JP2003298951A - Output correction apparatus for image sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To figure out a proper correction value corresponding to dispersion in an output depending on the characteristic of each pixel so as to correct the output of an image sensor with high accuracy without affected by a random noise included in an output signal of each pixel. <P>SOLUTION: An output correction apparatus for an image sensor reads a correction value corresponding to dispersion in the output characteristic of each pixel stored in advance in a memory and corrects according to prescribed arithmetic processing the output signal of each pixel in an image sensor using optical sensor circuits as the unit of pixels outputting a sensor signal corresponding to a current flowing to a photoelectric conversion element depending on the intensity of incident light. The output correction apparatus is provided with: a means for obtaining the average of outputs of each pixel of a plurality of frames in a dark state or a bright state of the image sensor; and a means for obtaining a correction value for each pixel so that the averages of the outputs of the pixels are equalized in a prescribed way. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output correction device for an image sensor for correcting variations in the output of each pixel in a MOS type image sensor.

[0002]

2. Description of the Related Art Conventionally, in a MOS type image sensor, an optical sensor circuit for one pixel is, as shown in FIG. 1, a photoelectric conversion element for generating a sensor current according to the amount of incident light Ls. Photodiode PD, a transistor Q1 for converting the sensor current flowing in the photodiode PD into a voltage signal Vpd with a logarithmic characteristic in a weak inversion state utilizing the characteristics of the subthreshold region, and the converted voltage signal Vpd is amplified. Transistor Q2
And a transistor Q3 that outputs the sensor signal Vo at the pulse timing of the read signal Vs, and is capable of expanding the dynamic range to detect an optical signal with high sensitivity. And
By setting the drain voltage VD of the transistor Q1 lower than the steady value for a predetermined time, the residual charge accumulated in the parasitic capacitance of the photodiode PD is discharged and initialized, thereby causing a rapid change in the sensor current. Even immediately, the voltage signal V corresponding to the light amount of the incident light Ls at that time
The pd is obtained so that an afterimage does not occur even when the amount of incident light is small (Japanese Patent Laid-open No. 2000-2000).
-329616 gazette).

Such an optical sensor circuit is shown in FIG.
As shown in, the photodiode PD
Shows a logarithmic output characteristic when there is a large amount of sensor current flowing in the photodiode PD.
A response delay occurs in charging of the parasitic capacitance C, and a substantially linear non-logarithmic output characteristic is exhibited. In the figure, WA indicates a non-logarithmic response region, and WB indicates a logarithmic response region.

However, in an image sensor using such an optical sensor circuit for each pixel, as shown in FIG.
The output characteristics vary due to the structure of each pixel, and it is necessary to correct the output of each pixel so that the output characteristics are uniform. In the figure, Io represents the sensor current in the dark, which corresponds to the dark current flowing in the photodiode PD when there is no incident light.

The cause of the variation in the output characteristic of each pixel is mainly to use the characteristic of the subthreshold region of the transistor Q1 to generate the voltage signal Vpd corresponding to the light quantity of the incident light Ls.
This is because the subthreshold value of is different for each pixel. Further, it is necessary to amplify the logarithmically converted voltage signal in each pixel with a high impedance and output the amplified voltage signal, but the nonuniformity of the characteristics of the amplifying transistor Q2 also causes a variation in the output of each pixel.

Therefore, it is proposed by the same applicant as the present application to correct the variation in the output characteristics of each pixel by the following method (Japanese Patent Application No. 2000-40493).
1, Japanese Patent Application 2000-404933, Japanese Patent Application 2001-75
035, Japanese Patent Application No. 2001-75036).

That is, the variation state of the output characteristic of each pixel is measured in advance, an offset correction value and a gain correction value that produce the predetermined output characteristic are created and stored in a memory, and the value is stored in the memory. Corresponding correction values are read and offset correction and gain correction of the output of each pixel are performed.

At this time, in an image sensor having a logarithmic output characteristic, a dark output cannot be obtained because basically the electric charge accumulated in the pixel is not forcibly reset. for that reason,
The following correction means are adopted.

First, in order to correct the dark level, the dark state (Io) of each pixel is set in the dark state in which the incident light is cut off.
Offset correction is performed so that the outputs of the two coincide with each other. Next, in order to correct the brightness level, the gain is corrected so that the inclinations of the output characteristics of the pixels are aligned in the bright state when uniform light is incident. Alternatively, in the reverse order of this procedure, offset correction is performed so that the output of each pixel is aligned in the bright state when uniform light is incident, and then in the dark state when the incident light is cut off. The gain correction is performed so that the dark outputs of the pixels are matched with each other, thereby correcting the variations in the output characteristics of the pixels.

