JPH07146437A - Light beam scanning optical system - Google Patents

Abstract

PURPOSE:To provide a light beam scanning optical system by which excellent constitution can be taken in respect of realization of high performance, realization of a high view angle, downsizing, an environmental variation resistant characteristic, a price or the like. CONSTITUTION:In a light beam scanning optical system arranged between a deflecting means to deflect a light beam and a surface to be scanned, the optical system is composed of two lenses of a first lens 6 composed of a cylindrical lens 4 having positive power only on a main scanning cross section or an a toric lens which has positive power on a main scanning cross section and whose sub-scanning cross section has only power close to zero and a second lens 7 which is arranged in the vicinity of a surface side to be scanned of the first lens 6 and which is composed of a toric lens whose both main and sub- scanning cross sections have positive power, and is characterized in that at least a single surface of the main scanning cross section of the first lens 6 is turned into an aspheric surface, and that at least a single surface of the main scanning cross section of the second lens 7 is turned into an aspheric surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light beam scanning optical system used in laser beam printers, digital copying machines and the like.

[0002]

2. Description of the Related Art Conventionally, in a light beam scanning optical system of this type, as described in Japanese Patent Publication No. 62-36210, a rotary polygon mirror is used to deflect and scan a light beam. It is common.

As described in the above publication,
In view of the fact that the rotating polygon mirror has an angle error (surface tilt) in the tilt direction of each reflecting surface with respect to the rotation axis, it causes a change in the scanning position of the light beam to be scanned, which adversely affects the final image output. In order to eliminate the influence of this surface tilt, it has already been proposed to use a spherical lens and a toric lens to place the rotary polygon mirror and the surface to be scanned in an optically conjugate relationship.

Further, as described in US Pat. No. 4,693,072, it is also proposed that a cylindrical lens is arranged in the vicinity of a surface to be scanned by a scanning beam to mitigate the influence of surface tilt.

[0005]

However, in the above-mentioned conventional example using the spherical lens and the toric lens, the shape of the toric lens in the main scanning cross section is a plano-convex lens in order to correct aberrations in order to maintain the optical performance. Further, the shape of the spherical concave lens arranged between the toric lens and the rotary polygonal mirror becomes close to plano-concave or biconcave, so that there is a limit to widening the angle of view. In other words, this means that in order to achieve a wide angle of view while maintaining the optical performance, the lens inevitably becomes thick and the device becomes large.

Further, a toric lens is difficult to process,
It is a factor of cost increase. In order to solve such a problem, it is conceivable to use a plastic toric lens, etc., but in the configuration in which concave and convex lenses are arranged in order from the rotary polygon mirror side as in the above conventional example, the power of the entire system is increased. Since the power of the toric lens becomes stronger, the influence of the power change of the plastic lens due to environmental changes cannot be ignored, and problems such as focus shift on the surface to be scanned occur.

On the other hand, in the conventional example in which the cylindrical lens is arranged in the vicinity of the surface to be scanned, although there is little problem due to the influence of the above-mentioned environmental change, for example, in a laser beam printer using an electrophotographic system, a developing device, a cleaner, etc. Since the process device is disposed in close contact with the photosensitive drum, it is not preferable to dispose an optical element such as a cylindrical lens in the vicinity of the photosensitive drum. Further, if such a cylindrical lens is arranged in the vicinity of the photoconductor drum, it is likely to be adversely affected by toner stains, heat, ozone and the like.

Therefore, in view of the above problems, an object of the present invention is to provide a light beam scanning optical system which has an excellent configuration in terms of high performance, wide angle of view, miniaturization, environmental resistance, and price. To provide.

[0009]

In order to achieve the above object, the present invention is directed to deflecting means for deflecting a light beam (rotating polygon mirror).
In a light beam scanning optical system arranged between a scanning surface and a deflecting means of a light beam scanning device that deflects light to scan a surface to be scanned (photosensitive drum or the like), a cylindrical body having a positive power only in a main scanning section. A lens, or a first lens consisting of a toric lens having a positive power in the main scanning section and a power in the sub-scan section close to zero,
And a main unit arranged near the surface to be scanned of the first lens,
Each of the sub-scanning cross sections is composed of two second lenses, which are toric lenses having positive power, and at least one of the first and second lenses in the main scanning cross section is aspherical.

