JP2004172925A - Dark current correction device and method for electronic camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dark current correction device and method for an electronic camera for relatively easily performing dark current correction. <P>SOLUTION: When a high sensitivity image pickup mode is set (YES in step 31), since it is considered that an object is dark, the influence of smears is less. For that, a processing of eliminating smear signal electric charges is stopped (step 32). Since the smear signal electric charges are not eliminated, the recombination of an electron and an electron hole to be the cause of a dark current generated in the vertical transfer path of a CCD is in a balanced state as well and a dark current value is turned to a fixed value. By subtracting the dark current value which is the fixed value from image data obtained by image pickup, the dark current correction is performed (step 33). <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
【Technical field】
The present invention relates to a dark current correction device for an electronic camera (including an electronic still camera, a digital still camera, a digital movie video camera, and a portable information device having a digital camera function) Regarding the method.
[0002]
BACKGROUND OF THE INVENTION
In a solid-state electronic image sensor such as a CCD, dark current is generated. Noise may appear in an image captured by the dark current. For this reason, it is considered that dark current is removed by correction (for example, Patent Document 1).
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 58-220576
However, removing dark current is cumbersome and often results in a relatively complex circuit.
[0005]
DISCLOSURE OF THE INVENTION
An object of the present invention is to remove dark current relatively easily.
[0006]
A dark current correction device for an electronic camera according to the present invention includes a solid-state electronic image pickup device in which a large number of photoelectric conversion elements are arranged and signal charges accumulated in the large number of photoelectric conversion elements are output as video signals through a transfer path. , Smear signal charge removal means for removing smear signal charges accumulated in the transfer path of the solid-state electronic image pickup device, first determination means for determining whether or not a high sensitivity imaging mode is set, and the first determination means The smear signal charge removal process is stopped in response to the determination that the high-sensitivity imaging mode is set, and the first determination unit determines that the high-sensitivity imaging mode is not set. The high-sensitivity imaging mode is set by the control means for controlling the smear signal charge removal means so as to perform the smear signal charge removal processing and the first determination means. In response to the determination and, characterized in that it comprises a first dark current removing means for removing the dark current component from the video signal output from the solid-state electronic image sensing device.
[0007]
The present invention also provides a method suitable for the dark current correction apparatus for the electronic camera. That is, this method includes a solid-state electronic imaging device in which a large number of photoelectric conversion elements are arranged, and signal charges accumulated in the large number of photoelectric conversion elements are output as video signals via a transfer path, and the solid-state electronic imaging device. In an electronic camera equipped with a smear signal charge removing means for removing smear signal charges accumulated in the transfer path, it is determined whether or not the high sensitivity imaging mode is set, and it is determined that the high sensitivity imaging mode is set. The smear signal charge removal process is stopped in response to the fact that the smear signal charge removal process is performed, and the smear signal charge removal process is performed in response to the high-sensitivity imaging mode not being set by the first determination unit. In response to determining that the high-sensitivity imaging mode is set by controlling the signal charge removal means, the video signal output from the solid-state electronic imaging device is detected. It is to remove the dark current component.
[0008]
Dark current is generated by recombination of electrons and holes in the transfer path. When the smear signal charge generated in the transfer path is not removed, the recombination of electrons and holes in the transfer path is in an equilibrium state, and the dark current component hardly increases with time. The dark current component converges to a constant value regardless of the timing output from the solid-state electronic image sensor. On the other hand, when smear signal charges generated in the transfer path are removed, recombination of electrons and holes in the transfer path is continued, and the dark current component often increases with time. The dark current component changes depending on the timing output from the solid-state electronic image sensor.
[0009]
According to the present invention, it is determined whether or not the high sensitivity imaging mode is set. Whether or not the high-sensitivity imaging mode is set can be determined according to the user's setting if the mode can be set by the user, or the high-sensitivity imaging mode can be determined depending on whether or not the high-sensitivity mode is set. It may be determined whether or not is set. For example, if the subject brightness is detected and the brightness is lower than a predetermined brightness, it is determined that the high sensitivity mode is set.
[0010]
When the high-sensitivity imaging mode is set, the process of removing smear signal charges accumulated in the transfer path of the solid-state electronic image sensor is stopped, and image data is output from the solid-state electronic image sensor. The dark current component is removed from the video signal (image data) output from the solid-state electronic image sensor. Since the smear signal charge removal process is stopped, the recombination of electrons and holes in the transfer path is maintained in an equilibrium state. Therefore, the dark current component included in the video signal output from the solid-state electronic image sensor is constant regardless of the output timing. The dark current component can be removed relatively easily.
[0011]
When the high sensitivity mode is set, the subject is often relatively dark. Therefore, the amount of smear signal charge is often small. Since the influence of the smear signal charge is relatively small, a relatively high quality image can be obtained without removing the smear signal charge.
[0012]
If the high-sensitivity imaging mode is not set, the smear signal charge accumulated in the transfer path of the solid-state electronic image sensor is removed. Since the smear signal charge is removed, it is possible to prevent the image quality from being deteriorated due to the influence of the smear signal charge.
[0013]
A dark current detecting means for detecting a dark current component included in a video signal output from the solid-state electronic image sensor in response to the high sensitivity imaging mode being determined by the first determining means; Second determining means for determining whether or not the dark current component detected by the dark current detecting means is equal to or greater than a predetermined threshold; and the dark current component contained in the video signal is determined by the second determining means Second dark current component removing means for removing the dark current component detected by the proposed current detecting means from the video signal output from the fixed electronic image pickup device when it is determined that the threshold current is greater than or equal to the threshold value. May be further provided.
[0014]
Since the dark current component removal process is performed when the dark current component is equal to or greater than the predetermined threshold value, the load on the second dark current component removal means can be reduced.
[0015]
The first dark current component removing unit and the second dark current removing unit may be the same.
[0016]
[Explanation of Examples]
FIG. 1 is a schematic diagram of a CCD 10 showing an embodiment of the present invention.
[0017]
A large number of photodiodes (photoelectric conversion elements) 1 are arranged in the horizontal direction and the vertical direction. A vertical transfer path 3 for transferring signal charges in the vertical direction is provided on the right side of the photodiode 1. Between the photodiode 1 and the vertical transfer path 3, a shift gate 2 for shifting the signal charge accumulated in the photodiode 2 to the vertical transfer path 3 is formed.
[0018]
Below the vertical transfer path 3, a horizontal transfer path 4 is provided for transferring the signal charges transferred from the vertical transfer path 3 in the horizontal direction. At the left end of the horizontal transfer path 4, a dummy addition circuit 5 for adding dummy signal charges serving as dummy pixel data is formed. An amplifier circuit 6 is connected to the dummy additional circuit 5.
[0019]
The signal charge accumulated in the photodiode 2 is given to the vertical transfer path 3 via the shift gate 2. The signal charges are transferred in the vertical direction on the vertical transfer path 3 and input to the horizontal transfer path 4. The signal charge is transferred along the horizontal transfer path 4 in the horizontal direction, and the dummy signal charge is added by the dummy addition circuit 5. The signal charge added with the dummy signal charge in the dummy addition circuit 5 is amplified in the amplification circuit 6 and output as a video signal.
[0020]
When a high-luminance object is imaged, the signal charge accumulated in the photodiode 1 may leak into the adjacent vertical transfer path 3 (this phenomenon is called smear, and the signal charge generated by smear is called smear signal charge). To do). The smear may cause vertical light streaks in the image represented by the video signal obtained from the CCD 1. In order to remove such light streaks, the smear signal charge accumulated in the vertical transfer path 3 may be removed before the signal charge accumulated in the photodiode 2 is shifted to the vertical transfer path 3.
[0021]
FIG. 2 is a time chart of the CCD 10 when the smear signal charge is removed.
[0022]
The vertical synchronization signal VD is supplied to the CCD 10 at a 1/60 second period (or a 1/30 second period). A mechanical shutter is provided in front of the CCD 10, and this mechanical shutter is opened. Substrate sweeping pulses are applied to the CCD 10 at 1H (one horizontal scanning period) period. The signal charge accumulated in the photodiode is swept out at a period of 1H (one horizontal scanning period).
[0023]
Finally, exposure of the CCD 10 starts from time T1 when the substrate sweeping pulse is applied to the CCD 10. At time T2, the mechanical shutter starts to be closed and light is completely shielded at time T3. Light enters the CCD 10 from time T1 to T3, and the exposure time is from time T1 to T3.
[0024]
When the mechanical shutter is completely closed at time T3, a high-speed vertical transfer pulse is applied to the vertical transfer path until time T4. Then, smear signal charges generated in the vertical transfer path 3 are swept out (smear sweep).
[0025]
At time T4, the timing gate pulse TG is applied to the shift gate 2. Then, the signal charge accumulated in the photodiode 2 due to the exposure is shifted from the photodiode 2 to the vertical transfer path 3. A vertical transfer pulse is given to the vertical transfer path 3, and the signal charge accumulated in the photodiode 2 is transferred in the vertical direction in the vertical transfer path 3. Thereafter, as described above, the image signal is output from the CCD 10.
[0026]
FIG. 3 is a time chart of the CCD 10 when the smear signal charge is not removed.
[0027]
Even when the smear signal charge is not removed, the process up to the exposure of the photodiode 2 is the same as when the smear signal charge is removed.
[0028]
When the smear signal charge is not removed, the timing gate pulse TG is given to the shift gate at the time T3 when the exposure ends. Then, the signal charge accumulated in the photodiode 2 is shifted to the vertical transfer path 3 at time T3. From time T3, vertical transfer of signal charges in the vertical transfer path 3 starts.
[0029]
FIG. 4 shows the relationship between the elapsed time from when the signal charge accumulated in the vertical transfer path 3 of the CCD 10 is reset and the dark current generated in the CCD 10.
[0030]
Dark current is generated by the generation of signal charges due to recombination of electrons and holes. Immediately after the signal charge accumulated in the vertical transfer path 3 is reset (elapsed time is 0), there is no signal charge due to recombination in the vertical transfer path 3, so the dark current level is also zero.
[0031]
As the signal charge in the vertical transfer path 3 is reset, the amount of signal charge generated by recombination of electrons and holes increases with time. That is, the level of dark current increases as time elapses from the time of reset.
[0032]
However, when a predetermined time t elapses after the signal charge in the vertical transfer path 3 is reset, the recombination of electrons and holes in the vertical transfer path 3 is in an equilibrium state. After the time t when the equilibrium state is reached, no recombination is performed even if the time elapses, so the dark current level is maintained at the level L when the equilibrium state is reached.
[0033]
As described above, when the smear signal charge removal process of the vertical transfer path 3 is performed, the signal charge of the vertical transfer path 3 is reset, so that the dark current contained in the video signal output from the CCD 10 is reset. The level increases in accordance with the elapsed time from the time of the smear signal charge removal process (time T4 in FIG. 2, reset). Since the dark current level changes according to the elapsed time, the process of removing the dark current becomes relatively complicated. On the other hand, if the smear signal charge removal process of the vertical transfer path 3 is not performed, the output time from the video signal from the CCD 10 is the time when the vertical transfer path 3 is reset (for example, the vertical transfer path in the previous photographing). 3), the dark current level contained in the video signal output from the CCD 10 is a constant level L. Since the dark current level is constant, it is relatively easy to remove the dark current component from the video signal.
[0034]
In this embodiment, when the high-sensitivity imaging mode is set, since the subject is relatively dark, the amount of smear signal charge generated is relatively small. The smear removal process is not performed because the smear has little influence on the obtained image. Since the dark current component included in the video signal output from the CCD 10 has a constant value, the dark current can be corrected only by subtracting the constant value from the video signal. Dark current correction can be realized relatively easily.
