JP2006343679A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image forming apparatus capable of accurately correcting unevenness in a first direction by decreasing influence by unevenness in a second direction crossing with the first direction even when the unevenness in the second direction is large. <P>SOLUTION: When forming a correction table used in an image signal conversion part 60, a test image T including a plurality of density patterns output from a test image data generation circuit 64 is repeatedly formed twice at the interval of 1/2 of the circumferential length of a photoreceptor (interval of 180° being the rotational phase difference of the photoreceptor), and read by an image reading part 12. A correction arithmetic calculation part 62 arithmetically calculates the average value AV(X) of output gradation values between the respective test images at respective pixel positions X in a main scanning direction for every density pattern based on the image data of the respective read test images. Based on the average value AV(X), the correction table is formed and output to the image signal conversion part 60. The image signal conversion part 60 corrects the input image data by using the correction table and outputs it. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that forms an image on a recording medium by electrophotography.

  An image formed on a recording medium by an electrophotographic method is inferior to other recording methods such as an ink jet recording method in terms of color uniformity in the recording surface. For example, nonuniformity in the main scanning direction (axial direction of the photoconductor) due to non-uniformity of the laser exposure amount for exposing the photoconductor, and nonuniformity in the sub-scanning direction (rotating direction of the photoconductor) due to nonuniformity of the photoconductor film thickness are mixed, This is significantly inferior to other recording methods. Further, not only the photoreceptor but also color unevenness may occur due to development unevenness or charging unevenness due to rotation of the developing roll or the charging roll.

In order to solve this problem, a technique for correcting the exposure amount (laser power) by changing it in the scanning direction based on the position information of the photoreceptor is known (see, for example, Patent Documents 1 and 2).
JP-A-9-197316 JP 2001-66835 A

  However, the conventional technique has a problem in that the unevenness in the main scanning direction cannot be accurately corrected because the unevenness in the sub-scanning direction is affected by the influence. In addition, when the high density part is made uniform, the low density part is not always uniform. Further, there is a problem that the level of the laser power does not respond to a very thin streak (for example, 0.5 mm or less), and the intensity changes discretely, so that a step is visible by correction.

  The present invention has been made in view of the above facts, and even when the unevenness in the second direction intersecting the first direction is large, the influence is reduced and the unevenness in the first direction is corrected with high accuracy. An object of the present invention is to provide an image forming apparatus capable of performing the above. It is another object of the present invention to provide an image forming apparatus that improves unevenness over the entire density range, not limited to a specific density.

  To achieve the above object, an image forming apparatus according to a first aspect of the present invention exposes a photoconductor, a charger for charging the photoconductor, and the photoconductor charged by the charger based on input image data. An exposure apparatus, a developer carrying member for carrying a developer, a developer for developing an electrostatic latent image formed by exposure of the exposure apparatus using the developer, and an image developed by the developer The image forming means having a transfer device for transferring the image to the transfer target and the test image having the same density in the first direction are repeatedly formed at a predetermined interval in the second direction intersecting the first direction. As described above, based on the image data output means for outputting the image data forming the test image to the image forming means, and the reading result obtained by reading each of the test images formed by the image forming means. The first A first calculation means for calculating an average value between the test images having a predetermined density for each pixel in the direction of the direction, and the density of the test image based on each of the average values calculated by the first calculation means. Second correction means for calculating a correction value for correcting color unevenness in the first direction with respect to each of the predetermined pixels, and a correction value calculated by the second calculation means. And correction means for correcting the image data.

  According to such a configuration, even when the unevenness in the second direction intersecting the first direction is large, the influence can be reduced and the unevenness in the first direction can be corrected with high accuracy.

  According to a second aspect of the present invention, there is provided an image forming apparatus comprising: a photosensitive member; a charger that charges the photosensitive member; an exposure device that exposes the photosensitive member charged by the charger based on input image data; And a developer that develops the electrostatic latent image formed by the exposure of the exposure apparatus using the developer, and the image developed by the developer is transferred to the transfer target. A test image comprising a plurality of density patterns having the same density in the first direction and different densities in the second direction intersecting the first direction. Image data output means for outputting image data for forming the test image to the image forming means so as to be repeatedly formed at predetermined intervals in the direction of the image, and each of the test images formed by the image forming means Read A first computing means for computing an average value between the test images of a predetermined density for each pixel in the first direction in each density pattern based on the obtained reading result; and the first computing means. Second calculation means for calculating a correction value for correcting the color unevenness in the first direction with respect to the density of the density pattern, for each predetermined pixel, based on each of the calculated average values; And correction means for correcting the input image data using the correction value calculated by the two calculation means.

  According to such a configuration, even when the unevenness in the second direction intersecting the first direction is large, the influence can be reduced and the unevenness in the first direction can be corrected with high accuracy. Furthermore, not only a specific density but also unevenness can be improved over the entire density range.

  In the first and second image forming apparatuses, the predetermined interval includes an average value of a predetermined density calculated for each pixel calculated by the first calculation means in a predetermined cycle in the second direction. The interval can be set to a value within a predetermined range including the center of density fluctuation.

  If the test image is repeatedly formed at such an interval, the average value of the density becomes a value within a predetermined range including the center of the density fluctuation. Therefore, the density fluctuation generated in the second direction at a predetermined cycle is generated. The correction value can be calculated with a reduction.

  Further, the density fluctuation generated in the second direction at a predetermined cycle is caused by any one of the photoconductor, the charger, the developer carrier, and the transfer unit rotating in the second direction. The predetermined interval may be an interval determined based on a peripheral length of any of the photosensitive member, the charger, the developer carrier, and the transfer unit.

  Taking a photoconductor as an example, if there is non-uniformity in the photoconductor film thickness or eccentricity of the rotation axis, as shown in FIG. 16, the circumference of the photoconductor is set as one cycle in the rotation direction of the photoconductor. Unevenness occurs. Therefore, even if a density pattern having the same density is formed in a direction intersecting with the rotation direction of the photoconductor to correct unevenness in the direction, the density varies depending on the position in the rotation direction of the photoconductor, A correction value for correcting unevenness that occurs in the intersecting direction cannot be calculated with high accuracy. Also, rotation of other rotating bodies such as the charger, developer carrier, transfer means, etc. causes charging unevenness, development unevenness, etc., and the correction value cannot be calculated with high accuracy as described above.

  On the other hand, in the present invention, the predetermined interval is set as an interval determined based on the peripheral length of any one of the photoconductor, the charger, the developer carrier, and the transfer unit. The correction value can be calculated with high accuracy while suppressing the influence of fluctuation caused by rotation.

  For example, the predetermined interval is based on the peripheral length of any of the photoconductor, the charger, the developer carrier, and the transfer unit, and the photoconductor, the charger, the developer carrier, and What is necessary is just to obtain an interval (rotation phase difference) that substantially cancels the density fluctuation caused by the rotation of the transfer means, and repeatedly forms a test image with the interval. The interval is not particularly limited, but it is particularly preferable that test images are repeatedly formed at the following intervals.

  That is, the image data output means of the first and second image forming apparatuses has a rotational phase difference 360 / N (N of any one of the photoconductor, the charger, and the developer carrying member rotating in the second direction. The image data of the test image can be output to the image forming means so that the test image is repeatedly formed N times at a position shifted by an integer of 2 or more.

  Thereby, the correction value can be calculated while minimizing the influence of density fluctuation.

  Further, the second calculation means of the first and second image forming apparatuses calculates an average value or mode value of the predetermined density for each pixel as a target density for the density of the test pattern. The correction value can be calculated for each predetermined pixel based on the target density and the predetermined density for each pixel.

  When calculating the average value as the target density, for example, when the color unevenness that occurs when the image data is not corrected is gentle, such color unevenness can be effectively reduced. Further, when the mode is calculated as the target density, for example, when the image data is not corrected, color unevenness occurs only in a part of the first direction, such as an end in the first direction. In such a case, such color unevenness can be effectively reduced.

  The predetermined pixel may be one pixel or a plurality of pixels.

  When the correction value is calculated and used for each pixel, the correction can be performed with high accuracy. When the correction value is calculated and used for each of the plurality of pixels, the same correction value can be used for each of the plurality of pixels. Therefore, the memory capacity for storing the correction value can be reduced.

  Further, the first and second image forming apparatuses may further include a reading unit that reads a test image formed by the image forming unit.

  For example, the reading unit may be provided on a paper conveyance path inside the image forming apparatus, or may be used as a reading unit that reads a document as long as the image forming apparatus has a copying function.

  In addition, when the correction value is calculated for each pixel located at a predetermined interval in the first direction, the correction unit calculates a correction value for a pixel other than the pixel corresponding to the correction value of peripheral pixels. It can be calculated by interpolation based on the correction value.

  Accordingly, it is possible to effectively reduce color unevenness while reducing the capacity of a memory that stores correction values.

  As described above, according to the present invention, even when the unevenness in the second direction intersecting the first direction is large, the influence can be reduced and the unevenness in the first direction can be corrected with high accuracy. , Has the effect. In addition, there is an effect that unevenness can be improved over the entire density range, not limited to a specific density.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an image forming apparatus 10 according to this embodiment.

  The image forming apparatus 10 includes an image reading unit 12, an image recording unit 14, and a control unit 16.

  The image reading unit 12 reads an image recorded on the paper P sandwiched between the mounting table 18 configured to include platen glass or the like and the cover 20. Specifically, the light from the light source 22 is irradiated onto the paper P, and the reflected light L is imaged on the image sensor 26 by the reflecting mirror 24 or a lens (not shown).

  The image sensor 26 includes line CCDs for R (red), G (green), and B (blue), signal processing circuits, A / D converters, and the like. Light corresponding to a line image in a predetermined direction of the paper P (direction perpendicular to the paper surface in FIG. 1) is formed on the light receiving surface of each color line CCD, and this is converted into an electric signal. Then, by performing predetermined signal processing and A / D conversion on the electric signal, digital data of a line image is obtained. By reading this line image along the direction of arrow A in FIG. 1, image data of the image recorded on the paper P (image data of each color of R, G, B) is obtained. Image data of the image read by the image sensor 26 is output to the control unit 16.

  The image recording unit 14 includes a light beam output unit 28. The light beam output unit 28 includes a light source and a lens (not shown), and the image data of each color Y (yellow), M (magenta), C (cyan), and K (black) output from the control unit 16. A light beam modulated based on the above is output. The light beams of the respective colors are output to the optical scanning devices 30Y, 30M, 30C, and 30K, respectively.

  Since each optical scanning device has the same configuration, only the optical scanning device 30Y will be described, and description of the other optical scanning devices will be omitted.

  The optical scanning device 30Y includes a polygon mirror 32, a folding mirror 34, an fθ lens (not shown), and the like. The light beam emitted from the light beam output unit 28 is deflected by the polygon mirror 32 in the main scanning direction (direction perpendicular to the paper surface in FIG. 1). The deflected light beam passes through an unillustrated fθ lens, is folded back by the folding mirror 34, and reaches the photosensitive drum 38 of the developing unit 36Y.

  The image recording unit 14 includes developing units 36Y, 36M, 36C, and 36K for each color. Since each developing unit has the same configuration, only the developing unit 36Y will be described, and description of the other developing units will be omitted.

  The developing unit 36Y includes a cylindrical photosensitive drum 38, and a charging device 40, a developing device 42, a primary transfer roller 44, a cleaner 46, and a charge eliminating device 48 are provided around the photosensitive drum 38. It has become.

  The photosensitive drum 38 rotates in the direction of arrow B in FIG. 1 and is uniformly charged by the charger 40. The light beam emitted from the optical scanning device 30Y is irradiated between the charger 40 and the developing device 42, and scans the photosensitive drum 38 in the main scanning direction (axial direction of the photosensitive drum 38). Sub-scanning is performed by rotating the photosensitive drum 38 in the arrow B direction. As a result, an electrostatic latent image corresponding to the image is formed on the photosensitive drum 38. The electrostatic latent image formed on the photosensitive drum 38 is developed with toner by a developing unit 42 having a developing roll carrying toner as a developer. As a result, a toner image corresponding to the image is formed on the photosensitive drum 38.

  The primary transfer roller 44 is disposed to face the photosensitive drum 38 with the intermediate transfer belt 50 interposed therebetween. The intermediate transfer belt 50 is conveyed in the direction of arrow C in FIG. The toner image formed on the photosensitive drum 38 is primarily transferred to the intermediate transfer belt 50 by the primary transfer roller 44.

  The other developing units also perform the same processing, and a color image is formed on the intermediate transfer belt 50 by sequentially transferring the toner images of the respective colors onto the intermediate transfer belt 50.

  On the other hand, the paper P discharged from the paper tray 54 is transported in the direction of arrow D in FIG. Then, when the paper P is conveyed to the secondary transfer roller 58 and the paper P is nipped with the intermediate transfer belt 50, a predetermined transfer voltage is applied, and the color image formed on the intermediate transfer belt 50 is transferred to the paper P. Is done.

  The sheet P on which the color image has been transferred is subjected to a fixing process by the fixing device 59 and is discharged to a discharge tray (not shown).

  The control unit 16 converts the image data output from other devices and the image data of the document read by the image reading unit 12 (image data of each color of CMYK) using a correction table described later. A conversion unit 60, a correction calculation unit 62 that creates a correction table, for example, a test image data generation circuit 64 that generates test image data of a test image T as shown in FIG. 2, and an image signal conversion according to the input operation mode The corrected image data output from the unit 60 is selected and output to the light beam output unit 28, or the test image data output from the test image data generation circuit 64 is selected and output to the light beam output unit 28. It includes a selector 66 for selecting the above.

  When the operation mode is the normal operation mode, the selector 66 selects the image data output from the image signal conversion unit 60 and outputs it to the light beam output unit 28. As a result, an image based on the image data input to the control unit 16 is printed on the paper P.

  When the operation mode is the test mode, the test image data output from the test image data generation circuit 64 is selected and output to the light beam output unit 28. Here, the test image is repeatedly printed twice with an interval of 1/2 of the peripheral length of the photosensitive drum 38 (a rotational phase difference of the photosensitive drum 38 rotating in the sub-scanning direction is 180 degrees). The test image data generation circuit 64 outputs the same test image data twice at a predetermined timing. As a result, as shown in FIG. 2, the same test image T is printed twice with an interval of ½ of the circumferential length of the photosensitive drum 38. Hereinafter, the test image printed for the first time is referred to as T1, and the test image printed at an interval corresponding to the rotational phase difference of the photosensitive drum 38 from the test image T1 is referred to as T2.

  The operation mode can be specified by, for example, the operator performing a predetermined operation from an operation unit (not shown).

  Hereinafter, the creation of the correction table in the test mode will be described. Here, the case of correcting any one of YMCK will be described as an example.

  A test image T shown in FIG. 2 is an image used when creating a correction table for correcting color unevenness in the main scanning direction M. The test image T has the same density in the main scanning direction M and the sub scanning direction S. Is configured to include a plurality of density patterns in which the density gradually increases step by step for each of a plurality of predetermined gradations from the lower limit to the upper limit of the gradation range. In the present embodiment, a case will be described in which the number of pixels in the main scanning direction M is 7000, the gradation range of each color is 0 to 255, that is, the image data is 8 bits.

  The test image T shown in FIG. 2 includes, for example, a belt-like density pattern having the main scanning direction M as the longitudinal direction for each of the densities N1 to N10 obtained by dividing the gradation range into 10 stages. Note that the test image is not limited to such a configuration, and any test image may be used as long as each density in the gradation range can be confirmed at each pixel position.

  The test image data of the test image T output from the test image data generation circuit 64 by the selector 66 is output to the light beam output unit 28 via the selector 66. As a result, the test image T is printed on the paper P by the image recording unit 14.

  The operator sets the paper P on which the test image T is printed on the image reading unit 12, and instructs the image reading by performing a predetermined operation from an operation unit (not shown). As a result, the test image T is read by the image reading unit 12, and the image data, that is, image data of each color of R, G, and B is output to the correction calculation unit 62.

  FIG. 3 shows a flowchart of a control routine executed in the correction calculation unit 62. In this control routine, a correction value (corrected gradation value) for correcting unevenness occurring in the main scanning direction is calculated for each input gradation value.

  First, in step 100, the correction calculation unit 62 inputs the reading results (density) of the test images T1 and T2 that are repeatedly formed twice and read by the image reading unit 12, and converts them into YMCK image data. This conversion can be performed using a predetermined conversion table or the like.

  In the following, the test value output from the test image data generation circuit 64 is the output tone value of the pixel value of each pixel constituting the YMCK image data of the test images T1 and T2 read by the image reading unit 12. A pixel value of each pixel constituting the image data is referred to as an input gradation value.

  In step 102, for each density pattern of the input tone values N1 to N10, the average value AV (X of the output tone value of the test image T1 and the output tone value of the test image T2 at each pixel position X in the main scanning direction. ) Is calculated. Here, X is a variable indicating each pixel position in the main scanning direction. Using this average value AV (X), as will be described later, a correction value (corrected gradation value) for correcting unevenness occurring in the main scanning direction is calculated. In this way, the average value of the density between the test images T1 and T2 formed with a predetermined interval (in this case, ½ of the peripheral length of the photosensitive drum 38) in the sub-scanning direction is used for the calculation of the correction gradation value. As a result, it is possible to reduce the influence of density fluctuations in the sub-scanning direction that are periodically generated by the rotation of the photosensitive drum 38 that rotates in the sub-scanning direction.

  Hereinafter, density fluctuations that occur in the sub-scanning direction will be described.

  If there is non-uniformity in the photosensitive member film thickness, eccentricity of the rotation axis, etc., as shown in FIG. It is known to occur in a substantially sinusoidal (periodically) manner.

  Therefore, even if a density pattern having the same density is printed in the main scanning direction, the output tone value of the density pattern varies depending on the position in the sub-scanning direction, and a correction value (correction) for correcting unevenness in the main scanning direction. (Gradation value) cannot be calculated with high accuracy.

  As shown in FIG. 4B, in a sine wave with an amplitude of 1, the average value of P1 and P2 whose phases are shifted by 180 degrees is equal to the center value (0.00) of the wave height of the sine wave. Similarly, the average value of Q1 and Q2 whose phases are shifted by 180 degrees is also the center value (0.00) of the wave height.

  From this, if the two-point average of the phase difference of 180 degrees is taken, the value at the approximate center of the periodic density fluctuation (wave height) with the peripheral length of the photosensitive drum 38 as one cycle can be obtained. By using this two-point average value as the output gradation value at each pixel position X, it is possible to calculate a corrected gradation value that is not affected by density fluctuations in the sub-scanning direction.

  Therefore, in the present embodiment, the same test image T (test images T1 and T2) is formed at an interval of ½ of the circumferential length of the photosensitive drum 38 (rotation phase difference 180 degrees), and the input gradation For each density pattern of values N1 to N10, the average value of the output tone value of the test image T1 and the output tone value of the test image T2 at each pixel position X in the main scanning direction is calculated. Then, the calculated average value is used for calculating the corrected gradation value.

  Here, the test images T1 and T2 are formed with a rotational phase difference of 180 degrees apart, but the formation interval of the test images T1 and T2 is not limited to this.

  FIG. 4A shows the amount of deviation from the center value of the above-mentioned wave height of the average of two points for each phase difference when the phase difference between two points is changed from 0 to 180 degrees in a sine wave having an amplitude of 1. It is the graph which showed the maximum value. As shown in the figure, when the phase difference between the two points is smaller than 180 degrees, the deviation amount from the center value of the wave height of the average of the two points is larger than zero. That is, the two-point average is a value away from the center of density fluctuation. When printing a test image twice, it is most preferable to set the interval between the test images to an interval of a phase difference of 180 degrees at which the deviation from the center value of the wave height is 0, but the phase difference is smaller than this. However, if the deviation is small, the influence of density fluctuation in the sub-scanning direction can be suppressed.

  Accordingly, the limit value of the deviation amount that can calculate the correction gradation value without being affected by the density fluctuation that occurs in the sub-scanning direction is obtained in advance by experiments or the like, The test image may be printed with a phase difference in which the maximum value is within the obtained limit value (that is, a phase difference in which the two-point average value is the center of density fluctuation ± a value within the range of the limit value). .

  In addition, here, the test image T is output twice with an interval of 180 degrees in phase difference, and the average value of each output gradation value is obtained, but is output three times with an interval of 120 degrees in phase difference. Then, an average value of the output gradation values may be obtained. That is, a test image having a phase difference of 360 degrees / N may be output N times, and an average value obtained by dividing the total of each output gradation value by N may be obtained. This can also reduce the influence of density fluctuation in the sub-scanning direction.

  In step 104, target gradation values for the input gradation values N1 to N10 are obtained based on the calculated average value AV (X) of the output gradation values at each pixel position X in the main scanning direction. Specifically, the average value AV (1) to AV (7000) is divided by 7000 for each density pattern, and the average value in the main scanning direction is obtained for each of the input gradation values N1 to N10. This is the target gradation value for the input gradation value. It should be noted that the target tone values for the input tone values other than the input tone values N1 to N10 may be obtained by interpolation from the target tone values for the input tone values before and after being obtained. For example, the target gradation value for the input gradation value between the input gradation values N1 and N2 may be interpolated based on the target gradation values obtained for the input gradation values N1 and N2.

  FIG. 5A shows actual gradation characteristics and target gradation characteristics for each pixel position X in the main scanning direction M. FIG. In FIG. 9A, the solid line is the actual gradation characteristic, and the dotted line is the target gradation characteristic. As shown in FIG. 5, when there is a difference between the actual gradation characteristic and the target gradation characteristic, it is necessary to correct the pixel position.

  Therefore, in step 106, for each pixel position X in the main scanning direction M, a correction gradation value for each input gradation value is calculated by a predetermined arithmetic expression, and the correction table, that is, the input gradation value at each pixel position is calculated. A correction table representing a correspondence relationship with the correction gradation value is created. The corrected gradation value calculated by the predetermined arithmetic expression is a value such that when the image of the corrected gradation value is printed on a sheet, the density becomes the density of the target gradation value. For example, when the input gradation value is DI, the average output gradation value at the pixel position X is AV (X), the target gradation value is DM, and the corrected gradation value at the pixel position X is DR (X), As an example, the following equation can be expressed by the following equation.

DR (X) = DI + (DM-AV (X)) (1)
FIG. 5B shows an example of the gradation characteristic (relationship between the input gradation value and the correction gradation value) at each pixel position X in the correction table. For example, as shown in FIG. 5B, a pixel whose output gradation value tends to be larger than the target gradation value as in the gradation characteristic at the pixel position X = 2 shown in FIG. Then, the correction table data is created so that the correction gradation value becomes smaller by that amount. For example, when the input gradation value DI is 50, the output gradation value AV (X) is 53, and the target gradation value DM is 52, the corrected gradation value DR (X) calculated by the above equation (1) is 49. Become.

  By converting the image data using the correction table created by calculating the correction gradation value in this manner, the output gradation value can be made to substantially match the target gradation value. The predetermined arithmetic expression is not limited to the above expression (1), and can be set as appropriate according to the characteristics of the apparatus.

  A correction table for each color can be created by performing the above processing for each color of C, Y, and M in the same manner.

  In step 108, the generated correction table data for each color is output to the image signal converter 60.

  As shown in FIG. 6, the image signal conversion unit 60 includes a position information output unit 70 and correction tables 72C, 72M, 72Y, and 72K for each color. The CMYK color image data constituting the image data, that is, input gradation values, are sequentially input to the position information output unit 70, and the main scanning of the input gradation values is performed based on the input order and the paper size information. The pixel position X in the direction M is determined and output to each correction table. Each correction table outputs a corrected gradation value corresponding to the input pixel position X and the input gradation value to the selector 66.

  FIG. 7A shows the configuration of the cyan correction table 72C, and FIG. 7B shows a numerical example of the correction table 72. The numerical value in the frame indicates the correction gradation value. In FIG. 5B, the pixel position and the input gradation value are thinned out. As shown in FIG. 5A, the cyan correction table 72C inputs the pixel position X in the main scanning direction M and the input gradation value Cin of the pixel position X, and corrects the input gradation value Cin. The corrected gradation value Cout is output. The other color correction tables have the same configuration.

  In the normal operation mode, the selector 66 selects the image data of each color composed of the corrected gradation value output from the image signal conversion unit 60 and outputs it to the light beam output unit 28. As a result, the image recording unit 14 prints an image based on the corrected image data on the paper P. The image printed on the paper P in this manner is an image in which the occurrence of color unevenness and fine stripes in the main scanning direction M is reduced as compared with the case where correction is not performed.

  In this embodiment, the test image T has a density pattern in which the density gradually increases in multiple gradations from the lower limit to the upper limit of the gradation range in the sub-scanning direction S. A density pattern in which the density gradually increases or decreases may be used. Thereby, color unevenness can be reduced more effectively.

  Further, in the present embodiment, the case where correction is performed for each pixel in the main scanning direction M has been described. However, correction may be performed for each of a plurality of pixels in the main scanning direction M. For example, when correcting every 10 pixels, an average value of output gradation values is obtained for each range of pixel positions X = 1 to 10, 11 to 20... 6991 to 7000, and based on the average value of each range. A target gradation value is obtained. Then, a correction table is created by obtaining corrected gradation values for each range based on the obtained target gradation values. Therefore, since the same correction gradation value can be used within each range, the capacity of the correction table can be reduced.

  FIG. 8 shows the results of experiments conducted by the present inventors on the relationship between the correction resolution (correction unit) and the appearance of color unevenness visually. As shown in FIG. 8, if the correction resolution is at least 50 lines / 1 inch (corrected every 12 pixels) or more, the color unevenness is not a concern, and the correction resolution is 100 lines / 1 inch (every 6 pixels). (Correction) Above, it was found that the color unevenness was not visible. Therefore, the correction resolution is set to 50 lines / inch or more, preferably 100 lines / inch or more.

  Further, when a correction table thinned out with respect to the pixel position X and the input gradation value is created as in the correction table shown in FIG. 9, when the correction gradation value is interpolated, the pixel position X and the input gradation value It is also possible to interpolate from the corrected gradation values around the area. For example, the pixel position to be interpolated is Xa, the input gradation value to be interpolated is Da, the pixel position where the corrected gradation value is calculated and the pixel positions before and after the pixel position Xa are X1, X2, and the corrected gradation value. Are the input gradation values calculated before and after the input gradation value Da, D1 and D2, the correction gradation value of the input gradation value D1 at the pixel position X1 is DH1, and at the pixel position X2. When the correction gradation value of the input gradation value D1 is DH2, the correction gradation value of the input gradation value D2 at the pixel position X1 is DH3, and the correction gradation value of the input gradation value D2 at the pixel position X2 is DH4, The corrected gradation value DHa of the input gradation value Da at the pixel position Xa can be obtained by the following equation.

DHa = A1 + (A2-A1) * (Da-D1) / (D2-D1) (2)
However, A1 = DH1 + (DH2-DH1) * (Xa-X1) / (X2-X1)
A2 = DH3 + (DH4-DH3) * (Xa-X1) / (X2-X1)
For example, in the correction table of FIG. 9, the corrected gradation value DHa of the input gradation value Da = 100 at the pixel position Xa = 4000 is the four corrected gradation values (93, 92, 109) shown in the rectangular frame of FIG. 108) from the above equation (2), in this case DHa = 98.

  In this way, by providing the interpolation means for interpolating the corrected gradation values for the pixel positions and input gradation values that do not exist in the correction table, for example, in the image signal converter 60, the capacity of the correction table can be reduced.

  In the present embodiment, the case where a correction table for reducing color unevenness in the main scanning direction M is created using the test image T as shown in FIG. 2 to correct the image data has been described. Is not limited to this. For example, the density is all the same in the sub-scanning direction S, and the density in the main scanning direction M is stepwise for each of a plurality of gradations set in advance from the lower limit to the upper limit of the gradation range. It is also possible to create a correction table for correcting color unevenness in S in the sub-scanning direction using a test image configured to include a plurality of density patterns that gradually become darker.

  In the present embodiment, an example in which the same test image T (test images T1 and T2) is formed at an interval of ½ of the peripheral length of the photosensitive drum 38 has been described. However, the present invention is not limited to this. For example, in the case where the photosensitive drum 38 is formed with an interval corresponding to the rotational phase difference of 180 degrees, it may be formed with an interval of ½ of the peripheral length of the photosensitive drum 38 as shown in FIG. Alternatively, the photosensitive drums 38 may be formed at intervals of 3/2, 5/2. For example, when the test image is formed twice and each correction gradation value for each of the colors Y, M, and C is calculated, first, as shown in FIG. 10, the C test image TC, the Y color test The first test image is formed by separating the images TY and M test images TM from each other by an interval of ½ of the peripheral length of the photosensitive drum 38, and the second test image is the first test image. It is formed at a distance of 3/2 of the peripheral length of the photosensitive member from the formed position. Even when formed in this way, each test image is formed with an interval corresponding to the rotational phase difference of 180 degrees of the photosensitive drum 38 for each color, so that the average value of each output gradation value is corrected. If the gradation value is calculated, a correction table free from the influence of density fluctuation in the sub-scanning direction can be obtained.

  In the present embodiment, the target gradation value for each input gradation value is the average value of the average value AV (X) of the output gradation values at all the pixel positions. The mode value of the average value AV (X) of the values may be set as the target gradation value. For example, when only the density at both ends in the main scanning direction tends to decrease due to the non-uniform contact of the secondary transfer roller 58 with the paper P, the output floor of each pixel position (X). Since the average value AV (X) of the tone values is significantly lower than the frequently output gradation value near the center in the main scanning direction, the pixel position (X) near the center is appropriate. In some cases, the corrected gradation value cannot be calculated. Accordingly, when the secondary transfer roller 58 has the above-described characteristics, the mode value of the average value AV (X) of the output gradation values is set as the target gradation value, so that both end sides of the paper P are set. Even if the density is low, color unevenness can be made inconspicuous.

  In the present embodiment, the correction is performed so that the color unevenness is reduced in the main scanning direction or the sub-scanning direction. However, the correction may be made so that the color unevenness is reduced in both directions. In this case, an image signal conversion unit including a correction table for the main scanning direction M and an image signal conversion unit including a correction table for the sub-scanning direction S may be connected in series. As a result, the corrected gradation value of each color output from the image signal conversion unit for the main scanning direction M is input as the input gradation value of the image signal conversion unit for the sub-scanning direction S. Can be reduced.

  In the present embodiment, when a test image is printed on the paper P, the operator sets the image on the image reading unit 12 to read the test image, and a correction table is created based on the image data of the read image. However, as in the image forming apparatus 10A shown in FIG. 11, a scanner 74 for reading a test image is provided at a position where the paper P is discharged, and is automatically read when the test image is discharged. May be. Thereby, an operator's effort can be saved.

  Further, as in the image forming apparatus 10B shown in FIG. 12, a scanner 76 for reading a test image formed on the intermediate transfer belt 50 is provided in front of the secondary transfer roller 58, and the test image read by the scanner 76 is provided. A correction table may be created based on the image data. Thereby, it is not necessary to print a test image on the paper P, and the paper can be saved.

  In the case where the image recording unit 14 is a device that can perform screen processing by switching between a plurality of screens, the color unevenness may vary depending on the screen. In this case, a correction table may be created according to the screen, and a correction table corresponding to each screen may be created according to the screen and used to correct the image data.

  In this embodiment, the test image T is exposed to light in order to reduce density fluctuation in the sub-scanning direction (rotating direction of the photosensitive member) due to non-uniformity of the photosensitive member film thickness of the photosensitive drum 38 and eccentricity of the rotating shaft. Although an example in which the average value of the density is obtained by forming a plurality of times at intervals corresponding to the peripheral length of the body drum 38 has been described, the present invention is not limited to this. For example, the density fluctuation may occur in the sub-scanning direction due to the rotation of another rotating body that rotates in the sub-scanning direction, such as the developing roll of the developing device 42, the primary transfer roller 44, the secondary transfer roller 58, or the like. Therefore, a corrected gradation value may be obtained by forming a test image a plurality of times at intervals based on the peripheral length of these rotating bodies, and obtaining an average value of density. In this embodiment, when the charger 40 is not a charger that charges the photosensitive drum 38 by corona discharge, but a charging roller that rotates and contacts the photosensitive drum 38, the charging roller 40 In some cases, the density fluctuation periodically occurs due to uneven charging in the sub-scanning direction of the charging roller. In this case, a test image is formed a plurality of times at intervals based on the peripheral length of the charging roller, and an average density value can be obtained to obtain a corrected gradation value.

(Second Embodiment)
In the first embodiment, a 1-input 1-output type correction table is created for each color and the image data is corrected for each color. In such a case, color unevenness can be reduced in the case of an image of only one color. However, color unevenness may not be reduced in the case of an image in which a plurality of colors are superimposed. In this embodiment, a correction table that can correct color unevenness of an image in which a plurality of colors are superimposed is created.

In the present embodiment, in addition to the image data of the test image T as shown in FIG. 2, as shown in FIG. 13, the test pattern 68PB of process black, which is a color obtained by combining all of yellow, magenta, and cyan in order from the top. The red test pattern 68R, which is a color combining yellow and magenta, the green test pattern 68G, which is a color combining yellow and cyan, and the blue test pattern 68B, which is a color combining cyan and magenta. The test image _TCMY image data is stored in the test image data generation circuit 64.

Similarly to the above, the test image T _CMY is repeatedly printed twice with a rotational phase difference of 180 degrees of the photosensitive drum 38, and each test image T _CMY printed on the paper P is read by the image reading unit 12. . The correction calculation unit 62 converts the RGB data of each density pattern of each read test image _TCMY into YMC data.

  Next, based on the image data (output gradation values of C, M, and Y colors) obtained by reading the process black test pattern 68PB, the average value of each (X) pixel position is obtained in the same manner as described above. After that, a target gradation value corresponding to each input gradation value is obtained, and a corrected gradation value is obtained based on the obtained target gradation value. By performing this similarly for the test patterns 68R, 68G, and 68B, when the C, M, and Y colors are overlapped, the correction gradation values when the two colors C and M, C and Y, and M and Y are overlapped are obtained. Can be sought.

Further, the same processing as described above is performed on the test image _TCMY to obtain a corrected gradation value for each color of YMCK.

  From these results, a three-input three-output correction table 80 as shown in FIG. 14A is created. FIG. 5B shows a numerical example of the correction table. In FIG. 5B, the pixel position and the input gradation value are thinned out. It should be noted that the correction gradation values for the input gradation values of each color that do not exist in the correction table can be interpolated using, for example, the same interpolation method as described in the first embodiment for each color.

  For black, a 1-input 1-output black correction table created based on image data obtained by reading the test image T in the same manner as described above is used.

FIG. 15 shows, as an example, pixel positions and lightness L ^* when an image in which cyan and yellow with a predetermined density are printed without correction and when corrected with a three-input three-output correction table and printed. Showed the relationship.

  As shown in the figure, when image data is corrected using a 3-input 3-output correction table, the brightness is almost the same at any pixel position, and correction is performed for each color using a 1-input 1-output correction table. Compared to the case, the color unevenness can be greatly reduced.

  In this way, by correcting the image data using the three-input three-output type correction table, it is possible to appropriately correct not only one color image but also two-color superimposed images and three-color superimposed images. Color unevenness can be effectively reduced.

1 is a schematic configuration diagram of an image forming apparatus. It is an image figure which shows an example of the test image used in order to reduce the color nonuniformity of the main scanning direction. It is a flowchart of the process performed in a correction | amendment calculating part. It is explanatory drawing explaining the density fluctuation which generate | occur | produces in a subscanning direction. It is a diagram which shows the gradation characteristic of each pixel position. It is a block diagram of an image signal converter. (A) is a figure which shows the correction table of 1 input 1 output type, (B) is a figure which shows the numerical example of the correction table of (A). (A) and (B) are a case where only cyan is printed with a density within a predetermined range without correcting the image data, a case where only cyan is printed with a density within a predetermined range by correcting the image data with a correction table, It is the figure which showed the relationship between the position of the main scanning direction M in L, and the lightness L ^* . It is a figure which shows the numerical example of the correction table of 1 input 1 output type | mold. This is a printed image when test images for each color of YMC are formed with an interval of 3/2 (rotational phase difference of 180 degrees) of the circumference of the photoreceptor. It is a schematic block diagram of the image forming apparatus which concerns on a modification. It is a schematic block diagram of the image forming apparatus which concerns on a modification. It is an image figure which shows an example of the test image used when creating a 3 input 3 output type | mold correction table. (A) is a figure which shows a 3 input 3 output type | mold correction table, (B) is a figure which shows the numerical example of the correction table of (A). It is a figure which shows the relationship between each pixel position and the lightness L ^* at the time of printing the image which superimposed cyan and yellow of predetermined density | concentration without a correction | amendment, and correcting and printing with the correction table of a 3 input 3 output type | mold. Unevenness in the sub-scanning direction (rotating direction of the photosensitive member) due to non-uniformity of the photosensitive member film thickness, eccentricity of the rotating shaft, etc., occurs periodically in a substantially sinusoidal shape with the peripheral length of the photosensitive drum as one cycle. It is explanatory drawing explaining these.

Explanation of symbols

10, 10A, 10B Image forming device 12 Image reading unit 14 Image recording unit 16 Control unit 28 Light beam output units 30Y, 30M, 30C, 30K Optical scanning devices 36Y, 36M, 36C, 36K Development unit 60 Image signal conversion unit 62 Correction Arithmetic unit 64 Test image data generation circuit 66 Selector 72Y, 72M, 72C, 72K Correction table 74, 76 Scanner 80 Correction table

Claims (9)

A photoconductor, a charger that charges the photoconductor, an exposure device that exposes the photoconductor charged by the charger based on input image data, and a developer carrier that carries a developer. An image forming means comprising: a developing unit that develops an electrostatic latent image formed by exposure of the apparatus using the developer; and a transfer unit that transfers the image developed by the developing unit to a transfer target;
Image data for forming the test image is formed in such a manner that test images having the same density in the first direction are repeatedly formed at predetermined intervals in a second direction intersecting the first direction. Image data output means for outputting to the means;
Based on a reading result obtained by reading each of the test images formed by the image forming means, a first value for calculating an average value between the test images having a predetermined density for each pixel in the first direction is calculated. One computing means;
A second value for calculating a correction value for correcting the color unevenness in the first direction with respect to the density of the test image for each predetermined pixel based on each of the average values calculated by the first calculation means. Computing means;
Correction means for correcting the input image data using the correction value calculated by the second calculation means;
An image forming apparatus including: A photoconductor, a charger that charges the photoconductor, an exposure device that exposes the photoconductor charged by the charger based on input image data, and a developer carrier that carries a developer. An image forming means comprising: a developing unit that develops an electrostatic latent image formed by exposure of the apparatus using the developer; and a transfer unit that transfers the image developed by the developing unit to a transfer target;
Test images composed of a plurality of density patterns having the same density in the first direction and different densities in the second direction intersecting the first direction are repeatedly formed at positions spaced by a predetermined interval in the second direction. Image data output means for outputting image data for forming the test image to the image forming means,
Based on the reading result obtained by reading each of the test images formed by the image forming means, the average value between the test images of the density for each predetermined pixel in the first direction in each density pattern First computing means for computing
A second value for calculating a correction value for correcting the color unevenness in the first direction with respect to the density of the density pattern for each predetermined pixel based on each of the average values calculated by the first calculation means. Computing means;
Correction means for correcting the input image data using the correction value calculated by the second calculation means;
An image forming apparatus including:   The predetermined interval is a value within a predetermined range in which a predetermined average density value for each pixel calculated by the first calculation means includes a center of density fluctuation generated in a predetermined cycle in the second direction. The image forming apparatus according to claim 1, wherein the distance is an interval. The density fluctuation generated in the second direction at a predetermined cycle is a fluctuation caused by any one of the photoconductor, the charger, the developer carrier, and the transfer unit rotating in the second direction. There,
The image forming apparatus according to claim 3, wherein the predetermined interval is an interval determined based on a peripheral length of any of the photosensitive member, the charger, the developer carrier, and the transfer unit.   The image data output means is located at a position shifted by a rotational phase difference of 360 / N (N is an integer of 2 or more) of any one of the photosensitive member, the charger, and the developer carrying member rotating in the second direction. The image forming apparatus according to claim 3, wherein image data of the test image is output to the image forming unit so that the test image is repeatedly formed N times.   The second calculation means calculates the average value or mode value of the predetermined density for each pixel as a target density for the density of the test pattern, and calculates the target density and the predetermined pixel-specific density. 6. The image forming apparatus according to claim 1, wherein the correction value is calculated for each predetermined pixel based on density.   The image forming apparatus according to claim 1, wherein the predetermined pixel is one pixel or a plurality of pixels.   The image forming apparatus according to claim 1, further comprising a reading unit that reads a test image formed by the image forming unit.   When the correction value is calculated for each pixel located at a predetermined interval in the first direction, the correction unit calculates a correction value for a pixel other than the pixel corresponding to the correction value as a correction value for peripheral pixels. The image forming apparatus according to claim 1, wherein the image forming apparatus is calculated by performing interpolation based on the image.