JP2006115191A - Imaging device, shading correction device, and shading correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a good image quality without luminance and color shading to every corner of a screen.
An imaging device having a light receiving portion that generates and accumulates photocharge according to the amount of received light, a temporary storage portion that temporarily stores the photocharge of the light receiving portion, and controls the start of photocharge accumulation of the imaging device. In the image pickup apparatus having the accumulation start control means and the accumulation end control means for controlling the accumulation end of the image pickup device by transferring the photoelectric charge of the light receiving part to the temporary accumulation part, the accumulation start control means and the accumulation end control means By changing one or both of the control timings for each pixel or line, the photocharge accumulation time is controlled for each pixel or line, thereby correcting the shading of the photographed image.
[Selection] Figure 4

Description

  The present invention relates to an image pickup apparatus, a shading correction apparatus, and a shading correction in which an image pickup apparatus and system using a solid-state image pickup device correct shading of a captured image by controlling a photocharge accumulation time for each pixel or line. Regarding the method.

  Conventionally, a mechanical shutter which is a mechanical light shielding member has been used as an exposure time control means of a camera using a film. In recent years, imaging devices using solid-state imaging devices such as digital cameras instead of film cameras are becoming mainstream, but many still use mechanical shutters as the exposure time control means of the light receiving unit.

  When a mechanical shutter is used, it is necessary to operate the shutter at a very high speed, particularly in exposure at a short time, and mechanical accuracy is required, and control becomes difficult. Although some single-lens reflex digital cameras use a focal plane type shutter with improved control accuracy, a complicated mechanism and accuracy are required, which increases costs.

  Unlike a film, a solid-state image sensor can inherently incorporate an exposure start control means and an exposure end control means inside an image sensor, and has the possibility of realizing an electronic shutter operation. Since this electronic shutter has advantages in terms of controllability and cost compared to a mechanical shutter, research and development are actively conducted.

  In the CCD type solid-state imaging device, an electronic shutter is realized by controlling the substrate voltage of the imaging device as an exposure start control means and performing transfer control from the photodiode to the VCCD as an exposure end control means. This method is characterized in that the exposure start and the exposure end control are performed collectively for all the pixels of the image sensor, so that the exposure timings of all the pixels are equal. On the other hand, in an image sensor such as a CMOS type that employs an XY address type scanning method, exposure is started by performing pixel reset control for removing unnecessary charges accumulated in the light receiving portion of the pixel for each pixel or for each line. It is possible to perform an electronic shutter operation for controlling the end of exposure by reading out signal charges after a predetermined time has elapsed for each pixel or line for each control. Such an electronic shutter is called a rolling electronic shutter.

  By using this rolling electronic shutter, it is possible to perform accumulation control with the timing freely changed for each pixel or for each line, which was impossible with the mechanical shutter or the electronic shutter for all pixel batch control.

  The structure and driving method of a conventional image sensor will be described with reference to FIGS. 1, 2 and 3, and the rolling electronic shutter operation will be described.

  FIG. 1 is a schematic diagram of a configuration of an image sensor that employs an XY address type scanning method. Reference numeral 101 denotes a unit pixel, and reference numeral 102 denotes a photodiode (hereinafter referred to as PD) that converts light into electric charges. Reference numeral 103 denotes a transfer switch that transfers the charge generated in the PD by the transfer pulse φTX to a floating diffusion region (hereinafter referred to as FD) 104 in which the charge is temporarily stored. Reference numeral 105 denotes an amplification MOS amplifier that functions as a source follower. Reference numeral 106 denotes a selection switch that selects a pixel by a selection pulse φSEL, and reference numeral 107 denotes a reset switch that removes charges accumulated in the FD by a reset pulse φRES. The FD 104, the amplification MOS amplifier 105, and a constant current source 109 described later constitute a floating diffusion amplifier, and the signal charge of the pixel selected by the selection switch 106 is converted into a voltage and output to the readout circuit 113 via the signal output line 108. The Reference numeral 109 denotes a constant current source serving as a load for the amplification MOS amplifier 105. Reference numeral 110 denotes a selection switch that selects an output signal from the readout circuit 113 and is driven by the horizontal scanning circuit 114. Reference numeral 112 denotes a vertical scanning circuit for selecting the switches 103, 106, and 107. The nth scanning line selected by the vertical scanning circuit 112 in each of φTX, φRES, and φSEL is defined as φTXn, φRESn, and φSELn.

  FIG. 2 is a diagram illustrating a circuit configuration of the digital camera. The overall control calculation unit 209 controls the timing generation unit 207 and the shutter driving device 204 to control the timing of the exposure / readout operation of the entire camera. The image sensor 205 is controlled by the overall control calculation unit 209 via the timing generation unit 207. Even in the rolling electronic shutter operation, when light is applied to the image sensor at a timing other than the exposure time, unnecessary photocharge is generated even in a portion that does not originally have a photoelectric conversion function such as PD or FD, resulting in image quality. Since it has an adverse effect, it is necessary to control the mechanical shutter 203 and shield it from light except during exposure.

  FIG. 3 shows a driving pulse and an operation sequence in the rolling electronic shutter operation. In FIG. 3, description is made with respect to n + 3 lines from n lines selected by the vertical scanning circuit 112.

  In the rolling electronic shutter operation, at time n31 to time t32, first, a pulse is applied to φRESn and φTXn to turn on transfer switch 103 and reset switch 107, and unnecessary information accumulated in PD102 and FD104 on the nth line is used. A reset operation for removing charges is performed. At time t32, the transfer switch 103 is turned off, and an accumulation operation for accumulating the photocharge generated in the PD is started. Next, at time t <b> 34, a pulse is applied to φTXn to turn on the transfer switch 103, and a transfer operation for transferring the photocharge accumulated in the PD 102 to the FD 104 is performed. Note that the reset switch 107 needs to be turned off prior to the transfer operation, and in this figure, the reset switch 107 is turned off simultaneously with the transfer switch 103 at time t32. Here, the accumulation time is from time t32 to time t35. After the transfer operation for the nth line is completed, a pulse is applied to φSELn and the selection switch 106 is turned on, whereby the charge held in the FD 104 is converted into a voltage and output to the reading circuit 113. The signals temporarily held by the reading circuit 113 are sequentially output from the horizontal scanning circuit 114 from time t36. In the n + 1 line, a reset operation, an accumulation operation, a transfer operation, and a read operation are performed at timings different from those of the n line. At this time, the reset operation, the accumulation operation, and the transfer operation are performed at an arbitrary timing different from the timing of the n line, although the time required for making the exposure time the same as that of the n line is the same. Can do. By performing the reset operation, the accumulation operation, and the transfer operation simultaneously on all the lines, the entire screen can be exposed at the same timing. However, since the read circuit 113 is used for the read operation, the n + 1 line read operation cannot be performed while the n line read operation is being performed. Usually, read operations are sequentially performed in the order of line numbers. For this reason, the reset operation, the accumulation operation, and the transfer operation are usually performed line-sequentially in accordance with the timing of the read operation.

  On the other hand, in a single-lens reflex type digital camera, a large-sized one close to a conventional 135 size film is used as an image sensor, and due to manufacturing variations compared to a small solid-state image sensor used in a consumer type digital camera, There was a problem that shading was likely to occur.

  Furthermore, since the image pickup device is large, incident light from the main lens does not enter the image pickup device perpendicularly at the periphery of the image pickup device, and there is a problem that the sensitivity is inferior to the image pickup device central portion due to the angle.

  Furthermore, there is a problem in that the intensity of light incident on the peripheral portion is lower than the intensity of light incident on the central portion of the image sensor due to the peripheral light amount drop characteristic of the main lens itself. In particular, interchangeable-lens digital single-lens reflex cameras can be equipped with lenses with various focal lengths, from super-wide-angle lenses to super-telephoto lenses. Depending on the situation.

  All of the above problems appear as shading of the photographed image and adversely affect the image quality. Since a solid-state image sensor generally has a lower dynamic range than a film, shading has a greater adverse effect on image quality than a film.

As a method of correcting such shading, there is a method of correcting by calculation after conversion to digital data (see, for example, Patent Document 1). However, calculation of digital data causes rounding errors or operations such as multiplication. As a result, noise increases and shading correction is performed, but the image quality is adversely affected.
JP-A-9-69980

  In order to solve the above-described problems, the present invention corrects the above-described shading by controlling the photocharge accumulation timing of the light receiving unit for each pixel or for each line using the electronic shutter operation of the solid-state imaging device. It is another object of the present invention to provide an imaging device, a shading correction device, and a shading correction method that can obtain a good image quality without luminance and color unevenness to every corner of the screen.

  In order to solve the above-described problem, the invention according to the application is directed to an image sensor including a light receiving unit that generates and accumulates photocharge according to the amount of received light, a temporary accumulation unit that temporarily stores the photocharge of the light receiving unit, In the imaging apparatus, comprising: an accumulation start control unit that controls an optical charge accumulation start of the image sensor; and an accumulation end control unit that performs an accumulation end control of the image sensor by transferring the photocharge of the light receiving unit to the temporary accumulation unit. By changing the control timing of one or both of the accumulation start control means and the accumulation end control means for each pixel or for each line, the photocharge accumulation time is controlled for each pixel or for each line, and the photographed image is obtained. Correct shading.

  According to the invention of the present application, an image sensor having a light receiving unit that generates and accumulates photocharge according to the amount of received light, a temporary storage unit that temporarily stores the photocharge of the light receiving unit, and the photocharge of the image sensor In an imaging apparatus having an accumulation start control means for controlling the accumulation start, and an accumulation end control means for controlling the accumulation end of the image sensor by transferring the photocharge of the light receiving section to the temporary accumulation section, the accumulation start control means By changing the control timing of either one or both of the storage end control means for each pixel or for each line, the photo charge accumulation time is controlled for each pixel or for each line, thereby shading the captured image. It is possible to obtain a good image without correcting the brightness and color balance to every corner of the screen.

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

  A first embodiment of the present invention will be described with reference to FIGS.

  In FIG. 2, when the camera starts a shooting operation by pressing the release button, the overall control calculation unit 209 performs the shading of the image sensor itself stored in advance, the lens information acquired from the lens control unit 202, the aperture From the information, pupil position information, etc., the shading amount of the captured image is calculated, and vertical shading correction coefficients for the total number of lines of the image are calculated so as to cancel the shading in the vertical direction. During the release time lag period from when the release button is pressed until the actual exposure operation starts, the overall control calculation unit 209 calculates the exposure time determined by exposure control such as AE to the timing generation unit 207. Set the vertical shading correction information for the total number of lines. The image sensor 205 is an image sensor that adopts an XY address type scanning method, and the timing generation unit 207 controls accumulation and reading operations.

  FIG. 4 is a shutter sequence diagram in the first embodiment. First, by controlling the front curtain of the mechanical shutter, the exposure surface of the image sensor is physically switched from the light emission state to the exposure state. Next, by performing a batch reset operation for all lines at the same timing, unnecessary charges of all PDs of the image sensor are reset, and an accumulation operation is started at the same timing for the entire screen. At this time, the timing generation unit 207 determines the accumulation time in each line, that is, the timing of the transfer operation in each line, according to the preset exposure time and the vertical shading correction coefficient for the total number of lines. Perform the transfer operation.

  When all the lines have been transferred, the rear curtain of the mechanical shutter is controlled to physically shield the image sensor. Since the mechanical shutter shields light from the smallest line number at the top of the screen and operates at a speed faster than the transfer operation of the image sensor, reading the image from the first line is started simultaneously with the mechanical shutter control, and all image data is Repeat until read.

  The output image of the image sensor obtained in this way is subjected to shading correction as shown in FIG. 5 when a uniform luminance surface is photographed. FIG. 5A shows the shading in the vertical direction of the image sensor obtained when the entire screen is exposed for a uniform time. The shading correction coefficient is calculated so as to cancel this shading, and the exposure time for each line is controlled as in b). Therefore, when a uniform luminance surface is photographed, the shading is corrected at the output stage of the sensor. Thus, a uniform sensor output as shown in c) can be obtained. In the method of controlling the exposure time, the resolution can be improved by increasing the frequency of the reference clock, and smoother correction is possible compared to the case of performing shading correction with digital data. In addition, when shading correction is performed on digital data, multiplication or the like is used, which may cause rounding errors due to computation and noise may also increase due to computation. However, there is no rounding error in the method of controlling the exposure time, and image processing Since the correction is completed before performing the process, the amount of noise does not change depending on the screen area. Therefore, the image quality after correction is superior to the method of performing shading correction by calculating digital data.

  Furthermore, in this embodiment, the shading correction only in the vertical direction is performed by controlling the transfer timing for each line. Similarly, the horizontal shading correction can also be performed by controlling the transfer timing in the horizontal direction. It is.

  A second embodiment of the present invention will be described with reference to FIGS.

  In the first embodiment, the vertical shading correction is performed by controlling the reset operation for each line of the image sensor, but here the vertical shading correction is performed by controlling the transfer operation for each line.

  As in the first embodiment, the overall control calculation unit 209 uses a pre-stored shading of the image sensor itself, lens information acquired from the lens control unit 202, aperture information, pupil position information, and the like to take a captured image. Are calculated, and vertical shading correction coefficients corresponding to the total number of lines of the image are calculated so as to cancel the shading in the vertical direction. During the release time lag period from when the release button is pressed until the actual exposure operation starts, the overall control calculation unit 209 calculates the exposure time determined by exposure control such as AE to the timing generation unit 207. Set the vertical shading correction information for the total number of lines.

  FIG. 6 is a shutter sequence diagram in the second embodiment. First, by controlling the front curtain of the mechanical shutter, the exposure surface of the image sensor is physically switched from the light emission state to the exposure state. Next, the timing generation unit 207 determines the reset operation timing in each line according to the preset exposure time and the vertical shading correction coefficients for the total number of lines, and performs the reset operation for each line separately. That is, the exposure time of each line is controlled by resetting unnecessary charges of all PDs of the image sensor for each line and entering the accumulation operation at different timings. Next, the collective charge transfer timing is determined according to the exposure time, and the charge is transferred from the PD to the FD at the same timing all over the screen.

  Next, the rear curtain of the mechanical shutter is controlled to physically shield the image sensor. Since the mechanical shutter shields light from the smallest line number at the top of the screen and operates at a speed faster than the transfer operation of the image sensor, reading the image from the first line is started simultaneously with the mechanical shutter control, and all image data is Repeat until read.

  With the operation as described above, the vertical shading is corrected by changing the accumulation time for each line according to the shading correction coefficient, and a good image without unevenness can be obtained.

  Furthermore, as a method of changing the accumulation time for each line, not only one of the reset timing and the charge transfer timing may be changed for each line, but the accumulation time may be changed by changing both, and the calculated shading correction coefficient If the accumulation time for each line is controlled according to the above, shading correction can be performed.

Schematic diagram of the configuration of an image sensor that employs an XY address type scanning method Digital camera circuit configuration Rolling electronic shutter timing chart Shutter drive timing chart of Embodiment 1 Diagram showing how shading correction works Shutter drive timing chart of Embodiment 2

Explanation of symbols

101 Unit Pixel 102 Photodiode 103 Transfer Switch 104 Storage Area 105 Amplification MOS Amplifier 106 Selection Switch 107 Reset Switch 108 Signal Output Line 109 Load Current Source 110 Horizontal Selection Switch 111 Output Amplifier 112 Vertical Scan Circuit 113 Read Circuit 114 Horizontal Scan Circuit 201 Lens Unit 202 Lens control unit 203 Shutter 204 Shutter driving device 205 Imaging element 206 Imaging signal processing circuit 207 Timing generation unit 208 Memory unit 209 Overall control calculation unit 210 Recording medium control I / F unit 211 Recording medium 212 External I / F unit 213 Photometry Device 214 Ranging device

Claims (4)

  An image sensor having a light receiving unit that generates and accumulates photocharge according to the amount of received light, and a temporary storage unit that temporarily stores the photocharge of the light receiving unit, and an accumulation start control that controls the start of photocharge accumulation of the image sensor And an accumulation end control means for controlling the accumulation end of the image pickup device by transferring the photoelectric charge of the light receiving section to the temporary accumulation section, either the accumulation start control means or the accumulation end control means An image pickup apparatus that controls the photocharge accumulation time for each pixel or for each line by changing one or both of the control timings for each pixel or for each line.   By changing the control timing of one or both of the accumulation start control means and the accumulation end control means for each pixel or for each line, by controlling the accumulation time for each pixel or for each line, shooting is performed. The imaging apparatus according to claim 1, wherein the image shading is corrected.   By changing the control timing of one or both of the accumulation start control means and the accumulation end control means for each pixel or for each line, by controlling the accumulation time for each pixel or for each line, shooting is performed. A shading correction device that corrects shading of images.   By changing the control timing of one or both of the accumulation start control means and the accumulation end control means for each pixel or for each line, by controlling the accumulation time for each pixel or for each line, shooting is performed. A shading correction method for correcting image shading.