JP2006189789A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing the occurrence of a false outline, etc., in an intermediate density region by correcting deterioration in gradation due to the variation in image forming characteristic of a dark toner and a light toner when the image is formed by using the dark toner and the light toner. <P>SOLUTION: A first patch pattern for the dark and light toners is formed on recording paper according to data after LUT conversion and the density is read. A second patch pattern is formed for the light toner according to the LUT through data and the density is read. Furthermore, a third patch pattern for the dark toner is formed according to the LUT through data and the density is read. A γ table for the light toner and a γ table for the dark toner are corrected according to the obtained density data. When the γ table for the light toner is corrected, correction is carried out by changing the slope of the gradation characteristic of the image data for the light toner with zero level as base point based on the density characteristics of the read patch pattern and the ratio of the amounts of the dark toner and the light toner that have been used. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus that forms an image using toners having substantially the same hue and different densities, and a control method thereof.

  An image forming apparatus that forms an image by electrophotography includes a charging device that uniformly charges a photosensitive surface of a photosensitive drum. The image forming apparatus further includes a latent image forming apparatus that forms an electrostatic latent image corresponding to image information on the charged photosensitive surface, and a developing apparatus that develops the electrostatic latent image with a developer. The image forming apparatus includes a transfer device that transfers the developed latent image to a recording material, and a fixing device that fixes the transferred image to the recording material.

In general, as the developer (toner), one type of toner of each color and predetermined density such as cyan, magenta, yellow, and black is used. However, when these one type of toner is used, the amount of toner is reduced in the highlight area (low density area), and the reproducibility of gradation (density gradation) for image data is difficult. In recent years, the level of user needs has increased, and an image forming apparatus in which the number of developer colors is increased has been proposed as compared to the conventional image forming apparatus that forms an image with four colors of toner. That is, an electrophotographic image forming apparatus using toners having different densities as toners of substantially the same hue is proposed. (See Patent Documents 1 and 2)
Many image forming apparatuses of this type use six types of developer including a cyan, a magenta, a yellow, and a black developer, and a thin cyan and a light magenta developer. The light cyan and light magenta developers are developers in which the spectral characteristics of the pigments contained are the same as those of normal cyan and magenta, respectively, but the content thereof is small. In the following, normal cyan and magenta developers are referred to as dark toner, and light cyan and light magenta developers are referred to as light toner. Further, during development, an image signal using dark toner is referred to as an image signal for dark toner, and an image signal using light toner is referred to as an image signal for light toner.

FIG. 19 is a graph showing the relationship between the image density of the input image signals for dark toner and light toner, the toner adhesion amount, and the output density. The characteristics of the solid line T1 and the wavy line T2 shown in the figure indicate the relationship between the dark toner and the light toner, and the adhesion amount of the toner on each recording paper with respect to the image density of the input image signal. Further, the characteristics of the straight line m indicate ideal output density characteristics with respect to the image density of the input image signal, and the adhesion amounts of the dark toner and the light toner with respect to the image density of the input image signal are dark toner. And the output density of the image formed with the light toner is determined to be an ideal linear. When the maximum value of the image density of the input image signal is 1.8, the graininess of the image is reduced in the area from the highlight portion (low density portion) to the intermediate density portion, which is 0.9 or less. Therefore, an image is formed using only light toner. In an intermediate density to high density region where the image density is 0.9 or more, in order to reduce the amount of applied toner, the amount of light toner used is reduced, dark toner is added, and both dark toner and light toner are used. Form.
JP 2004-145137 A JP 2001-290319 A

  However, in an image forming apparatus that forms an image using dark toner and light toner, the following problems occur when the output characteristics of the dark toner and light toner change.

  For example, when the resistance value of the surface layer of the photosensitive drum or the charge amount (tribo) of the developer is lowered due to the use environment or use conditions of the image forming apparatus, the contrast potential (Vcont) becomes small, and as a result, the toner adhesion amount Changes, the output density decreases.

  This point will be described in more detail. A curve l in FIG. 19 shows a case where the adhesion amount of the dark toner is reduced, and the characteristic of the output density at this time is the curve n. As is apparent from this curve n, the output density changes abruptly at an intermediate density portion (image density is near 0.9) where dark toner is added to start image formation. For this reason, an image using an intermediate density has problems such as unnatural gradation and generation of pseudo contours.

  In view of the above-described conventional problems, the present invention suppresses the density fluctuation at the joint between the dark toner and the light toner to prevent the occurrence of pseudo contours in the intermediate density region, and enables stable and high-quality image output at all times. It is an object of the present invention to provide an image forming apparatus and an image forming method.

  In order to achieve the above object, the present invention provides an image forming apparatus for forming an image using dark and light toners having substantially the same hue, pattern forming means for forming a pattern using dark toner and light toner, and a recording paper. Based on the pattern reading means for reading the density of the formed pattern, and the density characteristics of the pattern read by the pattern reading means and the usage ratio of the dark and light toners, the gradation characteristics of the image data for the light toner are Gradation correction means for correcting by changing the inclination with the zero level as a base point.

  According to another aspect of the present invention, there is provided an image forming apparatus that forms an image using dark and light toners having substantially the same hue, and a pattern forming unit that forms a pattern using dark toner and light toner; Based on the pattern reading means for reading the density of the pattern, and the density characteristics of the pattern read by the pattern reading means and the usage ratio of the dark and light toners, the image data gradation characteristics for the dark toner are based on the maximum density level. And a tone correction means for correcting by changing the inclination.

  According to another aspect of the present invention, there is provided an image forming apparatus that forms an image using dark and light toners having substantially the same hue, and a pattern forming unit that forms a pattern using dark toner and light toner; Based on the pattern reading means for reading the density of the pattern, and the density characteristics of the pattern read by the pattern reading means and the usage ratio of the dark and light toners, the gradation characteristics of the image data for the light toner are based on the zero level. In addition to correcting by changing the inclination, the image forming apparatus has gradation correction means for correcting the gradation characteristics of the image data for dark toner by changing the inclination with the maximum density level as a base point.

