JPH045167B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a constant velocity scanning lens used in a scanning optical system of a printer or the like.

(Structure of conventional example and its problems) An optical system as shown in FIG. 1 is used as a scanning optical system that deflects a laser beam whose brightness is modulated by a signal and scans and forms an image on a recording medium. A beam 2 emitted from a light source 1 is deflected by a rotating polygon mirror 3 rotating in the direction of an arrow 7, and a spot 6 is scanned and imaged onto a scanning surface 5 by an imaging lens 4. Normally, a rotating polygon mirror rotates at a constant angular velocity, so the deflection angular velocity is constant.In order to keep the scanning velocity of a spot on the scanning surface constant, an imaging lens is used to change the angle of incidence of the light beam to the lens by θ. When the focal length of the lens is f and the image height is y, it is preferable to use an imaging lens, that is, a constant velocity scanning lens, which has distortion characteristics such that y=f·θ.

The constant velocity scanning lens is required to have good scanning linearity with respect to the ideal image height y=fθ, a wide angle of view, and a spot diameter that is close to the diffraction limit. In order to satisfy these requirements, it is generally possible to eliminate various aberrations by increasing the number of lenses, using glass with a high refractive index, etc., but this results in a significant increase in cost. Japanese Patent Application Laid-Open No. 57-10109 is an example of a lens that aims to reduce costs by having a small number of components, but it has not been possible to obtain a lens with high performance in terms of distortion characteristics, such as a deviation in scanning linearity of 1% or more. Nakatsuta. Alternatively, even if good distortion characteristics were provided, correction of various aberrations would be insufficient, resulting in deterioration of image performance.

(Object of the Invention) The present invention has been made in view of the above points, and it is an object to provide a constant velocity scanning lens that achieves high performance and low cost.

(Structure of the Invention) The constant velocity scanning lens of the present invention has a two-lens structure of two groups consisting of a negative first lens and a positive second lens in order from the entrance pupil (position where the beam is deflected), It has good distortion characteristics and diffraction-limited imaging performance.

(Description of Examples) Hereinafter, the constant velocity scanning lens of the present invention will be specifically described with reference to Examples.

The necessary angle of view of the constant velocity scanning lens is determined by the determined scanning width and the focal length of the lens, which determines the size of the scanning optical system. Generally, a wide angle of view of around 32 degrees is required at half angle, but since the diameter of the incident light beam is small and the F-number is large, there is no need to worry too much about spherical aberration. All you need to do is to take into consideration improving the flatness of the image plane and correcting coma aberration. In addition, as a method for generating a power arrangement that reduces the Petzval sum and a negative distortion characteristic of y = fθ, taking into account that the ray height through which off-axis rays pass becomes higher as the distance from the entrance pupil increases, It is desirable to have a configuration in which a negative lens and a positive lens are arranged in this order from the pupil side, and the positive lens generates negative distortion.

In the constant velocity scanning lens according to the present invention,
It is composed of a negative first lens and a positive second lens in order from the human pupil side, and satisfies the following conditions.

(1) n ₁ ＜1.6 (2) n ₂ ＜1.6 (3) −12＜r ₂ /r ₁ ＜−6 (4) 3f＜r ₃ ＜5f However, n ₁ and n ₂ are the refractive index of each lens. , r ₁ , r ₂ , r ₃ ,
r ₄ is the radius of curvature of each refractive surface, and f is the combined focal length of the entire system.

Conditions (1) and (2) are intended to reduce the cost of Petzval sum and glass materials. As mentioned above, negative distortion characteristics can be produced by increasing the positive power of the second lens, but the Petzval sum of the entire system is positively biased and the image plane is under-biased, resulting in a wide field of view. Corners become difficult. In order to reduce this Petzval sum, it is preferable to use a glass material with a low refractive index for the negative first lens and a high refractive index for the positive second lens.
Specifically, if the refractive index of the first lens is greater than 1.6, the Petzval sum becomes too large and the flatness of the image plane deteriorates. If the second lens has a large refractive index, the Petzval sum can be reduced, but since the cost of the glass material generally increases, it is desirable to keep the refractive index to 1.6 or less for the purpose of cost reduction.

Condition (3) indicates the relationship between the radius of curvature of the first surface and the second surface of the negative first lens, and determines the sharing of power between the first surface and the second surface of the first lens for correcting the Petzval sum. This is a condition for properly correcting astigmatism and coma aberration. Considering the correction of astigmatism and coma aberration, this means reducing the absolute value of the radius of curvature _r1 of the first surface of the first lens and making the incidence of off-axis rays on this surface as close to normal incidence as possible. For example, the occurrence of astigmatism and coma can be reduced.
When the lower limit of the condition is exceeded, the distortion occurring at the second surface of the first lens becomes small, and the distortion characteristics of the entire system become negative. If the upper limit is exceeded, astigmatism and coma aberration occurring on the first surface become large and difficult to correct. Condition (4) indicates the relationship between the radius of curvature of the third surface of the second lens, and is a condition for satisfactorily correcting astigmatism and distortion. When the lower limit is exceeded, the sagittal image plane moves positively, the astigmatism difference increases, and the flatness of the image plane deteriorates. When the upper limit is exceeded, the occurrence of negative distortion at the third surface decreases, and the distortion characteristics of the entire system shift toward the positive.

Furthermore, since the present invention aims to achieve high performance while reducing costs, the number of components is small, and in order to further improve performance, it is desirable to make at least one surface aspherical. On the other hand, it is extremely difficult to achieve both asphericity and cost reduction using a glass lens, so it is more desirable to use a plastic lens.

Examples of the constant velocity scanning lens according to the present invention described above will be shown below.

Example 1 f=235 F80 Angle of view 2ω=64° Entrance pupil is in front of the first lens 15.00 r ₁ −64.249 r ₂ 488.451 ^*1 r ₃ 846.790 r ₄ −57.162 ^*2 d ₁ 1.00 d ₂ 14.726 d ₃ 11.00 n ₁ 1.488 n ₂ 1.582 However, r ₁ , r ₂ , r ₃ , r ₄ are the radius of curvature of each surface, d ₁ , d ₂ ,
d ₃ is the distance between the surfaces of each lens, n ₁ and n ₂ are the refractive index of each lens at a wavelength of 780 nm, and each acrylic resin,
It is polycarbonate resin. * is an aspherical surface, and when X is the deviation from the lens apex at the position of the radial distance Y of the aperture from the optical axis of the lens, It is indicated by. However, AD, AE, AF, and AG are aspheric coefficients. (The same applies to the following examples) *1 *2 AD 7.34712×10 ^-7 −6.67243×10 ^-8 AE 3.37367×10 ^-9 −5.83094×10 ^-11 AF −1.01671×10 ^-11 −1.04272×10 ^-14 AG −2.66026×10 ^-14 7.78683×10 ^-17 r ₂ /r ₁ = −7.60 r ₃ =3.60f Example 2 f=235 F80 Angle of view 2ω=64° Entrance pupil is 15.00 r ₁ −64.319 r in front of the first lens ₂ 652.991 ^*1 r ₃ 984.540 r ₄ −57.641 ^*2 d ₁ 1.00 d ₂ 14.726 d ₃ 11.00 n ₁ 1.488 n ₂ 1.582 *1 *2 AD 3.54090×10 ^-7 −1.97677×10 ^-8 AE 4.731 81×10 ^-10 -1.94567×10 ^-11 AF 4.79894×10 ^-12 2.94903×10 ^-15 AG −1.59411×10 ^-14 2.74076×10 ^-17 r ₂ /r ₁ =-10.15 r ₃ =4.19f Constant speed scanning of Example 1 above A lens layout diagram of the lens is shown in FIG. The third one shows astigmatism, where the solid line shows the aberration in the sagittal (S) direction and the dotted line shows the aberration in the meridional (M) direction. FIG. 4 shows the linearity of scanning expressed as a percentage deviation from the ideal line height y=fθ. Similarly for Example 2, FIG. 5 shows the lens arrangement, and FIG. 6 shows the astigmatism.
FIG. 7 shows the linearity of each scan. The maximum astigmatism is about 3 mm, which is well within the depth of focus determined by the brightness of the constant-velocity scanning lens system, and the deviation in scanning linearity is about 0.7% at most, which reduces various aberrations. It has been well corrected.

(Effect of the invention) As described above, the present invention has a wide angle of view, and
This is a constant-velocity scanning lens that has a two-element structure and uses inexpensive plastic as a glass material, reducing costs and providing good correction of various aberrations.
It has great industrial value.

[Brief explanation of drawings]

Figure 1 is a diagram showing the configuration of a scanning and optical system in which the lens of the present invention is used, Figure 2 is a lens arrangement diagram of Example 1 based on the present invention, and Figures 3 and 4 are Example 1.
Similarly, FIG. 5 is a lens arrangement diagram of the second embodiment, and FIGS. 6 and 7 are aberration diagrams of the second embodiment. 1... Light source, 2... Beam, 3... Rotating polygon mirror, 4... Imaging lens, 5... Scanning surface.

Claims (1)

[Claims] 1 A negative first lens and a positive second lens in order from the exit pupil side.
A constant velocity scanning lens is an optical system consisting of a lens, and is characterized by satisfying the following conditions: (1) n ₁ < 1.6 (2) n ₂ < 1.6 (3) −12 < r ₂ / r ₁ <-6 (4) 3f < r ₃ < 5f However, n ₁ and n ₂ are the refractive index of each lens, r ₁ , r ₂ , r ₃ ,
r ₄ is the radius of curvature of each refractive surface, and f is the composite focal length of the entire system. 2. The constant velocity scanning lens according to claim 1, wherein at least one surface is an aspherical surface. 3. A constant velocity scanning lens according to claim 1, characterized in that at least one lens is a plastic lens.