JP2553882B2 - Scanning optical system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION (Industrial application field) The present invention relates to a scanning imaging optical system used for a laser printer or the like, and in particular, a mirror surface of a rotary polygon mirror for reflecting a light beam and a rotation axis or a mounting axis thereof. The present invention relates to an improvement in a scanning image forming optical system for correcting a parallelism error (plane tilt error) with the above.

(Prior Art) Conventionally, in an optical scanning system in which a light source, a rotary polygon mirror, and a scanning imaging lens (fθ lens) are arranged in this order, for example, laser light emitted from a light source is mechanically rotated by rotating the rotary polygon mirror. It is known that the image plane is beam-scanned by scanning and condensing with a scanning image-forming lens. At this time, due to a parallelism error between the rotary polygon mirror and its rotation axis or mounting axis, a so-called surface tilt in which the reflecting surface of the rotary polygon mirror tilts occurs, which affects the quality of the scanning line image.

Therefore, the following scanning imaging optical system has been proposed as a scanning imaging optical system that corrects and improves this surface tilt. JPA
Japanese Patent Laid-Open No. 48-98844 (hereinafter referred to as the first example) proposes a scanning imaging optical system in which a cylindrical lens having a generatrix in a direction parallel to the scanning direction is arranged between a spherical lens system and an imaging scanning surface. Japanese Patent Application Laid-Open No. 58-132719 (hereinafter, referred to as a second example) proposes a scanning imaging optical system in which a toroidal lens is arranged between a deflecting reflection surface and a scanning lens. Japanese Laid-Open Patent Publication No. 58-49315 (hereinafter referred to as the third example) proposes a scanning image forming optical system in which a toroidal lens is arranged between a scanning lens and an image forming scanning surface. Japanese Patent No. 36622 (hereinafter, referred to as a fourth example) proposes a scanning imaging optical system using a toric lens as a scanning lens.

(Problems to be Solved by the Invention) By the way, in the scanning image forming optical system of the first example, when an attempt is made to dispose the cylindrical lens away from the image forming scanning surface and close to the spherical lens system, sagittal field curvature is generated. The field curvature in the direction perpendicular to the scanning direction (hereinafter referred to as the sub-scanning direction) corresponding to
The lens must be located close to the imaging scan plane, and the distance between the cylindrical lens and the imaging scan plane cannot be increased. For this reason, there is a disadvantage that the cylindrical lens needs to be as long as the scanning length.

In addition to the drawback that the toroidal lens is very difficult to manufacture, the scanning image forming optical system of the second example has an image forming magnification of −10 × between the deflective reflecting surface and the image forming scanning surface in the sub-scanning direction cross section. −
The enlargement magnification is about 20 ×, and there is a drawback that the positional accuracy of the deflective reflection surface is required to be very high.

The scanning image forming optical system of the third example improves the amount of field curvature in the sub-scanning direction, but has the drawback that the size of the toroidal lens is quite large and very difficult to manufacture.

The scanning and imaging optical system of the fourth example is dimensionally compact, and the imaging magnification between the deflective reflection surface and the imaging scanning surface in the cross section in the sub-scanning direction is an enlargement magnification of about -3 × -5 ×. Therefore, the positional accuracy of the deflecting / reflecting surface is not required as much as that of the scanning optical system of the second example, but it has a drawback that the toric lens is not easily manufactured.

Therefore, an object of the present invention is to provide a surface that is rotationally symmetric with respect to the optical axis such as a mirror surface and a cylindrical surface without using a toroidal lens or toric lens that is difficult and expensive to manufacture in a scanning imaging gastric optical system. Provided is a compact and inexpensive scanning and image forming optical system having a surface tilt correction function, which is configured as described above, and which reduces distortion as well as a focused spot to reduce astigmatism to flatten the image plane. To do.

(Means for Solving the Problems) The scanning and imaging optical system according to the present invention is a scanning and imaging optical system for scanning a light beam reflected and deflected by a deflector onto an imaging surface. In order from the deflector, a first lens group having a negative focal length as a whole, a second lens group having a positive focal length as a whole, and a main line having a generatrix in a direction parallel to the main scanning direction of the image plane. And a concave optical reflection element having a quadratic curve in a cross section in the direction perpendicular to the scanning direction. The cross section in the direction perpendicular to the main scanning makes the deflective reflection surface of the deflector and the image formation surface have an optical conjugate relationship. It is a scanning and imaging optical system that satisfies the above conditions.

_{(1) -2.523f ≦ f 1 ≦} -1.083f (2) 0.557f ≦ f 2 ≦ 0.821f (4) -0.579f ≤ γ ≤ -0.321f (5) 0.108f ≤ d ≤ 0.432f (6) 0.229f ≤ | R ・ cosψ | ≤ 0.813f where f: focal length of cross section of main system in main scanning direction f ₁ : focal length of the first lens group f ₂ : focal length of the second lens group d: length on the optical axis from the first surface of the first lens group to the last surface of the second lens group γ: second Radius of curvature of the final surface (surface to be scanned) of the lens group R: Radius of curvature of the cross section of the concave optical reflection element in the direction perpendicular to the main scanning direction ψ: Optical axes of the first lens group and the second lens group, and the optical axis The angle formed by the normal line on the intersection of the concave optical reflection elements.

