JP3725182B2

JP3725182B2 - Optical scanning device

Info

Publication number: JP3725182B2
Application number: JP14778093A
Authority: JP
Inventors: 雄二郎野村
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 1993-06-18
Filing date: 1993-06-18
Publication date: 2005-12-07
Anticipated expiration: 2020-12-07
Also published as: JPH0713097A

Description

【０００１】
【産業上の利用分野】
本発明はレーザープリンタ、ファクシミリなどの画像形成装置およびそれに用いられる光走査装置に関する。
【０００２】
【従来の技術】
従来のレーザービームプリンタ等に用いられている回転多面鏡には、通常、多面鏡の製作過程で生じる各面の平行度の誤差および回転軸と多面鏡の取り付け誤差があり、反射された光束の進行方向が偏向面に垂直な面内で変化して、走査ピッチむらが生じる。また回転多面鏡の回転軸に生じる歳差運動等の回転不良も同様な走査ピッチむらの原因となる。回転多面鏡や偏向装置全体の精度をピッチむらが画像上で問題のないレベルにまで高めるには製造に長時間を要して、装置が極めて高価なものになってしまう。また一方、これを補正するための方法、いわゆる面倒れ補正光学系が種々考案されている。
【０００３】
特開昭４８−４９３１５号、特開昭５６−３６６２２号に示されたものは共役型面倒れ補正方式と呼ばれるもので、主走査面と主走査面に垂直で光軸を含む面（以下副走査面と記す）のパワーの異なるアナモルフィックな走査光学系を用い、副走査面では回転多面鏡の反射面に光束を結像させ、反射面と感光体面が光学的に共役となるようにしたものであり、特開昭５２−１５３４５６号、特開昭５８−１３４６１８号に示されたものは緩和型面倒れ補正方式と呼ばれるもので、感光体面近傍に長尺のシリンドリカルレンズを配して、面倒れによる副走査面の偏向方向変化を緩和するようにしたものである。
【０００４】
回転多面鏡を用いた光学系の場合は図２３（ａ）に示すように光学系の副走査断面を光軸に沿って展開して考えると、回転多面鏡の偏向面７に面倒れがある場合、光源１から出射しコリメートレンズ２により平行にされた光束は偏向面７と光軸との交点Ｐで図に示す破線のように副走査方向に偏向され、ｆθレンズ４を通過し、被走査面５上で光軸を含む主走査面から外れ走査ピッチ誤差ΔＸが生じる。以下点Ｐのように偏向面の面倒れによって副走査方向に光束が偏向される点を面倒れ偏向点と呼ぶと、回転多面鏡を用いた光学系の場合は面倒れ偏向点は偏向面上にあるため、図２３（ｂ）に示す共役型面倒れ補正方式のように偏向面７とコリメートレンズ２の間に副走査方向だけにパワーを持つシリンドリカルレンズ６を配して光源１、偏向面７、被走査面５を光学的に共役関係とすることで偏向面７に面倒れがあっても、ほぼ完全に面倒れを補正することができる。
【０００５】
【発明が解決しようとする課題】
ところが、これらのものは、走査ピッチむらがなく高精度な光走査が可能なものの、これらの装置に用いられている回転多面鏡は光源部から照射される光束を偏向するだけで、光束の収差補正や光束を被走査面に結像させるといった機能を持つことができないため、レンズ枚数が増え装置の大型化、高価格化を招いてしまうといった問題点を有しており、また装置が大型、高価格にならざるを得ず、近年のパソコン等のＯＡ機器のダウンサイジング、パーソナルユースへの流れの妨げとなっていた。
【０００６】
一方、本出願人は特願平４−１６６０４２号において、回転軸とともに回転する反射面と所定の収差補正を行うごとく形状を定められた入射面と射出面とからなる光学素子（以下回転レンズ鏡と称す）を用いた光走査方式を提案した。この方式では、偏向面を１面とすることで面倒れ補正光学系をなくし、光束を偏向するポリゴンミラーと収差を補正するレンズの２つの部材の機能を回転レンズ鏡だけで済ませることができるため、部品点数が少ない小型低価格な光走査装置を構成することができる利点を有しているが、偏向面が１面の場合には回転レンズ鏡１回転で１回の走査しかできず、走査効率が悪いといった点でさらに改善の余地が残されている。
【０００７】
本発明は上述の課題を解決するため特願平４−１６６０４２に改良を加え、複数の回転レンズ鏡を用いた走査光学系に面倒れ補正機能を付加することで、小型低価格化と高速高解像度化を同時に実現する光走査装置を提供するものである。
【０００８】
【課題を解決するための手段】
本発明の１つの光走査装置は、入射屈折面と平面反射面と射出屈折面とを有する複数の光学素子を回転させることによって光束を偏向走査する光走査装置において、
前記光学素子の副走査方向のパワーが負で、
前記射出屈折面の副走査方向の面形状が凹であり、
前記入射屈折面の主走査方向の面形状が凹あるいは平面であり、
前記光学素子と被走査面の間に結像レンズが配置されていることを特徴とするものである。
本発明のもう１つの光走査装置は、入射屈折面と平面反射面と射出屈折面とを有する複数の光学素子を回転させることによって光束を偏向走査する光走査装置において、
前記光学素子の副走査方向のパワーが正で、
前記入射屈折面の副走査方向の面形状が凸で、前記射出屈折面の副走査方向の面形状が凹であり、
前記入射屈折面の主走査方向の面形状が凹あるいは平面であり、
前記光学素子と被走査面の間に配置される結像レンズが２枚で構成されていることを特徴とするものである。
【０００９】
【実施例】
本発明の光走査装置の基本構成は、複数の回転レンズ鏡をモータ等の駆動装置によって１つの回転軸のまわりを一体で回転させることにより、複数の回転レンズ鏡のそれぞれを光束が通過することによって回転軸１回転で複数本の走査を行うことができるようにしたもので、このように回転レンズ鏡を複数配置するとき、それぞれの回転レンズ鏡の取り付け誤差（面倒れ）により走査ピッチむらを生じるが、本発明は複数の回転レンズ鏡を用いた光走査装置において光学的に走査ピッチむらを補正するようにしたものである。図１は本発明の光学系を用いたレーザ走査装置についての代表的な実施例を示したものである。この装置は光源である半導体レーザ１より出射した光束がコリメータレンズ２によって平行な光束とされ、光束は回転レンズ鏡３の入射面Ｓ₁に入射した後、反射面Ｓ₂で反射され、射出面Ｓ₃から出射する。入射面Ｓ₁、射出面Ｓ₃は、走査中心を走査する光束がそれらの面を垂直に通過するように、また反射面Ｓ₂は走査中心を走査する光束が９０°以下の角度で反射面Ｓ₂に入射するように設定されている。回転レンズ鏡３は同形状なものが２枚用いられていて、回転軸Ｏに対して回転対称で反射面Ｓ₂どうしが背中合わせで接触するように配置されて一体的に回転し１回転当たり２回の光走査を行う。回転レンズ鏡３の回転軸Ｏは、反射面Ｓ₂に内包され、走査中心を走査するビームが反射する点を通る。光束は回転レンズ鏡３の回転に伴って偏向され、偏向された光束は結像レンズ４を通過し被走査面５上にスポットを形成する。
【００１０】
ここで、回転レンズ鏡を用いた走査光学系の面倒れについて説明すると、回転レンズ鏡を用いた走査光学系の回転レンズ鏡の面倒れを考えた場合、反射面の面倒れだけでなく入射面と射出面の面倒れも考慮しなければならない。回転レンズ鏡の面倒れを検討するため図２に示すように回転レンズ鏡３に固定した座標系Ｘ、Ｙ、Ｚを定義し、Ｘ軸は主走査面に垂直で反射面Ｓ₂に含まれる直線、Ｙ軸は主走査面と平行で反射面Ｓ₂に含まれる直線、Ｚ軸は反射面Ｓ₂の垂線とし、Ｘ軸は回転軸に一致し、Ｘ軸回りの回転レンズ鏡３の回転角度をωとし、Ｙ軸まわりの回転レンズ鏡３の回転角度をφとすると、図から解るように回転レンズ鏡３の面倒れはＹ軸回りの回転とＺ軸回りの回転の２軸存在する。Ｙ軸回りの回転では入射面Ｓ₁、反射面Ｓ₂、射出面Ｓ₃が面倒れし、Ｚ軸回りの回転では反射面Ｓ₂は面倒れせず、入射面Ｓ₁と射出面Ｓ₃のみ面倒れする。入射面Ｓ₁、射出面Ｓ₃の面倒れによる光束の副走査方向の偏向作用は、反射面Ｓ₂の面倒れによるそれよりもわずかであるので、回転レンズ鏡３の面倒れは面倒れ角がある程度小さければＹ軸回りの倒れのみ考慮すれば良い。そこで以下では回転レンズ鏡の面倒れはＹ軸回りの倒れのみ考え、Ｙ軸回りの回転を回転レンズ鏡の面倒れ、その回転角を面倒れ角φとする。
【００１１】
次に、回転レンズ鏡に面倒れがある場合の光束の様子を回転レンズ鏡を用いた走査光学系の副走査断面を光軸に沿って展開した図３を用いて説明する。図３（ａ）は光学系全体を図３（ｂ）は回転レンズ鏡だけを拡大して示している。回転レンズ鏡３が反射面Ｓ₂と光軸との交点Ｐを中心にＹ軸回りに角度φ回転すると入射面Ｓ₁と射出面Ｓ₃も点Ｐを中心に回転し、角度φが微小であれば近似的に入射面Ｓ₁と射出面Ｓ₃の光軸は、副走査断面で見ると主走査面に対してそれぞれφｓｉｎβ、−φｓｉｎβ回転する。ここで角度βは図２に示すように入射面Ｓ₁、射出面Ｓ₃の光軸が反射面Ｓ₂となす角度を示す。この面倒れがある回転レンズ鏡３に入射する光束は副走査方向に図中破線で示すように、まず入射面Ｓ₁で入射面Ｓ₁の倒れによりに偏向され、反射面Ｓ₂で反射面Ｓ₂の倒れによって角度２φ偏向され、そして射出面Ｓ₃でも射出面Ｓ₃の倒れにより偏向され回転レンズ鏡３から光軸とφ’の角度をなして出射する。回転レンズ鏡３から出射した光束は入射面Ｓ₁、射出面Ｓ₃での偏向作用のため見かけ上、反射面Ｓ₂上の点Ｐではなく光束の主光線と光軸との交点Ｐ’で偏向された如く振る舞い、点Ｐ’が回転レンズ鏡３の見かけ上の面倒れ偏向点となる。
【００１２】
次に面倒れがある回転レンズ鏡がＸ軸回りに回転して光束を偏向走査した場合を考える。図４（ａ）は光束が被走査面の走査中心を走査する時の回転レンズ鏡のＸ軸まわりの回転角度を０として回転レンズ鏡がＸ軸まわりに角度−ω、０、ω回転した時の光束の主光線を回転レンズ鏡を固定して示した回転レンズ鏡の主走査断面図である。回転レンズ鏡３に入射する主光線は回転レンズ鏡３の回転に伴い入射面Ｓ₁から入射面Ｓ₁の光軸に対しそれぞれ−ω、０、ωの角度をなして入射し、それぞれ反射面Ｓ₂上の点Ｐ₁、Ｐ₀、Ｐ₂で偏向され射出面Ｓ₃から出射する。図４（ａ）を図４（ｂ）に示すように副走査断面で光軸に沿って展開してみると偏向点Ｐは回転レンズ鏡の回転角により点Ｐ₁、Ｐ₀、Ｐ₂と光軸上を移動する。回転レンズ鏡３に倒れがあると図３（ｂ）で説明したように回転レンズ鏡３に入射する光束は入射面Ｓ₁で偏向され、次に回転角に対応したそれぞれの反射面Ｓ₂上の点Ｐ₁、Ｐ₀、Ｐ₂で角度２φ偏向され、そして射出面Ｓ₃でも偏向され出射する。射出面Ｓ₃から出射したそれぞれの光束は、出射光束の主光線と光軸とのそれぞれの交点Ｐ₁'、Ｐ₀'、Ｐ₂'を面倒れ偏向点として偏向されたように振る舞う。このように回転レンズ鏡では回転レンズ鏡の回転に伴い面倒れ偏向点が移動し、回転レンズ鏡からの出射光束の見かけ上の面倒れ偏向点も、回転レンズ鏡の回転により大きく移動してしまう。以下このような面倒れ偏向点の移動を面倒れ偏向点変位と呼び、その移動量を面倒れ偏向点変位量Δｄとし、また回転レンズ鏡から出射した光束の見かけ上の面倒れ偏向点の移動量を見かけ上の面倒れ偏向点変位量Δｄ’とする。
【００１３】
以上のような面倒れ偏向点変位がある走査光学系では、前に図２３を用いて述べた回転多面鏡を用いた走査光学系とは異なり、光源、偏向面、被走査面が共役関係となる光学系を構成しても、偏向面が変位し共役点からずれてしまうため面倒れを完全に補正することはできず、被走査面上の走査ピッチ誤差ΔＸは、次のような式で表される。
【００１４】
ΔＸ＝ｍ・Δｄ’・φ’ （１）
ここで、φ’は回転レンズ鏡から出射する光束の主光線が主走査面と直交する面内において光軸となす角を、ｍは結像レンズの副走査方向の光学倍率を、Δｄ’は回転レンズ鏡から出射する光束の見かけ上の面倒れ偏向点変位量を示す。上式でφ’は主に回転レンズ鏡の面倒れ角φに依存し、面倒れ角φは回転レンズ鏡取り付けの機械的精度で決められてしまう。このため光学的に走査ピッチ誤差ΔＸを小さくするためには、面倒れ偏向点変位量Δｄ’を小さくするか、あるいは結像レンズの副走査方向の光学倍率ｍを小さくする必要がある。このように回転レンズ鏡を用いた走査光学系で走査ピッチ誤差ΔＸを小さくする構成としては、主に面倒れ偏向点変位量Δｄを小さくする方式と、主に結像レンズの光学倍率ｍを小さくする方式に分類される。以下にそれぞれの方式について説明する。
【００１５】
図５は、面倒れ偏向点変位を小さくする方式についての本発明の説明図で、図５（ａ）は回転レンズ鏡を用いた走査光学系の副走査断面図であり、回転レンズ鏡は光軸に沿って展開して示してある。回転レンズ鏡３は副走査方向の入射面Ｓ₁が凸面、射出面Ｓ₃が凹面であり、全体として副走査方向に負のパワーを持っている。図に示すようにこの回転レンズ鏡３に入射する平行光束は入射面Ｓ₁で正のパワーにより集束光束となり、射出面Ｓ₃で負のパワーにより発散光束となって結像レンズを通って被走査面にスポットを形成する。この場合、光束はコリメータレンズから被走査面までの間では結像せず被走査面との共役点を持たないものの回転レンズ鏡３から出射した光束は、出射光束を回転レンズ鏡３側に延長して光軸と交わる点Ｑで見かけ上結像したように振る舞い、点Ｑが見かけ上の被走査面との共役点となっている。この回転レンズ鏡３が面倒れ角φで面倒れすると回転レンズ鏡３に入射した光束の主光線は図に点線で示すように反射面Ｓ₂で角度２φ偏向され、射出面Ｓ₃で光軸となす角度が大きくなるように偏向され出射する。この出射した光束の主光線を延長し光軸と交わる点Ｐ’が回転レンズ鏡３の見かけ上の面倒れ偏向点となり、前述の見かけ上の共役点Ｑの近傍に位置する。さらに、図５（ｂ）に示すように回転レンズ鏡３のＸ軸回りの回転により面倒れ偏向点がＰ₁、Ｐ₂のように変位する場合においては、回転レンズ鏡３から出射した光束の見かけ上の面倒れ偏向点は図に示すようにそれぞれの光束の主光線と光軸との交点Ｐ₁’、Ｐ₂’となる。この図から射出面Ｓ₃のパワーが負であるため実際の面倒れ偏向点変位量Δｄよりも見かけ上の面倒れ偏向点変位量Δｄ’の方が小さくなっていることが解る。このように回転レンズ鏡３の副走査方向のパワーが負で出射面Ｓ₃の副走査断面を凹の構成とすることで共役点からの面倒れ偏向点のずれ量Δｄを小さくすることができ前述の式（１）から走査ピッチ誤差ΔＸを低減させることができる。
【００１６】
次に結像レンズの副走査方向の光学倍率ｍを小さくする方式を図６を用いて説明する。図６は回転レンズ鏡を用いた走査光学系を主走査方向から見た図であり、回転レンズ鏡３は光軸に沿って展開して示してある。回転レンズ鏡３は副走査方向の入射面Ｓ₁が凸面、射出面Ｓ₃が凹面であり副走査方向は全体として正のパワーを持っている。結像レンズは、回転レンズ鏡３の近くに配置された第１の結像レンズ４１と被走査面付近に配置された第２の結像レンズ４２の２枚で構成されている。図に示すようにこの回転レンズ鏡３に平行光束が入射すると回転レンズ鏡３の正のパワーにより光束は第１の結像レンズ４１の手前点Ｑで結像し、第１の結像レンズ４１および第２の結像レンズ４２により被走査面５上にスポットを形成する。このように結像レンズを２枚で構成することにより、結像レンズの光学倍率ｍを小さく設定することができ前述の式（１）から走査ピッチ誤差ΔＸを低減させることができる。
【００１７】
ところで、このように面倒れによる走査ピッチ誤差を完全に補正できない光学系においては面倒れの補正効果を定量化しその値が一定以上あるいは以下となるよう光学設計をする必要がある。そこで、光学系の面倒れ補正効果を定量化した面倒れ補正率Ｈを以下のように定義する。
【００１８】
Ｈ＝ΔＸ（φ）／ΔＸ₀（φ）
＝ΔＸ（φ）／２Ｌ／φ （２）
ここでφは回転レンズ鏡の面倒れ角を、ΔＸ（φ）は回転レンズ鏡を用いた光学系において回転レンズ鏡が角度φ面倒れしたとき生じる副走査方向の最大のピッチ誤差を、Ｌは回転レンズ鏡の回転中心から被走査面までの距離を、ΔＸ₀（φ）は偏向面と被走査面の間にレンズ等の光学素子が存在しない場合に偏向面の倒れφによって生じる副走査方向のピッチ誤差すなわち２Ｌ・φを示す。面倒れ角度φがある程度小さい場合ΔＸ（φ）とφの関係は式（１）に示したような線形となり、面倒れ補正率Ｈは面倒れ角度φに依存しない回転レンズ鏡を含む光学系の関数となる。
【００１９】
さて、本発明人は面倒れ補正率Ｈの許容量を見積もるため面倒れ補正率が異なる走査光学系での印字シミュレーションを行った。シミュレーション条件を以下に示す。
【００２０】
面倒れ補正率：０、０．２５、０．５０、０．７５
面倒れ角φ ：１０秒
解像度：３００ｄｐｉ
スポット径：１００μｍ
光路長：２００ｍｍ
印字パターン：黒べた
偏向面面数：２
実際の面倒れ角φは、偏向モータの回転レンズ鏡取付面の回転軸に対する角度誤差で決まり、偏向モータでの取付面精度の限界値は通常±１０秒程度であるため、シミュレーションでの面倒れ角φは１０秒とした。図７は、上記の条件でのシミュレーションの印字結果を拡大して示したものである。この図から面倒れ補正率Ｈが０．５以上になると２走査おきに副走査方向のスポットとスポットの間に露光されない領域ができ、この領域は画像に主走査方向の白線として現れていることが解る。このような白線は、画像濃度を低下させ画質を著しく劣化させてしまう。さらに、本出願人は上記のシミュレーション結果に基づき、上記シミュレーションの条件で面倒れ補正率が約０．５、走査ピッチ誤差ΔＸに換算して約１０μｍの場合で印字実験を行った。その結果、主走査方向の白線さえ現れなければ、著しい画質の低下は認められないことが解った。これは１０μｍ程度の走査ピッチ誤差は、通常の人間の目の分解能以下であるため肉眼では判別できないためと考えられる。以上から補正率Ｈは０．５以下であるならば実用上問題の無い画像を得ることができ、面倒れ補正率Ｈの許容量は０．５以下と設定した。
【００２１】
回転レンズ鏡３、結像レンズ４の材質については、光学ガラス、光学プラスチックのいずれでも良いが、レンズの入射面あるいは射出面が非球面形状を有する場合は光学プラスチックの方が低価格で製造が可能なため望ましい。
【００２２】
次に、本発明の具体的な幾つかの実施例について、これらの構成を光学諸元、諸収差を含めて説明する。いずれの実施例においても、一走査の走査開始から走査終了までの回転レンズ鏡の回転角を２ωとし、各光学諸元の記号は、第ｉ面Ｓ_iの曲率半径をｒ_i、第ｉ面から次の面までの軸上間隔をｄ_iとする。さらに、当該面がトーリック面の場合には、副走査断面、主走査断面の曲率半径をそれぞれｒ_xi、ｒ_yiとし、トーリック面の主走査断面あるいは副走査断面が非円弧形状の場合には、次式で表される非球面係数を主走査断面ではＫ_yi、Ａ_yi、Ｂ_yi、副走査断面ではＫ_xi、Ａ_xi、Ｂ_xiとする。
【００２３】
【数１】

【００２４】
ただし、ｚ_iは光軸からの非円弧断面内の高さｈにおける非円弧断面上の点の、非円弧頂点の接平面からの距離であり、ｎ₁、ｎ₂はいずれも回転レンズ鏡の屈折率である。結像レンズの入射面、射出面をそれぞれＳ_c、Ｓ_dで、屈折率をｎ_cで表す。この他コリメートレンズと回転レンズ鏡の間に設けられるシリンドリカルレンズ、第２の結像レンズ、トーリック反射ミラーに関する光学諸元をも示す実施例では、シリンドリカルレンズ入射面、射出面、第２の結像レンズ入射面、射出面、トーリック反射面をそれぞれＳ_a、Ｓ_b、Ｓ_e、Ｓ_f、Ｓ_gで表し、シリンドリカルレンズ、第２の結像レンズの屈折率をそれぞれｎ_a、ｎ_eで表す。なお、回転レンズ鏡の反射面は平面である。Ｈは各実施例の面倒れ補正率を表す。
【００２５】
収差図については、像面湾曲は破線が主走査方向、実線が副走査方向の収差を表している。走査直線性は、ｆθレンズの通例では理想像高ｙ＝ｆθからの像高のずれを％で表すが、本発明では回転レンズ鏡の入射面と射出面とが回転するため理想像高がｆθとならない。従って、等価な表示方法として、光軸近傍の光線について、回転レンズ鏡の回転角に対する像高の変化率をζとして、理想像高Ｙ＝ζθからのずれを％で表示している。なお設計波長は７８０ｎｍとしている。
【００２６】
（実施例１）
次に本発明の第１の実施例について以下に詳述する。
【００２７】
図８は本実施例の断面の模式図を示し、（ａ）が主走査断面を（ｂ）が光軸に沿っ展開した副走査断面を示す。図９に本実施例の光走査装置の斜視図を示す。本実施例では同形状の２枚の回転レンズ鏡３が回転軸に対して回転対称で反射面Ｓ₂どうしが背中合わせに接触するように配置されており、回転レンズ鏡３は入射面Ｓ₁、射出面Ｓ₃ともトーリック面であって副走査方向に負のパワーを持つ。回転レンズ鏡３の他、結像レンズ４を１枚有する構成である。本実施例の代表的な設計例の光学諸元を以下に示す。
【００２８】

図１０はこの設計例の収差図である。
【００２９】
本実施例で示すように、回転レンズ鏡３の副走査方向のパワーを負とすることにより射出面Ｓ₃から出射した光線は、見かけ上回転レンズ鏡３の内部に共役点が有るように振る舞い、また射出面Ｓ₃の副走査断面は凹面であるため、回転レンズ鏡３の見かけの面倒れ偏向点は見かけの共役点付近となり、回転レンズ鏡３が回転しても偏向点変位量を小さくする効果がある。このため、面倒れ補正率Ｈを０．２１と小さくすることができる。さらに、図１０の収差図から解るように像面湾曲量は±１．０ｍｍ以内に抑えられており、スポット径で４０〜５０μｍ、解像度で６００〜８００ｄｐｉ程度の高精度な光走査が可能であり、走査直線性も０．５％以下と良好である。図９に示すように本実施例の光走査装置では２種類のしかも小型の光学素子のみで、歪曲特性、収差補正、面倒れ補正の機能をはたしており、極めて小型の面倒れによる走査ピッチ誤差の小さい高画質な光走査装置を構成することができる。
【００３０】
（実施例２）
図１１は本実施例の断面の模式図を示し、（ａ）が主走査断面を（ｂ）が光軸に沿って展開した副走査断面を示す。図１２に本実施例の光走査装置の斜視図を示す。本実施例では同形状の２枚の回転レンズ鏡３が回転軸に対して回転対称で反射面Ｓ₂どうしが背中合わせに接触するように配置されており、回転レンズ鏡３は入射面Ｓ₁、射出面Ｓ₃ともトーリック面であって副走査方向に負のパワーを持つ。回転レンズ鏡３の他、ミラーレンズ４を１枚有する構成である。本実施例の代表的な設計例の光学諸元を以下に示す。ただし、Ｘ_diは面Ｓ_iの副走査方向への偏心量を、Ｂ_diは面Ｓ_iと光軸との交点を中心として主走査面内の光軸と直交する方向を軸に面Ｓ_iが回転した回転角を示す。
【００３１】

図１３はこの設計例の収差図である。
【００３２】
通常実施例１に示した結像レンズのようにトーリック面を有するレンズでは材質に樹脂を用い成形で製作するのが一般である。しかし、樹脂には環境温度による屈折率の変動がガラスに比べ大きいという欠点があり、装置の使用環境の変動により結像性能が劣化してしまう。本実施例では光束の結像作用を凹面のトーリック反射鏡４に持たせることにより、トーリック反射鏡４を樹脂で成形したとしても環境温度の変動による結像性能の劣化がない高精度な光走査装置を構成することができる。また、図１３の収差図から解るように像面湾曲量は±１．５ｍｍ以内に抑えられてられており、６００ｄｐｉ程度の高精度な光走査が可能であり、走査直線性も０．５％以下と良好である。さらに小型な光走査装置には通常小型化のため光束の光路の途中に折り返しミラーが設けられるが、本実施例の構成とすることでトーリック反射ミラーが折り返しミラーの役割を兼ねるため、装置の部品点数を削減し、低価格な光走査装置を構成することができる。
【００３３】
（実施例３）
図１４は本実施例の断面の模式図を示し、（ａ）が主走査断面を（ｂ）が光軸に沿って展開した副走査断面を示す。本実施例では同形状の２枚の回転レンズ鏡３が回転軸に対して回転対称で反射面Ｓ₂どうしが背中合わせに接触するように配置されており、回転レンズ鏡３は入射面Ｓ₁、射出面Ｓ₃ともにトーリック面である。回転レンズ鏡３の他、回転レンズ鏡３手前に副走査方向のみパワーを有するシリンドリカルレンズ６と、結像レンズ４を１枚有する構成である。本実施例の代表的な設計例の光学諸元を以下に示す。
【００３４】

図１５は本実施例の設計例の収差図を示す。
【００３５】
本実施例では、回転レンズ鏡３の手前にシリンドリカルレンズ６を設け、光束を副走査方向にのみ集束させながら回転レンズ鏡３に入射させている。回転レンズ鏡を用いた光学系では、偏向点が回転レンズ鏡内部に存在するため共役点を回転レンズ鏡内部に設けることにより偏向点と共役点との距離である面倒れ偏向点変位を小さくすることができ、面倒れ補正効果を向上させることができる。回転レンズ鏡の手前にシリンドリカルレンズを設けずに回転レンズ鏡内部に共役点を持たせようとすると回転レンズ鏡入射面の副走査方向の曲率半径をかなり小さくする必要がある。しかし入射面の副走査方向の曲率半径が極端に小さいと回転レンズ鏡が回転したときに光束の入射面への入射角の違いによる共役点の変動が大きくなってしまい、かえって面倒れ偏向点変位が大きくなってしまう。また、曲率半径が他の面よりも極端に小さいと、収差が悪化したりその面の面精度が厳しくなってしまう。本実施例では、回転レンズ鏡３の手前にシリンドリカルレンズ６を設けることにより、回転レンズ鏡３の入射面Ｓ₁の曲率半径をそれほど小さくせずに回転レンズ鏡３内部に共役点を持たせることができため、面倒れ補正率Ｈを実施例１の半分程度にすることができ面倒れ補正効果を向上させることができる。さらに、回転レンズ鏡３に入射する光束は副走査方向に細いため、回転レンズ鏡３を副走査方向に薄くでき、回転レンズ鏡３の製造コストを下げることができる。
【００３６】
（実施例４）
図１６は本実施例の断面の模式図を示し、（ａ）が主走査断面を（ｂ）が光軸に沿って展開した副走査断面を示す。本実施例では同形状の２枚の回転レンズ鏡３が回転軸に対して回転対称で反射面Ｓ₂どうしが背中合わせに接触するように配置されており、回転レンズ鏡３は入射面Ｓ₁が平面、射出面Ｓ₃がトーリック面である。回転レンズ鏡３の他、回転レンズ鏡３手前に副走査方向のみパワーを有するシリンドリカルレンズ６と第１の結像レンズを４１と第２の結像レンズ４２を有する構成である。本実施例の代表的な設計例の光学諸元を以下に示す。
【００３７】

図１７はこの設計例の収差図である。
【００３８】
本実施例では、回転レンズ鏡３の手前にシリンドリカルレンズ６と第１の結像レンズ４１と第２の結像レンズ４２を設けている。このように回転レンズ鏡３の手前にシリンドリカルレンズ６を設けることにより、回転レンズ鏡の内部に共役点を持たせることができ、さらに回転レンズ鏡３の入射面Ｓ₁を平面とすることで回転レンズ鏡３が回転しても偏向点変位量をほぼ０とすることができる。このため本実施例では、光源１、偏向面Ｓ₂、被走査面５を常に光学的に共役関係とすることができ、通常の多面鏡に用いられる面倒れ補正光学系と同様、走査ピッチ誤差をほぼ完全に補正することがきる。さらに、結像レンズを２枚設けることで光学設計の自由度が増し、図１６の収差図から解るように像面湾曲量を±０．７ｍｍ以内とすることができ、８００ｄｐｉ以上の解像度にも対応可能であり、直線走査性も０．８％以下と良好である。このように本実施例では高精度、高解像度な走査光学系を構成することができる。
【００３９】
（実施例５）
図１８は本実施例の断面の模式図を示し、（ａ）が主走査断面を（ｂ）が光軸に沿って展開した副走査断面を示す。本実施例では同形状の２枚の回転レンズ鏡３が回転軸に対して回転対称で反射面Ｓ₂どうしが背中合わせに接触するように配置されており、回転レンズ鏡３は入射面Ｓ₁、射出面Ｓ₃ともにトーリック面である。回転レンズ鏡３の他、第１の結像レンズ４１と像面付近に副走査方向にのみパワーを有する第２の結像レンズであるシリンドリカルレンズ４２を設けた構成である。本実施例の代表的な設計例の光学諸元を以下に示す。
【００４０】

図１９は本実施例の設計例の収差図である。
【００４１】
本実施例では、像面付近にシリンドリカルレンズ４２を設けることで、結像レンズが１枚だけの場合と比較して、第１と第２の結像レンズを合成した光学倍率を小さくすることができ、式（１）から面倒れ補正効果を高めることができる。また、第１の結像レンズ４１の手前に副走査方向の共役点があるため第１の結像レンズ４２を通過する光束は副走査方向に細くなっており、第２の結像レンズ４２が被走査付近に配置されているため第２の結像レンズを通過する光束も細い。このため２つの結像レンズ４１、４２の厚さを薄くすることができ、結像レンズ４１、４２の製造コストを下げることができる。
【００４２】
（実施例６）
図２０は本実施例の断面の模式図を示し、（ａ）が主走査断面を（ｂ）が光軸に沿って展開した副走査断面を示す。本実施例では同形状の２枚の回転レンズ鏡３が回転軸に対して回転対称で反射面Ｓ₂どうしが背中合わせに接触するように配置されており、回転レンズ鏡３は入射面Ｓ₁、射出面Ｓ₃ともにトーリック面である。回転レンズ鏡３の他、第１の結像レンズ４１と像面付近に第２の結像レンズ４２を設けた構成である。本実施例の代表的な設計例の光学諸元を以下に示す。
【００４３】

図２１はこの設計例の収差図である。
【００４４】
本実施例では、像面付近に第２の結像レンズ４２を設けることで、結像レンズが１枚だけの場合と比較して、第１と第２の結像レンズを合成した光学倍率を小さくすることができ、式（１）から面倒れ補正効果を高めることができる。さらに第２の結像レンズ４２をトーリックレンズにすることで図２１に示すように像面湾曲量を±１．４ｍｍ以内に、直線走査性を０．７％以下と小さくすることができ高精度な光走査装置を構成することができる。
【００４５】
以上の実施例は、回転レンズ鏡が２枚の構成で説明したが、図２２に示すように回転レンズ鏡３を３枚以上用いて回転軸Ｏに対して回転対称な位置に配置し、走査効率をさらに高めることも可能である。また、回転レンズ鏡１枚でも構成することができ、回転レンズ鏡の駆動モータに回転不良がある場合等、走査ピッチむらを低減させる効果がある。
【００４６】
【発明の効果】
以上説明したように、本発明によれば複数の回転レンズ鏡を用いた光走査装置において、光学素子の副走査方向のパワーが負で、射出屈折面の副走査方向の面形状が凹であり、入射屈折面の主走査方向の面形状が凹あるいは平面であり、その光学素子と被走査面の間に結像レンズが配置されているようにするか、光学素子の副走査方向のパワーが正で、入射屈折面の副走査方向の面形状が凸で、射出屈折面の副走査方向の面形状が凹であり、入射屈折面の主走査方向の面形状が凹あるいは平面であり、回転レンズ鏡と被走査面の間に配置される結像レンズを２枚で構成することで、走査効率を高め走査ピッチむらを低減した小型低価格で高精度な光走差装置を構成することができるという効果を有する。さらに、本発明の光走査装置を用いたレーザプリンタ、デジタル複写機、ファクシミリ、レーザ走査ディスプレイ等の画像形成装置に対しても装置の小型低価格化に大きく寄与することは自明である。
【図面の簡単な説明】
【図１】本発明の走査光学系の構成を示す主走査断面図。
【図２】回転レンズ鏡の座標系を示す斜視図。
【図３】回転レンズ鏡の面倒れを示す副走査断面図。
【図４】回転レンズ鏡が回転した場合の面倒れを示す主走査および副走査断面図。
【図５】本発明の第１の面倒れ補正方法を示す回転レンズ鏡の副走査断面図。
【図６】本発明の第２の面倒れ補正方法を示す回転レンズ鏡の副走査断面図。
【図７】面倒れ補正率が変化した場合の印字シミュレーション結果を示す図。
【図８】本発明の第１の実施例を示す主走査および副走査断面の模式図。
【図９】本発明の第１の実施例の光走査装置の斜視図。
【図１０】本発明の第１の実施例の収差図。
【図１１】本発明の第２の実施例を示す主走査および副走査断面の模式図。
【図１２】本発明の第２の実施例の光走査装置の斜視図。
【図１３】本発明の第２の実施例の収差図。
【図１４】本発明の第３の実施例を示す主走査および副走査断面の模式図。
【図１５】本発明の第３の実施例の収差図。
【図１６】本発明の第４の実施例を示す主走査および副走査断面の模式図。
【図１７】本発明の第４の実施例の収差図。
【図１８】本発明の第５の実施例を示す主走査および副走査断面の模式図。
【図１９】本発明の第５の実施例の収差図。
【図２０】本発明の第６の実施例を示す主走査および副走査断面の模式図。
【図２１】本発明の第６の実施例の収差図。
【図２２】回転レンズ鏡を複数用いた場合の主走査断面図。
【図２３】従来の回転多面鏡を用いた光走査装置の面倒れ補正を示す副走査断面図。
【符号の説明】
１光源
２コリメータレンズ
３回転レンズ鏡
４結像レンズ
５被走査面
Ｓ₁ 入射面
Ｓ₂ 反射面
Ｓ₃ 射出面[0001]
[Industrial application fields]
The present invention relates to an image forming apparatus such as a laser printer and a facsimile, and an optical scanning device used therefor.
[0002]
[Prior art]
A rotating polygon mirror used in a conventional laser beam printer or the like usually has an error in parallelism of each surface and an attachment error between the rotating shaft and the polygon mirror that occur during the manufacturing process of the polygon mirror. The advancing direction changes in a plane perpendicular to the deflection surface, resulting in uneven scanning pitch. In addition, rotation failure such as precession generated on the rotation axis of the rotary polygon mirror also causes the same unevenness in scanning pitch. In order to increase the accuracy of the entire rotary polygon mirror and the deflection apparatus to a level where the pitch unevenness is not problematic on the image, it takes a long time to manufacture the apparatus, and the apparatus becomes extremely expensive. On the other hand, various methods for correcting this, so-called surface tilt correction optical systems, have been devised.
[0003]
JP-A-48-49315 and JP-A-56-36622 are referred to as a conjugate type surface tilt correction method, which is a main scanning plane and a plane perpendicular to the main scanning plane and including the optical axis (hereinafter referred to as sub-scanning). Using an anamorphic scanning optical system with different power (scanning surface), the sub-scanning surface forms a light beam on the reflecting surface of the rotating polygon mirror so that the reflecting surface and the photoreceptor surface are optically conjugate. JP-A-52-153456 and JP-A-58-134618 are called relaxation type surface tilt correction methods, and a long cylindrical lens is arranged in the vicinity of the surface of the photoreceptor. The change in the deflection direction of the sub-scanning surface due to the surface tilt is alleviated.
[0004]
In the case of an optical system using a rotating polygon mirror, when the sub-scan section of the optical system is developed along the optical axis as shown in FIG. 23A, the deflection surface 7 of the rotating polygon mirror is tilted. In this case, the light beam emitted from the light source 1 and made parallel by the collimator lens 2 is deflected in the sub-scanning direction as shown by the broken line in the figure at the intersection P between the deflecting surface 7 and the optical axis, passes through the fθ lens 4, A scanning pitch error ΔX occurs on the scanning surface 5 out of the main scanning surface including the optical axis. Hereinafter, the point at which the light beam is deflected in the sub-scanning direction due to the tilting of the deflection surface as point P is referred to as a plane tilting deflection point. In the case of an optical system using a rotating polygon mirror, the plane tilting deflection point is on the deflection surface. Therefore, a cylindrical lens 6 having power only in the sub-scanning direction is arranged between the deflecting surface 7 and the collimating lens 2 as in the conjugate type surface tilt correction method shown in FIG. 7. By making the scanned surface 5 optically conjugate, even if the deflection surface 7 is tilted, the tilt can be corrected almost completely.
[0005]
[Problems to be solved by the invention]
However, although these devices can perform high-precision optical scanning without uneven scanning pitch, the rotating polygon mirrors used in these devices only deflect the light beam emitted from the light source unit, and the aberration of the light beam. Since it cannot have functions such as correction and imaging of the light beam on the surface to be scanned, there is a problem that the number of lenses increases, leading to an increase in the size and cost of the device. Inevitably, the price has been high, which has hindered the recent trend toward downsizing and personal use of OA equipment such as personal computers.
[0006]
On the other hand, in Japanese Patent Application No. Hei 4-166042, the applicant of the present invention is an optical element (hereinafter referred to as a rotating lens mirror) composed of a reflecting surface that rotates with a rotating shaft, and an entrance surface and an exit surface that are shaped to perform predetermined aberration correction. We proposed an optical scanning method using This system eliminates the surface tilt correction optical system by making one deflection surface, and the function of two members, that is, a polygon mirror that deflects the light beam and a lens that corrects the aberration, can be performed only by the rotating lens mirror. This has the advantage that a small and inexpensive optical scanning device with a small number of parts can be constructed. However, when there is only one deflection surface, only one scan can be performed with one rotation of the rotating lens mirror. There is still room for improvement in terms of inefficiency.
[0007]
The present invention improves the Japanese Patent Application No. Hei 4-166042 in order to solve the above-mentioned problems, and adds a surface tilt correction function to a scanning optical system using a plurality of rotating lens mirrors. The present invention provides an optical scanning device that realizes resolution at the same time.
[0008]
[Means for Solving the Problems]
One optical scanning device of the present invention is an optical scanning device that deflects and scans a light beam by rotating a plurality of optical elements having an incident refracting surface, a plane reflecting surface, and an exit refracting surface.
The power in the sub-scanning direction of the optical element is negative,
The surface shape in the sub-scanning direction of the exit refracting surface is concave,
The surface shape in the main scanning direction of the incident refracting surface is concave or flat,
An imaging lens is disposed between the optical element and the surface to be scanned.
Another optical scanning device of the present invention is an optical scanning device that deflects and scans a light beam by rotating a plurality of optical elements having an incident refracting surface, a plane reflecting surface, and an exit refracting surface.
The power of the optical element in the sub-scanning direction is positive,
The surface shape in the sub-scanning direction of the incident refracting surface is convex, and the surface shape in the sub-scanning direction of the exit refracting surface is concave,
The surface shape in the main scanning direction of the incident refracting surface is concave or flat,
The imaging lens disposed between the optical element and the surface to be scanned is composed of two lenses.
[0009]
【Example】
The basic configuration of the optical scanning device of the present invention is that a plurality of rotating lens mirrors are integrally rotated around one rotating shaft by a driving device such as a motor so that light beams pass through each of the rotating lens mirrors. Thus, when a plurality of rotating lens mirrors are arranged in this way, scanning pitch unevenness is caused by mounting errors (surface tilt) of the respective rotating lens mirrors. However, the present invention optically corrects unevenness in scanning pitch in an optical scanning apparatus using a plurality of rotating lens mirrors. FIG. 1 shows a typical embodiment of a laser scanning device using the optical system of the present invention. In this apparatus, a light beam emitted from a semiconductor laser 1 as a light source is converted into a parallel light beam by a collimator lens 2, and the light beam is incident on an incident surface S of the rotating lens mirror 3. ₁ Is incident on the reflection surface S ₂ Reflected by the light exit surface S _Three Exits from. Incident surface S ₁ , Injection surface S _Three Is such that the light beam that scans the scanning center passes perpendicularly through these surfaces, and the reflecting surface S ₂ Is the reflecting surface S at an angle of 90 ° or less of the light beam that scans the scanning center. ₂ It is set to enter. Two rotating lens mirrors 3 having the same shape are used, and are rotationally symmetric with respect to the rotation axis O and have a reflecting surface S. ₂ They are arranged so that they touch each other back to back, and rotate together to perform two optical scans per revolution. The rotation axis O of the rotating lens mirror 3 is the reflecting surface S. ₂ And passes through a point where a beam scanning the scanning center is reflected. The light beam is deflected as the rotating lens mirror 3 rotates, and the deflected light beam passes through the imaging lens 4 to form a spot on the scanned surface 5.
[0010]
Here, the surface tilt of the scanning optical system using the rotating lens mirror will be described. When the surface tilting of the rotating lens mirror of the scanning optical system using the rotating lens mirror is considered, not only the surface tilt of the reflecting surface but also the incident surface. And it is necessary to consider the fall of the exit surface. In order to examine the surface tilt of the rotating lens mirror, a coordinate system X, Y, Z fixed to the rotating lens mirror 3 is defined as shown in FIG. 2, the X axis is perpendicular to the main scanning plane, and the reflecting surface S is defined. ₂ The straight line and the Y axis included in are parallel to the main scanning plane and the reflecting surface S ₂ The straight line and the Z axis are included in the reflection surface S ₂ If the rotation angle of the rotating lens mirror 3 around the X axis is ω and the rotation angle of the rotating lens mirror 3 around the Y axis is φ, the rotation is as shown in the figure. The surface tilt of the lens mirror 3 exists in two axes: rotation about the Y axis and rotation about the Z axis. In the rotation around the Y axis, the incident surface S ₁ , Reflective surface S ₂ , Injection surface S _Three Is tilted, and the reflection surface S is not rotated when rotating around the Z axis. ₂ Does not fall down, incident surface S ₁ And exit surface S _Three Only to be troubled. Incident surface S ₁ , Injection surface S _Three The deflection effect of the light beam in the sub-scanning direction due to the tilting of the surface of the reflection surface S ₂ Therefore, it is sufficient to consider only the tilt around the Y axis if the angle of tilt of the rotating lens mirror 3 is small to some extent. Therefore, in the following description, only the tilt of the rotating lens mirror is considered only about the Y axis, and the rotation of the rotating lens mirror is the tilt of the rotating lens mirror, and the rotation angle is the tilt angle φ.
[0011]
Next, the state of the light beam when the rotating lens mirror is tilted will be described with reference to FIG. 3 in which the sub-scanning section of the scanning optical system using the rotating lens mirror is developed along the optical axis. 3A shows the entire optical system, and FIG. 3B shows only the rotating lens mirror in an enlarged manner. Rotating lens mirror 3 is reflective surface S ₂ When the angle φ is rotated about the Y axis around the intersection P between the optical axis and the optical axis, the incident surface S ₁ And exit surface S _Three Is rotated about the point P, and the incident surface S is approximately approximated if the angle φ is small. ₁ And exit surface S _Three When viewed in the sub-scanning section, the optical axes of are rotated by φ sin β and −φ sin β, respectively, with respect to the main scanning plane. Where angle β is incident surface S as shown in FIG. ₁ , Injection surface S _Three Is the reflective surface S ₂ The angle between The light beam incident on the rotating lens mirror 3 with the surface tilt is first incident surface S as indicated by a broken line in the figure in the sub-scanning direction. ₁ The incident surface S ₁ Reflected by the fall of the reflection surface S ₂ Reflective surface S ₂ Is deflected at an angle of 2φ by the fall of the surface, and the exit surface S _Three But injection surface S _Three The light is deflected by the tilt of the light and is emitted from the rotating lens mirror 3 at an angle of φ ′ with the optical axis. The light beam emitted from the rotating lens mirror 3 is incident on the incident surface S. ₁ , Injection surface S _Three Reflective surface S due to the deflection effect at ₂ It behaves as if it is deflected not at the upper point P but at the intersection P ′ between the principal ray of the light beam and the optical axis, and the point P ′ becomes the apparent surface tilt deflection point of the rotating lens mirror 3.
[0012]
Next, consider a case where a rotating lens mirror with surface tilt rotates around the X axis to deflect and scan a light beam. FIG. 4A shows a case where the rotation lens mirror rotates around the X axis by an angle −ω, 0, ω around the X axis when the light beam scans the scanning center of the surface to be scanned. FIG. 6 is a main scanning sectional view of the rotating lens mirror showing the principal ray of the luminous flux with the rotating lens mirror fixed. The principal ray incident on the rotating lens mirror 3 is incident on the incident surface S as the rotating lens mirror 3 rotates. ₁ To entrance surface S ₁ Are incident at angles of -ω, 0, and ω, respectively, with respect to the optical axis of ₂ Top point P ₁ , P ₀ , P ₂ The exit surface S deflected by _Three Exits from. When FIG. 4A is developed along the optical axis in the sub-scan section as shown in FIG. 4B, the deflection point P depends on the rotation angle of the rotating lens mirror. ₁ , P ₀ , P ₂ And move on the optical axis. When the rotating lens mirror 3 is tilted, the light beam incident on the rotating lens mirror 3 is incident on the incident surface S as described with reference to FIG. ₁ And then each reflecting surface S corresponding to the rotation angle. ₂ Top point P ₁ , P ₀ , P ₂ At an angle of 2φ and the exit surface S _Three But it is deflected and emitted. Ejection surface S _Three Each light beam emitted from the light beam has an intersection P between the principal ray of the emitted light beam and the optical axis. ₁ ', P ₀ ', P ₂ It behaves as if it is deflected with 'turning down' as a deflection point. Thus, in the rotating lens mirror, the surface tilt deflection point moves with the rotation of the rotating lens mirror, and the apparent surface tilt deflection point of the light beam emitted from the rotating lens mirror also moves greatly due to the rotation of the rotating lens mirror. . Hereinafter, such movement of the surface tilt deflection point is referred to as surface tilt deflection point displacement, the amount of movement is defined as the surface tilt deflection point displacement amount Δd, and the apparent surface tilt deflection point movement of the light beam emitted from the rotating lens mirror The amount is assumed to be an apparent surface tilt deflection point displacement amount Δd ′.
[0013]
Unlike the scanning optical system using the rotary polygon mirror described above with reference to FIG. 23, the scanning optical system having the surface tilt deflection point displacement as described above has a conjugate relationship between the light source, the deflection surface, and the scanned surface. Even if the optical system is configured, the deflection surface is displaced and deviates from the conjugate point, so that the surface tilt cannot be completely corrected, and the scanning pitch error ΔX on the surface to be scanned is expressed by the following equation: expressed.
[0014]
ΔX = m · Δd ′ · φ ′ (1)
Here, φ ′ is the angle formed by the principal ray of the light beam emitted from the rotating lens mirror and the optical axis in the plane orthogonal to the main scanning plane, m is the optical magnification in the sub-scanning direction of the imaging lens, and Δd ′ is The apparent surface tilt deflection point displacement amount of the light beam emitted from the rotating lens mirror is shown. In the above equation, φ ′ mainly depends on the surface tilt angle φ of the rotating lens mirror, and the surface tilt angle φ is determined by the mechanical accuracy of mounting the rotating lens mirror. Therefore, in order to optically reduce the scanning pitch error ΔX, it is necessary to reduce the surface tilt deflection point displacement amount Δd ′ or reduce the optical magnification m in the sub-scanning direction of the imaging lens. As described above, the configuration in which the scanning pitch error ΔX is reduced in the scanning optical system using the rotating lens mirror is mainly a method in which the surface tilt deflection point displacement amount Δd is reduced, and the optical magnification m of the imaging lens is mainly reduced. Is classified into Each method will be described below.
[0015]
FIG. 5 is an explanatory diagram of the present invention regarding a method for reducing the surface tilt deflection point displacement. FIG. 5 (a) is a sub-scan sectional view of a scanning optical system using a rotating lens mirror. Expanded along the axis. The rotating lens mirror 3 has an incident surface S in the sub scanning direction. ₁ Is convex, exit surface S _Three Is a concave surface and has negative power in the sub-scanning direction as a whole. As shown in the figure, the parallel light beam incident on the rotating lens mirror 3 is incident on the incident surface S. ₁ With positive power, it becomes a converged light beam and exit surface S _Three Thus, a negative power causes a divergent light beam to pass through the imaging lens to form a spot on the surface to be scanned. In this case, although the light beam does not form an image between the collimator lens and the surface to be scanned and does not have a conjugate point with the surface to be scanned, the light beam emitted from the rotating lens mirror 3 extends the emitted light beam to the rotating lens mirror 3 side. Then, it behaves as if an image is formed at a point Q that intersects the optical axis, and the point Q is an apparent conjugate point with the surface to be scanned. When the rotating lens mirror 3 tilts at a tilt angle φ, the principal ray of the light beam incident on the rotating lens mirror 3 is reflected by the reflecting surface S as shown by the dotted line in the figure. ₂ At an angle of 2φ and the exit surface S _Three The light is deflected and emitted so that the angle formed with the optical axis becomes large. A point P ′ that extends the principal ray of the emitted light beam and intersects the optical axis becomes an apparent surface tilting deflection point of the rotating lens mirror 3, and is located in the vicinity of the aforementioned apparent conjugate point Q. Furthermore, as shown in FIG. 5 (b), the surface tilted deflection point P is caused by the rotation of the rotating lens mirror 3 around the X axis. ₁ , P ₂ In this case, the apparent surface tilt deflection point of the light beam emitted from the rotating lens mirror 3 is the intersection P between the principal ray of each light beam and the optical axis as shown in the figure. ₁ ', P ₂ 'Become. From this figure, exit surface S _Three It can be seen that the apparent surface tilt deflection point displacement amount Δd ′ is smaller than the actual surface tilt deflection point displacement amount Δd. Thus, the power in the sub-scanning direction of the rotating lens mirror 3 is negative and the exit surface S _Three By making the sub-scanning cross section of this concave structure, the amount of deviation Δd of the plane tilt deflection point from the conjugate point can be reduced, and the scanning pitch error ΔX can be reduced from the above equation (1).
[0016]
Next, a method of reducing the optical magnification m in the sub-scanning direction of the imaging lens will be described with reference to FIG. FIG. 6 is a view of a scanning optical system using a rotating lens mirror as viewed from the main scanning direction, and the rotating lens mirror 3 is developed along the optical axis. The rotating lens mirror 3 has an incident surface S in the sub-scanning direction. ₁ Is convex, exit surface S _Three Is concave and has a positive power as a whole in the sub-scanning direction. The imaging lens is composed of two lenses, a first imaging lens 41 arranged near the rotating lens mirror 3 and a second imaging lens 42 arranged near the surface to be scanned. As shown in the figure, when a parallel light beam enters the rotating lens mirror 3, the light beam forms an image at a point Q in front of the first imaging lens 41 by the positive power of the rotating lens mirror 3, and the first imaging lens 41. A spot is formed on the scanned surface 5 by the second imaging lens 42. By configuring the imaging lens with two lenses in this way, the optical magnification m of the imaging lens can be set small, and the scanning pitch error ΔX can be reduced from the above equation (1).
[0017]
By the way, in such an optical system that cannot completely correct the scanning pitch error due to the surface tilt, it is necessary to quantify the effect of correcting the surface tilt and to perform an optical design so that the value becomes a certain value or less. Therefore, the surface tilt correction rate H obtained by quantifying the surface tilt correction effect of the optical system is defined as follows.
[0018]
H = ΔX (φ) / ΔX ₀ (Φ)
= ΔX (φ) / 2L / φ (2)
Where φ is the surface tilt angle of the rotating lens mirror, ΔX (φ) is the maximum pitch error in the sub-scanning direction that occurs when the rotating lens mirror tilts the surface φ in the optical system using the rotating lens mirror, and L is The distance from the rotation center of the rotating lens mirror to the surface to be scanned is expressed as ΔX ₀ (Φ) indicates a pitch error in the sub-scanning direction, that is, 2L · φ caused by tilting of the deflection surface when there is no optical element such as a lens between the deflection surface and the surface to be scanned. When the surface tilt angle φ is small to some extent, the relationship between ΔX (φ) and φ is linear as shown in Equation (1), and the surface tilt correction factor H is an optical system including a rotating lens mirror that does not depend on the surface tilt angle φ. It becomes a function.
[0019]
Now, in order to estimate the allowable amount of the surface tilt correction rate H, the present inventor has performed a printing simulation with scanning optical systems having different surface tilt correction rates. The simulation conditions are shown below.
[0020]
Surface tilt correction rate: 0, 0.25, 0.50, 0.75
Face tilt angle φ: 10 seconds
Resolution: 300 dpi
Spot diameter: 100 μm
Optical path length: 200mm
Print pattern: Solid black
Number of deflection surfaces: 2
The actual surface tilt angle φ is determined by the angle error with respect to the rotation axis of the rotation lens mirror mounting surface of the deflection motor, and the limit value of the mounting surface accuracy with the deflection motor is usually about ± 10 seconds. The angle φ was 10 seconds. FIG. 7 is an enlarged view of a simulation printing result under the above conditions. From this figure, when the surface tilt correction rate H is 0.5 or more, an area that is not exposed between the spots in the sub-scanning direction is formed every two scans, and this area appears as a white line in the main scanning direction in the image. I understand. Such a white line lowers the image density and significantly deteriorates the image quality. Further, based on the above simulation results, the present applicant conducted a printing experiment when the surface tilt correction rate was about 0.5 under the conditions of the above simulation and the scanning pitch error ΔX was about 10 μm. As a result, it has been found that unless a white line in the main scanning direction appears, no significant deterioration in image quality is observed. This is probably because a scanning pitch error of about 10 μm is less than the resolution of the normal human eye and cannot be discriminated with the naked eye. From the above, if the correction rate H is 0.5 or less, an image having no practical problem can be obtained, and the allowable amount of the surface tilt correction rate H is set to 0.5 or less.
[0021]
The material of the rotating lens mirror 3 and the imaging lens 4 may be either optical glass or optical plastic. However, when the entrance surface or exit surface of the lens has an aspherical shape, the optical plastic is manufactured at a lower cost. This is desirable because it is possible.
[0022]
Next, regarding some specific examples of the present invention, these configurations will be described including optical specifications and various aberrations. In any of the embodiments, the rotation angle of the rotating lens mirror from the start of scanning to the end of scanning is 2ω, and the symbol of each optical element is the i-th surface S. _i The radius of curvature of r _i , The axial distance from the i-th surface to the next surface is d _i And Further, when the surface is a toric surface, the curvature radii of the sub-scanning section and the main scanning section are set to r, respectively. _xi , R _yi When the main scanning section or sub-scanning section of the toric surface is a non-arc shape, the aspheric coefficient expressed by the following equation is K for the main scanning section: _yi , A _yi , B _yi , K in the sub-scan section _xi , A _xi , B _xi And
[0023]
[Expression 1]

[0024]
Where z _i Is the distance from the tangent plane of the non-arc vertex to the point on the non-arc cross section at height h in the non-arc cross section from the optical axis, n ₁ , N ₂ Is the refractive index of the rotating lens mirror. The entrance surface and exit surface of the imaging lens are S _c , S _d And the refractive index is n _c Represented by In addition, in the embodiment also showing the optical specifications regarding the cylindrical lens, the second imaging lens, and the toric reflecting mirror provided between the collimating lens and the rotating lens mirror, the cylindrical lens entrance surface, the exit surface, the second imaging image The lens entrance surface, exit surface, and toric reflection surface are respectively S _a , S _b , S _e , S _f , S _g The refractive indexes of the cylindrical lens and the second imaging lens are represented by n, respectively. _a , N _e Represented by The reflecting surface of the rotating lens mirror is a flat surface. H represents the surface tilt correction rate of each embodiment.
[0025]
In the aberration diagrams, the field curvature indicates the aberration in the main scanning direction and the solid line in the sub-scanning direction. In the scan linearity, the deviation of the image height from the ideal image height y = fθ is expressed in% in the usual case of the fθ lens. However, in the present invention, since the incident surface and the exit surface of the rotating lens mirror rotate, the ideal image height is fθ. Not. Therefore, as an equivalent display method, the deviation from the ideal image height Y = ζθ is displayed in% for the light rays in the vicinity of the optical axis, where ζ is the rate of change in image height with respect to the rotation angle of the rotating lens mirror. The design wavelength is 780 nm.
[0026]
(Example 1)
Next, the first embodiment of the present invention will be described in detail below.
[0027]
FIG. 8 is a schematic diagram of a cross section of the present embodiment, in which (a) shows a main scanning section and (b) shows a sub-scanning section developed along the optical axis. FIG. 9 is a perspective view of the optical scanning device of this embodiment. In the present embodiment, two rotating lens mirrors 3 having the same shape are rotationally symmetric with respect to the rotation axis and have a reflecting surface S. ₂ The rotating lens mirrors 3 are arranged so that they are in contact with each other back to back. ₁ , Injection surface S _Three Both are toric surfaces and have negative power in the sub-scanning direction. In addition to the rotating lens mirror 3, the image forming lens 4 is provided. The optical specifications of a typical design example of this embodiment are shown below.
[0028]

FIG. 10 is an aberration diagram of this design example.
[0029]
As shown in this embodiment, the exit surface S is obtained by setting the power of the rotating lens mirror 3 in the sub-scanning direction to be negative. _Three The light beam emitted from the surface behaves as if there is a conjugate point inside the rotating lens mirror 3, and the exit surface S _Three Since the sub-scan section of FIG. 2 is a concave surface, the apparent surface tilt deflection point of the rotating lens mirror 3 is in the vicinity of the apparent conjugate point, and there is an effect of reducing the deflection point displacement amount even if the rotating lens mirror 3 rotates. For this reason, the surface tilt correction rate H can be reduced to 0.21. Furthermore, as can be seen from the aberration diagram of FIG. 10, the amount of curvature of field is suppressed within ± 1.0 mm, and high-precision optical scanning with a spot diameter of 40 to 50 μm and a resolution of about 600 to 800 dpi is possible. The scanning linearity is also good at 0.5% or less. As shown in FIG. 9, the optical scanning apparatus of the present embodiment has functions of distortion characteristics, aberration correction, and surface tilt correction with only two types of small optical elements, and a scanning pitch error caused by extremely small surface tilt. A small high-quality optical scanning device can be configured.
[0030]
(Example 2)
FIG. 11 is a schematic diagram of a cross section of the present embodiment, where (a) shows a main scanning section and (b) shows a sub-scanning section developed along the optical axis. FIG. 12 is a perspective view of the optical scanning device of this embodiment. In the present embodiment, two rotating lens mirrors 3 having the same shape are rotationally symmetric with respect to the rotation axis and have a reflecting surface S. ₂ The rotating lens mirrors 3 are arranged so that they are in contact with each other back to back. ₁ , Injection surface S _Three Both are toric surfaces and have negative power in the sub-scanning direction. In addition to the rotating lens mirror 3, the mirror lens 4 is provided. The optical specifications of a typical design example of this embodiment are shown below. However, X _di Is surface S _i The amount of eccentricity in the sub-scanning direction is _di Is surface S _i The surface S about the direction perpendicular to the optical axis in the main scanning plane with the intersection of the optical axis and the optical axis as the center _i Indicates the angle of rotation.
[0031]

FIG. 13 is an aberration diagram of this design example.
[0032]
In general, a lens having a toric surface like the imaging lens shown in Embodiment 1 is generally manufactured by molding using a resin as a material. However, the resin has a drawback in that the refractive index varies greatly with the environmental temperature compared to glass, and the imaging performance deteriorates due to variations in the operating environment of the apparatus. In the present embodiment, the concave toric reflector 4 is provided with the imaging function of the light beam, so that even if the toric reflector 4 is formed of resin, the imaging performance is not deteriorated due to the fluctuation of the environmental temperature. A device can be configured. Further, as can be seen from the aberration diagram of FIG. 13, the amount of curvature of field is suppressed within ± 1.5 mm, high-precision optical scanning of about 600 dpi is possible, and scanning linearity is also 0.5%. Good with: Further, a small-sized optical scanning device is usually provided with a folding mirror in the middle of the optical path of the light beam for miniaturization. However, since the toric reflection mirror also serves as the folding mirror in the configuration of this embodiment, the parts of the device The number of points can be reduced, and an inexpensive optical scanning device can be configured.
[0033]
(Example 3)
FIG. 14 is a schematic diagram of a cross section of the present embodiment, where (a) shows a main scanning section and (b) shows a sub-scanning section developed along the optical axis. In the present embodiment, two rotating lens mirrors 3 having the same shape are rotationally symmetric with respect to the rotation axis and have a reflecting surface S. ₂ The rotating lens mirrors 3 are arranged so that they are in contact with each other back to back. ₁ , Injection surface S _Three Both are toric surfaces. In addition to the rotating lens mirror 3, a cylindrical lens 6 having power only in the sub-scanning direction and one imaging lens 4 are provided in front of the rotating lens mirror 3. The optical specifications of a typical design example of this embodiment are shown below.
[0034]

FIG. 15 shows aberration diagrams of the design example of this embodiment.
[0035]
In the present embodiment, a cylindrical lens 6 is provided in front of the rotating lens mirror 3, and the light beam is incident on the rotating lens mirror 3 while being focused only in the sub-scanning direction. In an optical system using a rotating lens mirror, since the deflection point exists inside the rotating lens mirror, the displacement of the surface tilt deflection point, which is the distance between the deflection point and the conjugate point, is reduced by providing the conjugate point inside the rotating lens mirror. It is possible to improve the effect of correcting the tilting. If a cylindrical lens is not provided in front of the rotating lens mirror and a conjugate point is to be provided inside the rotating lens mirror, it is necessary to considerably reduce the radius of curvature in the sub-scanning direction of the rotating lens mirror entrance surface. However, if the radius of curvature of the incident surface in the sub-scanning direction is extremely small, when the rotating lens mirror rotates, the variation of the conjugate point due to the difference in the incident angle of the light beam on the incident surface becomes large, and instead the surface tilt deflection point displacement Will become bigger. Further, if the radius of curvature is extremely smaller than other surfaces, aberrations are deteriorated and surface accuracy of the surfaces becomes severe. In the present embodiment, by providing the cylindrical lens 6 in front of the rotating lens mirror 3, the incident surface S of the rotating lens mirror 3 is provided. ₁ Therefore, the surface tilt correction rate H can be reduced to about half that of the first embodiment, and the effect of surface tilt correction can be improved. Can do. Furthermore, since the light beam incident on the rotating lens mirror 3 is thin in the sub-scanning direction, the rotating lens mirror 3 can be thinned in the sub-scanning direction, and the manufacturing cost of the rotating lens mirror 3 can be reduced.
[0036]
(Example 4)
FIG. 16 is a schematic diagram of a cross section of the present embodiment, where (a) shows a main scanning section and (b) shows a sub-scanning section developed along the optical axis. In the present embodiment, two rotating lens mirrors 3 having the same shape are rotationally symmetric with respect to the rotation axis and have a reflecting surface S. ₂ The rotating lens mirrors 3 are arranged so that they are in contact with each other back to back. ₁ Is flat, exit surface S _Three Is the toric surface. In addition to the rotating lens mirror 3, a cylindrical lens 6 having a power only in the sub-scanning direction, a first imaging lens 41, and a second imaging lens 42 are provided in front of the rotating lens mirror 3. The optical specifications of a typical design example of this embodiment are shown below.
[0037]

FIG. 17 is an aberration diagram of this design example.
[0038]
In this embodiment, a cylindrical lens 6, a first imaging lens 41, and a second imaging lens 42 are provided in front of the rotating lens mirror 3. By providing the cylindrical lens 6 in front of the rotating lens mirror 3 in this way, a conjugate point can be provided inside the rotating lens mirror, and the incident surface S of the rotating lens mirror 3 can be provided. ₁ By making the plane flat, the deflection point displacement amount can be made substantially zero even when the rotating lens mirror 3 rotates. For this reason, in this embodiment, the light source 1 and the deflection surface S are used. ₂ The surface to be scanned 5 can always be optically conjugate, and the scanning pitch error can be almost completely corrected as in the case of the surface tilt correction optical system used in a normal polygon mirror. Furthermore, by providing two imaging lenses, the degree of freedom in optical design is increased, and as can be seen from the aberration diagram of FIG. 16, the amount of curvature of field can be within ± 0.7 mm, and the resolution can be 800 dpi or higher. The linear scanability is also good at 0.8% or less. In this way, in this embodiment, a scanning optical system with high accuracy and high resolution can be configured.
[0039]
(Example 5)
FIG. 18 is a schematic diagram of a cross section of the present embodiment, where (a) shows a main scanning section and (b) shows a sub-scanning section developed along the optical axis. In the present embodiment, two rotating lens mirrors 3 having the same shape are rotationally symmetric with respect to the rotation axis and have a reflecting surface S. ₂ The rotating lens mirrors 3 are arranged so that they are in contact with each other back to back. ₁ , Injection surface S _Three Both are toric surfaces. In addition to the rotating lens mirror 3, the first imaging lens 41 and a cylindrical lens 42, which is a second imaging lens having power only in the sub-scanning direction, are provided in the vicinity of the image plane. The optical specifications of a typical design example of this embodiment are shown below.
[0040]

FIG. 19 is an aberration diagram of a design example of this example.
[0041]
In this embodiment, by providing the cylindrical lens 42 in the vicinity of the image plane, the optical magnification obtained by combining the first and second imaging lenses can be reduced as compared with the case where only one imaging lens is provided. Therefore, it is possible to enhance the surface tilt correction effect from the equation (1). In addition, since there is a conjugate point in the sub-scanning direction in front of the first imaging lens 41, the light beam passing through the first imaging lens 42 is narrowed in the sub-scanning direction. Since it is arranged in the vicinity of the scanning target, the light beam passing through the second imaging lens is also thin. For this reason, the thickness of the two

imaging lenses

41 and 42 can be reduced, and the manufacturing cost of the

imaging lenses

41 and 42 can be reduced.
[0042]
(Example 6)
FIG. 20 is a schematic diagram of a cross section of the present embodiment, where (a) shows a main scanning section and (b) shows a sub-scanning section developed along the optical axis. In the present embodiment, two rotating lens mirrors 3 having the same shape are rotationally symmetric with respect to the rotation axis and have a reflecting surface S. ₂ The rotating lens mirrors 3 are arranged so that they are in contact with each other back to back. ₁ , Injection surface S _Three Both are toric surfaces. In addition to the rotating lens mirror 3, the first imaging lens 41 and the second imaging lens 42 are provided in the vicinity of the image plane. The optical specifications of a typical design example of this embodiment are shown below.
[0043]

FIG. 21 is an aberration diagram of this design example.
[0044]
In this embodiment, by providing the second imaging lens 42 in the vicinity of the image plane, the optical magnification obtained by combining the first and second imaging lenses can be increased as compared with the case where only one imaging lens is provided. It can be made small, and the surface tilt correction effect can be enhanced from Equation (1). Further, by making the second imaging lens 42 a toric lens, as shown in FIG. 21, the amount of field curvature can be made within ± 1.4 mm, and the linear scanning performance can be reduced to 0.7% or less. A simple optical scanning device can be configured.
[0045]
The above embodiment has been described with the configuration of two rotating lens mirrors. However, as shown in FIG. 22, three or more rotating lens mirrors 3 are used and arranged at a rotationally symmetric position with respect to the rotation axis O, and scanning is performed. It is possible to further increase the efficiency. Moreover, it can also be configured with a single rotating lens mirror, and has an effect of reducing unevenness of the scanning pitch when there is a rotation failure in the driving motor of the rotating lens mirror.
[0046]
【The invention's effect】
As described above, according to the present invention, in the optical scanning device using a plurality of rotating lens mirrors, the power of the optical element in the sub-scanning direction is negative, and the surface shape of the exit refracting surface in the sub-scanning direction is concave. The surface shape of the incident refracting surface in the main scanning direction is concave or flat, and an imaging lens is disposed between the optical element and the surface to be scanned, or the power of the optical element in the sub-scanning direction is Positive, the surface shape of the incident refracting surface in the sub-scanning direction is convex, the surface shape of the exit refracting surface in the sub-scanning direction is concave, and the surface shape of the incident refracting surface in the main scanning direction is concave or flat, rotating By forming the imaging lens arranged between the lens mirror and the surface to be scanned with two lenses, it is possible to construct a small, low-cost, high-accuracy optical running device that increases scanning efficiency and reduces scanning pitch unevenness. It has the effect of being able to. Further, it is obvious that image forming apparatuses such as a laser printer, a digital copying machine, a facsimile machine, and a laser scanning display using the optical scanning apparatus of the present invention greatly contribute to the reduction in size and cost of the apparatus.
[Brief description of the drawings]
FIG. 1 is a main scanning sectional view showing a configuration of a scanning optical system of the present invention.
FIG. 2 is a perspective view showing a coordinate system of a rotating lens mirror.
FIG. 3 is a sub-scanning sectional view showing surface tilt of a rotating lens mirror.
FIGS. 4A and 4B are main scanning and sub-scanning sectional views showing surface tilt when a rotating lens mirror rotates. FIGS.
FIG. 5 is a sub-scan sectional view of a rotating lens mirror showing a first method of correcting surface tilt according to the present invention.
FIG. 6 is a sub-scan sectional view of a rotating lens mirror showing a second method of correcting surface tilt according to the present invention.
FIG. 7 is a diagram illustrating a printing simulation result when a surface tilt correction rate changes.
FIG. 8 is a schematic diagram of main scanning and sub-scanning cross sections showing the first embodiment of the present invention.
FIG. 9 is a perspective view of the optical scanning device according to the first embodiment of the present invention.
FIG. 10 is an aberration diagram of the first example of the present invention.
FIG. 11 is a schematic diagram of main scanning and sub-scanning cross sections showing a second embodiment of the present invention.
FIG. 12 is a perspective view of an optical scanning device according to a second embodiment of the present invention.
FIG. 13 is an aberration diagram of the second example of the present invention.
FIG. 14 is a schematic diagram of cross sections of main scanning and sub-scanning showing a third embodiment of the present invention.
FIG. 15 is an aberration diagram of the third example of the present invention.
FIG. 16 is a schematic diagram of cross sections of main scanning and sub-scanning showing a fourth embodiment of the present invention.
FIG. 17 is an aberration diagram of the fourth example of the present invention.
FIG. 18 is a schematic diagram of cross sections of main scanning and sub scanning showing a fifth embodiment of the present invention.
FIG. 19 is an aberration diagram of the fifth example of the present invention.
FIG. 20 is a schematic diagram of cross sections of main scanning and sub-scanning showing a sixth embodiment of the present invention.
FIG. 21 is an aberration diagram of the sixth example of the present invention.
FIG. 22 is a main scanning sectional view when a plurality of rotating lens mirrors are used.
FIG. 23 is a sub-scan sectional view showing surface tilt correction of an optical scanning device using a conventional rotary polygon mirror.
[Explanation of symbols]
1 Light source
2 Collimator lens
3 Rotating lens mirror
4 Imaging lens
5 Scanned surface
S ₁ Incident surface
S ₂ Reflective surface
S _Three Ejection surface

Claims

In an optical scanning device that deflects and scans a light beam by rotating a plurality of optical elements having an incident refracting surface, a plane reflecting surface, and an exit refracting surface,
The power in the sub-scanning direction of the optical element is negative,
Ri sub-scanning direction of the surface shape concave der of the exit refractive surface,
The surface shape in the main scanning direction of the incident refracting surface is concave or flat,
Optical scanning apparatus according to claim Rukoto imaging lens is arranged between the optical element and the surface to be scanned.

In an optical scanning device that deflects and scans a light beam by rotating a plurality of optical elements having an incident refracting surface, a plane reflecting surface, and an exit refracting surface,
The power of the optical element in the sub-scanning direction is positive,
The surface shape in the sub-scanning direction of the incident refracting surface is convex, and the surface shape in the sub-scanning direction of the exit refracting surface is concave,
The surface shape in the main scanning direction of the incident refracting surface is concave or flat,
2. An optical scanning device comprising two imaging lenses disposed between the optical element and a surface to be scanned.