JP2008172522A - Image reader, control method and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image reader capable of reducing the occurrences of moire by reducing nonuniformity in the same surface of an original of a modulation transfer function characteristic without depending upon reading resolution. <P>SOLUTION: Each reading range of a CCD line sensor 126 and a CIS (Contact Image Sensor) 128 (B701a, b) for respectively reading each face of a double-sided original is respectively divided into a plurality of areas. Each filter coefficient of the first and second MTF (Modulation Transfer Function) correction filters is set about each area (B704) in order to make each MTF characteristic of image data respectively outputted from the CCD line sensor 126 and the CIS 128 almost the same. Each filter coefficient can be changed in accordance with reading resolution in the case of reading an original image. The first and second MTF correction filters are respectively made to correct MTF characteristics about image data of each area outputted respectively from the CCD line sensor 126 and the CIS 128 by using the set filter coefficient of a correspondence area (B705a, b). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image reading apparatus, a control method, and a program suitable for reducing the occurrence of a stripe pattern due to light interference.

  2. Description of the Related Art Conventionally, as an image reading apparatus mounted on a copying machine or the like, an apparatus that reads a document by so-called “flow-reading” is known (for example, see Patent Document 1). In this type of image reading apparatus, an original is conveyed one by one on an original platen glass by an original conveying apparatus, and an image is read by exposing the original by an exposure apparatus fixed to the conveying path.

  In addition, there is an image reading apparatus that is equipped with two image reading units, and improves the image reading speed by reading both the front and back sides of the document with each image reading unit with a single conveyance of the document (for example, see Patent Document 2). An image reading apparatus that reads the front and back of a document with a single conveyance of the document uses a CCD (Charge Coupled Device) line sensor, a reduction optical system using a reduction lens, and a contact image sensor (CIS). Some systems have a double optical system. In this type of image reading apparatus, a difference occurs between the front and back of the document in MTF (Modulation Transfer Function) characteristics and density characteristics.

  Here, the MTF characteristics refer to the degree to which the contrast of a light and dark stripe pattern is lowered due to diffraction and aberration of the optical system when a bright and dark stripe pattern is formed through an optical system. As the frequency (spatial frequency) of the bright and dark stripe pattern increases, the contrast greatly decreases.

  Further, in the image reading apparatus of the above-described type, when the original to be read is an original having a mixed density reproduction method such as a continuous image or a halftone dot image, the density difference is noticeable due to a slight difference in MTF characteristics. .

  Further, even when the same type of optical system is used for reading the front and back sides of the document, there is a difference in the reading characteristics between the front and back sides of the document as described above due to the assembly of the components of the optical system and the difference in spectral characteristics.

Regarding the MTF characteristics among the above-described differences in reading characteristics, conventionally, there has been proposed an image reading apparatus that performs adjustment of a reading optical system, MTF filter processing, smoothing filter processing, and the like on read image data (for example, And Patent Document 3). In this type of image reading apparatus, unevenness in MTF characteristics within the same surface of a document is reduced, and a difference in MTF characteristics between the front and back of the document is reduced.
JP 2001-285595 A JP 2004-187144 A JP 2001-157052 A (Patent No. 3660180)

  However, the conventional image reading apparatus disclosed in Patent Document 3 is premised on performing uniform MTF filter processing and smoothing filter processing within the same document surface. If the MTF characteristics vary widely between the same side of the document or between the front and back sides of the document even if changes are made in the reading mode, such as “Pressure plate reading” or “Flowing reading”, while the document is fixed and read by the pressure plate, It is difficult to strictly match the MTF characteristics between the front and back sides of the document. For this reason, there has been a limit to the reduction of moire within the same surface of the document and the color difference between the front and back of the document.

  Further, an image having a low resolution has a lower MTF characteristic than an image having a high resolution without performing MTF filter processing. For this reason, when uniform MTF filter processing is performed on the same surface of an original for images having different reading resolutions when reading an original image, an image with a low resolution has an excessively low MTF characteristic. That is, the image quality level of the read image is lowered.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image reading apparatus, a control method, and a program capable of reducing unevenness of a modulation transfer function characteristic on the same surface of a document without depending on reading resolution and reducing the occurrence of moire. And

  In order to achieve the above-described object, an image reading apparatus of the present invention includes first and second reading units that respectively read a first surface and a second surface of a document and output image data based on the document reading. , First and second correction filters for correcting modulation transfer function characteristics for image data output from the first and second reading units, respectively, and reading by the first and second reading units The changing means for changing the filter coefficients of the first and second correction filters according to the resolution, and the reading ranges of the first and second reading means are divided into a plurality of areas, respectively. Setting means for setting each filter coefficient of the first and second correction filters, and the first and second for image data of each region output from the first and second reading means, respectively. Correction fill In, and having a control means for causing each of the correction of the modulation transfer function characteristics by using the filter coefficients of the corresponding region set by said setting means.

  According to the present invention, the image data of each region output from the first and second reading means is modulated and transmitted to the first and second correction filters using the set filter coefficients of the corresponding region. Each function characteristic is corrected. As a result, when reading a double-sided original, it is possible to reduce the unevenness of the modulation transfer function characteristic within the same original without depending on the reading resolution at the time of reading the original image, and to reduce the occurrence of moire.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a configuration diagram showing a configuration of an image reading apparatus according to an embodiment of the present invention. FIG. 2 is a flowchart showing a procedure of a double-sided original reading operation executed by the image reading apparatus. In the present embodiment, the configuration of the image reading apparatus will be described with reference to FIG. 1, and the double-sided document reading operation executed by the image reading apparatus will be described with reference to the steps shown in FIG. Note that a CPU (not shown) of the image reading apparatus executes processing shown in each flowchart in FIG.

  In FIG. 1, an image reading device 117 has an automatic document feeder 100 mounted on the top of a housing, and a lamp 119, mirrors 120 to 122, a lens 125, a CCD line sensor 126, and the like inside the housing. Yes. In the present embodiment, the image reading device 117 constitutes a first image reading unit, and a CIS 128 described later constitutes a second image reading unit.

  A document 102 is stacked on the document tray 101 of the automatic document feeder 100. A paper feed roller 103 is disposed on the document feed side of the document tray 101. The paper feed roller 103 is connected to the same drive source as the separation / conveyance roller 104, rotates as the separation / conveyance roller 104 rotates, and feeds the document 102 (step S201). The paper feed roller 103 is normally set to be retracted to a home position (a position above the original surface) so as not to disturb the setting operation of the original 102. When the document feeding operation is started, the sheet feeding roller 103 descends and comes into contact with the upper surface of the document 102. The paper feed roller 103 is pivotally supported by an arm (not shown), and moves up and down as the arm swings.

  The separation conveyance driven roller 105 is disposed on the side opposite to the separation conveyance roller 104 and is pressed toward the separation conveyance roller 104 side. The separation conveyance driven roller 105 is formed of a material such as a rubber material that is slightly less frictioned than the separation conveyance roller 104, and cooperates with the separation conveyance roller 104 to copy the document 102 fed by the paper supply roller 103. Separate and feed one sheet at a time.

  The registration roller 106 and the registration driven roller 107 align the leading ends of the documents fed by the separation conveyance roller 104 and the separation conveyance driven roller 105. That is, the leading edge of the fed document is abutted against the nip portion between the stationary registration roller 106 and the registration driven roller 107. As a result, a loop occurs in the document and the leading ends thereof are aligned. Thereafter, the registration roller 106 and the registration driven roller 107 are rotated, and the read roller 108 and the read driven roller 109 flow the original toward the reading glass 116. A platen roller 110 is disposed on the opposite side of the flow reading glass 116.

  Image information on the surface of the document passing over the flow reading glass 116 is read by the CCD line sensor 126 of the image reading device 117 (first image reading unit) (step S202). When the reading of the surface image of the document by the CCD line sensor 126 is completed, the lead discharge roller 111 and the lead discharge driven roller 112 convey the document to the CIS 128 side. The jump stand 115 scoops up the original from the flow reading glass 116. A flow reading glass 129 is attached to the CIS 128. A platen roller 127 is disposed on the opposite side of the flow reading glass 129.

  Image information on the back side of the document passing over the flow reading glass 129 is read by the CIS 128 (second image reading unit) (step S203). When the reading of the back side image of the original with the CIS 128 is completed, the paper discharge roller 113 discharges the original to the paper discharge tray 114 (step S204).

  An image reading device 117 serving as a first image reading unit includes a lamp 119 that irradiates light on a read original surface, and mirrors 120, 121, and 122 that guide reflected light from the original to a lens 125 and a CCD line sensor 126. Is provided. The lamp 119 and the mirror 120 are attached to the first mirror base 123. The mirrors 121 and 122 are attached to the second mirror base 124.

  The mirror tables 123 and 124 are coupled to a drive motor (not shown) by wires and are moved in parallel along the document table glass 118 by the rotational drive of the drive motor. The reflected light from the document is guided to the lens 125 through the mirrors 120, 121, and 122, and is imaged on the light receiving portion of the CCD line sensor 126 by the lens 125. The CCD line sensor 126 photoelectrically converts the formed reflected light and outputs an electrical signal corresponding to the amount of incident light.

  Similarly, the CIS 128 serving as the second image reading unit photoelectrically converts the reflected light from the document by the light receiving element, and outputs an electrical signal corresponding to the incident light amount.

  The image reading apparatus having the above-described configuration can read a document in two modes: a document fixed reading mode and a flow reading mode. In the fixed document reading mode, the document 102 is placed on the document table glass 118, and the document is read while moving the first mirror table 123 and the second mirror table 124 in the sub-scanning direction (right direction in the figure).

  In the flow-reading mode, the original is read at the position of the flow-reading glass 116 while the original 102 is being conveyed by the automatic document feeder 100 while the first mirror stage 123 and the second mirror stage 124 are stopped. In the flow reading mode, image information on the back surface of the original 102 can be read by the CIS 128 through the flow reading glass 129.

  When the image information on the front and back sides of the document is read using this flow reading mode, the difference between the reading characteristics of the CCD line sensor 126 and the reading characteristics of the CIS 128 appears as a difference in density characteristics and MTF characteristics. . Therefore, even when both the front and back sides of the same document are read, there may be a large difference in the read images.

  In particular, in the case of a document on which offset printing or the like has been performed and a document composed of an image composed of halftone dots, a slight difference in MTF characteristics appears remarkably in the density difference. Further, when this original is a color original, if such an MTF characteristic difference exists between the front and back of the original, it will appear as a color difference between the front and back of the original. Further, if the MTF characteristics vary within the same surface of the document, density unevenness and color unevenness occur within the same surface of the document, which adversely affects the image.

  Further, when using the flow-reading mode, productivity may be improved by changing the flow-reading speed to lower the reading resolution when reading a document image. If the reading resolution when reading a document image is lowered, the MTF characteristic is also lowered. Therefore, if the reading resolution of the front and back of the document is different, a difference in MTF characteristics occurs between the front and back of the document.

  Therefore, if the MTF characteristic difference between the front and back sides when scanning the front and back images in the scanning mode simultaneously and the MTF characteristic unevenness on the same side of the original are reduced at the same time, only the color difference within the same side of the original can be obtained. In addition, the color difference between the front and back sides of the document can be reduced.

  Hereinafter, a method for reducing the difference in MTF characteristics between the front and back sides of the document and the MTF characteristic unevenness in the same side of the document will be described using specific examples.

  In general, the MTF characteristic represents the relationship between spatial frequency and contrast (light / dark ratio). That is, an image having a certain spatial frequency (a wavy striped image) is read, the contrast of the image reproduced from the obtained image data is measured, and the measured contrast is associated with the corresponding spatial frequency. If the MTF characteristic is low, the image is blurred, and if the MTF characteristic is high, the sharpness of the image increases.

  FIG. 3 is a diagram illustrating an example of an MTF characteristic evaluation chart used when measuring the MTF characteristic. FIG. 3A is an example of a chart having a low spatial frequency, and FIG. It is an example of the chart with a spatial frequency.

  For example, when the resolution of an image reading apparatus mounted on a copying machine or the like is 600 dpi, the MTF characteristic is measured using an MTF characteristic evaluation chart having a line number of 6 [lp / mm]. Here, [lp / mm] is a unit indicating the number of black lines per mm in the MTF characteristic evaluation chart, and “lp” is an abbreviation for line pair.

  The number of lines of the MTF characteristic evaluation chart to be used (number of chart lines) satisfies the following formula (1).

  According to the sampling theorem, since a line of 1 [lp / mm] needs to be projected over at least four pixels of the CCD line sensor 126, the limit value that can be measured in principle in the above equation (1) is 4 * Number of chart lines [lp / mm].

  Next, a method for measuring the MTF characteristics of the image reading apparatus will be described.

  FIG. 4 is a diagram illustrating an example of an MTF characteristic evaluation chart used when measuring the MTF characteristic.

  In FIG. 4, reference numeral 401 denotes a portion used when measuring the MTF characteristics in the main scanning direction. Reference numeral 402 denotes a portion used when measuring the MTF characteristics in the sub-scanning direction. Reference numeral 403 denotes a black and white reference, and includes a white reference 403a and a black reference 403b. In the portion 403, the white reference 403a is a background portion of the MTF characteristic evaluation chart, and the black reference 403b is a solid black ink on which a line described in the MTF characteristic evaluation chart is printed.

  FIG. 5 is a flowchart showing a procedure for measuring the MTF characteristics.

  In FIG. 5, first, the CPU of the image reading apparatus performs white reference / black reference verification (step S501). That is, the image of the portion 403 shown in FIG. 4 is read, and the reading setting of the image reading apparatus is changed based on the obtained histograms of the white reference 403a and the black reference 403b. For example, when black reference and white reference histograms (horizontal axis: read luminance, vertical axis: number of pixels having corresponding read luminance) as shown in FIGS. 6 (a) and 6 (b) are obtained, From these, the maximum value and the minimum value of the reading luminance are extracted.

