JP2003066355A - Scanning optical device and image forming device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a compact and accurate scanning optical device where the deviation of a printing position in a main scanning direction and jitter are reduced, and an image forming device using the scanning optical device. SOLUTION: This scanning optical device is equipped with a 1st optical device for changing luminous flux emitted from a light source means into nearly parallel beams, a deflection means for deflecting the luminous flux to perform scanning, and a 2nd optical device for making the deflected luminous flux form an image in a spot state on a surface to be scanned. In the device, the 2nd optical device is constituted of a single lens, and both surfaces on a main scanning cross section of the single lens are constituted in the shape of aspherical surface, and at least either lens surface is the shape of the toric surface and satisfies one or more conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical device and an image forming apparatus using the same, and is particularly suitable for an image forming apparatus such as a laser beam printer and a digital copying machine.

[0002]

2. Description of the Related Art Conventionally, a scanning optical device used in a laser beam printer, a digital copying machine or the like deflects a light beam emitted from a light source means by a deflecting means and scans the deflected light beam. Thus, an image is formed in a spot shape on the surface of the photosensitive drum which is the surface to be scanned, and the surface to be scanned is optically scanned. This type of scanning optical device is disclosed in, for example, Japanese Patent Laid-Open No. 8-760.
It is proposed in Japanese Patent Publication No. 11 and the like.

FIG. 12 is a schematic view of a main part of a scanning optical device proposed in the publication.

In the figure, a light beam emitted from a light source 91 composed of a semiconductor laser is converted into a convergent light beam by a condenser lens (collimator lens) 92, shaped into an optimum beam shape by an aperture stop 93, and then in the sub-scanning direction. It is incident on the cylindrical lens 94 having only power. Of the light beam incident on the cylindrical lens 94, the light beam is emitted as it is in the main scanning cross section. Further, in the sub-scanning cross section, the light is focused and deflected by the deflecting surface 95a
The image is formed as a line image (a line image elongated in the main scanning direction). Then, the light beam deflected and reflected by the deflecting surface 95a of the optical deflector 95 is passed through a single fθ lens (scanning optical means) 96 having an fθ characteristic to a photosensitive drum 97 as a surface to be scanned.
An image is formed in a spot shape on the surface, and the optical deflector 95 is moved to the arrow A
By rotating in the direction, the surface of the photosensitive drum 97 is optically scanned in the direction of arrow B. As a result, an image is recorded on the photosensitive drum surface 97 as a recording medium.

[0005]

Some of the scanning optical means in this type of scanning optical device use a single-lens optical element to achieve cost reduction and compactness. Further, by performing injection molding with a plastic material, it is possible to perform good aberration correction by using an aspherical shape.

In Japanese Patent Laid-Open No. 8-76011, a single lens is used as the scanning optical means, and both lens surfaces are aspherical in the main scanning section, whereby good aberration correction is achieved. Further, the light flux emitted from the light source means is converted into a convergent light flux by the first optical element including a collimator lens (condensing lens). As a result, the optical path length from the deflecting means to the surface to be scanned is shortened and the overall size of the apparatus is reduced.

However, when the light flux converted into the convergent state by the collimator lens is incident on the deflecting means, for example, the rotary polygon mirror, if the rotary polygon mirror has a mounting error or the like, the rotary polygon mirror is decentered. The rotation causes the jitter, which is a print position deviation in the main scanning direction, to occur in the surface period, which affects the image.

In general, an optical deflector depends on a deflection surface used even when deflecting a light beam at the same deflection angle as shown in FIG. 13 due to a fitting error with a motor rotation axis or a variation in a distance from a rotation center to a deflection surface. , Its deflection point changes back and forth. At this time, when the light beam deflected by the deflection surface 95a of the optical deflector and incident on the scanning optical means is a parallel light beam, the light beam is imaged at the same point on the photosensitive drum surface which is the image surface.

However, when the light flux from the collimator lens is a convergent light flux or a divergent light flux, the light flux is not imaged at the same point on the surface of the photosensitive drum, which is the image surface, and becomes a jitter of the deflection surface period, so that an image is formed. There is a problem of deterioration.

Here, for example, in the case of a convergent light beam, as shown in FIG. 14, the jitter amount J (mm) at this time is the deviation amount of the two light beams after deflection by h (mm), and the lateral magnification in the main scanning direction. Is expressed as J = mh, and the lateral magnification m is ft (mm) in the main scanning direction of the scanning optical means 96 as shown in FIG. The distance to Sk
(Mm), m = 1-Sk / ft. Therefore, the jitter amount J (mm) can be expressed as J = (1-Sk / ft) h. Further, as shown in FIG. 13, the deviation amount h of the two light beams is determined by using the incident angle θi of the light beam on the deflecting surface, the exit angle θe of the light beam from the deflecting surface, and the eccentric amount d (mm) of the deflecting surface as parameters. Quantity, h = d * sin ([theta] e- [theta] i) / cos [theta] e * cos
It can be expressed as ((θe−θi) / 2), but the value that can be taken for each parameter is limited, and any amount is large with the increase in speed in recent years, and the shift amount h is approximately 0.01 to 0. It is within the range of 05 (mm).

Generally, when the interval between two dots of an image is shifted by more than half of one dot, the jitter becomes visually conspicuous. However, when three colors are further overlapped to form a color image. Affects the image even if it is shifted by ¼ or more of 1 dot. For example, in the case of a scanning optical device of a printer having a resolution of 600 dpi, when the jitter amount J becomes J = 25.4 / 600/4 = 0.01 (mm) or more, the jitter becomes conspicuous, so that the quality is high. In order to form an image, the lateral magnification m in the main scanning direction needs to be suppressed to J = mh 0.01 ≧ m × 0.05 m ≦ 0.2 0.2 or less.

Similarly, in the case of a divergent system, when J = −0.01 (mm) or less, jitter becomes conspicuous. Therefore, in order to form a high quality image, the lateral magnification m in the main scanning direction is J = mh −0.01 ≦ m × 0.05 m ≧ −0.2 −0.2 or more.

In the above-mentioned Japanese Patent Laid-Open No. 8-76011, the permissible amount of jitter is set to half of one dot. However, as described above, a stricter permissible amount is required to achieve higher definition. There is.

When used in a multi-beam scanning optical system for scanning a convergent light beam or a divergent light beam with a plurality of light beams,
A print position deviation of each beam in the main scanning direction occurs, which affects the image as main scanning jitter. For example, if it is assumed that the number of luminous fluxes is 2 for the sake of simplicity, it will be 2 as shown in FIG.
In the multi-beam scanning optical system in which the two light emitting parts A and B are arranged to be inclined with respect to the sub-scanning cross section, the reflection angles after the two light beams A and B are deflected and reflected by the rotating polygon mirror as shown in FIG. Since they are different from each other, spots are formed on the surface of the photosensitive drum at positions separated from each other in the main scanning direction. Therefore, in order to match the image forming position, the image data is transmitted to one of the light emitting units with a timing shift of a predetermined time δT.

