JP2012249272A - Image reading device and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image reading device for removing influence by frequency diffusion of drive clocks with a smaller number of components, and an image forming device provided with the image reading device.SOLUTION: An image reading device comprises: characteristic acquisition means for acquiring a time change characteristic of frequency diffusion on the basis of a first image signal obtained by operating an imaging device with a timing signal generated using a reference signal to read a reference object having a prescribed reference color and a second image signal obtained by operating the imaging device with a timing signal generated using a frequency diffusion signal to read the reference object; and correction means for correcting a third image signal obtained by operating the imaging device with the timing signal generated using the frequency diffusion signal to read an object to be read with a correction value in which the time change characteristic was reflected and outputting the corrected image signal as a fourth image signal.

Description

  The present invention is directed to an image reading apparatus in which measures against EMI (Electro Magnetic Interference) caused by a clock are taken, and an image forming apparatus equipped with the image reading apparatus.

  With the recent increase in speed and capacity of information processing technology, the clock frequency for operating various devices that process these information is also increasing faster. As the clock frequency increases, EMI countermeasures are often required. As such EMI suppression means, a method of frequency-spreading a drive clock for operating an electronic device may be employed.

  Such EMI countermeasures are also applied to apparatuses having an image reading function (hereinafter collectively referred to as “image reading apparatuses”) such as copying machines, facsimiles, image scanners, and industrial cameras. Such an image reading apparatus has an imaging device for reading an image. As such an imaging device, typically, a CCD (Charge Coupled Device) image sensor, a CMOS (Complementary Metal Oxide Semiconductor) image sensor, an image reading sensor such as a CIS (Contact Image Sensor), and these image reading sensors. And an AD (Analog to Digital) converter for outputting an image signal.

  When the frequency of the drive clock as described above is spread, the frequency of the drive clock varies periodically, so that the image signal output from the imaging device includes a periodic variation. As a result, periodic density unevenness occurs in the read image. This is considered to be caused in particular by fluctuations in the sampling timing of the AD converter.

  By the way, in the image reading apparatus as described above, processing for correcting unevenness of sensitivity of the image reading sensor and unevenness of illumination of the original illumination generated by the optical component, that is, so-called shading correction is generally performed. In such shading correction, typically, a white plate for sensitivity correction provided in the image reading apparatus is read by the image reading sensor, and the sensitivity of the image reading sensor is corrected from density unevenness in the read image. To generate information (sensor sensitivity correction value). Then, when reading the document, image density correction is performed on the image data of the document while referring to the previously generated correction information.

  Such shading correction may be affected by the EMI countermeasures described above. Specifically, the density unevenness in the image caused by the frequency diffusion of the driving clock periodically occurs in the main scanning direction of the image reading sensor, but the sub-scanning period (one line reading period) and the frequency spreading When the frequency variation period is different, the behavior of the drive clock due to frequency spreading differs between lines. That is, if the periods of the two are different, the phase of the periodic density unevenness is shifted by a certain amount for each line, and it is necessary to prepare a different correction pattern for each line and perform shading correction. Therefore, simply using information (sensor sensitivity correction value) generated by reading the white plate for sensitivity correction with the image reading sensor cannot correctly remove the influence of the frequency diffusion of the drive clock.

In consideration of such issues, the following proposals have been made.
Japanese Patent Laid-Open No. 2001-339580 (Patent Document 1) discloses a method for removing periodic noise of an image signal by frequency spreading. The method disclosed in Patent Document 1 synchronizes the change in the frequency spreading factor by the frequency spreader with a main scanning synchronization signal that synchronizes in the main scanning direction when an image is read by the photoelectric conversion element. That is, in the method disclosed in Patent Document 1, the frequency spreader is reset in synchronism with the sub-scanning period so that the fluctuation phase of the image density unevenness caused by the frequency spread clock is matched between the lines.

  Japanese Patent Application Laid-Open No. 2008-113303 (Patent Document 2) discloses a document reading apparatus that strictly adjusts the timing of a clock or eliminates the need for complicated noise removal processing when taking measures against electromagnetic radiation noise by clock modulation. ing. In the method disclosed in Patent Document 2, the difference between the charge accumulation time is made constant by setting the relationship between the transport period Y and the diffusion period X to Y = aX + α (a is an integer).

JP 2001-339580 A JP 2008-113303 A

  However, in reality, it is not easy to synchronize the change of the frequency spreading factor by the frequency spreader disclosed in Patent Document 1 described above. This is because a general frequency spreader does not have a function to synchronize the change of the frequency spreading ratio, that is, to perform phase adjustment of the frequency fluctuation, and the frequency fluctuation period itself is different for each frequency spreader. Since there are individual differences, it is difficult to fix the phase of periodic density unevenness.

  In addition, the method disclosed in Patent Document 2 described above requires a mechanism for changing the document conveyance cycle, which may cause problems such as an increase in cost.

  That is, in order to remove density unevenness in the image due to frequency spreading of the drive clock without using a special frequency spreader or transport mechanism, the frequency variation due to frequency spreading does not match the sub-scanning cycle, that is, It is necessary to perform shading correction in a state in which the phase of periodic density unevenness changes with time. Such a method is not disclosed in the above prior art.

  Accordingly, the present invention has been made to solve such a problem, and an object of the present invention is to reduce the influence of the frequency diffusion of the drive clock with less configuration and to form an image on which the image reading apparatus is mounted. Is to provide a device.

  An image reading apparatus according to an aspect of the present invention includes a signal generation unit that generates a timing signal using one of a reference signal and a frequency spread signal obtained by frequency-spreading the reference signal according to a predetermined rule, and a timing signal from the signal generation unit An image pickup device for reading an image that operates according to the above, and a first image signal obtained by operating the image pickup device with a timing signal generated using the reference signal and reading a reference object having a predetermined reference color Acquisition of frequency spread time-varying characteristics based on the second image signal obtained by operating the imaging device with the timing signal generated using the frequency spread signal and reading the reference object And reading the object to be read by operating the imaging device with the timing signal generated using the frequency spread signal The third image signals, and a correcting means for outputting a fourth image signal on corrected using the correction value that reflects the time change characteristics.

  Preferably, the characteristic acquisition unit includes an eigenfunction including a constant indicating the time variation characteristic of the reference signal and the periodicity of the frequency spread, and is included in the eigenfunction based on the first image signal and the second image signal. By determining the constant, a correction value reflecting the time change characteristic is calculated.

  More preferably, the eigenfunction includes a constant indicating the amplitude, and the characteristic acquisition unit is configured to generate the eigenfunction based on a maximum value and a minimum value of the amplitude appearing in a difference signal between the first image signal and the second image signal. A constant indicating the amplitude included in the function is determined.

  Alternatively, more preferably, the eigenfunction includes a constant indicating a phase, and the characteristic acquisition unit has a maximum value and / or a minimum value of an amplitude appearing in a difference signal between the first image signal and the second image signal. A constant indicating the phase included in the eigenfunction is determined based on the position.

  More preferably, the correction unit updates the value indicating the phase of the time change characteristic according to the reading operation of the imaging device.

  More preferably, the imaging device has a line-shaped detection surface extending in the first direction, and the relative position between the imaging device and the object to be read is changed in a second direction orthogonal to the first direction. The correction means is configured to sequentially scan the image line by line, and the correction unit changes the phase of the time change characteristic by a certain amount for each line scan by the imaging device.

  An image forming apparatus according to another aspect of the present invention is a signal generator that generates a timing signal using one of an image forming unit that forms an image, and a reference signal and a frequency spread signal obtained by frequency spreading a reference signal according to a predetermined rule. A reference object having a predetermined reference color by operating the imaging device with a timing signal generated using a reference signal and an imaging device for reading an image that operates according to a timing signal from the signal generator Based on the first image signal obtained by reading and the second image signal obtained by operating the imaging device with the timing signal generated using the frequency spread signal and reading the reference object, the frequency Read by operating the imaging device with the characteristic acquisition means for acquiring the time-varying characteristics of the spread and the timing signal generated using the frequency spread signal. A third image signal obtained by reading the target object, and a correcting means for outputting a fourth image signal on corrected using the correction value that reflects the time change characteristics.

  According to the present invention, it is possible to remove the influence of the frequency spread of the drive clock with a smaller configuration.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. It is a block diagram for realizing an image reading function of the image forming apparatus according to the embodiment of the present invention. It is a figure which shows an example of the frequency spread of a drive clock, and the image signal from a corresponding image sensor. It is a figure explaining the principle which a density nonuniformity produces by carrying out frequency spreading of the drive clock. It is a figure which shows an example of the density nonuniformity produced by carrying out frequency spreading of the drive clock. It is a figure for demonstrating the calculation process of the image data according to embodiment of this invention. It is a schematic diagram which shows the imaging device and its periphery structure according to embodiment of this invention. It is a figure for demonstrating the whole process of the image reading operation according to embodiment of this invention. It is a figure which shows the function for calculating the image density nonuniformity correction value according to embodiment of this invention. FIG. 6 is a diagram illustrating an example of an image signal obtained by correcting an image signal obtained by operating an imaging device with a frequency-spread drive clock and reading a white plate with a shading correction value in the embodiment of the present invention. is there. It is a figure which shows the image density nonuniformity correction value in each line according to embodiment of this invention. It is a flowchart which shows the process sequence of image reading operation in the image forming apparatus according to the embodiment of the present invention.

  Embodiments of the present invention will be described in detail with reference to the drawings. Note that the same or corresponding parts in the drawings are denoted by the same reference numerals and description thereof will not be repeated.

<A. Overview>
In the image reading apparatus according to the present embodiment, the influence of density unevenness that appears due to frequency diffusion of the drive clock is removed with a correction value that is dynamically changed for each line, instead of a fixed correction value as in the prior art. . Hereinafter, such correction is also referred to as “image density unevenness correction”.

  As a method of making the correction value different for each line in this image density unevenness correction, an eigenfunction indicating a time change characteristic (period etc.) of a reference signal (reference clock) and a periodicity (phase etc.) of frequency spread is prepared in advance. deep. This eigenfunction includes the number of lines and the pixel position in each line as variables, and also includes a constant for reflecting the physical state. Then, by calculating the values of these constants based on the image signal obtained at the time of shading correction, a correction value reflecting the influence of frequency spreading is determined in the form of a function. Further, at the time of subsequent document reading, the determined function is updated according to each line, thereby sequentially calculating correction values suitable for each line. By executing the image density unevenness correction using the sequentially calculated correction values, the influence of frequency spreading of the drive clock is removed.

<B. Overall configuration>
The present invention can be applied to any apparatus having an image reading function, and typically includes a multi-function peripheral (MFP), a copier, a facsimile, an image scanner, an industry, and the like. Applicable to cameras. Hereinafter, as a typical example of the image forming apparatus according to the present invention, a multi-function machine equipped with a plurality of functions such as a copying function, a printing function, a facsimile function, and an image scanner function will be described.

  FIG. 1 is a schematic configuration diagram of an image forming apparatus MFP according to an embodiment of the present invention. Referring to FIG. 1, image forming apparatus MFP includes an automatic document feeder 2, an image scanner 3, a print engine 4, and a paper feeder 5.

  The automatic document transport unit 2 is for performing continuous scanning of a document, and includes a document feed table 21, a delivery roller 22, a registration roller 23, a transport drum 24, and a sheet discharge table 25. . The document to be scanned is placed on the document feeding table 21 and is sent out one by one by the operation of the sending roller 22. Then, the fed document is temporarily stopped by the registration roller 23 and the leading edge is adjusted, and then conveyed to the conveyance drum 24. Further, the original rotates integrally with the drum surface of the transport drum 24, and the image surface is scanned by an image scanner 3 described later in the process. Thereafter, the document is separated from the drum surface at a position approximately half the circumference of the drum surface of the transport drum 24 and is discharged to the discharge table 25.

  The image scanner 3 includes an imaging device 33 and a document table 35. The imaging device 33 reads an image of a document by temporally changing the relative position with respect to the document that is the subject. The imaging device 33 includes, as main components, a light source that irradiates light on a document, an image sensor that acquires an image generated by reflection of light irradiated from the light source, and an image signal output from the image sensor. An AD (Analog to Digital) converter, and an imaging optical system disposed in front of the image sensor. The document to be scanned can also be placed on the document table 35.

  In this specification, the direction in which the image scanner 3 electrically scans the document is defined as the main scanning direction (X direction shown in FIG. 1), and the direction in which the image scanner 3 and the document move relative to each other is the sub-scanning direction (see FIG. 1). 1 in the Y direction). The Z direction shown in FIG. 1 indicates a direction orthogonal to the document surface.

  For example, the print engine 4 executes an electrophotographic image forming process. Specifically, full-color print output is possible. Specifically, the print engine 4 includes imaging (imaging) units 44Y, 44M, 44C, which generate toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Includes 44K. The imaging units 44Y, 44M, 44C, and 44K are arranged in that order along the transfer belt 27 that is stretched and driven in the print engine 4.

  The imaging units 44Y, 44M, 44C, and 44K include image writing units 43Y, 43M, 43C, and 43K, and photosensitive drums 41Y, 41M, 41C, and 41K, respectively. Each of the image writing units 43Y, 43M, 43C, and 43K includes a laser diode that emits a laser beam corresponding to each color image included in the target image data, and a corresponding photoconductive drum 41Y, 41M by deflecting the laser beam. , 41C, 41K, and polygon mirrors that expose the surfaces in the main scanning direction.

  Electrostatic latent images are formed on the surfaces of the photosensitive drums 41Y, 41M, 41C, and 41K by the above-described exposure by the image writing units 43Y, 43M, 43C, and 43K, and the electrostatic latent images correspond respectively. A toner image is developed by toner particles supplied from the toner units 441Y, 441M, 441C, and 441K.

  The color toner images developed on the surfaces of the photosensitive drums 41Y, 41M, 41C, and 41K are sequentially transferred to the transfer belt 27. Further, the toner image superimposed on the transfer belt 27 is further transferred to the recording paper supplied from the paper supply unit 5 in time.

  The toner image transferred onto the recording paper is fixed at a fixing portion disposed at the downstream portion and then discharged onto the tray 57.

  In parallel with the operations of the photosensitive drums 41Y, 41M, 41C, and 41K described above, the feeding rollers 52, 53, and 54 and the manual paper feeding unit 26 respectively corresponding to the paper feeding cassette of the paper feeding unit 5 that stores the recording paper. Among these, the part corresponding to the recording paper to be used for image formation operates to supply the recording paper. The supplied recording paper is conveyed by the conveying rollers 55 and 56 and the timing roller 51, and is fed to the photosensitive drum 41 so as to be synchronized with the toner image formed on the photosensitive drum 41.

  The transfer unit 45 applies a voltage of opposite polarity to the photosensitive drum 41 to transfer the toner image formed on the photosensitive drum 41 to a recording sheet. The static eliminator 46 removes the recording paper from which the toner image has been transferred, thereby separating the recording paper from the photosensitive drum 41. Thereafter, the recording paper on which the toner image is transferred is conveyed to the fixing device 47. As the transfer device 45, a transfer method using a transfer charger or a transfer roller may be adopted instead of the transfer method using a transfer belt as shown in FIG. Alternatively, instead of the direct transfer method in which the toner image is directly transferred from the photosensitive drum 41 to the recording paper, an intermediate transfer member such as a transfer roller or a transfer belt is disposed between the photosensitive drum 41 and the recording paper. You may make it perform transfer by the process of a step or more.

  The fixing device 47 includes a heating roller 471 and a pressure roller 472. The heating roller 471 heats the recording paper to melt the toner transferred thereon, and the melted toner is fixed on the recording paper by the compression force between the heating roller 471 and the pressure roller 472. The Then, the recording paper is discharged to the tray 57. The fixing device 47 may employ a fixing method using a fixing roller or a non-contact fixing method instead of the fixing method using a fixing belt as shown in FIG.

  On the other hand, in the photosensitive drum 41 from which the recording paper has been separated, after the residual potential is removed, the residual toner is removed and cleaned by the cleaning unit 48. Then, the next image forming process is executed. For example, the cleaning unit 48 removes and cleans residual toner by using a cleaning blade, a cleaning brush, a cleaning roller, or a combination thereof. Alternatively, a cleaner-less system that collects residual toner using the photoconductive drums 41Y, 41M, 41C, and 41K instead of the cleaning unit 48 may be employed.

  The IDC sensor 49 detects the density of the toner image formed on the photosensitive drum 41. The IDC sensor 49 is a light intensity sensor typically composed of a reflection type photosensor, and detects the intensity of reflected light from the surface of the photosensitive drum 41.

<C. Main part of image reading function>
Next, a main configuration for realizing the image reading function installed in image forming apparatus MFP according to the present embodiment will be described.

  FIG. 2 is a block diagram for realizing the image reading function of image forming apparatus MFP according to the embodiment of the present invention. FIG. 2 shows a configuration of an electric circuit for realizing the image reading function. Referring to FIG. 2, image forming apparatus MFP has, as its image reading function, reference clock generator 102, frequency spreader 104, switch 106, drive clock generator 108, imaging device 33, and image processing. Part 120 and memory 122.

  The reference clock generator 102 generates a clock having a predetermined period. This clock may have any waveform as long as it has periodicity. Typically, a square (pulse) wave or a sawtooth wave is used as the clock. The clock from the reference clock generator 102 is input to the frequency spreader 104 and the switch 106.

  The frequency spreader 104 spreads the frequency of the clock from the reference clock generator 102 at a predetermined period, and outputs the clock after the frequency spread to the switch 106. In the frequency spreading, when viewed in the frequency domain, the frequency (period) of the clock from the reference clock generator 102 is temporally changed according to a predetermined rule.

  The switch 106 outputs one of the clock from the reference clock generator 102 and the frequency spread clock from the frequency spreader 104 to the drive clock generator 108. The switch 106 switches the clock according to a command from the image processing unit 120.

  The drive clock generator 108 generates a drive clock according to the clock from the switch 106 or the frequency spread clock. The drive clock generated by the drive clock generator 108 is output to the image sensor 110, the AD converter 112, and the image processing unit 120. Therefore, the presence / absence of frequency spreading for the image sensor 110, the AD converter 112, and the image processing unit 120 can be changed by the switching operation of the switch 106. As described above, the drive clock generator 108 is a signal generator that generates a timing signal (drive clock) using one of the reference signal (reference clock) and the frequency spread signal obtained by frequency-spreading the reference signal according to a predetermined rule. Equivalent to.

  The imaging device 33 is an electronic component for reading an image, and includes an image sensor 110 and an AD converter 112 as main components. All of these operate according to the drive clock from the drive clock generator 108.

  The image sensor 110 is typically composed of a charge coupled device (CCD) image sensor, a complementary metal oxide semiconductor (CMOS) image sensor, a contact image sensor (CIS), and the like.

  The AD converter 112 outputs an image signal from the image sensor 110. That is, the AD converter 112 samples the output signal of the image sensor 110 (potential appearing according to the electric charge) and outputs it as a digital value.

  The image processing unit 120 performs image processing (image density unevenness correction), which will be described later, on the image signal (output signal) from the imaging device 33, thereby correcting density unevenness that appears by frequency spreading the drive clock. Correct the effect. The image processing unit 120 also performs so-called shading correction. Then, the image processing unit 120 outputs the corrected image data to a print engine (not shown) or the like.

  The image processing unit 120 includes a CPU (Central Processing Unit), a work memory, and the like as main components. Typically, image processing as will be described later is realized by the CPU developing and executing a program stored in advance on a work memory. Alternatively, part or all of the functions realized by the CPU executing the program may be configured by dedicated hardware. The program executed by the CPU is a program module provided as a part of a computer operating system (OS), which calls necessary modules in a predetermined arrangement at a predetermined timing to execute processing. May be. In this case, the program itself does not include the module, and the process is executed in cooperation with the OS.

  The memory 122 holds various data for performing image processing as will be described later in the image processing unit 120. The memory 122 holds an eigenfunction (image unevenness correction function F (t)) for realizing the image density unevenness correction according to the present embodiment and various constants included therein.

<D. Density unevenness and problems due to frequency spreading>
First, the effect of density unevenness that appears by frequency spreading the drive clock will be described.

  FIG. 3 is a diagram illustrating an example of frequency spreading of the drive clock and a corresponding image signal from the image sensor 110. FIG. 4 is a diagram for explaining the principle of density unevenness caused by frequency spreading of the drive clock. FIG. 5 is a diagram illustrating an example of density unevenness caused by frequency spreading the drive clock.

  The frequency-spread drive clock output from the drive clock generator 108 changes its frequency (cycle) with time as shown in FIG. If the drive clock is a square (pulse) wave, the time (cycle) from the appearance of the preceding pulse to the appearance of the subsequent pulse increases or decreases in a constant cycle. This increase / decrease method (repetition pattern) differs depending on the frequency spreader 104.

  The image signal output from the image sensor 110 fluctuates when the imaging device 33 is driven with a frequency-spread drive clock as shown in FIG. That is, even when an object having the same color (for example, a white plate for sensitivity correction) is read by the imaging device 33, the image signal from the image sensor 110 is frequency spread as shown in FIG. It periodically changes in conjunction with the generated drive clock. As can be seen by comparing FIG. 3A and FIG. 3B, the repetition period of the image density unevenness coincides with the diffusion period of the drive clock.

  Such fluctuations in the image signal are considered to be mainly caused by fluctuations in the sampling timing of the AD converter 112. FIG. 4 shows an example of a temporal change in the output signal (voltage) of the image sensor 110.

  When the image sensor 110 reads an image, the output circuit is sequentially switched to the reset mode, the black level output mode, and the image level output mode according to the drive pulse, and the output voltage changes. More specifically, by switching the internal circuit of the CCD included in the image sensor 110 and switching modes, three types of voltages are repeatedly output. In the black level output mode, a predetermined voltage is output, and in the image level output mode, a voltage proportional to the amount of charge accumulated in the photodiode is output.

  Since the output voltage (analog waveform) has delay and dullness, the AD converter 112 samples the value after a certain period (after counting a certain clock) after switching the mode in the CCD. For example, it is assumed that the AD converter 112 is configured to sample at time t1 and time t2 specified by the number of pulses included in the drive clock. Time t1 and time t2 correspond to a certain period after the switching timing from the reset mode to the black level output mode and the switching timing from the black level output mode to the image level output mode, respectively. In this case, referring to FIG. 4A, the AD converter 112 outputs a differential voltage ΔV between the voltage Vt1 at time t1 and the voltage Vt2 at time t2 as an image signal detected by the image sensor 110.

  If the cycle of the drive clock fluctuates due to frequency spreading, the timing from when the CCD mode is switched to when sampling is performed changes. This change in timing causes fluctuations in the sampling voltage of the black level and the image level, and image density unevenness occurs. Note that the delay and dullness (falling) of the voltage waveform output from the CCD is determined by the output circuit inside the CCD and the analog amplification circuit outside the CCD, and thus is substantially unaffected by frequency spreading.

  More specifically, as shown in FIG. 4B, the timing of sampling each image signal with the cycle of the drive clock fluctuating is the time t1 ′ (= t1 + Δt1) and the time t2 ′ (= t2 + Δt2). Suppose that each changed. In this case, the AD converter 112 detects the difference voltage ΔV ′ between the voltage Vt1 ′ (= Vt1−ΔVt1) at time t1 ′ and the voltage Vt2 ′ (= Vt2−ΔVt2) at time t2 ′ by the image sensor 110. Output as an image signal.

  Although Δt1 (= t1′−t1) and Δt2 (= t2′−t2) shown in FIG. 4B have substantially the same time width, the slope characteristics of the CCD output voltage at the corresponding times are different. The voltage fluctuations ΔVt1 and ΔVt2 generated as image signals do not coincide with each other, and as a result, the difference ΔVt1−ΔVt2 between them becomes density unevenness caused by frequency spreading of the drive clock.

  For the reasons described above, frequency fluctuation of the drive clock causes periodic fluctuations in the image signal output from the imaging device 33.

  Such periodic fluctuations included in the image signal appear as image density unevenness as shown in FIG. That is, FIG. 5A shows a conceptual diagram of image density unevenness generated in one line in the main scanning direction by the frequency spread clock. Note that one square shown in FIG. 5 represents one pixel. 5B and 5C show an example in which the horizontal direction on the paper surface is the main scanning direction and the vertical direction on the paper surface is the sub-scanning direction.