Conventionally, a computer takes in output data of each pixel for one frame (one screen) in an image sensor, and calculates an offset correction value and a gain correction value according to variations in the output of each pixel. ing.

[0011]

The shin problem to be solved is to correct the output of an image sensor using a photosensor circuit for outputting a sensor signal corresponding to a current flowing through a photoelectric conversion element in accordance with incident light in pixel units. When performing the above, it is necessary to calculate the offset correction value and the gain correction value according to the variation in the output of each pixel based on the output data of each pixel for one frame of the image sensor. Therefore, the variation in the output of each pixel becomes indefinite, and the offset correction value and the gain correction value that are sequentially obtained become improper.

In this case, between pixels that include random noise, respectively, a large output variation will occur due to the addition of both random noises.

[0013]

According to the present invention, an output signal of each pixel in an image sensor in which an optical sensor circuit for outputting a sensor signal according to a current flowing through a photoelectric conversion element in response to incident light is used for each pixel. , Read the correction value according to the variation of the output characteristics of each pixel stored in advance in the memory, and when performing the correction by the predetermined arithmetic processing, the output that comes from the characteristics of each pixel without being affected by the random noise. In order to be able to calculate an appropriate correction value according to the variation of, the average value of the output of each pixel for multiple frames in the dark state or the bright state of the image sensor is calculated, and the output of each pixel is A means for obtaining a correction value for each pixel is provided so that the average value is adjusted to a predetermined value.

[0014]

The image sensor according to the present invention is basically
The above-described photosensor circuit shown in FIG. 1 is used for each pixel.

As the optical sensor circuit, a photodiode PD as a photoelectric conversion element which generates a sensor current according to the amount of incident light Ls, and a sensor current flowing in the photodiode PD are utilized in the characteristics of the subthreshold region. The voltage signal Vpd having a logarithmic characteristic in the weak inversion state
, A transistor Q1 for amplifying the converted voltage signal Vpd, and a read signal V1.
and a transistor Q3 that outputs a sensor signal Vo at a pulse timing of s.

In that case, the value of the gate voltage VG of the transistor Q1 is set to be equal to or lower than its drain voltage VD.

In the optical sensor circuit, when the incident light Ls is incident on the photodiode PD with a sufficient amount of light, a sufficient sensor current flows in the transistor Q1 and the resistance value of the transistor Q1 is not so large. Therefore, the optical sensor can detect the optical signal with a sufficient response speed that does not cause an afterimage as an image sensor.

However, the incident light L of the photodiode PD is
When the amount of light of s decreases and the sensor current flowing through the transistor Q1 decreases, the transistor Q1 is set to operate such that its resistance value increases by one digit when the current flowing through it decreases by one digit. The resistance value of Q1 increases, the time constant with the parasitic capacitance C of the photodiode PD increases, and it takes time to discharge the charge accumulated in the parasitic capacitance C. Therefore, an afterimage is observed for a long time as the amount of the incident light Ls decreases.

Therefore, the saturation time of the voltage signal Vpd corresponding to the sensor current when the light quantity of the incident light Ls of the photodiode PD is small becomes long, so that the sensor signal Vo is generated at the pulse timing of the read signal Vs as shown in FIG. When the reading is performed, the output of a level as high as at the beginning appears as an afterimage. In FIG. 5, Vpd 'represents the voltage signal inverted and amplified by the amplifying transistor Q2.

In such an optical sensor circuit, prior to reading the sensor signal Vo, the drain voltage VD of the transistor Q1 is set lower than the steady value for a predetermined time and stored in the parasitic capacitance C of the photodiode PD. In the case where the quantity of incident light Ls is small, a voltage signal corresponding to the quantity of incident light at that time is immediately obtained by discharging the generated electric charge and initializing it, even if a sudden change occurs in the sensor current. But I try not to cause afterimages.

FIG. 2 shows a time chart of signals at various parts in the optical sensor circuit at that time. Where t1
Indicates the initialization timing, and t2 indicates the optical signal detection timing. Drain voltage VD of transistor Q1
The predetermined time tm for switching from a steady value (high level H) to a low voltage (low level L) is, for example, 5 μse when the reading speed for one pixel is about 100 nsec.
It is set to about c. In the figure, T is a photodiode PD
Shows the accumulation period of the parasitic capacitance C of the
Is 1/30 sec for NTSC signals (or 1/60
sec).

In such a case, when the drain voltage VD of the transistor Q1 is switched to the low level L at the time of initialization, the potential difference between the gate voltage VG and the drain voltage VD at that time is the threshold of the transistor Q1. If it is larger than the value, the transistor Q1 is in a low resistance state.
As a result, the potential on the source side at that time becomes the same as the drain voltage VD (in the n-MOS transistor, source voltage = drain voltage), and the junction capacitance C of the photodiode PD is discharged.

When the drain voltage VD is switched to the steady high level H after the tm time has elapsed and the optical signal is detected, the potential on the source side changes to the drain voltage V.
If it becomes lower than D and the potential difference between the gate voltage VG and the drain voltage VD at that time is larger than the threshold value, the MOS transistor Q1 is in a low resistance state, and the junction capacitance C of the photodiode PD is charged. Be started.

As described above, when the junction capacitance C of the photodiode PD is discharged and initialized prior to the detection of the optical signal and then the parasitic capacitance C is charged, a fixed time has elapsed from the initialization timing. The output voltage (terminal voltage of the photodiode PD) Vpd at the time point has a value according to the light amount of the incident light Ls. That is, after the initialization, the discharge characteristic with a constant time constant following the change in the light amount of the incident light Ls can be obtained.

At this time, if left for a long time, the drain voltage V
Although the current supplied from D through the transistor Q1 is the same as the current flowing through the photodiode PD, the same discharge characteristic is always obtained if there is no charge remaining before, so that an afterimage does not occur.

Therefore, if the optical signal is detected at a fixed time after the initialization, it is possible to obtain the sensor signal Vo having no afterimage according to the light quantity of the incident light Ls.

FIG. 6 shows an image sensor in which a plurality of pixels are arranged in a matrix with such an optical sensor circuit as a pixel unit to perform a time-series read-out scanning of a sensor signal of each pixel. An example of one configuration is shown.

The basic structure of the image sensor is that, for example, 4 × 4 pixels D11 to D44 are arranged in a matrix, and a pixel column for each one line is output from the pixel column selection circuit 1. Selection signals LS1 to LS4 that are sequentially output
Selection signal DS1 sequentially output from the pixel selection circuit 2 for each pixel in the selected pixel column.
.. to DS4 sequentially turn on the corresponding switches SW1 to SW4 in the switch group 3, whereby the sensor signal Vo of each pixel is read out in time series. In the figure, 4 is a power supply for the gate voltage VG of the transistor Q1 in each pixel, and 6 is a power supply for the drain voltage VD.

In such an image sensor, when selecting a pixel row for each one line, the drain voltage VD of the transistor Q1 of each pixel in the selected pixel row is steady at a predetermined timing. A voltage switching circuit 5 for switching between a high level H and a low level L at the time of initialization is provided.

The operation of the image sensor according to the present invention having the above-mentioned structure will be described below with reference to the time chart of the signals of the respective parts shown in FIG.

First, when the pixel column selection signal LS1 goes to high level H, D11, D12, D13,
The first pixel column consisting of D14 is selected. And L
During a certain period T1 in which S1 is at high level H, the pixel selection signals DS1 to DS4 sequentially become high level H, and the sensor signals Vo of the pixels D11, D12, D13, D14 are sequentially read.

Next, when the next LS2 becomes high level H at the time when the pixel column selection signal LS1 becomes low level L, the second pixel column composed of D21, D22, D23 and D24 corresponding thereto is selected. . Then, the pixel selection signals DS1 to DS4 are sequentially set to the high level H during the fixed period T1 in which the LS2 is set to the high level H, and the sensor signals Vo of the pixels D21, D22, D23, D24 are sequentially read.

Similarly, the pixel column selection signals LS3 and LS4 are continuously set to the high level H and the corresponding third columns are selected.
And the fourth pixel column are sequentially selected, and LS3 and LS
4 are at the high level H, the pixel selection signals DS1 to DS4 are sequentially at the high level H during the fixed period T1.
Then, the sensor signals Vo of the pixels D31, D32, D33, D34 and D41, D42, D43, D44 are sequentially read.

Further, when the pixel column selection signal LS1 falls to the low level L after the period T1, each pixel D11, D12, in the first pixel column selected at that time
Initialization of each pixel is performed by switching the drain voltage VD1 of D13 and D14 from the high level H until then to the low level L for a predetermined time T2, and the sensor signal in the next cycle performed after the elapse of one cycle period T3. Prepare for reading.

Next, when the pixel column selection signal LS2 falls to the low level L after the period T1, each pixel D21, D2 in the second pixel column selected at that time.
2, by switching the drain voltage VD1 of D23, D24 from the high level H until then to the low level L for a predetermined time T2, initialization of each pixel is performed, and in the next cycle performed after the elapse of one cycle period T3. Prepare to read the sensor signal.

Similarly, when the pixel column selection signals LS3 and LS4 fall to the low level L after the period T1 respectively, the drain voltages VD3 respectively corresponding to the third and fourth pixel columns selected at that time. Is switched to the low level L to initialize each pixel, and the sensor signal is read out in the next cycle performed after the elapse of one cycle period T3.

Here, the pixel column selection signal LSX (X
1 to 4) fall to the low level L after the period T1, the drain voltage VDX is switched to the low level L to perform initialization. The initialization timing is the pixel column selection signal LSX. Is in the low level L state during the pause period T4 for pixel column selection.

The timing of generation of the signals of the respective parts as described above is determined by driving the pixel column selection circuit 1, the pixel selection circuit 2 and the voltage switching circuit 5 under the control of an ECU (not shown). ing.

As described above, by initializing each pixel at an appropriate timing according to the reading scanning of the sensor signal of each pixel, it becomes possible to reduce the excess or deficiency of the accumulation time of the image sensor as a whole.

Further, it is possible to realize an image sensor having a logarithmic output characteristic having a wide dynamic range and no afterimage.

In the present invention, in the image sensor configured as described above, the variation state of the sensor signal Vo of each pixel is measured in advance in order to correct the variation of the output characteristic of the sensor signal Vo of each pixel. , Create an offset correction value and a gain correction value such that they have predetermined output characteristics, store them in a memory, and read the corresponding correction value from the memory to perform offset correction and gain correction of each pixel. I am trying.

In this case, particularly in the present invention, in order to eliminate the influence of random noise contained in the sensor signal Vo output from each pixel, the output of each pixel for a plurality of frames in the dark state and the bright state of the image sensor is removed. A means for obtaining the average value and a means for obtaining the offset correction value and the gain correction value for each pixel are provided so that the average values of the outputs of the pixels are predetermined.

FIG. 8 shows an example of the configuration of the image sensor output correction apparatus according to the present invention.

Here, under the address control by the ECU 9, the sensor signal of each pixel output from the image sensor 8 in a time-sequential manner during normal photographing is amplified to a predetermined level by the amplifier 13 and then digitalized by the AD converter 10. Converted into a signal, the digitized data of each pixel is converted into a buffer memory (SRAM).
Etc.) 14. And the buffer memory 1
4 and the offset correction value and the gain correction value of each corresponding pixel, which are written in advance in the correction data memory (EEPROM or the like) 11, are sequentially read out to the output correction circuit 12. The offset correction and the gain correction of each pixel data are given by given arithmetic processing.

A conventional method is used as the offset correction and the gain correction of each pixel data, which is performed by a predetermined calculation process using the offset correction value and the gain correction value set in advance according to the variation of the output of each pixel. Applied.

In the offset correction value setting mode, the sensor signal output from the image sensor 8 in the dark state in which the incident light is cut off is stored in the buffer memory 14, and then the microcomputer 15 is stored in the buffer memory 14. Each pixel data is read, and the offset correction value is calculated according to the variation of the output of each pixel so that the output of each pixel is predetermined, and the offset correction value of each pixel thus extracted is nonvolatile. It is adapted to be written in the memory 11.

At this time, the microcomputer 15 fetches the data of each pixel for a plurality of frames sequentially accumulated in the buffer memory 14 and obtains the average value of the output for each pixel to include it in the output of each pixel. The random noise generated is reduced, and an offset correction value is calculated so that the average value of the random noise becomes uniform.

In the gain correction value setting mode, the sensor signal output from the image sensor 8 in a bright state where uniform light is incident is stored in the buffer memory 14,
The microcomputer 15 reads each pixel data accumulated in the buffer memory 14 and responds to the variation in the output of each pixel so that the output of each pixel is predetermined according to the data for offset correction. The gain correction value is calculated, and the calculated gain correction value of each pixel is written in the nonvolatile memory 11.

At this time, the microcomputer 15 fetches the data of each pixel for a plurality of frames sequentially accumulated in the buffer memory 14 and obtains the average value of the output for each pixel to include it in the output of each pixel. The random noise generated is reduced, and a gain correction value is calculated so that the average value of the random noise becomes uniform.

FIG. 9 shows another example of the configuration of the output correction device for an image sensor according to the present invention.

Here, as in the case described above, when the output correction circuit 12 is in the inoperative state without providing the buffer memory for accumulating the data for one frame of each pixel output from the image sensor 8. The microcomputer 15 directly takes in the output data of each pixel that has not been corrected and writes it in the internal memory.

FIG. 10 shows still another configuration example of the output correction device for an image sensor according to the present invention.

Here, unlike the case described above, the built-in CP is used without using the external microcomputer 15.
U16 takes in the data of each pixel accumulated in the buffer memory 14 and calculates the offset correction value and the gain correction value.

Further, according to the present invention, when the output data of each pixel in the dark state and the bright state of the image sensor 8 is obtained, the incident light of the image sensor 8 is actually cut off,
The dark state and the bright state of each pixel are artificially created without entering uniform light.

Specifically, the gate voltage VG of the transistor Q1 for logarithmic characteristic conversion in the photosensor circuit in the pixel unit of the image sensor 8 is switched to a value higher than a steady value at the time of photographing, and the transistor Q1 is in a conductive state. By using the sensor signal in this case, a means for correcting the variation in the output of each pixel is provided.

At this time, the gate voltage V of the transistor Q1
G is switched to a value higher than the steady-state value at the time of shooting to make it conductive so that the sensor signal when the drain voltage VD of the transistor Q1 is at a steady value corresponds to the sensor output in the dark at the time of shooting and the transistor Q1 The output voltage is corrected so that the sensor signal when the drain voltage VD of the pixel is switched to a value lower than the steady value corresponds to the sensor output at the time of light at the time of photographing, and the output levels at the time of darkness and light at each pixel are aligned. Let it be done.

However, by providing such means, the output state at the time of darkness and at the time of brightness can be artificially created without interrupting the light which actually enters the image sensor. Thus, it becomes possible to correct the variation in the output characteristic of each pixel.

Now, in the optical sensor circuit shown in FIG.
When the gate voltage VG of the transistor Q1 for logarithmic characteristic conversion is switched to a value higher than the steady-state value at the time of shooting and becomes conductive, the drain voltage VD of the transistor Q1 is directly applied to the gate of the transistor Q2 for amplification in the next stage. By being applied, the variation in the threshold voltage of the transistor Q1 is canceled. Then, the sensor output at that time corresponds to the output at dark.

At this time, a transistor Q for converting logarithmic characteristics
When 1 is turned on and it is assumed that the sensor output when the drain voltage VD of the transistor Q1 is directly applied to the gate of the amplifying transistor Q2 of the next stage corresponds to the output in the dark, the following is performed. Problems arise.

That is, it is assumed that the transistor Q1 and the photodiode PD have ideal characteristics. In reality, a dark current flows in the photodiode PD even if no light is incident thereon. As shown in FIG. 11, the sensor output when the transistor Q1 is in the conductive state is different from the actual dark output. In the figure, a indicates an ideal dark output and b indicates an actual dark output.

As shown in FIG. 12, the difference in the dark output is that the drain voltage VD of the transistor Q1 is set lower than the steady value for a predetermined time before reading the sensor signal Vo in order to suppress the afterimage. If the settings are made and initialization is performed, the difference becomes even larger. In the figure, c indicates the dark output when the afterimage is suppressed by the initialization.

Therefore, particularly in the present invention, when the gate voltage VG of the transistor Q1 for logarithmic characteristic conversion is switched to a value higher than a steady value at the time of photographing to make it conductive, a sensor corresponding to an appropriate dark output. So that you can get the output
The drain voltage VD of the transistor Q1 is variably adjusted.

Specifically, the value of the actual sensor output in the dark state before the output correction is stored, and the gate voltage VG of the transistor Q1 is switched to a value higher than the steady-state value at the time of photographing so that the transistor is switched. The drain voltage VD of the transistor Q1 is set so that the sensor output when Q1 is brought into the conducting state has the value stored previously. At that time, even if the sensor output when the transistor Q1 is turned on is substantially the same as the previously stored value, and if the output correction is repeated, the average value of the values stored at each correction is used. May be the same as. And
After that, when the output correction is performed, the sensor output when the gate voltage VG of the transistor Q1 is switched to a value higher than the steady value at the time of shooting to make it conductive by using the set state of the drain voltage VD is used. Offset correction of the output variation is performed.

By calculating the offset correction value based on the sensor output at this time, it becomes possible to perform output correction in the dark when the initial values are uniform. Further, changing the drain voltage VD of the transistor Q1 also matches the operating point for the amplifying transistor Q2 in the next stage.

When correcting the output in the dark, the transistor Q
Means for switching the gate voltage VG of 1 to a value higher than the steady value at the time of shooting, and the actual sensor output value in the dark state before the output correction is stored, and the gate voltage VG of the transistor Q1 is set to As a means for setting the drain voltage VD of the transistor Q1 so that the sensor output when the transistor Q1 is turned on by switching to a value higher than the steady-state value at the time of shooting becomes the previously stored value, FIG.
In the configuration of the image sensor shown in FIG.
Under the control of U, it is executed by driving the voltage switching circuit 7 and the voltage switching circuit 5 provided so as to switch the power source voltages of the VG power source 4 and the VD reading power source 6. .

