JP2015090998A - Imaging apparatus and image correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging apparatus capable of fast and accurately removing a dark current nonuniformity component even in the case where clamp processing is performed, and an image correction method.SOLUTION: A CCD image sensor 104 stores power of beams inputted via a lens 101 and performs the clamp processing to bring a black level into a constant value. A fixed value removal part 106 subtracts a fixed value from normal photographic image data which are obtained in the state where there is incident light into the CCD image sensor 104, and reference dark black image data which are obtained in the state where there is no incident light. A correction coefficient calculation part 133 calculates a correction coefficient on the basis of a photography condition of the reference dark black image data and a photography condition of the normal photographic image data. A dark current correction part 134 uses the correction coefficient to correct the reference dark black image data after the subtraction of the fixed value and uses the corrected reference dark black image data to perform dark current correction on the normal photographic image data after the subtraction of the fixed value.

Description

  The present invention relates to an imaging apparatus and an image correction method, and more particularly to an imaging apparatus and an image correction method that perform image correction related to dark current.

  When the temperature of the image sensor surface is not uniform in the imaging apparatus, the dark current amount fluctuates and unevenness occurs. The main shooting conditions related to the increase / decrease in the dark current amount include ISO sensitivity (Gain), exposure time, and housing (image sensor) temperature. The dark current amount increases in proportion to the ISO sensitivity (Gain). Similarly, the dark current amount increases in proportion to the exposure time. Further, the dark current amount increases as the casing temperature increases. The housing temperature varies depending on the environmental temperature outside the imaging apparatus in addition to the influence of heat generation in the internal peripheral circuits.

Since the amount of dark current generated in each region in the image data is not constant, dark current unevenness cannot be removed by OB subtraction processing that uniformly subtracts a constant value. For this reason, Dark Frame subtraction processing for subtracting reference dark image data (light-shielded image data, Dark Frame) captured under the same exposure conditions from normal captured image data (Image Frame) in units of pixels may be executed. FIG. 1 is a timing chart showing the flow of Dark Frame subtraction processing.

As shown in FIG. 1, the Dark Frame is acquired under the same exposure conditions after the Image Frame is acquired. After that, the dark current unevenness component is removed by subtracting the Dark Frame from the Image Frame. However, since the Dark Frame subtraction process needs to perform exposure of the Dark Frame, there is a problem that the photographing time becomes long.

  Patent document 1 is mentioned as a technique relevant to this problem. The imaging device described in Patent Document 1 stores imaging conditions (exposure time, temperature, gain) when imaging reference dark image data (light-shielded image data), and stores actual imaging conditions and the above-described imaging conditions (references). The correction data is created from the reference dark image data using the shooting conditions when shooting the dark image data. The imaging apparatus corrects actual captured image data using the correction data. This eliminates the shooting of the reference dark image data (light-shielded image data) every time, thereby realizing a reduction in shooting time.

JP 2010-141583 A JP 2011-135377 A

  In a general imaging device, the imaging device performs a clamping process so that the dark level is fixed to a constant value. However, in the technique of Patent Document 1, correction data is created without considering this clamping process. Therefore, the method of Patent Document 1 has a problem that the dark current unevenness component cannot be accurately removed.

One aspect of the imaging apparatus according to the present invention is:
An image sensor that stores a light beam input through a lens and performs a clamping process to set a black level to a constant value;
A fixed value removal unit that subtracts a fixed value from normal captured image data obtained in a state where incident light is present on the image sensor and reference dark image data obtained in a state where no incident light exists in the image sensor;
A correction coefficient calculation unit that calculates a correction coefficient based on the shooting conditions of the reference dark image data and the shooting conditions of the normal shooting image data;
The reference dark image data after the fixed value subtraction is corrected using the correction coefficient, and the dark current correction is performed on the normal photographed image data after the fixed value subtraction using the corrected reference dark image data. A dark current correction unit to be performed;
It is provided.

One aspect of the image correction method according to the present invention is:
An imaging step of performing a clamping process of storing a light beam input through a lens using an imaging device and setting a black level to a constant value;
A fixed value removal step of subtracting a fixed value from normal captured image data obtained in a state where incident light is present on the image sensor and reference dark image data obtained in a state where no incident light is present in the image sensor;
A correction coefficient calculating step for calculating a correction coefficient based on the shooting condition of the reference dark image data and the shooting condition of the normal shooting image data;
The reference dark image data after the fixed value subtraction is corrected using the correction coefficient, and the dark current correction is performed on the normal photographed image data after the fixed value subtraction using the corrected reference dark image data. Dark current correction step to be performed;
It is provided.

  The present invention can provide an imaging apparatus and an image correction method that can remove dark current unevenness components at high speed and with high accuracy even when clamping processing is performed.

It is a timing chart which shows the flow of a general Dark Frame subtraction process. FIG. 3 is a diagram showing a periphery (OB area) of an active area of the CCD image sensor 104 according to the first exemplary embodiment. 1 is a block diagram illustrating a configuration of an imaging apparatus 100 according to a first embodiment. 3 is a flowchart showing a flow of processing of a correction coefficient calculation unit 133 according to the first embodiment. 3 is a flowchart showing a flow of processing of a dark current correction unit 134 according to the first exemplary embodiment.

<Embodiment 1>

  First, an outline of the operation of the imaging apparatus 100 according to the present embodiment will be described. The image capturing apparatus 100 stores an image frame that is normal captured image data and a dark frame that is reference dark image data acquired under exposure conditions different from the image frame in an image memory 120 described later. The Dark Frame is image data captured in a light-shielded state. A general clamping process is performed when the Image Frame and the Dark Frame are acquired. The imaging apparatus 100 performs a subtraction process that excludes the influence of the clamp process on each frame (Image Frame, Dark Frame). The imaging apparatus 100 calculates a correction coefficient using the imaging conditions (ISO sensitivity, exposure time, and housing temperature) at the time of acquisition of each frame, and performs dark current correction of the Image Frame. The processing is represented by the following formula (1). Further, symbols used in the formula (1) are shown in Table (1).

  The fixed value component is removed by the subtraction of the OB (OB ') value described above. The OB area is an area around the active area of the CCD image sensor 104 as shown in FIG.

  The details of the embodiments of the present invention will be described below with reference to the drawings. FIG. 3 is a block diagram illustrating a configuration of the imaging apparatus 1 according to the present embodiment. The imaging apparatus 100 includes a lens 101, an aperture 102, a shutter 103, a CCD image sensor 104, a signal processing unit 105, a fixed value removal unit 106, a preprocessing unit 107, a post processing unit 108, an RGB-YCC conversion unit 109, and a data compression unit. 110, a memory card connection unit 111, a memory card 112, an image memory 120, an appropriate AE calculation unit 131, an AWB processing unit 132, a correction coefficient calculation unit 133, a dark current correction unit 134, an exposure control unit 141, and a flash 142.

  The light that has passed through the lens 101, the diaphragm 102, and the shutter 103 forms an image on the CCD image sensor 104. The CCD image sensor 104 is an image sensor that stores light that has passed through the shutter 103 as an electrical signal. The CCD image sensor 104 is an example of an image sensor, and may be a CMOS image sensor or the like. The CCD image sensor 104 performs a clamping process for setting the black level to a constant value. The clamping process is a process represented by the following formula (2). Further, symbols used in the formula (2) are shown in Table (2).

  The significance of the above formula (2) will be described. In principle, the dark current input to the CCD image sensor 104 (SensorInput Dark Current) and the dark current calculated by the CCD image sensor 104 (SensorDarkCurrent) match. However, when photographing is performed under a high ISO sensitivity state or a long exposure time, the dark current (Dark Current) of the sensor output value (SensorOutput) increases. For example, when photographing with a long exposure time is performed, image data with a very large dark current is acquired. Therefore, addition processing is performed so that the pixel value of the pixel in which the dark current is generated becomes a fixed value. By this addition processing, the pixel value of the pixel in which the dark current is generated becomes a fixed value (pedestal). As a result, the output of the CCD image sensor 104 is not dependent on the exposure time or ISO sensitivity. The CDD image sensor 104 inputs the signal after the clamp processing to the signal processing unit 105.

The signal processing unit 105 performs color separation processing, A / D conversion, and the like on the signal from the CCD image sensor 104. An RGB image signal corresponding to the Image Frame and the Dark Frame is generated by the processing of the signal processing unit 105. As described above, the signal processing unit 105 performs the above-described processing (for example, color separation processing) on the electrical signal that does not depend on the exposure time or ISO sensitivity.

The fixed value removing unit 106 performs a process of removing the above-described fixed value component from the Image Frame and Dark Frame output from the signal processing unit 105. This fixed value removal processing is realized by subtraction of the OB (OB ′) value in the equation (1). The fixed value removal unit 106 stores the image frame and dark frame after removal of the fixed value component in the image memory 120. Dark Frame is light-shielded image data (reference dark image data) obtained by photographing in a light-shielded state, and may be acquired once by the imaging apparatus 100 in principle. When the Dark Frame is stored in the image memory 120, the imaging conditions (case temperature, ISO sensitivity, exposure time) are also stored in the image memory 120.

  The preprocessing unit 107 performs processing such as defective pixel correction and shading correction on the image frame corrected by the dark current correction unit 134 described later. Further, the preprocessing unit 107 calculates an AE evaluation value based on the RGB image signal of Image Frame, and supplies the calculated AE evaluation value to the appropriate AE calculation unit 131. The appropriate AE calculation unit 131 obtains an AE calculated value from the AE evaluation value using a general method. When shooting is actually performed, the exposure control unit 141 adjusts the aperture 102, the shutter 103, the CCD image sensor 104, and the flash 142 based on the AE calculation value calculated by the appropriate AE calculation unit 131.

  The image signal processed by the preprocessing unit 107 is subjected to various images such as white balance correction processing, Bayer color interpolation (demosaic) processing, color correction processing, edge enhancement processing, gamma correction processing, and noise reduction processing by the postprocessing unit 108. Processing is performed. Here, the white balance correction process may use the white balance gain calculated by the AWB processing unit 132 described later. Thereafter, the RGB-YCC conversion unit 109 converts the RGB signal into a YCC signal, and the data compression unit 110 performs compression processing in a format such as JPEG. Then, image data is recorded on the memory card 112 via the memory card connection unit 111 which is an interface.

  The AWB processing unit 132 uses the AE calculation value calculated by the appropriate AE calculation unit 131, the captured image data captured when the flash is not emitted, and the white balance used during the white balance correction process using the captured image data captured when the flash is emitted. Calculate the gain. The calculation method may be the same method as in Patent Document 2, for example.

  The image memory 120 is an example of a storage unit that stores RGB signals that have been subjected to various types of preprocessing by the preprocessing unit 107. The post-processing unit 108 reads out the RGB signals stored in the image memory 120 and performs various processes. In addition, the image memory 120 stores a Dark Frame and its imaging conditions (case temperature, ISO sensitivity, exposure time).

  Although not shown, the imaging apparatus 100 includes a temperature sensor that acquires the casing temperature of the imaging apparatus 100, a timer that measures the exposure time, and the like.

  The correction coefficient calculation unit 133 calculates a correction coefficient using the imaging conditions of each frame (Image Frame, Dark Frame). Details of the correction coefficient calculation process will be described below with reference to the flowchart of FIG.

The correction coefficient calculation unit 133 calculates the ISO sensitivity coefficient GainCoefficient based on the ISO sensitivity of the Image Frame and the ISO sensitivity of the DarkFrame (S11). When the ISO sensitivity of the Image Frame is TargetSV and the ISO sensitivity of the Dark Frame is CurrentSV, the correction coefficient calculation unit 133 calculates the ISO sensitivity coefficient GainCoefficient based on the following equation (3). In addition, the symbol used in Formula (3) is shown in Table (3).

Next, the correction coefficient calculation unit 133 calculates an exposure time coefficient ExposureCoefficient based on the exposure time of the Image Frame and the exposure time of the DarkFrame (S12). When the exposure time of the Image Frame is TargetTV and the exposure time of the Dark Frame is CurrentTV, the correction coefficient calculation unit 133 calculates an exposure time coefficient ExposureCoefficient based on the following equation (4). In addition, the symbol used in Formula (4) is shown in Table (4).

  Next, the correction coefficient calculation unit 133 calculates a temperature coefficient TemperatureCoefficient based on the image frame casing temperature and the dark frame casing temperature (S13). When the housing temperature at the time of acquiring the image frame is v ′, the housing temperature at the time of acquiring the dark frame is v, and the doubling temperature is kd, the correction coefficient calculating unit 133 calculates the temperature coefficient temperature coefficient based on the following equation (5). calculate. In addition, the symbol used in Formula (5) is shown in Table (5).

The correction coefficient calculation unit 133 calculates a correction coefficient DarkFrameCoefficient that is multiplied by the Dark Frame based on the coefficients calculated in S11 to S13 described above (S14). The correction coefficient calculation unit 133 multiplies each coefficient calculated in S11 to S13 described above, that is, calculates the correction coefficient DarkFrameCoefficient according to the following equation (6).

  Refer to FIG. 3 again. The dark current correction unit 134 performs a correction process using the calculated correction coefficient on the Image Frame and Dark Frame after the fixed value component is removed. The processing of the dark current correction unit 134 will be described with reference to the flowchart of FIG.

  The dark current correction unit 134 reads out the Image Frame and Dark Frame from which the fixed value component has been removed from the image memory 120. Then, the dark current correction unit 134 multiplies the read Dark Frame by the correction coefficient DarkFrameCoefficient calculated by the correction coefficient calculation unit 133 (S21). When the Dark Frame before multiplication is the Dark Frame and the Dark Frame after multiplication is the Dark Frame ′, the dark current correction unit 134 calculates the Dark Frame ′ based on the following formula (7). In addition, the symbol used in Formula (7) is shown in Table (6).

  The dark current correction unit 134 subtracts the Dark Frame (Dark Frame 'in the above equation (7)) to which the correction coefficient is applied from the Image Frame from which the fixed value component has been removed (S22). When the pre-subtraction Image Frame is ImageFrame and the post-subtraction Image Frame is ImageFrame ′, the dark current correction unit 134 calculates ImageFrame ′ based on the following equation (8). In addition, the symbol used in Formula (8) is shown in Table (7).

  As described above, the imaging apparatus 100 calculates the correction coefficient, and outputs the image frame that has been corrected for the dark current to an arbitrary processing unit (the preprocessing unit 107 in the above example).

  Next, effects of the imaging device 100 according to the present embodiment will be described. As described above, the imaging apparatus 1 according to the present embodiment can perform appropriate correction even when the Dark Frame acquired under the exposure condition different from the Image Frame is used. Even when the clamping process is performed in the imaging apparatus 100, the imaging apparatus 100 performs a correction process after performing a fixed value subtraction process. Since the correction is performed after the subtraction process corresponding to the clamp process is performed, the correction can be performed with higher accuracy than the conventional technique (for example, Patent Document 1).

  The acquisition of the Dark Frame is not performed immediately after the acquisition of the Image Frame, but the Dark Frame may be acquired in advance at specific exposure conditions (temperature, ISO sensitivity, exposure time) in a factory production line or the like. Even in this case, the Dark Frame having the same exposure condition as the Image Frame can be generated by calculating the correction coefficient described above.

  Furthermore, it is desirable that the exposure time of the Dark Frame is shorter than the exposure time of the Image Frame. This is because the correction coefficient can be accurately calculated by the above-described correction coefficient calculation process, and therefore the overall photographing time can be kept short by shortening the exposure time of the Dark Frame.

Although the present invention has been described with reference to the above-described embodiment, the present invention is not limited to the configuration of the above-described embodiment, and those skilled in the art within the scope of the invention of the appended claims. It goes without saying that various modifications, corrections, and combinations that can be made are included.

DESCRIPTION OF SYMBOLS 100 Imaging device 101 Lens 102 Diaphragm 103 Shutter 104 CCD image sensor 105 Signal processing part 106 Fixed value removal part 107 Pre-processing part 108 Post-processing part 109 RGB-YCC conversion part 110 Data compression part 111 Memory card connection part 112 Memory card 120 Image Memory 131 Proper AE calculation unit 132 AWB processing unit 133 Correction coefficient calculation unit 134 Dark current correction unit 141 Exposure control unit 142 Flash

Claims (6)

An image sensor that stores a light beam input through a lens and performs a clamping process to set a black level to a constant value;
A fixed value removal unit that subtracts a fixed value from normal captured image data obtained in a state where incident light is present on the image sensor and reference dark image data obtained in a state where no incident light exists in the image sensor;
A correction coefficient calculation unit that calculates a correction coefficient based on the shooting conditions of the reference dark image data and the shooting conditions of the normal shooting image data;
The reference dark image data after the fixed value subtraction is corrected using the correction coefficient, and the dark current correction is performed on the normal photographed image data after the fixed value subtraction using the corrected reference dark image data. A dark current correction unit to be performed;
An imaging apparatus comprising:   The imaging apparatus according to claim 1, wherein an exposure time of the reference dark image data is shorter than an exposure time of the normal captured image data. The correction coefficient calculation unit
Calculating an exposure time coefficient calculated based on an exposure time at the time of shooting the normal shot image data and an exposure time at the time of shooting the reference dark image data;
Calculating a temperature coefficient calculated based on the case temperature at the time of shooting the normal image data and the case temperature at the time of shooting the reference dark image data;
An ISO sensitivity coefficient calculated based on the ISO sensitivity at the time of shooting of the normal captured image data and the ISO sensitivity at the time of shooting of the reference dark image data is calculated,
Calculating the correction coefficient by multiplying the exposure time coefficient, the temperature coefficient, and the ISO sensitivity coefficient;
The imaging device according to claim 1 or 2. An imaging step of performing a clamping process of storing a light beam input through a lens using an imaging device and setting a black level to a constant value;
A fixed value removal step of subtracting a fixed value from normal captured image data obtained in a state where incident light is present on the image sensor and reference dark image data obtained in a state where no incident light is present in the image sensor;
A correction coefficient calculating step for calculating a correction coefficient based on the shooting condition of the reference dark image data and the shooting condition of the normal shooting image data;
The reference dark image data after the fixed value subtraction is corrected using the correction coefficient, and the dark current correction is performed on the normal photographed image data after the fixed value subtraction using the corrected reference dark image data. Dark current correction step to be performed;
An image correction method comprising:   The image correction method according to claim 4, wherein an exposure time of the reference dark image data is shorter than an exposure time of the normal captured image data. In the correction coefficient calculation step,
Calculating an exposure time coefficient calculated based on an exposure time at the time of shooting the normal shot image data and an exposure time at the time of shooting the reference dark image data;
Calculating a temperature coefficient calculated based on the case temperature at the time of shooting the normal image data and the case temperature at the time of shooting the reference dark image data;
An ISO sensitivity coefficient calculated based on the ISO sensitivity at the time of shooting of the normal captured image data and the ISO sensitivity at the time of shooting of the reference dark image data is calculated,
Calculating the correction coefficient by multiplying the exposure time coefficient, the temperature coefficient, and the ISO sensitivity coefficient;
The image correction method according to claim 4 or 5.

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