Image forming device

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 common in the offset printing market. In addition, inkjet image forming devices have been successful 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 (hereafter referred to as "image quality") of the previous image forming devices that dominated the market must be maintained.

[0003] Image quality includes gradation, graininess, in-plane uniformity, character quality, and color reproducibility (including color stability). Of these, color reproducibility is said to be the most important. Humans have a memory of expected colors (especially human skin, sky, metal, etc.) based on experience, and colors that exceed the tolerance range of this memory feel unnatural. These memorized colors are called "memory colors." The reproducibility of memory colors is important when outputting photographs, etc. In addition, office users who feel unnatural when there is a color difference between printed business documents and the color displayed on their monitor, and graphic arts users who work with computer graphics, among others, have high demands for color reproducibility, including stability, from on-demand image forming devices.

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

[0005] The gradation adjustment image is formed, for example, 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 paper waste. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-107648 Summary of the Invention [Problem to be solved by the invention]

[0007] The image density on paper can vary even when the amount of toner used for printing is the same due to factors such as the paper's resistance value and surface properties (surface irregularities). Maintaining a target gradation (synonymous with target density) for each type of paper to accommodate variations in image density for each type of paper would incur costs such as increased memory storage capacity. Therefore, to improve color stability for each print job, one method is to use the gradation characteristics of the first sheet of paper (leading sheet) in a print job as the target gradation. The same type of paper is used in a single print job. Therefore, by using the gradation characteristics of the leading sheet as the target gradation and performing relative control of the image quality of the leading sheet, image quality can be maintained without increasing memory storage capacity.

[0008] The reading results of 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 can lead to discrepancies in the results of monitoring and adjusting the gradation characteristics, making it impossible to continuously output images with appropriate gradations over 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 that can acquire a target gradation required for gradation correction at an appropriate timing and output 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: for an image forming means for forming an image on paper; a conveying means for conveying paper from the image forming means in order to discharge the paper on which the image is formed by the image forming means; and a conveying means for conveying the paper on which the test image is formed by the image forming means while the conveying means is conveying the paper. reading means for reading an image; a storage means for storing a target reading result of the test image; and a control means for controlling the density of the image to be formed by the image forming means based on the reading result of the test image read by the reading means and the target reading result stored in the storage means. and a control means for ,of Preparation ,before The control means In a print job in which a plurality of images are formed on a plurality of sheets of paper, the reading result of a test image formed on the first sheet of paper is regarded as the target reading result of a test image to be formed on the second or subsequent sheets of paper of the print job, which are of the same type as the first sheet of paper. It is characterized by: [Effects of the Invention]

[0011] According to the present invention, it is possible to acquire the 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 explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming system. [Figure 2] FIG. [Figure 3] FIG. 10 is an explanatory diagram of an image for gradation adjustment. [Figure 4] 10 is a flowchart showing an image forming process. [Figure 5] An explanatory diagram of the effect. [Figure 6] FIG. 10 is a diagram illustrating an example of paper classification information. [Figure 7] Illustrative diagram of physical properties. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The embodiments described below are subject to various limitations that are technically preferable 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) FIG. 1 is a configuration diagram of an image forming system including an image forming apparatus according to 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 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 fuser 184, multiple (here, two) paper feed cassettes 185, and an inverting 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 multiple rollers and rotates in a predetermined direction (clockwise in this embodiment). The secondary transfer roller 183 and the fuser 184 are arranged on the transport path of 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 developer 18c, a charger 18d, and a cleaner 18e. The photoconductor 18b is a drum-shaped image carrier with a photosensitive layer on its surface. The photoconductor 18b rotates counterclockwise around the drum axis. The charger 18d applies a voltage to the photosensitive layer of the rotating photoconductor 18b, uniformly charging 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 irradiating the laser beam. The exposure devices 18a of the imaging units for other colors irradiate laser beams corresponding to the gradation value of each pixel of the corresponding color image. As a result, electrostatic latent images of the corresponding colors are formed on the photoconductors 18b of the imaging units for other 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 one on top of the other. 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 unit that transfers the full-color image from the intermediate transfer belt 182 to the paper. The paper with the transferred image is transported to the fuser 184. The fuser 184 heats and presses the paper with the transferred image, thereby fixing the image to the paper. When an image is formed on one side of the paper, the image formation 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 fuser 184 to the reversing mechanism 186 and turned over. The paper with the turned over is transported again to the secondary transfer roller 183, and an image is formed on the back side in the same procedure.

[0021] The image forming apparatus 10 performs the image forming process (printing process) 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 photosensitive member 18b with a laser beam in accordance with the print job. The image forming apparatus 10 forms an image on paper and transports the resulting printout to the reading device 50.

[0022] As will be described later, the image forming apparatus 10 determines whether to form a gradation adjustment image in addition to the image formed on 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 in the print job. The image data is data including the gradation values ​​of each pixel of the image for 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 results of reading the gradation adjustment image.

[0023] The reading device 50 includes a first conveying unit 51, a second conveying unit 52, a first reading sensor 53, a second reading sensor 54, and a colorimetric unit 55. The first conveying unit 51 includes a plurality of conveying roller pairs 511 that convey printed materials supplied from the image forming apparatus 10. The second conveying unit 52 includes a plurality of conveying roller pairs 521 that convey printed materials obtained from the first conveying unit 51. The first reading sensor 53 and the second reading sensor 54 are disposed on opposite sides of a conveying path along which the printed materials are conveyed. This allows the reading device 50 to read images on both sides of the printed materials in a single conveying operation using the first reading sensor 53 and the second reading sensor 54.

[0024] The first reading sensor 53 is disposed in the first conveying section 51 and reads an image formed on one side of a printed material passing through the first conveying section 51. The first reading sensor 53 outputs reading signals (brightness values) for each 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 conveying direction of the printed material (paper). The second reading sensor 54 is disposed in the second conveying section 52 and reads the image formed on the other side of the printed material passing through the second conveying section 52. The second reading sensor 54 has the same configuration as the first reading sensor 53 and outputs reading signals (brightness values) for each of the colors R (red), G (green), and B (blue) as the reading results. The line sensor reads the image in a direction perpendicular to the transport direction (paper feed direction) of the printed material, counting each line as one line. Therefore, the direction perpendicular to the transport direction of the printed material is the main scanning direction, and the transport direction of the printed material 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 gradation 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 generation unit 16, an image processing unit 17, a job analysis unit 20, an image formation unit 19, a gradation adjustment image addition unit 30, and a front / back adjustment image addition 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 unit 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 unit 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 for 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 written 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 to be printed from the print job, the number of copies to be printed, the output tray designation, the binder order for a job consisting of multiple binders, and the paper type.

[0031] The image generation unit 16 performs a rasterization process on the PDL data acquired via the communication unit 15 to generate bitmap image data 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, if the data value is 8 bits, the gradation value can represent shading from 0 to 255.

[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 generation 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 image reading by the reading device 50 into color, and generate image data of each color of yellow, magenta, cyan, and black.

[0033] The gradation correction process converts the gradation values ​​of each color contained in the image data so that the gradation characteristics of the image formed on paper match the target gradation characteristics. The gradation correction process uses a one-dimensional gradation correction table in which output gradation values ​​corresponding to input gradation values ​​are defined. The gradation correction table defines conversion conditions for converting the input gradation values ​​(input values) of the image data to output gradation values ​​(output values). A gradation correction table for the yellow color component is used to convert the input gradation values ​​of the magenta color component contained in the image data. A gradation correction table for the cyan color component is used to convert the input gradation values ​​of the cyan color component contained in the image data. A gradation correction table for the black color component is used to convert the input gradation values ​​of the black color component contained in the image data. The image processing unit 17 converts not only the image data of the user image (the image specified in 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. Further, types of halftone processing include, for example, error diffusion processing and screen processing using ordered dithering.

[0034] Under the control of the control unit 70, 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 fuser 184, the paper feed cassette 185, and the reversing mechanism 186. The image forming unit 19 performs an image forming process on paper. The image forming unit 19 forms an image made up of multiple colors on paper based on the gradation values ​​of each pixel of the image data that has been image-processed by the image processing unit 17. In this embodiment, the image forming unit 19 performs the image forming process using image data that has been image-processed by the image processing unit 17 and that has been added with gradation-adjusted image data and image data for front and back adjustment images (hereinafter referred to as "front and back adjustment image data"). 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 later) is formed. The target gradation determination image is a test image for determining target gradation characteristics. 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 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 paper by the image forming unit 19. The target gradation determination image is an image formed based on the same input gradation values ​​as the gradation adjustment image. FIG. 3 illustrates an example of an image formed on one side of a paper sheet 80. The paper sheet 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 paper sheet 80. A user image based on image data processed by the image processing unit 17 is formed in the image area 81. Gradation adjustment images 851Y, 851M, 851C, and 851K for each color (yellow, magenta, cyan, and black), front and back adjustment images 841 and 842, and cutting marks 83 are formed in the non-image area 82. When the colors are not distinguished, the gradation adjustment images 851Y, 851M, 851C, and 851K will be referred to 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 to update the gradation correction table. The read data of the target gradation determination image read by the reading device 50 (target read data) is referred to as the 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 explanatory purposes 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 is preferably formed on both ends of the paper 80 in a direction perpendicular to the paper feed direction, as shown in FIG. 3 . That is, gradation adjustment images 851 of two colors, yellow, magenta, cyan, and black, are formed in one end region in a direction perpendicular to the paper feed direction, and gradation adjustment images 851 of the remaining two colors are formed in the other end region in a direction perpendicular to the paper feed direction. In this embodiment, gradation adjustment images 851C and 851M are formed in one end region in a direction perpendicular to the paper feed direction, and gradation adjustment images 851Y and 851K are formed in the other end region in a direction perpendicular to the paper feed direction. Because the gradation adjustment image 851 is not formed at the leading edge of the paper 80 in the paper feed direction, it is possible to prevent the paper 80 from wrapping around the fixing process.

