JP3034565B2 - Telecentric fθ lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a telecentric fθ lens.

[Prior Art] A light beam from a light source device is deflected at a uniform angular velocity by a deflecting 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 in order to realize uniform optical scanning.

The generally known 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 fθ lens,
There is one disclosed in JP-A-62-299927.

[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 disposing a surface tilt correction lens.

 For this reason, face tilt correction cannot be performed.

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

[Means for Solving the Problems] Hereinafter, the present invention will be described.

The fθ lens of the present invention provides a light source that deflects a light beam from a light source device at a constant angular velocity by a deflecting device, and forms an image of 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 includes a first group to a fourth group sequentially arranged from the object side to the image side.

The “first group” may be a positive lens or a negative lens.

The “second group” is a negative meniscus lens having a concave surface facing the object side, the “third group” is a positive meniscus lens having a concave surface facing the object side, and the “fourth group” is a positive lens having a convex image-side lens surface. It is.

The combined focal length of the entire system is f, the focal length of the third group is f ₃ , the radii of curvature of the object-side and image-side lens surfaces of the second group lens are R _III , R _IV , and the second and third groups, respectively. On-axis air spacing
D _IV , when the refractive index of the material of the third lens unit is n _III ,
These are (I) 0.4 <f / f ₃ <0.95 (II) −0.3 <R _III /f<−0.2 (III) −0.4 <R _IV /f<−0.3 (IV) 0 <D _IV /f<0.06 (V) The condition 1.6 <n _III is satisfied.

 The first group includes one or two lenses.

FIGS. 1 to 3 show the lens structure of the fθ lens according to claims 2 to 4, respectively. In these figures, reference numeral G2 denotes a second lens unit, reference numeral G3 denotes a third lens unit, and reference numeral G4 denotes a fourth lens unit. The left side of the figure is the object side, that is, the deflecting 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 according to claim 2, the first group is a negative meniscus lens having a convex surface facing the object side.
G1.

As shown in FIG. 2, in the lens configuration of claim 3, the first group includes a positive lens G11 having a convex surface facing the object side,
The negative lens G12 is cemented to the image side of the positive lens G11 and has a concave surface facing the image side.

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

Therefore, in the lens arrangement of the second aspect, the fθ lens has four elements in four groups, and in the lens arrangement of the third and fourth aspects, the fθ lens has four elements in four groups.

[Operation] The conditions (I) to (V) will be described.

Conditions (I) to (III) are all fθ characteristics (the ideal image 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 H ′, {H′−fθ} · 100 / f
θ (%)) and coma aberration are corrected. If the upper limit of these conditions is exceeded, the fθ characteristic and coma aberration are over-corrected, and if the lower limit is exceeded, the correction is under-corrected.

Condition (IV) is a condition for correcting the fθ characteristic,
If the upper limit is exceeded, the correction will be over.

The condition (V) is a condition for correcting the fθ characteristic and the field curvature in the main scanning direction. Outside the condition range, the fθ characteristic is undercorrected, and the residual field curvature increases.

Since the fθ lens of the present invention is used together with the surface tilt correction lens, and the surface tilt correction lens has a function of removing the field curvature in the sub scanning direction, the field curvature correction in the sub scanning direction of the fθ lens is a problem. No need.

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

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

In FIG. 4, the 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 converged in the sub-scanning corresponding direction by the cylinder lens 2 having a positive refractive power in the sub-scanning corresponding direction, and a linear image long in the sub-scanning corresponding direction is formed on the deflecting / reflecting surface 4 of the rotary polygon mirror 3 as a deflecting device. As an image. The light beam reflected by the deflecting reflection surface 4 becomes a deflecting light beam. The light is deflected at a constant angular velocity with the rotation of the rotary polygon mirror 3.

The deflected light beam then passes through the fθ lens 5 and the surface tilt correction lens 6, and forms an image as a light spot on the scanning surface 7 by the action of these lenses.

FIG. 5 is a diagram in which the area from the light source device to the scanning surface is developed along the optical path, and the sub-scanning direction is the vertical direction. As shown in the figure, in this example, the fθ lens 5 and the surface tilt correction lens 6 make the deflecting reflecting surface 4 and the scanning surface 7 substantially geometrically optically conjugate with respect to the sub-scanning corresponding direction. Even when the surface 4 is tilted as shown by the broken line, the image forming position of the light spot does not move in the sub-scanning direction. Therefore, the tilting is corrected.

As the deflecting device, a pyramidal mirror can be used in addition to the rotary polygon mirror. In addition, as the surface tilt correction lens, the fourth and fifth lenses are used.
In addition to the long cylinder lens shown in the figure, a long toroidal lens or the like can be used.

Further, in using the fθ lens of the present invention, it is not always necessary to form a light beam from the light source device into a linear image near the deflecting / reflecting surface. Is not necessarily required to have a geometrical conjugate relationship with respect to the sub-scanning corresponding direction.

EXAMPLES Seven specific examples will be given below.

In each embodiment, f indicates the combined focal length of the entire system, and 2θ indicates the deflection angle.

The radius of curvature of the i-th lens surface counted from the object side is R _i
(I = 1 to 10), the i-th and the distance on the optical axis between the (i + 1) th lens surface D _{i (i} = 1~9), the material of the j-th lens counted from the object side The refractive index is n _j (j = 1 to 5)
Expressed by

Further, K ₁ , K ₂ , K ₃ , K ₄ , and K ₅ are respectively used as conditions (I) to
(V) represents each parameter.

The incident light beam in each embodiment is a parallel light beam in the main scanning corresponding direction, an optical axis distance between the deflection reflecting surface and the first lens surface and D _0.

The first to third embodiments are embodiments based on the lens configuration of claim 2.

Example 1 f = 50, 2θ = 48 degrees, K ₁ = 0.848, K ₂ = −0.259, K ₃ = −0.
359, K ₄ = 0.017, K ₅ = 1.82802 i R _i D _i ij n _j 0 12.690 1 38.247 5.984 1 1.51390 2 30.573 13.056 3 -12.927 5.440 2 1.83486 4 -17.975 0.870 5 -105.846 8.704 3 1.82802 6 -34.648 0.544 7 2003.630 6.528 4 1.79929 8 -106.038 Example 2 f = 50,2θ = 48 degrees, K ₁ = 0.818, K ₂ = −0.252, K ₃ = −0.
367, K ₄ = 0.017, K ₅ = 1.82802 i R _i D _i ij n _j 0 11.720 1 44.541 5.984 1 1.51390 2 31.763 13.056 3 -12.587 5.440 2 1.61420 4 -18.353 0.870 5 -77.306 8.704 3 1.82802 6 -32.149 0.544 7 −588.653 6.528 4 1.79929 8 −74.204 Example 3 f = 50, 2θ = 48 degrees, K ₁ = 0.778, K ₂ = −0.263, K ₃ = −0.
_{371, K 4 = 0.017, K} 5 = 1.79929 i R i D i j n j 0 8.456 1 29.446 5.984 1 1.51390 2 25.097 16.320 3 -13.153 5.440 2 1.83486 4 -18.557 0.870 5 -69.614 8.704 3 1.79929 6 -31.194 0.544 7 238.045 6.528 4 1.79929 8 -130.703 The following Embodiments 4 and 5 are embodiments based on the lens configuration of Claim 3.

Example 4 f = 50, 2θ = 48 degrees, K ₁ = 0.536, K ₂ = −0.260, K ₃ = −0.
360, K ₄ = 0.017, K ₅ = 1.82802 i R _i D _i j n _j 0 11.286 1 30.384 4.896 1 1.82802 2 -35.417 2.089 2 1.74601 3 24.409 12.832 4 -13.024 3.264 3 1.83486 5 -18.012 0.870 6 -36.364 8.704 4 1.82802 7 -27.399 0.870 8 300.145 9.792 5 1.82802 9 -62.923 Example 5 f = 50,2θ = 48 degrees, K ₁ = 0.811, K ₂ = −0.250, K ₃ = −0.
342, K ₄ = 0.017, K ₅ = 1.82802 i R _i D _i ij n _j 0 12.134 1 53.194 4.896 1 1.79465 2 380.799 1.629 2 1.61420 3 32.428 13.056 4 -12.497 3.264 3 1.83486 5 −17.088 0.870 6 −51.338 8.160 4 1.82802 7−27.436 0.870 8−463.494 9.792 5 1.82802 9−60.789 Finally, the sixth and seventh embodiments are embodiments based on the lens configuration of claim 4.

Example 6 f = 50, 2θ = 48 degrees, K ₁ = 0.558, K ₂ = −0.252, K ₃ = −0.
344, K ₄ = 0.017, K ₅ = 1.82802 i R _i D _i ij n _j 0 10.732 1 66.716 4.896 1 1.82802 2 -127.315 1.632 3 -271.999 1.629 2 1.74601 4 37.719 10.931 5 -12.587 3.264 3 1.83486 6 -17.197 0.870 7 −36.166 9.556 4 1.82802 8 −27.223 0.870 9 400.541 9.792 5 1.82802 10 −62.349 Example 7 f = 50, 2θ = 48 degrees, K ₁ = 0.511, K ₂ = −0.251, K ₃ = −0.
344, K ₄ = 0.017, K ₅ = 1.82802 i R _i D _i ij n _j 0 12.456 1 62.955 4.896 1 1.82802 2 701.244 1.632 3 271.599 1.629 2 1.61420 4 33.247 10.931 5 -12.552 3.264 3 1.83486 6 -17.206 0.870 7-32.289 6.528 4 1.82802 8 -25.196 0.870 9 1191.683 9.792 5 1.82802 10 -50.379 FIGS. 6 to 12 show aberration diagrams for Examples 1 to 7. The broken line in astigmatism is meridional, and the solid line is sagittal. Aberration is well corrected in each embodiment.

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

Since the fθ lens is well corrected for various aberrations and is telecentric, an appropriate fθ characteristic can be realized even if the distance between the scanning surface and the fθ lens is shifted in the optical axis direction. Further, since the distance from the last lens surface to the scanning surface is large, it can be used together with a surface tilt correction lens.

[Brief description of the drawings]

1 to 3 are views for explaining the lens configuration of the fθ lens of the present invention, and FIGS. 4 and 5 are fθ lenses of the present invention.
FIG. 6 to FIG. 12 are diagrams illustrating an example of use of a lens, and FIG. 6 to FIG. G1 ... Negative meniscus lens of the first group, G2 ... Second group, G3
…… Third group, G4 …… Fourth group

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (4)

(57) [Claims] A light beam from a light source device is deflected at a constant angular velocity by a deflecting 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. A lens system used as an imaging lens system in an optical scanning device, comprising a first group to a fourth group sequentially arranged from the object side to the image side, and a second group having a concave surface facing the object side. A negative meniscus lens, the third group is a positive meniscus lens having a concave surface facing the object side,
The group is a positive lens having a convex image-side lens surface. The combined focal length of the entire system is f, the focal length of the third group is f ₃ ,
The radii of curvature of the object-side and image-side lens surfaces of the group lens are R _III and R _IV , respectively, the on-axis air gap between the second and third groups is D _IV ,
Assuming that the refractive index of the material of the third lens unit is n _III , they are (I) 0.4 <f / f ₃ <0.95 (II) −0.3 <R _III /f<−0.2 (III) −0.4 <R _IV /f<-0.3 (IV) 0 <D _IV /f<0.06 (V) 1.6 <n _III A telecentric fθ lens characterized by satisfying the following condition: 2. The telecentric fθ lens according to claim 1, wherein the first group is a negative meniscus lens having a convex surface facing the object side. 3. The method according to claim 1, wherein the first lens unit includes a positive lens having a convex surface facing the object side, and a negative lens having a concave surface facing the image side of the positive lens joined to the image side. A telecentric fθ lens having a configuration of five elements in four groups. 4. The positive lens according to claim 1, wherein the first lens unit includes a positive lens having a convex surface facing the object side, and a negative lens disposed on the image side of the positive lens and having a concave surface facing the image side. A telecentric fθ lens having a configuration of five elements in four groups.