JP3668373B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus capable of improving image quality by making the density uniform.
[0002]
[Prior art]
In a conventional image forming apparatus employing an electrophotographic method, when an image is exposed on an image carrier, a portion where no toner is carried is exposed on the photosensitive member in the case of the forward development method, while in the case of the reverse development method, the toner is exposed. The exposure light source is composed of laser light, LED (light emitting diode), etc., and is exposed to a pixel unit according to the image data to form an image. In a typical image forming apparatus, for example, a target image density is a so-called Id density (logarithm with the reciprocal 10 of the reflectance being a base 10 logarithm, hereinafter simply referred to as density), and a square image having a side of 20 mm is imaged. When exposing, the entire image area was irradiated with a uniform exposure light amount.
[0003]
Then, when the electrostatic latent image on the image carrier is visualized (developed) with a developing device, a square peripheral portion is formed by the edge effect and development characteristics of the latent image as shown by the thick line portion shown in FIG. The image has a high density, and the center portion is relatively thin, resulting in an uneven density image. Further, in the portion where the low density portion and the high density portion are adjacent as in the photographic image, the density is reduced in the vicinity of the boundary between the low density portion and the high density portion as shown by the thick line portion in FIG.
[0004]
Therefore, in order to solve the above-mentioned problem, Japanese Patent Laid-Open No. 61-242468 has been proposed. In this publication, digital image information read by a scanner in units of pixels and subjected to multilevel quantization is proposed. When the image is recorded by the electrophotographic printer according to the binarized image information after expressing the gradation, the digital image information is enhanced in the low frequency region in the spatial frequency of the image. An image processing method for correcting an edge effect using a low-pass digital filter is disclosed.
[0005]
In addition, the adjacent pixel data can be simulated by alternately drawing black lines and white lines (no developer is attached) in the sub-scanning direction (transfer sheet transport direction, that is, rotation direction of the photoreceptor). A method for expressing halftones (hereinafter abbreviated as 1by1 format) is known, and a technique for reducing the above-described density unevenness when a photo mode is selected with a digital copying machine is known.
[0006]
As described above, by expressing the low density area in the 1by1 format, when the high density area and the low density area of the image are adjacent to each other, for example, in a normal PWM (pulse width modulation) exposure, It is known that there is a certain effect on the problem that the density of the adjacent portion is lowered.
[0007]
[Problems to be solved by the invention]
As described above, in Japanese Patent Application Laid-Open No. 61-24268, by performing image processing using low-pass digital filter processing, the peripheral portion of the image, high density and low density portions are obtained due to the edge effect and development characteristics of the latent image. Is effective in that the density drop at the adjacent boundary portion can be somewhat mitigated.
[0008]
However, according to the technique described in the publication, an attempt is made to make the density uniform by using the edge effect per pixel. For example, the processing data of the target pixel has a density value of 1 for the peripheral pixel. / 16, 2/16 is added to obtain, and the peripheral effect data is weighted average to reduce the edge effect. Therefore, white data may be inserted into the black data, and the image is very rough. There is a problem that the image quality is rough and the image quality is remarkably lowered.
[0009]
Therefore, in the method of the publication, since the image quality is deteriorated and the image quality is deteriorated and the impression is poor, it is not an effective solution to the above problem, and the problem of obtaining a smooth and uniform image quality is solved. If the resolution cannot be improved and the resolution is further increased, the effect expected in the publication cannot be obtained.
[0010]
In addition, as described above as another prior art, when a photo mode is selected with a digital copying machine, etc., a technique for reducing density non-uniformity by expressing adjacent pixel data in the 1-by1 format in the sub-scanning direction. However, when a low density part and a high density part are adjacent to each other in the image area as in a photographic document, for example, the image density is reduced in the vicinity of the adjacent part in the low density part, resulting in a white outline, and the image quality is remarkably improved. There is a problem that the image is reduced, and the expression in the 1by1 format cannot be made completely uniform, and since the image quality is configured in a line shape, there is a problem that the line is cut off, and the image density is not constant. Thus, only a limited effect on the improvement of the image quality can be expected.
[0011]
The present invention has been made in view of the above-described problems of the prior art, and the object of the present invention is to be a uniform image, which is originally intended to be a central portion due to an edge effect or a defect caused by a development method. It is an object of the present invention to provide an image forming apparatus that solves the conventional problem that the density of the peripheral portion is different from that of the peripheral portion.
[0012]
[Means for Solving the Problems]
The image forming apparatus according to claim 1 forms an image for each pixel data after image processing, image data input means for inputting image data, image processing means for performing predetermined image processing for each input pixel. Image density setting means for setting density, image exposure means for forming an electrostatic latent image corresponding to the image data on an image carrier based on the processing results of the image processing means and the image density setting means, and the image In an image forming apparatus provided with a developing means for developing an electrostatic latent image on a carrier,
Based on the density value of each pixel set by the image density setting means, an area extracting means for extracting a pixel area having substantially the same density, and among the pixel areas extracted by the area extracting means, adjacent to each other and between the areas A high density area extracting means for extracting a high density area having a density difference exceeding a predetermined value, and an image carrier corresponding to the high density area based on an extraction result of the high density area extracting means. Exposure intensity control means for changing the exposure intensity in the image carrier area to a value corresponding to the shortest distance among the distances from the pixel of interest to the image edge when performing image exposure of the area. An image forming apparatus.
[0013]
3. The image forming apparatus according to claim 2, wherein the developing method of the electrostatic latent image is a reversal developing method, and the exposure intensity control unit is configured to apply the image exposure unit when the size of the high density area is equal to or less than a predetermined value. 2. The image forming apparatus according to claim 1, wherein the exposure intensity of the image forming apparatus is lowered, and when the exposure intensity is greater than or equal to a predetermined value, the exposure intensity is changed according to the position of the pixel to be exposed and the end of the high density region. .
[0014]
The image forming apparatus according to claim 3, wherein the exposure intensity control means increases the exposure intensity of the image exposure means at the center of the high density area when the size of the high density area is equal to or greater than a predetermined value. The image forming apparatus according to claim 2, wherein the exposure intensity is lowered.
[0015]
5. The image forming apparatus according to claim 4, wherein the exposure intensity control means exposes the low density region in the vicinity of the boundary with the low density region adjacent to the high density region and the density difference between the regions exceeds a predetermined value. 4. The image forming apparatus according to claim 2, wherein the light amount is increased and the exposure light amount in the high density region is decreased.
[0016]
The image forming apparatus according to claim 5, wherein the exposure intensity control unit can change a control range of an exposure light amount according to a density difference between regions. It is.
[0017]
6. The image forming apparatus according to claim 6, wherein the exposure intensity control means can change either or both of the correction amount and the correction range of the light amount according to the direction of density change of the image. This is an image forming apparatus.
[0018]
The image forming apparatus according to claim 7, wherein the charging unit that charges the image carrier includes a charging potential setting unit that can change a charging potential, and the exposure intensity control unit is set to the charging potential setting unit. 7. The image forming apparatus according to claim 5, wherein a correction range of the exposure light amount can be varied according to a set value of the charging potential.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an overall cross-sectional view showing the periphery of a copying process section when the present invention is adopted in a digital copying machine. In this embodiment, the case where the present invention is applied to a digital copying machine has been described. However, the present invention includes a printer using a normal electrophotographic system or an image forming unit thereof. Needless to say, it can also be applied to plain paper facsimiles. In this embodiment, the case where the reversal development method is employed has been described. However, the present invention is not only for reversal development but also for the normal development method by considering the polarity. Of course, it can be similarly implemented.
[0020]
In FIG. 1, a placed document is exposed by a scanner 1 having a light source 10, reflected light is incident on a CCD 11 via a mirror and a lens, and image data is output. The image data read and generated by the CCD 11 is input to the image processing board 2 that performs image processing.
[0021]
The overall operation of the image forming apparatus is sequence-controlled by a controller board 3 composed of a microcomputer, ROM, RAM, etc., and image data processed by an image processing circuit of the image processing board 2 is an exposure system using a laser scanner. 6, the photoconductor 7 uniformly charged by the charging charger 12 is subjected to writing exposure, and an electrostatic latent image corresponding to the image data is formed.
[0022]
Next, the electrostatic latent image is visualized by the developer 4 in which the developer is stored, and the visualized developer image is transferred to a sheet-like transfer material via the transfer charger 8 and transferred. The developer remaining on the photoreceptor 7 after the process is removed by the cleaner 5.
[0023]
Here, the writing resolution at which the exposure system 6 writes laser light onto the photoconductor 7 is 400 dpi (dot per inch), for example, a semiconductor laser is used as the exposure light source, and the laser power of the semiconductor laser is controlled in 32 steps by PWM control. Can be changed. Here, the laser power changeable range is 0.20 mW to 0.60 mW, the standard laser power is adjusted to be 0.30 mW on the image carrier, and the standard laser power is to print a line image. It is determined on the assumption.
[0024]
Also, the brightness is changed by changing the laser lighting time (hereinafter referred to as DUTY) in 256 steps by PWM control. For example, when DUTY = 0, the lighting time for one pixel is 0 and printing is not performed, and DUTY = 255 means that all the time corresponding to one pixel is lit.
Note that the photoreceptor 7 as an image carrier is uniformly charged by the charger 12, but the reference surface potential is set to -600V.
[0025]
The developing system is a two-component developing system composed of a toner having an average particle diameter of 8 μm and a ferrite carrier having an average particle diameter of 90 μm, and in the developing device 4 that holds the image carrier and the developer at the closest point to the image carrier 7. The sleeve rotates in the same direction. The toner concentration in the developer was adjusted based on 4%, and the developing bias voltage was set to -450V.
[0026]
This potential is set by a control means (not shown) provided to maintain and stabilize the initial image density even when a change in the surrounding environment, process conditions, and a change with time of the photoconductor or developer occurs. It is fluctuated while maintaining the difference (150 V) between the surface potential and the developing bias.
[0027]
First, as a preliminary test, a circular image having an image size of about φ2 (30 pixels) to about φ20 (300 pixels) was created by changing the laser power, and the density and the size of the edge effect were examined. An example of the result is shown in FIGS.
13 is a schematic diagram of the density distribution of the image in the sub-scanning direction passing through the center of the circle in the prior art, and FIG. 14 shows the relationship between the image size in FIG. 13 and the image density measured in the sub-scanning direction. .
[0028]
As is apparent from FIG. 14, when the image is small, the edge effect is effective in the entire image area, so that there is no difference in density between the central portion and the peripheral portion of the image.
As the image becomes larger, the image density at the center decreases, and when the number of pixels is 200 pixels or more, the difference between the density at the center and the periphery does not change, and the density at the periphery is almost constant regardless of the size of the image. became.
[0029]
From this result, it was found that the central portion does not reach the maximum density when the image has 100 pixels or more at the standard laser power of 0.3 mW.
And it was found that changing the laser power is an effective means for this response. Further, FIG. 15 shows the result of measuring the distance from the end of the position where the image density is lowest.
[0030]
As an example of a method for determining the size of the image, 100 lines before and after the pixel of interest are stored in the memory, the image density data of the input data is represented by 256 gradations, gradation 0 is white, gradation 255 Is black. Then, the gradation of 200 pixels in total in the sub-scanning direction is compared, the density distribution is calculated, and the size of the image is determined by looking at the range where the data including the gradation is within ± 10. The size can be similarly determined in the main scanning direction. Here, it is determined that the range where the gradation is ± 10 is the same density, but the value is not limited to this value, and an appropriate value may be set as appropriate depending on the image forming conditions.
[0031]
More specifically, a microcomputer is mounted on the controller board 3, the scanner 1, the laser power control unit 26, etc. as a region extracting unit for extracting pixel regions having substantially the same density and a high concentration region extracting unit, It can be configured by reading and determining pixel data. The extracted region data (for example, boundary pixel data) is temporarily stored in the same memory or a separately provided memory, and a laser power control unit is used during image exposure. It can be easily realized by reading in
[0032]
(First embodiment of the present invention)
FIG. 2 shows a processing block diagram according to the first embodiment of the present invention. The original placed by the light source 10 of the scanner 1 is exposed, and the reflected light is read by the CCD 11 via the lens and mirror (CCD reading unit 21). The image data incident on the CCD 11 is input to the image processing substrate 2 and converted from an analog value to a digital value, and then image processing such as visibility correction, region separation, and filter processing is performed by the image processing unit 22. The gradation correction unit 23 performs gradation correction, the size determination unit 24 determines the size of the image, the density determination unit 25 determines the image density, and the laser power control unit 26 controls the output of the semiconductor laser. The processing shifts to the writing optical system.
[0033]
After the image carrier 7 is exposed to laser light and an electrostatic latent image is formed in accordance with the image size and image density determined in the image processing circuit in the density determination unit 25 from the image processing unit 22. The electrostatic latent image is visualized by the developing device 4.
[0034]
FIG. 3 shows the distance relationship between the pixel of interest A in an image and the image edges (ends) in the main scanning direction and the sub-scanning direction. An example will be described.
First, after determining the position of the target pixel A with respect to the edge of the image in the main scanning direction and the sub-scanning direction, the laser power is set to the value of the shortest part.
[0035]
Specifically, with respect to the target pixel A of the image in FIG. 3, the distances from the edge of the image are L = 30 pixels, M = 80 pixels, N = 60 pixels, and K = 150 pixels, respectively. The laser power setting is varied depending on the distance from the end as shown in FIG.
[0036]
For example, as shown in FIG. 4, the basic setting of the laser power is 0.30 mW for 50 pixels from the edge of the image, 0.40 mW for 101 pixels or more, and a linear function between 51 and 100 pixels in between. When the power is changed, for each distance, 0.30 mW when L = 30 pixels, 0.36 mW when M = 80 pixels, 0.32 mW when N = 60 pixels, and K = In the case of 150 pixels, the value is 0.40 mW, and the value of the shortest part, 0.30 mW, is used as the power of the pixel of interest A.
[0037]
In order to explain specifically, the control contents of the image size and the exposure light quantity are shown in FIG. In the image processing circuit, as shown in FIG. 5A, when the short side of the image is 100 pixels or less, the distance from the edge to the target pixel is 50 pixels at the maximum. For example, in a line drawing such as a character Performs image formation by exposing with a standard light amount of 0.30 mW.
[0038]
On the other hand, when it is determined that one side of the image is 300 pixels and the side is parallel to the main / sub-scanning direction, the following control is performed on the image of FIG. In other words, the standard light quantity is 0.30 mW from the edge to the 50th pixel when viewed in the direction perpendicular to the side A (arrow H direction), and the laser power is controlled by PWM control for controlling the exposure light quantity from the next 51 pixels to 100 pixels. The linear function was raised to 0.40 mW at the 100th pixel.
[0039]
And in the range between 100 pixels-200 pixels from an edge part, the exposure light quantity was maintained at 0.40 mW, and the said exposure control was implemented also to 100 pixels of the other side. Note that the same control as in the direction of the arrow H is also performed in the direction of the arrow V, and finally the laser power is controlled as shown in FIG. 6 for the main image. As a result, it was possible to obtain a 300 pixel angle image having a sufficient density and uniform density distribution from the edge to the center.
[0040]
As a result, the amount of exposure light for a line drawing such as a character (corresponding to FIG. 5A) and an image having an area (corresponding to FIG. 5B) is controlled separately, and the characters are compared. Therefore, exposure can be performed with a low amount of light, and as a result, toner consumption can be suppressed.
When the length of one side of the image is larger than 200 pixels, it is possible to cope with images of different sizes by increasing / decreasing the range of laser power = 0.40 mW.
Here, the control of the amount of exposure light is changed using a linear function, but the amount of light is changed stepwise according to the characteristics of the electrophotographic process to be used, and other functions such as a quadratic function and a nonlinear function are used. The amount of exposure light may be changed accordingly.
[0041]
(Second embodiment of the present invention)
In the first embodiment, the method of controlling the brightness of the light source itself is exemplified to change the amount of exposure light. However, in this embodiment, a case where the lighting time is changed will be described.
[0042]
As a method of controlling the laser lighting time, a method of selecting an LUT that matches an image size from a plurality of density correction curve look-up tables (hereinafter abbreviated as LUT) for controlling density gradation is adopted. FIG. 7 shows a plurality of density correction curves LUT in the present embodiment. In the figure, the horizontal axis represents a factor corresponding to the original density, and the vertical axis represents the laser lighting time.
[0043]
First, as shown in FIG. 7, data related to 16 density correction curves are stored in a storage element of the image processing circuit (in FIG. 7, only six representatives are displayed for simplicity). In FIG. 7, the DUTY increases as it goes to the left of the page, resulting in a darker image, and these are used properly according to the distance from the edge of the image.
[0044]
That is, since the density is relatively easy from the edge of the image to 50 pixels, a low density setting LUT (L in the figure) is used, and the central portion of 100 to 200 pixels from the edge is used as in the first embodiment. In the LUT (M in the figure) that is darkly set, in the change region from 50 to 100 pixels from the end, a plurality of LUTs that are located between L and M corresponding to the distance from the end are prepared. In the figure, N groups of LUTs were selected for exposure.
[0045]
In this manner, by determining the size of the image and controlling the amount of exposure light by changing the DUTY, it was possible to obtain an image quality in which the density at the solid portion was uniform.
[0046]
In the past, the amount of exposure light was changed in association with only the size of the image, so there are cases where these processes need not be performed depending on the target density.
That is, in the present embodiment, the above-described correction is performed only when the target image density is 1.0 or less. However, the image density is set to, for example, 0.9 or 1.1 depending on the image forming conditions. It is also possible to distinguish between these values.
[0047]
(Third embodiment of the present invention)
In the first and second embodiments described above, the independent image size and density have been described, but images of various densities continuously change during actual copying. An example of the target density distribution and the actual density distribution at that time is shown in FIG.
Since the image data is processed on the basis of the input image, such a density distribution is obtained when the processing considering density unevenness occurring in the printer unit is not performed. Therefore, in this embodiment, in order to express the density distribution as a target density distribution, the size of the image is measured and the light amount is corrected.
[0048]
FIG. 8 shows that two types of images having different densities are arranged adjacent to each other at the boundary line C. The left side of the paper surface shows the low density portion and the right side of the paper surface shows the high density portion image. , P and Q are the target pixels of each image portion.
The control contents in the adjacent portion will be described with reference to FIG. 8. First, the distance to the place where the density changes (the end of the image) is obtained for the target pixel P in the low density portion, and the boundary line C is obtained as a result of the measurement. Is closest to the laser, the laser power is corrected as shown in FIG. Similarly, for the target pixel Q on the high density side, the distance to the end is measured and the laser power is controlled.
[0049]
Actually, the laser power changes as shown in FIG. As the target image density, 0.5 and 1.0 are adjacent to each other, each having a size of 300 pixels, and the processing at the end is in accordance with the first and second embodiments.
In other words, the adjacent portion has a target density of 0.5 and the laser power is increased from 0.4 mW to 0.5 mW from the 100th pixel to the 50th pixel from the boundary, and 0.5 mW is maintained from the 50th pixel to the boundary. To do. Next, in the region where the target image density is 1.0, the laser power is lowered to 0.3 mW from the boundary to 50 pixels, and is returned to 0.4 mW from 50 pixels to 100 pixels, and 0.4 mW at further distances. It becomes. By this method, the decrease or increase in density due to the edge effect at the boundary portion was alleviated, and a distribution close to the target print density was shown. In FIG. 10, the laser power is changed by changing the brightness of the laser according to the density by PWM control.
[0050]
(Fourth embodiment of the present invention)
In the third embodiment, the case where the target density is 0.5 and 1.0 is confirmed. However, the deviation width from the target density is changed due to the difference between adjacent densities. The correction range was changed.
[0051]
That is, FIG. 11 shows the amount of correction when an image having a size of 100 pixels square or more changes in the difference from the adjacent density. Since the edge effect works greatly when the density difference is large, when the correction is not performed, the density drop is particularly conspicuous on the low density side, so the correction amount is set large. Accordingly, the correspondence relationship of FIG. 11 is stored in advance in the laser power control unit, etc., and after detecting the density difference with the adjacent region, the laser power suitable for the density difference is read out, according to the above embodiment. Just control.
[0052]
The relationship shown in FIG. 11 may be configured so that basic settings are made when the device is shipped from the factory. However, depending on the characteristics of each device, the setting can be changed so that a serviceman or the like can make fine adjustments in the market. (Updating and writing are possible).
[0053]
(Fifth embodiment of the present invention)
As a case where different densities are adjacent, there are typically a case where they are adjacent in the main scanning direction and a case where they are adjacent in the sub-scanning direction. As a method for further improving the third embodiment, changes in density in the main scanning direction and the sub-scanning direction were examined.
Then, the lowest density was reached at 100 pixels in the sub-scanning direction, and the lowest density was reached at 50 pixels in the main scanning direction. Therefore, the method according to the third embodiment is used in the sub-scanning direction, and data in which the size axis is halved is used in the main scanning direction.
[0054]
(Sixth embodiment of the present invention)
The degree of non-uniform density varies depending on the surface potential level of the photoreceptor. When the surface potential of the photoreceptor changes from −400 V to −800 V by the above-described process control for maintaining the initial image quality while maintaining the optimum state, the correction amount of the laser power is as shown in FIG. Multiply the correct factor. With this configuration, an image having a uniform density was stably obtained even when the charging potential was changed.
[0055]
It should be noted that the relationship shown in FIG. 12 may be configured so that the basic setting is made at the time of shipment of the device from the factory. However, depending on the characteristics of each device, a serviceman or the like can be finely adjusted in the market. The setting can be changed (updated and writable).
[0056]
【The invention's effect】
According to the image forming apparatus of claim 1, when the image exposure unit performs image exposure on the image carrier area corresponding to the high density area, the distance becomes the shortest distance from the target pixel to the image edge. Since the exposure intensity of the image carrier area is changed to the corresponding value, the density of the high density area (solid area) can be made uniform in the center / peripheral area without sacrificing the image quality degradation. Thus, a special effect that a high-quality image can be obtained is obtained.
[0057]
According to a second aspect of the present invention, in the first aspect, the electrostatic latent image developing method is a reversal developing method, and the exposure intensity control unit is configured such that the size of the high density region is a predetermined value or less. In this case, the exposure intensity of the image exposure means is lowered, and when it is equal to or greater than a predetermined value, the exposure intensity is changed according to the position between the pixel to be exposed and the end of the high density region. It is possible to appropriately control the exposure of a line drawing and an image having a relatively large area such as a photograph, and to obtain an optimum image quality for each type of image such as a character and a photographic image while suppressing an increase in toner consumption. The effect that it can be obtained.
[0058]
According to a third aspect of the present invention, in the second aspect, when the size of the high density area is equal to or greater than a predetermined value, the exposure intensity control means exposes the image exposure means at the center of the area. Since it is characterized by increasing the intensity and decreasing the exposure intensity at the edge, when the image size does not show the edge effect or is inconspicuous, the exposure control can be canceled to improve the processing efficiency. In the case of the image size in which the edge effect appears, there is an effect that the optimum exposure control can be performed to improve the image quality.
[0059]
According to an image forming apparatus of a fourth aspect of the present invention, in the second or third aspect, the exposure intensity control unit is connected to a low density region adjacent to the high density region and having a density difference between the regions exceeding a predetermined value. In the vicinity of the boundary, the amount of exposure light in the low density area is increased and the amount of exposure light in the high density area is decreased, so when the image area such as a photographic image is large, due to the area density difference in the image, It is possible to prevent the image on the low density side from becoming thinner, and to improve the image quality in the image portion where the high density area and the low density area are adjacent, while avoiding the serious problem of missing information. Can do.
[0060]
According to an image forming apparatus of a fifth aspect, in the first to fourth aspects, the exposure intensity control unit can vary a control range of the exposure light amount in accordance with a density difference between regions. The occurrence of image shading due to the edge effect is more likely to occur as the density difference of the image increases. However, by changing the exposure light amount control range according to the density difference, an image with a more uniform density can be formed.
[0061]
According to an image forming apparatus of a sixth aspect, in the first to fifth aspects, the exposure intensity control means can change both or one or both of the light amount correction amount and the correction range according to the direction of density change of the image. Therefore, when the information missing range has directionality, considering the directionality, the range for controlling the amount of exposure light can be varied to obtain more uniform image quality. In addition, as a method of changing the amount of light, by combining both the method of changing the brightness of the light source itself and the method of changing the lighting time for one pixel of the light source, the resolution can be partially changed and the resolution can be partially changed. Also, the degree of freedom in design can be improved and an image having a uniform density can be formed.
[0062]
According to an image forming apparatus of a seventh aspect, in the fifth or sixth aspect, the charging unit that charges the image carrier includes a charging potential setting unit that can change a charging potential, and the exposure intensity control unit includes: Since the exposure light amount correction range can be varied according to the set value of the charging potential set in the charging potential setting means, the charging potential is changed when the charging potential is changed so as to optimize the image forming process conditions. By adjusting the image exposure control width in accordance with the potential, there is an effect that the density of the solid image can be prevented from becoming non-uniform even when the charging potential changes. The same effect can be obtained even if the correction range of the exposure light quantity is variable (larger) in accordance with the absolute value of the charging potential of the image carrier.
[Brief description of the drawings]
FIG. 1 is an overall cross-sectional view for explaining a copying process of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a processing block diagram showing an overview of an image forming method according to an embodiment of the present invention.
FIG. 3 is a diagram illustrating a relationship between a target pixel and a distance from an image edge according to the embodiment of the present invention.
FIG. 4 is a diagram illustrating the relationship between the distance from the edge of an image and the amount of exposure light according to the embodiment of the present invention.
5A and 5B are diagrams illustrating a relationship between an image shape and a scanning direction according to the embodiment of the present invention, in which FIG. 5A is an image in which the sub-scanning direction is 100 pixels or less, and FIG. 5B is a main / sub-scanning direction. Each 300-pixel square image is represented.
FIG. 6 is a diagram illustrating a relationship between an image and a laser power according to the embodiment of the present invention.
FIG. 7 is a diagram illustrating a relationship (lookup table) between a laser emission time (DUTY) and an input density according to another embodiment of the present invention.
FIG. 8 is a diagram illustrating a relationship between a target image and an image edge when a low density image and a high density image are adjacent to each other according to another embodiment of the present invention.
9 is a diagram illustrating the relationship between the boundary line C and the laser power correction amount in the case of FIG. 8, according to another embodiment of the present invention.
10 is a diagram illustrating the relationship between the pixel position and the laser power in the case of FIG. 8, according to another embodiment of the present invention.
FIG. 11 is a diagram illustrating a relationship of a light amount correction coefficient with respect to a density difference, a laser power, and a density difference between adjacent images according to another embodiment of the present invention.
FIG. 12 is a diagram illustrating a relationship between a charging potential, a laser power correction coefficient, and an exposure light amount correction coefficient with respect to a change in surface potential according to another embodiment of the present invention.
FIG. 13 is a diagram illustrating an image size and an image density distribution in the related art.
FIG. 14 is a diagram illustrating a distance from an image end portion, a density transition, and a density difference in the related art.
FIG. 15 is a diagram illustrating the relationship between the image size and the distance to the lowest density portion in the prior art.
FIG. 16 is a diagram illustrating a relationship between a pixel position and a density when a high density image and a low density image are adjacent to each other in the related art.
[Explanation of symbols]
1 Scanner section
2 Image processing board
3 Controller board
4 Developer
5 Cleaner
6 Exposure system
7 Image carrier
8 Transcription charger
10 Light source
11 CCD (photoelectric conversion element)
12 Charging charger
21 CCD reading section
22 Image processing unit
23 Tone correction part
24 Size determination unit
25 Density judgment unit
26 Laser power control unit

Claims (7)

Image data input means for inputting image data, image processing means for performing predetermined image processing for each input pixel, image density setting means for setting density for image formation for each pixel data after image processing, Based on the processing results of the image processing means and the image density setting means, an image exposure means for forming an electrostatic latent image corresponding to the image data on the image carrier and the electrostatic latent image on the image carrier are developed. In an image forming apparatus comprising a developing unit,
Based on the density value of each pixel set by the image density setting means, an area extracting means for extracting a pixel area having substantially the same density and a pixel area extracted by the area extracting means and adjacent to each other and between the areas A high density area extracting means for extracting a high density area having a density difference exceeding a predetermined value, and an image carrier corresponding to the high density area based on an extraction result of the high density area extracting means. Exposure intensity control means for changing the exposure intensity in the image carrier area to a value corresponding to the shortest distance among the distances from the pixel of interest to the image edge when performing image exposure of the area. An image forming apparatus. The electrostatic latent image development method is a reversal development method,
The exposure intensity control means lowers the exposure intensity of the image exposure means when the size of the high density area is a predetermined value or less, and exposes the exposure intensity when the size of the high density area is a predetermined value or more The image forming apparatus according to claim 1, wherein the image forming apparatus is changed according to the position up to. The exposure intensity control means increases the exposure intensity of the image exposure means at the center of the high density area and lowers the exposure intensity at the edge when the size of the high density area is a predetermined value or more. The image forming apparatus according to 2. The exposure intensity control means increases the exposure light amount in the low density region and increases the exposure light amount in the high density region in the vicinity of the boundary with the low density region adjacent to the high density region and the density difference between the regions exceeds a predetermined value. The image forming apparatus according to claim 2, wherein the image forming apparatus is lowered. The image forming apparatus according to claim 1, wherein the exposure intensity control unit can change a control range of an exposure light amount according to a density difference between regions. 6. The image forming apparatus according to claim 1, wherein the exposure intensity control unit can change either or both of the light amount correction amount and the correction range depending on the direction of density change of the image. The charging means for charging the image carrier includes a charging potential setting means capable of changing a charging potential,
The image forming apparatus according to claim 5, wherein the exposure intensity control unit can vary a correction range of the exposure light amount according to a set value of the charging potential set in the charging potential setting unit.

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