JPH09133888A - Optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the degree of freedom of aberration compensation and improve image forming performance and speed equality by making at least one surface of a long-sized toroidal lens of a correction optical system aspherical. SOLUTION: Luminous flux from a light source 1 after passing through a condenser lens 2 passes through a linear image forming optical element to form a long linear image nearby the deflecting and reflecting surface of an optical deflector 4 along a horizontal scarining corresponding direction. The optical deflector 4 reflects this luminous flux and deflects the reflected luminous flux at an equal angular velocity. An fθ lens 5 and a long-sized optical element 6 are arranged between the optical deflector 4 and a scanned surface 7 and a composite system of respective lenses and a return mirror 8 bend the optical path to form an imaging spot on the scanned surface 7. The surface of this long-sized optical element 6 on the side of the deflector 4 consists of a barrel-shaped toroidal surface which gradually decreases in the radius of curvature in a vertical scanning direction with the distance in the horizontal scanning corresponding direction. In this case, the lens is made aspherical in the main scanning corresponding direction of the long-sized optical element 6 as the correction optical system, specially, in the horizontal scanning corresponding direction of the barrel-shaped toroidal surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device, and more particularly to an optical writing lens system thereof, which is applicable to a laser printer, a laser copying machine, a laser facsimile and the like.

[0002]

2. Description of the Related Art An optical scanning device which deflects a light beam by an optical deflecting device to optically scan a surface to be scanned is well known as an optical writing device in laser printers, laser copying machines, laser facsimiles and the like. When scanning the surface to be scanned with the light beam, if the image forming spot diameter of the light beam fluctuates or the image forming position, that is, the image height fluctuates with the light scanning, the dots written on the surface to be scanned are Since the sizes are non-uniform, a good image cannot be formed. Therefore, various measures have been taken to stabilize the diameter of the image forming spot formed by the light beam on the surface to be scanned. A conventional optical scanning device related to the present invention will be briefly described below.

Japanese Unexamined Patent Publication (Kokai) No. 3-33712 has a plurality of fθ lenses and a long toroidal lens, and the toroidal lens has a curvature radius in the sub-scanning direction that becomes smaller as it goes away from the optical axis in the main scanning corresponding direction. A scanning optical system is described which comprises a barrel-shaped toroidal surface as a concave lens surface. Japanese Patent Application Laid-Open No. 61-175607 discloses an optical scanning optical system including an fθ lens which is a convex meniscus lens having two convex lenses, and the surface closest to the entrance pupil is concave on the pupil surface side. The imaging optical system described in (1), in which an anamorphic lens having a weak refracting power in the main scanning direction is disposed on the scanning surface side of the imaging lens, and at least one surface thereof is made aspheric. Japanese Examined Patent Publication (Kokoku) No. 3-49407 describes a scanning optical system including an aspherical surface in a main scanning section in an element near a scanning surface among elements having a surface tilt correction effect in an imaging lens. There is.

[0004]

In recent years, in image forming apparatuses such as laser printers, laser copying machines, and laser facsimiles that use an optical scanning device, there is a demand for higher density and higher image quality. It is necessary to improve the imaging performance by reducing the curvature of field and improve the uniform speed of the scanning light on the surface to be scanned, that is, the linearity. If the field curvature is large, the beam spot diameter between image heights becomes uneven,
The pixel size also becomes non-uniform, leading to a reduction in resolution. If the linearity is poor, the size of the image in the partial portion on the surface to be scanned will differ, which becomes a particular problem when a drawing or the like is output.

However, JP-A-3-33712, JP-A-61-175607 and JP-B-3-4.
Nothing disclosed in Japanese Patent No. 9407 satisfies the currently desired performances in field curvature and linearity.

The present invention has been made in view of the above problems of the prior art. In order to obtain a high-density and high-quality image, the invention according to claim 1 is characterized by field curvature and constant velocity performance. It is an object of the present invention to provide an optical scanning device that is improved.

It is an object of the present invention to provide an optical scanning device in which constant velocity is improved by making a long toroidal lens constituting a correction optical system aspherical in the main scanning direction. And

According to the third aspect of the present invention, by making the aspherical surface of the long toroidal lens the aspherical surface of the barrel-shaped toroidal surface, a normal toroidal surface can be used on the opposite surface. An object is to provide an optical scanning device.

It is an object of the present invention to provide an optical scanning device capable of improving the easiness of processing by forming only one surface of the long toroidal lens as an aspherical surface.

According to a fifth aspect of the present invention, by controlling the amount of change of the aspherical surface within a predetermined range, it is possible to achieve a good balance between correction of field curvature and improvement of scanning constant velocity. The purpose is to provide.

According to a sixth aspect of the present invention, there is provided an optical scanning device capable of constructing an optical system having a lens configuration which is easy to manufacture and which can maintain high performance with a minimum number of lenses. The purpose is to

According to a seventh aspect of the present invention, there is provided an optical scanning device capable of constructing an optical system with a minimum number of lens components while achieving higher performance than that of the sixth aspect and achieving achromatization. With the goal.

It is an object of the present invention to provide an optical scanning device capable of facilitating the handling of the light source device by converting the light beam emitted from the light source device into a parallel light beam.

It is an object of the present invention to provide an optical scanning device in which the number of revolutions of the deflector can be lowered by using the deflector as a rotary polygon mirror.

It is an object of the present invention to provide an optical scanning device capable of downsizing the light source device by using a semiconductor laser as the light source.

[0016]

In order to achieve the above object, the invention according to claim 1 provides a light beam deflected by the deflector between the deflector and the surface to be scanned onto the surface to be scanned. An optical system is provided that forms an image as a spot, and has a deflective reflection surface of the deflector and a surface to be scanned in a geometrically-optically substantially conjugate relationship with respect to the sub-scanning direction. The optical system is deflected by the deflector. It is composed of an optical scanning lens that scans the light beam on the surface to be scanned at a substantially constant speed and forms an image on the surface to be scanned, and a surface tilt correction optical system, and the optical scanning lens is composed of a plurality of lenses.
The correction optical system is arranged between the optical scanning lens and the surface to be scanned, and has a long barrel-shaped toroidal surface as a concave lens surface whose radius of curvature in the sub-scanning direction becomes smaller as it goes away from the optical axis in the main scanning corresponding direction. The long toroidal lens is a toroidal lens, and at least one surface of the long toroidal lens is aspherical.

It is preferable that the long toroidal lens is aspherical in the main scanning direction as in the second aspect of the invention, and the aspherical surface of the long toroidal lens is the barrel in the third aspect of the invention. It is preferable that the mold toroidal surface is an aspherical surface, and it is more preferable that only one of the long toroidal surfaces is an aspherical surface as in the invention of claim 4.

When only one of the long toroidal surfaces is made aspherical as in the fourth aspect of the invention, the maximum angle-of-view ray of the light beam deflectively scanned is non-spherical as in the fifth aspect of the invention. The aspherical surface variation ΔX of the aspherical surface at the position intersecting the spherical surface is −0.0015 <ΔX, where fm is the focal length of the entire system in the main scanning direction and + is the scanning surface direction from the deflector. It is more preferable to satisfy /fm<0.01.

In the invention described in claim 5, as in the invention described in claim 6, the scanning lens is composed of two lenses, and the lens on the deflector side is a meniscus lens having a concave surface facing the deflector side, The lens on the scanned side may be composed of a convex lens, and the scanning lens may be composed of three lenses, and the lens on the deflector side may be a concave lens and the remaining two lenses. It is more preferable to use a convex lens.

According to the invention described in claim 8, the light beam emitted from the light source device may be a substantially parallel light beam.
As in the invention described above, the deflector for deflecting the light beam may be constituted by a rotating polygon mirror, and as in the invention described in claim 10, the light source in the light source device may be a semiconductor laser.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an optical scanning device according to the present invention will be described with reference to the drawings. In FIG. 1, reference numeral 1 indicates a light source. In this example, the light source 1 is a semiconductor laser and emits a divergent light beam. As the light source 1, an LED or the like can be used instead. After the light flux from the light source 1 passes through the condenser lens 2,
A long line image is formed in the main scanning corresponding direction in the vicinity of the deflection reflection surface of the optical deflector 4 through the line image forming optical element 3 formed of a cylindrical lens. The light deflector 4 reflects the incoming light flux and deflects the reflected light flux at a constant angular velocity. Optical deflector 4
In this example, a rotary polygon mirror (polygon mirror) is used. The fθ lens 5 and the long optical element 6 are arranged between the optical deflector 4 and the surface to be scanned 7, and the optical path is bent by the folding mirror 8 by the combination system of these lenses, and the surface to be scanned 7 is scanned. An imaging spot is formed at. Due to the rotation of the deflector 4, the imaging spot is moved to the scanned surface 7
Scan above. In FIG. 1, reference numeral 100 indicates a synchronization signal detection system for synchronizing the optical scanning.

