JP2000059678A - Imaging device - Google Patents

Abstract

(57) [Problem] To reduce the performance degradation of automatic control of an apparatus due to output saturation in an image sensor, and to enable a wide dynamic range of the image sensor. SOLUTION: A limited short period in which an input to an image sensor is made smaller than in a normal shooting state is set intermittently during a shooting period. Even for a subject that exceeds the dynamic range of the image sensor in a normal shooting state, the input to the image sensor in the short period should be sufficient to fall within the dynamic range of the image sensor in this short period. Keep small. For automatic control, both the image sensor output in a normal shooting state and the image sensor output in a short period set intermittently are used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup apparatus using a photoelectric conversion element such as a CCD image pickup element, and more particularly to an image pickup apparatus for controlling a dynamic range of an image pickup element to be used widely.

[0002]

2. Description of the Related Art Generally, when the output signal level of a device is constant, the S / N of an output signal can be improved as the dynamic range of an image sensor is widened and the input of the device is increased. Japanese Patent Application Laid-Open No. 7-107364 is an example of this type of imaging apparatus. From the viewpoint of S / N, it is desirable to control the input of the image sensor so that the dynamic range of the image sensor is always widened.

[0003] On the other hand, however, wide use of the dynamic range of the image sensor leads to a high possibility that the input when the subject changes exceeds the dynamic range of the image sensor. When an input exceeding the dynamic range of the image pickup device is input, saturation occurs in the image pickup device, so that an input beyond the dynamic range does not appear as a change in its output. Exposure control, auto white balance, in the general function of automatic control of the imaging device such as auto focus, in the correct transmission of the subject change to the automatic control system is an important point to bring out its performance, Output saturation in the image sensor causes performance degradation.

As an example, a description will be given of exposure control. For simplicity, exposure control is performed to control the maximum value of the input to a level that fills the dynamic range. When the subject changes to be dark, the input of the image sensor decreases and the output decreases. Therefore, the exposure state can be controlled so as to increase the input of the image sensor until the output returns to the initial level.
However, if the subject changes to become brighter,
Although the input of the image pickup device increases, the dynamic range is exceeded, so that saturation occurs in the device and the output does not change. For this reason, even though the subject is too bright than an appropriate state, it cannot be detected in the exposure control, and a malfunction that the input of the image sensor is not reduced is caused. The only way to prevent this is to abandon exposure control of the maximum value of the input to a level that fills the dynamic range, and to predict the change of the subject in advance and control the exposure to a level sufficiently smaller than the dynamic range.

As described above, in order to prevent the performance of the automatic control from being impaired in the conventional image pickup apparatus, a change in the subject is anticipated to some extent, and the input is reduced beforehand so as not to exceed the dynamic range. Needed.

[0006]

As described above, in the conventional imaging apparatus, there is a trade-off between widening the dynamic range and increasing the S / N ratio and the performance of automatic control. There were limits to the widespread use of.

SUMMARY OF THE INVENTION It is an object of the present invention to suppress deterioration in performance of automatic control due to output saturation in an image sensor, and to enable a wide dynamic range of the image sensor.

[0008]

In order to achieve the above object, in an image pickup apparatus according to the present invention, a limited short period of time during which an input to an image pickup device is made smaller than in a normal photographing state is set as a photographing period. Set intermittently inside. Even for a subject that exceeds the dynamic range of the image sensor in a normal shooting state, the input to the image sensor in the short period should be sufficient to fall within the dynamic range of the image sensor in this short period. Keep small. For the automatic control, both the output of the image sensor in a normal shooting state and the output of the image sensor in a short period set intermittently are used. As means for reducing the input to the image sensor in a short period, an image sensor capable of controlling the accumulation time is used, and the accumulation time in the short period is made shorter than in a normal shooting state. Alternatively, the aperture amount is adjusted to reduce the amount of light incident on the image sensor in a short period of time in a normal shooting state.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of an imaging apparatus according to a first embodiment of the present invention using an imaging element capable of controlling a charge accumulation time. In the figure, 1 is a lens, 2 is an aperture, 3 is a CCD image sensor, 4 is a signal processing circuit, 5 is an exposure detection circuit, and 6 is a microcomputer.

The incident light passing through the lens 1 and the stop 2 forms an image on the image pickup device 3. The image sensor 3 converts the input light into electric charge by photoelectric conversion and outputs the electric charge as an electric signal. In the present embodiment, a CCD image sensor capable of controlling the charge accumulation time is used as the image sensor 3, and the output increases when the accumulation time is long and the output decreases when the accumulation time is short even with the same incident light amount. Further, the dynamic range of the image pickup device 3 is determined by how much charge can be stored in the pixel for photoelectric conversion.

The signal processing circuit 4 performs a process of converting an output signal of the image sensor 3 into a video signal such as NTSC and outputting the video signal. The exposure detection circuit 5 extracts the luminance information of the subject from the output signal of the image sensor 3 and sends the information to the microcomputer 6. The microcomputer 6 controls the aperture amount of the aperture 2 and the charge accumulation time of the image sensor 3 based on this information, and performs exposure control.

FIG. 2 is an explanatory diagram showing the level of the output from the image sensor 3. In the same figure, the waveform when a gray scale chart is photographed is described as an example. At this time,
100% white and 50% gray are at the levels shown in the figure. The upper part (1) in FIG. 2 represents the output from the image sensor 3 in a steady state. From the image sensor 3,
Take out 60 images per second. In the normal photographing period (hereinafter, referred to as period a), the charge storage time of the image sensor is set to 1/60 seconds, which is the maximum value. The microcomputer 6 uses the output from the image sensor 3 during the period a for exposure control, and controls the aperture 2 so that the subject currently being photographed fills the dynamic range of the image sensor.

Here, for at least one image (a certain small period) out of 60 images per second in the normal photographing period, the accumulation time of the image sensor 3 is set to 1/180 second (this period is defined as:
Hereinafter, this is referred to as a period b). In the period b, since the accumulation time is set to 1/3 of the period a, the amount of accumulated charge for the same incident light is 1/3 of the period a, that is, it is used in 1/3 of the dynamic range. become. The exposure detection circuit 5 constantly monitors the output in the period b, and when it is determined that there is an input exceeding the dynamic range in the period a, performs exposure control using the output in the period b.

Now, it is assumed that the brightness of the subject changes twice and the input to the image sensor 3 increases. The output from the image sensor 3 at this time is shown in (2) on the lower side in FIG. In the period a, since the image sensor is used in the full dynamic range, the output is saturated and the change is very small. If exposure control is performed using a saturated output, the upper limit of the signal level cannot be detected, so that a change in a subject cannot be detected accurately, and accurate exposure control cannot be performed. However, in the period b, since the change of the subject is within three times that in the case of (1), the output saturation does not occur, and the change of the subject can be correctly transmitted to the exposure detection circuit 5.

As described above, the exposure detection circuit 5 constantly monitors the output during the period b. When the state (2) occurs, the output of the image sensor greatly exceeds the exposure control level and becomes twice the level in the period b. Thereby, it is determined that there is an input exceeding the dynamic range in the period a. With this determination, the exposure control uses the output from the image sensor 3 in the period b instead of the period a. In the exposure control, the aperture 2 is reduced so that the output of the image sensor in the period b becomes the exposure control level in the period b. Accordingly, the ratio of output saturation in the period a decreases. Finally, the input of the element becomes sufficiently small, and the control is stabilized in a new state in which the diaphragm 2 is stopped down to a point not exceeding the dynamic range even in the period a.

Note that the output of the period b is checked, and when it is determined that the input has been reduced to a level not exceeding the dynamic range in the period a, the exposure control uses the output from the image sensor in the period a. To If the input is at a level that does not exceed the dynamic range in period a,
The reason why the output of the period b is not used for the exposure control is as follows.
This is because the output in the period b is smaller than the period a and the output frequency is low, and thus the output is inferior to the period a in accuracy.

When the subject changes and the input to the image sensor 3 decreases, the aperture 2 is opened based on the exposure detection result in the period a, and the light amount is adjusted so that the dynamic range becomes full.

The output of the image sensor 3 during the period a is greater than the period b, which is advantageous in terms of control accuracy, but the output is likely to be saturated. Conversely, although the output in the period b is inferior in accuracy, the output in the period a
There is an advantage that the output does not saturate even when the subject is saturated. Therefore, when the input is near or smaller than the dynamic range, the exposure detection result in the period a is weighted.
When the exposure control is performed by combining the two in such a manner that the weight is assigned to the exposure detection result, more excellent control can be realized.

In the above description, the aperture (aperture value) is controlled using the outputs of the periods a and b as the exposure control. However, even if the exposure time (shutter time) is controlled as the exposure control. Good. For example, when the output of the period a exceeds the dynamic range, the exposure time that maximizes the dynamic range by the output of the period b is calculated,
The exposure time of the period a may be determined. In addition, as the exposure control, both the aperture and the exposure time may be controlled.

Here, in this embodiment, the period a and the period b
(1/60): (1/18)
0), that is, 3: 1, but this ratio is set according to the subject to be targeted by the apparatus. When the expected rate of change is small, the ratio is reduced. Also, the ratio of inserting the period b is set according to the subject to be targeted by the apparatus. When the change is frequent, a longer period b is inserted. FIG. 2 shows an example in which only one period b is provided.
Two or more periods b may be provided continuously. If two or more are continuously provided, the output in two or more periods b can be measured, so that the reliability of data in the period b can be improved. However, it is desirable that the period b is provided so as not to be visually detected.

According to the above-described embodiment, even when the dynamic range is widened and the S / N ratio is improved in the normal photographing period a, the change of the subject exceeding the dynamic range is always monitored in the period b. Since this can be reflected in the exposure control, the performance degradation of the exposure control can be suppressed. If the outputs of the period a and the period b are used for other automatic control such as auto white balance and auto focus, similarly, performance degradation can be suppressed with respect to a change of a subject exceeding the dynamic range.

When the outputs of the periods a and b are used for the automatic white balance control, the following may be performed, for example.

As in the case of the exposure control, when it is determined that the output in the period a is saturated, a white balance control signal is generated using the output in the period b, and the output level in the period a does not exceed the dynamic range. The white balance control signal generated using the output in the period b is held until the range is reached, and is used as the white balance control signal at the time of shooting in the period a. If the white balance control is performed using the output of the period a when the output level of the period a exceeds the dynamic range, accurate white balance can be performed because the reference signal for the white level is saturated. However, by using the output in the period b, it is possible to use an output in which the signal is not saturated,
Accurate white balance can be performed.

Similarly, in the case of the auto focus control, when it is determined that the output in the period a is saturated, the focus state is detected using the output in the period b, and the position of the focus lens is controlled. When the output level of the period a exceeds the dynamic range, the period a
When autofocus is performed using the output of the above, accurate autofocus cannot be performed because the saturated signal does not have a high-frequency component, but accurate autofocus control can be performed by using the output of the period b. It can be carried out.

When the output in the period a does not exceed the dynamic range, auto white balance and auto focus are performed using the output in the period a.

In addition, instead of using only the output of the period b for control when the output of the period a is saturated, automatic control may be performed by weighting the outputs of the periods a and b.

Incidentally, as a method of lowering the input of the image pickup element in the period b, it is possible to cope with changing the stop amount of the stop 2 instead of changing the storage time of the element. For example, by controlling the diaphragm 2 so that the amount of light incident on the imaging element in the period b becomes 1/3 in the period b, the same effect as in the above-described example can be obtained.

By the way, in the first embodiment described above, the output of the image sensor in the period b is 1/3 of the period a. In this state, the output of the apparatus during period b becomes 1/3 of period a, and the image changes momentarily. Therefore, if the signal processing circuit 5 is provided with a gain control function so that the output gain is tripled only during the period b, that is, the output gain is increased only during the period b (output / output of the period a). The output of the device can be made constant by making it twice as long as the output of the period b). However, in the period b, the S / N is deteriorated because the dynamic range is used narrowly.

In the case of a device for recording a still image, the period b
Even if an image change or S / N deterioration occurs for a moment, the recording may be prohibited only at this moment. It is also possible to use a memory so as not to make a momentary change in the output.
FIG. 3 illustrates an embodiment using such a memory.

FIG. 3 is a block diagram of an imaging apparatus according to a second embodiment of the present invention using an imaging element capable of controlling the charge accumulation time. The imaging device of the present embodiment is the first device of FIG.
The difference from the embodiment is that a memory circuit 7 and a switch 8 are provided before the signal processing circuit 4.

In the configuration shown in FIG.
Operates so as to store the image sensor output for one screen and output the screen stored in the next one screen period. In the period a, the switch 8 connects the output of the image sensor 3 to the signal processing circuit 4. In the period b, the switch 8 connects the output of the memory circuit 7 to the signal processing circuit 4. Accordingly, in the period b, the output of the period a immediately before the period b is supplied to the signal processing circuit 4. That is, the image in the period b is replaced with the image in the period a. Since the change of the two adjacent images of the imaging screen that takes out 60 images per second is slight, even if the same screen is continuously output for a certain moment period b,
It will not be detected.

The output of the period a immediately before the period b and the output of the period a immediately after the period b are averaged, or the output of the period a immediately before the period b and the output of the period a immediately after the period b are calculated from the output of the period a. b
May be generated and recorded by estimating the output of the period b. By using both the output of the period a immediately before the period b and the output of the period a immediately after the period b,
It is possible to obtain a more accurate output of the period b.

In this embodiment having such a configuration, in addition to the effects of the above-described first embodiment, there is an effect that an image change or S / N degradation in the period b in the device output can be prevented.

In the above, the period b is provided in a 1/60 second cycle. However, the period b can be provided only when the subject changes. Hereinafter, this will be described.

When the exposure detection circuit 5 extracts luminance information, the ratio of the output of the image sensor 3 which exceeds the dynamic range to the screen is detected. When the subject changes and exceeds the dynamic range of the image sensor 3, the output peak level is saturated, and therefore does not exceed the saturated output level. However, the area of the peak value per screen, that is, the area of saturation, should increase. Therefore, the exposure detection circuit 5 is provided with a circuit for counting the area of the detection value exceeding a certain threshold for each screen. The threshold to be set is set slightly lower than the saturation output of the image sensor. The area exceeding the threshold is over a certain value,
For example, the number of pixels whose luminance signal level exceeds the threshold level is counted, and when the number of pixels occupying the threshold level in one screen exceeds a predetermined value, the subject changes. Is determined to be bright, and a period b is set. The point that the amount of change exceeding the dynamic range is detected in the output in the period b and exposure control is performed is as follows.
This is as described in the previous embodiment. When the input of the image sensor falls within the dynamic range and becomes stable according to the result of the exposure control, the area of saturation becomes equal to or less than a predetermined value.

According to this method, since the period b is not set when the subject is stable, the number of image changes or S / N deterioration during the period b can be reduced.

[0037]

As described above, according to the present invention, performance degradation of automatic control such as exposure control, auto white balance, and auto focus due to wide use of the dynamic range of the image sensor can be suppressed. A wider dynamic range can be used, and thus the S / N can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a configuration of an imaging device according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating an output level from an imaging element in the imaging device according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram illustrating a configuration of an imaging device according to a second embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 lens 2 aperture 3 image sensor 4 signal processing circuit 5 exposure detection circuit 6 microcomputer 7 memory circuit 8 switch

Claims (16)

[Claims] 1. An image pickup apparatus having an image pickup device and means for controlling an input to the image pickup device, wherein a limited short period of time during which an input to the image pickup device is made smaller than a normal photographing state during a photographing period. Even if the input is set intermittently and the input exceeds the dynamic range of the image sensor due to a change in the subject in the normal shooting state, the input falls within the dynamic range of the image sensor within the short period. As described above, the imaging apparatus is characterized in that the input to the imaging element in the short period is sufficiently smaller than the input in a normal shooting state. 2. An image pickup apparatus comprising: an image sensor for photoelectrically converting an optical image to generate an electric signal; and means for controlling the amount of photoelectric conversion of the image sensor. An imaging apparatus characterized in that a shorter period is provided intermittently during a shooting period. 3. The imaging method according to claim 2, wherein the photoelectric conversion amount of the imaging element during a period in which the photoelectric conversion amount is smaller than a normal shooting state is within a dynamic range of the imaging element. apparatus. 4. The imaging apparatus according to claim 1, wherein an input to the imaging element in the short period is reduced by controlling a charge accumulation time in the imaging element. 5. The input device according to claim 1, wherein an input to the image sensor in the short period is reduced by controlling an aperture value of an aperture for variably controlling the amount of light incident on the image sensor. Imaging device. 6. The imaging device according to claim 1, wherein in a shooting period other than the short period, when an input exceeds a dynamic range of the imaging device due to a change in a subject, an output of the imaging device in the short period is used. An image pickup apparatus for performing exposure control. 7. The exposure control according to claim 6, wherein the exposure control using an output of the image sensor in the short period is performed as follows:
An image pickup apparatus, which is performed at least by controlling a charge accumulation time in an image pickup device. 8. The exposure control according to claim 6, wherein the exposure control using the output of the image sensor in the short period is performed as follows:
An image pickup apparatus characterized in that the control is performed at least by controlling the aperture value of a stop that variably controls the amount of light incident on the image pickup device. 9. The imaging device according to claim 1, wherein during a shooting period other than the short period, when an input exceeds a dynamic range of the imaging device due to a change in a subject, an output of the imaging device in the short period is used. An image pickup apparatus for performing automatic white balance control by using the image pickup apparatus. 10. The imaging device according to claim 1, wherein, during an imaging period other than the short period, when an input exceeds a dynamic range of the imaging device due to a change in a subject, an output of the imaging device in the short period is used. An image pickup apparatus that performs auto-focus control by using an automatic focus control. 11. The imaging device according to claim 1, wherein an output level of the image sensor in the short period is substantially equal to an output level of the imaging device in a normal period other than the short period. An imaging apparatus, wherein gain control of an output of an imaging element is performed. 12. The image sensor according to claim 1, wherein the output of the image sensor in the period before the short period is used without using the output of the image sensor in the short period. An imaging device characterized by being used in place of an output. 13. The signal according to claim 1 or 2, wherein a signal obtained by using an output of the image sensor in a period before and after the small period without using an output of the image sensor in the small period is: An imaging apparatus characterized in that it is used in place of the output of the imaging element during the short period. 14. The imaging apparatus according to claim 1, wherein an output of the imaging element in the short period is not recorded as an image. 15. The imaging apparatus according to claim 1, wherein the short period for reducing the input to the imaging element is set only when a change in the subject is detected. 16. The imaging apparatus according to claim 15, wherein the short period is set when the degree of saturation of the luminance signal per screen is equal to or more than a predetermined value.