Further, according to the present invention, in the image sensor configured as described above, the sensor at the time of light when light in each pixel is incident due to the variation in the output characteristics due to the configuration of the photo sensor circuit. In order to correct the unevenness of the output level of the signal Vo, the sensor output when the transistor Q1 is turned on and the drain voltage VD of the transistor Q1 is switched to a value lower than the steady value (or the set value) at the time of photographing The output correction is performed so that the output levels of the pixels at the time of light are aligned in accordance with the sensor signal at the time of light at.

By providing such means, however, the output state at the time of bright light is artificially created and the variation in the output characteristic of each pixel is corrected regardless of the incident state of light in the image sensor. You will be able to.

When correcting the output at the time of light, the transistor Q
The means for switching the gate voltage VG1 of 1 to a value higher than the steady value at the time of shooting and the means for switching the drain voltage VD to a value lower than the steady value at the time of shooting have the configuration of the image sensor shown in FIG. The voltage switching circuits 5 and 7 are driven under the control of an ECU (not shown).

As described above, according to the present invention, the variation in the output characteristics of each pixel in the image sensor is corrected without using any light source for correction.
As in the case of correcting the variation in the output characteristics of each pixel by obtaining each sensor signal at the time of darkness and lightness of each pixel while blocking or entering the light to the image sensor as in the conventional case. It is possible to accurately correct the variation in the output characteristics of each pixel without causing the problem of unevenness in illuminance due to the light source, in which uniform light cannot be frequently switched and incident on the pixel.

Then, not only at the time of shipment, but also the variation of the output characteristics of each pixel due to a change with time can be corrected at any time without using a light source.

Further, according to the present invention, when the output corrections of a large number of image sensors are simultaneously performed, a large number of light sources are not prepared and the equipment is not increased.

FIG. 13 shows a concrete configuration for correcting the variation in the output characteristic of each pixel in the image sensor.

The ECU 9 performs drive control for reading the sensor signals of the image sensor 8 and each pixel in time series, and the sensor signal Vo of each pixel output from the image sensor 8 in time series is converted into a digital signal. AD to do
A converter 10 and an offset correction value OFS and a multiplier MLT for gain correction according to the characteristics of each pixel are set in advance, and a signal of a pixel address (X, Y) given from the ECU 9 at the time of reading a sensor signal is provided. ADDRE
A memory 11 for reading a predetermined offset correction value OFS and a multiplier MLT according to SS, and an offset correction of a sensor signal DS converted into a digital signal based on the offset correction value OFS and the multiplier MLT read from the memory 11 and The output correction circuit 12 performs each calculation process of gain correction.

As described above, as the sensor signal Vo of each pixel output from the image sensor 8 in time series, the gate voltage VG of the transistor Q1 in each pixel is switched to a value higher than the steady value at the time of photographing. And the output at light when the gate voltage VG and the drain voltage VD of the transistor Q1 in each pixel in the state where light is cut off are switched to values lower than their steady values at the time of shooting. Is adopted.

FIG. 15 shows an example of a variation state of the output characteristics of the sensor signals A, B, C coming from the structure of three pixels. Here, the value Im of the sensor current according to the threshold value H of the pixel output is the sensor signal signals A, B,
The point at which C switches from the non-logarithmic response area WA to the logarithmic response area WB is shown. Further, Io represents the sensor current in the dark.

Here, such a non-logarithmic response area WA
The output characteristics of the image signals of the pixels are substantially the same, and the output characteristics of the image sensor are corrected when the inclinations of the output characteristics of the sensor signals of the pixels in the logarithmic response region WB are different. As a parameter of each pixel, information of a point at which each sensor signal switches from the non-logarithmic response area WA to the logarithmic response area WB,
The pixel output in the dark is used.

FIG. 14 shows a processing flow in the output correction circuit 12.

The memory 11 stores an offset correction value OF such that the pixel output becomes H when the sensor current has a value of Im.
S is set. Then, the offset correction unit 121
16, when the offset correction of the sensor signal DS converted into the digital signal of each pixel is performed by performing the addition / subtraction process using the offset correction value OFS, as shown in FIG. The characteristics of the non-logarithmic response area WA in B and C are matched.

Next, based on the offset-corrected sensor signal DS1, the gain correction unit 122 performs multiplication processing for gain correction on the logarithmic response region WB equal to or greater than the threshold value H.

Specifically, it is judged whether the offset-corrected sensor signal DS1 is equal to or more than the threshold value H,
If it is equal to or more than the threshold value H, that is, if the sensor signal DS1 is in the logarithmic response region WB, the output ← H + (sensor signal DS1− is used by using the predetermined multiplier MLT for gain correction read from the memory 10. H) × multiplier is calculated, and the calculation result is output as the output-corrected sensor signal DS2.

The sensor signals A, B, C of such pixels are
As a result of the gain correction of, the characteristics of the logarithmic response region WB are matched as shown in FIG.

At this time, if the offset-corrected sensor signal DS1 is smaller than the threshold value H, that is, if the sensor signal DS1 is in the non-logarithmic response area WA, the offset-corrected sensor signal DS1 is output-corrected. It outputs as the sensor signal DS2.

FIG. 19 shows another example of the variation state of the output characteristics of the sensor signals A, B, C coming from the structure of three pixels.

Here, the output of the image sensor when the slopes of the output characteristics of the sensor signals in the logarithmic response area WB are almost the same and the shapes of the output characteristics of the sensor signals in the non-logarithmic response area WA are different from each other. The case where correction is performed is shown.

FIG. 18 shows a processing flow in the output correction circuit 12.

The memory 11 stores the offset correction value OF such that the pixel output becomes H when the sensor current has a value of Im.
S is set. Then, the offset correction unit 121
20, when the offset correction of the sensor signal DS converted into the digital signal of each pixel is performed by performing the addition / subtraction process using the offset correction value OFS, as shown in FIG. The characteristics of the logarithmic response region WB in B and C become the same.

Then, based on the offset-corrected sensor signal DS1, the gain correction unit 112 performs multiplication processing for gain correction on the non-logarithmic response area WA equal to or less than the threshold value H.

Specifically, it is determined whether the offset-corrected sensor signal DS1 is less than or equal to the threshold value H,
If it is equal to or less than the threshold value H, that is, if the sensor signal DS1 is in the non-logarithmic response area WA, output ← H- (H- using the predetermined multiplier MLT for gain correction read from the memory 10. The sensor signal DS1) × multiplier is calculated, and the calculation result is output and corrected.
Output as.

The sensor signals A, B, C of such pixels are
As a result of the gain correction of No. 2, the characteristics of the non-logarithmic response area WA are matched as shown in FIG.

At this time, if the offset-corrected sensor signal DS1 is larger than the threshold value H, that is, if the sensor signal DS1 is in the logarithmic response region WB, the offset-corrected sensor signal DS1 is output-corrected. Output as a sensor signal DS2.

FIG. 23 shows still another example of the variation state of the output characteristics of the sensor signals A, B, C coming from the configuration of each pixel in the image sensor 8.

Here, the output characteristics of the sensor signals A, B, C in the logarithmic response area WB have different slopes, and the shapes of the output characteristics of the sensor signals A, B, C in the non-logarithmic response area WA are respectively different. It shows different cases.

In such a case, as shown in the flow of processing in the output correction circuit 12 in FIG. 22, the sensor signals A, A and B are generated by combining the respective processes shown in FIGS. The offset correction and the gain correction of B and C are sequentially performed, and finally, the sensor signal DS2 'in which the characteristics of the non-logarithmic response area WA and the logarithmic response area WBA are matched can be obtained.

[0094]

As described above, according to the present invention, the output signal of each pixel in the image sensor in which the photosensor circuit for outputting the sensor signal according to the current flowing through the photoelectric conversion element according to the incident light is used for each pixel is previously set. In an image sensor output correction apparatus that reads out a correction value stored in a memory according to variations in output characteristics of each pixel and corrects it by a predetermined calculation process, a plurality of image sensors in a dark state or a bright state are used. The means for obtaining the average value of the output for each pixel of the frame and the means for obtaining the correction value for each pixel so that the average value of the output of each pixel is predetermined are provided. The image sensor can calculate an appropriate correction value according to the variation in the output due to the characteristics of each pixel, without being affected by the random noise contained in the image sensor. The output correction performed accurately, has the advantage that it is possible to obtain a good imaging quality image.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing an optical sensor circuit for one pixel used in an image sensor according to the present invention.

FIG. 2 is a time chart of signals of respective parts in the optical sensor circuit.

FIG. 3 is a diagram showing an output characteristic of a sensor signal with respect to a sensor current flowing in a photodiode of the optical sensor circuit.

FIG. 4 is a diagram showing an example of a variation state of output characteristics of each pixel in an image sensor using the photosensor circuit for the pixel.

FIG. 5 is a diagram showing an output characteristic of a sensor signal read out at a predetermined timing when the amount of incident light in the optical sensor circuit when initialization is not performed is small.

FIG. 6 is a block diagram showing a configuration example of an image sensor according to the present invention.

FIG. 7 is a time chart of each signal in the image sensor.

FIG. 8 is a block diagram showing a configuration example of an output correction device for an image sensor according to the present invention.

FIG. 9 is a block diagram showing another configuration example of the image sensor output correction apparatus according to the present invention.

FIG. 10 is a block diagram showing still another configuration example of the output correction device for an image sensor according to the present invention.

FIG. 11 is a characteristic diagram showing a difference between a sensor output when a transistor for converting logarithmic characteristics in an optical sensor circuit is in a conductive state and an actual sensor output when dark.

FIG. 12 shows a difference between a sensor output when a transistor for converting logarithmic characteristics in an optical sensor circuit is in a conductive state and an actual sensor output during darkness when initialization for suppressing an afterimage is performed. It is a characteristic diagram.

FIG. 13 is a block diagram showing a specific configuration example for correcting variations in the output of each pixel in the image sensor.

14 is a diagram showing an example of the flow of processing in the output correction circuit in the configuration of FIG.

FIG. 15 is a characteristic diagram showing an example of a variation state of the output characteristics of the sensor signal coming from the configuration of each pixel in the image sensor.

16 is a characteristic diagram showing the result of offset correction of the sensor signal of each pixel having the output characteristic variation shown in FIG.

FIG. 17 is a characteristic diagram showing the result of offset correction and gain correction of the sensor signal of each pixel having the output characteristic variation shown in FIG. 15.

18 is a diagram showing another example of the flow of processing in the output correction circuit of FIG.

FIG. 19 is a characteristic diagram showing another example of a variation state of the output characteristics of the sensor signal coming from the configuration of each pixel in the image sensor.

20 is a characteristic diagram showing the result of offset correction of the sensor signal of each pixel having the output characteristic variation shown in FIG.

FIG. 21 is a characteristic diagram showing a result of offset correction and gain correction of the sensor signal of each pixel having the output characteristic variation shown in FIG. 19;

22 is a diagram showing still another example of the flow of processing in the output correction circuit of FIG.

FIG. 23 is a characteristic diagram showing still another example of the variation state of the output characteristics of the sensor signal coming from the configuration of each pixel in the image sensor.

[Explanation of symbols]

8 image sensor 9 ECU 10 AD converter 11 Memory for correction data 12 Output correction circuit 121 Offset correction unit 122 gain correction unit 14 buffer memory 15 Microcomputer 16 CPU

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4M118 AA02 AA05 CA02 CA09 DB09                       DD12 DD20 FA06 FA42                 5C024 CX27 GY31 GZ31 GZ36 HX40                       HX55                 5C051 AA01 BA02 DA06 DB01 DB07                       DB18 DC03 DC07 DE03 DE13

Claims (8)

[Claims] 1. An output signal of each pixel in an image sensor using a photosensor circuit for outputting a sensor signal corresponding to a current flowing through a photoelectric conversion element according to incident light in pixel units is stored in advance in a memory. In the output correction device of the image sensor, which reads the correction value according to the variation of the output characteristics of each pixel and corrects it by the predetermined arithmetic processing, for each pixel for a plurality of frames in the dark state or the bright state of the image sensor. An output correction device for an image sensor, which is provided with a means for obtaining an average value of the outputs of the above and a means for obtaining a correction value for each of the pixels so that the average values of the outputs of the respective pixels are predetermined. 2. The output correction device for an image sensor according to claim 1, wherein the correction value obtained in the dark state of the image sensor is an offset correction value for the output of each pixel. 3. The output correction device for an image sensor according to claim 1, wherein the correction value obtained when the image sensor is in a bright state is a gain correction value for the output of each pixel. 4. The output correction device for an image sensor according to claim 1, wherein the output of each pixel in the dark state of the image sensor is the output of each pixel in the image sensor when the incident light is cut off. . 5. The output correction of the image sensor according to claim 1, wherein the output of each pixel in the bright state of the image sensor is an output of each pixel in the image sensor when uniform light is irradiated. apparatus. 6. An image sensor converts a sensor current flowing through a photoelectric conversion element according to an amount of incident light at the time of photographing into a voltage signal with a logarithmic characteristic in a weak inversion state utilizing characteristics of a subthreshold region of a transistor, The image sensor output correction apparatus according to claim 1, wherein an optical sensor circuit that outputs a sensor signal corresponding to the converted voltage signal is used for each pixel. 7. A pixel signal in a dark state when a sensor signal when the gate voltage of a transistor for logarithmic characteristic conversion in a photo sensor circuit is switched to a value higher than a steady value at the time of photographing and the transistor is made conductive. The output correction device for an image sensor according to claim 6, wherein: 8. A gate voltage of a transistor for converting logarithmic characteristics in an optical sensor circuit is switched to a value higher than a steady value at the time of photographing to make the transistor conductive, and a drain voltage of the transistor is set to be smaller than a steady value. 7. The output correction device for an image sensor according to claim 6, wherein the sensor signal when switched to a low value is the output of the pixel in the bright state.