JP2006279690A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus capable of simplifying manufacturing steps by omitting the step of forming a light shielding film in an OPB area of a solid-state imaging device. <P>SOLUTION: In an imaging apparatus for endoscope, during a term E1 while turning off an LED, a state is provided in which pixels of an imaging section of a CCD image sensor can be accumulated in signal charges. An image signal read from the CCD image sensor by a charge accumulating operation in such a dark state is stored in a memory as a black level. When imaging an object, an LED 2 is turned on (term E2), and signal charges corresponding to incident light from the object are accumulated in the pixels of the imaging section. These signal charges are read out during a term R2. The black level stored in the memory is subtracted from the image data read out during the term R2, thereby producing corrected image data from which the black level is removed (term D). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image pickup apparatus using a solid-state image pickup device, and more particularly to correction of a black level corresponding to dark current or the like in a photoelectric conversion pixel.

  A light receiving pixel of a solid-state imaging device such as a CCD (Charge Coupled Device) image sensor generates a signal charge by photoelectric conversion according to incident light from a subject, and an image signal is generated based on the signal charge. Here, the light receiving pixel generates a charge due to a dark current or the like even in the absence of incident light, and a signal offset due to this charge can be a noise component with respect to the original image signal. Therefore, an optical black (OPB) region composed of light receiving pixels shielded by an aluminum (Al) layer or the like is provided at the end of the imaging unit of the conventional solid-state imaging device. The imaging device acquires a black level (dark signal level) representing an offset component due to dark current or the like from the image signal corresponding to the pixel (OPB pixel) in the OPB region, and subtracts this from the image signal corresponding to the normal pixel. Thus, the image signal is corrected.

  In a solid-state imaging device in which an OPB region is provided in the imaging unit, the OPB pixel basically accumulates signal charges over an exposure period that is common to the normal pixels, and the signal charges of the OPB pixel and the normal pixels are one frame image signals. It is taken out at the same time. The exposure period in this operation is started, for example, from the timing at which the signal charges of all the pixels of the imaging unit are once discharged by an electronic shutter operation. On the other hand, for example, in the frame transfer type CCD image sensor, the end of the exposure period is determined by the timing at which the signal charges are transferred from the image pickup unit to the storage unit at a high speed, and in the interline transfer type CCD image sensor, from the photodiode to the vertical CCD shift register. This is the timing when the signal charge is read out.

  Now, since the solid-state image sensor is small and can obtain a high-resolution image, it is used in a wide field. For example, use for an endoscope is one of them. In recent years, capsule endoscopes have been developed and used for observation applications such as the digestive organs of the human body. In this case, a solid-state imaging device, a light source, a drive circuit thereof, a battery, and the like are built in a small capsule. The subject swallows the capsule endoscope, and the capsule endoscope wirelessly transmits the captured image to the outside of the body while moving in the digestive system.

This endoscope imaging apparatus has a characteristic that the exposure period can be controlled by the light emission period of the light source. In addition, an imaging apparatus is also used in which a mechanical shutter is disposed in front of a solid-state imaging device and an exposure period is controlled by opening and closing the mechanical shutter.
Japanese Patent Application No. 2004-262251

  In order to form the OPB region, a step of forming a light shielding film with an Al layer or the like different from the wiring layer is required. Simplification of the manufacturing process is a common problem in various products. Even in a solid-state imaging device, omitting the light shielding film forming process for the OPB region has the effect of improving yield, shortening the manufacturing period, and reducing costs. This is a promising issue. However, from the viewpoint of maintaining the image quality, there is a problem that the OPB region cannot be eliminated simply by omitting the light shielding film.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an imaging apparatus that can ensure the image quality while omitting the OPB region and simplifying the manufacturing process of the solid-state imaging device. And

  An image pickup apparatus according to the present invention includes a solid-state image sensor that can receive light from a subject on each photoelectric conversion pixel, an exposure unit that controls the presence or absence of incident light on the solid-state image sensor, and a dark state without the incident light. A storage unit that stores a dark signal level corresponding to the signal charge of the photoelectric conversion pixel in the pixel, and the exposure unit sets the dark state, reads a dark image signal corresponding to the dark state from the solid-state imaging device, and A dark signal level acquisition operation for acquiring the dark signal level based on an image signal and storing the dark signal level in the storage unit; and a bright state in which the incident light is present by the exposure unit; and a bright image signal corresponding to the bright state And a control unit that performs a subject imaging operation read from the imaging device and a correction operation for correcting the bright image signal based on the dark signal level stored in the storage unit.

  In a preferred aspect of the present invention, the imaging apparatus is an endoscope imaging apparatus, the exposure unit is a light source that irradiates the subject with illumination light, and the control unit blinks the exposure unit. The image pickup apparatus is configured to control the bright state and the dark state of the solid-state image pickup device by controlling the solid state image pickup device.

  In another preferred aspect of the present invention, the solid-state imaging device has a plurality of vertical shift registers each bit constituting the photoelectric conversion pixel arranged in a row direction, and the vertical shift register causes the photoelectric conversion pixel to be An image pickup unit that accumulates signal charges and performs vertical transfer, a horizontal transfer unit that receives the signal charges vertically transferred by the vertical shift register in units of rows from the image pickup unit, and horizontally transfers the signal charges, and outputs from the horizontal transfer unit And an output unit that generates the image signal based on the signal charge.

  In a further preferred aspect of the present invention, the control unit causes the solid-state imaging device to be in the dark state by the exposure unit when the bright image signal is read out in the subject imaging operation.

  According to the present invention, since the dark state is realized by the exposure means, the dark signal level of the solid-state image sensor can be acquired regardless of the OPB area, so that the OPB area of the solid-state image sensor can be omitted and the image quality can be reduced. Can be obtained.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

  The present embodiment is a capsule endoscope, and FIG. 1 is a schematic diagram showing a schematic configuration of the capsule endoscope according to the present embodiment. This capsule endoscope is for observing the internal surface of the digestive system of the examinee, for example, and includes an LED (Light Emitting Diode) 2, a CCD image sensor 4, a control unit 6, a memory 8, and a battery. 10 is included in a capsule-shaped housing 12. The control unit 6 includes a drive circuit 14, a signal processing circuit 16, a transmission circuit 18, and a control circuit 20 that controls them.

  The LED 2 is a light source that emits light according to a voltage signal supplied from the drive circuit 14. The light emitted from the LED 2 is applied to a subject outside the housing 12 through a transparent window provided in the housing 12. The reflected light from the irradiated subject enters from the window of the housing 12.

  The CCD image sensor 4 is an image sensor that generates an image signal corresponding to a subject. The CCD image sensor 4 operates based on various clocks from the drive circuit 14. An optical system (not shown) such as a lens is disposed in front of the light receiving surface of the CCD image sensor 4. This optical system forms an optical image on the light receiving surface based on the reflected light from the subject, and the CCD image sensor 4 converts this optical image into an image signal Vout and outputs it.

  As will be described later, the memory 8 stores the black level acquired by the control unit 6 based on the output of the CCD image sensor 4.

  The drive circuit 14 receives power supply from the battery 10 and generates various signals for driving the LED 2 and the CCD image sensor 4 as described above.

  The signal processing circuit 16 receives an analog image signal Vout from the CCD image sensor 4 and performs various signal processing. The signal processing circuit 16 will be described in detail later.

  The transmission circuit 18 is a circuit that wirelessly transmits an image signal, generates a radio wave signal that is modulated based on the output of the signal processing circuit 16, and transmits the radio signal from the antenna to a receiving device outside the body.

  The control circuit 20 gives operation instructions to the drive circuit 14, the signal processing circuit 16, and the transmission circuit 18, and monitors their operations.

  The battery 10 supplies power to each unit of the control unit 6 in addition to the drive circuit 14.

  The housing 12 is made of a material that is not eroded by gastric juice or the like, and is configured in a cylindrical shape with a watertight structure. By adopting the cylindrical shape, the housing 12 is easily moved in the body in the axial direction with the end of the cylinder as the head. Therefore, for example, the LED 2 and the CCD image sensor 4 are arranged so as to face the outside from the end portion, and configured to obtain an image in the traveling direction. Incidentally, the shape of the housing 12 in FIG. 1 schematically represents a cross section along the central axis of the cylinder, and the left and right ends correspond to the ends of the cylinder. As shown in this figure, the end cross section of the cylinder is rounded so that the casing 12 smoothly advances in the body in the axial direction.

  FIG. 2 is a schematic plan view showing a schematic configuration of the CCD image sensor 4. The image sensor 4 includes an imaging unit 4i, a horizontal transfer unit 4h, and an output unit 4d formed on the semiconductor substrate surface.

  The imaging unit 4i includes a plurality of vertical CCD shift registers (vertical shift registers 4v) arranged in the row direction (horizontal direction). The vertical shift register 4v includes a plurality of gate electrodes provided in a row direction on a semiconductor substrate, and these gate electrodes control the potential of a transfer channel formed on the semiconductor substrate. For example, the drive circuit 14 supplies a three-phase clock φi to the imaging unit 4i. By this clock φi, the gate electrodes are driven in three phases, one potential well is formed for every three gate electrodes, signal charges are accumulated in the potential wells, and signal charges are vertically transferred along the transfer channel. . That is, the area for every three gate electrodes of each vertical shift register 4v constitutes one bit of the shift register. For example, the gate electrode is formed of a material such as polysilicon that transmits visible light. Further, the CCD image sensor 4 is not formed with a light shielding film, and is configured such that light from the subject can enter the semiconductor substrate at each bit of the vertical shift register 4v. Accordingly, each bit of the vertical shift register 4v functions as a light receiving pixel that generates a signal charge corresponding to the amount of incident light, and a plurality of light receiving pixels are arranged in a matrix in the imaging unit 4i. That is, OPB pixels are not provided in the imaging unit 4i.

  The horizontal transfer unit 4h is composed of a CCD shift register, and each bit thereof is connected to each output of a plurality of vertical shift registers 4v of the imaging unit 4i. The horizontal transfer unit 4h receives signal charges from the vertical shift register 4v in units of one row, and sequentially transfers the signal charges for one row to the output unit 4d.

  The output unit 4d includes an electrically independent capacitor and an amplifier that extracts the potential change thereof. The signal charge output from the horizontal transfer unit 4h is received by the capacitor in units of 1 bit and converted into a voltage value. Output as a signal.

  FIG. 3 is a schematic block diagram illustrating the configuration of the signal processing circuit 16. The output Vout of the CCD image sensor 4 is input to the analog signal processing circuit 30. The analog signal processing circuit 30 performs processing such as correlated double sampling (CDS) and automatic gain control (AGC). An A / D conversion circuit (Analog-to-Digital Conversion: ADC) 32 converts an analog image signal as an output result of the analog signal processing circuit 30 into a digital signal and outputs the digital signal. The switch 34 switches whether the output of the A / D conversion circuit 32 is input to one terminal of the subtraction circuit 36 or stored in the memory 8. The contents stored in the memory 8 can be read out and input to the other terminal of the subtraction circuit 36. The control circuit 20 controls the switching operation of the switch 34 and the read / write operation of the memory 8. The subtracting circuit 36 subtracts the stored value of the memory 8 from the image signal value input from the switch 34 and outputs it to the digital signal processing circuit 40. The digital signal processing circuit 40 can perform various digital signal processing on the digital image signal input from the subtraction circuit 36.

  FIG. 4 is a schematic timing chart for explaining an imaging operation by this apparatus. FIG. 4 shows the emission period of the LED 2, the charge accumulation control for the CCD image sensor 4, the readout control of the image signal from the CCD image sensor 4, the control for the memory 8, and the output period of the corrected image data from the subtraction circuit 36. Has been.

  In the charge accumulation period E1 from time t1 to time t2, the charge accumulation operation of the CCD image sensor 4 is performed with the LED 2 turned off. This charge accumulation operation is intended to acquire the black level superimposed on the image signal when the subject is imaged. The start of the charge accumulation period E1 is defined by the electronic shutter operation. In the electronic shutter operation, all the transfer clocks φi1 to φi3 are turned off, and all the potential wells of each pixel of the imaging unit 4i are extinguished for a predetermined period. Thereby, the signal charges accumulated in the potential well are discharged from the transfer channel to the back surface of the substrate.

  When the electronic shutter operation is completed, a clock signal having a predetermined phase of φi, for example, φi2 is turned on. As a result, a potential well is formed under the gate electrode corresponding to φi2, and the signal charge can be accumulated, and the charge accumulation period E1 starts from this timing t1.

  Here, in the charge accumulation period E1, as described above, the LED 2 is turned off, and basically no light is incident on the CCD image sensor 4 of the present apparatus thrown into the body. Therefore, the signal charge accumulated in each pixel of the imaging unit 4i during the charge accumulation period E1 is basically caused by dark current or the like.

  The end timing t2 of the charge accumulation period E1 is defined by the start of the reading operation (period R1) of the CCD image sensor 4. In the read operation period R1, a line transfer operation in which the signal charges accumulated in the imaging unit 4i are vertically transferred one row at a time, and one row of signal charges transferred to the horizontal transfer unit 4h by the line transfer is horizontally transferred to the output unit 4d. Are alternately performed.

  This readout operation is paired with the charge accumulation operation (period E1), and the purpose is to acquire the black level in each pixel of the imaging unit 4i. Therefore, in this readout operation, it is possible to read out all the pixels of the imaging unit 4i and acquire the black level for each pixel, but it is considered that the variation of each black level pixel is small. In this case, it is possible to determine the black level based only on a part of the pixels of the imaging unit 4i. For example, in the circuit configuration shown in FIG. 3, the output destination of the switch 34 is switched to the memory 8 when the image signal corresponding to a certain row near the horizontal transfer unit 4h is output, and the memory 8 stores the image signal of that one row. Is stored as a black level. In addition, the read operation for several rows may be performed in the read period R1, and the signal processing circuit 16 may be configured to store the average value thereof in the memory 8.

  When a potential well is formed in the horizontal transfer unit 4h in the charge accumulation period E1, signal charges due to dark current can be accumulated in each bit of the shift register constituting the horizontal transfer unit 4h. In that case, the horizontal transfer unit 4h is driven prior to line transfer in the read operation, and the signal charge of the horizontal transfer unit 4h is discharged or the image signal corresponding to the first row read from the imaging unit 4i. Can not be used for black level acquisition, and black level accuracy can be ensured.

  The black level is stored in the memory 8 by the charge accumulation operation (period E1) and the read operation (period R1) described above. In the state where the black level is stored in the memory 8, the following imaging operation of the subject is performed.

  The charge accumulation operation when the subject is imaged, that is, the exposure operation, is performed in a state where the LED 2 is lit in the exposure period E2 from time t3 to time t4. The start of the exposure period E2 is defined by the electronic shutter operation and the lighting of the LED2. For example, in the operation shown in FIG. 4, the LED 2 is turned on immediately after the electronic shutter operation, whereby the start time t3 of the exposure period E2 is defined by the lighting timing. On the other hand, an electronic shutter operation may be performed after the LED 2 is turned on, and the exposure period E2 may be started from the completion of the electronic shutter operation. The driving of the CCD image sensor 4 after the electronic shutter operation is the same as the operation in the above-described period E1.

  In the exposure period E2, reflected light from the subject illuminated by the illumination light from the LED 2 enters each pixel of the imaging unit 4i, and signal charges corresponding to the amount of light accumulate in the potential well. As already described, since there is basically no light source other than LED 2 in the body, the turn-off timing t4 of LED 2 is the end of the exposure period E2.

  When the exposure period E2 ends, a read operation (period R2) of the signal charge accumulated in the imaging unit 4i is started. Similar to the operation in the period R1, the readout operation in the period R2 is a line transfer operation in which the signal charges accumulated in the imaging unit 4i are vertically transferred one row at a time, and one row transferred to the horizontal transfer unit 4h by the line transfer. The operation of horizontally transferring the signal charge to the output unit 4d is performed alternately. Since the current readout operation is for the purpose of acquiring an image of the subject, the line transfer operation and the horizontal transfer operation in the period E2 are basically performed until the readout of the signal charges from the imaging unit 4i, that is, the vertical shift register 4v. It is repeated as many times as the number of bits.

  The subtraction circuit 36 operates in conjunction with this readout operation, and performs a process of subtracting the black level read from the memory 8 from the image signal output from the CCD image sensor 4. As a result, in the period D corresponding to the readout period R2, the subtracting circuit 36 outputs an image signal (corrected image data) that has been corrected to remove the black level, and is used for processing in the digital signal processing circuit 40. It is made.

  If a potential well is formed in the horizontal transfer unit 4h during the exposure period E2, signal charges can be accumulated in each bit of the shift register constituting the horizontal transfer unit 4h. In that case, the horizontal transfer unit 4h is driven prior to line transfer in the read operation (period R2), and the signal charges of the horizontal transfer unit 4h are discharged.

  In the signal processing circuit 16 shown in FIG. 3, the value stored in the memory 8 is simply subtracted from the value corresponding to the image signal read in the period R2, and in this case, the charge accumulation period E1 and the exposure are set. The period E2 is basically set to the same length.

  On the other hand, since dark current and the like are weak, it is possible to improve the black level accuracy by setting the charge accumulation period E1 longer than the exposure period E2 to ensure the amount of signal charge accumulated. In that case, the black level scaled according to the ratio between E1 and E2 may be subtracted from the image signal by the subtracting circuit 36. When E2 is fixed, this scaling can be performed in advance and the scaled value can be stored in the memory 8. On the other hand, when E2 can be expanded and contracted by exposure control according to the brightness of the subject, scaling is performed in accordance with E2 for each exposure operation in period E2.

  Here, as an embodiment, an endoscope imaging apparatus that can control the incidence of light on the imaging unit 4i by the blinking of the LED 2 has been described. On the other hand, the present invention can also be applied to other imaging devices having means capable of controlling the incidence of light on the imaging unit 4i. For example, an imaging apparatus having a mechanical shutter can be configured basically in the same manner as the apparatus of the above-described embodiment using a solid-state imaging element having no OPB pixel. The solid-state imaging device to which the present invention is applied may be a CMOS sensor in addition to a CCD image sensor.

1 is a schematic diagram illustrating a schematic configuration of a capsule endoscope according to an embodiment. 1 is a schematic plan view showing a schematic configuration of a CCD image sensor according to an embodiment. It is a typical block diagram explaining the structure of a signal processing circuit. It is a typical timing diagram explaining the imaging operation of the capsule endoscope according to the embodiment.

Explanation of symbols

  2 LED, 4 CCD image sensor, 4i imaging unit, 4h horizontal transfer unit, 4d output unit, 6 control unit, 8 memory, 10 battery, 12 housing, 14 drive circuit, 16 signal processing circuit, 18 transmission circuit, 20 control Circuit, 30 analog signal processing circuit, 32 A / D conversion circuit, 34 switch, 36 subtraction circuit, 40 digital signal processing circuit.

Claims (4)

A solid-state imaging device capable of receiving light from a subject on each photoelectric conversion pixel; and
Exposure means for controlling the presence or absence of incident light on the solid-state imaging device;
A storage unit for storing a dark signal level corresponding to a signal charge of the photoelectric conversion pixel in a dark state without the incident light;
Dark signal level acquisition that stores the dark signal level on the basis of the dark image signal by reading out the dark image signal corresponding to the dark state from the solid-state imaging device and setting the dark signal level in the storage unit. Based on the operation, a subject imaging operation in which a bright image signal having the incident light is provided by the exposure unit, and reading a bright image signal corresponding to the bright state from the solid-state imaging device, and the dark signal level stored in the storage unit A control unit that performs a correction operation for correcting the bright image signal;
An imaging device comprising: The imaging device according to claim 1,
The imaging device is an endoscope imaging device,
The exposure means is a light source that illuminates the subject with illumination light,
The control unit controls blinking of the exposure unit to switch between the bright state and the dark state of the solid-state imaging device;
An imaging apparatus characterized by the above. In the imaging device according to claim 1 or 2,
The solid-state imaging device is
An imaging unit in which each bit has a plurality of vertical shift registers constituting the photoelectric conversion pixel arranged in a row direction, and the signal charge is accumulated and vertically transferred for each photoelectric conversion pixel by the vertical shift register;
A horizontal transfer unit that receives the signal charges vertically transferred by the vertical shift register from the imaging unit in units of rows, and horizontally transfers them;
An output unit that generates the image signal based on the signal charge output from the horizontal transfer unit;
An imaging device comprising: The imaging device according to claim 3.
The control unit is configured to set the solid-state imaging device to the dark state by the exposure unit when reading the bright image signal in the subject imaging operation.

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