JPH07287163A - Telecentric ftheta lens - Google Patents

Abstract

PURPOSE:To decrease an F value and reduce the diameter of a spot and to adapt the telecentric ftheta lens to a scanning optical system with a rotary polygon mirror of equal-angular-speed rotation and a surface tilt correcting mechanism for a cylindrical lens by employing five-lens constitution and satisfying specific requirements. CONSTITUTION:This lens consists of a positive meniscus lens as a 1st lens L1 which has its concave surface on the object field side, a negative lens as a 2nd lens L2, a positive meniscus lens as a 3rd lens L3 which has its concave surface on the object field side, a positive meniscus lens as a 4th lens L4 which has its concave surface on the object field side, and a positive lens as a 5th lens L5 in order from the object field side. Further, conditions 1.7<n1, 1<r5/r6<1.1, and 1<d8<f<1.1 are satisfied, where n1 is the refractive index of the 1st lens L1, r5 and r6 the radii of curvatures of the 5th and 6th lens surfaces counted from the object-field side, d8 the air gap on the optical axis between the 4th lens L4 and 5th lens L5, and (f) the focal length of the whole system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a telecentric f.theta. Lens system used in an optical scanning device such as a laser printer.

[0002]

2. Description of the Related Art A scanning optical system such as a laser printer, which uses a rotating polygon mirror rotating at a constant angular velocity and a cylindrical lens having a surface tilt correction function, is often used. As the imaging lens, an fθ lens that can scan a beam on the surface of the photoconductor in the main scanning direction at a constant speed is used.

Since a generally known fθ lens is not telecentric, an error in image height easily occurs due to a displacement of a surface to be scanned, and in order to prevent this, a strict machining precision is required for a rotary polygon mirror. There is. Heretofore, as a telecentric fθ lens, Japanese Patent Laid-Open No. Sho 59 has been proposed.
-195211, JP-A-62-299927, JP-A-2
Although a lens as shown by −83511 is known, these lenses have a large F value.

Since the F value and the spot diameter are in a proportional relationship, when the F value is large, the spot diameter is also large and the resolution is lowered. Further, since the entrance pupil position of these lenses is close to the first surface of the lens (the most object side, that is, the surface on the incident light side), and the distance between the final lens surface and the scanning surface is small, rotation at a constant angular velocity It is difficult to arrange the polygon mirror and a lens having a surface tilt correction function such as a cylindrical lens in the optical path, and it is difficult to perform stable scanning.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has a small F value to reduce the diameter of a spot, and a rotary polygon mirror and a cylindrical mirror that rotate at a constant angular velocity. It can be adapted to a scanning optical system using a lens having a surface tilt correction mechanism.
In addition, the telecentric f with well-corrected aberrations
It is intended to provide a θ lens.

[0006]

In order to achieve the above object, in the present invention, in order from the object side, the first lens L ₁ of the positive meniscus lens with the concave surface facing the object side, the second lens of the negative lens L ₁ A lens L ₂ , a third lens L ₃ which is a positive meniscus lens having a concave surface facing the object side, a fourth lens L _{4 which is} a positive meniscus lens having a concave surface facing the object side, and a fifth lens L _{5 which is} a positive lens.
The refractive index of the first lens L ₁ is n ₁ , the curvature radii of the fifth and sixth lens surfaces counted from the object side are r ₅ ,
When r ₆ , the air distance between the fourth lens L ₄ and the fifth lens L ₅ on the optical axis is d ₈ , and the focal length of the entire system is f, the following conditions (1) to (3) are satisfied.

(1) 1.7 <n ₁ (2) 1 <r ₅ / r ₆ <1.1 (3) 1 <d ₈ /f<1.1

[0008]

In the fθ lens, the ideal image height is given by Y = fθ, but in order to realize this, it is necessary to intentionally generate a negative distortion aberration. Further, in order to be telecentric, the entrance pupil position and the front focal position must be matched, and if this entrance pupil position is too close to the first surface of the lens, a rotary polygon mirror corresponding to the entrance pupil surface is formed. The first surface of the lens comes into contact. When the entrance pupil position is close to the first lens surface, arranging the rotary polygon mirror so as not to contact the lens inevitably increases the angle of incidence on the rotary polygon mirror with respect to the optical axis of the telecentric fθ lens. Then,
Since the area of the reflecting portion of the rotary polygon mirror needs to be large, the rotary polygon mirror itself becomes large.

Further, if the distance between the final surface of the lens and the scanning surface is too close, there will be no space for disposing an element having a surface tilt correction function such as a cylindrical lens. Conditional expressions (1) to (3) are conditional expressions that take account of such a fact in addition to the aberration correction, and will be described below.

Conditional expression (1) relates to the refractive index of the first lens L ₁ and is a condition for favorably correcting spherical aberration. Below this range, not only spherical aberration will be undercorrected, but also coma will be adversely affected and other lenses will not be able to correct it. In addition, the amount of negative distortion that should be intentionally generated is small, and the performance as an fθ lens is not satisfied.

Conditional expression (2) relates to the radius of curvature of the third lens L ₃ , and is a condition for balancing the curvature of field. More preferably, 1.05 <r ₅ / r ₆ <
1.1. First, below the range of conditional expression (2),
As the distance between the entrance pupil position and the first surface of the lens becomes narrower,
The positive refractive power of the third lens L ₃ becomes very weak, and the spherical aberration is overcorrected. On the other hand, if it exceeds this range, the distance between the final surface of the lens and the scanning surface becomes narrow, and the working distance becomes short. Further, a large field curvature is generated that cannot be corrected.

Conditional expression (3) relates to the air distance between the fourth lens L ₄ and the fifth lens L ₅ on the optical axis and the focal length of the entire system, and reduces coma and balances them. It is a condition for. More preferably, 1 <d
_{8 /} f <1.05. When the value goes below the lower limit of the conditional expression (3), a large under-coma aberration is generated with respect to a light beam near the maximum angle of view, which makes correction difficult. On the other hand, when the value exceeds the upper limit, negative distortion occurs too much, and it becomes difficult to obtain the ideal image height Y = fθ required for the fθ lens. Also,
The bulge of spherical aberration becomes large, which adversely affects coma.

[0013]

EXAMPLES Next, examples of the telecentric fθ lens of the present invention will be described. As shown in FIG. 1, the lens configurations of Examples 1, 2, and 3 are, in order from the object side, a positive meniscus first lens L ₁ having a concave surface facing the object side, a negative second lens L ₂ , and an object. The third lens L _{3 is} a positive meniscus lens having a concave surface facing the field side, the fourth lens L _{4 is} a positive meniscus lens having a concave surface facing the object side, and the fifth lens L _{5 is} a positive lens.

2, 3 and 4 are the first, second and third embodiments, respectively.
Represents spherical aberration, astigmatism, distortion, and lateral aberration of
It The data of each Example are shown below. f is the focal length of the entire system
Away, F _NO.Is the F number, λ is the wavelength, 2θ is the deflection angle, and r
Is the radius of curvature, d is the distance between the lens surfaces, both in millimeters.
Heart, (however, d₀Is the sky on the optical axis between the entrance pupil and the first surface
Interval), n_dAnd ν_dIs the refractive index and
It is a base number. [Example 1] f = 100 F_NO． = 14 λ = 488 nm 2θ = 3
3.6 ° d₀= 15.475 NO. R d n_d ν_d 1 -98.756 2.652 1.90265 35.724 L₁ 2 -55.809 10.000 3 -23.886 3.217 1.74077 27.631 L₂ 4 ∞ 3.000 5 -38.355 5.261 1.90265 35.724 L₃ 6 -35.809 0.087 7 -631.766 7.044 1.78797 47.465 L_Four 8 -37.634 101.130 9 ∞ 7.522 1.90265 35.724 L_Five 10 -181.021 r_Five/ R₆= 1.071 d₈/F=1.011 [Example 2] f = 100 F_NO． = 14 λ = 488 nm 2θ = 3
3.6 ° d₀= 15.339 NO. R d n_d ν_d 1 -111.700 2.662 1.74950 35.191 L₁ 2 -53.153 9.789 3 -23.426 2.531 1.74077 27.631 L₂ 4 ∞ 3.022 5 -37.563 5.858 1.86994 39.816 L₃ 6 -35.229 0.506 7 -505.762 6.858 1.78797 47.465 L_Four 8 -37.359 101.834 9 ∞ 7.918 1.90265 35.724 L_Five 10 -184.154 r_Five/ R₆= 1.066 d₈/F=1.018 [Example 3] f = 100 F_NO． = 14 λ = 488 nm 2θ = 3
3.6 ° d₀= 15.833 NO. R d n_d ν_d 1 -142.581 2.709 1.84042 43.347 L₁ 2 -58.631 9.505 3 -24.440 1.790 1.80458 25.499 L₂ 4 ∞ 3.218 5 -38.741 6.290 1.90265 35.724 L₃ 6 -36.198 1.289 7 -434.460 7.017 1.79631 40.897 L_Four 8 -37.939 101.909 9 ∞ 7.946 1.90265 35.724 L_Five 10 -189.165 r_Five/ R₆= 1.070 d₈/F=1.019 From each aberration diagram, the F value is 14 and the spot diameter is
As a reduced telecentric fθ lens, spherical surface
The difference, distortion, and coma are well corrected.
I understand.

[0015]

As described above, according to the present invention, it is possible to provide a telecentric fθ lens having a small F value, a wide distance between the final lens surface and the scanning surface, and good aberration correction. As a result, the scanning spot can be made smaller,
Further, it can be adapted to a scanning optical system using a rotary polygon mirror rotating at a constant speed and a lens having a surface tilt correction function such as a cylindrical lens, and a good image can be obtained.

[Brief description of drawings]

FIG. 1 is an arrangement configuration diagram of a telecentric fθ lens of the present invention.

FIG. 2 is a diagram of various types of aberration in the first example.

FIG. 3 is a diagram of various types of aberration of the second embodiment.

FIG. 4 is a diagram of various types of aberration of the third example.

Claims (1)

[Claims] 1. A positive surface with a concave surface facing the object side in order from the object side.
First lens of meniscus lens, second lens of negative lens
3rd lens of positive meniscus lens with concave surface facing the object side
No. 4 of positive meniscus lens with concave surface facing the object side
The lens consists of the fifth lens, which is a positive lens, and the first lens
Refractive index n₁, The 5th and 6th lenses from the object side
The radius of curvature of the surface is r_Five, R₆, 4th lens and 5th lens
The air space on the optical axis of d ₈, F is the focal length of the entire system
In this case, the following conditions (1) to (3) are satisfied.
Telecentric fθ lens. (1) 1.7 <n₁ (2) 1 <r_Five/ R₆<1.1 (3) 1 <d₈/F<1.1