JP2014170192A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce user stress when performing main scanning shading correction.
An image forming apparatus forms a test pattern on a sheet fed from a storage and discharges the sheet. In addition, the image forming apparatus 100 feeds the discharged sheet 110 from the storage 133 in a state where the test pattern formed along the main scanning direction is rotated so that the test pattern is formed along the sheet conveyance direction. . The color sensor 200 measures a test pattern on the fed sheet 110. If the printer controller 103 determines that the direction of the sheet 110 set in the storage 133 is upside down based on the measurement result of the color sensor 200, the printer controller 103 arranges the order of the measurement values at a plurality of positions in the main scanning direction. Perform the replacement process.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus having a function of correcting unevenness of an image in a main scanning direction.

  Image quality (hereinafter referred to as image quality) of the image forming apparatus includes graininess, in-plane uniformity, character quality, color reproducibility (including color stability), and the like. As factors affecting image quality, in an image forming apparatus using an electrophotographic method, charging unevenness due to deterioration of a charger for charging a photosensitive drum, exposure unevenness such as a laser scanner for forming an electrostatic latent image on the photosensitive drum, Or there is uneven development of a developing device for developing the electrostatic latent image.

  These unevenness causes image unevenness (density unevenness and color unevenness) in the main scanning direction (direction orthogonal to the sheet conveying direction) of the image formed on the sheet, and as a factor that impairs in-plane uniformity. As a major issue.

  Therefore, in Patent Document 1, a sheet (printing a plurality of test patterns in the main scanning direction) is output, and the density of the test patterns is measured with a handy densitometer or the like to correct density unevenness in the main scanning direction (main scanning) Scanning shading correction) has been proposed.

  On the other hand, as a method of performing this main scanning shading correction using a color sensor mounted inside the image forming apparatus, a technique described in Patent Document 2 has been proposed.

  Patent Document 2 describes a technique for forming a strip-shaped test pattern with the same image signal value in the main scanning direction of a sheet. In Patent Document 2, a sheet on which a test pattern is formed is rotated 90 degrees and set in a sheet feeding unit, the sheet is fed again, and a test pattern is printed using a color sensor in the image forming apparatus. It is described to measure.

JP 2004-163216 A JP 2006-58565 A

  However, depending on the paper feed unit on which the sheet on which the test pattern is formed is set, it is set face up, set face down, rotated 90 ° to the right, or set 90 ° to the left. The user had to determine whether to do it.

  For this reason, the user has to set the sheet in consideration of the front and back as well as the left and right directions, and the operability is poor. In addition, if the setting direction in the sheet feeding unit is wrong, the test pattern cannot be measured accurately, and it is necessary to set the sheet again in the sheet feeding unit, resulting in user stress.

  Accordingly, an object of the image forming apparatus according to the present invention is to reduce user stress when performing main scanning shading correction.

  In order to achieve the above object, an image forming apparatus according to the present invention includes a conveying unit that conveys a sheet, and an image that forms a measurement image on a sheet along a main scanning direction orthogonal to a sheet conveying direction by the conveying unit. Forming means; paper discharge means for discharging the sheet on which the measurement image is formed by the image forming means; and a sheet once discharged by the paper discharge means is formed along the main scanning direction. A sheet feeding unit that feeds the measurement image in a state in which the image is rotated in a sheet conveyance direction, and irradiates the measurement image on the sheet fed by the sheet feeding unit with light, Measuring means for measuring reflected light from the measurement image and outputting measured values at a plurality of positions in the main scanning direction, and a direction of the sheet set on the paper feeding means based on the measurement result of the measuring means Determined to be upside down In this case, the processing means for rearranging the order of the measurement values at a plurality of positions in the main scanning direction and the unevenness of the image in the main scanning direction based on the measurement values rearranged by the processing means. Correction means for performing a process for correcting.

  According to the present invention, it is possible to reduce user stress when performing main scanning shading correction.

2 is a cross-sectional view showing the structure of the image forming apparatus 100. FIG. 2 is a diagram illustrating a color sensor 200. FIG. 1 is a block diagram showing a system configuration of an image forming apparatus 100. FIG. It is an image figure which shows the chart for color measurement. 1 is a schematic diagram of a color management environment. It is a figure which shows the operation part. It is a display screen when the user mode key 410 is selected. 3 is a flowchart showing the operation of the image forming apparatus 100. It is a flowchart which shows the operation | movement of maximum density adjustment. It is a flowchart which shows the operation | movement of a gradation adjustment. It is a flowchart which shows operation | movement of multi-order color correction processing. It is a flowchart which shows operation | movement of the main scanning shading correction | amendment. It is a figure which shows the detail of a test pattern. It is a figure which shows the display screen which alert | reports the setting direction of a chart. It is a figure for demonstrating the judgment method of the direction of the front and back of a chart. It is a figure which shows the spectral reflectance of the test pattern of each color. It is a figure which shows the density distribution of the main scanning direction of a test pattern. (A) is a diagram showing the relationship between the density ratio α (x) and the correction coefficient β (x) in the main scanning direction. FIG. 6B is a diagram illustrating the relationship between the density ratio α (x) and the correction coefficient γ (x) in the main scanning direction.

(Image forming device)
In this embodiment, a solution to the above problem will be described using an electrophotographic laser beam printer. Here, as an example, an electrophotographic system is adopted as an image forming system. However, the present invention can also be applied to an ink jet method and a sublimation method.

  FIG. 1 is a cross-sectional view illustrating the structure of the image forming apparatus 100. The image forming apparatus 100 includes a housing 101. The casing 101 is provided with various mechanisms for configuring the engine unit and a control board storage unit 104. The control board storage unit 104 stores an engine control unit 102 that performs control related to each printing process process (for example, a paper feed process) by each mechanism, and a printer controller 103.

  As shown in FIG. 1, the engine unit is provided with four stations 120, 121, 122, 123 corresponding to YMCK. Stations 120, 121, 122, and 123 are image forming units that transfer toner onto the sheet 110 to form an image. Here, YMCK is an abbreviation for yellow, magenta, cyan, and black. Each station is composed of almost common parts. The photosensitive drum 105 is a kind of image carrier and is charged to a uniform surface potential by a primary charger 111. An electrostatic latent image is formed on the photosensitive drum 105 by the laser light output from the laser 108. The laser exposure amount for the gradation of each pixel is changed by pulse width modulation (PWM).

  The developing device 112 develops the latent image using a color material (toner) to form a toner image. The toner image (visible image) is transferred onto the intermediate transfer member 106. The visible image formed on the intermediate transfer member 106 is transferred by the transfer roller 114 to the sheet 110 conveyed from the storage 113a or the storage 113b. Further, cleaning mechanisms 118 and 119 are in contact with the intermediate transfer member 106 and the transfer roller 114, respectively, so that the toner attached to the intermediate transfer member 106 and the transfer roller 114 can be removed.

  The fixing processing mechanism of this embodiment includes a first fixing device 150 and a second fixing device 160 that heat and press the toner image transferred to the sheet 110 and fix the toner image on the sheet 110. The first fixing device 150 includes a fixing roller 151 for applying heat to the sheet 110, a pressure belt 152 for pressing the sheet 110 against the fixing roller 151, and a first post-fixing sensor 153 for detecting the completion of fixing. Including. These rollers are hollow rollers and each have a heater inside.

  The second fixing device 160 is disposed downstream of the first fixing device 150 in the sheet conveying direction. The second fixing device 160 imparts gloss (gloss) to the toner image on the sheet fixed by the first fixing device 150 or secures the fixing property. Similar to the first fixing device 150, the second fixing device 160 also has a fixing roller 161, a pressure roller 162, and a second post-fixing sensor 163. Depending on the type of the sheet 110, it is not necessary to pass the second fixing device 160. In this case, the sheet 110 passes through the conveyance path 130 without passing through the second fixing device 160 for the purpose of reducing energy consumption.

  For example, when setting is made to add a lot of gloss to the image on the sheet 110, or when the sheet 110 requires a large amount of heat for fixing, such as thick paper, the sheet 110 that has passed through the first fixing device 150 is used. Is also conveyed to the second fixing device 160. On the other hand, when the sheet 110 is plain paper or thin paper and the setting for adding a large amount of gloss is not made, the sheet 110 is conveyed on the conveyance path 130 that bypasses the second fixing device 160. Whether the sheet 110 is conveyed to the second fixing device 160 or whether the sheet 110 is conveyed bypassing the second fixing device 160 is controlled by the switching member 131.

  The paper discharge conveyance path 139 is a conveyance path for discharging the sheet 110 to the outside. The switching member 132 switches between guiding the sheet 110 to the conveyance path 135 or guiding the sheet 110 to the paper discharge conveyance path 139. The leading edge of the sheet 110 guided to the conveyance path 135 passes through the reversal sensor 137 and is conveyed to the reversing unit 136. When the reverse sensor 137 detects the trailing edge of the sheet 110, the conveyance direction of the sheet 110 is switched. The switching member 133 switches whether the sheet 110 is guided to the conveyance path 138 for double-sided image formation or to the conveyance path 135.

  A color sensor 200 that detects a patch image on the sheet 110 is disposed in the conveyance path 135. The color sensor 200 includes four sensors 200a to 200d arranged in a direction orthogonal to the conveyance direction of the sheet 110, and can detect four rows of patch images. When measurement is instructed by an instruction from the operation unit 180, the engine control unit 102 executes main scanning shading correction, maximum density adjustment, gradation adjustment, multi-order color correction processing, and the like.

  The switching member 134 is a guide member that guides the sheet 110 to the paper discharge conveyance path 139. The sheet 110 conveyed on the paper discharge conveyance path 139 is discharged to the outside of the image forming apparatus 100.

(Color sensor)
FIG. 2 is a diagram illustrating the structure of the color sensor 200. Inside the color sensor 200, a white LED 201, a diffraction grating 202, a line sensor 203, a calculation unit 204, and a memory 205 are provided. The white LED 201 is a light emitting element that irradiates the patch image 220 on the sheet 110 with light. The light reflected from the patch image 220 passes through the window 206 made of a transparent member.

  The diffraction grating 202 separates the reflected light from the patch image 220 for each wavelength. The line sensor 203 is a light detection element that includes n light receiving elements that detect light decomposed for each wavelength by the diffraction grating 202. The calculation unit 204 performs various calculations from the light intensity value of each pixel detected by the line sensor 203.

  The memory 205 stores various data used by the calculation unit 204. The calculation unit 204 includes, for example, a spectral calculation unit that calculates the spectral reflectance from the light intensity value. Further, a lens for condensing the light emitted from the white LED 201 onto the patch image 220 on the sheet 110 and condensing the light reflected from the patch image 220 onto the diffraction grating 202 may be further provided. In addition, the measurement area where the color sensor 200 measures the patch image on the sheet 110 is equal to the irradiation area (spot diameter) of the white LED 201, and in this embodiment is φ5 mm.

  FIG. 3 is a block diagram illustrating a system configuration of the image forming apparatus 100. The maximum density adjustment, gradation adjustment, and multi-order color correction processing will be described with reference to this drawing. In FIG. 3, the inside of the printer controller 103 is represented by blocks in order to make the processing performed by the printer controller 103 easier to understand.

(Maximum density adjustment)
First, the printer controller 103 instructs the engine control unit 102 to output a test chart used for maximum density adjustment. At this time, a patch image for maximum density adjustment is formed for each color of CMYK on the sheet 110 with the charging potential, the exposure intensity, and the developing bias set in advance or set at the previous maximum density adjustment. Thereafter, the engine control unit 102 issues a patch image measurement instruction to the color sensor control unit 302.

  When the color sensor 200 measures the patch image, the measurement result is sent to the density conversion unit 324 as spectral reflectance data. The density conversion unit 324 converts the spectral reflectance data into CMYK density data, and sends the converted density data to the maximum density correction unit 320.

  The maximum density correction unit 320 calculates the correction amount of the charging potential, the exposure intensity, and the development bias so that the density when the image data having the maximum density is output as a toner image has a desired value, and the calculated correction. The amount is transmitted to the engine control unit 102. The engine control unit 102 uses the transmitted charging potential, exposure intensity, and development bias correction amount for the next and subsequent image forming operations. With the above operation, the maximum density of the output image is adjusted.

(Gradation adjustment)
When the maximum density adjustment process is completed, the printer controller 103 instructs the engine control unit 102 to form a 16-gradation patch image on the sheet 110. For example, the image signal of the 16 gradation patch image may be 00H, 10H, 20H, 30H, 40H, 50H, 60H, 70H, 80H, 90H, A0H, B0H, C0H, D0H, E0H, FFH. .

  At this time, a patch image having 16 gradations is formed for each color of CMYK on the sheet 110 using the correction amount of the charging potential, the exposure intensity, and the developing bias calculated by the maximum density adjustment. When a 16-gradation patch image is formed on the sheet 110, the engine control unit 102 instructs the color sensor control unit 302 to measure the patch image.

  When a patch image is measured by the color sensor 200, the measurement result is sent to the density conversion unit 324 as spectral reflectance data. The density conversion unit 324 converts the spectral reflectance data into CMYK density data, and sends the converted density data to the density gradation correction unit 321. The density gradation correction unit 321 calculates a correction amount for the exposure amount so that a desired gradation property is obtained. Then, the LUT creation unit 322 creates a single color gradation LUT and sends it to the LUT unit 323 as the signal value of each color CMYK.

(Profile)
In performing the multi-order color adjustment process, the image forming apparatus 100 creates an ICC profile described later from the measurement result of the patch image including the multi-order color, and converts the input image using the profile to form an output image. .

  Here, in the patch image 220 including multi-order colors, the dot area ratio is changed in three stages (0%, 50%, 100%) for each of the four colors of CMYK, and all the dot area ratios for each color are displayed. A combined patch image is formed. As shown in FIG. 4, the patch images 220 are formed in four rows so as to be read by the color sensors 200a to 200d.

  Here, an ICC profile accepted in the market in recent years is used as a profile for realizing excellent color reproducibility. However, the present invention is not an invention that can be applied only to an ICC profile. The present invention can also be applied to CRD (Color Rendering Dictionary) adopted from Level 2 of PostScript proposed by Adobe, and a color separation table in Photoshop (registered trademark).

  When replacing parts by a customer engineer, before a job that requires color matching accuracy, or when you want to know the color of the final output at the design concept stage, the user operates the operation unit 180 to perform color Instructs the profile creation process.

  The profile creation process is performed in the printer controller 103 shown in the block diagram of FIG. The printer controller 103 has a CPU, and reads a program for executing a flowchart described later from the storage unit 350 and executes it.

  When the operation unit 180 receives a profile creation instruction, the profile creation unit 301 outputs the CMYK color chart 210, which is an ISO12642 test form, to the engine control unit 102 without passing through the profile. The profile creation unit 301 sends a measurement instruction to the color sensor control unit 302. The engine control unit 102 controls the image forming apparatus 100 to execute processes such as charging, exposure, development, transfer, and fixing. As a result, an ISO 12642 test form is formed on the sheet 110.

  The color sensor control unit 302 controls the color sensor 200 to measure the ISO12642 test form. The color sensor 200 outputs spectral reflectance data, which is a measurement result, to the Lab calculation unit 303 of the printer controller 103. The Lab calculation unit 303 converts the spectral reflectance data into L * a * b * data and outputs it to the profile creation unit 301. The L * a * b * data is transferred via the color sensor input ICC profile storage unit 304. Note that the Lab calculation unit 303 may convert the spectral reflectance data into the CIE 1931XYZ color system that is a color space signal independent of the device.

  The profile creation unit 301 creates an output ICC profile from the relationship between the CMYK color signal output to the engine control unit 102 and the L * a * b * data input from the Lab calculation unit 303. The profile creation unit 301 stores the created output ICC profile in the output ICC profile storage unit 305.

  The ISO12642 test form includes patches of CMYK color signals that cover a color reproduction range that can be output by a general copying machine. Therefore, the profile creation unit 301 creates a color conversion table from the relationship between each color signal value and the measured L * a * b * value. That is, a conversion table of CMYK → Lab is created. Based on this conversion table, an inverse conversion table is created.

  Upon receiving a profile creation command from the host computer via the I / F 308, the profile creation unit 301 outputs the created output ICC profile to the host computer via the I / F 308. The host computer can execute color conversion corresponding to the ICC profile with an application program.

(Color conversion processing)
In color conversion in normal color output, an image signal input assuming an RGB signal value input from the scanner unit via the I / F 308 or a standard print CMYK signal value such as Japan Color is an input for external input. It is sent to the ICC profile storage unit 307. The input ICC profile storage unit 307 performs RGB → Lab or CMYK → Lab conversion according to the image signal input from the I / F 308. The input ICC profile stored in the input ICC profile storage unit 307 includes a plurality of LUTs (lookup tables).

  These LUTs are, for example, a one-dimensional LUT that controls the gamma of the input signal, a multi-order color LUT called direct mapping, and a one-dimensional LUT that controls the gamma of the generated conversion data. The input image signal is converted from device-dependent color space to device-independent L * a * b * data using these LUTs.

  The image signal converted into L * a * b * coordinates is input to the CMM 306. CMM is an abbreviation for color management module. The CMM 306 performs various color conversions. For example, the CMM 306 executes GAMUT conversion for mapping a mismatch between a reading color space such as a scanner unit as an input device and an output color reproduction range of the image forming apparatus 100 as an output device. In addition, the CMM 306 performs color conversion that adjusts a mismatch between a light source type at the time of input and a light source type when observing an output (also referred to as a color temperature setting mismatch).

  In this way, the CMM 306 converts the L * a * b * data into L ′ * a ′ * b ′ * data and outputs the data to the output ICC profile storage unit 305. A profile created by measurement is stored in the output ICC profile storage unit 305. Therefore, the output ICC profile storage unit 305 performs color conversion on the L ′ * a ′ * b ′ * data using the newly created ICC profile, converts the data into CMYK signals depending on the output device, and outputs the CMYK signal to the engine control unit 102. To do.

  In FIG. 3, the CMM 306 is separated from the input ICC profile storage unit 307 and the output ICC profile storage unit 305. However, as shown in FIG. 5, the CMM 306 is a module that manages color management, and performs color conversion using an input profile (print ICC profile 501) and an output profile (printer ICC profile 502).

  The shading correction amount determination unit 319 determines a correction amount in the main scanning shading mode. Details of the main scanning shading mode will be described later.

(Operation section)
FIG. 6 is a diagram illustrating an operation unit 180 for performing an operation input to the image forming apparatus 100. The operation unit 180 is provided with a soft switch 400 for turning on / off the power of the image forming apparatus 100, a copy start key 401 for instructing start of copying, and a reset key 402 for returning to the standard mode. . The standard mode is set to “Full Color / Single Side”.

  The operation unit 180 is also provided with a numeric keypad 403 for inputting a numerical value such as a set number of sheets, a clear key 404 for clearing the numerical value, and a stop key 405 for stopping copying during continuous copying. .

  On the left side of the operation unit 180, a touch panel display 406 for displaying various mode settings and printer status is provided. At the right end of the operation unit 180, an interrupt key 407 for interrupting and copying during an image forming operation, a secret key 408 for managing the number of copies for each individual or department, and a guidance key to be pressed when using a guidance function 409 is provided.

  Below that, there is provided a user mode key 410 for entering a user mode in which the user manages and sets the image forming apparatus 100 such as calibration mode designation, main scanning shading mode designation, and sheet information registration. Yes.

  The touch panel display 406 is provided with a full color image formation mode selection key 412 and a monochrome image formation mode selection key 413.

(Calibration mode)
Next, the calibration mode in this embodiment will be described. First, when the user selects the user mode key 410 on the operation unit 180 in FIG. 6, the screen in FIG. 7 is displayed on the touch panel display 406.

  The calibration mode key 421 is a key for instructing execution of calibration for improving image density and color stability. The main scanning shading mode key 422 is a key for instructing execution of main scanning shading correction for correcting density unevenness and color unevenness in the main scanning direction (direction orthogonal to the sheet conveying direction) of the image formed on the sheet 110. is there.

  The calibration here refers to the above-described maximum density adjustment, gradation adjustment, and multi-order color correction processing. When the calibration mode key 421 is selected, the calibration operation starts. Hereinafter, a series of calibration processes will be described with reference to a flowchart.

  FIG. 8 is a flowchart showing the operation of the image forming apparatus 100. This flowchart is executed by the printer controller 103. First, the printer controller 103 determines whether there is an image formation request from the operation unit 180 and whether there is an image formation request from the host computer via the I / F 308 (S801).

  If there is no image formation request, the printer controller 103 determines whether there is an instruction for main scanning shading from the operation unit 180 (S802). The main scanning shading instruction is performed by selecting the main scanning shading mode key 422 as described above. If there is an instruction for main scanning shading, main scanning shading correction described later with reference to FIG. 12 is performed (S803).

  Next, the printer controller 103 determines whether there is a calibration instruction from the operation unit 180 (S804). The calibration instruction is performed by selecting the calibration mode key 421 as described above.

  When the calibration is instructed, the maximum density adjustment described later in FIG. 9 is performed (S805), and the gradation adjustment described later in FIG. 10 is performed (S806). Thereafter, multi-order color correction processing described later with reference to FIG. 11 is performed (S807). If there is no calibration instruction in step S804, the process returns to step S801 described above. The reason why the maximum density adjustment and the gradation adjustment are performed before performing the multi-order color correction process is to perform the multi-order color correction process with high accuracy.

  If it is determined in step S801 that there is an image formation request, the printer controller 103 instructs the engine control unit 102 to feed the sheet 110 from the storage 113 (S808). Thereafter, the printer controller 103 instructs the engine control unit 102 to form a toner image on the sheet 110 (S809).

  Then, the printer controller 103 determines whether image formation for all pages has been completed (S810). If image formation for all pages has been completed, the process returns to step S801. If not, the process returns to step S808 to perform image formation for the next page.

  FIG. 9 is a flowchart showing the maximum density adjustment operation. This flowchart is executed by the printer controller 103. Note that the control of the image forming apparatus 100 is executed by the engine control unit 102 according to an instruction from the printer controller 103.

  First, the printer controller 103 instructs the engine control unit 102 to feed the sheet 110 from the storage 113 (S901), and instructs the sheet 110 to form a patch image for maximum density adjustment (S901). S902). Next, when the sheet 110 reaches the color sensor 200, the printer controller 103 causes the color sensor 200 to measure a patch image (S903).

  Then, the printer controller 103 converts the spectral reflectance data output from the color sensor 200 into CMYK density data using the density converter 324 (S904). Thereafter, the printer controller 103 calculates the correction amount of the charging potential, the exposure intensity, and the developing bias based on the converted density data (S905). The correction amount calculated here is stored in the storage unit 350 and used.

  FIG. 10 is a flowchart showing the gradation adjustment operation. This flowchart is executed by the printer controller 103. Note that the control of the image forming apparatus 100 is executed by the engine control unit 102 according to an instruction from the printer controller 103.

  First, the printer controller 103 instructs the engine control unit 102 to feed the sheet 110 from the storage 113 (S1001), and forms a patch image for gradation adjustment (16 gradations) on the sheet 110. (S1002). Next, when the sheet 110 reaches the color sensor 200, the printer controller 103 causes the color sensor 200 to measure a patch image (S1003).

  Then, the printer controller 103 converts the spectral reflectance data output from the color sensor 200 into CMYK density data using the density converter 324 (S1004). Thereafter, the printer controller 103 calculates an exposure intensity correction amount based on the converted density data, and creates an LUT for correcting gradation (S1005). The LUT calculated here is set in the LUT unit 323 and used.

  FIG. 11 is a flowchart showing the operation of the multi-order color correction process. This flowchart is executed by the printer controller 103. Note that the control of the image forming apparatus 100 is executed by the engine control unit 102 according to an instruction from the printer controller 103.

  First, the printer controller 103 instructs the engine control unit 102 to feed the sheet 110 from the storage 113 (S1101), and instructs the sheet 110 to form a patch image for multi-color correction processing. (S1102). Next, when the sheet 110 reaches the color sensor 200, the printer controller 103 causes the color sensor 200 to measure a patch image (S1103).

  The printer controller 103 uses the Lab calculator 303 to calculate color value data (L * a * b *) from the spectral reflectance data output from the color sensor 200. Based on the color value data (L * a * b *), the printer controller 103 creates an ICC profile by the above-described processing (S1104) and stores it in the output ICC profile storage unit 305 (S1105).

  As described above, by performing a series of calibrations such as maximum density adjustment, gradation adjustment, and multi-order color correction processing, image density, gradation, and color stability in the image forming apparatus 100 can be realized. High-precision color matching is possible.

(Main scanning shading mode)
FIG. 12 is a flowchart showing the main scanning shading correction operation. This flowchart is executed by the printer controller 103. Note that the control of the image forming apparatus 100 is executed by the engine control unit 102 according to an instruction from the printer controller 103.

  In the following, although correction of density unevenness will be described, color unevenness in the main scanning direction may be measured from L * a * b * data measured using the color sensor 200, and the color unevenness may be corrected. Absent.

  When the start of main scanning shading is instructed, the printer controller 103 instructs the engine control unit 102 to feed the sheet 110 from the storage 113 and form a measurement image (hereinafter referred to as a test pattern). (S1201).

  As shown in FIG. 13A, the test pattern of the present embodiment is a strip pattern extending in the main scanning direction, and is formed on the sheet 110 for each color of CMYK. The image density of the test pattern is 100% output.

  The sheet size used in this embodiment is A4 (210 mm × 297 mm), and the test pattern size is 40 mm × 260 mm for each color. Each color test pattern has 40 mm × 10 mm trigger patterns (hereinafter referred to as “triggers”) TR1 and TR2. The trigger is a pattern serving as a reference for measurement timing, and measurement of a test pattern is started a predetermined time after the trigger is detected.

  Color sensors 200b and 200c are arranged at a distance A from the center of the sheet 110 indicated by a dotted line. In addition, color sensors 200a and 200d are arranged at a distance B from the center of the sheet 110. In addition, test patterns are formed at regular intervals so that each color sensor 200a to 200d can measure.

  The reason for this arrangement is that, as shown in FIG. 13B, even if the user sets the seat 110 upside down (upside down), it is This is because the test pattern can be measured using different sensors. In addition, in order to display the position of the test pattern in an easy-to-understand manner, a separator line is shown, but it is not actually printed.

  When measuring the test pattern, it is necessary to set the sheet 110 in the measurement paper feeding unit so that the formed test pattern is along the sheet conveyance direction. For this reason, a message “Please rotate 90 degrees and set in cassette” is printed on the sheet 110.

  In this embodiment, as long as the sheet 110 is rotated 90 degrees and set, the test pattern can be measured regardless of whether the user sets the sheet 110 in the front, back, left or right direction. For this reason, a message is printed on the sheet 110 that says “You can set it in any direction, front and back, left and right.”

  Next, the printer controller 103 instructs the engine control unit 102 to temporarily discharge the sheet 110 (hereinafter referred to as a chart) on which the test pattern is formed to the outside of the image forming apparatus 100 (S1202).

  Since the test pattern is a strip-like pattern that is long in the main scanning direction, when the measurement is performed using the color sensor 200 along the longitudinal direction of the test pattern, the chart is rotated 90 degrees and set in the measurement paper feeding unit. There is a need. As shown in FIG. 13A, if the chart rotated 90 degrees clockwise is fed, the entire area of the CMYK test pattern can be measured by the color sensors 200a to 200d.

  For this reason, once the chart discharge is completed, the printer controller 103 displays the screen shown in FIG. 14 on the touch panel display 406 of the operation unit 180 (S1203). Note that the storage 113 may be used as the measurement paper feed unit for setting the chart, or a so-called manual feed tray may be used.

  Next, the printer controller 103 waits until the OK key in FIG. 14 is pressed, that is, until the chart setting is completed (S1204). When the chart setting is completed, the printer controller 103 instructs the engine control unit 102 to start feeding the chart (S1205). When the chart is fed, the printer controller 103 measures the CMYK test pattern using the color sensors 200a to 200d (S1206).

  Next, the printer controller 103 determines whether the orientation of the front and back of the chart is correct (S1207). This determination is made based on whether or not the color sensor 200 has detected the triggers TR1 and TR2. Details of this determination method will be described later.

  For example, when the chart is set on the measurement paper feeding unit in the reverse direction, the color sensor 200 measures the surface opposite to the surface on which the chart is formed, and the triggers TR1 and TR2 cannot be detected.

  In such a case, the printer controller 103 instructs the engine control unit 102 to reverse the front and back of the chart by the reversing unit 136 (S1208). Thereafter, the chart is guided to the conveyance path 138 for double-sided image formation, and is measured again in the above-described step S1206.

  If it is determined in step S1207 that the front and back directions are correct, the printer controller 103 determines whether the vertical direction of the chart is correct based on the measurement value of the color sensor 200 (S1207). This determination process will be described later.

  For example, when the chart is set upside down, the test pattern is measured in the state shown in FIG. That is, the color sensors 200a to 200d measure from the trigger TR2 side, not from the trigger TR1 side, and measure the test pattern in the reverse direction. In such a case, the printer controller 103 rearranges the order of the measurement values of the color sensors 200a to 200d from ascending order to descending order (S1210).

  If YES in step S1209 or if the order of measurement values is rearranged in step S1210, the printer controller 103 uses the density conversion unit 324 to convert the measurement results of the color sensors 200a to 200d into CMYK density values. (S1211).

  Next, the printer controller 103 calculates density unevenness in the main scanning direction from the CMYK density values obtained by measuring the test pattern (S1212). Details of the method for calculating density unevenness in the main scanning direction will be described later.

  Then, the printer controller 103 determines the shading correction amount based on the calculated density unevenness in the main scanning direction by the shading correction amount determination unit 319 (S1213). Details of the method for determining the shading correction amount will be described later. Thereafter, the printer controller 103 discharges the chart (S1214) and ends the processing according to this flowchart.

  In this embodiment, when the chart is set upside down, the order of the measurement values of the color sensors 200a to 200d is rearranged from the ascending order to the descending order, but the order of the correction values (shading correction amounts) is ascending. You may arrange them in descending order.

(Judgment method of front and back direction)
Next, a method for determining the front / back orientation of the chart in step S1207 will be described with reference to FIG. FIG. 15 shows a state in which the cyan test pattern of FIG. 13A is measured by the color sensor 200a when the chart is set up correctly on the front and back sides. The horizontal axis represents the time corresponding to the position of the test pattern, and the vertical axis represents the measurement result of the spectral reflectance at a wavelength of 400 nm measured by the color sensor 200a.

  When the measurement is started from the tip trigger TR1 side, the color sensor 200a detects the low-reflectance trigger TR1 after detecting the high-reflectance paper (white). Thereafter, after measuring the cyan test pattern, the color sensor 200a finally detects the trailing edge trigger TR2 to detect the paper background.

  The leading edge trigger TR1 is detected in response to the time from the timing T1 when the measured value falls below the trigger detection threshold Th to the timing T2 when the measured value exceeds the threshold continues for a predetermined time Tx or more. On the other hand, the rear end trigger TR2 is detected at a timing T3 when the measured value exceeds the threshold Th after the test pattern measurement.

  As described above, when the leading edge trigger TR1 and the trailing edge trigger TR2 are detected, the printer controller 103 determines that the orientations of the front and back sets of the chart are correct. When it is not determined that the measurement has been normally performed (for example, when the triggers TR1 and TR2 cannot be detected), the printer controller 103 determines that the chart is set upside down.

  Although the description has been given here taking the cyan test chart and the reflectance at a wavelength of 400 nm as an example, the wavelength to be measured may be changed for each color. However, it is necessary to use a wavelength that is the absorption sensitivity region of the color material used for the trigger, and to select a wavelength that can ensure contrast with the reflectance from the paper.

  Further, in the present embodiment, as the colors of the triggers TR1 and TR2, color materials of 100% black and 50% cyan are used, and the trigger TR1 and TR2 can be used in any wavelength band as long as they are visible light of 380 to 730 nm. TR2 can be detected. Therefore, if an image having absorption sensitivity over the entire visible light region as in the present embodiment is used as a trigger, the triggers TR1 and TR2 may be detected using an average reflectance over the entire visible light region.

(How to determine the vertical orientation)
Next, a method for determining the vertical direction of the chart in step S1209 will be described. As shown in FIG. 13A, when the chart is set in the correct vertical direction, the color sensor 200a measures cyan, the color sensor 200b measures magenta, the color sensor 200c measures yellow, and the color sensor 200d measures black.

  However, as shown in FIG. 13B, when the chart is set upside down, the color sensor 200a measures black, the color sensor 200b measures yellow, the color sensor 200c measures magenta, and the color sensor 200d measures cyan. To do.

  Therefore, if it is determined whether at least one of the color sensors 200a and 200d has measured black or cyan, it is determined whether the chart is set in the correct vertical direction or in the reverse direction. be able to. Further, by determining whether at least one of the color sensors 200b and 200c has measured yellow or magenta, it is determined whether the chart is set in the correct vertical direction or in the reverse direction. be able to.

  FIG. 16 shows the spectral reflectance of each color test pattern, where (a) shows cyan, (b) shows magenta, (c) shows yellow, and (d) black. The arrows in FIG. 16 are wavelength bands to be noted in order to determine the difference between cyan and black and the difference between magenta and yellow. The characteristics of the reflectance in this wavelength band are analyzed, and the color of the measured test pattern is detected. Based on the detected color, the printer controller 103 can determine whether the chart is set in the correct vertical direction.

  For example, when the color sensor 200a or 200d is used, it is only necessary to be able to determine whether cyan or black is detected. The spectral reflectance of cyan is shown in FIG. 16A, and the spectral reflectance of black is shown in FIG. As is clear from these figures, when attention is paid to specific wavelengths 470 nm and 650 nm indicated by arrows, the characteristics of the two are different.

  That is, the reflectance of cyan having a wavelength of 470 nm has a high reflectance of about 50%, and black has a low reflectance of about 10%. On the other hand, the reflectance of cyan at a wavelength of 650 nm is 5%, and the reflectance of black is 10%. By detecting this feature, the printer controller 103 determines whether the chart is set in the correct vertical direction.

  When the color sensor 200b or 200c is used, it is only necessary to be able to determine whether magenta or yellow is detected. If attention is paid to the characteristics of the wavelength of 440 nm and the wavelength of 560 nm, it can be determined whether it is magenta or yellow. Thus, even when the color sensor 200b or 200c is used, the printer controller 103 can determine whether the chart is set in the correct vertical direction.

(Density unevenness calculation method and shading correction amount determination method)
Next, the density unevenness calculation method in step S1212 of FIG. 12 and the shading correction amount determination method in step S1213 will be described.

  FIG. 17 is a diagram showing the density distribution in the main scanning direction of the test pattern obtained in step S1208. In this example, the measurement result of the C (cyan) test pattern is shown, the horizontal axis indicates the position X in the main scanning direction, and the vertical axis indicates the optical density. The test pattern is formed with a density of 100%.

  Note that C (cyan) is described here as an example, but similar processing may be performed for M (magenta), Y (yellow), and K (black).

  As a correction method, there are known a method of changing the degree of modulation of pulse width modulation (PWM) of the laser 108 according to the position in the main scanning direction, and a method of changing the intensity of light irradiated by the laser 108 according to the position in the main scanning direction. Although the above two methods will be described here, the correction method is not limited to these two methods.

(1) Correction of PWM of Laser 108 When correcting the modulation degree of PWM of the laser 108, the modulation degree after correction is obtained by the following equation.
M′PWM = MPWM × β (x)
M′PWM: Modulation degree after correction MPWM: Modulation degree before correction β (x): Correction coefficient in main scanning direction x: Position in main scanning direction

Here, how to obtain the correction coefficient β (x) in the main scanning direction will be described. In the measurement result of the color sensor 200 illustrated in FIG. 17, the lowest density value is Dmin, the density value at the position x in the main scanning direction is D (x), and the printer controller 103 sets the density ratio α (x) as follows. Based on the formula of
α (x) = Dmin / D (x)

  The printer controller 103 corrects the density ratio α (x) in the main scanning direction based on the relationship between the density ratio α (x) and the correction coefficient β (x) in the main scanning direction (FIG. 18A). Convert to coefficient β (x). The relationship between α (x) and β (x) shown in FIG. 18A is stored in advance in the storage unit 350 in the form of an expression, a table, or the like. The correction coefficient for the portion between the measurement positions of the test pattern is calculated by interpolation calculation.

  In this manner, the printer controller 103 can correct the density unevenness in the main scanning direction by obtaining the corrected modulation degree M′PWM and modulating the exposure light so that the modulation degree becomes M′PWM. it can.

(2) Correction of intensity of irradiation light by laser 108 Instead of correcting PWM of the laser 108, the intensity of irradiation light by the laser 108 may be corrected. Therefore, a case where the intensity of light irradiated by the laser 108 is corrected will be described. In this case, the intensity of the irradiation light after correction is obtained by the following equation.
P ′ = P × γ (x)
P ′: intensity of irradiation light after correction P: intensity of irradiation light before correction γ (x): correction coefficient in main scanning direction x: position in main scanning direction

Here, how to obtain the correction coefficient γ (x) in the main scanning direction will be described. In the measurement result of the color sensor 200 illustrated in FIG. 17, the lowest density value is Dmin, the density value at the position x in the main scanning direction is D (x), and the printer controller 103 sets the density ratio α (x) as follows. Based on the formula of
α (x) = Dmin / D (x)

  The printer controller 103 corrects the density ratio α (x) in the main scanning direction based on the relationship between the density ratio α (x) and the correction coefficient γ (x) in the main scanning direction (FIG. 18B). Convert to coefficient γ (x). The relationship between α (x) and γ (x) shown in FIG. 18B is stored in advance in the storage unit 350 in the form of an expression or a table. The correction coefficient for the portion between the measurement positions of the test pattern is calculated by interpolation calculation.

