JP2005140833A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner capable of reducing the diameter of a beam spot while an easily machinable, thin and compact scanning lens is used. <P>SOLUTION: The optical scanner has: a light source 1; a coupling optical system 2 which couples a luminous flux emitted from the light source; a line picture optical system 4 which makes the luminous flux emitted from the coupling optical system into a line picture long in a main scanning direction; a deflection means 5 which deflects and scans with the luminous flux emitted from the line picture optical system; and a scanning optical system which guides the luminous flux emitted from the deflection means onto a plane to be scanned. The scanning optical system is composed of a plurality of scanning lenses 7 and 8 under the conditions that "at least a scanning lens is composed of an aspherical face of symmetry of revolution and a special toric face in which the subscanning radius of curvature varies from the optical axis of the lens face toward the periphery in the main scanning direction", and "the face among all faces of scanning lens which has a region of the largest tilt angle within an effective region for the face perpendicular to the optical axis is an aspherical face of symmetry of revolution". <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical scanning apparatus and an electrophotographic image forming apparatus such as a laser printer, a digital copying machine, a facsimile, and a laser plotter using the optical scanning apparatus.

In recent years, in an electrophotographic image forming apparatus such as a laser printer, the density of formed images has been accelerated, and in order to increase the density of formed images, image formation of an image carrier such as a photoconductor is performed. It is necessary to reduce the diameter of the beam spot that performs optical scanning on the surface, and an optical scanning device is required to realize this.
Therefore, in order to reduce the beam spot diameter, a special toric surface in which the radius of curvature in the sub-scanning section changes asymmetrically from the optical axis of the lens surface toward the periphery in the main scanning direction is used. A two-lens scanning lens composed of a toric surface has been proposed (see, for example, Patent Document 1).

  In addition, in order to correct the wavefront aberration satisfactorily and realize a stable and good light spot, at least one surface of the lens included in the scanning imaging optical system has an arc or non-arc shape in the main scanning plane, A sub-arc surface whose shape in the sub-scanning surface is a non-arc shape, and this sub-non-arc surface is a lens of the principal ray of the deflected light beam incident on each surface of the lens of the scanning imaging optical system. There has been proposed an optical scanning device formed so that the incident angle with respect to the normal of the surface is 25 degrees or less in the entire lens effective region (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 2001-324689 JP 2001-21824 A

However, the scanning lens using the conventional special toric surface as described in Patent Document 1 is a scanning lens composed of an anamorphic surface on both sides,
・ The beam spot diameter increases due to eccentricity.
-If the special toric surface is used for a surface with a large inclination angle, the processing accuracy deteriorates, and a beam spot diameter increases due to a shape error such as waviness, and image defects such as vertical stripes occur.
There was an unresolved issue.

Moreover, the scanning lens using the conventional special toric surface as described in Patent Document 1 and Patent Document 2 has a thick lens, and even if this is a plastic molded product, the molding time becomes longer. There was a problem that the cost of parts was high. Also, no consideration is given to shading or ghost light.
Furthermore, the scanning lenses described in Patent Document 1 and Patent Document 2 both employ a surface whose sub-scanning curvature radius is asymmetric in the main scanning direction, but does not use a rotationally symmetric aspherical surface.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical scanning device that is excellent in processability and can realize a reduction in beam spot diameter.
Another object of the present invention is to provide an optical scanning device capable of realizing a reduction in beam spot diameter while using a thin and small scanning lens.
Still another object of the present invention is to provide an image forming apparatus capable of forming an image with fine granularity and high density and excellent gradation.

The present invention adopts the following configurations as means for achieving the above object.
A first configuration of the present invention is an optical scanning device, and includes a light source, a coupling optical system for coupling a light beam from the light source, and a line image in which the light beam from the coupling optical system is long in the main scanning direction. A linear image optical system, a deflecting unit that deflects and scans the light beam from the linear image optical system, and a scanning optical system that guides the light beam from the deflecting unit to a surface to be scanned. Consisting of multiple scanning lenses,
At least one scanning lens is composed of a rotationally symmetric aspherical surface and a special toric surface whose sub-scanning curvature radius changes from the optical axis of the lens surface toward the periphery in the main scanning direction.
A surface having a region having the largest inclination angle within an effective range with respect to a surface perpendicular to the optical axis among all scanning lens surfaces is a rotationally symmetric aspheric surface.
The above condition is satisfied (claim 1).

According to a second configuration of the present invention, in the optical scanning device according to the first configuration, the surfaces of all the scanning lenses of the scanning optical system are configured by rotationally symmetric aspheric surfaces or the special toric surfaces. (Claim 2).
Further, according to a third configuration of the present invention, in the optical scanning device having the first or second configuration, the surface having the largest effective width in the main scanning direction among the surfaces of all the scanning lenses of the scanning optical system is: It is the special toric surface (claim 3).

According to a fourth configuration of the present invention, in the optical scanning device having any one of the first to third configurations, all the scanning lenses of the scanning optical system are lenses within an effective range with respect to a plane perpendicular to the optical axis. The tilt angle of the surface is 30 degrees (deg) or less, and all the scanning lenses have a special toric surface whose sub-scanning curvature radius changes according to the lens height in the main scanning direction, and the special toric surface is a main scanning direction lens. The amount of change in the sub-scanning curvature per 1 mm height is smaller than 1.5 × 10 ⁻⁴ (1 / mm) (Claim 4).

According to a fifth configuration of the present invention, in the optical scanning device having any one of the first to fourth configurations, the scanning optical system includes two scanning lenses,
The outermost luminous flux of the effective writing width is inclined in the same direction in the deflection rotation plane with respect to the normal of the first surface of the two scanning lenses;
-The scanning lens closest to the deflecting means has a concave meniscus shape on the deflecting means side in the main scanning direction.
The first surface of the scanning lens closest to the deflecting means has a special toric surface that has negative power in the sub-scanning direction, and the sub-scanning curvature changes from the optical axis of the lens surface toward the periphery in the main scanning direction.
The first surface of the scanning lens farthest from the deflection means has a positive power in the sub-scanning direction;
The above condition is satisfied (claim 5).

  According to a sixth configuration of the present invention, in an image forming apparatus that forms an image on a transfer material by performing each process of charging, exposure, development, and transfer, first to fifth as means for executing the exposure process. An optical scanning device having any one of the constitutions is provided (claim 6).

  In the optical scanning device of the first configuration, a scanning lens having excellent workability is used as a combination of a special toric surface and a rotationally symmetric aspherical surface for at least one scanning lens of the scanning optical system. It is possible to reduce the beam spot diameter. It is also effective for multi-beams, and the number of rotations of deflecting means such as a polygon scanner can be reduced, realizing low power consumption, low noise, and high durability.

In the optical scanning device of the second configuration, in addition to the effect of the first configuration, all surfaces are configured by a special toric surface and a rotationally symmetric aspherical surface, so that the workability is excellent and the tolerance for eccentricity is large. An optical scanning device with excellent characteristics can be provided.
In the optical scanning device of the third configuration, in addition to the effects of the first or second configuration, the surface having the longest effective width in the main scanning direction is the second surface of the scanning lens, and the special toric surface is used here. By using this, it is possible to finely correct field curvature in the sub-scanning direction.

In the optical scanning device of the fourth configuration, in addition to the effects of any of the first to third configurations, the wavefront aberration can be improved, a stable beam spot diameter can be obtained, and the beam spot due to eccentricity can be obtained. Diameter degradation can be reduced.
Further, in the optical scanning device having the fifth structure, in addition to the effect of any of the first to fourth structures,
-Wavefront aberration is good and the beam spot diameter is stable.
・ Sub-scanning beam pitch deviation is small,
・ Ghost light can be removed.
An optical scanning device having the characteristics can be provided.

  The image forming apparatus having the sixth configuration includes any one of the first to fifth configurations by including the optical scanning device having any one of the first to fifth configurations as means for executing the exposure process. It is possible to provide an image forming apparatus that is effective and has excellent granularity and gradation.

The best mode and embodiments of an optical scanning device and an image forming apparatus according to the present invention will be described below with reference to the drawings.
FIG. 1 shows an embodiment of an optical scanning device according to the present invention. In FIG. 1, reference numeral 1 denotes a light source. The light source 1 is composed of, for example, a semiconductor laser (LD), but in addition, a multi-beam light source (for example, a plurality of LDs or a plurality of light sources arranged at equal intervals). LD array etc.). The light beam emitted from the light source 1 is a divergent light beam and is coupled to the subsequent optical system by the coupling lens 2 constituting the coupling optical system. The light beam coupled by the coupling lens 2 may be a parallel light beam, a weak divergent light beam, or a weak convergent light beam. This light beam is shaped by the aperture 3, converged only in the sub-scanning direction by the action of the cylindrical lens 4, and is a long line in the main scanning direction in the vicinity of the deflection reflection surface of the polygon mirror 5 which is a deflecting means via a folding mirror or the like. It is formed as an image.

  The polygon mirror 5 is driven to rotate at a high speed at a constant speed by a polygon motor (not shown), and deflects and scans the direction of the light beam at a constant angular speed by each deflecting reflection surface. A scanning optical system is disposed on the path of the light beam deflected at a constant angular velocity. In this embodiment, the scanning optical system is constituted by two scanning lenses 7 and 8. The scanning lenses 7 and 8 have a function of guiding the light beam from the polygon mirror 5 to the scanned surface 10 which is the surface of the photosensitive member, for example, and forming an image on the scanned surface 10 as a beam spot. The scanning lenses 7 and 8 also have an fθ function, as is well known, in order to cause the light beam deflected at a constant angular velocity by the polygon mirror 5 to scan at a constant speed on the linear scanned surface 10. .

  Here, the “main scanning direction” is a direction corresponding to a plane formed by the light beam deflected and scanned by the polygon mirror 5, and the “sub-scanning direction” is a direction orthogonal to the main scanning direction. That is. In FIG. 1, reference numeral 6 denotes a soundproof glass, and reference numeral 9 denotes a dustproof glass. The polygon mirror 5 and the polygon motor are covered with a soundproof cover so that noise generated by high-speed rotation does not leak to the outside, and light flux enters and exits through the soundproof glass 6 provided on the cover. . Further, the optical scanning device is incorporated as one unit in one housing provided in the image forming apparatus, sealed so that dust does not enter the housing, and the deflected light beam is emitted to the outside through the dustproof glass 9. It is configured.

