JPH0720398A - Plane tilt correction scanning optical system - Google Patents

Abstract

PURPOSE:To make a device more compact and inexpensive by making a field angle wide in a main scanning direction while excellently maintaining image- formation characteristic and plane tilt correction function. CONSTITUTION:A laser beam B emitted from a semiconductor laser 1 is changed to the parallel rays of light by a collimator lens 2, and linearly converged by a cylindrical lens 3, and forms an image on the deflecting reflection surface 4a of a polygonal mirror 4. An ftheta lens 5 is constituted of a 1st lens having a cylindrical surface and a 2nd lens having a toric surface in this order from the polygonal mirror 4 side, and provided with at least one optical axis symmetric aspherical surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface tilt correction scanning optical system which is mainly used in a laser beam printer or the like and removes unevenness in pitch of scanning lines in the sub-scanning direction. More specifically, a linear imaging optical system that linearly images the light beam emitted from the light source on the deflective reflection surface of the deflector, and the light beam reflected and deflected by the deflector onto the object to be scanned. The present invention relates to a surface tilt correction scanning optical system including a scanning image forming optical system for forming an image.

[0002]

2. Description of the Related Art Recently, laser beam printers have been gradually reduced in size and cost in addition to the advantage that recording can be performed at extremely high speed. With the diversification and development of OA equipment, laser beam printers have been increasing. The demand is increasing.

For example, in such a laser beam printer, a deflection reflection surface of a deflector such as a polygon mirror used for scanning a light beam from a light source has a manufacturing error, a mounting error, or a vibration during rotation. Therefore, there is a slight tilt error with respect to the direction orthogonal to the scanning plane.

Therefore, the light beam reflected by the deflecting and reflecting surface having such a tilt error shifts the image forming position on the object to be scanned in the sub-scanning direction, resulting in uneven scanning line pitch.
Then, the unevenness of the pitch of the scanning lines causes a deterioration in recording image quality in a recording apparatus such as a laser beam printer.

The above-described surface tilt correction scanning optical system is for removing such unevenness of the scanning line pitch, and a ray bundle from the light source is once made orthogonal to the scanning surface by the linear imaging optical system. The beam bundle from the deflective reflection point is restored in this direction by the scanning and image forming optical system and is conjugated to the object to be scanned. By forming an image, the influence of the tilt error of the deflective reflection surface is prevented. On the other hand, in the scanning plane, in order to make the scanning speed of the light flux on the object to be scanned constant, the scanning imaging optical system changes the light flux from the deflecting and reflecting surface to the incident angle to this optical system. The image is formed on the object to be scanned so that the image height becomes proportional.

In the present specification, the term "scanning plane" means a plane formed by a time-series aggregate of a bundle of rays to be scanned, that is, a main scanning line in an object to be scanned and this plane tilt correction scanning. It means a plane including the optical axis of the optical system.

Conventionally, various structures have been proposed as the above-described surface tilt correction scanning optical system. As an example thereof, as disclosed in Japanese Examined Patent Publication No. 52-28666, the scanning image-forming optical system is configured such that a beam bundle of rays linearly imaged by the linear image-forming optical system is once reflected by a deflector. There is a system that includes a beam shaping optical system such as a cylindrical lens that restores and shapes the light in a circular shape, and a focusing optical system that converges and forms an image of the restored and shaped light beam on the object to be scanned.

In this case, if the beam shaping optical system is configured to perform restoration shaping of the light beam, a constraint condition for restoring the beam into a circular beam is imposed on the beam shaping optical system. The degree of freedom for improving the distortion characteristic of the converging optical system and the image forming characteristic on the object to be scanned is reduced. Therefore, in order to obtain the scanning imaging optical system having the above-mentioned various excellent characteristics,
Since many lenses are required, there arises a problem that the configuration of the optical system becomes complicated.

As an improvement plan, as disclosed in Japanese Patent Laid-Open No. 50-93720, a beam shaping optical system such as the above-mentioned cylindrical lens is interposed between a converging optical system and an object to be scanned. There is.

In the case of such a construction, in order to obtain a good quality image, the beam shaping optical system must be provided close to the object to be scanned. Therefore, this beam shaping optical system needs to be long in the main scanning direction, and it is difficult to make it compact.

Further, as disclosed in Japanese Patent Laid-Open No. 56-36622, there is also a scanning imaging optical system in which a spherical single lens and a single lens having a toric surface are arranged in order from the deflector side. Are known. Further, this scanning image forming optical system has a distortion characteristic for obtaining a constant speed scanning property of a light beam bundle and a function for correcting the tilt error of the deflecting / reflecting surface in cooperation with the linear image forming optical system. Have both.

[0012]

However, in this configuration, although the optical system is compact,
If the degree of freedom is low and both the distortion characteristics for obtaining the uniform speed scanning of the light flux and the correction function for the tilt error of the deflecting / reflecting surface are maintained well, the scanning range of the light flux becomes wide and the angle of view becomes wide. There is a problem that it will be difficult to expand.

The surface tilt correction scanning optical system disclosed in Japanese Patent Application Laid-Open No. 63-19617 is made in view of the above-mentioned circumstances, and while maintaining good imaging characteristics and surface tilt correction function, The present invention has been proposed for the purpose of providing a surface tilt correction scanning optical system capable of widening the angle of view in the main scanning direction and making a device such as a laser beam printer more compact.

Another object of the present invention is to provide a surface tilt correction scanning optical system which can be made more compact and can be manufactured at low cost.

[0015]

In order to achieve this object, a surface tilt correction scanning optical system according to the present invention is a scanning imaging optical system for forming an image of a light beam reflected and deflected by a deflector on an object to be scanned. The system is characterized in that it comprises, in order from the deflector side, a first lens having a cylindrical surface and a second lens having a toric surface, and has at least one optical axis symmetric aspherical surface.

The term "toric surface" used here and hereinafter will be referred to as a "toric surface" in the main scanning direction in which a light beam is scanned in a plane orthogonal to the optical axis of the plane tilt correction scanning optical system. And a refracting surface having different refracting powers in the sub-scanning direction orthogonal to the main scanning direction. Further, the “cylindrical surface” means a refractive surface having a refractive power in only one of the main scanning direction and the sub-scanning direction and not having a refractive power in the other direction.

In the surface tilt correction scanning optical system according to the present invention, by providing the cylindrical surface and the toric surface on different lenses, respectively, the imaging characteristics in the direction orthogonal to the scanning surface can be improved over a wide range. Therefore, the degree of freedom in design can be increased. Then, by this, the restrictions on the design in the direction along the scanning surface are alleviated, and in this direction, while having the distortion characteristic that the constant speed scannability of the light beam on the object to be scanned can be maintained sufficiently high, It is possible to maintain good imaging characteristics and widen the angle of view.

Further, in the direction orthogonal to the scanning plane, the deflection reflection point of the deflector and the image forming point on the object to be scanned are maintained in a conjugate relationship with respect to the scanning image forming optical system, and the magnification is in a wide range. Can be set with. The lower the magnification is, the higher the effect of the face tilt correction is, and the higher the magnification is, the better the imaging characteristics are. Therefore, it is possible to set the magnification in accordance with the required performance. .

By increasing the degree of freedom in designing in this way, it becomes possible to relatively easily design even an optical system having a small size in accordance with desired performance. It became possible to realize a compact structure by widening the angle.

Further, according to the present invention, by making at least one surface an optical axis symmetric aspherical surface, the uniform velocity scanning property of the light flux on the object to be scanned can be maintained sufficiently high in the direction along the scanning surface. It becomes easy to maintain good imaging characteristics while having possible distortion characteristics.

As a result, the angle of view can be made wider, and the size can be further reduced. Further, even if an optical material having a low refractive index is used, sufficient performance can be obtained, so that the cost can be reduced.

Particularly, on the cylindrical surface of the first lens,
By giving negative refracting power in the direction orthogonal to the scanning plane,
The imaging characteristics in this direction can be improved over a wide range.

Further, it is preferable that the constitution is such that the following conditional expression (1) is satisfied. This condition should be satisfied in order to maintain good imaging characteristics when carrying out the present invention. | F _1H / f _1V |> 5 (1) where f _{1H is} the focal length of the first lens in the direction along the scanning plane f _{1V is} the focal length of the first lens in the direction orthogonal to the scanning plane.

This condition is mainly for correcting spherical aberration and field curvature in the direction orthogonal to the scanning plane. If this condition is not satisfied, it is difficult to correct both aberrations in a wide field angle in a well-balanced manner. In particular, if the field curvature is not sufficiently corrected, the spot diameter will fluctuate on the scanning line, and similarly, the image quality will be deteriorated.

[0025]

The present invention will be specifically described below. The surface tilt correction scanning optical system according to the present invention is used, for example, in a laser scanning device such as a laser beam printer as shown in FIG. This laser scanning device is composed of a semiconductor laser serving as a light source, a collimator lens 2, a cylindrical lens 3, a polygon mirror 4, an fθ lens 5, a photosensitive drum 6 and the like.

The semiconductor laser 1 emits a laser beam B directly modulated according to image information. This laser beam B, which is an example of a light beam bundle, is used by the collimator lens 2
Is shaped into parallel light. Then, the light is converged linearly by a cylindrical lens 3 which is an example of a linear imaging optical system, and an image is formed on a deflecting reflection surface 4a of a polygon mirror 4 which is an example of a deflector. The laser beam B after being reflected by the deflecting / reflecting surface 4a is deflected as the polygon mirror 4 rotates, and is imaged on the photoconductor drum 6 by an fθ lens 5 which is an example of a scanning and imaging optical system. Scanning is performed in the A direction in FIG.

The surface tilt correction scanning optical system according to the present invention comprises the linear imaging optical system (cylindrical lens) 3 and the scanning imaging optical system (fθ lens) 5 described above, and a deflector (polygon mirror) 4 The deviation of the scanning line pitch caused by the tilting of the deflective reflection surface 4a is removed.

The specifications of the specific example of the scanning and imaging optical system 5 are shown in Tables 2 to 12 below. Table 1 shows Examples 1 to 11
Are shown in correspondence with the lens configuration diagrams shown in FIGS. 1 to 3 and the aberration diagrams shown in FIGS. 4 to 25, respectively. That is, FIG. 1 is a lens configuration diagram of Examples 1 to 4, FIG. 2 is a lens configuration diagram of Examples 5 to 10, and FIG. 3 is Example 1
FIG. 1 is a lens configuration diagram of No. 1 and shows characteristics of each corresponding embodiment. Further, FIGS. 1 to 3A are sectional views showing the lens arrangement when the lens is cut in the direction along the scanning surface, and FIGS. 1 to 3B are orthogonal to the scanning surface. It is a lens block diagram which shows in cross section the lens arrangement when it cut | disconnects in the direction. In each lens configuration diagram (FIGS. 1 to 3) and lens data (Tables 2 to 12), the surface marked with [*] on the shoulder of the lens surface is a cylindrical surface,
Surfaces marked with [**] indicate toric surfaces, and surfaces marked with [+] indicate axisymmetric aspherical surfaces.

Further, in the aberration diagram in the direction along the scanning plane, the distortion aberration is expressed by f.θ (where θ is the incident angle [deflected, Angle formed by ray bundle with lens optical axis] f: focal length of all scanning imaging optical systems in the direction along the scanning plane), and the formula: {(y'-f.θ) / f · θ } × 100 (%), which is expressed as a percentage of the deviation of the actual image height y ′ from the ideal image height f · θ.

In addition, in the specifications of each embodiment, 2ω: maximum incident angle n ₁ : refractive index of the optical material constituting the first lens (G1) [780 nm
] N ₂ : Refractive index of the optical material constituting the second lens (G2) [780 nm
] F _1H : focal length of the first lens (G1) in the direction along the scanning plane f _2H : focal length of the second lens (G2) in the direction along the scanning plane f _V : in the direction orthogonal to the scanning plane Focal lengths of all scanning imaging optical systems f _1V : Focal length of the first lens (G1) in the direction orthogonal to the scanning plane f _2V : Focal length of the second lens (G2) in the direction orthogonal to the scanning plane .

A ₄ , A ₆ , and A ₈ are coefficients of the optical axis symmetric aspherical surface [+], and the shape of the aspherical surface is positive from the vertex of the surface on the optical axis in the direction of the scanned object. When the X axis is taken and the Y axis perpendicular to the X axis is taken at the same vertex, it is expressed by the following formula 1. However,
In the formula of Formula 1, C ₀ is the reciprocal of the radius of curvature (r ₁ , r ₂ , ...).

[0032]

[Equation 1]

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

[0040]

[Table 8]

[0041]

[Table 9]

[0042]

[Table 10]

[0043]

[Table 11]

[0044]

[Table 12]

[0045]

As described above, according to the present invention, the scanning and imaging optical system comprises, in order from the deflector side, the first lens having the cylindrical surface and the second lens having the toric surface, and Since it has at least one optical axis symmetric aspherical surface, it is possible to achieve a wider angle of view in the main scanning direction and further downsize the apparatus while maintaining good imaging characteristics and surface tilt correction function. Moreover, even if a low refractive index optical material is used, sufficient performance can be obtained,
It is possible to realize a surface tilt correction scanning optical system at low cost.

Furthermore, by giving the cylindrical surface of the first lens a negative refracting power in the direction orthogonal to the scanning surface, the image forming characteristic in the direction orthogonal to the scanning surface can be improved over a wide range. . Further, by forming the first lens so as to satisfy the conditional expression (1), it is possible to improve the imaging characteristics.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram corresponding to Examples 1 to 4 of the present invention.

FIG. 2 is a lens configuration diagram corresponding to Examples 5 to 10 of the present invention.

FIG. 3 is a lens configuration diagram corresponding to Example 11 of the present invention.

FIG. 4 is an aberration diagram in the direction along the scanning surface according to the first embodiment of the present invention.

FIG. 5 is an aberration diagram in the direction along the scanning surface according to the second embodiment of the present invention.

FIG. 6 is an aberration diagram in the direction along the scanning surface according to the third embodiment of the present invention.

FIG. 7 is an aberration diagram in the direction along the scanning surface according to the fourth embodiment of the present invention.

FIG. 8 is an aberration diagram in the direction along the scanning surface of Example 5 of the present invention.

FIG. 9 is an aberration diagram in the direction along the scanning surface according to the sixth embodiment of the present invention.

FIG. 10 is an aberration diagram in the direction along the scanning surface according to the seventh embodiment of the present invention.

FIG. 11 is an aberration diagram in the direction along the scanning surface according to the eighth embodiment of the present invention.

FIG. 12 is an aberration diagram in the direction along the scanning surface of Example 9 of the present invention.

FIG. 13 is an aberration diagram in the direction along the scanning surface according to the tenth embodiment of the present invention.

FIG. 14 is an aberration diagram in the direction along the scanning surface of Example 11 of the present invention.

FIG. 15 is an aberration diagram in the direction orthogonal to the scanning plane according to the first embodiment of the present invention.

FIG. 16 is an aberration diagram of a second embodiment of the present invention in a direction orthogonal to the scanning surface.

FIG. 17 is an aberration diagram of a third embodiment of the present invention in a direction orthogonal to the scanning surface.

FIG. 18 is an aberration diagram of a fourth embodiment of the present invention in a direction orthogonal to the scanning surface.

FIG. 19 is an aberration diagram of a fifth embodiment of the present invention in a direction orthogonal to the scanning surface.

FIG. 20 is an aberration diagram in the direction orthogonal to the scanning surface according to the sixth embodiment of the present invention.

FIG. 21 is an aberration diagram in the direction orthogonal to the scanning surface according to Example 7 of the present invention.

FIG. 22 is an aberration diagram in a direction orthogonal to the scanning surface according to Example 8 of the present invention.

FIG. 23 is an aberration diagram in the direction orthogonal to the scanning surface according to Example 9 of the present invention.

FIG. 24 is an aberration diagram of a tenth embodiment of the present invention in a direction orthogonal to the scanning surface.

FIG. 25 is an aberration diagram in the direction orthogonal to the scanning surface of Example 11 of the present invention.

FIG. 26 is a schematic configuration diagram of a scanning device of a laser beam printer in which an embodiment of the present invention is used.

[Explanation of symbols]

 1 ... Light source (semiconductor laser) 3 ... Linear imaging optical system (cylindrical lens) 4 ... Deflector (polygon mirror) 4a ... Deflection / reflection surface 5 ... Scanning imaging optical system (f.theta. Lens) 6 ... Scanned object (photosensitive) Body drum) B ... Ray bundle (laser beam)

Claims (3)

[Claims] 1. A linear imaging optical system for linearly imaging a light beam emitted from a light source on a deflecting and reflecting surface of a deflector, and a light beam reflected and deflected by the deflector onto an object to be scanned. A surface tilt correction scanning optical system including a scanning imaging optical system for forming an image, wherein the scanning imaging optical system has, in order from the deflector side, a first lens having a cylindrical surface and a toric surface. A surface tilt correction scanning optical system comprising a second lens and at least one optical axis symmetric aspherical surface. 2. The surface tilt correction scanning optical system according to claim 1, wherein the cylindrical surface of the first lens has a negative refractive power in a direction orthogonal to the scanning surface. 3. The surface tilt correction scanning optical system according to claim 1, wherein the first lens satisfies the following condition: | f _1H / f _1V |> 5 Here, f _{1H is} the focal length of the first lens in the direction along the scanning plane, and f _{1V is} the focal length of the first lens in the direction orthogonal to the scanning plane.