  In this way, the printer controller 103 can correct the density unevenness in the main scanning direction by obtaining the intensity P ′ of the irradiation light from the laser 108 and correcting the intensity of the irradiation light to be P ′. .

  When performing maximum density adjustment, gradation adjustment, and multi-order color correction processing, a patch image is formed in a state in which density unevenness is eliminated using the correction result of main scanning shading correction.

  As described above, according to the present embodiment, the test pattern can be measured even when the chart is set upside down and up and down, so that the user at the time of performing the main scanning shading correction Stress can be reduced.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 102 Engine control part 103 Printer controller (correction means, processing means)
110 sheets 113 storage (paper feeding means)
139 Paper delivery path (paper delivery means)
180 operation unit (display means)
200 Color sensor (measuring means)

Claims (13)

Conveying means for conveying the sheet;
An image forming unit that forms a measurement image on the sheet along a main scanning direction orthogonal to a sheet conveying direction by the conveying unit;
A paper discharge means for discharging the sheet on which the measurement image is formed by the image forming means;
A sheet feeding unit that feeds the sheet once discharged by the sheet discharging unit in a state in which the measurement image formed along the main scanning direction is rotated in a direction along the sheet conveying direction;
Measurement that irradiates the measurement image on the sheet fed by the sheet feeding unit with light, measures reflected light from the measurement image, and outputs measurement values at a plurality of positions in the main scanning direction. Means,
When it is determined that the direction of the sheet set on the sheet feeding unit is upside down based on the measurement result of the measuring unit, a process of rearranging the order of the measurement values at a plurality of positions in the main scanning direction is performed. Processing means to perform;
Calculation means for calculating a correction value for correcting unevenness of the image in the main scanning direction based on the measurement values rearranged by the processing means;
An image forming apparatus comprising: The measurement image is formed for each of a plurality of colors used for image formation,
The measuring means is provided corresponding to each of the plurality of colors,
The processing unit determines whether or not the direction of the sheet set on the sheet feeding unit is upside down by determining the color of the measurement image measured by the measurement unit. The image forming apparatus according to claim 1. The measuring means splits the reflected light according to a wavelength, receives and measures the split light,
The image forming apparatus according to claim 2, wherein the processing unit determines a color of the measurement image based on a measurement value at a specific wavelength of reflected light from the measurement image. Further comprising a reversing means for reversing the front and back of the sheet,
When the processing unit determines that the direction of the sheet set on the sheet feeding unit is reverse based on the measurement result of the measurement unit based on the measurement result of the measurement unit, the processing unit 2. The image forming apparatus according to claim 1, wherein the measurement image is measured by the measurement unit after reversing the front and back of the sheet. The measurement image has a trigger pattern serving as a timing reference for measuring the measurement image at an end in the main scanning direction,
5. The processing unit according to claim 4, wherein when the measurement unit cannot detect the measurement image, the processing unit determines that the direction of the sheet set on the sheet feeding unit is reverse. Image forming apparatus.   2. The image forming apparatus according to claim 1, further comprising display means for displaying the sheet discharged by the paper discharge means so as to be set on the paper supply means in a state rotated by 90 degrees.   The image forming apparatus according to claim 1, wherein the calculation unit calculates a correction value for density unevenness of the image in the main scanning direction.   The image forming apparatus according to claim 1, wherein the calculation unit calculates a correction value for color unevenness of the image in the main scanning direction.   The image forming apparatus according to claim 1, wherein the measurement image is a belt-like pattern extending in the main scanning direction.   The image forming unit forms an electrostatic latent image by exposing a photosensitive drum whose surface is charged in the main scanning direction, and forms the measurement image by developing the electrostatic latent image with toner. The image forming apparatus according to claim 1.   The calculation unit corrects unevenness of an image in the main scanning direction by changing a modulation degree of pulse width modulation of light when exposing the photosensitive drum according to a position in the main scanning direction. The image forming apparatus according to claim 10.   11. The unevenness of the image in the main scanning direction is corrected by changing the light intensity when exposing the photosensitive drum according to the position in the main scanning direction. The image forming apparatus described. Conveying means for conveying the sheet;
An image forming unit that forms a measurement image on the sheet along a main scanning direction orthogonal to a sheet conveying direction by the conveying unit;
A paper discharge means for discharging the sheet on which the measurement image is formed by the image forming means;
A sheet feeding unit that feeds the sheet once discharged by the sheet discharging unit in a state in which the measurement image formed along the main scanning direction is rotated in a direction along the sheet conveying direction;
Measurement that irradiates the measurement image on the sheet fed by the sheet feeding unit with light, measures reflected light from the measurement image, and outputs measurement values at a plurality of positions in the main scanning direction. Means,
Calculation means for calculating a correction value for correcting unevenness of the image in the main scanning direction based on the measurement value output by the measurement means;
When it is determined that the direction of the sheet set on the sheet feeding unit is upside down based on the measurement result of the measuring unit, a process of rearranging the order of correction values at a plurality of positions in the main scanning direction is performed. Processing means to perform;
An image forming apparatus comprising:

Images