Image forming apparatus

Abstract

Because the correction reflection rate is a fixed value, it is not possible to correct density unevenness in the main scanning direction with high precision. [Solution] The printer unit 100B forms patterns 91 and 92 based on the density profile in the main scanning direction detected from pattern 50 and a plurality of gains, and the control unit 110 determines the gain for determining the exposure compensation amount ΔLPW based on the reading results of patterns 91 and 92 by the reader unit 100A, and the printer control unit 109 controls the exposure amount of the exposure device 3 based on the exposure compensation amount ΔLPW.

Description

Technical Field

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

Background Art

[0002] In an electrophotographic image forming apparatus, density unevenness may occur in the main scanning direction of an output image due to various factors in the image forming process (for example, uneven sensitivity of a photosensitive drum or uneven exposure by an exposure device). As a technique for correcting such density unevenness in the main scanning direction of an output image, control for correcting the density unevenness of the output image by light amount correction of an exposure device based on the density unevenness detected using a detection pattern is known. This control is called main scanning shading by laser power (hereinafter referred to as LPWSHD).

[0003] In Patent Document 1, a plurality of test patterns having different image densities are formed on a recording sheet as a recording medium, each test pattern formed on the recording sheet is read by an image reading unit, density data of each test pattern is acquired, and the average image density of each test pattern is obtained. Then, among these test patterns, the one closest to the target image density is selected. A method of determining the light amount correction amount in an optimal density range by calculating an exposure correction value based on the density data of the selected test pattern has been proposed.

[0004] Also, as a density unevenness correction technique in the main scanning direction, control for correcting density unevenness by generating a plurality of conversion conditions corresponding to different positions in the main scanning direction is known. This control is called main scanning shading by a lookup table (hereinafter referred to as LUTSHD).

[0005] In Patent Document 2, a technique for achieving both prevention of pseudo contours due to overcorrection and correction accuracy by increasing the correction reflection rate according to the number of test patterns in LUTSHD has been proposed.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-2981 [Patent Document 2] Japanese Patent Publication No. 2021-24249 [Overview of the project] [Problems that the invention aims to solve]

[0007] In the technology described in Patent Document 1, the relationship between the difference from the target density and the light intensity correction amount of the exposure apparatus is predetermined in order to determine the exposure compensation amount. Therefore, depending on the state of the image forming apparatus, it was possible that density unevenness in the corrected output image could not be corrected with high precision.

[0008] Furthermore, in the technology described in Patent Document 2, the correction reflection rate depends on the number of test patterns, so there was a possibility that density unevenness in the corrected output image could not be corrected with high accuracy.

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

[0010] The present invention provides an image forming apparatus comprising: a photoreceptor; an exposure unit for exposing the photoreceptor to form an electrostatic latent image; and a developing means for developing the electrostatic latent image; an image forming means for forming an image on a sheet; a reading means for reading a pattern image formed on the sheet for detecting the density in the scanning direction in which light from the exposure unit scans the photoreceptor; a detection means for causing the image forming means to form a first pattern image, causing the reading means to read the first pattern image, and detecting information regarding the density in the scanning direction based on the reading result of the reading means; and a detection means for causing the image forming means to detect the detection means. The system is characterized by comprising: causing a second pattern image to be formed based on the result and a first correction condition; causing the image forming means to form a third pattern image based on the detection result of the detection means and a second correction condition different from the first correction condition; causing the reading means to read the second pattern image and the third pattern image; determining a third correction condition based on the reading result of the second pattern image and the reading result of the third pattern image by the reading means; 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 condition.