  Here, when the maximum value of the white reference reading luminance is 255, the reading setting of the image reading apparatus is changed so that the white reference maximum value is 255 or less. If the minimum value of the black reference reading brightness is 0, the reading setting of the image reading apparatus is changed so that the black reference minimum value is 1 or more.

  Next, the CPU reads an MTF characteristic evaluation chart having a line number of 6 [lp / mm] (step S502). That is, it is assumed that a histogram as shown in FIG. 6C is obtained as a result of reading an MTF characteristic evaluation chart having a line number of 6 [lp / mm], for example.

  Next, the CPU calculates an MTF characteristic value (step S503). For example, when a histogram as shown in FIG. 6C is obtained as described above, the maximum value and the minimum value in the histogram are extracted. That is, since two peaks appear, the peak value of the peak with the higher reading luminance is extracted as the maximum value (max), and the peak value of the peak with the lower reading luminance is extracted as the minimum value (min).

  Then, the MTF characteristic value is calculated based on the following equation (2).

  However, the calculation of the MTF characteristic value based on the above equation (2) is made on condition that the gamma value of the image reading apparatus is 1.0. If the gamma value of the image reading device is a value other than 1.0, it is necessary to apply the above equation (2) after normalizing once based on the following equation (3). is there.

In Expression (3), g represents a gamma value.

  The above MTF characteristic measurement method is a measurement method performed using a black and white line chart (MTF characteristic evaluation chart) as illustrated in FIG. 4 and is accurately called CTF (Contrast Transfer Function). The strict MTF characteristic measurement method is performed using a chart having a density period represented by a trigonometric sine function sin.

  Next, image processing performed by the image reading apparatus will be described with reference to FIG.

  FIG. 7 is a functional block diagram illustrating the image processing performed by the image reading apparatus shown in FIG. The image reading apparatus includes a CPU, ROM, RAM, and an information processing unit (not shown) having an input / output unit, and the image processing is performed by the CPU of the information processing unit executing a control program. .

  In FIG. 7, the image reading apparatus includes CCD blocks B701a and B701b, A / D conversion blocks B702a and B702b, and shading correction blocks B703a and B703b. The image reading apparatus further includes an MTF correction calculation block B704, MTF correction blocks B705a and B705b, and a transmission block B706 to the image forming apparatus.

  In blocks B701a and B701b, the image data read from the document by the CCD line sensor 126 and the CIS 128 are output as analog signals, respectively. In blocks B702a and B702b, these analog signals are A / D converted into digital signals, respectively. In blocks B703a and B703b, the image data that has become digital signals is subjected to shading correction.

  In the shading correction, as will be described below, a gain adjustment value and an offset adjustment are performed for each pixel in order to correct the pixel-to-pixel variation in image data that occurs when an image sensor such as the CCD line sensor 126 or the CIS 128 reads an image. Set the value.

  First, a shading white plate (not shown) attached to the back surface of the platen glass 118 is read by the CCD line sensor 126 with the lamp 119 turned on. Thus, shading data on the image reading device 117 (first image reading unit) side is obtained. Next, the shading white board affixed on the flow-reading glass 116 is irradiated with the light source built in CIS128, and is read by CIS128. Thereby, the shading data on the CIS128 (second image reading unit) side is acquired.

  The gain value is adjusted for each pixel so that the pixel values of both the shading data of the CCD line sensor 126 and the CIS 128 acquired as described above become a predetermined target value (for example, the luminance value 245). The gain adjustment values thus obtained are stored as shading correction data.

  Subsequently, data output from the CCD line sensor 126 and the CIS 128 is acquired in a state where the lamps corresponding to the CCD line sensor 126 and the CIS 128 are turned off. Next, offset adjustment is performed for each pixel so that each pixel value (black offset value) of the data becomes a predetermined target value (for example, luminance value 5). The offset adjustment values thus obtained are stored as shading correction data.

  In blocks B703a and B703b, shading correction is performed on normal image data obtained from the CCD line sensor 126 and the CIS 128 using the shading correction data stored as described above.

  In block B704, the filter coefficient of an MTF correction filter (not shown) used for MTF characteristic correction is calculated based on both image data subjected to shading correction. A method for calculating the filter coefficient will be described later with reference to FIG.

