Image forming device

Abstract

To solve a problem in which: image density cannot be accurately controlled due to an error that occurs in a result of measurement of an image fixed to a sheet.SOLUTION: An image forming apparatus 100 has: an image forming unit 123 that forms an image based on image data representing a user image input to a host I / F unit 302; an intermediate transfer body 106; a transfer roller 114 that transfers the image from the intermediate transfer body 106 to a sheet 110; a fuser 150 that fixes the image to the sheet 110; an image density sensor 117 that measures a test image 1061 on the intermediate transfer body 106; a line sensor 138 that reads a gradation correction pattern 1104 on the sheet 110; and a CPU 314 that forms the test image 1061 and the gradation correction pattern 1104 while a plurality of images are continuously formed on a plurality of sheets, and controls the density of an image to be formed based on a result of measurement performed by the image density sensor 117 and a result of measurement performed by the line sensor 138.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to density control of an image formed by an image forming apparatus. [Background technology]

[0002] A full-color image forming apparatus employing electrophotography forms an image by forming an electrostatic latent image on a photoreceptor, developing the electrostatic latent image, transferring the image to a sheet, and fixing the transferred image to the sheet. The image density of the image formed on the sheet varies depending on environmental conditions such as temperature and humidity, as well as deterioration of the developer used for development. To achieve this, 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 process is called "calibration." Calibration can be performed using the results of reading the test image formed on the sheet, or using the results of reading the test image on the image carrier before it is transferred to the sheet.

[0003] A known example of sheet-based calibration is one that uses an inline sensor located downstream of a fixing unit to read a test image formed on the same sheet as an image (user image) in response to a user's instructions (see Patent Document 1). According to the image forming apparatus described in Patent Document 1, calibration is performed so that the density of the test image printed on the sheet matches the target density, which is believed to enable more accurate control of image density than by reading the test image on the image carrier. The area where the test image is formed is a marginal area where no user image is formed. This marginal area is the area where the outer edge of the sheet is trimmed. Furthermore, this image forming apparatus can reduce downtime by performing calibration without interrupting image formation operations. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-030288 Summary of the Invention [Problem to be solved by the invention]

[0005] However, for example, variations in thickness or color that occur during the sheet manufacturing process can cause errors in the measurement results of the in-line sensor, which can lead to a problem that the density of the output image cannot be controlled with high precision when density control is performed using only the measurement results of the in-line sensor.

[0006] An object of the present invention is to control the density of an image formed by an image forming apparatus with high precision. [Means for solving the problem]

[0007] In order to solve the above problem, the image forming apparatus according to claim 1 , enter a detecting means for detecting a test image formed on the intermediate transfer body by the image forming means; a reading means provided downstream of the fixing means in a conveying direction in which the sheet is conveyed, the reading means reading a pattern image formed on the sheet by the image forming means and fixed by the fixing means; and a control means for controlling image forming conditions of the image forming means to adjust image density based on a detection result of the test image detected by the detecting means and a reading result of a first image of the pattern image read by the reading means. without using the detection result of the test image by the detection means, The pattern image read by the reading means Statue and generating means for generating the conversion conditions based on the reading result. Furthermore, in order to solve the above problem, an image forming apparatus according to another independent claim includes a conversion unit that converts input image data based on conversion conditions, an image forming unit that forms an image based on the image data converted by the conversion unit, an intermediate transfer body onto which the image formed by the image forming unit is transferred, a transfer unit that transfers the image from the intermediate transfer body to a sheet, a fixing unit that fixes the image transferred by the transfer unit to the sheet, a detection unit that detects a test image on the intermediate transfer body that is formed by the image forming unit, a reading unit that is provided downstream of the fixing unit in the conveying direction of the sheet, and that reads a pattern image on the sheet that is formed by the image forming unit and fixed by the fixing unit, and a control unit that controls image forming conditions of the image forming unit to adjust image density based on the detection result of a first image in the test image detected by the detection unit and the reading result of the pattern image read by the reading unit. without using the result of reading the pattern image by the reading means, The test image detected by the detection means Statue and generating means for generating the conversion conditions based on the detection results. [Effects of the Invention]

[0008] According to the present invention, the density of an image formed by an image forming apparatus can be controlled with high precision. [Brief explanation of the drawings]

[0009] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Figure 2] Schematic diagram of the main parts of the image density sensor [Figure 3] Control block diagram of an image forming apparatus [Figure 4] A 4th period chart explaining how the tone of the output image is corrected [Figure 5] Schematic diagram of a user image and a test image formed on an intermediate transfer body [Figure 6] Image density control flowchart [Figure 7]Schematic diagram of a user image and a gradation correction pattern formed on a sheet [Figure 8] Schematic diagram of density conversion table [Figure 9] Feedback coefficient ratio table [Figure 10] Diagram explaining the concept of generating a γLUT [Figure 11] Flowchart of image density control according to another embodiment [Figure 12] Schematic diagram of a correction amount determination table for determining the correction amount of exposure intensity. DETAILED DESCRIPTION OF THE INVENTION

[0010] (First embodiment) 1 is a schematic cross-sectional view of an image forming apparatus 100. The image forming apparatus 100 is composed of a printer 101 and a processing device 600. The image forming apparatus 100 (printer 101) forms an image on a sheet 110 by electrophotography. Note that the printer 101 of this embodiment may be an inkjet printer or a dye-sublimation printer.

[0011] The image forming apparatus 100 includes within the printer 101 mechanisms that constitute an engine unit for image formation, an engine control unit 102 that controls the operation of each mechanism, and a control board housing unit 104 that houses a printer controller 300. An operation panel 180 is provided on top of the printer 101. The operation panel 180 is a user interface, and includes an input device that accepts instructions from the user and an output device that displays screens such as an operation screen. The input device includes various key buttons and a touch panel. The output device includes a display and a speaker.

[0012] 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 sheet 110 feeding processing mechanism, and a sheet 110 transport processing mechanism. The charging / exposure processing mechanism forms an electrostatic latent image by scanning with laser light. The developing processing mechanism visualizes the electrostatic latent image. The transfer processing mechanism transfers the toner image generated by the visualization onto the sheet 110. The fixing processing mechanism fixes the toner image transferred to the sheet 110.

[0013] Each of these mechanisms is composed of image forming units 120, 121, 122, and 123, an intermediate transfer body 106, a fixing unit 150, a paper feed cassette 113, and the like within the printer 101. The image forming units 120, 121, 122, and 123 have the same configuration and perform the same operations, except for the colors of the images they form. The image forming unit 120 forms yellow (Y) images. The image forming unit 121 forms magenta (M) images. The image forming unit 122 forms cyan (C) images. The image forming unit 123 forms black (K) images. The image forming units 120, 121, 122, and 123 each include a photosensitive drum 105, a charger 111, a laser scanner 107, and a developer 112.

[0014] The charging and exposing 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 having 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.

[0015] The laser scanner 107 includes a light-emitting unit 108 that scans a laser beam emitted from a semiconductor laser in one direction, and a reflection mirror 109 that reflects the laser beam from the light-emitting unit 108 toward the photosensitive drum 105. The laser scanner 107 is equipped with a laser driver that drives the laser beam emitted from the light-emitting unit 108 in accordance with image data supplied from the printer controller 300. The laser beam emitted from the semiconductor laser is swung in one direction in accordance with the rotation of a rotary polygonal mirror in the light-emitting unit 108. The laser beam swung in one direction irradiates the photosensitive drum 105 via the reflection mirror 109. As a result, the laser beam scans the surface of the photosensitive drum 105 in one direction (the drum axial direction) to form an electrostatic latent image.

[0016] The developing mechanism visualizes the electrostatic latent image with toner supplied from the developing unit 112 to form a toner image on the photosensitive drum 105. The toner image on the photosensitive drum 105 is transferred to the intermediate transfer body 106. When forming a color image, toner images are sequentially transferred to the intermediate transfer body 106 from the photosensitive drums 105 of the image forming units 120, 121, 122, and 123 corresponding to each color so as to be superimposed on each other. In this embodiment, the intermediate transfer body 106 rotates clockwise in the drawing, and 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 unit 112 are detachable from the housing of the printer 101.

[0017] The transfer mechanism transfers a visible image (toner image) formed on the intermediate transfer body 106 onto a sheet 110 fed from a 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 sheet 110. The toner images transferred onto the intermediate transfer body 106 from each of the image forming units 120, 121, 122, and 123 are transported to the transfer roller 114 as the intermediate transfer body 106 rotates clockwise in the drawing. The sheet 110 is transported to the transfer roller 114 in accordance with the timing at which the toner image is transported to the transfer roller 114. The transfer roller 114 presses the sheet 110 against the intermediate transfer body 106 and simultaneously applies a bias of opposite polarity to that of the toner image, thereby transferring the toner image onto the sheet 110. The feeding processing mechanism includes a paper feed cassette 113 that stores sheets 110, a conveying path along which the sheets 110 are fed, and various rollers for conveying the sheets 110. The sheets 110 are fed from the paper feed cassette 113 and conveyed to a transfer roller 114 along the conveying path.

[0018] The sheet 110 onto which the toner image has been transferred is transported to a fixing mechanism. The fixing mechanism of this embodiment includes a fixing unit 150. The fixing unit 150 includes a fixing roller 151 for heating the sheet 110 in order to thermally fix the toner image onto the sheet 110, and a pressure belt 152 for pressing the sheet 110 against the fixing roller 151. The fixing roller 151 is a hollow roller that has an internal heater and is configured to convey the sheet 110 by rotating. The pressure belt 152 presses the sheet 110 against the fixing roller 151.

[0019] The sheet 110 on which the image has been fixed by the fixing device 150 may be transported to a transport path 131 or a transport path 135. A flapper 132 guides the sheet 110 to either the transport path 131 or the transport path 135. The flapper 132 guides the sheet 110 to the transport path 131 when the sheet 110 on which the image has been fixed is to be discharged face-up, and guides the sheet 110 to the transport path 135 when the sheet 110 on which the image has been fixed is to be discharged face-down. Furthermore, when double-sided printing is instructed, the flapper 132 guides the sheet 110 on which the image has been fixed on the first side to the transport path 135.

[0020] The sheet 110 is conveyed from the conveying path 131 or the conveying path 135 to a sheet conveying path 201 that conveys the sheet 110 to the post-processing device 600 side. Conveying rollers 140 and 141 convey the sheet 110 along the conveying path 201. Line sensors 138 and 139 that read the image on the sheet 110 are provided on the conveying path 201.

[0021] Each of the line sensors 138 and 139 is an optical sensor such as a CMOS line sensor or a CCD line sensor. The line sensor 138 reads one side of the sheet 110, and the line sensor 139 reads the other side of the sheet 110. The line sensors 138 and 139 read an image formed on the sheet 110, which is conveyed along a conveying path 201 by conveying rollers 140 and 141. Each of the line sensors 138 and 139 outputs a read signal including luminance values ​​of red (R), green (G), and blue (B) as the reading result. The luminance values ​​of these read signals are converted into density values ​​of cyan (C), magenta (M), yellow (Y), and black (K). Generally, a density value A is calculated from the luminance value of the red sensor for cyan, the luminance value of the green sensor for magenta, the luminance value of the blue sensor for yellow, and the luminance value of the green sensor for black. At this time, the luminance values ​​are converted into density values ​​A using a look-up table (LUT) 1 that is generated by previously acquiring the relationship between each luminance value of RGB and each density value of CMYK. Such an LUT 1 is stored in advance in a memory in the image forming apparatus 100.

[0022] The conveying path 135 is a path for conveying the sheet 110 to a reversing path 136 used to reverse the front and back sides of the sheet 110. A reversing sensor 137 that detects the sheet 110 is provided in the reversing path 136. When the reversing sensor 137 detects the trailing edge of the sheet 110, the conveying direction of the sheet 110 is reversed in the reversing path 136. The sheet 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 the sheet 110 is conveyed to the conveying path 135, the flapper 133 guides the sheet 110 to the conveying path 135, and further guides it to the conveying path 201 by the flapper 134. As a result, the sheet 110 is reversed (with the side on which the image is formed facing downwards) and is discharged from the printer 101 to the processing device 600. When the sheet 110 is conveyed to the reverse path 139, the sheet 110 is guided to the reverse path 139 by the flapper 133. The sheet 110 guided to the reverse path 139 is reversed and conveyed again to the transfer roller 114. As a result, an image is formed on the reverse side of the sheet 110.

[0023] (Image density sensor) 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 image density sensor 117 is used to measure the image density of a test image 1061 (FIG. 5) formed on the intermediate transfer body during image density control.

[0024] 2 is an explanatory diagram of the configuration of the image density sensor 117. As described above, the image density sensor 117 detects the test image 1061 (FIG. 5) 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.

[0025] 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 1061 (FIG. 5) at the specular reflection angle. The light receiving unit 1173 receives diffusely reflected light from the reflected light of the light irradiated from the LED 1171 onto the intermediate transfer body 106 or the test image 1061 (FIG. 5). The electrical board is mounted with a drive circuit that supplies current to the LED 1171 to cause the LED 1171 to emit light, and a light receiving circuit having an IV conversion function that converts the current generated in accordance with the amount of reflected light received by the light receiving units 1172 and 1173 into a voltage.

[0026] The image density sensor 117 configured as described above can measure both specularly reflected light and diffusely reflected light. A light-receiving unit 1172 that receives specularly reflected light and a 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 1061 (FIG. 5), respectively.

[0027] The voltage output from the image density sensor 117 is input to the engine control unit 102 (FIG. 1). The engine control unit 102 (FIG. 1) converts the output voltage corresponding to the measurement result of the test image 1061 (FIG. 5) into a density value.

[0028] At this time, for the black toner test image 1061 (FIG. 5), the output voltage based on the light reception result of the light receiving unit 1172 is converted into a density value. For the cyan, magenta, and yellow test images 1061 (FIG. 5), the output voltage based on the light reception result of the light receiving unit 1173 is converted into a density value. The engine control unit 102 (FIG. 1) converts the measurement result of the test image 1061 (FIG. 5) into a density value using LUT2 for converting the output voltage into a density value.

[0029] The image density sensor 117 is not limited to the configuration shown in this embodiment. For example, the light receiving unit 1173 may be disposed 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 1061 (FIG. 5) is formed. The light receiving units 1172 and 1173 may each be configured to include a polarizing filter.

[0030] (printer controller) 3 is a control block diagram of the image forming apparatus 100. The printer controller 300 is communicably connected to a host computer 301, which is a device provided external to the image forming apparatus 100. The host computer 301 and the image forming apparatus 100 are communicably connected via a communication line such as USB 2.0 High-Speed, 1000Base-T / 100Base-TX / 10Base-T (IEEE 802.3 compliant), or wirelessly. The printer controller 300 is also communicably connected to a reader 400, which is a device provided external to the image forming apparatus 100.

[0031] The printer controller 300 controls the overall operation of the image forming apparatus 100. To this end, the printer controller 300 is connected to the operation panel 180 and the printer 101. The printer 101 includes an engine control unit 102. The engine control unit 102 controls the operation of each mechanism within the printer 101 in response to instructions from the printer controller 300, and performs an image forming process on a sheet 110.

[0032] Furthermore, the engine control unit 102 transmits the measurement results of the test image 1061 (FIG. 5) obtained by the image density sensor 117 to the printer controller 300. Similarly, the engine control unit 102 transmits the reading results of the gradation correction pattern 1104 (FIG. 7) obtained by the line sensors 138 and 139 to the printer controller 300. The engine control unit 102 includes a CPU (Central Processing Unit).

[0033] 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 unit 302 is a communication interface with the host computer 301. The panel I / F unit 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 printer 101. The input / output buffer 303 is a temporary storage area for sending and receiving control codes and data via each interface.

[0034] The printer controller 300 includes a CPU 314, an image processing unit 200, 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 when it executes processing. The RAM 310 also includes a table storage unit 311 that stores a γLUT, an ICC profile, and a density conversion table (described later).

[0035] The image processing unit 200 includes a RIP (Raster Image Processor) unit 315, a color processing unit 316, a tone correction unit 317, and a halftone processing unit 318. The RIP unit 315 converts image objects (image data) into bitmap images. The color processing unit 316 performs multi-color color conversion processing on the image data converted into bitmap images 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 γLUT is an example of a conversion condition for converting image data. The halftone processing unit 318 performs halftone processing, such as a dither matrix or error diffusion method, on the image data that has been tone-corrected by the halftone processing unit 317. The image data that has been subjected to the halftone processing by the halftone processing unit 318 is sent to the printer 101 via an engine I / F 319. The engine control unit 102 of the printer 101 performs image formation processing based on image data acquired from the engine I / F 319 .

[0036] Each unit of the printer controller 300 described above is connected to a system bus 320 and can communicate via the system bus 320. The CPU 314 updates the ICC profile, γLUT, and density conversion 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.

[0037] (γLUT) FIG. 4 is a quadrant chart illustrating how gradation is reproduced. Quadrant I represents the relationship between the density of the image to be printed (original image) and the input value of image data (density signal) representing that image. Quadrant II represents the conversion characteristics (data characteristics) for converting the density signal into a laser output signal representing the amount of laser light output from the laser scanner 107. Quadrant III represents the gradation characteristics (printer characteristics) of the printer 101, which represent the relationship between the laser output signal and the density of the image formed on the sheet (image density). Quadrant IV represents the relationship between the density of the original image and the density of the image formed on the sheet (image density). The image forming apparatus 100 generates a γLUT for adjusting the data characteristics in quadrant II so that the density of the original image and the image density have an ideal relationship (linear relationship). The image forming apparatus 100 generates a γLUT for each color and stores it in RAM 310.

[0038] The gradation correction unit 317 converts the image data (laser output signal) based on the γLUT. The converted image data is converted into a pulse signal corresponding to the dot width by a pulse width modulation (PWM) circuit of the laser driver and sent to the laser driver that drives and controls the laser scanner 107. By changing the dot area using the laser scanner 107, an electrostatic latent image having desired gradation characteristics is formed on the photosensitive drum 105. This electrostatic latent image is developed and visualized as a toner image.

[0039] Next, the test image 1061 formed on the intermediate transfer belt 106 will be described with reference to Fig. 5. Fig. 5 is a schematic diagram of the test image 1061 of each color formed on the intermediate transfer belt 106 while a user image based on image data transferred from the host computer 301 (or the reader 400) is being formed on the intermediate transfer belt 106. The test image 1061 on the intermediate transfer belt 106 is formed at a position that passes the detection position of the image density sensor 117 as the intermediate transfer body 106 rotates. Therefore, four image density sensors 117 are arranged side by side in a direction perpendicular to the rotation direction of the intermediate transfer body 106.

[0040] The test image 1061 also includes multiple measurement images with different gradation values ​​for each color. Two of each measurement image are formed between images on different pages. The test image in this embodiment is composed of measurement images with 10 gradations. The gradation values ​​are, for example, 0, 16, 32, 64, 86, 104, 128, 176, 224, and 255. Therefore, measurement images with gradation values ​​of 0 and 16 are formed between the user image on the Nth page and the user image on the N+1th page, and measurement images with gradation values ​​of 32 and 64 are formed between the user image on the N+1th page and the user image on the N+2th page. After the user image on the N+4th page is formed, measurement images with gradation values ​​of 244 and 255 are formed. Thus, the image forming apparatus 100 reacquires measurement data for the measurement images with 10 gradations every time five pages of user images are formed. The measurement data for the measurement images with 10 gradations is used to adjust the exposure intensity and generate a γLUT.

[0041] Next, the test image 1104 formed on the sheet 110 will be described with reference to Fig. 7. Fig. 7 is a schematic diagram of the test image 1104 of each color formed on the sheet 110 while a user image based on image data transferred from the host computer 301 (or the reader 400) is being formed. The test image 1104 on the sheet is read by the line sensor 138 (or 139) at the reading position on the conveying path 201.

[0042] The sheet 110 is conveyed in the direction of the arrow (conveyance direction) shown in FIG. 7. The gradation correction pattern 1104 formed on the sheet 110 is formed in an edge region (non-image region 1102) of the sheet 110 in the conveyance direction. In FIG. 7, an image region 1101 on the recording material 110 where a user image is to be formed is indicated by dots. Cutting marks 1103 are also provided on the sheet 110 in advance. The cutting marks 1103 are configured by combining two L-shaped marks and are provided at the four corners of the image region 1101. The sheet 110 is cut at the cutting marks 1103.

[0043] The gradation correction patterns 1104 are formed for each color on one side of the sheet 110. The gradation correction patterns 1104 are also formed on both ends of the sheet 110 in a direction perpendicular to the conveyance direction of the sheet 110. The gradation correction patterns 1104 for two colors are formed in one end region of the sheet 110, and the gradation correction patterns 1104 for the other two colors are formed in the other end region of the sheet 110. The gradation correction patterns 1104 are composed of a plurality of measurement images (10 gradations in FIG. 7) in which the gradation values ​​of each color are gradually changed. The plurality of measurement images are each, for example, a square with a side length of approximately 8 mm, and are arranged in a row in the conveyance direction.

[0044] The measurement images for each color are formed such that measurement images for detecting the texture of the sheet 110 (i.e., measurement images with a gradation value of 0) sandwich other measurement images in the conveyance direction of the sheet 110. Nine measurement images with different gradation values ​​are arranged between gradation patches with a gradation value of 0. When the gradation values ​​are expressed as 0 to 255, the gradation correction pattern 1104 is composed of measurement images for each color with gradation values ​​of 0, 16, 32, 64, 86, 104, 128, 176, 224, 255, and 0. This allows the image forming apparatus 100 to obtain measurement data for 10 gradations of the measurement images each time one sheet of recording material 110 passes the reading position of the line sensor 138 (or 139). The measurement data for the 10 gradations of the measurement images is used to adjust the exposure intensity and generate a γLUT.

[0045] Next, the conversion process of the measurement results of the line sensor 138 (or 139) will be described below. The image forming apparatus 100 obtains a density value A from the reading results of the line sensor 138 (or 139) and converts this density value A into the image density detected by the image density sensor 117 based on a density conversion table. This is because the density value A obtained from the measurement results of the line sensor 138 (or 139) differs from the density value obtained from the measurement results of the image density sensor 117. The image on the recording material 110 is more affected by the transfer process and the fixing process than the image on the intermediate transfer body 106. Therefore, if the target density of the image on the recording material 110 is the same as the target density of the image on the intermediate transfer body 106, it may not be possible to control the image density with high precision.

[0046] Therefore, the image forming apparatus 100 of this embodiment controls the image density read by the line sensor 138 (or 139) after conversion so that the image density detected by the image density sensor 117 becomes the same target density. This configuration eliminates the need to set target densities separately, and makes it possible to suppress the complexity of image density control.

[0047] 8 is a schematic diagram of a density conversion table for converting the reading result (density value) of the gradation correction pattern 1104 on the sheet 110 by the line sensor 138 into the measurement result (density value) of the gradation correction pattern 1104 by the image density sensor 117. The density value A obtained from the reading result of the line sensor 138 is converted into the density value A' obtained from the measurement result of the image density sensor 117 based on the density conversion table. The density conversion table is stored in the table storage unit 311.

[0048] Next, the image density control executed by the CPU of engine control unit 102 and CPU 314 of printer controller 300 in cooperation with each other will be described with reference to the flowchart of Fig. 6. When image data for printing is input via host I / F unit 302, printer controller 300 starts the image forming operation by engine control unit 102. Here, image data representing a user image is subjected to various image processing by image processing unit 200 and then transferred to engine control unit 102.

[0049] First, the engine control unit 102 determines whether or not the exposure intensity needs to be changed (S100). Here, the exposure intensity for forming the user image and the gradation correction pattern is different from the exposure intensity for forming the test image. The exposure intensity for forming the user image and the gradation correction pattern is the exposure intensity used when forming the previous 10-level test image 1061. On the other hand, the exposure intensity for forming the test image is the exposure intensity corrected based on the measurement results of the previous 10-level test image 1061 and the reading results of the gradation correction pattern 1104 stored in RAM 310. This is to prevent a mismatch between the latest γLUT used to form the user image and the exposure intensity. The process of determining the exposure intensity correction amount will be described in detail later.

[0050] If the correction amount is other than 0 in the process of determining the correction amount of exposure intensity in step S100, the engine control unit 102 determines that it is necessary to change the exposure intensity for forming the test image 1061. Note that the exposure intensity for forming the user image and gradation correction pattern is the exposure intensity used when the previously formed test image 1061 was formed. If it is necessary to change the exposure intensity in step S100, the engine control unit 102 changes the exposure intensity for forming the test image 1061 based on the correction amount (S101).

[0051] Next, the engine control unit 102 causes the printer 101 to form a user image and a gradation correction pattern 1104 based on the image data that has been image processed by the image processing unit 200 (S102). As a result, the gradation correction pattern 1104 and the test image 1061 are formed while the printer 101 is continuously forming multiple images on multiple sheets 110. Here, the engine control unit 102 acquires the reading result (read data) of the line sensor 138 (or 139) every time the recording material 110 on which the gradation correction pattern 1104 has been formed reaches the reading position of the line sensor 138 (or 139). Then, the engine control unit 102 transmits the reading result (read data) of the gradation correction pattern 1104 to the printer controller 300.

[0052] The engine control unit 102 causes the printer 101 to form a test image 1061 based on the measurement image data so that the test image 1061 is formed in the area between the user image formed in step S102 and the user image of the subsequent page (S103). Furthermore, the image forming apparatus 100 of this embodiment forms two measurement images for each color each time one page of image is formed. The printer controller 300 selects two measurement image data for forming the test image 1061 in order from the above-mentioned gradation values ​​(0, 16, 32, 48, 64, 86, 104, 128, 176, 224, 255). The printer controller 300 causes the gradation correction unit 317 to convert the measurement image data based on the γLUT and transfers the converted measurement image data to the engine control unit 102. The engine control unit 102 causes the image forming units 120 , 121 , 122 , and 123 to form a test image 1061 on the intermediate transfer belt 106 based on the measurement image data acquired from the printer controller 300 .

[0053] Next, when the measurement image of the test image 1061 reaches the measurement position of the image density sensor 117, the engine control unit 102 controls the image density sensor 117 to measure the measurement image (S104). The engine control unit 102 transfers the measurement results of the test image 1061 by the image density sensor 117 to the printer controller 300 as measurement data. The CPU 314 of the printer controller 300 stores the transferred measurement data in the RAM 310. The CPU 314 then determines whether measurement data for 10 gradations has been collected (S105). In step S105, the CPU 314 does not correct the gradation correction table until measurement data for 10 gradations has been collected. In this case, the engine control unit 102 proceeds to step S108.

[0054] In step S105, the CPU 314 determines that the measurement data is complete each time measurement data is acquired for the test image 1061 with a gradation value of 255. This is to prevent the exposure intensity from being changed midway through the 10-level test image 1061. To prevent image density from being controlled based on measurement data for test images with different exposure intensities, the CPU 314 determines that the measurement data is not complete until measurement data is acquired for the measurement image with a gradation value of 255, which is formed last among the test images 1061.

[0055] Furthermore, if measurement data for 10 gradations is available in step S105, CPU 314 generates a gradation correction table based on the measurement data (S106) and determines the amount of exposure intensity correction based on the measurement results of the measurement image with a gradation value of 255 (S107). In step S105, CPU 314 also determines whether the read results (read data) of gradation correction pattern 1104 are stored in RAM 310. This is because the timing at which recording material 110 reaches the read position of line sensor 138 (or 139) is after the timing at which the test image on intermediate transfer body 106 reaches the detection position of image density sensor 117. Therefore, in step S105, if measurement data for 10 gradations is available and the read results (read data) of gradation correction pattern 1104 are stored in RAM 310, CPU 314 transitions the process to step S106. The method for generating the tone correction table and the method for determining the amount of exposure intensity correction will be described in detail later.

[0056] Next, the engine control unit 102 determines whether or not all images based on the image data have been formed (S108). If all images have not been formed in step S108, the engine control unit 102 shifts the process to step S100.

[0057] Here, the timing at which the engine control unit 102 changes the exposure intensity is not the same as the timing at which the γLUT is updated. This is because if the exposure intensity is changed even though the γLUT has been generated based on the density of the test image 1061, a mismatch will occur between the exposure intensity and the γLUT. If the exposure intensity and the γLUT are changed simultaneously, the image density cannot be controlled appropriately. For this reason, once the γLUT has been generated based on the density of the test image 1061 for 10 levels, the γLUT is updated from the next image to be formed, and the exposure intensity is switched only when the test image is formed.

[0058] Therefore, the exposure intensity for forming the user image and the gradation correction pattern is different from the exposure intensity for forming the test image. The exposure intensity for forming the user image and the gradation correction pattern is the same as the exposure intensity used when forming the previous 10-level test image 1061. The exposure intensity for forming the test image is corrected based on the measurement results of the previous 10-level test image 1061 and the reading results of the gradation correction pattern 1104 stored in RAM 310.

[0059] Furthermore, the exposure intensity of the 10-level test image 1061 is not changed midway. If a γLUT were generated based on the measurement results of the test image 1061 at different levels using different exposure intensities, the γLUT would not be generated appropriately. As a result, the density of the output image would deviate from the target value. Therefore, to prevent a γLUT from being generated based on the density values ​​of the test image 1061 formed using different exposure intensities, the CPU 314 does not change the exposure intensity until measurement data for 10 levels is collected.

[0060] Returning to the description of the flowchart, if all images have been formed in step S108, the engine control unit 102 ends the image forming process including the image density control.

[0061] Next, a method for generating exposure intensity and a γLUT based on the density read by the image density sensor 117 and the line sensor 138 (or 139) will be described. The CPU 314 obtains a density value A from the gradation correction pattern 1104 on the sheet 110. The CPU 314 stores the density values ​​A obtained from the gradation correction patterns 1104 on multiple sheets 110 in the RAM 310. Meanwhile, measurement data for 10 gradations of the test image 1061 is acquired every time five pages of images are formed. Therefore, each time measurement data for 10 gradations of the test image 1061 is completed, the CPU 314 reads from the RAM 310 multiple density values ​​A obtained from the gradation correction pattern 1104 formed based on the previous γLUT. The CPU 314 then obtains an average density of the multiple density values ​​A, converts the average value of the density values ​​A to a density value A' based on a conversion table, and combines the density value A' and density value B to obtain the gradation characteristics (density characteristics) of the printer 101. The CPU 314 determines the amount of exposure intensity correction and generates a γLUT so that the tone characteristics (density characteristics) become ideal tone characteristics.

[0062] The CPU 314 calculates the composite density C based on the density values ​​A and B acquired by the image density sensor 117 and the line sensor 138 (or 139) using the formula (1). C(i)=Fa(i)×A'(i)+Fb(i)×B(i)...Equation (1) Here, i is the number of the measurement image. The i corresponding to the gradation value 0 is 1, and the i corresponding to the gradation value 255 is 10. Furthermore, Fa and Fb are feedback coefficients as determination conditions, A' is a value obtained by converting the density value obtained from the reading result of line sensor 138 (or 139) based on the density conversion table, and B is a density value obtained from the measurement result of image density sensor 117.

[0063] The feedback coefficient in equation (1) is calculated by dividing the value in the table shown in FIG. 9(a) by 100. The values ​​in the table shown in FIG. 9(a) are feedback rates expressed as percentages of the feedback coefficients. Because the line sensor 138 (or 139) reads the fixed toner image, the reading result of the line sensor 138 (or 139) is affected by the thickness and color of the sheet 110. Therefore, the feedback coefficient Fa(i) in the low-density region of the composite density C is set smaller than the feedback coefficient Fa(i) in the high-density region. On the other hand, because the fixed toner image is the density of the final product, the feedback coefficient Fa(i) in the high-density region of the composite density C is set larger than the feedback coefficient Fa(i) in the low-density region. In this way, the CPU 314 calculates the composite density C from the densities of the image density sensor 117 and the line sensor 138 using equation (1) and Table 1.

[0064] FIG. 10 is a diagram illustrating the concept of how the CPU 314 generates a γLUT. The horizontal axis represents gradation values ​​(image signal values), and the vertical axis represents density values. The solid line represents ideal gradation characteristics that indicate the correspondence between image signals and target densities. The dashed line represents actual gradation characteristics calculated by linear interpolation from composite densities C for 10 gradations. To convert the density Dx of the image signal value x to the target density Dxtgt, the image signal value x simply needs to be converted to an image signal value xtgt that corresponds to the target density Dxtgt of the image signal value x. The CPU 314 generates a table that converts the image signal value x to the image signal value xtgt as a γLUT.

[0065] Next, we will explain how to determine the exposure intensity correction amount shown in step S107 of Figure 6. The CPU 314 determines the exposure intensity correction amount based on the difference between the composite density C10 of the gradation value 255 and the target density C10tgt. Note that in the following explanation, the smaller the composite density C10 value, the lighter the density, and the larger the composite density C10 value, the darker the density.

[0066] If the composite density C10 is lighter than the lower limit, the CPU 314 increases the exposure intensity by two levels. In other words, the correction amount of the exposure intensity is +2. When the exposure intensity increases by two levels, the intensity of the laser emitted from the laser scanner 107 increases, and therefore the density of the image formed on the photosensitive drum 102 increases. On the other hand, if the composite density C10 is darker than the upper limit, the CPU 314 decreases the exposure intensity by two levels. The correction amount of the exposure intensity is -2. When the exposure intensity decreases by two levels, the intensity of the laser emitted from the laser scanner 107 decreases, and therefore the density of the image formed on the photosensitive drum 102 decreases.

[0067] Furthermore, if the composite density C10 is higher than the lower limit and lower than the low density threshold, the CPU 314 increases the exposure intensity by one level. In other words, the correction amount of the exposure intensity is +2. If the composite density C10 is lower than the upper limit and higher than the high density threshold, the CPU 314 decreases the exposure intensity by one level. Note that if the composite density C10 is higher than the low density threshold and lower than the high density threshold, the CPU 314 controls the exposure intensity so that it does not change.

[0068] As described above, the gradation correction pattern 1104 is formed in the non-image area 1102 of the sheet 110. Therefore, the image forming apparatus 100 can maintain appropriate gradation characteristics without stopping the print job for gradation correction.

[0069] (Variation) The feedback coefficients used to calculate the composite density described above use different values ​​for each gradation value. Furthermore, the feedback coefficient Fa in the low-density range is lower than the feedback coefficient Fa in the high-density range. However, the feedback coefficients Fa and Fb are not limited to the values ​​in the table of FIG. 9(a). For example, the feedback coefficients Fa and Fb may be calculated by dividing the values ​​in the table shown in FIG. 9(b) by 100. The values ​​in the table shown in FIG. 9(b) are also feedback rates that represent the feedback coefficients as percentages.

[0070] 9(b), the feedback coefficient Fa is higher than the feedback coefficient Fb regardless of the gradation value. This is because the density A obtained from the measurement results of the line sensor 138 (or 139) is closer to the density of the product than the density B obtained from the measurement results of the image density sensor 117, and so the idea is to make the influence of the density A higher than that of the density B.

[0071] Even with this modified image formation, it is possible to maintain appropriate gradation characteristics without stopping the print job for gradation correction.

[0072] (Second embodiment) The image forming apparatus 100 of the first embodiment is configured so that the exposure intensity and γLUT are updated each time measurement data for 10 gradations of the test image 1061 on the intermediate transfer body 106 is completed. However, with this configuration, the test image 1061 is formed between two gradations between multiple user images, resulting in a decrease in productivity of the image forming apparatus 100. Therefore, a configuration in which the test image 1061 is formed between multiple user images one gradation at a time is being considered. In this case, the productivity of the image forming apparatus 100 is increased compared to the first embodiment, but it takes twice as long to complete measurement data for 10 gradations. As a result, the frequency of updating the exposure intensity and γLUT decreases, which may make it impossible to stabilize image density with high precision.

[0073] Therefore, the image forming apparatus 100 of this embodiment generates the exposure intensity and the γLUT when measurement data for five sheets of the gradation correction pattern 1104 on the sheet 110 is collected. If the image forming apparatus 100 is configured to generate the exposure intensity and the γLUT when measurement data for five sheets of the gradation correction pattern 1104 is collected, the productivity of the image forming apparatus 100 can be improved compared to the image forming apparatus 100 of the first embodiment, and the image density can be stabilized with high accuracy.

[0074] Image density control of this embodiment, which is executed by the CPU of engine control unit 102 and CPU 314 of printer controller 300 in cooperation with each other, will be described with reference to the flowchart of Fig. 11. Note that the processes of steps S100, S101, and S108 in the flowchart of Fig. 11 are the same as those in Fig. 6, and therefore will not be described here. Also, while five pages of images are being formed on intermediate transfer body 106, test images 1061 are formed in two gradations between the images formed on each page, and the measurement results of test images 1061 by image density sensor 117 are notified to printer controller 300. The measurement results of test images 1061 are stored in RAM 310.

[0075] The engine control unit 102 causes the printer 101 to form the user image and the gradation correction pattern 1104 on the same sheet 110 (S200). Although not shown in the flowchart of Fig. 11, the printer 101 of this embodiment forms both the gradation correction pattern 1104 and the test image 1061 while continuously forming multiple images on multiple sheets 110.

[0076] Next, when the sheet 110 on which the user image and the gradation correction pattern 1104 are formed is conveyed to the line sensor 138 (or 139), the engine control unit 102 causes the line sensor 138 (or 139) to read the gradation correction pattern 1104 (S201). The reading result of the line sensor 138 (or 139) is notified to the printer controller 300. The CPU 314 of the printer controller 300 obtains a density value A from the reading result and converts the density value A to a density value A' based on the density conversion table LUT1.

[0077] The CPU 314 determines whether the density values ​​A' of the five gradation correction patterns 1104 are complete (S202). If the density values ​​A' of the five gradation correction patterns 1104 are complete in step S202, the CPU 314 generates a γLUT based on the average value of the density values ​​A' of the five gradation correction patterns 1104 without using the measurement results of the test image 1061 (S203). In this embodiment, the test images 1061 are formed one gradation at a time to increase the productivity of the image forming apparatus 100. Therefore, if the γLUT is not updated until measurement data for the test images 1061 for 10 gradations is complete, the image density may deviate from the target density. To generate the γLUT frequently, the CPU 314 generates a γLUT each time measurement data for the five gradation correction patterns 1104 is complete, without using the measurement data of the test image 1061.

[0078] Next, the CPU 314 determines the exposure intensity correction amount based on the average of the latest density value B of the gradation value 255 stored in the RAM 310 and the density value A' of the gradation value 255 (S204). While the measurement data of the test image 1061 was not used in the γLUT in step S203, the exposure intensity correction amount is determined using both the measurement data of the test image 1061 and the measurement data of the gradation correction pattern 1104. This is because exposure intensity has a smaller effect on image density than the γLUT. If the γLUT is determined with high accuracy, density fluctuations caused by deviations in exposure intensity can be suppressed. Therefore, the image forming apparatus 100 of this embodiment uses both the measurement data of the test image 1061 and the measurement data of the gradation correction pattern 1104 when determining the exposure intensity correction amount.

[0079] A method for determining exposure intensity in this embodiment will be described. The CPU 314 sums the calculated value obtained by multiplying the density A' of the gradation correction pattern 1104 with a gradation value of 255 by a feedback coefficient of 0.7 and the calculated value obtained by multiplying the density B of the test image 1061 with a gradation value of 255 by a feedback coefficient of 0.3. This sum corresponds to the composite density C. Then, similar to the method for determining exposure intensity in the first embodiment, the CPU 314 compares the composite density C with multiple threshold values ​​to determine the amount of exposure intensity correction.

[0080] (Third embodiment) In the image forming apparatus 100 of the first and second embodiments, a gradation correction pattern 1104 is formed in the cutting area (edge ​​area 1102) of the sheet 110. This is called real-time mode. However, it is also possible to form an image without using a sheet 110 having an edge area 1102 large enough to form the gradation correction pattern 1104. In this case, the image forming apparatus 100 forms the gradation correction pattern 1104 on the sheet 110 every predetermined number of sheets, and controls the image density by reading the gradation correction pattern 1104 with the line sensor 138 (or 139). This is called interrupt mode. In interrupt mode, no user image is formed on the sheet 110 on which the gradation correction pattern 1104 is formed.

[0081] However, if the specified number of sheets is too small in the interrupt mode, the image forming apparatus 100 will consume a large number of sheets 110 to form the gradation correction pattern 1104. For this reason, the specified number may be set to 20 or more. This increases the interval between formation of the gradation correction pattern 1104, which may reduce the stability of the image density. Therefore, the image forming apparatus 100 of this embodiment controls the image density in the same way as in the second embodiment when the real-time mode is executed, and generates a γLUT based on the density B of the test image 1061 when the interrupt mode is executed.

[0082] The density control of this embodiment will be described below. Each time 105 pages of user images are formed, the CPU 314 can acquire measurement data for 10 gradations of the test image 1061. Once the measurement data for 10 gradations has been acquired, the CPU 314 generates a γLUT based on the measurement data of the test image 1061 without using the measurement data of the gradation correction pattern 1104.

[0083] In interrupt mode, the CPU 314 determines the exposure intensity correction amount based on the average of the density value B of the gradation value 255 and the density value A' of the previous gradation value 255 stored in the RAM 310. Although the measurement data of the gradation correction pattern 1104 was not used in generating the γLUT, the exposure intensity correction amount is determined using both the measurement data of the test image 1061 and the measurement data of the gradation correction pattern 1104. This is because exposure intensity has a smaller effect on image density than the γLUT. If the γLUT is determined with high accuracy, density fluctuations caused by deviations in exposure intensity can be suppressed. Therefore, the image forming apparatus 100 of this embodiment uses both the measurement data of the test image 1061 and the measurement data of the gradation correction pattern 1104 when determining the exposure intensity correction amount.

[0084] A method for determining exposure intensity according to this embodiment will now be described. If the density value A' of the gradation correction pattern 1104 with a gradation value of 255 is less than 1.6, the CPU 314 determines the amount of exposure intensity correction from the density B of the test image 1061 with a gradation value of 255 based on correction amount determination table T1. On the other hand, if the density A' of the gradation correction pattern 1104 with a gradation value of 255 is greater than 1.75, the CPU 314 determines the amount of exposure intensity correction from the density B of the test image 1061 with a gradation value of 255 based on correction amount determination table T2. If the density A' of the gradation correction pattern 1104 with a gradation value of 255 is greater than or equal to 1.6 and less than 1.75, the CPU 314 determines the amount of exposure intensity correction from the density B of the test image 1061 with a gradation value of 255 based on correction amount determination table T0. The correction amount determination tables T1, T2, and T0 correspond to other conditions for determining the correction amount. Here, correction amount determination tables T1, T2, and T0 are shown in FIG.

[0085] (Variation) The image forming apparatus 100 of the third embodiment switches the correction amount determination table for determining the correction amount based on the density value A' detected during the interrupt mode. However, when the interrupt mode is executed, the CPU 314 may determine the correction amount of the exposure intensity from the composite density C of the gradation value 255 based on the correction amount determination table T0.

[0086] In this case, the CPU 314 sums the calculated value obtained by multiplying the density value A' of the gradation correction pattern 1104 with a gradation value of 255 by a feedback coefficient of 0.3 and the calculated value obtained by multiplying the density value B of the test image 1061 with a gradation value of 255 by a feedback coefficient of 0.7. This sum corresponds to the composite density C. Then, similar to the method for determining exposure intensity in the first embodiment, the CPU 314 compares the composite density C with multiple threshold values ​​to determine the amount of exposure intensity correction.

[0087] As described above, according to the present invention, the density of an image formed by image forming apparatus 100 can be controlled with high precision based on both the measurement results of line sensor 138 (or 139) and image density sensor 117.

[0088] Furthermore, in the first to third embodiments, the correction amount of exposure intensity is determined based on the density value A of the gradation correction pattern 1104 with a gradation value of 255 and the density value B of the test image 1061 with a gradation value of 255. However, the gradation values ​​of the first and second measurement images used to determine the correction amount of exposure intensity are not limited to the above-mentioned values. For example, the correction amount may be determined based on the density value of the gradation correction pattern 1104 with a gradation value of 244 and the density value of the test image 1061 with a gradation value of 244. Furthermore, the gradation values ​​do not need to be the same. For example, the correction amount may be determined based on the density value of the gradation correction pattern 1104 with a gradation value of 255 and the density value of the test image 1061 with a gradation value of 128. [Explanation of symbols]

[0089] 302 Host I / F section 123 Image forming unit 117 Image density sensor 138 Line Sensor 314 CPU

Claims

1. a conversion means for converting input image data based on conversion conditions; an image forming means for forming an image based on the image data converted by the converting means; an intermediate transfer body onto which the image formed by the image forming means is transferred; a transfer means for transferring the image from the intermediate transfer body to a sheet; a fixing unit that fixes the image transferred by the transfer unit onto the sheet; a detection means for detecting the test image formed on the intermediate transfer body by the image forming means; a reading unit that is provided downstream of the fixing unit in the conveying direction in which the sheet is conveyed, and that reads a pattern image formed on the sheet by the image forming unit and fixed by the fixing unit; a control means for controlling the image forming conditions of the image forming means to adjust the image density based on the detection result of the test image detected by the detection means and the reading result of the pattern image read by the reading means; a generating unit that generates the conversion conditions based on the reading results of the pattern image read by the reading unit, without using the detection results of the test image by the detecting unit.

2. The pattern image includes a first image and a second image having a different density from the first image, 2. The image forming apparatus according to claim 1, wherein said image forming means forms said first image and said second image on the same sheet.

3. 3. The image forming apparatus according to claim 2, wherein the gradation value of the first image is higher than the gradation value of the second image.

4. the image forming means includes a photosensitive member, an exposure means for exposing the photosensitive member to light in order to form an electrostatic latent image on the photosensitive member, and a development means for developing the electrostatic latent image with toner; the image forming condition is the exposure intensity of the exposure unit, 4. The image forming apparatus according to claim 1, wherein the conversion conditions are a gradation correction table for correcting gradation characteristics of the image formed by the image forming unit.

5. a conversion means for converting input image data based on conversion conditions; an image forming means for forming an image based on the image data converted by the converting means; an intermediate transfer body onto which the image formed by the image forming means is transferred; a transfer means for transferring the image from the intermediate transfer body to a sheet; a fixing unit that fixes the image transferred by the transfer unit onto the sheet; a detection means for detecting the test image formed on the intermediate transfer body by the image forming means; a reading unit that is provided downstream of the fixing unit in the conveying direction in which the sheet is conveyed, and that reads a pattern image formed on the sheet by the image forming unit and fixed by the fixing unit; a control means for controlling the image forming conditions of the image forming means to adjust the image density based on the detection result of the test image detected by the detection means and the reading result of the pattern image read by the reading means; a generating means for generating the conversion conditions based on the detection results of the test image detected by the detecting means, without using the reading results of the pattern image by the reading means.

6. The image forming apparatus described in Claim 5, characterized in that the test image includes a first image and a second image having a density different from that of the first image.

7. 7. The image forming apparatus according to claim 6, wherein the gradation value of the first image is higher than the gradation value of the second image.

8. the image forming means includes a photosensitive member, an exposure means for exposing the photosensitive member to light in order to form an electrostatic latent image on the photosensitive member, and a development means for developing the electrostatic latent image with toner; the image forming condition is the exposure intensity of the exposure unit, 8. The image forming apparatus according to claim 5, wherein the conversion conditions are a gradation correction table for correcting gradation characteristics of the image formed by the image forming unit.

Images