JPH0493910A - Telecentric ftheta lens - Google Patents

Abstract

PURPOSE:To excellently improve various aberrations by arranging 1st - 4th lens groups in order in a lens system which images deflected luminous flux as a light spot on a scanned surface and determining the composite focal length of the whole system, the focal length of the 3rd group, the radius of curvature of the 2nd group lens surface, the on-air interval between the 2nd and 3rd groups, and the refractive index of the 3rd group lens under specific conditions. CONSTITUTION:The luminous flux from a light source device 1 is deflected as a rotary polygon mirror 3 rotates. Then the deflected luminous flux is imaged as the light spot on the scanned surface 7 through the ftheta lens 5 and a surface tile correcting lens 6. The ftheta lens 5 is an image forming lens system and constituted by arranging the 1st - 4th groups in order from the object side to the image side. Then 0.4<f/f3<0.95, -0.3<RIII/f<-0.2, -0.4<RIVC/f<-0.3, 0<DIV/f<0.06, and 1.6<nIII hold, where (f) is the composite focal length of the whole system, f3 the focal length of the 3rd group, RIII and RIV the radii of curvature of the object-side and image-side lens surfaces of the 2nd group lens, DIV the on-axis air interval between the 2nd and 3rd groups, and nIII the refractive index of the material of the 3rd group lens. Then the ftheta, theta, or comatic aberration is overcorrected or undercorrected outside those condition ranges.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a telecentric fθ lens.

[Prior art] A light beam from a light source device is deflected at a constant angular velocity by a deflection device, and the deflected light beam is imaged as a light spot on a scanning surface by an imaging lens system and a surface tilt correction lens to perform optical scanning. Optical scanning devices are known. In such an optical scanning device, an fθ lens is used as an imaging lens system to realize uniform speed optical scanning.

Generally, a cutout fθ lens is not telecentric, and therefore, if the distance between the scanning plane position and the fθ lens in the optical axis direction deviates from the designed distance, there is a problem that the designed fθ characteristics cannot be obtained.

Conventionally, as a telecentric f-theta lens, JP-A-6
There is one disclosed in Publication No. 2-299927.

[Problems to be Solved by the Invention] In this fθ lens, the distance between the final lens surface and the scanning surface is narrow, and there is no room for a surface tilt correction lens.

For this reason, it is not possible to perform surface tilt correction.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a telecentric fθ lens that can be used together with a lens for correcting surface tilt and has various aberrations well corrected.

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

The fθ lens of the present invention deflects a light beam from a light source device at a constant angular velocity using a deflection device, and images the deflected light beam as a light spot on a scanning surface by an imaging lens system and a surface tilt correction lens. A lens system used as an imaging lens system in an optical scanning device that performs scanning, and has first to fourth groups sequentially arranged from the object side to the image side.

The "first group" can be a positive lens or a negative lens.

The "2nd group" is a negative meniscus lens with a concave surface facing the object side, the "3rd group" is a positive meniscus lens with a concave surface facing the object side, and the "4th group J is a positive lens with a convex lens surface on the image side. It is.

The combined focal length of the entire system is f, the focal length of the third group is f3, and the radius of curvature of the object-side and image-side lens surfaces of the second group lens is R□I++RIVs2. The axial air distance between the third group is Dlv, and the refractive index of the material of the third group lens is 01□1
, these are (I) 0.4 < f/f3 < 0.95 (
II) -0,3<R,□,/f<-0,2(I
II) -0,4<RIV/f<-0,3(IV)
O<D, v/f<0.06(V)
1.6 <nt 1 , the following condition is satisfied.

The first group is composed of one or two lenses.

FIGS. 1 to 3 show lens structures of fθ lenses according to claims 2 to 4, respectively. In these figures, the symbol G2 indicates the second group, the symbol G3 indicates the third group, and the symbol G4 indicates the fourth group. The left side of the figure is the object side, that is, the deflection device side, and the right side is the image side, that is, the image plane S side.

As shown in FIG. 1, in the lens configuration of claim 2,
The first group is a negative meniscus lens G1 with a convex surface facing the object side.
It is.

As shown in FIG. 2, in the lens configuration of claim 3,
The first group is composed of a positive lens Gll with a convex surface facing the object side, and a negative lens G12 which is cemented to the image side of the positive lens Gll and has a concave surface facing the image side.

As shown in FIG. 3, in the lens configuration of claim 3,
The first group is composed of a positive lens G13 having a convex surface facing the object side, and a negative lens G14 disposed on the image side of the positive lens G13 and having a concave surface facing the image side.

Therefore, in the lens arrangement of claim 2, the FE lens has 4 groups, 4
In the lens configuration of claim 3.4, the number of fθ lenses is 4.
It consists of 5 elements in a group.

[For production] The above conditions (I) to (V) will be explained.

Conditions (I) to (III) are all fθ characteristics (the ideal height for a light beam incident at an angle of θ with respect to the optical axis is fθ,
When the actual image height is Ho, (Ho-fθ)・100
/fθ(z)) and coma aberration, and if the upper limits of these conditions are exceeded, the fO characteristics
Both coma aberration will be over-corrected, and if the lower limit is exceeded, it will be under-corrected.

