JPH0447286B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a scanning lens used in a scanning optical system such as a laser beam printer. Prior Art FIG. 1 shows a conventional example of a scanning optical system for a laser beam printer. In this figure, a laser beam generated by a semiconductor laser generator 1 is reflected by a rotating polygon mirror 2 and is reflected by a scanning lens 3.
The photosensitive drum 4 is reached through the photosensitive drum 4. Then, the deflection angle is changed by the rotation of the rotating polygon mirror 2, and the photosensitive drum 4 is scanned in the direction of the arrow a, and this is repeated. Here, the scanning lens 3 is a rotating polygon mirror 2.
This lens has a distortion aberration corresponding to the rotational characteristics of the laser beam, and makes the scanning speed of the laser beam on the photosensitive drum 4 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 sinusoidal oscillating mirror, the ideal height y is f·arcsinθ. This is an arcsine lens. Distortion aberration Dis of these lenses is expressed as Dis = y' - y/y x 100 (%), where y' is the actual image height. As shown in Fig. 1, the conventional scanning lens consists of a circular lens when viewed from the optical axis direction.
It had a truncated conical or cylindrical shape as a whole. However, as other elements of the scanning optical system, such as the use of semiconductor lasers, have become more compact, the space occupancy of such a shaped scanning lens has increased, and it has been desired to make the scanning lens more compact. In a scanning lens composed of a circular lens, the light beam actually passes only through the rectangular portion parallel to the scanning direction of the light beam, indicated by hatching in Figure 2A, and the other portions are covered by the lens. This is an unnecessary part. Therefore, as shown in Figure 3,
It is conceivable to flatten the scanning lens 3' in a direction perpendicular to the optical axis direction and the scanning direction. That is,
The scanning lens is shaped along the beam passage area (hatched area) as shown by the solid line in FIG. 2A. However, if this flattening is simply carried out, the following problems will arise. In other words, conventional scanning lenses were positioned and fixed by making circumferential line contact with the inner surface of the lens barrel, as shown in Figure 2B. When you try to do this,
As shown in Figure 2C, the arc-shaped abutting surface P when viewed from the optical axis direction must be machined to an accurate position and shape with respect to the center of the lens, and such processing is extremely difficult and increases the number of steps. do. Further, even if processing can be performed with a certain degree of accuracy, the contact surface is small, so each lens of the assembled scanning lens will be decentered, making it impossible to obtain sufficient performance. FIG. 2D shows a case where the lens barrel is milled, but in this case, it is supported at several points, making it impossible to obtain sufficient performance as in the above case. Purpose/Summary The present invention has been made in view of the above,
The purpose of the present invention is to provide a flat scanning lens that is easy to manufacture, can be assembled with high precision, and has sufficient lens performance. The above purpose is to provide a scanning lens that is composed of a plurality of lenses having a flat shape along the light beam passage area and scans the light beam deflected by a deflector at a constant speed on the imaging plane. are, in order from the incident side, a plano-concave lens, a plano-convex lens, and a plano-convex lens, and are provided with a lens holding member having a plurality of abutment surfaces against which the planar side lens surfaces of each constituent lens abut, respectively, and the condition -0.14≦d ₂ /f ₁ ≦−0.05 0.028≦d ₂ /f≦0.19 d ₂ : Distance between the second and third lens surfaces f ₁ : Focal length of the first lens f : A scanning lens that satisfies the focal length of the entire lens system. It is achieved by doing so. Embodiment FIG. 4 shows a flat single lens L whose one surface is flat. Both end portions (hatched portions P) of the plane of this lens L are used as contact surfaces for positioning. The other surface of the lens L is supported in point or line contact. FIG. 5 shows a further improved flat single lens L' of the present invention. This lens L' has one lens surface that is flat, and the side edge Q in the scanning direction that is also flat (Fig. 5A), and both ends (hatched portion R) of the other lens surface in the scanning direction are chamfered. (Figure 5B). Incidentally, FIG. 5C shows a plano-concave lens processed in the same manner. Flattening both ends can control the eccentricity of the lens around the optical axis, and when manufacturing such a lens L′, it is very easy to manufacture because it is only necessary to process one side of the rectangular parallelepiped to the lens surface. becomes easier. 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 ₁ , a plano-convex lens L ₂ , and a plano-convex lens L ₃ from the incident side, and these lenses are attached to holding members 10 and 1.
1. Spring holding members 12a, 12b, 13a,
It is held and positioned by 13b, 14a, 14b and upper and lower lid members (not shown). That is, the lens
The plane side lens surfaces of L ₁ , L ₂ , and L ₃ are held by the holding member 10,
11 contact surface 10-1, 11-1, 10-2, 1
1-2, 10-3, 11-3, respectively, and the curved lens surface is pressed by pressing members 12a, 12b, 1, respectively.
It is held down by 3a, 13b, 14a, and 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, using too many lenses is effective in correcting aberrations, but
Since there are disadvantages of reduced transmitted beam intensity and high cost, 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. From the object side, the f.
In the case of a θ lens, the following conditions are required. −0.14≦d ₂ /f ₁ ≦−0.05 …(1) 0.028≦d ₂ /f≦0.19 …(2) d ₂ : Distance between the second and third lens surfaces f ₁ : First lens f: Focal length of the entire lens Here, if _d2 is too small, distortion and astigmatism will both be under, and if d2 is too large, they will be over, and the same can be said for _f1 . However, since d ₂ and f ₁ are related to each other, defining them completely separately would result in extensive conditions.
Therefore, by defining the ratio of d ₂ and f ₁ , which are related to each other, as in condition (1), distortion and astigmatism can be corrected in a well-balanced manner. If the upper or lower limit of this condition (1) is exceeded, even if one of distortion aberration or astigmatism can be corrected, the other cannot be completely corrected. On the other hand, if the lower limit of condition (2) is exceeded, d ₂ will be too small, so that both distortion and astigmatism will be undervalued as described above. Also, the upper limit of condition (2) is
This defines a range narrower than the allowable value for aberration correction, and is set in consideration of problems in lens processing. In other words, if this upper limit is exceeded,
Since d ₂ is too large, the second and third lenses must be made large, which is disadvantageous in lens processing. 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, F _NO is 80, and a semiconductor laser with a wavelength of 780 mm is used as a light source.

【table】

【table】

[Table] Effects As is clear from the above description, according to the present invention, the scanning lens is composed of a flat lens, which makes it possible to make the scanning lens compact, and allows each component lens to be held and positioned with high precision. Further, the scanning lens of the present invention has a simple configuration and corrects distortion and astigmatism in a well-balanced manner.

[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. 4 and 5 are configuration diagrams of the flat lens in the embodiment, FIG. 6 is a diagram showing an assembly example of the scanning lens in the embodiment, and FIGS. 7 to 12 are lenses of the scanning lens in the configuration example. It is a figure showing a composition and aberration. Furthermore, Figures 2 and 4
5A, the left side is a front view seen from the optical axis direction, and the right side is a side view seen from a direction parallel to the scanning direction.

Claims (1)

[Scope of Claims] 1. A scanning lens that is composed of a plurality of flat-shaped lenses along a light beam passage area and that scans a light beam deflected by a deflector at a constant speed on an imaging plane, The above-mentioned constituent lenses are, in order from the incident side, a plano-concave lens, a plano-convex lens, and a plano-convex lens, and are provided with a lens holding member having a plurality of abutment surfaces against which the plane-side lens surfaces of each constituent lens respectively abut, and the condition is -0.14≦ d ₂ /f ₁ ≦−0.05 0.028≦d ₂ /f≦0.19 d ₂ : Distance between the second and third lens surfaces f ₁ : Focal length of the first lens f: Satisfy the focal length of the entire lens system A scanning lens characterized by: