JP2006324983A - Electronic camera suppressing image noise by using dark image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for suppressing an influence of random noise with respect to noise suppression using a dark image. <P>SOLUTION: The electronic camera includes an imaging part, a light shielding mechanism, a control part, and a noise suppression part. The imaging part generates image data by photoelectric conversion, The light shielding mechanism shields the imaging part from light. The control part drives the imaging part to image a subject and generates image data. The control part drives the imaging part put into a light shielding state by the light shielding mechanism, to generate dark image data. The noise suppression part suppresses fixed pattern noise in the image data on the basis of the dark image data. Especially, the control part sets a preliminarily determined imaging condition of increasing a level difference between the fixed pattern noise and random noise when generating the dark image data. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electronic camera that suppresses image noise using a dark image.

In general, when long-time shooting is performed with an electronic camera, fixed pattern noise appears in image data. As a device for removing this type of noise, a conventional device of Patent Document 1 is known.
In this conventional apparatus, first, normally captured image data and dark image data captured with the shutter closed are prepared. The conventional apparatus subtracts the dark image data from the image data in units of pixels, thereby removing the in-phase fixed pattern noise.
JP 2000-125204 A (Claims 1 and 2 etc.)

By the way, the dark image data taken for a long time includes random noise in addition to fixed pattern noise. In the subtraction processing of the conventional device described above, this random noise is reversed in phase and added to the image data, so that the noise of the image data is increased in reverse.
Accordingly, an object of the present invention is to provide a technique for suppressing the influence of random noise in noise suppression using a dark image.

<< 1 >> The electronic camera of the present invention includes an imaging unit, a light shielding mechanism, a control unit, and a noise suppression unit.
The imaging unit generates image data by photoelectric conversion.
The light shielding mechanism shields the imaging unit.
The control unit drives the imaging unit to capture an image of the object scene and generate image data. In addition, the control unit drives the imaging unit that is shielded by the light shielding mechanism, and generates dark image data.
The noise suppression unit suppresses fixed pattern noise in the image data based on the dark image data.
In particular, the above-described control unit sets “predetermined imaging conditions” for expanding the level difference between fixed pattern noise and random noise when generating dark image data.

<< 2 >> Preferably, the control unit enlarges the level difference between the fixed pattern noise and the random noise by setting the charge accumulation time of the dark image data longer than the charge accumulation time of the image data.

<< 3 >> Preferably, the control unit sets the signal gain of dark image data (hereinafter referred to as “imaging sensitivity”) to be higher than the imaging sensitivity of the image data, thereby reducing the level difference between fixed pattern noise and random noise. Expanding.

<< 4 >> Preferably, the control unit has a function of adjusting the temperature of the imaging unit. When generating dark image data, the control unit raises the temperature more than when generating image data. Thereby, the level difference between fixed pattern noise and random noise is expanded.

<< 5 >> Preferably, the noise suppressing unit corrects a level difference of fixed pattern noise caused by changing the setting of the imaging condition between the dark image data and the image data. After this level correction, the noise suppression unit suppresses the fixed pattern noise in the image data by subtracting the dark image data from the image data.

<< 6 >> Preferably, the noise suppression unit discriminates between fixed pattern noise and random noise in the dark image data at a threshold level according to the imaging condition of the dark image data. The noise suppression unit suppresses the fixed pattern noise in the image data based on the identified fixed pattern noise.

In conventional noise processing, it is common knowledge of those skilled in the art to align the imaging conditions of both data as much as possible in order to align the generation amount of fixed pattern noise in dark image data and image data.
For this reason, it has naturally been preferable to equalize the charge accumulation time in the dark image data and the image data. Further, it has been preferable to continuously generate image data and dark image data so that the temperature of the imaging unit does not change greatly. In addition, it has been said that it is preferable that the imaging sensitivities of both data are aligned.

