JP2013058857A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce deterioration of a picture quality caused by a dark current, without prolonging a period for taking an image of one frame.SOLUTION: An imaging apparatus comprises: an imaging device 4 comprising plural sets of first and second photoelectric converters PDA and PDB adjacent to each other; and control means which performs control such that, after simultaneous and temporary resetting of the first photoelectric converters PDA of the respective sets and exposing of the first and second photoelectric converters PDA and PDB of the respective sets, the second photoelectric converters PDB of the respective sets are reset simultaneously and temporarily without resetting the first photoelectric converters PDA of the respective sets and thereafter, signals accumulated in the first and second photoelectric converters PDA and PDB of the respective sets are read.

Description

  The present invention relates to an imaging apparatus.

  In an imaging device such as a digital camera, an XY address type solid-state imaging device such as a CMOS image sensor is generally used as an imaging device. As a characteristic of the image pickup device, it is known that dark current becomes fixed pattern noise and deteriorates the image quality of the picked up image.

  Patent Document 1 below discloses an electronic still camera that can reduce image quality degradation due to dark current. That is, in the following Patent Document 1, an XY address type solid-state imaging device having a plurality of pixels and capable of reset scanning and readout scanning of the plurality of pixels, and a light beam incident on the XY address type solid-state imaging device are transmitted or transmitted. In the electronic still camera, comprising: a shutter to be shut off; a drive system that resets and / or reads and scans a plurality of pixels of the XY address type solid-state imaging device; and a control system that controls the shutter and the drive system. The system resets the XY address type solid-state image sensor by setting the shutter in a state of blocking light rays incident on the XY address type solid-state image sensor, and then the light beam incident on the XY address type solid-state image sensor. The XY address type solid-state image sensor is predetermined in the order of reset and readout. Electronic still camera, wherein the controller controls the shutter and the drive system so as to shoot a still image of one frame by scanning at time intervals is disclosed.

JP 2003-18464 A

  However, in the conventional electronic still camera disclosed in Patent Document 1, an XY address type solid-state image sensor is scanned at a predetermined time interval in the order of reset and readout, and a still image of one frame is photographed. The time for capturing the frame image becomes long, and for example, the frame speed decreases.

  The present invention has been made in view of such circumstances, and it is desirable to provide an imaging apparatus capable of reducing image quality degradation due to dark current without lengthening the time for capturing an image of one frame. Objective.

  The following aspects are presented as means for solving the problems. The image pickup apparatus according to the first aspect resets the image pickup device having a plurality of sets of first and second photoelectric conversion units adjacent to each other and the first photoelectric conversion unit of each set at the same time, and then After exposing the first and second photoelectric conversion units of the set, after resetting the second photoelectric conversion unit of each set at the same time without resetting the first photoelectric conversion unit of each set, Control means for performing control to read out signals accumulated in the first and second photoelectric conversion units of each set.

  The imaging device according to a second aspect is the first aspect, wherein the image reading device reads out from the first photoelectric conversion unit based on a signal read out from the second photoelectric conversion unit for each of the sets. And a correction means for correcting the dark signal so that the dark current component is reduced.

  In the imaging device according to a third aspect, in the second aspect, either one of the light receiving area of the first photoelectric conversion unit and the light receiving area of the second photoelectric conversion unit is larger than the other, or both are the same. When the light receiving area of the first photoelectric conversion unit is expressed as a times the light receiving area of the second photoelectric conversion unit, the correction unit may perform the first operation for each set. The signal read from the first photoelectric conversion unit is corrected by subtracting a signal obtained by multiplying the signal read from the second photoelectric conversion unit by a from the signal read from the photoelectric conversion unit. Is.

  In the imaging device according to a fourth aspect, in any one of the first to third aspects, the control unit outputs the signals accumulated in the first and second photoelectric conversion units of the respective sets. Control is performed so that the data is sequentially read for each row of the set.

  In the imaging device according to a fifth aspect, in any one of the first to fourth aspects, the imaging device is provided in common with respect to the first and second photoelectric conversion units for each of the sets and is transferred. A charge-voltage conversion unit that converts the generated charge into a voltage, and a signal corresponding to the potential of the charge-voltage conversion unit that is provided in common to the first and second photoelectric conversion units for each set An amplifying unit, and a transfer switch that is provided corresponding to each of the first and second photoelectric conversion units for each set, and transfers charges from the first and second photoelectric conversion units to the charge voltage conversion unit, respectively. And a reset switch that is provided in common to the first and second photoelectric conversion units for each set and resets the potential of the charge-voltage conversion unit.

  The imaging device according to a sixth aspect is the imaging device according to any one of the first to fifth aspects, wherein the imaging element is provided corresponding to each column of the plurality of sets, and the first and second sets of corresponding columns. A vertical signal line to which a signal accumulated in the second photoelectric conversion unit is supplied and a signal corresponding to the signal of the vertical signal line are sampled and held according to a sampling control signal, and the held signal is horizontally scanned. A sample-and-hold unit that supplies a signal to a horizontal signal line according to a signal, and the sample-and-hold unit is provided corresponding to each of the vertical signal lines and stores a signal read from the first photoelectric conversion unit. And a second storage capacitor provided corresponding to each vertical signal line and storing a signal read from the second photoelectric conversion unit.

  According to the present invention, it is possible to provide an imaging apparatus capable of reducing image quality degradation due to dark current without increasing the time for capturing an image of one frame.

1 is a schematic block diagram illustrating an imaging apparatus according to an embodiment of the present invention. It is a circuit diagram which shows schematic structure of the image pick-up element in FIG. It is a schematic plan view which shows typically a part of pixel area | region of the image pick-up element in FIG. 2 is a timing chart schematically showing an example of the operation of the imaging apparatus shown in FIG. 1 and signals obtained thereby. 5 is a timing chart showing a part of FIG. 4 in detail. It is a timing chart which shows typically operation of an imaging device by a comparative example, and a signal obtained by this. It is a timing chart which shows a part in FIG. 6 in detail.

  Hereinafter, an imaging device according to the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic block diagram showing an imaging apparatus 1 according to an embodiment of the present invention. The imaging device 1 according to the present embodiment is configured as an electronic camera.

  A photographing lens 2 is attached to the imaging apparatus 1 according to the present embodiment. The photographing lens 2 is driven by the lens control unit 5 for focus and diaphragm. In the image space of the photographic lens 2, the imaging surface of the imaging element 4 is arranged. A mechanical shutter (mechanical shutter) 3 is provided between the photographic lens 2 and the image sensor 4 and is controlled by the shutter control unit 6 to open / close (pass and block) the incident light from the photographic lens 2 with respect to the image sensor 4. ) In the present embodiment, a vertical running shutter is used as the mechanical shutter 3, but a horizontal running shutter or the like may be used. Further, instead of the mechanical shutter 3, other shutters such as a liquid crystal shutter can be used.

  The imaging device 4 is driven by a control signal output from the imaging control unit 7 and outputs a pixel signal. The pixel signal output from the image sensor 4 is processed through the signal processing unit 8 and the A / D conversion unit 9 and then temporarily stored in the memory 10. The memory 10 is connected to the bus 17. Also connected to the bus 17 are a lens control unit 5, a shutter control unit 6, an imaging control unit 7, a microprocessor 11, a recording unit 12, a focus calculation unit 13, an image processing unit 14, an image compression unit 15, and the like. An operation unit 16 such as a release button is connected to the microprocessor 11. A recording medium 12a is detachably attached to the recording unit 12. The signal processing unit 8 and the A / D conversion unit 9 may be mounted on the image sensor 4.

  FIG. 2 is a circuit diagram showing a schematic configuration of the image sensor 4 in FIG. The image sensor 4 is configured as an XY address type solid-state image sensor. In the present embodiment, the imaging device 4 includes a plurality of pixels PX (only 2 × 2 pixels PX shown in FIG. 2) arranged in a two-dimensional manner, a vertical scanning circuit 21, and a horizontal scanning circuit. 22, a vertical signal line 23 provided corresponding to each column of the pixels PX and supplied with an output signal (pixel signal) of the pixel PX in the corresponding column, and a constant current source 24 connected to each vertical signal line 23. And. Needless to say, the number of pixels PX is not limited.

  Each pixel PX is provided in common to two photodiodes PDA and PDB that are close to each other as first and second photoelectric conversion units that generate and store charges according to incident light, and the photodiodes PDA and PDB. The floating capacitance unit FD as a charge-voltage conversion unit that converts the charges transferred from the photodiodes PDA and PDB into a voltage, and the charge is transferred from the photodiode PDA to the floating capacitance unit FD provided corresponding to the photodiode PDA. Common to the photodiodes PDA and PDB, the transfer transistor TXA serving as the transfer switch to be transferred, the transfer transistor TXB serving as the transfer switch provided corresponding to the photodiode PDB and transferring the charge from the photodiode PDB to the floating capacitor FD Provided in the flow A reset switch for resetting the potential of Ingu capacitor portion FD, and a selection transistor SEL serving as a selector for selecting the pixels PX, and is connected as shown in FIG. In the present embodiment, the transistors AMP, TX, RES, and SEL of the pixel PX are all nMOS transistors. In FIG. 2, VDD is a power supply voltage.

