Patent Publication Number: US-2023164453-A1

Title: Imaging device

Description:
TECHNICAL FIELD 
     The present disclosure relates to an imaging device that images a subject. 
     BACKGROUND ART 
     An imaging device includes, for example, a line sensor including one or more pixel lines. For example, PTL 1 discloses a line sensor including two or more pixel lines, in which the pixel lines have different sensitivities. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. H09-162381 
       
    
     SUMMARY OF THE INVENTION 
     In a line sensor, high image quality is desired, and further improvement in image quality is expected. 
     It is desirable to provide an imaging device that is able enhance image quality. 
     An imaging device according to an embodiment of the present disclosure includes a pixel array, an exposure controller, and a processor. The pixel array includes a plurality of light-receiving pixels. The plurality of light-receiving pixels is separated into a plurality of pixel lines and accumulates electric charge corresponding to a light-receiving amount in an accumulation period. The plurality of pixel lines includes a first pixel line and a second pixel line arranged in parallel in a first direction. The exposure controller sets a time length of the accumulation period in the plurality of light-receiving pixels to one of a plurality of time lengths including a first time length and a second time length in a manner that the plurality of time lengths repeat in predetermined order. The processor generates image data on a basis of an accumulation result in the plurality of light-receiving pixels. The accumulation period includes a first accumulation period and a second accumulation period each having the first time length, and a third accumulation period and a fourth accumulation period each having the second time length. The processor generates the image data by adding a plurality of pixel values based on the accumulation result in the first pixel line in the first accumulation period and a plurality of pixel values based on the accumulation result in the second pixel line in the second accumulation period, and adding a plurality of pixel values based on the accumulation result in the first pixel line in the third accumulation period and a plurality of pixel values based on the accumulation result in the second pixel line in the fourth accumulation period. 
     In the imaging device according to the embodiment of the present disclosure, the time length of the accumulation period in the plurality of light-receiving pixels is set to one of the plurality of time lengths including the first time length and the second time length in the manner that the plurality of time lengths repeat in predetermined order. In the light-receiving pixel, the electric charge corresponding to the light-receiving amount is accumulated in the accumulation period. The plurality of light-receiving pixels is separated into the plurality of pixel lines including the first pixel line and the second pixel line arranged in parallel in the first direction. In the processor, the image data is generated by adding the plurality of pixel values based on the accumulation result in the first pixel line in the first accumulation period having the first time length and the plurality of pixel values based on the accumulation result in the second pixel line in the second accumulation period having the first time length, and adding the plurality of pixel values based on the accumulation result in the first pixel line in the third accumulation period and the plurality of pixel values based on the accumulation result in the second pixel line in the fourth accumulation period. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an explanatory diagram illustrating a configuration example of an inspection system including an imaging device according to an embodiment of the present disclosure. 
         FIG.  2    is a block diagram illustrating a configuration example of the imaging device illustrated in  FIG.  1   . 
         FIG.  3    is an explanatory diagram illustrating an example of a TDI process to be performed in the imaging device illustrated in  FIG.  2   . 
         FIG.  4 A  is another explanatory diagram illustrating an example of the TDI process to be performed in the imaging device illustrated in  FIG.  2   . 
         FIG.  4 B  is another explanatory diagram illustrating an example of the TDI process to be performed in the imaging device illustrated in  FIG.  2   . 
         FIG.  5    is a timing diagram illustrating an operation example of the imaging device illustrated in  FIG.  2   . 
         FIG.  6 A  is another explanatory diagram illustrating an example of the TDI process to be performed in the imaging device illustrated in  FIG.  2   . 
         FIG.  6 B  is another explanatory diagram illustrating an example of the TDI process to be performed in the imaging device illustrated in  FIG.  2   . 
         FIG.  7    is an explanatory diagram illustrating an example of an image generated by the imaging device illustrated in  FIG.  2   . 
         FIG.  8    is an explanatory diagram illustrating an example of a TDI process to be performed in an imaging device according to a first comparative example. 
         FIG.  9    is a timing diagram illustrating an operation example of the imaging device according to the first comparative example. 
         FIG.  10    is an explanatory diagram illustrating an example of an image generated by the imaging device according to the first comparative example. 
         FIG.  11    is a timing diagram illustrating an operation example of an imaging device according to a second comparative example. 
         FIG.  12    is a timing diagram illustrating an operation example of an imaging device according to a modification example. 
         FIG.  13    is an explanatory diagram illustrating an example of a TDI process to be performed in the imaging device according to the modification example. 
         FIG.  14    is a timing diagram illustrating an operation example of an imaging device according to another modification example. 
         FIG.  15    is a timing diagram illustrating an operation example of an imaging device according to another modification example. 
         FIG.  16    is a block diagram illustrating a configuration example of an imaging device according to another modification example. 
         FIG.  17    is a block diagram illustrating a configuration example of an imaging device according to another modification example. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     In the following, some embodiments of the present disclosure are described in detail with reference to the drawings. 
     EMBODIMENTS 
     Configuration Example 
       FIG.  1    illustrates a configuration example of an inspection system  90  including an imaging device (an imaging device  1 ) according to an embodiment. The inspection system  90  is configured to, for example, image an industrial product being conveyed by a belt conveyor, and perform inspection of the industrial product by utilizing an image that has been obtained. The inspection system  90  includes a belt conveyor  91 , a conveyance controller  92 , a lens system  93 , the imaging device  1 , an HDR (High Dynamic Range) processor  94 , and an inspection processor  95 . 
     The belt conveyor  91  is configured to convey the industrial product which is to be inspected along a conveyance direction F. The industrial product is a subject  9  which is to be imaged by the imaging device  1 . 
     The conveyance controller  92  is configured to control a conveyance operation to be performed by the belt conveyor  91 . Further, the conveyance controller  92  generates a synchronization signal SYNC corresponding to a conveying speed of the belt conveyor  91 . The synchronization signal SYNC includes a plurality of pulses provided at intervals corresponding to the conveying speed. In this example, a pitch of the pulses is set in such a manner as to be time that is the same as time in which an image of the subject  9  progresses by one pixel line L on an imaging plane S of the imaging device  1 , as will be described later. Thereafter, the conveyance controller  92  supplies the imaging device  1  with the synchronization signal SYNC. 
     The lens system  93  is configured to direct the image of the subject  9  to the imaging plane S of the imaging device  1 . The lens system  93  is a Keplerian-type system in this example. 
     The imaging device  1  is a linear sensor and is configured to image the subject  9  conveyed by the belt conveyor  91  via the lens system  93 . In this example, the lens system  93  is a Keplerian-type system. Thus, the subject  9  moves in a direction opposite to the conveyance direction F on the imaging plane S of the imaging device  1 . The imaging device  1  performs an imaging operation in response to each of a plurality of pulses included in the synchronization signal SYNC. The imaging device  1  includes eight pixel lines L in this example as will be described later. The imaging device  1  performs the imaging operation each time the image of the subject  9  progresses by one pixel line L on the imaging plane S on the basis of the synchronization signal SYNC. The imaging device  1  is adapted to provide image data DT indicating an imaging outcome to the HDR processor  94 . 
     The HDR processor  94  is configured to generate image data including an image having a wider dynamic range by performing an HDR process on the basis of the image data DT supplied by the imaging device  1 . The HDR processor  94  thereafter supplies the inspection processor  95  with the generated image data. 
     The inspection processor  95  is configured to inspect the industrial product (the subject  9 ) on the basis of the image data supplied by the HDR processor  94 . 
       FIG.  2    illustrates a configuration example of the imaging device  1 . The imaging device  1  includes a pixel array  11 , an exposure controller  12 , a pixel controller  13 , a memory  14 , a TDI (Time Delay Integration) processor  15 , a TDI memory  16 , and an output processor  17 . 
     The pixel array  11  includes a plurality of light-receiving pixels PIX arranged in a matrix. The light-receiving pixel PIX includes a photodiode. The plurality of light-receiving pixels PIX is separated into eight pixel lines L in this example. In  FIG.  2   , the eight pixel lines L are arranged in parallel in a vertical direction. The direction of the parallel arrangement (the vertical direction) is a direction in which the image of the subject  9  moves. Each of the eight pixel lines L includes a predetermined number of light-receiving pixels PIX. Each of the plurality of light-receiving pixels PIX is configured to perform an accumulation operation of accumulating electric charge corresponding to a light-receiving amount in an accumulation period P, and perform an AD (Analog to Digital) conversion operation of converting an analog signal corresponding to the electric charge accumulated in the accumulation period P into a pixel value which is a digital value. The accumulation operation and the AD conversion operation in the plurality of light-receiving pixels PIX are performed on the basis of an instruction from the pixel controller  13 . 
     The exposure controller  12  is configured to control the accumulation operation and the AD conversion operation to be performed in the pixel array  11  on the basis of the synchronization signal SYNC supplied from the conveyance controller  92  ( FIG.  1   ). Specifically, the exposure controller  12  sets, in a time-division and alternate manner, the accumulation period P (an accumulation period PL) having a long time length TL and the accumulation period P (an accumulation period PS) having a short time length TS on the basis of the synchronization signal SYNC. The exposure controller  12  thereafter generates an exposure-start signal SST indicating a start timing of the accumulation period P and an exposure-end signal SED indicating an end timing of the accumulation period P, generates a conversion-start signal SAD indicating a start timing of the AD conversion operation, and supplies the pixel controller  13  with those signals. Further, the exposure controller  12  generates an exposure-kind signal EX indicating which of a process with respect to an accumulation result in the accumulation time PL or a process with respect to an accumulation result in the accumulation time PS is to be performed, and supplies the TDI processor  15  with the exposure-kind signal EX. 
     The pixel controller  13  is configured to control the accumulation operation by driving the plurality of light-receiving pixels PIX included in the pixel array  11  on the basis of the exposure-start signal SST and the exposure-end signal SED supplied from the exposure controller  12 , and to control the AD conversion operation by driving the plurality of light-receiving pixels PIX included in the pixel array  11  on the basis of the conversion-start signal SAD supplied from the exposure controller  12 . Further, after the AD conversion operation is completed, the pixel controller  13  also has a function of controlling an operation of transferring the respective pixel values generated by the plurality of light-receiving pixels PIX to the memory  14 . 
     The memory  14  is provided in the vicinity of pixel array  11  and is configured to temporarily store the plurality of pixel values supplied from the plurality of light-receiving pixels PIX included in the pixel array  11 . The memory  14  is a logic memory that stores the same number of pixel values as the number of the plurality of light-receiving pixels PIX included in the pixel array  11 . 
     The TDI processor  15  is configured to read the plurality of pixel values stored in the memory  14  in units of a plurality of pixel values related to one pixel line L, and to perform the TDI process on the basis of the plurality of pixel values that has been read. Specifically, the TDI processor  15  performs a TDI process  100 L using pixel values based on the accumulation result in the accumulation period PL and a TDI process  100 S using pixel values based on the accumulation result in the accumulation period PS. 
     The TDI memory  16  is a memory used by the TDI processor  15  in performing the TDI process. The TDI memory  16  includes a TDI memory  16 L and a TDI memory  16 S. The TDI memories  16 L and  16 S are each a logic memory, and are each able to store pieces of data, the number of which is the same as the number of the light-receiving pixels PIX in the pixel array  11 . The TDI memory  16 L stores the data generated when the TDI processor  15  performs the TDI process  100 L, and the TDI memory  16 S stores the data generated when the TDI processor  15  performs the TDI process  100 S. 
     The output processor  17  is configured to generate the image data DT by reading the data from the TDI memories  16 L and  16 S on the basis of an instruction from the TDI processor  15 . 
     With this configuration, the exposure controller  12  sets the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS in the time-division and alternate manner. Then, in each accumulation period P, the plurality of light-receiving pixels PIX performs the accumulation operation, and thereafter performs the AD conversion operation, to thereby generate the pixel values. The TDI processor  15  performs the TDI process  100 L using the pixel values based on the accumulation result in the accumulation period PL and the TDI process  100 S using the pixel values based on the accumulation result in the accumulation period PS. The imaging device  1  is thus able to widen the dynamic range. 
     The pixel array  11  corresponds to a specific example of a “pixel array” according to the present disclosure. The light-receiving pixel PIX corresponds to a specific example of a “light-receiving pixel” according to the present disclosure. The pixel line L corresponds to a specific example of a “pixel line” according to the present disclosure. The exposure controller  12  corresponds to a specific example of an “exposure controller” according to the present disclosure. The TDI processor  15 , the TDI memory  16 , and the output processor  17  correspond to a specific example of a “processor” according to the present disclosure. The TDI memory  16 S corresponds to a specific example of a “first storage” according to the present disclosure. The TDI memory  16 L corresponds to a specific example of a “second storage” according to the present disclosure. The accumulation period P corresponds to a specific example of an “accumulation period” according to the present disclosure. The time length TS corresponds to a specific example of a “first time length” according to the present disclosure. The time length TL corresponds to a specific example of a “second time length” according to the present disclosure. The synchronization signal SYNC corresponds to a specific example of a “synchronization signal” according to the present disclosure. 
     Operations and Workings 
     Next, operations and workings of the imaging device  1  according to the present embodiment will be described. 
     (Outline of Overall Operation) 
     First, with reference to  FIG.  2   , an outline of overall operation of the imaging device  1  will be described. The exposure controller  12  sets, in the time division manner, the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS on the basis of the synchronization signal SYNC. Thereafter, the exposure controller  12  generates the exposure-start signal SST, the exposure-end signal SED, the conversion-start signal SAD, and the exposure-kind signal EX. The pixel controller  13  controls the accumulation operation of the plurality of light-receiving pixels PIX included in the pixel array  11  on the basis of the exposure-start signal SST and the exposure-end signal SED, and controls the AD conversion operation the plurality of light-receiving pixels PIX included in the pixel array  11  on the basis of the conversion-start signal SAD. Each of the plurality of light-receiving pixels PIX included in the pixel array  11  performs the accumulation operation of accumulating the electric charge corresponding to the light-receiving amount in the accumulation period P, and performs the AD conversion operation of converting the analog signal corresponding to the accumulated electric charge into the pixel value which is the digital value. The memory  14  temporarily stores the plurality of pixel values supplied from the plurality of light-receiving pixels PIX included in the pixel array  11 . The TDI processor  15  reads the plurality of pixel values stored in the memory  14  in units of the plurality of pixel values related to one pixel line L, and performs the TDI process  100 L using the pixel values based on the accumulation result in the accumulation period PL and the TDI process  100 S using the pixel values based on the accumulation result in the accumulation period PS. The TDI memory  16 L stores the data generated when the TDI processor  15  performs the TDI process  100 L, and the TDI memory  16 S stores the data generated when the TDI processor  15  performs the TDI process  100 S. The output processor  17  generates the image data DT by reading the data from the TDI memories  16 L and  16 S on the basis of the instruction from the TDI processor  15 . 
     (Detailed Operation) 
       FIGS.  3 ,  4 A, and  4 B  each illustrate an example of the TDI process to be performed in the TDI processor  15 .  FIG.  3    illustrates an example of the TDI process for a line image related to one pixel line L,  FIG.  4 A  illustrates an example of the TDI process  100 S based on the accumulation result in the accumulation time PS, and  FIG.  4 B  illustrates an example of the TDI process  100 L based on the accumulation result in the accumulation time PL. 
     The exposure controller  12  sets the accumulation period P each time the image of the subject  9  progresses by one pixel line L on the imaging plane S on the basis of the synchronization signal SYNC. As illustrated in  FIG.  3   , the pixel array  11  generates images P 1  to P 8  on the basis of the respective accumulation results in the eight accumulation periods P that have been set sequentially. In the images P 1  to P 8 , the subject  9  progresses rightward in  FIG.  3    in units of one pixel line L as time elapses. For example, the line image related to the pixel line L 1  in the image P 1 , the line image related to the pixel line L 2  in the image P 2 , the line image related to the pixel line L 3  in the image P 3 , the line image related to the pixel line L 4  in the image P 4 , the line image related to the pixel line L 5  in the image P 5 , the line image related to the pixel line L 6  in the image P 6 , the line image related to the pixel line L 7  in the image P 7 , and the line image related to the pixel line L 8  in the image P 8  each indicate the same portion in the subject  9 . 
     The exposure controller  12  sets the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS in the time-division and alternate manner. In  FIG.  3   , the images P 1 , P 3 , P 5 , and P 7  each enclosed by a solid line are each an image generated on the basis of the accumulation result in the accumulation period PS having the short time length TS, and the images P 2 . P 4 , P 6 , and P 8  each enclosed by a dashed line are each an image generated on the basis of the accumulation result in the exposure time PL having the long time length TL. 
     The TDI processor  15  performs the TDI process  100 S using the pixel values based on the accumulation result in the accumulation period PS. In this example, the TDI processor  15  performs the TDI process  100 S by adding, in units of pixels, the line image related to the most upstream pixel line L 1  in the image P 1 , the line image related to the third pixel line L 3  in the image P 3 , the line image related to the fifth pixel line L 5  in the image P 5 , and the line image related to the seventh pixel line L 7  in the image P 7 . This allows the imaging device  1  to obtain a line image having high sensitivity related to the accumulation period PS having the short time length TS. 
     In a similar manner, the TDI processor  15  performs the TDI process  100 L using the pixel values based on the accumulation result in the accumulation period PL. In this example, the TDI processor  15  performs the TDI process  100 L by adding, in units of pixels, the line image related to the second pixel line L 2  in the image P 2 , the line image related to the fourth pixel line L 4  in the image P 4 , the line image related to the sixth pixel line L 6  in the image P 6 , and the line image related to the most downstream pixel line L 8  in the image P 8 . This allows the imaging device  1  to obtain a line image having high sensitivity related to the accumulation period PL having the long time length TL. 
     In the above description, one pixel line L is focused on each of the images P 1  to P 8 ; however, it is similar for other pixel lines L. 
     For example, as illustrated in  FIG.  4 A , the TDI processor  15  performs the TDI process  100 S on the basis of images P 1 , P 3 , P 5 , P 7 , P 9 , P 11 , and P 13  related to the accumulation period PS having the short time length TS. Specifically, the TDI processor  15  performs the TDI process  100 S on the basis of the line image related to the first pixel line L 1  in the image P 1 , the line image related to the third pixel line L 3  in the image P 3 , the line image related to the fifth pixel line L 5  in the image P 5 , and the line image related to the seventh pixel line L 7  in the image P 7 . Thereafter, the TDI processor  15  performs the TDI process  100 S on the basis of the line image related to the second pixel line L 2  in the image P 1 , the line image related to the fourth pixel line L 4  in the image P 3 , the line image related to the sixth pixel line L 6  in the image P 5 , and the line image related to the eighth pixel line L 8  in the image P 7 . In this manner, the TDI processor  15  generates a two-line line image on the basis of the images P 1 , P 3 , P 5 , and P 7 . In a similar manner, the TDI processor  15  generates the two-line line image on the basis of the images P 3 , P 5 , P 7 , and P 9 , generates the two-line line image on the basis of the images P 5 , P 7 , P 9 , and P 11 , and generates the two-line line image on the basis of the images P 7 , P 9 , P 11 , and P 13 . In this manner, the imaging device  1  is able to obtain an image DS having high sensitivity related to the accumulation period PS having the short time length TS. 
     In a similar manner, for example, as illustrated in  FIG.  4 B , the TDI processor  15  performs the TDI process  100 L on the basis of images P 2 , P 4 , P 6 , P 8 , P 10 , P 12 , and P 14  related to the accumulation period PL having the long time length TL. Specifically, the TDI processor  15  performs the TDI process  100 L on the basis of the line image related to the first pixel line L 1  in the image P 2 , the line image related to the third pixel line L 3  in the image P 4 , the line image related to the fifth pixel line L 5  in the image P 6 , and the line image related to the seventh pixel line L 7  in the image P 8 . Thereafter, the TDI processor  15  performs the TDI process  100 L on the basis of the line image related to the second pixel line L 2  in the image P 2 , the line image related to the fourth pixel line L 4  in the image P 4 , the line image related to the sixth pixel line L 6  in the image P 6 , and the line image related to the eighth pixel line L 8  in the image P 8 . In this manner, the TDI processor  15  generates the two-line line image on the basis of the images P 2 , P 4 , P 6 , and P 8 . In a similar manner, the TDI processor  15  generates the two-line line image on the basis of the images P 4 , P 6 , P 8 , and P 10 , generates the two-line line image on the basis of the images P 6 , P 8 , P 10 , and P 12 , and generates the two-line line image on the basis of the images P 8 , P 10 , P 12 , and P 14 . In this manner, the imaging device  1  is able to obtain an image DL having high sensitivity related to the accumulation period PL having the long time length TL. 