Condition (■v) is a condition for correcting the fθ characteristic,
If the upper limit is exceeded, the compensation will be over-compensated.

Condition (V) is a condition for correcting the fθ characteristic and the curvature of field in the main scanning direction. Outside the condition range, the fθ characteristic becomes under-corrected and the residual curvature of field becomes large.

It should be noted that since it is used together with the fθ cylinder surface tilt correction lens of the present invention and the surface tilt correction lens has a function of removing field curvature in the sub-scanning direction, correction of the field curvature in the sub-scanning direction of the fθ cylinder does not need to be a problem. There is no.

Further, as shown in FIGS. 1 to 3, there is sufficient room between the final lens surface and the image surface to provide a surface tilt correction lens.

Hereinafter, an example of using the fθ cylinder of the present invention will be briefly described with reference to FIGS. 4 and 5.

In FIG. 4, a light source device 1 is composed of, for example, a semiconductor laser and a collimating lens, and emits a substantially parallel light beam.

The parallel light beam is focused in the direction corresponding to the sub-scanning by a cylinder lens 2 having a positive refractive power in the direction corresponding to the sub-scanning, and forms a long line image in the direction corresponding to the sub-scanning on the deflecting reflection surface 4 of the rotating polygon mirror 3 which is a deflection device. imaged as. The light beam reflected by the deflection reflecting surface 4 becomes a deflected light beam. As the rotating polygon mirror 3 rotates, it is deflected at a constant angular velocity.

The deflected light beam then passes through the fθ cylinder and the surface tilt correction lens 6.
and is imaged as a light spot on the scanning surface 7 by the action of these lenses.

Figure 3 is a diagram showing the optical path from the light source device to the scanning surface, developed along the optical path, with the sub-scanning direction being the vertical direction.

As shown in the figure, in this example, due to the FE cylinder and the surface tilt correction lens 6, the deflection reflection surface 4 and the scanning surface 7 have a substantially conjugate relationship in geometrical optics with respect to the direction corresponding to the sub-scanning, and the deflection reflection surface As shown by the broken line in 4, even if the surface is tilted, the imaging position of the light spot does not move in the sub-scanning direction. Therefore, the surface inclination is corrected.

As a deflection device, a pyramidal mirror can be used in addition to a rotating polygon mirror. In addition to the long cylindrical lens shown in FIG. 4.5, a long toroidal lens or the like can be used as the surface tilt correction lens.

In addition, when using the fθ cylinder of the present invention, it is not necessarily necessary to form a line image of the light beam from the light source device near the deflection reflection surface, and the deflection reflection surface and the scanning surface are subdivided by the fθ cylinder surface tilt correction lens. It is not always necessary to establish a geometrically optical conjugate relationship with respect to the scan-corresponding directions.

[Example] Seven specific examples are listed below.

In each example, f represents the composite focal length of the entire system, and 2θ represents the deflection angle.

The radius of curvature of the first lens surface counting from the object side is R,
(i・1~10), the distance on the optical axis between the 1st and i+1th lens surfaces is D, (1=1~9), the material of the jth lens counting from the object side. Let the refractive index be nj (j=1
~5).

Also 1. 2. K3. K4. 5 represents each parameter of conditions (1) to (V), respectively.

Further, in each embodiment, the incident light beam is a parallel light beam in the direction corresponding to the main scanning, and the distance on the optical axis between the deflection reflection surface and the lens surface is equal to the distance between the deflection reflection surface and the lens surface. shall be.

Examples 1 to 3 mentioned first are examples based on the lens configuration of claim 2.

Example 1 f = 50, 2 0248 degrees, Kl = 0.848. 2=-0,259°K3=-0,359, 4=0.0
17. 5=1.82802i RHD,
, j n, 0 12, 690 1 38.247 5.984 1 1.51
3902 30.573 13.056 3 -12.927 5.440 2 1.83
4864 -17.975 0.870 5-105.846 8.704 3 1.82
8026 -34.648 0.544 7 2003.630 6.528 4 1.7
99298 -106.038 Example 2 f=50, 2θ=48 degrees, K, =0.818. K2
=-0,252°K3=-0,367, 4=0.01
7. 5=1.828021 R+D
lJ njQ 11.720 1 44.541 5.984 1 1.51
3902 31.768 13.056 3 -12.587 5.440 2 1.61
4204 -18.353 0.870 5 -77.306 8.704 3 1.82
8026 -32.149 0.544 7-588.653 6.528 4 1.79
9298 -74.204 Example 3 f=50, 2θ=48 degrees, K, =0.778. 2=-
0,263°K3=-0,371, 4=0.017.
5”1.799291 RID r J
njo 8, 456 1 29.446 5.984 1 1.51
3902 25.097 16.320 3 -13.153 5.4404 -18.
557 0.8705 -69.614 8
．． 7046 -31.194 0.5447
238.045 6.5288 -130.703 The following example has a 4° configuration.

