JP2007003781A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To highly accurately control image quality even in an image forming apparatus that has a relatively low-cost configuration. <P>SOLUTION: In each of image forming sections, the image forming apparatus forms an evaluation image corresponding to evaluation image data D1. The evaluation images include correction reference patches R12 to R16 and R22 to R26, besides Y, M, C, and K reference patches. In image reading section, the image forming apparatus reads the formed evaluation images. Based on the results of reading the correction reference patches, the image forming apparatus corrects the results of reading the Y, M, C and K reference patches. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that corrects image quality by performing so-called calibration, and more particularly to a technique for improving calibration accuracy.

  In an electrophotographic image forming apparatus, various image quality controls are performed for the purpose of maintaining an image quality of a certain level or higher. For example, in an image forming apparatus equipped with an image reading device such as a scanner, so-called a copying machine or a multifunction machine, an image serving as a reference for correction by arranging a plurality of patch-like images (hereinafter referred to as “evaluation image”). Image quality control is performed to compare the color when the evaluation image is read by the image reader and the color of the original evaluation image and correct the difference (hereinafter, this image quality control is referred to as “ This is called “calibration”, and a patch-like image formed by this calibration is called “reference patch”).

  As described above, calibration means that the image forming apparatus can reproduce the color more faithfully by correcting the difference between the “color of the evaluation image formed by the image forming apparatus” and the “original color of the evaluation image”. It is what you want to do. Therefore, in calibration, an evaluation image is formed under the same conditions as other images, and the formed evaluation image needs to be read as accurately as possible. However, there is a problem that the evaluation image is affected by the background color of the paper to be formed, and an error occurs in the reading result of the evaluation image. In particular, the reference patch in the highlight area having a low image density is strongly influenced by the background color of the paper, which may cause a large error in the reading result.

  To cope with this problem, conventionally, countermeasures have been taken such as limiting the paper for forming the evaluation image to a specific dedicated paper, or detecting the background color of the paper and removing the contribution of the background color from the read result. (For example, refer to Patent Document 2). Although different from the calibration using the above-described copying machine, for example, in Patent Document 3, before reading the “reference image” corresponding to the reference patch, the background color portion of the paper immediately before each reference image ( A technique is described in which a white reference pattern) is read by an internal sensor, and the reading result of each reference image is corrected each time with a white reference pattern. By using these techniques, it is possible to eliminate an error in the reading result caused by the background color of the paper.

  Incidentally, in an image forming apparatus such as a multifunction peripheral, an image is read at high speed in accordance with an image forming operation, and image data is generated. For this reason, a memory for temporarily storing image data is required to have a high-speed reading / writing performance. However, such a memory is generally expensive and therefore has a limited storage capacity. In many cases, it is less than one page of image data that can be handled. Therefore, in an image forming apparatus in which the memory capacity of the memory is less than one page of image data, the image data in the area read from the memory is sequentially deleted, and the image data in the area where the image is to be formed is vacated. The operation of sequentially writing to the storage area is performed.

When calibration is performed in such an image forming apparatus, image data for the entire evaluation image cannot be held in the memory. Therefore, in such a case, the image forming apparatus reads and stores as many reference patches as can be held in the memory, generates correction data obtained from the read reference patches, and releases the storage area. The operation of reading as many reference patches as can be held in the memory is repeated. That is, at this time, the image reading apparatus repeats an intermittent operation of reading a part of the evaluation image, temporarily stopping it, and reading it again when the correction data is generated (hereinafter, the evaluation image is read in this way). The reading method is called “inching measurement method”).
Japanese Patent Application Laid-Open No. 64-041375 Japanese Patent Laid-Open No. 10-145598 JP-A-10-063045 (paragraph 0021 etc.)

However, when calibration is performed using the inching measurement method, there is a problem that an error occurs in the reading result of the evaluation image due to a factor different from the above-described background color of the paper.
When calibration is performed using the inching measurement method, an image reading operation is performed for a long time in the image reading apparatus. Then, the light quantity of the light source that irradiates light to the evaluation image varies, and the reading condition of the evaluation image changes with time. As a result, the reading condition changes greatly between the reference patch read first and the reference patch read last, resulting in an error. Such fluctuations in the amount of light are particularly noticeable when a relatively inexpensive light source such as a fluorescent lamp is used.

  In order to eliminate such inconvenience, for example, the calibration is performed after waiting until the light amount of the light source is stabilized, or the well-known shading correction (the built-in white reference plate is replaced each time the reference patch is read). It is also conceivable to perform correction of white level by reading. However, since both methods are very time-consuming, the time required for the entire calibration increases to be impractical, and there is a problem that other normal image formation is hindered.

  The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a technique that enables high-precision image quality control even in an image forming apparatus having a relatively inexpensive configuration.

In order to achieve the above object, an image forming apparatus according to the present invention includes an image forming unit that forms an image on a recording material by applying predetermined image forming conditions, and an image while moving a reading position in the sub-scanning direction. An image reading unit to be read and an evaluation image formed on a recording material by the image forming unit, and a plurality of patch regions having different colors in the main scanning direction and the sub-scanning direction, and a reference of a reference color different from the patch region Storage means for storing an evaluation image having at least two patch areas in the sub-scanning direction, and reading results of a plurality of reference patch areas included in the read evaluation image when the image reading means reads the evaluation image , And calculating a fluctuation amount in the sub-scanning direction of the reading result by the image reading unit from the plurality of reading results referred to, and based on the calculated fluctuation amount Applying in the image forming unit based on the difference between the correction unit that corrects the reading result of the patch area, the reading result of the evaluation image corrected by the correcting unit, and the evaluation image stored in the storage unit And image quality control means for changing the image forming conditions to be in a direction to eliminate the difference.
According to such an image forming apparatus, it is possible to correct the light amount fluctuation in the sub-scanning direction in the image reading unit based on the reading results of a plurality of reference color patch areas.

In the present invention, when the evaluation image is read, it is desirable to read the entire image continuously. However, as described above, the entire image is restricted due to the storage capacity of the memory mounted in the image forming apparatus. Often have to be read intermittently. In such a case, the image reading means reads the image in either the first mode for intermittently reading the entire image or the second mode for continuously reading the entire image, and the evaluation When reading an image, the first mode may be used.
In the case of such a configuration, in the first mode, since it takes time to read the entire image, the temporal light amount fluctuation also increases. Therefore, in such a configuration, the correction effect becomes more remarkable.

In the present invention, it is preferable that the image reading unit simultaneously reads the patch area at the same position in the main scanning direction as the reference patch area when reading the reference patch area.
With such a configuration, the patch area and the reference patch area are read at the same timing, so that the correction accuracy can be improved.

In the present invention, it is preferable that the correction unit averages the reading results of the plurality of reference patch areas and corrects the reading results of the patch areas using the average value.
By adopting such a configuration, it is possible to suppress density unevenness due to dirt on the surface of the recording material.

In the present invention, the correction unit may perform a linear regression calculation using a plurality of reading results of the reference patch area, and correct the reading result of the patch area using the calculation result.
By adopting such a configuration, even if the reference patch area is not formed in every row, correction can be performed for the patch area in each row.

In the present invention, the evaluation image has at least two reference patch regions in the main scanning direction, and the correction unit performs main scanning of a reading result by the image reading unit from the plurality of reading results referred to. It is desirable to calculate the amount of variation in the direction.
At this time, the correction unit may average the reading results of the plurality of reference patch areas in the main scanning direction, and correct the reading results of the patch area using the average value.
With such a configuration, it is possible to correct not only the light amount fluctuation in the sub-scanning direction of the image reading unit but also the light amount fluctuation in the main scanning direction.

In the present invention, it is preferable that the correction unit performs weighting according to the density of the patch area when correcting the reading result of the patch area.
By adopting such a configuration, it is possible to suppress fluctuations in the light amount decrease depending on the density.

In the present invention, the correction means corrects the reading results of all the patch areas when the correction amount to be corrected for the reading result of the patch area of the first row read by the image reading means is a non-zero value. It is desirable to subtract the value from the quantity.
By adopting such a configuration, even when an offset occurs in the correction result, the offset can be canceled out.

  As described above, according to the present invention, it is possible to perform high-accuracy image quality control even in an image forming apparatus having a relatively inexpensive configuration using a light source that varies in light quantity such as a fluorescent lamp.