  According to another aspect of the present invention, there is provided an image forming method for forming an image using dark and light toners having substantially the same hue, a pattern forming step for forming a pattern using dark toner and light toner, and the above-mentioned pattern formed on a recording paper. Based on the pattern reading process for reading the pattern density, and the density characteristics of the pattern read in the pattern reading process and the usage ratio of the dark and light toners, the gradation characteristics of the image data for the light toner are based on the zero level. And a gradation correction step of correcting by changing the inclination.

  According to another aspect of the present invention, there is provided an image forming method for forming an image using dark and light toners having substantially the same hue, a pattern forming step for forming a pattern using dark toner and light toner, and the above-mentioned pattern formed on a recording paper. Based on the pattern reading process for reading the density of the pattern, the density characteristics of the pattern read in the pattern reading process, and the usage ratio of the dark and light toners, the image data gradation characteristics for the dark toner are based on the maximum density level. And a gradation correction step of correcting by changing the inclination.

  According to another aspect of the present invention, there is provided an image forming method for forming an image using dark and light toners having substantially the same hue, a pattern forming step for forming a pattern using dark toner and light toner, and the above-mentioned pattern formed on a recording paper. Based on the pattern reading process for reading the density of the pattern, and the density characteristics of the pattern read in the pattern reading process and the usage ratio of the dark and light toners, the gradation characteristics of the image data for the light toner are based on the zero level. And a gradation correction step of correcting the gradation characteristics of the image data for dark toner by changing the inclination with the maximum density level as a base point.

  According to the present invention, fluctuations in the density of the seam between the dark and light toners having substantially the same hue are suppressed, so that it is possible to prevent the occurrence of pseudo contours in the halftone part and to achieve stable and high image quality. Image output is possible.

  FIG. 1 is a schematic plan view illustrating the configuration of an electrophotographic image forming apparatus according to an embodiment of the present invention. This color image forming apparatus forms an image by electrophotography using a dark toner (hereinafter referred to as “dark toner”) and a thin toner (hereinafter referred to as “light toner”) having substantially the same hue and different densities. Is.

  That is, the color image forming apparatus 100 includes six developing devices 41, 42, 43, 44, 45, and 46. Developing unit 41 is light cyan toner, developing unit 42 is yellow toner, developing unit 43 is magenta toner, developing unit 44 is light magenta toner, developing unit 45 is cyan toner, and developing unit 46 is black toner. Are loaded.

  Incidentally, toners having substantially the same hue and different concentrations are usually toners having the same spectral characteristics and different amounts of color components (pigments) contained in a toner based on resin and color components (pigments). Say. The light toner is a toner having a relatively low density among a combination of toners having the same hue and different densities.

  In addition, as described above, “substantially the same hue” refers to those having the same spectral characteristics of the coloring component (pigment). However, even if they are not exactly the same, generally magenta, cyan, yellow, black, etc. Thus, it is set as a range that can be called the same color in the normal color concept.

  In the present invention, a toner (light toner) having substantially the same hue and a low density has an optical density after fixing of less than 1.0 for a toner amount of 0.5 mg / cm 2 on the recording material, and a dark toner (dark toner). Toner) has an optical density of 1.0 or more after fixing per 0.5 mg / cm 2 of toner on the recording material.

  In the present embodiment, the amount of pigment in the dark toner is adjusted so that the optical density after fixing is 1.6 when the amount of toner on the recording material is 0.5 mg / cm 2. The light toner is designed so that the applied density is 0.5 mg / cm 2 and the optical density after fixing is 0.8. The two gradations of toner are mixed well to reproduce the color gradation of each color toner.

  In this color image forming apparatus 100, two photoconductors that are drum-shaped image carriers, that is, a first photoconductor drum 1a and a second photoconductor drum 1b are arranged. Each photosensitive drum 1a, 1b is rotationally driven in the direction of the arrow.

  Around the photosensitive drum 1a, a pre-exposure lamp 11a, a corona charger 2a, a laser exposure device 3a, a potential sensor 12a, a developing rotary 4a containing developing devices 41 to 43, a primary transfer roller 5a, and a cleaning device 6a are arranged. ing. Here, these are collectively referred to as the first image forming unit Sa. The periphery of the photosensitive drum 1b is also arranged in the same manner as the second image forming unit Sb. These image forming portions Sa and Sb have substantially the same configuration (shape) from the viewpoint of cost reduction. For example, the configuration and shape of the developing device are almost the same. As a result, the developing devices 41 to 46 can be interchanged with each other.

  In the present embodiment, a belt-shaped intermediate transfer body, that is, an intermediate transfer belt 5 adjacent to the first and second photosensitive drums 1a and 1b functions as primary transfer means. The primary transfer rollers 5a and 5b, the driving roller 51, and the roller 52 are wound around and provided. The first and second primary transfer rollers 5a and 5b are in contact with the first and second photosensitive drums 1a and 1b, respectively, to form a primary transfer unit. Further, a secondary transfer roller 54 is disposed so as to face the roller and sandwich the intermediate transfer belt 5 to constitute a secondary transfer unit. The secondary transfer roller 54 can be separated from and brought into contact with the intermediate transfer belt 5. Further, a cleaner 50 for removing the transfer residual toner on the intermediate transfer belt 5 is provided so as to be detachable from the intermediate transfer belt 5.

  Next, an image forming operation in the image forming apparatus will be described.

  Based on the image signal of the original image read by the reader unit 300, an image formation start signal is issued. In addition to the image signal from the reader unit 300, an image signal from a computer or an image signal from a facsimile is also transferred. Here, an image transferred from the reader unit 300 is used. An image forming operation based on the signal will be described.

  As a result, the photosensitive drums 1a and 1b of the image forming units Sa and Sb that are rotationally driven at a predetermined process speed are neutralized by the pre-exposure lamps 11a and 11b, respectively, and uniformly negative by the corona chargers 2a and 2b. Is charged. Then, the laser exposure devices 3a and 3b radiate color-separated image signals input from the outside from the semiconductor laser 36, and pass through the polygon mirror 35, the reflection mirror 37, and the like on the photosensitive drums 1a and 1b. An electrostatic latent image of each color is formed.

  The subsequent operation will be described separately for the high-quality color mode and the normal color mode.

  The operation in the high image quality color mode (when an image is formed using six colors) will be described.

  A developing rotary 41a is rotated on the electrostatic latent image formed on the upstream photosensitive drum 1a so that a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1a is applied to the developing device 41. It is brought into contact with 1a, and a light cyan toner is attached to make the toner image visible.

  The light cyan toner image on the photosensitive drum 1a is transferred by a transfer roller 5a to which a primary transfer bias (opposite polarity (positive polarity) from toner) is applied at a primary transfer portion between the photosensitive drum 1a and the transfer roller 5a. The primary transfer is performed on the intermediate transfer belt 5.

  In parallel with the upstream image forming operation, a light magenta latent image is formed on the second, ie, downstream, photosensitive drum 1b by the corona charger 2b as the primary charging means and the laser exposure device 3b. By rotating the developing rotary 4b, the developing unit 44 containing light magenta toner is brought into contact with the downstream photosensitive drum 1b to develop a light magenta image. The light magenta image on the downstream photosensitive drum 1b is transferred to the intermediate transfer belt 5 by a second transfer bias applied to the downstream transfer roller 5b.

  As the intermediate transfer belt 5 rotates in the arrow direction B, the image on the intermediate transfer belt 5 passes through the secondary transfer roller 54 separated from the intermediate transfer belt 5 and the intermediate transfer belt cleaner 50 separated from the intermediate transfer belt 5, and again the primary transfer portion. Return to.

  Therefore, by rotating the developing rotary 4a, the developing device 43 containing yellow toner is brought into contact with the photosensitive drum 1a, and a yellow image is formed and transferred to the intermediate transfer belt 5. The image on the intermediate transfer belt 5 moves to the downstream photosensitive drum 1b as it is, and forms a cyan image as in the upstream.

  The above operation is repeated, and magenta and black toner images are similarly transferred onto the intermediate transfer belt 5, and the image moves to the secondary transfer portion with all the colors on. The transfer material (paper) is fed in accordance with the timing at which the front end of the full-color toner image on the intermediate transfer belt 5 is moved to the secondary transfer portion between the secondary transfer counter roller and the secondary transfer roller 54. Paper is selected from the cassettes 71 to 74 and fed through the transport path. Then, the transfer material is conveyed to the secondary transfer portion by the registration roller 85. A full-color toner image is secondarily transferred collectively by a secondary transfer roller 54 to which a secondary transfer bias (opposite polarity (positive polarity) to toner) is applied to a transfer material conveyed to the secondary transfer unit. . The residual toner on the intermediate transfer belt 5 is cleaned by bringing the intermediate transfer belt cleaner 50 into contact with the intermediate transfer belt 5 after the secondary transfer.

  The transfer material on which the full-color toner image is formed is conveyed to the fixing device 9 where the toner image is heated and pressed at the fixing nip portion between the fixing roller and the pressure roller to be thermally fixed on the surface of the transfer material. Is done. Thereafter, the sheet is discharged onto a sheet discharge tray 89 on the upper surface of the main body by a sheet discharge roller, and a series of image forming operations is completed.

  Next, the operation in the normal color mode (when using 4 colors to form an image) without using light toner will be described.

  A yellow latent image is formed on the upstream photosensitive drum 1a by a corona charger 2a as a primary charging unit and a laser exposure device 3a, and by rotating the developing rotary 4a, a light cyan developing device 41 is skipped, thereby causing yellow The developing device 43 is brought into contact with the photosensitive drum 1a to develop a yellow image. The yellow image on the photosensitive drum 1a is transferred to the intermediate transfer belt 5 by a transfer bias applied to the transfer roller 5a.

  In parallel with the upstream image forming operation, a cyan latent image is formed on the downstream photosensitive drum 1b by the corona charger 2b and the laser exposure device 3b as primary charging means. By rotating the developing rotary 4b, the light magenta developing device 44 is skipped and the cyan developing device 46 is brought into contact with the photosensitive drum 1b to develop a cyan image. The cyan image on the photosensitive drum 1b is transferred to the intermediate transfer belt 5 by a transfer bias applied to the transfer roller.

  As the intermediate transfer belt 5 rotates in the arrow direction B, the image on the intermediate transfer belt 5 passes through the secondary transfer roller 54 separated from the intermediate transfer belt 5 and the intermediate transfer belt cleaner 50 separated from the intermediate transfer belt 5, and again the primary transfer portion. Return to.

  Therefore, by rotating the developing rotary 4a, the magenta developing device 42 is brought into contact with the photosensitive drum 1a, and a magenta image is formed and transferred to the intermediate transfer belt 5. The image on the intermediate transfer belt 5 moves to the photosensitive drum 1b as it is, and forms a black image as in the upstream.

  When the four-color transfer is completed by the above operation and the image moves to the secondary transfer portion with all the colors on, the secondary transfer roller 54 comes into contact with the intermediate transfer belt 5 and is applied to the secondary transfer roller. The image is transferred to the transfer material P by the transfer bias and fixed on the transfer material P by fixing means (not shown). The residual toner on the intermediate transfer belt 5 is cleaned by bringing the intermediate transfer belt cleaner 50 into contact with the intermediate transfer belt after the secondary transfer.

  As described above, the two development rotarys 4a and 4b are provided, so that, for example, when a six-color image using light toner is formed as a high image quality mode, compared with a conventional rotary multicolor image forming apparatus. It is possible to output an image without reducing the throughput and without causing an increase in size and cost of the apparatus as in the inline method.

  For example, when a four-color image is formed without using light toner in the normal mode, the light-toner developing device is faster than the conventional multi-color image forming apparatus having only one developing rotary. Can output an image without using it wastefully.

  Note that the user can switch the mode between the high-quality color mode (when 66-color images are formed) / normal color mode (when 6-color images are formed) on the operation unit 1508. A description of the operation unit will be described later.

  The image forming apparatus 100 includes an automatic adjustment function that adjusts the voltage values of the primary chargers and primary transfer rollers of the image forming units Sa and Sb in order to obtain higher quality images. This automatic adjustment function includes DMAX control for determining the maximum image density for determining the gradation of the toner image, and gradation correction control for realizing the gradation. A patch detection sensor 53 that reads a patch image of a predetermined density and size that has been imaged to execute this automatic adjustment function is provided. In this automatic adjustment function, the patch detection sensor 53 reads the density of the patch image of each color and performs an adjustment so that the density of the toner image developed with the toner of each color is optimized.

  Further, density control using the patch detection sensor 53 is performed by forming a patch image on an intermediate transfer member or a drum and detecting it. Regarding the change in the color balance of the image due to the subsequent transfer and fixing to the recording material, Not controlling. The color balance also changes depending on the transfer efficiency in transferring the toner image to the recording material, and heating and pressurization by fixing. This change cannot be handled by density control using the patch detection sensor 53.

  Therefore, a gradation patch for each color of cyan, magenta, yellow, black, light cyan, and light magenta, or a mixed color patch of C, M, and Y is formed on the recording material. A sensor 99 for detecting chromaticity (hereinafter referred to as a color sensor) is installed.

  In this color image forming apparatus, the detection result is fed back to a calibration table or the like for correcting the exposure amount in the image forming unit, the process conditions, and the density-gradation characteristics. Image density or chromaticity can be controlled.

  FIG. 2 is a schematic diagram showing the configuration of the post-fixing sensor 99 of the color image forming apparatus 100 in FIG. As the light source of the post-fixing sensor 99, an LED 201 having a light emission peak wavelength in the range of 400 nm to 700 nm according to the color of the pattern to be measured is used. The LED 201 is disposed at an angle inclined by 45 ° with respect to the normal line N of the measurement opening 202 and emits light to the pattern 205 formed on the recording paper P conveyed to the measurement opening 202. Irradiate. Further, an imaging lens 203 and a light receiving unit 204 are disposed on 2n of the measurement opening 202. The light emitted from the LED 201 is reflected by the pattern 205 on the recording paper P, and the component in the normal N direction of the reflected light is imaged on the light receiving surface of the light receiving unit 204 by the imaging lens 203. The light receiving unit 204 is configured by arranging photoelectric conversion elements such as photodiodes. A glass 206 is installed between the post-fixing sensor 99 and the recording paper, and the recording paper is conveyed so as to be in close contact with the glass 206, and measurement is performed while the optical path length with respect to the recording paper is always constant.

  Two density sensors 99 are arranged between the heat roller fixing device 9 and the paper discharge tray 89 in a direction perpendicular to the conveyance direction of the recording material, and on the recording sheet fixed by the heat roller fixing device 9. The cyan, magenta, yellow, and black patterns can be simultaneously measured as necessary. Here, two are arranged side by side, but may be three or four.

  FIG. 15 is a block diagram illustrating a main configuration of a control unit 1501 that controls the operation of the image forming apparatus 100. The control unit 1501 includes a digital image processing unit 1503, a CPU 1506 having an I / F that exchanges information for performing control with respect to the printer control I / F 1505 and the external I / F 1504, an operation unit 1508, and a memory 1507. Has been. The memory 1507 includes a RAM 1510 that provides a work area to the CPU 1506 and a ROM 1509 that stores the CPU control program. The ROM 1509 is operated in various modes such as an automatic color selection (ACS) mode for automatically switching between color image formation and monochrome image formation, which will be described later, a high image quality color mode, a normal color mode, and a monochrome image formation mode. The control program for executing is stored. Further, a control program for controlling the entire image forming apparatus 100 is stored. Further, the operation unit 1508 is configured by a liquid crystal with a touch panel for notifying the operator of input of contents to be processed / executed by the operator and information on processing, warnings, and the like.

  FIG. 16 is a diagram illustrating a configuration of the operation unit 1508. The operation unit 1508 includes a numeric keypad 1601, a start key 1602, a stop key 1603, an LCD 1604, and a user mode key 1605. Here, the numeric keypad 1601 is a key used by the user when inputting the number of copies and the amount of image movement when copying. A start key 1602 is a key that the user presses when starting a copy job. A stop key 1603 is a key to be pressed by the user when the started job is stopped halfway. The LCD 1604 is a display unit that displays an operation state of the image forming apparatus 100. Further, the LCD 1604 is provided with a panel switch, and the user can set a copy job mode via the panel switch.

  A user mode key 1605 is a key that the user presses when displaying a user mode screen on the LCD 1604. On the user mode screen, specifications for each function of the image forming apparatus 100 can be set. For example, when the user does not explicitly specify any one of a high-quality color mode, a normal color mode, and a black and white image formation mode (also referred to as a black and white mode), the image to be formed is automatically determined, and a color image This is an automatic color selection (ACS) mode for switching between formation and monochrome image formation.

  If the paper size is irregular size when forming a black-and-white image, specify whether or not to enter the vertical / horizontal size of the paper size, or if the paper size is irregular size paper in the automatic color selection mode. In some cases, the user can set the standard operation of the copier, such as setting to specify whether to input the vertical and horizontal sizes of the paper first or when the color document is detected.

  An instruction to start the operation in the adjustment mode for controlling the density or gradation of the output image formed on the recording material, which is a feature of the present invention, can be given.

  FIG. 18 is a diagram showing a display screen of the adjustment mode on the LCD 1604. This screen is displayed when the user mode key 1605 is pressed to display the user mode screen on the LCD 1604 and the adjustment mode is selected. If 1801 is selected here, the adjustment mode is started, and if 1802 is selected, the screen returns to the user mode screen.

  FIG. 17 is a diagram showing a display screen in the standard state on the LCD 1604. On the screen 1700, reference numerals 1701 and 1702 denote buttons for setting a magnification for image formation. Reference numeral 1703 denotes a paper selection button, which is a button for designating paper sizes such as various standard sizes and irregular size paper. Reference numerals 1704, 1705, 1706, and 1714 denote buttons for forming an image in an automatic color selection (ACS) mode, a high-quality color mode, a normal color mode, and a monochrome image formation mode, respectively. Only one of these four buttons is selected exclusively and cannot be selected simultaneously. 1707, 1708, and 1709 are buttons for adjusting the print density of the image. Reference numeral 1711 denotes a button for designating processing such as stapling performed on a bundle of recording sheets by a paper discharge processing device (not shown). Reference numeral 1712 denotes a button for designating whether an image is to be arranged in a format from one side to one side, from one side to both sides, or from both sides to one side, or from both sides to both sides when forming an image from a document to recording paper. Reference numeral 1713 denotes a button for designating various application modes.

  FIG. 3 is a block diagram showing the flow of image signals of the image processing unit in the reader unit 300 of the image forming apparatus 100 in FIG.

  The image signal output from the CCD sensor 34 of the reader unit 300 and the signal output from the post-fixing sensor (density sensor) 99 are input to the analog signal processing unit 301 where gain adjustment and offset adjustment are performed. The A / D converter 302 converts each color signal into 8-bit digital image signals R1, G1, and B1. Thereafter, the signal is input to the shading correction unit 303, and known shading correction is performed using a read signal of the reference white plate for each color.

  Since the line sensors of the CCD sensor 34 are arranged at a predetermined distance from each other, the line delay unit 304 needs to correct the spatial deviation in the sub-scanning direction. The input masking unit 305 is a part that converts a reading color space determined by the spectral characteristics of the R (red), G (green), and B (blue) filters of the CCD sensor 34 into an NTSC standard color space, and is a 3 × 3 matrix. Perform the operation. The LOG conversion unit 306, which is a light amount / density conversion unit, includes a look-up table (LUT) RAM, and converts the luminance signals of R4, G4, and B4 into density signals. The image signals output from the LOG conversion unit 306, cyan C0, magenta M0, and yellow Y0 are supplied to the line delay memory 307, and are output to the printer control unit shown in FIG. 4 as the image signals C1, M1, and Y1. . In the following, it is assumed that C represents cyan, M represents magenta, Y represents yellow, and Bk represents a black image signal. In addition, image signals R4, G4, and B4 from the external input unit in the figure indicate image signals from a computer or image signals from a facsimile.

  FIG. 4 is a block diagram showing the flow of image signals in the printer controller 400 that controls the image forming apparatus 100 in FIG.

  The masking and UCR unit 408 extracts a black signal (BK) from the input three primary color signals of Y1, M1, and C1. Further, a calculation for correcting the color turbidity of the recording colorant in the image forming apparatus 100 is performed, and Y2, M2, C2, and BK2 signals are sequentially output with a predetermined bit width (8 bits) for each reading operation. To do.

  A spatial filter unit (output filter) 409 performs edge enhancement or smoothing processing. The image memory unit 410 temporarily stores Y3, M3, C3, and BK3 processed as described above and output from the spatial filter unit 409, and the grayscale data generation unit 411 and the line delay are synchronized with the image forming operation. Send to part 412.

  The density data generation unit 411 has a function of inputting the image signals C4 and M4 and converting them into cyan and magenta dark toner image signals DC5 and DM5 and light toner image signals PC5 and PM5, respectively. This conversion process is performed using a predetermined conversion table. The configuration of the predetermined conversion table is changed depending on whether the input image is a halftone image or a character image. In other words, the use of light toner is increased for halftone images to reduce the roughness in highlight areas, and the use of dark toner is increased for character images to limit the amount of toner applied. Then, the composition ratio of the image data for dark toner and light toner is changed.

  The line delay unit 412 inputs Y4 and BK4 for DC5, PC5, DM5, and PM5 generated by data conversion in the grayscale data generation unit 411 in order to synchronize the image data of each of the six colors input to the LUT 414 described later. This is to correct the deviation. The LUT 414 performs density correction (tone correction) to match the ideal tone characteristics of the image forming apparatus 100, and includes a γ table for light toner and a γ table for dark toner. The six color image data (DC5, PC5, DM5, PM5, Y5, BK5) output from the grayscale data generation unit 411 and the line delay unit 412 are supplied to the LUT 414 and subjected to gradation correction.

  Signals (DC6, PC6, DM6, PM6, Y6, BK6) output from the LUT 414 are sequentially sent to the PWM unit 415, and the laser driver 416 performs semiconductor lasers 417 to 422 for each color and light color (semiconductor laser 36 in FIG. 1). And a latent image is formed on the photosensitive drums 1a and 1b.

  FIG. 5 is a graph showing output characteristics of the light / dark data (the dark toner image signal and the light toner image signal) generated by the light / dark data generation unit 411 in FIG. The relationship between an input signal X (0 to 255) input to the density data generation unit 411 (for density toner) and an output signal output from the density data generation unit 411 at this time is shown. When the input signal X is in the range of 0 to 128, only the light toner is formed. When the input signal X is in the range of 128 to 255, the light toner is reduced and the dark toner is gradually added. Form.

  Thus, only the light toner outputs the output signal 0-255 until the input signal X = 0-128, and the output signal 0-255 is output for both the light toner and the dark toner when the input signal X = 128-255. As a result, at the input signal X = 128, the light toner has the same input value and the output value of 255, and the dark toner has the same input value and the output value of 0.

  In addition, a pattern generation unit 413 is arranged in the image forming apparatus of the present embodiment. As shown in FIG. 6A, the pattern generation unit 413 includes a first patch pattern 601 of magenta (M) and cyan (C) using dark toner and light toner, magenta (M) and cyan using light toner. The second patch pattern 602 of (C) or the third patch pattern 603 of magenta (M) and cyan (C) using dark toner is formed on the recording paper. For this purpose, for example, first, second and third pattern data 601a, 602a and 603a as shown in FIG. 6B are registered. Further, the first, second and third pattern data 601a, 602a and 603a are inputted to the input signal X. Outputs are made corresponding to Xp and Xd, respectively. Note that the first, second, and third patch patterns 601, 602, and 603 shown in FIG. 6 may be formed on the same recording paper or different recording papers.

  The pattern data output from the pattern generation unit 413 can be supplied to the grayscale data generation unit 401 and the line delay unit 412 via the image memory unit 410 or directly to the PWM unit 415 via the LUT 414. Yes. Accordingly, the printer control unit 400 shown in FIG. 4 can output both the pattern data converted by the gray data generation unit 411 and the LUT 414 and the pattern data not converted by the gray data generation unit 411 and the LUT 414. ing.

  The image signals of DC6, PC6, DM6, PM6, Y6, and BK6 processed in this way and output from the LUT 414 are sent to the PWM unit 415 and converted into laser light by the semiconductor lasers 417 to 422 via the laser driver 416. Is done.

<Tone correction method according to the present embodiment>
With reference to FIG. 7, a description will be given of gradation correction processing for light and light cyan and magenta performed in the adjustment mode of the color image forming apparatus configured as described above.

  FIG. 7 is a flowchart showing tone correction processing in the adjustment mode according to the present embodiment.

  The CPU 1506 that controls the overall operation of the full-color image forming apparatus according to the present embodiment executes the adjustment mode when the user instructs the start of the adjustment mode. The adjustment mode can be performed at an arbitrary timing selected by the user, such as before, during, or after execution of an image forming job.

The control unit 1501 is instructed by the user to start the adjustment mode. (Step S700)
I. Measurement of deviation amount ΔDn of output density (steps S701 and S702)
The first pattern data 601 a is output from the pattern generation unit 413 to the image memory 410. As a result, converted data (image signal for dark and light toner) of the first pattern data 601a is obtained through the dark and light data generation unit 411 and the LUT 414, and the first patch pattern 601 with dark and light toner as shown in FIG. (Step S701).

  The first patch pattern 601 is a pattern of magenta M and cyan C using dark toner and light toner. Of the 256-tone input image signal, 17 points (17 tones) divided at equal intervals. The input signal X (X = 0, 16, 32, 48, 64,..., 255) is input to the pattern generation unit 413 and formed on the recording paper. The first patch pattern 601 formed on the recording paper is such that the recording paper once output to the density sensor 99 or the paper discharge tray 89 disposed downstream of the fixing roller 9 is placed on the platen glass of the reader unit 300. Reading is performed by the CCD sensor 34 of the reader unit 300 (step S702).

  FIG. 8 is a graph showing the output density of dark and light toner with respect to the input signal X for dark and light toner, and shows the output density characteristics obtained by reading the first patch pattern 601.

  A curve Pa in the figure shows an actually obtained output density characteristic, and a straight line Pb shows a target reference output density characteristic. Here, data obtained by performing a smoothing process after interpolating a portion having no input data with respect to the density data of the read 17-gradation first patch pattern 701 is shown, and reference output density characteristics at each input point are shown. The actual output density deviation with respect to Pb is represented by ΔDn (n = 0 to 16).

  Hereinafter, a method of correcting the output density deviation amount ΔDn by adjusting the γ tables for light toner and dark toner will be described.

II. Measurement of output density characteristics of dark toner and light toner (steps S703 to S706)
First, in order to measure the output density characteristic of the light toner, the pattern generation unit 413 outputs the second pattern data 602a for the light toner to the LUT 414. At this time, the second patch pattern 602 is formed on the recording paper without passing the second pattern data 602a through the grayscale data generation unit 411 and the LUT 414 (step S703).

  Here, the input signal Xp for forming the second patch pattern 602 of the light toner is the input signal X (X = 0, 16, 32, 48, 64,...) Used when obtaining the output characteristics of FIG. , 255), and 9 points of Xp = 0, 32, 64, 96, 128,..., 255 based on the output characteristics of the light toner image signal in FIG. Similar to the first patch pattern 701, the second patch pattern 602 formed on the recording paper is read by the density sensor 99 or the CCD sensor 34 after fixing (step S704).

  Next, in order to measure the output density characteristics of dark toner, the formation and reading of the third patch pattern 603 of dark toner are performed in the same manner as the formation and reading of the second patch pattern 602 of light toner (step S705) and reading. Is performed (step S706). Here, the input signal Xd for forming the third patch pattern 603 of dark toner is the input signal X (X = 128, 144, 160, 176,...) Used when obtaining the output density characteristics of FIG. Corresponding to 255), it is composed of 9 points of Xd = 0, 32, 64, 96,..., 255 based on the output characteristics of the dark toner image signal shown in FIG.

III. Correction of γ table for light toner (steps S707 and S708)
Next, a method for correcting the light toner γ table for correcting the output density in the range of the input signal X = 0 to 128 will be described with reference to FIGS. 9 and 10. FIG. FIG. 9 is a graph for explaining the density gradient processing of the light toner according to the present embodiment. FIG. 10 is a graph showing correction processing of the light toner output density with respect to the light toner input signal Xp.

  In FIG. 9, the curve Pc shows the output density characteristic of the light toner actually obtained by reading the second patch pattern 702, and the straight line Pd shows the reference output density characteristic. In the image forming apparatus according to the present embodiment, the maximum density of the light toner is adjusted to 0.9. This value assumes a case where the switching position is fixed so that the usage ratio of the light toner and the dark toner is switched at the half of the maximum density as a boundary.

  In the range of the input signal X = 0 to 128, as shown in FIG. 5, since the image is formed only with the light toner from the usage ratio of the light toner and the dark toner, the light toner density correction value ΔDpn is the output density deviation. It becomes equal to the quantity ΔDn (n = 0 to 8). In the present embodiment, in particular, the purpose is to suppress the density difference between the light toner and the halftone portion, which is a mixed portion of the light toner and the dark toner. In order to suppress the difference in the vicinity of the input signal X = 128, control is first performed so that the density difference of ΔDp8 = ΔD8 is set to zero.

  From the output density characteristics of the light toner shown in FIG. 9, the density curve shown in FIG. 9 is such that the density difference of ΔDp8 = ΔD8 becomes 0 with the input signal Xp = 0 (output density D = 0) as a base point. The inclination curve is swung up and down, and the density curve is moved until the density level of ΔDp8 becomes 0 (that is, the white circle portion “◯” shown in the curve Pc (dotted line portion) in FIG. 9 becomes the curve Pe (solid line portion)). To the black circle “●” shown).

  As the density curve is moved, the output density value of the input signal X = 0 to 128 except for 0 and 128 is changed. Therefore, in order to replace the previously obtained output density value ΔDn (n = 1 to 7) with the density value difference ΔDnew (n) after the change, and further to correct this ΔDnew (n), FIG. As shown in FIG. 10, the input signal correction value ΔXpn (n = 1 to 7) is obtained by multiplying by an inverse function.

  The correction value ΔXpn (n = 1 to 7) is intended to suppress the difference in density at the halftone boundary, and therefore a value other than 0 and 128 of the input signal X (0 <X <128). ) Is not intended to strictly correct the density. When the density correction is strictly performed for the input value X in the range of 0 <X <128, the previously obtained output density value ΔDn (n = 1 to 7) and the data after moving the density curve are used. The difference may be calculated to calculate the input signal correction value ΔXpn (n = 1 to 7) (step S707).

  FIG. 11 is a graph showing a light toner γ table before and after correction. In the figure, a table before correction is indicated by a dotted curve gpo, and a table after correction is indicated by a dotted curve gpn. The light input signal Xp = 0, 32, 64, 96, 128,..., 255 in the γ table gpo is corrected by correcting the obtained input signal correction value ΔXpn (n = 0 to 8). A subsequent γ table gpn is created by interpolating each subsequent point and performing a smoothing process (step S708).

  By replacing the γ table gpo with the newly created γ table gpn, the density variation at the portion where the input signal X = 128 is the boundary (the junction with the halftone density region) is corrected, and as a result, the image quality is improved. The

IV. Correction of gamma table for dark toner (steps S709 and S710)
Next, a method for correcting the dark toner γ table for correcting the output density in the range of the input signal X = 128 to 255 will be described with reference to FIG. FIG. 12 is a graph showing the dark toner output density with respect to the dark toner input signal Xd, and the dark toner output density obtained by reading the density of the third patch pattern 703 formed on the recording paper. The characteristics are shown.

The range of the input signal X = 128 to 255 is obtained earlier in order to correct the gradation in this region with the dark toner γ table in order to form an image with light toner and dark toner as shown in FIG. In addition, the density correction by the gamma table gpn for light toner must be considered. Since the output characteristics of the light toner image signal in FIG. 5 are symmetrical with respect to the input signal X = 128, the dark toner density correction value ΔDdm is
ΔDdm = ΔD (7 + m) −ΔD (9−m) (m = 1 to 9)
It can be calculated by Here, ΔDdm is the density correction value of the dark toner, ΔD (7 + m) is the density correction value ΔDn (n = 8 to 16) for the intermediate to high density area, and ΔD (9−m) Is a density correction value corrected by the γ table gpn of light toner.

  Using ΔDdm calculated as described above, an input signal correction value ΔXdm (m = 1 to 9) corresponding to each point is obtained from the output density characteristics of FIG. 12 (step S709).

  FIG. 13 is a graph showing a dark toner γ table before and after correction. In the figure, the table before correction is indicated by a dotted curve gdo, and the table after correction is indicated by a dotted curve gdn. The obtained dark toner input signal correction value ΔXdm (n = 1 to 8) is corrected at 8 points of Xp = 0, 32, 64,..., 255 of the γ table gdo in FIG. By performing the smoothing process by interpolating the points, a solid-line γ table gdn is created (the intermediate density of the input image signal X = 128 to 255 by rewriting the γ table gdo to the newly created γ table gdn). As a result of correcting the density in the high density region, the gradation is improved (step S710).

  As described above, in the present embodiment, the density of the patch pattern formed with the dark toner and the light toner is read, and the γ table for controlling the gradation of the light toner and the dark toner according to the gradation characteristics is obtained. to correct. At this time, the gradation characteristic of the light toner is corrected by changing the slope so that a predetermined output characteristic can be obtained. As a result, the density fluctuation at the joint between the dark toner and the light toner is suppressed, so that it is possible to prevent the occurrence of a pseudo contour in the halftone portion and to always output a stable high quality image. Become.

  Further, since the object is to suppress the density difference at the halftone boundary, density correction can be easily performed for values other than 0 and 128 (0 <X <128) of the input signal X.

<Modification>
In this embodiment, as the first patch pattern 601, the output density was measured using a 17-point (17 gradation) pattern obtained by dividing an input image signal of 256 gradations at equal intervals. The gradation correction of the present invention can be performed with higher accuracy by increasing the number of pattern formation points (gradation) or adjusting the pattern formation interval according to the output characteristics.

  Also, in an image forming apparatus that performs image formation by switching the resolution according to the image, the gradation characteristics greatly differ depending on the resolution by forming the pattern of each resolution and performing the gradation correction of the present invention. However, stable and good image output is possible.

  In the above-described embodiment, in order to suppress the density fluctuation at the joint portion between the dark toner and the light toner and prevent the occurrence of the pseudo contour in the intermediate density region, the level of the input signal = 128 that becomes the mixed portion of the light and dark toner. The density of the light toner is corrected by changing the density gradient of the light toner from the 0 level as a base point. In the case of the dark toner, the maximum density (1.8 in the embodiment) is used. There is also a method in which the density correction of the light toner is performed after the density correction of the dark toner is performed by oscillating the density gradient from the base point. In this case, the density gradient correction method for dark toner differs only in that the position serving as the base point is the density 0 level in the density gradient correction method for the light toner described above, but the density maximum value Dmax. The correction processing method is the same as in the above-described embodiment.

  FIG. 14 is a plan view showing a schematic configuration of a tandem type image forming apparatus.

  The tandem type image forming apparatus forms an image using the number of image carriers (photosensitive members) corresponding to the type of toner. In the tandem type image forming apparatus 101 of this example, six image carriers 1a are used. 1b, 1c, 1d, 1e, and 1f. Each of the image carriers 1a to 1f is associated with one developer 41, 42, 43, 44, 45, and 46, each loaded with a developer having a different spectral characteristic. Image forming portions Sa, Sb, Sc, Sd, Se, and Sf each including a combination of one image carrier and one developing device are arranged in series.

  With this method, even when a six-color image forming apparatus is considered as a base, the image output speed can be made the same, and productivity can be improved.

  As a result, gradation degradation caused by changes in the image output characteristics of dark toner and light toner can be corrected, and the occurrence of pseudo contours in the intermediate density region can be prevented. It becomes possible to output.

  In addition, in an image forming apparatus that performs image formation by switching the resolution according to the image, by performing gradation correction by forming a pattern of each resolution, even if the gradation characteristics vary greatly due to the difference in resolution, the image forming apparatus is stable. A good image can be output.

  The present invention is not limited to the apparatus of the above-described embodiment, and may be applied to a system composed of a plurality of devices or an apparatus composed of one device. A storage medium storing software program codes for realizing the functions of the above-described embodiments is supplied to a system or apparatus, and a computer (or CPU or MPU) of the system or apparatus reads and executes the program codes stored in the storage medium. Needless to say, it will be completed by doing.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention. Examples of the storage medium for supplying the program code include a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, and a nonvolatile memory. Can be used. In addition, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also the OS running on the computer based on the instruction of the program code performs the actual processing. It goes without saying that a case where the functions of the above-described embodiment are realized by performing part or all of the processing, is also included.

  Furthermore, after the program code read from the storage medium is written to the memory provided in the function expansion board inserted in the computer or the function expansion unit connected to the computer, the program code is expanded based on the instruction of the next program code. It goes without saying that the functions of the embodiment described above are realized by performing some or all of the actual processing by the CPU or the like provided on the expansion board or expansion unit.

1 is a schematic plan view illustrating a configuration of an electrophotographic image forming apparatus according to an embodiment of the present invention. It is the schematic which shows the detailed structure of the post-fixing sensor periphery which concerns on embodiment of this invention. It is a block diagram which shows the flow of the image signal of the image process part in the reader part which concerns on embodiment of this invention. It is a block diagram which shows the flow of the image signal in the printer control part which concerns on embodiment of this invention. It is a graph which shows the output characteristic of the light / dark data produced | generated by the light / dark data generation part which concerns on embodiment of this invention. It is the schematic which shows the patch pattern used for the gradation correction process which concerns on embodiment of this invention. It is a flowchart of the adjustment mode which concerns on embodiment of this invention which concerns on embodiment of this invention. 6 is a graph showing the output density of dark and light toner with respect to an input signal for dark and light toner according to an embodiment of the present invention. 6 is a graph for explaining density gradient processing of a light toner according to an embodiment of the present invention. 6 is a graph showing a light toner output density correction process for a light toner input signal according to an embodiment of the present invention. 6 is a graph showing a γ table for light toner before and after correction according to an embodiment of the present invention. 5 is a graph showing an output density of dark toner with respect to an input signal for dark toner according to an embodiment of the present invention. 6 is a graph showing a dark toner γ table before and after correction according to an embodiment of the present invention. 1 is a plan view illustrating a schematic configuration of a tandem type image forming apparatus. It is a block diagram which shows roughly the structure of the control circuit which concerns on embodiment of this invention. FIG. 3 is a diagram illustrating an operation panel of the image forming apparatus according to the embodiment of the present invention. It is a figure which shows the display screen in the standard state in the operation panel which concerns on embodiment of this invention. It is a figure which shows the display screen of the adjustment mode in the operation panel which concerns on embodiment of this invention. 6 is a graph showing the relationship between the image density, toner usage, and output density of input image signals for dark toner and light toner.

Claims (10)

In an image forming apparatus that forms an image using light toner and dark toner having substantially the same hue,
Pattern forming means for forming a pattern using dark toner and light toner;
Pattern reading means for reading the density of the pattern formed on the recording paper;
A gradation that corrects the gradation characteristics of the image data for light toner by changing the inclination based on the zero level based on the density characteristics of the pattern read by the pattern reading means and the usage ratio of the dark and light toners. An image forming apparatus comprising: a correction unit. In an image forming apparatus that forms an image using dark and light toners and dark toners having substantially the same hue,
Pattern forming means for forming a pattern using dark toner and light toner;
Pattern reading means for reading the density of the pattern formed on the recording paper;
A gradation for correcting image data gradation characteristics for dark toner by changing the inclination based on the maximum density level based on the density characteristics of the pattern read by the pattern reading means and the usage ratio of dark and light toners An image forming apparatus comprising: a correction unit. In an image forming apparatus that forms an image using dark and light toners and dark toners having substantially the same hue,
Pattern forming means for forming a pattern using dark toner and light toner;
Pattern reading means for reading the density of the pattern formed on the recording paper;
Based on the density characteristics of the pattern read by the pattern reading means and the usage ratios of the dark and light toners, the gradation characteristics of the image data for light toner are corrected by changing the slope from the zero level as a base point, An image forming apparatus comprising: a gradation correction unit that corrects gradation characteristics of image data for toner by changing an inclination with a maximum density level as a base point. The pattern forming means includes
Forming one or more of a first pattern using a mixture of dark toner and light toner, a second pattern using light toner, and a third pattern using dark toner The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. Dark and light data generating means for generating dark toner image data and light toner image data from input image data;
First and second conversion means for converting the light and dark toner image data output from the light and dark data generation means so as to obtain predetermined output characteristics, respectively;
The pattern forming means includes
When forming the first pattern, the pattern data is input to the density data generation means, and the pattern is formed by the pattern data for the light and dark toners output from the first and second conversion means, respectively. ,
2. The method according to claim 1, wherein when forming the second and third patterns, pattern data for light and dark toner is input and the patterns are formed without using the first and second conversion means. The image forming apparatus according to any one of 1 to 4. The gradation correction means includes
Based on the density characteristics of the pattern read by the pattern reading means, a predetermined output characteristic can be obtained for the gradation characteristics of the light and dark toner image data to be converted by the first and second conversion means. Correct by changing the tilt so that
The image forming apparatus according to claim 5, wherein the first and second conversion units change conversion characteristics according to correction characteristics of the gradation correction unit.   The image forming apparatus according to claim 1, wherein the dark and light toners are dark and light developers having the same spectral characteristics and different amounts of pigments contained therein. In an image forming method of forming an image using dark and light toners having substantially the same hue,
A pattern forming step of forming a pattern using dark toner and light toner;
A pattern reading step for reading the density of the pattern formed on the recording paper;
A gradation that corrects the gradation characteristics of the image data for light toner by changing the inclination based on the zero level based on the density characteristics of the pattern read in the pattern reading process and the usage ratio of the dark and light toners. And an image forming method. In an image forming method of forming an image using dark and light toners having substantially the same hue,
A pattern forming step of forming a pattern using dark toner and light toner;
A pattern reading step for reading the density of the pattern formed on the recording paper;
A gradation that corrects the image data gradation characteristics for dark toner by changing the slope based on the maximum density level based on the density characteristics of the pattern read in the pattern reading process and the usage ratio of dark and light toners. And an image forming method. In an image forming method of forming an image using dark and light toners having substantially the same hue,
A pattern forming step of forming a pattern using dark toner and light toner;
A pattern reading step for reading the density of the pattern formed on the recording paper;
Based on the density characteristics of the pattern read in the pattern reading process and the usage ratios of the dark and light toners, the gradation characteristics of the image data for light toner are corrected by changing the inclination from the zero level as a base point. And a gradation correction step of correcting the gradation characteristics of the image data for toner by changing the inclination with the maximum density level as a base point.

Images