JPH09269451A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner suitable for compact and high definitive printing by properly setting a lens shape of one sheet of an fθ lens. SOLUTION: The light outgoing from a light source member 1 is converted into the converged light by a first optical element 2, and the converted light is image formed on a deflection surface of a deflection element 5 in a linear shape longitudinal in the main scan direction through a second optical element 4, and the light deflected by the deflection element 5 is image-formed on a surface to be scanned in a spot shape through a third optical element (fθlens) 6. Then, when the surface 7 to be scanned is scanned, the third optical element 6 consists of a single lens, and the single lens forms a meniscus shape projective to the deflection element 5 side in the main scan surface, and the first lens surface of the deflection element 5 side and the second lens surface of the surface 7 to be scanned are formed in an aspherical shape also, and the aspherical shape of the second lens surface forms the shape crossing in the peripheral part for the tangent of the radius of paraxial curvature and approaching to the deflection element 5 side in the outermost part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical device, and in particular, it has a fθ characteristic after a light beam optically modulated and emitted from a light source means is deflected and reflected by an optical deflector (deflecting element) composed of a rotating polygon mirror or the like. Suitable for a device such as a laser beam printer (LBP) having an electrophotographic process or a digital copying machine, which records an image information by optically scanning a surface to be scanned through an imaging optical system (fθ lens). Scanning optical device.

[0002]

2. Description of the Related Art Conventionally, in a scanning optical device used in a laser beam printer, a digital copying machine or the like, a light beam which is optically modulated and emitted from a light source means in accordance with an image signal is composed of, for example, a rotating polygon mirror. The light is deflected periodically by an optical deflector, and is focused into a spot on a surface of a photosensitive recording medium (photosensitive drum) by an image forming optical system having an fθ characteristic, and the surface is optically scanned to record an image. ing.

FIG. 7 is a schematic view of a main part of a conventional scanning optical device of this type.

In the figure, a divergent light beam emitted from a light source means 71 is made into a substantially parallel light beam by a collimator lens 72, and the light beam (amount of light) is limited by a diaphragm 73 to have a cylindrical power having a predetermined refracting power only in the sub-scanning direction. It is incident on the lens 74. Within the main scanning cross section, of the parallel light fluxes that have entered the cylindrical lens 74, they are emitted as they are in the state of parallel light fluxes. Further, in the sub-scanning cross section, they are converged and imaged as a substantially linear image on the deflecting surface (reflecting surface) 75a of the optical deflector 75 composed of a rotating polygon mirror (polygon mirror).

Then, the light beam deflected and reflected by the deflecting surface 75a of the optical deflector 75 is passed through an image forming optical system (f.theta. Lens) 76 having f.theta.
The light is guided to the surface and the optical deflector 75 is rotated in the direction of arrow A to optically scan the surface of the photosensitive drum 77 in the direction of arrow B. As a result, an image is recorded on the photosensitive drum 77 which is a recording medium.

In recent years, with the downsizing and cost reduction of laser beam printers, digital copying machines and the like, the same is required for scanning optical devices. As one of the solutions, a scanning optical device in which an image forming optical system having the fθ characteristic is composed of one lens is disclosed in, for example, Japanese Patent Publication No.
Japanese Patent No. 486884, Japanese Patent Application Laid-Open No. 63-157122, Japanese Patent Application Laid-Open No. 4-104213, and Japanese Patent Application Laid-Open No. 4-50908.
Various proposals have been made in Japanese publications.

Of these publications, Japanese Examined Patent Publication No. 61-4868
In Japanese Patent Laid-Open No. 4 and Japanese Patent Laid-Open No. 63-157122, a single lens having a concave surface is used as the fθ lens on the optical deflector side to focus the parallel light flux from the collimator lens on the recording medium surface.

In Japanese Unexamined Patent Publication No. 4-104213, a single lens having a concave surface on the optical deflector side and a toroidal surface on the image surface side is used as an fθ lens, and a light beam converted into convergent light by a collimator lens is incident on the fθ lens. I am letting you.

In Japanese Laid-Open Patent Publication No. 4-50908, a single lens having a high-order aspheric surface introduced as a lens surface is used as an fθ lens, and a light beam converted into a convergent light by a collimator lens is incident on the fθ lens.

[0010]

In order to record image information with high accuracy in this type of scanning optical apparatus, the field curvature should be well corrected over the entire surface to be scanned and the spot diameter should be uniform. It is necessary to have a distortion aberration (fθ characteristic) in which the angle of light and the image height are in a proportional relationship.

However, in the conventional scanning optical device described above, Japanese Patent Publication No. 61-48684 discloses that the field curvature in the sub-scanning direction remains and the parallel light flux is imaged on the surface to be scanned. Since the distance from the fθ lens to the surface to be scanned is equal to the focal length, there is a problem that it is difficult to construct a compact scanning optical device.

In JP-A-63-157122, fθ
Since the lens is thick, it is difficult to manufacture it by molding, which causes a problem of cost increase.

In Japanese Unexamined Patent Publication No. 4-104213, there is a problem that distortion aberration remains and jitter of the polygon surface period occurs due to an attachment error of a polygon mirror which is an optical deflector.

In Japanese Patent Application Laid-Open No. 4-50908, although a high-order aspherical fθ lens is used to correct aberrations satisfactorily, the unevenness of the magnification in the sub-scanning direction between the optical deflector and the surface to be scanned causes unevenness. There is a tendency that the spot diameter in the sub-scanning direction changes depending on the image height.

According to the present invention, when the converged light from the collimator lens is imaged on the surface to be scanned by one fθ lens through the optical deflector, the lens shape of the fθ lens is appropriately configured. This is a compact scanning optical device that corrects field curvature and distortion, prevents jitter due to optical polarizer mounting errors, and changes in spot diameter in the sub-scanning direction due to image height, and is suitable for high-definition printing. For the purpose of provision.

[0016]

In the scanning optical device of the present invention, (1) the light flux emitted from the light source means is converted into a convergent light flux by the first optical element, and the converted light flux is converted into the second optical element. An image is formed in a linear shape that is long in the main scanning direction on the deflecting surface of the deflecting element, and the light beam deflected by the deflecting element is imaged in a spot shape on the surface to be scanned through the third optical element. In the scanning optical device for scanning the surface to be scanned, the third optical element is composed of a single lens, and the single lens has a convex meniscus shape toward the deflecting element in the main scanning surface, and Both the first lens surface on the deflecting element side and the second lens surface on the scanned surface side are formed in an aspherical shape, and the aspherical shape of the second lens surface is a peripheral portion with respect to a tangent line of a paraxial radius of curvature. It is characterized by intersecting and forming a shape close to the deflection element side at the outermost part. .

In particular (1-1), the single lens constituting the third optical element has a paraxial radius of curvature of the first and second lens surfaces at the maximum effective diameter of the single lens in the main scanning plane. When the distances from the tangent line to the first and second lens surfaces in the optical axis direction are d1 and d2, respectively, and the central thickness of the single lens is S, 0.46 ≦ | d1 / S | ≦ 0.540 12 ≦ | d2 / S | ≦ 0.24, and (1-2) the aspherical shape of the second lens surface in the main scanning plane of the single lens that constitutes the third optical element. Indicates that there is a local point having a tangent line parallel to the tangent line of the paraxial radius of curvature, where X is the distance from the optical axis and Y is the maximum effective diameter of the single lens. Satisfying the condition of 48 ≦ X / Y ≦ 0.56, and (1-3) both lenses of a single lens constituting the third optical element Among them, the curvature in the sub-scanning surface of at least one lens surface is continuously changing in the effective portion of the single lens, and (1-4) the single lens forming the third optical element is It is characterized by being manufactured by plastic molding.

[0018]

1 is a sectional view (main scanning sectional view) of a main portion in a main scanning direction according to a first embodiment of the present invention.

In the figure, reference numeral 1 denotes a light source means, which is composed of, for example, a semiconductor laser. A collimator lens 2 as a first optical element converts a divergent light beam (light beam) emitted from the light source means 1 into a convergent light beam.
Reference numeral 3 denotes an aperture stop for adjusting the diameter of a passing light beam. Reference numeral 4 denotes a cylindrical lens as a second optical element, which has a predetermined refractive power only in the sub-scanning direction, and has an aperture stop 3
The light flux that has passed through is imaged as a substantially linear image on the deflection surface 5a of the optical deflector 5 described later in the sub-scanning cross section. Reference numeral 5 denotes an optical deflector as a deflecting element, which is composed of, for example, a polygon mirror (rotary polygon mirror), and a driving means such as a motor (not shown).
Thus, it is rotating at a constant speed in the direction of arrow A in the figure.

Reference numeral 6 denotes an fθ lens (imaging optical system) as a third optical element, which is composed of one lens having the fθ characteristic. As will be described later, the fθ lens 6 forms a convex meniscus shape on the optical deflector 5 side in the main scanning plane, and the first lens surface Ra on the optical deflector 5 side and the photosensitive drum 7 surface as the surface to be scanned. And the second lens surface Rb on the side are both composed of an aspherical toric surface, and the aspherical shape of the second lens surface Rb intersects the tangent of the paraxial radius of curvature at the peripheral portion, and at the outermost part. It has a shape close to the light deflector 5 side, forms a light beam based on the image information deflected and reflected by the light deflector 5 on the surface of the photosensitive drum 7, and deflects the light deflector 5. The face tangle is corrected.

Further, the fθ lens 6 in this embodiment is
The curvature in the sub-scanning direction of at least one of the lens surfaces Ra and Rb of the fθ lens is defined by the fθ lens 6
It is continuously changed in the effective part of and is manufactured by plastic molding.

In this embodiment, the light beam emitted from the semiconductor laser 1 is converted into a convergent light beam by the collimator lens 2, and the light beam (amount of light) is limited by the aperture stop 3 and is incident on the cylindrical lens 4. The light beam incident on the cylindrical lens 4 is emitted as it is in the main scanning section. In the sub-scan section, the light is converged and formed as an almost linear image (a linear image elongated in the main scanning direction) on the deflection surface 5a of the optical deflector 5. Then, the light beam deflected and reflected by the deflection surface 5a of the optical deflector 5 is guided to the surface of the photosensitive drum 7 through the fθ lens 6 having different refracting powers in the main scanning direction and the sub scanning direction, and the optical deflection is performed. Arrow A
By rotating it in the direction, the surface of the photosensitive drum 7 is scanned in the direction of arrow B. As a result, an image is recorded on the photosensitive drum 7, which is a recording medium.

Next, means for correcting distortion (f.theta. Characteristic) and field curvature in this embodiment will be described. In order to satisfy the distortion and field curvature of this device, fθ
In the main scanning plane of the lens 6, the optical axis from the tangent line of paraxial radius of curvature of the first and second lens surfaces at the maximum effective diameter of the fθ lens 6 to the first and second lens surfaces Ra and Rb. When the distances in the directions are d1 and d2 and the center thickness of the fθ lens 6 is S, 0.46 ≦ | d1 / S | ≦ 0.54 (1) 0.12 ≦ | d2 / S | ≦ The lens shape is set so as to satisfy the condition of 0.24 (2). It should be noted that d1 and d2 are positive when the traveling directions of the light rays are measured.

The above conditional expressions (1) and (2) are both the distance in the optical axis direction from the tangent line of the paraxial radius of curvature at the maximum effective diameter of the fθ lens 6 to the first and second lens surfaces Ra and Rb. , The ratio with respect to the central thickness of the fθ lens 6, and it is difficult to satisfactorily correct field curvature, distortion and the like in the main scanning direction if the conditional expression (1) is deviated. . If the conditional expression (2) is not satisfied, it becomes difficult to satisfactorily correct the field curvature in the main scanning direction, the distortion, and the uniformity of the spot diameter in the sub scanning direction, which is not preferable.

Further, the aspherical shape of the second lens surface Rb in the main scanning plane of the fθ lens 6 has a local point having a tangent line parallel to the tangent line of the paraxial radius of curvature, and the local point is the optical point. When the distance from the axis is X and the maximum effective diameter of the fθ lens 6 is Y, the lens shape is set so that the condition of 0.48 ≦ X / Y ≦ 0.56 (3) is satisfied. There is.

The above conditional expression (3) is the second condition of the fθ lens 6.
It relates to the ratio of the distance from the optical axis to the local point on the lens surface Rb to the maximum effective diameter of the fθ lens 6,
If conditional expression (3) is not satisfied, it becomes difficult to satisfactorily correct field curvature, distortion, and the like in the main scanning direction, which is not preferable.

In this embodiment, the lens shape of the fθ lens 6 is appropriately set so as to satisfy at least one conditional expression among the conditional expressions (1), (2) and (3). The uniformity of the spot diameter in the sub-scanning direction is improved while maintaining good field curvature and distortion.

In this embodiment, the lens shape of the fθ lens 6 is an aspherical shape which can be expressed by a function up to the 10th order in the main scanning direction, and the subscanning direction is composed of a spherical surface which continuously changes in the image height direction. . The lens shape is, for example, the origin at the intersection of the fθ lens 6 and the optical axis, the optical axis direction is the X axis, the axis orthogonal to the optical axis in the main scanning plane is the Y axis, and the optical axis is orthogonal to the optical axis in the sub scanning plane. When the axis to be set is the Z axis, the generatrix direction corresponding to the main scanning direction is X = (Y / R) / (1+ (1- (1 + K) (Y / R)
² ) ^1/2 + B ₄ Y ⁴ + B ₆ Y ⁶ + B ₈ Y ⁸ + B ₁₀ Y ¹⁰ (where R is the radius of curvature and K, B ₄ , B ₆ , B ₈ , B ₁₀ are aspherical coefficients) The sagittal direction corresponding to the sub-scanning direction (direction orthogonal to the main scanning direction including the optical axis) is S = (Z ² / r ′) / (1+ (1- (Z / r ') ² )
^1/2 (where r '= r (1 + D ₂ Y ² + D ₄ Y ⁴ + D ₆ Y ⁶
+ D ₈ Y ⁸ + D ₁₀ Y ¹⁰ )).

Table 1 shows the fθ lens 6 in this embodiment.
The aspherical coefficient of is shown. FIG. 2 is an explanatory view showing the aspherical shape of the fθ lens 6, where X1 is the first lens surface and X2 is the second lens surface.
It shows the shape of the lens surface.

[0030]

[Table 1] In the fθ lens 6 in the present embodiment, the distances in the optical axis direction from the tangent line of the paraxial curvature radius at the maximum effective diameter of the fθ lens 6 to the first and second lens surfaces Ra and Rb in the main scanning plane are d1 respectively. , D2, S is the center thickness of the fθ lens 6, S is the distance from the optical axis on the second lens surface Rb to the local point, and Y is the maximum effective diameter of the fθ lens 6.
The respective values are | d1 | = 5.15 mm, | d2 | = 1.97 mm S = 10 mm X = 22 mm, Y = 42 mm. From these values, | d1 / S | = 0.515 | d2 / S | = 0.197 X / Y = 0.524, and these values are the above-mentioned conditional expressions (1) to (3).
Are satisfied.

FIG. 3 is a diagram showing various aberrations showing the curvature of field and distortion (f.theta. Characteristic) in this embodiment. It can be seen from the figure that each aberration has been corrected to a level at which there is no practical problem.

FIG. 4 is a sectional view (main scanning sectional view) of a main part in the main scanning direction according to the second embodiment of the present invention. In FIG.
The same elements as those shown in are given the same reference numerals.

The present embodiment differs from the first embodiment described above in that the incident angle of the light beam incident on the deflection surface of the optical deflector is different, and accordingly, the lens shape of the fθ lens is different. That is. Other configurations and optical actions are substantially the same as those in the first embodiment, and the same effect is obtained.

That is, in the figure, 16 is an fθ lens, and the lens shape of the first lens surface Ra of the fθ lens 16 is, for example, the intersection of the fθ lens 16 and the optical axis as the origin, and the optical axis direction is X. Axis, the axis orthogonal to the optical axis in the main scanning plane is the Y axis, and the axis orthogonal to the optical axis in the sub scanning plane is the Z axis.
When the axis is used, the bus line direction corresponding to the main scanning direction is X = (Y ² / Ru) / (1+ (1- (1 + Kyu) (Y
/ Ru) ² ) ^1/2 ) + B ₂ uY ² + B ₄ uY ⁴ + B ₆ u
Y ⁶ + B ₈ uY ⁸ + B ₁₀ uY ¹⁰ (where Y ≧ 0, R is the radius of curvature, Kyu, B ₂ u, B _{4
u, B 6 u, B 8} u, B 10 u aspherical ^{coefficients) X = (Y 2 / Rl} ) / (1+ (1- (1 + Kyl) (Y
/ Rl) ² ) ^1/2 ) + B ₂ lY ² + B ₄ lY ⁴ + B ₆ l
Y ⁶ + B ₈ lY ⁸ + B ₁₀ lY ¹⁰ (where Y ≧ 0, R is the radius of curvature, Kyl, B ₂ l, B ₄
l, B ₆ l, B ₈ l, and B ₁₀ l can be represented by an expression of aspherical surface coefficient, and a child corresponding to the sub-scanning direction (direction orthogonal to the main scanning direction including the optical axis). The line direction is S = (Z ² / ru ') / (1+ (1- (Z + ru')
² ) ^1/2 (However, Y ≧ 0, 1 / ru ′ = 1 / ru + D ₂ uY ² +
D ₄ uY ⁴ + D ₆ uY ⁶ + D ₈ uY ⁸ + D ₁₀ uY ¹⁰ ) S = (Z ² / rl ′) / (1+ (1- (Z + rl ′)
² ) ^1/2 (However, Y <0, 1 / rl '= 1 / rl + D ₂ lY ² +
D ₄ lY ⁴ + D ₆ lY ⁶ + D ₈ lY ⁸ + D ₁₀ lY ¹⁰ ).

