JP2006181883A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent density unevenness of an image from generating and to prevent an image quality from worsening because the continuity of an image density is hindered for a photosensitive body in which charging unevenness is present, or a photosensitive body in which also sensitivity unevenness coexists in addition to that, while upsizing and cost increase of an apparatus are avoided. <P>SOLUTION: For each of a plurality of divided regions divided from a photosensitive body drum 1 surface, an exposure source 2 is controlled so that exposure is carried out by an exposure which has an increasing exposure Ea corresponding to a difference between an initial potential of the divided region and a reference initial potential V0 added to an exposure obtained by nearly linearly converting a pixel gradation, in all gradations of the pixel gradation including a gradation 0. Exposure control for the increasing exposure Ea is carried out by increasing an exposure time for each 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. 8 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. 8). 8A shows a case where exposure amount correction is not performed (represented by a thick broken line g01), and FIG. 8B 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. 8A has the pixel gradation as the horizontal axis, but the conversion from the pixel gradation to the exposure amount (the individual exposure amount conversion) is a certain conversion formula (coefficient Is fixed) or as long as it is performed based on the conversion table, the horizontal axis is equivalent to the exposure amount. That is, in FIG. 8A, 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. 8A, in general, the exposure characteristic representing the correspondence between the exposure amount and the potential after exposure on the photosensitive member (particularly, the 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. 8A 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 charging unevenness and sensitivity unevenness coexist on the photoconductor to be measured, as shown in FIG. 8A, an initial potential (charging potential before exposure, that is, a charging potential before exposure, that is, between the reference exposure characteristic g0 and 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. 8B, 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).
The graph g02 ′ in FIG. 9 sets all pixel gradations except for the 0 gradation when the surface of the photoconductor in which the charging 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. 9, the gap ΔV 0 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 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).
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. 9) between when the pixel gradation is 0 gradation and when it is 1 gradation is suppressed, the continuity of 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.

Further, for each of the divided regions, 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.
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.

Here, the exposure amount control means according to the difference information includes adjusting the exposure amount per pixel (exposure time control) according to the difference information by adjusting the exposure time per pixel by the exposure means. It is possible to do it.
In general, in the exposure means such as an LED array, the exposure is performed during a period from when a predetermined exposure start command is generated for each pixel until a periodic signal continuously generated at a predetermined period is generated a specified number of times. Many.
When such an exposure means is used, for example, a value corresponding to the pixel gradation (a value corresponding to the magnitude of the pixel gradation) is specified to the exposure means as the value of the count number as the exposure time control. And adjusting the time from generation of the exposure start command to generation of the first periodic signal according to the difference information, or setting a value according to the difference information to a value according to the pixel gradation. It is conceivable to adjust the exposure time per pixel by setting the added value as the number of counts specified to the exposure means.
In addition, when using the exposure means for performing exposure after a predetermined exposure start command is generated for each pixel until a predetermined exposure end command is generated, the exposure end command is generated from the generation of the exposure start command. It is also conceivable to adjust the exposure time per pixel by adjusting the time until occurrence of.
On the other hand, the control (adjustment) of the exposure amount that changes in accordance with the tilt information may be performed by adjusting the exposure intensity in the exposure means (adjustment of power supplied to the light emitting unit, etc.).

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. Specifically, in the a-Si photoconductor, the charging unevenness caused by the difference in film thickness is larger than the sensitivity unevenness caused by the film quality difference, so that it is effective to correct the charging unevenness. Furthermore, by adjusting the exposure time, it is possible to sufficiently adjust the sensitivity unevenness, and the best adjustment can be realized.

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, the unevenness of the post-exposure potential for each surface position of the photoconductor having uneven charging can be suppressed and the occurrence of uneven image density 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.
Further, the exposure amount is controlled in accordance with the initial potential distribution for each of the divided regions by adjusting the exposure time in the exposure means, and 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 tilt information that defines the tilt at the time, in the photoconductor in which the charging unevenness and the sensitivity nonuniformity coexist, after the exposure for each position of the surface of the photoconductor The variation in potential can be further reduced, and the effect of preventing the occurrence of uneven density 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. 4 shows the exposure amount control characteristic with respect to the pixel gradation in the image forming apparatus X. 5 is a graph showing an example of the relationship between the pixel gradation at that time and the potential after exposure, FIG. 5 is a time chart for explaining the first embodiment of exposure time control in the image forming apparatus X, and FIG. Is a time chart for explaining a second embodiment of exposure time control in the image forming apparatus X, FIG. 7 is a time chart for explaining a third embodiment of exposure time control in the image forming apparatus X, and FIG. Sensitivity that coexists with FIG. 9 is a graph showing an example of the relationship between the conventional pixel gradation on the surface and the potential after exposure, and FIG. 9 sets all the pixel gradations except for 0 when exposing the surface of the photoreceptor where charging unevenness and sensitivity unevenness coexist. 6 is a graph showing the relationship between pixel gradation and the potential after exposure when individual exposure amount conversion is performed so that the potential after exposure is matched with the reference characteristics.

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 control unit 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 possible to use a configuration in which a reference portion such as is provided, the passing position of the reference portion is detected by a contact-type switch, a photocoupler, or the like, and the elapsed time from the detection time is counted.
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. 8A 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. 8 (a), the tendency (exposure curve) of the exposure characteristics is determined to some extent, and it is common 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. 8 and FIG. 3 described above, the divided region on the surface of the a-Si photosensitive drum 1 has the characteristics shown in FIG. 8A, 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. 8A. 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. 8A 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. 8) 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, the raised exposure amount Ea (that is, the initial potential of the divided region) 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 exposure amount corresponding to the difference between the reference initial potential and the reference initial potential (first embodiment of the exposure amount control means).
Here, the exposure amount control by the control unit 10 (an example of the exposure amount control means) needs to be adjusted according to the exposure amount that needs to be adjusted according to the pixel gradation and the raised exposure amount Ea. This is performed by adjusting the exposure amount per pixel by adjusting the exposure time per pixel by the exposure source 2 (exposure means) together with the exposure amount (an example of exposure time control means). This will be described in detail later by showing an embodiment of exposure time control.
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. 9) between the gradation and the one gradation is suppressed, and the continuity of the halftone density is not hindered and the image quality is not deteriorated.
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. 8A, only when the pixel gradation is a gradation within a range showing a substantially linear characteristic from the 0 gradation (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>
The individual exposure amount will be described below with reference to FIGS. 8 and 4 described above, 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 areas having the exposure characteristics shown in the graph g01 of FIG. 8A 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 (including conversion approximate to linear conversion) for each of the divided areas, and the data storage unit 13 ( This is information stored in advance for each of the divided areas in an example of the individual inclination information storage unit.
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 exposure amount control by the control unit 10 (an example of the exposure amount control means) is the exposure amount that needs to be adjusted according to the pixel gradation and the raised exposure amount as in the first embodiment. The amount of exposure that needs to be adjusted according to Ea is adjusted by adjusting the exposure amount per pixel by adjusting the exposure time per pixel by the exposure source 2 (exposure means). (An example of exposure time control means).
On the other hand, for the adjustment (control) of the change in the exposure amount required for each of the divided areas in accordance with the tilt information, the power supplied to the light emitting unit in the exposure source 2 is controlled (ie, power modulation). This is performed by adjusting the exposure intensity (an example of exposure intensity control means).

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. .

<First Example of Exposure Time Control>
Next, a description will be given of a first embodiment of exposure time control by the control unit 10 that can be employed in the first or second embodiment of the exposure amount control.
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.
FIG. 5 is a time chart for explaining a first embodiment of exposure time control by the control unit 10.
As shown in FIG. 5, 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 specifying 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. However, here, for convenience of explanation, it is assumed that the set exposure intensity is set to a predetermined constant intensity. That is, the first embodiment of the exposure amount control described above (an example in which control based on the tilt information k1 is not performed) will be described.
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.
In the first embodiment of the exposure time control, the control unit 10 outputs the count signal Sg2 whose ON → OFF change period is a constant period to the exposure source 2, and the pixel gradation (here) In this case, a value obtained by adding 1 to an integer of 0 to 15 (an example of a value corresponding to the pixel gradation) is designated (set) as the value of the set count number Cs, and the exposure permission signal Sg1 is in a permission state ( The time ta from the first change of the count signal Sg2 (ON to OFF, which corresponds to the generation of a periodic signal) after the change to (OFF) (exposure start command is generated) to the first exposure signal Ea (difference) Adjust according to the information). Thereby, the exposure time per pixel by the exposure source 2 is adjusted.
The change from ON to OFF of the exposure permission signal Sg1 corresponds to the generation of an exposure start command, and the change to the number of changes in the count signal Sg2 from ON to OFF corresponds to the generation of a periodic signal.

More specifically, when performing exposure for each pixel, the control unit 10 first sets a value obtained by adding 1 to the pixel gradation value to the exposure source 2 as the set count number Cs. At the same time, the raised exposure amount Ea is converted into a corrected gradation (for example, an integer of 0 to 7) which is an index proportional to the value. The example shown in FIG. 5 represents an example where the correction gradation is 3 (+3).
Next, 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 then at a predetermined time t1 proportional to the correction gradation. Is detected as a reference time point P01. The detection of the reference time point P01 is, for example, counting that a timing signal (not shown) that changes at a constant period changes by “number of times corresponding to the value of the correction gradation + number of times corresponding to the delay time t0”. Etc.
Here, if the timing signal and the count signal are signals having the same cycle generated based on the same clock signal, the circuit configuration for generating each signal becomes simple.
Note that output signals Sg1 and Sg2 (control signals) to the exposure source 2 shown in the time charts of FIG. 5 and FIGS. 6 and 7 described later are generated by the pulse signal control circuit provided in the control unit 10.

Next, the control unit 10 starts the change (ON → OFF) of the count signal Sg2 at a constant period from the reference time point P01.
As a result, the time ta from when the exposure permission signal Sg1 is changed to the permitted state (generation of the exposure start command) until the first change of the count signal Sg2 (generation of the periodic signal) is the raised exposure amount. It is adjusted according to Ea.
By the function of the exposure source 2, the time ta from the reference time P01 to the reference time point P01 after the exposure permission signal Sg1 changes to the permission state and the time tb proportional to the pixel gradation from the reference time point P01 (the pixel Exposure is performed for a time (ta + tb) obtained by adding (gradation value × period of the count signal Sg2).
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 (t1 + tb).
As a result, as shown in FIG. 3B, with respect to the reference exposure amount (exposure amount during time tb) obtained by linearly converting the pixel gradation to the exposure amount for each divided region. , By the exposure source 2 with an exposure amount obtained by adding an exposure amount (exposure amount during time t1) corresponding to the raised exposure amount Ea in all gradations (entire range) of the pixel gradation including 0 gradations. Exposure amount control is performed so that exposure is performed.
If the exposure time control as shown in FIG. 5 is performed and the set exposure intensity is controlled by the control unit 10 so that the exposure intensity is proportional to the inclination coefficient for each of the divided areas, the control unit 10 in FIG. The exposure amount control characteristics shown in b) are obtained.
However, in this case, even if the raising exposure amount Ea is the same, it is necessary to adjust the correction gradation by an amount corresponding to the change in the set exposure intensity.

<Second Example of Exposure Time Control>
Next, a description will be given of a second embodiment of the exposure time control by the control unit 10 that can be employed in the first or second embodiment of the exposure amount control.
The function of the exposure source 2 in the second embodiment of the exposure time control is also the same as the function of the exposure source 2 in the first embodiment.
FIG. 6 is a time chart for explaining a second embodiment of the exposure time control by the control unit 10.
In the second embodiment of the exposure time control, the control unit 10 outputs the count signal Sg2 whose ON → OFF change period is a constant period to the exposure source 2, and the pixel gradation ( Here, a correction gradation which is an index proportional to the raised exposure amount Ea (an example of difference information) to a value obtained by adding 1 to an integer of 0 to 15 (an example of a value corresponding to a pixel gradation). A value obtained by adding a value (an example of a value corresponding to difference information) is designated (set) as the set count number Cs. Thereby, the exposure time per pixel by the exposure source 2 is adjusted.

More specifically, when performing exposure for each pixel, the control unit 10 first corrects the raised exposure amount Ea as an index proportional to the correction gradation (for example, an integer of 0 to 7). Convert to. The example shown in FIG. 6 represents an example where the correction gradation is 2 (+2).
Further, the control unit 10 sets a value obtained by adding the value of the correction gradation to the value obtained by adding 1 to the pixel gradation for the exposure source 2 as the set count number Cs.
Next, the control unit 10 changes the exposure permission signal Sg1 to be output to the exposure source 2 from ON to OFF (generation of an exposure start command), and the time when the delay time t0 has elapsed from that is the reference time P02. The count signal Sg2 is changed (ON → OFF) every time a fixed time (cycle) elapses from the reference time point P02 (at a fixed cycle). This delay time t0 is the same as the delay time in the first embodiment.
As a result, the pixel gradation is substantially reduced at time t1 corresponding to the correction gradation after the exposure permission signal Sg1 is changed to the permitted state (generation of an exposure start command) by the function of the exposure source 2. The exposure is performed for the time (ta + tb) obtained by adding the time ta corresponding to.
As a result, the exposure amount control showing the exposure amount control characteristic as shown in FIG. 3B is performed.
If the exposure time control as shown in FIG. 6 is performed and the set exposure intensity is controlled by the control unit 10 so that the exposure intensity is proportional to the slope coefficient for each of the divided areas, FIG. The exposure amount control characteristics shown in b) are obtained.
However, in this case as well, even if the raised exposure amount Ea is the same, it is necessary to adjust the correction gradation by an amount corresponding to the change in the set exposure intensity.

<Third Example of Exposure Time Control>
Next, a description will be given of a third embodiment of the exposure time control by the control unit 10 that can be employed in the first or second embodiment of the exposure amount control.
The function of the exposure source 2 in the third embodiment of the exposure time control is the same as the function of the exposure source 2 in the first embodiment.
FIG. 7 is a time chart for explaining a third embodiment of the exposure time control by the control unit 10.
In the third embodiment of the exposure time control, when the exposure of each pixel is performed, the control unit 10 sets 1 as the set count number Cs for the exposure source 2.
Thereby, the exposure source 2 causes the minimum change (ON → OFF, exposure) of the count signal Sg2 after the change of the exposure permission signal Sg1 (ON → OFF, an example of an exposure start command) occurs for each pixel. Exposure is performed until an example of an end command is generated.
Further, after changing the exposure permission signal Sg1 to the exposure permission state (OFF), the control unit 10 calculates the delay time t0 from the change, and the time t1 obtained by converting the raised exposure amount Ea into the exposure time. The count signal Sg2 that changes (ON → OFF) is output when a time (t0 + t1 + tb) obtained by adding the time ta corresponding to the pixel gradation has elapsed.
By such control of the control unit 10, the time from the change of the exposure permission signal Sg1 (generation of the exposure start command) to the change of the count signal Sg2 (generation of the exposure end command) is adjusted. The exposure time is adjusted.

In the example shown in FIG. 7, the exposure permission signal Sg1 and the count signal Sg2 are generated by the controller 10 (the pulse signal control circuit) as follows.
That is, the control unit 10 changes the exposure permission signal Sg1 (ON → OFF) at a certain reference time point P02 in synchronization with a predetermined reference clock signal Sg00 that changes with a sufficiently short period (high frequency).
Further, the control unit 10 divides the reference clock signal Sg00 to generate a pixel gradation reference signal Sg01 having a change period of an exposure time corresponding to a one-step change in the pixel gradation. To do. At that time, the phase adjustment is performed so that the time from the change (ON → OFF) of the exposure permission signal Sg1 to the first change (ON → OFF) of the pixel gradation reference signal Sg01 becomes the delay time t0.
Then, the control unit 10 counts the number of changes (ON → OFF) of the pixel gradation reference signal until the number of pixel gradation values + 1 is reached, and from that time further, the reference clock signal g00 is counted. The number of changes is counted until it reaches a number proportional to the raised exposure amount Ea (9 clocks in the example shown in FIG. 7), and the count signal Sg2 is changed at that time.
Thereby, the time from the change of the exposure permission signal Sg1 (generation of the exposure start command) to the change of the count signal Sg2 (generation of the exposure end command) is adjusted, and the exposure time per pixel is adjusted.
Further, when the exposure time control as shown in FIG. 7 is performed and the set exposure intensity is controlled by the control unit 10 so that the exposure intensity is proportional to the inclination coefficient for each of the divided areas, FIG. The exposure amount control characteristics shown in b) are obtained.
However, in this case, even if the raising exposure amount Ea is the same, the reference clock signal is counted (the number of times proportional to the raising exposure amount Ea (FIG. 7) by the amount corresponding to the change in the set exposure intensity. It is necessary to adjust 9))).
Even when a laser scanning device is used as the exposure source 2, the same control as the above-described exposure amount control is possible.

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, but is not limited thereto.
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 exposure time control in the image forming apparatus X; 7 is a time chart for explaining a second embodiment of exposure time control in the image forming apparatus X. FIG. 9 is a time chart for explaining a third embodiment of exposure time control 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

Claims (9)

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. Exposure amount control means for controlling the exposure means to perform exposure with an exposure amount obtained by adding an exposure amount corresponding to the difference information;
An image forming apparatus comprising:   2. The exposure amount control unit according to claim 1, wherein the exposure amount control unit includes an exposure time control unit that adjusts an exposure amount per pixel by adjusting an exposure time per pixel by the exposure unit according to the difference information. Image forming apparatus. The exposure means performs exposure after a predetermined exposure start command is generated for each pixel until a periodic signal continuously generated at a predetermined period is generated a specified number of times;
The exposure time control means designates a value according to the pixel gradation as the count number value to the exposure means, and sets a time from generation of the exposure start command to generation of the first periodic signal. The image forming apparatus according to claim 2, wherein an exposure time per pixel is adjusted by adjusting according to the difference information. The exposure means performs exposure after a predetermined exposure start command is generated for each pixel until a periodic signal continuously generated at a predetermined period is generated a specified number of times;
The exposure time control means adjusts the exposure time per pixel by setting a value obtained by adding a value corresponding to the difference information to a value corresponding to the pixel gradation to the exposure means. The image forming apparatus according to claim 2. The exposure means performs exposure after a predetermined exposure start command is generated for each pixel until a predetermined exposure end command is generated;
The image forming apparatus according to claim 2, wherein the exposure time control unit adjusts an exposure time per pixel by adjusting a time from generation of the exposure start command to generation of the exposure end command. 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;
6. The exposure amount adjusting unit according to claim 2, further comprising an exposure intensity adjusting unit that adjusts an exposure intensity in the exposure unit in accordance with a change in an exposure amount based on the tilt information for each of the divided areas. The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the divided area is an area obtained by dividing the surface of the drum-shaped photoconductor into one or both of an axial direction and a 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