JP4201315B2 - Scanning optical system, optical scanning device, and image forming apparatus - Google Patents

Description

（７）式において、Ｙの奇数次係数：Ｂ_１、Ｂ_３、Ｂ_５、・・の１以上が０でないとき、副走査断面内の曲率半径の変化は「主走査方向に非対称」となる。
【００９８】
特殊面の解析表現は上に挙げたものに限らず種々のものが可能であり、この発明における面形状が上記式による表現に限定されるものではない。
【００９９】
実施例１
「光源」
波長：６５５ｎｍ
「カップリングレンズ」
焦点距離：１５ｍｍ
カップリング作用：収束作用
自然集光点（カップリングレンズから射出した収束性の光束が、他の光学素子の屈折作用を受けないとした場合に、自然に集光する位置）は、偏向反射面から被走査面側へ向って６８６．７６ｍｍの位置にある。
【０１００】
「シリンドリカルレンズ」
副走査方向の焦点距離：７２ｍｍ
「ポリゴンミラー」
偏向反射面数：６
内接円半径：１６ｍｍ
光源側からの光束の入射角と走査光学系の光軸とがなす角：６０度
「ポリゴンミラーと被走査面との間にある光学系のデータ」
曲率半径を、主走査方向につき「Ｒ_ｍ」、副走査方向につき「Ｒ_ｓ」、屈折率を「ｎ」で表す。尚、以下のデータにおける「Ｒ_ｍ、Ｒ_ｓ」は、「近軸曲率半径」である。
【０１０１】

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＝０．７４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）のプラス方向（光走査の終了側）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。
【０１０２】
入射側面（面番号：ｉ＝１）は「副走査断面内の曲率が主走査方向において、光軸に対して非対称的に変化する特殊面（上記（７）式により表される）」である。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、光軸に関して対称的である。
この面の主走査・副走査方向の各係数を表１に挙げる。
【０１０３】
【表１】

【０１０４】
射出側面（面番号：ｉ＝２）は「特殊面」で、主走査断面内の形状は「光軸に関して対称的な非円弧形状」であり、副走査断面内の曲率は、主走査方向において光軸に関して対称的（（７）式右辺のＹの奇数次係数：Ｂ_１、Ｂ_３、Ｂ_５、・・が何れも０である）に変化する。
【０１０５】
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。
【０１０６】
【表２】

【０１０７】
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｜＝２．２３である。
【０１０８】
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。
【０１０９】
主走査方向：１．６６ｍｍ/２２０ｍｍ
副走査方向：０．１６８ｍｍ/２２０ｍｍ
リニアリティ：０．５６％/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
（３）０．１６８／２２０＝０．０００７６＜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、
０．９８７≦｜β_ｈ/β_２｜≦１．０１０
と、条件（２）を満足し、極めて良好に補正されている。
【０１１０】
図７は、実施例１の走査光学系のレンズ面の「副走査断面内における曲率の、主走査方向における変化」を示している。(ａ)は入射側面、（ｂ）は射出側面である。図７(ａ)に示すように、実施例１の走査光学系の入射側面では「副走査断面内における曲率」は、主走査方向(図の左右方向)に光軸を±像高側に離れるに従って単調に減少し、減少の途上で、符号が＋から―に反転している。
【０１１１】
図８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。
【０１１２】
即ち、実施例１のレンズは、光偏向器により偏向される光束を、被走査面上に光スポットとして集光させる走査光学系であって、主走査断面内の形状が両凸形状、副走査断面内の近軸形状が両凸形状で、両面共にアナモフィックな面の１枚のレンズにより構成され、少なくとも１面は、主走査断面内の形状が非円弧形状であり、少なくとも１面(入射側面)は、副走査断面内の曲率が主走査方向において連続的に変化する特殊面であって、且つ、上記曲率の主走査方向における変化の途中で曲率の符号が反転する面形状であり(請求項１)、両面共に特殊面で、１つの特殊面(入射側面)が、その曲率の主走査方向における変化の途中で上記曲率の符号が反転する面であり(請求項２)、特殊面の少なくとも１面(入射側面)は、副走査断面内の曲率の主走査方向における変化が光軸に対して非対称で(請求項３)、曲率の主走査方向における変化の途中で曲率の符号が反転する面である(請求項４)。
【０１１３】
また、中心像高の副走査方向の横倍率：β_２は、条件（１）を満足し(請求項５)、中心像高の副走査方向横倍率：β_２、任意像高の副走査方向横倍率：β_ｈが、条件（２）を満足する(請求項６)。さらに、有効書込幅：Ｗ、有効書込幅内における副走査像面湾曲の幅：Ｆ_Ｓは条件（３）を満足する(請求項７)。
【０１１４】
主走査断面内において、入射側面の近軸曲率半径：Ｒ_ｍ１＝２３５．６１、射出側面の近軸曲率半径：Ｒ_ｍ２＝−１６５．８３で条件（４）を満足し(請求項８)、光軸を含む副走査断面内において、入射側面の曲率半径：Ｒ_ｓ１＝１２７．４９、射出側面の曲率半径：Ｒ_ｓ２＝−２５．８５で条件（５）を満足し(請求項９)、光偏向器により偏向された「主走査方向に収束性の光束」を入射され(請求項１０)、同時に偏向される複数光束を、被走査面上に、副走査方向に分離した複数の光スポットとして集光することもでき(請求項１１)、樹脂製のレンズとして成形され(請求項１２)、主走査方向において、被走査面上の光スポットの光走査を等速化する機能を有し、副走査方向において、光偏向器の面倒れを補正する機能を有する(請求項１３)。
【０１１５】
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、副走査断面内の形状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。
【０１１６】
【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。
【０１１７】
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。
【０１１８】
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。
【０１１９】
図１１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。
【０１２０】
画像形成を行うときには、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。
【０１２１】
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、転写紙Ｐを転写部へ送り込む。
【０１２２】
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。
【０１２３】
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。
【０１２４】
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、走査光学系として実施例１のもの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。
【０１２５】
【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。
【０１２６】
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】実施例の走査光学系を用いる光走査装置の光学配置を示す図である。
【図５】実施例１の収差図（左図：像面湾曲と右図：等速度特性）である。
【図６】実施例１における倍率変化：｜β_ｈ/β_２｜を示す図である。
【図７】実施例１における各レンズ面((ａ)は入射側面、（ｂ）は射出側面)の副走査断面内における曲率の、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」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 machines 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. Of course, a single-lens configuration is advantageous in terms of cost, but a scanning optical system composed of a single lens has an extremely small number of design parameters and is simple compared to a single-lens configuration. It is difficult to satisfactorily correct aberration and constant velocity characteristics with a simple lens surface shape.
[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 9-281422). JP-A-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 such demands, the reduction and stabilization of the light spot has been pursued. In order to reduce and stabilize the diameter of the light spot, it is important not only to correct geometrical aberrations but also to correct wave optical aberrations. Not mentioned at all.
[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.
This lens has a biconvex shape in the main scanning section, a biconvex shape in the “paraxial” section in the sub-scanning section, and both surfaces are anamorphic surfaces.
At least one of the lens surfaces has a “non-arc shape in the main scanning section”.
[0009]
Also, The incident side where the deflected beam is incident is “A special surface in which the curvature in the sub-scanning section changes continuously in the main scanning direction, and a surface shape in which the sign of the curvature is reversed during the change of the curvature in the main scanning direction”. In this plane, the paraxial shape in the sub-scan section including the optical axis is a convex shape.
[0010]
The “main scanning section” is a virtual flat section that passes through the lens optical axis of the scanning optical system and is parallel to the main scanning direction. 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 that is the scanning optical system.
[0011]
The “paraxial shape” is a shape in a region including the vicinity of the optical axis.
[0012]
As described above, the “special surface” is a surface in which the curvature in the sub-scanning section changes continuously in the main scanning direction. Therefore, when the position of the sub-scanning section is changed in the main scanning direction, the position of the sub-scanning section In accordance with the lens surface, the curvature in the sub-scan section changes.
[0013]
The scanning optical system according to claim 1 “Both sides are special surfaces, and the exit side is a special surface whose curvature in the sub-scan section changes continuously in the main-scan direction.” (Claim 2).
[0014]
The scanning optical system according to claim 1 or 2 may be “at least one of the special surfaces has an asymmetric change in the main scanning direction of the curvature in the sub-scan section” with respect to the optical axis. 3), in this case, a special 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, “the sign of the curvature in the middle of the change in the main scanning direction of the curvature in the sub-scanning section. Can be inverted. ”(Claim 4).
[0015]
The scanning optical system according to any one of claims 1 to 4, wherein the lateral magnification of the central image height in the sub-scanning direction: β ₂ But the condition:
(1) 0.5 ≦ | β ₂ | ≦ 3.0
Is preferably satisfied (Claim 5).
[0016]
In this specification, “lateral magnification of scanning optical system in the sub-scanning direction: | β |” means the object point of the scanning optical system conjugate to the image point in the vicinity of the scanned surface and the vicinity of the scanned surface in the sub-scanning direction. Defined as lateral magnification with image point. Of the lateral magnifications: | β |, the light spot image height at 0 (center image height) is the above | β ₂ |.
[0017]
The scanning optical system according to any one of claims 1 to 5, wherein the 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 But the condition:
(2) 0.9 ≦ | β _h / β ₂ | ≦ 1.1
Is preferably satisfied (claim 6).
[0018]
The scanning optical system according to claim 1, wherein the effective writing width is W, and the width of the sub-scanning image field curvature is F within the effective writing width. _S Is the condition:
(3) F _S /W<0.001
Is preferably satisfied (claim 7).
[0019]
“Effective writing width: W” refers to an optical scanning area in which image writing can be performed effectively 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”.
[0020]
The scanning optical system according to any one of claims 1 to 7, wherein the curvature radius of the incident side surface in the main scanning section: R _m1 , Radius of curvature of injection side: R _m2 But the condition:
(4) | R _m1 ｜ ＞ ｜ R _m2 ｜
Is preferably satisfied (Claim 8), and the scanning optical system according to any one of Claims 1 to 8, in the sub-scan section, has a curvature radius of the incident side surface: R. _s1 , Radius of curvature of injection side: R _s2 But the condition:
(5) | R _s1 ｜ ＞ ｜ R _s2 ｜
Is preferably satisfied (claim 9).
[0021]
These radii of curvature: R _m1 , R _m2 , R _s1 , R _s2 Is a “paraxial curvature radius” when the shape of the lens surface in the main / sub-scan section is a non-arc shape.
[0022]
The scanning optical system according to any one of claims 1 to 9 can receive a “convex light beam deflected by an optical deflector and converged in the main scanning direction” (claim 10).
[0023]
The scanning optical system according to any one of claims 1 to 10, wherein the scanning optical system condenses a plurality of simultaneously deflected light beams as a plurality of light spots separated in the sub-scanning direction on the surface to be scanned. (Claim 11). 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.
[0024]
The scanning optical system according to any one of claims 1 to 11 may be “molded as a resin lens” (claim 12).
[0025]
The scanning optical system according to any one of claims 1 to 12 has a "function to make the optical scanning of the light spot on the surface to be scanned at a constant speed" in the main scanning direction, and "light deflection in the sub-scanning direction". It is possible to have a function of correcting the surface tilt of the vessel (claim 13).
[0026]
15. The optical scanning device according to claim 14, wherein a light beam from a light source is coupled to a subsequent optical system by a coupling lens, and the coupled light beam is deflected by a line image imaging optical system. A single beam that forms a long line image near the position in the main scanning direction, deflects it with an 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 The scanning optical system according to the thirteenth aspect is used as a scanning optical system.
[0027]
The optical scanning device according to claim 15, wherein the 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. Near the position of the deflecting reflection surface of the detector, it is formed as a plurality of line images that are long in the main scanning direction and separated in the sub-scanning direction, deflected simultaneously by the optical deflector, and the deflected light beam is scanned by a common scanning optical system A multi-beam optical scanning device that condenses light beams separated in the sub-scanning direction on the surface and simultaneously scans a plurality of scanning lines with the light spots, and has a common scanning optical system. The scanning optical system according to claim 13 is characterized in that "a plurality of light beams deflected simultaneously is condensed as a plurality of light spots separated in the sub-scanning direction on the surface to be scanned" is used. .
[0028]
The “coupling lens” in the optical scanning device according to claim 15 may be individual for a plurality of light beams or may be made common. 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 15 (claim 16).
[0029]
In each of the above optical scanning devices, as the “optical deflector”, a rotary polygon mirror that rotates a polygon mirror can be suitably used. A mirror or the like can be used.
[0030]
The image forming apparatus according to the present invention is an “image forming apparatus that forms an image by performing optical scanning with a light scanning device on a photosensitive image carrier”, and performs optical scanning of the image carrier. As defined in any one of claims 14 to 16, the invention described in any one of claims 14 to 16 is used (claim 17).
[0031]
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.
[0032]
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.
[0033]
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. You can also.
[0034]
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).
[0035]
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 transferring to the intermediate transfer medium, the intermediate transfer medium may be transferred to the sheet-like recording medium (intermediate transfer system).
Such an image forming apparatus can be implemented as an optical printer, an optical plotter, a digital copying apparatus, a facsimile apparatus, or the like.
[0036]
The image forming apparatus of the present invention also arranges a plurality of the photoconductors along the conveyance path of the sheet-like recording medium, and forms an electrostatic latent image for each photoconductor using a plurality of optical scanning devices. The toner image obtained by visualizing the toner image can be transferred and fixed to the same sheet-like recording medium, and can be implemented as a “tandem image forming apparatus” that synthetically obtains a color image or a multicolor image.
[0037]
Some supplementary explanation.
The scanning optical system according to the present invention is composed of one lens, and has a “biconvex shape (at least a paraxial shape is a biconvex shape in the subscanning section)” in both the main scanning section and the subscanning section. Therefore, it is possible to suppress “deterioration of wavefront aberration” by appropriately distributing the “positive power” in the main scanning and sub-scanning directions that the scanning optical system should have to the lens surfaces on the entrance side and exit side. Become. Further, since both surfaces are anamorphic surfaces, the positive power in the main scanning direction and the sub-scanning direction can be distributed to both surfaces independently of each other.
[0038]
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.
Further, since “the shape in the main scanning section is a non-arc shape” of at least one surface of the lens surface, by adjusting the shape of the non-arc, “the constant velocity characteristics such as“ main scanning field curvature ”and fθ characteristics are obtained. Can be corrected well independently of the characteristics in the sub-scanning direction.
[0039]
In order to perform good light 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 cannot be 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, which is particularly important in high-density optical scanning.
[0040]
Causes of fluctuation of the spot diameter in the sub-scanning direction depending on the image height include the presence of field curvature in the sub-scanning direction and the change in the lateral magnification in the sub-scanning direction due to the image height.
[0041]
Also, in multi-beam optical scanning, it is defined as “deviation of scanning line pitch (scanning line pitch) simultaneously scanned optically” (“difference between maximum and minimum values between image heights of adjacent scanning line pitches”). However, if the horizontal magnification in the sub-scanning direction varies depending on the image height, “the scanning line pitch of the multiple scanning lines that are simultaneously scanned by the multi-beam optical scanning is the image height. Problem that changes with.
[0042]
In order to effectively suppress such fluctuations in field curvature and lateral magnification in the sub-scanning direction, the scanning optical system according to the present invention requires that at least one surface “the curvature in the sub-scanning section changes continuously in the main scanning direction. It is a special surface and a surface shape in which the sign of the curvature is reversed during the change of the curvature in the main scanning direction.
[0043]
If a special surface is used, the refractive power in the sub-scanning direction on the special surface can be changed according to the position in the main scanning direction, and this is used to effectively correct field curvature and lateral magnification fluctuations. can do.
[0044]
In addition, when the sign of the special surface is reversed while the curvature in the sub-scanning section changes in the main scanning direction, the “main plane” in the sub-scanning direction of the lens constituting the scanning optical system is changed. The degree of freedom of position setting with respect to the image height is high, and the lateral magnification in the sub-scanning direction can be more easily “corrected between image heights”.
[0045]
In general, when a lens includes a planar shape, tolerance for mounting errors of the lens is large. Although the scanning optical system of the present invention is not flat on both sides, the special surface is a portion where the sign of curvature is reversed, and the shape in the sub-scanning direction is flat, so that the influence of mounting errors can be effectively reduced. Is possible. The more planes included in the cross-sectional shape in the sub-scanning direction, the closer the special surface is to a shape that is close to the plane while the curvature is changed, which is more effective in reducing the influence of mounting errors.
[0046]
The scanning optical system according to claim 2, wherein both surfaces are special surfaces, and at least one special surface is a surface in which the sign of the curvature is reversed in the middle of a change in the curvature in the main scanning direction. The effect of the above-described field curvature correction and lateral magnification equalization can be further enhanced, and the effect of reducing the influence of mounting errors is further enhanced.
[0047]
Since the rotation center of a general polygon mirror or rotating dihedral mirror as an optical deflector is set to be shifted from the optical axis of the scanning optical system, the reflection point on the deflecting reflecting surface is displaced along with the light beam deflection. An “optical sag” is generated in which the starting point of deflection of the deflected light beam fluctuates.
When a sag exists, the path through which the light beam passes differs 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 “changes asymmetrically in the main scanning direction”. In such a case, “asymmetrical lateral magnification change” means that at least one of the “special surfaces” is “a change in the curvature in the sub-scanning section in the main scanning direction is the same as in the scanning optical system according to claim 3. By making the surface asymmetric with respect to the axis, it can be corrected well.
[0048]
“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”
(A) Surface in which the radius of curvature in the sub-scanning cross section monotonously increases asymmetrically with increasing distance from the optical axis in the main scanning direction
(B) Surface whose radius of curvature in the sub-scanning section monotonously decreases asymmetrically as the distance from the optical axis in the main scanning direction
(C) The surface where the extreme value of the change in the main scanning direction of the radius of curvature in the sub-scanning section is outside the optical axis
(D) Surface in which the radius of curvature in the sub-scanning section monotonously increases from the + image height side to the −image height side in the main scanning direction.
(E) Surface in which the radius of curvature in the sub-scanning section monotonously decreases from the + image height side to the −image height side in the main scanning direction
(F) Surface where the change in the main scanning direction of the radius of curvature in the sub-scan section has two or more extreme values
Although various surfaces are conceivable, it refers to all surfaces such as these that do not have a general rotational symmetry axis as the optical axis. Which of these is adopted as the “surface with asymmetric curvature change” depends on the design conditions.
[0049]
In this specification, the “optical axis” in such an asymmetric lens refers to a “reference axis orthogonal to the main scanning / sub-scanning directions” in a reference coordinate system for determining the lens surface shape.
[0050]
Horizontal magnification in the sub-scanning direction at the center image height: β ₂ Is roughly β ₂ = (Distance from the exit-side lens surface of the scanning optical system to the scanned surface) / (Distance from the deflection starting point of the optical deflector to the entrance-side lens surface of the scanning optical system) ) Lower limit value: exceeding 0.5, the scanning optical system approaches the surface to be scanned, and the scanning optical system becomes larger, leading to an increase in cost.
[0051]
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 12), it is easily affected by the heat generated by the optical deflector. The optical performance is likely to deteriorate due to deformation. In addition, since the vertical magnification increases, the imaging position of the deflected light beam easily shifts with respect to the surface to be scanned and the spot diameter easily fluctuates due to the position variation in the optical axis direction of the scanning optical system, so that the processing accuracy of the parts The mounting accuracy becomes severe.
[0052]
Whether the optical scanning is a single beam system or a multi-beam system, the “lateral magnification in the sub-scanning direction: change in | β |” within the effective writing width is preferably 10% or less, and more preferably. Is preferably 7% or less. That is, the lateral magnification at the center image height: | β ₂ | And arbitrary image height: | β _h Ratio of |: β _h / Β ₂ | Preferably satisfies the condition (2), more preferably
0.93 ≦ | β _h / Β ₂ | ≦ 1.07
Is preferably satisfied.
[0053]
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.
[0054]
In order to perform good optical scanning, it is important that the spot diameter of the light spot (especially the spot diameter in the sub-scanning direction) does not vary depending on the image height. For this purpose, deflection that forms a light spot on the surface to be scanned is important. It is necessary that the beam waist position of the luminous flux 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, as described above, the sub-scanning field curvature of the scanning optical system may not vary greatly with the image height. is necessary.
[0055]
That is, the sub-scanning image field curvature (change) width: F within the effective writing width: W _S Satisfies the condition (3).
[0056]
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 influence of this “optical sag” generally does not necessarily coincide with the influence of “a change in lateral magnification of the scanning optical system in the sub-scanning direction”. For this reason, in order to correct the asymmetrical change of the lateral magnification and at the same time correct the asymmetry of the sub-scanning field curvature, “a special change in which the curvature in the sub-scanning section in the main scanning direction is asymmetric with respect to the optical axis” It may be preferred to employ "surface" on both sides of the scanning optical system.
[0057]
In order to effectively suppress the deterioration of the wavefront aberration, it is preferable to increase the curvature radius of the incident side surface with respect to the curvature radius of the exit side surface, and by satisfying the above conditions (4) and / or (5), the wavefront It is possible to effectively suppress the deterioration of aberration.
[0058]
In order to reduce the spot diameter of the light spot, there is a limit in performing “suppressing the deterioration of wavefront aberration” 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, thereby making it easy to regulate the deterioration of the wavefront aberration. 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 11, 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 iso-velocity characteristics while suppressing the deterioration.
[0059]
Since the scanning optical system of the present invention is “anamorphic surfaces for both lens surfaces”, “surface tilt of the optical deflector” is ensured in the sub-scanning direction while ensuring “equal velocity characteristics” such as fθ characteristics in the main scanning direction. It is possible to realize a function of correcting the above (claim 13).
[0060]
When the optical scanning device of the present invention is applied to the multi-beam method, the line image imaging is performed by “sharing the light beams from the line image imaging optical system to the scanning optical system to a plurality of light beams” for each coupled light beam. Since the optical system and below can be configured in the same manner as a single beam type optical scanning device, a multi-beam type optical scanning device with extremely good stability against mechanical fluctuations can be realized.
[0061]
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.
[0062]
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.
[0063]
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.
[0064]
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.
[0065]
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 (photosensitive image carrier) 8 which is bent and forms the substance of the “scanned surface”, and the scanned surface is optically scanned.
[0066]
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.
[0067]
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 the main scanning cross section and a biconvex shape in the sub scanning cross section, and both surfaces are anamorphic surfaces having different powers in the main scanning direction and the sub scanning direction.
[0068]
Further, at least one surface of the lens 6 in the main scanning section is a non-arc shape, and at least one of the lens surfaces is a special surface whose curvature in the sub-scanning section continuously changes in the main scanning direction. The surface shape is such that the sign of the curvature is reversed during the change of the curvature in the main scanning direction. The lens 6 preferably satisfies one or more of the above conditions (1) to (5).
[0069]
The lens 6 is an anamorphic optical system having a function of geometrically conjugate with respect to the sub-scanning direction between the vicinity of the position of the deflecting reflection surface and the photosensitive member 8 as the surface to be scanned. In the main scanning direction, the lens 6 is scanned. It has a function of making the optical scanning of the light spot on the surface constant, and has a function of correcting the surface tilt of the optical deflector in the sub-scanning direction.
[0070]
That is, the “single beam type optical scanning device” shown in FIG. 1 couples the light beam from the light source 1 to the subsequent optical system by the coupling lens 2, and the coupled light beam is the line image imaging optical system 4. Thus, a long line image in the main scanning direction is formed in the vicinity of the deflecting reflection surface of the optical deflector 5, deflected by the optical deflector 5, and the deflected light beam is formed as a light spot on the scanned surface 8 by the scanning optical system 6. A single-beam type optical scanning device that condenses and optically scans the surface to be scanned 8, and uses the scanning optical system according to claim 13 as the scanning optical system 6 (claim 14).
[0071]
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.
[0072]
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”.
[0073]
The four light beams emitted from the four light 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.
[0074]
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 position of the deflecting reflection surface of the polygon mirror 5 which 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.
[0075]
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”. The light is condensed on the body 8 as four light spots separated in the sub-scanning direction, and the four scanning lines on the scanned surface are simultaneously optically scanned.
[0076]
One of the deflected light beams is reflected by the mirror 9 prior to 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.
[0077]
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. The lens 6 is the same as that described with reference to FIG. 1 and preferably satisfies one or more of the above conditions (1) to (5).
[0078]
Also in this embodiment, the lens 6 is an anamorphic optical system having a function of geometrically conjugate with respect to the sub-scanning direction between the vicinity of the position of the deflecting reflection surface and the photoconductor 8 as the surface to be scanned. In the main scanning direction, it has a function of making the optical scanning of the light spot on the surface to be scanned uniform, and in the sub scanning direction, it has a function of correcting surface tilt of the optical deflector.
[0079]
That is, the multi-beam type optical scanning device shown in FIG. 2 is configured to couple light beams from a plurality of light emitting sources ch1 to ch4 to a subsequent optical system by the coupling lens 2, and to couple the light beams. Are formed 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 position of the deflecting reflection surface of the optical deflector 5 by the common line image forming optical system 4 and are simultaneously formed by the optical deflector 5. The deflected light beam is deflected and condensed as a plurality of light spots separated in the sub-scanning direction on the scanned surface 8 by the common scanning optical system 6, and a plurality of scanning lines are simultaneously optically scanned by the plurality of light spots. A scanning optical system according to claim 13, which is a multi-beam optical scanning device, wherein a plurality of light beams deflected simultaneously are separated on the surface to be scanned in the sub-scanning direction. Light spot As a light source, a monolithic semiconductor laser array 1 in which a plurality of light-emitting sources ch1 to ch4 are arranged in a row is used as a light source ( Claim 16).
[0080]
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. Also in this figure, in order to avoid complications, the same reference numerals as those in FIG.
[0081]
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.
[0082]
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.
[0083]
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 units 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.
[0084]
The two light beams synthesized by the beam are each in the main scanning direction near the position 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, the images are separated from each other in the sub-scanning direction.
[0085]
The two light beams deflected at a uniform 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.
[0086]
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.
[0087]
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. The lens 6 is the same as that described with reference to FIGS. 1 and 2, and preferably satisfies one or more of the above conditions (1) to (5).
[0088]
Also in this embodiment, the lens 6 is an anamorphic optical system having a function of geometrically conjugate with respect to the sub-scanning direction between the vicinity of the position of the deflecting reflection surface and the photoconductor 8 as the surface to be scanned. In the main scanning direction, it has a function of making the optical scanning of the light spot on the surface to be scanned uniform, and in the sub scanning direction, it has a function of correcting surface tilt of the optical deflector.
[0089]
That is, the multi-beam type optical scanning device shown in FIG. 3 is configured so that light beams from a plurality of light sources 1-1 and 1-2 are transmitted to the subsequent optical system by coupling lenses 2-1 and 2-2. A plurality of light beams coupled to each other by a common line image forming optical system 4 in the vicinity of the position of the deflecting reflection surface of the optical deflector 5 in the main scanning direction and separated in the sub-scanning direction. An image is formed and simultaneously deflected by the optical deflector 5, 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. 14. A multi-beam optical scanning apparatus for simultaneously scanning a plurality of scanning lines with a plurality of optical spots, wherein the scanning optical system according to claim 13 is used for multi-beam scanning as a common scanning optical system 6 (claim 11). (Claim 12) It is.
[0090]
The “spot diameter of the light spot” in this specification is 1 / e in the line spread function of the light intensity distribution in the light spot on the surface to be scanned. ² Defined by strength.
[0091]
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 determined, the line spread function in the Z direction: LSZ is
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 entire width of the light spot in the Z direction).
[0092]
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 1 / e of the maximum value ² It is given by the widths in the Y and Z directions of the above region.
[0093]
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.
[0094]
【Example】
Specific examples relating to the scanning optical system will be described below. The optical arrangement as the optical scanning device is as shown in FIG.
[0095]
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 the main scanning direction from the optical axis: Y, conic constant: K _m , Higher order coefficient: A ₁ , A ₂ , A ₃ , A ₄ , A ₅ , A ₆ ,... Depth in optical axis direction: X is represented by a well-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 equation (6), odd-order coefficient: A ₁ , A ₃ , A ₅ When one or more of... Is not 0, the shape is asymmetric in the main scanning direction.
[0096]
"Curvature in sub-scan section"
When the curvature in the sub-scan section (the reciprocal of the radius of curvature) changes in the main scanning direction (coordinates with the optical axis position as the origin: Y), the curvature in the sub-scan section: C _S (Y) is expressed by the following equation (7). R _S (0) represents “the radius of curvature on the optical axis” in the sub-scan section, and B ₁ , B ₂ ・ ・ Etc. represent higher-order coefficients.
[0097]

In the equation (7), odd-order coefficient of Y: B ₁ , B ₃ , B ₅ When one or more of... Is not 0, the change in the radius of curvature in the sub-scanning section becomes “asymmetric in the main scanning direction”.
[0098]
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 according to the above formula.
[0099]
Example 1
"light source"
Wavelength: 655nm
"Coupling lens"
Focal length: 15mm
Coupling action: convergence effect
The natural condensing point (the position where the converging light beam emitted from the coupling lens naturally converges when not receiving the refractive action of other optical elements) is from the deflecting / reflecting surface to the scanned surface side. It is at a position of 686.76 mm.
[0100]
"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 scanned surface"
The radius of curvature is “R” in the main scanning direction. _m ”,“ R ”in the sub-scanning direction _s ", And the refractive index is represented by" n ". In the following data, “R _m , R _s "Is a" paraxial radius of curvature ".
[0101]

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 = 0.74 in the surface number: 0 (deflection reflection surface) are the incident surface (surface number) of the lens 6 with respect to 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 positive direction (end side of the optical scanning) 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.
[0102]
The incident side surface (surface number: i = 1) is “a special surface in which the curvature in the sub-scan section changes asymmetrically with respect to the optical axis in the main scanning direction (expressed by the above equation (7))”. . The shape in the main scanning section is a non-arc shape represented by the above formula (6) and is symmetric with respect to the optical axis.
Table 1 lists the coefficients of this surface in the main scanning and sub-scanning directions.
[0103]
[Table 1]

[0104]
The exit side surface (surface number: i = 2) is a “special surface”, the shape in the main scanning section is “a non-arc shape symmetric with respect to the optical axis”, and the curvature in the sub-scanning section is in the main scanning direction. Symmetrical with respect to the optical axis (odd order coefficient of Y on the right side of equation (7): B ₁ , B ₃ , B ₅ ,... Are all 0).
[0105]
Table 2 lists the coefficients of the surface in the main scanning direction and the sub-scanning direction.
[0106]
[Table 2]

[0107]
Lateral magnification in the sub-scanning direction at the center image height of the scanning optical system of Example 1: | β ₂ | Is | β ₂ | = 2.23.
[0108]
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).
[0109]
Main scanning direction: 1.66 mm / 220 mm
Sub-scanning direction: 0.168mm / 220mm
Linearity: 0.56% / 220mm
Both the field curvature and constant velocity characteristics are corrected extremely well. In particular, the sub-scanning field curvature satisfies the condition (3).
(3) 0.168 / 220 = 0.00076 <0.001
FIG. 6 shows the horizontal magnification of the central image height in the sub-scanning direction: | β ₂ Horizontal magnification in the sub-scanning direction at an arbitrary image height with respect to | _h The change of | Magnification change: | β _h / Β ₂ |
0.987 ≦ | β _h / β ₂ | ≦ 1.010
And the condition (2) is satisfied and the correction is extremely good.
[0110]
FIG. 7 shows “change in curvature in the sub-scan section in the main scanning direction” of the lens surface of the scanning optical system of Example 1. (a) is an incident side surface, (b) is an emission side surface. As shown in FIG. 7A, on the incident side surface of the scanning optical system of the first embodiment, the “curvature in the sub-scanning section” moves the optical axis away from the ± image height side in the main scanning direction (left-right direction in the figure). As the rate decreases, the sign changes from + to-.
[0111]
FIG. 8 shows a “spot diameter depth curve (variation of spot shape with respect to 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, 1 / e of the line spread function ² The spot diameter defined by the intensity is intended to be about 50 μm. 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.
[0112]
That is, the lens of the first embodiment is a scanning optical system that condenses the light beam deflected by the optical deflector as a light spot on the surface to be scanned, and the shape in the main scanning section is a biconvex shape, and the sub-scanning. The paraxial shape in the cross section is a biconvex shape, and both surfaces are constituted by one lens having an anamorphic surface. At least one surface has a non-arc shape in the main scanning cross section, and at least one surface (incident side surface) ) Is a special surface in which the curvature in the sub-scan section changes continuously in the main scanning direction, and is a surface shape in which the sign of the curvature is reversed during the change of the curvature in the main scanning direction (claim) Item 1), both surfaces are special surfaces, and one special surface (incident side surface) is a surface in which the sign of the curvature is reversed during the change of the curvature in the main scanning direction (Claim 2). At least one surface (incident side surface) is the main run of curvature in the sub-scan section. The change in the inspection direction is asymmetric with respect to the optical axis (Claim 3), and the sign of the curvature is reversed during the change of the curvature in the main scanning direction (Claim 4).
[0113]
Also, the lateral magnification of the center image height in the sub-scanning direction: β ₂ Satisfies the condition (1) (Claim 5) and the lateral magnification in the sub-scanning direction of the center image height: β ₂ , Horizontal magnification in the sub-scanning direction at any image height: β _h Satisfies the condition (2) (Claim 6). Further, the effective writing width: W, the width of the sub-scanning image field curvature within the effective writing width: F _S Satisfies the condition (3) (Claim 7).
[0114]
In the main scanning section, the paraxial radius of curvature of the incident side surface: R _m1 = 235.61, paraxial radius of curvature of exit side: R _m2 = 165.83, which satisfies the condition (4) (Claim 8), and within the sub-scan section including the optical axis, the curvature radius of the incident side surface: R _s1 = 127.49, radius of curvature of exit side: R _s2 = 25.85, satisfying the condition (5) (Claim 9), the "convergent light beam in the main scanning direction" deflected by the optical deflector is incident (Claim 10), and a plurality of beams deflected simultaneously. The light beam can also be condensed as a plurality of light spots separated in the sub-scanning direction on the surface to be scanned (claim 11), molded as a resin lens (claim 12), and in the main scanning direction, It has a function of making the optical scanning of the light spot on the surface to be scanned uniform, and has a function of correcting surface tilt of the optical deflector in the sub-scanning direction.
[0115]
In the first embodiment, the lens 6 constituting the scanning optical system is made of a plastic material, but of course, a glass material may be used. In order to further reduce the beam spot diameter, the shape in the sub-scanning cross section may be a non-arc shape. In addition, it is possible to perform aberration correction more preferably by decentering the scanning optical system.
[0116]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 9 shows an embodiment of the image forming apparatus.
This image forming apparatus is a laser printer.
[0117]
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.
[0118]
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.
[0119]
In FIG. 11, 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.
[0120]
When forming an image, the image carrier 1110 which is a photoconductive photosensitive member is rotated at a constant speed in the clockwise direction, and the surface thereof is uniformly charged by the charging roller 1121, so that the optical writing of the laser beam LB of the optical scanning device 1171 An electrostatic latent image is formed upon exposure by. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed.
[0121]
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 sends the transfer paper P to the transfer unit in synchronization with the timing when the toner image on the image carrier 1110 moves to the transfer position.
[0122]
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.
[0123]
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.
[0124]
By using the “optical scanning device as shown in FIGS. 1, 2, and 3” as the optical scanning device 1171 and using the scanning optical system of Example 1, extremely good image formation can be executed.
[0125]
【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.
[0126]
The scanning optical system of the present invention can be manufactured inexpensively with a single lens configuration as described above, the optical scanning apparatus can be made compact, and not only geometrical aberration but also wave optical aberration can be corrected well. Yes, 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 showing an optical arrangement of an optical scanning device using the scanning optical system of the embodiment.
FIG. 5 is an aberration diagram of Example 1 (left diagram: field curvature and right diagram: constant velocity characteristics).
FIG. 6 shows a change in magnification in Example 1: | β _h / β ₂ FIG.
7 is a diagram illustrating a change in a main scanning direction of a curvature in a sub-scanning section of each lens surface ((a) is an incident side surface, and (b) is an exit side surface) in Embodiment 1. FIG.
8 is a diagram showing a “spot diameter depth curve” for each image height of a light spot in Example 1. FIG.
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 (17)

A scanning optical system that focuses a light beam deflected by an optical deflector as a light spot on a surface to be scanned,
The shape in the main scanning section is a biconvex shape, the paraxial shape in the sub-scanning section is a biconvex shape, and both surfaces are composed of one lens with an anamorphic surface,
At least one surface has a non-arc shape in the main scanning section,
On the incident side surface on which the light beam deflected by the optical deflector is incident, the curvature in the sub-scanning section continuously changes in the main scanning direction, and the sign of the curvature in the sub-scanning section is the main scanning A scanning optical system characterized by being a special surface having a surface shape that reverses in the middle of a change in direction. The scanning optical system according to claim 1.
A scanning optical system characterized in that the exit side surface is a special surface whose curvature in the sub-scanning section continuously changes in the main scanning direction . The scanning optical system according to claim 1 or 2,
At least one of the special surfaces is a scanning optical system characterized in that a change in curvature in the main scanning direction in the sub-scanning section is asymmetric with respect to the optical axis.   4. The scanning optical system according to claim 3, wherein a sign of the curvature in the course of the change of the curvature in the main scanning direction on a special surface where the change in the main scanning direction of the curvature in the sub-scan section is asymmetric with respect to the optical axis. A scanning optical system characterized in that is inverted. The scanning optical system according to any one of claims 1 to 4,
Sub-scanning direction lateral magnification of 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,
Sub-scanning direction lateral magnification of the central image height: β ₂ , and arbitrary image height lateral magnification in the sub-scanning direction: β _h are the conditions:
(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 6,
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 7,
In the main scanning section, the radius of curvature of the incident side surface: R _m1 and the radius of curvature of the exit side surface: R _m2 are the conditions:
(4) | R _m1 | >> _Rm2
A scanning optical system characterized by satisfying The scanning optical system according to any one of claims 1 to 8,
In the sub-scan section, the radius of curvature of the incident side surface: R _s1 and the radius of curvature of the exit side surface: R _s2 are the conditions:
(5) A scanning optical system satisfying | R _s1 |> | R _s2 |. The scanning optical system according to any one of claims 1 to 9,
A scanning optical system, wherein a convergent light beam deflected by an optical deflector is incident in a main scanning direction. The scanning optical system according to any one of claims 1 to 10,
A scanning optical system for condensing a plurality of simultaneously deflected light beams as a plurality of light spots separated in a sub-scanning direction on a surface to be scanned. The scanning optical system according to any one of claims 1 to 11,
A scanning optical system formed as a resin lens. The scanning optical system according to any one of claims 1 to 12,
A scanning optical system having a function of making the optical scanning of the light spot on the surface to be scanned uniform in the main scanning direction and a function of correcting the surface tilt of the optical deflector in the sub scanning direction . The light beam from the light source is coupled to a subsequent optical system by a coupling lens, and the coupled light beam is coupled as a long line image in the main scanning direction in the vicinity of the deflection reflection surface of the optical deflector by the line image imaging optical system. In an optical scanning device of a single beam system, the image is deflected by the optical deflector, 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.
An optical scanning apparatus using the scanning optical system according to claim 13 as the 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 formed and simultaneously deflected by the optical deflector, and the deflected light beam is separated in the sub-scanning direction on the surface to be scanned by the common scanning optical system. In a multi-beam optical scanning device that collects light as a plurality of light spots and simultaneously scans a plurality of scanning lines with the plurality of light spots.
14. The scanning optical system according to claim 13, wherein a plurality of light beams deflected simultaneously are condensed as a plurality of light spots separated in the sub-scanning direction on the surface to be scanned. A multi-beam optical scanning device characterized by the above. The multi-beam optical scanning device according to claim 15,
A multi-beam type optical scanning device characterized in that a monolithic semiconductor laser array in which a plurality of light emitting sources are arranged in a line is used as a light source. In an image forming apparatus for forming an image by performing optical scanning with an optical scanning device on a photosensitive image carrier,
An image forming apparatus using the optical scanning device according to any one of claims 14 to 16 as an optical scanning device for optically scanning an image carrier.

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