JPH02191911A - Scanning lens - Google Patents

Abstract

PURPOSE:To obtain a compact lens of good lens performance by constituting the lens of a plano-concave lens and plano-convex lenses in order from the incidence side and satisfying a prescribed condition. CONSTITUTION:This lens system consists of plural flat lenses L1, L2, and L3 in order from the incidence side, and the lens L1 is a plano-concave lens, and lenses L2 and L3 are plano-convex lenses. Holding members 10 and 11 are provided which have plural contact faces brought into contact with plane lens faces of respective constituting lenses. They satisfy the condition of an inequality I where d2 and f1 are the distance between the second face of the lens L1 and the third face of the lens L2 and the focal length of the first lens L1 respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a scanning lens used in a scanning optical system such as a laser beam printer.

BACKGROUND OF THE INVENTION FIG. 1 is a diagram showing a scanning optical system of a laser beam printer. In this figure, a semiconductor laser generator (1
) is reflected by a rotating polygon mirror (2), passes through a scanning lens (3), and is directed to a photosensitive drum (4).
reach. Then, by rotating the rotating polygon mirror (2), the deflection angle is changed and scanning is performed on the photosensitive drum (4) in the direction of arrow a, and this is repeated. Here, scan lens (3
) is a lens with a distortion aberration corresponding to the rotational characteristics of the rotating polygon mirror (2), and the photosensitive drum (
4) The scanning speed above is made constant. When the deflector is a rotating polygon mirror, such a scanning lens is an f-θ lens with the ideal height y as f-θ, and when the deflector is a sine-oscillating mirror, the ideal height y is f-arcsinθ. This is an arcsine lens. Distortion aberration Dis of these lenses is expressed as follows, where the actual image height is y'.

On the other hand, as shown in FIG. 1, a conventional scanning lens is composed of a circular lens when viewed from the optical axis direction, and has a frustoconical or cylindrical shape as a whole. However, when a scanning lens having such a shape is used in a scanning optical system, there is a problem that the scanning lens occupies a large space, and it has been desired to make the scanning lens more compact.

In a scanning lens composed of a circular lens,
The beam actually passes through only a rectangular portion parallel to the scanning direction of the beam, as shown by hatching in FIG. 2A, and the other portions are unnecessary as a lens. Therefore, as shown in FIG. 3, it is conceivable to flatten the scanning lens (3') in a direction orthogonal to the optical axis direction and the scanning direction to make it more compact. That is, the scanning lens is shaped along the beam passing region (hunting portion) shown by the solid line in FIG. 2 AI.

However, simply flattening the scanning lens brings about the following problems.

In other words, as shown in FIG. 2B, the scanning lens constituted by a circular lens was held by line contact between the outer periphery of the lens and the inner surface of the lens barrel. However, when the scanning lens is flattened, As shown in FIG. 2C, the contact surface P, which is arcuate when viewed from the optical axis direction, cannot be held with high precision unless it is machined into an accurate position and shape with respect to the center of the lens. However, it is very difficult to precisely process the arc-shaped contact surface CP with respect to the center of the lens, and the number of processing steps increases. Further, even if the scanning lens can be processed with a certain degree of precision, the contact surface is small, so each constituent lens of the assembled scanning lens will be decentered, making it impossible to obtain sufficient performance. Furthermore, the second diagram shows the case where the lens barrel is machined by 7 slices, but in this case, it is supported at several points, and as in the case described above, sufficient performance cannot be obtained.

The present invention aims to solve the above problems and provide a scanning lens that can obtain sufficient lens performance with a simple configuration.

Means for Solving the Problems In order to achieve the above object, the scanning lens of the present invention is a scanning lens composed of a plurality of flat-shaped lenses, and the constituent lenses are arranged in order from the incident side. concave lens,
The lens is a plano-convex lens, and includes a holding member having a plurality of abutment surfaces that abut on the plane-side lens surfaces of each component lens, and the condition -0.25≦dr/L≦-0.07d2: The second surface of the lens and the second surface of the lens The feature is that the distance f1 between the three surfaces satisfies the focal length of the first lens.

Example FIG. 4 shows a flat single lens (L) with one surface being flat.

Both end portions of the plane of this lens (L) (hatched portion P
) is used as a contact surface for positioning. Lens (L
) are supported by point or line contact.

FIG. 5 shows a further improved flattened single lens (L'). In this lens (L'), in addition to one lens m being flat, both sides (Q) in the scanning direction are also flat (Fig. 5A), and both ends of the other lens surface in the scanning direction (hatched portion R
) is chamfered (Fig. 5B). Incidentally, FIG. 5C shows a plano-concave lens processed in the same manner. Flattening both ends can control eccentricity of the lens around the optical axis, and when manufacturing such a lens (L'), it is only necessary to process one side of the rectangular parallelepiped into a lens surface. Manufacturing becomes very easy.

Chamfering both ends of the curved lens surface in the scanning direction has the advantage that the lens can be held on two planes, making it possible to more accurately and reliably position the lens.

Incidentally, such a lens can also be manufactured by plastic molding.

FIG. 6 shows an example of an assembly of a scanning lens in which three flat single lenses each having one of the lens surfaces described above are combined.

In FIG. 6, the scanning lens is composed of a plano-concave lens (L+), a plano-convex lens (L, ), and a plano-convex lens (L,) from the incident side, and these lenses are attached to a holding member (10) (
11) Spring holding members (12a) (12b)
(13a) (13b) (14a) (14b
) and upper and lower lid members (not shown) for holding and positioning.

That is, the plane side lens surface of the lens (Ll) (Ll) is the contact surface (10-) of the holding member (10) (11).
1) - 3), respectively, and the curved lens surface is pressed by pressing members (12a) (12b) (13a) (1), respectively.
3b) It is held down by (14a) (14b). The pressing member may be flat as shown, but it may also be formed into a concave shape so that it can press against the curved surface of the lens at at least two points.

Furthermore, when the lens is molded from plastic, a through hole may be provided at the end of the lens and screwed onto the contact surface.

The scanning lens constructed in this manner is very easy to assemble and can provide sufficient accuracy.

Since one of the lens surfaces of this scanning lens is flat, it is desirable to have a configuration of two or more lenses in order to obtain sufficient lens performance. However, if the number of lenses is too large, it is effective in correcting aberrations, but there are disadvantages such as a decrease in transmitted beam intensity and high cost, so a two or three lens configuration is practical.

In addition, since such a scanning lens includes many flat surfaces, the degree of freedom in correcting aberrations is reduced, and it is difficult to maintain uniform scanning speed and correct aberrations that are the main causes of laser beam distortion, namely distortion and astigmatism. Certain conditions must be met in order to achieve well-balanced control.

In the case of an f/theta lens consisting of three plano-concave and plano-convex lenses from the object side, the following conditions are required.

-0.14≦d*/L≦-0.05...(1) 0.0
28 ≦d, / f ≦ 0.19 ... (2
) d2: Distance between the second and third surfaces of the lens f1: Focal length of the first lens f: Focal length of the entire lens Here, if d is too small, both distortion and astigmatism will be undervalued. As it gets too much, it becomes over.

The same can be said about f. Therefore d2 and f
By setting , as in condition (1), both double-sided aberrations can be controlled in a well-balanced manner. That is, d2, f
If the upper or lower limit of condition (1) is exceeded due to the combination of , even if one of distortion or astigmatism can be controlled, the other becomes a large aberration.

The lower limit of condition (2) is also set to be advantageous in correcting aberrations. Further, the upper limit is for regulating the effective width of the lens, and is advantageous in processing the lens.

In the case of an f/theta lens consisting of two plano-concave and plano-convex lenses from the object side, the following condition (3) must be satisfied.

−0.25≦d/r+≦−0.07 (3) This condition (3) corresponds to the condition (1) for the three-sheet configuration. There is no need to specifically limit d and /f in condition (2).

An example of the configuration of an f/θ lens to which the present invention is applied will be shown below. In each configuration example, the focal length of the entire lens is 100 mm,
The FNO is 80, and a semiconductor laser with a wavelength of 780 mm is used as a light source.

Configuration example 1 (Figure 7) Radius of curvature r Surface spacing d Refractive index n Configuration example 2 (
(Fig. 8) Radius of curvature r Surface spacing d Refractive index n Configuration example 3 (Fig. 9) Radius of curvature r Surface spacing d Refractive index n r, -46,899 Configuration example 4 (Fig. 10) Radius of curvature r Surface spacing d Refractive index n Configuration example 7 (Figure 13) Radius of curvature r Surface spacing d Refractive index n Configuration example 5 (Figure 11) Radius of curvature r Surface spacing d Refractive index n Configuration example 8 (Figure 14) Radius of curvature r Surface Spacing d Refractive index n fl--63,250 Configuration example 6 (Fig. 12) Radius of curvature r Surface spacing d f,--282,440 Refractive index n Effect As is clear from the above description, the present invention For example, since the scanning lens is composed of a flat lens, it can be made compact, and each constituent lens can be held and positioned with high precision. Furthermore, a scanning lens with a simple configuration and good lens performance can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing a scanning optical system of a conventional laser printer, Fig. 2 is an explanatory diagram of flattening a scanning lens, and Fig. 3 is a schematic diagram showing a scanning optical system using a flat scanning lens. FIG. 4 and FIG. 5 are configuration diagrams of the flattened lens in the example,
FIG. 6 is a diagram showing an assembly example of the scanning lens in the embodiment, and FIGS. 7 to 14 are diagrams showing the lens configuration and aberration of the scanning lens in the configuration example. In FIG. 2, FIG. 4, and FIG. 5A, the left side is a front view viewed from the optical axis direction, and the right side is a side view viewed from a direction parallel to the scanning direction. Figure Patent Applicant: Minolta Camera Co., Ltd. Figure Figure A Figure 8 Figure A Figure 8 Figure 8 Figure B Figure 70 A, io Figure B , the acid base ball φ fits in Figure 11 A'! PI12 Figure A Figure 11 Figure 8 Figure B

Claims (1)

[Claims] (1) A scanning lens composed of a plurality of flat-shaped lenses, where the constituent lenses are, in order from the incident side, a plano-concave lens and a plano-convex lens, and are in contact with the plane-side lens surface of each constituent lens, respectively. A holding member having a plurality of contact surfaces is provided, and the condition -0.25≦d_2/f_1≦-0.07 d_2: Distance between the second and third lens surfaces f_1: Focal length of the first lens is satisfied. A scanning lens characterized by: