JP2013007799A - Image-forming device and image-forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image-forming device capable of further controlling a variation in density of an image to be formed by multiple light beams.SOLUTION: An image-forming device executes steps of: with respect to data for driving a first light-emitting element for emitting a laser beam L1, correcting a gradation value of data for driving a second light-emitting element for emitting a laser beam L2 using a graph 152 that shows correction data for correcting a density difference caused by a difference in a beam intensity distribution between the laser beams L1 and L2; and driving the second light-emitting element in a modulated manner on the basis of data showing the corrected gradation value, and causing the second light-emitting element to emit the laser beam L2.

Description

  The present invention relates to an image forming apparatus and an image forming method for forming an electrostatic latent image on an image carrier by exposing and scanning the same image carrier with a plurality of light beams.

A plurality of laser beams emitted from a plurality of light sources are deflected by a reflecting surface of a rotating polygon mirror, and the plurality of laser beams are simultaneously applied to the same image carrier such as a photosensitive drum via a scanning lens. A so-called multi-beam exposure type image forming apparatus has been developed in which exposure scanning is performed to form an electrostatic latent image on the image carrier.
In the multi-beam exposure type image forming apparatus, a plurality of laser beams are simultaneously deflected by one reflecting surface of a polygon mirror, so that the number of laser beams is several times faster than a configuration using one laser beam. Thus, an electrostatic latent image can be formed on the image carrier, and the time required for image formation can be shortened, and the productivity of image formation can be greatly improved.

In Patent Document 1, in a multi-beam exposure type image forming apparatus including two light sources LD1 and LD2, the current flowing to LD1 is fixed and the current flowing to LD2 is changed, so that the amount of light of LD2 with respect to the amount of light of LD1 is changed. A configuration is disclosed in which image patterns of isolated dots having different ratios are printed at different ratios.
For each image pattern of each ratio, the ratio at which the difference between the density of isolated dots by LD1 and the density of isolated dots by LD2 is minimized is determined as the adjustment ratio, and LD1 and LD2 are caused to emit light at the determined ratio. By controlling so that the dot reproduction by the laser beams from LD1 and LD2 is the same.

JP 2008-134326 A

In the above configuration, the light amount of the two light sources LD1 and LD2, for example, the difference between the maximum values can be reduced, but the light intensity distribution of each laser beam can be reduced even if the difference between the maximum values of the light amounts can be reduced. Are different from each other, there is a problem that reproduced images, in particular, the halftone density is likely to vary between the laser beams of LD1 and LD2.
That is, the laser beams emitted from the light sources have different light intensity distributions even though they are slight. If the light intensity distributions differ between the laser beams, the photosensitive member can be adjusted even if the maximum amount of light is adjusted to the same value. Of the exposure area of one dot by the laser beam on the drum, the depth of the electrostatic latent image (potential difference between the exposed part and the non-exposed part) around the maximum value (peak) of the light amount is the distance between the laser beams. It will be different from each other.

  If the depth of the electrostatic latent image is different between the laser beams, even if an attempt is made to form a halftone image having the same density by each laser beam from LD1 and LD2, the depth of the electrostatic latent image is reduced in the development process. Depending on the magnitude of the difference, there is a difference between the amount of toner adhering to the electrostatic latent image by the laser beam from LD1 and the amount of toner adhering to the electrostatic latent image by the laser beam from LD2, and there is a difference between the laser beams. Therefore, the density of the halftone reproduced image tends to vary.

Such density variations in the reproduced image between the laser beams are particularly likely to occur when multi-level exposure is performed by halftone reproduction such as dithering or error diffusion or pulse width modulation.
The present invention has been made in view of the above problems, and provides an image forming apparatus and an image forming method that can further suppress variation in density of images formed by a plurality of light beams. It is an object.

  In order to solve the above problems, an image forming apparatus according to the present invention emits first and second light beams from first and second light sources based on image data, and the emitted first and second light beams. An image forming apparatus that exposes and scans the same image carrier to form an electrostatic latent image on the image carrier, and develops the formed electrostatic latent image to form an image. Each of a plurality of different gradation values including halftone in the image data for driving the image is corrected to a gradation value for correcting a density difference of a reproduced image caused by a difference in light intensity distribution with the other light source. Generating means for generating correction data to be corrected, and correcting means for correcting a gradation value of image data for driving one light source by the correction data.

  In addition, the generation unit generates, on the image carrier or the recording sheet, a first pattern image including n patches corresponding to n (an integer of 1 or more) different gradation values by the first light beam. Pattern forming means for forming and forming a second pattern image including n patches corresponding to the same gradation value as the n by the second light beam, and the formed first and second patterns Detection means for detecting an image, and detection of patches formed by image data indicating the same gradation value among n patches included in each of the formed first and second pattern images The correction data may be generated based on the result.

  Here, the correction data is obtained from a difference between detection results of patches formed by image data having the same gradation value, and is an inverse characteristic of a characteristic indicating a relative light amount difference between one light source and the other light source. The correction means may correct the gradation value of the image data for driving one light source by using the data indicating the reverse characteristic so that the difference is eliminated.

Further, screen processing means for correcting the gradation value of the image data corrected by the correction means by screen processing using a predetermined gradation reproduction method, and driving the one light source based on the image data after the screen processing. The light source driving means for emitting a light beam from the one light source may be provided.
Further, the image processing apparatus includes screen processing means for correcting the gradation value of the image data for driving one light source by screen processing using a predetermined gradation reproduction method before driving, and an image for driving one light source. The data is image data after screen processing, and the gradation value of the image data after screen processing is an exposure step corresponding to the amount of light beam emitted from the one light source, and the correction means includes the exposure It may be light source driving means that varies the amount of light beam emitted from the one light source based on the step size, and the exposure step may be corrected using the correction data.

The n is a plurality, and each of the plurality of patches has a plurality of dots formed in a certain area on the image carrier, and each dot in the certain area has a different gradation value. At least one of the generation ratio and the generation frequency may be different.
Here, each of the first and second pattern images includes a plurality of rows in which a plurality of dots are connected in the main scanning direction and a linear first that is arranged at intervals in the sub-scanning direction. There are a plurality of patches, a single block formed by connecting a plurality of dots, a second patch in the form of dots scattered at regular intervals in the main scanning direction and the sub-scanning direction, and a single isolated dot. It is possible to include at least one of a plurality of dot-like third patches that are scattered at regular intervals in the main scanning direction and the sub-scanning direction.

  In addition, the first pattern image is formed by changing the amount of light emitted from the first light beam when forming one dot included in the patch for each patch having different gradation values. The pattern image is formed by varying the light emission amount of the second light beam when forming one dot included in the patch for each patch having different gradation values, and the first and second patterns. Of the plurality of patches included in each of the images, the light emission amount of the light beam may be set to be the same for patches formed by image data indicating the same gradation value.

  Here, screen processing means for correcting the gradation value of the image data for driving one light source by screen processing using a predetermined gradation expression method before the driving is provided, and the plurality of patches are provided with dot-like shapes. When the same patch value is included, the dot pattern in the dotted patch has the same or approximate pattern shape as the dot pattern after the screen processing when the same gradation value is reproduced by the screen processing. It is also good.

The n may be 1, and one gradation value may be a value corresponding to the maximum density, or a value in a range from a gradation value corresponding to the maximum density to an intermediate value.
Further, based on the detection result between the patches, a difference between the light amounts of the first light beam and the second light beam is obtained, and when the difference between the light amounts is larger than a predetermined value, a warning is issued to indicate that effect. Means may be provided.

  In the present invention, first and second light beams are emitted from first and second light sources based on image data, and the same image carrier is exposed and scanned by the emitted first and second light beams. An image forming method in an image forming apparatus for forming an electrostatic latent image on the image carrier and developing the formed electrostatic latent image to form an image, wherein image data for driving one light source A generation step of generating correction data for correcting each of a plurality of different gradation values including a halftone into a gradation value for correcting a density difference of a reproduced image caused by a difference in light intensity distribution with the other light source And a correction step of correcting the gradation value of the image data for driving one of the light sources using the correction data.

  By doing so, a plurality of different gradation values including halftones in the image data for driving one light source is eliminated so that there is no density difference in the reproduced image caused by the difference in light intensity distribution with the other light source. Compared to a configuration that adjusts so as to reduce the difference in the maximum amount of light between the first and second light beams, density variation including a halftone between the light beams is suppressed. Thus, the reproducibility of the formed image can be improved.

1 is a schematic configuration diagram illustrating a configuration of a printer according to a first embodiment. It is a block diagram which shows the structure of the control part with which a printer is equipped. It is a flowchart which shows the content of the correction data generation process for Y color. FIG. 4 is a diagram illustrating an example of a toner pattern formed on a surface of an intermediate transfer belt provided in a printer. FIG. 3 is an enlarged view showing a toner pattern. It is a figure which shows the example of the graph of the correction data for Y color. It is a block diagram which shows the structure of the gradation correction | amendment part with which a control part is provided. It is a figure which shows the structure of the gradation correction | amendment part as a comparative example. 6 is a block diagram illustrating a configuration of an LD driving unit according to Embodiment 2. FIG. It is a figure which shows the correction data for Y color in a table format. 6 is a diagram illustrating an example of a Y-color toner pattern according to Embodiment 3. FIG. FIG. 6 is a diagram illustrating another example of a Y toner pattern. FIG. 6 is a diagram illustrating still another example of a Y toner pattern. 10 is a flowchart showing the content of correction data generation processing for Y color according to the third embodiment. It is a figure which shows the example of the correction data for Y color. It is a figure which shows the example of the correction data which concern on a modification.

Hereinafter, embodiments of an image forming apparatus and an image forming method according to the present invention will be described by taking a tandem color printer (hereinafter simply referred to as “printer”) as an example.
<Embodiment 1>
(1) Overall Configuration of Printer FIG. 1 is a diagram illustrating the overall configuration of the printer 1.

  As shown in the figure, the printer 1 forms an image by a known electrophotographic method, and includes an image forming unit 10, a feeding unit 20, a control unit 30, and a fixing unit 40, and a network ( For example, when an execution instruction for a print (print) job is received from an external terminal device (not shown) connected to the LAN, yellow (Y), magenta (M), cyan (C), and cyan (C) are received based on the instruction. A color image formed of black (K) is executed.

The image forming unit 10 includes image forming units 10Y, 10M, 10C, and 10K corresponding to Y to K colors, an intermediate transfer belt 16, a print head 18, and the like.
The image forming unit 10Y includes a photosensitive drum 11Y, a charger 12, a developing unit 13, a primary transfer roller 14, and a cleaner 15 for cleaning the photosensitive drum 11 disposed around the photosensitive drum 11Y. A Y-color toner image is formed on the body drum 11Y. This configuration is the same for the other image forming units 10M to 10K, and the reference numerals are omitted in FIG. A toner image of a color corresponding to each image forming unit is formed on each of the photosensitive drums 11M, 11C, and 11K.

The intermediate transfer belt 16 is an endless belt, is stretched around a driving roller 161 and a driven roller 162, and runs around in a direction indicated by an arrow A in FIG.
The print head (light beam scanning unit) 18 is disposed below the image forming units 10Y to 10K and above the feeding unit 20, and has two laser beams for each of the photosensitive drums 11Y to 11K. Light emitting elements LD1Y and LD2Y that emit L1 and L2, LD1M and LD2M, LD1C and LD2C, and LD1K and LD2K are included.

The laser beams L1 and L2 emitted from the light emitting elements LD1Y to LD2K are deflected by a deflector such as a polygon mirror (not shown), then pass through a scanning lens such as an fθ lens, and are reflected by a reflecting mirror or the like, and corresponding photosensitive light. The body drums 11Y to 11K are scanned in the main scanning direction (direction perpendicular to the paper surface) to form electrostatic latent images on the photoreceptor drums 11Y to 11K.
The feeding unit 20 feeds the sheets S accommodated in the sheet feeding cassette 21 one by one onto the conveyance path 29, and drives the fed sheets S across the intermediate transfer belt 16 at the secondary transfer position 26. It is conveyed to the secondary transfer roller 25 that is in contact with 161.

The fixing unit 40 includes a fixing roller and a pressure roller, and heats and presses the paper S at a predetermined fixing temperature to fix the toner image on the paper S.
The control unit 30 converts the image signal from the external terminal device into a digital signal for Y to K colors, generates image data for driving the light emitting elements LD1Y to LD2K of the print head 18, and the image data Thus, the light emitting elements LD1Y to LD2K are driven. As a result, the laser beams L1 and L2 are emitted from the light emitting elements LD1Y to LD2K, and the photosensitive drums 11Y to 11K are exposed and scanned by the laser beams L1 and L2.

Prior to this exposure scanning, each of the photosensitive drums 11Y to 11K is uniformly charged by the charger 12, and electrostatic latent images are formed on the photosensitive drums 11Y to 11K by the exposure of the laser beams L1 and L2. Is formed.
The electrostatic latent images on the photoconductive drums 11Y to 11K are developed with toner by the corresponding developing unit 13. The toner images of the respective colors are primarily transferred onto the intermediate transfer belt 16 by the corresponding primary transfer rollers 14. At this time, the image forming operations for the respective colors are executed at different timings so that the toner images are superimposed and transferred at the same position on the intermediate transfer belt 16.

In synchronization with the timing of the image forming operation, the sheet S is fed from the feeding unit 20 on the conveyance path 29, and the sheet S and the intermediate transfer belt 16 that travels around and the secondary transfer are transferred. The toner image on the intermediate transfer belt 16 is secondarily transferred onto the paper S by the secondary transfer roller 25 at the secondary transfer position 26 while being sandwiched between the rollers 25.
The sheet S that has passed through the secondary transfer position 26 is conveyed to the fixing unit 40, where the toner image is heated and pressurized and fixed on the sheet S, and then discharged through the discharge roller 27, and the storage tray 28. Is housed in.

A toner pattern detection sensor 19 is disposed at a position between the image forming unit 10 </ b> K and the secondary transfer position 26 in the running direction of the intermediate transfer belt 16.
The toner pattern detection sensor 19 is a reflective optical sensor that detects toner patterns P1 and P2 (see FIG. 4) as pattern images formed on the surface of the intermediate transfer belt 16 by primary transfer. The toner pattern detection sensor 19 irradiates the toner patterns P1 and P2 formed on the surface of the intermediate transfer belt 16 with light, receives the reflected light, and converts it into an electrical signal indicating the amount of received light. This is sent to the control unit 30 as a detection signal.

  The control unit 30 obtains the densities of the toner patterns P1 and P2 actually formed on the surface of the intermediate transfer belt 16 based on the received detection signal, and includes a halftone from the obtained densities of the toner patterns P1 and P2. Correction data for correcting each of a plurality of different gradation values so as to eliminate a density difference of a reproduced image caused by a difference in light intensity distribution of each light emitting element is generated. In the actual printing operation, the gradation value is corrected using the correction data, thereby improving the gradation reproducibility of the output reproduced image.

As a halftone, for example, in the case of 256 gradations, gradation values belonging to a predetermined range before and after the intermediate value 128 can be used, but within an intermediate range excluding the minimum value and the maximum value. It can also be understood as a gradation value. Details of generation of correction data will be described later.
The toner patterns P1, P2 formed on the surface of the intermediate transfer belt 16 are removed by a cleaner (not shown) after detection by the toner pattern detection sensor 19.

(2) Configuration of Control Unit 30 FIG. 2 is a block diagram showing the configuration of the control unit 30.
As shown in the figure, the control unit 30 includes, as main components, a CPU 101, a communication interface (I / F) unit 102, an image processing unit 103, an image memory 104, a gradation correction unit 105, a screen processing unit 106, An LD driving unit 107, a ROM 108, a RAM 109, a pattern data storage unit 110, a γ correction data storage unit 111, a correction data generation unit 112, a first correction data storage unit 113, a second correction data storage unit 114, and the like are provided.

The communication interface unit 102 is an interface for connecting to a LAN such as a LAN card or a LAN board, receives print job data from the outside, and sends the received data to the image processing unit 103.
The image processing unit 103 converts the print job data from the communication interface unit 102 into image data of reproduction colors Y to K, outputs the image data to the image memory 104, and stores the image data for each reproduction color.

The CPU 101 reads out image data from the image memory 104 and sends the read image data to the gradation correction unit 105 when printing is performed.
The gradation correction unit 105 corrects the gradation value (for example, 256 gradations) of each pixel of the image data sent from the image memory 104 at the time of printing using the correction data, and after the correction. The image data is sent to the screen processing unit 106.

The screen processing unit 106 converts the image data from the gradation correction unit 105 into multi-value image data using a predetermined multi-value gradation reproduction method, for example, an 8-value dither method or an error diffusion method, The multi-valued image data is sent to the LD driving unit 107.
Based on the multivalued image data from the screen processing unit 106, the LD driving unit 107 modulates and drives the LD1Y to LD2K of the print head 18 for each color to emit the laser beams L1 and L2. Here, multi-valued image data consists of 8 values in pixel units, and the amount of light emitted from the light emitting element is 8 by the so-called laser pulse width modulation method (PWM) in which the light emission time (exposure area) for one pixel is variable. The exposure step can be switched to eight steps (including zero (extinguishment) and the maximum value) corresponding to the value.

The light emission amount (light emission time) changes by switching the exposure step to each stage, and the density of the reproduced image can be changed by changing the light emission amount unless changing the image formation conditions such as charging, development, and transfer other than exposure. Therefore, the exposure step can be regarded as a gradation value.
Note that the modulation method is not limited to the PWM method as long as the modulation method can realize multi-level gradation, and for example, a so-called laser intensity modulation method in which the light emission intensity is varied with a constant exposure time for one pixel may be used.

The pattern data storage unit 110 stores pattern data for forming the toner patterns P1 and P2.
The correction data generation unit 112 executes correction data generation processing for generating correction data for correcting the light amount of the laser beam L2 with respect to the laser beam L1 for each of Y to K colors.
FIG. 3 is a flowchart showing the content of the correction data generation process for Y color.

As shown in the figure, the pattern data is read from the pattern data storage unit 110 (step S1), the toner pattern P1 is formed on the surface of the intermediate transfer belt 16 using the light emitting element LD1Y (step S2), and the light emitting element LD2Y is formed. The toner pattern P2 is used to form the surface of the intermediate transfer belt 16 (step S3).
Specifically, the toner pattern forming image data stored in the pattern data storage unit 110 is sent to the LD drive unit 107 (signal line 115 in FIG. 2). Based on the image data, the LD drive unit 107 and The image forming unit 10Y forms toner patterns P1 and P2 on the surface of the intermediate transfer belt 16 through the same steps of charging, exposure, development, and primary transfer as in the printing operation.

FIG. 4 is a view showing an example of toner patterns P1, P2 formed on the surface of the intermediate transfer belt 16, and FIG. 5 is an enlarged view showing the toner patterns P1, P2. In both figures, only the Y color toner patterns P1 and P2 of Y to K are shown, and the other colors are not shown, but have the same pattern shape as the Y color.
The toner pattern P1 is formed by the laser beam L1 from the light emitting element LD1Y as shown in FIG. 5A, and the toner pattern P2 is formed by the laser beam L2 from the light emitting element LD2Y as shown in FIG. 5B. Is done. In both figures, one rectangle indicated by a broken line indicates the size of one pixel.

The toner pattern P1 includes a plurality of patches 81, 82, and 83 having different densities, and the patches 81, 82, and 83 include linear portions 91, 92, and 93 in which a plurality of pixels are continuous in the main scanning direction. A plurality of lines are formed at intervals of one scanning line (corresponding to one pixel) in the sub-scanning direction, and are formed by varying the exposure step for the light emitting element LD1Y for each patch.
The patch 81 is an exposure step 6, the patch 82 is an exposure step 4, and the patch 83 is an exposure step 1, and each is formed by a laser beam L 1 having a light amount corresponding to the exposure step. In the figure, for the sake of easy understanding that there is a difference in the exposure amount by the laser beam for each patch, for the sake of convenience, it is shown by the difference in density of the solid when the entire area for one pixel is exposed. . In the case of the PWM method, each patch is formed by actually changing the light emission time of the laser beam in accordance with the magnitude of the gradation value in units of one pixel (here, one dot). This is the same for the toner patterns described in each drawing described later.

Since the density of the reproduced image increases as the exposure step value increases, the density increases step by step in the order of the patches 83, 82, 81. It should be noted that the image forming conditions other than changing the light amount of the laser beam, such as charging and developing bias voltage, are the same. The same applies to the toner pattern P2.
The toner pattern P2 includes a plurality of patches 84, 85, and 86 having different densities, and the patches 84, 85, and 86 have linear portions 94, 95, and 96 in which a plurality of pixels are continuously arranged in the main scanning direction. A plurality of lines are formed at intervals of one scanning line in the sub-scanning direction, and are formed by varying the exposure step for the light emitting element LD2Y for each patch.

The patch 84 is the same exposure step 6 as the patch 81, the patch 85 is the same exposure step 4 as the patch 82, and the patch 86 is the same exposure step 1 as the patch 83, each of which corresponds to the exposure step. Formed by the laser beam L2. The density increases stepwise in the order of the patches 86, 85, 84.
As shown in FIG. 4, the patches 81 to 86 of the toner patterns P1 and P2 of the respective colors formed on the surface of the intermediate transfer belt 16 are rotated by the toner pattern detection sensor 19 along the broken lines in FIG. This detection signal is sent to the correction data generation unit 112.

  The correction data generation unit 112 receives the detection signal from the toner pattern detection sensor 19 in step S4 shown in FIG. 3, and in step S5, the toner pattern P1 formed on the surface of the intermediate transfer belt 16 from the received detection signal. , P2 patches 81 to 86 are obtained, and based on the obtained densities of the patches 81 to 86, correction data for correcting the light amount of the laser beam L2 with respect to the laser beam L1 is generated for the Y color.

FIG. 6 is a diagram illustrating an example of a graph of correction data for Y color. The horizontal axis of the graph in the figure represents the gradation value (0 to 255) before correction, and the vertical axis represents the gradation value after correction.
The graph 151 indicating the correction data for the laser beam L1 serves as a reference for the correction data for the laser beam L2. Here, the graph is a directly proportional linear shape in which the gradation value is the same before and after correction. It is represented by

On the other hand, the graph 152 indicating the correction data for the laser beam L2 is an example of two graphs 152a and 152b. Unlike the graph 151, the gradation value before correction and the gradation value after correction are shown. Is represented by a curve including a different portion, a so-called γ curve.
If the laser beams L1 and L2 exhibit the same light intensity distribution characteristics, there is no relative light amount difference between the laser beams L1 and L2 when reproducing an image having the same gradation value. L1 and L2 should have the same shape, but in this embodiment, since there is a certain difference in light intensity distribution characteristics, the graph 152 for the laser beam L2 is changed to the graph 151 for the laser beam L1. On the other hand, it has a different shape.

A graph 152a is an example in the case where the light emitting element LD2Y having the characteristic that the light amount of the laser beam L2 is smaller than the laser beam L1 in the middle tone, particularly in the range of about 128 in the gradation value, is used. An example of correction data for performing correction to increase the gradation level by the amount opposite to the characteristic, that is, the amount of light is reduced is shown.
On the contrary, the graph 152b is an example in the case where the light emitting element LD2Y having the characteristic that the light amount of the laser beam L2 is larger than that of the laser beam L1 in a halftone is used. That is, an example of correction data for performing correction to lower the gradation level by the amount of light is increased.

Specifically, the correction data is generated as follows.
That is, (a) from the density detection result of the toner pattern detection sensor 19, the density difference Δ1 between the patch 81 and the patch 84 having the same gradation value, the density difference Δ2 between the patch 82 and the patch 85 having the same gradation value, A density difference Δ3 between the patch 83 and the patch 86 having the same gradation value is obtained. The density difference Δ1 is positive if the patch 84 has a lower density than the patch 81, and negative if the density is high. The same applies to the density differences Δ2 and Δ3.

  (B) The correlation between the gradation value and the exposure step and the correlation between the gradation value and the density difference are obtained in advance, and the gradation value a corresponding to the exposure step 6 on the directly proportional graph 151 shown by the broken line (FIG. 6). ) Plus the gradation value a2 (FIG. 6) corresponding to the density difference Δ1 and the gradation value b corresponding to the exposure step 4 (FIG. 6) and the level corresponding to the density difference Δ2. The point b1 (FIG. 6) obtained by adding the tone values b2 (FIG. 6), and the gradation value c2 (FIG. 6) corresponding to the density difference Δ3 to the gradation value c (FIG. 6) corresponding to the exposure step 1 are added. The three points c1 (FIG. 6) are connected by a line by spline interpolation, polynomial approximation, or the like.

(C) The connected line is represented by a mathematical expression or a table in which gradation values before and after correction are associated, and the data represented by the represented mathematical expression or table is used as correction data.
This correction data is data indicating the reverse characteristic of the characteristic indicating the relative light amount difference between the light emitting elements LD1Y and LD2Y, and includes a plurality of different gradation values including halftones. Therefore, if this correction data is used, The gradation value can be corrected so that the relative difference disappears, and not only the maximum density but also the halftone can suppress the density difference of the reproduced image due to the difference in the beam intensity distribution of the laser beams L1 and L2. it can.

Returning to FIG. 3, in step S6, the generated correction data is stored as correction data for the laser beam L2 in the second correction data storage unit 114, and the process ends.
For example, when the power switch is turned on, every time a predetermined number of prints are executed, when the fluctuation amount of the environment surrounding the device (such as temperature and humidity) exceeds a predetermined range, or when returning from a trouble such as a failure It is executed each time. Each time this process is executed, new correction data is generated and overwritten and saved in the second correction data storage unit 114.

  Note that when the density difference Δ of patches having the same gradation value is obtained, a warning may be given to the user if the density difference Δ is larger than a predetermined value. The large density difference Δ means that there is a light amount difference between the laser beams L1 and L2, and if the light amount difference is too large, the gradation correction may not be able to be corrected. From this, the density difference can be regarded as a difference between the light amounts of the laser beams L1 and L2 and the difference between the light amounts is obtained. The predetermined value can be an upper limit value of the density difference Δ within a range where gradation correction can be performed, and can be obtained in advance by an experiment or the like. The warning to the user can be performed, for example, by displaying a warning message on an operation unit (not shown) or outputting a sound indicating the warning.

Further, data indicating a directly proportional straight line, which is the graph 151 for the laser beam L1, is stored in advance in the first correction data storage unit 113 as correction data for the laser beam L1. The correction data for Y color has been described above, but correction data for that color is generated in the same manner for other M to K colors.
The generation method of the correction data is not limited to the above method, and other approximate expressions may be used. Further, in the above description, the patches 81 to 83 and 84 to 86 are formed by the three exposure steps (6, 4, 1) as the toner patterns P1 and P2. Alternatively, a configuration may be used in which a plurality of patches having different gradation values are formed by, for example, four or more exposure steps. As the number of different gradation values increases, the number of points a1, b1,... Shown in FIG. 6 increases accordingly, and the shape of the γ curve can be brought closer to the actual exposure conditions, and the accuracy of correction using correction data. Will improve.

Although the method for generating correction data for Y color has been described above, correction data for that color is also generated for the other M to K colors by the same method as that for Y color.
Returning to FIG. 2, the γ correction data storage unit 111 stores γ correction data for γ correction for each of the image forming units 10Y to 10K. γ correction is a graph of a curve that deviates from the original proportional straight line graph when the gradation (density) to be reproduced and the gradation (density) of the output image that is actually reproduced are not exactly proportional. On the other hand, this is a process for improving gradation reproducibility by obtaining a graph showing the reverse characteristic and correcting the deviation from the original straight line using the obtained graph.

  A graph showing the reverse characteristic corresponds to γ correction data. Here, the γ correction data includes a plurality of toner patterns (not shown) having different gradation values using only the laser beam L1 for each of the colors Y to K. A plurality of toner patterns formed on the surface of the intermediate transfer belt 16 are detected by the toner pattern detection sensor 19, and a graph of a curve in which the density value deviates from the original proportional straight line is obtained from the detection result. The graph is generated by obtaining a graph of the inverse characteristic of the obtained graph. This inverse characteristic graph has a shape similar to the graph 152 shown in FIG.

The γ correction data is generated for each of the colors Y to K when the printer 1 is manufactured, and the generated γ correction data for each color is stored in the γ correction data storage unit 111.
The ROM 108 stores a program related to an image forming operation and a feeding operation in the image forming unit 10 and the feeding unit 20. The CPU 101 reads a necessary program from the ROM 108 and controls the operations of the image forming unit 10 and the feeding unit 20 in a unified manner with timing to execute a smooth printing operation. The RAM 109 is a work area for the CPU 101.

(3) Configuration of Tone Correction Unit 105 FIG. 7 is a block diagram showing the configuration of the tone correction unit 105.
As shown in the figure, the tone correction unit 105 includes a Y correction unit 121, an M correction unit 122, a C correction unit 123, and a K correction unit 124.
The Y correction unit 121 is a correction unit that corrects the gradation value of the Y color image data based on the Y color correction data. The M correction unit 122 is a correction unit that corrects the gradation value of the M color image data based on the correction data for M color, and the C correction unit 123 sets the gradation value of the C color image data for C color. The K correction unit 124 is a correction unit that corrects the gradation value of the K color image data based on the K color correction data. Since the Y correction unit 121 to the K correction unit 124 have basically the same configuration, only the configuration of the Y correction unit 121 will be described here, and the description of the configuration of the M correction unit 122 to K correction unit 124 will be omitted. To do.

The Y correction unit 121 includes a γ correction unit 131, a data separation unit 132, a first gradation correction unit 133, a second gradation correction unit 134, and a data synthesis unit 135.
The γ correction unit 131 corrects the tone value of the Y color image data using the Y color γ correction data stored in the γ correction data storage unit 111, and converts the corrected image data into the data separation unit 132. Send to.

The data separation unit 132 divides the first data into the first gray level by dividing the Y-color image data into first data for driving the light emitting element LD1Y and second data for driving the light emitting element LD2Y. The data is sent to the correction unit 133, and the second data is sent to the second gradation correction unit 134.
For example, when the image data for one page of the document has a data structure in which scanning line numbers are arranged in order of scanning lines 1, 2, 3,..., The first data is an odd number of 1, 3, 5,. The second scan line can be used for exposure scanning, and the second data can be used for exposure scanning of even-numbered scan lines 2, 4, 6,.

The first gradation correction unit 133 corrects the gradation value of each pixel of the first data using correction data for the Y-color laser beam L1 (data indicated by the graph 151 in FIG. 6). This correction data is read from the first correction data storage unit 113.
In the case of the graph 151 shown in FIG. 6, the first gradation correction unit 133 does not change the gradation value before and after correction, but since the γ correction unit 131 has already performed γ correction, the laser beam L1 is used. The gradation correction is actually executed.

The second gradation correction unit 134 corrects the gradation value of each pixel of the second data using correction data for the Y-color laser beam L2 (data indicated by the graph 152 in FIG. 6). This correction data is read from the second correction data storage unit 114.
Thereby, the gradation value of each pixel of the second data is eliminated so that the density difference of the reproduced image caused by the difference in the beam intensity distribution of the laser beam L2 with respect to the laser beam L1 that occurs when the reproduced image is output. It is corrected.

  The data synthesis unit 135 synthesizes the corrected first data and second data, returns the image data to the data structure before separation, and sends the image data to the screen processing unit 106. In this synthesis, the first data and the second data in the same page are synthesized so that the line numbers are in order in units of scanning lines. Therefore, the transmission of the first data and the second data from the first gradation correction unit 133 and the second gradation correction unit 134 to the data composition unit 135 is performed at substantially the same speed in units of scanning lines, and the first data and the first data are transmitted. When there is a difference between the transmission rates of the two data, it is not necessary to adopt a configuration in which the faster data is temporarily stored in the buffer memory or the like in order to match the faster data with the slower data.

  In the case of adopting a configuration in which a buffer memory is provided, for example, without providing the first gradation correction unit 133, the first data is sent as it is to the data synthesis unit 135 and is temporarily stored in the buffer memory. The second data is corrected by the second gradation correction unit 134, and the first data is sent from the buffer memory to the data synthesis unit 135 in units of scanning lines at the timing when the second data is output from the second gradation correction unit 134. Also good.

The screen processing unit 106 converts the Y-color image data into multi-value image data using a multi-value gradation reproduction method, in the above example, an 8-value dither method or an error diffusion method.
The multi-value image data of Y color is sent to the LD drive unit 107, and the LD drive unit 107 modulates and drives the Y color light emitting element LD1Y based on the image data corresponding to the first data to emit the laser beam L1. Then, based on the image data corresponding to the second data, the Y light emitting element LD2Y is modulated and driven to emit the laser beam L2.

FIG. 8 is a diagram illustrating a configuration of a gradation correction unit 190 as a comparative example.
As shown in the figure, the gradation correction unit 190 includes a Y correction unit to a K correction unit, and the Y correction unit includes only the γ correction unit 195, and the first gradation correction unit 133 and the above-described first correction unit 133. The second gradation correction unit 134 is not provided. The γ correction unit 195 corresponds to the γ correction unit 131 described above and can perform so-called γ correction. However, since the first gradation correction unit 133 and the second gradation correction unit 134 are not provided, the laser beam L1, The density difference of the reproduced image due to the difference in the light intensity distribution between L2 cannot be corrected, and in particular, the halftone gradation reproducibility cannot be improved.

  On the other hand, in the present embodiment, even if there is a difference in the beam intensity distribution between the laser beams L1 and L2, the first gradation correction unit 133 and the second gradation correction unit 134 shown in FIG. To reproduce the density image, the gradation value including the halftone can be corrected so that the density difference of the reproduced image caused by the difference in the beam intensity distribution of the laser beam L2 with respect to the laser beam L1 is eliminated. The gradation reproducibility can be further improved.

<Embodiment 2>
In the first embodiment, the gradation value of the image data is corrected by the correction data for each of the Y to K colors. However, in the second embodiment, the exposure of the light emitting elements LD2Y to LD2K is performed with respect to the light emitting elements LD1Y to LD1K. The step is corrected by the correction data, which is different from the first embodiment. Hereinafter, in order to avoid duplication of description, the description of the same contents as those of Embodiment 1 is omitted, and the same components are denoted by the same reference numerals.

FIG. 9 is a block diagram illustrating a configuration of the LD driving unit 207 according to the second embodiment.
As shown in the figure, the LD drive unit 207 includes a Y correction unit 211, an M correction unit 212, a C correction unit 213, and a K correction unit 214. Y to K color image data is input to the LD drive unit 207 from the image memory 104 via the gradation correction unit 190 and the screen processing unit 106.
Note that the gradation correction unit 190 is the same as that shown in FIG. 8 described above, and in the second embodiment, the first gradation correction unit 133 and the second gradation correction unit 134 of the first embodiment are provided. Absent.

Since the Y correction unit 211 to the K correction unit 214 have basically the same configuration, the configuration of the Y correction unit 211 will be described here, and the description of the configuration of the M correction unit 212 to the K correction unit 214 will be omitted. To do.
The Y correction unit 211 includes a data separation unit 220, a first exposure step conversion unit 221, a second exposure step conversion unit 222, and a correction unit 223.

  The data separation unit 220 divides the first data into the first exposure step by dividing the Y-color image data into first data for driving the light emitting element LD1Y and second data for driving the light emitting element LD2Y. The data is sent to the converter 221 and the second data is sent to the second exposure step converter 222. This data separation method may be the same method as the data separation unit 132 of the first embodiment.

The first exposure step conversion unit 221 converts the gradation value of each pixel of the first data into an exposure step value for LD1Y, and modulates and drives the light emitting element LD1Y in units of pixels based on the converted exposure step.
The second exposure step conversion unit 222 converts the gradation value of each pixel of the second data into an exposure step value for LD2Y, and sends the converted value for each pixel of the exposure step to the correction unit 223.

The correction unit 223 corrects the value of the exposure step after conversion for each pixel by using correction data for Y color, and modulates and drives the light emitting element LD2Y in units of pixels based on the corrected exposure step. .
FIG. 10A is a diagram showing correction data for Y color in the form of a table 250, which is data in which numerical values before and after correction of the exposure step are associated with each other and exposure steps 0 to 6 are associated with each other. In the example, the value of the exposure step is corrected to a value larger by one step in the range.

According to this correction data, the value of the exposure step for the pixel of the second data input to the light emitting element LD2Y is changed to a value one greater after correction than before correction within the range of exposure steps 0-6. Will be. Specifically, for example, if the value of the exposure step after conversion by the second exposure step conversion unit 222 is “1”, the correction unit 223 corrects it to “2”.
Since the exposure step is not corrected for the pixel of the first data, increasing the exposure step from the original “1” to “2” for the pixel of the second data means that the light emitting element LD2Y. Means that the light emission amount when emitting light in the same exposure step as that of LD1Y is reduced, and the exposure step is increased by one step to compensate for the reduction in light emission amount. The correction data in FIG. 10A indicates an exposure step having a reverse characteristic to the characteristic of the light emitting element LD2Y, and can be said to correspond to the graph 152a in FIG.

  On the other hand, the table 250 in FIG. 10B shows an example in which the value of the exposure step is corrected to a smaller value by one step in the range of the exposure steps 1-7. This means that the light emitting element LD2Y has a characteristic that the amount of light emitted when emitting light in the same exposure step is larger than that of LD1Y, and the amount of light emitted is increased by lowering the exposure step by one step. It can be said that it corresponds to the graph 152b of FIG.

  Since it can be said that the exposure step represents the gradation as described above, by taking the correlation between the gradation value and the exposure step, as in the first embodiment in which the gradation value of the image data is corrected, Correction data as shown in FIG. 10 can be generated from the detection results of the toner patterns P1 and P2. The generated correction data is overwritten and saved in the second correction data storage unit 114 every time it is generated.

In the above description, the exposure step correction for the Y color image data has been described. However, the exposure step correction for the M to K color image data is also performed in the same manner as for the Y color.
As described above, by adopting a configuration in which the exposure steps of the light emitting elements LD2Y to LD2K are corrected using the correction data, even if there is a difference in the beam intensity distribution between the laser beams L1 and L2, halftone adjustment is performed. The gradation reproducibility including can be further improved.

<Embodiment 3>
In the above embodiment, linear patches along the main scanning direction are formed as the toner patterns P1 and P2. However, in Embodiment 3, not only a linear patch but also a dotted patch is included. Is different in this respect. Further, in the first embodiment, the exposure step of the LD driving unit 107 has eight stages, but in the third embodiment, the exposure step can be switched to 16 stages.

FIGS. 11 to 13 are diagrams showing examples of the configuration of the Y toner pattern in the present embodiment. FIG. 11 shows the Y toner patterns A1 and A2, and FIG. 12 shows the Y toner pattern B1. , B2, and FIG. 13 shows Y toner patterns C1, C2, respectively.
[Toner patterns A1, A2]
As shown in FIG. 11A, the toner pattern A1 includes patches 301 to 304 having different densities, and the patches 301 to 304 each include a plurality of linear portions in which a plurality of pixels are continuous in the main scanning direction. These are formed at intervals of one scanning line in the sub-scanning direction, and are formed by varying the exposure step for the light emitting element LD1Y for each patch.

The patch 301 is an exposure step 4, the patch 302 is an exposure step 8, the patch 303 is an exposure step 12, and the patch 304 is an exposure step 16, each of which is formed by a laser beam L1 having a light amount corresponding to the exposure step. The
Since the density of the reproduced image increases as the value of the exposure step increases, the pattern image has a higher density in the order of the patches 301 to 304. Note that the image forming conditions such as charging and developing bias voltage are the same except that the amount of laser beam light is varied. This is the same for the following toner patterns.

As shown in FIG. 11B, the toner pattern A2 is composed of patches 305 to 308 having different densities, and the patches 305 to 308 have a plurality of linear portions in which a plurality of pixels are continuous in the main scanning direction. These are formed at intervals of one scanning line in the sub-scanning direction, and are formed by varying the exposure step for the light emitting element LD2Y for each patch.
The patch 305 is the same exposure step 4 as the patch 301, the patch 306 is the same exposure step 8 as the patch 302, the patch 307 is the same exposure step 12 as the patch 303, and the patch 308 is the same exposure step as the patch 304. 16, each of which is formed by a laser beam L2 having a light amount corresponding to the exposure step. The pattern data of the toner patterns A1 and A2 is stored in advance in the pattern data storage unit 110. This is the same for the pattern data of the toner patterns B1 to C2.

[Toner patterns B1, B2]
As shown in FIG. 12A, the toner pattern B1 includes patches 311 to 314 having different densities, and is formed by changing the exposure step for the light emitting element LD1Y for each patch.
The patch 311 has a plurality of columns in which blocks each composed of two pixels (dots) adjacent in the main scanning direction are arranged as a unit, and the blocks are arranged at intervals corresponding to two pixels in the main scanning direction. Positions in which the blocks included in one of the two adjacent columns and the blocks included in the other column are alternately shifted in the main scanning direction with an interval corresponding to one pixel in the sub-scanning direction Formed in a staggered relationship. The patches 312 to 314 are also formed in a staggered pattern similar to the patch 311.

The patch 311 is an exposure step 6, the patch 312 is an exposure step 9, the patch 313 is an exposure step 12, and the patch 314 is an exposure step 16, each of which is formed by a laser beam L1 having a light amount corresponding to the exposure step. The
As shown in FIG. 12B, the toner pattern B2 includes patches 315 to 318 having different densities, and is formed by varying the exposure step for the light emitting element LD2Y for each patch.

  The patches 315 to 318 are formed in a zigzag pattern having the same shape as the patches 311 to 314, the patch 315 is the same exposure step 6 as the patch 311, and the patch 316 is the same exposure step 9 and patch 317 as the patch 312. Are the same exposure step 12 as the patch 313, and the patch 318 is the same exposure step 16 as the patch 314, and each is formed by a laser beam L2 having a light quantity corresponding to the exposure step.

In toner patterns B1 and B2, the minimum value of the exposure step is 6, which is larger than the minimum value 4 for toner patterns A1 and A2. This is due to the following reason.
In other words, toner patterns B1 and B2 have only two dots that are continuous, whereas toner patterns A1 and A2 have a shape in which a large number of dots are continuous in a straight line.
In the shape where a large number of dots are continuous as in the toner patterns A1 and A2, a part of the beam spot by the laser beam for exposing one dot for each dot exposes each dot located on both sides of the dot. Therefore, the potential on the photosensitive drum 11Y is easily lowered by being superimposed on the beam spot by the laser beam for the purpose, and even if the minimum value of the exposure step is lowered, the density corresponding to the value is easily reproduced.

  On the other hand, in the shape in which only two dots are continuous as in the toner patterns B1 and B2, the area where the beam spot is superimposed by the laser beam is smaller than in the toner patterns A1 and A2, and therefore the exposure step is minimized. This is because if the value is lowered too much, it becomes difficult to reproduce the density corresponding to the value. The same applies to the next toner patterns C1 and C2. If there is no problem in reproducibility, the minimum value of the exposure step may be the same for each toner pattern.

[Toner patterns C1, C2]
As shown in FIG. 13A, the toner pattern C1 includes patches 321 to 324 having different densities, and is formed by varying the exposure step for the light emitting element LD1Y for each patch.
In the patch 321, one isolated pixel (dot) is scattered in a dot shape, and each of the dots is arranged in a plurality of rows, each having a row corresponding to three pixels in the main scanning direction. It is formed with an interval corresponding to three pixels in the scanning direction. The patches 322 to 324 have the same shape as the patch 321.

The patch 321 is an exposure step 8, the patch 322 is an exposure step 10, the patch 323 is an exposure step 12, and the patch 324 is an exposure step 16, each of which is formed by a laser beam L1 having a light amount corresponding to the exposure step. The
As shown in FIG. 13B, the toner pattern C2 includes patches 325 to 328 having different densities, and is formed by varying the exposure step for the light emitting element LD2Y for each patch.

  The patches 325 to 328 have the same shape as the patches 321 to 324, the patch 325 is the same exposure step 8 as the patch 321, the patch 326 is the same exposure step 10 as the patch 322, and the patch 327 is the same as the patch 323. The exposure step 12 and the patch 328 are the same exposure step 16 as the patch 324, and each is formed by a laser beam L2 having a light amount corresponding to the exposure step.

  In the toner patterns A1 and A2, a large number of dots continuously form a linear portion, and in the toner patterns B1 and B2, two dots continuously form one block, and the toner patterns C1 and C2 In this case, a point is composed of only one dot. When the straight line portion, the block, and the point are image portions, and the length of the image portion per unit length is the generation ratio, the generation ratio increases in the order of toner patterns C, B, and A. If the interval between adjacent image portions is a generated frequency, the generated frequency decreases as the interval increases, and therefore the generated frequency decreases in the order of toner patterns A, B, and C.

The toner patterns A1 and A2 are for generating correction data for a range A of gradation values 64 to 255 out of 256 gradations, and the toner patterns B1 and B2 are for a range B of gradation values 16 to 63. The toner patterns C1 and C2 are for generating correction data for the range C of gradation values 0 to 15.
The reason why the 256 gradations are divided into three ranges and the shape of the toner pattern for generating correction data is made different for each range is to obtain more suitable correction data.

That is, in the dither method and error diffusion method used for multi-level tone reproduction, a low-density image is usually reproduced so as to appear light to the human eye by interspersing a plurality of dots like toner patterns C1 and C2. Then, a method of reproducing a high-density image so that it looks darker to the human eye by using a plurality of dots in succession as in toner patterns A1 and A2 is used.
The fact that a low density image is reproduced with dot-like dots means that if the same dot-like toner pattern is detected and correction data is generated to correct the tone in the low density area, the output during actual printing Correction data can be generated under the same conditions as the image, and the correction by the correction data can be made more suitable for the actual output characteristics to further suppress the density difference of the reproduced image. The same applies to high density images. In this sense, the shape of each patch of the toner pattern to be formed is preferably the same or approximate to the pattern (dot generation ratio or frequency) that is actually reproduced by multi-value gradation reproduction.

  Specifically, for example, low-gradation pixels are continuous in a light background image region. Therefore, when reproducing multi-value gradation, the dot generation rate and generation frequency per unit area are roughly in accordance with the gradation value. Therefore, a pattern in which dots having the same generation ratio and generation frequency are scattered or a pattern approximate to this pattern is prepared. The shape to be approximated can be obtained from an experiment or the like to obtain an effect of suppressing the density difference by matching the actual output characteristics.

In the present embodiment, it is divided into high, medium, and low ranges A to C, and correction data for the ranges is generated using a toner pattern having a shape suitable for each range.
FIG. 14 is a flowchart showing the content of correction data generation processing for Y color in the present embodiment.
As shown in the figure, the pattern data of the toner patterns A1 and A2 are read from the pattern data storage unit 110 (step S31), and the toner pattern A1 is formed on the surface of the intermediate transfer belt 16 using the light emitting element LD1Y (step S32). ), A toner pattern A2 is formed on the surface of the intermediate transfer belt 16 using the light emitting element LD2Y (step S33).

  The toner patterns A1 and A2 formed on the surface of the intermediate transfer belt 16 are detected by the toner pattern detection sensor 19 (step S34), and the toner pattern A1 formed on the intermediate transfer belt 16 from the detection signal of the toner pattern detection sensor 19 is detected. , A2 patches 301 to 308, and the corrected gradation values a1, b1, c1, d1 with respect to the uncorrected gradation values a, b, c, d in the range A are obtained based on the obtained densities. Ask. Here, the gradation values a, b, c, and d before correction indicate gradation values corresponding to the exposure steps 16, 12, 8, and 4 when the toner patterns A1 and A2 are formed. The relationship between the gradation values a, b, c, d and a1, b1, c1, d1 will be described with reference to FIG.

FIG. 15 is a diagram showing an example of a graph 350 showing correction data for Y color.
In the graph 350 shown in the figure, a graph portion 351 belonging to the range A indicates correction data in the range A.
In the range A, the gradation values a, b, c, d indicate the gradation values corresponding to the exposure steps 16, 12, 8, 4 when the toner patterns A1, A2 are formed. On the graph 359 indicating the direct proportion of the broken line, for example, the gradation value a corresponding to the exposure step 16 and the gradation value a2 corresponding to the density difference between the patches 304 and 308 (in the figure, an example of a negative value is shown). The point a1 obtained by adding together is a gradation value a1 after correction with respect to the gradation value a before correction. Similarly, for example, a point d1 obtained by adding the gradation value d corresponding to the density difference between the patches 301 and 305 to the gradation value d corresponding to the exposure step 4 is a corrected level with respect to the gradation value d before correction. The adjustment value d1 is obtained. The method for calculating the gradation value is the same as in the first embodiment.

In this way, corrected gradation values a1, b1, c1, and d1 in the range A are obtained, respectively.
Returning to FIG. 14, in step S36, the pattern data of the toner patterns B1 and B2 are read. In steps S37 and S38, the toner pattern B1 is formed on the surface of the intermediate transfer belt 16 using the light emitting element LD1Y, and the light emitting element LD2Y. Is used to form a toner pattern B2 on the surface of the intermediate transfer belt 16.

  The toner patterns B1 and B2 formed on the surface of the intermediate transfer belt 16 are detected by the toner pattern detection sensor 19 (step S39), and the toner concentrations of the patches 311 to 318 of the toner patterns B1 and B2 are obtained from the detection signals. Based on the obtained density, corrected gradation values a1, b1, c1, and d1 for the gradation values a, b, c, and d in the range B (FIG. 15) are obtained. The gradation values a, b, c, and d in the range B indicate the gradation values corresponding to the exposure steps 16, 12, 9, 6 when forming the toner patterns B1, B2, and the gradation values a1, b1, c1 and d1 represent the gradation values after correction for the gradation values a, b, c, and d (FIG. 15). The calculation method of the gradation values a1 to d1 is the same as the range A.

Subsequently, in step S41, the pattern data of the toner patterns C1 and C2 are read. In steps S42 and S43, the toner pattern C1 is formed on the surface of the intermediate transfer belt 16 using the light emitting element LD1Y, and the light emitting element LD2Y is used. A toner pattern C <b> 2 is formed on the surface of the intermediate transfer belt 16.
The toner patterns C1 and C2 formed on the surface of the intermediate transfer belt 16 are detected by the toner pattern detection sensor 19 (step S44), and the toner density of the patches 321 to 328 of the toner patterns C1 and C2 is obtained from the detection signal. Based on the obtained density, the corrected gradation values a1, b1, c1, d1 for the gradation values a, b, c, d in the range C are obtained (step S45). This calculation method is the same as the ranges A and B.

The corrected gradation values a1, b1, c1, d1 (total of 16 points) calculated in each of the ranges A to C are connected by lines by spline interpolation, polynomial approximation, etc., and data indicating this is connected. It generates as correction data (step S46).
A graph 350 shown in FIG. 15 corresponds to a curve as a result of connecting the corrected gradation values a1, b1, c1, and d1 in each of the areas A to C.

The generated correction data is stored in the second correction data storage unit 114 as correction data for the laser beam L2 (step S47), and the process ends.
In this way, by dividing the 256 gradations into a plurality of ranges, here three, and forming the toner pattern with a pattern that approximates the dot pattern that is actually used to reproduce the gradation for each range, Correction data can be generated under the same conditions as in printing, and the density difference between reproduced images including halftones can be further suppressed.

In the above description, 256 gradations are divided into three ranges. However, the present invention is not limited to this, and a plurality of, for example, four or more ranges may be used.
Further, the exposure step is switched to four stages for the toner patterns A1 and A2, and each of the four patches is formed. However, the number of patches is not limited to this, and one or a plurality of, for example, five exposure steps are used. It is also possible to form five or more patches by switching to more stages. As the number of patches increases, correction data more suitable for actual output characteristics can be generated.

Furthermore, although both the dot generation ratio and the generation frequency are different for each patch, at least one of them may be different. Further, the values of the generation ratio and the generation frequency are not limited to the above, and appropriate values may be set according to the device configuration. The same applies to toner patterns B1, B2 and C1, C2 including point-like patches.
The present invention is not limited to an image forming apparatus, and may be an image forming method for generating correction data. Furthermore, the method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like. In some cases, some or all of the processing may be executed using dedicated hardware or may be executed by software.

<Modification>
Although the present invention has been described based on the above embodiment, it is needless to say that the present invention is not limited to the above embodiment. The following cases are also included in the present invention.
(1) In the above embodiment, n patches corresponding to n (integers of 2 or more) different gradation values are formed for each of the laser beams L1 and L2, and the n patches formed are formed. Although n corrected gradation values for the density of each patch are obtained from the detection result, and correction data is generated by interpolating the n corrected gradation values, a plurality of different gradation values including halftones are generated. If correction data for correcting each of the gradation values can be generated, the value of n is not limited to a plurality, and may be 1, for example.

FIG. 16 is a diagram illustrating correction data graphs 401 and 402 for the laser beam L2 according to the present modification.
In the figure, for each of the laser beams L1 and L2, only a patch having a light amount corresponding to the exposure step 7 is formed, and a corrected gradation value a1 is obtained with respect to the gradation value a before correction of the patch. In this example, the straight line connecting the gradation value a1 and the origin 0 is used as the correction data.

  Similar to the graph 152a in FIG. 6, a graph 401 is an example in which a light emitting element LD2Y having a characteristic that the light amount of the laser beam L2 is less than that of the laser beam L1 in a halftone is used. 6 shows an example in which a light emitting element LD2Y having a halftone and a characteristic that the amount of light of the laser beam L2 is larger than that of the laser beam L1 is used as in the graph 152b of FIG.

If the method of this modification is used, it is only necessary to form one patch for each of the laser beams L1 and L2, and the time and toner required for forming the patch can be improved compared to a configuration in which a plurality of patches having different gradation values are formed. Arithmetic processing for generating consumption and correction data can be reduced.
In the above description, a patch having a gradation value corresponding to the exposure step 7 is formed as one specific gradation value a. However, the present invention is not limited to this. For example, the gradation value corresponding to the maximum density or the maximum density is set. A value within a range from the corresponding gradation value to the intermediate value (128) may be used. If a value lower than the intermediate value is taken, the variation in the high density region increases when an error is included. Therefore, it is desirable to take a gradation value corresponding to the high density region as much as possible.

(2) In the above embodiment, the exposure step is switched to a plurality of stages as multi-value exposure. However, the present invention is not limited to this. For example, a binary (light emission and extinguishing) configuration may be adopted. When the binary configuration is used, patches having different densities can be formed by switching light emission and extinction in units of pixels in accordance with the magnitude of the gradation value using the same method as the actual print output.
In the above description, halftone gradation reproduction is performed by the screen processing unit 106. However, a configuration in which this is not performed may be employed.

  (3) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a tandem type color printer has been described. However, the present invention is not limited to this. Any image forming apparatus that forms an electrostatic latent image by exposing and scanning the same image carrier with a plurality of light beams, regardless of color or monochrome image formation, and developing the image to form an image. For example, the present invention can be applied to a copying machine, FAX, MFP (Multiple Function Peripheral), and the like. The image carrier is not limited to the photosensitive drum, and may be a belt-like one.

Further, the number of light beams is not limited to two and may be two or more. The light emitting element is not limited to a semiconductor laser, and an element capable of emitting a light beam suitable for exposing an image carrier such as a photosensitive drum, such as an LED or a surface emitting (two-dimensional multipoint light emitting) element, is used. It may be used.
Further, in the above description, the photosensitive drums 11Y to 11K and the intermediate transfer belt 16 are used as the image carrier, and the toner pattern formed on the intermediate transfer belt 16 is detected. However, the present invention is not limited to this. In an apparatus that is not provided, a toner pattern formed on an image carrier such as a photosensitive drum may be detected by a detection sensor.

  Further, the toner pattern formed on the image carrier is detected by the reflective optical sensor. However, the present invention is not limited to this, and other sensors may be used. Needless to say, the shape and number of patches included in the toner pattern are not limited to those described above. Depending on the configuration of the apparatus, for example, the toner patterns B1 and B2 of the third embodiment may be used instead of the toner patterns P1 and P2 of the first embodiment, or C1 and C2 may be used.

  Further, the configuration is not limited to the configuration in which the toner pattern formed on the image carrier is detected. For example, the toner pattern is formed on the recording sheet, and the toner pattern formed on the recording sheet is detected by another sensor in the printer 1. The configuration may be such that, after the recording sheet on which the toner pattern is formed is discharged from the printer 1, the toner pattern on the recording sheet is detected by reading with a scanner. In addition, although one pixel is exposed in an exposure unit of one dot, a configuration in which one pixel is exposed with a plurality of dots may be employed.

  The contents of the above embodiment and the above modification may be combined.

  The present invention can be widely applied to image forming apparatuses that expose and scan the same image carrier with a plurality of light beams.

DESCRIPTION OF SYMBOLS 1 Printer 11Y, 11M, 11C, 11K Photosensitive drum 18 Print head 19 Toner pattern detection sensor 81-86, 301-308, 311-318, 321-328 Patch 105 Gradation correction part 106 Screen processing part 107 LD drive part 110 Pattern data storage unit 112 Correction data generation unit 113 First correction data storage unit 114 Second correction data storage unit 131 γ correction unit 133 First gradation correction unit 134 Second gradation correction unit 221 First exposure step conversion unit 222 Second 2 exposure step conversion unit 223 correction unit L1, L2 laser beam LD1Y, LD2Y, LD1M, LD2M, LD1C, LD2C, LD1K, LD2K
Light emitting element P1, P2 Toner pattern

Claims (12)

First and second light beams are emitted from the first and second light sources based on the image data, and the same image carrier is exposed and scanned by the emitted first and second light beams. An image forming apparatus for forming an electrostatic latent image on the surface and developing the formed electrostatic latent image to form an image,
Tone for correcting the density difference in the reproduced image caused by the difference in light intensity distribution with the other light source for each of a plurality of different tone values including halftones in the image data for driving one light source Generating means for generating correction data to be corrected to a value;
Correction means for correcting the gradation value of the image data for driving one light source by the correction data;
An image forming apparatus comprising: The generating means includes
A first pattern image including n patches corresponding to n (an integer of 1 or more) different gradation values is formed on the image carrier or the recording sheet by the first light beam, and the second pattern image Pattern forming means for forming a second pattern image including n patches corresponding to the same gradation value as the n by a light beam;
Detecting means for detecting the formed first and second pattern images,
Generating the correction data based on a detection result of patches formed by image data indicating the same gradation value among n patches included in each of the formed first and second pattern images. The image forming apparatus according to claim 1. The correction data is
It is data indicating a reverse characteristic of a characteristic indicating a relative light amount difference between one light source and the other light source, obtained from a difference between detection results of patches formed by image data indicating the same gradation value,
The correction means includes
The image forming apparatus according to claim 2, wherein a gradation value of image data for driving one of the light sources is corrected using data indicating the reverse characteristic so that the difference is eliminated. Screen processing means for correcting the gradation value of the image data corrected by the correction means by screen processing using a predetermined gradation reproduction method;
A light source driving means for driving the one light source based on the image data after the screen processing and emitting a light beam from the one light source;
The image forming apparatus according to claim 2, further comprising: Screen processing means for correcting the gradation value of the image data for driving one light source by screen processing using a predetermined gradation reproduction method before the driving;
Image data for driving one light source is image data after screen processing,
The gradation value of the image data after the screen processing is an exposure step corresponding to the amount of light beam emitted from the one light source,
The correction means includes
Light source driving means for varying the amount of light beam emitted from the one light source based on the size of the exposure step;
The image forming apparatus according to claim 2, wherein the exposure step is corrected using the correction data. N is plural,
Each of the plurality of patches is
A plurality of dots are formed in a certain area on the image carrier,
6. The image forming apparatus according to claim 2, wherein at least one of a dot generation ratio and a generation frequency within a certain region is different for each different gradation value. Each of the first and second pattern images is
A plurality of rows in which a plurality of dots are connected along the main scanning direction, a first linear patch that is arranged at intervals in the sub-scanning direction;
A plurality of one block formed by connecting a plurality of dots, a point-like second patch formed by being scattered at regular intervals in the main scanning direction and the sub-scanning direction;
7. The isolated one dot includes at least one of a plurality of dot-like third patches, each of which is a plurality of dots that are scattered at regular intervals in the main scanning direction and the sub-scanning direction. The image forming apparatus described. The first pattern image is
Each patch having different gradation values is formed by changing the amount of light emitted from the first light beam when forming one dot included in the patch,
The second pattern image is
Each patch having different gradation values is formed by changing the amount of light emitted by the second light beam when forming one dot included in the patch,
Of the plurality of patches included in each of the first and second pattern images, the amount of light beam emission is set to be the same for patches formed by image data having the same gradation value. The image forming apparatus according to claim 6 or 7. Screen processing means for correcting the gradation value of the image data for driving one light source by screen processing using a predetermined gradation expression method before the driving;
When the plurality of patches include point-like patches,
8. The dot pattern in the dotted patch has a pattern shape that is the same as or approximate to the dot pattern after the screen processing when the same gradation value is reproduced by the screen processing. The image forming apparatus described in 1. N is 1;
6. One gradation value is a value corresponding to the maximum density, or a value within a range from a gradation value corresponding to the maximum density to an intermediate value. The image forming apparatus described in the item.   Warning means for obtaining a difference between the light amounts of the first light beam and the second light beam based on the detection result between the patches and issuing a warning indicating that the difference between the light amounts is larger than a predetermined value. The image forming apparatus according to claim 2, wherein the image forming apparatus is provided. First and second light beams are emitted from the first and second light sources based on the image data, and the same image carrier is exposed and scanned by the emitted first and second light beams. An image forming method in an image forming apparatus for forming an electrostatic latent image on the substrate and developing the formed electrostatic latent image to form an image,
Tone for correcting the density difference in the reproduced image caused by the difference in light intensity distribution with the other light source for each of a plurality of different tone values including halftones in the image data for driving one light source A generation step for generating correction data to be corrected to a value;
A correction step of correcting a gradation value of image data for driving one light source by the correction data;
An image forming method comprising: executing a step including:

Images