     If the accumulation period P is the accumulation period PS having the short time length TS, the pixel array  11  is able to image a bright portion of the subject  9  with high image quality, for example. Further, if the accumulation period P is the accumulation period PL having the long time length TL, the pixel array  11  is able to image a dark portion of the subject  9  with high image quality, for example. Accordingly, the imaging device  1  is able to achieve a wide dynamic range. 
     The imaging device  1  outputs, as the image data DT, the image DS related to the accumulation period PS and the image DL related to the accumulation period PL that have been generated by such a TDI process. Thereafter, the HDR processor  94  combines the image DS related to the accumulation period PS and the image DL related to the accumulation period PL by the HDR process. In other words, in the image DS related to the accumulation period PS having the short time length TS, for example, information of the dark portion in the subject  9  is missing, and in the image DL related to the accumulation period PL having the long time length TL, for example, information of the bright portion in the subject  9  is missing. The HDR processor  94  is able to combine the images DS and DL by, for example, causing the two images DS and DL to mutually compensate for the missing information, and selecting image portions whose S/N ratio is satisfactory from the two images DS and DL. This makes it possible for the HDR processor  94  to generate an image having a wide dynamic range. Thereafter, the inspection processor  95  inspects the industrial product (the subject  9 ) on the basis of the image having the wide dynamic range combined by the HDR processor  94 . As a result, the inspection system  90  is able to increase accuracy of the industrial-product inspection. 
     Next, more detailed operation of the imaging device  1  will be described. 
       FIG.  5    illustrates an operation example of the imaging device  1 , in which (A) indicates a waveform of the synchronization signal SYNC, (B) indicates a waveform of the exposure-start signal SST, (C) indicates a waveform of the exposure-end signal SED, (D) indicates a waveform of the exposure-kind signal EX, (E) indicates the accumulation operation performed in the pixel array  11 , and (F) indicates the processes performed in the TDI processor  15 . 
     The conveyance controller  92  ( FIG.  1   ) controls the conveyance operation in the belt conveyor  91  and generates the synchronization signal SYNC corresponding to the conveying speed of the belt conveyor  91  ( FIG.  5 (A) ). A pitch (a synchronization interval Tf) of the plurality of pulses in the synchronization signal SYNC is set in such a manner as to be the time that is the same as the time in which the image of the subject  9  progresses by one pixel line L on the imaging plane S of the imaging device  1 . The conveyance controller  92  maintains the conveying speed of the belt conveyor  91  at a constant speed and generates the pulses in the synchronization signal SYNC at a constant pitch (the synchronization interval Tf). 
     In this example, as illustrated in  FIG.  5 (A) , the conveyance controller  92  generates the pulse of the synchronization signal SYNC at timing t 1 , generates the next pulse of the synchronization signal SYNC at timing t 11  at which time has elapsed from timing t 1  by a synchronization interval Tf( 1 ), and generates the next pulse of the synchronization signal SYNC at timing t 21  at which time has elapsed from timing t 11  by a synchronization interval Tf( 2 ). 
     The exposure controller  12  generates the exposure-start signal SST, the exposure-end signal SED, and the exposure-kind signal EX, on the basis of a timing at which a delay time Td has elapsed from a timing of a rising edge of the synchronization signal SYNC as a reference timing. The delay time Td includes a processing delay time necessary for implementation. The delay time Td may be intentionally set to a predetermined time. Specifically, for example, it is possible to set the delay time Td in such a manner that the timing of the rising edge of synchronization signal SYNC coincides with the central timing of the accumulation period P. 
     First, the exposure controller  12  sets, as the reference timing, timing t 12  at which the delay time Td has elapsed from timing t 11  which is the rising edge of the synchronization signal SYNC, generates a pulse of the exposure-start signal SST at timing t 13  at which time Tls( 1 ) has elapsed from the reference timing ( FIG.  5 (B) ), and generates a pulse of the exposure-end signal SED at timing t 14  at which time tle( 1 ) has elapsed from the reference timing ( FIG.  5 (C) ). In this manner, the accumulation period PL having the long time length TL is set in a period from timing t 13  to timing t 14  ( FIG.  5 (E) ). In the pixel array  11 , the accumulation period PL is set at the same timing in all pixel lines L from the most upstream pixel line L 1  to the most downstream pixel line L 8 . Each light-receiving pixel PIX performs the accumulation operation of accumulating the electric charge corresponding to the light-receiving amount in the accumulation period PL, and performs the AD conversion operation of converting the analog signal corresponding to the accumulated electric charge into the pixel value which is the digital value in a predetermined period after the accumulation period PL is completed. The memory  14  stores the plurality of pixel values related to the accumulation period PL obtained in the plurality of light-receiving pixels PIX. 
     The exposure controller  12  generates the exposure-kind signal EX by performing a toggle operation at a timing of a rising edge of the exposure-end signal SED ( FIG.  5 (D) ). In this example, the exposure controller  12  changes the exposure-kind signal EX from a low level to a high level on the basis of the rising edge of the exposure-end signal SED at timing t 14 . On the basis of the exposure-kind signal EX, the TDI processor  15  performs the TDI process  100 L using the plurality of pixel values related to the accumulation period PL stored in the memory  14  in a period from timing t 14  to timing t 24  ( FIG.  5    (F)). 
       FIG.  6 A  illustrates an example of the TDI process  100 L. The TDI processor  15  sequentially reads the plurality of pixel values of one line corresponding to one pixel line L in the memory  14  in units of lines. Further, the TDI processor  15  reads a plurality of values of one line in the TDI memory  16 L corresponding to data of one line read from the memory  14 . The data of the one line read from the memory  14  and the data of the one line read from the TDI memory  16 L are each data indicating the same portion of the subject  9 . The TDI processor  15  generates a plurality of values by adding the plurality of pixel values of the one line read from the memory  14  and the plurality of values of the one line read from the TDI memory  16 L in units of pixels. The TDI processor  15  thereafter writes the generated plurality of values in the line read in the TDI memory  16 L. The TDI processor  15  thus updates data of eight lines in the TDI memory  16 L. 
     In this example, the TDI processor  15  processes data of one line in the TDI memory  16 L by reading the plurality of pixel values for each one line from the memory  14 ; however, the example is not limited thereto. Alternatively, for example, the TDI processor  15  may process data of two lines in the TDI memory  16 L by reading the plurality of pixel values for each two lines from the memory  14 . 
     When such a TDI process  100 L is completed, data of eight lines in the TDI memory  16 L includes data of two lines that has been added four times, data of two lines that has been added three times, data of two lines that has been added twice, and data of two lines that has been added once. The output processor  17  reads the data of two lines that has been added four times among the data of eight lines, and outputs the read data as the image data DT. In other words, as illustrated in  FIGS.  3  and  4 A , four line images are added to thereby complete the TDI process  100 L related to the line images, and thus, the output processor  17  outputs the data of the two lines added four times as the image data DT. Thereafter, the TDI processor  15  resets the data of the two lines in the TDI memory  16 L read by the output processor  17 . It is to be noted that, in this example, the data of the two lines has been reset; however, the example is not limited thereto. Alternatively, for example, instead of resetting, next, when updating the data of the two lines in the TDI memory  16 L, the data read from the memory  14  may not be added and may be written in the TDI memory  16 L as it is. 
     In a similar manner, as illustrated in  FIG.  5   , the exposure controller  12  sets, as the reference timing, timing t 22  at which the delay time Td has elapsed from timing t 21  which is the rising edge of the synchronization signal SYNC, generates a pulse of the exposure-start signal SST at timing t 23  at which time Tss( 2 ) has elapsed from the reference timing ( FIG.  5 (B) ), and generates a pulse of the exposure-end signal SED at timing t 24  at which time tse( 2 ) has elapsed from the reference timing ( FIG.  5 (C) ). In this manner, the accumulation period PS having the short time length TS is set in a period from timing t 23  to timing t 24  ( FIG.  5 (E) ). In the pixel array  11 , the accumulation period PS is set at the same timing in all pixel lines L from the most upstream pixel line L 1  to the most downstream pixel line L 8 . Each light-receiving pixel PIX performs the accumulation operation of accumulating the electric charge corresponding to the light-receiving amount in the accumulation period PS, and performs the AD conversion operation of converting the analog signal corresponding to the accumulated electric charge into the pixel value which is the digital value in a predetermined period after the accumulation period PS is completed. The memory  14  stores the plurality of pixel values related to the accumulation period PS obtained in the plurality of light-receiving pixels PIX. 
     The exposure controller  12  changes the exposure-kind signal EX from the high level to the low level on the basis of the rising edge of the exposure-end signal SED at timing t 24 . On the basis of the exposure-kind signal EX, the TDI processor  15  performs the TDI process  100 S using the plurality of pixel values related to the accumulation period PS stored in the memory  14  in a period from timing t 24  to timing t 34  ( FIG.  5    (F)). 
       FIG.  6 B  illustrates an example of the TDI process  100 S. The TDI processor  15  sequentially reads the plurality of pixel values of one line corresponding to one pixel line L in the memory  14  in units of lines. Further, the TDI processor  15  reads a plurality of values of one line in the TDI memory  16 S corresponding to data of one line read from the memory  14 . The data of the one line read from the memory  14  and the data of the one line read from the TDI memory  16 S are each data indicating the same portion of the subject  9 . The TDI processor  15  generates a plurality of values by adding the plurality of pixel values of the one line read from the memory  14  and the plurality of values of the one line read from the TDI memory  16 S in units of pixels. The TDI processor  15  thereafter writes the generated plurality of values in the line read in the TDI memory  16 S. The TDI processor  15  thus updates data of eight lines in the TDI memory  16 S. 
     When such a TDI process  100 S is completed, data of eight lines in the TDI memory  16 S includes data of two lines that has been added four times, data of two lines that has been added three times, data of two lines that has been added twice, and data of two lines that has been added once. The output processor  17  reads the data of two lines that has been added four times among the data of eight lines, and outputs the read data as the image data DT. Thereafter, the TDI processor  15  resets the data of the two lines in the TDI memory  16 S read by the output processor  17 . 
     In this manner, the imaging device  1  sets the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS in the time-division and alternate manner, and performs the TDI process  100 L using the pixel values based on the accumulation result in the accumulation period PL and the TDI process  100 S using the pixel values based on the accumulation result in the accumulation period PS, thereby generating the image data DT. 
     The exposure controller  12  uses, for example, the following expressions to calculate time Tls( 1 ), time Tle( 1 ), time Tss( 2 ), and time Tse( 2 ) indicated in  FIG.  5   . 
         Tls ( n )= Tf ( n )− TL/ 2− TS/ 2
 
