JP2006208612A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the density unevenness of an image from occurring and the degradation in image quality due to the obstruction of the continuity of image density with a photoreceptor in which electrostatic charge unevenness exists and in addition photosensitivity unevenness exists in parallel thereto while avoiding upsizing and cost increase of an image forming apparatus. <P>SOLUTION: An exposure source 2 is so controlled that exposure is performed at the amount of the exposure obtained by adding only the raised amount of the exposure Ea corresponding to the amount of the exposure V0 between the initial potential of the divided regions and a reference initial potential V0 in the entire gradation of pixel gradations including 0 gradation with respect to the amount of the exposure obtained by performing pixel gradation of the pixel region for each divided region obtained by dividing the surface of a photoreceptor drum 1 to a plurality of pieces. The adjustment of the amount of the exposure for the raising portion of the amount of the exposure Ea is performed by adjusting the exposure intensity of every pixel. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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.
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. 9 shows a 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. 9). 9A shows a case where exposure amount correction is not performed (shown by a thick broken line g01), and FIG. 9B shows a case where exposure amount sensitivity unevenness correction is performed (shown by a 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. 9A 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. 9A, 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. 9A, 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, in the exposure characteristic g01 of the photoconductor to be measured in FIG. 9A, a substantially linear exposure characteristic is shown in a range equal to or less than the charge amount E2 when the pixel gradation is I2, and the photoconductor as a reference. 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 charging unevenness and sensitivity unevenness coexist on the photoconductor to be measured, as shown in FIG. A difference in y intercept (corresponding to charging unevenness) and a difference in inclination of exposure characteristics (corresponding to sensitivity unevenness) occur. When exposure sensitivity unevenness correction (correction for matching inclinations) is performed on such a photoconductor, as shown in FIG. 9B, a potential difference (initial potential difference) corresponding to charging unevenness 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. 10 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 the pixel gradation is converted into the exposure amount 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. 10, 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 set. 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 object of the present invention is to provide an image forming apparatus capable of preventing image density unevenness from occurring on a coexisting photoconductor and preventing deterioration in image quality by inhibiting continuity of image density.

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). For each of the divided areas obtained by dividing the surface of the photosensitive member into a plurality of divided areas. Difference information corresponding to the difference between the initial potential (the potential after charging and before exposure) and the reference initial potential common to all the divided areas is recorded in the storage means (individual difference information storage means). In addition, with respect to the exposure amount obtained by substantially linearly converting the pixel gradation determined by the image processing means into an exposure amount, the pixel gradation is within a predetermined range including 0 gradation for each divided region. In some cases, the exposure unit controls the exposure with the exposure amount added by the exposure amount corresponding to the difference information (hereinafter referred to as the post-addition exposure amount). Here, the exposure amount is determined by the product of the exposure intensity and the exposure time. The exposure time in the exposure means is adjusted based on at least the pixel gradation, and the exposure intensity in the exposure means is determined based on the difference information. Adjust.
In this way, by performing exposure with the post-addition exposure amount in which the exposure amount is added (raised) according to the difference between the initial potential and the reference initial potential for each divided region, charging unevenness ( With respect to the photoconductor (charged photoconductor) having an initial potential distribution), the characteristics of the potential after exposure with respect to the pixel gradation approach or match the reference characteristics having the reference initial potential as the initial potential. As a result, variations in post-exposure potential for each position on the surface of the photoreceptor can be suppressed, and the occurrence of uneven density in the image can be prevented as much as possible. In particular, when the image processing means performs gradation expression by the area gradation method, even if there is a charging unevenness with a relatively large spatial period, it is prevented from appearing as an image unevenness density. It is preferable in that it can be performed.
Furthermore, since exposure is performed with an exposure amount that is additively corrected (correction with respect to the exposure amount after the substantially linear conversion) even for pixels in which the pixel gradation that is not conventionally exposed is 0 gradation, Since the gap in the post-exposure potential (ΔV0 in FIG. 10) between when the pixel gradation is 0 gradation and when it is 1 gradation is suppressed, the continuity of the halftone density is not hindered and the image quality is not deteriorated. .
In addition, since it can be realized by adjusting the exposure amount (adjustment of conversion from the pixel gradation to the exposure amount) of the existing exposure unit for writing the electrostatic latent image without adding a new exposure unit, etc. Increase in cost and cost.

Here, if the exposure intensity in the exposure means is controlled to be constant during exposure for one pixel, for example, during exposure for one pixel, exposure for adjusting the exposure amount according to the difference information, This is suitable when the exposure based on the pixel gradation is divided in time and it is difficult to switch the exposure intensity of each. In this case, even if the difference information is the same, it is needless to say that the exposure intensity needs to be set for each pixel gradation because the exposure time differs if the pixel gradation is different.
It is also conceivable to adjust the exposure time in the exposure means based on the pixel gradation and the difference information.
In this case, with respect to the exposure amount control based on the difference information, the exposure intensity of the exposure means is adjusted within a predetermined allowable range, and an exposure amount that is insufficient for adjusting the exposure intensity within the allowable range is obtained. By adjusting according to the exposure time of the exposure means, it is possible to prevent the adjustment range of the exposure intensity from becoming too large and returning to deteriorate the image quality. In particular, when the exposure intensity is constant during exposure for one pixel, the exposure time is shortened when the pixel gradation value is small, and the exposure intensity is set to add the exposure amount according to the difference information. The inconvenience of having to be remarkably strengthened can be prevented.

On the other hand, for each of the divided areas, inclination information that defines an inclination when the pixel gradation is substantially linearly converted to the exposure amount is stored in advance in storage means (individual inclination information storage means), and the image processing means Control is performed so that exposure by the exposure means is performed with an exposure amount obtained by adding an exposure amount corresponding to the difference information to an exposure amount obtained by substantially linearly converting the pixel gradation determined in accordance with the tilt information. It is also possible.
In this case, in the exposure amount control, the exposure intensity in the exposure unit may be adjusted based on the tilt information for each divided region.
As a result, as will be described later, with respect to a photoconductor in which charging unevenness and sensitivity unevenness coexist, the characteristics of the potential after exposure with respect to the pixel gradation over the entire gradation range showing linear characteristics including 0 gradation. This substantially matches the reference characteristic. As a result, in the photoreceptor 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 image density unevenness can be further enhanced.

The divided area includes an area obtained by dividing the surface of the drum-shaped photosensitive member into a plurality of parts in the axial direction or the circumferential direction (one-dimensional division), or a plurality of areas divided in both directions (two-dimensional division). Conceivable. For example, it is conceivable to divide in units of width or height of one pixel, or to divide in units of width or height for a plurality of pixels.
Here, it is needless to say that the exposure by the exposure means needs to be performed by recognizing each position of the divided area. In general, with respect to the exposure position in the axial direction (that is, the main scanning direction) of the surface of the photoreceptor, the writing position is recognized (detected) at least in pixel units in the exposure means (or its control means). On the other hand, since the absolute position in the circumferential direction (sub-scanning direction) of the surface of the photoconductor is not information directly necessary for image formation, it is necessary to provide means for detecting the rotational position of the photoconductor.
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, for each divided area obtained by dividing the surface of the photosensitive member into a plurality of areas, the exposure amount including the case where the pixel gradation is 0 gradation according to the difference between the initial potential and the reference initial potential. Therefore, variation in post-exposure potential for each surface position of the photosensitive member having uneven charging can be suppressed, and image density unevenness can be prevented.
Further, even for a pixel in which the pixel gradation is 0 gradation which is not conventionally exposed, the exposure correction is added (raised) according to the difference in the initial potential, so the pixel gradation is 0. The gap of the post-exposure potential between the gradation and the gradation is suppressed, and the continuity of the halftone density is not hindered and the image quality is not deteriorated.
In addition, since it can be realized by adjusting the exposure amount of the existing exposure means for writing an electrostatic latent image without adding new exposure means or the like, the apparatus is not increased in size and cost.
In addition, while performing exposure amount addition control in accordance with the distribution of the initial potential for each of the divided regions, tilt information that defines a tilt at which the pixel gradation is substantially linearly converted to the exposure amount for each of the divided regions. If the control for adjusting the exposure intensity in the exposure means is also performed based on the above, in the photoconductor in which the charging unevenness and the sensitivity nonuniformity coexist, the variation in the post-exposure potential for each position on the surface of the photoconductor is further suppressed. Thus, the effect of preventing density unevenness in the image can be further enhanced.

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 pixel floor in the image forming apparatus X. FIG. 4 is a graph showing an example of the relationship between the first embodiment of the exposure amount control characteristic with respect to the tone and the relationship between the pixel gradation at that time and the potential after exposure, and FIG. FIG. 5 is a graph showing an example of the relationship between the second embodiment and the pixel gradation at that time and the potential after exposure, and FIG. 5 is a time for explaining the first embodiment of control of exposure intensity and exposure time in the image forming apparatus X. FIG. 6 is a time chart for explaining the second embodiment of the control of the exposure intensity and the exposure time in the image forming apparatus X, and FIG. 7 shows the third embodiment of the control of the exposure intensity and the exposure time in the image forming apparatus X. Time chart FIG. 8 is a time chart for explaining the function of the exposure source in the image forming apparatus X, and FIG. 9 is an example of the relationship between the conventional pixel gradation and the potential after exposure on the surface of the photoconductor on which charging unevenness and sensitivity unevenness coexist. FIG. 10 is a graph showing the exposure amount conversion so that the potential after exposure after setting and exposing all pixel gradations except for 0 when exposing the surface of the photoreceptor where charging unevenness and sensitivity unevenness coexist is matched. It is a graph showing the relationship between the pixel gradation at the time of performing and the electric potential after exposure.

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 unit α1 has four photosensitive drums 1 (1BK for black, 1M for magenta, 1Y for yellow, 1C for cyan) carrying the images of the above four colors, and the surface of each of the photosensitive drums 1 is integrated. 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 3 is set 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) a corresponding exposure amount of light, and the electrostatic latent image The toner images formed on the surfaces of the developing drums 5 (5BK, 5M, 5Y, 5C) and the photosensitive drums 1 are developed in order to develop toner images by supplying toner to the recording paper. Intermediate transfer belt 7 for transfer, transport roller 8 for transporting the recording paper, fixing device 9 for heating and fixing the toner image transferred on the recording paper, and neutralizing the surface of the photosensitive drum 1 after transferring the toner image to the recording paper. It is schematically configured to include a static eliminating device 4 (4BK, 4M, 4Y, 4C) to be performed.

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 the like, which is a readable / writable storage means, and includes a data storage unit 13 for storing various data, a rotation position detection unit 14 for detecting the position of each of the photosensitive drums 1 in the rotation direction, and the like.
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.
The data storage unit 13 is used in advance to control the exposure amount of the exposure source 2 based on the pixel gradation for each of the divided areas obtained by dividing the surface of each of the photosensitive drums 1 into a plurality of areas. As information, information obtained by converting a difference (V) between an initial potential of each divided region and a reference initial potential common to all the divided regions (hereinafter referred to as a reference initial potential) into an exposure amount (μJ / cm ² ). Are stored separately (an example of individual difference information storage means). The specific contents will be described later.
Here, the divided region is, for example, a region corresponding to each pixel (a width of one pixel (axial direction) × a height of one line (circumferential direction)) or an area floor in the image processing unit 12. A region corresponding to the unit pixel group employed in the image processing in the gradation method may be considered.

Then, the control unit 10 acquires the pixel gradation determined by the image processing unit 12, and for each of the photoreceptors 1 and for each divided region based on the pixel gradation and the raised exposure amount. The exposure amount in the exposure source 2 is individually 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, etc., and measure the elapsed time from the detection time.
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. This will be described later.

Next, the raising exposure amount (an example of difference information) will be described.
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 therein in the manufacturing stage. 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. 9A 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. 9 (a), the tendency of exposure characteristics (curve shape) 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.

<First Example of Individual Exposure Amount Control>
Hereinafter, with reference to FIG. 9 and FIG. 3 described above, a certain divided region on the surface of the a-Si photosensitive drum 1 has the characteristics shown in FIG. 9A, that is, charging unevenness and sensitivity unevenness. The first embodiment of the individual exposure amount control will be described by taking the case of having the exposure characteristic (g0) as an example.
FIG. 3B is a graph showing a first example of the individual exposure amount control characteristic (pixel gradation-exposure amount characteristic) for the divided region having the exposure characteristic shown in the graph g01 of FIG. 9A. FIG. 3A is a graph showing the relationship between the pixel gradation and the potential after exposure (post-exposure potential) when the individual exposure amount control according to the characteristics of FIG. 3B is performed. It is.
Here, the characteristics (E = k0 · I, E is the exposure amount, I is the pixel gradation, and k0 is the inclination) shown by the one-dot broken line in FIG. 3B is a reference from the pixel gradation to the exposure amount. This represents a (standard) linear conversion characteristic (hereinafter referred to as a reference exposure amount conversion characteristic). The characteristics shown in the graphs g0 and g01 in FIG. 9A are the reference exposure amount conversion characteristics (slope = k0, y intercept). = 0)), it is assumed that the exposure is performed with the exposure amount according to the characteristics.
Also, the conversion characteristics (E = k0 · I + Ea, where E is the exposure amount, I is the pixel gradation, k0 is the inclination, and Ea is the above-mentioned raising exposure amount) indicated by the solid line in FIG. A characteristic of quantity control is obtained, and a y-intercept Ea in this characteristic is stored in advance in the data storage unit 13 as the raised exposure amount for each of the divided areas.
Further, the raised exposure dose Ea is necessary for lowering the potential by the difference (Vx0−V0) between the initial potential Vx0 and the reference initial potential V0 in the exposure characteristics (g01 in FIG. 9) of the divided area. Exposure amount Ea.

As shown in FIG. 3B, the control unit 10 executes a predetermined control program to expose the pixel gradation I determined by the image processing unit 12 according to the reference exposure amount conversion characteristics. With respect to the exposure amount obtained by linear conversion to the amount E (exposure amount Ei when the pixel gradation is i), the raised exposure is performed in the entire gradation range of the pixel gradation including 0 gradation for each divided region. The exposure source 2 is controlled so that the exposure is performed with the exposure amount added by the amount Ea (that is, the exposure amount corresponding to the difference between the initial potential of the divided area and the reference initial potential) (the first of the exposure amount control means). 1 example).
Here, the control unit 10 (an example of exposure amount control means) controls the exposure amount Ei that needs to be adjusted according to the pixel gradation and the raised exposure amount Ea for each of the divided regions. How to reflect in the control of the light source 2 will be described later.
When the exposure source 2 is controlled by such individual exposure amount control, the characteristics of the potential after exposure with respect to the pixel gradation are as shown by a graph gx1 in FIG.
The individual exposure amount control shown in FIG. 3B, that is, the exposure in which the exposure amount Ea is added (raised) according to the difference between the initial potential Vx0 and the reference initial potential V0 for each divided region. By performing exposure and controlling by the amount, as shown in the graph gx1 in FIG. 3A, the photosensitive member (charged photosensitive member) in which charging unevenness exists has been subjected to the pixel gradation after the exposure. The potential characteristics generally approach the reference characteristic g0 having the reference initial 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.
Further, even in the case where the pixel gradation which is conventionally not exposed is 0 gradation, the pixel gradation is 0 because the exposure with the raised exposure amount Ea corresponding to the difference in the initial potential is performed. The gap between the post-exposure potentials (ΔV0 in FIG. 10) between the gradation and the one gradation is suppressed, and the continuity of the halftone density is not hindered to deteriorate the image quality.
In the example shown in FIG. 3, the exposure control based on the raised exposure amount is performed when the pixel gradation is the entire range including the 0 gradation, but the pixel gradation is 0 gradation. The exposure amount correction based on the raised exposure amount may be performed only in a part of the range including.
For example, among the characteristics g01 shown in FIG. 9A, only when the pixel gradation is a gradation within the range from the 0 gradation to a substantially linear characteristic (that is, the potential after exposure is the residual potential and its potential. Even if the exposure amount control based on the raised exposure amount is performed, the exposure characteristics in each of the divided regions substantially coincide with the reference exposure characteristics. The same applies to the second embodiment of the next individual exposure amount control.

<Second Example of Individual Exposure Control>
Hereinafter, the individual exposure amount will be described by taking as an example the case where a certain divided area on the surface of the a-Si photosensitive drum 1 has the characteristic g0 shown in FIG. A second embodiment of control will be described.
FIG. 4B is a graph showing the characteristics of the second embodiment of the individual exposure amount control for the divided area having the exposure characteristics shown in the graph g01 of FIG. 9A as in FIG. FIG. 4A is a graph showing the relationship between the pixel gradation and the potential after exposure (post-exposure potential) when the individual exposure amount control according to the characteristics of FIG. 3B is performed. It is.
Here, the characteristic indicated by the dashed line in FIG. 4B represents the same characteristic as the dashed line in FIG.
Also, the conversion characteristics (E = k1 · I + Ea, where E is the exposure amount, I is the pixel gradation, k1 is the inclination, and Ea is the above-mentioned raising exposure amount) indicated by the solid line in FIG. A characteristic of quantity control is obtained, and a y-intercept Ea in this characteristic is stored in advance in the data storage unit 13 as the raised exposure amount for each of the divided areas.
Further, k1 is inclination information that defines an inclination when linearly converting the pixel gradation into the exposure amount for each of the divided areas, and the data storage unit 13 (an example of individual inclination information storage means) This information is stored in advance for each divided region.
Further, as described above, the raised exposure amount Ea is the exposure amount Ea necessary for lowering the potential by the difference (Vx0−V0) between the initial potential Vx0 and the reference initial potential V0 in the divided region. .
Here, the control unit 10 (an example of an exposure amount control unit) requires an exposure amount Ei that needs to be adjusted according to the pixel gradation, the raised exposure amount Ea for each divided region, and the divided region for each divided region. How to control the exposure amount Eki required according to the tilt information in the control of the exposure source 2 will be described later.

As shown in FIG. 4B, the control unit 10 executes the predetermined control program to change the pixel gradation I determined by the image processing unit 12 for each of the divided areas to the inclination. Exposure is performed with an exposure amount obtained by adding the raised exposure amount Ea (that is, an exposure amount corresponding to the difference between the initial potential of the divided area and the reference initial potential) to the exposure amount obtained by linear conversion according to the information k1. The exposure source 2 is controlled so that the exposure is performed (second embodiment of exposure amount control means).
When the exposure source 2 is controlled by the individual exposure amount control as described above, the variation in inclination in the exposure characteristics, that is, the variation in the exposure characteristics due to the sensitivity unevenness is corrected. The characteristic of the potential substantially coincides with the reference characteristic g0 as shown by a graph gx2 in FIG.
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. .
As described above, as shown in FIGS. 3 and 4, the control unit 10 determines that the pixel gradation is 0 for each divided region with respect to the exposure amount Ei obtained by linearly converting the pixel gradation to the exposure amount. Control is performed so that exposure by the exposure means is performed with an exposure amount (Ei + Ea) added by the raised exposure amount Ea (exposure amount corresponding to the difference in initial potential) in the case of the entire range including gradation. .
Further, when control based on the tilt information k1 is also performed, control is performed so that exposure by the exposure means is performed with an exposure amount (Ei + Ea + Eki) added with an exposure amount adjustment Eki based on the tilt information k1. is there.
At that time, the control unit 10 determines the exposure time in the exposure source 2 (exposure means) based on the pixel gradation or both the pixel gradation and the raised exposure amount Ea (an example of difference information). The exposure intensity is adjusted based on the raised exposure amount Ea (an example of exposure amount control means).
When the control based on the tilt information k1 is also performed, the exposure intensity in the exposure source 2 is adjusted based on the tilt information k1 in addition to the raised exposure amount Ea for each divided region.
Hereinafter, an embodiment of controlling the exposure intensity and the exposure time will be described.

<First Example of Control of Exposure Intensity and Exposure Time>
First, a description will be given of a first embodiment of exposure intensity and exposure time control (hereinafter referred to as intensity / time control) in the exposure source 2 performed by the control unit 10 executing a predetermined control program.
FIG. 5 is a time chart for explaining a first embodiment of the intensity / time control. Here, FIG. 5A shows a case where control based on the tilt information k1 is not performed (corresponding to FIG. 3B), and FIG. 5B shows a case where control based on the tilt information k1 is performed (FIG. 5). 4 (b)).
As shown in FIG. 5A, in this embodiment, the control unit 10 performs exposure for the raised exposure amount Ea by adjusting the exposure intensity P2 during a certain exposure time t1. Further, exposure for the exposure amount Ei based on the pixel gradation is performed by adjusting the exposure time t2 proportional to the pixel gradation i while maintaining a constant exposure intensity P1 (ie, Ea = P2 · t2, Ei = P1 · t1). Here, the exposure intensity P1 is an exposure intensity that is a reference common to all pixels, and the exposure intensity P1 is an exposure intensity that is proportional to the raised exposure amount Ea preset for each of the divided areas.
Thereby, the exposure time in the exposure source 2 (exposure means) is adjusted based on the pixel gradation, and the exposure intensity in a part of the time zone is adjusted based on the raised exposure amount Ea. As a result, it is possible to control the exposure amount with the characteristics shown in FIG.
Further, when the control based on the tilt information k1 is also performed, the exposure amount adjustment Eki (see FIG. 3B) based on the tilt information k1 is the exposure when performing exposure according to the pixel gradation. The exposure amount is adjusted by exposing the intensity P1 with the exposure intensity (P1 + P3) corrected by P3 (that is, P1 + P3 = P1 · k1 / k0).
Thereby, the exposure intensity in the exposure source 2 is adjusted based on the inclination information k1 in addition to the raised exposure amount Ea for each of the divided regions in a part of the time zone for exposure of one pixel. The As a result, it is possible to control the exposure amount with the characteristics shown in FIG.
For exposure based on the raising exposure amount Ea, the exposure time t2 is changed continuously or stepwise depending on the magnitude of the raising exposure amount Ea, for example, and Ea = P2 · t2 is maintained. The exposure intensity P2 may be set as described above. This is an example in which the exposure time in the exposure source 2 (exposure means) is adjusted based on both the pixel gradation and the raised exposure amount Ea (an example of difference information).
As described above, the exposure based on the pixel gradation and the tilt information and the exposure based on the raised exposure amount are individually performed at different times, so that the control is easy to understand.

<Second embodiment of control of exposure intensity and exposure time>
Next, a second embodiment of the intensity / time control by the control unit 10 will be described.
FIG. 6 is a time chart for explaining a second embodiment of the intensity / time control.
As shown in FIG. 6, in the present embodiment, the controller 10 performs an exposure time proportional to the pixel gradation i, except for the case where the pixel gradation is 0 gradation, for each exposure of one pixel. During t1, exposure is performed with an exposure amount obtained by adding a correction exposure intensity P2 'adjusted according to the raised exposure amount Ea and t1 (that is, the pixel gradation) to a predetermined reference exposure intensity P1. . Here, Ei = P1 · t1, Ea = P2 ′ · t1.
On the other hand, when the pixel gradation is 0 gradation, as shown in FIG. 5, the exposure intensity P2 and the exposure time t2 are set so that the raised exposure amount Ea is secured (Ea = P2 · t2). Here, it is conceivable that the exposure time t2 is set to a predetermined fixed time or variable according to the magnitude of the raised exposure amount Ea.
Thus, the exposure intensity is constant (= P1 + P2 ′) during exposure for one pixel, and the exposure amount control with the characteristics shown in FIG. 3B is possible.
Such control is suitable when it is difficult to switch the exposure intensity during the exposure for one pixel.
However, in this case, even if the raising exposure amount Ea is the same, if the pixel gradation is different, the exposure time t1 is different, so the exposure intensity P2 ′ is set for each pixel gradation (Ea = P2 ′ · t1).
Further, when the control based on the tilt information k1 is also performed, the exposure adjustment amount Eki (see FIG. 3B) based on the tilt information k1 is similar to that shown in FIG. 5B. When the pixel gradation is other than 0 gradation, the exposure amount is adjusted by exposing the exposure intensity (P1 + P2 ′) with an exposure intensity (P1 + P2 ′ + P3) corrected by P3 (not shown) (ie, P1 + P3). = P1 · k1 / k0).
Thereby, the exposure intensity in the exposure source 2 is adjusted based on the tilt information k1 in addition to the raised exposure amount Ea for each of the divided areas in the exposure for one pixel. As a result, it is possible to control the exposure amount with the characteristics shown in FIG.

<Third embodiment of control of exposure intensity and exposure time>
Next, a third embodiment of the intensity / time control by the control unit 10 will be described.
FIG. 7 is a time chart for explaining a third embodiment of the intensity / time control.
As shown in FIG. 7, also in this embodiment, the control unit 10 controls the exposure intensity to be constant (= P1 + P2 ″) during exposure for one pixel.
Here, the control unit 10 controls the raising exposure amount Ea (an example of an exposure amount based on difference information) for each exposure of one pixel, except when the pixel gradation is 0 gradation. , The exposure intensity P2 ″ is adjusted within a predetermined allowable range (0 to P2 ″ max), and the exposure amount insufficient for adjusting the exposure intensity P2 ″ within the allowable range is adjusted by the exposure time t3. That is, Eki = P1 · t2, Ea = P2 ″ · t2 + (P1 + P2 ″) · t3 (provided that 0 ≦ P2 ″ ≦ P2 ″ max and t3 = 0 within the range of Ea ≦ P2 ″ max · t2) It is.
On the other hand, when the pixel gradation is 0 gradation, although not shown in FIG. 7, as shown in FIG. 5, the exposure intensity P2 and the exposure time t2 are ensured so as to ensure the raised exposure amount Ea. Is set (Ea = P2 · t2). In this case, the exposure time t2 is made variable according to the magnitude of the raised exposure dose Ea, and the exposure intensity P2 is controlled so as not to exceed the allowable upper limit (P1 + P2 ″ max).
This makes it possible to control the exposure amount with the characteristics shown in FIG.
Such control is suitable when it is difficult to switch the exposure intensity during the exposure for one pixel.
Further, since the exposure intensity is adjusted within a range within the upper limit (P1 + P2 ″ max), the adjustment range of the exposure intensity is excessively increased and the image quality is deteriorated or the exposure intensity of the exposure source 2 is adjusted. It is possible to prevent exceeding the possible range.
Further, when the control based on the tilt information k1 is also performed, the exposure adjustment amount Eki (see FIG. 3B) based on the tilt information k1 is similar to that shown in FIG. 5B. The exposure amount is adjusted by exposing the exposure intensity (P1 + P2 ″) with an exposure intensity (P1 + P2 ″ + P3 ′) corrected by P3 ′ (not shown). That is, when the pixel gradation is other than 0 gradation, P1 + P3 ′ = P1 · t1 · (k1 / k0) / (t1 + t3).
Thereby, the exposure intensity in the exposure source 2 is adjusted based on the tilt information k1 in addition to the raised exposure amount Ea for each of the divided areas in the exposure for one pixel. As a result, it is possible to control the exposure amount with the characteristics shown in FIG.

Next, the function of the exposure source 2 will be described with reference to the time chart of FIG.
The exposure source 2 provided in the image forming apparatus X is an LED array type exposure unit in which one LED lamp (light emitting unit) is arranged for each pixel in the main scanning direction.
As shown in FIG. 8, the exposure source 2 generates a predetermined exposure permission signal Sg1 and a pulse signal once or continuously from the control unit 10 for each LED lamp (that is, for each pixel). The count signal Sg2 to be transmitted and the set count number Cs for designating the number of counts of the change of the count signal Sg2 from ON to OFF are input.
Further, the exposure source 2 receives a set exposure intensity for designating the light emission intensity (exposure intensity) for each LED lamp (for each pixel) from the control unit 10, so Exposure is performed with an exposure intensity (light emission intensity) corresponding to the set exposure intensity by the action of the supply power adjustment unit. The exposure intensity P1, P2, (P1 + P3), (P1 + P2 ′), (P1 + P2 ″) is adjusted according to the set exposure intensity.
Then, after the exposure permission signal Sg1 changes to the permission state (changes from ON to OFF), the exposure source 2 changes from the ON to OFF of the count signal Sg2 until the set count times Cs. During the exposure (between ta + tb in the figure), the LED lamp is turned on to perform exposure.

On the other hand, the control unit 10 outputs the count signal Sg2 whose ON → OFF change period is a constant period to the exposure source 2 and sets 1 to the pixel gradation (here, an integer of 0 to 15). Is specified (set) as the value of the set count number Cs, and the exposure permission signal Sg1 is changed to a permission state (OFF) (generation of an exposure start command). The exposure time t2 (FIG. 5) and t3 (FIG. 7) are adjusted by adjusting the time ta from the first change of the count signal Sg2 (ON to OFF, generation of a periodic signal). .
At that time, the control unit 10 changes the exposure permission signal Sg1 output to the exposure source 2 from ON to OFF (generation of an exposure start command), and from there, a time tc corresponding to the exposure time t2 or t3. In addition, a time point when a predetermined delay time t0 is added is detected as a reference time point Pt0. The detection of the reference time point Pt0 is performed, for example, by counting a timing signal (not shown) that changes at a constant cycle.
On the other hand, the exposure time t1 (FIGS. 5 to 7) is adjusted by adjusting the set count number Cs.
Here, the delay time t0 is a time for supplementing the exposure amount corresponding to the startup loss of the LED lamp in the exposure source 2 (exposure amount that is insufficient with respect to the exposure amount that should originally be due to the operation delay at the start of light emission). In this time chart, the exposure amount for the delay time t0 is regarded as 0 (zero). Therefore, the exposure time is substantially (tc + tb).
By controlling the exposure source 2 as described above, the intensity and time can be controlled.
Even when a laser scanning device is used as the exposure source 2, the exposure amount can be controlled similarly.

In the embodiment described above, the divided region is a region obtained by dividing the surface of the photosensitive drum 1 into a plurality of portions both in the axial direction and in 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 embodiment and the example, when the raised exposure amount Ea is converted as the difference information of the initial potential of each divided region with respect to the reference initial potential, and the pixel gradation is converted into the exposure amount as the inclination information. However, the present invention is not limited to this. For example, other information may be used as long as it is information that can identify the difference information and the inclination. For example, it is conceivable that information such as the difference between the initial potentials of the respective divided regions with respect to the reference initial potential and a value obtained by converting the difference into the correction gradation in advance is stored in the data storage unit 13 as the difference information. .
Similarly, a conversion table from the pixel gradation to the exposure amount, coordinate information for specifying an inclination with respect to a coordinate system composed of the axis of the pixel gradation and the axis of the exposure amount, etc. are stored as the inclination information in the data storage. It is conceivable to store in the unit 13.

In the embodiments and examples described above, the image forming apparatus X in which the raised exposure amount Ea and the inclination information are stored in advance in the data storage unit 13 is shown. However, the raised exposure amount Ea (difference) An example of information) and an image forming apparatus provided with means for calculating the tilt information are also conceivable as embodiments.
For example, information relating to exposure characteristics in each of the divided areas of the photosensitive drum 1 mounted on the image forming apparatus X and information relating to exposure characteristics serving as a common reference for all the divided areas are stored in the data storage unit 13 in advance. There is also provided an image forming apparatus configured to provide means for calculating the raised exposure amount Ea and the inclination information k1 based on the stored information, and to perform the exposure amount control as described above based on the calculation result. Conceivable. In this case, it can be said that the information regarding the exposure characteristics in each of the divided areas is basic information including the difference information and the inclination information.
Accordingly, it is possible to save the trouble of calculating and storing the raised exposure amount and the tilt information individually for each apparatus at the manufacturing stage of the image forming apparatus.

  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. 6 is a graph showing an example of the relationship between the pixel gradation at that time and the potential after exposure in the first embodiment of the exposure amount control characteristic with respect to the pixel gradation in the image forming apparatus X; 7 is a graph showing an example of the relationship between the pixel gradation at that time and the potential after exposure in the second embodiment of the exposure amount control characteristic with respect to the pixel gradation in the image forming apparatus X. 3 is a time chart for explaining a first embodiment of control of exposure intensity and exposure time in the image forming apparatus X. FIG. 9 is a time chart for explaining a second embodiment of control of exposure intensity and exposure time in the image forming apparatus X. 9 is a time chart for explaining a third embodiment of control of exposure intensity and exposure time in the image forming apparatus X. 3 is a time chart for explaining the function of an exposure source in the image forming apparatus X. 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

Claims (8)

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 photoreceptor by exposing each pixel with a corresponding exposure amount;
Individual difference information storage means for storing difference information according to the difference between the initial potential of the divided area and the reference initial potential common to all the divided areas, for each divided area obtained by dividing the surface of the photosensitive member into a plurality of areas; ,
In the case where the pixel gradation is within a predetermined range including 0 gradation for each of the divided areas with respect to the exposure amount obtained by substantially linearly converting the pixel gradation determined by the image processing unit into an exposure amount. Control is performed so that exposure by the exposure unit is performed with an exposure amount added according to the difference information, and the exposure time in the exposure unit is adjusted based on at least the pixel gradation, and the exposure unit Exposure amount control means for adjusting the exposure intensity in the light source based on the difference information;
An image forming apparatus comprising:   2. The image forming apparatus according to claim 1, wherein the exposure control means controls the exposure intensity in the exposure means to be constant during exposure for one pixel.   The image forming apparatus according to claim 1, wherein the exposure amount control unit adjusts an exposure time in the exposure unit based on the pixel gradation and the difference information.   The exposure amount control means adjusts the exposure intensity of the exposure means within a predetermined allowable range for controlling the exposure amount based on the difference information, and the adjustment of the exposure intensity within the allowable range is insufficient. The image forming apparatus according to claim 3, wherein the exposure amount is adjusted by the exposure time of the exposure means. For each of the divided areas, comprising individual inclination information storage means for storing inclination information defining an inclination when the pixel gradation is substantially linearly converted into the exposure amount;
The image forming apparatus according to claim 1, wherein the exposure amount control unit adjusts an exposure intensity in the exposure unit based on the tilt information for each of the divided regions.   The image forming apparatus according to claim 1, wherein the divided region is a region obtained by dividing the surface of the drum-shaped photoconductor into a plurality of one or both of the axial direction and the circumferential direction.   The image forming apparatus according to claim 1, wherein the photoconductor is an a-Si photoconductor.   The image formation according to claim 1, wherein the image processing unit performs gradation expression by an area gradation method that determines an array of the pixel gradations of a plurality of pixels based on the image data. apparatus.

Images