Table 2 shows the fθ lens 1 according to this embodiment.
The aspherical coefficient of 6 is shown. FIG. 5 is an explanatory view showing the aspherical shape of the fθ lens 16, where X1 represents the shape of the first lens surface and X2 represents the shape of the second lens surface.

[0036]

[Table 2] In the main scanning direction, the fθ lens 16 in the present embodiment has a distance d1 in the optical axis direction from the tangent of the paraxial radius of curvature at the maximum effective diameter of the fθ lens 16 to the first and second lens surfaces Ra and Rb, respectively. d2, the central thickness of the fθ lens 16 is S, the distance from the optical axis on the second lens surface Rb to the local point is X, and the maximum effective diameter of the fθ lens 16 is Y, the respective values are | d1 | = 5.77 mm, | d2 | = 1.84 mm S = 12 mm X = 24 mm, Y = 46 mm, and from these values, | d1 / S | = 0.482 | d2 / S | = 0.153 X / Y = 0.522, and these values correspond to the conditional expressions (1) to (3) described above.
Are satisfied.

FIG. 6 is a diagram of various aberrations showing the curvature of field and distortion (f.theta. Characteristic) in this embodiment. It can be seen from the figure that each aberration has been corrected to a level at which there is no practical problem.

[0038]

As described above, according to the present invention, when the converged light from the collimator lens is imaged on the surface to be scanned by one fθ lens through the optical deflector, the shape of the fθ lens is appropriate. By setting to 1, the field curvature, the distortion aberration (fθ characteristic), etc. are satisfactorily corrected and the influence of the jitter due to the optical deflector mounting error and the change of the spot diameter in the sub-scanning direction due to the image height are suppressed to a small level. This makes it possible to achieve a compact scanning optical device suitable for high-definition printing.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part in a main scanning direction according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an aspherical shape of the fθ lens according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram showing field curvature and distortion in the first embodiment of the present invention.

FIG. 4 is a sectional view of a main part in a main scanning direction according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an aspherical shape of the fθ lens according to the second embodiment of the present invention.

FIG. 6 is an aberration diagram showing field curvature and distortion in the second embodiment of the present invention.

FIG. 7 is a sectional view of a main part of a conventional scanning optical device in a main scanning direction.

[Explanation of symbols]

 Reference Signs List 1 light source means 2 first optical element (collimator lens) 3 aperture stop 4 second optical element (cylindrical lens) 5 deflection element (optical deflector) 6,16 third optical element (fθ lens) 7 scanned surface (Photosensitive drum surface)

Claims (5)

[Claims] 1. A light flux emitted from a light source means is converted into a convergent light flux by a first optical element, and the converted light flux is converted into a second light flux.
Image on the deflecting surface of the deflecting element in the form of a line elongated in the main scanning direction via the optical element, and the light beam deflected by the deflecting element is formed into a spot on the surface to be scanned via the third optical element. In the scanning optical device for forming an image and scanning the surface to be scanned, the third optical element comprises a single lens, and the single lens has a meniscus shape convex toward the deflecting element in the main scanning surface. Further, both the first lens surface on the deflecting element side and the second lens surface on the scanned surface side are formed in an aspherical shape, and the aspherical shape of the second lens surface is with respect to a tangent line of a paraxial radius of curvature. A scanning optical device, characterized in that the scanning optical device intersects at a peripheral portion and has a shape closest to the deflecting element side at the outermost portion. 2. The single lens constituting the third optical element is arranged from the tangent line of paraxial curvature radii of the first and second lens surfaces at the maximum effective diameter of the single lens in the main scanning plane to the first lens. The distance in the optical axis direction to the first and second lens surfaces is d
1, d2, where S is the center thickness of the single lens, 0.46 ≦ | d1 / S | ≦ 0.54 0.12 ≦ | d2 / S | ≦ 0.24 is satisfied. The scanning optical device according to claim 1. 3. The aspherical shape of the second lens surface in the main scanning plane of the single lens constituting the third optical element has a local point having a tangent line parallel to the tangent line of the paraxial radius of curvature. The local point is present, and when the distance from the optical axis is X and the maximum effective diameter of the single lens is Y, 0.48 ≦ X / Y ≦ 0.56 is satisfied. Item 2. The scanning optical device according to Item 1. 4. The curvature in the sub-scanning surface of at least one of the lens surfaces of the single lens forming the third optical element continuously changes in the effective portion of the single lens. The scanning optical device according to claim 1, wherein: 5. The scanning optical device according to claim 1, wherein the single lens forming the third optical element is manufactured by plastic molding.