FIG. 17 is an explanatory diagram showing how the principal rays of the two light beams at the scanning end are reflected by the deflecting surface. In the figure, the light flux first emitted from the light emitting portion A is reflected by the deflection surface 95a of the optical deflector and reflected in the direction A1 in the figure, and is imaged on the surface of the photosensitive drum by the scanning optical means. Next, the deflecting surface 95a ′ when it is deviated by a predetermined time δT
The luminous flux emitted from the light emitting portion B is reflected by the light emitting portion B, and is reflected in the direction of B1 'in the figure, that is, in the same direction as A1 in the figure, so that the respective image forming positions coincide.

However, 2 after being deflected and reflected by the rotary polygon mirror
Although the principal rays of the two light beams are incident on the scanning optical system at the same angle, the principal rays of the respective luminous fluxes are displaced from each other by δys in the main scanning direction and enter the scanning optical system. When the two light beams incident on the scanning optical system are convergent light beams, if the principal rays of the respective light beams deviate by δys in the main scanning direction, the image formation position of the spot on the photosensitive drum surface in the main scanning direction also deviates. Occurs.

Similarly, FIG. 18 is an explanatory view showing a state at the opposite scanning end. In the figure, the light flux firstly emitted from the light emitting section A is reflected by the deflecting surface 95a and reflected in the direction of A1 in the figure, and is imaged on the surface of the photosensitive drum by the scanning optical system. Next, the light beam emitted from the light emitting portion B is reflected by the deflecting surface 95a 'when the time is deviated by the predetermined time δT, and is reflected in the direction of B1' in the figure, that is, the same direction as A1 in the figure. The principal ray of the light flux of is shifted by δye in the main scanning direction and enters the scanning optical system. As can be seen from FIGS. 16 and 18, the light beam B ′ emitted from the light emitting unit B is displaced toward the optical axis side of the scanning optical system from the light beam A ′ emitted from the light emitting unit A. As shown in FIG. 19, the line scanned with the light beam B ′ is shorter than the line scanned with the light beam A ′.

That is, when the two light fluxes incident on the scanning optical system are convergent light fluxes or divergent light fluxes, positional deviations in the main scanning direction at the image forming points of the respective light fluxes on the surface of the photosensitive drum and main scanning jitter- Will occur.

It is an object of the present invention to provide a compact and high-definition scanning optical device that reduces print position deviation and jitter in the main scanning direction and an image forming apparatus using the same.

[0020]

According to another aspect of the present invention, there is provided a scanning optical device including: a first optical element for converting a light beam emitted from a light source means into a substantially parallel light beam; and a deflecting means for deflecting and scanning the light beam. A second optical element for forming the deflected light beam into a spot image on the surface to be scanned, wherein the second optical element comprises a single lens, and the second optical element is in a main scanning section of the single lens. Both lens surfaces are aspherical, and at least one of the lens surfaces is a toric surface, and within the main scanning cross section of the lens surface a on the deflecting means side and the lens surface b on the scanned surface side of the single lens. Paraxial radius of curvature (m
m) respectively, R1 and R2 (mm), the maximum effective image height in the main scanning direction from the optical axis of the single lens is Ymax (mm), from the paraxial radius of curvature at the image height Ymax of the lens surfaces a and b. The aspherical surface amounts are S1 and S2 (mm), the focal length of the second optical element in the main scanning section is ft (mm), and the scanned surface is scanned from the lens surface on the scanned surface side of the second optical element. When the distance to the surface is Sk (mm), R2 <0 (mm) <R1 (R1 ² −Ymax ² ) ^1/2 −R1 <S1 <0 (mm) (R2 ² −Ymax ² ) ^1/2 + R2 <S2 <0 (mm) -0.2 <1-Sk / ft <0.2 One or more of the conditional expressions are satisfied.

The invention of claim 2 is characterized in that, in the invention of claim 1, the second optical element satisfies the condition of | S1−S2 | ≦ 0.5 (mm).

According to a third aspect of the present invention, in the first aspect of the invention, the lens thickness on the optical axis of the second optical element is d (m
m), when the distance from the deflecting surface of the deflecting means to the lens surface a of the second optical element is L (mm), the condition of 0.2 <d / L <0.4 is satisfied. It has a feature.

According to a fourth aspect of the present invention, in the first aspect of the invention, the light source means is a multi-beam light source for emitting a plurality of light beams.

According to a fifth aspect of the invention, in the first aspect of the invention, the second optical element has at least one surface asymmetric with respect to the optical axis in the main scanning section.

According to a sixth aspect of the invention, in the first aspect of the invention, the second optical element has an axis of symmetry in the main scanning direction,
The axis of symmetry is tilted with respect to the perpendicular bisector of the surface to be scanned or /
It is characterized by being shifted.

According to a seventh aspect of the invention, in the first aspect of the invention, the radius of curvature of both lens surfaces of the second optical element continuously changes as the distance from the optical axis in the main scanning direction increases in both lens surfaces. It is characterized by that.

In the scanning optical apparatus according to the present invention, the first optical element for converting the luminous flux emitted from the light source means into the weakly divergent luminous flux, the deflecting means for deflecting and scanning the luminous flux, and the deflected luminous flux to be scanned. In a scanning optical device including a second optical element for forming a spot-shaped image on a surface, the second optical element includes a single lens, and both lens surfaces in a main scanning section of the single lens are both At least one of the lens surfaces has an aspherical shape and has a toric surface shape, and the paraxial radius of curvature of the lens surface a on the deflecting means side of the single lens and the lens surface b on the scanned surface side in the main scanning section ( mm) respectively R1 and R2
(Mm), Ymax (mm) is the maximum effective image height in the main scanning direction from the optical axis of the single lens, and Y is the image height Y of the lens surfaces a and b.
The aspherical amount from the paraxial radius of curvature at max is S
1, S2 (mm), the focal length in the main scanning cross section of the second optical element is ft (mm), and the distance from the lens surface on the scanned surface side of the second optical element to the scanned surface is Sk
(Mm) R2 <0 (mm) <R1 (R1 ² −Ymax ² ) ^1/2 −R1 <S1 <0 (mm) (R2 ² −Ymax ² ) ^1/2 + R2 <S2 <0 (mm ) -0.2 <1-Sk / ft <0 One or more of the conditional expressions are satisfied.

The invention of claim 9 is characterized in that, in the invention of claim 8, the second optical element satisfies the condition of | S1−S2 | ≦ 0.5 (mm).

According to a tenth aspect of the present invention, in the eighth aspect, the lens thickness on the optical axis of the second optical element is d (m
m), when the distance from the deflecting surface of the deflecting means to the lens surface a of the second optical element is L (mm), the condition of 0.2 <d / L <0.4 is satisfied. It has a feature.

The invention of claim 11 is characterized in that, in the invention of claim 8, the light source means is a multi-beam light source for emitting a plurality of light beams.

According to a twelfth aspect of the invention, in the eighth aspect, the second optical element has one or more surfaces asymmetric with respect to the optical axis in the main scanning section.

According to a thirteenth aspect of the present invention, in the eighth aspect, the second optical element has an axis of symmetry in the main scanning direction,
The axis of symmetry is tilted with respect to the perpendicular bisector of the surface to be scanned or /
It is characterized by being shifted.

According to a fourteenth aspect of the invention, in the eighth aspect of the invention, the radius of curvature of both lens surfaces of the second optical element continuously changes as the distance from the optical axis in the main scanning direction increases in both lens surfaces. It is characterized by that.

According to the fifteenth aspect of the present invention, in the scanning optical device, the first optical element for converting the light beam emitted from the light source means into the weakly convergent light beam, the deflecting means for deflecting and scanning the light beam, and the deflected light beam to be scanned. In a scanning optical device including a second optical element for forming a spot-shaped image on a surface, the second optical element includes a single lens, and both lens surfaces in a main scanning section of the single lens are both At least one of the lens surfaces has an aspherical shape and has a toric surface shape, and the paraxial radius of curvature of the lens surface a on the deflecting means side of the single lens and the lens surface b on the scanned surface side in the main scanning section ( mm) respectively R1 and R2
(Mm), Ymax (mm) is the maximum effective image height in the main scanning direction from the optical axis of the single lens, and Y is the image height Y of the lens surfaces a and b.
The aspherical amount from the paraxial radius of curvature at max is S
1, S2 (mm), the focal length in the main scanning cross section of the second optical element is ft (mm), and the distance from the lens surface on the scanned surface side of the second optical element to the scanned surface is Sk
(Mm) R2 <0 (mm) <R1 (R1 ² −Ymax ² ) ^1/2 −R1 <S1 <0 (mm) (R2 ² −Ymax ² ) ^1/2 + R2 <S2 <0 (mm ) One or more of the conditional expressions 0 <1-Sk / ft <0.2 are satisfied.

According to a sixteenth aspect of the invention, in the fifteenth aspect of the invention, the second optical element satisfies the condition of | S1−S2 | ≦ 0.5 (mm).

According to a seventeenth aspect of the present invention, in the fifteenth aspect of the invention, the lens thickness on the optical axis of the second optical element is d (m
m), when the distance from the deflecting surface of the deflecting means to the lens surface a of the second optical element is L (mm), the condition of 0.2 <d / L <0.4 is satisfied. It has a feature.

The invention of claim 18 is characterized in that, in the invention of claim 15, the light source means is a multi-beam light source for emitting a plurality of light beams.

A nineteenth aspect of the invention is characterized in that, in the fifteenth aspect of the invention, the second optical element has one or more surfaces asymmetric with respect to the optical axis in the main scanning section.

According to a twentieth aspect of the invention, in the fifteenth aspect of the invention, the second optical element has a symmetry axis in the main scanning direction, and the symmetry axis is tilted with respect to a perpendicular bisector of the surface to be scanned. / And is characterized by shifting.

According to a twenty-first aspect of the invention, in the fifteenth aspect of the invention, the radius of curvature of both lens surfaces of the second optical element continuously changes as the distance from the optical axis in the main scanning direction increases in both lens surfaces. It is characterized by that.

An image forming apparatus according to a twenty-second aspect of the present invention is a scanning optical apparatus according to any one of the first to twenty-first aspects, a photoconductor disposed on the surface to be scanned, and a scanning optical apparatus for scanning. Developing device that develops the electrostatic latent image formed on the photoconductor as a toner image by the generated light flux, a transfer device that transfers the developed toner image to a transfer material, and the transferred toner image is transferred. It is characterized by having a fixing device for fixing the material.

An image forming apparatus according to a twenty-third aspect of the present invention is the scanning optical apparatus according to any one of the first to twenty-first aspects, and the scanning optical apparatus by converting code data input from an external device into an image signal. It is characterized by having a printer controller for inputting to.

[0043]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] FIG. 1 is a sectional view (main scanning sectional view) of a main portion in a main scanning direction according to Embodiment 1 of the present invention.

In this specification, the direction in which the light beam is reflected and deflected (deflected and scanned) by the deflecting means is defined as the main scanning direction, and the direction orthogonal to the optical axis of the scanning lens and the main scanning direction is defined as the sub scanning direction.

In the figure, reference numeral 1 is a light source means, which is composed of, for example, a semiconductor laser. In the present embodiment, the light beam emitted from the light source means 1 may be one beam,
Two or more multi-beams (multi-beam light sources) may be used.

Reference numeral 2 denotes a condenser lens (collimator lens) which is a first optical element, and converts the divergent light beam emitted from the light source means 1 into a substantially parallel light beam. 3 is an aperture stop,
The light flux emitted from the condenser lens 2 is shaped into a desired optimum beam shape. 4 is a cylindrical lens,
A light beam having a predetermined power only in the sub-scanning direction and formed from the aperture stop 3 is imaged in the sub-scanning cross section in the vicinity of the deflecting surface 5a of the deflecting means 5 described later (long line image in the main scanning cross section). To do. Each element such as the condenser lens 2, the aperture stop 3, and the cylindrical lens 4 is the incident optical means 1.
It constitutes one element of 1.

Reference numeral 5 denotes an optical deflector as a deflecting means, which is composed of, for example, a rotary polygon mirror (polygon mirror), and is rotated at a constant speed in the direction of arrow A in the figure by driving means (not shown) such as a motor. There is.

Reference numeral 6 denotes a scanning lens as a second optical element having a condensing function and fθ characteristic, which is composed of a single lens. The scanning lens 6 is an anamorphic lens having different convex powers in the main scanning cross section and the sub scanning cross section, and has an entrance surface (first surface) 6a and an exit surface (second surface).
Both 6b have a toric surface shape, and both lens surfaces have an aspherical shape in the main scanning cross section.
In the sub-scanning cross section, the radius of curvature of both lens surfaces continuously changes as the distance from the optical axis in the main scanning direction increases. This enables field curvature correction and uniform magnification. The scanning lens 6 is made of a plastic material,
It is molded by injection molding. Further, the scanning lens 6 makes the deflecting surface 5a and the surface to be scanned 7 conjugate in the sub-scanning direction, so that even if the deflecting surface 5a is tilted in the sub-scanning section, the scanning line on the surface to be scanned 7 can be changed. It has a tilt correction function that suppresses the deviation amount of

Reference numeral 7 is a photosensitive drum surface as a surface to be scanned.

