Abstract:
An imaging device including an imaging element having a plurality of pixels for switching a linear conversion mode for linearly converting incident light to an electric signal and a logarithm conversion mode for logarithmically converting incident light to an electric signal on the basis of incident light intensity,
       a conversion unit for converting and outputting a reference electric signal converted logarithmically and outputted from the imaging element to an electric signal obtained by linearly converting an electric signal before logarithm conversion,   a correction unit, when an electric signal converted logarithmically and outputted from the imaging element is varied from the reference electric signal, for correcting it so as to coincide with the reference electric signal, and a circuit for giving the corrected electric signal to the conversion unit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an imaging device having an imaging element for converting incident light to an electric signal. 
       BACKGROUND 
       [0002]    Conventionally, in an imaging device such as a digital camera, an imaging element having a plurality of pixels for converting incident light to an electric signal is installed. These plurality of pixels switch the conversion mode to the electric signal on the basis of the incident light intensity and more in detail, switches the linear conversion mode for linearly converting the incident light to an electric signal and the logarithm conversion mode for logarithmically converting it. Further, at the later stage of the imaging element, a signal processing unit for performing characteristic conversion for converting the electric signal obtained by the logarithm conversion mode to a state obtained by the linear conversion mode or converting the electric signal obtained by the linear conversion mode to a state obtained by the logarithm conversion mode is installed, thus all the electric signals are unified to a state obtained by the linear conversion mode or logarithm conversion mode and the processing of the electric signals is simplified. 
         [0003]    According to such an imaging element, compared with an imaging element for performing only the linear conversion mode, the timing range of an electric signal is extended, so that even if an object having a wide brightness range is imaged, all the brightness information can be expressed by an electric signal. 
         [0004]    On the other hand, the plurality of pixels aforementioned have variations in the I/O characteristic due to differences between the pixels. Therefore, as a method for canceling such variations, there is a method available for correcting output from each pixel and making it coincide with a reference output value (for example, refer to Patent Documents 1 and 2). 
         [0005]    Patent Document 1: Japanese Patent Application Hei 11-298799 
         [0006]    Patent Document 2: Japanese Patent Application Hei 5-30350 
       DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention  
       [0007]    However, in the correction methods disclosed in the patent documents aforementioned, when the I/O characteristic is fluctuated due to the drive conditions such as the imaging conditions and environmental conditions, a variation between the reference output value under the reference conditions and an actual pixel output value cannot be corrected. Therefore, all the electric signals cannot be unified exactly to a state obtained by the linear conversion mode or logarithm conversion mode. 
         [0008]    A problem of the present invention is to provide an imaging device capable of exactly unifying electric signals to a state obtained by the linear conversion or logarithm conversion. 
       Means for Solving the Problems  
       [0009]    The invention stated in Item  1  is characterized in that an image device comprises:
   an imaging element having a plurality of pixels for switching a linear conversion mode for linearly converting incident light to an electric signal and a logarithm conversion mode for logarithmically converting incident light to an electric signal on the basis of incident light intensity,   a conversion unit for converting and outputting a reference electric signal converted logarithmically and outputted from the imaging element to an electric signal obtained by linearly converting an electric signal before logarithm conversion,   a correction unit, when an electric signal converted logarithmically and outputted from the imaging element is varied from the reference electric signal, for correcting it so as to coincide with the reference electric signal, and a circuit for giving the corrected electric signal to the conversion unit.
 
Further, the invention stated in Item  8  is characterized in that an image device comprises:
   an imaging element having a plurality of pixels for linearly converting and outputting an electric signal based on incident light intensity until predetermined light intensity is obtained, thereafter logarithmically converting and outputting the electric signal based on the incident light intensity,   a conversion unit for converting and outputting a reference electric signal converted logarithmically and outputted from the imaging element to an electric signal obtained by linearly converting an electric signal before logarithm conversion,   a correction unit, when an electric signal converted logarithmically and outputted from the imaging element is varied from the reference electric signal, for correcting it so as to coincide with the reference electric signal, and a circuit for giving the corrected electric signal to the conversion unit.   