[0035]
On the other hand, when the high-sensitivity imaging mode is not set, smear removal processing is performed because the influence of smear on the obtained image cannot often be ignored. Furthermore, in this embodiment, when the smear removal process is performed, the dark current component included in the video signal is detected. When the detected dark current component is equal to or higher than a predetermined threshold level Th, correction for removing the dark current component from the video signal is performed. The correction to remove the dark current component is not always performed, but the dark current component is corrected when the dark current component exceeding the predetermined threshold is included, so the dark current component is removed. This reduces the burden on the correction circuit.
[0036]
FIG. 5 is a block diagram showing an electrical configuration of the digital still camera.
[0037]
The overall operation of the digital still camera is controlled by the control device 25.
[0038]
The digital still camera includes a mode switch 23 for setting the above-described high sensitivity imaging mode. The high-sensitivity imaging mode setting signal output from the mode switch 23 is input to the control device 25. By setting the high-sensitivity imaging mode, the smear signal removal process in the CCD 10 is stopped as described above. Further, dark current correction is performed in which a predetermined dark current value is subtracted from the image data. When the high-sensitivity imaging mode is not set, smear removal processing is performed in the CCD 10, and correction for removing dark current according to the dark current value is performed when the dark current component is equal to or greater than a predetermined threshold value.
[0039]
The digital still camera includes a mechanical shutter 9 whose opening / closing is controlled by a drive circuit 21. The light that has passed through the mechanical shutter 9 enters the light receiving surface of the CCD 10. A light image representing the subject image is formed on the light receiving surface of the CCD 10. The drive circuit 22 controlled by the control device 25 outputs the above-described signals such as the vertical synchronization signal VD, timing gate pulse, and vertical transfer pulse, and supplies them to the CCD 10.
[0040]
The video signal output from the CCD 10 is subjected to correlated double sampling processing in a CDS (correlated double sampling) circuit 11. The video signal output from the CDS circuit 11 is converted into digital image data by the analog / digital conversion circuit 12 and input to the dark current detection circuit 13.
[0041]
In the dark current detection circuit 13, a dark current component is detected based on the difference between the dummy pixel data obtained based on the dummy signal charge added by the dummy pixel addition circuit 5 included in the CCD 10 and the image data. A signal indicating the detected dark current value and the input digital image data are input to the dark current correction circuit 14. A signal indicating the detected dark current value is also input to the control device 25.
[0042]
As described above, when the high-sensitivity imaging mode is set, the dark current component included in the digital image data can be regarded as a constant value regardless of the passage of time, and thus indicates the dark current value detected from the digital image data The dark current correction circuit 14 is controlled so that the data is subtracted regardless of the output timing from the CCD 10. When the high-sensitivity imaging mode is not set, the dark current component included in the digital image data changes with the passage of time, so the dark current value is detected sequentially, and the detected dark current value is digitally displayed each time. The dark current correction circuit 14 is controlled so as to be subtracted from the image data. When the detected dark current value is subtracted from the digital image data each time and dark current correction is performed, the burden on the dark current correction circuit 14 increases. In this embodiment, the detected dark current value is a predetermined value. The dark current correction circuit 14 is controlled so that dark current correction is performed when the value Th is greater than or equal to Th. The burden on the dark current correction circuit 14 is reduced.
[0043]
The digital image data corrected by the dark current correction circuit 14 is gamma corrected by the gamma correction circuit 15. The color balance adjustment circuit 16 adjusts the color balance of the gamma-corrected digital image data.
[0044]
The image data adjusted in color balance is given to the display control circuit 17, so that an image obtained by imaging is displayed on the display screen of the display device 18.
[0045]
When a shutter release button (not shown) is pressed, the image data output from the color balance adjustment circuit 16 is input to the compression circuit 19 as described above. The compression circuit 19 performs compression based on JPEG (joint photographic expert group) or the like. The compressed image data is given to the memory card 20 and recorded.
[0046]
In the embodiment described above, dark current correction is performed both when an image is displayed on the display screen of the display device 18 and when image data is recorded on the memory card 20. For example, dark current correction may be performed when image data is recorded in the memory card 20.
[0047]
FIG. 6 is a flowchart showing a dark current correction processing procedure.
