Patent Publication Number: US-11665436-B2

Title: Imaging device and camera system

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS 
     This application is a Continuation of Ser. No. 17/015,784, filed on Sep. 9, 2020, which is a Continuation of Ser. No. 16/131,270, filed on Sep. 14, 2018, now U.S. Pat. No. 10,805,554 issued on Oct. 13, 2020, which claims the benefit of Japanese Application No. 2017-188375, filed on Sep. 28, 2017, the entire disclosures of which applications are incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an imaging device and a camera system that can electrically change sensitivity. 
     2. Description of the Related Art 
     In recent years, when adjusting illuminance, a so-called electronic neutral density (ND) filter has been proposed as well as an aperture and a shutter (see, for example, Japanese Unexamined Patent Application Publication No. 2013-88596). In this method, a voltage is applied to set transmittance corresponding to illuminance, and the illuminance is electrically adjusted. 
     SUMMARY 
     One non-limiting and exemplary embodiment provides an imaging device that can more finely adjust sensitivity with respect to a change in brightness. 
     In one general aspect, the techniques disclosed here feature a camera system including: a photoelectric converter including a first electrode, a second electrode, and a photoelectric conversion layer between the first electrode and the second electrode, the photoelectric conversion layer converting incident light into electric charge; voltage application circuitry; and a controller that derives a duty cycle corresponding to an attenuation ratio set for a first frame and that causes the voltage application circuitry to apply a pulse voltage having the duty cycle between the first electrode and the second electrode in the first frame. 
     A comprehensive or specific aspect may be realized by an element, a device, a module, a system, an integrated circuit, or a method. A comprehensive or specific aspect may be realized by any combination of the element, the device, the module, the system, the integrated circuit, and the method. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a functional block diagram illustrating the configuration of a camera system according to a first embodiment; 
         FIG.  2    is diagram illustrating an example of a user I/F unit; 
         FIG.  3    is a diagram for explaining the configuration of a first conversion unit and a second conversion unit in the camera system according to the first embodiment; 
         FIG.  4    is a diagram for explaining a sensitivity adjusting method of the first conversion unit and the second conversion unit in the camera system according to the first embodiment; 
         FIG.  5    is a schematic sectional view of a frequency filter; 
         FIGS.  6 A to  6 D  are graphs for explaining frequency characteristics of the frequency filter; 
         FIG.  7    is a functional block diagram illustrating the configuration of a camera system according to a modification 1 of the first embodiment; 
         FIG.  8    is a table illustrating control patterns of a first conversion unit and a second conversion unit in the modification 1 of the first embodiment; 
         FIG.  9    is a functional block diagram illustrating the configuration of a camera system according to a second embodiment; 
         FIG.  10    is a diagram for explaining the configuration of a first conversion unit and a second conversion unit in the camera system according to the second embodiment; 
         FIG.  11    is a graph illustrating a relation between an applied voltage to a photoelectric conversion unit and an output voltage from a light receiving unit; 
         FIG.  12    is a flowchart for explaining a procedure for correcting light sensitivity of a photoelectric conversion unit when the photoelectric conversion unit is used as an electronic ND filter; 
         FIG.  13    is an example of a timing chart for adjusting the light sensitivity according to a voltage control scheme; 
         FIG.  14    is another example of the timing chart for adjusting the light sensitivity according to the voltage control scheme; 
         FIG.  15    is an example of a timing chart for adjusting the light sensitivity according to a pulse duty control scheme; 
         FIG.  16    is another example of the timing chart for adjusting the light sensitivity according to the pulse duty control scheme; 
         FIG.  17    is a functional block diagram illustrating the configuration of a camera system according to a modification 1 of the second embodiment; 
         FIG.  18    is a diagram for explaining a sensitivity adjusting method of a first conversion unit and a second conversion unit in the modification 1 of the second embodiment; 
         FIG.  19    is a diagram for explaining a light receiving unit in a modification 2 of the second embodiment; 
         FIG.  20    is a diagram for explaining the configuration of a photoelectric conversion unit in the modification 2 of the second embodiment; 
         FIG.  21    is a diagram for explaining a sensitivity adjusting method of the photoelectric conversion unit in the modification 2 of the second embodiment; 
         FIG.  22    is a diagram for explaining the sensitivity adjusting method of the photoelectric conversion unit in the modification 2 of the second embodiment; 
         FIG.  23    is a functional block diagram illustrating the configuration of a camera system according to a modification 3 of the second embodiment; 
         FIG.  24    is a graph illustrating a relation between an applied voltage to a photoelectric conversion unit and a spectral characteristic; 
         FIG.  25    is a functional block diagram illustrating the configuration of a camera system according to a third embodiment; 
         FIG.  26    is a diagram for explaining the configuration of a first conversion unit and a second conversion unit in the camera system according to the third embodiment; and 
         FIG.  27    is a diagram for explaining a sensitivity adjusting method of the first conversion unit and the second conversion unit in the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     It is known that an electronic ND filter can uniformly control the brightness of a captured image compared with an aperture. The electronic ND filter can reduce deterioration in image quality because a diffraction phenomenon seen in the aperture does not occur. However, it is requested that sensitivity can be more finely adjusted with respect to a change in brightness. The present disclosure provides an imaging device that can more finely adjust sensitivity with respect to a change in brightness. 
     An overview of an aspect of the present disclosure is as described below. 
     [Item 1] 
     An imaging device including: 
     a photoelectric converter including a pixel electrode, a counter electrode, and a photoelectric conversion layer between the pixel electrode and the counter electrode, the photoelectric conversion layer converting incident light into an electric charge; and 
     a voltage application circuit that applies a first voltage between the pixel electrode and the counter electrode in a first frame and that applies a second voltage between the pixel electrode and the counter electrode in a second frame different from the first frame, the first voltage being a constant voltage, the second voltage being a pulse-shaped voltage. 
     [Item 2] 
     An imaging device including: 
     a photoelectric converter including a photoelectric conversion layer that includes a first surface and a second surface opposite to the first surface and that converts incident light into an electric charge, a pixel electrode on the first surface, a counter electrode on the second surface, and an auxiliary electrode on the second surface; and 
     a voltage application circuit that applies a first voltage between the pixel electrode and the auxiliary electrode in a first frame and that applies a second voltage between the pixel electrode and the auxiliary electrode in a second frame different from the first frame, the first voltage being a constant voltage, the second voltage being a pulse-shaped voltage. 
     [Item 3] 
     The imaging device according to Item 1 or 2, wherein the voltage application circuit applies either the first voltage or the second voltage according to an ND value set for each frame. 
     [Item 4] 
     The imaging device according to Item 3, wherein the voltage application circuit applies the first voltage in a frame set to an ND value equal to or larger than a first value, and applies the second voltage in a frame set to an ND value smaller than the first value. 
     [Item 5] 
     The imaging device according to Item 4, wherein the voltage application circuit applies, in the frame set to the ND value equal to or larger than the first value, the first voltage having a voltage value corresponding to the ND value. 
     [Item 6] 
     The imaging device according to Item 4 or 5, wherein the voltage application circuit applies, in the frame set to the ND value smaller than the first value, the second voltage having a duty cycle corresponding to the ND value. 
     [Item 7] 
     The imaging device according to Item 4 or 5, wherein the voltage application circuit applies, in the frame set to the ND value smaller than the first value, the second voltage having a voltage value corresponding to the ND value and a duty cycle corresponding to the ND value. 
     [Item 8] 
     The imaging device according to any one of Items 1 to 7, further including a controller that switches an automatic mode for changing, according to intensity of the incident light, a voltage applied between the pixel electrode and the counter electrode by the voltage application circuit, and a manual mode for changing, according to an instruction from a user, a voltage applied between the pixel electrode and the counter electrode by the voltage application circuit. 
     [Item 9] 
     A camera system including: 
     the imaging device according to claim  1 ; 
     a display; and 
     a graphic interface unit that causes the display to selectively display a first image for receiving an instruction for continuously changing a sensitivity and a second image for receiving an instruction for changing a sensitivity stepwise. 
     An overview of another aspect of the present disclosure is as described below. 
     A camera system according to an aspect of the present disclosure includes a light receiving unit that converts incident light into an electric signal, a first conversion unit and a second conversion unit that change, according to an applied voltage, a gain in a process from incidence of the incident light until the conversion of the incident light into the electric signal by the light receiving unit, and an applied-voltage control unit that applies a voltage to the first conversion unit and the second conversion unit. The second conversion unit is disposed in a position where light passed through the first conversion unit is made incident. 
     Because the camera system includes the two conversion units in this way, the conversion units can change the gain independently from each other. Therefore, sensitivity can be more finely adjusted with respect to a change in brightness. Because the camera system includes the two conversion units, the two conversion units perform the change of the gain performed by one conversion unit. Therefore, the size of one conversion unit can be reduced. Further, because the applied voltage for controlling the respective conversion units can be reduced, the change of the gain can be performed at higher speed than when the camera system includes only one conversion unit. 
     For example, in the camera system according to the aspect of the present disclosure, the first conversion unit may be a filter that changes the gain by attenuating a light amount of the incident light according to the applied voltage, the second conversion unit may be a filter on which the light passed through the first conversion unit is made incident and that changes the gain by attenuating the light amount of the incident light according to the applied voltage, and the light receiving unit may receive the light passed through the second conversion unit as the incident light and convert the incident light into the electric signal. 
     Consequently, the sensitivity can be more finely adjusted with respect to the change in the brightness simply by, for example, adding another electronic ND filter to a normal imaging device including only one electronic ND filter. 
     For example, in the camera system according to the aspect of the present disclosure, the first conversion unit and the second conversion unit may be filters having different types of neutral density (ND) mechanisms. 
     Consequently, the light amount of the incident light can be adjusted making use of merits of the different types of the ND mechanisms. 
