Scanning lens and scanning optical device

A scanning lens molded with resin is disclosed. The lens has a first lens surface which a light beam incident to, and a second lens surface which emits the light beam. The lens further includes a first lens frame formed to project from the first lens surface, the first lens frame is provided on at least a part of the periphery of the first lens surface. The lens further includes a second lens frame formed to project from the second lens surface. The second lens frame is provided on at least a part of the periphery of the second lens surface. At least one of the first and second lens frames has approximately constant projection amount along with the main-scanning direction of the scanning lens.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-375926 filed on Dec. 26, 2002;

the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a scanning lens, and more particularly to a lens which has a lens frame to protect the lens surface. The invention further relates to a scanning optical device having a scanning lens.

DESCRIPTION OF THE BACKGROUND

FIG. 11Ashows a schematic plane view of an inside of a known optical scanning device such as an electronic copier, a laser beam printer, or a laser facsimile equipment.FIG. 11Bshows an oblique drawing of a scanning lens being used in the optical scanning device.

As shown inFIG. 11A, the optical scanning device is provided with a light source unit1, a rotary polygon mirror2, and a scanning lens4.

Light source unit1includes elements such as a semiconductor laser or a collimate lens. Rotary polygon mirror2as a scanning. means scans a parallel laser beam L1emitted from light source unit1. The scanned laser beam L1passes through a reflection mirror. Scanning lens4focuses the parallel beam L1to form an image on a photosensitive body on a rotary drum (not shown).

Rotary polygon mirror2and scanning lens4are housed inside an optical box5, and light source unit1is secured to a sidewall of optical box5.

After assembling the parts including light source unit1, rotary polygon mirror2and so forth inside optical box5, an upper-part opening of optical box5is capped with a lid (not shown).

A window6is provided on a sidewall of optical box5. The light beam L1scanned by rotary polygon mirror2goes through window6toward rotary drum (not shown) arranged out of optical box5.

A collimate lens (not shown) collimates laser beam L1emitted from a semiconductor laser of light source unit1. A cylindrical lens la converges the collimated laser beam L1into a linear light beam on a reflecting surface of rotary polygon mirror2. Reflection mirror3reflects laser beam L1from rotary polygon mirror2and scanning lens4to rotary drum (not shown). The reflected laser beam L1go through window6of optical box5.

Consequently, the light beam L1is focused into images on the photosensitive body on the rotary drum. The light beams form electrostatic latent images as the main-scanning rotation by rotary polygon mirror2and the sub-scanning rotation by the rotary drum are carried out.

Scanning lens4corrects distortions of point images which are formed on the photosensitive body. It is because scanning lens4performs as a so-called f-theta lens. Scanning lens4is an axially asymmetrical aspherical lens and is integrally molded with plastic.

As shown inFIG. 11B, a lens frame4xis arranged on the bottom portion of scanning lens4. Lens frame4xprojects from both sidewalls of scanning lens4respectively, and another lens frame4yis arranged on the upper portion of scanning lens4. Lens frame4yprojects from both sidewalls of scanning lens4respectively. A lens surface (effective-surface)4cof scanning lens4is provided between lens frame4xand lens frame4y.

Furthermore, scanning lens4has a pair of projecting parts11, at the center of flanges4xand4yin the longitudinal direction of flange4xand4y. Flanges4xand4yrespectively projects in the direction of the optical axis of scanning lens4. One of projecting parts11is held between a pair of positioning members16aand16b. Positioning members16aand16bprojects from the bottom wall of optical box5, and position scanning lens4in the main-scanning direction of scanning lens4e.

In addition, a bottom wall13of scanning lens4is finished with high flatness as a horizontal reference surface for accurate positioning.

Bottom wall13of scanning lens4contacts with a pair of horizontal pedestal parts (not shown) which project from the bottom wall. Bottom wall13and the pair of horizontal pedestal parts positions scanning lens4in the sub-scanning direction of scanning lens4.

A pair of positioning ribs14and15are formed on both the side edges of scanning lens4respectively. Positioning ribs14and15have vertical reference surface parts which are perpendicular to the optical-axis direction of scanning lens4. Positioning ribs14and15contact with a pair of vertical surface (not shown) provided on a sidewall or a partition wall of optical box5. Positioning ribs14and15position scanning lens4in the optical-axis direction of scanning lens4.

When scanning lens4is assembled with in optical box5, one of projecting parts11is hold between positioning members16aand16bwhich are provided at the bottom of optical box5. The horizontal reference surface provided at bottom wall13of scanning lens4is contacted with the horizontal pedestal parts of optical box5. Positioning ribs14and15are respectively contacted with the vertical surfaces parts of optical box5. Thus, scanning lens4is positioned in the main-scanning direction, in the sub-scanning direction, and in the optical-axis direction. After the positioning, scanning lens4is fixed on the bottom wall of optical box5with conventional ways, such as using adhesives or springs.

It is important to position scanning lens4in the main-scanning direction, the sub-scanning direction, and the optical-axis direction against rotary polygon mirror4in order to provide good images. The scanning lens mentioned above is described in Japanese Patent Publication (Kokai) No. 09-329755.

FIG.12A andFIG. 12Brespectively show a plane and front view of another type of known scanning lens. As shown inFIG. 12A, a positioning protrusion11amay be provided on only single sidewall of scanning lens4aat the center of scanning lens4a. End parts25a, which are provided at both ends in the main-scanning direction of scanning lens4amay be positioned by pins (not shown), for example, provided on an optical box5. The pins serves to prevent end parts25afrom moving to position scanning lens4ain the predetermined direction.

Further,FIG. 13Ais a cross section of another type of known lens, andFIG. 13Bshows a plane view of the lens.

A lens flame19is provided to surround lens part18. Projection11aprovided at the center in the scanning direction of lens4bpositions a scanning lens4b. Moreover, lens flame19prevents lens surface from being damaged in transporting, and from varying the optical characteristics owing to water vapor absorption from the sidewalls of lens4b.

It is difficult for the scanning lens shown inFIGS. 12A and 12Bto protect a lens surface in transporting and to prevent optical characteristics from varying owing to water vapor absorption.

The scanning lens shown inFIGS. 13A and 13Bcauses a problem, which is described below.

Firstly, the thickness of lens part18is not uniform in the scanning direction, while the thickness of lens frame19is uniform in the scanning direction. Thus, in comparing the cross sectional area ratio of lens part18with that of lens frame19, the cross sectional area of lens part18relative to lens flame19at the center in the scanning direction is larger than that at the end parts in the scanning direction.

FIG. 15describes resin fluidity in resin molding depending on cross sectional area as mentioned above.

When resin is inserted through a gate G, and pressure is applied to the resin, the tip portion of inserted resin flow ununiformly in the scanning direction. The tip shape of the inserted resin is quite different at the end part from that at the center part.

For example, at the end part in the scanning direction, the resin in lens frame19flows faster than the resin in lens part18, while at the center part in the scanning direction where lens part18thickens, the resin flows faster in lens part18than in lens flame19.

Consequently, ununiform fluidity of resin tip produces flow marks FM or rapid change of internal strain, etc., which deteriorate optical characteristics of lenses.

Moreover, providing a projection11ashown in FIG.12A andFIG. 13Bwidens and heightens a lens.

Further, scanning lens4shown inFIG. 11has lens frames4xand4ywhose projection amount from the lens surface partially vary relatively much in the main-scanning direction as shown in FIG.11A. Consequently, flow marks or rapid change of internal strain, etc. may occur which deteriorate optical characteristics of lenses.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide a scanning lens, which comprises a first a first lens surface for receiving a light beam; a second lens surface for emitting the light beam; a first lens frame that projects an approximately constant amount from a substantial part of the periphery of the first lens surface along a main scanning direction of the scanning lens; and a second lens frame that projects from a periphery of the second lens surface.

Another aspect of the present invention is to provide an optical scanning device for producing image information, the device comprising: a light source which emits a light beam; a scanning element which scans the light beam; a scanning lens which refracts the light beam, the scanning lens having a first lens surface; and a first lens frame that projects an approximately constant amount from a substantial part of the periphery of the first lens surface.

Further another aspect of the present invention is to provide a method of making a scanning lens comprising: forming a lens part having a first lens surface and a second lens surface; forming a first lens frame to project an approximately constant amount from a substantial part of the periphery of the first lens surface along a main scanning direction; and forming a second lens frame to project from the periphery of the second lens surface.

DETAILED DESCRIPTION OF THE INVENTION

A scanning lens and an optical scanning device according to a first embodiment in accordance with the present invention, will be explained below with reference toFIGS. 1to3.

The optical scanning device of the first embodiment has structures which are common with the optical scanning device shown inFIG. 11A, except the structure of the scanning lens.

Thus, detailed description about the optical scanning device is omitted except for the scanning lens to avoid repeated description. Some of the reference marks used in the description on the background of the invention are also cited in this modification.

FIG. 15shows a plan view of an optical scanning device according to the present invention.

As shown inFIG. 15, the optical scanning device is provided with a light source unit1, a rotary polygon mirror2, and a scanning lens4.

Rotary polygon mirror2as a scanning element scans a parallel laser beam L1emitted from light source unit1. The scanned laser beam L1passes through a reflection mirror. Scanning lens4focuses the parallel beam L1to form an image on a photosensitive body on a rotary drum (not shown).

Rotary polygon mirror2and scanning lens4are housed inside an optical box5, and light source unit1is secured to a sidewall of optical box5.

A collimate lens (not shown) collimates laser beam L1emitted from a semiconductor laser of light source unit1. A cylindrical lens la converges the collimated laser beam L1into a linear light beam on a reflecting surface of rotary polygon mirror2. Reflection mirror3reflects laser beam L1from rotary polygon mirror2and scanning lens4to rotary drum (not shown). The reflected laser beam L1go through window6of optical box5.

Consequently, the light beam L1is focused into images on the photosensitive body on the rotary drum. The light beams form electrostatic latent images as the main-scanning rotation by rotary polygon mirror2and the sub-scanning rotation by the rotary drum are carried out.

The optical scanning device of the first embodiment may have two scanning lens4nand4m, which have what is called the “f-theta characteristic” as shown in FIG.1.

Resin lens4nmay be a scanning spherical lens (“θ1lens”) with positive refracting power. Resin lens4m(“θ2lens”) may be a resin aspherical toric lens. Lenses4nand4mare simply called as scanning lenses in general. Lens4mhas refracting powers of the main-scanning direction different from of the sub-scanning direction.

A light beam modulated according to image information is scanned by a rotary polygon mirror2. The two lenses4nand4mfocus into images on a surface of a photosensitive drum9as a photosensitive body , the lenses4nand4mcompensate for plane tilt of polygon mirror2.

Scanning lens may be made of plastic such as PNMA which is cheap plastic. The scanning lens further may be a compound lens with the f-theta function, which uniforms the scanning speed of a point image focused on the surface of the photosensitive body.

Scanning lenses4nand4mare fixed on an optical box adhesives, for example, and are strictly positioned with respect to the optical path of the light beam which is reflected by polygon mirror2.

Detail description on scanning lenses applicable to the optical scanning device of the first embodiment is mentioned below.

FIG. 2Ais a cross section of scanning lens40aof the first embodiment of the present invention, andFIG. 2Bshows a plane view of the same. As shown in2B, scanning lens4B is almost rectangular having two long sides and short sides. An arrow A and an arrow B respectively show the main-scanning direction and the optical-axis direction of scanning lens40a. In a molding process, resin flows in the main-scanning direction inside a die to produce scanning lens40a. A lens part21and a lens flame22are integrally molded with resin.

In the present invention, “the main-scanning direction” means the direction in which scanning means such as a polygon mirror scans a light beam. The main-scanning direction is consistent with the longitudinal direction of scanning lens40a. Further, “the sub-scanning direction” means the direction perpendicular to the main-scanning direction. The sub-scanning direction is consistent with the direction parallel with the short sides of scanning lens40a. Furthermore, the optical-axis direction of scanning lens40ais perpendicular to both the main-scanning direction and sub-scanning direction.

As shown inFIG. 2A, the center part of lens part21in the main scanning direction is thicker than the end parts of lens part21. A lens surface23as a first lens surface23is a surface for receiving a light beam. A second lens surface24as a second lens surface24is a surface which emits the light beam. Both of the lens surfaces23and24form free curved surfaces.

It is difficult to represent a free curved surface, including surfaces23or24, with a single mathematical model. A free curved surface can be represented by interpolating with many mathematical models.

The center part in the sub-scanning direction of lens surface23is recessed, while the center part in the main-scanning direction of lens surface23is protruded. Lens surface23has a fine convexo-concave as a whole. Lens surface24is convex in both the main-scanning direction and sub-scanning direction. Lens surface24has a fine convexo-concave as a whole.

As shown inFIG. 2B, lens frames22xas a first lens frames22xare provided on two parallel sides of the periphery of lens surface23. The both sides are parallel with main-scanning direction A. Lens frames22xare formed to project from lens surface23, while a lens frames22yas a second lens frames22yare provided on two parallel sides of the periphery of lens surface24, and are formed to project from lens surface24. The both sides are parallel with main-scanning direction A.

Further, outer edges of lens frames22xand22yrespectively form a curved line with a constant curvature along with main-scanning direction A. The curvature of lens frame22xis approximately the same as the curvature of a curved line which approximates lens surface23. The curvature of lens frame22yis approximately the same as the curvature of a curved line which approximates second lens surface24.

The curvature of lens frames22xis slightly smaller than the curvature of a line which approximates the center section line of lens surface23a free curved line.

On the other hand, the curvature of lens frame of a beam output side is slightly larger than the curvature of a line which approximates the center section line of lens surface24with a free curved line.

Both lens frames22xand22yhave an inflection point near the center part in the main-scanning direction. The curvature is approximately zero at the vicinity of the center part in the main-scanning direction. Consequently, lens frame22xhas the most recessed position at the vicinity of the center part in the main-scanning direction, while lens frame22yhas the most protruded position at the vicinity of the center part in the main-scanning direction.

Consequently, each frame of lens frames22xand22yprojects an approximately constant amount from each lens surface. Each lens frame22xor22yis provided on the lens surface along with main-scanning direction A. At any position in the main-scanning direction, lens frame22xhas its height higher than the highest point of lens surface in the main-scanning direction by 2 mm or less. At any position in the main-scanning direction, lens frame22yhas its height higher than the highest point of lens surface in the main-scanning direction by 2 mm or less

Thereby, the change of perpendicular sectional area ratio between lens part21and lens frames22in the main-scanning direction can be within a certain predetermined range.

Hence, the tip part of injected resin flows uniformly inside the die to make molding scanning lens40a. Consequently, lens part21, which is a part of the molded article, is prevented from producing flow marks or from rapid change of internal strain. As a result, a scanning lens with good optical characteristics is provided.

If a lens frame has its height higher than the highest point of a lens surface by over 2 mm, the structure causes a large difference between a flow tip position apart from the lens frame and a flow tip position close to the lens frame in a molding process. It may cause defects such as flow marks, etc.

In addition, because lens frames22xand22yform a curved line extending to the main-scanning direction with a constant curvature, it is easy to produce a die to fabricate the scanning lens4. Consequently, the cost for producing a scanning lens may be reduced.

End parts25having a flat surface parallel to the main-scanning direction (shown as an arrow mark A inFIG. 2B) are provided on both ends in the direction A of lens part21. Because the surface of end parts25is formed so as to be orthogonal or perpendicular to the optical-axis direction, lens part21is positioned with optically high precision by making use of end parts25as a guide surface in attaching scanning lens4.

A positioning protrusion26with a surface parallel with the direction A may be provided at the center part in the scanning direction of first lens frame22xor at an original position for optical design.

As shown inFIG. 2A, in the optical-axis direction, positioning protrusion26is formed on lens frame22xso that the height of any portion of lens frame22xis lower than the end parts of end parts25extending in the main-scanning direction. In other words, positioning protrusion26is between the periphery of lens surface23and a plane including opposite end points of the lens surface23.

Thus, the height of scanning lens40adepends on the height of lens frame22in the optical axis direction. This makes it possible to reduce the height of scanning lens40a. As a result, both downsizing of scanning lens40aand accurate positioning of scanning lens40aare made possible.

Moreover, making use of both positioning projection26and end parts25as guides makes possible more accurate positioning.

Making the height of positioning projection26same as the height of end parts25may simplify structures of a housing to which the scanning lens40ais attached, and may makes it possible to position much more accurately by making use of both parts25and26as guides.

The scanning lens may be formed without end part25or protrusion26. Moreover, positioning protrusion26may be provided on another portion of the scanning lens in place of the center of the scanning lens. For example, protrusion26may be provided on a sidewall of lens frame22projecting in the sub-scanning direction. The lens frame may be formed into other kinds of lines with a curvature along with the main-scanning direction. For example, a curved line which is represented by higher order polynomials, such as a two-order curved line (expressed by a quadratic equation), a three-order curved line (expressed by a cubic equation), a four-order curved line (expressed by a biquadratic expression) , or a part of ellipse, etc. may be applicable.

Lens part21and lens frames22may be made without using resin. For example, Lens part21may be made of glass.

Lens frames22may project no more than 2 mm from lens surface23or24, or lens frames22may project over 2 mm.

Further, another scanning element instead of polygon mirror2may be applicable.

Further another, a scanning mirror instead of scanning lens40amay be applicable. In detail, a scanning mirror comprising a first lens surface for reflecting a light beam, and a first lens frame that projects an approximately constant amount from a substantial part of the periphery of the first lens surface along a main-scanning direction of the mirror, may be applicable.

The optical scanning device of the present invention may have a structure such as the structures of various known devices except for the scanning lens.

Modifications of the first embodiment of scanning lens40aare described below. In the modifications the shape of lens part21may be the same as that of the first embodiment mentioned above.

FIG. 3is a cross section of a modification of scanning lens40a. In scanning lens40amentioned above, lens frames22xand22yform a curved line with a constant curvature along with the main-scanning direction.

In this modification, a lens frame22ais formed with plural line segments with different angles along the main-scanning direction. Each segment approximates a section line in the main-scanning direction of lens surfaces23, which is a free curved surface. Thus, a die for a scanning lens40bof the first modification, is easily made. Consequently, cost for producing a scanning lens may be reduced.

Moreover, the change of perpendicular sectional area ratio between lens part21and lens frame22amay be restricted within a predetermined range in the main-scanning direction in this modification, too. Hence, lens part21, which is a part of the molded article, is prevented from causing a flow mark and rapid change of internal strain. As a result, a lens with good optical characteristics is provided.

In addition, end parts25having a surface orthogonal to the optical-axis direction and parallel to the main-scanning direction are provided on both ends of lens frame22a. Because end parts25are perpendicular to the optical-axis direction (shown as an arrow mark B), lens part21is positioned with optically precision by making use of end parts25as a guide surface in attaching scanning lens40b.

Moreover, positioning protrusion26is provided at the center in the scanning direction of lens frame22aor at an original position for optical design. As shown inFIG. 3, positioning protrusion26is provided not to project from the highest part, which is end parts25, in the optical-axis direction of lens frame22.

Consequently, the height of scanning lens40bin the optical axis direction depends on the height of lens part21, which makes it possible to reduce height of lens unit and which makes possible both downsizing of scanning lens40band accurate positioning of scanning lens40b.

FIG. 4is the cross section of the second modification of the scanning lens40a. In the first modification described above, lens frame22ais formed with plural line segments of different angles, while in this modification, a lens frame22breplaces a free curved surface of lens part21with a stepped approximation. Hence, a lens frame22bis formed with plural steps.

According to the second modification, a die with which a scanning lens40bis molded is easily worked and cost of the die may be reduced.

A positioning protrusion26is provided at the center in the scanning direction of lens frame22bor at an original position for optical design. Further, flat surface parts28parallel with the main-scanning direction are provided on the both sides of positioning protrusion26.

In the modification, the change of perpendicular sectional area ratio between lens part21and lens frame22bmay be restricted to the predetermined range in the main-scanning direction, too. Thus, a flow mark and rapid change of internal strain are prevented. As a result, a lens with good optical characteristics is provided.

