JP2512251B2 - Telecentric scanning optical system - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a telecentric scanning optical system, and more particularly to a telecentric scanning optical system for parallel scanning a light beam reflected by a sinusoidal vibrating mirror.

[0002]

2. Description of the Related Art In a telecentric scanning optical system used when a polygon mirror is rotated to scan / image light, the light reflected by the polygon mirror rotated at a constant speed and high speed enters a scanning lens. Changes in proportion to time.

That is, since the incident angle θ changes constantly with time, the central portion of the image plane has a slow scanning speed and the peripheral portion has a high scanning speed. Was used.

Further, in the case where the rotary polygon mirror is not used, a scanning beam is obtained by using an AOD which polarizes light by ultrasonic waves, but in any case, there is a problem that the apparatus becomes large and expensive. there were.

Further, there is a method in which a galvanometer is used instead of the rotary polygon mirror, and this galvanometer is combined with an arc sine θ lens, but it is not so general.

Now, the rotation angle φ of the deflecting mirror vibrating with a constant amplitude φ 0 is expressed by the following equation, where ω is the angular frequency and t is the time.

Φ ≦ φ 0 · sin ωt At this time, the deflection angle θ of the beam is given by the following equation: θ = 2φ = 2φ 0 · sin ωt A beam is scanned at a constant speed on the image plane of the scanning lens. , It is necessary to use a scanning lens whose image height y ′ is represented by the following equation.

Y ′ = 2φ 0 · sin ⁻¹ (θ / 2φ 0) A lens satisfying the above relation is called an arcsine θ lens.

The high-speed galvanometer may use a saw-tooth-shaped high-frequency current when vibrating at high speed, but since it is difficult to obtain a saw-tooth-shaped high-frequency current, a sine-wave current is used. By combining the arc sine θ lens, it is possible to obtain a small and inexpensive optical system that scans the image plane at a constant speed with high accuracy.

[0010]

DISCLOSURE OF THE INVENTION In the arcsine θ lens as described above, the distortion characteristics are such that the mirror contact width φ0
It changes with (the deflection angle of the mirror centered around the time when the light beam heading to the center of the mirror is reflected by the mirror and the reflected light coincides with the optical axis of the imaging lens), and f θ = ∞ and f θ
Become a lens. The arc sine θ lens is generally used when the mirror amplitude φ 0 is 20 ° or less,
In this case, the distortion aberration in f tan θ display is positive.

When the distortion aberration is corrected to a positive value, particularly in a pre-aperture lens such as a telecentric system having a one-group structure as a whole, the curvature of field tends to increase greatly in the positive direction.

As shown in FIG. 5, the mirror amplitude is set to 20.
F tan θ
The distortion on the display becomes 0 to negative, and the aberration correction becomes very easy.

In FIG. 5, when y ′ = f tan θ is the ideal image height, the image height y ′ = 2φ 0 f sin ⁻¹ (θ / 2φ 0) is ft.
It is displayed as distortion aberration with respect to anθ ({2φ 0 f si
n ⁻¹ (θ / 2φ 0) − f tan θ} / f tan θ × 100
(%)).

In this case, however, the mirror has to be swung beyond the required polarization angle, and the durability problem arises.

On the other hand, if the use angle is made small, there is a problem that a lens having a long focal length must be used to scan the same image height, and the optical system becomes large.

The present invention has been made in view of the above-mentioned conventional problems, and corrects the distortion aberration positively, and
It is an object of the present invention to provide a telecentric scanning optical system that uses an arc sine θ lens whose field curvature is corrected and that is inexpensive, compact and highly accurate.

[0017]

SUMMARY OF THE INVENTION The present invention is a telecentric scanning optical system in which a light beam is directed to a sine wave drive scanning mirror and the light beam reflected by the scanning mirror is imaged on a scanning surface by an imaging lens. , The imaging lens,
It is composed of a first lens group and a second lens group as a whole, the first lens group is a convex lens, the second lens group is composed of a concave lens and a convex lens and has a positive refracting power as a whole, F is a total focal length, and e1 is a first lens group. And the principal point distance between the second lens group and f1 and f2 are the focal lengths of the first lens group and the second lens group, the relationship of 0.3F <e1 <0.95F 1.1F <f1 1.1F <f2 is satisfied. In addition, the scanning mirror is arranged at the front focal point of the first group to achieve the above object.

[0018]

According to the present invention, F is the entire focal length, e1 is the principal point distance between the first and second lens groups, f1 and f2 are the first lens group,
Assuming that the focal length of the second lens unit is 0.3F <e1 <0.95
Since it is set to F, the distortion and the field curvature can be corrected in a well-balanced manner. Increasing e1 beyond the upper limit of 0.95F is good for correcting distortion and field curvature,
The front focus position and the image point position are close to the lens, and the scanning mirror cannot be placed, or the lens and the image forming position are too close to each other, which causes difficulty in workability and scannability.

