JP4343466B2 - Optical scanning device and image forming apparatus using the same - Google Patents

Description

【００４５】
走査終了側７ｂの面形状は、
【００４６】
【数２】

【００４７】
で表される。
【００４８】
但し，Ｒは曲率半径、Ｋ，Ｂ₄、Ｂ₆、Ｂ₈、Ｂ₁₀は非球面係数である。係数のサフィックスsは走査開始側、サフィックスeは走査終了側を表している。
【００４９】
また副走査断面内は光軸に対して走査開始側と走査終了側で第１、第２のfθレンズ６ａ，６ｂのうち、少なくとも２面の副走査方向の曲率半径がレンズの有効部内において連続的に変化しており、また第１、第２のfθレンズ６ａ，６ｂの主走査断面内における形状を光軸に対して対称に形成している。
【００５０】
副走査断面内の形状は図２に示すように光軸に対して走査開始側と走査終了側で、光軸をＸ軸、主走査断面内において光軸と直交する方向をＹ軸、副走査断面内で光軸と直交する方向をＺ軸としたとき、以下の連続関数で表せる。
【００５１】
走査開始側７ａの面形状は
【００５２】
【数３】

【００５３】
走査終了側７ｂの面形状は
【００５４】
【数４】

【００５５】
で表される。
【００５６】
但し、ｒは副走査方向の曲率半径、Ｄ₂、Ｄ₄、Ｄ₆、Ｄ₈、Ｄ₁₀は係数である
係数のサフィックスsは走査開始側、eは走査終了側を表している。
【００５７】
尚、副走査方向の曲率半径とは主走査方向の形状(母線)に直交する断面内における曲率半径のことである。
【００５８】
本実施形態では副走査方向のピント補正(像面湾曲補正)と走査レンズ系６における副走査方向の倍率の一様性(スポット径の像高による変動)を補正するために、第１のfθレンズ６ａの両レンズ面Ｒ１、Ｒ２と第２のfθレンズ６ｂの両レンズ面Ｒ３、Ｒ４の副走査方向の曲率半径をレンズ有効部内で連続的に変化させ、かつ第１のfθレンズ６ａの射出面Ｒ２と第２のfθレンズ６ｂの射出面Ｒ４の曲率半径を光軸に対して非対称に変化させている。
【００５９】
次に本発明の目的を達成するための手段と効果について説明する。
【００６０】
本実施形態の光源手段１は前述の如く同一のチップに複数の発光部を備えたモノリシックなマルチビーム光源で構成されており、第１、第２のfθレンズ６ａ，６ｂは共にプラスチックレンズで作製されており、成形上有利なように主走査断面内の形状が、光軸に対して走査開始側と終了側で対称に形成されている。
【００６１】
更に副走査方向の曲率半径は少なくとも２面がレンズの有効部内において連続的に変化しており、これにより像面湾曲、波面収差、スポット径の変動を補正している。
【００６２】
本実施形態では光源手段１から出射した光束が主走査断面内において、光軸に対して角度α(≠０)で光偏向器５の偏向面５ａに入射しているため、該光偏向器５の回転に伴う面の出入り(サグ)が、走査開始側と終了側で非対称に発生する。この非対称なサグにより、像面湾曲、波面収差、スポット径の変動が光軸に対して主走査方向に非対称に変化するのを良好に補正するために、走査レンズ系６は副走査方向の曲率半径が光軸に対して主走査方向に沿って非対称に変化する面(以下「副走査曲率半径非対称面」と称す。)を少なくとも１面有している。
【００６３】
本実施形態では装置全体をコンパクト化するために後述する条件式（８）を満足する折り返しミラー８を走査レンズ系６と感光ドラム面７との間の光路内に配置し、光路を折りたたんで該感光ドラム面７へ光束を導光している。本実施形態ではこの折り返しミラー８を最も被走査面７側の第２のfθレンズ６ｂに近づけることにより、走査レンズ系６をコンパクトにし、画像形成装置へ配置する際の自由度を高めている。
【００６４】
また本実施形態では副走査曲率半径非対称面を最も光偏向器５側の第１のfθレンズ６ａに形成することにより、波面収差のサグによる非対称性を良好に補正している。
【００６５】
一方、副走査方向の曲率半径を少なくとも２面以上変化させないと、像面湾曲とスポット径の変動の両者を同時に補正できないので良くない。
【００６６】
表−１に本実施形態の光学パラメータを示す。図３に本実施形態の光学特性を示す。
【００６７】
【表１】

【００６８】
本実施形態の第１、第２のｆθレンズ６ａ，６ｂは共に正のメニスカス形状より成り、また第１，第２のｆθレンズ６a、６bのうち、少なくとも一面の副走査方向の曲率半径を、該レンズの有効部内において連続的、かつ光軸に対して主走査方向に非対称に変化させ、像面湾曲の非対称性とスポット径の変動を同時に補正している。
【００６９】
本実施形態では、副走査曲率半径非対称面が一面で構成されているため、主走査方向の対称軸（ｆθレンズの光軸）を被走査面７の垂直二等分線に対してシフトかつチルトしている。これにより副走査曲率半径非対称面（副走査方向の曲率半径を光軸に対して主走査方向に沿って非対称に変化させた面）を追加することと同様の効果が得られ、該副走査曲率半径非対称面を一面のみで非対称性を補正することが可能であり、レンズ成形上有利になるという特徴を有する。
【００７０】
尚、本実施形態では走査レンズ系６の光軸を主走査方向における被走査面の垂直二等分線と一致させている。
【００７１】
本実施形態では走査レンズ系６の光軸方向における光偏向器５の偏向点５ｂから被走査面７までの距離をＴ、走査レンズ系６の光軸方向における該光偏向器５の偏向点５ｂから最も被走査面７側の第２のfθレンズ６ｂのレンズ面までの距離をＳ、走査レンズ系６の副走査断面内の光軸上の横倍率をβｓ（βｓ≦０）とするとき、
Ｓ／Ｔ≦０.３ ‥‥（１）
｜βｓ｜≦３ ‥‥（２）
なる条件を満たすように各要素を設定している。
【００７２】
本実施形態では上記条件式（１）を満たすように
Ｓ／Ｔ＝０.２５
と各要素を配置し、これによりレンズバックを確保し、折り返しミラー８の配置自由度を高めると共に、走査レンズ系６のｆθレンズの主走査方向の最大有効径Ｙmaxを小さくして、コンパクト化を達成している。
【００７３】
ここでｆθレンズの主走査方向の最大有効径Ｙmaxとは、図４に示すように、被走査面の走査開始位置又は走査終了位置に向かう光線が、ｆθレンズと交わる点の光軸との距離の最大値である。
【００７４】
本実施形態ではｆθレンズの主走査方向の最大有効径Ｙmaxを、
Ｙmax≦６０ｍｍ ‥‥（３）
なる条件を満たすように設定している。
【００７５】
副走査断面内の横倍率βsは走査レンズ系６の副走査方向の前側主平面をΔ、後側主平面をΔ'、第１，第２のｆθレンズ６a、６bの肉厚を各々ｄ₁、ｄ₂、第１，第２のｆθレンズ６a、６b間の距離（レンズ間隔）をＵとするとき、
【００７６】
【数５】