         Tle ( n )= Tf ( n )+ TL/ 2− TS/ 2
 
         Tss ( n )= Tf ( n )− TS  
 
         Tse ( n )= Tf ( n ) 
     Here, n is a number indicating the number of cycles. Tf(n) is a synchronization interval illustrated in  FIG.  5    and is measured by the exposure controller  12 . TL is a time length of the accumulation period PL and is stored, for example, in a control register. TS is a time length of the accumulation period PS and is stored, for example, in the control register. Data of the time lengths TL and TS stored in the control register is desirably changeable. This makes it possible to adjust the dynamic range depending on, for example, brightness of a subject. 
     Using those equations, time Tls( 1 ), time Tle( 1 ), time Tss( 2 ), and time Tse( 2 ) are as follows. 
         Tls (1)= Tf (1)− TL/ 2− TS/ 2
 
         Tle (1)= Tf (1)+ TL/ 2− TS/ 2
 
         Tss (2)= Tf (2)− TS  
 
         Tse (2)= Tf (2) 
     For example, the exposure controller  12  measures the synchronization interval Tf( 1 ) of between timing t 1  and timing t 11  on the basis of the synchronization signal SYNC, and calculates time Tls( 1 ) and Tle( 1 ) using the synchronization interval Tf( 1 ) and the time lengths TL and TS stored in the control register. The exposure controller  12  thus sets the accumulation period PL having the long time length TL. In a similar manner, the exposure controller  12  measures the synchronization interval Tf( 2 ) of between timing t 11  and timing t 21  on the basis of the synchronization signal SYNC, and calculates time Tss( 2 ) and time Tse( 2 ) using the synchronization interval Tf( 2 ) and the time length TS stored in the control register. The exposure controller  12  thus sets the accumulation period PS having the short time length TS. 
     Here, central timing tlc(n) of the accumulation period PL and central timing tsc(n) of the accumulation period PS are expressed as follows. 
     
       
         
           
             
               T 
               ⁢ 
               1 
               ⁢ 
               
                 c 
                 ⁡ 
                 ( 
                 n 
                 ) 
               
             
             = 
             
               
                 
                   ( 
                   
                     
                       T 
                       ⁢ 
                       1 
                       ⁢ 
                       
                         s 
                         ⁡ 
                         ( 
                         n 
                         ) 
                       
                     
                     + 
                     
                       T 
                       ⁢ 
                       1 
                       ⁢ 
                       
                         e 
                         ⁡ 
                         ( 
                         n 
                         ) 
                       
                     
                   
                   ) 
                 
                 / 
                 2 
               
               = 
               
                 
                   Tf 
                   ⁡ 
                   ( 
                   n 
                   ) 
                 
                 - 
                 
                   TS 
                   / 
                   2 
                 
               
             
           
         
       
       
         
           
             
               Tsc 
               ⁡ 
               ( 
               n 
               ) 
             
             = 
             
               
                 
                   ( 
                   
                     
                       Tss 
                       ⁡ 
                       ( 
                       n 
                       ) 
                     
                     + 
                     
                       Tse 
                       ⁡ 
                       ( 
                       n 
                       ) 
                     
                   
                   ) 
                 
                 / 
                 2 
               
               = 
               
                 
                   Tf 
                   ⁡ 
                   ( 
                   n 
                   ) 
                 
                 - 
                 
                   TS 
                   / 
                   2 
                 
               
             
           
         
       
     
