JP6493500B2 - Image sensor, image reading apparatus, image forming apparatus, and image reading method - Google Patents

Description

  The present invention relates to an image sensor, an image reading apparatus, an image forming apparatus, and an image reading method.

  For example, a CMOS image sensor is used in an image reading apparatus that reads a document image by irradiating a document with a light source such as an LED and converting reflected light from the document into an image signal with an image sensor. Here, fixed pattern noise generated due to pixel-to-pixel variations is known as a factor that degrades an image read by a CMOS image sensor. When there is fixed pattern noise, vertical stripes occur in the read image.

  As variations among pixels in a CMOS image sensor, for example, variations in dark current of a floating diffusion (FD) that is a charge detection unit, variations in offset voltage of a pixel transistor, and dark current of a photodiode (even when light is not exposed) Current). As a method for correcting fixed pattern noise, a correlated double sampling (CDS) technique is known in which a black level is corrected by taking a difference between an optical signal level and a reference level due to dark current or the like.

  Further, in Patent Document 1, a pixel pulse is reset by outputting a substrate pulse from a timing generator immediately before reading out a pixel signal, and then an operation pulse is immediately set up in a state where there is no signal due to light incidence. By reading out the pixel signal through the operation switch and holding it in the capacitor, and then outputting a horizontal scanning pulse from the horizontal shift register in the horizontal video period to sequentially turn on the horizontal switch, and sequentially outputting the capacitor signal, A solid-state imaging device that obtains a reference signal for correcting a fixed pattern noise in a shape is disclosed.

  However, in the conventional area sensor, R, G, and B spectral filters for each photodiode are attached by a Bayer array method or the like. Therefore, since the conventional area sensor cannot read data of a plurality of colors at the same pixel position, the data of a plurality of colors read at different positions are combined as data of one pixel by image processing. The conventional area sensor cannot process the data of a plurality of colors synthesized for the same pixel position by the same black correction circuit. There was a problem that a correction difference occurred.

  The present invention has been made in view of the above, and an imaging device, an image reading device, and an image reading device that can correct a black level without generating a correction difference between colors for substantially the same pixel position. An object is to provide an image forming apparatus and an image reading method.

In order to solve the above-described problems and achieve the object, the present invention arranges the reflected light of each color from a common pixel position in the reading target for each color by being arranged in one direction for each color to receive light. A plurality of photoelectric conversion elements that are capable of photoelectric conversion in units of pixels, and the photoelectric conversion elements of the pixels of each color that photoelectrically convert reflected light from the common pixel position for each color in the plurality of photoelectric conversion elements. and a pixel group, for each of the pixel groups, the first output result wherein an output result of the photoelectric conversion element is obtained by photoelectrically converting the reflected light of each color of the pixel in each pixel group, the first output result identical to the A black correction unit that corrects a black level of a reading target by subtracting a second output result obtained as an output result from the pixel, which is obtained as an output independent of the reflected light.

  According to the present invention, it is possible to perform black level correction without causing a correction difference between colors for substantially the same pixel position.

FIG. 1 is a configuration diagram illustrating a configuration example of an image forming apparatus according to an embodiment. FIG. 2 is a configuration diagram illustrating a configuration example of the image reading apparatus and the ADF. FIG. 3 is a block diagram illustrating the configuration of the CMOS image sensor and its periphery. FIG. 4 is a block diagram showing an outline of the conversion processing unit. FIG. 5 is a diagram illustrating details of the transfer unit and the PD. FIG. 6 is a diagram illustrating a configuration example of the black correction unit. FIG. 7 is a timing chart showing the operation when the CMOS image sensor corrects the black level. FIG. 8 is a diagram schematically showing black level correction performed by the CMOS image sensor. FIG. 9 is a block diagram illustrating an outline of a first modification of the conversion processing unit. FIG. 10 is a block diagram illustrating an outline of a second modification of the conversion processing unit. FIG. 11 is a block diagram illustrating an outline of a third modification of the conversion processing unit. FIG. 12 is a diagram illustrating a configuration example of the black correction unit. FIG. 13 is a timing chart showing an operation when the image reading apparatus corrects the black level. FIG. 14 is a block diagram illustrating an outline of a fourth modification of the conversion processing unit. FIG. 15 is a diagram schematically illustrating black level correction performed by an image reading apparatus having a CMOS image sensor including a conversion processing unit. FIG. 16 is a block diagram showing an outline of a modification of the CMOS image sensor. FIG. 17 is a flowchart illustrating an operation in which the image reading apparatus performs gain adjustment. FIG. 18 is a block diagram illustrating an outline of a modification of the second black correction unit.

(Embodiment)
An image forming apparatus according to an embodiment will be described below with reference to the accompanying drawings. FIG. 1 is a configuration diagram illustrating a configuration example of an image forming apparatus 300 according to the embodiment. The image forming apparatus 300 is a digital copier that includes a paper feeding unit 303 and an image forming apparatus main body 304 and has an image reading apparatus 100 and an automatic document feeder (ADF) 200 mounted thereon.

  In the image forming apparatus main body 304, a tandem image forming unit 305, a registration roller 308 that conveys recording paper supplied from the paper supply unit 303 via the conveyance path 307 to the image forming unit 305, and an optical writing device 309, a fixing conveyance unit 310, and a double-sided tray 311 are provided.

  In the image forming unit 305, four photosensitive drums 312 are arranged side by side corresponding to toners of four colors Y, M, C, and K. Around each photosensitive drum 312, image forming elements including a charger, a developing unit 306, a transfer unit, a cleaner, and a static eliminator are arranged.

  In addition, an intermediate transfer belt 313 stretched between the driving roller and the driven roller is disposed between the transfer unit and the photosensitive drum 312 while being sandwiched between the two nips.

  The tandem image forming apparatus 300 configured as described above performs optical writing on the photosensitive drum 312 corresponding to each color for each of Y, M, C, and K colors, and develops each toner of each color by the developing unit 306. Then, primary transfer is performed on the intermediate transfer belt 313 in the order of, for example, Y, M, C, and K.

  Then, the image forming apparatus 300 forms a full-color image on the recording paper by secondarily transferring the full-color image superimposed on the four colors by the primary transfer onto the recording paper, and then fixing and discharging. The image forming apparatus 300 forms an image read by the image reading apparatus 100 on a recording sheet.

  FIG. 2 is a configuration diagram illustrating a configuration example of the image reading apparatus 100 and the ADF 200. The image reading apparatus 100 is a scanner device mounted on an image forming apparatus such as a digital copying machine, a digital multifunction peripheral, or a facsimile machine. Further, the image reading apparatus 100 may be a single scanner apparatus. Then, the image reading apparatus 100 illuminates a document that is a subject (reading target) with irradiation light from a light source, processes the reflected light from the document with a CMOS image sensor, and processes image data of the document. read.

  Specifically, as shown in FIG. 2, the image reading apparatus 100 includes a contact glass 101 on which a document is placed, a first carriage 106 including a light source 102 for document exposure and a first reflection mirror 103, a first carriage 106. And a second carriage 107 having a second reflecting mirror 104 and a third reflecting mirror 105. The image reading apparatus 100 includes a CMOS image sensor (imaging device) 109, a lens unit 108 for forming an image on the CMOS image sensor 109, and a reference white plate used for correcting various distortions caused by a reading optical system. A (white reference plate) 110 and a sheet through reading slit 111 are also provided.

  The image reading apparatus 100 has an ADF 200 mounted thereon, and is connected via a hinge (not shown) or the like so that the ADF 200 can be opened and closed with respect to the contact glass 101.

  The ADF 200 includes a document tray 201 as a document placing table on which a document bundle composed of a plurality of documents can be placed. The ADF 200 also includes separation / feeding means including a feeding roller 202 that separates documents one by one from a bundle of documents placed on the document tray 201 and automatically feeds them toward the sheet-through reading slit 111. ing.

  The image reading apparatus 100 scans the image surface of the document to scan the image of the document and scans the image of the document by the first carriage 106 and the second carriage 107 by a stepping motor (not shown) in the direction indicated by the arrow A (secondary The document is scanned in the scanning direction. At this time, the second carriage 107 moves at a speed half that of the first carriage 106 in order to keep the optical path length from the contact glass 101 to the CMOS image sensor 109 constant.

  At the same time, the image surface which is the lower surface of the document set on the contact glass 101 is illuminated (exposed) by the light source 102 of the first carriage 106. Then, the reflected light image from the image plane is transferred to the CMOS image sensor 109 via the first reflecting mirror 103 of the first carriage 106, the second reflecting mirror 104 and the third reflecting mirror 105 of the second carriage 107, and the lens unit 108. Images are sent sequentially.

  Then, a signal is output by photoelectric conversion of the CMOS image sensor 109, and the output signal is converted into a digital signal. In this way, the image of the document is read, and digital image data is obtained.

  On the other hand, in the sheet-through mode in which the document is automatically fed and the image of the document is read, the first carriage 106 and the second carriage 107 move below the sheet-through reading slit 111. Thereafter, the document placed on the document tray 201 is automatically fed in the arrow B direction (sub-scanning direction) by the feeding roller 202, and the document is scanned at the position of the sheet through reading slit 111.

  At this time, the lower surface (image surface) of the automatically fed document is illuminated by the light source 102 of the first carriage 106. Therefore, the reflected light image from the image plane is sent to the CMOS image sensor 109 via the first reflecting mirror 103 of the first carriage 106, the second reflecting mirror 104 and the third reflecting mirror 105 of the second carriage 107, and the lens unit 108. Images are sent sequentially. Then, a signal is output by photoelectric conversion of the CMOS image sensor 109, and the output signal is converted into a digital signal. In this way, the image of the document is read, and digital image data is obtained. The document whose image has been read is discharged to a discharge port (not shown).

  The reflected light from the reference white plate 110 is converted into an analog signal by the CMOS image sensor 109 and then converted into a digital signal by illumination by the light source 102 started before reading an image in the scan mode or the sheet through mode. . In this way, the reference white plate 110 is read, and shading correction at the time of reading the image of the document is performed based on the reading result (digital signal).

  Further, when the ADF 200 is provided with a conveyance belt, the document can be automatically fed to the reading position on the contact glass 101 by the ADF 200 and the image of the document can be read even in the scan mode.

  Next, the CMOS image sensor 109 and its periphery will be described in detail. FIG. 3 is a block diagram illustrating the configuration of the CMOS image sensor 109 and its periphery. As shown in FIG. 3, a lighting circuit 40 and a control unit 42 are provided around the CMOS image sensor 109.

  The lighting circuit 40 includes a light source 102 such as an LED, and lights the light source 102 illustrated in FIG. 2 in accordance with the control of the control unit 42. The control unit 42 controls each part of the image reading apparatus 100, and includes a CPU (central processing unit) (not shown), a ROM that stores a program executed by the CPU, and a RAM that temporarily stores data. I have.

  The CMOS image sensor 109 includes a timing generator (TG) 50, a conversion processing unit 52a, and an interface (I / F) unit 54. The timing generator 50 generates a timing signal for setting operation timings of the lighting circuit 40, the conversion processing unit 52a, and the I / F unit 54. Various timings are set by control signals from the control unit 42.

  The lighting circuit 40 turns on the light source 102 according to the lighting timing signal from the timing generator 50 when the lighting ON signal from the control unit 42 is asserted. The lighting circuit 40 turns on the light source 102 when reading the document, so that the reflected light from the document enters the CMOS image sensor 109.

  The conversion processing unit 52 a includes a photoelectric conversion element such as a photodiode (PD), which will be described later. The conversion processing unit 52 a accumulates charges corresponding to reflected light from the document, and sends a signal corresponding to the charges to the I / F unit 54. Output.

  The I / F unit 54 transmits an image signal of each pixel (described later with reference to FIG. 4) to a subsequent image processing unit (not shown). The image processing unit performs shading correction for normalizing the image data of the document based on the image data read from the reference white board, and performs main-scanning direction such as sensitivity variation of the CMOS image sensor 109 and uneven light distribution of the optical system. Correct variations.

  FIG. 4 is a block diagram showing an outline of the conversion processing unit 52a. As shown in FIG. 4, the conversion processing unit 52a includes photodiodes PD1 (R) to PDn (R) provided with filters that receive R light, and photodiodes PD1 provided with filters that receive G light. Photodiodes PD1 (B) to PDn (B) each provided with a filter for receiving (G) to PDn (G) and B light are arranged in one direction for each color. Specifically, n pixel PDs are arranged in the main scanning direction for each of RGB colors. In addition, each pixel is provided with a transfer unit 520 that transfers, as a current, the charge accumulated by the photodiode (PD) through photoelectric conversion.

  Also, pixels of each color having the same position in the main scanning direction are grouped as one pixel group (column), and are subjected to black correction (black level correction: see FIG. 8) by a common black correction unit 522. It is comprised so that. For example, PD1 (R), PD1 (G), and PD1 (B) are combined into one column. The black correction unit 522-1 sequentially performs black correction on the signals photoelectrically converted by the PD1 (R), PD1 (G), and PD1 (B). That is, the conversion processing unit 52a is configured such that n pixels for each color are arranged in one direction, and the n black correction units 522 perform black correction. It should be noted that a plurality of substantially identical functional configurations such as PD1 (R) to PDn (R) and black correction units 522-1 to 522-n are simply PDs if they are not specified. And a black correction unit 522 and the like.

  As described above, in the conversion processing unit 52a, the common black correction unit 522 performs black correction for each column including pixels of the respective colors whose positions in the main scanning direction are the same (substantially the same). Therefore, the conversion processing unit 52a can correct the black level for the same pixel position in the reading target without generating a correction difference between colors caused by variations in the black correction units 522. ing. Each color pixel having the same position in the main scanning direction outputs a signal synthesized to indicate the color of the common (same) pixel position in the reading target.

  FIG. 5 is a diagram illustrating details of the transfer unit 520 and the PD. The transfer unit 520 includes a transfer transistor, an amplification transistor, and the like. When the <tx> signal is asserted, the transfer unit 520 outputs a signal photoelectrically converted by the PD to the black correction unit 522. Further, when the <rx> signal is asserted, the transfer unit 520 outputs the reference signal level to the black correction unit 522. Further, when the tx signal and the rx signal are simultaneously asserted, the transfer unit 520 resets the signal accumulated in the PD.

  The transfer unit 520 performs signal transfer at the same timing for each column. The timing signals for the R pixel are rx_r and tx_r, the timing signals for the G pixel are rx_g and tx_g, and the timing signals for the B pixel are rx_b and tx_b.

  FIG. 6 is a diagram illustrating a configuration example of the black correction unit 522. The black correction unit 522 includes an optical signal level holding unit 530, a black level holding unit 532, and a subtraction unit 534. The optical signal level holding unit 530 holds the optical signal level photoelectrically converted by the PD. The black level holding unit 532 holds a black level that is a reference level. The subtraction unit 534 calculates a difference between the optical signal level and the black level, and outputs the calculated difference to the I / F unit 54 as a corrected black level.

  FIG. 7 is a timing chart showing the operation when the CMOS image sensor 109 corrects the black level. The CMOS image sensor 109 operates according to each timing signal output from the timing generator 50 as described above.

  The CMOS image sensor 109 sequentially transfers and processes the pixel reference level and the optical signal level for each RGB color for one line in the main scanning direction of the image. The black level holding unit 532 takes in a signal at a timing at which the reference signal level is transferred, and sequentially updates each color. The optical signal level holding unit 530 captures a signal at the timing when the optical signal level obtained by photoelectric conversion of the PD is transferred, and sequentially updates the color for each color. The subtraction unit 534 calculates a difference between signals updated by the black level holding unit 532 and the optical signal level holding unit 530 (difference between the optical signal level and the black level). Then, the CMOS image sensor 109 sequentially outputs the corrected signal level of each color.

  FIG. 8 is a diagram schematically illustrating black level correction performed by the CMOS image sensor 109. The level held by the optical signal level holding unit 530 includes a read level corresponding to the reflected light from the document and fixed pattern noise. The black level holding unit 532 holds a fixed pattern noise level. Here, the fixed pattern noise refers to variations in dark current of floating diffusion (FD) which is a charge detection unit (not shown) of CMOS image sensor 109, variations in offset voltage of pixel transistors, variations in dark current of photodiodes, and the like. including.

  The CMOS image sensor 109 obtains a corrected document reading level by subtracting the fixed pattern level from the reading level including fixed pattern noise. Further, the CMOS image sensor 109 performs black correction processing by the common black correction unit 522 on the output of each color pixel having the same position in the main scanning direction. A correction difference between colors caused by variations in each of the black correction units 522 is not generated. That is, since the synthesized pixels at the same position are processed by the common black correction unit 522, there is no error between colors. Further, the CMOS image sensor 109 does not process different colors at the same timing, so that it is possible to suppress coloring and unevenness due to crosstalk between colors. Further, the CMOS image sensor 109 is controlled to perform black correction at least once every time one line of an image is read, for example, so that the image quality of the read image is reduced even if the temperature changes with time. Make it possible to prevent.

  In contrast, a conventional area sensor (such as a Bayer array two-dimensional CMOS image sensor) as a comparative example is provided with a circuit (black correction circuit) that performs black correction for each row or column of pixels. A black correction circuit that is different for each color performs black correction on the same pixel position in FIG. In other words, the conventional area sensor indicates the color of the same pixel position in the reading target by synthesizing the pixel data of each color in which a correction difference between colors caused by variations of the black correction circuits and the like occurs. Like the CMOS image sensor 109, black correction cannot be performed with high accuracy. Further, in the conventional area sensor, since different colors are processed at the same timing, coloring and unevenness due to crosstalk between colors occur.

  In the above-described CMOS image sensor 109, the case where the exposure timing is different in the column by the rolling shutter method is described. The CMOS image sensor 109 may be applied with a global shutter system in which exposure is performed on all pixels at the same timing by providing a memory that holds a signal of each pixel, for example, in the previous stage of the black correction unit 522.

(First Modification of Conversion Processing Unit 52a: Conversion Processing Unit 52b)
Next, a first modification of the conversion processing unit 52a will be described. FIG. 9 is a block diagram illustrating an overview of a first modification (conversion processing unit 52b) of the conversion processing unit 52a. In the following, components that are substantially the same as the components described are given the same reference numerals.

  The conversion processing unit 52b is configured such that the output of two PDs (for a total of six pixels) for each color is black-corrected by the black correction unit 523. Similar to the black correction unit 522, the black correction unit 523 performs black correction using an input signal. That is, in a CMOS image sensor (imaging device) having n pixels (n is an even number) photodiode for each color, six pixels including even and odd pixels of RGB are configured as one column. , N / 2 (pieces) of black correction units 523 perform black correction for each column. As a result, the circuit area of the black correction unit and the like can be reduced.

  When pixels of each color having the same position in the main scanning direction are set as one column and the black correction unit 522 (or the black correction unit 523) performs black correction for each column, the number of pixels included in the column is 3 It is not limited to pixels or 6 pixels.

(Second Modification of Conversion Processing Unit 52a: Conversion Processing Unit 52c)
Next, a second modification of the conversion processing unit 52a will be described. FIG. 10 is a block diagram showing an overview of a second modification (conversion processing unit 52c) of the conversion processing unit 52a. In the conversion processing unit 52c, pixels of respective colors having the same position in the main scanning direction are grouped as one column. The conversion processing unit 52c amplifies a signal that each PD photoelectrically converts and transfers by the transfer unit 520 by a common analog programmable gain amplifier (APGA) 524 for each column, and converts it into a digital signal by the AD conversion unit (ADC) 525. After conversion, the black correction unit 526 is configured to perform black correction. N black correction units 526 are provided for each column, and perform black correction on the digital signal in the same manner as the black correction unit 522.

  That is, the conversion processing unit 52c can obtain a corrected document reading level by subtracting fixed pattern noise caused by variations in the FD, the pixel transistor, and the ADC 525. In addition, the conversion processing unit 52c can arbitrarily amplify the signal sequentially transferred from each transfer unit 520 by the APGA 524 and effectively use the dynamic range of the ADC 525. Makes it possible to enhance.

  Next, correction of fixed pattern noise caused by variations in the dark current of the photodiode (current generated even when light is not exposed) will be described. As a technique for correcting fixed pattern noise due to dark current variations of photodiodes, there is black shading correction. Specifically, the image is read with the light source turned off, held as a reference black level for each pixel, and correction is performed by subtracting the reference black level from the image level obtained by reading the document.

  However, since the black shading correction needs to read the image with the light source turned off before the image reading (scanning), the productivity is lowered when continuous scanning is performed. If the generation of the reference black level for black shading correction is performed only once, the productivity of continuous scanning can be improved.However, when the fixed pattern noise changes due to temperature changes after a lapse of time, etc. Cannot correct normally.

  As a method of correcting the fixed pattern noise due to a temperature change after elapse of time while ensuring the productivity, the fluctuation of the reading level of the light-shielding pixel (OPB) of the image sensor is detected to detect the reference black level. A technique for correcting the value is already known. However, in the method of detecting the variation in the reading level of OPB and correcting the value of the reference black level, if the change in temperature over time is not uniform for each pixel, the change in fixed pattern noise varies from pixel to pixel. A vertical streak may occur in the image without correction.

(Third Modification of Conversion Processing Unit 52a: Conversion Processing Unit 52d)
Next, a description will be given of a third modification of the conversion processing unit 52a that can correct the fixed pattern noise caused by the variation in the dark current of the photodiode. FIG. 11 is a block diagram showing an overview of a third modification (conversion processing unit 52d) of the conversion processing unit 52a. As illustrated in FIG. 11, the conversion processing unit 52d includes an APGA 524, an ADC 525, and a black correction unit 527 for each of n columns including three pixels. Further, the conversion processing unit 52d is configured to use the signal level of the extinguishing period during the period of reading one line of the image in the main scanning direction as the reference signal level for black correction.

  FIG. 12 is a diagram illustrating a configuration example of the black correction unit 527. As illustrated in FIG. 12, the black correction unit 527 includes an optical signal level holding unit 530, a black level holding unit 532, and a subtraction unit 534 for each of RGB signals. That is, the black correction unit 527 outputs the signal after the black level correction for each color by taking the difference between the optical signal level and the black level for each color.