[0011] Furthermore, another image forming apparatus of the present invention is characterized by comprising: a photoreceptor; an exposure unit for exposing the photoreceptor to form an electrostatic latent image; and a developing means for developing the electrostatic latent image, wherein the image forming means forms an image on a sheet; a reading means for reading a pattern image formed on the sheet for detecting the density in the scanning direction in which light from the exposure unit scans the photoreceptor; a detection means for causing the image forming means to form a first pattern image, causing the reading means to read the first pattern image, and detecting information regarding the density in the scanning direction based on the reading result of the reading means; causing the image forming means to form a second pattern image based on the detection result of the detection means and a first correction condition, causing the image forming means to form a third pattern image based on the detection result of the detection means and a second correction condition different from the first correction condition, receiving user instruction information based on the comparison result of the second pattern image and the third pattern image, and controlling the density unevenness in the scanning direction of the image formed by the image forming means 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 drawing]

[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 γ characteristics depending on the state of the image forming apparatus. [Figure 4] Flowchart showing the correction process for density unevenness. [Figure 5] Schematic diagram of a pattern for detecting concentration unevenness. [Figure 6] Example diagram of the screen displayed on the display unit. [Figure 7] Table showing the difference in exposure compensation amount depending on different gain levels. [Figure 8]Table showing examples of exposure correction amounts obtained from different gains and density profiles [Figure 9] Schematic diagram of a pattern for determining the correction amount [Figure 10] Figure showing the effect of the correction process [Figure 11] Explanatory diagram of the LUT for each position in the main scanning direction [Figure 12] Explanatory diagram of the interpolation of the correction amount for each pixel [Figure 13] Schematic diagram of a pattern for detecting other density non-uniformities [Figure 14] Exemplary diagram of a density profile [Figure 15] Figure showing an example of the density difference for each gradation [Figure 16] Schematic diagram of another pattern for determining the correction amount [Figure 17] Figure showing an example of a density profile where pseudo-contours may occur [Figure 18] Figure showing the effect of another correction process

Best Mode for Carrying Out the Invention

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

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

[0016] The image forming apparatus 100 comprises four image forming units P (image forming units PY, PM, PC, PK) that form images using toners of different colors (Y, M, C, K). The image forming units PY, PM, PC, PK are included in the printer unit 100B. As shown in Figure 1, the image forming apparatus 100 is configured as a tandem-type intermediate transfer color printer, with the image forming units PY, PM, PC, PK arranged sequentially along the direction of movement of the intermediate transfer belt 6. The image forming units PY, PM, PC, PK employ similar configurations, except that the toner colors used by the developing units 4Y, 4M, 4C, 4K differ. In Figure 1, for simplification, the designation of reference numerals has been omitted for some components of the image forming units PM, PC, PK corresponding to M, C, and K.

[0017] (Leadership Department) 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 transported by the ADF 220, and "document glass reading," which reads the document G placed on the document glass 102.

[0018] The reader unit 100A comprises a light source 103, an optical system 104, and a reading sensor 105. The light source 103 irradiates light onto the document G. The irradiated light is reflected by the document G. The optical system 104 has lenses and the like, and images the reflected light from the document G onto 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 has been imaged onto the light-receiving surface. The reader unit 100A generates image data representing the image of the document 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 Figure 1. As a result, the entire image of the document G is read by the reading sensor 105.

[0019] The reading sensor 105 outputs a brightness value corresponding to the received reflected light. When shading is performed, this brightness value is converted into a density value by the image processing unit 108.

[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 operation of the entire image forming apparatus 100. The control unit 110 includes a CPU 111, RAM 112, and 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 comprises a photosensitive drum 1 (photoreceptor) and 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 arranged around the photosensitive drum 1. 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 apparatus 3 emits a laser beam based on an input image signal (input image data) and exposes the photosensitive drum 1 by scanning its surface with the laser beam. This forms an electrostatic latent image on the photosensitive drum 1 based on the input image data. The exposure apparatus 3 has a rotating polyhedron mirror (polygon mirror) for scanning the laser beam. The polygon mirror deflects the laser beam so that it scans the surface of the photosensitive drum 1 when the laser beam is irradiated onto one of its multiple reflective surfaces. The exposure apparatus 3 functions as an exposure unit that exposes the photosensitive drum 1 to form an electrostatic latent image.

[0024] The developing device 4 develops the electrostatic latent image on the photosensitive drum 1 by depositing toner onto 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 by 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, forming a primary transfer nip T1 between the photosensitive drum 1 and the intermediate transfer belt 6. When a transfer bias voltage is applied, 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 the toner remaining on the photosensitive drum 1 after the toner image has been transferred to the intermediate transfer belt 6.

