JPH11129535A - Apparatus and method for forming image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus and a method for forming images whereby optimum images of an optimum pixel size can be formed in accordance with image information. SOLUTION: A laser light source 253 is fitted to a laser substrate 252 of a laser exposure optical system of an image-forming apparatus. On an optical path 220 in an emergence direction of laser beams inside a case 250 are sequentially arranged a lens barrel 255 where a collimator lens 254 is set, a diaphragm plate 260 which has an aperture determining a shape of a luminous flux and a diaphragm diameter of which can be changed by the rotation of a blade plate of a motor, a cylindrical lens 256 and a polygon mirror 258. An fθlens 259 and a toric lens 270 are sequentially arranged on an optical path of the polygon mirror 258 in a reflection direction. A shape of pixels projected on a photosensitive drum 201 where an image is finally projected is proportional to a shape of the diaphragm plate 260.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital image forming apparatus and an image forming method, and more particularly, to an image forming apparatus having a pixel density changing means and an image forming method.

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine, a printer, a facsimile, etc., which forms an image digitally by a set of pixels, it is not easy to change the pixel size. In a structure that can be formed, in the case of a type of image in which large pixels are more desirable, for example, in the case of a photograph or a solid image, a large image is represented by collecting the smallest image according to the required image properties. I was FIG. 10 is a schematic top view of a latent image having a pixel size of 300 dpi and 600 dpi formed on the photosensitive drum, FIG. 10A is a 300 dpi latent image, and FIG.
Is a 600 dpi latent image. Reference numeral 41 in the figure is 300d
A latent image of a pixel of pi, 42 is a latent image of a pixel of 600 dpi.

Originally, even if it is desired to form an image with a pixel size of 300 dpi as shown in FIG.
In the case where the pixel size is fixed to four pixels, as shown in FIG. 10B, four half pixels have the same effect as a pixel twice as large in size (four times in area).

[0004]

In a dry electrophotographic system, the larger the pixel size, the stronger the potential and the stronger the attraction of the toner, so that the developed image is smooth and clear. In electrophotography, there is a method of modulating one pixel and forming an image by changing the lighting time of a light source such as a laser when forming a latent image, for example, to obtain an image with rich floor adjustment. FIG. 11 is a schematic top view showing the state of the latent image formed on the photosensitive drum when the lighting time is reduced to a quarter, where (a) is a 300 dpi latent image and (b) is 600 dpi. Is a latent image. Reference numeral 51 in the figure
Is a latent image of 300 dpi pixels, and 52 is a latent image of 600 dpi pixels.

As an example, 300 dpi or 84.7 μm
When the lighting time of the light source is set to の 一 so that the density of the pixel is 四, an image having a width of about 20 μm is formed as shown in FIG. When an image is formed with 42.3 μm pixels and the lighting time of the light source is set to の 一 in order to obtain a quarter image density, an image density corresponding to FIG. If desired, the latent image on the image carrier, for example, the photoreceptor, is shown in FIG.
Is an area having a length of about 10 μm. Since the diameter of a commonly used toner is about 7 μm, the area of one latent image is small, so the amount of charge on the image carrier, which is a force for attracting toner, is not stable. Although it had to be, it varied in the range of 0 to 3 pieces, and the image did not become rich in gradation.

Further, when outputting signals having different pixel densities, for example, when outputting a facsimile,
The main scanning direction, that is, the pixel size is 8 pels (about 20 pixels).
0 dpi), the sub-scanning direction is 3.85 pel (about 98 dpi)
In the conventional image forming apparatus which is easy to develop but has only a pixel density of, for example, 600 dpi, a conversion circuit for adjusting to the input pixel density requires an extremely high cost and is not a proper multiple. Therefore, it is not always the case that the input signal becomes the same as that of the input signal.

It is an object of the present invention to provide an image forming apparatus and an image forming method capable of forming an optimum image with an optimum pixel size according to image information.

[0008]

According to an image forming apparatus of the present invention, an image is formed by a set of pixels in a main scanning direction and a sub-scanning direction by forming a pixel on an image carrier with light from a light source according to an input signal. The image forming apparatus includes an exposure optical system capable of adjusting a light amount of a light source and continuously changing a size of a pixel formed on an image carrier by changing an optical path of the light source.

The method of changing the pixel size of the exposure optical system may be performed by continuously changing the opening area of a diaphragm mechanism provided in the optical path from the light source, and the lens provided in the optical path from the light source. May be performed by moving the focal position by adjusting the distance.

The image forming method of the present invention is an image forming method in which pixels are formed on an image carrier by an input signal with light from a light source, and an image is formed by a set of pixels in a main scanning direction and a sub-scanning direction. Using an exposure optical system capable of adjusting the light amount of the light source and continuously changing the size of the pixel formed on the image carrier by changing the optical path of the light source, the image information of the input signal is used. The size of the pixel to be imaged on the carrier is changed.

When the image information of the input signal is an image, the size of the pixel may be increased. When the image information of the input signal is a character or a line drawing, the size of the pixel may be reduced. May be increased, and the pixel size may be decreased when the pixel is black.

The light amount of the light source may be changed in accordance with the change of the pixel size to be imaged on the image carrier, and the change of the light amount of the light source may be the change of the maximum light amount. The maximum light intensity of the light source as well as the light intensity of the light source may be changed in proportion to.

The pixel pitch in the main scanning direction and the sub-scanning direction may be changed in proportion to the change in the size of the pixel formed on the image carrier. This may be performed by changing the scanning speed.

Further, according to the image forming method of the present invention, an image is formed by forming a pixel on the image carrier with light from a light source according to an input signal, and forming an image by a set of pixels in the main scanning direction and the sub-scanning direction. It is possible to adjust the light amount of the light source,
In addition, by using an exposure optical system capable of continuously changing the size of pixels formed on the image carrier by changing the optical path of the light source, the pixel density of the input signal in the main scanning direction and the pixel density in the sub-scanning direction are reduced. Correspondingly, the pixel size to be imaged on the image carrier is changed, the scanning speed of the scanning unit is changed, and the lighting interval of the light source is changed.

In the image forming apparatus of the present invention, the pixel size is made variable, and the pixel size is selectively used according to the type of the required image. The pixel pitch in the feed direction is properly used depending on the type. As a specific application example, in an image forming apparatus using a laser scanner, an image is formed by arbitrarily changing a laser spot diameter and a spot pitch.

By changing the pixel size and the pixel pitch in accordance with the required pixel density, that is, the pixel density of the input signal, it is possible to form an image faithful to the input.

When a laser beam is used to form an image and the lighting time is changed for each pixel to form an image with rich gradations, the pixel size should be increased and a line image that requires high sharpness should be used. The pixel size is reduced.

Supplementing the above description with experimental data, it is possible to stably develop with a toner having a diameter of 7 μm up to a latent image of about 4 toner particles.
If it is not more than μm, stable development cannot be performed. In principle, it can be said that a latent image having a diameter smaller than 7 μm cannot be developed. Therefore, at 600 dpi, the pixel size is 4
If the lighting time is less than 1/6 because of 2 μm, stable development cannot be performed. In other words, the number of tones that can be output from one pixel is limited to 10 tones even if white is included, based on the area ratio between 42 μm and 14 μm. At 300 dpi, since the pixel size is 85 μm, up to 38 gradations are possible.

[0019]

Next, embodiments of the present invention will be described with reference to the drawings. In the present embodiment, a color image forming apparatus having a copying function will be described. However, the present invention is not limited to this, and can be widely applied to image forming apparatuses such as a printer and a facsimile. First, a color image forming apparatus according to an embodiment of the present invention will be described. FIG. 1 is a sectional view showing the internal structure of a color image forming apparatus according to an embodiment of the present invention.

The color image forming apparatus has a digital color image reader section at the top and a digital color image printer section at the bottom.

In the reader section, the original 130 is placed on an original platen glass 131 and is exposed and scanned by an exposure lamp 132, so that a reflected light image from the original 130 is condensed on a full-color sensor 134 by a lens 133, and a color image is formed. Obtain a decomposed image signal. The color-separated image signal passes through an amplifier circuit (not shown), and is then processed by a video processing unit (not shown)
And sent to the printer unit.

In the printer section, a photosensitive drum 101 as an image carrier is rotatably supported in the direction of an arrow. Around the photosensitive drum 101, a pre-exposure lamp 111, a corona charger 102, a laser exposure optical system 103, a potential sensor 11
2. Four developing units 104y, 104c, and 10 having different colors
4 m · 104 Bk, on-drum light amount detection means 113, transfer device 105, and cleaning device 106 are arranged.

In the laser exposure optical system 103, image information of an input signal from a reader unit is converted into an optical signal by a laser output unit (not shown), and the converted laser light is reflected by a polygon mirror 103a, The light is projected on the surface of the photosensitive drum 101 through the mirror 103b and the mirror 103c.

At the time of image formation in the printer unit, the photosensitive drum 101 is rotated in the direction of the arrow, and the photosensitive drum 101 after having been neutralized by the pre-exposure lamp 111 is uniformly charged by the charger 102. The light image E is irradiated to form a latent image.

Next, predetermined developing units 104y, 104c,
By operating 104m · 104Bk, the photosensitive drum 101 is operated.
The upper latent image is developed, and a toner image based on a resin is formed on the photosensitive drum 101. The developing device has an eccentric cam 124.
The operations of y, 124c, 124m, and 124Bk make it possible to selectively approach the photosensitive drum 101 in accordance with each separation color.

Further, the toner image on the photosensitive drum 101 is transferred from the paper supply cassette 150 to a sheet supplied to a position facing the photosensitive drum 101 via the transport system 151 and the transfer device 105. In the present embodiment, the transfer device 105 includes a transfer drum 105a, a transfer charger 105b, an attraction roller 105g opposed to an attraction charger 105c for electrostatically attracting a sheet, an inner charger 105d, and an outer charger 1
A transfer sheet 105f made of a dielectric material is integrally stretched in a cylindrical shape in a peripheral opening area of the transfer drum 105a which is rotatably driven. The carrier sheet 105f uses a dielectric sheet such as a polycarbonate film.

As the transfer device in the form of a drum, that is, the transfer drum 105a is rotated, the toner image on the photosensitive drum is transferred to the recording material carrying sheet 10 by the transfer charger 105b.
Transfer onto the sheet carried on 5f.

A desired number of color images are transferred to the sheet conveyed by suction on the supporting sheet 105f in this manner, and a full-color image is formed. In the case of full-color image formation, when the transfer of the toner images of four colors is completed in this way, the sheet is separated from the transfer drum 105a by the action of the separation claw 108a, the separation push-up roller 108b, and the separation charger 105h. Is discharged to the tray 110 via the.

On the other hand, after the transfer, the photosensitive drum 101 is subjected to an image forming process again after the residual toner on the surface is cleaned by the cleaning device 106.

When images are formed on both sides of a sheet,
Immediately after the sheet is ejected from the fixing device 109, the conveyance path switching guide 119 is driven, and once guided to the reversing path 121a via the conveying vertical path 120, the reversing roller 121b
, The sheet is discharged in the opposite direction to the direction in which the sheet is fed, with the rear end of the sheet being sent in the leading direction.
To be stored. Thereafter, an image is formed on the other surface again by the above-described image forming step.

The transfer sheet 105 of the transfer drum 105a
The fur brush 114 and the supporting sheet 1 are used to prevent scattering of powder on 05f, oil on the sheet, and the like.
Cleaning is performed by the action of a backup brush 115 facing the brush 114 through the oil removal roller 05f and a backup brush 117 facing the roller 116 through the oil removal roller 116 and the supporting sheet 105f. Such cleaning is performed before or after image formation, and is performed whenever a jam (paper jam) occurs.

In the present embodiment, the eccentric cam 125 is operated at a desired timing, and the cam follower 105i integrated with the transfer drum 105a is operated, so that the gap between the carrier sheet 105f and the photosensitive drum 101 can be arbitrarily set. It has a configuration that can be set. For example, the interval between the transfer drum and the photosensitive drum can be increased during standby or when the power is off.

When a manual sheet is used, the uppermost sheet of the sheet 170 placed on the manual sheet feeding section 160 is fed to the sheet feeding roller 1.
61, the sensor S0 detects whether the sheet has been normally sent, and detects the length of the sheet 170.
The paper is fed at a predetermined timing at 62, and is further fed by the transport roller 163 to join the transport system 151 of the paper feed cassette 150.

The first embodiment of the image forming apparatus of the present invention described above.
2 and 3 are detailed views of the laser exposure optical system according to the embodiment.
And FIG. FIG. 2 is a schematic top view of the laser exposure optical system according to the first embodiment, FIG. 3 is a schematic side view of the laser exposure optical system according to the first embodiment, and FIG. FIG. 3 is a schematic partial side view of a diaphragm plate portion of the laser exposure optical system according to the embodiment. In the figure, reference numeral 201 denotes a photosensitive drum, 21
0 is a stage, 220 is an optical path, 250 is a case, 251
Is a holder, 252 is a laser substrate, 253 is a laser light source,
254 is a collimator lens, 255 is a lens barrel, 25
6 is a cylindrical lens, 257 is a motor, 258 is a polygon mirror, 259 is an fθ lens, 260 is an aperture plate, 260a is an opening, 261 and 262 are blades, 261
a, 262a is a long hole, 263 is a shaft, 264 is an arm,
64a is a sector gear, 264b is a dowel, 265 is a central axis, 266 is a motor, 267 is a gear, 269 is a CCD,
270 is a toric lens, 271 is a cover, 272 is a circuit board, 277 is a BD mirror, and 278 is a beam detector.

The case 250 is mounted on the stage 210, and the laser substrate 252 held by the holder 251 attached to the case 250 includes the laser substrate 25.
Laser light source 253 such as a semiconductor laser driven by
Is attached.

As shown in FIG. 2, a lens barrel 255 provided with a collimator lens 254 for converting the laser beam into parallel light is provided on the optical path 220 in the emission direction of the laser beam inside the case 250. An aperture plate 260 having an opening for determining the shape of a light beam, a cylindrical lens 256 for flattening parallel light, and a polygon mirror 258 rotated by a motor 257 and scanning laser light are sequentially arranged.

Further, on the optical path in the reflection direction of the polygon mirror 258, an fθ lens 259, a toric lens 2
The toric lens 270 performs correction for moving the laser beam scanned by the polygon mirror 258 at a constant speed on the photosensitive drum. The shape of the pixel finally projected on the photosensitive drum 201 on which the image is projected is projected in a shape proportional to the shape of the aperture plate 260.

The case 250 is hermetically sealed by a cover 271. A circuit board 272 for controlling a laser scanner is attached to the cover 271. The laser board 252 is connected to the circuit board 272. .

Here, the aperture plate 260 will be described in detail with reference to FIG. The aperture plate 260 has an opening 260a, and the blade plates 261 and 262 are supported so as to be rotatable about a shaft 263. The blades 261 and 262 are elongated holes 261a and 2
An arm 264 having a dowel 264b which has a dovetail 264b which is inserted into the elongated holes 261a and 262a is rotatable about a center shaft 265.
1 and 262 are moved, the aperture diameter D changes, and the diameter of the laser beam changes. The rotation of the dowel 264b is performed by the motor 266.
The sector gear 264a formed on the arm 264 meshes with a gear 267 fixed to the output shaft of the motor 26.
The stop diameter D changes in accordance with the rotation of No. 6. Reference numeral 269 denotes a CCD, which can detect the aperture diameter D by observing the position of the blade plate 262. As a drive source for changing the aperture diameter, a solenoid can also be used when the diameter to be changed is two kinds, and in that case, there is no need to detect the aperture diameter with a CCD or the like, so that the configuration can be simplified.

In the first embodiment, since the laser light, which is the light source, is cut off by the aperture, all of the light emission amount can be used when the pixel diameter is large, but almost all of the light amount is cut off when the pixel diameter is small. Will not be used. However, since the light intensity is the same when viewed from a light amount corresponding to a unit area on the photosensitive drum 201, the control circuit is easy.

Next, a second embodiment will be described with reference to FIGS. 5, 6 and 7. FIG. FIG. 5 is a schematic top view of a laser exposure optical system according to the second embodiment, and FIG. 6 is a schematic side sectional view of a projection lens unit of the laser exposure optical system according to the second embodiment. FIG. 7 is a schematic side view of the light receiving element array of the laser exposure optical system according to the second embodiment and the irradiation state of laser light. In the figure, reference numeral 301 denotes a photosensitive drum, 320 denotes an optical path, 350 denotes a case, 351 denotes a holder, 352 denotes a laser substrate, 353 denotes a laser light source, 355a denotes a projection lens,
56 is a cylindrical lens, 357 is a motor, 358
Is a polygon mirror, 359 is an fθ lens, 360a is an aperture plate, 370 is a toric lens, 373 is a helicoid motor, 374 is an output shaft, 375 is a worm, 376 is a lens barrel, 376a is a helical gear, 377 is a BD mirror, 3
Reference numeral 78 denotes a beam detector, 379 denotes a light receiving element array, and 380 denotes a laser beam.

The second embodiment will be described mainly on the points different from the first embodiment.
Has a fixed shape close to the upper limit of the pixels used, and the shape of the pixels on the photosensitive drum is the projection lens 355a.
Is changed, the pixel is made smaller by sharply connecting the focus position when projecting the shape of the laser beam projected from the laser light source 353 onto the photosensitive drum 301, and the pixel is made larger by blurring the focus. .
The pixel size may be changed by changing the focal length so that the lens has zoom characteristics. Explaining a specific operation, when changing the pixel size, first, the laser is always turned on. Output shaft 374 of helicoid motor 373
The worm 375 fixed to the lens is meshed with a helical gear 376a integral with a lens barrel 376 containing a lens 355a, and the position of the lens 355a is changed by rotating the helicoid motor 373, so that the pixel of the laser on the photosensitive drum is changed. Changes in size. Normally, a worm wheel is preferably used as a partner to be engaged with the worm. However, in this embodiment, since the lens barrel itself moves, the meshing state does not change even if the position of the lens barrel 376 moves as a helical gear. I have to. The moving amount is about 2 mm.

The laser light reflected by the polygon mirror 358 is reflected by a BD mirror 377 and enters a beam detector (hereinafter referred to as a BD) 378. In the case of normal image formation, an image is started to be written after a time corresponding to the length Lb to the image end of the photosensitive drum has elapsed after light enters the BD 378. Therefore, when the laser is turned on, it is turned on when the image signal is present in the regions B and C in FIG. 5 when converted on the photosensitive drum. However, when performing the operation of changing the pixel size, it is necessary to keep the light on at all times, or at least at the timing when light enters the light receiving element array 379, that is, before and after A.

The laser light reflected by the BD mirror 377 is
The light enters a light receiving element array 379 such as a CCD. Since the light receiving element array 379 is arranged at a position where the optical path length of the laser beam becomes the same as the virtual irradiation position A on the photosensitive drum, FIG.
7, which shows a state in which the laser beam incident on the light receiving element array 379 from the D direction is viewed from the side, the diameter l of the laser light 380 applied to the light receiving element array 379 is the diameter of the pixel on the photosensitive drum 301. And the light receiving element array 37
9 can be read directly. Therefore, the helicoid motor 373 may be rotated so as to have a desired pixel size. Specifically, at the time of 300 dpi, 84.6 μm, 600
At the time of dpi, the light receiving element row 3 is set to 42.3 μm.
At 79, the helicoid motor 37 is detected while detecting the magnitude of l.
The rotation of the helicoid motor 373 is stopped when the roller 3 is rotated to a proper size.

In the second embodiment, since the size of the pixel connected to the photosensitive drum by the laser light is changed, the laser light is not cut by the aperture, and the entire amount of emitted light is used, so that there is no waste. In other words, when used with a small pixel size, the power supplied to the laser can be reduced.

As a matter of course, also in the first embodiment, instead of detecting the aperture diameter D with the CCD 269, a light receiving element array 379 may be provided to directly measure the pixel diameter l.

The change in the pixel size in the main scanning direction (the direction in which the polygon mirror is scanned) has been described above, but the feed pitch in the sub-scanning direction (the rotation direction of the photosensitive drum) also needs to be changed. This is because if the rotation of the photosensitive drum is kept constant, the rotation speed of the polygon motor 57 may be changed.
Specifically, when the rotational speed at 300 dpi is R, the rotational speed may be 2R at 600 dpi. If the frequency at which the laser is turned on is changed without changing the rotation speed of the polygon motor, the pixel pitch in the main scanning direction changes, but the pixel pitch in the sub-scanning direction remains unchanged, and the image becomes strange. When the pixel size is changed, the pixel pitch of both the main scanning and the sub-scanning is changed at the same time simply by changing the rotation speed of the polygon motor accordingly. Naturally, the main scanning and the sub-scanning can be changed separately.

If the G3 fax is taken as an example, the main scanning is 8 pels and the sub-scanning is 7.7 pels.
5 μm, in practice, about 150 μm in consideration of development characteristics. When the laser is turned on at a resolution of 600 dpi, the laser is turned on at a pitch of 125 μm instead of 42 μm on the photosensitive drum. With 652R, the information from the G3 fax can be output without conversion of the pixel density.

If the pixel size is different, even if the photosensitive drum is irradiated with light (laser light) with the same amount of light per unit area, the potential of the photosensitive drum does not always become the same, and even if it becomes the same, The concentrations are not always the same. This is because pixels having a small area are difficult to develop as described above. Therefore, even in the case of the first embodiment, it is necessary to optimize the light amount of the light source in order to make the density after development the same.

Therefore, in the present invention, when the pixel is further reduced, the light amount is increased. Further, in order to improve the developing characteristics, when the size of the pixel is reduced, the developing bias applied to the developing device is changed. Specifically, the absolute value of the DC component of the developing bias is increased, the frequency of the AC component is increased,
The effect is great when used together. It is desirable not to change these individually, but to perform them in conjunction with a change in pixel size.

In some cases, the pixel size is changed to match the input signal. In this case, the matching circuit is not required in the present invention.

In some cases, this is performed to obtain high image quality. In particular, when a color image is obtained, the present invention is applied to M,
Images with C, Y, and C colors are often images that require gradation, so that a large pixel density, for example, 300
Since the image of dpi and black is often an image requiring sharpness of characters and the like, a small pixel density, for example, 600d
When an image is formed at pi, it is easy to obtain an image having both gradation and sharpness.

FIG. 8 is a conceptual block diagram showing the adjustment relationship of the pixel size, laser output, polygon motor rotation speed, and developing bias of the image forming apparatus of the present invention.
6 is a flowchart of the adjustment operation of FIG.

The flowchart will be briefly described. The pixel density of the input signal is 600 dpi or 300 dpi.
The light amount of the laser beam, the pixel size, the number of rotations of the polygon motor, the type of the developing bias, and the like are switched in accordance with i, (or black or color, or character or image).

[0055]

As described above, according to the present invention, the image size, the pixel pitch in the main scanning direction and the pixel pitch in the sub-scanning direction can be easily changed according to the input signal, so that an expensive conversion circuit is not required. In addition, there is an effect that there is no possibility of missing or changing image information. Furthermore, according to the present invention, since any of the pixel size, the pitch of the main scanning and the sub-scanning, and the rotation speed of the polygon motor can be controlled steplessly, a halfway pixel pitch, for example, 360 dpi or 16 pel (1 per 1 mm)
(6 pixels).

Furthermore, since an image can be formed with an optimum pixel size according to the type and color of the image, an optimum image can be formed in consideration of development characteristics.

When an image is formed with a large pixel size, the life of a light source, for example, a laser can be extended compared to an image forming apparatus having only a small pixel size. , The number of revolutions of the motor can be reduced, and the life of the motor can be extended.

Similarly, in order to extend the life of the motor, the number of revolutions of the image forming apparatus is reduced to about half during stamping and the number of revolutions is increased during image formation. It often took a long time to reach the rotation speed. Therefore, it was necessary to wait for a while when receiving a fax or the like, but in the image forming apparatus according to the present invention, if the polygon mirror driving motor is rotated near the number of rotations corresponding to the pixel density of the fax, Since an image can be formed at a pixel density corresponding to the input information, reception and printing can be performed immediately.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an internal structure of a color image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic top view of the laser exposure optical system according to the first embodiment.

FIG. 3 is a schematic side view of the laser exposure optical system according to the first embodiment.

FIG. 4 is a schematic partial side view of a diaphragm plate portion of the laser exposure optical system according to the first embodiment.

FIG. 5 is a schematic top view of a laser exposure optical system according to a second embodiment.

FIG. 6 is a schematic side sectional view of a projection lens unit of a laser exposure optical system according to a second embodiment.

FIG. 7 is a schematic side view of a light receiving element array of a laser exposure optical system according to a second embodiment and an irradiation state of laser light.

FIG. 8 is a conceptual block diagram illustrating an adjustment relationship among a pixel size, a laser output, a polygon motor rotation speed, and a developing bias of the image forming apparatus of the present invention.

FIG. 9 is a flowchart of the adjustment operation of FIG. 8;

FIG. 10 shows a pixel size 300 formed on a photosensitive drum.
It is a typical top view of a latent image of dpi and 600 dpi.
(A) is a latent image of 300 dpi. (B) is 600d
pi is a latent image.

FIG. 11 is a schematic top view showing a state of a latent image formed on a photosensitive drum when a lighting time is reduced to a quarter.
(A) is a latent image of 300 dpi. (B) is 600d
pi is a latent image.

[Explanation of symbols]

Reference Signs List 101 photosensitive drum 102 corona charger 103 laser exposure optical system 103a polygon mirror 103b lens 103c mirror 104y, 104c, 104m, 104Bk developing device 105 transfer device 105a transfer drum 105b transfer charger 105c adsorption charger 105d inner charger 105e outer charger 105f Carrying sheet 105g Suction roller 105h Separation charger 105i Cam follower 106 Cleaning device 108a Separation claw 108b Separation push-up roller 109 Fixing unit 110 Tray 111 Pre-exposure lamp 112 Potential sensor 113 Drum light amount detecting means 114 Fur brush 115 Backup brush 116 Oil removing roller 117 Backup brush 119 Switching guide 120 Paper path 121a Reverse path 121b Reverse roller 22 Intermediate tray 124y, 124c, 124m, 124Bk Eccentric cam 125 Eccentric cam 130 Document 131 Document table glass 132 Exposure lamp 133 Lens 134 Full color sensor 150 Paper feed cassette 151 Transport system 160 Manual feed unit 161 Paper feed roller 162 Registration roller 163 Transport Roller 170 Sheet 201, 301 Photosensitive drum 210 Stage 220, 320 Optical path 250, 350 Case 251, 351 Holder 252, 352 Laser board 253, 353 Laser light source 254 Collimator lens 255 Lens barrel 256, 356 Cylindrical lens 257, 357 Motor 258, 358 Polygon mirror 259, 359 fθ lens 260 Aperture plate 260a Opening 261, 262 Blade plate 261a, 262a Slot 2 3-axis 264 Arm 264a Sector gear 264b Dowel 265 Center axis 266 Motor 267 Gear 269 CCD 270, 370 Toric lens 271 Cover 272 Circuit board 277, 377 BD mirror 278, 378 Beam detector 360a Aperture plate 373 Helicoid motor 374 Output mirror 375 U Cylinder 376a Helical gear 379 Light receiving element array 380 Laser beam 41, 51 Latent image of pixel at 300 dpi 42, 52 Latent image of pixel at 600 dpi

Claims (12)

[Claims] 1. An image forming apparatus which forms an image by a light of a light source on an image carrier by an input signal and forms an image by a set of the pixels in a main scanning direction and a sub-scanning direction. An image forming apparatus comprising: an exposure optical system that is adjustable and that can continuously change the size of the pixel formed on the image carrier by changing the optical path of the light source. 2. The image forming apparatus according to claim 1, wherein the method of changing the size of the pixel of the exposure optical system is performed by continuously changing the opening area of a diaphragm mechanism provided in an optical path from the light source. . 3. The image forming apparatus according to claim 1, wherein the method of changing the size of the pixel of the exposure optical system is performed by moving a focal position by adjusting a lens provided in an optical path from the light source. 4. An image forming method for forming pixels by light from a light source on an image carrier according to an input signal and forming an image by a set of the pixels in a main scanning direction and a sub-scanning direction. Using an exposure optical system capable of adjusting the amount of light, and capable of continuously changing the size of the pixel that forms an image on the image carrier by changing the optical path of the light source, the image information of the input signal An image forming method, wherein a size of a pixel to be imaged on an image carrier is changed. 5. The image forming method according to claim 4, wherein the pixel size is increased when the image information of the input signal is an image, and the pixel size is decreased when the image information is a character or a line drawing. 6. The image forming method according to claim 4, wherein the pixel size is increased when the image information of the input signal is color, and the pixel size is decreased when the image information is black. 7. The image forming method according to claim 4, wherein the light amount of the light source is changed in accordance with a change in the size of a pixel formed on the image carrier. 8. The image forming method according to claim 7, wherein the change of the light amount of the light source is a change of a maximum light amount. 9. The image forming method according to claim 7, wherein the light amount of the light source and the maximum light amount of the light source are changed in proportion to the change in the size of the pixel. 10. The image forming method according to claim 4, wherein the pixel pitches in the main scanning direction and the sub-scanning direction are also changed in proportion to the change in the size of the pixels formed on the image carrier. 11. The image forming method according to claim 10, wherein the change in the pixel pitch in the sub-scanning direction is performed by changing the scanning speed in the main scanning direction. 12. An image forming method for forming pixels by light from a light source on an image carrier in response to an input signal and forming an image by a set of the pixels in a main scanning direction and a sub-scanning direction. An exposure optical system capable of adjusting the amount of light, and capable of continuously changing the size of the pixel that forms an image on the image carrier by changing the optical path of the light source, in the main scanning direction of the input signal. Pixel density and corresponding to the pixel density in the sub-scanning direction, while changing the size of the pixel imaged on the image carrier, changing the scanning speed of the scanning means, changing the lighting interval of the light source. Image forming method.