JPS585084A - Solid-state image pickup device - Google Patents

Abstract

PURPOSE:To eliminate variation in black level by shielding part of a solid-state image pickup element optically, subtracting a signal from the shield part from an output signal, and then clamping the output. CONSTITUTION:Several columns of picture elements of an image pickup element 101 are shielded optically. The output of the image pickup element 101 is led through a preamplifier 102 and a low-pass filter 103 to a clamping circuit 104 and a subtracting circuit 107. A signal, which corresponds to the optical shield part, of the output of the clamping circuit 104 is written in a memory 105 and supplied as a vertical smear signal to the subtracting circuit 107. The output of the circuit 107 is clamped by a clamping circuit 108.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an imaging device, and more particularly to suppression of variations in black level caused by changes in dark current and vertical smear in a solid-state imaging device.

In imaging devices that convert images into electrical signals, solid-state imaging devices obtained by semiconductor integrated circuit technology have recently been used as photoelectric conversion devices. By using a solid-state image sensor, an image pickup device can be made smaller, more reliable, and have a longer lifespan than an image pickup tube type.

Incidentally, in general, a photoelectric conversion device generates a dark current that increases or decreases depending on the temperature, and when this dark current increases or decreases in the signal current, the black level of the video signal may fluctuate.

In order to suppress this, conventionally, a black mask as shown in FIG. 1 has been used, which covers a part of the photoelectric conversion section and blocks the incidence of light. In the figure, reference numeral 1 denotes a portion of the photoelectric conversion section that scans during the image period of the video signal, and is optically exposed. Reference numeral 2 denotes a portion of the photoelectric conversion section that scans during a part of the horizontal blanking period of the video signal, and is optically masked. FIG. 2 shows an example of a signal obtained when such a photoelectric conversion section is scanned. In the figure 3
The period indicated by is one horizontal scanning period of the video signal, the period indicated by 4 is a horizontal blanking period, the period indicated by 5 is a period during which the nine element conversion unit is covered with a black mask, and the period indicated by 6 is an optical period. The period indicated by 7, which is a video period in which the normal photoelectric conversion section is scanned, is a period in which the photoelectric conversion section is not scanned.

5. Since the photoelectric conversion unit is scanned in both periods indicated by 6, the signal obtained here includes the dark current generated in the photoelectric conversion unit, whereas the signal obtained during the period indicated by 7 does not include the dark current generated in the photoelectric conversion unit. do not have. In other words, the dark current generated in the photoelectric conversion section is I4,
When the signal current corresponding to the subject image is calculated, 7. s,
Signal Iy=Is, I obtained in each period indicated by 6
・ are as follows.

■、=0...0
) I s ” I-...(2) Engineering・"' I a + I-...(3) As is well known, the video signal has a DC component, and unless this is reproduced, the brightness and darkness will not be correct. I cannot get a playback image. Usually, a pulse clamp circuit is used to regenerate the DC component. Since the pulse clamp circuit is widely used, a detailed explanation will be omitted, but DC reproduction is performed by fixing the DC level of the video signal during the period in which the pulse is input to a reference value. Normally, the pulse position is placed within the horizontal blanking period, but if this is selected to be the period shown by 5 in FIG. 2, the dark current I4 is fixed at the reference DC level. As a result,
The DC level with respect to the reference of the video period is the signal current 1. The black level of the video signal is not affected by dark current.

Conventionally, in solid-state imaging devices, similar effects have been obtained by optically covering the training μ located at the position corresponding to the beginning of scanning in a horizontal pixel array with a black mask. An example of the configuration of a solid-state image sensor with a black mask is shown in FIG. 3, taking a MOS type as an example. In the figure, the vertical scanning circuit (
The MOS transistor 12 is opened and closed by the vertical scanning pulse output from the shift register 9 to the signal line 16.
The photo-charge weight accumulated in the photodiode IIK and the dark current generated in the photodiode I4. is transferred to the vertical signal line 13. The dark current I4v generated in the vertical signal line is added to this, and the MOS transistor 14 is opened and closed by the horizontal scanning pulse outputted from the horizontal scanning circuit (shift register) 8, so that the dark current I4v generated in the vertical signal line is sequentially transmitted to the horizontal output line 15. taken out,
Here, the dark current l4IK generated in the horizontal output line is added and output from the output terminal 17. The portion designated by 10 indicates the portion of the black mask that is optically covered. Output terminal 17
An example of the signal obtained from is shown in Figure #g4. Similar to Fig. 2, 3 is one horizontal scanning period, 4 is horizontal blanking period,
5 is a period in which picture elements covered with a black mask are scanned, 6 is a period in which normal picture elements are scanned, and 7 is a period in which picture elements are not scanned. ? , 5.6 Signal LqZ obtained in each period
II'+1. / are as follows.

Iy'=Ia violet
-(4) I,'=I-Iris +
I a-+ I am
・・・Can) Is""Ia 冨+I a-+ I
Direct, +■, ...(6
) Here, set the pulse position of the pulse clamp circuit to 5 in Figure 4.
If the period indicated by is selected, the dark current Iam+Iav+Iip is fixed at the reference DC level. As a result of this, the DC level of the video period with respect to the reference is determined by the signal current 1. The zero level of the video signal is not affected by dark current.

However, in solid-state imaging devices, there are factors other than dark current that cause the black level of the video signal to vary. This will be explained next.

In a solid-state image sensor, it is desirable that the part waiting for photosensitivity is only the photodiode 11 shown in Figure 3 (11/), but in reality, it is the peripheral part of the photodiode, for example, the MO
S) The drain of the transistor 12 can also be photosensitive. The photocharge generated at the drain is transferred to the vertical signal line 13 regardless of whether the MOS transistor is on or off, but since all the drains of hundreds of MOS transistors lined up above and below are connected to the non-electrical signal line, , the RT nine-light information projected onto each transistor is mixed and added to the vertical signal line and stored. That is, a signal current corresponding to the integrated light amount in one direction of the projected subject image is accumulated in each vertical signal line.

Since the signal is added to the normal signal current for each horizontal scanning period, for example, if you capture an object image that includes a bright area as shown in Fig. 5(a), the signal shown in Fig. 5 will appear on the playback screen. ), a trailing false signal is generated in the vertical direction. This kind of noise component specific to solid-state image sensors is called vertical smear. Due to the presence of vertical smear, the black level of the video signal changes in a vertical line shape as the integrated light amount of the subject in the vertical direction increases.

In our company's model a (Utility Model Application Publication No. 11968/1983), which was devised to remove vertical smear, the vertical smear is equal in each horizontal scanning period as is clear from the generation process, so one The signal corresponding to the vertical smear for the horizontal scanning period is stored in the t-IH memory (memory that can record the signal for the horizontal scanning period), and this is subtracted from the video signal obtained in each horizontal scanning period. It was proposed to remove it. The embodiment of the earlier application is shown in FIG. In the figure, the signal output from the solid-state image element 18 is transmitted to a preamplifier 19,
It is added to the subtraction circuit 23, the memo +721, and f'L6° via the low frequency three-wave generator 20t. At this time, the vertical smear of one horizontal scanning period is recorded in the memory in the vertical blanking period by the control signal from the synchronization circuit 22. .

The recorded signal is read out during a normal horizontal scanning period, is applied to a subtraction circuit 23, and is subtracted from the signal from the n1 low-frequency three wave filter. Here, in order to obtain a signal corresponding to the vertical smear multiplied by 11 on the vertical signal line 13 shown in FIG.
If the MOS) transistor 12 shown in the figure is turned off, no signal will be mixed in, but the dark current I4 generated in the photodiode. is not included. As a result, the signals "M41 IMs" obtained during the horizontal blanking period and the period of scanning the vertical signal lines written in the memory are respectively Ii
z4xI4M...(
7) IM6-Iam+Ia-+I-- (8). Here, Ls is a signal corresponding to the vertical smear.

On the other hand, an example of a signal obtained during a horizontal scanning period during normal scanning is shown in FIG. In the figure, a period indicated by 3 is a horizontal scanning period, a period indicated by 4 is a horizontal blanking period, and a period indicated by 6 is a period for scanning picture elements. Signal 14# obtained in each period shown in 4.6, 1. # is as follows.

Engineering,'=I4g...
(9) I@“=I-Tomi+Ia-+Isu+1., +1.
...or the output current IO4e Io obtained by subtracting IM41 IM6 from Ia' e I@'
s is as follows.

l04-0...II
os=Iip+Ia...a
As a result of this, the signal corresponding to the vertical smear is removed from the output current, as is clear from the above equation. If the pulse clamp circuit's pulse position is selected during the period indicated by 4 in Figure 7, the zero level will be fixed at the reference DC level, but the dark current generated in the photodiode during the period when the picture element is scanned will ,
Since I-2 is mixed in, the island level of the video signal is affected.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an imaging device using a solid-state Ilkl element image, which is stable and unaffected by low levels of dark current and vertical smear.

The present invention will be explained in detail below using examples.

First, the position of the black mask on the image sensor used in the embodiment of the present invention will be explained. In the present invention, as shown in FIG. 8, a black mask for obtaining a reference black level is used at a portion of the photosensitive section 81 of the image sensor corresponding to several scanning lines at the beginning or end of vertical scanning. 82. 83 is a horizontal shift register, and 84 is a vertical shift register. When a black mask is provided in this manner, a signal as shown in FIG. 9 is obtained as an output signal of the image sensor. That is,
The period 91 in Figure 9 is the part covered by the black mask,
Output signal Io Fi Io=Lt+Ia-+I=+I
am appears. The period 92 in FIG. 9 is a signal period, and I
o plus signal 1. Needless to say, is added.

Next, FIG. 10 shows an embodiment of the present invention.

After the output of the image sensor 101 is amplified by a preamplifier 102, unnecessary lock components and the like are removed by a low-pass filter 103, resulting in a video signal. One of the video signals is guided to a clamp circuit 104 and subjected to DC reproduction. The period used as a reference level during calibration is the horizontal blanking period. That is, by clamping, the dark current In+ of the horizontal signal line is removed. From this clamped video signal, a vertical smear signal obtained during the vertical retrace period is written into the one-line memory 105 as described in FIG. These timing control pulses are generated by a synchronous generator 106. The vertical smear signal written into the memory 105 once per vertical scanning period is read out as the same waveform every horizontal scanning period within the field period. demand.

Here, the signal components stored in the memory 105 will be described in detail again. As mentioned above, the vertical smear component 1. , and the dark current Iav of the vertical signal line is obtained. That is, the signal II stored in the memory 105 is in the section = Ia-+I-. Therefore, in the output of the subtraction circuit 106, the period covered by the black mask is Io It, and the video period is I-+IO.
It becomes II. That is, Io Ix=Iap+Li l-+IOIK=I-+14. +I411, and the dark current component Lv and vertical smear component Lm of the vertical signal line are removed. As can be seen from the above equation, if DC reproduction can be performed using the potential during the video period of the silently masked scanning line, dark current and vertical smear are isometrically subtracted from the output, and only the signal component component is obtained. I understand. Therefore, the output of the subtraction circuit 107 is connected to the above DC regeneration circuit (hereinafter referred to as vertical clamp circuit).
If DC regeneration is performed at 108, the object of the present invention can be achieved. Next, the vertical clamp circuit will be briefly explained. 1st
Figure 1 shows an example of a vertical clamp circuit. The input video signal 111 is first applied to a normal pulse clamp circuit 112, and subjected to DC reproduction during a horizontal blanking period. The waveform of the clamp pulse 113 is shown in FIG. 11(b). The DC reproduced signal is then guided to a sample and hold circuit 115. Here, the switch 116 is opened and closed by the waveform shown at 117 in FIG. 11(b), that is, the black level signal applied several times in the vertical period, and the pulse to be sampled, and the black level signal is held in the capacitor 118 for one vertical period. Do the action. The black level potential held for one vertical cycle period and the reference potential 119 are transferred to a differential amplifier 120.
This output is negatively fed back to the clamp potential of the clamp circuit 112. By doing the above, an output is obtained at the output terminal 114 in which the signal is clamped to the vertically inserted black level.

By combining the vertical smear correction circuit and the vertical clamp circuit as described above, it is possible to eliminate all factors that cause variations in the black level, and it is possible to obtain stable image quality.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the position of a conventional black mask for dark current correction, Figure 2 is a waveform diagram of an output signal obtained by the black mask in Figure 1, and Figure 3 is a diagram of the configuration of a solid-state image sensor. A diagram showing an example, FIG. 4 is an explanatory diagram of the signal and dark current of a solid-state image sensor, FIG. 5 is a diagram explaining vertical smear, FIG. 6 is a block diagram of a conventional vertical smear correction circuit, and FIG. 7 is an explanatory diagram of a conventional vertical smear correction circuit. 8 is an explanatory diagram of signals during one horizontal scanning period, FIG. 8 is a diagram illustrating the position of the black mask used in the present invention, FIG. 9 is a diagram representing signals obtained from the image sensor of FIG. 8, and FIG. A diagram showing one embodiment of the present invention, FIG. 11 is a vertical crane
tb)=59 4th 2nd 7th ward 1 Figure 83 1 % 9th 7D 1I area σ) 〃2

Claims (1)

[Claims] 1. In a solid-state imaging device in which photosensitive elements are arranged in an array in the horizontal and vertical directions, and a video signal is obtained by scanning each photosensitive element sequentially, means for optically blocking one or more columns; means for clamping the output signal of the solid-state image pickup device with a horizontal period; and l obtained from the vertical side line period.
means for storing signals in the horizontal scanning period; means for subtracting the signal clamped in the horizontal period from the signal stored in the memory; and converting the subtracted signal back into the signal of the optically suppressed portion. A solid-state imaging device characterized by being provided with means for clamping to a level. −