JP2877390B2 - Fθ lens system in optical scanning device - Google Patents

Description

The present invention relates to an fθ lens system in an optical scanning device.

2. Description of the Related Art An optical scanning device is known as a device for writing and reading information by scanning a light beam, and is used for a laser printer, a facsimile, and the like.

Among such optical scanning devices, `` a substantially parallel light beam from the light source device is formed into a long line image in the main scanning corresponding direction, and a rotating polygon mirror having a reflecting surface near the image forming position of the image is used. There is an apparatus of the type "deflecting the light beam at a uniform angular velocity, forming an image of the deflected light beam in a spot shape on the surface to be scanned by an imaging lens system, and optically scanning the surface to be scanned".

An optical scanning device using a rotary polygon mirror has a problem of so-called tilting, and a light beam to be deflected has a constant angular velocity of the rotary polygon mirror. Is not performed at a constant speed. Therefore, a device for scanning at a constant speed is required. The fθ lens system is a lens system that optically realizes constant-speed scanning of the surface to be scanned. The image height of a light beam incident at an angle of θ with respect to the lens optical axis is represented by fθ where f is the focal length. Fθ
Has functions.

In order to solve the problem of surface tilt, the lens system provided between the rotating polygon mirror and the surface to be scanned is an anamorphic system, and the reflection position of the rotating polygon mirror and the surface to be scanned are conjugated in the sub-scanning direction. There are known ways to connect to relationships.

[Problems to be Solved by the Invention] There are various known fθ lens systems that use an anamorphic lens to perform constant-speed scanning and solve the problem of surface tilt. For example, Japanese Patent Laid-Open Publication No. 63-19617 discloses a two-sheet configuration.

However, the fθ lens system does not always sufficiently correct the curvature of field, and the diameter of the image spot on the surface to be scanned fluctuates considerably depending on the scanning position, so that it is difficult to realize high-density optical scanning. Japanese Patent Application Laid-Open No. 61-120112 discloses a two-element fθ lens system using a so-called saddle-shaped toric surface in order to satisfactorily correct field curvature, but this fθ lens system has an aspherical surface. There is a problem that processing is difficult because two surfaces are used.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a novel fθ that enables sufficient correction of curvature of field in the main and sub-scanning directions and a solution to the problem of surface tilt in a rotating polygon mirror. The purpose is to provide a lens system.

[Means for Solving the Problems] Hereinafter, the present invention will be described.

The fθ lens system according to any one of claims 1 to 6, "forms a substantially parallel light flux from the light source device into a long line image in the main scanning corresponding direction, and has a reflecting surface near the image forming position of the line image. An optical scanning device that deflects the light beam at a constant angular velocity by a rotating polygon mirror, forms an image of the deflected light beam in a spot shape on the surface to be scanned by an imaging lens system, and optically scans the surface to be scanned at a substantially constant speed. , An imaging lens system for forming an image of a light beam deflected by the rotating polygonal mirror on the surface to be scanned '', wherein in the sub-scanning direction, the reflection position of the rotating polygonal mirror and the surface to be scanned are geometrically , And a “fθ function with respect to the main scanning direction”.

Each of these six types of fθ lens systems is composed of a first lens group and a second lens element arranged in the first and second order from the rotating polygon mirror toward the surface to be scanned.
It is a two-element configuration, and when each lens surface is defined as the first to fourth surfaces counted from the rotating polygon mirror, a “barrel-shaped toric surface” is used among these four lens surfaces. In common.

Considering a light beam that is ideally deflected by the rotating polygon mirror, a virtual plane formed by sweeping the principal ray of the ideal deflected light beam is referred to as a “deflection surface”. A plane parallel to the optical axis of the imaging optical system, that is, the fθ lens, and orthogonal to the deflecting surface is referred to as a “deflection orthogonal surface”.

In the fθ lens system according to the first aspect, the shape of each lens surface in the deflecting surface is an arc, a straight line, a straight line, and an arc sequentially from the first surface to the fourth surface, and in a plane parallel to the deflecting surface. The first lens has a negative refractive power and the second lens has a positive refractive power.

The first surface is a concave barrel-shaped toric surface in which the radius of curvature in the plane orthogonal to the deflection becomes smaller as the distance from the optical axis increases, the second surface is a convex cylinder surface or plane having a refractive power only in the plane orthogonal to the deflection, and the third surface. The surface is a concave cylinder surface having a refracting power only in the deflecting direct light surface, and the fourth surface is a convex toric surface having a strong curvature in the deflecting orthogonal surface.

The composite focal length in the orthogonal plane of deflection is f _s , and the radius of curvature of the third surface in the orthogonal plane including the optical axis is r ′.
When they are ₃ , these satisfy the condition of 1.6 ≦ | r ′ ₃ / f _S | ≦ 3.1 (1-I).

In the fθ lens system according to the second aspect, the shape of each lens surface in the deflecting surface is an arc, a straight line, a straight line, and an arc sequentially from the first surface to the fourth surface, and in a plane parallel to the deflecting surface. The first lens has a negative refractive power and the second lens has a positive refractive power.

The first surface is a concave barrel-shaped toric surface in which the radius of curvature in the plane orthogonal to the deflection becomes smaller as the distance from the optical axis increases, the second surface is a concave cylinder surface having a refractive power only in the plane orthogonal to the deflection, and the third surface.
The surface is a concave cylinder surface that has refractive power only in the plane orthogonal to the deflection,
The fourth surface is a convex toric surface having a strong curvature in a plane orthogonal to the deflection.

The combined focal length in the orthogonal plane of deflection is f _S , and the radii of curvature of the first, second, third, and fourth surfaces in the orthogonal plane including the optical axis are r ′ ₁ , r ′ ₂ , r ′, respectively. ₃ , r ′ ₄ , they are <2.0 <| [{(1 / r ′ ₁ ) − (1 / r ′ ₂ )} − ｛(1 / r ′ ₃ ) − (1 / r ′ ₄ ) ｝] · F _S | <9.8 (2-I)

In the fθ lens system according to the third aspect, the shape of each lens surface in the deflecting surface is an arc, an arc, a straight line, and an arc sequentially from the first surface to the fourth surface, and in a plane parallel to the deflecting surface. The first lens has a positive or negative refractive power, and the second lens has a positive refractive power.

The first surface is a spherical surface, the second surface is a convex barrel-shaped toric surface in which the radius of curvature in the plane orthogonal to the deflection decreases as the distance from the optical axis increases, and the third surface is a concave cylinder having a refractive power only in the plane orthogonal to the deflection. The fourth surface is a convex toric surface having a strong curvature in a plane orthogonal to the deflection.

When the composite focal length in the orthogonal plane of deflection is f _S , and the radii of curvature of the second and fourth surfaces in the orthogonal plane including the optical axis are r ′ ₂ and r ′ ₄ , these are 0.3 <| r ′ ₂ / r ′ ₄ | <1.0 (3-I) 0.03 <| r ′ ₂ / f _S | <0.54 (3-II)

In the fθ lens system according to the fourth aspect, the shape of each lens surface in the deflecting surface is a straight line, an arc, a straight line, and an arc sequentially from the first surface to the fourth surface, and in a plane parallel to the deflecting surface. The first lens has a negative refractive power and the second lens has a positive refractive power.

The first surface is a plane, the second surface is a concave barrel-shaped toric surface in which the radius of curvature in the plane orthogonal to the deflection becomes smaller as the distance from the optical axis increases, and the third surface is a concave cylinder having a refractive power only in the plane orthogonal to the deflection. The fourth surface is a convex toric surface having a strong curvature in a plane orthogonal to the deflection.

When the combined focal length and lateral magnification in the deflection orthogonal plane are f _S and β, respectively, and the radii of curvature of the third and fourth surfaces in the deflection orthogonal plane including the optical axis are r ₃ and r ₄ , respectively. these are _{0.4 <| {(1 / r} '3) - (1 / r' 4)} · f S · β | <
2.0 (4-I) The following condition is satisfied.

In the fθ lens system according to claim 5, the shape of each lens surface in the deflecting surface is an arc, an arc, a straight line, and an arc sequentially from the first surface to the fourth surface, and in a plane parallel to the deflecting surface. The first lens has a negative refractive power and the second lens has a positive refractive power.

The first surface is a concave barrel-shaped toric surface in which the radius of curvature in the plane perpendicular to the deflection becomes smaller as the distance from the optical axis is increased, the second surface is a convex spherical surface, and the third surface is a concave surface having a refractive power only in the plane orthogonal to the deflection. And the fourth surface are convex toric surfaces having a strong curvature in the plane orthogonal to the deflection.

When the combined focal length and lateral magnification in the deflection orthogonal plane are f _S and β, respectively, and the radii of curvature of the third and fourth surfaces in the deflection orthogonal plane including the optical axis are r ₃ and r ₄ , respectively. And 0.1 <| {(1 / r ′ ₃ ) − (1 / r ′ ₄ )} · f _S · β | <
5.4 (5-I) The following condition is satisfied.

In the fθ lens system according to claim 6, the shape of each lens surface in the deflecting surface is a straight line, an arc, an arc, and an arc sequentially from the first surface to the fourth surface, and in a plane parallel to the deflecting surface. First,
The second lens also has a positive refractive power.

The first surface is a concave cylinder surface having a refractive power only in the plane orthogonal to the deflection, the second surface is a convex barrel-shaped toric surface in which the radius of the curve in the plane orthogonal to the deflection becomes smaller as the distance from the optical axis increases,
The surface is a concave spherical surface, and the fourth surface is a convex toric surface having a strong curvature in a plane orthogonal to the deflection.

Assuming that the combined focal length in the plane orthogonal to the deflection is f _s , and the radii of curvature of the first and second surfaces in the plane orthogonal to the deflection including the optical axis are r ′ ₁ and r ′ ₂ , these are 0.5 <| {(1 / r ′ ₁ ) − (1 / r ′ ₂ )} · f _S | <3.8
(6-I) The following condition is satisfied.

Here, each lens surface of the fθ lens system of the present invention will be described with reference to FIG.

FIGS. 1 (I) to 1 (VI) show fθ in the claims 1 to 6 in sequence.
3 shows the shape of a lens system.

In each figure of FIG. 1, the left side of the figure is the side of the rotating polygonal mirror, and the right side is the side of the surface to be scanned. Therefore, the left lens represents the first lens and the right side represents the second lens. The lens surfaces are first to fourth surfaces sequentially from left to right.

In addition, the upper part of each drawing in FIG. 1 shows the lens shape in the deflection plane of the fθ lens system, and the lower part shows the lens shape in the deflection orthogonal plane including the optical axis. Since the crossing line of the deflection surface with the scanned surface corresponds to the ideal main scanning direction,
The upper part of FIG. 1 is labeled "main". At the same time, since the plane orthogonal to the deflection corresponds to the sub-scanning direction, the lower drawing of FIG. 1 is indicated as "sub".

FIG. 1 also shows whether the function of each lens in the deflecting plane / deflection orthogonal plane is “convex” or “concave”.

In the lower view of FIG. 1 (I), the second lens of the first lens
The surface is represented by a straight line (solid line) and an arc (dashed line). In the fθ lens system according to the first aspect, the second surface has a refractive power only within a “plane” or “deflection orthogonal plane”. With a convex cylinder surface ".

[Operation] Hereinafter, conditions regarding the fθ lens system of each claim will be described.

When the fθ lens thread is configured with the lens surface configuration as described in claims 1 to 6, excellent correction of the field curvature in the main and sub scanning directions can be performed.

In a state in which “surface tilt” is corrected and the field curvature in the main scanning direction is satisfactorily corrected, in order to satisfactorily correct the field curvature in the sub-scanning direction, each fθ lens system is also required. The conditions described in section must be satisfied.

The condition (1-I) for the fθ lens system according to claim 1 is a condition for satisfactorily correcting field curvature in the sub-scanning direction while correcting field tilt and satisfactorily correcting field curvature in the main scanning direction. When the lower limit of the condition (1-I) is exceeded, the curvature of field in the sub-scanning direction is significantly generated on the over side, and when the upper limit is exceeded, it is significantly generated on the under side.

In the fθ lens system according to claim 2, the condition (2-I) is satisfied.
If the lower limit is exceeded, the curvature of field in the sub-scanning direction will significantly occur on the over side, and if it exceeds the upper limit, it will significantly occur on the under side.

In the fθ lens system according to claim 3, the condition (3-I) is satisfied.
If the lower limit is exceeded, the curvature of field in the sub-scanning direction will significantly occur on the over side, and if it exceeds the upper limit, it will significantly occur on the under side. When the value exceeds the upper limit of the condition (3-II), the curvature of field in the sub-scanning direction is remarkably generated on the over side, and when the value exceeds the lower limit, the image surface is remarkably generated on the under side.

In the fθ lens system according to claim 4, the condition (4-1) is satisfied.
If the lower limit is exceeded, the curvature of field in the sub-scanning direction will significantly occur on the over side, and if it exceeds the upper limit, it will significantly occur on the under side.

In the fθ lens system according to claim 5, the condition (5-I) is satisfied.
If the lower limit is exceeded, the curvature of field in the sub-scanning direction will significantly occur on the over side, and if it exceeds the upper limit, it will significantly occur on the under side.

In the fθ lens system according to claim 6, the condition (6-I) is satisfied.
If the lower limit is exceeded, the field curvature in the sub-scanning direction occurs on the under side, and if the upper limit is exceeded, it occurs on the over side.

Therefore, in each of the fθ lens systems according to the first to sixth aspects, if the respective conditions are not satisfied, the imaging performance is degraded, the fluctuation of the light spot diameter in the sub-scanning direction becomes large, and it becomes difficult to realize good optical scanning. Conversely, when each of the above conditions is satisfied, the "field curvature correction function in the sub-scanning direction" of the barrel-shaped toric surface is favorably exhibited.

