JP2018121330A - Photoelectric conversion device, method for determining defective pixel, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the accuracy of a photoelectric conversion element in detecting a defective pixel.SOLUTION: A photoelectric conversion device acquires dark time pixel values that are pixel values obtained while light is not made incident on a photoelectric conversion element that creates an image signal according to the quantity of received light, at least respectively in a first time and a second time longer than the first time, takes the difference between dark time pixel values acquired in the second time and dark time pixel values acquired in the first time to detect the amount of noise during the dark time. The photoelectric conversion device then determines a pixel having an amount of noise equal to or larger than a predetermined first threshold as a defective pixel.SELECTED DRAWING: Figure 12

Description

  The present invention relates to a photoelectric conversion device, a defective pixel determination method, and an image forming apparatus.

  A linear image sensor formed by a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) or the like includes a defective pixel that causes a saturation charge amount abnormality or dark current abnormality of a photodiode (PD). It has been known. A defective pixel has a higher pixel value or a lower pixel value than a normal pixel, which causes a reduction in image quality. For this reason, it is necessary to detect a defective pixel and correct the pixel value.

  Patent Document 1 (Japanese Patent Application Laid-Open No. 2014-110622) determines whether or not there is linearity with respect to the time of the pixel value of the target pixel obtained at two different exposure times, and the pixel having no linearity in the exposure time and the pixel value. An image processing apparatus that detects a pixel as a defective pixel is disclosed.

  However, the image processing apparatus disclosed in Patent Document 1 uses an image at the time of exposure for detecting defective pixels. Since the change in the pixel value due to light is larger than the change in the pixel value due to the dark current, there is a problem that it is difficult to detect defective pixels having an abnormality in the dark current.

  The pixel value includes an offset component of the circuit. The offset component of the circuit is a non-negligible magnitude for dark current. For this reason, the image processing apparatus of Patent Document 1 that detects a defective pixel using linearity as a determination condition has a problem that it is difficult to accurately detect a defective pixel having an abnormality in dark current.

  SUMMARY An advantage of some aspects of the invention is that it provides a photoelectric conversion device, a defective pixel determination method, and an image forming apparatus that can accurately detect defective pixels of an image sensor.

  In order to solve the above-described problems and achieve the object, the present invention provides a photoelectric conversion element that generates an image signal corresponding to the amount of received light, and a pixel value obtained in a state where no light is incident on the photoelectric conversion element. A certain dark pixel value is acquired at least at a first time and a second time longer than the first time, and is acquired at each dark pixel value acquired at the first time and the second time. A noise detection unit that detects a noise amount in the dark time by taking a difference between the pixel values in the dark time, and a determination unit that determines a pixel having a noise amount equal to or greater than a predetermined first threshold as a defective pixel Have.

  According to the present invention, it is possible to accurately detect defective pixels of an image sensor.

FIG. 1 is a cross-sectional view of the MFP according to the embodiment. FIG. 2 is a cross-sectional view of a reading device provided in the MFP according to the embodiment. FIG. 3 is a hardware configuration diagram of the MFP according to the embodiment. FIG. 4 is a block diagram of a photoelectric conversion element provided in the MFP according to the embodiment. FIG. 5 is a circuit diagram of a photoelectric conversion element provided in the MFP according to the embodiment. FIG. 6 is a diagram for explaining image noise generated in a read image due to defective pixels. FIG. 7 is a diagram illustrating image noise generated in a read image when defective pixels are generated in the linear sensor. FIG. 8 is a diagram for explaining a defective pixel detection method as a reference example. FIG. 9 is a diagram for explaining a problem of a defective pixel detection method as a reference example. FIG. 10 is a block diagram of an image reading unit provided in the MFP according to the embodiment. FIG. 11 is a block diagram of a noise detection unit provided in the image reading unit. FIG. 12 is a diagram illustrating pixel values from which circuit offset components have been removed by the noise detection unit provided in the MFP according to the embodiment. FIG. 13 is a block diagram of the image reading unit when the black shading correction unit is provided in the MFP according to the embodiment. FIG. 14 is a diagram illustrating an example of defective pixels detected by a noise detection unit and a determination unit provided in the MFP according to the embodiment. FIG. 15 is a diagram for explaining an operation of storing defective pixels detected by the noise detection unit and the determination unit based on a predetermined priority in the MFP according to the embodiment. FIG. 16 is a flowchart for explaining the detection timing of a defective pixel when the MFP according to the embodiment is activated. FIG. 17 is a flowchart for explaining detection timing of a defective pixel at the time of reading a document using a pressure plate in the MFP according to the embodiment. FIG. 18 is a flowchart for explaining detection timing of a defective pixel at the time of reading a document using an ADF in the MFP according to the embodiment. FIG. 19 is a block diagram of a defective pixel correction unit provided in the MFP according to the embodiment. FIG. 20 is a diagram for explaining defective pixel correction processing of the MFP according to the embodiment. FIG. 21 is a flowchart illustrating a flow of defective pixel correction processing of the MFP according to the embodiment.

  First, the field of application will be described. The photoelectric conversion device and the defective pixel determination method can be applied not only to a device that reads an image, but also to a device that senses the presence or absence of light and performs predetermined information processing. Specifically, a photoelectric conversion device and a photoelectric conversion method are provided on a multifunction peripheral (MFP) linear sensor, an autofocus line sensor of a camera device or a video camera device, and an interactive whiteboard device (electronic blackboard). The present invention can be applied to a line sensor or the like that reads characters, symbols, or figures written on the. Hereinafter, as an example, an MFP to which a photoelectric conversion device and a photoelectric conversion method are applied will be described.

(MFP configuration)
First, FIG. 1 shows a view of the MFP according to the embodiment as viewed from the side. FIG. 1 shows a state in which the main body of the MFP is seen through. As shown in FIG. 1, the MFP includes a reading device 1 and a main body 2. The reading device 1 includes an automatic document feeder (ADF) 3 and a scanner mechanism 4.

  In the main body 2, a tandem type image forming unit 5, a registration roller 7 that supplies recording paper from the paper supply unit 13 to the image forming unit 5 through a conveyance path 6, an optical writing device 8, a fixing conveyance unit 9, and The double-sided tray 10 is provided. The image forming unit 5 includes four photosensitive drums 11 corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K). Around each photosensitive drum 11, image forming elements including a charger, a developing device 12, a transfer device, a cleaner, and a static eliminator are arranged. Further, an intermediate transfer belt 14 is provided between the transfer unit and the photosensitive drum 11 so as to be stretched between the driving roller and the driven roller while being sandwiched between the nips thereof.

  In the tandem-type image forming apparatus configured as described above, optical writing is performed on the photosensitive drum 11 corresponding to each color of YMCK, and development is performed for each color toner by the developing unit 12, for example, Y, M, C, K. In this order, primary transfer is performed on the intermediate transfer belt 14. Then, a full-color image in which four colors are superimposed by primary transfer is secondarily transferred onto a recording sheet, and then fixed and discharged. Thereby, a full-color image is formed on the recording paper.

(Configuration of ADF and scanner mechanism)
FIG. 2 is a cross-sectional view of the ADF 3 and the scanner mechanism 4. The scanner mechanism 4 includes a contact glass 15 on which an original is placed. The scanner mechanism 4 includes a first carriage 18 having a light source 16 for document exposure and a first reflection mirror 17, and a second carriage 24 having a second reflection mirror 19 and a third reflection mirror 20. Yes. Further, the scanner mechanism 4 includes a lens unit 22 for forming an image of the light reflected by the third reflecting mirror 20 on the light receiving region of the photoelectric conversion element 21. The scanner mechanism 4 includes a reference white plate 23 for correcting various distortions by a reading optical system and the like, and a sheet-through reading slit 24. The scanner mechanism 4 receives reflected light from the original illuminated by the light emitted from the light source 16 by the photoelectric conversion element 21, converts it into an electrical signal (image data), and outputs it.

  The ADF 3 is connected to the main body 2 via a hinge member (not shown) so that the ADF 3 can be opened and closed with respect to the contact glass 15. The ADF 3 includes a document tray 28 on which a document bundle 27 composed of a plurality of documents can be placed. The ADF 3 also includes a separation feeding unit including a feeding roller 29 that separates documents one by one from the document bundle 27 placed on the document tray 28 and automatically feeds them toward the sheet-through reading slit 25. It also has.

(Original scanning operation)
Such a reading apparatus 1 has a scan mode for reading a document placed on the contact glass 15 and a sheet-through mode for reading a document automatically fed by the ADF 3. Before reading the image in the scan mode or the sheet through mode, the reference white plate 23 is illuminated by the light source 16 that is turned on, and the image by the reflected light is read by the photoelectric conversion element 21. Then, shading correction data is generated and stored so that each pixel of the image data for one line has a uniform level. The stored shading correction data is used for shading correction of image data read in a scan mode or a sheet through mode described below.

  In the scan mode, the first carriage 18 and the second carriage 24 are moved in the arrow A direction (sub-scanning direction) by a stepping motor (not shown) to scan the document. At this time, the second carriage 24 moves at half the speed of the first carriage 18 in order to keep the optical path length from the contact glass 15 to the light receiving region of the photoelectric conversion element 21 constant.

  At the same time, the image surface, which is the lower surface of the document placed on the contact glass 15, is illuminated (exposed) by the light source 16 of the first carriage 18. Then, the reflected light from the image plane is sequentially reflected by the first reflecting mirror 17 of the first carriage 18, the second reflecting mirror 19 of the second carriage 24, and the third reflecting mirror 20. Then, the reflected light beam from the third reflecting mirror 20 is focused by the lens unit 22 and imaged on the light receiving region of the photoelectric conversion element 21. The photoelectric conversion element 21 generates image data by photoelectrically converting the light received for each line. The image data is digitized, gain-adjusted, and output. The document whose image has been read is discharged to a discharge port (not shown).

  In the sheet through mode, the first carriage 18 and the second carriage 24 move to the lower side of the sheet through reading slit 25 and stop. Thereafter, the lowermost originals of the original bundle 27 placed on the original tray 28 of the ADF 3 are sequentially automatically fed in the arrow B direction (sub-scanning direction) by the feeding roller 29, and the sheet through reading slit 25 When the document passes through the position, the document is scanned.

  At this time, the lower surface (image surface) of the automatically fed document is illuminated by the light source 16 of the first carriage 18. Then, the reflected light from the image plane is sequentially reflected by the first reflecting mirror 17 of the first carriage 18, the second reflecting mirror 19 of the second carriage 24, and the third reflecting mirror 20. Then, the reflected light beam from the third reflecting mirror 20 is focused by the lens unit 22 and imaged on the light receiving region of the photoelectric conversion element 21. The photoelectric conversion element 21 generates image data by photoelectrically converting the light received for each line. The image data is digitized, gain-adjusted, and output. The document whose image has been read is discharged to a discharge port (not shown).

(Hardware configuration of MFP)
Next, FIG. 3 shows a hardware configuration of the MFP. As shown in FIG. 3, the MFP includes a CPU 41, ROM 42, RAM 43, HDD (hard disk drive) 44, and flash memory 45. The MFP includes a FAX modem 46, an operation panel 47, an engine 48, an ADF 49 (corresponding to ADF 3 shown in FIGS. 1 and 2), a connection interface (connection I / F) 50, an image reading unit 52, and a network such as the Internet. A communication I / F 51 that performs wired communication or wireless communication via the communication interface 40 is provided. The CPU 41 to the image reading unit 52 are connected to each other via the system bus 18 shown in FIG.

  The CPU 41 controls the overall operation of the MFP. The CPU 41 uses the RAM 43 as a work area (work area) to execute programs stored in the ROM 42 or the HDD 44, thereby controlling the operation of the entire MFP, and various functions such as a copy function, a scanner function, a facsimile function, and a printer function. Is realized.

  The engine 48 is hardware that performs processing other than general-purpose information processing and communication for realizing a copy function, a scanner function, a printer function, and the like. The engine 48 includes a scanner that scans and reads characters and images such as a document or a business card, and a plotter that prints on a sheet material such as paper. The FAX modem 46 has a facsimile communication function for performing facsimile communication.

(Configuration of photoelectric conversion element)
FIG. 5 is a detailed block diagram of the photoelectric conversion element 21. As an example, as shown in FIG. 5, the photoelectric conversion element 21 is a linear sensor in which pixels are arranged one-dimensionally along the main scanning direction. The direction in which the pixels are arranged one-dimensionally is the main scanning direction, and the direction two-dimensionally orthogonal to the main scanning direction is the sub-scanning direction.

  That is, the photoelectric conversion element 21, together with the timing signal generation unit 60, a pixel signal generation circuit 61 formed by arranging pixels one-dimensionally, and an amplifier (PGA: programmable gain amplifier) that amplifies each image signal with a predetermined gain 62, and an analog-digital converter (ADC) 63 for digitizing the image signal from each PGA.

  Further, the photoelectric conversion element 21 includes a parallel / serial conversion unit 64 that converts an image signal supplied in parallel from the ADC 63 into a serial image signal and supplies the serial image signal to a subsequent processing unit. Output timings of the pixel signal generation circuit 61 to the parallel / serial conversion unit 64 are controlled based on a clock signal from the timing signal generation unit 60.

  When the photoelectric conversion element 21 is a color-compatible photoelectric conversion element, the pixel signal generation circuit 61 is provided for each color channel such as red, green, blue (RGB) or yellow, magenta, cyan (YMC), for example. . The PGA 62 and the ADC 63 are also provided for each color channel.

