JPH09105859A - Scanning optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an fθ lens which can easily be increased in resolution even when the lens consists of one element without providing a slit by specifying the constitution of an aspherical surface type used for designing. SOLUTION: In this scanning optical system which reflects beam light emitted by a light source by a rotating polygon mirror and forms an image on a photosensitive body by scanning, the fθ lens of single-element constitution which is arranged between the polygon mirror and an image formation plane and makes fθ corrections is made aspherical in the vertical scanning direction (X axis) and horizontal scanning direction (Y axis). Further, plural terms determining the respective aspherical surfaces are designed by the aspherical surface type defined with >=8 terms including odd and even terms. This aspherical surface expression is as shown by the equation, where the intersection of the fθ lens and optical axis is the origin, the optical-axis direction a Z axis, the main scanning direction an X axis, and the subscanning direction a Y axis. In the equation, Rx , Ry , K, An , and Bm are arbitrary coefficients. Consequently, optical performance needed for high resolution, specially, a very small spot diameter can easily be obtained even when the fθ lens consists of one element.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to f for scanning optical systems.
Regarding the θ lens.

[0002]

2. Description of the Related Art A scanning optical system A used in a printing unit such as a laser printer, a copying machine, a facsimile or the like generally has a configuration as shown in FIG.

In FIG. 8, reference numeral 1 denotes an LD unit incorporating a semiconductor laser or the like, which emits a beam light 2 which is narrowed down. Reference numeral 3 denotes a first optical system, which focuses the beam light 2 only in the sub-scanning direction (rotational direction of the photosensitive drum). Reference numeral 4 denotes a polygon mirror which, when rotated, distributes the light beam in the main scanning direction. Reference numeral 5 denotes a second optical system including an fθ lens, which converges in the main scanning direction (axial direction of the photosensitive drum) and the sub-scanning direction, and forms an electrostatic latent image on the photosensitive body 6 such as the photosensitive drum by the converged image formation. To form.

Here, in addition to the converging action, the second optical system 5 corrects fθ (correction for matching the rotational angular velocity of the polygon mirror with the scanning speed of the image formation on the photosensitive drum).
Must have the function of. Therefore, usually 2 to 3
Often combined with a single lens.

On the other hand, in order to reduce the number of parts and reduce the size and weight, a single-lens fθ lens is also considered (Japanese Patent Laid-Open No. 5-323223). This fθ lens is

(Equation 2)

It is designed by using the equation expressing the toric surface. Further, in the invention of this publication, a slit-shaped diaphragm extending in the main scanning direction is provided immediately before the fθ lens so that the F value in the sub-scanning direction is constant regardless of the position of the spot.

According to the publication, the spot diameter on the photoconductor is 120 μm or more in the optical performance of the scanning optical system when an fθ lens having a single aspherical lens structure is used.

[0008]

In the fθ lens using the above-mentioned conventional design formula, the aspherical formula corresponds only to the main scanning direction. The sub-scanning direction is a simple toric surface,
The F value is made constant by combining with the slit-shaped diaphragm. However, this conventional configuration has a problem that it is difficult to reduce the performance required for higher resolution, particularly, to reduce the spot diameter.

Therefore, the present invention provides an f.theta. Lens in which a high resolution can be easily achieved without a slit even by devising the aspherical surface type used in the design. With the goal.

[0010]

SUMMARY OF THE INVENTION The present invention is a scanning optical system in which a light beam emitted from a light source is reflected by a rotating polygon mirror to scan and form an image on a photoconductor.

A single-element fθ lens arranged between the polygon mirror and the image forming surface for performing fθ correction has aspherical surfaces in the main scanning direction (X axis) and the sub scanning direction (Y axis).
Further, it is characterized in that a plurality of terms for determining each aspherical surface are designed by an aspherical surface formula defined by eight or more terms including an odd term and an even term.

This aspherical expression is specifically as follows.

When the intersection of the fθ lens and the optical axis is the origin, the optical axis direction is the Z axis, the main scanning direction is the X axis, and the sub-scanning direction is the Y axis (in FIG. 3, the X, Y, and Z coordinate axes are shown). Shows the relationship between the lens shape and).

(Equation 3) _{However, R x, R y, K} , A n, B m represents an arbitrary coefficient.

In the above scanning optical system, as shown in FIG. 1, when the light beam emitted from the light source is incident on the polygon mirror at an angle in the main scanning direction with respect to the optical axis of the fθ lens, the photosensitive member is passed through the polygon mirror. The left and right shapes of the fθ lens seen in the scanning direction can be made asymmetric by designing the aspherical expressions individually and having different coefficients.

[0015]

The above-mentioned aspherical expression is designed so that the main scanning direction and the sub-scanning direction can be made aspherical surfaces at the same time, and that waviness is generated in the lens shape, and the number of coefficient terms that produce a desired shape by the combination thereof is designed. With a high degree of freedom, even with a single-element fθ lens, the optical performance required for high resolution,
In particular, miniaturization of the spot diameter can be easily achieved.

When the beam light is incident on the polygon mirror from the direction intersecting the optical axis of the fθ lens, the fθ lens is left-right asymmetrical and aspherical expressions are separately set to reflect the polygon mirror. It is possible to appropriately remove the shift of the image formation point due to the change of the position depending on the reflection angle.

[0017]

FIG. 1 shows the overall construction of a scanning optical system which is an embodiment of the present invention. In this embodiment, an fθ lens designed by using the aspherical expression of the present invention is incorporated.

In FIG. 1, reference numeral 7 denotes an LD unit which incorporates a light source and produces a light beam that is narrowed down. Reference numeral 8 denotes a first optical system which converges the beam light mainly in the sub-scanning direction (rotational direction of the photosensitive drum), and also in the main scanning direction with a smaller convergence rate. Reference numeral 9 denotes a polygon mirror as an optical polarizer, 10 denotes a second optical system including an fθ lens, and 11 denotes a photoconductor such as a printing drum. FIG. 2 is rewritten to clarify the part to which various values of the design example are applied and to show FIG.

As shown in FIG. 2, the LD unit 7 is composed of a semiconductor laser 12, a collimator lens 13 that brings the laser light close to parallel light (including a case where the light is completely parallel light), and a diaphragm 14. The first optical system 8 includes a toroidal lens that converges the light beam in the main scanning direction and the sub scanning direction (the rotation direction of the photosensitive drum).

The polygon mirror 9 has a reflecting surface formed in a polygonal shape, and when it is rotated, it deflects the incident beam light so as to distribute it to the main scanning direction (axial direction of the photosensitive drum).

The second optical system 10 is an f.theta. Lens designed by using the aspherical expression of the present invention. It converges in both the main scanning direction and the sub scanning direction and, at the same time, performs f.theta. Correction and convergence. The light beam is scanned and imaged on the photoconductor 11.

Next, two specific design examples of each part of the above-mentioned structure will be given, and their optical performances will be described respectively.

[Design Example 1] The first optical system 8 formed of a toroidal lens and the second optical system 10 formed of an fθ lens are
Each of them is composed of one sheet, and the f of the fθ characteristic is f
Is 160 mm, and the scanning width L on the photoconductor 6 is ± 110 mm.

The LD unit 7 uses a semiconductor laser 12 having a wavelength of 780 nm (refractive index of each optical system is 1.51863), which is converged by using a collimator lens 13 of f = 8 mm. , Main scanning direction is 3.6
The truncate ratio (LD waveform) is 0.44 in the main scanning direction and 0.844 in the sub scanning direction by using an elliptical diaphragm 14 having a size of mm and a sub scanning direction of 2.2 mm.

The polygon mirror 9 has an inscribed circle with a diameter of φ3.
At 4.6, the outer peripheral reflecting surface is a hexahedron.

The toroidal lens which is the first optical system 8 is
In FIG. 2, the entrance surface R ₁ and the exit surface R ₂ have a toroidal shape, and the shape is defined by the following radius of curvature R and aspherical surface coefficient A _n .

R ₁ (TR) X direction R = −78.41536 Y direction 1 / R = 0.15778977522210E-01 R ₂ (TR) X direction R = −59.559976 Y direction 1 / R = −0.1613664358233E _{-01 A 4 = 0.6883591488656E-03 A} 6 = -0.1995713227015E-03 A 8 = -0.1211908173700E-03 A 10 = 0.1078233022719E-03 K = 0

Of the above numerical values, the X direction is shown in FIG.
Means the sub-scanning direction, and is the same as the notation in the figure for the second optical system 10, but when viewed from the first optical system 8, the direction orthogonal to the optical axis parallel to the paper surface. Becomes

The shape of the exit surface R _{2 in} the Y direction is obtained by applying the above numerical values to the following equation.

(Equation 4)

The fθ lens, which is the second optical system 10, has the right and left portions from the polygon mirror 9 toward the photosensitive member 11,
Designed separately according to the aspherical formula of the present invention (each coefficient of the aspherical formula is shown in the following [Table 1] and [Table 2]), it is left-right asymmetric. This is because the LD unit 7 is arranged laterally and the positional deviation of the image forming point on the photoconductor 11 due to the movement of the reflection point on the polygon mirror 9 is absorbed and corrected.

In the following table, R ₁ indicates the entrance side and R ₂ indicates the exit side. When the photoreceptor 11 is viewed from the polygon mirror 9, the minus side means the left side and the plus side means the right side (X, Y, (See FIGS. 2 and 3 for the correspondence relationship between the Z axis and each surface of the fθ lens).

[Table 1]

[Table 2]

The optical performance obtained by the scanning optical system of the design example 1 is as follows.

As shown in FIG. 4, the spot diameter of the image formed on the photosensitive member remains constant at 60 μm in the main scanning direction and becomes 60 to 70 μm in the sub scanning direction, and the variation thereof is ± 5.
μm or less.

Here, the spot diameter is a range in which the amount of energy irradiation is reduced to 1 / e ² ≈13.5% when the maximum energy portion is 100%.

As described above, since the spot diameter can be made considerably smaller than 120 μm of the prior art described above, high resolution printing of, for example, 600 dpi becomes possible.

Further, the scanning width L = −110.04 mm to +
As shown in FIG. 5, the linearity at 110.03 mm is within 0.129 mm, and it can be seen that high accuracy can be obtained with the single-lens fθ lens.

[Design Example 2] This is the same as the design example 1 except that
The polygon mirror 9 is changed to a tetrahedron whose inscribed circle has a diameter of φ12 mm, and the arrangement of the second optical system 10 is adapted to this. Other elements are common to the first design example.

The optical performance of the scanning optical system of design example 2 is as follows. As shown in FIG. 6, the spot diameter of the image formed on the photoconductor is 60 to 65 μm in the main scanning direction and 60 to 70 μm in the sub scanning direction, and the variation is ± 5 μm.
It is as follows. Further, the scanning width L = −108.31 mm to +
The linearity at 107.77 mm is within 0.31 mm as shown in FIG.

Therefore, similarly to the design example 1, the design example 2 has a spot diameter of 120 μm of the prior art described above.
Since it can be made smaller than the above, high-resolution printing of, for example, 600 dpi is possible.

[0040]

According to the present invention, since the fθ lens designed by using the aspherical expression having a high degree of design freedom is incorporated in the scanning optical system, the spot diameter required for high resolution can be obtained with a single fθ lens. And the linearity can be improved.

Further, by making the shape asymmetrical to the left and right, it is possible to eliminate the displacement of the image forming position due to the movement of the reflection point when the beam light is irradiated from the light source arranged laterally to the polygon mirror.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of a scanning optical system of the present invention.

FIG. 2 is a diagram in which FIG. 1 is rewritten to clarify the dimensions and the like of each part.

FIG. 3 is a diagram showing a relationship between an aspherical coordinate axis used in the present invention and a lens shape.

FIG. 4 is a diagram showing a spot diameter obtained in design example 1 of the present invention in a main scanning direction and a sub scanning direction.

FIG. 5 is a linearity characteristic diagram actually measured in design example 1 of the present invention.

FIG. 6 is a diagram showing a spot diameter obtained in a design example 2 of the present invention in a main scanning direction and a sub scanning direction.

FIG. 7 is a linearity characteristic diagram actually measured in design example 2 of the present invention.

FIG. 8 is a diagram showing a general configuration of a scanning optical system.

[Explanation of symbols]

 7 LD Unit 8 First Optical System 9 Polygon Mirror 10 Second Optical System (fθ Lens) 11 Photosensitive Member

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04N 1/113 H04N 1/04 104A

Claims (2)

[Claims] 1. A scanning optical system in which a light beam emitted from a light source is reflected by a rotating polygon mirror to scan / image on a photoconductor, and the beam is converged by being arranged between the polygon mirror and the photoconductor. And the fθ lens that performs fθ correction, with the intersection of the fθ lens and the optical axis as the origin, the optical axis direction as the Z axis,
When the main scanning direction is the X axis and the sub scanning direction is the Y axis, _{However, R x, R y, K} , A n, B m represents an arbitrary coefficient. A scanning optical system characterized in that it is designed with a single element according to the aspherical expression. 2. When the light beam emitted from the light source is incident on the polygon mirror at an angle in the main scanning direction with respect to the optical axis of the fθ lens, the left and right sides of the fθ lens in the scanning direction viewed from the polygon mirror are seen. 2. The scanning optical system according to claim 1, wherein the shapes are separately designed by applying the aspherical expressions individually and having different coefficients to make them bilaterally asymmetric.