Image forming device

Abstract

To solve the problem in which: density of an image formed by an image forming apparatus is not stabilized.SOLUTION: An image forming apparatus has: line sensors 138 and 139; and a printer controller CPU 314 that converts a result of reading performed by the line sensor 138 concerning a gradation correction pattern 1104 on a recording medium 110 on the basis of a conversion table 140 to control density of an output image, or converts a result of reading performed by the line sensor 139 concerning the gradation correction pattern 1104 on the recording medium 110 on the basis of a conversion table 141 to control the density of the output image. The printer controller CPU 314 generates the conversion table 140 on the basis of, a result of measurement performed by a density sensor 117 concerning a gradation correction pattern 1061 on an intermediate transfer body 106 and the result of reading performed by the line sensor 138, and generates the conversion table 141 on the basis of the result of measurement performed by the density sensor 117 concerning the gradation correction pattern 1061 on the intermediate transfer body 106 and the result of reading performed by the line sensor 139.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to image density control for controlling the density of an image formed on a recording medium by an image forming apparatus. [Background technology]

[0002] An electrophotographic image forming apparatus forms an image by forming an electrostatic latent image on a photoreceptor, developing the electrostatic latent image, transferring the image to a recording medium, and fixing the image to the recording medium. The density of the image formed on the recording medium varies depending on environmental conditions such as temperature and humidity, and the state of the developer (toner) used for development. For this reason, the image forming apparatus forms a test image, and adjusts the density of the image formed by the image forming apparatus based on the result of reading this test image with a sensor. This is called "calibration." There are two types of calibration: calibration that uses the reading result of the test image formed on the recording medium, and calibration that uses the measurement result of the test image before it is fixed to the recording medium.

[0003] The image forming apparatus disclosed in Patent Document 1 performs calibration by forming a test image in a marginal area on a recording medium on which an image (user image) in accordance with an instruction from a user is formed. This allows calibration to be performed in real time. This image forming apparatus is superior to conventional calibration in that the formation of the user image is not interrupted by the execution of calibration. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2014-107648 A Summary of the Invention [Problem to be solved by the invention]

[0005] The image forming apparatus of Patent Document 1 can perform calibration using the results of reading a test image on a recording medium, as well as calibration using the results of measuring the test image before it is fixed on the recording medium. However, when performing calibrations using different methods, there is a possibility that the correction amounts determined in these calibrations will differ. This is due, for example, to differences in the sensors used for calibration and the fact that the target values ​​do not necessarily match for each calibration. This hinders the stability of the density of the image formed by the image forming apparatus.

[0006] SUMMARY OF THE PRESENT DISCLOSURE In view of the above problems, an object of the present invention is to stabilize the density of an image formed by an image forming apparatus. [Means for solving the problem]

[0007] The image forming apparatus of the present invention includes an image forming means for forming an image on a recording medium based on image formation conditions for controlling image density, a transport path along which the recording medium is transported, a first reading unit for reading a pattern image formed on the recording medium transported on the transport path from a first side relative to the transport path, and a second reading unit for reading the pattern image formed on the recording medium transported on the transport path from a second side opposite to the first side relative to the transport path; an intermediate transfer body onto which a measurement image formed by the image forming means is transferred; and a measuring means for measuring the measurement image transferred to the intermediate transfer body; a generating unit that causes the image forming unit to form a pattern image on the recording medium and generates the image forming conditions based on a reading result of the pattern image by the reading unit; another generating means for generating the first conversion condition and the second conversion condition; the generation means, when the first reading unit reads a pattern image formed on a surface of the recording medium facing the first reading unit, converts a first reading result of the pattern image by the first reading unit based on a first conversion condition and generates the image formation conditions based on the converted first reading result, and when the second reading unit reads a pattern image formed on a surface of the recording medium facing the second reading unit, converts a second reading result of the pattern image by the second reading unit based on a second conversion condition different from the first conversion condition and generates the image formation conditions based on the converted second reading result. The other generating means generates the first conversion condition based on the measurement result of the measurement image by the measuring means and the reading result of the measurement image on the recording medium read by the first reading unit, and generates the second conversion condition based on the measurement result of the measurement image by the measuring means and the reading result of the measurement image on the recording medium read by the second reading unit. The present invention is characterized by the above. Effect of the Invention

[0009] According to the present invention, it is possible to stabilize the density of an image formed by an image forming apparatus. [Brief description of the drawings]

[0010] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Diagram 2] Schematic cross-sectional view of a concentration sensor [Diagram 3] Control block diagram of an image forming apparatus [Figure 4] A four-level chart to explain tone correction [Diagram 5] Schematic diagram of a gradation correction pattern formed in a first gradation correction process; [Figure 6] Schematic diagram of a gradation correction pattern formed in the second gradation correction process; [Figure 7] Flowchart showing the process of generating a conversion table [Figure 8] FIG. 1 is a flowchart showing an image forming process including a gradation correction process. [Figure 9] Illustrative diagram of a conversion table [Figure 10] Schematic diagram of the output side setting screen [Figure 11] FIG. 1 is a flowchart showing an image forming process including a gradation correction process. [Figure 12] Selection screen diagram [Figure 13] FIG. 1 is a flowchart showing an image forming process including a gradation correction process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Example 1 (Configuration of Image Forming Apparatus) FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100. The image forming apparatus 100 includes an image forming engine unit 101 (also called a printer), a reader unit 400 for reading an original, a finisher 600, and an operation panel 180. The image forming engine unit 101 has image forming stations 120, 121, 122, and 123. The image forming station 120 forms an image of yellow (Y). The image forming station 121 forms an image of magenta (M). The image forming station 122 forms an image of cyan (C). The image forming station 120 forms an image of black (Bk). A printer controller 300 for controlling each unit of the image forming apparatus 100 is housed in a controller box 104 of the image forming apparatus 100.

[0012] The image forming station 120 includes a photosensitive drum 105, a charger 111, a laser scanner unit 107, and a developing unit 112. Since the other image forming stations 121, 122, and 123 have the same configuration as the image forming station 120, the configuration of the image forming station 120 will be described below.

[0013] The photosensitive drum 105 is a metal cylinder having a photosensitive layer on its surface. The photosensitive layer of the photosensitive drum 105 functions as a photoconductor. The charger 111 is a corona charger having a wire. A charging voltage is applied to the charger 111 to uniformly charge the photosensitive drum 105. Note that the charger 111 is not limited to a corona charger, and may be configured to have, for example, a charging roller that rotates while in contact with the photosensitive drum 105.

[0014] The laser scanner unit 107 has a semiconductor laser 108, a mirror 109, and a laser driver that controls the laser light emitted from the semiconductor laser 108. The laser driver drives the semiconductor laser 108 based on a laser output signal supplied from the printer controller 300. The semiconductor laser 108 of the laser scanner unit 107 exposes the photosensitive drum 105 to light so that an electrostatic latent image is formed on the photosensitive drum 105.

[0015] The developer 112 of the image forming station 120 contains yellow toner and uses the yellow toner to develop the electrostatic latent image on the photosensitive drum 105. The developers 112 of the other image forming stations 121, 122, and 123 contain toner of the corresponding color.

[0016] The intermediate transfer body 106 is a belt that rotates in a predetermined direction as the drive roller rotates. Around the intermediate transfer body 106, a sensor 115 that detects the image on the intermediate transfer body 106 and a sensor 116 that detects the recording medium 110 are provided so that the image on the intermediate transfer body 106 is transferred to a desired position on the recording medium 110. Also, around the intermediate transfer body 106, a density sensor 117 that detects a measurement image on the intermediate transfer body 106 is arranged.

[0017] In order to transfer the image transferred to the intermediate transfer body 106 onto the recording medium 110, a transfer nip is formed between the intermediate transfer belt 106 and a transfer roller 114. A transfer voltage is applied to the transfer roller 114 while the image on the intermediate transfer belt 106 and the recording medium 110 fed from a storage 113 pass through the transfer nip, so that the image on the intermediate transfer belt 106 is transferred to the recording medium 110. The recording medium 110 that has passed through the transfer nip is transported to a fixing device 105, which will be described later.

[0018] The fixing device 150 includes a fixing roller 151 having a heater, a pressure belt 152 for pressing the recording medium 110 against the fixing roller 151, and a sensor 153 for detecting that the recording medium 110 has been discharged from the fixing device 150. The recording medium 110 conveyed to the fixing device 150 is subjected to heat and pressure by the fixing roller 151 and the pressure belt 152, and the toner on the recording medium 110 is melted to fix the image on the recording medium 110.

[0019] The image forming engine unit 101 includes a conveying mechanism 130 that controls the conveying direction of the recording medium 110, downstream of the fixing unit 150 in the conveying direction in which the recording medium 110 is conveyed. The conveying mechanism 130 includes flappers 132, 133, and 134, a conveying path 135, an inverting unit 136, and a sensor 137. The flapper 132 is driven to switch whether the recording medium 110 is conveyed to the conveying path 135 or to the conveying path 201. Downstream of the conveying path 135 in the conveying direction is the inverting unit 136. The sensor 137 detects the end of the recording medium 110. The timing to stop conveying the recording medium 110 is controlled based on the detection result of the sensor 137.

[0020] A line sensor 138 that reads one side of the recording medium 110 of the image forming apparatus 100 and a line sensor 139 that reads the other side of the recording medium 110 are arranged on the conveying path 201. The recording medium 110 conveyed via the conveying path 201 is conveyed to the finisher 600 and discharged to an arbitrary tray of the finisher 600.

[0021] Next, the operation of the image forming engine unit 101 to form an image will be described. The image forming apparatus 100 forms an image on a recording medium 110 based on a job. When a job is executed, the photosensitive drum 105 rotates, and the charger 111 charges the photosensitive drum 105. The laser driver controls the semiconductor laser 108 based on a laser output signal generated from image data included in the job. In order to form an electrostatic latent image on the photosensitive drum 105, the laser scanner unit 107 exposes the surface of the photosensitive drum 105 charged by the charger 111. The developer 112 develops the electrostatic latent image on the photosensitive drum 105 with toner. As a result, the photosensitive drum 105 carries an image visualized using toner. The toner of each color on the photosensitive drum 105 is transferred to the intermediate transfer body 106 so as to be superimposed.

[0022] The recording media 110 in the storage 113 are fed one by one by rollers (not shown) and transported to the transfer nip. When the recording medium 110 and the image on the intermediate transfer body 106 pass through the transfer nip, a transfer voltage is applied to the transfer roller 114, so that the image on the intermediate transfer body 106 is transferred to the recording medium 110. The recording medium 110 to which the image has been transferred is transported to the fixing device 150. The fixing device 150 fixes the image on the recording medium 110 and transports the recording medium 110 to the transport mechanism 130. The recording medium 110 transported to the transport path 210 in the transport mechanism 130 is discharged to an arbitrary tray of the finisher 600.

[0023] Here, for example, when an image is formed on both sides of the recording medium 110, the recording medium 110 with the image formed on the first side is conveyed to the conveying path 135 by the flapper 132, and is conveyed to the inversion section 136 by the flapper 133. After the end of the recording medium 110 is detected by the sensor 137, the conveying of the recording medium 110 is stopped once at the position where the end of the recording medium 110 passes the sensor 137. Then, the conveying direction is controlled to the reverse direction by a roller (not shown). The flapper 133 conveys the recording medium 110, whose conveying direction is controlled to the reverse direction, to the double-sided conveying path. As a result, the recording medium 110 passes through the transfer nip in a state where the front and back are inverted. Then, the recording medium 110 with the image formed on the second side is conveyed to the conveying path 201 by the flapper 132.

[0024] Furthermore, the image forming apparatus 100 can control whether to discharge the recording medium 110 with the image formed side facing up or with the image formed side facing down based on the print setting (paper discharge side setting) when an image is formed on only one side. Face-up paper discharge is a paper discharge side setting in which the recording medium 110 is discharged with the image formed side facing up, and face-down paper discharge is a paper discharge side setting in which the recording medium 110 is discharged with the image formed side facing down.

[0025] Here, for example, when face-up discharge is performed, the recording medium 110 on which an image has been formed is transported to a transport path 201 by the flapper 132. As a result, the recording medium 110 discharged onto the tray of the finisher 600 is stacked with the image-formed surface facing upward. The transport path 201 is a common transport path through which both the recording medium 110 that has been inverted in the inverting unit 136 and the recording medium 110 that is not inverted pass.

[0026] On the other hand, when face-down discharge is performed, the recording medium 110 on which an image has been formed is conveyed to a conveying path 135 by a flapper 132, and after an end of the recording medium 110 is detected by a sensor 137, the conveying of the recording medium 110 is temporarily stopped. After that, the conveying direction is controlled to be reversed by a roller (not shown). Then, the recording medium 110 is conveyed to a conveying path 201 by a flapper 134. As a result, the recording medium 110 discharged onto the tray of the finisher 600 is stacked with the image-formed surface facing downward.

[0027] (Concentration sensor) FIG. 2 is a schematic cross-sectional view of the density sensor 117. The density sensor 117 is an optical sensor that measures a measurement image formed on the intermediate transfer body 106. The density sensor 117 includes a light-emitting diode (hereinafter, LED) 1171 and photodiodes (hereinafter, PD) 1172 and 1173. The LED 1171 irradiates the intermediate transfer body 106 with infrared light at an incident angle of 15°. The PD 1172 receives reflected light from the intermediate transfer body 106 and reflected light from the measurement image at a position with a reflection angle of 15° (specular reflection angle). The PD 1173 receives reflected light from the intermediate transfer body 106 and reflected light from the measurement image at a reflection angle of −35° (diffuse reflection angle). The density sensor 117 further includes a drive circuit that supplies a current to the LED 1171 in order to drive the LED 1171, and a conversion circuit that converts the current output according to the amount of light received by the PDs 1172 and 1173 into a voltage.

[0028] The density sensor 117 can measure both the specular reflected light and the diffuse reflected light. The PD 1172 that receives the specular reflected light is used to measure the black measurement image. The output value (voltage) of the PD 1172 corresponding to the reflected light from the black measurement image is converted into a density (or toner adhesion amount) value of the black measurement image. The PD 1173 that receives the diffuse reflected light is used to measure the color (yellow, cyan, magenta) measurement images. The output value (voltage) of the PD 1173 that corresponds to the reflected light from the yellow measurement image is converted into a density (or toner adhesion amount) value of the yellow measurement image. The magenta measurement image and the cyan measurement image are converted in the same way. A density conversion table for converting the output value (voltage) into a density (or toner adhesion amount) is prepared in advance for each color.

[0029] (Line sensor) Line sensors 138 and 139 that read the image formed on recording medium 110 are, for example, a CMOS sensor or a CCD. Line sensor 138 is disposed on the upper surface side of conveying path 201, and line sensor 139 is disposed on the lower surface side of conveying path 201, and each reads images formed on the upper surface and lower surface of recording medium 110 when recording medium 110 is conveyed. Line sensors 138 and 139 output read data (luminance signal) based on the read result.

[0030] A gradation correction pattern 1104 (FIG. 6) is formed on the recording medium 110 as a measurement image. When the measurement images of each color are read, the luminance signal of the measurement image is converted into a density value based on a luminance density conversion table corresponding to each color. The line sensors 138 and 139 are provided with color filters and can extract red, green, and blue light. The density value of the cyan measurement image is converted from an output value (luminance signal) corresponding to the luminance of the complementary color red based on the cyan luminance density conversion table. The density value of the magenta measurement image is converted from an output value (luminance signal) corresponding to the luminance of the complementary color green based on the magenta luminance density conversion table. The density value of the yellow measurement image is converted from an output value (luminance signal) corresponding to the luminance of the complementary color blue based on the yellow luminance density conversion table. The density value of the black measurement image is converted from an output value (luminance signal) corresponding to the luminance of the complementary color blue based on the yellow luminance density conversion table. The density value of the black measurement image is converted from an output value (luminance signal) corresponding to the luminance of the green based on the black luminance density conversion table.

[0031] (Configuration of image processing unit) 3 is a control block diagram of the image forming apparatus 100. The host computer 301 and the image forming apparatus 100 are connected by a communication line such as USB 2.0 High-Speed ​​or 1000Base-T / 100Base-TX / 10Base-T (compliant with IEEE 802.3).

[0032] A printer controller CPU 314 of the printer controller 300 controls the operation of the image forming apparatus 100. The printer controller 300 includes a host I / F unit 302 which is an interface used for communicating with a host computer 301, an input / output buffer 303 used for sending and receiving data, and a RAM 310 which functions as a system work memory. In addition, a program ROM 304 stores a control program and control data for the printer controller CPU 314.

[0033] Printer controller CPU 314 executes a program stored in program ROM 304 by loading it into RAM 310. As a result, printer controller CPU 314 functions as an image information generation unit 305, a main scanning unevenness correction table generation unit 306, an automatic gradation correction generation unit 307, a multidimensional table generation unit (ICC profile) 308, and an image defect detection unit 309.

[0034] The printer controller CPU 314 functioning as an image information generating unit 305 generates an image object based on data received from the host computer 301. The printer controller CPU 314 functioning as a main scanning unevenness correction table generating unit 306 corrects uneven density of an image formed by the image forming engine unit 101. The printer controller CPU 314 functioning as an automatic gradation correction generating unit 307 generates a γLUT for correcting the gradation characteristics of images of each color of yellow, cyan, magenta, and black. The printer controller CPU 314 functioning as a multi-color table generating unit 308 generates a color profile for controlling the color of an image of mixed colors. The printer controller CPU 314 functioning as an image defect detecting unit 309 detects image defects in an output image read by the line sensor 138.

[0035] The image data transferred to the image forming apparatus 100 is subjected to image processing in a RIP unit 315, a color processing unit 316, a gradation correction unit 317, and a pseudo-halftone processing unit 318. The RIP (Raster Image Processor) unit 315 develops an image object into a bitmap image. The color processing unit 316 performs color conversion processing of the image data based on a color profile. The gradation correction unit 317 converts the image data based on a γLUT to correct the gradation characteristics of the image to ideal gradation characteristics. The pseudo-halftone processing unit 318 performs pseudo-halftone processing on the image data. The engine I / F unit 319 transfers the image data processed by the pseudo-halftone processing unit 318 to the image forming engine unit 101. The RIP unit 315, the color processing unit 316, the gradation correction unit 317, and the pseudo-halftone processing unit 318 are realized by an ASIC.

[0036] Measurements using the line sensor 138 and density sensor 117 are controlled by an engine control CPU 102 in the image forming engine unit 101 .

[0037] The table storage unit 311 stores an ICC profile as a color profile and a γLUT. The operation panel 180 has a display and functions as an interface that accepts user instruction information. The panel I / F unit 312 is electrically connected to the operation panel 180 having a display and functions as an interface that accepts user instruction information. The reader unit 400 reads an original document using an image sensor (not shown) and transfers the read image to the printer controller 300. The reader I / F unit 313 functions as an interface that accepts the read image transferred from the reader unit 400. The printer controller 300 transmits and receives information between each unit via the system bus 320.

[0038] (Explanation of tone correction) Image processing (called gradation correction) for correcting the gradation characteristics of an image formed by the image forming apparatus 100 to ideal gradation characteristics will be described with reference to the quadrant chart shown in FIG. 4. FIG. 4 is a quadrant chart showing how gradation is reproduced. Quadrant I shows the reading characteristics of a sensor that converts the density of an original image into a density signal. Quadrant II shows the conversion characteristics (data characteristics of the γLUT) for converting the density signal into a laser output signal. Quadrant III shows the gradation characteristics (printer characteristics) of an image formed by the image forming engine unit 101. Quadrant IV shows the relationship between the density of an original image and the density of an output image. Note that in FIG. 4, the density signal and the laser output signal are processed as 8-bit digital signals. Therefore, the number of gradations in FIG. 4 is 256.

[0039] The gradation correction unit 317 (FIG. 3) converts the laser output signal to adjust the gradation characteristics (printer characteristics) of the image formed by the image forming engine unit 101 to ideal gradation characteristics (quadrant IV). The gradation correction unit 317 (FIG. 3) uses a γLUT (quadrant II) to convert a density signal (input value) into a laser output signal (output value). The image data (laser output signal) converted based on the γLUT is sent to a laser driver that controls the driving of the laser light source 108. Then, an electrostatic latent image is formed on the photosensitive drum 105 by scanning the photosensitive drum 105 with the laser light output from the laser light source 108. This electrostatic latent image is developed to form an image with ideal density.

[0040] The image forming apparatus 100 is capable of carrying out a first gradation correction process using the measurement results of the measurement image on the intermediate transfer body 106 and a second gradation correction process using the reading results of the measurement image on the recording medium 110 .

[0041] (First tone correction process) The first gradation correction process is realized by cooperation between the engine control CPU 102, which controls the density sensor 117, and the printer controller CPU 314 of the printer controller 300. When the first gradation correction process is executed, the engine control CPU 102 causes the image forming engine unit 101 to form a gradation correction pattern 1061, and causes the density sensor 117 to measure the gradation correction pattern 1061. Then, the printer controller CPU 314, which functions as the automatic gradation correction generation unit 307, generates a γLUT based on the measurement result of the gradation correction pattern 1061 by the density sensor 117.

[0042] The gradation correction unit 317 converts the image data based on the γLUT generated by the printer controller CPU 314. This makes it possible to control the gradation characteristics of the image formed by the image forming engine unit 101 to ideal gradation characteristics. The first gradation correction process is performed every time the image forming engine unit 101 forms a predetermined number of pages of images. Here, the predetermined number of pages is, for example, 100 sheets.

[0043] 5 is a schematic diagram showing a gradation correction pattern 1061 formed on the intermediate transfer body 106. The gradation correction pattern 1061 is formed so that the gradation correction pattern 1061 passes through a position where the LED 1171 of the density sensor 117 irradiates light. The gradation correction pattern 1061 has a plurality of gradation patches with gradually different gradation values ​​of C, M, Y, and K, respectively.

[0044] Each of the multiple gradation patches is a square with sides of 10 mm. The multiple gradation patches are arranged in a line parallel to the moving direction (rotation direction) of the intermediate transfer body 106. The multiple gradation patches on the intermediate transfer body 106 are multiple measurement images of different gradations. The density signals for forming the multiple gradation patches are, for example, 16, 32, 64, 86, 104, 128, 176, 224, and 255. Note that, although the gradation correction patterns 1061 are arranged in a line in FIG. 5, they may be arranged in multiple rows when multiple density sensors 117 are installed in the main scanning direction.

[0045] The gradation correction pattern 1061 on the intermediate transfer body 106 is formed at a timing when no image to be transferred to the recording medium 110 is formed. Note that, other than the timing when no image to be transferred to the recording medium 110 is formed, the gradation correction pattern 1061 may be formed after all images based on a job are formed.

[0046] Next, a method of updating the γLUT on the recording medium 110 from the density value of the gradation correction pattern 1061 on the intermediate transfer body 106 will be described. The target density of the gradation correction pattern 1061 on the intermediate transfer body 106 is stored in advance in the program ROM 304. The printer controller CPU 314 functioning as the automatic gradation correction generation unit 307 generates a γLUT so that the density value of the gradation correction pattern 1061 acquired using the density sensor 117 becomes the above-mentioned target density.

[0047] (2nd tone correction process) FIG. 6 is a schematic diagram showing a gradation correction pattern 1104 formed on a recording medium 110. Here, the gradation correction pattern 1104 is formed together with a user image. A marginal area in which the gradation correction pattern 1104 is formed is called a non-image area 1102. Here, the marginal portion (non-image area 1102) on the recording medium 1101 in which the user image is not formed is cut. In other words, the gradation correction pattern 1104 formed in the marginal portion (non-image area 1102) does not remain as a final product. The cutting mark 1103 is a mark indicating the position where the cutting process is performed. The cutting mark 1103 is composed of two overlapping L-shaped marks and is formed near the four corners of the image portion 1101, and the outside of the area surrounded by the four cutting marks 1103 indicates the cutting area. An operator cuts the marginal portion according to the cutting marks 1103 using, for example, a cutting device.

[0048] Note that the dots indicating the image area 1101 in Fig. 6 are shown for the purpose of explanation, and are not actually formed on the recording medium 110. Also, the recording medium 110 shown in Fig. 6 is described taking as an example a case where the recording medium 110 is transported along the longitudinal direction of the recording medium 110.

[0049] The gradation correction patterns 1104 are formed for each of the colors C, M, Y, and K on the first surface of the recording medium 110. The first surface of the recording medium 110 is the surface (image forming surface) that comes into contact with the intermediate transfer body 106 when the recording medium 110 first passes through the transfer nip.

[0050] 6, the gradation correction pattern 1104 is preferably formed in an end region in a direction perpendicular to the transport direction in which the recording medium 110 is transported. For example, if the gradation correction pattern 1104 is formed at the end of the transport direction (i.e., the leading end side and trailing end side of the recording medium 110 in the transport direction), the toner may become an adhesive and cause the recording medium 110 to wrap around the fixing roller 151 of the fixing device 150. If the gradation correction pattern 1104 is formed in the end region of the recording medium 110 in the direction perpendicular to the transport direction, the recording medium 1104 can be prevented from wrapping around the fixing roller 151.

[0051] The gradation correction pattern 1104 has a plurality of gradation patches with gradually different gradation values ​​for C, M, Y, and K. Each of the plurality of gradation patches has a square shape with one side measuring 8 mm. The plurality of gradation patches are arranged in a line parallel to the transport direction of the recording medium 110. Yellow and black gradation patches are arranged in one end region, and magenta and cyan gradation patches are arranged in the other end region.

[0052] The multiple gradation patches on the recording medium 110 are multiple measurement images of different gradations. The density signals for forming the multiple gradation patches are, for example, 16, 32, 64, 86, 104, 128, 176, 224, and 255. The gradation patches of the gradation correction pattern 1104 are not limited to the colors C, M, Y, and K, but may be mixed-color images created from multiple colors of cyan, magenta, and yellow. Alternatively, the gradation patches of the gradation correction pattern 1104 may be gray images (called process gray or process black) made by mixing yellow, cyan, and magenta.

[0053] Furthermore, the gradation pattern 1104 on the recording medium 110 is read by the line sensor 138 or the line sensor 139. When face-up paper ejection is set, the gradation pattern 1104 on the recording medium 110 is read by the line sensor 138. On the other hand, when face-down paper ejection is set, the gradation pattern 1104 on the recording medium 110 is read by the line sensor 139. Also in a double-sided mode in which images are formed on both sides, the gradation pattern 1104 on the recording medium 110 is read by the line sensor 139.

[0054] When the line sensor 138 reads the gradation correction pattern 1104, the yellow and cyan gradation correction patterns 1104 on the recording medium 110 pass through the reading area of ​​the line sensor 138. When the line sensor 138 reads the gradation correction pattern 1104, the magenta and black gradation correction patterns 1104 pass through the reading area of ​​the line sensor 138 after the yellow and cyan gradation correction patterns 1104. On the other hand, when the line sensor 139 reads the gradation correction pattern 1104, the magenta and black gradation correction patterns 1104 on the recording medium 110 pass through the reading area of ​​the line sensor 139. When the line sensor 139 reads the gradation correction pattern 1104, the yellow and cyan gradation correction patterns 1104 pass through the reading area of ​​the line sensor 138 after the magenta and black gradation correction patterns 1104.

[0055] (Create a conversion table) In the second gradation correction process, the measurement result of the gradation correction pattern 1104 is converted into a density value of the image formed on the intermediate transfer body 106. This is because the image density is not stable if the target density of the first gradation correction process differs from the target density of the second gradation correction process. By controlling the density value acquired in the gradation correction to be the target density on the intermediate transfer body 106, it is possible to easily achieve a stable image density even when the gradation correction to be performed is switched. Therefore, the printer controller CPU 314 generates a conversion condition for converting the reading result of the measurement image on the recording medium 110 into the density of the measurement image on the intermediate transfer body 106.

[0056] Furthermore, the image forming apparatus 100 has two line sensors 138 and 139. Here, there are individual differences between the line sensors 138 and 139. Therefore, in order to stabilize the image density with high accuracy, the printer controller CPU 314 generates conversion conditions for converting the reading results of the measurement image on the recording medium 110 into the density of the measurement image on the intermediate transfer body 106 for each of the line sensors 138 and 139. In other words, the printer controller CPU 314 generates conversion conditions for converting the reading results of the measurement image by the line sensor 138 into the measurement results of the density sensor 117, and conversion conditions for converting the reading results of the measurement image by the line sensor 139 into the measurement results of the density sensor 117.