FIG. 2A is a plan view for explaining the function in the deflection scanning plane (main scanning section) of the optical scanning device. The light flux from the light source 1 passes through the condenser lens 2, the linear image forming optical element 3, and then is reflected by the deflection reflection surface 4 a of the deflector 4, and the reflected light flux is deflected as the deflector 4 rotates. To be done. Further, the deflected light beam is imaged on the surface to be scanned 7 by the synthetic system of the fθ lens 5 composed of two lenses and the long optical element 6, and the scanning speed of the imaged spot is kept constant. The fθ lens 5 and the long optical element 6 cooperate with each other to cause the light beam deflected by the deflector 4 to scan the surface to be scanned 7 at a substantially constant speed and form an image on the surface to be scanned. And an optical scanning lens and a correction optical system.

FIG. 2B is a side view for explaining the function in the direction (sub-scan section) orthogonal to the deflection scanning plane of the optical scanning device, that is, the function of correcting the influence of the plane tilt of the deflector. Is. The light beam emitted from the light source 1 passes through the condenser lens 2 and is linearly imaged by the line image imaging optical element 3 in the vicinity of the deflection reflection surface 4a of the deflector 4. A light spot is formed on the surface to be scanned 7 through the fθ lens 5 composed of two spherical single lenses and the surface tilt correction long optical element 6 having different refracting powers in the main scanning direction and the sub scanning direction. However, the fθ lens 5 and the surface tilt correction long optical element 6 have a power arrangement and a positional relationship in the sub-scanning cross-section, and the deflection reflection surface 4a and the surface to be scanned 7 are different.
And are distributed so that they are geometrically and optically conjugate.
And it is arranged. Therefore, the deflecting / reflecting surface 4 of the deflector 4
When a is tilted in the direction orthogonal to the deflection scanning plane and changes to the position indicated by 4a ′, the light flux passing through the fθ lens 5 and the long optical element 6 changes as indicated by the broken line, but on the scanned surface 7 The image-forming position on the screen hardly changes, and the deflective reflection surface 4
The influence of the fact that a falls to the position indicated by reference numeral 4a 'is corrected.

The long optical element 6 has an anamorphic surface. Therefore, the anamorphic surface forming the long optical element 6 will be described. The surface of the long optical element 6 on the deflector 4 side is composed of a barrel-shaped toroidal surface whose radius of curvature in the sub-scanning direction becomes smaller as the optical axis moves away from the optical axis in the main scanning corresponding direction. FIG. 3 illustrates the barrel-shaped toroidal surface. In FIG. 3A, when the center of curvature of the arc AB is C1, and when the straight lines X1 and Y1 are between the point V on the arc AB and the center of curvature C1 of the arc AB, the straight lines X1 and Y1 are rotation axes. As the arc AB is rotated, a surface such as the side surface of the barrel is formed as shown in FIG. 3 (b). This is a barrel-shaped toroidal surface, and the long optical element 6 uses this barrel-shaped toroidal surface as a concave surface. Further, the surface of the long optical element 6 on the scanned surface side is generally called a toroidal surface, and in FIG. 3A, straight lines X1, Y1 are used.
Is on the right side of C1 (not between the point V and C1), it is constituted by a plane formed with X1 and Y1 as rotation axes.

When the light beam emitted from the light source device composed of the light source 1 and the condenser lens 2 is a parallel light beam, the positional relationship (distance) between the light source device and the line image forming optical element 3 can be freely set. Therefore, the degree of freedom in the layout design is increased and the design is facilitated. As the deflector 4, a galvanometer mirror, a single-sided mirror, a pyramidal mirror,
Although it can be considered variously as a rotary polygon mirror (polygon mirror), the use of the rotary polygon mirror is advantageous in reducing the rotation speed of the motor. Further, when the number of rotations is the same, the scanning speed can be improved, which is advantageous for increasing the speed.

In the present invention, the above-mentioned Japanese Patent Laid-Open No. 3-337 is used.
In addition to improving the image forming performance, mainly in the main scanning direction, as compared with the conventional example described in Japanese Patent Publication No. 12 or the like,
In order to improve the scanning speed (also called linearity or magnification error), the main scanning direction of the long optical element 6 which is the correction optical system, especially the main scanning direction of the barrel-shaped toroidal surface is made aspheric. There is. The reason why the barrel-shaped toroidal surface is made aspherical is that when the opposite toroidal surface is made aspherical, the degree of freedom in aberration correction in the sub-scanning direction is reduced, and performance is deteriorated. Since the present invention is intended to improve the scanning uniform velocity, it is necessary to make the long optical element 6 aspheric in the main scanning direction, and basically it is impossible to make the toroidal aspheric in only the main scanning direction. Is.

Therefore, in the present invention, the main scanning direction of the barrel-shaped toroidal surface is made aspherical to improve the scanning uniform velocity, and in cooperation with the toroidal surface, the deflective reflection surface and the scanned surface are formed in the sub-scanning direction. It maintains a geometrically-optically substantially conjugate relationship and corrects field curvature.

As the optical scanning lens, a single lens has been recently seen (see Japanese Patent Laid-Open No. 7-113950), but it is necessary to make the optical scanning lens aspherical.
The optical performance is inferior to that of the two-sheet structure, so higher density
It is impossible to improve the image quality. Also in terms of lens processing, it is difficult because it uses an aspherical surface. Therefore, in the present invention, two optical scanning lenses are provided (provided that each surface is a spherical surface).
As a result, the aim was to construct an optical system with a simple configuration while preventing deterioration of optical performance. The lens on the deflector side was a meniscus lens with a concave surface facing the deflector side to correct the field curvature, and the lens on the scanned surface side was a convex lens to improve the scanning constant velocity.

Next, in recent years, there has been a great demand for higher density, and for that purpose, correction of chromatic aberration is required. In order to correct chromatic aberration and to make the optical performance sufficiently compatible with high density, it is necessary to configure the optical scanning lens with three lenses. In order to correct chromatic aberration, a combination of a concave lens and a convex lens is required. In another embodiment of the present invention shown in FIG. 4, in order to correct chromatic aberration, the field curvature and scanning constant velocity are corrected. In addition, the optical scanning lens 5 is composed of three lenses, and the lens on the deflector 4 side among the three lenses is a concave lens, and the remaining two lenses are arranged as convex lenses.

First, an embodiment in which two optical scanning lenses are constructed will be described below. The barrel-shaped toroidal surface is a general formula in which the coordinate in the optical axis direction is X and the coordinate in the optical axis orthogonal direction with the optical axis position as the origin is Y in the main scanning corresponding direction. (However, Rm: radius of curvature on the optical axis, K: conical constant, A
2, A3, A4 ...: Aspherical coefficient), and with respect to the sub-scanning direction, the light is separated from the surface by Rs on the optical axis and the light is deflected in the scanning plane (in the main scanning plane). It is a concave barrel-shaped toroidal surface formed by rotating the above-described shape with an axis orthogonal to the axis as a rotation axis. Here, Rm is shown in FIG.
In (a), it corresponds to VC1 and Rs corresponds to VC2.

Further, the aspherical surface of the barrel-shaped toroidal surface in the main scanning corresponding direction has the maximum angle of view of the light beam deflected by the deflector at the position (height) where the ray intersects the aspherical surface. The amount of deviation from the axial curvature radius Rm, and the direction of deviation from the paraxial spherical surface to the surface to be scanned are defined as + transition directions), but the combined focal length of the entire optical system after the deflector in the main scanning direction is fm. At this time, when the aspherical surface change amount ΔX is set to satisfy −0.0015 <ΔX / fm <0.01, both the field curvature in the main scanning direction and the scanning uniform velocity are well balanced with good balance. Can be corrected.

[0032]

EXAMPLES Various examples of the optical scanning apparatus according to the present invention will be described below. The reference numerals used in each embodiment are the same as those used in FIGS. However, the curvature of each surface is represented by r in FIGS. 2 and 4, whereas it is represented by R in each embodiment. The barrel-shaped toroidal surface is r5 surface. In each embodiment, the combined focal length fm in the deflection scanning plane (main scanning plane) is standardized to 100. The deflection angle (half angle of view) by the deflector 4 is θ.

Examples 1 to 5 show cases in which two optical scanning lenses are used. Moreover, Examples 1 to 5 are shown in FIGS.
The aberration chart of is shown. The field curvature in each figure is the dynamic field curvature when the deflector is rotated, and the dotted line shows the image formation state in the deflection scanning direction (main scanning direction) and the solid line shows the deflection orthogonal direction (sub scanning direction). In the aberration diagram, the dotted line shows the fθ characteristic and the solid line shows the linearity. As is well known, the fθ characteristic is that when the ideal image height is Hi (θ) and the actual image height is Hr (θ) (fθ characteristic) = {Hr (θ) / Hi (θ) −1} × 10
It is defined as 0 (%). Further, the scanning uniform velocity (linearity) is (linearity) = {dHr (θ) / dHi (θ) −
It is represented by 1} × 100 (%).

Examples 6 to 8 show examples in which three optical scanning lenses are used. Aberration diagrams of the respective examples are shown in FIGS. The reference numerals in the specific example in the case where the optical scanning lens is composed of three lenses are in accordance with FIG. The barrel-shaped toroidal surface is the r7 surface. The combined focal length fm of the entire optical system after the deflector in the deflection scanning plane (in the main scanning plane) of each embodiment is standardized to 100 as in the example in which two optical scanning lenses are configured. .

(Example 1)

(Example 2)

(Example 3)

(Example 4)

(Example 5)

(Example 6)

(Example 7)

(Embodiment 8)

[0043]

According to the first aspect of the present invention, an optical system is provided between the deflector and the surface to be scanned for forming an image of the light beam deflected by the deflector on the surface to be scanned as a light spot. This optical system is composed of an optical scanning lens and a correction optical system for scanning the light beam deflected by the deflector on the surface to be scanned at a substantially constant speed and forming an image on the surface to be scanned. The optical scanning lens is composed of a plurality of lenses, and the correction optical system is arranged between the optical scanning lens and the surface to be scanned, and the radius of curvature in the sub-scanning direction becomes smaller as the distance from the optical axis in the main scanning corresponding direction increases. Since this is a long toroidal lens having a barrel-shaped toroidal surface as a concave lens surface, and at least one surface of this long toroidal lens is made aspherical, the degree of freedom in aberration correction is increased and the imaging performance and constant velocity are improved. Providing a designed optical scanning device It is possible.

According to the second aspect of the present invention, the long toroidal lens that constitutes the correction optical system is made aspheric in the main scanning direction, which is effective in improving the uniform velocity, so that the uniform velocity is improved. be able to.

According to the third aspect of the present invention, the barrel-shaped toroidal surface of the long toroidal lens is made aspheric to improve the uniform velocity, and the surface on the opposite side is image-forming performance in the sub-scanning direction. To improve the performance of the optical system, a toroidal surface can be used to improve the performance of the optical system as a whole.

According to the fourth aspect of the present invention, the long toroidal lens can be easily processed by forming only one aspherical surface of the long toroidal lens.

According to the fifth aspect of the invention, the amount of change of the aspherical surface is regulated within a predetermined range, so that the field curvature (mainly in the main scanning direction) is corrected and the scanning uniform velocity is improved. It is well-balanced and effective.

According to the sixth aspect of the present invention, the minimum number is required when the optical scanning lens is constituted by a spherical surface which is easily processed while achieving the high density and high image quality desired in the market. By configuring the optical system with two sheets, a high-performance and low-cost optical scanning device can be provided.

In order to achieve ultra high density, the influence of chromatic aberration becomes a problem, and at least two lenses or more are required to correct chromatic aberration. If two lenses are used, the imaging performance and the correction of chromatic aberration are required. According to the invention of claim 7, since the optical scanning lens is composed of three lenses, it is difficult to achieve both at the same time. An optical system can be configured.

According to the eighth aspect of the invention, by making the luminous flux emitted from the light source device a parallel luminous flux, the positional relationship between the light source device and the cylinder lens can be freely set, and the optical layout can be facilitated.

According to the ninth aspect of the invention, a galvanometer mirror or the like can be considered as the deflector. However, by using a rotary polygon mirror, the number of rotations of the motor can be reduced or the scanning speed can be increased. it can.

According to the tenth aspect of the present invention, the light source device can be miniaturized by using the semiconductor laser as the light source in the light source device.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of an optical scanning device according to the present invention.

FIG. 2 is an optical layout diagram of the optical scanning device of the above, FIG.
FIG. 3B is a diagram showing a main scanning section, and FIG.

FIG. 3 is an explanatory diagram of a barrel-shaped toroidal surface used in the optical scanning device.

4A and 4B are optical layout diagrams showing another embodiment of the optical scanning device according to the present invention, in which FIG. 4A is a main scanning cross section and FIG.
FIG. 6 is a diagram showing a sub-scanning section.

FIG. 5 shows field curvature and linearity according to the first embodiment of the present invention,
It is a diagram showing an fθ characteristic.

FIG. 6 shows field curvature and linearity according to the second embodiment of the present invention,
It is a diagram showing an fθ characteristic.

FIG. 7 shows field curvature and linearity according to the third embodiment of the present invention,
It is a diagram showing an fθ characteristic.

FIG. 8 is a field curvature and linearity of Example 4 of the present invention,
It is a diagram showing an fθ characteristic.

FIG. 9 shows field curvature and linearity according to the fifth embodiment of the present invention,
It is a diagram showing an fθ characteristic.

FIG. 10 is a diagram showing field curvature, linearity, and fθ characteristic according to the sixth embodiment of the present invention.

FIG. 11 is a diagram showing field curvature, linearity, and fθ characteristic of Example 7 of the present invention.

FIG. 12 is a diagram showing field curvature, linearity, and fθ characteristic of Example 8 of the present invention.

[Explanation of symbols]

 1 Light Source 4 Deflector 5 Optical Scanning Lens 6 Correction Optical System 7 Scanned Surface

Claims (10)

[Claims] 1. An optical scanning device for deflecting a light beam by a deflector to optically scan a scanned surface, wherein a light beam deflected by the deflector is disposed between the deflector and the scanned surface. Image it as a light spot on the top,
Further, with respect to the sub-scanning direction, it has an optical system in which the deflective reflection surface of the deflector and the surface to be scanned are in a geometrically-optically substantially conjugate relationship, and the optical system described above makes the light beam deflected by the deflector the surface to be scanned. The optical scanning lens comprises a plurality of optical scanning lenses for scanning the upper surface at a substantially constant speed and forming an image on the surface to be scanned, and the optical scanning lens is composed of a plurality of lenses. A long toroidal lens that is disposed between the scanning lens and the surface to be scanned, and has a barrel-shaped toroidal surface as a concave lens surface that has a radius of curvature in the sub-scanning direction that decreases with distance from the optical axis in the main scanning corresponding direction, An optical scanning device, wherein at least one surface of the long toroidal lens is aspherical. 2. The optical scanning device according to claim 1, wherein the long toroidal lens is aspherical in the main scanning direction. 3. The optical scanning device according to claim 1, wherein the aspherical surface of the long toroidal lens is a barrel-shaped toroidal surface. 4. The optical scanning device according to claim 3, wherein only one of the long toroidal surfaces is aspherical. 5. An aspherical surface change amount Δ of the aspherical surface at a position where the maximum angle of view ray of the light beam deflectively scanned crosses the aspherical surface.
X is characterized by satisfying −0.0015 <ΔX / fm <0.01 when fm is the focal length of the entire system in the main scanning direction and + is the direction of the surface to be scanned from the deflector. The optical scanning device according to claim 4. 6. The optical scanning lens comprises two lenses, the lens on the deflector side is a meniscus lens having a concave surface facing the deflector side, and the lens on the scanned side is a convex lens. The optical scanning device according to claim 5. 7. The optical scanning device according to claim 5, wherein the optical scanning lens is composed of three lenses, the deflector side lens is a concave lens, and the remaining two lenses are convex lenses. . 8. The optical scanning device according to claim 6, wherein the light beam emitted from the light source device is a substantially parallel light beam. 9. The optical scanning device according to claim 8, wherein the deflector for deflecting the light beam is a rotary polygon mirror. 10. The optical scanning device according to claim 8, wherein the light source in the light source device is a semiconductor laser.