JPH0746170B2 - f / θ lens - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an f.theta. Lens used in an apparatus for recording and reading with a laser beam subjected to luminance modulation such as a facsimile and a laser printer.

2. Description of the Related Art The f.theta. Lens is used in printers and facsimiles that use a laser beam. The principle of the f.theta. Lens will be described with reference to FIG. The beam is expanded by the beam expander 13, the expanded beam is subsequently reflected by the reflection mirror 14 to be expanded by the beam expander 13, and the expanded beam is rotated by a polyhedral mirror 15 or the like. A deflection angle θ is given by the deflector. A laser beam given this deflection angle θ is recorded by an f · θ lens (constant speed scanning lens) 16 having a focal length f using a reflection mirror 17, or a surface to be scanned such as a reading medium.
Focus on 18 The f.theta. Lens 16 is further arranged so that the deflection angle .theta. Having a constant angular velocity and the scanning speed on the surface to be scanned 18 which is a recording or reading image plane orthogonal to the optical axis are in a proportional relationship. The lens 16 has a distortion aberration, and the deflection angle θ and the image height Y of the image formed from the optical axis are set so that a uniform scanning speed is always obtained.
It is designed such that Y = f · θ during the period.

As a conventional f / θ lens, for example, an ortho-meter type lens described in JP-A-55-53308 or JP-A-55-53308
There are known telecentric types described in JP-A-18613 and JP-A-51-37245.

Problems to be Solved by the Invention However, when recording or reading with a scanning length of 500 mm or more, the conventional telecentric f.theta.
Although the aberration is very good and a good spot can be obtained, it requires a lens diameter equal to or longer than the scanning length, which results in an enormous size of the lens, which is not practical.

Also, the angle of view of the ortho-meter type f / θ lens is 60
Since the lens has a wide angle of about 0 °, it has a compact lens shape even if the scanning length becomes large, and is the most suitable lens for a scanning system having a long scanning length. When it is 500 mm or more, there is a drawback that high-order astigmatism increases, and a uniform beam spot is not formed in the scanning direction.Moreover, even for a scanning length of 500 mm or less, the amount of curvature of the image in the sagittal direction is large, If the spot diameter is 21 μm or less, there is a drawback that the spot is not uniform in the scanning direction.

Therefore, the present invention solves the above-mentioned problems of the prior art, minimizes the image curvature in the sagittal direction even at a long scanning length, has a constant speed scanning property, and is bright and has a high resolution. It is intended to provide an image f / θ lens.

Means for Solving the Problems In order to solve the above problems, the present invention provides a positive meniscus lens having a concave surface on the light incident side and a negative lens having a concave surface on the light incident side. A lens system having a five-group, five-element configuration including a second lens, which is a meniscus lens, and a third lens, which is a positive meniscus lens with a concave surface facing the light incident side, a fourth lens, and a fifth lens. It is characterized by satisfying each condition of.

(1) 0.90 <| r ₂ / r ₃ | <1.02 (2) 0.007f <d ₄ <0.050f (3) −1.3f <r ₅ <−0.43f (4) 0.008f <| d ₃ / n ₂ ＋ d ₄ ＋ d ₅ / n ₃ ＋ d ₆ ＋ d ₇ / n ₄ ＋ d ₈ ＋ d ₉
/ n ₅ | <0.12f (where r ₂ , r ₃ and r ₅ are the exit surface of the first lens, the entrance surface of the second lens, the entrance surface of the third lens, d ₃ , d ₅ , d ₇ , and d ₉ are the lens thickness of the second lens, the lens thickness of the third lens, the lens thickness of the fourth lens, the lens thickness of the fifth lens, and n ₂ , n ₃ , n ₄ , n ₅ are The refractive index of the second lens, the refractive index of the third lens, the refractive index of the fourth lens, and the fifth lens, respectively.
The refractive indices of the lenses, d ₄ , d ₆ , and d ₈ , are the second lens and the third lens, respectively.
Lens surface (air) distance, third lens and fourth lens surface (air) distance, fourth lens and fifth lens surface (air) distance, and f is the focal length of the entire system. The f.theta. Lens of the present invention produces negative distortion which is a characteristic of the f.theta. Lens, f is the focal length of the entire system, .theta. Is the deflection angle, and Y'is from the optical axis of the image forming position. Shows constant velocity scanning when distance The value of is close to the atmosphere. Moreover,
In order to obtain a spot diameter close to the diffraction limit at a small F number, spherical aberration is satisfactorily corrected, and even if the depth of focus is small, the image in the sagittal direction has a uniform spot shape in the scanning direction. This is a correction of the strain.

Therefore, the f.theta. Lens of the present invention satisfies the above four conditions.

Under the condition (1), if | r ₂ / r ₃ | exceeds the upper limit of 1.02, the spherical aberration becomes excessive and falls in the positive direction, and conversely, if it becomes lower than 0.90, the spherical aberration becomes too under.

When d ₄ exceeds the upper limit of 0.050f in the condition (2), the meridional image is tilted in the positive direction, the high-order astigmatism in the sagittal direction is increased, and the amount of curvature of the image is increased. If the lower limit of 0.007f is not reached, the meridional image will tilt in the negative direction, increasing the astigmatism with respect to the sagittal direction and further reducing distortion.

When r ₅ is equal to or lower than the lower limit of −1.3f in the condition (3), high-order astigmatism in the sagittal direction increases, and the amount of image curvature increases. When the upper limit of −0.43f is exceeded, the astigmatic difference between the meridional direction and the sagittal direction increases, and the distortion aberration further decreases.

In condition (4), when the entire length from r ₃ to r ₁₀ is replaced with air, the sagittal curvature increases when the length exceeds the upper limit of 0.12f, and the distortion decreases when the length becomes less than the lower limit of 0.08f. , Fθ characteristics cannot be obtained.

Action The present invention has a five-group, five-element configuration including a negative meniscus lens,
By concentrating the aperture and shortening the overall length, the image curvature in the sagittal direction can be minimized even at a long scanning length, and it has a constant speed scanning property and a bright high resolution. You can get a statue.

Examples Examples of the present invention will be shown below.

FIG. 1 shows the basic configuration of the f.theta. Lens of the present invention. The f.theta. Lens of the present invention is a positive meniscus lens having a concave surface on the light incident side, which is a first lens 1, a third lens 3, and a fourth lens.
A lens 5 including a lens 4 and a fifth lens 5 and a second lens 2 which is a negative meniscus lens having a concave surface facing the light incident side.
It consists of 5 pieces. Details are shown below. However,
r ₁ is the entrance side surface of the first lens 1, r ₂ is the exit side surface of the first lens 1, r ₃ is the entrance side surface of the second lens 2, and r ₄ is the exit side surface of the second lens 2. Surface, r ₅ is the surface on the incident side of the third lens 3,
r ₆ is the exit side surface of the third lens 3, r ₇ is the entrance side surface of the fourth lens 4, r ₈ is the exit side surface of the 4th lens 4, and r ₉ is the entrance side of the 5th lens 5. Surface, r ₁₀ is the surface on the exit side of the fifth lens 5, and d ₁ , d ₃ , d ₅ , d ₇ , and d ₉ are the first, second, third, and
The lens thickness of the fourth and fifth lenses 1, ₂ , 3, 4, and 5, d ₂ ,
d ₄ , d ₆ and d ₈ are the surface spacing between the first lens 1 and the second lens 2, the surface spacing between the third lens 3 of the second lens 2 and the surface spacing between the third lens 3 and the fourth lens 4, respectively. The surface distance between the fourth lens 4 and the fifth lens 5, n ₁ , n ₂ , n ₃ , n ₄ and n ₅ are respectively the first and
Refractive indices of the second, third, fourth, and fifth lenses 1, 2, 3, 4, and 5, f is the focal length of the entire system, F is the F number of the lens system, Y is the image height, and λ is the wavelength. is there.

<Example _{1> r 1 = -0.14191 d 1} = 0.011785 n 1 = 1.63613 r 2 = -0.11008 d 2 = 0.014000 r 3 = -0.10999 d 3 = 0.024934 n 2 = 1.56459 r 4 = -0.69993 d 4 = 0.015998 r _{5 = -0.65254 d 5 = 0.023998 n} 3 = 1.61985 r 6 = -0.29656 d 6 = 0.001315 r 7 = -0.67047 d 7 = 0.03000 n 4 = 1.46300 r 8 = -0.29104 d 8 = 0.0004 r 9 = -1.65163 d 9 = 0.0334 n ₅ = 1.49668 r ₁₀ = -0.36443 f = 1, F / 23.7, Y = 0.526, λ = 632.8nm | r ₂ / r ₃ | = 1.0008 | d ₃ / n ₂ + d ₄ + d ₅ / n ₃ + d _{6 + d 7 / n 4 +} d 8 + d 9 / n 5 | = 0.0913 FIG. 2 (a) ~ (c) shows spherical aberration of example 1, astigmatism, the correction state of the distortion.

<Example 2> r ₁ = -0.14778 d ₁ = 0.016187 n ₁ = 1.63613 r ₂ = -0.11102 d ₂ = 0.012378 r ₃ = -0.10963 d ₃ = 0.011426 n ₂ = 1.57216 r ₄ = -0.48942 d ₄ = 0.034216 r _{5 = -0.57131 d 5 = 0.021900 n} 3 = 1.63613 r 6 = -0.31700 d 6 = 0.000952 r 7 = -0.77127 d 7 = 0.036183 n 4 = 1.51509 r 8 = -0.29422 d 8 = 0.000952 r 9 = -1.80914 d 9 _{= 0.033136 n 5 = 1.51509 r 10} = -0.48847 f = 1, F / 23.06, Y = 0.526, λ = 632.8nm | r 2 / r 3 | = 1.0127 | d 3 / n 2 + d 4 + d 5 / n 3 + d _{6 + d 7 / n 4 +} d 8 + d 9 / n 5 | = 0.1025 FIG. 3 (a) ~ (c) shows spherical aberration of example 2, the astigmatism, the correction state of the distortion.

