JPH10161017A - Plastic lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plastic-made f.θ lens which has small internal distortion in an optical plane by securing the thickness of the end part of even a thin lens. SOLUTION: The plastic lens 6 has both its incidence surface 6b and projection surface 6a made convex and the incidence-side horizontal-scanning directional optical surface width S1 is different from the laser-projection side horizontal-scanning directional optical surface width S2. Here, the laser incidence-side horizontal-scanning directional optical width S1 is made less than the laser projection-side horizontal-scanning directional optical width S2 so that 1<S2/Sl<1+Tc/L. Here, Tc is the center thickness of the lens and L is the distance from a polygon mirror reflecting surface to a beam incidence surface lens. This lens 6 is nearly convex and can be made small in thickness (t) at the lens end part 6t correspondingly by making the incidence-side optical surface width 6b smaller than the projection-side optical surface width 6a in the horizontal scanning direction, and the plastic lens which has no distortion in the optical surface is obtained although the lens is a thin type.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic lens used in a laser beam polarizing optical system of an LBP, a digital copying machine, etc., and more particularly to a plastic lens for preventing internal distortion during injection molding.

[0002]

2. Description of the Related Art In a laser printer or a digital copier, the surface of a charged cylindrical photoreceptor is selectively irradiated while being scanned in parallel by a light beam to remove static electricity.
An electrostatic latent image is formed on the surface of the photoconductor. The electrostatic latent image on the photoreceptor surface is visualized into a toner image by a developing device, and printing is performed by transferring the toner image to recording paper. Here, a scanning optical system for scanning the surface of the photoreceptor with a light beam emits a beam by a light beam generating means such as a semiconductor laser, and condenses the beam into a line image. And a f / θ lens that has the function of keeping the scanning speed of the light beam on the photoreceptor surface constant while forming an image of the light beam polarized and scanned by the polarizer as a spot on the surface of the photoreceptor. You.

[0003] The spot on the surface of the photoreceptor moves parallel to the surface of the cylindrical photoreceptor along its axis to perform main scanning, while rotating the photoreceptor to perform sub-scanning. As a result, an electrostatic latent image is formed on the surface of the photoconductor, and printing is performed. The f · θ lens of such a scanning optical system uses a plastic lens to reduce cost and size.

Here, in plastic lens molding, there is a problem that optical performance is deteriorated due to distortion inside the lens during molding. As a measure against internal distortion, for example, a rib is provided on the lens to make the longitudinal cross-sectional area uniform,
A method for suppressing the generation of distortion by eliminating the difference in cooling rate (see Japanese Patent Application Laid-Open No. Hei 5-188285), and preventing the generation of distortion in the optically effective surface by providing a negative portion in the thickness direction (see Japanese Patent Application Laid-Open No. Hei 5-18885). No. 93804)
And so on. However, these methods are not practical because they unnecessarily complicate the lens shape.

Further, as a method of reducing the influence of lens distortion when distortion cannot be prevented by all means, a proposal has been made to reduce the influence of internal distortion by disposing the gate portions of two plastic lenses in opposite directions. JP 6
No. 347713.

[0006]

According to experiments conducted by the present applicant, as a measure against distortion inside the lens, the thickness of the lens end is increased to suppress the internal distortion generated during molding into the end. It has been found that a technique for preventing the occurrence of distortion in the optical surface and preventing the deterioration of the optical performance is the most effective. This method can easily obtain a lens without distortion without complicating the lens shape. However, recently, plastic lenses have become thinner in order to achieve higher functionality and shorter cycle times. As the thickness of the lens becomes thinner, the thickness of the lens end is becoming thinner, and it is difficult to drive the distortion occurrence position to the lens end. Therefore, the present invention secures the thickness of the end portion of the lens even in a lens having a small thickness, and has a plastic f.
It is intended to provide a θ lens.

[0007]

In order to achieve the above-mentioned object, the plastic lens of the present invention has a convex shape on both the entrance surface and the exit surface, and has an optical surface width S in the main scanning direction on the laser incident side. ₁ and the laser emission side main scanning direction optical surface width S ₂ is provided with a different configuration. Also, the optical surface width S _{1 in} the main scanning direction on the incident side and the optical surface width S _{2 in} the main scanning direction on the emission side.
Satisfies the following equation. _{1 <S 2 / S 1 <} 1 + Tc / L However, Tc: Lens center thickness L: distance from the polygon mirror reflective surface to the beam entrance surface lens

[0008]

Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a plan view showing a schematic configuration of a scanning optical system of an LBP and a digital copying machine. The scanning device includes a light source 1, a collimator lens 2, an image forming optical system 3 before a polygon, a deflector 4, an f · θ optical system 8 (first lens 5, second lens 6), and a scanned object 7. The light source 1 is made of a semiconductor laser in the form shown in the drawing, and the light beam 9 from the light source 1 is emitted as divergent light. The collimator lens 2 collimates the light beam 9 incident as divergent light into a substantially parallel light beam, and adjusts the position of the light beam 9 in the direction of the optical axis so that the light beam 9 is focused on the scanning surface of the scanned object 7 on the main scanning surface. Focus adjustment for convergence is performed.
In this embodiment, the pre-polygon imaging optical system 3 includes a cylindrical lens, has power in the sub-scanning plane,
The light beam 9 is once converged on the reflecting mirror 4a in the deflector.

At this time, since the cylindrical lens of the pre-polygon imaging optical system 3 has no power in the scanning plane, the light beam 9 forms a line image along the main scanning plane on the reflecting mirror 4a. The polygon mirror 4a of the deflector is rotated by a scanner motor, and deflects the light beam 9 by changing the angle of the reflection surface 4a. The light beam 9 is deflected and scanned in a plane (main scanning plane) perpendicular to the rotation axis of the deflector 4, and
To the pre-polygon optical system 3 are arranged such that their optical axes are in the main scanning plane.

The f · θ optical system 8 has the function of finally converging the light beam 9 to one point on the surface to be scanned of the object 7 to be scanned, and the reflecting mirror 4a of the deflector and the object to be scanned on the sub-scanning surface. 7 has a function of correcting a tilt error of the reflecting mirror 4a by adjusting the scanning mirror 7 so as to have a conjugate relationship with the scanning surface. In the case of a laser printer or the like, for example, a photosensitive drum is in charge of the scanned object 7, and a signal is exposed by a light beam 9.

The f · θ optical system 8 comprises one or more plastic lenses or a combination of a glass lens and a plastic lens. The f · θ optical system 8 scans the light beam 9 deflected by the deflector 4 at a constant speed in the main scanning direction while forming a uniform spot on the scanned surface of the scanned object 7. Further, the f · θ optical system 8 has a function of correcting the inclination of each reflecting mirror 4a of the deflector 4 in the sub-scanning direction. Therefore, the light beam emitted from the light source 1 passes through the collimator lens 2 and the image forming optical system 3 before the polygon, is deflected by the reflecting mirror 4a by the rotation of the deflector 4, and becomes an image forming optical system after the polygon (f · θ optical system). The light is condensed by 8 and is irradiated on the surface (photosensitive surface) of the scanned object 7.

Here, a conventional example of an f.theta. Lens used in a post-polygon imaging optical system will be described with reference to FIG.
The f · θ lens 60 shown in the figure is designed so that the width S ₁ of the beam incident side optical surface 6b in the lens main scanning direction is equal to the width S _{2 of} the emission surface side optical surface 6a. In the case of this lens 60,
If the thickness Tc at the center of the lens is reduced without changing the polarities between the entrance surface and the exit surface, the thickness t at the lens end is also reduced, and it is difficult to concentrate distortion at the lens end.

FIG. 2 is a perspective view of the plastic lens of the present invention. This plastic lens 6 is configured such that the width S ₁ of the incident side optical surface 3b in the main scanning direction is smaller than the width S ₂ of the exit side optical surface 3a. With this configuration, it is possible to increase the thickness t of the lens end. Here, the width S ₁ of the incidence-side optical surface 3b of the lens 6 will be described with reference to FIG 4 the relationship between the width S ₂ of the exit-side optical surfaces 3a. Here, for convenience, the description will be made on the upper part of the lens 6 in the direction perpendicular to the scanning direction. From the polygon mirror 4 to the incident side optical surface 3b
Is L, and the polarization angle is θ. Incident optical surface 3
The width S ₁ of b is given by S ₁ = L × tan θ Equation 1. The width S ₂ of the emission-side optical surface 3a is given by S ₂ <(L + t) × tan θ Equation 2.

[0014] Therefore, from equation (2), and applying the formula 1 to _{S 2 <L · tanθ + t} · tanθ S 1, S 2 <S 1 + t · tanθ _{Thus, S 2 / S 1 <1} + t · tanθ / S 1 Equation 3 S ₂ / S ₁ <1 + t / L Here, since the relationship of t <Tc is satisfied in the convex lens, Equation 3 becomes S ₂ / S ₁ <1 + t / L <1 + Tc / L Equation 4.

Further, in this plastic lens scanning optical system, since S ₂ > S ₁ ,
The relationship of S ₂ / S ₁ > 1 holds. Therefore, the width S ₁ of the entrance-side optical surface 3 b of the lens 6 at this time and the exit-side optical surface 3
The relationship of a with the width S ₂ is as follows: 1 <S ₂ / S ₁ <1 + Tc / L Equation 5

(Operation) As described above, the conventional plastic lens 60 has the incident side optical surface width 6b in the main scanning direction.
And the output side optical surface width 6a are designed to be equal, but in the plastic lens 6 according to the present invention, the incident side optical surface width 3b is formed smaller than the output side optical surface width 3a in the main scanning direction. Furthermore, since the lens 6 has a substantially convex shape, the thickness t of the lens end 6t can be increased by reducing the optical surface width 3b. As described above, even when the thickness Tc at the center of the lens is reduced, the thickness t of the lens end 6t can be sufficiently ensured, and the lens internal distortion during molding can be driven into the lens end. . Therefore, the plastic lens 6 is a thin plastic lens, but has no distortion in the optical surface.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to plastic lenses shown in FIGS. Example 1 (see FIG. 5) Plastic lens 600 shown in FIGS. 5 (a) and 5 (b)
a and 600b indicate that the radii of curvature of the incident surfaces 20b and 30b are 42
3 mm, the radius of curvature of the exit surfaces 20a, 30a is 440.1
This is a spherical lens having a thickness of 17 mm and a central portion thickness Tc of 20 mm. This lens has a distance L from the polygon mirror reflecting surface of 115 mm for optical design. Under the above conditions, the plastic lens 600a shown in (a) has a lens optical surface width S ₁ , S ₂ of 168 mm and an end thickness t.
Is 3.538 mm. Under the same conditions, the thickness t at the end of the plastic lens 600b shown in FIG. Plastic lens 600b _{is, 1 <S 2 / S 1} <1 + Tc
/ L was satisfied, the incident light surface width S _{1 was} smaller than the outgoing light surface width S ₂ , and the outgoing light surface width S _{2 was} 168 mm, whereas the incident light surface width S _{1 of the} lens was 162 mm. From S ₂ / S ₁ = 168/162 = 1.037 Tc = 20, L = 115, 1 + Tc = 1.174
And this lens has the following relationship: 1 <S ₂ / S ₁ <1 + Tc /
L is satisfied.

The plastic lens 600b
Was t = 4.135 mm.
That is, the change Δt between the thickness t of the end of the plastic lens 600b of the present invention and the thickness t of the end of the conventional plastic lens 600a is the change Δt = 4.135−3.538 = 0. .597 mm, and the thickness t at the end can be increased by 0.597 mm. As described above, the thickness t of the end portion of the plastic lens 600b can be increased by 0.597 mm without changing the thickness Tc at the center portion of the conventional lens.

Embodiment 2 (see FIG. 6) Plastic lens 650 shown in FIGS. 6 (a) and 6 (b)
a and 650b indicate that the radii of curvature of the incident surfaces 25b and 35b are
8.820 mm, the radius of curvature of the exit surfaces 25a and 35a is 4
This is a spherical lens having a thickness of 31.773 mm and a center thickness Tc of 20 mm. This lens has a distance L from the polygon mirror reflecting surface of 135 mm for optical design.

Under the above conditions, the plastic lens 650a shown in (a) has the lens optical surface widths S ₁ and S ₂ of 1
The end portion thickness t is 5.622 mm for 68 mm.
Under the same conditions, the plastic lens 65 shown in FIG.
Look at the end thickness t of 0b. Plastic lens 650b
Is the incident light surface width S ₁ <the outgoing light surface width S ₂ , and the outgoing light surface width S
_{For 2} = 168 mm, the incident light surface width S _{1 of the} lens = 162
mm. From _{S 2 / S 1 = 168/} 162 = 1.037 Tc = 20, L = 135, 1 + Tc = 1.148
And this lens has the following relationship: 1 <S ₂ / S ₁ <1 + Tc /
L is satisfied. The thickness of the plastic lens 650b at the lens end is t = 6.054 mm.
Becomes That is, the plastic lens 65 of the present invention
0b and the thickness t of the conventional plastic lens 65
The amount of change Δt from the thickness t at the end of 0a is the amount of change Δt = 6,054−5.622 = 0.432 mmm, and the thickness t at the end can be increased by 0.432 mm.

As described above, the plastic lens 650b
The thickness t of the end portion can be increased by 0.432 mm without changing the thickness Tc at the center portion of the conventional lens.
Although the above embodiment shows an example of a spherical lens, the present invention is applicable to a toric surface, an axisymmetric aspheric surface, an axisymmetric aspheric surface, or a combination of these surfaces in addition to a spherical lens. is there.

[0022]

As described above, in the plastic lens of the present invention, the thickness of the lens end can be easily secured without complicating the lens shape. As a result, distortion inside the lens can be driven into the lens edge, and even when the center thickness of the lens is reduced, it is possible to suppress the occurrence of distortion in the optical surface of the lens. It can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a scanning optical system.

FIG. 2 is a schematic view of a plastic f / θ lens according to the present invention.

FIG. 3 is a schematic view of a conventional plastic f / θ lens.

FIG. 4 is an explanatory diagram of the relationship among S ₁ , S ₂ , L, and Tc in the scanning optical system.

FIG. 5 is an explanatory diagram of a plastic lens according to the first embodiment.

FIG. 6 is an explanatory diagram of a plastic lens according to a second embodiment.

[Explanation of symbols]

1 light source, 2 collimator lens, 3 cylinder lens, 4 polygon mirror, 5 f / θ lens 1,
6 f · θ lens 2, 6 a Emission-side optical surface, 6 b
Incident optical surface, 6t end, 9 light beams.

Claims (2)

[Claims] 1. A plastic lens used for a laser beam polarization scanning optical system such as an LBP or a digital copier, wherein the plastic lens has a convex shape on both an entrance surface and an exit surface, and a laser beam in a laser beam scanning direction. A plastic lens, wherein the width of the optical surface on the incident side is different from the width of the optical surface on the laser light emission side. 2. The plastic lens according to claim 1, wherein the width of the optical surface on the incident side and the width of the optical surface on the output side satisfy the following relationship. 1 <S ₂ / S ₁ <1 + Tc / L, where S ₁ : entrance-side optical surface width S ₂ : exit-side optical surface width Tc: lens center thickness L: laser light incident side optical surface from the polygon mirror reflection surface to the lens Distance to