JP2009069232A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus that stably outputs a special color without generating gradation whitening and gradation black crush by density-correcting prescribed special color in the same control as a single color. <P>SOLUTION: When the special color is set by a user, special color decomposition patch images ms, cs, ys, and ks of magenta, cyan, yellow and black obtained to decompose the special color are added to density correction patterns M, C, Y, K. For instance, two patches m4, m5 whose densities are adjacent to the special color decomposition patch ms in the density correction pattern M are density-modified so as to halve a density difference between the special color decomposition patch ms whose density is adjacent to the patches m4, m5 and a patch m3, and a density difference between the special color decomposition patch ms and a patch m6, respectively. The density correction patterns C, Y, K are also modified. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using electrophotography, and more particularly to a density correction method for an output image in a color image forming apparatus.

  In an image forming apparatus using an electrophotographic process, the density is corrected by transferring the toner directly onto the toner carrier when the apparatus is activated or when a mode (calibration mode) for setting the image density appropriately is set. A pattern (reference image) is formed, and its density is detected to perform density correction. For example, in Patent Document 1, a first correction for correcting the density of a solid image and a second correction for correcting the density of a halftone image are sequentially executed, so that an image including a halftone can be stably displayed with high quality. An image forming apparatus that can output the output is disclosed.

  As a method for adjusting the image density, a method for adjusting the charging potential of the photosensitive member, a developing bias potential, an exposure amount by the exposure unit based on the detected image density, a method for directly changing the density data of the image signal, or both However, it is difficult to change each parameter value used for density correction such as the charge amount, exposure amount, development bias value, etc. of the photoconductor according to the density stage. In the case of performing precise density correction, a method of changing density data of an image signal is used.

  In the case of a color image forming apparatus, a magenta, cyan, yellow, and black image forming unit forms a reference image of each color on an image carrier, and the density is corrected by detecting the density of the reference image by a detecting unit. In addition, a so-called color matching technique is used in which a specific color is used to determine the mixing amount of the four colors of toner to match the spot color with the monitor screen or the original color. However, with unique color matching, it was difficult to accurately reproduce the logo color of the company and the corporate color that symbolizes organizations such as companies and organizations.

Therefore, a method for improving the accuracy of density correction for a desired color has been proposed. For example, Japanese Patent Application Laid-Open No. H10-228667 presents and outputs several types of color samples for a predetermined color (spot color) registered by the user. An image forming system is disclosed in which the specified spot color is separated into a single color to form a patch image so that the density correction of the spot color is performed simultaneously with the density correction of each normal color.
Japanese Patent No. 2972254 JP 2006-287708 A

  However, even when the image forming conditions such as γ correction are optimally adjusted using the method of Patent Document 1, the density printed as the density correction pattern is almost always different from the density of the single color constituting the predetermined color. There is no guarantee that the density of a given color is accurately adjusted. In the method of Patent Document 2, since the spot color and the single color are corrected using different LUTs (look-up tables), the spot color and its vicinity are affected by the secular change of the process unit such as the photosensitive drum and the developing device. There is a risk of image defects such as gradation skipping or gradation collapse in the color. In other words, when the density of magenta, cyan, yellow, and black to be output for a specific color is specified, the target color may change from the target color over time even if the target color is output at the beginning of setting. was there.

  In view of the above-described problems, the present invention corrects a predetermined color and a single color by the same control, so that a color image that can stably output a predetermined color without causing gradation skipping or gradation collapse is generated. An object is to provide a forming apparatus.

  To achieve the above object, the present invention provides an image carrier, an image forming unit capable of forming density correction patterns of a plurality of colors composed of single color patches having different densities on the image carrier, and the image forming unit. In the color image forming apparatus, comprising: a detecting unit that detects the density of the density correction pattern that has been detected; and a control unit that corrects the density of each color based on the detection result of the detecting unit. When the spot color is set, a spot color separation patch obtained by separating the spot color into a single color is added to the density correction pattern, and at least density differences among a plurality of single color patches including the spot color separation patch are substantially equalized. As described above, the density correction pattern is changed.

  According to the present invention, in the image forming apparatus having the above-described configuration, the control unit may include two single-color patches whose densities are adjacent to the spot color separation patch, and another density whose density is adjacent to the spot color separation patch and the two single-color patches. The density correction pattern is changed so as to divide the density difference from the single color patch into two equal parts.

  Further, according to the present invention, in the image forming apparatus having the above-described configuration, the control unit causes the density correction pattern to include a first area on a lower density side than the spot color separation patch and a first area on the higher density side than the spot color separation patch. In the first and second areas, the density correction pattern is changed so that the density difference between the monochrome patches whose densities are adjacent to each other is equalized.

  According to the first configuration of the present invention, the spot color specified by the user can be directly corrected simultaneously with the density correction of the single color, and the color tone of the spot color is set to a high level regardless of the secular change of the process unit used for image formation. Can be maintained. In addition, since the density difference between patches is substantially equalized in the vicinity of the spot color separation patch, it is possible to suppress deterioration in halftone density correction accuracy due to a reading error of the detection means.

  Further, according to the second configuration of the present invention, in the image forming apparatus having the first configuration, the spot color separation patch is centered only by changing the density of two single color patches whose density is adjacent to the spot color separation patch. The density difference between the five patches can be easily equalized on the low density side and the high density side of the spot color separation patch.

  According to the third configuration of the present invention, in the image forming apparatus having the first configuration, the first area on the lower density side than the spot color separation patch and the second area on the higher density side than the spot color separation patch. Since the density difference between the monochrome patches whose densities are adjacent in the region is equalized, the density difference between the patches is equalized over the entire density correction pattern, and halftone density correction can be performed more accurately. .

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a color image forming apparatus of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and magenta, cyan, and yellow are respectively subjected to charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto an intermediate transfer belt 8 that moves adjacent to each image forming unit while rotating clockwise in FIG. 1 by a driving means (not shown). The image is transferred (secondary transfer) onto the sheet S at the same time by the next transfer roller 9, and further fixed on the sheet S by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper S on which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing units 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing units 3a to 3d includes a developing roller (developer carrying member) disposed opposite to the photosensitive drums 1a to 1d, and each of magenta, cyan, yellow, and black toners is supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing units 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure unit 4. A toner image is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred onto the intermediate transfer belt 8 by the transfer rollers 6a to 6d. The primary transfer. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet S is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet S is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto S. The sheet S on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet S conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet S, and a predetermined full-color image is formed. It is formed. The sheet S on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the sheet S, it is discharged to the discharge tray 17 by the discharge roller 15 as it is.

  On the other hand, when forming images on both sides of the sheet S, a part of the sheet S that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the sheet S is distributed to the sheet conveyance path 18 by the branching portion 14 by rotating the discharge roller 15 in the reverse direction, and is conveyed again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the sheet S where the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  A density detection sensor 21 is disposed on the downstream side of the image forming unit Pd and the upstream side of the secondary transfer roller 9. The density detection sensor 21 irradiates the density correction pattern formed on the intermediate transfer belt 8 in the image forming units Pa to Pd with measurement light, and detects the amount of reflected light from each patch image constituting the density correction pattern. A detection result is transmitted to the control part 32 mentioned later as a light reception output signal. As the density detection sensor 21, an optical sensor including a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the density of the density correction pattern, when the measurement light is sequentially irradiated from the light emitting element to each patch image on the intermediate transfer belt 8, the measurement light is reflected as light reflected by the toner and light reflected by the belt surface. Incident on the light receiving element.

  When the toner adhesion amount is large, the reflected light from the belt surface is blocked by the toner, so that the light reception amount of the light receiving element is reduced. On the other hand, when the adhesion amount of toner is small, conversely, the amount of reflected light from the belt surface increases, resulting in an increase in the amount of light received by the light receiving element. Accordingly, the toner adhesion amount (image density) of each color patch image is detected from the output value of the received light signal based on the received reflected light amount, and the development bias characteristic value and the like are adjusted in comparison with a predetermined reference density. Thus, density correction is performed for each color.

  Since the density detection sensor 21 needs to strictly define the distance to the measurement object, as shown in FIG. 1, it opposes the driving roller 11 having a small distance variation to the surface of the intermediate transfer belt 8. The intermediate transfer belt 8 is positioned in the width direction according to the density correction pattern formation position on the intermediate transfer belt 8.

  The density detection sensor 21 may be disposed at another position where the density correction pattern on the intermediate transfer belt 8 can be detected. However, when the density detection sensor 21 is disposed downstream of the secondary transfer roller 9, for example, the image forming unit Pa. The time from when the density correction pattern is formed by .about.Pd to when the density detection is performed becomes longer, and the density correction pattern may come into contact with the secondary transfer roller 9 to change the surface state of the density correction pattern. . Therefore, as shown in FIG. 1, it is preferable to dispose on the downstream side of the image forming unit Pd and the upstream side of the contact position of the secondary transfer roller 9.

  FIG. 2 is a block diagram showing a control path of the color image forming apparatus of the present invention. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, an image input unit 30, an AD conversion unit 31, a control unit 32, a storage unit 33, an operation panel 34, a fixing unit 7, an intermediate transfer belt 8, a density detection sensor 21, and the like. It is the composition which includes.

  When the image forming apparatus 100 is a color copying machine, the image input unit 30 includes a scanning lamp mounted with a scanner lamp that illuminates the original during copying and a mirror that changes the optical path of reflected light from the original, and reflection from the original. 1 is an image reading unit that includes a condenser lens that collects light to form an image and a CCD that converts the formed image light into an electrical signal. In the case of a printer, the receiving unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 30 is converted into a digital signal by the AD conversion unit 31 and then sent to the image memory 40 in the storage unit 33.

  The storage unit 33 includes an image memory 40, a RAM 41, and a ROM 42. The image memory 40 stores an image signal input from the image input unit 30 and digitally converted by the AD conversion unit 31, and is stored in the control unit 32. Send it out. The RAM 41 and the ROM 42 store a processing program, processing content, and the like of the control unit 32.

  The RAM 41 stores a patch density-sensor output value lookup table (density-sensor output value LUT) and density data in which the density of each patch image in the density correction pattern and the output value of the density detection sensor 21 are stored in association with each other. Lookup table (data input value-sensor output value LUT) in which the input value of the sensor and the target value of the sensor output value corresponding thereto are stored in association with each other, and the γ table in which the density input value and actual density output value are stored in association with each other Is stored.

  The operation panel 34 includes an operation unit composed of a plurality of operation keys, and a display unit (none of which is shown) that displays setting conditions, the state of the apparatus, and the like, and the user sets printing conditions and the like. In addition, for example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 33 and reading or rewriting the registered transmission destination. It is also used when the user sets a desired color (hereinafter referred to as a spot color). When the image forming apparatus is connected online to an external input device such as a personal computer, the spot color may be set from the external input device. The set spot color is stored in the RAM 41 of the storage unit 33.

  The control unit 32 is, for example, a central processing unit (CPU), and according to a set program, the image input unit 30, the image forming units Pa to Pd, the fixing unit 7, and the sheet S from the sheet cassette 16 (see FIG. 1). In addition to overall control of conveyance and the like, the image signal input from the image input unit 30 is converted into image data by scaling processing or gradation processing as necessary. The exposure unit 4 irradiates a laser beam based on the processed image data, and forms latent images on the photosensitive drums 1a to 1d.

  Further, when the calibration mode is set by a key operation or the like on the operation panel 34, the control unit 32 receives an output signal from the density detection sensor 21, and uses the sensor output value of each patch image of the density correction pattern as a target value. It has a function of performing density correction for each color by adjusting the density data of the image signal stored in the image memory 40 according to the comparison function and the comparison result. The calibration mode may be automatically set when the apparatus is turned on or when a predetermined number of image forming processes are completed, in addition to manual setting by the user.

  Next, density correction in the color image forming apparatus of the present invention will be described. FIG. 3 shows an example of a density correction pattern used for density correction control of the present invention. In the present invention, when a predetermined color (hereinafter referred to as a spot color) is set by the user, the spot color is separated into single colors of magenta (M), cyan (C), yellow (Y), and black (K), The obtained four color patches are added to the normal density correction pattern.

  FIG. 3A is an example of a density correction pattern (tone pattern) when no spot color is set. For example, in the magenta density correction pattern M, patch images m1 to m9 having nine levels of density from the white solid image (m1) to the darkest image (m9) are formed in order from the advancing direction. Each is formed in a single color so that the density changes. The density correction patterns C, Y, and K for cyan, yellow, and black are similarly composed of nine levels of patch images.

  When the spot color is set by the user, as shown in FIG. 3B, the magenta, cyan, yellow and black spot color separation patch images ms, cs, ys and ks obtained by separating the spot colors are density correction patterns. Added to M, C, Y, and K, each density correction pattern M to K includes a patch image having 10 levels of density. FIG. 4 shows an enlarged view of the magenta density correction pattern M in FIG. Here, as shown in FIG. 4, the spot color separation patch image ms is interrupted between the patch images m4 and m5, and the patch images in the density correction pattern M are arranged in order of density. The same applies to the other density correction patterns C to K, but the spot color separation patch images ms to ks may be arranged at other locations (for example, immediately after each pattern).

  Next, density correction control of the first embodiment in the image forming apparatus of the present invention will be described. FIG. 5 is a graph showing the relationship between the density of each patch image forming the density correction pattern and the output value of the density detection sensor. FIG. 6 shows a method for changing the density correction pattern in the density correction according to the first embodiment. It is a graph to show. Here, the description will be given by taking the magenta density correction pattern M as an example, but the other density correction patterns C, Y, and K are also explained in exactly the same manner.

  Since the density of each patch image and the sensor output value are theoretically proportional, the target density-output value characteristic is a straight line L as shown by a broken line in FIG. However, the density detection sensor 21 detects the density of the 10-step patch image in which ms is added to m1 to m9 as shown in FIG. 4, and the horizontal axis indicates the density (%) and the vertical axis indicates the sensor output value. When the detection results are plotted, the curve S deviates from the straight line L. For example, if the density input value of the spot color separation patch image ms is D%, the sensor output value Q when the actually formed spot color separation patch image ms is detected is the target value Q ′ (the X coordinate on the straight line L is D (Y coordinate of a point that is%)).

  Therefore, when the density of the spot color separation patch image ms is D%, in order to set the sensor output value to the target value Q ′, the input value of the density of the spot color separation patch image ms is set to D ′% (Y on the curve S). X coordinate of the point whose coordinates are Q ′), and the density data of the image signal stored in the image memory 40 is directly set so that the density becomes D ′% (so that the sensor output value becomes Q ′). Change it.

  As described above, the input value of each density is set using the curve S and the straight line L, and the density data is adjusted so that the set density (or sensor output value) is obtained, so that the photosensitive drum, the developing device, etc. Regardless of the change in concentration due to secular change, a constant concentration can always be maintained. Further, by adding the special color separation patches ms to ks, the single color density used for the special color can be directly controlled, so that the density stability of the special color is also improved. The input value of the patch density may be set by using two types of density-output value LUT in which the patch density and the sensor output value are associated with each other based on the straight line L and the curve S, or may be derived from the curve S and the straight line L. You may calculate by calculation using the calculated numerical formula.

  By the way, depending on the density of the added spot color separation patch, the density may be close to the original patch. For example, in FIG. 5, the densities of the spot color separation patch ms and the patch m5 are close to each other. In this case, the sensor output values of the spot color separation patch ms and the patch m5 are reversed due to the reading error of the density detection sensor 21, and there is a possibility that gradation skip or gradation collapse may occur in the vicinity of the spot color.

  Therefore, as shown in FIG. 6, the density difference Δa between the patches m3 and m4 is equal to the density difference Δb between the patch m4 and the spot color separation patch ms, the density difference Δc between the spot color separation patch ms and the patch m5, and the patch m5. The densities of the patches m4 and m5 are changed so that the density difference Δd of m6 becomes equal. That is, two patches m4 and m5 whose densities are adjacent to the spot color separation patch ms are respectively equal in density difference between the spot color separation patch ms and patch m3 and the spot color separation patch ms and patch m6 whose density is adjacent to the patches m4 and m5, respectively. The density correction pattern is changed so that it can be divided.

  For example, the set densities of m1 to m9 are m1 = 0%, m2 = 12.5%, m3 = 25%, m4 = 37.5%, m5 = 50%, m6 = 62.5%, m7 = 75%, respectively. , M8 = 87.5%, m9 = 100%, and the set density of the special color separation patch ms is 46%. At this time, the density of the patch m4 whose density is adjacent to the spot color separation patch ms is (25 + 46) /2=35.5%, and the density of the patch m5 is (46 + 62.5) /2=54.25%. Thereby, the density difference between patch images is substantially equalized in the vicinity of the spot color separation patch ms, so that the density of the spot color can be stabilized without degrading the halftone density correction accuracy.

  FIG. 7 is a flowchart showing the density correction procedure of the first embodiment. A calibration execution procedure will be described with reference to FIGS.

  When the calibration mode is set by a user operation or after printing a predetermined number of sheets, the control unit 32 instructs the formation of a density correction pattern (step S1). Next, it is determined whether or not a spot color is set from the operation panel 34 or the personal computer (step S2). If the spot color is set by the user, the spot color is separated into magenta, cyan, yellow and black single colors (step S3). Then, the special color separation patches ms, cs, ys and ks obtained by the separation are added to the density correction patterns M, C, Y and K (step S4).

  Next, regarding the added single color patches (patches m4 and m5 in FIG. 5) whose densities are adjacent to the added special color separation patches ms, cs, ys, and ks, the adjacent single color patches (special color separation patches ms in FIG. 5). And the patch m3, and the density is changed to an intermediate density between the spot color separation patch ms and the patch m6) (step S5). The target value of the sensor output value for the single color patch after the density change is set using the density-output value LUT stored in the storage unit 33.

  Then, the changed density correction pattern (see FIG. 3B) is formed on the intermediate transfer belt 8 to execute density correction for each color (step S6). Specifically, the density detection sensor 21 detects the density of each patch constituting the density correction pattern, and transmits a detection signal to the control unit 32. The control unit 32 compares the sensor output value of each patch with the target value, creates a γ table for each color, and executes density correction. Thereafter, the density correction patterns M to K on the intermediate transfer belt 8 are removed by the cleaning blade 19, and the calibration is completed.

  On the other hand, if no special color is set in step S2, a normal density correction pattern (see FIG. 3A) without adding the special color separation patches ms to ks is formed on the intermediate transfer belt 8 to correct the density of each color. Is executed (step S7). Since a specific method of density correction is the same as that in step 6, description thereof is omitted.

  By performing the density correction according to the above procedure, the spot color specified by the user can be corrected directly at the same time as the single color density correction, and the color tone of the spot color is maintained at a high level regardless of the aging of the process unit used for image formation. can do. In addition, since the density difference between patches is substantially equalized in the vicinity of the spot color separation patch, it is possible to suppress deterioration in halftone density correction accuracy due to a reading error of the density detection sensor.

  Next, density correction control of the second embodiment in the image forming apparatus of the present invention will be described. FIG. 8 is a graph showing a density correction pattern changing method in the density correction of the second embodiment, and FIG. 9 is a flowchart showing the density correction procedure of the second embodiment. With reference to FIG. 1 to FIG. 5 and FIG. 8, the calibration execution procedure will be described according to the steps of FIG. 9. Here, the description will be given by taking the magenta density correction pattern M as an example, but the other density correction patterns C, Y, and K are also explained in exactly the same manner.

  In this embodiment, as shown in FIG. 9, when a spot color is set in step S2, the spot color is separated into a single color and a spot color separation patch is added to the density correction pattern of each color (steps S3 and S4). ) Is the same as the control in the first embodiment. Thereafter, as shown in FIG. 8, the density correction pattern is divided into a region (first region) R1 on the lower density side than the special color separation patch ms, and a region (second region) on the higher density side than the special color separation patch ms. The density correction pattern is changed so that the density difference between the patches in each of the regions R1 and R2 is equal (Δe and Δf) (step S5).

  For example, the set densities of m1 to m9 are m1 = 0%, m2 = 12.5%, m3 = 25%, m4 = 37.5%, m5 = 50%, m6 = 62.5%, m7 = 75%, respectively. , M8 = 87.5%, m9 = 100%, and the set density of the special color separation patch ms is 48%. At this time, since Δe = (48−0) / 4 = 12% in the first region R1, the concentrations of m2, m3, and m4 are changed to 12%, 24%, and 36%, respectively. On the other hand, since Δe = (100−48) /5=10.4% in the second region R2, m5 = 48 + 10.4 = 58.4%, m6 = 48 + 10.4 × 2 = 68.8% M7 = 48 + 10.4 × 3 = 79.2% and m8 = 48 + 10.4 × 4 = 89.6%.

  As a result, the density difference between patch images is substantially equalized not only in the vicinity of the spot color separation patch ms but also in the entire density correction pattern, so that halftone density correction can be performed more accurately than in the first embodiment. it can. In addition, since the control procedure after step S5 is the same as that of 1st Embodiment, description is abbreviate | omitted.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the density correction pattern shown in the above embodiment is merely an example, and other patterns can be used.

  In the above-described embodiment, the intermediate transfer type tandem color image forming apparatus that transfers the full color image formed by sequentially laminating the toner images of the respective colors on the intermediate transfer belt 8 onto the paper at the same time has been described. The present invention relates to a direct transfer tandem color image forming apparatus that sequentially transfers toner images of respective colors onto a sheet carried and conveyed on a conveying belt, and a plurality of developing devices around one photosensitive drum. The present invention can also be applied to a one-drum type color image forming apparatus in which the above is disposed. Further, if the density correction pattern formed on the photosensitive drum is detected without transferring the density correction pattern onto the intermediate transfer belt or the conveyance belt, high accuracy can be obtained regardless of the surface state of the belt. Concentration detection is possible.

  The present invention relates to an image carrier, an image forming unit capable of forming a plurality of color density correction patterns composed of single-color patches having different densities on the image carrier, and the density of the density correction pattern formed by the image forming unit. A spot color separation patch for separating a spot color into a single color when a predetermined spot color is set by a user, and a control means for correcting the density of each color based on the detection result of the detection means Is added to the density correction pattern, and at the same time, the density correction pattern is changed so that the density differences of at least a plurality of single color patches including the spot color separation patch are substantially equalized.

  This allows the spot color specified by the user to be corrected directly at the same time as the single color density correction, so that the color tone of the spot color can be maintained at a high level regardless of the aging of the process unit. A color image forming apparatus capable of being suppressed can be provided.

  In addition, since the two single-color patches whose densities are adjacent to the special color separation patch are changed so that the density difference between the special color separation patch and the other single-color patches whose density is adjacent to the two single-color patches is bisected, 2 By simply changing the patch density at a point, the density difference between the single color patches in the vicinity of the spot color separation patch can be easily and substantially equalized.

  Further, the density correction pattern is divided into a first area on the lower density side and a second area on the higher density side than the spot color separation patches, and between the single-color patches whose densities are adjacent in the first and second areas. Therefore, the density difference between patches is equalized over the entire density correction pattern, and the halftone density correction accuracy is further improved.

FIG. 1 is a schematic diagram showing an overall configuration of a color image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the color image forming apparatus of the present invention. These are figures which show an example of the density correction pattern when a spot color is not set (FIG. 3A) and when a spot color is set (FIG. 3B). FIG. 4 is an enlarged view of a magenta density correction pattern in FIG. These are graphs showing the relationship between the density of each patch image forming the density correction pattern and the output value of the density detection sensor. These are the graphs which show the change method of the density correction pattern in the density correction of the first embodiment. These are the flowcharts explaining the density correction procedure of the first embodiment. These are graphs showing a method of changing the density correction pattern in the density correction of the second embodiment. These are the flowcharts explaining the density correction procedure of the second embodiment.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
2a to 2d Charger 3a to 3d Developing unit 4 Exposure unit 6a to 6d Transfer roller 7 Fixing unit 8 Intermediate transfer belt (image carrier)
9 Secondary transfer roller 21 Concentration detection sensor (detection means)
32 Control unit (control means)
33 Storage Unit 34 Operation Panel 100 Image Forming Apparatus M to K Density Correction Pattern ms to ks Special Color Separation Patch R1 First Area R2 Second Area

Claims (3)

An image carrier;
An image forming unit capable of forming density correction patterns of a plurality of colors composed of single color patches having different densities on the image carrier;
Detecting means for detecting the density of the density correction pattern formed by the image forming unit;
A color image forming apparatus comprising: a control unit that performs density correction of each color based on a detection result of the detection unit;
When a predetermined spot color is set by a user, the control means adds a spot color separation patch obtained by separating the spot color into a single color in the density correction pattern, and includes a plurality of single color patches including at least the spot color separation patch. A color image forming apparatus, wherein the density correction pattern is changed so that density differences are substantially equalized.   The control means is configured so that two monochrome patches whose density is adjacent to the spot color separation patch bisects a density difference between the spot color separation patch and another monochrome patch whose density is adjacent to the two monochrome patches. The color image forming apparatus according to claim 1, wherein the density correction pattern is changed.   The control means divides the density correction pattern into a first area on the lower density side than the spot color separation patch and a second area on the higher density side than the spot color separation patch. 2. The color image forming apparatus according to claim 1, wherein the density correction pattern is changed so that the density differences between the single color patches adjacent to each other in the area of 2 are equal.