 
         [0016]    Further, the invention stated in Item 12 is characterized in that an image device comprises:
   an imaging element having a plurality of pixels for linearly converting and outputting an electric signal based on incident light intensity until predetermined light intensity is obtained, thereafter logarithmically converting and outputting the electric signal based on the incident light intensity,   a derivation unit for deriving an inflection point signal at the point where the linear conversion is switched to the logarithm conversion,   a comparison unit for comparing the inflection point signal derived by the derivation unit with the electric signal outputted from the imaging element,   a conversion unit, as a result of comparison, when the inflection point signal is large, for converting and outputting a reference electric signal converted logarithmically and outputted from the imaging element to an electric signal obtained by linearly converting an electric signal before logarithm conversion,   a correction unit, when an electric signal converted logarithmically and outputted from the imaging element is varied from the reference electric signal, for correcting it so as to coincide with the reference electric signal, and a circuit for giving the corrected electric signal to the conversion unit.   
 
       EFFECTS OF THE INVENTION 
       [0022]    According to the present invention, electric signals converted logarithmically can be exactly unified to a state obtained from the linear conversion. Furthermore, according to the present invention, electric signals converted logarithmically can be unified to a state obtained from the linear conversion without using a complicated circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a block diagram showing the schematic constitution of the imaging device relating to the preset invention. 
           [0024]      FIG. 2  is a block diagram showing the constitution of the imaging element. 
           [0025]      FIG. 3  is a drawing for explaining the operation of a pixel and the linearization unit. 
           [0026]      FIG. 4  is a drawing for showing the relationship between the exposure time and the inflection point. 
           [0027]      FIG. 5  is a drawing for showing the relationship between the control voltage and the inflection point. 
           [0028]      FIG. 6  is a circuit diagram showing the constitution of a pixel. 
           [0029]      FIG. 7  is a block diagram showing the signal processing unit and inflection point signal derivation unit. 
           [0030]      FIG. 8  is a drawing showing a correction factor α. 
           [0031]      FIG. 9  is a flow chart showing the processes of fluctuation correction and characteristic conversion. 
       
    
    
     DESCRIPTION OF NUMERALS 
       [0032]      1  Imaging device 
         [0033]      2  Imaging element 
         [0034]      3  Signal processing unit 
         [0035]      30  Fluctuation correction unit 
         [0036]      31  Linearization unit (characteristic conversion unit) 
         [0037]      32  Factor derivation unit 
         [0038]      32   a  Lookup table 
         [0039]      33  Processing unit 
         [0040]      34  Inflection signal derivation unit 
         [0041]      34   a  Lookup table 
         [0042]    G 11  to Gmn Pixel 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment  
       [0043]    Hereinafter, the embodiment of the present invention will be explained with reference to the accompanying drawings. 
         [0044]      FIG. 1  is a block diagram showing the schematic constitution of an imaging device  1  relating to the preset invention. 
         [0045]    As shown in the drawing, an imaging device  1  has an imaging element  2  for receiving incident light via a lens group  10  and an aperture diaphragm  11 . For these lens group  10  and aperture diaphragm  11 , well-known ones are used conventionally. 
         [0046]    The imaging element  2 , as shown in  FIG. 2 , has a plurality of pixels G 11  to Gmn (n and m are integers of 1 or larger) arranged in a matrix shape. 
         [0047]    Each of the pixels G 11  to Gmn converts photoelectrically incident light and outputs an electric signal. The pixels G 11  to Gmn switch the conversion mode to the electric signal on the basis of the incident light intensity and in this embodiment, as indicated by a solid line in  FIG. 3 , for incident light intensity lower than a predetermined incident light intensity th, the linear conversion mode for linearly converting incident light is performed and for incident light intensity of the predetermined incident light intensity th or higher, the logarithm conversion mode for logarithmically converting incident light is performed. 
         [0048]    Here, the boundary where the linear conversion mode and logarithm conversion mode are switched, that is, the so-called inflection point varies with the drive conditions of the pixels G 11  to Gmn of the imaging element  2 , for example, the exposure time and control voltage during imaging. Concretely, as shown in  FIG. 4 , as the exposure time is shortened in the order of t 1  to t 3 , the output signal value at the inflection point (hereinafter, referred to as an inflection output signal value H) and the predetermined incident light intensity th increase in the order of I to III. Further, as shown in  FIG. 5 , as the control voltage is reduced in the order of V 1  to V 3 , the inflection output signal value H of the pixels G 11  to Gmn increases in the order of IV to VI. Further, in  FIGS. 4 and 5 , a 1  to a 3 , b to d, a, and d 1  to d 3  are respectively constants. Among them, the inclinations a 1  to a 3  of the I/O characteristic in the linear conversion mode under the drive condition of the exposure time t 1  to t 3  are in proportion to the exposure time t 1  to t 3 . Further, the sections d 1  to d 3  of the I/O characteristic in the logarithm conversion mode under the drive condition of the control voltages V 1  to V 3  are in proportion to the control voltages V 1  to V 3 . Hereinafter, when the predetermined incident light intensity th is minimum, that is, when the rate of performance of the linear conversion mode is minimum and the rate of performance of the logarithm conversion mode is maximum, the exposure time t 1  is assumed as reference exposure time. 
         [0049]    On the side of the lens group  10  of the pixels G 11  to Gmn, a filter (not drawn) of any one color of red, green, and blue is arranged. Further, to the pixels of G 11  to Gmn, as shown in  FIG. 2 , a power source line  20 , signal impression lines LA 1  to LAn, LB 1  to LBn, and LC 1  to LCn, and signal reading lines LD 1  to LDm are connected. Further, to the pixels G 11  to Gmn, the lines such as a clock line and a bias supply line are connected, though the illustration of the lines is omitted in  FIG. 2 . 
         [0050]    The signal impression lines LA 1  to LAn, LB 1  to LBn, and LC 1  to LCn give signals φv and φVPS to the pixels G 11  to Gmn (refer to  FIG. 6 ). To the signal impression lines LA 1  to LAn, LB 1  to LBn, and LC 1  to LCn, a vertical scanning circuit  21  is connected. The vertical scanning circuit  21 , on the basis of a signal from a signal generation unit  48  (refer to  FIG. 1 ) which will be described later, impresses a signal to the signal impression lines LA 1  to LAn, LB 1  to LBn, and LC 1  to LCn and switches sequentially the signal impression lines LA 1  to LAn, LB 1  to LBn, and LC 1  to LCn which are subjects of impression of a signal in the X direction. 
         [0051]    To the signal reading lines LD 1  to LDm, electric signals generated by the pixels G 11  to Gmn are derived. To the signal reading lines LD 1  to LDm, constant current sources D 1  to Dm and selection circuits S 1  to Sm are connected. 
         [0052]    To one ends (the lower ends shown in the drawing) of the constant current sources D 1  to Dm, a DC voltage VPS is impressed. 
         [0053]    The selection circuits S 1  to Sm sample-hold a noise signal given from the pixels G 11  to Gmn via the signal reading lines LD 1  to LDm and an electric signal at time of imaging. To the selection circuits S 1  to Sm, a horizontal scanning circuit  22  and a correction circuit  23  are connected. The horizontal scanning circuit  22  switches sequentially the selection circuits S 1  to Sm for sample-holding an electric signal and transmitting it to the correction circuit  23  in the Y direction. Further, the correction circuit  23 , on the basis of a noise signal transmitted from the selection circuits S 1  to Sm and an electric signal at time of imaging, removes the noise signal from the electric signal. 
         [0054]    Further, for the selection circuits S 1  to Sm and correction circuit  23 , the ones disclosed in Japanese Patent Application Hei 2001-223948 can be used. Further, in this embodiment, the example that for all the selection circuits S 1  to Sm, one correction circuit  23  is used is explained, though for each of the selection circuits S 1  to Sm, one correction circuit  23  may be used. 
         [0055]    To the imaging element  2  aforementioned, as shown in  FIG. 1 , via an amplifier  12  and an A-D converter  13 , a black reference setting unit  14  and a signal processing unit  3  are connected in this order. 
         [0056]    The black reference setting unit  14  sets a lowest level of a digital signal. 
         [0057]    The signal processing unit  3  performs the signal processing for an electric signal outputted from the imaging element  2  in the logarithm conversion mode and has a linearization unit  31  and a fluctuation correction unit  30 . 
         [0058]    The linearization unit  31  is a characteristic conversion unit of the present invention and unifies output signals from the imaging element  2  to a state obtained by the linear conversion mode. The linearization unit  31 , as shown in  FIG. 7 , includes a selector  31   b,  a reference conversion table  31   a,  and an output unit  31   c.  Further, in  FIG. 7 , the A-D converter  13  and a controller  46  are not drawn. 
         [0059]    The selector  31   b  discriminates the magnitude relation between an electric signal from the imaging element  2  and the inflection output signal value H aforementioned, and when the electric signal from the imaging element  2  is larger than the inflection output signal value H, that is, an electric signal obtained by the logarithm conversion mode is outputted from the imaging element  2 , outputs the output signal from the imaging element  2  to the reference conversion table  31   a,  and when it is the inflection output signal value H or smaller, outputs the output signal from the imaging element  2  to the output unit  31   c.    
         [0060]    The reference conversion table  31   a,  as shown by an arrow Z in  FIG. 3 , among electric signals outputted from the imaging element  2 , characteristic-converts the electric signal obtained by the logarithm conversion mode to the state linearly converted from the incident light, that is, the state obtained by the linear conversion mode. The conversion characteristic of the reference conversion table  31   a,  when the drive conditions of the imaging element  2  are the predetermined reference conditions, in this embodiment, when the exposure time of the pixels G 11  to Gmn is the reference exposure time t 1  aforementioned, is set so that the electric signal outputted from the imaging element  2  in the logarithm conversion mode is exactly put into the state obtained by the linear conversion mode. 
         [0061]    The output unit  31   c  outputs the electric signal inputted from the selector  31   b  or the reference conversion table  31   a.    
         [0062]    The fluctuation correction unit  30 , when the I/O characteristic of the imaging element  2  is fluctuated due to the drive conditions of the imaging element  2 , that is, in this embodiment, the exposure time of the pixels G 11  to Gmn, performs fluctuation correction of the electric signal outputted from the imaging element  2 . 
         [0063]    The fluctuation correction unit  30 , as shown in  FIG. 7 , includes a factor derivation unit  32  and a processing unit  33 . 
         [0064]    The factor derivation unit  32 , on the basis of the exposure time information on the exposure time of the pixels G 11  to Gmn and pixel information on the pixels G 11  to Gmn, derives correction factors α 11  to αmn for each of the pixels G 11  to Gmn. And in this embodiment, the factor derivation unit  32  has a lookup table  32   a  for calculating the correction factors α 11  to αmn by input of the exposure time information and pixel information. 
         [0065]    Here, for example, when the exposure time during imaging is the exposure time t 2  (variable) aforementioned (refer to  FIG. 4 ), the correction factor α is a value indicated as α=cLn(a 1 /a 2 )=cLn(t 1 /t 2 ), in other words, as shown in  FIG. 8 , it is a distance on the input axis (the x axis shown in the drawing) between a virtual conversion table (refer to the dotted line shown in the drawing) having a conversion characteristic such that an electric signal outputted in the logarithm conversion mode from the imaging element  2  under the drive condition of the exposure time t 2  is exactly put into the state obtained by the linear conversion mode and the reference conversion table  31   a  (refer to the solid line shown in the drawing). Further, such a virtual conversion table can be obtained by experiments or theoretical calculations and the virtual conversion table and reference conversion table  31   a  are in the mutual parallel relationship. Further, in  FIG. 8(   a ), illustration of each conversion table for the linear region is omitted. 
         [0066]    Further, as pixel information, intrinsic information such as the ID number of each of the pixels G 11  to Gmn and position information in the imaging element are used. 
         [0067]    The processing unit  33 , on the basis of the correction factors α 11  to αmn derived by the factor derivation unit  32 , performs the fluctuation correction aforementioned for each of the pixels G 11  to Gmn and in this embodiment, from an electric signal outputted from each of the pixels G 11  to Gmn in the logarithm conversion mode, the correction factors α 11  to αmn are subtracted. By doing this, the fluctuation-corrected electric signal obtained by the logarithm conversion mode enters the state that it can be exactly characteristic-converted to the electric signal obtained by the linear conversion mode by the reference conversion table  31   a.    
         [0068]    Concretely, for example, as shown in  FIG. 8(   a ) aforementioned, the signal value of electric signals outputted in the logarithm conversion mode from the pixels G 11  to Gmn under the drive condition of the exposure time t 2  (refer to  FIG. 4)  is assumed as X 2 . In this case, when the signal value X 2  obtained by the logarithm conversion mode is exactly characteristic-converted to the electric signal in the state obtained by the linear conversion mode, that is, when the signal value X 2  is characteristic-converted by the virtual conversion table, the output signal value after characteristic conversion is Y 2 . On the other hand, when the signal value X 2  is characteristic-converted straight by the reference conversion table  31   a,  the signal value after characteristic conversion is Y 1 , though when the signal value X 1  (=X 2 −α) obtained by subtracting the correction factor α from the signal value X 2  is characteristic-converted by the reference conversion table  31   a,  the signal value after characteristic conversion is Y 2 . Namely, by subtracting the correction factor α from the signal value X 2  obtained by the logarithm conversion mode, the electric signal after subtraction enters the state that it can be exactly characteristic-converted to the electric signal obtained by the linear conversion mode by the reference conversion table  31   a.    
