JP2006121292A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus which is free from a deterioration in picture quality due to luminance unevenness and color unevenness of an image and due to a partial increase in defect pixels while maintaining a frame rate and continuous shooting performance even when an imaging device partially rises in temperature. <P>SOLUTION: Temperatures are measured for each block (per unit area) by finding the numbers of defect pixels for each of the blocks and the output of the imaging device is corrected for each of the blocks according to the temperatures for each of the blocks. The temperatures for each of the blocks are measured by finding output levels of the defect pixels for each of the blocks and the output of the imaging device is corrected for each of the blocks according to the temperatures for each of the blocks. Dark current levels corresponding to the temperatures of each of the blocks are found by measuring the temperatures for each of the blocks and the output of the imaging device is corrected for each of the blocks. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image pickup apparatus using an image pickup element such as a CCD or a CMOS image sensor, and more particularly to improvement of image quality of a picked up image.

  In an imaging apparatus such as a digital camera or a video camera, a CCD or a CMOS image sensor is generally used as an imaging element.

  It is well known that the characteristics of the image sensor include fixed pattern noise due to dark current variation of the photodiode for each pixel and defective pixels due to dark current, which deteriorate the image quality of the captured image. ing.

  The dark current is generally said to increase by a factor of 2 with a temperature increase of 8 ° C. The fixed pixel noise due to the dark current variation of the photodiode and the defective pixel due to the dark current have temperature characteristics. It is also well known that fixed pattern noise and defective pixels increase at higher temperatures.

  As described above, as a method for reducing fixed pattern noise due to dark current of a photodiode having temperature characteristics, Patent Document 1 previously stores fixed pattern noise for each predetermined temperature range and detects the temperature of the image sensor, A technique is disclosed in which fixed pattern noise stored in advance corresponding to a detected temperature is read out and subtracted from an actual captured image signal to correct the fixed pattern noise.

  As for a defective pixel caused by dark current, a technique for storing the address of the defective pixel and interpolating the defective pixel information based on the surrounding pixel information is known. It is also conceivable to correct the defective pixel by storing the address of the defective pixel.

  Also, by performing the imaging operation in a state where the entire imaging device is shielded immediately before or after the actual imaging operation, a captured image of only the dark current component is obtained and subtracted from the actual captured image signal. There is known a technique for reducing a fixed pattern noise caused by dark current including a temperature change and a reduction in image quality due to defective pixels caused by dark current (correction by actual dark subtraction).

In addition, Patent Document 2 discloses a method of detecting the ambient temperature of the image sensor based on the average value of the optical black region of the image sensor and suppressing fixed pattern noise.
Japanese Unexamined Patent Publication No. 1-147973 JP 2001-174329 A

  However, in Patent Document 2, it is very effective when the entire image sensor undergoes a uniform temperature change. However, for example, if a long-time accumulation operation of about 30 seconds is continued or continuous shooting is repeated, Circuits and peripheral circuits outside the image sensor generate heat, and the temperature of a part of the adjacent image sensor partially rises, resulting in a difference in dark current between the high temperature part and the low temperature part in the image sensor. Therefore, the fixed pattern noise cannot be reduced appropriately. In addition, the captured image at this time causes a deterioration in image quality such as luminance unevenness and color unevenness due to a dark current difference in the screen.

  Further, when there is a temperature difference in the image sensor as described above, if the entire screen is corrected with a defective pixel address for each predetermined temperature range as in Patent Document 1, an excessively corrected or insufficiently corrected portion is generated. Since defective pixel correction is often performed by interpolation from surrounding pixel information, it is well known that when the correction is excessive, the reproducibility of the pattern deteriorates when a fine pattern is imaged. If the correction is insufficient, the noise on the screen increases and the image quality deteriorates.

  In recent years, as the number of pixels of an image sensor has increased, the chip size of the image sensor itself has increased. Some imaging devices such as digital still cameras that are particularly required to have high image quality have a chip size that is equivalent to the screen size of a 35 mm film of a silver salt camera. And the problem of defective pixels becomes particularly noticeable.

  Also, with the correction method using actual dark subtraction, since the imaging operation is performed twice in one scene, the shooting time of one scene is doubled, and the frame rate of the video camera and the continuous shooting performance of the digital still camera are greatly reduced. become.

  In order to solve the above problems, the object of the present invention is to measure the pixel portion temperature of the image sensor, and maintain the frame rate and continuous shooting performance even when the image sensor partially rises in temperature. An object of the present invention is to provide an imaging device in which image quality does not deteriorate due to luminance unevenness, color unevenness, and a partial increase in defective pixels.

  To correct brightness unevenness, color unevenness, and defective pixels even when there is a temperature difference in the image sensor by continuing a long-time accumulation operation as described above or repeating continuous shooting. Means for dividing the pixel portion of the image sensor into a plurality of blocks and obtaining the number of defective pixels for each block; and a first table for converting the number of defective pixels into a temperature for each block of the image sensor. The output value of the image sensor is corrected for each block according to the temperature obtained by the first table means, that is, the number of defective pixels (per unit area) for each block is obtained. By measuring the temperature and correcting the output of the image sensor for each block according to the temperature of each block, it is possible to provide an image pickup apparatus that does not deteriorate image quality even when the temperature of the image sensor partially rises Unisuru.

  Means for dividing the pixel portion of the image sensor into a plurality of blocks, storing an address and output level of the defective pixel for each block, and a second for converting the output level of the defective pixel into a temperature for each block of the image sensor. And the output value of the image sensor is corrected for each block according to the temperature obtained by the second table means, that is, the output level of the defective pixel is obtained for each block. By measuring the temperature of the image sensor and correcting the output of the image sensor for each block according to the temperature of each block, it is possible to provide an image pickup apparatus that does not deteriorate the image quality even when the temperature of the image sensor partially rises. .

  Further, as correction of the output of the image pickup device, correction of defective pixels is performed for each block according to the temperature of each block, so that an image pickup apparatus that does not deteriorate image quality due to partial increase of defective pixels can be provided. Like that.

  Third table means for converting the converted temperature into a dark current level of the image sensor, and means for obtaining a dark current level for each block from the third table means, the third table means The output value of the image sensor is corrected for each block according to the obtained dark current level, that is, the dark current level corresponding to the temperature of each block is obtained by measuring the temperature for each block, and for each block. In addition, by correcting the output of the image sensor, it is possible to provide an image pickup apparatus in which the image quality does not deteriorate even when the temperature of the image sensor partially increases.

  In addition, by correcting luminance unevenness or color unevenness due to the dark current of the image sensor as correction of the output of the image sensor, an image pickup apparatus that does not deteriorate image quality due to uneven brightness or color unevenness of the image can be provided. To.

  The temperature of each block is measured by calculating the number of defective pixels (per unit area) for each block, and the output of the image sensor is corrected for each block according to the temperature of each block. It is possible to provide an imaging device that does not deteriorate image quality even when the temperature rises.

  Measure the temperature of each block by determining the output level of defective pixels for each block, and correct the output of the image sensor for each block according to the temperature of each block, so that when the temperature of the image sensor partially rises In addition, it is possible to provide an imaging device in which image quality does not deteriorate.

  Further, as correction of the output of the image pickup device, correction of defective pixels is performed for each block according to the temperature of each block, so that an image pickup apparatus that does not deteriorate image quality due to partial increase of defective pixels can be provided. Like that.

  By measuring the temperature for each block, the dark current level corresponding to the temperature of each block is obtained, and by correcting the output of the image sensor for each block, the image quality deteriorates even when the temperature of the image sensor partially rises It is possible to provide an imaging device that does not.

  In addition, by correcting luminance unevenness or color unevenness due to the dark current of the image sensor as correction of the output of the image sensor, an image pickup apparatus that does not deteriorate image quality due to uneven brightness or color unevenness of the image can be provided. To.

  Further, since the frame rate and the continuous shooting performance are not sacrificed while performing the above correction, it is possible to provide an imaging apparatus with less photographer stress.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to examples.

  FIG. 1 shows a pixel circuit of a CMOS area sensor in an embodiment of the present invention.

  In the figure, a photodiode 1, a transfer switch 2, a reset switch 3, a pixel amplifier 10, and a row selection switch 6 are provided in the pixel, and the gate of the transfer switch 2 is ΦTX (n, n + 1) from the vertical scanning circuit 14. ), The gate of the reset switch 3 is connected to ΦRES (n, n + 1) from the vertical scanning circuit 14, and the gate of the row selection switch 6 is connected to ΦSEL (n, n + 1) from the vertical scanning circuit 14. Yes.

  Photoelectric conversion is performed by the photodiode 1, and the transfer switch 2 is in an off state during the accumulation period of the light amount charge, and the charge photoelectrically converted by the photodiode 1 is transferred to the gate of the source follower constituting the pixel amplifier. Not. The gate 11 of the source follower constituting the pixel amplifier is initialized to an appropriate voltage by turning on the reset switch 3 before starting the accumulation. That is, this is a dark level. Next or simultaneously, when the row selection switch 6 is turned on, the source follower circuit composed of the load current source 7 and the pixel amplifier 10 is in an operating state, and the transfer switch 2 is turned on here to turn on the photodiode 1. The charge accumulated in the pixel amplifier is transferred to the gate of the source follower constituting the pixel amplifier. Here, 4 is a reset power source, and 5 is a power source for driving the source follower.

  Here, the output of the selected row is generated on the vertical output line 13. This output is stored in the signal storage unit 15 through the transfer gates 15a and 15b. The output temporarily stored in the signal storage unit 15 is sequentially read out to the output amplifier unit by the horizontal scanning circuit 16.

  FIG. 2 is an operation timing chart of the CMOS area sensor of FIG. At the timing of the all-pixel reset period T1, ΦTX (n) and ΦTX (n + 1) become active, and the charge of the photodiode 1 of all the pixels is transferred to the gate of the source follower via the transfer switch 2, The photodiode 1 is reset. This state is a state in which the cathode charge of the photodiode 1 is transferred to the gate of the source follower and is averaged. By increasing the capacitance component of the capacitor 9 at the gate of the source follower, the cathode of the photodiode 1 is reset. Similar to level.

  At this time, a mechanical shutter (not shown) that guides the amount of light of the target image is open, and at the end of time T1, accumulation starts for all pixels simultaneously. The mechanical shutter remains open during the period T3, and this period is the accumulation period of the photodiode 1.

  After the elapse of time T3, the mechanical shutter is closed at the timing T4, and the accumulation of photocharges in the photodiode 1 is completed. In this state, charges are accumulated in the photodiode 1. Next, reading starts for each line. That is, the Nth row is read after the N-1th row is read.

  The period ΦSEL (n) at time T5 becomes active, the row selection switch 6 is turned on, and the pixel amplifiers 10 of all the pixels connected to the nth row are configured.

  The source follower circuit is activated. Here, the ΦRES (n) of the source follower gate configured by the pixel amplifier 10 becomes active in the period T2, the reset switch 3 is turned on, and the gate 11 of the source follower is initialized. That is, this dark level signal is output to the vertical output line 13.

  Next, ΦTN (n) becomes active, the transfer gate 15 b is turned on, and is held in the signal storage unit 15. This operation is executed simultaneously in parallel for all pixels connected to N rows. When the transfer of the dark level signal storage unit 15 is completed, the signal charge stored in the photodiode 1 is made active by making ΦTX (n) active, so that the transfer switch 2 is turned on. Transfer to the configured source follower gate 11. At this time, the potential of the gate 11 of the source follower constituted by the pixel amplifier 10 varies from the reset level by an amount corresponding to the transferred signal charge, and the signal level is determined.

  Here, ΦTS becomes active, the transfer gate 15 a is turned on, and the signal level is held in the signal storage unit 15. This operation is executed simultaneously in parallel for all pixels connected to N rows. Here, the signal storage unit 15 holds the dark level and the signal level of all the pixels connected to the N rows. By taking the difference between the dark level and the signal level between the pixels, the source follower The fixed pattern noise (FPN) due to the threshold voltage Vth variation and the KTC noise generated when the reset switch 3 is reset can be canceled to obtain a signal from which a noise component having a high S / N is removed. Here, the output from the pixel in which the photodiode 1 is optically shielded is only a dark current component caused by a surface defect or the like of the photodiode 1.

  The horizontal scanning circuit 16 horizontally scans the difference signal between the dark level and the signal level stored in the signal storage unit 15 by the horizontal scanning circuit 16, and outputs the signal in time series at the timing of T7. This completes the output of N rows. Similarly, by driving ΦSEL (n + 1), ΦRES (n + 1), ΦTX (n + 1), ΦTN, and ΦTS in the same manner as the Nth row as shown in FIG.

  FIG. 3 schematically shows the chip 20 of the CMOS area sensor of FIG. Here, 20a indicates an effective pixel portion of the area sensor, and 20b is an OB pixel portion where the surface of the pixel portion is shielded.

  The figure also shows an image output from the CMOS type area sensor of FIG. 1, where the uniform luminance surface is slightly under-exposed under the conditions of an ambient environment temperature of 20 ° C. and an accumulation time of 0.1 second. It is an image. During sensor accumulation, the output amplifier unit (not shown) that is an amplifier for amplifying and outputting the signal of the signal accumulation unit 15 shown in FIG. 2 is continuously driven, but the accumulation time is as short as 0.1 second. The output amplifier unit (not shown) that is an amplifier for amplifying and outputting the signal of the signal storage unit 15 shown in FIG. 2 generates little heat. At this time, the temperature in the screen is almost 20 ° C. and there is no luminance unevenness or color unevenness. In the effective portion 20a, defective pixels caused by dark current are shown as white spots.

  FIG. 4 is an output image when the ambient temperature is the same as FIG. 3 at 20 ° C., but the accumulation time is 30 seconds, and for amplifying and outputting the signal of the signal accumulation unit 15 shown in FIG. The state where the output amplifier unit (not shown), which is an amplifier for amplifying and outputting the signal of the signal storage unit 15 shown in FIG. Is shown.

  2 is an amplifier for amplifying and outputting a signal of the signal storage unit 15 shown in FIG. The luminance level is apparently high, resulting in luminance unevenness and color unevenness, and a large number of defective pixels due to dark current.

  On the other hand, the lower right side of the screen is closer to the ambient temperature of 20 ° C., the dark current is lower and the luminance level is lower and the number of defective pixels is lower than the upper left of the screen. That is, when the entire screen is divided into 6 blocks from a to f, there is a large difference between the luminance level of the a portion, the number of defective pixels, the luminance level of the f portion, and the number of defective pixels.

  FIG. 5 is an image under conditions when the ambient environment temperature changes with an accumulation time of 0.1 seconds in a state where the entire CMOS area sensor of FIG. 1 is shielded from light. Similar to the description of FIG. 3, the image at this time is not affected by heat generated by an output amplifier (not shown) that is an amplifier for amplifying and outputting the signal of the signal storage unit 15 shown in FIG. = Area sensor pixel part temperature, there is no output level unevenness in the screen, and defective pixels caused by dark current according to the ambient environment temperature are shown as white spots. Since the pixel is a defective pixel due to dark current, the level and number of white spots increase as the temperature increases.

  FIG. 6 illustrates the description of FIG. 5 more specifically. In FIG. 6, the number of white spots in each block corresponding to the area sensor pixel unit temperature and the level of the representative white spot in the block are shown in a table. It is a thing.

  Here, a pixel that outputs a value exceeding a predetermined level with respect to the average output level of the entire screen is defined as a white spot. For example, when the average output level of the entire screen is 100 mV and the predetermined level is 10 mV, 110 mV or more Pixels that output the value of are used as white spots. The representative white spot level is a level of a white spot showing the maximum value among the defined white spots, and is defined as a value obtained by subtracting the average output level of the entire screen. At this time, the address of the representative white spot of the same block is assumed to be fixed, and the address is also described in the table.

  As shown in the table of FIG. 6, it is generally known that the number of white spots increases as the temperature rises, and the representative white spot level changes with a temperature coefficient of about 2/8 ° C.

  If there is a table as shown in FIG. 6, by measuring the number of white spots or the representative white spot level of each block, the temperature of the area sensor can be measured for each block as follows.

Example of measuring temperature according to the number of white spots Number of white spots in part a = 80 ⇒ Temperature in part a = 36 ° C.
Example of measuring temperature by representative white spot level Representative white spot level of part b = 60 mV = temperature of part b = 28 ° C.

  Here, since there are individual variations in area sensors, the table in FIG. 6 must be created for each area sensor. Images are acquired under various temperature conditions in the area sensor manufacturing process and the camera manufacturing process. Create it.

  However, as for the level of the representative white spot, there is no actual harm even if the representative white spot level is measured under a specific reference temperature and a table with a temperature coefficient of 2/8 ° C. is created.

  Next, the table of FIG. 7 will be described. This table shows the dark current output of the normal pixel when the accumulation time is 0.1 second when the ambient temperature of the area sensor changes. Since the dark current output of normal pixels also varies from area sensor to area sensor, it is necessary to obtain this table for each area sensor as well. However, the dark current output is measured at a specific temperature, and it is 2/8 ° C. Creating a table with a temperature coefficient is not harmful.

Specifically explaining the table of FIG.
The dark current output at 20 ° C. is 2 mV / s and the shooting condition is 30 s 2 mV / s × 30 s = 60 mV
Thus, the area sensor output is an output having an offset of 60 mV.

  That is, the dark current correction of normal pixels can be performed by subtracting this offset value from all pixels in each block from the signal of the area sensor that actually captured the subject.

  FIG. 8 is a flowchart for explaining correction of luminance unevenness and color unevenness of the image as shown in FIG. 4 as an operation of the camera.

  In step 101, a shooting operation is started by a switch ON operation (not shown). In step 102, a shutter (not shown) that shields the surface of the area sensor 20 is opened to photograph the subject.

  Thereafter, in step 103, the area sensor 20 is accumulated for 30 seconds, and then the shutter is closed in step 104. The signal accumulated in step 105 is read and stored in a temporary memory (not shown). The image stored here becomes an image with many luminance unevenness and color unevenness as shown in FIG. 4 due to the heat generation of the circuit due to the long exposure of 30 seconds.

  Subsequently, in step 106, the accumulation of the area sensor 20 is performed for 0.1 second with the shutter closed for the purpose of obtaining an image for correcting the image output obtained in steps 102 to 105. The signal accumulated in step 107 is read out and stored at a different address in the temporary memory.

  In step 108, the number of white spots or the representative white spot level of the 0.1 second accumulated image acquired in steps 106 to 107 is obtained and compared with the table of FIG. Find the sensor temperature.

  As described above, the number of white spots counts the pixels that output the average output level of the entire screen +10 mV or more, and the representative white spot level is calculated from the pixel output of the representative white spot address described in the table of FIG. The total average output level is subtracted.

  In step 109, the temperature for each block obtained in step 108 is compared with the table of FIG. 7, and the dark current correction value of the normal pixel for each block is obtained from the storage time of 30 seconds executed in step 103.

  In step 110, the luminance unevenness and color unevenness of the image are corrected by subtracting the dark current correction value obtained in step 109 from all the pixels in each block from the images having the accumulation time of 30 seconds acquired in steps 102 to 105. To do.

  In step 111, the image with the accumulation time of 30 seconds corrected in step 110 is stored in the external memory, and in step 112, the series of operations is terminated.

  What is important here is that the accumulation time of the area sensor in the state where the shutter is closed in step 106 is 0.1 second, and can be ignored with respect to the accumulation time 30 seconds for photographing the subject in step 103. Short accumulation time. Therefore, since steps 101 to 112 require only approximately the same time as the storage time of 30 seconds for shooting an object, continuous shooting performance can be maintained.

  In steps 109 to 110, the dark current of normal pixels in each block is corrected. However, correction of defective pixels output as white spots may be performed simultaneously.

  Specifically, the address of the white spot number is stored in the table of FIG. Since the temperature of each block is obtained in step 108, the number of white spots corresponding to the temperature of each block and its address are known from the table of FIG. The defective pixel can be corrected by using the pixel at the address as a defective pixel and performing known interpolation based on the peripheral pixel output value.

  Although the CMOS type area sensor has been described in the above example, the present invention may be applied to area sensors having other structures such as a CCD type. Further, the present invention is not limited to an area sensor, and can be similarly applied to a line sensor, particularly a sensor having a plurality of line sensors.

Pixel part circuit diagram of CMOS area sensor in an embodiment of the present invention Operation timing diagram of the CMOS area sensor of FIG. Chip schematic diagram of the CMOS area sensor of FIG. Chip schematic diagram of the CMOS area sensor of FIG. 1 is an image diagram under conditions in which the ambient temperature of the CMOS area sensor in FIG. 1 has changed. First and second tables for the CMOS area sensor of FIG. 3rd table regarding CMOS type area sensor of FIG. The flowchart figure explaining operation | movement of the imaging device of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photodiode 2 Transfer switch 3 Reset switch 4 Reset power supply 5 Power supply which drives a source follower 6 Row selection switch 7 Load current source 9 Capacitor 10 of a source follower 10 Pixel amplifier 11 Gate 13 of a source follower 13 Vertical output line 14 Vertical scanning circuit DESCRIPTION OF SYMBOLS 15 Signal storage part 15a, 15b Transfer gate 16 Horizontal scanning circuit 20, 30 CMOS type | mold area sensor chip | tip 20a, 30a Effective part 20b of CMOS type | mold area sensor, 30b Shading pixel (OB) part of CMOS type | mold area sensor

Claims (5)

  Means for dividing the pixel portion of the image sensor into a plurality of blocks and obtaining the number of defective pixels for each block; and a first table for converting the number of defective pixels into a temperature for each block of the image sensor. An image pickup apparatus, wherein the output value of the image pickup device is corrected for each block in accordance with the temperature obtained by the first table means.   Means for dividing the pixel portion of the image sensor into a plurality of blocks, storing an address and output level of the defective pixel for each block, and a second for converting the output level of the defective pixel into a temperature for each block of the image sensor. An image pickup apparatus comprising the table means, wherein the output value of the image pickup element is corrected for each block in accordance with the temperature obtained by the second table means.   The imaging apparatus according to claim 1, wherein the correction is correction of a defective pixel caused by a dark current of the imaging element.   Third table means for converting the converted temperature into a dark current level of the image sensor, and means for obtaining a dark current level for each block from the third table means, the third table means The imaging apparatus according to claim 1, wherein an output value of the imaging element is corrected for each of the blocks according to a dark current level obtained more. The imaging apparatus according to claim 4, wherein the correction is correction of luminance unevenness or color unevenness caused by a dark current of the image sensor.