  Here, as disclosed in the above-mentioned Patent Document 1, the change in the frequency spreading factor by the frequency spreader is synchronized with a main scanning synchronization signal that synchronizes in the main scanning direction when an image is read by the photoelectric conversion element. If it is possible, as shown in FIG. 5B, the image density unevenness position (repetitive phase) can be matched between the lines. That is, if means for synchronizing the spreading cycle of the frequency spreading clock and the sub-scanning cycle is used, it is sufficient to use an image signal for one line shown in FIG. 5B as a correction pattern for correcting image density unevenness. .

  In the case shown in FIG. 5B, the sensitivity correction white board provided in the image forming apparatus MFP is read by the imaging device 33 and corrected based on the density unevenness in the read image. In addition, it is possible to simultaneously remove unevenness in sensitivity of the image sensor 110, unevenness in illumination of optical components, and unevenness in density caused by frequency diffusion of the drive clock.

  However, as described above, realizing the method disclosed in Patent Document 1 is not easy in practice.

  Therefore, in the normal configuration, the image density unevenness in the image as shown in FIG. In general, since the spreading cycle by the frequency spreader 104 and the scanning cycle for one line by the image sensor 110 do not coincide with each other, the occurrence position (repetitive phase) of unevenness is shifted by a certain amount in each line. This amount of phase shift is related to the sub-scanning period and the frequency spreading period.

  Therefore, in the present embodiment, the density unevenness in the image shown in FIG. 5C is defined as a function including the pixel position as a variable, and image density unevenness correction is performed using this function.

<E. Overall processing>
Next, overall processing of the image reading function according to the present embodiment will be described. In the present embodiment, the shading correction value X (t) for correcting the sensitivity unevenness of the image sensor 110 and the illuminance unevenness of the optical component (hereinafter also referred to as “shading correction”) and the image density unevenness correction are performed. Therefore, correction processing is performed using the image density unevenness correction value Y ^(s) (t). As shown in FIG. 1, the image sensor 110 is a line sensor in which a plurality of pixels are arranged in the main scanning direction, and the variable t of these correction values is a pixel number in the main scanning direction (t = 1, 2,... , N), and the variable s of the image density unevenness correction value is a scanning (line) number (s = 1, 2,..., M) in the sub-scanning direction.

FIG. 6 is a diagram for describing calculation processing of image data according to the embodiment of the present invention. Referring to FIG. 6, shading is performed on the image signal V ^(s) (t) (FIG. 6A) acquired by operating the imaging device 33 with the frequency-spread drive clock and reading the document. Correction is performed by correcting (typically calculating the difference) with the correction value X (t) (FIG. 6B) and the image density unevenness correction value Y ^(s) (t) (FIG. 6C). Later image data is calculated.

  As will be described later, the shading correction value X (t) is obtained by operating the imaging device 33 with a drive clock that is not frequency-spread and reading the white plate for sensitivity correction with the imaging device 33. This shading correction value X (t) is an eigenvalue in the t-th pixel of the imaging device 33.

In conjunction with the acquisition of the shading correction value X (t), the imaging device 33 is operated with the frequency-spread drive clock, and the imaging device 33 reads the white plate. Then, the shading correction value X (t) read by operating the imaging device 33 with the drive clock that has not been frequency spread, and the image signal Y ₀ read by operating the imaging device 33 with the drive clock that has been frequency spread. Using (t), a function for calculating the image density unevenness correction value Y ^(s) (t) is determined.

Subsequently, the image signal V ^(s) (t) is acquired by operating the imaging device 33 with the frequency-spread drive clock and reading the document. Further, for each line, the image density unevenness correction value Y ^(s) (t) is calculated sequentially, and the image signal V ^(s) (t) is converted into the shading correction value X (t) and the corresponding image density unevenness correction value Y. ^{(S) The} corrected image data is calculated by correcting in (t).

  FIG. 7 is a schematic diagram showing the imaging device 33 and its peripheral configuration according to the embodiment of the present invention. FIG. 8 is a diagram for describing the overall processing of the image reading operation according to the embodiment of the present invention.

  FIG. 7 schematically shows a plan view of the image scanner 3 in FIG. 1 as viewed along the Z direction from below in the drawing. The image scanner 3 includes an original platen 35 on which platen glass is disposed, and a white plate 37 for sensitivity correction provided at a position away from the original platen 35 along the Y direction of the original platen 35 by a predetermined distance. In addition, a window 38 is provided in association with the document transport path by the automatic document transport unit 2.

  The imaging device 33 is disposed at the position of the white plate 37 in the initial stage of the image reading operation. When reading a document placed on the platen glass, the imaging device 33 sequentially moves to the right side of the drawing along the Y direction. When reading a document automatically conveyed by the automatic document conveyance unit 2, the imaging device 33 moves to the left side of the drawing along the Y direction and stands by at the position of the window unit 38.

  The imaging device 33 reads an image for each line and outputs it as an image signal.

  Thus, the imaging device 33 has a linear detection surface extending in the first direction (X direction), and the relative position between the imaging device 33 and the object to be read (document) is defined as the first direction. Is configured to sequentially scan an image line by line while changing in a second direction (Y direction) perpendicular to each other.

As shown in FIG. 7, in the imaging device 33 according to the present embodiment, since the white plate 37 enters the detection range during the reading operation, the image signal at this time is used in addition to the shading correction value X (t). Then, information for calculating the image density unevenness correction value Y ^(s) (t) is acquired.

  That is, as shown in FIG. 8A, the imaging device 33 according to the present embodiment first passes through the area of the white plate 37 and enters a document reading operation. At the time of reading the white plate 37 by the imaging device 33, as shown in FIG. 8B, the drive clock is switched between a state where the frequency is not spread and a state where the frequency is spread. Note that the switching order is not particularly limited, but a frequency-spread drive clock is used at the time of subsequent document reading, so that the frequency spread state is set first and then the frequency spread state is switched. It is preferable.

As shown in FIG. 8C, when the imaging device 33 is present at the position of the white plate 37, the image signal (shading correction value X ( t)) is read, and the image signal Y ₀ (t) is read by operating the imaging device 33 with the frequency-spread drive clock.

Furthermore, a function that defines the image density unevenness correction value Y ^(s) (t) is determined based on these image signals.

Subsequently, when the imaging device 33 is present at the position of the document, the imaging device 33 is driven by the frequency-spread drive clock, and the image signal V ^(s) (t) obtained by reading the document. Are output line by line.

First, when the image signal V ⁽¹⁾ (t) of the first line is output, the corresponding phase shift amount is calculated, and then the corresponding image density unevenness correction value Y ⁽¹⁾ (t) is determined. The Then, after the image signal V ⁽¹⁾ (t) is corrected using the determined image density unevenness correction value Y ⁽¹⁾ (t) and the shading correction value X (t), the corrected image data Is output as

  Thereafter, the same processing is repeated until the reading of the second line, the third line, and the original is completed.

As described above, the series of processing shown in FIGS. 6 and 8 is executed by the image processing unit 120 (FIG. 2). That is, the image processing unit 120 operates the imaging device 33 with a timing signal (driving clock that is not frequency-spread) generated using a reference signal (reference clock), and a reference object (white plate) having a predetermined reference color The imaging device 33 is operated with the first image signal (shading correction value X (t)) obtained by reading the image) and the timing signal (frequency-spread drive clock) generated using the frequency spread signal. , And functions as a characteristic acquisition unit that acquires time-varying characteristics of frequency spread based on the second image signal (image signal Y ₀ (t)) obtained by reading the reference object (white plate). Furthermore, the image processing unit 120 operates the imaging device 33 with a timing signal (frequency-spread drive clock) generated using the frequency spread signal to obtain a third object obtained by reading the object to be read (original). The image signal (image signal V ^(s) (t)) is corrected using the correction value (Y ^(s) (t)) reflecting the time change characteristic and then the fourth image signal (corrected image data). ) Function as correction means for outputting as

<F. Image density unevenness correction value calculation processing>
(F1: Eigenfunction)
As a function for defining the above-described image density unevenness correction value Y ^(s) (t), the following general formula is used. That is, the correction value Y ^(s) (t) for the t-th (t = 1, 2,..., N) pixel for the image data for one line of the s-th line (s = 1, 2,..., M). Is
Y ^(s) (t) = B × F (t−nT−A (s))
Can be expressed as

  Here, T is a frequency spreading period in the frequency spreader 104, F is a function indicating a time waveform of a clock generated by the reference clock generator 102 (or the drive clock generator 108), and A (s ) Is the phase shift amount for the function F in the s-th line (s = 1, 2,..., M), and B is the amplitude of the sensitivity unevenness correction value.

FIG. 9 is a diagram showing a function for calculating the image density unevenness correction value according to the embodiment of the present invention. In the image reading function according to the present embodiment, the image signal (shading correction value X (t)) read by operating the imaging device 33 with a drive clock that is not frequency spread, and the imaging device with a frequency spread drive clock. Using the image signal Y ₀ (t) read by operating No. 33, the phase shift amount A (s) and the sensitivity unevenness correction coefficient B which is the amplitude shown in FIG. 9 are determined.

  FIG. 9 illustrates a case where the function F is a sawtooth wave corresponding to the drive clock shown in FIG. 3, but the time waveform of the function F differs depending on the type of the drive clock. In addition, the sensitivity unevenness correction coefficient B differs depending on the apparatus due to individual differences of the imaging devices 33 or the like.

  Since the function F has periodicity, the phase shift amount A (s) is determined so that the conditions of t−nT−A (s) <T and A (s) <T are satisfied. The function F is a repetitive pattern indicating repetitive density unevenness specific to the apparatus depending on the sub-scanning period, the diffusion period of the frequency spreader, and the characteristics of the imaging device 33.

  The function F and the period T that define the eigenfunction as described above are stored in the memory 122 in advance, and the image processing unit 120 reads out these values and executes the processing.

As described above, the image processing unit 120 uses the first image signal (for the eigenfunction including constants indicating the time variation characteristic (function F) of the reference signal (reference clock) and the periodicity of the frequency spread (period T) ( By determining a constant included in the eigenfunction based on the shading correction value X (t)) and the second image signal (image signal Y ₀ (t)), the image density unevenness correction value Y, which is a time-varying characteristic, is determined. ^(S) Calculate (t).

(F2: Determination of sensitivity unevenness correction coefficient B)
First, a method of determining the sensitivity unevenness correction coefficient B that is the amplitude of the image density unevenness correction value Y ^(s) (t) will be described. This sensitivity unevenness correction coefficient B corrects the image signal Y ₀ (t) obtained by operating the imaging device 33 with the frequency-spread drive clock and reading the white plate 37 with the shading correction value X (t). It is determined based on the image signal obtained in (1).

FIG. 10 shows an image signal Y ₀ (t) obtained by operating the imaging device 33 with a frequency-spread drive clock and reading the white plate 37 in the embodiment of the present invention as a shading correction value X (t). It is a figure which shows an example of the image signal obtained by correct | amending. More specifically, the image signal shown in FIG. 10 corresponds to the image signal Y ₀ (t) minus the shading correction value X (t).

Then, processing for fitting the image density unevenness correction value Y ^(s) (t) to the image signal shown in FIG. 10 is performed. As an example, the maximum value and the minimum value are extracted from the image signal shown in FIG. 10, and the sensitivity unevenness correction coefficient B is calculated from the difference between the extracted maximum value and minimum value. That is, the sensitivity unevenness correction coefficient B (amplitude) = {(maximum value) − (minimum value)} / 2 is calculated.

As described above, the eigenfunction for calculating the image density unevenness correction value Y ^(s) (t) includes a constant (sensitivity unevenness correction coefficient B) indicating the amplitude, and the image processing unit 120 uses the first image signal (shading). A constant indicating the amplitude included in the eigenfunction based on the maximum value and the minimum value of the amplitude appearing in the difference signal between the correction value X (t)) and the second image signal (image signal Y ₀ (t)). decide.

(F3: Determination of initial phase shift amount A (0))
Furthermore, among the image signals shown in FIG. 10, the pixel having the maximum value or the pixel having the minimum value is specified, and the initial phase is determined from at least one of these specified positions (pixel number). A deviation amount A (0) is determined.

