JP3806973B2 - Dark current correction device for photoelectric converter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for correcting dark current included in an output of a photoelectric converter.
[0002]
[Prior art]
An image sensor is used for a focus detection device such as a camera. This image sensor has a plurality of charge storage photoelectric conversion pixels arranged one-dimensionally. The output of the image sensor reflects the light luminance distribution of the subject, and the focus detection device detects the focus adjustment state of the photographing lens based on the output of the image sensor.
[0003]
By the way, when the subject has low luminance, the accumulation time is lengthened in order to obtain an image sensor output of a predetermined level or more, but this also increases the dark current and lowers the S / N ratio. The magnitude of the dark current varies from one photoelectric conversion pixel to another, and if the dark current of a certain pixel is significantly larger than the dark current of the surrounding pixels, it looks as if the subject has high contrast. (See FIG. 4A). In particular, when the luminance is low, the output level of all the photoelectric conversion pixels is low, the dark current component is relatively large, and the focus detection performance is greatly adversely affected. Further, since the magnitude of the dark current depends not only on the accumulation time but also on the temperature, the dark current increases and the S / N ratio decreases at high temperatures.
[0004]
In order to solve such a problem, a method of correcting a dark current included in the output of an image sensor has been proposed. For example, Japanese Patent Laid-Open No. 3-10473 discloses an image sensor provided with a light-shielding pixel portion for monitoring a dark current in addition to an aperture pixel portion. By calculating the dark current increase / decrease rate from the ratio of the shaded pixel output during the focus detection operation and the stored shaded pixel output, and multiplying the stored aperture pixel output by the ratio, The dark current for each pixel is obtained. Here, the aperture pixel is a pixel on the image sensor to which a light beam from a subject is irradiated by the focus detection optical system.
[0005]
[Problems to be solved by the invention]
However, the conventional dark current correction device for a photoelectric converter needs to store dark current of all pixels, and has a large capacity when many photoelectric converters are used like a multipoint ranging type focus detection device. Memory is required, and the correction time becomes long.
Further, the dark pixel output for dark current monitoring may not necessarily reflect the temperature depending on the configuration of the image sensor. For example, when the environmental temperature of the image sensor is about 50 ° C. or less due to the influence of the heat of the peripheral circuit, the light-shielded pixel output becomes substantially constant with respect to the temperature. For this reason, there is also a problem that optimum correction cannot be performed for the output of the aperture pixel including the dark current component depending on the temperature.
[0006]
An object of the present invention is to provide a dark current correction device for a photoelectric converter that does not require a large-capacity memory and corrects the dark current accurately in a short time.
[0007]
[Means for Solving the Problems]
(1) According to the first aspect of the present invention, at the time of dark current measurement , the difference between the dark current of a specific pixel of a photoelectric converter composed of a plurality of pixels and the average dark current of pixels other than the specific pixel is determined by the shading pixel. The value divided by the output is stored in advance as a dark current correction value during dark current measurement, and during use, the dark current correction during use is based on the output of the light-shielded pixels and the dark current correction value during dark current measurement. The value is obtained, and the dark current correction value is subtracted from the output current of a specific pixel of the photoelectric converter to correct the dark current.
(2) dark current correction apparatus of the photoelectric converter according to claim 2, stores the temperature information at the time of the dark current sensing, the use time based on the temperature information at the time of use and the temperature information at the time of the dark current measurement The dark current correction value is corrected for temperature , and the dark current correction value after use for temperature correction is subtracted from the output current of the specific pixel of the photoelectric converter to correct the dark current.
(3) The dark current correction device for a photoelectric converter according to claim 3 preferentially corrects the dark current for pixels included in a region where the photoelectric converter is frequently used.
(4) The dark current correction device for a photoelectric converter according to claim 4 is a specific pixel that performs dark current correction by extracting a predetermined number in order from the largest dark current among the plurality of pixels of the photoelectric converter. Is.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
-First embodiment of the invention-
FIG. 1 shows the configuration of the first embodiment of the present invention.
The light beam from the subject that has passed through the objective lens 1 is guided to the image sensor 3 by the focus detection optical system 2 and imaged. The image sensor 3 performs photoelectric conversion according to the light intensity distribution of the subject image and outputs a subject image signal. This subject image signal is sent to the A / D conversion circuit 4, converted into a digital signal, and output to the dark current correction circuit 5. Data relating to dark current correction is stored in the correction data storage unit 6, and the correction data is sent to the dark current correction circuit 5. The temperature monitor unit 10 measures the temperature of the image sensor 3 and outputs temperature data to the dark current correction circuit 5. Note that the ambient temperature of the image sensor 3 may be measured by the temperature monitor 10.
The dark current correction circuit 5 corrects the dark current included in the subject image signal of the image sensor 3 based on the dark current correction data and the temperature data, and sends it to the calculation unit 7. The calculation unit 7 performs focus detection calculation based on the subject image signal with the dark current corrected, and calculates the defocus amount of the objective lens 1. The drive control unit 8 drives and controls the motor 9 according to the calculated defocus amount, and moves the objective lens 1.
[0009]
FIG. 2 is a flowchart showing dark current correction data measurement and storage processing in the manufacturing process of the apparatus. FIG. 3 is a flowchart showing dark current correction processing when the dark current correction apparatus of this embodiment is applied to a camera.
In this embodiment, an image sensor in which 100 photoelectric conversion pixels (photoelectric conversion elements) are arranged one-dimensionally will be described as an example. Then, α pixels are extracted from the photoelectric conversion pixel array having the larger dark current, and the dark current is corrected.
[0010]
FIG. 4 shows the dark current for each pixel.
Here, as shown in (a), a case is considered where the dark current of the (i + 1) th pixel is larger than the dark current of peripheral pixels such as the i-th and (i + 2) -th pixels. If the dark current is simply corrected for the (i + 1) -th pixel having a large dark current, only the dark current of the (i + 1) -th pixel becomes almost zero as shown in FIG. The difference from the dark current increases. This state may be mistaken as if the contrast of the subject is large at the (i + 1) th pixel portion, which may reduce the focus detection accuracy.
Therefore, in this embodiment, as shown in (c), for the (i + 1) th pixel having a large dark current, the dark current is an average dark current level of peripheral pixels (indicated by a broken line in the figure). Correct until. In this way, the dark current levels of all pixels after dark current correction are averaged, and high focus detection accuracy can be maintained.
[0011]
First, the measurement and storage processing of dark current correction data in the manufacturing process of the apparatus will be described with reference to FIG.
Prior to measuring and storing dark current correction data, the camera is installed in a dark, room temperature environment. In step 100, parameters such as the accumulation time and gain of the image sensor 3 are set to predetermined values for measuring dark current data. At this time, in order to make the dark current difference between the pixels clearly appear to some extent, it is preferable to lengthen the accumulation time and increase the gain. In step 101, the charge accumulation operation of the image sensor 3 is performed according to the dark current data measurement parameters, the output of the image sensor 3 is A / D converted by the A / D conversion circuit 4, and output to an external computer.
[0012]
In the correction value calculation operation in the external computer, first, in step 102, the correction pixel number x is initialized. Next, in step 103, a pixel having the maximum dark current among the 100 pixels is detected, and the dark current value is stored in D (x) and the pixel number is stored in P (x). The i-th pixel output is expressed as Y (i), and the maximum value among 100 pixel outputs is expressed as Max (Y (i)). Further, the output Y (i) is rewritten to 0 so that the pixel in which the dark current D (x) and the pixel number P (x) are stored is not detected again assuming that the dark current is the maximum. In steps 104 and 105, the process of step 103 is repeated until the corrected pixel number x reaches α, and the dark current D (x) of the α pixels and the pixels in order from the largest dark current among the 100 pixels. The number P (x) is stored.
[0013]
When the dark current data of α pixels having a large dark current is measured, the process proceeds to step 106, and an average value Av of dark currents of pixels other than α is calculated. In step 107, x is initialized again. In the next step 108, the difference between the x-th dark current D (x) and the average dark current Av is taken, and the difference is divided by the light-shielded pixel output OPB to calculate the dark current correction value H (x). In steps 109 and 110, the process of step 108 is repeated until the number of corrected pixel numbers x becomes α, and the dark current correction values H (x) of α pixels in order from the largest dark current among the 100 pixels. Is calculated. In step 111, the dark current correction value H (x) of the α pixels, the pixel number P (x), and the temperature T at the time of correction data measurement are stored in the correction data storage unit 6.
[0014]
Here, the charge accumulation operation in step 101 may be performed a plurality of times in order to avoid the influence of random noise, and the average data may be acquired to perform the processing after step 102.
[0015]
Next, temperature correction data is obtained.
For example, when the dark current D (1) of the pixel of pixel number 1 is measured while changing the temperature t, the dark current temperature characteristic as shown in FIG. 6 is obtained. A function of this temperature characteristic is obtained and used as temperature correction data f (t). In order to avoid an increase in storage capacity and an increase in calculation time, the temperature correction data f (t) may be approximated to a quadratic function or a linear function. The temperature correction data f (t) is stored in the correction data storage unit 6.
Note that the temperature correction data f (t) is obtained by measuring the temperature characteristics of the dark currents D (1) to D (α) of α pixels included in one image sensor and taking an average, or several pixels. The temperature characteristics of the dark current may be measured and averaged. In any case, it is not necessary to obtain for each image sensor.
[0016]
The dark current correction process when the dark current correction apparatus of this embodiment is applied to a camera will be described with reference to FIG.
In step 200, the charge accumulation operation of the image sensor 3 is performed, and the output is A / D converted by the A / D conversion circuit 4 and output to the dark current correction circuit 5. The dark current correction circuit 5 first initializes the correction pixel number x in step 201. In subsequent step 202, the dark current correction value H (x), the correction pixel number P (x), the temperature T at the time of correction data measurement, and the temperature correction data f (t) are read from the correction data storage unit 6 and the temperature monitor unit. The temperature t during use is input from 10 and the dark current correction value DK (x) under the use environment is calculated based on these data and the light-shielded pixel output OPB during use. Here, in order for the temperature correction coefficient to be 1 at the temperature T ° C. when the correction data is measured, the temperature correction coefficient is f (t) / f (T). In step 203, the dark current correction value DK (x) is subtracted from the output Y (P (x)) to perform correction processing to obtain the dark current correction pixel output YH (P (x)). In steps 204 and 205, the processing in steps 202 and 203 is repeated until the pixel number x reaches α, and dark current correction processing for α pixels is performed.
[0017]
-Second embodiment of the invention-
In focus detection, focus detection may be performed based on the output of some photoelectric conversion pixels without using the output of all photoelectric conversion pixels on the image sensor. Depending on the relative arrangement of the objective lens, focus detection optical system, and image sensor, the light beam may not reach the photoelectric conversion pixels at the edge of the image sensor. No correction is necessary even if the current is large.
In addition, when a focus detection region is switched and used in one image sensor, photoelectric conversion pixels included in a narrow focus detection region are frequently used. For example, when a narrow focus detection area N and a wide focus detection area W are set in the shooting screen as shown in FIG. 5A, the narrow focus detection area N is narrow on the image sensor shown in FIG. A wide focus detection area W corresponding to the area N ′ corresponds to a wide area W ′ on the image sensor shown in FIG. In this case, the photoelectric conversion pixels included in the narrow area N ′ on the image sensor are used for focus detection regardless of whether the focus detection area is narrow or wide.
In such a case, a second embodiment will be described in which dark current correction is performed preferentially from photoelectric conversion pixels that are frequently used. The configuration of the second embodiment is the same as that of the first embodiment shown in FIG.
[0018]
7 to 9 are flowcharts showing dark current correction data measurement and storage processing in the manufacturing process of the apparatus.
The outline will be described. First, in steps 300 to 304, a dark current D (x) and a pixel number P (x) are obtained in a narrow region N ′ on the image sensor 3 as in the first embodiment. In subsequent steps 305 to 307, the dark current D (x) and the pixel number P (x) are obtained in the wide area W ′ excluding the narrow area N ′ on the image sensor 3. Next, in steps 308 to 311, a dark current correction value H (x) is obtained for a pixel in which the dark current D (x) is equal to or greater than a predetermined value β in the narrow region N ′. At this time, when the number of pixels to be corrected whose dark current correction value D (x) is equal to or larger than β is less than a predetermined α, in Steps 313 to 314, the shortage that is less than α is reduced to a narrow region N ′. It is determined from the wide area W ′ that is excluded.
[0019]
This will be described in detail with reference to FIGS.
In steps 300 and 301, the charge accumulation operation is performed in the same manner as in the first embodiment shown in FIG. 2 to obtain dark current data. In step 302, the narrow area N ′ on the image sensor 3 is set to a predetermined range, and in step 303, the subroutine A shown in FIG. 8 is executed.
[0020]
In the subroutine A of FIG. 8, in steps 400 to 403, α dark currents D (x) and pixel numbers PL (in order of the output in the descending order as in the first embodiment for the pixels included in the predetermined range N ′. x). In step 404, the average dark current Av for the remaining pixels in the predetermined range N ′ is obtained, and in steps 405 to 408, the difference DL (x) between the α dark currents D (x) and the average dark current Av is obtained.
[0021]
Returning to step 304 in FIG. 7, the difference DL (x) and the pixel number PL (x) are saved to DN (x) and PN (x), respectively. Next, in steps 305 to 307, (W′−N ′) is set to a predetermined range, and similarly, a difference DL (x) and a pixel number PL (x) are obtained, and DW (x) and PW (X), respectively. Evacuate. In step 308, α correction values H (x) and pixel number P (x) are reset to 0, and in step 309, the count number j of the correction target pixel is set to 0, and then saved in step 310 first. The values of DN (x) and PN (x) are returned to DL (x) and PL (x) again. In step 311, subroutine B shown in FIG. 9 is executed.
[0022]
In subroutine B of FIG. 9, first, at step 500, the correction pixel number x is initialized. If the difference DL (x) protruding with respect to the peripheral pixel is larger than the predetermined value β in step 501, j is incremented in step 502 and the dark current correction value is determined in step 503. H (x) and pixel number P (x) are obtained. Note that if the difference DL (x) is equal to or smaller than the predetermined value β, the processing in steps 502 and 503 is not performed. After the determination in step 501 is performed on α differences DL (x), the process returns to FIG.
[0023]
If the number of pixels determined to be corrected in subroutine B is less than α, j <α, so the process proceeds to steps 313 to 314 in FIG. 7, and this time in the range of (W′−N ′). The correction target pixel is determined. After returning the saved values of DW (x) and PW (x) to the difference DL (x) and the pixel number PL (x), respectively, the subroutine B is executed again and the above-described determination is performed. Before determining all the α differences DL (x), if the number of correction target pixels reaches α, j = α, and the process returns from step 505 to FIG. If all the α differences DL (x) are determined, x = α, and the process returns from step 504 to FIG. 7 even if the correction target pixel does not reach α. In this way, α or less correction values H (x) and pixel numbers P (x) can be obtained preferentially from the range of N ′, and in step 315, the correction data storage unit 6 together with the temperature T at the time of correction data measurement. To remember.
[0024]
In this way, by setting priority to the pixel range and focus detection area in the image sensor, and correcting in order from the pixel with higher priority and the dark current protruding from the surrounding pixels, the light flux at the end of the photoelectric converter Unnecessary dark current correction for pixels that do not reach is avoided, and appropriate correction processing is performed with a limited storage capacity.
[0025]
In the above-described embodiment, correction is not performed for pixels whose difference DL (x) is equal to or less than the predetermined value β even when the number of correction target pixels is less than α. It is not limited to. If less than α, the predetermined value β may be set smaller than the previous time, and the process may return to step 302 again, and the correction data measurement operation may be repeated until the number of correction target pixels reaches α.
When a plurality of focus detection areas are provided, the correction target pixel number α may be changed for each area depending on the frequency of use of each area.
[0026]
【The invention's effect】
(1) According to the first aspect of the invention, during dark current measurement, the difference between the dark current of a specific pixel of a photoelectric converter composed of a plurality of pixels and the average dark current of pixels other than the specific pixel is shielded. The value divided by the pixel output is stored in advance as a dark current correction value at the time of dark current measurement, and at the time of use, based on the output of the light-shielded pixel and the dark current correction value at the time of dark current measurement, Since the current correction value is obtained and the dark current correction value is subtracted from the output current of a specific pixel of the photoelectric converter to correct the dark current, the memory for storing the dark current correction data is stored. The storage capacity can be reduced and the correction processing time can be shortened. In addition, since the dark current of a specific pixel having a large dark current is reduced to the average dark current level of surrounding pixels, the dark current level of all pixels after dark current correction is averaged, and this dark current correction device is focused. If used in the photoelectric converter of the detection device, the accuracy and reliability in focus detection can be improved. Further, since the dark current correction is not performed by the dark pixel output for monitoring the dark current as in the prior art, the optimum correction can be performed at any temperature.
(2) According to the invention of claim 2, temperature information at the time of dark current measurement is stored, and the dark current correction value at the time of use is based on the temperature information at the time of dark current measurement and the temperature information at the time of use. In addition to the effect of claim 1, the dark current is corrected by subtracting the dark current correction value at the time of use after temperature correction from the output current of the specific pixel of the photoelectric converter. The fluctuation of the dark current due to the temperature change can be removed.
(3) According to the invention of claim 3, since the dark current correction is preferentially performed on the pixels included in the region where the photoelectric converter is frequently used, the light flux at the end of the photoelectric converter. Unnecessary dark current correction for pixels that do not reach is avoided, and appropriate correction processing is performed.