【００７７】
と近似できる。そのため上式（４）から副走査横倍率|βs|を低減し、ピッチムラを小さくするためには、
(a)走査レンズ系６の副走査方向の前側主平面及び後側主平面を被走査面７側に配置する、
(b)走査レンズ系６の２枚のｆθレンズ６a、６ｂ間の距離Ｕを小さくする、ことが必要である。
【００７８】
そこで本実施形態では上記(a)を達成するために、
最も光偏向器５側の第１のｆθレンズ６ａの副走査方向の屈折力φ１、
最も被走査面７側の第２のｆθレンズ６ｂの副走査方向の屈折力φ２、
とするとき、
副走査方向の曲率半径を上記表−１の如く設定し、即ち
φ１=−０.０２２＜０
φ２=０.０３９＞０
とし、走査レンズ系６の副走査方向の後側主平面を被走査面７に近づけるよう配置することにより、副走査横倍率|βs|を
|βs|＝２.８９
として上記条件式（２）を満たしている。
【００７９】
また上記(b)を達成するために、第１、第２のｆθレンズ６a、６ｂのレンズ間の距離をＵ、光偏向器５の偏向点５ｂから最も被走査面７側の第２のｆθレンズ６ｂのレンズ面までの距離をＳとするとき、
Ｕ／Ｓ≦０.６ ‥‥（５）
なる条件を満たすように各要素を設定している。本実施形態では上記条件式（５）を満たすように、
Ｕ／Ｓ＝０.１８
と各要素を配置し、これにより副走査横倍率|βs|を低減させて、ピッチムラを小さくしている。
【００８０】
また第１、第２のｆθレンズ６a、６ｂ間の距離Ｕを短縮し、最も光偏向器５側の第１のｆθレンズ６ａを光偏向器５から遠ざけることにより、該光偏向器５の回転に伴い発生する発熱や振動がレンズ等の光学素子に与える影響(例えば、ピント変動)を軽減する効果をも有する。
【００８１】
副走査横倍率|βs|をさらに低減させるためには最も被走査面７側のｆθレンズの光偏向器５側のレンズ面が正の屈折力を有するレンズ面であることが望ましい。本実施形態では第２のｆθレンズ６ｂの副走査断面内の入射面Ｒ３を正の屈折力のレンズ面にしている。
【００８２】
また高精細な画像形成装置やカラー出力可能な画像形成装置に用いられる光走査装置においては、被走査面上でのスポット径を７０μm以下に抑えることが望ましい。そこで本実施形態では副走査断面内において被走査面７上での有効Ｆ値（有効Ｆナンバー）をＦＳｎｏとするとき、有効走査域全域において、
ＦＳｎｏ≦６５ ‥‥（６）
なる条件を満たすように各要素を設定している。
【００８３】
また主走査断面内において、被走査面７上での有効Ｆ値をＦＭｎｏとするとき、有効走査域全域において、
ＦＭｎｏ≦５５ ‥‥（７）
なる条件を満たすように各要素を設定している。
【００８４】
また本実施形態では第３の光学系６を構成する１つの光学素子としての折り返しミラー８の位置を次の如く設定している。
【００８５】
即ち、光偏向器５の偏向点５ｂから折り返しミラー８までの光軸上の距離をＰ、該折り返しミラー８から被走査面７までの距離をＱとするとき、
１．５≦Ｑ／Ｐ ‥‥（８）
なる条件を満たすように各要素を設定している。
【００８６】
一般的に光走査装置は図２に示すように光路を折り返しミラー等で折り返し、本体（画像形成装置）に配置されるため、光走査装置を小型化するためには光偏向器の偏向点から折り返しミラーまでの距離を短くする必要がある（距離Ｐを小さくする必要がある。）。また光走査装置が本体に配置されるときは他の部材、例えば現像器等を排泄するスペースを確保する必要があり、ミラーバック（Ｑ）を長くする必要がある。また折り返しミラーで折り返す角度も鈍角である方が、よりスペースを確保しやすく、また他部材との距離を確保でき、光走査装置が昇温の影響を受けづらい等のメリットがある。
【００８７】
本実施形態では上記条件式（８）を満たすように、
Ｐ＝５３、Ｑ＝１２０、Ｑ／Ｐ＝２．２６
と各要素を配置し、これにより前述した効果を得ている。
【００８８】
このように本実施形態では上述の如く各条件式を満足させ、最も被走査面側の第２のｆθレンズ６ｂを光偏向器５側に近づけることにより、折り返しミラー８を配置する際の自由度を高め、さらに第１、第２のｆθレンズ６ａ、６ｂの副走査方向の曲率半径を適切に設定することにより、副走査横倍率の絶対値を低下させ、ピッチムラの少ない高精細な光走査装置を得ている。
【００８９】
尚、本実施形態では上記の条件式（１），（５）のうち少なくとも一方を満たし、かつ条件式（２）を満たせば所望の効果は得られる。
【００９０】
また本実施形態では光源手段１としてモノリシックなマルチビーム光源を用いたが、これに限らず、例えば異なる複数の光源から出射した光束をプリズム等で合成した光源手段を用いてもよく、もちろん光源手段を単一の光源より構成しても良い。
【００９１】
また本実施形態では走査レンズ系６を２枚のレンズ６ａ，６ｂより構成したが、これに限らず、例えば単一、もしくは３枚以上で構成してもよく、さらにfθミラーや回折光学素子等の光学素子を用いても全く同様の効果を得ることができる。
【００９２】
尚、走査レンズ系６を１枚のレンズより構成した場合は、上記条件式（１），（２）を満たせば所望の効果は得られる。
【００９３】
また本実施形態においては集光レンズ２やシリンドリカルレンズ３等を省略し、光源手段１から出射した光束を開口絞り４を介して直接光偏向器５の偏向面５ａに導光してもよい。
【００９４】
［実施形態２］
図５は本発明の実施形態２の主走査方向の要部断面図（主走査断面図）である。同図において前記図１に示した要素と同一要素には同符番を付している。
【００９５】
表―２に本実施形態の光学パラメータを示す。図６に本実施形態の光学特性を示す。
【００９６】
【表２】

【００９７】
本実施形態において前述の実施形態１と異なる点は走査レンズ系１６を構成する第１、第２のｆθレンズ１６ａ，１６ｂのレンズ形状を異ならせて形成し、かつ上記の各条件式を満たすように各要素を設定したことであり、その他の構成及び光学的作用は実施形態１と略同様であり、これにより同様な効果を得ている。
【００９８】
即ち、本実施形態の第１、第２のfθレンズ１６ａ，１６ｂは共にプラスチックレンズで作製されており、主走査断面内の形状がサグ(偏向面の出入り)による被走査面７の像面湾曲、fθ特性の非対称性を良好に補正するために、走査開始側と終了側で光軸(被走査面の垂直二等分線)に対して非対称に形成されている。
【００９９】
また副走査方向の曲率半径は少なくとも２面がレンズの有効部内において連続的に変化しており、これにより像面湾曲、スポット径の変動を補正している。
【０１００】
また非対称なサグにより像面湾曲、スポット径の変動が光軸に対して非対称に変化するのを良好に補正するために、走査レンズ系１６は副走査方向の曲率半径が光軸に対して主走査方向に沿って非対称に変化する面(副走査曲率半径非対称面)を少なくとも１面有している。
【０１０１】
さらに本実施形態では前記条件式（１）を満足するように、
Ｓ／Ｔ＝０.２６
と各要素を配置し、これによりレンズバックを確保し、折り返しミラー８の配置自由度を高めると共に、走査レンズ系１６のfθレンズの主走査方向の最大有効径Ｙmaxを小さくして、コンパクト化を達成している。
【０１０２】
また本実施形態では副走査方向の曲率半径を上記表-２の如く設定し、即ち
φ１=−０.０１４＜０
φ２=０.０２５＞０
とし、走査レンズ系１６の副走査方向の後側主平面を被走査面７に近づけるよう配置することにより、副走査横倍率|βs|を
|βs|＝２.６９
として上記条件式（２）を満たしている。
【０１０３】
また本実施形態では前記条件式（５）を満たすように、
Ｕ／Ｓ＝０．３４
と各要素を配置し、これにより副走査横倍率|βs|を低減させて、ピッチムラを小さくしている。
【０１０４】
さらに本実施形態では前記条件式（８）を満たすように、
Ｐ＝８０、Ｑ＝１３１．３５、Ｑ／Ｐ＝１．６４
と各要素を配置し、これにより前述した効果を得ている。
【０１０５】
本実施形態は副走査曲率半径非対称面（子線非対称面）が２面で構成されており、レンズ作製上有利であると共に第２のfθレンズ１６ｂの主走査方向の対称軸を被走査面７の垂直二等分線に対して対称に構成しているため、取り付け誤差による性能の劣化を低減できるという特徴を有する。
【０１０６】
このように本実施形態では上述の如く各条件式を満足させ、最も被走査面７側の第２のｆθレンズ１６ｂを光偏向器５側に近づけることにより、折り返しミラー８を配置する際の自由度を高め、さらに第１、第２のｆθレンズ１６ａ、１６ｂの副走査方向の曲率半径を適切に設定することにより、副走査横倍率の絶対値を低下させ、ピッチムラの少ない高精細な光走査装置を得ている。
【０１０７】
［実施形態３］
図７は本発明の実施形態３の主走査方向の要部断面図（主走査断面図）である。同図において前記図１に示した要素と同一要素には同符番を付している。
【０１０８】
表―３に本実施形態の光学パラメータを示す。図８に本実施形態の光学特性を示す。
【０１０９】
【表３】

