JP2006058565A - Image forming apparatus and correction method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct a density or chromaticity unevenness in a scanning direction even in a color image forming apparatus equipped with one color sensor.
An image forming apparatus that includes a color sensor provided at a predetermined position in a scanning direction and having a detection range narrower than an image formable region and forms a color image on a recording medium. The test pattern 70 is formed along the scanning direction at a position that passes the detection range of the color sensor when the recording medium on which the test pattern is formed is fed in a state rotated 90 degrees in a predetermined direction. When a recording medium on which a test pattern is formed is fed in a state rotated 90 degrees in a predetermined direction, parameters relating to image formation in the scanning direction are corrected based on the test pattern detection result by the color sensor. To do.
[Selection] Figure 8

Description

  The present invention relates to an image forming apparatus such as a color printer and a color copying machine, and a correction method thereof. More specifically, the present invention is provided at a predetermined position in a scanning direction substantially orthogonal to the conveyance direction of a recording medium, The present invention relates to correction for improving image quality in a scanning direction in an image forming apparatus that has a color sensor having a narrow detection range and forms a color image on a recording medium.

  In recent years, color image forming apparatuses employing an electrophotographic system such as a color printer or a color copying machine, an ink jet system, and the like have been required to improve the output image quality.

  However, in the color image forming apparatus, when there is a change in each part of the apparatus due to a change in environment or long-term use, the color of the obtained image changes. In particular, in the case of an electrophotographic color image forming apparatus, even slight environmental fluctuations may cause color fluctuations, which may cause the color balance to be lost. Therefore, there is a means for stably reproducing colors and color gradations. is doing.

  For example, for each color toner, the absolute humidity measured by the temperature / humidity sensor has gradation correction means such as several kinds of exposure amount and development bias according to the absolute humidity, process conditions such as a developing bias, and a look-up table (LUT). In general, a configuration is used in which the process conditions at that time and the optimum value for tone correction are selected. In addition, a density detection toner patch is formed on the intermediate transfer member, the photosensitive member, etc. with a single color toner so that a constant color and gradation of the color can be obtained even if variations in each part of the apparatus occur. The density of an unfixed single color toner patch is detected by a density detection sensor for unfixed toner (hereinafter referred to as a density sensor), and feedback is provided to process conditions such as exposure amount and development bias and tone correction means such as LUT based on the detection result. By performing density control over time, stable colors and gradation of colors can be obtained.

  However, the density control using the above-described density sensor is to detect a patch formed on an intermediate transfer body or a drum, and the subsequent change in image color balance due to transfer and fixing to a recording medium. Has no control. The color balance also changes depending on the transfer efficiency in transferring the toner image to the recording medium, and heating and pressurization by fixing. This change cannot be dealt with by density control using such a density sensor.

  Therefore, cyan (C), magenta (M), yellow (Y), and black (K) single-color gradation patches and C, M, and Y mixed color patches are formed on the recording medium, and after fixing, on the recording medium. There has been proposed a color image forming apparatus provided with a sensor (hereinafter referred to as a color sensor) for detecting the density or chromaticity of the patch.

  In this color image forming apparatus, the detection result is fed back to an exposure amount of the image forming unit, process conditions, a calibration table for correcting density-gradation characteristics, and the like, and finally formed on a recording medium. It is possible to control the density or chromaticity of the output image. Although it is possible to detect the output image of the color image forming apparatus with an external image reading device or a densitometer / chromaticity meter and perform the same control, this method completes the control within the color image forming apparatus. Is excellent.

  As a color sensor of this color image forming apparatus, for example, three or more light sources having different emission spectra such as red (R), green (G), and blue (B) are used as the light emitting elements, or the light emitting elements are white ( A light source that emits W) is used to form three or more filters having different spectral transmittances such as red (R), green (G), and blue (B) on the light receiving element. As a result, three or more different outputs such as RGB output can be obtained.

Even in an ink-jet color image forming apparatus, it is difficult to keep the density-gradation characteristics constant because the color balance changes due to changes in ink discharge amount with time, environmental differences, and individual differences among head cartridges. Therefore, a color image forming apparatus has also been proposed in which the head cartridge is replaced with a color sensor, the density or chromaticity of the patch on the recording medium is detected, and the density or chromaticity is controlled.
JP 2003-084532 A JP2003-107830 JP2003-107835

  However, since the detection range of a color sensor that is normally used is not so wide, if only one color sensor is mounted, the color sensor in a direction substantially orthogonal to the recording medium conveyance direction (hereinafter referred to as a scanning direction). It is necessary to match the formation position in the scanning direction of the patch with the mounting position.

  On the other hand, in order to be able to detect patches formed over the entire scanning direction, it is necessary to mount a large number of color sensors in the scanning direction, which increases the cost.

  On the other hand, an electrophotographic color image forming apparatus has a problem that unevenness occurs in density and chromaticity even when patches of the same image signal are formed in the scanning direction. This is because the transfer characteristics of the toner and the fixing characteristics of the fixing device that fixes the toner transferred to the recording medium by heating and pressurizing the recording medium are slightly different between the central portion and the end portion in the scanning direction. Is attributed.

  Therefore, in a conventional color image forming apparatus equipped with a color sensor, it is possible to improve the stability of color and color gradation, but it is effective for density and chromaticity unevenness in the scanning direction of the recording medium. There is no coping method.

  Inkjet color image forming apparatuses that employ a system that replaces the head cartridge with a color sensor can move the color sensor in the scanning direction in the same way as the recording head, so it detects density or chromaticity unevenness in the scanning direction. Is possible.

  However, in order to realize the same configuration in other types of color image forming apparatuses, it is necessary to newly provide a scanning mechanism for moving the color sensor in the scanning direction, which increases costs. Further, when moving the color sensor in the scanning direction, it is necessary to stop the recording medium with the recording medium completely coming out of the fixing device. For this purpose, the color sensor is at least as long as the length of the longest recording medium. Cannot be realized in a color image forming apparatus having a short distance from the fixing device to the paper discharge unit.

  There is a color sensor that can detect density or chromaticity over the entire scanning direction, such as a line type sensor, but naturally it has a large irradiation area and a wide light receiving area, so it is a type of color that detects only one location. Compared with the sensor, the cost increases accordingly.