  In blocks B705a and B705b, the MTF characteristic correction is performed on the image data after the shading correction, using the filter coefficient calculated in block B704. This MTF characteristic correction can reduce the MTF characteristic difference not only on the same plane of the document but also on the front and back of the document. In block B706, the image data corrected in this way by the image reading apparatus is sent to the image forming apparatus (in the case of a copying machine).

  Next, a calculation method of the filter coefficient of the MTF correction filter used for the MTF characteristic correction and the correction effect by the method will be described.

  FIG. 8 is a diagram showing an MTF characteristic evaluation chart in which an image region is divided into three along the main scanning direction.

  In FIG. 8, the MTF characteristic evaluation chart has an image area divided into three along the main scanning directions of the CCD line sensor 126 and the CIS 128, and is composed of a left area, a center area, and a right area. Each region is divided with an equal width along the dividing direction. Each region has the same configuration as the MTF characteristic evaluation chart shown in FIG. The MTF characteristic evaluation chart is read in the main scanning direction indicated by the arrow in FIG.

  FIG. 9 is a diagram showing the results of measuring the MTF characteristics of the CCD line sensor and the CIS mounted image reading apparatus using the MTF characteristic evaluation chart shown in FIG. The MTF characteristic evaluation chart used here is an MTF characteristic evaluation chart of 6 [lp / mm].

  9, the MTF characteristic values of the CCD line sensor 126 and the CIS 128 in the main scanning direction and the sub-scanning direction in the left region, the central region, and the right region when the MTF characteristic evaluation chart shown in FIG. 8 is read in the flow reading mode. Is shown.

  As can be seen from the MTF characteristic values shown in FIG. 9, MTF characteristic unevenness occurs in the same plane of the original (between the left area, the central area, and the right area), and between the front and back of the original (the mutual relationship between the CCD line sensor and CIS) MTF characteristic difference occurs between the two).

  As shown in FIG. 10, the MTF characteristic unevenness in the same surface of the document is generated due to a large fluctuation of the MTF characteristic value depending on the adjustment position. FIG. 10 is a diagram illustrating a change in the MTF characteristic value with respect to the adjustment position and the scanning direction. In addition, the MTF characteristic unevenness in the same plane occurs due to the fact that the MTF characteristic value tends to decrease at the end in the main scanning direction due to the influence of the reduction lens in the reduction optical system using the CCD line sensor. Further, in a 1 × optical system using CIS, the focus position of the lens varies greatly depending on the adjustment position of the SLA (selfoc lens array), so that the MTF characteristics are likely to fluctuate. As a result, MTF characteristic unevenness occurs in the same plane.

  In order to reduce the MTF characteristic unevenness in the same surface of the document and the difference in MTF characteristics between the front and back of the document, the filter coefficient of the MTF correction filter is switched in the same surface of the document, and the document image is read. Switching is also performed according to the reading resolution. This will be described below.

  FIG. 11 is a flowchart showing a procedure of calculation processing for calculating the filter coefficient of the MTF correction filter. This process corresponds to the process performed in block B704 shown in FIG.

  In FIG. 11, first, the CPU of the image reading apparatus selects a reference MTF characteristic value in each region in the main scanning direction and the sub scanning direction on the front and back sides of the document (step S1101). In this embodiment, the minimum MTF characteristic value in each region is selected. For example, in the case of the MTF characteristic values shown in FIG. 9, 60% of the central area on the CIS side is selected in the main scanning direction, and 53% of the central area on the CCD line sensor side is selected in the sub-scanning direction.

  In this embodiment, the minimum value on the front and back sides of the document is selected as the reference MTF characteristic value. However, an arbitrary value may be specified for the reference MTF characteristic value. Further, an average value of MTF characteristic values in the main scanning direction may be selected.

  Next, the CPU calculates the MTF correction rate in each region based on the following equation (4) (step S1102).

  FIG. 12 is a diagram showing the MTF correction rate in each region calculated according to the above equation (4) based on the MTF characteristic value shown in FIG.

  Next, the CPU calculates a filter coefficient (correction coefficient) of the MTF correction filter for realizing the MTF correction rate calculated in step S1102 (step S1103).

  FIG. 13 is a diagram illustrating an example of filter coefficients of the MTF correction filter.

  Note that a smoothing filter (LPF: low-pass filter) or an edge enhancement filter (HPF: high-pass filter) is generally used as the MTF correction filter. The method for deriving the filter coefficients of the smoothing filter and the edge enhancement filter is described in detail in other documents (for example, “Practical digital video processing learned in C language” by Seiki Inoue et al., Published by Ohm). The description is omitted.

  Next, the CPU performs linear interpolation calculation of the filter coefficient of the MTF correction filter between the regions (adjacent regions) based on the following formula (5) (step S1104).

  Here, POINT1DATA and POINT2DATA are data values after 5 × 5 filter calculation at the filter coefficient switching point 1 and switching point 2, respectively. POINT is the number of pixels from the switching point 1 to the target pixel. WIDTH is the number of pixels from switching point 1 to switching point 2.

  FIG. 14 is a diagram illustrating a change in the MTF correction rate when the filter coefficient (correction coefficient) of the MTF correction filter is switched along the main scanning direction.

  For each pixel, the filter coefficient (correction coefficient) after linear interpolation calculated by the above equation (5) is continuously applied to each switching point. As a result, it is possible to eliminate moire, image streaks, and the like due to a rapid change in MTF characteristics.

  FIG. 15 is a diagram showing MTF correction values obtained by performing MTF characteristic correction on the correction values shown in FIG. 9 using the filter coefficients calculated by the calculation processing shown in FIG.

  FIG. 15 shows MTF characteristic values after correcting the MTF characteristics of the CCD line sensor 126 and the CIS 128 in the main scanning direction and the sub scanning direction in the left region, the central region, and the right region. According to FIG. 15, it can be seen that the MTF correction can be performed almost as intended.

  In this way, by reducing the MTF characteristic unevenness in the same surface of the original and the difference in MTF characteristics between the front and back of the original, the generation pattern of moire and the density (color) unevenness in the same surface of the original are made inconspicuous in the same surface of the original. And the difference between the front and back sides of the document can be reduced.

  Furthermore, the MTF characteristics also vary depending on the reading resolution when reading the document image. FIG. 16 is an example showing the relationship between the reading resolution and the MTF characteristics when reading a document image. In the graph of FIG. 16, the horizontal axis indicates the reading magnification, and the vertical axis indicates the MTF value in the sub-scanning direction. When the reading magnification is 100, reading is performed at a resolution of 600 dpi, and when the reading magnification is 50, reading is performed at a resolution of 300 dpi. In other words, when the scanning magnification is 100, the scanning magnification is set to half the image size in the sub-scanning direction when the scanning magnification is 50 and the scanning scanning speed is doubled.

  As for the image size in the main scanning direction, magnification (resolution) adjustment can be performed by performing a process of averaging two pixels into one pixel. Here, the magnification (resolution) conversion in the sub-scanning direction can be performed by changing the flow reading speed, but can also be achieved by a method similar to the averaging process in the main scanning direction. FIG. 16 shows that the MTF value decreases as the magnification (resolution) decreases. This indicates that the resolution and the MTF value are closely related.

  FIG. 17 is a diagram showing the relationship between the scanning magnification (resolution) and the sub-scanning direction MTF when an MTF correction filter for applying 45% MTF on the back side of the document when a magnification of 100 is applied. As described above, when an MTF correction filter that obtains a predetermined effect at a reading magnification of 100 is set, if the reading magnification (resolution) is reduced, the correction effect is too effective, and a desired MTF value cannot be obtained. become.

  Therefore, it is necessary to set an optimum MTF correction filter corresponding to the reading magnification (resolution). The following two methods are conceivable.

  <Method 1> An optimum MTF correction filter coefficient is calculated for each reading magnification (50, 60,...). In this case, since the filter coefficient becomes very large, the memory capacity may be reduced. However, since an optimum filter can be set, it is suitable for strict matching of the MTF characteristics.

  <Method 2> An optimum MTF correction filter coefficient is calculated for two points (for example, 50 and 100) from each reading magnification, and linearly interpolated coefficients are applied to other magnifications. In this case, the memory capacity for storing the filter coefficient is saved, but the accuracy of the MTF characteristic matching is not as strict as the method 1.

  In order to perform linear interpolation of the MTF correction filter, the following equation (6) is used.

here,
Kn: Coefficient after interpolation (however, K1>Kn> K2)
K1, K2: Coefficient before interpolation (however, K1 correction rate (B1)> K2 correction rate (B2))
Bn: Target correction rate (B1>Bn> B2)
B1, B2: Correction rate before interpolation. The above interpolation calculation may be performed for all the filter coefficients.

  When the MTF correction is performed on the reading characteristic of the back side of the document as shown in FIG. Compared with FIG. 17, it can be seen that even when the reading magnification (resolution) is low, the MTF characteristic does not become too low, and it keeps the same relationship as the MTF characteristic of the document surface.

  Therefore, it can be seen that by changing the coefficient setting of the MTF correction filter according to the reading magnification (reading resolution), it is possible to reduce the difference in MTF characteristics between the front and back sides of the document at an arbitrary resolution. The coefficient setting of the MTF correction filter may be changed according to the reading speed of the CCD line sensor 126 and the CIS 128.

  By performing MTF correction in combination with the above method, it is possible to make moiré patterns and density (color) unevenness in the same surface of the document less noticeable in the same surface of the document without depending on the reading resolution. The difference between the front and back sides can be reduced.

  In this embodiment, the MTF characteristic evaluation chart is equally divided into three along the main scanning direction. However, the present invention is not limited to this. In order to perform more precise adjustment of the MTF characteristics, the MTF characteristic evaluation chart may be divided into 5 divisions, 7 divisions,... Along the main scanning direction.

  Further, when the MTF characteristic evaluation chart is divided along the sub-scanning directions of the CCD line sensor 126 and the CIS 128, it may be considered in the same manner as when dividing along the main scanning directions of the CCD line sensor 126 and the CIS 128. . For example, FIG. 19 is a diagram showing an MTF characteristic evaluation chart divided into three along the main scanning direction and the sub-scanning direction.

  Further, for a color document, the color document may be divided into RGB three-color documents, and the above-described MTF characteristic correction may be performed on each color document.

  When two CCD line sensors are used instead of CIS when reading the front and back of a document, there are variations in the characteristics of not only the CCD line sensor but also each component (lamp, mirror, lens, etc.) of the reading optical system. is there. Further, there is a limit to adjustment such as 6-axis adjustment in the manufacturing process of each component of the reading optical system. Therefore, as in the case where the CCD line sensor and the CIS are used to read the front and back of the document, a difference in MTF characteristics occurs in the same surface of the document or between the front and back of the document. It is valid.

  As described above, according to the present embodiment, the reading ranges of the CCD line sensor 126 and the CIS 128 (B701a, B701b) for reading each side of a double-sided document are divided into a plurality of areas. In order to make the MTF characteristics of the image data output from the CCD line sensor 126 and the CIS 128 substantially the same, the filter coefficients of the first and second MTF correction filters are set for each region (B704). Each filter coefficient can be changed according to the reading resolution when reading the document image. The first and second MTF correction filters are used to correct the MTF characteristics of the image data output from the CCD line sensor 126 and the CIS 128, respectively, using the set filter coefficients of the corresponding areas ( B705a, B705b).

  Accordingly, when reading a double-sided original, unevenness in the same side of the original having the MTF characteristic can be reduced without depending on the reading resolution, so that the occurrence of moire can be reduced. Further, by making the MTF characteristics between the front and back sides of the document the same, it is possible to prevent the occurrence of density differences and color differences between the front and back sides of the document.

[Other Embodiments]
Although the image reading apparatus has been described in the above embodiment, the present invention is not limited to application to a single image reading apparatus. The present invention can also be applied to various image processing apparatuses (copiers, multifunction machines, facsimiles, etc.) equipped with an image reading apparatus.

  The object of the present invention is achieved by executing the following processing. That is, a storage medium recording software program codes for realizing the functions of the above-described embodiments is supplied to a system or apparatus, and a computer (or CPU, MPU, etc.) of the system or apparatus is stored in the storage medium. This is a process of reading the program code.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code and the storage medium storing the program code constitute the present invention. .

  Moreover, the following can be used as a storage medium for supplying the program code. For example, floppy (registered trademark) disk, hard disk, magneto-optical disk, CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, DVD-RW, DVD + RW, magnetic tape, nonvolatile memory card, ROM or the like. Alternatively, the program code may be downloaded via a network.

  Further, the present invention includes a case where the function of the above-described embodiment is realized by executing the program code read by the computer. In addition, an OS (operating system) running on the computer performs part or all of the actual processing based on the instruction of the program code, and the functions of the above-described embodiments are realized by the processing. Cases are also included.

  Furthermore, the present invention includes a case where the functions of the above-described embodiment are realized by the following processing. That is, the program code read from the storage medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer. Thereafter, based on the instruction of the program code, the CPU or the like provided in the function expansion board or function expansion unit performs part or all of the actual processing.

1 is a configuration diagram illustrating a configuration of an image reading apparatus according to an embodiment of the present invention. 2 is a flowchart showing a procedure of a double-sided document reading operation executed by the image reading apparatus shown in FIG. It is a figure which shows the example of the chart for MTF characteristic evaluation used when measuring an MTF characteristic. It is a figure which shows the example of the chart for MTF characteristic evaluation used at the time of MTF characteristic measurement. It is a flowchart which shows the procedure of a MTF characteristic measurement. FIG. 5 is a diagram showing a histogram obtained by reading the MTF characteristic evaluation chart shown in FIG. 4. It is a figure which shows the image process performed with the image reading apparatus shown in FIG. 1 with a functional block along the flow. It is a figure which shows the chart for MTF characteristic evaluation which divided the image area | region into 3 along the main scanning direction. It is a figure which shows the result of having measured the MTF characteristic of the CCD line sensor and the image reader mounted with CIS using the chart for MTF characteristic evaluation shown in FIG. It is a figure which shows the change of the MTF characteristic value with respect to an adjustment position and a scanning direction. It is a flowchart which shows the procedure of the calculation process which calculates the filter coefficient of an MTF correction filter. It is a figure which shows the MTF correction factor in each area | region calculated based on the MTF characteristic value shown in FIG. It is a figure which shows the example of the filter coefficient of an MTF correction filter. It is a figure which shows the MTF correction factor change at the time of switching the filter coefficient (correction coefficient) of an MTF correction filter along the main scanning direction. It is a figure which shows the MTF correction value obtained by performing MTF characteristic correction | amendment using the filter coefficient calculated by the calculation process shown in FIG. 11 with respect to the correction value shown in FIG. It is a figure showing the relationship between reading magnification (resolution) and subscanning direction MTF. It is a figure showing the relationship of the subscanning direction MTF at the time of applying the same MTF correction filter irrespective of reading magnification (resolution) and reading magnification. It is a figure showing the relationship of the subscanning direction MTF at the time of applying an appropriate MTF correction filter according to reading magnification (resolution) and reading magnification. It is a figure which shows the chart for MTF characteristic evaluation divided | segmented into 3 each along the main scanning direction and the subscanning direction.

Explanation of symbols

100 automatic document feeder 117 image reading device 126 CCD line sensor (first reading means)
128 CIS (second reading means)
B701a, B701b block (first and second reading means)
B702a, B702b A / D conversion block B703a, B703b Shading correction block B704 MTF correction calculation block (change means, setting means, control means)
B705a, B705b MTF correction block (first and second correction filters)
B706 Transmission block to image forming apparatus

Claims (8)

First and second reading means for respectively reading one side and the other side of the document and outputting image data based on the document reading;
First and second correction filters for correcting modulation transfer function characteristics for image data respectively output from the first and second reading means;
Changing means for changing each filter coefficient of the first and second correction filters according to the reading resolution of the first and second reading means;
Setting means for dividing each reading range of the first and second reading means into a plurality of areas, and setting each filter coefficient of the first and second correction filters for each area;
The image data of each area output from each of the first and second reading means is modulated using the filter coefficient of the corresponding area set by the setting means for the first and second correction filters. A control means for correcting the transfer function characteristics,
An image reading apparatus comprising:   The plurality of regions are regions obtained by dividing each reading range of the first and second reading units along each main scanning direction of the first and second reading units. The image reading apparatus according to 1.   The plurality of regions are regions obtained by dividing each reading range of the first and second reading units along each sub-scanning direction of the first and second reading units. The image reading apparatus according to 1.   4. The image reading apparatus according to claim 2, wherein the reading ranges of the first and second reading units are divided with the same width along the dividing direction.   The control means performs linear interpolation calculation of each filter coefficient in an adjacent region, and corrects the modulation transfer function characteristic using the filter coefficient obtained by the linear interpolation calculation for the first and second correction filters. The image reading apparatus according to claim 1, wherein the image reading apparatus performs each of the following.   2. The image reading apparatus according to claim 1, wherein the changing unit changes each filter coefficient of the first and second correction filters according to each reading speed of the first and second reading units. apparatus. First and second reading means for reading the first side and the second side of the original document and outputting image data based on the original reading, and image data output from the first and second reading means, respectively. A control method for an image reading apparatus comprising first and second correction filters that respectively correct modulation transfer function characteristics with respect to
A change step of changing each filter coefficient of the first and second correction filters according to the reading resolution of the first and second reading means;
A setting step of dividing each reading range of the first and second reading means into a plurality of areas, and setting each filter coefficient of the first and second correction filters for each area;
The image data of each area output from each of the first and second reading means is modulated using the filter coefficients of the corresponding area set by the setting step in the first and second correction filters. A control step for each of the corrections of the transfer function characteristics;
A control method comprising: First and second reading means for reading the first side and the second side of the original document and outputting image data based on the original reading, and image data output from the first and second reading means, respectively. A program for causing a computer to execute a control method of an image reading apparatus including first and second correction filters that respectively correct modulation transfer function characteristics with respect to
A change module for changing each filter coefficient of the first and second correction filters according to the reading resolution of the first and second reading means;
A setting module that divides each reading range of the first and second reading means into a plurality of areas, and sets each filter coefficient of the first and second correction filters for each area;
The image data of each area output from each of the first and second reading means is modulated using the filter coefficients of the corresponding area set by the setting module in the first and second correction filters. A control module that each corrects the transfer function characteristics;
A program comprising: