JP4333434B2 - Electronic camera - Google Patents

Description

  The present invention relates to a dark current correction technique for an electronic camera.

  2. Description of the Related Art Conventionally, it has been known that an imaging sensor of an electronic camera has a dark current component noise included in a charge accumulated in a light receiving element. As a noise correction means for this dark current component, an optical black region for detecting the dark current level is formed by shielding the light receiving elements in a part of the image sensor, and the dark current in the optical black region is determined from the image level of the effective pixel region. A method of subtracting the level is known.

  By the way, when strong light is incident near the optical black area of the image sensor, for example, charges leaked from the light receiving element in the effective pixel area, unnecessary charges generated deep in the substrate, or stray light entering the back surface of the light shielding film As a result, the dark current level detected from the optical black region may increase from the original value. In this case, since the dark current level is excessively subtracted from the image level, the corrected captured image sinks relatively darkly.

With respect to this point, Patent Document 1 discloses that unnecessary charge leaking from the periphery of the optical black region is reduced by lowering the emitter potential of the light receiving element adjacent to the effective pixel region in the optical black region. A technique for blocking is disclosed.
Japanese Patent Laid-Open No. 5-130520

  However, in Patent Document 1, there is room for improvement in that, for example, when the stray light that has entered from the back surface of the light shielding film reaches the depth of the optical black region, the dark current level increases from the original value. . In addition, depending on the sensitivity setting of the image sensor and the like, the influence of the high-brightness subject may be small. Therefore, it is desirable to consider conditions such as the sensitivity setting of the image sensor when correcting the dark current.

  The present invention is intended to solve the above-described problems of the prior art, and an object of the present invention is to enable highly accurate dark current correction even when there is a high-luminance subject in the vicinity of the optical black region.

  According to a first aspect of the present invention, there is provided an electronic camera comprising an imaging sensor, a divided photometric unit, a position determining unit, a limit determining unit, and a dark current correcting unit. The imaging sensor includes an effective pixel region that photoelectrically converts a subject image to generate an image signal, and an optical black region that is formed around the effective pixel region and generates a dark current level reference signal. The split photometry means acquires luminance information of the object scene. The position determining means determines the position of the high brightness subject in the effective pixel area based on the output of the divided photometry means. The restriction determination unit determines whether the reference signal is restricted based on sensitivity information of the imaging sensor and luminance information of the high-luminance subject when the high-luminance subject is located in the vicinity of the optical black area. The dark current correction unit limits the upper limit value of the reference signal with a threshold value corresponding to the sensitivity information based on the determination result of the limit determination unit, and darkens the image signal based on the reference signal after the limitation. Perform current correction.

  According to a second aspect of the present invention, there is provided an electronic camera comprising an imaging sensor, a divided photometric unit, a position determining unit, and a dark current correcting unit. The imaging sensor includes an effective pixel region that photoelectrically converts a subject image to generate an image signal, and an optical black region that is formed around the effective pixel region and generates a dark current level reference signal. The split photometry means acquires luminance information of the object scene. The position determining means determines the position of the high brightness subject in the effective pixel area based on the output of the divided photometry means. The dark current correction unit limits the upper limit value of the reference signal with a threshold value corresponding to sensitivity information of the imaging sensor when the high-luminance subject is located in the vicinity of the optical black region, and the reference signal after the limitation Based on the above, dark current correction of the image signal is performed.

  In the present invention, when a high-brightness subject is located in the vicinity of the optical black area, the dark current level is limited by a threshold corresponding to the sensitivity information of the imaging sensor, so that excessive dark current level due to the influence of the high-brightness subject is prevented. And accurate dark current correction can be easily realized.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(Configuration of the first embodiment)
FIG. 1 is a schematic diagram showing the configuration of an electronic camera 100 according to the first embodiment of the present invention (corresponding to the electronic camera of claim 1). The electronic camera 100 according to the first embodiment includes an imaging sensor 1, a photographing optical system 2 that forms an object image on the imaging sensor 1, a CDS circuit 3, a gain circuit 4, a clamp circuit 5, and A / D conversion. The circuit 6 includes a dark current correction circuit 7, an image processing circuit 8, an AE sensor 9, an image sensor driving circuit (TG) 10, and a CPU 11.

  A light receiving element (photodiode) that performs photoelectric conversion is two-dimensionally arranged on the light receiving surface of the image sensor 1. In the region where the light receiving elements of the image sensor 1 are arranged, vertical readout lines (not shown) are provided for each column of the light receiving elements. A horizontal readout line 14 is provided at the output end of these vertical readout lines, and the output of the horizontal readout line 14 is connected to the CDS circuit 3.