     Thus, as illustrated in  FIG.  5 (E) : time between the central timing of the accumulation period PL having the long time length TL of between timing t 13  and timing t 14  and the central timing of the accumulation period PS having the time length TS of between timing t 23  and timing t 24  is equal to time of the synchronization interval Tf; and time between the central timing of the accumulation period PS having the time length TS of between timing t 23  and timing t 24  and the central timing of the accumulation period PL having the long time length TL of between timing t 33  and timing t 34  is equal to time of the synchronization interval Tf. In other words, the time length between the respective central timings of the accumulation periods P (the accumulation periods PL and PS) that are adjacent to each other on a time axis is equal to the time of the synchronization interval Tf and is constant. As a result, in the imaging device  1 , it is possible to suppress deviation between a position of the center of gravity of the subject  9  in the image DL generated by the TDI process  100 L and a position of the center of gravity of the subject  9  in the image DS generated by the TDI process  100 S, as will be described below. 
       FIG.  7    is an example of the image DL generated by the TDI process  100 L and the image DS generated by the TDI process  100 S. As illustrated in  FIG.  5   , the time length TL of the accumulation period PL is longer than the time length TS of the accumulation period PS. Thus, in the image DL generated by the TDI process  100 L, the subject  9  is slightly wider in a moving direction (a horizontal direction in  FIG.  7   ) of the subject  9  as compared with the image DS generated by the TDI process  100 S. As illustrated in  FIG.  7   , the position of the center of gravity of the subject  9  in the image DL generated by the TDI process  100 L and the position of the center of gravity of the subject  9  in the image DS generated by the TDI process  100 S coincides with each other in the moving direction of the subject  9 . In other words, as illustrated in  FIG.  5   , in the imaging device  1 , the time length between the respective central timings of the accumulation periods P that are adjacent to each other on the time axis is equal to the time of the synchronization interval Tf and is constant. Accordingly, it is possible to make the position of the center of gravity of the subject  9  in the image DL and the position of the center of gravity of the subject  9  in the image DS to coincide with each other. Thus, the HDR processor  94  at a subsequent stage in the imaging device  1  is able to combine the image DL and the image DS whose positions of the center of gravity coincide with each other as they are by the HDR process. This makes it possible to enhance the quality of the image generated by the HDR process as compared with comparative examples described below. 
     Next, effects of the present embodiment will be described in comparison with some comparative examples. 
     First Comparative Example 
     First, an imaging device  1 R according to a first comparative example will be described. The imaging device  1 R is configured to set the accumulation period PS having the short time length TS for the pixel lines L 1  to L 4  of the pixel array  11 , and set the accumulation period PL having the long time length TL for the pixel lines L 5  to L 8  of the pixel array  11 . The imaging device  1 R includes an exposure controller  12 R and a TDI processor  15 R in a manner similar to the imaging device  1  ( FIG.  2   ) according to the present embodiment. 
       FIG.  8    is an example of the TDI process to be performed in the imaging device  1 R. The exposure controller  12 R sets the accumulation period PS having the short time length TS for the pixel lines L 1  to L 4  of the pixel array  11 , and sets the accumulation period PL having the long time length TL for the pixel lines L 5  to L 8  of the pixel array  11 . Accordingly, the images P 1  to P 8  each include: an image which is enclosed by the solid line in  FIG.  8    and is generated on the basis of the accumulation result in the accumulation period PS having the short time length TS; and an image which is enclosed by the dashed line in  FIG.  8    and is generated on the basis of the accumulation result in the accumulation period PL having the long time length TL. 
     The TDI processor  15 R performs a TDI process  100 RS using the pixel values based on the accumulation result in the accumulation period PS. In this example, the TDI processor  15 R performs the TDI process  100 RS by adding, in units of pixels, the line image related to the most upstream pixel line L 1  in the image P 1 , the line image related to the second pixel line L 2  in the image P 2 , the line image related to the third pixel line L 3  in the image P 3 , and the line image related to the fourth pixel line L 4  in the image P 4 . 
     In a similar manner, the TDI processor  15 R performs a TDI process  100 RL using the pixel values based on the accumulation result in the accumulation period PL. In this example, the TDI processor  15 R performs the TDI process  100 RL by adding, in units of pixels: the line image related to the fifth pixel line L 5  in the image P 5 , the line image related to the sixth pixel line L 6  in the image P 6 , the line image related to the seventh pixel line L 7  in the image P 7 , and the line image related to the most downstream pixel line L 8  in the image P 8 . 
       FIG.  9    illustrates an example of the accumulation period P in the imaging device  1 R, in which (A) indicates the waveform of the synchronization signal SYNC, and (B) indicates the accumulation operation performed in the pixel array  11 . The exposure controller  12 R sets the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS on the basis of the synchronization signal SYNC. Specifically, for example, the exposure controller  12 R sets the accumulation period PL that starts at timing t 41  and ends at timing t 43 , and sets the accumulation period PS that starts at timing t 42  and ends at timing  43 . Also in this example, the time length between the respective central timings of the accumulation periods PL that are adjacent to each other on the time axis is equal to the time of the synchronization interval Tf and is constant, and the time length between the respective central timings of the accumulation periods PS that are adjacent to each other on the time axis is equal to the time of the synchronization interval Tf and is constant. 
     However, in this example, the central timing of the accumulation period PL and the central timing of accumulation period PS are deviated from each other. As a result, in the imaging device  1 R, as illustrated in  FIG.  10   , a deviation Δ occurs between the position of the center of gravity of the subject  9  in the image DL generated by the TDI process  100 RL and the position of the center of gravity of the subject  9  in the image DS generated by the TDI process  100 RS. Accordingly, the HDR processor  94  at a subsequent stage in the imaging device  1 R first has to perform an image correction process on the basis of the images DL and DS in such a manner that the positions of the center of gravity of the subject  9  coincide with each other, and to combine the two corrected images by the HDR process. Thus, the image correction process may reduce the image quality. 
     Further, the imaging device  1 R sets the accumulation period PS having the short time length TS for the pixel lines L 1  to L 4  of the pixel array  11 , and sets the accumulation period PL having the long time length TL for the pixel lines L 5  to L 8  of the pixel array  11 . Accordingly, the pixel controller  13  has to control, for example, the accumulation operation of the plurality of light-receiving pixels PIX included in the pixel lines L 1  to L 4  and the accumulation operation of the plurality of light-receiving pixels PIX included in the pixel lines L 5  to L 8  by using different control signals. As a result, for example, a wiring line of the control signal in the pixel array  11  can become complicated. In particular, in a linear sensor, it is desirable that the light-receiving pixels PIX are disposed closely to each other. In this case, in the pixel array  11 , space for the wiring line between the light-receiving pixels PIX decreases, for example. It is thus difficult to dispose the wiring line. For example, in a case where a complicated wiring line is provided, this can affect the image quality. 
     In contrast, the imaging device  1  according to the present embodiment is able to make the position of the center of gravity of the subject  9  in the image DL and the position of the center of gravity of the subject  9  in the image DS to coincide with each other. The HDR processor  94  is thus able to combine the image DL and image DS whose positions of the center of gravity coincide with each other as they are by the HDR process, without performing the image correction process. This makes it possible to enhance the quality of the image generated by the HDR process. Further, the imaging device  1  sets the accumulation period PL or the accumulation period PS in a time-division manner for all pixel lines L 1  to L 8  included in the pixel array  11 . The pixel controller  13  is thus able to control the accumulation operation of all light-receiving pixels PIX using the same control signal, for example. This makes it possible to simplify the wiring line of the control signal in the pixel array  11 , and to reduce a possibility of decreasing the image quality due to the complicated wiring line. 
     Second Comparative Example 
     Next, an imaging device  1 S according to a second comparative example will be described. In the imaging device  1 S is obtained by changing timings of the exposure periods PL and TS in the imaging device  1 R according to the first comparative example. The imaging device  1 S includes the exposure controller  12 S and the TDI processor  15 S in a manner similar to the imaging device  1  ( FIG.  2   ) according to the present embodiment. 
       FIG.  11    is an example of the accumulation period P in the imaging device  1 S. The exposure controller  12 S sets the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS on the basis of the synchronization signal SYNC. Specifically, for example, the exposure controller  12 S sets the accumulation period PL that starts at timing t 51  and ends at timing t 54 , and sets the accumulation period PS that starts at timing t 52  and ends at timing t 53 . In this example, the central timing of the accumulation period PL and the central timing of the accumulation period PS are made to coincide with each other. Accordingly, in the imaging device  1 S, it is possible to make the position of the center of gravity of the subject  9  in the image DL and the position of the center of gravity of the subject  9  in the image DS to coincide with each other in a manner similar to the case of the imaging device  1  ( FIG.  7   ) according to the present embodiment. 
     However, in this case, the plurality of light-receiving pixels PIX included in the pixel lines L 1  to L 4  starts the AD conversion operation from timing t 53  at which the accumulation period PS ends, and the plurality of light-receiving pixels PIX included in the pixel lines L 5  to L 8  starts the AD conversion operation from timing t 54  at which the accumulation period PL ends. Accordingly, the pixel controller  13  has to control, for example, the AD conversion operation of the plurality of light-receiving pixels PIX included in the pixel lines L 1  to L 4  and the AD conversion operation of the plurality of light-receiving pixels PIX included in the pixel lines L 5  to L 8  by using different control signals. Thus, for example, a wiring line of the control signal in pixel array  11  can become complicated. For example, in a case where a complicated wiring line is provided, this can affect the image quality. Further, since the timings of the AD conversion operation are different from each other in this manner, for example, power supply noise caused by the AD conversion operation related to the plurality of light-receiving pixels PIX included in the pixel lines L 1  to L 4  can affect the accumulation operation of the plurality of light-receiving pixels PIX included in the pixel lines L 5  to L 8 , and the image quality can be reduced. 
     In contrast, the imaging device  1  according to the present embodiment sets the accumulation period PL or the accumulation period PS in a time-division manner for all pixel lines L 1  to L 8  included in the pixel array  11 , and is thus able to make timings of the AD conversion operation in the plurality of light-receiving pixels PIX included in all pixel lines L 1  to L 8 . The pixel controller  13  is thus able to control the AD conversion operation of all light-receiving pixels PIX using the same control signal, for example. This makes it possible to simplify the wiring line of the control signal in the pixel array  11 , and to reduce a possibility of decreasing the image quality due to the complicated wiring line. In addition, since it is possible to make the timings of the AD conversion operation to coincide with each other in this manner, the power supply noise generated by the AD conversion operation of a certain light-receiving pixel PIX does not affect the accumulation operation of another light-receiving pixel PIX. Thus, it is possible to decrease the possibility of the image quality being reduced. 
     In this manner, the imaging device  1  sets the time length of the accumulation period P to one of the short time length TS or the long time length TL in the time-division and alternate manner, and performs the accumulation operation in which each of the plurality of light-receiving pixels PIX included in the pixel array  11  accumulates the electric charge corresponding to the light-receiving amount in the accumulation period P. Thereafter, for example, as illustrated in  FIG.  4 A , the plurality of pixel values based on the accumulation result in the pixel line L 1  in the accumulation period PS having the short time length TS and the plurality of pixel values based on the accumulation result in the pixel line L 3  in another accumulation period PS are added to each other, and, for example, as illustrated in  FIG.  4 B , the plurality of pixel values based on the accumulation result in the pixel line L 1  in the accumulation period PL having the long time length TL and the plurality of pixel values based on the accumulation result in the pixel line L 3  in another accumulation period PL are added to each other, thereby generating the image data DT. If the accumulation period P is the accumulation period PS having the short time length TS, the pixel array  11  is able to image the bright portion of the subject  9  with high image quality, for example, and if the accumulation period P is the accumulation period PL having the long time length TL, the pixel array  11  is able to image a dark portion of the subject  9  with high image quality, for example. Accordingly, the imaging device  1  is able to achieve the wide dynamic range. Further, the imaging device  1  adds the plurality of pixel values by the TDI process, and is thus able to enhance the sensitivity. As a result, the imaging device  1  is able to enhance the image quality. 
     In particular, the imaging device  1  alternately sets the time length of the accumulation time P to one of the short time length TS or the long time length TL. The imaging device  1  is thus able to simplify the wiring line in the pixel array  11 , and to decrease the possibility of the image quality being reduced due to the complicated wiring line. In addition, since the power supply noise generated by the AD conversion operation does not affect the accumulation operation, it is possible to enhance the image quality. 
     Further, in the imaging device  1 , the time length between the respective central timings of two accumulation periods P that are adjacent to each other on the time axis is made constant. The imaging device  1  is thus able to make the position of the center of gravity of the subject  9  in the image DL generated by the TDI process  100 L and the position of the center of gravity of the subject  9  in the image DS generated by the TDI process  100 S to coincide with each other. Accordingly, for example, in the case of combining the image DL and the image DS by the HDR process, it is possible to combine the image DL and the image DS as they are without performing the image correction process of adjusting the position of the center of gravity position. This makes it possible to enhance the image quality of the image generated by the HDR process. 
     Effects 
     As described above, in the present embodiment, the time length of the accumulation period is alternately set to one of the short time length or the long time length, and the plurality of light-receiving pixels included in the pixel array performs the accumulation operation of accumulating the electric charge corresponding to the light-receiving amount in the accumulation period. Thereafter, for example, the plurality of pixel values based on the accumulation result in the pixel line L 1  in the accumulation period PS having the short time length and the plurality of pixel values based on the accumulation result in the pixel line L 3  in another accumulation period PS are added to each other, and, for example, the plurality of pixel values based on the accumulation result in the pixel line L 1  in the accumulation period PL having the long time length and the plurality of pixel values based on the accumulation result in the pixel line L 3  in another accumulation period PL are added to each other, thereby generating the image data. As a result, it is possible to enhance the image quality. 
     In the present embodiment, the time length between the respective central timings of two accumulation periods P that are adjacent to each other on the time axis is made constant. This makes it possible to enhance the image quality. 
     Modification Example 1 
     In the embodiment described above, the accumulation period PS having the short time length TS and the accumulation period PL having the long time length TL are set alternately; however, the present disclosure is not limited thereto, and, for example, it is possible to set the accumulation period PS and the accumulation period PL in predetermined order. Hereinafter, an imaging device  1 A according to the present modification example will be described in detail. The imaging device  1 A includes an exposure controller  12 A and a TDI processor  15 A in a manner similar to the imaging device  1  ( FIG.  2   ) according to the present embodiment. 
       FIG.  12    illustrates an example of the accumulation period P of the imaging device  1 A.  FIG.  13    illustrates an example of the TDI process to be performed by the imaging device  1 A. 
     As illustrated in  FIG.  12   , the exposure controller  12 A sets, on the basis of the synchronization signal SYNC, the accumulation period PS, the accumulation period PS, the accumulation period PL, and the accumulation period PL, in this order repeatedly. Specifically, for example, the exposure controller  12 A sets the accumulation period PS that starts at timing t 61  and ends at timing  162 , sets the accumulation period PS that starts at timing t 63  and ends at timing t 64 , sets the accumulation period PL that starts at timing t 65  and ends at timing t 66 , and sets the accumulation period PL that starts at timing t 67  and ends at timing t 68 . Also in this example, the time length between the respective central timings of the accumulation periods P that are adjacent to each other on the time axis is equal to the time of the synchronization interval Tf and is constant. 
     In  FIG.  13   , the images P 1 , P 2 , P 5 , and P 6  each enclosed by the solid line are each the image generated on the basis of the accumulation result in the accumulation period PS having the short time length TS, and the images P 3 , P 4 , P 7 , and P 8  each enclosed by the dashed line are each the image generated on the basis of the accumulation result in the exposure time PL having the long time length TL. 
     The TDI processor  15 A performs the TDI process  100 S using the pixel values based on the accumulation result in the accumulation period PS. In this example, the TDI processor  15 A performs the TDI process  100 S by adding, in units of pixels, the line image related to the most upstream pixel line L 1  in the image P 1 , the line image related to the second pixel line L 2  in the image P 2 , the line image related to the fifth pixel line L 5  in the image P 5 , and the line image related to the sixth pixel line L 6  in the image P 6 . 
     In a similar manner, the TDI processor  15 A performs the TDI process  100 L using the pixel values based on the accumulation result in the accumulation period PL. In this example, the TDI processor  15 A performs the TDI process  100 L by adding, in units of pixels, the line image related to the third pixel line L 3  in the image P 3 , the line image related to the fourth pixel line L 4  in the image P 4 , the line image related to the seventh pixel line L 7  in the image P 7 , and the line image related to the most downstream pixel line L 8  in the image P 8 . 
     Modification Example 2 
     In the embodiment described above, the two accumulation periods PS and PL are set alternately; however, the present disclosure is not limited thereto, and, for example, three or more accumulation periods P may be set in predetermined order. Hereinafter, an imaging device  1 B according to the present modification example will be described in detail. The imaging device  1 B includes a pixel array  11 B, and an exposure controller  12 B, and a TDI processor  15 B in a similar manner to the imaging device  1  ( FIG.  2   ) according to the present embodiment. The plurality of light-receiving pixels PIX included in the pixel array  11 B is separated into six pixel lines L in this example. 
       FIG.  14    illustrates an example of the accumulation period P of the imaging device  1 B.  FIG.  15    illustrates an example of the TDI process to be performed by the imaging device  1 B. 
     As illustrated in  FIG.  14   , the exposure controller  12 A sets, on the basis of the synchronization signal SYNC, the accumulation period PS having the short time length TS, an accumulation period PM having a medium time length TM, and the accumulation period PL having the long time length TL in this order repeatedly. Also in this example, the time length between the respective central timings of the accumulation periods P that are adjacent to each other on the time axis is equal to the time of the synchronization interval Tf and is constant. 
     In  FIG.  15   , the images P 1  and P 4  each enclosed by the solid line are each the image generated on the basis of the accumulation result in the accumulation period PS having the short time length TS, the images P 2  and P 5  each enclosed by a long dashed line are each an image generated on the basis of an accumulation result in the exposure time PM having the medium time length TM, and the images P 3  and P 6  each enclosed by a short dashed line are each the image generated on the basis of the accumulation result in the accumulation period PL having the long time length TL. 
     The TDI processor  15 B performs the TDI process  100 S using the pixel values based on the accumulation result in the accumulation period PS. In this example, the TDI processor  15 B performs the TDI process  100 S by adding, in units of pixels, the line image related to the most upstream pixel line L 1  in the image P 1  and the line image related to the fourth pixel line L 4  in the image P 4 . 
     In a similar manner, the TDI processor  15 B performs a TDI process  100 M using the pixel values based on the accumulation result in the accumulation period PM. In this example, the TDI processor  15 B performs the TDI process  100 M by adding, in units of pixels, the line image related to the second pixel line L 2  in the image P 2  and the line image related to the fifth pixel line L 5  in the image P 5 . 
     In a similar manner, the TDI processor  15 B performs the TDI process  100 L using the pixel values based on the accumulation result in the accumulation period PL. In this example, the TDI processor  15 B performs the TDI process  100 L by adding, in units of pixels, the line image related to the third pixel line L 3  in the image P 3  and the line image related to the sixth pixel line L 6  in the image P 6 . 
     Modification Example 3 
     In the embodiment described above, the light-receiving pixel PIX performs the accumulation operation and the AD conversion operation; however, the present disclosure is not limited thereto. Alternatively, for example, a circuit that performs the AD conversion operation may be provided separately from the light-receiving pixel. Hereinafter, an imaging device  1 C according to the present modification example will be described in detail. 
       FIG.  16    illustrates a configuration example of the imaging device  1 C. The imaging device  1 C includes a pixel array  11 C, an exposure controller  12 C, a pixel controller  13 C, a GS (Global Shutter) memory  14 C, and an AD converter  18 C. 
     The pixel array  11 C includes a plurality of light-receiving pixels PIXC arranged in a matrix. Each of the plurality of light-receiving pixels PIXC performs the accumulation operation of accumulating the electric charge corresponding to the light-receiving amount in the accumulation period P. In other words, although the light-receiving pixel PIX according to the above-described embodiment performs the accumulation operation and the AD conversion operation, the light-receiving pixel PIXC according to the present modification example performs only the accumulation operation. The accumulation operation in the plurality of light-receiving pixels PIX is to be performed on the basis of an instruction from the pixel controller  13 C. 
     The exposure controller  12 C is configured to control the accumulation operation to be performed in the pixel array  11 C on the basis of the synchronization signal SYNC supplied from the conveyance controller  92  ( FIG.  1   ). Specifically, the exposure controller  12 C sets, in the time-division and alternate manner, the accumulation period PL having the long time length TL and the accumulation period PS having the short time length TS on the basis of the synchronization signal SYNC. The exposure controller  12 C thereafter generates the exposure-start signal SST indicating the start timing of the accumulation period P and the exposure-end signal SED indicating the end timing of the accumulation period P, and supplies the pixel controller  13 C with those signals. Further, the exposure controller  12 C generates the exposure-kind signal EX indicating which of the process with respect to the accumulation result in the accumulation time PL or the process with respect to the accumulation result in the accumulation time PS is to be performed, and supplies the TDI processor  15  with the exposure-kind signal EX. 
     The pixel controller  13 C is configured to control the accumulation operation by driving the plurality of light-receiving pixels PIXC included in the pixel array  11 C on the basis of the exposure-start signal SST and the exposure-end signal SED supplied from the exposure controller  12 C. Further, after the accumulation operation is completed, the pixel controller  13 C also has a function of controlling an operation of transferring the electric charge accumulated in the plurality of light-receiving pixels PIXC to the GS memory  14 C. 
     The GS memory  14 C is an analog memory, is provided in the vicinity of the pixel array  11 C, and is configured to temporarily store the electric charge supplied from the plurality of light-receiving pixels PIX included in the pixel array  11 C. 
     The AD converter  18 C is configured to generate a plurality of pixel values of one line by performing AD conversion on the basis of the electric charge of the one line which is read from the GS memory  14 C. Here, the AD converter  18 C corresponds to a specific example of an “AD converter” according to the present disclosure. 
     Modification Example 4 
     In the embodiment described above, the imaging device  1  outputs the image data DT including the image DL generated by the TDI process  100 L and the image DS generated by the TDI process  100 S, and the HDR processor  94  that is provided separately from the imaging device  1  performs the HDR process on the basis of the image data DT; however, the present disclosure is not limited thereto. Alternatively, for example, as in an imaging device  1 D illustrated in  FIG.  17   , an HDR processor  19 D may be provided in the imaging device  1 D, and the HDR processor  19 D may perform the HDR process on the basis of the image data DT and output image data DT 2  generated by the HDR process. 
     Other Modification Examples 
     Further, two or more of the modification examples may be combined. 
     Although the disclosure is described hereinabove with reference to the example embodiments and modification examples, these embodiments and modification examples are not to be construed as limiting the scope of the disclosure and may be modified in a wide variety of ways. 
     For example, in the above-described embodiment, although the pixel array  11  is provided with eight pixel lines L, the present disclosure is not limited thereto, and instead, for example, the number of the plurality of pixel lines L may be seven or less, or may be nine or more. 
     For example, in the above-described embodiment, the imaging device  1  performs the imaging operation each time the image of the subject  9  progresses by one pixel line L on the imaging plane S on the basis of the synchronization signal SYNC; however, the present disclosure is not limited thereto. Alternatively, for example, the imaging device may perform the imaging operation each time the image of the subject  9  progresses by two or more pixel lines L on the imaging plane S. 
     For example, in the above-described embodiment, as illustrated in  FIG.  1   , the conveyance controller  92  generates the synchronization signal SYNC, and the imaging device  1  performs the imaging operation on the basis of the synchronization signal SYNC; however, the present disclosure is not limited thereto. Alternatively, for example, the imaging device may generate the synchronization signal SYNC and perform the imaging operation on the basis of the synchronization signal SYNC, and the conveyance controller may control the conveyance operation of the belt conveyor  91  on the basis of the synchronization signal SYNC. 
     For example, in the above-described embodiment, as illustrated in  FIG.  1   , the imaging device  1  is fixed and the subject  9  is moved; however, the present disclosure is not limited thereto. Alternatively, for example, the subject  9  may be fixed and the imaging device  1  may be moved. 
     It should be appreciated that the effects described herein are mere examples and are not limited to those described herein. The disclosure may further include any effects other than those described herein. 
     It is to be noted that the present technology may have the following configurations. According to the present technology having the following configurations, it is possible to enhance image quality. 
     (1) 
     An imaging device including: 
     a pixel array including a plurality of light-receiving pixels, the plurality of light-receiving pixels being separated into a plurality of pixel lines and accumulating electric charge corresponding to a light-receiving amount in an accumulation period, the plurality of pixel lines including a first pixel line and a second pixel line arranged in parallel in a first direction; 
     an exposure controller that sets a time length of the accumulation period in the plurality of light-receiving pixels to one of a plurality of time lengths including a first time length and a second time length in a manner that the plurality of time lengths repeat in predetermined order; and 
     a processor that generates image data on a basis of an accumulation result in the plurality of light-receiving pixels, in which 
     the accumulation period includes a first accumulation period and a second accumulation period each having the first time length, and a third accumulation period and a fourth accumulation period each having the second time length, and 
     the processor generates the image data by adding a plurality of pixel values based on the accumulation result in the first pixel line in the first accumulation period and a plurality of pixel values based on the accumulation result in the second pixel line in the second accumulation period, and adding a plurality of pixel values based on the accumulation result in the first pixel line in the third accumulation period and a plurality of pixel values based on the accumulation result in the second pixel line in the fourth accumulation period. 
     (2) 
     The imaging device according to (1), in which a time length between respective central timings of two of the accumulation periods that are adjacent to each other is constant. 
     (3) 
     The imaging device according to (1) or (2), in which 
     the exposure controller changes the time length of the accumulation period each time a predetermined period elapses, and 
     a timing difference between a central timing of the first accumulation period and a central timing of the third accumulation period is equal to a time length of the predetermined period. 
     (4) 
     The imaging device according to (3), in which the exposure controller sets the time length of the accumulation period on a basis of a synchronization signal supplied from an outside. 
     (5) 
     The imaging device according to (4), in which 
     the synchronization signal includes a plurality of pulses including a first pulse and a second pulse, and 
     the exposure controller
         sets a start timing of the first accumulation period and an end timing of the first accumulation period on a basis of the first pulse, and   sets a start timing of the third accumulation period and an end timing of the third accumulation period on a basis of the second pulse.
 
