JP2014095741A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the color of an image for measurement from being changed due to the influence of heat from fixing means, when a reference chart is measured by measuring means.SOLUTION: An image forming apparatus 100 is configured to select one of a first mode where a patch image is formed on a sheet, the sheet passes through a fixing unit, and then, the patch image is measured by a color sensor 200 and a second mode where the sheet on which the patch image has already been formed is fed and conveyed without an image forming process and passes through the fixing unit, and then, the patch image is measured by the color sensor 200. The temperature of the fixing unit when the second mode is selected is made lower than that when the first mode is selected, to measure the patch image by the color sensor 200.

Description

  The present invention relates to an image forming apparatus having a function of measuring the color of a measurement image.

  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. In today's widespread use of multicolor image forming apparatuses, the most important image quality is sometimes referred to as color reproducibility.

  Humans have a memory of expected colors (especially human skin, blue sky, metal, etc.) based on experience, and feel uncomfortable when the tolerance is exceeded. These colors are called memory colors, and their reproducibility is often asked when outputting photographs.

  Not only for photographic images but also for document images, color reproducibility (stable) for image forming devices, such as office user groups who feel uncomfortable with the color difference from the monitor, and graphic arts user groups who pursue color reproducibility of CG images. (Including sex) is increasing.

  Therefore, an image forming apparatus that reads a measurement image (patch image) formed on a sheet by a color sensor provided in the sheet conveyance path has been proposed in order to satisfy the user's color reproducibility requirements (for example, , See Patent Document 1).

  This image forming apparatus forms a measurement image on a sheet, and applies feedback to process conditions such as exposure amount and development bias based on the result of reading the measurement image by a color sensor, thereby providing a constant density and gradation. It becomes possible to reproduce the nature and color.

JP 2004-086013 A

  However, when a color sensor is arranged in an image forming apparatus and used as an in-line sensor, there is a problem that the measured value fluctuates due to various factors such as the mounting accuracy when mounted on the apparatus, the environment in the apparatus, and changes over time. Occur.

  Therefore, it is necessary to correct the variation of the measurement value of the color sensor. For this reason, it is necessary to measure a reference chart (hereinafter referred to as a reference chart) with a color sensor to correct a variation in the measured value.

  However, if a dedicated storage for storing the reference chart or a dedicated transport path is provided, the apparatus becomes large and complicated. On the other hand, if a storage or transport path used for normal image formation is used without newly providing a dedicated storage or transport path, the reference chart will be heated again by the fixing device, and the heat from the fixing device will There is a problem that the color of the patch image changes.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of preventing the color of a measurement image from changing due to the influence of heat from a fixing unit when measuring a reference chart by a measuring unit. To do.

  To achieve the above object, an image forming apparatus according to the present invention includes an image forming unit that forms a measurement image on a sheet, and a fixing that heats and fixes the measurement image on the sheet formed by the image forming unit. A measurement means for irradiating the measurement object with light and measuring the amount of light at each wavelength of reflected light from the measurement object; and a measurement image on the sheet formed by the image formation means is measured by the measurement means A selection unit that selects one of a first mode to be performed, and a second mode in which the measurement unit measures a measurement image on a sheet without going through an image forming step by the image forming unit, and the selection unit When the second mode is selected, the measurement unit measures the measurement image on the sheet by lowering the temperature of the fixing unit when the second mode is selected. line And having a control unit for, a.

  According to the present invention, when the reference chart is measured by the measuring unit, it is possible to prevent the color of the measurement image from being changed due to the influence of heat from the fixing unit.

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 display screen when a colorimetry mode selection key 414 is selected. It is a flowchart which shows the process in reference | standard chart measurement mode. 6 is a screen showing measurement results of the color sensor 200. It is a figure which shows a direct mapping table.

(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. A latent image is formed on the photosensitive drum 105 by the laser light output from the laser 108. 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 body 106 is transferred by the transfer roller 114 to the sheet 110 conveyed from the storage 113.

  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 switching member 132 switches whether to guide the sheet 110 to the conveyance path 135 or to the conveyance path 139 to the outside. 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 measurement image (hereinafter referred to as 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 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 conveyance path 139 to the outside. The sheet 110 conveyed on the 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.

  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 maximum density of the output image becomes a desired value, and sends the calculated correction amount to the engine control unit 102. Send. 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 detection result of the patch image including the multi-order color, converts the input image using the profile, and forms 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. 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 instead of the output ICC profile stored 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.

  The first fixing heater is a heater provided in the first fixing device 150, the second fixing heater is a heater provided in the second fixing device 160, and these heaters are controlled by the engine control unit 102. Is done. The temperature sensor 154 as a temperature measuring unit is a sensor that measures the temperature of the first fixing device 150. The engine control unit 102 also controls a conveyance roller drive motor 311 for driving various conveyance rollers in the image forming apparatus 100.

(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).

(Operation section)
FIG. 6 is a diagram illustrating the operation unit 180. 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, a user mode key for entering a user mode in which the user manages and sets the image forming apparatus 100, such as specifying a calibration mode, registering sheet information, and changing a setting time until entering an energy saving mode. 410 is provided.

  The touch panel display 406 is provided with a full color image formation mode selection key 412, a monochrome image formation mode selection key 413, and a color measurement mode selection key 414.

(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 sensor adjustment mode key 422 is a key for instructing execution of a sensor adjustment mode for adjusting the output value of the color sensor 200. A clustering mode key 423 is a key for instructing execution of a clustering mode for matching colors with other image forming apparatuses.

  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 a calibration instruction from the operation unit 180 (S802). 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 (S803), and the gradation adjustment described later in FIG. 10 is performed (S804). Thereafter, multi-order color correction processing described later with reference to FIG. 11 is performed (S805). If there is no multi-order color correction instruction in step S802, 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 feeds the sheet 110 from the storage 113 (S806), and forms a toner image on the sheet 110 (S807). Then, the printer controller 103 determines whether or not image formation for all pages has been completed (S808). If image formation for all pages has been completed, the process returns to step S801. If not, the process returns to step S806 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 feeds the sheet 110 from the storage 113 (S901), and instructs the engine control unit 102 to form a patch image for maximum density adjustment on the sheet 110 (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 feeds the sheet 110 from the storage 113 (S1001), and instructs the engine control unit 102 to form a patch image (16 gradations) for gradation adjustment 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 feeds the sheet 110 from the storage box 113 (S1101), and instructs the engine control unit 102 to form a patch image for multi-order color correction processing on the sheet 110 (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.

(Reference chart measurement mode)
Next, the reference chart measurement mode will be described. In the reference chart measurement mode, a color value and density are measured using the color sensor 200 with a chart output from another image forming apparatus as a reference chart. In this mode, the reference chart can be measured using the chart once output by the image forming apparatus 100 as a reference chart.

  The color value and density of the reference chart are measured by an instruction from the host computer or the operation unit 180. In the present embodiment, a method of issuing a measurement instruction from the operation unit 180 and displaying the measurement result on the touch panel display 406 will be described, but the present invention is not limited to this.

  The reference chart is prepared in advance so that the location corresponding to the position of the color sensor 200 is measured, and the patch image on the reference chart is arranged at a position corresponding to this measurement position.

  First, when measuring the color of the patch image on the reference chart, the color measurement mode selection key 414 on the operation unit 180 shown in FIG. 6 is selected by the user. When the colorimetry mode selection key 414 is selected, a screen shown in FIG. 12A is displayed. On this screen, a message “Please set the chart to be measured in the cassette X” is displayed.

  The user who has confirmed the screen of FIG. 12A sets a reference chart to be measured in the storage (cassette) 113 and selects an OK key. In response to this instruction, the color measurement operation of the chart by the color sensor 200 is started. Details of the color measurement mode will be described below with reference to flowcharts.

  FIG. 13 is a flowchart showing processing in the reference chart measurement mode. This flowchart is executed by the printer controller 103 in response to selection of the OK key on the screen of FIG.

  First, the printer controller 103 stops power supply to the first fixing device 150 until the temperature of the first fixing device 150 becomes a predetermined temperature or lower (60 ° C. or lower) based on the output of the temperature sensor 154. Wait (S1301). In the present embodiment, since the toner having a softening point temperature required to soften the toner is 73.9 ° C., the predetermined temperature is set to 60 ° C. If the temperature is 60 ° C. or less from the beginning, the process proceeds to the next step without supplying power to the first fixing device 150 without waiting in step S1301.

  Note that the temperature of the first fixing device 150 in the multi-order color correction mode (first mode) shown in FIG. 11 is set to 150 ° C. Therefore, the temperature (60 ° C.) of the first fixing device 150 in the reference chart measurement mode (second mode) is higher than that of the first fixing device 150 in the multi-color correction mode (first mode). That is, it is set lower than the temperature (150 ° C.).

  In a state where the temperature of the first fixing device 150 is 60 ° C. or less, the printer controller 103 instructs the engine control unit 102 to start feeding the chart set in the storage 113 (S1302). . Then, based on an instruction from the printer controller 103, the engine control unit 102 conveys the chart and passes the fixing device 150 (S1303).

  Here, the temperature of the first fixing device 150 is lowered to less than 60 ° C., and even if the chart passes through the first fixing device 150, the color value and density value of the chart are hardly affected. Note that the pressing force of the first fixing device 150 only needs to be sufficient to sandwich and convey the chart, and may be sufficiently lower than the normal image forming mode. The pressure is changed by moving one of a plurality of rollers that convey the pressure belt 152 in a direction away from the fixing roller 151 using a cam mechanism. The reference chart passes through the fixing device 150 and then passes through a conveyance path that bypasses the second fixing device 160.

  Next, the printer controller 103 waits until the chart reaches the measurement position of the color sensor 200 (S1304). When the chart reaches the measurement position, the printer controller 103 causes the color sensor 200 to measure the chart (S1305). At this time, the color sensor 200 outputs spectral reflectance data of the patch image on the chart. During the measurement operation by the color sensor 200, a screen informing that the measurement is being performed as shown in FIG. 12B is displayed.

  Next, the printer controller 103 converts the spectral reflectance data output from the color sensor 200 into color value data (L * a * b * data) using the Lab calculation unit 303 (S1306). Further, the printer controller 103 converts the spectral reflectance data output from the color sensor 200 into density data using the density conversion unit 324 (S1307).

  Next, the printer controller 103 displays the screen shown in FIG. 14 on the touch panel display 406 of the operation unit 180 based on the calculated color value data and density data (S1308). On the screen shown in FIG. 14, color values (L, a, b) and densities (D_C, D_M, D_Y, D_K) are displayed for each of a plurality of patch images on the chart.

  The measured color value data and density data can be transferred to a host computer or stored in an external memory. Thereafter, the printer controller 103 discharges the chart to the outside of the image forming apparatus 100 (S1309), and ends the processing according to this flowchart.

(Correction coefficient calculation method)
The measurement value (color value data) of the color sensor 200 must be equal to the measurement value of the external measuring instrument serving as a reference for color measurement, but the color value data calculated in step S1306 is the environment in the apparatus and the change over time. For this reason, there is a possibility that the measured value of the reference external measuring instrument is deviated. For this reason, it is necessary to match the measurement value (color value data) of the color sensor 200 with the measurement value of the external measuring device using the correction coefficient.

  In the present embodiment, the color value data (L * a * b * data) of the reference chart measured by the color sensor 200 is converted into L * ′ a * ′ b * ′ data of the reference chart measured by an external measuring instrument. As a correction coefficient for this, a direct mapping table is used.

  As the reference chart, a chart suitable for direct mapping is used, and the Lab color space region is divided into grid points and the distance between grid points is substantially the same. In this embodiment, the number of grid points is 216, and the grid points are evenly arranged in the color space region. Note that the number of grid points of the reference chart is not limited to this, and the number may be appropriately increased or decreased according to the performance of the printer.

  The printer controller 103 associates the L * a * b * data and L * ′ a * ′ b * ′ data for 216 colors, and creates a direct mapping table as shown in FIG. In the present embodiment, linearly interpolated values are used for values between lattice points. Although a method for creating a direct mapping table has been described here, correction may be performed using a transformation matrix or the like.

  In the present embodiment, control is performed to lower the temperature of the fixing device 150 to 60 ° C. or less so as not to affect the color value and density value of the chart once output. However, in a state where the temperature of the fixing device 150 is low, such as immediately after the main power supply of the image forming apparatus 100 is turned on, it is not necessary to wait until the temperature of the fixing device 150 decreases, and measurement can be started immediately. Therefore, it is convenient for the user to display on the operation unit 180 that the chart can be measured immediately after the main power is turned on until the temperature of the first fixing device 150 reaches 60 ° C. The sex is further improved.

  As described above, according to the present embodiment, when the reference chart is measured by the color sensor 200, it is possible to prevent the color of the patch image from changing due to the influence of heat from the fixing device. Color stability can be improved.

  Furthermore, according to the present embodiment, there is no need to provide a dedicated storage for storing the reference chart or a dedicated transport path, so that the apparatus can be prevented from becoming large and complicated.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 102 Engine control part 103 Printer controller (control means)
110 sheet 150 first fixing device (fixing means)
154 Temperature sensor (temperature measuring means)
160 Second fixing device (fixing means)
200 Color sensor (measuring means)
303 Lab calculator (first calculator)
324 Density converter (second computing means)

Claims (8)

An image forming means for forming an image for measurement on a sheet;
Fixing means for heating and fixing the measurement image on the sheet formed by the image forming means;
Measuring means for irradiating the measurement object with light and measuring the light quantity at each wavelength of the reflected light from the measurement object;
A first mode in which a measurement image is formed on the sheet by the image forming unit, the sheet is transported and passed through the fixing unit, and then the measurement image on the sheet is measured by the measurement unit; The sheet on which the measurement image has already been formed is fed, and the sheet is conveyed without passing through the image forming unit and passed through the fixing unit. Selection means for selecting one of the second modes to be measured by the measurement means;
When the second mode is selected by the selection unit, the temperature of the fixing unit is lowered and the measurement image on the sheet is measured by the measurement unit when the second mode is selected. Control means for performing the measurement of
An image forming apparatus comprising:   2. The image forming apparatus according to claim 1, wherein the sheet conveyance path in the first mode is the same as the sheet conveyance path in the second mode. The fixing unit includes a first fixing device and a second fixing device provided downstream of the first fixing device in the sheet conveying direction,
In the second mode, the control unit conveys the sheet on which the measurement image is formed and passes the sheet through the first fixing device, and then passes the sheet to a conveying path that bypasses the second fixing device. The image forming apparatus according to claim 1, wherein the image for measurement is guided and then the measurement image is measured by the measurement unit.   2. The image forming apparatus according to claim 1, further comprising a display unit that displays a measurement value of the measurement unit in the second mode. A temperature measuring means for measuring the temperature of the fixing means;
When the second mode is selected by the selection unit, the image forming unit waits until the temperature of the fixing unit becomes a predetermined temperature or less, and then causes the measurement unit to measure the measurement image. The image forming apparatus according to claim 1. The image forming unit forms an image for measurement on a sheet using toner,
6. The image forming apparatus according to claim 5, wherein the predetermined temperature is set to a temperature lower than a softening point temperature of the toner.   The image forming apparatus according to claim 1, wherein the measurement image formed by the image forming unit is a multi-order color image formed by overlapping a plurality of color materials. First calculation means for calculating a color value of the measurement image based on the measurement value of the measurement means;
Second calculating means for calculating the density of the measurement image based on the measurement value of the measuring means;
The image forming apparatus according to claim 1, further comprising:

Images