Next, referring to FIG. 2, this figure schematically illustrates an example of an optical scanning apparatus using an fθ lens system. FIG. 3 shows the optical arrangement of FIG. 2 as viewed from the sub-scanning direction, that is, the state in the deflection plane.

In FIG. 2, a parallel light beam from a light source device 1 comprising a light source and a light source and a light condensing device is brought into the vicinity of a reflecting surface 3a of a rotary polygon mirror 3 by a cylinder lens 2 which is a line image forming optical system. An image is formed as a line image LI substantially parallel to the deflection surface. This line image
The longitudinal direction of the LI is a main scanning corresponding direction.

The light beam reflected by the rotating polygon mirror 3 is formed into an image of a spot on the surface 6 to be scanned by the fθ lens system. Scan.

The fθ lens system includes a first lens 4 and a second lens 5, and the lens 4 is located on the side of the rotary polygon mirror 3 and the lens 5.
Are disposed on the side of the surface 6 to be scanned. As viewed in the deflection plane, as shown in FIG. 3, the fθ lens system including the lenses 4 and 5 links the infinity on the light source device 1 side and the position of the surface 6 to be scanned to a geometric conjugate relationship.

On the other hand, when viewed in the plane orthogonal to the deflection, that is, in the sub-scanning direction, the fθ lens system links the reflection position of the rotary polygon mirror 3 and the surface 6 to be scanned to a substantially optically conjugate relationship. Therefore, even if the reflection surface 3a is tilted as shown by the reference numeral 3a 'as shown in FIG. 4, the image position on the surface 6 to be scanned by the fθ lens system is in the sub-scanning direction (vertical direction in FIG. 4). Hardly move to). Therefore, the tilting is corrected.

When the rotary polygon mirror 3 rotates, the reflecting surface 3a rotates about the axis 3A. Therefore, as shown in FIG. 5, the reflecting surface 3a rotates between the imaging position P of the line image and the reflecting surface 3a. A position shift ΔX occurs, and the position P ′ of the conjugate image of the line image by the fθ lens system is shifted from the scanned surface 6 by ΔX ′. This deviation amount Δ
X ′ is β, where x is the lateral magnification of the fθ lens system in the sub-scanning direction.
As is well known, it is given by ΔX ′ = β ² × ΔX.

FIGS. 6 and 7 show the relationship between θ and ΔX when the angle between the lens optical axis of the fθ lens system and the principal ray of the deflected light beam is θ in the deflection plane. FIG. 6 depicts the specific incident angle α (see FIG. 8) as 90 degrees and the radius R ′ of the inscribed circle of the rotating polygon mirror 3 as a parameter. In FIG. 7, the radius R 'of the inscribed circle is 40 mm, and the specific incident angle α
Is drawn as a parameter. As can be seen from FIGS. 6 and 7, ΔX increases as the radius of the inscribed circle R ′ increases and as the specific incident angle α decreases.

Further, the relative displacement between the position of the line image and the reflecting surface due to the rotation of the reflecting surface occurs two-dimensionally in the deflecting surface and moves asymmetrically with respect to the lens optical axis. Therefore, in the optical scanning device as shown in FIG. 2, it is necessary to satisfactorily correct the field curvature of the fθ lens system in the main and sub scanning directions. Needless to say, the fθ characteristic must be corrected well in the main scanning direction.

Here, the specific incident angle α will be described. In FIG. 8, a symbol a indicates a principal ray of a light beam incident on the rotating polygon mirror, and a symbol b indicates an optical axis of the fθ lens system. The specific incident angle α is defined as the intersection angle between the principal ray a and the optical axis b as shown in the figure.

The X and Y axes are determined as shown in the figure, using the position of the intersection of the principal ray a and the optical axis b as the origin, and the coordinates of the rotational axis position of the rotary polygon mirror 3 are represented by X _C ,
Y _C.

As is well known, R is defined as the radius of the circumscribed circle of the rotary polygon mirror, as described above, in order to minimize the variation in ΔX of the positional shift amount between the line image position and the reflecting surface, as described above. 0 <Xc <Rcos (α / 2) The condition of <Yp <Rsin (α / 2) may be imposed on X _C and Y _C.

In order to prevent the principal ray a of the incident light beam from existing outside the effective main scanning area and prevent the return light from the scanned surface 6 from re-entering the main scanning area on the scanned surface as ghost light, a rotating polygon mirror must be used. Assuming that the number of surfaces is N and the deflection angle is θ, a condition of θ <α <(4π / N) −θ may be imposed on α.

Next, the barrel-shaped toric surface forming one end of the features of the present invention will be described.

As is well known, a toric surface is a surface obtained by rotating an arc around a "straight line that is within a plane containing the arc and does not pass through the center of curvature of the arc".

Curve passing through AVB Referring to FIG. 9 is a circular arc whose center of curvature position C _1. When this arc is rotated around a straight line X ₁ Y ₁ which is in the same plane as the arc, a barrel-shaped curved surface TT as shown in FIG. 10 is obtained. This surface TT is a barrel-shaped toric surface. When this surface TT is used as a lens surface, it can be used as a convex surface or as a concave surface.

In the fθ lens systems of claims 1, 2, and 5, a barrel-shaped toric surface having a concave surface on the first surface is used. In the fθ lens systems of claims 3 and 6, a barrel-shaped toric surface having a convex surface on the second surface is provided. In the fθ lens system No. 4, a concave barrel-shaped toric surface is used for the second surface.

Incidentally, in the "convex toric surface" commonly used for the fourth surface in the fθ lens system of claims 1 to 6, the arc is in the plane orthogonal to the deflection including the optical axis, and the rotation axis is in this plane. Are parallel to the sub-scanning direction and are on the opposite side of the arc with respect to the center of curvature of the arc. Therefore, this toric surface has a strong curvature in a plane orthogonal to the deflection including the optical axis.

Looking at the radius of curvature of the barrel-shaped toric surface in X ₁ Y ₁ perpendicular to the axis to the plane, which is smaller with increasing distance to C ₂ points in the axial direction, the radius of curvature the axis X ₁ Y ₁ and arc AVB Equal to the distance to

In the present invention, the direction of the axis X ₁ Y ₁ is made parallel to the main scanning direction within the deflection plane in the “barrel-shaped toric surface” in each of the fθ lens systems of the respective claims.

[Examples] Specific examples will be given below for the fθ lens system of each claim.

In each embodiment, f _M represents a combined focal length in the main scanning direction of the fθ lens system, that is, a combined focal length in a plane parallel to the deflection surface, and this value is normalized to 100.

The f _S represents the composite focal length of the synthesis focal distance or the sub-scanning direction in the deflection plane perpendicular. 2θ is the deflection angle (unit:
Degree), α is a specific incident angle (unit: degree), and β is a lateral magnification in a plane orthogonal to the deflection.

r _ix is the radius of curvature in the deflection plane of the i-th lens surface counted from the side of the rotating polygon mirror, that is, the radius of curvature of the lens surface shape appearing in the figures indicated as “main” in each of FIGS. _iY is the radius of curvature of the i-th lens surface in the plane orthogonal to the deflection, that is, the radius of curvature of the lens surface shape that appears in the figure indicated as “sub” in each of FIGS. ₁ and ₂ , and r _1Y , r _2Y , r _3Y and r _4Y have been described as r ′ ₁ , r ′ ₂ , r ′ ₃ and r ′ ₄ in relation to the above-mentioned conditions.

Regarding the barrel-shaped toric surface, r _ix represents the distance between VC _{1 and riY} represents the distance between VC _{2 in} FIG. d _i is the i-th lens surface distance, d _o is the distance from the reflecting surface of the rotary polygon mirror to the first lens surface, n _j denotes the refractive index of the j-th lens.

Embodiments 1 to 9 are embodiments relating to the fθ lens system of claim 1.

Embodiments 10 to 18 are embodiments relating to the fθ lens system of claim 2.

Embodiments 19 to 21 are embodiments relating to the fθ lens system of claim 3.

Embodiments 22 to 24 are embodiments relating to the fθ lens system of claim 4.

Embodiments 25 to 34 are embodiments relating to the fθ lens system of claim 5.

Embodiments 35 to 38 are embodiments relating to the fθ lens system of claim 6.

In each of the first to ninth embodiments relating to the fθ lens system of the _{first aspect} , K ₁ represents a parameter under the above-mentioned condition (1-I), that is, | r ′ ₃ / f _S |.

Example _{1 f M = 100, f S} = 16.59, β = -2.009, α = 54,2θ = 63.1 K 1 = 3.073, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -3.607 5.531 1 1.71221 2 ∞ ∞ 10.943 3 ∞ -50.986 6.373 2 1.67500 4 -46.055 -10.39 FIG. 11 shows aberration diagrams and fθ characteristics of the first embodiment.

In each of the following embodiments, the curvature of field is associated with the rotation of the rotary polygon mirror, and the broken lines indicate the main scanning direction and the solid lines indicate the sub-scanning direction.

In the fθ characteristic, the ideal image height is f _M · θ, and the actual image height is h _M
When a is defined by _{(h-f M · θ)} · 100 / (f M · θ).

Example _{2 f M = 100, f S} = 21.695, β = -3.963, α = 54,2θ = 63.1 K 1 = 1.868, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -96.199 5.531 1 1.71221 2 ∞ ∞ 10.943 3 ∞ -40.524 6.373 2 1.67500 4 -46.055 -11.295 FIG. 12 shows aberration diagrams and fθ characteristics of the second embodiment.

Example _{3 f M = 100, f S} = 21.634, β = -3.565, α = 54,2θ = 63.1 K 1 = 2.023, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -24.05 5.531 1 1.71221 2 ∞ ∞ 10.943 3 ∞ -43.771 6.373 2 1.67500 4 -46.055 -11.159 FIG. 13 shows aberration diagrams and fθ characteristics of the third embodiment.

Example _{4 f M = 100, f S} = 20.345, β = -3.682, α = 54,2θ = 63.4 K 1 = 1.814, d o = 5.411 i r ix r iY d i j n i 1 -113.348 -18.037 4.209 1 1.71221 2 ∞ ∞ 13.468 3 ∞ −36.916 6.012 2 1.67500 4 −46.536 -10.702 FIG. 14 shows aberration diagrams and fθ characteristics of Example 4.

Example _{5 f M = 100, f S} = 17.419, β = -2.788, α = 54,2θ = 63.4 K 1 = 2.209, d o = 5.411 i r ix r iY d i j n i 1 -113.348 -5.531 4.209 1 1.71221 2 ∞ ∞ 13.468 3 ∞ −38.48 6.012 2 1.67500 4 −46.536 -10.434 FIG. 15 shows aberration diagrams and fθ characteristics of the fifth embodiment.

Example _{6 f M = 100, f S} = 20.967, β = -4.088, α = 54,2θ = 63.4 K 1 = 1.703, d o = 5.411 i r ix r iY d i j n i 1 -113.348 -60.125 4.209 1 1.71221 2 ∞ ∞ 13.468 3 ∞ −35.714 6.012 2 1.67500 4 −46.536 -10.798 FIG. 16 shows aberration diagrams and fθ characteristics of the sixth embodiment.

Example _{7 f M = 100, f S} = 21.051, β = -4.175, α = 54,2θ = 63.4 K 1 = 1.679, d o = 5.411 i r ix r iY d i j n i 1 -113.348 -108.224 4.209 1 1.71221 2 ∞ ∞ 13.468 3 ∞ −35.353 6.012 2 1.67500 4 −46.536 −10.811 FIG. 17 shows aberration diagrams and fθ characteristics of the seventh embodiment.

Example _{8 f M = 100, f S} = 19.704, β = -2.71, α = 54,2θ = 63.1 K 1 = 2.374, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -6.012 5.531 1 1.71221 2 ∞ -51.707 10.943 3 ∞ -46.777 6.373 2 1.67500 4 -46.055 -10.841 FIG. 18 shows aberration diagrams and fθ characteristics of the eighth embodiment.

Example _{9 f M = 100, f S} = 18.803, β = -3.69, α = 54,2θ = 63.4 K 1 = 1.746, d o = 5.416 i r ix r iY d i j n i 1 -113.348 -5.531 4.209 1 1.71221 2 ∞ -12.025 13.468 3 ∞ -44.853 6.012 2 1.67500 4 -46.536 -10.841 FIG. 19 shows aberration diagrams and fθ characteristics of the ninth embodiment.

In each of the embodiments, the aberration is good, and particularly, the field curvature is mainly
It is corrected very well in the sub-scanning direction. Fθ
The characteristics are also good.

The following Examples 10 to 18 relating to the fθ lens system according to claim 2
Parameters in the condition (2-I) with a K ₁ is at the _{| [{(1 / r '} 1) - (1 / r' 2)} - {(1 /
r ′ ₃ ) − (1-r ′ ₄ )}] · f _S |.

Example _{10 f M = 100, f S} = 20.938, β = -3.757, α = 54,2θ = 63.4 K 1 = 2.27, d o = 5.411 i r ix r iY d i j n i 1 -113.349 -48.100 4.209 1 1.71221 2 ∞ 48.100 13.468 3 ∞ −37.277 6.012 2 1.67500 4 −46.536 -10.670 FIG. 20 shows aberration diagrams and fθ characteristics of Example 10.

Example _{11 f M = 100, f S} = 17.314, β = -2.700, α = 54,2θ = 63.4 K 1 = 4.47, d o = 5.411 i r ix r iY d i j n i 1 -113.349 -6.012 4.209 1 1.71221 2 ∞ 48.100 13.468 3 ∞ −39.081 6.012 2 1.67500 4 −46.536 -10.364 FIG. 21 shows aberration diagrams and fθ characteristics of Example 11.

Example _{12 f M = 100, f S} = 19.113, β = -2.589, α = 54,2θ = 63.4 K 1 = 4.47, d o = 5.411 i r ix r iY d i j n i 1 -113.349 -120.249 4.209 1 1.71221 2 ∞ 6.012 13.468 3 ∞ −40.283 6.012 2 1.67500 4 −46.536 −10.036 FIG. 22 shows aberration diagrams and fθ characteristics of Example 12.

Example _{13 f M = 100, f S} = 17.280, β = -1.972, α = 54,2θ = 63.1 K 1 = 5.72, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -6.012 5.531 1 1.71221 2 ∞ 12.025 10.943 3 ∞ −56.156 6.373 2 1.67500 4 −46.055 −10.087 FIG. 23 shows aberration diagrams and fθ characteristics of the thirteenth embodiment.

Example _{14 f M = 100, f S} = 20.452, β = -2.258, α = 54,2θ = 63.1 K 1 = 5.53, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -48.100 5.531 1 1.71221 2 ∞ 6.012 10.943 3 ∞ −58.201 6.373 2 1.67500 4 −46.055 −9.957 FIG. 24 shows aberration diagrams and fθ characteristics of Example 14.

Example _{15 f M = 100, f S} = 18.502, β = -1.824, α = 54,2θ = 63.0 K 1 = 6.91, d o = 7.816 i r ix r iY d i j n i 1 -112.654 -120.249 5.531 1 1.71221 2 ∞ 3.607 10.943 3 ∞ −57.118 6.373 2 1.67500 4 −46.055 −9.498 FIG. 25 shows aberration diagrams and fθ characteristics of Example 15.

Example _{16 f M = 100, f S} = 13.341, β = -1.210, α = 54,2θ = 63.0 K 1 = 7.34, d o = 9.019 i r ix r iY d i j n i 1 -109.496 -6.012 6.133 1 1.71221 2 ∞ 3.487 9.740 3 ∞ −78.162 6.373 2 1.67500 4 −45.454 −9.084 FIG. 26 shows aberration diagrams and fθ characteristics of Example 16.

Example _{17 f M = 100, f S} = 17.411, β = -1.507, α = 54,2θ = 63.0 K 1 = 8.09, d o = 9.019 i r ix r iY d i j n i 1 -109.496 -48.100 6.133 1 1.71221 2 ∞ 2.886 9.740 3 ∞ −79.605 6.373 2 1.67500 4 −45.454 −9.082 FIG. 27 shows aberration diagrams and fθ characteristics of Example 17.

Example _{18 f M = 100, f S} = 17.712, β = -1.519, α = 54,2θ = 63.0 K 1 = 9.61, d o = 9.019 i r ix r iY d i j n i 1 -109.496 -120.249 6.133 1 1.71221 2 ∞ 2.766 9.740 3 ∞ −79.485 6.373 2 1.67500 4 −45.454 −9.059 FIG. 28 shows aberration diagrams and fθ characteristics of Example 18.

In all of the tenth to eighteenth embodiments, the aberration is good, and particularly, the curvature of field is corrected very well in both the main and sub-scanning directions. Also, the fθ characteristics are good.

The following Examples 19 to 21 relating to the fθ lens system according to claim 3
In the above condition (3-I) with K ₁
and orchestra to the conditions (3-II) with a _{K 2 | | r '2 /} r' 4 r '2 /
f _S |

Example _{19 f M = 100, f S} = 17.991, β = -6.127, α = 54,2θ = 63.1 K 1 = 0.752, K 2 = 0.421, d o = 7.816 i r ix r iY d i j n i 1 −16.358 −16.358 3.968 1 1.71221 2 −18.398 −7.582 14.67 3 ∞−14.069 4.329 2 1.67500 4 −69.023 −10.087 FIG. 29 shows aberration diagrams and fθ characteristics of the nineteenth embodiment.

Example _{20 f M = 100, f S} = 151.4, β = -4.846, α = 54,2θ = 63.7 K 1 = 0.339, K 2 = 0.0349, d o = 3.607 i r ix r iY d i j n i 1 −30.303 −30.303 3.487 1 1.71221 2 −29.341 −5.291 38.961 3 ∞ −33.61 6.012 2 1.67500 4 −80.794 −15.619 FIG. 30 shows aberration diagrams and fθ characteristics of Example 20.

Example _{21 f M = 100, f S} = 21.774, β = -4.478, α = 54,2θ = 63.1 K 1 = 0.98, K 2 = 0.53, d o = 12.025 i r ix r iY d i j n i 1 −25.373 −25.373 6.133 1 1.71221 2 −27.778 −11.424 11.905 3 −1−15.067 6.012 2 1.67500 4 −75.242 −11.671 FIG. 31 shows aberration diagrams and fθ characteristics of Example 21.

In all of Examples 19 to 21, the aberration is good, and particularly, the curvature of field is corrected very well in both the main and sub scanning directions. Also, the fθ characteristics are good.

The following Examples 22 to 24 relating to the fθ lens system according to claim 4.
, K ₁ represents a parameter | {(1 / r ′ ₃ ) − (1 / r ′ ₄ )} · f _S · β | in the condition (4-I).

Example _{22 f M = 100, f S} = 17.459, β = -1.354, α = 54,2θ = 60.6 K 1 = 1.51, d o = 7.816 i r ix r iY d i j n i 1 ∞ ∞ 2.645 1 1.71221 2 190.636 -6.012 20.563 3 ∞ -61.327 4.329 2 1.67500 4 -53.391 -12.45 FIG. 32 shows aberration diagrams and fθ characteristics of Example 22.

Example _{23 f M = 100, f S} = 17.851, β = -1.936, α = 54,2θ = 60.6 K 1 = 1.94, d o = 3.607 i r ix r iY d i j n i 1 ∞ ∞ 6.012 1 1.71221 2 180.374 6.012 19.48 3 4 -48.1 9.62 2 1.67500 4 -53.212 -13.001 FIG. 33 shows aberration diagrams and fθ characteristic diagrams for Example 23.

Example _{24 f M = 100, f S} = 10.085, β = -0.603, α = 54,2θ = 63.4 K 1 = 0.41, d o = 12.025 i r ix r iY d i j n i 1 ∞ ∞ 6.012 1 1.74405 2 195.345 1.864 16.691 3 ∞ -127.464 9.62 2 1.70217 4 −55.176 -13.194 FIG. 34 shows aberration diagrams and fθ characteristic diagrams of Example 24.

In all of Examples 22 to 24, the aberration is good, and particularly, the curvature of field is corrected very well in both the main and sub scanning directions. Also, the fθ characteristics are good.

The following Examples 25 to 34 relating to the fθ lens system according to claim 5
In the above, K ₁ represents the parameter | {(1 / r ′ ₃ ) − (1 / r ′ ₄ )} · f _S · β | in the condition (5-I).

Example _{25 f M = 100, f S} = 14.385, β = -1.542, α = 54,2θ = 63.7 K 1 = 0.96, d o = 3.607 i r ix r iY d i j n i 1 -30.303 -6.012 3.487 1 1.71221 2 −29.341 −29.341 38.961 3 ∞−57.72 6.012 2 1.675 4 −80.794 −16.497 FIG. 35 shows aberration diagrams and fθ characteristics of Example 25.

Example _{26 f M = 100, f S} = 3.616, β = -0.701, α = 54,2θ = 63.7 K 1 = 0.11, d o = 3.607 i r ix r iY d i j n i 1 -30.303 -1.202 3.487 1 1.71221 2 −29.341 −29.341 38.961 3 ∞ −57.72 6.012 2 1.675 4 −80.794 -16.216 FIG. 36 shows aberration diagrams and fθ characteristics of Example 26.

Example _{27 f M = 100, f S} = 29.686, β = -1.988, α = 54,2θ = 63.7 K 1 = 2.72, d o = 3.607 i r ix r iY d i j n i 1 -30.303 -24.05 3.487 1 1.71221 2 −29.341 −29.341 38.961 3 ∞ −57.118 6.012 2 1.675 4 −80.794 −16.611 FIG. 37 shows aberration diagrams and fθ characteristics of Example 27.

Example _{28 f M = 100, f S} = 9.014, β = -0.702, α = 54,2θ = 63.1 K 1 = 0.48, d o = 12.025 i r ix r iY d i j n i 1 -25.373 -1.202 6.133 1 1.71221 2 -27.778 -27.778 11.905 3 ∞ -44.372 6.012 2 1.675 4 -75.242 -10.222 FIG. 38 shows aberration diagrams and fθ characteristics of Example 28.

Example _{29 f M = 100, f S} = 24.389, β = -2.771, α = 54,2θ = 63.1 K 1 = 3.68, d o = 12.025 i r ix r iY d i j n i 1 -25.373 -12.025 6.133 1 1.71221 2 -27.778 -27.778 11.905 3 ∞ -32.948 6.012 2 1.675 4 -75.242 -11.794 FIG. 39 shows aberration diagrams and fθ characteristics of Example 29.

Example _{30 f M = 100, f S} = 23.789, β = -3.768, α = 54,2θ = 63.1 K 1 = 4.06, d o = 12.025 i r ix r iY d i j n i 1 -25.373 -60.125 6.133 1 1.71221 2 −27.778 −27.778 11.905 3 ∞−31.265 6.012 2 1.675 4 −75.242 −12.943 FIG. 40 shows aberration diagrams and fθ characteristics of Example 30.

Example _{31 f M = 100, f S} = 23.381, β = -3.979, α = 54,2θ = 63.1 K 1 = 4.04, d o = 12.025 i r ix r iY d i j n i 1 -25.373 -144.299 6.133 1 1.71221 2 -27.778 -27.778 11.905 3 ∞ -31.145 6.012 2 1.675 4 -75.242 -13.239 FIG. 41 shows aberration diagrams and fθ characteristics of Example 31.

Example _{32 f M = 100, f S} = 21.59, β = -4.295, α = 54,2θ = 63.1 K 1 = 5.05, d o = 7.816 i r ix r iY d i j n i 1 -16.358 -24.05 3.968 1 1.71221 2 −18.398 −18.398 14.67 3 ∞−27.537 4.329 2 1.675 4 −69.023 −11.016 FIG. 42 shows aberration diagrams and fθ characteristics of Example 32.

Example _{33 f M = 100, f S} = 20.608, β = -2.861, α = 54,2θ = 63.1 K 1 = 3.67, d o = 7.816 i r ix r iY d i j n i 1 -16.358 -6.012 3.968 1 1.71221 2 −18.398 −18.398 14.67 3 ∞−27.778 4.329 2 1.675 4 −69.023 −37.95 FIG. 43 shows aberration diagrams and fθ characteristics of Example 33.

Example _{34 f M = 100, f S} = 20.676, β = -5.001, α = 54,2θ = 63.1 K 1 = 5.25, d o = 7.816 i r ix r iY d i j n i 1 -16.358 -144.299 3.968 1 1.71221 2 -18.398 -18.398 14.67 3 ∞ -27.778 4.329 2 1.675 4 -69.023 -11.528 FIG. 44 shows aberration diagrams and fθ characteristics of Example 34.

In all of the embodiments 25 to 34, the aberration is good, and particularly, the curvature of field is corrected very well in both the main and sub-scanning directions. Fθ is also good.

The following Examples 35 to 48 relating to the fθ lens system according to Claim 6.
In the formula, K ₁ represents the parameter | {(1 / r ′ ₁ ) − (1 / r ′ ₂ )} · f _S | under the above condition (6-I).

Example _{35 f M = 100, f S} = 15.34, β = -3.834, α = 54,2θ = 60 K 1 = 3.76, d o = 5.411 i r ix r iY d i j n i 1 ∞ -3.607 1.509 1 1.71221 2 −954.892 −31.264 7.245 3 −36.527 −36.527 4.637 2 1.675 4 −25.487 −7.237 FIG. 45 shows aberration diagrams and fθ characteristic diagrams relating to Example 35.

Example _{36 f M = 100, f S} = 17.016, β = -4.58, α = 54,2θ = 60.4 K 1 = 1.95, d o = 9.018 i r ix r iY d i j n i 1 ∞ -5.05 3.716 1 1.71221 2 −478.035 −12.024 0.602 3 −40.653 −40.653 8.779 2 1.675 4 −28.998 −8.37 FIG. 46 shows aberration diagrams and fθ characteristic diagrams relating to Example 36.

Example _{37 f M = 100, f S} = 14.92, β = -5.528, α = 54,2θ = 60.4 K 1 = 1.59, d o = 7.816 i r ix r iY d i j n i 1 ∞ -6.012 3.367 1 1.71221 2 −623.55 −16.654 1.202 3 −36.795 −36.795 6.012 2 1.675 4 −26.335 −7.034 FIG. 47 shows aberration diagrams and fθ characteristic diagrams relating to Example 37.

Example _{38 f M = 100, f S} = 16.104, β = -5.425, α = 54,2θ = 60.4 K 1 = 0.57, d o = 9.018 i r ix r iY d i j n i 1 ∞ -26.964 3.716 1 1.71221 2 −478.035 −601.224 0.602 3 −40.653 −40.653 8.779 2 1.675 4 −28.998 −8.329 FIG. 48 shows aberration diagrams and fθ characteristic diagrams relating to Example 38.

In all of Examples 35 to 38, the aberration is good, and particularly, the curvature of field is corrected very well in both the main and sub scanning directions. Also, the fθ characteristics are good.

[Effects of the Invention] As described above, according to the present invention, a novel fθ lens system can be provided. Since this fθ lens system has the above-described configuration, it is possible to realize optical scanning by satisfactorily correcting surface tilt of the rotating polygon mirror and satisfactorily correcting field curvature in the main and sub scanning directions. Therefore, high-density optical scanning becomes possible.

[Brief description of the drawings]

FIG. 1 is a view for explaining the shape of the fθ lens system of the present invention, FIGS. 2 to 8 are views for explaining an optical scanning device, and FIGS. 9 and 10 are barrel-shaped toric surfaces. FIGS. 11 to 48 are aberration diagrams and fθ characteristic diagrams relating to each example. DESCRIPTION OF SYMBOLS 1 ... Light source device, 2 ... Cylinder lens, 3 ... Rotating polygon mirror, 4, 5 ... 1st and 2nd lens which comprises ftheta lens system

Continued on the front page (31) Priority claim number Japanese Patent Application No. 1-137805 (32) Priority date Hei 1 (1989) May 31 (33) Priority claim country Japan (JP) (31) Priority claim number Special No. Hei 1-156554 (32) Priority date Hei 1 (1989) June 19 (33) Priority claiming country Japan (JP) (31) Priority claim number Japanese Patent Application No. 1-218055 (32) Priority date Hei 1 (1989) August 24 (33) Countries claiming priority Japan (JP)

Claims (6)

(57) [Claims] 1. A method according to claim 1, further comprising the step of: forming a substantially parallel light beam from the light source device into a long linear image in the direction corresponding to the main scanning, and rotating the light beam at a constant angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the linear image. In a light scanning device that forms a spot on the surface to be scanned by the imaging lens system and optically scans the surface to be scanned at a substantially constant speed, the light is deflected by the rotating polygon mirror. An imaging lens system that forms an image of a light beam on a surface to be scanned.The imaging lens system has a function of coupling the reflection position of the rotary polygon mirror and the surface to be scanned in a sub-scanning direction to a substantially optically conjugate relationship. A two-group, two-lens configuration having first and second lenses that has an fθ function in the scanning direction and is provided in the first and second order from the side of the rotary polygon mirror toward the surface to be scanned. Of the light beam ideally deflected by the rotating polygon mirror. When a surface formed by sweeping light rays is a deflecting surface, and a surface parallel to the optical axis of the imaging lens system and orthogonal to the deflecting surface is a deflecting orthogonal surface, each lens surface is counted from the rotating polygon mirror side. Assuming the first to fourth surfaces, the shapes of these lens surfaces in the deflection surface are sequentially from the first surface to the fourth surface, such as an arc, a straight line, a straight line, and the like.
The first lens has a negative refractive power and the second lens has a positive refractive power in a plane parallel to the deflecting surface, and the first surface becomes smaller as the radius of curvature in the deflecting orthogonal plane moves away from the optical axis. A concave barrel-shaped toric surface, a second surface is a convex cylinder surface or a plane having a refractive power only in the plane orthogonal to the deflection, a third surface is a concave cylinder surface having a refractive power only in the plane orthogonal to the deflection, and a fourth surface. The surface is a convex toric surface having a strong curvature in the orthogonal plane of deflection. The composite focal length in the orthogonal plane of deflection is f _s , and the radius of curvature of the third surface in the orthogonal plane including the optical axis is r. ' ₃
Where f satisfies the following condition: 1.6 ≦ | r ′ ₃ / f _S | ≦ 3.1 (1-I) 2. The method according to claim 1, wherein the substantially parallel light beam from the light source device is formed into a long line image in the main scanning direction, and the light beam is subjected to a uniform angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the line image. In a light scanning device that forms a spot on the surface to be scanned by the imaging lens system and optically scans the surface to be scanned at a substantially constant speed, the light is deflected by the rotating polygon mirror. An imaging lens system that forms an image of a light beam on a surface to be scanned.The imaging lens system has a function of coupling the reflection position of the rotary polygon mirror and the surface to be scanned in a sub-scanning direction to a substantially optically conjugate relationship. A two-group, two-lens configuration having first and second lenses that has an fθ function in the scanning direction and is provided in the first and second order from the side of the rotary polygon mirror toward the surface to be scanned. Of the light beam ideally deflected by the rotating polygon mirror. When a surface formed by sweeping light rays is a deflecting surface, and a surface parallel to the optical axis of the imaging lens system and orthogonal to the deflecting surface is a deflecting orthogonal surface, each lens surface is counted from the rotating polygon mirror side. Assuming the first to fourth surfaces, the shapes of these lens surfaces in the deflection surface are sequentially from the first surface to the fourth surface, such as an arc, a straight line, a straight line, and the like.
The first lens has a negative refractive power and the second lens has a positive refractive power in a plane parallel to the deflecting surface, and the first surface becomes smaller as the radius of curvature in the deflecting orthogonal plane moves away from the optical axis. A concave barrel-shaped toric surface, a second surface is a concave cylinder surface having a refractive power only in a plane orthogonal to deflection, a third surface is a concave cylinder surface having a refractive power only in a plane orthogonal to deflection, and a fourth surface is A convex toric surface having a strong curvature in a plane orthogonal to the deflection, the composite focal length in the plane orthogonal to the deflection is f _S , and the first, second, third, and fourth planes in the plane orthogonal to the deflection including the optical axis. Assuming that the radii of curvature of the surfaces are r ′ ₁ , r ′ ₂ , r ′ ₃ , r ′ ₄ , these are 2.0 <| [{(1 / r ′ ₁ ) − (1 / r ′ ₂ )} − ｛ (1 / r ′ ₃ ) − (1 / r ′ ₄ )｝] · f _S | <9.8 (2-I) An fθ lens system characterized by satisfying the following condition: 3. A substantially parallel light beam from the light source device is formed into a long linear image in the main scanning direction, and the light beam is subjected to a uniform angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the linear image. In a light scanning device that forms a spot on the surface to be scanned by the imaging lens system and optically scans the surface to be scanned at a substantially constant speed, the light is deflected by the rotating polygon mirror. An imaging lens system that forms an image of a light beam on a surface to be scanned.The imaging lens system has a function of coupling the reflection position of the rotary polygon mirror and the surface to be scanned in a sub-scanning direction to a substantially optically conjugate relationship. A two-group, two-lens configuration having first and second lenses that has an fθ function in the scanning direction and is provided in the first and second order from the side of the rotary polygon mirror toward the surface to be scanned. Of the light beam ideally deflected by the rotating polygon mirror. When a surface formed by sweeping light rays is a deflecting surface, and a surface parallel to the optical axis of the imaging lens system and orthogonal to the deflecting surface is a deflecting orthogonal surface, each lens surface is counted from the rotating polygon mirror side. In FIGS. 1 to 4, the shapes of these lens surfaces in the deflecting surface are sequentially changed from the first surface to the fourth surface in the form of an arc, an arc, a straight line, and the like.
The first lens is positive or negative in a plane parallel to the deflection surface
A second lens having a positive refractive power, the first surface is a spherical surface, and the second surface is a convex barrel-shaped toric surface in which a radius of curvature in a plane orthogonal to the deflection decreases as the distance from the optical axis increases.
Cylindrical surface of the concave third surface having a refractive power only in the deflection plane orthogonal, the fourth surface is a toric surface of a convex having a strong curvature on the deflection plane perpendicular, the combined focal length of the deflection plane orthogonal f _S , second in the deflecting perpendicular plane including the optical axis, when the curvature of the fourth surface radius and r _'2, r' ₄ each, they _{0.3 <| r '2 / r} ' 4 | <1.0 ( 3-I) 0.03 <| r ′ ₂ / f _S | <0.54 (3-II) An fθ lens system characterized by satisfying the following condition: 4. A substantially parallel light beam from a light source device is formed into a long linear image in a direction corresponding to the main scanning, and the light beam is subjected to uniform angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the linear image. In a light scanning device that forms a spot on the surface to be scanned by the imaging lens system and optically scans the surface to be scanned at a substantially constant speed, the light is deflected by the rotating polygon mirror. An imaging lens system that forms an image of a light beam on a surface to be scanned.The imaging lens system has a function of coupling the reflection position of the rotary polygon mirror and the surface to be scanned in a sub-scanning direction to a substantially optically conjugate relationship. A two-group, two-lens configuration having first and second lenses that has an fθ function in the scanning direction and is provided in the first and second order from the side of the rotary polygon mirror toward the surface to be scanned. Of the light beam ideally deflected by the rotating polygon mirror. When a surface formed by sweeping light rays is a deflecting surface, and a surface parallel to the optical axis of the imaging lens system and orthogonal to the deflecting surface is a deflecting orthogonal surface, each lens surface is counted from the rotating polygon mirror side. Assuming the first to fourth surfaces, the shapes of these lens surfaces within the deflection surface are linear, arc, linear, and so on in order from the first surface to the fourth surface.
The first lens has a negative refractive power and the second lens has a positive refractive power in a plane parallel to the deflection surface. The first surface has a flat surface, and the second surface has a radius of curvature in a plane orthogonal to the deflection. A concave barrel-shaped toric surface that becomes smaller as it moves away from the optical axis, a third surface is a concave cylinder surface having refractive power only in a plane orthogonal to the deflection, and a fourth surface is a convex toric surface having a strong curvature in the plane orthogonal to the deflection. And the combined focal length and lateral magnification in the plane orthogonal to the deflection
When the radii of curvature of the third and fourth surfaces in the deflecting direct light plane including f _S , β and the optical axis are r ′ ₃ and r ′ ₄ , respectively, they are 0.4 <│ ｛(1 / r ′ _{3) - (1 / r '} 4)} · f S · β | <2.0 (4-I) comprising fθ lens system, characterized by satisfying the condition. 5. A method according to claim 1, wherein the substantially parallel light beam from the light source device is formed into a long line image in the main scanning direction, and the light beam is subjected to a constant angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the line image. In a light scanning device that forms a spot on the surface to be scanned by the imaging lens system and optically scans the surface to be scanned at a substantially constant speed, the light is deflected by the rotating polygon mirror. An imaging lens system that forms an image of a light beam on a surface to be scanned.The imaging lens system has a function of coupling the reflection position of the rotary polygon mirror and the surface to be scanned in a sub-scanning direction to a substantially optically conjugate relationship. A two-group, two-lens configuration having first and second lenses that has an fθ function in the scanning direction and is provided in the first and second order from the side of the rotary polygon mirror toward the surface to be scanned. Of the light beam ideally deflected by the rotating polygon mirror. When a surface formed by sweeping light rays is a deflecting surface, and a surface parallel to the optical axis of the imaging lens system and orthogonal to the deflecting surface is a deflecting orthogonal surface, each lens surface is counted from the rotating polygon mirror side. Assuming the first to fourth surfaces, the shapes of these lens surfaces in the deflection surface are, in order from the first surface to the fourth surface, an arc, an arc, a straight line, and the like.
The first lens has a negative refractive power and the second lens has a positive refractive power in a plane parallel to the deflecting surface, and the first surface becomes smaller as the radius of curvature in the deflecting orthogonal plane moves away from the optical axis. A concave barrel-shaped toric surface, a second surface is a convex spherical surface, a third surface is a concave cylinder surface having refractive power only in a plane orthogonal to the deflection, and a fourth surface is a convex cylinder having a strong curvature in the plane orthogonal to the deflection. It is a toric surface, and the combined focal length and lateral magnification in the plane orthogonal to the deflection
Assuming that the radii of curvature of the third and fourth surfaces in the deflecting direct light plane including f _S , β and the optical axis are r ′ ₃ and r ′ ₄ , respectively, these are 0.1 <| ｛(1 / r ′). _{3) - (1 / r '} 4)} · f S · β | <5.4 (5-I) becomes fθ lens system, characterized by satisfying the condition. 6. A substantially parallel light beam from the light source device is formed into a long linear image in the main scanning direction, and the light beam is subjected to a uniform angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the line image. In a light scanning device that forms a spot on the surface to be scanned by the imaging lens system and optically scans the surface to be scanned at a substantially constant speed, the light is deflected by the rotating polygon mirror. An imaging lens system that forms an image of a light beam on a surface to be scanned.The imaging lens system has a function of coupling the reflection position of the rotary polygon mirror and the surface to be scanned in a sub-scanning direction to a substantially optically conjugate relationship. A two-group, two-lens configuration having first and second lenses that has an fθ function in the scanning direction and is provided in the first and second order from the side of the rotary polygon mirror toward the surface to be scanned. Of the light beam ideally deflected by the rotating polygon mirror. When a surface formed by sweeping light rays is a deflecting surface, and a surface parallel to the optical axis of the imaging lens system and orthogonal to the deflecting surface is a deflecting orthogonal surface, each lens surface is counted from the rotating polygon mirror side. Assuming the first to fourth surfaces, the shapes of these lens surfaces within the deflection surface are linear, circular, circular, arc, in that order from the first surface to the fourth surface.
The first and second lenses have a positive refractive power in a plane parallel to the deflecting surface, and the first surface is a concave cylinder surface and a second surface having a refractive power only in a plane orthogonal to the deflecting surface. Is a convex barrel-shaped toric surface in which the radius of curvature in the plane perpendicular to the deflection decreases as the distance from the optical axis increases, the third surface is a concave spherical surface, and the fourth surface is a convex toric surface having a strong curvature in the plane orthogonal to the deflection. When the combined focal length in the orthogonal plane of deflection is f _S , and the radii of curvature of the first and second surfaces in the orthogonal plane including the optical axis are r ′ ₁ and r ′ ₂ , these are 0.5 < | {(1 / r ′ ₁ ) − (1 / r ′ ₂ )} · f _S | <3.8 (6-I) An fθ lens system characterized by satisfying the following condition: