Patent Publication Number: US-2020292906-A1

Title: Electrochromic element

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an electrochromic element. 
     Description of the Related Art 
     An electrochromic (hereinafter, which may be referred to as “EC”) element is an element having a pair of electrodes and an EC layer arranged between the electrodes. Further, the EC element is an optical element that adjusts a color phase or a light amount in a visible light region by applying a voltage between the pair of electrodes to oxidize or reduce a compound in an EC layer. In recent years, there is an increasing demand for a moving-image capturing device using an image pickup device on which a variable ND filter capable of adjusting the optical density in a stepless manner is mounted. Since an EC element using an organic EC material has a wide range of light amount adjustment and design of spectral transmittance thereof is relatively easy, the EC element is a promising candidate for a variable ND filter mounted on a moving-image capturing device. On the other hand, a so-called solution-type EC element configured such that a solution in which an organic EC material is dissolved in a solvent is arranged between a pair of electrodes generates an inhomogeneous potential distribution in an electrode face because of a relatively large current flow, which may cause inhomogeneity of transmittance in a coloring region face. Japanese Patent Application Laid-Open No. 2017-187684 discloses a technique in which current is limited by adding a cyanoethylated polymer as a thickening agent to an EC layer. 
     As a scheme for directly improving an inhomogeneous potential distribution that is a cause of inhomogeneity of transmittance, two methods, namely, decreasing the  electrode resistance and reducing the element current are considered. However, when the former is performed, for example, when an electrode resistance is reduced due to a thickened film of a transparent electrode, light absorption at the electrode part increases, and transmittance of the whole element is reduced. Further, when the latter is performed, when a current is limited by adding a thickening agent as with the case of Japanese Patent Application Laid-Open No. 2017-187684, material transfer is inhibited, and the response speed of the element is delayed. As described above, it is difficult to obtain a sufficient effect without compromising a basic performance of an EC element such as transmittance or response performance by using the traditional methodology of reducing inhomogeneity of transmittance by improving an inhomogeneous potential distribution. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems and intends to provide an EC element in which inhomogeneity of transmittance is reduced by control of an optical path length in an element sectional direction. 
     An electrochromic element of the present disclosure includes: a pair of electrodes; and an electrochromic layer arranged between the pair of electrodes, the electrochromic element has a modulation range of an optical density ΔOD that is 0≤ΔOD≤D, and when a maximum optical density and a minimum optical density in a coloring region face when an inter-electrode distance of the pair of electrodes is constant are ΔOD max  and ΔOD min , respectively, an inter-electrode distance d′ of the pair of electrodes is represented by a following equation at a position providing the minimum optical density ΔOD min : 
     
       
      
       d′=d+δd  
      
         
         
           
             d: an inter-electrode distance when the inter-electrode distance of the pair of electrodes is constant 
             δd: an inter-electrode distance correction amount, and  
             when an optimal inter-electrode distance correction amount δd 0  calculated when an optical density difference between the maximum optical density ΔOD max  and the minimum optical density ΔOD min  is completely eliminated at the position providing the minimum optical density ΔOD min  is defined as a following equation: 
           
         
       
    
       δd 0 (ΔOD)= d ×(ΔOD max /ΔOD min −1),
         the inter-electrode distance correction amount δd at the position providing the minimum optical density ΔOD min  is smaller than or equal to a maximum value δd 0, MAX  of the optimal inter-electrode distance correction amount δd 0  (0&lt;ΔOD&lt;D) at 0&lt;ΔOD&lt;D and larger than or equal to the optimal inter-electrode distance correction amount δd 0  (ΔOD=D) at ΔOD=D.       

     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic plan view illustrating one embodiment of an EC element according to the present invention. 
         FIG. 1B  is a schematic sectional view illustrating one embodiment of the EC element according to the present invention. 
         FIG. 2A  is a graph illustrating a potential distribution in an element when an inter-electrode distance is constant. 
         FIG. 2B  is a graph illustrating transmittance distribution in the element when an inter-electrode distance is constant. 
         FIG. 3A  is a graph illustrating an inter-electrode distance (X-axis reference) determined using  FIG. 2A  and  FIG. 2B . 
         FIG. 3B  is a graph illustrating an inter-electrode distance (Y-axis reference) determined using  FIG. 2A  and  FIG. 2B . 
         FIG. 4A  is a graph illustrating a potential distribution in an element of an  inter-electrode distance d′ (x, y). 
         FIG. 4B  is a graph illustrating a transmittance distribution in an element of an inter-electrode distance d′ (x, y). 
         FIG. 5  is a graph illustrating dependency of the number of optical steps of inhomogeneity of transmittance in an element in which an inter-electrode distance is constant. 
         FIG. 6A  is a graph illustrating inhomogeneity of transmittance when correction of an inter-electrode distance is performed at the time of coloring at an optical density of 0.3. 
         FIG. 6B  is a graph illustrating inhomogeneity of transmittance when correction of an inter-electrode distance is performed at the time of coloring at an optical density of 1.0. 
         FIG. 6C  is a graph illustrating inhomogeneity of transmittance when correction of an inter-electrode distance is performed at the time of coloring at an optical density of 2.0. 
         FIG. 7  is a graph illustrating a relationship between an optical density and an optimal inter-electrode distance correction amount δd 0  in a modulation range of an optical density ΔOD. 
         FIG. 8  is a graph illustrating dependency of an optical density of inhomogeneity of transmittance when an inter-electrode distance correction amount δd=1.35 μm is employed. 
         FIG. 9  is a graph illustrating dependency of an optical density of inhomogeneity of transmittance when an inter-electrode distance correction amount δd=1.73 μm is employed. 
         FIG. 10  is a graph illustrating dependency of an optical density of inhomogeneity of transmittance when an inter-electrode distance correction amount δd=1.50 μm is employed.  
         FIG. 11  is a graph illustrating dependency of an optical density of inhomogeneity of transmittance when an inter-electrode distance correction amount δd=0.84 μm is employed. 
         FIG. 12  is a graph illustrating dependency of an optical density of an inhomogeneity of transmittance when an inter-electrode distance correction amount δd=2.02 μm is employed. 
         FIG. 13A  is a schematic diagram illustrating one embodiment of an imaging device having a lens unit with an optical filter of the present disclosure. 
         FIG. 13B  is a schematic diagram illustrating another embodiment of an imaging device having an optical filter of the present disclosure. 
         FIG. 14A  is a schematic diagram illustrating an example of one embodiment of a window member (perspective view) of the present disclosure. 
         FIG. 14B  is a schematic diagram illustrating an example of one embodiment of a window member (sectional view) of the present disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     1. Electrochromic Element (EC Element) 
     The preferable embodiments for the configuration of the EC element according to the present disclosure will be illustratively described below in detail with reference to the drawings. Note that a configuration, relative arrangement, and the like described in the embodiment do not limit the scope of the present invention unless otherwise specified. 
     First, the configuration of the EC element of the present disclosure will be briefly described with reference to  FIG. 1A  and  FIG. 1B .  FIG. 1A  is a schematic plan view illustrating one embodiment of an EC element  5  of the present disclosure.  FIG. 1B  is a schematic sectional view of the EC element  5  taken along a line A-A′ passing through the element center O of  FIG. 1A . In  FIG. 1A  and  FIG. 1B , the EC element  5  is formed  of a glass substrate as a pair of substrates  1   a  and  1   b  on which transparent electrodes are formed as a pair of electrodes  2   a  and  2   b , respectively, and an EC layer  4  arranged in a space defined by the pair of electrodes  2   a  and  2   b  and a seal  3 . As illustrated in  FIG. 1A , a region surrounded by the seal  3  is a coloring region  6 , and the element center O is the center of the coloring region  6  in  FIG. 1A . 
     An inter-electrode distance of the pair of electrodes  2   a  and  2   b  of the EC element  5  of the present disclosure is characterized by being determined based on an optical density distribution when the inter-electrode distance is constant. The EC element  5  in  FIG. 1A  and  FIG. 1B  is configured such that the inter-electrode distance at the element center is the largest. As long as the electrode arrangement and the power supply configuration that are symmetrical with respect to the element center are employed, the element center has always the highest transmittance, and the transmittance distribution can be corrected and improved by maximizing the inter-electrode distance at the element center. 
     More specifically, an inter-electrode distance d′ of the pair of electrodes  2   a  and  2   b  of the EC element  5  of the present invention is represented by the following equation (a) at the position providing the minimum optical density ΔOD min . Herein, the minimum optical density ΔOD min  is the minimum optical density in the coloring region face when the inter-electrode distance of the pair of electrodes  2   a  and  2   b  is constant. 
         d′=d+δd   (a)
         d: an inter-electrode distance when the inter-electrode distance of the pair of electrodes  2   a  and  2   b  is constant   δd: an inter-electrode distance correction amount       

     Control of the inter-electrode distance can be preferably realized by weight control or the like when the pair of substrates  1   a  and  1   b  are attached to each other. 
       FIG. 2A  and  FIG. 2B  are graphs illustrating a potential distribution E (x, y) and a transmittance distribution T (x, y) in an element in which the inter-electrode distance  of the pair of electrodes  2   a  and  2   b  is constant in a coloring region face when colored to the optical density ΔOD=2.0 by applying an external voltage of 0.661 V. The coloring region face is a rectangle in  FIG. 2A  and  FIG. 2B  but may be another shape such as a circle. As illustrated in  FIG. 2A , the potential is the maximum value of 0.649 V at the four corners of the coloring region face and is the minimum value of 0.640 V at the center of the coloring region face. On the other hand, as illustrated in  FIG. 2B , the transmittance is the minimum value of 0.92% at the four corners of the coloring region face in which the potential is the largest and is the maximum value of 1.12% at the element center in which the potential is the smallest. The transmittance variation at this time is around ±10% in the plane and is a significantly large value for a use in a variable ND filter. 
     Herein, the inhomogeneity of transmittance due to a potential distribution can be completely canceled by using Lambert-Beer law in which the optical density (the logarithmic value of the transmittance) is proportional to an optical path length so that an inter-electrode distance d′ (x, y) at a position (x, y) satisfies the following equation (b), 
         d ′( x, y )= d ×(ΔOD max /ΔOD ( x, y ))  (b)
         d: an inter-electrode distance when the inter-electrode distance of the pair of electrodes  2   a  and  2   b  is constant   ΔOD (x, y): an optical density at a position (x, y) when the inter-electrode distance is constant   ΔOD max : the maximum optical density in the coloring region face when the inter-electrode distance is constant (the optical density at the four corners in the coloring region face in  FIG. 2A  and  FIG. 2B ).       

       FIG. 3A  and  FIG. 3B  are graphs illustrating a new inter-electrode distance d′ (x, y) that can cancel the inhomogeneity of transmittance determined by applying the relationship illustrated in  FIG. 2A  and  FIG. 2B .  FIG. 3A  is a graph based on the X-axis direction as a reference, and  FIG. 3B  is a graph based on the Y-axis direction as a reference. It is indicated that convex shape correction of 1.35 μm at the maximum is required at the  element center for an inter-electrode distance d=30 μm when the inter-electrode distance is constant. Further, the new inter-electrode distance d′ (x, y) that can cancel the inhomogeneity of transmittance is the same quadratic curve as a catenary with fixed both ends. Accordingly, it is also indicated that, when the inter-electrode distance is corrected by only focusing on the position providing the maximum transmittance/the position providing the minimum optical density ΔOD min  (the element center in  FIG. 2A  and  FIG. 2B ), the inter-electrode distance can cancel inhomogeneity of transmittance over the whole coloring region can be formed. 
       FIG. 4A  and  FIG. 4B  are graphs illustrating a potential distribution E (x, y) and a transmittance distribution T (x, y) in the coloring region face at the time of coloring in the same condition as in  FIG. 2A  and  FIG. 2B  in the element of the new inter-electrode distance d′ (x, y) illustrated in  FIG. 3A  and  FIG. 3B  in which the element center has a convex shape. As illustrated in  FIG. 4A , the potential distribution is not changed at all from  FIG. 2A . On the other hand, as illustrated in  FIG. 4B , it can be seen that the transmittance is  0 . 92 % over the whole region, and the inhomogeneity of transmittance is eliminated at the minimum value. 
     As described above, by newly determining the inter-electrode distance d′ (x, y) based on the optical density distribution of the EC element whose inter-electrode distance is constant, the inhomogeneity of transmittance can be reduced. 
     Next,  FIG. 5  illustrates dependency of the number of optical steps of the inhomogeneity of transmittance (modulation range of the optical density ΔOD: 0≤ΔOD≤2.0) at the position providing the minimum optical density ΔOD min  in the element that has a constant inter-electrode distance and is not corrected. Since the current that flows into the element increases as the number of optical stages increases, it can be seen that the inhomogeneity of transmittance also increases due to a voltage drop. The inhomogeneity of transmittance at the optical density ΔOD=2.0 is +10.3%/−9.5%, which corresponds to around 0.3 steps of exposure difference and thus is large value for a use in  a variable ND filter of an imaging device. Further, the fact that the inhomogeneity of transmittance significantly changes depending on the optical density means that the brightness of the acquired image changes in a use for a variable ND filter of an imaging device. Thus, it is also necessary to suppress optical density dependency of inhomogeneity of transmittance to a range that does not affect an image. 
     On the other hand,  FIG. 6A ,  FIG. 6B , and  FIG. 6C  illustrate change of the inhomogeneity of transmittance at the time of coloring at the optical density ΔOD=0.3, 1.0, and 2.0, respectively, when the inter-electrode distance is corrected. The inter-electrode distance correction amount δd of  FIG. 6A ,  FIG. 6B , and  FIG. 6C  represents a difference between an inter-electrode distance of the position providing the maximum optical density ΔOD max /the position providing the minimum transmittance (four corners of the coloring region face in  FIG. 2A  and  FIG. 2B ) and an inter-electrode distance of the position providing the minimum optical density ΔOD min /the position providing maximum transmittance (the element center in  FIG. 2A  and  FIG. 2B ). That is, the inter-electrode distance correction amount δd of  FIG. 6A ,  FIG. 6B , and  FIG. 6C  is an inter-electrode distance correction amount at the position providing the minimum optical density ΔOD min  and is defined by the following equation (c). 
       δ d=d ×(ΔOD max /ΔOD min )− d ×(ΔOD max /ΔOD min )= d ×(ΔOD max /ΔOD min −1)   (c)
         d: an inter-electrode distance when the inter-electrode distance of the pair of electrodes  2   a  and  2   b  is constant   ΔOD max : the maximum optical density in the coloring region face when the inter-electrode distance is constant   ΔOD min : the minimum optical density in the coloring region face when the inter-electrode distance is constant       

     From  FIG. 6A  illustrating the relationship between the inter-electrode distance correction amount and the inhomogeneity of transmittance at the time of coloring  at ΔOD=0.3, first, the inhomogeneity of transmittance when no correction is performed (δd=0) is +1.1%/−1.1%. The inhomogeneity of transmittance then gradually decreases as the inter-electrode distance correction amount increases from δd=0, and the inhomogeneity of transmittance becomes zero at the inter-electrode distance correction amount δd=0.84 μm. The inter-electrode distance correction amount od, that is, the inter-electrode distance correction amount δd calculated when the difference of an optical density between the maximum optical density ΔOD max  and minimum optical density ΔOD min  is completely eliminated is the optimal inter-electrode distance correction amount δd 0  at the time of coloring at ΔOD=0.3 and is represented by the following equation (d). 
       δ d   0 (ΔOD=0.3)= d ×(ΔOD max /ΔOD min −1)  (d)
         d: an inter-electrode distance when the inter-electrode distance of the pair of electrodes  2   a  and  2   b  is constant   ΔOD max : the maximum optical density in the coloring region face when the inter-electrode distance is constant   ΔOD min : the minimum optical density in the coloring region face when the inter-electrode distance is constant       

     The inhomogeneity of transmittance gradually increases as the inter-electrode distance correction amount further increases from the optimal inter-electrode distance correction amount δd 0 , and the density gradient for the position is reversed. That is, the transmittance on the element center is lower than the transmittance on the four corners of the coloring region face in  FIG. 2A  and  FIG. 2B . 
     Similarly, as illustrated in  FIG. 6B , at the time of coloring at ΔOD=1.0, the inhomogeneity of transmittance when no correction is performed (δd=0) is +6.7%/−6.6%. The inhomogeneity of transmittance then gradually decreases as the inter-electrode distance correction amount increases from δd=0, and the inhomogeneity of transmittance becomes zero at the inter-electrode distance correction amount δd=1.73 μm. That is, the optimal inter-electrode distance correction amount δd 0 =1.73 μm. Moreover, as  illustrated in  FIG. 6C , at the time of coloring at ΔOD=2.0, the inhomogeneity of transmittance when no correction is performed (δd=0) is +10.3%/−9.5%. The inhomogeneity of transmittance then gradually decreases as the inter-electrode distance correction amount increases from δd=0, and the inhomogeneity of transmittance becomes zero at the inter-electrode distance correction amount δd=1.35 μm. That is, the optimal inter-electrode distance correction amount δd 0 =1.35 μm. 
     In such a way, due to the non-linearity of the current-voltage property and the voltage-optical density property of the element, since the optimal inter-electrode distance correction amount δd 0  at which the inhomogeneity of transmittance becomes zero due to the optical density ΔOD is different, various correction specifications can be considered in accordance with how to use the element. 
       FIG. 7  is a graph illustrating a relationship between the optical density ΔOD and the optimal inter-electrode distance correction amount δd 0  in a modulation range (0≤ΔOD≤2.0) of the optical density ΔOD. Assuming a quadratic curve passing through the origin, the optimal inter-electrode distance correction amount δd 0  is the maximum value δd 0, MAX  (0≤ΔOD≤D)=1.81 μm at the optical density ΔOD=1.3. Further, δd 0  (ΔOD=D)=1.35 μm at ΔOD=2.0 that is the end of the modulation range of the optical density ΔOD. 
     As described below, the preferable range of the inter-electrode distance correction amount δd at the position providing the minimum optical density ΔOD min  of the element is between the above two values, and thereby it is possible to suppress the inhomogeneity of transmittance to a small value throughout the modulation range of the optical density ΔOD. 
     Thus, in the EC element of the present invention, the inter-electrode distance correction amount δd at the position providing the minimum optical density ΔOD min  satisfies the following equation (e). 
       δ d≤δd   0, MAX  (0&lt;ΔOD&lt;D) at the position providing δd 0  (ΔOD=D)≤ΔOD min   (e)
          δd 0  (ΔOD=D): the optimal inter-electrode distance correction amount at ΔOD=D   ΔOD min : the minimum optical density in the coloring region face when the inter-electrode distance is constant   δd: an inter-electrode distance correction amount   δd 0, MAX  (0&lt;ΔOD&lt;D): the maximum value of the optimal inter-electrode distance correction amount at 0&lt;ΔOD&lt;D       

     (1) Case of inter-electrode distance correction amount δd=1.35 μm 
     An example in which an element is fabricated by employing the optimal inter-electrode distance correction amount δd 0 =1.35 μm at the maximum value ΔOD=2.0 (end of the modulation range) in the modulation range (0≤ΔOD≤2.0) of the optical density ΔOD is provided.  FIG. 8  illustrates optical density dependency of the inhomogeneity of transmittance at a position providing the minimum optical density ΔOD min  at this time. The inhomogeneity of transmittance on the lower density (ΔOD=0.3) side and the inhomogeneity of transmittance on the higher density (ΔOD=2.0) side are suppressed to a small value, and the inhomogeneity of transmittance at the intermediate density (ΔOD=1.0) is relatively larger but is around ±1.5% as the absolute value and is significantly small. Further, on the lower density (ΔOD=0.3) side, the density gradient reverses due to excess correction (see  FIG. 6A ) but is less than 1% as the absolute value and is not notable. This is a correction specification having good balance in the whole optical density modulation range. 
     (2) Case of inter-electrode distance correction amount δd=1.73 μm 
     An example in which an element is fabricated by employing the optimal inter-electrode distance correction amount δd 0 =1.73 μm at the intermediate value ΔOD=1.0 in the modulation range (0≤ΔOD≤2.0) of the optical density ΔOD is provided.  FIG. 9  illustrates optical density dependency of the inhomogeneity of transmittance at a  position providing the minimum optical density ΔOD min at this time. The inhomogeneity of transmittance is suppressed to a small value from the lower density (ΔOD=0.3) side to the intermediate density (ΔOD=1.0), and the inhomogeneity of transmittance on the higher density (ΔOD=2.0) side is relatively larger but is around ±2% as the absolute value and is significantly small. This correction specification is preferable for frequent use in the intermediate density region. 
     (3) Case of inter-electrode distance correction amount δd=1.50 μm 
     An example in which an element is fabricated by employing the optimal inter-electrode distance correction amount δd 0 =1.50 μm at the intermediate values ΔOD=0.8 and 1.8 in the modulation range (0≤ΔOD≤2.0) of the optical density ΔOD is provided.  FIG. 10  illustrates optical density dependency of the inhomogeneity of transmittance at a position providing the minimum optical density ΔOD min  at this time. Excess correction occurs (see  FIG. 6A  and  FIG. 6C ) on the lower density (ΔOD=0.3) side and the higher density (ΔOD=2.0) side, and optical density gradient reverses. However, the absolute values of the inhomogeneity of transmittance are around ±0.8% and around ±1.1%, respectively, and the maximum density difference is less than 1/32 steps, which is in a range where visual recognition is not possible. Further, at the intermediate density (ΔOD=1.0), the absolute value of the inhomogeneity of transmittance is around ±0.9% and is significantly small. 
     (4) Case of inter-electrode distance correction amount δd=0.84 μm 
     An example in which an element is fabricated by employing the optimal inter-electrode distance correction amount δd 0 =0.84 μm at the intermediate values ΔOD=0.3 in the modulation range (0≤ΔOD≤2.0) of the optical density ΔOD is provided. The optimal inter-electrode distance correction amount δd 0 =0.84 μm is smaller than the optimal inter-electrode distance correction amount δd 0 =1.35 μm at the end of the optical density modulation range (ΔOD=2.0).  FIG. 11  illustrates optical density dependency of the inhomogeneity of transmittance at a position providing the minimum optical density  ΔOD min  at this time. Since correction is insufficient in the whole modulation range of the optical density ΔOD, the inhomogeneity of transmittance on the lower density (ΔOD=0.3) side is sufficiently small. However, the inhomogeneity of transmittance is slightly large from the intermediate density (ΔOD=1.0) to the higher density (ΔOD=2.0) side and is around ±5% as the absolute value. Since this corresponds to an exposure difference of around 1/8 steps and a brightness change is visible, it is not suitable for a use in a variable ND filter of an imaging device. 
     (5) Case of inter-electrode distance correction amount δd=2.02 μm 
     An example in which an element is fabricated by employing the inter-electrode distance correction amount δd=2.02 μm that is larger than the maximum value δd 0, MAX =1.81 μm of the optimal inter-electrode distance correction amount δd 0  in the modulation range (0≤ΔOD≤2.0) of the optical density ΔOD is provided.  FIG. 12  illustrates optical density dependency of the inhomogeneity of transmittance at a position providing the minimum optical density ΔOD min  at this time. Excess correction occurs in the whole modulation range of the optical density ΔOD (see  FIG. 6A ,  FIG. 6B  and  FIG. 6C ), and the inhomogeneity of transmittance at the intermediate density (ΔOD=1.0) is around ±1.1% and is sufficiently small. However, the inhomogeneity of transmittance is slightly large on the higher density (ΔOD=2.0) side and is around ±5% as the absolute value. As with “(4) Case of inter-electrode distance correction amount δd=0.84 μm”, this is not suitable for a use in a variable ND filter of an imaging device. 
     As described above, when the inter-electrode distance correction amount δd at the position providing the minimum optical density ΔOD min  satisfies the above equation (e), an excellent correction specification having a good balance in the whole optical density modulation range and a significantly small absolute value of the inhomogeneity of transmittance is obtained. 
     Herein, although it is preferable that the optical density gradient in the radius direction of the element (the direction from the center point toward to the edge) do not  reverse, reverse of the optical density gradient may be allowed in a range where no substantial recognition is possible. It is assumed that the reference of brightness that is difficult for a human to recognize is 1/16 steps, preferable application is possible when the maximum density difference in the coloring region face is smaller than or equal to (log2)/16, preferably, the optical density difference is smaller than or equal to (log2)/16 between any regions in the coloring region face. 
     Further, the inter-electrode distance in a region between a position providing the maximum optical density ΔOD max  and a position providing the minimum optical density ΔOD min  may be larger than or equal to the inter-electrode distance at the position providing the maximum optical density ΔOD max  and may be smaller than or equal to the inter-electrode distance at the position providing the minimum optical density ΔOD min . 
     The inter-electrode distance in a region between a position providing the maximum optical density ΔOD max  and a position providing the minimum optical density ΔOD min  may be a value on a catenary where the inter-electrode distance at the position providing the maximum optical density ΔOD max  corresponds to both ends and the inter-electrode distance at a position providing the minimum optical density ΔOD min  corresponds to an apex. 
     The coloring region face has a rectangular shape, and the position providing the maximum optical density ΔOD max  may include four corners of the coloring region face, and the position providing the minimum optical density ΔOD min  may include the center of the coloring region face. 
     1.1 Member Forming EC Element 
     The EC element  5  of the present embodiment has the pair of substrates  1   a  and  1   b , the pair of electrodes  2   a  and  2   b , and the EC layer  4  arranged between the pair of electrodes  2   a  and  2   b . The pair of electrodes  2   a  and  2   b  are attached to each other by a seal  3 , and the EC layer  4  having an EC compound is arranged in a space defined by the pair of electrodes  2   a  and  2   b  and the seal  3 . Members forming the EC element  5  will be  described below in detail. 
     1.1.1 Electrochromic Layer (EC Layer)  4   
     The EC layer  4  may be a solid layer formed by a deposition method or the like or may be a solution layer in which an EC compound is dissolved in an electrolyte solution. A method for forming the EC layer  4  may be a method in which a liquid containing a pre-prepared EC compound by a vacuum injection method, an atmospheric injection method, a meniscus method, or the like is injected into a gap provided between the pair of electrodes  2   a  and  2   b.    
     The EC compound may be an organic compound or an inorganic compound, or may be an anodic compound that is colored by oxidation reaction from a transparent state or a cathodic compound that is colored by reduction reaction from a transparent state. Further, both an anodic compound and a cathodic compound may be used. In particular, when an organic compound is used, it is preferable that both an anodic organic compound and a cathodic organic compound be used because coloring efficiency for current is higher. In this specification, an element having both an anodic compound and a cathodic compound is referred to as a complementary EC element, and an element having any one of an anodic compound and a cathodic compound is referred to as a unipolar EC element. The anodic compound is also referred to as an anode material, and the cathodic compound is also referred to as a cathode material. 
     When a complementary EC element is driven, electrons are extracted from the EC compound by oxidation reaction in one electrode, and an EC compound receives the electrons by reduction reaction in the other electrode. A radical cation may be generated from a neutral molecule by oxidation reaction. Further, a radical anion may be generated from a neutral molecule by reduction reaction, or a radical cation may be generated from a dication molecule by reduction reaction. Since the EC compound is colored in both of the pair of electrodes  2   a  and  2   b , a complementary EC element is preferably used when a large change in the optical density is required at the time of  coloring. On the other hand, a unipolar EC element is preferably used because power consumption of the unipolar EC element can be suppressed compared to power consumption of the complementary EC element. This is because the complementary EC element has a self-erasing reaction in which the colored anodic compound and the colored cathodic compound exchange electrons, and a large current is required for maintaining a coloring state. 
     When the EC compound is an inorganic compound, an electrolyte layer may be provided between the EC layer  4  and at least one of the pair of electrodes  2   a  and  2   b . On the other hand, when the EC compound is an organic compound, an electrolyte layer may be provided as with the case of an inorganic compound, or an electrolyte solution may be provided with an organic compound. 
     The organic EC compound may be a conductive polymer such as polythiophene or polyaniline, an organic low-molecular compound such as a viologen-based compound, an anthraquinone-based compound, an oligothiophene derivative, a phenazine derivative, or the like. The inorganic EC compound may be a metallic oxide material such as NiO x  or WO 3 . 
     The EC layer  4  may have a stacked configuration of an electrolyte layer including an electrolyte and a layer including an EC compound. The EC layer  4  may have only the single type of EC compound or may have multiple types of EC compounds. When the EC layer  4  includes multiple types of EC compounds, it is preferable that the difference in the oxidation-reduction potential of the EC compound be small. When the EC layer  4  has multiple types of EC compounds, the EC layer  4  may have four or more types of EC compounds including an anodic compound and a cathodic compound. The EC element of the present invention may have five or more types of EC compounds. When the EC layer  4  has multiple types of EC compounds, the oxidation-reduction potential of a plurality of anode materials may be within 60 mV, and the oxidation-reduction potential of a plurality of cathode materials may be within 60 mV. When the  EC layer  4  has multiple types of EC compounds, the multiple types of EC compounds may include a compound having an absorption peak that is higher than or equal to 400 nm and lower than or equal to 500 nm, a compound having an absorption peak that is higher than or equal to 500 nm and lower than or equal to 650 nm, and a compound having an absorption peak that is higher than or equal to 650 nm. The absorption peak means that the full width half maximum is larger than or equal to 20 nm. Further, a state of a material when light is absorbed may be an oxidation state, a reduction state, or a neutral state. 
     An electrolyte is not limited as long as it is an ionic dissociable salt and has a good solubility for a solvent or high compatibility in a solid electrolyte. Specifically, an electrolyte having electron donating is preferable. These electrolytes may be referred to as a supporting electrolyte. An electrolyte may be, for example, inorganic ion salts such as various alkali metal salts, or alkaline earth metal salts, quaternary ammonium salts, cyclic quaternary ammonium salts, or the like. Specifically, an alkali metal salt of Li, Na, or K such as LiClO 4 , LiSCN, LiBF 4 , LiAsF 6 , LiCF 3 SO 3 , LiPF 6 , LiI, NaI, NaSCN, NaClO 4 , NaBF 4 , NaAsF 6 , KSCN, or KCl, a quaternary ammonium salt such as (CH 3 ) 4 NBF 4 , (C 2 H 5 ) 4 NBF 4 , (n-C 4 H 9 ) 4 NBF 4 , (n-C 4 H 9 ) 4 NPF 6 , (C 2 H 5 ) 4 NBr, (C 2 H 5 ) 4 NClO 4 , or (n-C 4 H 9 ) 4 NClO 4 , a cyclic quaternary ammonium salt, or the like may be used. 
     As a solvent used for dissolving an EC compound and an electrolyte, while the solvent is not particularly limited as long as it can dissolve an EC compound or an electrolyte, the polar solvent is preferable in particular. Specifically, water or an organic polar solvent such as methanol, ethanol, propylene carbonate, ethylene carbonate, dimethyl sulfoxide, dimethoxyethane, γ-butyrolactone, γ-valerolactone, sulfolane, dimethylformamide, dimethoxyethane, tetrahydrofuran, acetonitrile, propionnitrile, 3-methoxypropionnitrile, benzonitrile, dimethylacetamide, methylpyrrolidinone, dioxolan, or the like may be used. 
     The EC layer  4  may further include a polymer matrix or a gelling agent. In  such a case, the EC layer  4  is a higher viscous liquid or a gel in some cases. For example, a polymer may be polyacrylonitrile, carboxymethylcellulose, pullulan-based polymer, polyvinyl chloride, polyethylene oxide, polypropylene oxide, polyurethane, polyacrylate, polymethacrylate, polyamide, polyacrylamide, polyester, Nafion (registered trademark), or the like, and PMMA is preferably used. 
     1.1.2 Substrates  1   a  and  1   b    
     As the substrates  1   a  and  1   b , a colorless or colored glass, a tempered glass, or the like may be used, for example. As such glass materials, an optical glass substrate such as Corning #7059 or BK-7 may be preferably used. Moreover, as the substrates  1   a  and  1   b , a material having high rigidity and causing less distortion is preferable. Note that, in present embodiment, “transparent” means that the transmittance of visible light is larger than or equal to 50%. 
     1.1.3 Electrodes  2   a  and  2   b    
     The electrodes  2   a  and  2   b  may be, for example, a metal or metallic oxide such as an indium tin oxide (ITO) alloy, fluorine-doped tin oxide (FTO), tin oxide (NESA), indium zinc oxide (IZO), silver oxide, vanadium oxide, molybdenum oxide, gold, silver, platinum, copper, indium, or chromium, a silicon-based material such as polycrystalline silicon, or amorphous silicon, a carbon material such as carbon black, graphite, or glassy carbon, or the like. Further, a conductive polymer whose conductivity is improved by doping treatment or the like, for example, polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, a complex of polyethylenedioxythiophene (PEDOT) and polystyrenesulfonate, or the like are also preferably used. 
     Since the EC element  5  according to the present invention preferably has a high transmittance in a decolored state, the electrodes  2   a  and  2   b  are particularly preferably formed of a transparent material such as ITO, IZO, NESA, PEDOT: PSS, or graphene, for example. These materials can be used in various forms such as bulk or fine particles. Note that each of these electrodes may be used alone, or a plurality of these  electrodes may be used in combination. 
     1.1.4 Seal  3   
     As the seal  3 , a material that is chemically stable, does not permeate a gas or a liquid, and does not inhibit oxidation-reduction reaction of the EC compound is preferable. The seal  3  may be, for example, an inorganic material such as a glass frit, an organic material such as an epoxy resin, a metal material, or the like. 
     1.1.5 Spacer 
     The EC element  5  according to the present invention may have a spacer. The spacer has a function of defining the distance between the electrodes  2   a  and  2   b . The seal  3  may have the function of the spacer. The spacer may be formed of an inorganic material such as silica beads, or a glass fiber or an organic material such as polydivinylbenzene, polyimide, polytetrafluoroethylene, fluororubber, or an epoxy resin. 
     2. Use of EC Element 
     By driving the EC element according to the present embodiment, the EC element can adjust the amount of a light passing through the EC element and can be used for an optical filter such as a variable ND filter, a lens unit, an imaging device, or the like. 
     2.1 Optical Filter 
     An optical filter of the present invention has the EC element of the present invention and an active element connected to the EC element. The optical filter of the present invention may include a peripheral device. The active element may be directly connected to the EC element or may be indirectly connected via other elements. The active element may be a TFT element, a MIM element, or the like, for example. The optical filter of the present invention adjusts the light amount of a light passing through the EC element by an active element driving the EC element. The optical filter of the present invention may be used for an imaging device such as a camera. When used for an imaging device, the optical filter may be provided in an imaging device body or may be provided in a lens unit.  
     2.2 Lens Unit and Imaging Device 
     The lens unit of the present invention has an optical filter of the present invention described above and an imaging optical system having a plurality of lenses. The lens unit of the present invention may be arranged so that a light that has passed through the optical filter of the present invention passes through the imaging optical system or may be arranged so that a light that has passed through the imaging optical system passes through the optical filter of the present invention. 
     Further, the imaging device of the present invention has the optical filter of the present invention and the image pickup device that receives a light that has passed through the optical filter. 
       FIG. 13A  is a schematic diagram illustrating an imaging device using the optical filter of the present invention and illustrates an imaging device having a lens unit  102  using an optical filter  101  of the present invention, and  FIG. 13B  illustrates an imaging device having the optical filter  101  of the present invention. As illustrated in  FIG. 13A  and  FIG. 13B , the lens unit  102  is connected in a removable manner to an imaging unit  103  via a mount member (not illustrated). 
     The lens unit  102  is a unit having a plurality of lenses or lens groups. For example, in  FIG. 13A , the lens unit  102  represents a zoom lens of a rear focus scheme that performs focusing on the post-stage of an aperture. The lens unit  102  has four lens groups: a first lens group  104  of positive refractive power, a second lens group  105  of negative refractive power, a third lens group  106  of positive refractive power, and a fourth lens group  107  of positive refractive power in this order from the object side (the left side to the sheet). The magnification is changed by changing the gap between the second lens group  105  and the third lens group  106 , and focusing is performed by moving some of the lens of the fourth lens group  107 . For example, the lens unit  102  has an aperture  108  between the second lens group  105  and the third lens group  106  and has the optical filter  101  of the present invention between the third lens group  106  and the fourth lens group   107 . The arrangement is such that a light passing through the lens unit  102  passes through each of the lens groups  104  to  107 , the aperture  108 , and the optical filter  101  of the present invention, and a light amount can be adjusted by using the aperture  108  and the optical filter  101  of the present invention. 
     Further, the configuration inside the lens unit  102  can be changed as appropriate. For example, the optical filter  101  of the present invention can be arranged in front of the aperture  108  (on the object side) or behind the aperture  108  (on the imaging unit  103  side), and may be arranged in front of the first lens group  104  or may be arranged behind the fourth lens group  107 . With the optical filter  101  of the present invention being arranged at a position where a light converges, there is an advantage of the reduced area of the optical filter  101  of the present invention or the like. Further, the form of the lens unit  102  can be selected as appropriate, which may be an inner focus scheme that performs focusing in front of the aperture or other schemes instead of the rear focus scheme. Further, a special lens such as a fisheye lens, a macro-lens, or the like instead of the zoom lens can be selected as appropriate. 
     A glass block  109  in the imaging unit  103  is a glass block such as a low-pass filter, a phase plate, a color filter, or the like. Further, the image pickup device  110  is a sensor unit that receives a light that has passed through the lens unit  102 , and a CCD, a CMOS, or the like can be used. Further, the image pickup device  110  may be an optical sensor such as a photodiode, and an element that acquires and outputs information on the intensity or the wavelength of a light can be used as appropriate. 
     As illustrated in  FIG. 13A , when the optical filter  101  of the present invention is embedded in the lens unit  102 , a drive unit may be arranged inside the lens unit  102 , for example, may be arranged outside the lens unit  102 , such as inside the imaging unit  103 . When the drive unit is arranged outside the lens unit  102 , the EC element and the drive unit inside and outside the lens unit  102  are connected via a wiring for drive control.  
     As illustrated in  FIG. 13B , the imaging unit  103  may have the optical filter  101  of the present invention. The optical filter  101  of the present invention may be arranged at a suitable part inside the imaging unit  103 , and the image pickup device  110  is arranged so as to receive a light that has passed through the optical filter  101  of the present invention. In  FIG. 13B , the optical filter  101  of the present invention is arranged closely in front of the image pickup device  110 , for example. When the optical filter  101  of the present invention is built in the imaging unit  103 , since the connected lens unit  102  is not required to have the optical filter  101  of the present invention, it is possible to configure a dimmable imaging device using an existing lens unit. 
     Such an imaging device is applicable to a product having a combination of a light amount adjustment function and an image pickup device. For example, the imaging device can also be used in a camera, a digital camera, a video camera, or a digital video camera and can be applied to a product such as a mobile phone or a smartphone, a PC, a tablet, or the like which the imaging device is built in. 
     By using the optical filter  101  of the present invention as a dimming member, it is possible to appropriately change a dimming amount by a single filter, and there is an advantage of the reduced number of components or space-saving. 
     2.3 Window Member 
     The window member of the present invention has the EC element  5  of the present invention. The window member of the present invention preferably has a driving unit that drives the EC element  5 .  FIG. 14A  and  FIG. 14B  are diagrams illustrating the window member of the present invention.  FIG. 14A  is a perspective view, and  FIG. 14B  is a sectional view taken along a line X-X′ of  FIG. 14A . 
     A window member  111  illustrated in  FIG. 14A  and  FIG. 14B  is a dimming window and is formed of the EC element  5 , transparent plates  113  interposing the EC element, and a frame  112  surrounding and integrating the entirety. The driving unit may be integrated inside the frame  112  or may be arranged outside the frame  112  and  connected to the EC element  5  through a wiring. The transparent plate  113  is not particularly limited as long as it is a material having a high light transmittance and is preferably a glass material when a use as a window is considered. While the EC element  5  is a separate component from the transparent plates  113  in  FIG. 14A  and  FIG. 14B , the substrates  1 a and  1 b of the EC element  5  may be the transparent plates  113 , for example. Any material may be used for the frame  112 , and a general member that covers at least a part of the EC element  5  and has an integrated form can be a frame. 
     Such a dimming window can be applied to a use to adjust the incidence amount of the sunlight in daytime into a room, for example. Such dimming window can also be applied to adjustment of a heat amount as well as a light amount of the sun and thus can be used for control of the brightness or the temperature inside a room. Further, such a dimming window is applicable to a use to block a view from the outside of a room to the inside of the room as a shutter. Such a dimming window is applicable not only to a glass window for a building but also to a window of a vehicle such as an automobile, a train, an airplane, a ship, or the like or a filter on the display face of a watch or a mobile phone. 
     According to the EC element of the present disclosure, it is possible to provide an EC element in which inhomogeneity of transmittance is reduced by control of an optical path length in an element sectional direction. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2019-048245, filed Mar. 15, 2019, which is hereby incorporated by reference herein in its entirety.