Image forming apparatus

Abstract

To provide an image forming apparatus that acquires a target gradation at appropriate timing.SOLUTION: An image forming apparatus includes: an image processing unit 17 that converts image data on the basis of conversion conditions; an image forming unit 19 that forms an image on a sheet on the basis of the converted image data; a reading device 50 that reads the image formed on the sheet; a control unit 70 that causes the image forming unit 19 to form a user image and a first test image on a first sheet, causes the image forming unit 19 to form the user image and a second test image on a second sheet, causes the reading device 50 to obtain reading results of the first test image and the second test image, and generates conversion conditions on the basis of the reading result of the first test image and the reading result of the second test image; and a storage unit 12 that stores the reading result of the first test image corresponding to the type of a sheet. When predetermined conditions are satisfied by the type of a sheet on which the image forming unit 19 forms the user image, the control unit 70 forms the first test image on the sheet.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a multifunction machine, or a printer. [Background technology]

[0002] In recent years, the market for on-demand image forming devices has been expanding. For example, electrophotographic image forming devices are becoming more widespread in the offset printing market. In addition, there are inkjet image forming devices that have succeeded in cultivating a wide range of markets due to their large format, low initial cost, and ultra-high speed. However, market expansion is not easy, and the image quality (hereinafter referred to as "image quality") of the previous image forming devices that have dominated that market must be maintained.

[0003] Image quality includes gradation, graininess, in-plane uniformity, character quality, color reproducibility (including color stability), etc. Among these, color reproducibility is said to be the most important. Humans have memories of colors they expect based on experience (especially human skin, sky, metal, etc.), and feel discomfort when they see colors that are beyond the tolerance range of this memory. Such memorized colors are called "memory colors." The reproducibility of memory colors becomes important when outputting them as photographs, etc. In addition, office users who feel discomfort when they see differences in color between printed business documents and their monitors, and graphic arts users who handle computer graphics, etc., have high demands for color reproducibility, including stability, for on-demand image forming devices.

[0004] Electrophotographic image forming apparatuses generally correct the gradation characteristics so that the gradation characteristics of an image to be formed match target gradation characteristics (target gradation). A gradation correction table in which a corrected gradation value is set for each gradation value is used to correct the gradation characteristics. The gradation characteristics of an image vary due to changes in the installation environment of the image forming apparatus and changes over time. For this reason, the image forming apparatus periodically adjusts (calibrates) the gradation characteristics to optimize the gradation correction table. The calibration is performed by forming an image for gradation adjustment on paper. The image for gradation adjustment formed on the paper is read by a reading device such as a scanner. The gradation correction table is updated according to the difference between the gradation characteristics obtained from the read image for gradation adjustment and the target gradation.

[0005] For example, the gradation adjustment image is formed in a non-image area on the paper excluding the image area where the image corresponding to the print job is formed (Patent Document 1). This eliminates the need to form the gradation adjustment image on a separate sheet of paper from the image desired by the user, eliminates the need to interrupt the print job, and prevents the generation of paper waste. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2014-107648 A Summary of the Invention [Problem to be solved by the invention]

[0007] Due to the resistance value and surface properties (irregularities of the surface) of the paper, the image density on the paper may change even if the amount of toner used for printing is the same. If a target gradation (synonymous with target density) is stored for each type of paper in order to accommodate the change in image density for each type of paper, costs such as an increase in memory storage capacity will be incurred. Therefore, in order to improve color stability on a print job basis, there is a method of setting the gradation characteristics of the first sheet of paper (leading sheet) of a print job as the target gradation. The same type of paper is used in one print job. For this reason, the gradation characteristics of the leading sheet are used as the target gradation, and relative control is performed with respect to the image quality of the leading sheet, thereby making it possible to maintain image quality without increasing memory storage capacity.

[0008] The results of the reading by the reading device when determining the target gradation may vary depending on the repeatability of the image reading by the reading device. This causes the target gradation to vary even for the same type of paper. This may cause deviations in the results of monitoring and adjusting the gradation characteristics, making it impossible to continuously output images with appropriate gradations for a long period of time.

[0009] In view of the above problems, the present invention has as its main object to provide an image forming apparatus capable of acquiring a target gradation required for gradation correction at an appropriate timing and outputting images with appropriate colors for a long period of time. [Means for solving the problem]

[0010] The image forming apparatus of the present invention comprises a conversion means for converting image data based on conversion conditions, an image forming means for forming an image on paper based on the image data converted by the conversion means, a reading means for reading the image formed on the paper, a control means for forming a first test image together with a user image on a first sheet of paper by the image forming means, acquiring the reading result of the first test image by the reading means, forming a second test image together with the user image on a second sheet of paper by the image forming means, acquiring the reading result of the second test image by the reading means, and generating the conversion conditions based on the reading result of the first test image and the reading result of the second test image, and a memory means for storing the reading result of the first test image corresponding to a type of paper, wherein the control means forms the first test image on the paper when a predetermined condition related to the type of paper on which the user image is formed by the image forming means is satisfied. Effect of the Invention

[0011] According to the present invention, it is possible to obtain a target gradation required for gradation correction at an appropriate timing, and to output an image with appropriate colors for a long period of time. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing the configuration of an image forming system. [Diagram 2] FIG. [Diagram 3] FIG. [Figure 4] 4 is a flowchart showing an image forming process. [Diagram 5] An explanatory diagram of the effect. [Figure 6] FIG. [Figure 7] Illustrative diagram of physical properties. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The embodiments of the present invention will be described with reference to the drawings. The embodiments described below are subject to various technically preferable limitations for carrying out the present invention, but the scope of the invention is not limited to the following embodiments and illustrated examples.

[0014] (Image forming system) 1 is a configuration diagram of an image forming system including an image forming apparatus of this embodiment. The image forming system 1 includes an image forming apparatus 10, a reading device 50, and an operation unit 180. The image forming apparatus 10 forms (prints) an image on paper. The reading device 50 reads an image for gradation adjustment, which is a test image for gradation adjustment formed on the paper. The operation unit 180 is a user interface having an input device and an output device. The input device is various key buttons and a touch panel. The output device is a display and a speaker.

[0015] The image forming apparatus 10 includes four imaging units 181Y, 181M, 181C, and 181K, an intermediate transfer belt 182, a secondary transfer roller 183, a fixing device 184, a plurality of (here, two) paper feed cassettes 185, and a reversing mechanism 186. The imaging units 181Y, 181M, 181C, and 181K are arranged along the belt surface of the intermediate transfer belt 182. The intermediate transfer belt 182 is an endless transfer body that is wound around a plurality of rollers and rotates in a predetermined direction (clockwise direction in the present embodiment). The secondary transfer roller 183 and the fixing device 184 are arranged on a transport path of the paper transported from the paper feed cassette 185. The paper feed cassette 185 contains paper of a predetermined size. In this embodiment, two paper feed cassettes 185 are provided, and the paper contained in each paper feed cassette 185 may be the same type or different types.

[0016] The imaging units 181Y, 181M, 181C, and 181K have the same configuration and only differ in the colors of the images they form. Imaging unit 181Y forms a yellow (Y) image. Imaging unit 181M forms a magenta (M) image. Imaging unit 181C forms a cyan (C) image. Imaging unit 181K forms a black (K) image. Here, the configuration of imaging unit 181Y will be described, and descriptions of the configurations of the other imaging units 181M, 181C, and 181K will be omitted.

[0017] The imaging unit 181Y includes an exposure device 18a, a photoconductor 18b, a developing device 18c, a charger 18d, and a cleaner 18e. The photoconductor 18b is a drum-shaped image carrier having a photosensitive layer on its surface. The photoconductor 18b rotates counterclockwise around the drum axis in the drawing. The charger 18d applies a voltage to the photosensitive layer of the rotating photoconductor 18b to uniformly charge the surface of the photoconductor 18b. The exposure device 18a irradiates the charged surface of the photoconductor 18b with a laser beam corresponding to the gradation value of each pixel of the yellow image. An electrostatic latent image is formed on the surface of the photoconductor 18b by the irradiation of the laser beam. The exposure devices 18a of the imaging units of the other colors irradiate laser beams corresponding to the gradation value of each pixel of the image of the corresponding color. As a result, the photoconductors 18b of the imaging units of the other colors form electrostatic latent images of the corresponding colors.

[0018] The developing unit 18c develops the electrostatic latent image formed on the photoconductor 18b with a color material such as yellow toner. By developing the electrostatic latent image, a yellow image is formed on the photoconductor 18b. The developing units 18c of the image creating units of other colors develop the electrostatic latent images with color materials of the corresponding colors. As a result, images of the corresponding colors are formed on the photoconductors 18b of the image creating units of the other colors.

[0019] The images formed on the photoconductors 18b of the imaging units 181Y, 181M, 181C, and 181K are transferred to the intermediate transfer belt 182 so as to be superimposed in sequence. A full-color image is formed on the intermediate transfer belt 182 to which the images of each color have been transferred. Any color material remaining on the photoconductors 18b after transfer is removed by the cleaner 18e.

[0020] The paper is transported from the paper feed cassette 185 to the secondary transfer roller 183 in accordance with the timing at which the image formed on the intermediate transfer belt 182 is transported to the secondary transfer roller 183 by the rotation of the intermediate transfer belt 182. The secondary transfer roller 183 functions as a transfer section that transfers a full-color image from the intermediate transfer belt 182 to the paper. The paper onto which the image has been transferred is transported to the fixing device 184. The fixing device 184 fixes the image onto the paper by heating and pressing the paper onto which the image has been transferred. When an image is formed on one side of the paper, the image forming process is completed as described above. When an image is formed on both sides of the paper, the paper with the image formed on one side is transported from the fixing device 184 to the reversing mechanism 186 and reversed. The paper with the reversed front and back sides is transported again to the secondary transfer roller 183, and an image is formed on the back side in the same manner.

[0021] The image forming apparatus 10 performs the image forming process (printing process) as described above in accordance with a print job. The print job includes data representing the image to be formed and image forming conditions such as the type of paper to be used. The exposure device 18a irradiates the photoconductor 18b with a laser beam in accordance with the print job. The image forming apparatus 10 forms an image on paper and conveys the resulting printout to the reading device 50.

[0022] As will be described later, the image forming apparatus 10 judges whether or not to form a gradation adjustment image in addition to the image formed on the paper in accordance with the print job. When forming an additional gradation adjustment image, image data of the gradation adjustment image (hereinafter referred to as "gradation adjustment image data") is added to the image data specified by the print job. The image data is data including the gradation values ​​of each pixel of the image of each color. The printed matter on which the gradation adjustment image has been formed is read by the reading device 50. The gradation characteristics are monitored and adjusted based on the result of reading the gradation adjustment image.

[0023] The reading device 50 includes a first conveying section 51, a second conveying section 52, a first reading sensor 53, a second reading sensor 54, and a color measurement section 55. The first conveying section 51 includes a plurality of conveying roller pairs 511 that convey the printed material supplied from the image forming apparatus 10. The second conveying section 52 includes a plurality of conveying roller pairs 521 that convey the printed material acquired from the first conveying section 51. The first reading sensor 53 and the second reading sensor 54 are disposed at positions sandwiching a conveying path along which the printed material is conveyed. Therefore, the reading device 50 can read images on both sides of the printed material in a single conveying operation by the first reading sensor 53 and the second reading sensor 54.

[0024] The first reading sensor 53 is disposed in the first transport section 51 and reads an image formed on one side of a printed material passing through the first transport section 51. The first reading sensor 53 outputs reading signals (brightness values) of the colors R (red), G (green), and B (blue) as reading results. The first reading sensor 53 is, for example, an optical sensor. A line sensor such as a CMOS (Complementary Metal Oxide Semiconductor) line sensor or a CCD (Charge Coupled Device) line sensor is used as the first reading sensor 53. By using a line sensor, the first reading sensor 53 can read the entire printed material in a direction perpendicular to the transport direction of the printed material (paper). The second reading sensor 54 is disposed in the second transport section 52 and reads the image formed on the other side of the printed material passing through the second transport section 52. The second reading sensor 54 has a configuration similar to that of the first reading sensor 53, and outputs reading signals (brightness values) of each of the colors R (red), G (green), and B (blue) as reading results. The line sensor reads the image in a direction perpendicular to the transport direction (paper feed direction) of the printed matter as one line. Therefore, the direction perpendicular to the transport direction of the printed matter is the main scanning direction. The transport direction of the printed matter is the sub-scanning direction.

[0025] The colorimetric unit 55 is disposed downstream of the second reading sensor 54 in the transport direction of the printed material. The colorimetric unit 55 reads the image on the other side of the printed material passing through the second transport unit 52. The colorimetric unit 55 spectrally measures the color of the tone adjustment image formed on the printed material to obtain colorimetric data. The colorimetric data is expressed in a color system such as XYZ.

[0026] (controller) 2 is an explanatory diagram of a controller that controls the operation of the image forming system 1. The controller includes a control unit 70, a storage unit 12, a communication unit 15, an image generating unit 16, an image processing unit 17, a job analysis unit 20, an image forming unit 19, a gradation adjustment image adding unit 30, and a front / back adjustment image adding unit 40. The control unit 70 is also connected to the operation unit 180 and the reading device 50 described above. The operation unit 180 includes an input device 13 and an output device 14.

[0027] The control unit 70 controls the operation of each part constituting the image forming system 1. The control unit 70 includes a CPU (Central Processing Unit) 71, a RAM (Random Access Memory) 72, and a ROM (Read Only Memory) 73. The CPU 71 controls the operation of each part of the image forming system 1 by executing computer programs stored in the ROM 73 and the storage unit 12. The RAM 72 provides a work area when the CPU 71 executes processing, and temporarily stores various programs, data, etc.

[0028] The storage unit 12 stores computer programs executed by the control unit 70 (CPU 71), data used in processing, etc. The storage unit 12 stores values ​​used for monitoring and adjusting gradation characteristics and front / back adjustment, which will be described later. A large-capacity storage device such as a hard disk can be used as the storage unit 12.

[0029] The communication unit 15 is a communication interface that controls communication with an external device, such as a computer, provided outside the image forming system 1. For example, the communication unit 15 receives data described in a page description language (PDL) (hereinafter, referred to as "PDL data") from the external device via a network. The PDL data is included in, for example, a print job that instructs image formation.

[0030] The job analysis unit 20 analyzes the image formation conditions of the print job acquired via the communication unit 15. The job analysis unit 20 analyzes details of job setting information related to the entire print job, such as the document name and number of copies to be printed from the print job, the output destination paper output tray designation, the binder order for a job consisting of multiple binders, and the paper type.

[0031] The image generating unit 16 performs a rasterization process on the PDL data acquired via the communication unit 15, and generates image data in a bitmap format for each of the colors yellow, magenta, cyan, and black. The image data includes a gradation value for each pixel. The gradation value is a data value that represents the shading of the image. For example, when the data value is 8 bits, the gradation value can represent shading from 0 to 255 gradations.

[0032] The image processing unit 17 performs image processing such as gradation correction processing and halftone processing on the image data generated by the image generating unit 16. The image processing unit 17 can also convert the read signals of each color of R (red), G (green), and B (blue) that are the results of reading the image by the reading device 50, to generate image data of each color of yellow, magenta, cyan, and black.

[0033] The gradation correction process is a process of converting the gradation values ​​of each color included in the image data so that the gradation characteristics of the image formed on the paper become the target gradation characteristics. The gradation correction process uses a one-dimensional gradation correction table in which an output gradation value corresponding to an input gradation value is determined. The gradation correction table is a conversion condition for converting the input gradation value (input value) of the image data to the output gradation value (output value). To convert the input gradation value of the yellow color component included in the image data, a gradation correction table for the yellow color component is used. To convert the input gradation value of the magenta color component included in the image data, a gradation correction table for the magenta color component is used. To convert the input gradation value of the cyan color component included in the image data, a gradation correction table for the cyan color component is used. To convert the input gradation value of the black color component included in the image data, a gradation correction table for the black color component is used. The image processing unit 17 converts not only the image data of the user image (the image specified by the print job) but also the gradation-adjusted image data of the gradation adjustment image added to the user image based on the gradation correction table. Types of halftone processing include, for example, error diffusion processing and screen processing using an ordered dither method.

[0034] The image forming unit 19 controls the operations of the imaging units 181Y, 181M, 181C, 181K, the intermediate transfer belt 182, the secondary transfer roller 183, the fixing device 184, the paper feed cassette 185, and the reversing mechanism 186 under the control of the control unit 70. The image forming unit 19 performs an image forming process on the paper. The image forming unit 19 forms an image consisting of multiple colors on the paper based on the gradation value of each pixel of the image data image processed by the image processing unit 17. In this embodiment, the image forming unit 19 performs an image forming process using image data in which gradation adjustment image data and image data for front and back adjustment images (hereinafter referred to as "front and back adjustment image data") are added to the image data image processed by the image processing unit 17. The image is formed centered on the paper.

[0035] The gradation adjustment image adding unit 30 adds the gradation adjustment image data to the image data so that a gradation adjustment image is formed. The gradation adjustment image adding unit 30 also adds image data of a target gradation determination image (hereinafter referred to as "target gradation image data") to the image data so that a target gradation determination image described below is formed. The target gradation determination image is a test image for determining a target gradation characteristic. Here, the image processing unit 17 converts the target gradation image data based on a gradation correction table. The image forming unit 19 forms the target gradation determination image on the paper 80 based on the converted target gradation image data. The front and back adjustment image adding unit 40 adds the image data of the front and back adjustment image to the image data so that a front and back adjustment image is formed.

[0036] (Image for tone adjustment) FIG. 3 is an explanatory diagram of a gradation adjustment image formed on a sheet by the image forming unit 19. The target gradation determination image is an image formed based on the same input gradation value as the gradation adjustment image. FIG. 3 illustrates an image formed on one side of a sheet of paper 80. The sheet of paper 80 has an image area 81 in the center, and a non-image area 82 between the periphery of the image area 81 and the edge of the sheet of paper 80. In the image area 81, a user image based on image data processed by the image processing unit 17 is formed. In the non-image area 82, gradation adjustment images 851Y, 851M, 851C, and 851K for each color of yellow, magenta, cyan, and black, front and back adjustment images 841 and 842, and a cutting mark 83 are formed. When the colors are not distinguished, the gradation adjustment images 851Y, 851M, 851C, and 851K are described as "gradation adjustment images 851". The read data of the target gradation determination image read by the reading device 50 and the read data of the gradation adjustment image 851 read by the reading device 50 are used in an update process for updating the gradation correction table. The read data of the target gradation determination image read by the reading device 50 (target read data) is called a target gradation. The cutting marks 83 are provided in advance by the user. The cutting marks 83 are composed of two overlapping L-shaped marks, and are formed near the four corners of the image area 81. The area surrounded by the four cutting marks 83 (area surrounded by dashed lines) forms the cutting position of the paper 80. Note that the diagonal lines indicating the image area 81 are shown for the purpose of explanation, and are not actually formed on the paper 80.

[0037] The gradation adjustment image 851 may be formed anywhere on the periphery of the paper 80, but as shown in FIG. 3, it is preferable to form it on both ends of the paper 80 in a direction perpendicular to the paper feed direction of the paper 80. That is, the gradation adjustment image 851 of two colors, yellow, magenta, cyan, and black, is formed in one end area in a direction perpendicular to the paper feed direction, and the gradation adjustment image 851 of the remaining two colors is formed in the other end area in a direction perpendicular to the paper feed direction. In this embodiment, the gradation adjustment image 851C and the gradation adjustment image 851M are formed in one end area in a direction perpendicular to the paper feed direction, and the gradation adjustment image 851Y and the gradation adjustment image 851K are formed in the other end area in a direction perpendicular to the paper feed direction. Since the gradation adjustment image 851 is not formed at the leading end of the paper 80 in the paper feed direction, it is possible to suppress the occurrence of wrapping of the paper 80 during the fixing process.

[0038] The gradation adjustment images 851Y, 851M, 851C, and 851K are each composed of patch images of multiple gradations with gradation values ​​that are gradually changed. In FIG. 3, the gradation adjustment image 851 is composed of patch images of 10 gradations. For example, each patch image has a length of 8 [mm] in the main scanning direction and a length of 20 [mm] in the sub-scanning direction. The main scanning direction is a direction perpendicular to the paper feed direction, and the sub-scanning direction is the paper feed direction. The multiple patch images are arranged in a line in the paper feed direction of the paper 80.

[0039] When the gradation value is expressed in 255 gradations, the gradation values ​​of the patch images arranged in a row in gradation adjustment image 851 are set to any value between 0 and 255 so that the difference in gradation value between adjacent patch images is equal. The gradation values ​​of the patch images at both ends are set to 0 and 255, respectively. Note that the patch images constituting gradation adjustment image 851 are not limited to yellow, magenta, cyan, and black, and may be formed of R, G, B, process Bk, etc. In this embodiment, the size of the gradation adjustment image 851 is determined so that all four color gradation adjustment images 851Y, 851M, 851C, and 851K can fit on a single A3 size sheet of paper.

[0040] The image formed on the paper 80 is read by the reading device 50. The results of reading the gradation adjustment images 851Y, 851M, 851C, and 851K and the front and back adjustment images 841 and 842 by the reading device 50 are stored in the memory unit 12 or the RAM 72 of the control unit 70. The stored reading results are analyzed by the control unit 70.

[0041] The control unit 70 monitors and adjusts the gradation characteristics of the image formed by the image forming unit 19. The control unit 70 causes the reading device 50 to read the gradation adjustment image 851 formed on the paper 80, and generates a correction table for correcting the gradation correction table based on the reading result. The control unit 70 then generates a new gradation correction table by combining the gradation correction table stored in advance with the correction table. Note that the present invention is not limited to a configuration in which the correction table is generated and the gradation correction table is updated by combining the gradation correction table with the correction table. For example, the control unit 70 may be configured to directly generate a new gradation correction table from the read data of the gradation adjustment image 851.

[0042] The control unit 70 alternately monitors and adjusts the gradation characteristics for each sheet of paper 80. Note that the present invention is not limited to a configuration in which monitoring and adjustment of the gradation characteristics are alternately performed for each sheet. For example, the control unit 70 may be configured to update the gradation correction table based on the read data of n sheets of gradation adjustment images 851 every time the read data of n sheets is acquired.

[0043] (Image formation processing) The control unit 70 judges whether to acquire the target gradation characteristic (target gradation) each time the type of paper used for printing is changed. When the type of paper is changed, the target gradation is acquired from the first printed sheet after the change. In order to enhance color stability under the image forming conditions set in the print job, the image forming device 10 is configured to be independent of the paper characteristics by controlling the target gradation relative to the target gradation. In addition, the control unit 70 acquires the target gradation again after the time elapsed since the target gradation was acquired reaches a predetermined time. Alternatively, the control unit 70 acquires the target gradation again after the number of sheets of paper on which images have been formed by the image forming device 10 reaches a predetermined number after the target gradation was acquired. This is because the more time has passed since the target gradation was acquired, the lower the accuracy of the target gradation becomes. Even if the print job is changed, if there is no change in the type of paper used for printing and the elapsed time is less than the predetermined time (and the number of sheets on which images have been formed is less than the predetermined number), the control unit 70 does not acquire a new target gradation. The type of paper used for printing is the type of paper quality, and is set depending on the print job as plain paper, recycled paper, coated paper, etc. Paper types are distinguished by, for example, basis weight, thickness, Beck smoothness, volume resistivity, paper size, and whiteness.

[0044] Since there are various paper types on the market, the same number of paper types may be set for a print job. The paper types that can be used in the image forming system 1 can be registered in advance by the user using the input device 13. The registration information for the paper types is stored in the storage unit 12, for example.

[0045] The reading device 50, which has a first reading sensor 53 and a second reading sensor 54 that are line sensors, reads the image for gradation adjustment, for example, at 600 [dpi] with a range of 4 [mm] * 4 [mm] as one unit. In this case, the reading result of the patch image of one gradation becomes two data that are the reading result of two units because the width of 2 [mm] at both ends of the patch image in the sub-scanning direction is not read due to the influence of optical flare and toner edge effect. The average value of the two data is adopted as the reading signal (brightness value) of the patch image. In addition to the average value, the maximum value and the minimum value may be adopted as the reading signal (brightness value). The calculation of the reading signal is determined by the configuration of the reading device 50 and the characteristics of the image forming apparatus 10.

