JP3320239B2 - Scanning optical device - Google Patents

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 more particularly to a scanning optical device which deflects a light beam emitted from a light source means after being modulated by an optical deflector (optical deflecting element) comprising a rotary polygon mirror or the like.
For example, a laser beam printer (LBP) having an electrophotographic process or digital copying, in which image information is recorded by optically scanning the surface to be scanned through an imaging element including at least two mirrors having fθ characteristics. The present invention relates to a scanning optical device suitable for a device such as a machine.

[0002]

2. Description of the Related Art Conventionally, in a scanning optical device such as a laser beam printer, a light beam which is light-modulated from a light source means in accordance with an image signal and emitted is periodically transmitted by an optical deflector comprising a rotating polygon mirror (polygon mirror). Deflect,
The light is focused in the form of a spot on a photosensitive recording medium (photosensitive drum) surface by an imaging optical system having fθ characteristics, and the surface is optically scanned to record an image.

FIG. 5 is a schematic view of a main part of a conventional scanning optical device.

In FIG. 1, a divergent light beam emitted from a light source means 51 is converted into a substantially parallel light beam by a collimator lens 52, and the light beam (light amount) is restricted by a stop 53, and the cylindrical light beam has a predetermined refractive power only in the sub-scanning direction. The light is incident on the lens 54. Of the parallel light beams incident on the cylindrical lens 54, they are emitted as parallel light beams as they are in the main scanning section. In the sub-scanning section, the light is converged and formed as a substantially linear image on the deflection surface (reflection surface) 55a of the optical deflector 55 composed of a rotating polygon mirror (polygon mirror).

The light beam deflected and reflected by the deflecting surface 55a of the optical deflector 55 is guided through an imaging optical system (fθ lens) 56 having fθ characteristics onto a surface of a photosensitive drum 58 as a surface to be scanned. By rotating the optical deflector 55 in the direction of arrow A, the surface of the photosensitive drum 58 is optically scanned in the direction of arrow B to record image information.

[0006]

In recent years, an imaging lens (f) used as an imaging element in such a scanning optical device has been proposed.
For the θ lens), a lens (plastic lens) made of a plastic material is often used. The plastic lens has features such as improvement of the optical performance of the scanning optical device by making the surface shape aspherical, and low cost and good productivity.

[0007] On the other hand, however, there is a problem that environmental characteristics are inferior to lenses made of glass material (glass lenses), and that a change in refractive index with respect to temperature change is particularly large.

For this reason, in a conventional scanning optical device using a plastic lens, the imaging lens (fθ lens) undergoes a change in refractive power due to temperature rise due to heat generated from the periphery of the device, and as a result, the image plane shifts. And there is a problem that image field curvature and the like occur. In particular, in a scanning optical device having a high imaging magnification in the sub-scanning direction as in the above-described conventional example, the image plane movement (defocus) in the sub-scanning direction becomes large. For example, when the refractive power of the fθ lens decreases due to the temperature rise, the imaging point of the fθ lens in the sub-scanning direction is moved to the rear of the scanned surface, so that the spot shape on the scanned surface deteriorates, As a result, it was difficult to obtain high quality printing.

Further, as shown in FIG. 6, in the conventional scanning optical apparatus in which an imaging optical system 70 is constituted by combining one fθ lens 66 made of a plastic material and a long cylindrical mirror 67, the above-described sub-scanning is performed. Image plane movement (defocus) due to environmental changes in the direction is reduced, but on the other hand, there are problems such as bending of the scanning line by the long cylindrical mirror 67 and rotation of the spot on the surface to be scanned. In FIG. 6, the same elements as those shown in FIG. 5 are denoted by the same reference numerals.

For this reason, in the conventional scanning optical apparatus, for example, a method of correcting the bending of the scanning line by rotating or eccentrically rotating the fθ lens with respect to the optical axis or using parallel eccentricity has been used. It is very difficult to correct the image at the same time, and this has been a serious problem in performing highly accurate image formation.

According to the present invention, when the light beam from the deflection element is imaged on the surface to be scanned by the second optical element having the fθ characteristic, the second optical element is constituted by at least two mirrors, By properly setting the shape and arrangement position of the mirror, the influence of changes in refractive power due to environmental fluctuations can be eliminated, and the bending of the scanning line, which has been a problem when using a mirror as an element of an imaging element in the past, It is an object of the present invention to provide a scanning optical device capable of satisfactorily correcting spot rotation on a scanning surface or the like and obtaining high-quality printing.

[0012]

According to a first aspect of the present invention, there is provided a scanning optical device which guides a light beam emitted from a light source means onto a deflection element surface via a first optical element, and is deflected by the deflection element. In a scanning optical device that scans the surface to be scanned by forming a focused light beam on the surface to be scanned through the second optical element in a spot shape, the second optical element includes first and second mirrors. ,
An optical path from the deflecting element to the first mirror and an optical path from the second mirror to the surface to be scanned are arranged so as to intersect with each other, and are incident on the optical axis of the first mirror in the sub-scanning surface. The angle between the light beam and the incident light beam is θ ₁ , and the angle between the optical axis of the second mirror and the incident light beam in the sub-scanning plane is θ ₂ .
When the displacement of the most off-axis light flux of the first mirror from the plane at the deflection point is L ₁ , and the displacement of the most off-axis light flux of the second mirror from the plane at the deflection point is L ₂ (where θ ₁ ,
The signs of θ ₂ , L ₁ , and L ₂ are all positive. | 2 (L ₁ sin θ ₁ −L ₂ sin θ ₂ ) | <0.5 (1) . According to a second aspect of the present invention, in the first aspect, the first and second mirrors both have a positive refractive power in the main scanning plane.

According to a third aspect of the present invention, there is provided a laser beam printer including the scanning optical device according to the first or second aspect and a photosensitive drum on the surface to be scanned.

[0014]

FIG. 1 is a schematic view of a main part of a first embodiment of the present invention, and FIG.
3A and 3B are main-portion cross-sectional views of a main scanning direction and a sub-scanning direction of Embodiment 1 of the present invention, respectively.

In FIG. 1, reference numeral 1 denotes light source means, which is made of, for example, a semiconductor laser.

Reference numeral 2 denotes a collimator lens as a first optical element, which converts a divergent light beam (light beam) emitted from the light source means 1 into a parallel light beam.

Reference numeral 3 denotes a stop, which limits a light flux (light quantity).

4 is a cylindrical lens (cylinder)
And has a predetermined refractive power only in the sub-scanning direction,
The light beam that has passed through the stop 3 is formed as a substantially linear image on a deflecting surface 5a of an optical deflector 5 described later in the sub-scanning cross section.

Reference numeral 5 denotes an optical deflector as a deflecting element, for example, a polygon mirror (rotating polygon mirror), which is rotated at a constant speed in a direction indicated by an arrow A in the figure by a driving means (not shown) such as a motor.

Reference numeral 10 denotes an image forming optical system (image forming element) having fθ characteristics as a second optical element, which serves as a first mirror having a positive refractive power in a main scanning plane (deflection scanning plane). It has two mirrors, a cylindrical mirror 6 and a toric mirror 7 as a second mirror.
The two mirrors 6 and 7 are respectively angled with respect to the incident light beam in the sub-scanning plane such that an optical path S1 from the optical deflector 5 to the cylindrical mirror 6 and an optical path S2 from the toric mirror 7 to the scanning surface 8 cross each other. θ ₁ , θ ₂
(Θ ₁ > 0, θ ₂ > 0) and are decentered (inclined), and a light beam based on image information deflected and reflected by the optical deflector 5 is transferred onto the surface of the photosensitive drum 8 as a surface to be scanned. Image.

In this embodiment, a divergent light beam emitted from a semiconductor laser 1 is converted into a parallel light beam by a collimator lens 2, and the light beam (light amount) is restricted by a stop 3 before being incident on a 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 converges and forms a substantially linear image (a linear image elongated in the main scanning direction) on the deflecting surface 5a of the optical deflector 5. The light beam deflected and reflected by the deflecting surface 5a of the optical deflector 5 passes through a cylindrical mirror 6 having a refractive power only in the main scanning direction to a toric mirror 7 having different refractive powers in the main scanning direction and the sub-scanning direction. Incident. The shape of these two mirrors 6, 7 is such that the origin is an intersection point with the optical axis, the optical axis direction is the X axis, the axis perpendicular to the optical axis in the main scanning direction is the Y axis, and the optical axis is orthogonal to the sub scanning direction. When the axis is the Z axis, the main scanning direction (general line direction) XY plane

[0022]

(Equation 1) Where R is the radius of curvature on the optical axis, K, B ₄ , B ₆ , B
_8, B ₁₀ are aspherical coefficients subscanning direction (sagittal direction) XZ plane

[0023]

(Equation 2) Where _{r'= (1 + D 2 Y} 2 + D 4 Y 4 + D 6 Y 6 + D
₈ Y ⁸ + D ₁₀ Y ¹⁰ ) where r is the radius of curvature D ₂ , D ₄ , D ₆ , D ₈ , and D ₁₀ on the optical axis can be expressed by the following equation.

In the above equations (a) and (b), the main scanning direction is 1
The aspheric surface can be represented by a function up to the 0th order, and the sub-scanning direction indicates a toric surface having a different curvature depending on the value of Y.

The light beam based on the image information via the two mirrors 6 and 7 having such a shape is guided on the surface of the photosensitive drum 8 and is rotated by rotating the light deflector 5 in the direction of arrow A. Optical scanning is performed in the direction of arrow B on the surface of the photosensitive drum 8. Thereby, image recording is performed.

FIG. 3 is an optical path diagram in the vicinity of the cylindrical mirror 6 as the first mirror, and is a cross-sectional view of a main part in the sub-scanning direction (only the principal ray of light rays is shown).

In FIG. 1, reference numeral 21 denotes an on-axis light beam (light beam deflected parallel to the optical axis), 22 denotes the most off-axis light beam, and 6a and 6a.
Reference numeral 6b denotes a deflecting surface (reflection surface) of the cylindrical mirror 6, respectively.
6a is a deflection surface for the on-axis light beam, and 6b is a deflection surface for the most off-axis light beam.

Since the cylindrical mirror 6 has a positive refractive power in the main scanning direction as described above, the deflection point of the most off-axis light beam is a distance (displacement) L ₁ (mm) from the deflection point of the on-axis light beam. ) Is on the near side (the optical deflector 5 side). The distance between the between the deflection point and the optical axis of the outermost off-axis light beam in the distance L ₁ is a main scanning plane Y
_m (mm)

[0029]

(Equation 3) Where R is the radius of curvature on the optical axis, K, B ₄ , B ₆ , B
_8, B ₁₀ is expressed by the aspherical surface coefficients.

In the present embodiment, as described above, the optical axis (the axis passing through the center of curvature) of the cylindrical mirror 6 in the sub-scanning plane is angled by θ ₁ (deg) (θ ₁ ) with respect to the incident light beam.
> 0), the scanning line after deflection has a curvature of B1. Here the amount of flexure B1 may be represented by B1 = 2L ₁ sinθ _1.

On the other hand, since the toric mirror 7 as the second mirror can be handled almost in the same manner as the cylindrical mirror 6, the amount of bending B2 of the scanning line caused by the toric mirror 7 can be expressed as B2 = 2L ₂ sin θ _2. it can. Here, θ ₂ is the angle (d) between the optical axis of the toric mirror 7 and the incident light beam in the sub-scanning plane.
eg), L ₂ is the displacement (mm) from the plane (the deflection point of the on-axis light beam) at the deflection point of the most off-axis light beam of the toric mirror 7, and is expressed in the same manner as the above equation (c). Can be.

At this time, in the toric mirror 7, the scanning line is also bent by the curvature in the sagittal direction, but since the curvature in the sagittal direction is small, it can be ignored here.

Therefore, the amount of bending B of the scanning line in the entire apparatus is
Is B = B1-B2 = 2 ( L 1 sinθ 1 -L 2 sinθ 2) next, in order to perform recording of small high-precision image information with bending of the scanning line is _{| 2 (L 1 sinθ 1 -L} 2 sinθ ₂ ) | <0.5 ‥‥ (1) (However, the signs of θ ₁ , θ ₂ , L ₁ , L ₂ are all positive.) It is good to set the position.

Therefore, in this embodiment, conditional expressions (1) are satisfied by appropriately setting each element as shown in numerical embodiments described later, which causes a problem when a mirror is used as an imaging element. It is possible to satisfactorily correct the scanning line bending and the rotation of the spot on the surface to be scanned, and obtain high-quality printing.

Conditional expression (1) relates to the shape and arrangement of each of the mirrors 6 and 7, and if conditional expression (1) is not satisfied, bending of the scanning line, rotation of the spot on the surface to be scanned, etc. will be satisfactory. Correction becomes difficult, and high-quality printing cannot be obtained.

Next, numerical examples according to the present invention will be described.

Effective scanning area A3 size equivalent (± 148.5 mm) Focal length f = 213 mm (combined focal length of first and second mirrors in the main scanning direction) Cylindrical lens r1 = 30.789 mm r2 = ∞d = 6 mm Glass material OHARA BSL7 (registered trademark) Optical deflector φ50 6-sided polygon First mirror (cylindrical mirror) Generating line direction R = -1017 K = -1.028 B4 = -1.314E-9 Child direction r = ∞ Second mirror ( Toric mirror) Generating direction R = −613.3 K = −0.989 B4 = 1.871E−9 Child direction r = −181.9 D2 = −2.248E−6 D4 = 2.163E−11 Between polygons 48.3 mm Polygon-Between first mirrors 108.9 mm Between first mirror and second mirror 65.1 mm Second mirror- Inclination of the first mirror 6 to the inclination angle 5 ° embodiment of inclination 20 ° second mirror of 186.9mm first mirror between scan plane shown in (theta ₁₎ the 20 °, the inclination angle of the second mirror 7 (Θ ₂ ) is 5 °
And the optical path from the optical deflector 5 to the first mirror 6 and the optical path from the second mirror 7 to the surface 8 to be scanned intersect. The bending amount B of the scanning line at this time
From L ₁ = 3.879 and L ₂ = 1.974, B = B1−B2 = 2 * (3.879 * sin20 ° −10.974 * sin5 °) = 0.370, which is the above-mentioned conditional expression (1). Are satisfied.

FIG. 4 is a diagram showing various aberrations showing the curvature of field, the curvature of the scanning line, and the distortion in this embodiment. As is apparent from these aberration diagrams, it is understood that each aberration is satisfactorily corrected in the present embodiment.

As described above, in the present embodiment, the light beam from the deflection element 5 is focused on the surface 8 to be scanned 8 as described above.
The optical element 10 is composed of two mirrors 6 and 7, and the refractive power of the main scanning surface (deflection scanning surface) of the two mirrors 6 and 7 is both positive. By arranging the two mirrors 6 and 7 so that the optical path going to the optical path and the optical path going from the second mirror 7 to the surface 8 to be scanned intersect with each other, and satisfying the conditional expression (1), the environmental Eliminates the effects of changes in refractive power and satisfactorily corrects scanning line bending and spot rotation on the scanned surface, which were problems when using mirrors as an element of the imaging element. As a result, high quality printing is obtained.

[0040]

According to the present invention, when the light beam from the deflection element is imaged on the surface to be scanned by the second optical element having the fθ characteristic as described above, the second optical element is moved in the main scanning plane. And at least two mirrors each having a positive refractive power, and the two mirrors are arranged such that an optical path from the deflecting element to the first mirror and an optical path from the second mirror to the surface to be scanned intersect each other. By setting each element appropriately so as to satisfy the conditional expression (1), the image forming apparatus is resistant to environmental fluctuations such as temperature rise and the like, and is capable of forming a high-precision image with less scanning line bending and spot rotation. Can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic view of a main part of a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a main part in a main scanning direction and a sub-scanning direction according to the first embodiment of the present invention.

FIG. 3 is a sectional view of a main part in the sub-scanning direction near a first mirror according to the first embodiment of the present invention.

FIG. 4 is a diagram showing various aberrations of the first embodiment of the present invention.

FIG. 5 is a schematic view of a main part of an optical system of a conventional scanning optical device.

FIG. 6 is a schematic view of a main part of an optical system of a conventional scanning optical device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source means 2 1st optical element (collimator lens) 3 Aperture 4 Cylindrical lens 5 Deflection element (optical deflector) 10 2nd optical element 6 1st mirror 7 2nd mirror 8 Scanning surface (photosensitive drum)

Claims (3)

(57) [Claims] 1. A light beam emitted from a light source means is guided to a deflecting element surface via a first optical element, and a light beam deflected by the deflecting element is projected onto a surface to be scanned via a second optical element. In a scanning optical device that forms an image in a spot shape and scans the surface to be scanned, the second optical element includes first and second mirrors , and an optical path from the deflecting element to the first mirror and the second optical element. An optical path from the second mirror to the surface to be scanned intersects with each other, and an optical axis of the first mirror and an incident light beam in the sub-scanning surface are arranged .
The angle formed is θ ₁ , and the light of the second mirror in the sub-scanning plane is
The angle formed between the axis and the incident light beam is θ ₂ , and the highest axis of the first mirror
The displacement amount of the external light beam from the plane at the deflection point is L ₁ ,
Displacement of the most off-axis light flux of two mirrors from the plane at the deflection point
When the amount was L ₂ (where, theta _1, of θ _2, L _1, L ₂
All codes are _{positive) | 2 (L 1 sinθ 1} -L 2 sinθ 2) | < scanning optical apparatus, characterized by satisfying 0.5 following condition. 2. The scanning optical device according to claim 1, wherein the first and second mirrors both have a positive refractive power in a main scanning plane. 3. The scanning optical device according to claim 1 or 2,
Laser having a photosensitive drum on a surface to be scanned
Beam printer.