Image forming apparatus

Abstract

To solve the problem in which: with a correction reflection rate being a fixed value, density unevenness in a main scanning direction cannot be accurately corrected.SOLUTION: An image forming apparatus has: a control unit 110 that causes a printer unit 100B to form patterns 91 and 92 based on a plurality of gains and a density profile in a main scanning direction detected from a pattern 50, and based on a result of reading of the patterns 91 and 92 performed by a reader unit 100A, determines a gain for determining an exposure correction amount ΔLPW; and a printer control unit 109 that controls the amount of exposure of an exposure device 3 based on the exposure correction amount ΔLPW.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to shading correction for correcting density unevenness in the main scanning direction. [Background technology]

[0002] In electrophotographic image forming apparatuses, density unevenness in the main scanning direction of an output image can occur due to various factors in the image formation process (e.g., uneven sensitivity of the photosensitive drum and uneven exposure by the exposure device). A known technique for correcting such density unevenness in the main scanning direction of an output image is a control that corrects density unevenness in the output image by correcting the light amount of an exposure device based on density unevenness detected using a detection pattern. This control is called laser power main scanning shading (hereinafter referred to as LPWSHD).

[0003] Patent Document 1 proposes a method in which multiple test patterns with different image densities are formed on a recording sheet, and each test pattern formed on the recording sheet is read by an image reader to obtain density data for each test pattern and calculate the average image density for each test pattern. Then, from these test patterns, the one closest to the target image density is selected. Based on the density data of the selected test pattern, an exposure correction value is calculated to determine the amount of light intensity correction in the optimal density range.

[0004] Another known technique for correcting density unevenness in the main scanning direction is control that corrects density unevenness by generating multiple conversion conditions corresponding to different positions in the main scanning direction. This control is called main scanning shading using a lookup table (hereinafter referred to as LUTSHD).

[0005] Patent Document 2 proposes a technique for achieving both prevention of false contours due to overcorrection and high correction accuracy by increasing the correction reflection rate in LUTSHD according to the number of test patterns. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-2981 [Patent Document 2] Patent Publication No. 2021-24249 Summary of the Invention [Problem to be solved by the invention]

[0007] In the technology of Patent Document 1, in order to calculate the exposure correction amount, the correspondence between the difference from the target density and the light intensity correction amount of the exposure device is predetermined, so depending on the state of the image forming device, there is a possibility that density unevenness in the output image after correction cannot be corrected with high precision.

[0008] Furthermore, in the technique of Patent Document 2, the correction reflection rate depends on the number of test patterns, so there is a possibility that density unevenness in the output image after correction cannot be corrected with high precision.

[0009] In view of the above problems, an object of the present invention is to correct density unevenness in the main scanning direction with high precision. [Means for solving the problem]

[0010] The image forming apparatus of the present invention has a photosensitive member, an exposure unit that exposes the photosensitive member to light to form an electrostatic latent image, and a developing unit that develops the electrostatic latent image, and includes an image forming unit that forms an image on a sheet, a reading unit that reads a pattern image formed on the sheet for detecting density in a scanning direction in which light from the exposure unit scans the photosensitive member, a detecting unit that causes the image forming unit to form a first pattern image, causes the reading unit to read the first pattern image, and detects information about density in the scanning direction based on the reading result of the first pattern image by the reading unit, and a detecting unit that causes the image forming unit to detect the density in the scanning direction based on the reading result of the first pattern image by the reading unit. and a control means for controlling the density unevenness in the scanning direction of the image formed by the image forming means based on the information and the third correction conditions, causing the image forming means to form a second pattern image based on the detection result of the detection means and second correction conditions different from the first correction conditions, causing the reading means to read the second pattern image and the third pattern image, determining third correction conditions based on the reading result of the second pattern image by the reading means and the reading result of the third pattern image by the reading means, and controlling the density unevenness in the scanning direction of the image formed by the image forming means based on the information and the third correction conditions.

[0011] Another image forming apparatus of the present invention has a photosensitive member, an exposure unit that exposes the photosensitive member to light to form an electrostatic latent image, and a development unit that develops the electrostatic latent image, and is characterized by having: an image forming unit that forms an image on a sheet; a reading unit that reads a pattern image formed on the sheet to detect density in a scanning direction in which light from the exposure unit scans the photosensitive member; a detection unit that causes the image forming unit to form a first pattern image, causes the reading unit to read the first pattern image, and detects information regarding density in the scanning direction based on the reading result of the first pattern image by the reading unit; and a control unit that causes the image forming unit to form a second pattern image based on the detection result of the detection unit and first correction conditions, causes the image forming unit to form a third pattern image based on the detection result of the detection unit and second correction conditions different from the first correction conditions, receives user instruction information based on the comparison result between the second pattern image and the third pattern image, and controls density unevenness in the scanning direction of the image formed by the image forming unit based on the information and the user instruction information. [Effects of the Invention]

[0012] According to the present invention, density unevenness in the main scanning direction can be corrected with high precision. [Brief explanation of the drawings]

[0013] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Figure 2] Control block diagram of an image forming apparatus [Figure 3] A diagram showing the difference in gamma characteristics depending on the state of the image forming apparatus [Figure 4] Flowchart showing the process of correcting density unevenness [Figure 5] Schematic diagram of a pattern for detecting density unevenness [Figure 6] Illustrative diagram of a screen displayed on the display unit [Figure 7] Table showing the difference in exposure correction amount for different gains [Figure 8]Table showing examples of exposure correction amounts obtained from different gain and density profiles [Figure 9] Schematic diagram of the pattern for determining the amount of correction [Figure 10] A diagram showing the effect of correction processing [Figure 11] An explanatory diagram of the LUT for each position in the main scanning direction [Figure 12] An explanatory diagram of interpolation of correction amount for each pixel [Figure 13] Schematic diagram of other patterns for detecting density unevenness [Figure 14] Concentration profile illustration [Figure 15] An example of density differences for each gradation [Figure 16] Schematic diagram of another pattern for determining the amount of correction [Figure 17] A diagram showing an example of a density profile that may cause false contours. [Figure 18] FIG. 10 is a diagram showing the effect of other correction processes. DETAILED DESCRIPTION OF THE INVENTION

[0014] Example 1 (Image forming device) 1 is a schematic cross-sectional view of an image forming apparatus. Image forming apparatus 100 includes an operation unit 20, an image reading unit (reader unit) 100A that reads an image of an original G, and a printer unit 100B that forms an image based on image data. Operation unit 20 has a display unit 218. Operation unit 20 is connected to control unit 110 and image reading unit 100A. Operation unit 20 is used to accept operations by a user.

[0015] Image forming apparatus 100 is a color laser printer that forms images using yellow (Y), magenta (M), cyan (C), and black (K) developers (toners). Image forming apparatus 100 may be configured as, for example, a printing device, printer, copier, multifunction peripheral (MFP), or facsimile machine. Note that the suffixes Y, M, C, and K at the end of reference numerals indicate that the developer (toner) colors targeted by the corresponding components are yellow, magenta, cyan, and black, respectively. In the following description, reference numerals without the suffixes Y, M, C, and K will be used when it is not necessary to distinguish between colors.

[0016] The image forming apparatus 100 includes four image forming units P (image forming units PY, PM, PC, and PK) that form images using toner of different colors (Y, M, C, and K). The image forming units PY, PM, PC, and PK are included in the printer unit 100B. As shown in FIG. 1, the image forming apparatus 100 is configured as a tandem intermediate transfer color printer in which the image forming units PY, PM, PC, and PK are arranged in order along the direction of movement of the intermediate transfer belt 6. The image forming units PY, PM, PC, and PK have the same configuration except that the colors of toner used by the developing devices 4Y, 4M, 4C, and 4K are different. Note that, for simplicity, reference numerals have been omitted in FIG. 1 for some of the components of the image forming units PM, PC, and PK corresponding to M, C, and K.

[0017] (Leader section) The reader unit 100A includes a document scanner 210 and an automatic document feeder (hereinafter referred to as ADF) 220. The document scanner 210 can perform image reading by "ADF reading" which reads the document G conveyed by the ADF 220, and by "document table reading" which reads the document G placed on the document table glass 102.

[0018] The reader unit 100A includes a light source 103, an optical system 104, and a reading sensor 105. The light source 103 irradiates light onto the original G. The irradiated light is reflected by the original G. The optical system 104 has a lens and the like, and forms an image of the light reflected by the original G on the light-receiving surface of the reading sensor 105. The reading sensor 105 is, for example, a CCD (Charge-Coupled Device) sensor, and receives the reflected light that forms an image on its light-receiving surface. The reader unit 100A generates image data representing an image of the original G according to the reflected light received by the reading sensor 105, and transmits the generated image data to the printer unit B. The light source 103, the optical system 104, and the reading sensor 105 are integrally configured as a reading unit, and move in the direction of the arrow shown in FIG. 1 . As a result, the entire image of the original G is read by the reading sensor 105.

[0019] The reading sensor 105 outputs a luminance value corresponding to the received reflected light. When shading is performed, the luminance value is converted into a density value by the image processing unit .

[0020] (Printer section) The printer unit 100B forms an image based on the image data generated by the reader unit 100A. The printer unit 100B can also form an image based on image data received from an external device via a network or telephone line.

[0021] The printer unit 100B includes a control unit 110 that controls the overall operation of the image forming apparatus 100. The control unit 110 includes a CPU 111, a RAM 112, and a ROM 113. The printer unit 100B further includes a printer control unit 109 that controls the image forming operation (printing operation) using the image forming units PY, PM, PC, and PK.

[0022] The image forming unit P includes a photosensitive drum 1 (photoconductor), and, arranged around the photosensitive drum 1, a charging device 2, an exposure device 3, a developing device 4, a potential sensor 5, a primary transfer roller 7, and a cleaning device 8. The photosensitive drum 1 rotates in the direction of arrow R1. The charging device 2 charges the surface of the photosensitive drum 1 to a predetermined potential.

[0023] The exposure device 3 emits a laser beam (light beam) based on an input image signal (input image data) and exposes the photosensitive drum 1 by scanning the surface of the photosensitive drum 1 with the laser beam. As a result, an electrostatic latent image is formed on the photosensitive drum 1 based on the input image data. The exposure device 3 has a rotating polygon mirror for scanning the laser beam. The polygon mirror deflects the laser beam so that the laser beam scans the surface of the photosensitive drum 1 when it is irradiated onto one of its multiple reflecting surfaces. The exposure device 3 functions as an exposure unit that exposes the photosensitive drum 1 to light in order to form an electrostatic latent image.

[0024] The developing device 4 develops the electrostatic latent image on the photosensitive drum 1 by adhering toner to the electrostatic latent image. This forms a toner image on the photosensitive drum 1. A potential sensor 5 is provided near the photosensitive drum 1, between the exposure position of the exposure device 3 and the developing device 4. The potential sensor 5 can detect the potential of the electrostatic latent image formed on the photosensitive drum 1.

[0025] The primary transfer roller 7 presses against the inner surface of the intermediate transfer belt 6 to form a primary transfer nip T1 between the photosensitive drum 1 and the intermediate transfer belt 6. When a transfer bias voltage is applied to the primary transfer roller 7, the primary transfer roller 7 transfers the toner image on the photosensitive drum 1 to the intermediate transfer belt 6. The cleaning device 8 collects toner remaining on the photosensitive drum 1 after the toner image has been transferred to the intermediate transfer belt 6.

[0026] The four-color toner images formed on the photosensitive drums 1Y, 1M, 1C, and 1K in the image forming units PY, PM, PC, and PK, respectively, are transferred in order onto the intermediate transfer belt 6 in a superimposed manner (primary transfer). As a result, a multi-color toner image consisting of Y, M, C, and K is formed on the intermediate transfer belt 6.

[0027] The intermediate transfer belt 6 is supported by a tension roller 61, a drive roller 62, and an opposing roller 63, and is driven by the drive roller 62 to rotate in the direction of arrow R2 at a predetermined speed. As the intermediate transfer belt 6 rotates, the toner image formed on the intermediate transfer belt 6 is transported to a secondary transfer nip T2 between the intermediate transfer belt 6 and a secondary transfer roller 64. The toner image on the intermediate transfer belt 6 is transferred to a sheet S by the secondary transfer roller 64. A cleaning device 68 collects toner remaining on the intermediate transfer belt 6 after the toner image has been transferred from the intermediate transfer belt 6 to the sheet S.

[0028] The sheets S are fed and conveyed from a paper feed cassette 65 in accordance with the timing at which the toner image on the intermediate transfer belt 6 reaches the secondary transfer nip portion T2. ​​The sheets S may also be called recording paper, recording material, recording medium, paper, transfer material, transfer paper, etc. The sheets S are separated one by one by a separation roller 66 and fed to a conveyance path, and conveyed along the conveyance path toward a pair of registration rollers 67. The pair of registration rollers 67 stops the sheets on the conveyance path and waits, and is driven to send the sheets S into the secondary transfer nip portion T2 in accordance with the timing at which the toner image on the intermediate transfer belt 6 is conveyed. As a result, the toner image on the intermediate transfer belt 6 is transferred to the sheets S at the secondary transfer nip portion T2 (secondary transfer).

[0029] The sheet S onto which the toner image has been transferred is transported by a transport belt 10 to a fixing device 11. The fixing device 11 applies heat and pressure to the toner image transferred onto the sheet S, thereby fixing the toner image to the sheet S. After the fixing process by the fixing device 11, the sheet S is discharged outside the image forming apparatus 100 (for example, to an output tray).

[0030] (Control configuration for correcting density unevenness in the main scanning direction using exposure amount) 2 is a block diagram showing an example of the control configuration of the image forming apparatus 100. The printer control unit 109 has a light amount control circuit 190, a pulse width modulation circuit 191, and a pattern generator 192. The image processing unit 108 has a gamma correction circuit 209.

[0031] The light intensity control circuit 190 controls the light intensity (power) of the laser light output from the exposure device 3. The light intensity control circuit 190 determines the light intensity of the laser light output from the exposure device 3 so as to obtain a desired image density in response to a laser drive signal. The light intensity of the laser light corresponds to the exposure amount of the exposure device 3 and is an example of an image formation condition. The pattern generator 192 holds image data for forming a test pattern, which is a pattern image for density measurement, which will be described later.

[0032] The gamma correction circuit 209 converts the input image signal (input value) contained in the input image data into an output image signal (output value) by referring to a gradation correction table (γLUT). The gradation correction table is a conversion table for converting the input value of the image data in order to correct the gradation characteristics of the image formed by the image forming unit P to ideal gradation characteristics. The correspondence between the output image signal and the density level is determined in advance and stored in the ROM 113. The gradation correction table generated based on this correspondence is saved in the gamma correction circuit 209.

[0033] The pulse width modulation circuit 191 generates a laser drive signal based on the light intensity determined by the light intensity control circuit 190 and the image signal converted based on the gradation correction table and output from the gamma correction circuit 209. The laser drive signal is a PWM (pulse width modulation) signal used to modulate the laser light output from the exposure device. The pulse width modulation circuit 191 outputs, as the laser drive signal, a pulse signal having a pulse width (time width) corresponding to the density indicated by the input image signal for each pixel. The laser drive signal has a wide pulse width for high-density pixels, a narrow pulse width for low-density pixels, and an intermediate pulse width for medium-density pixels.

[0034] The exposure device 3 forms an image (electrostatic latent image) on the photosensitive drum 1, in which gradation is expressed by area gradation in accordance with the pulse width of the laser drive signal. Specifically, the laser light source (semiconductor laser) of the exposure device 3 emits light for a period corresponding to the pulse width of the supplied laser drive signal. The laser light source is driven for a longer period of time as the density of the pixel to be formed increases, and for a shorter period of time as the density of the pixel to be formed decreases. As a result, the dot size (area) of the electrostatic latent image formed on the photosensitive drum 1 varies depending on the pixel density. In other words, the exposure device 3 exposes a longer range in the main scanning direction for high-density pixels and a shorter range in the main scanning direction for low-density pixels. In this embodiment, the main scanning direction is a direction perpendicular to the sheet transport direction, and the sub-scanning direction is the sheet transport direction (direction perpendicular to the main scanning direction).

[0035] Further, a density sensor 12 is provided near the photosensitive drum 1, between the developing device 4 and the primary transfer nip T1. The density sensor 12 is a photosensor for detecting the density of the toner image formed on the photosensitive drum 1. The control unit 110 (CPU 111) can measure the density of the toner image formed on the photosensitive drum 1 using the density sensor 12.

[0036] In this embodiment, the control unit 110 (CPU 111) uses the reader unit 100A to perform "platen reading" or "ADF reading" to acquire measurement data indicating the measurement results of the density of the test pattern and perform processing based on the measurement data. The control unit 110 (CPU 111) also controls the formation of a test pattern for density unevenness correction by the printer unit 100B and the correction of the exposure amount of the exposure device 3 during image formation by the printer unit 100B.

[0037] (Shading function) The image forming apparatus 100 corrects density unevenness in the main scanning direction that occurs in an image (output image) formed by the printer unit 100B using a shading function of the exposure device 3. The exposure device 3 can adjust the intensity (exposure amount) of the laser light output from the laser light source using the shading function. By adjusting (correcting) the light amount of the laser light in accordance with the density unevenness in the main scanning direction that occurs in the output image, it is possible to correct the density unevenness in the main scanning direction that occurs in the output image.

[0038] In the image forming apparatus 100, the image forming area where an image is formed by scanning with a laser beam in the main scanning direction is divided into a plurality of areas at equal intervals in the main scanning direction, and the light intensity setting (LPW) of the exposure device 3 is determined for each divided area. For example, the image forming area is divided into 14 areas A to N (see FIG. 5). The length of each area in the main scanning direction is set to, for example, 23.59 mm. The image forming apparatus 100 corrects the light intensity setting (LPW) for each of the areas A to N. The correction amount of the laser beam intensity (exposure correction amount ΔLPW) used for density unevenness correction is generated for each of the above-mentioned areas (for each of the areas A to N) by a correction process described below.

[0039] The exposure correction amount ΔLPW described above is calculated based on the gamma characteristic of the image forming engine, which corresponds to the density characteristic that indicates the correspondence relationship between the light intensity setting value of the exposure device 3 and the density value of the output image. When the exposure correction amount ΔLPW for each of the above-described regions is determined using a predetermined gamma characteristic, any change in the gamma characteristic of the image forming engine may result in a decrease in the accuracy of density unevenness correction.

[0040] FIG. 3 illustrates the difference in gamma characteristics depending on the state of the image forming apparatus 100. The state of the image forming apparatus 100 refers to the potential characteristics of the photosensitive drum, the charging characteristics of the toner, the mounting tolerances of each component, the potential of the photosensitive drum during image formation, and the exposure amount of the laser beam during image formation. For example, as shown in FIG. 3, if the density difference ΔD to be corrected for density unevenness in the main scanning direction is the same in state A and state B, which have different gamma characteristics, the required exposure correction amount ΔLPW should be different for each state. However, if the exposure correction amount ΔLPW corresponding to the density difference ΔD is determined using a fixed, predetermined gamma characteristic, the light intensity setting value of the exposure device 3 cannot be appropriately controlled, resulting in a decrease in the accuracy of exposure unevenness correction.

[0041] Therefore, the image forming apparatus 100 first outputs a pattern for detecting density unevenness and determines multiple exposure correction amounts ΔLPW based on the measurement results of the pattern.The image forming apparatus 100 then outputs multiple patterns corresponding to the multiple exposure correction amounts ΔLPW and selects an appropriate exposure correction amount ΔLPW from the multiple patterns, thereby accurately correcting the density unevenness.Here, the multiple patterns corresponding to the multiple exposure correction amounts ΔLPW are patterns for determining the correction amount and are used to determine a gain as a correction condition for increasing or decreasing the exposure correction amount.

[0042] (Density unevenness correction process) Next, a correction process for correcting density unevenness in the main scanning direction that occurs in an image (output image) formed by printer unit 100B will be described. This correction process is executed by control unit 110 in image forming apparatus 100.

[0043] Fig. 6 is a view showing an example of an operation screen for accepting execution of the aforementioned correction process. The operation screen is displayed on the display of the display unit 218 of the operation unit 200. By selecting a button displayed on the operation screen using the operation unit 200, the user can instruct the image forming apparatus 100 to execute the process associated with that button. Fig. 4 is a flowchart showing the procedure for the density unevenness correction process. The density unevenness correction process will be described below in accordance with the flowchart.

[0044] First, the control unit 110 causes the display unit 218 to display a screen 231 as shown in Fig. 6(a). A button 241 for instructing printing of a pattern 50 for detecting density unevenness is displayed on the screen 231. When the user presses the button 241 on the screen 231, the control unit 110 controls the printer unit 100B to form the pattern 50 including the Y, M, C, and K band images shown in Fig. 5 on the sheet S (S101).

[0045] FIG. 5 is a schematic diagram of the density unevenness detection pattern 50 output in step S101. As shown in FIG. 5, the density unevenness detection pattern 50 has a constant width in the conveyance direction (also called the sub-scanning direction) in which the sheet S is conveyed, and includes multiple band images extending in a band shape in the main scanning direction. Each of these multiple band images is formed across the entire image formation area in the main scanning direction based on a uniform image signal value. In this embodiment, four band images, one for each color, are arranged in the sub-scanning direction perpendicular to the main scanning direction. Each band image is formed as an image with uniform density if density unevenness does not occur. Note that the image signal for forming each band image has a value indicating a density level of 40% of the maximum density level, for example. Once the density unevenness detection pattern 50 is formed on the sheet S, the control unit 110 causes the display unit 231 to display a screen 232, as shown in FIG. 6B. A button 242 for instructing reading is displayed on the screen 232.

[0046] When the user places sheet S, on which pattern 50 for detecting density unevenness is formed, on reader unit 100A and presses button 242 on screen 232, control unit 110 controls reader unit 100A to read pattern 50 on sheet S (S102). As a result, control unit 110 obtains the reading result of the image (pattern 50) on sheet S by reading sensor 105. Here, the user can select the reading mode by reader unit 100A from "platen reading" and "ADF reading."

[0047] Next, the control unit 110 acquires the density values ​​of regions A to N in the main scanning direction for each band image in the pattern 50 for detecting density unevenness (S103), and stores the density values ​​of regions A to N in the main scanning direction of each band image as a density profile in RAM 112. The density profile is a detection result detected as information related to the density of the pattern 50. In step S103, the signal output from the reading sensor 105 is a signal value (luminance value) corresponding to the luminance, so the control unit 110 acquires the density value converted from the luminance value in the image processing unit 108.

[0048] The following describes a method for correcting density unevenness in a yellow image based on the density profile of the yellow band image among the density profiles of the four-color (Y, M, C, K) band images. Note that the same method applies to correcting density unevenness in images of other colors based on the density profiles of the band images of other colors.

[0049] Based on the density profile stored in RAM 112, the control unit 110 calculates an average density value, which is the average value of the density values ​​of regions A to N, and also calculates the difference (density difference ΔD) between the density value of each region and the average density value (S104). The distribution of the density differences ΔD of each band image corresponds to the distribution of density unevenness in the main scanning direction that occurs in the output image. The density differences ΔD of each region A to N correspond to the amount of density correction in the process of correcting density unevenness in the output image. The average density value is the target value of density in the correction process.

[0050] Next, as shown in FIG. 7, the control unit 110 determines multiple exposure correction amounts ΔLPW (ΔLPW1, ΔLPW2, ΔLPW3, ΔLPW4, ΔLPW5) from the density difference ΔD based on multiple gains (Gain1, Gain2, Gain3, Gain4, Gain5). Here, the exposure correction amount ΔLPW is a correction amount of LPW that is added to LPW during image creation. Then, as shown in FIG. 8, the control unit 110 determines five exposure correction amounts ΔLPW (ΔLPW1, ΔLPW2, ΔLPW3, ΔLPW4, ΔLPW5) for the density difference ΔD between regions A to N (S105). Here, in order to print a pattern for determining the correction amount, the control unit 110 causes the display unit 218 to display a screen 233 as shown in FIG. 6(c). A button 243 for instructing printing of a pattern for determining the correction amount is displayed on the screen 233.

[0051] After the user presses button 243 on screen 233, control unit 110 controls printer unit 110B based on exposure correction amount ΔLPW determined in step S105 so that correction amount determination patterns 91 to 95 (FIG. 9) are printed on sheets S. Here, correction amount determination patterns 91 to 95 are formed on separate sheets S (five sheets in total) for each gain, as shown in FIG. 9(A). Furthermore, each pattern 91 to 95 may have a mark formed thereon to indicate which gain, Gain1 to Gain5, the pattern reflects.

[0052] Furthermore, instead of the patterns 91 to 95 for determining the correction amount, a pattern 96 for determining the correction amount based on a plurality of gains as shown in Fig. 9(B) may be formed on a single sheet S. The pattern 96 for determining the correction amount shown in Fig. 9(B) has five band images corresponding to five gains for each of the four colors (Y, M, C, and K).

[0053] After the patterns 91 to 95 (or pattern 96) are printed, the control unit 110 causes the display unit 218 to display a screen 234 as shown in Fig. 6(d). On the screen 234, a button 244 for instructing automatic adjustment of the correction amount and a button 245 for instructing manual adjustment of the correction amount are displayed so that the user can select them. Then, the control unit 110 determines the method for adjusting the exposure correction amount ΔLPW based on whether the user has selected button 244 or button 245 on the screen 234 (S107).

[0054] When button 244 is pressed on screen 234, control unit 110 determines that automatic adjustment of exposure correction amount ΔLPW has been selected. When automatic adjustment is selected, control unit 110 causes display unit 218 to display screen 235 as shown in FIG. 6(e). Screen 235 has button 246 for instructing reading of patterns for determining the correction amount. When a user places multiple sheets S on reader unit 100A on which patterns 91 to 95 are formed and presses button 244 on screen 234, control unit 110 controls reader unit 100A to read patterns 91 to 95 on sheets S (S108). As a result, control unit 110 obtains the reading results of images (patterns 91 to 95) on sheets S by reading sensor 105. Similarly, when pattern 96 is used, the user places a sheet S on which pattern 96 is formed on the reader unit 100A, and when the user presses button 244 on screen 234, the control unit 110 controls the reader unit 100A to read the pattern 96 on the sheet S.

[0055] Here, the user may select either platen reading or ADF reading of the sheet S on which the patterns 91 to 95 (or pattern 96) are formed by the reader 100A. If platen reading is selected by the user, the reader 100A reads the patterns 91 to 95 (or pattern 96) on the sheet S placed on the platen glass 102. On the other hand, if ADF reading is selected by the user, the reader 100A reads the patterns 91 to 95 (or pattern 96) on the sheet S while transporting the sheet S placed on the tray of the ADF 220. Therefore, ADF reading is suitable when patterns 91 to 95 for determining the correction amount are formed on multiple sheets S.

[0056] 10A shows density profiles calculated from the density values ​​of the regions A to N acquired in step S103. FIG. 10B shows density profiles of the patterns 91 to 95 (or pattern 96) acquired in step S108. In FIG. 10B, profile 1 is the density profile (corrected density profile) of pattern 91 formed based on the exposure correction amount ΔLPW1 determined based on the density profile before correction and Gain 1. In FIG. 10B, profile 2 is the density profile (corrected density profile) of pattern 92 formed based on the exposure correction amount ΔLPW2 determined based on the density profile before correction and Gain 2. In FIG. 10B, profile 3 is the density profile (corrected density profile) of pattern 93 formed based on the exposure correction amount ΔLPW3 determined based on the density profile before correction and Gain 3. In Fig. 10(B), profile 4 is a density profile (corrected density profile) of pattern 94 formed based on an exposure correction amount ΔLPW4 determined based on the density profile before correction and Gain 4. In Fig. 10(B), profile 5 is a density profile (corrected density profile) of pattern 95 formed based on an exposure correction amount ΔLPW5 determined based on the density profile before correction and Gain 5. Note that even when pattern 96 is formed, profiles (profiles 1 to 5) can be created from the reading results of pattern 96.

[0057] The control unit 110 determines (S109) from the multiple corrected density profiles (profiles 1 to 5) which profile has the smallest difference between the maximum and minimum densities, and determines (S111) the exposure correction amount ΔLPW used to generate that profile. For example, in FIG. 10B, the exposure correction amounts ΔLPW1 and ΔLPW2 are insufficient compared to the exposure correction amount ΔLPW3. Therefore, the difference between the maximum and minimum densities of profiles 1 and 2 is smaller than the difference between the maximum and minimum densities of profile 3. Also, for example, in FIG. 10B, the exposure correction amounts ΔLPW4 and ΔLPW5 are excessively large compared to the exposure correction amount ΔLPW3. Therefore, the difference between the maximum and minimum densities of profiles 4 and 5 is also smaller than the difference between the maximum and minimum densities of profile 3. In other words, among the exposure correction amounts ΔLPW1 to ΔLPW5, the exposure correction amount ΔLPW3 is the optimal correction amount.

[0058] Next, a case where manual adjustment is selected in step S107 will be described. When button 245 is pressed on screen 234, control unit 110 determines that manual adjustment of exposure correction amount ΔLPW has been selected. When manual adjustment is selected, control unit 110 causes display unit 218 to display screen 236 as shown in FIG. 6(f). Screen 236 has check boxes 247 for selecting the exposure correction amount ΔLPW that will result in the least density unevenness. The user visually compares patterns 91 to 95 (or pattern 96) for determining the correction amount, and selects the pattern with the least density unevenness from the comparison results using check box 247. After check box 247 is checked, control unit 110 proceeds to step S111 and determines exposure correction amount ΔLPW based on user instruction information regarding the comparison results entered by the user using check box 247.

[0059] After the exposure correction amount ΔLPW is determined in step S111, the control unit 110 ends the processing of the flowchart in Fig. 4. Thereafter, when the printer unit 100B forms an image (output image), the light amount control circuit 190 controls the exposure amount of the exposure device 3 as an image formation condition based on the exposure correction amount ΔLPW for each of the regions A to N determined in step S111. This allows appropriate correction of density unevenness in the main scanning direction of the output image.

[0060] (Effect of density unevenness correction) Using a plurality of states X and Y of the image forming apparatus 100, the effect of correcting exposure unevenness in the main scanning direction that occurs in an output image when the above-described correction process is applied to each state will be described.

[0061] FIG. 10C shows the density profile (before correction) of an image formed by the image forming apparatus 100 in state Y, which is different from state X in FIG. 10A. FIG. 10D shows the density profile (after correction) of the patterns 91-95 (or pattern 96) for determining the correction amount, which are formed based on the density profile in FIG. 10C. Compared to state X, state Y is a state in which, for example, the amount of wear of the photosensitive drum 1 is different or the amount of charge of the toner is different. Therefore, the gradation characteristics (also referred to as density characteristics) of the image formed by the image forming apparatus 100 in state Y are different from the gradation characteristics (density characteristics) of the image formed by the image forming apparatus 100 in state X. As a result, the amount of density change with respect to the exposure correction amount ΔLPW is different. Therefore, if the gain value is a predetermined fixed value, the exposure correction amount ΔLPW cannot be corrected appropriately for each of state X and state Y, and density unevenness in the main scanning direction cannot be corrected with high precision.

[0062] On the other hand, in the correction process described in this embodiment, the optimum correction amount is selected using patterns 91 to 95 (or pattern 96) for determining the correction amount. Therefore, for example, in state X, the exposure correction amount ΔLPW3 is selected, and in state Y, the exposure correction amount ΔLPW2 is selected. Therefore, in the correction process, it is possible to appropriately suppress density unevenness in the main scanning direction in either state X or state Y.

[0063] Example 2 The image forming apparatus 100 described in the first embodiment performs main scanning shading (called LPWSHD), which corrects density unevenness by varying the laser power irradiating multiple areas on the photosensitive drum 1 in the main scanning direction. In addition to LPWSHD, there is another main scanning shading method for correcting density unevenness in the main scanning direction (called LUTSHD), which corrects density unevenness by generating a lookup table corresponding to multiple areas on the photosensitive drum 1 in the main scanning direction. Compared to LPWSHD, LUTSHD can correct density unevenness of different gradations with higher accuracy. The following describes the image forming apparatus 100 that performs LUTSHD as a correction process. Note that components similar to those in the first embodiment are assigned the same numbers, and detailed descriptions thereof will be omitted.

[0064] (Control configuration of image forming device that executes LUTSHD) The control unit 110 converts image data to correct density unevenness in the main scanning direction of the image to be formed. To correct density unevenness in the main scanning direction, the control unit 110 converts the image data based on multiple LUTs (Look Up Tables) corresponding to multiple positions in the main scanning direction (each main scanning position). The LUT is a one-dimensional table that indicates the correspondence between the input image signal (image signal value before conversion) and the output image signal (image signal value after conversion). FIG. 11 is an explanatory diagram of the LUT for each position in the main scanning direction.

[0065] 11(a) is an LUT (image signal value correction table) showing the correspondence between the input image signal (image signal value before conversion) and the output image signal (image signal value after conversion) for each main scanning position when divided into 32 in the main scanning direction. This image signal value correction table is generated based on the reading results of the density unevenness detection pattern for LUTSHD, which will be described later. The density unevenness detection pattern for LUTSHD is formed as a four-level test image in the sub-scanning direction, which is perpendicular to the main scanning direction. The image signal value correction table has a correction table for each of the 32 divided areas in the main scanning direction.

[0066] Figure 11(b) shows the result of linearly interpolating the image signal value correction table of Figure 11(a). Figure 11(c) shows an LUT corresponding to multiple regions in the main scanning direction, created by interpolating between 32 divisions in the main scanning direction of the linearly interpolated image signal value correction table of Figure 11(b). This image signal value correction table is generated for each position in the main scanning direction. Furthermore, these image signal value correction tables are provided for each color. For example, if the length in the main scanning direction in which the image forming apparatus 100 can form an image is 320 mm, the number of pixels will be 15,118 at a resolution of 1200 dpi. When divided into 32 regions in the main scanning direction, the distance between the centers of adjacent regions is 472 pixels. At the center of each region where the image density of the density unevenness detection pattern 51 is measured, the image signal correction amount created based on the image density measurement results is used as is. At other main scanning positions, the correction amounts of the image signals on both sides are interpolated and used. Figure 12 is an explanatory diagram of the interpolation of correction amounts for each pixel. By using the LUTs corresponding to the multiple regions in the main scanning direction in FIG. 11(c), the correction of density unevenness is realized in pixel units.

[0067] However, the density unevenness detection pattern for LUTSHD has a limited number of gradations, and if correction is made using these measurement results, the continuity of density and gradation may be lost in gradations that have not been detected, which could result in false contours appearing in the formed image.

[0068] To prevent this, it is necessary to suppress the correction result using a correction reflection rate as a correction condition. The correction reflection rate is a coefficient that represents the ratio of the image density measurement result to the correction amount. If the correction reflection rate is 100% (coefficient is 1), the converted image signal value is determined so that the difference between the density as the read result and the ideal density becomes 0. Also, if the correction reflection rate is 80%, the converted image signal value is determined so that 80% (coefficient is 0.8) of the difference between the density as the read result and the ideal density is corrected. The correction reflection rate is stored in, for example, ROM 113 and is used when measuring image density.

[0069] In this way, lowering the correction reflection rate can suppress the occurrence of false contours. However, this reduces the effectiveness of density unevenness correction. Therefore, the image forming apparatus 100 forms a pattern for detecting density unevenness for LUTSHD and calculates LUTs corresponding to multiple regions in the main scanning direction for each of multiple correction reflection rates based on the measurement results of the pattern. Then, it outputs a pattern for determining the correction amount, reflecting the LUTs corresponding to the multiple regions in the main scanning direction. By selecting from the pattern an LUT corresponding to the appropriate multiple regions in the main scanning direction, the occurrence of false contours is suppressed and the accuracy of density unevenness correction is improved.

[0070] (Correction of density unevenness by converting image data) Next, a description will be given of a correction process for correcting density unevenness in the main scanning direction that occurs in an image (output image) formed by the printer unit 100B. This correction process is executed by the control unit 110 in the image forming apparatus 100. In the LUTSHD, a pattern 51 is formed on the sheet S as a pattern for detecting density unevenness.

[0071] FIG. 13 is a schematic diagram of a density unevenness detection pattern 51 for LUTSHD. The density unevenness detection pattern 51 is an image (band image) having a constant width in the sub-scanning direction and extending in a band shape in the main scanning direction. These band images are formed across the entire image formation area in the main scanning direction based on uniform image signal values. Each band image is formed as an image with uniform density if density unevenness does not occur. Note that the image signal for forming each band image has values ​​indicating density levels of 25%, 50%, 75%, and 100% of the maximum density level, for example. When the density unevenness detection pattern 50 is formed on the sheet S, the control unit 110 causes the display unit 231 to display a screen 232 as shown in FIG. 6(b). A button 242 for instructing reading is displayed on the screen 232.

[0072] The control unit 110 stores the density values ​​calculated from the density values ​​of regions 1 to 32 (FIG. 13) for each band image as a density profile in RAM 112. The density profile is a detection result detected as information related to the density of pattern 51. Note that the following describes processing of one color (Y) out of the four colors (Y, M, C, K), but processing of the other colors is also performed in the same way. FIG. 14 shows an example of a density profile before correction obtained from pattern 51.

[0073] Based on the density profile stored in RAM 112, the control unit 110 determines an average density value, which is the average value of the density values ​​of regions 1 to 32, and a density difference ΔD, which is the difference between the density value of each region and the average density value. The distribution in the main scanning direction of the density difference ΔD from the average density value in each band image corresponds to the distribution of density unevenness in the main scanning direction that occurs in the output image. The density difference ΔD of regions 1 to 32 corresponds to the density correction amount in the correction process for density unevenness in the output image, and the average density value corresponds to the target density value in the correction process. Figure 15 is an example diagram of the calculated density difference. The calculated density difference is the density unevenness in the main scanning direction. This density unevenness becomes the correction target (required correction amount).

[0074] The control unit 110 generates LUTs corresponding to multiple regions in the main scanning direction based on the density profile and multiple correction reflection rates (60%, 70%, 80%, 90%, 100%). Patterns for determining the correction amount are formed on multiple sheets S based on the LUTs corresponding to the multiple regions in the main scanning direction. FIG. 16 shows an example of a pattern for determining the correction amount for LUTSHD. The pattern 70 for determining the correction amount shown in FIG. 16(A) includes a gradation pattern for each color, and patterns 70 with different correction reflection rates are output on separate sheets S. Each sheet S also has a mark formed thereon to indicate which of the multiple regions in the main scanning direction the LUTs corresponding to are reflected in the pattern 70 formed on that sheet S. Patterns 80 and 81 for determining the correction amount for LUTSHD shown in FIG. 16(B) include eight-tone band patterns for each color, and are formed by reflecting the correction reflection rate of one correction amount on two sheets S. Therefore, the resulting pattern is composed of ten sheets S, each reflecting an LUT corresponding to five main scanning direction regions on two sheets S. The number of gradations in the correction amount determination pattern 70 is greater than the number of gradations in the LUTSHD density unevenness detection pattern 50, allowing the user to visually check whether false contours occur after density unevenness correction. Each pattern also includes marks to indicate which main scanning direction region the LUT corresponds to. Gradation patterns are easier for users to visually determine whether false contours occur than uniform density patterns. Because uniform density patterns have a wider range of identical signal values ​​than gradation patterns, uniform density patterns allow for accurate calculation of density profiles at each gradation level from the scan results. However, gradation patterns may be used for automatic adjustment, or users may visually correct uniform density patterns.