     For example, in the camera system according to the aspect of the present disclosure, the first conversion unit may be a filter that changes the gain by attenuating a light amount of the incident light according to the applied voltage, the light receiving unit may include a second light amount conversion layer that receives light passed through the first conversion unit as the incident light and converts the incident light into the electric signal and two second electrodes that sandwich the second photoelectric conversion layer, and the second photoelectric conversion layer may function as the second conversion unit and change the gain by changing, according to the applied voltage, quantum efficiency in converting the incident light into the electric signal. 
     Consequently, a function of an electronic ND filter can be realized by the light receiving unit. Therefore, the camera system can be made compact compared with when two electronic ND filters are disposed. 
     For example, in the camera system according to the aspect of the present disclosure, the light receiving unit may include a first photoelectric conversion layer that converts the incident light into the electric signal and two first electrodes that sandwich the first photoelectric conversion layer and a second photoelectric conversion layer that receives light passed through the first conversion unit as the incident light and converts the incident light into the electric signal and two second electrodes that sandwich the second photoelectric conversion layer, the first photoelectric conversion layer may function as the first conversion unit and change the gain by changing, according to the applied voltage, quantum efficiency in changing the incident light into the electric signal, and the second photoelectric conversion layer may function as the second conversion unit and change the gain by changing, according to the applied voltage, the quantum efficiency in converting the incident light into the electric signal. 
     Because the first conversion unit and the second conversion unit are provided in the light receiving unit in this way, functions of two electronic ND filters can be realized by the light receiving unit. Therefore, the camera system can be made more compact than when an electronic ND filter is provided separately from the light receiving unit. 
     For example, in the camera system according to the aspect of the present disclosure, the applied-voltage control unit may change, according to the quantum efficiency, a duty cycle of a pulse voltage applied to the second photoelectric conversion layer. 
     Consequently, for example, the gain can be changed by changing the duty cycle of the applied voltage. 
     For example, the camera system according to the aspect of the present disclosure may further include an automatic/manual control unit that switches an automatic mode for changing, according to intensity of the incident light, the voltage applied by the applied-voltage control unit and a manual mode for changing, according to an instruction from a user, the voltage applied by the applied-voltage control unit. 
     Consequently, the user can freely select a desired mode according to a photographing situation. 
     For example, the camera system according to the aspect of the present disclosure may further include a white-balance control unit that adjusts, according to a voltage applied to the second photoelectric conversion layer, a white balance with respect to the electric signal converted by the light receiving unit. 
     Consequently, because the white balance is kept even if the applied voltage is changed, a balanced image can be obtained. 
     For example, in the camera system according to the aspect of the present disclosure, the light receiving unit may further include a third electrode for applying a voltage to the second photoelectric conversion layer, and the applied-voltage control unit may change, according to the quantum efficiency, a voltage applied to the third electrode. 
     Because the camera system includes the third electrode in this way, the quantum efficiency can be adjusted simply by adjusting the voltage of the third electrode. 
     For example, in the camera system according to the aspect of the present disclosure, at least one of the first conversion unit and the second conversion unit may change a characteristic of a frequency filter according to the applied voltage. 
     Because the camera system includes the frequency filter in this way, light in a specific wavelength region can be selectively caused to pass and an image having a desired tone can be obtained. 
     For example, the camera system according to the aspect of the present disclosure may further include a display and a graphic interface unit that switches and displays, on the display, a first image for receiving an instruction for continuously changing the gain and a second image for receiving an instruction for changing the gain stepwise. 
     Consequently, a desired user interface can be realized. 
     Note that these comprehensive or specific aspects may be realized by a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM or may be realized by any combination of the system, the method, the integrated circuit, the computer program, and the recording medium. 
     Embodiments of the present disclosure are specifically explained below with reference to the drawings. 
     However, detailed explanation more than necessary is sometimes omitted. For example, detailed explanation of already well-known matters and redundant explanation concerning substantially the same components are sometimes omitted. This is to prevent the following explanation from becoming unnecessarily redundant and facilitate understanding of those skilled in the art. Note that the accompanying drawings and the following explanation are for those skilled in the art to sufficiently understand the present disclosure. The accompanying drawings and the following explanation are not intended to limit the subject described in the claims. 
     Note that, in the drawings, elements representing substantially the same configurations, operations, and effects are denoted by the same reference numerals and signs. All numerical values described below are illustrated to specifically explain the present disclosure. The present disclosure is not limited by the illustrated numerical values. Further, a connection relation among constituent elements is illustrated to specifically explain the present disclosure. A connection relation for realizing the functions of the present disclosure is not limited to this connection relation. 
     A camera system according to an aspect of the present disclosure includes a light receiving unit that converts incident light into an electric signal, a first conversion unit and a second conversion unit that change, according to an applied voltage, a gain in a process from incidence of the incident light until the conversion of the incident light into the electric signal by the light receiving unit, and an applied-voltage control unit that applies a voltage to the first conversion unit and the second conversion unit. The second conversion unit is disposed in a position where light passed through the first conversion unit is made incident. For example, the camera system is a camera that captures a moving image and a still image. 
     In the present disclosure, the light receiving unit is an imaging element or an imaging device. The light receiving unit may be, for example, a laminated organic solid-state imaging device or may be a photodiode. 
     The first conversion unit and the second conversion unit change the gain by changing the applied voltage in the process from the incidence of the incident light until the conversion of the incident light into the electric signal by the light receiving unit. The gain in the process from the incidence of the incident light until the conversion of the incident light into the electric signal by the light receiving unit includes not only a conversion ratio for converting the incident light into the electric signal but also a conversion ratio from light into light, that is, an attenuation ratio for attenuating a light amount of the incident light. For example, as an example in which the gain is reduced, when the camera system receives strong light such as direct sunlight, by attenuating the light amount of the incident light, an excessively large voltage is prevented from being generated in the light receiving unit. As an example in which the gain is changed, by changing quantum efficiency in converting the incident light into the electric signal, light sensitivity of the light receiving unit can be adjusted. 
     Because the camera system of the present disclosure has the configuration explained above, the sensitivity of the conversion unit can be more finely adjusted with respect to a change in brightness. 
     As specific examples, a first embodiment to a fourth embodiment are explained blow. 
     First Embodiment 
     An overview of a camera system according to a first embodiment of the present disclosure is explained below.  FIG.  1    is a functional block diagram illustrating the configuration of a camera system  70 A according to this embodiment. 
     In the camera system  70 A according to this embodiment, a first conversion unit  12 A is a filter that changes a gain by attenuating a light amount of incident light according to an applied voltage. A second conversion unit  13 A is a filter on which light passed through the first conversion unit  12 A is made incident and that changes the gain by attenuating a light amount of the incident light according to the applied voltage. A light receiving unit  11 A may receive the light passed through the second conversion unit  13 A as incident light and convert the incident light into an electric signal. 
     As illustrated in  FIG.  1   , the camera system  70 A according to this embodiment includes an imaging unit  10 A that images an object, a graphic interface unit (hereinafter referred to as graphic I/F unit)  30  that processes an electric signal output from the imaging unit  10 A, a system control unit  20 A that controls operations of components in the imaging unit  10 A, a recording medium  40  that stores image data processed by the graphic I/F unit  30 , a display  50  that displays an image, and a user interface unit (hereinafter referred to as user I/F unit)  60  that transmits an input instruction from a user to the system control unit  20 A. 
     The imaging unit  10 A includes, in order from a side on which light is made incident on the imaging unit  10 A, a lens  14 , a mechanical shutter  15 , a focus lens  16 , the first conversion unit  12 A, the second conversion unit  13 A, and the light receiving unit  11 A. That is, the first conversion unit  12 A and the second conversion unit  13 A are provided separately from the light receiving unit  11 A. The second conversion unit  13 A is disposed in a position where light passed through the first conversion unit  12 A is made incident. 
     The first conversion unit  12 A in this embodiment is a filter that attenuates a light amount of incident light according to an applied voltage to thereby change a gain in a process from incidence of the incident light until conversion of the incident light into an electric signal by the light receiving unit  11 A. The first conversion unit  12 A is a so-called electronic ND filter. 
     The second conversion unit  13 A is a filter on which the light passed through the first conversion unit  12 A is made incident and that attenuates a light amount of the incident light according to the applied voltage to thereby change a gain in a process from the incidence of the incident light until conversion of the incident light into an electric signal by the light receiving unit  11 A. The second conversion unit  13 A is a so-called electronic ND filter. 
     That is, the camera system  70 A according to this embodiment includes the first conversion unit (a first electronic ND filter)  12 A and the second conversion unit (a second electronic ND filter)  13 A capable of changing sensitivity according to the applied voltage. 
     Note that, in  FIG.  1   , an example is illustrated in which the camera system  70 A includes two electronic ND filters of the first electronic ND filter  12 A and the second electronic ND filter  13 A. However, the camera system  70 A may include three or more electronic ND filters. 
     In this embodiment, the light receiving unit  11 A is a so-called imaging element that receives light passed through the second conversion unit  13 A as incident light and converts the incident light into an electric signal. In this embodiment, the light receiving unit  11 A may be configured from, for example, a laminated solid-state imaging element including a photoelectric conversion film that performs photoelectric conversion according to incident light. The photoelectric conversion film may include, for example, amorphous silicon. Note that the light receiving unit  11 A may be an imaging device such as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor. 
     Further, the camera system  70 A according to this embodiment may further include the display  50  and the graphic I/F unit  30  that switches and displays, on the display  50 , a first image  60   a  for receiving an instruction for continuously changing the gain and a second image  60   b  for receiving an instruction for changing the gain stepwise. 
     The display  50  is a display device that displays an image and is, for example, a liquid crystal (LC) display or an organic electro luminescence (EL) display. 
     The graphic I/F unit  30  receives digitally processed imaging data output from the light receiving unit  11 A, compresses the imaging data in a predetermined compression scheme such as a JPEG format, and stores the compressed imaging data in the recording medium  40 . The recording medium  40  is a nonvolatile memory and is, for example, a flash memory. 
     The graphic I/F unit  30  reduces the digitally processed imaging data to an image size adjusted to a size of the display  50  and displays the imaging data on the display  50 . 
     The graphic I/F unit  30  transmits, to the system control unit  20 A, a result of a feature value calculated from the digitally processed imaging data output from the light receiving unit  11 A. The feature value is information indicating a characteristic of an image and is, for example, a numerical value representing a defocus amount, an exposure amount, a tone, or the like of the image. 
     The system control unit  20 A receives the feature value of the imaging data calculated by the graphic I/F unit  30  and determines an appropriate imaging condition from the feature value. The system control unit  20 A transmits a control signal to the imaging unit  10 A based on the determined imaging condition. 
     The system control unit  20 A in this embodiment includes a lens control unit  22 , an exposure control unit  23 , and an applied-voltage control unit  24 . 
     The lens control unit  22  controls, based on the defocus amount calculated by the graphic I/F unit  30 , a position to which the focus lens  16  is moved. In  FIG.  1   , the imaging unit  10 A includes only one focus lens  16 . However, the imaging unit  10 A may include a plurality of focus lenses  16 . When the imaging unit  10 A includes the plurality of focus lenses  16 , the focus lenses  16  are less easily affected by an aberration. A focus can be more finely adjusted. 
     The exposure control unit  23  controls an aperture value and shutter speed of the mechanical shutter  15  based on the exposure amount calculated by the graphic I/F unit  30 . The mechanical shutter  15  adjusts a light amount (illuminance) of light made incident on the imaging unit  10 A. 
     The applied-voltage control unit  24  controls an applied voltage to the first conversion unit  12 A (e.g., a variable transmittance dimming filter) and the second conversion unit  13 A based on the feature value of the imaging data calculated by the graphic I/F unit  30 . Consequently, the applied-voltage control unit  24  adjusts a conversion ratio from light into light in the first conversion unit  12 A and the second conversion unit  13 A, that is, an attenuation ratio of a light amount. 
     Note that the system control unit  20 A may include an electronic-shutter control unit that controls start timing of charge accumulation of the light receiving unit  11 A. In embodiments and modifications explained below, similarly, a system control unit may include an electronic-shutter control unit. 
     The user interface (user I/F) unit  60  includes an operation unit that receives operation from the user and transmits an operation signal received from the user to the system control unit  20 A. The user I/F unit  60  is, for example, a touch panel, a dial, or a push button. The touch panel is disposed on the display  50 . The touch panel detects a position touched by a finger of the user on the touch panel and transmits position information to the system control unit  20 A. The user can easily designate desired setting parameters by operating the touch panel while confirming display items such as setting parameters displayed on the display  50 . 
     The user I/F unit  60  is explained with reference to a specific example.  FIG.  2    is diagram illustrating an example of the user I/F unit  60 . 
     As illustrated in  FIG.  2   , as means for the user to change setting of an ND value, there are a touch type in which the user designates a parameter displayed on the display  50  and a mechanical type in which the user designates the parameter with a dial, a push button, and the like. There are multistage control in which the parameter can be adjusted in multiple stages and no-stage control in which the parameter can be continuously adjusted. The parameter is the ND value. 
     In the touch type no-stage ND control illustrated in  FIG.  2 ( a ) , a bar indicating gradation of the ND value, which is the first image  60   a , is displayed on the display  50 . When the user traces, with a finger, the bar  60   a  indicating the gradation of the ND value displayed on the display  50 , the touch panel detects the position of the finger that traces the bar  60   a . The touch panel transmits information concerning the detected position of the finger to the system control unit  20 A. Consequently, the user can instruct the system control unit  20 A to change the ND value to a desired ND value. At this time, when the user traces, with the finger, the bar indicating the gradation of the ND value of the first image  60   a , an image corresponding to transmittance touched by the finger is displayed on the display  50 . 
     In the touch type multistage ND control illustrated in  FIG.  2 ( b ) , icons indicating ND values in stages, which are the second image  60   b , are displayed on the display  50 . When the user touches, with a finger, the icon  60   b  indicating the ND values in the multiple stages displayed on the display  50  and selects the icon  60   b , the touch panel detects a position touched by the finger. The touch panel transmits information concerning the detected position of the finger to the system control unit  20 A. Consequently, the user can instruct the system control unit  20 A to change the ND value to the desired ND value. At this time, when the user touches, with the finger, the icon indicating the ND value in each stage, which is the second image  60   b  and selects the icon, an image corresponding to the ND value touched by the finger is displayed on the display  50 . 
     In the mechanical multistage ND control illustrated in  FIG.  2 ( c ) , the user can instruct the system control unit  20 A to change the ND value to the desired ND value by turning a dial  60   c  disposed in a camera body. 
     In all the cases explained above, because the user can adjust the ND value while viewing the image displayed on the display  50 , it is possible to more finely and quickly perform sensitivity adjustment continuously or in multiple stages. Consequently, it is possible to prevent the display of the display  50  from suddenly changing according to a sudden change of exposure. Therefore, it possible to reduce, for example, fatigue of the eyes of the user. 
     Subsequently, an ND mechanism in the camera system  70 A according to this embodiment is explained in detail with reference to  FIG.  3   .  FIG.  3    is a diagram for explaining the configuration of the first conversion unit  12 A and the second conversion unit  13 A in the camera system  70 A according to the first embodiment. Note that, in  FIG.  3   , only components necessary for explaining the ND mechanism are described. In this embodiment, both of the first conversion unit  12 A and the second conversion unit  13 A are electronic ND filters. In the following explanation, in this embodiment, the first conversion unit  12 A and the second conversion unit  13 A are respectively referred to as first electronic ND filter  12 A and second electronic ND filter  13 A. 
     As illustrated in  FIG.  3   , incident light is made incident on the imaging unit  10 A through the lens  14 . The light made incident on the imaging unit  10 A passes through the first electronic ND filter  12 A and thereafter passes through the second electronic ND filter  13 A. Consequently, the light made incident on the imaging unit  10 A is attenuated to a desired light amount and is made incident on the light receiving unit  11 A. 
     The first electronic ND filter  12 A includes a pair of electrodes  120 A and  122 A and a first ND filter layer  121 A sandwiched by the pair of electrodes  120 A and  122 A. The second electronic ND filter  13 A includes a pair of electrodes  130 A and  132 A and a second ND filter layer  131 A sandwiched by the pair of electrodes  130 A and  132 A. 
     In the camera system  70 A according to this embodiment, the first electronic ND filter  12 A and the second electronic ND filter  13 A may be filters including different types of ND mechanisms. 
     For example, the first electronic ND filter  12 A is a liquid crystal-type electronic ND filter including a liquid crystal layer as the first ND filter layer  121 A. The second electronic ND filter  13 A is a precipitation-type electronic ND filter including a reactant layer as the second ND filter layer  131 A. 
     A sensitivity adjusting method of the first electronic ND filter  12 A and the second electronic ND filter  13 A is explained.  FIG.  4    is a diagram for explaining a sensitivity adjusting method of the first electronic ND filter  12 A and the second electronic ND filter  13 A in the camera system  70 A according to this embodiment. A voltage power supply  80  illustrated in  FIG.  4    is a power supply that applies a voltage to an electrode of a filter. A pulse-duty-voltage power supply  81  is a power supply that applies a voltage converted into a pulse voltage to the electrode of the filter. 
     The first electronic ND filter  12 A is the liquid crystal-type electronic ND filter. The second electronic ND filter  13 A is the precipitation-type electronic ND filter. These electronic ND filters change the transmittance of light by adjusting an applied voltage with the voltage power supply  80 . Because liquid crystal includes a dimming material that absorbs light, the liquid crystal-type electronic ND filter adjusts the applied voltage to thereby change a tilt of liquid crystal molecules and change a degree of dimming, that is, the transmittance of light. In the precipitation-type electronic ND filter, a reaction solution including a reactant that reversibly performs oxidation-reduction reaction is filled on the inside of a cell, on the inner surface of which an electrode is provided. The precipitation-type electronic ND filter adjusts the applied voltage to thereby control the oxidation-reduction reaction of the reactant in the cell. More specifically, the precipitation-type electronic ND filter adjusts the applied voltage, whereby silver chloride, which is the reactant, causes the oxidation-reduction reaction and precipitates or dissolves silver in the reaction solution. 
     It is known that, although the liquid crystal-type and the precipitation-type electronic ND filters can uniformly change the transmittance of light, response speed to a light amount change of incident light is relatively low. However, as in the camera system  70 A according to this embodiment, by using the two electronic ND filters in combination, the respective electronic ND filters can further reduce an attenuated light amount than when one electronic ND filter is used. Therefore, in this embodiment, a response time until incident light is attenuated to a desired light amount can be reduced to approximately ½ times compared with when the camera system  70 A includes only one electronic ND filter. That is, in response to a sudden change in an exposure amount of an object, response speed until the electronic ND filter changes to light transmittance for attenuating the incident light to the desired light amount can be reduced. 
     Note that the ND mechanism illustrated in  FIGS.  3  and  4    is an example. The ND mechanism in the present disclosure may be configured from two or more electronic ND filters. In this embodiment, an example is explained in which the respective electronic ND filters have different configurations such as the liquid crystal type and the precipitation type. However, all of the two or more electronic ND filters may have the same configuration and may be configured by any combination. 
     In this embodiment, the camera system  70 A may include a filter having characteristic other than the characteristics of the liquid crystal-type and the precipitation-type electronic ND filters. For example, in the camera system  70 A according to this embodiment, in at least one of the first electronic ND filter  12 A and the second electronic ND filter  13 A, a characteristic of the first electronic ND filter  12 A and the second electronic ND filter  13 A functioning as a frequency filter may be changed according to an applied voltage. The frequency filter is explained below. 
       FIG.  5    is a schematic sectional view of a frequency filter  300 . The frequency filter  300  includes an electrode pair including an anode  301  and a cathode  302  as a pair, a filter layer  303  sandwiched by the electrode pair, and a spacer  304  that surrounds the outer periphery of a filter layer to seal the filter layer while retaining an interval between the electrode pair. 
     The filter layer  303  is disposed in a closed space surrounded by the electrode pair and the spacer  304 . The filter layer  303  includes a substance, a tone and a color of which change when a voltage is applied to the electrode pair. In general, such a color change is called electrochromism and is a reversible change of a tone and a color of a substance caused by electrochemical oxidation-reduction reaction. That is, the color change is caused because an oxidation-reduction state of molecules and metal ions included in the substance changes and an absorption band of light changes. The object showing such electrochromism is called electrochromic substance. Note that the electrochromic substance is explained below. 
     When a constituent material of an electrolyte layer including the electrochromic substance is liquid or gel, the constituent material may be sealed by, for example, a conducive film and disposed between electrodes or may be directly injected into a space sealed by the electrode pair and the spacer. When the constituent material of the electrolyte layer including the electrochromic substance is a solid state such as a powder state, a particle state, a film state, a layer state, or a plate state, the constituent material may be molded by press or may be kneaded with resin or the like and molded. The constituent material only has to be molded in a state without fluidity generally called solid state. In this case, the spacer  304  that seals the filter layer  303  between the electrode pair may be present or may be absent. 
     As explained above, the tone and the color of the frequency filter  300  change when the applied voltage to the frequency filter  300  is adjusted. A mechanism for causing the color change is the reversible oxidation-reduction reaction of the electrochromic substance in the electrode pair. Decoloring and coloring can be switched by adjusting an applied voltage to the electrode pair. 
     The anode  301  and the cathode  302  are desirably transparent electrodes. Consequently, light transmitted through the frequency filter  300  can be made incident on the light receiving unit  11 A. Note that the transparent electrode only has to be a transparent electrode used in general. A specific material and a definition of transparency are explained below in explanation of a counter electrode in a second embodiment. 
     The electrochromic substance is specifically explained. The electrochromic substance is present in both of an organic compound and an inorganic compound. When the electrochromic substance is the organic compound and a voltage is applied to the electrode pair, molecules change to other substances according to the oxidation-reduction reaction or changes to a radical state, whereby a color of the electrochromic substance changes. When the electrochromic substance is the inorganic compound and a voltage is applied to the electrode pair, the molecules change according to the oxidation-reduction reaction, whereby the color of the electrochromic substance changes. Examples of the inorganic compound include pentacarbonyl ferrate. The pentacarbonyl ferrate is colored in blue during oxidation and is colorless during reduction. Nickel oxide (NiOx) is brown during oxidation and is colorless during reduction. These inorganic compounds are an oxidation color development type. Examples of the organic compound include heptyl viologen. The heptyl viologen is colored in purple during reduction and is colorless during oxidation. That is, the heptyl viologen is a reduction color development type. 
     Subsequently, a frequency characteristic of the frequency filter  300  is explained.  FIGS.  6 A to  6 D  are graphs for explaining the frequency characteristic of the frequency filter  300 . In  FIGS.  6 A to  6 D , the vertical axis indicates an output and the horizontal axis indicates a wavelength of light transmitted through a filter. 
       FIG.  6 A  is a graph illustrating a frequency characteristic of a high-pass filter having high transmittance for only light having a wavelength in a short wavelength region, that is, a high frequency. In the high-pass filter, the pentacarbonyl ferrate is used as the electrochromic substance. As explained above, the pentacarbonyl ferrate is colored in blue when being oxidized. Therefore, as illustrated in  FIG.  6 A , the high-pass filter increases transmittance only for light having a wavelength in a short wavelength region including a wavelength of blue. 
       FIG.  6 B  is a graph illustrating a frequency characteristic of a low-pass filter having high transmittance for only light having a wavelength in a long wavelength region, that is, a low frequency. In the low-pass filter, the nickel oxide (NiOx) is used as the electrochromic substance. As explained above, the nickel oxide (NiOx) is colored in brown when being oxidized. Therefore, the low-pass filter increases transmittance only for light having a wavelength in a long wavelength region such as infrared and near infrared regions including a wavelength of brown. 
       FIG.  6 C  is a graph illustrating a frequency characteristic of a band-pass filter having high transmittance for only light having a wavelength in an intermediate wavelength region. In the band-pass filter, the electrochromic substances having the low-pass characteristic and the high-pass characteristic are combined, whereby the transmittances of lights having wavelengths in the short wavelength region and the long wavelength region are reduced and the transmittance of light having a wavelength in the intermediate wavelength region such as a visible light region is improved. Consequently, the band-pass filter absorbs lights having wavelengths in an infrared region and an ultraviolet region and increases transmittance only for light having a wavelength in the visible light region. 
       FIG.  6 D  is a graph illustrating a frequency characteristic of a band elimination filter having high transmittance only for lights having wavelengths in the short wavelength region and the long wavelength region contrary to the band-pass filter. The band elimination filter improves the transmittances of lights in the short wavelength region and the long wavelength region and reduces the transmittance of light in the intermediate wavelength region such as the visible light region. Consequently, it is possible to use the band elimination filter to transmit only lights having wavelengths in the infrared region and the ultraviolet region. 
     In this way, by using the frequency filter as the electronic ND filter, it is possible to change a wavelength distribution (a color temperature) of light made incident on the light receiving unit  11 A more quickly and easily than attaching and detaching a color conversion filter to and from a lens. 
     Modification 1 of the First Embodiment 
     Subsequently, a camera system according to a modification 1 of the first embodiment is explained.  FIG.  7    is a functional block diagram illustrating the configuration of a camera system  70 B according to this modification. Only differences from the camera system  70 A according to the first embodiment are explained below. 
     The camera system  70 B according to this modification may further include an automatic/manual control unit  26  that switches an automatic mode for changing, according to intensity of incident light, a voltage applied by the applied-voltage control unit  24  and a manual mode for changing, according to an instruction from the user, the voltage applied by the applied-voltage control unit  24 . 
     As illustrated in  FIG.  7   , the camera system  70 B according to this modification is different from the camera system  70 A according to the first embodiment in that the camera system  70 B includes the automatic/manual control unit  26  in a system control unit  20 B. 
     The automatic/manual control unit  26  is a circuit that is connected to the first electronic ND filter  12 A and the second electronic ND filter  13 A and informs one of automatic control and manual control selected by the user I/F unit  60  to the first electronic ND filter  12 A and the second electronic ND filter  13 A. 
     The automatic/manual control unit  26  receives a feature value of imaging data calculated by the graphic I/F unit  30 , determines an appropriate imaging condition from the feature value, and performs automatic control for transmitting a control signal to the imaging unit  10 A under the determining imaging condition. Further, the automatic/manual control unit  26  also performs manual control in which the user confirms imaging data output from the graphic I/F unit  30  to the display  50  and the automatic/manual control unit  26  transmits a control signal to the imaging unit  10 A based on an imaging condition input to the user I/F unit  60  by the user. 
     For example, when the user selects the automatic control of the electronic ND filter, the automatic/manual control unit  26  selects a voltage having a voltage value corresponding to a recommended ND value, or a pulse voltage having a voltage value and a duty cycle corresponding to the recommended ND value. The selected voltage or pulse voltage is supplied to each of the first electronic ND filter  12 A and the second electronic ND filter  13 A. When the user selects the automatic control of the electronic ND filter, the user selects an ND value with the user I/F unit  60 . A voltage having a voltage value corresponding to the selected ND value or a pulse voltage having a voltage value and a duty cycle corresponding to the selected ND value is supplied to each of the first electronic ND filter  12 A and the second electronic ND filter  13 A. 
       FIG.  8    is a table illustrating control patterns of the first electronic ND filter  12 A and the second electronic ND filter  13 A in the camera system  70 B according to this modification. 
     The user can select control patterns in which both of the first electronic ND filter  12 A and the second electronic ND filter  13 A are always fixed to the automatic control or the manual control like a pattern  1 , a pattern  3 , a pattern  7 , and a pattern  9  illustrated in  FIG.  8   . Alternatively, as in the pattern  5  illustrated in  FIG.  8   , the user can select a control pattern in which both of the first electronic ND filter  12 A and the second electronic ND filter  13 A can always be switched to the automatic control or the manual control. The user can select a control pattern in which one of the first electronic ND filter  12 A and the second electronic ND filter  13 A can be switched to the automatic control or the manual control and the other of the first electronic ND filter  12 A and the second electronic ND filter  13 A is always fixed to the manual control or the automatic control. 
     Second Embodiment 
     An overview of a camera system according to a second embodiment is explained below.  FIG.  9    is a functional block diagram illustrating the configuration of a camera system  70 C according to this embodiment. Note that, in  FIG.  9   , only components different from the components of the camera system  70 A according to the first embodiment are denoted by different reference numerals and signs. 
     In the camera system  70 C according to this embodiment, a first conversion unit  12 C may be a filter that changes a gain by attenuating a light amount of incident light according to an applied voltage. A light receiving unit  110  includes a photoelectric conversion unit. The photoelectric conversion unit may include a photoelectric conversion layer that converts light passed through the first conversion unit  12 C into an electric signal and two electrodes sandwiching the photoelectric conversion layer. The photoelectric conversion unit may function as a second conversion unit  13 C and change a gain by changing, according to the applied voltage, quantum efficiency in converting the incident light into the electric signal. 
     Only components different from the components of the camera system  70 A according to the first embodiment are explained below. 
     As illustrated in  FIG.  9   , the camera system  70 C according to this embodiment is different from the camera system  70 A in the first embodiment in that the photoelectric conversion unit in the light receiving unit  11 C functions as a second conversion unit  13 C. At this time, the first conversion unit  12 C is disposed separately from the light receiving unit  11 C. 
     Subsequently, an ND mechanism in the camera system  70 C according to this embodiment is explained in detail with reference to  FIG.  10   .  FIG.  10    is a diagram for explaining the configuration of the first conversion unit  12 C and the second conversion unit  13 C in the camera system  70 C according to this embodiment. Note that, in  FIG.  10   , only components necessary for explaining the ND mechanism are described. In this embodiment, the first conversion unit  12 C is an electronic ND filter. In the following explanation, in this embodiment, the first conversion unit  12 C is referred to as first electronic ND filter  12 C. Note that, in this embodiment, an example is explained in which the first electronic ND filter  12 C is provided separately from the light receiving unit  11 C. However, two or more electronic ND filters may be provided separately from the light receiving unit  110 . The electronic ND filters may be filters including different ND mechanisms. 
     As illustrated in  FIG.  10   , incident light is made incident on an imaging unit  100  through the lens  14 . The light made incident on the imaging unit  100  passes through the first electronic ND filter  12 C and is thereafter made incident on the second conversion unit  13 C in the light receiving unit  110 . The first electronic ND filter  12 C is, for example, a liquid crystal-type electronic ND filter. The second conversion unit  13 C is a photoelectric conversion unit in the light receiving unit  110 . The first electronic ND filter  12 C includes the pair of electrodes  120 A and  122 A and the first ND filter layer  121 A sandwiched by the pair of electrodes  120 A and  122 A. The photoelectric conversion unit includes a pair of electrodes  105  and  101  and a photoelectric conversion layer  104  sandwiched between the pair of electrodes  105  and  101 . The photoelectric conversion unit converts light made incident on the photoelectric conversion unit into an electric signal. 
     The graphic I/F unit  30  transmits, to the system control unit  20 A, a feature value calculated from imaging data, which have been output from the light receiving unit  110  and digitally processed. The applied-voltage control unit  24  transmits, to the imaging unit  100 , an instruction for adjusting an applied voltage to the photoelectric conversion unit, which is the first electronic ND filter  12 C and the second conversion unit  13 C, based on the feature value received by the system control unit  20 A. 
     In this embodiment, the applied-voltage control unit  24  adjusts an attenuation ratio of light by controlling an applied voltage of the first electronic ND filter  12 C and adjusts light sensitivity of the photoelectric conversion unit by controlling an applied voltage to the photoelectric conversion unit, which is the second conversion unit  13 C. Note that an adjusting method for the light sensitivity of the photoelectric conversion unit is explained below with reference to  FIG.  12   . 
     The camera system  70 C according to this embodiment has such a configuration. Therefore, sensitivity can be more finely and more quickly adjusted with respect to a change in brightness. 
     The photoelectric conversion unit is explained. The photoelectric conversion unit includes a counter electrode  105 , a pixel electrode  101  and a photoelectric conversion layer  104  sandwiched between the counter electrode  105  and the pixel electrode  101 . 
     The counter electrode  105  is typically a transparent electrode formed of a transparent conductive material. The counter electrode  105  is disposed on a side on which light is made incident in the photoelectric conversion unit. Therefore, light passed through the counter electrode  105  is made incident on the photoelectric conversion layer  104 . Note that the light detected by the imaging unit  100  is not limited to light in a wavelength region of visible light (e.g., 380 nm or more and 780 nm or less). “Transparent” in this specification means that at least a part of light in a wavelength region to be detected is transmitted. It is not essential to transmit light over the entire wavelength region of the visible light. In this specification, an entire electromagnetic wave including an infrared ray and an ultraviolet ray is represented as “light” for convenience. As the counter electrode  105 , a transparent conducting oxide (TCO) such as indium tin oxide (ITO), indium zinc oxide (IZO), aluminum-doped zinc oxide (AZO), fluorine-doped tin oxide (FTO), SnO 2 , TiO 2 , or ZnO 2  can be used.  FIG.  11    illustrates a relation between an applied voltage to a photoelectric conversion unit with ITO and an output voltage of the light receiving unit  110 . 
     The photoelectric conversion layer  104  receives incident light and generates a hole-electron pair. An electric charge of one of the generated hole-electron pair is captured by the pixel electrode  101  as a signal charge. The electric charge captured by the pixel electrode  101  is detected by a known signal detection circuit. The signal detection circuit may include an amplification transistor that amplifies a signal corresponding to the signal charge and outputs the signal. A gate of the amplification transistor may be electrically connected to the pixel electrode  101 . The photoelectric conversion layer  104  is typically formed of an organic semiconductor material such as quinacridone. 
     Subsequently, an overview of a method of causing the photoelectric conversion unit as an electronic ND filter is explained. 
       FIG.  11    is a graph illustrating a relation between an applied voltage to the photoelectric conversion unit and an output voltage from the light receiving unit  110 . The vertical axis of  FIG.  11    indicates a normalized output voltage. The normalized output voltage is a normalized value. The output voltage from the light receiving unit  110  when the applied voltage to the photoelectric conversion unit is 10 V is set to 1.0 as a reference of the normalization. Note that the output voltage at the time when the applied voltage is 10 V does not need to be set as a reference of the normalization. The reference of the normalization may be changed as appropriate. Further, a range where linearity is secured can be different from the range illustrated in this figure since a threshold can also be changed according to a calculation method and a calculation range for an approximation straight line. 
     It is seen from  FIG.  11    that the output voltage from the light receiving unit  110  can be adjusted by adjusting the applied voltage to the photoelectric conversion unit. For example, in the graph illustrated in  FIG.  11   , a normalized output voltage equivalent to ND2, which represents a filter that reduces an amount of incident light by half, is 0.5, which is a half of 1.0. Therefore, it is seen that, by applying a voltage for adjusting the normalized output voltage to 0.5 to the photoelectric conversion unit, an ND function equivalent to the ND2 can be realized by the photoelectric conversion unit. A normalized output voltage equivalent to ND4, which represents a filter that reduces an amount of incident light to ¼, is 0.25, which is a quarter of 1.0. A normalized output voltage equivalent to ND8, which represents a filter that reduces an amount of incident light to ⅛, is 0.125, which is one eighth of 1.0. Concerning these NDs, ND functions can be realized by the photoelectric conversion unit, by applying a voltage corresponding to each normalized output voltage to the photoelectric conversion unit. 
     Therefore, by applying a voltage corresponding to a normalized output voltage equivalent to a desired ND function to the photoelectric conversion unit, the photoelectric conversion unit can function as an electronic ND filter. However, linearity may not be always secured over an entire range of the applied voltage to the photoelectric conversion unit. 
     For example, as illustrated in  FIG.  11   , a range where linearity is secured between the applied voltage to the photoelectric conversion unit and the output voltage (here, normalized output voltage) from the light receiving unit  11 C, and a range where linearity is not secured are present. In a range where the applied voltage is larger than a threshold, that is, in a voltage range where linearity is secured between the applied voltage and the normalized output voltage, a linear correlation is obtained between the applied voltage and the normalized output voltage. Therefore, the light sensitivity of the photoelectric conversion unit can be easily adjusted by adjusting the magnitude of the applied voltage to the photoelectric conversion unit. On the other hand, in a range where the applied voltage is smaller than the threshold, that is, a voltage range where linearity is not secured between the applied voltage and the normalized output voltage, an applied voltage corresponding to a desired normalized output voltage is calculated from a function representing a relation between the applied voltage to the photoelectric conversion unit and the normalized output voltage. Therefore, it is necessary to cause the camera system to store the function representing the relation between the applied voltage and the normalized output voltage. 
     Therefore, when the light sensitivity of the photoelectric conversion unit is adjusted to perform a desired ND function, an applied voltage in a range where linearity is secured between the applied voltage and the normalized output voltage may be used. This will make it easy to control the light sensitivity of the photoelectric conversion unit. Consequently, it is unnecessary to cause the camera system to store the function. The light sensitivity of the photoelectric conversion unit can be simply adjusted and the ND function can be realized by the photoelectric conversion unit. 
     Subsequently, a procedure for controlling the light sensitivity of the photoelectric conversion unit used as an electronic ND filter is explained.  FIG.  12    is a flowchart for explaining a procedure for correcting the light sensitivity of the photoelectric conversion unit when the photoelectric conversion unit is used as an electronic ND filter. 
     In the camera system  70 C according to this embodiment, the applied-voltage control unit  24  may change, according to quantum efficiency, a duty cycle of a pulse voltage applied to the photoelectric conversion unit. In this embodiment, the photoelectric conversion unit is the second conversion unit  13 C. 
     As illustrated in  FIG.  12   , first, the applied-voltage control unit  24  obtains an output voltage of the light receiving unit  11 C corresponding to the desired ND value, and determines whether an applied voltage corresponding to the output voltage is larger than the threshold (see  FIG.  11   ) (step S 1 ). When the applied voltage is larger than the threshold (YES in step S 1 ), a linear correlation is obtained between the applied voltage to the photoelectric conversion unit and the output voltage. Therefore, the applied-voltage control unit  24  determines an applied voltage corresponding to the desired ND value (step S 2 ) and adjusts a voltage value of the voltage applied to the photoelectric conversion unit (step S 3 ). Consequently, the photoelectric conversion unit can be changed to light sensitivity corresponding to the desired ND value (step S 4 ). 
     Subsequently, an adjusting method for the light sensitivity of the photoelectric conversion unit in a voltage range where linearity is not secured between the applied voltage and the normalized output voltage (NO in step S 1 ) is explained. 
     First, the applied-voltage control unit  24  obtains an output voltage of the light receiving unit  110  corresponding to the desired ND value, and determines whether an applied voltage corresponding to the output voltage is larger than the threshold (see  FIG.  11   ), for example, 9 V (step S 1 ). When the applied-voltage control unit  24  determines that the applied voltage is equal to or smaller than the threshold 9 V (NO in step S 1 ), the applied-voltage control unit  24  determines whether correction of the ND value by adjustment of the applied voltage is possible (step S 5 ). In step S 5 , the applied-voltage control unit  24  determines whether the correction of the ND value by the adjustment of the applied voltage is possible, based on whether the applied voltage is larger than a second threshold. The second threshold in step S 5  is 7V, for example. 
     In a range where the applied voltage is equal to or smaller than the threshold 9 V and in the vicinity of 9V in step S 1 , linearity is sometimes obtained depending on a calculation method for an approximation straight line. Accordingly, the light sensitivity of the photoelectric conversion unit can also be controlled in the range, by determining an applied voltage corresponding to the desired ND value and applying the determined voltage to the photoelectric conversion unit. For example, in a range larger than 7 V and equal to or smaller than 9 V in  FIG.  11   , linearity may be obtained depending on a calculation method for an approximation straight line, though linearity is not obtained in this range. In such a range, a pulse voltage having a certain voltage value and a certain duty cycle may be used to adjust a light sensitivity corresponding to the desired ND value. The pulse voltage will be described later. 
     Therefore, when determining that the correction of the ND value by the adjustment of the applied voltage is possible (YES in step S 5 ), the applied-voltage control unit  24  determines a combination of a voltage value and a duty cycle (step S 6 ). Subsequently, the applied-voltage control unit  24  adjusts the voltage value determined in step S 6  (step S 7 ) and adjusts the duty cycle determined in step S 6  (step S 8 ). Consequently, the photoelectric conversion unit can be changed to have a light sensitivity corresponding to the desired ND value (step S 4 ). 
     On the other hand, when it is determined that the correction of the ND value by the adjustment of the applied voltage is not possible (NO in step S 5 ), that is, the applied voltage corresponding to the desired ND value is 7 V or smaller, linearity is not obtained between the applied voltage and the normalized output voltage. Further, in this case, when the applied voltage corresponding to the desired ND value is directly applied to the photoelectric conversion unit, a white balance is deteriorated as explained below with reference to  FIG.  24   . Therefore, the applied-voltage control unit  24  determines a duty cycle that can realize the desired ND value (step S 9 ), based on a relation between an applied voltage and the normalized output voltage in a range where the applied voltage is larger than the threshold 9 V in step S 1 , that is, in a range where linearity is secured. The applied-voltage control unit  24  sets a voltage value of the applied voltage to a predetermined value and adjusts a duty cycle of the applied voltage to the determined duty cycle (step S 10 ). The predetermined value may be a value larger than the threshold 9 V. Consequently, the photoelectric conversion unit can be changed to have a light sensitivity corresponding to the desired ND value (step S 4 ). The deterioration in the white balance can be reduced by setting the voltage value of the applied voltage to a value larger than the threshold 9 V. 
     Note that, in the explanation in this embodiment, the range of threshold 9 V or smaller is divided into the vicinity region (e.g., larger than 7 V and 9 V or smaller) of the threshold and the range equal to or smaller than the vicinity region (e.g., 7 V or smaller). However, the range of threshold 9 V or smaller does not have to be finely divided as explained above. For example, in a range where the applied voltage corresponding to the desired ND value is larger than the threshold 9 V, the light sensitivity may be adjusted by the adjustment of the applied voltage in steps S 1  to S 4 . In a range where the applied voltage corresponding to the desired ND value is 9 V or smaller, the light sensitivity may be adjusted by the adjustment of the duty cycle in steps S 9  to S 10 . When the light sensitivity is adjusted by the adjustment of the duty cycle, the voltage value of the applied voltage may be adjusted, or the voltage value of the applied voltage may be fixed to a predetermined value. 
     As explained above, in  FIG.  12   , an example is explained in which the threshold is provided between the range where linearity is secured between the applied voltage to the photoelectric conversion unit and the normalized output voltage and the range where linearity is not secured. However, irrespective of the threshold, a correlation is obtained between a voltage control scheme for controlling the voltage value of the applied voltage and a pulse duty control scheme for converting the applied voltage into pulse voltage. The correlation between these control schemes is explained below. 
       FIG.  13    is an example of a timing chart for adjusting light sensitivity according to the voltage control scheme.  FIG.  14    is another example of the timing chart for adjusting light sensitivity according to the voltage control scheme. 
     In  FIGS.  13  and  14   , the vertical axis indicates an applied voltage to the photoelectric conversion unit and the horizontal axis indicates an application time. Changes in applied voltages in any three frames, that is, an N−1-th frame, an N-th frame, and an N+1-th frame are illustrated. Note that the same applies in  FIGS.  15  and  16    referred to below. 
     When a light amount of incident light on an imaging unit  10 D from an object is stable, as illustrated in  FIG.  13   , the applied voltages to the photoelectric conversion unit in the N−1-th frame, the N-th frame, and the N+1-th frame are constant. In this way, when the light amount of the incident light is stable, it is unnecessary to adjust an ND value for each frame. A stable moving image can be acquired even if the applied voltage has a fixed value (V 0 ). 
     When the light amount of the incident light on the imaging unit  10 D from the object fluctuates, that is, when brightness of the object repeats an increase and a decrease, as illustrated in  FIG.  14   , the applied voltages are adjusted in the respective frames of the N−1-th frame, the N-th frame, and the N+1-th frame. When the light amount of the incident light fluctuates in this way, the applied voltages are adjusted for each frame to, for example, an applied voltage V 1  in the N−1-th frame, an applied voltage V 2  (&gt;V 1 ) in the N-th frame, and an applied voltage V 3  (&lt;V 2 ) in the N+1-th frame. This changes the output voltage to be equivalent to the desired ND value. Consequently, the light sensitivity of the photoelectric conversion unit can be appropriately adjusted according to a change in the light amount of the incident light. Therefore, a moving image having a stable light amount can be acquired. Note that, in  FIG.  14   , it is assumed that V 1 , V 2 , and V 3  [V] are applied voltages equal to or higher than the threshold voltage illustrated in  FIG.  11   . 
     Subsequently, an example is explained in which the light sensitivity of the photoelectric conversion unit is adjusted according to the pulse duty control scheme.  FIG.  15    is an example of the timing chart for adjusting the light sensitivity according to the pulse duty control scheme. 
     In the timing chart illustrated in  FIG.  14   , when V 1  and V 3  are smaller than the threshold voltage illustrated in  FIG.  11   , a method of adjusting the light sensitivity according to the pulse duty control scheme is also effective. In the following explanation, it is assumed that V 1  and V 3  are smaller than the threshold voltage illustrated in  FIG.  11   . 
     In the timing chart illustrated in  FIG.  15   , a voltage value of an applied voltage is fixed at V 2  and a duty cycle of the applied voltage is changed for each frame. At this time, applied voltages in the frames illustrated in  FIG.  15    respectively have correlations with the applied voltages in the frames illustrated in  FIG.  14   . Specifically, for example, in a pulse period T 1  [s] of the applied voltage in the N−1-th frame, a length of a High period is set based on V 1 /V 2  and a length of a Low period is set based on (V 2 −V 1 )/V 2 . That is, the duty cycle in the N−1-th frame is set to V 1 /V 2 . Consequently, an applied voltage equivalent to the applied voltage V 1  in the N−1-th frame in  FIG.  14    can be reproduced. Similarly, in a pulse period T 2  [s] of the applied voltage in the N+1-th frame illustrated in  FIG.  15   , a length of a High period is set based on V 3 /V 2  and a length of a Low period is set based on (V 2 −V 3 )/V 2 . That is, the duty cycle in the N+1-th frame is set to V 3 /V 2 . Consequently, an applied voltage equivalent to the applied voltage V 3  in the N+1-th frame illustrated in  FIG.  14    can be reproduced. Quantum efficiency of the photoelectric conversion unit at the time when the light sensitivity of the photoelectric conversion unit, that is, the ND value is adjusted only with a voltage value of the applied voltage illustrated in  FIG.  14    is the same as quantum efficiency of the photoelectric conversion unit at the time when the ND value is adjusted with only the duty cycle illustrated in  FIG.  15   . 
       FIG.  16    is another example of the timing chart for adjusting light sensitivity according to the pulse duty control scheme. In the timing chart illustrated in  FIG.  16   , both of a voltage value and a duty cycle of an applied voltage value are changed for each frame. At this time, applied voltages in frames illustrated in  FIG.  16    respectively have correlations with the applied voltages in the frames illustrated in  FIG.  14   . Specifically, a voltage value of an applied voltage in the N−1-th frame in  FIG.  16    is set to V 4  (&lt;V 2 ) [V] and, in a pulse period T 3  [s], a length of a High period is set based on V 1 /V 4  and a length of a Low period is set based on (V 4 −V 1 )/V 4 . That is, the duty cycle in the N−1-th frame is set to V 1 /V 4 . Consequently, an applied voltage equivalent to the applied voltage V 1  in the N−1-th frame in  FIG.  14    can be reproduced. Similarly, a voltage value of an applied voltage in the N+1-th frame in  FIG.  16    is set to V 5  (&lt;V 2 ) [V] and, in a pulse period T 4  [s], a length of a High period is set based on V 3 /V 5  and a length of a Low period is set based on (V 5 −V 3 )/V 5 . That is, the duty cycle in the N+1-th frame is set to V 3 /V 5 . Consequently, an applied voltage equivalent to the applied voltage V 3  in the N+1-th frame in  FIG.  14    can be reproduced. Quantum efficiency of the photoelectric conversion unit at the time when the ND value is adjusted only with the voltage value of the applied voltage illustrated in  FIG.  14    is the same as quantum efficiency of the photoelectric conversion unit at the time when the ND value is adjusted with the voltage value of the applied voltage and the duty cycle illustrated in  FIG.  16   . 
     Note that, in  FIGS.  15  and  16   , it is assumed that V 2 , V 4 , and V 5  [V] are voltage values of applied voltages, at which linearity is secured, equal to or higher than the threshold voltage illustrated in  FIG.  11   .  FIGS.  13  to  16    are examples in which one frame is 1/60 [s]. However, in the present disclosure, the same effect can be expanded at other frame rates such as 1/120 [s] and 1/180 [s] of one frame. 
     Note that, in this embodiment, as explained below in a modification 3 of the second embodiment, the system control unit  20 A may include a white-balance control unit and a white balance table. Further, the system control unit  20 A may include an automatic/manual control unit. 
     In the examples illustrated in  FIGS.  11  and  24   , deterioration in linearity between the applied voltage and the normalized output voltage and breakage of white balance occur when the applied voltage is lowered. The same effects as the effects explained above can be obtained in a photoelectric conversion unit in which deterioration in linearity and breakage of white balance occur when the applied voltage is raised. For example, in the photoelectric conversion unit in which linearity is deteriorated when the applied voltage is raised, a constant voltage may be applied for a light sensitivity corresponding to a voltage range with high linearity. A pulse voltage may be applied for a light sensitivity corresponding to a voltage range with low linearity. 
     Modification 1 of the Second Embodiment 
     Subsequently, a camera system according to a modification 1 of the second embodiment is explained.  FIG.  17    is a functional block diagram illustrating the configuration of a camera system  70 D according to this modification. In the following explanation, only differences from the camera system  70 C according to the second embodiment are explained. 
     As illustrated in  FIG.  17   , the camera system  70 D according to this modification is different from the camera system  70 C according to the second embodiment in that the camera system  70 D includes a pulse-duty control unit  29  in a system control unit  20 D. 
     The pulse-duty control unit  29  controls the light sensitivity of the photoelectric conversion unit based on a feature value of imaging data transmitted from the graphic I/F unit  30 . In the second embodiment, the applied-voltage control unit  24  controls the applied voltages to the first electronic ND filter  12 C and the photoelectric conversion unit. The first electronic ND filter  12 C is disposed separately from the light receiving unit  11 C, and the photoelectric conversion unit is the second conversion unit  13 C. In this modification, the applied-voltage control unit  24  controls an applied voltage to the first electronic ND filter  12 C. The pulse-duty control unit  29  controls the applied voltage to the photoelectric conversion unit in the same manner as the control explained above in the second embodiment. 
     The camera system  70 D is more specifically explained with reference to  FIG.  18   .  FIG.  18    is a diagram for explaining a sensitivity adjusting method of the first electronic ND filter  12 C and the second conversion unit  13 C in the camera system  70 D according to this modification. In the figure, only components necessary for the explanation are described. 
     As illustrated in  FIG.  18   , incident light is made incident on the imaging unit  100  through the lens  14 . The light made incident on the imaging unit  100  passes through the first electronic ND filter  12 C and is thereafter made incident on the second conversion unit  13 C, which is the photoelectric conversion unit, in the light receiving unit  110 . The first electronic ND filter  12 C is, for example, a liquid crystal-type electronic ND filter. 
     In the first electronic ND filter  12 C, an applied voltage of the voltage power supply  80  is controlled by the applied-voltage control unit  24 . Consequently, an attenuation ratio of light made incident on the first electronic ND filter  12 C is adjusted. 
     The pulse-duty control unit  29  controls a voltage value and a duty cycle of an output voltage of combined power supply of the voltage power supply  80  and the pulse-duty-voltage power supply  81 , for the photoelectric conversion unit, which is the second conversion unit  13 C. When the photoelectric conversion unit is caused to function as an electronic ND filter, sensitivity can be quickly adjusted since a voltage in a range where linearity between the applied voltage to the photoelectric conversion unit and the output voltage of the light receiving unit  11 C is secured is used. At this time, the usable voltage range is limited. However, the applied voltage can be adjusted so as to correspond to the desired ND value by being converted into a pulse voltage. Consequently, the light sensitivity of the photoelectric conversion unit can be more finely adjusted. 
     Note that, depending on the ND value selected by the user I/F unit  60 , according to necessity, the applied-voltage control unit  24  may adjust the applied voltage to the first electronic ND filter  12 C and the pulse-duty control unit  29  may adjust an applied voltage to the photoelectric conversion unit, which is the second conversion unit  13 C. In this way, when the photoelectric conversion unit is caused to function as an electronic ND filter, the pulse-duty control unit  29  can reduce a load on the applied-voltage control unit  24  by controlling the applied voltage to the photoelectric conversion unit. 
     Modification 2 of the Second Embodiment 
     An overview of a camera system according to a modification 2 of the second embodiment is explained below. 
       FIG.  19    is a diagram for explaining a light receiving unit  11 E according to this modification. The light receiving unit  11 E in this modification includes, as illustrated in  FIG.  19   , a plurality of pixels  100  and a peripheral circuit formed on a semiconductor substrate. The pixels  100  include photoelectric conversion units disposed above the semiconductor substrate. For example, the pixels  100  are arranged in a matrix shape of m rows and n columns (m and n are integers equal to or larger than 2) to form an imaging region  200 . The peripheral circuit includes a row scanning circuit  201 , a signal processing circuit  202 , a control unit  203 , and an output circuit  204 . The row scanning circuit  201  reads out respective pixel signals and performs scanning. The signal processing circuit  202  performs signal processing of the read-out pixel signal. The control unit  203  generates control signals for controlling the row scanning circuit  201  and the signal processing circuit  202 . The output circuit  204  outputs a signal of the signal processing circuit  202 . 
     The camera system according to this modification is common to the camera system  70 C according to the second embodiment and a camera system  70 G (see  FIG.  25   ) according to a third embodiment in that the photoelectric conversion unit functions as an electronic ND filter. However, an electrode structure of the photoelectric conversion unit is different. 
       FIG.  20    is a diagram for explaining the configuration of the photoelectric conversion unit in this modification. Note that, in the figure, for simplification of explanation, only different components are described. 
     As illustrated in  FIG.  20   , the photoelectric conversion unit in this modification is different from the photoelectric conversion unit in the second embodiment and the third embodiment in that the photoelectric conversion unit includes a third electrode  102  disposed to surround the periphery of the pixel electrode  101  in the pixel  100 . 
     A sensitivity adjusting method of the photoelectric conversion unit in this modification is explained below.  FIGS.  21  and  22    are diagrams for explaining the sensitivity adjusting method of the photoelectric conversion unit in this modification. 
     In the camera system according to the modification 2 of the second embodiment, the light receiving unit  11 E may further include the third electrode  102  for applying a voltage to the photoelectric conversion layer  104 . The applied-voltage control unit  24  (not illustrated in  FIGS.  21  and  22   ) may change a voltage applied to the third electrode  102  according to necessary quantum efficiency. 
     As illustrated in  FIG.  21   , the pixel  100  includes the counter electrode  105 , the photoelectric conversion layer  104 , the pixel electrode  101 , the third electrode  102  disposed to surround the periphery of the pixel electrode  101 , an interlayer insulation layer  107  in which wires are disposed, and a semiconductor substrate  108  on which a readout circuit is disposed, in that order from a light incident side. The third electrode  102  is also referred to as auxiliary electrode. 
     A photoelectric conversion unit  12 E is configured from the counter electrode  105 , the photoelectric conversion layer  104 , the pixel electrode  101 , and the third electrode  102 . Light sensitivity of the photoelectric conversion unit  12 E is adjusted according to a relation between the strength of an electric field generated by a potential difference between the pixel electrode  101  and the counter electrode  105  and the strength of an electric field generated by a potential difference between the third electrode  102  and the counter electrode  105 . 
     For example, when there is no potential difference between the third electrode  102  and the counter electrode  105 , a region where the third electrode  102  can capture electric charges  106  is small (a region  104 A). Therefore, the electric charges  106  generated in the photoelectric conversion layer  104  near the third electrode  102  are not captured by the third electrode  102  and flow into the pixel electrode  101 . Therefore, the electric charges  106  contribute to the light sensitivity of the photoelectric conversion unit  12 E. 
     On the other hand, as illustrated in  FIG.  22   , when there is a potential difference between the third electrode  102  and the counter electrode  105 , the region (i.e., a region  104 B) where the third electrode  102  can capture the electric charges  106  expands according to an increase in the potential difference. Therefore, more electric charges  106  generated in the photoelectric conversion layer  104  near the third electrode  102  are captured compared with when the potential difference is small. Therefore the light sensitivity of the photoelectric conversion unit  12 E decreases. 
     An amount of the captured electric charges  106  is proportional to the strength of the electric field generated by the potential difference between the third electrode  102  and the counter electrode  105 . That is, the light sensitivity of the photoelectric conversion unit  12 E is adjusted by a voltage of the third electrode  102 . For example, when the voltage of the third electrode  102  is raised, the light sensitivity of the photoelectric conversion unit  12 E can be improved. When the voltage of the third electrode  102  is lowered, the light sensitivity of the photoelectric conversion unit  12 E can be reduced. 
     As explained above, the camera system according to the modification 2 of the second embodiment can adjust the light sensitivity of the photoelectric conversion unit by adjusting the applied voltage to the third electrode  102 . Consequently, the sensitivity of the light receiving unit  11 E is optimized. A dynamic range is expanded. 
     Modification 3 of the Second Embodiment 
     A camera system according to a modification 3 of the second embodiment is explained.  FIG.  23    is a functional block diagram illustrating the configuration of a camera system  70 F according to this modification. Only differences from the camera system  70 C according to the second embodiment are explained. 
     The camera system  70 F according to this modification may further include a white-balance control unit  27 . The white-balance control unit  27  adjusts a white balance of an electric signal converted by the light receiving unit  11 C, according to a voltage applied to the photoelectric conversion unit. In this modification, the photoelectric conversion unit is a photoelectric conversion unit in the light receiving unit  11 C. 
     As illustrated in  FIG.  23   , the camera system  70 F according to this modification is different from the camera system  70 C according to the second embodiment and the camera system  70 D according to the modification 1 of the second embodiment in that the camera system  70 F includes the white-balance control unit  27  and a white balance table  28  in a system control unit  20 F. 
     Control of a white balance is necessary when light sensitivity is adjusted by adjusting an applied voltage to the photoelectric conversion unit. For example, the control of the white balance is necessary when the photoelectric conversion unit in the light receiving unit  11 C is caused to function as a second conversion unit as in this modification. 
     The control of the white balance is specifically explained below.  FIG.  24    is a graph illustrating a relation between an applied voltage to the photoelectric conversion unit and a spectral characteristic. 
     As illustrated in  FIG.  24   , as the applied voltage to the photoelectric conversion unit is higher, quantum efficiency of the photoelectric conversion unit is higher. However, a difference in the quantum efficiency occurs among wavelength regions of lights of red (R), green (G), and blue (B), depending on a magnitude of a voltage value of the applied voltage to the photoelectric conversion unit. For example, in the graph of  FIG.  24    according to the highest applied voltage, the quantum efficiency of the wavelength region of blue (B) is high. The quantum efficiency of the wavelength region of green (G) and the quantum efficiency of the wavelength region of red (R) are the same degree. In this case, an obtained image has a bluish tone unless the white balance is adjusted. On the other hand, in the graph of  FIG.  24    according to the lowest applied voltage, the quantum efficiency of the wavelength region of red (R) is high and the quantum efficiency of the wavelength region of blue (B) is low. In this case, an obtained image has a reddish tone unless the white balance is adjusted. 
     In this way, the quantum efficiency of the wavelength region of red (R), the quantum efficiency of the wavelength region of green (G), and the quantum efficiency of the wavelength region of blue (B) are different from one another according to a magnitude of the applied voltage to the photoelectric conversion. Therefore, it is necessary to adjust the white balance by adjusting light sensitivity in each wavelength regions. 
     As illustrated in  FIG.  24   , a spectral characteristic of the photoelectric conversion unit changes according to a magnitude of the applied voltage to the photoelectric conversion unit. The white-balance control unit  27  is a circuit for correcting the characteristic of the photoelectric conversion unit. 
     In the white balance table  28 , applied voltages and coefficients are saved as a table such that image signals of RGB can be respectively multiplied by the coefficients to keep the white balance even if the applied voltage changes. As another method, the white-balance control unit  27  may store a function to obtain the above coefficients. The white balance of RGB may be kept constant by using the function even if the applied voltage to the photoelectric conversion unit changes to obtain a desired transmittance. 
     Consequently, in this modification, even if the applied voltage to the photoelectric conversion unit changes, the white balance can be adjusted by the function saved in the white-balance control unit  27 . Alternatively, the white balance can be adjusted by causing the white-balance control unit  27  and the white balance table  28  to mutually function. Therefore, it is possible to control the white balance of pixel signals of RGB, variably control the sensitivity of the light receiving unit  11 A, and capture a satisfactory image. Because optimum sensitivity adjustment is possible for each color, it is possible to capture a color image having a satisfactory white balance. 
     The white balance table  28  does not need to be disposed in the system control unit  20 F and may be disposed in another place such as the recording medium  40 . Note that, as explained above in the modification 1 of the second embodiment, when the photoelectric conversion unit in the light receiving unit  11 C is caused to function as the electronic ND filter, the camera system  70 F may include the pulse-duty control unit  29  (see  FIG.  17   ). The light sensitivity of the photoelectric conversion unit can be more finely adjusted by converting a voltage applied to the photoelectric conversion unit into pulse voltage. 
     Third Embodiment 
     An overview of a camera system according to a third embodiment is explained.  FIG.  25    is a functional block diagram illustrating the configuration of the camera system  70 G according to this embodiment. Note that, in  FIG.  25   , only components different from the components of the camera system  70 A according to the first embodiment and the camera system  70 C according to the second embodiment are denoted by different reference numerals and signs. 
     In the camera system  70 G according to this embodiment, a light receiving unit  11 G may include a first photoelectric conversion unit including a first photoelectric conversion layer that converts incident light into an electric signal and two first electrodes that sandwich the first photoelectric conversion layer. The light receiving unit  11 G may include a second photoelectric conversion unit including a second photoelectric conversion layer that receives light passed through a first conversion unit  12 G as incident light and converts the incident light into an electric signal and two electrodes that sandwich the second photoelectric conversion layer. The first photoelectric conversion unit may function as the first conversion unit  12 G and change a gain by changing, according to an applied voltage, quantum efficiency in converting the incident light into the electric signal. The second photoelectric conversion unit may function as a second conversion unit  13 G and change a gain by changing, according to the applied voltage, quantum efficiency in converting the incident light into the electric signal. 
     Note that, in this embodiment, an example is explained in which the camera system  70 G includes the first conversion unit  12 G and the second conversion unit  13 G in the light receiving unit  11 G. However, the camera system  70 G may further include an electronic ND filter separately from the light receiving unit  11 G. 
     In the following explanation, only components different from the components of the camera system  70 A according to the first embodiment and the camera system  70 C according to the second embodiment are explained. 
     As illustrated in  FIG.  25   , the camera system  70 G according to this embodiment is different from the camera system  70 A according to the first embodiment and the camera system  70 C according to the second embodiment in that the first conversion unit  12 G and the second conversion unit  13 G are present on the inside of the light receiving unit  11 G. 
     Subsequently, an ND mechanism in the camera system  70 G according to this embodiment is explained in detail with reference to  FIG.  26   .  FIG.  26    is a diagram for explaining the configuration of the first conversion unit  12 G and the second conversion unit  13 G in the camera system  70 G according to this embodiment. 
     As illustrated in  FIG.  26   , incident light is made incident on an imaging unit  10 G through the lens  14 . The light made incident on the imaging unit  10 G is made incident on the photoelectric conversion unit, which is the first conversion unit  12 G in the light receiving unit  11 G. The first conversion unit  12 G includes a first photoelectric conversion layer  1041  and two first electrodes  1051  and  1011  that sandwich the first photoelectric conversion layer  1041 . The first photoelectric conversion layer  1041  converts the light made incident on the first conversion unit  12 G into an electric signal. The incident light passed through the first conversion unit  12 G is made incident on the photoelectric conversion unit, which is the second conversion unit  13 G. The second conversion unit  13 G includes a second photoelectric conversion layer  1042  and two second electrodes  1052  and  1012  that sandwich the second photoelectric conversion layer  1042 . The second photoelectric conversion layer  1042  converts the light made incident on the second conversion unit  13 G into an electric signal. 
     In this embodiment, a conversion ratio from light into electricity is changed by changing quantum efficiency of these two photoelectric conversion units. The photoelectric conversion units are more specifically explained with reference to  FIG.  27   .  FIG.  27    is a diagram for explaining a sensitivity adjusting method of the first conversion unit  12 G and the second conversion unit  13 G in this embodiment. 
     As illustrated in  FIG.  27   , an applied voltage of the voltage power supply  80  is controlled for the photoelectric conversion unit, which is the first conversion unit  12 G, by the applied-voltage control unit  24 . As in steps S 1  to S 3  explained with reference to  FIG.  12   , the applied-voltage control unit  24  changes the quantum efficiency of the photoelectric conversion unit by adjusting the applied voltage to the photoelectric conversion unit. That is, the applied-voltage control unit  24  adjusts the light sensitivity of the photoelectric conversion unit by applying a voltage corresponding to the desired ND value to the photoelectric conversion unit. For example, when the light sensitivity is adjusted by the voltage control scheme illustrated in  FIGS.  13  and  14   , in a voltage range where linearity is secured between the applied voltage to the photoelectric conversion unit and the output voltage, the light sensitivity of the photoelectric conversion unit can be easily adjusted by adjusting the magnitude of the applied voltage to the photoelectric conversion unit. Note that, in the voltage range where the linearity is secured, the light sensitivity of the photoelectric conversion unit can be more quickly adjusted by adjusting the magnitude of the applied voltage to the photoelectric conversion unit than by converting the applied voltage into pulse voltage as described later. 
     The applied-voltage control unit  24  controls an applied voltage and duty cycle of a combined power supply of the voltage power supply  80  and the pulse-duty-voltage power supply  81 . The voltage output from the power supply is provided to the photoelectric conversion unit, which is the second conversion unit  13 G. As in steps S 5  to S 7  and steps S 9  to S 10  explained with reference to  FIG.  12   , the applied-voltage control unit  24  changes the quantum efficiency of the photoelectric conversion unit by adjusting an applied voltage and a duty cycle of the voltage provided to the photoelectric conversion unit. At this time, a voltage corresponding to the desired ND value may be in a voltage range where linearity is not secured between the applied voltage to the photoelectric conversion unit and the output voltage from the light receiving unit  110 . In such a case, an applied voltage needs to be converted into a pulse voltage that has a voltage value in a voltage range where linearity is secured. For example, when the light sensitivity is adjusted by the pulse duty control scheme illustrated in  FIGS.  15  and  16   , the applied voltage can be converted into a pulse voltage having a voltage value in the range where the linearity is secured, for example, a voltage value of 9 V or more, to obtain an applied voltage equivalent to the desired ND value. Consequently, the photoelectric conversion unit can be adjusted to light sensitivity equivalent to the desired ND value. Note that, the light sensitivity of the photoelectric conversion unit can be more finely adjusted by converting an applied voltage to the photoelectric conversion unit into a pulse voltage than by adjusting only a voltage value of an applied voltage. 
     The imaging device according to the present disclosure is explained above based on the embodiments and the modifications. However, the present disclosure is not limited to the embodiments and the modifications. Various modifications conceived by those skilled in the art applied to the embodiments and the modifications and other forms constructed by combining a part of the constituent elements in the embodiments and the modifications are also included in the scope of the present disclosure as long as the modifications and the forms do not deviate from the spirit of the present disclosure. 
     In the embodiments, a part of the constituent elements may be realized by executing a software program suitable for the constituent elements. The constituent elements may be realized by a program executing unit such as a CPU or a processor reading out and executing a software program recorded in a recording medium such as a hard disk or a semiconductor memory. 
     The camera system according to the present disclosure can be used in various camera systems such as a digital still camera, a camera for broadcasting/business use, a camera for medical use, a camera for monitoring, an on-vehicle camera, a digital single-lens reflex camera, and a digital mirrorless single-lens reflex camera and sensor systems.