Thus, the eigenfunction for calculating the image density unevenness correction value Y ^(s) (t) includes a constant indicating the phase (phase shift amount A (s)), and the image processing unit 120 uses the first image signal. Based on the position where the amplitude appearing in the difference signal between the (shading correction value X (t)) and the second image signal (image signal Y ₀ (t)) becomes the maximum value and / or the minimum value, A constant indicating the phase to be included is determined.

(F4: Update of phase shift amount)
As described above, since the phase shift amount A (s) of the function F varies for each line, the phase shift amount A (s) is updated for each line. More specifically, since the phase fluctuation amount between the lines is an eigenvalue determined by the frequency spreading period and the sub-scanning period in the frequency spreader 104, if this phase fluctuation quantity is acquired in advance as a, The phase shift amount A (s) can be calculated sequentially from the phase shift amount A (0).

  That is, after specifying the initial phase shift amount A (0), it can be updated by sequentially adding the phase fluctuation amount a stored in the memory 122 in advance every time one line is scanned. Note that if there is any blank time after the initial phase shift amount A (0) is specified and before the first line of the document is scanned, the blank time is measured and the phase of the first line is measured. What is necessary is just to calculate deviation | shift amount A (1).

FIG. 11 shows an image density unevenness correction value in each line according to the embodiment of the present invention. FIG. 11A shows the waveform of the image density unevenness correction value Y ⁽¹⁾ (t) of the first line, and FIG. 11B shows the image density unevenness correction value Y ⁽ 2) of the second line following that. ²⁾ The waveform of (t) is shown. When the phase fluctuation amount between the lines is a, between the phase shift amount A (1) of the first line and the phase shift amount A (2) of the second line,
A (2) = A (1) + a
The relationship is established.

Similarly, the image density unevenness correction value Y ⁽³⁾ (t) for the third line shown in FIG.
A (3) = A (2) + a = A (1) + a + a
This is determined using the phase shift amount A (3) of the third line calculated according to the calculation formula.

  As described above, the image processing unit 120 updates the value indicating the phase of the time change characteristic (phase shift amount A (s)) according to the reading operation of the imaging device 33.

(F5: Summary)
As described above, the image density unevenness correction value Y ^(s) (t) = B × F (t−nT−A ( ^{) in} order to perform the image density unevenness correction generated by frequency spreading the drive clock before reading the document. s)) and the shading correction value X (t) and the image density unevenness correction value Y ^(s) (t) with respect to the image signal V ^(s) (t) read line by line during the subsequent document reading. By performing the two types of corrections described above, it is possible to remove the sensitivity unevenness of the image sensor 110, the illuminance unevenness of the optical component, and the density unevenness caused by frequency diffusion of the drive clock.

<G. Processing procedure>
FIG. 12 is a flowchart showing a processing procedure of an image reading operation in image forming apparatus MFP according to the embodiment of the present invention.

  Referring to FIG. 12, when an image reading instruction is received from a user or the like (step S2), image forming apparatus MFP starts generating a drive clock that is not frequency spread (step S4). Subsequently, image forming apparatus MFP positions imaging device 33 on white plate 37 for image correction, and reads white plate 37 with imaging device 33 to obtain shading correction value X (t) (step S6).

Subsequently, image forming apparatus MFP restarts the frequency spreading of the drive clock (step S8). In other words, the frequency-spread drive clock is supplied to the imaging device 33. Subsequently, image forming apparatus MFP obtains image signal Y ₀ (t) by reading white plate 37 with imaging device 33 (step S10).

Thereafter, image forming apparatus MFP corrects image signal Y ₀ (t) acquired in step S10 with shading correction value X (t) acquired in step S6 (step S12), and the correction obtained by the correction in step S12. From the subsequent image signal, the sensitivity unevenness correction coefficient B included in the image density unevenness correction value Y ^(s) (t) is determined (step S14). Further, image forming apparatus MFP determines initial phase shift amount A (0) included in image density unevenness correction value Y ^(s) (t) from the corrected image signal obtained by the correction in step S12. (Step S16). Through the above processing, a function that defines the image density unevenness correction value Y ^(s) (t) is determined.

  Thereafter, image forming apparatus MFP positions imaging device 33 at the document reading position, and starts a document reading operation with imaging device 33 (step S18).

Image forming apparatus MFP obtains image signal V ^(s) (t) indicating the original for one line by imaging device 33 (step S20), and calculates phase shift amount A (s) in the line (step S22). ). Subsequently, image forming apparatus MFP calculates image density unevenness correction value Y ^(s) (t) using phase shift amount A (s) determined in step S22 (step S24). Thereafter, image forming apparatus MFP uses image signal V ^(s) (t) acquired in step S20, shading correction value X (t) acquired in step S6, and image density unevenness correction value Y calculated in step S24. ^(S) Correction is performed at (t) (step S26). Image forming apparatus MFP outputs the corrected image signal (step S28).

  Thereafter, image forming apparatus MFP determines whether or not reading of the entire area of the document has been completed (step S30). If reading of the document has not been completed (NO in step S30), image forming apparatus MFP positions imaging device 33 at the reading position of the next line, and starts a document reading operation with imaging device 33 (step S32). ). And the process after step S20 is repeated.

  Image forming apparatus MFP completes the image reading process when reading of the entire area of the document has been completed (YES in step S30).

<H. Other Embodiments>
In the above-described embodiment, the processing example in the case where the imaging device 33 including the line sensor is used has been described. In this processing example, the image correction is performed for each line. However, since the correction value for each pixel can be calculated as described above, the image correction may be performed for one pixel or a plurality of pixels.

<I. Another aspect>
As another aspect, the present invention can be further expressed as follows.

  An image reading apparatus according to still another aspect of the present invention includes an image reading unit that reads an image of a document, an image correction white plate for correcting image density unevenness, and image data output from the image reading unit. An image arithmetic device for performing correction, a reference clock generating means for generating a reference clock, a frequency spreader for periodically spreading the frequency of the clock based on the reference clock, and an image reading device for the frequency spread clock or the reference clock The image correction value X (t) for one line calculated from the image data obtained when the reference clock is supplied to the image reading means and the frequency spread clock are obtained when the frequency spread clock is supplied to the image reading means. And an image correction value Y (t) for correcting image density unevenness by the frequency spreader calculated from the obtained image data.

  Preferably, the image reading device has an image correction pattern F (t) unique to the image reading device stored in the image calculation device, and an image obtained by applying a correction value X (t) to the image data obtained when the frequency spread clock is supplied. From the data. The maximum value and the minimum value of density unevenness are extracted, and the image correction value Y (t) is calculated by applying the aforementioned maximum value and minimum value to the image correction pattern F (t).

  More preferably, the image reading device specifies the position where the image data is maximized or minimized from the image data obtained by applying the correction value X (t) to the image data obtained when the frequency spread clock is supplied, and performs image correction. A phase shift amount A of the image correction pattern F (t) at the value Y (t) is calculated.

  Alternatively, more preferably, the image reading device calculates the image correction values X (t) and Y (t) while scanning the image correction white board, and after the calculation, the image correction is performed by a fixed amount a for each line scan. The phase shift amount A of the value Y (t) is changed.

<J. Advantage>
As described above, in the image reading mechanism according to the present embodiment, when the white plate for sensitivity correction is read, the frequency diffusion is temporarily stopped, and the image signal acquired by the imaging device 33 in a state where there is no density unevenness due to the frequency diffusion. From V (t), the correction value X (t) of the sensitivity unevenness of the image sensor 110 and the illuminance unevenness of the optical component is calculated, and then the image data V (t) acquired by the imaging device 33 in a state where the frequency diffusion is resumed. And the correction value X (t), a correction value Y (t) that varies from line to line for correcting image density unevenness that occurs periodically due to frequency spreading and that shifts in phase of each line is calculated. Two types of corrections X (t) and Y (t) are performed on the image data V (t) at the time of document reading.

  In this way, by performing two types of correction, that is, a correction value X (t) that is constant for each line and Y (t) that varies from line to line, the sensitivity unevenness of the image sensor 110, the illumination unevenness of the optical component, In addition, it is possible to remove density unevenness caused by frequency diffusion of the drive clock and obtain an image without image density unevenness.

  Thus, according to the present embodiment, an image from which image density unevenness has been removed can be obtained even with an image reading apparatus or an image forming apparatus that has taken EMI countermeasures.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  2 Automatic Document Feeder, 3 Image Scanner, 4 Print Engine, 5 Paper Feeder, 21 Document Feeder, 22, 52, 53, 54 Delivery Roller, 23 Registration Roller, 24 Transport Drum, 25 Paper Discharge Stand, 26 Hands Differential feeding section, 27 transfer belt, 33 imaging device, 35 document table, 37 white plate, 38 window section, 41, 41Y, 41M, 41C, 41K photosensitive drum, 43Y, 43M, 43C, 43K image writing section, 44Y , 44M, 44C, 44K Imaging (image forming) unit, 45 transfer device, 46 static eliminator, 47 fixing device, 48 cleaning unit, 49 IDC sensor, 51 timing roller, 55 transport roller, 57 tray, 102 reference clock generator, 104 frequency spreaders, 106 switches, 108 drive clock generators, 10 image sensor 112 converter, 120 an image processing unit, 122 memory, 441Y, 441M, 441C, 441K toner unit, 471 heating roller, 472 a pressure roller, MFP image forming apparatus.

Claims (7)

A signal generator that generates a timing signal using one of a reference signal and a frequency spread signal obtained by frequency spreading the reference signal according to a predetermined rule;
An imaging device for reading an image that operates according to the timing signal from the signal generator;
A first image signal obtained by operating the imaging device with a timing signal generated using the reference signal and reading a reference object having a predetermined reference color, and generated using the frequency spread signal. Characteristic acquisition means for acquiring a time-varying characteristic of frequency spreading based on a second image signal obtained by operating the imaging device with a read timing signal and reading the reference object;
A third image signal obtained by operating the imaging device with a timing signal generated using the frequency spread signal and reading an object to be read is corrected using a correction value reflecting the time-varying characteristics. An image reading apparatus comprising: correction means for outputting as a fourth image signal.   The characteristic acquisition means, for the eigenfunction including a constant indicating the time variation characteristic of the reference signal and the periodicity of the frequency spread, to the eigenfunction based on the first image signal and the second image signal The image reading apparatus according to claim 1, wherein a correction value that reflects the time change characteristic is calculated by determining an included constant. The eigenfunction includes a constant indicating amplitude,
The characteristic acquisition means determines a constant indicating the amplitude included in the eigenfunction based on a maximum value and a minimum value of an amplitude appearing in a difference signal between the first image signal and the second image signal. The image reading apparatus according to claim 2. The eigenfunction includes a constant indicating a phase;
The characteristic acquisition unit may include the phase included in the eigenfunction based on a position where an amplitude appearing in a difference signal between the first image signal and the second image signal is a maximum value and / or a minimum value. The image reading apparatus according to claim 2, wherein a constant indicating the number is determined.   The image reading apparatus according to claim 4, wherein the correction unit updates a value indicating a phase of the time change characteristic in accordance with a reading operation of the imaging device. The imaging device has a linear detection surface extending in a first direction, and the relative position between the imaging device and the object to be read is changed in a second direction orthogonal to the first direction. It is configured to sequentially scan the image line by line,
The image reading apparatus according to claim 5, wherein the correction unit changes the phase of the time change characteristic by a certain amount for each line scan by the imaging device. An image forming means for forming an image;
A signal generator that generates a timing signal using one of a reference signal and a frequency spread signal obtained by frequency spreading the reference signal according to a predetermined rule;
An imaging device for reading an image that operates according to the timing signal from the signal generator;
A first image signal obtained by operating the imaging device with a timing signal generated using the reference signal and reading a reference object having a predetermined reference color, and generated using the frequency spread signal. Characteristic acquisition means for acquiring a time-varying characteristic of frequency spreading based on a second image signal obtained by operating the imaging device with a read timing signal and reading the reference object;
A third image signal obtained by operating the imaging device with a timing signal generated using the frequency spread signal and reading an object to be read is corrected using a correction value reflecting the time-varying characteristics. An image forming apparatus comprising: a correcting unit that outputs the fourth image signal.