  The present invention has been made in view of the above situation. Even in a color image forming apparatus equipped with a single color sensor that detects only a small range in the scanning direction, density or chromaticity unevenness in the scanning direction is provided. It is an object of the present invention to enable detection of color density and correction of density or chromaticity unevenness based on the detection result.

An image forming apparatus according to an aspect of the present invention that achieves the above object is an image forming apparatus that forms a color image on a recording medium.
A color sensor provided at a predetermined position in the scanning direction substantially perpendicular to the conveyance direction of the recording medium and having a detection range narrower than the image formable region;
When the test pattern for image quality correction in the scanning direction is fed in a state in which the recording medium on which the test pattern is formed is rotated 90 degrees in a predetermined direction, the test pattern is set to a position that passes the detection range of the color sensor. Test pattern forming means for forming along the scanning direction;
Parameters related to image formation in the scanning direction based on the detection result of the test pattern by the color sensor when the recording medium on which the test pattern is formed is fed in a state rotated 90 degrees in the predetermined direction. And a correction means for correcting.

  That is, according to the present invention, an image that has a color sensor that is provided at a predetermined position in the scanning direction substantially orthogonal to the conveyance direction of the recording medium and that has a detection range narrower than the image formable area and forms a color image on the recording medium. In the forming apparatus, when a test pattern for image quality correction in the scanning direction is fed in a state in which a recording medium on which the test pattern is formed is rotated 90 degrees in a predetermined direction from the direction in which the pattern is formed When the recording medium formed along the scanning direction at a position passing the detection range of the color sensor and having the test pattern formed thereon is fed in a state rotated 90 degrees in a predetermined direction, the color sensor Based on the test pattern detection result obtained by the above, parameters relating to image formation in the scanning direction are corrected.

  In this way, even in a color image forming apparatus having a color sensor that detects only a narrow range in the scanning direction, the formed test pattern is rotated 90 degrees in a predetermined direction and fed, and image formation is performed based on the detection result. By correcting these parameters, it is possible to correct unevenness in image quality in the scanning direction.

  Therefore, for example, in a color image forming apparatus in which a color sensor is already mounted, it is not necessary to add a new configuration or mechanism, and the image quality of the color image forming apparatus can be improved without increasing the cost of the apparatus.

  Note that the test pattern may include a plurality of sets of patches each made up of patches of colorants used for forming a color image, and each patch may be formed under the same conditions.

  Alternatively, the test pattern forming means forms a test pattern composed of patches formed using one colorant used for forming a color image on a number of recording media corresponding to the number of colorants used for forming the color image. May be.

  The test pattern may be a test pattern for correcting unevenness in image density or chromaticity in the scanning direction.

  The image forming apparatus may be an electrophotographic system or an inkjet system.

  The parameters relating to image formation can be applied to the PWM modulation degree of exposure light or the modulation degree of exposure light intensity.

  When forming a second test pattern that is different from the test pattern for purposes other than image correction in the scanning direction, a patch that allows the user to visually determine whether image correction in the scanning direction is necessary is added. May be.

  In this case, the purpose other than the image correction in the scanning direction is correction for obtaining a stable color and color gradation, and the correction for obtaining the stable color and color gradation is performed in the scanning direction. It is preferable that the setting is made so as to be performed more frequently than the image correction.

  Information indicating the relationship between the parameter value relating to image formation and the characteristic value of the image to be formed may be recorded in advance in the storage means.

  Alternatively, the test pattern forming means adds a patch in which the value of the parameter relating to image formation is changed in a plurality of stages to the test pattern at a position passing through the detection range of the color sensor, and detects the patch by the color sensor. Based on the result, the relationship between the value of the parameter relating to image formation and the characteristic value of the image to be formed may be obtained.

According to another aspect of the present invention for achieving the above object, a correction method for an image forming apparatus is provided at a predetermined position in a scanning direction substantially perpendicular to the recording medium conveyance direction and is narrower than an image formable area. A correction method for an image forming apparatus for forming a color image on a recording medium,
When the test pattern for image quality correction in the scanning direction is fed in a state in which the recording medium on which the test pattern is formed is rotated 90 degrees in a predetermined direction, the test pattern is set to a position that passes the detection range of the color sensor. , A test pattern forming step for forming along the scanning direction;
Parameters related to image formation in the scanning direction based on the detection result of the test pattern by the color sensor when the recording medium on which the test pattern is formed is fed in a state rotated 90 degrees in the predetermined direction. And a correction step for correcting.

  Further, the above object can be achieved by a computer program that causes a computer apparatus to execute the correction method for the image forming apparatus, and a storage medium that stores the computer program.

  According to the present invention, even in a color image forming apparatus having a color sensor that detects only a narrow range in the scanning direction, the formed test pattern is rotated 90 degrees in a predetermined direction and fed, and an image is obtained based on the detection result. By correcting the formation parameters, it is possible to correct unevenness in image quality in the scanning direction.

  Therefore, for example, in a color image forming apparatus in which a color sensor is already mounted, it is not necessary to add a new configuration or mechanism, and the image quality of the color image forming apparatus can be improved without increasing the cost of the apparatus.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the components described in the following embodiments are merely examples, and are not intended to limit the scope of the present invention only to them.

  Further, in this specification, “correction” means that the original performance of the apparatus is removed except for errors due to external factors, and is used in the same meaning as “calibration”.

  Further, in this specification, the “scanning direction” means a direction substantially perpendicular to the conveyance direction of the recording medium, and the image forming apparatus moves a recording unit such as a recording head to form an image. It does not matter whether or not an image is formed.

(First embodiment)
FIG. 1 is a functional block diagram showing the overall configuration of a first embodiment of a color image forming apparatus according to the present invention. The color image forming apparatus of the present embodiment includes an image processing unit and an image forming unit.

  First, processing in the image processing unit will be described. The color matching table 111 converts an RGB signal representing the color of an image sent from a host device such as a personal computer into a device RGB signal (hereinafter referred to as DevRGB) that matches the color reproduction range of the color image forming apparatus. Next, the converted DevRGB signal is converted by the color separation table 112 into a CMYK signal which is a toner color material color of the color image forming apparatus. The calibration table 113 is a table for correcting the density-gradation characteristics specific to each color image forming apparatus, and converts the CMYK signal into a C′M′Y′K ′ signal with density-gradation characteristics corrected. To do. Further, a PWM (Pulse Width Modulation) table 114 converts the C′M′Y′K ′ signal into the exposure times Tc, Tm, Ty, and Tk of the corresponding scanner units 24C, 24M, 24Y, and 24K.

  Next, the image forming unit will be described. Main means relating to image formation are a charging unit 122, an exposure unit 123, a developing unit 124, a transfer unit 125, and a fixing unit 126. These are controlled by the CPU 121. Further, the color sensor 42 is also connected to the CPU.

  FIG. 2 is a cross-sectional view showing a mechanical configuration of the color image forming apparatus of the present embodiment. As shown in the figure, the apparatus of the present embodiment is a tandem color image forming apparatus that employs an intermediate transfer member 28 that is an example of an electrophotographic color image forming apparatus. The operation of the image forming unit in the electrophotographic color image forming apparatus will be described with reference to FIG.

  The image forming unit forms an electrostatic latent image by turning on exposure light based on the exposure time converted by the image processing unit, and develops the electrostatic latent image to form a single color toner image. The images are superimposed to form a multicolor toner image, the multicolor toner image is transferred to the recording medium 11, and the multicolor toner image on the recording medium is fixed.

  As the charging means 122, four injection charges for charging the respective photoreceptors 22Y, 22M, 22C, and 22K for each of the yellow (Y), magenta (M), cyan (C), and black (K) stations. Each of the charging chargers is provided with sleeves 23YS, 23MS, 23CS, and 23KS.

  The photoconductors 22Y, 22M, 22C, and 22K are configured by applying an organic optical transmission layer to the outer periphery of an aluminum cylinder, and are rotated by the driving force of a drive motor (not shown) being transmitted. 22M, 22C, and 22K are rotated counterclockwise according to the image forming operation.

  The exposure unit 123 irradiates the photoconductors 22Y, 22M, 22C, and 22K with exposure light from the scanner units 24Y, 24M, 24C, and 24K, and selectively exposes the surfaces of the photoconductors 22Y, 22M, 22C, and 22K. The electrostatic latent image is formed.

  The developing unit 124 includes four developing units 26Y, 26M that develop yellow (Y), magenta (M), cyan (C), and black (K) for each station in order to visualize the electrostatic latent image. Each developing device is provided with a sleeve 26YS, 26MS, 26CS, and 26KS. Each developing device 26 is detachable.

  The transfer means 125 rotates the intermediate transfer body 28 clockwise to transfer the single color toner image from the photoconductor 22 to the intermediate transfer body 28, and is positioned opposite to the photoconductors 22Y, 22M, 22C, 22K. As the primary transfer rollers 27Y, 27M, 27C, and 27K rotate, the monochrome toner image is transferred. By applying an appropriate bias voltage to the primary transfer roller 27 and making a difference between the rotation speed of the photosensitive member 22 and the rotation speed of the intermediate transfer body 28, the monochromatic toner image is efficiently transferred onto the intermediate transfer body 28. This is called primary transfer.

  Further, the transfer means 125 superimposes a single color toner image on the intermediate transfer body 28 for each station, conveys the superimposed multicolor toner image to the secondary transfer roller 29 as the intermediate transfer body 28 rotates, and further records the recording medium. 11 is nipped and conveyed from the paper feed tray 21 to the secondary transfer roller 29, and the multicolor toner image on the intermediate transfer member 28 is transferred to the recording medium 11. An appropriate bias voltage is applied to the secondary transfer roller 29 to electrostatically transfer the toner image. This is called secondary transfer. The secondary transfer roller 29 contacts the recording medium 11 at a position 29a while transferring the multicolor toner image onto the recording medium 11, and is separated to a position 29b after the printing process.

  The fixing unit 126 adds a fixing roller 32 for heating the recording medium 11 and a pressure roller for pressing the recording medium 11 against the fixing roller 32 in order to melt and fix the multicolor toner image transferred to the recording medium 11 to the recording medium 11. A pressure roller 33 is provided. The fixing roller 32 and the pressure roller 33 are formed in a hollow shape, and heaters 34 and 35 are incorporated therein. The fixing device 31 conveys the recording medium 11 holding the multicolor toner image by the fixing roller 32 and the pressure roller 33 and applies heat and pressure to fix the toner on the recording medium 11.

  After the toner fixing, the recording medium 11 is discharged to a discharge tray (not shown) by a discharge roller (not shown), and the image forming operation is finished.

  The cleaning unit 30 cleans the toner remaining on the intermediate transfer body 28. The waste toner after transferring the four-color multicolor toner image formed on the intermediate transfer body 28 to the recording medium 11 is: Stored in a cleaner container.

  The density sensor 41 is disposed toward the intermediate transfer body 28 in the color image forming apparatus of FIG. 2 and measures the density of the toner patch formed on the surface of the intermediate transfer body 28. An example of the configuration of the density sensor 41 is shown in FIG.

  The density sensor 41 includes an infrared light emitting element 51 such as an LED, a light receiving element 52 such as a photodiode or CdS, an IC (not shown) that processes received light data, and a holder (not shown) that accommodates these. The infrared light emitting element 51 irradiates the toner patch 64 formed on the intermediate transfer body 28, the light receiving element 52 a detects the intensity of irregularly reflected light from the toner patch 64, and the light receiving element 52 b is regularly reflected from the toner patch 64. Detect the light intensity. Thus, by detecting both the regular reflection light intensity and the irregular reflection light intensity, it is possible to detect the density of the toner patch from a high density to a low density. An optical element such as a lens (not shown) may be used for optical coupling between the light emitting element 51 and the light receiving element 52.

  The color sensor 42 is arranged downstream of the fixing device 31 in the recording medium conveyance path toward the image forming surface of the recording medium 11, detects the color of the mixed color patch after fixing formed on the recording medium 11, Outputs RGB values. By disposing the image inside the color image forming apparatus, it becomes possible to automatically detect the fixed image before discharging it to the paper discharge unit.

  The detection range of the color sensor 42 is narrower than the area where the image forming apparatus can form an image. For example, if the maximum recording size of the image forming apparatus is A4 size, the detection range is less than the width of the A4 size.

