JP3287111B2 - Scanning optical system - 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 system , and more particularly to a scanning optical system used for a print head of a laser printer.

[0002]

2. Description of the Related Art Hitherto, various types of scanning lenses comprising a single fθ lens for forming an image of a light beam deflected by a deflector on a surface to be scanned have been proposed (see, for example, Japanese Patent Application Laid-Open No. HEI 3-3-1). 213812, JP-A-4-50908, JP-A-62-139520, JP-A-63-157122
issue). In an apparatus such as a laser printer using these scanning lenses, the light is emitted from a light source, converted into a parallel light beam or a convergent light beam by an optical system such as a collimator lens, and deflected at a constant angular velocity by a deflector such as a polygon mirror. The luminous flux is
An image is formed on the surface to be scanned by the scanning lens, and scanning is performed at a substantially constant speed.

[0003]

However, according to the above conventional example, it is difficult to reduce the size of the entire apparatus while maintaining the optical performance. If the scanning optical path is shortened by widening the angle of view of the scanning lens, that is, the deflection angle, the entire apparatus can be reduced in size.However, if an attempt is made to widen the deflection angle while maintaining optical performance, As the distance from the deflection point to the image plane and the length of the lens in the main scanning direction increase, the size of the entire apparatus increases.

For example, in JP-A-3-213812,
Since the distance from the deflector to the scanning lens is more than 60 mm, the lens length must be increased to widen the deflection angle, and conversely, if the lens length is reduced, the range of the deflection angle is increased. You have to make it narrow. Therefore, there is a problem that the deflection angle cannot be widened unless the distance from the deflection surface to the image surface is increased.

In Japanese Patent Application Laid-Open No. 4-50908, although the distance from the deflecting point to the scanning lens is as small as 15 mm, it has been attempted to increase the deflection angle by bringing the scanning lens closer to the deflector. It does not directly contribute to shortening of the lens length in the direction. This is because the curvature of the first surface in the vicinity of the optical axis is convex toward the deflection point, so that the light flux incident position on the lens surface when the deflection angle is large is smaller than the lens surface position on the optical axis. Because it is on the surface side. Furthermore, since the aspheric surface of the scanning lens in the main scanning direction has a surface shape having two or more inflection points, there is a problem that it is difficult to process, measure, and manage a mold.

JP-A-62-139520, JP-A-63-139520
In -157122, since the light beam incident on the deflector is a parallel light beam, the design parameter of the convergence position of the incident light beam is lacking. According to the scanning lens using the parallel light beam, the aberration is deteriorated when the deflection angle is large. Therefore, the deflection angle cannot be increased, which leads to an increase in the size of the entire apparatus. However, Japanese Patent Application Laid-Open No. 62-139520
No. is designed to ignore coma. Also,
In Japanese Patent Application Laid-Open No. 63-157122, although the distance from the deflector to the scanning lens is as small as about 10 mm, the center thickness of the lens is very large, 30 mm to 60 mm, and the deflection angle is further increased. Despite the distance, the distance from the scanning lens to the image plane is 170 mm
For the reasons described above, the fact that the deflection angle is substantially increased does not contribute to downsizing of the apparatus.

SUMMARY OF THE INVENTION The present invention has been made in view of these points, and has been made to reduce the distance from the deflecting point to the image plane, the lens length in the main scanning direction, and the wide angle of deflecting angle while maintaining optical performance. It is an object of the present invention to provide a scanning optical system in which the image formation is achieved.

[0008]

In order to achieve the above object, a scanning optical system according to the present invention forms an image of a convergent light beam from a deflecting point on a surface to be scanned by a scanning lens comprising one fθ lens . A scanning optical system , wherein a first
Both the surface and the second surface are aspherical surfaces, and the following conditional expression (1)
~ (7) is satisfied. The material of the scanning lens is preferably a resin that is advantageous in terms of cost and mass production.

R ₁ <0 (1) r ₁ / r ₂ > 0 (2) _aymax <a ′ (3) 0 <a / G <0.15 (4) 0 <t / G <0.25 (5) 1.1 <L / G <1.5 (6) | f | / G <1.5 (7)

Here, r ₁ : paraxial radius of curvature of the first surface r ₂ : paraxial radius of curvature of the second surface a _ymax : light flux passing from the deflection point position to the upper limit position of the lens effective height passes through the first surface. the position to the passing distance measured parallel to the optical axis a ': extending from the deflection point position, the light flux passing through the upper limit position of the lens effective height from the axis of the first surface with the paraxial curvature radius r ₁ G: distance from deflection point to image plane a: distance from deflection point to first surface t: lens center thickness (lens core thickness) ) L: scanning width (width of image plane) f: focal length

FIG. 1 shows positions, distances and the like corresponding to the symbols in the above conditional expression, and FIG. 2 shows a schematic configuration of an entire scanning optical system using the scanning lens of the present invention. The symbols in FIG. 1 are defined below.

H: Front principal point position of the scanning lens H ': Rear principal point position of the scanning lens OS: Natural convergence point position of the light beam incident on the scanning lens
(That is, the position of the convergence point on the optical axis of the convergent light beam when there is no scanning lens.) DS: Deflection point position IS: Image plane (scanned plane) position C: Distance from deflection point position DS to front principal point position H S: Distance from front principal point position H to natural convergence point position OS S1: Distance from deflection point position DS to natural convergence point position OS y _max : Effective lens height

As shown in FIG. 2, a light source {for example, LD (lase
r diode), LED (light emitting diode), etc.} 1 is adjusted by the condenser lens 2 so as to converge to the natural convergence point position OS (FIG. 1). After passing through the cylindrical lens 3, the convergent light beam is reflected by the deflector 4 (4 a is a deflection point), and is scanned by the scanning lens 5.
Incident on. Note that the cylindrical lens 3 has power only in the sub-scanning direction for correcting surface tilt of the deflector 4.

At the position IS (FIG. 1) of the surface to be scanned, the scanning lens 5 forms a beam spot having excellent image surface properties and linearity and in which coma has been well corrected. Although the deflection point 4a slightly moves in the main scanning plane due to the operation of the deflector 4, the radius S1 about the deflection point 4a is substantially the center.
Scanning is performed on the image plane (scanned surface) 6 so as to draw the arc of FIG.

The beam incident on the deflector 4 is a convergent light beam in the main scanning direction. Thereby, the deflection angle by the deflector 4 (that is, the angle of view between the deflected light beam and the optical axis AX)
When θ is large, the amount of aberration on the image plane 6 becomes better than when parallel light flux is incident. Therefore, the deflection angle θ can be increased, and the entire device can be reduced. JP-A-62-139520, JP-A-63-15
In the conventional example of No. 7122, since the light beam incident on the deflector is a parallel light beam, there is no design parameter for the convergence position of the incident light beam. Therefore, such an effect cannot be obtained.

The optical performance required for the single-lens scanning lens 5 disposed between the deflection point 4a and the image plane 6 is linearity and image plane property in the main scanning plane. The linearity refers to the ability to scan a light beam deflected at a constant angular speed by the deflector 4 on the image plane 6 at a constant speed. In order to improve the linearity, it is necessary to reduce the values represented by the following equation 1 for the image height y and the deflection angle θ. The image plane property refers to the ability to always converge the deflected light beam on the image plane 6 within the effective deflection angle.
In order to obtain good optical performance, not only the paraxial performance of the light beam but also the coma aberration must be suppressed.

[0017]

(Equation 1)

Even if the incident light beam is made a convergent light beam in order to improve the above-mentioned linearity and image plane property, the degree of freedom in design is small. Therefore, if the first surface or both surfaces of the scanning lens 5 are not made aspherical, the distance G Is difficult to reduce.
Moreover, if the deflection angle θ is to be increased in order to reduce the distance G, it is an essential condition that both surfaces of the scanning lens 5 be aspherical as in the present invention.

Conditional expressions (1) and (2) indicate that the scanning lens 5 is a meniscus lens concave on the deflection point 4a side near the optical axis on both the first surface and the second surface. Also, the conditional expression
(3) is the position where the light beam passing through the upper limit position of the lens effective height y _max passes through the first surface, and the light beam passing through the upper limit position of the lens effective height is the paraxial radius of curvature r from the axis of the first surface. _With respect to the position passing through the spherical surface extended by _1, it indicates that the former position is closer to the deflection point 4a side than the latter position.

By satisfying conditional expressions (1) to (3), the first surface and the second surface (the paraxial radii of curvature are r ₁ and r ₂ ) of the scanning lens 5 are both located around the periphery. An aspherical shape in which the curvature becomes steeper as it goes. Accordingly, when the deflection angle θ is larger than in the vicinity of the optical axis, the first surface is located on the deflector 4 side. It does not grow. In addition, when the scanning lens has a meniscus shape, uneven thickness is reduced, and thus there is also an effect that moldability is improved.

When the conditional expressions (1) to (3) are out of the range, the fact that the scanning lens 5 is moved closer to the deflection point 4a does not contribute to shortening of the lens length in the main scanning direction at all. Further, if the shape is out of the meniscus shape, the uneven thickness is increased, and the moldability is deteriorated. In order to improve image quality and linearity, the first surface and the second surface must satisfy conditional expressions (1) to (3).
And it is preferable that they have the same shape.
In that case, the scanning lens 5 will have a shape that results in less uneven thickness.

Conditional expression (4) shows the relationship between the distance a and the distance G from the deflection point 4a to the first surface. When the distance a is reduced, the scanning lens 5 can be reduced in the main scanning direction, so that it is not necessary to increase the size of the scanning lens 5 even if the deflection angle is widened. When out of the range of conditional expression (4),
The scanning lens 5 approaches the image plane 6 side, and as a result, the scanning lens 5 becomes large in the main scanning direction.

Conditional expression (5) shows the relationship between the center thickness t of the scanning lens 5 and the distance G. If the value falls outside the range of the conditional expression (5), the lens center thickness t becomes abnormally large. An increase in the lens center thickness t results in deterioration of moldability and an increase in the size of the entire apparatus.

Conditional expression (6) shows the relationship between the scanning width L and the distance G. If the lower limit of conditional expression (6) is exceeded, the size of the apparatus will be large. If the upper limit of conditional expression (6) is exceeded, performance (linearity, image surface properties) cannot be satisfied with the lens of this embodiment.

Conditional expression (7) shows the relationship between the focal length f and the distance G. In a configuration that satisfies conditional expression (7),
Further, it is preferable that the following conditional expression (7 ′) is satisfied. 1.2 <| f | / G <1.5 ... (7 ')

When the value exceeds the upper limit of the conditional expression (7 ') (that is, when the conditional expression (7) is not satisfied), the linearity increases in the minus direction at the center of the angle of view and in the plus direction at the end face of the angle of view,
fθ performance cannot be obtained. If the lower limit of the conditional expression (7 ') is exceeded, the linearity and the image plane property are kept good, but the distance G from the deflecting point 4a to the image plane 6 becomes large, so that the apparatus becomes large. Further, since the distance a from the deflecting point 4a to the first surface of the scanning lens 5 becomes small, there is also a problem that a polarizer such as a polygon mirror in which the rotation center axis and the deflecting surface are separated cannot be used. appear. In addition, the coma aberration increases, but F _NO =
If it is about 70, there is no particular problem. Examples 1 and 2 described below are examples satisfying conditional expression (7 ′).
Is | f | on the assumption that a galvanomirror or the like is used.
/G=0.87.

By satisfying conditional expressions (4) to (7) as described above, the distance a is made very small, the distance G is made small, and the size of the scanning lens 5 in the main scanning direction is made small. It is possible to realize good correction of the image surface property and the linearity with a single lens while keeping the above-mentioned conditions possible. Also, the distance G
, The size of the entire device (for example, a print head of a laser printer) using the scanning lens 5 can be reduced in size.

In the present invention, it is preferable to further satisfy the following conditional expression (8). Conditional expression (8) shows the relationship between the length (2 · y _max ) of the scanning lens 5 in the main scanning direction and the scanning width L. If the upper limit of conditional expression (8) is exceeded, the scanning lens 5
Becomes large. 0 <2 · y _max /L<0.4 (8)

In the sub-scanning direction, the light is condensed on the image plane 6 by continuously changing the curvature in one or both of the first and second surfaces in the y-direction. It is possible. Therefore, as shown in FIG. 3, a surface in which the curvature in the main scanning direction is an aspheric surface and the curvature in the sub-scanning direction continuously changes with respect to the displacement in the y direction (herein referred to as a deformed toric surface). ) Is preferably used for the first surface and the second surface. This deformed toric surface is defined by the following equation.

X = vy ² / {1+ (1-μv ² y ² ) ^1/2 } + ρ
+ A where v = V / (1−Vρ) ρ = Cz ² / {1+ (1−C ² z ² ) ^1/2 } A = a ₂₂ y ² z ² + a ₄₂ y ² z ² + a ₆₂ y ² z ² + a ₈₂ y ² z ²
+ ... + a ₀₄ y ² + a ₀₆ y ² + a ₀₈ y ² + ... x: optical axis direction y: main scanning direction z: sub-scanning direction 1 / C: radius of curvature of profile curve C _p (surface vertex of main curve C _m 1 / V: radius of curvature of the surface vertex of the main curve C _m a _i2 : coefficient representing the change in the radius of curvature in the y direction (angle of view) in the sub scanning direction a _0j : y direction of the main curve A coefficient μ representing a change in the radius of curvature of (angle of view): a quadratic curve parameter in the main curve direction.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the scanning lens according to the present invention will be described below. Here, n is the refractive index of the scanning lens with respect to light having a wavelength of 780 nm, and 2ω is the maximum angle of view with respect to the scanning lens (that is, the maximum deflection angle × 2).

[0032] <Example 1> a = 18.08mm t = 28.67mm b = 108mm S1 = 241.52mm f = 199.01mm c = 44.04mm G = 154.75mm a ymax = 15.75mm y max = 36mm L = 221mm n = 1.5722 2ω = 103.2 ゜

[First surface] r ₁ = -233.9 mm μ = -46.819 a ₀₄ = -0.72053 × 10 ^-5 a ₀₆ = 0.12274 × 10 ^-8 a ₀₈ = 0.41432 × 10 ^-12 a ₀₁₀ = 0.93289 × 10 ^{− 16} a ₀₁₂ = 0.34668 × 10 ^-18 a ₀₁₄ = 0.10868 × 10 ^-22

[Second surface] r ₂ = −80.007 mm μ = −7.5331 a ₀₄ = −0.37404 × 10 ⁻⁵ a ₀₆ = 0.41983 × 10 ⁻⁹ a ₀₈ = −0.55201 × 10 ⁻¹² a ₀₁₀ = −0.3793 × 10 ^-16 a ₀₁₂ = 0.13604 × 10 ^-18 a ₀₁₄ = -0.8481 × 10 ^-22

[0035] <Example 2> a = 19.9mm t = 17.74mm b = 108mm S1 = 241.52mm f = 218.54mm c = 32.2mm G = 154.64mm a ymax = 16.8mm y max = 31mm L = 218mm n = 1.5722 2ω = 100 ゜

[First surface] r ₁ = −1300.7 mm μ = 55.853 a ₀₄ = −0.11714 × 10 ⁻⁴ a ₀₆ = −0.23242 × 10 ⁻⁸ a ₀₈ = 0.24488 × 10 ⁻¹² a ₀₁₀ = −0.10597 × 10 ^-15 a ₀₁₂ = -0.14867 × 10 ^-16 a ₀₁₄ = 0.93616 × 10 ^-22

[Second surface] r ₂ = −114.65 mm μ = 12.53 a ₀₄ = −0.403 × 10 ⁻⁵ a ₀₆ = −0.54552 × 10 ⁻¹⁰ a ₀₈ = −0.17216 × 10 ⁻¹¹ a ₀₁₀ = −0.3793 × 10 ^-5 a ₀₁₂ = 0.20358 × 10 ^-17 a ₀₁₄ = -0.19845 × 10 ^-20

[0038] <Example 3> a = 5.523mm t = 42.67mm b = 148mm S1 = 241.52mm f = 170.51mm c = 71.26mm G = 196.20mm a ymax = 4.90mm y max = 29mm L = 217.5mm n = 1.57222 2ω = 104 ゜

[First surface] r ₁ = −42.17 mm μ = -10.491 a ₀₄ = −0.23381 × 10 ⁻⁴ a ₀₆ = 0.96019 × 10 ⁻⁹ a ₀₈ = 0.50438 × 10 ⁻¹² a ₀₁₀ = 0.92630 × 10 ^{− 16} a ₀₁₂ = -0.38919 × 10 ^-16 a ₀₁₄ = 0.10917 × 10 ^-22

[Second surface] r ₂ = -40.286 mm μ = -0.71817 a ₀₄ = -0.37401 × 10 ^-5 a ₀₆ = -0.46958 × 10 ^-9 a ₀₈ = -0.88085 × 10 ^-12 a ₀₁₀ = -0.37930 × 10 ^-16 a ₀₁₂ = 0.38461 × 10 ^-18 a ₀₁₄ = -0.65222 × 10 ^-21

FIGS. 4, 8 and 12 show Embodiments 1 to 3 in cross section in the main scanning direction. In each of the above embodiments, since the material is a resin, it can be mass-produced at low cost and has one inflection point.
Processing is easier than in the conventional example of JP-A-4-50908.

FIGS. 5 to 7, FIGS. 9 to 11, and FIGS.
FIG. 5 shows aberration diagrams of the first to third embodiments. Of which
FIGS. 5, 9 and 13 show Examples 1 to 3, respectively.
3 shows the main scanning field curvature corresponding to the main scanning direction. 6 and 10
14 shows distortions corresponding to Examples 1 to 3, respectively. 7, 11, and 15 show lateral aberration curves (lateral aberrations on a Gaussian surface with a meridional ray) corresponding to Examples 1 to 3, respectively. K attached to the lateral aberration diagram indicates the amount of aberration on the image plane of a light beam having a certain deflection angle θ represented by the formula: K = S1 × sin θ.

[0043]

As described above, according to the present invention, a convergent light beam from a deflection point is scanned by a scanning lens comprising a single fθ lens.
A scanning optical system for imaging the surface to be scanned on the lens, before
Both the first surface and the second surface of the scanning lens are aspherical surfaces, and are configured to satisfy the above conditional expressions (1) to (7) and (7 ′). The distance from the deflection point to the image plane and the length of the lens in the main scanning direction can be reduced, and the deflection angle can be widened.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an optical path and performance of a scanning lens according to the present invention.

FIG. 2 is a diagram showing a schematic configuration and an optical path of a scanning optical system using the scanning lens according to the present invention.

FIG. 3 is a diagram for explaining a deformed toric surface used for the scanning lens according to the present invention.

FIG. 4 is a sectional view of a lens according to a first embodiment of the present invention.

FIG. 5 is an aberration diagram of main scanning field curvature in the first embodiment of the present invention.

FIG. 6 is a distortion diagram of the first embodiment of the present invention.

FIG. 7 is a lateral aberration diagram of the first embodiment of the present invention.

FIG. 8 is a sectional view of a lens according to a second embodiment of the present invention.

FIG. 9 is an aberration diagram of main-scanning field curvature according to the second embodiment of the present invention.

FIG. 10 is a distortion diagram of the second embodiment of the present invention.

FIG. 11 is a lateral aberration diagram according to the second embodiment of the present invention.

FIG. 12 is a sectional view of a lens according to a third embodiment of the present invention.

FIG. 13 is an aberration diagram of main-scanning field curvature according to the third embodiment of the present invention.

FIG. 14 is a distortion diagram of the third embodiment of the present invention.

FIG. 15 is a lateral aberration diagram according to the third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Condensing lens 3 ... Cylindrical lens 4 ... Deflector (polygon mirror) 5 ... Scanning lens 6 ... Image plane (scanned surface) DS ... Deflection point position OS ... Natural convergence point position IS ... Image plane ( (Scanned surface) position

Claims (1)

(57) [Claims] 1. An fθ lens for converging a luminous flux from a deflection point
Optics that forms an image on the surface to be scanned with a scanning lens composed of
A system, a first and second surfaces are both aspherical the scanning lens, the scanning optical system, characterized by further satisfying the following _{condition; r 1 <0 r 1 /} r 2> 0 a _ymax <a ′ 0 <a / G <0.15 0 <t / G <0.25 1.1 <L / G <1.5 1.2 < | f | / G <1.5 where r ₁ : paraxial radius of curvature of the first surface r ₂ : _Paraxial radius of curvature of the second surface _aymax : distance measured parallel to the optical axis from the deflection point position to the position where the light beam passing through the upper limit position of the effective lens height passes through the first surface a ' : from deflection point position, to the position where the light flux passing through the upper limit position of the lens effective height passes extended spherical paraxial curvature radius r ₁ from the axis of the first surface, measured parallel to the optical axis Distance G: Distance from deflection point to image plane a: Distance from deflection point to first surface t: Lens center thickness L: Scan width f: Focal length