JP4626981B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus that appropriately adjusts the excess or deficiency of a post-exposure potential caused by uneven exposure sensitivity or uneven charging on the surface of a photoreceptor.

In electrophotographic image forming apparatuses (copiers, printers, facsimile machines, etc.), the surface of the photoreceptor is uniformly charged to a predetermined initial potential by a charging device (charging means), and the charged photoreceptor surface is An electrostatic latent image is written by exposure with exposure means such as laser light scanning or an LED array.
Here, when image formation is performed, first, a pixel gradation representing a gray level for each pixel is determined based on image data to be image formed by a predetermined image processing unit, and charged in advance by a charging device. The pixel gradation determined by the image processing means is converted into an exposure amount based on predetermined conversion information (usually linear conversion), and the surface of the photoconductor is exposed by the exposure means according to the exposure amount obtained thereby. The
By the way, there are individual differences due to variations in film thickness and material characteristics of the surface layer of the photoconductor. Even if the surface is uniformly charged by a charging device under a certain condition, a unique potential is generated for each photoconductor. Distribution occurs. This is so-called charging unevenness. Further, even if each region having the same initial potential is exposed with the same exposure amount, it does not necessarily decrease to the same potential, but causes variations. That is, there is a distribution (unevenness) in the ratio (slope) of the difference in potential drop amount to the difference in exposure amount, which is so-called sensitivity unevenness.
When the conversion from the pixel gradation to the exposure amount is performed based on the same (common) conversion information for each region of the surface of the photoconductor having such inherent charging unevenness and sensitivity unevenness, the same exposure is performed. Even if the exposure is performed in an amount, the potential after exposure varies from region to region, and the density developed by the toner (development density) becomes excessive or insufficient with respect to the original density, resulting in development unevenness (density unevenness). appear.
In general, in the case of an apparatus (so-called digital machine) that performs gradation expression by an area gradation method that expresses the lightness and darkness of an image by the array of pixel gradations of a plurality of pixels, an apparatus that expresses the lightness and darkness of an image only by the lightness and darkness of a pixel unit Compared to (so-called analog machines), even though minute sensitivity unevenness and charging unevenness are less likely to appear as image density unevenness, if there is charging unevenness with a relatively large spatial period, a digital machine that expresses gradation using the area gradation method In this case, uneven density cannot be prevented.
In particular, in a color image forming apparatus that superimposes four color toner images of CMYK (cyan, magenta, yellow, and black), a mixed color gray image is formed by superimposing the three color toner images of CMY. If there is uneven charging on the surface, the CMY balance is lost and a uniform mixed color gray image is not formed (density unevenness occurs).

For example, according to Patent Document 1, if there is a potential unevenness of 5 V or more in the potential after exposure, the density unevenness appears remarkably. Such a phenomenon is particularly remarkable in a so-called tandem color image forming apparatus. In addition, since an a-Si photosensitive member (photosensitive member whose photosensitive layer is made of amorphous silicon) generally has larger charging unevenness than an OPC photosensitive member, unevenness in image density becomes more remarkable. However, in the a-Si photosensitive member, if the charging irregularity is 5 V or less as the quality standard (acceptable level), the yield is remarkably deteriorated, which is not realistic.
On the other hand, Patent Document 1 discloses a technique of providing auxiliary exposure means for correcting the distribution of the initial potential before exposure for writing an electrostatic latent image.
Patent Document 2 discloses a technique for correcting the exposure amount based on the sensitivity information of the photoconductor. Patent Document 3 discloses a technique for correcting sensitivity unevenness for each rotational position of the photoconductor. A technique for correcting the sensitivity unevenness for each exposure position of the photosensitive member is disclosed in Patent Document 5, and a technique for correcting the sensitivity unevenness according to the sensitivity distribution data of the photosensitive member is disclosed.
JP 2003-154706 A JP 10-31332 A JP 2000-162834 A JP 2004-61860 A JP 2004-233694 A

However, as disclosed in Patent Document 1, providing an exposure unit that is independent from the exposure unit for writing an electrostatic latent image leads to an increase in the size and cost of the apparatus, and is difficult to apply. There was a problem that there were many. In particular, in the case of a tandem type color image forming apparatus, it is necessary to provide a new exposure unit for each of a plurality of (usually four) photoconductors, and space and cost problems become more prominent.
In the techniques disclosed in Patent Documents 1 to 5, the exposure amount is adjusted in units of pixels. In this case, the exposure amount is adjusted by adjusting the exposure time or exposure intensity for each pixel, or a combination thereof. In such exposure amount adjustment, the exposure amount can be adjusted with high accuracy due to the limitation of the adjustment resolution. There was a problem that it was not possible.
The techniques disclosed in Patent Documents 2 to 5 all correct the sensitivity unevenness of the photoconductor, that is, the exposure characteristics (relationship between the exposure amount and the potential decrease amount) of the photoconductor as a reference, and the control target. This is to correct the difference in slope (ratio of the difference in potential drop to the difference in exposure amount) from the exposure characteristics of the photosensitive member, so that there is a distribution in the initial potential of the charged photoreceptor before exposure (charging) In the case of unevenness), the potential distribution remains as an offset as it is, and there is a problem that the density unevenness of the image cannot be resolved.

FIG. 10 shows the relationship between the pixel gradation in the a-Si photosensitive member in which charging unevenness and sensitivity unevenness coexist and the potential of the photosensitive member after exposure with an exposure amount corresponding to the pixel gradation (FIG. 10). 10A shows a case where exposure amount correction is not performed (represented by a thick broken line g01), and FIG. 10B shows a case where exposure amount sensitivity unevenness correction is performed (thick solid line g02). Represent each characteristic. In the figure, the characteristic represented by the thick solid line (g0) represents the characteristic of the reference (standard) exposure object (hereinafter referred to as the reference characteristic).
Here, the graph shown in FIG. 10A has the pixel gradation as the horizontal axis, but the conversion from the pixel gradation to the exposure amount is converted into a certain conversion formula (coefficient is fixed) or a conversion table. As long as it is performed based on this, it is equivalent even if the horizontal axis is regarded as the exposure amount. That is, in FIG. 10A, the graph line g0 representing the characteristics of the photoconductor as the reference and the graph line g01 representing the characteristics of the photoconductor as the measurement target to be controlled are both the same conversion formula ( In other words, since the conversion from the pixel gradation to the exposure amount is performed according to (no correction), the horizontal axis is replaced with the exposure amount, and the exposure characteristic (characteristic of the potential after exposure with respect to the exposure amount) is assumed. It is equivalent to see.
As shown in FIG. 10A, in general, the exposure characteristic representing the correspondence between the exposure amount and the potential after exposure on the photosensitive member (particularly, a-Si photosensitive member) is almost linear as the exposure amount increases. After the exposure, the potential drops after exposure, and the area other than the convergence area (the range where the slope at which the potential decreases with increasing exposure dose) becomes very small is almost the same as the residual potential (the potential remaining after exposure at the maximum exposure dose). Linear exposure characteristics are shown. For example, the exposure characteristic g01 of the photoconductor to be measured in FIG. 10A shows a substantially linear exposure characteristic in a range equal to or less than the charge amount E2 when the pixel gradation is I2, and serves as a reference photoconductor. The exposure characteristic g0 shows a substantially linear exposure characteristic in a range equal to or less than the charge amount Es2 when the pixel gradation is Is2.
Further, in the case where the unevenness of charging and the unevenness of sensitivity coexist on the photoconductor to be measured, as shown in FIG. 10A, the initial potential (the charging potential before exposure, that is, the charging potential before exposure, i.e. A difference in y intercept (corresponding to charging unevenness) and a difference in inclination of exposure characteristics (corresponding to sensitivity unevenness) occur. When exposure unevenness correction (correction for matching the inclinations) is performed on such a photoconductor, as shown in FIG. 10B, a potential difference corresponding to charging unevenness (difference in initial potential) is used as an offset. This will cause uneven density in the image.

By the way, at each position on the photosensitive member, the correspondence characteristic (correspondence) between the pixel gradation and the potential after exposure with the exposure amount obtained by converting the pixel gradation is expressed as follows. Conversion from the pixel gradation to the exposure amount so as to coincide with a predetermined reference characteristic over a range of all other gradations except for the case of a key (when no exposure is performed) (determination of the exposure amount) To do).
A graph g02 ′ in FIG. 11 sets all pixel gradations except for the 0 gradation when the surface of the photoconductor in which the electric unevenness and the sensitivity unevenness shown in the graph g0 in FIG. It is a graph showing the relationship between the pixel gradation and the potential after exposure when conversion from the pixel gradation to the exposure amount is performed so that the potential after exposure matches a predetermined reference characteristic.
However, when exposure amount conversion is performed so as to obtain the result shown in FIG. 11, the gap ΔV0 between the initial potential before exposure and the potential after exposure with the pixel gradation set to 1 (minimum value excluding 0). Is particularly large. If the gap ΔV0 is too large, there is a problem that the image quality deteriorates because the density continuity is inhibited when the image is expressed in halftone.
Therefore, the present invention has been made in view of the above circumstances, and the object of the present invention is to avoid the increase in size and cost of the apparatus, and also in the photosensitive member in which charging unevenness exists and in addition to the sensitivity unevenness. An image forming apparatus capable of preventing the occurrence of image density unevenness and the deterioration of image quality by inhibiting the continuity of image density by adjusting the exposure amount with high precision (high resolution) for the coexisting photoconductor It is to provide.

In order to achieve the above object, the present invention provides a pixel gradation representing a gray level for each pixel based on predetermined image data, for example, image data read from an original in a copying machine or image data such as a print job in a printer. Is determined by the image processing means, and the surface of the photosensitive member charged in advance by the charging means is exposed according to the exposure amount obtained by converting the pixel gradation determined by the image processing means (electrostatic latent image writing). And an image forming apparatus that writes an electrostatic latent image on the photosensitive member by exposing the photosensitive member to a plurality of pixels, each of which is obtained by dividing the surface of the photosensitive member into a plurality of pixels. For each divided area, a difference that is a difference between an exposure characteristic indicating a correspondence between an average exposure amount of the entire divided area and a potential after exposure and a reference characteristic common to all the divided areas. Difference information storage means for storing information, an array of the correction target pixels determined in advance by area gradation method image processing based on the difference information stored in the difference information storage means for each of the divided regions, and Correction pixel & gradation determination means for determining correction pixel & gradation information indicating the correction gradation of the pixel gradation in each correction target pixel, and in each of the divided areas according to the determination result of the correction pixel & gradation determination means The pixel gradation correcting means for correcting the pixel gradation of each pixel, and exposure for controlling the exposure means so that exposure is performed with an exposure amount corresponding to the pixel gradation after the correction by the pixel gradation correcting means. In the image forming apparatus comprising the amount control means, the average value of the correction gradation in the correction pixel & gradation information of each of the divided areas is an average exposure amount and exposure of the entire divided area. A value corresponding to a difference between an exposure characteristic indicating a correspondence with a later potential and a reference characteristic common to all the divided areas, and the pixel gradation correction unit is configured to output the pixel gradation of all pixels in the divided area. The image forming apparatus is characterized in that the pixel gradation is corrected based on the correction pixel & gradation information even when is 0 gradation .

Here, as the area gradation method used for determining the correction pixel & gradation information, an error diffusion method, a screen method, or the like can be considered.
Further, it is conceivable that the correction pixel & gradation information is stored in advance in a storage means for each of the divided areas and used, or determined by image processing based on predetermined information.
For example, for each of the divided areas, storage means stores difference information relating to a difference between an exposure characteristic indicating a correspondence between an average exposure amount of the entire divided area and a potential after exposure and a reference characteristic common to all the divided areas. The correction pixel & gradation information is determined for each divided region based on the difference information, and the pixel gradation of each pixel in each divided region is corrected according to the determination result. Can be considered.
Further, the correction pixel & gradation information determined in advance for each divided area may be stored in a storage means, and the pixel gradation of each pixel in each divided area may be corrected according to the stored information. .
In this way, by performing exposure amount correction by dispersing correction target pixels by area gradation method image processing in units of the divided regions, which are regions for a plurality of pixels, the resolution for adjusting the exposure amount of each pixel can be reduced. Even if it is low, the exposure amount adjustment for the entire divided area is “resolution = m · (n−1) +1, where n is the resolution of the pixel gradation, m according to the number of pixels constituting the divided area. The resolution is improved in relation to “the number of pixels in the divided area” (for each divided area). As a result, the exposure amount can be adjusted with high accuracy for the entire divided area.
In particular, the image processing means performs gradation expression by an area gradation method in which the pixel gradation arrangement is determined for each unit pixel group composed of a plurality of pixels based on the image data (so-called digital machine image processing). In other words, even if there are charging unevenness having a relatively large spatial period, it is possible to suppress minute sensitivity unevenness and charging unevenness from appearing as image density unevenness. It is preferable in that it is possible to prevent the charging unevenness from appearing as image density unevenness by adjusting the exposure amount by the adjustment method.

Here, as the correction pixel & gradation information of each of the divided areas, the average value of the correction gradation in the information represents the correspondence between the average exposure amount of the entire divided area and the potential after exposure. The correction pixel & level is used even when the pixel gradation of all the pixels in the divided area is 0 gradation, with information having a value corresponding to the difference between the exposure characteristic and the reference characteristic common to all the divided areas. It is conceivable to correct the pixel gradation based on the key information. For example, the average value of the correction gradation in each of the divided areas is a value corresponding to the difference between the initial potential in the average exposure characteristic of the entire divided area and the initial potential in the reference characteristic (for example, the difference is The value to be raised) is considered.
As a result, since the pixel gradation is corrected (that is, the exposure amount is corrected) including the case where the pixel gradation that has not been conventionally exposed is 0 gradation, variations in the initial potential (reference initial value) are performed. The exposure amount is corrected (raised) according to the difference (potential difference), and the gap between the post-exposure potentials when the pixel gradation is 0 gradation and 1 gradation is suppressed. The image quality is not deteriorated by inhibiting the continuity of density.
In addition, the divided region, which is a unit of area gradation method in exposure amount correction, and the unit pixel group, which is a unit of area gradation method for determining a pixel gradation for image formation (electrostatic latent image writing) If the number of pixels in the vertical direction and the number of pixels in the horizontal direction are the same between each other, the spatial period of the gradation arrangement determined by each area gradation method is the same. Can be prevented.
In addition, an area gradation method used for determining the correction pixel & gradation information and an area gradation method used for determining the pixel gradation by the image processing unit are shifted from each other by about 15 ° or more. Even with the screen system, the occurrence of moiré can be similarly prevented.
In addition, when the photoconductor is an a-Si photoconductor, charging unevenness is particularly noticeable in many cases, which is suitable for application of the present invention.

According to the present invention, in accordance with the information on the pixel arrangement (corrected gradation arrangement) determined by the area gradation method image processing in units of divided areas corresponding to a plurality of pixels obtained by dividing the surface of the photosensitive member into a plurality of pixels. Since pixel gradation correction for image formation is performed, even if the resolution for adjusting the exposure amount of each pixel is low, the exposure amount adjustment for the entire divided region can be performed according to the number of pixels constituting the divided region. In particular, the resolution is improved, and the exposure amount can be adjusted with high accuracy for the entire divided area.
In particular, the image processing means performs gradation expression by an area gradation method in which the pixel gradation arrangement is determined for each unit pixel group composed of a plurality of pixels based on the image data (so-called digital machine image processing). In other words, even if there are charging unevenness having a relatively large spatial period, it is possible to suppress minute sensitivity unevenness and charging unevenness from appearing as image density unevenness. It is preferable in that it is possible to prevent the charging unevenness from appearing as image density unevenness by adjusting the exposure amount by the adjustment method.
In addition, the average value of the correction gradation in the correction pixel & gradation information of each of the divided areas, the exposure characteristics representing the correspondence between the average exposure amount of the entire divided area and the potential after exposure, Even if the pixel gradation of all the pixels in the divided area is 0 gradation, the value of the pixel gradation based on the correction pixel & gradation information is set to a value corresponding to the difference from the reference characteristic common to the divided area. If correction is performed, correction of the pixel gradation (that is, correction of the exposure amount) is performed including the case where the pixel gradation that is not conventionally exposed is 0 gradation. The exposure amount is corrected (raised) according to the difference from the reference initial potential, and the gap between the post-exposure potentials when the pixel gradation is 0 gradation and 1 gradation is suppressed. Do not deteriorate the image quality by inhibiting the continuity of halftone density. .
In addition, the divided region, which is a unit of area gradation method in exposure amount correction, and the unit pixel group, which is a unit of area gradation method for determining a pixel gradation for image formation (electrostatic latent image writing) The number of pixels in the vertical direction and the number of pixels in the horizontal direction are made the same between each other, the area gradation method used for determining the correction pixel & gradation information, and the determination of the pixel gradation by the image processing means The area gradation method used in the above is a screen method in which the screen angles are shifted from each other by about 15 ° or more, so that the generation of moire due to the mutual interference of the gradation arrangement determined by each of the area gradation methods. Can be prevented.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that the present invention can be understood. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
1 is a schematic sectional view of an image forming apparatus X according to an embodiment of the present invention, FIG. 2 is a block diagram showing a schematic configuration of a main part of the image forming apparatus X, and FIG. 3 is a divided region in the image forming apparatus X. Pixel gradation average and the corresponding pixel gradation after correction based on the first embodiment of the difference information and the original pixel gradation average and exposure when exposure is performed according to the corrected pixel gradation FIG. 4 is a graph showing the relationship with the post-potential. FIG. 4 shows the correspondence between the pixel gradation average of the divided regions in the image forming apparatus X and the pixel gradation after correction based on the second embodiment of the difference information, and the correction thereof. FIG. 5 is a graph showing the relationship between the original pixel gradation average and the post-exposure potential when exposure is performed according to the post-pixel gradation, and FIG. 5 is a diagram showing a first example of correction pixel & gradation information in the image forming apparatus X; FIG. 6 is a second example of correction pixel & gradation information in the image forming apparatus X. FIG. 7 is a diagram illustrating a third example of correction pixel & gradation information in the image forming apparatus X, FIG. 8 is a diagram illustrating a fourth example of correction pixel & gradation information in the image forming apparatus X, and FIG. FIG. 10 is a diagram showing an arrangement of pixel gradations based on image data in the image forming apparatus X and a fifth example of correction pixel & gradation information. FIG. 10 shows a conventional pixel level on the surface of the photosensitive member where uneven charging and uneven sensitivity exist. FIG. 11 is a graph showing an example of the relationship between the tone and the potential after exposure, and FIG. 11 shows a state after exposure by setting all the pixel gradations except for 0 in the exposure of the surface of the photoreceptor where charging unevenness and sensitivity unevenness coexist. It is a graph showing the relationship between the pixel gradation and the potential after exposure when the exposure amount conversion is performed so that the potential matches the reference characteristic.

First, the overall configuration of the image forming apparatus X according to the embodiment of the present invention will be described using the cross-sectional view shown in FIG.
The image forming apparatus X is a printer that is an example of a tandem type image forming apparatus that uses toner of four colors of black (BK), magenta (M), yellow (Y), and cyan (C).
The image forming apparatus X includes an image forming unit α1 that forms a toner image and forms an image on a recording sheet, a paper feeding unit α2 that supplies the recording sheet to the image forming unit α1, and a recording sheet on which image formation has been performed. Is discharged.
Image information (print job) received from a communication unit (not shown) from an external device such as a personal computer is black (BK), magenta (M), yellow (Y), cyan (C) by an image processing unit 12 to be described later. Are converted into pixel gradations which are grayscale information for each pixel for each of the four colors.

  The image forming portion α1 has four photosensitive drums 1 (1BK for black, 1M for magenta, 1Y for yellow, and 1C for cyan) carrying the images of the four colors, and the surface of each of the photosensitive drums 1. The charging device 3 (3BK, 3M, 3Y, 3C) to be charged in this manner, and the surface of each of the photosensitive drums 1 charged in advance by the charging device 4 to the pixel gradation determined by the image processing unit 12 described later. An exposure source 2 (2BK, 2M, 2Y, 2C, an example of exposure means) that writes an electrostatic latent image on the photosensitive drum 1 by irradiating (exposing) light with a corresponding exposure amount for each pixel, The toner image formed on the surface of each of the developing drums 5 (5BK, 5M, 5Y, 5C) and the photosensitive drum 1 for developing the toner image by supplying toner to the electrostatic latent image is sequentially transferred to the toner. An intermediate transfer belt 7 for transferring the toner image onto the recording paper, a conveying roller 8 for conveying the recording paper, a fixing device 9 for heating and fixing the toner image transferred onto the recording paper, and the photosensitive drum 1 after transferring the toner image onto the recording paper. It is schematically configured to include a static eliminator 4 (4BK, 4M, 4Y, 4C) or the like that performs static elimination on the surface.

The photosensitive drum 1 is, for example, an a-Si photosensitive member that is excellent in durability because of its high hardness and stable properties, while charging unevenness is likely to appear relatively remarkably in addition to sensitivity unevenness.
The charging device 3 uniformly charges the surface of the photosensitive drum 1 along its axial direction. If the photosensitive drum 1 is unevenly charged, Distribution occurs in the potential (initial potential) before exposure.
The exposure source 2 shown in FIG. 1 shows an example in which the exposure source 2 is constituted by an LED array in which a plurality of LEDs are arranged for each pixel in the axial direction (main scanning direction) of the photosensitive drum 1. In addition, the exposure source 2 may be constituted by a laser scanning device or the like that scans laser light in the axial direction of the photosensitive drum 1.
The developing device 5 includes a developing roller that supplies toner to the photosensitive drum 1, and corresponds to a potential gap between a potential (developing bias potential) applied to the developing roller and a potential on the surface of the photosensitive drum 1. The toner on the developing roller is attracted onto the surface of the photosensitive drum 1, and the electrostatic latent image is visualized as a toner image.
The paper feeding unit α2 is roughly configured to include a paper feeding cassette 20, a paper feeding roller 6, and the like. The recording paper previously stored in the paper feed cassette 20 is conveyed to the image forming unit α1 when the paper feed roller 6 is driven to rotate.
The recording paper delivered from the paper feeding unit α2 is transferred by the transfer roller 8 and the toner image is transferred from the intermediate transfer belt 7. Then, the recording paper on which the toner image has been transferred is conveyed to the fixing device 9, and is heated and fixed on the recording paper by, for example, a heating roller, and then conveyed to the paper discharge unit α3 and discharged.

FIG. 2 is a block diagram illustrating a schematic configuration of a main part of the image forming apparatus X.
The image forming apparatus X includes an MPU and its peripheral devices such as a ROM and a RAM in addition to the charging device 3, the exposure source 2, the developing device 5, and the charge eliminating device 4. A control unit 10 that controls the components, a display operation unit 11 such as a liquid crystal touch panel that is a unit for inputting information according to a user operation, and an image processing unit 12 that performs various types of image processing. , An EEPROM or other readable / writable storage means for storing various data, a rotational position detector 14 for detecting the position of each photosensitive drum 1 in the rotational direction, and image processing based on differential information described later. A correction image processing unit 15 is provided.
The image processing unit 12 calculates a pixel gradation representing a gray level for each pixel for each color of toner based on predetermined image data (print job or the like) input from an external device via a communication control unit (not shown). The process determined by the digital method is executed.
Here, based on the image data, the image processing unit 12 arranges pixels to be printed (drawing pixels) in units of a pixel group composed of a plurality of pixels (hereinafter referred to as a unit pixel group) and the pixels to be printed. The density gradation representation of the image is performed by an area gradation method such as an error diffusion method or a screen method for determining the pixel gradation.
In the data storage unit 13, the average exposure of the entire divided area for each divided area composed of a plurality of pixels, which is an area obtained by dividing the surface of each of the photosensitive drums 1 in advance. An exposure characteristic (for example, the characteristic of the graph g01 in FIG. 10A) indicating the correspondence between the amount and the potential after exposure, and a reference characteristic common to all the divided regions (for example, the graph g0 in FIG. 10A). Difference information, which is information for specifying the difference between the first and second characteristics), is stored in advance (an example of difference information storage means). The contents of the difference information will be described later.
Here, the divided regions are, for example, pixels in the vertical direction (sub-scanning direction) between the divided region and the unit pixel group employed in the image processing in the area gradation method in the image processing unit 12. It is conceivable that the number of pixels and the number of pixels in the horizontal direction (main scanning direction) are the same.
The correction image processing unit 15 obtains correction pixel & gradation information to be described later by performing area gradation image processing based on the difference information.

Then, the control unit 10 acquires the pixel gradation determined by the image processing unit 12, and individually for each of the photoreceptors 1 and for each divided region based on the pixel gradation and the difference information. The exposure amount in the exposure source 2 is controlled (an example of exposure amount control means). Hereinafter, this exposure amount control is referred to as individual exposure amount control. As a result, each of the divided regions is exposed by the exposure source 2 with the exposure amount according to the individual exposure amount control.
The controller 10 recognizes each position (exposure position) of the divided area and controls exposure by the exposure source 2.
That is, when an LED array is used for the exposure source 2, the LEDs are arranged for each pixel, so that the control unit 10 determines the exposure position in the axial direction (main scanning direction) of the surface of the photosensitive drum 1. Recognized by the array position (array number, etc.) of the LEDs to be lit.
On the other hand, with respect to the exposure position in the circumferential direction (sub-scanning direction) of the surface of the photosensitive drum 1, any position on the surface of the photosensitive drum 1 by the rotational position detector 14 is the light irradiation position of the exposure source 2. The control unit 10 recognizes by acquiring the detection result.
On the other hand, a combination of LED identification information (LED array number, etc.) and detection value of the rotational position detection unit 14 is stored in the data storage unit 13 as identification information for each of the divided regions. The raised exposure amount is stored in association with each combination (identification information of each divided region).
Further, the control unit 10 determines the raised exposure amount used for the individual exposure amount control based on the position of the LED to be lit (array number, etc.) and the detection result of the rotational position detection unit 14. Data is extracted (searched) from the data storage unit 13 and read out.
The rotational position detector 14 may be configured to detect a rotational position by providing a rotary potentiometer on the rotating shaft of the photosensitive drum 1 or a protrusion on the rotating shaft of the photosensitive drum 1. It is conceivable to provide a reference portion such as the above, detect the passing position of the reference portion with a contact-type switch, a photocoupler, or the like, and measure the elapsed time from the detection point.
When a laser scanning device is used as the exposure source 2, the rotational position of the polygon mirror used for scanning the laser beam is detected for the exposure position in the axial direction (main scanning direction) of the surface of the photosensitive drum 1. Alternatively, it may be detected by measuring the elapsed time after the light receiving element detects that the laser beam has been deflected to the predetermined base position.
Further, the control unit 10 includes a pulse signal control circuit (not shown) that divides and counts a predetermined clock signal, and outputs various signals generated thereby to the exposure source 2, thereby The exposure time of the exposure source 2 is controlled. Here, the exposure time is set in proportion to the pixel gradation. As a result, as long as the exposure intensity is constant, exposure is performed with an exposure amount proportional to the pixel gradation.

By the way, the image forming apparatus X is subjected to a characteristic evaluation test for obtaining the exposure characteristics of each of the photosensitive drums 1 incorporated in the image forming apparatus X in a manufacturing stage or the like. More specifically, the characteristic evaluation test (preliminarily measured) is performed on the photosensitive drum 1 charged (charged) by the charging device 3 under a condition of a plurality of exposure amounts for each divided region. The exposure source 2 performs exposure and the initial potential before exposure and the potential after exposure for each divided region are measured, and the exposure characteristics of each divided region, that is, the exposure amount and the potential after exposure, The characteristics representing the relationship (hereinafter referred to as measured exposure characteristics) are clarified. A thick broken line graph g0 shown in FIG. 10A is an example of the exposure characteristic thus clarified.
Here, as a method for measuring the exposure characteristics of each of the divided areas, for example, the exposure characteristics of each of the divided areas are densely switched and exposed, and the potential after exposure is measured to measure accurate exposure characteristics. it can. In addition, as shown in FIG. 10 (a), the tendency (exposure curve) of the exposure characteristics is determined to some extent, and it is general that it can be formulated by a common equation if only the coefficient is changed. The exposure characteristics may be estimated based on the result of measuring the potential after exposure with a plurality of representative exposure amounts.
For example, in the case of an a-Si photosensitive drum, the residual potential is almost constant regardless of the position of the surface of the photosensitive drum 1, so that the exposure is performed with the initial potential and one exposure amount within the range of the substantially linear characteristic. By measuring the potential after the exposure, the exposure characteristics can be estimated with sufficient accuracy.
The difference information is determined (set) based on the exposure characteristics for each of the divided regions measured (or estimated based on the measurement).

<First Example of Difference Information>
10 and 3 described above, the average exposure characteristic of a certain divided area on the surface of the a-Si photosensitive drum 1 is the characteristic shown in FIG. The first example of the difference information will be described by taking as an example the case where the exposure characteristic (g0) coexists with sensitivity unevenness.
Here, FIG. 3B shows an average of the pixel gradations determined by the image processing unit 12 based on image data for the divided regions having the exposure characteristics shown in the graph g01 in FIG. Value (hereinafter, simply referred to as pixel gradation average or original pixel gradation average) and the average value of pixel gradations after correction based on the first embodiment of the difference information (hereinafter referred to as corrected pixel gradation) 3 (a) is a graph showing a correspondence relationship with the average), and FIG. 3 (a) shows the original when the exposure is performed with an exposure amount obtained by proportionally converting the corrected pixel gradation average shown in FIG. 3 (b). It is a graph showing the relationship between pixel gradation average and the potential after exposure (potential after exposure).
Here, the one-dot broken line shown in FIG. 3B corresponds to the case where the original pixel gradation average Iave and the corrected pixel gradation average Ix are equal (Ix = Iave), that is, when correction is not performed. Represents a relationship. Note that the characteristics shown in the graphs g0 and g01 in FIG. 10A show that the original pixel gradation average is not corrected as it is, and the average exposure amount in the divided area is directly proportional to the original pixel gradation average. This is a characteristic when the exposure is performed so that the converted exposure amount is obtained.
Also, the solid line shown in FIG. 3B is a correspondence relationship between the original pixel gradation average Iave and the corrected pixel gradation average Ix (Ix = Iave + Ia, where Ia is set for each of the divided regions. A constant (a raised gradation described later). The y-intercept Ia in this correspondence relationship is stored in advance in the data storage unit 13 as “raised tone” that is an example of the difference information for each of the divided areas.
Here, the raised gradation Ia has the initial potential Vx0 in the average exposure characteristic (graph g01 in FIG. 10A) and the reference characteristic (FIG. This is a value obtained by converting the difference (Vx0−V0) [V] from the initial potential V0 (reference initial potential) in the graph g0) of a) into the pixel gradation (an example of a value corresponding to the difference).
That is, as shown in FIG. 10A, the raised pixel gradation Ia has an average exposure characteristic g01 of the divided area, and a conversion coefficient (divided area) that determines the raised pixel gradation Ia in advance. If an exposure amount obtained by linear conversion based on a coefficient (slope) common to all is applied, the pixel gradation is such that the potential after exposure drops by the difference (Vx0−V0).

Here, how the control unit 10 (an example of exposure amount control means) specifically reflects the pixel gradation and the raised gradation Ia (difference information) in the exposure amount control of each pixel. As will be described later, for each of the divided regions, the average exposure amount is added by the exposure amount corresponding to the raised gradation Ia to the exposure amount proportional to the pixel gradation average (raising). By performing exposure with the exposure amount thus obtained, the characteristics of the potential after exposure with respect to the average pixel gradation become as shown by a graph gx1 in FIG.
As shown in the graph gx1 in FIG. 3A, the characteristics of the potential after exposure with respect to the pixel gradation average of the photoconductor (charged photoconductor) in which charging unevenness exists are generally the reference initial value. It approaches the reference characteristic g0 having the potential V0 as the initial potential. As a result, variations in the post-exposure potential for each position on the surface of the photoreceptor 1 can be suppressed, and the occurrence of uneven density in the image can be prevented as much as possible.
Furthermore, since exposure is performed with an exposure amount corresponding to the raised pixel gradation Ia corresponding to the difference in the initial potential even for a pixel whose pixel gradation is 0 gradation, which is not conventionally exposed, The gap in the post-exposure potential (ΔV0 in FIG. 11) when the pixel gradation is 0 gradation and 1 gradation is suppressed, and the image quality is not deteriorated by inhibiting the continuity of halftone density. .
In the example shown in FIG. 3, correction based on the raised gradation Ia is performed when the pixel gradation average is the entire range including 0 gradation, but the pixel gradation is 0 gradation. The correction based on the raised gradation Ia may be performed only in a part of the range including.
For example, among the characteristics g01 shown in FIG. 10 (a), only when the pixel gradation average is a gradation within a range from 0 gradation to a substantially linear characteristic (that is, the potential after exposure is the residual potential and Even if the exposure amount control for adding the exposure amount corresponding to the raised gradation Ia is performed, the exposure characteristic in each of the divided areas is almost the same as the reference exposure characteristic. Match.

<Second Example of Difference Information>
10 and 3 described above, the average exposure characteristic of a certain divided area on the surface of the a-Si photosensitive drum 1 is the characteristic shown in FIG. The second embodiment of the difference information will be described by taking as an example the case where the exposure characteristic (g0) coexists with sensitivity unevenness.
FIG. 4B shows the original pixel tone average and the difference information of the difference information for the divided region having the exposure characteristics shown in the graph g01 of FIG. FIG. 4A is a graph showing a correspondence relationship with the corrected pixel gradation average corrected based on Example 2, and FIG. 4A is a proportional conversion of the corrected pixel gradation average shown in FIG. It is a graph showing the relationship between the said original pixel gradation average at the time of exposing with exposure amount, and the electric potential after exposure (post-exposure electric potential).
Here, the characteristic indicated by the dashed line in FIG. 4B represents the same characteristic as the dashed line in FIG.
Also, the solid line shown in FIG. 4 (b) indicates the correspondence between the original pixel gradation average Iave and the corrected pixel gradation average Ix (Ix = k1 · Iave + Ia, where k1 is the slope, and Ia is the bulk. Represents a raised gradation). k1 is inclination information that defines an inclination when linearly converting the original pixel gradation average into the corrected pixel gradation average for each of the divided regions.
The raised gradation Ia and the inclination information k1 in this correspondence relationship are stored in advance in the data storage unit 13 for each of the divided areas as an example of the difference information.

Here, the control unit 10 (an example of exposure amount control unit) specifically determines how the pixel gradation, the raised gradation Ia, and the inclination information k1 (difference information) are applied to the exposure amount of each pixel. Whether to reflect in the control will be described later. For each of the divided regions, the average exposure amount is determined by the raised gradation Ia and the inclination information k1 with respect to the exposure amount proportional to the pixel gradation average. After exposure with respect to the pixel gradation average, exposure is performed with an exposure amount corresponding to the gradation (Ia + Iki) obtained by adding the specified correction gradation Iki (= Iave · (k1-1)). The characteristic of the potential is as shown by a graph gx2 in FIG.
As shown in the graph gx2 in FIG. 4A, in the photosensitive member (charged photosensitive member) in which charging unevenness and sensitivity unevenness coexist, in addition to initial potential variation (charging unevenness), variation in inclination in exposure characteristics. That is, since the exposure characteristic variation due to the sensitivity unevenness is also corrected, the characteristic of the potential after the exposure with respect to the pixel gradation average substantially matches the reference characteristic g0.
As a result, in the photosensitive drum 1 in which charging unevenness and sensitivity unevenness coexist, variation in the post-exposure potential for each position on the surface can be almost eliminated, and the effect of preventing the occurrence of uneven image density can be further enhanced. .

Next, how the difference information (raised gradation Ia and inclination information k1) is reflected in the exposure amount control of each pixel will be described.
In the image forming apparatus X, the correction image processing unit 15 performs area gradation type image processing based on the difference information in advance for each of the divided regions before performing image processing based on image data. The correction pixel & gradation information indicating the correction gradation of the pixel gradation in each of the correction target pixels is determined by the correction target pixel array (an example of correction pixel & gradation determination means).
In general, in area gradation image processing, for each of a plurality of pixel groups (unit pixel group) as a processing unit, a set value of density gradation of the entire unit pixel group (hereinafter referred to as set density gradation). ), The average value of the pixel gradation of each pixel in the unit pixel group becomes the given set density gradation, and the pixel gradation of the pixel gradation is scattered so that the drawing pixels are scattered. The sequence is determined. The decision rule is predetermined.
Accordingly, the area of the unit pixel group of the area gradation method image processing based on the difference information (the processing of the correction image processing unit 15) is set so as to correspond to each of the divided areas. If an average corrected gradation level (Ia or (Ia + Iki)) to be corrected for the entire divided area is determined based on the difference information and the corrected gradation level is given as the set density gradation, An array of correction gradations for each pixel based on the area gradation method is determined so that an average value of correction gradations (corresponding to pixel gradations) of each pixel becomes a correction gradation level corresponding to the difference information. . The correction gradation array determined in this way is the correction pixel & gradation information (correction target pixel and correction gradation of each pixel).

For example, when the correction pixel & gradation information is determined based on the raised gradation Ia, the raised gradation Ia is supplied from the control unit 10 to the correction image processing unit 15 for each divided region. Is given as the set density gradation. Thereby, the correction gradation of each pixel in the divided area is determined so as to be scattered in the divided area. The correction gradation array thus determined is the correction pixel & gradation information, and the average value of the correction gradations is the raised gradation Ia.
Similarly, when the correction pixel & gradation information is determined based on the raised gradation Ia and the inclination information k1, first, all the pixels included in the divided area are determined by the control unit 10 for each divided area. The average value Iave of the pixel gradation is obtained, and further, a gradation correction amount Iki (= Iave · (k1-1)) based on the inclination information k1 is obtained.
Next, a gradation (Ia + Iki) obtained by adding the raised gradation Ia and the corrected gradation Iki based on the inclination information k1 from the control unit 10 to the correction image processing unit 15 is set as the set density gradation. Give as. As a result, the correction gradation of each pixel in the divided area is determined to be scattered in the divided area, and the average value of the correction gradation is (Ia + Iki).
Hereinafter, the overall correction gradation (Ia or (Ia + Iki)) based on the difference information in each of the divided areas is referred to as an average correction gradation.

FIG. 5 is a diagram illustrating a first example of the correction pixel & gradation information determined by the correction image processing unit 15.
The example shown in FIG. 5 is an example of the correction pixel & gradation information determined by the correction image processing unit 15 that performs image processing using an error diffusion method (two-dimensional error diffusion method) as an area gradation method. It is.
Each square in the correction pixel & gradation information shown in FIG. 5 represents one pixel, and a numerical value in the square represents a correction gradation of each pixel. In this example, the divided region, that is, the region of the unit pixel group of the area gradation method is a region corresponding to 10 pixels in the main scanning direction × 10 pixels in the sub-scanning direction = 100 pixels. The average correction gradation (the average value of the correction gradation of each pixel) is 0.36. As described above, the average correction gradation is determined by the exposure characteristic (g01 in FIG. 10) indicating the correspondence between the average exposure amount of the entire divided area and the potential after exposure, and a reference common to all the divided areas. It is a value (Ia or (Ia + Iki)) corresponding to the difference from the characteristic (g0 in FIG. 10). This also applies to the examples shown in FIGS. 6 to 8 and FIG. 9B described later.
The control unit 10 corrects the pixel gradation determined based on the image data by the image processing unit 12 according to the correction pixel & gradation information determined in this way (an example of a pixel gradation correction unit). . Specifically, correction is performed by adding the correction gradation of each pixel in the correction pixel & gradation information to the corresponding pixel gradation. Therefore, the control unit 10 corrects the pixel gradation based on the correction pixel & gradation information even when the original pixel gradation of all the pixels in the divided region is 0 gradation (capacity). Will be raised).
As a result, for each of the divided regions, the average exposure amount of the entire region is corrected (added) by an exposure amount corresponding to the average correction gradation, and the exposure for the pixel gradation average for each of the divided regions is performed. The characteristic of the subsequent potential approaches or substantially matches the reference characteristic g0 as a whole (see FIGS. 3 (a) and 4 (a)).
In addition, in the unit of the divided region, which is a region for a plurality of pixels, the correction target pixels (pixels of “1” gradation in FIG. 5) are dispersed by area gradation method image processing to correct the pixel gradation ( By increasing the exposure amount), even if the resolution of adjusting the exposure amount of each pixel is low, the exposure amount adjustment of the entire divided area is dramatically increased according to the number of pixels constituting the divided area. Resolution is improved. In the example shown in FIG. 5, if the resolution of the pixel gradation in pixel units is 16, the resolution for each divided area is 100 × (16−1) + 1 = 1501 (gradation). As a result, the exposure amount can be adjusted with high accuracy for the entire divided area.
In particular, when the image processing unit 12 performs gradation expression by an area gradation method based on image data (so-called image processing means of a digital machine), minute sensitivity unevenness and charging unevenness are image density unevenness. In addition, even if there is charging irregularity with a relatively large spatial period, the charging irregularity becomes image density irregularity by adjusting the exposure amount by the area gradation method in the unit of the divided area. It is preferable in that it can be prevented from appearing.

FIG. 6 is a diagram illustrating a second example of the correction pixel & gradation information determined by the correction image processing unit 15.
The example shown in FIG. 6 is an example of the correction pixel & gradation information determined by the correction image processing unit 15 that performs image processing using the screen method as the area gradation method.
Also in this example, the divided area is an area corresponding to 100 pixels, and the average correction gradation is 2.40.
Further, the pixel gradation correction method based on the correction pixel & gradation information determined in this way is the same as the method described in FIG. This also applies to the examples shown in FIGS.
Thus, even if the screen method is adopted as the area gradation method, the same effect can be obtained.

FIG. 7 is a diagram illustrating a third example of the correction pixel & gradation information determined by the correction image processing unit 15.
In the example shown in FIG. 7, the correction image processing unit 15 uses the error diffusion method in which the correction gradation is diffused only in the main scanning direction (the axial direction of the photosensitive drum 1) as the area gradation method. It is an example of the determined correction pixel & gradation information.
FIG. 8 is a diagram illustrating a fourth example of the correction pixel & gradation information determined by the correction image processing unit 15.
In the example shown in FIG. 8, the correction image processing unit 15 uses the error diffusion method in which the correction gradation is diffused only in the sub-scanning direction (the circumferential direction of the photosensitive drum 1) as the area gradation method. It is an example of the determined correction pixel & gradation information.
In the examples shown in FIGS. 7 and 8, the divided region is a region corresponding to 100 pixels, and the average correction gradation is 2.40.
As described above, even if the one-dimensional error diffusion method is adopted as the area gradation method, the same effect can be obtained.

FIG. 9 is a diagram illustrating the pixel gradation array (a) based on the image data determined by the image processing unit 12 and the fifth example (b) of the correction pixel & gradation information.
The example shown in FIG. 9A is an example of the array of pixel gradations determined by the image processing unit 12 when the screen method is adopted as the area gradation method used for image processing based on image data. .
The unit pixel group is 10 pixels × 10 pixels.
Further, the example shown in FIG. 9B shows the correction pixel & floor determined by the correction image processing unit 15 when the screen method is used as the area gradation method used for the image processing based on the difference information. It is an example of key information.
Also in this example, the divided area is an area corresponding to 100 pixels, and the average correction gradation is 2.40.
Here, the thick alternate long and short dash line shown in FIGS. 9A and 9B represents the direction of the screen angle in the image processing of each screen method. As shown in FIG. 9, the area gradation method used for determining the correction pixel & gradation information by the correction image processing unit 15 and the area gradation used for determining the pixel gradation by the image processing unit 12. The adjustment method is a screen method in which the screen angles deviate from each other.
If the screen angle deviation is 15 ° or more, when the pixel gradation and the correction gradation are added (overlapped), it is possible to prevent the occurrence of moire due to mutual interference between the gradations. FIG. 9 shows an example in which the screen angles are shifted from each other by about 37 °.
The correction image processing unit 15 is used to determine the correction pixel & gradation information, the divided region which is an area gradation method processing unit, and the image processing unit 12 to determine the pixel gradation. If the number of pixels in the vertical direction and the number of pixels in the horizontal direction are the same among the unit pixel groups that are processing units of the area gradation method, the gradations determined by each of the area gradation methods are the same. The spatial period of the array is the same. This can also prevent the occurrence of moire.

In the embodiment described above, for each of the divided areas, the difference information regarding the difference between the average exposure characteristic of the entire divided area and the common reference characteristic is stored in the data storage unit 13 in advance, and the divided area is stored. An example in which the correction pixel & gradation information is determined for each region based on the difference information is shown.
However, the present invention is not limited to this. For example, the correction pixel & gradation information obtained in advance for each of the divided regions is stored in the data storage unit 13 (an example of correction pixel & gradation information storage unit), and the control unit A configuration in which the pixel gradation of each pixel in each of the divided regions is corrected according to the stored information by 10 (an example of pixel gradation correction means) is also conceivable. Accordingly, although the capacity of the data storage unit 13 is increased, the calculation load can be reduced and the configuration can be simplified because the processing of the correction image processing unit 15 at the time of image formation becomes unnecessary.

In the above-described embodiment, the divided area is an area obtained by dividing the surface of the photosensitive drum 1 into a plurality of both the axial direction and the circumferential direction. However, the present invention is not limited to this.
For example, when charging unevenness or sensitivity unevenness mainly in the axial direction or circumferential direction of the photosensitive drum 1 is a problem, the divided region is divided into a plurality of parts only on the surface of the photosensitive drum 1 in the axial direction. It is also conceivable to use a region obtained by dividing the photosensitive drum 1 in a ring shape or a region obtained by dividing the photosensitive drum 1 in the circumferential direction.
In the embodiments and examples, the raised gradation Ia and the inclination information k1 are shown as examples of the difference information. However, the present invention is not limited to this, and the average of the entire divided area for each divided area is not limited thereto. Other information may be used as long as the information relates to the difference between the exposure characteristic indicating the correspondence between the exposure amount and the potential after exposure and the reference characteristic common to all the divided areas.
For example, instead of the raised gradation Ia, the difference between the initial potentials of the divided areas with respect to the reference initial potential is stored, and the difference in the initial potential is stored in the raised gradation Ia when forming an image. Conversion is also possible.
Further, instead of the tilt information k1, coordinate information or the like for specifying a tilt for a coordinate system including the original pixel gradation axis and the corrected pixel gradation axis is stored in the data storage unit 13. It can be considered.

  The present invention can be used for an image forming apparatus.

1 is a schematic sectional view of an image forming apparatus X according to an embodiment of the present invention. 2 is a block diagram illustrating a schematic configuration of a main part of the image forming apparatus X. FIG. Correspondence between the pixel gradation average of the divided areas in the image forming apparatus X and the pixel gradation after correction based on the first embodiment of the difference information, and the original when the exposure is performed according to the corrected pixel gradation The graph showing the relationship between the pixel gradation average of and the electric potential after exposure. Correspondence between the pixel gradation average of the divided areas in the image forming apparatus X and the pixel gradation after correction based on the second embodiment of the difference information, and the original when exposure is performed according to the corrected pixel gradation The graph showing the relationship between the pixel gradation average of and the electric potential after exposure. 6 is a diagram illustrating a first example of correction pixel & gradation information in the image forming apparatus X. FIG. FIG. 10 is a diagram illustrating a second example of correction pixel & gradation information in the image forming apparatus X. FIG. 10 is a diagram illustrating a third example of correction pixel & gradation information in the image forming apparatus X. FIG. 10 is a diagram illustrating a fourth example of correction pixel & gradation information in the image forming apparatus X. 10 is a diagram illustrating an array of pixel gradations based on image data and a fifth example of correction pixel & gradation information in the image forming apparatus X. FIG. The graph showing an example of the relationship between the conventional pixel gradation and the potential after exposure on the surface of the photoreceptor where charging unevenness and sensitivity unevenness coexist. When the exposure on the surface of the photoconductor in which charging unevenness and sensitivity unevenness coexist, the individual exposure amount conversion is performed so that all pixel gradations except for 0 are set and the potential after exposure is matched with the reference characteristics. The graph showing the relationship between a pixel gradation and the electric potential after exposure.

Explanation of symbols

X: Image forming apparatuses 1BK, 1M, 1Y, 1C according to embodiments of the present invention: Photosensitive drums 2BK, 2M, 2Y, 2C ... Exposure sources 3BK, 3M, 3Y, 3C ... Charging devices 4BK, 4M, 4Y, 4C ... Static elimination devices 5BK, 5M, 5Y, 5C... Development device 6... Feed roller 7. Intermediate transfer belt 8. Conveyance roller 9. Fixing device 10 Control unit 11 Display operation unit 12 Image processing unit 13 Data storage unit 14: Rotation position detector 15 ... Image processor for correction

Claims (7)

An image processing means for determining a pixel gradation representing a light and shade level for each pixel based on predetermined image data; and a surface of the photosensitive member charged in advance by a charging means to the pixel gradation determined by the image processing means. An image forming apparatus comprising: an exposure unit that writes an electrostatic latent image on the photoconductor by exposing each pixel with an exposure amount based on
For each divided area composed of a plurality of pixels each obtained by dividing the surface of the photosensitive member into a plurality of areas, the exposure characteristics indicating the correspondence between the average exposure amount of the whole divided area and the potential after exposure and all the divisions Difference information storage means for storing difference information that is a difference from a reference characteristic common to the area;
For each of the divided regions, the correction target pixel array determined in advance by the area gradation method image processing based on the difference information stored in the difference information storage unit, and the pixel gradation of each of the correction target pixels. Correction pixel & gradation determination means for determining correction pixel & gradation information indicating correction gradation;
The pixel gradation correction means for correcting the pixel gradation of each pixel in each of the divided regions according to the determination result of the correction pixel & gradation determination means;
Exposure amount control means for controlling the exposure means so that exposure is performed at an exposure amount corresponding to the pixel gradation after correction by the pixel gradation correction means;
In images forming apparatus ing comprises a,
The average value of the correction gradation in the correction pixel & gradation information of each of the divided areas is an exposure characteristic indicating a correspondence between an average exposure amount of the entire divided area and a potential after exposure, and all the divided areas. Is a value according to the difference from the standard characteristics common to
The pixel gradation correction means corrects the pixel gradation based on the correction pixel & gradation information even when the pixel gradation of all the pixels in the divided region is 0 gradation. An image forming apparatus. An average value of the correction gradations in the correction pixel & gradation information of each of the divided areas is a value corresponding to a difference between an average initial potential in the exposure characteristic and an initial potential in the reference characteristic of the entire divided area. The image forming apparatus according to claim 1 , wherein 3. The image forming apparatus according to claim 1, wherein the area gradation method used for determining the correction pixel & gradation information is an error diffusion method or a screen method. Any said image processing means, according to claim 1 to 3 in which the gradation expressed by an area gradation method to determine the sequence of the pixel gray scale for each unit pixel group composed of a plurality of pixels based on the image data An image forming apparatus according to claim 1. The image forming apparatus according to claim 4 , wherein the number of pixels in the vertical direction and the number of pixels in the horizontal direction are the same between the divided region and the unit pixel group. A screen in which an area gradation method used for determination of the correction pixel & gradation information and an area gradation method used for determination of the pixel gradation by the image processing means deviate from each other by about 15 ° or more. the image forming apparatus according to claim 4 or 5 is a scheme. The image forming apparatus according to any one of claims 1 to 6, wherein the photoconductor is an a-Si photosensitive member.

Images