[0026] In the image forming sections PY, PM, PC, and PK, the four toner images formed on the photosensitive drums 1Y, 1M, 1C, and 1K, respectively, are sequentially transferred onto the intermediate transfer belt 6 (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 at a predetermined speed in the direction of arrow R2. The toner image formed on the intermediate transfer belt 6 is conveyed to the secondary transfer nip T2 between the intermediate transfer belt 6 and the secondary transfer roller 64 as the intermediate transfer belt 6 rotates. The toner image on the intermediate transfer belt 6 is transferred to the sheet S by the secondary transfer roller 64. The cleaning device 68 collects the toner remaining on the intermediate transfer belt 6 after the transfer of the toner image from the intermediate transfer belt 6 to the sheet S.

[0028] Sheet S is fed and transported from the 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 section T2. ​​Sheet S may also be referred to as recording paper, recording material, recording medium, paper, transfer material, transfer paper, etc. Sheet S is separated one by one by the separation roller 66 and fed into the transport path, and transported along the transport path toward the registration roller pair 67. The registration roller pair 67 keeps the sheets on the transport path in a stopped state and is driven to feed the sheets S to the secondary transfer nip section T2 in accordance with the transport timing of the toner image on the intermediate transfer belt 6. As a result, the toner image on the intermediate transfer belt 6 is transferred to the sheet S in the secondary transfer nip section T2 (secondary transfer).

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

[0030] (Control configuration for correcting density unevenness in the main scanning direction based on exposure amount) Figure 2 is a block diagram showing an example of the control configuration of the image forming apparatus 100. The printer control unit 109 includes a light intensity control circuit 190, a pulse width modulation circuit 191, and a pattern generator 192. The image processing unit 108 includes 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 that a desired image density is obtained in response to the 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 the image formation conditions. The pattern generator 192 holds image data for forming a test pattern, which is a pattern image for density measurement 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 the tone correction table (γLUT). The tone correction table is a conversion table for converting the input values ​​of the image data in order to correct the tone characteristics of the image formed by the image forming unit P to ideal tone characteristics. The correspondence between the output image signal and the density level is determined in advance and stored in the ROM 113. The tone correction table generated based on this correspondence is stored 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 output from the gamma correction circuit 209, which has been converted based on the grayscale correction table. The laser drive signal is a PWM (pulse width modulation) signal and is used to modulate the laser light output from the exposure apparatus. For each pixel, the pulse width modulation circuit 191 outputs a pulse signal as the laser drive signal, which has a pulse width (time width) corresponding to the density shown by the input image signal. 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 intermediate-density pixels.

[0034] The exposure apparatus 3 forms an image (electrostatic latent image) on the photosensitive drum 1, in which the gradation is expressed by area gradation according to the pulse width of the laser drive signal. Specifically, the laser light source (semiconductor laser) of the exposure apparatus 3 emits light for a time corresponding to the pulse width of the supplied laser drive signal. The laser light source is driven for a longer time the higher the density of the pixels to be formed, and for a shorter time the lower the density of the pixels to be formed. As a result, the dot size (area) of the electrostatic latent image formed on the photosensitive drum 1 will be different in size according to the density of the pixels. That is, the exposure apparatus 3 exposes a longer area in the main scanning direction for high-density pixels and a shorter area in the main scanning direction for low-density pixels. In this embodiment, the main scanning direction is the direction perpendicular to the sheet transport direction, and the sub-scanning direction is the sheet transport direction (the direction perpendicular to the main scanning direction).

[0035] Furthermore, a density sensor 12 is provided near the photosensitive drum 1, between the developing device 4 and the primary transfer nip section 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 "document reading" or "ADF reading" to acquire measurement data indicating the measurement result of the density of the test pattern, and to perform processing based on the measurement data. The control unit 110 (CPU 111) further 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 in 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 the image (output image) formed by the printer unit 100B using the shading function of the exposure apparatus 3. The exposure apparatus 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 amount of 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 laser light in the main scanning direction, is divided into multiple areas at equal intervals in the main scanning direction, and the light intensity setting (LPW) of the exposure apparatus 3 is determined for each divided area. For example, the image forming area is divided into 14 areas A to N (see Figure 5). The length of each area in the main scanning direction is, for example, 23.59 mm. The light intensity setting (LPW) of each area A to N is corrected in the image forming apparatus 100. The amount of correction for the laser light intensity (exposure correction amount ΔLPW), which is used for density uniformity correction, is generated for each of the above-mentioned areas (for each of areas A to N) by the correction process described later.

[0039] The exposure compensation amount ΔLPW described above is determined based on the γ characteristics of the image forming engine, which correspond to the density characteristics showing the relationship between the light intensity setting value of the exposure device 3 and the density value of the output image. When determining the exposure compensation amount ΔLPW for each region described above using a predetermined γ characteristic, a change in the γ characteristics of the image forming engine may reduce the accuracy of density unevenness correction.

[0040] Here, Figure 3 illustrates the differences in γ 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 when forming an image, and the exposure amount of laser light when forming an image. For example, as shown in Figure 3, if the density difference ΔD to be corrected for density unevenness correction in the main scanning direction is the same in states A and B, which have different γ characteristics, the required exposure compensation amount ΔLPW should be different for each state. However, if the exposure compensation amount ΔLPW corresponding to the density difference ΔD is determined using fixed, predetermined γ characteristics, the light intensity setting value of the exposure apparatus 3 cannot be appropriately controlled, and the accuracy of exposure unevenness correction decreases.

[0041] Therefore, the image forming apparatus 100 first outputs a pattern for detecting density unevenness and determines multiple exposure compensation amounts ΔLPW based on the measurement results of the said pattern. Then, the image forming apparatus 100 outputs multiple patterns corresponding to the multiple exposure compensation amounts ΔLPW and accurately corrects the density unevenness by selecting an appropriate exposure compensation amount ΔLPW from among the multiple patterns. Here, the multiple patterns corresponding to the multiple exposure compensation amounts ΔLPW are patterns for determining the compensation amount and are used to determine the gain as a compensation condition for increasing or decreasing the compensation amount of exposure.

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

[0043] Figure 6 is an example of an operation screen that accepts the execution of the correction process described above. The operation screen is displayed on the display unit 218 of the operation unit 200. The user can instruct the image forming apparatus 100 to execute the process associated with a button by selecting the button displayed on the operation screen using the operation unit 200. Figure 4 is a flowchart of the procedure for density unevenness correction processing. The density unevenness correction processing will be explained below in accordance with the flowchart.

[0044] First, the control unit 110 displays a screen 231 on the display unit 218, as shown in Figure 6(a). The screen 231 displays a button 241 that instructs the printing of a pattern 50 for detecting density unevenness. When the user presses button 241 on the screen 231, the control unit 110 controls the printer unit 100B to form a pattern 50, which includes the Y, M, C, and K band images shown in Figure 5, on the sheet S (S101).

[0045] Figure 5 is a schematic diagram of the density unevenness detection pattern 50 output in step S101. As shown in Figure 5, the density unevenness detection pattern 50 has a certain width in the transport direction (also called the sub-scanning direction) in which the sheet S is transported, and has multiple band images that extend in a band shape in the main scanning direction. These multiple band images are each formed over the entire image formation area in the main scanning direction based on uniform image signal values. In this embodiment, a total of 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 no density unevenness occurs. The image signal for forming each band image is, for example, a value indicating a density level of 40% of the maximum density level. When the density unevenness detection pattern 50 is formed on the sheet S, the control unit 110 displays the screen 232 on the display unit 231, as shown in Figure 6(b). A button 242 that instructs reading is displayed on the screen 232.

[0046] When a user places a sheet S on which a pattern 50 for detecting density unevenness has been formed onto the reader unit 100A, and the user presses button 242 on the screen 232, the control unit 110 controls the reader unit 100A to read the pattern 50 on the sheet S (S102). As a result, the control unit 110 acquires the reading result of the image (pattern 50) on the sheet S by the reading sensor 105. Here, the user can select the reading mode of the 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 density unevenness detection (S103), and stores the density values ​​of regions A to N in the main scanning direction for each band image as a density profile in the RAM 112. The density profile is a detection result that is detected as information about the density of 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 how to correct density unevenness in a yellow image based on the density profile of the yellow band image, one of the four color (Y, M, C, K) band images. The method for correcting density unevenness in images of other colors based on the density profiles of other color band images is similar.

[0049] The control unit 110 calculates an average density value, which is the average of the density values ​​of regions A to N, based on the density profile stored in the RAM 112, and also calculates the difference (density difference ΔD) of the density values ​​of each region relative to the average density value (S104). The distribution of density differences ΔD for 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 for each region A to N correspond to the amount of density correction in the density unevenness correction process of the output image. The average density value is the target density value in the said correction process.

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

[0051] After the user presses button 243 on screen 233, the control unit 110 controls the printer unit 110B based on the exposure compensation amount ΔLPW determined in step S105 so that the compensation amount determining patterns 91 to 95 (Figure 9) are printed on sheet S. Here, the compensation amount determining patterns 91 to 95 are formed on separate sheets S (a total of 5 sheets) for each gain, as shown in Figure 9(A). In addition, each pattern 91 to 95 may have a mark formed to determine which of the gains Gain1 to Gain5 the pattern reflects.

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

[0053] After patterns 91-95 (or pattern 96) are printed, the control unit 110 displays screen 234 on the display unit 218, as shown in Figure 6(d). Screen 234 displays a selectable button 244 for instructing automatic adjustment of the compensation amount and a button 245 for instructing manual adjustment of the compensation amount. The control unit 110 then determines how to adjust the exposure compensation amount ΔLPW based on whether button 244 or 245 was selected by the user on screen 234 (S107).

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

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

[0056] Figure 10(A) shows the density profiles obtained from the density values ​​of each region A to N acquired in step S103. Figure 10(B) shows the density profiles of each pattern 91 to 95 (or pattern 96) acquired in step S108. In Figure 10(B), profile 1 is the density profile of pattern 91 (corrected density profile) formed based on the exposure compensation amount ΔLPW1 determined based on the density profile before correction and Gain1. In Figure 10(B), profile 2 is the density profile of pattern 92 (corrected density profile) formed based on the exposure compensation amount ΔLPW2 determined based on the density profile before correction and Gain2. In Figure 10(B), profile 3 is the density profile of pattern 93 (corrected density profile) formed based on the exposure compensation amount ΔLPW3 determined based on the density profile before correction and Gain3. In Figure 10(B), Profile 4 is the density profile of Pattern 94 (corrected density profile) formed based on the exposure compensation amount ΔLPW4 determined based on the density profile before correction and Gain 4. In Figure 10(B), Profile 5 is the density profile of Pattern 95 (corrected density profile) formed based on the exposure compensation amount ΔLPW5 determined based on the density profile before correction and Gain 5. Note that even if Pattern 96 is formed, profiles (Profiles 1-5) can be created from the reading results of Pattern 96.

[0057] The control unit 110 determines from a plurality of corrected density profiles (profiles 1 to 5) the profile with the smallest difference between the maximum and minimum densities (S109), and determines the exposure compensation amount ΔLPW used to generate that profile (S111). For example, in Figure 10(B), exposure compensation amounts ΔLPW1 and ΔLPW2 are insufficient compared to exposure compensation 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 Figure 10(B), exposure compensation amounts ΔLPW4 and ΔLPW5 are excessively large compared to exposure compensation 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 exposure compensation amounts ΔLPW1 to ΔLPW5, exposure compensation amount ΔLPW3 is the optimal compensation amount.

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

[0059] After the exposure compensation amount ΔLPW is determined in step S111, the control unit 110 completes the processing shown in the flowchart of Figure 4. Subsequently, when the printer unit 100B forms an image (output image), the light intensity control circuit 190 controls the exposure amount of the exposure device 3 as an image formation condition based on the exposure compensation amount ΔLPW for each region A to N determined in step S111. This appropriately corrects the density unevenness in the main scanning direction of the output image.

[0060] (Effect of density unevenness correction) This section describes the effect of correcting exposure unevenness in the main scanning direction that occurs in the output image when the above-described correction process is applied to each of the multiple states X and Y used as the image forming apparatus 100.

[0061] Figure 10(C) 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 Figure 10(A). Figure 10(D) shows the density profiles (after correction) of the correction amount determination patterns 91-95 (or pattern 96) formed based on the density profile in Figure 10(C). State Y is different from state X, for example, in which the amount of wear on the photosensitive drum 1 is different, or the amount of charge on the toner is different. Therefore, the gradation characteristics (also called 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 compensation amount ΔLPW is different. Therefore, with a predetermined fixed Gain value, it is not possible to correct the exposure compensation amount ΔLPW without excess or deficiency for each of states X and Y, and it is not possible to correct density unevenness in the main scanning direction with high precision.

[0062] On the other hand, in the correction process described in this embodiment, the optimal correction amount is selected using the correction amount determination patterns 91 to 95 (or pattern 96). For example, in state X, the exposure correction amount ΔLPW3 is selected, and in state Y, the exposure correction amount ΔLPW2 is selected. Thus, in the correction process, density unevenness in the main scanning direction can be appropriately suppressed regardless of whether it is state X or state Y.

[0063] (Example 2) The image forming apparatus 100 described in Example 1 performs main scan shading (called LPWSHD) to correct density unevenness by varying the laser power that irradiates multiple areas on the photosensitive drum 1 in the main scan direction. In addition to LPWSHD, there is a method of correcting density unevenness in the main scan direction by generating a lookup table corresponding to multiple areas on the photosensitive drum 1 in the main scan direction (called LUTSHD). Compared to LPWSHD, LUTSHD can correct density unevenness of different gradations with higher precision. The image forming apparatus 100 that performs LUTSHD as a correction process will be described below. Note that the same numbering will be used for configurations similar to those in Example 1, and their detailed descriptions will be omitted.

[0064] (Control configuration of an image forming apparatus that performs LUTSHD) The control unit 110 transforms the 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 transforms the image data based on multiple LUTs (Look Up Tables) corresponding to multiple positions in the main scanning direction (each main scanning position). A LUT is a one-dimensional table that shows the correspondence between the input image signal (image signal value before transformation) and the output image signal (image signal value after transformation). Figure 11 is an explanatory diagram of the LUTs for each position in the main scanning direction.

[0065] Figure 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 scan position when the main scan direction is divided into 32 sections. 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 by creating a 4-level test image in the sub-scan direction orthogonal to the main scan direction. The image signal value correction table has a correction table for each of the 32 divided regions in the main scan direction.

[0066] Figure 11(b) shows the result of linear interpolation of the image signal value correction table in Figure 11(a). Figure 11(c) is a LUT corresponding to multiple regions in the main scanning direction, created by interpolating between the 32 divisions of the linearly interpolated image signal value correction table in Figure 11(b) in the main scanning direction. 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 of the main scanning direction in which the image forming apparatus 100 can form an image is 320 [mm], the number of pixels will be 15118 at a resolution of 1200 dpi. When the main scanning direction is divided into 32 regions, the distance between the center positions of adjacent regions is 472 pixels. At the center position of each region where the image density of the pattern 51 for density unevenness detection is measured, the correction amount of the image signal created based on the image density measurement result is used as is, and 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 the correction amount for each pixel. By using LUTs corresponding to multiple regions in the main scanning direction as shown in Figure 11(c), density unevenness can be corrected on a pixel-by-pixel basis.

[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 the gradations that were not detected, potentially resulting in false contours in the resulting image.

[0068] To suppress this, it is necessary to limit the correction result using a correction reflection rate as a correction condition. The correction reflection rate is a coefficient that represents the ratio to which the measurement result of image density is reflected in 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 a reading 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% of the difference between the density as a reading result and the ideal density (coefficient is 0.8) is corrected. The correction reflection rate is stored in ROM 113, for example, and used when measuring image density.

[0069] Thus, lowering the correction reflection rate can suppress the generation of false contours. However, this presents the challenge that the correction effect on density unevenness is reduced accordingly. Therefore, the image forming apparatus 100 forms a density unevenness detection pattern for LUTSHD and calculates LUTs corresponding to multiple regions in the main scanning direction for each of the multiple correction reflection rates based on the measurement results of the pattern. Then, it outputs a correction amount determination pattern that reflects the LUTs corresponding to multiple regions in the main scanning direction, and by selecting an appropriate LUT corresponding to multiple regions in the main scanning direction from among them, it improves the accuracy of density unevenness correction while suppressing the generation of false contours.

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

[0071] Figure 13 is a schematic diagram of the pattern 51 for detecting density unevenness for LUTSHD. The pattern 51 for detecting density unevenness is an image (band image) that has a constant width in the sub-scanning direction and extends in a band shape in the main scanning direction. These band images are formed over 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 no density unevenness occurs. The image signals for forming each band image are, for example, values ​​that represent density levels of 25%, 50%, 75%, and 100% of the maximum density level. When the pattern 50 for detecting density unevenness is formed on the sheet S, the control unit 110 displays the screen 232 on the display unit 231, as shown in Figure 6(b). A button 242 that instructs reading is displayed on the screen 232.

[0072] The control unit 110 stores the density values ​​obtained from the density values ​​of regions 1 to 32 (Figure 13) for each band image as a density profile in the RAM 112. The density profile is a detection result that is detected as information about the density of pattern 51. Note that the processing of one of the four colors (Y, M, C, K) (Y) will be described below, but the processing of the other colors will be performed in the same way. Figure 14 shows an example of the uncorrected density profile obtained from pattern 51.

[0073] The control unit 110 determines the average density value, which is the average of the density values ​​in regions 1 to 32, and the density difference ΔD, which is the difference between the density values ​​of each region and the average density value, based on the density profile stored in the RAM 112. The distribution of the density difference ΔD with respect to the average density value in each band image in the main scanning direction 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 amount of density correction in the density unevenness correction process of the output image, and the average density value corresponds to the target density value in the said 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 target of correction (amount of correction required).

[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%). The patterns for determining the correction amount are formed on multiple sheets S based on the LUTs corresponding to multiple regions in the main scanning direction. Figure 16 shows an example of a pattern for determining the correction amount for LUTSHD. The correction amount determination pattern 70 shown in Figure 16(A) includes a gradient pattern for each color, and patterns 70 with different correction reflection rates are output separately on different sheets S. In addition, each sheet S has a mark formed to determine which of the multiple regions in the main scanning direction's LUTs is reflected in the pattern 70 formed on that sheet S. The correction amount determination patterns 80 and 81 for LUTSHD shown in Figure 16(B) include an 8-level grayscale pattern for each color, and are formed by reflecting the correction reflection rate of one correction amount on two sheets S. Therefore, the pattern consists of a total of 10 sheets S, each reflecting two LUTs corresponding to multiple regions in the five main scanning directions. The number of gradations in pattern 70 for determining the correction amount is greater than the number of gradations in pattern 50 for detecting density unevenness for LUTSHD, allowing the user to visually confirm whether or not false contours have occurred after density unevenness correction. Each pattern also has a mark to indicate which LUTs corresponding to multiple regions in the main scanning direction are reflected in the pattern. The gradient pattern makes it easier for the user to visually determine the presence or absence of false contours than the uniform density pattern. Since the uniform density pattern has a wider area of ​​the same signal value than the gradient pattern, the uniform density pattern allows for more accurate calculation of the density profile at each gradation from the reading results. However, the gradient pattern may be used for automatic adjustment, or the user may visually correct using the uniform density pattern.

[0075] The control unit 110 creates a density profile based on each correction reflection rate of LUTSHD. Then, it determines whether or not false contours occur in the created density profile for each correction reflection rate. Figure 17 shows an example of a density profile in which false contours may occur. In the area enclosed by the dotted line in the figure, the density difference between grayscale 5 and grayscale 6 is below the threshold (0.05 in this embodiment), raising concerns about the occurrence of false contours. For this reason, the control unit 110 selects the highest correction reflection rate among those lower than the LUTSHD correction reflection rate as the LUTSHD correction reflection rate. This suppresses the occurrence of false contours and allows for the determination of a correction reflection rate that corrects density unevenness in the main scanning direction with high accuracy.

[0076] (Effect of density unevenness correction through image data conversion) Next, we will describe the correction effect when the image forming apparatus 100 uses multiple states α and β, and the LUTSHD correction process is applied to each state, thereby suppressing the generation of false contours and improving the accuracy of density unevenness correction.

[0077] Figure 18 shows examples of the presence or absence of pseudo-contours in the correction reflection rate and the pattern 70 for determining the correction amount for LUTSHD for states α and β. State α is a state in which, for example, the amount of wear on the photosensitive drum 1 is different from that of state β, or the amount of charge on the toner 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 pseudo-contours in both states α and β, the correction reflection rate needs to be set to 70%. However, in state β, further correction is not possible even though it is feasible.

[0078] On the other hand, in the correction process, for each state of the image forming apparatus 100, a correction amount with a higher correction reflection rate can be selected while suppressing the generation of false contours. In other words, in state α, a correction reflection rate of 70% can be selected, and in state β, a correction reflection rate of 90% can be selected. Therefore, in the correction process, for both state α and state β, the generation of false contours can be suppressed, and each density value in the corrected density profile can be corrected to a value close to the average density value.

[0079] Furthermore, although the density profiles described in Examples 1 and 2 are explained as values ​​obtained by converting the luminance values ​​of the pattern to density, a luminance profile may be used instead of a density profile. [Explanation of Symbols]

[0080] 100A Leader Unit 100B Printer Unit 110 Control Unit 50 Patterns for detecting concentration variations 91-95 Patterns for determining the correction amount

Claims

1. Image forming means comprising a photoreceptor, an exposure unit for exposing the photoreceptor to form an electrostatic latent image, and a developing means for developing the electrostatic latent image, wherein an image is formed on a sheet. A reading means for reading a pattern image formed on a sheet for detecting the density in the scanning direction in which light from the exposure area scans the photoreceptor, A detection means that causes the image forming means to form a first pattern image, causes the reading means to read the first pattern image, and detects information regarding the density in the scanning direction based on the reading result of the reading means of the first pattern image, An image forming apparatus characterized by comprising: causing the image forming means to form a second pattern image based on the detection result of the detection means and a first correction condition; causing the image forming means to form a third pattern image based on the detection result of the detection means and a second correction condition different from the first correction condition; causing the reading means to read the second pattern image and the third pattern image; determining a third correction condition based on the reading result of the reading means for the second pattern image and the reading result of the reading means for the third pattern image; 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 condition.

2. Image forming means comprising a photoreceptor, an exposure unit for exposing the photoreceptor to form an electrostatic latent image, and a developing means for developing the electrostatic latent image, wherein an image is formed on a sheet. A reading means for reading a pattern image formed on a sheet for detecting the density in the scanning direction in which light from the exposure area scans the photoreceptor, A detection means that causes the image forming means to form a first pattern image, causes the reading means to read the first pattern image, and detects information regarding the density in the scanning direction based on the reading result of the reading means of the first pattern image, An image forming apparatus characterized by comprising: an image forming means that causes the image forming means to form a second pattern image based on the detection result of the detection means and a first correction condition; an image forming means that causes the image forming means to form a third pattern image based on the detection result of the detection means and a second correction condition different from the first correction condition; a control means that receives user instruction information based on the comparison result of the second pattern image and the third pattern image; and controls the density unevenness in the scanning direction of the image formed by the image forming means based on the information and the user instruction information.

3. The image forming apparatus according to claim 1 or 2, characterized in that the control means controls the density unevenness by controlling the intensity of the light used by the exposure unit to expose the photoreceptor based on the information and the third correction condition.

4. The image forming apparatus according to claim 1 or 2, characterized in that the reading means reads the pattern image while transporting the sheet on which the pattern image is formed.

5. The image forming apparatus according to claim 1 or 2, characterized in that the pattern image includes images with multiple grayscale levels.

6. The first pattern image includes an image with a first number of grayscale levels, The image forming apparatus according to claim 1 or 2, characterized in that the second pattern image and the third pattern image each include an image with a second number of grayscale levels greater than the first number of grayscale levels.

Images