JP3075056B2 - 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 scanning method for forming convergent light deflected at a constant angular speed by a deflector such as a polygon mirror on a surface to be scanned, and for scanning the surface to be scanned at a substantially constant speed. Optical system.

[0002]

2. Description of the Related Art Conventionally, in a laser scanning optical system used for a laser beam printer or the like, a divergent beam emitted from a laser light source is converged by a collimator lens or the like, and then deflected at a constant angular velocity by a deflector such as a polygon mirror. There is known a device which scans, forms an image of the beam as a laser beam spot on a surface to be scanned by a scanning lens system, and scans the surface to be scanned at substantially constant speed. Many scanning optical systems using a plurality of lenses as this scanning lens system have been proposed and put into practical use.

On the other hand, as a simpler, a scanning optical system using a single-lens scanning lens is disclosed in Japanese Patent Laid-Open Publication No. Hei 3-2138.
No. 12 and JP-A-4-50908. The scanning optical system disclosed in JP-A-3-213812 uses a single biconvex lens or a positive meniscus lens as a scanning lens. In the example using a biconvex lens, the biconvex lens causes a frame near the maximum deflection angle (the deflection angle is the angle of view formed by the light beam reflected by the deflection surface and the optical axis, and the maximum deflection angle is the maximum angle of view). Aberration is suppressed, and by making the lens second surface an aspheric surface, distortion,
In addition, the field curvature in the main scanning direction is corrected. Also,
In the example using the positive meniscus lens, the distortion and the curvature of field in the main scanning direction are corrected by using a meniscus lens having a convex surface directed toward the deflector and an aspheric second surface.

On the other hand, the scanning optical system disclosed in JP-A-4-50908 uses a scanning lens having aspherical surfaces on both sides. In the scanning optical systems disclosed in both publications, the convergent light beam is made incident on the scanning lens because, when a parallel light beam is made incident, a surface in the main scanning direction is obtained in order to obtain a refractive power in the main scanning direction. The shape must be specified, distortion,
Also, this is because the curvature of field in the main scanning direction cannot be sufficiently corrected.

[0005]

However, the scanning optical system disclosed in Japanese Unexamined Patent Publication No. 3-213812 discloses a second type of scanning lens.
Although the surface is made aspherical, the curvature of field in the main scanning direction is sufficiently corrected, but the distortion is insufficiently corrected and the performance is poor. In addition, there is a problem that the deviation in thickness of the scanning lens (the difference between the core thickness of the lens near the maximum deflection angle and the core thickness of the lens on the optical axis) is large. In other words, when this scanning lens is formed and processed, the difference in wall thickness is large, so that the lens surface accuracy after processing cannot be obtained, the physical properties inside the lens cannot be homogenized, and the surface shape of the lens when the temperature and humidity rise. There is a problem that the shift amount of the image plane caused by the change in the refractive index and the change in the refractive index increases. When the scanning lens is made of resin,
In particular, these problems become remarkable. Also, Japanese Patent Application Laid-Open No.
If a glass lens is used like the scanning lens of No. 3812, there is also a problem that the cost of the lens increases.
On the other hand, the scanning optical system disclosed in JP-A-4-50908 is
Since a scanning lens having a very complicated aspherical surface on both sides is used, there is a problem that molding is very difficult.

[0006]

An object of the present invention is to provide a scanning optical system having a low-cost scanning lens which can be easily formed, suppresses uneven thickness, and has a low cost. The purpose is. A further object of the present invention is to provide a scanning optical system in which distortion and curvature of field in the main scanning direction are satisfactorily corrected.

[0007]

In order to solve the above problems, a scanning optical system according to the present invention forms an image on a surface to be scanned while forming a deflector and convergent light deflected by the deflector at an equal angular velocity. A single scanning lens that scans the scanning surface at a substantially uniform speed, wherein the scanning lens is a biconvex lens made of a material having a refractive index of 1.6 or less; The main curve in the main scanning direction on the first surface on the deflector side is a circle, and the main curve in the main scanning direction on the second surface on the scanned surface is a curve in which the radius of curvature increases as the angle of view in the main scanning direction increases. And satisfy the following conditions. (1) -5 <(-nf / c) + (f / s) <-2 (2) -0.41 <(-c / f) <-0.25 (3) d1 / d3 <1 f: focal length of the scanning lens in the main scanning direction n: refractive index of the scanning lens d1: axial top surface distance from the deflecting surface of the deflector to the first surface of the scanning lens d3: from the second surface of the scanning lens to the surface to be scanned S: The distance from the front principal point of the scanning lens to the natural convergence point of the convergent light c: The distance from the deflecting surface of the deflector to the front principal point of the scanning lens The scanning optical system according to the present invention includes a deflector and a single scan that forms convergent light deflected by the deflector at a constant angular speed on the surface to be scanned and scans the surface to be scanned at substantially the same speed. A scanning optical system comprising: a lens having a refractive index of 1.6 or less; A positive meniscus lens having a convex surface facing the director, the main curve in the main scanning direction on the first surface on the deflector side is a circle, and the main curve in the main scanning direction on the second surface on the scanned surface is the main scanning direction. Is a curve in which the radius of curvature becomes smaller as the angle of view becomes larger, and further satisfies the following conditions. (4) −7 <(− nf / c) + (f / s) <− 3 (5) −0.41 <(− c / f) <− 0.13 (6) d1 / d3 <1 f: focal length of the scanning lens in the main scanning direction n: refractive index of the scanning lens d1: axial top surface distance from the deflecting surface of the deflector to the first surface of the scanning lens d3: from the second surface of the scanning lens to the surface to be scanned S: Distance from the front principal point of the scanning lens to the natural convergence point of the convergent light c: Distance from the deflecting surface of the deflector to the front principal point of the scanning lens

[0008]

According to the above arrangement, distortion and curvature of field in the main scanning direction are satisfactorily corrected. Moreover, since the surface shape of the scanning lens is simple, processing is easy.

[0009]

Embodiments of the present invention will be described below. 1 and 7 are diagrams showing a configuration of a scanning optical system to which the present invention is applied. Reference numeral 1 denotes a light source including a light emitting element such as an LD or an LED, and emits a divergent light beam. Reference numeral 2 denotes a collimator lens that converges a divergent light beam from the light source 1, 3 denotes a cylindrical lens that converges the converged light beam linearly, and 4 denotes a polygon mirror that deflects and scans the converged light beam by rotation.

The light beam deflected at a constant angular velocity by the polygon mirror 4 passes through the scanning lens 5 and forms an image on the surface 6 to be scanned. The scanning lens 5 acts so that the light beam deflected at a constant angular velocity by the polygon mirror 4 scans the scanned surface 6 at a substantially constant velocity. Here, the moving direction of the light beam (light spot) on the scanned surface 6 due to the rotation of the polygon mirror 4 is called a main scanning direction, and a direction perpendicular to the main scanning direction is called a sub-scanning direction.

The first and second embodiments will be described below. First,
The scanning lens 5 in the second embodiment is located between the polygon mirror 4 and the surface to be scanned 6 as shown in FIGS. The scanning lens 5 is a biconvex lens in the main scanning direction, and the first surface r1 on the polygon mirror 4 side is spherical.
The second surface r2 on the surface 6 to be scanned is an aspheric surface. That is, the scanning lens 5 has a simple shape that is easy to process. Further, as shown in conditional expressions (3) and (6) described later, the scanning lens 5 has an axial upper surface distance d1 between the deflecting surface of the polygon mirror 4 and the first surface r1 of the scanning lens 5.
The distance d3 between the second surface r2 and the surface to be scanned 6 is smaller than the distance d3. With this arrangement, the size of the scanning optical system can be reduced, and the size of the print head can be reduced.

The scanning lens 5 in the first and second embodiments
Is a single biconvex lens, and the main curve of the first surface r1 in the main scanning direction (the curve along the lens surface passing through the vertex of the lens surface on the scanning surface) is formed to be a circle. . The main curve in the main scanning direction of the second surface r1 is formed as a curve in which the radius of curvature in the main scanning direction increases as the angle of view in the main scanning direction increases. With this configuration, when the scanning lens 5 is arranged at a position closer to the polygon mirror 4 than the surface 6 to be scanned, the field curvature and distortion in the main scanning direction are satisfactorily corrected. Further, since the scanning lens 5 is made of a resin having a refractive index of 1.6 or less, homogeneity of physical properties in the lens and accuracy of the lens surface pose a problem as compared with ordinary glass. In order to solve such a problem, the scanning lens 5 is required to set the distance t between the upper surfaces of the lenses to 25.
mm, the core thickness tmin of the lens near the maximum deflection angle is 4 m
m or more, and the thickness deviation ratio (ratio between the core thickness tmin and the core thickness d2 of the lens on the optical axis) of the scanning lens 5 is reduced. The thickness deviation ratio (d2 / tmin) is preferably 4 or less.

Further, the scanning optical systems of the first and second embodiments are
The following conditional expressions (1) to (3) are satisfied. (1) -5 <(-nf / c) + (f / s) <-2 (2) -0.41 <(-c / fH) <-0.25 (3) d1 / d3 <1 The above condition In the equation, n is the refractive index of the scanning lens, f is the focal length of the scanning lens, d1 is the axial upper surface distance between the deflecting surface of the polygon mirror 4 and the first surface r1 of the scanning lens 5, and d3 is the second distance of the scanning lens 5. The axial upper surface distance between the surface r2 and the surface 6 to be scanned,
c is the distance from the deflecting surface of the polygon mirror 4 to the front principal point H of the scanning lens 5, and s is the natural convergence point of the convergent light flux from the front principal point H of the scanning lens 5 (convergence of the convergent light flux when there is no scanning lens 5). Point). If the upper and lower limits of conditional expression (1) are exceeded, it will not be possible to correct the field curvature even if the aspherical shape of the second surface r2 is changed. Also, conditional expression (2)
Exceeds the upper and lower limits, it becomes difficult to correct distortion with the scanning lens 5 having a low refractive index and a reduced thickness ratio.

Next, third to eighth embodiments will be described. 3rd ~
The scanning lens 5 in the eighth embodiment is located between the polygon mirror 4 and the surface 6 to be scanned, as shown in FIGS. The scanning lens 5 is a positive meniscus lens having a convex surface facing the polygon mirror 4 in the main scanning direction. The first surface r1 on the polygon mirror 4 side is spherical, and the second surface r2 on the scanned surface 6 side is aspheric. is there. That is, the scanning lens 5 has a simple shape that is easy to process. Further, as shown in conditional expressions (3) and (6) described later, the axial distance d1 between the deflecting surface of the polygon mirror 4 and the first surface r1 of the scanning lens 5 is equal to the second distance of the scanning lens 5. It is arranged to be smaller than the distance d3 between the surface r2 and the surface 6 to be scanned. With this arrangement, the size of the scanning optical system can be reduced, and the size of the print head can be reduced.

The scanning lens 5 in the third to eighth embodiments
Is composed of a single positive meniscus lens with the convex surface facing the polygon mirror 4 side, and the first surface r1 is formed such that the main curve in the main scanning direction is a circle. The main curve in the main scanning direction of the second surface r2 is formed as a curve in which the radius of curvature in the main scanning direction decreases as the angle of view in the main scanning direction increases. With this configuration, even when the scanning lens 5 is closer to the polygon mirror 4 than the surface to be scanned 6, the field curvature and distortion in the main scanning direction can be corrected well. Further, since the scanning lens 5 is made of a resin having a refractive index of 1.6 or less, homogeneity of physical properties in the lens and accuracy of the lens surface pose a problem as compared with ordinary glass. In order to solve such a problem, the scanning lens is
The distance t between the upper surfaces of the lenses is set to 25 mm or less, the core thickness tmin of the lens near the maximum deflection angle is set to 4 mm or more, and the thickness deviation ratio (ratio between the core thickness tmin and the core thickness d2 on the optical axis) of the scanning lens is reduced. ing. The thickness deviation ratio (d2 / tmin) is preferably 4 or less.

Further, the scanning optical systems of the third to eighth embodiments are
The following conditional expressions (4) to (6) are satisfied. (4) −7 <(− nf / c) + (f / s) <− 3 (5) −0.41 <(− c / f) <− 0.13 (6) d1 / d3 <1 Each symbol is the same as in the first and second embodiments. If the upper and lower limits of conditional expression (4) are exceeded, it is no longer possible to correct the field curvature with the scanning lens 5 having a low refractive index and a reduced thickness ratio. If the upper and lower limits of conditional expression (5) are exceeded, it becomes difficult to correct distortion.

Hereinafter, each embodiment will be specifically described. In each embodiment, d1 is the axial upper surface distance between the deflection surface of the polygon mirror 4 and the first surface r1 of the scanning lens 5, and d2 is the scanning lens 5
, The axial distance between the first surface r1 and the second surface r2, d3 is the axial distance between the second surface r2 of the scanning lens 5 and the scanned surface 6, and r
1 is a radius of curvature of the first surface r1 of the scanning lens 5 in the main scanning direction (on the optical axis), r2 is a radius of curvature of the second surface r2 of the scanning lens 5 in the main scanning direction (on the optical axis), and n is a scanning lens. 5, f is the focal length of the scanning lens 5 in the main scanning direction, and c is the front principal point H of the scanning lens 5 from the deflection surface of the polygon mirror 4.
, S is the distance from the front principal point H of the scanning lens 5 to the natural convergence point of the convergent beam, and s1 is the polygon mirror 4
, L is the distance from the deflecting surface of the polygon mirror 4 to the surface 6 to be scanned, Y is the scanning width, and ω is the angle of view of the scanning lens 5.

The shape of the aspherical surface when an aspherical surface is used for the surface of the scanning lens 5 is expressed by the following equation. x = (CoΦ2) / (1+ (1-εCo2Φ2) 1/2) + AiΦi, where x is a coordinate from the vertex of the lens surface to the optical axis, Co is a paraxial curvature of the aspherical lens surface, and Φ is a value from the optical axis. Height, Ai
(I = 2 to 10) indicates a higher-order parameter, and ε indicates a quadratic surface parameter.

In each embodiment, the lens surface shape in the sub-scanning direction is not specified, but the first surface r of the scanning lens 5 is not defined.
Reference numeral 1 denotes a spherical surface having a radius of curvature in the main scanning direction, and the second surface r2 of the scanning lens 5 becomes larger as the angle of view in the main scanning direction increases.
An aspherical surface whose radius of curvature in the sub-scanning direction changes may be used. With this configuration, it is possible to correct the curvature in the sub-scanning direction that changes depending on the angle of view in the main scanning direction.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

[0025]

[Table 6]

[0026]

[Table 7]

[0027]

[Table 8]

In each embodiment, the conditional expressions (1), (2),
The numerical values in (4) and (5) are summarized in the following table. For reference, the uneven thickness ratio (t / tmin) is also shown in the following table.

[0029]

[Table 9]

[0030]

As is clear from the above description, according to the present invention, distortion can be obtained even in a single scanning lens having a low refractive index resin, which can be easily formed and has a small thickness deviation ratio. Thus, it is possible to favorably correct the curvature of field in the main scanning direction. Further, since the axial distance d1 between the deflecting surface and the first surface of the scanning lens is smaller than the distance d3 between the second lens surface and the surface to be scanned, the size of the scanning optical system can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a scanning optical system according to first and second embodiments of the present invention.

FIG. 2 is a diagram showing a configuration and an arrangement of a scanning lens according to the first and second embodiments of the present invention.

FIG. 3 is an aberration diagram of a scanning lens in the first embodiment.

FIG. 4 is a diagram showing a configuration of a scanning lens in the first embodiment.

FIG. 5 is an aberration diagram of a scanning lens in the second embodiment.

FIG. 6 is a diagram illustrating a configuration of a scanning lens according to a second embodiment.

FIG. 7 is a diagram showing a configuration of a scanning optical system according to third to eighth embodiments of the present invention.

FIG. 8 is a diagram showing a configuration and an arrangement of a scanning lens according to third to eighth embodiments of the present invention.

FIG. 9 is an aberration diagram of a scanning lens in the third embodiment.

FIG. 10 is a diagram showing a configuration of a scanning lens in a third embodiment.

FIG. 11 is an aberration diagram of a scanning lens in the fourth embodiment.

FIG. 12 is a diagram illustrating a configuration of a scanning lens according to a fourth embodiment.

FIG. 13 is an aberration diagram of a scanning lens in the fifth embodiment.

FIG. 14 is a diagram illustrating a configuration of a scanning lens according to a fifth embodiment.

FIG. 15 is an aberration diagram of a scanning lens in the sixth embodiment.

FIG. 16 is a diagram illustrating a configuration of a scanning lens according to a sixth embodiment.

FIG. 17 is an aberration diagram of a scanning lens in the seventh embodiment.

FIG. 18 is a diagram illustrating a configuration of a scanning lens according to a seventh embodiment.

FIG. 19 is an aberration diagram of a scanning lens in the eighth embodiment.

FIG. 20 is a diagram showing a configuration of a scanning lens in an eighth embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Collimator lens 3 ... Cylindrical lens 4 ... Polygon mirror 5 ... Scanning lens 6 ... Scanning surface

──────────────────────────────────────────────────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 G02B 26 / Ten

Claims (2)

(57) [Claims] 1. A deflector and a single scanning lens that forms convergent light deflected by the deflector at a constant angular speed on a surface to be scanned and scans the surface to be scanned at a substantially constant speed. A scanning optical system, wherein the scanning lens is a biconvex lens made of a material having a refractive index of 1.6 or less, the main curve in the main scanning direction on the first surface on the deflector side is circular, A scanning optical system, characterized in that a main curve in the main scanning direction on the second surface on the surface side is a curve in which the radius of curvature increases as the angle of view in the main scanning direction increases, and further satisfies the following conditions. (1) -5 <(-nf / c) + (f / s) <-2 (2) -0.41 <(-c / f) <-0.25 (3) d1 / d3 <1 f: focal length of the scanning lens in the main scanning direction n: refractive index of the scanning lens d1: axial top surface distance from the deflecting surface of the deflector to the first surface of the scanning lens d3: axis from the second surface of the scanning lens to the surface to be scanned Upper surface interval s: distance from the front principal point of the scanning lens to the natural convergence point of the convergent light c: distance from the deflecting surface of the deflector to the front principal point of the scanning lens 2. A deflector, and a single scanning lens that forms convergent light deflected by the deflector at a constant angular speed on a surface to be scanned and scans the surface to be scanned at a substantially constant speed. A scanning optical system comprising: a scanning lens made of a material having a refractive index of 1.6 or less;
A positive meniscus lens having a convex surface facing the deflector side, the main curve in the main scanning direction on the first surface on the deflector side is a circle, and the main curve in the main scanning direction on the second surface on the scanned surface side is the main scanning direction. A scanning optical system characterized in that the curvature radius decreases as the angle of view increases, and further satisfies the following conditions. (4) −7 <(− nf / c) + (f / s) <− 3 (5) −0.41 <(− c / f) <− 0.13 (6) d1 / d3 <1 f: focal length of the scanning lens in the main scanning direction n: refractive index of the scanning lens d1: axial top surface distance from the deflecting surface of the deflector to the first surface of the scanning lens d3: from the second surface of the scanning lens to the surface to be scanned S: distance from the front principal point of the scanning lens to the natural convergence point of the convergent light c: distance from the deflecting surface of the deflector to the front principal point of the scanning lens