Further, the second lens has a concentric shape in which the center of the radius of curvature exists on the deflecting means side. Further, the focal length in the main scanning cross section of the second lens is f2a, the focal length in the sub scanning cross section of the second lens is f2b, the combined focal length of the main scanning cross sections of the first and second lenses is fa, and the first focal length is The focal length of the lens in the sub-scan section is f1b, the distance between the second lens and the surface to be scanned is L, and the maximum thickness of the second lens in the optical axis direction is d.
(1) 0.1 <fa / f2a <0.3 (2) 0.25 <f2b / fa <0.5 (3) 0.6 <L / fa <1 (4) dmax / f2b <0.2 (5) | f2b / f1b | <0.35 At least one of the conditions is satisfied.

The first lens is made of a plastic material or the like, and in particular, the conditional expression (5) among the above five conditional expressions is important when the first lens is made of plastic.

The second lens is also made of a plastic material or the like, and in particular, among the above five conditional expressions (3) and (4).
The conditional expression is important when forming the second lens with a plastic material.

With respect to the significance of the above-mentioned concrete constitution, etc.,
It is described in the description of the examples below.

[0014]

Embodiments of the optical system for scanning a light beam of the present invention will be described below with reference to the drawings.

1 and 2 show a first embodiment of the present invention,
FIG. 1 illustrates a state in a main scanning section, and FIG. 2 illustrates a state in a sub scanning section which is perpendicular to the main scanning section and includes an optical axis. The main scanning surface refers to a light flux plane formed by the light beam deflectively scanned by the rotary polygon mirror 5 with time.

In FIG. 1, reference numeral 1 denotes a semiconductor laser which is a light source. A light beam emitted from the semiconductor laser 1 is made into substantially parallel light by a collimator lens 2 and its cross section is adjusted in size by an aperture stop 3. It enters the cylindrical lens 4. The cylindrical lens 4 has power in the sub-scanning section, but does not have power in the main-scanning section. Therefore, the light beam is parallel light in the main-scanning section, and is formed into a substantially linear shape in the sub-scanning section, so that the rotating polyhedral surface is formed. Mirror 5
Incident on.

The rotating polygonal mirror 5 rotates at a constant speed and at a high speed in the direction of the arrow, and the light beam incident on the rotating polygonal mirror 5 is reflected here and deflected and scanned in the main scanning section at a high speed.

In this way, the light beam deflected and scanned by the uniform angular velocity movement has a cylindrical lens having a positive power only in the main scanning section, or has a positive power in the main scanning section, and the sub-scanning section is close to zero. A first lens 6, which is a toric lens having only power, and a main scanning section,
After passing through the second lens 7, which is a toric lens having a positive power in both sub-scanning cross sections, an image is formed on the photosensitive drum 8 and scanned with a substantially uniform linear motion.

In FIG. 2, P indicates the position of the reflecting surface of the rotary polygonal mirror 5, and in the sub-scanning cross section, the light beam is focused at this position P as described above. Here, since the reflecting surface P and the photosensitive drum 8 are set in a substantially optically conjugate relationship via the first lens 6 and the second lens 7, even if the reflecting surface P is tilted in the sub-scan section ( That is, the light beam is imaged on the same scanning line on the photosensitive drum 8 even if the surface is tilted. In this way, a so-called plane tilt correction system of the rotary polygon mirror 5 is configured.

Here, the first lens 6 and the second lens 7 are constructed as follows so as to obtain good field curvature and fθ characteristics over a wide angle of view in the main scanning section.

First, the first lens 6 is a cylindrical lens having a positive power only in the main scanning section or a toric lens having a positive power in the main scanning section and a power in the sub-scanning section close to zero. In order to correct the field curvature in the meridional direction (main scanning direction) and obtain a good fθ characteristic over a wider angle of view, at least one of the main scanning cross sections is an aspherical surface.

