JP2012125953A - Image forming apparatus, and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce, by a simple structure, deterioration of image quality resulting from variation in emission intensity and scanning interval errors, which are caused in an electrophotographic process of a multibeam scanning system.SOLUTION: A chart recording control unit 15 retains chart image data for repeatedly forming, in a sub-scanning direction, a recording line for superimposing and recording main scanning lines of a plurality of colors and a blank line. The chart image data is formed on a recording medium while changing, for each color, a combination of writing laser light sources for which exposure is started first among a plurality of laser light sources in a laser array L, whereby a plurality of test charts are obtained. A chart evaluation unit 16 calculates an evaluation value which indicates the level of image quality deterioration for each of the plurality of test charts. A laser combination selection unit 17 selects a combination of writing laser light sources which corresponds to a test chart with the evaluation value being in an allowable range. An image formation control unit 11 in an image forming unit of each color performs image forming while controlling laser light sources so as to attain the selected combination of laser light sources.

Description

  The present invention relates to an image forming apparatus that forms an image by an electrophotographic process that performs multi-beam scanning using a plurality of laser light sources, and a control method therefor.

  2. Description of the Related Art Conventionally, so-called electrophotographic image forming apparatuses such as a laser beam printer and a copying machine that form an image by scanning an image carrier with laser light have become widespread. In general, an electrophotographic image forming apparatus forms an image through a plurality of processes such as charging, exposure, development, transfer, fixing, and cleaning.

  Here, a general electrophotographic process will be described first using an image forming apparatus that forms a single color image as an example. After the photosensitive member is uniformly charged by the charging means provided in the apparatus, the photosensitive member is exposed in accordance with the image signal by the exposure means including a laser light source. An electrostatic latent image is formed. Thereafter, the electrostatic latent image on the photoconductor is developed by the developing unit to become a toner image, and the toner image on the photoconductor is transferred to the recording medium by the transfer unit. The toner image transferred to the recording medium is fixed by a fixing unit to obtain a single color image. The transfer residual toner remaining on the photoreceptor is collected by a cleaning unit.

  An image forming apparatus that forms a color image on a recording medium by superimposing toner images of a plurality of colors takes the following process. First, an electrostatic latent image is formed for each color on the photosensitive member, and a toner image is obtained by sequentially developing the latent image with a developing means of a plurality of colors, for example, C, M, Y, and K, The toner image is transferred onto an intermediate transfer belt by primary transfer means. By repeating this process for each color, a plurality of color toner images are superimposed on the intermediate transfer belt. Further, a plurality of color toner images superimposed on the intermediate transfer belt are collectively transferred onto a recording medium by a secondary transfer unit, and fixed by a fixing unit to obtain a color image.

  Further, in recent years, so-called tandem multiple transfer systems have been widely adopted in which process units such as a photoconductor and a developing device are independently provided for each color and a full color image can be formed in one pass. In the tandem multiple transfer system, a process unit is provided for each of a plurality of colors, for example, C, M, Y, and K, and charging, exposure, development, and transfer are performed. The toner image developed for each color is superimposed and transferred onto the intermediate transfer belt by the primary transfer unit for each color, further transferred to the recording medium by the secondary transfer unit, and fixed by the fixing unit, thereby obtaining a color image. It is done. The transfer residual toner remaining on the photoconductors of the respective colors is removed from the intermediate transfer belt by the cleaning unit and collected.

  In the general electrophotographic process as described above, there is a multi-beam scanning method in which scanning is simultaneously performed by a plurality of laser beams. In the multi-beam scanning method, a laser array including a plurality of laser light sources is provided in one process unit, and different positions on the photosensitive member are collectively scanned in parallel by the laser light of each laser light source. As an example, when three laser light sources are provided in one process unit, three scanning lines are formed by one scan.

  In such a multi-beam scanning method, there is a problem in that image quality deterioration occurs due to the overlap of various errors of the laser array. The various errors of the laser array include processing variations of the laser light source, mounting errors when the laser light source is installed in the laser array, mounting errors of the laser array, processing variations and mounting errors of the optical elements constituting the exposure means, etc. It is.

  For example, a plurality of laser light sources constituting a laser array generally have variations in emission intensity. This variation in light emission intensity appears as a shading unevenness with the number of laser light sources as a period in the formed image as it is, which causes a problem that the image quality deteriorates.

  As another example, the scanning interval between a plurality of laser light sources generally includes an error and is non-uniform. If an error occurs in the scanning interval, the formed image becomes non-uniform, causing density unevenness and color unevenness, resulting in a problem that the image quality deteriorates.

  As a means for solving the problems in the multi-beam scanning method, the following method has been proposed. First, each laser light source is used individually to form an image on the photoconductor, the image density is detected by an optical density sensor, and light is emitted for each laser light source so as to reach the target density based on the detection result. There is a method for controlling the amount (see, for example, Patent Document 1). In addition, there is a technique in which part of laser light is divided in a plurality of laser light paths, the divided laser light is received by a light receiving element, and the amount of light emitted from the laser light source is controlled according to the output of the light receiving element (for example, , See Patent Document 2).

JP 2004-341171 A Japanese Patent Laid-Open No. 06-031980

  However, the above conventional technique for solving the problems in the multi-beam scanning method has the following problems. First, in the technique described in Patent Document 1, the laser light amount is corrected based on images individually formed for each laser light source. Therefore, although it is possible to correct variations in the emission intensity of each laser light source, it is not possible to reduce image quality degradation due to scanning interval errors between a plurality of laser light sources. In the technique described in Patent Document 2, since laser light is individually received for each laser light source to correct the laser light amount, the scanning interval error between a plurality of laser light sources is the same as in the technique described in Patent Document 1. It is not possible to reduce image quality degradation due to. Furthermore, since a light receiving element that receives laser light and a circuit that corrects the amount of laser light are required, the manufacturing cost increases and the processing becomes complicated.

  The present invention has been made to solve the above-described problems, and an image forming apparatus including an exposure unit capable of forming a plurality of main scanning lines by one exposure scanning with a plurality of laser light sources has the following functions. It aims at realizing. In other words, image quality deterioration due to emission intensity variations and scanning interval errors in a plurality of laser light sources is reduced with a simple configuration.

  As a means for achieving the above object, an image forming apparatus of the present invention comprises the following arrangement.

  That is, an exposure unit capable of forming a plurality of main scanning lines by one exposure scanning with a plurality of laser light sources in a main scanning direction orthogonal to the sub scanning direction is used for an image carrier that moves in the sub scanning direction. An image forming apparatus that forms an image for each of a plurality of colors and forms a color image by superimposing the images of the plurality of colors on a recording medium. By changing the combination of each of the plurality of colors of the writing laser light source that performs the exposure of the first main scanning line, and forming an image according to predetermined chart image data on the recording medium, a plurality of each combination And a recording unit for selecting the combination of the writing laser light sources corresponding to one selected from the plurality of test charts. The write laser light source for each color of the plurality of colors, characterized in that and a control means for controlling said exposure means so that the combination of the write laser light source selected by said selection means.

  The present invention relates to an image forming apparatus including an exposure unit capable of forming a plurality of main scanning lines by one exposure scanning with a plurality of laser light sources, and image quality resulting from variations in emission intensity and scanning interval errors in the plurality of laser light sources. There is an effect of reducing deterioration with a simple configuration.

FIG. 3 is a diagram illustrating a configuration of an image forming apparatus according to the first embodiment. The figure explaining a multi-beam scanning system, The figure which shows the block structure for exposure control in this embodiment, The figure explaining the image quality degradation which occurs in the multi-beam scanning method, A diagram showing the correspondence (for one unit) between the scanning line and the laser used, A diagram showing the correspondence (for 4 units) between the scanning line and the laser used, The figure which shows the first scanning position when the writing laser is different, Schematic diagram of a recording medium on which a plurality of test charts are recorded, A diagram showing a configuration example of each test chart, The figure which shows the main scanning line density example in a test chart, The flowchart which shows the exposure control processing in this embodiment, The figure which shows the block structure for exposure control in 2nd Embodiment. The flowchart which shows the exposure control processing in 2nd Embodiment, A diagram showing an example of a test chart selected by the user, FIG. 10 is a diagram showing a configuration of an image forming apparatus having four color developing devices and one photoconductor as another embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments do not limit the present invention related to the scope of claims, and all combinations of features described in the present embodiments are essential to the solution means of the present invention. Not necessarily.

<First Embodiment>
Apparatus Configuration FIG. 1 is a diagram showing an electrophotographic process configuration in the image forming apparatus of this embodiment. This configuration shows a so-called tandem multiple transfer system, which is provided with units a, b, c, and d corresponding to the number of colors that form an image of each color, that is, one unit per color.

  In the figure, reference numerals 1a, 1b, 1c and 1d denote photoconductors, 2a, 2b, 2c and 2d denote chargers, 3a, 3b, 3c and 3d denote exposure units, and 4a, 4b, 4c and 4d denote developing units. 51 is an intermediate transfer belt, 53a, 53b, 53c and 53d are primary transfer sections, 55 is an intermediate transfer belt cleaner, 56 and 57 are secondary transfer sections, 6a, 6b, 6c and 6d are cleaners, and 7 is a fixing device. , P is a recording medium.

  After the photoreceptors 1a, 1b, 1c, 1d rotating at a constant speed are uniformly charged by the chargers 2a, 2b, 2c, 2d, the exposure units 3a, 3b, 3c, 3d are exposed according to the image signal. As a result, electrostatic latent images are formed on the photoreceptors 1a, 1b, 1c, and 1d. Thereafter, the electrostatic latent images are developed by the developing devices 4a, 4b, 4c, and 4d to obtain toner images. The toner images on the four photoconductors 1a, 1b, 1c, and 1d are multiplex-transferred to the intermediate transfer belt 51 by the primary transfer portions 53a, 53b, 53c, and 53d, and further, the recording medium P is transferred by the secondary transfer portions 56 and 57. Is transcribed. Transfer residual toner remaining on the photoreceptors 1a, 1b, 1c, and 1d is collected by the cleaners 6a, 6b, 6c, and 6d. Further, the transfer residual toner remaining on the intermediate transfer belt 51 is collected by the intermediate transfer belt cleaner 55. Then, the toner image transferred to the recording medium P is fixed by the fixing device 7, whereby a color image is obtained.

  Generally, four colors of cyan (C), magenta (M), yellow (Y), and black (K) are used as color toners used in the image forming apparatus. For example, C toner can be mounted on the developing unit 4a, M on the developing unit 4b, Y on the developing unit 4c, and K on the developing unit 4d. The order of colors may be arbitrary.

  The image forming apparatus of this embodiment forms an electrostatic latent image by a multi-beam scanning method. That is, a plurality of laser beams (multi-beams) are used to perform multiple exposure scans with a plurality of laser light sources in the main scanning direction orthogonal to the sub-scanning direction on the image carrier that moves in the sub-scanning direction. It has a laser array capable of forming main scan lines. Then, for each of a plurality of colors, an image is formed by multi-beam scanning, and the color image is formed by superimposing the images of the plurality of colors on a recording medium.

  FIG. 2 is a diagram illustrating the configuration of the exposure unit according to the present embodiment in which multi-beam scanning is performed using the exposure unit 3a as an example. In the multi-beam scanning method, a plurality of laser light sources are installed in one unit, and different positions on the photosensitive member are collectively scanned in parallel by the laser light of each laser light source. In this embodiment, a semiconductor laser array (hereinafter simply referred to as a laser array) is used as a laser light source that emits a plurality of laser beams. This is achieved by forming a plurality of laser diodes (three in this embodiment) on a single element substrate and individually controlling the currents flowing to the individual laser diodes, thereby allowing individual semiconductor lasers (hereinafter simply referred to as lasers). Can be controlled independently. The three laser beams emitted from the laser array La are shaped into parallel laser beams by the collimator lens 31a. These laser beams are deflected periodically and repeatedly by the rotating polygon mirror PMa. The deflected laser light forms an imaging spot on the photoreceptor 1a by the scanning lens 32a. Each of the three laser beams is referred to as LB1a, LB2a, LB3a in order from the one that scans the upstream side in the sub-scanning direction, and LBa is used as a general term for LB1a, LB2a, LB3a. A laser that emits the laser beam LB1a is referred to as L1a, a laser that emits the laser beam LB2a is referred to as L2a, and a laser that emits the laser beam LB3a is referred to as L3a.

  FIG. 3 is a diagram showing a block configuration for exposure control in the image forming apparatus of the present embodiment. Here, in order to simplify the description, the block configuration of the unit a for one color will be described in detail, but the other units b, c, and d have the same configuration.

  The chart recording control unit 15 in the controller outputs a test chart formation signal for detecting an optimum combination of lasers used in each unit based on the same chart image data for each unit of the present embodiment. . Details of this test chart will be described later. The chart evaluation unit 16 measures and evaluates a plurality of test charts formed on the recording medium under the control of the chart recording control unit 15. Based on the evaluation result by the chart evaluation unit 16, the laser combination selection unit 17 selects an optimum combination of write lasers in the laser arrays La, Lb, Lc, and Ld of the units a, b, c, and d. Here, the writing laser is one of a plurality of lasers in the laser array L that performs exposure of the first main scanning line at the time of image formation. Hereinafter, the write laser combination for each unit is simply referred to as a laser combination, and details of the selection method will be described later. Then, according to the selected combination, for example, information for designating the writing laser and the sub-scanning direction writing timing is given to the image forming control unit 11a of the unit a. Details of the sub-scanning direction writing timing will be described later.

  As shown in FIG. 3, the unit a includes an exposure unit 3a for the photoreceptor 1a. In the exposure unit 3a, a plurality of laser beams LBa emitted from each laser of the laser array La are reflected by a plurality of reflecting surfaces of a polygon mirror PMa that can rotate at a constant speed, so that different positions of the photoconductor 1a Are simultaneously scanned in parallel. The main scanning direction synchronization timing in the scanning is detected by the main scanning direction synchronization timing detection sensor BDSa, and the main scanning direction synchronization control unit 10a determines the main scanning direction writing timing based on the detection signal.

  The image formation control unit 11a controls the laser control unit 12a to emit the laser light LBa based on the image information from the laser array La. Further, the polygon mirror PMa is rotated at a constant speed by controlling the polygon motor control unit 13a. The laser beam LBa is reflected on the reflection surface of the polygon mirror PMa, and scans on the photoreceptor 1a.

  The image formation control unit 11a is based on information for designating the writing laser and the sub-scanning direction writing timing given by the laser combination selecting unit 17, and the main scanning direction writing timing determined by the main scanning direction synchronization control unit 10a. Perform the above control.

  The configuration of the unit a shown in FIGS. 2 and 3 may be provided for each of the units a, b, c, and d as shown, but may be a common configuration for each unit. In FIG. 3, “a” is added to the number of each component to indicate that the unit a is configured. However, by omitting this “a”, each component provided in each unit, Or a common structure shall be shown. For example, 'laser array' is a generic term for laser arrays a, b, c, d, and 'polygon motor control unit 13' is a generic term for polygon motor control units 13a, 13b, 13c, 13d in each unit.

Image Quality Degradation Next, image quality degradation caused by light emission intensity variation and scanning interval error, which occurs in the multi-beam scanning method, will be described with reference to FIG. FIG. 4A shows an example in which three laser beams are scanned at equal intensity and at equal intervals in the multi-beam scanning method. It is desirable that the three laser beams be scanned uniformly in this way. However, the plurality of lasers constituting the laser array generally vary in emission intensity. FIG. 4B shows an example of image quality deterioration in which shading unevenness occurs due to variations in emission intensity of each laser. In addition, it is difficult to adjust the scanning interval between a plurality of lasers with high accuracy. FIG. 4C shows an example of image quality deterioration in which scanning intervals between a plurality of lasers are not uniform.

Writing Laser In this embodiment, a writing laser that starts exposure first among the three lasers in the laser array is set in each unit. FIG. 5 is a diagram showing a correspondence relationship (for one unit) between each scanning line continuous in the sub-scanning direction and a laser used for the scanning. In this figure, the scanning line numbers are assigned to each scanning line from the upstream side (starting side) in the sub-scanning direction as 1, 2, 3, 4,..., And the writing laser is L1, L2, L3, respectively. Is a laser used for scanning each scanning line. Here, the writing laser is a laser used for scanning a scanning line whose scanning line number is 1. As shown in the figure, L1, L2, and L3 are sequentially used repeatedly when the writing laser is any of L1, L2, and L3.

  FIG. 6 is a diagram illustrating a correspondence relationship (for four units) between each scanning line continuous in the sub-scanning direction and a laser combination of each unit used for the scanning. The writing laser can be arbitrarily set in the laser array of each unit. FIG. 6A shows an example in which the writing lasers of the units a, b, c, and d are L1a, L2b, L3c, and L1d, respectively. At this time, as shown in the figure, the laser combinations of the units a, b, c, d are (L1a, L2b, L3c, L1d), (L2a, L3b, L1c, L2d), (L3a, L1b, L2c, L3d) The scanning lines are sequentially and repeatedly recorded. On the other hand, FIG. 6B shows an example in which the write lasers of the units a, b, c, and d are L2a, L3b, L1c, and L2d, respectively. The laser combination in this case is the same as the example shown in FIG. 4A except for the difference in scanning line number. Similarly, when the writing lasers of units a, b, c, and d are L3a, L1b, L2c, and L3d, respectively, the same laser combination is used except for the difference in scanning line numbers. As described above, among all laser combinations, there are three types of write laser settings that realize substantially the same laser combination.

  Therefore, in the present embodiment, the writing laser of the unit d is fixed to L1d, and control is performed so that the writing lasers are selected for the remaining three units a, b, and c. At this time, since the writing laser can be selected from the three lasers in each of the three units, the total number of selectable combinations is 27 (3 to the third power). An optimum combination is selected from these 27 combinations, and image formation is performed by the selected combination.

Subscanning direction writing timing The setting of the subscanning direction writing timing in the present embodiment will be described below. In the image forming apparatus of this embodiment, the toner images formed by the units are superimposed on the intermediate transfer belt 51 to form a single color image. At that time, in order to accurately superimpose the toner images, it is necessary to adjust the area where the photosensitive member 1 is exposed to form an electrostatic latent image (hereinafter referred to as an image forming area) in each unit with different writing lasers. There is.

  FIGS. 7A to 7C show the initial scanning positions when the writing lasers are LB1, LB2, and LB3, respectively. In the present embodiment, collective scanning with three laser beams is performed. Here, the “scanning position” is defined as the position in the sub-scanning direction on the photoconductor 1 scanned by LB1. As shown in the figure, the initial scanning position varies depending on the setting of the writing laser. Specifically, when the ideal interval between the scanning lines is g, the write laser shown in FIG. 7B is LB2 with reference to the case where the write laser shown in FIG. 7A is LB3. In this case, the initial scanning position is shifted upstream in the sub-scanning direction by g. Similarly, when the writing laser shown in FIG. 7C is LB1, the initial scanning position is shifted to the downstream side in the sub scanning direction by 2 × g.

  In order to vary the initial scanning position on the photosensitive member 1 rotating at a constant speed, the timing of emitting the laser beam may be varied. Specifically, assuming that the time required for one scan is 3 × t, when the writing laser is LB3 as a reference, only t when the writing laser is LB2, and 2 when the writing laser is LB1. The timing of irradiating the laser beam is delayed by xt. At that time, the timing for starting the rotation of the polygon mirror PM prior to image formation is similarly delayed. Hereinafter, “sub-scanning direction writing timing” is used as a general term for the timing of emitting the first laser beam described above and the timing of starting the rotation of the polygon mirror PM. In the present embodiment, by appropriately setting the sub-scanning direction writing timing, a good color image can be formed regardless of the setting of the writing laser. Hereinafter, the sub-scanning direction writing timing is simply referred to as “writing timing”.

  In the image forming apparatus according to the present exemplary embodiment, the toner image formed by each unit is shifted and overlapped on the intermediate transfer belt 51 due to uneven rotation speed of the photoreceptor 1 or uneven movement speed of the intermediate transfer belt 51. Deterioration may occur. The image forming apparatus of the present embodiment can be configured to detect such image quality deterioration and control the writing timing in the sub-scanning direction so as to cancel it.

Test Chart In this embodiment, a test chart is recorded to select an optimal laser combination prior to actual image formation. Hereinafter, this test chart will be described in detail.

  FIG. 8 is a schematic diagram showing a state in which a plurality of test charts are recorded on a recording medium. Each square part in the figure represents each test chart. Each test chart is recorded with all the laser combinations of each unit different, and there are 27 combinations as described above. Therefore, the total number of test charts is equal to the total number of laser combinations, ie 27. Note that there are no particular restrictions on how to select the 27 laser combinations used for chart recording and the arrangement of each test chart on the recording medium, and it is arbitrary.

  FIG. 9 is a diagram showing a chart configuration example common to all 27 test charts shown in FIG. As shown in the figure, the test chart of the present embodiment has a pattern in which one recording line + 3 blank lines are repeated in the sub-scanning direction based on the chart image data. In FIG. 9, the notation (n, n + 1, n + 2,...) On the left side of the pattern is a scanning line number assigned to each scanning line, and n is an integer of 1 or more. The test chart of this embodiment is common to four colors C, M, Y, and K. Therefore, the recording lines that are actually recorded in the test chart on the recording medium are C, M, Y, and K. It is a line where all colors are recorded overlapping. A blank line is a line on which nothing is recorded. At this time, the resolution in the sub-scanning direction of the test chart is equal to the scanning line interval in the sub-scanning direction, the recording duty of the recording line portion is 100%, and the recording duty of the blank line portion is 0%. The size of the test chart is assumed to be 15 mm × 15 mm, for example. In one test chart, the pattern repeats every four scanning lines, whereas there are three lasers installed in each unit, so the recording lines in the test chart are shifted by three lasers one by one. However, it is used and recorded sequentially.

  In this embodiment, as a test chart, one recording line + 3 blank lines, that is, an example in which a repetition of a pattern in which the same number of blank lines as the number of lasers in the laser array follows one recording line is used for all colors. Indicated. However, the test chart used in the present invention is not limited to this example, and other types of charts may be used as long as differences in image quality due to laser combinations of the respective colors can be detected.

Evaluation of Test Chart In this embodiment, the image quality is measured and evaluated for each of the plurality of test charts formed as described above as follows. Specifically, first, a recording medium on which a plurality of test charts are recorded is read by reading means (not shown) such as a scanner provided inside or outside the image forming apparatus. For each test chart, the average of the density values of each pixel (hereinafter referred to as the main scanning line density) is obtained for each line in the main scanning direction, thereby shifting the recording line, that is, the deviation from the ideal position based on the chart image data. The amount is evaluated as a value indicating the degree of image quality degradation. Specifically, the main scanning line density variation σ in the sub-scanning direction is calculated for each test chart, and this is used as an evaluation value indicating the amount of positional deviation in the sub-scanning direction. Here, the main scanning line density variation σ indicates a variation in the difference between the actually detected main scanning line density value (actually measured density value) and the ideal density value that is an ideal density value for the main scanning line. It may be calculated as a value (variance or standard deviation). The ideal density value can be calculated in advance as an ideal value of the main scanning line density from chart image data, for example. At this time, tilt correction or magnification conversion accompanying reading may be performed as necessary.

  Here, FIG. 10 shows a detection example of the main scanning line density in the sub-scanning direction of the test chart shown in FIG. 9 formed by a certain laser combination. In the figure, the horizontal axis indicates the position of the test chart in the sub-scanning direction, and the vertical axis indicates the calculation result of the average density (main scanning line density) for each main scanning. FIG. 10 is an enlarged view of the vicinity of a certain position in the sub-scanning direction with respect to the main scanning line density obtained for a certain three combinations A, B, and C among the 27 combinations of lasers. Each recording line in each test chart is formed so that all colors overlap, but the laser combinations used at that time are different from each other. Therefore, as shown in FIG. 10, the main scanning line densities in the respective test charts do not necessarily match at each position in the sub-scanning direction, and the degree of overlap of each color in the recording line portion and the interval (blank line portion) are not uniform. The features such as are different.

  In the present embodiment, as described above, a combination that will form an image with little positional deviation in the sub-scanning direction and consequently less unevenness and color unevenness is selected from the laser combinations of each color. Since recording is performed with the selected combination at the time of actual image formation, it is possible to reduce unevenness and color unevenness in the sub-scanning direction due to laser light emission intensity variation and scanning interval error.

  In this embodiment, since it is necessary to detect high-frequency unevenness in the sub-scanning direction, it is possible to ensure a sufficient reading resolution with respect to the scanning resolution in the sub-scanning direction in the image forming apparatus when measuring the test chart. desirable. Therefore, in this embodiment, it is assumed that the resolution in the sub-scanning direction of the image forming apparatus, that is, the resolution in the sub-scanning direction of the test chart is 600 dpi, and the reading resolution of the scanner that reads this is 1200 dpi. When it is desired to detect macroscopic image stripe unevenness, it is possible to detect a relatively low frequency stripe unevenness component in the sub-scanning direction by reducing the reading resolution of the scanner to, for example, about 150 dpi. Alternatively, a low-pass filter corresponding to visual characteristics may be applied to an image after reading to evaluate an image corresponding to human vision. In addition, other well-known method for evaluating a stripe with respect to an image can be applied.

  In addition, as density information for each pixel used for calculating an evaluation value for a test chart, parameters that can express density and color, such as brightness, luminance, optical density, and L * a * b * values, are generally used. Is possible. Therefore, in this embodiment, for example, even if the test chart is monochrome, it is possible to appropriately evaluate the unevenness.

  Here, in order to compare the positional deviation in the sub-scanning direction for each test chart, the method using the variation σ of the main scanning line density is shown, but the comparison may be performed by other methods. For example, a comparison method using various parameters such as the difference between the maximum value and the minimum value of the main scanning line density and the integrated value of the power spectrum when Fourier transformed is applicable.

Exposure Control Process The exposure control process in this embodiment will be described in detail below using the flowchart of FIG.

  First, in S1001, the chart recording control unit 15 selects one of the 27 laser combinations described above, and in S1002, the chart recording control unit 15 forms a test chart on the recording medium by the selected combination. To control. That is, the image forming control unit 11 of each unit is notified of the writing laser and its writing timing according to the selected combination. At this time, the chart image data held in advance in the chart recording control unit 15 is sent to the image formation control unit 11 of each unit. Then, each unit operates according to the notified writing laser and the writing timing, thereby recording a full-color test chart as shown in FIGS. 8 and 9 on the recording medium. At this time, the chart recording control unit assigns a predetermined identifier to each test chart, and holds information indicating a correspondence relationship between the identifier and the laser combination as a table, for example.

  Next, in S1003, the chart recording control unit 15 determines whether or not a test chart has been recorded in S1002 for all 27 laser combinations. If an unprocessed combination remains, the process returns to S1001 to select the next laser combination, and a test chart is recorded in S1002. Note that the order of selecting combinations in S1001 is not particularly defined. On the other hand, if all combinations are selected in S1003, the process proceeds to S1004.

  In S1004, the chart evaluation unit 16 measures and evaluates the image quality of each of the plurality of test charts recorded on the recording medium. That is, as described above, the recording medium on which a plurality of test charts are recorded is read by a scanner or the like, and the main scanning line density is calculated for each test chart, thereby obtaining the measurement result as shown in FIG. Then, as described above, the variation σ of the main scanning line density in the sub-scanning direction is calculated as the evaluation value. The calculated evaluation value is held in a table indicating the relationship between the identifier of the test chart and the laser combination.

  Next, in S1005, the laser combination selection unit 17 selects, from the table, a test chart that causes less unevenness and color unevenness based on the evaluation value for each test chart obtained in S1004. Specifically, one test chart is selected in which the variation σ of the main scanning line density in the sub-scanning direction is not more than a predetermined value as an assumed allowable range. Ideally, the test chart selected here has the smallest variation σ in all the test charts. However, even if the variation σ is not necessarily the minimum, the test chart may be selected as long as it is within an allowable range in which it is determined that the occurrence of unevenness and color unevenness is sufficiently small. Then, the laser combination selection unit 17 selects a laser combination when the selected test chart is recorded from the above table, and writes information on the write laser of each unit and the write timing corresponding to the combination to the image formation of each unit. Notify the control unit 11.

  In S1006, actual image recording by the image formation control unit 11 of each unit is executed by the laser combination selected in S1005. That is, the image formation control unit 11 of each unit controls the laser control unit 12 and the polygon motor control unit 13 so that an image is formed by the write laser notified in S1005 and its write timing. In this way, the laser combination of each unit is set so that the formed image has a high image quality. Therefore, in the image recorded here, the variation in the laser emission intensity and the scanning interval error in the laser array of each unit. The resulting image quality degradation is minimized.

  As described above, according to the present embodiment, by performing test chart printing using a combination of a plurality of lasers, a laser capable of reducing image quality degradation caused by light emission intensity variation and scanning interval error, which occurs in a multi-beam scanning method. Combinations can be detected. Further, there is no need to provide a special sensor for directly measuring each laser beam or each scanning line of the formed image, so there is no adverse effect such as high cost. Further, a good image can be formed without performing complicated processing such as correction of the light emission amount of each laser.

  Furthermore, according to the present embodiment, in the manufacturing stage of the laser and the laser array, even if a member that has been regarded as a defective product due to insufficient precision is not made unusable uniformly, it can be used by the method of the present invention. There is also. Therefore, an effect of improving the yield rate when manufacturing the laser and the laser array can be obtained.

Second Embodiment
Hereinafter, a second embodiment according to the present invention will be described. In the first embodiment described above, an example is shown in which the image quality of a plurality of test charts recorded by changing the laser combination of each unit is measured and evaluated using a reading unit such as a scanner. The second embodiment shows an example in which the image quality determination of the test chart is left to the user of the image forming apparatus, and the laser combination selected by the user is acquired by a user instruction via a user interface (UI).

  Since the configuration of the image forming apparatus according to the second embodiment is the same as that of the first embodiment described above, the description thereof is omitted. Only portions that are particularly different from the first embodiment will be described below.

  FIG. 12 is a diagram showing a block configuration for exposure control in image formation according to the second embodiment. As shown in FIG. 12, the unit a in the second embodiment is different from the configuration shown in FIG. 3 in the first embodiment described above in that the chart evaluation unit 16 in the controller is replaced with a chart evaluation UI unit 26. Yes, the other configurations are the same. The chart evaluation UI unit 26 receives evaluation results by the user for each of the test charts recorded on the recording medium by the chart recording control unit 15 for each image quality, that is, the degree of occurrence of uneven stripes and color unevenness. The

  Hereinafter, the exposure control processing in the second embodiment will be described with reference to the flowchart of FIG. In FIG. 13, steps similar to those in FIG. 11 described in the first embodiment are given the same step numbers, and description thereof is omitted.

  First, as in the first embodiment described above in S1001 to S1003, a test chart for the laser combination of each unit is printed out on a recording medium.

  Then, in S2004, the user evaluates the image quality of the plurality of test charts, selects one test chart with little unevenness and color unevenness, and specifies identification information indicating the test chart via the chart evaluation UI unit 26 . Then, the laser combination selection unit 17 notifies the image formation control unit 11 of each unit of the write laser of each unit and the information of the write timing according to the laser combination when the test chart designated by the user is recorded. .

  In S1006, actual image recording by the image formation control unit 11 of each unit is executed by the laser combination designated by the user in S2004. That is, the image formation control unit 11 controls the laser control unit 12 and the polygon motor control unit 13 so that an image is formed based on the writing laser notified in S2004 and its writing timing.

  Hereinafter, the test chart designation process in S2004 will be specifically described. In the second embodiment as well, as in the first embodiment described above, a plurality of test charts are output in the format shown in FIG. 8, and the user evaluates them to select the optimum one. At this time, as the identification information for designating the selected test chart, it is possible to use the information of the writing laser of each unit corresponding to the test chart. As an example, a case where the user selects a test chart surrounded by a thick frame shown in FIG. 14 will be described. Since the writing laser of each unit is L2a for unit a, L3b for unit b, and L2c for unit c, the user designates this as (L2a, L3b, L2c) from the chart evaluation UI unit 26. Or you may specify like (2, 3, 2) more simply. As a method for expressing the writing laser combination, other symbols such as XYZ and Aiu can be used. In the second embodiment as well, since the writing laser of unit d is fixed to L1d, the user need not specify unit d.

  Note that the association between the test chart and the laser combination is not limited to the example described above. For example, by attaching an ID to each test chart and maintaining a table that associates the ID with the laser combination in the apparatus, the user can only specify the ID of the test chart by the user, and the corresponding laser combination Can be obtained. The chart evaluation UI unit 26 is not necessarily provided in the image forming apparatus. If the input can be referred from the apparatus, the chart evaluation UI unit 26 is provided on an external apparatus such as a PC connected to the apparatus. May be. Note that the user can easily determine the image quality of the test chart by visual observation, but it is also possible to determine the image quality using a measuring instrument or the like. That is, it suffices if information on the test chart that the user has determined to be optimal can be input from the UI.

  As described above, according to the second embodiment, even when the image forming apparatus does not include an image reading unit such as a scanner, the combination of polygon mirror use start surfaces is determined according to a user instruction. The same effects as those of the first embodiment described above can be obtained.

<Other embodiments>
In the first and second embodiments described above, as a configuration of the image forming apparatus, a so-called tandem-type multiplex capable of forming a full-color image in one pass by independently providing a process unit such as a photoconductor and a developing device for each color. An example of the structure of the transfer system is shown. However, the present invention is not limited to such a configuration, and can be applied to image forming apparatuses having other configurations as long as a color image is formed using a plurality of lasers. For example, even in a configuration in which laser arrays are shared between colors, for example, a configuration in which Y and K, and C and M each have a common exposure unit, a laser combination that can reduce image quality degradation can be obtained.

  This configuration is also applicable to a configuration in which one laser array is shared by each color, for example, a configuration in which an electrostatic latent image is formed on one photoconductor using a four-color developer as shown in FIG. The invention is applicable. In FIG. 15, the same components as those in FIG. That is, according to the configuration shown in FIG. 15, after the photosensitive member 1 rotating at a constant speed is uniformly charged by the charger 2, the exposure unit 3 performs exposure according to the image signal, thereby causing the photosensitive member to be exposed. An electrostatic latent image is formed on 1. This is developed by any of the four color developing devices 4a, 4b, 4c, and 4d to obtain a toner image, and the toner image is transferred onto the intermediate transfer belt 51 by the primary transfer unit 53. By repeating this process for each color, a plurality of color toner images are superimposed on the intermediate transfer belt 51. Further, these are collectively transferred onto the recording medium P by the secondary transfer unit 57 and fixed by the fixing device 7, whereby a color image is obtained. The developing devices 4a, 4b, 4c, and 4d perform development using toners of colors C, M, Y, and K, respectively, but the order of these colors is not limited.

  In the configuration shown in FIG. 15, since there is only one image forming unit common to each color, a common laser array is used for all four colors. However, even in such a configuration, the test chart can be formed by changing the writing laser for each color. Therefore, even in a configuration in which image formation is performed using one laser array, it is possible to determine a laser combination for each color so that unevenness in the sub-scanning direction and color unevenness are reduced. Thus, it is possible to obtain a laser combination that can reduce image quality degradation caused by light emission intensity variation and scanning interval error that occurs in the multi-beam scanning method.

  The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and the processor (or CPU, MPU, etc.) of the system or apparatus reads the program. It is a process to be executed.

Claims (10)

Using an exposure means capable of forming a plurality of main scanning lines by one exposure scanning with a plurality of laser light sources in a main scanning direction orthogonal to the sub-scanning direction with respect to the image carrier moving in the sub-scanning direction, An image forming apparatus that forms an image for each of a plurality of colors and forms a color image by superimposing the plurality of colors on a recording medium,
Of the plurality of laser light sources in the exposure unit, the combination of each of the plurality of colors of the writing laser light source that performs exposure of the first main scanning line at the time of image formation is changed, and an image corresponding to predetermined chart image data is recorded. Chart recording control means for obtaining a plurality of test charts for each combination by forming on a medium;
Selecting means for selecting a combination of the writing laser light sources corresponding to one selected from the plurality of test charts;
Control means for controlling the exposure means so that the writing laser light source for each color of the plurality of colors is a combination of the writing laser light sources selected by the selection means;
An image forming apparatus comprising: Furthermore, it has an evaluation means for evaluating the degree of image quality deterioration for each of the plurality of test charts,
The said selection means selects the combination of the said writing laser light source corresponding to the test chart evaluated by the said evaluation means that the degree of the said image quality degradation is in the predetermined tolerance. The image forming apparatus according to 1.   The evaluation means, based on reading values of the plurality of test charts, for each of the test charts, the amount of positional deviation of the main scanning line from the ideal position of the main scanning line calculated from the chart image data, The image forming apparatus according to claim 2, wherein the image forming apparatus calculates an evaluation value indicating a degree of image quality deterioration.   The evaluation means, based on the density values obtained by measuring the plurality of test charts, for each of the test charts, a density value for each main scanning line and an ideal density value of the main scanning line calculated from the chart image data 4. The image forming apparatus according to claim 2, wherein a value indicating variation in the difference between the two is calculated as an evaluation value indicating the degree of image quality deterioration. 5. The chart recording control means holds a correspondence relationship between the combination of the writing laser light sources and a test chart formed by the combination in a table,
The evaluation means holds an evaluation value calculated for each of the plurality of test charts in the table,
The said selection means selects the combination of the said write-out laser light source corresponding to the test chart with which the said evaluation value becomes the minimum among these test charts using the said table. 5. The image forming apparatus according to 4. The evaluation means inputs a user instruction indicating one of the plurality of test charts,
The image forming apparatus according to claim 1, wherein the selection unit selects a combination of the writing laser light sources corresponding to the test chart indicated by the user instruction.   The chart image data is data for repeatedly forming, in the sub-scanning direction, a recording line for recording a line by overlapping main scanning lines corresponding to the plurality of colors and a blank line for recording none of the plurality of colors. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided.   The chart image data is data that repeats, in the sub-scanning direction, a pattern in which the same number of blank lines as the number of the laser light sources included in the exposure unit follows the one recording line. Item 8. The image forming apparatus according to Item 7. An exposure means capable of forming a plurality of main scanning lines by one exposure scanning by a plurality of laser light sources in a main scanning direction orthogonal to the sub-scanning direction on an image carrier that moves in the sub-scanning direction, and chart recording A control unit, a selection unit, and an image formation control unit. The exposure unit is used to form an image for each of a plurality of colors, and the plurality of colors are superimposed on a recording medium to form a color image. An image forming method in an image forming apparatus,
The chart recording control unit changes a combination of each of the plurality of colors of the writing laser light source that performs exposure of the first main scanning line at the time of image formation among the plurality of laser light sources in the exposure unit, thereby changing a predetermined chart image By forming an image according to data on a recording medium, a plurality of test charts for each combination are obtained,
The selection means selects a combination of the writing laser light sources corresponding to one selected from the plurality of test charts;
The image formation control unit performs image formation by controlling the exposure unit so that the writing laser light source for each of the plurality of colors is a combination of the writing laser light sources selected by the selection unit. An image forming method.   A program for causing a processor of an image forming apparatus to function as each unit of the image forming apparatus according to any one of claims 1 to 8 when executed by a processor of the image forming apparatus.

Images