JP2006256086A - Image forming apparatus and exposure control method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reflect an influence of unevenness of color in a sub-scanning direction, in the correction of exposure in a main scanning direction. <P>SOLUTION: The result of printing is read by printing a test chart at exposure based on each exposure correction pattern in the main scanning direction, and the worst value of a main-scanning-direction difference between colors in the test chart is determined in terms of each color in the test chart from the read result (S10-S14). Thus, the distribution of the worst value of the difference between the colors when the exposure correction pattern is varied can be obtained in terms of each color (S16). After the test chart for the sub-scanning direction is printed (S18), the result of the printing is read (S20), and the worst value of a sub-scanning-direction difference between the colors in the test chart is determined in terms of each color in the test chart from the read result (S22). Subsequently, the distribution of the worst value of the main-scanning-direction difference between the respective colors is corrected by the worst value of the sub-scanning-direction difference between the colors, which is determined in S22 (S24). An exposure correction amount pattern, in which the worst value of the difference between the respective colors is traded off, is identified from the corrected distribution of the worst value of the difference between the respective colors. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to exposure correction control thereof.

  Regarding exposure correction of an electrophotographic image forming apparatus, the apparatus disclosed in Patent Document 1 prints a test image of a predetermined density and reads it in the main scanning direction of the photosensitive drum (axial direction of the photosensitive element). The density distribution along the main scanning direction is created, correction data for the inverse profile of the density distribution is created, and the exposure intensity at each position along the main scanning direction is corrected according to the correction data, thereby suppressing density unevenness in the main scanning direction. ing.

  The apparatus disclosed in Patent Document 2 applies the same method as described above in the sub-scanning direction (the rotation direction of the photosensitive drum), and suppresses density unevenness in the sub-scanning direction.

  In each of these conventional techniques, the exposure amount is corrected so that the density of each point in the main scanning direction or the sub-scanning direction is constant at a specific gradation (for example, Cin (input density) = 60%). It is.

  However, the density distribution does not always have the same slope at all gradations. For example, it is well known that density unevenness caused by the distance between the developing roll and the photosensitive drum is reversed depending on gradation. Accordingly, with the exposure correction amount determined based on a certain gradation, the uneven density cannot be eliminated with other gradations having different tendency of uneven density.

  Patent Document 3 by the present applicant proposes the following technique in order to deal with such a problem. In this method, a test pattern for each of a plurality of gradations is printed while changing the exposure amount pattern in the main scanning direction in various ways, and the print result is read, so that a main pattern for each exposure amount pattern is obtained for each gradation. The maximum color difference in the scanning direction is obtained. Then, based on the information on the maximum color difference, an exposure amount pattern having the best color difference in the whole (total) of the plurality of gradations is obtained, and exposure control in the main scanning direction is performed using the exposure amount pattern.

JP-A-11-112809 JP-A-11-112810 JP 2004-138609 A

  In-plane density unevenness in the electrophotographic system exists not only in the main scanning direction but also in the sub-scanning direction. The technique of Patent Document 3 is effective in reducing density unevenness in the main scanning direction, but does not consider density unevenness in the sub-scanning direction. Note that although the method of Patent Document 2 reduces density unevenness in the sub-scanning direction, total correction cannot be performed for a plurality of gradations.

  The present invention makes it possible to reflect the influence of uneven printing in the sub-scanning direction on the exposure correction in the main scanning direction.

  An image forming apparatus according to the present invention is an image forming apparatus provided with an electrophotographic print engine, which exposes an image of each color in each main scanning direction exposure amount pattern for each of a plurality of colors to be inspected. A color difference index indicating a color difference in the main scanning direction that occurs when an image is formed is obtained, and a plurality of inspection objects are determined based on the color difference index for each exposure amount pattern in the main scanning direction for each of the plurality of colors. An image forming apparatus including a main scanning exposure control unit that obtains a main scanning direction exposure amount pattern having the best color difference index for all colors and causes the print engine to perform exposure based on the obtained main scanning direction exposure amount pattern The test chart for the sub-scanning direction is read from the image formed by the print engine, and each of the plurality of colors to be inspected is read based on the read result. A sub-scanning color difference calculating means for obtaining a color difference index indicating a color difference in the sub-scanning direction that occurs when an image of the color is exposed to form an image, and the main scanning direction exposure for each of the plurality of colors Main scanning color difference correction means for correcting the color difference index in the main scanning direction for each quantity pattern using the color difference index in the sub-scanning direction for the color, and the main scanning exposure control means includes the Based on the color difference index for each main scanning direction exposure amount pattern for each of the plurality of colors corrected by the main scanning color difference correction means, the main scanning direction exposure amount pattern that provides the best color difference index for the plurality of colors as a whole. It is characterized by seeking.

  The “color” in the present invention is a color represented in input image information for the image forming apparatus. A single color expressed by one toner having different input gradations (density) is a different “color”. In addition, in the case of multiple colors represented by a plurality of color toners, if the combination of the input gradations of each single color constituting the multiple colors is different, it is treated as a different “color”. Therefore, “a plurality of colors to be inspected” is not limited to a plurality of “colors” having different hues, and may be composed of a plurality of “colors” having different gradations of a single color.

  According to the present invention, a color difference index indicating a color difference of each color in the sub-scanning direction is obtained from the reading result of the test chart, and the color difference index of each color for each exposure amount pattern in the main scanning direction is determined in the sub-scanning direction of the corresponding color. Therefore, the best exposure pattern in the main scanning direction can be determined by reflecting the influence of the color difference in the sub-scanning direction.

  The best mode for carrying out the present invention (hereinafter referred to as “embodiment”) will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention.

  The image forming apparatus shown in FIG. 1 is an apparatus that prints a full-color image by an electrophotographic method, and has four photosensitive elements corresponding to four colors of Y (yellow), M (magenta), C (cyan), and K (black). A body 2 is provided. The control unit 10 is a device that controls the image forming apparatus, and includes a hardware such as a CPU, an image processor, and a memory, and various control programs. The control unit 10 serially outputs image data to be printed to the laser driver 12 according to the raster scanning order. The control unit 10 includes an exposure correction calculation unit 102 and an exposure control unit 104 as mechanisms for controlling exposure by the laser driver 12, which will be described later. The laser driver 12 drives the laser diode in each exposure device 14 according to the image data signal supplied from the control unit 10. The combination of the laser beam output control, beam scanning by an optical system such as a polygon mirror built in the exposure device 14 and the rotation of the photosensitive member 2 allows the photosensitive member 2 uniformly charged by the charger 16 to be rotated. An electrostatic latent image is formed on the surface. Further, the toner stored in the developing device 3 is transferred to each photoconductor 2, whereby the electrostatic latent image on each photoconductor 2 is developed, and a toner image is formed on each photoconductor 2. The toner images of the respective colors on the photoconductor 2 are sequentially transferred onto the intermediate transfer belt 4 by the corresponding transfer sections 6. As a result, a full-color toner image is formed on the intermediate transfer body belt 4. This full-color toner image is transferred from the intermediate transfer belt 4 onto the sheet fed on the sheet conveyance path 8 by the secondary transfer unit 7, and the toner image is fixed to the sheet by the fixing unit 9. The As described above, a color image is printed on the paper.

  The reading unit 18 is provided downstream of the fixing unit 9 on the paper conveyance path 8 and optically reads the printed paper 22 on which the toner image is fixed by the fixing unit 9. The reading unit 18 includes a set of image sensors for primary colors of R (red), G (green), and B (blue), for example, and can read full-color images. The image sensor may be an area sensor in which the light detection elements are two-dimensionally arranged, or a one-dimensional line sensor arranged in a direction perpendicular to the paper conveyance direction on the paper conveyance path 8. In the case of a line sensor, it is possible to read a full-color image on the entire surface of the paper 22 by repeatedly reading and scanning the light detection element array in synchronization with the conveyance of the paper 22 on the paper conveyance path 8. . The reading unit 18 is used for reading a test chart in exposure correction control.

  When the image forming apparatus is a copying machine or a multifunction machine, a scanner for reading a document is provided. Therefore, the reading unit 18 can be omitted by using the scanner instead of the reading unit 18. However, in this case, it is necessary for a person to set the printed paper on the scanner.

  Further, instead of reading the test chart printed on the paper, the exposure correction can be performed by reading the test chart printed on the intermediate transfer belt 4. In this case, the reading unit may be provided in the intermediate transfer belt 4 at a position downstream of the transfer unit for all colors of YMCK.

  The image forming apparatus also has a tone correction function for printing a test pattern on the intermediate transfer belt 4 or paper and reading it, creating a tone reproduction curve from the read result, and correcting the input tone. However, since this is a well-known thing, description is abbreviate | omitted.

  Next, exposure control in this embodiment will be described. In the exposure control of the present embodiment, correction in consideration of unevenness in the sub-scanning direction is added to the method disclosed in Patent Document 3 described above.

  In the control unit 10, an exposure correction calculation unit 102 is a functional module that controls processing for obtaining an exposure correction pattern. Further, the exposure control unit 104 controls the exposure amount (exposure intensity) of the photosensitive member 2 of each color by controlling the level of a signal supplied to the laser driver 12 according to the obtained exposure amount correction pattern. It is. Hereinafter, processing performed by the exposure correction calculation unit 102 will be described.

  FIG. 2 shows an example of the procedure for determining the exposure amount correction pattern in the main scanning direction performed by the exposure correction calculation unit 102. The exposure correction calculation unit 102 controls each unit of the image forming apparatus and executes the following procedure.

  Among these procedures, the processing of steps S10 to S16 is the same as the processing in the method proposed by the present applicant in Patent Document 3, and the outline thereof will be described below. For details, see Patent Document 3.

  First, steps S10 to S14 are repeated for all exposure amount correction patterns in the main scanning direction that can be selected by the image forming apparatus.

  Here, the exposure amount correction pattern in the main scanning direction includes, for example, a linear pattern as shown in FIG. In this linear pattern, the exposure correction amount linearly changes from one end (x = x0) to the other end (x = x1) in the main scanning direction of the photosensitive member 2. The exposure correction amount is a correction amount that is added to or subtracted from the reference exposure amount. If the exposure correction amount changes linearly, the exposure amount as a whole also changes linearly. In this linear pattern, for example, the exposure correction amount at one end is fixed (= R0), and the exposure correction amount 200 at the other end is changed at predetermined intervals from R1 to R2, thereby correcting the exposure amount with various inclinations. A pattern can be obtained. In this case, the value of the exposure correction amount 200 at the other end serves as a parameter that defines the exposure correction pattern of the linear pattern. Conversely, various linear patterns can be expressed by the value of the exposure correction amount 200 at the other end. it can.

  Further, the exposure correction pattern in the main scanning direction includes a V-shaped pattern as shown in FIG. In this V-shaped pattern, the exposure correction amount decreases linearly from one end (x = x0) in the main scanning direction of the photoreceptor 2 to a predetermined intermediate position (x = x2), and from the intermediate position to the other end ( In this pattern, the exposure correction amount increases linearly up to x = x1). In this V-shaped pattern, for example, the exposure correction amount at both ends is fixed (= R0), and the exposure correction amount 202 at the intermediate position is changed from R1 to R2 at predetermined intervals, so that various exposure amount correction patterns can be obtained. Obtainable. In this case, the value of the exposure correction amount 202 at the intermediate position is a parameter that defines the exposure amount correction pattern.

  In addition, as shown in Patent Document 3, by combining a linear pattern and a V-shaped pattern, a V-shaped exposure correction pattern having different exposure correction amounts at both ends in the main scanning direction may be formed. it can.

  These are merely examples, and the exposure amount correction pattern in the main scanning direction may include still another pattern shape. In any case, in this embodiment, the procedure of steps S10 to S14 is repeated for all main scanning direction exposure correction patterns that can be used in the image forming apparatus.

  That is, each time an exposure correction pattern is selected, a predetermined test chart is printed on a sheet (or intermediate transfer belt) while changing the exposure along the main scanning direction according to the pattern (S10). The printing result is read (S12). In this case, regions of several different gradations (for example, Cin = 20% and 60%, etc.) for each toner single color (Y, M, C, K), and multiple colors (for example, Y and Y) that can be formed by combining these gradations with a single color. A test chart including an area of secondary color G (green) 20%, etc., where C is both Cin = 20% is printed. In this processing procedure, a single color toner is handled as a different “color” for each gradation. For example, even for the same yellow (Y), Cin = 20% and 60% are treated as different “colors”. The multiple colors are also assumed to have different “colors” for each combination of each single color and its gradation. For example, a color in which Y20% and C20% are superimposed and a color in which Y60% and C60% are superimposed are different colors, and a color in which Y20% and C60% are superimposed are further different colors. The test chart may be such that each “color” has a region of “color” extending from one end to the other end of the photoconductor 2 in the main scanning direction. For example, for each “color” to be inspected, a test chart may be obtained by printing the entire surface of one sheet of the color. In this case, in step S10, pages for the number of colors are printed. Further, the number of test charts can be reduced by arranging a plurality of colors on one sheet as a strip-like region (stripe) extending in the main scanning direction for each “color”.

  In step S14, a color difference index representing the degree of color difference along the main scanning direction in the printing result of “color” is calculated for each “color” from the read result of the test chart. In the calculation of the color difference index, for example, a colorimetric value of the printing result of the color at each position along the main scanning direction is obtained from the reading result of one “color” region. When the reading unit 18 is a general optical scanner, the reading value is represented by a combination of R, G, and B, and this is represented by a uniform perceptual color space such as L *, a *, b * (of course, this). Color space conversion to a value of (not limited to), and the conversion result is used as a colorimetric value. When the reading unit 18 is a color system, such conversion is not necessary. Then, for example, the maximum color difference is obtained as a color difference index in the distribution of the colorimetric values at each position along the main scanning direction. Here, the color difference can be defined as the distance between two colorimetric values (points in the space) in the uniform perceptual color space. Various other color differences are known and any of them may be used. In addition, the larger the color difference of the print result for the input indicating the same “color” is, the larger the unevenness is. Therefore, the maximum value of the color difference in the main scanning direction is, in other words, the worst color difference in the main scanning.

  In the above, the worst color difference value in the main scanning direction in the printing result of one “color” is used as the color difference index, but this is only an example. This color difference index only needs to indicate the degree of color unevenness in the main scanning direction when the same “color” is printed. As shown in Patent Document 3, the color difference at each point in the main scanning direction is used. Other values such as the average value of these and the square sum of the color differences of these points may be used as the color difference index. For example, if the maximum value of color unevenness is to be suppressed, the maximum value of color difference may be used as the color difference index, and the average value of color difference may be used to suppress the average of color unevenness.

  By repeating the above processing, the color difference index in the main scanning direction of each color can be obtained for each main scanning direction exposure amount correction pattern. By arranging this for each color, it is possible to obtain a color difference index distribution indicating how the color difference index in the main scanning direction obtained from the print result of each color changes according to the exposure amount correction pattern (S16). ).

  For example, FIG. 5 shows that multiple colors of process black 20% (3C 20%) in which all of YMC are Cin = 20% and K is Cin = 0%, Y and C are Cin = 60%, and the others are 0 An example of color difference index distribution when the main scanning direction exposure amount correction pattern is changed in a V-shaped pattern with respect to a multiple color of G (green) 60%, which is%. This example is a distribution obtained when the exposure correction amount at the valley bottom point of the V-shaped pattern is changed from −9% to + 11% in increments of 1% as a parameter, and the worst color difference value is used as the color difference index on the vertical axis. Used.

  In this figure, when attention is paid to process black (3C) 20%, point A (exposure correction amount -5%) at which the worst value of color difference becomes the minimum value is the optimum exposure correction parameter, and attention is paid to green (G) 60%. In this case, the point B (exposure correction amount + 1%) is the optimum exposure correction parameter in the same manner. However, since these colors are used together in actual printing, the overall optimum exposure correction parameter combining these two colors is used. It is necessary to ask. Therefore, in the method of Patent Document 3, a point C1 (exposure correction amount of about −2%) at which the color difference worst value trades off in the color difference index distribution for each color is adopted as the optimum exposure correction parameter as a whole. .

  The distribution for multiple colors in FIG. 5 is obtained when the exposure amount correction pattern for three of YMCKs is fixed and only one (for example, C) is changed. The specified optimum exposure correction parameter (and thus the exposure correction pattern) is applied to the exposure device 14 for the toner color whose exposure amount correction pattern is changed. Information on the color difference index distribution obtained when the exposure amount correction pattern is changed for a single toner color is naturally used to determine the exposure amount correction pattern for the toner color.

  In general, each toner color with different gradations obtained when the exposure amount correction pattern for the photoconductor 2 of a certain toner color is changed variously (the exposure amount correction pattern for other toner colors is fixed). In consideration of all of the color difference index distribution and the multiple color difference index distribution including the toner color as an element, an exposure amount correction pattern with the best color difference index is obtained.

  However, since the exposure unevenness factor in the main scanning direction and the exposure unevenness factor in the sub-scanning direction are independent, the exposure amount correction pattern is determined only from the information on color unevenness in the main scanning direction as described above. The color unevenness of the image cannot be reduced and remains. Accordingly, in the present embodiment, an exposure amount correction pattern that reflects the influence of unevenness in the sub-scanning direction is determined by the following procedure.

  That is, first, a test chart for the sub-scanning direction is printed (S18), and is read by the reading unit 18 (S20). In the test chart for the sub-scanning direction, for each “color” to be inspected, the color area extends over the entire circumference of the photosensitive member 2 at the same position in the main scanning direction of the photosensitive member 2. Use the distribution. One “color” may be printed on one sheet of paper, or a band-like region extending in the sub-scanning direction may be arranged for each “color”. In the former case, it can also be used as a test chart for the main scanning direction (of course, only in the case of the one-color test chart for the main scanning direction). In the test chart for the main scanning direction described above, the same “color” is formed several times while changing the exposure correction pattern for the main scanning direction in various ways. Since the exposure amount correction pattern in the direction is basically independent, the test chart for the sub-scanning direction may form an image with one standard main scanning direction exposure amount pattern.

  When the reading of the test chart (S20) is completed, the distribution of the colorimetric values along the sub-scanning direction is obtained for each “color” to be inspected based on the reading result, and the sub-scanning is performed from the distribution of the colorimetric values. A color difference index indicating the degree of color unevenness in the direction is calculated (S22). The distribution of the colorimetric values of the printing result of a certain “color” is the reading value at each position in the sub-scanning direction in the “color” area of the read image (this reading value is the sub-scanning direction of the “color” area). Can be obtained by converting color values in a uniform perceptual color space such as L * a * b * as necessary. As in the case of the main scanning direction, the color difference index may be determined according to the component to be suppressed, such as the worst value or average value of the color difference in the colorimetric value distribution in the sub-scanning direction.

  The color difference index distribution in the main scanning direction of each “color” already calculated in step S16 is corrected using the color difference index in the sub-scanning direction of each “color” obtained in this way (S24). In this correction, for example, the color difference index distribution of each “color” in the main scanning direction is raised by the color difference index of the “color” in the sub scanning direction (that is, the color difference index in the sub scanning direction is added to each point of the distribution). You can do that. This is because the cause of color unevenness in the main scanning direction and the sub-scanning direction is independent, and in the worst case, there is a possibility that a color difference of the sum of the worst values of both may occur.

  For example, as shown in FIG. 5, when obtaining an optimal exposure correction pattern from the worst color difference values in the main scanning direction of process black (3C) 20% and green (G) 60%, in S22, as shown in FIG. For each of 20% and green 60%, the worst color difference value in the sub-scanning direction is obtained, and each value may be added to the value of each point of the color difference index distribution in the main scanning direction of the corresponding color.

  In step S26, based on the color difference index distribution in the main scanning direction of each color corrected in this way, an optimum exposure amount correction pattern for all the colors is determined. This optimum pattern is the same as the method disclosed in Patent Document 3, in which the value of the color difference index distribution of each color (however, corrected in consideration of the effect of the sub-scanning direction in this case) trades off (for example, both distribution curves) And the point at which the sum of squares of the color difference indices of both colors at the same exposure correction amount is minimized).

  For example, when the color difference index distribution of each color obtained in step S16 is as shown in FIG. 5 and the corresponding color difference index in the sub-scanning direction is as shown in FIG. 6, the corrected color difference The index distribution is as shown in FIG. This is merely a translation of the distribution upward for each “color”, and the optimum exposure correction pattern (parameter) for each “color” is the same as in FIG. However, if the color difference index in the sub-scanning direction is different for each “color”, the relationship between both “colors” changes, so the trade-off point (exposure correction amount parameter) between both “colors” is the same as in FIG. Changes to point C2 (exposure correction amount −4%).

  In the above description, the best exposure correction pattern between two colors (process black 20% and green 60%) is obtained as a specific example, but the same applies when obtaining the best pattern for three or more colors. Just do it. The example of FIG. 7 is an example of obtaining the best exposure correction pattern of a V-shaped pattern. However, other shapes of exposure correction patterns such as a straight line pattern and a mixed pattern of a straight line and a V shape are available. In consideration of this, the color difference index distribution of each color is obtained for all the patterns, and for example, a pattern that minimizes the sum of squares of the color difference indices for all colors to be inspected may be adopted.

  When the best exposure correction pattern for the entire “color” as the target is thus obtained, the pattern is set in the exposure control unit 104, and in the subsequent printing processing, the exposure amount in the main scanning direction according to this pattern. Is controlled.

  As described above, in the present embodiment, when determining an exposure correction pattern in the main scanning direction that is generally good across a plurality of colors, the color difference index distribution in the main scanning direction of each color that is the basis thereof is determined. Each color was corrected by a color difference index in the sub-scanning direction. As a result, it is possible to obtain a main-scanning direction exposure amount correction pattern that can suppress overall color unevenness including color unevenness in the sub-scanning direction.

  In the above example, in step S26, the best exposure correction pattern is obtained from the intersection of the color difference index distributions of the respective colors or the point where the square sum of the color difference indices of the respective colors is the smallest, but this is only an example. For example, instead of a simple sum of squares of the color difference index of each color, the best exposure amount correction pattern may be obtained by weighting the color difference index of each color and minimizing the square sum. For example, in the example of FIG. 7, when it is desired to suppress the color unevenness of 20% of process black than that of 60% of green, the former color difference index distribution (corrected) is weighted larger than that of the latter. And the square sum of the values of both colors at the same exposure correction amount may be obtained.

  The execution order of the steps in the procedure of FIG. 2 is merely an example, and the execution order of one to a plurality of steps can be changed as long as similar processing can be realized. For example, steps S18 to S22 can be executed first because they can be executed independently of steps S10 to S16 in principle.

  5 to 7 are examples in the case of multiple colors, but there may be a case where an exposure correction pattern that trades off between a plurality of “colors” having a single toner color and different gradations may be obtained. The same method as described above may be used.

  When comprehensively considering a single toner color and multiple colors, the color difference index distribution obtained for each gradation of the single color and the color difference index distribution obtained for each multiple color are treated in the same row, and the same as in the example of FIG. The trade-off between these distributions may be obtained simply, but the following method is also conceivable. That is, first, an optimum exposure correction pattern for only a single color is obtained for each toner color, and an optimum exposure correction pattern in consideration of multiple colors is obtained using that as a starting point. In this method, when changing the exposure correction parameter for one toner color in the multiple color inspection, an exposure amount correction pattern obtained from only a single color is adopted for the other toner colors.

  The test chart for the sub-scanning direction may include all the multiple colors to be inspected, but this increases the number of test charts. As a countermeasure against this, there is a method in which the test chart includes only one having different gradations for each toner single color (Y, M, C, K). The colorimetric value distribution of multiple colors is obtained by color space conversion from the combination of color distributions obtained from the print results of the test charts for each single color and each gradation.

  For example, on one test chart, stripes of Y60% and 20%, C60% and 20% are printed as shown in FIG. 8, and M60% and 20% and K60% and 20% stripes are printed on the other sheet. Is printed, a plurality of colorimetric value distributions of multiple colors composed of combinations of the respective single colors and gradations are obtained with two test charts. For example, a process is performed by color space converting a combination (Y, M, C) of density reading results at each position along the sub-scanning direction of each stripe of Y20%, M20%, and C20% into a uniform perceptual color space. A distribution of colorimetric values along the sub-scanning direction of black 20% can be obtained, and a color difference index can be obtained from the colorimetric value distribution as described above. Similarly, the colorimetric value distribution of green 60% can be estimated from the density distribution of Y60% and C60%.

  Thus, the multi-color distribution can be estimated by the combination of the single-color density distributions only in the sub-scanning direction. That is, the pressing force of the transfer member is different from each other at each position along the axial direction, but is nearly constant in the rotation direction. For this reason, in the main scanning direction (axial direction), density distribution mismatch due to transfer failure occurs between the single color and multiple colors, so the combination of the single color results cannot be multiple colors, but the sub-scan direction (rotation direction) Since there is no inconsistency due to such transfer, the value of multiple colors can be accurately estimated by combining the results of single colors.

  The detailed procedure of step S22 (see FIG. 2) in this case is shown in FIG. In this procedure, first, the density distribution in the sub-scanning direction in the test chart for each single color and each gradation is obtained (S30). Then, for each multiple color to be inspected, the density distribution in the sub-scanning direction of each single color and each gradation constituting the multiple color is extracted, and a set of values (Y, M) at the same position in the sub-scanning direction in each density distribution. , C, K) is changed to a uniform perceptual color space to obtain a distribution of colorimetric values along the sub-scanning direction of the multiple colors (S32). Then, a color difference index is calculated from the colorimetric value distribution of the multiple colors (S34).

  In the example of FIG. 8, correction in the sub-scanning direction may be performed for each single color based on the density distribution in the sub-scanning direction of each single color obtained in step S30. The correction in this case may be basically performed by correcting the exposure amount and the input tone value according to the inverse distribution of the obtained density distribution. When obtaining the density distribution of a plurality of gradations for the same single color as in the example of FIG. 8, one of them is used as a basis for correction, the average distribution of both is used, or the weighted average of both (more A distribution that increases the weight of the gradation to be emphasized may be used.

  In this case, even if the inverse distribution of the obtained density distribution is applied as it is, correction cannot be performed and an error remains. This is because, in the density distribution in the sub-scanning direction, random components with low periodicity are mixed in addition to the rotation cycle components of various rotating members in the transfer unit, fixing unit, and the like starting with the photoconductor. This is because the effects of random components and the like remain if the method is applied as it is. As a countermeasure against this, the following method can be considered.

  That is, this method does not use the inverse distribution of the density distribution in the sub-scanning direction as it is, but calculates the superposition of the components that change with the rotation period of the rotating member on the paper conveyance path and the harmonic components in the density distribution. Then, correction is performed according to the inverse distribution. In the case of exposure correction, since the rotation period of the photoconductor 2 is important, it is preferable to perform correction using an inverse distribution of superposition of the component of the period and the harmonic component. What is necessary is just to obtain | require the component of each period by Fourier-transform. In addition, sine waves of each period are overlaid while changing the phase and amplitude in various ways. Among them, the one with the smallest difference from the actual concentration distribution is adopted, the inverse distribution is obtained, and correction is performed according to this inverse distribution. Also good.

  If the actual density distribution and the inverse distribution used for correction are added together, the density error distribution remaining by this correction can be obtained. This can be obtained for each combination of single color and gradation. If this density error distribution for each single color and gradation is used instead of the above-described density distribution, a colorimetric value distribution can be obtained for a component that cannot be corrected by the correction by the reverse distribution in the sub-scanning direction. It is possible to obtain a color difference index in the sub-scanning direction for a component that cannot be corrected. The color difference index distribution in the main scanning direction may be corrected using this.

1 is a diagram illustrating a configuration of a main part of an image forming apparatus according to an embodiment. It is a flowchart which shows an example of the procedure of exposure amount correction pattern determination processing. It is a figure for demonstrating a linear pattern among exposure amount correction patterns. It is a figure for demonstrating a V-shaped pattern among exposure amount correction patterns. It is a figure which shows the example of the color difference parameter | index distribution about a some color. It is a figure which shows the example of the color difference worst value about the subscanning direction of each color. It is a figure which shows the example of the color difference parameter | index distribution of each color corrected in consideration of the influence of the sub-scanning direction. It is a figure which shows an example of the test chart for subscanning directions. It is a flowchart which shows an example of the procedure which calculates | requires the color difference parameter | index of the subscanning direction of multiple colors.

Explanation of symbols

  2 Photoconductor, 3 Developer, 4 Intermediate transfer belt, 5 ADC sensor, 6 Primary transfer unit, 7 Secondary transfer unit, 8 Paper transport path, 9 Fixing unit, 10 Control unit, 12 Laser driver, 14 Exposure device, 16 charger, 18 reading unit, 102 exposure correction calculation unit, 104 exposure control unit.

Claims (3)

An image forming apparatus including an electrophotographic print engine, wherein a main scanning direction occurs when an image of the color is exposed with each main scanning direction exposure amount pattern for each of a plurality of colors to be inspected. A color difference index indicating a color difference at each of the plurality of colors is obtained, and based on the color difference index for each of the main scanning direction exposure amount patterns for each of the plurality of colors, the color difference index for the plurality of colors to be inspected is the best. In the image forming apparatus provided with the main scanning exposure control means for causing the print engine to perform exposure based on the obtained main scanning direction exposure amount pattern.
This occurs when the test chart for the sub-scanning direction is imaged by the print engine and the image is formed by exposing an image of the color for each of the plurality of colors to be inspected based on the read result. Sub-scanning color difference calculating means for obtaining a color difference index indicating a color difference in the sub-scanning direction;
Main scanning color difference correction means for correcting a color difference index in the main scanning direction for each exposure amount pattern in the main scanning direction for each of the plurality of colors using a color difference index in the sub scanning direction for the color; ,
And the main scanning exposure control means is based on a color difference index for each of the plurality of colors corrected by the main scanning color difference correction means for each exposure amount pattern in the main scanning direction. An image forming apparatus characterized in that a main scanning direction exposure amount pattern having the best color difference index is obtained. The test chart for the sub-scanning direction includes each single-color region that constitutes the multiple colors to be inspected,
The sub-scanning color difference calculating means obtains a density distribution in the sub-scanning direction of each single color region in the print result of the test chart for the sub-scanning direction, and combines the multiple colors of the inspection object by a combination of the density distributions of the respective single colors. A color value distribution in the sub-scanning direction is obtained, and a color difference index in the sub-scanning direction for the multiple color is obtained from the distribution.
The image forming apparatus according to claim 1. An exposure control method in an image forming apparatus including an electrophotographic print engine,
(A) For each of a plurality of colors to be inspected, a step of obtaining a color difference index indicating a color difference in the main scanning direction that occurs when an image of the corresponding color is exposed with each main scanning direction exposure amount pattern to form an image. When,
(B) reading a result of image formation of the test chart for the sub-scanning direction by the print engine;
(C) obtaining a color difference index indicating a color difference in the sub-scanning direction that occurs when an image of the color is exposed to form an image for each of the plurality of colors to be inspected based on the reading result; ,
(D) The color difference index in the main scanning direction for each exposure amount pattern in the main scanning direction for each of the plurality of colors obtained in step (a) is used as the sub color for the color obtained in step (c). Correcting using the color difference index in the scanning direction;
(E) Based on the color difference index for each exposure amount pattern in the main scanning direction for each of the plurality of colors corrected in step (d), the color difference index for the entire plurality of colors to be inspected is the best. Obtaining a main scanning direction exposure amount pattern;
(F) performing main scanning direction exposure control for causing the print engine to perform exposure based on the main scanning direction exposure amount pattern obtained in step (e);
An exposure control method for an image forming apparatus.

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