(6)
       

     The imaging device according to any one of (1) to (5), in which the processor includes
         a first storage associated with the first time length, and   a second storage associated with the second time length,       

     uses the first storage to add the plurality of pixel values based on the accumulation result in the first pixel line in the first accumulation period and the plurality of pixel values based on the accumulation result in the second pixel line in the second accumulation period, and 
     uses the second storage to add the plurality of pixel values based on the accumulation result in the first pixel line in the third accumulation period and the plurality of pixel values based on the accumulation result in the second pixel line in the fourth accumulation period. 
     (7) 
     The imaging device according to any one of (1) to (6), in which the image data includes first image data based on the accumulation result in the accumulation period having the first time length and second image data based on the accumulation result in the accumulation result having the second time length. 
     (8) 
     The imaging device according to any one of (1) to (7), in which each of the plurality of light-receiving pixels converts an analog signal corresponding to the electric charge accumulated in the accumulation period into the pixel value which is a digital value. 
     (9) 
     The imaging device according to (8), in which each of the plurality of light-receiving pixels accumulates the electric charge corresponding to the light-receiving amount in the accumulation period, and converts, in a predetermined period after the accumulation period, the analog signal corresponding to the electric charge accumulated in the accumulation period into the pixel value which is the digital value. 
     (10) 
     The imaging device according to any one of (1) to (7), further including an AD converter that converts an analog signal corresponding to the electric charge accumulated in each of the plurality of light-receiving pixels into the pixel value which is a digital value. 
     (11) 
     The imaging device according to any one of (1) to (10), in which the exposure controller sets the accumulation period each time an image of a subject progresses in the first direction by a predetermined number of pixel lines on an imaging plane of the pixel array. 
     This application claims the benefit of Japanese Priority Patent Application JP2020-108113 filed with the Japan Patent Office on Jun. 23, 2020, the entire contents of which are incorporated herein by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations, and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.