[0038] The gradation adjustment images 851Y, 851M, 851C, and 851K are each composed of patch images with multiple gradations that have gradually varying gradation values. In FIG. 3, the gradation adjustment image 851 is composed of patch images with 10 gradations. Each patch image has, for example, 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 values ​​are expressed in 255 gradations, the gradation values ​​of the multiple 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 values ​​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 that make up 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 reading results of 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 a pre-stored gradation correction table with the correction table. Note that the present invention is not limited to a configuration in which a correction table is generated and then combined with the gradation correction table to update the gradation correction table. For example, the control unit 70 may be configured to generate a new gradation correction table directly 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 each time read data for n sheets is acquired.

[0043] (Image formation processing) The control unit 70 determines whether to acquire the target gradation characteristics (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. To enhance color stability under the image formation conditions set in the print job, the image forming apparatus 10 is configured to be independent of the paper characteristics by controlling the target gradation relative to the target gradation. The control unit 70 also acquires the target gradation again after a predetermined time has elapsed since the target gradation was acquired. 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 apparatus 10 since the target gradation was acquired reaches a predetermined number. This is because the reliability of the target gradation decreases as time passes since the target gradation was acquired. Even if the print job changes, the control unit 70 will not acquire a new target gradation if the type of paper used for printing remains unchanged 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 type of paper used for printing is the type of paper quality, and is set depending on the print job, such 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 a wide variety of paper types on the market, it may be possible to set as many paper types as there are types of paper in 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 registered paper type information is stored in the memory 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 gradation adjustment image, for example, at 600 dpi, with a range of 4 mm x 4 mm as one unit. In this case, the reading result of a patch image with one gradation is two data points, which are reading results of two units, because 2 mm widths at both ends of the patch image in the sub-scanning direction are not read due to the influence of optical flare and toner edge effect. The average value of the two data points is used as the read signal (brightness value) of the patch image. Note that, in addition to the average value, the maximum or minimum value may also be used as the read signal (brightness value). The calculation of the read 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] 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 apparatus 10, the CPU 71 reads and executes the image forming process program stored in the ROM 73. In this process, the gradation adjustment image 851 in FIG. 3 is used to update 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] If gradation adjustment is to be 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 for 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 for 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 for 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 for which the target gradation is set, the CPU 71 executes the target gradation determination process. Alternatively, even if the type of paper on which the image is formed matches the type of paper for which the target gradation is set, the CPU 71 executes the target gradation determination process if a predetermined time or more has elapsed since the target gradation was generated.

[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 using 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 then causes the image forming unit 19 to perform an image formation process on paper based on the image data to which the target gradation image data has been added (S104). As a result, as shown in FIG. 3, a printed matter is generated on paper 80, on which a gradation adjustment image 851 and an image specified in the print job are formed. The printed matter is transported 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 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 sets, excluding the upper and lower limits, as the luminance value of the patch image. This makes it possible to accurately determine the luminance value of the patch image, and to determine the target gradation with high precision.

[0052] When the target gradation determination process is completed, the CPU 71 starts updating the gradation correction table. When starting the updating process of the gradation correction table, the CPU 71 first causes the gradation adjustment image adding unit 30 to add gradation adjustment image data to the image data generated by the image generating unit 16 (S106). The CPU 71 then causes the image forming unit 19 to perform an image formation process on paper based on the image data to which the gradation adjustment image data has been added (S107). As a result, as shown in FIG. 3, a printed matter is generated on paper 80, on which a gradation adjustment image 851 and an image specified in the print job are formed. The printed matter is transported 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 the images included in the print job have not been formed on paper (S109: No), the CPU 71 repeatedly executes the processes from S102 onwards until all the images included in the print job have been formed on paper.

[0054] If 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 generation unit 16 (S110), and proceeds to step S109. If 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 gradation adjustment image 851 on paper, whether or not to perform target gradation determination processing is determined based on whether the paper type is the same as that of the previous print job. By not updating the target gradation if the paper type is the same and the time that has passed since the target gradation was generated is less than a predetermined time, deviations in the target gradation obtained from the reading results due to measurement errors caused by the reading device 50 or fluctuations during paper transport are suppressed. This makes it possible to suppress fluctuations in image quality when forming images using different print jobs on the same paper type.