Embodiments of the present invention will be described below with reference to the drawings.
[1. Constitution]
FIG. 1 is a diagram showing a device configuration of an image forming apparatus 100 according to an embodiment of the present invention. This image forming apparatus 100 is a so-called tandem type color image forming apparatus provided with an intermediate transfer belt, and transfers yellow (Y), magenta (M), cyan (C), and black (K) toner images to a sheet. A color image is formed by forming it on a recording material such as. The image forming apparatus 100 is a so-called multi-function machine having functions of a scanner, a printer, and a copier. Here, the image forming apparatus 100 is roughly divided into an image forming unit 1, an image reading unit 2, a paper feeding unit 3, and an operation unit 4, and the configuration of each unit will be described.

  The image forming unit 1 includes toner image forming units 10Y, 10M, 10C, and 10K, an intermediate transfer belt 11, a drive roll 12, a secondary transfer roll 13, a backup roll 14, and a fixing device 15. Note that the alphabets attached to the end of each symbol of the toner image forming units 10Y, 10M, 10C, and 10K merely indicate the color of the toner. There is no big difference. Therefore, in the following description, these alphabets may be omitted as appropriate and may be collectively referred to as “toner image forming unit 10”. Similarly, each part of the toner image forming unit 10 will be described by omitting the alphabet as appropriate.

  The toner image forming unit 10 includes a photosensitive drum 101, a charger 102, an exposure device 103, a developing device 104, a primary transfer roll 105, a drum cleaner 106, and a charge removal device 107. The photosensitive drum 101 is an image carrier having a photoconductive layer formed on the surface thereof, and is rotated in the direction of arrow A in the figure. The charger 102 generates a predetermined potential difference on the surface of the photosensitive drum 101 and uniformly charges it. The exposure device 103 is provided with a beam emission source such as a laser diode, and the charged light on the surface of the photosensitive drum 101 is irradiated with beam light so that the charge of the irradiated portion disappears, and according to the input image data of each color. An electrostatic latent image is formed. The developing device 104 includes toners of Y, M, C, and K colors, and forms a toner image by applying toner to the electrostatic latent image on the surface of the photosensitive drum 101. The primary transfer roll 105 faces the photoconductive drum 101 via the intermediate transfer belt 11, and transfers the toner image to the intermediate transfer belt 11 at a position where these faces.

  The drum cleaner 106 collects untransferred toner remaining on the surface of the photosensitive drum 101 after the toner image is transferred. The neutralization device 107 neutralizes the surface of the photosensitive drum 101. That is, the drum cleaner 106 and the charge removal device 107 remove unnecessary toner and electric charges from the photosensitive drum 101, and prepare the photosensitive drum 101 for the next image forming process.

  The intermediate transfer belt 11 is an endless belt member. The intermediate transfer belt 11 is rotated in the direction of arrow B in the figure by the drive roll 12 and conveys the toner image formed in the image forming unit 10 to the nip region formed by the secondary transfer roll 13. The secondary transfer roll 13 is electrically grounded (grounded) via its shaft, and generates a potential difference with the backup roll 14 connected to a power source (not shown). The toner image on the surface of the intermediate transfer belt 11 is moved by this potential difference and adheres to the surface of the paper supplied from the paper supply unit 3.

  The fixing device 15 includes a fixing roll 151 provided with a heat source therein and a pressure pad 152, and heats and pressurizes the paper on which the toner image is transferred, thereby fixing the toner on the paper. As a result, an image corresponding to the image data is formed on the paper with toner. The paper on which the image is formed is discharged to the outside.

  The image reading unit 2 includes a full rate carriage 20, a half rate carriage 21, an imaging lens 22, a line sensor 23, a platen glass 24, a platen cover 25, and a white reference plate 26. The full-rate carriage 20 includes a light source such as a fluorescent lamp and a mirror, and reads a document placed on the upper surface of the platen glass 24 while moving in the sub-scanning direction (arrow C direction in the figure). The half-rate carriage 21 moves in the sub-scanning direction at half the speed of the full-rate carriage 20, and guides reflected light of the original from the full-rate carriage 20 to the imaging lens 22. The imaging lens 22 forms an image so that the reflected light of the document is satisfactorily read on the surface of the line sensor 23. The line sensor 23 includes a large number of image sensors such as a CCD (Charge Coupled Device) arranged in the main scanning direction (vertical direction in FIG. 1), and represents an original based on reflected light from the received original. Generate data. The platen glass 24 is a transparent glass plate on which an original is placed, and the platen cover 25 covers the platen glass 24 and blocks outside light. The white reference plate 26 is a white plate-like member provided along one side of the platen glass 24, and reflects most of the light when irradiated with light from the light source. The white reference plate 26 is provided for performing shading correction.

