JP2013088475A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of precisely detecting chromaticity of an image for measurement by reducing chromaticity variation due to influence of thermochromism and capable of improving productivity.SOLUTION: An image forming device 100 executes multi-color correction processing in which an image formation condition is controlled on the basis of a chromaticity value of a patch image detected by a color sensor 200, and target value arithmetic processing in which a target value of density of the patch image is calculated from a density value of the patch image detected by a density sensor 170. During execution of the multi-color correction processing, the image forming device 100 performs the target value arithmetic processing in a time period from after passing the recording paper 110 through a fixing device until measuring color by the color sensor 200.

Description

  The present invention relates to an image forming apparatus having a colorimetric function.

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

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

JP 2004-086013 A

  However, in the invention of Patent Document 1, since a color sensor is arranged in the conveyance path in the vicinity of the fixing device, a phenomenon called “thermochromism” in which the chromaticity of a patch image to be measured changes with temperature is a problem. Become. This is a phenomenon caused by a change in the molecular structure forming a color material such as toner or ink due to “heat”.

  Here, in order to measure the color of the patch image inside the image forming apparatus, it is necessary that the color mixture is completed after the color material is placed on the recording paper. In an image forming apparatus that uses ink as a color material, it is necessary to perform color measurement after drying by heating with a drying device. In an image forming apparatus using toner as a color material, it is necessary to measure the color after the toner is heated and melted and mixed by a fixing device. Therefore, the color sensor needs to be disposed downstream of the drying device and the fixing device in the recording paper conveyance direction.

  On the other hand, in order to make the image forming apparatus compact, the length of the conveyance path from the drying device or the fixing device to the color sensor needs to be kept to the minimum necessary. Therefore, the recording paper and the color material heated by the drying device or the fixing device are conveyed to the color sensor without being cooled to room temperature. Further, the temperature of the recording paper becomes higher than the normal temperature also due to the temperature rise of the members inside the image forming apparatus such as the recording paper conveyance guide and the atmosphere inside.

  As described above, in an image forming apparatus including a color sensor therein, a color measurement result different from the chromaticity under a normal environment (normal temperature environment) may be obtained under the influence of thermochromism. Therefore, in order to detect the chromaticity of the measurement image with high accuracy, it is necessary to reduce the influence of thermochromism by radiating or cooling the recording paper until it is close to the normal environment and then measuring the color with a color sensor. There is.

  However, if the image forming apparatus waits for the operation until the heat radiation or cooling of the recording paper is completed, a downtime is caused correspondingly, and productivity is lowered. In order to perform the chromaticity correction processing with high accuracy, it is necessary to perform density adjustment in advance. However, if a part of the density adjustment operation is performed using the standby time in the chromaticity correction processing, As a result, the overall processing time can be shortened and productivity can be improved.

  Therefore, the present invention provides an image forming apparatus capable of reducing chromaticity fluctuations due to the effect of thermochromism, detecting the chromaticity of a measurement image with high accuracy, and improving productivity. Objective.

  In order to achieve the above object, an image forming apparatus according to the present invention includes an intermediate transfer body for transferring an image formed of a color material onto a conveyed recording paper, and the intermediate transfer body or the recording paper depending on the color material. An image forming means for forming a measurement image on the sheet, a density detection means for detecting the density of the measurement image formed on the intermediate transfer member, and a fixing for heating and fixing the measurement image formed on the recording paper Means, a chromaticity detecting means for detecting the chromaticity of the measurement image formed on the recording paper downstream of the fixing means in the conveyance direction of the recording paper, and the measurement detected by the chromaticity detecting means A first control for controlling an image forming condition based on a chromaticity value of the image, and a second control for calculating a target value of the density of the measurement image from the density value of the measurement image detected by the density detection means; Control hand to perform And the control means during the execution of the first control until the color measurement by the chromaticity detection means is performed after the recording paper passes through the fixing means. The target value in the second control is calculated.

  According to the present invention, it is possible to reduce chromaticity variation due to the influence of thermochromism, to accurately detect the chromaticity of a measurement image, and to improve productivity.

2 is a cross-sectional view showing the structure of the image forming apparatus 100. FIG. 2 is a diagram illustrating a structure of a color sensor 200. FIG. 1 is a block diagram showing a system configuration of an image forming apparatus 100. FIG. 1 is a schematic diagram of a color management environment. It is a figure which shows the tendency of the chromaticity change for every coloring material. It is a figure which shows the tendency of the density change for every coloring material. This is spectral reflectance data at each temperature when the color sensor 200 measures a magenta patch image. It is a figure which shows the filter sensitivity characteristic used for a density | concentration calculation process. 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 a target value calculation process.

(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. This is because the present invention is effective in an image forming apparatus in which a thermochromism phenomenon in which the chromaticity of an object to be measured changes with temperature can occur. In the ink jet system, an image forming unit that forms an image on recording paper by discharging ink and a fixing unit (drying unit) that dries the ink are used.

  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, and 124 corresponding to YMCK. Stations 120, 121, 122, and 124 are image forming units that transfer toner onto the recording paper 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 member 106 is transferred to the recording paper 110 conveyed from the storage 113 by the transfer roller 114.

  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 recording paper 110 and fix the toner image on the recording paper 110. The first fixing device 150 includes a fixing roller 151 for applying heat to the recording paper 110, a pressure belt 152 for pressing the recording paper 110 against the fixing roller 151, and a first post-fixing sensor for detecting completion of fixing. 153. These rollers are hollow rollers and each have a heater inside.

  The second fixing device 160 is arranged downstream of the first fixing device 150 in the conveyance direction of the recording paper 110. The second fixing device 160 imparts gloss (gloss) to the toner image on the recording paper 110 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 recording paper 110, it is not necessary to pass through the second fixing device 160. In this case, the recording paper 110 passes through the transport path 130 without going through the second fixing device 160 for the purpose of reducing energy consumption.

  For example, when the setting is made to add a lot of gloss to the image on the recording paper 110, or when the recording paper 110 requires a large amount of heat for fixing like a thick paper, it passes through the first fixing device 150. The recording paper 110 is also conveyed to the second fixing device 160. On the other hand, when the recording paper 110 is plain paper or thin paper and the setting for adding a lot of gloss is not made, the recording paper 110 is transported through a transport path 130 that bypasses the second fixing device 160. Whether the recording paper 110 is conveyed to the second fixing device 160 or whether the recording paper 110 is conveyed bypassing the second fixing device 160 is controlled by switching the flapper 131.

  The conveyance path switching flapper 132 is a guide member that guides the recording paper 110 to the discharge path 135 or guides the recording paper 110 to the discharge path 139 to the outside. The leading edge of the recording paper 110 guided to the discharge 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 recording paper 110, the conveyance direction of the recording paper 110 is switched. The transport path switching flapper 133 is a guide member that guides the recording paper 110 to the transport path 138 for forming a double-sided image or guides the recording paper 110 to the discharge path 135.

  A color sensor 200 that detects a patch image on the recording paper 110 is disposed in the discharge path 135. Four color sensors 200 as chromaticity detection means are arranged side by side in a direction orthogonal to the conveyance direction of the recording paper 110, and can detect four rows of patch images. When color detection 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 conveyance path switching flapper 134 is a guide member that guides the recording paper 110 to the discharge path 139 to the outside. The recording paper 110 conveyed through the discharge path 139 is discharged outside the image forming apparatus 100.

  A density sensor 170 serving as a density detection unit is provided to face the intermediate transfer member 106. The density sensor 170 is a regular reflection type sensor that includes a light emitting element 171 composed of a light emitting diode (LED) and a light receiving element 172. In the present embodiment, a regular reflection type sensor is used, but a diffuse reflection type sensor or a combination of a regular reflection type and an irregular reflection type may be used, but the present invention is not limited to this.

(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 emits light to the patch image 220 on the recording paper 110. The diffraction grating 202 separates the light reflected 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 performs a spectral calculation from a light intensity value, a Lab calculation unit that calculates a Lab value, and the like. Further, a lens 206 for condensing the light emitted from the white LED 201 onto the patch image 220 on the recording paper 110 or condensing the light reflected from the patch image 220 onto the diffraction grating 202 may be further provided. .

(Maximum density adjustment)
FIG. 3 is a block diagram illustrating a system configuration of the image forming apparatus 100. 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 adjusting the maximum density of each color of YMCK is formed on the recording paper 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 instructs the color sensor control unit 302 to perform color measurement of the patch image.

  When the color sensor 200 performs color measurement of the patch image, the color 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 recording paper 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 of 16 gradations of each color of YMCK is formed on the recording paper 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 recording paper 110, the engine control unit 102 instructs the color sensor control unit 302 to perform color measurement of the patch image.

  When the color sensor 200 performs color measurement of the patch image, the color 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.

(Paper patch control)
Since the above-described gradation adjustment takes time to control, it is not performed during a job. Therefore, during the job, control is performed to form a patch image at an interval between the images on the intermediate transfer member 106 (between sheets), measure the density variation of the patch image, and reduce the density variation.

  In this inter-sheet patch control, a patch image having a specific halftone density (40H in this embodiment) is formed on the intermediate transfer member 106 among the patch images formed at the time of gradation adjustment described above, and the patch image The density is detected by the density sensor 170. The density sensor 170 is driven by the density sensor control unit 328 based on an instruction from the engine control unit 102. The output signal of the density sensor 170 is sent to the density conversion unit 325.

  The density conversion unit 325 converts the output signal from the density sensor 170 into CMYK density data, and the single color set in the LUT unit 323 so that the density data becomes the target value T set in the target value storage unit 327. The gradation LUT is corrected. By controlling the image forming conditions in this way, desired gradation characteristics can be obtained.

(Profile)
In performing the multi-order color correction process, the image forming apparatus 100 creates a profile 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, 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, or a color separation table in Photoshop.

  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. 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 color 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 recording paper 110. The color sensor control unit 302 controls the color sensor 200 to measure the color of the ISO12642 test form. The color sensor 200 outputs spectral reflectance data, which is a color 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.

(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 → L * a * b * or CMYK → L * a * b * 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 * chromaticity 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 GUMAT 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 colorimetry 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, and converts it into CMYK signals depending on the output device.

  The LUT unit 323 corrects the gradation of the CMYK signal using the LUT set by the LUT creation unit 322 described later. The CMYK signal whose gradation has been corrected is output to the engine control unit 102.

  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. 4, 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).

(Thermochromism color characteristics)
Next, thermochromism characteristics for each color will be described. When the molecular structure forming the color material such as toner or ink is changed by heat, the reflection / absorption characteristic of light is changed and the chromaticity is changed. As a result of experimentation and verification, it was found that the tendency of chromaticity change differs for each color material as shown in FIG. In this figure, the horizontal axis indicates the temperature of the patch image, and the vertical axis indicates the chromaticity change ΔE with 15 ° C. as a reference.

  ΔE can be expressed by the following three-dimensional distance between two points (L1, a1, b1) and (L2, a2, b2) in the L * a * b * color space defined by CIE. .

  In FIG. 5, C: cyan 100%, M: magenta 100%, Y: yellow 100%, K: black 100%, W: paper white. As shown in this figure, the magenta chromaticity change is particularly large. As the temperature of the patch image increases, the chromaticity change of the patch image increases, and an error occurs in the ICC profile to be created.

  As an index for color matching accuracy and color stability, in the color matching accuracy standard described in ISO 12647-7 (IT 8.7 / 4 (ISO 12642: 1617 patch) [4.2.2]), ΔE average is 4 .0. Further, reproducibility [4.2.3], which is a stability standard, defines that ΔE ≦ 1.5 of each patch. In order to satisfy this condition, the detection accuracy of the color sensor 200 is desirably ΔE ≦ 1.0. As shown in FIG. 5, in order to realize ΔE ≦ 1.0 for all colors of YMCK, it is necessary to dissipate the temperature of the patch image to 34 ° C. or lower.

(Relationship between temperature and concentration value)
As described above, the chromaticity value (Lab value) varies with temperature. On the other hand, as a result of examination by the present applicant, it has been found that the concentration value hardly changes even when the temperature changes, and has no correlation with the temperature. The result is shown in FIG.

  The phenomenon that the chromaticity value changes but the density value does not change when the temperature changes can be explained from the region where the spectral reflectance changes, and the difference in the calculation method when calculating the chromaticity value and density value. it can. This will be described by taking magenta (M) having a large chromaticity change ΔE with respect to a temperature change as an example.

  FIG. 7 shows spectral reflectance data at each temperature when the color sensor 200 measures a magenta patch image. 7A is an enlarged view of the entire wavelength region of 400 to 700 nm, FIG. 7B is an enlarged view of the wavelength region of 550 to 650 nm, and FIG. 7C is an enlarged view of the wavelength region of 500 to 580 nm.

  As shown in FIG. 5, when the temperature of the patch image changes in the range of 15 ° C. to 60 ° C., magenta has a chromaticity change ΔE of about 2.0, and this chromaticity change ΔE has a spectral reflectance. Because it changes. Although it is difficult to understand the change in the spectral reflectance in FIG. 7A, it can be seen from FIG. 7B in which the wavelength region of 550 to 650 nm is enlarged that the spectral reflectance changes due to the temperature change of the patch image. This is because the Lab calculator 303 calculates the chromaticity using the spectral reflectance for the entire wavelength region, so that the chromaticity value changes due to the change in the spectral reflectance.

  On the other hand, as shown in FIG. 6, even if the temperature of the patch image changes in the range of 15 ° C. to 60 ° C., the density hardly changes. This is because the density conversion unit 324 calculates the density using the spectral reflectance for a specific wavelength region. Specifically, the density conversion unit 324 converts spectral reflectance data into density data for cyan, magenta, and yellow using the filter shown in FIG. In addition, for black, the density conversion unit 324 converts spectral reflectance data into density data using the visibility spectral characteristics as shown in FIG. 8B.

  It can be seen that there is almost no change in the spectral reflectance in the wavelength region in FIG. The area shown in FIG. 7C is an area having a green sensitivity characteristic in the wavelength range on the horizontal axis shown in FIG. 8A. For magenta, the density is obtained using the green sensitivity characteristic which is a complementary color. A value is calculated. Therefore, in this region, there is almost no change in the density value because there is almost no change in the spectral reflectance even if the temperature changes.

  As described above, the chromaticity of the patch image changes due to the temperature change, while the density of the patch image hardly changes due to the temperature change. Therefore, in this embodiment, at the time of multi-order color correction (ICC profile creation), the recording paper 110 heated by the fixing device is radiated and then the color measurement is performed by the color sensor 200. However, at the time of maximum density adjustment or gradation adjustment, color measurement is performed by the color sensor 200 without radiating the recording paper 110.

(Thermochromism technology)
FIG. 9 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 (S901).

  If there is no image formation request, the printer controller 103 determines whether there is a multi-order color correction instruction from the operation unit 180 (S902). When there is a multi-order color correction instruction, the maximum density adjustment described later in FIG. 10 is performed (S903), and the gradation adjustment described later in FIG. 11 is performed (S904). Thereafter, multi-order color correction processing, which will be described later with reference to FIG. 12, is performed (S905). If there is no multi-order color correction instruction in step S902, the process returns to step S901 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 S901 that there is an image formation request, the printer controller 103 feeds the recording paper 110 from the storage 113 (S906), and forms a toner image on the recording paper 110 (S907). Then, the printer controller 103 determines whether image formation for all pages has been completed (S908). If image formation for all pages has been completed, the process returns to step S901. If not, the process returns to step S906 to perform image formation for the next page. Each time a predetermined number of images are formed, the above-described inter-sheet patch control is executed in order to stabilize the density.

  FIG. 10 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 recording paper 110 from the storage 113 (S1001), and forms a patch image for adjusting the maximum density of each color of YMCK on the recording paper 110 (S1002). Next, when the recording paper 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 the correction amount of the charging potential, the exposure intensity, and the developing bias based on the converted density data (S1005). The correction amount calculated here is stored in the storage unit 350 and used.

  FIG. 11 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 recording paper 110 from the storage 113 (S1101), and forms a patch image (16 gradations) for gradation adjustment of each color of YMCK on the recording paper 110 (S1102). Next, when the recording paper 110 reaches the color sensor 200, the printer controller 103 causes the color sensor 200 to measure a patch image (S1103).

  Then, the printer controller 103 uses the density conversion unit 324 to convert the spectral reflectance data output from the color sensor 200 into CMYK density data (S1104). Thereafter, the printer controller 103 creates an LUT for correcting gradation based on the converted density data (S1105). The LUT calculated here is set in the LUT unit 323 and used.

  FIG. 12 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 recording paper 110 from the storage 113 (S1201), and forms a patch image for multi-order color correction processing on the recording paper 110 (S1202). Next, the printer controller 103 waits until the reversing sensor 137 detects the trailing edge of the recording paper 110 and detects that the recording paper 110 has reached the reversing unit 136 (S1203). When the recording paper 110 reaches the reversing unit 136, the printer controller 103 controls the conveyance roller drive motor 311 to stop conveyance of the recording paper 110 (S1204).

  When the conveyance of the recording paper 110 is stopped in step S1204, the printer controller 103 performs a target value calculation process which will be described later with reference to FIG. 13 (S1205). This target value calculation process is a process for calculating the target value T used in the above-described inter-sheet patch control. Since both the maximum density adjustment and the gradation adjustment have been executed at this time, the output image has been adjusted so that the density becomes a desired density. Accordingly, it is necessary to form a patch image on the intermediate transfer member 106 at this time and set the density value of the patch image to the target value T in the inter-sheet patch control.

  When the target value calculation process ends, the printer controller 103 determines whether or not a predetermined time (40 seconds in this embodiment) has elapsed since the conveyance of the recording paper 110 was stopped in step S1204 (S1206). Whether or not the predetermined time has passed is determined based on the count value of the timer started after the conveyance of the recording paper 110 is stopped. In this way, the patch image on the recording paper 110 is radiated by stopping the conveyance of the recording paper 110 for a predetermined time. As a result, the change in chromaticity due to the influence of thermochromism can be reduced.

  As shown in FIG. 5, in order to realize ΔE ≦ 1.0 for all colors of YMCK, it is necessary to dissipate the temperature of the patch image to 34 ° C. or lower. In this embodiment, 40 seconds is set as the time required for this heat dissipation. If the recording paper 110 is stopped for 40 seconds, both the first fixing heater 312 provided in the first fixing device 150 and the second fixing heater 313 provided in the second fixing device 160 are used. Even if it exists, it can thermally radiate to 34 degrees C or less.

  When the stop time of 40 seconds elapses, the printer controller 103 controls the conveyance roller drive motor 311 to resume conveyance of the recording paper 110 (S1207). At this time, the printer controller 103 conveys the recording sheet 110 toward the color sensor 200 with the conveying direction of the recording sheet 110 reversed.

  When the recording paper 110 reaches the color sensor 200, the printer controller 103 causes the color sensor 200 to measure a patch image (S1208). Then, the printer controller 103 uses the Lab calculator 303 to calculate chromaticity data (L * a * b *) from the spectral reflectance data output from the color sensor 200. Based on the chromaticity data (L * a * b *), the printer controller 103 creates an ICC profile by the above-described processing (S1209) and stores it in the output ICC profile storage unit 305 (S1210).

  FIG. 13 is a flowchart showing the operation of the target value calculation 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. As shown in FIG. 12, the target value calculation process is executed during a period in which the conveyance of the recording paper 110 is stopped and the heat is dissipated.

  First, the printer controller 103 forms a patch image for inter-sheet patch control on the intermediate transfer member 106 (S1301). The signal value of the patch image formed here is 40H as described above. Next, the printer controller 103 measures the density of the patch image using the density sensor 170 (S1302).

  The printer controller 103 calculates the density of the patch image as the target value T used for the inter-sheet patch control (S1303), and stores it in the target value storage unit 327 (S1304). That is, in this flowchart, the output signal from the density sensor 170 that has detected the patch image is converted into YMCK density data, and this density data is stored in the target value storage unit 327 as the target value T.

  In the inter-sheet patch control, the printer controller 103 compares the measured value of the patch image formed on the intermediate transfer member 106 during the continuous job with the target value T stored in the target value storage unit 327 in step S1304. , LUT correction is performed.

(Explanation of effect)
By performing the above control, it is possible to reduce the chromaticity variation due to the influence of thermochromism, to detect the chromaticity of the patch image with high accuracy, and to improve the productivity. Table 1 shows a comparison of the total time when all of the maximum density adjustment, gradation adjustment, multi-order color correction processing, and target value calculation processing are executed in the present embodiment and the comparative example. . In the comparative example, the target value calculation process is performed after the multi-order color correction process is completed.

  In the target value calculation process for patch control between sheets, a patch image of a 40H image signal is formed on the intermediate transfer member 106, and the density of this patch image is measured. This takes about 30 seconds. there were. Note that the target value T is not limited to a patch image with an image signal of 40H. In order to further improve the accuracy, the target value of other image signals may be added to form and measure a patch image. In this case, the target value calculation process is longer than 30 seconds.

  As can be seen from Table 1, the present embodiment can reduce the total time by 30 seconds, that is, 20% by executing the target value calculation process in the multi-order color correction process as compared with the comparative example. .

  As described above, in the present embodiment, during the execution of the multi-order color correction process, the target value calculation is performed after the recording paper 110 passes through the fixing device and before the color measurement is performed by the color sensor 200. Processed. In particular, in the present embodiment, the target value calculation process used in the inter-sheet patch control is performed while the conveyance of the recording paper 110 is stopped and the patch image is dissipated. Accordingly, in the present embodiment, chromaticity variation due to the effect of thermochromism can be reduced, the chromaticity of the measurement image can be detected with high accuracy, and productivity can be improved.

  In the above description, the recording paper 110 is temporarily stopped until the color measurement is performed by the color sensor 200 after the recording paper 110 passes through the fixing device, so that the recording paper 110 is dissipated. did. However, instead of temporarily stopping the recording sheet 110, the colorimetric timing may be delayed by reducing the conveyance speed of the recording sheet 110.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 102 Engine control part (control means)
103 Printer controller (control means)
106 Intermediate transfer member 110 Recording paper 150 First fixing device (fixing means)
160 Second fixing device (fixing means)
170 Concentration sensor (concentration detection means)
200 Color sensor (chromaticity detection means)

Claims (5)

An intermediate transfer body for transferring an image formed of a color material to a recording paper to be conveyed;
Image forming means for forming an image for measurement on the intermediate transfer member or the recording paper with a color material;
Density detecting means for detecting the density of the measurement image formed on the intermediate transfer member;
Fixing means for heating and fixing the measurement image formed on the recording paper;
Chromaticity detection means for detecting the chromaticity of the measurement image formed on the recording paper downstream of the fixing means in the conveyance direction of the recording paper;
A first control for controlling an image forming condition based on a chromaticity value of the measurement image detected by the chromaticity detection unit; and a density of the measurement image based on the density value of the measurement image detected by the density detection unit. Control means for executing a second control for calculating the target value of
The control means is configured to execute the first control in the second control during the period from when the recording paper passes through the fixing means to when the color measurement by the chromaticity detection means is performed. An image forming apparatus that performs calculation of a target value.   In the first control, the control unit temporarily stops the recording sheet from the time when the recording sheet passes through the fixing unit to the time when the color measurement by the chromaticity detection unit is performed. The image forming apparatus according to claim 1, wherein a timing at which color measurement is performed by the chromaticity detection unit is delayed.   In the first control, the control means decelerates the conveyance speed of the recording paper after the recording paper passes through the fixing means and reaches the chromaticity detection means, thereby detecting the chromaticity detection. 2. The image forming apparatus according to claim 1, wherein the timing at which the color measurement is performed by the means is delayed.   The control means executes a third control for controlling an image forming condition based on the density value of the measurement image detected by the density detection means and the target value calculated in the second control. The image forming apparatus according to claim 1. The image forming means is means for transferring the toner onto the recording paper to form the image;
The image forming apparatus according to claim 1, wherein the fixing unit is a unit that heats the toner and fixes the toner onto the recording sheet.

Images