JP2009143188A - Image-forming apparatus and calibration method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image-forming apparatus which avoids an influence due to a variation of a white reference plate while moderating an increase in cost by eliminating the need of the white reference plate for calibration and realizes a good image density control by improving a color-detecting accuracy of a sensor. <P>SOLUTION: The image-forming apparatus comprises a detection part for detecting a density or a chromaticity of an image and a control part for controlling an image-forming conditions based on a detected density or chromaticity. The detection part comprises a light-emitting part for emitting light and a first light-receiving part for receiving a reflected light from an image fixed on a recording medium of the emitted light, and a density or a chromaticity of the image is detected with a signal output from the first light-receiving part. The image-forming apparatus further comprises a second light-receiving section provided separately from the first light-receiving part for receiving a light other than the reflected light of the light emitted from the light-emitting part, and a calibration part for calibrating the detection part based on a signal output from the second light-receiving part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention generally relates to an image forming apparatus, and more particularly to a calibration method for improving image quality in an image forming apparatus.

  2. Description of the Related Art Image forming apparatuses such as printers and copiers that employ an electrophotographic method or an ink jet method have become widespread. With the widespread use, quality requirements for output images have become stricter year by year. However, it is not easy to improve the quality of the output image. This is because the state of each part of the image forming apparatus fluctuates due to environmental changes and long-term use, so that the density and color of the output image also fluctuate. In particular, in the case of an electrophotographic color image forming apparatus, even a slight environmental change may cause a color variation, which may result in a loss of color balance.

  Conventionally, an invention for improving the quality of an image by obtaining a certain color and gradation of color has been proposed. Patent Document 1 discloses an invention in which a toner patch is formed with a single color toner on an intermediate transfer member or a photosensitive member, and the density is controlled by feeding back process conditions and a LUT (look-up table) according to the density of the toner patch. is suggesting. Patent Document 2 proposes a density measurement method for density detection images.

  By the way, the color balance also changes depending on the transfer efficiency when transferring the toner image to the recording medium and the heating and pressurization by fixing. In the invention described in Patent Document 1, since the density of an unfixed toner image is detected, it is impossible to cope with a change in color balance due to transfer to a recording medium and fixing.

Patent Document 3 discloses the density or color of cyan (C), magenta (M), yellow (Y), black (K) single-color gradation patches or C, M, Y mixed color patches fixed on a recording medium. A color image forming apparatus having a color sensor for detecting the degree is proposed. This color image forming apparatus feeds back a detection result to a calibration table for correcting an exposure amount, process conditions, and density-gradation characteristics.
Japanese Patent No. 3430702 Japanese Patent Application Laid-Open No. 7-055703 JP 2003-084532 A

  According to the invention described in Patent Document 3, since density or chromaticity is detected from a fixed image by a color sensor, there is an advantage that exposure amount and process conditions can be controlled in consideration of fluctuation due to transfer or fixing. However, the characteristics of the color sensor may change, and there are also differences in characteristics among individuals in the color sensor. Causes of changes and differences in characteristics include variations in the spectral characteristics of the light-emitting elements that make up the color sensor, changes in the light-emitting elements over time and changes in ambient temperature, and a decrease in sensor output due to adhesion of paper dust, toner, or ink. is there. Such a change or difference in characteristics leads to a decrease in density gradation and color balance control accuracy, which is not preferable.

  If the output of the color sensor is calibrated using a white reference plate, the density gradation and color reproducibility can be improved. However, since the white reference plate is expensive, the cost of the image forming apparatus is increased. Also, if paper dust or toner adheres to the white reference plate, the calibration accuracy will be reduced.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. The present invention, for example, eliminates the effect of fluctuations in the white reference plate while reducing the cost increase by eliminating the need for a white reference plate for calibration, and improves the color detection accuracy of the sensor. The object is to realize image density control (tone control). Other issues can be understood throughout the specification.

  The image forming apparatus of the present invention includes a detection unit that detects the density or chromaticity of an image, and a control unit that controls image forming conditions based on the detected density or chromaticity. The detection unit includes a light emitting unit that emits light and a first light receiving unit that receives reflected light from an image fixed on the recording medium among the emitted light, and a signal output from the first light receiving unit. Detect the density or chromaticity of the image. In particular, the image forming apparatus is provided separately from the first light receiving unit, and is based on a second light receiving unit that receives light that is not reflected light out of light emitted from the light emitting unit, and a signal output from the second light receiving unit. And a calibration unit that calibrates the detection unit.

