JP2008011007A - Imaging apparatus and exposure time control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus and an exposure time control method capable of preventing the luminance level differences from being produced in an acquired image, even if exposure time is changed, when a rolling shutter is used. <P>SOLUTION: The imaging apparatus is provided with an imaging device which has a plurality of pixels 601 to be specified by row addresses and column addresses, and includes an imaging region 602 and an invalid pixel region 101; an initialization means for scanning the row addresses and initializing pixels belonging to the row addresses; a read means for scanning the row addresses and reading pixels belonging to the row addresses; and an exposure time control means for performing control so as to perform switching of the exposure time, starting from initialization of a pixel, by the initialization means, up to the reading of the pixel by the reading means, during a period in which the initialization means scans the invalid pixel region. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exposure time control method for an image sensor, and more particularly to electronic shutter exposure time control for an XY address type image sensor.

  The CMOS image sensor, which is one of the XY address type image sensors, consumes less power than a CCD image sensor, can be driven by a low-voltage single power supply, and can integrate peripheral circuits on-chip. In recent years, its range of use has been expanded.

  FIG. 6 is a schematic diagram of an XY address type image sensor. A plurality of pixels 601 are arranged in a matrix to form an imaging region 602. A row address selection unit 603 selects a row to be initialized or read. Here, initialization means resetting the potential of the pixel 601, and reading means reading out the potential of the pixel 601.

  The same exposure time is given to all the pixels 601 constituting the imaging region 602. Accordingly, after initialization, reading is sequentially performed from a row after a certain time has elapsed. The read potential is sequentially output from the column address selection unit 604 to the pixel data output terminal 606 via the output buffer 605.

  When the illuminance of the environmental light source changes, such as when moving from outdoors to indoors, the exposure time, that is, the time from initialization to read-out is changed to obtain the brightness of the image Is controlled so as not to change drastically. Since such control is performed electronically, it is referred to as an electronic shutter. In particular, a system that always repeats initialization and reading during imaging is referred to as a rolling shutter.

  When the image sensor is built into an image pickup device such as a digital still camera, all pixels of the image sensor are used to generate an image due to the size of the image circle of the image pickup optical system, distortion, and peripheral light loss. Do not mean. Of the imaging area 602, an area used for image generation is called an effective pixel area 607, and an area not used for image generation is called an invalid pixel area. The invalid pixel area may include an OB (Optical Black) area where the pixel is shielded to define the black level.

  FIG. 7 is a circuit diagram illustrating a configuration of the pixel 601 arranged in the m-th row and the n-th column of the imaging region 602. The potential of the photodiode 704 changes according to the amount of received light. The MOS transistor 702 amplifies current at the time of reading. The MOS transistor 701 becomes conductive when the initialization signal line Im connected to the row address selection unit 603 becomes a high potential, and resets the potential of the photodiode 704. The MOS transistor 703 becomes conductive when the read signal line Rm connected to the row address selection unit 603 becomes a high potential, and the potential of the photodiode 704 is read to the column address selection unit 604 via the output signal line Cn. . Vdd is a power supply line of the image sensor, and Vss is a ground line of the image sensor.

  FIG. 8 is a timing chart of initialization and reading. The horizontal axis is time, and the vertical axis is row number. The dotted line indicates the progress of initialization 801, and the solid line indicates the progress of reading 802. Initialization 801 starts at time t0 and ends at time t2. Reading 802 starts at time t1 and ends at time t3. Accordingly, initialization 801 and reading 802 are performed in parallel between time t1 and time t2. Further, the period between time t3 and time t4 is a vertical blanking interval, and neither initialization 801 nor reading 802 is performed.

  As can be seen from FIG. 8, during the driving of the XY address type image sensor, the initialization 801 and the reading 802 are performed in parallel during the period in which only the initialization 801 is performed (between time t0 and time t1). Period (between time t1 and time t2), period during which only reading 802 is performed (between time t2 and time t3), period during which neither initialization 801 nor reading 802 is performed (time t3 and time t4) Between).

  Then, the period in which the initialization 801 and the reading 802 are performed in parallel is compared with the period in which only the initialization 801 and the reading 802 are performed or the period in which neither the initialization 801 nor the reading 802 is performed. As a result, the load on the power supply increases. For this reason, it is known that a band-shaped so-called electronic shutter noise is generated in the obtained image only during a period in which the potential of the power source is lowered and the initialization 801 and the reading 802 are performed in parallel.

In order to solve this problem, a dummy area is provided in the image sensor separately from the imaging area. For example, during the period in which only the initialization is performed, the dummy area is read in parallel, An imaging apparatus with a uniform load has been proposed (see Patent Document 1).
JP 2001-8109 A

  However, in the imaging apparatus disclosed in Patent Document 1, when the imaging apparatus detects a change in the illuminance of the environmental light source and changes the exposure time, there may be a luminance step in the obtained image.

  The present invention has been made in view of the above problems, and provides an imaging apparatus and an exposure time control method that do not cause a luminance step in an obtained image even when the exposure time is changed during a rolling shutter. Objective.

  In order to achieve the above object, an imaging apparatus of the present invention scans the row address, an imaging device having a plurality of pixels specified by a row address and a column address, and including an effective pixel region and an invalid pixel region. Initialization means for initializing pixels belonging to the row address, reading means for scanning the row address to read out pixels belonging to the row address, and reading out after the initialization means initializes the pixels. Exposure time control means for controlling the initialization time until the initialization means scans the invalid pixel area until the means reads out the pixels.

  According to the present invention, since the exposure unit is changed during the period in which the initialization unit scans the invalid pixel region, the imaging apparatus and the exposure time in which a luminance difference due to the change of the exposure time does not occur in the obtained image. A control method can be provided.

(1. Configuration)
(1.1 Digital still camera configuration)
An embodiment of the present invention will be described using a digital still camera as an example. FIG. 2 is a block diagram of a digital still camera which is an embodiment of the present invention. The light collected by the lens 201 is incident on the CMOS image sensor 202 driven by the CMOS image sensor driving circuit 203. The CMOS image sensor 202 generates an electrical signal corresponding to the amount of incident light. The electrical signal generated by the CMOS image sensor 202 is converted into a digital signal by the A / D conversion circuit 204. The digital signal converted by the A / D conversion circuit 204 is stored in the SDRAM 205.

  When the user is displaying an image of a subject on the LCD 208 in preparation for shooting, the following processing is performed. That is, the image signal processing unit 206 generates display image data to be displayed on the LCD 208 from the digital signal stored in the SDRAM 205 and outputs the display image data to the LCD 208. The generation of electrical signals by the CMOS image sensor 202 and the display of display image data by the LCD 208 are continuously performed at a cycle of 30 fps (30 frames per second).

  The exposure meter 211 measures the illuminance of the subject. The illuminance of the subject measured by the exposure meter 211 is input to the CPU 207. The CPU 207 instructs the exposure time control means 203a of the CMOS image sensor drive circuit 203 to change the exposure time according to the illuminance of the subject measured by the exposure meter 211. The exposure time control unit 203a changes the time from when the initialization unit 203b performs initialization until the reading unit 203c performs reading.

When the user presses the shutter button 209, the CPU 207 detects it and instructs the CMOS image sensor driving circuit 203 to perform a photographing operation. The image signal processing unit 206 generates recording image data for recording from the digital signal stored in the SDRAM 205 by the photographing operation, and records it in the memory card 210.
(1.2 Configuration of CMOS image sensor)
FIG. 1 is a schematic diagram of a CMOS image sensor 202. The same components as those in FIG. 6 are denoted by the same reference numerals as those in FIG. In this CMOS image sensor 202, it is assumed that the invalid pixel area 101 exists below the imaging area 602 in order to avoid complicated explanation. An initialization signal line and a read signal line are connected from the row address selection unit 603. An output signal line is connected to the column address selection unit 604.

Note that the imaging region 602 is composed of 6 rows and 5 columns, that is, 30 pixels, but this is for avoiding the complexity of the figure. In practice, pixels of about several hundred thousand to ten million are used. An image pickup device having the above has been put into practical use. The invalid pixel area 101 may include an OB area.
(2. Operation)
(2.1 Generation of luminance step)
FIG. 3A is a timing chart of initialization and reading when a luminance step occurs. The horizontal axis is time, and the vertical axis is row number. The dotted line indicates the progress of initialization, and the solid line indicates the progress of reading. FIG. 3B is a waveform diagram of the vertical synchronization signal VD. B1 to B4 are vertical blanking periods. Four frame periods from F1 to F4 are shown. FIG. 3C is a screen image when a gray card with a reflectance of 18% is photographed and displayed on the LCD 208 under a uniform environmental light source.

  Assume that an instruction to change the exposure time from E1 to E3 is given from the CPU 207 to the exposure time control means 203a at time t5. However, the illuminance of the environmental light source is not changed.

  The exposure time control unit 203a instructs the initialization unit 203b to stop initialization. In order to display the entire region of the frame F3, the reading unit 203c continues reading. As a result, assuming that the effective pixel region 607 of the CMOS image sensor 202 is composed of m2 rows, the x rows from the first row to the m1th row are exposed at the exposure time E1, and the (m1 + 1) th row to the m2th row are exposed. The y rows up to the row are exposed with the exposure time E2.

  From the frame F4, initialization by the initialization unit 203b is started earlier than the frames F1 to F3 with respect to the rising edge of the vertical blanking period B3 of the vertical synchronization signal VD, and all the rows of the CMOS image sensor 202 are The exposure is performed with an exposure time E3.

  Here, paying attention to the screen image of the LCD 208 shown in FIG. 3C, in the frames F1 and F2, a slightly dark image is displayed on the entire screen corresponding to the exposure time E1. In the frame F2, a slightly dark image is displayed in the upper x row of the screen as in the frames F1 and F2, but a very bright image is displayed in the lower y row of the screen corresponding to the exposure time E2. . In the frame F4, a slightly bright image is displayed on the entire screen corresponding to the exposure time E3. Note that a relationship of E1 << E3 << E2 exists between the exposure times E1 to E3.

Therefore, in the frame F3, a luminance step is confirmed at the boundary between the upper x row of the slightly dark screen and the lower y row of the very bright screen. When the screen is updated at 30 fps, this brightness step is displayed only for 1/30 second, but the brightness of a part of the screen is very high, so that it is noticeable and the quality of the display image is greatly impaired.
(2.2 Resolution of brightness difference)
FIG. 4A is a timing chart of initialization and reading when the luminance step is eliminated by the imaging apparatus or the exposure time control method of the present invention. The horizontal axis is time, and the vertical axis is row number. The dotted line indicates the progress of initialization, and the solid line indicates the progress of reading. FIG. 4B is a waveform diagram of the vertical synchronization signal VD. B1 to B4 are vertical blanking periods. Four frame periods from F1 to F4 are shown. FIG. 4C is a screen image when a gray card with a reflectance of 18% is photographed and displayed on the LCD 208 under a uniform environmental light source.

  Assume that an instruction to change the exposure time from E1 to E3 is given from the CPU 207 to the exposure time control means 203a at time t5. However, the illuminance of the environmental light source is not changed.

  FIG. 5 is a flowchart showing processing of the exposure time control unit 203a when an instruction to change the exposure time is given from the CPU 207 to the exposure time control unit 203a. In the exposure time control unit 203a, the initialization unit 203b acquires the line number m1 currently being initialized (S501). Referring to FIG. 4A, since m1 is smaller than m2 which is the maximum row of the effective pixel area 607 (S502), the process returns to S501 again to acquire the currently initialized row number.

  When initialization by the initialization unit 203b proceeds and the row number being initialized becomes larger than m2, which is the maximum row of the effective pixel region 607, at time t6 in FIG. 4A (S502), the exposure time control unit 203a. Instructs the initialization unit 203b to cancel initialization (S503). The reading unit 203c continues reading after time t6, completes reading of the effective pixel area 607 at time t7, and completes reading of the invalid pixel area 101 corresponding to the z row at time t8. Here, the CMOS image sensor 202 has m2 rows of effective pixel regions 607 and z rows of invalid pixel regions 101.

  Next, the exposure time control unit 203a calculates an initialization start time corresponding to the changed exposure time E3 (S504). When the calculated initialization start time is reached, the initialization unit 203b is instructed to start initialization (S505).

  As described above, in the frame F3, the initialization by the initialization unit 203b is performed for all the rows constituting the effective pixel region 607. Therefore, the exposure time for all the rows is E1, which is shown in FIG. Such a luminance step does not appear. After the frame F4, a slightly bright image corresponding to the exposure time E3 is displayed.

  In step S503, instead of instructing the initialization unit 203b to cancel the initialization, when the initialization reaches the m3rd row, which is the maximum row of the invalid pixel region 101, the first row of the invalid pixel region 101 is again displayed. The initialization may be continued from the (m2 + 1) th row. As a result, initialization and reading are performed in parallel, so that the load fluctuation of the power source is reduced.

  In addition to the imaging area 602, a dummy area may be provided. When the number of invalid pixel areas 101 is small, a dummy area provided with a free size can be used. The dummy area may be provided with the pixel 601 similarly to the imaging area 602, or may be configured with a dummy load having a load similar to that of the pixel 601.

  According to the present invention, it is possible to provide an imaging apparatus and an exposure time control method that do not cause a luminance difference in the obtained image even if the exposure time is changed during the rolling shutter. It is suitable as an imaging apparatus such as a camera or a camera-equipped mobile phone and its exposure time control method.

Schematic diagram of CMOS image sensor Digital still camera block diagram Explanatory diagram of initialization and readout when a luminance step occurs Explanatory diagram of initialization and reading when the brightness difference is resolved Flow chart showing processing of exposure time control means Schematic of XY address type image sensor Circuit diagram showing pixel configuration Initialization and readout timing chart

Explanation of symbols

101 Invalid pixel region 601 Pixel 602 Imaging region 603 Row address selection unit 604 Column address selection unit 605 Output buffer 606 Pixel data output terminal 607 Effective pixel region

Claims (10)

An image sensor having a plurality of pixels specified by a row address and a column address, and including an effective pixel area and an invalid pixel area;
Initialization means for scanning the row address to initialize pixels belonging to the row address;
Reading means for scanning the row address and reading out pixels belonging to the row address;
Exposure for controlling the switching of the exposure time from when the initialization unit initializes the pixel to when the reading unit reads out the pixel during the period during which the initialization unit scans the invalid pixel area. A time control means;
An imaging apparatus having The invalid pixel area includes an OB (Optical Black) area.
The imaging apparatus according to claim 1. The initialization means initializes the pixels belonging to the invalid pixel region a plurality of times during a readout period of one frame;
The imaging apparatus according to claim 1 or 2, wherein An image sensor having a plurality of pixels specified by a row address and a column address, and including an imaging region and a dummy region;
Initialization means for scanning the row address to initialize pixels belonging to the row address;
Reading means for scanning the row address and reading out pixels belonging to the row address;
An exposure time for controlling the switching of the exposure time from when the initialization unit initializes the pixel to when the readout unit reads out the pixel during the period during which the initialization unit scans the dummy area. Control means;
An imaging apparatus having The initialization means initializes the pixels belonging to the dummy region a plurality of times during a readout period of one frame;
The imaging apparatus according to claim 4. An image sensor having a plurality of pixels specified by a row address and a column address and including an effective pixel region and an invalid pixel region,
An initialization step of scanning the row address to initialize pixels belonging to the row address;
A reading step of scanning the row address and reading out pixels belonging to the row address;
Exposure for controlling the switching of the exposure time from the initialization process to pixel initialization until the readout process to read out pixels during the period during which the initialization process scans the invalid pixel area. Time control method. The invalid pixel area includes an OB (Optical Black) area.
The exposure time control method according to claim 6. The initialization step initializes the pixels belonging to the invalid pixel region a plurality of times during a reading period of one frame.
8. The exposure time control method according to claim 6, wherein the exposure time is controlled. An image sensor having a plurality of pixels specified by a row address and a column address, and including an imaging area and a dummy area,
An initialization step of scanning the row address to initialize pixels belonging to the row address;
A reading step of scanning the row address and reading out pixels belonging to the row address;
Exposure time for controlling the switching of the exposure time from the initialization process to pixel initialization until the readout process reads out the pixels during the period during which the initialization process scans the dummy area. Control method. The initialization step initializes the pixels belonging to the dummy region a plurality of times during a reading period of one frame.
The exposure time control method according to claim 9.

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