[0046] As described above, in this embodiment, the size of the gradation adjustment image 851 is determined so that all four color gradation adjustment images 851 can fit on one sheet of A3 size paper fed vertically. A normal gradation adjustment image 851 is printed on each sheet, and gradation correction is performed based on the difference between the gradation obtained from the read signal and the target gradation. If the target gradation registered last time differs from the target gradation registered this time, the effect of a reading error of the gradation adjustment image 851 will appear in the gradation of the formed image.

[0047] Fig. 4 is a flowchart showing the image forming process of this embodiment. This image forming process is performed for each print job. When a print job is transferred to the image forming device 10, the CPU 71 reads out and executes a program for the image forming process stored in the ROM 73. In this process, the gradation adjustment image 851 in Fig. 3 is used for updating the gradation correction table.

[0048] When a print job is transferred to the image forming apparatus 10, the CPU 71 first determines whether or not to perform gradation adjustment (S101). Whether or not to perform gradation adjustment is set in advance by the user using the input device 13. The setting contents are stored in the RAM 72, and the CPU 71 makes this determination by referring to the RAM 72.

[0049] When gradation adjustment is performed (S101: Yes), the CPU 71 determines whether or not it is necessary to generate a target gradation (S102). In the process of S102, the CPU 71 determines whether or not the type of paper on which the image is formed matches the type of paper on which the target gradation stored in the storage unit 12 is set. If the type of paper on which the image is formed matches the type of paper on which the target gradation is set, the CPU 71 determines whether or not the time that has elapsed since the target gradation corresponding to that type of paper was generated is less than a predetermined time. If the type of paper on which the image is formed matches the type of paper on which the target gradation stored in the storage unit 12 is set and the time that has elapsed since the target gradation was generated is less than the predetermined time, the CPU 71 determines that it is unnecessary to generate a target gradation. On the other hand, if the type of paper on which the image is formed does not match the type of paper on which the target gradation is set, the CPU 71 executes the target gradation determination process. Or, even if the type of paper on which the image is formed matches the type of paper on which the target gradation is set, if the time that has elapsed since the target gradation was generated is equal to or longer than a predetermined time, the CPU 71 executes the target gradation determination process.

[0050] When the target gradation determination process is started (S102: Yes), the CPU 71 first adds target gradation image data for determining the target gradation of the print job to the image data for the first sheet generated by the image generation unit 16 by the gradation adjustment image addition unit 30 (S103). The target gradation image data is the same as the gradation adjustment image data. The CPU 71 performs an image formation process on a sheet of paper based on the image data to which the target gradation image data has been added by the image formation unit 19 (S104). As a result, as illustrated in FIG. 3, a printed matter is generated on the sheet of paper 80 on which the gradation adjustment image 851 and the image specified by the print job are formed. The printed matter is conveyed to the reading device 50 and read. The CPU 71 acquires a luminance value from the reading result (read signal) of the gradation adjustment image 851 by the reading device 50. The CPU 71 determines the acquired luminance value as the target gradation and registers it in the RAM 72 (S105).

[0051] The read result of the gradation adjustment image 851 is nine data for each patch image, that is, for each gradation of one color. The CPU 71 determines the average value of seven data, excluding the upper and lower limits, from the nine data, as the luminance value of the patch image. This makes it possible to accurately determine the luminance value of the patch image, and the target gradation is determined with high accuracy.

[0052] When the target gradation determination process is completed, the CPU 71 starts the update process of the gradation correction table. When the update process of the gradation correction table is started, the CPU 71 first adds gradation adjustment image data to the image data generated by the image generation unit 16 by the gradation adjustment image adding unit 30 (S106). The CPU 71 performs an image formation process on a sheet based on the image data to which the gradation adjustment image data has been added by the image forming unit 19 (S107). As a result, as illustrated in FIG. 3, a printed matter is generated on the sheet 80, on which the gradation adjustment image 851 and the image specified by the print job are formed. The printed matter is conveyed to the reading device 50 and read. The CPU 71 acquires a luminance value from the reading result of the gradation adjustment image 851 by the reading device 50. The CPU 71 updates the gradation correction table based on the difference between the acquired luminance value and the target gradation (S108).

[0053] The CPU 71 determines whether all images included in the print job have been formed on paper (S109). If all images included in the print job have been formed on paper (S109: Yes), the CPU 71 ends the image forming process. If all images included in the print job have not been formed on paper (S109: No), the CPU 71 repeats the process from S102 onwards until all images included in the print job have been formed on paper.

[0054] When gradation adjustment is not performed (S101: N), the CPU 71 causes the image forming unit 19 to perform image formation processing on paper based on the image data generated by the image generating unit 16 (S110), and moves the process to step S109. When gradation adjustment is not performed, the gradation adjustment image 851 is not printed on paper.

[0055] As described above, when performing gradation adjustment by forming the gradation adjustment image 851 on paper, whether or not to perform the target gradation determination process is determined based on whether or not the paper type is the same as that of the previous print job. By not updating the target gradation when the paper type is the same and the elapsed time since the target gradation was generated is less than a predetermined time, deviations in the target gradation obtained from the reading result due to measurement errors caused by the reading device 50 and fluttering during paper transport can be suppressed. This makes it possible to suppress fluctuations in image quality when forming images by different print jobs on the same paper type.

[0056] 5 is an explanatory diagram of the effect of this embodiment. Here, the print job prints 500 sheets, and the conditions are registered for obtaining the target gradation from the first printed sheet. Two consecutive print jobs using the same paper type are performed. In FIG. 5, the same print job prints 500 sheets twice, so the total number of output sheets is 1000 sheets.

[0057] Conventionally, the target gradation is registered from the first printed sheet and the 501st printed sheet at the start of each print job. As a result, there is a difference of about 0.01 in the target gradation between the first print job and the next print job (image density of 0.86 and 0.87). In this way, conventionally, different target gradations are set for the same type of paper between print jobs.

[0058] In this embodiment, the target gradation is set based on the first printed page of the first print job, but the target gradation is not set from the first printed page of the second print job. This suppresses the effect of reading variations in the reading device 50, and prevents deviations in image density. This keeps the image quality of the prints constant. Even if there are three or more consecutive print jobs, if the paper type is the same, the target gradation is set based on the first printed page of the first print job, and the target gradation is not set for the subsequent print jobs.

[0059] It is preferable that the condition for re-registering the target gradation is other than the first sheet of paper in a print job. Re-registration of the target gradation is a relative control that is completed within a print job, and is performed when the installation environment (temperature, humidity) of the image forming apparatus 10 changes significantly or when the power is turned off. Therefore, re-registration of the target gradation is performed after a predetermined time has elapsed since the target gradation was generated, or after the number of images formed since the target gradation was generated reaches a predetermined number. In addition, re-registration of the target gradation is also performed when gradation correction is performed based on an absolute standard.

[0060] The image forming apparatus 10 of the present embodiment as described above determines whether or not to obtain a new target gradation depending on whether or not the type of paper used is the same. For this reason, the target gradation is set at an optimal timing. Once the target gradation is determined, the image forming apparatus 10 performs gradation correction based on the difference between the result of reading the gradation adjustment image and the target gradation, thereby making it possible to output images of appropriate colors for a long period of time.

[0061] (Modification) The target gradation may be acquired taking into consideration not only the paper type and the timing at which the paper type is specified, but also the paper settings. For example, if any of the three conditions of the paper type, the timing at which the paper type is specified, and the paper settings are the same in the preceding print job and the subsequent print job, the target gradation is not acquired again in the subsequent print job. These conditions can be set by the operation unit 180. Usability is improved by customizing the conditions for whether or not to acquire the target gradation so that they can be added or deleted by the operation unit 180.

[0062] The timing of paper type instruction is the timing at which the paper type to be used for printing is first specified within a predetermined period. The predetermined period is appropriately set by the operation unit 180 according to the user's usage status, such as 8 hours, 24 hours (1 day), 1 week, 1 hour, etc. If the type of paper for which the target gradation has been obtained within the predetermined period is used for printing again within the predetermined period, the target gradation is not obtained. In other words, if an image is formed on paper of the same paper type or paper settings as paper used in a print job within the predetermined period, the target gradation is not obtained.

[0063] The paper settings are more detailed than the paper type and the physical characteristics of the paper. For example, even plain paper has various surface properties. The surface properties of the paper affect the image density characteristics of the image to be formed. The paper settings are input from the operation unit 180. The user inputs the paper settings according to the paper settings screen displayed on the output device 14 of the operation unit 180. For example, the paper types registered in the image forming device 10 are displayed in a list on the paper settings screen, and the paper settings can be made by selecting from the list using the input device 13. The paper settings may also be made based on information for identifying the type of paper, such as the product code of the paper.

[0064] The image forming apparatus 10 can access an external database through the communication unit 15. The communication unit 15 communicates using an existing communication network such as a LAN (Local Area Network), the Internet, or a dedicated line. If the usable paper types are not registered in advance, the image forming apparatus 10 cannot display a list of registered paper types. In this case, the image forming apparatus 10 accesses the database and displays a list of paper types registered in the database on the output device 14. When the database server acquires the paper type selected by the input device 13 from the image forming apparatus 10, it transmits the physical characteristics of the paper type to the image forming apparatus 10. The physical characteristics include, for example, basis weight, thickness, Beck smoothness, volume resistivity, sizing degree, whiteness, etc.

[0065] FIG. 6 is an example diagram of paper classification information stored in the database. In the paper classification information, physical characteristics of categories 1 to 5 are set. FIG. 7 is an example diagram of physical characteristics searched for by the database server according to the paper type acquired from the image forming device 10. The data server, which has acquired "plain paper" as the paper type, extracts the categories of basis weight, thickness, Beck smoothness, and volume resistance from the paper classification information. The image forming device 10 acquires the physical characteristics of the extracted category from the database.

[0066] If the paper type is the same even between different print jobs, the control unit 70 determines that it is the same job and does not acquire the target gradation. The control unit 70 can acquire the target gradation at a more appropriate timing based on whether the paper type specified in the print job is a paper type used for the first time.

[0067] In the above process, in the case of a print job in which the tone adjustment image 851 is printed on the paper 80 and the tone correction table is updated, if the paper type is the same and the time elapsed since the timing when the paper type was specified is less than a predetermined time, a new target tone is not acquired. This makes it possible to acquire an appropriate target tone while suppressing the influence on the reading result of measurement errors caused by the reading device 50, unevenness due to unevenness of the paper surface, and fluttering of the paper due to paper transport. Once the target tone is determined, the image forming device 10 updates the tone correction table based on the difference between the reading result of the tone adjustment image 851 and the target tone, thereby making it possible to output images of appropriate colors for a long period of time.

[0068] The image forming apparatus 10 described above does not update the target gradation if the paper type does not differ significantly, determining that there is no change in the paper characteristics. Also, paper fed from the same paper feed cassette 185 is likely to be the same paper if the paper feed cassette 185 is not opened or closed. For this reason, if paper from the same paper feed cassette 185 is used for printing, even if the print job is different, the target gradation may not be updated.

[0069] Furthermore, the control unit 70 does not acquire the target gradation if the type of paper on which the image is formed matches the type of paper for which the target gradation stored in the storage unit 12 is set and if the time that has elapsed since the target gradation was generated is less than a predetermined time. However, the control unit 70 may be configured not to acquire the target gradation if the type of paper on which the image is formed matches the type of paper for which the target gradation stored in the storage unit 12 is set.

[0070] In the above description, the image forming apparatus 10 and the reading device 50 are separate devices, but these devices may be configured as one unit. For example, the first reading sensor 53 and the second reading sensor 54 may be disposed downstream of the fixing device 184 in the paper transport direction within the image forming apparatus 10.

[0071] The image forming system 1 may be configured such that a paper processing device that performs paper processing such as stapling, punching, folding, and bookbinding is provided downstream of the reading device 50 in the paper transport direction. The paper processing device may execute paper processing only when instructed to do so by the control unit 70. When there is no instruction from the control unit 70 and no paper processing is to be performed, the paper processing device simply ejects the transported paper.

[0072] In addition, although the first reading sensor 53 reads the image on the paper from below, and the second reading sensor 54 and color measurement unit 55 read the image on the paper from above, this may be reversed. Also, although the image formed based on the image data includes the cutting marks 83, the image may not include the cutting marks 83. In such a case, the image forming apparatus 10 may be configured to add cutting mark information to the image data so that the cutting marks 83 are formed at the cutting positions of the paper on which the image is formed based on the image data.

Claims

1. An image forming means for forming an image on a sheet of paper; a conveying means for conveying paper from the image forming means to discharge the paper on which the image is formed by the image forming means; a reading means for reading the test image formed by the image forming means while the conveying means is conveying the paper; a storage means for storing a target reading result of the test image; a control means for controlling a density of an image to be formed by the image forming means based on a reading result of the test image read by the reading means and the target reading result stored in the storage means, the control means sets a reading result of a test image formed on a first sheet of paper in a print job in which a plurality of images are formed on a plurality of sheets of paper as a target reading result of a test image to be formed on a second or subsequent sheet of paper of the print job which is of the same type as the first sheet of paper. Image forming device.

2. the storage means stores a target reading result for each type of paper on which the test image is formed in the print job. The image forming apparatus according to claim 1 .

3. When the reading result of a test image formed on a first sheet of paper in the print job is stored in the storage means as the target reading result, the control means sets the reading result of a test image formed on a sheet of paper of the same type as the first sheet of paper as the target reading result after a predetermined time has elapsed since the test image was formed. The image forming apparatus according to claim 1 .

4. When the reading result of the test image formed on the first sheet of paper in the print job is stored in the storage means as the target reading result, the control means sets the reading result of the test image formed on the same type of paper as the first sheet of paper as the target reading result after the number of image formation sheets since the test image was formed reaches a predetermined number. The image forming apparatus according to claim 1 .

5. The test image has a plurality of images with different gradations. The image forming apparatus according to claim 1 .

6. The image forming means has a conversion unit that converts image data based on a conversion condition, and forms the image based on the image data converted by the conversion unit, the control means generates the conversion condition based on the reading result of the test image and the target reading result, thereby controlling the density of the image to be formed by the image forming means. The image forming apparatus according to claim 1 .

Images