JP2014107738A - Imaging device and control method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce deterioration in image quality due to an increase in dark current caused by heat generation in reading operation by an image sensor.SOLUTION: A correction amount in horizontal OB clamp processing is changed in consideration of temperature of an image sensor and set photographing sensitivity.

Description

  The present invention relates to an imaging apparatus and a control method thereof.

  An imaging apparatus such as a digital camera or a video camera includes an imaging sensor (imaging device) such as a CMOS image sensor. In recent years, the number of pixels of an image sensor has been increased, and the size of each pixel of the image sensor has been reduced accordingly. When the pixel size is reduced, the area of the light receiving surface of the photoelectric conversion unit (for example, photodiode) in each pixel is also reduced, so that the level of the optical signal corresponding to the charge generated in the photoelectric conversion unit is reduced. And a noise level becomes large relatively and S / N deteriorates. In order to improve the S / N, it is necessary to reduce fixed pattern noise and dark shading.

  By the way, in the imaging apparatus as described above, the OB clamp processing for correcting the dark shading component in the image signal of the effective pixel region is performed using the output signal of the light shielding pixel region of the imaging sensor. In the OB clamp processing, the processing accuracy may be reduced due to the influence of temperature, photographing sensitivity, and the like, which may cause image quality degradation. As a countermeasure, Patent Document 1 proposes to change the OB clamp region or the OB clamp cycle in accordance with the temperature and the imaging sensitivity in an imaging device including a CCD image sensor capable of multi-field readout. .

JP 2007-36332 A

  As the OB clamping process, there is a first horizontal OB clamping process in which the black level reference signal and the offset of the corresponding row are gradually corrected at an arbitrary interval. In addition, there is a second horizontal OB clamping process for adjusting to the black level when the black level reference signal and the offset of the corresponding row exceed an arbitrary threshold value. There are image pickup apparatuses that perform these two types of horizontal OB clamping.

  In recent years, an increase in dark current due to a rise in temperature of an image sensor has become remarkable, and there is a concern about the influence on image quality. In particular, there may be a case where image quality deterioration due to an increase in dark current is more conspicuous as the readout timing is delayed as a result of the image sensor generating heat during the signal readout operation. In that case, the offset of the black level reference signal increases in the row where the dark current component of the image sensor exceeds the correction step amount of the first horizontal OB clamp process. When the black level reference signal and the offset of the corresponding row exceed an arbitrary threshold value, correction by the second horizontal OB clamping process is performed. However, performing the second horizontal OB clamping process may increase the horizontal stripes appearing in the image and degrade the image quality.

  An object of the present invention is made in view of the above problems, and an imaging apparatus capable of forming a high-quality image by performing OB clamping in consideration of temperature, imaging sensitivity, and the like in the imaging apparatus, and driving thereof It aims to provide a method.

  The present invention has been made to achieve the above object, and the imaging device according to claim 1 is a light-shielding region in which a plurality of pixels are arranged in a row direction and a column direction, and light-shielded pixels are arranged. An imaging device having an effective area in which pixels that are not shielded are arranged, a first generator that generates an offset signal based on a signal read from the pixels in the shielded area, and the offset signal Second generating means for generating first correction data for correcting the black level using the first correction means for correcting the black level of the image signal using the first correction data, and the temperature of the image sensor. Correction of black level in the correction means taking into account the temperature detection means for detecting the image, the sensitivity setting means for setting the imaging sensitivity, the temperature detected by the temperature detection means, and the imaging sensitivity set by the sensitivity setting means Change quantity Characterized by comprising control means for controlling the so that, a.

  According to a fourth aspect of the present invention, there is provided a method for controlling an imaging apparatus, wherein a plurality of pixels are arranged in a row direction and a column direction, a light shielding region where light-shielded pixels are arranged, and a pixel where non-light-shielded pixels are arranged. An image sensor having a region, a first generation unit that generates an offset signal based on a signal read from a pixel in the light shielding region, and first correction data that corrects a black level using the offset signal A second generation unit that generates the first correction data, a first correction unit that corrects the black level of the image signal using the first correction data, a temperature detection unit that detects the temperature of the image sensor, and an imaging sensitivity. A method for controlling an imaging apparatus including a sensitivity setting unit, wherein the black level in the first correction unit is calculated by taking into account the temperature detected by the temperature detection unit and the imaging sensitivity set by the sensitivity setting unit. Change the correction amount It is characterized in.

  According to the present invention, since the image sensor generates heat during the read operation, the OB clamp can follow the black level variation caused by the dark current component that increases as the read timing is delayed.

1 is an overall block diagram of an imaging apparatus. The CMOS image sensor whole layout figure. CMOS image sensor pixel circuit diagram. The figure which shows the structure of AFE. The figure which shows the horizontal OB clamp process which concerns on 1st Embodiment. 3 is a flowchart relating to operation control of the imaging system according to the first embodiment. The figure which shows the matrix table of the step amount of the 1st horizontal OB clamp which concerns on 1st Embodiment. 9 is a flowchart relating to operation control of the imaging system according to the second embodiment. The figure which shows the horizontal OB clamp process which concerns on 2nd Embodiment. The figure which shows the matrix table of the step amount of the 1st horizontal OB clamp which concerns on 2nd Embodiment. The figure which shows the horizontal OB clamp process which concerns on 2nd Embodiment.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(First embodiment)
FIG. 1 is an overall block diagram of an imaging apparatus according to the first embodiment of the present invention. The photographing lens 1 forms an image of a subject on the imaging surface of an imaging sensor (imaging device) 2. A shutter and a diaphragm (not shown) are provided between the photographing lens 1 and the image sensor 2. The imaging sensor 2 such as a CMOS image sensor generates and outputs an image signal corresponding to the image of the subject formed on the imaging surface. In addition, the image sensor 2 is provided with a light-shielded pixel region as described later, and a black level reference signal is output from the light-shielded pixel region.

  The analog front end (AFE) 3 generates an offset signal based on a black level reference signal read from the light-shielded pixel area of the image sensor 2 and performs a horizontal OB clamping process, which will be described later, to generate a dark shading component of the image signal. Remove. Then, the AFE 3 performs AD conversion on the image signal that has been subjected to the horizontal OB clamping process, and outputs a digital signal.

  The timing generator (TG) 8 generates various drive signals for driving the image sensor 2 and supplies the generated drive signals to the image sensor 2. Examples of the drive signal to be supplied include a timing signal for controlling the start and end of charge accumulation, a signal readout control signal (horizontal synchronization signal, vertical synchronization signal, transfer signal, etc.) for each pixel. The temperature detection unit 9 detects the temperature of the imaging sensor 2 and outputs temperature information to the control unit 7.

  The control unit 7 performs overall control of each unit. Further, the temperature information output from the temperature detection unit 9 is notified to the AFE 3 and the image processing unit 6. The image processing unit 6 performs various correction processes and development processes on the digital signal output from the AFE 3 to generate image data. The memory unit 4 is used as a working memory in the developing process by the image processing unit 6 and a buffer memory when the developing process of the image processing unit 6 is not in time when the imaging operation is continued. The display unit 5 converts the image data into an analog signal for display, and displays an image corresponding to the converted signal on the display.

  Next, the configuration of the image sensor 2 will be described. In the present embodiment, an example in which a CMOS image sensor is used as the imaging sensor 2 will be described. FIG. 2 is a diagram showing an overall layout of the CMOS image sensor. In FIG. 2, the CMOS image sensor 101 has a plurality of pixels arranged in the row direction and the column direction, and an effective pixel area (hereinafter referred to as an effective area) and a light-shielded pixel area (hereinafter referred to as a light-shielded area). Have. The effective area 104 is an area where pixels that are not shielded from light are arranged. The light shielding area is an area where a light-shielded pixel among a plurality of pixels is arranged, and includes a vertical optical black area (hereinafter referred to as a VOB area) 103 and a horizontal optical black area (hereinafter referred to as a HOB area) 102. Including. The signals read from the VOB area 103 and the HOB area 102 include a dark current component or a signal component due to a shift in the reference level (black level) due to temperature fluctuation. Therefore, it is used to correct the dark shading component in the subject exposure signal read from the effective area 104. The signal read from the VOB area 103 is used for correcting the dark shading component in the horizontal direction. The signal read from the HOB area 102 is used for correcting dark shading components in the vertical direction.

  Next, FIG. 3 is a circuit configuration diagram of each pixel in the pixel array of the CMOS image sensor of FIG. In the pixel 110, a photodiode (hereinafter referred to as PD) 111 generates and accumulates charges corresponding to incident light. A transfer switch (hereinafter referred to as “TX”) 112 transfers charges generated in the PD 111 to a floating diffusion (hereinafter referred to as “FD”) 114. The FD 114 is equivalently a capacitor, and converts the charge transferred from the PD 111 into a voltage. The amplifier 115 is configured by a MOS transistor, and outputs a signal corresponding to the voltage of the FD 114 to the column signal line 117 by performing a source follower operation together with a constant current source (not shown) connected to the column signal line 117. Turning on the selection switch 116 brings the pixel 110 into a selected state, and turning it off turns the pixel 110 into a non-selected state. The reset switch 113 resets the FD 114.

  Returning to FIG. 2, the vertical scanning circuit 108 selects a row from which a signal in the pixel array is to be read, and drives the pixels in each column in the selected row so that the signal in the selected row is read out to the reading circuit 105. The readout circuit 105 performs CDS processing for obtaining a difference between an optical signal (S signal) and a noise signal (N signal) output from a pixel in each column of the row selected by the vertical scanning circuit 108. By this processing, the readout circuit 105 obtains and holds an image signal of each column of pixels from which fixed pattern noise inherent in the CMOS image sensor is removed. The fixed pattern noise includes noise generated when the reset switch 113 resets the FD 114, noise due to variation in the threshold voltage of the MOS transistor constituting the amplifier 115 for each pixel, and the like. The horizontal scanning circuit 107 sequentially selects the pixel signals of each column held in the readout circuit 105 and transfers them to the output amplifier 106. The output amplifier 106 amplifies and outputs the transferred pixel signal.

  FIG. 4 is a block diagram of the configuration of the AFE 3 shown in FIG. The AFE 3 performs horizontal OB clamping processing of the image signal output from the effective area based on the output signal of the HOB area 102. An analog signal output from the image sensor 2 is amplified by a programmable gain amplifier (PGA) 801. The analog-digital converter (ADC) 802 converts the signal amplified by the PGA 801 from an analog format into, for example, a 14-bit digital format and outputs the converted signal.

  When the output signal of the HOB area, which is a light-shielding area for each row of the pixel array, is input, the OB clamp unit 803 samples a signal for a predetermined number of pixels from that point. This operation is performed while the signal of the clamp signal generator 806 is input. The OB clamp unit 803 receives a clamp target value from the target level setting unit 805. The offset in which the difference between the output signal of the HOB area and the clamp target value becomes zero, that is, the output signal of the HOB area approaches the clamp target value by a value obtained by multiplying the aforementioned difference by a predetermined gain. A signal (horizontal OB clamp correction amount) is generated.

  The offset signal generated by the OB clamp unit 803 is converted from a digital format to an analog format by a digital-analog converter (DAC) 804 and fed back to the PGA 801. In the PGA 801, the offset adjustment of the image signal is performed by the offset signal (horizontal OB clamp correction amount), and the dark level of the image signal output from the effective area is adjusted to the clamp target value. Then, the PGA 801 amplifies the image signal subjected to the offset adjustment with the gain adjusted by the control unit 7 and outputs it. By adjusting the gain of the PGA 801, it is adjusted to the imaging sensitivity set by the sensitivity setting unit (not shown).

  Here, since the offset signal (horizontal OB clamp correction amount) is integrated as the row for reading the signal in the pixel array advances, it follows only a loose change. As described above, the target level setting unit 805 inputs the clamp target value to the OB clamp unit 803, but the value can be set arbitrarily.

  FIG. 5 illustrates an output level when the horizontal OB clamping process is performed in the imaging system according to the present embodiment. As shown in FIG. 5A, in the horizontal OB clamp process, the calculation unit 807 refers to the cumulative step amount of the horizontal OB clamp correction amount from the row located on the screen to the previous row sequentially. Then, the horizontal OB clamp correction amount of the target row is calculated by adding / subtracting the correction amount up to the previous row to the integration result of the pixel output of the HOB area 102 of the target row.

  In the target row, after the addition / subtraction, a comparison is made with a clamp target value that is a dark level as a reference, and it is determined whether the image signal of the corresponding row has an offset component in the positive or negative direction. Then, the deviation direction (positive / negative) between the image signal after applying the correction amount up to the previous line and the dark level is determined, and a preset step amount is determined with respect to the image signal after addition / subtraction. Add / subtract. In this way, the black level (dark level) is gradually approached each time the line moves down the screen. The step amount set in advance is a value that does not cause overcorrection due to the influence of random noise or the like, and is generally set to a value of 1 LSB or less.

  FIG. 6 is a flowchart relating to operation control of the imaging system according to the present embodiment. FIG. 7 illustrates a matrix table of the set imaging sensitivity and the temperature of the imaging sensor 2 for selecting the step amount of the horizontal OB clamp unit 33 in the imaging system according to the present embodiment. . The table shown in FIG. 7 is stored in the memory unit 4.

  In FIG. 6, when the power supply is turned on and the imaging sequence is started (F201), the shutter is opened and the imaging sensor 2 is exposed (F202). Next, the set imaging sensitivity is confirmed (F203), and further the temperature of the imaging sensor 2 is confirmed (F204). Based on these pieces of information, the table shown in FIG. 7 is referred to and it is determined whether or not to change the step amount of the horizontal OB clamp unit 33 (F205). For example, when the imaging sensitivity is ISO 6400 or lower and the temperature of the imaging sensor 2 is 40 ° C. or lower, as shown in FIG. 5A, the horizontal level is set to the initial horizontal OB clamp unit 33 step amount (1/8 LSB). OB clamping operation is performed.

  On the other hand, in some cases, the horizontal OB clamp operation in the initial setting cannot follow the fluctuation of the output level accompanying the increase in the dark current component due to the temperature rise of the image sensor 2. That is, as shown in FIG. 5B, in the operation of sequentially reading the pixel signals of the image sensor 2 in the vertical direction, the dark current component increases and the influence becomes larger in the lower row of the screen where the readout timing is delayed. The image signal fluctuates. Therefore, the row average value in the lower area of the screen shifts in the positive direction with respect to the black level. 5B, even if the horizontal OB clamping operation is performed with a preset step amount, the change in the image signal of the image sensor 2 due to the influence of the dark current component is more than the preset step amount. Becomes larger and it becomes difficult to adjust to the black level.

  For example, when the imaging sensitivity is ISO 25600 or higher and the temperature of the imaging sensor is 41 ° C. or higher, the table shown in FIG. 7 is referred to. Then, as shown in FIG. 5C, the step amount of the horizontal OB clamp unit 33 is changed to a step amount (1/4 LSB) that is twice a preset value (F207), and the horizontal OB clamp operation is performed ( F208). As can be seen from FIG. 5C, the HOB clamp operation can follow the change in the pixel signal of the image sensor 2 due to the influence of the dark current component, and can be adjusted to the black level (dark level). . Then, development processing including correction processing for image signals other than the horizontal OB clamping processing is performed (F209), and the imaging sequence is terminated (F210).

  As described above, according to the first embodiment, during the reading operation, the temperature, imaging sensitivity, and the like are taken into account (considering the fluctuation of the black level accompanying the increase in the dark current component due to the temperature rise of the imaging sensor 2). ) To perform horizontal OB clamping. And it becomes possible to follow a horizontal OB clamp operation. Accordingly, it is possible to provide an imaging apparatus capable of forming a higher quality image and a driving method thereof.

(Second Embodiment)
In the first embodiment described above, the horizontal OB clamping operation performed for noise such as horizontal stripe noise and horizontal smear having a large offset from the black level reference signal is not considered. Whether the horizontal OB clamping process in the first embodiment is the first horizontal OB clamping process, and in this embodiment, a threshold is provided for the offset from the black level reference signal, and the second horizontal OB clamping process is performed. Determine whether. The second horizontal OB clamping process does not gradually follow the black level reference signal as in the first horizontal OB clamping process, but is adapted to the black level reference signal at once.

  In the imaging system that performs these two types of horizontal OB clamping processing, the step amount by the first horizontal OB clamping processing is not appropriate in a situation where the dark current component is increased due to temperature rise during the reading operation of the imaging sensor 2. There is a possibility of degrading image quality. For example, when the change in the dark current component is larger than the step amount by the first horizontal OB clamp, the second horizontal OB clamp process is performed, and the horizontal stripe noise of the image is deteriorated. Therefore, a method for appropriately controlling the first horizontal OB clamping process while considering the second horizontal OB clamping process will be described in the second embodiment. The configuration of the imaging device is the same as that of the first embodiment.

  FIG. 8 is a flowchart illustrating operation control of the imaging system according to the second embodiment. FIG. 9 illustrates an output level when the first horizontal OB clamping process is performed in consideration of the operation of the second horizontal OB clamping process in the imaging system according to the second embodiment. The operation for confirming the imaging sensitivity set from the start of the imaging sequence and the temperature of the imaging sensor 2 is the same as F201 to F204 of the first embodiment shown in FIG.

  Here, as a result of confirming the set imaging sensitivity and the temperature of the imaging sensor 2, if it is determined that the step amount of the first horizontal OB clamping operation needs to be changed, the following control is performed. . That is, a threshold value for performing the second horizontal OB clamping operation (a black level reference signal obtained from the HOB area 102 in each row and an allowable value of the black level offset amount in the image signal after the first HOB clamping processing operation is performed) ) Is confirmed (F305). Note that the second horizontal OB clamping process is performed by the image processing unit 6. As shown in FIG. 10, the step amount of the first horizontal OB clamping process is determined from the set imaging sensitivity and the temperature table of the imaging sensor 2 for each threshold value at which the second horizontal OB clamping operation is performed. Is done. As in FIG. 7, the matrix table is stored in the memory unit 4.

  In the second horizontal OB clamping process, as in the first horizontal OB clamping process, the image signal is not corrected by a preset step amount with reference to the step amount up to the previous row. That is, the black level reference signal and the black level offset amount in the image signal after the first HOB clamp processing are set as the step amount for each corresponding row. Therefore, it is easily affected by random noise or the like, and it is necessary to limit the application by providing the threshold value. That is, the first horizontal OB clamping process is intended to correct a slowly changing vertical shading component, and the second horizontal OB clamping process is intended to correct a sharp component such as horizontal stripe noise and lateral smear. It is different in point.

  For example, as shown in FIG. 11, first, the first HOB clamping process is performed, and the black level is gradually adjusted from the line located on the screen. Then, in each row, the image processing unit 6 determines whether the black level offset amount in the image signal after the first HOB clamp processing operation is performed exceeds an arbitrary threshold value. In a row where the offset amount is determined to exceed an arbitrary threshold (fourth row in FIG. 11), the offset amount is added / subtracted from the image signal of the corresponding row, and is adjusted to the black level.

  FIG. 9A shows a case where the step amount of the first horizontal OB clamping operation is set without considering the threshold value for performing the second horizontal OB clamping operation.

  In the lower row of the screen, which is later than the start of the read operation on the first row, the image signal fluctuates due to the dark current component of the image sensor 2, and the threshold value of the second horizontal OB clamp operation is set to the black value of the image signal. The level offset amount will be exceeded. As a result, the second horizontal OB clamping operation is performed in the lower area of the screen. As described above, the second horizontal OB clamp is different from the first horizontal OB clamp in that the black level in the image signal is adjusted to the black level reference signal in each row at once, so that the influence of random noise or the like is in each row. If they are different, they are visually recognized as horizontal stripe noise, which may cause image quality degradation. Therefore, as described above, the optimum step of the first horizontal OB clamping operation is considered in consideration of the imaging sensitivity and temperature state set in the imaging sensor 2 and the threshold value for performing the second horizontal OB clamping operation. Make a quantity selection. As a result, as shown in FIG. 9B, the optimum first horizontal OB clamping operation can be performed without being affected by the second horizontal OB clamping operation.

  As described above, according to the second embodiment, during the reading operation of the imaging sequence, the temperature, the imaging sensitivity, and the second sensitivity against the fluctuation of the black level accompanying the increase in the dark current component due to the temperature rise of the imaging sensor 2. In consideration of the horizontal OB clamping operation, the first horizontal OB clamping is performed. In this way, the first horizontal OB clamping operation can be made to follow the reference black level. Accordingly, it is possible to provide an imaging apparatus capable of forming a higher quality image and a driving method thereof.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

(Other examples)
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

2 Imaging sensor 3 AFE
DESCRIPTION OF SYMBOLS 7 Control part 8 Timing generation part 9 Temperature detection part 101 CMOS image sensor 102 Horizontal optical black area | region 103 Vertical optical black area | region 104 Effective area | region 801 Gain control amplifier 802 AD converter 803 OB clamp part 804 DA converter

Claims (6)

A plurality of pixels arranged in a row direction and a column direction, an image sensor having a light shielding region in which light-shielded pixels are arranged, and an effective region in which pixels that are not light-shielded are arranged;
First generation means for generating an offset signal based on a signal read from a pixel in the light shielding region;
Second generation means for generating first correction data for correcting the black level using the offset signal;
First correction means for correcting the black level of the image signal using the first correction data;
Temperature detecting means for detecting the temperature of the image sensor;
Sensitivity setting means for setting the imaging sensitivity;
Control means for controlling to change the correction amount of the black level in the correction means in consideration of the temperature detected by the temperature detection means and the imaging sensitivity set by the sensitivity setting means;
An imaging apparatus comprising: Third generation means for generating second correction data different from the first correction data;
Second correction means for correcting the image signal using the second correction data;
A determination unit that determines whether to implement the second correction unit by setting a threshold value with respect to the signal amount of the offset signal generated by the first generation unit;
The control means changes a black level correction amount in the first correction means in consideration of a threshold value of the offset signal for determining whether or not to execute the second correction means. Item 2. The imaging device according to Item 1.   The control means considers the threshold value of the offset signal for determining whether or not to implement the second correction means, the temperature detected by the temperature detection means, and the photographing sensitivity set by the sensitivity setting means. The imaging apparatus according to claim 1, wherein the black level correction amount in the first correction unit is changed. A plurality of pixels arranged in a row direction and a column direction, an image sensor having a light shielding region in which light-shielded pixels are arranged, and an effective region in which pixels that are not light-shielded are arranged;
First generation means for generating an offset signal based on a signal read from a pixel in the light shielding region;
Second generation means for generating first correction data for correcting the black level using the offset signal;
First correction means for correcting the black level of the image signal using the first correction data;
Temperature detecting means for detecting the temperature of the image sensor;
A method for controlling an imaging apparatus including sensitivity setting means for setting imaging sensitivity,
A control method for an image pickup apparatus, wherein the black level correction amount in the first correction means is changed in consideration of the temperature detected by the temperature detection means and the image pickup sensitivity set by the sensitivity setting means. The imaging apparatus includes: a third generation unit that generates second correction data different from the first correction data; a second correction unit that corrects an image signal using the second correction data; And a determination unit that determines whether to perform the second correction unit by providing a threshold value with respect to the signal amount of the offset signal generated by the first generation unit,
5. The black level correction amount in the first correction unit is changed in consideration of a threshold value of the offset signal for determining whether or not to execute the second correction unit. Control method of imaging apparatus.   Considering the threshold value of the offset signal for determining whether to execute the second correction unit, the temperature detected by the temperature detection unit, and the imaging sensitivity set by the sensitivity setting unit, the first 5. The method of controlling an imaging apparatus according to claim 4, wherein the correction amount of the black level in the correcting means is changed.

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