JP6221916B2 - Image forming apparatus and exposure control method - Google Patents

Description

  The present invention relates to an image forming apparatus and an exposure control method.

An electrophotographic image forming apparatus forms an electrostatic latent image by exposing a photoconductor in accordance with image data, supplies a coloring material such as toner, and develops the electrostatic latent image, thereby generating an image. Is forming.
At the time of exposure, the laser beam is deflected by a polygon mirror and scanned on the photoconductor. In order to speed up image formation, a plurality of laser beams are irradiated in parallel to irradiate a plurality of lines in one scan. An image is also formed.

When an image of a screen pattern is formed by a plurality of laser beams in order to reproduce the intermediate gradation, unevenness in density may occur in an image formed by interference between the screen pattern having periodicity and unevenness in the amount of light of each laser beam. .
As a countermeasure against such density unevenness, conventionally, a period in which density unevenness occurs is calculated from the number of screen lines and the screen angle, and the number of laser beams used for image formation is controlled so that the period becomes a predetermined value or less. (For example, refer to Patent Document 1).

JP 2008-175859 A

However, the mirror surface of the polygon mirror has a considerable variation in shape, and jitter occurs in the laser beam deflected by the polygon mirror. When a plurality of laser beams irradiated in parallel are deflected, jitter is repeatedly generated in the sub-scanning direction. Therefore, the jitter and the screen pattern interfere with each other, resulting in density unevenness.
Therefore, even if the number of laser beams is controlled in consideration of only the screen pattern as in the prior art, density unevenness due to jitter cannot be eliminated.

  An object of the present invention is to reduce unevenness in the density of a visible image.

According to the invention of claim 1,
A plurality of writing units that irradiate a plurality of laser beams in parallel, deflect each laser beam in the main scanning direction by a polygon mirror, expose the photosensitive member, and then develop, and each screen has a different color screen pattern An image forming unit that forms and superimposes the images of
A control unit that changes the number of laser beams irradiated in parallel during the exposure when the screen pattern of any two of the different color images is axisymmetric through the main scanning direction; Prepared,
The control unit generates density unevenness from a screen period in which the images of the screen patterns of the two colors are repeatedly overlapped in the sub-scanning direction and a jitter period in which jitter of the laser beam deflected by the polygon mirror is repeatedly generated in the sub-scanning direction. There is provided an image forming apparatus characterized in that the period is determined, and the number of the laser beams is changed so that the density unevenness period is outside a visible range determined in advance according to visual sensitivity.

According to invention of Claim 2,
The controller determines one or a plurality of jitter periods to be predicted by a product of the number of the plurality of laser beams and each of one or a plurality of natural numbers that can divide the number of mirrors of the polygon mirror, 2. The image forming apparatus according to claim 1, wherein the number of the laser beams is changed so that all the density unevenness periods determined from the screen period and each jitter period are outside the visible range. Provided.

According to invention of Claim 3,
The control unit changes the exposure condition of the laser beam according to the number of laser beams after the change so that the time required for image formation does not change before and after changing the number of laser beams. An image forming apparatus according to claim 1 or 2 is provided.

According to invention of Claim 4,
The image forming apparatus according to claim 1, wherein the control unit changes the number of the laser beams in both of the writing units that respectively form the two-color images. Provided.

According to the invention of claim 5,
The image forming apparatus according to any one of claims 1 to 3, wherein the control unit changes the number of the laser beams in a writing unit that forms one of the two color images. The

According to the invention of claim 6,
An exposure control method for a plurality of writing units that irradiates a plurality of laser beams in parallel, deflects each laser beam in a main scanning direction by a polygon mirror, exposes a photosensitive member, and develops the photosensitive member.
When the screen pattern of any two colors among the images of different color screen patterns formed by the plurality of writing units is axisymmetric with respect to the main scanning direction, irradiation is performed in parallel during the exposure. Including changing the number of laser beams,
The process includes
Determining a screen period in which the images of the two-color screen patterns repeatedly overlap in the sub-scanning direction;
Determining a jitter period in which jitter of the laser beam deflected by the polygon mirror repeatedly occurs in the sub-scanning direction;
Determining a density unevenness period from the screen period and the jitter period;
Changing the number of the laser beams such that the period of density unevenness is outside a visible range determined in advance according to visual sensitivity;
An exposure control method is further provided.