In this embodiment, the divergent light beam emitted from the semiconductor laser 1 is converted into a substantially parallel light beam by the condenser lens 2 and shaped into a desired beam shape by the aperture stop 3. Then, the luminous flux is imaged by the cylindrical lens 4 in the vicinity of the deflection surface 5a of the optical deflector 5 in the sub-scanning direction (long line image in the main scanning direction). Thereafter, the light beam reflected and deflected by the deflecting surface 5a is scanned by the scanning lens 6
An image is formed in a spot shape on the photosensitive drum surface 7 by rotating the optical deflector 5 in the direction of arrow A in the figure at a constant speed in the direction of arrow B (main scanning direction) in the figure. Optically scanned.

The shape of the scanning lens 6 in this embodiment is such that the intersection of the lens surface and the optical axis is the origin, and the optical axis direction is X.
Axis, the direction perpendicular to the optical axis in the main scanning cross section is the Y axis, and the direction perpendicular to the optical axis in the sub scanning cross section is the Z axis,
Each can be expressed as follows.

Main scanning direction ... Aspherical surface shape expressed by a function up to the tenth order in the following equation

[0053]

[Equation 1]

Where R is the radius of curvature, k, B4, B6, B
8 and B10 are aspherical coefficients (when a coefficient has a subscript u, when it has an anti-laser side 1 from the optical axis, when it has a laser side from the optical axis) Sub-scanning direction ... A spherical surface whose radius of curvature continuously changes in the Y-axis direction shape

[0055]

[Equation 2]

However, r is the radius of curvature, D2, D4, D6,
D8 and D10 are aspherical coefficients (when the coefficient has a subscript u,
In the case where the side opposite to the laser 1 is attached to the optical axis, the optical arrangement in the present embodiment is shown in Table 1 and the aspherical coefficients of the scanning lens 6 in the main scanning direction and the sub scanning direction are shown in Table 2. .

[0057]

[Table 1]

[0058]

[Table 2]

In this embodiment, the paraxial radius of curvature of the lens surface (first surface) 6a on the optical deflector 5 side of the scanning lens 6 and the lens surface (second surface) 6b on the scanned surface 7 side in the main scanning section. (M
m) is R1 and R2 (mm), the scanning lens 6 has R1 = 236.9 (mm) and R2 = −191.1 (mm).
Is a biconvex shape and satisfies the following conditional expression (1).

R2 <0 (mm) <R1 (1) If the conditional expression (1) is not satisfied, the fθ characteristic and the field curvature in the main scanning direction are deteriorated.

In this embodiment, the maximum effective image height (maximum effective diameter) in the main scanning direction from the optical axis of the scanning lens 6 is Yma.
When x (mm), the first surface: Ymax = 47.9 (mm) and the second surface: Ymax = 49.8 (mm). The maximum effective image height referred to here is the passage position of the beam principal ray at the maximum scanning angle.

Here, (R1 ² −Ymax ² ) ^1/2 −R1 = −4.9 (mm).

Here, the aspherical amount S1 of the first surface is shown in FIG.
As shown in FIG. 20A, when the X-coordinates of the spherical surface and the aspherical surface at the height Ymax from the optical axis are R1x and X1, respectively, S1 = X1-R1x and the aspherical surface amount S2 of the second surface is shown in FIG. ), When the X-coordinates of the spherical surface and the aspherical surface at the height Ymax from the optical axis are R2x and X2, respectively, S2 = X2-R2x.

At this time, the first surface 6a at the Ymax position
The aspherical amount S1 (mm) from the paraxial radius of curvature of is S1 = -2.5 (mm), and therefore satisfies the following conditional expression (2).

(R1 ² −Ymax ² ) ^1/2 −R1 <S1 <0 (mm) (2) Further, (R2 ² −Ymax ² ) ^1/2 + R2 = −6.6 (mm) Become. Further, the aspherical amount S2 (mm) from the paraxial radius of curvature of the second surface 6b at the Ymax position is S2 = −2.2 (mm), which satisfies the following conditional expression (3).

(R2 ² −Ymax ² ) ^1/2 + R2 <S2 <0 (mm) (3) From the above, | S1−S2 | = 0.3 (mm), so the following conditional expression Satisfies (4).

| S1−S2 | ≦ 0.5 (mm) (4) Conditional expressions (2), (3), and (4) favorably correct the fθ characteristic and the field curvature in the main scanning direction. However, if at least one of the above conditional expressions is not satisfied, various aberrations will worsen, which is not good.

When the lens thickness on the optical axis of the scanning lens 6 is d and the distance from the deflection surface 5a of the optical deflector 5 to the first surface 6a of the scanning lens 6 is L, d = 15.11 (m)
m) and L = 50.16 (mm), d / L = 0.30, which satisfies the following conditional expression (5).

0.2 <d / L <0.4 (5) If the conditional expression (5) is not satisfied, the fθ characteristic and the field curvature in the main scanning direction cannot be satisfactorily corrected, or In injection molding using a plastic material, problems such as molding conditions and molding cost may occur.

Furthermore, in this embodiment, when the focal length of the scanning lens 6 in the main scanning section is ft (mm) and the lens back is Sk (mm), ft = 204.26.
(Mm) and Sk = 199.36 (mm).

The lens back is the distance from the lens surface 6b on the scanned surface 7 side of the scanning lens 6 to the scanned surface 7.

In this embodiment, since the light beam emitted from the condenser lens 2 is a substantially parallel light beam, 1-Sk / ft = 0.02, which satisfies the following conditional expression (6).

-0.2 <1-Sk / ft <0.2 (6) If conditional expression (6) is not satisfied, jitter in the main scanning direction due to the optical deflector 5 and multi-beam are generated as described above. The effect of jitter due to multiple light sources at the time of scanning becomes remarkable, and the image is affected, which is not preferable.

In this embodiment, the above conditional expression (1),
If one or more of the conditional expressions (2), (3), and (6) are satisfied, the intended purpose can be achieved.

The optical performances of this embodiment satisfying the above conditions are shown in FIGS. FIG. 2 shows field curvature in the main scanning direction and the sub-scanning direction, and FIG. 3 shows distortion.
From both figures, it can be seen that the optical performance of this embodiment is corrected well.

Further, in this embodiment, in order to satisfactorily correct the asymmetric component of the aberration, one or more surface shapes in the main scanning section of the scanning lens 6 are made asymmetric with respect to the optical axis (symmetry axis), or the scanning lens is The axis of symmetry 6 in the main scanning direction may be tilted and / or shifted with respect to the perpendicular bisector of the photosensitive drum surface 7.

Here, the optical axis (symmetry axis) in the main scanning cross section of the scanning lens 6 refers to a line Lab connecting the centers of curvature of paraxial curvature radii of the lens surface a and the lens surface b as shown in FIG. The lens surface a is asymmetric with respect to the optical axis Lab in the main scanning section.

As described above, in the present embodiment, it is possible to reduce the jitter in the main scanning direction due to the optical deflector 5, the jitter due to a plurality of light sources during multi-beam scanning, and the field curvature and the fθ characteristic. It is possible to provide a scanning optical device in which the above is corrected well.

In this embodiment, the light flux from the light source means 1 may be directly guided to the optical deflector 5 without using the condenser lens 2 and the cylindrical lens 4.

Further, in the present embodiment, the scanning optical means 6
Although it is composed of one lens, it is not limited to this and may be composed of, for example, two or more lenses.

[Second Embodiment] FIG. 5 shows a second embodiment of the present invention.
FIG. 4 is a cross-sectional view of a main part in the main scanning direction (main scanning sectional view). In the figure, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

The present embodiment is different from the first embodiment described above in that the condenser lens 22 as the first optical element is used.
It is that the light beam from the semiconductor laser 1 is converted into a weakly divergent light beam, that the numerical range of conditional expression (6) is changed accordingly, and that the shape of the scanning lens 26 is made different. Other configurations and optical functions are substantially the same as those of the first embodiment, and the same effect is obtained.

That is, in the figure, reference numeral 22 denotes a condenser lens (collimator lens) which is a first optical element, and converts the light beam emitted from the light source means 1 into a weakly divergent light beam.

Reference numeral 26 denotes a scanning lens as a second optical element having a light condensing function and fθ characteristic, which is composed of a single lens. The scanning lens 26 is an anamorphic lens having different powers in the main scanning cross section and the sub scanning cross section, and both the entrance surface (first surface) 26a and the exit surface (second surface) 26b have a toric surface shape. In the main scanning section, both lens surfaces have an aspherical shape, and both lens surfaces are asymmetric with respect to the optical axis. By making the shape asymmetric, it becomes possible to satisfactorily correct the asymmetry of aberration. In the sub-scanning cross section, the radius of curvature of both lens surfaces continuously changes as the distance from the optical axis in the main scanning direction increases. This enables field curvature correction and uniform magnification. The scanning lens 26 is made of a plastic material and is molded by injection molding. Further, the scanning lens 26 makes the deflection surface 5a and the surface to be scanned 7 conjugate with each other in the sub-scanning direction, so that even if the deflection surface 5a is tilted in the sub-scanning cross section, the scanning line on the surface to be scanned 7 is scanned. It has a tilt correction function that suppresses the deviation amount of

In this embodiment, the divergence is set so that the distance from the deflecting surface to the apparent object point is 1100 (mm). By designing the optical path length (distance from the deflecting surface to the surface to be scanned) equivalent to that of the present embodiment by using a weakly divergent light beam, the power of the scanning lens 26 as the second optical element can be increased. Since this is possible, the configuration is particularly advantageous for correcting the fθ characteristic.

Table 3 shows the optical arrangement in this embodiment.
Table 4 shows the aspherical surface coefficients of the scanning lens 26 in the main scanning direction and the sub scanning direction.

[0087]

[Table 3]

[0088]

[Table 4]

The scanning lens 26 of this embodiment has the first surface 26.
The paraxial radius of curvature of a is R1 = 333.5 (mm), the second surface 26
The paraxial radius of curvature of b is R2 = −130.9 (mm) and is a biconvex shape, which satisfies the conditional expression (1).

In the present embodiment, the maximum effective image height Ymax (mm) in the main scanning direction from the optical axis of the scanning lens 26 is as follows: First surface: Ymax = 47.6 (mm) Second surface: Ymax = 49. It is 6 (mm). Here, (R1 ² −Ymax ² ) ^1/2 −R1 = −3.4 (mm). Further, the aspherical amount S1 (mm) from the paraxial radius of curvature of the first surface 26a at the Ymax position is S1 = -1.9 (mm), which satisfies the conditional expression (2).

Further, (R2 ² −Ymax ² ) ^1/2 + R2 = −9.8 (mm). The aspherical amount S2 (mm) from the paraxial radius of curvature of the second surface 26b at the Ymax position is S2 = -1.6 (mm), which satisfies the conditional expression (3).

From the above, | S1-S2 | = 0.3 (mm) Therefore, the conditional expression (4) is satisfied.

Further, the lens thickness d on the optical axis of the scanning lens 26
= 17.08 (mm), the distance L from the deflecting surface 5a of the optical deflector 5 to the first surface 26a of the scanning lens 26 L = 50.71 (mm)
As a result, d / L = 0.34, which satisfies the conditional expression (5).

Further, in the present embodiment, the focal length ft = 181.58 (mm) in the main scanning section of the scanning lens 26,
The lens back Sk is 212.18 (mm).

In this embodiment, since the light beam emitted from the condenser lens 22 is a weakly divergent light beam, 1-Sk / ft = -0.17, which satisfies the following conditional expression (7).

-0.2 <1-Sk / ft <0 (7) If the conditional expression (7) is not satisfied, jitter in the main scanning direction due to the optical deflector 5 or multi-beam scanning occurs as described above. This is not good because the effect of jitter due to multiple light sources becomes noticeable and the image is affected.

In the present embodiment, the above conditional expression (1),
If at least one of the conditional expressions (2), (3), and (7) is satisfied, the intended purpose can be achieved.

The optical performances of this embodiment satisfying the above conditions are shown in FIGS. FIG. 6 shows field curvature in the main scanning direction and the sub-scanning direction, and FIG. 7 shows distortion.
From both figures, it can be seen that the optical performance of this embodiment is corrected well.

Further, in this embodiment, the surface shape of the scanning lens 26 in the main scanning cross section is made asymmetric with respect to the optical axis (symmetry axis) in order to satisfactorily correct the asymmetric component of the aberration. The axis of symmetry in the main scanning direction may be tilted and / or shifted with respect to the perpendicular bisector of the photosensitive drum surface 7.

In the present embodiment, the influence of the jitter due to the weakly divergent light flux is suppressed to a small extent, and the power of the scanning lens 26 is strengthened to realize the good correction of the fθ characteristic.

As described above, in the present embodiment, it is possible to reduce the jitter in the main scanning direction due to the optical deflector 5 and the jitter due to a plurality of light sources during multi-beam scanning, and further the field curvature and the fθ characteristic. It is possible to provide a scanning optical device in which the above is corrected well.

[Third Embodiment] FIG. 8 shows a third embodiment of the present invention.
FIG. 4 is a cross-sectional view of a main part in the main scanning direction (main scanning sectional view). In the figure, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

The present embodiment differs from the first embodiment described above in that the light flux from the semiconductor laser 1 is converted into a weakly convergent light flux by the condenser lens (collimator lens) 32 which is the first optical element. Accordingly, the numerical range of conditional expression (6) is changed and the shape of the scanning lens 36 is changed. Other configurations and optical functions are substantially the same as those of the first embodiment, and the same effect is obtained.

That is, in the figure, reference numeral 32 denotes a condenser lens (collimator lens) which is the first optical element, and converts the light beam emitted from the light source means 1 into a weakly convergent light beam.

Reference numeral 36 denotes a scanning lens as a second optical element having a condensing function and fθ characteristic, which is composed of a single lens. The scanning lens 36 is an anamorphic lens having different convex and convex powers in the main scanning cross section and the sub scanning cross section, and both the entrance surface (first surface) 36a and the exit surface (second surface) 36b are formed in a toric surface shape. Both lens surfaces are aspherical in the main scanning section, and the axis of symmetry in the main scanning direction is the intersection of the first surface 36a and the axis of symmetry with respect to the perpendicular bisector of the surface 7 to be scanned. 1.5 at the center
It is tilted counterclockwise by the minute. Thereby, the asymmetry of the field curvature is corrected well. In the sub-scanning cross section, the radius of curvature of both lens surfaces continuously changes as the distance from the optical axis in the main scanning direction increases. This enables field curvature correction and uniform magnification. The scanning lens 36 is made of a plastic material and is molded by injection molding. Further, the scanning lens 36 has a conjugate relationship between the deflecting surface 5a and the surface to be scanned 7 in the sub-scanning direction, so that even if the deflecting surface 5a is tilted in the sub-scanning section, the scanning line on the surface to be scanned 7 is scanned. It has a tilt correction function that suppresses the deviation amount of

In this embodiment, the degree of convergence is set so that the distance from the deflection surface to the natural convergence point is 1700 (mm). By designing the optical path length (distance from the deflecting surface to the surface to be scanned) equivalent to that of the first embodiment by using a weakly converging light flux, the power of the scanning lens 36, which is the second optical element, can be weakened. Since this is possible, it is advantageous in reducing the lens thickness of the scanning lens 36 on the optical axis. Further, the main scanning jitter and the multi-beam jitter due to the eccentricity of the rotating polygonal mirror, which has been a problem in the converging system, can be suppressed minutely by using the weak converging system.

Table 5 shows the optical arrangement in this embodiment.
Table 6 shows the aspherical coefficients of the scanning lens 36 in the main scanning direction and the sub scanning direction.

[0108]

[Table 5]

[0109]

[Table 6]

The scanning lens 36 of this embodiment has the first surface 36.
The paraxial radius of curvature of a is R1 = 177.9 (mm), the second surface 36
The paraxial radius of curvature of b is R2 = −340.1 (mm) and is a biconvex shape, which satisfies the conditional expression (1).

In this embodiment, the maximum effective image height Ymax (mm) in the main scanning direction from the optical axis of the scanning lens 36 is as follows: First surface: Ymax = 49.3 (mm) Second surface: Ymax = 51. It is 0 (mm). Here, (R1 ² −Ymax ² ) ^1/2 −R1 = −7.0 (mm). The aspherical amount S1 (mm) from the paraxial radius of curvature of the first surface 36a at the Ymax position is S1 = -3.7 (mm), which satisfies the conditional expression (2).

Furthermore, (R2 ² −Ymax ² ) ^1/2 + R2 = −3.8 (mm). The aspherical amount S2 (mm) from the paraxial radius of curvature of the second surface 36b at the Ymax position is S2 = −3.2 (mm), which satisfies the conditional expression (3).

From the above, | S1-S2 | = 0.5 (mm) Therefore, the conditional expression (4) is satisfied.

Further, the lens thickness d on the optical axis of the scanning lens 36
= 14.15 (mm), the distance L from the deflecting surface 5a of the optical deflector 5 to the first surface 36a of the scanning lens 36 L = 50.75 (mm)
As a result, d / L = 0.28, which satisfies the conditional expression (5).

Further, in the present embodiment, the focal length ft = 224.97 (mm) in the main scanning section of the scanning lens 36,
The lens back Sk is 191.26 (mm).

In this embodiment, the light flux emitted from the condenser lens 32 is a weakly convergent light flux, and therefore 1-Sk / ft = 0.15, which satisfies the following conditional expression (8).

0 <1-Sk / ft <0.2 (8) If the conditional expression (8) is not satisfied, jitter in the main scanning direction due to the optical deflector 5 or multi-beam scanning is caused as described above. This is not good because the effect of jitter from multiple light sources becomes noticeable and the image is affected.

In this embodiment, the above conditional expression (1),
If one or more of the conditional expressions (2), (3), and (8) are satisfied, the intended purpose can be achieved.

The optical performances of this embodiment satisfying the above conditions are shown in FIGS. FIG. 9 shows field curvature in the main scanning direction and the sub-scanning direction, and FIG. 10 shows distortion. From both figures, it can be seen that the optical performance of this embodiment is corrected well.

Further, in the present embodiment, in order to satisfactorily correct the asymmetric component of the aberration, the axis of symmetry of the scanning lens 36 in the main scanning direction is tilted with respect to the perpendicular bisector of the photosensitive drum surface 7. The scanning lens 6 may be shifted in the main scanning direction, or the surface shape in the main scanning direction may be asymmetric with respect to the optical axis as described above.

In this embodiment, the power of the scanning lens 36 is weakened and the lens thickness is made thinner while the influence of the jitter due to the weakly converged light beam is suppressed to a small extent.

As described above, in the present embodiment, it is possible to reduce the jitter in the main scanning direction due to the optical deflector 5 and the jitter due to a plurality of light sources at the time of multi-beam scanning. It is possible to provide a well-corrected scanning optics.

[Image Forming Apparatus] FIG. 11 is a sectional view of a main part in a sub-scan section showing an embodiment of an image forming apparatus (electrophotographic printer) using the scanning optical system according to the first, second or third embodiment. Is. In FIG. 11, reference numeral 104
Indicates an image forming apparatus. The image forming apparatus 104 includes
Code data Dc is input from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) Di by the printer controller 111 in the apparatus. This image data D
i is input to the optical scanning unit 100 having the configuration shown in each of the first, second, and third embodiments. Then, a light beam (light flux) 103 modulated according to the image data Di is emitted from the optical scanning unit (scanning optical system) 100,
The light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.

The photosensitive drum 101, which is an electrostatic latent image carrier (photoconductor), is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves with respect to the light beam 103 in the sub scanning direction orthogonal to the main scanning direction. A charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided above the photosensitive drum 101 so as to contact the surface.
Then, the surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100.

As described above, the light beam 103 is
The light beam 103 is modulated based on the image data Di, and an electrostatic latent image is formed on the surface of the photosensitive drum 101 by irradiating the light beam 103. This electrostatic latent image is more sensitive to the photosensitive drum 101 than the irradiation position of the light beam 103.
The toner is developed as a toner image by the developing device 107 arranged so as to come into contact with the photosensitive drum 101 on the downstream side in the rotational cross section.

The toner image developed by the developing device 107 is transferred onto the paper 112, which is the material to be transferred, by the transfer roller (transfer device) 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. To be done. Paper 1
12 is the front of the photosensitive drum 101 (right side in FIG. 11)
Although the paper is stored in the paper cassette 109, the paper can be manually fed. A paper feed roller 110 is arranged at the end of the paper cassette 109,
The paper 112 inside is sent to the transport path.

The sheet 112 to which the unfixed toner image has been transferred as described above is further rearward of the photosensitive drum 101 (see FIG. 1).
1 to the fixing device on the left side). The fixing device includes a fixing roller 113 having a fixing heater (not shown) inside.
And a pressure roller 114 arranged so as to be in pressure contact with the fixing roller 113, and the sheet 112 sent from the transfer portion is fixed to the fixing roller 113 and the pressure roller 1.
The unfixed toner image on the sheet 112 is fixed by heating while applying pressure at the pressure contact portion 14 of the sheet. Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and discharges the fixed paper 112 to the outside of the image forming apparatus.

Although not shown in FIG. 11, the print controller 111 performs not only the above-described data conversion but also the motor 115, each part in the image forming apparatus, the polygon motor in the optical scanning unit 100, and the like. Control.

[0129]

According to the present invention, the luminous flux converted by the condenser lens as the first optical element as described above is passed through the optical deflector to the surface to be scanned by the scanning lens as the second optical element. When forming an image, by appropriately setting the paraxial radius of curvature in the main scanning direction of the scanning lens, the amount of aspherical surface, the focal length, and the distance to the surface to be scanned, field curvature, distortion, etc. It is possible to satisfactorily correct the error and to suppress the effects of the jitter (main scanning jitter and multi-beam jitter) due to the optical deflector mounting error and the change of the spot diameter in the sub-scanning direction due to the image height. A scanning optical device suitable for fine printing and an image forming apparatus using the same can be achieved.

[Brief description of drawings]

FIG. 1 is a main-scan sectional view of a first embodiment of the present invention.

FIG. 2 is a field curvature diagram in a main scanning direction and a sub scanning direction according to the first embodiment of the present invention.

FIG. 3 is a distortion diagram of the first embodiment of the present invention.

FIG. 4 is a main scanning sectional view of the scanning lens according to the first embodiment of the present invention.

FIG. 5 is a main-scan sectional view of a second embodiment of the present invention.

FIG. 6 is a field curvature diagram in a main scanning direction and a sub scanning direction according to the second embodiment of the present invention.

FIG. 7 is a distortion diagram of the second embodiment of the present invention.

FIG. 8 is a main-scan sectional view of a third embodiment of the present invention.

FIG. 9 is a field curvature diagram in a main scanning direction and a sub scanning direction according to the third embodiment of the present invention.

FIG. 10 is a distortion diagram of the third embodiment of the present invention.

FIG. 11 is a sub-scanning sectional view showing a configuration example of an image forming apparatus (electrophotographic printer) using the scanning optical system of the present invention.

FIG. 12 is a main-scan sectional view of a conventional scanning optical device.

FIG. 13 is an enlarged view of a part of the deflector.

FIG. 14 is an explanatory diagram showing a correlation between a two-beam deviation and a jitter amount.

FIG. 15 is an explanatory diagram showing a positional relationship from a deflector to a surface to be scanned.

FIG. 16 is an explanatory diagram of a multi-beam light source unit.

FIG. 17 is an explanatory diagram of beam deflection reflection at the scanning end in the focusing system.

FIG. 18 is an explanatory diagram of beam deflection reflection at the opposite scanning end in the converging system.

FIG. 19 is an explanatory diagram of multi-beam main scanning jitter.

FIG. 20 is an explanatory diagram of an aspherical amount of a scanning lens.

[Explanation of symbols]

1 light source means 2, 22.32 First optical element (condensing lens) 3 aperture stop 4 Cylindrical lens 5 Deflection means (rotary polygon mirror) 5a Deflection surface 6, 26, 36 Second optical element (scanning lens) 7 Scanned surface (photosensitive drum surface) 11 Incident optical system 100 scanning optical system 101 photosensitive drum 102 charging roller 103 light beam 104 image forming apparatus 107 developing device 108 transfer roller 109 paper cassette 110 paper feed roller 111 Printer controller 112 Transfer material (paper) 113 fixing roller 114 pressure roller 115 motor 116 paper ejection roller 117 External device

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H045 AA01 BA02 CA04 CA34 CA55                       CA68 CB15                 2H087 KA19 LA22 PA01 PA17 PB01                       RA05 RA08 RA12 RA13                 5C072 AA03 BA01 BA04 DA02 HA02                       HA06 HA13 RA12 XA01 XA05

Claims (23)

[Claims] 1. A first optical element for converting a light flux emitted from a light source means into a substantially parallel light flux, a deflecting means for deflecting and scanning the light flux, and a deflected light flux for forming a spot-like image on a surface to be scanned. And a second optical element, wherein the second optical element comprises a single lens, both lens surfaces in a main scanning section of the single lens both have an aspherical shape, and at least one of them Has a toric surface shape, and the paraxial curvature radii (mm) in the main scanning cross section of the lens surface a on the deflecting means side and the lens surface b on the scanned surface side of the single lens are R1 and R2 (mm), respectively. ), The maximum effective image height in the main scanning direction from the optical axis of the single lens is Ymax (mm), and the aspherical amounts from the paraxial radius of curvature at the image height Ymax of the lens surfaces a and b are S1 and S2 ( mm) in the main scanning section of the second optical element The kick focal length ft (mm), when the distance from the lens surface of the scanned surface side of the optical element of the second up to the surface to be scanned and Sk (mm) R2 <0 ( mm) <R1 (R1 2 -Ymax ² ) ^1/2 -R1 <S1 <0 (mm) (R2 ² -Ymax ² ) ^1/2 + R2 <S2 <0 (mm) -0.2 <1-Sk / ft <0.2 A scanning optical device satisfying at least one of the conditions. 2. The scanning optical device according to claim 1, wherein the second optical element satisfies a condition of | S1−S2 | ≦ 0.5 (mm). 3. The lens thickness on the optical axis of the second optical element is d (mm), and the distance from the deflection surface of the deflection means to the lens surface a of the second optical element is L (mm). The scanning optical device according to claim 1, wherein the condition of 0.2 <d / L <0.4 is satisfied. 4. The scanning optical device according to claim 1, wherein the light source means is a multi-beam light source that emits a plurality of light beams. 5. The scanning optical device according to claim 1, wherein the second optical element has at least one surface asymmetric with respect to the optical axis in the main scanning section. 6. The second optical element has an axis of symmetry in the main scanning direction, and the axis of symmetry is tilted and / or shifted with respect to a perpendicular bisector of the surface to be scanned. The scanning optical device according to claim 1. 7. The scanning according to claim 1, wherein, in the sub-scanning cross section, the radius of curvature of both lens surfaces of the second optical element continuously changes as the distance from the optical axis in the main scanning direction increases. Optical device. 8. A first optical element for converting a light beam emitted from a light source means into a weakly divergent light beam, a deflecting means for deflecting and scanning the light beam, and an image of the deflected light beam in the form of a spot on a surface to be scanned. And a second optical element, wherein the second optical element comprises a single lens, both lens surfaces in a main scanning section of the single lens both have an aspherical shape, and at least one of them Has a toric surface shape, and the paraxial curvature radii (mm) in the main scanning cross section of the lens surface a on the deflecting means side and the lens surface b on the scanned surface side of the single lens are R1 and R2 (mm), respectively. ), The maximum effective image height in the main scanning direction from the optical axis of the single lens is Ymax (mm), and the aspherical amounts from the paraxial curvature radius at the image height Ymax of the lens surfaces a and b are S1 and S2 ( mm) in the main scanning section of the second optical element The kick focal length ft (mm), when the distance from the lens surface of the scanned surface side of the optical element of the second up to the surface to be scanned and Sk (mm) R2 <0 ( mm) <R1 (R1 2 -Ymax ² ) ^1/2 -R1 <S1 <0 (mm) (R2 ² -Ymax ² ) ^1/2 + R2 <S2 <0 (mm) -0.2 <1-Sk / ft <0 A scanning optical device characterized by satisfying at least one of these conditions. 9. The scanning optical device according to claim 8, wherein the second optical element satisfies a condition of | S1−S2 | ≦ 0.5 (mm). 10. The lens thickness on the optical axis of the second optical element is d (mm), and the distance from the deflection surface of the deflection means to the lens surface a of the second optical element is L (mm). 9. The scanning optical device according to claim 8, wherein the condition: 0.2 <d / L <0.4 is satisfied. 11. The scanning optical device according to claim 8, wherein the light source means is a multi-beam light source that emits a plurality of light beams. 12. The scanning optical device according to claim 8, wherein the second optical element has at least one surface asymmetric with respect to the optical axis in the main scanning section. 13. The second optical element has an axis of symmetry in the main scanning direction, and the axis of symmetry is tilted and / or shifted with respect to a perpendicular bisector of the surface to be scanned. The scanning optical device according to claim 8. 14. The scanning according to claim 8, wherein the curvature radius of both lens surfaces of the second optical element continuously changes with distance from the optical axis in the main scanning direction in the sub-scanning cross section. Optical device. 15. A first optical element for converting a light beam emitted from a light source means into a weakly convergent light beam, a deflecting means for deflecting and scanning the light beam, and an image of the deflected light beam in the form of a spot on a surface to be scanned. And a second optical element, wherein the second optical element comprises a single lens, both lens surfaces in a main scanning section of the single lens both have an aspherical shape, and at least one of them Has a toric surface shape, and the paraxial curvature radii (mm) in the main scanning cross section of the lens surface a on the deflecting means side and the lens surface b on the scanned surface side of the single lens are R1 and R2 (mm), respectively. ), The maximum effective image height in the main scanning direction from the optical axis of the single lens is Ymax (mm), and the aspherical amounts from the paraxial radius of curvature at the image height Ymax of the lens surfaces a and b are S1 and S2 ( mm) in the main scanning section of the second optical element The definitive focal length ft (mm), when the distance from the lens surface of the scanned surface side of the optical element of the second up to the surface to be scanned and Sk (mm) R2 <0 ( mm) <R1 (R1 2 -Ymax ² ) ^1/2 -R1 <S1 <0 (mm) (R2 ² -Ymax ² ) ^1/2 + R2 <S2 <0 (mm) 0 <1-Sk / ft <0.2 A scanning optical device characterized by satisfying one or more conditions. 16. The scanning optical device according to claim 15, wherein the second optical element satisfies a condition of | S1−S2 | ≦ 0.5 (mm). 17. The lens thickness on the optical axis of the second optical element is d (mm), and the distance from the deflection surface of the deflection means to the lens surface a of the second optical element is L (mm). 16. The scanning optical device according to claim 15, wherein the condition: 0.2 <d / L <0.4 is satisfied. 18. The scanning optical device according to claim 15, wherein the light source means is a multi-beam light source that emits a plurality of light beams. 19. The scanning optical device according to claim 15, wherein the second optical element has at least one surface asymmetric with respect to the optical axis in the main scanning section. 20. The second optical element has an axis of symmetry in the main scanning direction, and the axis of symmetry is tilted and / or shifted with respect to a perpendicular bisector of the surface to be scanned. The scanning optical device according to claim 15. 21. The scanning according to claim 15, wherein the radius of curvature of both lens surfaces of the second optical element continuously changes as the distance from the optical axis in the main scanning direction increases in the sub-scanning cross section. Optical device. 22. The scanning optical device according to claim 1, a photosensitive member arranged on the surface to be scanned, and a light beam scanned by the scanning optical device onto the photosensitive member. It has a developing device for developing the formed electrostatic latent image as a toner image, a transfer device for transferring the developed toner image to a transfer material, and a fixing device for fixing the transferred toner image on the transfer material. An image forming apparatus characterized by the above. 23. A scanning optical device according to claim 1, and a printer controller for converting code data input from an external device into an image signal and inputting the image signal into the scanning optical device. An image forming apparatus characterized by the above.