JP4298222B2 - Scanning optical system, optical scanning device, and image forming apparatus - Google Patents

Description

上記において、Ｘ、Ｙは、面番号：ｉ〜ｉ＋１における頂点間の光軸方向および主走査方向の距離を表す。例えば、面番号：０（偏向反射面）におけるＸ＝４７．８９、Ｙ＝０．４７は、偏向反射点位置（像高：０を与える反射位置）に対してレンズ６の入射面（面番号：１）の頂点が、光軸方向（Ｘ方向）に４７．８９ｍｍ、主走査方向（Ｙ方向）に０．７４ｍｍ、それぞれ離れていることを意味する。面番号：１におけるＸ＝２０．８１はレンズ６の光軸上の肉厚を与える。
【０１００】
入射面（面番号：ｉ＝１）は「曲率一定面」であり、かつ、主走査断面内の形状は上記（６）式で表される非円弧形状である。
この面の主走査方向の各係数を表１に挙げる。
【０１０１】
【表１】

【０１０２】
射出面（面番号：ｉ＝２）は「特殊面」で、主走査断面内の形状は光軸に対称的な非円弧形状である。
この面の主走査方向と副走査方向の係数を表２に挙げる。
【０１０３】
【表２】

【０１０４】
実施例１の走査光学系の中心像高における副走査方向の横倍率：β_２は、β_２＝２．３である。
【０１０５】
図５に、実施例１の像面湾曲（左図 実線は副走査像面湾曲、破線は主走査像面湾曲）と等速度特性（右図 実線はリニアリティ、破線はｆθ特性）を示す。
【０１０６】
主走査方向：１．６６６ｍｍ/２２０ｍｍ
副走査方向：０．２１ｍｍ/２２０ｍｍ
リニアリティ：０．５５５％/２２０ｍｍ
と、像面湾曲・等速度特性ともに極めて良好に補正されている。特に、副走査像面湾曲は条件（３）を満足する。即ち、
（３） ０．２１／２２０＝０．０００９５＜０．００１
図６には、中心像高の副走査方向横倍率：β_２に対する、任意像高の副走査方向横倍率：β_ｈの変化を示す。倍率変化：｜β_ｈ/β_２｜は、
０．９９８≦｜β_ｈ/β_２｜≦１．０１３
と、条件（２）を満足し、極めて良好に補正されている。
【０１０７】
図７は、レンズ６の入射面（図７（ａ））、射出面（図７（ｂ））の曲率変化を示している。本実施例では、入射面を「曲率一定面」としている。
【０１０８】
図８には、実施例１における光スポットの各像高ごとの「スポット径の深度曲線（光スポットのデフォーカスに対するスポット形の変動）」を示す。（ａ）は主走査方向、（ｂ）は副走査方向に関するものである。実施例１では、ラインスプレッド関数の１/ｅ^２強度で定義されるスポット径として５０μｍ程度を意図している。図に示されているように、主走査・副走査方向とも良好な深度を有しており、被走査面の位置精度に対する許容度が高い。
【０１０９】
実施例１では、走査光学系をなすレンズ６をプラスチック材料で構成しているが、勿論、ガラス材料を使ってもよい。また、更なるビームスポット径の小径化を狙うために、副走査方向を非円弧形状としてもよい。また、走査光学系を偏心させることで、より好ましく収差補正を行うことが可能であることも付記しておく。
【０１１０】
【発明の実施の形態】
図９に画像形成装置の実施の１形態を示す。
この画像形成装置はレーザプリンタである。
【０１１１】
レーザプリンタ１０００は感光性の像担持体１１１０として「円筒状に形成された光導電性の感光体」を有している。像担持体１１１０の周囲には、帯電手段としての帯電ローラ１１２１、現像装置１１３１、転写ローラ１１４１、クリーニング装置１１５１が配備されている。帯電手段としては「コロナチャージャ」を用いることもできる。
【０１１２】
レーザ光束ＬＢにより光走査を行う光走査装置１１７１が設けられ、帯電ローラ１１２１と現像装置１１３１との間で「光書込による露光」を行うようになっている。
【０１１３】
図９において、符号１１６１は定着装置、符号１１８１はカセット、符号１１９１はレジストローラ対、符号１２０１は給紙コロ、符号１２１１は搬送路、符号１２２１は排紙ローラ対、符号１２３１はトレイ、符号Ｐはシート状記録媒体としての転写紙を示している。
【０１１４】
画像形成を行うときは、光導電性の感光体である像担持体１１１０が時計回りに等速回転され、その表面が帯電ローラ１１２１により均一帯電され、光走査装置１１７１のレーザ光束ＬＢの光書込による露光を受けて静電潜像が形成される。形成された静電潜像は所謂「ネガ潜像」であって画像部が露光されている。
【０１１５】
この静電潜像は現像装置１１３１により反転現像され、像担持体１１１０上にトナー画像が形成される。転写紙Ｐを収納したカセット１１８１は、画像形成装置１０００本体に脱着可能であり、図のごとく装着された状態において、収納された転写紙Ｐの最上位の１枚が給紙コロ１２０１により給紙され、給紙された転写紙Ｐは、その先端部をレジストローラ対１１９１に銜えられる。レジストローラ対１１９１は、像担持体１１１０上のトナー画像が転写位置へ移動するのにタイミングを合わせて、転写紙Ｐを転写部へ送り込む。
【０１１６】
送り込まれた転写紙Ｐは、転写部においてトナー画像と重ね合わせられ転写ローラ１１４１の作用によりトナー画像を静電転写される。トナー画像を転写された転写紙Ｐは定着装置１１６１へ送られ、定着装置１１６１においてトナー画像を定着され、搬送路１２１１を通り、排紙ローラ対１２２１によりトレイ１２３１上に排出される。
【０１１７】
トナー画像が転写された後の像担持体１１１０の表面は、クリーニング装置１１５１によりクリーニングされ、残留トナーや紙粉等が除去される。
【０１１８】
光走査装置１１７１として「図１、２、３のごとき光走査装置」を用いることにより、極めて良好な画像形成を実行することができる。
【０１１９】
【発明の効果】
以上に説明したように、この発明によれば、新規な、走査光学系および光走査装置および画像形成装置を実現できる。
【０１２０】
この発明の走査光学系は、上記の如く、レンズ１枚構成で安価に作製でき、光走査装置をコンパクト化でき、なおかつ、幾何光学的な収差のみならず波動光学的な収差も良好に補正可能であり、光スポットの小径化に適している。このような走査光学系を用いる走査光学系は、小径化した光スポットによる良好な光走査が可能となり、従って、かかる光走査装置を用いる画像形成装置は、精細な画像の形成が可能である。
【図面の簡単な説明】
【図１】シングルビーム方式の光走査装置の実施の１形態を説明するための図である。
【図２】マルチビーム方式の光走査装置の実施の１形態を説明するための図である。
【図３】マルチビーム方式の光走査装置の実施の１形態を説明するための図である。
【図４】光走査装置の実施例の光学配置を示す図である。
【図５】実施例の収差図（像面湾曲と等速度特性）である。
【図６】実施例における倍率変化：｜β_ｈ/β_２｜を示す図である。
【図７】実施例の走査光学系の入射側面（曲率一定面）と射出側面（特殊面）の、副走査断面内の曲率の主走査方向における変化を示す図である。
【図８】実施例における光スポットの各像高ごとの「スポット径の深度曲線」を示す図である。
【図９】画像形成装置の実施の１形態を説明するための図である。
【符号の説明】
５ 光偏向器（ポリゴンミラー）
６ 走査光学系[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a scanning optical system, an optical scanning device, and an image forming apparatus.
[0002]
[Prior art]
Optical scanning devices are widely known in connection with digital copying devices and laser printers. The scanning optical system is an optical system that is used in an optical scanning device and collects a light beam deflected by an optical deflector as a light spot on a surface to be scanned.
[0003]
Conventionally, the number of lenses in a scanning optical system configured as a lens system varies from one to a plurality. In terms of cost, a single-lens configuration is advantageous, but a scanning optical system composed of a single lens has an extremely small number of design parameters compared to a single-lens configuration and has a simple lens surface shape. “Good aberration correction” is difficult.
[0004]
It has been proposed to satisfactorily correct aberrations and constant velocity characteristics in a single-lens scanning optical system by adopting a special lens surface shape (Japanese Patent Laid-Open Nos. 9-33850 and 10-90620). JP-A-10-148755).
[0005]
In recent years, optical scanning devices have been strongly demanded to increase the density of optical scanning, and in order to meet this demand, reduction in the diameter and stabilization of the light spot has been pursued. Such light spot diameter reduction / stabilization is not only correction of geometric optical aberrations, but correction of wave optical aberrations is important. The above publications do not mention such wave optical aberration correction.
[0006]
[Problems to be solved by the invention]
In view of the above, the present invention is a novel scanning optical system capable of satisfactorily correcting not only geometrical aberration but also wave optical aberration while having a single lens configuration, and light having such a scanning optical system. An object is to realize a scanning device and an image forming apparatus.
[0007]
[Means for Solving the Problems]
The scanning optical system of the present invention is a “scanning optical system for condensing a light beam deflected by an optical deflector as a light spot on a surface to be scanned”, and has the following characteristics (claim 1).
[0008]
That is, the scanning optical system is composed of one lens, has a biconvex shape in the main scanning direction and a biconvex shape in the sub-scanning direction, and both surfaces are anamorphic surfaces, and one surface is a “curvature constant surface”. It is.
[0009]
The “curvature constant surface” is a lens surface in which the curvature in the sub-scan section is constant in the main scanning direction, that is, the curvature in the sub-scan section does not change depending on the position in the main scan direction.
[0010]
The “sub-scanning cross section” is a virtual flat cross section orthogonal to the main scanning direction at an arbitrary position in the main scanning direction of the lens of the scanning optical system. In addition, a virtual flat section that passes through the lens optical axis and is parallel to the main scanning direction is referred to as a “main scanning section”.
[0011]
The other surface of the scanning optical system according to claim 1 is a “special surface”.
[0012]
The “special surface” is a lens surface whose curvature in the sub-scanning section continuously changes in the main scanning direction. That is, it is a lens surface whose curvature in the sub-scanning direction changes according to the position of the sub-scanning cross section when the position of the sub-scanning cross section is changed in the main scanning direction.
[0013]
The “special surface” of the scanning optical system according to claim 1 is “the curvature in the sub-scan section changes asymmetrically with respect to the optical axis in the main scanning direction” .
Then, the scanning optical system of claim 1, wherein the sub-scanning direction lateral magnification at the central image height: beta ₂ is the condition:
(1) 0.5 ≦ | β ₂ | ≦ 3.0
Satisfied.
[0014]
In the scanning optical system according to the first aspect, “the shape in the main scanning section of at least one surface is a non-arc shape” can be achieved .
[0015]
In this specification, “horizontal magnification in the sub-scanning direction of the scanning optical system: β” means the object point of the scanning optical system conjugate to the image point near the surface to be scanned and the image point near the surface to be scanned in this specification. And the horizontal magnification.
[0016]
In the first aspect, wherein the scanning optical system, the sub-scanning direction lateral magnification at the central image height: and beta _2, sub-scanning direction lateral magnification at any image height: beta _h the condition:
(2) 0.9 ≦ | β _h / β ₂ | ≦ 1.1
Is preferably satisfied ( Claim 3 ).
[0017]
In the scanning optical system according to any one of the claims 1 to 3, the effective writing width: W, in the effective writing width sub-scanning curvature Width: F _S the condition:
(3) F _S /W<0.001
Is preferably satisfied ( claim 4 ). “Effective writing width” refers to an optical scanning area where image writing can be effectively performed by optical scanning. “Sub-scanning field curvature” refers to field curvature in the sub-scanning direction. The field curvature in the main scanning direction is referred to as “main scanning field curvature”.
[0018]
Further, in the scanning optical system according to any one of claims 1 to 4 , in the main scanning direction, the curvature radius of the entrance surface: R _m1 and the curvature radius of the exit surface: R _m2 are as follows:
(4) | R _m1 | >> _Rm2
Preferably satisfies (Claim 5), any of the scanning optical system according to one of claims 1 to 5, in the sub-scanning direction, the incident surface curvature radius: R _s1, the exit surface of the curvature radius: R _s2 is the condition:
(5) | R _s1 | >> | R _s2 |
Is preferably satisfied ( claim 6 ).
[0019]
The scanning optical system according to any one of claims 1 to 6 can receive “a light beam deflected by a light deflector and convergent in the main scanning direction” ( claim 7 ).
[0020]
The scanning optical system according to any one of claims 1 to 7 , wherein “the scanning optical system has a function of optically scanning a light spot on the surface to be scanned in the main scanning direction at a constant speed, and the surface of the optical deflector is complicated in the sub scanning direction. It preferably has a function of correcting this ( claim 8 ).
[0021]
The scanning optical system according to any one of claims 1 to 8 may be "molded with a resin" ( claim 9 ).
[0022]
The scanning optical system according to any one of claims 1 to 9 , wherein the scanning optical system condenses a plurality of light beams deflected simultaneously as a plurality of light spots separated in the sub-scanning direction on the surface to be scanned. ( Claim 10 ). That is, the scanning optical system of the present invention can be used as a scanning optical system of a multi-beam type optical scanning device.
