JP2002333746A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device in which occurrence of image irregularity based on a characteristics of a light scanning system is securely prevented at a low cost. SOLUTION: The image forming device has an image carrier for forming a toner image thereon, an image processing section for patterning image data by use of a particular threshold matrix, thereby converting it into data indicating the gradation of a toner image, and an exposure device for emitting exposure light based upon the gradation data and illuminating the surface of the image carrier, thereby forming an electrostatic latent image. The image forming device finally forms an image onto a recording medium. In the image forming device, a light quantitative adjusting means is provided. The means adjusts the value of a threshold matrix of the image processing section in accordance with a picture depicting position within a depicting plane, thereby adjusting the quantity of light thrown onto the surface of the image carrier.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus used for a copying machine, a printer and the like.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus used in a copying machine, a printer, or the like, a gradation mirror or the like which converts image-processed gradation data for each pixel into an optical signal and rotates the optical signal. There is used an exposure apparatus that forms a latent image by performing a raster scan by reflecting the light with a light scanning system, and irradiating the surface of the photoreceptor with an optical signal obtained by the raster scan.

The latent image formed by the exposure device is developed with toner of a developing device to form a toner image, and the toner image is finally transferred and fixed on a sheet to form an image.

FIG. 1 is a schematic view showing an optical scanning system of an electrophotographic image forming apparatus.

An optical scanning system 10 shown in FIG. 1 includes a semiconductor laser 11 for emitting an optical signal, a polygon mirror 12 composed of a rotating polygon mirror, an F-θ lens 13, and a reflection mirror 14.
It is composed of

The optical signal emitted from the semiconductor laser 11 is reflected by the rotating polygon mirror 12 to become an optical signal for raster scanning at an equal angular velocity, and is converted into an optical signal for linear scanning at an equal speed by the F-θ lens 13 and reflected. The light is reflected by the mirror 14 to form a latent image on the surface of the photoconductor 15.

FIG. 2 is a diagram showing an optical signal incident on the polygon mirror and an optical signal reflected therefrom.

In FIG. 2, when a laser beam emitted from a semiconductor laser is reflected by a polygon mirror and raster-scanned, when the incident angle of the incident light 16 is small θ1, the intensity of the reflected light 17 is strong and the incident light is high. When the incident angle 16 is large θ2, the intensity of the reflected light 17 is low.

FIG. 3 is a diagram showing optical characteristics of a lens.

In FIG. 3, the central portion 19 of the lens 18 has high light intensity, and the end portions 20 have low light intensity.

As described above, in the optical scanning system, due to the optical characteristics, a difference occurs in the potential of the latent images formed at the center and both ends of the surface of the photosensitive member due to the difference in the scanning angle. When transferred to a color image, the color difference is 2
About 3 color unevenness is caused.

FIGS. 4 and 5 show a latent image formed on the photoreceptor surface by continuously raster-scanning the potential difference appearing on the photoreceptor surface by an optical scanning system with exposure light from an exposure device at a constant intensity. Was measured by measuring the density of an image formed by transferring and fixing a developed toner image on paper.

FIG. 4 is a diagram showing measured values obtained by measuring the density of an image transferred and fixed on a sheet with a density sensor.

In FIG. 4, the vertical axis represents the amount of light received by the sensor output from the density measuring sensor, and the horizontal axis represents the position where the image is printed.

As can be seen from the figure, the received light amount differs by about 15% between the center and both ends of the sheet, and the density is lower at both ends than at the center.

FIG. 5 shows measured values obtained by measuring the density of a color image transferred and fixed on a sheet using a CCD sensor.

In FIG. 5, the vertical axis represents the measured value of the image density as a color difference with respect to the center of the paper.

As can be seen from the figure, there is a color difference of about 2 to 3 at the center and both ends of the sheet, and it is understood that the color is lighter at both ends than at the center.

In the above, the color unevenness in the image plane caused by the difference in light intensity due to the difference in the scanning angle of the optical scanning system has been described. Even in an LED array exposure system in which LEDs are arranged in an array, similar color unevenness occurs if there is unevenness in the amount of light among a plurality of light emitting elements.

In addition to the exposure system, the same unevenness in charging of the photosensitive member in the axial direction of the photosensitive member, in-plane sensitivity unevenness of the photosensitive member itself, and similar color unevenness due to development unevenness of the developing device have been known. Are known.

[0021]

However, the quality of an image printed by an image forming apparatus has been improving year by year, and it has become an important issue to remove density unevenness and the like due to these optical scanning systems.

As a technique for removing density unevenness and the like in the optical scanning system of high-end products such as DTP, a light profile formed by the optical scanning system is obtained by increasing the light beam intensity at both ends from the center rather than the central portion, and thereby forming a profile in the opposite direction. Japanese Patent Application Laid-Open No. 8-52901 discloses a technique for removing unevenness in density by so-called smile correction.

However, in order to perform the correction with the signal of the profile in the opposite direction by the exposure apparatus, the correction by the analog signal is necessary, and there is a problem that the cost becomes high.

Further, in order to suppress the difference in light amount between the elements of the LED array, the difference in charge of the photosensitive member charger, the difference in photosensitive member sensitivity, the difference in development, etc. to a level at which the problem of color unevenness does not occur, there is a problem of manufacturing cost and There is a problem of dealing with changes over time.

In view of the above circumstances, it is an object of the present invention to provide an image forming apparatus in which density unevenness based on the characteristics of an optical scanning system is reliably reduced at low cost.

[0026]

According to the image forming apparatus of the present invention which achieves the above object, a toner image is formed by forming an electrostatic latent image on the surface and developing the electrostatic latent image with toner. An image carrier, an image processing unit for patterning the image data with a predetermined threshold matrix and converting the image data into gradation data representing the density of a toner image, emitting exposure light based on the gradation data, An exposure device for irradiating the surface of the carrier to form an electrostatic latent image, developing the latent image on the surface of the image carrier to form a toner image, and finally transferring and fixing the toner image on a recording medium In an image forming apparatus for forming an image composed of a fixed toner image on the recording medium by passing through an image forming process, a threshold value of a threshold value matrix of the image processing unit is set in a drawing position within an image drawing surface. Increase or decrease according to By, characterized by comprising a light amount adjusting means for adjusting the amount of light applied to the carrier surface.

Here, the exposure device is an exposure device for raster-scanning the surface of the image carrier with a laser beam, and the light amount adjusting means is a light amount generated by a difference in scanning angle of the exposure device on the image carrier. It is preferable that the light amount is adjusted according to the drawing position based on the scanning angle so as to correct the unevenness.

A density measuring toner image forming means for forming a density measuring toner image through the image forming process; and a density measuring means for measuring a density distribution of the density measuring toner image. Preferably, the means adjusts the threshold value of the threshold value matrix of the image processing unit according to a drawing position based on the density distribution measured by the density measuring means.

Further, the apparatus further comprises potential measuring means for measuring a potential distribution of the electrostatic latent image formed on the surface of the image carrier, wherein the light quantity adjusting means is adapted to detect the surface potential distribution measured by the potential measuring means. The threshold value of the threshold value matrix of the image processing unit may be increased or decreased based on the drawing position.

The image processing unit has a function of interpreting a command described in a page description language, classifies the image data into predetermined objects based on the command described in the page description language, It is preferable that the light amount adjusting means adjusts the threshold value matrix of the image processing unit in accordance with a drawing position based on image data classified for each object.
Further, the light amount adjusting means is configured to measure a difference between a density or a potential at a central portion of the drawing and a density or a potential at each drawing position, which is measured by the density measuring means or the potential measuring means, It has a correspondence table with the threshold matrix,
The threshold matrix of the image processing unit may be increased or decreased based on the correspondence table.

In a preferred embodiment, the light amount adjusting means varies the amount of increase or decrease of the threshold value in accordance with the magnitude of the threshold value in the threshold value matrix of the image processing section.

[0032]

Embodiments of the present invention will be described below.

FIG. 6 is a schematic configuration diagram showing a color printer which is a first embodiment of the image forming apparatus of the present invention.

The color printer shown in FIG. 6 has a photosensitive member 22 which rotates in a predetermined direction A and forms a toner image.
The photoconductor 22 is uniformly charged by the charger 21.

The engine control unit 23 converts the pixel data into gradation data using a threshold value matrix, and pulse-modulates the gradation data of each color and sends it to the exposure device 24.

The exposure device 24 irradiates exposure light onto the surface of the photoreceptor 22 based on the pulse-modulated gradation data to form an electrostatic latent image of each color.

The latent image of each color formed on the photoconductor 22 is developed by a developing device 25 having a color toner corresponding to the electrostatic latent image, and a color toner image is formed on the photoconductor.

Thereafter, the exposure by the exposure device 24 and the development by the developing device 25 are repeated on the photoconductor 22 for the number of toner colors, and are superimposed, so that a plurality of color toner images are formed on the photoconductor 22. .

The paper conveyed from the paper tray 26 is transferred to a nip 28 formed by the photosensitive body 22 and the transfer body 27.
, The multi-color toner images formed on the photoconductor 22 are transferred.

The fixing device 29 fixes the toner image transferred onto the paper onto the paper, and the printed paper is stacked on the discharge tray 30.

On the downstream side of the fixing device 29 in the process direction, there is provided a density sensor 31 for measuring the density of the fixed toner image fixed on the sheet.

The engine controller 23 forms a fixed toner image for density measurement on a sheet of paper, and the density sensor 31 measures the density of the fixed toner image for density measurement.

The density measurement value measured by the density measurement sensor 31 is sent to the printer engine control unit 23, and the printer engine control unit 23 adjusts the threshold value of the threshold matrix based on the density measurement value. .

Here, the embodiment of the color printer has been described. However, the image forming apparatus does not necessarily have to be a color printer, and may be one that forms a monotone image. An indirect transfer type using an intermediate transfer member may be used.

FIG. 7 is a schematic diagram showing a signal processing system of a color printer.

The signal processing system of the color printer shown in FIG. 7 uses a controller 37 including a PDL interpreting unit 25 for interpreting a page description language (PDL), a drawing unit 36 for performing color conversion, and a threshold matrix. Processing unit 3 that determines the gradation of the image output from the printer
8 and a printer engine control unit 40 comprising a pulse width modulator 39 for converting the gradation data into multi-valued data in order to increase the resolution of the gradation data.

The signal obtained by pulse-width-modulating the gradation data is
The exposure light is sent to the exposure device 24, and the exposure device 24 emits exposure light based on the pulse width-modulated signal, and scans the photoconductor with an optical scanning system.

The PDL interpreter 35 interprets a page description language (PDL) sent from the PC.

The drawing unit 36 classifies the image data into objects, for example, images, characters, graphics, etc., and generates image data using R (red), G (green), and B (blue) as parameters for each object. For Y (yellow), C (sian), M (magenta),
It is converted into gradation data using K (black) as a parameter.

Although this conversion is performed using an LUT (look-up table) here, it is not necessary to limit the conversion to a method using an LUT, for example, RGB and YMC.
The relationship of K may be regressed to a determinant and processed by a masking method of performing color correction by the matrix operation.

The converted gradation data is developed into raster data of 600 dpi which is the resolution of the printer engine, and the screening processing unit 38 performs screening by a dither pattern according to the object.

In this embodiment, YM
The dither pattern is screened so that the image density of each color of CK becomes a halftone of, for example, 20% of the entire surface, and is output by a printer for each color.

At this time, the threshold values of all the pixels are set from 0 to 10
The offset is incremented by 5 to 0, and based on the offset, 21 patches are printed as shown in FIG. 13 and the color is measured by the density sensor, and the relationship between the offset amount and the color difference is stored as an LUT. . Here, as shown in FIG. 13, the 21 patches are randomly arranged with respect to the offset amount in order to avoid the influence of the in-plane unevenness of the density. FIG. 8 is a diagram illustrating an example of a relationship between the offset amount and the color difference stored in the LUT of the screening processing unit.

When the power of the printer is turned on, the printer engine control unit 40 prints a 20% halftone image of each color toner on the paper, and the density sensor detects the image in the width direction crossing the process direction of the printed paper. The density distribution is measured, and the measurement result is sent to the printer engine controller 40.
Send to

Consider the case where the density distribution is as high as 15% in the density sensor light receiving amount and about 2 to 3 in the color difference at the center of the paper as shown in FIGS.

The screening processing unit 38 of the printer engine control unit 40 converts the transmitted density measurement result into an LUT
, The offset amount is determined so that both ends of the paper have the same color as the center.

Although the PDL interpreter 35 is provided here, the image forming apparatus other than the printer may not include the PDL interpreter 35.

FIG. 8 is a diagram showing an example of the relationship between the offset amount and the color difference stored in the LUT of the screening processing unit.

In FIG. 8, the vertical axis represents the color difference, and the horizontal axis represents the offset amount.

The graph in the figure is a graph showing the relationship between the offset amount and the color difference.
, The color difference increases in a quadratic curve.

In the example shown in the figure, when the color difference is 2.5, which is the average of 2 to 3, the offset amount is 20. Therefore, the offset amount at both ends is set to 0, and the offset amount at the center portion is set to 20. If the threshold value of the position is determined, a signal having a profile in an opposite direction is added, and a change in density caused by a difference in scanning angle of the optical scanning system can be suppressed.

FIG. 9 is a diagram showing a method of obtaining a threshold value of a threshold value matrix at each print position of a print.

In FIG. 9, the width is 25.4 cm (10
Inch) paper 41, and paper 41 has 150 lpi.
(150 lines per inch) 1500 cells 42
And cell positions are allocated from addresses 0 to 1500 from the left side of the drawing of the sheet.

Now, the cell position is corrected to C, the total number of cells of the main scanning line to CN, the length of the main scanning line to M, the number of lines to L (150 lines), and the threshold value of image data to S. Assuming that the threshold is NS and the offset obtained in FIG. 8 is OF, CN = M × L NS = 2 × OF × (− | CN / 2−C | + CN / 2) /
By CN + S, a corrected threshold value can be obtained.

However, if the corrected threshold value exceeds 255, it is rounded to 255.

FIG. 10 shows 150 lpi (600 dpi).
FIG. 7 is a diagram showing a threshold matrix corrected for each printing position from the default value of FIG.

Now, as an example, when the offset amount is 20, the printing position is the cell at both ends, the cell at the center, 1 /
Find the threshold value of the 4-position cell.

In the cell where the printing position is at both ends, C is 0 or 15
Since it is 00, NS described in FIG. 9 is S.

Accordingly, the corrected threshold value becomes the same as the threshold value before the correction, and the threshold value matrix becomes the same as the default value.

In the cell where the print position is at the center, C is 75.
Since it is 0, NS is 20 + S.

Therefore, since the corrected threshold value may be obtained by adding 20 to the threshold value before the correction, the threshold value matrix is obtained by adding 20 to each of the default values.

In the cell where the printing position is 1/4 position, C is 375
Therefore, NS is 10 + S.

Therefore, since the corrected threshold value can be obtained by adding 10 to the threshold value before the correction, the threshold value matrix is obtained by adding 10 to each of the default values.

As described above, the pixel data is converted into gradation data by using the corrected threshold value matrix, and the exposure light is irradiated from the exposure apparatus based on the corrected gradation data. It is possible to correct the difference in the potential of the photoconductor surface caused by the difference in the angle, and to improve the density unevenness of the image finally formed on the paper.

Next, a second embodiment of the image forming apparatus of the present invention will be described.

The second embodiment is different from the first embodiment in that the PDL interpretation unit uses a photograph (image), graphic,
Pixel data such as characters (characters) are classified by object, and for photos (images) and graphics,
The threshold value is corrected in the same manner as in the first embodiment, but the threshold value is not corrected for characters.

Therefore, although the flow of the threshold value correction processing is different, the other steps are the same, and the illustration and description of the image forming apparatus and the like are omitted.

FIG. 11 is a diagram showing a processing flow for correcting the threshold value matrix.

In FIG. 11, the upper part shows the first step, which is a processing flow when the image forming apparatus is offline.
In the middle stage, the power supply of the image forming apparatus, which is the second step, is turned off.
The lower part shows the processing flow when the print operation of the image forming apparatus, which is the third step, is started.

In the first step, the printer engine control unit shown in FIG. 7 forms a halftone toner image having an image density of 20% on the photosensitive drum, and measures the toner image density with a density sensor. Then, the operation of storing the measured color values in the memory is repeated for 21 patches in which the screen offset amount is increased from 0 to 100 by 5 as shown in FIG.

As a result, the density of the toner image of each color formed by changing the offset amount of the screen from 0 to 100 is measured in the memory, and the density difference between the both ends and the center obtained from the measured value is measured. A correspondence table between the offset value and the offset amount is created and stored.

In the second step, a halftone toner image of each color having an image density of 20% is printed with an offset of 0, and the densities at both ends and the center of the printed sheet are measured. And the color difference.

From the obtained color difference, the offset amount is determined based on the correspondence table stored in the memory.

In the third step, the PD of the controller
The L interpreting unit classifies the image data into photographs, graphics and characters, and the screen unit of the engine control unit corrects the threshold value for each printing position based on the offset amount determined at the time of turning on the power. As for the graphic data, the photograph and the graphic data are screened based on the threshold value and converted into gradation data. On the other hand, character data is screened using an uncorrected threshold value.

The screened gradation data is subjected to pulse width modulation to become a pulse modulated signal.

As described above, the photograph and the graphic data are converted into gradation data using the corrected threshold value matrix, and the exposure device irradiates the exposure light based on the gradation data. The difference in the surface potential of the photosensitive member caused by the difference in the angle is corrected and equalized, so that the image finally formed on the paper has improved density unevenness.

Although the first and second embodiments have described the density unevenness due to the light amount unevenness caused by the difference in the angle of the optical scanning system, other causes such as the light amount difference between the elements of the LED array, the light sensitivity, It is also effective against density unevenness caused by a charge difference of the body charger, a difference in photoreceptor sensitivity, a difference in development, and the like. Further, the LUT in FIG. 8 shows a positive offset relationship with respect to the color difference on the dark side, but of course, the same effect can be obtained by storing a negative offset relationship on the color difference on the low density side. Is obtained.

Next, a third embodiment of the present invention will be described.

FIG. 12 is a view showing a third embodiment of the image forming apparatus of the present invention.

The third embodiment is effective when the cause of the density unevenness is related to the latent image potential of the photoconductor, that is, when it is caused by the exposure system, the photoconductor, or the charging system.

In the first embodiment, the threshold of the threshold matrix is increased or decreased by measuring the density of the fixed toner image for density measurement. In the third embodiment, the scanning angle of the optical scanning system is adjusted. Differences, differences between LED array elements,
Equipped with an electrometer that measures the surface potential of the photoconductor or the photoconductor whose difference has occurred due to unevenness of the charging system along the axial direction of the photoconductor. The difference is that the distribution is measured and the threshold value of the threshold matrix is increased or decreased based on the relationship between the potential distribution and the offset amount. The description is omitted.

In FIG. 12, downstream of the exposure device 24 in the rotation direction A of the photoconductor and upstream of the developing device 25,
A potential sensor 31 for measuring the potential distribution of the photoconductor 22 along the axial direction of the photoconductor is provided.

The exposure device 24 adjusts the image density of each color to 2 with an offset of 0 according to a command from the printer engine control unit 23.
The exposure light set to be 0% is emitted, and the surface of the photosensitive drum 22 is scanned by an optical scanning system to form an electrostatic latent image.

The potential sensor 32 measures the potential distribution on the surface of the photosensitive member 22 on which the electrostatic latent image has been formed.

The measured values are sent to the printer engine control unit 23.

In the screening processing section of the printer engine control section 23, a correspondence table between the potential distribution on the photosensitive member surface and the offset amount of the toner image is prepared and stored in advance, so that the power supply of the image forming apparatus is turned on. Then, by measuring the surface potential of the photoconductor on which the latent image is formed, the offset amount can be obtained.

Therefore, similarly to the first embodiment, the threshold value of the threshold value matrix of the screening processing unit can be increased or decreased based on the offset amount, to prepare for the start of printing.

Next, a fourth embodiment will be described.
Until the third embodiment, the offset amount of the entire threshold matrix is determined based on one image density of each color of YMCK, that is, the density of the entire halftone of 20%.
That is, all the density correction amounts were determined from 0% to 100% of the image density according to the density difference of 20%.

However, among the causes of the density unevenness, there are some systems in which the tendency of the density unevenness differs depending on the image density.

For example, in the two-component magnetic brush developing system,
The distance between the developing roll and the photoconductor (Drum to Roll)
If Space (hereinafter DRS) is close to the standard setting,
The higher the image density, the higher the developability and the higher the density, but the lower the image density, the more the toner is written by the magnetic brush, the lower the density. Therefore, as shown in FIG. 14, there is a difference in DRS in the photoconductor axis direction,
If the far side is far, the 20% density is 7
0% concentration is reversed.

In the fourth embodiment, offset values of different threshold values are determined according to the image density so as to cope with the case where the tendency of the density unevenness differs according to the image density.

More specifically, as in the first embodiment, a dither pattern is screened in advance so that the image density of each color of YMCK is a halftone of 20%, and output by a printer for each color. 70%
The dither pattern is screened so that the entire surface becomes halftone, and the color is output by the printer.

At this time, at the time of printing of both 20% and 70%, the threshold values of all the pixels are offset by increasing the threshold value from 0 to 100 in increments of 5 and based on the offset, FIG.
As shown in FIG. 3, 21 patches are printed for both 20% and 70%, and the color is measured with a density sensor. The relationship between the offset amount and the color difference is set as an LUT for both 20% and 70% as shown in FIG. Save it.

When the power of the printer is turned on, the printer engine control unit 40 prints two halftone images of 20% and 70% of each color toner on paper at offset 0, and the density sensor The density distribution in the width direction intersecting with the process direction of the formed sheet is measured, and the measurement result is sent to the printer engine control unit 40.

As shown in FIG. 16, consider the case where a 20% image has a color difference of about 2.5 at the center of the sheet with a color difference as shown in FIG.

The screening processing unit 38 of the printer engine control unit 40 converts the transmitted density measurement result into an LUT
, The offset amounts are determined to be 20% and 70%, respectively, so that both ends of the sheet have the same color as the center.

In this case, the offset of the 20% image is 2
The offset of the 0, 70% image is minus 30 because the density is lighter.

Next, in the fourth embodiment, based on the offset amounts of these two points, the offset amounts of the entire image density, that is, 16 threshold values existing in the default threshold matrix shown in FIG. The offset amount to be calculated is calculated.

In 8-bit 256 gradation, 20% is 51,
Since 70% is equivalent to 178, 5 to 0-255
The offset amounts for the 16 default threshold matrices in 0-255 are calculated by the first-order linear approximation of offset 20 with 178 and offset -30 with 178.

20 = 51A + B-30 = 178A + B Therefore, A = -0.41 and B = 41, so that the offset amount = -0.41 * threshold value + 41.

Therefore, the following table shows the default offset amounts of the 16 threshold matrixes.

In the present example, the density difference is almost constant at 70% or more, and the offset is set to be the same as 70% at 70% or more. However, the present invention is not limited to this.

[0113]

[Table 1]

Then, the offset amounts at both ends are set to 0 for each default threshold value matrix, and the offset amounts at the center are set to the values in the above table. If the threshold value of each print position is determined, a signal having a profile in the opposite direction is added for each image density, so that a change in density due to a DRS difference can be suppressed.

However, the corrected threshold value is 0-25.
If it exceeds 5, round to 0-255.

FIG. 17 shows a case where 150 lpi (600 dpi) is used.
FIG. 10 is a diagram showing threshold matrices corrected for each printing position for each of 16 matrices from the default values of FIG.

[0117]

As described above, according to the image forming apparatus of the present invention, the difference in the scanning angle of the light scanning system of the exposure apparatus, the difference between the elements of the LED array, the photoconductor, the charging system, or the like. Density unevenness caused by unevenness of the developing system is adjusted at the digital signal stage by increasing or decreasing the threshold value of a threshold value matrix for converting pixel data into gradation data, so that image quality is reliably improved. be able to.

Further, it is not necessary to add expensive optical equipment and a correction circuit, to manage severe sensitivity unevenness in the photoconductor manufacturing process, or to increase the mechanical accuracy of the charger and the developing device. It can be realized with little cost because it can be handled by processing.

Further, in the fourth embodiment, a similar effect can be obtained even for a device having a different tendency of density unevenness according to the image density.

Further, in the first to fourth embodiments, only the correction of the density unevenness in the axial direction of the photosensitive member has been described. Is sent to the printer engine control unit, and the processing described in FIGS. 9 and 10 is performed in the process direction.

It is also possible to perform two-dimensional correction by performing both the photosensitive member axial direction and the process direction.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a light scanning system of an electrophotographic image forming apparatus.

FIG. 2 is a diagram showing an optical signal incident on a polygon mirror and an optical signal reflected therefrom.

FIG. 3 is a diagram showing optical characteristics of a lens.

FIG. 4 is a diagram showing measured values obtained by measuring the density of an image transferred and fixed on a sheet with a density sensor.

FIG. 5 shows measured values obtained by measuring the density of a color image transferred and fixed on a sheet using a CCD sensor.

FIG. 6 is a schematic configuration diagram of a color printer showing a first embodiment of the image forming apparatus of the present invention.

FIG. 7 is a schematic diagram illustrating a signal processing system of the color printer.

FIG. 8 is a diagram illustrating an example of a relationship between an offset amount stored in an LUT of a screening unit and a color difference.

FIG. 9 is a diagram illustrating a method of calculating a threshold value of a threshold value matrix at each print position of a print.

FIG. 10 is a diagram showing a threshold matrix in which a corrected threshold value for each printing position is obtained from a default value of 150 lpi (600 dpi).

FIG. 11 is a diagram showing a processing flow for modifying a threshold matrix.

FIG. 12 is a diagram illustrating a second embodiment of the image forming apparatus of the present invention.

FIG. 13 is a diagram showing 21 patches formed by offsetting the threshold values of all pixels from 0 to 100 by 5 in increments of 20 when printing both image densities of 20% and 70%.

FIG. 14 shows a difference in DRS in the axial direction of the photoreceptor.
It is a figure which shows the density | concentration in the case of%.

FIG. 15 shows an image density of 20% which is stored as an LUT.
It is a figure showing the relationship of the offset amount and color difference with respect to both 70%.

FIG. 16 is a diagram showing a case where the density distribution of a 20% image is about 2.5 in color difference at the center of the sheet with a color difference, and the density distribution of a 70% image is about 4 with a color difference at the center of the sheet with a light color.

FIG. 17 is a diagram illustrating a threshold matrix corrected from a default value of 150 lpi (600 dpi) for each printing position for each of 16 matrices.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical scanning system 11 Semiconductor laser 12 Polygon mirror 13 F-theta lens 14 Reflection mirror 15, 22 Photoconductor 16 Incident light 17 Reflected light 18 Lens 19 Central part 20 Both ends 21 Charger 23, 40 Printer engine control part 24 Exposure device Reference Signs List 25 developing device 26 paper tray 27 transfer body 28 nip portion 29 fixing device 30 discharge tray 31 density sensor 32 electrometer 35 PDL interpreter 36 drawing unit 37 controller 38 screening processing unit 39 pulse width modulator 41 paper 42 cell

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 1/401 H04N 1/40 101A 5C077 1/405 C (72) Inventor Takayuki Sunaga 2274 Hongo, Ebina-shi, Kanagawa Address Fuji Xerox Co., Ltd. Ebina Office (72) Inventor Yasuyuki Tanaka 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd. Ebina Office F term (reference) 2C362 AA24 AA54 AA55 AA63 AA65 AA66 AA68 CA03 CA04 CA06 CA14 2H027 DA02 DA09 DA23 DE02 DE05 DE07 DE10 EA02 EB04 EC03 EC06 EC07 EC11 EC19 ZA07 2H076 AB05 AB12 AB76 DA31 5B057 AA11 BA02 CA01 CA02 CA08 CA12 CA16 CB01 CB02 CB08 CB12 CB16 CC01 CE11 CE18 CH07 CH08 5C074 AA20 CC02 CC26 DD26 PP32 PP33 PP37 PQ12 PQ23 SS02 TT03

Claims (7)

[Claims] An image carrier on which an electrostatic latent image is formed on a surface and the electrostatic latent image is developed with toner to form a toner image, and image data is patterned by a predetermined threshold matrix. An image processing unit that converts the image data into gradation data representing the density of the toner image, and an exposure device that emits exposure light based on the gradation data and irradiates the surface of the image carrier to form an electrostatic latent image. A toner image formed by developing the latent image on the surface of the image carrier, and finally transferring and fixing the toner image on a recording medium, thereby fixing the toner image on the recording medium from the fixed toner image. In an image forming apparatus for forming an image, a threshold value of a threshold value matrix of the image processing unit is
An image forming apparatus comprising: a light amount adjusting unit that adjusts the amount of light applied to the surface of the carrier by increasing or decreasing the amount in accordance with a drawing position in an image drawing surface. 2. The exposure device according to claim 1, wherein the exposure device performs a raster scan of the laser beam onto the surface of the image carrier. 2. The image forming apparatus according to claim 1, wherein the light amount is adjusted in accordance with the drawing position based on the scanning angle so as to correct the light amount. 3. A light quantity adjusting device comprising: a density measuring toner image forming means for forming a density measuring toner image through the image forming process; and a density measuring means for measuring a density distribution of the density measuring toner image. The means for adjusting a threshold value of a threshold value matrix of the image processing unit according to a drawing position based on a density distribution measured by the density measuring means. 3. The image forming apparatus according to 1 or 2. 4. An electric potential measuring unit for measuring a potential distribution of the electrostatic latent image formed on the surface of the image carrier, wherein the light amount adjusting unit is adapted to detect the surface potential distribution measured by the potential measuring unit. The image forming apparatus according to claim 1, wherein a threshold value of a threshold value matrix of the image processing unit is increased or decreased based on a drawing position. 5. The image processing unit has a function of interpreting a command described in a page description language, classifying the image data for each predetermined object based on the command described in the page description language, 5. The apparatus according to claim 1, wherein the light amount adjusting unit adjusts a threshold value matrix of the image processing unit in accordance with a drawing position based on image data classified for each object. The image forming apparatus as described in the above. 6. The image processing device according to claim 6, wherein the light amount adjusting unit is configured to determine a difference between a density or a potential at a central portion of the drawing and a density or a potential at each position of the drawing, measured by the density measuring unit or the potential measuring unit. 5. The image forming apparatus according to claim 3, further comprising a correspondence table with a threshold matrix of the image processing unit, wherein the threshold matrix of the image processing unit is increased or decreased based on the correspondence table. . 7. The light amount adjusting unit according to claim 1, wherein the amount of increase / decrease of the threshold value varies according to the magnitude of the threshold value of the threshold value matrix of the image processing unit.
7. The image forming apparatus according to claim 6, wherein