If e1 is made smaller than the lower limit of 0.3F, when the distortion is corrected positively, the curvature of field is also overcorrected in the positive direction, and a wide scanning range cannot be obtained.

Further, the front focus position and the image forming position are defined as 1.1F <f1 and 1.1F <f2. f1 is 1.1F
If it becomes smaller, the image point can be formed in the vicinity of the second group, and the workability, the scannability, etc. are deteriorated.

When f2 is smaller than 1.1F, the first
The front focus position of the group can be near the first group,
It becomes impossible to arrange the scanning mirror.

[0022]

EXAMPLES Examples of the telecentric scanning optical system of the present invention will be described below.

Imaging lens 1 of the first embodiment shown in FIG.
0 is composed of the first group 12 and the second group 14 as a whole.

The first group 12 is composed of one convex lens L1, and the second group 14 is constructed by cementing a concave lens L2 and a convex lens L3, and has a positive refracting power as a whole.

Reference numeral 16 in FIG. 1 designates a sine wave driving scanning mirror composed of a high-speed galvanometer mirror or the like, and 18 designates a light beam from a light source 19 to make it a parallel light beam, and said sine wave driving scanning mirror 1
6A and 6B respectively show collimator lenses for making the light incident on the lens 6.

The lens specifications are as shown in Table 1 below.

Here, r is the radius of curvature (mm), d is the lens thickness or interval (mm), n670 is the refractive index for light of wavelength λ = 670 nm, mirror amplitude φ 0 = ± 12 °, polarization angle θ used =
It is ± 6.089 °.

<Table 1> r d n670 1 56.875 2.5 1.70465 L1 2 -117.07 22 1. 3 20.08 1.2 1.83488 L2 4 9.387 4 1.5139 L3 5 -28.26

Here, the total focal length F = 30.01 mm
Thus, the principal point spacing e1 = 24. Between the first group 12 and the second group 14.
45mm = 0.81F, f1 = 48.48mm = 1.62F,
f2 = 39.06 mm = 1.3 F, which satisfies all the above three conditional expressions.

The cubic Seidel coefficient is I ... 0.94
6, II ... 0.009, III ... -0.498, P ... 1.0
18, V ... -0.995, and as shown in the aberration diagram of FIG. 2, it can be seen that all the aberrations are well corrected in the use state.

Next, the image forming lens of the second embodiment shown in FIG. 3 will be described.

The image forming lens 20 of the second embodiment has a first
It is composed of a group 22 and a second group 24, the first group is composed of one convex lens L1, and the second group is composed of a convex lens L2 and a concave lens L1.
It is constructed by joining 3 and has a positive refracting power as a whole.

The specifications of the lens are as shown in Table 2 below.

[0034]

Also in this second embodiment, e1 = 26.31 mm = 0.8 for the entire focal length F = 30.0 mm.
8F, f1 = 38.72mm = 1.29F, f2 = 42.73
mm = 1.42F, which satisfies all the above three conditions.

The third-order Seidel coefficient is I ... 0.845, II
...- 0.315, III ...- 0.630, P ... 1.05
3, V ... -0.999, and as shown in the aberration diagram of FIG. 4, it can be seen that all the aberrations are well corrected in use.

In the above embodiment, the second groups 14 and 24 are both cemented with a concave lens and a convex lens, but the present invention is not limited to this, and the concave lens and the convex lens are separated. Even in this case, it goes without saying that a predetermined aberration correction can be obtained.

[Brief description of drawings]

FIG. 1 is an optical layout diagram showing an embodiment of a telecentric scanning optical system according to the present invention.

FIG. 2 is a diagram showing aberrations of the lens of the same example.

FIG. 3 is an optical layout diagram showing an imaging lens according to Example 2 of the present invention.

FIG. 4 is a diagram showing aberrations of the lens of the example.

FIG. 5 is a diagram showing a relationship between a mirror amplitude φ0 in an arcsine θ lens and a distortion aberration in ftan θ display.

[Explanation of symbols]

 10, 20 ... Imaging lens, 12, 22 ... First group, 14, 24 ... Second group, 16 ... Sine wave drive scanning mirror, 18 ... Collimator lens, 19 ... Light source.

Claims (1)

(57) [Claims] 1. A light beam is directed to a sinusoidal drive scanning mirror,
In a telecentric scanning optical system in which a light beam reflected by the scanning mirror is imaged on a scanning surface by an imaging lens, the imaging lens is composed of a first group and a second group as a whole, and the first group is The second lens group is composed of a concave lens and a convex lens and has a positive refracting power as a whole. F is the overall focal length, e1 is the principal point distance between the first and second lens groups, and f1 and f2 are the first lens group. When the focal length of the second lens unit is set, the relationship of 0.3F <e1 <0.95F 1.1F <f1 1.1F <f2 is satisfied, and the scanning mirror is arranged at the front focal point of the first lens unit. A telecentric scanning optical system characterized by the above.