(Circuit configuration of pixel signal generation circuit)
FIG. 5 shows a circuit diagram of a portion corresponding to each pixel of the pixel signal generation circuit 61. As shown in FIG. 5, each pixel of the pixel signal generation circuit 61 has a photodiode PD which is a light receiving element, and a floating diffusion FD which converts charges accumulated corresponding to the amount of received light into a voltage. Each pixel has a charge transfer switch TX for transferring the charge accumulated in the photodiode PD to the floating diffusion FD, and a reset switch RT for resetting the potential of the floating diffusion FD to the reset potential AVDD_RT. Each pixel has a source follower SF inserted and connected between the power supply voltage AVDD_PIX of the source follower SF and the current source DRG, and a switch SL for transferring the voltage-converted image signal (Pix_out) to the PGA 62 in the subsequent stage. have.

  Since the threshold voltage of the source follower SF transistor varies, a circuit offset that does not depend on the amount of incident light occurs in the output signal. The circuit offset generates FPN (Fixed pattern noise) in the image and causes a reduction in image quality.

  In addition, in the manufacture of the photoelectric conversion element 21, pixels with an abnormally large or small dark current of the photodiode PD are generated with a certain probability due to heavy metal contamination or crystal defects. A pixel in which the dark current of the photodiode PD is abnormally large is referred to as a “white scratch pixel”. In addition, a pixel having a small capacitance of the photodiode PD and a small saturation charge amount is generated. Such a pixel is called a “black scratch pixel”. The white scratch pixel and the black scratch pixel appear on the image as image noise at a point having a pixel value different from that of a normal pixel. Further, the dark current noise component of the photodiode PD increases in proportion to time. In addition, the dark current noise component of the photodiode PD is temperature-dependent, and as an example, the dark current noise component doubles when the temperature rises by 10 ° C.

(Image noise caused by defective pixels)
FIG. 6 shows the state of occurrence of image noise when an image is read under a condition where the incident light intensity and the accumulation time are different in a nine-pixel area sensor including white defect pixels 301 and black defect pixels 302. . The horizontal axis in FIG. 6 is the light accumulation time of the photodiode PD, and the vertical axis is the incident intensity of light with respect to the photodiode PD.

  As shown in FIG. 6A or FIG. 6B, the white flaw pixel 301 does not become image noise because the signal component due to light becomes large and the influence of dark current becomes inconspicuous when the amount of incident light is large. . However, as shown in FIG. 6C or FIG. 6D, when the amount of incident light is small, the pixel value is higher than that of a normal pixel, so that it appears as image noise at a point brighter than the surroundings. Further, as shown in FIG. 6D, when the accumulation time is long, it appears as more conspicuous image noise.

  As shown in FIG. 6A or FIG. 6B, the black scratched pixel 302 appears as dark-point image noise because the pixel value is lower than the normal pixel when the amount of incident light is large. . Further, as shown in FIG. 6B, when the accumulation time is long, it appears as more conspicuous image noise. Note that, as shown in FIG. 6C or FIG. 6D, the black scratched pixel 302 does not cause image noise because the photodiode PD is not saturated when the amount of incident light is small.

  As described above, the white scratch pixel 301 causes a deterioration in image quality in a dark image, and the black scratch pixel 302 causes a decrease in image quality in a bright image. In order to prevent these image quality degradations, it is necessary to detect and correct defective pixels. For example, if the black scratch pixel 301 acquires image data when exposed for a long time, the normal pixel appears as a dark point in a saturated white image, and thus detection is easy.

(Image noise caused by defective pixels of linear sensor)
FIG. 7A is a diagram showing the position of white flaw pixels on the linear sensor, and the pixels indicated by diagonal lines are white flaw pixels. FIG. 7B is a diagram illustrating a dark image (an image acquired in a state where the linear sensor is not exposed) acquired by a linear sensor including white scratch pixels. The pixel value of the white flaw pixel is higher than that of the normal pixel. For this reason, if white scratches are included on the linear sensor, as shown in FIG. 7B, the image (pixel) at the position corresponding to the white scratch pixel in the entire image is more than the surrounding image. Bright vertical stripe image noise appears.

  In addition, when the black sensor pixel is included in the linear sensor, the opposite result is obtained when the white sensor pixel is included. That is, the pixel value of the black flaw pixel is lower than that of the normal pixel. For this reason, if the linear sensor includes black flaw pixels, vertical streak image noise in which an image (pixel) at a position corresponding to the black flaw pixel in the entire image becomes darker than the surrounding image appears.

  In this way, in the case of a linear sensor, an adverse effect due to a defective pixel appears as vertical streak image noise. Therefore, an adverse effect due to a defective pixel is greater than an area sensor where the adverse effect due to the defective pixel appears as a point image noise. The adverse effect of defective pixels becomes more prominent when a plurality of defective pixels are adjacent to each other.

(Reference example of defective pixel detection method)
Next, a reference example of the defective pixel detection method will be described. FIGS. 8A and 8B are diagrams illustrating pixel values when the same subject is imaged at different exposure times t1 and t2 (t2> t1). Among these, FIG. 8A shows pixel values when a subject is imaged with normal pixels. FIG. 8B shows pixel values when a subject is imaged with black flaw pixels.

  When the exposure time t = 0, it is assumed that the pixel value S = 0 and the pixel value S linearly changes with time, and after calculating the rate of change S1 / t1 with respect to time from the pixel value S1 at the exposure time t1, exposure is performed. An ideal pixel value S2 at time t2 is calculated. An arithmetic expression of such a pixel value S2 is shown in the following formula 1.

  S2 = (S1 / t1) × t2 (Expression 1)

  In practice, electrical noise is included in the pixel value. Therefore, the pixel value S3 at the exposure time t2 is considered to have an error (fixed value) of ± α with respect to the pixel value S2, and it is determined whether or not the pixel value S3 is within the range of the pixel value S2 ± α. Thus, it is determined whether or not the pixel is a defective pixel. Note that the dark current noise component increases in proportion to time in the same manner as the signal component. The circuit offset component is a fixed value that each pixel of the photoelectric conversion element has individually. However, since the dark current noise component and the circuit offset component are small values with respect to the signal component, there is no influence on the determination of the defective pixel.

  In the case of a normal pixel, the signal component changes linearly with respect to the exposure time as shown in FIG. For this reason, since the pixel value S3 falls within the range of the pixel value S2 ± α, it is determined as a normal pixel. On the other hand, in the case of a black flaw pixel, since the saturation charge amount is small, as shown in FIG. 8B, the charge accumulated at a low pixel value is saturated at the exposure time t2. For this reason, the value of the pixel value S3 is smaller than the pixel value S2 of the normal pixel. Therefore, since the pixel value S3 does not fall within the range of the pixel value S2 ± α, it is determined as a defective pixel.

(Problems of the defective pixel detection method in the reference example)
8A and 8B are examples of black scratched pixels, whereas FIGS. 9A and 9B are the same at different exposure times t1 and t2 (t2> t1). It is a figure which shows the pixel value of a white flaw pixel when a to-be-photographed object is imaged. Among these, FIG. 9A shows pixel values obtained when light is applied to white flaw pixels, and FIG. 9B shows a case where light is not applied to white flaw pixels (when dark). ) Shows the pixel values obtained.