  According to the present invention, even when the density unevenness occurs due to the polygon mirror, the generated density unevenness can be changed to the density unevenness with a period that is difficult to be visually recognized, and the density unevenness of the visible image can be reduced. .

1 is a functional block diagram illustrating a configuration of an image forming apparatus according to an exemplary embodiment for each function. It is a front view which shows schematic structure of an image formation part. It is a perspective view showing schematic structure of an exposure part. It is a schematic diagram showing the image of the two-color screen pattern superimposed. It is a graph which shows a visual transfer function. The processing procedure when the control unit changes the number of laser beams is shown. It is a schematic diagram showing the part which the image of 2 colors shown in FIG. 4 overlaps.

  Embodiments of an image forming apparatus and an exposure control method of the present invention will be described below with reference to the drawings.

FIG. 1 is a functional block diagram illustrating the configuration of the image forming apparatus G according to the present embodiment for each function.
As shown in FIG. 1, the image forming apparatus G includes a control unit 11, a storage unit 12, an operation unit 13, a display unit 14, a communication unit 15, an image generation unit 16, a memory control unit 17, an image memory 18, and an image processing unit. 19 and an image forming unit 20.

The control unit 11 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and the like. The control unit 11 controls each unit of the image forming apparatus G by reading and executing a program stored in the storage unit 12.
For example, the control unit 11 causes the image generation unit 16 to generate bitmap format image data, and causes the image processing unit 19 to perform image processing on the image data. The control unit 11 causes the image forming unit 20 to form an image on a sheet based on the image processed image data.

The control unit 11 can change the number of laser beams to be irradiated when forming an image of a different color in the image forming unit 20 in order to reduce the density unevenness of the visible image.
When the number of laser beams is changed, the control unit 11 may change the exposure condition of the laser beam according to the number of laser beams after the change so that the time required for image formation does not change before and after the change. it can.

The storage unit 12 stores programs, files, and the like that can be read by the control unit 11.
As the storage unit 12, a storage medium such as a hard disk or a ROM (Read Only Memory) can be used.

  The operation unit 13 includes an operation key, a touch panel configured integrally with the display unit 14, and the like, and outputs operation signals corresponding to these operations to the control unit 11. The user can perform input operations such as job setting and processing content change by the operation unit 13.

  The display unit 14 can be an LCD (Liquid Crystal Display) or the like, and displays an operation screen or the like according to instructions from the control unit 11.

  The communication unit 15 communicates with a computer on the network, for example, a user terminal, a server, another image forming apparatus, etc. according to an instruction from the control unit 11. For example, the communication unit 15 receives data described in PDL (Page Description Language) from a user terminal.

The image generation unit 16 rasterizes the data described in PDL received by the communication unit 15, and converts bitmap format image data having a gradation value for each pixel into C (cyan), M (magenta), Generated for each color of Y (yellow) and K (black). The gradation value is a signal value that represents the level of shading of the image within a range of 0 to 100%.
The image generation unit 16 includes a scanner, and can read image data set by a user using the scanner to generate image data of each color of R (red), G (green), and B (blue). . The image generation unit 16 performs color conversion processing on the image data of each color R, G, and B to generate image data of each color C, M, Y, and K.

The memory control unit 17 writes the image data generated by the image generation unit 16 in the image memory 18 and stores it. Further, the memory control unit 17 reads out the image data from the image memory 18 and outputs it to the image processing unit 19.
For example, a DRAM (Dynamic RAM) or the like can be used as the image memory 18.

The image processing unit 19 performs image processing such as gradation correction processing and screen processing on the C, M, Y, and K image data read from the image memory 18.
The gradation correction process is a process for converting the gradation value of each pixel of the image data into a gradation value corrected so that the density characteristic of the formed image matches the target density characteristic.
In the screen processing, a dither matrix in which a threshold value of n × m is set is collated with each pixel of the image data, and a threshold value corresponding to the position of each pixel of the image data is obtained from the dither matrix and compared with the threshold value. Thus, the gradation value of each pixel is binarized. Screen processing that multi-values the gradation value of each pixel may be used.

The image forming unit 20 forms an image of each color according to the image data of each color of C, M, Y, and K output from the image processing unit 19 and superimposes the image having a plurality of colors on the paper. To form.
FIG. 2 shows a schematic configuration of the image forming unit 20.
The image forming unit 20 includes four writing units 21, an intermediate transfer belt 26, a secondary transfer roller 27, and a fixing device 28. Each writing unit 21 is arranged in series along the belt surface of the intermediate transfer belt 26. The intermediate transfer belt 26 is wound and rotated by a plurality of rollers. One of the plurality of rollers constitutes a secondary transfer roller 27. The secondary transfer roller 27 and the fixing device 28 are arranged on a conveyance path of a sheet conveyed from a sheet feeding tray (not shown).

The four writing units 21 form images of different colors of C, M, Y, and K, respectively. Each writing unit 21 has the same configuration, and includes an exposure unit 30, a photoconductor 22, a development unit 23, a charging unit 24, and a cleaning unit 25.
At the time of image formation, the charging unit 24 applies a voltage to the photosensitive member 22 to charge it, and then irradiates a laser beam modulated according to image data of each color of C, M, Y, and K, thereby forming the photosensitive member. 22 is exposed to form an electrostatic latent image. Thereafter, a color material such as toner is supplied onto the photoconductor 22 by the developing unit 23 to develop the electrostatic latent image, and an image of each color is formed on the photoconductor 22 of each writing unit 21.

The image forming unit 20 forms an image composed of a plurality of colors on the intermediate transfer belt 26 by sequentially transferring the images on the respective photoreceptors 22 on the intermediate transfer belt 26, and secondary-transfers the images. The image is transferred onto the paper by the roller 27. After the transfer, in each writing unit 21, the color material remaining on the photosensitive member 22 is removed by the cleaning unit 25.
The image forming unit 20 conveys the sheet to the fixing device 28 by the secondary transfer roller 27 and heats and pressurizes the sheet by the fixing device 28 in order to fix the image on the sheet.

FIG. 3 shows details of the exposure unit 30 in the writing unit 21.
As shown in FIG. 3, the exposure unit 30 includes a laser beam light source 31, a slit 32, a collimator lens 33, a polygon mirror 34, an fθ lens 35, a cylindrical lens 36, a mirror 37, and a sensor 38.
The exposure unit 30 includes a control unit 41 that controls driving of the light source 31, the polygon mirror 34, and the photosensitive member 22, and driving units 42 to 44.

The light source 31 includes a plurality of light emitting elements, and each light emitting element can irradiate a plurality of laser beams in parallel. As the light source 31, a semiconductor laser array or the like can be used.
The slit 32 shapes each laser beam emitted from the light source 31 into a predetermined spot diameter.
The collimator lens 33 converts the laser beam that has passed through the slit 32 into parallel light.

The polygon mirror 34 is a rotating body provided with a plurality of mirrors on each side of a regular polygonal column. The polygon mirror 34 reflects the laser beam converted into parallel light on each mirror surface and guides it to the photosensitive member 22. The laser beam is deflected by the rotation of the polygon mirror 34 and scans on the photoconductor 22 along the axial direction of the photoconductor 22. The direction in which the laser beam scans is the main scanning direction x, and the direction perpendicular to the main scanning direction x on the photoconductor 22 is the sub-scanning direction y.
The fθ lens 35 is disposed on the optical path between the polygon mirror 34 and the photoconductor 22 and equalizes the scanning speed of the laser beam that scans the photoconductor 22.
The cylindrical lens 36 images the laser beam that has passed through the fθ lens 35 on the photosensitive member 22.

The mirror 37 guides the first laser beam reflected by each mirror surface of the polygon mirror 34 to the sensor 38. When the sensor 38 detects the laser beam, it outputs a detection signal to the control unit 41.
The control unit 41 generates a horizontal effective signal HSYNC indicating the effective area of the image in the main scanning direction x in accordance with the detection signal from the sensor 38. In response to the horizontal effective signal HSYNC, image data is transferred from the image memory 18 to the image forming unit 20. The control unit 41 outputs the transferred image data to the drive unit 42. The drive unit 42 drives each light emitting element of the light source 31 to emit light, and irradiates a laser beam modulated according to image data.

The control unit 41 instructs the drive unit 42 to irradiate the laser beam according to the exposure conditions such as the modulation frequency of the image data, the number of laser beams, and the light amount determined by the control 11.
Further, the control unit 41 instructs each of the drive units 43 and 44 to rotate the photoconductor 22 and the polygon mirror 34 at the rotation speed determined by the control unit 11. The drive units 43 and 44 rotate and drive the photosensitive member 22 and the polygon mirror 34, respectively, at the rotation speed designated by the control unit 41.

Next, an exposure control method of the image forming apparatus G will be described.
When the image forming apparatus G forms an image of a screen pattern for reproduction of intermediate gradations, the image of each color is superimposed by changing the screen angles of the screen patterns of C, M, Y, and K colors. This reduces interference between screen patterns.
However, when the screen patterns of at least two colors of C, M, Y, and K are in a line-symmetric relationship with the main scanning direction x as the axis of symmetry, the period in which the images of the screen patterns overlap and the polygon mirror In some cases, unevenness of density may occur due to interference with a period in which jitter occurs in a plurality of laser beams deflected due to distortion of the mirror surface 34 and variations in shape such as unevenness. Jitter is fluctuation, and the jitter of the laser beam causes a shift in the scanning position, resulting in unevenness in the amount of light.

FIG. 4 is a schematic diagram showing an image of a screen pattern of two colors superimposed. In FIG. 4, the solid line represents a C color image, and the dotted line represents an M color image.
The screen patterns of the C-color and M-color images have the same number of screen lines, and are in a line-symmetric relationship with the main scanning direction x as the axis of symmetry. Intersections where the images overlap are periodically located in the main scanning direction x and the sub-scanning direction y.

On the other hand, if there is a variation in the shape of each mirror surface of the polygon mirror 34, fluctuation of the laser beam reflected by each mirror surface, that is, jitter occurs. When a plurality of laser beams are deflected by the polygon mirror 34, scanning of a plurality of lines is performed in parallel by one mirror surface, and jitter of one pattern is generated. Since jitter of the same pattern is generated by the same mirror surface, jitter of the same pattern is repeatedly generated in the sub-scanning direction y by rotation of the polygon mirror 34.
Since the scanning direction of the laser beam and the direction in which the intersections of the two-color images are arranged are the same main scanning direction x, the period in which the laser beam jitter occurs in the sub-scanning direction y overlaps the period of the intersections of the two-color images. In FIG. 4, uneven density occurs at intervals indicated by arrows.

Not all density unevenness that occurs is visually recognized by the user, and may not be visually recognized by human visual sensitivity.
As a visual transfer function (hereinafter referred to as VTF: Visual Transfer Function) with respect to lightness fluctuation, the following Approximate equation is known.
VTF = 5.05exp (−0.138u) {1-exp (−0.1u)}
u = (πlf) / 180
In the above formula, l represents the observation distance (mm), and f represents the spatial frequency (cycles / mm).

FIG. 5 shows the VTF when l = 300 mm.
This VTF indicates that when the spatial frequency is in the vicinity of 1.0 (cycles / mm), the visual sensitivity with respect to the brightness variation is maximized and is most easily recognized. Moreover, it represents that visual sensitivity decreases and it becomes difficult to visually recognize, so that it becomes larger than 1.0 (cycles / mm) and it becomes small.

In the image forming apparatus G, even when the above-described density unevenness occurs due to the polygon mirror 34, the control unit 11 performs the irradiation in parallel at the time of exposure so that the density unevenness of the spatial frequency that is difficult to be visually recognized by the user occurs. By changing the number of laser beams to be generated, the period of density unevenness to be generated is changed.
FIG. 6 shows a processing procedure when the control unit 11 changes the number of laser beams. This processing procedure is executed when the screen patterns of at least two of the superimposed images have a line-symmetric relationship with the main scanning direction x as the axis of symmetry.

As shown in FIG. 6, the control unit 11 determines a cycle in which the images of the two-color screen patterns having a line-symmetric relationship are repeatedly overlapped in the sub-scanning direction y as a screen cycle (step S1).
FIG. 7 shows a portion where the two color images shown in FIG. 4 overlap.
As shown in FIG. 7, the control unit 11 obtains the number of pixels in the sub-scanning direction y between the intersection P1 and the intersection P3 of the image of each color or between the intersection P2 and the intersection P4 and sets it as the screen period A.

In the M-color screen pattern, when the distance between two adjacent screen lines is represented by B (pixel) and the screen angle is represented by θ (°), the screen period A can be represented by the following formula (1). The screen angle is an angle formed by each screen line with the main scanning direction x.
A = {B / sin θ} / 2 (1)

When the basic movement amounts of the M-color screen pattern in the main scanning direction x and the sub-scanning direction y are respectively expressed as dx and dy (pixels), the distance B can be expressed by the following equation (2). The basic movement amount refers to the movement amount in the main scanning direction x and the sub-scanning direction y from one halftone dot forming the screen pattern to the adjacent halftone dot, and dx and dy are positive integers.
B = √ (dx ² + dy ² ) (2)

Further, the screen angle θ can be expressed by the following formula (3).
θ = tan ⁻¹ (dy / dx) (3)

From the above formulas (1) to (3), the screen period A can be expressed by the following formula (4).
A = {√ (dx ² + dy ² ) / sin (tan ⁻¹ (dy / dx))} / 2 (4)
The control unit 11 can obtain the screen period A by substituting the basic movement amounts dx and dy of the M color screen pattern into the above equation (4).
For example, when the basic movement amount dx is 2 pixels and the basic movement amount dy is 6 pixels, the result that the screen period A is 10 pixels is obtained by the above equation (4).

Since the C-color screen pattern is in a line-symmetric relationship with the M color, the basic movement amounts of the C-color screen pattern in the main scanning direction x and the sub-scanning direction y can be expressed as dx and -dy, respectively. Even if the screen period A is determined based on the basic movement amount of the C color screen pattern, the same result can be obtained.
Note that the calculation method is not limited to the above-described calculation method as long as the screen period A can be calculated. Further, the screen cycle A of each screen pattern that can be used in the image forming apparatus G is stored in the storage unit 12 in advance, and the control unit 11 acquires the screen cycle A of the screen pattern to be used from the storage unit 12. Also good.

  Next, the control unit 11 determines a period in which the jitter of the laser beam is repeatedly generated in the sub-scanning direction y due to the polygon mirror 34 as a jitter period. If there is one mirror having a variation in shape, jitter of at least one pattern is generated by one rotation of the polygon mirror 34. However, if some of the jitter characteristics of each mirror are the same, the jitter becomes shorter in a shorter period. Will occur. Since each period of jitter that can occur depending on the number of mirrors of the polygon mirror 34 can be predicted, the control unit 11 determines all the predicted jitter periods (step S2).

In general, the polygon mirror 34 is a regular polygonal column, and among the plurality of mirrors arranged on each side of the regular polygon, the mirror surfaces arranged at equal intervals often have the same jitter characteristics. . For this reason, jitter tends to occur at a natural number greater than 1 among the natural numbers that can reduce the number of mirrors.
Therefore, the control unit 11 calculates the product of the number of the plurality of laser beams irradiated in parallel at the time of exposure and one or a plurality of natural numbers that can reduce the number of mirrors, as one or a plurality of jitter periods to be predicted. Respectively.

  For example, when the polygon mirror 34 is a regular hexagonal cylinder, among natural numbers other than 1, the natural numbers that can be used to divide the number 6 of mirrors are 6, 3 and 2, and the distance between the mirror surfaces of 6, 3 and 2 is It is predicted that jitter will occur repeatedly. When the number of laser beams is 4, the product of 4 which is the number of laser beams and 6, 3 and 2 which are natural numbers which can divide the number of mirrors 6 is 24, 12 and 8 pixels, respectively. Therefore, each period of 24, 12, and 8 pixels is determined as a jitter period of all the predicted jitters.

  When the polygon mirror 34 is a regular heptagonal prism, out of natural numbers other than 1, the number 7 of mirrors can only be divisible, so it is predicted that jitter will occur at the interval between the 7 mirror surfaces. The Therefore, 28 pixels, which is a product of the number 4 of laser beams, is determined as the jitter period of all predicted jitters.

  In addition, when the polygon mirror 34 is a regular dodecagonal column, natural numbers other than 1 that can divide the number of mirrors 12 are 12, 6, 4, 3, and 2, and the jitter is determined by the distance between these mirror surfaces. It is expected to occur. Therefore, the periods of 48, 24, 16, 12, and 8 pixels, which are the products of each of these five natural numbers and the number of laser beams 4, are determined as the jitter periods of all the predicted jitters.

Next, the control unit 11 determines a period in which the screen period and each jitter period overlap as a period of density unevenness caused by interference between the screen pattern and jitter (step S3).
Specifically, the control unit 11 determines the least common multiple of the screen period and each jitter period as a period in which the screen period and each jitter period overlap, that is, a period of density unevenness.
For example, if the screen period is 10 pixels and each jitter period is 24, 12 and 8 pixels, 120, 60 and 40 pixels which are the least common multiple of these are determined as the period of density unevenness.

Table 1 below shows a list of screen periods, jitter periods, and density unevenness periods when the number of polygon mirrors is 6 and the image resolution is 1200 dpi. The period of density unevenness in mm is calculated assuming that the size of one pixel is 21.2 μm.

The controller 11 changes the number of laser beams so that all the density unevenness periods are outside the visible range determined in advance according to the visual sensitivity (step S4).
The visible range can be arbitrarily determined according to the visual sensitivity described above. For example, based on the VTF shown in FIG. 5, the period corresponding to the spatial frequency of 0.6 to 1.4 (cycles / mm) including the vicinity of 1.0 where the visual sensitivity is maximum may be determined as the visible range. it can. In order to reduce the density unevenness that can be visually recognized, the visible range may be further expanded.

When the resolution of the image is 1200 dpi, since the size of one pixel is 21.2 μm, the range of the period corresponding to the spatial frequency range of 0.6 to 1.4 (cycles / mm) is 0.71 to 0.71. The range is 1.67 mm.
As shown in Table 1 above, when the number of polygon mirrors 34 is 6, the periods of 120, 60 and 40 pixel density unevenness in mm units are 2.54, 1.27 and 0.84 mm, respectively. . Since the density unevenness period of 60 and 40 pixels is in the visible range, it is necessary to change the number of laser beams.

The control unit 11 reduces the number of laser beams by 1 and recalculates the density unevenness period.
For example, when the number of laser beams is changed from 4 to 3, the screen period, jitter period, and density unevenness period shown in Table 1 above change as shown in Table 2 below by recalculation.

  As shown in Table 2 above, since all the density unevenness periods can be outside the visible range of 0.71 to 1.67 mm, the control unit 11 determines the number of laser beams after the change to 3.

If the number of laser beams can be increased, the number of laser beams can be changed from 4 to 5. In this case, the screen period, jitter period, and density unevenness period shown in Table 1 above change as shown in Table 3 below by recalculation.

As shown in Table 3 above, since the period of all density unevenness can be outside the visible range of 0.71 to 1.67 mm, the control unit 11 determines that the number of laser beams after the change is 5 You can also.
In this way, the control unit 11 changes the number of laser beams and recalculates each period of density unevenness, and obtains the number of laser beams when all the recalculated periods are outside the visible range.

Next, the control unit 11 changes the exposure condition of the laser beam according to the number of laser beams after the change so that the time required for image formation does not change before and after the change of the number of laser beams (step) S5).
Thereby, it is possible to prevent the time required for image formation from varying due to the change in the number of laser beams. In particular, when the number of laser beams is reduced, the time for forming an image becomes longer. However, by changing the exposure conditions, the image can be formed in the same time as before the number is reduced.

Examples of laser beam exposure conditions that can be changed include the rotational speed (rpm) of the polygon mirror 34, the modulation frequency (MHz) of the image data, the amount of light of the laser beam, and the like.
The control unit 11 can determine the exposure conditions after each change according to the following formulas (a) to (c).
(A) Polygon mirror rotation speed (rpm)
= {60 × photosensitive member rotational speed (mm / s) × image resolution (dpi)} / (25.4 × number of mirrors × number of laser beams after change)
(B) Modulation frequency (MHz)
= {Width in the main scanning direction x (mm) × photosensitive member rotational speed (mm / s) × (image resolution) ² } / {25.4 ² × scanning efficiency (%) × number of laser beams after change }
(C) Light quantity = (number of laser beams before change × light quantity before change) / number of laser beams after change Note that the scanning efficiency (%) is the main scan with respect to the full width (mm) in the main scan direction x. The ratio (%) of the width (mm) in which image formation is effective in the direction x.

When the controller 11 changes the number of laser beams and the exposure conditions in both of the writing units 21 that respectively form two-color images, the controller 11 changes the number of laser beams after the change and Instruct to expose according to exposure conditions. Thereafter, this process is terminated.
The control unit 11 changes the laser beam after changing the writing unit 21 so as to change the number of laser beams and the exposure condition in the writing unit 21 that forms one of the two color images. It can also be instructed to perform exposure according to the number of beams and exposure conditions. Also in this case, as in the case where the change is made in both writing units 21, the effect of reducing the visible density unevenness can be obtained.