  FIG. 4 is a block diagram showing the configuration of the color sensor and its peripheral devices. The color sensor and its peripheral devices include a light emitting element 101, a light receiving element 102, an A / D converter 103, and a CPU 121. The light emitting element 101 is a light source of a color sensor and irradiates light to the measuring object 103. Then, irregularly reflected light whose reflectance depends on the object color of the measurement object is reflected. The irregularly reflected light is incident on the light receiving element 102 that converts the light into an analog electric signal. Further, the analog electric signal is converted into a digital electric signal by the A / D converter 104. Then, the digital electrical signal is captured by the CPU 121, processed by primary conversion or a lookup table, and the chromaticity is output.

  FIG. 5A is a cross-sectional view showing the configuration of the color sensor 42 of the present embodiment. The color sensor 42 uses a white LED 53 as the light emitting element 101 and a charge storage type sensor 54 with an on-chip filter of three or more colors such as RGB as the light receiving element 102. The white LED 53 is incident on the recording medium 11 on which the patch 71 after fixing is formed at an angle of 45 degrees, and the intensity of diffuse reflected light in the 0 degree (vertical) direction is detected by the charge storage sensor 54 with an RGB on-chip filter. . The light receiving portion of the charge storage type sensor 54 with the RGB on-chip filter is a pixel with independent RGB as shown in FIG.

  The light receiving element 102 may be a photodiode. A plurality of RGB three-pixel sets may be arranged. Further, a configuration in which the incident angle is 0 degree and the reflection angle is 45 degrees may be employed. Furthermore, the color sensor 42 may be constituted by LEDs of three or more colors such as RGB as light emitting elements and a filterless sensor as a light receiving element.

  FIG. 6 is a flowchart showing a procedure for correcting density unevenness in the scanning direction in the present embodiment. An outline of the correction procedure will be described with reference to this drawing, and details of some steps will be described later.

  In the following description, all density is used for explanation, but the same calibration effect can be obtained even if all density is replaced with L * of chromaticity (L * a * b * color system).

  First, in step S211, correction of density unevenness in the scanning direction is started. The start timing may be when a user of the color image forming apparatus (hereinafter simply referred to as a user) desires correction of density unevenness in the scanning direction, or when the color image forming apparatus includes an environment detection unit, the ambient temperature The timing at which the density unevenness state may change, such as when a change in humidity or humidity is detected, or when a predetermined number of images are formed, may be used.

  In step S212, an image of a predetermined scanning direction density unevenness correction test pattern (hereinafter also simply referred to as a test pattern) is formed on a recording medium by a color image forming apparatus and output to a paper discharge tray. The configuration of the test pattern will be described later. A recording medium on which an image of this test pattern is formed is also simply referred to as a test pattern below.

  In step S213, the user takes the test pattern output to the paper discharge tray, and rotates the direction of the test pattern output to the paper discharge tray 90 degrees clockwise relative to the direction during image formation (recording). Thereafter, it is set on the paper feed tray of the color image forming apparatus. The user's work will be described later.

  In step S214, after confirming that the formed test pattern is set on the paper feed tray, the test pattern is conveyed to the position of the color sensor. At that time, no image is formed. Confirmation of the setting is performed by determining whether or not the user has pressed a button on the control panel of the color image forming apparatus.

  In step S215, the density or chromaticity of the patch formed in the test pattern is detected using the color sensor 42, and the degree of density unevenness in the scanning direction is calculated.

  In step S216, based on the calculation result in step S215, the color image forming apparatus is corrected so as to eliminate density unevenness. The correction method will be described later.

  In step S217, the correction of density unevenness in the scanning direction ends.

  Next, the test pattern 70 formed in step S212 will be described using FIG. The scanning direction when the pattern is formed on the recording medium 11 and the conveyance direction of the recording medium 11 are as indicated by arrows in the figure.

  In one row along the scanning direction, patches 71 of the same image data are formed alternately in a single color of C, M, Y, K. For example, the image data may have a gradation of 100% or 50%. As for the number of patches, C, M, Y, and K are formed as one set, and n sets (n is an integer of 2 or more), and the number of patches is as large as possible that can be detected by the color sensor 42. . Accordingly, the scanning direction width of each patch to be formed is set to be small within a range that can be detected by the color sensor 42.

  In addition, in order to correctly guide the user's work to be described later, it is preferable to print a comment 72 on the setting direction and operation of the test pattern 70 on which the image has been formed on the paper feed tray.

  In the illustrated example, the arrangement order of the C, M, Y, and K patches is in the order of C, M, Y, and K from the left end in the figure and the right end in the figure, and the scanning direction. Although it is symmetrical with respect to the central portion, it is not always necessary to do so.

  The formation position of the patch 71 is limited by the installation position of the color sensor 42 in the scanning direction. Including this, the user's work in step S213 will be described with reference to FIG. FIG. 8 shows a side view and a top view of the color image forming apparatus in (a) and (b), respectively.

  The test pattern 70 is discharged to the paper discharge tray 36 of the color image forming apparatus 20 in the direction as shown in FIG. The user rotates the discharged test pattern 70 by 90 degrees clockwise as indicated by a dotted arrow, and then sets the test pattern 70 on the paper feed tray 21.

  FIG. 9A shows the positional relationship between the color sensor 42 rotated 90 degrees clockwise by the user and set on the paper feed tray and the recording medium 11 on which the test pattern is formed, and FIG. The state of the test pattern formed on the recording medium 11 is shown.

  As shown in FIG. 9, the range detectable by the color sensor 42 is limited to a predetermined range in the scanning direction. Accordingly, not all the patches 71 can be detected when an image of the test pattern 70 is formed. The user can detect all the patches 71 by transporting the test pattern 70 set on the upper paper feed tray rotated 90 degrees clockwise as described above to the position of the color sensor.

  That is, the patch 71 is formed on the test pattern 70 at a position where at least a part of the patch 71 is detected by the color sensor 42 when the test pattern 70 is rotated 90 degrees clockwise with respect to the image forming direction. It is necessary to set the position so as to pass through. This is a limitation on the position where the patch 71 is formed.

  Next, the density unevenness correction method in the scanning direction of the color image forming apparatus in step S216 will be described. FIG. 11 is a graph showing an example of the result of detecting the patch 71 of the test pattern 70 using the color sensor 42 in step S215. This example shows the detection result for a certain color patch, the patch is formed with a gradation of 100%, the horizontal axis is the position in the scanning direction, that is, the position of the patch, and the vertical axis is the optical density. Is shown. From the example shown in the figure, it can be seen that the density at the center in the scanning direction is low and the density at both ends is high. In the following, a correction method for the result of only one color among C, M, Y, and K will be described, but the same correction may be made for the results of other colors.

  As a correction method, a method of changing the modulation degree of the PWM modulation of the exposure light depending on the scanning position, and a method of changing the modulation degree of the intensity of the exposure light depending on the scanning position are known. Although the above two methods will be described here, the correction method is not limited to these two methods.

  FIG. 10 is a block diagram showing a configuration of means for realizing correction of density unevenness in the scanning direction. In the present embodiment, density unevenness correcting means controlled by the CPU 121 is provided in the image forming unit. Then, the CPU 121 controls the density unevenness correcting unit 131 according to the detection result by the color sensor 42 to correct the density unevenness by changing the PWM modulation degree of the exposure unit 122 or the modulation degree of the exposure light intensity. A specific correction method by the density unevenness correction unit 131 will be described below.

When changing the PWM modulation degree of exposure light, the corrected PWM modulation degree is expressed by the following equation:
M ' _PWM = M _PWM × β (x)
Follow. However,
M ' _PWM : PWM modulation degree after correction
M _PWM : PWM modulation degree before correction
β (x): Correction factor in the scanning direction
x: Scanning position.

  Here, how to obtain the correction coefficient β (x) in the scanning direction will be described.

In the detection result of the color sensor 42 illustrated in FIG. 11, the density value of the lowest density is Dmin, the density value at the position x in the scanning direction is D (x), and the density ratio α (x) is
α (x) = Dmin / D (x)
Calculate according to

  Based on the relationship between the density ratio α (x) and the correction coefficient β (x) in the scanning direction, the density ratio α (x) is converted into the correction coefficient β (x) in the scanning direction. FIG. 12 is a graph showing an example of the relationship between the two. The relationship between the two is stored in advance in the color image forming apparatus in the form of a formula or a table. Further, in the example shown in FIG. 12, the relationship between the two is linear, but of course, it may have a relationship other than linear.

  Further, since the position x in the scanning direction corresponds to the position where the patch is formed, the correction coefficient for the portion in between is calculated by interpolation calculation.

  In this way, the corrected PWM modulation degree M ′ is obtained, and the exposure light is modulated so that the modulation degree becomes M ′, whereby density unevenness in the scanning direction can be corrected.

Next, a case where the modulation degree of the intensity of exposure light is changed will be described. In this case, the degree of modulation of the exposure light intensity after correction is as follows:
P ′ = P × γ (x)
Follow. However,
P ′: degree of modulation of exposure light intensity after correction
P: Degree of modulation of exposure light intensity before correction
γ (x): Correction factor in the scanning direction
x: Scanning position.

  Here, how to obtain the correction coefficient γ (x) in the scanning direction will be described.

In the detection result of the color sensor 42 illustrated in FIG. 11, the density value of the lowest density is Dmin, the density value at the position x in the scanning direction is D (x), and the density ratio α (x) is
α (x) = Dmin / D (x)
Calculate according to

  Based on the relationship between the density ratio α (x) and the correction coefficient γ (x) in the scanning direction, the density ratio α (x) is converted into a correction coefficient γ (x) in the scanning direction. FIG. 13 is a graph showing an example of the relationship between the two. The relationship between the two is stored in advance in the color image forming apparatus in the form of a formula or a table. In the example shown in FIG. 13, the relationship between the two is linear, but it is needless to say that the relationship may be other than linear.

  Further, since the position x in the scanning direction corresponds to the position where the patch is formed, the correction coefficient for the portion in between is calculated by interpolation calculation.

  In this way, the density unevenness in the scanning direction can be corrected by obtaining the corrected degree P ′ of the exposure light intensity and modulating the exposure light intensity so that the degree of modulation becomes P ′.

  In the present embodiment, the density unevenness correction control in the scanning direction is completed with a single test pattern. Therefore, C, M, Y, and K single-color patches 71 are alternately arranged in the test pattern 70 for density unevenness correction in the scanning direction. However, one test pattern may be used for each color.

  FIG. 14 shows an example of a test pattern in which a cyan patch 71C is formed. In this way, one test pattern in which a single color patch is formed for each color may be formed, and the density unevenness in the scanning direction may be corrected using the four test patterns. It is preferable to print a comment 72 regarding the setting direction of the test pattern on which an image has been formed on the paper feed tray and its operation also on this test pattern.

  In this case, the density can be measured at many points in the scanning direction as compared with the case where a single test pattern in which patches of four colors are alternately formed is used, so that an effect of improving the correction accuracy can be obtained.

  As described above, a test pattern having a patch as described in the present embodiment is formed, and after the user has set the test pattern by rotating it 90 degrees clockwise from the paper discharge tray to the paper feed tray, By performing detection with the sensor, even in an image forming apparatus having a low-cost color sensor with a narrow detection range in the scanning direction, density unevenness in the scanning direction can be detected and corrected, and color can be increased without increasing the cost of the apparatus. The image quality of the image forming apparatus can be improved.

(Second Embodiment)
Hereinafter, a second embodiment of the color image forming apparatus according to the present invention will be described. The second embodiment is also a color image forming apparatus having the same configuration as that of the first embodiment. Hereinafter, description of the same parts as those of the first embodiment will be omitted, and characteristic features of the second embodiment will be omitted. The explanation will focus on these parts.

  The second embodiment allows the user to easily determine the presence or absence of density unevenness in the scanning direction so that the density unevenness correction in the scanning direction described in the first embodiment can be performed at an optimal timing. Is.

  In general, the recommended execution frequency of density or chromaticity control described in the background section is higher than the recommended execution frequency of density unevenness correction in the scanning direction described in the first embodiment. In the present embodiment, a test pattern (patch) that allows the user to determine whether or not there is density unevenness in the scanning direction in a test pattern that is output when density or chromaticity control is performed using such a frequency difference. Is added.

  In this embodiment, all the density is described, but the same calibration effect can be obtained even if all the density is replaced with L * of chromaticity (L * a * b * color system). .

  FIG. 15 is a diagram illustrating an example of a test pattern formed on a recording medium when the density or chromaticity control for obtaining a stable color and color gradation described in the background art section is performed. is there. Patches 74 are formed in a line along the recording medium conveyance direction at positions that can be detected by the color sensor 42 in the scanning direction of the recording medium 11. The patch 74 includes at least one of C, M, Y, and K single-color gradation patches and CMY mixed-color gradation patches.

  FIG. 16 is a diagram illustrating an example of a test pattern 77 formed on a recording medium when performing density or chromaticity control for obtaining a stable color and color gradation in the second embodiment. is there. In addition to the test pattern 74 used for density or chromaticity control shown in FIG. 15, a plurality of patches 75 for checking density unevenness in the scanning direction are formed on both the left and right sides in the scanning direction. The scanning direction density unevenness confirmation patch 75 includes C, M, Y, and K single-color patches having an arbitrary gradation, and includes a plurality of sets of patches. The patches of C, M, Y, and K are included. Are the same gradation. For example, all the C patches have a gradation of 50%, all the M patches have a gradation of 75%, all of the Y patches have a gradation of 90%, and all of the K patches have a gradation of 100%. To. Of course, the gradations of all the C, M, Y, and K patches may be the same.

  Further, in the test pattern 77, it is preferable to print a comment 76 which describes that the density unevenness correction in the scanning direction is performed when there is a density difference in each color patch of the patch 75 for checking density unevenness in the scanning direction. .

  Next, a procedure for allowing the user to determine whether there is density unevenness in the scanning direction in the present embodiment will be described with reference to the flowchart of FIG.

  In step S221, density or chromaticity control for obtaining stable color and color gradation is started. The start timing may be when the user desires correction of density or chromaticity, or in a color image forming apparatus provided with environment detection means, when a change in ambient temperature or humidity is detected, or when a predetermined number of images are It may be the timing when the color and gradation of the output image may change, such as when it is formed.

  In step S222, the density or chromaticity control test pattern 77 described above with reference to FIG. 16 is formed.

  In step S223, after the density or chromaticity control test pattern 77 is fixed by the fixing unit, the color sensor 42 detects the patch 74, and the conventional density or chromaticity control is performed, and then the test is performed on the paper discharge tray. The pattern 77 is discharged.

  In step 224, the user visually inspects the discharged test pattern 77. The user visually compares the patches of the same color in the scanning direction density unevenness confirmation patch 75 to determine whether there is a density difference.

  If it is determined in step S225 that the user has a density difference between patches of the same color arranged in the scanning direction, the density unevenness correction control in the scanning direction described in the first embodiment is executed in step S226, and then the step In S227, the printer returns to a state where normal image formation is possible.

  If it is determined in step S225 that there is no density difference, the process returns to a state where normal image formation is possible in step S227.

  As described above, according to the present embodiment, when performing density or chromaticity control for obtaining a predetermined color and color gradation, a test for allowing the user to determine whether there is density unevenness in the scanning direction. By forming a pattern, density unevenness control in the scanning direction can be performed at an appropriate timing, and the image quality of the color image forming apparatus can be further improved without increasing costs.

(Third embodiment)
Hereinafter, a third embodiment of the color image forming apparatus according to the present invention will be described. The third embodiment is also a color image forming apparatus having the same configuration as that of the first embodiment. Hereinafter, the description of the same parts as those of the first embodiment will be omitted, and the characteristic of the third embodiment will be described. The explanation will focus on these parts.

  In the third embodiment, the relationship between the density ratio α (x) used for correcting the density unevenness in the scanning direction and the correction coefficient β (x) or γ (x) in the scanning direction is shown in the first embodiment. Instead of storing in advance as described above, it is obtained by actual measurement when correction is performed.

  In this embodiment, all the density is described, but the same calibration effect can be obtained even if all the density is replaced with L * of chromaticity (L * a * b * color system). .

  FIG. 18 is a diagram illustrating an example of a scanning direction density unevenness correction test pattern (hereinafter simply referred to as a test pattern) 79 used in the third embodiment. A difference from the test pattern 70 used in the first embodiment is that a patch 78 for obtaining a relationship between the density ratio α (x) and the correction coefficient β (x) or γ (x) in the scanning direction is added. It is a point.

  This patch 78 forms n sets (n is an integer of 2 or more), with one set of each of C, M, Y, and K single color patches as one set. If the density unevenness in the scanning direction is corrected by the PWM modulation degree of the exposure light, the PWM modulation degree is set to a different value for each set. If the density unevenness in the scanning direction is corrected by modulating the exposure light intensity, the degree of modulation of the exposure light intensity is set to a different value for each set.

  The density of the patch 78 is detected by the color sensor 42 when an image of the test pattern 79 is formed. Accordingly, as shown in FIG. 18, the patch 78 for obtaining the relationship between the density ratio α (x) and the correction coefficient β (x) or γ (x) in the scanning direction is formed at a position that can be detected by the color sensor 42. There is a need to.

  FIG. 19 is a graph showing an example of the relationship between the density ratio α (x) and the correction coefficients β (x) and γ (x) in the scanning direction obtained from the detection result of the patch 78. The relationship between the density ratio α (x) and the correction coefficient β (x) in the scanning direction, and (b) shows the relationship between the density ratio α (x) and the correction coefficient γ (x) in the scanning direction.

  The example shown in FIG. 19 is a case where the number of sets of C, M, Y, and K single-color patches is n = 4, and the results of four points are linearly interpolated. Of course, the interpolation method is not limited to linear interpolation, and may be spline interpolation.

  Note that the patch 78 for obtaining the relationship between the density ratio α (x) and the correction coefficient β (x) or γ (x) in the scanning direction is located at a position in the scanning direction that can be detected by a color sensor such as the center of the recording medium. Although it is necessary to form the relationship, the relationship between the density ratio α (x) and the correction coefficient β (x) or γ (x) in the scanning direction hardly depends on the position in the scanning direction, so that accuracy is not so required.

  After that, as in the first embodiment, the test pattern 79 is rotated 90 degrees clockwise and set on the paper feed tray, the patch 71 is detected by the color sensor 42, and density unevenness in the scanning direction is corrected. To do. At that time, as illustrated in FIG. 19, the relationship between the density ratio α (x) obtained by actual measurement and the correction coefficient β (x) or γ (x) in the scanning direction is used.

  As described above, according to the present embodiment, the relationship between the density ratio α (x) and the correction coefficient β (x) or γ (x) in the scanning direction is obtained with one test pattern, and the density in the scanning direction is determined. Unevenness can be corrected, the correction coefficient can be determined based on the data actually measured immediately before correction, and the image quality of the color image forming apparatus can be further improved without increasing the cost. it can.

<Other embodiments>
In the embodiment described above, the color image forming apparatus adopting the electrophotographic method as the image forming method has been described as an example, but the present invention is also applied to a color image forming device adopting another image forming method such as an ink jet method. Is applicable. In that case, “ink” is used instead of “toner” used as a colorant in each of the above embodiments.

  Further, the configuration of the color sensor to be used is not limited to that illustrated in the above embodiment, and various configurations can be used. Similarly, the mounting position of the color sensor in the scanning direction is not limited to the mounting position in the above embodiment.

  As the shape of the storage medium used for forming the test pattern, after the paper is discharged once, the angle is changed by 90 degrees from the direction of image formation (recording), and the operation is performed to set on the paper feed tray. For this reason, the length in the transport direction needs to be a shape that is equal to or less than the length in the scanning direction, and it is preferable that both sizes be close to a square. The rotation direction of 90 degrees is not limited to the clockwise direction, and may be counterclockwise.

  In addition, in the above embodiment, the case where the test pattern is formed in one row has been described as an example. However, the test pattern passes through the detection range of the sensor when the paper is rotated 90 degrees from the time of formation and fed. However, it is not always necessary to form in one row.

  Furthermore, in the above-described embodiment, the color image forming apparatus has been described as an example. However, the present invention can be applied to an apparatus that forms an image having a single color and different levels of gradation (density). In addition, it is not always necessary to use a color sensor, and a single-color density sensor can also be used as long as density or chromaticity unevenness can be detected with a desired sensitivity.

  The present invention may be applied to a system including a plurality of devices including the image forming apparatus, or may be applied to an image forming apparatus including a single device.

  In the present invention, a software program (in this embodiment, a program corresponding to at least a part of the flowcharts shown in FIGS. 6 and 17) that realizes at least a part of the functions of the above-described embodiment is stored in a system or apparatus. This includes a case where the program is supplied directly or remotely, and is achieved also by the computer of the system or apparatus reading and executing the supplied program code. In that case, as long as it has the function of a program, the form does not need to be a program.

  Accordingly, since the functions of the present invention are implemented by computer, the program code installed in the computer also implements the present invention. That is, the claims of the present invention include the computer program itself for realizing the functional processing of the present invention.

  In this case, the program may be in any form as long as it has a program function, such as an object code, a program executed by an interpreter, or script data supplied to the OS.

  As a recording medium for supplying the program, for example, flexible disk, hard disk, optical disk, magneto-optical disk, MO, CD-ROM, CD-R, CD-RW, magnetic tape, nonvolatile memory card, ROM, DVD (DVD-ROM, DVD-R).

  As another program supply method, a client computer browser is used to connect to an Internet site, and the computer program itself of the present invention or a compressed file including an automatic installation function is downloaded from the site to a recording medium such as a hard disk. Can also be supplied. It can also be realized by dividing the program code constituting the program of the present invention into a plurality of files and downloading each file from a different site. That is, a WWW server that allows a plurality of users to download a program file for realizing the functional processing of the present invention on a computer is also included in the scope of the present invention.

  In addition, the program of the present invention is encrypted, stored in a storage medium such as a CD-ROM, distributed to users, and key information for decryption is downloaded from the site via the Internet to users who have cleared predetermined conditions. It is also possible to execute the encrypted program by using the key information and install the program on a computer.

  In addition to the functions of the above-described embodiments being realized by the computer executing the read program, the OS running on the computer based on the instruction of the program is a part of the actual processing. Alternatively, the functions of the above-described embodiment can be realized by performing all of them and performing the processing.

  Furthermore, after the program read from the recording 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, the function expansion board or The CPU or the like provided in the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

1 is a functional block diagram showing an overall configuration of a first embodiment of a color image forming apparatus according to the present invention. It is sectional drawing which shows the mechanical structure of the color image forming apparatus of FIG. It is a figure which shows an example of a structure of a density sensor. It is a block diagram which shows the structure of a color sensor and its peripheral device. It is a figure which shows the structure of a color sensor. 6 is a flowchart illustrating a procedure for correcting density unevenness in the scanning direction in the first embodiment. It is a figure explaining the test pattern of 1st Embodiment. It is a figure for demonstrating a user's work. FIG. 4 is a diagram illustrating a positional relationship between a color sensor and a recording medium on which a test pattern is formed. It is a block diagram which shows the structure of a means which implement | achieves the correction | amendment of a scanning direction density nonuniformity. It is a graph which shows the example of the result of having detected the patch of the test pattern using the color sensor. It is a graph which shows the relationship between density ratio (alpha) (x) and the correction coefficient (beta) (x) of a scanning direction. It is a graph which shows the relationship between density ratio (alpha) (x) and the correction coefficient (gamma) (x) of a scanning direction. It is a figure which shows the example of the test pattern in which only the patch for cyan was formed. It is a figure which shows the example of the test pattern used for the density | concentration or chromaticity control for obtaining the stable color and the gradation of a color. It is a figure explaining the test pattern used in 2nd Embodiment. It is a flowchart which shows the procedure which makes a user judge the presence or absence of the density nonuniformity of a scanning direction in 2nd Embodiment. It is a figure explaining the test pattern used in 3rd Embodiment. It is a graph which shows the example of the relationship between density | concentration ratio (alpha) (x) and the correction coefficients (beta) (x) and (gamma) (x) of a scanning direction obtained from the detection result.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Recording medium 22 Photoconductor 23 Primary charging means 24 Scanner part 25 Toner cartridge 26 Developing means 27 Primary transfer roller 28 Intermediate transfer body 29 Secondary transfer roller 30 Cleaning means 31 Fixing device 41 Density sensor 42 Color sensor 70, 77, 79 Test Pattern 71, 74, 75 patch

Claims (26)

An image forming apparatus for forming a color image on a recording medium,
A color sensor provided at a predetermined position in the scanning direction substantially perpendicular to the conveyance direction of the recording medium and having a detection range narrower than the image formable region;
When the test pattern for image quality correction in the scanning direction is fed in a state in which the recording medium on which the test pattern is formed is rotated 90 degrees in a predetermined direction, the test pattern is set to a position that passes the detection range of the color sensor. Test pattern forming means for forming along the scanning direction;
Parameters related to image formation in the scanning direction based on the detection result of the test pattern by the color sensor when the recording medium on which the test pattern is formed is fed in a state rotated 90 degrees in the predetermined direction. An image forming apparatus comprising: a correcting unit that corrects the image.   The image forming apparatus according to claim 1, wherein the test pattern includes a plurality of sets of patches each made of a colorant used for forming a color image, and each patch is formed under the same conditions.   The test pattern forming unit forms a test pattern composed of patches formed using one colorant used for forming a color image on a number of recording media corresponding to the number of colorants used for forming a color image. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein the test pattern is a test pattern for correcting unevenness in density or chromaticity of an image in a scanning direction.   The image forming apparatus according to claim 1, wherein a color image is formed according to an electrophotographic system.   6. The image forming apparatus according to claim 5, wherein the parameter relating to image formation is a PWM modulation degree of exposure light.   6. The image forming apparatus according to claim 5, wherein the parameter relating to image formation is a degree of modulation of exposure light intensity.   When forming a second test pattern different from the test pattern for purposes other than image correction in the scanning direction, a patch is added that allows the user to visually determine whether or not image correction in the scanning direction is necessary. The image forming apparatus according to claim 1, further comprising a second test pattern forming unit. The purpose other than the image correction in the scanning direction is correction for obtaining a stable color and color gradation.
The image forming apparatus according to claim 8, wherein the correction for obtaining the stable color and the gradation of the color is set to be performed at a frequency higher than the image correction in the scanning direction. .   The image forming apparatus according to claim 1, further comprising a storage unit that stores information indicating a relationship between a parameter value relating to the image formation and a characteristic value of the image to be formed. The test pattern forming means adds a patch in which the value of the parameter relating to the image formation is changed in a plurality of stages to the test pattern at a position passing through the detection range of the color sensor,
The coefficient calculation means for obtaining a relationship between a value of a parameter relating to the image formation and a characteristic value of an image to be formed based on a detection result of the patch by the color sensor. Image forming apparatus. A correction method for an image forming apparatus that includes a color sensor provided at a predetermined position in a scanning direction substantially perpendicular to the conveyance direction of a recording medium and having a detection range narrower than an image formable area, and forms a color image on the recording medium. There,
When the test pattern for image quality correction in the scanning direction is fed in a state in which the recording medium on which the test pattern is formed is rotated 90 degrees in a predetermined direction, the test pattern is set to a position that passes the detection range of the color sensor. , A test pattern forming step for forming along the scanning direction;
Parameters related to image formation in the scanning direction based on the detection result of the test pattern by the color sensor when the recording medium on which the test pattern is formed is fed in a state rotated 90 degrees in the predetermined direction. And a correction step of correcting the image forming apparatus.   13. The image forming apparatus according to claim 12, wherein, in the test pattern forming step, a plurality of sets of patches each composed of patches of colorants used for forming a color image are formed, and each patch is formed under the same conditions. Correction method.   In the test pattern forming step, a test pattern composed of patches formed using one colorant used for forming a color image is formed on a number of recording media corresponding to the number of colorants used for forming a color image. The correction method for an image forming apparatus according to claim 12.   13. The correction method for an image forming apparatus according to claim 12, wherein in the correction step, unevenness in image density or chromaticity in the scanning direction is corrected.   13. The image forming apparatus correction method according to claim 12, which is applied to an image forming apparatus that forms a color image in accordance with an electrophotographic system.   17. The correction method for an image forming apparatus according to claim 16, wherein the parameter relating to image formation is a PWM modulation degree of exposure light.   17. The image forming apparatus correction method according to claim 16, wherein the parameter relating to image formation is a degree of modulation of exposure light intensity.   When forming a second test pattern different from the test pattern for purposes other than image correction in the scanning direction, a patch is added that allows the user to visually determine whether or not image correction in the scanning direction is necessary. The image forming apparatus correction method according to claim 12, further comprising a second test pattern forming step. The purpose other than the image correction in the scanning direction is correction for obtaining a stable color and color gradation.
The image forming apparatus according to claim 19, wherein the correction for obtaining the stable color and the gradation of the color is set to be performed at a frequency higher than the image correction in the scanning direction. Correction method.   13. The correction method for an image forming apparatus according to claim 12, further comprising a storage step of storing information indicating a relationship between a parameter value relating to the image formation and a characteristic value of the image to be formed. In the test pattern forming step, a patch in which parameter values relating to the image formation are changed in a plurality of stages is formed by adding to the test pattern at a position passing through the detection range of the color sensor,
13. The coefficient calculating step according to claim 12, further comprising a coefficient calculating step for obtaining a relationship between a value of the parameter relating to the image formation and a characteristic value of the image to be formed based on a detection result of the patch by the color sensor. A correction method for an image forming apparatus.   23. A computer program for causing a computer device to execute the image forming apparatus correction method according to claim 12.   A storage medium storing the computer program according to claim 23. A color sensor provided at a predetermined position in the scanning direction substantially orthogonal to the conveyance direction of the recording medium and having a detection range narrower than the image formable area, and in the scanning direction of an image forming apparatus that forms a color image on the recording medium. A test pattern used for image quality correction,
The test pattern is formed at a position that passes the detection range of the color sensor when the recording medium on which the test pattern is formed is fed in a state rotated 90 degrees in a predetermined direction. . An image forming unit for forming an image on a recording medium;
A sensor provided in a recording medium conveyance path and having a detection range narrower than an image formable region in a direction perpendicular to the recording medium conveyance direction;
Test pattern forming means for forming a test pattern along a direction perpendicular to the conveyance direction of the recording medium;
Based on the result of detection of the test pattern by the color sensor when the recording medium on which the test pattern is formed is re-feeded in a state rotated 90 degrees from the direction at the time of test pattern formation. An image forming apparatus comprising: correction means for correcting parameters relating to image formation in a vertical direction.

Images