(4) According to the invention of claim 4, since a predetermined number is extracted in order from the largest dark current among the plurality of pixels of the photoelectric converter and dark current correction is performed, the dark current correction is performed. Correction accuracy can be improved.
[0027]
【The invention's effect】
(1) According to the invention of claim 1, the difference between the dark current of a specific pixel of a photoelectric converter composed of a plurality of pixels and the average dark current of pixels other than the specific pixel is stored, and photoelectric conversion is performed. Since the dark current is corrected by subtracting the difference from the output current of a specific pixel of the device, the storage capacity of the memory for storing the dark current correction data can be reduced, and the correction processing time can be shortened. In addition, since the dark current of a specific pixel having a large dark current is reduced to the average dark current level of surrounding pixels, the dark current level of all pixels after dark current correction is averaged, and this dark current correction device is focused. If used in the photoelectric converter of the detection device, the accuracy and reliability in focus detection can be improved. Further, since the dark current correction is not performed by the dark pixel output for monitoring the dark current as in the prior art, the optimum correction can be performed at any temperature.
(2) According to the invention of claim 2, the difference between the dark current of a specific pixel of the photoelectric converter composed of a plurality of pixels and the average dark current of pixels other than the specific pixel, and the temperature at the time of dark current measurement Information is stored, the temperature of the difference is corrected based on the temperature information at the time of dark current measurement and the temperature information at the time of dark current correction, and after temperature correction from the output current of the specific pixel of the photoelectric converter Since the dark current is corrected by subtracting the difference between the two, in addition to the effect of the first aspect, the fluctuation of the dark current due to the temperature change can be eliminated.
(3) According to the invention of claim 3, since dark current correction is preferentially performed on pixels included in a region where the photoelectric converter is frequently used, the light flux at the end of the photoelectric converter. Unnecessary dark current correction for pixels that do not reach is avoided, and appropriate correction processing is performed.
(4) According to the invention of claim 4, since a predetermined number is extracted in order from the largest dark current among the plurality of pixels of the photoelectric converter and the dark current correction is performed, the dark current correction is performed. Correction accuracy can be improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a first embodiment.
FIG. 2 is a flowchart showing correction data measurement and storage processing according to the first embodiment;
FIG. 3 is a flowchart showing dark current correction processing of a dark current correction device for a photoelectric converter applied to a camera.
FIG. 4 is a diagram showing dark current before and after correction.
FIG. 5 is a diagram showing a narrow focus detection area and a wide focus detection area.
FIG. 6 is a diagram showing changes in dark current due to temperature.
FIG. 7 is a flowchart showing correction data measurement and storage processing according to the second embodiment;
FIG. 8 is a flowchart illustrating correction data measurement and storage processing according to the second embodiment, following FIG. 7;
FIG. 9 is a flowchart showing correction data measurement and storage processing of the second embodiment, following FIG. 8;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Objective lens 2 Focus detection optical system 3 Image sensor 4 A / D conversion circuit 5 Dark current correction circuit 6 Correction data storage part 7 Calculation part 8 Drive control part 9 Motor

Claims (4)

During dark current measurement, a value obtained by dividing the difference between the dark current of a specific pixel of a photoelectric converter composed of a plurality of pixels and the average dark current of pixels other than the specific pixel by the output of the light-shielded pixel is measured. Storage means for storing in advance as a dark current correction value of
In use, the calculated dark current correction value at the time of use on the basis of the output of light-shielded pixel and the dark current correction value when the dark current measurement, when the use of the output current of the particular pixel of said photoelectric converter And a correction means for correcting the dark current by subtracting the dark current correction value . In the dark current correction device for a photoelectric converter according to claim 1,
The storage means stores temperature information at the time of dark current measurement,
The correction means performs temperature correction of the dark current correction value at the time of use based on temperature information at the time of dark current measurement stored in the storage means and temperature information at the time of use, and the photoelectric converter A dark current correction device for a photoelectric converter, wherein a dark current is corrected by subtracting a dark current correction value in use after temperature correction from an output current of the specific pixel. In the dark current correction device for a photoelectric converter according to claim 1 or 2,
The dark current correction device for a photoelectric converter, wherein the correction means preferentially corrects dark current for pixels included in a region where the photoelectric converter is frequently used. In the dark current correction device for a photoelectric converter according to any one of claims 1 to 3,
The specific current pixel is a dark current correction device for a photoelectric converter, wherein a predetermined number of pixels are extracted in descending order of the dark current among the plurality of pixels of the photoelectric converter.

Images