JP2016174246A - Solid-state imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid-state imaging apparatus capable of reducing influences on an image, of variation in the characteristics of the solid-state imaging apparatus in a pixel region.SOLUTION: The solid-state imaging apparatus comprises a first calculation circuit, a second calculation circuit and a third calculation circuit. An approximation calculation circuit 31 that is the first calculation circuit calculates a first approximate function that represents a relationship between a position of a light-shielding pixel in a first direction in a light-shielding pixel region and a pixel value according to a signal from the light-shielding pixel. The first calculation circuit calculates an estimation value for a pixel value that is a gradation reference in the effective pixel, on the basis of the first approximate function. A linear correction circuit 32 that is the second calculation circuit performs a calculation for correction of the estimation value using a second approximate function that uses a position of a pixel in a second direction vertical to the first direction as a variable. A clamp circuit 33 that is the third calculation circuit corrects a pixel value according to a signal from the effective pixel on the basis of the result of the calculation performed in the second calculation circuit.SELECTED DRAWING: Figure 4

Description

  The present embodiment relates to a solid-state imaging device.

  An image signal obtained by the solid-state imaging device may include noise due to dark current. As one means for reducing such noise, black level correction for correcting the black level of an image signal to an ideal level is known. The black level is a signal level used as a reference when the luminance level is expressed as a gradation. The solid-state imaging device is required to reduce the influence on the image due to the variation in characteristics in the pixel region by highly accurate black level correction.

JP 2011-71709 A

  An object of one embodiment is to provide a solid-state imaging device that can reduce an influence on an image due to variation in characteristics in a pixel region.

  According to one embodiment, the solid-state imaging device includes a pixel region and a circuit unit. Pixels are arranged in a matrix in the pixel region. The pixel includes a photoelectric conversion element. The circuit unit processes a signal transmitted from the pixel region. The pixel area includes an effective pixel area and a light-shielded pixel area. Effective pixels are arranged in the effective pixel region. An effective pixel is a pixel for capturing light from the subject. The light shielding pixels are arranged in the light shielding pixel region. A light-shielded pixel is a pixel that is shielded from light. The circuit unit includes a first arithmetic circuit, a second arithmetic circuit, and a third arithmetic circuit. The first arithmetic circuit obtains a first approximate function. The first approximate function represents the relationship between the position of the light-shielded pixel in the first direction of the light-shielded pixel region and the pixel value corresponding to the signal from the light-shielded pixel. The first arithmetic circuit calculates an estimated value of a pixel value that is a reference for gradation in the effective pixel based on the first approximation function. The second arithmetic circuit performs a calculation for correcting the estimated value using the second approximation function. The second approximation function uses the position of the pixel in the second direction as a variable. The second direction is a direction perpendicular to the first direction. The third arithmetic circuit corrects the pixel value corresponding to the signal from the effective pixel based on the calculation result in the second arithmetic circuit.

FIG. 1 is a diagram illustrating a block configuration of the solid-state imaging device according to the first embodiment. FIG. 2 is a diagram illustrating a block configuration of a camera system including the solid-state imaging device illustrated in FIG. FIG. 3 is a schematic plan view of the pixel region shown in FIG. FIG. 4 is a block diagram showing the configuration of the black level correction circuit shown in FIG. FIG. 5 is a diagram for explaining calculation in the black level correction circuit shown in FIG. FIG. 6 is a block diagram illustrating a configuration of a black level correction circuit provided in the solid-state imaging device according to the second embodiment. FIG. 7 is a diagram for explaining the calculation in the black level correction circuit shown in FIG.

  Exemplary embodiments of a solid-state imaging device will be described below in detail with reference to the accompanying drawings. Note that the present invention is not limited to these embodiments.

(First embodiment)
FIG. 1 is a diagram illustrating a block configuration of the solid-state imaging device according to the first embodiment. FIG. 2 is a diagram illustrating a block configuration of a camera system including the solid-state imaging device illustrated in FIG.

  The camera system 1 is an electronic device that includes a camera module 2. The camera system 1 is a mobile terminal with a camera, for example. The camera system 1 may be an electronic device such as a digital camera.

  A camera system 1 shown in FIG. 2 includes a camera module 2 and a post-processing unit 3. The camera module 2 includes an imaging optical system 4 and a solid-state imaging device 5. The post-processing unit 3 includes an image signal processor (ISP) 6, a storage unit 7, and a display unit 8.

  The imaging optical system 4 captures light from the subject. The imaging optical system 4 includes a lens that forms a subject image. The solid-state imaging device 5 is a CMOS image sensor. The solid-state imaging device 5 captures a subject image. The ISP 6 performs signal processing of an image signal obtained by imaging with the solid-state imaging device 5. The storage unit 7 stores an image that has undergone signal processing in the ISP 6. The storage unit 7 outputs an image signal to the display unit 8 in accordance with a user operation or the like.

  The display unit 8 displays an image according to the image signal input from the ISP 6 or the storage unit 7. The display unit 8 is, for example, a liquid crystal display. The camera system 1 performs feedback control of the camera module 2 based on data that has undergone signal processing in the ISP 6.

  A solid-state imaging device 5 illustrated in FIG. 1 includes a pixel region 11, a control circuit 12, a row scanning circuit 13, a column scanning circuit 14, a column processing circuit 15, and a signal processing circuit 16. The control circuit 12, the row scanning circuit 13, the column scanning circuit 14, the column processing circuit 15, and the signal processing circuit 16 constitute a peripheral circuit unit integrated on a chip on which the pixel region 11 is mounted.

  The pixel area 11 is an area including pixels arranged in a matrix. Each pixel includes a photodiode that is a photoelectric conversion element. The photoelectric conversion element generates a signal charge corresponding to the amount of incident light. The pixel accumulates signal charges generated according to the amount of incident light.

  Various data and clock signals for driving the solid-state imaging device 5 are supplied to the control circuit 12 from outside the chip. The control circuit 12 generates various pulse signals for controlling driving of the peripheral circuit unit in accordance with a clock signal supplied from the outside of the solid-state imaging device 5. The control circuit 12 supplies a pulse signal instructing drive timing to each of the row scanning circuit 13, the column scanning circuit 14, the column processing circuit 15, and the signal processing circuit 16.

  The row scanning circuit 13 includes a shift register and an address decoder. The row scanning circuit 13 which is a pixel driving circuit supplies a driving signal to the pixels in the pixel region 11. The control circuit 12 supplies a pulse signal corresponding to the vertical synchronization signal to the row scanning circuit 13. The row scanning circuit 13 sequentially selects pixel rows from which pixel signals are read according to the pulse signal from the control circuit 12. The row scanning circuit 13 performs readout scanning by sequentially supplying a readout signal for each pixel in the selected pixel row. The read signal is a drive signal for reading a pixel signal generated according to the amount of incident light from the pixel.

  The row scanning circuit 13 performs sweep-out scanning by supplying a reset signal to each pixel prior to supplying a readout signal to each pixel. The reset signal is a drive signal for discharging the charge remaining in the photoelectric conversion element. Each pixel accumulates signal charges generated according to the amount of incident light from when the reset signal is supplied to when the readout signal is supplied.

  The drive signal is transmitted from the row scanning circuit 13 to each pixel through the pixel drive line 18. The pixel drive line 18 is provided for each pixel row in the pixel region 11. A pixel row consists of pixels arranged in the row direction (horizontal direction).

  The pixel signal is transmitted from each pixel to the column processing circuit 15 through the vertical signal line 19. The vertical signal line 19 is provided for each pixel column in the pixel region 11. The pixel column is composed of pixels arranged in the column direction (vertical direction).

  The column processing circuit 15 processes the pixel signal transmitted through the vertical signal line 19 by a unit circuit (not shown) provided for each pixel column. The column processing circuit 15 performs correlated double sampling processing (CDS) for reducing fixed pattern noise on the pixel signal. The column processing circuit 15 performs AD conversion, which is conversion from an analog signal to a digital signal, on the pixel signal. The column processing circuit 15 may perform processing other than CDS and AD conversion. The column processing circuit 15 holds the pixel signal that has undergone CDS and AD conversion for each unit circuit.

  The column scanning circuit 14 includes a shift register and an address decoder. The control circuit 12 supplies a pulse signal corresponding to the horizontal synchronization signal to the column scanning circuit 14. The column scanning circuit 14 sequentially selects pixel columns from which pixel signals are read according to the pulse signal from the control circuit 12. The column processing circuit 15 sequentially outputs pixel signals held in each unit circuit in accordance with the selective scanning by the column scanning circuit 15.

  The signal processing circuit 16 performs various kinds of signal processing on the image signal including the pixel signal from the column processing circuit 15 as a component. The signal processing circuit 16 includes a black level correction circuit 17. The black level correction circuit 17 corrects the black level of the image signal input to the signal processing circuit 16. The black level is a signal level used as a reference when the luminance level is expressed as a gradation.

  The signal processing circuit 16 performs defect correction, gain adjustment, white balance adjustment and the like in addition to the image signal correction in the black level correction circuit 17. In the signal processing circuit 16 shown in FIG. 1, the illustration of components other than the black level correction circuit 17 is omitted. The solid-state imaging device 5 outputs an image signal that has been processed by the signal processing circuit 16.