(Embodiment) Hereinafter, a scanning imaging optical system according to the present invention will be described in detail with reference to an embodiment shown in the accompanying drawings.

FIG. 1 is a perspective view of a scanning image forming optical system according to the present invention, and FIG. 2 is a direction perpendicular to the main scanning direction (sub scanning direction) of FIG.
FIG. The scanning and imaging optical system according to the present invention includes a first lens group 6 having a negative refracting power as a whole in the main scanning direction section, and a second lens group 2 having a positive refracting power as a whole in the main scanning direction section
The lens group 7 and a cylindrical mirror 8 having a generatrix in a direction parallel to the main scanning direction. In FIG. 1, a substantially circular or elliptical light beam emitted from a laser light source 1 passes through an optical system 2 including a collimator lens, a beam expander, or a combination thereof, and a cylindrical lens 3 rotates a polygon mirror. The light is focused on the deflecting / reflecting surface 5 of the rotating polygon mirror 4 or in the vicinity thereof as a line image perpendicular to the rotation axis 20 of 4. This line image is reflected and deflected by the deflecting / reflecting surface 5, then refracted and transmitted by the first lens group 6 and the second lens group 7 of the scanning and imaging optical system of the present invention, further reflected by the cylindrical mirror 8 and then scanned by the scanning surface 9. The light is focused and imaged in a spot shape on the top. The condensed beam is rotated by the rotating polygonal mirror 4 in the C direction to scan the scanning surface 9
The upper part is main-scanned in the A direction. At this time, with respect to the light flux in the sub-scanning B direction perpendicular to the main scanning A direction, the rotary polygon mirror 4
Since the deflecting / reflecting surface 5 and the scanning surface 9 have a substantially optically conjugate positional relationship, the deflecting / reflecting surface 5 is inclined from the normal state due to the inclination of the rotation axis 20 of the rotary polygonal mirror 4 and the like to cause surface tilt. However, the position of the condensed beam in the sub-scanning direction B on the scanning surface 9 from the predetermined position hardly occurs. Further, the imaging magnification between the deflective reflection surface 5 and the scanning surface 9 is −
Since the magnification is as low as 0.3 × −0.5 ×, high precision is not required for the position accuracy of the deflective reflection surface 5.

3 to 8 are configuration diagrams of a scanning image forming optical system showing an embodiment of the present invention. In FIG. 3, the first lens group 6 has a negative meniscus with a concave surface facing the deflective reflection surface 5 side.
The second lens group 7 is composed of a lens 6a and a positive meniscus lens 6b having a concave surface facing the deflective reflection surface 5, and the second lens group 7 is composed of a biconvex lens. In FIG. 4, the first lens group 6 is a biconcave lens, and the second lens group 7 is a biconvex lens. In FIG. 5, the first lens group 6 is composed of a negative meniscus lens having a concave surface facing the deflective reflecting surface 5, and the second lens group 7 is a biconvex lens 7a and a negative lens having a concave surface facing the deflective reflecting surface 5 side. It consists of a cemented lens with the meniscus lens 7b. Sixth
In the figure, the first lens group 6 is composed of a negative meniscus lens having a convex surface facing the deflective reflection surface 5, and the second lens group 7 is composed of a biconvex lens. In FIG. 7, the first lens group 6 is a biconcave lens, and the second lens group 7 is a positive plano-convex lens. In FIG. 8, the first lens group 6 is composed of a negative meniscus lens having a convex surface facing the deflective reflection surface 5, and the second lens group 7 is composed of a positive plano-convex lens.

In order to obtain a good imaging performance, each scanning imaging optical system,
The following conditions are satisfied.

_{(1) -2.523f ≦ f 1 ≦} -1.083f (2) 0.557f ≦ f 2 ≦ 0.821f (4) -0.579f ≤ γ ≤ -0.321f (5) 0.108f ≤ d ≤ 0.432f (6) 0.229f ≤ | R ・ cosψ | ≤ 0.813f where f: focal length of cross section of main system in main scanning direction f ₁ : focal length of the first lens group f ₂ : focal length of the second lens group d: length on the optical axis from the first surface of the first lens group to the last surface of the second lens group γ: second Radius of curvature of the final surface (surface to be scanned) of the lens group R: Radius of curvature of the cross section of the concave optical reflection element in the direction perpendicular to the main scanning direction ψ: Optical axes of the first lens group and the second lens group, and the optical axis The angle formed by the normal line on the intersection of the concave optical reflection elements.

Conditions (1) to (4) are conditions relating to the fθ characteristic that the scanning speed of the spot on the scanning surface is constant when the field curvature aberration in the main scanning direction is corrected and the deflection angular velocity of the deflector is constant. If the conditions (1) to (4) are not satisfied, it becomes difficult to correct the field curvature aberration in the main scanning direction and keep the fθ characteristic excellent.

The condition (5) is a condition regarding the lens shape. When the upper limit of the condition (5) is exceeded, the lens shape becomes large, which makes it impractical in terms of weight and economy. When the lower limit of the condition (5) is exceeded, the field curvature aberration in the main scanning direction and the fθ characteristic are obtained. Is difficult to correct.

The condition (6) relates to the correction of the field curvature aberration and the distance between the cylindrical mirror and the scanning surface. When the value exceeds the upper limit of the condition (6), the field curvature aberration in the sub-scanning direction deteriorates, and it becomes impossible to make the size of the image forming light spot on the scanning surface uniform over the entire screen. When the value goes below the lower limit of the condition (6), the field curvature aberration in the sub-scanning direction becomes small,
Since the cylindrical mirror and the scanning surface are too close to each other, it is difficult to properly dispose the photosensitive material on the scanning surface.

In the above-described embodiment, the concave optical reflection element provided between the second lens group 7 and the surface 9 to be scanned has a generatrix in a direction parallel to the main scanning A direction and has a direction perpendicular to the main scanning A direction. An optical reflection element whose cross section has a quadratic curve such as a parabola may be used.

Numerical examples corresponding to FIGS. 3 to 8 of the scanning image forming optical system of the present invention are shown in Tables 1 to 6, respectively.
In each table, d ₀ is the distance from the deflective reflection surface 5 to the first surface of the scanning imaging optical system first lens group 6, λ is the wavelength of light emitted from the light source 1, γ is the radius of curvature of the lens, and Is the distance between the optical surfaces or the air gap, and N is the refractive index of the glass material of the optical element.

However, the 7th surface is a cylindrical mirror surface.

However, the fifth surface is a cylindrical mirror surface.

However, the seventh surface is a cylindrical mirror surface.

However, the fifth surface is a cylindrical mirror surface.

However, the fifth surface is a cylindrical mirror surface.

However, the fifth surface is a cylindrical mirror surface.

9 to 14 are aberration curve diagrams corresponding to the numerical examples of Tables 1 to 6, respectively. In these aberration curve diagrams, H represents the field curvature aberration in the main scanning direction, V represents the field curvature aberration in the sub scanning direction, and the distortion aberration is the ideal image when the ideal image position with respect to the scanning angle θ is f · θ. The amount of deviation from the position is displayed in%. The scanning angle θ represents a half angle of view.

(Effect of the Invention) The scanning and imaging optical system of the present invention comprises, in order from the rotary polygonal mirror, a first lens group having a negative refractive index as a whole, a second lens group having a positive refractive index as a whole, and a main lens in the main scanning direction. Since it is configured with a concave cylindrical mirror having a generatrix in a parallel direction and satisfies the above-mentioned conditions, it can be inexpensively constructed without using an expensive toroidal lens or toric lens that is difficult to manufacture. It is possible to provide good imaging performance.

[Brief description of drawings]

FIG. 1 is a perspective view of a scanning image forming optical system according to the present invention, FIG. 2 is a sectional view in a direction perpendicular to the main scanning direction of FIG. 1, and FIGS.
FIG. 9 is a configuration diagram of a scanning imaging optical system showing an embodiment of the present invention, and FIGS. 9 to 14 are aberration curve diagrams corresponding to FIGS. 3 to 8, respectively. 1 ... Light source, 2 ... Optical system, 3 ... Cylindrical lens, 4 ... Rotating polygonal mirror, 5 ... Deflection / reflection surface, 6 ... First lens group, 7 ... Second lens group, 8 ... ... Cylindrical mirror, 9 ... Scanning plane, A ... Main scanning direction, B ...
Sub scanning direction.

Claims (1)

(57) [Claims] 1. A scanning imaging optical system for scanning an image plane with a light beam reflected and deflected by a deflector, wherein the scanning imaging optical system has a negative focal length as a whole in order from the deflector. A second lens group, which has a positive focal length as a whole
The deflector is composed of a lens group and a concave optical reflecting element having a generatrix in a direction parallel to the main scanning direction of the image plane and having a quadratic curve in the cross section in the main scan orthogonal direction. (1) -2.523f ≤ f ₁ ≤ -1.083f (1)-(23) ≤ f ₁ ≤ -1.083f ( 2) 0.557f ≦ f ₂ ≦ 0.821f (4) -0.579f ≤ γ ≤ -0.321f (5) 0.108f ≤ d ≤ 0.432f (6) 0.229f ≤ | R ・ cosψ | ≤ 0.813f where f: focal length of cross section of main system in main scanning direction f ₁ : focal length of the first lens group f ₂ : focal length of the second lens group d: length on the optical axis from the first surface of the first lens group to the last surface of the second lens group γ: second Radius of curvature of the final surface (surface to be scanned) of the lens group R: Radius of curvature of the cross section of the concave optical reflection element in the direction perpendicular to the main scanning direction ψ: Optical axes of the first lens group and the second lens group, and the optical axis The angle formed by the normal line on the intersection of the concave optical reflection elements.