JP3198685B2 - Light beam scanning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light beam scanning device used for a laser beam printer or the like, and more particularly to a light beam scanning device using a cylindrical mirror for correcting a tilt of a rotary polygon mirror.

[0002]

2. Description of the Related Art In a scanning optical system used for a laser printer or the like, a rotary polygon mirror is often used to deflect a light beam at a high speed. There is a problem of uneven pitch of the scanning spot. As a method for solving this problem, a surface tilt correction optical system in which the reflection surface of the rotary polygon mirror and the surface to be scanned are substantially conjugated with each other is widely used, and a system using a cylindrical lens (Japanese Patent Laid-Open No. 58-93021). Gazette) or using a toric surface (JP-A-57-145)
No. 15, JP-A-61-175616, etc.).

FIG. 5 shows a deflection part of a scanning optical system using a rotating polygon mirror. The light beam L from the light source 26 is converged only in the sub-scanning direction (the direction perpendicular to the paper surface in FIG. 5) by the line image forming optical system 27, and is formed as a line image on almost the surface of the rotary polygon mirror 28. . The reflected light beam L is scanned from 23 to 24 and then 25 by the rotation of the rotary polygon mirror 28, and both the scanning of the light beam L and the incidence on the rotary polygon mirror 28 are in the YZ plane (in FIG. (In an included plane), the incident angle α of the light beam L to the rotary polygon mirror 28 (the angle formed with the optical axis Z) becomes larger than the maximum value of the scanning angle θ.

Here, the maximum value of the scanning angle θ of the rotary polygon mirror 28 is θmax =
360 / N, the actual effective scanning range is smaller than this. For example, in the case of six surfaces, α is required to be about 70 °, and when θ is increased to make the entire apparatus compact, α increases. Have a relationship. FIG. 6 is an explanatory diagram of the rotation position of the deflection surface (reflection surface) of the rotary polygon mirror 28, the reflection position of the light beam L, and the image formation position. That is, in FIG. 6, one deflecting surface of the rotary polygon mirror 28 is 33, 34,
The position changes to the position indicated by 35. At this time, the reflection point of the light beam L changes to 33a, 34a, and 35a, and the reflection direction also changes to 3a.
3b, 34b, and 35b. That is, the light beam L
Is obliquely incident on the optical axis Z, the reflection position of the rotating polyhedron 28 is changed from 33a to 34 with rotation as shown in FIG.
The light beam L moves forward and backward with respect to the incident light beam L from a, 34a to 35a. As a result, the position of the line image formation also shifts from the surface of the rotary polygon mirror 28 as it rotates.

FIG. 7 shows a movement locus 41 of a reflection point accompanying the rotation when the line image forming position of the light beam L reflected in the optical axis Z direction coincides with the reflection surface 28a of the rotary polygon mirror 28.
And a movement locus 42 of the line image forming position. The parameters are
The maximum scanning angle 44 °, the inscribed diameter of the rotating polygon mirror 28 is 50 mm,
This is the case of the incident angle α70 °. As the rotary polygon mirror 28 rotates, the reflection point moves on a straight line 41 from 41a to 28a. The trajectory 42 of the line image forming position moves from 42a to 28a and further from 28a to 42b as the rotary polygon mirror 28 rotates, and moves asymmetrically with respect to the optical axis Z. Become. The line image forming position is determined by the rotating polygon mirror 28.
8 shows the conjugate relationship between the reflection surface 28a and the scanned surface, and when 51 is moved to 55 by rotation,
The image point 52 having a conjugate relationship also moves to 56.

As described above, when the light beam L is incident on the scanning optical system in which the reflecting surface of the rotary polygon mirror and the surface to be scanned are conjugated with each other from a direction oblique to the optical axis, the reflecting point moves. The line image forming position moves asymmetrically with respect to the optical axis, and the spot image forming position in the sub-scanning direction, which has a conjugate relationship therewith, also becomes asymmetrical with respect to the optical axis. There is a problem that the spot size becomes uneven depending on the scanning position. This problem does not occur when the reflection point coincides with the rotation axis as in a galvanometer mirror or when a parallel light beam is incident. This is a problem that is peculiar to the combination of a rotary polygon mirror and a surface tilt correction optical system.

As a method for solving this problem, an aspherical surface whose curvature radius in the sub-scanning direction is monotonically increased as the distance from the optical axis is increased is used for the imaging lens, and the aspherical surface is made asymmetric with respect to the optical axis. One using a lens (Japanese Patent Laid-Open No. 23233/1990) has been proposed.

[0008]

However, in order to machine a lens that is asymmetrical with respect to the optical axis, dedicated machining equipment is required, and if an imaging lens is provided with a tilt correction function, it becomes compact. In order to form an imaging lens, it is necessary to set the lateral magnification to 3 or more, and there is a problem that the processing accuracy of parts becomes severe. Furthermore, when this lens is made of plastic, there is a problem that the imaging characteristics change due to a change in the refractive index with a change in temperature.

In view of the above-mentioned circumstances, the present invention provides a light beam scanning apparatus that uses a rotating polygon mirror to make a light beam incident from a direction oblique to the optical axis of an imaging lens.
The contents described in 1) shall be the subject. (O01) To provide a light beam scanning device which has a flattening function in a sub-scanning direction, has a uniform spot diameter irrespective of a scanning position, and is easy to manufacture, while having a surface tilt correcting function.

[0010]

Next, the present invention devised to solve the above-mentioned problems will be described. Elements of the present invention are used to facilitate correspondence with elements of the embodiments described later. , The reference numerals of the elements of the embodiment are enclosed in parentheses. The reason why the present invention is described in correspondence with the reference numerals of the embodiments described below is to facilitate understanding of the present invention, and not to limit the scope of the present invention to the embodiments.

In order to solve the above-mentioned problem, the first application of the present application
The light beam device according to the present invention includes a rotary polygon mirror (4) for deflecting and scanning the light beam, and an optical system in which the rotary polygon mirror (4) and the surface to be scanned (7a) are substantially conjugated in the sub-scanning direction. Wherein the optical system includes an imaging lens (5) for converging the light beam on the surface to be scanned (7a) and a cylindrical mirror (6) having a curvature only in the sub-scanning direction. (Y01) The cylindrical mirror (6), characterized in that:
The monotonically increasing the radius of curvature of the section in the sub-scanning direction along the main scanning direction.

(Explanation of Terms) The “imaging lens (5)”
Uses an fθ lens (5). fθ lens (5)
Is to correct the deflection of the light beam at a constant angular velocity so that the light spot moves at a constant speed on the surface to be scanned (7a), and enters from the deflection point at an angle of θ from the optical axis. From the optical axis of the imaging lens (5) having the focal length f
It is referred to by this name to mean that light is condensed at an image height position of xθ. The optical system constituted by the fθ lens (5) and the cylindrical mirror (6) constitutes a surface tilt correction optical system in which the reflection surface of the rotary polygon mirror (4) and the position to be scanned have a substantially conjugate relationship. The pitch unevenness of the scanning line due to the tilt of the rotary polygon mirror (4) is corrected.

The "cylindrical mirror (6)" used in the scanning optical system according to the present invention is desirably formed of a plastic material. Since the cross-sectional radius of curvature of the cylindrical mirror (6) in the sub-scanning direction changes depending on the position in the main scanning direction, the cylindrical mirror (6) is rubbed with a polishing plate having a required optical surface shape and a concavo-convex surface, as conventionally used. Polishing is difficult. However, in the case of injection molding or the like, if the difference in the cross-sectional radius of curvature depending on the position is small, the same process as in the case of molding the conventional cylindrical mirror (6) can be employed.

The lateral magnification β of the “conjugate relationship” of the “optical system in which the rotary polygon mirror (4) and the surface to be scanned (7a) are substantially conjugate in the sub-scanning direction” of the scanning optical system according to the present invention.
Is preferably 0.4 <β <1.4. If the lateral magnification is 0.4 or less, the distance from the fθ lens (5) to the cylindrical mirror (6) increases, and the size of the cylindrical mirror (6) increases, which is not desirable. When the horizontal magnification is 1.4 or more, the vertical magnification is 2 or more, and the required accuracy of the parts becomes severe. In addition, when the cylindrical mirror (6) of the present invention is formed of plastic, the imaging characteristics due to environmental changes are reduced. Changes cannot be ignored.

(Embodiment 1 of the present invention) Embodiment 1 of the light beam scanning apparatus of the present invention is characterized by satisfying the following requirement (Y011) in the present invention (Y011).
The cylindrical mirror (6) is formed of a plastic material.

(Embodiment 2 of the Present Invention) The embodiment 2 of the light beam scanning device of the present invention is the light beam scanning device of the present invention and the embodiment 1 described above, in which the following requirement (Y01) is satisfied.
(Y012) The lateral magnification of the conjugate relationship is set to 0.4 or more and 1.4 or less.

[0017]

Next, the operation of the present invention having the above-mentioned features will be described. According to the light beam apparatus of the present invention having the above-mentioned features, the light beam deflected and scanned by the rotating polygon mirror (4) is such that the rotating polygon mirror (4) and the surface to be scanned (7a) are moved in the sub-scanning direction. It is narrowed down on the surface to be scanned (7a) by the imaging lens (5) and the cylindrical mirror (6) of the optical system having a substantially conjugate relationship. Since the cross-sectional radius of curvature of the cylindrical mirror (6) in the sub-scanning direction monotonically increases in the main scanning direction, the rotating polygon mirror (4)
The curvature of the image plane in the sub-scanning direction caused by the movement of the reflection position due to the rotation of.

Next, the reason why the curvature of the image plane can be corrected by the cylindrical mirror (6) will be described with reference to FIG. 9 and FIG. 3 for describing an embodiment described later. The reflecting surface of the rotating polygon mirror (4) and the surface to be scanned (7a) are in a conjugate relationship, and the image plane curvature in the sub-scanning direction when the deflection point (reflection position on the rotating polygon mirror (4)) does not move. In a light beam scanning apparatus in which a rotating polygon mirror (4) is combined with an fθ lens (5) and a cylindrical mirror (6) corrected as indicated by a solid line in FIG. 9, a light beam is incident from a direction oblique to the optical axis. In this case, one side of the imaging position in the sub-scanning direction is largely separated from the position to be scanned as shown by a broken line in FIG. 9 due to the movement of the reflection point accompanying the rotation of the rotary polygon mirror (4). If it is assumed that the deflection point does not move in this manner, the image forming position in the sub-scanning direction of the light beam far away from the scanned position coincides with the scanned surface (7a) as shown by the broken line in FIG. This can be achieved by increasing the curvature of the reflecting surface of the cylindrical mirror (6) corresponding to the one side portion (that is, by decreasing the radius of curvature).

However, in practice, the deflection point moves. For this reason, in general, the rotating polygon mirror (4) and the imaging lens (5) are used so that the imaging position of the light beam in the sub-scanning direction coincides with the scanned surface (7a) even by the movement of the deflection point. Are adjusted. So in general,
The imaging position of the light beam in the sub-scanning direction on the surface to be scanned (7a) is qualitatively indicated by a solid line in FIG. The solid line in FIG. 3 indicates that the imaging position of the light beam in the sub-scanning direction is
a) It shows that it moves forward along the main scanning direction. Therefore, in order to bring the image position of the light beam in the sub-scanning direction closer to the surface to be scanned (7a), the curvature of the cylindrical mirror (6) in the sub-scanning direction may be reduced along the main scanning direction. That is, by increasing the cross-sectional radius of curvature of the cylindrical mirror (6) in the main scanning direction, it is possible to make the image position of the light beam in the sub-scanning direction closer to the surface to be scanned (7a). That is, the deformation in which the sectional radius of curvature of the cylindrical mirror (6) is monotonously increased along the sub-scanning direction so that the correction effect for the curvature of the maximum angle of view in which the amount of movement of the imaging position is the largest is maximized. By using the cylindrical mirror (6), the amount of the asymmetrical curvature can be reduced as a whole, and the spot size depending on the scanning position can be corrected.

[0020]

Next, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. FIG. 1 is an explanatory view of the configuration of an embodiment of the scanning optical system according to the present invention. In FIG. 1, 1 is a laser light source, 2 is a collimating lens, 3 is an imaging lens for forming a line image near the reflection surface of the rotary polygon mirror 4, 4 is a rotary polygon mirror, and 5 is f
The .theta. lens 6, 6 is a modified cylindrical mirror according to the present invention, 7 is a photosensitive drum, and 7a is a scanned surface (photosensitive drum surface).

The laser light emitted from the laser light source 1 is converted into a parallel light beam by the collimator lens 2, converged only in the sub-scanning direction by the cylindrical lens 3, and conjugated with the surface of the photosensitive drum 7, that is, the scanned surface 7 a in the sub-scanning direction. A line image is formed near the reflecting surface of the rotating polygon mirror 4 in the relationship. The reflected light is deflected and scanned by the rotating polygon mirror 4 rotating at a constant speed, and is incident on the fθ lens 5. Lens 5 includes a first lens 5a and a second lens 5b.
When the focal length of the θ lens in the main scanning direction is f, the laser beam deflected at an angle θ is formed at an image height position of f × θ. The laser beam that has passed through the fθ lens 5 is converged only in the sub-scanning direction by the deformed cylindrical lens, and the left-right asymmetric image plane shape in the sub-scanning direction is corrected to be spotted on the surface (scanned surface) 7 a of the photosensitive drum 7. It is imaged.

Table 1 shows an example of specific lens data of the present embodiment described above.

[Table 1] In Table 1, 0 represents the reflecting surface of the rotary polygon mirror, 1, 2 represents the lens surface of the fθ lens 5a, 3, 4 represents the lens surface of the fθ lens 5b, and 5 represents the reflecting surface of the modified cylindrical mirror 6. Indicates the radius of curvature in the sub-scanning direction. The angle formed by the normal line of the cylindrical mirror 6 and the incident beam on the optical axis in the sub-scanning direction is 7.5 °, and the wavelength of the laser beam is 780 nm. FIG. 2 is a view showing the positional relationship of the rotary polygon mirror and the fθ lens 5 (only the first lens 5a is shown in FIG. 2) with respect to the light beam L in Table 1. In FIG. 2, the incident position of the light beam L on the rotating polygon mirror is shifted by 3 mm from the center of the mirror surface, and the optical axis of the fθ lens 5 is 1.7 mm from the scanning reference (center) line Z of the light beam.
The deviation is for optimal adjustment of the image surface curvature in the main scanning direction.

In the above lens data exemplified in Table 1, when the sectional radius of curvature of the cylindrical mirror is constant irrespective of the position, the image plane shape in the sub-scanning direction is, as shown by the solid line in FIG. It becomes an asymmetrical shape shifted before and after the scanning position. In order to make this a flat scanning surface shape, the cross-sectional radius of curvature of the scanning beam on the cylindrical mirror surface of the scanning beam spot-formed on the front side of the scanned position is compared with the cross-sectional radius of curvature on the optical axis. You just need to increase it. Conversely, at the position on the cylindrical mirror surface where the scanning beam at the scanning position where the spot image is formed on the far side of the scanned position passes, the sectional radius of curvature may be smaller than that on the optical axis.

In the case of the image plane shape shown by the solid line in FIG. 3, in order to make the spot image forming position in the sub-scanning direction at each scanning angle coincide with the position to be scanned, the sectional curvature as shown by the solid line in FIG. A deformed cylindrical mirror having a radius distribution is required. If a mirror having such a shape is used, the image plane shape in the sub-scanning direction can be almost completely corrected, but processing and inspection are difficult and expensive, and practical use is difficult. Therefore, in the present embodiment, as shown by a broken line in FIG. In this embodiment, the cross-sectional radius of curvature on the optical axis is 117.6, the inclination is 0.00927, and the in-plane difference of the cross-sectional radius of curvature is 1.25 mm at the maximum.
The image forming characteristic when using the deformed cylindrical mirror having the distribution of the cross-sectional radius of curvature shown by the broken line in FIG. 4 is 1.5 mm or less as shown by the broken line in FIG. The amount of curvature of the image plane when the cylindrical mirror is used is 4.5 mm (see the solid line in FIG. 3), which is less than half, and it can be seen that there is a sufficient correction effect.

The reason why it is easy to manufacture a deformed cylindrical mirror having a shape in which the cross-section radius of curvature is linearly changed (that is, a cylindrical mirror having not a cylindrical reflection surface but a conical reflection surface having a different radius of curvature in the axial direction). This will be described below. When such a shape is processed by plastic molding, a necessary mold is a mold having a conical convex surface with a diameter of about 250 mm. The mold having such a shape can be manufactured relatively easily by using a normal NC lathe and cutting the cutting tool by a change in the cross-sectional radius of curvature while feeding the cutting tool in the straight direction. It should be noted that it is clear that molding of a mirror structure that requires molding accuracy at a wavelength level only on one side is more preferably made of plastic than a lens that requires precision on both sides.

(Modifications) Although the embodiments of the present invention have been described in detail, the present invention is not limited to the above-described embodiments, but falls within the scope of the present invention described in the appended claims. Thus, various changes can be made. A modified embodiment of the present invention is illustrated in the following (H01).

(H01) When molding a shape such as a cylindrical mirror having a longer length than the width, it is common to inject the resin from the longitudinal end. It is possible to change from part to tip. By utilizing this pressure change, it is also possible to form a cylindrical mirror whose cross-sectional radius of curvature changes continuously using a mold having a cylindrical upper surface.

[0028]

According to the present invention, the following effect (E01) can be obtained. (E01) By using a cylindrical mirror whose cross-sectional radius of curvature in the sub-scanning direction is monotonically increased along the main scanning direction, a light beam is obliquely incident on the rotating polygon mirror of the scanning device having the tilt correction optical system. In this case, it is possible to easily correct a field curvature that is asymmetrical with respect to the optical axis generated in the sub-scanning direction, thereby enabling high resolution and high angle of view scanning.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a light beam scanning device of the present invention.

FIG. 2 is a diagram showing a positional relationship between a rotating polygon mirror of the embodiment, a light beam incident on the rotating polygon mirror, and an fθ lens.

FIG. 3 is a diagram showing a comparison of image plane shapes between the embodiment and a conventional example.

FIG. 4 is a view showing a distribution of a sectional radius of curvature of a cylindrical mirror used in the embodiment.

FIG. 5 is a diagram showing a relationship between a scanning angle of a light beam scanning device to which the present invention is applied and an incident angle on a rotary polygon mirror.

FIG. 6 is a diagram showing a movement of a reflection position of a rotary polygon mirror of a light beam scanning device to which the present invention is applied.

FIG. 7 is a diagram showing a movement locus of a reflection position and a line image formation position accompanying rotation of a rotary polygon mirror of a light beam scanning device to which the present invention is applied.

FIG. 8 is a diagram showing a conjugate relationship between a rotary polygon mirror and a scanned position of the light beam scanning device to which the present invention is applied.

FIG. 9 is a diagram showing the relationship between the movement of the reflection position of the rotary polygon mirror of the light beam scanning device to which the present invention is applied and the curvature of the image plane.

[Explanation of symbols]

4: rotating polygon mirror, 5: imaging lens (fθ lens), 6:
Cylindrical mirror, 7a: scanned surface,

Claims (1)

(57) [Claims] A rotating polygon mirror for deflecting and scanning the light beam; and an optical system in which the rotating polygon mirror and the surface to be scanned have a substantially conjugate relationship in a sub-scanning direction. A light beam scanning device having an imaging lens for focusing on a surface to be scanned and a cylindrical mirror having a curvature only in the sub-scanning direction, wherein the light beam device has the following requirements: (Y01) the cylindrical mirror. The monotonically increasing the radius of curvature of the section in the sub-scanning direction along the main scanning direction.