         [0069]    To the signal processing unit  3 , as shown in  FIG. 1 , an inflection signal derivation unit  34  and an image processing unit  4  are connected respectively. 
         [0070]    The inflection signal derivation unit  34 , on the basis of the exposure time information and pixel information, derives the inflection output signal value H and in this embodiment, as shown in  FIG. 7 , has a lookup table  34   a  for deriving the inflection output signal value H by input of the exposure time information and pixel information. 
         [0071]    The image processing unit  4  performs the image process for image data composed of all the electric signals from the pixels G 11  to Gmn and includes an AWB (auto white balance) processing unit  40 , a color interpolation unit  41 , a color correction unit  42 , a gradation conversion unit  43 , and a color space conversion unit  44 . These AWB (auto white balance) processing unit  40 , color interpolation unit  41 , color correction unit  42 , gradation conversion unit  43 , and color space conversion unit  44  are connected to the signal processing unit  3  in this order. 
         [0072]    The AWB processing unit  40  performs the white balance process for the image data and the color interpolation unit  41 , on the basis of electrical signals from a plurality of proximity pixels in which the filters of the same color are installed, performs interpolation operations for the electric signal for the pixels positioned between the proximity pixels. The color correction unit  42  corrects the color balance of the image data and more in detail, corrects the electric color of each color for each of the pixels G 11  to Gmn on the basis of the electric signals of the other colors. The gradation conversion unit  43  performs gradation conversion for the image data and the color space conversion unit  44  converts R, G, and B signals to Y, Cb, and Cr signals. 
         [0073]    Further, to the signal processing unit  3 , an evaluation value calculation unit  5  and the controller  46  are connected in this order. 
         [0074]    The evaluation value calculation unit  5  calculates an AWB evaluation value used in the white balance process (AWB process) by the AWB processing unit  40  and an AE evaluation value used in the exposure control process (AE process) by an exposure control processing unit  47 . 
         [0075]    The controller  46  controls each unit of the imaging device  1  and as shown in  FIG. 1 , is connected to the amplifier  12 , black reference setting unit  14 , signal processing unit  3 , inflection signal derivation unit  34 , AWB processing unit  40 , color interpolation unit  41 , color correction unit  42 , gradation conversion unit  43 , and color space conversion unit  44  which are described above. Further, the controller  46  is connected to the aperture diaphragm  11  via the exposure control processing unit  47  and is connected to the imaging element  2  and A-D converter  13  via a signal generation unit  48 . 
         [0076]    Then, the pixels G 11  to Gmn of this embodiment will be explained. 
         [0077]    Each of the pixels G 11  to Gmn, as shown in  FIG. 6 , includes a photodiode P and transistors T 1  to T 3 . Further, the transistors T 1  to T 3  are a MOS transistor of a channel N with the back gate grounded. 
         [0078]    On the photodiode P, light passing the lens group  10  and aperture diaphragm  11  strikes. To a cathode Pk of the photodiode P, a DC voltage VPD is impressed and to an anode PA, a drain T 1 D and a gate T 1 G of the transistor T 1  and a gate T 2 G of the transistor T 2  are connected. 
         [0079]    To a source T 1 S of the transistor T 1 , a signal impression line LC (equivalent to LC 1  to LCn shown in  FIG. 2 ) is connected and from the signal impression line LC, a signal φVPS is inputted. Here, the signal φVPS is a binary voltage signal and more in detail, it takes two values of a voltage VH for operating the transistor T 1  in the sub-threshold region when the incident light intensity exceeds a predetermined value and a voltage VL for putting the transistor T 1  into the continuity state. 
         [0080]    Further, to a drain T 2 D of the transistor T 2 , the DC voltage VPD is impressed and a T 2 S of the transistor T 2  is connected to a drain T 3 D of the transistor T 3  for row selection. 
         [0081]    To the gate T 3 G of the transistor T 3 , a signal impression line LA (equivalent to LA 1  to LAn shown in  FIG. 2 ) is connected and from the signal impression line LA, a signal φV is inputted. Further, a source T 3 S of the transistor T 3  is connected to a signal reading line LD (equivalent to LD 1  to LDm shown in  FIG. 2 ). 
         [0082]    Further, for the pixels G 11  to Gmn aforementioned, the ones disclosed in Japanese Patent Application 2002-77733 can be used. 
         [0083]    Here, the reason that as shown in  FIG. 4  aforementioned, as the exposure time is shortened, the rate of the linear conversion mode is increased is that as the exposure time is shortened, the potential difference between the gate T 2 G of the transistor T 2  and the source T 2 S thereof is increased and the rate of the object brightness when the transistor T 2  is operated in the cut-off state, that is, the rate of the object brightness converted linearly is increased. Further, although not illustrated in  FIG. 4 , when the control voltage for the imaging element  2 , that is, the difference between the voltages VL and VH of the signal φVPS is increased or even when the temperature lowers, the rate of the object brightness converted linearly is increased. Therefore, by changing the control voltage, exposure time, and temperature, the dynamic range of an image signal, the predetermined incident light intensity th at the inflection point, and the inflection output signal value H can be controlled. Concretely, for example, when the brightness range of an object is narrow, the voltage VL is lowered and the brightness range converted linearly is widened, and when the brightness range of the object is wide, the voltage VL is increased and the brightness range converted logarithmically is widened, thus the photoelectric conversion characteristic of the pixels G 11  to Gmn can be fit to the characteristic of the object. Furthermore, when minimizing the voltage VL, the pixels G 11  to Gmn can be always put into the linear conversion state and when maximizing the voltage VL, the pixels G 11  to Gmn can be always put into the logarithm conversion state. 
         [0084]    Then, the imaging operation of the imaging device  1  will be explained. 
         [0085]    Firstly, the imaging element  2  converts photoelectrically incident light to each of the pixels G 11  to Gmn and outputs an electric signal obtained by the linear conversion mode or logarithm conversion mode as an analog signal. Concretely, as mentioned above, when each of the pixels G 11  to Gmn outputs an electric signal to the signal reading line LD, the electric signal is amplified by the constant current source D and is sample-held sequentially by the selection circuit S. And, when the sample-held electric signal is sent from the selection circuit S to the correction circuit  23 , the correction circuit  23  removes noise and outputs the electric signal. 
         [0086]    Next, the analog signal outputted from the imaging element  2  is amplified by the amplifier  12  and is converted to a digital signal by the A-D converter  13 . Next, the black reference setting unit  14  sets the lowest level of the digital signal and as shown in  FIG. 9 , transmits the digital signal to the linearization unit  31  and fluctuation correction unit  30  of the signal processing unit  3  (Steps T 1  and U 1 ). Further, the controller  46  transmits the exposure time information and pixel information of each of the pixels G 11  to Gmn of the imaging element  2  to the fluctuation correction unit  30  and inflection signal derivation unit  34  (Steps U 1  and S 1 ). 
         [0087]    Upon receipt of the exposure time information and pixel information, the inflection signal derivation unit  34  derives the inflection output signal value H by the lookup table  34   a  (Step S 2 ) and transmits it to the fluctuation correction unit  30  and the selector  31   b  of the linearization unit  31  (Step S 3 ). As mentioned above, the lookup table  34   a  derives the inflection output signal value H on the basis of the exposure time and pixel information, so that the inflection output signal value H is derived exactly. Further, the inflection output signal value H is derived by the lookup table  34   a,  so that compared with the case of derivation by operations, the constitution of the inflection signal derivation unit  34  is simplified and the derivation processing is speeded up. 
         [0088]    Upon receipt of the inflection output signal value H from the inflection signal derivation unit  34  (Step U 2 ), the fluctuation correction unit  30  compares the magnitude between the signal values of the output signals from the pixels G 11  to Gmn and the inflection output signal value H (Step U 3 ) and when the signal values of the output signals from the pixels G 11  to Gmn are the inflection output signal value H or smaller, that is, when the output signals from the pixels G 11  to Gmn are the electric signal obtained by the linear conversion mode (Yes at Step U 3 ), the fluctuation correction unit  30  finishes the process. On the other hand, at Step U 3 , when the output signals from the pixels G 11  to Gmn are larger than the inflection output signal value H (No at Step U 3 ), the fluctuation correction unit  30  derives the correction factors α 11  to αmn for each of the pixels G 11  to Gmn by the lookup table  32   a  (Step U 4 ), performs fluctuation correction for each of the pixels G 11  to Gmn by the processing unit  33  (Step U 5 ), and then transmits the electric signal after fluctuation correction to the selector  31   b  of the linearization unit  31  (Step U 6 ). 
         [0089]    As mentioned above, when the I/O characteristic of the pixels G 11  to Gmn is fluctuated due to the exposure time of the pixels G 11  to Gmn, fluctuation correction of electric signals outputted from the pixels G 11  to Gmn is performed by the fluctuation correction unit  30 , so that even if the I/O characteristic is fluctuated depending on the drive conditions, variations between the output value at the reference exposure time t 1  and the actual output values of the pixels G 11  to Gmn are corrected. Further, the factor derivation unit  32  derives the correction factors α 11  to αmn on the basis of the exposure time and pixel information during imaging, so that the derived correction factors α 11  to αmn are used by the processing unit  33 , thus the fluctuation of the I/O characteristic of the imaging element  2  due to the exposure time and pixels G 11  to Gmn is corrected exactly. Further, the correction factors α 11  to αmn are derived by the lookup table  32   a,  so that compared with the case that the correction factors α 11  to αmn are derived by operations, the constitution of the factor derivation unit  32  is simplified and the derivation processing is speeded up. Further, fluctuation correction is performed only when the output signals from the pixels G 11  to Gmn are the electric signal obtained by the logarithm conversion mode, so that when the output signals are the electric signal obtained by the linear conversion mode, that is, when there is no need to convert the electric signal obtained by the logarithm conversion mode to a state obtained by another conversion mode, fluctuation correction is not performed uselessly, so that the signal processing is speeded up. 
         [0090]    Further, upon receipt of the inflection output signal value. H from the inflection signal derivation unit  34  (Step T 2 ), the selector  31   b  of the linearization unit  31  compares the magnitude between the signal values of the output signals from the pixels G 11  to Gmn and the inflection output signal value H (Step T 3 ) and when the output signals from the pixels G 11  to Gmn are the inflection output signal value H or smaller (Yes at Step T 3 ), outputs straight the output signals from the pixels G 11  to Gmn via the output unit  31   c  (Step T 4 ). On the other hand, when the output signals from the pixels G 11  to Gmn are larger than the inflection output signal value H (No at Step T 3 ), the selector  31   b  receives the electric signal after fluctuation correction from the fluctuation correction unit  30  (Step T 5 ), permits the reference conversion table  31   a  to perform characteristic conversion for the electric signal (Step T 6 ), and outputs it via the output unit  31   c  (Step T 7 ) 
         [0091]    As mentioned above, only when the output signals from the pixels G 11  to Gmn are the electric signal obtained by the logarithm conversion mode, characteristic conversion is performed, so that when the output signals are the electric signal obtained by the linear conversion mode, that is, when there is no need to convert the electric signal obtained by the logarithm conversion mode to the state obtained by another conversion mode, fluctuation correction is not performed uselessly, so that the signal processing is speeded up. 
         [0092]    Next, on the basis of the electric signal outputted from the linearization unit  31 , the evaluation value calculation unit  5  calculates the AWB evaluation value and AE evaluation value. 
         [0093]    Next, on the basis of the AE evaluation value calculated, the controller  46  controls the exposure control processing unit  47  and permits it to adjust the amount of the exposure for the imaging element  2 . 
         [0094]    Further, on the basis of the AWB evaluation value and the minimum level set by the black reference setting unit  14 , the controller  46  controls the AWB processing unit  40  and permits it to perform the white balance process for image data outputted from the signal processing unit  3 . 
         [0095]    And, on the basis of the image data outputted from the AWB processing unit  40 , the color interpolation unit  41 , color correction unit  42 , gradation conversion unit  43 , and color space conversion unit  44  perform respectively the image processing and then output image data. 
         [0096]    According to the imaging device  1  aforementioned, even if the I/O characteristic is fluctuated depending on the drive conditions, unlike the conventional way, variations between the output value at the reference exposure time t 1  and the actual output values can be corrected, so that by characteristic conversion by the linearization unit  31 , electric signals can be exactly unified to the state obtained by the linear conversion mode. 
         [0097]    Further, for the plurality of pixels G 11  to Gmn, only one fluctuation correction unit  30  is installed, so that compared with the case that a plurality of fluctuation correction units  30  are installed in correspondence to the respective pixels G 11  to Gmn, the constitution of the imaging device  1  can be simplified. 
       Modification of Embodiment 
       [0098]    Next, a modification of the embodiment aforementioned will be explained. Further, to the same components as those of the embodiment aforementioned, the same numerals are assigned and the explanation thereof will be omitted. 
         [0099]    The fluctuation correction unit  30  of this modification, when the I/O characteristic of the pixels G 11  to Gmn is fluctuated due to the control voltage for each of the pixels G 11  to Gmn, performs fluctuation correction of the electric signals outputted from the pixels G 11  to Gmn. 
         [0100]    Concretely, the factor derivation unit  32  of the fluctuation correction unit  30 , as shown in  FIGS. 5 and 8(   b ) assumes the control voltage V 1  when the predetermined incident light intensity th is minimum as a reference control voltage and uses a distance of “X2−X1”=d 2 −d 1 =m 2 V 2 −m 1 V 1 (m 2 =d 2 /V 2 , m 1 =d 1 /V 1 ) on the input axis (the x axis shown in  FIG. 8(   b )) between a virtual conversion table corresponding to the drive condition of the control voltage V 2  (variable) and the reference conversion table  31   a  corresponding to the drive condition of the reference control voltage V 1  as a correction factor α. 
         [0101]    Even in such a case, the same effect as that of the first embodiment aforementioned can be obtained. 
         [0102]    Further, in the first embodiment and modification aforementioned, it is explained that the fluctuation correction unit  30  is arranged at the preceding stage of the linearization unit  31 , though it may be arranged at the later stage thereof or it is possible to install the factor derivation unit  32  at the preceding stage of the linearization unit  31  and the processing unit  33  at the later stage thereof. 
         [0103]    Further, as drive conditions for the imaging element  2 , use of the exposure time and control voltage is explained, though temperature may be used. 
         [0104]    Further, it is explained that the fluctuation correction unit  30  has the processing unit  33  for deriving an electric signal after fluctuation correction, though it may have a lookup table, by input of the drive conditions, pixel information, and an electric signal outputted from the imaging element  2 , for deriving the electric signal after fluctuation correction. In this case, the same effect as that of the embodiment aforementioned can be obtained and compared with the case that an electric signal after fluctuation correction is derived by operations, the constitution of the fluctuation correction unit  30  can be simplified.  32   7498   
         [0105]    Further, it is explained that only one fluctuation correction unit  30  and one linearization unit  31  are installed, though a plurality of units may be installed in correspondence to each of the pixels G 11  to Gmn. Particularly, when a plurality of linearization units  31  are installed, even if the pixels G 11  to Gmn are different in the conversion characteristic of photoelectric conversion from each other, all the electric signals can be exactly unified to the state obtained by the linear conversion mode or the logarithm conversion mode. Further, when a plurality of fluctuation correction units  30  are installed, even if the fluctuation amount of the I/O characteristic is different for each of the pixels G 11  to Gmn, fluctuation correction can be performed exactly. 
         [0106]    Further, it is explained that the factor derivation unit  32 , on the basis of the drive conditions and pixel information, derives the correction factors α 11  to αmn for each of the pixels G 11  to Gmn, though on the basis of only the drive conditions, the correction factor α common to the pixels G 11  to Gmn may be derived. 
         [0107]    Further, it is explained that the factor derivation unit  32  has the reference conversion table  31   a  for deriving  33   7498  the correction factor, though it may install an operational unit for deriving a correction factor by input of the drive conditions and others. 
         [0108]    Further, it is explained that the characteristic conversion unit of the present invention is the linearization unit  31  for characteristic-converting the electric signal obtained by the logarithmic conversion mode to the state generated by linear conversion, though the electric signal obtained by the linear conversion mode may be characteristic-converted to the state obtained by the logarithm conversion mode. 
         [0109]    Further, it is explained that the inflection signal derivation unit  34  derives the inflection output signal value H on the basis of the drive conditions and pixel information, though it may be derived on the basis of only the drive conditions. Further, it is explained that the inflection signal derivation unit  34  has the lookup table  34   a  for deriving the inflection output signal value H, though it may have an operational unit for deriving the inflection output signal value H. 
         [0110]    Further, it is explained that the linearization units  31  and  36  perform characteristic conversion by the reference conversion table  31   a,  though the units may perform it by operations such as exponential conversion. 
         [0111]    Further, if is explained that the pixels G 11  and Gmn have the constitution as shown in  FIG. 6 , though if the linear conversion mode and logarithm conversion mode can be switched, the pixels may have the constitution as disclosed in Patent Document 1 aformentioned. 
         [0112]    Further, it is explained that the pixels G 11  and Gmn are equipped with R, G, and B filters, though the pixels may be equipped with filters of other colors such as cyan, magenta, and yellow.