[0048]
As described above, it is determined whether or not the high-sensitivity imaging mode is set (step 31).
[0049]
If the high-sensitivity imaging mode is set (YES in step 21), the smear signal charge sweeping process in the CCD 10 is stopped (step 32). Since the smear signal charge sweep-out process is stopped, the dark current component included in the digital image data obtained by imaging becomes a constant value. Dark current correction is performed by subtracting a predetermined dark current value from the digital image data (step 33). Since the dark current value subtracted from the digital image data is a constant value, dark current correction can be performed relatively easily.
[0050]
If the high-sensitivity imaging mode is not set (NO in step 21), the smear signal charge sweeping process in the CCD 10 is performed (step 34). Of the image data obtained by imaging, a dark current value is detected corresponding to time (step 35). If the detected dark current value is equal to or greater than a predetermined threshold value, the dark current value that changes with time is subtracted from the digital image data to perform dark current correction (step 37).
[0051]
In the above-described embodiment, when the high-sensitivity imaging mode is not set, dark current correction is performed when the dark current value is equal to or greater than a predetermined threshold, but the high-sensitivity imaging mode is set. If not, dark current correction may be stopped.
[0052]
Each processing described above may be realized by hardware or software.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a CCD.
FIG. 2 is a time chart of a CCD.
FIG. 3 is a time chart of a CCD.
FIG. 4 shows the relationship between the elapsed time from the reset of the vertical transfer path and the dark current level.
FIG. 5 is a block diagram showing an electrical configuration of a digital still camera.
FIG. 6 is a flowchart showing dark current correction processing;
[Explanation of symbols]
1 Photodiode 3 Vertical Transfer Path 4 Horizontal Transfer Path 5 Dummy Pixel Additional Circuit 10 CCD
13 Dark Current Detection Circuit 14 Dark Current Correction Circuit 23 Mode Setting Switch 25 Control Device

Claims (3)

A solid-state electronic imaging device in which a large number of photoelectric conversion elements are arranged, and signal charges accumulated in the large number of photoelectric conversion elements are output as video signals via a transfer path;
Smear signal charge removing means for removing smear signal charge accumulated in the transfer path of the solid-state electronic image sensor;
First determination means for determining whether the high-sensitivity imaging mode is set;
The smear signal charge removal process is stopped in response to determining that the high-sensitivity imaging mode is set by the first determination unit, and the high-sensitivity imaging mode is not set by the first determination unit. The high-sensitivity imaging mode is determined to be set by the control means for controlling the smear signal charge removal means so as to perform the smear signal charge removal processing in response to the determination, and the first determination means. In response, first dark current removing means for removing dark current components from the video signal output from the solid-state electronic image sensor,
A dark current correction device for an electronic camera equipped with A dark current detecting means for detecting a dark current component included in a video signal output from the solid-state electronic image sensor in response to the high sensitivity imaging mode being determined by the first determining means;
Second determining means for determining whether or not the dark current component detected by the dark current detecting means is equal to or greater than a predetermined threshold; and the dark current component contained in the video signal is determined by the second determining means Second dark current component removing means for removing the dark current component detected by the proposed current detecting means from the video signal output from the fixed electronic image pickup device when it is determined that the threshold current is greater than or equal to the threshold value. ,
The dark current correction apparatus for an electronic camera according to claim 1, further comprising: A large number of photoelectric conversion elements are arranged, and the signal charges accumulated in the large number of photoelectric conversion elements are accumulated in the transfer path of the solid-state electronic image pickup element that is output as a video signal through the transfer path. In an electronic camera having a smear signal charge removing means for removing the smear signal charge,
Determine whether the high-sensitivity imaging mode is set,
The smear signal charge removal process is stopped when it is determined that the high-sensitivity imaging mode is set, and according to the determination that the high-sensitivity imaging mode is not set by the first determination unit. Controlling the smear signal charge removing means to perform the smear signal charge removing process;
Removing the dark current component from the video signal output from the solid-state electronic image sensor in response to determining that the high-sensitivity imaging mode is set;
Electronic camera dark current correction method.

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