  As shown in FIG. 9A, in the case of a white flaw pixel, the dark current noise component increases according to the exposure time. However, since the dark current noise component is smaller than the signal component due to the incident light, the pixel value S3 of the white scratch pixel is within the range of the pixel value S2 ± α. For this reason, there is a problem that the white defect pixel is erroneously determined as a normal pixel.

  Further, as shown in FIG. 9B, the pixel value of the white flaw pixel in the dark has a circuit offset component larger than the dark current noise component. For this reason, if the defective pixel is determined based on the pixel value S2 on the assumption that the pixel value S = 0 when the exposure time t = 0 and the pixel value S linearly changes with time, even if the pixel is a normal pixel, The pixel value S3 does not fall within the range of the pixel value S2 ± α, which causes a problem that a normal pixel is erroneously determined as a defective pixel. Therefore, the defective pixel detection method in the reference example has a problem that it is difficult to detect white flaw pixels even though black flaw pixels can be detected.

(Configuration and operation of main part of MFP of embodiment)
In the MFP of the embodiment, in order to accurately detect such defective pixels, the image reading unit 52 has a configuration shown in FIG. That is, in the MFP according to the embodiment, the image reading unit 52 includes a light shielding unit 71 that shields light from entering the photoelectric conversion element 21, a noise detection unit 72 and a determination unit 83 that detect defective pixels, and noise. A defective pixel correction unit 73 that corrects the defective pixels detected by the detection unit 72 and the determination unit 83 is provided.

  As an example, the light shielding unit 71 is a light shielding mechanism that covers the light receiving surface of the photoelectric conversion element 21 with a light shielding member that does not transmit light so that light does not enter the photoelectric conversion element 21. Note that the ADF 3, the pressure plate, or the reference white plate 23 may be used for shielding the photoelectric conversion element 21.

  When the defective pixel is corrected, the CPU 41 controls the light shielding portion 71 so that the photoelectric conversion element 21 is covered with the light shielding member. The photoelectric conversion element 21 has the above-described accumulation times t1 and t2 (t2) in “dark time”, which is a time during which the photoelectric conversion element 21 is covered with a light shielding member (a time during which light is not incident on the photoelectric conversion element 21). Pixel data of> t1) is acquired.

  The noise detection unit 72 and the determination unit 83 detect defective pixels based on the pixel data acquired from the photoelectric conversion element 21. Based on the detection result of the defective pixel detected by the noise detection unit 72 and the determination unit 83, the defective pixel correction unit 73 performs predetermined processing on the pixel data of the defective pixel among the pixel data supplied from the photoelectric conversion element 21. Is supplied to an external device such as a monitor device or a storage device.

  The noise detection unit 72, the determination unit 83, and the defective pixel correction unit 73 will be described as being realized by hardware, but any one or all of them may be realized by software. In this case, for example, the CPU 41 reads out the defective pixel correction program stored in the HDD 44 or the flash memory 45 shown in FIG. 3 and develops the noise detection unit 72, the determination unit 83, and the defective pixel correction unit 73 in the RAM 43 in software. . Then, the CPU 41 executes a defective pixel detection process and a defective pixel correction process as described below.

(Configuration and operation of noise detection unit and determination unit)
FIG. 11 shows a block diagram of the noise detection unit 72. As shown in FIG. 11, when detecting noise, the noise detecting unit 72 stores a pixel value St1x of the charge accumulation time t1 in the dark, a pixel value St1x stored in the memory 81, and a dark value. And a subtractor 82 for detecting a difference between the accumulation time t2 (t2> t1) and the pixel value St2x. Note that the configuration for detecting the difference between the pixel value St1x and the pixel value St2x is not limited to the subtractor. For example, the same result can be obtained by a configuration of a multiplier that inverts the pixel value St1x to a negative value and an adder that adds the multiplication result and the pixel value St2x. The memory 81 may use, for example, the RAM 43 or the flash memory 45 shown in FIG. 3 provided outside the noise detection unit 72.

  For example, in the MFP according to the embodiment, photoelectric conversion elements 21 for each color are provided such as RGB or YMC. For this reason, each pixel data (pixel value) of RGB from the photoelectric conversion element 21 for each color is supplied to the noise detection unit 72. The noise detection unit 72 stores, in the memory 81, pixel values St1x for all the pixels of the photoelectric conversion elements 21 for each color acquired at the accumulation time t1 in the dark when noise detection is performed.

  In this example, the pixel value St1x of the accumulation time t1 is stored in the memory 81. However, the pixel value St2x for each color acquired at the accumulation time t2 longer than the accumulation time t1 may be stored in the memory 81. .

  When the pixel value St2x acquired for each color at the accumulation time t2 longer than the accumulation time t1 is supplied to the subtractor 82 in the dark when the defective pixel is detected, the CPU 41 is the same as the pixel position of each pixel value St2x. The pixel value St1x at the pixel position is read from the memory 81 and supplied to the subtractor 82. The subtractor 82 calculates a difference value St2x−St1x for each color.

  The determination unit 83 compares the difference value St2x−St1x at each pixel position with a predetermined threshold value Dth, and determines a pixel having a pixel value at which the difference value St2x−St1x is equal to or greater than the threshold value Dth as a defective pixel. Further, when at least one pixel value equal to or greater than the threshold value Dth exists among the pixel values of the respective colors at the same pixel position, the determination unit 83 determines all the color pixels at the same pixel position as defective pixels (first pixel). Judgment processing). The threshold value Dth may be a fixed value or may be set to an arbitrary value by the user.

  The defective pixel determination operation will be specifically described. The pixel values of the accumulation times t1R, t1G, and t1B shown in FIG. 12A indicate the pixel values of the normal pixels of each color at the accumulation time t1, and the pixel values of the accumulation times t2R, t2G, and t2B are the accumulation time t2. The pixel value of the normal pixel of each color is shown. Subtracting the pixel values of the accumulation times t1R, t1G, and t1B from the pixel values of the accumulation times t2R, t2G, and t2B removes the circuit offset component that becomes a substantially fixed value regardless of the passage of time, and dark current noise Each pixel value that is a component value is generated. In the case of a normal pixel, each pixel value indicating the value of the dark current noise component of each pixel is less than the threshold value Dth as shown in FIG. In this case, the determination unit 83 determines each color pixel as a normal pixel.

  In contrast, the pixel values of the accumulation times t1R, t1G, and t1B shown in FIG. 12B indicate the pixel values of the normal pixels of the respective colors at the accumulation time t1, and the pixel values of the accumulation times t2R, t2G, and t2B. Indicates pixel values of defective pixels of each color at the accumulation time t2. As described above, when the pixel values of the accumulation times t1R, t1G, and t1B are subtracted from the pixel values of the accumulation times t2R, t2G, and t2B, the circuit offset component that becomes a substantially fixed value regardless of the passage of time is removed. Then, each pixel value that is the value of the dark current noise component is generated.