However, the present inventor has noticed that the ratio between the fixed pattern noise and the random noise is changed by intentionally operating the imaging condition in the imaging experiment of dark image data.
The present invention actively utilizes this phenomenon to change the imaging condition against the common sense of those skilled in the art when generating dark image data. Thereby, the level difference between the fixed pattern noise and the random noise in the dark image data is enlarged.
By such an increase in level difference, random noise in dark image data becomes relatively smaller than fixed pattern noise. Therefore, it is possible to reduce the adverse effects of random noise in suppressing noise in image data using dark image data.

<< Description of Configuration of this Embodiment >>
FIG. 1 is a block diagram showing the electronic camera 11.
In FIG. 1, a photographing lens 12 is attached to the electronic camera 11. The lens control unit 12a performs focus driving, aperture control, and the like of the photographing lens 12. In the image space of the photographic lens 12, the light receiving surface of the image sensor 13 is disposed via the shielding mechanism 12b. The shielding mechanism 12b may be used as a mechanical shutter or a diaphragm, or may be a dedicated shielding mechanism. The shielding mechanism 12 b has a function of shielding the light receiving surface of the imaging element 13 by blocking the incident optical path of the photographing lens 12 with a signal from the imaging control unit 14.
On the other hand, the image sensor 13 is integrally provided with a temperature adjustment unit 13a. The imaging control unit 14 controls the temperature of the imaging element 13 by driving the temperature adjustment unit 13a.

In addition, as this temperature adjustment part 13a, the mechanism which can be heated and cooled like a Peltier device or a Carnot heat engine is preferable. Further, as the temperature adjusting unit 13a, a mechanism capable of increasing the temperature, such as a resistor, an electric circuit, or a light emitter (such as a light emitting element for a monitor screen of the electronic camera 11) is also preferable. The temperature adjustment unit 13a may be a circuit directly formed on the semiconductor substrate of the image sensor 13. Further, the temperature adjustment unit 13a may be integrated with the package of the image sensor 13.
In particular, in the case of a Peltier element, it is preferable to arrange the Peltier element so as to be sandwiched between the imaging element 13 and the monitor screen (light emitting element) because of the arrangement relationship of the internal components of the electronic camera 11. In such a configuration, it is possible to raise the temperature of the image sensor 13 in a short time by transferring the heat of the light emitting element to the image sensor 13 using a Peltier element.

Further, the image sensor 13 or the temperature adjustment unit 13a is provided with a temperature sensor 13b. The temperature detected by the temperature sensor 13b is transmitted to the imaging control unit 14.
On the other hand, the imaging element 13 is driven by the imaging control unit 14 and outputs image data. The image data is processed via the signal processing unit 15 and the A / D conversion unit 16 and then temporarily stored in the memory 17.
This memory 17 is connected to a bus 18. The bus 18 is also connected to a lens control unit 12a, an imaging control unit 14, a microprocessor 19, an image processing unit 20, a recording unit 22, and a monitor display unit 23 that drives a monitor screen (light emitting element).
The microprocessor 19 is connected to an operation unit 19a such as a release button. The recording unit 22 is loaded with a recording medium 22a.

<< Explanation of operation of this embodiment >>
FIG. 2 is a flowchart for explaining the operation of the present embodiment.
Hereinafter, the operation will be described along the step numbers shown in FIG.

Step S1: When the temperature adjustment unit 13a is an element that can be selected from heating and cooling, such as a Peltier element, the imaging control unit 14 preferably lowers the temperature of the temperature adjustment unit 13a prior to a still image imaging operation. The imaging control unit 14 detects the temperature at the time of imaging of the image data with the temperature sensor 13 b and transmits information to the microprocessor 19.

Step S2: When the microprocessor 19 detects a release instruction from the user using the operation unit 19a or the like, the microprocessor 19 instructs the imaging control unit 14 about imaging conditions (charge accumulation time, imaging sensitivity, etc.) of the image data. The imaging control unit 14 causes the imaging device 13 to start photoelectric conversion of the subject image with the shielding mechanism 12b opened, and waits for the instructed charge accumulation time to read out the image data from the imaging device 13.

Step S3: In preparation for the generation of dark image data, the imaging control unit 14 drives the temperature adjustment unit 13a to start the temperature increase of the imaging element 13. At this time, it is preferable that the imaging control unit 14 performs temperature monitoring by the temperature sensor 13b and raises the temperature of the imaging device 13 to a predetermined temperature. The temperature after the increase is transmitted from the imaging control unit 14 to the microprocessor 19.

Step S4: The signal processing unit 15 amplifies the read image data according to the imaging sensitivity set for the image data. The amplified image data is digitized by the A / D converter 16 and then temporarily stored in the memory 17.

Step S5: The imaging control unit 14 sets the charge accumulation time of the dark image data as long as possible, with “additional time allowed for noise removal” custom-set in the electronic camera 11 as an upper limit. The charge accumulation time is preferably set as long as possible within a time range (such as experimental values) in which the dark image data (fixed pattern noise) is not saturated inside the image sensor 13. With this setting, the charge accumulation time of dark image data is set longer than the charge accumulation time of most image data.

Step S6: The imaging control unit 14 sets the imaging sensitivity (signal gain) of the signal processing unit 15 as high as possible within a range where the signal processing unit 15 and the A / D conversion unit 16 do not saturate the signal. With this setting, the imaging sensitivity of dark image data is set higher than the imaging sensitivity of most image data.

Step S7: The imaging control unit 14 shields the shielding mechanism 12b and keeps the light receiving surface of the imaging element 13 in a dark state and a temperature rising state.

Step S8: The imaging control unit 14 accumulates charges in the image sensor 13 according to the charge accumulation time set in step S5, and then reads dark image data from the image sensor 13.

Step S9: The signal processing unit 15 amplifies the generated dark image data according to the imaging sensitivity set in Step S6. The amplified dark image data is digitized by the A / D converter 16 and then stored in the memory 17.
Note that it is preferable to suppress fluctuations in the level range (generally a small level region) regarded as random noise (non-fixed pattern noise) for the dark image data at this stage. Further, this level range may be replaced with a fixed value (virtual signal level when random noise is not generated). In this case, if random noise does not change conspicuously due to a change in imaging conditions, the level range regarded as random noise may be constant regardless of the change in imaging conditions. Further, when the random noise changes due to a change in the imaging condition, it is preferable to change the setting of the level range regarded as the random noise according to the change in the imaging condition.

Step S10: The microprocessor 19 acquires information about the imaging conditions (here, charge accumulation time, imaging sensitivity, temperature, etc.) of the dark image data from the imaging control unit 14.
The microprocessor 19 refers to the correspondence relationship stored in advance in “Change in imaging condition between image data and dark image data”, and acquires information on the increase rate of the fixed pattern noise accompanying the change in the imaging condition. .
This correspondence is obtained by experimentally obtaining “change in imaging condition” and “accompanying increase rate of fixed pattern noise” in advance.

Step S11: The microprocessor 19 selects a gradation correction table for correcting the level of this increase rate, and transmits it to the image processing unit 20.
The image processing unit 20 performs dark image data level correction according to the selected gradation correction table. In this level correction, a correction gain that is inversely proportional to the increase rate is applied to the fixed pattern noise in the dark image data. As a result, the fixed pattern noise in the dark image data is reduced in level, and is at the same level as the fixed pattern noise in the image data.
Note that it is preferable to suppress non-linear fluctuations in a level range (generally a small level region) that is regarded as random noise (non-fixed pattern noise) for dark image data after level correction. In addition, this level range may be replaced with a fixed value (virtual signal level when random noise is not generated). In this case, it is preferable to change the setting of the level range regarded as random noise in accordance with a change in imaging conditions and a level correction associated therewith.

Step S12: The image processing unit 20 subtracts the dark image data after level correction from the image data in the memory 17 in units of pixels. This subtraction process suppresses fixed pattern noise in the image data.

