JP4532979B2 - Image forming apparatus and control method thereof - Google Patents

Description

  The present invention relates to a color image forming apparatus and a control method thereof.

  In recent years, color image forming apparatuses employing an electrophotographic system such as a color printer and a color copying machine, an inkjet system, and the like have become possible to achieve an image quality comparable to that of a photograph.

  However, in the color image forming apparatus, when there is a change in the state of each part of the apparatus due to a change in environment or long-term use, the color of the obtained image also changes. In particular, in the case of an electrophotographic color image forming apparatus, even a slight environmental change may cause a color change, which may cause a loss of color balance. For this reason, it has means for stably reproducing the color and the gradation of the color. For example, for each color toner, there are several kinds of exposure amount and development bias depending on the absolute humidity, development bias and other gradation correction means such as an image data conversion table, based on the absolute humidity measured by the temperature and humidity sensor Thus, the optimum process conditions and gradation correction values are selected. In addition, a density detection toner image (patch) is formed on an intermediate transfer member, a photosensitive member, or the like with a single color toner so that a constant color and gradation of color can be obtained even if fluctuations occur in each part of the apparatus. Then, the density of the unfixed single-color patch is detected by a density detection sensor for unfixed toner (hereinafter referred to as a density sensor). By providing feedback to the correction means and performing density control, a stable color and color gradation are obtained.

  However, density control using a density sensor detects patches by forming patches on an intermediate transfer member, drum, etc., and controls changes in image color balance due to subsequent transfer and fixing to a recording medium. The current situation is not. However, it has been found that the density and color balance also change depending on the transfer efficiency in transferring a patch to a recording medium, and heating and pressurization by fixing. Such a point cannot be dealt with by the conventional density control using the density sensor.

  Therefore, cyan (C), magenta (M), yellow (Y), and black (K) single tone toner images and C, M, and Y mixed color toner images are formed on the recording medium, and the recording medium is fixed after fixing. There has been proposed a color image forming apparatus provided with a sensor (hereinafter referred to as a color sensor) for detecting the density or chromaticity of the upper toner image (Patent Document 1).

  In a color image forming apparatus equipped with a color sensor, the detection result is fed back to a calibration table for correcting the exposure amount of the image forming unit, process conditions, density-gradation characteristics, etc., and formed on a recording medium. Thus, density control or chromaticity control of the final output image can be performed. Although it is possible to detect the output image of the color image forming apparatus with an external image reading device or a densitometer / colorimeter and implement the same control, it is excellent in that the control is completed within the color image forming apparatus. ing. This color sensor uses, for example, three or more light sources having different emission spectra such as red (R), green (G), and blue (B) as light emitting elements, or the light emitting element emits white light (W). Is used, and three or more types of filters having different spectral transmittances such as R, G, and B are formed on the light receiving element. As a result, three or more different outputs such as RGB output can be obtained.

Even in an ink-jet color image forming apparatus, the color balance changes due to changes in ink discharge amount with time, environmental differences, and individual differences of ink cartridges, and density-gradation characteristics cannot be kept constant. In view of this, there is also an ink jet type color image forming apparatus that performs density control or chromaticity control by detecting the density or chromaticity of a patch on a recording medium by providing a color sensor.
JP 2003-084532 A

  When density control and chromaticity control are performed using a color sensor, image formation is performed by forming several discrete tone patches on a recording medium and measuring and interpolating each density and each chromaticity of the discrete patch. The tone characteristics of the device were sought. However, with a discrete patch, it is difficult to accurately determine the shape of the gradation characteristics, and it is very difficult to capture subtle hue fluctuations. In particular, the portion that is lighter than the first patch created on the highlight side is “0”, and since interpolation can only be performed from the density of the first patch, the portion where the gradation starts to appear cannot be accurately known. If the gradation characteristics on the highlight side are not accurately known, an image with gradation, such as a pseudo contour, or an originally faint part will appear dark will result in an unnatural gradation. Further, if the method of selecting the image data value of the patch is not always appropriate, an erroneous gradation characteristic may be obtained when the gradation characteristic changes greatly due to environmental fluctuations.

  In view of the circumstances as described above, the present invention intends to provide a technique capable of accurately detecting the shape of gradation characteristics and always obtaining a high-quality image.

In order to solve this problem, for example, an image forming apparatus of the present invention has the following configuration. That is,
An image forming apparatus for forming a color image on a record medium,
A pattern forming unit that fixes and forms a gradation pattern consisting of patches that continuously and uniformly change along the conveyance direction of the recording medium on the recording medium when a predetermined instruction is input;
Detecting means for irradiating the formed gradation pattern with light having a spot diameter straddling a plurality of patches and detecting the density or color of the gradation pattern according to the reflected light ;
Setting means for setting printing conditions based on the detected density or color ;
Means for detecting a recordable size in the transport direction of the recording medium;
Comparing means for comparing the detected recordable size with the length required for recording the continuous and uniformly changing gradation pattern,
If the comparison result of the comparison means indicates that the recordable size is less than the length required for recording the gradation pattern, the pattern forming means determines the gradation pattern according to the recordable size. A gradation pattern is formed so as to include a predetermined number of patches that overlap each other in the divided portions of the divided gradation pattern, and each divided floor is formed on a plurality of recording media. A tone pattern is formed.
An image forming apparatus as another invention has the following configuration. That is,
An image forming apparatus for forming a color image on a recording medium,
A pattern forming unit that fixes and forms a gradation pattern consisting of patches that continuously and uniformly change along the conveyance direction of the recording medium on the recording medium when a predetermined instruction is input;
Detecting means for irradiating the formed gradation pattern with light having a spot diameter straddling a plurality of patches and detecting the density or color of the gradation pattern according to the reflected light;
Setting means for setting printing conditions based on the detected density or color;
Means for detecting the size of the recording medium,
The pattern forming means divides the gradation pattern into a plurality of divisions when the recordable size in the conveyance direction of the recording medium is less than the length required for recording the gradation pattern by the detection. Forming a gradation pattern to include a predetermined number of gradation patches that overlap each other in the divided portion of the gradation pattern, and forming each divided gradation pattern on a plurality of recording media. Features.

  As described above, according to the present invention, in an image forming apparatus capable of halftone expression, a continuous tone pattern is actually output onto a recording medium to measure density or chromaticity. The shape of the characteristic can be accurately known, and appropriate gradation correction can be performed.

  Hereinafter, the present invention will be described in detail according to preferred embodiments with reference to the accompanying drawings.

<First Embodiment>
FIG. 1 is a configuration diagram relating to image formation in the color image forming apparatus according to the first embodiment. FIG. 2 is a cross-sectional view of the color image forming apparatus. As shown in the figure, this apparatus is a tandem color image forming apparatus employing a belt-like intermediate transfer member 28 which is an example of an electrophotographic color image forming apparatus. The operation of the electrophotographic color image forming apparatus will be described with reference to FIG.

  As charging means 122, four injection chargers 23Y for charging the photoreceptors 22Y, 22M, 22C, and 22K for each station of Y (yellow), M (magenta), C (cyan), and K (black). , 23M, 23C, 23K, and each injection charger is provided with sleeves 23YS, 23MS, 23CS, 23KS.

  The photoconductors 22Y, 22M, 22C, and 22K are configured by applying an organic optical transmission layer to the outer periphery of an aluminum cylinder, and are rotated by the driving force of a drive motor (not shown) being transmitted. 22M, 22C, and 22K are rotated counterclockwise in FIG. 2 according to the image forming operation.

  The exposure unit 123 irradiates the photoconductors 22Y, 22M, 22C, and 22K with exposure light from the scanner units 24Y, 24M, 24C, and 24K, and selectively exposes the surfaces of the photoconductors 22Y, 22M, 22C, and 22K. The electrostatic latent image is formed.

  The developing unit 124 includes four developing units 26Y, 26M, 26C, and 26K that develop Y, M, C, and K for each station in order to visualize the electrostatic latent image. Are provided with sleeves 26YS, 26MS, 26CS, and 26KS. Each developing device 26 is detachable.

  The transfer unit 125 transfers the single color toner adhered by developing the photoconductor 22 to the intermediate transfer body 28, and further transfers each color toner transferred to the intermediate transfer body 28 to the recording medium 11.

  Therefore, the intermediate transfer body 28 is rotated (conveyed) in the clockwise direction in FIG. 2, and the primary transfer rollers 27Y, 27M, 27C, and 27K positioned opposite to the photosensitive bodies 22Y, 22M, 22C, and 22K are rotated. A single color toner image is transferred. By applying an appropriate bias voltage to the primary transfer roller 27 and making a difference between the rotation speed of the photosensitive member 22 and the rotation speed of the intermediate transfer body 28, the monochromatic toner image is efficiently transferred onto the intermediate transfer body 28. The above transfer is called primary transfer.

  Further, the transfer means 125 superimposes a single color toner image on the intermediate transfer body 28 for each station, conveys the superimposed multicolor toner image to the secondary transfer roller 29 as the intermediate transfer body 28 rotates, and further records the recording medium. 11 is nipped and conveyed from the paper feed tray 21a (or the hand tray 21b) to the secondary transfer roller 29, and the multicolor toner image on the intermediate transfer member 28 is transferred to the recording medium 11. An appropriate bias voltage is applied to the secondary transfer roller 29 to electrostatically transfer the toner image. This transfer is called secondary transfer. The secondary transfer roller 29 contacts the recording medium 11 at a position 29a while transferring the multicolor toner image onto the recording medium 11, and is separated to a position 29b after the printing process.

  The transfer means 125 performs the above primary transfer and secondary transfer.

  The fixing unit 126 adds a fixing roller 32 for heating the recording medium 11 and a pressure roller for pressing the recording medium 11 against the fixing roller 32 in order to melt and fix the multicolor toner image transferred to the recording medium 11 to the recording medium 11. A pressure roller 33 is provided. The fixing roller 32 and the pressure roller 33 are formed in a hollow shape, and heaters 34 and 35 are incorporated therein. The fixing device 31 conveys the recording medium 11 holding the multicolor toner image by the fixing roller 32 and the pressure roller 33 and applies heat and pressure to fix each color toner on the recording medium 11.

  Thereafter, the recording medium 11 after toner fixing is discharged to a discharge tray (not shown) by a discharge roller (not shown), and the image forming operation is completed.

  The cleaning unit 30 cleans the residual toner stuck on the intermediate transfer member 28. That is, the waste toner after transferring the four-color multicolor toner image formed on the intermediate transfer body 28 to the recording medium 11 is stored in the cleaner container.

  The density sensor 41 is disposed toward the intermediate transfer body 28 in the color image forming apparatus of FIG. 2 and measures the density of the toner patch formed on the surface of the intermediate transfer body 28. An example of the configuration of the density sensor 41 is shown in FIG. An infrared light emitting element 51 such as an LED, a light receiving element 52 such as a photodiode or CdS, an IC (not shown) that processes received light data, and a holder (not shown) that accommodates these elements. The light receiving element 52a detects the intensity of irregularly reflected light from the toner patch, and the light receiving element 52b detects the intensity of regular reflected light from the toner patch. By detecting both the regular reflection light intensity and the irregular reflection light intensity, it is possible to detect the density of the toner patch from a high density to a low density. An optical element such as a lens (not shown) may be used for coupling the light emitting element 51 and the light receiving element 52.

  The color sensor 42 is located downstream of the fixing device 31 in the recording medium conveyance path and is disposed toward the image forming surface of the recording medium 11, and detects the color of the mixed color patch after fixing formed on the recording medium 11. Then, RGB values are output. The spot diameter (diameter) of the irradiation light of the color sensor 42 on the recording medium 11 is about 5 mm. In addition, a white reference plate 43 for calibrating the sensor output for obtaining the absolute density and absolute chromaticity of the patch is provided opposite to the color sensor 42. When the patch forming process is performed, before the recording medium 11 reaches the color sensor 42, the sensor output calibration white reference plate 43 is detected by the color sensor 42, so that the absolute density and absolute chromaticity serving as a reference are detected. Will be detected. Note that when the direction in which the recording medium is conveyed is defined as the y-axis and the direction perpendicular thereto is defined as the x-axis, the positions of the density sensor 41 and the color sensor 42 on the x-axis are the same.

  FIG. 4 is a diagram illustrating a configuration of a peripheral device of the color sensor 42. The color sensor 42 includes a light emitting element 101, a light receiving element 102, an A / D converter 103, and a CPU 121. The light emitting element 101 is a light source of a color sensor and makes light incident on the measurement object 103. Then, irregularly reflected light whose reflectance depends on the object color of the measurement object is reflected. The irregularly reflected light is incident on the light receiving element 102 that converts the light into an analog electric signal. Further, the analog electric signal is converted into a digital electric signal by the A / D converter 104. Then, the digital electrical signal is captured by the CPU 121, processed by primary conversion or a lookup table, and the chromaticity is output.

  FIG. 5 shows a cross-sectional view of the color sensor 42. The color sensor 42 uses a white LED 53 as the light emitting element 101 and a charge storage type sensor 54 a with an on-chip filter of three or more colors such as RGB as the light receiving element 102. The white LED 53 is incident on the recording medium 11 on which the patch after fixing is formed at an angle of 45 degrees, and the intensity of diffuse reflected light in the 0 degree direction is detected by the charge storage sensor 54a with an RGB on-chip filter. The light receiving portion of the charge storage type sensor 54a with RGB on-chip filter has a filter for each RGB like its light receiving surface 54b, and is constituted by an independent light receiving element for each color component. The light receiving element 102 may be a photodiode. A plurality of RGB three-pixel sets may be arranged. Further, a configuration in which the incident angle is 0 degree and the reflection angle is 45 degrees may be employed. Furthermore, you may comprise by LED which emits light of 3 or more colors, such as RGB, and a sensor without a filter.

  The image memory 127 is for developing image data for each color component for print output when print data is received from the host computer, and the interface 128 is for communicating with the host computer. Data and color correction instruction information (supplied with a command that can be distinguished from print data) are received.

  When the general print data is received, the CPU 121 controls the charging unit 122 to the fixing unit 126 and the paper feeding mechanism according to the process of developing the image based on the print data into the image memory 127 and the printing conditions at that time. Although printing is performed, this point is not a main part of the present invention, and thus description thereof will be omitted, and control processing for determining printing conditions will be described.

  This process is executed when a predetermined instruction command is received from the host computer or when a predetermined switch on an operation panel (not shown) of the image forming apparatus is operated.

  A feature of this embodiment is that a continuous tone pattern is used as a patch detected by the color sensor 42 located downstream of the fixing unit 126 when tone correction is performed. In the present embodiment, the color component of each pixel is represented by 8 bits (256 gradations).

  In the case of an 8-bit image signal, image data of 256 levels from 00H to FFH (H means hexadecimal display) can be generated, and therefore a continuous tone pattern composed of image data of 256 levels is used. The spot diameter of the irradiation light of the color sensor 42 used in the present embodiment on the recording medium 11 is set to about 5 mm, and the image data for about three gradations is averaged in the spot so as to be stably measured. Furthermore, the size of each data was set to a width of 7 mm (direction orthogonal to the recording paper conveyance direction) and a length of 1 mm (recording paper conveyance direction). FIG. 6 shows a schematic diagram of a continuous tone pattern.

  When the CPU 121 detects an appropriate timing such as an instruction from the host computer or the user, the CPU 121 starts a continuous tone pattern measurement sequence for each color. Prior to measuring the continuous tone pattern by the color sensor 42, the color sensor 42 is calibrated by measuring the white reference plate 43 for output calibration. The CPU 121 sends appropriate signals to the charging unit 122, the exposure unit 123, the developing unit 124, the transfer unit 125, and the fixing unit 126 to form a continuous tone pattern on the recording medium 11, and the recording in which the continuous tone pattern is formed. The medium 11 is conveyed to a position where the continuous tone pattern can be measured by the color sensor 42, and accurately measures the shape of the tone characteristics of the continuous tone pattern. Based on the shape of the obtained gradation characteristic, the image data conversion table is rewritten so as to obtain a desired gradation characteristic.

  In addition, if unevenness occurs in the continuous tone pattern for some reason, a thinner part than the original part is formed, and the density measurement value may partially vary. In such a case, the measured data may be first smoothed by interpolating the measurement data with a polynomial of the fifth order. Since there are 256 pieces of measurement data, accurate interpolation can be performed even if there is some variation. Note that the polynomial may have other orders, or spline interpolation or the like. In this way, not only the gradation characteristics of Y, M, C, K single color toner images but also the shape of the gradation characteristics of mixed color toner images such as R, G, B, etc. can be accurately measured to produce subtle color fluctuations. It became possible to capture.

  FIG. 11 shows a processing procedure of the CPU 121 in the embodiment. Hereinafter, although there is a part which overlaps with the above, it demonstrates according to the same figure as a procedure of a process.

  First, in step S1, a paper feed instruction is issued, and conveyance of the recording paper 11 from the paper feed tray 21a (or the hand tray 21b) to the secondary transfer roller 29 is started.

  Next, the process proceeds to step S2, and the continuous tone as described above is located at a position in the image memory 127 corresponding to the main scanning direction of the color sensor 452 (the x axis or the direction orthogonal to the recording paper conveyance direction). A pattern is generated in the image memory 127, and image formation and output are performed. As a result, the yellow toner is transferred to the recording paper, and the chromaticity (R, G, B value) of the yellow component image on the recording paper that has passed through the fixing device is detected by the color sensor 42 (step S3). The detected value is stored in the memory as digital data in accordance with the sampling timing). Since the patch length of each gradation is 1 mm, the color sensor 42 samples every time the object moves at least 1 mm (actually, the distance may be shorter than that, for example, 0.5 mm, 0.1 mm interval). To do.

  The above is the processing of the yellow patch image, but the same applies to magenta, cyan, and black (steps S4 to S6). When the patch output and detection of all the recording color components are completed, the printing conditions are updated (the image data conversion table is rewritten) in step S7.

  As described above, according to the present embodiment, printing conditions that take into account the conditions of the fixing device are set by detecting the continuous tone patches actually recorded on the recording paper downstream from the fixing unit 126. It becomes possible to do. Further, since the change axis of the gradation patch is the same as the conveyance direction of the recording medium, all the gradations can be detected by one fixed color sensor 42.

<Second Embodiment>
When the length of each image data of the continuous tone pattern is set to 1 mm, since there are 256 levels of data for an 8-bit image signal, the length of the continuous tone pattern is 256 mm. That is, it is impossible to form a continuous tone pattern of one color component in one sheet on the recording medium 11 having a length in the transport direction of less than 256 mm. In addition, when the length is 256 mm or more, a blank area that is not printed is wasted. Therefore, in the second embodiment, the length of each data is varied according to the length of the recording medium 11, and control is performed so that the blank area that is not printed becomes as small as possible.

  When the CPU 121 detects an appropriate timing such as an instruction from the host computer or the user, the CPU 121 starts a continuous tone pattern measurement sequence. Prior to measuring the continuous tone pattern by the color sensor 42, the color sensor 42 is calibrated by measuring the white reference plate 43 for output calibration. Then, the CPU 121 acquires size information (length in the transport direction) of the recording medium 11 to be fed. Acquisition of the size information of the recording medium may be obtained from the detection timing and conveyance speed of the leading end position and the trailing end position during conveyance, or may be obtained by detecting a marker or the like provided in the paper feed cassette. Good. In any case, the printable area can be determined from the detected size information. The CPU 121 sets the length of the continuous tone pattern so that there is no margin in the printable area (the length per tone can be obtained by dividing the length of the recording medium in the transport direction by 255 equal parts. it can). The CPU 121 sends appropriate signals to the charging unit 122, the exposure unit 123, the developing unit 124, the transfer unit 125, and the fixing unit 126 to form a continuous tone pattern on the recording medium 11, and the recording in which the continuous tone pattern is formed. The medium 11 is conveyed to a position where the continuous tone pattern can be measured by the color sensor 42, and measures the density or chromaticity of the continuous gray pattern of process gray. Further, since the length of one level patch is determined, the sampling intervals of the density sensor and the color sensor are changed accordingly.

  As described above, it is possible to accurately determine the shape of the gradation characteristics without waste regardless of the size of the recording medium 11.

<Third Embodiment>
When the length of each image data of the continuous tone pattern is set to 1 mm, since there are 256 levels of data for an 8-bit image signal, the length of the continuous tone pattern is 256 mm. In other words, it is impossible to form a continuous tone pattern within one sheet on the recording medium 11 having a length of less than 256 mm. If the length of each data is shortened, it can be accommodated in one sheet, but if it is too short, the detection accuracy will be deteriorated. If the continuous tone pattern is divided into a plurality of recording media 11 and output, it is possible to determine the shape of the tone characteristics without deteriorating the detection accuracy even with a short recording medium 11, but from the spot diameter. Since each data length is short, when the continuous tone pattern is divided into a plurality of parts and outputted, the division point is averaged with a portion not including the toner image of the tone data. That is, the density or chromaticity at the division point is output small.

  FIG. 7 is a sensor output obtained when measuring the gradation pattern of continuous image data that is not divided. FIG. 8 is a graph showing the sensor output of each continuous image data gradation pattern when divided into two. It is shown above. It can be seen that at the division point, the detected output value is smaller than when the original output is not divided (broken line portion).

  Therefore, in the third embodiment, even when the length of the recording medium 11 is shorter than the length of the continuous tone pattern, the shape of the tone characteristics of the continuous tone pattern can be accurately measured.

  First, when the CPU 121 detects an appropriate timing such as an instruction from the host computer or the user, the CPU 121 starts a continuous tone pattern measurement sequence. Prior to measuring the continuous tone pattern by the color sensor 42, the color sensor 42 is calibrated by measuring the white reference plate 43 for output calibration. Then, the CPU 121 acquires size information (length in the transport direction) of the recording medium 11 to be fed. When the length of the recording medium is longer than the continuous tone pattern, the continuous tone pattern is output in one recording medium 11. When the length of the recording medium 11 in the transport direction is shorter than the continuous tone pattern, the continuous tone pattern is divided into a plurality of pieces. At this time, the same data overlaps at the rear end of the divided continuous tone pattern of the first half portion and the front end of the continuous tone pattern of the second half portion so that there are several tones. The amount of overlap may be half or more of the spot diameter. In the embodiment, since the spot diameter is 5 mm and the length for one gradation is 1 mm, the overlap amount is set to 3 mm (for three gradations).

  FIG. 9 shows the sensor output of each continuous image data gradation pattern when divided into two in the third embodiment on the same graph. There are two outputs before and after the division point, but the smaller value of the two is the inaccurate value. If the larger value is selected, the continuous tone pattern is detected without being divided. Almost the same value is obtained.

  As described above, even when the recording medium 11 is short and divided into a plurality of recording media 11, the shape of the gradation curve of the continuous gradation pattern can be accurately determined.

  In the present embodiment, the case where there is one division point has been described, but it goes without saying that the same effect can be obtained by performing the same processing at each division point even if there are two or more division points.

<Fourth Embodiment>
When the length of each image data in the continuous tone pattern is smaller than the spot diameter of the irradiation light of the color sensor 42 on the recording medium 11, the measurement is performed with a mixture of a plurality of gradations. Chromaticity data cannot be measured accurately. In order to measure the density or chromaticity for one gradation, the length of each data may be made larger than the spot diameter, and if the spot diameter is about 5 mm, it may be set to about 7 mm. However, in the case of an 8-bit image signal, since there are 256 levels of data, the length of the gradation pattern is about 1800 mm, making it almost impossible to fit in one recording medium 11. Further, even if the continuous tone pattern is divided into a plurality of parts, the number of recording media 11 increases, and an increase in processing time is inevitable.

  Therefore, in the fourth embodiment, in addition to a short continuous tone pattern, a discrete tone patch is output and detected, thereby reducing the number of recording media 11 used and reducing the shape of the continuous tone pattern. In addition, the chromaticity of each gradation data can be accurately obtained.

Since the spot diameter of the irradiation light of the color sensor 42 on the recording medium 11 is about 5 mm, the size of the patch needs to be larger than that in order to accurately obtain the value of the patch density or chromaticity. Therefore, a square with a side of 7 mm is used as the size of the discrete tone patch. From these, seven types of image data S1 to S7 suitable for reproducing the correspondence relationship between image data versus density or image data versus chromaticity are selected as data for P1 to P7. The values of S1 to S7 in the present embodiment are as follows.
S ₁ S ₂ S ₃ S ₄ S ₅ S ₆ S ₇
10H 20H 50H 80H A0H B0H D0H

  Following the creation of seven discrete tone patches, a continuous tone pattern using all data from 01H to FFH (H represents a hexadecimal number) is created. This continuous tone pattern has a width of 7 mm and a length of 1 mm for each data.

  When the CPU 121 detects an appropriate timing such as an instruction from the host computer or the user, the patches P1 to P7 formed on the recording medium 11 are measured by the color sensor 42.

  Next, the density measurement data of the continuous tone pattern is corrected based on the measurement data of the discrete tone patch.

  The correction method will be described with reference to FIG. The solid line in FIG. 10 is the measurement data of the continuous tone pattern after smoothing, the black circle is the measurement data of the discrete tone patch, the left axis is the sensor output value of the discrete tone patch, and the right axis is the patch data The sensor output value of the continuous tone pattern is shown.

For example the sensor output value continuous tone pattern corresponding to the image data S ₃ is so B _50H, to convert the sensor output value D ₃ of the discrete tone patches may multiply the length D ₃ / B _50H as a correction factor. On the other hand, the sensor output value continuous tone pattern corresponding to S ₄ may be multiplied by D ₄ / B _80H as a correction factor to convert B _80H So, the sensor output value D ₄ of the discrete tone patches. The continuous tone pattern data in the section from S ₃ to S ₄ may be corrected while changing the correction coefficient stepwise from D ₃ / B _50H to D ₄ / B _80H . That is, assuming that the image data is X, the sensor output value B _X of the continuous tone pattern may be corrected to B ′ _X by the following equation.
B ′ _x = {(D ₄ / B _80H −D ₃ / B _50H ) / (S ₄ −S ₃ ) × (X−S ₃ ) + D ₃ / B _50H } × B _x
More generally, the sensor output value BX in the sections S _{1 to} S ₇ is B ′ _x = {(D _{i + 1} / B _{Si + 1} −D _i / B _Si ) / (S _{i + 1} −S _i ). × (X−S _i ) + D _i / B _i } × B _x
(Where i = 1, 2,..., 6)
Can be corrected.

Further, the correction coefficient so as to remain in the sensor output value B _00H when the image data between 00H～S ₁ image data 00H is a 1, the correction coefficient so B _10H becomes D ₁ when S ₁ D ₁ / B _10H is used. Therefore, the correction coefficient is changed stepwise from 1 to D ₁ / B _10H ,
B′x = {(D ₁ / B _10H −1) / S ₁ × X + 1} × Bx
It is corrected.

During the image data S ₇ to FFH is a correction coefficient as B _D0H is D ₇ when the image data S ₇ and D ₇ / B _D0H, the sensor output value remains at B _FFH when the image data FFH Thus, the correction coefficient is set to 1, and the correction coefficient is changed stepwise from D ₇ / B _D0H to 1,
B′x = {(1−D ₇ / B _D0H ) / (FFH−S ₇ ) × (X−S ₇ )} × Bx
It is corrected.

  In addition to being able to accurately measure the shape of the gradation characteristics by correcting over the entire area of the image data as described above, the density or chromaticity of each data is also suppressed while suppressing the measurement time and the number of recording media 11 used. It becomes possible to measure accurately.

<Fifth Embodiment>
When a color image is created by superimposing toners and inks of four colors C, M, Y, and K, for example, the balance of color tone such as strong redness and lack of yellowness is C, M, and Y. Determined by the balance (gray balance). Therefore, maintaining a good gray balance leads to stabilization of the color tone.

  When detecting a gray pattern of process gray consisting of three colors Y, M, and C using a color sensor, correction of variations in spectral characteristics of the light-emitting elements and light-receiving elements constituting the sensor, correction of changes over time, and changes in ambient temperature Therefore, a reference with a known absolute value of density or chromaticity such as a white reference plate for sensor output calibration is required. However, the white reference plate used as a reference for sensor output calibration is not only expensive, but paper dust, toner, or ink may be scattered on the white reference plate in the same manner as the sensor, and may not be used as a reference plate. .

  Therefore, a control method for maintaining a good gray balance without using a white reference plate in the fifth embodiment will be described below.

  The control continuous tone pattern is composed of a continuous K single-color gray tone pattern consisting of only K and a continuous process gray tone pattern consisting of three color components mixed with Y, M, and C ( The length per gradation is the same). These continuous tone patterns are detected by the color sensor 42.

  When an instruction from the host computer or the user is sent to the CPU 121, the K gray gradation pattern and the process gray gradation pattern formed on the recording medium 11 are recorded by the color sensor 42, and the chromaticity is adjusted by the color sensor 42. Measured. Since the detected chromaticity does not use the sensor output calibration reference, the absolute accuracy of the chromaticity is not questioned.

Here, the K gray gradation pattern is substantially achromatic color, that is, the color of the K gray gradation pattern using the fact that the values of a ^* and b ^* are almost zero in the L ^* a ^* b ^* space. Relative comparison is made if the degree curve and the chromaticity curve of the process gray gradation pattern are the same.

  If the chromaticities of the two are different, it can be determined that the process gray gradation pattern is a chromatic color and the gray balance is lost. If the chromaticities of the two coincide, it can be determined that the process gray gradation pattern is achromatic and the gray balance is maintained.

  When comparing the chromaticity curve of the K gray gradation pattern and the chromaticity curve of the process gray gradation pattern, even if they do not completely match, there is no difference within the color difference indicated by a predetermined threshold allowed by humans. You may judge that it is coloring.

  According to the color identification method of the color image forming apparatus described above, it is possible to determine whether the process gray patch is an achromatic color without using the sensor output calibration reference of the color sensor, and to know the lightness level. It is inexpensive because it does not require a standard, and it does not detect absolute chromaticity, but by comparing the K gray gradation pattern with the process gray gradation pattern, sensor contamination and color due to scattering of paper dust, toner, or ink High-precision density or chromaticity control can be performed without being affected by variations in temperature characteristics and spectral characteristics of the sensor 42.

  In the embodiment, the printer connected to the host computer has been described as an example. However, the present invention is not limited to the above because it can be similarly realized by a color copying machine.

  In the embodiments, the spot diameter of the color sensor is described as being 5 mm, and the length of the pattern of one gradation is basically 1 mm. However, the present invention is not limited to these.

1 is a diagram illustrating an overall configuration of a color image forming apparatus. 1 is a cross-sectional configuration diagram of a color image forming apparatus. It is a figure which shows the structure of a density sensor. It is a figure which shows the structure of a color sensor and its peripheral device. It is a section lineblock diagram of a color sensor. It is a figure which shows a continuous tone pattern. It is a figure which shows the reading result of a continuous tone pattern. It is a figure which shows the reading result of the divided | segmented continuous tone pattern. It is a figure which shows the reading result of the continuous tone pattern in 3rd Embodiment. It is a conceptual diagram which shows the method of matching the measured value of the continuous tone pattern with the measured value of a patch pattern in 5th Embodiment. It is a flowchart which shows the patch recording & measurement process in embodiment.

Claims (6)

An image forming apparatus for forming a color image on a record medium,
A pattern forming unit that fixes and forms a gradation pattern consisting of patches that continuously and uniformly change along the conveyance direction of the recording medium on the recording medium when a predetermined instruction is input;
Detecting means for irradiating the formed gradation pattern with light having a spot diameter straddling a plurality of patches and detecting the density or color of the gradation pattern according to the reflected light ;
Setting means for setting printing conditions based on the detected density or color ;
Means for detecting a recordable size in the transport direction of the recording medium;
Comparing means for comparing the detected recordable size with the length required for recording the continuous and uniformly changing gradation pattern,
If the comparison result of the comparison means indicates that the recordable size is less than the length required for recording the gradation pattern, the pattern forming means determines the gradation pattern according to the recordable size. A gradation pattern is formed so as to include a predetermined number of patches that overlap each other in the divided portions of the divided gradation pattern, and each divided floor is formed on a plurality of recording media. An image forming apparatus for forming a tone pattern . An image forming apparatus for forming a color image on a recording medium,
A pattern forming unit that fixes and forms a gradation pattern consisting of patches that continuously and uniformly change along the conveyance direction of the recording medium on the recording medium when a predetermined instruction is input;
Detecting means for irradiating the formed gradation pattern with light having a spot diameter straddling a plurality of patches and detecting the density or color of the gradation pattern according to the reflected light;
Setting means for setting printing conditions based on the detected density or color;
Means for detecting the size of the recording medium,
The pattern forming means divides the gradation pattern into a plurality of divisions when the recordable size in the conveyance direction of the recording medium is less than the length required for recording the gradation pattern by the detection. Forming a gradation pattern to include a predetermined number of gradation patches that overlap each other in the divided portion of the gradation pattern, and forming each divided gradation pattern on a plurality of recording media. An image forming apparatus. 3. The image forming apparatus according to claim 1, wherein the number of patches to be overlapped is a number of patches having a length equal to or more than half of the spot diameter. A method of controlling an image forming apparatus for forming a color image on a record medium,
A pattern forming step of fixing and forming a gradation pattern consisting of patches that continuously and uniformly change along the conveyance direction of the recording medium on the recording medium when a predetermined instruction is input;
A detection step of irradiating the formed gradation pattern with light having a spot diameter straddling a plurality of patches, and detecting the density or color of the gradation pattern according to the reflected light;
A setting step for setting printing conditions based on the detected density or color ;
Detecting a recordable size in the transport direction of the recording medium;
A comparison step for comparing the detected recordable size with a length required for recording the continuous and uniformly changing gradation pattern, and
In the pattern formation step, when the comparison result of the comparison step indicates that the recordable size is less than the length required when the gradation pattern is recorded, the gradation pattern is changed according to the recordable size. A gradation pattern is formed so as to include a predetermined number of patches that overlap each other in the divided portions of the divided gradation pattern, and each divided floor is formed on a plurality of recording media. A control method for an image forming apparatus, wherein a tone pattern is formed . A control method of an image forming apparatus for forming a color image on a recording medium,
A pattern forming step of fixing and forming a gradation pattern consisting of patches that continuously and uniformly change along the conveyance direction of the recording medium on the recording medium when a predetermined instruction is input;
A detection step of irradiating the formed gradation pattern with light having a spot diameter straddling a plurality of patches, and detecting the density or color of the gradation pattern according to the reflected light;
A setting step for setting printing conditions based on the detected density or color;
Detecting the size of the recording medium, and
In the pattern forming step, when the size in the conveyance direction of the recording medium is less than the length required for recording the gradation pattern by the detection, the gradation pattern is divided into a plurality of parts. The gradation pattern is formed so as to include a predetermined number of gradation patches that overlap each other in the divided portion of the gradation pattern, and each divided gradation pattern is formed on a plurality of recording media. Control method for image forming apparatus. 6. The method of controlling an image forming apparatus according to claim 4, wherein the number of patches to be overlapped is the number of patches having a length equal to or more than half of the spot diameter.

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