  The optical scanning device according to the present invention is characterized by the configuration of the scanning optical system. Then, next, the structure of the said scanning optical system is demonstrated concretely. The surface of the lens constituting the scanning optical system on the side close to the polygon mirror 5, that is, the deflecting unit is referred to as a first surface, and the surface far from the deflecting unit is referred to as a second surface.

(First configuration)
In the optical scanning device according to the present invention, the scanning optical system includes a plurality of scanning lenses 7 and 8,
At least one scanning lens is composed of a rotationally symmetric aspherical surface and a special toric surface whose sub-scanning curvature radius changes from the optical axis of the lens surface toward the periphery in the main scanning direction.
A surface having a region having the largest inclination angle within an effective range with respect to a surface perpendicular to the optical axis among all scanning lens surfaces is a rotationally symmetric aspheric surface.
It satisfies the condition.
More specifically, the first scanning lens 7 closest to the deflecting / reflecting surface of the scanning optical system has a special toric in which the radius of the sub-scanning curvature changes as the first surface moves from the optical axis of the lens surface to the periphery in the main scanning direction. The second surface is a rotationally symmetric aspherical surface. Here, the rotationally symmetric aspherical surface is an aspherical surface that is rotationally symmetric with respect to the optical axis of the lens.

  By using the special toric surface as the first surface of the first scanning lens 7 in this way, the design wavefront aberration can be reduced, and the difference in the F-number in the sub-scanning direction on the scanned surface due to the image height. , The deviation of the sub-scanning beam spot diameter due to the image height can be reduced, and the beam pitch deviation between the image heights when the beam is made into a multi-beam can be reduced. Of all the scanning lenses, the surface having the largest maximum tilt angle with respect to the optical axis is the second surface of the first scanning lens 7, and a rotationally symmetric aspherical surface is used for this surface. It is possible to reduce the beam spot diameter increase due to the eccentricity and to stabilize the beam spot diameter. In addition, the scanning lens is processed by cutting or molding using plastic or the like, but the mold at the time of molding is also processed by cutting. At that time, if it is a rotationally symmetric aspherical surface, both the processing time and processing accuracy are improved, and as a result, quality is improved.

(Second configuration)
In the optical scanning device according to the present invention, in addition to the above-described configuration, the surfaces of all the scanning lenses 7 and 8 of the scanning optical system are composed of rotationally symmetric aspherical surfaces or special toric surfaces.
The rotationally symmetric aspherical surface described above is a surface having a large tolerance for decentration and excellent workability, and the special toric surface is a surface excellent in correcting aberrations in design. There are other cylindrical lens surfaces, ordinary toric surfaces, etc., but none of them has a large tolerance for decentration, and the design aberration correction capability is not as great as that of special toric surfaces. Accordingly, by forming the surfaces of all the scanning lenses 7 and 8 of the scanning optical system with a special toric surface and a rotationally symmetric aspherical surface, the optical scanning device is excellent in workability, has a high tolerance for decentration, and has excellent optical characteristics. Can be provided.
Here, the second scanning lens 8 has the special toric surface on both surfaces, thereby reducing the design wavefront aberration and reducing the difference in the F-number image height in the sub-scanning direction on the scanned surface. Reduced.

(Third configuration)
In the optical scanning device according to the present invention, in addition to the above configuration, the surface having the largest effective width in the main scanning direction among all the scanning lenses 7 and 8 of the scanning optical system is the special toric surface. .
Here, the surface having the longest effective width in the main scanning direction among the surfaces of the scanning lenses 7 and 8 is the second surface of the second scanning lens 8, and the special toric surface is used here, so that the sub-scanning direction is used. Can be corrected finely.

(Fourth configuration)
In the optical scanning device according to the present invention, in addition to the above configuration, all the scanning lenses 7 and 8 of the scanning optical system have an inclination angle of the lens surface within an effective range with respect to the surface perpendicular to the optical axis (30 degrees). deg), and all the scanning lenses 7 and 8 have a special toric surface whose sub-scanning curvature radius changes according to the lens height in the main scanning direction, and the special toric surface per 1 mm in the main scanning direction lens height. The amount of change in the sub-scanning curvature is smaller than 1.5 × 10 ⁻⁴ (1 / mm).
Here, the processing method of the scanning lens is roughly divided into cutting and molding, and the mold is also manufactured by cutting. The inclination angle of the lens surface within the effective range is greatly related to the accuracy of the cutting process, and in order to reduce the waviness, the inclination angles of all the scanning lenses 7 and 8 need to be 30 degrees or less. In the present invention, a special surface whose sub-scanning curvature changes according to the lens height in the main scanning direction is used. However, it is necessary to make the change in the lens height in the main scanning direction as small as possible. Therefore, in order to improve the wavefront aberration and obtain a stable beam spot diameter, the amount of change in the sub-scanning curvature per 1 mm in the main scanning direction lens height is 1.5 × 10 ^{− on the} special toric surface. ^It is necessary to make it smaller than ⁴ (1 / mm). Further, when the change in the sub-scanning curvature per 1 mm of the lens height in the main scanning direction is large, the beam spot diameter is greatly deteriorated due to the arrangement error of the lens. The present invention is also effective in this respect.

(Fifth configuration)
In the optical scanning device according to the present invention, in addition to the above configuration, the scanning optical system includes two scanning lenses 7 and 8,
The outermost luminous flux of the effective writing width is inclined in the same direction within the deflection rotation plane with respect to the normal line of the first surface of the two scanning lenses 7 and 8;
The scanning lens 7 closest to the polygon mirror 5 that is a deflection unit has a meniscus shape that is concave on the deflection unit side in the main scanning direction.
The first surface of the scanning lens 7 closest to the polygon mirror 5 has a negative power in the sub-scanning direction, and from a special toric surface whose sub-scanning curvature changes from the optical axis of the lens surface toward the periphery in the main scanning direction. Become,
The first surface of the scanning lens 8 farthest from the polygon mirror 5 has a positive power in the sub-scanning direction;
It satisfies the condition.

  Here, the first scanning lens 7 has a concave meniscus shape toward the polygon mirror 5 in the main scanning direction, whereby the first scanning lens 7 can be thinned, and the first scanning lens. 7 can reduce the normal angle between the luminous flux incident on the surface 7 and the lens surface, thereby correcting the wavefront aberration. Also, by making the power in the sub-scanning direction of the first surface of the first scanning lens 7 negative, the absolute value of the lateral magnification in the sub-scanning direction of the scanning optical system can be reduced, and the optical element placement error, The tolerance of component errors can be expanded. At this time, the sub-scanning radius of curvature decreases from the optical axis toward the periphery in the main scanning direction, and a special toric surface that increases from the extreme value as a boundary is used. The difference in direction F-number can be reduced, the sub-scanning beam spot diameter deviation due to the image height can be reduced, and the beam pitch deviation between the image heights when the beam is made multi-beam can be reduced.

  However, when the power in the sub-scanning direction of the first surface of the first scanning lens 7 is negative, wavefront aberration remains even if a special toric surface is used. Therefore, as shown in FIG. 1, the most peripheral light flux in the scanning optical system is configured to be inclined in the same direction with respect to the normal line of the first surface of the first scanning lens 7 and the first surface of the second scanning lens 8. did. Thereby, the tolerance with respect to the eccentricity of the first scanning lens 7 and the second scanning lens 8 is increased. As described in Patent Document 2, there is a method of reducing the angle formed between the incident light beam and the scanning lens. However, in this case, since the scanning lens needs to be greatly curved in the deflection rotation surface, not only the workability of the scanning lens is reduced, but also the tolerance for decentration is significantly reduced. Therefore, in the above configuration, the power in the sub-scanning direction of the first surface of the second scanning lens 8 is positive, and the wavefront aberration generated on the first surface of the first scanning lens 7 is the first of the second scanning lens 8. It was made to offset in terms of surface.

  Further, when the second scanning lens 8 has a concave meniscus shape toward the polygon mirror in the main scanning direction as shown by a one-dot chain line in FIG. 2, the first surface of the second scanning lens 8. The ghost light reflected by is reflected again by the first surface or the second surface of the first scanning lens 7 as indicated by the dotted arrow, and reaches the scanned surface 10 made of a photosensitive member, and ghost light is generated. there is a possibility. Further, the ghost light on the first surface and the second surface of the first scanning lens 7 may be combined, and the ghost light intensity on the scanned surface 10 may increase. However, as shown by the solid line in FIG. 2, if the power of the first surface in the sub-scanning direction is positive, in other words, if the second scanning lens 8 is a convex surface toward the deflecting means side, then the second scanning lens 8 The ghost light reflected by the first surface of the scanning lens 8 diverges to a level with no problem. Note that the direction of the light beam reflected by the first scanning lens 7 and returning to the polygon mirror is not a problem because the direction changes greatly.

(Sixth configuration)
The optical scanning device described above can be applied as an exposure unit or a writing device of an image forming apparatus such as a printer or a copying machine. FIG. 3 schematically shows an example of an image forming apparatus to which the optical scanning device is applied as an exposure unit. In FIG. 3, in the image forming apparatus 100, a charging unit 112 and an exposure unit 117 are used to perform an electrophotographic process such as charging, exposure, development, transfer, and cleaning around the photosensitive drum 111. The developing unit 113, the transfer unit 114, and the cleaning unit 115 are arranged in this order in the rotation direction of the photosensitive drum 111. The exposure unit 117 includes the optical scanning device, and the laser beam LB deflected and scanned as described above is emitted from the exposure unit 117 toward the surface of the photosensitive drum 111 that is the surface to be scanned. The surface of the body drum 111 is scanned by a laser beam spot.

  The surface of the photosensitive drum 111 is uniformly charged by the charging unit 112 in advance, and an electrostatic latent image is formed on the surface of the photosensitive drum 111 by scanning a beam spot modulated in accordance with the image signal. The The electrostatic latent image is visualized as a toner image by supplying toner from the developing unit 113. This toner image is transferred by the transfer unit 114 to a transfer material (transfer paper, various sheets) P supplied one by one from the paper feed cassette 118 by the paper feed roller 120 and by the registration roller 119 with timing. It is comprised so that. The surface of the photosensitive drum 111 after the transfer is configured to be neutralized and cleaned by the cleaning unit 115 and charged again. On the other hand, the transfer paper P onto which the toner image has been transferred is heated and fixed by the fixing unit 116 and is discharged to the paper discharge tray 123 via the paper discharge passage 121 and the paper discharge roller 122.

In the image forming apparatus configured as described above, the optical scanning apparatus configured as described above is provided as means for performing the exposure process, so that the wavefront aberration can be improved and a stable beam spot diameter can be obtained. In addition, beam spot diameter deterioration due to eccentricity can be reduced, and ghost light can be removed, so that the diameter of the beam spot can be reduced, and image formation with excellent granularity and gradation can be performed. .
Although FIG. 3 shows an example of an image forming apparatus that forms a single-color image, an image forming unit having a structure including the photosensitive drum 111 and its surrounding charging, exposing, developing, transferring, and cleaning units is used as a transfer material. By using a configuration (so-called tandem configuration) arranged in parallel in the transport direction, an image forming apparatus that forms a multicolor or full-color image can be obtained. Further, if a single photosensitive drum is provided with a plurality of color developing units and an intermediate transfer member, a one-drum / intermediate transfer type color image forming apparatus can be obtained. Then, by using the optical scanning device of the present invention for these color image forming apparatuses, it is possible to form a color image having excellent granularity and gradation.

  Next, specific examples of the optical scanning device according to the present invention will be described.

(Example 1)
Specifications from the light source to the polygon mirror 5 as the deflecting means are as follows.
-Wavelength of the light source 1: 655 nm
-Focal length of coupling lens 2: 27 mm
Coupling action: Collimating action Polygon mirror 5: Number of deflecting reflecting surfaces: 5
Inscribed circle radius: 18mm
The angle formed by the incident angle of the beam from the light source side and the optical axis of the scanning optical system: 58 degrees

Lens data after the deflecting means is shown below.
The first surface of the first scanning lens 7 and both surfaces of the second scanning lens 8 are expressed by the following equations (1) and (2).

[Main scanning non-arc type]
The surface shape in the main scanning plane is a non-arc shape, the paraxial radius of curvature in the main scanning plane on the optical axis is Rm, the distance in the main scanning direction from the optical axis is Y, the cone constant is K, and higher order Are represented by the following polynomial expression, where X is the depth in the optical axis direction: A1, A2, A3, A4, A5, A6,.
X = (Y ^ 2 / Rm) / [1 + √ {1- (1 + K) (Y / Rm) ^ 2}]
+ A1 ・ Y + A2 ・ Y ^ 2 + A3 ・ Y ^ 3 + A4 ・ Y ^ 4
+ A5 ・ Y ^ 5 + A6 ・ Y ^ 6 + ... (1)
Here, when a numerical value other than zero is substituted for odd-order A1, A3, A5,..., It has an asymmetric shape in the main scanning direction.
In the first embodiment, only the even order is used, and the system is symmetric in the main scanning direction. Also in Example 2 described later, only the even order is used, and the system is symmetric in the main scanning direction.

[Sub-scanning curvature formula]
An expression for changing the sub-scanning curvature according to the main scanning direction is shown by (2).
Cs (Y) = 1 / Rs (0) + B1 · Y + B2 · Y ^ 2
+ B3 ・ Y ^ 3 + B4 ・ Y ^ 4 + B5 ・ Y ^ 5 + ... (2)
Here, when a numerical value other than zero is substituted for B, B3, B5... Of Y, the sub-scanning curvature radius becomes asymmetric in the main scanning direction.
Further, the second surface of the first scanning lens 7 is a rotationally symmetric aspheric surface, and is expressed by the following equation.

[Rotationally symmetric aspheric surface]
When the paraxial radius of curvature in the optical axis is R, the distance in the main scanning direction from the optical axis is Y, the cone constant is K, and the higher order coefficients are A1, A2, A3, A4, A5, A6,. The depth in the optical axis direction is represented by X and is expressed by the following polynomial.
X = (Y ^ 2 / R) / [1 + √ {1- (1 + K) (Y / Rm) ^ 2}]
+ A1 ・ Y + A2 ・ Y ^ 2 + A3 ・ Y ^ 3 + A4 ・ Y ^ 4
+ A5 ・ Y ^ 5 + A6 ・ Y ^ 6 + ... (3)

Specific numerical data is shown below. In the following numerical data, “× 10 ⁺¹ ” is expressed as “E + 01”, “× 10 ⁻⁷ ” is expressed as “E-07”, and the other is the same.

[Shape of the first surface of the first scanning lens 7]
Rm = −279.9, Rs = −61.0
K = -2.900,000E + 01
A4 = 1.755765E-07
A6 = -5.491789E-11
A8 = 1.087700E-14
A10 = -3.183245E-19
A12 = −2.6635276E-24
B1 = −2.066347E-06
B2 = 5.727737E-06
B3 = 3.1252201E-08
B4 = 2.280241E-09
B5 = −3.729852E-11
B6 = -3.283274E-12
B7 = 1.765590E-14
B8 = 1.732959E-15
B9 = −2.889722E-18
B10 = -1.984531E-19

[Shape of the second surface of the first scanning lens 7]
R = −83.6
K = −0.549157
A4 = 2.748446E-07
A6 = −4.502346E-12
A8 = -7.366455E-15
A10 = 1.803003E-18
A12 = 2.727900E-23

[Shape of the first surface of the second scanning lens 8]
Rm = 6950, Rs = 110.9
K = 0.000000E + 00
A4 = 1.549648E-08
A6 = 1.292741E-14
A8 = −8.811446E-18
A10 = −9.182312E-22
B1 = −9.593510E-07
B2 = -2.135322E-07
B3 = −8.079549E-12
B4 = 2.390609E-12
B5 = 2.881396E-14
B6 = 3.693775E-15
B7 = -3.258754E-18
B8 = 1.814487E-20
B9 = 8.72085E-23
B10 = −1.3340807E-23

[Shape of the second surface of the second scanning lens 8]
Rm = 766, Rs = −68.22
K = 0.000000E + 00
A4 = -1.150396E-07
A6 = 1.096926E-11
A8 = −6.5542135E-16
A10 = 1.9844381E-20
A12 = −2.411512E−25
B2 = 3.644079E-07
B4 = −4.847051E-13
B6 = −1.666159E-16
B8 = 4.534859E-19
B10 = −2.819319E-23
Further, the refractive indices of the scanning lenses at the used wavelength are all 1.52724.

Specific numerical values of the optical arrangement are shown below.
Distance d1: 64 mm from the deflection surface to the first surface of the first scanning lens
Center thickness d2 of the first scanning lens 7: 22.6 mm
The distance d3 from the second surface of the first scanning lens to the first surface of the second scanning lens: 75.9 mm
Center wall thickness d4 of second scanning lens: 4.9 mm
Distance d2: 158.7 mm from the second surface of the second scanning lens to the surface to be scanned
A soundproof glass 6 and a dustproof glass 9 having a refractive index of 1.514 and a thickness of 1.9 mm are arranged as shown in FIG. 1, and the soundproof glass 6 is arranged in a direction parallel to the main scanning direction in the deflection rotation plane. It is tilted 10 degrees.

The F number in the sub-scanning direction at the outermost periphery of the scanning optical system and the central image height is shown below.
Image height 150 mm: 41.5
Image height 0 mm: 40.4
Image height -150 mm: 41.0

FIG. 4 shows aberration diagrams of the above example. FIG. 4A shows field curvature, where the X axis is defocus (mm) and the Y axis is image height (mm). The solid line indicates the field curvature in the sub-scanning direction, and the dotted line indicates the field curvature in the main scanning direction. FIG. 4B shows constant velocity, where the X axis is% and the Y axis is the image height (mm). The solid line indicates linearity, and the dotted line indicates fθ characteristics.
FIG. 5 shows the radius of curvature of the first surface (R1) of the first scanning lens (L1) 7 in the sub-scanning direction with respect to the lens height in the main scanning direction.
FIG. 6 shows the radius of curvature in the sub-scanning direction with respect to the main scanning direction lens height on the first surface (R1) of the second scanning lens (L2) 8.
FIG. 7 shows the radius of curvature in the sub-scanning direction with respect to the main scanning direction lens height on the second surface (R2) of the second scanning lens (L2) 8.
FIG. 8 shows the beam spot diameter with respect to defocusing, where (a) shows the beam spot diameter in the main scanning direction and (b) shows the beam spot diameter in the sub-scanning direction.

The maximum inclination angle of all the lens surfaces is the second surface of the first scanning lens 7 and is 28 degrees.
Further, on the first surface of the first scanning lens 7 and the first surface and the second surface of the second scanning lens 8 which are special toric surfaces, the amount of change in the sub-scanning curvature per 1 mm of the lens height of each surface is in order,
7.9E-05 (1 / mm),
7.1E-05 (1 / mm),
1.4E-04 (1 / mm),
It becomes.

(Example 2)
Next, a second embodiment of the optical scanning device will be shown.
・ Light source wavelength: 655 nm
・ Coupling lens focal length: 27mm
-Coupling action: Collimating action-Polygon mirror: Number of deflecting reflecting surfaces: 5
Inscribed circle radius: 18mm
The angle formed by the incident angle of the beam from the light source side and the optical axis of the scanning optical system: 58 degrees

Lens data after the deflecting means is shown below.
The first surface of the first scanning lens 7 and both surfaces of the second scanning lens 8 are expressed by the following equations (1) and (2).

[Main scanning non-arc type]
The surface shape in the main scanning plane is a non-arc shape, the paraxial radius of curvature in the main scanning plane on the optical axis is Rm, the distance in the main scanning direction from the optical axis is Y, the cone constant is K, and higher order Are represented by the following polynomial expression, where X is the depth in the optical axis direction: A1, A2, A3, A4, A5, A6,.
X = (Y ^ 2 / Rm) / [1 + √ {1- (1 + K) (Y / Rm) ^ 2}]
+ A1 ・ Y + A2 ・ Y ^ 2 + A3 ・ Y ^ 3 + A4 ・ Y ^ 4
+ A5 ・ Y ^ 5 + A6 ・ Y ^ 6 + ... (1)
Here, when a numerical value other than zero is substituted for odd-order A1, A3, A5,..., It has an asymmetric shape in the main scanning direction.

[Sub-scanning curvature formula]
An expression for changing the sub-scanning curvature according to the main scanning direction is shown by (2).
Cs (Y) = 1 / Rs (0) + B1 · Y + B2 · Y ^ 2
+ B3 ・ Y ^ 3 + B4 ・ Y ^ 4 + B5 ・ Y ^ 5 + ... (2)
Here, when a numerical value other than zero is substituted for B, B3, B5... Of Y, the sub-scanning curvature radius becomes asymmetric in the main scanning direction.
Further, the second surface of the first scanning lens 7 is a rotationally symmetric aspheric surface, and is expressed by the following equation.

[Rotationally symmetric aspheric surface]
When the paraxial radius of curvature in the optical axis is R, the distance in the main scanning direction from the optical axis is Y, the cone constant is K, and the higher order coefficients are A1, A2, A3, A4, A5, A6,. The depth in the optical axis direction is represented by X and is expressed by the following polynomial.
X = (Y ^ 2 / R) / [1 + √ {1- (1 + K) (Y / Rm) ^ 2}]
+ A1 ・ Y + A2 ・ Y ^ 2 + A3 ・ Y ^ 3 + A4 ・ Y ^ 4
+ A5 ・ Y ^ 5 + A6 ・ Y ^ 6 + ... (3)

Specific numerical data is shown below. In the following numerical data, “× 10 ⁺¹ ” is expressed as “E + 01”, “× 10 ⁻⁷ ” is expressed as “E-07”, and the other is the same.

[Shape of the first surface of the first scanning lens 7]
Rm = −303.54, Rs = −61.0
K = -2.900,000E + 01
A4 = 2.28E-07
A6 = −6.57E-11
A8 = 1.18E-14
A10 = -2.10E-19
A12 = 8.00E-24
B1 = -1.00E-06
B2 = 5.22E-06
B3 = 1.70E-08
B4 = −5.06E-11
B5 = -6.80E-12
B6 = -9.46E-14
B7 = −7.34E-16
B8 = -2.10E-17
B9 = −5.03E-19
B10 = 7.51E-21

[Shape of the second surface of the first scanning lens 7]
R = −85.6
K = −0.549157
A4 = 2.83E-07
A6 = 6.04E-12
A8 = -1.18E-14
A10 = 2.26E-18
A12 = 6.61E-23

[Shape of the first surface of the second scanning lens 8]
Rm = 6950, Rs = 94.4
K = 0.000000E + 00
A4 = 1.13E-08
A6 = 9.27E-14
A8 = -2.16E-19
A10 = -9.18E-22
B1 = −4.41E-07
B2 = −6.96E−08
B3 = -7.45E-11
B4 = 1.37E-11
B5 = −6.44E-16
B6 = −3.81E-15
B7 = 3.04E-18
B8 = 4.21E-19
B9 = -2.33E-22
B10 = -1.55E-23

[Shape of the second surface of the second scanning lens 8]
Rm = 781.2, Rs = −76.09
K = 0.000000E + 00
A4 = -1.14E-07
A6 = 9.25E-12
A8 = -3.65E-16
A10 = 9.51E-22
A12 = 2.38E-25
B2 = 4.91E-07
B4 = -1.64E-11
B6 = 7.96E-16
B8 = 1.30E-19
B10 = 1.47E-23
Further, the refractive indices of the scanning lenses at the used wavelength are all 1.52724.

The specifications of the optical arrangement are shown below.
Distance d1: 64.1 mm from the deflection surface to the first surface of the first scanning lens 7
Center wall thickness d2 of the first scanning lens 71: 22.5 mm
Distance d3 from the second surface of the first scanning lens to the first surface of the second scanning lens d3: 76 mm
Center wall thickness d4 of second scanning lens: 4.9 mm
Distance from second scanning lens second surface to surface to be scanned d5: 158.6 mm

  The soundproof glass 6 and the dustproof glass 9 having a refractive index of 1.514 and a thickness of 1.9 mm are arranged as shown in FIG. 1, and the soundproof glass 6 is parallel to the main scanning direction in the deflection rotation plane. Is tilted 10 degrees.

The F number in the sub-scanning direction at the outermost periphery of the scanning optical system and the central image height is shown below.
Image height 150 mm: 41.8
Image height 0 mm: 40.8
Image height -150 mm: 41.1

FIG. 9 shows aberration diagrams of the above example. FIG. 9A shows field curvature, where the X axis is defocused (mm) and the Y axis is image height (mm). The solid line indicates the field curvature in the sub-scanning direction, and the dotted line indicates the field curvature in the main scanning direction. FIG. 9B shows constant velocity, where the X axis is% and the Y axis is the image height (mm). The solid line indicates linearity, and the dotted line indicates fθ characteristics.
10 shows the sub-scanning curvature radius of the first surface (R1) of the first scanning lens (L1) 7, and FIG. 11 shows the sub-scanning curvature radius of the first surface (R1) of the second scanning lens (L2) 8. FIG. 12 shows the sub-scanning radius of curvature of the second surface (R2) of the second scanning lens (L2) 8, which has the shape as described above.
FIG. 13 shows the beam spot diameter with respect to defocusing, where (a) shows the beam spot diameter in the main scanning direction and (b) shows the beam spot diameter in the sub-scanning direction.

  In the embodiments described above, the light source is shown as an example of one semiconductor laser (LD). However, the present invention uses a plurality of semiconductor lasers (LD), an LD array having a plurality of light emitting points, etc. as the light source. The present invention can also be applied to conventional multi-beam optical systems. Further, by using a resin lens as the scanning lens, a scanning lens having a special toric surface or a rotationally symmetric aspherical surface can be mass-produced by molding and cost reduction can be realized.

  The optical scanning device of the present invention can improve the wavefront aberration, obtain a stable beam spot diameter, reduce beam spot diameter deterioration due to decentration, and remove ghost light. The beam spot can be reduced in diameter, and it can be used in the exposure unit and optical writing unit of electrophotographic image forming devices such as laser printers, digital copying machines, facsimiles, and laser plotters. It is possible to perform image formation with excellent properties. Further, the optical scanning device of the present invention can be used for a scanning image display device and the like in addition to the above-described image forming device, and further, a scanning type for measuring the length and shape of the object to be measured. It can also be used for measuring devices.

1A and 1B are diagrams illustrating a configuration of an optical scanning device according to an embodiment of the present invention, in which FIG. 1A illustrates an optical system arrangement in a main scanning corresponding direction, and FIG. 2B illustrates an optical system arrangement in a sub scanning corresponding direction; is there. 2A and 2B are diagrams for explaining a ghost light reduction effect in an optical scanning apparatus having the same configuration as in FIG. 1, in which FIG. 1A is a diagram illustrating an optical system arrangement in a main scanning correspondence direction, and FIG. It is a figure which shows system | strain arrangement | positioning. 1 is a schematic configuration diagram of an image forming apparatus showing an embodiment of the present invention. The aberrational diagram in Example 1 is shown, (a) is a field curvature, (b) is a figure which shows constant velocity. 7 is a graph showing a change in radius of curvature with respect to the lens height in the main scanning direction of the surface on the deflection unit side of the scanning lens closest to the deflection unit in Example 1; 6 is a graph showing a change in radius of curvature with respect to the lens height in the main scanning direction of the surface on the deflection unit side of the scanning lens farthest from the deflection unit in Example 1; 6 is a graph showing a change in a curvature radius with respect to a lens height in a main scanning direction of a second surface of a scanning lens farthest from the deflecting unit in the first embodiment. FIG. 7 is a graph showing the beam spot diameter with respect to defocus in Example 1, where (a) is a graph showing the beam spot diameter in the main scanning direction, and (b) is a graph showing the beam spot diameter in the sub-scanning direction. The aberrational diagram in Example 2 is shown, (a) is a field curvature, (b) is a figure which shows isokineticity. 10 is a graph showing a change in a radius of curvature with respect to a lens height in a main scanning direction of a surface on the deflection unit side of a scanning lens closest to the deflection unit in Example 2. 10 is a graph showing a change in radius of curvature with respect to the lens height in the main scanning direction of the surface on the deflection unit side of the scanning lens farthest from the deflection unit in Example 2; 6 is a graph showing a change in a curvature radius with respect to a lens height in a main scanning direction of a second surface of a scanning lens farthest from the deflecting unit in the first embodiment. FIG. 7 is a graph showing the beam spot diameter with respect to defocus in Example 1, where (a) is a graph showing the beam spot diameter in the main scanning direction, and (b) is a graph showing the beam spot diameter in the sub-scanning direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Coupling lens 3 Aperture 4 Cylindrical lens 5 Polygon mirror which is a deflection means 6 Soundproof glass 7 First scanning lens 8 Second scanning lens 9 Dustproof glass 10 Scanned surface (photoreceptor surface)

Claims (6)

A light source;
A coupling optical system for coupling a light beam from the light source;
A line image optical system in which a light beam from the coupling optical system is a line image long in the main scanning direction;
Deflection means for deflecting and scanning the light beam from the line image optical system;
A scanning optical system for guiding the light beam from the deflecting means to the surface to be scanned,
The scanning optical system comprises a plurality of scanning lenses,
At least one scanning lens is composed of a rotationally symmetric aspherical surface and a special toric surface whose sub-scanning curvature radius changes from the optical axis of the lens surface toward the periphery in the main scanning direction.
A surface having a region having the largest inclination angle within an effective range with respect to a surface perpendicular to the optical axis among all scanning lens surfaces is a rotationally symmetric aspheric surface.
An optical scanning device characterized by satisfying the following condition. The optical scanning device according to claim 1,
The surface of all scanning lenses of the scanning optical system comprises a rotationally symmetric aspherical surface or the special toric surface. The optical scanning device according to claim 1 or 2,
Of the surfaces of all scanning lenses of the scanning optical system, the surface having the largest effective width in the main scanning direction is the special toric surface. In the optical scanning device according to any one of claims 1 to 3,
All the scanning lenses of the scanning optical system have an inclination angle of the lens surface within an effective range with respect to a surface perpendicular to the optical axis of 30 degrees or less,
All the scanning lenses have a special toric surface whose sub-scanning curvature radius changes depending on the lens height in the main scanning direction, and the special toric surface has a sub-scanning curvature change amount of 1 mm per 1 mm in the main scanning direction lens height. An optical scanning device characterized by being smaller than 5 × 10 ⁻⁴ (1 / mm). In the optical scanning device according to any one of claims 1 to 4,
The scanning optical system comprises two scanning lenses,
The outermost luminous flux of the effective writing width is inclined in the same direction in the deflection rotation plane with respect to the normal of the first surface of the two scanning lenses;
-The scanning lens closest to the deflecting means has a concave meniscus shape on the deflecting means side in the main scanning direction.
The first surface of the scanning lens closest to the deflecting means has a special toric surface that has negative power in the sub-scanning direction, and the sub-scanning curvature changes from the optical axis of the lens surface toward the periphery in the main scanning direction.
The first surface of the scanning lens farthest from the deflection means has a positive power in the sub-scanning direction;
An optical scanning device characterized by satisfying the following condition. In an image forming apparatus that forms an image on a transfer material by performing each process of charging, exposure, development, and transfer,
An image forming apparatus comprising the optical scanning device according to claim 1 as means for executing the exposure process.

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