Patent Publication Number: US-10791594-B2

Title: Light beam direction control device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2018-222198 filed in Japan on Nov. 28, 2018, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     This disclosure relates to a light beam direction control device. 
     Flat-panel display devices are used as display devices in various information processing devices such as mobile phones, personal digital assistants (PDAs), automatic teller machines (ATMs), and personal computers. 
     A commonly known configuration for such flat-panel display devices includes a built-in optical element for adjusting the exit direction of the light that has entered from the backside, a backlight for emitting light uniformly toward the optical element, and a liquid crystal display for displaying an image. 
     The trend of increase in size and usage of display screens is producing demands for various light distribution characteristics to the flat-panel display devices. Particularly from the viewpoint of information leakage, there is a demand to limit the viewable range to prevent peeking or a demand not to provide light in unnecessary directions. As an optical element meeting this demand, an optical film capable of limiting the viewable range of the display (or the range of outgoing light) has been proposed and commercialized. 
     A flat-panel display device with this optical film requires removing the optical film each time when people want to see the display from multiple directions together. Because of such bothersome operations and waste of time for the user, there is an increasing demand to achieve a wide viewable range state and a narrow viewable range state at desired times without a bothersome operation of removing the film. 
     To meet this demand, an optical element capable of switching the viewable range of the display between a wide view mode and a narrow view mode has been proposed (for example, U.S. Pat. No. 7,751,667 B). U.S. Pat. No. 7,751,667 B discloses a light beam direction control element which includes light transmissive regions and electrophoretic elements for controlling the direction of light provided on a transparent substrate and controls the viewing angle of the light transmitted through the light transmissive regions. According to U.S. Pat. No. 7,751,667 B, transparent electrodes are provided on both ends of each electrophoretic element in which liquid including colored (black) charged particles is encapsulated. To achieve a wide viewing angle state, a direct voltage is applied across the transparent electrodes to move the colored charged particles. To achieve a narrow viewing angle state, an alternating voltage is applied across the transparent electrodes to disperse the colored charged particles within the electrophoretic elements. 
     SUMMARY 
     An aspect of this disclosure is a light beam direction control device comprising: a light beam direction control panel; and a control circuit configured to control the light beam direction control panel, wherein the light beam direction control panel includes: a first transparent substrate having a first main face; a second transparent substrate having a second main face opposed to the first main face, a plurality of light transmissive regions provided between the first main face and the second main face, the plurality of light transmissive regions being arrayed along the first main face; a plurality of light absorbing regions provided between the first main face and the second main face, each of the plurality of light absorbing regions including light-absorptive electrophoretic particles having charges of a specific polarity and light-transmissive dispersion medium and being disposed between light transmissive regions adjacent to each other; and a first transparent electrode and a second transparent electrode provided on the first main face of the first transparent substrate and the second main face of the second transparent substrate, respectively, in such a manner that the first transparent electrode and the second transparent electrode sandwich the plurality of light absorbing regions, and wherein the control circuit is configured to: change a dispersion state of the electrophoretic particles by controlling voltage across the first transparent electrode and the second transparent electrode to change a range of exit direction of light transmitted through the light transmissive regions and the dispersion medium; apply direct voltage at a first voltage value across the first transparent electrode and the second transparent electrode to change the range of exit direction from a narrow range to a wide range; measure luminance of light transmitted through the light beam direction control panel during application of the voltage at the first voltage value; and increase the voltage value to be applied across the first transparent electrode and the second transparent electrode in a case where the measured luminance of the transmitted light is lower than a target value. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of a light beam direction control device; 
         FIG. 2  is a plan diagram illustrating an example of the layout of light transmissive regions and light absorbing regions when viewed in the direction normal to the main face of the light beam direction control panel; 
         FIG. 3  is a cross-sectional diagram of a light beam direction control panel cut along the line III-III in  FIG. 2  to illustrate another configuration example; 
         FIG. 4A  illustrates a configuration example of a display device including a light beam direction control device in a narrow viewing angle state and a display panel; 
         FIG. 4B  illustrates a configuration example of a display device including a light beam direction control device in a wide viewing angle state and a display panel; 
         FIG. 5A  is a cross-sectional diagram illustrating the angle of light that is emitted by a light source, enters a light beam direction control panel from its entrance face, and goes out from its exit face in a narrow viewing angle state; 
         FIG. 5B  illustrates a relation between the angle of outgoing light and the transmittance; 
         FIG. 6A  is a cross-sectional diagram illustrating the angle of light that is emitted by a light source, enters a light beam direction control panel from its entrance face, and goes out from its exit face in a wide viewing angle state; 
         FIG. 6B  illustrates a relation between the angle of outgoing light and the transmittance; 
         FIG. 7  is a graph illustrating the variation in transmittance between a narrow viewing angle state and a wide viewing angle state; 
         FIG. 8A  schematically illustrates the state of an electrophoretic element when voltage is applied across the transparent electrodes; 
         FIG. 8B  schematically illustrates the state of an electrophoretic element when no voltage is applied across the transparent electrodes; 
         FIG. 9  illustrates the behavior of electrophoretic particles; 
         FIG. 10  schematically illustrates examples of the response characteristic of transmittance to applied voltage desired for a light beam direction control panel and a response characteristic of transmittance changed because of the condition of the environment or the electrophoretic elements; 
         FIG. 11A  illustrates an example of feedback control of the voltage to be applied that is performed by a control device; 
         FIG. 11B  provides an example of a look-up table to be used for the feedback control of the voltage to be applied; 
         FIG. 12  illustrates another example of feedback control of the voltage to be applied that is performed by a control device; 
         FIG. 13  illustrates still another example of feedback control of the voltage to be applied that is performed by a control device; 
         FIG. 14  illustrates still another example of feedback control of the voltage to be applied that is performed by a control device; 
         FIG. 15  illustrates an example of the disposition of a photosensor for measuring the luminance of transmitted light from the light beam direction control panel; 
         FIG. 16  illustrates another example of the disposition of a photosensor for measuring the luminance of transmitted light through the light beam direction control panel; 
         FIG. 17  schematically illustrates an example of the circuit configuration of the light beam direction control device; 
         FIG. 18  illustrates a configuration example of a display device including a thermo-sensor for measuring temperature; 
         FIG. 19  schematically illustrates another example of the circuit configuration of the light beam direction control device; 
         FIG. 20  is a perspective diagram of a configuration example of opposed electrodes of a light beam direction control panel; 
         FIG. 21  is a plan diagram illustrating an example of the disposition of photosensors for measuring transmitted light from the light beam direction control panel and light sources for the measurement; 
         FIG. 22A  is a cross-sectional diagram of  FIG. 21  cut along the line XXII-XXII when the light beam direction control panel is in a narrow viewing angle state; 
         FIG. 22B  is a cross-sectional diagram of  FIG. 21  cut along the line XXII-XXII when the light beam direction control panel is in a wide viewing angle state; 
         FIG. 23  is a plan diagram illustrating another example of the disposition of transparent segmented electrodes, photosensors, and measurement light sources of a light beam direction control panel; 
         FIG. 24  illustrates still another example of transparent segmented electrodes of a light beam direction control panel; 
         FIG. 25  schematically illustrates an example of a method of configuring the control of the voltages to be applied to individual transparent segmented electrodes; 
         FIG. 26  illustrates an example of the feedback control of the voltages to be applied in accordance with the look-up table configured by the method illustrated in  FIG. 25 ; and 
         FIG. 27  schematically illustrates an example of the circuit configuration of a light beam direction control device. 
     
    
    
     EMBODIMENTS 
     Hereinafter, embodiments of this disclosure are described with reference to the accompanying drawings. It should be noted that the embodiments are merely examples to implement this disclosure and not to limit the technical scope of this disclosure. Elements common to drawings are assigned the same reference signs. 
     The light beam direction control device in this disclosure measures the light transmitted through its light beam direction control panel after starting application of voltage to the light beam direction control panel to change the light beam direction control panel from a narrow viewing angle state to a wide viewing angle state. The narrow viewing angle state is a state where the directions of outgoing light are in a narrow range and the wide viewing angle state is a state where the directions of outgoing light is in a wide range. The light beam direction control device controls the voltage to be applied to the light beam direction control panel based on the measurement result. As a result, the light beam direction control panel can speedily change from a narrow viewing angle state to a wide viewing angle state. 
     Embodiment 1 
     Device Configuration 
       FIG. 1  is a block diagram illustrating an example of a light beam direction control device. A light beam direction control device  1  includes a light beam direction control panel  10  for controlling the viewing angle and a control device  2 . The light beam direction control panel  10  includes a transparent electrode  12  and another transparent electrode  15  each made of a sheet of transparent conductive film. 
     The control device  2  includes a power circuit  3  and a control circuit  4 . The control circuit  4  receives a light beam direction control signal from a host control device (not shown). The control circuit  4  controls the potentials to be applied from the power circuit  3  to the transparent electrode  12  and the transparent electrode  15  (the voltage across the transparent electrode  12  and the transparent electrode  15 ), based on the light beam direction control signal. 
       FIG. 1  also illustrates an example of the cross-sectional structure of the light beam direction control panel  10 . The transparent electrode  12  (second transparent electrode) is on the main face of a transparent substrate  11  (second transparent substrate) and the transparent electrode  15  (first transparent electrode) is on the main face of a transparent substrate  16  (first transparent substrate). The transparent substrate  11  and the transparent substrate  16  are disposed in such a manner that their main faces are opposed to each other. Between the transparent electrode  12  and the transparent electrode  15  opposed to each other, light transmissive regions  13  and light absorbing regions  14  are disposed. The light transmissive regions  13  and the light absorbing regions  14  are disposed alternately along the main faces of the transparent substrates  11  and  16 . 
     The transparent substrates  11  and  16  can be made of glass, polyethylene terephthalate (PET), polycarbonate (PC), or polyethylene naphthalate (PEN), for example. The transparent electrodes  12  and  15  can be made of ITO, ZnO, or IGZO, for example. 
       FIG. 2  is a plan diagram illustrating an example of the layout of the light transmissive regions  13  and the light absorbing regions  14  when viewed in the direction normal to the main face of the light beam direction control panel  10  (the transparent substrates  11  and  16 ), or when viewed planarly. Rectangular light transmissive regions  13  are disposed at predetermined intervals in the vertical direction and the horizontal direction of the drawing. 
     Light absorbing regions  14  are provided between light transmissive regions  13  adjacent to each other. Note that the layout of the light transmissive regions  13  and the light absorbing regions  14  is not limited to the example of  FIG. 2 . 
     A light transmissive region  13  is made of a transparent material (for example, resin) that transmits light. The height thereof can be within the range from 3 μm to 300 μm. The width of a light transmissive region  13  (light transmissive pattern width) can be within the range from 1 μm to 150 μm. Furthermore, the width between light transmissive regions  13  (light blocking pattern width) can be within the range from 0.25 μm to 40 μm. 
     As illustrated in  FIG. 1 , an electrophoretic element is encapsulated in each light absorbing region  14 . The electrophoretic element is a mixture of light-blocking electrophoretic particles  140  having charges of a specific polarity and dispersion medium  141 . To achieve a light blocking function, the electrophoretic particles  140  can be black in color that absorbs light. 
     The electrophoretic particles  140  can be charged microparticles of carbon black. The example described in the following employs negatively charged carbon black microparticles. The dispersion medium  141  is transparent to transmit light and has a refractive index substantially equal to the refractive index of the transparent material of the light transmissive regions  13 . This configuration minimizes the interfacial reflection between the dispersion medium  141  and the light transmissive regions  13 . 
       FIG. 3  is a cross-sectional diagram of a light beam direction control panel  10  cut along the line III-III in  FIG. 2  to illustrate another configuration example. The light beam direction control panel  10  includes insulating films  17  and  18  covering the transparent electrodes  12  and  15  in addition to the configuration illustrated in  FIG. 1 . The other elements are the same as those in  FIG. 1 . 
     A transparent insulating film  17  is disposed between the transparent electrode  12  and the layer of the light transmissive regions  13  and the light absorbing regions  14 . A transparent insulating film  18  is disposed between the transparent electrode  15  and the layer of the light transmissive regions  13  and the light absorbing regions  14 . The insulating films  17  and  18  are made of SiO 2 , for example. That is to say, the insulating films  17  and  18  are disposed between the main faces of the transparent substrates  11  and  16  opposed to each other in such a manner that each insulating film is interposed between a transparent substrate and the light absorbing regions  14 . 
     The material of the transparent insulating films  17  and  18  is not limited to SiO 2  and can be a different transparent insulating material. The insulating film  17  ( 18 ) can be provided only between the transparent electrode  12  ( 15 ) on the main face of the transparent substrate  11  ( 16 ) and the light absorbing regions  14  and does not need to be provided between the light transmissive regions  13  and the transparent electrode  12  ( 15 ). 
     The insulating films  17  ( 18 ) interposed between the transparent electrode  12  ( 15 ) and the light absorbing regions  14  prevents the electrophoretic particles  140  from firmly sticking to the transparent electrode  12  ( 15 ) even after the electrophoretic particles  140  are collected around the transparent electrode  12  ( 15 ) for a long time. As a result, the transition characteristics between a wide viewing angle state and a narrow viewing angle state are more stabilized. 
     In another configuration example, either one or both of the transparent electrodes  12  and  15  can be a patterned electrode. Its pattern can be identical to the pattern of the light absorbing regions  14  when viewed planarly. The patterned transparent electrode achieves reduction in area of the transparent electrode to improve the transmissivity of the light beam direction control panel  10 . In still another example, the patterned electrode that collects the electrophoretic particles  140  in a wide viewing angle state can be made of a light reflective metal. 
       FIGS. 4A and 4B  illustrate a configuration example of a display device including a light beam direction control device  1  and a display panel  5 . The display panel  5  can be a liquid crystal display panel, an organic EL panel, an inorganic EL panel, or an LED panel.  FIG. 4A  illustrates a display device in a narrow viewing angle state and  FIG. 4B  illustrates the display device in a wide viewing angle state. In this configuration example, the display panel  5  is disposed behind the light beam direction control panel  10 . The user sees the image displayed on the display panel  5  through the light beam direction control panel  10 . 
     Hereinafter, the side on which the display device displays an image for the user or the side on which the image is seen by the user is referred to as front side and the opposite side as back side. The opposite face to the face on the front side or the front face is referred to as rear face or back face. 
     The light beam direction control panel  10  may be disposed in front of a planar light source. In a display device including a backlight (planar light source) like a liquid crystal display device, the light beam direction control panel  10  may be disposed between the liquid crystal display panel and the backlight. 
       FIG. 4A  illustrates a narrow viewing angle state; the electrophoretic particles  140  are uniformly dispersed in the dispersion medium  141 .  FIG. 4B  illustrates a wide viewing angle state; the electrophoretic particles  140  are gathered in the proximity of one transparent electrode  15 . The control device  2  changes the distribution of the electrophoretic particles  140  in the light beam direction control panel  10  to switch the viewing angle that allows observation of the displayed image between a narrow state and a wide state. 
     Transmissivity of Light Beam direction Control Panel 
     The transmissivity in a narrow viewing angle state and the transmissivity in a wide viewing angle state are described.  FIG. 5A  is a cross-sectional diagram illustrating the angle of light that is emitted by a light source, enters the light beam direction control panel  10  from its entrance face  161 , and goes out from its exit face  111  in a narrow viewing angle state.  FIG. 5B  illustrates a relation between the angle of outgoing light and the transmittance. The angle is the angle with respect to the normal to the light beam direction control panel  10 . In a narrow viewing angle state, the control device  2  does not apply voltage across the transparent electrode  12  and the transparent electrodes  15 , so that no electric field is applied to the electrophoretic elements. 
     When the narrow viewing angle state is stable, the electrophoretic particles  140  are completely dispersed in the light absorbing regions  14 . Since the electrophoretic particles  140  are in a color having a light-blocking property, such as black, the light that hits the electrophoretic particles  140  in the light that comes from the entrance face  161  of the light beam direction control panel  10  is absorbed and does not go out from the light beam direction control panel  10 . Accordingly, the transmittance with respect to the angle of outgoing light is as indicated in  FIG. 5B . 
       FIG. 6A  is a cross-sectional diagram illustrating the angle of light that is emitted by a light source, enters the light beam direction control panel  10  from its entrance face  161 , and goes out from its exit face  111  in a wide viewing angle state.  FIG. 6B  illustrates a relation between the angle of outgoing light and the transmittance. The angle is the angle with respect to the normal to the light beam direction control panel  10 . In a wide viewing angle state, the control device  2  applies voltage across the transparent electrode  12  and the transparent electrode  15  to apply an electric field to the electrophoretic elements. 
     In response to application of voltage such that the transparent electrode  15  will have a potential higher than the potential of the transparent electrode  12 , the negatively charged electrophoretic particles  140  are collected to the proximity of the transparent electrode  15  having a positive potential. Accordingly, as illustrated in  FIG. 6A , the light that hits the electrophoretic particles  140  in the light that comes from the entrance face  161  is a little, compared to the case of  FIG. 5A . Since the dispersion medium  141  is transparent as described above, the incident light at an angle that is blocked in the narrow viewing angle state passes through the light beam direction control panel  10 . Accordingly, the transmittance with respect to the angle of outgoing light is as indicated in  FIG. 6B . 
       FIG. 7  is a graph illustrating the variation in transmittance between a narrow viewing angle state and a wide viewing angle state. In the graph of  FIG. 7 , the solid line  511  represents the relation between the angle and the transmittance in a wide viewing angle state and the broken line  512  represents the relation between the angle and the transmittance in a narrow viewing angle state. As the viewing angle changes from a narrow viewing angle or a wide viewing angle to the other in accordance with the potential control of the transparent electrode  12  and the transparent electrode  15 , the transmittance at each angle changes from the value of the line  512  to the value of the line  511  or from the value of the line  511  to the value of the line  512 . For example, at an angle α, the transmittance changes as indicated by the arrowed line  513  between the wide viewing angle state and the narrow viewing angle state. 
     Behavior of Electrophoretic Particles 
     Hereinafter, the behavior of electrophoretic particles  140  in an electrophoretic element is described more specifically. The electrophoretic element is designed so that the repulsion between electrophoretic particles  140  generated by their electric charge is higher than the attraction acting thereon. 
       FIG. 8A  schematically illustrates the state of an electrophoretic element when voltage (electric field) is applied across the transparent electrodes  12  and  15 .  FIG. 8B  schematically illustrates the state of the electrophoretic element when no voltage (electric field) is applied across the transparent electrodes  12  and  15 . 
     When an electric field exists between the transparent electrodes  12  and  15 , the negatively charged electrophoretic particles  140  gather to the proximity of the electrode having a higher potential or the transparent electrode  15 , as schematically illustrated in  FIG. 8A . When no electric field exists, the most stable state of the electrophoretic particles  140  is a state (completely dispersed state) where the electrophoretic particles  140  are dispersed within the dispersion medium in a macroscopically uniform density because of the repulsion on one another, as schematically illustrated in  FIG. 8B . 
     The light beam direction control panel  10  achieves its wide viewing angle state with the electrophoretic particles  140  collected by application of an electric field and achieves its narrow viewing angle state with the electrophoretic particles  140  diffused under no electric field. As described above, the most stable state in a narrow viewing angle state is a state where the density of the electrophoretic particles  140  are macroscopically uniform. However, the electrophoretic particles  140  behave intricately until reaching the stable state because of the hydrodynamic effects or electrostatic interaction. 
     Presuming the damping vibration behavior of the electrophoretic particles  140  as a simple model, the behavior of the electrophoretic particles  140  is illustrated in  FIG. 9 . Upon vanishment of the electric field applied across the electrodes, the electrophoretic particles  140  start diffusing toward the electrode opposite from the electrode around which the electrophoretic particles  140  have been collected (T11). Subsequently, the density of the electrophoretic particles  140  becomes higher in the area closer to the opposite electrode than the area closer to the original electrode. For this reason, some electrophoretic particles  140  move toward the original electrode because of the repulsion (T12). 
     Some time later, even if all electrophoretic particles  140  look like uniformly distributed, it can be considered that the individual electrophoretic particles  140  are vibrating (T13, T14). In other words, even if the angular distribution of the transmitted light has become unchanged (a narrow viewing angle state) under the condition of no electric field, it can be considered that each electrophoretic particle  140  keeps vibrating minutely. 
     Accordingly, in re-applying an electric field to change the electrophoretic particles  140  (light beam direction control panel  10 ) from the narrow viewing angle state to a wide viewing angle state, the response time is different depending on the magnitude of the vibration. Specifically, an electrophoretic particle  140  at least moving a little moves faster than a completely static electrophoretic particle  140 . That is to say, in changing the light beam direction control panel  10  that has stayed in a narrow viewing angle state for a long time to a wide viewing angle state, applying the same voltage for the same time as those to change the light beam direction control panel  10  that has been in a narrow viewing angle state for a short time to a wide viewing angle state cannot attain the desired transmittance. 
     From another point of view, the response characteristic of an electrophoretic element could change with temperature. The motion of an electrophoretic particle  140  in an electric field depends on the mobility (electrophoretic mobility μ). The motion of an electrophoretic particle (charged particle)  140  having an electric charge amount q is accelerated by receiving a force qE from an electric field E but eventually, becomes uniform motion because of the balance with the viscous resistance of the liquid (dispersion medium  141 ). When a charged particle having a radius a moves in a liquid having a viscosity η at a velocity v, it receives a resistive force of 6πηav. The value obtained by dividing the velocity v by E is the electrophoretic mobility μ. Accordingly, the following formulae (1) to (3) can be obtained: 
     
       
         
           
             
               
                 
                   qE 
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                     6 
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                     ⁢ 
                     
                         
                     
                     ⁢ 
                     av 
                   
                 
               
               
                 
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                   1 
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                   v 
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                     qE 
                     
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                       πη 
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                       a 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   μ 
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                       v 
                       E 
                     
                     = 
                     
                       1 
                       
                         6 
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                         πη 
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                         a 
                       
                     
                   
                 
               
               
                 
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                   3 
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     The viscosity of the liquid depends on the temperature of the liquid. Accordingly, the mobility of an electrophoretic particle  140  has temperature dependency. For this reason, in changing an electrophoretic element (the light beam direction control panel  10 ) from a narrow viewing angle state to a wide viewing angle state, the response time is different depending on the environmental temperature. Specifically, in changing the light beam direction control panel  10  to a wide viewing angle state under a low environmental temperature, the desired transmittance cannot be attained by applying the same voltage for the same time as those to change the light beam direction control panel  10  to a wide viewing angle state under a high environmental temperature. 
     The response characteristic of transmittance to applied voltage of an electrophoretic element (light beam direction control panel  10 ) can be different depending on not only the temperature and the period without application of voltage but also the humidity and the frequency of operation of the electrophoretic element. Accordingly, in changing the light beam direction control panel  10  from a narrow viewing angle state to a wide viewing angle state, the response characteristic of the transmittance of the light beam direction control panel  10  to the voltage applied across the transparent electrodes  12  and  15  can be different depending on the conditions of the environment or the electrophoretic elements. 
     Response Characteristic of Transmittance 
       FIG. 10  schematically illustrates examples of the response characteristic of transmittance to applied voltage desired for the light beam direction control panel  10  and a response characteristic of transmittance changed because of the condition of the environment or the electrophoretic elements. Specifically,  FIG. 10  includes a graph providing a relation between elapsed time in a certain period and applied voltage and a graph providing relations between the elapsed time and transmittance. The time axes of the two graphs are the same. In the graph of transmittance, the broken line  512  represents the desired transmittance response characteristic and the solid line  522  represents the transmittance response characteristic changed from the desired one. 
     The voltage  311  applied to the light beam direction control panel  10  is changed from 0 to V 1  at a time T 0  and thereafter, maintained at V 1 . The voltage value V 1  can be +5 V. For example, the transparent electrode  12  is provided with the ground potential of a reference potential and the transparent electrode  15  is provided with a potential of +5 V. 
     In the desired transmittance response characteristic  521 , the transmittance starts increasing at the time T 0  and reaches the target value TR 0  at a time T 1 . In the transmittance response characteristic  522  changed from the desired one, the transmittance starts increasing at the time T 0  and reaches a value TR 1  at a time T 2 . The time T 2  is later than the time T 1  and the transmittance TR 1  is lower than the transmittance TR 0 . Because of a long period without application of voltage or a low-temperature environment, the transmittance response characteristic of the light beam direction control panel  10  may change from the desired transmittance response characteristic  521  to the transmittance response characteristic  522 . 
     Feedback Control Based on Measured Luminance of Transmitted Light 
     The control device  2  in this disclosure measures the light transmitted through the light beam direction control panel  10  after starting application of voltage to the light beam direction control panel  10  to change the viewing angle from a narrow viewing angle to a wide viewing angle and controls the voltage to be applied based on the measurement result. As a result, the light beam direction control panel  10  can speedily change from a narrow viewing angle state to a wide viewing angle state. 
       FIG. 11A  illustrates an example of the feedback control of the voltage to be applied that is performed by the control device  2 .  FIG. 11A  includes a graph providing relations between elapsed time in a certain period and applied voltage and a graph providing relations between the elapsed time and transmittance. The time axes of the two graphs are the same. The solid line  312  in the graph of applied voltage represents the applied voltage feedback-controlled by the control device  2 . The solid line  525  in the graph of transmittance represents the transmittance response characteristic to the applied voltage  312 . 
     The control device  2  starts applying a positive voltage V 1  (direct voltage at a first voltage value) to the light beam direction control panel  10  at a time T 0  to change the light beam direction control panel  10  from a narrow viewing angle state to a wide viewing angle state. The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 A. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 A, the control device  2  maintains the voltage V 1 . 
     The control device  2  holds information on the target value for the luminance of the transmitted light in advance. The information on the target value can directly indicate the luminance value of the transmitted light to be measured or indicate the required transmittance value. The control device  2  calculates the transmittance of the light beam direction control panel  10  from the measured luminance of the transmitted light and the luminance of the original light from the light source and compares the calculated value with the required transmittance. Whether the luminance of the transmitted light has reached the target value can be determined by such calculation of the transmittance of the light beam direction control panel  10 . The same applies to the other examples of feedback control. 
     In the example of  FIG. 11A , when the applied voltage corresponds to the broken line  311 , the measured luminance of the transmitted light is lower than the target value. The control device  2  increases the voltage to be applied from V 1  to V 2  as indicated by the line  312  in the graph of applied voltage. For example, the voltage values V 1  and V 2  are +5 V and +10 V, respectively. 
     In the transmittance response characteristic  525  to the applied voltage  312 , the transmittance increases from the time T 0  to the time T 1 A; the rate of increase rises from the time T 1 A. This is caused by the increase in applied voltage from the value V 1  to the value V 2 . The transmittance response characteristic  525  reaches a transmittance TR 0  in the desired transmittance response characteristic  521  at a time T 1 . 
     This control of increasing the voltage to be applied if the measured luminance of the transmitted light from the light beam direction control panel  10  has not reached a predetermined value expedites the response of the transmittance, while saving the power consumption. 
     The control device  2  can be configured to determine the increment for the voltage based on the measured luminance of the transmitted light. The control device  2  can have information for relating the measured luminance of the transmitted light to the increased voltage V 2 . For example, the control device  2  can have a function or a table indicating the relation between the difference of the measured luminance of the transmitted light from the target value and the increment to the applied voltage V 1 . The control device  2  determines the voltage V 2  to be applied in accordance with this information. Higher voltage V 2  is assigned to lower measured luminance of transmitted light. 
     A specific example of the information is provided in  FIG. 11B  in the form of a look-up table (LUT) and operation using the LUT is described. The luminance rate of transmitted light in  FIG. 11B  is a value obtained by dividing the luminance of the transmitted light at the time T 1 A by the target luminance. When the luminance rate of the transmitted light is lower, it indicates that the luminance is more deviated from the target value. In the example of  FIG. 11A , when the luminance rate is 20%, the increment to the applied voltage is determined to be 5 V with reference to the LUT of  FIG. 11B . The control device  2  adds this increment to the voltage being applied and applies the increased voltage across the transparent electrodes  12  and  15 . 
       FIG. 12  illustrates another example of the feedback control of the voltage to be applied that is performed by the control device  2 .  FIG. 12  includes a graph providing relations between elapsed time in a certain period and applied voltage and a graph providing relations between the elapsed time and transmittance. The time axes of the two graphs are the same. The solid line  313  in the graph of applied voltage represents the applied voltage feedback-controlled by the control device  2 . The solid line  526  in the graph of transmittance represents the transmittance response characteristic to the applied voltage  313 . 
     The control device  2  starts applying a positive voltage V 1  to the light beam direction control panel  10  at a time T 0  to change the light beam direction control panel  10  from a narrow viewing angle state to a wide viewing angle state. The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 A. The luminance of the transmitted light can be expressed in luminance. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 A, the control device  2  maintains the voltage V 1 . 
     In the example of  FIG. 12 , the measured luminance of the transmitted light is lower than the target value. The control device  2  oscillates the voltage as indicated by the solid line  313  in the graph of the applied voltage. The control device  2  applies direct voltage to the light beam direction control panel  10  from the time T 0  to the time T 1 A and applies alternating voltage including negative voltage from the time T 1 A to a time T 1 . After the time T 1 , the control device  2  maintains a constant voltage. 
     In the example of  FIG. 12 , the voltage  313  applied from the time T 1 A to the time T 1  is a non-sinusoidal alternating voltage having an amplitude of V 1 . Specifically, the applied voltage  313  is a rectangular voltage; the periods at a positive voltage V 1  and the periods at a negative voltage −V 1  are successively alternating. In the example of  FIG. 12 , each period at the positive voltage V 1  is equal to each period at the negative voltage −V 1 . These periods can be different; for example, each period at the negative voltage −V 1  can be shorter than each period at the positive voltage V 1 . 
     The applied voltage  313  can be a non-sinusoidal alternating voltage other than a rectangular voltage or a sinusoidal alternating voltage. The absolute values of the maximum value and the minimum value of the voltage can be different; for example, the absolute value of the minimum value can be smaller than the absolute value of the maximum value. In this disclosure, the amplitude of the alternating voltage is the difference between the average value and the maximum value or the minimum value. 
     In a period at a negative voltage of −V 1 , the electrophoretic particles  140  move in the opposite direction, compared to a period at a positive direction of +V 1 ; accordingly, the transmittance  526  drops once. However, this movement causes the electrophoretic particles  140  to move easily; the electrophoretic particles  140  move quicker in response to the next application of the positive voltage V 1 , accelerating the increase of the transmittance  526 . This application of alternating voltage improves the transmittance response characteristic. 
     The control device  2  can be configured to determine the amplitude of the alternating voltage based on the measured luminance of the transmitted light. The control device  2  can have information for relating the measured luminance of the transmitted light to the amplitude of the voltage. For example, the control device  2  can have a function or a table indicating the relation between the difference of the measured luminance of the transmitted light from the target value and the increment for the amplitude. The control device  2  determines the amplitude of the alternating voltage in accordance with this information. Larger amplitude is assigned to lower measured luminance of transmitted light. 
       FIG. 13  illustrates still another example of the feedback control of the voltage to be applied that is performed by the control device  2 .  FIG. 13  provides a relation  314  between elapsed time and applied voltage. In this example, the control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  twice and determines the voltage to be applied based on the measurement results. 
     More specifically, the control device  2  starts applying a positive voltage V 1  to the light beam direction control panel  10  at a time T 0  to change the light beam direction control panel  10  from a narrow viewing angle state to a wide viewing angle state. The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 A. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 A, the control device  2  maintains the voltage V 1 . 
     In the example of  FIG. 13 , the measured luminance of the transmitted light is lower than the target value. The control device  2  increases the voltage to be applied from V 1  (the first voltage value) to V 2  (the second voltage value). For example, the voltage values V 1  and V 2  are +5 V and +10 V, respectively. 
     The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 B. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 B, the control device  2  maintains the voltage V 2 . In the example of  FIG. 13 , the measured luminance of the transmitted light is lower than the target value. The control device  2  increases the voltage to be applied from V 2  to V 3 . For example, the voltage value V 3  is +15 V. Thereafter, the voltage to be applied is maintained at V 3 . 
     As described above, the control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  for multiple times and at each time, increases the voltage to be applied if the measured luminance is lower than the target value. This control enables the transmittance to reach the target value more speedily, while saving the power consumption. The luminance of the transmitted light can be measured three times or more. 
     As described with reference to  FIG. 11A , the control device  2  can determine the increment for the voltage based on the measured luminance of the transmitted light at each measurement occasion. A larger increment is assigned to lower measured luminance of transmitted light. 
       FIG. 14  illustrates still another example of the feedback control of the voltage to be applied that is performed by the control device  2 .  FIG. 14  provides a relation  315  between elapsed time and applied voltage. In this example, the control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  twice to determine the amplitude of the alternating voltage based on the measurement results. 
     The control device  2  starts applying a positive voltage V 1  to the light beam direction control panel  10  at a time T 0  to change the light beam direction control panel  10  from a narrow viewing angle state to a wide viewing angle state. The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 A. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 A, the control device  2  maintains the voltage V 1 . 
     In the example of  FIG. 14 , the measured luminance of the transmitted light is lower than the target value. The control device  2  oscillates the voltage. The control device  2  applies direct voltage to the light beam direction control panel  10  from the time T 0  to the time T 1 A and applies alternating voltage including negative voltage from the time T 1 A to a time T 1 . The alternating voltage is a rectangular voltage and the maximum value is V 1  and the minimum value is −V 1 . The amplitude is V 1 . 
     The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 B. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 B, the control device  2  maintains the alternating voltage having an amplitude of V 1 . 
     In the example of  FIG. 14 , the measured luminance of the transmitted light is lower than the target value. Therefore, the control device  2  increases the amplitude of the alternating voltage. In the example of  FIG. 14 , the amplitude is increased from V 1  to V 2 . For example, V 1  can be +5 V and V 2  can be +10 V. The control device  2  applies a rectangular voltage having an amplitude of V 2  to the light beam direction control panel  10  from the time T 1 B to the time T 1  and applies a direct voltage V 2  from the time T 1 . 
     As described above, the control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  for multiple times and at each time, increases the amplitude of the alternating voltage if the measured luminance is lower than the target value. This control enables the transmittance to reach the target value more speedily, while saving the power consumption. The luminance of the transmitted light can be measured three times or more. 
     As described with reference to  FIG. 12 , the control device  2  can determine the increment for the amplitude based on the measured luminance of the transmitted light at each measurement occasion. A larger increment is assigned to lower measured luminance of transmitted light. 
     Disposition of Photosensor 
       FIG. 15  illustrates an example of the disposition of a photosensor for measuring the luminance of transmitted light from the light beam direction control panel  10 . The display device in  FIG. 15  includes a display panel  5  and a light beam direction control panel  10  disposed in front of the display panel  5 . The photosensor  71  is disposed on the front face of the light beam direction control panel  10 . The light receiving face of the photosensor  71  is on the back side; in other words, it is facing the main face of the light beam direction control panel  10 . 
     The photosensor  71  measures the light emitted from the display panel  5  and transmitted through the light beam direction control panel  10 . That is to say, the photosensor  71  measures the luminance of the light transmitted through the light beam direction control panel  10  out of the light emitted from the display panel  5 . 
     In the example of  FIG. 15 , the photosensor  71  is disposed outside the display region  56  of the display panel  5  when viewed planarly. This disposition prevents the photosensor  71  from becoming an obstacle for the displayed image. In the example of  FIG. 15 , the photosensor  71  is disposed within the light beam direction controllable region  105  of the light beam direction control panel  10  when viewed planarly. The light beam direction controllable region  105  is a region where the angle range of the transmitted light is controllable. The photosensor  71  can be disposed outside the light beam direction controllable region  105 . 
     For example, the control device  2  calculates the transmittance of the light beam direction control panel  10  from the intensity of light measured by the photosensor  71 . The control device  2  calculates the transmittance of the light beam direction control panel  10  from the information on the luminance of the display panel  5  acquired from the host control device and the value measured by the photosensor  71 . The information on the luminance of the display panel  5  can be an average value of the luminance of the entirety or a predetermined partial region of the display region  56 . 
     In another example, a light source for the measurement can be provided within the display panel  5  or between the display panel  5  and the light beam direction control panel  10 . The photosensor  71  measures the light emitted from the measurement light source and transmitted through the light beam direction control panel  10 . Since the luminance of the measurement light source is known in advance, the control device  2  can easily identify the relation between the value measured by the photosensor  71  and the target value. The light beam direction control device  1  can include multiple photosensors. The control device  2  controls the voltage to be applied to the light beam direction control panel  10  based on the average of the measurement results of the multiple photosensors. This configuration enables more accurate control. 
       FIG. 16  illustrates another example of the disposition of a photosensor for measuring the luminance of transmitted light from the light beam direction control panel  10 . The display device in  FIG. 16  includes a backlight  81 , a display panel  5  disposed in front of the backlight  81 , and a light beam direction control panel  10  disposed between the backlight  81  and the display panel  5 . 
     The display panel  5  is a display panel that displays an image by controlling transmission of the light emitted from the backlight  81 ; it can be a liquid crystal display panel. The light beam direction control panel  10  controls the angle range of the light emitted from the backlight  81  to control the viewing angle of the display panel  5 . 
     The photosensor  71  is disposed on the front face of the light beam direction control panel  10 . The light receiving surface of the photosensor  71  is on the back side. In other words, the photosensor  71  is facing the main face of the light beam direction control panel  10 . The photosensor  71  measures the light emitted from the backlight  81  and transmitted through the light beam direction control panel  10 . In other words, the photosensor  71  measures the luminance of the light transmitted through the light beam direction control panel  10  out of the light emitted from the backlight  81 . 
     In the example of  FIG. 16 , the photosensor  71  is disposed outside the display region  56  of the display panel  5  when viewed planarly. This disposition prevents the photosensor  71  from becoming an obstacle for the displayed image. In the example of  FIG. 16 , the photosensor  71  is disposed within the light beam direction controllable region  105  of the light beam direction control panel  10  when viewed planarly. The photosensor  71  can be disposed outside the light beam direction controllable region  105 . 
     In the case where the luminance of the backlight  81  is fixed, the control device  2  can directly compare the value measured by the photosensor  71  with a predetermined target value and control the voltage for the light beam direction control panel  10  based on the comparison result. In the case where the luminance of the backlight  81  is variable, the control device  2  calculates the transmittance of the light beam direction control panel  10  from the information on the luminance of the backlight  81  acquired from the host control device and the value measured by the photosensor  71 . The control device  2  compares the calculated transmittance with the target value. The control device  2  can also calculate the transmittance in the case where the luminance of the backlight  81  is fixed. 
     Circuit Configuration 
       FIG. 17  schematically illustrates an example of the circuit configuration of the light beam direction control device  1 . The light beam direction control device  1  includes a plurality of photosensors  71 . In  FIG. 17 , only one of the photosensors is provided with a reference sign  71  by way of example. The control circuit  4  includes a controller  41  and a look-up table (LUT)  42 . The power circuit  3  includes a DA converter  31 . The output of the DA converter  31  is provided to the transparent electrode  15 . The power circuit  3  provides the ground potential to the transparent electrode  12  as a reference potential. 
     Each photosensor  71  measures transmitted light from the light beam direction control panel  10 . The controller  41  determines a potential to be given to the transparent electrode  15  based on the information in the look-up table  42  and the values measured by the photosensors  71 , and inputs data indicating the determined potential to the DA converter  31 . The DA converter  31  provides the potential specified by the controller  41  to the transparent electrode  15 . Since the transparent electrode  12  in this example is at the ground potential, the potential of the transparent electrode  15  corresponds to the voltage across the transparent electrodes  12  and  15 . 
     Specific operation is described as follows. The control circuit  4  calculates an average of the measured values acquired from the plurality of photosensors  71 . The controller  41  consults the LUT  42  with the calculated average, selects a potential associated with the average as the potential to be provided to the transparent electrode  15 , and sends a potential signal to the power circuit  3 . The power circuit  3  outputs a potential from the DA converter  31  based on the potential signal to apply voltage across the transparent electrodes  15  and  12  of the light beam direction control panel  10 . 
     The look-up table  42  provides information to determine the potential to be provided to the transparent electrode  15  from the values measured by the photosensors  71 . For the example described with reference to  FIG. 11A , the look-up table  42  may provide relations of ranges of the difference between the measured transmittance and the target value to voltages to be applied to the light beam direction control panel  10 . For the example described with reference to  FIG. 12 , the look-up table  42  may provide relations of ranges of the difference between the measured transmittance and the target value to amplitudes of the alternating voltage to be applied to the light beam direction control panel  10 . 
     For the example described with reference to  FIG. 13 , the look-up table  42  may provide relations of ranges of the difference between the measured transmittance and the target value to increments for the voltage applied to the light beam direction control panel  10  in individual measurement occasions. For the example described with reference to  FIG. 14 , the look-up table  42  may provide relations of ranges of the difference between the measured transmittance and the target value to increments for the amplitude of the alternating voltage applied to the light beam direction control panel  10  in individual measurement occasions. 
     In place of the look-up table  42 , the controller  41  can internally hold the information in the look-up table  42 . The information can be in the form of a table or a function. The look-up table  42  can be updated in accordance with an instruction from the external such as the manufacturer or the user. The power circuit  3  can include a voltage-dividing circuit for applying multiple potentials and a selector for selecting one of the potentials, in place of the DA converter  31 . The controller  41  selects a potential to be applied to the transparent electrode  15  by controlling the selector. 
     In the example in  FIG. 17 , the potential of the transparent electrode  12  is fixed. Unlike this example, the controller  41  may change the potentials of both transparent electrodes  12  and  15  depending on the transmitted light from the light beam direction control panel  10 . The behavior of the electrophoretic particles  140  is determined by the voltage across the transparent electrodes  12  and  15 . 
     Embodiment 2 
     Feedback Control Based on Temperature 
     Hereinafter, temperature-based feedback control of the voltage to be applied is described.  FIG. 18  illustrates a configuration example of a display device including a thermo-sensor  75  for measuring temperature. Hereinafter, differences from the configuration illustrated in  FIG. 15  are mainly described. The thermo-sensor  75  is disposed on the front face of the light beam direction control panel  10  to measure the environmental temperature through the light beam direction control panel  10 . 
     In the example illustrated in  FIG. 18 , the thermo-sensor  75  is disposed outside the display region  56  and the light beam direction controllable region  105  when viewed planarly. This disposition prevents the thermo-sensor  75  from becoming an obstacle for the displayed image. The thermo-sensor  75  can be disposed within the light beam direction controllable region  105 . The thermo-sensor  75  is disposed at a desired place to measure the environmental temperature. Typically, the thermo-sensor  75  is disposed in the proximity of the light beam direction controllable region  105 . 
       FIG. 19  schematically illustrates another example of the circuit configuration of the light beam direction control device  1 . Hereinafter, differences from the configuration illustrated in  FIG. 17  are mainly described. The light beam direction control device  1  includes a thermo-sensor  75 . Although the light beam direction control device  1  includes one thermo-sensor  75 , the light beam direction control device  1  can include multiple thermo-sensors. In that case, the controller  41  can be configured to use the average of the temperatures measured by the plurality of thermo-sensors. 
     The thermo-sensor  75  measures the temperature around the thermo-sensor  75 . The controller  41  determines a potential to be given to the transparent electrode  15  based on the information in the look-up table  43  and the values measured by the photosensors  71  and the thermo-sensor  75 , and inputs data indicating the determined potential to the DA converter  31 . 
     As described above, the responding speed of the light beam direction control panel  10  to the applied voltage slows down when the temperature falls. The controller  41  provides a higher voltage to the light beam direction control panel  10  when the temperature measured by the thermo-sensor  75  is lower. For example, the look-up table  43  includes information to determine the potential to be provided to the transparent electrode  15  from the measurement values of the photosensors  71  in each of a plurality of temperature ranges. 
     The controller  41  determines a temperature range including the temperature measured by the thermo-sensor  75  and consults the look-up table  43  with the temperature range. In application to the examples described with reference to  FIGS. 11A and 13 , the increment to the voltage at the time T 1 A and T 1 B is larger for a lower temperature range. 
     In application to the example described with reference to  FIG. 11A , the look-up table  43  can be configured to indicate the voltage to be applied at the time T 0  and the voltage to be applied (or the increment to the applied voltage) at the time T 1 A if the luminance of the transmitted light has not reached the target value, for each temperature range. In the case where the controller  41  controls the applied voltage at the time T 1 A depending on the measured luminance of the transmitted light, the look-up table  43  includes information for determining an increment to the applied voltage to meet the measured luminance of the transmitted light, such as information for relating the transmittance to the increment to the applied voltage, in each temperature range. 
     In application to the example described with reference to  FIG. 13 , the look-up table  43  can be configured to indicate the voltage to be applied at the time T 0 , the voltage to be applied (or the increment for the voltage) at the time T 1 A in the case where the luminance of the transmitted light has not reached the target value, and the voltage to be applied (or the increment for the voltage) at the time T 1 B in the case where the luminance of the transmitted light has not reached the target value, for each temperature range. In the case where the controller  41  controls the applied voltage at the time T 1 A depending on the measured luminance of the transmitted light, the look-up table  43  includes information for determining an increment to the applied voltage to meet the measured luminance of the transmitted light in each temperature range. 
     In application to the example described with reference to  FIGS. 12 and 14 , the increment for the amplitude of the alternating voltage is larger for a lower temperature range. In application to the example described with reference to  FIG. 12 , the look-up table  43  can be configured to indicate the amplitude of the alternating voltage to be applied (or the variation from the voltage applied at the time T 0 ) at the time T 1 A if the luminance of the transmitted light has not reached the target value, for each temperature range. In the case where the controller  41  controls the amplitude depending on the measured luminance of the transmitted light, the look-up table  43  includes information for determining an amplitude to meet the measured luminance of the transmitted light, such as information for relating the transmittance to the amplitude, in each temperature range. 
     In application to the example described with reference to  FIG. 14 , the look-up table  43  can be configured to indicate the amplitude of the alternating voltage to be applied at the time T 1 A if the luminance of the transmitted light has not reached the target value and the amplitude of the alternating voltage to be applied at the time T 1 B if the luminance of the transmitted light has not reached the target value, for each temperature range. In the case where the controller  41  controls the amplitude depending on the measured luminance of the transmitted light, the look-up table  43  provides information for determining an amplitude to meet the measured luminance of the transmitted light at the times T 1 A and T 1 B, in each temperature range. 
     As described above, controlling the light beam direction control panel based on the measured value of the thermo-sensor in addition to the measured values of the photosensors enables the light beam direction control panel to change more speedily from a narrow viewing angle state to a wide viewing angle state. 
     Embodiment 3 
     Segmented Electrodes 
     Hereinafter, a configuration example of a light beam direction control device including a plurality of individually controllable transparent segmented electrodes on a transparent substrate is described.  FIG. 20  is a perspective diagram of a configuration example of opposed electrodes of a light beam direction control panel  10 . A sheet of transparent electrode  12  is opposed to a plurality of transparent segmented electrodes (a plurality of first transparent electrodes)  15 A to  15 D. The transparent electrode  12  is disposed on a transparent substrate  11  and the transparent segmented electrodes  15 A to  15 D are disposed on a transparent substrate  16 . The part composed of a transparent segmented electrode, the part of the transparent electrode  12  opposed to the transparent segmented electrode, and the region sandwiched therebetween is called a segment. A segment consists of a transparent electrode pair consisting of transparent electrodes opposed to each other and the region sandwiched therebetween. The part of the transparent electrode  12  can be regarded as one transparent electrode. 
     In  FIG. 20 , the X-axis and the Y-axis are parallel to the main faces of the transparent substrates  11  and  16  and orthogonal to each other. The Z-axis is along the normal to the main faces of the transparent substrates  11  and  16  and orthogonal to the X-axis and the Y-axis. 
     Each of the transparent segmented electrodes  15 A to  15 D is shaped like a strip extending along the Y-axis. The transparent segmented electrodes  15 A to  15 D are separate and disposed side by side along the X-axis. The transparent segmented electrodes  15 A to  15 D are opposed to the transparent electrode  12 . A plurality of light transmissive regions  13  and light absorbing regions  14  are provided between a transparent segmented electrode and the transparent electrode  12 . 
     The control device  2  can provide potentials to the transparent segmented electrodes  15 A to  15 D individually. Each of the transparent segmented electrodes  15 A to  15 D is connected with the power circuit  3  through a different line. 
       FIG. 21  is a plan diagram illustrating an example of the disposition of photosensors for measuring transmitted light from the light beam direction control panel  10  and light sources for the measurement. The light beam direction control device  1  includes a plurality of photosensors  71 A to  71 D and a plurality of measurement light sources  78 A to  78 D. The plurality of photosensors  71 A to  71 D are disposed along the X-axis. The plurality of measurement light sources  78 A to  78 D are disposed along the X-axis. 
     The photosensor  71 A and the measurement light source  78 A are opposed to each other along the Y-axis and sandwich the transparent segmented electrode  15 A when viewed planarly. The photosensor  71 A and the measurement light source  78 A are to measure the light transmitted through between the transparent segmented electrode  15 A and the transparent electrode  12 . The photosensor  71 B and the measurement light source  78 B are opposed to each other along the Y-axis and sandwich the transparent segmented electrode  15 B when viewed planarly. The photosensor  71 B and the measurement light source  78 B are to measure the light transmitted through between the transparent segmented electrode  15 B and the transparent electrode  12 . 
     The photosensor  71 C and the measurement light source  78 C are opposed to each other along the Y-axis and sandwich the transparent segmented electrode  15 C when viewed planarly. The photosensor  71 C and the measurement light source  78 C are to measure the light transmitted through between the transparent segmented electrode  15 C and the transparent electrode  12 . The photosensor  71 D and the measurement light source  78 D are opposed to each other along the Y-axis and sandwich the transparent segmented electrode  15 D when viewed planarly. The photosensor  71 D and the measurement light source  78 D are to measure the light transmitted through between the transparent segmented electrode  15 D and the transparent electrode  12 . 
       FIGS. 22A and 22B  are cross-sectional diagram of  FIG. 21  cut along the line XXII-XXII.  FIG. 22A  is a diagram in a narrow viewing angle state and  FIG. 22B  is a diagram in a wide viewing angle state. The light receiving face of the photosensor  71 D is opposed to the light emission face of the measurement light source  78 D and they are located between the transparent segmented electrode  15 D and the transparent electrode  12  along the Z-axis. 
     The photosensor  71 D measures the light transmitted along the Y-axis between the transparent segmented electrode  15 D and the transparent electrode  12  out of the light from the measurement light source  78 D. The light to be measured travels along the main faces of the transparent substrates  11  and  16  and is measured. The luminance of the transmitted light is different depending on the distribution of electrophoretic particles  140  between the transparent segmented electrode  15 D and the transparent electrode  12 . The control device  2  can determine the transmittance between the transparent segmented electrode  15 D and the transparent electrode  12  from the value measured by the photosensor  71 D. 
     The same explanation is applicable to the pair of the photosensor  71 A and the measurement light source  78 A, the pair of the photosensor  71 B and the measurement light source  78 B, and the pair of the photosensor  71 C and the measurement light source  78 C. The control device  2  determines the transmittance between the transparent segmented electrode  15 A and the transparent electrode  12 , the transmittance between the transparent segmented electrode  15 B and the transparent electrode  12 , and the transmittance between the transparent segmented electrode  15 C and the transparent electrode  12  from the values measured from these pairs. 
     The control device  2  individually controls the potentials to be provided to the transparent segmented electrodes  15 A to  15 D based on the measurement values of the photosensors  71 A to  71 D. Hence, the segments of the light beam direction control panel  10  can be controlled individually. The ways of control described with reference to  FIGS. 11A to 14  are applicable to the control of each segment; control information is prepared for each segment. 
     The measurement light sources  78 A to  78 D can be LEDs that output infrared or ultraviolet light, for example. Using light outer than the visible light range enables the user not to see the light from the measurement light sources  78 A to  78 D. This is applicable to any embodiment using a measurement light source. 
     The control device  2  can modulate the output of the measurement light sources  78 A to  78 D. For example, the control device  2  controls the measurement light sources  78 A to  78 D to output pulsed light at a predetermined frequency. The control device  2  filters the light receiving signals from the photosensors  71 A to  71 D to extract light receiving signals corresponding to the light from the measurement light sources  78 A to  78 D. This configuration reduces the effects of the environmental light to measure the light from the measurement light sources  78 A to  78 D more accurately. In this connection, LEDs that output visible light can be employed as the measurement light sources  78 A to  78 D if they are configured to output pulsed light having shorter emission time in addition to the modulation of their output. 
       FIG. 23  is a plan diagram illustrating another example of the disposition of transparent segmented electrodes, photosensors, and measurement light sources of a light beam direction control panel  10 . Differences from  FIG. 21  are mainly described. In the configuration example of  FIG. 23 , each of the transparent segmented electrodes  15 A to  15 D is shaped like a strip extending along the X-axis. The transparent segmented electrodes  15 A to  15 D are separate and disposed one above another along the Y-axis. 
     The control device  2  can provide potentials to the transparent segmented electrodes  15 A to  15 D individually. Each of the transparent segmented electrodes  15 A to  15 D is connected with the power circuit  3  through a different line. 
     The plurality of photosensors  71 A to  71 D are disposed along the Y-axis. The plurality of measurement light sources  78 A to  78 D are disposed along the Y-axis. The photosensor  71 A and the measurement light source  78 A are opposed to each other along the X-axis and sandwich the transparent segmented electrode  15 A when viewed planarly. The photosensor  71 A and the measurement light source  78 A are to measure the light transmitted through between the transparent segmented electrode  15 A and the transparent electrode  12 . The photosensor  71 B and the measurement light source  78 B are opposed to each other along the X-axis and sandwich the transparent segmented electrode  15 B when viewed planarly. The photosensor  71 B and the measurement light source  78 B are to measure the light transmitted through between the transparent segmented electrode  15 B and the transparent electrode  12 . 
     The photosensor  71 C and the measurement light source  78 C are opposed to each other along the X-axis and sandwich the transparent segmented electrode  15 C when viewed planarly. The photosensor  71 C and the measurement light source  78 C are to measure the light transmitted through between the transparent segmented electrode  15 C and the transparent electrode  12 . The photosensor  71 D and the measurement light source  78 D are opposed to each other along the X-axis and sandwich the transparent segmented electrode  15 D when viewed planarly. The photosensor  71 D and the measurement light source  78 D are to measure the light transmitted through between the transparent segmented electrode  15 D and the transparent electrode  12 . 
     The shapes and the layout of the transparent segmented electrodes are not limited to the foregoing examples. The number of transparent segmented electrodes can be as desired. Each of the transparent segmented electrodes can have a different shape. The transmitted light from one segment can be measured with two or more pairs of photosensors and light sources. Multiple transparent segmented electrodes can be controlled in accordance with the measurement result of one pair of a photosensor and a measurement light source. The transparent substrates  11  and  16  can both have separate transparent segmented electrodes thereon. The minimum combination is one segment and one pair of a photosensor and a measurement light source. Explaining it with  FIG. 21 , a light beam direction control panel  10  including the transparent electrode  15 A and a pair of the photosensor  71 A and the measurement light source  78 A can constitute a combination. 
       FIG. 24  illustrates still another example of transparent segmented electrodes in a light beam direction control panel  10 . A sheet of transparent electrode  12  (not shown in  FIG. 24 ) is opposed to a transparent segmented electrode set  155  composed of a plurality of transparent segmented electrodes  551 . The transparent electrode  12  is disposed on the transparent substrate  11  and the transparent segmented electrodes  551  are disposed on the transparent substrate  16 . 
     In  FIG. 24 , only one of the transparent segmented electrodes is provided with a reference sign  551  by way of example. Instead of one sheet of transparent electrode  12 , a plurality of transparent segmented electrodes can be disposed on the transparent substrate  11 . Each transparent segmented electrode on the transparent substrate  11  is opposed to one of the transparent segmented electrodes  551  on the transparent substrate  16 . 
     The transparent segmented electrodes  51  are disposed in a matrix. Specifically, four transparent segmented electrodes  551  are disposed along the X-axis and three transparent segmented electrodes  551  are disposed along the Y-axis. The transparent segmented electrodes  551  are separate and they are opposed to the transparent electrode  12 . A plurality of light transmissive regions  13  and a plurality of light absorbing regions  14  are provided between a transparent segmented electrode  551  and the transparent electrode  12 . 
     In the example of  FIG. 24 , one measurement light source  78  and one photosensor  71  are disposed to sandwich the transparent segmented electrode set  155  when viewed planarly. The photosensor  71  and the measurement light source  78  are opposed to each other along the X-axis. The photosensor  71  and the measurement light source  78  measure the light transmitted through between the transparent segmented electrode set  155  and the transparent electrode  12 . 
     The control device  2  can use the average of the measurement results from a plurality of pairs of photosensors and measurement light sources. The photosensor  71  and the measurement light source  78  can be disposed at other locations. The photosensor  71  can be disposed on the front face of the light beam direction control panel  10  and the measurement light source  78  can be omitted as described with reference to  FIG. 15 or 16 . 
     Each transparent segmented electrode  551  is connected with the power circuit  3  through a different line. The control device  2  can provide potentials to the transparent segmented electrodes  551  individually. The control device  2  individually controls the potentials of the transparent segmented electrodes  551  in accordance with the measurement result of the photosensor  71 . 
     The shapes and the layout of the transparent segmented electrodes  551  are not limited to the example illustrated in  FIG. 24 . The transparent segmented electrodes  551  can have a desired shape and/or different shapes. The transparent segmented electrodes  551  can be disposed in a layout different from a matrix. 
       FIG. 25  schematically illustrates an example of a method of configuring the control of the voltages to be applied to the individual transparent segmented electrodes  551 . This configuration method registers information for individually controlling the voltages to be applied to the transparent segmented electrodes  551  to a look-up table  44 . The registration to the look-up table  44  can be done by the control device  2  or in accordance with a signal from an external control device that is independent from the light beam direction control device  1 . 
     This configuration method individually measures the transmittance responses of the segments (S 10 ). Specifically, the method selects one segment (transparent segmented electrode  551 ) from the transparent segmented electrode set  155  and makes the selected segment to a narrow viewing angle state and the other segments to a wide viewing angle state. Specifically, the method provides the transparent segmented electrode  551  of the selected segment with 0 V and all the other transparent segmented electrodes  551  with a predetermined potential, for example 10 V. The transparent electrode  12  is maintained at the ground potential. As a result, all transparent segmented electrodes  551  except for the selected transparent segmented electrode  551  are maintained in a wide viewing angle state. 
     Subsequently, the method changes the selected segment from the narrow viewing angle state to a wide viewing angle state. Specifically, the method increases the voltage to be applied to the segmented electrode  551  of the selected segment from 0 V to a predetermined potential, for example +5 V. The method measures the transmitted light after the voltage is raised to +5 V. This configuration method selects the transparent segmented electrodes  551  one after another to measure the transmittance response of each segment. 
     Next, this configuration method analyzes the measurement results to determine the value for each segment (S 20 ). Specifically, the method compares the luminance of the transmitted light from each segment with a threshold after a predetermined time has elapsed since the applied voltage is increased to + 5  V and determines the voltage to be applied to each segment based on the difference. For example, in application to the example described with reference to  FIG. 11A , the method determines the voltages to be applied at the times T 0  and T 1 A; in application to the example described with reference to  FIG. 12 , the method determines the voltage to be applied at the time T 0  and the amplitude of the alternating voltage. 
     Application to the example described with reference to  FIG. 11A  is described as follows. If the measured luminance of the transmitted light has reached a first threshold, the method determines that the voltage to be applied at the time T 0  is +5 V and the increased voltage to be applied after the time T 1 A is +10 V. If the measured luminance of the transmitted light is lower than the first threshold and the difference from the first threshold is smaller than a second threshold, the method determines that the voltage to be applied at the time T0 is +6 V and the increased voltage to be applied after the time T 1 A is +11 V. If the measured luminance of the transmitted light is lower than the first threshold and the difference is larger than the second threshold, the method determines that the voltage to be applied at the time T 0  is +7 V and the increased voltage to be applied after the time T 1 A is +12 V. 
     Application to the example described with reference to  FIG. 12  is described as follows. If the measured luminance of the transmitted light has reached a first threshold, the method determines that the voltage to be applied at the time T 0  and the amplitude of the alternating voltage are +5 V. If the measured luminance of the transmitted light is lower than the first threshold and the difference from the first threshold is smaller than a second threshold, the method determines that the voltage to be applied at the time T 0  and the amplitude of the alternating voltage are +6 V. If the measured luminance of the transmitted light is lower than the first threshold and the difference is larger than the second threshold, the method determines that the voltage to be applied at the time T 0  and the amplitude of the alternating voltage are +7 V. 
     In the example of  FIG. 25 , the responses of the segment in the first row and the fourth column and the segment in the second row and second column are slow. The segment in the first row and the fourth column is determined to be provided with +6 V and the segment in the second row and the second column is determined to be provided with +7 V. Lastly, the configuration method registers the determined configuration information to the look-up table  44  in the control device  2  (S 30 ). The look-up table  44  stores configuration information for each segment. 
       FIG. 26  illustrates an example of the feedback control of the voltages to be applied in accordance with the look-up table  44  configured by the method illustrated in  FIG. 25 . Differences from the example described with reference to  FIG. 11A  are mainly described. 
     The solid lines  317 ,  318 , and  319  in the graph of applied voltage represent the applied voltages feedback-controlled by the control device  2 . The applied voltage  318  is for the segment in the first row and the fourth column in the example of  FIG. 25 ; the applied voltage  319  is for the segment in the second row and the second column in the example of  FIG. 25 ; and the applied voltage  317  is for the other segments in the example of  FIG. 25 . 
     The control device  2  applies +6 V to the segment in the first row and the fourth column, +7 V to the segment in the second row and the second column, and +5 V to the other segments at a time T 0  to change the viewing angle from a narrow viewing angle state to a wide viewing angle state. The control device  2  measures the luminance of the transmitted light from the light beam direction control panel  10  at a time T 1 A. If the luminance of the transmitted light from the light beam direction control panel  10  has reached the target value at the time T 1 A, the control device  2  maintains the voltage applied to each segment. 
     In the example of  FIG. 26 , the measured luminance of the transmitted light is lower than the target value. The control device  2  increases the voltages to be applied to the segments. The control device  2  applies +11 V to the segment in the first row and the fourth column, +12 V to the segment in the second row and the second column, and +10 V to the other segments. 
     The transmittance response characteristics can be equalized among individual segments by predetermining the voltages to be applied in accordance with the measurement results of the transmittance response characteristic of the segments as described above. 
       FIG. 27  schematically illustrates an example of the circuit configuration of the light beam direction control device  1 . Differences from the example illustrated in  FIG. 17  are mainly described. The light beam direction control device  1  includes a configuration for individually controlling the plurality of segments. The light beam direction control device  1  includes a plurality of measurement light sources  78  each paired with a photosensor  71 . In  FIG. 27 , only one of the measurement light sources is provided with a reference sign  78 . The controller  41  controls the measurement light sources  78 . 
     For example, in the example described with reference to  FIGS. 20 to 23 , each pair of a photosensor  71  and a measurement light source  78  measures transmitted light from a segment. In the example described with reference to  FIGS. 24, 25, and 26 , the average value of the values measured by the pairs of a photosensor  71  and a measurement light source  78  is used to control the voltages to be applied. The number of photosensors  71  and the number of measurement light sources  78  can be one. 
     The control circuit  4  includes a look-up table (LUT)  44 . The look-up table  44  stores configuration information for each segment. The power circuit  3  includes a plurality of DA converters  31 . In  FIG. 27 , only one of the DA converters is provided with a reference sign  31 . The outputs of the DA converters  31  are provided to the associated transparent segmented electrodes. The power circuit  3  provides the transparent electrode  12  with the ground potential as reference potential. 
     The controller  41  controls the outputs of the DA converters  31  in accordance with the consultation results of the look-up table  44  with the values measured by the photosensors  71  to control the voltages to be applied to individual segments (transparent segmented electrodes). The specific control method has been described with reference to  FIGS. 20 to 26 . The power circuit  3  can include a voltage-dividing circuit for applying multiple potentials and selectors for selecting one of the potentials, in place of the DA converters  31 . 
     As set forth above, embodiments of this disclosure have been described; however, this disclosure is not limited to the foregoing configuration examples. Those skilled in the art can easily modify, add, or convert each element in the foregoing configuration examples within the scope of this disclosure. A part of one configuration example can be replaced with a part of another configuration example or a part of a configuration example can be incorporated into another configuration example.