JP2020057882A - Imaging apparatus - Google Patents

Abstract

To solve the problem in which, while turning on a transfer switch that transfers charges from a charge holding unit to an amplifier, when an imaging method is performed in which the transfer switch that transfers charges from a photoelectric conversion unit to the charge holding unit is turned on a plurality of times, when a flash such as strobe light is generated, color balance of an image may be lost.SOLUTION: A time at which a first transfer switch is turned on is changed according to whether strobe light is emitted.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a digital single-lens reflex camera, a digital still camera, a digital video camera, and the like, and more particularly to an operation of an imaging device that captures moving images.

  2. Description of the Related Art A CMOS image sensor (hereinafter, referred to as a CMOS sensor) is used as an imaging element of an imaging device represented by a video camera. Most CMOS sensors employ a rolling shutter system in which horizontal lines are sequentially exposed and a video signal is sequentially read out for each line to generate one frame. The use of a global shutter system for reading images simultaneously in rows has been increasing.

  In general, in the rolling shutter method, rolling shutter distortion occurs in which the exposure time is shifted in each row. However, in the global shutter method, exposure is performed simultaneously, so that rolling shutter distortion does not occur and when a moving object is photographed. It is said to be valid.

  In order to realize the global shutter method, one of the features is to have a holding unit for holding electric charges in addition to the photoelectric conversion unit. However, as compared with the rolling shutter method, there is a problem in that the area of the photoelectric conversion unit is reduced by having the holding unit, and the amount of saturated charges that can be handled by one pixel is reduced.

  Patent Literature 1 discloses a photoelectric conversion unit, a holding unit that holds charges, a first transfer switch that transfers charges from the photoelectric conversion unit to the charge holding unit, and a first transfer switch that transfers charges from the charge holding unit to the amplification unit. The second transfer switch is turned on, and the first transfer switch is turned on a plurality of times while the second transfer switch is turned on, whereby the charge is transferred from the photoelectric conversion unit to the charge holding unit a plurality of times, thereby increasing the saturation charge amount. The method is described.

JP 2015-177349 A

  However, when an imaging method is performed in which the transfer switch for transferring the charge from the photoelectric conversion unit to the charge holding unit is turned on a plurality of times while the transfer switch for transferring the charge from the charge holding unit to the amplifying unit is turned on, a strobe light is used. When flash light such as light is generated, there is a problem that the color balance of an image is lost.

In order to solve the above-described problems, an imaging device according to the present invention includes:
A photoelectric conversion unit that accumulates charges generated by incident light, a holding unit that holds the charges, an amplification unit that outputs a signal based on the charges, and transfers the charges from the photoelectric conversion units to the holding units A plurality of pixels each having a first transfer switch and a second transfer switch for transferring the charge from the holding unit to the amplifying unit, and signals from the amplifying unit of the plurality of pixels are output. An output line, an amplifying means for amplifying a signal output from the output line, and a timing control unit for controlling a timing at which the strobe emits light. The time is changed.

  ADVANTAGE OF THE INVENTION According to the imaging device which concerns on this invention, when transfer from a photoelectric conversion part to a holding | maintenance part is performed several times, the image quality degradation which arises at the time of flash generation can be suppressed.

FIG. 2 is a diagram illustrating a typical configuration of an imaging device. FIG. 3 is a diagram illustrating a detailed example inside an image sensor. FIG. 3 is a diagram illustrating a detailed example of a pixel unit of the imaging element. FIG. 5 is a diagram illustrating operation timing of a transfer switch. FIG. 6 is a diagram illustrating the operation of the transfer switch and the transition of the charge amount. 6 is a flowchart in the embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram of an imaging apparatus according to an embodiment of the present invention.

  Each block in FIG. 1 will be described below.

  The optical lens 101 captures light from a subject, and typically has a focus mechanism for focusing, a diaphragm mechanism for adjusting the amount of light and depth of field, and a zoom mechanism for changing the focal length. And an optical lens for forming an image on the image sensor and entering the image.

  The image sensor 102 receives incident light from the optical lens 101, converts it into an electric signal, and outputs it.

  The configuration of the present image sensor will be described later with reference to FIG.

  The image processing unit 103 acquires the video signal from the image sensor 102 as a digital signal. If the output from the image sensor is an analog signal, it includes an analog / digital front end. The image processing unit performs arbitrary image processing on the video signal.

  The exposure control unit 104 calculates a set value of each exposure, such as an image sensor, a lens, and a strobe, from the analysis of the image signal obtained by the image processing unit.

  The image sensor control unit 105 and the lens control unit 106 transmit the set values calculated by the exposure control unit to the image sensor and the lens.

  The strobe control unit 107 receives a strobe light emission command from the exposure control unit, and controls the strobe 108 to control a light emission amount and a light emission timing.

  Note that the image processing unit, the exposure control unit, the image sensor control unit, the lens control unit, and the strobe control unit may be integrated by one LSI, and this division does not limit the form.

  Next, the configuration of the image sensor and the image processing unit will be described with reference to FIG.

  Each block in FIG. 2 will be described below.

  Reference numeral 20 in FIG. 2 denotes an image sensor (CMOS image sensor) on which a parallel type AD converter is mounted.

  Reference numeral 21 in FIG. 2 denotes an image processing LSI which performs development processing such as white balance processing and gamma processing on image data output from the image sensor 1, and finally outputs the image data to a monitor or records the image data on a recording medium. Or

  Further, the image processing LSI has a built-in CPU, and the CPU performs communication (for example, serial communication) with the image pickup device according to the operation mode of the image pickup device to perform control.

  2 includes a timing control unit 200, a pixel unit 210, a vertical scanning circuit 220, a column circuit 230, a horizontal transfer circuit 240, a signal processing circuit 250, an external output circuit 260, and a controller circuit 270.

  The controller circuit 270 is an I / F unit with the image processing LSI 21, and receives control of the image sensor 1 via the image sensor controller using serial communication means or the like.

  The timing control unit 200 supplies an operation CLK to each block of the image sensor, and supplies a timing signal to each block to control the operation.

  The pixel unit 210 is a photoelectric conversion element that performs photoelectric conversion according to the amount of incident light and outputs a voltage. On the surface of the photoelectric conversion element (photodiode), a color filter and a microlens are respectively mounted, and by using three color filters of R (red), G (green), and B (blue), a so-called Although a periodic structure of a Bayer array using RGB primary color filters is generally used, this is not necessarily the case.

  In some cases, the structure of the pixel portion is changed from the rolling shutter type in order to realize the global shutter system. The structure of the imaging apparatus will be described later with reference to FIG.

  The vertical scanning circuit 220 performs timing control for sequentially reading out pixel signal voltages of the pixel units 210 arranged two-dimensionally in one frame. Generally, a video signal is sequentially read out from one row to another row in one frame.

  The column circuit 230 includes an amplifier for electrically amplifying a signal read from the pixel unit 210 for each column, and a circuit for converting a signal from analog to digital.

  The horizontal transfer circuit 240 is a circuit that transmits a pixel signal from a column circuit in the horizontal direction. The output is input to the signal processing circuit 250. The signal processing circuit 250 is a circuit that digitally performs signal processing. In addition to adding a fixed amount of offset value by digital processing, a gain operation can be easily performed by performing a shift operation or a multiplication. In addition, by providing the pixel portion 210 with a pixel region that is intentionally shielded from light, a digital black level clamping operation using this may be performed.

  The output of the signal processing circuit 250 is passed to the external output circuit 260. The external output circuit 260 has a serializer function, and converts a multi-bit input parallel signal from the signal processing circuit 250 into a serial signal. Further, the serial signal is converted into, for example, an LVDS signal or the like, and output as an external device (in this case, image information exchange with the image processing LSI 21).

  Next, a configuration per unit pixel in the pixel unit 210 in FIG. 2 will be described with reference to FIG.

  The photoelectric conversion unit 31 accumulates charges generated from incident light.

  Reference numeral 35 in the figure denotes a transfer switch 1 (TX1 in the drawing), which transfers the electric charge accumulated in 31 to the holding unit 32.

  Reference numeral 36 in the figure denotes a transfer switch 2 (TX2 in the drawing), which transfers the charge held by 32 to the amplifier 33.

  In the figure, a reset transistor 37 resets the voltage of the amplifier 33.

  In the figure, reference numeral 38 denotes a transistor for selecting a pixel, which outputs a signal to a vertical output line 39.

  Reference numeral 34 in the figure denotes a transistor for resetting unnecessary charges of the photoelectric conversion unit 31.

  By providing the holding unit 32, a signal can be stored until a signal is output to the vertical output line 39, and the photoelectric conversion unit 31 can accumulate charges during that period.

  Therefore, a global shutter operation for simultaneous accumulation of all pixels becomes possible.

  FIG. 4 is a timing chart showing the timing at which the transfer switches of the OFD and TX1 and TX2 are turned on.

  After the electric charge of the photoelectric conversion unit is removed by the OFD switch, the accumulation of the electric charge in the photoelectric conversion unit starts. In TX1, transfer to the holding unit is performed at least once before the accumulation of the photoelectric conversion unit ends.

FIG. 4A is a diagram in which transfer to the holding unit is performed twice during the accumulation period of the photoelectric conversion unit.
Even if the amount of charge accumulated in the photoelectric conversion unit is smaller than the amount of charge accumulated in the holding unit, the charge can be accumulated to the full amount of charge saturation of the holding unit by performing the transfer a plurality of times.

  On the other hand, in FIG. 4B, transfer to the holding unit is performed once at the end of the accumulation period of the photoelectric conversion unit. In this case, only the saturated charge amount of the photoelectric conversion unit can be transferred to the holding unit. .

  Next, the charges transferred to the holding unit are transferred from the pixel unit of the image sensor to the amplifying unit, and sequentially output to the vertical output line via the selection transistor for each row.

  FIG. 5 is a diagram illustrating an example of the change in the charge amount of the photoelectric conversion unit and the change in the charge amount of the holding unit when the transfer to the holding unit is performed twice during the accumulation period of the photoelectric conversion unit.

  FIG. 5A is a diagram illustrating a transition of the charge when the charge amount of the photoelectric conversion unit does not reach the saturation charge amount depending on the brightness of the subject.

  After the OFD is turned off at time t1, the photoelectric conversion unit performs the first exposure until time t2 when the first transfer to the holding unit starts.

  From time t2 to time t3, the transfer switch from the photoelectric conversion unit to the holding unit is turned on, and charges are transferred during this period.

  From time t3 to t4, the second exposure is performed in the photoelectric conversion unit.

  From time t4 to time t5, the transfer switch from the photoelectric conversion unit to the holding unit is turned on again, and the charge is transferred again in this period. After the transfer to the holding unit, the OFD is turned on until the next accumulation starts in order to reset the electric charge of the photoelectric conversion unit. The time at which the OFD is turned off depends on the set accumulation time.

  The electric charge for two times transferred from the photoelectric conversion unit is accumulated in the holding unit, and the electric charge is held until the time t6 when the electric charge is transferred to the amplifying unit. At the time t6, the electric charge is transferred to the amplifying unit.

  FIG. 5B shows the same driving as that of FIG. 5A except that at time t7, a flash light such as a strobe light has a light emission time sufficiently short with respect to the exposure time and a bright subject is generated. Is shown.

  In this case, the charge is accumulated up to the saturation charge of the photoelectric conversion unit at time t7.

  The charge amount of the holding unit is accumulated up to the saturation charge amount of the photoelectric conversion unit at the first transfer in t2 to t3. If no flash is generated during the next exposure of the photoelectric conversion unit, the light is accumulated by the normal brightness of the subject.

  In such a case, when a normal subject is photographed, the amount of light corresponding to the color of the subject enters through the R, G, and B color filters provided in the pixel portion, and the accumulated charge changes. During the second exposure, the saturation charge of the photoelectric conversion unit is accumulated due to the effect of the flash, and the balance of the total charge transferred to the holding unit is distorted from the actual color balance of the subject. It will be deteriorated.

  Due to the above-described problem, the imaging device controls the timing of emitting the strobe light.

  FIG. 5C shows the light emission period of the strobe in addition to FIG. 5B.

  The period during which the strobe emits light can be calculated from the type of strobe and the set light emission amount.

  If the time period during which the strobe emits light is before (from time t1 to t2) or after (from time t3 to t4) from time t2 when TX1 is on, the above-described problem occurs.

  Therefore, the control is performed so that the time from the time t8 when the strobe light emission is started to the time t9 when the strobe light emission is completed includes the time from the time t2 when the TX1 is turned on to the time t3.

  FIG. 6A is a flowchart illustrating the operation of the imaging apparatus according to the present embodiment.

  The imaging apparatus determines whether or not there is a strobe light emission based on exposure conditions and the like, and if there is a strobe light emission, calculates an emission time from the amount of strobe light emission.

  Then, the setting values of the OFD and the strobe light emission trigger of the next imaging operation are set, and the imaging operation is performed.

  As described above, the time during which the TX1 is ON is included in the strobe light emission time, so that the strobe light can be emitted during the first exposure period and the second exposure period, thereby preventing the image quality from deteriorating when the strobe light is emitted. be able to.

  In the present embodiment, the number of transfers to the holding unit by the TX1 until the accumulation of the photoelectric conversion unit ends is controlled based on the presence or absence of strobe light emission.

  This embodiment will be described with reference to FIG.

  When strobe light emission is performed by the exposure control unit in the next imaging operation, the number of transfers of TX1 is one. Assuming that the number of transfers is one, if the saturation charge amount of the holding unit is larger than the saturation charge amount of the photoelectric conversion unit, the strobe light does not accumulate the saturation charge amount of the holding unit.

  Therefore, by increasing the gain in the subsequent stage, coloring that causes image quality deterioration is prevented. The latter stage refers to a block capable of amplifying a video signal, such as an amplifier gain in an image sensor, a gain of an AD converter, a gain of an image processing unit, and the present embodiment does not particularly designate the block. Absent.

By setting the number of transfers of TX1 to one and further increasing the gain, the exposure control unit calculates the most suitable exposure condition in that state again and sets it in each block of the imaging device.
Then, an imaging operation is performed.

  By performing such an imaging operation, even if the amount of charge transferred from the photoelectric conversion unit is smaller than the saturation charge amount of the holding unit, coloring can be suppressed by applying a gain at a subsequent stage, and image quality degradation at the time of flashing is reduced. Can be prevented.

101 optical lens, 102 image sensor, 103 image processing unit,
104 exposure control unit, 105 image sensor control unit, 106 lens control unit,
107 strobe control unit, 108 strobe

Claims (3)

A photoelectric conversion unit that accumulates charges generated by incident light,
A holding unit for holding the charge,
An amplifier that outputs a signal based on the charge;
A plurality of pixels each having a first transfer switch for transferring the charge from the photoelectric conversion unit to the holding unit, and a second transfer switch for transferring the charge from the holding unit to the amplification unit;
An output line from which a signal from the amplifying unit of the plurality of pixels is output,
Amplifying means for amplifying a signal output from the output line;
It has a timing control unit that controls the timing of firing the strobe,
An image pickup apparatus, wherein the time at which the first transfer switch is turned on is changed in accordance with the presence or absence of strobe light.   2. The imaging apparatus according to claim 1, wherein a time at which the first transfer switch is turned on is included in a time when the strobe light is emitted.   The imaging apparatus according to claim 1, wherein the number of times the first transfer switch is turned on is changed according to whether or not the strobe emits light.