JP2000235156A - Optical scanner and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the curvature of a field from occurring without spoiling the degree of freedom for arranging a photosensitive drum in an over-field type scanning optical system. SOLUTION: By inserting a plate mirror 9 in an over-field type image forming optical system 5 including an f θ lens 7 and a cylindrical mirror 8, the bending angle θ of the mirror 8 is formed to be the acute angle being <=30 deg. so as to prevent the deterioration of image quality such as the curvature of the field from occurring. Thus, the degree of freedom for arranging the photosensitive drum 6 is enhanced in comparison with the case that the image is directly formed on the drum 6 from the mirror 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used in a scanning optical system such as a laser beam printer or a digital copier, and more particularly to an optical scanning device which converts an incident light beam into a scanning surface with respect to a reflecting surface such as a rotary polygon mirror. The present invention relates to an optical scanning device and an image forming apparatus equipped with a scanning optical system such as an overfield type that obliquely enters from below.

[0002]

2. Description of the Related Art In a conventional scanning optical system, a rotary polygonal mirror serving as a rotary deflecting means is rotated, a light beam such as a laser beam is incident on a plane perpendicular to the rotation axis, and a light beam reflected on the same plane is reflected. The so-called underfield type in which an image is formed on an image forming surface such as a photosensitive drum by an image forming optical system has been mainly used.

FIGS. 3 and 4 show a cross section in the main scanning direction (Y-axis direction) including the optical axis (X-axis direction) and a cross section in the sub-scanning direction (Z-axis direction) of an underfield scanning optical system according to a conventional example. Are respectively shown. A light beam emitted from the light source unit 101 as a substantially parallel light beam forms a focal line on the reflection surface 103a of the rotary polygon mirror 103 by a cylindrical lens 102 having optical power in the sub-scanning direction. Reflective surface 10
The light beam deflected and reflected at 3a is scanned in the main scanning direction by the rotation of the rotary polygon mirror 103, and forms an image on the image forming surface 106a of the photosensitive drum 106 via the image forming optical system 105.

The imaging optical system 105 includes a two-element fθ lens 107 having a function of correcting in the main scanning direction, a reflecting surface 103a of the rotary polygon mirror 103 and an imaging surface of the photosensitive drum 106 in a cross section in the sub-scanning direction. 106a having a cylindrical mirror 108 conjugated with the cylindrical mirror 108a.
Is disposed near the photosensitive drum 106, and the rotating polygon mirror 1
The light beam deflected and scanned by 03, that is, the scanning light is turned back and guided to the image forming surface 106a of the photosensitive drum 106.

The reflecting surface of the cylindrical mirror 108 is formed of a cylindrical surface having a spherical center O as shown in FIG.

The light beam formed on the image forming surface 106a of the photosensitive drum 106 is mainly scanned by rotating the polygon mirror 103 in the direction of the arrow in FIG. 3, and the light beam is rotated in the direction of the arrow in FIG. 2 along with the sub-scan
Form a two-dimensional image.

In this scanning optical system, the light beam deflected by the rotary polygon mirror 103 is scanned in a plane (XY plane) perpendicular to the reflecting surface 103a, so that the height of the light beam reflected by the cylindrical mirror 108 Is constant in the sub-scanning direction. Therefore, even if the bending angle θ of the cylindrical mirror 108 is changed, the reflecting surface 103a of the rotating polygon mirror 103 is changed.
And the scanned surface, that is, the image forming surface 106a of the photosensitive drum 106
Does not change on the optical axis and off the optical axis, and the arrangement of the photosensitive drum 106 can be freely selected while keeping the optical path length constant.

On the other hand, an overfield type (Over Filled Scann) for the purpose of increasing the scanning speed
5), the optical system aiming at miniaturization of the main scanning cross section, as shown in FIG. 5, emits light from below the main scanning plane (XY plane) without setting the angle of the incident light beam in the deflection direction. Obliquely incident along the axis. That is, in order to separate the scanning light deflected and reflected by the rotating polygon mirror 203 from the incident light, the scanning light is arranged so as to be incident on the reflecting surface 203a of the rotating polygon mirror 203 at an angle in the sub-scanning plane (XZ plane). Things.

A light beam emitted from the light source unit 201 as a substantially parallel light beam forms a focal line on a reflection surface 203a of a rotary polygon mirror 203 by a cylindrical lens 202 having optical power in the sub-scanning direction. The light beam deflected and reflected by the reflecting surface 203a is scanned in the main scanning direction by the rotation of the rotary polygon mirror 203, and forms an image on the image forming surface 206a of the photosensitive drum 206 via the image forming optical system 205.

The imaging optical system 205 includes a two-element fθ lens 207 having a function of correcting in the main scanning direction and the like, a reflecting surface 203 a of the rotary polygon mirror 203 and an imaging surface of the photosensitive drum 206 in a section in the sub-scanning direction. A cylindrical mirror 208 having a cylindrical mirror 208
Is disposed in the vicinity of the photosensitive drum 206, and the rotating polygon mirror 2
The light flux deflected and scanned by 03, that is, the scanning light is turned back and guided to the image forming surface 206 a of the photosensitive drum 206.

The light beam formed on the image forming surface 206a of the photosensitive drum 206 is divided into a main scan by rotating the polygon mirror 203 around the rotation axis and a sub-scan by rotating the photosensitive drum 206 in the direction of the arrow. Accordingly, a two-dimensional image is formed.

In such an obliquely incident scanning optical system, the scanning line trajectory of the deflected light beam (scanning light) accompanying the rotation of the rotary polygon mirror 203 has a conical surface, so that the scanning line is bent on the photosensitive drum 206, The eccentricity of the rotary polygon mirror 203 causes a shift in the imaging position. As a countermeasure against this, a cylindrical mirror 208 with a reduced image forming magnification in the sub-scanning direction is used.

[0013]

However, according to the above-mentioned prior art, in an overfield type optical system which obliquely enters the XY plane, the height of light rays incident on the cylindrical mirror is not on the same plane. In addition, the optical power of the cylindrical mirror acting on the image changes on the optical axis and off the optical axis in proportion to the deflection angle. That is, as shown in FIG. 6A, the image forming position coincides with the image forming surface 206a of the photosensitive drum 206 on the optical axis.
As shown in (b) of FIG.
06 does not reach the imaging plane 206a. If the bending angle θ by the cylindrical mirror 208 is large, the difference between the optical power on the optical axis and the optical power off the optical axis is further increased, and the image performance is remarkably deteriorated as the field curvature.

This development becomes remarkable especially when the optical power of the cylindrical mirror is large, that is, when the magnification in the sub-scanning direction is small. Therefore, the bending angle of the cylindrical mirror must be limited to a small angle. As a result, the position of the photosensitive drum is restricted. In other words, when the degree of freedom in the arrangement of the photosensitive drums is given, the imaging magnification in the sub-scanning direction cannot be reduced, and the result is that the original scanning line bending and the deviation of the imaging position cannot be solved.

Specifically, the oblique incidence angle on the rotating polygon mirror is 2
°, when the scanning angle of view is 54 °, for example, when the bending angle of the cylindrical mirror is 90 ° and the sub-scanning magnification is 0.4, the field curvature in the sub-scanning direction is as large as about 2 mm,
Very unacceptable. To suppress the field curvature to 0.5 mm or less, the bending angle of the cylindrical mirror should be 30 °.
However, if the sheet is bent at such an acute angle, there is a disadvantage that a space for disposing the photosensitive drum is lost.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned unresolved problems of the prior art, and avoids deterioration of image performance such as curvature of field due to a cylindrical mirror in an overfield type scanning optical system or the like. It is another object of the present invention to provide an optical scanning device and an image forming apparatus capable of greatly improving the degree of freedom of arrangement of the photosensitive drum.

[0017]

In order to achieve the above object, an optical scanning device according to the present invention comprises: a rotary deflecting means for reflecting and deflecting a light beam obliquely incident on a plane perpendicular to a rotation axis;
An image-forming optical system that forms an image of the light beam on a predetermined image-forming surface via the rotary deflecting unit; the image-forming optical system includes a condensing and reflecting unit having optical power in a sub-scanning direction; In order to reduce a bending angle of the light beam by the light reflecting means, a mirror provided on an optical path of the light beam is provided.

Preferably, the condensing and reflecting means is a cylindrical mirror.

It is preferable that the bending angle of the condensing / reflecting means is 30 ° or less.

It is preferable that the light collecting and reflecting means and the mirror are integrally supported by the housing.

Further, a rotary deflecting means for reflecting and deflecting a light beam obliquely incident on a plane perpendicular to the rotation axis, and an image forming optical system for forming the light beam on a predetermined image forming surface via the rotary deflecting means. Wherein the imaging optical system includes an elliptical mirror that conjugates the reflection surface of the rotary deflecting means with the imaging surface.

[0022]

The light beam is bent by the mirror on the upstream side of the condensing and reflecting means such as a cylindrical mirror, so that the bending angle of the reflecting means is reduced to 30 ° or less.

That is, by adding a mirror, the degree of freedom in the arrangement of the photosensitive drum having an image forming surface is greatly improved, and the bending angle of the condensing / reflecting means is reduced to 30 ° or less, and the sub-scanning direction is reduced. It is possible to suppress the field curvature and secure a selection range of the magnification in the sub-scanning direction.

When an elliptical mirror in which the reflecting surface and the image forming surface of the rotary deflecting means are conjugated to each other is used instead of a cylindrical mirror or the like, even if the height of the incident position of the light beam with respect to the elliptical mirror changes, the field curvature will be increased. Because there is no risk of
All problems such as curvature of field in the sub-scanning direction can be eliminated, and the degree of freedom in the arrangement of the photosensitive drum can be greatly improved.

[0025]

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

FIG. 1 shows a first embodiment. A light beam emitted from the light source unit 1 as a substantially parallel light beam forms a focal line on a reflection surface 3a of a rotary polygon mirror 3, which is a rotary deflecting unit, by a cylindrical lens 2 having optical power in the sub-scanning direction. The light beam deflected and reflected by the reflecting surface 3a is scanned in the main scanning direction (Y-axis direction) by the rotation of the rotary polygon mirror 3, passes through the image forming optical system 5, and forms an image forming surface 6a of the photosensitive drum 6.
Image.

The imaging optical system 5 includes a two-element fθ lens 7 having a function of correcting in the main scanning direction, a reflecting surface 3a of the rotary polygon mirror 3 and a photosensitive drum in a cross section in the sub-scanning direction (Z-axis direction). 6 has a cylindrical mirror 8 as a condensing and reflecting means having a conjugate image forming surface 6a, and a flat mirror 9 as a mirror. The cylindrical mirror 8 and the flat mirror 9 are light beams deflected and scanned by the rotary polygon mirror 3. That is, the scanning light is turned back and guided to the image forming surface 6 a of the photosensitive drum 6.

The reflecting surface of the cylindrical mirror 8 is formed by a cylindrical surface having a spherical center in the Y-axis direction.

The light beam that forms an image on the image forming surface 6a of the photosensitive drum 6 is divided into a main scan caused by the rotation of the rotary polygon mirror 3 around the Z axis and a sub-scan caused by the rotation of the photosensitive drum 6 around the Y axis. A two-dimensional image is formed with the scanning.

Since the flat mirror 9 is inserted (added) as a new member in the optical path in front of the cylindrical mirror 8, the bending angle θ of the light beam from the cylindrical mirror 8 to the photosensitive drum 6 is kept at an acute angle of 30 ° or less. be able to. If the bending angle θ of the cylindrical mirror 8 is set to 30 ° or less in this manner, no remarkable field curvature occurs, and the arrangement of the photosensitive drum 6 can be freely selected.

That is, a light beam (scanning light) can be guided to a predetermined image plane while reducing the magnification in the sub-scanning direction.
It is also possible to suppress the deviation of the imaging position due to the curvature of the scanning line due to oblique incidence and the eccentricity of the rotary polygon mirror.

Since two mirrors, ie, the cylindrical mirror 8 and the flat mirror 9, are required as the reflecting members, it is necessary to control the posture for holding them with high precision. Therefore,
It is desirable that the two mirrors 8 and 9 be integrally supported by the same support member so that their relative positions do not change.

When the two mirrors 8 and 9 are directly supported on an optical box as a housing together with other optical components constituting the scanning optical system, it is easy to manage the holding posture.

By mounting such an optical scanning device,
This can greatly contribute to improving the image quality of the image forming apparatus.

FIG. 2 shows a second embodiment. this is,
An elliptical mirror 10 is used in place of the cylindrical mirror 8 which is a condensing and reflecting means for bending a light beam. Even if the height of the light beam incident on the elliptical mirror 10 on the optical axis or off the optical axis changes, the curvature of the elliptical mirror 10 changes according to the light beam height, so that the light always forms on the imaging surface 6a of the photosensitive drum 6. Can be imaged.

Further, if the focal point of the elliptical mirror 10 is made substantially coincident with the vicinity of the reflecting surface 3a of the rotary polygon mirror 3 and the image forming surface 6a of the photosensitive drum 6, the deviation of the image forming position is compensated by the properties of the elliptical mirror 10. be able to. The light source unit 1, the rotary polygon mirror 3, the photosensitive drum 6, and the fθ lens
Since the third embodiment is the same as the first embodiment, the same reference numerals are used, and the description is omitted.

If an elliptical mirror is used, the arrangement of the photosensitive drum can be freely selected without causing curvature of field even if the bending angle is increased in order to reduce the magnification in the sub-scanning direction. Other points are the same as those of the first embodiment.

[0038]

Since the present invention is configured as described above, it has the following effects.

A mirror such as a flat mirror is arranged in front of a cylindrical mirror having optical power in the sub-scanning direction, and the bending angle of the cylindrical mirror is set to an acute angle of 30 ° or less. As a result, it is possible to realize a scanning optical system capable of suppressing occurrence of field curvature in the sub-scanning direction and freely selecting the arrangement of the photosensitive drums.

Also, by using an elliptical mirror instead of a cylindrical mirror, the occurrence of field curvature in the sub-scanning direction is suppressed in the same manner as described above without increasing the number of assembly parts as in the case of adding a flat mirror. And other effects can be expected.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an optical scanning device according to a first embodiment.

FIG. 2 is a schematic diagram illustrating an optical scanning device according to a second embodiment.

FIG. 3 is a schematic diagram illustrating an underfield type optical scanning device.

FIG. 4 is a schematic diagram showing the apparatus of FIG. 3 in another cross section.

FIG. 5 is a schematic diagram illustrating an overfield optical scanning device according to a conventional example.

FIG. 6 is a diagram for explaining that an image forming position is shifted in the apparatus of FIG. 5;

[Explanation of symbols]

 Reference Signs List 1 light source unit 3 rotating polygon mirror 5 imaging optical system 6 photosensitive drum 7 fθ lens 8 cylindrical mirror 9 flat mirror 10 elliptical mirror

Claims (6)

[Claims] 1. A rotary deflecting means for reflecting and deflecting a light beam obliquely incident on a plane perpendicular to a rotation axis, and an image forming optical system for forming the light beam on a predetermined image forming surface via the rotary deflecting means. Wherein the imaging optical system is disposed in the light path of the light beam in order to reduce a bending angle of the light beam by the light collecting and reflecting device, which has optical power in the sub-scanning direction. An optical scanning device comprising a mirror. 2. The optical scanning device according to claim 1, wherein the condensing and reflecting means is a cylindrical mirror. 3. The optical scanning device according to claim 1, wherein a bending angle of the condensing / reflecting means is 30 ° or less. 4. The optical scanning device according to claim 1, wherein the condensing / reflecting means and the mirror are integrally supported by a housing. 5. A rotary deflecting means for reflecting and deflecting a light beam obliquely incident on a plane perpendicular to a rotation axis, and an image forming optical system for forming an image of the light beam on a predetermined image plane via the rotary deflecting means. Wherein the imaging optical system includes an elliptical mirror that conjugates the reflection surface of the rotary deflecting means with the imaging surface. 6. An image forming apparatus comprising: the optical scanning device according to claim 1; and a photosensitive drum having an image forming surface.