  Based on the above configuration, the image reading unit 2 reads a document placed on the platen glass 24 and generates image data corresponding to the image of the document. Further, the image reading unit 2 performs known shading correction at an appropriate timing before reading the document. When reading an image, the image reading unit 2 has two types of operation modes. When the image forming apparatus 100 causes the image reading unit 2 to read a document other than the evaluation image, the image reading unit 2 scans the entire surface of the platen glass 24 and reads the document. Hereinafter, this operation mode is referred to as “normal mode”.

  On the other hand, when the image forming apparatus 100 is executing calibration and causes the image reading unit 2 to read the evaluation image, the image reading unit 2 can read the evaluation image placed on the platen glass 24. The evaluation image is intermittently read such that the scanning is temporarily stopped and the scanning is temporarily stopped, and the scanning is performed again when the image processing for the read region is completed. That is, the image forming apparatus 100 of the present embodiment reads the evaluation image by the inching measurement method when performing calibration. Hereinafter, this operation mode is referred to as “calibration mode”.

  The paper feed unit 3 includes a plurality of paper trays 30, a plurality of transport rolls 31, and a registration roll 32. Each of the plurality of paper trays 30 stores different types of paper, and the paper in each tray has a different size and quality. The transport roll 31 transports the paper supplied from the paper tray 30 to the image forming unit 1. The registration roll 32 controls the timing at which the sheet enters the secondary transfer position.

  The operation unit 4 includes a display, a plurality of buttons, and the like (all not shown), and notifies the user of various information and accepts instructions from the user. The user uses the operation unit 4 to input, for example, start of image formation or calibration or change of various conditions such as image density.

The apparatus configuration of the image forming apparatus 100 is as described above. In the image forming apparatus 100, the operation of each unit is controlled by a control unit (not shown).
FIG. 2 is a block diagram showing the configuration of the control unit 5. The control unit 5 includes a central processing unit 51, a main storage unit 52, an auxiliary storage unit 53, an image processing unit 54, and a communication unit 55. Hereinafter, these functions will be described.

  The central processing unit 51 is a processing device such as a CPU (Central Processing Unit) or an MPU (Micro Processing Unit), and executes basic arithmetic processing related to the operation of each unit of the image forming apparatus 100. The main storage unit 52 is a so-called memory such as a RAM (Random Access Memory), and temporarily stores various data as a working area of the central processing unit 51.

  The main storage unit 52 includes an image memory that temporarily stores the image data generated by the image reading unit 2. The image memory is configured so that the image data read by the central processing unit 51 can be sequentially deleted, and new image data can be sequentially written in the empty area. In this way, it is possible to handle image data having a capacity that can be stored in a single image memory at a high speed.

  The auxiliary storage unit 53 is a storage device such as an HDD (Hard Disk Drive) or a flash memory, and stores a program executed by the central processing unit 51 and evaluation image data D1 used for calibration. The image processing unit 54 is an integrated circuit such as an ASIC (Application Specific Integrated Circuit), and executes arithmetic processing specialized for image formation such as resolution conversion and screen processing (hereinafter referred to as “image processing”). Image processing performed by the image processing unit 54 includes gradation correction processing using a lookup table. The communication unit 55 is an interface device that transmits / receives data to / from an external device (not shown).

Here, the evaluation image data D1 stored in the auxiliary storage unit 53 will be described. The evaluation image data D1 is reference image data that is formed and compared in the calibration, and includes a plurality of patch areas.
FIG. 3A is a diagram illustrating an example of the evaluation image data D1 of the present embodiment. FIG. 3B is a diagram showing evaluation image data for performing conventional calibration for comparison. The evaluation image data D1 is data indicating an evaluation image that is read while being moved in the direction of arrow S in the drawing in the image reading unit 2. That is, the row direction of the arranged reference patches is the main scanning direction in the image reading unit 2, and the column direction is the sub-scanning direction in the image reading unit 2.

  As shown in the figure, the evaluation image data D1 is composed of reference patches of 7 rows and 14 columns. Here, the second to fourth columns are yellow (Y), the fifth to seventh columns are magenta (M), the eighth to tenth columns are cyan (C), and the eleventh to thirteenth columns are black (K) reference patches. The first column and the fourteenth column are a colorless reference patch for correction processing and a reference patch for detection of placement error. Here, the values of the reference patches for each of the colors Y, M, C, and K are determined so that the input image density gradually changes between 0 and 100%. Alphabets and numbers shown inside the reference patch in the figure indicate the color of the reference patch and the input image density, respectively. That is, the reference patch described as “Y35” in the third row and first column means that the reference patch is yellow with an input image density of 35%. The “input image density” referred to here is an input value (input coverage) indicating the toner coverage.

  The reference patches R12 to R16 and R22 to R26 for correction processing are formed in the 1st and 14th columns in the 2nd to 6th rows. Since the color of this reference patch (reference color) is the background color of the paper, that is, colorless, no image density is actually specified for the area corresponding to this reference patch. That is, in the present embodiment, the image density of the reference patch for correction processing is 0% in terms of data.

  The reference patches D1 to D4 for mounting error detection are reference patches having a predetermined color and shape. When the user executes calibration, the evaluation patches placed on the image reading unit 2 by the user are placed. This is for detecting whether or not the placement direction is correct. For example, the reference patches D1 to D4 can be recognized in the main scanning direction and the sub-scanning direction by making each of them different colors or different shapes.

  Next, the tone correction process in this embodiment will be described. The tone correction processing of the present embodiment corrects the input image density value using a lookup table. Specifically, the lookup table of the present embodiment is a correction value for each gradation of 0 to 100% of the input image density, and the image formed by the image forming apparatus 100 by the image processing unit 54 has a desired gradation property. As shown, the input image density is corrected. The correction value of each gradation, that is, the target image density can change before and after calibration. For example, when the gradation of an image to be formed changes due to a change with time of the image forming apparatus 100 itself or an environmental change such as temperature and humidity, the target image density is updated according to the result of reading the reference patch by calibration. In this case, the central processing unit 51 calculates the correction value of the lookup table for each gradation and supplies it to the image processing unit 54 as a new target image density.

[2. Operation]
Based on the above configuration, the image forming apparatus 100 forms a toner image based on the input image data, and transfers and fixes the toner image on a sheet to form an image. The image data may be generated based on an image read by the image reading unit 2 or may be input from an external device via the communication unit 55. At this time, the image reading unit 2 operates in the “normal mode”.
Further, the image forming apparatus 100 executes calibration according to an instruction from the user, and corrects part of the image forming conditions as necessary. The specific operation of this calibration is as follows.

  FIG. 4 is a flowchart showing the flow of calibration executed in the present embodiment. Hereinafter, description will be made with reference to FIG. First, the control unit 5 receives an input of a calibration execution instruction from the user via the button of the operation unit 4 (step S1). When this instruction is input, the control unit 5 causes the image forming unit 1 to form an evaluation image using the evaluation image data D1 stored in the auxiliary storage unit 53 (step S2). When the evaluation image is formed, the control unit 5 determines whether or not there is an instruction from the user by displaying a screen for reading the evaluation image on the display of the operation unit 4 (step S3).

  When the user gives an instruction to start scanning (step S3; YES), the control unit 5 switches the operation mode of the image reading unit 2 from “normal mode” to “calibration mode”, and the image reading unit 2 reads the evaluation image. (Step S4). At this time, the image reading unit 2 reads the evaluation image line by line.

  When the reference patch in the first row is read, the control unit 5 determines whether or not the mounting direction of the evaluation image is correct (step S5). Specifically, the control unit 5 determines the placement direction by recognizing the color or shape of the reference patch for placement error detection at both ends of the generated image data for one row. If it is determined that the placement direction is not correct (step S5; NO), the control unit 5 notifies the user of a warning or the like via the operation unit 4 (step S6) and prompts another scan.

  On the other hand, when it is determined that the placement direction is correct (step S5; YES), the control unit 5 reads the reference patch of the evaluation image and updates each value in the lookup table to change the image forming condition (step S7). ). When the update of the lookup table is completed, the control unit 5 notifies the user of the end of calibration via the operation unit 4 (step S8). At the same time, the control unit 5 switches the operation mode of the image reading unit 2 from “calibration mode” to “normal mode”.

The outline of the calibration operation in the present embodiment is as described above. Next, the lookup table update process performed in step S7 will be described in more detail.
FIG. 5 is a flowchart showing the look-up table update process executed in the present embodiment. Hereinafter, description will be given with reference to FIG. First, the control unit 5 measures the density of each reference patch based on the result of reading the evaluation image by the image reading unit 2 (step S71). The measurement value is extracted for each row of the evaluation image. Here, the measurement value is 8 bits, that is, any value from 0 to 255, 0 is “white”, and 255 is the so-called “solid density”. The value measured at this time is used as the “input image density” described above.

When the densities of all the reference patches have been measured, the control unit 5 calculates a correction amount for the read result of each row using the measurement result of the reference patch for correction processing (step S72). The correction amount for each row is calculated as follows.
FIG. 6 is a flowchart showing a process for calculating a correction amount for the reading result. Hereinafter, description will be given with reference to FIG. First, the control unit 5 obtains the average value of the reading results of the correction processing reference patches (total of six) in the second to fourth rows, and sets this as the average density _GL of the background color (step S721). Subsequently, the control unit 5 obtains the average value of the reading results of the correction processing reference patches (total of six) in the fourth to sixth rows, and sets this as the average density _GH of the background color (step S722).

Here, the average value is used as the background color in consideration of the case where there is a stain or the like at a position corresponding to the reference patch for the correction processing of the paper of the evaluation image, and density unevenness due to this stain is given to the correction result. This is to reduce the influence. Since the density _GL is the average of the second to fourth rows, this is assumed to be the background color density of the third row. Similarly, since the density _GH is the average of the 4th to 6th rows, this is assumed to be the background color density of the 5th row. In addition, since the light quantity of the light source of the image reading unit 2 decreases with time, G _H > G _L is set.

Subsequently, the control unit 5 calculates the average value ΔG of the light amount fluctuation per row using the densities _GH and _GL (step S723). In the above description, since the density _GL is regarded as the background color density of the third row and the density _GH is regarded as the background color density of the fifth row, the light amount variation ΔG per row is
ΔG = ( _GH − _GL ) / 2
(Equation 1).

After determining the amount variation .DELTA.G per line using Equation 1, the control unit 5 calculates a correction reference value G _i of the reading results in each row by using the value of this .DELTA.G (step S724). Here, i is a value of 1 to 6 indicating the number of rows. That is, the correction reference value Gi of the _i- th row is
G _i = (G _L −2 × ΔG) + (i−1) × ΔG
(Equation 2). In this equation, (G _L −2 × ΔG) is a predicted value of the background color density in the first row. When the above equation 2 is transformed,
G _i = G _L + (i−3) × ΔG
(Equation 3)

The process for calculating the correction amount is as described above. The correction reference value G _i obtained here is merely a reference value for the correction amount in each row, and does not necessarily match the actual correction amount. This is because the actual correction amount varies depending on the density of the reference patch. Hereinafter, a specific correction method will be described.

Returning to the description of FIG. Subsequently, the control unit 5 corrects the input image density of each reference patch on the basis of the corrected reference value G _i calculated using Equation 3 (step S73). Specifically, when the input image density of the solid density reference patch is d _max and the input image density of the reference patch in the i-th row and j-th column is d _ij , the control unit 5 performs the corrected i-th row and j-th row. The image density D _{ij of the} column is
D _ij = d _ij − [G _i × (d _max −d _ij ) / {d _max − (G _L −2 × ΔG)}]
(Equation 4).

That is, the control unit 5 performs a process of subtracting a value corresponding to the density from the input image density _dij . As is clear from Equation 4, the control unit 5 decreases the correction amount as the reference patch is close to solid density, that is, as the density is high, and conversely increases as the density is low. Weighting is performed. This is because the reference patch having a lower density is more strongly affected by the light amount of the light source. When such weighting is performed, the correction amount of the input image density is different even for the reference patch in the same row.

  After correcting the input image density, the control unit 5 supplies this value to the image processing unit 54 (step S74). The image processing unit 54 updates the look-up table based on this value and applies it in the gradation correction process. This completes the lookup table update process.

Next, the effect of performing the process of correcting the input image density as described above will be described with a specific example.
FIG. 7 shows the same procedure as in the present embodiment, that is, when the reference patch of a certain color (black) is read by the inching measurement method, the measured value and correction value of the input image density, and the reference patch for correction processing. It is the figure which showed the actual measurement value of the input image density. At this time, the light source used in the image reading unit 2 of the image forming apparatus 100 is a fluorescent lamp used in a normal scanner. For ease of explanation, only one column of the measurement results of the reference patch is shown here.

  As can be seen from the figure, the input image density of the reference patch for correction processing increases almost linearly. The fact that the density of the background color that should be the same color is increasing means that the amount of light of the image reading unit 2 decreases with time, and the read density is relatively dark (black). is doing. The error due to the decrease in the amount of light increases as the measurement time, that is, the continuous lighting time of the light source increases. Therefore, when the document is scanned for a long time as in this embodiment, the error is remarkably read. Appear in the results. In particular, as in the present embodiment, when the original is an evaluation image used for calibration, the calibration execution result is affected by errors, and the color balance of the image formed thereafter is lost. Degraded image quality.

  However, as described in the present embodiment, an error is included in the read density by reading an image with reference to the background color and correcting the light amount reduction of the light source using the background color reading result. It becomes possible. As a result, more accurate color reproduction can be performed when an image is copied, and more accurate calibration can be performed.

[3. Modified example]
In addition, this invention is not limited to the aspect mentioned above in the implementation, A various deformation | transformation is possible. Below, a part of the modification is demonstrated.
First, a modified example of the evaluation image will be described. In the above-described embodiment, the reference patch for detecting the placement error is formed in the evaluation image. However, the evaluation image may not include the reference patch for detecting the placement error. A reference patch for correction processing may be provided at the position of the reference patch for mounting error detection. In addition, the number and arrangement pattern of the reference patches for each color may be different from those shown in FIG.

  Further, in the above-described embodiment, the reference patch for correction processing is formed at both ends of the evaluation image, but is not limited to this position. Of course, it may be between the reference patches of each color Y, M, C, K. Further, the reference patch for correction processing is not limited to two rows, but may be one row or three or more rows.

  It should be noted that by forming a plurality of reference patches for correction processing, it is also possible to correct light quantity fluctuations in the main scanning direction of the image reading unit 2. Here, as an example, a case will be described as an example in which reference patches for correction processing are provided at both ends and the center of the evaluation image, that is, at three locations. In such a case, for example, if the degree of light intensity reduction at the center is larger than that at both ends, a larger correction amount is applied to the reading result of the reference patch formed in the column near the center of the evaluation image. Good.

  Further, in the above-described embodiment, five reference patches for correction processing are formed per row, but of course the number is not limited to this. If the reference patch for correction processing is not formed in all rows, the correction amount may be obtained by performing processing such as interpolation for that row. However, at least two or more reference patches for correction processing are required in the column direction in order to perform processing for correcting the light amount decrease.

  In the above-described embodiment, the color of the reference patch for correction processing has been described as the background color of the paper, that is, the surface color, but is not limited thereto. As long as the colors of the respective reference patches are substantially the same and can detect a decrease in the amount of light, the reference patch for correction processing may be any color.

  Subsequently, a modified example of the image reading unit will be described. In the above-described embodiment, the image reading unit 2 includes the platen glass 24, and the original placed on the platen glass 24 is read while the full-rate carriage 20 moves. It may be configured such that a fixed image pickup device picks up a document to be read. In short, the present invention is a technique applicable to an image forming apparatus including an image reading unit configured to read an image while relatively moving the reading position of a document (evaluation image) in the sub-scanning direction.

In the above-described embodiment, the image reading unit 2 has been described as having a configuration in which the reference patch of the evaluation image is read line by line. However, depending on the capacity of the image memory, a configuration in which the reference patch is read a plurality of lines at a time may be employed. .
In the above-described embodiment, it has been described that the image reading unit 2 outputs the measurement value of the reference patch in 8 bits.

Next, a modified example related to the correction process will be described. In the above-described embodiment, the corrected image density is calculated by the method shown in Formulas 1 to 4, but the present invention is not limited to this method. For example, in the above-described embodiment, a regression line may be obtained by the least square method or the like using the read input image density of the reference patch for correction processing, and the light amount fluctuation may be estimated from the regression line. Also, in the above-described calculation of the densities _GL and _GH , the average of the reading results at two locations in the same row may be used instead of the average of a plurality of rows.

Also, when calculating the correction reference value G ₁ of the first row using Equation 3 above, is not necessarily coincide with the input image density of the reference patch for correction processing of the first row produces a slight offset Sometimes. However, since the shading correction is performed immediately before reading the first-line reference patch, it is considered that the reading result of the first-line reference patch contains almost no error. Therefore, in such a case, a second correction process for subtracting the above-described offset from the corrected image density may be performed. This makes it possible to match the input image density of the reference patch for correction processing in the first row with the corrected image density. Such processing is effective even when shading correction is not performed.

  In the above-described embodiment, the correction target is described as the lookup table. However, the correction target is not limited to the lookup table, and the charging potential, the exposure amount, the developing bias potential, and the toner are similar to the known calibration. Image forming conditions such as density (mixing ratio of toner and carrier) may be the correction target. Even when the correction target is a lookup table, processing for correcting the target image density may be performed instead of correcting the input image density as in the above-described embodiment.

1 is a diagram illustrating an apparatus configuration of an image forming apparatus according to an embodiment of the present invention. 2 is a block diagram illustrating a configuration of a control unit of the image forming apparatus. FIG. It is the figure which showed an example of evaluation image data. It is the flowchart which showed the flow of the calibration in the same embodiment. It is the flowchart which showed the update process of the look-up table in the same embodiment. It is the flowchart which showed the calculation process of the correction amount in the same embodiment. FIG. 6 is a diagram showing measured values and correction values of input image density and measured values of input image density of a reference patch for correction processing when a reference patch of a certain color is read by an inching measurement method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus, 1 ... Image forming part 10, 10Y, 10M, 10C, 10K ... Toner image formation unit, 101 ... Photosensitive drum, 102 ... Charger, 103 ... Exposure device, 104 ... Developing device, 105 ... Primary transfer roll, 106 ... drum cleaner, 107 ... neutralization device, 11 ... intermediate transfer belt, 12 ... drive roll, 13 ... secondary transfer roll, 14 ... backup roll, 15 ... fixing device, 2 ... image reading unit, 20 ... Full-rate carriage, 21 ... half-rate carriage, 22 ... imaging lens, 23 ... line sensor, 24 ... platen glass, 25 ... platen cover, 26 ... white reference plate, 3 ... feed section, 30 ... paper tray, 31 ... conveyance Roll, 32 ... Registration roll, 4 ... Operation unit, 5 ... Control unit, 51 ... Central processing unit, 52 ... Main storage unit, 53 ... Auxiliary storage unit, 54 ... Image Processing section, 55 ... communication unit

Claims (9)

Image forming means for forming an image on a recording material by applying predetermined image forming conditions;
Image reading means for reading an image while moving the reading position in the sub-scanning direction;
An evaluation image formed on a recording material by the image forming unit, having a plurality of patch regions of different colors in the main scanning direction and the sub-scanning direction, and at least sub-scanning a reference patch region of a reference color different from the patch region Storage means for storing evaluation images having two or more in the direction;
When the image reading unit reads the evaluation image, the reading result of the plurality of reference patch regions included in the read evaluation image is referred to, and the reading result of the image reading unit is obtained from the plurality of read reading results. A correction unit that calculates a fluctuation amount in the sub-scanning direction, and corrects the reading result of the patch area based on the calculated fluctuation amount;
Based on the difference between the read result of the evaluation image corrected by the correction unit and the evaluation image stored in the storage unit, the image forming condition applied in the image forming unit is changed in a direction to eliminate the difference. And an image quality control means. The image reading means
When reading an image in either the first mode for intermittently reading the entire image or the second mode for continuously reading the entire image and reading the evaluation image, use the first mode. The image forming apparatus according to claim 1. The image reading means
The image forming apparatus according to claim 1, wherein when the reference patch area is read, the patch area located at the same position in the main scanning direction as the reference patch area is simultaneously read. The correction means includes
The image forming apparatus according to claim 1, wherein the reading results of the plurality of reference patch areas are averaged, and the reading results of the patch areas are corrected using the average value. The correction means includes
The image forming apparatus according to claim 1, wherein linear regression calculation is performed using a plurality of reading results of the reference patch area, and the reading result of the patch area is corrected using the calculation result. The evaluation image is
Having at least two reference patch regions in the main scanning direction;
The correction means includes
The image forming apparatus according to claim 1, wherein a fluctuation amount in a main scanning direction of a reading result obtained by the image reading unit is calculated from the plurality of read reading results. The correction means includes
The image forming apparatus according to claim 6, wherein the reading results of the plurality of reference patch areas are averaged in the main scanning direction, and the reading results of the patch areas are corrected using the average value. The correction means includes
The image forming apparatus according to claim 1, wherein when the reading result of the patch area is corrected, weighting is performed according to the density of the patch area. The correction means includes
When the correction amount to be corrected for the read result of the patch area in the first row read by the image reading means is a non-zero value, the value is subtracted from the correction amount for the read result of all the patch areas. The image forming apparatus according to claim 1.

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