JP2001260413A - Image-forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an image quality decrease because of a surface tilt of a polygon mirror without newly adding parts of a complicate constitution, etc. SOLUTION: In this image-forming apparatus of a system in which a beam light corresponding to image data scans a plurality of mirror faces of the polygon mirror 53 to irradiate an image carrier, a detecting means 5 is set for detecting a rotation angle of the polygon mirror 53. Image data are processed on the basis of outputs of the detecting means 5 so as to reduce effects of a scanning position deviation by the tilting mirror face when the beam light scans the tilting mirror face of the polygon mirror 53.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic process, such as a printer or a copying machine, and more particularly, to an image forming apparatus which emits a beam corresponding to image data by rotating a polygon mirror (rotating polygon mirror). The present invention relates to an image forming apparatus that scans on a carrier.

[0002]

2. Description of the Related Art In recent years, demands for output image quality of image forming apparatuses such as printers and copiers have become strict, and high-resolution machines have been commercialized. With higher resolution of image quality,
Accuracy requirements for the deviation of the inclination of each mirror surface of the polygon mirror with respect to the rotation axis, that is, the deviation of the exposure position in the sub-scanning direction due to the so-called surface tilt, are becoming stricter.

[0003] When the surface is tilted during image formation, the position of the main scanning line scanned by the tilted mirror surface shifts back and forth with respect to the rotation direction of the photosensitive member. If the shift amount is large, the image output using the polygon mirror will be recognized by the naked eye as unevenness even in a general image.

To solve this problem, Japanese Patent Laid-Open No. 5-1677 is disclosed.
In the image forming apparatus described in Japanese Patent No. 93, a mirror for reflecting laser light is newly provided between the laser light source and the image carrier, and the displacement of the exposure position in the sub-scanning direction of the laser light is reduced.
Measurement is performed for each mirror surface of the polygon mirror, and based on the measurement result, the polygon mirror is displaced by a small angle by an actuator, thereby correcting the exposure position deviation.

[0005]

SUMMARY OF THE INVENTION Incidentally, Japanese Patent Application Laid-Open No.
In the image forming apparatus described in Japanese Patent No. 167793, a complicated configuration such as a polygon mirror, an actuator for driving the polygon mirror, and a control circuit for controlling the actuator in accordance with the amount of tilting is required. There is a problem of being connected.

The present invention has been made in view of such circumstances, and an image forming apparatus capable of suppressing a decrease in image quality due to a face-down of a polygon mirror without newly adding a component having a complicated configuration. The purpose is to provide.

[0007]

SUMMARY OF THE INVENTION The present invention relates to an image forming apparatus in which a beam light corresponding to image data is scanned by a plurality of mirror surfaces of a polygon mirror to irradiate an image carrier. Based on the detection means for detecting the angle, and based on the output of this detection means, at the time of light beam scanning of the mirror surface on which the polygon mirror has been tilted,
An image data processing means for processing image data is provided so as to reduce the influence of the scanning position shift due to the mirror surface.

According to the image forming apparatus of the present invention, the light beam scans the image carrier in the main scanning direction by rotating the polygon mirror, and further moves by the relative movement of the image carrier with respect to the belt-shaped scanning area. By scanning in the scanning direction, 2
Form a two-dimensional image. During the two-dimensional image formation,
The print quality is enhanced by processing the image data so that the deviation of the exposure position in the sub-scanning direction on the photosensitive member due to the mirror surface having the surface tilt is not conspicuous.

In the image forming apparatus of the present invention, as a specific process executed by the image data processing means, one pixel of the multi-tone original image is made to correspond to a sub-matrix having a predetermined number of dots, and By using a gradation expression method in which the density of the black dots in the sub-matrix is represented by the area ratio of the black dots in the sub-matrix, the priority of the arrangement of the black dots with respect to the scanning lines formed by the mirror surface where the surface is tilted is lowered, A process of reducing the influence of the displacement can be given.

When such data processing is performed, a scanning line formed by a mirror surface on which a surface is tilted is not used in a low density region where the density is less than a certain value and the influence of a scanning position shift is conspicuous. That is, black dots are not formed in the low-density area, the density becomes equal to or higher than a certain value, and the black dots are formed after the influence of the scanning position shift becomes less noticeable. As described above, by using the density pattern in which the exposure position shift is not conspicuous in the low density region, it is possible to specifically suppress the influence of the scanning position shift due to the mirror surface.

As a more specific image data processing, a dot distribution type gradation expression method is used in which a sub-matrix is divided into a plurality of blocks, and black dots are arranged in each of the blocks to express gradation. And a process of dividing a sub-matrix such that a scanning line formed by a mirror surface on which a surface is tilted becomes a boundary region of each block.

By performing such image data processing, it is possible to reduce the influence of density deviation. That is, conventionally, the number of surfaces of the polygon mirror and 1
Independent of the number of sub-scan lines in the block,
As in the present invention, by dividing the blocks by providing a correspondence between the number of surfaces of the polygon mirror and the number of sub-scanning lines of the sub-matrix, the boundaries of blocks originally having a low possibility of black dots being formed are obtained. A scanning line formed by a mirror surface on which surface tilt has occurred can correspond to the region. As a result, it is possible to narrow the density region where the density shift due to the exposure position shift occurs, and at the same time,
The effect of the density shift can be suppressed to a small level.

In the image data processing as described above, if the number of lines in the sub-scanning direction of the sub-matrix is set to an integral multiple of the number of surfaces of the polygon mirror, the number of surfaces of the polygon mirror and the number of sub-scanning lines of the sub-matrix are reduced. Since there is a correspondence between the two, the use of only one sub-matrix can prevent image degradation due to face-to-face collapse without reducing the number of gradations that can be expressed.

When the number of lines in the sub-matrix in the sub-scanning direction is set to an integral multiple of the number of surfaces of the polygon mirror, the area of the sub-matrix required for the number of tones to be expressed is secured by the size in the main scanning direction. In this way, the main scanning line in each matrix and the mirror surface of the polygon mirror can be in a one-to-one correspondence regardless of which sub-matrix is used. By using only one sub-matrix, it is possible to prevent the image quality from deteriorating due to the trouble.

On the other hand, in the above image data processing,
When the least common multiple of the size (s) of the sub-matrix in the sub-scanning direction and the number of surfaces of the polygon mirror is N, (N /
s) If a pattern sequence for expressing the number of gradations is used for the sub-matrix, even if the size of the sub-matrix in the sub-scanning direction is not an integral multiple of the number of polygon mirror surfaces, the exposure position shift due to surface tilting occurs. Image quality deterioration can be prevented.

Here, in the image forming apparatus of the present invention,
The criterion for determining the surface tilt of the polygon mirror is such that when the resolution of the image forming apparatus is A [dpi], the deviation of the scanning line is equal to or more than (2.54 / A) × 0.2 [cm]. Set.

As described above, the criterion for determining the surface tilt is set to 20% of the dot diameter, which is a level at which pitch unevenness can be visually confirmed. If the exposure position shift is less than the criterion, a normal shift is not considered. By performing the control, it is possible to prevent the image quality from deteriorating due to unnecessary control.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus (electrophotographic process) to which the present invention is applied.

In the electrophotographic process, an electrostatic latent image corresponding to a document image or image data from a host computer is generally formed on a photosensitive member 1 as an image carrier.
The electrostatic latent image is visualized by the developing device 2 and transferred onto a recording medium 3 such as paper to form an image. In addition,
The developing device 2 includes a developing roller 21, a supply roller 22, a screw 23, a toner tank 24, and the like.

Around the photosensitive member 1, a charging member 4, an exposing member 5, the developing device 2, a transfer member 6, a cleaning member 7, and a charge removing member 8 are arranged in order from the upstream side in the rotation direction of the photosensitive member 1. Have been. Further, a fixing member 9 is disposed downstream of the photoconductor 1.

The photoreceptor 1 has a photoconductive layer 12 such as amorphous silicon (a-Si), selenium (Se), or an organic optical semiconductor (OPC) formed on a peripheral surface of a metal drum 11 made of, for example, aluminum. Are formed in a thin film shape.

The charging member 4 is composed of, for example, a charging wire such as a tungsten wire, a corona charger made of a metal shield plate and a grit plate, or a charging roller and a charging brush. The transfer member 6 is, for example, a corona charger.
It is composed of a charging roller, a charging brush, and the like.

The photosensitive member 1 is first uniformly charged by the charging member 4 and then irradiated with light by the exposing member 5 according to the image data. The electrostatic latent image thus formed on photoconductor 1 is developed between photoconductor 1 and developing device 2 by a developing electric field formed by a bias power supply (not shown) provided in developing device 2. The developer (toner) in the developing device 2 moves and is visualized as a toner image. This toner image is transferred onto the recording medium 3 by the transfer member 6 and is fixed by heating at the fixing member 9.

After the transfer of the toner image, the toner remaining on the photoreceptor 1 is removed by a cleaning member 7 such as a cleaning blade, and the electric charge remaining on the photoreceptor 1 is removed by a discharging member 8 such as a discharging lamp. The charge is removed.

FIG. 2 is a perspective view schematically showing the structure of the exposure member 5.

The exposing member 5 of this embodiment is called a laser beam scanning unit (LSU). A laser diode 51 outputs a laser beam in accordance with image data, and the output laser beam is collected by a cylindrical lens 52. The light is radiated so as to be focused on the polygon mirror 53 in the sub-scanning direction.

The polygon mirror 53 is a polygon mirror that is rotatable around an axis perpendicular to the rotation center of the photoconductor 1. The laser beam is emitted from the cylindrical lens 52 to irradiate a predetermined range on the photoconductor 1 with laser light. The laser beam is reflected while rotating.

The reflected light deflected at a constant angular velocity by the polygon mirror 53 is corrected by the f-θ lens 54 so as to be deflected at a constant velocity on the photosensitive member 1,
The light path is changed by the reflection mirror 55, and the light is irradiated onto the photoconductor 1.

On the optical path from the f-θ lens 54 to the reflection mirror 55, a start sensor 56 and a start sensor mirror 57 facing each other are arranged. The start sensor 56 is realized by a photodiode having a slit, for example.

Then, the laser light is scanned on the photoreceptor 1 in the arrow X direction by the rotation of the polygon mirror 53 (rotation in the arrow φ direction). In this laser beam scanning, the laser beam is reflected by the start sensor
Is a start position in the main scanning direction. Thereby, the position of the mirror surface of the polygon mirror 53 and the period in the main scanning direction are detected,
A start position in the main scanning direction is determined.

On the other hand, on the upper surface of the polygon mirror 53, a reference position mark 53b is formed corresponding to the reference surface 53a. The reference position mark 53b is formed, for example, by increasing the surface roughness of the end of the polygon mirror 53 (a region close to the adjacent surface) or the upper surface of the reference surface of the polygon mirror 53, or is formed by blackening the irradiation light. It is formed by applying a paint or the like. This reference position mark 53b is
The reference surface 53a can be detected by the detection by the reference position detection means 58 including a photo interrupter or the like.

The polygon mirror 53 detected by the reference position detecting means 58 and the start sensor 56
The polygon mirror driving motor (not shown) is driven so that the rotation angle of the laser beam coincides with the input image data, and the laser diode 51 is turned on / off so as to coincide with the image data. The scanning of the laser beam corresponding to the image data on the body 1 is performed.

Control unit 1 for controlling laser diode 51
00, as shown in FIG. 3, a processor 101, a ROM 102 in which sub-matrix information described later is stored,
A RAM 103, a trigger pulse generator 104, and a buffer 105 are provided. The RAM 103 stores the tilting information obtained from the reference position detecting means 58 described above.

The processor 101 starts processing the image data starting from the pulse generated by the trigger pulse generator 104 in accordance with the output of the start sensor 56. The image data processing is executed by the processing described later based on the sub-matrix information stored in the ROM 102 and the tilt information in the RAM 103.

The details of the image data processing applied to the embodiment of the present invention will be described below.

In the following description, the polygon mirror 5
As shown in FIG. 4A, the shift of the scanning line due to the surface tilt of No. 3 is shifted to the rear side from the original position (broken line) as “shift to the downstream side” (shift to the minus side). As shown in FIG. 4 (b), a case where the position is shifted forward from the original position (broken line) is referred to as a "shift to the upstream side" (a shift to the plus side).

FIG. 5 to FIG. 7 are views showing the state of the exposure position shift due to the surface tilt.

In the examples shown in FIGS. 5 to 7, one pixel of the original image of 8 bits, that is, 256 gradations is made to correspond to a sub-matrix of 16.times.16 dots, and the density of each pixel is determined by the black dot in the sub-matrix. A dot distribution type gradation expressing method is used in which the sub-matrix is divided into four blocks, and black dots are arranged in each block to express the gradation by expressing the gradation. Also, FIG.
FIG. 7 shows an example in which the 0th density is solid black, the 255th density is white solid, the polygon mirror 53 has eight surfaces, and the third surface is tilted.

In this example, at the 248th density,
As shown in FIG. 5 (a1), eight black dots are evenly distributed two in each of the four blocks. The black dots are present on the second line and the sixth line of each block, and therefore, are shown in FIGS. 5A2 and 5A3.
As shown in (1), there is no black dot on the third line, which is affected by surface tilt, due to the shift to the plus side and the shift to the minus side, and the image quality is not affected.

On the other hand, at the 224th density, as shown in FIG. 6B1, the 24th density shown in FIG.
A cluster of four dots is formed, each having eight black dots at the eighth density as nuclei. At this time, black dots exist on the third and seventh lines in addition to the second and sixth lines.

For this reason, as shown in FIG. 6 (b2), a black dot is normally formed on the seventh line with respect to the shift to the plus side, but the chunk is separated on the third line. (Shaded area in the figure). Further, as shown in FIG. 6B3, depending on the shift to the minus side, the black dot overlaps with the nucleus of the second line, and as shown in FIG. 6B3, the shift is ３. In the case of the line portion, the area of the black dot is reduced by 25%.

Further, as shown in FIG. 7 (c1),
Even at the 208th density, the 24th density shown in FIG.
A lump of six dots is formed, each having eight black dots at the eighth density as nuclei. At this time, FIG.
As shown in 2), with respect to a shift to the plus side, the chunks are separated in the third line (hatched portion in the figure). In addition, as shown in FIG. 7C3, depending on the shift to the minus side, the dot overlaps with the black dot on the second line serving as a nucleus.

Therefore, in this embodiment, first, as described above, a sub-matrix of M × N dots is taken to represent one original pixel, and the density gradation is expressed by the ratio of black dots in the sub-matrix. Method. Then, according to the rotation angle of the polygon mirror 53 detected by the reference position detection means 58 and the start sensor 56, the polygon mirror 53 corresponds to the mirror surface on which the surface has been tilted out of the plurality of (eight) mirror surfaces. For the scanning line to be changed, the priority of the arrangement of the black dots is lowered.

For example, as shown in FIG. 8, when the scanning line corresponding to the mirror surface on which the surface is tilted is denoted by L1, as the density increases, FIGS. 8 (a), 8 (b) and 8
As shown in FIG. 8C, black dots are arranged from the remaining scanning lines L2 to L5 where no surface tilt has occurred, as shown in FIG. 8D and FIG. 8E. When the remaining scanning lines L2 to L5 are full of black dots, the image data is processed so as to arrange the black dots on the scanning line L1 on the mirror surface on which the mirror has been tilted.

Therefore, the scanning line L1 on the mirror surface on which the surface is tilted is not used in a low density region where the density is less than a certain value and the influence of the scanning position shift is conspicuous. That is, black dots are not formed in the low-density region, the density becomes equal to or higher than a certain value, and the black dots are formed after the influence of the scanning position shift becomes less noticeable. As described above, by using the density pattern in which the exposure position deviation is not conspicuous in the low density region, the influence of the scanning position deviation can be reduced, and the print quality can be improved.

In the density pattern method, for example,
Bayer (diffusion) type in which the formation order of black dots is set in a form as shown in FIG. 9A, spiral type in which the formation order of black dots is set in a form as shown in FIG. 9B, and In the case of a halftone dot type or the like in which the formation order of black dots is set in a form as shown in FIG. 9C, the formation order of the dots on the scanning line on the mirror surface on which the surface is tilted is changed to the remaining scanning line. A similar effect can be obtained by lowering the order of formation so that the formation is performed after the dot is formed.

FIG. 10 is a diagram for explaining a method of processing image data according to another embodiment of the present invention.

In the image data processing method shown in FIG. 10, the reference surface 53a of the polygon mirror 53 is referred to as a first surface, and is used for gradation expression according to the number of a mirror surface having a predetermined amount or more of inclination. It changes the pattern. Specifically, a method of dividing the sub-matrix into blocks, a method of arranging nuclei, and a method of thickening dots (a method of arranging dots) are defined.

In the example of FIG. 10, the number of polygon mirrors 53 is eight, and the third and sixth surfaces of the polygon mirror 53 are tilted as shown in FIG. 10 (a).

First, in a scanning line in the sub-scanning direction,
As shown in FIG. 10B, the sub-matrix is divided into blocks such that the third line and the sixth line corresponding to the surface tilt are located at the boundaries of the blocks. Next, for example, a black dot serving as a nucleus is arranged every other block in the vertical and horizontal directions, and as the density increases, black dots are preferentially arranged from the dots adjacent to the black dot serving as the nucleus. We make black dot fat.

As described above, by dividing the line corresponding to the surface inclination so as to be at the position of the boundary of the block, the influence of the density deviation can be reduced.

That is, in the prior art, the number of surfaces of the polygon mirror and the number of sub-scanning lines of one block in the sub-matrix are irrelevant. By performing block division with a correspondence relationship with the number of lines, it is possible to correspond a scan line that is tilted to the boundary region of a block where a black dot is unlikely to be originally formed, and the exposure position shift In addition, it is possible to narrow the density region in which the density shift occurs due to the influence of the density shift.

Further, by setting the number of sub-scanning lines in the sub-matrix to be an integral multiple of the number of surfaces of the polygon mirror 53, it is possible to prevent the image quality from deteriorating by using only one sub-matrix.

FIG. 11 is a diagram for explaining a method of processing image data according to still another embodiment of the present invention.

In the above embodiment, since the size of the sub-matrix in the sub-scanning direction is an integral multiple of the number of surfaces of the polygon mirror 53, a simple example in which mirrors on the same surface correspond to the writing lines for all matrices Met.

This embodiment is an example in which the size of the sub-matrix in the sub-scanning direction is not an integral multiple of the number of surfaces of the polygon mirror 53. When the least common multiple of the number of surfaces (m) is (N), (N / s) pattern sequences for gradation expression are used for the sub-matrix.

In the example of FIG. 11, s = 16, m = 6, its least common multiple is N = 48, and (N / s) = 3.
Therefore, three density pattern sequences corresponding to the densities from 0 to 255 are prepared, and these are A, B, and C, respectively.

In the example shown in FIG. 11, the sixth surface is tilted. When the block in the sub-scanning direction is divided by using the line scanned by the sixth surface as a boundary, the matrix A has six lines from the upstream side. The eyes and the twelfth line are boundaries. In the matrix B, the boundaries are the second, eighth, and fourteenth lines. In the matrix C, the fourth, tenth, and sixteenth lines are provided.
B and C are mutually different block divisions.

Here, for example, if only a block division pattern created based on the matrix of A is applied to all the matrices, a density pattern of blocks which are not divided by a mirror surface that is scanned by a mirror surface that is tilted is used. However, the image quality cannot be improved.

Therefore, as described above, the least common multiple (N) of the size (s) of the sub-matrix in the sub-scanning direction and the number of surfaces (m) of the polygon mirror 53 is calculated as the matrix size (s).
By preparing a series of density patterns by the number of values (N / s) divided by, the size (s) of the sub-matrix in the main scanning direction is not an integral multiple of the number of surfaces (m) of the polygon mirror 53. Even at this time, it is possible to prevent the image quality from deteriorating due to the exposure position shift due to the surface tilt.

That is, each line of the sub-matrix of the fourth block and the mirror surface of the polygon mirror 53 for scanning the same are the same as the matrix of the first block.
The sequence pattern A may be used again. Thereafter, similarly,
The sequence pattern of C is used repeatedly in sequence.

FIG. 12 is a diagram for explaining a method of processing image data according to still another embodiment of the present invention.

In the example of FIG. 12, the size of the sub-matrix in the sub-scanning direction is set to an integral multiple of the number of surfaces of the polygon mirror 53 (six in this example), and the area of the sub-matrix required for the number of tones to be expressed is determined. The size is secured by the size in the main scanning direction.

More specifically, since the number of surfaces of the polygon mirror 53 is 6, the size of the sub-matrix in the sub-scanning direction is a multiple of 6. By doing so, the main scanning line in each matrix and the surface of the polygon mirror 53 have a one-to-one correspondence regardless of which sub-matrix is taken.

In the example of FIG. 12A, the size in the sub-scanning direction is set to 12, so that the size in the main scanning direction required to express 256 gradations is from 22 ＝ 12 = 21.3 to 22 And In the example of FIG. 12B, the size in the sub-scanning direction is set to 18, so that 256 ÷ 18
From = 14.2, the size in the main scanning direction is set to 15. With this configuration, it is possible to prevent the image quality from deteriorating due to the trouble, without reducing the number of gradations that can be expressed and using only one sub-matrix. Here, the two-dimensional image obtained by the image forming apparatus of the present invention was evaluated. The evaluation results are shown in Table 1 below. Table 1 shows image evaluation results when a pattern in which resolution and tilting are taken into consideration is used and when patterns are not taken into account. 3 shown in Table 1
In the judgment item of the column, the upper column shows the pitch unevenness when scanning every other line, the middle column evaluates the collapse in the dark area by a 256 density gradation pattern, and the lower column shows the human upper body image. Was evaluated.

[0067]

[Table 1]

As is apparent from Table 1, when the density pattern in which the surface tilt of the polygon mirror 53 is considered is used, the image quality is improved as compared with the case where the density pattern in which the polygon mirror 53 is not considered is used. It can be seen that a large effect is obtained for a typical continuous line.

LSUs 1, 2 and 3 in Table 1 are all eight-sided polygon mirrors, and only one surface is tilted. The deviation of the exposure position due to each tilt is LSU.
1: 4 μm, LSU2: 5 μm, LSU3: 5 μm, L
SU4: 7 μm. The size of the sub-matrix is 16 × 16, and the sub-matrix is divided into four 8 × 8 blocks.

When the tilting is not taken into consideration, the mirror surface for scanning the divided line is arbitrary, and when the pattern taking the tilting is taken into consideration, the mirror for scanning the dividing line is not tilted. Dots are arranged on the surface and on the line in a non-priority manner. In addition, the determination of the surface inclination is performed when the resolution of the image forming apparatus is A [dpi] and the deviation of the scanning line is (2.54 / A) × 0.2 [cm] or more.

Table 2 below shows an evaluation of a human upper body image, and shows the relationship between pitch unevenness (pitch unevenness is used because it is easy to determine the minimum) and subjective evaluation of image quality. Table 2 (a) shows the exposure position shift amount and the image quality evaluation result with the 1200 dpi machine, and Table 2 (b) shows the exposure position shift amount and the image quality evaluation result with the 600 dpi machine. From these results, it is understood that when the pitch unevenness is 20% or more of the dot diameter, the pitch unevenness can be visually confirmed.

[0072]

[Table 2]

Therefore, by selecting the criterion for determining the tilt of the surface as described above, if the exposure position is less than the criterion, the normal control without considering the deviation is performed. Can be prevented from decreasing.

As a method of measuring the inclination of the surface of the photosensitive member 1 at the time of shipment of the image forming apparatus having the structure of the present invention, for example, as described in the above-mentioned Japanese Patent Application Laid-Open No. 5-167793. A CCD is set next to the CCD, and it is determined whether or not the irradiation position of the laser beam is within the specified range based on the output of the CCD. If there is a deviation, this is realized by simultaneously measuring the deviation amount.

From the measured data, the number of the face on which the face has been tilted is input on the operation panel of the machine, and the data is stored in the RAM. Image formation that suppresses the above-mentioned effects by creating a pattern corresponding to each density based on the data or selecting an optimal pattern from the patterns stored in the ROM from the beginning and using it It can be performed.

[0076]

As described above, according to the image forming apparatus of the present invention, the influence of the scanning position shift due to the mirror surface during the light beam scanning of the mirror surface on which the polygon mirror is tilted is reduced. As described above, since the image data processing is performed, it is possible to form a high-quality image in which a decrease in image quality due to the tilting of the polygon mirror is suppressed without newly adding a component having a complicated configuration.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus to which the present invention is applied.

FIG. 2 is a perspective view schematically showing a configuration of an exposure member of the image forming apparatus shown in FIG.

FIG. 3 is a block diagram illustrating a configuration of a control unit of the laser diode.

FIG. 4 is a diagram that defines a direction of displacement of an exposure position.

FIG. 5 is a diagram illustrating a state of displacement of an exposure position.

FIG. 6 is a view for explaining a state of a shift of an exposure position.

FIG. 7 is a view for explaining a state of a shift of an exposure position.

FIG. 8 is an explanatory diagram of an example of image data processing applied to the embodiment of the present invention.

FIG. 9 is an explanatory diagram of another example of image data processing applied to the embodiment of the present invention.

FIG. 10 is an explanatory diagram of another example of image data processing applied to the embodiment of the present invention.

FIG. 11 is an explanatory diagram of still another example of the image data processing applied to the embodiment of the present invention.

FIG. 12 is an explanatory diagram of still another example of the image data processing applied to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Developing device 3 Recording medium 4 Charging member 5 Exposure member 51 Laser diode 52 Cylindrical lens 53 Polygon mirror 53a Reference surface 53b Reference position mark 54 f-θ lens 55 Reflection mirror 56 Start sensor 57 Start sensor mirror 58 Reference position detection Means 6 Transfer member 7 Cleaning member 8 Static elimination member 9 Fixing member 100 Control unit 101 Processor 102 ROM 103 RAM 104 Trigger pulse generator

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) G03G 21/00 510 G03G 15/04 120 5C072 H04N 1/113 H04N 1/04 104A 5C074 1/23 103 9A001 F term (reference) ) 2C262 AA05 AA17 AA24 AA26 AA27 AB05 BB14 BB23 GA34 2C362 BB05 CA04 CA05 2H027 EF09 FD08 HB06 ZA07 2H045 AA01 CA01 DA28 2H076 AB05 AB12 AB16 AB76 5C072 AA03 BA02 BA17 HA02 A13 HB02B03 H03 HAB H02A13A02 KK16 KK42

Claims (7)

[Claims] 1. An image forming apparatus in which a beam light corresponding to image data is scanned by a plurality of mirror surfaces of a polygon mirror to irradiate an image carrier, detecting means for detecting a rotation angle of the polygon mirror, Image data processing means for processing image data based on the output of the detection means so as to reduce the influence of the scanning position shift due to the mirror surface during beam light scanning of the mirror surface on which the polygon mirror has been tilted. An image forming apparatus comprising: 2. The image data processing means associates one pixel of a multi-tone original image with a sub-matrix having a preset number of dots, and expresses the density of each pixel by the area ratio of black dots in the sub-matrix. By using a gradation expression method, the priority of the arrangement of the black dots with respect to the scanning line formed by the mirror surface on which the surface is tilted is lowered, so that the influence of the scanning position shift is reduced. The image forming apparatus according to claim 1, wherein: 3. The image data processing means uses a dot dispersion type gradation expression method of dividing a sub-matrix into a plurality of blocks and arranging black dots for each block to express gradation. 3. The image forming apparatus according to claim 2, wherein the sub-matrix is divided so that a scanning line formed by the mirror surface having the surface tilt becomes a boundary region of each block. 4. The image forming apparatus according to claim 2, wherein the number of lines in the sub-scanning direction of the sub-matrix is set to an integral multiple of the number of surfaces of the polygon mirror. 5. The image forming apparatus according to claim 4, wherein the area of the sub-matrix required for the number of gradations to be expressed is secured by the size in the main scanning direction. 6. When the least common multiple of the size (s) of the sub-matrix in the sub-scanning direction and the number of surfaces of the polygon mirror is N, a pattern sequence for expressing (N / s) gradations is represented by a sub-matrix. The image forming apparatus according to claim 2, wherein the image forming apparatus is used for: 7. A criterion for determining the inclination of a polygon mirror is as follows:
When the resolution of the image forming apparatus is A [dpi], a shift of a scanning line is set to a value of (2.54 / A) × 0.2 [cm] or more. 7. The image forming apparatus according to 1, 2, 3, 4, 5, or 6.