminous intensity exceeds the dazzling intensity l e  is the maximal value of threshold voltages U ref1 and U ref2 , and the deactivation threshold voltage when the luminous intensity becomes less than the dazzle threshold l e  is the minimal value of the voltages U ref1 and U ref2 . 
         [0079]    For example, U ref1 is strictly greater than U ref2 , and U ref1  is the activation voltage threshold, and U ref2  is the deactivation voltage threshold. 
         [0080]    U ref1 and U ref2  are distinct by some tens of volts, and in particular they are selected preferably so that the polarisation voltage U LCD  is strictly less than these two voltages. Here strictly means that U ref2  is greater than U LCD  with some tens of volts added, and U ref1 is greater than U ref2  with one ten or some tens of volts added. 
         [0081]    Finally, the level of opacity OP 2  of the glasses, when the voltage U e  at the terminals of the shutter  30  is greater than the polarisation voltage U LCD , can be constant, and is defined originally by the equipment used, or a posteriori by the definition of the threshold voltage U ref . 
         [0082]    Alternatively, the optical filter can also comprise a manual control device of the opacity of the lenses to adjust the opacity OP 2  as a function of the wishes of users. For example, a voltage booster regulator can be used, whereof the output voltage is adjustable, to adjust the threshold voltage delivered to the liquid-crystal shutters. 
         [0083]    The optical filter according to the invention can also comprise an additional device for manually deactivating and where appropriate manually reactivating the electronic control module. 
         [0084]    This invention embodies numerous advantages. 
         [0085]    First, the use of voltage thresholds avoids any risk of variegations associated with poor orientation of molecules in the liquid crystals, especially since these variegations are generally persistent, even if the voltage at the terminals of the liquid-crystal shutter again drops below the polarisation voltage. 
         [0086]    Also, since the sole source of supply of the electronic control module and of the liquid-crystal shutter is the photosensitive sensor, the filter has no battery which could fail the user when he needs it. The operation of the filter is conditioned solely to the presence or not of sun or another light source. 
         [0087]    This operation with threshold based solely on the output voltage of the photosensitive sensor allows quasi-instantaneous adaptation of the darkening of the lens. Quasi-instantaneous is understood as time of the order of one hundredth of a second, and which in particular is less than the persistence of vision time. This allows the user to perceive the adaptation as instantaneous, and even for sequences rapid change in luminosity, such as for example in some tunnels. 
         [0088]    Finally, this extremely simple optical filter electronic circuit can be miniaturised and can be inserted very discretely into a cavity of a support  50 . 
         [0089]    Whatever the support  50 , two configurations are possible for integration of the optical filter  1  into the support. According to a first embodiment, and in reference to  FIG. 3   a , the photosensitive sensor  10  and the electronic control module  20  can be inserted into the support  50  (not shown in the figure), and be connected to the shutter  30  which can be integrated into a lens  40  via the electrodes  310 ,  311  and which is connected to the electronic module  20  via its terminals  31 . 
         [0090]    Alternatively, these days it is possible to use an optically transparent support surface made of glass or another material (for example some plastics) to integrate all the optical filter  1 , as illustrated in  FIGS. 3   b  and  3   c.    
         [0091]    In  FIG. 3   b , the photosensitive sensor  10  and the electronic control module  20  are integrated into a support surface  40  on which the shutter  30  is arranged without covering the photosensitive sensor  10  or the electronic control module  20 . 
         [0092]    In  FIG. 3   c , the photosensitive sensor  10  and the electronic control module  20  are placed on a support surface  40  or integrated into the latter, and are covered by the liquid-crystal shutter  30 . Where appropriate, the optically transparent screens  330  and  331  of the optical shutter can form the lens  40  acting as support surface to the optical filter. Alternatively, a support surface  40  can be covered by the liquid-crystal shutter  30 . 
         [0093]    In these latter two cases, photosensitive sensors of photovoltaic cell type integrated into the lens are preferably used. Also, in this case the control electronics are preferably made in a printed circuit created by silkscreen printing on the glass. 
         [0094]    Also in these cases, the liquid-crystal shutter  30  can cover only one part or some parts of the support surface  40 . 
         [0095]    In reference to  FIG. 4   a , the support  50  can be a glasses mount comprising two lenses  40 A and  40 B forming the support surfaces  40  mentioned hereinabove. A photosensitive sensor  10  sufficiently small to be integrated very discretely in the mount  50  of the lenses can be used, as shown in the figure. Also, the electronic control module is simple enough to be miniaturised and also be integrated into the mount  50 . 
         [0096]    Also, the optical filter  1  comprises two liquid-crystal shutters  30 A,  30 B. The optically transparent screens  330 A,  331 A,  330 B,  331 B of the shutter  30  can constitute the lenses  40 A,  40 B of the glasses. Alternatively, at least one additional lens  41  can be adjoined to the shutters  30 A,  30 B so that the shutters  30 A,  30 B are integrated into the support surfaces  40  of the glasses. 
         [0097]    Also, integration of a liquid-crystal shutter  30  in glasses of lenses  40 A,  40 B can be combined with the fact that the glasses lend optical correction to the wearer. 
         [0098]    Where appropriate, and in reference to  FIG. 5 , the additional glass  41  can be a glass slide worked according to known techniques to lend optical correction to the wearer, and placed upstream or optionally downstream of the shutter relative to the path of incident light. 
         [0099]    Alternatively, the shutter is placed between two glass slides  41  machined to lend optical correction to the wearer, the shutters then being curved so as to join the contact surfaces of the glass slides. 
         [0100]    Also, and again in reference to  FIG. 5 , the lenses  40  of the glasses can optionally comprise one or more anti-UV filters  42 . 
         [0101]    The invention applies similarly to the case of a monocle which has just one glass  40  and in this case the optical filter  1  comprises just a single liquid-crystal shutter  30  integrated into the lens. 
         [0102]    The result is a unique product comprising a sun protection which deploys automatically as and when needed, and an optical correction, which is permanent. This unique product therefore eliminates any risk linked to rapid transition phases of luminous intensity mentioned hereinabove. 
         [0103]    In reference to  FIG. 4   b , the support  51  can be a helmet, such as for example a helmet for two-wheeled vehicles, the helmet comprising a visor forming the lens  40  of the optical filter  1 . 
         [0104]    Also, the optically transparent screens  330 ,  331  are not limited to glass screens but can also be soft screens, for example made of soft plastic, so as to be able to deform to exhibit the preferred curvature. 
         [0105]    The applications of the optical filter according to the invention are however not limited to this embodiment, but can also relate to window panes used in buildings, for example on windows, doors, or bay windows, or any type of vitreous surface whereof partial, temporary or permanent blocking can be realised by means of the optical filter according to the invention. 
         [0106]    Irrespective of the application of the optical filter, the automatic character of its activation makes it more practical and of less risk to use than a traditional sun-protection device.