  FIG. 13 is a timing chart showing an operation when the image reading apparatus 100 having the CMOS image sensor 109 having the conversion processing unit 52d shown in FIG. 11 performs black level correction. Also in this case, the CMOS image sensor 109 operates in accordance with each timing signal output from the timing generator 50 as described above.

  The lighting circuit 40 (FIG. 3) causes the light source 102 to sequentially switch on and off when reading a document. For example, the lighting circuit 40 sets the lighting time Ton and the turn-off time Toff to the same time in the reading period of one line in the main scanning direction.

  During the extinguishing period, the control unit 42 first resets the PD optical signal level of the conversion processing unit 52d in order for each color. Then, after a predetermined time has elapsed, the conversion processing unit 52d transfers the optical signal level accumulated during the extinguishing period to the black level holding unit 532 (FIG. 12).

  During the lighting period, the control unit 42 first resets the PD optical signal level of the conversion processing unit 52d in order for each color. Then, after a predetermined time has elapsed, the conversion processing unit 52 d transfers the optical signal level accumulated during the lighting period to the optical signal level holding unit 530.

  Then, the conversion processing unit 52d performs black correction by calculation by the subtraction unit 534 for each color, and sequentially outputs the corrected signals. The signals of the respective colors are transferred as image signals by the I / F unit 54 to the subsequent image processing unit.

  As described above, the image reading apparatus 100 having the CMOS image sensor 109 including the conversion processing unit 52d subtracts fixed pattern noise caused by variations in dark current of the FD, the pixel transistor, the APGA 524, the ADC 525, and the PD. The corrected original reading level can be obtained. In addition, since the image reading apparatus 100 performs black correction every time an image is read for one line, even if a temperature change occurs with time, the image quality of the read image can be prevented. In addition, since the image reading apparatus 100 performs signal transfer at the same timing for each color, coloration (false color) or color unevenness due to crosstalk that affects other colors via the power supply or GND is generated. Can be prevented.

(Fourth Modification of Conversion Processing Unit 52a: Conversion Processing Unit 52e)
Next, a fourth modification of the conversion processing unit 52a will be described. FIG. 14 is a block diagram showing an outline of a fourth modification (conversion processing unit 52e) of the conversion processing unit 52a. As illustrated in FIG. 14, the conversion processing unit 52 e includes a digital programmable gain amplifier (DPGA) 528, a first black correction unit 529 a that performs two black corrections, and a conversion processing unit 52 d illustrated in FIG. 11. The difference is that the second black correction unit 529b is provided for each column of the same number as the number of pixels of each color.

  The DPGA 528 digitally amplifies the digital signal A / D converted by the ADC 525 with an arbitrary gain. The first black correction unit 529a has the same configuration as the black correction unit 522 illustrated in FIG. The first black correction unit 529a is used to correct fixed pattern noise caused by variations in the FD, the pixel transistor, and the ADC 525. The second black correction unit 529b has the same configuration as the black correction unit 527 shown in FIG. The second black correction unit 529b is used to correct fixed pattern noise caused by the dark current variation of the PD.

  FIG. 15 is a diagram schematically illustrating black level correction performed by the image reading apparatus 100 including the CMOS image sensor 109 including the conversion processing unit 52e. The conversion processing unit 52e acquires the optical signal level and the black level in the lighting period and the extinguishing period, respectively.

  The optical signal level acquired by the conversion processing unit 52e during the lighting period includes a read level corresponding to the reflected light from the document and fixed pattern noise due to the FD, the pixel transistor, the APGA 524, and the PD. The black level acquired by the conversion processing unit 52e during the lighting period is fixed pattern noise caused by the FD, the pixel transistor, and the APGA 524.

  Further, the optical signal level acquired by the conversion processing unit 52e during the extinguishing period includes fixed pattern noise due to the FD, the pixel transistor, the APGA 524, and the PD. Here, the fixed pattern noise due to the dark current of the PD is at a level smaller than the lighting period because the light extinction period is short. The black level acquired by the conversion processing unit 52e during the extinguishing period is fixed pattern noise caused by the FD, the pixel transistor, and the APGA 524.

  Here, the DPGA 528 changes the gain in the lighting period and the extinguishing period in order to make the fixed pattern noise due to the dark current of the PD in the lighting period and the extinguishing period the same. For example, the DPGA 528 matches the fixed pattern noise due to the dark current of the PD by setting the gain during the lighting period to 1 and setting the gain during the extinguishing period as a gain corresponding to the ratio between the lighting time Ton and the extinguishing time Toff.

  The first black correction unit 529a corrects the fixed pattern noise caused by the FD, the pixel transistor, and the APGA 524 by taking the difference between the optical signal level and the black level in each of the lighting period and the extinguishing period. Further, the second black correction unit 529b corrects the fixed pattern noise due to the dark current of the PD by taking the difference between the light-on period and the light-off period signal. As described above, the image reading apparatus 100 including the CMOS image sensor 109 including the conversion processing unit 52e can reduce the ratio of the turn-off time Toff as compared with the CMOS image sensor 109 including the conversion processing unit 52d. To do.

  In the second black correction unit 529b, it is necessary to set the gain of the DPGA 528 so that the dark current noise matches. For the gain of DPGA 528, there are a method for calculating the setting according to the driving condition and a method for calculating the setting by adjustment. When calculating the gain setting of the DPGA 528 according to the driving conditions, the setting is calculated based on, for example, the ratio between the lighting time and the lighting time of the light source 102. Since the dark current noise is proportional to the accumulation time, the dark current noise of the corrected level a and the corrected level b can be matched by setting the gain of the DPGA 528 to the ratio of the lighting time to the extinguishing period. c can be calculated. Here, the gain of the DPGA 528 is calculated by the following equation 1.

  Gain of DPGA528 = lighting period / lighting period (1)

(Modification of CMOS image sensor 109)
Next, a modified example of the CMOS image sensor 109 will be described. FIG. 16 is a block diagram showing an outline of a modified example (CMOS image sensor 109a) of the CMOS image sensor 109. As shown in FIG. The CMOS image sensor 109a has an adjustment unit 56 that adjusts the gain in the conversion processing unit 52e in addition to the functions of the CMOS image sensor 109 shown in FIG. In addition to the functions of the I / F unit 54, the I / F unit 55 outputs a signal received from the conversion processing unit 52 e to the adjustment unit 56.

  The adjustment unit 56 adjusts the gain setting for each APGA 524 and each DPGA 528 according to the signal received via the I / F unit 55.

  FIG. 17 is a flowchart illustrating an operation in which the image reading apparatus 100 having the CMOS image sensor 109a performs gain adjustment. The image reading apparatus 100 having the CMOS image sensor 109a fixes the gain of the DPGA 528 during the lighting period and adjusts the gain of the DPGA 528 during the extinguishing period.

  In step 100 (S100), the control unit 42 turns off the light source 102 by negating the lighting ON signal.

  In step 102 (S102), the adjustment unit 56 sets (changes) the gain of the DPGA 528 during the extinguishing period. The first gain setting value of DPGA 528 is set as an initial value.

  In step 104 (S104), the adjustment unit 56 acquires and holds the gain setting value of the DPGA 528 and the corrected level corresponding to the gain setting value. Here, since the light source 102 is turned off, both the optical signal level and the black level input to the second black correction unit 529b are dark current noise. Therefore, when the gain is set correctly, the corrected level is a value close to zero.

  In step 106 (S106), the adjustment unit 56 determines whether or not the absolute value of the corrected level is equal to or less than a predetermined threshold value. When the adjustment unit 56 determines that the absolute value of the corrected level is equal to or less than the predetermined threshold (S106: Yes), the process ends. If the adjustment unit 56 determines that the absolute value of the corrected level is not equal to or less than the predetermined threshold (S106: No), the process proceeds to S108.

  In step 108 (S108), the adjustment unit 56 determines whether or not all settable gains have been tried. When it is determined that all the settable gains have not been tried (S108: No), the adjustment unit 56 proceeds to the process of S102. If it is determined that all the settable gains have been tried (S108: Yes), the adjustment unit 56 proceeds to the process of S110.

  In step 110 (S110), the adjustment unit 56 sets the gain that minimizes the absolute value of the corrected level, and ends the adjustment.

  Note that the image reading apparatus 100 having the CMOS image sensor 109a can set the gain according to the state of the CMOS image sensor 109a and the like by performing the operation shown in FIG. 17 when the power is turned on.

(Modification of second black correction unit 529b)
Next, a modified example of the second black correction unit 529b will be described. FIG. 18 is a block diagram showing an outline of a modified example (second black correction unit 529c) of the second black correction unit 529b. As shown in FIG. 18, the second black correction unit 529c is different from the black correction unit 527 (second black correction unit 529b) shown in FIG. 12 for each color by a subtraction unit 535, an attenuator 536, and an addition unit 537. Is different in that it is further provided.

  The subtracting unit 535 takes the difference between the value currently held by the black level holding unit 532 and the new value input and outputs the difference to the attenuator 536. The attenuator 536 attenuates the value input from the subtracting unit 535 and outputs the attenuated value to the adding unit 537. The attenuation factor of the attenuator 536 is set to a value that can follow the fluctuation of the dark current noise due to the temperature change. The adding unit 537 adds the value currently held by the black level holding unit 532 and the value attenuated by the attenuator 536 and outputs the result to the black level holding unit 532. As described above, the second black correction unit 529c smoothes the updated value of the post-correction level b illustrated in FIG. 15 by updating the value held by the black level holding unit 532.

  That is, the second black correction unit 529c makes it less likely to be affected by random noise or sudden noise even when dark current noise is small due to the short turn-off time of the light source 102 for obtaining the black level. to enable.

40 lighting circuit 42 control unit 50 timing generators 52a, 52b, 52c, 52d, 52e conversion processing units 54, 55 I / F unit 56 adjustment unit 100 image reading device 102 light source 109, 109a CMOS image sensor 200 automatic document feeder (ADF)
300 Image forming apparatus 520 Transfer unit 522, 523, 526, 527 Black correction unit 524 APGA
525 ADC
528 DPGA
529a First black correction unit 529b, 529c Second black correction unit 530 Optical signal level holding unit 532 Black level holding unit 534, 535 Subtraction unit 536 Attenuator 537 Addition unit

JP 10-1226697 A

Claims (12)

A plurality of photoelectric conversion elements that are arranged in one direction for each color to receive light, and are capable of photoelectrically converting reflected light of each color from a common pixel position in a reading target in units of pixels for each color ;
A pixel group of the reflected light from the common pixel position with the photoelectric conversion element of each color of pixels for photoelectrically converting each said color in said plurality of photoelectric conversion elements,
For each of the pixel groups, an output result from the same pixel as the first output result from a first output result that is an output result of photoelectric conversion of reflected light by the photoelectric conversion elements of the pixels of each color in each pixel group A black correction unit for correcting the black level of the reading target by subtracting the second output result obtained as an output independent of the reflected light;
An image pickup device comprising: The black correction unit is
The image sensor according to claim 1, wherein a black level to be read is corrected for each of the colors arranged in one direction. An AD conversion unit that digitizes an output result obtained by photoelectric conversion of the photoelectric conversion element for each pixel group;
The black correction unit is
The imaging device according to claim 1, wherein the black level of the reading target is corrected using an output result digitized by the AD conversion unit. The output unit obtained by photoelectric conversion of the photoelectric conversion element for each pixel group is amplified according to a setting, and further includes an amplification unit that outputs the amplified output result to the AD conversion unit. The imaging device described in 1. The imaging device according to any one of claims 1 to 4,
A light source that emits light to a document to be read;
A control unit that controls the black correction unit to correct the black level of the document every time the image sensor receives reflected light from one line in the main scanning direction of the document;
An image reading apparatus comprising: The controller is
Within the period in which the image sensor receives reflected light from one line in the main scanning direction of the document, the light source is sequentially turned on and off,
The black correction unit is
The black level of the reading target is corrected by subtracting the output result of the photoelectric conversion element during the period when the light source is turned off from the output result of the photoelectric conversion element photoelectrically converting the reflected light for the pixel group. The image reading apparatus according to claim 5. The image reading apparatus according to claim 5, wherein the image sensor is a CMOS sensor. According to the ratio of the lighting time of the light source and the turn-off time, further has an adjustment unit for adjusting the black level of the photoelectric conversion element in the period when the light source is turned off,
The black correction unit is
The black level of a reading target is corrected by subtracting the output result adjusted by the adjustment unit from the output result obtained by photoelectrically converting reflected light from the photoelectric conversion element for the pixel group. The image reading apparatus described in 1. The image reading apparatus according to claim 8, wherein the adjustment unit performs adjustment when power is turned on. The black correction unit is
The image reading apparatus according to claim 5, wherein the black level of the photoelectric conversion element is smoothed, and the reflected light is subtracted from an output result obtained by photoelectric conversion of the reflected light. An image reading apparatus according to any one of claims 5 to 10,
An image forming apparatus comprising: an image forming unit that forms an image read by the image reading apparatus. A step of photoelectrically converting the reflected light of each color from a common pixel position in the reading target in units of pixels for each color by a plurality of photoelectric conversion elements arranged in one direction for each color to be received;
For each pixel group that includes the photoelectric conversion element of each color pixel that photoelectrically converts reflected light from the common pixel position for each color, the photoelectric conversion element of each color pixel in each pixel group photoelectrically reflects the reflected light. By subtracting the second output result obtained from the same pixel as the first output result and obtained as an output independent of the reflected light from the first output result which is the converted output result Correcting the black level of the reading object;
An image reading method including:

Images