JP5129650B2 - Imaging device - Google Patents

Description

  The present invention relates to an imaging apparatus.

  In recent years, imaging sensors such as CMOS image sensors have been used for digital single-lens reflex cameras and video cameras. In an image sensor, the size of a pixel decreases as the number of pixels increases. When the pixel size is reduced, the area of the light receiving surface of the photoelectric conversion unit (for example, a photodiode) in the pixel is also reduced, so that the level of the optical signal corresponding to the charge generated in the photoelectric conversion unit is also reduced. As a result, it has become increasingly necessary to reduce fixed pattern noise and dark shading so as not to deteriorate the S / N.

  A technique for reducing fixed pattern noise and dark shading will be described with reference to FIGS. 9 and 10 while explaining the configuration of the CMOS image sensor 70. FIG. 9 is a diagram showing an overall layout of the CMOS image sensor 70. FIG. 10 is a circuit configuration diagram of a pixel.

  The CMOS image sensor 70 includes the following components.

  In the pixel array PA, a plurality of pixels are arranged in the row direction and the column direction. The pixel array PA has an effective area EA and a light shielding area SA. The effective area EA is an area 73 where pixels that are not shielded from light are arranged. The light shielding area SA is an area where a light-shielded pixel among a plurality of pixels is arranged, and includes a vertical optical black area (VOB area) 72 and a horizontal optical black area (HOB area) 71. Since the signals read from the VOB area 72 and the HOB area 71 include a dark current component or a deviation of the reference level (black level) due to temperature fluctuation and do not include a signal component, dark shading in the signal read from the effective area Used to correct the component. The signal read from the VOB area 72 is used for correcting the dark shading component in the horizontal direction. The signal read from the HOB area 71 is used for correcting the dark shading component in the vertical direction.

  Each pixel 60 in the pixel array PA includes a photodiode (hereinafter referred to as PD) 61, a transfer switch (hereinafter referred to as TX) 62, a floating diffusion (hereinafter referred to as FD) 64, an amplifier 65, a selection switch 66, and a reset switch. 63.

  The PD 61 generates and accumulates charges corresponding to light. The TX 62 transfers the charge generated in the PD 61 to the FD 64. The FD 64 is equivalently a capacitor, and converts the transferred charge into a voltage. The amplifier 65 is composed of a MOS transistor, and outputs a signal corresponding to the voltage of the FD 64 to the column signal line 67 by performing a source follower operation together with a constant current source connected to the column signal line 67. The selection switch 66 puts the pixel 60 into a selected state when turned on, and puts the pixel 60 into a non-selected state when turned off. The reset switch 63 resets the FD 64.

  The vertical scanning circuit 76 selects a pixel row from which a signal is to be read out in the pixel array PA, and drives the pixels in each column of the selected row so that the signal is read out to the reading circuit 75.

  The readout circuit 75 performs CDS processing for obtaining a difference between the optical signal (S signal) output from the pixel in each column of the selected row and the noise signal (N signal). As a result, the readout circuit 75 obtains the image signal of the pixel in each column from which the fixed pattern noise unique to the CMOS image sensor is removed. The readout circuit 75 holds the obtained image signal of the pixel in each column.

  This fixed pattern noise includes noise generated when the reset switch 63 resets the FD 64, noise due to variation in the threshold voltage of the amplifier (MOS transistor) 65 from pixel to pixel, and the like. This fixed pattern noise is almost removed (corrected) by performing the CDS process.

  The horizontal scanning circuit 77 sequentially selects the image signals of the pixels in each column held in the readout circuit 75 and transfers them to the output amplifier 74.

  The output amplifier 74 amplifies and outputs the transferred image signal. As a result, in the subsequent stage of the output amplifier 74, the dark shading component in the image signal of the pixel in the effective region is corrected (OB correction is performed) using the image signal of the pixel in the light shielding region.

  This dark shading includes, in addition to dark current shading, voltage shading based on the impedance of the power supply line, as inherent to the CMOS image sensor. Since the dark shading component in the vertical direction changes gently in the pixel array PA and may cause line flaws in the resulting image if there are flawed pixels, a low pass filter is applied to the output signals from multiple rows It is corrected by.

  On the other hand, the power line may receive external noise as an antenna. As a result, the power supply voltage supplied to the pixel via the power supply line may fluctuate in a predetermined cycle, and noise that fluctuates in a predetermined cycle may be mixed in a signal read from the pixel. In this case, different levels of noise are mixed in the S signal and N signal read out from the pixel at different timings. Therefore, even if the CDS process is performed, the noise cannot be completely removed.

On the other hand, in Patent Document 1, it is assumed that the sampling cycle of the N signal and the S signal is sufficiently short with respect to the cycle of fluctuation of the power supply voltage, and the N signal is acquired a plurality of times at a predetermined time interval, It has been proposed to acquire the S signal at the predetermined time interval for the signal. Thus, according to Patent Document 1, it is said that an image signal from which temporally varying noise is removed can be obtained.
JP 2008-42573 A

  Noise that fluctuates in a predetermined cycle when a noise source is mounted on a power supply is a type of voltage shading in the sense that it is caused by fluctuations in the power supply voltage supplied to the pixel via the power supply line. You can think of it. In the following description, it is assumed that the noise that fluctuates in the predetermined cycle is voltage shading.

  In Patent Document 1, it is assumed that only low frequency components are included in voltage shading. However, voltage shading includes a component having a high frequency. Since the high frequency component fluctuates in a cycle shorter than the sampling cycle of the N signal and the S signal, it cannot be removed even using the technique of Patent Document 1. Thereby, horizontal stripe noise may be conspicuous in the obtained image.

  On the other hand, a signal read from the HOB area 71 in the pixel array PA shown in FIG. 9 is subtracted from the signal read from the effective area EA (by performing OB correction), thereby including a high-frequency component. It is also conceivable to correct the voltage shading.

  However, when the noise source is mounted on the power supply, the pixels in the light shielding area SA shielded by the metal layer that can be capacitively coupled to the power supply line and the pixels in the effective area EA not shielded by such a metal layer are: The magnitude of the voltage shading received may vary. For this reason, even if a signal read from a pixel in the light shielding area SA is subtracted by a signal read from a pixel in the effective area EA, a shading component may remain in the corrected signal. As a result, horizontal stripe noise may be noticeable in the obtained image.

  An object of the present invention is to suppress horizontal stripe noise in an image obtained by performing OB correction.

An imaging apparatus according to the present invention includes a light-shielding region in which a plurality of pixels are arranged in a row direction and a column direction, and a light-shielded pixel among the plurality of pixels is disposed, and a pixel that is not light-shielded among the plurality of pixels. A pixel array having an effective area, and a generation unit that generates an offset signal based on a signal read from a pixel in the light shielding area, a black level in the light shielding area, and a black level in the effective area A correction signal is generated by applying a coefficient determined so as to reduce the difference from the offset signal to generate a correction signal, and using the generated correction signal, correction for correcting the black level of the signal read from the pixel in the effective area and a section, wherein the correcting unit, the offset signal into a plurality of frequency components, added by multiplying a coefficient corresponding to the frequency in each of the separated plurality of frequency components By causing I, and obtaining the correction signal.

  According to the present invention, horizontal stripe noise in an image obtained by performing OB correction can be suppressed.

  In the present specification, the description will be made assuming that the noise that fluctuates in the predetermined cycle is a component of voltage shading in the sense that it is caused by fluctuations in the power supply voltage supplied to the pixels via the power supply line.

  The present inventor subtracts the signal read from the HOB area 71 in the pixel array PA shown in FIG. 9 from the signal read from the effective area EA in order to correct voltage shading including a high frequency component. As a result, OB correction was performed. However, horizontal stripe noise was still conspicuous in the image obtained by the signal.

  As a result of the study by the present inventor, the reason why the horizontal stripe noise in the image cannot be suppressed is that the influence of the noise source, that is, the voltage shading differs between the light shielding area SA and the effective area EA depending on where the noise source is placed. I found out that it was the cause.

