JPH0252307A - Ftheta lens - Google Patents

Abstract

PURPOSE:To obtain the bright high-resolution ftheta lens which has superior ftheta characteristics and its curvature of field compensated sufficiently in the sagittal and meridional directions by employing four-group, four-element constitution which use specific lenses. CONSTITUTION:The 1st-4th groups are arrayed in this order from the object side to the image side. Then the 1st group consists of a lens 10 which has its concave surface on the object side and negative refracting power and the 2nd group consists of a meniscus lens 12 which has its concave surface on the object side and has positive refracting power, and the 3rd group consists of a lens 14 which has its convex surface on the image side and has positive refracting power and the 4th group consists of a lens 16 which has its concave surface on the image side and nearly no refracting power. Here, inequalities I-IV hold for the radii ri (i=1-8) of curvature of i-th lens surfaces from the object side, the distance d0 from the deflection surface of a deflecting device to the 1st lens surface, and the focal length (f) of the whole system. Consequently, the bright high-resolution ftheta lens is obtained which has the excellent ftheta characteristics and its curvature of field compensated sufficiently and can makes a high-density optical scan.

Description

[Detailed description of the invention] [Industrial application fields] The present invention relates to an fθ lens.

[Prior Art] Optical scanning devices are known in connection with laser printers, facsimile machines, digital copying machines, digital plate making machines, and the like. As is well known, an fθ lens is a lens used in such an optical scanning device.

FIG. 7 schematically shows an example of an optical scanning device.

A light beam from a light source device 1 consisting of a light source or a light source and a condensing optical system is reflected by a deflection surface 3 of a deflection device 2 and is deflected as the deflection device 2 rotates.

The deflected light beam passes through the fθ lens 4, is reflected by the reflecting mirror 6, and enters the scanning surface 5.

Note that the light source device 1 usually includes a laser diode and a collimator lens, and a substantially parallel light beam is emitted from the light source device 1. Further, as the deflection device 2, a rotating polygon mirror as shown in the figure is generally used, so that the deflected light beam is deflected at a constant angular velocity.

The fθ lens 4 is disposed between the deflection device 2 and the scanning surface 5 as described above, and its basic characteristics are the image height when the light beam is incident on the fθ lens at an angle of θ with respect to the optical axis. The purpose is to establish a proportional relationship between Y and the angle θ, where the focal length is f, and Y=f・θ.With this characteristic, the deflected light beam is imaged as a spot on the scanning surface 5, and the spot is Make the displacement constant.

Various types of f& lenses that are required to have such functions have been proposed in the past. (For example, JP-A-55-533
Publication No. 08, Publication No. 60-135914, Publication No. 60-1
35915).

[Problem to be solved by the invention] Among the fθ lenses proposed in the past, Japanese Patent Application Laid-Open No. 55-533
Although the lens disclosed in No. 08 has a wide angle and the curvature of field is well corrected, there is still room for improvement in the fθ characteristics.
The number of lens components is also large, at 5.

JP 60-135914, JP 60-13591
The fθ lens described in Publication No. 5 is also wide-angle, but it suffers from curvature of field,
There is still room for improvement in the fO characteristics.

The present invention has been made in view of the above-mentioned circumstances, and aims to provide brightness, high resolution, and
An object of the present invention is to provide an f.theta. lens which has excellent .theta. characteristics and has sufficiently corrected field curvature in the sagittal and meridional directions.

[Means to solve the problem] The present invention will be explained below.

The f& lens of the present invention is a lens for imaging a deflected light beam deflected at a constant angular velocity by a deflection surface of a deflection device as a spot on a scanning plane, and for making the displacement of the spot constant, The first to fourth groups are arranged in this order from the object side to the image side.

As shown in Figure 1, the first group has a negative refractive power with a concave surface facing the object side (left side in Figure 1), and the second group has a positive refractive power with a concave surface facing the object side. A meniscus lens 12, the third group has a positive refractive power with a convex surface facing the image side (right side in Figure 1)]4, a fourth group has a concave surface facing the image side and has almost refractive power. Therefore, the fD lens of the present invention is composed of four elements in four groups.

The radius of curvature of the first lens surface from the object side is r.

(1 to 8), the distance from the deflection surface of the deflection device to the first lens surface is d. , if the focal length of the gold thread is f, then
These are (1) -1,2<rl/dO<-1,0(
II) -2,0<r7/ri<-1,0(III
) −0.8f<rs<−0.7f(IV) −
The following four conditions are satisfied: 0.5f<r6<-0.35f.

[Operation] If the upper limits of the above conditions (I) and (III) are exceeded, the curvature of field in the main scanning direction and the fθ characteristic become under, and the curvature of field in the sub-scanning direction becomes over. Also, if the lower limit is exceeded, the curvature of field in the main scanning direction and the fθ characteristics will be excessive.
The curvature of field in the sub-scanning direction becomes under.

If the upper limit of condition (II) is exceeded, the field curvature becomes under in both the main and sub-scanning directions, and the fθ characteristic becomes over. Conversely, when the lower limit is exceeded, the curvature of field becomes over in both the main and sub-scanning directions, and the fθ characteristic becomes under.

If the upper limit of condition (rv) is exceeded, the curvature of field in the main scanning direction and the fO characteristic become under, and the curvature of field in the sub-scanning direction becomes over. Furthermore, coma aberration in the meridional direction is directed inward. When the lower limit of condition (v) is exceeded, the curvature of field in the main scanning direction and the fθ characteristic become over, the curvature of field in the sub-scanning direction becomes under, and the coma aberration in the meridional direction becomes outward.

[Example code] Hereinafter, description will be given based on specific examples.

As shown in FIG.
= 1 to 7), and the refractive index of the j-th lens is nJ (j
=1 to 4). Further, f represents the focal length of the gold thread, FNO represents the F number, 20 represents the effective deflection angle, and dO represents the tenor distance from the deflection surface to the first lens surface. Further conditions (■), OI), (or)
, (IV), in order, K, , 2
．． 3. 4.

Example 1 f=1.00, P=13.134, 2 fl=4
8 degrees rId+ J 21.0503 2.0073 1 1.51118 1.9031 5.7747 2 1.51118 0.4097 7.7877 3 1.76605 0.1589 5.0563 4 1.63552 ] -23,8139 2 ■ 3 -74.0181 4 -34.5329 5 ψ 6 -46.3236 7 69.8806 8 72.6481 KI=-1,131, 2=-1,509, 3=-0
．． 740. 4 = -0.463 Example 2 f = 100, F = 13.134, 2θ = 42 degrees dt J nj 20.9521 1.8014 1 2.0823 6.2510 2 0.2582 6.5805 3 0. 8200 4.4801 4 23.5759 74.5947 -34.6941 -46.0393 68.7333 71.2003 ni, = -1,125, ni 2 Example 3 f=100, F 1.51118 1.5. 1. l]8 1.63552 1.493. 3 = -0.746, 4 = -0.460
23,7257 13,13420 d.

21.6551 2.3876 2.0179 40 degrees j 1.51118 3 -75.1796 6.3638 2
1.511184 -35.3141
0.23975 oo 5.68
97 3 1.766056 -46.21
50 0; 59327 69.4431
4.5623 4 1.635528
72.5374, -1,098, 2=-1,503, 3=-0
．． 752, 4 = -0.462 Example 4 f, 100, F = 13.134, 2θ = 40 degrees i
r4 d+ Jo
21.5451 1 -23.3672 L9:306 1 1.
．． 572192 oo2.1365 3 -73.5918 4.9001 2 1.5
72194 -34.3430 0.72285
oo6.5937 3 1.766056 -4
2.7995 0.42117 62.1147
3.9131 4 1.635528 64.0
599 Kl"-1,085, 2=-1,451, 3"-0
．． 736, 4=-0.428 Example 5 f=100, F=13.134, 2θ=40 degrees i
l'J d+ j
njo 2
1.54001 -23.3605 1.94
67 1],,5721.92
ω 2.13473 -73.5720
4.9218 2 1.572194
-:34.3694 0.68475
ω 6.5651 3 1.7
66056 -42.7757 0.4135
7 6], 9] 43 3.8] 55 4
1.635528 63.9056 Kl=-1,085, 2=-1,447, 3=-0
．． 736, 4=-0.428 Aberration diagrams regarding the above-mentioned Examples 1 to 5 are sequentially shown in FIGS. 2 to 6. The solid line in the field curvature indicates the imaging position in the sub-scanning direction, and the broken line indicates the imaging position in the main scanning direction. Further, the specific value of the focal length f of the lens gold thread is f=3]0.352 in Example 1, f-354,688 in Example 2, and f=372 in Examples 3 and 4.
．． 423, and f=372.424 in Example 5.

[Effects of invention As described above, the present invention can provide a novel fO lens.

Although this fO lens has a compact structure with 4 elements in 4 groups, it is bright, has high resolution, and has good fO characteristics. Field curvature is sufficiently corrected in both the sagittal and meridional directions, and it can handle high-density light. Scanning becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the lens configuration of the present invention, FIGS. 2 to 6 are aberration diagrams corresponding to embodiments to 5, and FIG. 7 is for explaining the optical scanning device. This is a diagram for 10., first group lens, 12. ,, second group lens, 1
4. . SA S, A Ah? (Inuse I row i) θ 24' OMA DS, DM form σ illusion (゛) S no 1 shi itaku Z ) θ=21゜ OMA DS, DFI θ=24” DIST (〆) (fθ21≧;) Hino 1 /i) θ=?B. DIST (%) (f θ n ) l) S, A Houki b (actual Subi V Mi row 4) θ Zo. OMA Dδ, DI θ π. DIST ('1) (fe 4 Potatoes 1 Bin SA Anzu C Warm (Tsu O/y-Ishio゛j 5) θ=n. OMA DS, D/Vl e=ZO” D/ST('l) (fθ special note)

Claims (1)

[Scope of Claims] An fθ lens for imaging a deflected light beam deflected at a constant angular velocity by a deflection surface of a deflection device as a spot on a scanning plane, and for making the displacement of the spot constant, The first to fourth groups are arranged in this order from the object side to the image side. A meniscus lens with a positive refractive power with a concave surface facing the image side, the third group has a positive refractive power with a convex surface facing the image side, and a fourth group has a concave surface facing the image side and has almost no refractive power. The lens is composed of 4 elements in 4 groups, and the radius of curvature of the i-th lens surface from the object side is r_i(i
=1 to 8), when the distance from the deflection surface of the deflection device to the first lens surface is d_o, and the focal length of the gold thread is f, these are (I)-1.2<r_1/d_0<- 1.0(II)-
2.0<r_7/r_6<-1.0(III)-0.8f
An fθ lens that satisfies the following condition: <r_3<-0.7f (IV)-0.5f<r_6<-0.35f.