  FIG. 3 is a schematic plan view schematically showing a part of the pixel region (region where the pixels PX are two-dimensionally arranged) of the image sensor 4 in FIG. In FIG. 3, the X-axis direction indicates the row direction (horizontal direction), and the Y-axis direction orthogonal to the X-axis direction indicates the column direction (vertical direction). In FIG. 3, 3 × 3 pixels PX are shown, and among the components of the pixel PX, only the photodiodes PDA and PDB and the microlenses MLA and MLB that collect the incident light to the photodiodes PDA and PDB, respectively, Simplified and shown. In the following description, it is assumed that the light receiving area of the photodiode PDA is a times the light receiving area of the photodiode PDB. In the present embodiment, the light receiving area of the photodiode PDA is larger than the light receiving area of the photodiode PDB, and a> 1. However, the light receiving area of the photodiode PDA may be the same as the light receiving area of the photodiode PDB or may be smaller than the light receiving area of the photodiode. That is, a = 1 may be sufficient and a <1 may be sufficient.

  Note that the arrangement of the two photodiodes PDA and PDB in the pixel PX is not necessarily limited to the oblique arrangement as shown in FIG. Further, instead of providing the microlenses MLA and MLB for the two photodiodes PDA and PDB, respectively, one microlens may be provided in common for the two photodiodes PDA and PDB.

  Referring to FIG. 2 again, the gate of the transfer transistor TXA is connected to a signal line for guiding the control signal φTXA from the vertical scanning circuit 21 for each pixel row. The gate of the transfer transistor TXB is connected to a signal line that guides the control signal φTXB from the vertical scanning circuit 21 for each pixel row. The gate of the reset transistor RES is connected to a signal line that guides the control signal φRES from the vertical scanning circuit 21 for each pixel row. The gate of the selection transistor SEL is connected to a signal line that guides the control signal φSEL from the vertical scanning circuit 21 for each pixel row.

  The photodiodes PDA and PDB generate signal charges according to the amount of incident light (subject light). The transfer transistor TXA is turned on during the high level period of the control signal φTXA, and transfers the charge accumulated in the photodiode PDA to the floating capacitor FD. The transfer transistor TXB is turned on during the high level period of the control signal φTXB, and transfers the charge accumulated in the photodiode PDB to the floating capacitor FD. The reset transistor RES is turned on during the high level period of the control signal φRES and resets the floating capacitor FD. By simultaneously setting the control signal φTXA and the control signal φRES to the high level and simultaneously turning on the transfer transistor TXA and the reset transistor RES, the charge accumulated in the photodiode PDA is reset. By simultaneously setting the control signal φTXB and the control signal φRES to the high level and simultaneously turning on the transfer transistor TXB and the reset transistor RES, the charge accumulated in the photodiode PDB is reset. Here, simultaneously resetting the photodiodes PDA of all the pixels PX is referred to as global resetting of the photodiodes PDA, and simultaneously resetting the photodiodes PDB of all the pixels PX is referred to as global resetting of the photodiodes PDB.

  The amplification transistor AMP includes a source follower circuit having a drain connected to the power supply voltage VDD, a gate connected to the floating capacitor FD, a source connected to the drain of the selection transistor SEL, and the constant current source 24 as a load. It is composed. The amplification transistor AMP outputs a signal corresponding to the voltage value of the floating capacitor unit FD to the vertical signal line 23 via the selection transistor SEL. The selection transistor SEL is turned on during the high level period of the control signal φSEL, and connects the source of the amplification transistor AMP to the vertical signal line 23.

  The vertical scanning circuit 21 outputs control signals φSEL, φRES, φTXA, and φTXB for each pixel row under the control of the imaging control unit 7 to control vertical scanning. In FIG. 2, (n) attached to the control signal indicates that the control signal is a signal in the nth row. The horizontal scanning circuit 22 controls the horizontal scanning by outputting a horizontal scanning signal φH for each column under the control of the imaging control unit 7. (m) attached to φH indicates that the signal is in the m-th column.

  The image pickup device 4 samples and holds signals according to the signals of the vertical signal lines 23 according to the sampling control signals φTVN and φTVS, and holds the held signals according to the horizontal scanning signal φH. A sample hold unit 25 for supplying to 26S is provided. In the present embodiment, each vertical signal line 23 is directly connected to the sample hold unit 25. However, even if each vertical signal line 23 is connected to the sample hold unit 25 via a column amplifier (not shown). Good.

  In the present embodiment, the sample hold unit 25 stores the storage capacitors CS and CN provided corresponding to each vertical signal line 23 and the signal corresponding to the signal of each vertical signal line 23 according to the sampling control signal φTVS. Sampling switch TVS for accumulating in CS, sampling switch TVN for accumulating a signal corresponding to the signal of each vertical signal line 23 in accumulating capacitor CN according to sampling control signal φTVN, and a signal stored in accumulating capacitor CS for horizontal scanning signal φH The horizontal transfer switch THS for supplying the horizontal signal line 26S to the horizontal signal line 26S and the horizontal transfer switch THN for supplying the signal accumulated in the storage capacitor CN to the horizontal signal line 26N according to the horizontal scanning signal φH. Output amplifiers APS and APN are connected to the horizontal signal lines 26S and 26N, respectively. In this embodiment, the switches TVS, TVN, THS, and THN are all nMOS transistors.

  The gates of the sampling switches TVS are connected in common, and a sampling control signal φTVS is supplied from the imaging control unit 7 thereto. When the sampling switch TVS is turned on according to the sampling control signal φTVS, a signal corresponding to the signal on the vertical signal line 23 is stored in the corresponding storage capacitor CS. The gates of the sampling switches TVN are connected in common, and a sampling control signal φTVN is supplied from the imaging control unit 7 thereto. When the sampling switch TVN is turned on according to the sampling control signal φTVN, a signal corresponding to the signal on the vertical signal line 23 is stored in the corresponding storage capacitor CS.

  For each column, the gates of the horizontal transfer switches THS and THN are connected in common, and the horizontal scanning signal φH of the corresponding column is supplied from the horizontal scanning circuit 22 to the gate. When the horizontal transfer switches THS and THN of each column are turned on in response to the horizontal scanning signal φH of each column, the signals stored in the storage capacitors CS and CN of the corresponding column are respectively transmitted to the horizontal signal lines 26S and 26N. 1 is output to the signal processing unit 8 in FIG. 1 via the output amplifiers APS and APN, respectively. Although not shown in the drawing, the signal processing unit 8 performs correction processing to be described later under the control of the microprocessor 11.

  Further, the imaging device 4 includes horizontal line reset transistors RSTS and RSTN for resetting the horizontal signal lines 26S and 26N to a predetermined potential VREF at a predetermined timing in accordance with the horizontal line reset control signal φRSTH.

  FIG. 4 is a timing chart schematically showing an example of the operation of the imaging apparatus 1 according to the present embodiment and signals obtained thereby. In the following description, the accumulation operation of the photodiode in the exposure state (the state in which the mechanical shutter 3 is opened) reflected in the read signal is referred to as main exposure, and the signal obtained by the main exposure is referred to as the main signal. Further, the accumulation operation of the photodiode in the light-shielded state (the state where the mechanical shutter 3 is closed) reflected in the read signal is referred to as dark exposure, and the signal obtained by dark exposure is referred to as a dark signal. In the dark exposure, dark current is accumulated in the photodiode.

  FIG. 4A shows the travel positions of the front and rear curtains of the mechanical shutter 3 with time as the horizontal axis and the vertical (Y-axis direction, column direction) pixel position (that is, pixel row position) as the vertical axis. 4 schematically shows the global reset timing of the photodiode PDA, the readout timing of the pixel PX, and the main exposure and dark exposure periods of the photodiode PDA. FIG. 4B shows the travel positions of the front and rear curtains of the mechanical shutter 3 with time as the horizontal axis and the vertical (Y-axis direction, column direction) pixel position (ie, pixel row position) as the vertical axis. 4 schematically shows the global reset timing of the photodiode PDB, the readout timing of the pixel PX, and the dark exposure period of the photodiode PDB.

  FIG. 4C schematically shows the main signal and the dark signal constituting the readout signal of the photodiode PDA of the pixel PX at the pixel row position K (position on the final readout row) with the horizontal axis as time. . FIG. 4D schematically shows a dark signal constituting the readout signal of the photodiode PDB of the pixel PX at the pixel row position K with the horizontal axis as time. FIG. 4E schematically shows the main signal and the dark signal constituting the signal after correction of the photodiode PDA of the pixel PX at the pixel row position K with the horizontal axis as time.

  FIG. 4F schematically shows the main signal and the dark signal constituting the readout signal of the photodiode PDA of the pixel PX at the pixel row position J (position on the first readout row side) with the horizontal axis as time. Yes. FIG. 4G schematically shows a dark signal constituting the readout signal of the photodiode PDB of the pixel PX at the pixel row position J with the horizontal axis as time. FIG. 4H schematically shows the main signal and the dark signal constituting the signal after correction of the photodiode PDA of the pixel PX at the pixel row position K with the horizontal axis as time.

  In the present embodiment, as shown in FIG. 4, both photodiodes PDA and PDB are globally reset at time t0. This global reset is performed by the imaging control unit 7 controlling the imaging device 4 under the control of the microprocessor 11. Note that since the photodiode PDB is globally reset at time t100, which will be described later, the photodiode PDB does not necessarily need to be globally reset at time t0.

  The photodiode PDA is darkly exposed from time t0. As with the photodiode PDA, dark current is accumulated in the photodiode PDB from the time point t0. However, since the photodiode PDB is globally reset at a time point t100 described later, dark exposure is not performed before the time point t100.

  Subsequently, the mechanical shutter 3 is controlled by the shutter control unit 6 under the control of the microprocessor 11 so that the front curtain of the mechanical shutter 3 travels and the main exposure of the photodiode PDA is started. As the front curtain of the mechanical shutter 3 travels, charge accumulation by incident light is started in the photodiode PDB as in the photodiode PDA. However, since the photodiode PDB is globally reset at a time point t100 described later, The main exposure is not performed before t100.

  Next, the mechanical shutter 3 is controlled by the shutter control unit 6 under the control of the microprocessor 11, so that after the exposure time has elapsed since the front curtain of the mechanical shutter 3 traveled, the rear curtain of the mechanical shutter 3 traveled. Then, the main exposure of the photodiode PDA is completed. At the time t100 when the main exposure of the photodiodes PDA of all the pixels PX is completed, only the photodiode PDB is globally reset without resetting the photodiode PDA of any pixel PX. This global reset is performed by the imaging control unit 7 controlling the imaging device 4 under the control of the microprocessor 11.

  Thereafter, the imaging control unit 7 controls the imaging device 4 under the control of the microprocessor 11 to sequentially read out the pixels PX for each pixel row from the pixel PX in the first row to the pixel PX in the last row. The signal readout of the pixel PX (photodiode PDA, PDB signal readout) will be described later with reference to FIG. Note that the reading order of pixel rows is not necessarily limited to the order of arrangement of pixel rows.

  The period in which the charge constituting the signal of the photodiode PDA to be read is actually accumulated is the period from the time point t0 when the photodiode PDA is globally reset to the time point when the photodiode PDA is read. ) And FIG. 4F, the photodiode PDA of the pixel PX at the position (row) K to be read later is more than the photodiode PDA of the pixel PX at the position (row) J to be read earlier. The amount of dark signal accumulation increases.

  Since the traveling speed of the mechanical shutter 3 is generally sufficiently faster than the pixel readout, the dark signal amount during the period from the time point t0 to the front curtain traveling timing of the mechanical shutter 3 is from the rear curtain traveling timing of the mechanical shutter 3 to the pixel readout time point. It can be said that it is extremely small compared to the dark signal amount in the period. Difference between the amount of dark signal in the signal of the photodiode PDA of the pixel PX at the position (row) K and the amount of dark signal in the signal of the photodiode PDA of the pixel PX at the position (row) K (that is, each pixel For the case where the signal of the PX photodiode PDA is used as an image signal as it is, the amount of dark signal during the period from the rear curtain running timing of the mechanical shutter 3 to the pixel readout time is dominant.

  On the other hand, the period in which the charge constituting the signal of the photodiode PDB to be read is actually accumulated is the period from the time t100 when the photodiode PDB is globally reset immediately before the signal reading to the time when the photodiode PDB is read. Therefore, the readout signal of the photodiode PDB of the pixel PX at each position (row) K, J is only a dark signal as shown in FIGS. 4 (d) and 4 (g).

  Since the photodiodes PDA and PDB of the same pixel PX are arranged close to each other, the amount of charge of the dark signal component accumulated in the photodiode PDA of the pixel PX from the time point t100 to the reading time of the pixel PX is It can be said that this is proportional to the amount of total charges accumulated in the photodiode PDB of the pixel PX. In the present embodiment, since the light receiving area of the photodiode PDA is a times the light receiving area of the photodiode PDB, the dark signal accumulated in the photodiode PDA of the pixel PX from the time t100 to the reading time of the pixel PX. It can be said that the amount of charge of the component is equal to a times the amount of total charge accumulated in the photodiode PDB of the pixel PX.

  Therefore, for each pixel PX, by subtracting a signal obtained by multiplying the readout signal of the photodiode PDB of the pixel PX by a from the readout signal of the photodiode PDA of the pixel PX, the time t100 accumulated in the photodiode PDA. To the dark signal component from when the pixel PX is read out. Therefore, for each pixel PX, a signal obtained by subtracting a signal obtained by multiplying the readout signal of the photodiode PDB of the pixel PX by a from the readout signal of the photodiode PDA of the pixel PX is a corrected signal of the pixel PX. Then, as shown in FIGS. 4E and 4H, the dark signal component included in the corrected signal of the pixel PX at any position (row) is reduced and becomes substantially the same. Vertical dark shading caused by dark current is reduced, and image quality deterioration due to dark current can be reduced.

  Here, the reading operation of the pixel PX after the time t100 in FIG. 4 will be described with reference to FIG.

  After time t100 in FIG. 4, the pixels PX are sequentially selected row by row, and the same operation is sequentially performed for each row. FIG. 5 is a timing chart showing a part of FIG. 4 in detail, and shows the operation when the pixel PX in the n-th row is selected.

  A period t1-t14 is a selection period of the pixels PX in the n-th row. In a period t1-t14, φSEL (n) is set to a high level, and the selection transistor SEL in the n-th row is turned on.

  When the selection period of the pixel PX in the n-th row is started at time t1, first, φRES (n) is set to a high level during the period t2-t3, the reset transistor RES in the n-th row is turned on, and the n-th row The floating capacitor FD is reset. Next, in a period t4-t5, φTXB (n) is set to the high level, and the transfer transistor TXB in the n-th row is turned on. As a result, the accumulated charge (dark signal) of the photodiode PDB of the pixel PX in the n-th row is transferred to the floating capacitor FD of the pixel PX in the n-th row. Next, in the period t6 to t7, φTVN is set to the high level, and the signal of the photodiode PDB of the pixel PX in the n-th row is accumulated in the storage capacitor CN.

  Thereafter, in a period t8-t9, φRES (n) is set to a high level, the n-th row reset transistor RES is turned on, and the n-th row floating capacitor FD is reset. Next, in a period t10 to t11, φTXA (n) is set to a high level, and the transfer transistor TXA in the n-th row is turned on. As a result, the accumulated charge (main signal + dark signal) of the photodiode PDA of the pixel PX in the n-th row is transferred to the floating capacitor FD of the pixel PX in the n-th row. Next, in the period t12 to t13, φTVS is set to the high level, and the signal of the photodiode PDA of the pixel PX in the n-th row is accumulated in the storage capacitor CS.

  Next, in a period t13 to t14, φH of each column is sequentially set to the high level to perform horizontal scanning, and the signal (dark signal) of the photodiode PDB and the photodiode PDA stored in the storage capacitors CN and CS, respectively. Signals (main signal + dark signal) are output to the horizontal signal lines 26N and 26S, respectively, and output to the signal processing unit 8 in FIG. 1 via the output amplifiers APN and APS, respectively.

  The signal processing unit 8 subtracts a signal obtained by multiplying the readout signal of the photodiode PDB of the pixel PX output from the output amplifier APN by a from the readout signal of the photodiode PDA of each pixel PX output from the output amplifier APS. A signal is obtained as a signal after correction of the pixel PX. The correction of the photodiode PDA based on the readout signal of the photodiode PDB is not necessarily limited to subtracting a signal obtained by multiplying the readout signal of the photodiode PDB by a from the readout signal of the photodiode PDA. Based on the read signal of the PDB, the read signal of the photodiode PDA may be corrected so that the dark current component is reduced.

  The corrected signal of each pixel PX obtained by the signal processing unit 8 is converted into a digital signal by the A / D conversion unit 9 and further temporarily stored in the memory 10. When the entire image based on the corrected signal is stored in the memory 10, the microprocessor 11 performs desired processing in the image processing unit 14 or the image compression unit 15 as necessary based on a command from the operation unit 16. The processed image is output to the recording unit 12 and recorded on the recording medium 12a.

  Instead of obtaining a corrected signal for each pixel PX by the signal processing unit 8, the readout signal of the photodiode PDA of each pixel PX output from the output amplifier APS and the photo of the pixel PX output from the output amplifier APN. After the signal read from the diode PDB is amplified by the signal processing unit 8, the signal is converted into a digital signal by the A / D conversion unit 9, temporarily stored in the memory 10, and then stored in the memory 10 by the image processing unit 14. An image obtained by multiplying the image by the readout signal from the photodiode PDB in the memory 10 by a times (the level by a times) may be subtracted from the image by the readout signal from the photodiode PDA.

  Here, an imaging device according to a comparative example compared with the imaging device 1 according to the present embodiment will be described. FIG. 6 is a timing chart schematically showing the operation of the image pickup apparatus according to this comparative example and signals obtained thereby, and corresponds to FIG. FIG. 7 is a timing chart showing a part of FIG. 6 in detail, showing the operation when the pixel PX in the n-th row is selected, and corresponds to FIG.

  The imaging device according to this comparative example is different from the imaging device 1 according to the present embodiment in that the photodiode PDB and the transfer transistor TXB are removed from each pixel PX in the imaging device 4 and the operation shown in FIG. Instead, the operation shown in FIG. 6 is performed, the operation shown in FIG. 7 is performed instead of the operation shown in FIG. 5 when the pixel PX in the n-th row is selected, and the readout signal of the photodiode PDA Is used as it is as a captured image without being corrected.

  In this comparative example, the operation shown in FIG. 6A is performed instead of the operation shown in FIG. The operation shown in FIG. 4A is different from the operation shown in FIG. 6A. In this comparative example, since there is no photodiode PDB, only the photodiode PDA is globally reset at time t0 and at time t100. Then, the photodiode PDB is not globally reset. The signal in FIG. 6B is the same as the signal in FIG. 4C, and the signal in FIG. 6C is the same as the signal in FIG.

  The operation shown in FIG. 7 differs from the operation shown in FIG. 5 in this comparative example in that there is no φTXB (n) because there is no transfer transistor TXB, and in the selection period of the pixel PX in the n-th row, the period t8-t9. In this case, φRES (n) is maintained at the low level, the reset transistor RES in the n-th row is maintained in the off state, and the same applies to the selection periods of the pixels PX in the other rows. Thus, in this comparative example, a general correlated double sampling (CDS) process is realized by taking the difference between the signals stored in the storage capacitors CN and CS, respectively.

  In this comparative example, as can be seen from the above description, the photodiode PDA is read from the time point t0 when the photodiode PDA is globally reset during the period in which the charge constituting the signal of the photodiode PDA to be read is actually accumulated. 6 (b) and FIG. 6 (c), the photodiode PDA of the pixel PX at the position (row) K to be read later is the position to be read first (see FIG. 6B and FIG. 6C). Row) The dark signal accumulation amount is larger than that of the photodiode PDA of the pixel PX of J.

  However, in this comparative example, since the readout signal of the photodiode PDA is used as it is as a captured image without being corrected, the dark signal component is not reduced at all, and the vertical dark shading caused by the dark current is reduced at all. In other words, image quality deterioration due to dark current cannot be reduced at all.

  On the other hand, in this embodiment, as described above, the readout signal of the photodiode PDA is corrected for each pixel PX by the readout signal of the photodiode PDB indicating only the dark signal. ), The dark signal component included in the corrected signal of the pixel PX is also reduced to be approximately the same, so that the vertical dark shading caused by the dark current is reduced, and image quality deterioration due to the dark current can be reduced.

  In this embodiment, since the readout signal of the photodiode PDA is corrected for each pixel PX by the readout signal of the photodiode PDB, not only when the dark current has a distribution in the vertical direction, but also the dark current is horizontal. Even when there is a distribution in the direction, the influence of the dark current can be reduced, and also from this point, image quality deterioration due to the dark current can be further reduced.

  Furthermore, in the present embodiment, unlike the imaging device disclosed in Patent Document 1 described above, a single frame of still images is not captured by scanning at predetermined time intervals in the order of reset and readout. The time for capturing an image of one frame does not become long, and the frame speed does not decrease.

  As described above, according to the present embodiment, it is possible to reduce image quality degradation due to dark current without increasing the time for capturing an image of one frame.

  In the operation shown in FIGS. 4 and 5, as described above, the readout signal of the photodiode PDA is corrected based on the readout signal of the photodiode PDB, and the corrected signal is mainly the main signal of the photodiode PDA. Consists of. Conversely, the readout signal of the photodiode PDB can be corrected based on the readout signal of the photodiode PDA, and the corrected signal can be mainly composed of the main signal of the photodiode PDB. In this case, the operation shown in FIGS. 4 and 5 may be modified as follows. That is, in this case, at least the photodiode PDB is globally reset at time t0, the photodiode PDA is globally reset without globally resetting the photodiode PDB at time t100, and φTXA ( The high level period of n) and the high level period of φTXB (n) are interchanged, and the same applies to the selection periods of the pixels PX in the other rows, and the signal processing unit 8 performs the photo of the pixel PX for each pixel PX. The corrected signal of the pixel PX may be obtained by subtracting a signal obtained by multiplying the read signal of the photodiode PDA of the pixel PX by 1 / a from the read signal of the diode PDB.

  In the present embodiment, the light receiving area of the photodiode PDA is larger than the light receiving area of the photodiode PDB. Therefore, in the operation shown in FIGS. 4 and 5, since the corrected signal mainly consists of the main signal of the photodiode PDA, the sensitivity to incident light is increased. Therefore, the operations shown in FIGS. 4 and 5 are suitable for imaging a low-luminance subject. On the other hand, in the operation obtained by modifying the operation shown in FIGS. 4 and 5 as described above, the corrected signal mainly consists of the main signal of the photodiode PDB, so that the sensitivity to the incident light is lowered, and the luminance of the subject is reduced. Since it does not saturate even if it is high, it is suitable for imaging a high-luminance subject. Therefore, it is preferable that the operation mode shown in FIGS. 4 and 5 and the operation mode modified as described above can be switched. This switching may be performed, for example, by a command from the operation unit 16 or may be automatically performed by the microprocessor 11 based on the luminance of the subject.

  In other operation modes, if the charge stored in the photodiode PDA and the charge stored in the photodiode PDB are simultaneously transferred to the floating capacitor FD, and the both charges are added and read, the image quality degradation due to dark current is reduced. Although it cannot be avoided, it is possible to perform imaging with higher sensitivity than when the photodiodes PDA and PDB are alone.

  As mentioned above, although embodiment of this invention and its modification were demonstrated, this invention is not limited to these. For example, in the above-described embodiment, all the pixels PX have two photodiodes PDA and PDB, but some of the pixels PX (for example, pixels having a low dark current component) You may have only a diode.

DESCRIPTION OF SYMBOLS 1 Imaging device 4 Imaging element 7 Imaging control part 8 Signal processing part 11 Microprocessor PDA, PDB Photodiode

Claims (6)

An imaging device having a plurality of sets of first and second photoelectric conversion units adjacent to each other;
After resetting the first photoelectric conversion units of each set at the same time and then exposing the first and second photoelectric conversion units of each set, the first photoelectric conversion units of each set are reset. Control means for performing control to read out the signals accumulated in the first and second photoelectric conversion units of each set, after simultaneously resetting the second photoelectric conversion units of each set without simultaneously
An imaging apparatus comprising:   Correction for correcting the signal read from the first photoelectric conversion unit for each set based on the signal read from the second photoelectric conversion unit so that the dark current component is reduced. The imaging apparatus according to claim 1, further comprising means.   Either one of the light receiving area of the first photoelectric conversion unit and the light receiving area of the second photoelectric conversion unit is larger than or equal to the other, and the light receiving area of the first photoelectric conversion unit is the first. When the light receiving area of the second photoelectric conversion unit is expressed as a times, the correction unit is configured to output the second photoelectric conversion signal from the signal read from the first photoelectric conversion unit for each set. The imaging apparatus according to claim 2, wherein the signal read from the first photoelectric conversion unit is corrected by subtracting a signal obtained by multiplying the signal read from the conversion unit by a.   4. The control unit according to claim 1, wherein the control unit performs control so that signals accumulated in the first and second photoelectric conversion units of each set are sequentially read out for each row of each set. 5. The imaging device according to any one of the above.   The image sensor includes a charge-voltage conversion unit that is provided in common for the first and second photoelectric conversion units for each of the groups and converts the transferred charge to a voltage, and the first for each of the groups. And an amplifier that is provided in common to the second photoelectric converter and outputs a signal corresponding to the electric potential of the charge-voltage converter, and corresponds to the first and second photoelectric converters for each set. A transfer switch that transfers charges from the first and second photoelectric conversion units to the charge-voltage conversion unit, and is common to the first and second photoelectric conversion units for each set. The imaging apparatus according to claim 1, further comprising: a reset switch that is provided and resets a potential of the charge-voltage conversion unit. The imaging device is provided corresponding to each column of the plurality of sets, and a vertical signal line to which signals accumulated in the first and second photoelectric conversion units of the corresponding column set are supplied, and the vertical A sample hold unit that samples and holds a signal according to the signal of the signal line according to the sampling control signal, and supplies the held signal to the horizontal signal line according to the horizontal scanning signal,
The sample-and-hold unit is provided corresponding to each vertical signal line, and corresponds to each first storage capacitor in which a signal read from the first photoelectric conversion unit is stored, and each vertical signal line. The imaging apparatus according to claim 1, further comprising a second storage capacitor that is provided and stores a signal read from the second photoelectric conversion unit.

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