Next, the second lens 7 is arranged near the rear of the first lens 6 and is a toric lens having a positive power in both the main scanning section and the sub-scanning section, and corrects the field curvature over a wide angle of view. As described above, the concentric shape is provided (the centers of the radii of curvature of both surfaces of the toric lens 7 are close to the rotary polygon mirror side), and at least one surface of the main scanning cross section is aspheric.

Further, if the power of the second lens 7 in the main scanning section is made too strong, the fθ characteristic and the field curvature in the meridional direction cannot be corrected while being balanced. Therefore, the focal length in the main scanning section of the second lens 7 cannot be corrected. F2
a, and the composite focal length of the first lens 6 and the second lens 7 in the main scanning cross section is fa, the second lens 7 in the main scanning cross section should satisfy the relationship of 0.1 <fa / f2a <0.3.
It is good to weaken the power of.

That is, if the lower limit is exceeded, f2a becomes large, which is advantageous for aberration correction, but the second lens 7 approaches the surface to be scanned (photosensitive drum 8 surface) side and becomes large.
Further, when the upper limit is exceeded, f2a becomes small, which is advantageous for making the device compact, but it becomes difficult to correct the fθ characteristic and the field curvature in a well-balanced manner.

On the other hand, the focal length f2b of the second lens 7 in the sub-scan section is 0 so that the field curvature in the sagittal direction (the direction in the sub-scan section and perpendicular to the optical axis) is sufficiently corrected. It is preferable to satisfy the relationship of 0.25 <f2b / fa <0.5.

That is, when the upper limit is exceeded, f2b becomes large, which is advantageous for aberration correction, but the second lens 7 approaches the surface to be scanned, which is not preferable as in the above-mentioned conventional example. Further, if the lower limit is exceeded, f2b becomes small and it becomes difficult to correct the field curvature in the meridional direction and the sagittal direction in a well-balanced manner.

Further, it is preferable that the second lens 7 is arranged at a position satisfying the relationship of 0.6 <L / fa <1 where L is the distance between the second lens 7 and the surface to be scanned.

That is, when the value goes below the lower limit, the apparatus becomes large in size and the effect of compensating for face tilt decreases. Further, if the upper limit is exceeded, the power of the second lens 7 becomes strong, and especially when the second lens 7 is formed of a plastic material by taking advantage of the cost, the focus on the surface to be scanned due to the influence of environmental changes and the like. Becomes unacceptable.

When the maximum thickness of the second lens 7 in the optical axis direction is dmax, it is preferable to make the second lens 7 thin so as to satisfy the relationship of dmax / f2b <0.2.

As a result, environmental changes, especially the second lens 7
It is possible to reduce the focus shift on the surface to be scanned due to moisture absorption and to facilitate the molding with plastic.

When the focal length of the sub-scan section of the first lens 6 is f1b, the power of the sub-scan section of the first lens 6 is zero so that the relationship of | f2b / f1b | <0.35 is satisfied. It is desirable to set a small power close to.

As a result, when the first lens 6 is formed of a plastic material by taking advantage of the cost, it is possible to reduce the focus shift of the sub-scanning section due to environmental changes (especially refractive index change due to temperature change). .

With the structure as described above, the performance is high over a wide angle of view, and even if both the first lens 6 and the second lens 7 are made of plastic or the like, they are inexpensive and less affected by environmental changes. An optical system for scanning a light beam can be realized.

FIG. 3 shows an aberration diagram showing field curvature in the first embodiment, and FIG. 4 shows an fθ characteristic.

Specific numerical examples of the first embodiment are shown below.

Focal length of the entire system 189 mm Maximum scanning angle 90 ° Deflection point to R1 surface 42.49 mm R1 = 762.93 D1 = 16.0 N1 = 1.
519 K = −2.703328E-03 B = −1.472224E-07 C = 7.02329E-12 D = 1.18196E-14 E = −1.06494E-18 R1 ′ = ∞ R2 = −142.70 D2 = 21.84 K = -4.29792E-01 B = -1.34291E-07 C = 3.72520E-11 D = -2.355595E-14 E = 5.43226E-18 R2 '= ∞ R3 = -1202 .21 D3 = 10.0 N3 =
1.519 K = -3.62143E-03 B = -3.998368E-08 C = 7.24592E-13 D = -1.02261E-15 E = 5.13177E-20 R3 '=-52.7468 y ≧ 0 = b = -1.07193E-04 c = 4.51879E-08 d = -1.11655E-11 e = 1.51246E-15 f = -8.75587E-20 y <0 b = -1 .15230E-04 c = 4.66555E-08 d = -1.13154E-11 e = 1.55414E-15 f = -9.35603E-20 R4 = -328.27 D4 = 165.22 K = 9.435576 B = −5.52070E−09 C = D = E = 0.0 R4 ′ = − 20.0 fa = 189.25 fa / f2a = 0.218 f1b = ∞ f2b / a = 0.297 f2a = 866.709 L / fa = 0.873 L = 165.22 f2b = 56.207 dmax / f2b = 0.1
78 dmax = 10.0 | f2b / f1b | = 0.
0

Here, in the above numerical example, FIGS.
As shown in FIG. 5, the radii of curvature in the main scanning cross section of each surface are R1 to R4 from the rotary polygon mirror 5 side, the radii of curvature in the sub scanning cross section are R1 ′ to R4 ′ from the rotary polygon mirror 5 side, and the distances between the respective surfaces are It is shown by D1-D4.

The refractive index of each lens at a wavelength of 780 nm is represented by N1 and N3 from the side of the rotary polygon mirror 5.

K and B to E are relational expressions between the height y of the lens surface on the xy plane and the distance x shown below.

[0040]

[Outer 1] The aspherical coefficient of each order of is shown.

The shape of the sub-scanning section of the second lens 7 on the side of the first lens 6 is, as shown in FIG. 5, within a section including a normal line set in the main scanning section and orthogonal to the main scanning section. It is an arc having a measured radius of curvature r '.

When the Z axis is taken perpendicularly to the main scanning section, the surface shape S in this section is expressed by the following equation.

[0043]

[Outside 2] r'is represented by the following formula.

R ′ = R ′ · (1 + by ² + cy ⁴ + dy ⁶ + ey ⁸ + fy ¹⁰ ).

In the first embodiment of the present invention, the radius of curvature r'in the sub-scan section of the surface of the second lens 7 on the first lens 6 side is asymmetric with respect to the optical axis and varies depending on the lens position. There is.

The first lens 6 is a cylindrical lens which has no power in the sub-scan section.

Next, FIG. 6 shows a second embodiment of the present invention, FIG. 7 is an aberration diagram showing field curvature, and FIG. 8 is a diagram showing f.theta. Characteristics.

In the second embodiment of the present invention, the first lens 16 is not a cylindrical lens having power only in the main scanning section, but the main scanning section has positive power.
A toric lens having only a positive power close to zero is used in the sub-scanning section, and the field curvature in the sagittal direction has been successfully corrected. The second lens 17 has almost the same configuration as that of the first embodiment.

Numerical examples in the second embodiment will be shown below.

Focal length of the entire system 189 mm Maximum scanning angle 90 ° Deflection point to R1 surface 42.41 mm R1 = 298.26 D1 = 16.0 N1 =
1.519 K = -2.23743E + 03 B = -1.15510E-07 C = 6.73662E-12 D = 1.116333E-14 E = -1.18599E-18 R1 '= 500.0 R2 = -125. 03 D2 = 21.72 K = −4.25693 B = −1.50034E-07 C = 2.54255E-11 D = −2.05626E-14 E = 4.46555E-18 R2 ′ = − 200.0 R3 = -1157.99 D3 = 10.0 N3 =
1.519 K = -1.000606E + 03 B = -7.37408E-09 C = -5.40177E-12 D = -1.02261E-15 E = 5.13177E-20 R3 '=-44.6693 y ≧ 0 When b = −8.62997E-05 c = 3.20619E-08 d = −5.000789E-12 e = 1.91740E-16 f = 1.02273E-20 When y <0 b = −9.04427E -05 c = 3.20222E-08 d = -5.15838E-12 e = 2.75519E-16 f = 1.71002E-21 R4 = -331.78 D4 = 165.45 K = 6.31853 B =- 1.81135E-08 C = -5.61162E-12 D = E = 0.0 R4 '=-20.0 fa = 189.17 fa / f2a = 0.2 2 f1b = 277.421 f2b / fa = 0.324 f2a = 892.288 L / fa = 0.875 L = 165.45 f2b = 61.289 dmax / f2b = 0.1
63 dmax = 10.0 | f2b / f1b | = 0.
221

[0051]

As described above, according to the present invention, the light beam is disposed between the deflecting means and the surface to be scanned of the light beam scanning device for deflecting the light beam by the deflecting means to scan the surface to be scanned. In the scanning optical system, a first lens composed of a cylindrical lens having a positive power only in the main scanning section or a toric lens having a positive power in the main scanning section and having a power close to zero in the sub scanning section. ,
And a second lens composed of a toric lens having a positive power in both the main scanning section and the sub-scanning section, which is arranged in the vicinity of the surface to be scanned of the first lens. By making at least one of the main scanning cross-sections aspherical for both lenses, even if both the first lens and the second lens are made of inexpensive plastic material, they are not easily affected by environmental changes and have a wide angle of view and high performance. An optical system for scanning a light beam can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a state in a main scanning section of a first embodiment of the present invention.

FIG. 2 is a diagram showing a state in a sub-scan section of the first embodiment.

FIG. 3 is an aberration diagram illustrating field curvature of Example 1.

FIG. 4 is an aberration diagram illustrating an fθ characteristic of the first example.

FIG. 5 is a diagram for explaining an aspherical expression of the first embodiment.

FIG. 6 is a diagram showing a state in a main scanning section of a second embodiment.

FIG. 7 is an aberration diagram illustrating a field curvature of Example 2.

FIG. 8 is an aberration diagram illustrating fθ characteristics of the second example.

[Explanation of symbols]

 1 Semiconductor Laser 2 Collimator Lens 3 Aperture Stop 4 Cylindrical Lens 5 Rotating Polygonal Mirror 6 First Lens 7 Second Lens 8 Photosensitive Drum

Claims (9)

[Claims] 1. An optical system for scanning a light beam, which is arranged between the deflecting means and a surface to be scanned of a light beam scanning device for deflecting a light beam by a deflecting means to scan the surface to be scanned. A first lens comprising a cylindrical lens having a positive power only in the main scanning section, or a toric lens having a positive power in the main scanning section and a sub-scanning section having a power close to zero, and the first lens The second lens is arranged in the vicinity of the surface to be scanned of the lens, and is composed of two second lenses each of which is a toric lens having a positive power in both main and sub-scanning cross sections. An optical system for scanning a light beam, wherein the optical system is spherical, and at least one of the main scanning cross sections of the second lens is aspherical. 2. The optical system for scanning a light beam according to claim 1, wherein the second lens has a concentric shape in which the center of the radius of curvature exists on the deflecting means side. 3. When the focal length of the second lens in the main scanning section is f2a and the combined focal length of the first and second lenses in the main scanning section is fa, then 0.1 <fa / f2a <0. The optical system for scanning a light beam according to claim 1, wherein the condition 3 is satisfied. 4. When the focal length of the second lens in the sub-scan section is f2b and the combined focal length of the main scan sections of the first and second lenses is fa, 0.25 <f2b / fa <0. The optical system for scanning a light beam according to claim 1, wherein the condition 5 is satisfied. 5. When the composite focal length of the main scanning cross sections of the first and second lenses is fa and the distance between the second lens and the surface to be scanned is L, 0.6 <L / fa <1 The optical system for scanning a light beam according to claim 1, which satisfies: 6. The focal length of the sub-scan section of the second lens is f2b, and the maximum thickness of the second lens in the optical axis direction is dm.
The optical system for scanning a light beam according to claim 1, wherein dmax / f2b <0.2 when ax is satisfied. 7. The focal length of the sub-scan section of the first lens is f1b, and the focal length of the sub-scan section of the second lens is f1.
2b, the optical system for scanning a light beam according to claim 1, wherein | f2b / f1b | <0.35 is satisfied. 8. The optical system for scanning a light beam according to claim 1, wherein the first lens is made of plastic. 9. The optical system for scanning a light beam according to claim 1, wherein the second lens is made of plastic.