[0057] The above-mentioned conversion conditions are a conversion table for converting the luminance signal of line sensor 138 into a density value, and a conversion table for converting the luminance signal of line sensor 139 into a density value. Note that the conversion conditions may be a conversion table for converting the luminance signal of line sensor 138 into an output value (voltage) of density sensor 117, and a conversion table for converting the luminance signal of line sensor 139 into an output value (voltage) of density sensor 117. Alternatively, the conversion conditions may be a conversion table for converting a density value calculated from the luminance signal of line sensor 138 (or line sensor 139) into a density value calculated from the output value of density sensor 117.

[0058] (Creating and selecting a conversion table) In image forming apparatus 100, the orientation of gradation correction pattern 1104 formed on recording medium 110 passing through conveyance path 201 changes depending on the paper discharge surface setting. This is because gradation correction pattern 1104 is formed on the first surface. Therefore, printer controller CPU 314 selects whether to use a conversion table for line sensor 138 or a conversion table for line sensor 139 depending on the paper discharge surface setting. Furthermore, printer controller CPU 314 controls whether to generate a conversion table for line sensor 138 or a conversion table for line sensor 139 depending on the paper discharge surface setting.

[0059] Next, the process of generating a conversion table will be described with reference to the flowchart of Fig. 7. When the printer controller CPU 314 receives an instruction to execute the process of generating a conversion table, it reads out a program from the program ROM 304 and loads it in the RAM 310 to execute each step of Fig. 7.

[0060] First, the printer controller CPU 314 forms the gradation correction pattern 1104 on the recording medium 1104 (S70). Since the formation of the gradation correction pattern 1104 is executed based on an instruction from the engine control CPU 102, the printer controller CPU 314 instructs the engine control CPU 102 to form the gradation correction pattern 1104 in step S70.

[0061] The gradation correction pattern 1104 formed in step S70 is a measurement image measured to generate a conversion table. Therefore, the gradation correction pattern 1104 formed in step S70 may be an image with a different gradation from the gradation correction pattern 1104 formed to generate the γLUT shown in FIG.

[0062] Next, the printer controller CPU 314 determines whether the paper ejection side setting is set to face-up ejection (S71). Here, FIG. 10 shows a setting screen for setting the paper ejection side setting displayed on the operation panel 180. The user can select face-up ejection or face-down ejection on the setting screen shown in FIG. 10. If no setting is made in the print settings, face-down ejection is selected. The printer controller CPU 314 accepts user instruction information regarding the paper ejection side setting from the operation panel 180.

[0063] If face-up paper ejection is set in step S71, printer controller CPU 314 causes line sensor 138 to read gradation correction pattern 1104 (S72). In step S72, engine control CPU 102 controls line sensor 138 based on instructions from printer controller CPU 314 to read gradation correction pattern 1104 on recording medium 110. The luminance signal of line sensor 138 is converted to density value A1, and printer controller CPU 314 acquires density value A1 transmitted from engine control CPU 102.

[0064] Next, the printer controller CPU 314 causes the intermediate transfer body 106 to form the gradation correction pattern 1061 (S73). Since the gradation correction pattern 1061 is formed based on an instruction from the engine control CPU 102, the printer controller CPU 314 instructs the engine control CPU 102 to form the gradation correction pattern 1061 in step S73.

[0065] Here, the gradation correction pattern 1061 formed in step S73 is a measurement image measured to generate a conversion table. Therefore, the gradation correction pattern 1061 formed in step S73 may be a gradation image different from the gradation correction pattern 1061 formed to generate the γLUT shown in FIG.

[0066] Next, the printer controller CPU 314 causes the density sensor 117 to read the gradation correction pattern 1106 (S74). In step S74, the engine control CPU 102 controls the density sensor 117 based on instructions from the printer controller CPU 314 to read the gradation correction pattern 1061 on the intermediate transfer body 106. The output value (voltage) of the density sensor 117 is converted to a density value B1, and the printer controller CPU 314 acquires the density value B1 transmitted from the engine control CPU 102.

[0067] Then, printer controller CPU 314 generates conversion table 140-140 based on density value A1 acquired in step S72 and density value B1 acquired in step S74 (S75). Conversion table 140 generated in step S75 is stored in table storage unit 311, and printer controller CPU 314 ends the conversion table generation process.

[0068] Furthermore, if face-down discharge is set in step S71, the engine control CPU 102 causes the conveying mechanism 130 to stop the recording medium 110 at the inversion unit 136 and convey the recording medium 110 from the inversion unit 136 to the conveying path 201. As a result, the recording medium 110 is conveyed so that the measurement image 1104 faces the line sensor 139.

[0069] Then, printer controller CPU 314 causes line sensor 139 to read gradation correction pattern 1104 (S76). In step S76, engine control CPU 102 controls line sensor 139 based on instructions from printer controller CPU 314 to read gradation correction pattern 1104 on recording medium 110. The luminance signal of line sensor 139 is converted to density value A2, and printer controller CPU 314 acquires density value A2 transmitted from engine control CPU 102.

[0070] Next, the printer controller CPU 314 causes the intermediate transfer body 106 to form the gradation correction pattern 1061 (S77). Since the gradation correction pattern 1061 is formed based on an instruction from the engine control CPU 102, the printer controller CPU 314 instructs the engine control CPU 102 to form the gradation correction pattern 1061 in step S77.

[0071] Here, the gradation correction pattern 1061 formed in step S77 is a measurement image measured to generate a conversion table. Therefore, the gradation correction pattern 1061 formed in step S77 may be a gradation image different from the gradation correction pattern 1061 formed to generate the γLUT shown in FIG.

[0072] Next, the printer controller CPU 314 causes the density sensor 117 to read the gradation correction pattern 1106 (S78). In step S78, the engine control CPU 102 controls the density sensor 117 based on instructions from the printer controller CPU 314 to read the gradation correction pattern 1061 on the intermediate transfer body 106. The output value (voltage) of the density sensor 117 is converted to a density value B2, and the printer controller CPU 314 acquires the density value B2 transmitted from the engine control CPU 102.

[0073] Then, printer controller CPU 314 generates conversion table 141 based on density value A2 acquired in step S76 and density value B2 acquired in step S78 (S79). Conversion table 141 generated in step S79 is stored in table storage unit 311, and printer controller CPU 314 ends the conversion table generation process.

[0074] 7, the gradation correction pattern 1061 is formed after the gradation correction pattern 1104 is formed, but the order of forming the gradation correction pattern 1104 and the gradation correction pattern 1061 is not limited to the above order. The gradation correction pattern 1601 may be formed first, and then the gradation correction pattern 1104 may be formed. In this case, it is sufficient to obtain user instruction information regarding the paper discharge surface setting in advance.

[0075] Moreover, it is preferable that the conversion table 140 (or 141) is generated when it is determined that the recording medium is to be used for the first time. The conversion table 140 (or 141) may be generated based on user instruction information from the operation panel 180, for example.

[0076] Fig. 9 is an example diagram of conversion tables 140 and 141. In Fig. 9, the vertical axis represents density values ​​calculated from the measurement results of density sensor 117, and the horizontal axis represents density values ​​calculated from the reading results of line sensor 138 or 139. It can be seen from Fig. 9 that there are individual differences between line sensor 138 and line sensor 139. In the gradation correction performed after the conversion table generation process is executed, printer controller CPU 314 converts the reading results of gradation correction pattern 1104 into density values ​​based on the corresponding conversion table 140 or 141.

[0077] (Tone correction control) The second gradation correction process is realized by the cooperation of the engine control CPU 102, which controls the line sensors 138 and 139, and the printer controller CPU 314. When the second gradation correction process is executed, the printer controller CPU 314, which functions as the automatic gradation correction generation unit 307, generates a γLUT based on the reading result of the gradation correction pattern 1104 by the line sensor 138 or 139. Then, the gradation correction unit 317 converts image data (density signal) based on the γLUT generated by the automatic gradation correction generation unit 307. Then, the image forming engine unit 101 forms an image based on the image data (laser output signal) converted by the gradation correction unit 317. As a result, the gradation characteristics of the image formed by the image forming engine unit 101 are controlled to ideal gradation characteristics. In the second gradation correction process, the gradation correction pattern 1104 is repeatedly formed on the multiple recording media 110. Then, the γLUT is generated based on the average value of the density values ​​of the gradation correction patterns 1104 formed on the multiple recording media 110.

[0078] Next, the image forming process including the gradation correction process will be described with reference to the flowchart of Fig. 8. When the image forming apparatus 100 receives an instruction to execute the image forming process for forming an image based on a job, the printer controller CPU 314 reads a program from the program ROM 304 and loads it in the RAM 310 to execute each step of Fig. 8.

[0079] First, the printer controller CPU 314 instructs the engine control CPU 102 to start image formation processing, and starts job printing (S81). The printer controller CPU 314 determines whether execution of the second gradation correction processing is enabled or not based on user instruction information related to gradation correction input from the operation panel 180 (S82). If execution of the second gradation correction processing is permitted in step S82, the printer controller CPU 314 checks the setting contents of the paper discharge side setting (S83). The printer controller CPU 314 determines whether a conversion table corresponding to the paper discharge side setting checked in step S83 is stored in the table storage unit 311 (S84).

[0080] If a conversion table corresponding to the discharge side setting is not stored in the table storage unit 311 in step S84, the printer controller CPU 314 executes the conversion table generation process shown in Fig. 7 (S85). In step S85, a conversion table corresponding to the currently set discharge side setting is stored in the table storage unit 311. After the conversion table generation process is executed, the printer controller CPU 314 increments the count value for counting the number of output sheets by 1, and proceeds to step S86 described below.

[0081] On the other hand, if a conversion table corresponding to the discharge surface setting is stored in table storage unit 311 in step S84, printer controller CPU 314 causes gradation correction pattern 1104 to be formed on recording medium 110 together with a user image based on the job (S86). In step S86, printer controller CPU 314 transmits a laser output signal to engine control CPU 102, which controls image forming engine unit 101, to cause engine control CPU 102 to form the user image and gradation correction pattern 1104 on recording medium 110.

[0082] The printer controller CPU 314 causes the line sensor 138 or 139 to read the gradation correction pattern 1104 (S87). In step S86, the engine control CPU 102 controls the line sensor 138 or 139 based on an instruction from the printer controller CPU 314 to read the gradation correction pattern 1104 on the recording medium 110. The luminance signal of the line sensor 138 or 139 is converted into a density value. The printer controller CPU 314 acquires the density value transmitted from the engine control CPU 102. Then, the printer controller CPU 314 converts the density value based on the conversion table 140 or 141 corresponding to the line sensor 138 or 139 that read the gradation correction pattern 1104 (S88).

[0083] The printer controller CPU 314 generates a γLUT based on the density values ​​converted in step S88 (S89), and determines whether the count value of the number of output sheets has reached the number of output sheets set in the job (S90). If the count value has reached the number of output sheets set in the job in step S90, it is determined that the last image based on the job has been formed, and the printer controller CPU 314 ends the image formation process.

[0084] On the other hand, if the count value does not reach the number of output sheets set in the job in step S90, the printer controller CPU 314 shifts the process to step S86. As a result, the image forming apparatus 100 continues to repeatedly form the gradation correction pattern 1104 together with the user image on the recording medium 110 until the count value reaches the number of output sheets set in the job.

[0085] If execution of the second gradation correction process is not permitted in step S82, the printer controller CPU 314 forms the gradation correction pattern 1061 at a predetermined timing in order to execute the first gradation correction process (S90). The predetermined timing is, for example, the timing when 20 pages of images are formed. In step S90, the printer controller CPU 314 transmits a laser output signal of the gradation correction pattern 1061 to the engine control CPU 102 that controls the image forming engine unit 101 so as to form the gradation correction pattern 1061.

[0086] The printer controller CPU 314 causes the density sensor 117 to measure the gradation correction pattern 1061 (S92). In step S92, the engine control CPU 102 controls the density sensor 117 based on instructions from the printer controller CPU 314 to measure the gradation correction pattern 1061 on the recording medium 110. The output value (voltage) of the density sensor 117 is converted into a density value. The printer controller CPU 314 acquires the density value transmitted from the engine control CPU 102.

[0087] Printer controller CPU 314 generates a γLUT based on the density value acquired in step S92 (S93), and causes the process to proceed to step S90. If the count value has not reached the number of output sheets set in the job in step S90, printer controller CPU 314 causes the process to proceed to step S91. As a result, image forming apparatus 100 forms tone correction pattern 1061 at each predetermined timing until the count value reaches the number of output sheets set in the job.

[0088] Since the gradation correction pattern 1104 is formed in a non-image area 1102 of the recording medium 110 (an edge area in a direction perpendicular to the conveying direction in which the recording medium 110 is conveyed), the formation of the user image is not interrupted for gradation correction. Therefore, downtime can be suppressed in the second gradation correction process. The density value acquired in the second gradation correction process is converted into the measurement result of the measurement image on the intermediate transfer body 106, and a γLUT is generated based on the converted density value. Therefore, it is possible to easily achieve stable image density regardless of the type of gradation correction.

[0089] In addition, it is preferable to reacquire the conversion table according to various conditions. The control parameters of each process of image transfer and fixing are changed according to the environmental conditions (temperature and humidity) in which the image forming apparatus 100 is installed. The control parameters are, for example, the value of the transfer voltage and the fixing temperature. This may cause a change in the correspondence relationship between the density (toner adhesion amount) of the image on the recording medium 110 and the density (toner adhesion amount) of the image on the intermediate transfer body 106. Therefore, the printer controller CPU 314 executes a conversion table generation process to update the conversion table, for example, every time 10,000 pages of images are formed.

[0090] In the image forming process shown in FIG. 8, only one of the first tone correction process and the second tone correction process is executed. However, the printer controller CPU 314 may control whether to execute only the first tone correction process without executing the second tone correction process, or whether to execute both the first tone correction process and the second tone correction process. If the execution of the second tone correction process is permitted in step S82, the printer controller CPU 314 executes both the first tone correction process and the second tone correction process. On the other hand, if the execution of the second tone correction process is not permitted in step S82, the printer controller CPU 314 executes only the first tone correction process. Since the target density of the first tone correction process and the target density of the second tone correction process are the same, even if both the first tone correction process and the second tone correction process are executed, it is possible to suppress a change in image density when the tone correction process is switched.

[0091] Furthermore, according to the image forming apparatus 100, the reading result of the line sensor 138 or 139 is converted based on a conversion table corresponding to the paper discharge surface setting, so that the density of the image formed by the image forming apparatus 100 can be controlled with high precision.

[0092] Example 2 The image forming apparatus 100 of this embodiment is similar to that of the embodiment 1. The image forming apparatus 100 of this embodiment creates and selects a conversion table in response to a user's selection of a line sensor to be used, rather than the setting of the discharge surface of the recording medium 110.

[0093] The output of line sensors 138 and 139 decreases due to, for example, the adhesion of toner or paper powder. When line sensors 138 and 139 are so contaminated with toner or paper powder that they cannot perform highly accurate measurements, it is possible to adopt a configuration in which an available line sensor is used.

[0094] At this time, as described in the first embodiment, regardless of the discharge surface setting selected by the user, it is necessary to create and select a conversion table corresponding to the line sensor 138 or 139.

[0095] When one of the line sensors cannot be used and the conversion table of the available line sensor is valid, the image forming apparatus 100 selects the conversion table corresponding to the available line sensor. Also, the conversion table corresponding to the available line sensor may be retaken due to a change in the environment where the image forming apparatus 100 is installed or an update of the control parameters of the image forming apparatus 100. In this case, regardless of the paper discharge surface setting selected by the user, a density value conversion table corresponding to the available line sensor is created.

[0096] The image forming process including the gradation correction process will be described with reference to the flowchart of Fig. 11. Note that the processes of steps S101 and S102 shown in Fig. 11 are the same as the processes of steps S81 and S82 shown in Fig. 8. Also, the processes of steps S105 to S110 and step S118 shown in Fig. 11 are the same as the processes of steps S84 to S90 shown in Fig. 8. Furthermore, the processes of steps S119 to S121 shown in Fig. 11 are the same as the processes of steps S91 to S93 shown in Fig. 8. Therefore, the description of each of the above steps will be omitted.

[0097] If execution of the second gradation correction process is permitted in step S102, printer controller CPU 314 determines whether both line sensors 138 and 139 are available (S103). If both line sensors 138 and 139 are available in step S103, printer controller CPU 314 checks the paper discharge side setting (S104).

[0098] On the other hand, if both line sensors 138 and 139 are unavailable in step S103, printer controller CPU 314 determines whether either line sensor 138 or 139 is available (S111). If line sensor 138 or 139 is available in step S111, printer controller CPU 314 checks the paper discharge side setting (S112). Then, printer controller CPU 314 determines whether line sensor 138 or 139 used in the selected paper discharge side setting is capable of reading gradation correction pattern 1104 (S113). If line sensor 138 or 139 used in step S113 is capable of reading gradation correction pattern 1104, printer controller CPU 314 shifts the process to step S105.

[0099] Furthermore, if neither line sensor 138 nor 139 can be used in step S111, printer controller CPU 314 causes operation panel 180 to display a screen for notifying the user that the second gradation correction process (on-paper correction) cannot be executed (S114). Then, printer controller CPU 314 shifts the process to step S119 and executes the first gradation correction process.

[0100] Furthermore, if the gradation correction pattern 1104 cannot be read in step S113, the printer controller CPU 314 causes the operation panel 180 to display a selection screen for selecting whether or not to execute the second gradation correction process (S115). The selection screen is a screen where the user can select whether or not to change the paper discharge surface setting. The image forming apparatus 100 cannot execute the second gradation correction process unless the paper discharge surface setting is changed. The user can select on the selection screen whether not to execute the second gradation correction process or to change the paper discharge surface setting.

[0101] Next, printer controller CPU 314 obtains user instruction information related to the content selected by the user on the selection screen from operation panel 180, and determines whether the second gradation correction process (on-paper correction) has been prioritized (S116). If the second gradation correction process (on-paper correction) has not been prioritized in step S116, that is, if a change in the paper discharge surface setting has not been instructed, printer controller CPU 314 transitions the process to step S119 and executes the first gradation correction process.

[0102] On the other hand, if the second gradation correction process (on-paper correction) is given priority in step S116, that is, if an instruction to change the paper discharge side setting is given, printer controller CPU 314 determines whether or not a conversion table is stored in table storage unit 311 (S117). If conversion table 140 or 141 for usable line sensor 138 or 139 is not stored in table storage unit 311 in step S117, printer controller CPU 314 shifts the process to step S118. As a result, a conversion table generation process is executed, and the conversion table for usable line sensor 138 or 139 is stored in table storage unit 311.

[0103] Furthermore, if conversion tables 140 or 141 for usable line sensors 138 or 139 are stored in table storage unit 311 in step S117, printer controller CPU 314 advances the process to step S106.

[0104] When the reading accuracy of one of line sensors 138 and 139 cannot be compensated for, image forming apparatus 100 notifies the user that the second gradation correction process cannot be executed with the paper discharge surface setting selected by the user. Furthermore, the user can select whether to execute the second gradation correction process using the other available line sensor in the subsequent operation, or to execute the image formation process by prioritizing the paper discharge surface setting, making it possible to perform an operation according to the situation. As described above, according to image forming apparatus 100 described in this embodiment, it is possible to display to the user the option of prioritizing the paper discharge surface setting or the second gradation correction process.

[0105] Example 3 The image forming apparatus 100 described in the first and second embodiments is described only in the case where a user image and a gradation correction pattern 1104 are formed on one side of the recording medium 110. In other words, only a job in which the image forming apparatus 100 forms an image on one side of the recording medium 100 (hereinafter referred to as a single-sided job) is described. However, the image forming apparatus 100 can also form images on both sides of the recording medium 110. Therefore, hereinafter, a gradation correction process when an image is formed based on a job in which the image forming apparatus 100 forms images on both sides of the recording medium 110 (hereinafter referred to as a double-sided job) will be described.

[0106] Here, in a single-sided job, the recording medium 110 passes through the fixing device 150 only once, whereas in a double-sided job, the recording medium 110 passes through the fixing device 150 twice. In other words, the influence of heat from the fixing device 150 differs between a single-sided job and a double-sided job. Therefore, the gradation correction pattern 1104 and the density of the user image may also differ between a single-sided job and a double-sided job. Therefore, the image forming apparatus 100 of this embodiment is characterized by having a single-sided conversion table and a double-sided conversion table in order to control the density of the output image with high accuracy in both a single-sided job and a double-sided job.

[0107] The process of generating the double-sided conversion table is performed when the recording medium 110 is output for the first time by the image forming apparatus 100. First, the engine control CPU 102 causes the image forming engine unit 101 to form a gradation adjustment pattern 1061 on the intermediate transfer body 106 based on an instruction from the printer controller CPU 314. Next, the engine control CPU 102 causes the density sensor 117 to measure the gradation correction pattern 1061 on the intermediate transfer body 106 based on an instruction from the printer controller CPU 314. As a result, the printer controller CPU 314 acquires the density value B of the gradation correction pattern 1061. After that, the engine control CPU 102 causes the image forming engine unit 101 to form a gradation correction pattern 1104 on the first side of the recording medium 110 based on an instruction from the printer controller CPU 314. When a double-sided job is executed, the conveying mechanism 130 conveys the recording medium 110 to the double-sided conveying path. As a result, the first side of the recording medium 110 conveyed on the conveying path 201 is read by the line sensor 139. Based on instructions from the printer controller CPU 314, the engine control CPU 102 causes the line sensor 139 to read the gradation correction pattern 1104 on the recording medium 110. The printer controller CPU 314 acquires a density value A of the gradation correction pattern 1104. The printer controller CPU 314 generates a double-sided density conversion table T3 based on the density value A and the density value B. When a double-sided job is executed, the printer controller CPU 314 converts the reading result of the gradation correction pattern 1104 into a density value based on the double-sided conversion table T3, and generates a γLUT based on the converted density value.

[0108] (Tone correction control) Next, the image forming process including the gradation correction process will be described with reference to the flowchart of Fig. 13. When the image forming apparatus 100 accepts an instruction to execute the image forming process based on a job, the printer controller CPU 314 reads out a program from the program ROM 304 and loads it in the RAM 310 to execute each step of Fig. 13.

[0109] Note that the processes of steps S901 and S902 shown in Fig. 13 are the same as the processes of steps S81 and S82 shown in Fig. 8. Also, the processes of S908 to S915 shown in Fig. 13 are the same as the processes of S86 to S93 shown in Fig. 8. Therefore, the description of each of the above steps will be omitted.

[0110] If execution of the second gradation correction process is permitted in step S902, printer controller CPU 314 determines whether it is a double-sided job or not (S903). Note that the user selects whether it is a single-sided job or a double-sided job in the print settings when transmitting the job to image forming apparatus 100. Printer controller CPU 314 can determine whether it is a double-sided job or a single-sided job by acquiring information indicating whether it is a single-sided job or a double-sided job.

[0111] If it is determined in step S903 that the job is a double-sided job, the printer controller CPU 314 determines whether or not the double-sided conversion table T3 is stored in the table storage unit 311 (S904). If the double-sided conversion table T3 is stored in the table storage unit 311 in step S904, the printer controller CPU 314 shifts the process to step S908. As a result, the image forming engine unit 101 forms a user image and a gradation correction pattern 1104 on the recording medium 1104 based on the double-sided job.

[0112] On the other hand, if the double-sided conversion table T3 is not stored in the table storage unit 311 in step S904, the printer controller CPU 314 executes the above-mentioned process of generating the double-sided conversion table T3. As a result, the double-sided conversion table T3 is stored in the table storage unit 311. Then, the printer controller CPU 314 shifts the process to step S908.

[0113] Furthermore, if it is determined in step S903 that the job is a single-sided job, printer controller CPU 314 determines whether a conversion table (single-sided conversion table) corresponding to the paper discharge side setting described in the first embodiment is stored in table storage unit 311 (S906). If a single-sided conversion table is stored in table storage unit 311 in step S906, printer controller CPU 314 shifts the process to step S908. As a result, image forming engine unit 101 forms a user image and gradation correction pattern 1104 on recording medium 1104 based on the single-sided job.

[0114] On the other hand, if a single-sided conversion table is not stored in table storage unit 311 in step S906, printer controller CPU 314 executes the conversion table generation process shown in Fig. 7. As a result, a single-sided conversion table is stored in table storage unit 311. Then, printer controller CPU 314 shifts the process to step S908.

[0115] According to the image forming apparatus 100 described in the third embodiment, both a single-sided conversion table and a double-sided conversion table are generated, so that the image density can be controlled with high precision taking into account the density fluctuations caused by the heat influence of the fixing device 150.

[0116] In the image forming apparatus 100 described in the third embodiment, the density value converted based on the single-sided conversion table is the same as the density value converted based on the double-sided conversion table. This makes it possible to suppress the variation in image density by using the single-sided conversion table and the double-sided conversion table even when an image is formed based on a job in which single-sided jobs and double-sided jobs are mixed. [Explanation of symbols]

[0117] 106 Intermediate transfer body 110 Recording media 138 Line Sensor 139 Line Sensor 140 Conversion Table 141 Conversion Table 314 Printer controller CPU 1061 Gradation Correction Pattern 1104 Gradation correction pattern

Claims

1. an image forming means for forming an image on a recording medium based on image forming conditions for controlling image density; a reading means including a conveying path along which the recording medium is conveyed, a first reading unit that reads a pattern image formed on the recording medium conveyed on the conveying path from a first side with respect to the conveying path, and a second reading unit that reads the pattern image formed on the recording medium conveyed on the conveying path from a second side opposite to the first side with respect to the conveying path; an intermediate transfer body onto which a measurement image formed by the image forming means is transferred; a measuring means for measuring the measurement image transferred to the intermediate transfer body; a generating unit that causes the image forming unit to form a pattern image on the recording medium and generates the image forming conditions based on a reading result of the pattern image by the reading unit; another generating means for generating the first conversion condition and the second conversion condition; having The generating means includes: When the first reading unit reads a pattern image formed on a surface of the recording medium facing the first reading unit, a first reading result of the pattern image by the first reading unit is converted based on a first conversion condition, and the image forming condition is generated based on the converted first reading result; when the second reading unit reads a pattern image formed on a surface of the recording medium facing the second reading unit, a second reading result of the pattern image by the second reading unit is converted based on a second conversion condition different from the first conversion condition, and the image forming condition is generated based on the converted second reading result; The other generating means is generating the first conversion condition based on a measurement result of the measurement image by the measurement means and a reading result of the measurement image on the recording medium read by the first reading unit; generating the second conversion condition based on a measurement result of the measurement image by the measuring means and a reading result of the measurement image on the recording medium read by the second reading unit; 1. An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein the reading unit is disposed downstream of the image forming unit in a transport direction in which the recording medium is transported.

3. the image forming unit is capable of switching between face-up paper ejection, in which the surface on which the image is formed faces upward, and face-down paper ejection, in which the surface on which the image is formed faces downward; the first reading unit reads the pattern image of the recording medium when the face-up paper ejection is performed; 3. The image forming apparatus according to claim 2, wherein the second reading section reads the pattern image on the recording medium when the face-down paper ejection is performed.

4. the image forming means has a fixing section that heats the recording medium to fix the image on the recording medium, When the image forming unit executes a double-sided job for forming images on both sides of the recording medium, the pattern image on the recording medium passes through the fixing unit twice, the generating means, when the second reading unit reads the pattern image of the recording medium in the double-sided job, converts a second reading result of the pattern image by the second reading unit based on a third conversion condition, and generates the image forming condition based on the converted second reading result; 2. The image forming apparatus according to claim 1, wherein the third conversion condition is different from both the first conversion condition and the second conversion condition.

5. the image forming means has an image processing section which performs image processing on image data based on a gradation correction condition, and forms the image based on the image data which has been subjected to the image processing by the image processing section; 2. The image forming apparatus according to claim 1, wherein the image forming conditions are the tone correction conditions.

Images