JPH037082B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention reflects a light beam from a light source on a rotating polygon mirror and applies it to a scanned surface via a condensing lens.
The present invention relates to a light beam scanning device that performs scanning.

This type of light beam scanning device can be summarized as follows.
As shown in the figure, the light beam from the laser 2, etc. that was originally incident on the rotating polygon mirror 1 is reflected by the mirror surface of the rotating polygon mirror 1 rotating in the direction of the arrow, and the light beam is reflected by the condensing lens (f/θ lens). The light spot is configured to connect light spots moving at a uniform speed in the scanning direction (x direction) along the scanning line SP1 on the scanned surface SP through 3. However, if each mirror surface is not parallel to the rotation axis of the rotating polygon mirror 1 and there are irregular angles (inclination angle error),
For example, if the inclination angle error of the reflective surface 1a is Δθ as shown in FIG. 2, then Δd=2f・Δθ (where f is the focused This results in a shift amount (focal length of the lens 3). Since this inclination angle error and rotational shaft wobbling cause unevenness in the pitch of the scanning lines, it is necessary to take some countermeasures. One way to do this is to improve machining accuracy and minimize inclination angle errors, but in reality this is close to the limit in terms of machining accuracy, and even if machining could be done, it would take a lot of man-hours and would be difficult to mass produce. The problem is that it is expensive. or,
Conventionally, as shown in FIGS. 3 and 4, a light beam is incident on the rotating polygon mirror 1 as a linear beam parallel to the scanning direction using a cylindrical lens 4, etc. A cylindrical lens (or toroidal lens) 5 and a condensing lens 3 arranged between the surface SP and the linear beam and the scanning position are corrected as optical conjugates in a direction perpendicular to the scanning direction (special No. 49315 (1977).
However, if the cylindrical lens 5 of this shape is used, it is not possible to obtain a uniform spot size over the entire scanning width, and if a toroidal lens having a constant curvature also in the x direction is used instead of the cylindrical lens 5. , a nearly uniform spot size can be obtained, but
The high cost of toroidal lenses creates a new problem in that the optical beam scanning device is expensive.
Furthermore, as another conventional example, as shown in FIG. 5 and FIG.
The figure shows the beam shape on one surface 1a of the rotating polygon mirror 1), and a light beam that is nearly parallel to the optical axis direction is incident on the rotating polygon mirror 1, and the condenser lens 3
There is also one in which a cylindrical lens 8 is simply provided between the scanning surface SP and the surface to be scanned SP. This device also has the problem of not being able to obtain a uniform spot size over the entire scanning width.

The present invention was made in view of the above-mentioned problems, and reflects a light beam from a light source with a rotating polygon mirror.
In a light beam scanning device that performs scanning by applying a light beam to a surface to be scanned through a condensing lens, a light beam is curved between the condensing lens and the surface to be scanned so that a longitudinal end approaches the surface to be scanned. By disposing such cylindrical lenses, a uniform spot size can be obtained over the entire scanning width, and a light beam scanning device that does not cause scanning line pitch unevenness can be realized with a simple structure and at low cost. The position of the cylindrical lens is, for example, near the surface to be scanned, and the light beam incident on the rotating polygon mirror is wide and flat in the scanning direction, and approximately parallel to the optical axis direction. It is preferable to use

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 8 is a schematic configuration diagram of the light beam and optical system between the rotating polygon mirror and the surface to be scanned in the light beam scanning device according to the present invention, b is a top sectional view, and a is a side sectional view at the center of scanning. Figure 1c shows a side sectional view at the scanning end. In the figure, 11 is a rotating polygon mirror, and in this embodiment, a light beam that is wide in the scanning direction, not flat, and nearly parallel to the optical axis direction is incident from a beam shaping means (not shown) onto its reflecting surface (mirror surface). are doing. 12 is a condenser lens (an f/θ lens is used here) that receives the reflected beam from the rotating polygon mirror 11, and 13 is a surface to be scanned (usually a plane or a surface that can be considered to be almost a plane with respect to the scanning line). ) A convex cylindrical lens placed near SP, with the end facing toward the scanned surface SP (one end 13 is shown in Figure 8b).
(only a is shown) has a curved shape so that it approaches. This convex cylindrical lens 13 is arranged in a direction (y direction) perpendicular to the scanning direction (x direction).
It has refractive power.

In the device of the present invention, the reflecting surface of the rotating polygon mirror 11 and the scanned surface SP are almost geometrically optically conjugate with respect to the y direction in this embodiment, and the light beam passing through the condenser lens 12 is directed in the x direction. about,
The light beam uniformly converges from the condensing lens 12 toward the scanned surface SP, and in the y direction, it begins to converge from the condensing lens 12 and, after passing through the cylindrical lens 13, converges particularly rapidly. In this embodiment, the cylindrical lens 13 is provided near the scanned surface SP, but
By changing the curvature of the condenser lens 12
It is also possible to arrange it closer to the side.

By the way, the effective focal length of the cylindrical lens 13 becomes shorter when the light beam is incident obliquely, so if a cylindrical lens 13 that is straight with respect to the x direction is used, the light beam will move away from the scanning center. As a result, the beam waist of the light beam generated after passing through the cylindrical lens 13 becomes closer to the scanned surface.
However, in the present invention, as described above, since a curved cylindrical lens 13 is used, the lens moves near the scanned surface SP (on the scanned surface SP). ) may cause a beam waist. This point will be explained below using equations.

First, if the radius of curvature of the convex side of the refractive surface of the plano-convex cylindrical lens 13 used in this example is R, the focal length is f, and the refractive index is n, then f=R/(n- 1) ...(1) The following relationship holds true.

Also, if the incident angle ψ and refraction angle when the light beam leaves the scanning center and enters the cylindrical lens 13 obliquely are ψ', sinψ=nsinψ' according to the law of refraction. (2) A relationship holds true.

Furthermore, if the effective focal length when the light beam is obliquely incident on the cylindrical lens 13 is f', and the radius of curvature at a minute portion on the optical axis is R', then R'=R ² /(R/cosψ ′) = Rcosψ′ ...(3) f′=R′/(n-1) ...(4) holds.

Therefore, f/f' becomes f/f'=R/R'=1/cosψ'=1/√1-() ² (5) from equations (1) to (4).

Here, since √1−() ² ≦1,
f≧f', and as described above, when the light beam obliquely enters the cylindrical lens 13, the focal length becomes short. Therefore, in the present invention, the cylindrical lens 13 is bent so that the size (including shape) of the beam spot on the scanned surface SP becomes uniform by convergence of the light beam in the y direction by the cylindrical lens 13. It is set to . In particular, in this embodiment, the beam waist is generated near (including coinciding with) the scanned surface SP, and the reflective surface 1a of the rotating polygon mirror 1 and the scanned surface
It is configured so that SP is conjugate.

Therefore, by appropriately determining the relative positions of the rotating polygon mirror 11, the condensing lens 12, the cylindrical lens 13, and the scanned surface SP, the focal length, the diameter of the light beam incident on this system, the radius of wavefront curvature, etc. A desired spot can be obtained on the scanning plane SP, and the shape of this spot (not necessarily a perfect circle, but may be an ellipse) can be made uniform over the entire scanning width.

Specifically speaking, as an example, the cylindrical lens 13
The focal length of the condensing (f/θ) lens 12 is 350 mm, the scanning length is 280 mm, and the cylindrical lens 13 is curved in an arc shape with a radius of approximately 2100 mm.
Using the optical system selected as the main scanning means, photographic paper was attached to a table that moved at a constant speed, and characters were recorded, and almost good recording was achieved over the entire scanning width. On the other hand, in the conventional optical system as shown in FIG. 5, for example, good recording can be obtained near the center of the scan, but deterioration such as thickening of lines and image breakage occurs at the ends of the scan.

If more uniformity of the spot diameter is required, instead of bending into an arc shape, it is possible to bend into various other shapes, such as a quartic curve, to meet the above requirement.

Further, although the beam shaping means is not shown in FIG. 8, this beam shaping means may include, for example, a beam expanding means 15 using a spherical lens that receives the laser beam from the laser 14, as shown in FIGS. 9 and 10. , a long focus cylindrical lens 16 and a short focus cylindrical lens 17 into which the laser beam after passing through the beam expanding means 15 is incident. Furthermore, as shown in FIG. 11, a beam expansion device is formed by a single cylindrical lens 19 that receives the laser beam emitted from the laser 14 via the modulator 18, and a spherical lens into which the laser beam after passing through the cylindrical lens 19 is incident. It can also be constructed from the means 20. The reason for using such a beam shaping means is that the y-direction wavefront curvature radius of the light beam incident on the rotating polygon mirror 11 is very large and the light beam width is small. This is to obtain a luminous flux having desired parameters while avoiding an extremely long length.

As explained in detail above, in the present invention, the reflecting surface of the rotating polygon mirror and the scanning surface are an optical system consisting of a condensing lens and a cylindrical lens, so that they are almost geometrically conjugate in the y direction. None.This prevents the occurrence of scan pitch unevenness due to tilt angle error of the rotating polygon mirror, and at the same time, by using a curved cylindrical lens, the size of the spot on the scanned surface can be reduced. It is made uniform. For this reason, the effect of tilt angle correction is greater than in the past, and an almost perfectly uniform spot can be obtained on the scanning surface. Objects can be constructed at low cost without using toroidal lenses or the like.

[Brief explanation of the drawing]

Fig. 1 is a basic configuration diagram of the main parts of a light beam scanning device, Fig. 2 is an explanatory diagram of inclination angle error, Figs. 3 to 7 are explanatory diagrams of a conventional device, and Fig. 8 is an inventive device 9 to 11 are explanatory diagrams of the beam shaping means. 1, 11... Rotating polygon mirror, 3, 12... Condensing lens (f/θ lens), 4 to 8, 13, 16, 1
7, 19... Cylindrical lens, 2, 14... Laser,
15, 20...Beam expansion means, 18...Modulator, SP...Scanned surface.

Claims (1)

[Claims] 1. Reflecting a light beam from a light source with a rotating polygon mirror,
In a light beam scanning device that performs scanning by applying a light beam to a surface to be scanned through a condensing lens, a light beam is provided between the condensing lens and the surface to be scanned such that a longitudinal end thereof approaches the surface to be scanned. A light beam scanning device characterized by having a curved cylindrical lens. 2. The light beam scanning device according to claim 1, wherein the cylindrical lens is arranged near the surface to be scanned between the condenser lens and the surface to be scanned. 3 As a light beam incident on the rotating polygon mirror,
The light beam scanning device according to claim 1 or 2, characterized in that a light beam that is wide and flat in the scanning direction and substantially parallel to the optical axis direction is used. 4. Claims 1 to 3, characterized in that the reflective surface of the rotating polygon mirror and the scanned surface are configured to have a geometrically optical conjugate relationship in a direction substantially perpendicular to the scanning direction. The light beam scanning device according to any one of the above. 5. The light beam scanning device according to any one of claims 1 to 4, characterized in that the beam waist is formed in the vicinity of the scanned surface. 6. The light beam scanning device according to any one of claims 1 to 5, wherein the light beam is a laser beam. 7. A patent claim characterized in that the beam shaping means includes a beam expanding means using a spherical lens, and a long focal length cylindrical lens and a short focal length cylindrical lens into which the light beam after passing through the beam expanding means is incident. A light beam scanning device according to any one of the ranges 1 to 6. 8. Claim 1, characterized in that the beam shaping means comprises a single cylindrical lens and an enlarging means made of a spherical lens into which the light beam after passing through the cylindrical lens enters. 7. The light beam scanning device according to any one of items 6 to 6.