JPH0364724A - F-theta lens - Google Patents

Abstract

PURPOSE:To make the f.theta lens bright and compensate various aberrations by composing the f.theta lens of a positive meniscus lens, a negative meniscus lens, and a positive meniscus lens which has a concave surface on a luminous flux incidence side, and a biconvex lens in order from the luminous flux incidence side and satisfying specific conditions. CONSTITUTION:The lens consists of a relatively small number, i.e. four lens elements which are the 1st positive meniscus lens L1 which has the concave surface on the luminous flux incidence side, the 2nd negative lens L2 which is concave to the luminous flux incidence side, the 3rd positive meniscus lens L3 which is concave to the luminous flux incidence side, and the 4th positive lens L4 which is convex to the luminous flux projection side in order from the luminous flux incidence side. Then conditions shown by inequalities are satisfied. Here, (f) is the composite focal length of the whole system, f2 is the focal length of the 2nd lens L2, f3 is the focal length of the 3rd lens L3, f4 is the focal length of the 4th lens L4, fI is the composite focal length of the 1st lens L3 and 2nd lens L2, and fII is the focal length of the 3rd lens L3 and 4th lens L4. Consequently, the lens which is bright and has various aberrations compensated excellently is obtained.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to an F/θ system used for scanning laser light at a constant speed in laser printers, plate making, etc.
It's about lenses.

[Conventional technology]

In laser printers, plate making, etc., a plane scanning optical system that scans a beam of light such as a laser beam at a constant speed is used, and as this optical system, an optical system as shown in FIG. 6 is used. In this optical system, a light beam 2 from a light m (which may include optical systems such as a collimating lens and a beam expander) 1 is directed to the surface of a rotating polygon mirror (or an optical deflector such as a galvanometer) 3. The light beam is reflected at 14, refracted through an imaging lens 4, and imaged onto a scanning surface 5''. now,
Assuming that the rotating polygon mirror 3 is rotating in the direction of arrow A in FIG. 6, the image forming point on the scanning surface 5 is scanned in the direction of arrow B. In this plane scanning optical system, the imaging point is on the scanning plane 5.
It is necessary to scan the top at a constant speed, and although means for controlling the rotational speed of the rotating polygon mirror 3 has been proposed as a means for this purpose, such control is extremely difficult.

Therefore, in practice, a method is mainly adopted in which an f/θ lens is used as the imaging lens 4 so that the imaging point scans the scanning surface 5 at a constant speed.

[Problem to be solved by the invention]

However, with this method, various restrictions arise in correcting the distortion aberration of the r/θ lens, and in order to satisfy these restrictions,
The disadvantage is that it requires an optical system with a large number of components.
Furthermore, the brightness, which is an important characteristic of this lens, was not sufficient.

The present invention has a relatively small number of four elements, is bright, and has a high performance f/- lens with various aberrations well corrected.
[Means for Solving the Problem] In order to achieve the above object, the f/theta lens according to the present invention has a concave surface facing the light flux incident side in order from the light flux incident side. a first lens L that is a positive meniscus lens; a second lens L2 that is a negative lens with a concave surface facing the light beam incident side; and a third lens L that is a positive meniscus lens that has a concave surface facing the light beam incident side. , and a fourth lens L4, which is a positive lens with a convex surface facing the light beam output side, f: composite focal length of the entire system f2: focal length of the second lens L2 f: third lens L, Focal length f4: Focal length of the fourth lens L4 f■: Composite focal length of the first lens L and second lens L2 "M: Composite focal length of the third lens L and fourth lens L4, C] -0.3 1 < f z / f <
0.2 3(2) 0.83 < f x /
f a < 2.1 (3) L0 7 < f
It is characterized by satisfying the following conditional expressions: t/f<0.99.

[Effect]

In the present invention, the first lens L is a positive meniscus lens having a concave surface facing the light flux incident side in order from the light flux incident side;
The second lens L2 is a negative lens with a concave surface facing the light beam incident side.
, a third lens L which is a positive meniscus lens with a concave surface facing the light beam incidence side, and a fourth lens L4 which is a positive lens with a convex surface facing the light beam exit side, and has a relatively small number of lenses. Moreover, conditional expressions (1), (2), (
By satisfying 3), it becomes possible to obtain a high-performance f/θ lens that is bright and has various aberrations well corrected.

The significance of each condition will be explained below.

Condition (1) defines the ratio of the second lens L2 to the composite focal length of the entire system, and condition (2) defines the ratio of the second lens L2 to the composite focal length of the entire system.
and the fourth lens L4, and these conditional expressions are conditions regarding field curvature and spherical aberration. Furthermore, condition (3) defines the ratio of the composite focal length f1 of the first lens L1 and the second lens L and the bottom focal length @rga of the third lens LY and fourth lens L4. The conditions relate to field curvature.

First, regarding spherical aberration, the question is whether conditions (1) and (2) are satisfied.If the lower limit of conditions (1) and (2) is exceeded, the spherical aberration becomes under-corrected.
If the upper limits of conditions (1) and (2) are exceeded, the spherical aberration will be overcorrected, and if the upper and lower limits are exceeded, it will be difficult to obtain a spot shape close to the diffraction limit.

Next, regarding the field curvature, all conditions (1) and (2)
, the question is whether or not (3) is satisfied. That is,
If the upper limit of condition (1), (2), or (3) is exceeded, the meridional image surface will sharply curve in the direction away from the lens at the periphery of the screen, and if the lower limit is exceeded, the meridional image surface will curve sharply toward the periphery of the screen. As a result, in both cases, the astigmatism at the periphery of the screen becomes large, making it difficult to obtain a homogeneous imaging point over the entire scanning surface.

〔Example〕

Hereinafter, the first to fourth embodiments of the present invention will be described in detail based on the basic structure of the 17 nons shown in FIG. 1.
~・The fourth embodiment consists of a positive meniscus lens L with its concave surface facing the light beam incident side from the light beam incident side, and a negative meniscus lens L with its concave surface facing the light beam incident side. A positive meniscus lens L with four sides,
Biconvex ω lens 17. In the following explanation, +, index, : 1st lens to 4th lens m: Surface number rl, rz, ... r, counted in order from the object side, : radius of curvature d of each lens surface, .

d2. ...d7: Thickness or air spacing of each lens nl, n2. n3゜n4: The refractive index of each lens with respect to the d-line.

The specific configuration of the first embodiment is as shown in the table below.

mrdnl-14,6572,6611,613082-1
4゜210 3.850 3 -15.998 L839 L627624
(63°140 0.451 5-86.177 4.250 L613086-
24.146 0.106 7 368.900 4.497 L613088
-42.736 Focal length f = 100 F number = 16.1 Half angle of view θ - 21, 1° Wavelength λ - 790 nm f z / f = 0.2840 f, /f, = 0.8501 f! / f w = L0434The aberration curve according to this specific configuration is as shown in FIG. In addition, the distortion aberration V
is expressed by the following equation.

However, y': Image height f: Focal length θ: Half angle of view Further, the specific +** of the second embodiment is as shown in the table below.

m v d nl −2
1,8752,6611,6i3082-18.992
3.850 3 -16.085 L839 L627624
316.460 L406 5 -7L720 2.750 L613086
-29.768 0.106? 306.510 5.997 L61308
8-26°821 Focal length f = 100 F number = i 6. i half angle of view θ-21, 1° wavelength λ-=790nrn fz/f=-0,24314 f3/fa=L9996 f-violet/f=-0,999 The aberration curve according to the specific configuration is the third It will look like the figure.

Furthermore, the specific configuration of the third embodiment is as shown in the table below.

m r d nl -15,
Oio 2.661 L613082 -14.
827 3°850 3 -17.427 L839 L627624-
207.390 0.582 5 -84.168 3°750 L613086
-24.777 0.106 7 225.49O4,9971,613088-45
, 741 Focal length f = 100 F number = 1 a 1 Half angle of view θ - 2161° Wavelength λ - 790 nm f2/f = -0, 3043 fz / fa = 0.8954 f I / f z = L0639 This specific configuration The aberration curve is shown in FIG.

Furthermore, the specific configuration of the fourth embodiment is as shown in the table below.

m r d nl -15,
5842, 6611, 613082-14, 6123,
850 3-15, 2001, 8391, 627624-228
,5400,545 5-90,8993,7501,613086-25,
8440,106 7(X) 4.997 L613088 -
32.301 Focal length f = 100 F number F = 16.1 Half angle of view θ = 2L1° Wavelength λ = 790 nm f z / f -0,2603 r3/ fJ = L0940 f I / f = -1,0224 This The aberration curve according to the specific configuration is as shown in FIG.

A second aberration curve diagram of each of the first to fourth embodiments described above.
．． Various aberrations such as spherical aberration in Figures 3 and 4.5 are well corrected.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, since the lens structure and conditions (1) to (4) are satisfied, the f/θ lens has excellent characteristics.
Spherical aberration, chromatic aberration of magnification, etc. can be well corrected to improve resolution, and the observation and diagnostic functions described above can be improved, and it has become possible to provide an f/θ lens that is lightweight and has good operability.

[Brief explanation of drawings]

Fig. 1 shows the lens composition countries of the f/θ lens, Fig. 2 shows the aberration curve of the first embodiment, Fig. 3 shows the aberration curve of the second embodiment,
FIG. 4 is an aberration curve diagram of the third embodiment, FIG. 5 is an aberration curve diagram of the fourth embodiment, and FIG. 6 is a schematic configuration diagram of a plane scanning optical system using an f/θ lens. Ll to L4: first lens to fourth lens X: optical axis rl, r2. ...rs: radius of curvature dl of each lens surface
, d2. ...d7: Thickness or air spacing of each lens Fig. 1

Claims (1)

[Claims] In order from the light flux incident side, a first lens L_1 is a positive meniscus lens with a concave surface facing the light flux incident side, a second lens L_2 is a negative lens with a concave surface facing the light flux incident side, and a light flux incident side. The third lens L_ is a positive meniscus lens with a concave surface facing the side.
3 and a fourth lens L_4, which is a positive lens with a convex surface facing the light beam exit side, f: composite focal length of the entire system f_2: focal length of the second lens L_2 f_3: third lens L_3 Focal length f_4: Focal length of the fourth lens L_4 f_1: Combined focal length of the first lens L_3 and second lens L_2 f_3: Combined focal length of the third lens L_3 and fourth lens L_4, (1) - 0.31<f_2/f<-0.23(2)0.
83<f_3/f_4<2.1 (3)-1.07<f_I /f_II<-0.99
・θ lens.