  Specifically, when the noise source is mounted on the power supply, the potential of the power supply line or the reference potential of the semiconductor substrate itself is fluctuated, and the fluctuation of the potential is a metal that shields the pixels in the light shielding region SA by capacitive coupling. Transmitted to the layer. The pixels in the light-shielding area SA that are shielded from light by a metal layer that can be capacitively coupled to the power supply line or the reference electrode of the semiconductor substrate, and the pixels in the effective area EA that are not shielded from light by such a metal layer are caused by potential fluctuations. There is a tendency that the magnitude of voltage shading is different. In this case, even when the signal read from the HOB area 71 is subtracted from the signal read from the effective area EA, the voltage shading cannot be removed.

  On the other hand, when the noise source is mounted on the readout circuit 75, the capacitive coupling between the readout circuit 75 and the metal layer that shields the pixels in the light shielding area SA is at a negligible level. Thereby, the magnitude of the voltage shading received due to the noise source is equal between the pixels in the light shielding area SA and the pixels in the effective area EA. In this case, the voltage shading can be removed by subtracting the signal read from the HOB area 71 from the signal read from the effective area EA.

  The difference in voltage shading between the light shielding area SA and the effective area EA when the noise source is mounted on the power supply will be described in more detail with reference to FIG. FIG. 11 is a diagram illustrating an example of a cross-sectional configuration of the CMOS image sensor 80.

  The CMOS image sensor 80 includes a pixel array PA. The pixel array PA includes a light shielding area SA and an effective area EA.

  In the light shielding area SA, the pixels P1 and P2 that are shielded by the metal layer 83a are arranged. In the pixels P1 and P2, the light that has passed through the microlens ML is blocked by the metal layer 83a and does not reach the photodiode 88a. The metal layer 83a functions as a light shielding layer.

  In the effective area EA, pixels P3 and P4 that are not shielded by the metal layer 83b are arranged. In the pixel P3 and the pixel P4, the light that has passed through the microlens ML passes through the opening region defined by the metal layers 83b, 84, and 85 and reaches the photodiode 88a.

  Here, when comparing the light shielding area SA and the effective area EA, the area of the metal layer 83a is larger than the area of the corresponding metal layer 83b. Thereby, the coupling capacitance C1 formed between the metal layer 83a and the FD 87a in the light shielding region SA becomes larger than the coupling capacitance C2 formed between the metal layer 83b and the FD 87b in the effective region EA. For this reason, even if the potential fluctuations of the metal layer 83a and the metal layer 83b are equivalent when the noise source is mounted on the power source, the fluctuation of the potential transmitted to the FD 87a via the capacitive coupling is not affected. , Larger than the fluctuation of the potential transmitted to the FD 87b. As a result, the voltage shading in the light shielding area SA becomes larger than the voltage shading in the effective area EA.

  As a result of experiments by the inventor, the degree of influence of noise such as a power supply depends on the layout, but the effective pixel EA is about 0.4 to 0.8 times the shading pixel SA.

  In this case, if the OB correction is performed by subtracting the signal read from the HOB area 71 in the pixel array PA shown in FIG. 9 from the signal read from the effective area EA, overcorrection occurs. For this reason, it is considered that the horizontal stripe noise was still conspicuous in the image obtained from the signal subjected to the OB correction.

  Therefore, the present inventor has examined which component in the voltage shading is different between the shading pixel SA and the effective pixel EA as shown in FIG. 12A to 12C, the vertical axis indicates the row position of the pixel in the pixel array PA, and the horizontal axis indicates the offset from the actual black level.

  FIG. 12B shows that dark shading in the vertical direction is included in the signal read from the effective area EA in the absence of light. The signal read from the effective area EA includes an offset component that does not change with time and a voltage shading component that changes gradually with time. The slowly changing voltage shading component is due to circuit impedance or the like.

  FIG. 12C shows an offset component and a gradual voltage shading component that are removed by horizontal OB clamping processing by AFE 2 described later.

  FIG. 12A compares the dark shading component in the vertical direction (broken line) in the signal read from the light shielding area SA with the dark shading component in the vertical direction (solid line) in the signal read from the effective area EA. As shown. When the broken line and the solid line are compared, the following can be understood. The offset component, the gradual voltage shading component, and the high-frequency voltage shading component are the same in the light shielding area SA and the effective area EA, whereas the low-frequency voltage shading component is in the light shielding area SA and the effective area EA. Different. The voltage shading component having a low frequency in the light shielding area SA tends to be larger than the voltage shading component having a low frequency in the effective area EA.

  Here, a temporally varying component in voltage shading, that is, a voltage shading component having a high frequency and a voltage shading component having a low frequency are one of the voltage shading received by the FD 64 of the pixel when the noise source is mounted on the power source. It is an ingredient.

  Thus, the present inventor has found that the voltage shading cannot be removed even if OB correction is performed uniformly because the magnitude of the low frequency component in the voltage shading is different between the light shielding area SA and the effective area EA. .

  Next, the imaging apparatus 100 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is an overall block diagram of an imaging apparatus 100 according to the first embodiment of the present invention.

  The shutter 12 is provided in front of the taking lens 11 on the optical path, and controls exposure.

  The photographing lens 11 forms an image of a subject on the imaging surface (pixel array PA) of the imaging sensor 1.

  The diaphragm 13 is provided between the photographing lens 11 and the image sensor 1 on the optical path, and adjusts the amount of light guided to the image sensor 1 after passing through the photographing lens 11.

  The imaging sensor 1 generates an image signal corresponding to the image of the subject formed in the pixel array PA. The pixel array PA includes a light shielding area SA and an effective area EA (see FIG. 9). The imaging sensor 1 reads the black level reference signal from the light shielding area SA, and reads the generated image signal from the effective area EA. The imaging sensor 1 outputs the read black level reference signal and the image signal. The image sensor 1 is, for example, a CMOS image sensor.

  The analog front end (AFE) (generation unit) 2 receives the black level reference signal and the image signal output from the image sensor 1 and performs predetermined analog signal processing. For example, the AFE 2 generates a later-described offset signal based on a black level reference signal read from a pixel in the light shielding area SA by performing a later-described horizontal OB clamping process (OB correction). For example, the AFE 2 generates an image signal from which a slowly changing voltage shading component is removed by performing horizontal OB clamping processing (OB correction). For example, the AFE 2 performs AD conversion on the signals that have been subjected to the horizontal OB clamping process, and generates a digital signal. The AFE 2 outputs the processed digital signal.

  The image processing device (correction unit) 5 receives the digital signal output from the AFE 2. The image processing unit 5 generates a correction signal by applying a coefficient determined so as to reduce the difference between the black level in the light shielding area SA and the black level in the effective area EA to the offset signal. The image processing unit 5 corrects the black level of the image signal read from the pixels in the effective area EA by using the generated correction signal (performs OB correction). Then, the image processing unit 5 performs development processing on the corrected image signal (digital signal) to generate image data. The image processing unit 5 supplies the image data to the display unit 8 or to the recording unit 9 via the control unit 6.

  The display unit 8 converts the image data into an analog signal for display, and displays an image corresponding to the converted signal on the display.

  The recording unit 9 converts the image data into compressed data for recording, and records the converted compressed data on a predetermined recording medium. The predetermined recording medium is, for example, a compact flash memory (registered trademark) detachably connected to the imaging apparatus 100.

  The memory unit 4 is used as a working memory in the development processing by the image processing unit 5 or as a buffer memory when imaging is continuously performed and the development processing by the image processing unit 5 is not in time.

  The operation unit 7 receives a predetermined instruction from the user. The operation unit 7 includes an operation member such as a shutter button.

  The timing generator 10 generates various drive signals that drive the image sensor 1. The timing generator 10 supplies the generated drive signal to the image sensor 1.

  The control unit 6 controls each unit as a whole.

  As described above, the imaging apparatus 100 generates a correction signal by applying the coefficient determined so as to reduce the difference between the black level in the light shielding area SA and the black level in the effective area EA to the offset signal. Then, the black level of the signal read from the pixel in the effective area EA is corrected by using the generated correction signal. Thereby, even if the voltage shading received by the light shielding area SA and the effective area EA is different, the voltage shading can be accurately removed from the image signal. That is, horizontal stripe noise in an image obtained by performing OB correction can be suppressed.

  Next, the configuration of the image sensor 1 will be described with reference to FIGS. 9 and 10.

  The imaging sensor 1 includes the following components.

  In the pixel array PA, a plurality of pixels are arranged in the row direction and the column direction. The pixel array PA has an effective area EA and a light shielding area SA. The effective area EA is an area 73 where pixels that are not shielded from light are arranged. The light shielding area SA is an area where a light-shielded pixel among a plurality of pixels is arranged, and includes a vertical optical black area (VOB area) 72 and a horizontal optical black area (HOB area) 71. Since the signals read from the VOB area 72 and the HOB area 71 include a dark current component or a deviation of the reference level (black level) due to temperature fluctuation and do not include a signal component, dark shading in the signal read from the effective area Used to correct the component. The signal read from the VOB area 72 is used for correcting the dark shading component in the horizontal direction. The signal read from the HOB area 71 is used for correcting the dark shading component in the vertical direction.

  Each pixel 60 in the pixel array PA includes a photodiode (hereinafter referred to as PD) 61, a transfer switch (hereinafter referred to as TX) 62, a floating diffusion (hereinafter referred to as FD) 64, an amplifier 65, a selection switch 66, and a reset switch. 63.

  The PD 61 generates and accumulates charges corresponding to light. The TX 62 transfers the charge generated in the PD 61 to the FD 64. The FD 64 is equivalently a capacitor, and converts the transferred charge into a voltage. The amplifier 65 is composed of a MOS transistor, and outputs a signal corresponding to the voltage of the FD 64 to the column signal line 67 by performing a source follower operation together with a constant current source connected to the column signal line 67. The selection switch 66 puts the pixel 60 into a selected state when turned on, and puts the pixel 60 into a non-selected state when turned off. The reset switch 63 resets the FD 64.

  The vertical scanning circuit 76 selects a pixel row from which a signal is to be read out in the pixel array PA, and drives the pixels in each column of the selected row so that the signal is read out to the reading circuit 75.

  The readout circuit 75 performs CDS processing for obtaining a difference between the optical signal (S signal) output from the pixel in each column of the selected row and the noise signal (N signal). As a result, the readout circuit 75 obtains the image signal of the pixel in each column from which the fixed pattern noise unique to the CMOS image sensor is removed. The readout circuit 75 holds the obtained image signal of the pixel in each column.

  This fixed pattern noise includes noise generated when the reset switch 63 resets the FD 64, noise due to variation in the threshold voltage of the amplifier (MOS transistor) 65 from pixel to pixel, and the like. This fixed pattern noise is almost removed (corrected) by performing the CDS process.

  The horizontal scanning circuit 77 sequentially selects the image signals of the pixels in each column held in the readout circuit 75 and transfers them to the output amplifier 74.

  The output amplifier 74 amplifies and outputs the transferred image signal. As a result, in the subsequent stage of the output amplifier 74, the dark shading component in the image signal of the pixel in the effective region is corrected (OB correction is performed) using the image signal of the pixel in the light shielding region.

  This dark shading includes, in addition to dark current shading, voltage shading based on the impedance of the power supply line, as inherent to the CMOS image sensor. Since the dark shading component in the vertical direction changes gently in the pixel array PA and may cause line flaws in the resulting image if there are flawed pixels, a low pass filter is applied to the output signals from multiple rows It is corrected by.

  Next, the configuration of the AFE 2 will be described with reference to FIG. FIG. 2 is a diagram illustrating the configuration of the AFE 2.

  As shown in FIG. 2, the AFE 2 includes a gain control amplifier (AMP) 21, an analog / digital (AD) converter 23, a horizontal OB clamp function 24, and a digital / analog (DA) converter 22.

  The AMP 21 receives a black level reference signal and an image signal. The AMP 21 includes a subtracter, and subtracts a predetermined correction amount from the image signal. The AMP 21 amplifies the black level reference signal and the subtracted image signal with a gain adjusted by the control unit 6 described later, and outputs the amplified signal. Thereby, the sensitivity is adjusted.

  The AD converter 23 AD-converts analog signals (black level reference signal and image signal) output from the AMP 21 to generate, for example, a 14-bit digital signal. The AD converter 23 outputs the converted digital signal to the subsequent stage or the horizontal OB clamp function 24.

  The horizontal OB clamp function 24 includes a low pass filter (LPF). The horizontal OB clamp function 24 gradually follows the black level reference signal (digital signal) and the black level of the image signal (digital signal) for each row of the pixel array PA. By performing such horizontal OB clamping processing, an offset signal that does not vary with time and a voltage shading component that changes gradually with time are removed from the black level reference signal to generate an offset signal. Further, by performing the horizontal OB clamping process, an offset component that does not vary with time and a voltage shading component that changes gradually with time are removed from the image signal. This horizontal OB clamping process is set so as not to be affected by noise in the light shielding area SA. The horizontal OB clamp function 24 outputs the processed digital signal to the DA converter 22.

  The DA converter 22 DA-converts the digital signal output from the horizontal OB clamp function 24 to generate an analog signal. The DA converter 22 feeds back the generated analog signal to the AMP 21. As a result, the correction amount subtracted from the image signal by the AMP 21 is integrated as the row position in the pixel array PA corresponding to the signal advances, and therefore follows only a loose change.

  Next, the shading correction process performed after the horizontal OB clamp process will be described with reference to the flowchart of FIG. FIG. 3 is a flowchart showing a shading correction process performed after the horizontal OB clamping process. The processing shown in the flowchart of FIG. 3 is performed in software on the image data temporarily stored in the memory unit 4 in order to process in real time the image reading speed. This process is mainly performed by the image processing unit 5 controlled by the control unit 6 into which the firmware has been read. The image processing unit 5 temporarily stores (writes) image signals (digital signals) or image data in the memory unit 4 and reads (reads) image signals (digital signals) or image data from the memory unit 4. .

  In step 1, the shutter 12 is opened and the image sensor 1 is exposed.

  In step 2, the shutter 12 is closed and the signal output from the image sensor 1 is converted into a digital signal by the AFE2. The AFE 2 outputs the converted digital signal (offset signal, image signal) to the image processing unit 5.

  In step 3, the image processing unit 5 temporarily stores the digital signal in the memory unit 4.

  In step 4, the characteristic shading correction process in the present embodiment is started.

  In step 5, n representing a line number is initialized to zero.

  In step 6, n is incremented by one.

  In step 7, the n rows of image signals stored in the memory unit 4 and the n rows of offset signals are averaged. The reason for averaging is to reduce the influence of random noise and scratched pixels.

  In step 8, it is determined whether all lines have been completed. If not completed (N), the process returns to step 6. If all lines have been completed (Y), the process proceeds to step 9.

  In step 9, n representing the row number is initialized to 0 again.

  In step 10, n is incremented by one.

  In step 11, an offset (A) from the black reference level is obtained with respect to the horizontal OB average.

  In step 12, the average (B) of the offset data of ± 3 rows with respect to n rows is calculated (moving average processing of 7 rows). By this processing, offset data can be separated into a plurality of frequency components, and components that change at a frequency lower than the reference frequency can be extracted.

  In step 13, the calculation of A−B + Bxk is performed to obtain a correction amount (X) for n rows.

  Here, the first coefficient (= 1) for the component AB = C changing at a frequency higher than the reference frequency, and the second coefficient k (a value smaller than 1) for the component_B changing at a frequency lower than the reference frequency. A correction amount (correction signal) obtained by adding the value multiplied by can be obtained.

  FIG. 4 is a diagram showing signals processed in step 13. In FIG. 4, D represents an offset component having gentle shading generated by impedance. A indicates a component having a relatively fast change cycle compared to D generated due to power supply fluctuation or the like. The A component includes a B component having a large amplitude difference between the light shielding region SA (HOB region 71) and the effective region EA, and a C component having a small amplitude difference. By calculating A−B + Bxk, that is, by multiplying A−B by the first coefficient and multiplying the B component by the second coefficient, the correction amount X corresponding to the amplitude in the region other than the horizontal OB can be obtained. it can.

  Here, the second coefficient k depends on the layout in the image sensor 1. The component B having a low frequency has a large difference between the light shielding area SA (HOB area 71) and the effective area EA. As a result, the second coefficient k for the component B having a frequency lower than the reference frequency becomes a value of 0.4 or more and 0.8 or less. If the second coefficient k is made smaller than 0.4, the residual voltage shading correction becomes large, and thus horizontal stripe noise may be conspicuous in the obtained image. If the second coefficient k is larger than 0.8, voltage shading overcorrection becomes large, and horizontal stripe noise may be conspicuous in the obtained image.

  When this process is compared with the horizontal OB clamp process in AFE2, the horizontal OB clamp process differs from the process in step 13 in that the coefficient to be applied to the component B having a low frequency is 1. That is, the horizontal OB clamp function 24 integrates by feeding back about several percent of the offset to the input of the AD converter 23 and applying LPF with several tens of lines. As a result, if the period is somewhat loose, the coefficient will be 1 to follow.

  On the other hand, in the processing in step 13, since it is performed independently for each row of the pixel array PA and no feedback is applied, the coefficient (second coefficient) is always constant and does not become 1 even in a loose period.

  On the other hand, the component C having a high frequency has a relatively small difference between the light shielding area SA (HOB area 71) and the effective area EA, so that the coefficient (first coefficient) to be applied to the component C is a value close to 1.

  In step 14, the correction amount X obtained in step 13 is subtracted from the image signal of the effective area EA in the same row, and the image signal after subtraction is written in the memory unit 4.

  In step 15, it is determined according to the variable n indicating the row position whether all image signals have been processed. If not completed (N), the process returns to Step 10, and if completed (Y), the process proceeds to Step 16.

  In step 16, development processing is performed.

  As described above, according to the present embodiment, the influence on the pixel output due to the fluctuation of the power supply is different between the light-shielding area and the effective area, and the light-shielding area and the effective area depend on where the power-supply fluctuation occurs. The correction is made taking into account that there is a difference in the amount of the region. As a result, it has become possible to remove horizontal stripe noise that could not be sufficiently removed. In addition, since dark shading that occurs due to circuit impedance in the image sensor or the like can be determined, dark shading can also be removed.

  In the above embodiment, the frequency classification of the shading change is performed using a moving average of a plurality of rows, but a frequency component may be extracted strictly by discrete cosine transform or the like.

  Further, as illustrated in FIG. 5, the imaging apparatus 100 a may further include a digital front end (DFE) 3. The DFE 3 receives an image signal (digital signal) of each pixel from the AFE 2 and digitally processes image signal correction, pixel rearrangement, and the like. Further, the horizontal OB clamping process may be performed digitally by DFE3 instead of being analogized by AFE2.

  Next, an imaging apparatus according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 6 is a diagram illustrating a sequence of shooting operations performed by the imaging apparatus according to the second embodiment of the present invention. FIG. 7 is a diagram illustrating a portion where horizontal stripe noise occurs in the pixel array. FIG. 8 is a flowchart showing the operation of the imaging apparatus.

  The basic configuration of the imaging apparatus according to the second embodiment is the same as that of the first embodiment, but the operation is different from that of the first embodiment.

  As shown in FIG. 6, the imaging apparatus performs mechanical operations (lens driving, mirror driving, charge driving, etc.) at each timing during the readout period. For this reason, the power source to be shaken is different even during the readout period, and therefore, a phenomenon occurs in which the period of noise (horizontal stripe noise) appearing in the image is different in the pixel array.

  Specifically, as shown in FIG. 7, the shaded portion of the pixel array is a light shielding region, and voltage shading that causes horizontal stripe noise occurs between the light shielding region (HOB region) and the effective region. Since the timing at which the signal is read is different between the upper part and the lower part of the pixel array, the period of the generated voltage shading differs. In addition, there is a large difference in the voltage shading amplitude between the light shielding region (HOB region) and the effective region above the pixel array PA, and the voltage shading amplitude between the light shielding region (HOB region) and the effective region below the pixel array PA. It can be seen that the difference is small.

  On the other hand, noise can be removed by dividing the pixel area PA into areas with different upper and lower or left and right readout timings, and calculating the subtraction amount by changing the coefficient for the offset for each divided area. Specifically, as shown in the flowchart of FIG. 8, processing different from that of the first embodiment is performed in the following points.

  In step 32, it is determined whether or not the number of n lines exceeds half of the total number of lines. If it is less than or equal to one half (Y), the process proceeds to step 33, and if it exceeds one half (N ), Go to step 34.

  In step 33, Axk is calculated to obtain a correction amount (X) for n rows.

  In step 34, A is a correction amount (X) for n rows.

  Thereby, the correction amount calculated | required with the different coefficient with respect to the HOB area | region can be set by the upper part and lower part of pixel array PA.

  In the present embodiment, the case where the readout timing is different in the vertical direction (column direction) of the pixel array PA has been described, but when the readout timing is different in the horizontal direction (row direction), different coefficients are set in the horizontal direction. It will be.

  Further, in the present embodiment, the pixel array is divided into two in different reading timing directions, but it may be divided into two or more, and a coefficient that changes linearly in the pixel array may be set instead of the division. .

1 is an overall block diagram of an imaging apparatus 100 according to a first embodiment of the present invention. The figure which shows the structure of AFE2. It is a flowchart which shows the shading correction process performed after a horizontal OB clamp process. The figure which showed the signal processed in step 13. FIG. The whole block diagram of the imaging device 100a which concerns on the modification of 1st Embodiment of this invention. The figure which shows the sequence of imaging | photography operation | movement by the imaging device which concerns on 2nd Embodiment of this invention. The figure which shows the generation | occurrence | production location of the horizontal stripe noise in a pixel array. 6 is a flowchart showing the operation of the imaging apparatus. The figure for demonstrating background art. The figure for demonstrating background art. The figure for demonstrating the problem which invention intends to solve. The figure for demonstrating the problem which invention intends to solve.

Explanation of symbols

1 Imaging sensor 100, 100a Imaging device

Claims (4)

A plurality of pixels are arranged in a row direction and a column direction, and a light shielding region where a light-shielded pixel among the plurality of pixels is arranged, and an effective region where a pixel that is not light-shielded among the plurality of pixels is arranged. A pixel array having
A generating unit that generates an offset signal based on a signal read from a pixel in the light shielding region;
A correction signal is generated by applying a coefficient determined so as to reduce the difference between the black level in the light-shielding region and the black level in the effective region to the offset signal, and a pixel in the effective region is generated by using the generated correction signal. and a correcting unit for correcting the black level of the signal read from,
The correction unit obtains the correction signal by separating the offset signal into a plurality of frequency components, and adding each of the separated frequency components by applying a coefficient corresponding to the frequency. An imaging apparatus characterized by the above. The correction unit applies a first coefficient to a component having a frequency higher than a reference frequency among the plurality of separated frequency components, and applies a second smaller than the first coefficient to a component having a frequency lower than the reference frequency. The imaging apparatus according to claim 1 , wherein a coefficient is applied. The first coefficient is 1,
The imaging apparatus according to claim 2 , wherein the second coefficient has a value not less than 0.4 and not more than 0.8. A plurality of pixels are arranged in a row direction and a column direction, and a light shielding region where a light-shielded pixel among the plurality of pixels is arranged, and an effective region where a pixel that is not light-shielded among the plurality of pixels is arranged. A pixel array having
A generating unit that generates an offset signal based on a signal read from a pixel in the light shielding region;
A correction signal is generated by applying a coefficient determined so as to reduce the difference between the black level in the light-shielding region and the black level in the effective region to the offset signal, and a pixel in the effective region is generated by using the generated correction signal. A correction unit for correcting the black level of the signal read from
Wherein the correction unit is configured by the coefficient corresponding to the timing at which signals are read out from the columns or rows of pixels in the light shielding region be subjected to the offset signal, the correction signal imaging device you and obtains the.

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