JP2000162537A - Scanning optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning optical system constituted so that the bending of a scanning line by plural light beams is suppressed and an accurate scanning action can be executed. SOLUTION: A cylindrical lens 50 is formed so that the incident surface IS1 at the center (incident angle β=0 deg.) is provided with a semi-circular cross section whose radius is (r0) but the radius of the curvature (r) of the incident surface IS2 whose cross section is vertical at the position of the incident angle βis r=r0.(1/cos β)m. Thus, when a focal distance by the incident angle β=0 deg. is defined as (f0), the focal distance (f) at the vertical cross section at the position of the incident angle βsatisfies the expression of f=f0.(1/cos β)m. Due to that, since the effective focal distance at the cross section at the position of the angle β becomes almost uniform in a scanning direction (Y direction), the bending of the scanning line is suppressed and the accurate scanning action can be executed with a multibeam scanning.

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 for scanning a surface to be scanned by irradiating a plurality of light beams in parallel.

[0002]

2. Description of the Related Art A scanning optical system for recording an image by irradiating a surface to be scanned with a light beam and performing a main scan by oscillating the light beam using a deflector has been conventionally known.
Among them, in a scanning optical system using a polygon mirror as a deflector, there is a problem in that the mirror surface is tilted due to blurring of the central axis of the polygon mirror, that is, so-called "surface tilt", and the scanning line fluctuates.

For this purpose, a cylindrical lens is provided between the polygon mirror and the surface to be scanned as a surface tilt correction lens, so that the mirror surface of the polygon mirror and the surface to be scanned are at least perpendicular to the main scanning direction (sub scanning direction). ) Is generally optically conjugated to suppress the effect of surface tilt.

[0004]

FIG. 5 shows the incident angle β of the light beam LB on the cylindrical surface 90a of the cylindrical lens 90.
FIG. As shown in the drawing, in the scanning optical system as described above, when the light beam LB is deflected, the incident angle β on the cylindrical surface 90a of the cylindrical lens 90 also changes. When the incident angle β with respect to the cylindrical surface 90a changes, the action of the cylindrical surface 90a on the light beam LB also changes. When the cylindrical surface 90a having the radius of curvature r0 is cut at an angle of the incident angle β, the cross-sectional shape has a major axis in the direction in which the light beam LB travels, and a minor axis (length 2) in the orthogonal direction (X direction). (R0) can be represented by (half of) an ellipse.

[0005] The length of the major axis of this elliptical cross section varies depending on the incident angle β, and β = 0 °, that is, the length of the major axis is 2 · r0 when the light beam is incident perpendicular to the cylindrical surface 90a.
(True circle), and becomes an ellipse having a long axis as the incident angle β increases. As a result, the effective focal length of the cylindrical lens 90 in the sub-scanning direction changes as the incident angle β changes, so that the imaging magnification of the image plane (scanned surface) with respect to the mirror surface of the polygon mirror is also changed to the incident angle β. Will change in response to this.

When the light beam is one on the optical axis, the object height is "0" and the image height is also "0", so that the scanning is performed on the generatrix of the cylindrical lens 90 regardless of the change in the imaging magnification. be able to. However, as can be seen from FIG. 6 showing the relationship between the object height h0 and the image height h in the scanning optical system, a so-called multi-beam type in which a plurality of light beams are arranged and irradiated in the sub-scanning direction (X-axis direction). In the scanning optical system, since the magnification varies depending on the incident angle β, the light beam LB incident on the polygon mirror 91 at a certain height h0 from the optical axis LA for scanning has an image plane (scanned surface). ) Also have different values depending on the incident angle β. That is, as can be seen from FIG. 7 showing an example of a scanning line by a multi-beam type scanning optical system, a scanning line L0 by a light beam passing through the optical axis is linear, while scanning by another light beam is performed. The line L1 does not have a straight line shape, and the scanning line is bent such that the image height h decreases as the distance from the center increases.

An object of the present invention is to overcome the above-mentioned problems in the prior art, and to provide a scanning optical system capable of suppressing a scanning line from being bent by a plurality of light beams.

[0008]

In order to achieve the above object, a scanning optical system according to a first aspect of the present invention performs a main scan while irradiating a plurality of light beams in parallel on a surface to be scanned. A scanning optical system that performs sub-scanning in a direction perpendicular to the light source, a light source that emits a plurality of light beams, a deflecting unit that deflects the plurality of light beams to perform main scanning, and a light source and the deflecting unit. A pre-stage optical system that is provided therebetween and guides the plurality of light beams to the deflecting unit, and a scanning lens that guides the plurality of light beams deflected by the deflecting unit to the surface to be scanned,
A surface tilt correction lens provided between the deflecting unit and the surface to be scanned and having power in at least the sub-scanning direction;
Wherein the focal length of the surface tilt correction lens in the sub-scanning direction is different depending on the incident angle with respect to the surface tilt correction lens.

A scanning optical system according to a second aspect of the present invention is the scanning optical system according to the first aspect, wherein the focal length and the incident angle of the surface tilt correction lens are f and β, respectively. When the focal length f at β = 0 ° is f0, the surface tilt correction lens has a characteristic of f = f0 (1 / cosβ) ^m , where 1 ≦ m ≦ 2.3.

The scanning optical system according to a third aspect of the present invention is the scanning optical system according to the second aspect, wherein the surface tilt correction lens has a characteristic of 1.5 ≦ m ≦ 1.8. It is characterized by the following.

According to a fourth aspect of the present invention, there is provided the scanning optical system according to the third aspect, wherein the surface tilt correction lens has a characteristic of m = 1.64. I do.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

[1. Device Configuration and Principle of Embodiment]
FIG. 1 is a perspective view showing the overall configuration of a scanning optical system according to an embodiment of the present invention. In each of the drawings, a three-dimensional coordinate system XY-having a horizontal plane as a YZ plane and a vertical direction as an X-axis direction is used.
Z is defined. Hereinafter, the overall configuration of the scanning optical system according to the embodiment of the present invention will be described with reference to FIG.

The scanning optical system according to the first embodiment of the present invention includes semiconductor lasers 10a and 10a as light sources, as shown in FIG.
b, 10c, a front optical system 20, a polygon mirror 30 as a deflecting means, a scanning lens 40, a cylindrical lens 50 as a surface tilt correction lens, and the like. The scanning surface SS is moved in the negative X-axis direction by a moving mechanism (not shown) while the main scanning is performed by deflecting and swinging the scanning surface SS in the Y-axis direction (changing the angle of deflection). This is a device that performs sub-scanning in this manner and thereby draws an image on the scanned surface SS.

The functions of each of the above-described units when drawing by the scanning optical system are as follows. First, the light source is composed of three semiconductor lasers 10a, 10b and 10c.
Each is directly modulated and driven by a control signal from a control unit (not shown), and emits a light beam LB based on the control signal. Note that the light beam LB from the semiconductor laser 10b is used for the following optical systems (including the cylindrical lens 50).
Has passed through the optical axis.

The emitted light beam LB is shaped by the lenses 21 and 22 constituting the pre-stage optical system 20 and is incident on the mirror surface of the polygon mirror 30 by the cylindrical lens 23 so as to be condensed. Then, as shown in FIG. 1, the light beam L reflected by the polygon mirror 30
B is incident on the scanning lens 40 while being swung in the YZ plane (hereinafter referred to as a main scanning plane) by the rotation of the polygon mirror 30 about the X-axis direction as a central axis.

A scanning lens 40 (for example, an fθ lens)
Is composed of four lenses 40a to 40d, and forms an image of the incident light beam LB on the surface SS to be scanned. Also,
In cooperation with the scanning lens 40 and the cylindrical lens 50 having power only in the sub-scanning direction, the mirror surface of the polygon mirror 30 and the surface SS to be scanned are optically substantially conjugated in the sub-scanning direction (X direction). You. As a result, the tilt of the mirror surface due to the deviation of the center axis of the polygon mirror 30 is corrected, and the distortion of the scanning line on the scanned surface SS is corrected. In this way, the image forming point of the light beam LB that forms an image on the surface to be scanned SS is repeatedly moved in the Y-axis direction by the rotation of the polygon mirror 30, so that the main scanning is repeatedly performed and the sub-surface of the surface to be scanned SS is Sub-scanning is performed by movement in the direction opposite to the scanning direction (the negative direction of the X-axis).

Next, the main part of this embodiment will be described.

FIG. 3 is a view showing an incident angle β of the light beam LB to the cylindrical lens 50. As shown in the figure, the angle of incidence of the light beam LB on the cylindrical lens 50 in the main scanning plane is expressed as an angle made with the Z axis.
It is.

As described above, in the ordinary cylindrical lens 50, the light beam LB on the incident surface depends on the incident angle β.
Although the effective radius of curvature in the passing path is different, it is assumed that the portion of the entrance surface of the cylindrical lens 50 where the light beam LB actually passes is about 20 μm even when the beam diameter on the surface SS to be scanned converges to the minimum. Then
Since the F number is up to about F20, it is only necessary to consider the properties of the incident surface IS near the optical axis only.

The effective radius of curvature re of the incident surface in such a cross section of the light beam LB near the optical axis (cross section obtained by cutting the cylindrical lens 50 at the incident angle β) is as follows: re = r0 · (cos β) It was confirmed that ^m could be approximated.

Therefore, the cylindrical lens 50 of this embodiment cancels the above-mentioned change in the effective radius of curvature re at any position (any incident angle β) within the scanning range in the main scanning direction, and In order to keep the effective focal length in the scanning direction constant, (X
The focal length f (on the −Z plane) is given by the following equation according to the incident angle β.

F = f0 · (1 / cosβ) ^m (1) where 1 ≦ m ≦ 2.3, and f0 is the focal length in the sub-scanning direction at β = 0 °.

Here, m takes a value of about 1.64 as an optimum value, and in this case, each light beam L in the multi-beam
It was confirmed that an almost linear scanning line could be obtained in any of B (detailed later). However, in practice, some degree of bending of the scanning line is allowed, and typically about 4% of the scanning height (height from the optical axis in the sub-scanning direction) is allowed. The range of values corresponds to this. Hereinafter, the derivation of the range of m will be described.

As described in the section of the problem to be solved by the invention, since the larger the incident angle β, the larger the curvature of the scanning line, the range of the incident angle β to be considered must be determined first. The incident angle β is the maximum emission angle of the scanning lens 40, and usually, β = ± 18 ° is the maximum. Further, as described above, as the absolute value of the incident angle β increases (ie, as the distance from the center of the scanning lens 40 increases), the curvature at the portion where the light beam LB passes increases. As the value increases, the difference from the height of the scanning line at β = 0 ° increases. Therefore, as an allowable range of the bending of the scanning line, β = β = ± 18 °
The difference in scanning height from 0 °, that is, the difference in imaging magnification in the sub-scanning direction is set to within 4%.

Although the imaging magnification in the sub-scanning direction is determined by the combination of the scanning lens 40 and the cylindrical lens 50, since the focal length of the scanning lens 40 is constant,
The change in the imaging magnification substantially corresponds to the change in the focal length of the cylindrical lens 50. As described above, the focal length f
Assuming that the allowable value of the variation from 0 is about 4%, (1 / cos18ｓ) ^1.64 = 1.0858, and the numerical range of ± 4% is 1.044 to 1.129, which is almost (1 / cos18 ゜).゜) ^1.0- (1 / co
s18 ゜) The range is ^2.3 . Therefore, the range of m = 1 to 2.3 is allowed.

Note that here, m only for β = ± 18 °
As described above with reference to FIGS. 6 and 7, since the change in the height of the scanning line (bending) increases monotonically with respect to the absolute value of the incident angle β, β = ± 18 °. Is the most severe condition (the range of m is narrow). For example, β =
At 0 ゜, cos β = 1, and is not dependent on m (that is, the allowable range of m is from “0” to infinity). From the functional form of cos θ, the above β =
It can be seen that the range of m at ± 18 ° is the most severe condition.

As described below, in the first and second embodiments, the radius of curvature and the refractive index of the cylindrical lens 50 are changed according to the incident angle β, respectively, so that the focal length f of the equation (1) is changed. Realizing the relationship.

With this configuration, according to this embodiment, the effective focal length can be kept constant irrespective of the angle of incidence β, and the coupling between the mirror surface of the polygon mirror 30 and the surface SS to be scanned can be maintained. Since the image magnification can be kept constant, even when scanning with a multi-beam is performed, even if the bending of the scanning line other than on the optical axis is large, even when the range of the incident angle β is as large as about −18 ° to about 18 °, both ends of the scanning range However, the scanning height can be made within ± 4% of the scanning height, and high-precision scanning can be performed.

<1-1. First Embodiment> FIG.
FIG. 9 is a cross-sectional view of the cylindrical lens 50 at the position of the incident angle β in the embodiment. As shown in FIG. 2, in the cylindrical lens 50 according to the first embodiment, the surface on the scanning surface SS side (the emission surface OS) is flat, and the surface on the opposite side (the incidence surface IS) is in the main scanning direction. (Therefore, the radius of curvature changes depending on the position (the incident angle β). That is, the cylindrical lens 50 is positioned at the center (incident angle β = 0
°), the radius of curvature r of the incident surface IS2 in a cross section perpendicular to the incident surface IS2 at the position of the incident angle β is: = R0 · (1 / cosβ) ^m Equation 2 where 1 ≦ m ≦ 2.3. As a result, the focal length f satisfies the relationship of Expression 1, and as described above, it is possible to suppress the bending of the scanning line and perform high-precision scanning even in multi-beam scanning.

Hereinafter, the present invention will be described specifically with reference to examples.

<< 1-1-1. First Embodiment >> In the first embodiment, the radius of curvature r of the entrance surface of the cylindrical lens 50 is
Is set to the above-mentioned optimum value m = 1.64 in Expression 2. However, in the first embodiment, r0 = 25 mm, the thickness along the optical axis is 5 mm, and the material is glass having a refractive index n = 1.511176.

FIGS. 3 and 4 show a polygon mirror 30 and a scanning lens 40 of the scanning optical system according to the first embodiment, respectively.
4A and 4B are a plan view and a side view illustrating a positional relationship between the cylindrical lens 50 and the lens. As shown in FIG. 3, the incident surface I of the cylindrical lens 50 extends from the final surface of the scanning lens 40.
By setting 449 mm to S and 49.735 mm from the exit surface (flat surface) of the cylindrical lens 50 to the surface SS to be scanned, as shown in FIG. The relationship is conjugate.

The polygon mirror 30 has a wavelength of 780n.
The three light beams LB of m are guided so as to converge and enter three positions of an incident height of 0 μm and ± 700 μm in the sub-scanning direction.

The scanning lens 40 has a focal length of 400 mm, a back focus (focal position on the surface to be scanned) = 500 mm (see FIG. 4), a front focus (focal position on the polygon mirror 30 side) = 240 mm, and a scanning lens from the polygon mirror 30. It is 50 mm up to the first surface of 40.

Since the polygon mirror 30 is located closer to the lens than the front focus position of the scanning lens 40, the principal ray of the light beam LB emitted from the scanning lens 40 is transmitted to the cylindrical lens 5 in the main scanning plane.
0.

With this scanning optical system, the polygon mirror 30 is
When rotated by 15.717 ° (β = 18 ° at this time) and scanned 220 mm, the height of the polygon mirror 30 in the sub-scanning direction = 700
The result of computer simulation of the trajectory drawn by the light beam LB incident at μm is shown.

[0038]

[Table 1]

However, each symbol is as follows.

Θ: rotation angle of polygon mirror 30 β: angle of incidence on incidence surface IS of cylindrical lens 50 in the main scanning plane r: radius of curvature of incidence surface IS of cylindrical lens 50 in the sub-scanning direction f: cylindrical lens 50; focal length of the light incident surface IS in the sub-scanning direction X; position of the light beam LB on the surface to be scanned SS in the sub-scanning direction Y; position of the light beam LB on the surface to be scanned SS in the main scanning direction At the incident angle β = 0.00 °, the position X in the sub-scanning direction is 100.00 μm, whereas at the incident angle β = 18.00 °, the position X is 99.88, and the change in the position X (the maximum bending of the scanning line) is 0.15 μm. , The incident angle β = 1
The maximum bend of the scanning line at 8.00 ° is not only less than 4%, but also when scanning with multiple beams,
It can be seen that an almost ideal straight scanning line can be obtained.

<< 1-1-2. Second Embodiment >> In the second embodiment, the radius of curvature r of the entrance surface IS of the cylindrical lens 50 is set to m = 1 in Equation 2. Further, in this case, in order to correct the curvature of field caused by the change of the effective focal length of the cylindrical lens 50, 9000 mm (the further away from the optical axis,
(Direction approaching the surface to be scanned). The cylindrical lens 50 in the second embodiment is made of glass having a refractive index n = 1.511176, as in the first embodiment. The result of the computer simulation in this case is shown.

[0042]

[Table 2]

In this result, since the position X = 100.00 μm in the sub-scanning direction at the incident angle β = 0.00 °, the position X = 95.80 at the incident angle β = 18.00 °
Maximum bending of scanning line at incident angle β = 18.00 ° is almost 4.2μ
It can be seen that m and 4% are suppressed. That is,
It is an example showing a limit of an allowable range.

<< 1-1-3. Third Embodiment >> In the third embodiment, the radius of curvature r of the entrance surface of the cylindrical lens 50 is set to m = 2.3 in Equation 2. Also in this case, in order to correct the field curvature caused by a change in the effective focal length of the cylindrical lens 50, the curvature in the main scanning plane is 9000 mm (the curvature increases in the direction away from the scanning surface as the distance from the optical axis increases. ) Has a curvature. In addition,
The cylindrical lens 50 in the third embodiment is made of glass having a refractive index n = 1.511176 as in the first embodiment. The result of the computer simulation in this case is shown.

[0045]

[Table 3]

In this result, since the position X = 104.03 at the incident angle β = 18.00 ° with respect to the position X = 100.00 μm in the sub-scanning direction at the incident angle β = 0.00 °, the incident angle β = 18.00 The maximum bend of the scan line in ° is almost
It can be seen that it is suppressed to 4.0 μm and about 4%. That is, this is an example showing almost the limit of the allowable range.

<< 1-1-4. Fourth Embodiment >> In the fourth embodiment, the radius of curvature r of the entrance surface of the cylindrical lens 50 is set to m = 1.5 in Equation 2. Further, in the fourth embodiment, similarly to the first embodiment, the cylindrical lens 50 is formed.
Is flat in the main scanning plane, but the field curvature is within the depth of focus. Incidentally, the cylindrical lens 50 in the fourth embodiment is made of glass having a refractive index n = 1.511176 as in the first embodiment. The result of the computer simulation in this case is shown.

[0048]

[Table 4]

In this result, since the position X = 100.00 μm in the sub-scanning direction at the incident angle β = 0.00 °, the position X = 98.51 at the incident angle β = 18.00 °.
Maximum bending of scan line at incident angle β = 18.00 ° is approximately 1.5μ
It can be seen that not only m and 4%, but also a scanning line close to a straight line can be obtained.

<<< 1-1-5. Fifth Embodiment >> In the fifth embodiment, the radius of curvature r of the entrance surface of the cylindrical lens 50 is set to m = 1.8 in Equation 2. Also, in the fifth embodiment, similarly to the first embodiment, the cylindrical lens 50 is formed.
Is flat in the main scanning plane, but the field curvature is within the depth of focus. The cylindrical lens 50 in the fifth embodiment is made of glass having a refractive index n = 1.511176, as in the first embodiment. The result of the computer simulation in this case is shown.

[0051]

[Table 5]

In this result, the position X = 100.00 μm in the sub-scanning direction at the incident angle β = 0.00 °, the position X = 101.66 at the incident angle β = 18.00 °, so that the incident angle β = 18.00 The maximum bend of the scan line in ° is almost
It can be seen that not only the 1.7 μm is suppressed to within 4%, but also a scanning line close to a straight line can be obtained.

As is clear from the first to fifth embodiments, the optimum condition is the case where m = 1.64 in the equation (2), and the allowable range is 1 ≦ m ≦ 2.3, preferably 1.5 ≦ m
≦ 1.8.

<1-2. Second Embodiment> In a second embodiment, the incident surface IS of the cylindrical lens 50 is formed by changing the radius of curvature r to a constant value of 25 mm regardless of the incident angle β (see FIG. 5). Instead of having the same shape as the ordinary cylindrical lens 50), the refractive index n is changed according to the incident angle β. That is, using the relational expression f = r / (n−1) (3) between the focal length f and the refractive index n, the refractive index n is changed with the incident angle β so that the focal length f satisfies the expression 1. And change it.

With such a configuration, in the second embodiment as well, in scanning by multi-beams, it is possible to suppress the bending of the scanning line and perform scanning with high accuracy.

Hereinafter, a specific example will be described.

<<<< 1-2-1. Example >> In the example of the second embodiment, similarly to the first embodiment, the polygon mirror 30 is rotated by 15.717 ° (β =
18 ゜) The results of computer simulation of the trajectory drawn by the light beam LB incident on the polygon mirror 30 at a height of 700 μm in the sub-scanning direction when scanning 220 mm are shown below. However, each symbol is the same as in the first embodiment, and n represents the refractive index of the cylindrical lens 50. Also, the value of m in Equation 1 is an optimal value.
1.64.

[0058]

[Table 6]

In this result, the position X in the sub-scanning direction at the incident angle β = 0.00 ° is X = 100.00 μm, and the position X is 100.37 at the incident angle β = 18.00 °. Therefore, the incident angle β = 18.00. Maximum bend of scan line in ° is 0.37
It can be seen that not only is it suppressed to 4 μm or less, but also an almost ideal straight scanning line can be obtained in multi-beam scanning.

[2. Modifications] Although an example of the scanning optical system has been described in the first and second embodiments, the present invention is not limited to this.

For example, in the first and second embodiments, the radius of curvature and the refractive index of the cylindrical surface of the cylindrical lens 50 are changed, but a lens having a curvature on both sides is used as a surface tilt correction lens. In this case, the surface thickness of the lens may be changed according to the incident angle β.

In the first and second embodiments, the curvature and the refractive index of the entire cylindrical surface of the cylindrical lens 50 are changed in accordance with the incident angle β. May be changed only in a limited range that affects the refractive index.

[0063]

As described above, according to the first to fourth aspects of the present invention, in a scanning optical system for scanning a surface to be scanned by deflecting a plurality of light beams by a deflecting means, a surface tilt correction lens is provided. Since the focal length in the sub-scanning direction varies depending on the angle of incidence with respect to the surface tilt correction lens, the scanning line can be prevented from being bent by a plurality of light beams, and scanning can be performed with high accuracy.

According to the second aspect of the present invention, the surface tilt correction lens has a characteristic of f = f0 (1 / cos β) ^m where 1 ≦ m ≦ 2.3. Scanning line bending can be suppressed to within about 4%, and scanning can be performed with high accuracy.

According to the third aspect of the present invention, since the surface tilt correction lens has a characteristic of 1.5 ≦ m ≦ 1.8, a scanning line close to a straight line is formed as a scanning line by a plurality of light beams. As a result, relatively high-precision scanning can be performed.

According to the fourth aspect of the present invention, since the surface tilt correction lens has a characteristic of m = 1.64, an ideal linear scanning line can be obtained as a scanning line by a plurality of light beams. Thus, high-precision scanning can be performed.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an overall configuration of a light beam scanning device according to an embodiment.

FIG. 2 is a cross-sectional view of the cylindrical lens according to the first embodiment at an incident angle β.

FIG. 3 is a plan view showing a positional relationship among a mirror surface of a polygon mirror, a scanning lens, and a cylindrical lens of the light beam scanning device.

FIG. 4 is a side view showing a positional relationship among a mirror surface of a polygon mirror, a scanning lens, and a cylindrical lens of the light beam scanning device.

FIG. 5 is a diagram showing an incident angle of a light beam on a cylindrical surface of a cylindrical lens.

FIG. 6 is a diagram showing a relationship between an object height h0 and an image height h in a conventional scanning optical system.

FIG. 7 is a diagram showing an example of a scanning line by a conventional multi-beam scanning optical system.

[Explanation of symbols]

 10a, 10b, 10c Semiconductor laser (light source) 20 Pre-stage optical system 30 Polygon mirror (deflecting means) 40 Scanning lens 50 Cylindrical lens (Slack correction lens) IS, IS1, IS2 Incident surface OS Outgoing surface LB light beam SS Scanned surface f, f0 Focal length of the cylindrical lens 50 n Refractive index of the cylindrical lens 50 r, r0 Curvature radius of the cylindrical lens 50 β Incident angle

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C362 BA58 BA86 BB04 2H045 CA03 2H087 KA19 RA07 5C072 AA03 BA04 BA17 HA02 HA06 HA08 HA13 RA12

Claims (4)

[Claims] 1. A scanning optical system for performing main scanning by irradiating a plurality of light beams in parallel to a surface to be scanned and performing sub-scanning in a direction perpendicular to the main scanning, comprising: a light source that emits a plurality of light beams; Deflecting means for deflecting the plurality of light beams to perform main scanning; a pre-stage optical system provided between the light source and the deflecting means for guiding the plurality of light beams to the deflecting means; A scanning lens that guides the plurality of light beams deflected by the scanning surface to the surface to be scanned, and a surface tilt correction lens that is provided between the deflecting unit and the surface to be scanned and has power in at least the sub-scanning direction.
A scanning optical system, characterized in that a focal length of the surface tilt correction lens in the sub-scanning direction differs according to an incident angle with respect to the surface tilt correction lens. 2. The scanning optical system according to claim 1, wherein the focal length of the surface tilt correction lens is f, the incident angle is β, and the focal length f at β = 0 ° is f0. Wherein the surface tilt correction lens has a characteristic of f = f0 (1 / cos β) ^m where 1 ≦ m ≦ 2.3. 3. The scanning optical system according to claim 2, wherein the surface tilt correction lens has a characteristic of 1.5 ≦ m ≦ 1.8. 4. The scanning optical system according to claim 3, wherein the surface tilt correction lens has a characteristic of m = 1.64.