[0075] The control unit 110 creates a density profile based on each correction reflection rate of LUTSHD. Then, for each created density profile, it determines whether or not a false contour occurs. Figure 17 shows an example of a density profile in which a false contour may occur. In the area surrounded by the dotted line in the figure, the density difference between gradation 5 and gradation 6 is below a threshold value (0.05 in this embodiment), raising concerns about the occurrence of a false contour. Therefore, the control unit 110 selects the highest correction reflection rate lower than the correction reflection rate of LUTSHD as the correction reflection rate of LUTSHD. This makes it possible to determine a correction reflection rate that suppresses the occurrence of a false contour and accurately corrects density unevenness in the main scanning direction.

[0076] (Effect of density unevenness correction by converting image data) Next, we will explain the correction effect of suppressing the occurrence of false contours and improving the accuracy of correcting density unevenness when using multiple states α and β as the image forming device 100 and applying LUTSHD correction processing to each state.

[0077] FIG. 18 shows an example of the correction reflection rate and the presence or absence of false contours in the pattern 70 for determining the correction amount for LUTSHD for states α and β. Compared to state β, state α is a state in which, for example, the amount of wear on the photosensitive drum 1 is different, or the amount of toner charge is different. In other words, the gradation characteristics (density characteristics) of the image forming apparatus 100 in state α are different from the gradation characteristics (density characteristics) of the image forming apparatus 100 in state β. Therefore, with a predetermined correction reflection rate, in order to suppress the occurrence of false contours in both state α and state β, the correction reflection rate needs to be set to 70%. However, in state β, further correction is not possible, even though it is possible.

[0078] On the other hand, the correction process can select a correction amount with a higher correction reflection rate while suppressing the occurrence of false contours for each state of the image forming apparatus 100. That is, in state α, the correction reflection rate can be selected as 70%, and in state β, the correction reflection rate can be selected as 90%. Therefore, in both state α and state β, the correction process can correct each density value in the post-correction density profile to a value close to the average density value while suppressing the occurrence of false contours.

[0079] Furthermore, the density profiles described in the first and second embodiments are explained as values ​​obtained by converting the luminance values ​​of the patterns into densities, but a luminance profile may be used instead of the density profile. [Explanation of symbols]

[0080] 100A leader 100B Printer unit 110 control section 50 Pattern for detecting density unevenness 91~95 Pattern for determining the amount of correction

Claims

1. An image forming means for forming an image based on image forming conditions; A reading means for reading a detection image used for detecting density unevenness; a control means for causing the image forming unit to form the detection image, detecting the density unevenness based on a reading result of the detection image read by the reading means, and controlling the image forming conditions based on the detected density unevenness to suppress density unevenness in the image to be formed by the image forming means, the control means causes the image forming means to form a plurality of pattern images based on the detected density unevenness, acquires selection information indicating a pattern image selected from the plurality of pattern images, and controls the image forming conditions based on the selection information; an image forming apparatus, characterized in that the plurality of pattern images are formed under a plurality of different image forming conditions under which the detected density unevenness is suppressed;

2. The image forming means has a photoconductor, an exposure section which exposes the photoconductor to light to form an electrostatic latent image, and a developing section which develops the electrostatic latent image; 2. The image forming apparatus according to claim 1, wherein the detection image is used to detect density unevenness in a main scanning direction in which the light from the exposure unit scans the photoconductor.

3. The image forming apparatus as described in Claim 2, characterized in that the image forming conditions are image forming conditions related to the intensity of the light corresponding to multiple different regions on the photosensitive body in the main scanning direction.

4. The image forming means has an image processing unit that converts image data based on a plurality of conversion conditions corresponding to a plurality of different regions on the photoconductor in the main scanning direction, The image forming means forms the image based on the image data converted by the image processing unit, 3. The image forming apparatus according to claim 2, wherein the image forming conditions are the plurality of conversion conditions.

5. The image forming means has a photoconductor, an exposure unit which exposes the photoconductor to light to form an electrostatic latent image, a developing means which develops the electrostatic latent image, and a conveying means which conveys a sheet, and forms the image on the sheet conveyed by the conveying means; 2. The image forming apparatus according to claim 1, wherein the detection image is used to detect density unevenness in a direction perpendicular to a conveying direction in which the conveying means conveys the sheet.

6. The image forming apparatus of claim 5, characterized in that the image forming conditions are image forming conditions regarding the intensity of the light corresponding to multiple different regions on the photosensitive body in the direction perpendicular to the conveying direction.

7. The image forming means has an image processing unit that converts image data based on a plurality of conversion conditions corresponding to a plurality of different regions on the photoconductor in the direction perpendicular to the transport direction, The image forming means forms the image based on the image data converted by the image processing unit, 6. The image forming apparatus according to claim 5, wherein the image forming conditions are the plurality of conversion conditions.

8. The image forming apparatus described in Claim 1, characterized in that the control means selects an image forming condition from the plurality of different image forming conditions based on the selection information, and controls the image forming condition to the selected image forming condition.

9. The image forming apparatus according to claim 1, characterized in that the reading means reads the detection image formed on a sheet.

10. The image forming apparatus of claim 1, characterized in that the multiple pattern images are formed on a single sheet.

11. An image forming apparatus as described in claim 1, characterized in that the multiple pattern images are formed on multiple sheets.

12. The plurality of pattern images include a first pattern image and a second pattern image, 2. The image forming apparatus according to claim 1, wherein each of the first pattern image and the second pattern image has a plurality of images of different colors.

13. The image forming apparatus of claim 1, wherein the detection image has a plurality of images of different colors.

14. The plurality of pattern images include a first pattern image and a second pattern image, 2. The image forming apparatus according to claim 1, wherein the control means determines image forming conditions for the first pattern image based on the detected density unevenness and a first coefficient, and determines image forming conditions for the second pattern image based on the detected density unevenness and a second coefficient different from the first coefficient.

Images