JPH05341215A - Temperature compensated scanning optical system - Google Patents

Abstract

PURPOSE:To obtain a scanning optical system which can reduce the influence caused by a temperature variation without increasing the number of component pieces of a scanning lens. CONSTITUTION:In the scanning optical system in which a luminous flux emitted from a light source 1 forms an image once only in the sub-scanning direction, and also, is deflected by a deflector P provided in the vicinity of the image formed position, and the deflected luminous flux is subjected to image formation on the image face by an anamorphic scanning lens 30 made of plastic, a positive lens 10, and a cylindrical lens 20 consisting of a single plastic lens having negative power against the sub-scanning direction without having roughly power against the main scanning direction are provided between the light source 1 and the reflector P, and the power of the cylindrical lens 20 is set so as to cancel the influence caused by a temperature variation generated by the scanning lens 30 in the sub-scanning face.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system used in, for example, a laser printer, and more particularly to a scanning optical system in which focus movement due to temperature change is suppressed.

[0002]

2. Description of the Related Art Laser scanning lenses include
Plastic lenses are often used to reduce cost and weight. However, compared with glass, plastic has a greater degree of linear expansion and refractive index change with respect to temperature change, and thus drawing quality is more likely to deteriorate due to defocus.

Further, a scanning optical system such as a laser printer generally employs a configuration in which a light beam is once focused in the sub-scanning direction in the vicinity of the polygon mirror in order to reduce the influence of the surface tilt error of the polygon mirror. In many cases, the anamorphic optical system has a power in the sub-scanning direction stronger than that in the main-scanning direction. Therefore, defocus due to temperature change is also a problem mainly only in the sub-scanning direction.

In order to reduce the influence of such temperature change, a method of combining a plurality of plastic lenses in a scanning lens and arranging them so as to cancel out the focal movements generated is known.

[0005]

However, in the above-mentioned conventional method, the number of scanning lenses must be increased for temperature compensation, which is against the trend of downsizing and cost reduction.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and provides a scanning optical system capable of reducing the influence of temperature change without increasing the number of scanning lenses. With the goal.

[0007]

The temperature-compensated scanning optical system according to the present invention achieves the above object.
The light beam emitted from the light source is formed into an image only in the sub-scanning direction, and is deflected by a deflector provided in the vicinity of this image formation position, and the deflected light beam is formed on the image plane by a plastic anamorphic scanning lens. In the scanning optical system for imaging, a positive lens is provided between the light source and the deflector,
A cylindrical lens made of a single plastic lens that has almost no power in the main scanning direction and has negative power in the sub scanning direction is provided, and the power of the cylindrical lens is set in the sub scanning plane. It is characterized in that it is set so as to cancel out the influence of the temperature change that occurs at.

[0008]

Embodiments of the present invention will be described below. Figure 1
FIG. 3 is a lens diagram of a scanning optical system according to an embodiment of the present invention, in which (a) shows a cross section in the main scanning direction and (b) shows a cross section in the sub scanning direction.

This optical system uses only an anamorphic positive lens 10 for converting the divergent light emitted from the semiconductor laser 1 as a light source into a parallel light beam in the main scanning direction and a convergent light in the sub scanning direction, and only in the sub scanning direction. A cylindrical lens 20 having a negative power, a deflector P for deflecting a light beam emitted from the cylindrical lens, and an anamorphic scanning lens 30 for forming the deflected light beam on an image plane are provided.

The light beam emitted from the light source is temporarily imaged only in the sub-scanning direction near the deflector P via the positive lens 10 and the cylindrical lens 20.

Since FIG. 1 is an exploded view of the optical path, the deflector P is shown by a straight line, but it is well known that a polygon mirror or the like is specifically used. In addition, in order to shorten the optical path, a part of the optical path is omitted in the part shown by the broken line.

The positive lens 10 is a rotationally asymmetric glass lens having positive power in both the main and sub-scanning directions and a power in the sub-scanning direction larger than that in the main-scanning direction.
Such a lens can be molded by, for example, a glass mold.

The cylindrical lens 20 is a plastic lens having a negative power in the sub scanning direction, and has almost no power in the main scanning direction.

The scanning lens 30 is a single-lens fθ lens, and is a plastic lens having a strong positive power in the sub-scanning direction as compared with the main scanning direction. Since the aspherical surface processing of the plastic lens is easy, it is possible to create an fθ lens that satisfies the performance of a laser printer or the like even with a single lens structure. However, since the influence of the temperature change cannot be canceled out in the scanning lens with one lens, the temperature is compensated by providing the plastic cylindrical lens 20.

In a scanning optical system such as a general laser printer, a rotationally symmetric collimating lens is arranged at the position of the positive lens 10, and the position of the negative cylindrical lens 20 has a positive power only in the sub-scanning direction. It has a cylindrical lens.

However, in order to cancel the influence of the temperature change of the scanning lens having a strong positive power in the sub scanning direction, it is necessary to provide a plastic lens having a negative power in the sub scanning direction. Therefore, in the scanning optical system of the present invention, a negative cylindrical lens 20 made of plastic is provided, and the positive power in the sub-scanning direction generated by the conventional positive cylindrical lens and the positive cylindrical lens 20 cancels the positive power. The positive lens 10 shares the power of the above. The main scanning direction is the same as the conventional configuration.

With the above arrangement, focus fluctuation due to temperature change hardly occurs in the main scanning direction. on the other hand,
Since the focus variation in the sub-scanning direction is generated by the cylindrical lens 20 and the scanning lens 30, the power is set so as to cancel the influence of the temperature change generated by these lenses.

Here, the distance from the cylindrical lens 20 to the focal point of the positive lens 10 in the sub scanning direction is A, the distance from the cylindrical lens 20 to the image forming position in the sub scanning direction is B, and the sub of the cylindrical lens 20 due to temperature change is sub. When the change in focal length in the scanning direction is Δf _20Z , the change in focal length of the scanning lens 30 in the sub-scanning direction is Δf _30Z, and the magnification of the scanning lens 30 in the sub-scanning direction is m, the amount of movement of the sub-scanning image plane due to temperature change ΔP _IM is expressed by Equation 1 below.

[0019]

## EQU1 ## ΔP _IM = (1- (B / A)) ² × f _20Z ² × m ² + (1-m) ² × Δf _30Z

Cylindrical lens 20 and scanning lens 3
When the same plastic is used for 0, the focus movement due to the temperature change of the scanning lens is corrected when the following expression 2 is established.

[0021]

(2) (1- (B / A)) ² × f _20Z ² × m ² = (1-m) ² × Δf _30Z

Positive lens 10 and cylindrical lens 20
Table 1 shows the specific numerical configuration of and. In the table, the surface number is the surface number counted from the object side on the left side in the figure, ry is the radius of curvature in the main scanning direction, rz is the radius of curvature in the sub-scanning direction (the surface not described is rotationally symmetric), d is the lens Thickness or air spacing,
n is the refractive index at a wavelength of 780 nm.

The first surface of the positive lens 10 is a cylindrical aspherical surface, and the second surface is a rotationally symmetric aspherical surface. For an aspherical surface, the distance from the tangent plane of the aspherical vertex of the coordinate point on the aspherical surface whose height from the optical axis is Y is X, and the curvature of the aspherical vertex (1 / r)
Is C, the conic coefficient is K, and the fourth-order aspherical coefficient is A ₄ , which is expressed by the following equation. The radius of curvature of the aspherical surface in Table 1 is the radius of curvature of the aspherical vertex, and the conical coefficient and the aspherical surface coefficient of these surfaces are shown in Table 2.

[0024]

[Formula 3] X = CY ² / (1 + √ (1- (1 + K) C ² Y ² )) + A ₄ Y ⁴

[0025]

[Table 1] f _30Z = 70 mm m = -2.5 Surface number ry rz dn 1 ∞ 26.280 3.00 1.76622 2 -6.896 31.20 3 ∞ -25.604 2.00 1.48617 4 ∞ 25.604

[0026]

[Table 2] First surface Second surface K = 0 K = -0.650 A4 = -4.50000 × 10 ^-4 A4 = 0

[0027]

As described above, according to the present invention, the number of scanning lenses having a relatively large diameter is increased by allowing the lens on the light source side of the deflector having a relatively small diameter to have a temperature compensation function. In this way, the influence of temperature change can be reduced, and the device can be downsized and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a development view of an optical system showing the configuration of an embodiment,
(a) shows the main scanning direction and (b) shows the sub scanning direction.

[Explanation of symbols]

 10 ... Positive lens 20 ... Cylindrical lens 30 ... Scanning lens

Claims (2)

[Claims] 1. A anamorphic scanning lens made of plastic, in which a light beam emitted from a light source is once imaged only in a sub-scanning direction and is deflected by a deflector provided in the vicinity of the image formation position. In the scanning optical system for forming an image on the image plane by means of a positive lens, there is almost no power in the main scanning direction and a negative power in the sub scanning direction between the light source and the deflector. And a cylindrical lens made of a single plastic lens having the power of the cylindrical lens is set so as to cancel the influence of the temperature change generated in the scanning lens in the sub-scanning plane. Compensated scanning optics. 2. The temperature according to claim 1, wherein the positive lens has positive power in the main scanning direction and the sub scanning direction, and the power in the sub scanning direction is larger than the power in the main scanning direction. Compensated scanning optics.