JP2010068228A - Image reading apparatus, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image reading apparatus which corrects density variation of image data caused by clock frequency variation without restricting scanning speed because of variation period of clock frequency, and to provide an image forming apparatus using the image reading apparatus. <P>SOLUTION: The image reading apparatus has a periodic signal generator 72 which generates a clock signal CLK using an SSCG 71, a CCD 512 which sequentially outputs signals that indicate a density value of each pixel of image data synchronizing with the clock signal CLK, a correction value acquisition part 102 which turns off an exposure lamp 511 and allows a RAM 517 to store the density value of each pixel, which is output from the CCD 512, by one period of variation period tcyc as a corrected value corresponding to each pixel, and a density corrector 514 which corrects the density value of each pixel, which is output from the CCD 512, based on each corrected value stored in the RAM 517 when reading a document using the CCD 512. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image reading apparatus that reads an image from a document and an image forming apparatus that forms an image on a sheet.

  Conventionally, in order to reduce electromagnetic radiation noise radiated from an electronic device, the frequency spectrum of the clock signal is spread by changing the clock frequency used inside the device using an SSCG (Spread Spectrum Clock Generator). Technology is used.

  In addition, a CCD (Charge Coupled Device) used as an image sensor in an image reading apparatus synchronizes with a clock signal, a reset signal synchronized with the clock signal, and a drive pulse that is a periodic signal such as a clamp signal, Each pixel value of the data is sequentially output as an analog signal pixel by pixel.

  In such an image reading apparatus using a CCD, when the clock frequency is varied using SSCG, the pulse width of the CCD drive pulse is changed by SSCG, and as a result, the output waveform of the CCD is distorted. For this reason, there is a disadvantage that the density value of the image data acquired from the CCD varies according to the variation cycle of the clock frequency by SSCG.

Therefore, there is a technique for adjusting the amount of charge accumulated in the CCD by adjusting the conveyance speed of the original to be read by the CCD according to the variation in the clock frequency due to the SSCG and compensating for the variation in the density of the image data caused by the variation in the clock frequency. It is known (for example, refer to Patent Document 1).
JP 2003-60862 A

  However, in the technique described in Patent Document 1, since the synchronization signal period in the main scanning direction is an integral multiple of the clock frequency fluctuation period by SSCG and correction is performed using correction data for one line, scanning at the time of image reading is performed. There was a disadvantage that the speed was limited by the fluctuation cycle of the clock frequency.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image reading apparatus capable of correcting density fluctuations in image data caused by clock frequency fluctuations without limiting the scanning speed by the clock frequency fluctuation period, and an image forming apparatus using the same. Is to provide.

  An image reading apparatus according to the present invention includes a periodic signal generation unit that generates a periodic signal whose frequency periodically fluctuates with a predetermined variation period, and obtains image data from a document, and is generated by the periodic signal generation unit. The image sensor that sequentially outputs a signal indicating the density value of each pixel of the image data in synchronization with the periodic signal, a storage unit that stores data, and the image sensor in a state where no light is applied to the image sensor. When the density value for each pixel to be output is stored in the storage unit as a correction value corresponding to each pixel for one period of the fluctuation period, and the image sensor acquires image data from the document A density correction unit that corrects the density value of each pixel output from the image sensor based on each correction value stored in the storage unit.

  According to this configuration, the image sensor is not exposed to light, that is, in a state where factors other than the frequency fluctuation of the periodic signal that affects the operation of the image sensor, such as the original image and the light distribution characteristics of the light source, are reduced. Is stored in the storage unit as a correction value corresponding to each pixel for one period of the fluctuation period. When reading the image of the document, the density correction unit corrects the density value of each pixel output from the image sensor based on each correction value stored in the storage unit. As a result, the density fluctuation caused by the frequency fluctuation of the periodic signal can be corrected regardless of the scanning speed when reading the image of the document, so that the scanning speed is not limited by the fluctuation period of the clock frequency, Electromagnetic radiation noise can be reduced by changing the frequency.

  Further, when trying to acquire image data from the original by the image sensor, the maximum value at which the density value output from the image sensor is maximized within one cycle of the fluctuation period in a state where the light is not applied. The timing is detected, and the correction values are sequentially read from the storage unit in synchronization with the periodic signal so that the maximum value of each correction value stored in the storage unit corresponds to the maximum value timing. Accordingly, it is preferable to further include a synchronization unit that synchronizes the density value of each pixel sequentially output from the image sensor and the correction value corresponding to each pixel.

  According to this configuration, when an image is read from an original by the image sensor, first, the maximum value at which the density value output from the image sensor is maximized within one period of the fluctuation period by the synchronization unit in a state where no light is applied. Timing is detected. Further, since each correction value is sequentially read from the storage unit in synchronization with the periodic signal so as to correspond the maximum value of each correction value stored in the storage unit to the maximum value timing by the synchronization unit, The density value of each pixel sequentially output from the image sensor and the correction value corresponding to each pixel can be synchronized.

  The correction unit further includes a maximum value address storage unit that stores an address of the storage unit in which the maximum value among the correction values is stored as a maximum value address, and the synchronization unit is configured to change the fluctuation period from the maximum value timing. When one period elapses, the correction value is sequentially read from the maximum value address of the storage unit in synchronization with the period signal, thereby obtaining the density value of each pixel and the correction value corresponding to the pixel. Are preferably synchronized.

  The synchronization unit detects the maximum value timing at which the density value output from the image sensor is maximized within one cycle of the fluctuation cycle, but when the maximum value timing is detected, the maximum value timing has already passed. Yes. Therefore, when one cycle of the fluctuation cycle has elapsed from the maximum value timing, that is, from the next maximum value timing, the correction value is sequentially read out from the maximum value address of the storage unit in synchronization with the periodic signal, whereby the density value of each pixel is read. And the correction value corresponding to each pixel can be synchronized.

  The image sensor further includes a light source unit that irradiates light on the document, the image sensor acquires the image data from reflected light of the document, and the correction value acquisition unit acquires the correction values. At this time, it is preferable to turn off the light source unit.

  According to this configuration, the correction value acquisition unit can turn off the light source unit so that the image sensor is not exposed to light when acquiring each correction value.

  Further, a light distribution correction coefficient storage unit that stores a light distribution correction coefficient set in advance so as to correct the light distribution characteristic of the light source by multiplying the density value of each pixel, and the density correction unit corrects the light distribution correction coefficient. It is preferable to further include a light distribution characteristic correction unit that performs correction by multiplying the density value of each pixel by the light distribution correction coefficient stored in the light distribution correction coefficient storage unit.

  The light source has light distribution characteristics, and its illuminance is not uniform throughout the document. Therefore, the image data acquired by the image sensor includes a density error due to the light distribution characteristics of the light source. Therefore, the light distribution correction coefficient storage unit stores a light distribution correction coefficient set in advance so as to correct the light distribution characteristic of the light source by multiplying the density value of each pixel, and is corrected by the density correction unit. By multiplying the density value of each pixel by the light distribution correction coefficient stored in the light distribution correction coefficient storage unit, it is possible to reduce the density error due to the light distribution characteristic of the light source and improve the image quality.

  At this time, if the light distribution correction coefficient is multiplied by the light distribution characteristic correction unit before the density value of each pixel is corrected by the density correction unit, the density error caused by the frequency variation of the periodic signal may occur. There is a possibility that the density error increases due to multiplication by the light distribution correction coefficient. However, since the light distribution characteristic correction unit multiplies the light distribution correction coefficient after the density correction unit corrects the density value of each pixel, the light distribution correction coefficient is added to the density error caused by the frequency variation of the periodic signal. The possibility that the density error increases due to multiplication is reduced.

  The density correction unit preferably performs the correction by subtracting a correction value corresponding to each pixel stored in the storage unit from a density value of each pixel output from the image sensor.

  According to this configuration, the correction of the density error caused by the frequency fluctuation of the periodic signal can be performed by a simple calculation process called subtraction, so that the density correction unit can be configured with a simple configuration.

  In addition, the correction value acquisition unit acquires the density value for each pixel output from the image sensor for a plurality of periods of the fluctuation period and averages it for each period in a state where the image sensor is not exposed to light. The average value is preferably stored in the storage unit as each correction value for the one cycle.

  According to this configuration, random noise included in the density value for each pixel output from the image sensor is averaged, so that the accuracy of the correction value is improved.

  An image forming apparatus according to the present invention includes the above-described image reading device and an image forming unit that forms an image on a sheet based on the density value of each pixel corrected by the density correction unit.

  According to this configuration, the scan speed is not limited by the clock frequency fluctuation period, and based on the image data indicated by the density value of each pixel in which the density fluctuation of the image data caused by the clock frequency fluctuation is corrected, Since the image can be formed on the paper, it is possible to reduce the electromagnetic radiation noise by changing the frequency of the periodic signal while ensuring the freedom of scanning speed of the document, and to further improve the image quality.

  The image reading apparatus having such a configuration is a state in which light is not applied to the image sensor, that is, a state in which factors other than the frequency fluctuation of the periodic signal that affects the operation of the image sensor, such as a document image and a light distribution characteristic of the light source, are reduced Thus, the density value for each pixel output from the image sensor is stored in the storage unit as a correction value corresponding to each pixel for one period of the fluctuation period. When reading the image of the document, the density correction unit corrects the density value of each pixel output from the image sensor based on each correction value stored in the storage unit. As a result, the density fluctuation caused by the frequency fluctuation of the periodic signal can be corrected regardless of the scanning speed when reading the image of the document, so that the scanning speed is not limited by the fluctuation period of the clock frequency, Electromagnetic radiation noise can be reduced by changing the frequency.

  In the image forming apparatus having such a configuration, the scan speed is not limited by the clock frequency fluctuation period, and the density value of each pixel is corrected for the density fluctuation of the image data caused by the clock frequency fluctuation. Since an image can be formed on paper based on image data, the frequency of the periodic signal is varied to reduce electromagnetic radiation noise while further ensuring the degree of freedom of scanning speed of the document, and the image quality is further improved. Is possible.

  Embodiments according to the present invention will be described below with reference to the drawings. In addition, the structure which attached | subjected the same code | symbol in each figure shows that it is the same structure, The description is abbreviate | omitted. FIG. 1 is a structural diagram schematically showing an internal configuration of a copying machine as an example of an image forming apparatus using an image reading apparatus according to an embodiment of the present invention.

  The image forming apparatus is not limited to a copying machine, but may be a printer, a facsimile, or a multifunction machine having these functions. Further, the image reading apparatus is not limited to an image forming apparatus such as a copying machine or a multifunction peripheral, and may be a scanner apparatus connected to a network, for example.

  The copying machine 1 includes a main body 2, a stack tray 3 disposed on the left side of the main body 2, a scanner device 5 disposed on the upper portion of the main body 2, and a scanner device 5. The original document feeding unit 6 is provided.

  A substantially rectangular operation panel 47 is provided at the front of the copying machine 1. The operation panel unit 47 includes a display unit 473 and an operation key unit 476. The display unit 473 is configured by, for example, a liquid crystal display having a touch panel function. The operation key unit 476 includes various key switches such as a start key for the user to input a print execution instruction and a numeric keypad for inputting the number of copies to be printed.

  FIG. 2 is a block diagram showing an example of the electrical configuration of the copying machine 1 shown in FIG. The copying machine 1 includes a scanner device 5, an SSCG 71, a periodic signal generation unit 72, an AFE (analog front end) circuit 513, a density correction unit 514, a shading correction unit 515 (light distribution characteristic correction unit), and a RAM (Random Access Memory) 517. (Storage unit), synchronization unit 518, HDD (Hard Disk Drive) 150 (light distribution correction coefficient storage unit), and control unit 100. The control unit 100 also functions as a copy control unit 101, a correction value acquisition unit 102, and a maximum value address acquisition unit 103.

  In this case, the image reading apparatus according to the claims includes the scanner device 5, the periodic signal generation unit 72, the density correction unit 514, the shading correction unit 515, the RAM 517, the synchronization unit 518, the HDD 150, the correction value acquisition unit 102, and the maximum value address. The acquisition unit 103 is configured.

  The SSCG 71 is a clock generation circuit with a frequency modulation function that generates the clock signal CK0. The frequency of the clock signal CK0 periodically fluctuates at a predetermined constant fluctuation period tcyc.

  The periodic signal generator 72 generates a periodic signal for operating a later-described CCD (Charge Coupled Device) 512, specifically, a clock signal CLK, a reset signal RS, and a clamp signal CLP. The SSCG 71 may be used as it is as the periodic signal generator.

  The scanner device 5 includes a scanner unit 51, a document table 52 and a document reading slit 53 made of a transparent member such as glass. The scanner unit 51 includes an exposure lamp 511 (light source unit) and a CCD 512 (image sensor). The exposure lamp 511 irradiates the original with light.

  The scanner unit 51 is configured to be movable by a driving unit (not shown). When reading a document placed on the document table 52, the scanner unit 51 is moved along the document surface at a position facing the document table 52, and the document Image data acquired by the CCD 512 is output to the AFE circuit 513 while scanning the image. When reading the document fed by the document feeding unit 6, the document is moved to a position facing the document reading slit 53, and the document is transported by the document feeding unit 6 through the document reading slit 53 by the CCD 512. The document image is acquired in synchronization with the image data, and the image data is output to the AFE circuit 513.

  The CCD 512 acquires image data by reflected light from the document, and sets the density value of each pixel of the image data in synchronization with the clock signal CLK, the reset signal RS, and the clamp signal CLP output from the periodic signal generator 72. The image signal D expressed by voltage is sequentially output to the AFE circuit 513. Note that the image sensor is not limited to the CCD, and any image sensor may be used as long as the detected density value varies according to the frequency variation of the clock signal.

  The document feeding unit 6 includes a document placing unit 61 for placing a document, a document discharge unit 62 for discharging a document whose image has been read, and a document placed on the document placing unit 61. A document transport mechanism 63 is provided that feeds the sheets one by one to transport them to a position facing the document reading slit 53 and discharges them to the document discharge section 62.

  The main body 2 includes a plurality of paper feed cassettes 461, a paper feed roller 462 that feeds the paper from the paper feed cassette 461 one by one and transports it to the image forming unit 40, and an image on the paper carried out from the paper feed cassette 461. And an image forming unit 40 that controls the operation of the entire apparatus.

  The image forming unit 40 includes a paper transport unit 411, an optical scanning device 42, a photosensitive drum 43, a developing unit 44, a transfer unit 41, and a fixing unit 45. The paper transport unit 411 is provided in the paper transport path in the image forming unit 40, and includes a transport roller 412 that supplies the paper transported by the paper feed roller 462 to the photosensitive drum 43, and a paper sheet that is discharged from the stack tray 3. Conveying rollers 463 and 464 that convey to the tray 48 are provided.

  The optical scanning device 42 forms an electrostatic latent image on the photosensitive drum 43 by outputting laser light or the like based on the image data output from the control unit 100 and exposing the photosensitive drum 43. The developing unit 44 develops the electrostatic latent image on the photosensitive drum 43 with toner to form a toner image. The transfer unit 41 transfers the toner image on the photosensitive drum 43 to a sheet. The fixing unit 45 heats the sheet on which the toner image is transferred to fix the toner image on the sheet.

  The control unit 100 includes, for example, a CPU (Central Processing Unit) that executes predetermined arithmetic processing, a ROM (Read Only Memory) that stores a predetermined control program, and a RAM (Random Access Memory) that temporarily stores data. And these peripheral circuits and the like. The control unit 100 is connected to the scanner device 5, RAM 517, image forming unit 40, operation panel unit 47, and HDD (Hard Disk Drive) 150.

  The control unit 100 functions as a copy control unit 101, a correction value acquisition unit 102, and a maximum value address acquisition unit 103 by executing a control program stored in the ROM, the HDD 150, or the like. Further, the HDD 150 stores in advance a light distribution correction coefficient set in advance so as to correct the light distribution characteristics of the exposure lamp 511, for example. In this case, the HDD 150 corresponds to an example of a light distribution correction coefficient storage unit.

  The copy control unit 101 controls the operation of each unit in the apparatus and executes copying of a document image.

  The correction value acquisition unit 102 turns off the exposure lamp 511 and corrects the density value for each pixel in the image signal output from the CCD 512 via the AFE circuit 513 for one period of the fluctuation period tcyc. Is stored in the RAM 517.

  The maximum value address acquisition unit 103 causes the RAM 517 to store the address where the maximum value among the correction values stored in the RAM 517 is stored as the maximum value address. In this case, the RAM 517 corresponds to an example of a maximum value address storage unit.

  The AFE circuit 513 samples the image signal D output from the CCD 512 for each pixel, converts it to a digital value, and outputs the digital value to the density correction unit 514.

  The synchronization unit 518 is configured using, for example, a comparison circuit that compares density values, a counter circuit that counts the clock signal CLK, other logic circuits, a state machine, and the like. Then, the synchronization unit 518 detects the maximum value timing at which the maximum value of the density value of each pixel is output from the CCD 512 via the AFE circuit 513, and the clock signal from when the fluctuation period tcyc has elapsed from the maximum value timing. The correction value is sequentially read from the maximum value address of the RAM 517 in synchronization with the CLK and output to the density correction unit 514, whereby the density value of each pixel output from the AFE circuit 513 and each correction value stored in the RAM 517 are displayed. Synchronize with.

  The density correction unit 514 performs correction by subtracting each correction value stored in the RAM 517 from the density value of each pixel output from the AFE circuit 513.

  The shading correction unit 515 corrects the light distribution characteristics of the exposure lamp 511 by multiplying the density value of each pixel corrected by the density correction unit 514 by the light distribution correction coefficient stored in the HDD 150.

  The image processing unit 516 performs, for example, image processing for improving the image quality, enlarging or reducing the image data corrected by the shading correction unit 515, or controlling the laser device of the optical scanning device 42, for example. For example, the signal is analog-modulated into a laser signal for output to the image forming unit 40. In the image forming unit 40, an image is formed on a sheet based on the signal output from the image processing unit 516.

  Next, an example of the operation of the copying machine 1 configured as described above will be described. FIG. 3 is a flowchart showing an example of the operation of the copying machine 1 shown in FIG. FIG. 3 shows a correction value acquisition operation by the correction value acquisition unit 102 shown in FIG.

  When the copying machine 1 is activated, the clock signal CK0 is generated by the SSCG 71. Then, the periodic signal generator 72 generates a clock signal CLK, a reset signal RS, and a clamp signal CLP based on the clock signal CK 0 and supplies them to the CCD 512. Then, since the frequency of the clock signal CK0 fluctuates in the fluctuation period tcyc, the period (or waveform) of the clock signal CLK, the reset signal RS, and the clamp signal CLP also fluctuates in the fluctuation period tcyc.

  The correction value acquisition unit 102 turns off the exposure lamp 511 (step S1). Then, the CCD 512 outputs the image signal D according to the clock signal CLK, the reset signal RS, and the clamp signal CLP without being affected by the light distribution characteristics of the exposure lamp 511 and the original image. Then, the density of the image signal D varies according to the cycle variation of the clock signal CLK, the reset signal RS, and the clamp signal CLP.

  FIG. 4 is a signal waveform diagram for explaining the operation of the CCD 512. First, when the clock signal CLK and the reset signal RS become high level, the CCD 512 starts to discharge the accumulated charge, and the voltage level of the image signal D rises. Next, when the reset signal RS falls and the clamp signal CLP becomes high level, the voltage level of the image signal D is maintained at a substantially constant clamp level.

  When the clamp signal CLP becomes low level, the image signal D decreases. Here, the difference between the lowest point at which the image signal D is lowered and the clamp level indicates the density of the pixel. The AFE circuit 513 acquires a difference between the lowest point and the clamp level as a density value and outputs the density value to the density correction unit 514.

  Next, when the clock signal CLK and the reset signal RS rise again, the CCD 512 starts discharging the accumulated charge of the next pixel, and thereafter repeats the same operation.

  FIG. 5 is an explanatory diagram for explaining a change in the image signal D accompanying a change in the period (or waveform) of the clock signal CLK, the reset signal RS, and the clamp signal CLP. As shown in FIG. 5, when the period (or waveform) of the clock signal CLK, the reset signal RS, and the clamp signal CLP fluctuates, the exposure lamp 511 is turned off and the amount of light received by the CCD 512 does not change. As indicated by symbols A and B in FIG. 5, the density value changes.

  FIG. 6 is an explanatory diagram showing an example of a change in density value obtained by the CCD 512. As shown in FIG. 6, the density value indicated by the image signal D changes periodically with a fluctuation period tcyc. For example, when the fluctuation cycle tcyc corresponds to 512 cycles of the clock signal CLK, the density value periodically changes every 512 pixels.

  The correction value acquisition unit 102 acquires pixel values for the fluctuation period tcyc (for 512 pixels), and stores them in the RAM 517 as correction values corresponding to the pixels for the fluctuation period tcyc (step S2). In this case, since the correction value stored in the RAM 517 may be the fluctuation cycle tcyc (for example, 512 pixels), the storage capacity for storing the correction value is reduced as compared with the case where the correction data for one line in Patent Document 1 is used. Can be made.