  As described above, the image forming apparatus G of the present embodiment irradiates a plurality of laser beams in parallel, deflects each laser beam in the main scanning direction x by the polygon mirror 34, and exposes the photosensitive member 22. The image forming unit 20 includes a plurality of writing units 21 to be developed, and each of the writing units 21 forms and superimposes a screen pattern image of a different color, and the image of any two colors among the different color images. When the screen pattern is line symmetric with respect to the main scanning direction x, a control unit 11 is provided that changes the number of laser beams irradiated in parallel during exposure. The control unit 11 determines the density from the screen period in which the images of the screen patterns of the two colors are repeatedly overlapped in the sub-scanning direction y and the jitter period in which the jitter of the laser beam deflected by the polygon mirror 34 is repeatedly generated in the sub-scanning direction y. The period of unevenness is determined, and the number of the laser beams is changed so that the period of uneven density is outside the visible range determined in advance according to the visual sensitivity.

  As a result, the number of laser beams can be changed so that the period of density unevenness occurring is outside the visible range. Even when the density unevenness caused by the polygon mirror 34 occurs, the generated density unevenness can be changed to the density unevenness of the spatial frequency that is difficult for the user to visually recognize, and the density unevenness of the visually recognizable image can be reduced.

The above embodiment is a preferred example of the present invention, and the present invention is not limited to this. Modifications can be made as appropriate without departing from the spirit of the present invention.
For example, as a computer-readable medium for a program for causing the control unit 11 to execute the above processing procedure, a non-volatile memory such as a ROM and a flash memory, and a portable recording medium such as a CD-ROM can be applied. is there. A carrier wave is also used as a medium for providing program data via a communication line.

G Image forming apparatus 11 Control unit 12 Storage unit 18 Image memory 20 Image forming unit 21 Writing unit 30 Exposure unit 31 Light source 34 Polygon mirror 22 Photoconductor

Claims (6)

A plurality of writing units that irradiate a plurality of laser beams in parallel, deflect each laser beam in the main scanning direction by a polygon mirror, expose the photosensitive member, and then develop, and each screen has a different color screen pattern An image forming unit that forms and superimposes the images of
A control unit that changes the number of laser beams irradiated in parallel during the exposure when the screen pattern of any two of the different color images is axisymmetric through the main scanning direction; Prepared,
The control unit generates density unevenness from a screen period in which the images of the screen patterns of the two colors are repeatedly overlapped in the sub-scanning direction and a jitter period in which jitter of the laser beam deflected by the polygon mirror is repeatedly generated in the sub-scanning direction. An image forming apparatus comprising: determining a period, and changing the number of the laser beams so that the period of density unevenness is outside a visible range determined in advance according to visual sensitivity.   The controller determines one or a plurality of jitter periods to be predicted by a product of the number of the plurality of laser beams and each of one or a plurality of natural numbers that can divide the number of mirrors of the polygon mirror, 2. The image forming apparatus according to claim 1, wherein the number of the laser beams is changed so that all the density unevenness periods determined from the screen period and each jitter period are outside the visible range.   The control unit changes the exposure condition of the laser beam according to the number of laser beams after the change so that the time required for image formation does not change before and after changing the number of laser beams. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein the control unit changes the number of the laser beams in both of the writing units that respectively form the two-color images.   The image forming apparatus according to claim 1, wherein the control unit changes the number of the laser beams in a writing unit that forms one of the two color images. An exposure control method for a plurality of writing units that irradiates a plurality of laser beams in parallel, deflects each laser beam in a main scanning direction by a polygon mirror, exposes a photosensitive member, and develops the photosensitive member.
When the screen pattern of any two colors among the images of different color screen patterns formed by the plurality of writing units is axisymmetric with respect to the main scanning direction, irradiation is performed in parallel during the exposure. Including changing the number of laser beams,
The process includes
Determining a screen period in which the images of the two-color screen patterns repeatedly overlap in the sub-scanning direction;
Determining a jitter period in which jitter of the laser beam deflected by the polygon mirror repeatedly occurs in the sub-scanning direction;
Determining a density unevenness period from the screen period and the jitter period;
Changing the number of the laser beams such that the period of density unevenness is outside a visible range determined in advance according to visual sensitivity;
An exposure control method, further comprising: