JPH0990216A - Telecentric ftheta lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an θ lens which is small-sized, light weight and low in costs, and has its aberrations excellently compensated and also has excellent fθcharacteristics and telecentric performance. SOLUTION: This lens consists of a 1st negative lens 1 meniscus lens which has its concave surface on an object side, a 2nd positive meniscus lens 2 which has its concave surface on the object side, a 3rd positive meniscus lens 3 which has its concave surface on the object side, and a 4th positive lens 4 in order from the object side; and -1.0<f1 /f<-0.4, 0.5<f2 /f<2.1, 1.4<f3 /f<2.1, and -0.9<r1 /d0 <-0.5 hold for the focal length (f) of the whole system, the focal lengths f1 , f2 , and f3 of the 1st, 2nd, and 3rd lenses, the distance d0 from the deflection surface of a deflecting device to the 1st lens 1, and the radius r1 of curvature of the incidence surface of the 1st lens 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a four-lens f.theta. Lens system having telecentric characteristics.

[0002]

2. Description of the Related Art Optical scanning devices are used in laser printers, facsimiles, laser inspection devices, laser processing devices and the like. In an optical scanning device in which a light beam from a light source is deflected by a deflecting device such as a rotating polygon mirror, and the deflected light beam is imaged as a spot on a scanning surface by an imaging lens system, an fθ lens is deflected at a constant angular velocity by a deflecting device. It is a lens that forms a deflected light beam as a spot on a scanning surface and enables uniform scanning of the spot.

At this time, assuming that the focal length is f, the image height Y on the scanning surface and the angle θ deflected from the optical axis by the deflecting device.
, And a relation of Y = f · θ, which is called fθ characteristic indicating constant velocity, is established.

[0006] A general fθ lens is not telecentric, and if the scanning surface is defocused, there arises a problem that the fθ characteristic cannot be obtained, which affects image formation, and particularly in a laser inspection device, a laser processing device, etc. There arise problems that the laser is obliquely incident and the inspection of the shadow of the protrusion becomes inadequate, or the deep machining is inclined.

An fθ lens having telecentricity is disclosed in, for example, Japanese Patent Application Laid-Open No. 62-254112 and Japanese Patent Application Laid-Open No. 2-93511. This product is configured to have at least 6 sheets so that desired characteristics can be obtained.

[0006]

However, if at least six lenses are used as in the above conventional lens, telecentricity can be obtained, and even if various aberrations are easily corrected, the entire length is long. It is difficult to meet today's demands for smaller size, lighter weight and lower cost. On the other hand, there has not yet been provided one having a four-lens structure and capable of sufficiently correcting various aberrations and sufficiently satisfying the telesetric property and the fθ property.

Therefore, as a result of various studies and investigations by the present inventors, in the fθ lens system having a four-element four-group configuration, the combination of various lenses, the mutual relationship between each lens and the focal length of the entire lens, and the deflection surface. From the first lens to the first lens and the mutual relationship of the radius of curvature of the entrance surface of the first lens, etc., are the main characteristics of the entire fθ lens system, such as spherical aberration, curvature of field,
It was found that the fθ characteristic and the telecentricity are greatly influenced.

An object of the present invention is to solve the problems of the prior art based on the above findings, and to provide an fθ lens having a four-lens configuration, which is excellent in correcting aberrations and which is excellent in fθ characteristics and telecentricity. And

[0009]

To achieve the above object, a telecentric f.theta. Lens of the present invention is a first lens of a negative meniscus lens having a concave surface facing the object side in order from the object side to the image side. And a second lens of a positive meniscus lens having a concave surface facing the object side, a third lens of a positive meniscus lens having a concave surface facing the object side, and a fourth lens of a positive lens. In this lens system, the focal length of the entire system is f, the focal lengths of the first, second, and third lenses are f ₁ , f ₂ , and f ₃ , and the distance from the deflecting surface of the deflecting device to the first lens is d _0. , The radius of curvature of the entrance surface of the first lens is r ₁
(1) -1.0 <f ₁ /f<-0.4 (2) 0.5 <f ₂ / f <2.1 (3) 1.4 <f ₃ / f <2. 1 (4) −0.9 <r ₁ / d ₀ <−0.5 is satisfied.

[0010]

The above conditions (1) to (4) are applicable to the above-mentioned specific 4 groups 4
It effectively works to keep the main characteristics such as spherical aberration, field curvature, fθ characteristic, and telecentricity of the lens system of the single-lens configuration within a required range. If the upper limit of the above condition (1) is exceeded, spherical aberration will be over , The field curvature in the main / sub scanning direction, f
The θ characteristic and telecentricity are under.

If the lower limit is exceeded, spherical aberration will be under, and fθ characteristic and telecentricity will be over.

If the upper limit of the condition (2) is exceeded, the field curvature in the main / sub scanning direction, the fθ characteristic, and the telecentricity will be under. If the lower limit is exceeded, the spherical aberration will be under, and the fθ characteristic and telecentricity will be over.

If the upper limit of the above condition (3) is exceeded, the fθ characteristic will become under. If the lower limit is exceeded, the curvature of field in the main and sub scanning directions will be under.

If the upper limit of the above condition (4) is exceeded, spherical aberration will be excessive, and field curvature in the main and sub-scanning directions, fθ characteristics, and telecentricity will be under. If the lower limit is exceeded, the spherical aberration will be under, and the fθ characteristic and telecentricity will be over.

Therefore, by satisfying the above conditions (1) to (4), it is possible to obtain an f.theta. Lens having a good f.theta.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings together with two examples.

In the embodiment of the present invention, as is apparent from the first and second embodiments shown in FIGS. 1 and 3, the negative surface having the concave surface facing the object side is arranged in order from the object side toward the image side. The first lens 1 of the meniscus lens, the second lens 2 of the positive meniscus lens having a concave surface facing the object side, the third lens 3 of the positive meniscus lens having a concave surface facing the object side, and the fourth lens of the positive lens 4 The basic configuration is a lens system of four groups and four lenses, which is composed of the lenses 4.

In the lens system having such a basic structure, the focal lengths f _{1 to} f _{4 of the first to} fourth lenses 1 to _{4 are set.}
And the focal length f of the entire lens system, and the distance d from the deflecting surface of the deflecting device, which is the entrance pupil, to the first lens 1.
₀ and relationships of the radius of curvature r ₁ of the entrance surface of the first lens 1, the main characteristics of the entire lens system, for example spherical aberration, curvature of field, f [theta] characteristic, a great influence telecentricity like.

Then, the inventors of the present invention manipulated various conditions variously and observed changes in various characteristics of the entire lens system. As a result, f ₁ / f, f ₂ / f, f ₃ / f, and r ₁ / d ₀
is important.

When f ₁ / f exceeds -0.4, spherical aberration becomes excessive, and field curvature in the main and sub scanning directions, fθ characteristic, and telecentricity become under.
Further, when it is less than -1.0, the spherical aberration is under, and fθ is
The characteristics and telecentricity are over.

Regarding f ₂ / f, if 2.1 is exceeded,
The field curvature in the main and sub-scanning directions, the fθ characteristic, and the telecentricity become under. On the other hand, when it is less than 0.5, the spherical aberration is under, and the fθ characteristic and the telecentricity are over.

When f ₃ / f exceeds 2.1, the θ characteristic becomes under. On the other hand, when it is less than 1.4, the field curvature in the main and sub scanning directions is under.

If r ₁ / d ₀ exceeds -0.5, the spherical aberration becomes excessive, and the field curvature in the main and sub scanning directions, the fθ characteristic, and the telecentricity become inferior.
Also, if it is less than −0.9, the spherical aberration is under, and fθ
The characteristics and telecentricity are over.

Therefore, (1) -1.0 <f ₁ /f<-0.4 (2) 0.5 <f ₂ / f <2.1 (3) 1.4 <f ₃ / f <2 1 (4) -0.9 <r ₁ / d ₀ <−0.5 is preferable, and the aberration is satisfactorily corrected, and fθ
It was possible to obtain an fθ lens having good characteristics and telecentricity with a four-lens configuration.

Next, the embodiments will be described. In each of the first and second embodiments shown in FIGS. 1 and 3, f is the focal length of the entire system, and f is the focal length of the i-th lens from the object side. ₁ , F is the F number of the lens system, 2ω is the deflection angle, and the radius of curvature of the i-th lens surface from the object side is r _i ( _i = 1 to 1
8), the distance from the deflection surface of the deflection device to the first lens is d
₀ , the i-th and (i + 1) -th surface spacings are defined as _di ( _i = 1 to 1
7), the refractive index of the j-th lens is n _j ( _j = 1 to 4)
And

The focal length of the entire system is f, the focal lengths of the first, second, and third lenses are f ₁ , f ₂ , and f ₃ , the distance from the deflecting surface of the deflecting device to the first lens is d ₀ , and the first lens is When the radius of curvature of the entrance surface of one lens is r ₁ , (Example 1)

[0027]

[Table 1]

FIG. 1 shows a lens cross section of the first embodiment, and FIGS. 2 (a), 2 (b) and 2 (c) show various aberrations of the lens.

(Example 2)

[0030]

[Table 2]

FIG. 3 shows the lens cross section of the second embodiment, and FIGS. 4 (a), 4 (b) and 4 (c) show various aberrations of the lens.

As is clear from the above, each of the embodiments shown in FIGS. 1 and 3 achieves the object of the present invention.

[0033]

As described above, according to the present invention, it is possible to realize an fθ lens having a four-lens structure, a small size, excellent correction of aberrations, and good fθ characteristics and telecentricity.

[Brief description of drawings]

FIG. 1 is a sectional view showing a lens system of Example 1 according to an embodiment of the present invention.

FIG. 2 is a diagram showing various aberrations of the lens system of FIG.

FIG. 3 is a sectional view of a lens system of Example 2 of the embodiment of the present invention.

FIG. 4 is a diagram showing various aberrations of the lens system of FIG. 1 1st lens 2 2nd lens 3 3rd lens 4 4th lens f Focal length of lens system f ₁ Focal length of 1st lens f ₂ Focal length of 2nd lens f ₃ Focal length of 3rd lens r ₁ 1st Radius of curvature of the entrance surface of the lens d ₀ Distance from the deflection surface of the deflection device to the first lens

Claims (1)

[Claims] 1. A first lens of a negative meniscus lens having a concave surface facing the object side in order from the object side to the image side,
A lens having a four-group four-element configuration including a second lens of a positive meniscus lens having a concave surface facing the object side, a third lens of a positive meniscus lens having a concave surface facing the object side, and a fourth lens of a positive lens. In the system, the focal length of the entire system is f, the focal lengths of the first, second, and third lenses are f ₁ , f ₂ , and f ₃ , the distance from the deflecting surface of the deflecting device to the first lens is d ₀ , and the first lens is When the radius of curvature of the entrance surface of one lens is r ₁ , (1) -1.0 <f ₁ /f<-0.4 (2) 0.5 <f ₂ / f <2.1 (3) _{1.4 <f 3 / f <2.1} (4) -0.9 < telecentric fθ lens satisfies the r ₁ / d ₀ <-0.5 becomes conditions.