<Example _{3> r 1 = -0.16353 d 1} = 0.025636 n 1 = 1.65575 r 2 = -0.15022 d 2 = 0.034608 r 3 = -0.15740 d 3 = 0.010254 n 2 = 1.57215 r 4 = -0.55501 d 4 = 0.009075 r _{5 = -0.60500 d 5 = 0.041530 n} 3 = 1.63613 r 6 = -0.33480 d 6 = 0.001282 r 7 = -0.71652 d 7 = 0.050143 n 4 = 1.63613 r 8 = -0.35890 d 8 = 0.001282 r 9 = -1.99112 d 9 _{= 0.042300 n 5 = 1.63613 r 10} = -0.62792 f = 1, F / 17.93, Y = 0.526, λ = 632.8nm | r 2 / r 3 | = 0.9544 | d 3 / n 2 + d 4 + d 5 / n 3 + d _{6 + d 7 / n 4 +} d 8 + d 9 / n 5 | = 0.1000 FIG. 4 (a) ~ (c) shows spherical aberration of example 3, the astigmatism, the correction state of the distortion.

<Example _{4> r 1 = -0.15710 d 1} = 0.022840 n 1 = 1.65575 r 2 = -0.17420 d 2 = 0.042600 r 3 = -0.15720 d 3 = 0.025700 n 2 = 1.57215 r 4 = -0.38420 d 4 = 0.009420 r _{5 = -0.46900 d 5 = 0.032000 n} 3 = 1.65575 r 6 = -0.37000 d 6 = 0.001000 r 7 = -0.64000 d 7 = 0.038000 n 4 = 1.65575 r 8 = -0.34080 d 8 = 0.001000 r 9 = -1.68000 d 9 _{= 0.053400 n 5 = 1.65575 r 10} = -0.57000 f = 1, F / 14.80, Y = 0.410, λ = 632.8nm | r 2 / r 3 | = 0.936 | d 3 / n 2 + d 4 + d 5 / n 3 + d _{6 + d 7 / n 4 +} d 8 + d 9 / n 5 | = 0.1023 FIG. 5 (a) ~ (c) shows spherical aberration of example 4, astigmatism, the correction state of the distortion.

EFFECTS OF THE INVENTION In summary, according to the present invention, by using a five-group, five-element configuration including a negative meniscus lens, a concentric shape with respect to the diaphragm and a shortened overall length, even in a long scanning length, the sagittal direction is reduced. Image curvature can be minimized, uniform scanning characteristics can be obtained, and bright high resolution can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of an f.theta. Lens according to an embodiment of the present invention, FIG. 2 is an aberration curve diagram of the f.theta. Lens according to the first embodiment of the present invention, and FIG. 3 is a second view of the present invention. 4 is an aberration curve diagram of the f.theta. Lens in the example of FIG.
FIG. 5 is an aberration curve diagram of the f.theta. Lens in the embodiment of FIG. 5, FIG. 5 is an aberration curve diagram of the f.theta. Lens in the fourth embodiment of the present invention, and FIG. 6 is a perspective view showing the scanning optical system. 1 ... First lens, 2 ... Second lens, 3 ... Third lens, 4 ... Fourth lens, 5 ... Fifth lens, 11 ... Laser light source, 12 ... Modulator, 13 ... Beam expander, 14 …… Reflecting mirror, 15 …… Polyhedral mirror, 16 …… f
・ Θ lens, 17 …… Reflecting mirror, 18 …… Surface to be scanned.

Claims (1)

[Claims] 1. A first lens, which is a positive meniscus lens having a concave surface on the light incident side, and a second lens, which is a negative meniscus lens having a concave surface on the light incident side, both having a concave surface on the light incident side. 5 group 5 consisting of a third lens, a fourth lens and a fifth lens, which are positive meniscus lenses facing each other
An f · θ lens characterized by satisfying the following conditions in a lens system having a single-lens configuration. (1) 0.90 <| r ₂ / r ₃ | <1.02 (2) 0.007f <d ₄ <0.050f (3) −1.3f <r ₅ <−0.43f (4) 0.08f <| d ₃ / n ₂ ＋ d ₄ ＋ d ₅ / n ₃ ＋ d ₆ ＋ d ₇ / n ₄ ＋ d ₈ ＋ d ₉ /
n ₅ | <0.12f (where r ₂ , r ₃ and r ₅ are the exit surface of the first lens, the entrance surface of the second lens, the entrance surface of the third lens, d ₃ and d ₅ , ₅ , d ₇ , and d ₉ are the lens thickness of the second lens, the lens thickness of the third lens, the lens thickness of the fourth lens, the lens thickness of the fifth lens, and n ₂ , n ₃ , n ₄ , n ₅ , respectively. Refractive index of second lens, third lens, fourth lens, fifth
The refractive indices of the lenses, d ₄ , d ₆ , and d ₈ , are the second lens and the third lens, respectively.
The surface distance between the lenses, the surface distance between the third lens and the fourth lens, the surface distance between the fourth lens and the fifth lens, and f are the focal lengths of the entire system. )