JP2003324657A - Output correction device for image sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image having proper image quality, wherein the logarithmic output characteristic lost upon low illuminance is recovered by initializing an optical sensor circuit, and the contrast will not deteriorate, even upon low illuminance. <P>SOLUTION: An image sensor is adapted as follows: a sensor current, conducted through a photoelectric converter in response to the quantity of incident light upon photographing is converted to a voltage signal with a logarithmic characteristic in a weak inversion state utilizing a sub-threshold region of a transistor; a sensor signal in response to the voltage signal thus converted is outputted; drain voltage of the transistor is changed to a value lower than the steady state value prior to the photographing to discharge electric charges stored in a parasitic capacitor of the photoelectric converter for initialization of an optical sensor circuit used as pixels. In the optical sensor circuit, a means is provided for converting the output characteristic of a non-logarithmic response region in each optical sensor circuit, into a logarithmic one. <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 a light for converting a sensor current flowing through a photoelectric conversion element according to the amount of incident light into a voltage signal with a logarithmic characteristic in a weak inversion state utilizing the characteristics of a subthreshold region of a transistor and outputting the voltage signal. The present invention relates to an output correction device for an image sensor using a sensor circuit for pixels.

[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 a sensor current flowing through the photodiode PD into a voltage signal Vpd with a logarithmic output characteristic in a weak inversion state, and a transistor Q for amplifying the voltage signal Vpd.
2 and the transistor Q3 that outputs the sensor signal Vo at the pulse timing of the read signal Vs. By providing the logarithmic output characteristic, the dynamic range can be expanded and the optical signal can be detected with high sensitivity. I have to.

In the image sensor having such a configuration, the photodiode P in the photosensor circuit of each pixel is
When the incident light Ls is incident on D 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, so that an afterimage may occur as an image sensor. The optical signal can be detected with a sufficient response speed that does not occur.

However, the incident light L of the photodiode PD
When the amount of light of s decreases and the current flowing through the transistor Q1 decreases, the transistor Q1 is set so that its resistance value increases by one digit when the current flowing through the transistor Q1 decreases by one digit. And 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.

FIG. 6 shows a change characteristic of the voltage signal Vpd when the sensor current of the photodiode PD suddenly changes from 1E-10A to 1E-15A.

From this characteristic, when the sensor current of about 1E-12A in which the light amount of the incident light Ls to the photodiode PD is small, when the sensor signal Vo is output every 1/30 sec, the voltage signal is output within that time. It can be seen that Vpd is not saturated.

Therefore, the saturation time of the voltage signal Vpd corresponding to the sensor current when the light quantity of the incident light Ls on the photodiode PD is small becomes long, so that the sensor signal Vo 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. 8, Vpd 'indicates a voltage signal that is inverted and amplified by the amplifying transistor Q2.

Therefore, conventionally, a pixel is provided with an optical sensor circuit in which a sensor current flowing in a photoelectric conversion element is converted into a voltage signal with a logarithmic output characteristic in a weak inversion state using a MOS transistor in accordance with the amount of incident light at the time of photographing. As a unit, in an image sensor in which a plurality of pixels are arranged in a matrix, by setting the drain voltage VD of the transistor Q1 in each photosensor circuit to a value lower than a steady value during shooting for a predetermined time prior to shooting. By discharging and initializing the residual charges accumulated in the parasitic capacitance C of the photodiode PD, even if a sudden change occurs in the sensor current, the voltage signal Vpd corresponding to the light amount of the incident light Ls at that time is immediately generated. So that an afterimage does not occur even when the amount of incident light is small.
000-329616).

In such an optical sensor circuit, as 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.

When an image sensor using such an optical sensor circuit as a pixel is mounted on a vehicle to detect a white line in front of a road on which the vehicle is traveling for the purpose of controlling the automatic traveling of the vehicle, it can be used for day and night. A wide dynamic range is required so that a white line can be projected with good contrast even under severe conditions such as shooting or shooting the bright part of the tunnel exit from the dark tunnel or the dark part of the tunnel from the bright tunnel entrance. .

At that time, there is always a constant brightness difference between the road surface and the white line regardless of day or night. When the sensor signal is logarithmically processed in the photosensor circuit, the brightness is always constant. This is convenient because there is a signal difference of.

However, in the photosensor circuit having the logarithmic output characteristic described above, in the case where the initialization means for suppressing the occurrence of the afterimage is taken, as shown in FIG. When the sensor current flowing through the diode PD is small (when the illuminance is low), there is a response delay in charging the parasitic capacitance C of the photodiode PD, and the logarithmic output characteristic is lost.

[0013]

The problem to be solved is that the sensor current flowing through the photoelectric conversion element has a logarithmic characteristic in a weak inversion state utilizing the characteristics of the subthreshold region of the transistor according to the amount of incident light at the time of photographing. The voltage signal is converted, and a sensor signal corresponding to the converted voltage signal is output, and the drain voltage of the transistor is switched to a value lower than a steady value prior to photographing,
In an image sensor that uses a photosensor circuit for pixels to discharge and initialize charges accumulated in the parasitic capacitance of the photoelectric conversion element, the logarithmic output characteristic of each sensor circuit is lost when the illuminance is low. It is to end up.

[0014]

According to the present invention, a sensor current flowing through a photoelectric conversion element is converted into a voltage signal with a logarithmic characteristic in a weak inversion state utilizing the characteristics of a subthreshold region of a transistor according to the amount of incident light at the time of photographing. Then, while outputting a sensor signal according to the converted voltage signal, the drain voltage of the transistor is switched to a value lower than a steady value prior to photographing, and the charge accumulated in the parasitic capacitance of the photoelectric conversion element is changed. In an image sensor that uses a photosensor circuit for pixels to discharge and initialize
A means for converting the output characteristic of the non-logarithmic response region in each photosensor circuit into a logarithmic characteristic is provided.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the image sensor according to the present invention, the photosensor circuit shown in FIG. 1 is basically used for each pixel.

In the photosensor circuit, prior to photographing, the drain voltage VD of the transistor Q1 for logarithmic characteristic conversion is set to be lower than the steady value for photographing for a predetermined time, and the parasitic of the photodiode PD is set. The charges accumulated in the capacitor C are discharged and initialized.

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, the MO
The drain voltage VD of the S transistor Q1 is low level L
If the potential difference between the gate voltage VG and the drain voltage VD at that time is larger than the threshold value of the transistor Q1, then the transistor Q1 enters the low resistance state. As a result, the potential on the source side at that time becomes the same as the drain voltage VD (actually, a potential difference corresponding to the threshold remains), and the junction capacitance C of the photodiode PD is discharged.

FIG. 4 shows the transistor Q at the time of initialization.
The operating state due to the flow of the electric charge q of 1 is simulated.

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.
When 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 the low resistance state, and the junction capacitance C of the photodiode PD is in the charged state. become.

FIG. 5 schematically shows an operating state due to the flow of charges q of the transistor Q1 at the time of detecting an optical signal.

As described above, if the junction capacitance C of the photodiode PD is discharged and initialized prior to the detection of the optical signal and then the junction capacitance C is charged, a certain time elapses from the initialization timing. The output voltage (terminal voltage of the photodiode PD) Vpd at that time has a value corresponding to the amount of 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 for a fixed time after the initialization, it is possible to obtain the sensor signal Vo having no afterimage according to the light amount of the incident light Ls.

FIG. 6 shows the change characteristics of the voltage signal Vpd when the sensor current of the photodiode PD changes abruptly from 1E-10A to 1E-15A. It shows a case where the timing of signal detection is set.

FIG. 7 shows the voltage signal Vp when the optical signal is repeatedly read at the timing of 1/30 sec.
The characteristic of the amplified signal of d is shown. According to this, 1 /
It can be seen that the signal characteristic obtained every 30 seconds corresponds to the sensor current according to the light amount of the incident light Ls on the photodiode PD, and there is no influence of an afterimage.

FIG. 9 shows the output characteristics of the sensor signal Vo with respect to the sensor current when the light quantity of the incident light Ls on the photodiode PD is changed. According to this, it can be seen that when the sensor current of the photodiode PD is 1E-13A or more, the logarithmic output characteristic is completely obtained. Further, it can be seen that in the region where the sensor current is 1E-13A or less, an output without an afterimage can be obtained although it deviates from the logarithmic characteristic.

Further, the drain voltage V of the transistor Q1
If you adjust the value of low level L when lowering D,
If the voltage is lowered until the transistor Q1 is brought into the low resistance state completely, the output characteristic as shown in FIG. 9A is obtained. However, if the control voltage VD is set to be the same as the gate voltage VG, a normal logarithmic output characteristic as shown by (b) in FIG. 9 is obtained.

Therefore, in the case of the output characteristic shown in FIG. 9A, there is no afterimage, but the sensitivity is reduced when the amount of light is small. In the case of the logarithmic output characteristic shown in (b) in FIG. 9, the afterimage becomes remarkable although the sensitivity is high even when the light amount is small. That is, a trade-off relationship is established between the sensitivity and the afterimage.

FIG. 10 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 so that the sensor signal of each pixel is read in time series. A specific configuration example 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 the image sensor, when selecting a pixel line for each one line, the drain voltage VD of the transistor Q1 of each pixel in the selected pixel line is set to a high level in a steady state at a predetermined timing. A voltage switching circuit 5 for switching to H and to a low level L at initialization is provided.

The operation of the image sensor configured as described above 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 becomes 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 corresponding second pixel column D21, D22, D23, D24 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.

When the pixel column selection signal LS1 falls to the low level L after the T1 period, 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, each of the pixels is initialized 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, and the next cycle performed after the passing of one cycle period T3. To read the sensor signal at.

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 reading of the sensor signal in the next cycle is performed after the one-cycle period T3 is exceeded.

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.

According to the present invention, there is provided means for converting the output characteristic of the non-logarithmic response region of each photosensor circuit in the image sensor thus constructed into a logarithmic characteristic.

Specifically, as shown in FIG. 12, after the sensor signal Vo of each pixel read out from the image sensor 8 in time series is converted into a digital signal by the AD converter 13, the digital signal of the digital signal is converted in advance. The output characteristic of the non-logarithmic response region is converted into the logarithmic characteristic by using the lookup memory 14 in which the conversion table of the input / output characteristic is set so that a predetermined output is obtained according to the value.

In FIG. 13, the output characteristic of the sensor signal Vo output from the optical sensor circuit in the non-logarithmic response region is converted into a logarithmic characteristic and corrected so as to show a logarithmic characteristic over the entire region from low illuminance to high illuminance. The sensor signal Vo 'is shown.

When converting the output characteristic of the non-logarithmic response area into the logarithmic characteristic using the lookup memory 14 according to the digital value of the sensor signal Vo, the conversion data is expressed by 1 byte (8 bits). In case of 8
Divide into areas that can be expressed in bits.

In that case, under the control of a controller (not shown), each divided signal is given an identification code so that the divided signal can be identified from the side using the converted signal. To do. Further, under the control of the controller, the divided areas can be switched according to the request from the signal utilizing side.

FIG. 14 shows a case where the sensor signal Vo output from the optical sensor circuit is divided into two with an arbitrary incident light amount as a boundary and the characteristics are switched to Vo1 'and Vo2'.

Further, when the sensitivity of the non-logarithmic response region of the sensor signal Vo is increased to convert the output characteristic into the logarithmic characteristic, the noise included in the sensor signal Vo is also amplified and becomes a sensor signal in which the noise is emphasized. It will be converted.

At this time, it is conceivable to filter the sensor signal Vo converted into the logarithmic characteristic to remove noises. At that time, if filtering is performed over the entire region,
The whole output image of the image sensor becomes blurred.

Therefore, in the present invention, the sensor signal Vo '
The noise is removed by filtering only the portion of the output signal at the time of low illuminance converted into the logarithmic characteristic in, that is, the portion corresponding to the non-logarithmic response region in the original sensor signal Vo.

By performing such filter processing, the noise component amplified when converting the output characteristic into the logarithmic characteristic by increasing the sensitivity of the non-logarithmic response region of the sensor signal Vo is effectively removed. Therefore, the same filtering process is not unnecessarily performed on the other portions where the noise is not amplified, the entire output image of the image sensor is not blurred, and the noise component is effectively removed and the contrast is good. The processed image can be obtained.

Further, according to the present invention, when noise is removed by filtering only the part of the output signal at low illuminance converted into the logarithmic characteristic in the sensor signal Vo ', the non-logarithmic response region in the photosensor circuit is removed. The filter characteristic is changed according to the degree to which the output characteristic is converted to the logarithmic characteristic, that is, the amplification factor in the memory 14 for input / output characteristic conversion.

As described above, by performing the noise removal with the filter characteristic according to the amplification factor of the memory 14, the noise component amplified at the time of converting the input / output characteristic of the sensor signal Vo without causing excess or deficiency of the noise removal. Can be properly removed.

Further, if the amplification factor at the time of input / output characteristic conversion is large, as shown in FIG. 15, the sensor signal between adjacent pixels due to the digital signal becomes discrete and the image quality of the output image of the image sensor is poor. However, by performing this filtering process, the sensor signal between adjacent pixels is smoothed and the continuity of the sensor signal is improved.

A general noise removing filter is widely used in the filtering process of the present invention.

FIG. 16 shows an example of the IIR low pass filter. Besides, a FIR filter or the like is used.

FIG. 17 shows an example of a filter processing circuit according to a method (moving average processing) of obtaining an average value of outputs of adjacent pixels and removing high frequency components. Since the image signal has continuity, a signal that changes abruptly as compared with an adjacent pixel can be treated as noise.

In FIG. 18, a plurality of frame memories FM are used to filter the output of the pixel at the same position. The filter used is as shown in Fig. 1.
6 and those shown in FIG. 17 are used.

19 to 22 show the sensor signal Vo '.
3A and 3B respectively show configuration examples of a concrete filter processing circuit when a noise is removed by filtering only a portion of the output signal at low illuminance converted into the logarithmic characteristic in FIG.

In the filter processing circuit shown in FIG. 19, a signal value serving as a switching point of the sensor signal Vo 'that has been input / output converted in advance is set in the comparison circuit 21, and is sent from each pixel of the image sensor in time series. While switching the signal switching circuit 22 according to the value of the received sensor signal Vo ′,
A signal obtained by processing only the output signal at low illuminance converted into the logarithmic characteristic by the low-pass filter 23 and the sensor signal V
It is configured to selectively output the signal which is not subjected to any filtering in the other part of o '.

In the filter processing circuit shown in FIG. 20, the value of the signal processed by the low pass filter 23 is compared with the value set in the comparison circuit 21 to switch the signal.

In the filter processing circuits shown in FIGS. 21 and 22, respectively, the sensor signal Vo 'is changed while changing the filter characteristic according to the amplification factor at the time of input / output characteristic conversion.
It is made possible to filter only the part of the output signal at low illuminance converted into the logarithmic characteristic in.

In the filter processing circuit shown in FIG. 21, a plurality of values are set in the comparison circuit 21 ', each set value is compared with the sensor signal Vo', and the filter characteristics are respectively changed in the signal switching circuit 22. Of the low-pass filters 23-1 to 23-4 different from each other, one of the processed signals is selectively output.

In the filter processing circuit shown in FIG. 22, the value of the signal processed by a specific low-pass filter 23-3 of the plurality of low-pass filters 23-1 to 23-4 provided and set in the comparison circuit 21. Compare with the value,
The signal switching circuit 22 is configured to selectively output one of the signals processed by the low-pass filters 23-1 to 23-4 having different filter characteristics.

In the filter processing circuits shown in FIGS. 21 and 22, one of the processed signals by the low pass filters 23-1 to 23-4 having different filter characteristics is selectively output. However, the constant of the filter characteristic in one low-pass filter 23 may be switched.

In the image sensor using the optical sensor circuit as shown in FIG. 1 for each pixel, as shown in FIG. 23, the output characteristics vary due to the structure of each pixel, and the output characteristics are uniform. Thus, it is necessary to correct the output of each pixel. 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 according 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.

Means for correcting variations in the output characteristics of each pixel have been proposed by the same applicant as the present application (Japanese Patent Application Nos. 2000-404931, 2000-40493).
3, Japanese Patent Application No. 2001-75035, Japanese Patent Application No. 2001-750
36).

According to the present invention, after correcting the variation in the output characteristic of each pixel, a process for converting the sensor signal Vo output from each pixel in time series into a complete logarithmic characteristic is performed, and the conversion is performed. The sensor signal Vo 'is filtered.

FIG. 24 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 out 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 the sensor signal Vo of each pixel output from the image sensor 8 in time series, as described above, 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. 26 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. 25 shows a flow of processing 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
27, 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 No. 2, 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. 30 shows another example of the variation state of the output characteristics of the sensor signals A, B, C coming from the configuration of three pixels.

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

FIG. 29 shows a flow of processing in the output correction circuit 12.

The memory 11 has 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
In FIG. 31, 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. 31, the sensor signal A of each pixel, 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, a predetermined multiplier MLT for gain correction read from the memory 10 is used to output ← H- (H- The calculation of sensor signal DS1) × multiplier is performed, 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 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. 34 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 slopes of the output characteristics of the sensor signals A, B, C in the logarithmic response area WB are different, 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. 33, the respective processings shown in FIG. 25 and FIG. 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.

[0091]

As described above, according to the present invention, the sensor current flowing in the photoelectric conversion element is converted into a voltage signal with a logarithmic characteristic in the weak inversion state utilizing the characteristic of the subthreshold region of the transistor according to the amount of incident light at the time of photographing. , Outputs a sensor signal according to the converted voltage signal, switches the drain voltage of the transistor to a value lower than a steady value prior to photographing, and discharges charges accumulated in the parasitic capacitance of the photoelectric conversion element. In an image sensor using a photosensor circuit that is adapted to be initialized as a pixel, a means for converting the output characteristic of the non-logarithmic response region in each photosensor circuit into a logarithmic characteristic is provided. It is possible to recover the logarithmic output characteristics lost in low illuminance and obtain a high quality image with no deterioration in contrast even in low illuminance. It has the advantage that it is.

Further, according to the present invention, by providing the filter processing means for removing the noise of the output portion converted into the logarithmic characteristic, the noise component amplified during the conversion of the logarithmic characteristic is effectively removed, and the image quality without blurring is obtained. It has an advantage that a processed image with good quality can be obtained.

[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 according to an amount of incident light when the optical sensor circuit is initialized.

FIG. 4 is a diagram schematically showing an operating state due to a flow of a charge q of a transistor Q1 at the time of initialization of an optical sensor circuit.

FIG. 5 is a diagram schematically showing an operating state due to a flow of a charge q of a transistor Q1 when an optical signal of the optical sensor circuit is detected.

FIG. 6 is a diagram showing a change characteristic of each voltage signal Vpd when a sensor current of a photodiode PD in an optical sensor circuit changes.

FIG. 7 is a voltage signal Vpd when an optical sensor circuit repeatedly reads an optical signal at a predetermined timing.
It is a figure which shows the characteristic of the amplified signal of.

FIG. 8 is a diagram showing an output characteristic of an image signal read at a predetermined timing when the amount of incident light in the photosensor circuit when initialization is not performed is small.

FIG. 9 is a diagram showing output characteristics of a sensor signal with respect to a sensor current when a drain voltage of a transistor for converting logarithmic characteristics in an optical sensor circuit is changed.

FIG. 10 is a block showing a specific configuration example of an image sensor according to the present invention.

FIG. 11 is a time chart of each signal in the image sensor.

FIG. 12 is a block diagram showing an embodiment of an output correction device for an image sensor according to the present invention.

FIG. 13 is a characteristic diagram in which the output of the sensor signal with respect to the sensor current according to the amount of incident light when the initialization is performed in the optical sensor circuit according to the present invention is converted into a logarithmic characteristic.

FIG. 14 is a characteristic diagram in which the output of the sensor signal with respect to the sensor current according to the amount of incident light when initialization is performed in the optical sensor circuit according to the present invention is divided at arbitrary points and converted into logarithmic characteristics.

FIG. 15 is a diagram showing an example of a state in which a digitized sensor signal of each pixel in an image sensor is discrete.

FIG. 16 is an electric circuit diagram showing a configuration example of a general low-pass filter.

FIG. 17 is a block diagram showing an example of a configuration for performing a filtering process by a method of obtaining an average value of outputs of adjacent pixels and removing a high frequency component.

FIG. 18 is a block diagram showing a configuration example when performing a filtering process on the same pixel using a plurality of frame memories.

FIG. 19 is a block diagram showing an example of a configuration for filtering a sensor signal converted into a logarithmic characteristic according to the present invention.

FIG. 20 is a block diagram showing another configuration example when filtering the sensor signal converted into the logarithmic characteristic according to the present invention.

FIG. 21 is a block diagram showing a configuration example when a sensor signal converted into a logarithmic characteristic is filtered while changing the filter characteristic according to the present invention.

FIG. 22 is a block diagram showing another configuration example when filtering the sensor signal converted into the logarithmic characteristic according to the present invention while changing the filter characteristic.

FIG. 23 is a diagram showing an example of a variation state of output characteristics of each pixel in the image sensor.

FIG. 24 is a block diagram showing a specific configuration example for correcting variations in the output of each pixel in the image sensor.

25 is a diagram showing an example of the flow of processing in the output correction circuit in the configuration of FIG. 24.

FIG. 26 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.

27 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. 26.

FIG. 28 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. 26.

FIG. 29 is a diagram showing another example of the processing flow in the output correction circuit of FIG. 24.

FIG. 30 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.

FIG. 31 is a characteristic diagram showing a result of offset correction of the sensor signal of each pixel having the output characteristic variation shown in FIG. 30.

32 is a characteristic diagram showing the results of offset correction and gain correction of the sensor signal of each pixel having the output characteristic variation shown in FIG. 30.

FIG. 33 is a diagram showing still another example of the processing flow in the output correction circuit in FIG. 24.

FIG. 34 is a characteristic diagram showing still another example of variations in output characteristics of sensor signals coming from the configuration of each pixel in the image sensor.

[Explanation of symbols]

8 image sensor 13 AD converter 14 Memory for input / output characteristic conversion table 21 Comparison circuit 21 'comparison circuit 22 Signal switching circuit 23 Low-pass filter 23-1 Low-pass filter 23-2 Low-pass filter 23-3 Low-pass filter 23-4 Low-pass filter

Claims (7)

[Claims] 1. A sensor current flowing through a photoelectric conversion element according to an amount of incident light at the time of photographing is converted into a voltage signal with a logarithmic characteristic in a weak inversion state utilizing characteristics of a subthreshold region of a transistor, and the converted voltage is obtained. A sensor signal corresponding to the signal is output, and the drain voltage of the transistor is switched to a value lower than a steady value prior to shooting so as to discharge and initialize charges accumulated in the parasitic capacitance of the photoelectric conversion element. An image sensor using the above-described optical sensor circuit as a pixel is provided with means for converting an output characteristic of a non-logarithmic response region in each optical sensor circuit into a logarithmic characteristic. 2. The output characteristic of the non-logarithmic response region is converted into a logarithmic characteristic by using an input / output characteristic conversion table which is set in advance so that a predetermined output is obtained according to an input. An output correction device for an image sensor according to claim 1. 3. The output correction device for an image sensor according to claim 1, further comprising a filter processing unit for removing noise of the sensor signal converted into a logarithmic characteristic. 4. A sensor signal value is compared with a preset value, and a filtered sensor signal is output in a region where the sensor signal value is equal to or less than the set value, and the sensor signal value is set to the set value. The output correction device for an image sensor according to claim 3, wherein the sensor signal is output as it is in a region exceeding the value. 5. The output correction of the image sensor according to claim 3, wherein the filter characteristic is changed according to the degree to which the output characteristic of the non-logarithmic response region in the optical sensor circuit is converted into the logarithmic characteristic. apparatus. 6. The image sensor output correction apparatus according to claim 5, wherein a plurality of filters each having a different filter characteristic are switched to change the filter characteristic. 7. The output correction device for an image sensor according to claim 5, wherein the filter characteristic is changed by switching the constant of the filter.