[0023]
12. The optical scanning device according to claim 11, wherein the light beam from the light source is coupled to a subsequent optical system by a coupling lens, and the coupled light beam is adjacent to a deflecting reflection surface of the optical deflector by a line image imaging optical system. The image is formed as a long line image in the main scanning direction, deflected at a constant angular velocity by an optical deflector, and the deflected light beam is condensed as a light spot on the surface to be scanned by the scanning optical system, and the surface to be scanned is optically scanned. A single beam type optical scanning device ”, wherein the scanning optical system according to any one of claims 1 to 9 is used.
[0024]
13. The optical scanning device according to claim 12 , wherein light beams from a plurality of light sources are coupled to a subsequent optical system by a coupling lens, and the coupled light beams are deflected by a common line image forming optical system. A common scanning optical system that forms multiple line images that are long in the main scanning direction and separated in the sub-scanning direction in the vicinity of the position of the deflecting reflection surface of the detector and simultaneously deflects them at the same angular velocity by the optical deflector. A multi-beam optical scanning device that condenses light onto the scanning surface as a plurality of light spots separated in the sub-scanning direction and simultaneously scans a plurality of scanning lines with the plurality of light spots, The common scanning optical system is the one described in claim 10 .
[0025]
The “coupling lens” may be individual for a plurality of light beams or may be shared.
[0026]
A “monolithic semiconductor laser array in which a plurality of light emitting sources are arranged in a line” can be used as a light source in the optical scanning device according to claim 12 ( claim 13 ).
[0027]
The image forming apparatus according to the present invention is an “image forming apparatus that forms a latent image by performing optical scanning on a photosensitive image carrier with an optical scanning device, and visualizes the latent image with a developing unit to obtain an image”. The optical scanning device according to any one of claims 11 to 13 is used as an optical scanning device for optically scanning an image carrier ( claim 14 ).
[0028]
Various types of “photosensitive image carrier” can be used. For example, a “silver salt film” can be used as the image carrier. In this case, a latent image is formed by writing by optical scanning, and this latent image can be visualized by processing by a normal silver salt photographic process. Such an image forming apparatus can be implemented as an “optical plate making apparatus” or an “optical drawing apparatus” for drawing a CT scan image or the like.
[0029]
As the photosensitive image carrier, a “coloring medium (positive printing paper) that develops color by the thermal energy of the light spot during optical scanning” can be used. In this case, a visible image is directly displayed by optical scanning. Can be formed.
[0030]
A “photoconductive photoreceptor” can also be used as the photosensitive image carrier. As the photoconductive photoconductor, a sheet-like material such as zinc oxide paper can be used, or a “selenium photoconductor or organic photo-semiconductor such as“ repetitively used in a drum shape or belt shape ”is used. Can do.
[0031]
When a photoconductive photoconductor is used as an image carrier, an electrostatic latent image is formed by uniform charging of the photoconductor and optical scanning by an optical scanning device. The electrostatic latent image is visualized as a toner image by development. The toner image is directly fixed on the photoconductor when the photoconductor is in the form of a sheet such as zinc oxide paper, and transfer paper or an OHP sheet when the photoconductor can be used repeatedly. It is transferred and fixed on a sheet-like recording medium such as (plastic sheet for overhead projector).
[0032]
The transfer of the toner image from the photoconductive photosensitive member to the sheet-like recording medium may be directly transferred from the photosensitive member to the sheet-like recording medium (direct transfer method), or may be temporarily transferred from the photosensitive member to an intermediate transfer belt or the like. After transfer to the intermediate transfer medium, transfer from the intermediate transfer medium to a sheet-like recording medium (intermediate transfer method) may be performed.
Such an image forming apparatus can be implemented as an optical printer, an optical plotter, a digital copying apparatus, or the like.
[0033]
In the image forming apparatus of the present invention, a plurality of the photoconductors are arranged along the conveyance path of the sheet-like recording medium, and an electrostatic latent image is formed for each photoconductor using a plurality of optical scanning devices. The toner image obtained by visualizing these can be transferred and fixed to the same sheet-like recording medium, and can be implemented as a “tandem type image forming apparatus” that synthetically obtains a color image or a multicolor image.
[0034]
Some supplementary explanation.
The scanning optical system having a single lens structure according to the present invention has a “biconvex shape” in both the main scanning and sub-scanning directions. Therefore, the positive power in the main scanning and sub-scanning directions that the scanning optical system should have is incident. It is possible to suppress “deterioration of wavefront aberration” by distributing to the lens surfaces on the side and the exit side.
[0035]
Since the deterioration of wavefront aberration can be suppressed in this way, the spot diameter of the light spot on the surface to be scanned can be reduced, and high-density writing can be handled.
In order to effectively suppress the deterioration of the wavefront aberration, it is preferable that the radius of curvature of the entrance surface is larger than the radius of curvature of the exit surface, and the wavefront aberration is satisfied by satisfying the above conditions (4) and / or (5). It is possible to effectively suppress degradation of the.
[0036]
In order to reduce the spot diameter of the light spot, it is necessary to suppress the deterioration of the wavefront aberration, but there is a limit to performing this only with the scanning optical system. In this case, the “form of the light beam” incident on the scanning optical system is also used as a parameter for correcting the wavefront aberration, so that the deterioration of the wavefront aberration can be easily regulated. If the light beam incident on the scanning optical system is “convergent in the main scanning direction” as in the scanning optical system according to claim 10, the curvature of the scanning optical system in the main scanning direction can be reduced. This makes it easy to achieve a balance of improving the constant velocity characteristics while suppressing the deterioration.
[0037]
Horizontal magnification in the sub-scanning direction at the center image height: β ₂ is roughly: | β ₂ | = (distance from the exit lens surface of the scanning optical system to the scanned surface) / (scanning from the deflection starting point of the optical deflector) The distance to the incident-side lens surface of the optical system can be expressed by the following formula. However, when the lower limit of condition (1) is decreased beyond 0.5, the scanning optical system approaches the scanned surface side, and the scanning optical system is Increases cost.
[0038]
On the contrary, when the value exceeds the upper limit: 3.0, the scanning optical system approaches the optical deflector side, and when this is a resin molded product ( Claim 9 ), it is easily affected by the heat generated by the optical deflector. The optical performance is likely to deteriorate due to deformation. In addition, the imaging position of the deflected light beam is likely to be shifted with respect to the surface to be scanned and the spot diameter is likely to fluctuate, so that the processing accuracy and mounting accuracy of the parts become severe.
[0039]
In the scanning optical system according to the present invention, both lens surfaces are anamorphic surfaces, so that “surface tilt of the optical deflector” occurs in the sub-scanning direction while ensuring “constant velocity characteristics” such as fθ characteristics in the main scanning direction. It is possible to realize a function to be corrected ( claim 8 ).
[0040]
By the way, in the multi-beam optical scanning, it is important to keep the “deviation of the pitch of scanning lines simultaneously scanned (referred to as scanning line pitch)” small.
[0041]
“Scanning line pitch deviation” is defined as “difference between maximum value and minimum value between image heights of adjacent scanning line pitches”.
[0042]
In order to perform good optical scanning, the diameter of the light spot on the surface to be scanned (the diameter in the main scanning direction can be dealt with to some extent by electrical correction of the signal, but the diameter in the sub-scanning direction is corrected in this way. Therefore, it is also important that the spot diameter in the sub-scanning direction does not change greatly depending on the image height, and is particularly important in high-density optical scanning.
[0043]
In order to “the diameter of the light spot in the sub-scanning direction does not change greatly with the image height on the surface to be scanned”, the lateral magnification: β in the sub-scanning direction of the scanning optical system may not change greatly with the image height. is necessary.
[0044]
Further, the fluctuation due to the image height of the lateral magnification β in the sub-scanning direction causes a problem that the scanning line pitch of the scanning lines simultaneously scanned with the image height changes in the multi-beam optical scanning. Therefore, it is necessary to “correct the lateral magnification in the sub-scanning direction of the scanning optical system to be constant between the image heights” in order to suppress “fluctuation due to the image height of the scanning line pitch” in multi-beam optical scanning. .
[0045]
This can be achieved by employing a special surface on at least one surface. In addition, a polygon mirror, which is a general optical deflector, is installed with its rotational center shifted from the optical axis of the scanning optical system. An “optical sag” is generated in which the deflection starting point fluctuates. When a sag exists, the path through which the light beam passes is different between the + image height side and the −image height side with respect to the optical axis of the scanning optical system, and the lateral magnification in the sub-scanning direction “asymmetrically changes in the main scanning direction”.
[0046]
This asymmetric lateral magnification change can be corrected by making the “special surface” a “surface in which the change in the main scanning direction of the curvature in the sub-scan section is asymmetric with respect to the optical axis”.
[0047]
“A surface in which the change in the main scanning direction of the curvature in the sub-scanning section is asymmetric with respect to the optical axis” means, for example, (a) that the radius of curvature in the sub-scanning section increases monotonically monotonically as the distance from the optical axis increases in the main scanning direction. Surface (b) Surface whose radius of curvature in the sub-scanning section monotonously decreases as the distance from the optical axis in the main scanning direction (c) Extreme value of change in the main scanning direction of the radius of curvature in the sub-scanning section (D) Surface in which the change in the main scanning direction of the radius of curvature in the sub-scanning section in the main scanning direction monotonically increases from the + image height side to the −image height side. (E) The curvature in the sub-scanning section. A surface in which the change in radius in the main scanning direction is monotonously decreased from the + image height side toward the −image height side. (F) A surface in which the change in the radius of curvature in the sub-scanning section has two or more extreme values. Various aspects can be considered, such as "optical axis and Te no common axis of rotational symmetry "refers to all aspects. Which of these is adopted as a “surface with an asymmetric curvature change” is determined by design conditions.
[0048]
In this specification, the “optical axis” in such a lens having an asymmetric shape refers to the “reference axis orthogonal to the main scanning / sub-scanning directions” in the reference coordinate system for determining the lens surface shape. To do.
[0049]
Whether the optical scanning is a single beam method or a multi-beam method, the “horizontal magnification in the sub-scanning direction: absolute value of β: change in | β |” within the effective writing width is 10% or less. Is preferable, and more preferably 7% or less.
[0050]
That is, the lateral magnification at the center image height: and beta _2, the lateral magnification at an arbitrary image height ratio of the absolute value of the _{β h: | β h / β} 2 | is preferable to satisfy the condition (2), more Preferably,
0.93 ≦ | β _h / β ₂ | ≦ 1.07
Is preferably satisfied.
[0051]
In particular, in the case of optical scanning in the multi-beam method, if the change in lateral magnification in the sub-scanning direction is 7% or less, even when “seventh interlaced optical scanning at 1200 dpi” is performed, the simultaneous scanning line pitch is 148.19 μm. The pitch variation is 10.37 μm, and the adjacent pitch at 1200 dpi is suppressed to approximately half of 21.17 μm.
[0052]
In order to perform good optical scanning, it is also important that the beam waist position of the deflected light beam that forms the light spot on the surface to be scanned does not vary greatly depending on the image height. In particular, in order that the beam waist position in the sub-scanning direction does not vary greatly with respect to the surface to be scanned depending on the image height, it is necessary that the sub-scanning field curvature of the scanning optical system does not vary greatly with the image height.
[0053]
In other words, effective writing width: the sub-scan field curvature in the W (change) Width: F _S, it is preferable to satisfy the condition (3).
[0054]
In order to satisfy the condition (3) while satisfying the condition (2), it is necessary to consider “optical sag” for the sub-scanning image surface curvature. The effect of this “optical sag” generally does not necessarily coincide with the effect of “scanning optical system change in lateral magnification in the sub-scanning direction”, but to what extent the aim of the spot diameter on the surface to be scanned is. Whether or not the scanning optical system is arranged can have a certain relationship between the correction of the sub-scanning image surface curvature and the correction of the change in the lateral magnification.
[0055]
In this case, it employs a "special surface" only one surface, preferably the curvature of the sub-scan section along a surface opposite to the main scanning direction and a constant "constant curvature surface" (claim 1 ).
[0056]
The “special surface” has a complicated shape, and in particular, if “change in the main scanning direction of the curvature in the sub-scan section is asymmetric in the main scanning direction”, it is difficult to find the optical axis position. If it is “surface”, it is difficult to maintain the performance of the scanning optical system. For this reason, it is preferable to minimize the use of the “special surface” in terms of ensuring the performance of the scanning optical system and reducing the processing cost. In claim 1 , the “special surface” is adopted for only one surface in consideration of the above matters.
[0057]
If the surface opposite to the special surface is a “curvature constant surface”, the lens surface shape in the main scanning section is “maintained regardless of the height in the sub-scanning direction”. Even if the deviation occurs, it is advantageous in that it does not cause any problem in optical performance. Furthermore, since the curvature in the sub-scan section is constant regardless of the position in the main scanning direction, there is also an advantage that the scanning optical system can be easily evaluated.
[0058]
In addition, when the optical scanning device of the present invention is applied to the multi-beam method, the line image imaging optical system to the scanning optical system are “shared by a plurality of light beams” for each coupled light beam, thereby Since the image forming optical system and below can be configured in the same manner as the single beam type optical scanning device, it is possible to realize a multi-beam type optical scanning device that is extremely stable against mechanical fluctuations.
[0059]
As the light source of the multi-beam type optical scanning device, an LD array type or a “beam combining type” can be used. In the case of using an LD array type light source, it is possible to effectively reduce the thermal and electrical influence between the light emitting sources and perform good multi-beam optical scanning by setting the interval between the light emitting sources to 10 μm or more. It becomes possible.
[0060]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments will be described.
FIG. 1 shows only a main part of one embodiment of the optical scanning device. This optical scanning device is of a single beam type.
[0061]
A divergent light beam emitted from a light source 1 which is a semiconductor laser is coupled to a subsequent optical system by a coupling lens 2. The form of the light beam converted by the coupling lens 2 can be a weak divergent light beam, a weak convergent light beam, or a parallel light beam, depending on the optical characteristics of the optical system thereafter.
[0062]
When the light beam that has passed through the coupling lens 2 passes through the opening of the aperture 3, the peripheral portion of the light beam is blocked and “beam-shaped”, and enters the cylindrical lens 4 that is a “line image imaging optical system”. The cylindrical lens 4 has a power-less direction in the main scanning direction and a positive power in the sub-scanning direction, and converges the incident light beam in the sub-scanning direction. The polygon mirror 5 is an “optical deflector”. Is condensed as a “line image long in the main scanning direction” in the vicinity of the deflecting reflection surface.
[0063]
The light beam reflected by the deflecting reflecting surface is deflected at a constant angular velocity as the polygon mirror 5 rotates at a constant speed, passes through one lens 6 forming a “scanning optical system”, and folds the optical path by the bending mirror 7. The light is condensed as a light spot on the photoconductive photosensitive member 8 which is bent and forms a substance to be scanned, and the surface to be scanned is optically scanned.
[0064]
Prior to optical scanning of the photoconductor 8, the deflected light beam is reflected by the mirror 9, and is condensed on the light receiving element 11 by the lens 10. Based on the output of the light receiving element 11, the writing start timing of the optical scanning is determined.
[0065]
The “scanning optical system” is an optical system that condenses the light beam deflected by the optical deflector 5 as a light spot on the surface to be scanned 8, and is configured by a single lens 6. The lens 6 has a biconvex shape in both the main scanning direction and the sub-scanning direction.
[0066]
Scanning optical system, the sub-scanning direction lateral magnification of the central image height: beta ₂ is the condition:
(1) 0.5 ≦ | β ₂ | ≦ 3.0
Those satisfying the above are used.
[0067]
In this embodiment, the lens 6 is an anamorphic optical system having a function of geometrically conjugate with respect to the sub-scanning direction in the vicinity of the deflecting / reflecting surface and the photoconductor 8 as the scanned surface.
[0068]
In this embodiment, a “curvature constant surface” is used for one surface of the lens 6 and a “special surface” is used for the other surface. In the special surface, “a change in the main scanning direction of the curvature in the sub-scan section” is adopted. “Asymmetric with respect to optical axis”, lateral magnification in the sub-scanning direction at the center image height: β ₂ , lateral magnification in the sub-scanning direction at an arbitrary image height: β _h , conditions:
(2) 0.9 ≦ | β _h / β ₂ | ≦ 1.1
Satisfied, effective writing width: W, effective in the write width of the sub-scan field curvature Width: F _S condition:
(3) F _S /W<0.001
And those satisfying the conditions (4) and (5) are used.
[0069]
The “single beam optical scanning device” shown in FIG. 1 couples a light beam from a light source to a subsequent optical system by a coupling lens 2, and deflects the coupled light beam by a line image imaging optical system 4. A line image that is long in the main scanning direction is formed in the vicinity of the deflecting and reflecting surface of the scanner 5, deflected at a constant angular velocity by the optical deflector 5, and the deflected light beam is formed as a light spot on the surface to be scanned 8 by the scanning optical system 6. The optical scanning device collects light and optically scans the surface to be scanned 8, and the scanning optical system 6 according to any one of claims 1 to 9 can be used as the scanning optical system 6.
[0070]
The optical scanning device shown in FIG. 2 is of a “multi-beam type”. In order to avoid complications, the same reference numerals as those in FIG.
[0071]
The light source 1 is a semiconductor laser array in which four light emitting sources ch1 to ch4 are arranged in a line at equal intervals. Here, an embodiment arranged in the sub-scanning direction is shown. Of course, the light emitting source arrangement direction of the semiconductor laser array may be “tilted with respect to the sub-scanning direction”.
[0072]
The four light beams emitted from the four light emission sources ch1 to ch4 are divergent light beams in which the major axis direction of the “elliptical far field pattern” is directed to the main scanning direction, but the coupling lens 2 is common to the four light beams. By this, it is coupled to the subsequent optical system. The form of each coupled light beam can be a weak divergent light beam, a weak convergent light beam, or a parallel light beam, depending on the optical characteristics of the optical system thereafter.
[0073]
The four light beams transmitted through the coupling lens 2 are “beam-shaped” by the aperture 3 and are converged in the sub-scanning direction by the action of the cylindrical lens 4 which is a “common line image imaging optical system”. In the vicinity of the deflecting and reflecting surface of the polygon mirror 5 that is a “device”, the images are separated in the sub-scanning direction from each other as line images that are long in the main scanning direction.
[0074]
The four light beams deflected at a constant angular velocity by the deflecting reflecting surface are transmitted through one lens 6 forming a “scanning optical system”, and the optical path is bent by a bending mirror 7 to form a “scanned surface” entity. The light is condensed as four light spots separated in the sub-scanning direction on the body 8, and the four scanning lines on the surface to be scanned are simultaneously optically scanned.
[0075]
One of the deflected light beams is reflected by the mirror 9 prior to the optical scanning, and is condensed on the light receiving element 11 by the lens 10. Based on the output of the light receiving element 11, the write start timing of the optical scanning of each of the four light beams is determined.
[0076]
The “scanning optical system” is an optical system that collects four light beams simultaneously deflected by the optical deflector 5 as four light spots on the surface to be scanned 8, and is configured by a single lens 6. This lens 6 is the same as that described with reference to FIG. 1, and the lateral magnification of the central image height in the sub-scanning direction: β ₂ is the condition:
(1) 0.5 ≦ | β ₂ | ≦ 3.0
Those satisfying the above are used.
[0077]
As in the embodiment of FIG. 1, a “curvature constant surface” is adopted on one surface of the lens 6 and a “special surface” is adopted on the other surface, and the “main curvature direction of curvature in the sub-scan section” of the special surface. Is asymmetric with respect to the optical axis, the horizontal magnification in the sub-scanning direction at the center image height: β ₂ , and the horizontal magnification in the sub-scanning direction at an arbitrary image height: β _h are the conditions:
(2) 0.9 ≦ | β _h / β ₂ | ≦ 1.1
Satisfied, effective writing width: W, effective in the write width of the sub-scan field curvature Width: F _S condition:
(3) F _S /W<0.005
And those satisfying the conditions (4) and (5) are used.
[0078]
The multi-beam optical scanning device shown in FIG. 2 is coupled by coupling a plurality of light beams from a plurality of light emitting sources ch1 to ch4 to a subsequent optical system by a common coupling lens 2. The plurality of light beams are imaged as a plurality of line images that are long in the main scanning direction and separated in the sub-scanning direction in the vicinity of the deflection reflection surface of the optical deflector 5 by the common line image imaging optical system 4. 5 is simultaneously deflected at a constant angular velocity, and the deflected light beam is condensed on the scanned surface 8 as a plurality of light spots separated in the sub-scanning direction by the common scanning optical system 6, and a plurality of light spots are collected by the plurality of light spots. A multi-beam optical scanning device that simultaneously scans scanning lines, using the scanning optical system according to claim 10 as the common scanning optical system 6 ( claim 12 ), and a plurality of light sources No light source ch A monolithic semiconductor laser array 1 in which 1 to ch4 are arranged in a line is used ( claim 13 ).
[0079]
The optical scanning device shown in FIG. 3 is also of the multi-beam type. In this optical scanning device, a beam combining type is used as a light source.
The light sources 1-1 and 1-2 are semiconductor lasers, each having a single light source. The light beams emitted from the light sources 1-1 and 1-2 are coupled by the coupling lenses 2-1 and 2-2. The form of each coupled light beam can be a weak divergent light beam, a weak convergent light beam, or a parallel light beam, depending on the optical characteristics of the optical system thereafter.
[0080]
The light beams transmitted through the coupling lenses 2-1 and 2-2 are “beam-shaped” by the apertures 3-1 and 3-2 and enter the beam combining prism 20. The beam combining prism 20 has a reflecting surface, a polarization separation film, and a half-wave plate. The light beam from the light source 1-2 is reflected by the reflecting surface of the beam combining prism 20 and the polarization separation film, and exits the beam combining prism 20.
[0081]
The light beam from the light source 1-1 is rotated 90 degrees on the polarization plane by the half-wave plate, passes through the polarization separation film, and exits from the beam combining prism 20. In this way, the two light beams are combined. Due to the “positional adjustment” of the light-emitting portions of the light sources 1-1 and 1-2 with respect to the optical axes of the coupling lenses 2-1 and 2-2, the two combined beams form a minute angle in the sub-scanning direction. ing.
[0082]
The two combined beams are combined in the main scanning direction in the vicinity of the deflecting reflection surface of the polygon mirror 5 that is an “optical deflector” by the action of the cylindrical lens 4 that is a “common line image forming optical system”. As long line images, they are separated from each other in the sub-scanning direction.
[0083]
The two light beams deflected at a constant angular velocity by the deflecting reflecting surface are transmitted through one lens 6 forming a “scanning optical system”, and the optical path is bent by a bending mirror 7 to form a “scanned surface” entity. The light is condensed on the body 8 as two light spots separated in the sub-scanning direction, and two scanning lines on the surface to be scanned are simultaneously optically scanned.
[0084]
One of the light beams is condensed on the light receiving element 11 prior to the optical scanning, and the writing start timing of the optical scanning of the two light beams is determined based on the output of the light receiving element 11. Instead of doing this, each of the two light beams is condensed on the light receiving element 11 prior to the optical scanning, and the writing start timing of the optical scanning of the two light beams is individually set based on the output of the light receiving element 11. You may make it decide to.
[0085]
The “scanning optical system” is an optical system that condenses two light beams simultaneously deflected by the optical deflector 5 as two light spots on the surface to be scanned 8, and is configured by a single lens 6. This lens 6 is the same as that described with reference to FIGS. 1 and 2, and the lateral magnification in the sub-scanning direction of the central image height: β ₂ is the condition:
(1) 0.5 ≦ | β ₂ | ≦ 3.0
Those satisfying the above are used.
[0086]
As in the embodiment of FIGS. 1 and 2, a “special surface” is employed on one surface of the lens 6, and the change in curvature in the sub-scanning section in the main scanning direction is relative to the optical axis. Asymmetrical, sub-scanning direction lateral magnification of center image height: β ₂ , arbitrary image height sub-scanning direction lateral magnification: β _h , conditions:
(2) 0.9 ≦ | β _h / β ₂ | ≦ 1.1
Satisfied, effective writing width: W, effective in the write width of the sub-scan field curvature Width: F _S condition:
(3) F _S /W<0.001
And further satisfying the conditions (4) and (5) are used.
[0087]
In the multi-beam optical scanning device shown in FIG. 3, the light beams from a plurality of light sources 1-1 and 1-2 are coupled to the subsequent optical system by coupling lenses 2-1 and 2-2. The coupled and coupled light beams are combined into a plurality of line images that are long in the main scanning direction and separated in the sub-scanning direction in the vicinity of the deflection reflection surface of the optical deflector 5 by the common line image forming optical system 4. And deflecting the deflected light beam as a plurality of light spots separated in the sub-scanning direction on the surface to be scanned 8 by the common scanning optical system 6. A multi-beam optical scanning device that simultaneously scans a plurality of scanning lines with a plurality of light spots, and uses the scanning optical system according to claim 10 as the common scanning optical system 6.
[0088]
The “spot diameter of the light spot” in this specification is defined by 1 / e ² intensity in the line spread function of the light intensity distribution in the light spot on the surface to be scanned.
[0089]
The “line spread function” is a light spot light intensity distribution: f (Y, Z) based on coordinates Y, Z in the main scanning direction and the sub-scanning direction with reference to the center coordinates of the light spot formed on the surface to be scanned. Is defined as LSZ (Z) = ∫f (Y, Z) dY (integration is performed for the full width of the light spot in the Y direction), and the line spread function in the Y direction. : LSY is
LSY (Y) = ∫f (Y, Z) dZ (Integration is performed for the full width of the light spot in the Z direction).
[0090]
These line spread functions: LSZ (Z), LSY (Y) are generally of a Gaussian distribution type, and the spot diameters in the Y direction and Z direction are determined by the line spread functions: LSZ (Z), LSY (Y ) Is given by the width in the Y and Z directions of a region that is 1 / e ² or more of the maximum value.
[0091]
The spot diameter defined above by the line spread function can be easily measured by scanning the light spot at a constant speed with a slit, receiving the light passing through the slit with a photodetector, and integrating the amount of light received. There are also commercially available devices for performing such measurements.
[0092]
【Example】
A specific example will be given below. The optical arrangement as the optical scanning device is as shown in FIG.
[0093]
The shape of the lens surface and the like are according to the following formula.
"Non-arc shape in main scanning section"
Paraxial radius of curvature in main scanning section: R _m , distance in main scanning direction from optical axis: Y, conic constant: K, higher order coefficients: A ₁ , A ₂ , A ₃ , A ₄ , A ₅ , A ₆ ,... Depth in the optical axis direction: X is used to represent a known polynomial (6).
X = (Y ² / R _m ) / [1 + √ {1- (1 + K _m ) (Y / R _m ) ² }] + A ₁ Y + A ₂ Y ² + A ₃ Y ³ + A ₄ Y ⁴ + A ₅ Y ⁵ + A ₆ Y ⁶ + ・ ・ （6）
In the equation (6), when one or more of odd-order coefficients: A ₁ , A ₃ , A ₅ ,... Is not 0, an asymmetric shape is formed in the main scanning direction.
[0094]
"Curvature in sub-scan section"
When the curvature in the sub-scan section (the reciprocal of the radius of curvature) changes in the main scan direction (coordinate with the optical axis position as the origin: Y), the curvature in the sub-scan section: C _S (Y) It is expressed by equation (7). R _S (0) represents the “radius of curvature on the optical axis” in the sub-scan section, and B ₁ , B ₂ ... Represent higher-order coefficients.
[0095]
C _S (Y) = {1 / R _S (0)} + B ₁ Y + B ₂ Y ² + B ₃ Y ³ + B ₄ Y ⁴ + B ₅ Y ⁵ + B ₆ Y ⁶ + (7)
In equation (7), when one or more of odd-order coefficients of Y: B ₁ , B ₃ , B ₅ ,... Is not 0, the change in the radius of curvature in the sub-scanning section becomes “asymmetric in the main scanning direction”. . When the coefficients of Y: B ₁ , B ₂ , B ₃ ... Are all 0, a “curvature constant surface” is obtained.
[0096]
The analytical expression of the special surface is not limited to the above-described ones, and various kinds are possible, and the surface shape in the present invention is not limited to the expression by the above formula.
[0097]
Example 1
"light source"
Wavelength: 655nm
"Coupling lens"
Focal length: 15mm
Coupling action: Converging action The natural condensing point (the position where the converging light beam emitted from the coupling lens naturally converges when not receiving the refracting action of other optical elements) is the deflecting reflecting surface. The position is 686.76 mm from the side toward the surface to be scanned.
[0098]
"Cylindrical lens"
Focal length in the sub-scanning direction: 72mm
"Polygon mirror"
Number of deflecting reflecting surfaces: 6
Inscribed circle radius: 16mm
Angle formed by the incident angle of the light beam from the light source side and the optical axis of the scanning optical system: 60 degrees “data of the optical system between the polygon mirror and the surface to be scanned”
The radius of curvature is represented by “R _m ” in the main scanning direction, “R _s ” by the sub scanning direction, and the refractive index by “n”. In the following data, “R _m , R _s ” is “paraxial curvature radius”.
[0099]

In the above, X and Y represent the distance between the vertices in the surface numbers: i to i + 1 in the optical axis direction and the main scanning direction. For example, X = 47.89 and Y = 0.47 at the surface number: 0 (deflection reflection surface) are the incident surface (surface number) of the deflection reflection point position (reflection position giving image height: 0). 1) is 47.89 mm away in the optical axis direction (X direction) and 0.74 mm away in the main scanning direction (Y direction). X = 20.81 in the surface number: 1 gives the thickness of the lens 6 on the optical axis.
[0100]
The incident surface (surface number: i = 1) is a “curvature constant surface”, and the shape in the main scanning section is a non-arc shape represented by the above equation (6).
Table 1 lists the coefficients of this surface in the main scanning direction.
[0101]
[Table 1]

[0102]
The exit surface (surface number: i = 2) is a “special surface”, and the shape in the main scanning section is a non-arc shape symmetrical to the optical axis.
Table 2 lists the coefficients of the surface in the main scanning direction and the sub-scanning direction.
[0103]
[Table 2]

[0104]
The lateral magnification in the sub-scanning direction at the center image height of the scanning optical system of the first embodiment: β ₂ is β ₂ = 2.3.
[0105]
FIG. 5 shows the curvature of field (the solid line on the left is the sub-scanning field curvature and the broken line is the main scanning field) and the constant velocity characteristics (the right line is the linearity and the broken line is the fθ characteristic).
[0106]
Main scanning direction: 1.666 mm / 220 mm
Sub-scanning direction: 0.21mm / 220mm
Linearity: 0.555% / 220mm
Both the field curvature and constant velocity characteristics are corrected extremely well. In particular, the sub-scan field curvature satisfies the condition (3). That is,
(3) 0.21 / 220 = 0.00095 <0.001
Figure 6 is a sub-scanning direction lateral magnification of the central image height: for beta _2, the sub-scanning direction lateral magnification of any image height: shows changes in beta _h. Magnification change: | β _h / β ₂ |
0.998 ≦ | β _h / β ₂ | ≦ 1.013
And the condition (2) is satisfied and the correction is extremely good.
[0107]
FIG. 7 shows a change in curvature of the entrance surface (FIG. 7A) and exit surface (FIG. 7B) of the lens 6. FIG. In this embodiment, the incident surface is a “curvature constant surface”.
[0108]
FIG. 8 shows a “spot diameter depth curve (a variation in the spot shape with respect to the defocusing of the light spot)” for each image height of the light spot in the first embodiment. (A) relates to the main scanning direction, and (b) relates to the sub-scanning direction. In Example 1, about 50 μm is intended as the spot diameter defined by the 1 / e ² intensity of the line spread function. As shown in the figure, both the main scanning and sub-scanning directions have good depth, and the tolerance for the positional accuracy of the surface to be scanned is high.
[0109]
In the first embodiment, the lens 6 constituting the scanning optical system is made of a plastic material, but a glass material may be used as a matter of course. In order to further reduce the beam spot diameter, the sub-scanning direction may be a non-arc shape. It is also noted that aberrations can be corrected more preferably by decentering the scanning optical system.
[0110]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 9 shows an embodiment of the image forming apparatus.
This image forming apparatus is a laser printer.
[0111]
The laser printer 1000 has a “photoconductive photoreceptor formed in a cylindrical shape” as a photosensitive image carrier 1110. Around the image carrier 1110, a charging roller 1121, a developing device 1131, a transfer roller 1141, and a cleaning device 1151 are arranged as charging means. A “corona charger” can also be used as the charging means.
[0112]
An optical scanning device 1171 that performs optical scanning with the laser beam LB is provided, and “exposure by optical writing” is performed between the charging roller 1121 and the developing device 1131.
[0113]
In FIG. 9, reference numeral 1161 denotes a fixing device, reference numeral 1181 denotes a cassette, reference numeral 1191 denotes a registration roller pair, reference numeral 1201 denotes a paper feeding roller, reference numeral 1211 denotes a conveyance path, reference numeral 1221 denotes a discharge roller pair, reference numeral 1231 denotes a tray, reference numeral P Indicates transfer paper as a sheet-like recording medium.
[0114]
When forming an image, an image carrier 1110 that is a photoconductive photosensitive member is rotated at a constant speed in the clockwise direction, and the surface thereof is uniformly charged by a charging roller 1121, so that the optical beam of the laser beam LB of the optical scanning device 1171 is written. An electrostatic latent image is formed upon exposure to the image. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed.
[0115]
This electrostatic latent image is reversely developed by the developing device 1131, and a toner image is formed on the image carrier 1110. The cassette 1181 storing the transfer paper P is detachable from the main body of the image forming apparatus 1000. When the cassette 1181 is mounted as shown in the drawing, the uppermost sheet of the stored transfer paper P is fed by the paper supply roller 1201. Then, the transferred transfer paper P is fed by the registration roller pair 1191 at the leading end. The registration roller pair 1191 feeds the transfer paper P to the transfer unit in time with the toner image on the image carrier 1110 moving to the transfer position.
[0116]
The transferred transfer paper P is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 1141. The transfer paper P onto which the toner image has been transferred is sent to the fixing device 1161, where the toner image is fixed by the fixing device 1161, passes through the conveyance path 1211, and is discharged onto the tray 1231 by the discharge roller pair 1221.
[0117]
The surface of the image carrier 1110 after the toner image has been transferred is cleaned by a cleaning device 1151 to remove residual toner, paper dust, and the like.
[0118]
By using the “optical scanning device as shown in FIGS. 1, 2, and 3” as the optical scanning device 1171, extremely good image formation can be performed.
[0119]
【The invention's effect】
As described above, according to the present invention, a novel scanning optical system, optical scanning device, and image forming apparatus can be realized.
[0120]
As described above, the scanning optical system of the present invention can be manufactured at a low cost with a single lens configuration, the optical scanning device can be made compact, and not only geometrical aberration but also wave optical aberration can be corrected well. It is suitable for reducing the diameter of the light spot. A scanning optical system using such a scanning optical system can perform good optical scanning with a light spot having a reduced diameter. Therefore, an image forming apparatus using such an optical scanning apparatus can form a fine image.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining an embodiment of a single beam type optical scanning device;
FIG. 2 is a diagram for explaining one embodiment of a multi-beam type optical scanning device;
FIG. 3 is a diagram for explaining one embodiment of a multi-beam type optical scanning device;
FIG. 4 is a diagram illustrating an optical arrangement of an embodiment of an optical scanning device.
FIG. 5 is an aberration diagram (curvature of field and constant velocity characteristics) of the example.
FIG. 6 is a diagram showing a change in magnification: | β _h / β ₂ | in an example.
FIG. 7 is a diagram illustrating a change in a main scanning direction of a curvature in a sub-scan section of an incident side surface (curvature constant surface) and an exit side surface (special surface) of a scanning optical system according to an embodiment.
FIG. 8 is a diagram illustrating a “spot diameter depth curve” for each image height of a light spot in the example.
FIG. 9 is a diagram for explaining one embodiment of an image forming apparatus.
[Explanation of symbols]
5 Optical deflector (polygon mirror)
6 Scanning optical system

Claims (14)

A scanning optical system for condensing a light beam deflected by an optical deflector as a light spot on a surface to be scanned,
Consists of one lens,
It has a biconvex shape in the main scanning direction and a biconvex shape in the sub scanning direction, and both surfaces are anamorphic surfaces, and one surface is a constant curvature surface in which the curvature in the sub scanning section is constant in the main scanning direction. The other surface is a special surface whose curvature in the sub-scanning section changes continuously in the main scanning direction and asymmetrically with respect to the optical axis,
Sub-scanning direction lateral magnification at the central image height: beta ₂ is the condition:
(1) 0.5 ≦ | β ₂ | ≦ 3.0
A scanning optical system characterized by satisfying The scanning optical system according to claim 1.
  A scanning optical system characterized in that at least one surface has a non-arc shape in a main scanning section. The scanning optical system according to claim 1 or 2,
Horizontal magnification in the sub-scanning direction at the center image height: β ₂ Lateral magnification in the sub-scanning direction at an arbitrary image height: β _h But the condition:
(2) 0.9 ≦ | β _h / Β ₂ | ≦ 1.1
A scanning optical system characterized by satisfying The scanning optical system according to any one of claims 1 to 3,
Effective writing width: W, effective writing width in the sub-scanning curvature in the width: F _S condition:
(3) F _S /W<0.001
A scanning optical system characterized by satisfying The scanning optical system according to any one of claims 1 to 4,
Radius of curvature of incident surface in main scanning direction: R _m1 , Radius of curvature of the exit surface: R _m2 But the condition:
  (4) | R _m1 ｜ ＞ ｜ R _m2 ｜
A scanning optical system characterized by satisfying The scanning optical system according to any one of claims 1 to 5,
  Radius of curvature of incident surface in sub-scanning direction: R _s1 , Radius of curvature of the exit surface: R _s2 But the condition:
  (5) | R _s1 ｜ ＞ ｜ R _s2 ｜
A scanning optical system characterized by satisfying The scanning optical system according to any one of claims 1 to 6,
  A scanning optical system which is deflected by an optical deflector and receives a convergent light beam in a main scanning direction. The scanning optical system according to any one of claims 1 to 7,
  In the main scanning direction, the light spot on the surface to be scanned has a function of optical scanning at a constant speed,
  A scanning optical system having a function of correcting surface tilt of an optical deflector in a sub-scanning direction. The scanning optical system according to any one of claims 1 to 8,
  A scanning optical system characterized by being molded of resin. The scanning optical system according to any one of claims 1 to 9,
  A scanning optical system that focuses a plurality of beams deflected simultaneously as a plurality of light spots separated in a sub-scanning direction on a surface to be scanned. The light flux from the light source is coupled to the subsequent optical system by a coupling lens, and the coupled light flux is coupled as a long line image in the main scanning direction near the deflection reflection surface of the optical deflector by the line image imaging optical system. A single beam type optical scanning device that images and deflects the deflected light beam at a constant angular velocity by the optical deflector, condenses the deflected light beam as a light spot on the surface to be scanned by the scanning optical system, and optically scans the surface to be scanned In
  An optical scanning apparatus using the scanning optical system according to any one of claims 1 to 9 as a scanning optical system. The light beams from multiple light sources are coupled to the subsequent optical system by a coupling lens, and the coupled multiple light beams are main-scanned near the position of the deflecting reflection surface of the optical deflector by a common line image imaging optical system. A plurality of line images that are long in the direction and separated in the sub-scanning direction are deflected at the same angular velocity simultaneously by the optical deflector, and a plurality of deflected light beams are scanned on the surface to be scanned by a common scanning optical system. In a multi-beam type optical scanning device that condenses as a plurality of light spots separated in the sub-scanning direction and simultaneously scans a plurality of scanning lines with the plurality of light spots,
  An optical scanning apparatus using the scanning optical system according to claim 10 as a common scanning optical system. The optical scanning device according to claim 12, wherein
  An optical scanning device characterized in that a monolithic semiconductor laser array in which a plurality of light emitting sources are arranged in a row is used as a light source. In an image forming apparatus that forms a latent image by performing optical scanning with a light scanning device on a photosensitive image carrier, and visualizes the latent image with a developing unit to obtain an image.
  An image forming apparatus using the optical scanning device according to any one of claims 11 to 13 as an optical scanning device for optically scanning an image carrier.

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