JPS6217720A - Equal speed scanning lens - Google Patents

Abstract

PURPOSE:To stabilize a value of MTF with regard to the whole scanning sur face by providing an entrance pupil on the object side of a lens system constitut ed of two groups and two pieces having two lenses of negative and positive from the object side, and making this optical system satisfy a prescribed condi tion. CONSTITUTION:This lens system is constituted of two groups and two pieces, and the first lens and the second lens from an object side are a negative lens and a positive lens, respectively. Also, an entrance pupil is provided in front of this lens system. This optical system is made to satisfy conditions of an expression I - an expression IV. In this regard, in the expressions, (f), r1-r4, and d1-d3 denote a focal distance of the whole system, radiuses of curvature of each surface of lenses in order from the pupil side, and thickness of the lens surface and an interval of the lens surface in order of the pupil side, respec tively.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a constant velocity scanning lens used in high-density recording and reproducing devices such as laser printers.

(Prior Art) An f-theta lens is known as a constant-velocity scanning lens that uses a semiconductor laser as a light source and is used to scan a surface to be scanned with a laser beam using a polarizer such as a rotating polygon mirror or a galvanometer mirror. There is.

(Problems to be Solved by the Invention) Generally, in a condensing optical system, an f-theta lens has a relationship of H=ftan theta, where H is the image height with respect to the incident angle theta. In such an f-theta lens, the reflection surface of the rotating polygon mirror is the entrance pupil, the scanning angle is θ, the focal length of the lens system is f, and the image height is H. It is required to obtain uniform velocity scanning and satisfy (%) (it is required to distort in the negative direction by the value of D obtained by f and θ).

In general, f-theta lenses with these basic features include the so-called telecentric type, in which the entrance pupil is located on the front focal plane, and the wide-angle type, but the wide-angle type is more suitable for specifications such as laser printers. It is commonly used.

Conventionally, this type of f-theta optical system must be corrected in terms of the angle of view regarding the scanning width as a printer, the linearity with respect to f and θ, and the spot size of imaging. And the lenses that make up the f-0 optical system are:
Generally, the number of constituent images changes in conjunction with the F-number, and as the F-number becomes smaller (brighter), the number of constituent images also increases. However, for printers and the like that aim to reduce costs, it is necessary to keep the required characteristics constant and to minimize the number of f-θ optical systems. Therefore, conventionally, f-
A θ lens has been proposed, but this is f, [? Problems remain regarding the linearity of the image and the flatness of the screen over the entire scanning angle, making it difficult to obtain precise hard copies.

(Means for solving the problem) Set the entrance pupil at the position of the rotating polygon mirror in front of the f-θ lens, and set the image height H with respect to the beam incident at the scanning angle θ with the reflecting surface of the rotating polygon mirror as the center. It is an f-theta lens that has negative distortion when = f・0 and has an image plane that is flat over the entire scanning width, and has a concave surface with a small radius of curvature facing the entrance pupil side. It consists of a negative lens and a positive second lens with a convex surface with a large radius of curvature facing the entrance pupil.The focal length of the entire f-θ lens system is f, and the curvature of each surface of each lens is The radius is r1 to r4 in order from the pupil side. The lens surface spacing is d, ~d in order from the pupil side.
(1) r,<f (2) 2 <r2/r1<2.6r.

(3) 2f<r, <4f (4) 0.04f<d, z<0.08f.

Here, condition (1) is particularly related to off-axis aberrations, and when this condition is exceeded, the image plane sharply increases in the negative direction at the farthest periphery of the scanning angle, and the image plane flatness as a wide-angle scanning lens increases. is inhibited.

Condition (2) defines the shape and power of the first negative lens, and if the upper limit of 2.6 is exceeded, the f/θ properties will be improved, but the curvature of field will sharply increase around the predetermined maximum angle of view. increase

Therefore, the image flatness of the wide-angle f/θ lens is impaired.

When the lower limit of 2.0 is exceeded, the flatness of the image plane improves, but f
・The linearity of θ deteriorates (in the positive direction).

Condition (3) is related to f/θ characteristics, field curvature, and spherical aberration; if the upper limit of 4f is exceeded, the image surface flatness improves, but f/θ characteristics worsen (distortion aberration is on the positive side). ), and the spherical aberration becomes insufficiently corrected. When the lower limit, 2f, is exceeded, the f/θ properties are improved.

The image plane increases in one direction.

Condition (4) relates to astigmatism and the linearity of f·θ; when the upper limit of 0.08 f is exceeded, astigmatism increases and off-axis coma aberration also increases. Also, if the lower limit of 0.04f is exceeded, f・
0 linearity deteriorates.

(Function) The maximum scanning width is approximately 63° by angle of view conversion, the incident beam diameter is 3,
5 mm, focal length of 250 mm, and wavelength used of 830 nm, the F number of the f-θ lens is calculated to be approximately 71. Assuming that the energy of the semiconductor laser used at this time has a Gaussian distribution, 1/e2 of the energy intensity
Let be the spot size of the beam.

The spot diameter due to diffraction is approximately 75 μm, which is sufficient resolution for a printer lens.

Therefore, the optical system used in laser printers is
The linearity of f and θ is sufficient and is a basic characteristic,
It is only necessary to consider the flatness of the entire scanning angle, and there is no need to spend a lot of effort on the focusing characteristics in terms of rotation.

According to the present invention, the required resolving power of the f-theta lens is approximately 14 lines of 71 mm from the spot diameter of 75 μm due to the above-mentioned diffraction.
Therefore, the characteristics of the focused light are corrected so that the MTF value at 15 lines/1 mm is stable over the entire scanning surface and is approximately 70% or more, and the linearity of f and θ is approximately 1% over the entire scanning area. f for printers consisting of two groups of two sheets within
A −θ lens is obtained.

(Example) Example 1゜d,,43.00 d4=286.4072 Wavelength used = 830nmf =249.998 Example 2゜do=ts,oo d4=289.7368 Wavelength used = 830nmf =250.003 However, in FIG. 1, P indicates the pupil position and A indicates the surface to be scanned. Note that the aberration curve of Example 2 is similar to that of Example 1, and therefore will be omitted.

[Brief explanation of the drawing]

Fig. 1 is an implantation diagram of a constant velocity scanning lens showing an embodiment of the present invention, Fig. 2 is an aberration diagram of the embodiment, and Fig. 3 is an M of the embodiment 1.
Indicates TF. fθ. Linearity Astigmatism □ Sagittal 111 Mesora Mamoru Onal 1 Bushi'tal - 11 - Meridional Procedural Correction Letter September 12, 1985

Claims (1)

[Claims] Two groups 2 in which the first lens is a negative lens and the second lens is a positive lens.
A lens system consisting of a lens system, which has an entrance pupil in front of the lens system, the focal length of the entire system is f, the radius of curvature of each surface of the lens is r_1 to r_4 in order from the pupil side, and the distance between lens surfaces is When d_1 to d_3 in order from the pupil side, (1) r_2
<f (2) 2<r_2/r_1<2.6 (3) 2f<r_3<4f (4) 0.04f<d_2<0.08f A constant velocity scanning lens that satisfies the following conditions.