Example 4 f=50, 2θ=48 degrees, KI=0.536. 2”-
0,260°K3 knee 0.360, 4” 0.017
．． 5”1.82802i R, Di J
njo 11, 286 1 30.834 4.896 2 -35.417 2.089 3 24.409 12.832 4 -i3.024 3.264 5 -18.012 0.870 6 -36.364 8.704 1.82802 1.83486 1.82802 1.74601 1.79929 1.79929 1.83486 5 is the lens 7 of claim 3 -27.399 0.8708 300
．． 145 9.792 5 1.828
02 Example 5 f=50, 2θ: 48 degrees, K1=0.811. K2=
-0,250°Ks” 0.342, K4”0.0
17. K5:1.828021 RI
Di jn, 0 12, 134 1 53.194 4.896 1 1.79
4652 380.799 1.629 2 1
．． 614203 32.428 13.056 4 -12.497 3.264 3 1.83
4865 -17.088 0.870 6 -51.338 8.160 4 1.82
8027 -27.436 0.870 8-463.494 9.792 5 1.82
8029 -60.789 The last examples 6 and 7 are examples based on the lens configuration of claim 4.

Example 6 f=50, 2θ=48 degrees, K□=0.558. K2
=-0,252°K3=-0,344, K4=0.01
7. K5=1.82802iRtDt
Jn.

010.732 1 66.716 4.896 1 1
．． 828022-127.315 1.632 3-271.999 1.629 2 1
．． 746014 37.791 10.9315
-12.587 3.264 3 1.
834866 -17.197 0.8707
-36.166 9.556 4 1.
828028 -27.223 0.8709
400.541 9.792 5 1.8
280210 -62.349 Example 7 f=50, 2θ=48 degrees, K,=0.511. K2=
-0,251°K3=-0,344, 4"0.017
．． 5=1.828021 R; D
, J n=0 12, 456 1 62.955 4.896 1 1.82
8022 701.244 1.6323 2
17.599 1.629 2 1614
204 33.247 10.9315 -
12.552 3.264 3 1.83
4866 -17.206 0.8707 -
32.289 6.528 4 1.82
8028 -25.196 0.8709 11
91.683 9.792 5 1.82
80210 -50.379 Aberration diagrams for Examples 1 to 7 are shown in FIGS. 6 to 12 in sequence. The broken line in astigmatism is meridional, and the solid line is sagittal. Aberrations are well corrected in each example.

[Effects of the Invention] As described above, according to the present invention, a telecentric fθ lens can be provided.

Since this f.theta. lens has various aberrations well corrected and is telecentric, it is possible to realize suitable f.theta. characteristics even if the distance between the scanning surface and the f.theta. lens is shifted in the optical axis direction. Furthermore, since the distance from the final lens surface to the scanning surface is large, it can be used together with a surface tilt correction lens.

[Brief explanation of drawings]

1 to 3 are diagrams for explaining the lens configuration of the fθ lens of the present invention, and FIGS. 4 and 5 are diagrams for explaining the lens configuration of the fθ lens of the present invention.
Figures 6 to 12 illustrating an example of how the lens is used
The figures are aberration diagrams for each example. G1. , , 1st group negative meniscus lens G2. ,
, 2nd astigmatism ticket FNo.: 13.5 Frame x and θ = Z4゜Naθ苧义11-1

Claims (1)

[Scope of Claims] 1. A light beam from a light source device is deflected at a constant angular velocity by a deflection device, and the deflected light beam is imaged as a light spot on a scanning surface by an imaging lens system and a surface tilt correction lens to generate light. A lens system used as an imaging lens system in an optical scanning device that performs scanning, in which first to fourth groups are sequentially arranged from the object side to the image side, and the second group is on the object side. The third group is a negative meniscus lens with the concave surface facing the object side, and the fourth group is a positive meniscus lens with the concave surface facing the object side.
The group is a positive lens whose image side lens surface is convex, and the composite focal length of the entire system is f, the focal length of the third group is f_3,
The radius of curvature of the object-side and image-side lens surfaces of the second group lens is R_ I _ I _ I and R_ I _V, respectively. The axial air distance between the second and third groups is D_ I _V, and the third group lens is When the refractive index of the material is n_ I _ I _ I, these are (I)0.4<f/f_3<0.95 (II)-0.3<R_ I _ I _ I /f<- 0.
2(III)-0.4<R_I_V/f<-0.3
A telecentric fθ lens that satisfies the following condition: (IV)0<D_I_V/f<0.06 (V)1.6<n_I_I_I. 2. A telecentric f-theta lens having four elements in four groups, as claimed in claim 1, wherein the first group is a negative meniscus lens with a convex surface facing the object side. 3. Claim 1 is characterized in that the first group is composed of a positive lens with a convex surface facing the object side, and a negative lens cemented to the image side of the positive lens and having a concave surface facing the image side. Telecentric f-theta lens with 5 elements in 4 groups. 4. Claim 1 is characterized in that the first group is composed of a positive lens with a convex surface facing the object side, and a negative lens disposed on the image side of the positive lens with a concave surface facing the image side. Telecentric f-theta lens with 5 elements in 4 groups.