[0056] FIG. 5 is an explanatory diagram of the effect of this embodiment. Here, a print job prints 500 sheets, and 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, resulting in a total of 1,000 sheets output.

[0057] Previously, the target gradation was registered from the first printed sheet and the 501st printed sheet at the start of each print job. As a result, there was a difference of approximately 0.01 in the target gradation between the first print job and the next print job (image densities of 0.86 and 0.87). In this way, previously, different target gradations were 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 onwards. This reduces the effects of variations in reading by the reading device 50, preventing deviations in image density. This maintains consistent image quality for printed pages. Even in the case of 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 target gradation is not set for subsequent print jobs.

[0059] It is preferable that the condition for re-registering the target gradation is one 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 this embodiment as described above determines whether to acquire a new target gradation depending on whether the type of paper used is the same. Therefore, the target gradation is set at the optimal timing. Once the target gradation is determined, the image forming apparatus 10 performs gradation correction based on the difference between the read result of the gradation adjustment image and the target gradation, thereby enabling the output of images with appropriate colors over a long period of time.

[0061] (Variation) 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 - paper type, timing at which the paper type is specified, and paper settings - are the same for the preceding print job and the subsequent print job, the target gradation will not be acquired again for the subsequent print job. These conditions can be set using 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 using the operation unit 180.

[0062] The timing for specifying the paper type is the timing when the paper type to be used for printing is specified for the first time within a predetermined period. The predetermined period is set appropriately by the operation unit 180 depending on the user's usage, 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 will not be 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 will not be obtained.

[0063] Paper settings are more detailed than paper type and refer to the type of paper and its physical characteristics. 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. Paper settings are input from the operation unit 180. The user inputs paper settings according to the paper setting screen displayed on the output device 14 of the operation unit 180. The paper setting screen displays, for example, a list of paper types registered in the image forming device 10, and paper settings can be made by selecting from the list using the input device 13. Paper settings may also be made using information for identifying the type of paper, such as the paper product code.

[0064] The image forming apparatus 10 can access an external database via 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 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 selected paper type to the image forming apparatus 10. The physical characteristics include, for example, basis weight, thickness, Beck smoothness, volume resistivity, sizing degree, and whiteness.

[0065] FIG. 6 is an example diagram of paper classification information stored in the database. The paper classification information has physical characteristics of categories 1 to 5 set therein. 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 the jobs are the same 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 that is being used for the first time.

[0067] In the above process, in the case of a print job that prints gradation adjustment image 851 on paper 80 and updates the gradation correction table, if the paper type is the same and the elapsed time from when the paper type was specified is less than a predetermined time, a new target gradation is not acquired. This makes it possible to acquire an appropriate target gradation while suppressing the effects on the reading results of measurement errors caused by reading device 50, unevenness due to unevenness on the paper surface, and flapping of the paper during paper transport. Once the target gradation is determined, image forming apparatus 10 updates the gradation correction table based on the difference between the reading result of gradation adjustment image 851 and the target gradation, making it possible to output images with appropriate colors over 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 paper characteristics. Furthermore, paper fed from the same paper feed cassette 185 is likely to be the same paper if the paper feed cassette 185 has not been opened or closed. Therefore, even if different print jobs are being performed, the target gradation may not be updated if paper from the same paper feed cassette 185 is used for printing.

[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 an integrated unit. For example, the first reading sensor 53 and the second reading sensor 54 may be disposed downstream of the fixing unit 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 binding is provided downstream of the reading device 50 in the paper transport direction. The paper processing device may perform paper processing only when instructed to do so by the control unit 70. If 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 colorimetric 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 cut marks 83, it may not include cut marks 83. In such a case, the image forming apparatus 10 may be configured to add cut mark information to the image data so that cut marks 83 are formed at the cut 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; a conveying means for conveying paper from the image forming means in order to discharge the paper on which the image is formed by the image forming means; a reading means for reading the test image formed on the paper by the image forming means while the conveying means is conveying the test image; a storage means for storing the target reading result of the test image; a control means for controlling the density of an image to be formed by the image forming means based on the reading result of the test image read by the reading means and the target reading result stored in the storage means, The control means is characterized in that the reading result of the test image formed on the first sheet in a print job in which a plurality of images are formed on a plurality of sheets of paper is set as the target reading result of the test image to be formed on the second or subsequent sheets of paper of the print job which are of the same type as the first sheet of paper. Image forming device.

2. the storage means stores the 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 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 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 an image of a plurality of 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 conversion conditions, and forms the image based on the image data converted by the conversion unit, the control means generates the conversion conditions 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