JP2001145028A - Image pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image pickup device that applies a prescribed stop to deterioration in the image quality due to defect compensation and prevents a system from being failed in the case of compensating a defect pixel of an image pickup element. SOLUTION: The image pickup device is provided with a CCD image pickup element 105 that picks up an image of an object, an exposure control device 103 that controls the exposure to this image pickup element 105, a CCD driver 106 that controls a charge storage time of the image pickup element 105, and a system controller 112 that controls the exposure control device 103 and the CCD driver 106, detects a defective pixel in a prescribed operating state of the image pickup element 105 through the comparison with a defect discrimination level, and compensates information of the detected defective pixel with pixel information of pixels around the defective pixel. The defect discrimination level is revised so that number of detected effective pixels is limited to a predetermined limit value in the detection of the defective pixel by the system controller 112.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging device such as a digital camera, and more particularly to an imaging device having a pixel defect compensation function.

[0002]

2. Description of the Related Art Imaging devices such as video cameras have been widely used. In recent years, electronic still cameras that mainly capture and record still images have also come into widespread use especially as digital cameras, and video movies mainly used for recording moving images also have a still image capturing and recording function. The long-time exposure used for still image shooting increases the exposure time by increasing the charge accumulation time in the image sensor, thereby enabling shooting without using auxiliary lighting such as a strobe even under low illumination. It is known as a technology.

On the other hand, a solid-state imaging device such as a CCD has a dark output due to the presence of a so-called dark current.
Since this is superimposed on the image signal, the image quality deteriorates. The presence of a pixel having a high dark output level is called a pixel defect, and it is widely practiced to supplement the information using the output information of neighboring pixels without using the output information of the pixel.

In the present specification, such a complementing process is referred to as pixel defect compensation. For example, a predetermined (NTSC) determined on the premise of driving a moving image at a used frame rate.
Then, the dark output is evaluated at the standard exposure time of 1/60 second or a predetermined margin based on this, for example, 1/15 second when the margin is 4 times, and a pixel having a large level is evaluated as a defective pixel. And the above-mentioned pixel defect compensation is applied.

Further, Japanese Patent Laid-Open No. 6-038113 discloses a technique for improving the problem that the evaluation of defective pixels is not sufficient just before shipment from the factory because pixel defects are accompanied by temperature dependence and aging. Is disclosed.
This publication describes a technique of closing a light receiving surface by closing an iris immediately after turning on a power supply, and detecting a defective pixel by evaluating a CCD dark output prior to use of a camera to perform defect compensation. ing.

[0006]

By the way, when a defective pixel is detected and compensated for in a camera device (as a completed product after shipment from the factory) as in the technique disclosed in the above-mentioned publication, the image quality is degraded in a recorded image. There is a fear. This is because, when a new defective pixel is detected due to an increase in dark current caused by temperature dependence or aging, the state of occurrence of the defective pixel is generally difficult to predict. This is a serious problem particularly when the above-mentioned conventional technology is further developed and applied to long-time exposure.

That is, as a new technical proposal, consider that the dark output level of a pixel is further increased due to the accumulation effect of dark current in the case of long-time exposure exceeding the standard exposure time. In this case, the degradation that occurs can be devised in the worst case, because the degradation that occurs in this case increases in proportion to the approximate exposure time due to the dark current accumulation effect. Means that a large number of pixels determined to be defective may occur.

[0008] If the number of defective pixels is too large, no matter how the above-mentioned pixel defect compensation is applied, there is a case where there is no longer a pixel having image information to be used for complementation in the vicinity. In such a case, there is a problem that an image of image quality that can withstand viewing cannot be obtained.

This problem is a serious problem particularly from the following points. That is, in order to actually perform the defect compensation, the address of the detected defective pixel is temporarily registered in a predetermined memory area in the camera, and a complementing process based on the peripheral pixel information is performed based on the registered address information. Will be. In this case, since the assigned area (capacity) of the address registration memory is limited, if more defective pixels than the capacity allows, a system error occurs at the time of defective pixel address registration, and the worst case occurs. In this case, the function of the camera cannot be maintained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to solve the problems associated with the large number of defective pixels in pixel defect compensation, and to prevent image quality from deteriorating. Another object of the present invention is to provide a high-performance imaging device that does not cause a system breakdown.

[0011]

(Structure) In order to solve the above problem, the present invention employs the following structure.

That is, the present invention provides an image pickup device for picking up a subject image, exposure control means for controlling exposure of the image pickup device, accumulation time control means for controlling a charge accumulation time in the image pickup device, A defective pixel detecting unit that controls a control unit and an accumulation time controlling unit to detect a defective pixel in a predetermined operation state of the image sensor; and information on the defective pixel detected by the defective pixel detecting unit includes information on the vicinity of the pixel. An image pickup apparatus comprising: pixel defect compensating means for compensating for this with pixel information, wherein said defective pixel detecting means has defective pixel number limiting means for limiting the number of defective pixels to be detected to a predetermined value or less. It is characterized by being.

Here, the defective pixel detecting means is a test imaging means for executing a charge accumulation and readout operation in the image pickup element in a state in which the exposure control means has cut off the exposure to the image pickup element. And a defect determining unit that determines a defective pixel by comparing the obtained image sensor output level with a predetermined pixel defect determining level. If the number of determined defective pixels exceeds the predetermined value, it is preferable that the pixel determination level is changed and then the determination by the defect determination unit is performed again.

It is preferable that an initial set value of the pixel defect determination level is 5 to 10% of a maximum value of an image handling level in the imaging device.

(Operation) According to the present invention, the defective pixel number limiting means for limiting the number of defective pixels detected by the defective pixel detecting means to a predetermined value or less is provided. In addition, it is possible to prevent the system from being broken by detecting more defects than can be stored.

Further, in order to limit the number of defective pixels, if the number of detections once performed by the defect determination exceeds a predetermined value, the determination level is changed and the defect determination is repeated again, thereby reducing the number of defective pixels to a predetermined value. Within a certain range, it is possible to prioritize defect compensation with priority from the one having a relatively large dark charge level, and it is possible to obtain almost the best image quality that can be obtained by simple processing.

Further, by setting the initial determination level to a value of 5 to 10% of the maximum value of the image handling level, a pixel having a sufficiently low dark current level is determined to be a defective pixel, and a dark current is large. Any problem of not detecting a pixel from the initial determination can be prevented, and appropriate pixel defect detection can be performed.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a block diagram showing a circuit configuration of a digital camera according to an embodiment of the present invention.

In the figure, reference numeral 101 denotes a lens system including various lenses; 102, a lens driving mechanism for driving the lens system 101; 103, an exposure control mechanism for controlling the diaphragm of the lens system 101; 104, a filter such as an LPF; System, 10
5 is a CCD color image pickup device having a built-in color filter, 10
6, a CCD driver for driving the image sensor 105;
107, a preprocessing circuit including an A / D converter and the like;
8 is a digital process circuit for performing color signal generation processing, matrix conversion processing, and other various digital processing.
Reference numeral 109 denotes a card interface, 110 denotes a memory card such as a CF, and 111 denotes an LCD image display system.

In the figure, reference numeral 112 denotes a system controller (CPU) for comprehensively controlling each unit, 113 denotes an operation switch system including various switches, and 114 denotes an operation display for displaying an operation state, a mode state, and the like. System, 115 is a lens driver for controlling the lens driving mechanism 102, 116 is a strobe as a light emitting means, 117 is an exposure control driver for controlling the strobe 116, 118
Is a nonvolatile memory (E) for storing various setting information and the like.
EPROM).

In the digital camera of the present embodiment,
A system controller 112 performs overall control, and particularly includes a shutter device included in the exposure control mechanism 103 and a CCD image pickup device 1 by a CCD driver 106.
Exposure (charge accumulation) and signal reading are performed by controlling the driving of the pixel 05, and the detection of the pixel defect described below is performed using the output level information stored in the digital process circuit 108 via the pre-process circuit 107. And so on. At that time, the number of defective pixels is evaluated together with a defective pixel number limiting means for limiting the number of defective pixels to a predetermined number or less.

Further, the digital camera of this embodiment has a pixel defect compensating means, and at the time of actual photographing, address data of a defective pixel relating to a normal defect (hereinafter referred to as a normal defect) stored in the EEPROM 118 in advance, At the time of long-time exposure, a defect as a result of detecting the above pixel defect before photographing (hereinafter referred to as a detected defect)
This pixel defect compensation processing is performed in the digital process circuit 108 based on the pixel address data of the defective pixel.

Next, processing directly related to detection and compensation of pixel defects and limitation of the number of defective pixels in the present embodiment will be described with reference to the flowchart of FIG. However, in this camera, the digital processing of the signal level is 8 bits (0 to 25 bits).
It shall be performed in 5). Also, description will be made assuming normal temperature, except for the parts described later.

Prior to photographing, an exposure time Ttotal required for photographing is set based on manual settings or photometric results (however, the longest exposure time is 10 seconds as a camera specification). This is the standard exposure time Tstd in the camera (can be set arbitrarily, but in this example it is 1/15
Seconds), ie, Ttotal ≦ Tstd
Is determined (S1).

(1) In the case of Ttotal ≦ Tstd, the apparatus waits for a shooting trigger command for main imaging, which is the same as the prior art, receives the command, and performs exposure based on a predetermined exposure control value (S2). Is read out and subjected to predetermined signal processing, and then recorded on the memory card 110 (S4). At this time, pixel defect compensation corresponding to the above, that is, only for the always defective pixel is accompanied (S3).

(2) If Ttotal> Tstd, pixel defect detection is first performed prior to actual photographing. This pixel defect detection includes the following test imaging and defect determination using the result (including processing for limiting the number of defective pixels).

More specifically, when a photographing trigger command is received, first, test photographing is performed in a state where the light receiving surface of the image pickup device 103 is shielded from light by a shutter device included in the exposure control mechanism 103. That is, the CCD image pickup operation is performed in the dark by the CCD driver 106 for the exposure time Ttotal to read out the test image pickup signal (dark output signal).
8 is stored.

Next, a defect determination is performed by examining each output level of the effective output pixels excluding at least the normal defective pixels in all the stored data and performing a digital comparison with a predetermined pixel defect determination level. The initial criteria are as follows. That is, if the output level of the pixel of interest is S, the defect is determined if S> TH, and the defect is determined to be non-defect otherwise (S ≦ TH). In this camera, the initial value of TH is 25
Is adopted.

This means that, at the time of the initial determination, the dark output level at the time of the main image pickup is 25 or less (about 1 of the full range 255).
0%) is allowed (S5). Here, the output level 25 (approximately 10%) is naturally not the only absolute level and can be arbitrarily set according to the circumstances at the time of design. If, for example, about 5% is also effective), the possibility that the influence of the dark output superimposed on the image becomes apparent becomes sufficiently low.

If this is selected to be 0%, it is possible to completely eliminate the pixel on which the dark output is superimposed. In this respect, this can be cited as one preferred embodiment. , The number of pixels that are regarded as defective pixels and are complemented (that is, the image information is completely invalidated) increases, and image quality is rather likely to deteriorate. In reality, the initial reference level is set in consideration of these trade-off factors. As a value at which the effect of the trade-off is normally sufficiently exhibited, a numerical value range of 5 to 10% is particularly effective.

Next, for the detected defect, S> TH is determined as described above, and the number of defective pixels is calculated (S6). Since a defect is compensated for both a normal defect and a detected defect in the same manner, in this determination, these defect addresses are all processed as one and the same defect address. Then, it is determined whether the number of defective pixels is equal to or less than a predetermined number (S7). In the camera of this embodiment, a memory area in the digital process circuit 108 is secured so that up to 255 defect addresses can be stored. Therefore, when the number of detected defective pixels is 255 or less, the defective pixel address is stored in the defective address storage area, and the pixel defect detection ends (S9).

If the number of detected defective pixels is 256 or more, TH = TH + 10, that is, TH = 35 (S8), and the second defect determination is performed. That is, in the case of the initial judgment, the allowable dark output level was set to 25 as described above. However, since the number of defective pixels did not fall within the predetermined value, it was unavoidable to raise the allowable dark output level and then determine the defective pixel. Is performed again. If the number of detected defective pixels is 255 or less, the defective pixel address at that time is stored in the defective address storage area, and the pixel defect detection ends.

Similarly, if the number of defective pixels detected in each determination is 256 or more, TH is increased by 10 and the next defect determination is performed.
In the following cases, the defective pixel address at that time is stored in the defective address storage area, and the pixel defect detection ends.

In the above processing, considering the pixel finally detected as a defective pixel, all pixels having a dark charge level equal to or higher than TH at that time are included. In other words, the dark charge level of any pixel left out of the compensation target is better than the dark charge level of the defective pixel to be compensated. Therefore, by the relatively simple processing as described above, at least almost the best image quality that can be obtained as a system is secured.

Apart from this, considering a case in which an image sensor having virtually extremely poor performance is exposed for an extremely long time at an extremely high temperature, even if the TH is increased, the number of defective pixels is not easily increased. Although it may be considered that the number does not fall below 255 or less, the number of detected defective pixels always becomes 0 when TH = 255 at the end. Of course, it is out of the question whether or not practical image quality can be obtained in such a case, but it is extremely significant in a real imaging apparatus that at least a system failure does not occur in such a case.

After the pixel defect detection is completed as described above, in the case of (2), based on the predetermined exposure control value, that is, the exposure time Ttotal, in the same manner as after the shooting trigger command of (1). The main exposure is performed (S2), the image pickup signal is read out, subjected to predetermined signal processing, and then recorded on the memory card 110 (S4). At this time, first, known pixel defect compensation is performed based on the defective pixel address data stored in the defective address storage area (S3). The video signal processing in the circuit at the subsequent stage after the defect compensation until the recording is performed is common to both (1) and (2), but is used as appropriate according to its necessity. Processing, conversion to a luminance-color difference signal by matrix operation or its inverse conversion processing, false color removal or reduction processing by band limitation, various nonlinear processing represented by γ conversion, various information compression processing, and the like.

As described above, according to the present embodiment, it is possible to limit the number of defective pixels to be detected when performing pixel defect detection, so that it is possible to prevent image quality degradation accompanying defect compensation to a certain extent. An imaging device that does not cause a system breakdown is obtained.

In the above description, the charge accumulation time and the exposure time are identified for the sake of simplicity. Strictly speaking, the charge accumulation is started before the start of exposure using a mechanical shutter. Alternatively, when the charge is transferred to the transfer path a predetermined time after the completion of the exposure or the transfer of the accumulated charge is transferred to the transfer path, and the transfer is started a predetermined time later, that is, when a so-called delayed read method is used, the two do not always match. is there. However, since the difference between the two is managed and recognized by the system controller 112, the above-described embodiment may be applied by specifically considering the difference as necessary.

Various modifications other than the above-described embodiment are possible.

First, as for the exposure time setting at the time of test imaging, the exposure time Ttotal of the main exposure is used as it is in the above embodiment, but for example, Ttotal / 2 or Ttotal / 10
To reduce the test exposure time. In this case, utilizing the fact that the dark charge is substantially proportional to the accumulation time, the determination level TH may be set to 1/2 or 1/10. If the same correction is applied to the determination method, it is also possible to adopt a fixed value for the test imaging exposure time at least up to a range where Ttotal is equal to or less than a certain value.

The timing of the test imaging is preferably immediately before the actual imaging, but is not limited to this. For example, when the power is turned on, when switching to the shooting mode in a camera that can be switched to the playback mode, when the camera is switched to the shooting mode in the two-stage release switch camera (the second stage is a shooting trigger)
It can be configured to be performed at any time according to the purpose, such as at the time of operating the stage, when operating a separately provided test imaging switch.

In addition, if the supplementary explanation is given regarding the quantization level of the A / D converter used in the above embodiment, in reality, A
The existence of error characteristics of the hardware of the / D converter,
Even if it is absent, in consideration of the fact that the quantization error is relatively 100% in principle near the minimum quantization level, the A / D conversion used for the actual quantization in the above embodiment is considered. It is more preferable to use a unit having a number of quantization bits larger than the number of quantization bits of the image processing system (8 bits in the embodiment), for example, about 10 bits or 12 bits (or more). In calculating the expression, the influence of the error can be sufficiently reduced.

Although several embodiments of the present invention have been specifically described above, the present invention is not limited to these embodiments, but may take any form as long as it is described in the appended claims. Needless to say.

[0045]

As described above in detail, according to the present invention, since there is provided a defective pixel number limiting means for limiting the number of defective pixels detected by the defective pixel detecting means to a predetermined value or less, deterioration in image quality is reduced. It is possible to prevent the system from breaking down due to detection of more defects than the number that can be stored, as well as being able to stop a certain amount. In other words, a high-performance imaging device that can halt the image quality degradation to a certain extent and that does not cause system breakdown can be obtained.

In order to limit the number of defective pixels, if the number of defects once detected by the defect determination exceeds a predetermined value, the determination level is changed and the defect determination is repeated again, so that the number of defective pixels is reduced to the predetermined value. It is possible to give priority to defect compensation from the one with a relatively large dark charge level, and to obtain almost the best image quality that can be obtained by simple processing. Further, by setting the initial determination level to a value of 5 to 10% of the maximum value of the image handling level, a pixel having a sufficiently low dark current level is determined as a defective pixel, and a pixel having a large dark current level is determined as an initial pixel. It is possible to eliminate any problem of not detecting from the determination, and it has an excellent effect that appropriate pixel defect detection can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of a digital camera according to an embodiment of the present invention.

FIG. 2 is a flowchart for explaining processing directly related to detection and compensation of pixel defects and limitation of the number of defective pixels in the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 101 ... Lens system 102 ... Lens drive mechanism 103 ... Exposure control mechanism 104 ... Filter system 105 ... CCD color image sensor 106 ... CCD driver 107 ... Pre-process part 108 ... Digital process part 109 ... Card interface 110 ... Memory card 111 ... LCD image Display system 112 ... System controller (CPU) 113 ... Operation switch system 114 ... Operation display system 115 ... Lens driver 116 ... Strobe 117 ... Exposure control driver 118 ... Non-volatile memory (EEPROM)

Claims (3)

[Claims] An image pickup element for picking up a subject image; an exposure control means for controlling exposure of the image pickup element; an accumulation time control means for controlling a charge accumulation time in the image pickup element; A defective pixel detecting means for controlling an accumulation time controlling means to detect a defective pixel in a predetermined operating condition of the image sensor; and information on the defective pixel detected by the defective pixel detecting means is determined by pixel information near the pixel. An image pickup apparatus comprising a pixel defect compensating unit for compensating for the defect, wherein the defective pixel detecting unit has a defective pixel number limiting unit for limiting the number of defective pixels to be detected to a predetermined value or less. An imaging device characterized by the above-mentioned. 2. The apparatus according to claim 1, wherein said defective pixel detecting means interrupts exposure of said image pickup device by said exposure controlling means.
Test imaging means for performing charge accumulation and readout operations in the image sensor; and defect determination means for determining a defective pixel by comparing an image sensor output level obtained in correspondence with a predetermined pixel defect determination level. When the number of defective pixels once determined by the defect determination means exceeds the predetermined value, the defective pixel number limiting means changes the pixel defect determination level and then re-determines the defective pixel. The imaging apparatus according to claim 1, wherein the determination is performed by a determination unit. 3. An initial set value of the pixel defect determination level is:
The maximum value of the image handling level in the imaging device is 5 to 5.
The imaging device according to claim 2, wherein the ratio is 10%.