JPH05161076A - Drive method for solid-state image pickup device and solid-state image pickup device - Google Patents

Abstract

PURPOSE:To display detailed information of an object with a large dynamic range and a simple device by changing a reset level of a photoelectric conversion element corresponding to a mean illuminance of a surrounding area so as to reset them photoelectric conversion element. CONSTITUTION:A sensor 4 including a lot of MOS photoelectric conversion elements is driven by a drive circuit 9 and its output voltage is fed to a head amplifier 6. The head amplifier 6 stores once the detected signal to a memory and gives it to a control circuit 7. The control circuit 7 uses an averaging circuit 7a to average the illuminance within a peripheral area by the number of sides and picture elements, for example, and to decide a reset level corresponding to the averaged illuminance by a lookup table 7b. As a result, a reset level in matching with a highlight portion and a reset level in matching with a dark portion are set and a picture whose detailed part is kept is picked up within respective regions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state image pickup device, and more particularly to a method of driving the solid-state image pickup device and a solid-state image pickup device which can deal with a wide dynamic range object.

[0002]

2. Description of the Related Art A photoelectric conversion element that performs photoelectric conversion and accumulates the generated charges has a limit on the amount of charges that can be accumulated. If there is a dark portion and a highlight portion in the screen, the image information of the other portion will be scarce even if the exposure is adjusted to either one.

Therefore, the exposure time is set to two levels, long and short,
Widening the dynamic range is being carried out. This conventional technique will be described with reference to FIG. In FIG. 2, the horizontal axis represents the illuminance on the screen and the vertical axis represents the terminal voltage. If the imaging device is reset under certain conditions,
The relationship between the terminal voltage and the illuminance changes like EA and EB depending on the exposure time.

When the exposure time is shortened, the response to the highlight portion like the straight line EA becomes satisfactory, but the image information cannot be sufficiently extracted from the portion with low illuminance.

When the exposure time is lengthened, the integrated light quantity increases with respect to the same illuminance, so that the characteristic becomes like a straight line EB. In this case, the highlight portion is likely to be white. Therefore, an exposure EA with a short exposure time and an exposure EB with a long exposure time are performed together, image information of two screens is stored, and these two screens are combined by signal processing in an external circuit to increase the dynamic range. To get That is, a screen having a wide dynamic range DRX is reproduced by synthesizing two screens of the dynamic ranges DRA and DRB.

Further, when reproducing a picked-up image, a CRT or the like is generally used. By the way, the CRT has only a reproduction range of about 8 bits. When a screen having a wide dynamic range is to be reproduced by a display means having only a reproduction area of about 8 bits, the central area is aligned with the desired illuminance portion and the upper and lower areas are compressed or cut.

[0007]

As described above, when trying to obtain a wide dynamic range by the conventional technique,
A complicated procedure, an image memory device, a signal processing device, etc. were required.

It is an object of the present invention to provide a solid-state image pickup device capable of displaying detailed information of an object having a wide dynamic range with a simple device.

[0009]

A method for driving a solid-state image pickup device according to the present invention is a method for driving a solid-state image pickup device including a large number of MOS photoelectric conversion elements, which comprises a step of reading accumulated charges from each photoelectric conversion element. For each photoelectric conversion element, the step of determining the reset level corresponding to the average illuminance in the partial area based on the accumulated charge read in the partial area in the screen including the photoelectric conversion element, and the determined reset level The method includes a step of resetting each photoelectric conversion element and a step of reading the accumulated charge after reset from each photoelectric conversion element.

[0010]

When the photoelectric conversion element is reset, the reset level is changed in accordance with the average illuminance of the peripheral area, so that imaging can be performed according to the illuminance of the peripheral area.

That is, the reset level set to the highlight is set in the highlight part, and the reset level set to the dark part is set in the dark part. For this reason, it is possible to capture an image in which details are retained in both the dark area and the highlight area.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically shows a method of driving a solid-state image pickup device and a structure of the solid-state image pickup device according to an embodiment of the present invention.

FIG. 1A is a graph for explaining resetting of the photoelectric conversion element in the solid-state image pickup device. The horizontal axis shows the illuminance in the screen, and the vertical axis shows the terminal voltage. The image sensor is, for example, a photodiode, and is charged with a reverse bias voltage before light irradiation.

The characteristic of the standardly charged photoelectric conversion element is shown by a straight line 1. Due to the charging, the terminal voltage of the photoelectric conversion element is V ₀ . When this photoelectric conversion element is irradiated with light, electron-hole pairs are generated, and these carriers move according to the electric field in the photoelectric conversion element, whereby the photoelectric conversion element is gradually discharged.

After a certain exposure time, the terminal voltage (output voltage) of the photoelectric conversion elements distributed in the screen changes as shown by a straight line 1 according to the illuminance. That is, in the high illuminance region, the discharge current is large and the terminal voltage is low. On the other hand, in the dark area where the illuminance is low, the discharge is slightly performed and the terminal voltage is maintained at a high value.

By the way, the photoelectric conversion element has a limited illuminance region showing a good response as shown by the dynamic range DR0. If there is a bright portion or a dark portion that does not fit within the dynamic range in the screen, the captured image will be missing information.

Therefore, after the image is once taken, the photoelectric conversion element is charged with a higher reset voltage V ₀ + ΔV so that the bright portion is indicated by the straight line 2, and the dark portion is charged with a lower voltage V ₀ −ΔV. The photoelectric conversion element is charged.

As described above, when the initial state is changed according to the illuminance, in the charge accumulation by the next light irradiation, ΔV is high in the bright portion of the terminal voltage and ΔV is low in the dark portion. Therefore, the terminal voltage obtained after the elapse of a predetermined exposure time corresponds to the dynamic range DR0, but the image information thereof corresponds to the wider dynamic range DR1.

FIG. 1B shows an example of such reset level setting. The horizontal axis represents the read voltage, and the vertical axis represents the reset level. After resetting the photoelectric conversion element with a uniform reset voltage and irradiating light to accumulate charges, the read voltage ΔV is generated by the capacitance C coupled to the photoelectric conversion element and incident light. The charge ΔQ determines ΔV = ΔQ / C.

A high read voltage indicates that a large amount of light is emitted, and a low read voltage indicates that a small amount of light is emitted. Therefore, the reset level is set so as to be almost proportional to the read voltage.

In order to realize a wider dynamic range, it is effective to loosen the gradient in the low read voltage portion and the high read voltage portion as shown in the figure. Although the case where the gradient is changed continuously is shown,
The gradient may be changed in a polygonal line.

FIG. 1C schematically shows the structure of a solid-state image pickup device for realizing the above-described method for driving the solid-state image pickup device. The sensor 4 including a large number of photoelectric conversion elements for detecting incident light is driven by the drive circuit 9 and supplies its output voltage to the head amplifier 6. Head amplifier 6
Is the control circuit 7 after the detected signal is temporarily stored in the memory.
And the averaging circuit 7a in the control circuit 7 averages the illuminance in the peripheral area for one side + pixels, for example.
The look-up table 7b determines the reset level corresponding to this illuminance.

The look-up table 7b supplies an output signal corresponding to the vertical axis to an input signal (address signal) corresponding to the horizontal axis of the graph of FIG. 1B, and is composed of, for example, a ROM. ..

The reset level determined by the look-up table 7b is from the control circuit 7 to the reset circuit 5.
The reset circuit 5 resets the photoelectric conversion element in the sensor 4 at a level corresponding to the illuminance.

Thus, as shown in FIG. 1A, the initial state of the image pickup apparatus which is reset by the reset voltage of several stages is realized. Then, the sensor 4 is irradiated with light, and after a lapse of a predetermined time, the drive circuit 9 drives the output signal to read out the output signal through the head amplifier 6, thereby keeping the image information in a wide dynamic range in detail. You can read it.

FIG. 3 shows in more detail the sequence of the driving method of the solid-state image pickup device according to the embodiment of the present invention. After the image pickup device is switched on, at time t1, charge is read out from each photoelectric conversion element to initialize it. In this initialization, each photoelectric conversion element is reset with a uniform reset voltage to set a uniform initial state.

After the charge is stored, preliminary reading is performed at time t2. This charge read is a normal read, and a read signal corresponding to the illuminance of the screen can be obtained. The reset level in each area in the screen is determined according to the read signal.

After the reset level is determined, each photoelectric conversion element is reset at the determined reset level at time t3. In this state, each photoelectric conversion element is given a reset level according to the illuminance of its surroundings, and therefore the state of charge of each photoelectric conversion element is not uniform.

Subsequently, after charge accumulation by light irradiation is performed, image data is read from each photoelectric conversion element at time t4. This reading process is performed by charging each photoelectric conversion element with a uniform reset voltage.

The charge is read from the photoelectric conversion element four times at times t1, t2, t3 and t4, but at time t2.
Is to detect the illuminance information on the screen. The charge reading at time t3 is for setting each photoelectric conversion element in a state corresponding to the illuminance, and the reading itself has no meaning.

Further, the reading of the time t1 is for the initialization of the solid-state image pickup device, and it is not necessary to repeat it in the case of continuously picking up an image in the movie mode or the like. However, it is preferable to perform initialization (empty reading) when an image is to be captured again after a certain amount of time has elapsed in the still mode or the like.

In the case of subsequently capturing an image, the preliminary read result or the determined reset level is stored in the memory, and the reset level may be corrected using the image information obtained as a result of the data read. .. In this case, the required number of times of reading charges can be twice per image. In addition, the charge reading for determining the reset level does not have to be performed for all pixels. For example,
One line may be used for every few lines.

By strictly resetting each photoelectric conversion element according to the image information in the preliminary reading in accordance with the illuminance at that point, the image information extracted in the data reading can be one color for the entire screen.

In order to take out the details of the image information, the average illuminance in the peripheral area of the target photoelectric conversion element is detected, and if reset at a reset level corresponding to the average illuminance, the image information is obtained as relative illuminance to the average illuminance. Can read. By selecting the size of the area for calculating the average illuminance, it is possible to select the dynamic range in the reproduced screen.

FIG. 4 is a diagram for explaining a method of driving a solid-state image pickup device according to another embodiment of the present invention. In this way, by performing resetting according to the average illuminance in each part of the screen, it is possible to perform imaging while retaining the details of the image information in each region. For example, in an industrial TV or a research imaging device, the inside of the screen is extremely bright,
In the case where the image is extremely dark, it is possible to reproduce the image without losing any of the image information.

As shown in FIG. 4A, the screen 21 is divided into sub areas 23 in advance. The number of sub-regions 23 may be arbitrary, but it is preferable that most of the image information is selected within each sub-region so that it falls within a dynamic range of a predetermined width.

After once detecting the image information of the screen 21,
The average illuminance in each sub-region 23 is calculated, and the reset level corresponding to the average illuminance is determined. FIG. 4B is a diagram showing the reset level determined in this way as viewed in the horizontal direction. The horizontal axis represents the horizontal position x, and the vertical axis represents the determined reset level VR1. The reset level is high in highlight areas and low in dark areas. At the boundary of each sub-region, the reset level changes abruptly.

When each photoelectric conversion element is reset at such a reset level, light is irradiated to accumulate charges, and image information is extracted, an image in a wide dynamic range is obtained with a signal intensity corresponding to a predetermined dynamic range. You can play a screen that retains the details of information.

However, a stripe pattern appears due to the difference in reset level between the sub areas. For some purposes, for example, for monitor use and microscopic observation, the appearance of such stripes may not cause much trouble.

FIG. 4C shows the correction of the reset level for preventing such a striped pattern. That is, the reset level is corrected so that the sub-regions are connected smoothly. In this way, by determining that the reset level changes smoothly on the screen, the stripes on the boundary of the sub-region 23 on the reproduction screen disappear.

The screen imaged in such a state is
It is possible to reproduce the degree of brightness and darkness according to the average illuminance in each sub-region without causing annoying stripes, and to reproduce the details.

When performing repeated image pickup, the reset level for each sub area may be held in the memory, and the reset level may be corrected when the result of data reading falls outside the predetermined dynamic range.

In the standard driving method of the solid-state image pickup device shown in FIG. 3, image reading is required four times to obtain image information of one screen. According to the conventional technique, the time required for imaging becomes longer than the case where it is sufficient to perform only the image reading for initialization and the image reading for data reading twice.

FIG. 5 schematically shows the structure of a solid-state image pickup device capable of shortening the time required for one image pickup.
FIG. 5A schematically shows a plane structure of the semiconductor chip.
A light receiving portion 32, in which a large number of photoelectric conversion elements are arranged, is formed in the central portion of the semiconductor chip 31. Horizontal shift registers HSRA35 and HSRB3 are provided above and below the light receiving unit 32.
6 are formed, and vertical shift registers VSRA33 and VSRB34 are arranged on the left and right of the light receiving unit 32. That is, the drive circuits for driving the light receiving section 32 are provided in duplicate.

FIG. 5B is an enlarged view of a circuit for reading out charges from the photodiode in the light receiving section 32. Two transfer gates Q1 and Q2 are connected to the photodiode PD, which is a photoelectric conversion element, and these transfer gates Q1 and Q2 are connected to vertical signal lines 41 and 42, respectively, and output via other transfer gates Q3 and Q4. It is connected to the signal line. Transfer gate Q1, Q
2, Q3 and Q4 are formed of MOS transistors, for example.

The gate electrodes of the transfer gates Q1 and Q2 are controlled by word lines 43 and 44, respectively. The gate electrodes of the transfer gates Q3 and Q4 are controlled by control lines 47 and 78, respectively. The word lines 43 and 44 are supplied with control signals by the vertical shift registers 34 and 33, and the control lines 47 and 48 are supplied with control signals from the horizontal shift registers 36 and 35.

Each photodiode PD has VSRB 34 and H
Under the control of the SRB 36, the transfer gates Q1 and Q3
Can also be read via VSRA33, HSRA
It can also be read out via the other transfer gates Q2 and Q4 under the control of 35.

The read circuits formed in duplicate can be controlled independently of each other, and the image information shown in FIG. 3 can be read in parallel. For example, the time required for imaging can be shortened by performing the preliminary reading shown at time t2 in FIG. 3 by one drive circuit and performing the reset operation shown at time t3 in FIG. 3 with a fixed time difference. You can

The present invention has been described above with reference to the embodiments.
The present invention is not limited to these. For example,
The method of determining the reset level corresponding to the illuminance distribution on the screen may be performed by another method such as selecting one of several preset levels. Further, the setting of the area for detecting the average illuminance can be arbitrarily selected.

It will be apparent to those skilled in the art that various other modifications, improvements, combinations and the like can be made.

[0051]

As described above, according to the present invention,
By performing a reset according to the average illuminance in each part of the screen, it is possible to obtain image information that retains details in a wide illuminance range for a subject having a wide dynamic range illuminance.

[Brief description of drawings]

FIG. 1 is a graph and a block diagram for explaining an embodiment of the present invention.

FIG. 2 is a graph for explaining a conventional technique.

FIG. 3 is a time chart for explaining the sequence of the embodiment of the present invention.

FIG. 4 is a schematic plan view and a graph for explaining an example of the present invention.

FIG. 5 is a schematic plan view and a circuit diagram for explaining an embodiment of the present invention.

[Explanation of symbols]

 1, 2, 3 Terminal voltage contrast 4 Sensor 5 Reset circuit 6 Head amplifier 7 Control circuit 7a Averaging circuit 7b Look-up table 9 Driving circuit 21 Screen 23 Sub area 31 Semiconductor chip 32 Light receiving part 33, 34 Vertical shift register 35, 36 Horizontal shift register PD Photodiode Q transistor

Claims (4)

[Claims] 1. A method of driving a solid-state imaging device including a large number of MOS photoelectric conversion elements, the method comprising: reading out accumulated charges from each photoelectric conversion element; and for each photoelectric conversion element, a screen including the photoelectric conversion element. Determining the reset level corresponding to the average illuminance in the partial region based on the accumulated charge read in the partial region, resetting each photoelectric conversion element at the determined reset level, and each photoelectric conversion element A method of driving a solid-state imaging device, the method comprising: 2. The method for driving a solid-state imaging device according to claim 1, wherein the screen is divided into a plurality of sub-regions in advance, and the sub-region to which each photoelectric conversion element belongs is used as the partial region. 3. The method of driving a solid-state imaging device according to claim 2, wherein the reset level is determined so that the reset level is smoothly connected around the boundary of the sub-region. 4. A large number of photoelectric conversion elements distributed in a screen, and a reset for each photoelectric conversion element, which is a part of the screen and corresponds to an average illuminance in a region including the photoelectric conversion elements. A solid-state imaging device comprising: a unit for determining a level; a unit for resetting each photoelectric conversion element using the reset level; and a unit for reading out accumulated charges from each photoelectric conversion element.