In the second modification, end parts25are provided on the both ends of lens frame22b. Because end parts25have a flat surface which is perpendicular to the optical-axis direction, so that lens part21is positioned with optical precision by making use of end parts25as a guide surface in attaching scanning lens40c.

Further, not only an affection of dimensional inaccuracy of a housing to which scanning lens40cis fixed but also a tilt or a deformation of lens40care prevented by making use of flat surface28as a guide surface in attaching scanning lens40c. It is because flat surface part28which is parallel with the main-scanning direction is provided on the both sides of positioning protrusion26.

FIG. 5Ashows a cross-sectional view of the third modification of scanning lens40a, andFIG. 5Bshows an enlarged view of the both end parts in the main-scanning direction of the third modification. The third modification of scanning lens40dhas a characteristic shape at the end parts. Lens part21and lens frame22cexcept the end parts may be applicable to the first embodiment, the first or the second modification of lens40amentioned above. Some of the reference marks used in the embodiment or the modifications are also cited in this modification.

As for the shape of the end parts, a flange27for guiding position, which flange has a flat surface orthogonal to the optical-axis direction and parallel to the main-scanning direction is provided only at one end part. Hence, any flange is not provided at the other end part of lens part23. Flange27on light-entering side has a flat surface27awith a predetermined area. Flat surface27ais used as a surface for guiding position. A light-exiting side of flange27does not need to be formed in a specific shape. For example, flat surface27amay be formed like a circular arch as ever.

In the modification, flat surface27afor guiding position is formed on light-entering side of flange27. Thus, facing flat surface27ato the mounting surface of housing-unit (not shown) makes scanning lens40dpossible to position with optical accuracy.

FIG. 6Ashows a cross-sectional view of the fourth modification of the scanning lens40a, andFIG. 6Bshows a plane view of the modification. The fourth modification40ehas a characteristic shape on the light-entering side of lens frame22d. Lens part21and lens frame22dexcept the shape on the light-entering side of lens frame22dcan be applicable to the first embodiment, the first, the second, or the third modification. Some of the reference marks used in the embodiment or the modifications are also cited in this modification.

In scanning lens40e, lens surface23, which has a free curved surface, is approximately flattened. On the other hand, an outer edge of lens frame22dof light-entering side is straight along with the main-scanning direction. Thus, the die for molding scanning lens40eis easily produced and cost for producing lens40ecan be reduced. The light-exit side of lens frame22dapproximates the second lens surface24.

Next, a scanning spherical lens with positive refracting power (“theta lens1”) is described below.FIG. 7shows an oblique view of a scanning spherical lens as a comparison example. The scanning spherical lens10has a shape basically the same as the scanning lens4shown FIG.13A andFIG. 13Bdescribed as the known technique. Scanning lens10has a lens frame structure which has a lens frame32so as to surround lens part31. The front shape of lens frame32is like trapezoidal-shape, and one side of lens frame32on the light-entering side is longer than the other side of frame32on light-exiting side. Further, positioning protrusion33is provided at the center in the scanning direction of lens frame32. The scanning spherical lens10is positioned with the positioning protrusion33in assembling.

FIG. 8is an oblique view which shows the shape of a scanning spherical lens10a(theta lens1) as another embodiment of the present invention.

The center part of lens part31of lens10ais formed thicker than the end part of lens part31. The first lens surface23and second lens surface24of lens part31has constant curvatures respectively.

The front shape of lens frame32aof lens10ahas the substantially a similar figure as the front shape of lens part31. Lens frame32ais a little larger than lens part31and has the curvature of approximately the same as a curvature of a line which approximates a centerline of the surface of lens part31.

The curvature of lens frame32aon the light-entering side is formed slightly smaller than the curvature of a line which makes approximation the center line of first lens surface23, while the curvature of lens frame32aon the light-exiting side is formed slightly larger than the curvature of a line which makes approximation the center line of second lens surface24.

Moreover, the change of cross sectional area ratio between lens part31and lens frame32ain scanning spherical lens10ais restricted within a predetermined range between the center part and the end part in the scanning direction in the modification, too. Thus, a tip part (a front surface) of injected resin flows uniformly inside the die to fill the die with resin in the molding process. Consequently, lens part31, which is a part of the molded article, is prevented from causing a flow mark and rapid change of internal strain. As a result, a lens with good optical characteristics is provided.

Moreover, an end part34having a surface that is parallel to the scanning direction is formed at the end part (or both the parts) of lens frame32a. Using end part34as a guide surface, in assembling, the scanning spherical lens10apositions lens part31with optical accuracy, because the surface is orthogonal or perpendicular to the optical-axis direction (allow mark B).

A positioning protrusion34is provided at the center part of lens frame32ain the scanning direction. Positioning protrusion33is provided not to project from lens frame32. Thus, the height of scanning lens in the optical-axis direction depends on the height of lens frame32, which makes it possible to reduce height of lens unit of scanning spherical lens10a. Therefore, both downsizing of scanning lens10aand accurate positioning of scanning lens10aare made possible.

Further, making use of both end parts34provided at the end part of lens frame32aand positioning protrusion33as guides makes the lens position with more accuracy. Making the height of projection33the same as the height of end parts34provided at end parts of lens frame32asimplifies the structure of housing to which the scanning lens10ais attached. Using end part34and positioning protrusion33as guides in assembling makes much accurate positioning of lens.

The light-entering side of the lens10ais formed in accordance with the curvature of the first lens surface23, while the light-exit side or light-outputting side of second lens surface24and lens frame32amay be formed into a flat surface. Thus, the die for the lens10acan be easily produced and cost for producing lens can be reduced.

Further another, one embodiment of an optical scanning device according to the invention is explained.

FIG. 9is a side sectional view showing an outline structure of an electronic copier.FIG. 10shows a plane view of the optical system which constitutes the electronic copier.

As shown inFIG. 9, a scanner42with a light source (not shown) moving along an original platen41is provided below original platen41on which an original copy is placed. An image processing part43which digitally processes the reflection light from the original copy exposed by scanner42and a laser optical system is placed under scanner42. As shown inFIG. 10, the laser optical system applies the digital optical signals processed at image processing part43to a photosensitive drum44. For this purpose, a laser diode45, a collimate lens46, a reflection mirror47, a cylindrical lens48, a polygon mirror49, a f-theta lens50and a f-theta lens51are serially arranged on the optical axis in the progressing direction of the light. In this case, the afore-mentioned scanning lens40a,40b,40c,40dor40emay be applicable to f-theta lens50and51.

Photosensitive drum44has a predetermined electrostatic potential as a surface potential by being charged uniformly by a charger52in advance. The resistance value of the area exposed by laser diode45is reduced. It is because the photosensitive body on photosensitive drum45is a photoconductive material, which lowers the surface potential of the area since the surface charges flows to ground at the back surface of photosensitive drum44. Consequently, an electrostatic latent image is formed by the electrostatic potentials made by light exposure.

Photosensitive drum44rotates, and magnetic developers (toners) are supplied to the electrostatic latent image by a fur brush, etc. at a development section provided around photosensitive drum44. Consequently, a development is executed by optionally attracting the developers corresponding to the electronic latent image.

A cleaner57to clean photosensitive drum44is provided around photosensitive drum44.

The original copy is transferred synchronously with the rotation of photosensitive drum44from a paper feed cassette to belt transfer part arranged around a photosensitive drum44. Then, a high voltage is applied to the original copy from the back surface of the original copy at the belt transfer part.

Therefore, the developers of developed image formed on the surface of photosensitive drum are sucked/transferred to the original copy. A roll heater of fix device56heats and fixes the developers of the image transferred on the surface of the original copy.

In the electronic copier above, visualizing process is executed after image processing part43digitally processes input signals from the original copy, while in case of a laser printer, input digital signals from an original may be transmitted to an optical system. An optical path after the optical system is like the same as the electronic copier.

In the electronic copier or laser printer, the optical system can be assembled with excellent accuracy because the optical scanning system or the lens mentioned above is assembled.

Numerous modifications and modifications of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the present invention can be practiced in a manner other than as specifically described herein.