  However, in the case of a white flaw pixel, a dark current noise component having a high value appears in spite of the dark time when no light is incident, like the pixel value of the accumulation time t2G in FIG. The example in FIG. 12B is an example in which the G pixel is a white scratch pixel. Therefore, among the dark current noise components of the pixels of the respective colors generated by subtracting the pixel values of the accumulation times t1R, t1G, and t1B from the pixel values of the accumulation times t2R, t2G, and t2B, FIG. As shown in FIG. 5, a pixel having a dark current noise component that is equal to or greater than the threshold value Dth is detected.

  In this case, the determination unit 83 determines a pixel having a dark current noise component that is equal to or greater than the threshold value Dth as a defective pixel, and also determines a pixel of another color at the same pixel position as a defective pixel. In other words, when at least one determination unit 83 determines that the pixel is a defective pixel, the determination unit 83 determines all color pixels at the same pixel position as the defective pixel.

  The temperature of the photoelectric conversion element 21 rises due to heat generated by driving. Therefore, the temperature increases as a longer time elapses from the start of driving. Since the dark current noise component increases as the temperature increases, white scratch pixel determination is performed based on the difference St2x−St1x between the pixel value St2x of the accumulation time t2 having a long accumulation time and the pixel value St1x having a short accumulation time t1. By doing so, white scratch pixels can be detected with high accuracy.

(Second determination process)
Further, as described above, the determination unit 83 determines that the pixel is a normal pixel when all the pixel values of the respective colors at the same pixel position are less than the threshold value Dth, but further performs a second determination process described below. May be.

  That is, when all the pixel values of each color at the same pixel position are less than the threshold value Dth, the determination unit 83 integrates all the pixel values of each color at the same pixel position before determining as a normal pixel. It is determined whether or not the integrated value is greater than or equal to a predetermined threshold value Dth2. When the integrated value of the pixel values of each color is equal to or greater than the predetermined threshold value Dth2, all color pixels at the same pixel position are determined as defective pixels (second determination process).

  Although the determination unit 83 may perform only the first determination process and determine the defective pixel, the determination accuracy of the defective pixel is improved by performing the second determination process together with the first determination process. Can do.

(Black shading correction process)
Here, a black shading correction unit 88 may be provided for the image reading unit 52 as shown in FIG. In this case, the black shading correction unit 88 is provided between the photoelectric conversion element 21 and the noise detection unit 72. The example in FIG. 13 is an example in which the black shading correction unit 88 is provided as hardware. However, the CPU 41 may implement a black shading correction unit 88 described below by software, for example, by executing a black shading correction program.

  The black shading correction unit 88 acquires the pixel values of all the pixels acquired from the photoelectric conversion element 21 in the dark. The black shading correction unit 88 stores each acquired pixel value in the dark in a storage unit provided inside the black shading correction unit 88 or outside the black shading correction unit 88. Then, the black shading correction unit 88 enters the light into the photoelectric conversion element 21 to read a document or the like, and from each pixel value at the time of light that is image data obtained at the time of reading, the dark time stored in the storage unit Each pixel value is subtracted.

  As a result, a circuit offset component, which is fixed pattern noise (FPN), can be removed from the imaging data, and the S / N ratio (Signal to Noise Ratio) of the imaging data can be improved.

  In addition, each dark pixel value stored in the storage unit for black shading correction can be used for the above-described white defect pixel determination process by the noise detection unit 72 in the subsequent stage. Therefore, when the black shading correction unit 88 is provided in the image reading unit 52, the noise detection memory 81 can be omitted. In other words, the storage unit for black shading correction and the storage unit for noise detection can be used together, and the number of parts can be reduced and the configuration can be simplified.

(Judgment result storage operation)
Next, the operation of storing the determination result from the determination unit 83 will be described. FIGS. 14A and 14B are histograms that are the determination results of defective pixels when the number of pixels of the RGB color photoelectric conversion elements 21 that are, for example, linear sensors is 20 pixels. is there. Among these, FIG. 14A is a histogram that is the N-1th determination result, and FIG. 14B is a histogram that is the Nth determination result. “N” is a natural number of 2 or more. In FIG. 14A and FIG. 14B, the hatched pattern histogram is the R pixel histogram, the dotted line pattern histogram is the G pixel histogram, and the white histogram is the B pixel histogram. It is a histogram of a pixel.

  In the example of FIGS. 14A and 14B, for example, a pixel having a pixel value equal to or greater than the first threshold Dth = 8 is determined as a defective pixel. In other words, the R pixel of the first pixel has a pixel value equal to or greater than the first threshold value Dth, which is the above-mentioned “pixel value St2x of accumulation time t2−pixel value St1x of accumulation time t1”. In this case, the determination unit 83 determines each RGB pixel of the first pixel as a defective pixel. If at least one of RGB is determined to be a defective pixel, the defective pixel correction process can be performed with high accuracy by determining three pixels of RGB as defective pixels. Similarly, in the fourth pixel G, the value of “pixel value St2x of accumulation time t2−pixel value St1x of accumulation time t1” described above is a pixel value equal to or greater than the first threshold value Dth. For this reason, the determination unit 83 determines each of the fourth RGB pixels as a defective pixel.

  The determination unit 83 stores, for example, determination information for six pixels among the determination information indicating the determination result of each pixel in the storage unit such as the RAM 43 illustrated in FIG. 3, the HDD 44, or the memory 81 illustrated in FIG. 11. . FIG. 15A to FIG. 15D are schematic diagrams of determination information stored in the storage unit. Among these, FIG. 15A is a schematic diagram of determination information that is an N-1th determination result, and FIGS. 15B to 15D are schematic diagrams of determination information that is an Nth determination result. FIG. N is a natural number of “2” or more.

  As shown in FIG. 15A, the determination unit 83 stores rank (priority order), pixel position, noise amount of each RGB pixel, and maximum value of noise amount of each RGB pixel, and determination information. Store in the department. The example of FIG. 15A is an example in which the determination information of each pixel is stored in descending order of the noise amount. Specifically, in the case of the example of FIG. 15A, it is determined that each pixel at pixel positions 13, 7, 19, 4, 17, and 11 has a larger noise amount in order. The pixels at the 13 pixel positions with the rank “1” have RGB noise amounts of 7, 13, and 7, respectively. Among these, the noise amount “13” is the maximum noise amount. Similarly, the pixels at 19 pixel positions with a rank of “3” have RGB noise amounts of 11, 6, and 6, respectively. Among these, the noise amount “11” is the maximum noise amount.

  FIG. 15B shows determination information that is an Nth determination result in an example in which the determination information of each pixel is stored in descending order of the noise amount. That is, the determination information in FIG. 15B is determination information obtained by the determination unit 83 performing determination again after the determination information illustrated in FIG. 15A is obtained. The determination unit 83 stores a maximum of six pieces of determination information as an example in the storage unit. Then, the determination information stored in the storage unit is rewritten according to the determination information obtained later.

  Specifically, in the case of the noise amount priority example shown in FIG. 15B, as can be seen from the comparison with FIG. 15A, the determination information corresponding to the rank “6” is RGB at 10 pixel positions. When new determination information with each noise amount of 8, 8, and 9 is obtained, the determination information of rank “6” is rewritten to the determination information of the ten pixel positions.

  Note that the RGB noise amounts of rank “6” shown in FIG. 15A are 8, 8, and 9, whereas the determination information of rank “6” is newly added as shown in FIG. Each of the RGB noise amounts of the determination information of the ten pixel positions to be rewritten is 8, 8, and 9 which is the same as the determination information of the eleven pixel positions.

  Here, the user of the MFP often places an imaging target such as a document at the center of the document table. For this reason, when the noise amount is the same, the determination unit 83 selects the determination information of the pixel position closer to the center pixel of the photoelectric conversion element 21 and stores (rewrites) the determination information in the storage unit. Accordingly, more effective determination information can be stored in the storage unit and used for the defective pixel correction process.

  Further, the above-described example is a storage example of determination information in the case of noise amount priority. However, when the adjacent pixels are both defective pixels, such as the pixels at the pixel positions 1 and 2 shown in FIG. 14B, the influence on the image quality is great. For this reason, as shown in FIG.15 (c), the determination part 83 may rank and memorize | store the determination information of an adjacent defective pixel in a memory | storage part. The example of FIG. 15C is an example in which the determination information of 10 adjacent pixel positions and 11 pixel positions, and the determination information of 2 adjacent pixel positions and 1 pixel position are preferentially stored. It is.

  Even if the noise level is equal to or less than that, if the plurality of color channels are defective pixels, the influence on the image quality is large. Therefore, as shown in FIG. 15D, the determination unit 83 may rank the determination information of defective pixels in a plurality of color channels and store them in the storage unit. The example of FIG. 15D is an example in which the determination information of the pixels at the respective pixel positions 15, 13, 7, 19, 4, and 17 is preferentially stored as pixels of a color channel having a plurality of defective pixels. It is.

  The ranking may be performed by comprehensively considering the above-described defective pixels having a large amount of noise, adjacent defective pixels, and defective pixels of a plurality of color channels.

(Detection processing operation of defective pixels at startup)
Next, the MFP according to the embodiment detects such defective pixels when the MFP is activated. The flowchart of FIG. 16 shows a flow of operations for detecting defective pixels at the time of activation. First, when the main power of the MFP is turned on, the CPU 41 shown in FIG. 3 applies a drive voltage (ON) to the photoelectric conversion element 21 for warm-up (step S1).

  In the first carriage 18 and the second carriage 24 of the MFP shown in FIG. 2, for example, a standby position that is a position at the time of document conveyance reading is set. For this reason, the CPU 41 performs homing control to move the first carriage 18 and the second carriage 24 to the standby position after applying the driving voltage to the photoelectric conversion element 21 (step S2). A state where the first carriage 18 and the second carriage 24 are waiting at the standby position is an example of reading standby.

  Next, the CPU 41 controls the movement of the first carriage 18 below the reference white plate 23 for adjustment (step S3). In order to prevent light from leaking outside the MFP, the CPU 41 moves the first carriage 18 below the reference color plate 23 and then drives the light source 16 to turn on (step S4). The light source 16 is driven to turn on, and the reference color plate 23 is used to perform adjustment operations of each part such as light amount adjustment of the light source 16 and gain adjustment of the PGA 62 shown in FIG. 4 (step S5), and then the light source 16 is turned off. (Step S6).

  Next, after turning off the light source 16, the CPU 41 checks whether there is any abnormality in the black level (step S7), and after checking the black level, performs the above-described defective pixel detection process (step S8).

  Finally, the CPU 41 performs the above-described homing control (step S9), and the activation process of the flowchart of FIG. 16 ends.

  The defective pixel has temperature dependency, and the detection accuracy increases as the temperature of the photoelectric conversion element 21 increases. Therefore, the defective pixel can be detected with high detection accuracy by performing the defective pixel detection process at the end of the MFP activation process shown in the flowchart of FIG. Further, in step S8, the difference in the amount of noise between pixels can be reduced by acquiring the pixel value with the longer accumulation time among the two dark pixel values with different accumulation times acquired for detecting defective pixels. Since it becomes large, a defective pixel can be detected with high accuracy.

  Further, if the first carriage 18 is positioned below the reference white plate 23 or the pressure plate (or ADF) is closed and the light source 16 is not turned off, data at the dark time cannot be obtained. For this reason, defective pixel detection is performed during the start-up process of the MFP that controls to move the first carriage 18 below the reference white plate 23. Thereby, the detection process of a defective pixel can be performed irrespective of the open / closed state of the pressure plate (or ADF). Further, the homing control for moving the first carriage 18 can be prevented every time defective pixel detection processing is performed, and the waiting time of the user until the document can be read can be shortened.

  Further, when the light source 16 is driven to be turned off and turned on, it takes time until the light source 16 is stabilized. For this reason, it is desirable that the number of times the light source 16 is turned off and turned on is small. Therefore, the defective pixel detection process (step S8) is performed together with the black level confirmation process (step S7) when the light source 16 is turned off during the MFP activation process. As a result, it is possible to prevent the disadvantage that the number of times the light source 16 is turned off and turned on due to the defective pixel detection process increases.

  As illustrated in FIG. 13, when the image reading unit 52 includes the shading correction unit 88, the noise detection unit 72 performs the above-described step S <b> 8 and uses the above-described black shading correction processing. Get the pixel value in the dark.

(Detection operation for defective pixels when reading a document using a pressure plate)
FIG. 17 is a flowchart for explaining the defective pixel detection processing operation at the time of document reading using the pressure plate. In this case, the defective pixel is detected after the image data of the document is acquired and the light source 16 is turned off.

  After the original is read using the pressure plate, there is a timing at which light does not enter the photoelectric conversion element 21 due to light shielding by the pressure plate or the reference white plate 23, and it is not necessary to individually create a state in which light does not enter the photoelectric conversion element 21. Further, the temperature of the photoelectric conversion element 21 is higher because the photoelectric conversion element 21 is driven for a longer time after the image data acquisition than in the start-up process described using the flowchart of FIG. A highly accurate defective pixel detection process can be performed.

  That is, in the flowchart of FIG. 17, when reading of a document is started using a pressure plate, the CPU 41 applies a drive voltage (ON) to the photoelectric conversion element 21 for warm-up (step S11), and the light source 16 Is driven to light (step S12).

  Next, the CPU 41 moves the first carriage 18 in the direction A shown in FIG. 2 to acquire document image data (step S13, step S14), and controls the light source 16 to be turned off (step S15). Next, the CPU 41 determines whether or not the pressure plate is closed (step S16).

  When the pressure plate is closed (step S16: Yes), the CPU 41 performs the above-described defective pixel detection process (step S17), and performs the defective pixel correction process of the image data detected in step S14 (step S18). . A specific example of the defective pixel correction process will be described later. And CPU41 performs the above-mentioned homing control by step S19, and complete | finishes the process of the flowchart of FIG.

  On the other hand, when the pressure plate is not closed (step S16: No), the CPU 41 performs the above-described homing control in step S20. In step S <b> 21, the CPU 41 determines whether or not the first carriage 18 is positioned below the reference white plate 23 by the homing control described above.

  When it is determined that the first carriage 18 is positioned below the reference white plate 23 (step S21: Yes), the CPU 41 performs the above-described defective pixel detection process in step S22. Then, the defective pixel of the image data detected in step S14 is corrected (step S23), and the process of the flowchart in FIG.

  When the pressure plate is closed, the photoelectric conversion element 21 is in a light-shielding state, so that defective pixels can be detected immediately after the light source 16 is controlled to be turned off. At this time, the difference in the amount of noise between pixels is increased by making the longer accumulation time longer than normal out of two dark image data with different accumulation times acquired for defective pixel detection. As described above, defective pixels can be detected well.

  Further, it is preferable that the defective pixel correction process (step S18, step S23) is performed using the latest determination information of the defective pixel. Therefore, the defective pixel correction process (step S18, step S23) is executed after the defective pixel detection process (step S17, step S22). In addition, immediately after performing homing control (step S20), you may perform the correction process of a defective pixel.

(Detection processing operation of defective pixels at the time of document reading using ADF)
FIG. 18 is a flowchart for explaining the defective pixel detection processing operation at the time of document reading using ADF3. In the flowchart of FIG. 18, when reading of a document using the ADF 3 is started, the CPU 41 applies a drive voltage (ON) to the photoelectric conversion element 21 for warm-up (step S31) and turns on the light source 16. The lighting is driven (step S32).

  Next, the CPU 41 starts conveyance control of the document set on the ADF 3 (step S33), and acquires image data of the document (step S34). Then, the CPU 41 uses the defective pixel determination information detected when the MFP described with reference to the flowchart of FIG. 16 is activated or when reading the document using the pressure plate described with reference to the flowchart of FIG. Correction processing of defective pixels of the acquired image data in S34 is performed (step S35).

  Next, in step S36, the CPU 41 determines whether or not reading of all the originals set on the ADF 3 has been completed. When it is determined that all the originals have been read (step S36: Yes), the CPU 41 controls the light source 16 to be turned off (step S37), performs the above-described defective pixel detection process (step S38), and FIG. The process of the flowchart ends.

  On the other hand, when it is determined that reading of all originals has not been completed (step S36: No), the CPU 41 controls the light source 16 to be turned off (step S39), and performs the above-described defective pixel detection process (step S39). Step S40). That is, when the CPU 41 continuously reads a document using the ADF 3, the CPU 41 performs a defective pixel detection process while reading the document and the next document. Thereby, the determination information of the defective pixel can be updated in real time, and the defective pixel correction process can be performed in accordance with the state. When the defective pixel detection process is completed, the CPU 41 turns on the light source 16 and returns the process to step S34.

  The defective pixel detection process in step S40 takes time until the light quantity is stabilized after the light source 16 is turned on, and the total reading time may be increased. It may be done every time.

  When reading an original using the ADF 3, the ADF 3 should be closed at the end of reading. For this reason, in step S37, as in reading a document using a pressure plate, of the two dark pixel values having different accumulation times acquired for detecting defective pixels, the longer accumulation time is set longer than usual. You may get it.

(Defect pixel correction processing operation)
Next, a defective pixel correction processing operation performed using the determination information acquired as described above will be described. FIG. 19 is a detailed block diagram of the defective pixel correction unit 73 of the MFP 1. As shown in FIG. 19, the defective pixel correction unit 73 includes a correlation degree calculation unit 101 and a replacement processing unit 102 together with the memory 100.

  As an example, as illustrated in FIG. 20, the defective pixel correction unit 73 uses 16 pixels before and after the defective pixel determined by the determination unit 83 and 3 lines before and after as pixels to be interpolation candidates as a search area (target pattern). Explore. Further, the defective pixel correction unit 73 sets a region corresponding to three lines formed by the defective pixel, two pixels before and after the defective pixel, and two pixels above and below the two pixels before and after the defective pixel as a template. Then, as shown in FIG. 20, the defective pixel correction unit 73 compares the target pattern (image data of 16 pixels before and after the defective pixel and 3 lines before and after) while moving the template pixel by pixel. The degree of correlation between each part and the template is calculated.

  For example, a SAD (Sum of Absolute Difference) value is used as the degree of correlation. For example, in the case of the example illustrated in FIG. 20, the defective pixel correction unit 73 calculates 24 SAD values based on the following mathematical formula (for example, when SAD values S1 to S24 are obtained). The defective pixel correction unit 73 then replaces the defective pixel with the central pixel of the portion of the target pattern for which the smallest SAD value (= maximum correlation) is calculated as a replacement pixel.

  In the following formula and FIG. 20, “S” means an SAD value. “S1” indicates the first SAD value, and S24 indicates the 24th SAD value. “P” means a pixel of the target pattern. “P11” means the first pixel of the first line of the target pattern, and “P321” means the 32nd pixel of the first line of the target pattern. Similarly, “P13” means the first pixel on the third line of the target pattern, and “P323” means the 32nd pixel on the third line of the target pattern. “T” means a pixel of the template. “T11” means the first pixel on the first line of the template, and “T41” means the fourth pixel on the first line of the template. Similarly, “P13” means the first pixel on the third line of the target pattern, and “P43” means the fourth pixel on the third line of the target pattern.

  S1 = (P11−T11) + (P12−T12) + (P13−T13) + (P21−T21) + (P22−T22) + (P23−T23) + (P31−T31) + (P32−T32) + (P33−T33)... S24 = (P281−T11) + (P282−T12) + (P283−T13) + (P291−T21) + (P292−T22) + (P293−T23) + (P301−T31) ) + (P302-T32) + (P303-T33)

  The defective pixel correction unit 73 calculates SAD values S <b> 1 to S <b> 24 by performing arithmetic processing based on such a mathematical expression. For example, when the value of the SAD value S22 is the minimum value, the pixel of P282 that is the central pixel of the portion of the target pattern for which the minimum value is calculated is used as a replacement pixel and replaced with a defective pixel.

  FIG. 21 is a flowchart showing the flow of such an interpolation processing operation. In step S51, the correlation calculation unit 101 shown in FIG. 19 sets “1” as the SAD value calculation count in the counter (n = 1: n is the calculation count). That is, in the case of the example shown in FIG. 20, the correlation degree calculation unit 101 calculates a total of 24 SAD values. For this reason, in step S51, the correlation degree calculation unit 101 first sets “1” in the counter, which indicates the first calculation operation of the SAD value. Each time the degree of correlation calculation unit 101 completes calculation of one SAD value, the value of the counter is incremented by one, such as “1” → “2” → “3”. Increment. The increment timing is a timing at which the template is moved by one pixel with respect to the target pattern.

  Next, in step S52, the correlation degree calculation unit 101 calculates an SAD value (Sn: S is an SAD value, and n is a number from 1 to 24 of the calculated SAD value) based on the above formula. To do. In step S <b> 53, the correlation degree calculation unit 101 determines whether or not the number of calculations is 24 (n = 24: n is the number of calculations). As described above, the correlation degree calculation unit 101 performs the above calculation while moving the template one by one with respect to the target pattern. For this reason, when the number of calculations is less than 24 (step S53: No), the process proceeds to step S55, the number of calculations is incremented by one (n = n + 1), and the process returns to step S52. In step S52, the SAD value is calculated again.

  When 24 SAD values are calculated in this way (step S53: Yes), the process proceeds to step S54, and the replacement processing unit 102 shown in FIG. 19 calculates the minimum value among the 24 SAD values. The central pixel of the pattern portion is used as a replacement pixel and replaced with a defective pixel (interpolation processing), and the processing of the flowchart in FIG.

  Finally, the above-described embodiment has been presented as an example, and is not intended to limit the scope of the present invention. The novel embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. Further, the embodiments and modifications of the embodiments are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

1 MFP (MFP) reader 2 MFP body 3 Automatic document feeder (ADF)
4 Scanner mechanism 21 Photoelectric conversion element 41 CPU
60 Timing Signal Generation Unit 61 Pixel Signal Generation Circuit 62 Amplifier (PGA)
63 Analog-to-digital converter (ADC)
64 Parallel / serial converter 64
71 Shading unit 72 Noise detection unit 73 Defective pixel correction unit 81 Memory 82 Subtractor 83 Determination unit 88 Black shading correction unit

JP, 2014-110622, A

Claims (17)

A photoelectric conversion element that generates an image signal corresponding to the amount of received light;
Obtaining a pixel value in the dark which is a pixel value obtained in a state where light is not incident on the photoelectric conversion element at least in a first time and a second time longer than the first time; A noise detection unit that detects a noise amount in the dark time by taking a difference between each pixel value in the dark time acquired in the first time and each pixel value in the dark time acquired in the second time;
And a determination unit that determines a pixel having the noise amount equal to or greater than a predetermined first threshold as a defective pixel. The image processing apparatus further includes a correction unit that corrects a pixel value of a pixel determined as the defective pixel among pixel values of each pixel of the photoelectric conversion element to a pixel value of a normal pixel and outputs the corrected pixel value. Item 2. The photoelectric conversion device according to Item 1. The photoelectric conversion device according to claim 1, further comprising: a light shielding portion that shields light from entering the photoelectric conversion element. As the photoelectric conversion element, a photoelectric conversion element for a predetermined plurality of colors,
When the determination unit determines at least one pixel of the photoelectric conversion element for each color as a defective pixel, the determination unit corresponds to the same pixel position as the pixel determined as the defective pixel. The photoelectric conversion device according to any one of claims 1 to 3, wherein pixels of photoelectric conversion elements for other colors are also determined as defective pixels. When the pixels of the photoelectric conversion elements for the respective colors corresponding to the same pixel position are each less than the first threshold value, the determination unit determines the pixel of the photoelectric conversion element for the respective colors corresponding to the same pixel position. An integrated value of pixel values is calculated, and when the integrated value is equal to or greater than a predetermined second threshold, the pixels of the photoelectric conversion elements for the respective colors corresponding to the same pixel position are determined as defective pixels, respectively. The photoelectric conversion device according to claim 4. By subtracting each dark pixel value stored in the storage unit from each pixel value at the time of imaging obtained by making light incident on the photoelectric conversion element, shading correction of each pixel value at the time of imaging is performed. A black shading correction unit that performs processing;
The noise detection unit detects the amount of noise using each dark pixel value stored in the storage unit as each dark pixel value acquired at the second time or the first time. The photoelectric conversion device according to any one of claims 1 to 5, wherein the photoelectric conversion device is performed. The determination unit stores determination information regarding the pixel determined as the defective pixel in a storage unit according to a predetermined priority, and rewrites each stored determination information according to the priority. The photoelectric conversion apparatus as described in any one of Claims 6. The photoelectric conversion apparatus according to claim 7, wherein the determination unit stores the determination information of the defective pixels in the storage unit in descending order of noise amount. The photoelectric conversion device according to claim 7 or 8, wherein the determination unit stores the determination information of adjacent defective pixels in the storage unit in a predetermined high order. As the photoelectric conversion element, a photoelectric conversion element for a predetermined plurality of colors,
The determination unit stores, in the storage unit, the determination information of a color channel having a large number of defective pixels among the color channels in which a predetermined number of pixels for each color are collected. The photoelectric conversion device according to claim 8 or 9. The determination unit is configured to detect the defective pixel in one or both of after a start process of a device to which the photoelectric conversion device is applied or when the photoelectric conversion element is read and the dark state is set. The photoelectric conversion device according to any one of claims 1 to 10, wherein determination is performed. 12. The determination unit according to claim 1, wherein the determination unit determines the defective pixel every time reading of the photoelectric conversion element is completed once or every time reading of a plurality of times is completed. A photoelectric conversion device according to claim 1. The noise detection unit acquires an order of pixel value acquisition in the dark for detecting the amount of noise at the second time after acquiring at the first time. The photoelectric conversion device according to any one of Items 1 to 12. The photoelectric conversion apparatus according to any one of claims 2 to 13, wherein the correction unit executes the correction processing when the photoelectric conversion element is on standby for reading. The correction unit arranges a template having a partial size of the target pattern from an end portion of the target pattern on a target pattern including a pixel column including the defective pixel and one or a plurality of pixel columns before and after. The degree of correlation between the target pattern and the template is calculated while moving at least one pixel along the direction, and the defective pixel is obtained by using the central pixel of the part of the target pattern for which the maximum degree of correlation is calculated as a replacement pixel. The photoelectric conversion device according to any one of claims 2 to 14, wherein the photoelectric conversion device is subjected to an interpolation process. The noise detection unit has a pixel value in the dark which is a pixel value obtained in a state where light is not incident on a photoelectric conversion element that generates an image signal corresponding to the amount of received light, at least for the first time, and An acquisition step of acquiring each in a second time longer than the first time;
Noise detection for detecting the amount of noise in the dark time by taking the difference between the pixel values in the dark time acquired in the second time and the pixel values in the dark time acquired in the first time Steps,
A determination step of determining a pixel having the noise amount equal to or greater than a predetermined first threshold as a defective pixel. An image forming apparatus that irradiates light on a document placed on a placement table, receives reflected light with a photoelectric conversion element, and reads the document,
Obtaining at least a first time and a second time longer than the first time, a pixel value in the dark which is a pixel value obtained in a state where light is not incident on the photoelectric conversion element;
Noise detection for detecting the amount of noise in the dark time by taking the difference between the pixel values in the dark time acquired in the second time and the pixel values in the dark time acquired in the first time And
A determination unit that determines a pixel having the noise amount equal to or greater than a predetermined first threshold as a defective pixel;
An image forming apparatus comprising: a correction unit that corrects a pixel value of a pixel determined as the defective pixel among pixel values of each pixel of the photoelectric conversion element to a pixel value of a normal pixel and outputs the corrected pixel value. apparatus.

Images