<< Effects of this embodiment >>
Hereinafter, the effect of this embodiment will be described.

(About the effect of extending the charge accumulation time)
FIG. 3 is a diagram in which the relationship between the charge accumulation time and the fixed pattern noise and random noise is actually measured. As shown in FIG. 3, the noise level of the fixed pattern noise increases significantly as the charge accumulation time increases. On the other hand, the noise level of random noise does not increase remarkably even if the charge accumulation time becomes longer.

Therefore, as in this embodiment, the level difference between the fixed pattern noise and the random noise can be increased by setting the charge accumulation time of dark image data longer than the charge accumulation time of image data. After that, by correcting the level of the dark image data, the noise level of the fixed pattern noise is almost equal to the noise level of the fixed pattern noise in the image data. At this time, the random noise in the dark image data becomes smaller as the level difference increases.
As a result, in the noise removal of the image data using the dark image data after the level correction, the influence of random noise is reduced, and a good noise removal effect can be obtained.

(About the sensitivity enhancement effect)
FIG. 4 is a diagram in which the relationship between imaging sensitivity, fixed pattern noise, and random noise is measured. As shown in FIG. 4, the noise level of the fixed pattern noise increases significantly as the imaging sensitivity increases. On the other hand, the noise level of random noise does not increase noticeably even when the imaging sensitivity is increased.

Therefore, the level difference between the fixed pattern noise and the random noise can be expanded by setting the imaging sensitivity of the dark image data higher than the imaging sensitivity of the image data as in the present embodiment. After that, by correcting the level of the dark image data, the noise level of the fixed pattern noise is almost equal to the noise level of the fixed pattern noise in the image data. At this time, the random noise in the dark image data becomes smaller as the level difference increases.
As a result, in the noise removal of the image data using the dark image data after the level correction, the influence of random noise is reduced, and a good noise removal effect can be obtained.

(Regarding the temperature rise effect)
FIG. 5 is a diagram in which the relationship between temperature and fixed pattern noise and random noise is measured. As shown in FIG. 5, the noise level of the fixed pattern noise increases as the temperature increases. On the other hand, the noise level of random noise does not increase noticeably even when the temperature increases.

Therefore, the level difference between the fixed pattern noise and the random noise can be expanded by setting the temperature at the time of capturing the dark image data higher than the temperature at the time of capturing the image data as in the present embodiment. After that, by correcting the level of the dark image data, the noise level of the fixed pattern noise is almost equal to the noise level of the fixed pattern noise in the image data. At this time, the random noise in the dark image data becomes smaller as the level difference increases.
As a result, in the noise removal of the image data using the dark image data after the level correction, the influence of random noise is reduced, and a good noise removal effect can be obtained.

(About the synergistic effect of multiple imaging conditions)
In particular, in the present embodiment, the level difference between the fixed pattern noise and the random noise in the dark image data is synergistically expanded by changing a plurality of imaging conditions such as (charge accumulation time, imaging sensitivity, temperature). Can do. Therefore, it is possible to suppress random noise in the dark image data by a synergistic enlargement of the level difference. Therefore, in the noise removal of the image data using the dark image data, the influence of the random noise can be reduced synergistically, and a much better noise removal effect can be obtained.

<< Additional items of embodiment >>
In the above-described embodiment, fixed pattern noise is removed by subtracting dark image data from image data. However, the embodiment is not limited to this.
For example, in the above-described embodiment, the level difference between the fixed pattern noise and the random noise in the dark image data increases due to the change in the imaging condition. Therefore, by setting the threshold level within this level difference according to the imaging condition of the dark image data, the image processing unit 20 can more accurately discriminate the occurrence location of the fixed pattern noise. The image processing unit 20 performs noise suppression processing with reference to surrounding pixels and the like on the pixel positions in the image data corresponding to these occurrence locations. Also in this case, it is possible to obtain a good noise removal effect while suppressing the influence of random noise.

  Furthermore, in the above-described embodiment, a plurality of imaging conditions such as (charge accumulation time, imaging sensitivity, temperature) are changed when generating dark image data. However, the embodiment is not limited to this. At least one of charge accumulation time, imaging sensitivity, or temperature may be changed.

  The imaging conditions are not limited to these three types. Generally, any imaging condition that enlarges the level difference between fixed pattern noise and random noise may be used.

  In the above-described embodiment, the image data is generated first, and the dark image data is generated later. However, the embodiment is not limited to this. The dark image data may be generated first. Further, the dark image data may be generated at a timing independent from the generation of the image data (such as when the power is turned on) (rather, since the above-described embodiment does not require the imaging conditions to be uniform, Easy to generate dark image data). In such an operation, it is possible to omit the dark image data shooting sequence after shooting the image data.

  The present invention can be implemented in various other forms without departing from the spirit or main features thereof. Therefore, the above-mentioned embodiment is only a mere illustration in all points, and should not be interpreted limitedly. The scope of the present invention is shown by the scope of claims, and is not restricted by the text of the specification. Further, all modifications and changes belonging to the equivalent scope of the claims are within the scope of the present invention.

  As described above, the present invention is a technique that can be used for image noise suppression and the like.

1 is a block diagram showing an electronic camera 11. FIG. It is a flowchart explaining operation | movement of this embodiment. It is a figure which shows the relationship between electric charge accumulation time, fixed pattern noise, and random noise. It is a figure which shows the relationship between imaging sensitivity, fixed pattern noise, and random noise. It is a figure which shows the relationship between temperature, fixed pattern noise, and random noise.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Electronic camera, 12 ... Shooting lens, 12a ... Lens control part, 12b ... Shielding mechanism, 13 ... Imaging element, 13a ... Temperature adjustment part, 13b ... Temperature sensor, 14 ... Imaging control part, 15 ... Signal processing part, 16 ... A / D conversion unit, 17 ... memory, 18 ... bus, 19 ... microprocessor, 20 ... image processing unit

Claims (6)

An imaging unit that generates image data by photoelectric conversion;
A light shielding mechanism for shielding the imaging unit;
A controller that drives the imaging unit to capture an image of the scene to generate image data, and drives the imaging unit that is shielded by the light shielding mechanism to generate dark image data;
A noise suppression unit that suppresses fixed pattern noise in the image data based on the dark image data;
The electronic controller according to claim 1, wherein the control unit sets “predetermined imaging condition” in which a level difference between fixed pattern noise and random noise is enlarged when the dark image data is generated. The electronic camera according to claim 1,
The electronic camera is characterized in that the control unit enlarges a level difference between fixed pattern noise and random noise by setting a charge accumulation time of the dark image data longer than a charge accumulation time of the image data. The electronic camera according to claim 1 or 2,
The control unit enlarges a level difference between fixed pattern noise and random noise by setting a signal gain (hereinafter referred to as “imaging sensitivity”) of the dark image data to be higher than an imaging sensitivity of the image data. A featured electronic camera. The electronic camera according to any one of claims 1 to 3,
The control unit has a function of adjusting the temperature of the imaging unit, and when generating the dark image data, the control unit raises the temperature more than the generation of the image data, thereby generating fixed pattern noise and random noise. An electronic camera characterized by expanding the level difference. The electronic camera according to any one of claims 1 to 5,
The noise suppression unit corrects a level difference of the fixed pattern noise caused by the setting change of the imaging condition between the dark image data and the image data, and the dark image data is converted into the image after the level correction. An electronic camera that suppresses fixed pattern noise in the image data by subtracting from the data. The electronic camera according to any one of claims 1 to 5,
The noise suppression unit discriminates the fixed pattern noise and the random noise in the dark image data at a threshold level according to the imaging condition of the dark image data, and based on the distinguished fixed pattern noise An electronic camera that suppresses fixed pattern noise in the image data.

Images