  FIG. 7 is an explanatory diagram for explaining correction values corresponding to the respective pixels for the variation period tcyc stored in the RAM 517. In the RAM 517, for example, density values for 512 pixels are stored in the order of addresses.

  In step S2, the correction value acquisition unit 102 acquires the density value for each pixel output from the AFE circuit 513 for a plurality of periods of the fluctuation period tcyc, averages the average value between the periods, and calculates the average value. You may make it memorize | store in RAM517 as each correction value for 1 period. In this case, since the random noise included in each density value is averaged, the accuracy of the correction value is improved.

  Next, the maximum value address acquisition unit 103 detects the maximum value Dmax among the correction values stored in the RAM 517 and stores the address where the maximum value Dmax is stored in the RAM 517 as the maximum value address Address_max ( Step S3).

  Steps S1 to S3 may be executed, for example, when the power of the copying machine 1 is turned on, may be executed immediately before the image formation is executed, or may be executed periodically. Also good.

  Next, an operation when reading an image from a document will be described. FIG. 8 is a flowchart illustrating an example of an operation when the scanner unit 51 reads an image from a document. First, the copy control unit 101 turns off the exposure lamp 511 (step S11).

  Next, the periodic signal generator 72 supplies the clock signal CLK, the reset signal RS, and the clamp signal CLP to drive the CCD 512 (step S12). Then, the image signal D that fluctuates at the fluctuation period tcyc is output from the CCD 512 to the AFE circuit 513 pixel by pixel in synchronization with the clock signal CLK.

  Next, for example, the synchronization unit 518 sequentially compares the pixel value sequentially output from the AFE circuit 513 with the previous pixel value, thereby determining the density value of each pixel from the AFE circuit 513 within the variation period tcyc. The timing Tp at which the maximum value is output is detected (step S13).

  Next, the copy control unit 101 turns on the exposure lamp 511 (step S14), and causes the scanner device 5 to read a document image (step S15). Then, the image signal D indicating the image of the document is output from the CCD 512 to the AFE circuit 513 pixel by pixel in synchronization with the clock signal CLK.

  Next, the synchronization unit 518 synchronizes with the clock signal CLK from the timing Ts of the 512th clock of the clock signal CLK after the detection of the timing Tp, that is, the timing Ts when the fluctuation cycle tcyc has elapsed from the timing Tp. While sequentially adding addresses from the value address Address_max, the correction value of each pixel is read and output to the density correction unit 514 (step S16). When the correction value is read to the end, it returns to the start address of the correction value again and repeats.

  Thus, the synchronization unit 518 can read out and synchronize the correction value corresponding to the density value of each pixel output from the AFE circuit 513 from the RAM 517.

  Next, the density correction unit 514 subtracts the correction value output from the synchronization unit 518 from the density value of each pixel sequentially output from the AFE circuit 513, and outputs the result to the shading correction unit 515 (step S17). As a result, an error component caused by the frequency fluctuation of the clock signal CK0 included in the image signal D is reduced, so that it is possible to reduce the periodic density fluctuation accompanying the frequency spreading as a measure against EMI. .

  Next, the shading correction unit 515 corrects the light distribution characteristics of the exposure lamp 511 by multiplying the density value of each pixel corrected by the density correction unit 514 by the light distribution correction coefficient stored in the HDD 150 ( Step S18). In this case, if the light distribution characteristics in step S18 are corrected before the density value correction in step S17, the light distribution correction coefficient is added to the periodic density fluctuation error included in each density value by shading correction. Therefore, even if the correction value is subsequently subtracted from each density value, the density value cannot be corrected correctly.

  However, since the copying machine 1 shown in FIG. 2 performs correction of light distribution characteristics after correcting periodic density fluctuations associated with frequency spreading, the density fluctuation error is multiplied by a light distribution correction coefficient. Thus, the density error does not increase.

  Next, the image processing unit 516 performs image processing on the image data corrected by the shading correction unit 515 and outputs the image data to the image forming unit 40 (step S19). Then, an image is formed on the paper by the image forming unit 40 (step S20). In this case, the image data is reduced in periodic density fluctuations due to frequency spreading, so that the quality of the image formed on the paper is improved.

1 is a structural diagram schematically showing an internal configuration of a copying machine as an example of an image forming apparatus using an image reading apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating an example of an electrical configuration of the copier illustrated in FIG. 1. 3 is a flowchart showing an example of the operation of the copying machine 1 shown in FIG. It is a signal waveform diagram for demonstrating operation | movement of CCD. It is explanatory drawing for demonstrating the change of the image signal D accompanying the fluctuation | variation of the period (or waveform) of the clock signal CLK, the reset signal RS, and the clamp signal CLP. It is explanatory drawing which shows an example of the change of the density value obtained by CCD. It is explanatory drawing for demonstrating the correction value corresponding to each pixel for the fluctuation period tcyc memorize | stored in RAM. 6 is a flowchart illustrating an example of an operation when an image is read from an original by a scanner unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Copy machine 2 Main body part 5 Scanner apparatus 40 Image formation part 51 Scanner part 71 SSCG
72 Periodic Signal Generation Unit 100 Control Unit 101 Copy Control Unit 102 Correction Value Acquisition Unit 103 Maximum Value Address Acquisition Unit 150 HDD
511 Exposure lamp 512 CCD
513 AFE circuit 514 Density correction unit 515 Shading correction unit 516 Image processing unit 517 RAM
518 Synchronizing section Address_max Maximum value address CK0, CLK Clock signal CLP Clamp signal D Image signal RS Reset signal tcyc Fluctuation period

Claims (8)

A periodic signal generating unit that generates a periodic signal whose frequency periodically changes at a preset changing period;
An image sensor that acquires image data from a document and sequentially outputs a signal indicating a density value of each pixel of the image data in synchronization with the periodic signal generated by the periodic signal generator;
A storage unit for storing data;
A correction value acquisition unit that stores a density value for each pixel output from the image sensor in the storage unit as a correction value corresponding to each pixel for one period of the variation period in a state where the image sensor is not exposed to light. When,
A density correction unit that corrects the density value of each pixel output from the image sensor based on each correction value stored in the storage unit when image data is acquired from the document by the image sensor; An image reading apparatus. When trying to acquire image data from the original by the image sensor, the maximum value timing at which the density value output from the image sensor is maximized within one cycle of the fluctuation period in a state where the light is not applied. By detecting and sequentially reading each correction value from the storage unit in synchronization with the periodic signal so as to correspond to the maximum value of each correction value stored in the storage unit with respect to the maximum value timing, The image reading apparatus according to claim 1, further comprising: a synchronization unit that synchronizes a density value of each pixel sequentially output from the image sensor and a correction value corresponding to the pixel. A maximum value address storage unit that stores, as a maximum value address, the address of the storage unit in which the maximum value of each correction value is stored;
The synchronization unit is
When one period of the fluctuation period has elapsed from the maximum value timing, the correction value is sequentially read out from the maximum value address of the storage unit in synchronization with the periodic signal, whereby the density value of each pixel and the The image reading apparatus according to claim 2, wherein the correction value corresponding to each pixel is synchronized. A light source unit for irradiating the original with light;
The image sensor acquires the image data from reflected light of the document,
The image reading apparatus according to claim 1, wherein the correction value acquisition unit turns off the light source unit when acquiring the correction values. A light distribution correction coefficient storage unit that stores a light distribution correction coefficient set in advance to correct the light distribution characteristic of the light source by multiplying the density value of each pixel;
A light distribution characteristic correction unit that performs correction by multiplying the density value of each pixel corrected by the density correction unit by a light distribution correction coefficient stored in the light distribution correction coefficient storage unit. The image reading apparatus according to claim 1. The density correction unit
6. The correction is performed by subtracting a correction value corresponding to each pixel stored in the storage unit from a density value of each pixel output from the image sensor. The image reading apparatus according to claim 1. The correction value acquisition unit
In a state where the image sensor is not exposed to light, the density value for each pixel output from the image sensor is acquired for a plurality of periods of the fluctuation period, averaged between the periods, and the average value is calculated for the one period. The image reading apparatus according to claim 1, wherein each of the correction values is stored in the storage unit. An image reading apparatus according to any one of claims 1 to 7,
An image forming apparatus comprising: an image forming unit that forms an image on a sheet based on a density value of each pixel corrected by the density correction unit.

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