【０１１０】
本実施形態において前述の実施形態１、２と異なる点は走査レンズ系２６の少なくとも一面を主走査断面内又は／及び副走査断面内において回折作用を有する回折面より構成し、かつ上記の各条件式を満たすように各要素を設定したことであり、その他の構成及び光学的作用は実施形態１と略同様であり、これにより同様な効果を得ている。
【０１１１】
即ち、２６は第３の光学系としての走査レンズ系であり、第１、第２の２枚の走査レンズ２６，２６ｂを有しており、第１の走査レンズ２６ａは両レンズ面共に屈折面で、非球面形状のトーリック面より成り、第２の走査レンズ２６bは光偏向器５側が屈折面でトーリック面より成り、被走査面７側が主走査断面内又は／及び副走査断面内において回折作用を有する回折面より成っている。
【０１１２】
本実施形態の回折面は、光軸をＸ軸、主走査断面内において光軸と直交する方向をＹ軸、副走査断面内で光軸と直交する方向をＺ軸とし、以下の連続関数で表せる。
【０１１３】
【数６】

【０１１４】
(φは位相関数、λは波長、ｍは回折次数、本実施形態ではｍ＝１を使用)
本実施形態の第１、第２のｆθレンズ２６，２６ｂは共にプラスチックレンズで作製されており、成形上有利なように主走査断面内の形状が走査開始側と終了側で光軸に対して対称に構成されている。
【０１１５】
また副走査方向の曲率半径は少なくとも２面がレンズの有効部内において連続的に変化しており、これにより像面湾曲、スポット径の変動を補正している。
【０１１６】
また本実施形態では回折面の位相関数を走査開始側と走査終了側で光軸に対して非対称に変化させることで、像面湾曲、スポット径の変動が光軸に対して非対称性に変化するのを補正している。
【０１１７】
さらに本実施形態では前記条件式（１）を満たすように、
Ｓ／Ｔ＝０.３０
と各要素を配置し、これによりレンズバックを確保し、折り返しミラー８の配置自由度を高めると共に、走査レンズ系２６のｆθレンズの主走査方向の最大有効径Ｙmaxを小さくして、コンパクト化を達成している。
【０１１８】
また本実施形態では副走査方向の曲率半径を上記表-３の如く設定し、即ち
φ１=−０.００６＜０
φ２= ０.０３０＞０
とし、走査レンズ系２６の副走査方向の後側主平面を被走査面に近づけるよう配置することにより、副走査横倍率|βs|を
|βs|＝２.６１
として上記条件式（２）を満たしている。
【０１１９】
また本実施形態では前記条件式（５）を満足するように、
Ｕ／Ｓ＝０．５１
と各要素を配置し、これにより副走査横倍率|βs|を低減させて、ピッチムラを小さくしている。
【０１２０】
さらに本実施形態では前記条件式（８）を満たすように、
Ｐ＝５５．５、Ｑ＝１０４．０、Ｑ／Ｐ＝１．８７
と各要素を配置し、これにより前述した効果を得ている。
【０１２１】
このように本実施形態では上述の如く各条件式を満足させ、最も被走査面７側の第２のｆθレンズ２６ｂを光偏向器５側に近づけることにより、折り返しミラー８を配置する際の自由度を高め、さらに第１、第２のｆθレンズ２６ａ、２６ｂの副走査方向の曲率半径を適切に設定することにより、副走査横倍率の絶対値を低下させ、かつ回折光学素子を用いることにより、環境変化による光学性能の劣化の少ない高性能な光走査装置を得ることが可能である。
【０１２２】
［画像形成装置］
図９は、前述した実施形態１、２又は３の光走査装置を用いた画像形成装置（電子写真プリンタ）の実施形態を示す副走査断面内における要部断面図である。
【０１２３】
図９において、符号１０４は画像形成装置を示す。この画像形成装置１０４には、パーソナルコンピュータ等の外部機器１１７からコードデータＤｃが入力する。このコードデータＤｃは、装置内のプリンタコントローラ１１１によって、画像データ（ドットデータ）Ｄｉに変換される。この画像データＤｉは、各実施形態１、２、３で示した構成を有する光走査ユニット１００に入力される。そして、この光走査ユニット（光走査装置）１００からは、画像データＤｉに応じて変調された光ビーム（光束）１０３が射出され、この光ビーム１０３によって感光ドラム１０１の感光面が主走査方向に走査される。
【０１２４】
静電潜像担持体（感光体）たる感光ドラム１０１は、モータ１１５によって時計廻りに回転させられる。そして、この回転に伴って、感光ドラム１０１の感光面が光ビーム１０３に対して、主走査方向と直交する副走査方向に移動する。感光ドラム１０１の上方には、感光ドラム１０１の表面を一様に帯電せしめる帯電ローラ１０２が表面に当接するように設けられている。そして、帯電ローラ１０２によって帯電された感光ドラム１０１の表面に、前記光走査ユニット１００によって走査される光ビーム１０３が照射されるようになっている。
【０１２５】
先に説明したように、光ビーム１０３は、画像データＤｉに基づいて変調されており、この光ビーム１０３を照射することによって感光ドラム１０１の表面に静電潜像を形成せしめる。この静電潜像は、上記光ビーム１０３の照射位置よりもさらに感光ドラム１０１の回転断面内における下流側で感光ドラム１０１に当接するように配設された現像器１０７によってトナー像として現像される。
【０１２６】
現像器１０７によって現像されたトナー像は、感光ドラム１０１の下方で、感光ドラム１０１に対向するように配設された転写ローラ（転写器）１０８によって被転写材たる用紙１１２上に転写される。用紙１１２は感光ドラム１０１の前方（図９において右側）の用紙カセット１０９内に収納されているが、手差しでも給紙が可能である。用紙カセット１０９端部には、給紙ローラ１１０が配設されており、用紙カセット１０９内の用紙１１２を搬送路へ送り込む。
【０１２７】
以上のようにして、未定着トナー像を転写された用紙１１２はさらに感光ドラム１０１後方（図９において左側）の定着器へと搬送される。定着器は内部に定着ヒータ（図示せず）を有する定着ローラ１１３とこの定着ローラ１１３に圧接するように配設された加圧ローラ１１４とで構成されており、転写部から撒送されてきた用紙１１２を定着ローラ１１３と加圧ローラ１１４の圧接部にて加圧しながら加熱することにより用紙１１２上の未定着トナー像を定着せしめる。更に定着ローラ１１３の後方には排紙ローラ１１６が配設されており、定着された用紙１１２を画像形成装置の外に排出せしめる。
【０１２８】
図９においては図示していないが、プリントコントローラ１１１は、先に説明したデータの変換だけでなく、モータ１１５を始め画像形成装置内の各部や、光走査ユニット１００内のポリゴンモータなどの制御を行う。
【０１２９】
【発明の効果】
本発明によれば前述の如く第３の光学系（走査レンズ系）を構成する光学素子のうち最も被走査面側の光学素子を光偏向器側に近づけることにより、折り返しミラーを配置する際の自由度を高めることができ、また光学素子の副走査方向の曲率半径を適切に設定することにより、副走査横倍率の絶対値を低下させ、ピッチムラの少ない高性能でコンパクトな光走査装置及びそれを用いた画像形成装置を達成することができる。
【図面の簡単な説明】
【図１】 本発明の実施形態１の主走査断面図
【図２】 本発明の実施形態１の副走査断面図
【図３】 本発明の実施形態１の光学特性を示す図
【図４】 本発明の実施形態１のＹmaxを示す図
【図５】 本発明の実施形態２の主走査断面図
【図６】 本発明の実施形態２の光学特性を示す図
【図７】 本発明の実施形態３の主走査断面図
【図８】 本発明の実施形態３の光学特性を示す図
【図９】 本発明の光走査装置を用いた画像形成装置（電子写真プリンタ）の構成例を示す副走査断面図
【図１０】 従来の光走査装置の主走査断面図
【図１１】 従来の光走査装置の副走査断面図
【符号の説明】
１ 光源手段（半導体レーザ）
２ コリメーターレンズ
３ 絞り
４ シリンドリカルレンズ
５ 偏向手段（ポリゴンミラー）
６，１６，２６ 走査レンズ系（ｆθレンズ系）
７ 被走査面
８ 折り返しミラー
１００ 光走査装置
１０１ 感光ドラム
１０２ 帯電ローラ
１０３ 光ビーム
１０４ 画像形成装置
１０７ 現像装置
１０８ 転写ローラ
１０９ 用紙カセット
１１０ 給紙ローラ
１１１ プリンタコントローラ
１１２ 転写材（用紙）
１１３ 定着ローラ
１１４ 加圧ローラ
１１５ モータ
１１６ 排紙ローラ
１１７ 外部機器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical scanning device and an image forming apparatus using the same, and in particular, increases the degree of freedom in disposing a folding mirror, reduces the absolute value of lateral magnification in the sub-scanning direction, and causes pitch unevenness (scanning lines due to surface tilting). It is suitable for an image forming apparatus such as a laser printer, a digital copying machine, or a multifunction printer (multifunctional printer).
[0002]
[Prior art]
Conventionally, in an optical scanning device used for a laser beam printer, a digital copying machine, a multifunction printer, etc., a light beam (light beam) emitted from a light source according to an image signal is periodically generated by an optical deflector such as a rotating polygon mirror. Using a scanning lens having an fθ characteristic, spots are formed while scanning the surface of the photosensitive drum, which is the surface to be scanned, at a substantially constant speed. As a result, image information is recorded on the surface of the photosensitive drum as a recording medium.
[0003]
FIG. 10 is a sectional view (main scanning sectional view) of the main part in the main scanning direction of the conventional optical scanning device, and FIG. 11 is a sectional view (sub scanning sectional view) of the relevant part in the sub scanning direction of the conventional optical scanning apparatus.
[0004]
10 and 11, the divergent light beam emitted from the light source means 91 is changed into a substantially parallel light beam or a convergent light beam by a condenser lens (collimator lens) 92, and the light beam is limited by a diaphragm 93 and has a refractive power in the sub-scanning direction. It is incident on the cylindrical lens 94. Since the substantially parallel light beam incident on the cylindrical lens 94 is imaged only in the sub-scanning direction, it is substantially imaged on the deflection surface 95a of the optical deflector 95 as a line image that is long in the main scanning direction. Then, the light beam deflected by the deflecting surface 95a of the optical deflector 95 scans the photosensitive drum surface 97, which is the surface to be scanned, through the scanning lens system (imaging optical system) 96 having the fθ characteristic to record an image. ing.
[0005]
In this type of optical scanning device, in order to record a high-definition image,
(Ar 1) the field curvature is well corrected over the entire surface to be scanned;
(Ar 2) The spot diameter and spot shape on the scanned surface are good,
(Ar 3) It has a tilt correction function for correcting so that the positional deviation of the scanning line does not occur even when the deflecting surface of the optical deflector falls.
(Ar 4) Distortion is corrected well,
Etc. are necessary.
[0006]
Furthermore, in recent years, with the miniaturization of image forming apparatuses, the optical scanning apparatus has to be miniaturized, and the scanning lens is also required to have a compact lens shape.
[0007]
[Problems to be solved by the invention]
In order to make the scanning lens system compact, it is necessary to shorten the focal length of the scanning lens system, that is, to shorten the distance from the deflection point of the optical deflector to the surface to be scanned. When the distance is shortened, the distance from the lens surface of the scanning lens closest to the scanning surface side to the scanning surface (lens back) is shortened, and the optical path between the scanning lens surface closest to the scanning surface side and the scanning surface is reduced. When a folding mirror or the like is disposed on the screen, the degree of freedom is reduced.
[0008]
Moreover, even if the lens back can be secured to some extent, the lateral magnification in the sub-scanning direction (sub-scanning magnification) becomes high, and the deterioration of the scanning line (pitch unevenness) due to the surface tilt increases, and a high-definition image forming apparatus or There is a problem that it cannot be applied to a color image forming apparatus capable of color output.
[0009]
Thus, in recent years, it has been difficult to obtain a desired optical performance by securing a lens back and reducing a sub-scanning magnification while a compact optical scanning device is required.
[0010]
The present invention is a compact, high-quality optical scanning device with a long lens back that can secure a lens back, reduce pitch unevenness, and can satisfactorily correct variations in curvature of field, distortion, and beam diameter due to image height. And an image forming apparatus using the same.
[0011]
[Means for Solving the Problems]
The optical scanning device according to claim 1 is a light source unit, a first optical system that converts a light beam emitted from the light source unit into another light beam, a deflecting unit, and a light beam that has passed through the first optical system. And a second optical system that forms an image on the deflecting surface of the deflecting means as a long line image in the main scanning direction, and a second optical system that images the light beam deflected by the deflecting surface of the deflecting means on the scanned surface. An optical scanning device having three optical systems,
The third optical system includes a first scanning optical element and a second scanning optical element in order from the deflecting unit,
In the optical path between the second scanning optical element and the scanned surface Plane mirror Is provided Plane mirror In the sub-scan section, the angle at which the incident light beam is folded is an obtuse angle,
In the optical axis direction of the third optical system The surface to be scanned from the deflection point of the deflection means Distance to T In the optical axis direction of the third optical system The exit surface of the second scanning optical element from the deflection point of the deflection means Distance to S in the sub-scan section of the third optical system On the optical axis The lateral magnification is βs (βs ≦ 0), and the deflection point of the deflection unit in the optical axis direction of the third optical system is Plane mirror P is the distance to the optical axis direction of the third optical system. Plane mirror Q from the scanning surface to the surface to be scanned in the optical axis direction of the third optical system Distance from the first scanning optical element to the second scanning optical element U, of the first scanning optical element On the optical axis The refractive power in the sub-scanning direction is φ1, and the second scanning optical element On the optical axis When the refractive power in the sub-scanning direction is φ2,
S / T ≦ 0.3,
1.5 ≦ Q / P,
U / S ≦ 0.6,
φ1 <0, φ2> 0,
| Βs | ≦ 3
It is characterized by satisfying the following condition.
[0012]
A second aspect of the present invention is characterized in that, in the first aspect of the invention, the light source means has a plurality of light emitting portions.
[0013]
In the invention of claim 3 in the invention of claim 1 or 2, when the effective F value on the surface to be scanned is FSno in the sub-scan section, in the entire effective scanning area,
FSno ≦ 65
It is characterized by satisfying the following condition.
[0014]
The invention of claim 4 is the invention of any one of claims 1 to 3, wherein in the main scanning section, when the effective F value on the surface to be scanned is FMNO, in the entire effective scanning area,
FMNO ≦ 55
It is characterized by satisfying the following condition.
[0015]
An image forming apparatus according to a fifth aspect of the present invention comprises 4 An electrostatic latent image formed on the photoconductor by the optical scanning device according to any one of the above, a photoconductor arranged on the surface to be scanned, and a light beam scanned by the optical scanning device. The image forming apparatus includes a developing device that develops the toner image, a transfer unit that transfers the developed toner image onto the transfer material, and a fixing device that fixes the transferred toner image onto the transfer material.
[0016]
An image forming apparatus according to a sixth aspect of the present invention is the first aspect. 4 The optical scanning device according to any one of the above, and a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
FIG. 1 is a sectional view (main scanning sectional view) of the main part in the main scanning direction according to Embodiment 1 of the present invention, and FIG. 2 is a sectional view (sub scanning sectional view) of the relevant part in the sub scanning direction according to Embodiment 1 of the present invention. is there.
[0035]
In this specification, the surface formed by the optical axis of the scanning lens system and the light beam deflected by the optical deflector is the main scanning section, and the surface including the optical axis of the scanning lens system and orthogonal to the main scanning section is sub-scanned. It is defined as a cross section.
[0036]
1 and 2, reference numeral 1 denotes a light source means, which comprises a monolithic multi-beam light source having a plurality of light emitting portions on the same chip. Reference numeral 2 denotes a condensing lens (collimator lens) as a first optical system, which converts a plurality of divergent light beams emitted from the light source means 1 into substantially parallel light beams or convergent light beams. A cylindrical lens 3 as a second optical system has a predetermined power only in the sub-scanning direction, and a light beam converted by the condensing lens 2 of an optical deflector 5 described later in the sub-scan section. A substantially linear image is formed on the deflection surface (reflection surface) 5a. Reference numeral 4 denotes an aperture stop which shapes the beam shape by limiting the passing light flux. Each element such as the collimator lens 2, the cylindrical lens 3, and the aperture stop 4 constitutes an element of incident optical means.
[0037]
An optical deflector 5 as a deflecting means is composed of, for example, a four-sided polygon mirror (rotating polygon mirror), and is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor. ing.
[0038]
Reference numeral 6 denotes a scanning lens system (fθ lens system) as a third optical system having a condensing function and an fθ characteristic, and the first and second fθ sheets made of a plastic material in order from the optical deflector 5 side. lens (Scanning optical element) 6a and 6b and a folding mirror 8, and a light beam based on the image information reflected and deflected by the optical deflector 5 is imaged in a spot shape on the photosensitive drum surface 7 as a surface to be scanned, and in the sub-scanning section. In FIG. 3, the tilting function is provided by providing a conjugate relationship between the deflecting surface 5a of the optical deflector 5 and the photosensitive drum surface 7.
[0039]
In the present embodiment, of the two fθ lenses 6a and 6b constituting the scanning lens system 6, at least one fθ lens has an aspheric surface (toric surface) in the main scanning section. The scanning lens system 6 has an fθ lens having a negative refractive power in the sub-scan section, and further has an fθ lens having a convex surface facing the deflecting surface.
[0040]
Reference numeral 7 denotes a photosensitive drum surface as a surface to be scanned. Reference numeral 8 denotes a folding mirror, which is a flat mirror. The folding mirror 8 may have refractive power in the main scanning section and / or in the sub-scanning section.
[0041]
In the present embodiment, a plurality of divergent light beams emitted from the multi-beam light source 1 are converted into substantially parallel light beams or convergent light beams by the condenser lens 2 and are incident on the cylindrical lens 3. Out of the light beam incident on the cylindrical lens 3, it exits as it is in the main scanning section and passes through the aperture stop 4 (partially shielded). In the sub-scan section, the light beam converges and passes through the aperture stop 4 (partially shielded) to form an almost linear image (a linear image longitudinal in the main scanning direction) on the deflecting surface 5a of the optical deflector 5. The plurality of light beams reflected and deflected by the deflecting surface 5a of the optical deflector 5 are imaged in the form of spots on the photosensitive drum surface 7 by the first and second fθ lenses 6a and 6b via the folding mirror 8. By rotating the optical deflector 5 in the direction of arrow A, optical scanning is performed on the photosensitive drum surface 7 in the direction of arrow B (main scanning direction) at a constant speed. As a result, an image is recorded on the photosensitive drum surface 7 as a recording medium.
[0042]
In the present embodiment, the shapes of the first and second fθ lenses 6a and 6b constituting the scanning lens system 6 are expressed by the following function.
[0043]
For example, the intersection point between the first and second fθ lenses 6a and 6b and the optical axis is the origin, and the surface in the main scanning section on the scanning start side 7a and the scanning end side 7b with respect to the optical axis as shown in FIG. When the X axis is the optical axis, the Y axis is the direction orthogonal to the optical axis in the main scanning section, and the Z axis is the direction orthogonal to the optical axis in the sub scanning section,
The surface shape of the scanning start side 7a is
[0044]
[Expression 1]

[0045]
The surface shape of the scanning end side 7b is:
[0046]
[Expression 2]

[0047]
It is represented by
[0048]
Where R is the radius of curvature, K, B _Four , B ₆ , B ₈ , B _Ten Is the aspheric coefficient. The coefficient suffix s represents the scanning start side, and the suffix e represents the scanning end side.
[0049]
In the sub-scan section, the curvature radius in the sub-scan direction of at least two surfaces of the first and second fθ lenses 6a and 6b is continuous in the effective portion of the lens on the scan start side and the scan end side with respect to the optical axis. The first and second fθ lenses 6a and 6b are formed symmetrically with respect to the optical axis in the main scanning section.
[0050]
As shown in FIG. 2, the shape in the sub-scan section is the X axis on the scan start side and the scan end side with respect to the optical axis, the Y axis is the direction perpendicular to the optical axis in the main scan section, and the sub scan. When the direction perpendicular to the optical axis in the cross section is taken as the Z axis, it can be expressed by the following continuous function.
[0051]
The surface shape of the scanning start side 7a is
[0052]
[Equation 3]

[0053]
The surface shape of the scanning end side 7b is
[0054]
[Expression 4]

[0055]
It is represented by
[0056]
Where r is the radius of curvature in the sub-scanning direction, and D ₂ , D _Four , D ₆ , D ₈ , D _Ten Is a coefficient
The coefficient suffix s represents the scanning start side, and e represents the scanning end side.
[0057]
The curvature radius in the sub-scanning direction is a curvature radius in a cross section perpendicular to the shape (bus line) in the main scanning direction.
[0058]
In the present embodiment, the first fθ is corrected in order to correct focus correction (field curvature correction) in the sub-scanning direction and uniformity of magnification in the sub-scanning direction in the scanning lens system 6 (variation due to image height of the spot diameter). The curvature radii in the sub-scanning direction of both lens surfaces R1, R2 of the lens 6a and both lens surfaces R3, R4 of the second fθ lens 6b are continuously changed within the lens effective portion, and the first fθ lens 6a is emitted. The curvature radii of the surface R2 and the exit surface R4 of the second fθ lens 6b are changed asymmetrically with respect to the optical axis.
[0059]
Next, means and effects for achieving the object of the present invention will be described.
[0060]
The light source means 1 of the present embodiment is composed of a monolithic multi-beam light source having a plurality of light emitting portions on the same chip as described above, and the first and second fθ lenses 6a and 6b are both made of plastic lenses. The shape in the main scanning section is formed symmetrically on the scanning start side and the end side with respect to the optical axis so as to be advantageous in forming.
[0061]
Further, at least two surfaces of the radius of curvature in the sub-scanning direction continuously change within the effective portion of the lens, thereby correcting field curvature, wavefront aberration, and spot diameter variation.
[0062]
In the present embodiment, the light beam emitted from the light source means 1 is incident on the deflection surface 5a of the optical deflector 5 at an angle α (≠ 0) with respect to the optical axis in the main scanning section. The entry / exit (sag) of the surface accompanying the rotation of the lens occurs asymmetrically on the scanning start side and the end side. The scanning lens system 6 has a curvature in the sub-scanning direction in order to satisfactorily correct the variation in the field curvature, wavefront aberration, and spot diameter in the main scanning direction with respect to the optical axis. It has at least one surface whose radius changes asymmetrically along the main scanning direction with respect to the optical axis (hereinafter referred to as “sub-scanning curvature radius asymmetric surface”).
[0063]
In this embodiment, in order to make the entire apparatus compact, a folding mirror 8 that satisfies conditional expression (8) described later is disposed in the optical path between the scanning lens system 6 and the photosensitive drum surface 7, and the optical path is folded. The light beam is guided to the photosensitive drum surface 7. In this embodiment, the folding mirror 8 is moved closer to the second fθ lens 6b closest to the surface to be scanned 7, thereby making the scanning lens system 6 compact and increasing the degree of freedom in arranging it in the image forming apparatus.
[0064]
Further, in this embodiment, the asymmetry due to the sag of the wavefront aberration is satisfactorily corrected by forming the sub-scanning curvature radius asymmetric surface on the first fθ lens 6a closest to the optical deflector 5.
[0065]
On the other hand, unless the curvature radius in the sub-scanning direction is changed by at least two or more surfaces, both the curvature of field and the variation of the spot diameter cannot be corrected at the same time.
[0066]
Table 1 shows the optical parameters of the present embodiment. FIG. 3 shows the optical characteristics of this embodiment.
[0067]
[Table 1]

[0068]
The first and second fθ lenses 6a and 6b of the present embodiment are both of a positive meniscus shape, and at least one of the first and second fθ lenses 6a and 6b has a radius of curvature in the sub-scanning direction. In the effective portion of the lens, it is continuously changed and asymmetric in the main scanning direction with respect to the optical axis to simultaneously correct the asymmetry of the field curvature and the variation of the spot diameter.
[0069]
In this embodiment, since the sub-scanning curvature asymmetric surface is composed of one surface, the axis of symmetry in the main scanning direction (the optical axis of the fθ lens) is shifted and tilted with respect to the perpendicular bisector of the surface to be scanned 7. is doing. Thus, an effect similar to that of adding a sub-scanning curvature radius asymmetric surface (a surface in which the curvature radius in the sub-scanning direction is changed asymmetrically with respect to the optical axis along the main scanning direction) can be obtained. It is possible to correct asymmetry with only one radial asymmetric surface, which is advantageous in terms of lens molding.
[0070]
In the present embodiment, the optical axis of the scanning lens system 6 is aligned with the perpendicular bisector of the surface to be scanned in the main scanning direction.
[0071]
In this embodiment In the optical axis direction of the scanning lens system 6 From the deflection point 5b of the optical deflector 5 to the scanned surface 7 Distance of T, In the optical axis direction of the scanning lens system 6 From the deflection point 5b of the optical deflector 5 to the lens surface of the second fθ lens 6b closest to the scanned surface 7 Distance of S in the sub-scan section of the scanning lens system 6 On the optical axis When the lateral magnification is βs (βs ≦ 0),
S / T ≦ 0.3 (1)
| Βs | ≦ 3 (2)
Each element is set to satisfy the following condition.
[0072]
In the present embodiment, the conditional expression (1) is satisfied.
S / T = 0.25
These elements are arranged, thereby securing a lens back, increasing the degree of freedom of arrangement of the folding mirror 8, and reducing the maximum effective diameter Ymax in the main scanning direction of the fθ lens of the scanning lens system 6, thereby reducing the size. Have achieved.
[0073]
Here, the maximum effective diameter Ymax of the fθ lens in the main scanning direction is the distance from the optical axis at the point where the light beam traveling toward the scanning start position or scanning end position of the surface to be scanned intersects with the fθ lens, as shown in FIG. Is the maximum value.
[0074]
In this embodiment, the maximum effective diameter Ymax of the fθ lens in the main scanning direction is
Ymax ≦ 60mm (3)
Is set to satisfy the following condition.
[0075]
The lateral magnification βs in the sub-scanning section is Δ for the front main plane in the sub-scanning direction of the scanning lens system 6, Δ ′ for the rear main plane, and d for the thicknesses of the first and second fθ lenses 6a and 6b. ₁ , D ₂ When the distance (lens interval) between the first and second fθ lenses 6a and 6b is U,
[0076]
[Equation 5]

[0077]
Can be approximated. Therefore, in order to reduce the sub-scanning lateral magnification | βs | from the above equation (4) and reduce the pitch unevenness,
(a) The front main plane and the rear main plane in the sub-scanning direction of the scanning lens system 6 are arranged on the scanned surface 7 side.
(b) It is necessary to reduce the distance U between the two fθ lenses 6a and 6b of the scanning lens system 6.
[0078]
Therefore, in the present embodiment, in order to achieve the above (a),
Refracting power φ1 in the sub-scanning direction of the first fθ lens 6a closest to the optical deflector 5,
Refracting power φ2 in the sub-scanning direction of the second fθ lens 6b closest to the scanned surface 7;
And when
The curvature radius in the sub-scanning direction is set as shown in Table 1 above.
φ1 = −0.022 <0
φ2 = 0.039> 0
By arranging the rear main plane of the scanning lens system 6 in the sub-scanning direction so as to approach the scanned surface 7, the sub-scanning lateral magnification | βs |
| βs | = 2.89
The above conditional expression (2) is satisfied.
[0079]
In order to achieve the above (b), the distance between the lenses of the first and second fθ lenses 6a and 6b is U, and the second fθ closest to the scanned surface 7 from the deflection point 5b of the optical deflector 5 is used. When the distance to the lens surface of the lens 6b is S,
U / S ≦ 0.6 (5)
Each element is set to satisfy the following condition. In the present embodiment, so as to satisfy the conditional expression (5),
U / S = 0.18
Each element is arranged, thereby reducing the sub-scanning lateral magnification | βs | and reducing the pitch unevenness.
[0080]
Further, by shortening the distance U between the first and second fθ lenses 6a and 6b and moving the first fθ lens 6a closest to the optical deflector 5 away from the optical deflector 5, the rotation of the optical deflector 5 is achieved. This also has the effect of reducing the influence (for example, focus variation) on the optical element such as a lens caused by the heat generation and vibration generated with the lens.
[0081]
In order to further reduce the sub-scanning lateral magnification | βs |, it is desirable that the lens surface on the optical deflector 5 side of the fθ lens closest to the scanned surface 7 is a lens surface having a positive refractive power. In the present embodiment, the incident surface R3 in the sub-scan section of the second fθ lens 6b is a lens surface having a positive refractive power.
[0082]
Further, in an optical scanning device used in a high-definition image forming apparatus or an image forming apparatus capable of color output, it is desirable to suppress the spot diameter on the surface to be scanned to 70 μm or less. Therefore, in this embodiment, when the effective F value (effective F number) on the surface to be scanned 7 is FSno in the sub-scan section,
FSno ≦ 65 (6)
Each element is set to satisfy the following condition.
[0083]
In the main scanning section, when the effective F value on the scanned surface 7 is FMNO, in the entire effective scanning area,
FMNO ≦ 55 (7)
Each element is set to satisfy the following condition.
[0084]
In this embodiment, the position of the folding mirror 8 as one optical element constituting the third optical system 6 is set as follows.
[0085]
That is, when the distance on the optical axis from the deflection point 5b of the optical deflector 5 to the folding mirror 8 is P and the distance from the folding mirror 8 to the scanned surface 7 is Q,
1.5 ≦ Q / P (8)
Each element is set to satisfy the following condition.
[0086]
In general, the optical scanning device is configured by folding the optical path with a folding mirror or the like as shown in FIG. 2 and arranged in the main body (image forming apparatus). Therefore, in order to reduce the size of the optical scanning device, the deflection point of the optical deflector is used. It is necessary to shorten the distance to the folding mirror (the distance P needs to be reduced). Further, when the optical scanning device is disposed in the main body, it is necessary to secure a space for excreting other members, such as a developing device, and it is necessary to lengthen the mirror back (Q). Further, when the angle turned back by the turning mirror is an obtuse angle, it is easier to secure a space, and it is possible to secure a distance from other members, and there is an advantage that the optical scanning device is not easily affected by the temperature rise.
[0087]
In the present embodiment, so as to satisfy the conditional expression (8),
P = 53, Q = 120, Q / P = 2.26
Each element is arranged to obtain the effect described above.
[0088]
As described above, in this embodiment, each conditional expression is satisfied as described above, and the degree of freedom in disposing the folding mirror 8 by bringing the second fθ lens 6b closest to the scanning surface closer to the optical deflector 5 side. And further, by appropriately setting the curvature radii in the sub-scanning direction of the first and second fθ lenses 6a and 6b, the absolute value of the sub-scanning lateral magnification is reduced, and a high-definition optical scanning device with less pitch unevenness Have gained.
[0089]
In the present embodiment, a desired effect can be obtained if at least one of the conditional expressions (1) and (5) is satisfied and the conditional expression (2) is satisfied.
[0090]
In the present embodiment, a monolithic multi-beam light source is used as the light source unit 1. However, the present invention is not limited to this. For example, a light source unit that combines light beams emitted from a plurality of different light sources with a prism or the like may be used. May be composed of a single light source.
[0091]
In this embodiment, the scanning lens system 6 is composed of two lenses 6a and 6b. However, the present invention is not limited to this. For example, the scanning lens system 6 may be composed of a single lens or three or more lenses. Even if this optical element is used, the same effect can be obtained.
[0092]
When the scanning lens system 6 is composed of a single lens, a desired effect can be obtained if the conditional expressions (1) and (2) are satisfied.
[0093]
In the present embodiment, the condensing lens 2 and the cylindrical lens 3 may be omitted, and the light beam emitted from the light source means 1 may be directly guided to the deflecting surface 5 a of the optical deflector 5 through the aperture stop 4.
[0094]
[Embodiment 2]
FIG. 5 is a sectional view (main scanning sectional view) of the main part in the main scanning direction according to the second embodiment of the present invention. In the figure, the same elements as those shown in FIG.
[0095]
Table-2 shows the optical parameters of this embodiment. FIG. 6 shows the optical characteristics of this embodiment.
[0096]
[Table 2]

[0097]
In this embodiment, the difference from the first embodiment is that the first and second fθ lenses 16a and 16b constituting the scanning lens system 16 are formed with different lens shapes and satisfy the above conditional expressions. The other elements and optical functions are substantially the same as those of the first embodiment, and the same effects are obtained.
[0098]
That is, the first and second fθ lenses 16a and 16b of the present embodiment are both made of plastic lenses, and the curved surface of the surface to be scanned 7 has a sag (the entrance and exit of the deflection surface) in the main scanning section. In order to satisfactorily correct the asymmetry of the fθ characteristics, they are formed asymmetrically with respect to the optical axis (vertical bisector of the surface to be scanned) on the scanning start side and the end side.
[0099]
Further, at least two surfaces of the radius of curvature in the sub-scanning direction continuously change in the effective portion of the lens, thereby correcting field curvature and spot diameter variation.
[0100]
In addition, the scanning lens system 16 has a curvature radius in the sub-scanning direction with respect to the optical axis in order to satisfactorily correct the variation of the field curvature and spot diameter due to the asymmetric sag. It has at least one surface (sub-scanning curvature radius asymmetric surface) that changes asymmetrically along the scanning direction.
[0101]
Furthermore, in this embodiment, so as to satisfy the conditional expression (1),
S / T = 0.26
These elements are arranged, thereby securing a lens back, increasing the degree of freedom of arrangement of the folding mirror 8, and reducing the maximum effective diameter Ymax in the main scanning direction of the fθ lens of the scanning lens system 16, thereby reducing the size. Have achieved.
[0102]
In this embodiment, the radius of curvature in the sub-scanning direction is set as shown in Table 2 above.
φ1 = −0.014 <0
φ2 = 0.025> 0
By arranging the rear main plane of the scanning lens system 16 in the sub-scanning direction so as to be close to the scanned surface 7, the sub-scanning lateral magnification | βs |
| βs | = 2.69
The above conditional expression (2) is satisfied.
[0103]
In this embodiment, so as to satisfy the conditional expression (5),
U / S = 0.34
Each element is arranged, thereby reducing the sub-scanning lateral magnification | βs | and reducing the pitch unevenness.
[0104]
Furthermore, in this embodiment, so as to satisfy the conditional expression (8),
P = 80, Q = 131.35, Q / P = 1.64
Each element is arranged to obtain the effect described above.
[0105]
In the present embodiment, the sub-scanning radius-of-curvature asymmetric surface (sub-wire asymmetric surface) is composed of two surfaces, which is advantageous in terms of lens fabrication, and the axis of symmetry of the second fθ lens 16b in the main scanning direction is the surface to be scanned 7. Since it is configured symmetrically with respect to the vertical bisector, it is possible to reduce deterioration in performance due to mounting errors.
[0106]
As described above, in the present embodiment, each conditional expression is satisfied as described above, and the second fθ lens 16b closest to the scanned surface 7 is brought closer to the optical deflector 5 side, thereby allowing freedom when the folding mirror 8 is arranged. By increasing the degree of curvature and appropriately setting the curvature radii of the first and second fθ lenses 16a and 16b in the sub-scanning direction, the absolute value of the sub-scanning lateral magnification is reduced and high-definition optical scanning with little pitch unevenness is achieved. I'm getting a device.
[0107]
[Embodiment 3]
FIG. 7 is a sectional view (main scanning sectional view) of the main part in the main scanning direction of Embodiment 3 of the present invention. In the figure, the same elements as those shown in FIG.
[0108]
Table 3 shows the optical parameters of this embodiment. FIG. 8 shows the optical characteristics of this embodiment.
[0109]
[Table 3]

[0110]
The present embodiment is different from the first and second embodiments described above in that at least one surface of the scanning lens system 26 is constituted by a diffractive surface having a diffractive action in the main scanning section and / or the sub-scanning section, and each of the above conditions. This is that each element is set so as to satisfy the formula, and other configurations and optical actions are substantially the same as those in the first embodiment, thereby obtaining the same effects.
[0111]
That is, reference numeral 26 denotes a scanning lens system as a third optical system, which has first and second scanning lenses 26 and 26b. The first scanning lens 26a has a refractive surface on both lens surfaces. The second scanning lens 26b has a refractive surface on the optical deflector 5 side and a toric surface, and the scanned surface 7 side has a diffractive action in the main scanning section and / or in the sub-scanning section. And a diffractive surface.
[0112]
The diffraction surface of the present embodiment has an optical axis as an X axis, a direction perpendicular to the optical axis in the main scanning section as a Y axis, and a direction perpendicular to the optical axis in the sub-scanning section as a Z axis. I can express.
[0113]
[Formula 6]

[0114]
(φ is a phase function, λ is a wavelength, m is a diffraction order, and m = 1 is used in this embodiment)
The first and second fθ lenses 26 and 26b of the present embodiment are both made of plastic lenses, and the shape in the main scanning section is relative to the optical axis on the scanning start side and the end side so as to be advantageous for molding. It is configured symmetrically.
[0115]
Further, at least two surfaces of the radius of curvature in the sub-scanning direction continuously change in the effective portion of the lens, thereby correcting field curvature and spot diameter variation.
[0116]
Further, in this embodiment, by changing the phase function of the diffractive surface asymmetrically with respect to the optical axis on the scanning start side and the scanning end side, the curvature of field and the variation in spot diameter change asymmetrically with respect to the optical axis. Is corrected.
[0117]
Furthermore, in this embodiment, so as to satisfy the conditional expression (1),
S / T = 0.30
These elements are arranged, thereby securing a lens back, increasing the degree of freedom of arrangement of the folding mirror 8, and reducing the maximum effective diameter Ymax of the fθ lens of the scanning lens system 26 in the main scanning direction, thereby reducing the size. Have achieved.
[0118]
In this embodiment, the radius of curvature in the sub-scanning direction is set as shown in Table 3 above.
φ1 = −0.006 <0
φ2 = 0.030> 0
By arranging the rear main plane of the scanning lens system 26 in the sub-scanning direction so as to be close to the surface to be scanned, the sub-scanning lateral magnification | βs |
| βs | = 2.61
The above conditional expression (2) is satisfied.
[0119]
In this embodiment, so as to satisfy the conditional expression (5),
U / S = 0.51
Each element is arranged, thereby reducing the sub-scanning lateral magnification | βs | and reducing the pitch unevenness.
[0120]
Furthermore, in this embodiment, so as to satisfy the conditional expression (8),
P = 55.5, Q = 104.0, Q / P = 1.87
Each element is arranged to obtain the effect described above.
[0121]
As described above, in the present embodiment, each conditional expression is satisfied as described above, and the second fθ lens 26b closest to the scanning surface 7 is brought closer to the optical deflector 5 side, so that the freedom in arranging the folding mirror 8 can be improved. By increasing the degree of curvature and appropriately setting the curvature radii of the first and second fθ lenses 26a and 26b in the sub-scanning direction, the absolute value of the sub-scanning lateral magnification is reduced and a diffractive optical element is used. Thus, it is possible to obtain a high-performance optical scanning device with little deterioration in optical performance due to environmental changes.
[0122]
[Image forming apparatus]
FIG. 9 is a cross-sectional view of an essential part in a sub-scanning cross section showing an embodiment of an image forming apparatus (electrophotographic printer) using the optical scanning device of the first, second or third embodiment described above.
[0123]
In FIG. 9, reference numeral 104 denotes an image forming apparatus. Code data Dc is input to the image forming apparatus 104 from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) Di by a printer controller 111 in the apparatus. The image data Di is input to the optical scanning unit 100 having the configuration shown in the first, second, and third embodiments. The light scanning unit (light scanning device) 100 emits a light beam (light beam) 103 modulated in accordance with the image data Di, and the light beam 103 causes the photosensitive surface of the photosensitive drum 101 to move in the main scanning direction. Scanned.
[0124]
The photosensitive drum 101 serving as an electrostatic latent image carrier (photoconductor) is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103. Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to contact the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100.
[0125]
As described above, the light beam 103 is modulated based on the image data Di, and by irradiating the light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101. This electrostatic latent image is developed as a toner image by a developing device 107 disposed so as to abut on the photosensitive drum 101 further downstream in the rotational section of the photosensitive drum 101 than the irradiation position of the light beam 103. .
[0126]
The toner image developed by the developing unit 107 is transferred onto a sheet 112 as a transfer material by a transfer roller (transfer unit) 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. The paper 112 is stored in the paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 9), but can be fed manually. A paper feed roller 110 is provided at the end of the paper cassette 109, and feeds the paper 112 in the paper cassette 109 into the transport path.
[0127]
As described above, the sheet 112 on which the unfixed toner image has been transferred is further conveyed to a fixing device behind the photosensitive drum 101 (left side in FIG. 9). The fixing device includes a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113 and has been fed from a transfer unit. The unfixed toner image on the sheet 112 is fixed by heating the sheet 112 while being pressed by the pressure contact portion between the fixing roller 113 and the pressure roller 114. Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and the fixed paper 112 is discharged out of the image forming apparatus.
[0128]
Although not shown in FIG. 9, the print controller 111 controls not only the data conversion described above but also each part in the image forming apparatus including the motor 115 and the polygon motor in the optical scanning unit 100. Do.
[0129]
【The invention's effect】
According to the present invention, as described above, the optical element constituting the third optical system (scanning lens system) is moved closer to the optical deflector side by bringing the optical element closest to the scanned surface closer to the optical deflector side. It is possible to increase the degree of freedom, and by appropriately setting the radius of curvature of the optical element in the sub-scanning direction, the absolute value of the sub-scanning lateral magnification is reduced, and a high-performance and compact optical scanning device with little pitch unevenness and the same An image forming apparatus using can be achieved.
[Brief description of the drawings]
FIG. 1 is a main scanning sectional view of Embodiment 1 of the present invention.
FIG. 2 is a sub-scan sectional view of Embodiment 1 of the present invention.
FIG. 3 is a diagram showing optical characteristics of Embodiment 1 of the present invention.
FIG. 4 is a diagram showing Ymax according to the first embodiment of the present invention.
FIG. 5 is a main scanning sectional view of Embodiment 2 of the present invention.
FIG. 6 is a diagram showing optical characteristics of Embodiment 2 of the present invention.
FIG. 7 is a main scanning sectional view of Embodiment 3 of the present invention.
FIG. 8 is a diagram showing optical characteristics of Embodiment 3 of the present invention.
FIG. 9 is a sub-scanning sectional view showing a configuration example of an image forming apparatus (electrophotographic printer) using the optical scanning device of the present invention.
FIG. 10 is a main scanning sectional view of a conventional optical scanning device.
FIG. 11 is a sub-scan sectional view of a conventional optical scanning device.
[Explanation of symbols]
1 Light source means (semiconductor laser)
2 Collimator lens
3 Aperture
4 Cylindrical lens
5 Deflection means (polygon mirror)
6, 16, 26 Scanning lens system (fθ lens system)
7 Scanned surface
8 Folding mirror
100 Optical scanning device
101 Photosensitive drum
102 Charging roller
103 Light beam
104 Image forming apparatus
107 Developing device
108 Transfer roller
109 Paper cassette
110 Paper feed roller
111 Printer controller
112 Transfer material (paper)
113 Fixing roller
114 Pressure roller
115 motor
116 Paper discharge roller
117 External equipment

Claims (6)

Light source means, a first optical system that converts a light beam emitted from the light source means into another light beam, a deflecting means, and a light beam that has passed through the first optical system is scanned on the deflection surface of the deflecting means. An optical scanning device comprising: a second optical system that forms a line image that is long in the direction; and a third optical system that forms an image of the light beam deflected by the deflecting surface of the deflecting unit on the surface to be scanned. Because
The third optical system includes a first scanning optical element and a second scanning optical element in order from the deflecting unit,
Plane mirror provided in an optical path between the surface to be scanned and the second scanning optical element, said plane mirror, in the sub-scan section, the angle of folding the light beam incident is obtuse,
The distance from the deflection point of the deflection unit in the optical axis direction of the third optical system to the scanned surface is T, and the second point from the deflection point of the deflection unit in the optical axis direction of the third optical system . The distance to the exit surface of the scanning optical element is S, the lateral magnification on the optical axis in the sub-scan section of the third optical system is βs (βs ≦ 0), and the optical axis direction of the third optical system is the optical axis direction. The distance from the deflection point of the deflecting means to the plane mirror is P, the distance from the plane mirror to the scanned surface in the optical axis direction of the third optical system is Q, the optical axis direction of the third optical system The distance from the first scanning optical element to the second scanning optical element is U, the refractive power in the sub-scanning direction on the optical axis of the first scanning optical element is φ1, and the second scanning optical element When the refractive power in the sub-scanning direction on the optical axis is φ2,
S / T ≦ 0.3,
1.5 ≦ Q / P,
U / S ≦ 0.6,
φ1 <0, φ2> 0,
| Βs | ≦ 3
An optical scanning device characterized by satisfying the following condition. The optical scanning device according to claim 1, wherein the light source unit includes a plurality of light emitting units. When the effective F value on the surface to be scanned is FSno in the sub-scanning cross section, in the entire effective scanning area,
FSno ≦ 65
The optical scanning device according to claim 1 , wherein the following condition is satisfied. In the main scanning section, when the effective F value on the surface to be scanned is FMNO, in the entire effective scanning area,
FMNO ≦ 55
The optical scanning device according to any one of claims 1 to 3, wherein the condition is satisfied to become. An optical scanning apparatus according to any one of claims 1 to 4, a photosensitive member disposed on said surface to be scanned, which is formed on said photosensitive member by a light beam scanned by said optical scanning device A developing unit that develops an electrostatic latent image as a toner image, a transfer unit that transfers the developed toner image onto a transfer material, and a fixing unit that fixes the transferred toner image onto the transfer material. An image forming apparatus. Wherein the optical scanning apparatus according to any one of claims 1 to 4, that converts code data inputted from an external device into an image signal and a printer controller for inputting the imagewise signal into said optical scanning device An image forming apparatus.

Images