  As shown in FIG. 2, the region where the light receiving elements of the image sensor 1 are arranged is partitioned into an effective pixel region 12 and an optical black region 13 formed adjacent to the upper portion of the effective pixel region 12. In the effective pixel region 12, signal charges are accumulated in each light receiving element in accordance with the brightness of the subject image formed by the photographing optical system 2, and an image signal for generating photographed image data based on the signal charges. Generated.

  On the other hand, the surface of the optical black region 13 is shielded by a light shielding film. The output of the optical black region 13 can be regarded as data corresponding to the portion where the subject light is not incident, that is, the black color of the subject image. In the optical black region 13, the charge (dark current component) accumulated in the light receiving element due to temperature change or the like is detected, and a dark current level reference signal is generated based on this charge.

  The CDS circuit 3 reduces noise components of the image signal and the reference signal by correlated double sampling. The gain circuit 4 amplifies the output of the image signal and the reference signal after noise removal. The clamp circuit 5 clamps a specific part of the waveform of the input signal to a constant voltage level. The A / D conversion circuit 6 performs A / D conversion of the image signal and the reference signal and outputs the result to the dark current correction circuit 7.

  The dark current correction circuit 7 averages the reference signal of the optical black area 13 in a predetermined processing unit, and calculates the average value of the reference signal. Then, dark current correction is performed by subtracting the dark current level (average value of the reference signal) from the image level of the image signal. The dark current correction circuit 7 has a function of limiting the upper limit value of the reference signal according to an instruction from the CPU 11. Specifically, when the high-brightness subject is located in the vicinity of the optical black area 13, the dark current correction circuit 7 calculates the average value of the reference signal after replacing the value exceeding the upper limit value of the reference signal with the upper limit value. .

The image processing circuit 8 performs processing such as gamma correction, color separation, and white balance on the image signal output from the dark current correction circuit 7 to generate captured image data.
The AE sensor 9 is composed of a light receiving element such as a CCD, and divides the object field into a plurality of areas and performs photometry. Then, the AE sensor 9 outputs luminance information for each area to the CPU 11. Each area of the AE sensor 9 corresponds to a part of the effective pixel region 12.

The image sensor drive circuit 10 generates drive signals necessary for the operations of discharging unnecessary charges, storing charges, and outputting accumulated charges of the image sensor 1, and supplies the drive signals to the image sensor 1. Further, the image sensor driving circuit 10 sets the amplification gain of the gain circuit 4 according to the sensitivity information (ISO sensitivity) of the image sensor 1 output from the CPU 11.
The CPU 11 controls the operation of each part of the electronic camera 100 and executes various arithmetic processes necessary for focus control and exposure control. For example, the CPU 11 instructs the image sensor driving circuit 10 to set the sensitivity of the image sensor 1 and to instruct the charge accumulation time of the image sensor 1. Further, the CPU 11 performs opening / closing control of a shutter mechanism (not shown) based on the setting for the shutter speed.

Further, the function of the CPU 11 in the first embodiment will be described below.
(1) The CPU 11 calculates and acquires subject luminance information (subject EV value) based on the output of the AE sensor 9. Then, the CPU 11 determines “the presence / absence of a high-brightness subject in the object scene” and “in which area of the effective pixel area the high-brightness subject is located” (function of “position determination unit”).

  (2) The CPU 11 has a restriction determination LUT in which data on the correspondence between the combination of the sensitivity information of the image sensor 1 and the luminance information of the subject and the presence / absence of restriction of the reference signal is recorded (see FIG. 3). Then, the CPU 11 determines whether or not the reference signal is limited by the limit determination LUT based on the sensitivity information of the image sensor 1 and the luminance information of the subject (function of “limit determination unit”).

(3) The CPU 11 has a threshold value LUT in which data on the correspondence between the sensitivity information of the image sensor 1 and the upper limit value (threshold value) of the reference signal is recorded (see FIG. 4). Then, the CPU 11 outputs the threshold value output from the threshold value LUT to the dark current correction circuit 7 based on the sensitivity information of the image sensor 1.
(Operation of the first embodiment)
Hereinafter, the operation of the electronic camera 100 of the first embodiment will be described with reference to the flowchart of FIG.

  Step S101: The user makes a predetermined input to the electronic camera 100 such as half-pressing a release button (not shown) after the electronic camera 100 is turned on. The CPU 11 of the electronic camera 100 takes in the luminance information of the object scene from the AE sensor 9 for each area. Then, the CPU 11 performs an exposure calculation based on the luminance information of the subject, and determines shooting conditions (aperture value, ISO sensitivity of the image sensor 1, shutter speed, etc.) at the time of shooting. Note that the CPU 11 may set shooting conditions based on a user's manual input.

Step S102: The CPU 11 determines whether or not a high brightness subject exists in the scene. If there is a high brightness subject (YES side), the CPU 11 proceeds to step S103. On the other hand, when there is no high brightness subject (NO side), the CPU 11 shifts to a normal shooting mode (S112).
Step S103: The CPU 11 determines whether or not the high-brightness subject is located in the vicinity of the optical black area 13 (upper part of the screen). If the high-luminance subject is located at the top of the screen (YES side), the CPU 11 proceeds to step S104. On the other hand, when there is no high brightness subject (NO side), the CPU 11 shifts to a normal shooting mode (S112).

  Step S104: The CPU 11 determines whether or not the shutter speed set in step S101 is equal to or longer than the predetermined shutter time (long time). If the shutter speed is a long time (YES side), the CPU 11 proceeds to step S105. On the other hand, when the shutter speed is not the long time (NO side), the CPU 11 shifts to the normal shooting mode (S112).

  Step S105: The CPU 11 inputs the sensitivity information of the image sensor 1 and the luminance information of the subject to the restriction determination LUT, and determines whether the reference signal is restricted based on the output result. If there is a restriction on the reference signal (YES side), the CPU 11 proceeds to step S106. on the other hand,. When there is no restriction on the reference signal (NO side), the CPU 11 shifts to the normal photographing mode (S112).

  For example, according to the restriction determination LUT shown in FIG. 3, when the sensitivity setting of the image sensor 1 is ISO400, if the EV value of the subject is +9 EV or more, the CPU 11 executes an operation of restricting the upper limit value of the reference signal. On the other hand, when the EV value of the subject is less than +9 EV, the dark current level in the optical black area 13 is not significantly affected, and the CPU 11 shifts to the normal shooting mode without performing the operation of limiting the upper limit value of the reference signal. . The EV value that limits the upper limit value of the reference signal also varies depending on the sensitivity setting of the image sensor 1.

  Step S106: The CPU 11 inputs sensitivity information of the image sensor 1 to the threshold value LUT, and acquires a threshold value (upper limit value of the reference signal) corresponding to the sensitivity information. Then, the CPU 11 outputs the acquired threshold value to the dark current correction circuit 7. For example, according to the threshold value LUT shown in FIG. 4, when the sensitivity setting of the image sensor 1 is ISO400, the threshold value that the CPU 11 outputs to the dark current correction circuit 7 is 205.

Step S107: The CPU 11 determines whether or not the shutter release has been executed by the user. If there is a shutter release (YES side), the CPU 11 proceeds to step S108. On the other hand, when there is no shutter release (NO side), the CPU 11 returns to step S101 and performs the photometric operation and the like again.
Step S108: With the shutter release, the CPU 11 controls the image sensor 1 via the image sensor driving circuit 10 to execute photoelectric conversion of the subject image. From the imaging sensor 1, the reference signal of the optical black area 13 and the image signal of the effective pixel area 12 are sequentially read out. The reference signal and the image signal pass through the CDS circuit 3, the gain circuit 4, the clamp circuit 5, and the A / D conversion circuit 6 and are input to the dark current correction circuit 7.

  Step S109: The dark current correction circuit 7 compares the input reference signal value with the threshold value (S106), and performs processing for replacing the reference signal value exceeding the threshold value with the threshold value. For example, when the output of the imaging sensor 1 is 8 bits (output value: 0 to 255) and the threshold value acquired in step S106 is 205, the dark current correction circuit 7 determines that the output value of the reference signal is 205 or less. The output value is not corrected. On the other hand, when the output value of the reference signal exceeds 205, the dark current correction circuit 7 replaces the output value with 205 and restricts it.

Step S110: The dark current correction circuit 7 averages the reference signal (S109) limited by the threshold value in a predetermined processing unit, and calculates the average value of the reference signal. Then, the dark current correction circuit 7 performs dark current correction by subtracting the dark current level (average value of the reference signal) from the image level of the image signal.
Step S111: The image processing unit 8 performs image processing on the image signal after dark current correction to generate captured image data. The captured image data output from the image processing unit 8 is compressed and encoded and then stored in a recording medium (not shown). Thereafter, the CPU 11 shifts to a standby state for the next shooting.

  Step S112: If any of the requirements from Step S102 to Step S105 is not satisfied, the CPU 11 shifts to a normal shooting mode. In this normal shooting mode, the electronic camera 100 generates shot image data without performing dark current correction according to the shooting conditions (S101). Alternatively, the electronic camera 100 performs dark current correction using the reference signal as it is, and generates captured image data.

(Effect of 1st Embodiment)
In the electronic camera according to the first embodiment, the upper limit value of the dark current level reference signal is limited when the high-brightness subject is located in the vicinity of the optical black region, so that excessive dark current level due to the influence of the high-brightness subject is prevented. it can. In addition, since the upper limit value of the reference signal changes corresponding to the sensitivity information of the image sensor, dark current correction with high accuracy according to the sensitivity of the image sensor can be realized.

(Configuration, operation and effect of the second embodiment)
The configuration of the electronic camera of the second embodiment (corresponding to the electronic camera of claim 2) is the same as that of the first embodiment shown in FIG. 1, and is the first only in that the CPU 11 does not have a restriction determination LUT. It is different from the embodiment. That is, in the second embodiment, the CPU 11 obtains the upper limit value of the reference signal based on the sensitivity information of the image sensor 1 without determining whether or not the reference signal is restricted. Then, the dark current correction circuit 7 compares and limits the upper limit value of the reference signal and the output value.

  FIG. 6 is a flowchart showing the operation of the electronic camera 100 of the second embodiment. The operation of the electronic camera of the second embodiment is the same as that of the electronic camera of the first embodiment except that there is no operation corresponding to step S105 (the operations of S201 to S204 in FIG. 5 corresponds to the operation from S <b> 101 to S <b> 104 in FIG. 6, and the operation after S <b> 205 in FIG. 6 corresponds to the operation after S <b> 106 in FIG. 5.

In the second embodiment, in addition to the same effects as those in the first embodiment, the operation for determining whether or not the reference signal is restricted based on the ISO sensitivity of the imaging sensor and the EV value of the subject is omitted. The circuit configuration of the CPU 11 is simplified.
(Correspondence between Claims and Embodiments)
Here, the correspondence between the claims and the embodiment is shown. Note that the correspondence shown below is an interpretation given for reference only, and does not limit the technical scope of the present invention.

  The imaging sensor 1 corresponds to “imaging sensor”. The “effective pixel region” and the “optical black region” correspond to the effective pixel region 12 and the optical black region 13, respectively. The AE sensor 9 corresponds to “divided photometry means”. The CPU 11 corresponds to “position determination means”. The CPU 11 corresponds to “limitation determining means” in claim 1. The dark current correction circuit 7 corresponds to the “dark current correction unit”.

(Supplementary items of the embodiment)
As mentioned above, although this invention has been demonstrated by said embodiment, the technical scope of this invention is not limited to the said embodiment. For example, in the above embodiment, the optical black region is formed above the effective pixel region, but the optical black region may be formed below or on the side of the effective pixel region. In the above embodiment, an XY address type (CMOS, LBCAST, etc.) image sensor is used, but the image sensor may be a CCD. Furthermore, the shutter of the electronic camera may be an electronic shutter by controlling the charge accumulation time of the image sensor. Furthermore, it is good also as a structure which performs division | segmentation photometry with an imaging sensor, without providing an AE sensor in an electronic camera.

  According to the present invention, it is possible to realize an electronic camera capable of highly accurate dark current correction even when there is a high-luminance subject near the optical black area.

It is a schematic diagram which shows the structure of the electronic camera of 1st Embodiment. It is a figure which shows the structure of an imaging sensor. It is a figure which shows the content of the restriction | limiting determination LUT. It is a figure which shows the content of the threshold value LUT. It is a flowchart which shows operation | movement of the electronic camera of 1st Embodiment. It is a flowchart which shows operation | movement of the electronic camera of 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Imaging sensor 2 Imaging optical system 3 CDS circuit 4 Gain circuit 5 Clamp circuit 6 A / D conversion circuit 7 Dark current correction circuit 8 Image processing circuit 9 AE sensor 10 Imaging sensor drive circuit (TG)
11 CPU
12 Effective pixel area 13 Optical black area 14 Horizontal readout line 100 Electronic camera

Claims (2)

An image sensor that includes an effective pixel region that photoelectrically converts a subject image to generate an image signal, and an optical black region that is formed around the effective pixel region and generates a dark current level reference signal;
Split photometry means for obtaining luminance information of the object scene;
Position determining means for determining the position of a high-luminance subject in the effective pixel area based on the output of the divided photometry means;
Limit determination means for determining whether or not the reference signal is limited based on sensitivity information of the imaging sensor and luminance information of the high-brightness subject when the high-brightness subject is located in the vicinity of the optical black region;
A dark current that limits the upper limit value of the reference signal with a threshold value corresponding to the sensitivity information based on the determination result of the limit determination means, and performs dark current correction of the image signal based on the reference signal after the limitation And an electronic camera. An image sensor that includes an effective pixel region that photoelectrically converts a subject image to generate an image signal, and an optical black region that is formed around the effective pixel region and generates a dark current level reference signal;
Split photometry means for obtaining luminance information of the object scene;
Position determining means for determining the position of a high-luminance subject in the effective pixel area based on the output of the divided photometry means;
When the high-brightness subject is located in the vicinity of the optical black region, the upper limit value of the reference signal is limited by a threshold value corresponding to sensitivity information of the imaging sensor, and the image signal is based on the reference signal after the limitation An electronic camera comprising: a dark current correction unit that performs dark current correction.

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