JP7596163B2 - Image forming device - Google Patents

Description

The present invention relates to image forming devices such as copiers, multifunction machines, and printers.

A full-color image forming apparatus that employs electrophotography forms an image on an image carrier, transfers the image from the image carrier to a recording material, and fixes the transferred image to the recording material to form an image. The image density of the image formed on the recording material changes depending on environmental conditions such as temperature and humidity, and on the deterioration of the developer used for development. To this end, the image forming apparatus forms a test image to adjust the image density, and adjusts the image formation conditions and creates a gradation correction table based on the results of reading this test image with a sensor, thereby stabilizing the image density. This is called "calibration." Calibration can be performed using the results of reading the test image formed on the recording material, or using the results of reading the test image on the image carrier before it is transferred to the recording material.

The image forming apparatus disclosed in Patent Document 1 performs calibration by forming a test image in a margin area on a recording material where an image (user image) in accordance with instructions from a user is formed. This allows calibration to be performed in real time during a print job. The margin area where the test image is formed is an area where the outer edge of the recording material is cut. This type of image forming apparatus maintains appropriate image density characteristics (tone characteristics) and prevents a decrease in productivity by not interrupting the image formation operation.

JP 2014-107648 A

The image forming apparatus of Patent Document 1 can perform calibration using the results of reading a test image formed on a recording material, as well as calibration using the results of reading a test image on an image carrier. However, when performing calibration using different methods like these, the target value may differ for each calibration. This may hinder the stability of high-precision image density (image quality).

In view of the above problems, the present invention aims to provide an image forming device that can form images with stable image quality even when calibration is performed using different methods.

The image forming apparatus of the present invention includes an image carrier on which an image is formed, a transfer means for transferring the image formed on the image carrier to a sheet, and a fixing means for fixing the image on the sheet, and is equipped with an image forming means for forming an image on the sheet based on image density conditions for controlling image density , a tray to which the sheet is discharged, a transport means for transporting the sheet from the fixing means to the tray, a detection means for detecting a pattern image on the image carrier, a reading means arranged downstream of the fixing means in a conveying direction of the sheet by the transport means and for reading the pattern image on the sheet, and a control means for generating the image density conditions, wherein the control means generates the image density conditions based on a detection result by the detection means of the pattern image on the image carrier formed by the image forming means, and the control means converts a reading result by the reading means of the pattern image on the sheet formed by the image forming means based on a conversion condition for converting a reading result by the reading means of the pattern image on the sheet formed by the image forming means into a detection result by the detection means, and generates the image density conditions based on the converted reading result .
Another image forming apparatus of the present invention includes an image carrier on which an image is formed, a transfer means for transferring the image formed on the image carrier to a sheet, and a fixing means for fixing the image on the sheet, and includes an image forming means for forming an image on the sheet based on an image density condition for controlling an image density, a tray on which the sheet is placed, a transport means for transporting the sheet from the fixing means to the tray, a detection means for detecting a pattern image on the image carrier, a reading means arranged downstream of the fixing means in the sheet transport direction by the transport means and for reading the pattern image on the sheet, and and a control means for generating image density conditions, wherein the control means generates the image density conditions based on a detection result by the detection means of the pattern image on the image carrier formed by the image forming means, and the control means determines a relationship between the detection result and the reading result based on the detection result of the pattern image on the image carrier detected by the detection means and the reading result of the pattern image on the sheet read by the reading means, and generates the image density conditions based on the relationship and the reading result by the reading means of the pattern image on the sheet.

According to the present invention, it is possible to stabilize the image quality of the image formed on the recording material.

FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. FIG. 2 is a diagram illustrating the configuration of an image density sensor. FIG. A 4th period chart explaining how gradation is reproduced. 13A and 13B are diagrams illustrating test images for tone correction. 13A and 13B are diagrams illustrating test images for tone correction. 13 is a flowchart showing a process for creating a density conversion table. FIG. 4 is an explanatory diagram of a density conversion table. 4 is a flowchart showing a tone correction process. 5A to 5C are diagrams illustrating the effect of tone correction processing. 4 is a flowchart showing a tone correction process. FIG. 11 is an explanatory diagram of information used to determine regeneration. 13A and 13B are diagrams illustrating the effect when a density conversion table is regenerated. 4 is a flowchart showing a tone correction process. 6 is an explanatory diagram of a comparison process between a density conversion table and a comparison table.

The following describes an embodiment of the present invention with reference to the drawings. However, the embodiment described below has various limitations that are technically preferable for implementing the present invention, but the scope of the invention is not limited to the following embodiment and illustrated examples.

First Embodiment
1 is a diagram showing the configuration of an image forming apparatus according to the present embodiment. The image forming apparatus 100 according to the present embodiment is composed of a printer 101, a reader 400, and a processing device 600. The image forming apparatus 100 (printer 101) forms an image on a sheet-shaped recording material 110 by an electrophotographic method. The printer 101 according to the present embodiment may be an inkjet printer or a dye-sublimation printer.

The image forming apparatus 100 comprises the various mechanisms constituting the engine section for image formation within the printer 101, an engine control section 102 that controls the operation of each mechanism, and a control board housing section 104 that houses the printer controller 300. An operation panel 180 is provided on the top of the printer 101. The operation panel 180 is a user interface, and comprises an input device that accepts instructions from the user, and an output device that displays a screen such as an operation screen. The input device is various key buttons, a touch panel, etc. The output device is a display and a speaker. The reader 400 is an image reading device that reads an image from a recording material (original) on which the image is formed.

The mechanisms that make up the engine section include a charging/exposure processing mechanism, a developing processing mechanism, a transfer processing mechanism, a fixing processing mechanism, a paper feed processing mechanism for the recording material 110, and a transport processing mechanism for the recording material 110. The charging/exposure processing mechanism forms an electrostatic latent image by scanning with a laser beam. The developing processing mechanism visualizes the electrostatic latent image. The transfer processing mechanism transfers the toner image generated by the visualization to the recording material 110. The fixing processing mechanism fixes the toner image transferred to the recording material 110.

Each of these mechanisms is composed of image forming units 120, 121, 122, 123, intermediate transfer body 106, fixing unit 150, paper feed cassette 113, etc. in printer 101. Image forming units 120, 121, 122, 123 have the same configuration and perform the same operations, but only differ in the color of the images they form. Image forming unit 120 forms a yellow (Y) image. Image forming unit 121 forms a magenta (M) image. Image forming unit 122 forms a cyan (C) image. Image forming unit 123 forms a black (K) image. Image forming units 120, 121, 122, 123 each include a photosensitive drum 105, a charger 111, a laser scanner 107, and a developer 112.

The charging and exposure processing mechanism uniformly charges the surface of the photosensitive drum 105 with a charger 111, and forms an electrostatic latent image on the surface of the photosensitive drum 105 with a laser scanner 107. The photosensitive drum 105 is a drum-shaped photosensitive body with a photosensitive layer on its surface, and rotates around the drum axis. The charger 111 uniformly charges the photosensitive layer on the surface of the rotating photosensitive drum 105.

The laser scanner 107 includes a light-emitting unit 108 that scans the laser light emitted from the semiconductor laser in one direction, and a reflection mirror 109 that reflects the laser light from the light-emitting unit 108 toward the photosensitive drum 105. The laser scanner 107 includes a laser driver that drives the laser light emitted from the light-emitting unit 108 in accordance with image data supplied from the printer controller 300. The laser light emitted from the semiconductor laser is swung in one direction in accordance with the rotation of a rotating polygon mirror in the light-emitting unit 108. The laser light swung in one direction irradiates the photosensitive drum 105 via the reflection mirror 109. As a result, the laser light scans the surface of the photosensitive drum 105 in one direction (drum axis direction) to form an electrostatic latent image. The one direction (depth direction in the figure) in which the laser scanner 107 scans the photosensitive drum 105 is the main scanning direction.

The developing mechanism visualizes the electrostatic latent image with toner supplied from the developing device 112 to form a toner image on the photosensitive drum 105. The toner image on the photosensitive drum 105 is transferred onto the intermediate transfer body 106 to which a voltage with the opposite characteristics to that of the toner image is applied. When forming a color image, the toner images are transferred from the photosensitive drums 105 of the image forming units 120, 121, 122, and 123 corresponding to each color to the intermediate transfer body 106 in a sequential superimposed manner. In this embodiment, the intermediate transfer body 106 rotates clockwise in the figure, and the toner images are transferred in the order of the image forming unit 120 (yellow), the image forming unit 121 (magenta), the image forming unit 122 (cyan), and the image forming unit 123 (black). As a result, a full-color toner image (visible image) is formed on the intermediate transfer body 106. The photosensitive drum 105 and the developing device 112 are detachable from the housing of the printer 101.

The transfer mechanism transfers the visible image (toner image) formed on the intermediate transfer body 106 to the recording material 110 fed from the paper feed cassette 113. The transfer mechanism includes a transfer roller 114 for transferring the toner image from the intermediate transfer body 106 to the recording material 110. The toner images transferred from each image forming unit 120, 121, 122, and 123 to the intermediate transfer body 106 are conveyed to the transfer roller 114 by the intermediate transfer body 106 rotating clockwise in the figure. The recording material 110 is conveyed to the transfer roller 114 in accordance with the timing at which the toner image is conveyed to the transfer roller 114. The transfer roller 114 presses the recording material 110 against the intermediate transfer body 106 and simultaneously applies a bias with the opposite characteristics to the toner image, thereby transferring the toner image to the recording material 110.

An image formation start position detection sensor 115 and an image density sensor 117 are arranged around the intermediate transfer body 106. The image formation start position detection sensor 115 is used to determine the print start position during image formation. The image formation start position detection sensor 115 is provided upstream of the transfer roller 114 in the rotation direction of the intermediate transfer body 106. The image density sensor 117 is used to measure the image density of a test image for image density detection formed on the intermediate transfer body 106 during image density control. The image density sensor 117 is provided downstream of the image forming unit 123 in the rotation direction of the intermediate transfer body 106.

The feeding process mechanism includes a paper feed cassette 113 that stores the recording material 110, a transport path along which the recording material 110 is fed, and various rollers for transporting the recording material 110. The recording material 110 is fed from the paper feed cassette 113, and while being transported along the transport path, a toner image is transferred and fixed to form an image on the recording material 110, and the recording material 110 is discharged outside the printer 101. The transport direction of the recording material 110 is the sub-scanning direction that is perpendicular to the main scanning direction.

The recording material 110 is fed from a paper feed cassette 113 and transported along a transport path to a transfer roller 114. A paper feed timing sensor 116 is provided along the transport path from the paper feed cassette 113 to the transfer roller 114 to adjust the transport timing of the recording material 110. The timing at which the recording material 110 is transported to the transfer roller 114 is adjusted based on the timing at which the image formation start position detection sensor 115 detects the image on the intermediate transfer body 106 and the timing at which the paper feed timing sensor 116 detects the recording material 110. This causes the toner image to be transferred from the intermediate transfer body 106 to a predetermined position on the recording material 110.

The recording material 110 onto which the toner image has been transferred is transported to a fixing mechanism. The fixing mechanism of this embodiment includes a fixing device 150. The fixing device 150 includes a fixing roller 151 for heating the recording material 110 in order to thermally pressurize the toner image onto the recording material 110, a pressure belt 152 for pressing the recording material 110 against the fixing roller 151, and a post-fixing sensor 153 for detecting the completion of fixing. The fixing roller 151 is a hollow roller that has an internal heater and is configured to transport the recording material 110 by rotating. The pressure belt 152 presses the recording material 110 against the fixing roller 151. The post-fixing sensor 153 detects the recording material 110 after the image has been fixed.

The recording material 110 with the image fixed by the fixing device 150 may be discharged as is or transported to the transport path 135. For this reason, a flapper 132 is provided after the fixing device 150. The flapper 132 guides the recording material 110 to either the transport path 135 or the transport path 201. The transport path 201 includes transport rollers 140 and 141. The recording material 110 guided to the transport path 201 is transported by the transport rollers 140 and 141, and is discharged from the printer 101 to the processing device 600 with the side on which the image is formed facing up. Between the transport rollers 140 and 141 of the transport path 201, a line sensor 138 is provided at a position where the image on the recording material 110 can be detected.

The line sensor 138 is an optical sensor such as a CMOS line sensor or a CCD line sensor. The line sensor 138 reads an image formed on the recording material 110 conveyed along the conveying path 201 by the conveying rollers 140 and 141. The line sensor 138 outputs a read signal including the luminance values of the colors red (R), green (G), and blue (B) as the reading result. The luminance values of these read signals are converted into density values of the colors cyan (C), magenta (M), yellow (Y), and black (K) for use. Generally, cyan is calculated from the luminance value of the red sensor, magenta from the luminance value of the green sensor, yellow from the luminance value of the blue sensor, and black from the luminance value of the green sensor. At that time, the conversion from each luminance value to each color density value is performed using a look-up table (LUT) that is generated by acquiring in advance the relationship between each luminance value of RGB and each density value of CMYK. Such a LUT is stored in the image forming apparatus 100 in advance.

The conveying path 135 is a path for conveying the recording material 110 to the reversing path 136 used for reversing the front and back sides of the recording material 110. The reversing path 136 is provided with a reversing sensor 137 for detecting the recording material 110. When the reversing sensor 137 detects the rear end of the recording material 110, the conveying direction of the recording material 110 is reversed at the reversing path 136. The recording material 110 whose conveying direction has been reversed is conveyed to either the conveying path 135 or the reversing path 139. For this purpose, a flapper 133 is provided at the branch point between the conveying path 135 and the reversing path 139. When conveyed to the conveying path 135, the recording material 110 is guided to the conveying path 135 by the flapper 133, and further guided to the conveying path 201 by the flapper 134. As a result, the recording material 110 is discharged from the printer 101 to the processing device 600 with the front and back sides reversed (with the side on which the image is formed facing down). When the recording material 110 is transported to the reverse path 139, the recording material 110 is guided to the reverse path 139 by the flapper 133. The recording material 110 guided to the reverse path 139 is reversed and transported again to the transfer roller 114. As a result, an image is formed on the reverse side of the recording material 110.

(Image density sensor)
2 is a diagram illustrating the configuration of the image density sensor 117. As described above, the image density sensor 117 detects a test image for detecting image density formed on the intermediate transfer body 106. The image density sensor 117 includes an optical sensor including a light emitting diode (LED) 1171 as a light source and light receiving units 1172 and 1173, and an electric board (not shown) on which the optical sensor is mounted. The light receiving units 1172 and 1173 are, for example, photodiodes.

The LED 1171 irradiates the intermediate transfer body 106 with infrared light at a predetermined angle of incidence (15° in this example). The light receiving unit 1172 receives the reflected light of the light irradiated from the LED 1171 onto the intermediate transfer body 106 or the test image at the position of the specular reflection angle. The light receiving unit 1173 receives the diffuse reflected light of the reflected light of the light irradiated from the LED 1171 onto the intermediate transfer body 106 or the test image. The electric board is equipped with a drive circuit that supplies current to the LED 1171 to cause the LED 1171 to emit light, and a light receiving circuit with an IV conversion function that converts the current generated according to the amount of reflected light received by the light receiving units 1172 and 1173 into a voltage.

The image density sensor 117 configured as described above can measure both specularly reflected light and diffusely reflected light. The light receiving unit 1172 that receives specularly reflected light and the light receiving unit 1173 that receives diffusely reflected light measure the light reflected by the intermediate transfer body 106 and the light reflected by the test image, respectively.

At this time, the black toner test image is converted into a density value based on the detection result of the specular reflected light acquired by the light receiving unit 1172. The chromatic color test images of cyan, magenta, and yellow are converted into density values based on the detection result of the diffuse reflected light acquired by the light receiving unit 1173. As described above, the image forming apparatus 100 converts the brightness value included in the detection result into a density value using the LUT.

The image density sensor 117 is not limited to the configuration shown in this embodiment. For example, the light receiving unit 1172 or the light receiving unit 1173 may be arranged so that the optical axis for receiving reflected light is normal to the surface of the intermediate transfer body 106 on which the test image is formed. The light receiving unit 1172 and the light receiving unit 1173 may also be configured to include a polarizing filter. In this embodiment, the light receiving unit 1172 and the light receiving unit 1173 are described as being arranged in positions opposite to the position where the infrared light irradiated from the LED 1171 is reflected by the intermediate transfer body 106, but these can be arranged as appropriate.

(Printer Controller)
3 is an explanatory diagram of the printer controller 300 of this embodiment. The printer controller 300 is communicably connected to a host computer 301, which is a device provided outside the image forming apparatus 100. The host computer 301 and the image forming apparatus 100 are communicatively connected via a communication line such as USB 2.0 High-Speed or 1000Base-T/100Base-TX/10Base-T (IEEE 802.3 compliant) or wirelessly.

The printer controller 300 controls the overall operation of the printer 101. To this end, the printer controller 300 is connected to the operation panel 180, the reader 400, and the engine unit 1011. The engine unit 1011 controls the operation of each mechanism in the printer 101 in response to instructions from the printer controller 300, and performs image formation processing on the recording material 110. The engine unit 1011 includes an engine control unit 102. The engine control unit 102 controls the operation of each mechanism in the engine unit 1011. The engine control unit 102 also controls the detection operation of the test image by the image density sensor 117 and the line sensor 138. The engine control unit 102 is configured, for example, by a CPU (Central Processing Unit).

The printer controller 300 includes a host interface (I/F) 302, a panel interface (I/F) 312, a reader interface (I/F) 313, an engine interface (I/F) 319, and an input/output buffer 303. The host I/F 302 is a communication interface with the host computer 301. The panel I/F 302 is a communication interface with the operation panel 180. The reader I/F 313 is a communication interface with the reader 400. The engine I/F 319 is a communication interface with the engine unit 1011. The input/output buffer 303 is a temporary storage area for sending and receiving control codes and data via each interface.

The printer controller 300 includes a CPU 314, a program ROM (Read Only Memory) 304, and a RAM (Random Access Memory) 310. The CPU 314 controls the operation of the printer controller 300 by executing computer programs stored in the program ROM 304. The RAM 310 provides a working area for the printer controller 300 to execute processes.

The program ROM 304 has, as modules, an image information generating unit 305, a main scanning unevenness correction table generating unit 306, an automatic tone correction generating unit 307, a multi-color table generating unit 308, and an image defect detecting unit 309. The image information generating unit 305 generates various image objects by setting data acquired from the host computer 301. The main scanning unevenness correction table generating unit 306 generates a main scanning unevenness correction table for suppressing image density unevenness in the main scanning direction by correcting the laser emission intensity. The automatic tone correction generating unit 307 generates a tone correction table (γLUT) for performing density tone correction of a single color. The multi-color table generating unit 308 generates an ICC profile, which is a multidimensional LUT, in order to correct fluctuations in multi-colors. The image defect detecting unit 309 detects image defects in the image read by the line sensor 138.

The RAM 310 temporarily stores the results of processing by the image information generation unit 305, the main scanning unevenness correction table generation unit 306, the automatic tone correction generation unit 307, and the multi-color table generation unit 308. The RAM 310 has a table storage unit 311. The table storage unit 311 stores the main scanning unevenness correction table, the γLUT, the ICC profile, and the density conversion table described below.

The printer controller 300 includes a RIP (Raster Image Processor) unit 315, a color processing unit 316, a tone correction unit 317, and a pseudo-halftone processing unit 318. The RIP unit 315 develops an image object (image data) into a bitmap image. The color processing unit 316 performs multi-color color conversion processing on the image data developed into the bitmap image by the RIP unit 315 using an ICC profile. The tone correction unit 317 performs monochromatic tone correction processing on the image data that has been color converted by the color processing unit 316 using a γLUT. The pseudo-halftone processing unit 318 performs pseudo-halftone processing such as a dither matrix or error diffusion method on the image data that has been tone corrected by the tone correction unit 317. The image data that has been pseudo-halftone processed by the pseudo-halftone processing unit 318 is sent to the engine unit 1011 via the engine I/F 319. The engine control unit 102 of the engine unit 1011 performs image formation processing based on image data acquired from the engine I/F 319.

The above-described components of the printer controller 300 are connected to a system bus 320 and can communicate with each other via the system bus 320. The CPU 314 manages and updates the ICC profile, γLUT, and main-scan unevenness correction table used during image formation via the system bus 320. The CPU 314 makes it possible to output an image in the desired color by reflecting the latest tables in the color processing unit 316, tone correction unit 317, etc.

(γLUT)
A γLUT that does not depend on the location where a test image for tone correction is formed will be described. FIG. 4 is a four-limit chart for explaining how tone is reproduced. Quadrant I represents the reading characteristics of a sensor that reads an original image. This sensor converts the image density of the original image into a density signal. Quadrant II represents the conversion characteristics (data characteristics) of the γLUT for converting the density signal into a laser output signal that represents the amount of laser light output from the laser scanner 107. Quadrant III represents the recording characteristics of the printer 101 that converts the laser output signal into the image density of an image formed on a recording material. Quadrant IV represents the relationship between the image density of the original image and the image density of the image formed on a recording material. In other words, the four-limit chart represents the total tone reproduction characteristics of the image forming apparatus 100 shown in FIG. 1.

Figure 4 shows the case where 8-bit digital signals are processed, and the number of gradations is 256. Here, the sensors in quadrant I are the line sensor 138 that reads the test image for gradation correction on the recording material 110, and the image density sensor 117 that reads the test image for gradation correction on the intermediate transfer body 106. In order to make the total gradation characteristics of the printer 101, that is, the gradation characteristics of quadrant IV, linear, the non-linear printer characteristics of quadrant III are corrected by the γLUT in quadrant II. The image signal whose gradation characteristics have been converted by the γLUT is converted into a pulse signal corresponding to the dot width by a pulse width modulation (PWM) circuit of the laser driver, and is sent to the laser driver that drives and controls the light emitting unit 108. In this embodiment, a gradation reproduction method using pulse width modulation is used for all colors of yellow, magenta, cyan, and black.

By scanning the laser light output from the light emitting unit 108 of the laser scanner 107, an electrostatic latent image having a predetermined gradation characteristic is formed on the photosensitive drum 105, in which the gradation is controlled by changing the dot area. This electrostatic latent image is developed into a toner image, which is then transferred and fixed to the recording material 110 to reproduce the gradation image.

(Gradation correction using a test image for gradation correction formed on the intermediate transfer body 106)
The image forming apparatus 100 performs calibration using two different methods. That is, the image forming apparatus 100 performs calibration using the results of reading a test image printed in an edge area (non-image area) of the recording material 110, and calibration using the results of reading a test image formed on the intermediate transfer body 106. These are conventional calibrations.

The tone correction process (calibration), which is performed by forming a test image for tone correction on the intermediate transfer body 106, is performed in cooperation with the engine control unit 102, which controls the image density sensor 117, and the CPU 314. The tone correction unit 317 adjusts the tone characteristics of the image formed by the printer 101. After the color processing unit 316 performs initial adjustments for the color correction process, the tone correction unit 317 performs calibration at regular intervals (e.g., 100 sheets) after the printer 101 has processed a certain number of sheets.

Figure 5 is an example of a test image for tone correction formed on the intermediate transfer body 106. The test image for tone correction (tone correction pattern 1061) formed on the intermediate transfer body 106 is formed at a position that passes the detection position of the image density sensor 117 as the intermediate transfer body 106 rotates. The tone correction pattern 1061 is composed of multiple tone patches (11 tones in Figure 5) with gradually different tone values for each color. Each of the multiple tone patches is, for example, a square with one side measuring approximately 10 mm, and is arranged in a row in the direction of rotation of the intermediate transfer body 106.

For each color, a gradation patch for detecting the texture of the intermediate transfer body 106 (i.e., a gradation patch with a gradation value of 0) is arranged at both ends of the rotation direction of the intermediate transfer body 106. Nine gradation patches with equally allocated gradation values are arranged between gradation patches with a gradation value of 0. When the gradation value is expressed as 0 to 255, the gradation correction pattern 1061 is composed of gradation patches of each color with gradation values of 0, 16, 32, 64, 86, 104, 128, 176, 224, 255, and 0. Note that when multiple image density sensors 117 are provided in the main scanning direction (the direction perpendicular to the rotation direction of the intermediate transfer body 106), multiple gradation correction patterns 1061 may be formed corresponding to each image density sensor 117.

The gradation correction pattern 1061 is formed when the image forming apparatus 100 is not forming an image. The gradation correction pattern 1061 is formed by interrupting the print job when the image forming apparatus 100 has formed an image on a predetermined number of sheets of recording material 110, or after the print job is completed. In other words, calibration is performed using the results of reading the test image formed on the intermediate transfer body 106 when the image forming apparatus 100 has interrupted the print job when it has formed an image on a predetermined number of sheets of recording material 110, or after the print job is completed.

This section describes a method for updating the γLUT based on the density values read from the gradation correction pattern 1061 on the intermediate transfer body 106.

When the image formed on the recording material 110 has the target tone reproduction characteristics, such as immediately after the automatic tone correction process is performed, the value of the tone correction pattern 1061 read by the image density sensor 117 is held as a tone target on the intermediate transfer body 106. The automatic tone correction process is not a tone correction process performed during a print job, but a tone correction process performed by a user at a predetermined timing. The automatic tone correction process adjusts the maximum density of each color and the tone characteristics of each color and each screen pattern to a predetermined target value. The conversion LUT on the intermediate transfer body 106 created by comparing the tone target and the density value read from the tone correction pattern 1061 is synthesized with the γLUT for the recording material 110. Here, "synthesis" means creating a γLUT for the recording material 110 by linking the relationship between the tone target of the intermediate transfer body 106 when the target tone reproduction characteristics are obtained on the recording material 110 and the γLUT on the recording material 110. The γLUT for the recording material 110 is created from the density values of the gradation correction pattern 1061 on the intermediate transfer body 106.

(Test image for tone correction formed on the recording material 110)
FIG. 6 is an example diagram of a test image for gradation correction formed on the recording material 110 together with a user image according to an instruction from a user. The recording material 110 is conveyed in the direction of the arrow (conveying direction) shown in FIG. 6. The test image for gradation correction (gradation correction pattern 1104) formed on the recording material 110 is formed in an end region (non-image region 1102) of the recording material 110 except for an image region 1101 where a user image is formed. The gradation correction pattern 1104 of this embodiment is formed in an end region (non-image region 1102) of the recording material 110 in the conveying direction. The image region 1101 is an area indicated by dots in FIG. 6. A cutting mark 1103 is provided in advance on the recording material 110. The cutting mark 1103 is configured by combining two L-shaped marks and is provided at the four corners of the image region 1101. The recording material 110 is cut at the cutting mark 1103. It should be noted that the dots in the image area 1101 are shown for the purpose of explanation, and are not actually printed on the recording material 110 .

The gradation correction pattern 1104 is formed for each color on one side of the recording material 110. The gradation correction pattern 1104 is usually formed in the non-image area 1102 outside the image area 1101 so as not to overlap with it. However, if the CPU 314 determines that the gradation correction pattern 1104 is to be formed overlapping the image area 1101, the gradation correction pattern 1104 may be formed overlapping the image area 1101. In this embodiment, overlapping the image area 1101 includes not only the case where the gradation correction pattern is formed overlapping only the image area 1101, but also the case where the gradation correction pattern is formed across the image area 1101 and the non-image area 1102.

The gradation correction pattern 1104 may be formed on any of the periphery of the recording material 110. In this embodiment, the gradation correction pattern 1104 is formed on both ends of the recording material 110 in a direction (short side direction of the recording material 110) perpendicular to the conveying direction of the recording material 110 (longitudinal direction of the recording material 110). That is, the gradation correction pattern 1104 of two colors is formed on one end of the recording material 110 in the short side direction, and the gradation correction pattern 1104 of the remaining two colors is formed on the other end of the recording material 110 in the short side direction. In this embodiment, the gradation correction pattern 1104 of cyan and magenta is formed on one end of the recording material 110 in the short side direction, and the gradation correction pattern 1104 of yellow and black is formed on the other end of the recording material 110 in the short side direction. As a result, the gradation correction pattern 1104 is not formed on the leading end of the recording material 110 in the conveying direction, and the occurrence of wrapping of the recording material 110 during the fixing process can be more reliably suppressed.

The gradation correction pattern 1104 is composed of multiple gradation patches (11 gradations in FIG. 6) with gradation values for each color that are gradually changed. Each of the multiple gradation patches is, for example, a square with a side length of approximately 8 mm, and is arranged in a row in the transport direction.

For each color, tone patches for detecting the texture of the recording material 110 (i.e., tone patches with a tone value of 0) are arranged at both ends of the recording material 110 in the conveying direction. Nine tone patches with equally-distributed tone values are arranged between tone patches with a tone value of 0. When tone values are expressed as 0 to 255, the tone correction pattern 1104 is composed of tone patches of each color with tone values of 0, 16, 32, 64, 86, 104, 128, 176, 224, 255, and 0. Note that the tone correction pattern 1104 is not limited to yellow, magenta, cyan, and black, and may be composed of red, green, and blue colors or process black. Furthermore, the size and tone order are not limited.

(Density conversion table)
The target gradation of the recording material 110 and the target gradation of an image carrier such as the intermediate transfer body 106 do not have the same target value because they are measured at different locations. Therefore, if calibration is performed using a test image formed on the image carrier (intermediate transfer body 106) after calibration using the recording material 110, the image density of the image formed on the recording material 110 may change. This is because the change in image density of the test image on the recording material 110 and the change in image density of the test image on the image carrier are not the same.

The image forming apparatus 100 of this embodiment uses the target gradation of the image carrier as the target gradation for the calibration of the recording material 110. To this end, the image forming apparatus 100 requires a density conversion table as a conversion condition for replacing the image density of the image on the recording material 110 with the image density of the image on the image carrier. This density conversion table allows the image density of the image on the recording material 110 to be regarded as the image density of the image detected on the image carrier. For this reason, it is not necessary to set the target gradation for each recording material 110, and the target gradation of the image carrier can be used.

Figure 7 is a flowchart showing the process of creating a density conversion table. The density conversion table is created (generated) by forming a gradation correction pattern (pattern image) on each of the intermediate transfer body 106 and the recording material 110, and based on the relationship between the reading results of each gradation correction pattern. It is preferable to create the density conversion table when the image of that type of recording material is formed and output for the first time by the image forming device 100. The density conversion table is a lookup table that is created and stored for each type of recording material.

The density conversion table links the image density on the image carrier with the image density of the corresponding type of recording material. Since the density conversion table links the image density of the image on the recording material 110 with the image density of the image on the image carrier, the execution timing may be determined separately, or a button may be provided for the user to instruct execution. It is preferable that the density conversion table be regenerated in response to various conditions. For this reason, the density conversion table is regenerated at an appropriate timing based on a regeneration judgment, which will be described later.

When the process of creating the density conversion table is started, the CPU 314 controls the engine unit 1011 via the engine control unit 102 to form the gradation correction pattern 1104 (first pattern image) illustrated in FIG. 6 on the recording material 110 (S71). The CPU 314 controls the line sensor 138 via the engine control unit 102 to read the gradation correction pattern 1104 formed on the recording material 110 (S72). The reading result (sensor signal value) of the gradation correction pattern 1104 by the line sensor 138 is transmitted to the CPU 314 via the engine I/F 319. The CPU 314 converts the luminance value included in the sensor signal value into a density value.

The CPU 314 controls the engine unit 1011 via the engine control unit 102 to form the gradation correction pattern 1061 (second pattern image) illustrated in FIG. 5 on the intermediate transfer body 106 (S73). The CPU 314 controls the image density sensor 117 via the engine control unit 102 to read the gradation correction pattern 1061 formed on the intermediate transfer body 106 (S74). The reading result (sensor signal value) of the gradation correction pattern 1061 by the image density sensor 117 is transmitted to the CPU 314 via the engine I/F 319. The CPU 314 converts the luminance value included in the sensor signal value into a density value.

The CPU 314 creates a density conversion table based on the relationship between the density values obtained from the reading results of the gradation correction pattern 1104 and the density values obtained from the reading results of the gradation correction pattern 1061 (S75). Note that although the gradation patches of the gradation correction pattern 1104 and the gradation correction pattern 1061 have the same gradation value, this is not necessary. If the gradation patches of the gradation correction pattern 1104 and the gradation correction pattern 1061 have different gradation values, the density conversion table is created by linearly interpolating the density values obtained from the respective reading results and associating the density values after linear storage.

The image forming apparatus 100 of this embodiment uses a test image for tone correction (tone correction patterns 1104, 1061) to generate a density conversion table. Not limited to this, the test image for tone correction and the image (pattern image) for generating the density conversion table may be different images. The image (pattern image) for generating the density conversion table may be, for example, an image with fewer gradations than the test image for tone correction. In addition, the image forming apparatus 100 of this embodiment forms the tone correction pattern 1104 and the tone correction pattern 1061 separately to generate the density conversion table, but one type of pattern image may be detected by the density sensor 117 and the line sensor 138. In this case, after the pattern image formed on the intermediate transfer body 106 is transferred to the recording material 110, the line sensor 138 reads the pattern image on the recording material 110. Then, the density conversion table is generated based on the detection result of the line sensor 138 and the detection result of the density sensor 117.

Figure 8 is an explanatory diagram of the density conversion table. The density conversion table shows the relationship (conversion conditions) between the reading result (density value) of the gradation correction pattern 1061 on the intermediate transfer body 106 by the image density sensor 117 and the reading result (density value) of the gradation correction pattern 1104 on the recording material 110 by the line sensor 138. The created density conversion table is stored in the table storage unit 311. The density value A obtained by reading a specified image with the line sensor 138 is converted into the density value A', which is the reading result of the image density sensor 117, by passing it through the density conversion table.

(Gradation correction processing)
The image forming apparatus 100 executes calibration such as tone correction processing for each page using the recording material 110. The tone correction processing using the intermediate transfer body 106 is as described above. A characteristic feature of the present invention is the tone correction processing performed in real time using the recording material 110.

The gradation correction process, which forms a gradation correction pattern 1104 (detection image) on the recording material 110, is performed by the engine control unit 102, which controls the line sensor 138, and the CPU 314 in cooperation with each other. The gradation correction unit 317 adjusts the gradation characteristics for each recording material 110 on which an image is formed by the printer 101. In other words, the gradation correction unit 317 performs calibration each time a sheet of recording material 110 is passed through. As illustrated in FIG. 6, the gradation correction pattern 1104 of the recording material 110 is formed in a non-image area 1102 of the recording material 110. This makes it possible to perform gradation correction for each sheet of recording material 110.

Figure 9 is a flowchart showing the tone correction process. This tone correction process is performed for each recording material 110 while an image is being formed on the recording material 110 in accordance with a print job.

When the CPU 314 starts the printing process in response to a print job (S81), it determines whether or not the user has instructed gradation correction (on-paper correction) using the recording material 110 (S82).

If on-paper correction is not instructed (S82:N), the CPU 314 sets gradation correction by the intermediate transfer body 106 (image carrier). The CPU 314 controls the engine unit 1011 by the engine control unit 102 at a predetermined timing to form a gradation correction pattern 1061 on the intermediate transfer body 106 (S91). The gradation correction pattern 1061 is formed at intervals of, for example, 20 pages of the recording material 110 on which the user image is formed. When multiple pages of images are formed on the recording material 110, the gradation correction pattern 1061 is formed between the image on the Nth page and the image on the N+1th page. In this case, it is a multiple of 20. Note that the interval at which the gradation correction pattern 1061 is formed is not limited to 20 pages.

The CPU 314 detects the density value from the reading result of the gradation correction pattern 1061 on the intermediate transfer body 106 by the image density sensor 117 (S92). Based on the density value detected from the gradation correction pattern 1061, the CPU 314 creates a γLUT so that the image density of the image formed on the intermediate transfer body 106 becomes the target value (S93). The CPU 314 stores the created γLUT in the table storage unit 311. The γLUT is used for gradation correction processing by the gradation correction unit 317 at the timing of the next image formation.

If on-paper correction is instructed (S82: Y), the CPU 314 checks whether or not a density conversion table has already been stored in the table storage unit 311 (S83). If a density conversion table has not been stored (S83: N), the CPU 314 creates a density conversion table according to the process described in FIG. 7 and stores it in the table storage unit 311 (S84).

If a density conversion table is stored (S83: Y), or if a density conversion table has been created, the CPU 314 controls the printer 101 to form the user image and gradation correction pattern (detection image) specified in the print job on the recording material 110 (S85). As a result, the user image and gradation correction pattern 1104 are printed on the recording material 110, as shown in FIG. 6. At this time, the gradation correction unit 317 performs gradation correction using the γLUT created in the previous gradation correction process.

The CPU 314 detects a density value from the reading result of the gradation correction pattern 1104 (detection image) on the recording material 110 by the line sensor 138 (S86). The CPU 314 converts the detected density value of the gradation correction pattern 1104 on the recording material 110 into a density value of the image on the intermediate transfer body 106 based on the density conversion table stored in the table storage unit 311 (S87).

Based on the density value of the converted gradation correction pattern 1104, the CPU 314 creates a γLUT so that the image density of the image formed on the intermediate transfer body 106 becomes the target value (S88). The CPU 314 stores the created γLUT in the table storage unit 311. The γLUT is used for gradation correction processing by the gradation correction unit 317 at the timing of the next image formation.

After creating the γLUT, the CPU 314 determines whether image formation on the recording material 110 for the number of prints set in the print job has been completed (S89). If image formation on the recording material 110 for the number of prints set in the print job has not been completed (S89: N), the CPU 314 repeats the processes of S85 to S89 until image formation on the recording material 110 for the number of prints set in the print job has been completed. Note that in the process of creating the density conversion table in S84, the gradation correction pattern formed on the recording material 110 is used, so one sheet of recording material 110 is used. Therefore, the process of S85 is an operation for the second sheet. If image formation on the recording material 110 for the number of prints set in the print job has been completed (S89: Y), the CPU 314 ends the gradation correction process.

As described above, the gradation correction pattern 1104 is formed in the non-image area 1102 of the recording material 110. Therefore, the gradation correction process can be performed every time an image is formed. Therefore, the image forming apparatus 100 can maintain appropriate gradation characteristics without stopping a print job for gradation correction. Since the density values converted in the process of S87 correspond to the density values used in the gradation correction process by the intermediate transfer body 106, a γLUT is created in accordance with the gradation correction process by the intermediate transfer body 106.

Figure 10 is an explanatory diagram of the effect of the tone correction process described above. The horizontal axis is the number of sheets of recording material 110 on which an image is formed (number of output sheets), and the vertical axis is an index (D) indicating image density. The output conditions are as follows. The image print signal ratio is 8%. Images are formed on 2000 sheets of recording material 110, which is 128g coated paper. Up to the 1000th sheet, tone correction pattern 1061 is formed on the image carrier (on intermediate transfer body 106) for every 100 sheets, and tone correction process is performed. From sheet 1001 onwards, tone correction pattern 1104 is formed on the recording material 110 for each sheet, and tone correction process is performed.

The solid line represents the image density when the image density of the image on the recording material 110 is converted to the image density of the image on the image carrier, and calibration (tone correction) is performed based on that density information. The dotted line represents the image density when the target gradation of the image on the recording material 110 is determined at the 1001st sheet point and calibration (tone correction) is performed. The dotted line shows a difference from the original target gradation. Since the target gradation of the recording material 110 is determined and corrected at a predetermined point in time when it deviates from the target gradation on the image carrier, it can be seen that the image density of the image output as a whole deviates due to differences in the calibration set for the job when viewed on a job-by-job basis. As described above, the solid line showing this embodiment confirms that correction to the target gradation continues, and is found to be effective.

In this embodiment, tone correction is performed using a density conversion table for performing density conversion from the density value of the image on the recording material 110 to the density value of the image on the image carrier. In this case, the frequency of correction is improved by performing tone correction using the recording material 110. Therefore, tone correction can be performed in a finely detailed manner.

(Decision to regenerate density conversion table)
The density conversion table does not need to be updated (regenerated) if there is no change in the type of recording material or image forming conditions. Also, regenerating the density conversion table is not desirable from the viewpoint of productivity. However, even if the type of recording material is the same, if there is a change in the image forming conditions or fluctuations over time, the image density characteristics may change and the accuracy of gradation correction may decrease. For this reason, the image forming apparatus 100 judges whether to regenerate the density conversion table and regenerates the density conversion table at an appropriate timing.

Three examples of changes in image forming conditions, which are factors that cause the density conversion table (conversion conditions) to be regenerated, will be described below.
The first case is when the development contrast setting, transfer current setting, fixing setting, etc. are changed by user instruction information from a user or a serviceman and set to a target offset from the recommended setting. The development contrast is the difference between the potential of the electrostatic latent image at the time of development and the development bias applied to the developer 112. The transfer current is the current flowing through the transfer roller 114 when the toner image is transferred. The transfer current is one of the transfer conditions when the toner image is transferred to the recording material 110. The fixing setting is the fixing temperature and the pressure at which the fixing device 150 fixes the toner image to the recording material 110. The fixing setting is one of the fixing conditions of the toner image to the recording material 110. For example, in order to adjust the glossiness of the image formed on the recording material 110, the fixing condition is controlled (fixing control) when the temperature setting during the fixing process is adjusted. When the setting is changed in this way, even if the image density of the image on the intermediate transfer body 106 does not change, the image density of the image formed on the recording material 110 changes, so the inclination of the density conversion table changes.
The second is when the dither is changed. When the dither is changed, the density after fixing changes due to the dot gain depending on the number of lines and dither shape (dots, lines) even with the same toner amount. This causes the gradient of the density conversion table to change.
The third case is when the single-sided/double-sided printing setting is changed in the print job. When the single-sided/double-sided setting is changed, the number of times the recording material 110 passes through the fixing device 150 changes. As a result, even if the amount of toner applied is the same, the density after fixing changes, and the inclination of the density conversion table changes.

When the above-mentioned tone correction process is performed and the target tone is updated, the density conversion tables for all recording materials that are held are discarded, and new density conversion tables are obtained. This is because the image formation tone value and image formation conditions such as charging and development also change in accordance with the change in the target tone, and the slope of the density conversion table can change regardless of the recording material.

Two examples of temporal changes, which are one of the factors that cause the density conversion table to be regenerated, are explained below. "Temporal" refers to the time that has passed since the density conversion table was created and the number of images formed. In other words, the decision to regenerate the table is made based on the time that has passed since the density conversion table was created and the number of images formed. The density conversion table is regenerated if the time that has passed is greater than a predetermined time or if the number of images formed is greater than a predetermined number.

The first is the deterioration of toner (developer) and parts. When developer deteriorates, the triboelectricity changes, which changes the transfer efficiency and amount of scattering, and the image density after fixing. This causes the slope of the density conversion table to change. When parts deteriorate, for example when the resistance of the intermediate transfer body 106 changes, the transfer efficiency changes, and the image density after fixing changes. This causes the slope of the density conversion table to change. Note that when replacing parts due to changes over time, the slope of the density conversion table can change regardless of the type of recording material, so the density conversion tables for all recording materials that are held are discarded and new density conversion tables are obtained.

The second is a change in the detection characteristics due to deterioration of the light source of the image density sensor 117 or the line sensor 138, window dirt, etc. When the number of sheets of recording material on which images are formed by the image forming apparatus 100 increases, toner scatters inside the image forming apparatus 100 and adheres to the glass of the light receiving surface. Therefore, the image density sensor 117 and the line sensor 138 are controlled to keep the amount of light received constant by periodically adjusting the amount of light, and reading performance up to a certain concentration is guaranteed. However, the accuracy of the highlight and shadow parts changes depending on the absolute amount of light. For example, when the amount of light is large, the amount of light received in the highlight part is likely to saturate. In such a case, even if the relationship between the image density of the image on the intermediate transfer body 106 and the image density of the image on the recording material is constant, the slope of the density conversion table changes due to the change in the detection characteristics of the sensor.

(Gradation correction before regeneration)
If the image forming apparatus 100 has not performed gradation correction for a long time, the printer characteristics in quadrant III shown in FIG. 4 may vary significantly due to the influence of changes in the state of the developer, and the gradation characteristics in quadrant IV may not be linear. If a density conversion table is created in this state, the sampled gradation correction pattern will be biased, and the accuracy of the interpolated portion will decrease. Therefore, if a predetermined time has passed since the previous gradation correction, it is desirable to perform gradation correction before regenerating the density conversion table.

Figure 11 is a flowchart showing the tone correction process that involves regeneration of the density conversion table. As described above, regeneration of the density conversion table is determined according to changes in image formation conditions and fluctuations over time. The same steps as those in the tone correction process in Figure 9 are given the same step numbers.

As with the processes of S81 to S83 in FIG. 9, printing is started and it is determined whether or not a density conversion table exists (S81 to S83). If on-paper correction has not been instructed (S82: N), a process similar to the processes of S91 to S93 in FIG. 9 is performed to create a γLUT (S91 to S93). If a density conversion table has not been stored (S83: N), a process similar to the process of S84 in FIG. 9 is performed to create a density conversion table and store it in the table storage unit 311 (S84).

If a density conversion table is stored (S83: Y), the CPU 314 checks a regeneration flag indicating whether or not to regenerate the density conversion table (S101). If the regeneration flag is set to regenerate the density conversion table (S101: Y), the CPU 314 creates a density conversion table by processing similar to the processing of S84 in FIG. 9 (S84). If the regeneration flag is not set to regenerate the density conversion table (S101: N), the CPU 314 creates a γLUT by processing similar to the processing of S85 to S89 in FIG. 9 (S85 to S89).

Figure 12 is an explanatory diagram of the information used to determine regeneration. In this embodiment, the time the density conversion table was created, the number of images formed (number of outputs) after the density conversion table was created, the development contrast setting, the transfer current setting, the fixing setting, the dithering, the single-sided/double-sided printing setting, the sensor light amount on the image carrier, and the sensor light amount after fixing are used to determine regeneration.

When the time elapsed since the density conversion table creation time and the number of output sheets after the density conversion table creation exceed a predetermined value, it is determined that the density conversion table should be regenerated. In this embodiment, thresholds of one day or more and 50,000 sheets or more are set, respectively. When the time elapsed is one day or more or the number of output sheets is 50,000 or more, a density conversion table regeneration flag is set.

When a target offset from the recommended settings is set for the development contrast setting, transfer current setting, and fixing setting based on user instruction information from the user or service technician, it is determined that the density conversion table is to be regenerated. When a target offset from the recommended settings is set, a density conversion table regeneration flag is set. User instruction information is input from the operation panel 180.

If the dithering is changed by the user or service technician, it is determined that the density conversion table should be regenerated. If the dithering is changed, the density conversion table regeneration flag is set. If the print job is switched between single-sided and double-sided printing, it is determined that the density conversion table should be regenerated. If the setting is changed between single-sided and double-sided printing, the density conversion table regeneration flag is set.

For the image density sensor light amount and the line sensor light amount, if the light amount output from each sensor changes by more than a predetermined threshold, it is determined that the density conversion table should be regenerated. In this embodiment, a threshold of 10% is set for each. If at least one of the image density sensor light amount and the light amount output from the line sensor changes by more than 10%, a density conversion table regeneration flag is set.

Even if the conditions for each item change, a concentration conversion table with the same conditions is stored in the internal memory, and if the conditions such as elapsed time are not met, the stored concentration conversion table is used without being regenerated.

A method for resetting the density conversion table will be described. When a user or service technician replaces a part, or when a user performs a gradation correction process, the CPU 314 deletes the density conversion table stored in the table storage unit 311. This causes a new density conversion table to be created for the next print job. In this case, it is desirable to perform gradation correction before starting the print job. If gradation correction has not been performed, gradation correction may be performed before obtaining the density conversion table.

As explained above, by determining whether or not to obtain the density conversion table used for tone correction, the downtime required for obtaining it can be reduced, and by obtaining it at the appropriate time, a decrease in tone correction accuracy can be suppressed.

Figure 13 is an explanatory diagram of the effect of regenerating the density conversion table in response to changes in image formation conditions and fluctuations over time. In Figure 13 (a), the horizontal axis is the number of output sheets, and the vertical axis is an index (D) representing image density. The output conditions are as follows. The image print signal rate is 8%, and the recording material 110 is the same glossy paper. In addition, a gradation correction pattern is formed on the recording material 110 and gradation correction processing is performed up to the 500th sheet, and then the fixing temperature is set to be increased to adjust the glossiness, and a gradation correction pattern is formed on the recording material 110 and gradation correction processing is performed from the 501st sheet onwards. The solid line shows the result of regenerating the density conversion table after changing the setting of the fixing temperature, and represents the image density when calibration (gradation correction) is performed using the regenerated density conversion table. The dotted line represents the image density when calibration (gradation correction) is performed by continuing to use the density conversion table at the time of the first sheet without regenerating the density conversion table.

The reason why the dotted line shows a difference from the original target gradation will be explained. FIG. 13B is an explanatory diagram of a density conversion table showing the relationship between the image density of the image on the intermediate transfer body 106 and the image on the recording material 110. The solid line in FIG. 13B shows the density conversion table at the time of the first sheet, and the dotted line shows the density conversion table at the time of the 501st sheet after the setting of the fixing temperature is changed. It can be seen that the image density of the dotted line is higher than that of the image on the recording material 110, even though they are images on the same intermediate transfer body 106, due to the increase in the fixing temperature. If the density conversion table is not regenerated, the image density detected from the recording material 110 will be higher due to the density conversion error, so the γLUT is adjusted to lower the image density. Therefore, the dotted line in FIG. 13A will be lower than the original target gradation. The effectiveness of this embodiment is shown above.

Second Embodiment
In the first embodiment, a decision is made to regenerate the density conversion table based on the conditions linked to the density conversion table. However, even if the conditions are not met, there may be cases where the density conversion table needs to be regenerated. In the second embodiment, data for comparison with the density conversion table is obtained for the first sheet in a job or periodically, and the density conversion table is compared with the comparison data. If the difference in the comparison results is large, the density conversion table is updated. For the comparison data, a density conversion table created at a different time from the density conversion table used for the previous gradation correction is used. This enables more accurate gradation correction.

Figure 14 is a flowchart showing the tone correction process of the second embodiment. The same steps as those in the tone correction process of Figure 9 are given the same step numbers. As with the processes of S81 to S83 in Figure 9, printing starts and it is determined whether or not a density conversion table exists (S81 to S83). If on-paper correction has not been instructed (S82:N), a process similar to the processes of S91 to S93 in Figure 9 is performed to create a γLUT (S91 to S93). If a density conversion table has not been stored (S83:N), a process similar to the process of S84 in Figure 9 is performed to create a density conversion table and store it in the table storage unit 311 (S84).

If a density conversion table is stored (S83: Y), the CPU 314 determines whether the first page of the print job is being processed or whether a predetermined number of pages have been printed (S1301). If the first page of the job is not being processed or the predetermined number of pages have not been printed (S1301: N), the CPU 314 creates a γLUT by processing similar to S85-S88 in FIG. 9 (S85-S88).

When the first sheet of the job is being processed or a predetermined number of sheets are being printed (S1301: Y), the CPU 314 detects the density value of the gradation correction pattern formed on the recording material 110 by processing similar to S85 and S86 in FIG. 9 (S1302, S1303).

The CPU 314 controls the engine unit 1011 via the engine control unit 102 to form a gradation correction pattern on the intermediate transfer body 106 (S1304). This is not to create a density conversion table but to determine whether to regenerate. For this reason, the gradation correction pattern formed here should have fewer gradations than the gradation correction pattern 1061 for creating a density conversion table shown in FIG. 5. By using only specific gradations, toner consumption can be reduced. The CPU 314 controls the image density sensor 117 via the engine control unit 102 to read the gradation correction pattern formed on the intermediate transfer body 106. The CPU 314 detects the density value of the gradation correction pattern based on the detection result by the image density sensor 117 (S1305).

The CPU 314 creates a comparison table (S1306). The CPU 314 creates a density conversion table using the density values detected from the gradation correction pattern on the intermediate transfer body 106 in the process of S1305 and the density values detected from the gradation correction pattern on the recording material 110 in the process of S1303. At this time, the CPU 314 selects the density values of the gradation correction pattern formed on the recording material 110 corresponding to the density values of the gradation correction pattern formed on the intermediate transfer body 106, and creates the comparison table.

The CPU 314 compares the original density conversion table with the comparison table (S1307). FIG. 15 is an explanatory diagram of the comparison process between the original density conversion table and the comparison table. The CPU 314 compares the original density conversion table (solid line) with the comparison table (x mark) for each gradation to calculate the differences Δ1 to Δ4. If the result of the comparison shows that the sum of the differences Δ1 to Δ4 exceeds the threshold, the CPU 314 determines to update the density conversion table (S1307: Y). As shown by the dotted line in FIG. 15, the CPU 314 updates the density conversion table by propagating the difference ratio between the original density conversion table and the comparison table to other gradations (S1308). If the sum of the differences Δ1 to Δ4 does not exceed the threshold, the CPU 314 determines not to update the density conversion table (S1307: N). As soon as the creation of the density conversion table is completed, the CPU 314 resets the specified number counter (S1309).

The CPU 314 determines whether image formation has been completed (S89) in the same manner as in the process of S89 in FIG. 9. If image formation on the recording material 110 for the number of prints set in the print job has not been completed (S89: N (on-paper correction instruction present)), the CPU 314 repeats the process from S1301 onwards until image formation on the recording material 110 for the number of prints set in the print job has been completed. If image formation on the recording material 110 for the number of prints set in the print job has been completed (S89: Y), the CPU 314 ends the gradation correction process.

As explained above, by determining whether or not to obtain the density conversion table used for tone correction, the downtime required for obtaining it is reduced. In addition, by obtaining the density conversion table at the appropriate time, it is possible to suppress a decrease in the accuracy of tone correction.

In the image forming apparatus 100 of the first and second embodiments, when on-paper correction is enabled, both on-paper correction and gradation correction by the intermediate transfer body 106 (image carrier) may be performed. In this case, a gradation correction pattern 1104 is formed on each page of the recording material 110, and a gradation correction pattern 1061 is formed at a predetermined page interval.

In the first and second embodiments, a case where gradation correction is performed as the calibration has been described, but the present invention is not limited to this. The present invention is effective for calibrations that are performed using different methods using the recording material 110 and the image carrier.

Claims (11)

an image forming unit having an image carrier on which an image is formed, a transfer unit for transferring the image formed on the image carrier to a sheet, and a fixing unit for fixing the image on the sheet, and forming an image on the sheet based on an image density condition for controlling an image density;
a tray onto which the sheet is discharged;
a conveying means for conveying the sheet from the fixing means to the tray;
a detection means for detecting a pattern image on the image carrier;
a reading unit arranged downstream of the fixing unit in a sheet conveying direction by the conveying unit, the reading unit reading a pattern image on the sheet;
a control unit for generating the image density conditions;
the control unit generates the image density condition based on a detection result by the detection unit of the pattern image on the image carrier formed by the image forming unit,
The control means converts the reading result of the pattern image on the sheet formed by the image forming means by the reading means based on conversion conditions for converting the reading result by the reading means of the pattern image on the sheet formed by the image forming means into a detection result by the detection means, and generates the image density conditions based on the converted reading result. the image forming means has an image processing means for converting image data based on the image density condition,
2. The image forming apparatus according to claim 1, wherein said image forming means forms an image based on image data converted by said image processing means. The image forming apparatus according to claim 2, characterized in that the image density conditions include a gradation correction table. The image forming device according to claim 1, characterized in that the conversion conditions vary depending on the type of sheet. The image forming apparatus according to claim 1, characterized in that the pattern image is formed in the outer edge area of the sheet to be cut. The image forming device according to claim 1, characterized in that the pattern image includes a plurality of test images of different gradations. an image forming unit having an image carrier on which an image is formed, a transfer unit for transferring the image formed on the image carrier to a sheet, and a fixing unit for fixing the image on the sheet, and forming an image on the sheet based on an image density condition for controlling an image density;
A tray on which the sheet is placed;
a conveying means for conveying the sheet from the fixing means to the tray;
a detection means for detecting a pattern image on the image carrier;
a reading unit arranged downstream of the fixing unit in a sheet conveying direction by the conveying unit, the reading unit reading a pattern image on the sheet;
a control unit for generating the image density conditions;
the control means generates the image density condition based on a detection result by the detection means of a pattern image formed on the image carrier by the image forming means,
The control means determines a relationship between the detection result of the pattern image on the image carrier detected by the detection means and the reading result of the pattern image on the sheet read by the reading means, and generates the image density condition based on the relationship and the reading result of the pattern image on the sheet by the reading means. the image forming means has an image processing means for converting image data based on the image density condition,
8. The image forming apparatus according to claim 7, wherein said image forming means forms an image based on the image data converted by said image processing means. The image forming apparatus according to claim 8, characterized in that the image density conditions include a gradation correction table. The image forming apparatus according to claim 7, characterized in that the pattern image is formed in the outer edge area of the sheet to be cut. The image forming device according to claim 7, characterized in that the pattern image includes a plurality of test images of different gradations.

Images