  The camera system 1 may be configured such that the ISP 6 of the post-stage processing unit 3 performs at least one of the signal processing performed by the signal processing circuit 16 in the embodiment. In the camera system 1, at least one of the signal processing may be performed by both the signal processing circuit 16 and the ISP 6. The signal processing circuit 16 and the ISP 6 may perform signal processing other than the signal processing described in the embodiment. The function of the black level correction circuit 17 may be shared by the signal processing circuit 16 and the ISP 6.

  FIG. 3 is a schematic plan view of the pixel region shown in FIG. The pixel area 11 includes an effective pixel area 20, a vertical optical black area (VOB area) 21, and a horizontal optical black area (HOB area) 22.

  The effective pixel area 20 is an area where the effective pixels 23 are arranged in a matrix. The effective pixel 23 is a pixel for capturing light from the subject. The effective pixel 23 outputs an effective pixel signal that is an analog signal having a level corresponding to the amount of incident light. On the incident side of each effective pixel 23, a color filter (not shown) corresponding to the color arrangement is provided. Each effective pixel 23 shares and detects color light according to the color arrangement.

  Each of the VOB area 21 and the HOB area 22 is a light-shielded pixel area in which light-shielded pixels 24 are arranged in a matrix. The light shielding pixel 24 is a pixel that is shielded from light. The light shielding pixel 24 outputs an optical black signal (OB signal) which is an analog signal indicating a level in the dark. Each light shielding pixel 24 is provided with a light shielding film (not shown) that shields light. The light shielding film is made of a metal material such as aluminum.

  The HOB area 22 that is the first light-shielding pixel area is adjacent to the effective pixel area 20 in the row direction. The VOB region 21 that is the second light-shielding pixel region is adjacent to the effective pixel region 20 and the HOB region 22 in the column direction. The VOB area 21 is an area including a plurality of pixel rows selected by the row scanning circuit 13 in the first period of the frame period in the pixel area 11.

  The VOB region 21 includes light-shielding pixels 24 arranged from the first end to the second end in the row direction in the pixel region 21. The first end in the row direction is one end of the pixel region 21 in the row direction, for example, the left end in the figure. The second end in the row direction is an end of the pixel region 21 other than the first end in the row direction, for example, the right end in the figure.

  FIG. 4 is a block diagram showing the configuration of the black level correction circuit shown in FIG. The black level correction circuit 17 includes an approximate calculation circuit 31, a linear correction circuit 32, and a clamp circuit 33. These circuits included in the black level correction circuit 17 are configured by appropriately combining various logic circuits and storage elements for holding calculation results and various data. The storage element may be either a register or a memory.

  The effective pixel signal includes a noise component due to dark current. The amount of dark current varies depending on the temperature. The noise component included in the effective pixel signal may vary for each position of the effective pixel 23 in the effective pixel region 20 due to the influence of the ambient temperature of the solid-state imaging device 5 or the like. Such noise component variation appears as a black level shading characteristic in the effective pixel region 20.

  The approximate arithmetic circuit 31 that is the first arithmetic circuit performs an operation for obtaining an approximate function by the least square method for each of the VOB region 21 and the HOB region 22. The approximate calculation circuit 31 calculates a quadratic function, which is a first approximate function, for each region. The approximate calculation circuit 31 generates a first value that is an estimated value of the black level according to the first approximate function for each pixel position in the first direction by a calculation using the first approximate function.

  The approximate arithmetic circuit 31 is based on the pixel value corresponding to the OB signal from the VOB area 21, and based on the relationship between the position of the light-shielding pixel 24 and the pixel value in the row direction that is the first direction of the VOB area 21. Find a function. The pixel value is a digital value obtained by AD conversion to a pixel signal that is an analog signal. The approximate calculation circuit 31 obtains an estimated value of the black level for each pixel position in the row direction in the effective pixel region 20 using the obtained quadratic function. The black level correction circuit 17 performs such calculation by the approximate calculation circuit 31 in order to correct the black level shading characteristics in the row direction.

  The approximate calculation circuit 31 is based on the pixel value corresponding to the OB signal from the HOB area 22, and based on the relationship between the position of the light shielding pixel 24 and the pixel value in the column direction that is the first direction of the HOB area 22. Find a function. The approximate calculation circuit 31 obtains such a quadratic function using the pixel position in the column direction as a variable separately from the above-described quadratic function using the pixel position in the row direction as a variable. The approximate arithmetic circuit 31 obtains an estimated value of the black level for each pixel position in the column direction in the effective pixel region 20 using the obtained quadratic function. The black level correction circuit 17 performs such calculation by the approximate calculation circuit 31 in order to correct the black level shading characteristics in the column direction.

  The linear correction circuit 32, which is the second arithmetic circuit, performs an operation for correcting the calculation result in the approximate arithmetic circuit 31, using a linear function that is the second approximate function. The linear correction circuit 32 converts the first value generated by the approximate calculation circuit 31 into a second value, which is a correction value, using a linear function. The linear function uses the position of the effective pixel 23 in the second direction perpendicular to the first direction as one of the variables.

  The linear correction circuit 32 corrects the estimated value of the black level obtained for each pixel position in the row direction which is the first direction for the VOB region 21 based on the second approximate function. The second approximation function is a linear function in which the pixel position in the column direction, which is the second direction perpendicular to the first direction, is one of the variables. The black level correction circuit 17 performs this calculation by the linear correction circuit 32 in order to correct the output difference according to the distance between the VOB region 21 and the effective pixel 23.

  The linear correction circuit 32 corrects the estimated value of the black level obtained for each pixel position in the column direction that is the first direction for the HOB region 22 based on the second approximation function. The second approximation circuit is a linear function in which the pixel position in the row direction, which is the second direction perpendicular to the first direction, is one of the variables. The black level correction circuit 17 performs such calculation by the linear correction circuit 32 in order to correct the output difference according to the distance between the HOB area 22 and the effective pixel 23.

  The clamp circuit 33 that is the third arithmetic circuit subtracts the target value of the black level from the second value that is the estimated value that has been corrected by the linear correction circuit 32. The clamp circuit 33 obtains a black level correction value for each effective pixel 23 by such subtraction. As a result, the clamp circuit 33 calculates a correction value that cancels out the deviation between the estimated value of the black level that has undergone the correction by the linear correction circuit 32 and the target value for each effective pixel 23. The black level correction circuit 17 holds the black level target value in a register in advance.

  The clamp circuit 33 corrects the pixel value corresponding to the signal from each effective pixel 23 using the black level correction value for each pixel position. In this way, the clamp circuit 33 corrects the pixel value of the effective pixel 23 based on the calculation result in the linear correction circuit 32.

  FIG. 5 is a diagram for explaining calculation in the black level correction circuit shown in FIG. The approximate calculation circuit 31 uses the pixel value from the light shielding pixel 24 in the VOB region 21 to obtain A, B, and C that minimize Q in the following equation (1).

  In Equation (1), X is a variable representing the pixel position in the row direction. Y is a variable representing a pixel value. A, B and C represent constants of a quadratic function which is a first approximate function. In Expression (1) and Expressions (2) and (3) described below, the sum (Σ) represents the sum of the data of (X, Y) for each light-shielding pixel 24 included in the VOB region 21. .

  The approximate arithmetic circuit 31 receives the pixel value actually measured for each light-shielded pixel 24. The approximate calculation circuit 31 collects the coordinates indicating the pixel position in the row direction and the data of (X, Y) that is the actual measurement value of the pixel value for each light-shielding pixel 24 in the VOB region 21.

  The approximate arithmetic circuit 31 calculates A, B, and Q when Q is minimized from simultaneous equations in which equality is assumed that a term obtained by partial differentiation of Q with respect to A, B, and C is equal to 0. Find C.

When the numerator of A which is a constant of X ² is α and the denominator is β (A = α / β), the approximate arithmetic circuit 31 uses, for example, the following equations (2) and (3) to calculate A Ask for. Note that the calculation for obtaining A is not limited to the one using the following formula. The expression used for the calculation can be changed as appropriate.

  In equations (2) and (3), N is the number of light-shielding pixels 24 in the row direction of the VOB region 21. The total sum of X can be obtained in advance according to the number of pixels included in the effective pixel region 20. The approximate arithmetic circuit 31 holds a result calculated in advance in a register or memory for a term including the sum of X and not including Y among the terms included in the expressions (2) and (3). The approximate calculation circuit 31 calculates a value of a term including Y each time an actual measurement value is input, and holds the calculation result in a register or memory.

Similar to A, the approximate arithmetic circuit 31 obtains B, which is a constant of X, and C, which is a constant term, based on the simultaneous equations described above. The approximate calculation circuit 31 obtains A, B, and C when the data about the pixel values obtained by the reading scan to the VOB region 21 in one frame is prepared. Thereby, the approximate arithmetic circuit 31 determines constants A, B, and C of the quadratic function Y = AX ² + BX + C which is the first approximate function.

  The curve L1 is a graph in which the quadratic function obtained by the approximate arithmetic circuit 31 is represented with the coordinate X in the row direction as the first axis and the pixel value Y of the light-shielded pixel 24 as the second axis. The position of X = 0, which is the origin of the first axis, is the first end in the row direction in the VOB area 21.

  The approximate calculation circuit 31 substitutes the coordinates of the effective pixel 23 in the row direction for X of the quadratic function obtained based on the VOB region 21. Y obtained by substituting the coordinates for X is an estimated value of the black level in the effective pixel 23 located at the coordinates. Thereby, the approximate calculation circuit 31 calculates an estimated value of the black level in the effective pixel 23 at the coordinate.

  The approximate calculation circuit 31 uses the pixel value from the light-shielding pixel 24 in the HOB area 22 to obtain A, B, and C that minimize Q in the above equation (1). In this case, X shown in Expression (1) is a variable representing the pixel position in the column direction. The approximate calculation circuit 31 sets the pixel position in the row direction to X for the VOB region 21 described above, and sets the pixel position in the column direction to X for the HOB region 22.

  The approximate calculation circuit 31 performs the same calculation for the HOB area 22 as for the VOB area 21 except that the direction of X is changed. In the calculation for the HOB region 22, the sum (Σ) in the above-described equations (1) to (3) represents the sum of the data of (X, Y) for each light shielding pixel 24 included in the HOB region 22. To do.

  The approximate calculation circuit 31 collects coordinates indicating the pixel position in the column direction and data of (X, Y) that is an actual measurement value of the pixel value for each light-shielding pixel 24 in the HOB region 22. In equations (2) and (3), N is the number of light-shielding pixels 24 in the column direction of the HOB region 22.

The approximate calculation circuit 31 obtains A, B, and C when the data about the pixel values obtained by the reading scan to the HOB area 22 in one frame is prepared. Thereby, the approximate arithmetic circuit 31 determines constants A, B, and C of the quadratic function Y = AX ² + BX + C which is the first approximate function.

  The curve L2 is a graph in which the quadratic function obtained by the approximate arithmetic circuit 31 is expressed with the coordinate X in the column direction as the first axis and the pixel value Y of the light-shielded pixel 24 as the second axis. The position of X = 0, which is the origin of the first axis, is the end position of the HOB area 22 on the VOB area 21 side in the column direction.

  The approximate calculation circuit 31 substitutes the coordinates of the effective pixels 23 in the column direction for X of the quadratic function obtained based on the HOB region 22. Thereby, the approximate calculation circuit 31 calculates an estimated value of the black level in the effective pixel 23 at the coordinate.

  Next, the calculation in the linear correction circuit 32 that is the second calculation circuit will be described. The shading characteristic of the black level in the column direction of the effective pixel area 20 changes with respect to the shading characteristic of the black level in the VOB area 21 as the distance from the VOB area 21 increases. The linear correction circuit 32 corrects the estimated value obtained by the approximate arithmetic circuit 31 based on the second approximate function representing the characteristic change in the column direction, which is the second direction in the VOB region 21.

  The linear correction circuit 32 corrects the estimated value of the black level obtained from the quadratic function represented by the curve L1 for the VOB region 21 using a linear function Y = DX + E. In such a function, X is a variable representing the pixel position in the column direction. Y is a variable representing a correction amount for the estimated value.

  The linear correction circuit 32 holds a result calculated in advance in a register or memory for D which is a constant of X and E which is a constant term. D and E are obtained based on data of (X, Y) which are coordinates indicating pixel positions in the column direction and differences indicating black level characteristic errors between the VOB area 21 and the effective pixels 23. As a difference indicating an error in the black level characteristic, for example, a difference between an average value of pixel values for each pixel row of the effective pixels 23 and an average value of pixel values for each pixel row of the light shielding pixels 24 in the VOB region 21 is used. It is done. D and E are obtained by linearly approximating the data of (X, Y).

  The straight line L3 is a graph representing the linear function used in the linear correction circuit 32 with the coordinate X in the column direction as the first axis and the correction amount Y as the second axis. The position of X = 0, which is the origin of the first axis, is the end position on the VOB area 21 side in the column direction in the effective pixel area 20. The linear correction circuit 32 corrects the estimated value of the black level for each pixel position in the column direction in the effective pixel region 20 using such a linear function. The clamp circuit 33 obtains a black level correction value for each effective pixel 23 based on the estimated value that has been corrected for each pixel position in the column direction.

  The linear correction circuit 32 also uses the approximate arithmetic circuit 31 to change the shading characteristics at the black level in the HOB area 22 based on the second approximation function representing the characteristic change in the row direction, which is the second direction in the HOB area 22. The obtained estimated value is corrected.

  The linear correction circuit 32 corrects the estimated value of the black level obtained from the quadratic function represented by the curve L2 for the HOB region 22 using a linear function Y = DX + E. In such a function, X is a variable representing the pixel position in the row direction. Y is a variable representing a correction amount for the estimated value. The linear function having the pixel position in the row direction as a variable is obtained separately from the above linear function having the pixel position in the column direction as a variable.

  Even in this case, the linear correction circuit 32 holds a result calculated in advance in a register or memory for D which is a constant of X and E which is a constant term. D and E are obtained based on data of (X, Y) which are coordinates indicating pixel positions in the row direction and differences indicating black level characteristic errors between the HOB area 22 and the effective pixels 23. As a difference indicating an error in the black level characteristic, for example, a difference between an average value of pixel values for each pixel column of the effective pixels 23 and an average value of pixel values for each pixel column of the light-shielding pixels 24 in the HOB region 22 is used. It is done. D and E are obtained by linearly approximating the data of (X, Y).

  The straight line L4 is a graph representing the linear function used in the linear correction circuit 32 with the coordinate X in the row direction as the first axis and the correction amount Y as the second axis. The position of X = 0, which is the origin of the first axis, is the end position of the effective pixel area 20 on the HOB area 22 side in the row direction. The linear correction circuit 32 corrects the estimated value of the black level for each pixel position in the row direction in the effective pixel region 20 using such a linear function. The clamp circuit 33 obtains a black level correction value for each effective pixel 23 based on the estimated value that has been corrected for each pixel position in the row direction.

  The linear correction circuit 32 may update the linear function by observing a black level characteristic error between the light-shielding pixel region and the effective pixel 23 in a timely manner.

  According to the first embodiment, the solid-state imaging device 5 corrects the black level shading characteristics in the row direction and the column direction by performing a calculation in the first calculation circuit. The solid-state imaging device 5 can correct the black level by reducing the error of the black level characteristic caused by moving away from the light-shielded pixel region by performing the calculation in the second arithmetic circuit. Thereby, the solid-state imaging device 5 has an effect of being able to obtain an image in which the influence due to the characteristic variation in the pixel region 20 is reduced.

(Second Embodiment)
FIG. 6 is a block diagram illustrating a configuration of a black level correction circuit provided in the solid-state imaging device according to the second embodiment. The same parts as those in the first embodiment are denoted by the same reference numerals, and redundant description will be omitted as appropriate. The black level correction circuit 40 is provided in the signal processing circuit 16 of the solid-state imaging device 5 in the same manner as the black level correction circuit 17 shown in FIG. The black level correction circuit 40 corrects the black level of the image signal input to the signal processing circuit 16.

  The black level correction circuit 40 includes an approximate calculation circuit 31, a moving average calculation circuit 41, and a clamp circuit 33. These circuits included in the black level correction circuit 40 are configured by appropriately combining various logic circuits and storage elements for holding operation results and various data. The storage element may be either a register or a memory. A moving average calculation circuit 41 as a fourth calculation circuit calculates an exponential moving average of pixel values according to a signal from the light-shielded pixel 24.

  FIG. 7 is a diagram for explaining the calculation in the black level correction circuit shown in FIG. In the second embodiment, the approximate arithmetic circuit 31 that is the first arithmetic circuit performs an operation for obtaining an approximate function by the least square method for the VOB region 21. The approximate calculation circuit 31 obtains a quadratic function that is a first approximate function based on the pixel value corresponding to the OB signal from the VOB region 21. The approximate calculation circuit 31 obtains an estimated value of the black level for each pixel position in the row direction in the effective pixel region 20 using the obtained quadratic function. The clamp circuit 33 obtains a black level correction value for each pixel column of the effective pixels 23 based on the estimated value obtained by the approximate calculation circuit 31.

The moving average calculation circuit 41 calculates an exponential moving average of the pixel values of the light-shielded pixels 24 for the HOB region 22. The moving average calculation circuit 41 calculates an exponential moving average using, for example, the following equation (4) every time a pixel value obtained by reading scanning to the HOB region 22 is input.
S _t = K × Y _t−1 + (1−K) × S _t−1 (4)

S _t and S _t−1 indicate exponential moving average values at certain points in time t and t−1, respectively. Y _t−1 indicates the pixel value obtained at time t−1. K represents a smoothing coefficient. The smoothing coefficient is a constant and has a value between 0 and 1. The moving average calculation circuit 41 holds the value of K in a register or memory.

  The moving average calculation circuit 41 obtains an initial value of the exponential moving average based on a quadratic function that is the first approximation function obtained by the approximate calculation circuit 31 for the VOB region 21. The moving average calculation circuit 41 uses the initial value for the light-shielding pixels 24 in the first pixel row from the VOB area 21 side in the HOB area 22.

The moving average calculation circuit 41 substitutes the coordinates in the row direction for each light-shielded pixel 24 in the first row of the HOB region 22 into X of Y = AX ² + BX + C, which is a quadratic function for the VOB region 21. The moving average calculation circuit 41 sets the average value of the estimated values obtained for the respective light-shielding pixels 24 in the first row as the initial value S ₀ of the exponential moving average and the pixel value Y ₀ .

  The moving average calculation circuit 41 calculates the exponential moving average for the second and subsequent rows of the HOB area 22 using the above equation (4). The clamp circuit 33 obtains a black level correction value for each pixel row of the effective pixels 23 based on the exponential moving average obtained by the moving average calculation circuit 41.

  An exponential moving average reduces the weight exponentially over a time series of data. For this reason, in the calculation of the exponential moving average, the most recent data change greatly affects the calculation result. If the moving average calculation circuit 41 starts the calculation of the exponential moving average based on the actual measurement value, the calculation result may vary greatly due to the fluctuation of the actual measurement value. A curve L5 represents a state in which the calculation result varies in the portion on the VOB region 21 side in the HOB region 22. The curve L5 is a graph representing the coordinate X in the column direction as the first axis and the exponential moving average Y as the second axis.

  The moving average arithmetic circuit 41 can obtain an exponential moving average with high accuracy and stability by using the first approximate function obtained by the approximate arithmetic circuit 31 for calculating the initial value. Note that the range for obtaining the initial value based on the first approximation function is not limited to the case of the first pixel row in the HOB region 22. The moving average calculation circuit 41 may obtain initial values for the light-shielding pixels 24 in any range in the HOB area 22 close to the VOB area 21. The moving average calculation circuit 41 may apply any value other than the value obtained based on the first approximation function as the initial value. The moving average calculation circuit 41 may calculate a weighted moving average that is a moving average other than the exponential moving average.

  The black level correction circuit 40 may include a linear correction circuit 32 described in the first embodiment. The linear correction circuit 32 corrects the estimated value of the black level obtained based on the pixel value from the VOB area 21 based on a linear function that is a second approximation function.

  The black level correction circuit 40 may perform the calculation based on the pixel value from the HOB area 22 by the approximate calculation circuit 31 and the linear correction circuit 32. The black level correction circuit 40 calculates a black level correction value for each pixel row of the effective pixels 23 based on the calculation results of the approximate calculation circuit 31 and the linear correction circuit 32.

  The black level correction circuit 40 corrects the black level by using the pixel value from the HOB area 22, the first correction based on the calculation result in the moving average calculation circuit 41, and the approximation calculation circuit 31 and the linear correction circuit 32. You may switch to the 2nd correction | amendment by a calculation result. As an example, the black level correction circuit 40 performs a calculation by the moving average calculation circuit 41 on the image signal obtained in the first frame period from the start of the black level correction according to the image signal. The moving average calculation circuit 41 calculates an exponential moving average using the pixel value obtained by the reading scan to the HOB area 22 in the first frame period. Based on the calculated exponential moving average, the clamp circuit 33 corrects the pixel value obtained by the reading scan to the effective pixel region 20 in the frame period.

  On the other hand, the approximate calculation circuit 31 obtains a first approximate function by using the pixel value obtained by the reading scan to the HOB area 22 in the first frame period. The approximate calculation circuit 31 obtains an estimated value of the black level for each pixel position in the column direction using the obtained quadratic function. The linear correction circuit 32 corrects the estimated value obtained by the approximate calculation circuit 31. The clamp circuit 33 corrects the pixel value obtained by the reading scan to the effective pixel region 20 in the second frame period based on the estimated value that has been corrected by the linear correction circuit 32.

  As described above, in the second frame period, the black level correction circuit 40 corrects the black level based on the calculation results of the approximate calculation circuit 31 and the linear correction circuit 32 for the pixel value in the first frame period. In the second and subsequent frame periods, the black level correction circuit 40 corrects the black level based on the calculation results of the approximate calculation circuit 31 and the linear correction circuit 32 for the pixel values in the previous frame period.

  The black level correction circuit 40 corrects the black level of the image signal from the start of the black level correction by applying the calculation in the moving average calculation circuit 41 to the image signal obtained in the first frame period. can do.

  The black level correction circuit 40 may include a switching circuit for switching black level correction. The switching circuit switches the black level correction between the first correction based on the calculation result in the moving average calculation circuit 41 and the second correction based on the calculation result in the approximate calculation circuit 31 and the linear correction circuit 32. The switching circuit switches from the first correction to the second correction when shifting from the first frame period to the second frame period.

  The black level correction circuit 40 may perform corrections other than the first correction and the second correction as black level correction. For example, as a third correction, the black level correction circuit 40 performs black correction based on the average value of the pixel values obtained by the reading scan to the VOB area 21 and the average value of the pixel values obtained by the reading scan to the HOB area 22. Level correction may be performed.

  In this case, the black level correction circuit 40 includes a circuit that calculates the average value. The black level correction circuit 40 corrects the shading characteristics of the black level in the row direction based on the result of averaging the pixel values for each column of the VOB area 21. The black level correction circuit 40 corrects the shading characteristics of the black level in the column direction based on the result of averaging the pixel values for each row of the HOB area 22.

  The switching circuit may switch the black level correction to the first correction, the second correction, and the third correction. The switching circuit may switch the black level correction in accordance with, for example, a user operation on the camera system 1. The black level correction circuit 40 can perform various black level corrections according to the function selection by the user. The black level correction circuit 17 of the first embodiment may also include a circuit for performing the third correction and a switching circuit.

  According to the second embodiment, the solid-state imaging device 5 corrects the black level shading characteristics in the row direction and the column direction by performing calculations in the first calculation circuit and the fourth calculation circuit. The solid-state imaging device 5 can correct the black level of the image signal from the start of black level correction. Thereby, the solid-state imaging device 5 has an effect of being able to obtain an image in which the influence due to the characteristic variation in the pixel region 20 is reduced.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  5 solid-state imaging device, 11 pixel area, 16 signal processing circuit, 20 effective pixel area, 21 VOB area, 22 HOB area, 23 effective pixel, 24 shading pixel, 31 approximation arithmetic circuit, 32 linear correction circuit, 33 clamp circuit, 41 Moving average calculation circuit.

Claims (7)

A pixel region in which pixels including photoelectric conversion elements are arranged in a matrix; and
A circuit unit for processing a signal transmitted from the pixel region,
The pixel area includes an effective pixel area in which effective pixels that are pixels for capturing light from a subject are arranged, and a light-shielded pixel area in which light-shielded pixels that are light-shielded pixels are arranged,
The circuit section is
A first approximate function representing a relationship between the position of the light-shielded pixel in the first direction of the light-shielded pixel region and a pixel value corresponding to a signal from the light-shielded pixel is obtained, and based on the first approximate function, A first arithmetic circuit for calculating an estimated value of a pixel value serving as a gradation reference;
A second arithmetic circuit that performs a calculation for correcting the estimated value using a second approximation function having a variable of a pixel position in a second direction perpendicular to the first direction;
A solid-state imaging device, comprising: a third arithmetic circuit that corrects a pixel value corresponding to a signal from an effective pixel based on a calculation result in the second arithmetic circuit. The circuit unit further includes a fourth arithmetic circuit that calculates a moving average of pixel values according to a signal from the light-shielded pixel,
2. The solid-state imaging according to claim 1, wherein the third arithmetic circuit corrects a pixel value corresponding to a signal from the effective pixel based on the moving average obtained by the fourth arithmetic circuit. apparatus. The shading pixel region is
A first light-shielding pixel region adjacent to the effective pixel region in a row direction;
A second light-shielding pixel region adjacent to the effective pixel region and the first light-shielding pixel region in a column direction,
The first arithmetic circuit obtains the first approximate function representing the relationship between the position and pixel value of the light-shielded pixel in the column direction for the first light-shielded pixel region, and the light-shielding in the row direction for the second light-shielded pixel region. Obtaining the first approximation function representing a relationship between a pixel position and a pixel value;
The second arithmetic circuit corrects the estimated value obtained based on a pixel value in the first light-shielding pixel region using the second approximate function having a pixel position in a row direction as a variable, 2. The estimated value obtained based on a pixel value in a second light-shielding pixel region is corrected using the second approximate function having a pixel position in a column direction as a variable. Solid-state imaging device. The shading pixel region is
A first light-shielding pixel region adjacent to the effective pixel region in a row direction;
A second light-shielding pixel region adjacent to the effective pixel region and the first light-shielding pixel region in a column direction,
The first arithmetic circuit obtains the first approximate function representing the relationship between the position of the light-shielded pixel in the row direction and the pixel value for the second light-shielded pixel region,
The solid-state imaging device according to claim 2, wherein the fourth arithmetic circuit calculates the moving average of the light-shielded pixels in the first light-shielded pixel region.   5. The solid-state imaging device according to claim 2, wherein the fourth arithmetic circuit obtains an initial value of the moving average based on the first approximate function obtained by the first arithmetic circuit. 6. . The first approximate function is a quadratic function having the position of the pixel in the first direction as a variable,
6. The solid-state imaging device according to claim 1, wherein the first arithmetic circuit obtains the estimated value for each position in the first direction of the effective pixel region.   7. The solid-state imaging device according to claim 1, wherein the second approximate function is a linear function having a variable of a pixel position in the first direction. 8.

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