  According to the present invention, since the detection unit is calibrated based on the signal output from the second light receiving unit provided separately from the first light receiving unit, a white reference plate for calibration is not necessary. Therefore, it is possible to avoid the influence due to the fluctuation of the white reference plate while reducing the cost increase. Furthermore, since the color detection accuracy of the sensor is improved, good image density control can be realized.

  An embodiment of the present invention is shown below. The individual embodiments described below will help to understand various concepts, such as the superordinate concept, intermediate concept and subordinate concept of the present invention. Further, the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments.

(First embodiment)
FIG. 1 is a schematic cross-sectional view illustrating an example of a color image forming apparatus according to an embodiment. The image forming apparatus 100 employs a tandem method, but may employ a rotary method. Although an electrophotographic image forming apparatus will be described here, the present invention may be applied to an image forming apparatus that employs another image forming system such as an ink jet system. Note that the image forming apparatus is realized as, for example, a printing apparatus, a printer, a copier, a multifunction peripheral, or a facsimile.

  The paper feed unit 21 is a unit that feeds the recording medium 11 to the transport path. The recording medium is sometimes called, for example, a recording material, paper, sheet, transfer material, or transfer paper. The fed recording medium is transported to the transfer roller 28 along the transport path.

  On the other hand, the image forming unit includes a photosensitive drum 22, an injection charger 23, a scanner unit 24, a toner cartridge 25, a developing unit 26, an intermediate transfer member 27, a transfer roller 28, and the like. Usually, in a multicolor image forming apparatus, an image forming station is formed for each color of a developer (eg, toner). In this embodiment, four color toners such as yellow (Y), magenta (M), cyan (C), and black (K) are used, so the number of image forming stations is four.

  The photosensitive drum 22 carries an electrostatic latent image and a toner image, and is sometimes called a photosensitive member or an image carrier. The injection charger 23 is a unit that uniformly charges the surface of the photosensitive drum 22. The scanner unit 24 is a unit that forms an electrostatic latent image by exposing the uniformly charged surface of the photosensitive drum 22 in accordance with an image signal. The scanner unit is sometimes called an exposure device, a scanning optical device, an optical scanning device, or an optical scanner device. The toner cartridge 25 is a unit that supplies toner to the developing device 26. The developing device 26 is a unit that develops an electrostatic latent image into a toner image using toner.

  The intermediate transfer member 27 is a unit that carries a multicolor toner image formed by sequentially transferring a single color toner image from the photosensitive drum 22 of each image forming station. The multicolor toner image carried on the intermediate transfer member 27 is secondarily transferred to the recording medium 11 by the transfer roller 28. Next, the recording medium 11 is conveyed to the fixing unit 30.

  The fixing unit 30 is a unit that melts and fixes an unfixed toner image while conveying the recording medium 11. The fixing unit 30 includes a fixing roller 31 and a pressure roller 32. The fixing roller 31 and the pressure roller 32 are formed in a hollow shape, and heaters 33 and 34 are incorporated therein. The recording medium 11 on which the toner image is fixed is discharged to the outside.

  The density detection sensor 41 for unfixed toner is a sensor that detects the density of an unfixed multicolor toner image carried on the intermediate transfer body 27. The color sensor 42 is disposed downstream of the fixing unit 30 in the transport direction and detects the chromaticity of the fixed toner image. The density detection sensor 41 for unfixed toner is not an essential unit for the present invention and may be omitted.

  The image processing unit 43 is a unit that executes various types of image processing on input image data. The image processing unit 43 includes a color matching table 44, a color separation table 45, a calibration lookup table 46, and a halftone processing unit 47.

  The image processing unit 43 uses a color matching table 44 prepared in advance to convert an RGB signal representing the color of the image into a device RGB signal (hereinafter referred to as DevRGB) that matches the color reproduction range of the image forming apparatus 100. . The image processing unit 43 converts DevRGB signals into CMYK signals that are toner color material colors of the image forming apparatus 100 using a color separation table 45 prepared in advance. Next, the image processing unit 43 converts the CMYK signal into a C′M′Y′K ′ signal by using a calibration lookup table 46 for correcting density-gradation characteristics unique to the image forming apparatus. . Finally, the halftone processing unit 47 performs halftone processing such as dithering on the C′M′Y′K ′ signal.

  2 and 3 are diagrams illustrating an example of the density detection sensor 41 according to the embodiment. The density detection sensor 41 is disposed toward the intermediate transfer body 27 and measures the density of the toner patch formed on the surface of the intermediate transfer body 27.

  The light emitting element 51 is a light source such as an infrared LED. The light receiving element 52 is a light receiving element such as a photodiode or Cds. The optical elements 53 and 54 are optical components used for coupling the light emitting element 51 and the light receiving element 52. That is, the light emitted from the light emitting element 51 is reflected by the toner patch 29. The reflected light is received by the light receiving element 52 via the optical element 54. Although not shown in the figure, the density detection sensor 41 includes an IC that processes received light data and a holder that accommodates these components.

  Comparing FIG. 2 and FIG. 3, the position of the light receiving element 52 is different. The light receiving element 52 shown in FIG. 2 detects the sum of the regular reflection component and the irregular reflection component in the light emitted from the light emitting element 51. On the other hand, the light receiving element 52 shown in FIG. 3 detects only the irregular reflection component in the light emitted from the light emitting element 51. Whether the configuration shown in FIG. 2 or the configuration shown in FIG. 3 should be used may be appropriately determined depending on the optical reflection characteristics of the toner and the intermediate transfer member.

  The detection result (density signal) output from the density detection sensor 41 is sent to the image processing unit 43. The image processing unit 43 performs density correction by feeding back the received detection result to the calibration lookup table 46.

  FIG. 4 is a diagram for explaining an example of the color sensor 42 according to the embodiment. The parts described in FIGS. 2 and 3 are given the same reference numerals to simplify the description.

  The color sensor 42 is disposed toward the image forming surface of the recording medium 11. The color sensor 42 detects the density of the single color patch or the chromaticity of the mixed color patch fixed on the recording medium, and outputs the detection result (density signal or chromaticity signal) to the image processing unit 43. The color sensor 42 includes a light emitting unit and a light receiving unit, and is an example of a detection unit that detects the density or chromaticity of an image based on a signal output from the light receiving unit.

  The image processing unit 43 controls the density or chromaticity by correcting the calibration lookup table 46 according to the input detection result. The image processing unit 43 is an example of a control unit that controls image forming conditions based on detected density or chromaticity. The color sensor 42 can measure not only the actual density of the toner image formed on the recording medium but also the chromaticity of a mixed color of two or more colors. Therefore, the color sensor 42 can realize correction with higher accuracy than the density detection sensor 41.

  The light emitting element 51 used in the color sensor 42 is a light source (such as an LED) that emits white light (W) in order to detect chromaticity. The light emitting element 51 is an example of a light emitting unit that emits light. The output white light enters the recording medium 11 on which the patch 60 is fixed at an angle of 45 degrees. Of the incident light, the intensity of the irregularly reflected light diffusely reflected in the direction of 0 degrees is detected by the light receiving unit 56 for detection. The light receiving unit 56 includes a light receiving element 52 and an RGB filter 55. The light receiving unit 56 is an example of a first light receiving unit that receives reflected light from an image fixed on a recording medium among emitted light. Note that specific numerical values for the incident angle and the reflection angle are merely examples. The light receiving element 52 is divided into an R element, a G element, and a B element corresponding to the RGB filter 55.

  The light receiving unit 57 for calibration also includes a light receiving element 52 and an RGB filter 55, and is disposed on the same side as the light emitting element 51 with respect to the conveyance path of the recording medium 11. For example, in FIG. 4, when the conveyance path is divided into an upper side and a lower side with the boundary as a boundary, the calibration light receiving portion 57 and the light emitting element 51 are arranged on the same side. The calibration light-receiving unit 57 is an example of a second light-receiving unit that is provided separately from the first light-receiving unit and receives light that is not reflected light among the light emitted from the light-emitting unit. The light receiving portion 57 is less likely to be contaminated by the optical element 53 due to scattering of paper dust, toner or ink. It is assumed that the variation in the light receiving characteristic of the light receiving unit 56 and the variation in the light receiving characteristic of the light receiving unit 57 are negligible. In addition, the change with time and the change in ambient temperature of the light receiving unit 56 are similar to those of the light receiving unit 57, and the difference between them can be ignored.

<Calculation method of output correction coefficient used for color sensor calibration>
FIG. 5 is a diagram showing average spectral characteristics of a light source that emits white light used in a color sensor. The horizontal axis indicates the wavelength, and the vertical axis indicates the amount of emitted light. The amount of emitted light is normalized with the maximum value being 1. The spectral characteristics of the light emitting element 51 have a certain degree of variation with respect to the average characteristics shown in FIG.

  FIG. 6 is a diagram showing an average spectral characteristic of a light receiving element used in the color sensor. The horizontal axis indicates the wavelength, and the vertical axis indicates the amount of received light. The received light quantity is normalized with the maximum value of the received light quantity of the R element being 1, and the received light quantities of the R element, G element, and B element are normalized.

  Here, an average color sensor configured using the light emitting element having the average spectral characteristic shown in FIG. 5 and the light receiving element having the average spectral characteristic shown in FIG. 6 will be considered. . The sensor output value output by each element of the color sensor when the light receiving element directly detects the white light output by the light emitting element is as follows.

Output value of R element = Σ (spectral characteristic of light emitting element × spectral characteristic of R element)
Output value of G element = Σ (spectral characteristic of light emitting element × spectral characteristic of G element)
Output value of B element = Σ (spectral characteristics of light emitting element × spectral characteristics of B element)
Here, Σ means integration in the wavelength region of 400 to 700 nm.

When each output value is calculated using the spectral characteristics shown in FIGS.
R element: G element: B element = 0.83: 1.00: 0.70
It becomes.

  In this embodiment, when the white light output from the light emitting element 51 is detected by the calibration light receiving unit 57, the average output values of the elements are R = 2.07V, G = 2.50V, and B = 1.74V, respectively. The color sensor 42 is set so that These values will be referred to as color sensor reference values.

  However, as described above, the characteristics of the light emitting elements 51 of the color sensor 42 are accompanied by individual variations and changes with time, so that the output value from each element of the color sensor 42 is the same as the reference value described above. Don't be.

  Therefore, in this embodiment, the color sensor 42 is calibrated by the following procedure. First, white light from the light emitting element 51 is detected by the calibration light receiving unit 57. The output values at this time are R0, G0, and B0.

Next, an output correction coefficient of the color sensor is calculated. The output correction coefficient is determined for each of the R, G, and B output values of the color sensor 42. The correction coefficients are αR, αG, and αB, respectively. The calculation formula of the output correction coefficient is shown below.
αR = 2.07 ÷ R0
αG = 2.50 ÷ G0
αB = 1.74 ÷ B0
Calibration (output correction) of the color sensor 42 is executed using the calculated output correction coefficient. Actual output correction is preferably performed at the time of image density control using the color sensor 42. This is to reduce the downtime, which is a period during which image formation cannot be performed, as much as possible.

<Patch used for image density control>
FIG. 7 is a diagram showing an example of a patch pattern for density-gradation characteristic control fixed on a recording medium. The patch pattern for density-gradation characteristic control (hereinafter referred to as patch) is, for example, a gray gradation patch pattern. Gray is a color that is the center of the color gamut, so it is a very important color for achieving color balance. The patch 60 includes gray gradation patches 64a and 64b of black (K) and process gray gradation patches 65a and 65b in which cyan (C), magenta (M), and yellow (Y) are mixed. The gray gradation patch 64a and the process gray gradation patch 65a are close to each other in chromaticity. Similarly, the gray gradation patch 64b and the process gray gradation patch 65b are close to each other in chromaticity. In this way, the gray tone patches of black (K) and the gray tone patches of mixed colors that are close in chromaticity are arranged on the recording medium 11 in pairs. The color sensor 42 outputs each output value of RGB by detecting the reflected light from each gray tone patch by the light receiving unit 56 for detection.

<Image density control (tone control)>
FIG. 8 is a block diagram illustrating a part of a control unit that controls the image forming apparatus. The parts already described are given the same reference numerals. The calibration unit 801 is a unit that calibrates the color sensor 42 based on the signal output from the calibration light receiving unit 57. The calibration unit 801 is an example of a calibration unit that calibrates the detection unit based on the signal output from the second light receiving unit. The correction unit 802 is a unit that corrects the signal output from the detection light-receiving unit 56 in accordance with the signal output from the calibration light-receiving unit 57. The correction unit 802 is an example of a correction unit that corrects the signal output from the first light receiving unit in accordance with the signal output from the second light receiving unit.

  FIG. 9 is a flowchart showing an image density control using a color sensor and an output correction method of the color sensor. In step S <b> 901, the calibration light receiving unit 57 detects white light emitted from the light emitting element 51. In step S902, the correction unit 802 included in the calibration unit 801 calculates correction coefficients αR, αG, and αB using the above-described calculation formula. Therefore, the correction unit 802 is an example of a calculation unit that calculates a correction coefficient by dividing the intensity of the signal output from the second light receiving unit by a predetermined reference value.

  In step S903, the image processing unit 43 creates the patch 60 based on the calibration lookup table 46 prepared in advance. In step S <b> 904, the detection light receiving unit 56 detects reflected light from the patch 60 fixed on the recording medium 11. Here, among the light receiving elements 52 provided in the light receiving unit 56, the output value of the R element is R1, the output value of the G element is G1, and the output value of the B element is B1.

  In step S905, the correction unit 802 corrects the output value from each element of the light receiving unit 56 for detection according to the output value output from the light receiving unit 57 for calibration. For example, the correction unit 802 multiplies the output values from each element by correction coefficients αR, αG, and αB. The corrected output values are R ′, G ′, and B ′, respectively. Therefore, the correction unit 802 is an example of a multiplication unit that multiplies the calculated correction coefficient by the signal output from the first light receiving unit.

R ′ = R1 × αR
G ′ = G1 × αG
B ′ = B1 × αB
In step S906, the image processing unit 43 converts each corrected output value into Lab data that is a standard color space. In step S907, the image processing unit 43 corrects the calibration lookup table 46 according to the Lab data corresponding to each output value after correction. For example, the image processing unit 43 sets the calibration look-up table 46 so that K = CMY based on the difference between the Lab gray data of the gray gradation patch by black (K) and the Lab data of the process gray gradation patch. Correct.

  According to the present embodiment, in addition to the light receiving unit 56 for detecting density and chromaticity, the calibration light receiving unit 57 is provided, and the light receiving unit 56 is calibrated based on the signal output from the light receiving unit 57. The color detection accuracy of the color sensor 42 can be improved. That is, sensor stains due to scattering of paper dust, toner, or ink, variations in spectral characteristics of the color sensor, and influences due to changes in diameter are alleviated, and color balance is improved. If the color detection accuracy is improved, good image density control can be realized. The density-gradation characteristics change due to environmental changes and long-term use. Therefore, calibration at an appropriate timing is very effective. Furthermore, in this embodiment, since the calibration white reference plate is not necessary, the influence of the variation of the white reference plate can be avoided while reducing the cost increase.

  In the present embodiment, the light receiving unit 57 receives direct light that is not reflected light (indirect light) among the light emitted from the light emitting element 51. Therefore, the influence of other optical components can be reduced as compared with the case of receiving indirect light.

  In order to receive direct light, in the present embodiment, the light emitting element 51 and the light receiving portion 57 are provided on the same side with respect to the conveyance path for conveying the recording medium. Accordingly, the light receiving unit 57 can receive light directly with a simple configuration and arrangement.

  Various methods for configuring the color sensor 42 are also conceivable. Among them, in the present embodiment, the correction unit 802 that corrects the signal output from the light receiving unit 56 according to the signal output from the light receiving unit 57 has been described. Since such a correction unit 802 can be built in an IC provided inside the color sensor 42, an increase in the size and cost of the color sensor 42 may be easily suppressed. The correction unit 802 divides the intensity (R0, G0, B0) of the signal output from the light receiving unit 57 by a predetermined reference value (eg, 2.07V, 2.50V, 1.74V). Correction coefficients αR, αG, and αB are calculated. Furthermore, the correction unit 802 calibrates the output value by multiplying the calculated correction coefficient by the signal (R1, G1, B1) output from the light receiving unit 56. Therefore, calibration can be realized by comparatively simple calculation.

  In the present embodiment, a correction method using a gray tone patch has been described in order to enable highly accurate correction. However, this is only an example, and the chromaticity of a mixed color other than gray, which is a mixture of two or three colors of yellow, magenta, and cyan, may be measured.

(Second Embodiment)
In the first embodiment, a white light source is used as the light emitting unit, and the light receiving element 52 including three elements corresponding to the RGB filter 55 is used as the light receiving unit 57. In the first embodiment, the configurations of the light emitting unit and the light receiving unit are different from those of the first embodiment.

  FIG. 10 is a diagram illustrating the color sensor 42 according to the present embodiment. The light emitting part employs three light emitting elements each having a different emission spectrum. The light emitting element 1000R emits light having a red (R) spectrum. The light emitting element 1000G emits light having a green (G) spectrum. The light-emitting element 1000B emits light having a blue (B) spectrum. These light emitting elements are realized by a light source such as an LED. The RGB filter 55 described above is not used in the light receiving unit 56 for detection and the light receiving unit 57 for calibration. The light receiving element 52 used in the light receiving unit 56 for detection and the light receiving unit 57 for calibration may be a single element. In the light receiving unit 57, as in the first embodiment, the optical element 53 makes it difficult for sensor contamination due to scattering of paper powder, toner, or ink to occur.

  FIG. 11 is a diagram showing an average spectral characteristic of each light emitting element used in the color sensor. The horizontal axis indicates the wavelength, and the vertical axis indicates the amount of emitted light. The light emission amounts corresponding to R, G, and B are normalized with the maximum value being 1.

  FIG. 12 is a diagram illustrating an average spectral characteristic of a light receiving element used in the color sensor. The horizontal axis indicates the wavelength, and the vertical axis indicates the amount of received light. The amount of received light is normalized with a maximum value of 1.

  In the first embodiment, white light is simultaneously received by three light receiving elements corresponding to RGB. However, in this embodiment, each light emitting element corresponding to R, G, and B is turned on in order so that a single light receiving element that does not use an RGB filter can independently receive light of different spectra. ing.

  In the first embodiment, the output value output from the R light receiving element is set as the R output value, the output value output from the G light receiving element is set as the G output value, and the output output from the B light receiving element. The value was the output value of B. On the other hand, in the second embodiment, the output value from the light receiving element when the light emitting element 1000R is turned on is the output value of R. Further, an output value from the light receiving element when the light emitting element 1000G is turned on is set as an output value of G. The output value from the light receiving element when the light emitting element 1000B is turned on is defined as an output value of B. The light emitting elements 1000R, 1000G, and 1000B are alternatively turned on.

  In the present embodiment, the color sensor 42 is set so that the average output value when the light emitted from each light emitting element is received by the calibration light receiving unit 57 is 2.50V. This 2.50 V is set as a reference value for the color sensor. For example, the amplification degree of the amplifier circuit that amplifies the output value from the light receiving element is adjusted so that the output values are all 2.50V. The amplification degree is changed depending on which light emitting element is turned on.

  FIG. 13 is a flowchart illustrating an image density control and a color sensor output correction method according to the embodiment. Steps common to the steps described in FIG. 9 are given the same reference numerals.

  In step S1301, the calibration unit 801 causes the light emitting elements 1000R, 1000G, and 1000B to emit light in order, and the calibration light receiving unit 57 detects the light. In step S <b> 1302, the correction unit 802 adjusts the light emission amounts of the light emitting elements 1000 </ b> R, 1000 </ b> G, and 1000 </ b> B so that each output value corresponding to RGB becomes a reference value (eg, 2.50 V). Specifically, the value of the current supplied to each light emitting element is adjusted. The correction unit 802 is an example of a correction unit that corrects the amount of light emitted from the light emitting unit in accordance with a signal output from the second light receiving unit. The correcting unit 802 is also an example of an adjusting unit that adjusts the amount of light emitted from the light emitting unit so that the intensity of the signal output from the second light receiving unit becomes a predetermined reference value.

  Thereafter, steps S903, S904, S906, and S907 described above are executed. In the present embodiment, since the light emitting element is calibrated, the calibration of the output value of the light receiving element (step S905) becomes unnecessary.

  As described above, in the second embodiment, a color sensor is configured by a plurality of light emitting elements that emit light of different spectra and a single light receiving element, and an output value from the light receiving element becomes a reference value. The light emission quantity of each light emitting element is adjusted. Therefore, the second embodiment also has the same effect as the first embodiment.

  Note that the second embodiment and the first embodiment may be combined. For example, the calibration method of the first embodiment may be employed while employing the light emitting unit and the light receiving unit according to the second embodiment. In this case, S902 is adopted instead of step S1302, and S905 is also executed without being skipped. However, if the difference in the amount of light emitted by each light emitting element is too large, this method may cause a decrease in detection accuracy. Therefore, when the difference between the light emission amounts of the light emitting elements is too large, it is preferable to adopt the second embodiment.

(Third embodiment)
FIG. 14 is a diagram illustrating an example of a color sensor according to the embodiment. Compared with the first and second embodiments, the light receiving unit 57 for calibration is different from the light emitting element 51 with respect to the conveyance path 1400 of the recording medium. Since the light receiving unit 57 is located below the conveyance path 1400, there is an advantage that the light receiving unit 57 is less likely to be soiled by waste paper or toner.

  However, since the transport path 1400 exists between the light emitting element 51 and the calibration light receiving unit 57, direct light from the light emitting element 51 is blocked by the transport path. Therefore, in order to directly receive light, an opening 1401 is provided in the transport path at a portion that intersects the optical path from the light emitting element 51 to the light receiving unit 57. When the calibration light receiving unit 57 receives light from the light emitting element 51, the conveyance of the recording medium is controlled so that the recording medium does not pass over the opening.

  However, waste paper or toner may reach the light receiving unit 57 through the opening 1401. Therefore, a shutter 1402 is provided in the opening 1401. The shutter 1402 is opened when the light receiving unit 57 needs to receive light (when calibration is performed), and is closed when the light receiving unit 57 does not need to receive light (when calibration is not performed). This reduces the possibility that waste paper or toner will reach the light receiving unit 57 through the opening 1401.

  Note that instead of providing the shutter 1402, the opening 1401 may be covered with a light-transmitting material. In this case, there is an advantage that the shutter opening / closing mechanism can be omitted. Note that the shutter 1402 and the light-transmitting material are all examples of a protective member for protecting the second light receiving unit from dirt.

  In the first and second embodiments, it is assumed that the directivity of the light emitting element 51 is not so narrow and the light receiving unit 57 can sufficiently receive light directly. However, if the directivity of the light emitting element 51 is narrow, the light receiving unit 57 arranged at the position shown in the first or second embodiment will not be able to receive light sufficiently. On the other hand, with the arrangement described in the third embodiment, even if the directivity of the light emitting element 51 is narrow (sharp), the light can be sufficiently received.

  The configuration of the light emitting unit and the light receiving unit illustrated in FIG. 14 conforms to the first embodiment, but the light emitting unit and the light receiving unit illustrated in the second embodiment may be employed.

[Other Embodiments]
In the first to third embodiments, a multicolor image forming apparatus has been described as an example of an image forming apparatus. However, the present invention can also be applied to a monochrome image forming apparatus. As a gradation control method, the method of correcting the calibration lookup table 46 has been described in the above embodiment. However, the present invention is not limited to this. This is because the present invention is not limited by the gradation control method. For example, the gradation control method may be a method of correcting the color matching table 44 or the color separation table 45. Furthermore, the present invention is not limited to an electrophotographic image forming apparatus, but can also be applied to an image forming apparatus that employs another image forming system such as an inkjet system.

1 is a schematic cross-sectional view illustrating an example of a multicolor image forming apparatus. It is a figure which shows an example of a density | concentration detection sensor. It is a figure which shows an example of a density | concentration detection sensor. It is a figure which shows an example of a color sensor. It is a figure which shows the average spectral characteristic of the light source which light-emits white light used for a color sensor. It is a figure which shows the average spectral characteristic of the light receiving element used for a color sensor. FIG. 6 is a diagram illustrating an example of a patch pattern for density-gradation characteristic control fixed on a recording medium. 2 is a block diagram illustrating a part of a control unit that controls the image forming apparatus. FIG. 6 is a flowchart illustrating an image density control using a color sensor and an output correction method of the color sensor. It is a figure which shows an example of a color sensor. It is a figure which shows the average spectral characteristic of each light emitting element used for a color sensor. It is a figure which shows the average spectral characteristic of the light receiving element used for a color sensor. It is a flowchart which shows the image density control and the output correction method of a color sensor. It is a figure which shows an example of a color sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Recording medium 21 ... Paper feed part 22 ... Photosensitive drum (photoconductor)
23 ... Injection charger 23 ... Sleeve 24 ... Scanner unit 25 ... Toner cartridge 26 ... Developer 27 ... Intermediate transfer member 28 ... Transfer roller 30 ... Fixing unit 31 ... Fixing roller 32 ... Pressure roller 33, 34 ... Heater 41 ... TBD Density detection sensor 42 for contact toner ... Color sensor (density or chromaticity detection sensor)
43 Image processing unit (printer controller)
44 ... Color matching table 45 ... Color separation table 46 ... Calibration lookup table 47 ... Halftone processing unit 51 ... Light emitting element 52 ... Light receiving element 53 ... Optical element 54 ... Optical element 55 ... RGB filter 60 ... Patch 64 ... By black Gray tone patch 65 ... Process gray tone patch by YMC

Claims (12)

A light-emitting unit that emits light; and a first light-receiving unit that receives reflected light from an image fixed on a recording medium out of the emitted light, and the image is output by a signal output from the first light-receiving unit. A detector for detecting the density or chromaticity of
A control unit that controls image forming conditions based on the detected density or chromaticity;
A second light receiving unit that is provided separately from the first light receiving unit and receives light that is not reflected light among the light emitted from the light emitting unit;
An image forming apparatus comprising: a calibration unit that calibrates the detection unit based on a signal output from the second light receiving unit.   The image forming apparatus according to claim 1, wherein the second light receiving unit directly receives light out of the light emitted from the light emitting unit.   The image forming apparatus according to claim 2, wherein the light emitting unit and the second light receiving unit are provided on the same side with respect to a conveyance path for conveying the recording medium.   The image forming apparatus according to claim 2, wherein the light emitting unit and the second light receiving unit are provided on opposite sides of a conveyance path for conveying the recording medium.   The image forming apparatus according to claim 4, wherein the conveyance path is provided with an opening at a portion intersecting an optical path from the light emitting unit to the second light receiving unit.   The image forming apparatus according to claim 5, wherein the opening is provided with a protection member for protecting the second light receiving unit from contamination.   7. The shutter according to claim 6, wherein the protection member is a shutter that opens when the second light receiving unit needs to receive light and closes when the second light receiving unit does not need to receive light. The image forming apparatus described.   8. The correction unit according to claim 1, wherein the calibration unit includes a correction unit that corrects a signal output from the first light receiving unit according to a signal output from the second light receiving unit. The image forming apparatus described in 1. The correction unit is
A calculation unit that calculates a correction coefficient by dividing the intensity of the signal output from the second light receiving unit by a predetermined reference value;
The image forming apparatus according to claim 8, further comprising: a multiplication unit that multiplies the calculated correction coefficient by the signal output from the first light receiving unit.   The said calibration part contains the correction | amendment part which correct | amends the light quantity of the light emitted from the said light emission part according to the signal output from the said 2nd light-receiving part, The any one of Claim 1 thru | or 7 characterized by the above-mentioned. The image forming apparatus described in 1. The correction unit is
The image forming apparatus according to claim 10, further comprising an adjustment unit that adjusts a light emission amount of the light emitting unit so that an intensity of a signal output from the second light receiving unit becomes a predetermined reference value. apparatus. A light-emitting unit that emits light; and a first light-receiving unit that receives reflected light from an image fixed on a recording medium among the emitted light, and the image output by a signal output from the first light-receiving unit A detector for detecting the density or chromaticity of
A control unit that controls image forming conditions based on the detected density or chromaticity;
A calibration method in an image forming apparatus provided separately from the first light receiving unit, the second light receiving unit receiving light that is not reflected light among the light emitted from the light emitting unit,
A light emitting step in which the light emitting part emits light;
A light receiving step in which the second light receiving unit receives the emitted light;
A calibration method comprising a calibration step of calibrating the detection unit based on a signal output from the second light receiving unit.

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